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This is to certify that the
dissertation entitled

THE DEVELOPMENT OF MnOz COATED CERAMIC .
MEMBRANES FOR COMBINED CATALYTIC OZONATION AND
ULTRAFILTRATION OF DRINKING WATER

presented by

Lindsay Marie Corneal

has been accepted towards fulﬁllment
of the requirements for the

Ph.D. degree in Materials Science and
Engineering

 

 

 

MSU Is an Afﬂn'native Action/Equal Opportunity Employer

 

 

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.
MAY BE RECALLED with earlier due date if requested.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5/08 K:IProilec&Pres/ClRC/DateDue.indd

THE DEVELOPMENT OF Mn02 COATED CERAMIC MEMBRANES FOR
COMBINED CATALYTIC OZONATION AND ULTRAFILTRATION OF
DRINKING WATER
By

Lindsay Marie Corneal

A DISSERTATION
Submitted to
Michigan State University
in partial fulﬁllment Of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Materials Science and Engineering

2010

ABSTRACT

THE DEVELOPMENT OF Mn02 COATED CERAMIC MEMBRANES FOR
COMBINED CATALYTIC OZONATION AND ULTRAFILTRATION OF
DRINKING WATER
BY

Lindsay Marie Corneal

A novel method for the preparation Of hydrated Mn02 by the ozonation Of MnCl2

in water is described. The hydrated MnOz was used tO coat titania water ﬁltration
membranes using a layer-by-layer technique. The coated membranes were then

sintered in air at 500°C for 45 minutes. Upon sintering, the Mn02 is converted to

a-Mn203 (as characterized by x-ray and electron diffraction).

Atomic force microscopy (AFM) imaging showed no signiﬁcant change in the
roughness or height Of the surface features Of coated membranes, while
scanning electron miCroscopy (SEM) imaging showed an increase in grain size
with increasing number Of coating layers. Energy dispersive x-ray spectroscopy
(EDS) mapping and line scans revealed manganese present throughout the
membrane, indicating that manganese dispersed into the porous membrane
during the coating process and diffused into the titania grains during sintering.

Selected area diffraction (SAD) of the coated and sintered membrane was used
to index the surface layer as a-Mn203. The surface layer was uneven, although

there was a trend of increasing thickness with increasing coating layers.

The coating acts as a catalyst for the oxidation Of organic matter when coated
membranes are used in a hybrid ozonation-membrane ﬁltration system. A trend
of decreasing total organic carbon (TOC) in the permeate water was observed
with increasing number of coating layers. The catalytic activity also manifests
itself as improved recovery of the water ﬂux due to oxidation of foulants on the

membrane surface.

Ceramic nanoparticle coatings on ceramic water ﬁltration membranes must
undergo high temperature sintering. However, this means that the underlying
membrane, which has been engineered for a given molecular weight cut-Off
(MWCO), also undergoes a high temperature heat treatment that serves to
increase pore size that have resulted in increases in permeability Of titania
membranes. Coating the titania membrane with manganese oxide followed by
sintering in air at 500°C maintains the MWCO of the membranes, with high DI

water permeability, which may be favorable in terms of membrane use.

SEM micrographs of titania membrane samples sintered between 500°C to
900°C were analyzed to identify a statistically signiﬁcant increase in grain size
with increasing sintering temperature. The grains however, generally retain a
uniform shape until the 900°C sintering temperature, where large, irregularly
shaped grains were observed. AFM analysis showed a corresponding increase in

the surface roughness Of the membrane for the sample sintered at 900°C.

To Chris and Jacquie, for all of your love and support.

iv

ACKNOWLEDGMENTS

I have many people that I would like to thank for their help and support during my
doctoral studies. First, I would like to thank my advisor, Dr. Melissa Baumann
and the members of my guidance committee, Dr. Eldon Case, Dr. Susan Masten

and Dr. Volodymyr Tarabara, for their help, advice, time and support.

I could not have completed my research without the ﬁnancial support of the US
National Science Foundation (Grant NO. 0506828) Nanoscale Interdisciplinary
Research Team (NIRT) program. The other members of this NIRT group have
been a great assistance to me. I would like to thank Dr. Simon Davies for his help
and guidance, along with Dr. Jeonghwan Kim, Dr. Seokjong Byun and Ms. Alla
Alpatova for all of their help. I would also like to thank Mr. Minyoung Jeong for his
assistance in preparing manganese oxide and coating of membranes during the

early part Of my research study.

There have been many other individuals that have helped me along the way.
From the Center for Advanced Microscopy, I would like to thank Dr. Alicia Pastor,
Dr. Ewa Danielewicz, Dr. Xudong Fan, Dr. Stanley Flegler and Dr. Carol Flegler.
From the Department of Chemistry, I would like to thank Mr. Rui Huang. From
the Composite Materials and Structures Center, I would like to thank Dr. Mike

Rich, Dr. Per Askeland and Dr. Hazel-Ann Hosein.

Finally, I would like to thank my family for all Of their support. Chris, Jacquie, Dan,
Kelly, mom and dad, your love and encouragement has helped me more than I
could ever put into words. Audra, your smiles brighten even the most stressful

days.

There have been many others that have helped me along the way. I would like to

thank everyone that I may have inadvertently omitted.

vi

TABLE OF CONTENTS

List of Tables ........................................................................................ ix
List of Figures ........................................................................................ x
Chapter 1 — Introduction .......................................................................... 1
1.1 Introduction ...................................................................................... 1
1.2 Objectives ........................................................................................ 3
1.3 Structure of Dissertation ..................................................................... 4
Chapter 2 — Background .......................................................................... 7
2.1 Catalytic Membrane Filtration .............................................................. 7
2.2 Catalytic Properties Of Manganese Oxides ............................................ 9
2.3 Preparation of Manganese Oxide Nanoparticles .................................... 11
2.4 Layer-by-layer Coating .................................................................... 15
2.5 Sintering of Porous Ceramics ............................................................ 16

Chapter 3 — Preparation and Characterization of Manganese Oxide Coating

Material ............................................................................... 20
3.1 Introduction ................................................................................... 20
3. 2 Materials and Methods ..................................................................... 21
3.3 Results and Discussion .................................................................... 27
3.3.1 Characterization of Particles Prepared by Ozonation ........................... 27
3.3.2 Characterization of Sintered Particles ............................................... 30
3.3.3 Characterization Of Particles Prepared by Mixing Of KMnO4 and PAH ...... 34
3.4 Conclusions ................................................................................... 37

Chapter 4 - Catalytic Membrane Preparation and Characterization.................39

4.1 Introduction .................................................................................... 39
4.2 Materials and Methods .................................................................... 44
4.2.1 Layer-by-layer Coating of Membranes .............................................. 44
4.2.2 Microscopy Sample Preparation.. ................................................... 46
4.3 Results and Discussion .................................................................... 51
4.3.1 Atomic Force Microscopy (AFM) Analysis .......................................... 51
4.3.2 Scanning Electron Microscopy (SEM) Analysis ................................... 57
4.3.3 Energy Dispersive X-ray Spectroscopy (EDS) Analysis ........................ 61
4.3.4 Transmission Electron Microscopy (T EM) Analysis ............................. 69
4.4 Conclusions ................................................................................... 74

vii

Chapter 5 — Hybrid Ozonation-Membrane Filtration ...................................... 76

5.1 Introduction .................................................................................... 76
5.2 Materials and Methods ..................................................................... 80
5.3 Results and Discussion .................................................................... 83
5.3.1 Total Organic Carbon (TOC) Removal .............................................. 83
5.3.2 Flux Recovery ............................................................................. 87
5.4 Conclusions ................................................................................... 88
Chapter 6 - Thermal Stability of Ceramic Membranes ................................. 92
6.1 Introduction .................................................................................... 92
6.2 Materials and Methods ..................................................................... 97
6.3 Results and Discussion ............................................................... 101
6.3.1 X-ray Diffraction (XRD) Analysis .................................................... 101
6.3.2 Deionized Water Permeability .................................................... 104
6.3.3 Sintered Membrane Analysis ........................................................ 106
6.3.3.1 Scanning Electron Microscopy (SEM) Analysis ........................... 106
6.3.3.2 Atomic Force Microscopy (AFM) Analysis ..................................... 112
6.3.4 Coated Membrane Analysis .......................................................... 113
6.3.4.1 Scanning Electron Microscopy (SEM) Analysis ............................... 114
6.3.4.2 Molecular Weight Cut-off (MWCO) Measurements .......................... 119
6.5 Conclusions ................................................................................. 122
Chapter 7 - Summary and Conclusions ................................................... 125
7.1 Summary and Conclusions .............................................................. 125
Chapter 8 - Recommendations for Future Work ........................................ 130
8.1 Recommendations for Future Work ................................................... 130
Appendix A — Representative Fourier Transform Data ................................ 135
Appendix B — Diffusion Calculations ....................................................... 136

Appendix C - Scanning Electron Microscopy (SEM) Images of Membranes
After Use in Hybrid System ................................................. 138

References ....................................................................................... 140

viii

LIST OF TABLES

Table 1: Summary of AFM and SEM Observations for all membrane

samples ............................................................................................. 60
Table 2: DI water permeability of unsintered membrane sections

(ﬂow rate: 0.8 Umin, transmembrane pressure: 103 kPa) ........................... 105

Table 3: DI water permeability of sintered membrane sections
(flow rate: 0.8 L/min, transmembrane pressure: 103 kPa) ............................ 106

ix

 

LIST OF FIGURES

Figure 1. Schematic representation of densiﬁcation by a) grain
boundary diffusion and b) volume diffusion from the grain boundary
area to the neck area (Adapted from Figure 10.9b in [97]) .............................. 17

Figure 2. Schematic representation of a) pores at grain boundaries that
b) coalesce at the intersections of grains during grain growth ......................... 18

Figure 3. Schematic representation of apparatus used for preparation
Of MnOz colloid using ozonation ................................................................ 22

Figure 4. TEM micrograph of hydrated Mn02 colloid prepared using
ozonation, resulting in ﬂakes of a non—crystalline hydrated manganese
oxide, with an average particle size of 92 nm :I: 13 nm ................................... 28

Figure 5. UV-vis absorbence spectrum of manganese oxide prepared
by ozonation, conﬁrming the presence of Mn02 ........................................... 29

Figure 6. TGA of MnOz prepared using ozonation. Physisorbed water
is removed at approximately 80°C [112] ..................................................... 30

Figure 7. XPS results from the MnOx samples that were pressed and
sintered at 550°C for 30 minutes and at 500°C for 45 minutes showing
peaks at 641.7 eV and 642.3 eV, respectively ............................................. 31

Figure 8. XRD results from manganese oxide that was pressed and
sintered a) at 550°C for 30 minutes, b) at 500°C for 45 minutes, c)
purchased Mn203 that is unsintered and d) purchased MnOz that is
unsintered ............................................................................................ 32

Figure 9. TEM micrograph Of the manganese oxide prepared by
ozonation, sintered at 500°C for 45 minutes. Inset is the selected area
and diffraction pattern showing polycrystalline ring pattern (100 nm
aperture) .............................................................................................. 33

Figure 10. TEM micrograph of the manganese oxide prepared by
ozonation and sintered at 500°C for 45 minutes, with an average
particle size of 21 nm :t 5 nm ................................................................... 34

Figure 11. TEM image Of manganese oxide prepared by mixing of

KMnO4 and Poly(allylamine hydrochloride) (unsintered particles), with
an average particle size of 11 nm :I: 1 nm ................................................... 35

Figure 12. UV-vis absorbance spectrum of KMnO4 before mixing with
PAH and Mn02 prepared by mixing of KMnO4 and PAH. The broad
peak at 340 nm is indicative of Mn02 [45, 46, 49] ......................................... 36

Figure 13. Schematic representation Of the sample preparation for a)
AFM, b) SEM, C) EDS and d) TEM ............................................................. 47

Figure 14. Feature height data from AFM images Of membranes that
were uncoated or coated and sintered at various conditions. The
nomenclature for the naming of the samples is that the ﬁrst number is
the molecular weight cut-Off Of the membrane (3. g. 5 k0), the second
is the number of coating layers and the third is the sintering
temperature (in °C, where 0 indicates that the membrane was
unsintered). For the coated membranes, the word ozone after the
name of the sample indicates that the manganese oxide particles were
prepared using ozonation while PAH indicates that the manganese

oxide particles were prepared by mixing of KMnO4 and PAH. Error
bars indicate the standard deviation based on 5 different scans of each
sample ............................................................................................... 52

Figure 15. Roughness data based on AFM imaging of membranes that
were uncoated or coated and sintered at various conditions. The
nomenclature for the naming of the samples is the same as for Figure
14. Error bars indicate the standard deviation based on 5 different
scans of each sample ............................................................................ 55

Figure 16. AFM images of ceramic membranes a) 5-0-0, b) 5-0-500, c)
5—0-550, d) 5-20-500 Ozone, e) 5-20-550 Ozone, f) 5-30-500 Ozone, g)
5-40-500 Ozone, h) 5-20-500 PAH, i) 5-20-550 PAH and j) 5-30-550
PAH. The scale for all samples is x: 5.00 um/division and y: 3000
nm/division. The nomenclature for the naming of the samples is the
same as for Figure 14 ............................................................................. 56

Figure 17. SEM micrographs of ceramic membranes a) 5-0-0, b) 5-0-
500, c) 5—0-550, d) 5-20-500 Ozone, e) 5-20-550 Ozone, f) 5-30-500
Ozone, 9) 540-500 Ozone, h) 5-20-500 PAH, i) 5-20-550 PAH and j)
5-30-550 PAH. The nomenclature for the naming of the samples is the
same as for Figure 14 ............................................................................... 58

Figure 18. Comparison Of grain sizes (calculated using intercept
method) for uncoated and coated ceramic ﬁltration membranes. The
nomenclature of the naming of the samples is the same as for Figure
14 ...................................................................................................... 61

xi

Figure 19. EDS mapping image Of uncoated and unsintered ceramic
membrane. The ﬁltration surface is on the right hand side Of the
images ................................................................................................ 62

Figure 20. EDS mapping image of 5 kD ceramic membrane a) coated
with 40 layers of the MnOz coating prepared using ozone and sintered
at 500°C for 45 min and b) coated with 30 layers Of MnOz prepared by
mixing of KMnO4 and PAH and sintered at 550°C for 30 min. The
Mn02 coated ﬁltration surface is on the right hand side Of the images .............. 64

Figure 21. EDS line scan Of 5 k0 ceramic membrane coated with 30
layers of the manganese oxide coating prepared using ozone and
sintered at 500°C for 45 min and with 30 layers Of manganese oxide
prepared by mixing Of KMnO4 and PAH and sintered at 550°C for 30
min. EDS line scan counts were normalized based on Mn counts at the
surface of each membrane sample ............................................................ 65

 

Figure 22. SEM micrograph Of the coarse grained membrane support
layer with high porosity ............................................................................ 67

Figure 23. EDS mapping image of a 5 kDa ceramic membrane coated

with 10 layers Of the supernatant from the MnOz colloid and sintered at
500°C or 45 minutes, showing Mn present throughout the membrane.
The ﬁltration surface is on the right hand side of the images .......................... 68

Figure 24. a) TEM photomicrograph Of a cross section Of an as
received ceramic water ﬁltration membrane. Inset are the selected
area and the diffraction pattern showing an amorphous ring pattern
(100 nm aperture). b) TEM photomicrograph of cross section of a
membrane with 40 layers of manganese oxide, sintered at 500°C for
45 minutes. Inset are the selected area and the diffraction pattern
showing polycrystalline ring pattern (100 nm aperture) .................................. 70

Figure 25. TEM photomicrograph Of cross section of a membrane with
20 layers Of manganese oxide, sintered at 500°C for 45 minutes. ................... 72

Figure 26. TEM photomicrograph Of cross section of a membrane with
30 layers Of manganese oxide, sintered at 500°C for 45 minutes. ................... 73

Figure 27. Comparison of the Observed coating thickness to the
expected coating thickness for 20, 30 and 40 coating layers. The
expected coating thickness assumes a close-packed arrangement of

the Mn203 nanoparticles .......................................................................... 73

xii

Figure 28. Schematic representation of the hybrid ozonation-
membrane ﬁltration system ...................................................................... 82

Figure 29. Total organic carbon present in permeate samples after
combined ozonation and catalytic membrane ﬁltration. Initial feed water
TOC: 9.9 :I: 0.4 mg C/L, temperature: 22.5 :I: 0.5°C, transmembrane
pressure: 138 kPa ................................................................................. 84

Figure 30. Comparison Of grain size to the amount of total organic
carbon present in permeate samples after combined ozonation and
catalytic membrane ﬁltration. Initial feed water TOC: 9.9 :l: 0.4 mg ClL,
temperature: 22.5 d: 0.5°C, transmembrane pressure: 138 kPa ....................... 86

Figure 31. Normalized ﬂux for an uncoated membrane as well as
membranes coated with 20, 30 and 40 layers of the manganese oxide

prepared using ozonation. 03 gas ﬂow rate: 1x10'5 m3/min, crossﬂow
velocity: 0.47 mls, transmembrane pressure: 200-228 kPa, initial ﬂux:

104-115 le2-hr, temperature: 22.5 a: 05°C ............................................... 88

Figure 32. XRD spectra of a) the ﬁltration surface of the titania
membrane, indexed as the rutile phase, b) powdered titania
membrane, indexed as the rutile phase, 0) ﬁltration surface of the AZT
membrane, where the indexed pattern shows the alumina (A), zirconia
(Z) and titania (T) peaks and d) powdered AZT membrane, where the
indexed pattern shows the alumina (A), zirconia (Z) and titania (T)
peaks ............................................................................................... 103

Figure 33. SEM micrographs of the titania membrane a) unsintered, b)
sintered at 500°C for 30 min, c) sintered at 600°C for 30 min, cl)
sintered at 700°C for 30 min, e) sintered at 800°C for 30 min and f)
sintered at 900°C for 30 min. All scale bars are 100 nm ............................... 108

Figure 34. Grain size versus percent Of theoretical density for pure
coarsening, coarsening and densiﬁcation and densiﬁcation followed by
grain growth (Adapted from Figure 10.53 in [97]) ....................................... 110

Figure 35. Comparison of the average grain size and permeability for
the unsintered and sintered membranes. The average and standard
deviation for the grain size measurements were determined by
measurement at least 600 grains using the intercept method (ASTM
Standard E 112-96 (2004)). The average and standard deviation Of the
permeability measurements were determined by testing each
membrane at least 4 times until steady state conditions were achieved
(permeability test conditions: ﬂow rate: 0.8 L/min, transmembrane
pressure: 103 kPa) .............................................................................. 111

xiii

Figure 36. AFM average roughness (Ra) for the unsintered and
sintered membranes ............................................................................ 113

Figure 37. SEM micrographs of the titania membrane coated with 20
layers Of the manganese oxide coating and sintered at 3) 350°C for 45
min, b) sintered at 400°C for 45 min and c) sintered at 450°C for 45
min, each with a ramped heating/cooling proﬁle, as well as d) sintered
at 350°C for 30 min, e) sintered at 450°C for 30 min and f) sintered at
500°C for 30 min without a ramped heating/cooling proﬁle. All scale
bars are 100 nm .................................................................................. 115

Figure 38. A plot of the temperature versus time and diffusivity versus
time for the ramped heating and cooling proﬁle ........................................... 117

Figure 39. Comparison Of the average grain size and permeability for
the membrane samples coated with 20 layers Of manganese oxide and
sintered with and without a ramped heating and cooling proﬁle ..................... 118

Figure 40. Percent rejection of 1 kDa, 5 kDa, 25 kDa, 50 kDa and 80
kDa dextran for titania, 7-channel, 5 kDa membranes that are
uncoated and unsintered, uncoated and sintered at 500°C and coated
with 10, 20, 30 and 40 layers of manganese oxide and sintered in air at
500°C ............................................................................................... 119

Figure A1. Representative Fourier transform analysis of 30 AFM line
scans from a ceramic membrane coated with 20 layers of the
manganese oxide coating prepared using ozone and sintered at 500°C
for 45 minutes .................................................................................................... 135

Figure C1. SEM micrograph of a titania membrane coated with 40
layers of manganese oxide prepared by ozonation (sintered in air at
500°C for 45 minutes) and used in the ﬁltration system ................................ 138

Figure CZ. SEM micrograph of a titania membrane coated with 40
layers of manganese oxide prepared by ozonation (sintered in air at
500°C for 45 minutes), used in the ﬁltration system and then soaked in
water from 11/21/2008 to 09/30/2009 ....................................................... 139

xiv

CHAPTER 1

INTRODUCTION

 

1.1 INTRODUCTION

While ozone reacts with humic substances to produce oxygenated by-products, it
does not remove organic matter from water to any signiﬁcant extent, therefore,

the use Of a catalyst is required [1]. Kamik et al. [2-4] have shown that
nanolayers of Fe203 act as a catalyst in the oxidation of organic matter. Here,

ceramic water ﬁltration membranes coated with iron oxide nanoparticles were
shown to improve water quality by reducing the dissolved organic carbon (DOC)
and disinfection by—products (DBPs) in permeate samples when used in a hybrid
ozonation-membrane ﬁltration system. While the nanostructured iron oxide
coating (the colloids had an initial particle size of 4 to 6 nm) dramatically

improved water quality as well as prevented membrane fouling, other coated

membrane ﬁltration surfaces, in combination with ozonation, have not been

evaluated.

