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MICHIGAINIJ 1B ES
STATE UNIVERSITY
EAST LANSING, MICH 48824- 1048

This is to certify that the
dissertation entitled

SORPTION AND HYDROLYSIS OF CARBARYL
IN CLAY-WATER SUSPENSIONS

presented by

L. JACQUELINE ARROYO DAUL

has been accepted towards fulfillment
of the requirements for the

Crop and Soil Sciences and

 

 

Ph.D. degree in Environmental Toxicology

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6/01 clelFIC/DateDuepss-sz

SORPTION AND HYDROLYSIS OF CARBARYL
IN CLAY-WATER SUSPENSIONS

By

L. Jacqueline Arroyo Daul

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Crop and Soil Sciences
Center of Integrated Toxicology

2004

ABSTRACT

SORPTION AND HYDROLYSIS OF
CARBARYL IN CLAY-WATER SUSPENSIONS

By
L. Jacqueline Arroyo Daul

We studied the influence of clay preparation methods on sorption and
hydrolysis of carbaryl by reference clay K-SWy—2. Three different ways of
preparation were used: (1) unfractionated (whole clay). (2) Fractionated by low-
speed centrifugation (clay-cent). (3) Fractionated by sedimentation (clay-sed). All
clay preparations were K” saturated. Each preparation manifested mineral
fractions with significantly different abilities to hydrolyze carbaryl to 1-naphthol,
decreasing in the order: whole clay > > clay-sed. > clay-cent. The extent of 1-
naphthol disappearance from solution followed the order: whole clay >>> clay-
sed. > clay-cent. X-ray diffraction of the heavy fraction revealed peaks
corresponding to calcite and dolomite. Aqueous slurries of whole clay and clay-
sed. were alkaline, whereas the pH of slurried clay-cent. was neutral. Dissolution
of carbonates with sodium acetate buffer eliminated hydrolytic activity associated
with SWy-2. We recommend for particle size separation the low speed
centrifugation procedure followed by treatment with acetate buffer. Sorption of 1-
naphthol, by reference clay K-SWy—2, at high and low pH, was studied. We used:
(1) whole clay. (2) Carbonate-free clay. (3) Carbonate-free clay amended with
calcite. For whole clay and carbonate-free clay amended with calcite, ~80% and
90% of the initial concentration of 1-naphthol disappeared from solution within 24

h, corresponding to 2.0 and 2.3 mg/g of clay, respectively. In carbonate-free

clay, ~35% of the initial concentration of 1-naphthol was sorbed by the clay within
24 h. Negligibly amounts (>1%) of 1-naphthol could be recovered from clay after
methanol extractions. It is apparent that 1-naphthol is rendered unextractable by
K-SWy-2. XRD patterns of sorbed 1-naphthol-K-smectite suggested that 1-
naphthol was intercalated in whole and carbonate-free clay. The smectite basal
spacings increased implying some intercalation of 1-naphthol and/or its
transformation product(s) between smectite layers. FTIR spectra of sorbed 1-
naphthol-K-smectite complexes demonstrated structural Fe3+ reduction. FTIR
spectra also showed evidence of the transformation of 1-naphthol. It appears
that reduction of structural iron may be coupled to oxidation of 1-naphthol.
Unextractable oxidative products of 1-naphthol may be produced and caused the

darkening of color in presence of clay and carbonates.

To my son Daniel R. Merino, the sunshine of my life.

iv

ACKNOWLEDGEMENTS

I would like to express my infinite gratitude to my major professor Dr.
Stephen A. Boyd for this great and unique opportunity to study and conduct
research in such a wonderful environment at Michigan State University. I am
indebted to him for his guidance and financial support. Much of this study was
accomplished under his advice. I have learned and gained much knowledge
from him that I will apply it for the rest of my career.

I would like to thank Dr. Brian J. Teppen, member of my Graduate
Committee, for helping me to solve numberless problems during thousand hours
of research, his knowledge in clay science and environmental soil chemistry is
virtually unsurpassed.

I also extend my gratitude and appreciation to Dr. Thomas C. Voice and
Dr. Donald Penner, members of my Graduate Committee, for their careful and
thoughtful reviews and discussions of my research.

I am also deeply indebted to the Facultad de Ciencias Quimicas,
Universidad Central del Ecuador, for allowing me to study in USA; the Fulbright
Commission in Quito, Ecuador, for the Fulbright Scholar Award; Dr. Kirk and
Marjorie Lawton for the Graduate Student Lawton Award; the Clay Minerals
Society, for the Student Research Award; the Department of Crop and Soil
Sciences, for the Recruitment Fellowship; and the College of Agriculture and
Graduate School for the Graduate Completion Fellowship. Many thanks for the

financial support.

I owe special gratitude to my Professors Boyd G. Ellis, Stephen A. Boyd,
James Tiedje, Brian J. Teppen, Syed Hashsham, Thomas C. Voice, David T.
Long, Renate Snider, James G. Wagner, Lawrence J. Fischer, Karen Chou, and
Delbert Mockma for the excellence of courses offered.

I would like to express my gratitude to Dr. Max M. Mortland and Dr. Cliff T.
Johnston for their time and advice with my research.

Additionally, I owe much appreciation to the group of postdoctoral
researcher associates Hui Li, John Quensen and Guangyao Sheng, and to the
group of fine graduate students: Martha P. Perez, Michael G. Roberts, Simone
M. Charles, Tomeka Priloeau and to James F. Stoddard with whom I have
worked at the Department of Crop and Soil Sciences over the past five years for
their assistance with my research.

My interactions with Rita House, Darlene Johnson, Beverly Riedinger,
Jodie Schonfelder, Cal Brick, staff of the Department of Crop and Soil Sciences
and in particular with Michael Fisch, David Homer, and Peter Briggs, former and
current Directors of the International Center at Michigan State University were
particularly helpful and friendly.

Special thanks to my husband Ramiro H. Merino and son Daniel for the

love, support, and encouragement they gave me through these years.

To all of them, thanks very much.

Jacqueline

vi

TABLE OF CONTENTS

LIST OF TABLES ................................................................................................. ix

LIST OF FIGURES ............................................................................................... x

CHAPTER 1

HYDROLYSIS OF CARBARYL BY CARBONATE IMPURITIES lN REFERENCE

CLAY SWY-2 ........................................................................................................ 1
Abstract ...................................................................................................... 1
Introduction ................................................................................................ 3
Objective .................................................................................................... 9
Materials and Methods ............................................................................. 10
Results ..................................................................................................... 18
Discussion ................................................................................................ 24

REFERENCES ................................................................................................... 33

CHAPTER 2

A SIMPLE METHOD FOR PARTIAL PURIFICATION OF REFERENCE

CLAYS .............................................................................................. 38
Abstract .................................................................................................... 38
Introduction .............................................................................................. 40
Objective .................................................................................................. 42
Materials and Methods ............................................................................. 43
Results ..................................................................................................... 51
Discussion ................................................................................................ 57

REFERENCES ................................................................................................... 67

CHAPTER 3

REDUCTION OF STRUCTURAL FE3+ IN SMECTITE COUPLED TO

OXIDATION OF 1-NAPHTHOL .............................................................. 71
Abstract .................................................................................................... 71
Introduction .............................................................................................. 73
Objective .................................................................................................. 76
Materials and Methods ............................................................................. 77
Results .................................................................................................... 81
Discussion ................................................................................................ 88

REFERENCES ................................................................................................... 96

vii

CHAPTER 4

SUMMARY AND CONCLUSIONS .................................................................... 100
REFERENCES ................................................................................................. 102

viii

LIST OF TABLES

Table 1.1. Time-Dependant Distribution of 14C Activity in Whole
K-Smectite (SWy-2) Slurries Amended with [14C]Carbaryl. ................................ 31

Table 1.2. Time-Dependant Distribution of 14c Activity in Light
Fraction K-Smectite (SWy-2) Slurries Amended with [14C]Carbaryl .................... 32

Table 3.1. dam-Spacings (A) for K-SWy-2, and K-SWy-2

Extracted to Remove Carbonate Impurities. The clay films with

and without 1-naphthol sorption were analyzed by XRD after

air-dried, then exposed the air-dried clay films to 1 and 100%

relative humidity (RH) ............................................................................. 95

ix

Figure 2.5. EDS spectra of K-SWy-2: (a) whole clay, with elemental

composition of O (50.1%), Mg (0.6%), Al (4.9%), Si (35.6%),

K (2.7%), Fe (3.6%), Na (0.7%), Ca (1.2%), and Ti (0.5%),

and (b) “light fraction” with elemental composition of O (51.5%),

Mg (1.7%), Al (11.5%), Si (28.3%), K (3.5%), Fe (3.4%). While

qualitative, this analysis would result in a unit

CBII formula Of Ko.34(SI3.38AI0.12)(AI1_51Fe3+0.23Mgo.25)O10 H20. .......................... 66

Figure 3.1. Disappearance of 1-naphthol from aqueous solution

by (a) whole clay (carbonates-containing); (b) carbonate-free

clay; (c) carbonate-free clay, amended with calcite;

(d) disappearance of 1-naphthol in a 0.01% calcite solution. .............................. 89

Figure 3.2. X-ray diffraction patterns of oriented air-dried films of

1-naphthol and clay K-SWy—2: (a) whole clay (control) and

1-naphthol-whole-clay complex at 120h; (b) carbonate-free clay

(control) and 1-naphthoI-carbonate-free-clay complex at 120h ........................... 90

Figure 3.3. FTIR spectra of 1—naphthol sorbed to the carbonate-

containing SWy—2 and to carbonate-free SWy—2 clay in the 4000

to 400 cm'1 region, containing the most prominent bands from the

sorbed 1-naphthol and its transformation products ...................................... 91

Figure 3.4 An expanded region in the 2200 to 1350 cm'1 of the

FTIR spectra of whole K-SWy-2 (carbonate-containing clay,

represented by the dotted lines) shows additional band intensity

present as a broad spectral component under the 1701, 1636 and

1531 cm'1 bands, which is attributed to the transformation product(s)

of 1-naphthol ........................................................................................ 92

Figure 3.5. FI' IR spectra of the structural OH bending region of

the carbonate-containing and carbonate-free K-SWy-2 smectite

show a reduction in intensity of the AlFe3*OH bending mode of the

carbonate containing K-SWy—2 smectite relative to the intensity of

this band in carbonate-free clay and no change in the intensities of

the AlAIOH and AIMgOH bands .............................................................. 93

Figure 3.6. FTIR spectra of the structural OH stretching region of

the carbonate-containing and carbonate-free K-SWy-2 smectite

show a composite band that reflects the overall contribution of

AIAIOH, AlFe+3OH, AIMgOH bands. The v(OH) band of the whole

clay (carbonate-containing clay) shows a lower intensity in the lower

energy portion (3595 to 3572 cm") of this spectrum, which is the

area where AlFe3+OH and FeOHFe components occur. ................................. 94

xi

LIST OF FIGURES

Figure 1.1. Hydrolysis of carbaryl ............................................................ 6

Figure 1.2. XRD patterns of K-SWy-2: (a) whole clay, oriented

air-dried and (b) after ethylene glycol salvation; (c) light fraction,

oriented air-dried and (d) after ethylene glycol salvation; (e) clay-

sed., oriented air-dried; (f) clay-cent, oriented air-dried; (9)

heavy fraction, oriented air-dried. Lower case letters show the

probable mineral present in the heavy fraction, k = chlorite (?),

r = rutile, q = quartz, h = iron oxide, 0 = calcite, d = dolomite,

m = mica and/or illite, f= orthoclase feldspar, and p = plagioclase

feldspar ............................................................................................................... 28

Figure 1.3. Hydrolysis of carbaryl by smectite clay K-SWy-2:

(a) whole clay; (b) light fraction; (c) heavy fraction; (d) clay-cent;

(e) clay-sed.; (f) whole clay (now without carbonates);

(g) hydrolysis of carbaryl by calcite. .................................................................... 29

Figure 1.4. Amounts of methanol extractable carbaryl and
1-naphthol from K-SWy-2: (a) whole clay; (b) light fraction,
(c) heavy fraction; (d) clay-cent. .......................................................................... 30

Figure 2.1. Sorption and hydrolysis of carbaryl by clay K-SWy-2:
(a) whole clay; (b) clay-sed.; (c) clay-cent; (d) carbonate-free clay,
Method A; (e) carbonate-free clay, Method B. .................................................... 62

Figure 2.2. Amounts of methanol extractable carbaryl from
carbonate-free K-SWy-2: (a) Method A and (b) Method B. ................................. 63

Figure 2.3. X-ray map of the heavy fraction. Elements as 0, Fe, Na,
Mg, Al, Si, K, Ca, and Ti were identified. The brightness of pixel in
x-ray map is directly proportional to the quantity of element present .................. 64

Figure 2.4. SEM photomicrographs of the SWy—2 “light fraction”
(Arroyo et al., 2004), which occurs in a closed fine texture
showing a honeycombed arrangement of particles ............................................. 65

CHAPTER 1

HYDROLYSIS OF CARBARYL BY CARBONATE
IMPURITIES IN REFERENCE CLAY SWY-2

ABSTRACT
The influence of clay preparation methods on the sorption and hydrolysis
of carbaryl (1-naphthyl, N-methyl carbamate) by KI-saturated reference

smectite SWy-2 was studied. Four methods were utilized: (1) The reference (or

specimen) clay used as received was K+-saturated (hereafter referred to as

whole clay). (2) High-speed centrifugation (3295 g) of whole clay resulted in a
pellet with three discrete bands. The upper, light-colored low-density band was
obtained by manual separation (light fraction). The higher density, dark-colored
material comprising the lower band (heavy fraction) was also obtained
manually. (3) SWy-2 was subjected to overnight gravity sedimentation to
obtain the <2 um particles (clay-sed.) and then K+-saturated. (4) SWy-2 was
subjected to low-speed centrifugation (58-609) to separate the <2 um particle
size (clay-cent.) then K+-saturated. Each preparation of mineral fractions
manifested significantly different abilities to hydrolyze carbaryl to 1-naphthol,
decreasing in the order whole clay > heavy fraction >> clay-sed. > light clay >
clay-cent. The extent of 1-naphthol disappearance from solution, accompanied

by a progressive darkening of the clay, followed the order whole clay > heavy

fraction >>> light clay > clay-sed. > clay-cent. Using ring labeled [14C]carbaryl,

~61 and 15% of the total 140 activity added to the whole clay and light fraction,

respectively, remained unextractable. X-ray diffraction of the heavy fraction
revealed several peaks corresponding to minor impurities, including calcite and
dolomite. Aqueous slurries of whole clay, light fraction, clay-sed., and heavy
fraction were alkaline, whereas the pH of slurried clay-cent. was neutral. It was
concluded that dissolution of inorganic carbonate impurities in SWy—2 caused
alkaline conditions in the slurries leading to the hydrolysis of carbaryl.
Dissolution of carbonates with sodium acetate buffer eliminated hydrolytic
activity associated with SWy—2. None of the four preparation methods reliably
removed inorganic carbonates. The use of commercial or reference smectites
in surface chemistry studies should be accompanied by a treatment with

acetate buffer to remove carbonate impurities.

INTRODUCTION

Currently, ~2.04 billion kilograms of chemicals are used as pesticides
each year in the United States. Agricultural usage accounts for ~77% of the
total, much of which is applied to soil (Aspelin, 1997). Surface and ground
waters can be contaminated with pesticides because of the intensive use of
pesticides in agriculture, leading to the potential for exposure of nontarget
organisms (Zalkin et al., 1984; Barcelo and Hennion, 1997; Yen et al., 1997).
Sorption of soil-applied agrochemicals is a key determinant of environmental
fate and impact of pesticides, including mobility, bioavailability, and persistence.
Numerous studies of soil-applied pesticides have indicated the importance of
soil organic matter (SOM) as a dominant sorptive phase (Chiou, 1990; 1998).
Other studies have demonstrated the potential importance of clay minerals in
the retention of certain classes of pesticides (Sheng et al., 2001; Laird et al.,
1992; Laird and Fleming, 1999) and energetics (Weissmahr et al., 1997; 1999;
Haderlein and Schwarzenbach, 1993; Haderlein et al., 1996; Boyd et al., 2001;
Johnston et al., 2001), and that they may contribute to the degradation of
carbamate pesticides, for example, carbosulfan, carbofuran, aldicarb, pirimicarb
(Wei et al., 2001) and other pesticides, for example, triasulfuron (Pusino et al.,
2000)

Carbaryl is the common name for the carbamic acid derivative 1-
naphthyl N—methyl carbamate. Carbaryl has been used for about 47 years as a
contact and ingestion insecticide with slight systemic properties. The most

common trade name for carbaryl is Sevin. It is widely used to control several

species of insects that are pests in ornamentals, trees, and food crops.
Carbaryl is also used on animals and livestock as an acaricide and
molluscicide. The usual application rate for carbaryl is 0.25-2 kg/ha, but in fruit
trees application increases to 10 kg/ha (WHO, 1993; 1994; Howard, 1991).
Current carbaryl usages in the United States and the European Union are
50000-500000 kg/year (Barcelo and Hennion, 1997; Larson et al., 1996;
Barbash and Resek, 1996). In 1995 and 1996, exports of carbaryl from US.
ports to developing countries were 1.04 and 1.42 million kilograms, respectively
(FASE Research Report, 1998).

Technical grade carbaryl is a white crystalline solid with low volatility.

Carbaryl is poorly soluble in water (104 mg/L at 25 °C), but is soluble in many

organic solvents (log Kow = 1.6—2.36) (Howard, 1991; Montgomery, 1993;

Roberts, 1999). Carbaryl is readily hydrolyzed in alkaline media, but resistant
to hydrolysis at neutral and acidic pH (Kuhr and Dorough, 1976; Matsumura,
1985; Howard, 1991; WHO, 1993; 1994; Montgomery, 1993; Roberts, 1999).
The primary hydrolysis product of carbaryl in soil is 1-naphthol (Kuhr and
Dorough, 1976; Mount and Oehme, 1981; Rajagopal et al., 1983; Matsumura,
1985; Howard, 1991; WHO, 1993, 1994; Montgomery, 1993; Roberts, 1999),
which can be metabolized by soil microorganisms (Mount and Oehme, 1981;
Rajagopal et al., 1984; Racke and Coats, 1988; WHO, 1993, 1994), Figure 1.1.
The reported persistence of carbaryl in soils is considered to be short (e.g., 2
weeks) to moderate (e.g., 16 weeks) (Rajagopal et al., 1984), depending on
environmental factors such as pH and temperature, as well as the rate of

application, chemical formulation, and frequency of application (Rajagopal et

al., 1984). The half-life of carbaryl in soils was reported as 8-15 days,
depending substantially on the aeration status (flooded vs. unflooded) of the
soil (Howard, 1991; Rajagopal et al., 1983, 1984). Small amounts of carbaryl
present in soil solution may be absorbed by plant roots but are not expected to
persist in the dissolved state (Howard, 1991; Rajagopal et al., 1984; Jana and
Das, 1997).