A possible coating alternative to iron oxide is manganese dioxide. Manganese-
containing catalysts have been shown by Spasova et al. to be quite effective for
the decomposition of ozone [5]. Also, manganese-containing catalysts have been
found to catalyze the degradation of organic matter by ozone [6441. Studies
have shown that manganese-containing catalysts are effective for the oxidation
of pyruvic acid [9, 11], oxalic acid [8], metol [10], humic acids [12] and atrazine
[13, 14] by ozone in aqueous systems as well as for the oxidation of volatile

organic compounds (VOCs) in gas phase reactions [25, 30-32, 34-40, 43, 44].

Using MnOz nanoparticles to coat ceramic water ﬁltration membranes should

produce a catalytic surface to aid in the oxidation Of natural organic matter
(NOM), however the optimal number of coating layers, the sintering conditions
and the surface reactions that act to reduce the concentration of contaminants in
water must be determined. Layering of fine nanoparticles will help ensure an
even coating on the membrane surface. By varying the number of coating layers

and the sintering temperature and time, the Optimum coating conditions can be
determined based on improved water quality. The MnOz coated membranes
must also be characterized to determine the ﬁnal structure of the membrane and

coating layer. Finally, the thermal stability of the membranes during the heat

treatment required for the coating and sintering process must be determined.

1 .2 OBJECTIVES

The Objectives of this dissertation research were: 1) to produce and characterize
Mn02 nanoparticles, 2) to use these nanoparticles to coat ceramic water ﬁltration

membranes, 3) to evaluate the water quality using the coated membranes in
combination with ozonation 4) to characterize the coated membranes and 5) to

study the thermal stability of the ceramic membranes.

TO coat ceramic water ﬁltration membranes with MnOz nanoparticles, the
particles had to ﬁrst be synthesized. Many methods have been used to produce
MnOz nanoparticle colloids [45-49]. One of the Objectives Of this research was to

use ozone tO oxidize Mn based on a modiﬁcation Of the method used by Kijima et

al. [50].

Once the MnOz nanoparticles were synthesized, they were characterized to

determine their composition and particle size. The nanoparticles were
characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy
(XPS), energy dispersion spectroscopy (EDS), wet chemical analysis,

transmission electron microscopy (TEM) and selected area diffraction (SAD).

When suitable MnOz nanoparticles had been prepared, they were then used to

coat titania water ﬁltration membranes (INSIDE CéRAM, TAMI North America,

St. Laurent, Québec, Canada) using a layer-by-layer technique. The deposition

Of thin nanoparticle layers helps ensure even coating Of the membranes. The
coated membrane was then sintered to fuse the coating to the membrane to
obtain a durable coating. The Optimum sintering conditions of time and
temperature were determined by comparing water quality results using the
sintered coated membranes in an ozone-ﬁltration system. Additionally, the
number of coating layers yielding the optimum water quality was also assessed

in a similar manner.

Transmission electron microscopy (TEM), scanning electron microscopy (SEM)
and atomic force microscopy (AFM) were used to characterize the morphology

and microstructure of the coating layer, the surface and the pore structure of the

sintered Mn02 coated membranes.

Finally, the thermal stability of the titania membrane was studied. In this way, the
link between the membrane performance and the coating and sintering of the

membrane was determined.

1.3 STRUCTURE OF DISSERTATION

This dissertation has been organized in chapters. A detailed literature review of
relevant background information for this dissertation is presented in Chapter 2.

Also included in Chapter 2 is a review of catalytic membrane ﬁltration, the

catalytic properties of manganese oxides, manganese oxide nanoparticle
preparation methods, Iayer-by—Iayer coating and ﬁnally, sintering Of porous

ceramics.

The experimental work in this dissertation is covered in Chapters 3, 4, 5 and 6.
Chapter 3 presents the preparation and characterization of the manganese oxide
coating material. The characterization of the coating material was performed
using wet chemistry, x-ray photoelectron spectroscopy (XPS), x-ray diffraction
(XRD), therrnogravimetric analysis (TGA), UV-visible absorbance spectroscopy

and transmission electron microscopy (TEM) analysis.

Chapter 4 presents the catalytic membrane preparation, using a layer-by-layer
coating method, and coated membrane characterization. The characterization of
the coated membranes was performed using atomic force microscopy (AFM),
scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy

(EDS) and transmission electron microscopy (TEM) analysis.

Chapter 5 presents the use Of the catalytic coated membranes in a hybrid
ozonation-membrane ﬁltration system. The effect Of the manganese oxide and
the number of coating layers on the total organic carbon (T 00) removal and the

ﬂux recovery are discussed.

 

Chapter 6 presents the thermal stability of the ceramic membranes.
Characterization of uncoated membranes, under various sintering conditions,
was performed using x-ray diffraction (XRD), scanning electron microscopy
(SEM) and atomic force microscopy (AFM) analysis. Characterization of
manganese oxide coated membranes, under various sintering conditions, was
performed using scanning electron microscopy (SEM) analysis and by

performing molecular weight cut-Off (MWCO) measurements.

Chapter 7 presents a summary and conclusions, with recommendations for

future work being presented in Chapter 8.

CHAPTER 2

BACKGROUND

 

2.1 CATALYTIC MEMBRANE FILTRATION

Researchers have studied the use Of ozonation with polymeric membrane
ﬁltration [51-54]. However, polymeric membranes are susceptible to degradation
by ozone [52, 55] due to oxidative attack. Hence, ozone must be allowed to

decompose before the ozonated water contacts the polymeric membrane.

Polytetraﬂuoroethylene (PTFE) [56], polyvinylidene ﬂuoride (PVDF) [57-59] and
polysulfone (PS) [60] membranes have been shown to be sufﬁciently ozone
resistant to be used in a hybrid ozone-membrane process. More recently,
however, because Of their resistance to ozone and their longevity in water
ﬁltration systems, ceramic membranes have been used in combination with

ozonation [2-4, 57, 61-74]. Despite the beneﬁts of ceramic membranes,

polymeric membranes are still widely used for water ﬁltration since ceramic
membranes have not been studied as extensively for water treatment [70, 75, 76]
and ceramic membranes are more expensive than polymeric membranes [57,

77-80].

Kamik et al. have reported their extensive work with catalytic membranes for
drinking water treatment using ozonation and ultraﬁltratlon [2-4, 65-68]. Their

work used an alumina, zirconia and titania (AZT) ceramic membrane which was

then coated with a Fe203 nanoparticle coating. They demonstrated that the

Fe203 coated membrane improved the performance of a combined ozonation-
ﬁltration system. The enhanced performance was linked to the catalytic
decomposition Of ozone by the iron oxide to produce hydroxyl and other radicals
[66]. The radicals produced at the Fe203 surface degrade organic foulants,
resulting in a decrease in membrane fouling and a concomitant decrease in the
dissolved organic carbon in the permeate [66]. The grain size Of the Fe203
coating layer (as measured by SEM) increased with increasing number of coating

layers (from 21 :I: 0.24 nm for the uncoated membrane to a maximum of 66 t 23

nm for the membrane coated with 40 layers). The membrane coated with 40
layers of the F9203 coating showed the largest improvement in water quality, in

terms Of the removal of total trihalomethanes, halo acetic acids, aldehydes,

ketones and ketoacids [4].

 

2.2 CATALYTIC PROPERTIES OF MANGANESE OXIDES

Catalysis of organic matter by manganese oxide with ozone has been studied by
others [6-44], indicating that manganese oxide would be quite promising as a
coating for ceramic membranes. Naydenov and Mehandjiev used manganese
dioxide tO catalytically oxidize benzene by ozone [6]. They achieved complete
catalytic oxidation of benzene by ozone on MnOz in a temperature range of 10-
80°C (283-353 K). Einaga et al. [18-24] also studied the effects of catalyst
support, reaction conditions and manganese loading on the ability of manganese

oxide catalysts to catalyze the oxidation Of benzene. Andreozzi et al.
demonstrated the catalytic ability of MnOz for the ozonation of pyruvic acid [9,
11], oxalic acid [8] and metol [10]. They found that pyruvic acid did not react with
either Mn02 or 03, however, signiﬁcant oxidation was Observed when ozone was

used in combination with Mn02 [9, 11]. Andreozzi et al. found that oxalic acid

reacted only slightly with Mn02 in the absence Of ozone, however, the reactivity
was greatly increased with the addition of ozone [8]. Although these experiments
were done in a semibatch process with suspended MnOz particles, rather than
on a coated membrane, the catalytic properties of the Mn02 in the presence of

ozone seem quite promising.

Lee et al. examined the catalytic ozonation of humic acids [12]. Humic acid was

used as a model organic compound because it is a major constituent of natural

 

organic matter. The authors observed a signiﬁcant decrease in UV absorption

with Mn02 combined with ozone, indicating the oxidation of the humic acids [12].

Ma and Graham [13, 14] showed that manganese oxide catalysts enhanced the
oxidation Of atrazine by promoting ozone decomposition and hydroxyl radical
formation. Ma and Graham’s work also demonstrated that humic substances
have a positive effect on the oxidation of atrazine as compared to that Observed

with the manganese catalyst and ozone alone [13, 14].

MnOz can therefore be an effective catalyst for the ozonation Of organic matter in
water. The favorable catalytic properties Of manganese dioxide suggest that
experimentation with a MnOz coating on the ceramic ﬁltration membranes for the
use in combined ozonation-ﬁltration is logical. In order to coat the ceramic

filtration membranes it is necessary to produce Mn02 nanoparticles.

Additionally, Mn203 has been used as an effective catalyst in gas phase
reactions [81-83] and may be effective for the decomposition of ozone in an
aqueous system. The effectiveness of a Mn203 coating would need to be

evaluated using water quality measurements when the coated membrane is used

in the hybrid ozonation-membrane ﬁltration system since previous work has only

shown Mn203 to be effective in gas phase reactions.

10

2.3 PREPARATION OF MANGANESE OXIDE NANOPARTICLES

The oxides of manganese have been studied quite extensively for use in solid-

state batteries and fuel cells [84-86] and various methods have been used to

produce MnOz nanoparticle colloids.

Fujimoto et al. soniﬁed manganese dioxide nanoparticles with a diameter of

approximately 2 nm [45]. An argon-purged aqueous solution containing 0.1 mM
Of KMnO4 was sonicated in the presence of 8 mM sodium dodecylsulfate (SDS)
at 200 kHz, with an input power of 200 W. The UV-VIS spectra showed that the
absorbance peaks from the MnO4_ ions, which were originally at 525 nm and 545
nm, disappeared after 10 minutes Of sonication and were replaced by a peak

around 360 nm, indicating the presence Of MnOz.

Lume-Pereira et al. used v-irradiation [46] to produce manganese oxide

nanoparticles. Potassium permanganate (4 x 10'4 M) was irradiated at pH 10 and
a temperature Of 20°C. Using a dose rate Of 7 x 104 rad/h, the absorption of the
MnO4- ion at ~530 nm disappeared and was replaced by a broad band at ~336

nm, after a dose of 4.0 x 105 rad. Electron microscopic studies Of an evaporated

drop on a thin carbon carrier showed particles of 3-5 nm in diameter.

11

 

Lume-Pereira et al. described a “conventional” manganese oxide colloid

preparation method [46] in which 75 mL of an aqueous solution of 4 x 10‘4 M

Mn(CIO4)2 was added to 50 mL Of an aqueous solution of 4 x 10'4 M KMnO4 with

strong stirring. The colloid formed instantly. The exact size of the manganese
dioxide particles is not known, however, the solution could not be ﬁltered through
a 10 nm nucleopore ﬁlter. This indicates that the particles were signiﬁcantly

larger than the particles that were produced using the y-irradiation method [46].

Stadniychuk et al. [47] prepared a MnOz colloid using a modiﬁcation Of the Clorox

method described by Ulrich and Stone [48]. In this modiﬁed method, Stadniychuk
et al. mixed 15 mL of 1.0 M NaOH and 100 mL of 0.705 M NaOCl with 1620 mL
of deionized water and stirred for 5 min to obtain a pH of 12.5. Then 13 mL of
1.009 M MnCI2 - 4H20 were added and rapidly stirred for 90 min. The mixture
had a pH of 7-8. The solution was left to settle for approximately 24 hours.
Excess liquid was removed and deionized water was added to make 500 mL Of
suspension. The suspension was then ultrasonicated for 3 hours and dialyzed
against deionized water for 5 days. The water was changed at 1, 4, 16, 48, 72
and 96 hours to remove electrolyte ions. The sol was then removed from the
dialysis bag and ultrasonicated for 5 hours. The sol was next ﬁltered to remove

large particles to prevent Ostwald ripening. This method produced MnOz

particles with a mean diameter of 30 nm [47].

12

Stadniychuk et al. [47] also described a Mn02 preparation developed by Bach et
al. [87] which used a fumaric acid reduction method [87], in which 400 mL of
0.250 M NaMnO4-H20 were mixed with 100 mL of 0.333 M CZH204Na2 (fumaric
acid disodium salt) and stirred vigorously for 1 hour, then ultrasonicated for 3
hours. Then 10 mL Of 2.5 M H2804 were added and the solution was stirred for

3-5 hours. The solution was then put in dialysis bags having a molecular weight

cut-Off of 3500 Da and dialyzed for 3 days against deionized water, with the
water being Changed at 1, 4, 12, 24 and 48 hours. This method produced MnOz

particles with a mean diameter of 180 nm.

Kijima et al. [50] used ozonation to produce a—MnOz. Fourteen (14) g of

MnSO4'5H20 were dissolved in 600 mL Of 3 M H2804. The solution was then
heated to 80°C (353 K) while stirring at 200 rpm. When the solution reached

80°C, ozone was bubbled into the reactor for 3 hours at a rate of 40 cm3/min.

The 03 concentration was 135 mg/L.

Luo prepared MnOz nanoparticles by directly mixing potassium permanganate

and a polyelectrolyte aqueous solution [49]. At room temperature, 100 mL of a 20
mM potassium permanganate aqueous solution was mixed with equal amounts
of poly(allylamine hydrochloride) (PAH) aqueous solution at a molar ratio of 4:1
of PAH (repeating unit) to Mn. This method resulted in spherical MDOz

nanoparticles with diameters of 6 — 12 nm.

’13

Many of these methods (i) involve using equipment that is not readily available
[45, 46] or (ii) produce MnOz nanoparticles that are too large [47, 48]. To evenly

coat the membranes, particle sizes of less than 20 nm are required. The direct

mixing method by Luo [49], described above, appeared to be the best Option for

preparing Mn02 nanoparticles Of a ﬁne, uniform particle size.

Ozone is a strong oxidizing agent that reacts with Mn(ll) ions to form manganese

oxide according to equation (1) [88]:

an“ + 03 (aq) + H20 —+ Mn02 (s) + 02 (aq) + 2H“ (1)

TO produce nanoparticles of the desired size, however, particle growth and
agglomeration must be controlled. The particle growth and agglomeration can be
controlled for this ozonation method by adjusting the duration Of ozonation as

well as the pH of the resulting colloid.

Because Of the lack of mechanical stability, the use Of manganese oxide

nanoparticles as a coating layer for ceramic water ﬁltration membranes required

that the deposition layer on the membrane be sintered [3, 4, 66-68]. Mn02,

however, decomposes to Mn203 at approximately 550°C (823 K) [89, 90],

making it necessary to characterize the nanoparticles before and after sintering.

l4

2.4 LAYER-BY-LAYER COATING

Based on the work of Kamik et al [3, 4, 66-68] with iron oxide nanoparticles, a
layer-by-Iayer technique can be used to create a thin coating of the Mn02

particles on the ceramic ﬁltration membranes. This technique is used so that
reproducible layer thicknesses can be achieved [91]. The adsorption of the
charged molecules on the surface reverses the charge on the surface and
restricts the adsorption to a single layer [91-94]. Introducing molecules of
opposite charge then allows the adsorption of a second layer. Alternating
between the oppositely charged molecules results in a multilayer structure [3, 4,

68, 91-94].

Previous work has been done with layer-by-layer assembly of ultrathin ﬁlms of
MnOz on electrodes [91, 94]. To create the layers, the electrodes were ﬁrst
submerged in a colloidal solution of manganese dioxide for 15 minutes and then
rinsed with a pH 12 solution to remove any weakly adsorbed molecules. The
electrodes were then submerged in poly(dimethyldiallylammonium chloride)
(PDDA) (2 mg/mL, pH 12) for 15 minutes, then rinsed again with a pH 12

solution. This procedure was repeated for the desired number of layers.

15

2.5 SINTERING OF POROUS CERAMICS

As ceramic membranes become more widely used, there is a growing concern
about the effect of sintering temperature on the underlying membrane structure
[95, 96]. Although ceramic membranes have higher thermal stability than
polymeric membranes [96], there can still be structural changes that occur in
ceramic membranes at elevated temperatures [95, 96]. Lin et al. [95] found that
lanthanan—doped alumina and titania membranes and yttria-doped zirconia
membranes signiﬁcantly improved the thermal stability of the memberanes. The
doping raises the phase transformation temperatures in the materials and
decreases the reduction in surface area and pore growth below the phase
transformation temperatures [95]. However, doping of the ceramic membranes
does not completely retard the structure changes that occur at elevated
temperatures [95]. When ceramic membranes are coated with materials that
necessitate high temperature treatment, the morphology and microstructure of
the underlying membrane may be altered during the sintering of the coating. This
may in turn result in changes to the underlying membrane properties such as the

pore size and structure.

During sintering, the grains can either grow with very little densiﬁcation or there
can be an increase in the density, to approximately 95% of theoretical density,
followed by grain growth [97]. Alternatively, grain growth and densiﬁcation can

occur simultaneously [97]. During densiﬁcation, mass is transferred from the

16

grain boundary or region between the particles to the pores or the necks between
the particles (Figure 1) [97]. The driving force for the increase in grain size and
pore size is the reduction in the surface energy. By increasing the average grain
size, there is a reduction in the total surface area, resulting in an overall reduction

in the surface energy [97].

During the grain growth process, pores located at the grain boundaries can move
with the grain boundaries and coalesce into larger pores at the intersections of
the grains (Figure 2) [97, 98]. In ﬁne grained oxides, an increase in pore size is
observed in the initial stages of sintering. As sintering time continues, however,

there is a marked increase in grain size [97, 98].

 

Db

Figure 1. Schematic representation of densiﬁcation by a) grain boundary
diffusion and b) volume diffusion from the grain boundary area to the neck area
(Adapted from Figure 10.9b in [97]).

I7

 

a) b)

Figure 2. Schematic representation of a) pores at grain boundaries that b)
coalesce at the intersections of grains during grain growth.

Since the nanoparticle coated membranes examined in this study are sintered for
relatively short periods of time, the underlying membrane support only
experiences the early stages of sintering and is not able to completely eliminate
the agglomerated pores or further densify. An increase in the pore diameter of
porous ceramic membranes with increasing sintering temperature (when sintered

for short periods of time) has previously been observed by Baticle et al. [99].

Although purchased membranes were used in the work described in this
dissertation, it is possible to synthesize ceramic membranes [72-74, 100-103],
which could then be tailored for subsequent application of a catalytic coating and
sintering. Some possible membrane preparation methods include the sol-gel

process [73, 74, 100, 101] and slip casting [73, 102, 103].

18

In the sol-gel process, a colloid is prepared by dispersing the ceramic particles in
an electrolyte. Modifying the charge on the particles, by adjusting the pH of the
electrolyte, the colloidal suspension transforms to a gel structure [73]. The gel
consists of agglomerates or interlinked chains of particles [73]. Adjusting the
agglomerate size at the time of gelation, the density of the ﬁlm can be adjusted
[73]. This method can be used to produce membranes with narrow pore size

distributions [73].

In the slip casting method, a porous support is dipped into a colloidal ceramic
suspension of ceramic particles called a slip. The dispersion medium ﬂows into
the pores of the support and the solid particles remain at the entrance of the
pores. This forms a gel layer on the surface of the membrane. The viscosity of
the slip can be adjusted so as to ensure that the slip does not penetrate into the

porous support [73].

To achieve good reproducibility in the membranes as well as a narrow pore size
distribution, the optimum method for membrane fabrication would be the sol-gel
process [73, 74, 102]. Manufacturing the membranes so that the resulting
membranes are very reproducible would allow for comparisons to be made
between membranes and the effects of any coatings and heat treatments could

also be compared.

19

CHAPTER 3

PREPARATION AND CHARACTERIZATION OF MANGANESE OXIDE

COATING MATERIAL

 

3.1 INTRODUCTION

The oxides of manganese have been studied quite extensively for use as
catalysts for the oxidation of organic matter by ozone [6441. Preparing a
manganese oxide nanoparticle colloid and applying it to a ceramic water ﬁltration
membrane, should therefore, produce a catalytic coating to degrade organic
matter in water when used in a hybrid ozonation-membrane ﬁltration system [24,

65-68].

Various methods have been used to prepare manganese oxide nanoparticle
colloids [45-50]. Kijima et al. [50] oxidized Mn2+ to Mn4+ in H2804 (>2 M).

However, since ozone is a strong oxidizing agent, it should react with Mn2+ at

20

Circumneutral pH to produce Mn02 [88]. The MnOz prepared using this modiﬁed

ozonation method was characterized to conﬁrm that Mn02 was in fact present

and to determine the size of the colloidal particles.

3.2 MATERIALS AND METHODS

Manganese oxide nanoparticles were prepared using ozonation. This method is

signiﬁcantly different than that developed by Kijima et al. [50], where the
oxidation was performed in strong H2804 (> 2 M) solution and at elevated

temperatures (> 70°C). For the nanostructured manganese oxide used in our
study, the ozonation was performed in deionized (DI) water at room temperature.
The system used to fabricate the nanoparticles (Figure 3) included a 2 L covered
glass reactor, with inlet and outlet ports for the ozone gas and an injection port. A
fritted glass aerator was used to bubble ozone into the reactor. Pure oxygen,
from a compressed gas cylinder was used to generate ozone. The oxygen was
ﬁrst passed through a moisture trap containing anhydrous calcium sulfate
(Drierite, Xenia, OH), then through an ozone generator (Ozotech Inc. Model
OZZPCS-V, Yreka, CA) at a ﬂow rate of 50 mL/min. Ozone was bubbled into
1000 ml of deionized water (DI water) for 20 minutes. Following this, 100 mL of a
2 mM manganese (ll) Chloride (99+%, Sigma-Aldrich Co., St. Louis, MO)

aqueous solution was injected. The ozonation was stopped immediately after the

addition of MnCl2. Continuing to bubble the ozone into the sample after the

21

addition of MnCl2 resulted in particles that were visible to the naked eye. Ceasing

ozonation once the MnCl2 was added, resulted in a transparent, golden brown

colored colloidal suspension having a pH of 3.7. The vented gas was passed

through a 2% solution of potassium iodide (Kl) in order to destroy residual ozone.

 

 

 

 

 

 

 

MnClz l ﬂ Morsture 6:)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ozone Trap
-"- Generator
Ej O
Oxygen

V Kl Cylinder
Covered

Glass
Reactor

Figure 3. Schematic representation of apparatus used for preparation of MnOz
colloid using ozonation.

At pH 3.7, the manganese oxide particles rapidly agglomerated. The particles
were centrifuged (at 2500 rpm) for 30 min to settle the particles. The supernatant
was then decanted and discarded. The resulting particles were dispersed into
500 mL of a 10'4 N KN03 (2 99.0%, J.T. Baker, Phillipsburg, NJ) solution and
sonicated in an ice bath (Branson Ultrasonics Corporation Model 250 Soniﬁer,

Danbury, CT) for 30 minutes, at an output of 58 watts, to break up any

22

agglomerated particles. The ﬁnal pH of the resulting suspension was 6.3. At the

pH of zero point of charge (szpc) the net surface charge on a particle is zero,
and particles in a suspension will rapidly agglomerate [104, 105]. For pH>pHch
the net surface charge is negative and for pH<szpc the net surface charge is

positive. Therefore, as the pH is adjusted away from szpc the particles in a
suspension repel each other, resulting in a stable suspension [104, 105]. Since

the szpc for MnOz is 1.48 - 2.40 [106, 107], by raising the pH further from the

szpc (from 3.7 to 6.3), a stable colloidal suspension was able to be achieved.

The second approach to preparing manganese oxide nanoparticles was based

on the method described by Luo [49]. For this method, 100 mL of a 20 mM
KMnO4 (>99.0%, Sigma-Aldrich) aqueous solution was mixed with an equal
amount of an aqueous solution of 80 mM poly(allylamine hydrochloride) (PAH,
average molecular weight: 15,000, 295.0%, Aldrich) at a high speed for 5 min at

room temperature. No further treatment was necessary before the colloid could

be used.

Samples of the manganese oxide particles were prepared for TEM by dipping a

holey carbon coated TEM grid into both colloidal manganese oxide suspensions

prepared using both methods.

23

The manganese oxide colloid was used to coat ceramic water ﬁltration

membranes and then the coated membranes were sintered to adhere the coating
to the membranes [3, 4, 66-68]. Since Mn02 reduces to Mn203 at approximately
550°C [89, 90], a sintering temperature of 500°C was chosen to avoid this
reduction. Since nanoparticles have a greater reactivity than micron-sized

particles [108, 109], the MnOz nanoparticles that are prepared, may still be

reduced to Mn203 at a sintering temperature of 500°C. Therefore, it was

necessary to characterize the phase of the nanoparticles before and after

sintering.

To examine the effect of sintering on the manganese oxide, the centrifuged
particles from the sample prepared using ozonation were dried in air and the
dried particles were pressed into a 12 mm disk of approximately 1 mm thickness
using a uniaxial press at 31 kN. The disk was sintered (in an alumina dish) in air
at 500°C for 45 minutes. Using the same process, a second disk was prepared
and sintered at 550°C for 30 minutes to compare the oxidation state of the
sintered manganese oxide. Half of each sintered disk was used for XRD

analysis. The other half of each disk was ground with a mortar and pestle and
approximately 10 mg of the ground powder was added to 100 mL of 10“1 N KN03
(299.0%, J.T. Baker, Phillipsburg, NJ) and sonicated for 30 minutes (at an output

of 58 watts) in an ice bath in preparation for the TEM analysis. Samples of the

suspension of sintered manganese oxide powders were prepared for TEM

24

analysis by dipping a holey carbon-coated TEM grid into the sonicated

suspensions followed by air drying.

Characterization of the oxidation state of manganese in the sintered samples
prepared using ozonation was performed using the forrnaldoxime procedure
described by Morgan and Stumm [110]. The analytical procedures were as

follows: The manganese oxide was ground with a mortar and pestle and 10 mg
of the ground powder was added to 100 mL of 104 N KNO3 and sonicated for 30

minutes (at an output of 58 watts) in an ice bath. In a 100 mL volumetric ﬂask, 10
mL of the sonicated manganese oxide suspension was added (after being ﬁltered
through a 0.45 pm ﬁlter), along with 2 mL of the forrnaldoxime reagent (10 g of
paraforrnaldehyde and 23.5 g of reagent grade hydroxylamine sulfate dissolved
in DI water to give a ﬁnal volume of 100 mL), 5 mL of 5 M NaOH and DI water to
bring the ﬁnal volume to 100 mL. The optical absorbance was then measured
using the Spectronic Genesys spectrophotometer (Milton Roy, lveland, PA) at a
wavelength of 450 nm. In another volumetric ﬂask, 5 mL of the manganese
oxide colloid was added, along with 10 mL of 0.1% o-tolidine dihydrochloride, 25
mL of 3 M perchloric acid and sufﬁcient DI water to bring the ﬁnal volume to 100
mL. The absorbance was then measured at a wavelength of 440 nm. An aliquot
of the sample used for the o-tolidine determination was then analyzed using

atomic absorption (manganese atomic absorption standard solution, 1000 ug/mL

Mn+2 in 1 wt% HNO3, Aldrich, Milwaukee, WI) using the SpectrAA-200 Flame AA

25

Spectrometer (Varian, Inc., Palo Alto, CA) to determine the total Mn content of

the solution.

The Mn(ll) content of the sintered samples was determined by the forrnaldoxime
method [110], the concentration of oxidized Mn (=Mn(lll)+2Mn(lV)) was
determined using the o-tolidine procedure [110] and the total Mn
(=Mn(ll)+Mn(lll)+Mn(lV)) was measured by atomic absorption
spectrophotometry. The concentration of the Mn present in each of the three

oxidation states was determined by difference using these three measurements.

XRD was also used to determine the phases present in the sample prepared
using ozonation. Half of each sintered disk was analyzed by XRD using a Rigaku

Rotaﬂex RU-200B (45KV/100mA, copper radiation, Rigaku, Tokyo, Japan). High

purity samples of both Mn02 (Riedel-de Haén, 99% (complexometric)) and

Mn203 (Aldrich, 99.999% (metals basis)) were purchased and used for XRD

comparison.

TEM images were obtained using a JEOL 2200FS transmission electron
microscope (JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV.
Photomicrographs were collected at magniﬁcations between 150,000x an
250,000x using a Hamamatsu CCD digital camera (Hamamatsu City, Japan).
Particle sizes were measured using the Simple Measure program (JEOL, Tokyo,

Japan). Particle size measurements were made across the diagonal of the

26

particles and the average and standard deviation is calculated from
measurements of 25 particles (Simple Measure, JEOL, Tokyo, Japan). X-ray
photoelectron spectroscopy (XPS) was performed using a Perkin Elmer Phi 5400
electron spectroscopy for chemical analysis (ESCA) (magnesium KO X-ray
source, Perkin Elmer, Waltham, Massachusetts), thermal gravimetric analysis
(TGA) was performed using a TGA 050 (TA Instruments, New Castle, Delaware)
and the UV-vis absorbance spectrum was obtained using a MultiSpec-1501

spectrophotometer (Shimadzu Scientiﬁc Instruments, Kyoto, Japan).

3.3 RESULTS AND DISCUSSION

3.3.1 CHARACTERIZATION OF PARTICLES PREPARED BY OZONATION

Figure 4 is a TEM micrograph of the manganese oxide colloidal particles
prepared using ozonation. The TEM micrograph shows particles of approximately

92 nm :I: 13 nm, having a ﬂake-like morphology. Cheney et al., made similar

observations of amorphous MnOz with diameters of ~100 nm [111].

Ozonation of Mn(ll) should result in the formation of MnOz [88], however, to

conﬁrm that the manganese oxide prepared is in fact MnOz, UV-vis absorption

spectroscopy was used, since it was not possible to obtain selected area

diffraction (SAD) patterns using the TEM because these ﬂakes formed such a

27

thin layer on the TEM grids that they would not diffract the electron beam
sufﬁciently to index the resulting diffraction pattern. The UV-vis spectrum (in
Figure 5) showed a broad peak at 370 nm, indicative of MnOz [45, 46, 49]. Lume-
Pereira et al. observed a broad 340 nm adsorption peak for an MnOz colloid [46],
while Fujimoto et al. observed an absorption band for nanocrystalline MnOz (as
conﬁrmed by TEM and XRD) located around 360 nm [45] and Luo determined

the absorbance peak for Mn02 nanoparticles (as conﬁrmed by the x-ray

photoelectron spectra (XPS)) centered at 370 nm [49].

 

Figure 4. TEM micrograph of hydrated MnOz colloid prepared using ozonation,
resulting in ﬂakes of a non-crystalline hydrated manganese oxide, with an
average particle size of 92 nm 1 13 nm.

28

TGA results (Figure 6) conﬁrm that the manganese oxide prepared by ozonation

is hydrated, with physisorbed water that is driven off at approximately 80°C (353

K). Reddy and Reddy also found physisorbed water to be removed from Mn02
xerogel and ambigel between 30-250°C [112]. Since the MnOz particles prepared

by mixing of PAH and KMnO4 had a smaller particle size, they were used to coat

the membranes to determine how the smaller particle size would affect the

surface morphology of the coated membrane.

3.0
2.7
2.4
2.1 -
1.8 -
1.5 1
1.2 1
0.9 1
0.6 -

   
 
   
 
  
  
  
  

Broad absorbance peak for MnOz

Absorbance

 

 

 

0.3 ‘
o f T . . r . . . . . . ‘7
190 240 290 340 390 440 490 540 590 640 690 740 790

Wavelength (nm)

Figure 5. UV-vis absorbance spectrum of manganese oxide prepared by
ozonation, conﬁrming the presence of MnOz.

29

 

100

1
80 T
1

Weight (%)

Removal of physisorbed water

 

 

 

 

oi ' '160' 200 300 -40."- isoof ‘600
Temperature (°C)

Figure 6. TGA of MnOz prepared using ozonation. Physisorbed water is removed
at approximately 80°C [1 12].

3.3.2 CHARACTERIZATION OF SINTERED PARTICLES

Using the results of the formaldoxime, o-tolidine and atomic absorption analyses,
the average oxidation state of Mn in the sample sintered at 550°C for 30 minutes
was 3.88 :l: 0.06 and was 3.08 :I: 0.09 in the sample sintered at 500°C for 45

minutes.

Since the wet chemical analysis did not allow for the identiﬁcation of the sintered
samples as exactly MnOz or Mn203, XPS analysis was performed on the sintered

samples to determine the oxidation state of the Mn. The Mn 2p3 binding energy

30

peak for the sample sintered at 550°C for 30 minutes was 641.7 eV, while for the

sample sintered at 500°C for 45 minutes, was 642.3 eV (Figure 7). The Mn 2p3

peak for MnOz is at 642.5 eV, while for Mn203, the peak is at 641.6 eV.

 

 
    

 

 

 

 

 

30000
641 .7 V

25000 - e

20000 - ._.
3 . 3:
g 1 5000 - a; - 3

10°00 ' 642.3 eV

5000 - + MnOx sintered at 550°C - 30 min i
—-— MnOx sintered at 500°C - 45 min
0 1 e . . .
680 670 660 650 640 630 620

Binding Energy (eV)

Figure 7. XPS results from the MnOx samples that were pressed and sintered at

550°C for 30 minutes and at 500°C for 45 minutes showing peaks at 641.7 eV
and 642.3 eV, respectively.

Using XRD, all of the peaks for both sintered samples, as well as the purchased

Mn203 were indexed as a-Mn203, while the purchased Mn02 that was not
sintered was indexed as MnOz (pyrolusite), as shown in Figure 8. Therefore,

sintering of the hydrated MnOz at either 500°C for 45 minutes or 550°C for 30

minutes resulted in a reduction to a-anOg.

31

 

(25'2) MnOx sintered at 550°C - 30 min

    
  
   

 

(”04) (332) 341) (440)

 

(2 2I
MnOx sintered at 500°C - 45 min

 

  
   
    

 

 

 

b
3. ) (004) (332) (341) (44°)
'3 (2 2)
g Purchased Mn203
.E c)
“i” Pi" (3399:" f‘i”
(1 0)
Purchased "no;
(211)
d) (310) (3 1)
10 210 30 40 50 60 70 80

Figure 8. XRD results from manganese oxide that was pressed and sintered a)
at 550°C for 30 minutes, b) at 500°C for 45 minutes, c) purchased Mn203 that is
unsintered and d) purchased Mn02 that is unsintered.

TEM microscopy and selected area diffraction (SAD) were also used to
characterize the sintered particles. Figure 9 is a representative TEM micrograph
of the manganese oxide sintered at 500°C for 45 minutes. The inset (Figure 9,
inset at the top right) of the SAD pattern (100 nm aperture, 100 cm camera

length) shows a polycrystalline ring pattern. The ring pattern was indexed as d-

Mn203 (measurements were made with Simple Measure software, JEOL, Tokyo,
Japan). Although the sintering temperature was below 550°C, the MnOz was still

reduced to Mn203 during the sintering process, most likely due to the greater

reactivity of the nanoparticles as compared to typical micron-sized particulate

32

sintering behavior [108, 109]. As shown in Figure 10, the particles of the sintered
material are now smaller in size (average particle size 21 nm i 5 nm) than the
gel that was initially prepared. The removal of the physisorbed water and the
rearrangement of the atoms from the hydrated Mn02 gel structure to the
crystalline structure with the concomitant reduction to o-Mn203, results in the

decrease in particle size during sintering.

 

Figure 9. TEM micrograph of the manganese oxide prepared by ozonation,
sintered at 500°C for 45 minutes. Inset is the selected area and diffraction pattern
showing polycrystalline ring pattern (100 nm aperture).

33

 

Figure 10. TEM micrograph of the manganese oxide prepared by ozonation and
sintered at 500°C for 45 minutes, with an average particle size of 21 nm 1 5 nm.

3.3.3 CHARACTERIZATION OF PARTICLES PREPARED BY MIXING OF

KMnO4 AND PAH

To compare the effects of coating the membranes with Mn02 nanoparticles that

have a smaller initial particle size, a second method was used to prepare the
colloid. This second method for preparing manganese oxide involved the mixing

of KMnO4 and PAH solutions [49]. This procedure yielded uniform spherical

34

particles with a diameter of approximately 11 nm 1 1 nm (Figure 11). This is
consistent with the observations of Luo [49] who reported particles with a size

distribution from 6 - 12 nm with the mode for the distribution being 10 nm.

 

(,i a.
gs

Figure 11. TEM image of manganese oxide prepared by mixing of KMnO4 and
poly(allylamine hydrochloride) (unsintered particles), with an average particle
size of 11 nm :I:1nm.

To conﬁrm that the manganese oxide prepared using this method was in fact
MnOz, UV-vis absorption spectroscopy was used. The UV-vis spectmm (in

Figure 12) shows the initial peaks centered at 315 nm, 525 nm and 545 nm from

35

the KMnO4 (red curve), being replaced with a broad peak at 340 nm for the
sample that was mixed with the PAH (black curve). This is indicative of the
presence of MnOz [45, 46, 49]. These results are consistent with the results of
Luo [49] where the time-dependent UV-vis spectra showed a gradual decline in
the peaks at 315 nm, 525 nm and 545 nm with the gradual appearance of a new

peak at 370 nm.

3.5
:, —~— KMnO4 before mixing with PAH
3'0 i" + MnOz prepared by mixing of KMnO4 and PAH

2.5
2.0

1.5

Absorbance

1.0

0.5

 

o ‘11—‘7‘1T' ‘ 'T i ’ ’_T' T‘“ "1W. 7 V CIA" ' ' ’7'" V V l

190 240 290 340 390 440 490 540 590 640 690 740 790
Wavelength (nm)

Figure 12. UV-vis absorbance spectrum of KMnO4 before mixing with PAH and
MnOz prepared by mixing of KMnO4 and PAH. The broad peak at 340 nm is
indicative of Mn02 [45, 46, 49].

36

Both methods of preparing the manganese oxides resulted in nanoparticles of
MnOz. The method of mixing KMnO4 and PAH resulted in Mn02 nanoparticles of
a much ﬁner particle size (11 nm :I: 1 nm) than the particles produced by
ozonation (92 nm 1: 14 nm), however, the particles prepared by mixing of KMnO4

and PAH are highly agglomerated.

3.4 CONCLUSIONS

A novel method was used to produce manganese oxide nanoparticles. The

ozonation of Mn(ll) resulted in the formation of highly irregular ﬂakes of hydrated
MnOz, as conﬁrmed by TEM, UV-vis absorption spectroscopy and TGA. The

average size of the hydrated particles was 92 nm 1 13 nm. Physisorbed water

was driven off at approximately 80°C.

The determination of the phase of the pressed and sintered (at 500°C for 45 min

or 550°C for 30 min) particles was inconclusive using wet chemical analysis and
XPS. The manganese oxide in the sintered samples was identiﬁed as a—Mn203
using XRD and SAD. Although the 500°C sintering temperature was chosen to
avoid the reduction of MnOz, the nanoparticles did reduce to d-Mn203 upon

sintering, even at the lower temperature.

37

Removal of the physisorbed water and the rearrangement of the atoms from the
hydrated MnOz gel structure to the crystalline structure during the reduction to a-
Mn203, resulted in a decrease in the particle size during sintering. The average

size of the resulting sintered particles was 21 nm i 5 nm.

The mixing of KMnO4 and PAH solutions resulted in Mn02 nanoparticles, as

determined by UV-vis absorption spectroscopy. These particles were quite
uniform in size and shape, with an average diameter of 11 nm 1: 1 nm, although

highly agglomerated.

38

CHAPTER 4

CATALYTIC MEMBRANE PREPARATION AND CHARACTERIZATION

 

4.1 INTRODUCTION

Coating a membrane with manganese oxide is expected to produce a catalytic
surface, which in the presence of ozone, will reduce fouling caused by natural
organic matter (NOM) that deposits on the membrane surface. The deposition of
NOM on membranes changes the contact angle, surface charge and

hydrophilicity of the membranes [61 , 113-116].

Jucker and Clark [113] have shown that as the mass of hydrophilic humic
substances on the surface of a membrane increases, the contact angle
decreases. A decrease in the contact angle indicates an increase in the
membrane hydrophilicity [113]. However, Violleau et al. [116] have shown that

the wettability of fouled membranes follow the hydrophilic or hydrophobic nature

39

of the foulants. In addition to being inﬂuenced by the humic substances, the
wettability of fouled membranes is also inﬂuenced by the membrane material

[113,114,116-138].

Yuan and Zydney [115] have shown that the deposition of humic acid during
fouling occurs primarily on the surface of the polyethersulfone (PES) polymeric
membrane, rather than in the pore structure. This humic acid deposition on the
surface of the membrane has the effect of changing the wettability of the

membrane [1 15].

The ﬂux decline during ﬁltration of humic acids is due to several factors. It is a
result of the adsorption of humic acid on or within the pores of the membrane,
humic acid deposition during ﬁltration and humic acid concentration polarization
[115]. Yuan and Zydney [115], determined that the fouling is dominated by humic
acid deposition for higher molecular weight cutoff membranes while the effects of
concentration polarization dominate the fouling behavior for smaller pore size
membranes. They also determined that humic acid adsorption had only a small
contribution (<7%) to the ﬂux decline due to the electrostatic repulsion between
the negatively charged humic acid and the negatively charge PES membranes
(at pH 7). Although the humic acid adsorption did not result large ﬂux declines, it
did contribute to large changes in the contact angle of the membranes [115].
After humic acid ﬁltration, the contact angle increased 44° to 74°. These results

of Yuan and Zydney indicate that there is a thin layer of humic acid adsorption

40

within the membrane pores although the humic acid deposition is primarily on the

upper surface of the membrane [115].