Although carbaryl is not generally considered to be recalcitrant in the
environment (WHO, 1993, 1994), its transformation products (e.g., 1-naphthol)
have been found in California ground-water at concentrations up to 610 pg/L
(Barcelo and Hennion, 1997; Barbash and Resek, 1996). Howard (1991)
reported that carbaryl persisted in the environment (soil and water) from a few
weeks up to 3 months. Carbaryl persisted for 3 weeks in mud, but 1-naphthol
did not (Rajagopal et al., 1984). Rajagopal et al. (1984) reported that > 50% of
the soil-applied carbaryl accumulated as 1-naphthol, which was more toxic to
molluscs and three species of marine fish than the parent compound in 24 and
48 hours laboratory experiments (Mount and Oehme, 1981; Day, 1991).

Retention of carbaryl in soils may result from sorption by clay minerals.
Carbaryl sorption by bentonite clay (composed of a large amount of smectite)
was greater than by kaolinite (Rajagopal et al., 1984). The ability of smectite to
adsorb organic contaminants and pesticides from aqueous solution is
influenced by the exchangeable cation, as well as the extent of structural

charge and its origin (Haderlein et al., 1996; Boyd et al., 2001). Aly et al.

+
(1980) studied sorption isotherms of carbaryl and found that Ca2 -bentonite

exhibited the highest affinity for carbaryl, followed by an alluvial soil, 3

calcareous soil, and calcite. Although carbaryl is subject to alkaline
hydrolysis, its degradation by calcite was not noted (Aly et al., 1980). Sheng et

al., (2001) evaluated the potential contribution of smectite and organic matter to

+
pesticide retention in soils, and concluded that homoionic K -smectite was a

more effective sorbent than muck soil for carbaryl.

ii
O—C ——NHCH3 OH
O O —-> O O
carbaryl 1-naphthol

Figure 1.1. Hydrolysis of carbaryl.

The Source Clays Program of the Clay Minerals Society (CMS) was
initiated in 1972 to provide a common set of reference clays to researchers
(Costanzo, 2001). The initial descriptions of these materials were presented in
the Data Handbook for Clay Materials and Other Non-Metallic Minerals (Van

Olphen and Fripiat, 1979). An updated version of this book provides

descriptions of materials added to the original reference set and additional data
obtained with new analytical techniques (Costanzo and Guggenheim, 2001).
Chipera and Bish (2001) established that most of the Source Clays, which are
naturally occurring materials, contain discernible amounts of other materials,
which are considered to be impurities. The nature of clay impurities may vary
considerably among reference clays. Although the availability of reference
clays has greatly improved our ability to compare results from different studies
and laboratories, different sample preparation methods may undercut these
efforts to ensure the use of common standard materials in studies utilizing clay
minerals. A cursory examination of recent literature established that Source
Clays are treated in different fashions prior to use, for example to prepare
different homoionic forms of the clays or to remove impurities. Preparations
reported in the literature include a simple wet sedimentation, that is, gravity
settling overnight, to obtain the <2 pm fraction (Sheng et al., 2001; Laird et al.,
1992; Boyd et al., 2001; Wei et al., 2001; Pusino et al., 2000; Liu et al., 2002;
Hofstetter et al., 2003; Boyd et al., 1988; Li et al., 2003; Sheng et al., 2002;

Aguer et al., 2000; Garcia-Junco et al., 2003), and Na+-saturation followed by

low speed centrifugation to separate larger particles from the < 2 pm fraction

(Johnston et al., 2001, 2002; Stucki et al., 1991). Often, reference clays are
used without previous Na+-saturation, wet sedimentation, or centrifugation

(Haderlein et al., 1996). They are sometimes subject to atypical pretreatments

such as the removal of organic matter and metal oxides by 0.1 M KOH solution

and 0.1 M HNO3 solution, respectively (Schmitt et al., 2002) prior to cation

saturation, or do not have defined pretreatment procedures at all (Aly et al.,
1980, Mogyorosi et al., 2002; Kutsuna et al., 2002; Celis et al., 1997).
Experimental details such as dispersal method, sedimentation time,
centrifugation forces, and type of membranes used for dialysis are often not
given. Potentially these different methodological approaches may differentially
include/exclude impurities and particles of different sizes, which may influence

sorption and degradation processes.

OBJECTIVE

The objective of this study was to determine the influence of clay

preparation/purification methods on the sorption and hydrolysis of carbaryl by
reference clay SWy-2 in the K+-saturated form. The reference clay used
without purification was compared to clays subjected to centrifugation or gravity

sedimentation as essential purification steps. Each of the clay preparations

manifested significantly different degrees of carbaryl sorption and hydrolysis.

MATERIALS AND METHODS

The reference smectite clay, SWy-2, was used in the study. The clay
was obtained from the Source Clays Repository of the CMS (Columbia, MO)
and has been characterized previously (Grim and Guven, 1978; Van Olphen
and Fripiat, 1979; Mermut and Cano, 2001; Vogt et al., 2002). Four separate

procedures, detailed below, were used to prepare homoionic K-SWy—2.

Whole Clay
In this case, no preparation/purification step was utilized prior to making

the homoionic K-SWy—2. The reference clay used as received, was saturated

with K“ by mixing 5.0 g of clay with 200 mL of 0.1 M aqueous KCI in 250 mL

polypropylene screw-cap bottles and shaking on a reciprocating shaker for 24
hours. The mixture was centrifuged for 20 minutes at 32959 (4500 rpm) using
a Sorvall GSA rotor and centrifuge (DuPont 00., Wilmington, DE). The pellet

was collected and the supernatant discarded. The pellet was re-suspended in
150 mL of 0.1 M KCI and then shaken and centrifuged as above. This K+-

saturation process was repeated four times. The clay was then mixed with 150
mL of Milli-Q (deionized, d.i.) water (Millipore Corp.), shaken overnight, and
centrifuged as above, and the supernatant was discarded. Another 150 mL
portion of d.i. water was added to the clay pellet, and the whole was placed in
porous membrane tubing (four 30 cm x 50 mm of molecular weight cutoff

6000—8000) obtained from VWR Scientific-Spectrum Medical Industries, Inc.

10

(Laguna Hills, CA) and dialyzed against d.i. water until free of chloride as

indicated by AgNO3. The sample was quick-frozen, freeze-dried, and stored.

Light Fraction
A fraction of the whole clay was collected after the formation of three

distinctly colored bands was noted in the centrifuge pellet obtained after the

fourth K+-saturation step (from above). The light fraction was collected by

scraping the upper part (band) of the pellet, which presented a distinguishing
light-yellow color, with a metal spatula. The clay pellet was then washed with
d.i. water and dialyzed as above. The sample was quick-frozen, freeze-dried,

and stored.

Heavy Fraction

The pellet remaining after isolation of the light fraction was composed of
two distinctly colored bands. The upper dark-yellow band was manually
removed using a metal spatula. The remaining dark-brown material (heavy
fraction) was redispersed in d.i. water and centrifuged at 609 for 6 minutes and
the supernatant discarded. The sample was quick-frozen, freeze-dried, and

stored.

Clay-cent.
To obtain the clay-sized particles (<2 um), 25 g of SWy-2 was placed in
a 1 L beaker with 500 mL of d.i. water and stirred for 24 hours using a magnetic

stir-bar and plate to hydrate the clay and to form a stable suspension. The

11

suspension was then poured into four 250 mL polyethylene centrifuge bottles
and centrifuged for 6 minutes at 58—609 (~600 rpm) using a Sorvall GSA rotor
and centrifuge. The supernatant suspension containing the <2 um clay-sized
particles was then collected in six polyethylene centrifuge bottles and

centrifuged again for 30 minutes at 32959 (4500 rpm). The supernatant was
discarded. The clay pellet was K+ saturated (4x) with 0.1 M KCI, washed with

d.i. water, and dialyzed as above. The sample was then quick-frozen, freeze-

dried, and stored.

Clay-sed.

To obtain the clay-sized particles (<2 um), 25 g of SWy-2 was placed in
a 2 L beaker with 1.8 L of distilled water and stirred for 8 hours using a
magnetic stir bar and plate to hydrate the clay and to form a stable suspension.
After overnight (~18 hours) settling, the supernatant suspension containing the
<2 urn clay-sized particles was poured into 12 polyethylene centrifuge bottles
and then centrifuged for 30 minutes at 32959 (4500 rpm). The supernatant
was discarded. The clay pellet was K+ saturated (4x) with 0.1 M KCI. After K+-

saturation, the clay was washed with d.i. water until free of chloride as indicated

by AgN03. The sample was then quick-frozen, freeze-dried, and stored.