Ozone is a powerful oxidant that has a high reactivity with NOM, mainly reacting
with double bonds, activated aromatic systems and non-protonated amines [63,
139]. The reaction of ozone with NOM has been shown to reduce membrane

fouling [2, 3, 54, 57, 59, 65].

When ozone is added to NOM containing waters, the rate of ozone consumption
is faster than when ozone is added to non-NOM containing waters [140].
Westerhoff et al. [140] found that 60% of the ozone was consumed during the
ﬁrst minute when ozone was added to NOM containing water. The initial
oxidation reactions, such as ring cleavage, result in the formation of new
reactions sites during ozonation [140]. These oxidation products as well as the
slower-reacting NOM sites (such as carbon-carbon single bonds) are responsible

for the slower second-phase of ozone decomposition [140].

With low ozone doses, Zhu et al. [141] showed that the zeta potential of the
suspended particles in the source water (secondary efﬂuent from BeiXiaoHe
waste water treatment plant, Beijing, China) to be lower, resulting in aggregation
of particles. Increasing the ozone treatment time resulted in further oxygenation
of the organic matter, increasing the absolute value of the zeta potential,

decreasing the aggregation of the particles [141].

41

The degradation of organic matter by ozone and the breakdown of larger
particles into smaller ones results in decreased membrane fouling [63, 65, 139-
142]. The decrease in fouling is due to a reduction in the ﬁltration cake layer
formation [65, 141]. The resistance of the ﬁltration cake results in a reduction in

the ﬂux through the membranes [65, 141].

Surface modiﬁcation, such as the addition of coatings, affects the surface
morphology of coated ceramic membranes [4, 143-149]. An increase in the
roughness of the membrane surface has been shown to increase contact angles,
increasing the hydrophobic nature of the membrane [113, 150]. Also,
modiﬁcation of the surface charges on ceramic membranes will change the

contact angle as well as the separation performance of the membrane [151-153].

The surface of an oxide will contain a layer of hydroxyl groups, which will cause
the surface to acquire a positive charge in an aqueous solution that has a pH
less than the isoelectric point (IEP) of the oxide [154]. If the pH of the aqueous
solution is higher than the IEP of the oxide, the surface will acquire a negative
charge [154]. Since fouling is promoted by the absorption of ions or solute
components to the surface of the membrane [155], the surface charge of the

. membrane affects its fouling performance.

42

The streaming potential is a method of determining the surface charge on
membranes [155]. The streaming potential of multilayered membranes, however,
are a combination of the streaming potentials of the multiple layers of the
membrane since the different layers can have different charges in aqueous
solutions [155]. The addition of a coating layer to the ceramic membranes will
affect the overall streaming potential of the membrane. The work presented in
this chapter will describe the surface morphology characterization of the coated
and sintered ceramic membranes. The streaming potential was not measured in
this particular work. However, additional studies should be completed to
determine the effects of the surface coating on the streaming potential of the

membranes.

In addition to the surface changes that result from the application of a coating to
the membrane, the sintering of the coated membrane can also affect the surface
morphology of the membrane [4, 68]. Characterization of the coated membranes
is necessary [156-158] to determine the effects of the coating and sintering on
the roughness, grain size and pore size of the ﬁltration surface. Researchers
have characterized membranes using AFM, SEM and TEM to study pore size [4,

144-147], roughness [4, 144-146] and grain size [4, 147].

This Chapter discusses the effect of particle preparation method, the number of

coating layers and the sintering temperature and time on the structure of the

resulting catalytic coating. Atomic force microscopy (AFM), scanning electron

43

microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and
transmission electron microscopy were used to Characterize the surface
morphology and topography as well as the coating thickness of manganese

oxide coated ceramic membranes.

4.2 MATERIALS AND METHODS

Manganese oxide colloids were prepared by ozonation and by mixing of KMnO4

and PAH using the methods described in Chapter 3.

4.2.1 LAYER-BY-LAYER COATING OF MEMBRANES

The particles produced using ozonation were washed and dispersed before use.

First, centrifugation (ThermoForma General Purpose Centrifuge, Therrno
Scientiﬁc, Waltham, MA, 30 min at 2500 rpm) was used to settle the Mn02
particles and the supernatant was subsequently decanted and discarded. The
solids were then mixed with 104 N KNO3 (299.0%, J.T. Baker CO., Phillipsburg,

NJ), and sonicated (58 W, Branson Ultrasonics Corporation Model 250 Soniﬁer,

Danbury, CT) for 30 min in an ice bath to disperse the nanoparticles.

44

A layer-by-layer technique developed by Lvov et al. [91] and Espinal et al. [94]
was used to coat the ceramic membranes with manganese oxide nanoparticles.
The coating procedure was as follows: the ceramic membrane was immersed for
15 min in a 0.2 wt% poly(diallyldimethylammonium chloride) (PDDA) solution
(Aldrich, average MW < 100,000), then rinsed for 15 s with 0.01 M NaOH
(298.0%, J.T. Baker Co., pH 12), immersed for 15 min in the manganese oxide
suspension and rinsed for 15 s with 0.01 M NaOH. This sequence produced one
layer. The sequence was then repeated to obtain the desired number of layers of
nanoparticles. After coating, the membrane was sintered in air at either 550°C
for 30 min or 500°C for 45 min (temperature ramping up at a rate of

approximately 10°C/min and allowed to free cool over 8 hours).

The ceramic membrane (INSIDE CéRAM, TAMI North America, St. Laurent,
Quebec, Canada) had a tubular design with 7-channels, and as documented on
the TAMI website [159], composed of a titanium oxide ceramic support with a
titanium oxide ﬁltration layer (although EDS also shows zirconium oxide present
on the surface of the membrane). The outside diameter of the membrane was 1

cm with a length of 25 cm. The nominal molecular weight cut-off was 5 kDa.

45

4.2.2 MICROSCOPY SAMPLE PREPARATION

A schematic representation of the AFM sample preparation is given in Figure
13a. AFM sample preparation started with cutting a 1 mm thick cross section
from the coated membrane using a diamond-wafering saw. An arc-shaped
specimen taken from this section was mounted on a stainless steel disc (12 mm
in diameter) using double-sided adhesive tabs so that the manganese oxide

coated surface was pointing upward.

AFM images were obtained using a Nanoscope lIl Multimode Atomic Force
Microscope (Digital Instruments Inc.) in air, using the contact mode. A triangular
Si3N4 NP probe (Veeco Instruments, CA), with a nominal cantilever spring
constant of 0.12 N/m and nominal frequency of 20 kHz, was used. The tip height
was 2.5 pm to 3.5 pm, with a nominal radius of 20 nm and a side angle of 35°.
Five different areas, of dimension 20 um x 20 pm, were scanned at a rate of 0.5
Hz on each sample so that average values could be determined for the

roughness and height of each sample.

AFM roughness and height data was obtained using the Nanoscope program
(Nanoscope 5.30 r3 sr3, Veeco Instruments, CA). The average roughness (Ra)

for the image is deﬁned as the arithmetic average of the absolute values of the
surface height deviations measured from the mean plane, rather than the

frequency or spacing of the features. The height comparisons were performed

46

using the maximum roughness (Rmax), deﬁned as the maximum vertical distance

between the highest and lowest data points in the image.

Step 1 Ceramic membrane Slice of 7-channel
from SUPPIiel’ membrane

. Titanium oxide
Cut CTOSS 59°90” (563$ ceramic membrane

~ :1 O 1
132$ Mn02 coating layer

5‘ {.1

S 2 . Cross sectioned sample
tep SlIce Of membrane arc approximately 3 mm x 1 mm
approximately 1 mm thick

68% Cut small arc :MnOz coating layer

. ~
Q59») Titanium oxide

ceramic membrane

Step 3: Preparation of AFM samples Step 3: Preparation of SEM samples

For AFM surface For SEM characterization of
roughness analysis coated surface
Direction of viewing Direction of viewing
Steel disc

Aluminum stub
a) b)

Step 3: Preparation Of EDS samples Step 3: Preparation of TEM samples

For EDS of compositional changes For TEM coating thickness
with depth from coated surface measurements

Direction of viewing / Sample Direction of viewing
' Sam... U gig

- Copper rid
AlumInum stub Top view 9 Front view

C) d)

Figure 13. Schematic representation of the sample preparation for a) AFM, b)
SEM, c) EDS and d) TEM.

47

Fourier transform analysis (MATLAB R2008b, The MathWorks, Natick, MA) was
also used to analyze AFM images. To look for possible changes in amplitude or
frequency of the surface features, 30 AFM line scans were analyzed per sample
using the fast Fourier transform (FFT) function. The Fourier transforms were
plotted and the amplitude and frequencies of the surface features were

compared.

In a similar method as for the AFM samples, SEM samples were prepared by
cutting a 1 mm thick cross section from the coated membrane using a diamond-
wafering saw. This section was then cut into a small arc-shaped specimen of
approximately 3 mm in length. The arc-shaped specimen was then placed on an
aluminum SEM stub with carbon tape, so that the coated surface of the
membrane was pointing upward. A schematic representation of the SEM sample

preparation is given in Figure 13b.

A schematic representation of the EDS sample preparation is given in Figure
13C. Samples were prepared so that the small arc-shaped portions of the
membrane were placed ﬂat on the aluminum stub such that the cross sectional
surface of the membrane was pointing upward so that the depth of penetration of
the manganese into the membrane could be observed using EDS. In this
manner, a map of the spatial distribution of manganese within the sample was

determined.

48

Since the samples were non-conductive, they were gold-coated for SEM and
carbon-coated for the EDS analyses. A layer of gold was applied using a gold
sputter coater (Emscope SC 500, Ashford, Kent, Great Britain) at a rate of 7
nm/min, with a current of 20 mA, for a total thickness of approximately 21 nm.
For EDS analysis, the samples were carbon-coated using a carbon string
evaporator (EFFA Mk II carbon coater, Ernest Fullam Inc., Latham, NY), since

the gold coating would interfere with the detection of the surface elements.

SEM micrographs were obtained using a JEOL 6400V SEM (Japan Electron
Optics Laboratories, Tokyo, Japan) with a LaBs emitter (Noran EDS, Noran

Instruments Inc., Middleton, WI) at an accelerating voltage of 15 kV and
magniﬁcation of 25,000x. Average grain size measurements were calculated
from ﬁve SEM micrographs of each sample using the intercept method (ASTM
Standard E 112-96 (2004)) where an average of 200 grains were measured per
micrograph. A Noran EDS analyzer (Noran Instruments Inc., Middleton, WI,
accelerating voltage of 20 kV and magniﬁcation of 1,000x) was used for EDS

microanalysis.

TEM analysis was used to characterize the microstructure, the coating thickness
and the phase of manganese oxide present on the surface of the coated ceramic
membranes. XRD was not used to determine the phase of the manganese oxide
on the surface of the membrane (as it had been used to determine the phase of

the sintered particles in Chapter 3) since the penetration of the x-ray beam into

49

the sample would not allow for accurate determination of the very thin surface
layer of manganese oxide. A schematic representation of the process used to
prepare the TEM samples is given in Figure 13d. To prepare TEM grids, a
diamond-wafering saw was used to slice the membrane into a thin wafer,
approximately 1 mm thick. This thin wafer was then cut into an arc,
approximately 3 mm long, using a razor blade. The arc was mounted on a stub in
order to thin and was hand polished using 15 pm then 5 pm polishing paper,
followed by 6 pm, 3 pm and 1 pm diamond paste. The polished surface was then
mounted face down on a slotted copper TEM grid (3 mm outer diameter) and the
entire grid and sample were mounted on a stub for further polishing, using the
same sequence of successively ﬁner polishing paper and diamond paste, until a

thickness of approximately 100 um was achieved.

The sample was then dimpled (GATAN Precision Dimple Grinder, Model 656,
Pleasanton, CA) to thin the center to approximately 50 pm, while avoiding
damage to the outer portion of the sample and then ion milled (GATAN Precision
Polishing System Model 691, Pleasanton, CA) at an accelerating voltage of 4.0
keV and a beam inclination angle of 4° until a hole just started to form at the
center of the coated membrane surface. The ﬁnal specimens were examined
using a TEM (JEOL 2200FS, Tokyo, Japan, 200 kV accelerating voltage).
Photomicrographs were collected using a Hamamatsu CCD digital camera
(Hamamatsu City, Japan) to observe the layered structure of the ceramic

membranes. Selected area diffraction patterns were collected using a

50

nanoaperture of 100 nm in diameter and a camera length of 100 cm and indexed
(using the Simple Measure software, JEOL, Tokyo, Japan) to identify the

manganese oxide phase present on the surface of the coated membrane.

One-way ANOVA testing, with Tukey’s mean comparison tests (95% conﬁdence
interval) (Minitab statistical software, Minitab Inc.), was used to determine if
differences in feature heights, roughness and grain size measurements were

statistically signiﬁcant.

4.3 RESULTS AND DISCUSSION

4.3.1 ATOMIC FORCE MICROSCOPY (AF M) ANALYSIS

AFM analysis was performed on membranes that were 1) as received, 2)
uncoated and sintered at 500°C and at 550°C, 3) coated with manganese oxide
prepared using ozone with 20 layers and sintered at 500°C and 550°C, 4) coated
with 30 and 40 layers and sintered at 500°C, 5) coated with manganese oxide

prepared by mixing of KMnO4 and PAH with 20 layers and sintered at 500°C and

550°C and 6) coated with 30 layers and sintered at 550°C.

51

The feature height, determined by AFM imaging (the average and standard
deviation of the Rmx for 5 different 20 pm x 20 pm areas of each sample) is

shown in Figure 14.

 

3.5

 

3.0

 

2.5

 

 

 

 

 

 

Height. Rmax (um)

 

 

 

 

IIILJ lILILLI

IIIII IIIIIII
llIll IlLllIl

IIII.

 

 

 

 

 

 

 

IIIIIl IIIIIT]

111111 1111111

 

 

 

 

 

 

 

 

 

 

 

50 5—30—550
PAH

I\.)
C)
01

500 50—500 50650 520500 520—550 530500 540-500 520-500 5
Ozone Ozone Ozone Ozone PAH

U
>
I

Membrane

Figure 14. Feature height data from AFM images of membranes that were
uncoated or coated and sintered at various conditions. The nomenclature for the
naming of the samples is that the ﬁrst number is the molecular weight cut—off of
the membrane (e. g. 5 kDa), the second is the number of coating layers and the
third is the sintering temperature (in °C, where 0 indicates that the membrane
was unsintered). For the coated membranes, the word ozone after the name of
the sample indicates that the manganese oxide particles were prepared using
ozonation while PAH indicates that the manganese oxide particles were prepared
by mixing of KMnO4 and PAH. Error bars indicate the standard deviation based
on 5 different scans of each sample.

Although not statistically signiﬁcant, the means show differences in feature
heights. FOr example, upon sintering an uncoated sample (average height 1.64

pm :I: 0.72 pm), there is a decrease in the feature height (average heights for the

52

sample sintered at 500°C and at 550°C are 1.08 pm :I: 0.20 pm and 1.19 pm :I:
0.46 pm, respectively). This decrease in feature height is to be expected since
the partial pressure of the diffusing species is higher over a convex surface, such
that there is a driving force during sintering that causes atoms to move from the
convex to the concave areas [97]. As the number of manganese oxide layers
increases from 20 to 30, the feature height also increases, then decreases with
40 layers. The height is greatest for the membrane coated with 30 manganese

oxide layers prepared using ozone, with the average height for this sample being
2.21 pm 1: 0.82 pm. With the manganese oxide prepared by mixing of KMnO4

and PAH, the average height for the sample coated with 30 layers was 1.77 pm 1:
0.74 pm. This decrease in feature height with the application of 40 layers of

coating is consistent with what was observed by Kamik et al. [4] in their work with
Fe203 coated ceramic membranes. The decrease in feature height with the

application of 40 layers is likely due to the coating material ﬁlling the valleys
between the features, having the effect of reducing the overall feature height with

increasing number of coating layers.

The roughness data for all membranes is shown in Figure 15. Trends similar to
those observed for the maximum feature height were observed for the roughness
data. The differences in roughness were not statistically signiﬁcant. There was a
decrease in overall roughness when the uncoated membrane (average
roughness 150 nm 1: 57 nm) was sintered. The roughness of the membrane

sintered at 500°C (average roughness 111 nm 1 9 nm) was less than the

53

membrane sintered at 550°C (average roughness 146 nm :I: 56 nm). The
decrease in roughness due to sintering is also expected expected since the
partial pressure of the diffusing species is higher over a convex peaks on the
sample, with the driving force during sintering causing atoms to move from the
convex to the concave valleys [97]. Although the mean of the roughness for the
sample sintered at 550°C is higher than the sample sintered at 500°C, the
statistically there is no difference due to the high deviation in the roughness for
the sample sintered at 550°C. As the number of layers of the manganese oxide
particles was increased, there was little change in roughness of the samples. The
roughness was greatest for the membrane coated with 30 layers of the
manganese oxide particles prepared using ozone, with the average roughness

for this sample being 170 nm :I: 43 nm, while with the manganese oxide prepared
by mixing of KMnO4 and PAH, the average roughness for the sample coated with

30 layers is 150 nm :I: 59 nm. This is consistent with the results of Kamik et al. [4]
for iron oxide nanoparticle layers on ceramic water ﬁltration membranes and
Reilly et al. [160] found that for multilayer ﬁlms of polyelectrolyte molecules on
glass slides, there was little change in roughness with an increasing number of

coating layers.

Although the height and roughness of the uncoated membranes decrease after
sintering, the number of features appeared to increase (see Figure 16 a-c). The
number of features present after coating the membranes with manganese oxide

prepared using ozone (Figure 16 d-g) and with manganese oxide prepared by

54

mixing of KMnO4 and PAH (Figure 16 h-j) and sintering at 500°C and 550°C

appeared to increase with the number of coating layers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

250
200 I. , T I T
E
5 _
m 150
3' ::
g a as
I .-
8 mo -— e as _
3 E: :
g a: =
:5 E
50 ~— — 55 2
II :
E: :
E: :
E: :
0 E: :

500 50500 50550 520500 520550 530500 540500 520500
Ozone Ozone Ozone Ozone PAH

Membrane

US
)

0—550 5—30—550
PAH

Figure 15. Roughness data based on AFM imaging of membranes that were
uncoated or coated and sintered at various conditions. The nomenclature for the
naming of the samples is the same as for Figure 14. Error bars indicate the
standard deviation based on 5 different scans of each sample.

Fourier transform analysis [161] of line scans of the samples, however, did not
show any signiﬁcant change in the amplitude or frequency of the surface features
on any of the samples. In all samples, the highest amplitudes were at the lowest
frequencies with a steady decline in amplitude with increasing frequencies. A
representative ﬁgure of the Fourier transform data for the AFM samples is shown

in Appendix A.

55

 

Figure 16. AFM images of ceramic membranes a) 5-0-0, b) 5-0-500, c) 5-0-550,
d) 5-20-500 Ozone, e) 5-20-550 Ozone, f) 5-30-500 Ozone, g) 5-40-500 Ozone,
h) 5-20-500 PAH, i) 5-20-550 PAH and j) 5-30-550 PAH. The scale for all
samples is x: 5.00 um/division and y: 3000 nmldivision. The nomenclature for the
naming of the samples is the same as for Figure 14.

56

The manganese oxide coatings were applied to porous substrates. Other studies
of coatings on porous substrates have shown similar changes in surface
topography [162-164]. Cai et al. [162] and Shaoqiang et al. [163] observed a
decrease in AFM feature height with the application of coatings on porous silicon
substrates, due to the partial ﬁlling of the coating particles into the pores of the
substrate. Ghosh et al. [164] observed an initial increase in the average
roughness of a porous silicon substrate with the deposition of an indium tin oxide
coating and then a decrease in the average roughness with increasing deposition

of the coating.

4.3.2 SCANNING ELECTRON MICROSCOPY (SEM) ANALYSIS

The SEM micrographs of the uncoated and coated ceramic membranes are
shown in Figure 17 a-j. Although the difference in grain size is not statistically
signiﬁcantly, analysis of the SEM data showed a similar coarsening of the grains
as was seen in the roughness data from the AFM analysis (a summary of the
AFM and SEM observations is provided in Table 1). The average grain size for
the uncoated membrane increases from 71 nm :I: 5.1 nm to 76 nm :I: 5.5 nm when
the uncoated membranes were sintered at 500°C. The grain size did not change
when the uncoated membrane was sintered at 550°C, compared to the

unsintered membrane.

57

For both coating preparation methods, the grain size (where the average size
and standard deviation were calculated from ﬁve SEM micrographs with an
average of 200 grains measured per micrograph) increased as the number of
manganese oxide coating layers increased. A statistically signiﬁcant difference
was observed between the grain size for the membrane coated with 40 layers
(87nm i 7.6 nm) of the manganese oxide prepared by ozonation and all other
samples (shown in Figure 18). A statistically signiﬁcant difference was also
observed between the grain size for the membrane coated with 30 layers of the
manganese oxide prepared by mixing of KMnO4 and PAH (79 nm 1 3.8 nm) and
the uncoated and unsintered membrane (71 nm :I: 5.1 nm), the uncoated
membrane sintered at 550°C (71 nm 1 7.0 nm) and the membrane coated with
20 layers of the manganese oxide prepared by ozonation and sintered at 500°C
(72 nm 1: 5.8 nm). These observations were consistent with what was observed

by Kamik et al. [4] with the Fe203 coated ceramic membranes, where little

change in grain size was seen until 40 layers had been applied.

  

E is 1pm
Figure 17. SEM micrographs of ceramic membranes a) 5-0-0, b) 5-0-500, c) 5-0-
550, d) 5-20—500 Ozone, e) 5-20-550 Ozone, f) 5-30-500 Ozone, 9) 5—40-500

Ozone, h) 5-20-500 PAH, i) 5-20-550 PAH and j) 5-30-550 PAH. The
nomenclature for the naming of the samples is the same as for Figure 14.

58

Figure 17. Continued

 

59

Table 1: Summary of AF M and SEM Observations for all membrane samples.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Maximum Average Average
Feature Roughness Grain
Parameter Height (pm) (nm) Size (nm)
Method used AFM AFM SEM
5-0-0 1.64 1 0.72 150 1 57 71 1: 5.1
5-0-500 1.08 1: 0.20 111 1 9 76 1 5.5
5-0-550 1.19 1 0.46 146 1 56 71 1 7.0
g 5-20-500 Ozone 1.62 1 0.47 167 1 33 72 1 5.8
g 5-20-550 Ozone 1.55 1 0.19 163 1 43 74 :t 4.9
g 5-30-500 Ozone 2.21 :I: 0.82 170 1 43 75 1 6.3
5 5-40-500 Ozone 1.60 1 0.66 157 1 43 87 1 7.6
5-20-500 PAH 1.07 1 0.26 114 1 22 77 1 4.7
5-20-550 PAH 1.06 1 0.62 138 1 37 75 1 4.3
5-30-550 PAH 1.77 1: 0.74 150 1 59 79 :I: 3.8

 

 

When comparing membranes with the same sintering conditions, there is an
increase in grain growth with increasing coating layers. Since the colloids are
prepared and sonicated and then used to coat the membranes; with the
additional number of coating layers, the colloids are more likely to have
agglomerated by the time that the suspension is used to coat a membrane with a
greater number of coating layers (it takes over 20 hours to coat a membrane with
40 layers). The increase in agglomerates as additional material is applied results
in the changes in the sintering behavior. The agglomerated particles will sinter
together into the larger gains. Sintering of agglomerates does not allow the

material to densify and will maintain porosity [97].

During sintering, large grains will grow at the expense of smaller grains and will

result in grain coarsening [97, 98, 165, 166]. As the grain boundaries migrate, the

60

pores between the grains will move with the grain boundaries and coalesce into
larger pores at the intersection of grain boundaries [97]. Therefore, sintering of
the agglomerates has the effect of coarsening the grains, and subsequently the

pores, on the ﬁltration surface of the membrane.

 

120

 

100

 

            

 

Average Grain Size (nm)

 

 

 

 

 

TTTITTI IFTXLII 111111 1111111

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

._
._.
,—
,—
._
.—

111I1111 IIIIIII

.—
.—

 

 

 

0‘ .LIIIJII IIIIIIIIIIIIIIIIIFIIIII]

500 5—0- 500 5—0—550 5- 20—500 5—20- 550 530500 540500 520500 —20—
Ozone Ozone Ozone Ozone PAH PAH

50 5—30-550
PAH
Membrane
Figure 18. Comparison of grain sizes (calculated using intercept method) for
uncoated and coated ceramic ﬁltration membranes. The nomenclature of the

naming of the samples is the same as for Figure 14.

4.3.3 ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDS) ANALYSIS

Energy Dispersive X-ray Spectroscopy (EDS) microanalysis mapping was
performed on a cross section of an uncoated ceramic membrane. EDS mapping

of the as-received, uncoated and unsintered membrane (Figure 19) showed the

61

membrane composition to be a porous Ti02 support with ZrOz and TiOz on the
ﬁltration surface (the ﬁltration surface is on the right hand side of the images).
The membrane supplier’s product characteristics data, however, describes this
membrane as having only a Ti02 active layer for the 5 k0, INSIDE CéRAM
membranes used in this study [159]. The details of the membrane manufacturing
are not known, however, it is likely that the small amounts of zirconia were added

to aid in the manufacturing process of the filtration layer.

inYA’H' ‘ l I. ‘ "V .

 

E is 50 pm

Figure 19. EDS mapping image of uncoated and unsintered ceramic membrane.
The ﬁltration surface is on the right hand side of the images.

62

Cross-sectioned samples of the membranes coated with 40 layers of the
manganese oxide prepared using ozone and 30 layers of the manganese oxide
prepared by mixing of KMnO4 and PAH were analyzed with EDS mapping
(Figure 20). The Mn elemental map clearly demonstrates that Mn is present
throughout the membrane, rather than remaining on the ﬁltration surface. Thus,
the manganese oxide nanoparticles have penetrated into the membrane, adding

to the catalytic ability of the membrane.

Diffusion of the manganese oxide coatings into the membrane is similar to the
diffusion of Fe203 nanoparticles in the alumina-zirconia-titania (AZT) membranes
used by Kamik et al. [4]. The uniform diffusion of manganese into the membrane
was conﬁrmed by a quantitative analysis of the EDS line scans of the coated
membranes (Figure 21). The membrane coated with 30 layers of the manganese
oxide prepared using ozone tend to have higher Mn counts further into the
membrane (the data was normalized based on Mn counts at the surface of each

membrane sample) than the membrane coated with 30 layers of the manganese

oxide prepared by mixing of KMnO4 and PAH.

63

1
Electron Image

 

Z_ LoI1 M_ Kq1

 

E is 400 um

10540-500 Ozone

Electron Image

   

   

E is 400 pm
b) 530-550 PAH

Figure 20. EDS mapping image of 5 kDa ceramic membrane a) coated with 40
layers of the MnOz coating prepared using ozone and sintered at 500°C for 45
min and b) coated with 30 layers of Mn02 prepared by mixing of KMnO4 and
PAH and sintered at 550°C for 30 min. The MnOz coated ﬁltration surface is on
the right hand side of the images.

64

1.4

 

 

 

 

I
1.2 -
I
1.0 I A I
I A A I
2';
c 0.8 - A A A A
3 I
8
I I
I: 0.6 -
E
0.4 -
0 2 . I5-30500 Ozone
' A5-30-550 PAH
A
0.0 T I r T
0 200 400 600 800 1000

Distance from the coated surface (pm)

Figure 21. EDS line scan of 5 kDa ceramic membrane coated with 30 layers of
the manganese oxide coating prepared using ozone and sintered at 500°C for 45

min and with 30 layers of manganese oxide prepared by mixing of KMnO4 and
PAH and sintered at 550°C for 30 min. EDS line scan counts were normalized
based on Mn counts at the surface of each membrane sample.

Since Mn was seen throughout the membrane after coating and sintering, an
estimate of the diffusivity has been calculated. Using equation (2), the diffusivity,

D, can be estimated using the diffusion coefﬁcient, Do, and the activation energy,
Q, for a transition metal in an oxide host (D0 = 1.8 x 10'9 m2/s, Q = 202 x 103
J/mol [167], for Ni in M90) at a temperature of 500°C (773 K), the resulting

diffusivity is 4.0 x 10'23 m2/s (calculations provided in Appendix B).
— Q
D = D ex — 2
o p[RT) ( )

65

Using equation (3), an estimate of the diffusivity can be calculated for diffusion

across the entire membrane wall thickness (1 mm) for a sintering time of 45
minutes (2700 s). This would require a diffusivity of 3.7 x 10'7 m2/s (calculations

provided in Appendix B). This is 16 orders of magnitude higher than the diffusivity
estimated for a transition metal in an oxide host.
x z J'D’t (3)

If equation (3) is used to determine the diffusion distance using the diffusivity
estimated for a transition metal in an oxide host, for a sintering time of 45
minutes, the resulting diffusion distance is 0.33 nm (calculations provided in
Appendix B). This diffusion distance would not explain the presence of Mn
throughout the membrane. Therefore, rather than diffusing through the
membrane entirely by solid state diffusion, the coating must be dispersed through
the membrane during the coating process and then diffuses into the grains of the

membrane by solid state diffusion during sintering.

The ceramic membrane is composed of very ﬁne pores on the ﬁltration surface
and large pores within the support of the membrane. Liquid ﬁlling of a channel is
dominated by capillary action due to surface tension as the channel size
decreases to the nanoscale [168]. This capillary action draws the coating
material through the pores of the ﬁltration layer and into the pores of the support
layer of the membrane during the coating process. The coarse membrane
support, with high porosity, is shown in Figure 22. During sintering, solid state

diffusion allows the manganese oxide coating to diffuse from the pores into the

66

grains of the membrane, resulting in the distribution of manganese throughout

the cross section of the coated membrane.

 

high porosity.

The large particle size of the as prepared Mn02, however, would be too large to
ﬁt through the small pores (approximately 1.5 nm) on the ﬁltration surface of the
membrane. Therefore, for the Mn to be present thrOughout the membrane, there
must be some residual Mn ions present in the colloid that is used for coating the
membranes. To study this, after the MnOz was dispersed in the KN03 and

sonicated, rather than using it to coat a membrane, it was then separated by

67

centrifugation once again and the resulting supernatant was used to coat a
membrane. The EDS map of a membrane coated with 10 layers of this
supernatant and sintered at 500°C for 45 minutes is shown in Figure 23. The
presence of Mn throughout the membrane indicates that the Mn ions that are in
the MnOz suspension disperse into the pore structure of the membrane during

coating then diffuse into the grains of the membrane during sintering.

   

E is 400 um
Figure 23. EDS mapping image of a 5 kDa ceramic membrane coated with 10
layers of the supernatant from the MnOz colloid and sintered at 500°C or 45

minutes, showing Mn present throughout the membrane. The filtration surface is
on the right hand side of the images.

Since Mn is present throughout the membrane as well as on the surface, this
would serve to improve the long-term performance of the membrane. During
service the membrane may wear away [72] and while such wear is much less for

ceramic membranes as compared to that observed with polymeric membranes

68

[72], because Mn has diffused throughout the membrane, enhanced performance

would be expected to continue even if the coated surface layer has eroded.

Another advantage to having the catalyst throughout the membrane is that the
catalytic activity will be inside the membrane’s porosity, not only on the coated
surface. This would allow for smaller molecular weight compounds, which pass
through the membranes, to be catalytically oxidized by ozone within the pores of
the membranes. This will allow for the oxidation of smaller molecular weight
compounds without requiring the use of a membrane with a lower molecular
weight cut-off (MWCO), since membranes with lower MWCOs require higher

operating pressures, which increases the operating costs.

4.3.4 TRANSMISSION ELECTRON MICROSCOPY (T EM) ANALYSIS

Figure 24a shows a TEM photomicrograph of a cross section of an as received
membrane. This micrograph reveals the underlying Ti02 support and the outer

ﬁltration layer with a thickness varying from approximately 110 nm to 360 nm.
The inset diffraction pattern shows a diffuse ring pattern, indicating that the
ﬁltration layer is amorphous. Figure 24b is a TEM photomicrograph of a
membrane that was coated with 40 layers of the manganese oxide coating and

sintered at 500°C for 45 minutes. This micrograph reveals an uneven coating

layer, varying between approximately 20 nm to 73 nm in thickness, over the ZrOz

69

ﬁltration layer. The inset diffraction pattern shows a polycrystalline ring pattern,
which was indexed as a-Mn203, which is consistent with the XRD and SAD data

of the sintered particles (Figures 8 and 9).

TiOz support

 

Figure 24. a) TEM photomicrograph of a cross section of an as received ceramic
water ﬁltration membrane. Inset are the selected area and the diffraction pattern
showing an amorphous ring pattern (100 nm aperture). b) TEM photomicrograph
of cross section of a membrane with 40 layers of manganese oxide, sintered at
500°C for 45 minutes. Inset are the selected area and the diffraction pattern
showing polycrystalline ring pattern (100 nm aperture).

70

Figure 24. Continued

Filtration

 

When 20 layers of the manganese oxide were applied to a membrane and
sintered, the resulting coating thickness varied between approximately 14 nm to
54 nm (Figure 25). However, when 30 layers were applied, the resulting
thickness varies between approximately 19 nm to 110 nm (Figure 26). With an
approximately particle size of 20 nm, and assuming a close-packed arrangement
of spherical particles, the expected coating thicknesses would be 296 nm, 444
nm and 592 nm, for the 20, 30 and 40 layers, respectively, if the coating remains
only on the surface of the membrane (Figure 27). Since the ﬂakes of the

hydrated MnOz are not spherical, but rather have a higher aspect ratio, the

71

application of the hydrated MnOz ﬂakes does not result in uniform increases in
thickness with each coating layer, depending on how the ﬂakes lie on the surface
of the membrane. The coating thickness is therefore not directly related to the
number of layers applied in a linear fashion, however, there is a trend of

increasing coating thickness with increasing number of coating layers.

TiOz support

 

Figure 25. TEM photomicrograph of cross section of a membrane with 20 layers
of manganese oxide, sintered at 500°C for 45 minutes.

72

TiOz support

 

Figure 26. TEM photomicrograph of cross section of a membrane with 30 layers
of manganese oxide, sintered at 500°C for 45 minutes.

 

 

 

 

700
E 600 - +Observed
:
v +E I d
m 500 _ xpec e
m
3
x 400 -
.2
5 300 -
3’
z 200 "
3
o 100 - ;/+\71
0 I I 1
10 20 30 40 50

Number of coating layers
Figure 27. Comparison of the observed coating thickness to the expected

coating thickness for 20, 30 and 40 coating layers. The expected coating
thickness assumes a close-packed arrangement of the Mn203 nanoparticles.

73

The observations of the manganese oxide coated cross sections of the porous
membranes are consistent with the observations of other coatings on porous
substrates [162, 163]. The manganese oxide coatings were cohesive with the
underlying membranes. Cai et al. [162] made similar observations of a cross
section of a ZnO ﬁlm on a porous silicon substrate which revealed that the ﬁlm
was closely connected to the substrate, likely due to the partial ﬁlling of the
coating particles into the surface pores. Shaoqiang et al. [163] also made similar
observations of hydroxyapatite growth into the porous silicon substrate during

sintering to produce a well bonded coating on the substrate.

4.4 CONCLUSIONS

Coating a ceramic water ﬁltration membrane with these nanoparticles using a
layer-by-Iayer process, followed by sintering, yields an a-Mn203 surface layer as

well as evidence that the Mn is present in the interior of the membrane.

The AFM and SEM data showed that with increasing number of coating layers,
the roughness remains unchanged, the grain size increased and the feature
height decreased. The increase in grain size with increased number of coating
layers indicates that grain growth occurs as the thickness of the coating on the

surface of the membrane increases due to agglomeration of the coating material

74

with additional coating layers, since there was no signiﬁcant change in the grain

size due to sintering.

Since the as prepared Mn02 particles are too large to diffuse into the ﬁne pores

on the ﬁltration surface of the membranes, Mn ions that remain in the MnOz

coating suspension disperse into the pore network by capillary action during
coating. The Mn present within the pores then diffuses into the grains of the
membrane during sintering, as is indicated by the EDS mapping and line scans
of cross sectioned samples. Since Mn is present throughout the membrane as
well as on the surface, this would likely serve to improve the Iong-terrn
performance of the membrane, since the catalyst will still be present even if the
surface layer has eroded as well as allowing small molecular weight compound

contaminants to be oxidized as they pass through the membrane.

Although there is not a direct linear relationship between the number of layers

applied and the resulting coating thickness, the trend is that the coating thickness

does increase with increasing application of coating layers.

75

CHAPTER 5

HYBRID OZONATION-MEMBRANE FILTRATION

 

5.1 INTRODUCTION

The use of a hybrid ozonation-membrane ﬁltration system has been shown to
improve the water ﬂux recovery as compared to convention membrane ﬁltration
without ozonation [2, 3, 63, 65, 71]. Ozone decomposition, resulting in OH radical
formation, results in the decomposition of organic foulants on the surface of the

membrane, allowing for improved ﬂux through the membrane [65, 66].

Degradation of organic matter by manganese oxide catalysts with ozone has
been studied by others [644]. Naydenov and Mehandjiev used MnOz to
catalytically oxidize benzene by ozone [6]. They achieved complete oxidation of
benzene by ozone on Mn02 catalysts in a temperature range of 10-80°C (283-

353 K). Einaga et al. [18-24] also studied the effects of catalyst support, reaction

76

conditions and manganese loading on the ability of manganese oxide to catalyze

the oxidation of benzene in gas phase reactions. Einaga et al. determined that
alumina-supported manganese oxides catalyze the oxidation of benzene to CO2

and CO by ozone at room temperature although the rate of oxidation is
independent of the Mn loading (between 5-20 wt%) [18, 20, 24]. The alumina-
supported manganese oxide catalyst was the only catalyst to show a steady-
state activity for cyclohexane decomposition, compared to Ag, Fe, Co, Cu and Ni

catalysts on alumina supports at room temperature [19].

Einaga et at. also determined that the addition of water vapor to reaction gases
suppressed the deactivation of the Al203-supported [21] and SiO2-supported [23]
manganese oxide catalyst. The water vapor increased the oxidation of the

organic byproducts, such as formic acid, to C02.

Andreozzi et al. demonstrated the catalytic ability of Mn02 for the ozonation of
pyruvic acid [9, 11], oxalic acid [8] and metol [10]. They found that pyruvic acid
did not react with either MnO2 or 03, however, signiﬁcant oxidation was observed
when ozone was used in combination with Mn02 [9, 11]. Andreozzi et al. found
that oxalic acid reacted only slightly with Mn02 in the absence of ozone,

however, the reactivity was greatly increased with the addition of ozone [8].

Although these experiments were done in a semibatch process with suspended

77

Mn02 particles, rather than on a coated membrane, the catalytic properties of the

Mn02 in the presence of ozone seem quite promising.

Lee et al. examined the catalytic ozonation of humic acids [12]. Humic acid was
used as a model organic compound because it is a major constituent of natural

organic matter. The authors observed a signiﬁcant decrease in UV absOrption

with Mn02 combined with ozone, indicating the oxidation of the humic acids [12].

Ma and Graham [13, 14] showed that manganese oxide catalysts enhanced the
oxidation of atrazine by promoting ozone decomposition and hydroxyl radical
formation. Ma and Graham’s work also demonstrated that humic substances
have a positive effect on the oxidation of atrazine as compared to that observed

with the manganese catalyst and ozone alone [13, 14].

Manganese-containing catalysts have also been shown to be effective for the
decomposition of organic compounds in gas phase reactions [25, 30-32, 34-40,
43, 44]. Delagrange et al. [32] have shown that when non-thermal plasma
techniques are used to oxidize volatile organic compounds (VOCs), the formation
of CO, CO2 and 03 was observed. With the addition of manganese oxide based
catalysts, the ozone that is generated in the plasma reactor decomposes the

V003 and eliminates the ozone [32].

78

The catalytic oxidation of VOCs allows for the reduction in the reaction
temperature [36-38]. This reduction in the reaction temperature allows for air
pollution control in industrial applications without the need for heating and cooling
large volumes of air [32]. The lower reaction temperature also allows the
manganese catalysts to be effective for the decomposition of VOCs during

engine warrn-up, in automotive applications [38].

Mn02 can therefore be an effective catalyst for the ozonation of organic matter.
The favorable catalytic properties of manganese dioxide suggest that
experimentation with a MnO2 coating on the ceramic ﬁltration membranes for the

use in combined ozonation-ﬁltration is logical.

This chapter will describe the effect of the membrane coatings on the water ﬂux

and the quality of the water (using TOC measurements) treated by the hybrid
ozonation-membrane ﬁltration system. The optimum choice of the Mn02

preparation method, the number of coating layers, the sintering conditions and
the surface reactions that act to lower the concentrations of water contaminants

will be studied.

79

5.2 MATERIALS AND METHODS

Manganese oxide colloids were prepared by ozonation and by mixing of KMn04

and PAH using the methods described in Chapter 3. These colloids were used to
coat ceramic water ﬁltration membranes, as described in Chapter 4 and were

sintered in air at either 500°C for 45 minutes or 550°C for 30 minutes.

The membranes were used in a hybrid ozonation-membrane ﬁltration system
[63], operated in total recycling (permeate and retentate) mode. Figure 28 shows
a schematic representation of the hybrid ozonation-membrane ﬁltration system.
The membrane module is equipped with a stainless-steel housing (TAMI North
America, St. Laurent, QC, Canada). The permeate was recycled using a pump
(Masterﬂex, Cole-Parrner lnc., Vernon Hills, IL) into the feed tank at 15 min
intervals. The transmembrane pressure and crossﬂow velocity were controlled
using a recirculation pump (Gear Pump Drive, Micropump, Cole-Farmer Inc.,
Vernon Hills, IL) and a back pressure regulator (Swagelok, Solon, OH). The
temperature of the water was maintained at 22.5 1 05°C. The temperature,
crossﬂow rate and pressure were monitored in the recirculation line using a
multifunctional sensor (L Series, Alicat Scientiﬁc, Tucson, AZ) and recorded
using a Lab View data acquisition system (National Instruments, Austin, TX). The
permeate mass was measured at 60 second intervals using an electronic
balance (Adventurer Pro Analytical Balance, Ohaus Corporation, Pine Brook,

NJ).