X-ray Diffraction (XRD)

X-ray diffractograms of K-SWy-2 clays were determined by XRD

analysis. Suspensions of K-SWy—2 were dropped onto glass slides and allowed

12

to air-dry at ambient conditions to obtain oriented clay films. XRD patterns of
oriented films were analyzed before and after 24 hours of ethylene glycol
saturation at 40 °C. The XRD patterns were recorded using Cu-Ka radiation
and an XRD system consisting of a Philips 3100 X-ray generator (Philips
Electronic Instrument, lnc., Mahwah, NJ), a Philips 3425 wide-range
goniometer fitted with a 9-compensating slit, a 0.2 mm receiving slit, a
diffracted-beam graphite monochromator, and PW1877 automated powder
diffraction (Philips Electronics) control software. Diffraction patterns were

measured from 4 to 60° 29, in steps of 002° 29, at 2 s/step.

Chemicals

Carbaryl (>99% purity) was obtained from ChemService (West Chester,
PA), and 1-naphthol (>99% purity) was obtained from Sigma-Aldrich (St. Louis,
MO). Carbaryl and 1-naphthol solutions were prepared in 0.01 M KCI at pH 3.0
(adjusted with HCI) and used as standards for analysis by high pressure liquid
chromatography (HPLC). Carbaryl-naphthalene-1-14C with a specific activity of
8.4 mCi/mmol and a radiochemical purity of 97% was obtained from Sigma-
Aldrich. Solutions of 40 ug/mL of carbaryl in 0.01 M KCI solution at pH 6.5,
protected from light, were prepared and used immediately for the

sorption/degradation experiments. In some instances, the 40 ug/mL carbaryl
solution was amended with the 14C-ring-labeled carbaryl. The labeled
14
l

Cjcarbaryl solution was prepared as follows: 100 uCi of [14C]carbaryl (~2.4

mg of [14C]carbaryl) was dissolved in 1 mL of methanol, and then 100 mL of 40

13

ug/mL unlabeled carbaryl solution was amended with 10 (IL (~1 uCi) of labeled

carbaryl stock solution. The final carbaryl solution contained ~0.01 uCi/mL.

Kinetics

Kinetics data concerning carbaryl disappearance and 1-naphthol
appearance were determined directly in aqueous samples taken from slurries of
K-SWy—2 clays using a batch method. A certain amount (5.0 mL) of a 40 ug/mL
carbaryl solution (in 0.01 M KCI, pH 6.5) was pipetted into 7.4 mL borosilicate
amber glass vials from Supelco (Bellefonte, PA) containing 60 mg of clay. The
pH values of the clay slurries were not adjusted. Vials, in replicates of three,
were mixed briefly using a Fisher Vortex mixer from Fisher Scientific (Bohemia,
NY) and then mechanically rotated continuously at room temperature (23 :l: 1
°C). Vials were then centrifuged at ~35009 for 20 minutes (whole clay and
heavy fraction) or for 40 minutes (light fraction and clay-cent.) to separate solid
and liquid phases. Centrifugation occurred after 2, 4, 8, 12, 16, 24, 48, 72, and
96 hours reaction times. Methanol extractions of the centrifuged clay pellets
were conducted to determine (by HPLC) the amounts of adsorbed carbaryl.
Amounts of carbaryl extracted from the clay pellets were obtained by
subtracting the mass of carbaryl present in residual water associated with the
clay after centrifugation from the mass of carbaryl extracted by methanol. To

assess mass balance, experiments were run utilizing (otherwise identical)

solutions containing 14C-ring-labeled carbaryl, for the whole clay and the light-

fraction. Concentrations of carbaryl and 14C activity in the liquid phase were

determined by HPLC and liquid scintillation counting (LSC), respectively.

14

Water and methanol were used sequentially to extract the adsorbed 14C activity

from the clay, which was determined by LSC. To determine residual 14C
activities, the extracted clays were air-dried and combusted at 900 °C in an OX-

300 Harvey biological oxidizer. The 14C02 produced by combustion was

trapped and quantified by LSC.

In another set of experiments, the pH values of clay slurries used in the
kinetic experiments were measured. After it was determined that whole clay,
light fraction, clay-sed., and the heavy fraction slurries were alkaline, but the pH
of clay-cent. was neutral, two additional kinetic experiments were performed.
First, carbaryl hydrolysis was evaluated in a suspension of calcium carbonate
using the batch equilibration method. A certain amount (5.0 mL) of a 40 pg/mL
carbaryl solution (in 0.01 M KCI, pH 6.5) were pipetted into 7.4 mL borosilicate
amber glass vials containing 1.3 mg of calcium carbonate, which corresponded
to the amount of calcium carbonate present in 60 mg of whole clay (see above),
according to recent analysis of K-SWy-2 (Mermut and Cano, 2001). Vials, in
replicates of three, were mixed briefly using a Fisher Vortex mixer from Fischer
Scientific (Bohemia, NY) and then mechanically rotated continuously at room
temperature. Aqueous concentrations of carbaryl and 1-naphthol were _
sampled and determined by HPLC as above. Second, inorganic carbonates in
the whole clay were removed by incremental additions of a 0.5 M sodium

acetate buffer at pH 5.0 (Kunze and Dixon, 1986) until the clay suspension

reached pH 6.8. The Na+-saturated clay (whole clay, but now without solid-

phase carbonates) was then exchanged with K+ ions, which was prepared

15

using the method described above for the whole clay. Kinetics of carbaryl
sorption/hydrolysis in slurries of the whole K-SWy-2 (without carbonates, at pH
6.8) were obtained using the batch method as above. The percentage of
carbonate carbon present in clay samples used in our experiments was

determined titrimetrically by Huffman Laboratories, Inc. (Golden, CO) by

collecting the 002 evolved from inorganic carbonates.

Aqueous carbaryl and 1-naphthol concentrations were determined by a
Perkin-Elmer HPLC (Shelton, CT) consisting of a binary LC 250 pump, a UV-vis
detector set at 230 nm, and a series 200 autosampler. Chromatographic data
were acquired using Turbochrom software 6.1. Chromatography was carried
out using 3 150x46 mm Supelcosil-C18 column, of 5 pm particle size and 120 A
pore size. The mobile phase was a mixture of 71% methanol and 29% 0.025 M
KH2PO4 solution at pH 3.2. The injection volume was 25 uL, and the flow rate
was 1.0 mL/min. This chromatographic condition allowed baseline resolution
and well defined carbaryl and 1-naphthol peaks for identification and

quanfificaflon.

Half-Life Analysis
The rate law for dissipation of pesticides is often described as a pseudo-

first-order reaction:

In (Mt/M0) = -kobs t (1)

16

where Mo and M are the masses of carbaryl in the whole system at time zero

(applied) and time t, respectively, and kobs is the observed first order rate

constant (Schwarzenbach et al., 1993). A plot of the natural logarithm of the

mass of carbaryl in the system versus time yielded a straight line, with its slope
equal to the first order rate constant (k). The half-live (ti/2) for the dissipation of

carbaryl from solution in these clays were estimated by assuming first order

dissipation processes (Lee et al., 2003) using kobs from the equation 1.

In 2 = 0.693
k k

obs obs

 

(2)

ty2=

17

RESULTS

Clay samples obtained from commercial sources are often assumed to
be pure; hence, they are used without further purification. The attachment of
labels such as “reference clay”, “specimen clay”, or “clay mineral standard” to
the samples obtained from sources such as the CMS Repository may further
give the mistaken impression that these samples are free of impurities. We
prepared a homoionic K-smectite under the “assumption” of sample purity.
This involved mixing the reference smectite clay SWy-2 (as received from the
CMS Repository) with aqueous KCI (whole clay). Centrifugation of K-SWy-2 at
high speed immediately after saturation of the whole clay with K+ showed a
clear separation of particle sizes, as indicated by three distinctly colored bands
in the centrifuged pellet. The upper part of the pellet (referred to as the light
fraction), was a pale yellow color with a gel-like consistency. It comprised
~20% (w/w) of the whole clay. The second band in the pellet was darker
yellow, and a third (bottom) one was dark brown in color. The dark-brown

material comprised ~14% of the total weight of clay, and is referred to as the
heavy fraction. The ionic strength of the 0.1 M KCI solution used in the K‘-

saturation process inhibited clay particle dispersion, resulting in a distinct
separation of particle sizes after centrifugation (at 32959). This facilitated
physical separation of the light and heavy fractions.

Clay-sized particles (<2 um) were obtained from SWy-2 (prior to K-

saturation) by gravity sedimentation are referred to as clay-sed. According to

18

Stokes equation, the >2 um particles should settle 23.5 cm in 18 hours, which

forms the basis of the gravity sedimentation procedure (Moore and Reynolds,
1989). The fine clay-sed. was then K+ saturated with 0.1 M solution of KCI

solution. Some of very fine particles were lost in the subsequent washing
process. Removing excess salts by washing with d.i. water caused re-
suspension (dispersion) of some clay that could not be effectively separated
even after high speed centrifugation. The amount of this material discarded in
the washing water was minimized as much as possible. The clay-sed. obtained
corresponded to 60—70% (w/w) of the original SWy-2 and consisted almost
entirely of dark-yellow material plus a small amount of brown material.

Clay-sized particles were also obtained by low speed centrifugation to
separate coarser particles (Moore and Reynolds, 1989). The clay-cent,
obtained from the resultant aqueous suspension corresponded to ~70% (w/w)
of the original SWy-2. When this suspension was centrifuged at high speed
(32959) the pellet consisted almost entirely of an intermediate yellow material
plus a very small amount of brown material.