80

Pure oxygen from a compressed gas cylinder was passed through a moisture
trap containing anhydrous calcium sulfate (Drierite, Xenia, OH) into a high-
pressure ozone generator (Atlas Series, Absolute Ozone Generator, Absolute
System Inc., Edmonton, AB, Canada) and injected directly into the pressurized
system. The input ozone pressure (20 — 30 kPa greater than the transmembrane
pressure) was monitored using a pressure gauge (Ashcroft Inc., Stratford, CT).
The ozone gas ﬂow rate was regulated using a mass ﬂow controller (Model GFC
17, Aalborg, Orangeburg, NY) which was installed between the ozone generator
and the membrane module. The ozone concentration in the gas phase was
measured by an ozone gas monitor (Model 450H, Teledyne Instruments, City of
Industry, CA) and the dissolved ozone concentration in the retentate and
recirulation loop was continuously monitored using an amperometric ozone
microsensor (AMT Analysenmesstechnik GmbH, Rostock, Germany) and

recorded using a LabView data acquisition system.

TOC was measured using an Ol Analytical model 1010 analyzer (0| Analytical,
College Station, TX) using the UV/persulfate method [169] as a surrogate
parameter to estimate the potential for disinfection by-product (DBP) formation
during chlorination [170-172]. Suwannee River NOM (International Humic
Substances Society, St. Paul, MN) was used. This material was isolated from the
Suwannee River using reverse osmosis. The NOM solutions were prepared in

deionized water at a concentration of 9.9 1 0.4 mg C/L (pH 8.0).

81

Fume hood < ---------------------

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Residual ozone detector

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
  

 

Ozone monitor} Flow -------------------- ;
; control :
...... [I 0 ti [:1 “»->’ va'Ve ‘1 v gum-1E Data
_. : ‘ o o o 1 A : 1 . . .
g ; : Pressure P ;: acqursmon
Ozone ; ressure, , 1 system

i ' control ﬂowrate 8 3 '
generator '----J----Pressure 1 valve :I
: temperature :I
gauge 3 sensor i i
. . . ’ I
ReCIrculatIon , - g :
I . I
v pump ------- Eﬂm »:
02 tank Three valve Membrane module 5 ;
‘ Check valve | i E I
L\ . : L

Q Permeate

. .- reservoir

Timer 1‘» l .
gazées: Feed tank Electric balance

Figure 28. Schematic representation of the hybrid ozonation-membrane ﬁltation

system.

Water samples from Lake Lansing (Haslett, MI), a borderline eutrophic lake with

a dissolved organic carbon (DOC) of ~11 mg C/L were treated in the hybrid

ozonation-membrane ﬁltration system [63] (03 gas ﬂow rate: 1x10'5 m3/min,

crossﬂow velocity: 0.47 mls, transmembrane pressure: 200-228 kPa, initial ﬂux:

104-115 L/mz-hr, temperature: 22.5 1 05°C). The ﬂux through the membrane

was measured and normalized with respect to the initial Lake Lansing water

permeability.

82

One-way ANOVA testing, with Tukey‘s mean comparison tests (p s 0.05)
(Minitab statistical software, Minitab Inc.), was used to determine if differences in

measurements are statistically signiﬁcant.

5.3 RESULTS AND DISCUSSION

5.3.1 TOTAL ORGANIC CARBON (TOC) REMOVAL

Samples of the permeate (the water that has passed through the membrane)
were collected at 10, 40, 80, 120, 140 and 160 minutes of operation and
analyzed for total organic carbon (T OC). The average TOC was reported in
milligrams of carbon per liter of water (Figure 29) [173]. The effect of the
membrane sintering temperature on the average TOC in permeate samples from
the hybrid ozonation-membrane ﬁltration system was compared. For the
membranes coated with 20 layers of manganese oxide prepared using the
ozonation method, the average TOC in the permeate for the membrane sintered
at 500°C (5-20-500 Ozone) was 5.7 1 0.9 mg C/L, while for the membrane
sintered at 550°C (5-20-550 Ozone) it was 6.8 1 0.9 mg C/L. Since the average
TOC in the permeate collected using the membrane sintered at 500°C (5—20-500
Ozone) was lower than for the membrane sintered at 550°C (5-20-550 Ozone),
although not statistically signiﬁcant, the 500°C sintering temperature was used

for the membrane that was coated with 30 layers of the manganese oxide

83

prepared by ozonation (5-30-500 Ozone). The average TOC in the permeate

samples for the 5-30-500 Ozone membrane was 4.4 1 0.7 mg C/L.

 

 

 

 

 

 

 

 

9
:8

6 7 i

Es T

V "—1—

:51 + 4 .
E 4-— 1

0

0.3._,

.E

o

O

'—

  

 

 

 

 

 

 

lllllll 11] IlIIJIJ

1111111 IIL IIIIJII

 

 

r-
1..
I...
r-q
.—

 

 

 

 

Hp.

Unooated 5-20—500 5-20-550 5-30—500 5—20—500 20—550 5—30—550
OZONE OZONE OZONE PAH PAH PAH

 

 

E5
5.

Membrane

Figure 29. Total organic carbon present in permeate samples after combined
ozonation and catalytic membrane ﬁltration. Initial feed water TOC: 9.9 1 0.4 mg
C/L, temperature: 22.5 1 0.5°C, transmembrane pressure: 138 kPa.

For the membranes coated with 20 layers of the Mn02 prepared by mixing of
KMnO4 and PAH, the average TOC in the permeate for the membrane sintered
at 500°C (5-20-500 PAH) was 5.4 1 0.5 mg C/L, while for the membrane sintered
at 550°C (5-20-550 PAH) it was 4.6 1 0.4 mg C/L. Since the average TOC in the
permeate collected using the membrane sintered at 550°C (5-20-550 PAH) was
lower than for the membrane sintered at 500°C (5-20-500 PAH), although not

statistically signiﬁcant, the 550°C sintering temperature was used for the

84

membrane that was coated with 30 layers of the MnOz prepared by mixing of

KMnO4 and PAH (5-30-550 PAH). The average TOC in the permeate samples

for the 5-30-550 PAH membrane was 4.5 1 0.9 mg C/L. This trend of decreasing
organic content in permeate samples with increasing number of coating layers

was also seen in the work of Kamik et al. [3].

A trend of decreasing average TOC concentration was observed with increasing
average grain size (y=-2.0852x + 85.71, with an r2 of 0.4424, where x is the TOC

and y is the grain size), when comparing the grain size measurements made
using SEM micrographs in Chapter 4, to the average TOC in the permeate
samples. The largest average grain size for the catalytic coating of 79 nm (5-30-
500 PAH) resulted in only 4.5 mg C/L in the permeate sample, while the smallest
average grain size of 71 nm (5-0-0) resulted in 5.9 mg C/L in the permeate

sample, as shown in Figure 30.

Comparing the two different coating methods, treatment with the membrane that
was coated with 30 layers of the manganese oxide prepared by ozonation
resulted in a lower average TOC concentration in the permeate samples
(although not statistically signiﬁcant) than that observed with the membrane
coated with the manganese oxide prepared by mixing of KMnO4 and PAH (4.4 1
0.7 mg C/L vs. 4.5 1 0.9 mg C/L, respectively). The ﬂux recovery was measured

using an uncoated membrane as well as membranes coated with 20, 30 and 40

85

layers of the manganese oxide prepared by ozonation and sintered in air at

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

500°C for 45 minutes.
85
1 T
A 80 _ V
E :
5 .
a \ . )
'7) +‘\ﬁ L-
.E 75 -: 1 : E \
‘3 \
a: \ J
0 . .
D ._
3
< 70 ‘ \
65 I . d I I

 

0 2 4 6 8 10
TOC in permeate (mg CIL)

Figure 30. Comparison of grain size to the amount of total organic carbon
present in permeate samples after combined ozonation and catalytic membrane

ﬁltration. Initial feed water TOC: 9.9 1 0.4 mg C/L, temperature: 22.5 :I: 05°C,
transmembrane pressure: 138 kPa.

The decrease in the TOC in the permeate water samples indicate that the
manganese oxide coating does in fact act as a catalyst for the oxidation of

organic matter [174, 175] when used in a hybrid ozonation-membrane ﬁltration

system.

86

5.3.2 FLUX RECOVERY

Since the deposition of organic foulants on the membrane surface results in a
decrease in the water ﬂux through the membrane, the performance of the system
can be evaluated using water ﬂux measurements [173], as shown in Figure 31.
When the uncoated membrane was used in the hybrid ozonation-membrane
ﬁltration system, the ﬂux decreased steadily during the ﬁrst hour to approximately
60% of the clean water ﬂux with no recovery over the remaining 6.5 hours of
measurements. When the coated membranes were used in the hybrid system,
there was also an initial decrease in the ﬂux during the first hour due to fouling of
the membrane but then there was a steady recovery of the ﬂux. The membrane
coated with 20 layers achieved approximately a 95% recovery after 5.5 hours,
while the membrane coated with 30 layers achieved approximately a 90%
recovery after 7.5 hours and the membrane coated with 40 layers achieved
approximately an 85% recovery after 7.5 hours. This recovery of the permeate
ﬂux due to the oxidation of the organic foulants by ozone was also observed by

Kamik et al. [65].

The improved ﬂux recovery for the coated membranes, as compared to the
uncoated membrane, is the result of the catalytic activity of the coating for the
oxidation of the organic matter by ozone [3, 63, 65, 174]. This demonstrates the
ability of the manganese oxide coated membrane to catalyze the degradation of

foulants at the membrane surface when used in combination with ozonation.

87

Increasing the number of coating layers, however, does not result in an

improvement in the ﬂux recovery.

 

JIJO

  

o Uncoated
n 5—20-500

A 5—30-500
x 5-40-500

 

Time (hours)

Figure 31. Normalized ﬂux for an uncoated membrane as well as membranes
coated with 20, 30 and 40 layers of the manganese oxide prepared using

ozonation. 03 gas ﬂow rate: 1x10'5 m3/min, crossﬂow velocity: 0.47 m/s,

transmembrane pressure: 200-228 kPa, initial ﬂux: 104-115 L/mz-hr,
temperature: 22.5 1 05°C.

5.4 CONCLUSIONS

Surface modiﬁcation of ceramic water ﬁltration membranes, by coating with
manganese oxide and sintering, has been shown to reduce the average TOC in
the permeate when the membranes are used in a combined ozonation and

membrane ﬁltration system. Increasing the number of coating layers resulted in a

88

decrease in the TOC concentration in permeate samples. The optimum coating
material and sintering condition, in terms of TOC removal, was manganese oxide
prepared using ozonation that was sintered at 500°C for 45 minutes. This
decrease of organic content in permeate samples with increasing number of
manganese oxide coating layers on a ceramic membrane used in [a hybrid

ozonation—membrane ﬁltration system was also seen in the work of Kamik et al.

[3] with F9203 coated ceramic membranes.

The application of the manganese oxide coating results in improved water ﬂux
through the membrane, when compared to an uncoated membrane in a
combined ozonation and membrane ﬁltration system. Reed et al. [37] have
shown that increasing the loading of manganese oxide on a silica-supported
catalyst from 3% to 10% gave higher reaction rates for both acetone oxidation
and ozone decomposition in gas phase reactions. In our studies of manganese
oxide coatings on ceramic water ﬁltration membranes, increasing the amount of
manganese oxide, by increasing the number of coating layers, did not show an
increase in the oxidation of the foulants that result in the decreased water ﬂux
through the membrane. The membrane coated with 20 layers shows improved
ﬂux recovery over the uncoated membrane and the membranes coated with 30

or 40 layers.

Although the hydrated Mn02 was reduced to a-Mn203 during the sintering

process, the decrease in TOC demonstrates that d-Mn203 is an effective catalyst

89

for the oxidation of organic matter by ozone in aqueous solutions. In gas phase
reactions Radwan et al. [82] determined that the amount of Mn203 (between 5 to
20 wt.%) added to a cordierite support increased the catalytic activity towards CO

oxidation by 02 as well as H202 decomposition. In our studies of the manganese

oxide coating on the ceramic water ﬁltration membranes, the amount of Mn203

catalyst was adjusted by changing the number of coating layers applied to the
membrane. Increasing the amount of manganese oxide on the surface of the

membrane did improve the TOC removal from permeate water samples.

Fouling of water ﬁltration membranes is a result of the adsorption and deposition
of organic substances on the surface of the membrane, which results in a
decrease in the water ﬂux through the membrane [115]. You et al. [54]
determined that ozone reduces fouling of membranes by destroying aromatic
rings and forming linear chains, thereby changing the chemical structure and
characteristics of the ﬁltration cake. This change in the characteristics of the
ﬁltration cake allows the water ﬂowing across the membranes to ﬂush away loose
fragments, which decreases the fouling of the membrane [54]. The increase in

the ﬂux recovery with the use of the manganese oxide coated membranes in our
studydemonstrate that d-Mn203 is an effective catalyst for the catalytic oxidation
of organic matter by ozone. Mn203 has been shown to be an effective catalyst for
gas phase reactions [81-83]. The water ﬂux improvement and the reduction of

TOC in permeate samples in this work indicates that Mn203 is also an effective

90

catalyst for the degradation of organic compounds in aqueous systems [2, 3, 63,

65,174]

91

CHAPTER 6

THERMAL STABILITY OF CERAMIC MEMBRANES

 

6.1 INTRODUCTION

Ceramic membranes have been used for puriﬁcation of gases [176-180] as well
as water [2-4, 61-74]. In gas-phase applications, the ceramic membranes may be
exposed to elevated temperatures [176-178], requiring high thermal stability of
the membranes. In the high temperature gas-phase applications, silica
membranes [178] or membranes that are silicon based [176] or palladium based
[177] have been used. Where titania membranes have been used for high
temperature applications, they have been coated with an alumina separation

layer [181].

Ceramic membranes have also been studied for use in hybrid ozonation-

membrane ﬁltration systems due to their resistance to ozone [2-4, 61-71]. When

92

preparing catalytic membranes, the coating material is applied then the
membrane is sintered or calcined at elevated temperatures to ensure adherence
of the coating to the underlying membrane [3, 4, 182, 183]. As ceramic
membranes become more widely used, there is a growing concern about the

effect of sintering temperature on the underlying membrane structure [95, 96].

When ceramic membranes are coated with materials that necessitate high
temperature treatment, the morphology and microstructure of the underlying
membrane may be altered during the sintering of the coating. This may in turn
result in changes to the underlying membrane properties such as the pore size

and structure.

During sintering, the grains can either grow with very little densiﬁcation or there
can be an increase in the density, to approximately 95% of theoretical density,
followed by grain growth [97]. Alternatively, grain growth and densiﬁcation can
occur simultaneously [97]. The driving force for the increase in grain size is the
reduction in the surface energy. By increasing the average grain size, there is a

reduction in the total surface area, resulting in a reduction in the surface energy

[97].

When the iron oxide coating was applied to alumina-zirconia-titania (AZT)

ceramic membranes in the work of Kamik et al., the membranes were sintered in

air at 900°C [3, 4]. However, because of its signiﬁcantly lower melting point, the

93

manganese oxide coated titania membranes are sintered at SOD-550°C, also in
air. As the AZT membranes used in the work of Kamik et al. are no longer

available for purchase, during the course of experimentation, the titania
membranes were coated with the nano—Fe203 coating and sintered in air at
900°C. The deionized (DI) water permeability of these membranes increased

from 360 L/mz-hr-MPa (uncoated and unsintered) to 10250 L/mz-hr-MPa after

coating with 40 layers of the Fe203 nanoparticles and sintering at in air 900°C

(unpublished results). When these membranes were coated with 40 layers of

manganese oxide nanoparticles and sintered in air at 500°C, the permeability of

the membranes was only 650 L/mZ-hr-MPa.

Ahmad et al. [181] have also seen this increase in the permeability of titania
membranes when they were coated with an alumina separation layer and
sintered. Although the alumina separation layer allows for high temperature gas
separation, the titania membrane support undergoes pore coarsening when the
alumina is sintered to the titania support [181]. In the preparation of a titania
membrane with a titania support, Wang et al. [184] saw an increase in the mean
pore size distribution with increased sintering temperature and a concomitant

increase in the pure water permeability.

Kim et al. [185] used a hydrothermal process to improve the thermal stability of
titania membranes. In this hydrothermal process, a titania sol was autoclave

aged at 135°C for 12 hours before being used to prepare unsupported titania

94

membranes. The hydrothermal process retarded the anatase-to—rutile phase
transformation when the membranes were calcined at 600°C. However, the pore
size of the membranes prepared using the hydrothermal process shifted to a

larger pore size after calcination at 600°C due to crystallite growth [185].

During the grain growth process, pores located at the grain boundaries
can move with the grain boundaries and coalesce into larger pores at the

intersections of the grains [97, 98]. During sintering of ceramics, the velocity of
an inclusion, such as a pore, up, can be related to the pore mobility, Bp, and the
driving force, Fp, according to equation (4) [98]:

up = Bp Fp (4)
When the pore is dragged by the grain boundary, the velocities of the pore and

boundary are identical, however, unhindered, the grain boundary mobility, Bb, is

much greater than the pore mobility (Bb»Bp). Therefore, at some point when the

boundary moves beyond the pore, the pore becomes unpinned, resulting in an
isolated pore. This makes further densiﬁcation increasingly difﬁcult since pore

diffusion through the bulk is much slower than along the grain boundaries. The

resulting grain boundary velocity, ob, is controlled by the driving force on the

boundary, Fb, along with the pore mobility and the number of pores per grain

boundary, p, according to equation (5) [98]:

=35.
P

(5)

Ub

95

In ﬁne grained oxides, an increase in pore size is observed in the initial stages of
sintering. As sintering time continues, however, there is a marked increase in

grain size [97, 98].

Since the nanoparticle coated membranes examined in this study are sintered for
relatively short periods of time, the underlying membrane support only
experiences the early stages of sintering and is not able to completely eliminate
the agglomerated pores or further densify. An increase in the pore diameter of
porous ceramic membranes with increasing sintering temperature (when sintered

for short periods of time) has previously been observed by Baticle et al. [99].

The addition of zirconia (up to 20 mol%) to titania membranes by Sekulié et al.
[186] resulted in a smaller pore sizes when the membranes were calcined at
450°C. It is believe that this is a result of the supression of the anantase-to—rutile
phase transition by the zirconia forming clusters between anatase particles.
These clusters prevent the contact of the anatase particles, suppressing the
phase transformation to rutile [186]. Bae et al [187] also demonstrated that the

addition of Fe203, Ce02 and Pt dopants to titania membranes decreased the

average pore size of sintered membranes.
Different ceramic membranes, although possessing the same molecular weight

cut-off (MWCO), because of differences in membrane composition, will have

greater or less themal stability. The focus of this chapter, therefore, is to link the

96

application of high temperature coatings and/or high temperature sintering to

changes in the membrane performance.

6.2 MATERIALS AND METHODS

Two commercial titania membranes (INSIDE CéRAM, TAMI North America, St.
Laurent, Quebec, Canada) having a 7-channel, tubular design, MWCO of 5 kDa
with an outside diameter of 1 cm and length of 25 cm were each cut into three
equal pieces. The permeability of the six sections of membrane was measured
using DI water with a ﬂow rate of 0.8 L/min and transmembrane pressure of 103

kPa.

After testing their initial permeability, one sample was left unsintered and the
other ﬁve membrane samples were sintered (in air) at either 500°C, 600°C,
700°C, 800°C or 900°C by placing the sample in the furnace and increasing the
temperature at a rate of approximately 10°C/min to the desired sintering
temperature, holding that temperature for 30 minutes and then turning the
furnace off and allowing the samples to cool to room temperature in the furnace

over the next 8 hours.

Manganese oxide nanoparticles were prepared by ozonation using the method

described in Chapter 3. This nanoparticle colloid was used to coat ceramic water

97

ﬁltration membranes, as described in Chapter 4. Membranes were coated with
20 layers then sintered (in air) either using the same ramped heating and cooling
proﬁle as for the uncoated samples (with sintering temperatures of 350°C, 400°C
and 450°C for 45 minutes), or else the furnace was heated to the desired
sintering temperature, the samples were placed in the furnace and sintered for
30 minutes (at 350°C, 450°C and 500°C) and then they were removed from the

furnace to cool to room temperature.

XRD samples of the unsintered membrane were prepared by cutting a 5 mm
section from the membrane using a diamond-watering saw then further cutting
this section to expose the inner ﬁltration surface of the membrane. The pieces

were then adhered to a glass slide using a double-sided adhesive tab so that an
area of approximately 1 cm2 of the ﬁltration surface of the membrane was

exposed.

Another section of the unsintered membrane was also cut from the membrane

and it was ground into a powder with a mortar and pestle for XRD testing.

To compare to the AZT ceramic membrane that had been used in the work of
Kamik et al., an unsintered section of the AZT ceramic membrane was also
prepared in the same manner as described above to obtain XRD samples of the

ﬁltration surface as well as the powdered membrane.

98

XRD spectra were obtained using a Rigaku Rotaﬂex RU-200B XRD (Tokyo,

Japan, 45KV/100mA, copper radiation).