XRD patterns of both the whole clay and light fraction (Figure 1.2) show
the distinguishing smectite peaks in the air-dried and ethylene glycol-solvated
states. Basal spacings of 11.2 and 11.5 A in the air-dried state for whole clay
and light fraction, respectively, correspond to the dam XRD peak of smectite.
The 11.2 and 11.5 A peaks in the XRD patterns of the whole clay and the light
fraction shifted to 16.2 and 17.0 A, respectively, after ethylene glycol saturation,
which expands the smectite interlayers. The light-fraction and clay-cent.

appeared to be without crystalline impurities based on XRD analysis.

19

Additional peaks not assignable to smectite occurred in the XRD patterns
obtained for the whole clay and clay-sed., indicating the presence of crystalline
impurities. The 3.34 A peak indicates the presence of quartz, and the 3.24 A
peak may be due to feldspar. The XRD pattern for the heavy fraction is also
shown in Figure 1.2. Peaks corresponding to quartz, orthoclase and
plagioclase feldspar, calcite, dolomite, and rutile are apparent. Chipera and
Bish (2001) and Vogt et al. (2002) reported similar impurities in their
investigations of reference clays.

Batch equilibrium sorption experiments were conducted in an attempt to
measure the distribution of carbaryl between K-SWy-2 and water. During
preliminary kinetic studies of carbaryl sorption by the whole-clay, a color
change in the clay from pale yellow to gray was noted. It was clearly
observable within minutes after the whole clay had been mixed with aqueous
solutions of carbaryl, and the color intensified over time. Interestingly, the light
fraction did not manifest a similar change of color under otherwise identical
conditions. Slight color formation was observed with the clay-cent. but did not
change over time. Similar color formation was observed with the clay-sed., and
the color intensified slightly over time. When mixed with aqueous carbaryl, the
heavy fraction immediately developed the gray color, which intensified over
time as observed with the whole clay.

Analysis by HPLC of the aqueous phase from K-SWy-2 slurries clearly
demonstrated that carbaryl was hydrolyzed to 1-naphthol (Figure 1.3).
Hydrolysis of carbaryl in blanks not containing clay was minimal after 48 hours,

and the amount hydrolyzed remained constant at ~8% (of initial carbaryl

20

concentration) between 48 and 96 hours. Disappearance of carbaryl and the
formation of 1-naphthol in solution were evident in slurries of each clay
preparation, but to different extents. During the initial 24 hours reaction time,
the rate and extent of carbaryl degradation followed the order whole clay >
heavy fraction >> clay-sed. > light clay > clay-cent. Accumulation of 1-naphthol
in solution maximized at ~24 hours for the heavy fraction and the whole clay
and then declined steadily during the remainder of the experiment. The decline
in aqueous phase 1-naphthol was more gradual in the heavy fraction than in
the whole clay. Overall, it appeared that carbaryl hydrolysis and disappearance
of 1-naphthol (produced via carbaryl hydrolysis) were most rapid in slurries that
contained smectite and the heavy fraction together (i.e., the whole clay).
Carbaryl hydrolysis, as evidenced by 1-naphthol accumulation in solution, was
much slower in slurries of the light fraction than clay-sed. and negligible in the
clay-cent (Figure 1.3c—e).

In experiments utilizing the whole clay, ~80% of carbaryl plus 1-naphthol
(on a molar basis) had disappeared from the aqueous solution within 48 hours
(Figure 1.3a). lf sorbed, this would correspond to ~30 mg of carbaryl/g of clay.
This is in contrast to the light fraction and fine clay-cent, where ~80% of
carbaryl plus 1-naphthol (on a molar basis) was present in the aqueous solution
after 48 hours of reaction time. In these slurries, disappearance of carbaryl
from solution occurred only during the first 2 hours, after which its concentration
remained almost constant. The calculated half-lives of carbaryl were 9 and 10
hours in the presence of the whole clay and the heavy fraction, respectively.

Due to the slow hydrolysis of carbaryl in light fraction and clay-cent. slurries, its

21

half-life in these systems was estimated at 204 and 770 hours, respectively.
Although carbaryl disappeared completely, and 1-naphthol disappeared
substantially from solution in slurries of the whole clay, quantities of these
compounds that could be extracted from the whole clay (or the heavy fraction)
by methanol were small (<10% of amount presumed sorbed based on
disappearance from solution). Recoveries from extraction of the light fraction or
the clay-cent. with methanol were much higher, ca. 30 and 65%, respectively
(Figure 1.4). These extracts contained primarily carbaryl with very little 1-
naphthol present.

To achieve mass balance for carbaryl in the slurries of the whole clay

and light fraction, 14C-ring-labeled carbaryl was used. Total recoveries were
100 :t 6% (Tables 1.1 and 1.2). For the whole clay slurries after 48 hours, 14C
activity in the aqueous phase was ~16% of the total 14C activity added (Table
1.1). Water and methanol extracted 25% of the total 14C activity added (or

~29°/o of the activity adsorbed by the clay). Approximately 61% of the total 14C

activity added was sorbed by the whole clay but not extractable after 48 hours
of reaction time (Table 1.1). For the whole clay slurries in general, the amount
of solution phase activity decreased steadily over time (up to 48 hours), and the

amount of sorbed but not extractable activity increased. For the light fraction

slurries after 48 hours, 14C activity in the aqueous phase was ~50% of the total

14C activity (Table 1.2). Water and methanol extracted 31% of the total 14C

activity added (or ~67% of the activity sorbed by the clay). Approximately 15%

22

of the total carbaryl/naphthol activity was sorbed by the light fraction but not
extractable after 48 hours of reaction time.

The pH values of the various clay slurries increased in the order clay-
cent. (pH 6.7) < light fraction (pH 8.2) < clay-sed. (pH 8.5) < whole clay (pH 8.8)
< heavy fraction (pH 9.1 ). Note that this order correlates very well with rates of
carbaryl hydrolysis in these slurries (Figure 1.3). In an aqueous suspension of
calcium carbonate, the pH was 9.5, and hydrolysis of carbaryl proceeded
rapidly, similar to that observed in slurries of the whole clay and heavy fraction
(Figure 1.3a,b,f). Although hydrolysis of carbaryl was rapid and complete, 1-
naphthol accumulated in a nearly stoichiometric quantity in the calcium
carbonate suspension.

Sodium acetate buffer (pH 5) was used to remove carbonate impurities
from SWy-2 (Kunze and Dixon, 1986) by titrating the clay suspension until the
pH was 6.8. The Na+-saturated clay was subsequently exchanged with K+ and
then amended with carbaryl. After 48 h, ~30% of carbaryl had disappeared
from the aqueous solution. This corresponds to an amount sorbed of ~1.0
mg/g of clay (Figure 1.39). There was negligible formation of 1-naph‘thol in this

clay slurry.

23

DISCUSSION

The availability of specimen (or reference) clays from the Clay Minerals
Source Repository and their extensive characterization (van Olphen and Fripiat,
1979; Chipera and Bish, 2001; Grim and Guven, 1978; Mermut and Cano,
2001; Vogt et al., 2002) are important in providing standardized materials for
research in clay, soil, and environmental sciences. Although there are
published methods for the separation and purification of clays from soils and
sediments (Kunze and Dixon, 1986; Gee and Bauder,1986), and for purifying
clay samples (Moore and Reynolds, 1989), there is still no general recognition
of the need to purify reference (or commercial) clays prior to use (Haderlein and
Schwarzenbach, 1993; Haderlein et al., 1996). It is probably most common in
the fields of clay and soil science to purify clay mineral samples by fractionation
based on particle size, although this is not performed consistently or by any
uniform method. Clay—sized particles are generally defined as those <2 pm in
effective spherical diameter. These are usually obtained by gravity
sedimentation or by low-speed centrifugation to remove the >2 um particles.
The clay-sized particles remain suspended and are physically isolated by
separating the non-clay-sized sediment from the aqueous suspension
containing clay-sized particles. Frequently, however, no attempt is made to
purify or fractionate clay minerals obtained from commercial sources,
encouraged perhaps by monikers such as “reference” or “specimen” clays and,

perhaps, by the lack of a standard purification procedure.

24

There are a number of minor impurities in reference SWy-2 that could
reasonably lead to the hydrolysis of carbaryl, including mineral forms of Ti, Mn,
and Fe (Mermut and Cano, 2001; Vogt et al., 2002). However, in searching for
the causative hydrolytic agent, we noted a direct relationship between the pH of
the clay suspension and its hydrolytic activity. Only the clay-cent. had a pH of
<7 in aqueous suspension, and it was the only clay sample devoid of apparent
hydrolytic activity (i.e., no 1-naphthol formation). We hypothesized that
dissolution of some carbonate mineral in the aqueous clay suspensions may
raise the pH sufficiently to cause alkaline hydrolysis of carbaryl. Calcite and/or
dolomite were likely candidates because small peaks corresponding to these
minerals were resolved in the XRD pattern of the heavy fraction (Figure 1.2).
Direct measurement of inorganic carbonate showed it to be present in the
whole clay (0.12%), and heavy fraction (0.12%), but it was not detectable in the
light clay or fine clays (<0.02%). Previous authors have reported CaO contents
between 1.68 and 0.05% (Mermut and Cano, 2001; Vogt et al., 2002); the CaO
content in the bulk CMS sample of SWy—2 was higher than in the corresponding
clay-sized fraction (Chipera and Bish, 2001).

Adding carbaryl to an aqueous calcite suspension resulted in the rapid
and stoichiometric hydrolysis of carbaryl to 1-naphthol (Figure 1.3f).
Furthermore, the hydrolysis of carbaryl by whole clay suspensions was
eliminated when SWy-2 was subjected to carbonate dissolution by sodium
acetate buffer (Kunze and Dixon, 1986) prior to use (Figure 1.39). These

results seem to confirm our suspicion that inorganic carbonate impurities

25

present in SWy-2 were responsible for causing the alkaline pH, leading to
alkaline hydrolysis of carbaryl to 1—naphthol.

In slurries where the hydrolytic activity was high (e.g., whole clay),
recovery of sorbed substrate by extraction with methanol was low (Figure 1.4
and Tables 1.1 and 1.2), indicating some irreversible process. In contrast,
recoveries of sorbed substrate from slurries with low hydrolytic activity (e.g.,
light fraction or clay-cent.) were much higher and consisted almost entirely of
carbaryl. These results suggest some reaction of 1-naphthol after its adsorption
to smectite clay. This is consistent with our observation of a color change (to
dark gray) of the clay in the most hydrolytically active clay slurries.

This study provides a case-in-point that small amounts of impurities
present in reference clay mineral samples may substantially alter their apparent
sorptive properties or reactivities. These impurities may not be completely
removed using common purification procedures based on particle size
separation (Pusino, et al., 2000; Sheng et al., 2001; Wei et al., 2001). In this
study, even miniscule quantities of inorganic carbonates, carried through the
sedimentation procedure, were sufficient to cause substantial alkaline
hydrolysis of carbaryl. Such reactivity, present in the reference clay, as well in
the purified or fractionated reference clays, may mistakenly be attributed to the
clay mineral itself, rather than to the actual causative agent. It is advised that
reference or specimen clays should not be used in surface chemistry studies
without such prior treatment. We suggest adoption of some standard
fractionation/purification procedure for commercially available reference clays

prior to their use in studies on sorption and reactivity. The essential steps of

26

such a procedure would include (1) Hydration of the clay, (2) separation of the
<2 um clay-sized particles by low-speed centrifugation or gravity sedimentation,
(3) dissolution of carbonates and soluble salts with sodium acetate buffer, (4)
washing with water to remove residual salts and decrease ionic strength, and

(5) metal homoionic saturation.

27

b) whole
clay

plus EG

c) light
fraction

 

d) light
fraction
plus EG

Intensity

9) fine
clay-sad.

 

11.5A

f) flne
- clay-co nt.

45A
jaoA

 

 

g)heavy
fraction

 

m k
k I m q r I
4 8 12 16 20 24 28 32 36 40 44 48 52 56 60
Angle (Degrees 2 Theta)

 

Figure 1. 2. XRD patterns of K-SWy-2: (a) whole clay, oriented air-dried and (b)
after ethylene glycol salvation; (0) light fraction, oriented air-dried and (d) after
ethylene glycol salvation; (e) clay-sed., oriented air-dried; (f) clay-cent, oriented
air-dried; (9) heavy fraction, oriented air-dried. Lower case letters show the
probable mineral present in the heavy fraction, k = chlorite (?), r = rutile, q =
quartz, h = iron oxide, c = calcite, d = dolomite, m = mica and/or illite, f =
orthoclase feldspar, and p = plagioclase feldspar.

28

 

100 .

a) whole clay e) clay cent.

 

 

100 .
80 .
60 .

20*

0 l
100

 

 

 

c) light fraction

 

 

 

Carbaryl and naphthol in solution (% of moles added)

 

8O .
60 J ' 9) whole clay
4O ‘ wlo carbonates
20 1 L
0 . .W + fl
100 l a . - . . U , a ,
so 4 0') C'av sed- o 20 40 so so 100
60 Time (h)
40 .
+ carbaryl
2° ‘ ' + naphthol
o . I + sum

 

 

0 20 40 60 80 100
Time (h)
Figure 1.3. Hydrolysis of carbaryl by smectite clay K-SWy-2: (a) whole clay; (b)

light fraction; (c) heavy fraction; (d) clay-cent; (e) clay-sed.; (f) whole clay (now
without carbonates); (g) hydrolysis of carbaryl by calcite.

29

 

a) whole clay b) light fraction

+cabay

 

and 1-naphthol

(mole % of that sorbed y the clay

 

 

  

Methanol extractable carba

 

 

 

 

 

 

,i-fltp,b,l,ls
1ND 20 40

60 80 100
Time (h)

Figure 1.4. Amounts of methanol extractable carbaryl and 1-naphthol from K-
SWy—2: (a) whole clay; (b) light fraction; (0) heavy fraction; (d) clay-cent.

3O

Table 1.1. Time-Dependant Distribution of 14C Activity in Whole K-Smectite
(SWy-2) Slurries Amended with [14C]Carbaryl

 

1‘c activity“

time in solution sorbed by K-SWy-Z sorbed but sorbed but sorbed and total
water extractable MeOH extractable nonextractable recovery

 

 

 

 

(h) (%) so (°/.) so (%) so (°/.) so (‘70) so cm
2 80 1 25.9 0.1 4.0 0.3 4.6 0.2 17.3 02 106
8 59 1 43 2 2.0 0.1 11.3 0.4 30 2 101
12 53 4 49 2 2.1 0.3 13.4 0.3 34 2 102
16 49 10 53 3 3.0 0.4 12 4 38 2 102
24 41 2 62 2 3.1 0.3 14.0 02 4s 2 103
49 16 1 95.7 0.1 6.6 0.2 18.0 0.4 61.1 0.3 102

 

aExpressed as a percentage of the initial added 14C activity.

31

Table 1.2. Time-Dependent Distribution of 14C Activity in Light Fraction K-
Smectite (SWy—2) Slurries Amended with [14C]Carbaryl

 

1"C carbaryla

time in solution sorbed by K-SWy-2 sorbed but sorbed but sorbed and total
water extractable MeOH extractable nonextractable recovery

 

 

 

(h) °/. so °/. so % so °/. so °/. so °/.
2 75 3 19 2 2.0 0.4 8.1 0.1 9 1 94
8 73 2 24 3 1.8 0.3 12.2 0.3 10 2 97
12 70 1 29 3 1.4 02 18 1 10 3 99
24 63 2 35 2 2.5 0.4 19 1 13 3 96
48 52 1 46 2 3.2 0.5 26 2 15 1 99
72 47 2 54 4 3.7 02 35.4 0.4 15 4 101

 

aExpressed as a percentage of the initial added 14C activity.

32

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37

CHAPTER 2

A SIMPLE METHOD FOR PARTIAL
PURIFICATION OF REFERENCE CLAYS

ABSTRACT

The influence of clay preparation procedure on sorption and hydrolysis of
carbaryl (1-naphthyl, N—methyl carbamate) by the reference smectite clay SWy-2
was examined. For research purposes, reference clays are sometimes used
without purification, or more commonly, the <2 um size fraction is obtained by
gravity sedimentation or low-speed centrifugation. We determined that these
common methods were unreliable in removing inorganic carbonate impurities
present in SWy-2, and that these impurities caused alkaline conditions in
aqueous clay suspensions leading to the alkaline hydrolysis of carbaryl to 1-
naphthol. The hydrolytic activity of K-SWy-2 disappeared once carbonates were
eliminated. Two methods were evaluated for preparing K-SWy—2 devoid of
inorganic carbonates. In Method A inorganic carbonates were first removed by
incremental additions of a 0.5 M sodium acetate buffer (pH 5.0) until the clay
suspension reached pH 6.8, followed by low-speed centrifugation to obtain the
<2 um size fraction; in Method B the order of these steps was reversed. Carbaryl
hydrolysis was used as a probe to determine the effectiveness of the two clay
purification methods. Homoionic K-SWy-2 obtained by Methods A and B
produced near neutral pH when suspended in water and hydrolysis of carbaryl in

these suspensions was not evident. In this regard, both clay preparation

38

methods were acceptable. However, there were procedural advantages with
Method B, which is therefore recommended for the partial purification of

reference clays, as detailed in this paper.

39

INTRODUCTION

Our previous study showed that carbaryl (1-naphthyl, N-methyl
carbamate) hydrolysis in slurries of the reference clay SWy-2 was due to the
presence of inorganic carbonates impurities (e.g., calcite and dolomite) (Arroyo
et al., 2004). Dissolution of these carbonates created alkaline conditions leading
to the alkaline hydrolysis of carbaryl to 1-naphthol. Other organophosphorus
and carbamate pesticides are also unstable under alkaline conditions (Fukuto,
1987). Carbonates are often present in reference clays as sparingly soluble clay-
sized minerals (Grim and Guven, 1978; Mermut and Cano, 2001; van Olphen
and Fripiat, 1979; Vogt et al., 2002). Although such carbonates are not reliably
removed by common purification procedures based on particle size separation
(Arroyo et al., 2004), they are subject to selective dissolution (Kunze and Dixon,
1986; Gee and Bauder, 1986; Moore and Reynolds, 1989). Treatment of clays
with strong acids (e.g., HCI) to remove carbonates can cause destruction of the
crystalline lattice of clay minerals (Gee and Bauder, 1986; Loeppert and Suarez,
1996). Therefore, weak acid treatment with 0.5 M sodium acetate buffer at pH 5
is recommended for removal of carbonates from clay mineral samples (Kunze
and Dixon, 1986).