To study the morphological changes that occur in the titania membranes during
sintering at the different temperatures, SEM samples were prepared by cutting
an approximately 3 mm thick cross section from each sample using a diamond-
wafering saw. An arc of approximately 3 mm was cut from this cross section and
was adhered to an aluminum stub using carbon paste cement. Since the ceramic
samples are non-conductive, they were coated with osmium for SEM
observation. The conductive Os coating is completely amorphous, precisely
follows the contours of the specimen surface and shows no granularity, even

when viewed at high magniﬁcation.

SEM analysis was performed using a JEOL JSM-7500F SEM (Japan Electron
Optics Laboratories, Tokyo, Japan) at an accelerating voltage of 15 kV and
magniﬁcation of 75,000x. The average grain size for each sample was
determined by taking six micrographs from each sample and measuring an
average of at least 100 grains per micrograph using the mean linear intercept
method (ASTM Standard E 112-96 (2004)). To determine if differences were
statistically signiﬁcant, all measurements were analyzed using one-way ANOVA
testing, with Tukey’s mean comparison tests (p s 0.05, Minitab statistical

software, Minitab Inc.).

99

To study the topographical changes of the sintered titania membrane samples,
AFM samples were prepared in a similar manner to that used for SEM analysis.
Samples were adhered to a 12 mm diameter steel disc using double-sided

adhesive tabs, so that the ﬁltration surface was pointing upward.

A Nanoscope Ill Multimode Atomic Force Microscope (Digital Instruments Inc.),

operating in contact mode, was used for AFM imaging. The probe was a
triangular silicon nitride (Si3N4) NP probe (Veeco Instruments, CA, nominal

cantilever spring constant: 0.12 N/m, nominal frequency: 20 kHz, tip height: 2.5
pm to 3.5 pm, nominal radius: 20 nm, side angle: 35°). Each sample had ﬁve
different areas, of dimension 5 pm x 5 pm, scanned at a rate of 1.0 Hz to
determine the average roughness of each sample. To determine if differences
were statistically signiﬁcant, all measurements were analyzed using one-way
ANOVA testing, with Tukey’s mean comparison tests (p s 0.05, Minitab statistical

software, Minitab Inc.).

The MWCO was determined by measuring the rejection of 1, 5, 25, 50 and 80
kDa dextrans (Sigma-Aldrich Co., St. Louis, M0) by the ceramic membranes.
The MWCO is the lowest molecular weight that has a rejection of greater than

80% by the membrane [188, 189].

Titania membranes with a nominal MWCO of 5 kDa were coated with 10, 20, 30

and 40 layers of manganese oxide and sintered in air at 500°C for 45 min. One

100

uncoated membrane was also sintered in air at 500°C for 45 min and one
membrane was left uncoated and unsintered. Dextran feed solutions were
prepared using 1, 5, 25, 50 and 80 kDa dextran molecules (Sigma-Aldrich Co.,
St. Louis, MO), at a concentration of 10 mg/L [173]. The dextran solutions were
ﬁltered using 25 cm long ceramic membranes in a stainless-steel housing (TAMI
North America, St. Laurent, QC, Canada). Tests were conducted with DI water
using a crossﬁow velocity of 0.47 m/s and transmembrane pressure of 172-207

kPa (transmembrane pressure adjusted to give an initial permeate ﬂux of 110 1 5
lezhr) at a temperature of 25 1 0.5°C. Approximately 35 mL of permeate was

collected and the total organic carbon (TOC) content was measured using a Cl
Analytical model 1010 analyzer with a 1051 auto-sampler (O.l. Corporation,
College Station, TX). Only one sample was collected for each molecular weight
dextran sample and membrane combination, therefore, the statistical differences

between samples were not determined.

6.3 RESULTS AND DISCUSSION

6.3.1 X-RAY DIFFRACTION (XRD) ANALYSIS
Figure 323-d shows the intensity vs. 20 for the ﬁltration surface of the titania
membrane, the ground titania membrane, the ﬁltration surface of the AZT

membrane and the ground AZT membrane, where 9 is the angle between the

101

incident x-ray beam and the scattering planes, used to determine the atomic
plane spacing, d, according to Bragg’s law, equation (6):

nA=2d sin 0 (6)
where A is the x-ray wavelength. Indexing of the XRD spectra of the ﬁltration
surface (Figure 32a) and the powdered (Figure 32b) titania membrane
demonstrates that the titania is present as the rutile phase. The differences in the
intensities of the indexed planes between the two samples indicates that there is

a preferred orientation of the (110) and (211) planes on the ﬁltration surface.
Although rutile is a stable phase of Ti02 [190, 191], the rutile phase exhibits rapid

grain growth around 800°C [191]. The Zr that was observed from the EDS
mapping of the titania and AZT membranes, however, was not observed in the
XRD spectra. This is due to the penetration of the x-ray beam into the XRD

sample, such that the small amounts of Zr present on the ﬁltration surface were

not able to be detected. Adding Zr02 to Ti02 has the effect of inhibiting the phase
transformation of the Ti02 from anatase to rutile due to the substitution of Zr4+

ions for Ti4+ ions, causing an increase in the strain energy which lowers the

driving force for the phase transformation [98, 186]. While the manufacturer will
not disclose the rationale or the chemistry of the surface layers of either the AZT
or the titania membrane, the Zr may have been added to the ﬁltration surface to

promote the adsorption of viruses on the membrane [192].

Indexing of the XRD spectra of the ﬁltration surface (Figure 320) and the

powdered (Figure 32d) AZT membrane shows additional titania peaks for the

102

ﬁltration surface, indicating that the ﬁltration surface is composed of titania with
an alumina-zirconia-titania membrane support, as was described in the work of
Kamik et al. [68]. The presence of the alumina and zirconia with the titania in the
AZT membranes improves the high temperature stability [193, 194]. The melting
temperature of titania is 1840-1870°C [193, 194], while the melting temperatures
of alumina and zirconia are 2030-2050°C [181, 193] and 2700°C [193],
respectively. Since diffusion occurs at approximately half of the melting
temperature [195], sintering the titania membrane at 900°C facilitates grain
growth due to diffusion. With the essentially pure titania membrane, we saw

extensive grain growth upon sintering. However, in the membrane containing the

higher temperature Al203 and ZrOz phases, as expected, less grain growth was

 

 

 

 

 

 

observed.
a) (“I”) (211)
Q (101) ‘1
8 1 10 ’
31:3 (11112 ) .1920) (310)601)
_ ) ‘ (0021 £112)
0 10 20 30 4o 50 50 70 80

29 (degrees)

Figure 32. XRD spectra of a) the ﬁltration surface of the titania membrane,
indexed as the rutile phase, b) powdered titania membrane, indexed as the rutile
phase, c) ﬁltration surface of the AZT membrane, where the indexed pattern
shows the alumina (A), zirconia (Z) and titania (T) peaks and d) powdered AZT
membrane, where the indexed pattern shows the alumina (A), zirconia (Z) and
titania (T) peaks.

103

Figure 32. Continued

 

 

 

 

 

 

 

 

 

 

 

 

b) (101)
. (211)
3. (110)
'Z’
9 (111) (002) (301)
E 1610) (112)
. 1 L
0 10 20 70
c) T:
2?
'0‘)
c
.9
.5
Cl 10
d) A
I
Q T
a)
c A
g “T A AT A A -
- ‘ A zAZA T
U 10 20 30 40 50 60 70

20 (degrees)

6.3.2 DEIONIZED WATER PERMEABILITY

The deionized (DI) water permeability of the six sections cut from the two

uncoated and unsintered titania membranes is shown in Table 2.

104

Table 2: DI water permeability of unsintered membrane sections (ﬂow rate: 0.8
L/min, transmembrane pressure: 103 kPa).

 

 

 

 

 

 

Membrane section
1 2 3 4 5 6
Permeability
L/mz-hrMPa) 53 91 180 131 61 94
Standard Deviation
(L/mz-hr-MPa) 1 2 5 5 1 2

 

 

 

 

 

 

 

Since membrane section 3 had higher permeabilities than the others, presumably
due to discrepancies in the membrane manufacturing, that piece was left
unsintered. For comparison, the remaining pieces of membrane that were the
closest in permeabilities were then sintered as follows: section 1 at 500°C,
section 2 at 600°C, section 4 at 700°C, section 5 at 800°C and section 6 at
900°C. All samples were sintered in air. The samples were placed in the furnace
and the ramped heating proﬁle (approximately 10°C /min) was used and the
samples were sintered for 30 minutes, once the desired sintering temperature
was achieved. The samples were then allowed to cool in the furnace (to room

temperature) over 8 hours.

After sintering, the clean water permeability of the membrane samples was once

again tested and the results are shown in Table 3.

Although there is large variability in the permeabilities of the uncoated
membranes, there is an order of magnitude increase in the permeability from the

unsintered membrane to the membrane sintered at 500°C. There is a steady

105

increase in the permeability of the membranes as the sintering temperature is
increased. Since the ends of the membrane sections were resealed prior to the
second set of permeability tests, there were some small changes in the sealing
area. Even small changes in the sealing area can have a large effect on the
effective area of membrane ﬁltration since the membrane lengths used in these
tests is so small [173]. This change in the effective area resulted in the large
increase in the standard deviation between the ﬁrst set of tests and the second

set of tests.

Table 3: DI water permeability of sintered membrane sections (ﬂow rate: 0.8
L/min, transmembrane pressure: 103 kPa).

 

Membrane Section

 

 

 

 

 

 

1 2 3 4 5 6
Sintering Condition 500°C 600°C unsintered 700°C 800°C 900°C
Permeability
(Umz-hr-MPa) 1830 4150 152 5070 9600 11300
Standard Deviation
(between
experiments)
(L/mz-hr-MPa) 203 216 22 365 299 424

 

 

 

 

 

 

 

6.3.3 SINTERED MEMBRANE ANALYSIS

6.3.3.1 SCANNING ELECTRON MICROSCOPY (SEM) ANALYSIS

Since such large changes in permeabilities were observed, the microstructure of

the sintered and unsintered membranes were compared by SEM analysis. SEM

106

micrographs of the ﬁltration surface of the titania membrane samples (Figure
333-0 for membranes sintered at 500°C (33b), 600°C (33c) and 700°C (33d)
show little increase in grain size, while the grain size of the membrane sintered at
800°C (339) appears slightly larger, with a large increase in grain size for the
membrane sintered at 900°C (Figure 33f). However, grain size measurements
demonstrate that the unsintered sample has spherical grains with an average
grain size of 38.0 1 1.2 nm (statistically signiﬁcant difference from all other
measurements at 95% conﬁdence interval), which increases to 47.2 1 3.3 nm for
the sample‘sintered at 500°C, to 47.7 1 3.8 nm for the sample sintered at 600°C
and to 48.1 1 3.8 nm for the sample sintered at 700°C (no statistically signiﬁcant
difference between the samples sintered at 500°C, 600°C and 700°C at 95%
conﬁdence interval). The membrane section sintered at 800°C shows some
increase in irregularity in the shape of the grains (Figure 33e) and has an
average grain size of 56.3 1 4.8 nm (statistically signiﬁcant difference from all
other measurements at 95% conﬁdence interval). The sample sintered at 900°C
has very large, irregular grains (Figure 33f) and has an average grain size of 90.5
1 13.0 nm (statistically signiﬁcant difference from all other measurements at 95%
conﬁdence interval). This increase in grain size beginning at 800°C is well known
and has been described by others [190, 191]. The large increase in grain size is
also accompanied by an increase in pore size, as can be seen by the large pores

between the grains in the sample sintered at 900°C (Figure 33f).

107

 

Figure 33. SEM micrographs of the titania membrane a) unsintered, b) sintered
at 500°C for 30 min, c) sintered at 600°C for 30 min, d) sintered at 700°C for 30
min, e) sintered at 800°C for 30 min and f) sintered at 900°C for 30 min. All scale
bars are 100 nm.

At elevated temperatures (approximately half of the melting temperature and
above), the atoms that comprise the grains of the ceramic material have
sufﬁcient energy for diffusion to occur [195]. The melting temperature for TIOZ is
1840-1870°C [193, 194], therefore, atomic diffusion would be expected when the

titania membrane is sintered at 900°C, the sintering temperature used for the

108

Fe203 coating on the AZT membranes. In contrast, when the AZT membrane

was sintered by Kamik et al. [3, 4], this signiﬁcant increase in grain size was not
observed since the melting temperatures for alumina (2030-2050°C) [181, 193]
and zirconia (2700°C) [193] are significantly higher than that of titania. This
results in higher thermal stability of the AZT membrane than the titania

membrane.

The driving force during sintering is the reduction of surface energy. This can be
achieved by coarsening of the particles or by the reduction of solid/vapor
interfaces (due to pores) through the creation of grain boundary area, followed by
grain growth, resulting in densiﬁcation [97]. The increase in the membrane
permeability after sintering indicates that densiﬁcation has not occurred, rather,
the pores have coarsened during sintering. During sintering, the grains on the
ﬁltration surface of the membrane are believed to follow the pure coarsening
trajectory shown in the plot of grain size versus percent of theoretical density in
Figure 34. Following this trajectory, large grains are formed as well as large
pores. The mechanisms that lead to coarsening are evaporation—condensation,
surface diffusion and lattice diffusion from the surface to the neck area between
grains. These mechanisms do not lead to densiﬁcation because they do not allow
the particle centers to move closer together. As the grain boundaries migrate, the
pores between grains can move along the grain boundary and coarsen as well
[97]. For densiﬁcation to occur, the mechanisms involved are volume diffusion

from the grain boundary area to the neck area or grain boundary diffusion from

109

the grain boundary area to the neck area. This allows the particle centers to

move closer together [97] and follows the densiﬁcation followed by grain growth

trajectory shown in Figure 34.

 

 

  
 
 

 

A
I Pure Coarsening
Coarsening
+

Densification

a)

.5

a)

.E

E

(D

 

 
  
  

Densification
followed by
grain growth

 

 

 

Percent of theoretical density

100

Figure 34. Grain size versus percent of theoretical density for pure coarsening,
coarsening and densiﬁcation and densiﬁcation followed by grain growth (Adapted

from Figure 10.53 in [97]).

Measurement of the SEM micrographs of the sintered membrane surfaces along

with the permeability data indicates that grain and pore coarsening has occurred,

rather than densiﬁcation. Therefore, the prevailing sintering mechanisms are

surface diffusion rather than bulk diffusion.

A graph of the increase in grain size with increased sintering temperature can be

seen in Figure 35. The DI water permeability of the sintered and unsintered

110

membranes is also shown in Figure 35. Both the grain size and the DI water
permeability show a steady increase as the sintering temperature increases. This
increased permeability is due to an increase in the pores on the ﬁltration surface
of the membrane at the elevated sintering temperatures [72]. Even with the small
change in the grain size, and the accompanying coarsening of the pores, there
are large changes in the DI water permeability of the membranes. Since the
molecular weight of water is so small (18 Da), even slight changes in the pore

size have a pronounced effect on the permeability of the membranes.

 

120 8000

+Average Grain Size -- 7000
100 -1 +Permeability A
a
A -- 6000 D-
E ?
V 80 T” h
.c
g ~- 5000 “'8

(D
E“; 60 _. -- 4000 g:
h
(D E
o -- 3000 a
3’ 40 J- 3
.. E
g «5 2000 3
< n.
20 -1-
-- 1000
0 0

 

 

 

 

unsintered 500 600 700 800 900
Sintering Temperature (°C)

Figure 35. Comparison of the average grain size and permeability for the
unsintered and sintered membranes. The average and standard deviation for the
grain size measurements were determined by measurement at least 600 grains
using the intercept method (ASTM Standard E 112-96 (2004)). The average and
standard deviation of the permeability measurements were determined by testing
each membrane at least 4 times until steady state conditions were achieved
(permeability test conditions: ﬂow rate: 0.8 L/min, transmembrane pressure: 103
kPa)

111

6.3.3.2 ATOMIC FORCE MICROSCOPY (AFM) ANALYSIS

AFM analysis Of the average roughness (Ra) (Shown in Figure 36), which is the

arithmetic average Of the absolute values Of the surface height deviations as
measured from the mean plane, shows no statistically signiﬁcant difference (at
95% conﬁdence interval) in the average roughness values for the unsintered
sample (30.2 1 4.8 nm) and the samples sintered in air at 500°C (24.6 1 8.9 nm),
600°C (30.8 1 6.3 nm), 700°C (34.4 1 11.2 nm) or 800°C (32.4 1 4.4 nm).
However, there is a statistically Signiﬁcant difference between the sample
sintered in air at 900°C (64.5 1 26.6 nm) and all other samples. Comparing the
grain size changes to the average roughness, the larger grains on the surface of
the titania membrane sintered at 900°C correspond to the increased roughness
of the ﬁltration surface. As would be expected with the large increase in grain
Size, and the accompanying coarsening of the pores, this results in an increased
roughness of the ﬁltration surface. This increase in the roughness with an
increase in the grain Size for nanoparticle grains was Observed by Dias et al.

[195].

This Signiﬁcant increase in roughness, when the membrane was Sintered at
900°C, was not seen by Kamik et al. in their work with the AZT membranes [4].
The improved thermal stability of the AZT membranes did not result in the
topographical changes that the titania membranes underwent, when sintered at

900°C.

112

 

        

 

 

 

100
A 90 —

E

5 80—

a? 70 -

g 60 -

3 50

s _

g: 40 -

O

n: 30 —

8: 20

1

> 10 —

< 0 7 3 ‘

Lnsintered 500°C 600°C 700°C 800°C 900°C
Membrane

Figure 36. AFM average roughness (Ra) for the unsintered and sintered
membranes.

6.3.4 COATED MEMBRANE ANALYSIS

To study the effect of coating the membranes and sintering temperature, six
additional membrane sections were prepared. To study the effect of lowering the
sintering temperature, three membrane sections were coated with 20 layers of
the manganese oxide coating and then sintered in air at 350°C, 400°C and
450°C for 45 min using the same heating and cooling proﬁle that was used for
the uncoated samples. To investigate whether the grain growth can be
decreased by decreasing the time that the membranes are at the elevated
temperatures, three membrane sections were coated with 20 layers of the

manganese oxide coating and then sintered in air at 350°C, 450°C and 500°C for

113

30 minutes. Rather than using the previously described heating and cooling
proﬁle, the furnace was heated to the Sintering temperature, the samples were
inserted, sintered for 30 minutes and then removed from the furnace to cool to
room temperature. The maximum sintering temperature used with this stepped
sintering proﬁle was 500°C Since higher temperatures would cause thermal
shock. These samples were then analyzed using SEM and the grain sizes were
measured. TO determine the effect that the coating and sintering has on the
MWCO Of the membranes, the rejection of 1 kDa, 5 kDa, 25 kDa, 50 kDa and 80

kDa dextran was measured.

6.3.4.1 SCANNING ELECTRON MICROSCOPY (SEM) ANALYSIS

SEM micrographs Of the coated and Sintered samples (Figure 37) do not appear
to Show any changes in the grain size Of the coated membrane surface.
However, grain Size measurements of the samples sintered at 400°C (41.0 1 2.7
nm) and 450°C (43.5 1 3.1 nm) using the ramped heating and cooling method
demonstrate a statistically Signiﬁcant difference from the unsintered sample (38.0
1 1.2 nm). The grain size of the samples Sintered at 450°C (41.9 1 3.0 nm) and
500°C (43.5 1 2.7 nm) without the ramped heating and cooling proﬁle also
demonstrate a statistically signiﬁcant difference from the unsintered sample.

There is no statistical difference in grain Size between the samples Sintered at

114

350°C with the ramped (39.3 1 2.3 nm) or without the ramped (39.6 1 3.4 nm)

heating and cooling method and the unsintered sample.

 

Figure 37. SEM micrographs of the titania membrane coated with 20 layers of
the manganese oxide coating and sintered at a) 350°C for 45 min, b) sintered at
400°C for 45 min and c) sintered at 450°C for 45 min, each with a ramped
heating/cooling proﬁle, as well as d) sintered at 350°C for 30 min, e) sintered at
450°C for 30 min and f) sintered at 500°C for 30 min without a ramped
heating/cooling proﬁle. All scale bars are 100 nm.

115

Grain Size measurements indicate that it is possible to sinter the manganese
oxide coating onto the titania membrane without coarsening the grains by using a
sintering temperature Of 350°C (with or without a ramped heating and cooling

proﬁle).

Using the ramped heating and cooling proﬁle and calculating the resulting
diffusivity, using equation (2) (approximating using a transition metal in an oxide

host, D0 = 1.8 x 10'9 m2/s and Q = 202 x 103 J/mol for Ni in M90 [167]) the

diffusivity can be plotted with respect to time for the ramped proﬁle (Figure 38).
Although the sample experiences elevated temperatures for a longer period of
time when Sintered using the ramped heating and cooling proﬁle, the diffusivity
increases and decreases very rapidly during the heat up and cool down periods,
respectively. Therefore, very little diffusion occurs outside Of the hold time for the
Sintering process. This is why changing from a ramped to a stepped heating and
cooling proﬁle did not limit the amount Of grain growth experienced during
Sintering. Lowering the Sintering temperature, however, reduced the grain
growth, Since the diffusivity is function of the sintering temperature, as shown by

equation (2).

116

 

 

 

 

 

 

 

 

 

500 1.65-30

500. ~-1.4E-30

125-30
0 a A
L 40° ~1E-30 “3
g E.
.5 -3001 “BE-31 31
g E
"GE-31 m
g 2001 :12
" 145-31 a

100‘ +2E-31

o 7 ' ' I T' ’ I ' 1
o 100 200 300 400 500

Time (minutes)

Figure 38. A plot Of the temperature versus time and diffusivity versus time for
the ramped heating and cooling proﬁle.

Comparing the permeability Of the coated membranes to the grain size, a trend Of
increasing permeability with increasing grain size is Observed (Figure 39). This is
consistent with the results of the uncoated membranes. This correlation between
the grain size and permeability has been Observed by others [197, 198] and is
due to the coalescence of the pores during sintering [97]. At the low sintering
temperatures and short times used for the manganese oxide coated titania
membranes, the nanoparticle coating is able to sinter to the underlying
membrane because of the enhanced reactivity Of nanoparticles [199, 200], while
for the supporting membrane, the larger particles are only able to undergo the

initial stages of Sintering: grain growth and pore coarsening [98].

117

As would be expected, the grain sizes of the coated samples Sintered between
350°C-500°C, either with or without the ramped heating and cooling proﬁle (grain
Sizes range between 39.3 1 2.3 nm and 43.5 1 3.1 nm), are within the range of
the grain Sizes Observed for the uncoated and unsintered sample (38.01 1.2 nm)
and the uncoated sample that was sintered at 500°C (47.2 1 3.3 nm). Since there
was no statistically signiﬁcant difference (at 95% conﬁdence interval) in the AFM
roughness measurements between the unsintered sample and the uncoated
samples sintered up to 800°C, the roughness of the coated samples that were

sintered at or below 500°C was not measured, since no statistically signiﬁcant

difference would be expected.

 

 

 

 

 

 

60 250
1— Permeability - with ramp
1— Permeability - no ramp —’
55 - —~ 200 a
O.
E 5'"
5 50 - “ 150‘“E
1'1 a
a)
5 45 — 54 100 g
g n
O 8
1.
40 — “ 50 °-
-l- Grain size - with ramp
-- -EI—Grain Size - no ramp
35 f l I I 0
300 350 400 450 500 550

Sintering Temperature (°C)

Figure 39. Comparison Of the average grain size and permeability for the
membrane samples coated with 20 layers of manganese oxide and Sintered with
and without a ramped heating and cooling proﬁle.

118

6.3.4.2 MOLECULAR WEIGHT CUT-OFF (MWCO) MEASUREMENTS

Since an increase in the permeability of the membranes was observed at the
500°C sintering temperature that is used with the manganese oxide coating, the
effect of the increased permeability on the MWCO of the membranes was
studied. The rejection Of 1 kDa, 5 kDa, 25 kDa, 50 kDa and 80 kDa dextran was
measured using an uncoated and unsintered membrane, an uncoated membrane
that was sintered at 500°C, and membranes that were coated with 10, 20, 30 and

40 layers Of manganese oxide and sintered in air at 500°C (Figure 40).

  
 

 

 

100 —
80 ‘ +
x
I; 60 -
2: -<>-Uncoated unsintered
E —a—Uncoated sintered
O 40 — o
.33.), +5kD-10-500 C
II —<>— 5kD-20-500°C
20 — ‘X‘ 5kD-30-500°C
—+— 5kD—40-500°C
0 l l I 1
0.1 1 10 100 1000

Molecular weight cut-off (kDa)

Figure 40. Percent rejection of 1 kDa, 5 kDa, 25 kDa, 50 kDa and 80 kDa
dextran for titania, 7-channel, 5 kDa membranes that are uncoated and
unsintered, uncoated and sintered at 500°C and coated with 10, 20, 30 and 40
layers of manganese oxide and sintered in air at 500°C.

119

The uncoated membrane that was sintered in air at 500°C has lower rejection of
the 1 kDa and 5 kDa dextran (8.4% and 79.7%, respectively) than the uncoated
and unsintered membrane (43.0% and 88.5%, respectively). However, the
membranes that were coated with the manganese oxide coating all have higher
rejection of the 1 kDa dextran (69.5%, 71.1%, 67.5% and 78.7% for the 10, 20,
30 and 40 coating layers, respectively) than the uncoated and unsintered
membrane. The 5 kDa dextran rejection for the coated and sintered membranes
(89.6%, 94.5%, 91.2% and 88.7% for the 10, 20, 30 and 40 coating layers,
respectively) was also equal to or higher than the uncoated and unsintered
membrane (88.5%). The 25 kDa dextran rejection for the membranes coated with
10, 20 and 40 layers Of the manganese oxide and sintered at 500°C (87.8%,
89.9% and 86.1%, respectively) was Slightly lower than the uncoated and
unsintered membrane (93.1%), however, the membrane coated with 30 layers
had a 93.8% rejection of the 25 kDa dextran. With the 50 kDa dextran, the
rejection for the coated membranes were all lower (90.6%, 87.5%, 85.0% and
81.9% for the 10, 20, 30 and 40 coating layers, respectively) than the uncoated
and unsintered membrane (93.5%). The 80 kDa dextran rejection was slightly
lower than the uncoated and unsintered membrane (93.6%) for the membranes
coated with 20, 30 and 40 coating layers (91.7%, 87.3% and 82.9%,
respectively), however, the membrane coated with 10 coating layers had a

rejection of 94.2% for the 80 kDa dextran.

120

Sintering of the uncoated membrane resulted in an increase in the permeability
of the membrane. The rejection of 1 kDa and 5 kDa dextrans also decreased
when the uncoated membrane was sintered. However, the addition of the
manganese oxide coating resulted in an increase in the 1 kDa and 5 kDa dextran
rejection. Since the permeability was tested using DI water, which has a
molecular weight Of 18 Da, the increase in pore size due to sintering resulted in
an increase in its permeability. However, the dextrans having molecular weights
of 1 kDa and 5 kDa are rejected at percentages >69.5% with the coated and
Sintered membranes (the nominal MWCO of the membranes, as received from

the supplier, is 5 kDa).

The increase in the MWCO with sintering has been Observed by Schaep et al.
[201]. However, the increase in permeability of the membranes while maintaining
the molecular weight cut-Off with the coated membranes is unique. Ma et al. [202]
found that coating of Al203 membranes with a Si-doped Ti02 resulted in a
decrease in the MWCO Of the membrane with a concomitant decrease in the
permeability. However, the coatings on these membranes were calcined at
500°C and the alumina membrane has a higher thermal stability than the titania

membrane [193].

121

J

6.5 CONCLUSIONS

The large increase in DI water permeability Of the titania membranes is due to a
pore coalescence in the ﬁltration layer, marked by the increase in grain size Of
the ﬁltration surface of this membrane during Sintering. This signiﬁcant

coarsening was not seen with the AZT membranes since the alumina and
zirconia in the AZT membrane have higher melting temperatures than the TiOz.
The lower melting temperature TiOz allows for increased diffusion at the
Fe203/AZT Sinten'ng temperatures of 900°C. The large grains that result from

sintering the titania membrane at 900°C also increase the surface roughness of

the Sintered membrane.

Characterization Of the titania water ﬁltration membranes indicates that it has

lower thermal stability than the AZT membranes. This lower thermal stability

results in an increase in permeability from 152 Umz'hr-MPa for the unsintered

membrane tO 11,300 L/m2-hr-MPa for the membrane sintered at 900°C.

Therefore, when a coating is applied to the titania membrane, it cannot be
sintered at as high Of temperatures as the AZT membrane. This is consistent with
the work of Sekulié et al. [186] and Bae et al. [187], in which dopants had to be
added to titania membranes in order to suppress the increase in pore size during

high temperature treatments.

122

Since titania membranes have poor thermal stability above 500°C (due to the
anatase to rutile phase transformation) [187], dopants can be used to retard the

anatase to rutile transformation and any subsequent grain growth. The addition

of ZrO2 to TiO2 has been shown to inhibit this transformation by substituting Ti4+

. . 4+ . . . . . . . .
Ions WIth Zr Ions, caUSIng an Increase In straIn energy, thch lowers the dnvmg

force for the phase transformation [98, 186]. Further, Zr has been shown by
Wegmann et al. to have the added beneﬁt Of promoting virus adsorption onto the

surface Of the membranes [192].

Evaluation of the dextran rejection by the membranes coated with manganese
oxide and sintered at 500°C Shows that the nominal 5 kDa MWCO was
maintained, while the permeability Of the membranes actually increased
(because of the effects of membrane Sintering). This is of beneﬁt because it
allows for lower Operating pressures, and hence a lower cost of Operation, to

maintain a constant ﬂux through the membrane.

In the preparation of gas separation membranes, Ahmad et al. [181] determined
that the addition Of a Sintered alumina coating on a titania membrane increased
the selective permeability of the membrane. Alumina was used since it is stable
up to its melting temperature Of 2030-2050°C [181, 193]. Although an increase in
pore size was observed as a result of the Sintering (at 800°C and 1000°C) Of the

alumina to the base titania membrane, there was an improvement in the

separation Of gas mixtures containing H2 and N2. This improvement in

123

performance, despite the increase in pore size, is consistent with the results of
the improvement in low MW dextran rejection by the manganese oxide coated

titania membranes that were Sintered at 500°C.

124

CHAPTER 7

SUMMARY AND CONCLUSIONS

 

7.1 SUMMARY AND CONCLUSIONS

The engineering signiﬁcance Of the work performed is that it has shown that it is
possible to produce an effective catalytic manganese oxide coating on ceramic
water ﬁltration membranes for use in a hybrid ozonation-membrane ﬁltration
system. Although polymeric membranes are widely used for water puriﬁcation
due to their lower cost than ceramic membranes [57, 77-80], the longevity of
ceramic membranes and their resistance to degradation by ozone allows ceramic

membranes to be used in combination with ozonation [2-4, 57, 61-74].

A manganese oxide nanoparticle colloid for coating Of the ceramic membranes

was prepared by a novel method, the ozonation of Mn(ll). This resulted in a

colloidal suspension Of hydrated MnO2, with a ﬂake-like morphology and average

125

particle size of ~92 nm. Physisorbed water is driven Off the hydrated MnO2 at
approximately 80°C. Sintering of the hydrated MnO2, at either 500°C for 45
minutes or 550°C for 30 minutes, both result in the reduction Of the MnO2 to d-
Mn2O3. When the hydrated MnO2 colloid is applied to a ceramic water ﬁltration

membrane and sintered, the resulting o-Mn203 coating acts as a catalyst for the

oxidation Of NOM at the surface of the membrane when used in combination with
ozonation. The degradation of the NOM results in an improved water ﬂux through
the membrane as well as a decrease in the TOC concentration in permeate

samples, when compared to uncoated ceramic membranes.

With increasing number of coating layers, the roughness of the coated
membrane surface remains relatively unchanged, however the maximum feature

height decreases, likely due to the ﬁlling of the valleys between the features with
the MnO2 nanoparticles. The grain Size Of the coated membrane surface

increased with increasing number of coating layers, with a trend Of decreasing
TOC in permeate with an increase in the grain Size. This iS consistent with the

observations of Kamik et al. [4], where no statistically signiﬁcant change in
roughness was observed with increasing number of Fe203 coating layers on AZT

membranes, however, an increase in grain Size was Observed.

There was no change in the grain Size of the ﬁltration surface with the addition Of

20 or 30 coating layers. There was, however, an increase in the grain size of the

126

 

ﬁltration layer of the membrane that was coated with 40 layers Of the manganese
oxide and then Sintered at 500°C. Since the Sintering conditions remained the
same for the different number of coating layers, the increase in grain Size is likely
due to the agglomeration of the manganese oxide colloid resulting in increased
grain growth. Sintering of agglomerates results in the coarsening the grains, and

subsequently the pores, of the ﬁltration surface of the membranes.

EDS mapping showed Mn present throughout the coated and sintered
membrane. Since solid state diffusion across the entire cross section of the
membrane would not be possible at the Sintering temperatures and times used,
the Mn dispersed into the pores of the membrane during coating and then

diffused into the grains of the-membrane by solid state diffusion during Sintering.
The MnO2 particles, however, are too large to ﬁt into the pores Of the ﬁltration

layer, therefore, Mn ions that were present in the colloid dispersed into the
porous membrane by capillary action during coating. During the sintering
process, the Mn ions diffuse into the grains Of the membrane. The presence of
the manganese within the membrane will be of beneﬁt in the long term
performance of the membranes since the catalytic performance will be
maintained, even if there is wear at the surface Of the membrane. Additionally,
Since the manganese is present within the pore structure of the membrane, small
molecular weight compounds can be oxidized as they pass through the

membranes.

127

TEM micrographs show that the manganese oxide produced a cohesive coating
on the ceramic membrane substrate. This cohesion of nanoparticle coatings on
porous substrates was Observed by others [162, 163] and is likely due to the
partial ﬁlling Of the pores during the coating and sintering process. There was a
trend of increasing thickness of the manganese oxide coating with increasing

number of coating layers, however, the thicknesses varied considerably.

Increasing the number Of coating layers improved the TOC removal by the
membranes when used in the hybrid ozonation-membrane ﬁltration system. The
catalytic degradation of the organic foulants on the surface of the membrane was
Observed by improved water ﬂux through the coated membranes. The water ﬂux
through the membranes recovered more fully and quickly with the membrane
coated with only 20 layers, compared to the uncoated membrane and the
membranes coated with 30 or 40 layers. NO additional beneﬁt is seen by

increasing the number of coating layers beyond 20. Despite the reduction Of the
MnO2 to O-Mn2O3 during sintering, the a-Mn203 is an effective catalyst for the

degradation of organic matter in the water, as Observed by the decreased TOC in

the permeate and improved ﬂux through the membranes.

The titania membranes used in these studies have lower thermal stability than

alumina-zirconia-titania membranes. In the course of experimentation, the titania
membranes were coated with Fe203 nanoparticles (Since AZT membranes were

no longer available for purchase) to compare to the results of Kamik et al. [3, 4,

128

 

66-68] (where F9203 nanoparticle coatings were applied to AZT membranes and

sintered at 900°C). However, when these membranes were sintered at 900°C, a
large increase in the permeability was observed. The lower thermal stability of
the titania membranes, as compared to the AZT membranes resulted in
increasing grain Size during Sintering. This correlation between increased grain
size and increased permeability was Observed by others [197, 198] and is the

result of the coalescence Of pores during the sintering process [97].

Pore coarsening during grain growth causes increased DI water permeability.
Although the coated membranes that are Sintered at 500°C show an increase in
permeability, they are still able to maintain a MWCO of 5 kDa. The increase in
permeability Of the membranes while maintaining the molecular weight cut-Off
with the coated membranes is unique and would allow for high water production

without sacriﬁcing the rejection by the membrane.

129

CHAPTER 8

RECOMMENDATIONS FOR FUTURE WORK

 

8.1 RECOMMENDATIONS FOR FUTURE WORK

Although a 500°C sintering temperature was chosen to prevent the reduction Of
the MnO2 to Mn203, this reduction still occurred. However, the resulting a-Mn203

coating has been Shown to be an effective catalyst for the degradation of NOM in
the water samples when used in the hybrid ozonation-membrane ﬁltration
system. Since sintering the coated membranes at higher temperatures results in
higher permeability of the membranes (without sacriﬁcing the 5 kDa MWCO),
future work could study the use of a higher sintering temperature for the

manganese oxide coated membranes, since the reduction to a-Mn2O3 has

already occurred. Using the 500°C sintering temperature is not required to avoid

the reduction Of the coating material.

130

If a higher sintering temperature is used, the manganese oxide coated
membrane may be able tO maintain the 5 kDa MWCO while having higher
permeabilities. This would decrease the operating cost Of the hybrid ozonation-
membrane ﬁltration system if lower pressures could be used. Studies would have
to be completed to determine how high the permeability can be increased before
the MWCO of the membrane is compromised. Also, Since only one test Of the
dextran rejection was completed for each membrane, additional testing of the
MWCO should be completed for coated and Sintered membranes to determine

the reproducibility of the results.

If the manganese oxide colloids has agglomerated before it is applied to the
ceramic membrane, the agglomerates will sinter together into larger grains on the
surface of the membrane [97]. These larger grains will grow at the expense Of the
smaller grains and will result in grain coarsening [97, 98, 165, 166]. Since it is
believe that the manganese oxide colloid is agglomerating over time (resulting in
the increase in the grain size Of the coated surface for the membrane coated with
40 layers), a study should be completed to determine the extent Of agglomeration
over time. The manganese oxide colloid Should be prepared and sonicated and
then holey carbon coated TEM grids should be dipped into the suspension at
evenly spaced time intervals over the a 24 hour time period. Then TEM
micrographs Of these samples could be used to measure the change in

agglomerate size over time.

131

 

The streaming potential can be used to determine the surface Charge on a
membrane [155]. Changing the membrane’s surface charge will affect the
adsorption of foulants on the membrane surface [155]. Since the application Of
the manganese oxide coating will affect the surface Charge Of the membrane, the
streaming potential of the coated membranes should be determined. The work
completed has characterized the morphology Of the coated membrane surface,

however, the streaming potential of the coated membranes has not been studied.

SEM analysis of membranes that have been used in the ﬁltration system did not
Show any microcracking of the ﬁltration surface, even after the membranes were
left in water for over ten months (images Of these samples are shown in
Appendix C). A comprehensive study Of the long term performance of the
membranes should be completed to look at the durability Of the coated ceramic
membranes. This study Should include microscopy Characterization Of the coated
membranes after they have been used in the hybrid system for extended periods

of time.

Additionally, the long term performance of the coating should be studied to
ensure that the catalytic properties are maintained after extended use. These
studies should include the water quality measurements obtained from permeate
samples after extended use of the membrane in the hybrid ozonation-membrane
ﬁltration system. Also, any changes in the permeability of the membranes should

be monitored.

132

The manganese oxide coating has been Shown to reduce the TOC in permeate
samples. Additional work should be completed to study the reduction of
disinfection by-product formation, as well as the ability to remove other
contamintants in water such as pharmaceuticals, Chemicals, bacteria and

viruses.

Since AZT membranes are no longer available for purchase, the work completed
with the manganese oxide coatings was performed using titania membranes. The
lower thermal stability of the titania membranes limits the sintering temperatures,
and hence the type Of coating material that can be applied. Future work should
include the production Of the membrane so that the thermal properties can be
tailored to the application, without relying on commercially available membranes.
This would also allow for modiﬁcations, such as the addition Of the catalytic
coating layer to be made during the production Of the membrane, rather than as
a secondary process. Adding the manganese oxide during the initial fabrication
of the membrane would not only decrease the production time for the catalytic
membranes, but it would also allow for the manganese oxide to be combined
within the support Of the membrane in larger quantities than can disperse into the

membrane during the coating process.

Flat ceramic membranes could be produced using membrane preparation

methods such as the sol-gel process or Slip casting, as described in Chapter 2.

133

The preparation of alumina membranes has been studied by others [203-206]
and could be applied to this application. During the production of the membranes,
the pore sizes can be tailored SO that the desired MWCO can be Obtained. In
addition, AZT membranes could be prepared to have similar thermal properties

as the AZT membranes used by Kamik et al. [2-4, 65-68].

134

 

APPENDIX A

REPRESENTATIVE FOURIER TRANSFORM DATA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

10

Figure A1. Representative Fourier transform analysis of 30 AFM line scans from
a ceramic membrane coated with 20 layers of the manganese oxide coating
prepared using ozone and sintered at 500°C for 45 minutes.

135

APPENDIX B

DIFFUSION CALCULATIONS

 

Calculations for the estimation of the diffusivity, D, using Equation (2) and the

diffusion coefﬁcient, Do, and the activation energy. Q, for a transition metal in an
oxide host (DO = 1.8 x 10'9 m2/s, O = 202 x 11)3 J/mol [167], for Ni in M90) at a

temperature Of 500°C (773 K):

D = DoeXO(£]
RT

Do = 1.8 x 10'9 m2/s
Q = 202 kJ/mOI
= 202 x 103J/mol

R = 8.314 J/mol-K
T = 500°C
= 773 K

 

_ 3
D=( 1-8X10-9m2/s) exp 202x10 J/mol
( 8.314J/mOl-K) ( 773K)

D = 4.0 x 10'23 m2/S

136

.A.

Calculations for the estimation Of the diffusivity, D, using Equation (3), for
diffusion across the entire membrane wall thickness (1 mm) for a Sintering time of

45 minutes:

xzﬁ

x2 th

)(2

D ‘S’I—t—
t = 45min
= 2700 S

x=1mm
=1x103m

(1x10'3m)2
2700s

 

D

D~3.7x10‘7m2/s

Calculations for the estimate Of the diffusion distance using the diffusivity
estimated for a transition metal in an oxide host, for a sintering time of 45

minutes:

 

x z I 4.0X10_23m/SX27OOS)

x z 3.3x10’10m
x z 0.33nm

137

APPENDIX C

SCANNING ELECTRON MICROSCOPY (SEM) IMAGES OF MEMBRANES

AFTER USE IN HYBRID SYSTEM

 

 

Figure C1. SEM micrograph of a titania membrane coated with 40 layers Of
manganese oxide prepared by ozonation (sintered in air at 500°C for 45 minutes)
and used in the ﬁltration system.

138

 

.7”

Figure CZ. SEM micrograph of a titania membrane coated with 40 layers of
manganese oxide prepared by ozonation (sintered in air at 500°C for 45

minutes), used in the ﬁltration system and then soaked in water from 11/21/2008
to 09/30/2009.

139

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