Although several methods have been published for separation and
purification of clays from soils and sediments (Jackson, 1979; Kunze and Dixon,
1986; Gee and Bauder, 1986; Moore and Reynolds, 1989), and for purifying clay

samples (Moore and Reynolds, 1989), it appears that there is no general

40

recognition of the need to purify commercially available clay minerals prior to
use in surface chemistry studies, and certainly no uniform experimentally
validated method in use for doing so. Many previous studies of the complexation
polymerization, sorption and degradation of organic contaminants and pesticides
by clays have reported some type of clay purification method based on wet
sedimentation (Haderlein and Schwarzenbach, 1993; Aguer et al., 2000; Pusino
et al., 1988,1995, 2000, 2003; Mortland and Halloran, 1976; Sawhney et al.,
1984, Some et al., 1986; Sheng, et al., 2001; Mingelgrim et al., 1977; Fusi et al.,
1983; Wei et al., 2001), whereas other studies used clay samples without prior
purification (e.g.,Forteza et al., 1989; Haderlein et al., 1993, 1996; Li et al.,
2003). Experimental details regarding purification such as dispersal methods,
sedimentation times, centrifugation forces and times, and type of membranes
used for dialysis, etc, are often different or unspecified. This situation may have
been exacerbated by the commercial availability of “standard”, “reference” or
“specimen” clays, and the inference by some that such samples are free of
impurities. Certainly failure to purify clay minerals and/or utilization of nonuniform
purification procedures diminish our ability to compare and reproduce results
from different studies and laboratories even when standard clay mineral samples
are used. Consequently, there is a need to operationally standardize purification

procedures for reference clays.

41

OBJECTIVE

The objective of this study is to develop a convenient and reliable
procedure for the partial purification of reference clays, avoiding any harsh
chemical treatments that may cause alteration or destruction of the clay
structures. All relevant experimental details and observations regarding such a
method are described herein. The recommended method should advance the

use of standard materials in surface chemistry studies utilizing clay minerals.

42

MATERIAL AND METHODS

The reference smectite clay (SWy—2) used in this study was obtained from
the Source Clays Repository of the Clay Minerals Society (Columbia, MO). Milli-
Q (deionized, d.i.) water (Millipore Corp., Bedford, MA), was used for all sample

preparations.

Method A. Removal of carbonates prior to particle size fractionation
Sample dispersion

The reference clay, as received, was suspended in d.i. water for 24 hours
while stirring using a magnetic stir-bar and plate to hydrate the clay. The
suspension was obtained by mixing 5.0 g of clay with 100 mL of d.i. water in a 1

L beaker.

Chemical Removal of Carbonates.

After suspending the clay, the pH was measured (>7) and then the clay
suspension was titrated with 0.5 M sodium acetate buffer, pH 5.0 (Kunze and
Dixon, 1986), until the pH of the clay suspension reached 6.8. During the
titration, water was added to decrease the viscosity of the clay suspension,
which increased over time with the incremental addition of the sodium acetate
buffer. The total amount of water added to the clay was 800 mL (700 mL d.i.
water added + 100 mL d.i. water added initially). After a pH of 6.8 was reached

initially, the sample was stirred for one hour, and the pH increased to 8. Titration

43

with 0.5 M sodium acetate was then continued until pH 6.8 was again reached.
This step was repeated three times until pH remained constant at 6.8 for 30

minutes. The total volume of 0.5 M Na-acetate buffer added was 39.5 mL.

Removal of Soluble Salts.

The neutral clay suspension was poured into four 250 mL polyethylene
centrifuge bottles, closed with screw-on caps, and then centrifuged for 30
minutes at 32959 (4500 rpm) using a Sorvall GSA rotor and centrifuge (DuPont
Co., Wilmington, DE). The supernatant containing soluble salts was discarded.
The clay pellets from the four bottles were combined into two bottles, and then
washed by resuspending the clay in 200 mL of d.i. water. The two bottles were
laid on a rotary lnnova 2300 Platform Shaker (New Brunswick Scientific 00.,
Edison, NJ) and shaken at 120 rpm for 8 h or until well resuspended. The two

bottles were then centrifuged as above and the clear supernatant discarded.

Particle size fractionation.

The clay, now presumably free of carbonates and other soluble salts, was
resuspended in 200 mL d.i water by laying the bottles on a rotary shaker and
shaken at 120 rpm for 8 hours. The <2 um clay-sized particles were obtained by
separating the >2 um particles using low speed centrifugation (58-609 (~600
rpm) for 6 minutes (Costanzo, 2001). The supernatants, containing clay
suspensions of <2 um particles, were siphoned into separate 250 mL centrifuge

bottles. The >2 um particles (sediment) were collected, quick-frozen, freeze-

44

dried and stored. The clay suspension was subject again to high speed
centrifugation (32959 for 30 minutes) and the clear supernatant discarded. Clay-
size particles were combined in one 250 mL centrifuge bottle and then subjected

to K-saturation (see below).

Method B. Particle size fractionation and partial physical removal of

carbonates

Sample dispersion

The reference clay, as received, was suspended in d.i. water for 24 hours
while stirring using a magnetic stir-bar and plate to hydrate the clay. The
suspension was obtained by mixing 5.0 g of clay with 100 mL of d.i. water in a

250 mL polyethylene centrifuge bottle.

Particle size fractionation

To obtain the <2 um clay-sized particles, the clay suspensiOn was
centrifuged for 6 minutes at 58—609 (~ 600 rpm) using a Sorvall GSA rotor and
centrifuge (Costanzo, 2001). The supernatant suspension containing the <2 um
clay-sized particles was then siphoned and collected into a 250 mL polyethylene
centrifuge bottle. The pellet containing the >2 um particles was re-suspended
with 100 mL of d.i. water by mixing briefly using a Vortex mixer, and then
subjected to low speed centrifugation as before to recover additional clay-sized

particles. The supernatant suspension was siphoned and added to the

45

previously collected suspension of clay sized materials. The >2 um particles
(sediment) were collected, quick-frozen, freeze-dried and stored for scanning

electron microscopy (SEM) analysis.

Chemical Removal of Carbonates.

The clay suspension from above was then titrated with 0.5 M sodium
acetate buffer at pH 5.0 (Kunze and Dixon, 1986) to pH of 6.8. After the desired
pH was reached, the sample was stirred for one hour during which the pH
remained stable. The total volume of sodium-acetate buffer added was 6.5 mL.
Compared to Method A this process yielded a stable pH more quickly and did
not require numerous increments of water to reduce viscosity of the suspension.
The volume of acetate buffer used to remove carbonates was considerably less

in Method B than in Method A.

Removal of Soluble Salts

To promote flocculation of the clay, ~1.5 g NaCI was added to the single
bottle containing the clay suspension and stirred for approximately 5 minutes
until the salt was dissolved, then the suspension was centrifuged for 20 minutes
at 32959 (4500 rpm). The supernatant containing soluble salts was discarded.
The clay pellet was resuspended in 200 mL of d.i water. The centrifuge bottle
was laid on a rotary shaker and shaken at 120 rpm for 8 hours to wash out the

excess NaCl. The clay suspension was subject again to centrifugation at 32959

46

(4500 rpm) for 30 minutes and the supernatant containing soluble salts was

discarded. The clay-size particles were then subjected to K-saturation.

Preparation of homoionic clays for Methods A and B

The clay pellets contained in the 250 mL centrifuge bottles from Methods
A and B were each resuspended in 200 mL of 0.1 M KCI; the bottles were laid
on a rotary shaker and shaken at 120 rpm for 8 hours, then the clay suspension
was centrifuged for 20 minutes at 32959 (4500 rpm) and the supernatant
containing excess KCI discarded. This step was repeated four times. Then, the
clay was resuspended in 200 mL of d.i. water and shaken at 120 rpm on the
rotary shaker for 8 hours, then centrifuged at 32959 (4500 rpm) for 30 minutes.
The supernatant containing excess KCL was discarded. A 50 mL portion of d.i.
water was added to each clay pellet to resuspend the clay, and the whole was
placed in porous membrane tubing (two 30 cm x 50 mm sections of molecular
weight cut-off 6000—8000, obtained from VWR Scientific-Spectrum Medical

Industries, Inc., Laguna Hills, CA) and dialyzed against distilled water until free
of chloride as indicated by AgN03, Both samples were quick-frozen, freeze-dried

and stored in polyethylene bottles.

A description of materials and methods used to prepare other clay
samples referred to in this paper, i.e., whole clay (no purification at all is used),
clay-sed. (obtained by gravity sedimentation without carbonate dissolution), clay-
cent. (obtained by low speed centrifugation without carbonate dissolution), and

light fraction (a sub-fraction of SWy-2, comprised of the lightest colored material,

47

obtained manually). as well as characterization of mineral phases by x-ray
diffraction (XRD) were previously reported (Arroyo et al., 2004). Each of those

samples was K-saturated by the same procedure reported above.

Scanning electron microscopy (SEM)

The K-SWy—2 samples were studied by scanning electron microscopy
(SEM); the SEM analysis included qualitative analytical x-ray energy-dispersive
spectra (EDS) and imaging of gold/carbon-coated clay films using a JSM-6400V
Scanning Microscope equipped with a Noran Detector unit. The EDS spectra
were obtained using electron-beam spot sizes of ~2—10 um, and a lifetime of

120 seconds. Analyses were standardless.

Chemicals

Carbaryl (>99% purity) was obtained from ChemService (W. Chesnut,
PA) and 1-naphthol (>99% purity) was obtained from Sigma-Aldrich (St. Louis,
MO). Carbaryl and 1-naphthol solutions were prepared in 0.1 M KCI at pH 3.0
(adjusted with HCI) and used as standards for analysis by high pressure liquid
chromatography (HPLC) (see below). Sodium acetate trihydrate was A.C.S.
reagent and obtained from J.T. Baker (Phillisburg, NJ) and glacial acetic acid

was obtained from Mallinckrodt-Baker, Inc. (Paris, KY).

Sorption kinetics

Kinetics of carbaryl adsorption were measured in slurries of K-SWy-2 clays

48

using a batch method. Solutions of 40 ug/mL of carbaryl in 0.1 M KCI at ambient
pH 6.5, protected from light, were prepared and used immediately for the
sorption/degradation experiments. Five mL volumes of a 40 ug/mL carbaryl
solution (in 0.1 M KCI, pH 6.5) were pipetted into 7.4 mL borosilicate amber
glass vials (Supelco, Bellefonte, PA,) containing 60 mg of clay. Contents of the
vials, in replicates of three, were mixed briefly using a vortex mixer, then
continuously rotated for various time intervals at room temperature (23 d: 1 °C),
then centrifuged at ~35009 for 20 minutes to separate solid and liquid phases.
Centrifugation occurred after 2, 4, 24, 48, 72, and 96 hours reaction times.
Aqueous concentrations of carbaryl and 1-naphthol were determined by HPLC
using a Perkin Elmer (Shelton, CT) instrument consisting of a binary LC 250
pump, a UV-vis detector set at 230 nm, and a series 200 autosampler/injector.

Chromatographic data was acquired using Turbochrom software 6.1. Reverse-
phase chromatography was carried out using a 150 x 4.6 mm Supelcosil-C13

column, of 5pm particle size and 120A pore size (Supelco). The mobile phase
was an isocratic mixture of 71% methanol and 29% water at pH 3.2 (adjusted
with HCI). The injection volume was 25 (IL and the flow rate was 1.0 mL/min.
Identification and quantification of carbaryl and 1-naphthol were achieved using
analytic standards of known concentrations.

Methanol extractions of the centrifuged clay pellets were
conducted to determine (by HPLC) the amounts of adsorbed carbaryl. After
determining carbaryl concentrations in aqueous phase from kinetics

experiments, amounts of aqueous solutions from weighted vials were taken out

49

with a Pasteur pipette. Vials containing clay pellets and residual water were
weighted. Five mL of methanol minus the volume of residual water remaining in
clays were added to vials containing clays and residual water. Vials were well
mixed using a Vortex mixer and then rotated continuously at room temperature.
Vials were centrifuged at 24 hours to separate clay from methanol phase.
Methanol carbaryl concentrations were determined by HPLC as above. Amounts
of carbaryl extracted from clay were obtained by subtracting the mass of
carbaryl present in residual water (determined gravimetrically) associated with

the clay after centrifugation from the mass of carbaryl extracted by methanol.

50

RESULTS

Two methods were used to isolate carbonate-free clay-sized particles
from the reference smectite SWy—2. In Method A, carbonates were removed
prior to separating the <2 um particles and in Method B after separating the <2
um particles (Method B), by low-speed centrifugation. Clay samples prepared
by these methods were compared to the reference clay (SWy—2) as received
(whole clay) and to clays where the <2 pm particles were obtained, without
removal of carbonates, utilizing either gravity sedimentation (clay-sed.) or low
speed centrifugation (clay-cent); details in preparation of the latter three clays

can be found in Arroyo et al. (2004).

Method A

In Method A, clay-sized particles (<2 um) were obtained from SWy-2 after
removal of carbonates. To briefly re-cap, SWy—2, as received from the CMS
Repository, was mixed with d.i. water (1:20) to form a suspension. Carbonates
were removed by slow addition of sodium acetate solution (pH 5) to lower the
clay suspension pH to 6.8. During this operation the viscosity of the clay
suspension increased making it difficult to stir with a magnetic bar. Since the
clay suspension became too thick for effective mixing, addition of water was
necessary to allow effective stirring, causing substantial increase in total volume
of the suspension. By the end of the titration (pH 6.8) the volume of clay

suspension was four times greater than that in Method B. High speed

51

centrifugation was used to separate the clay from the aqueous phase containing
soluble salts. The clay-sized particles were then obtained by washing the clay to
remove soluble salts, resuspending the clay in d.i. water, and low-speed
centrifugation. The yield of clay-sized particles was ~74% (w/w) of the original
SWy-2. Visually, it consisted mostly of yellow colored material plus a small
amount of brown colored material. The dark-brown sediment (obtained during
low speed centrifugation) comprised about 10% of the original weight of the
SWy-2 clay, free of carbonates. In Method A, the total mass removed during
isolation of carbonate-free clay-sized particles (obtained by subtracting the mass
of the neutralized-clay and neutralized-sediment from the initial amount of clay)

was ~16% (w/w) of the whole clay.

Method B

In Method B, the reference clay was initially suspended in d.i. water as
above, and then clay-sized particles (<2 um) were obtained by low speed
centrifugation prior to removal of carbonates by titration with Na-acetate buffer.
The volume of acetate buffer needed to remove inorganic carbonates from the
<2 urn fraction (i.e., obtain stable pH = 6.8) was only one sixth that used in
Method A where the whole clay was titrated. After titration with Na-acetate
buffer, the <2 um fraction was isolated by high speed centrifugation, but washing
the clay pellet with deionized water caused dispersion of some clay particles.
Incomplete flocculation of the clay was apparent from the presence of turbidity

and the yellow color of the supernatant. The method was therefore modified by

52

increasing the ionic strength of the original <2 um clay suspension (acetate
buffer titrated) to 0.1 M by adding NaCI prior to high-speed centrifugation. The
previously suspended fine particles appeared to flocculate and be effectively
removed from suspension by centrifugation as indicated by loss of turbidity and
color in the supernatant. The material obtained in this fashion, after K“
saturation, corresponded to 77% (w/w) of the original SWy—2 and consisted
mostly of yellow colored material plus a very small amount of brown material.
The dark-brown sediment (obtained from the original low-speed centrifugation)
comprised about 18% of the total weight of clay, and is referred to as the >2 pm
fraction, and contains carbonates and other impurities. In Method B, the mass
removed by titration of the fractionated clay (obtained by difference from the
initial amount of clay minus the sum of neutralized-clay and sediment)
comprised about 5% (w/w) of the whole clay.

The time required to achieve a stable pH of 6.8 while titrating with the Na-
acetate buffer was ~3 times greater in Method A than in Method B. Additionally,
~6-fold greater volume of sodium acetate buffer was used to remove carbonates
in Method A as compared to Method B. These both represent procedural
disadvantages to Method A. The yields of clay-sized particles recovered from

Methods A and B were similar.
Carbaryl sorption

Batch equilibrium experiments were conducted to measure the

distribution of carbaryl between K-SWy—2 and water and to evaluate hydrolysis

53

of carbaryl to 1-naphthol. Hydrolysis of carbaryl was used as an ultra-sensitive
probe to detect the presence of carbonates impurities in the various clay
preparations. In our previous study (Arroyo, et al., 2004) we observed a color
change in SWy-2 from pale-yellow to gray when carbaryl was added to
suspensions of the unfractionated (whole) clay or to clays fractionated based on
conventional <2 um particle size separations, but where carbonates were not
removed using acetate buffer. In this study, the carbonate-free clays obtained by
Methods A and B did not manifest a change of color when mixed with aqueous
carbaryl solutions.

Two kinetic experiments of carbaryl sorption and hydrolysis were
performed utilizing the carbonate-free clay slurries prepared by Methods A and
B, and the results compared to our previous data (Arroyo, et al., 2004). The
previous study utilized whole unfractionated clay, whole clay subjected to gravity
sedimentation overnight (clay-sed.), and whole clay subjected to low-speed
centrifugation (clay-cent); the latter two methods are commonly used to obtain
the <2 um particle size fraction (Figure 2.1). Sorption in the carbonate-free clay
slurries appeared to reach equilibrium within 4 hours, ~30% of carbaryl was
removed from aqueous solution in these slurries during the 96 hours
equilibration. This corresponds to an amount sorbed of ca. 1.0 mg/g clay. No
appreciable production of 1-naphthol was noted in these slurries. This is in
contrast to the fate of carbaryl in suspensions of the whole clay and clay-sed.
(Arroyo et al., 2004), where disappearance of carbaryl from aqueous solution

was continuous; 80% and 40% of carbaryl had disappeared within 48 hours,

54

respectively. In slurries of the whole clay, 1-naphthol concentration maximized
at about 24 hours, then declined to nearly zero between 24 and 96 hours,
whereas with clay-sed., 1-naphthol concentrations were nearly constant
between 24 and 96 hours (Figures 2.1.a,b). There was negligible formation of
1-naphthol in slurries of the clay-cent, similar to the carbonate-free Methods A
and B clay slurries (Figures 2.1 .c—e).

Recovery of sorbed carbaryl by methanol extraction of the carbonate-free
clays was ~100% (Figures 2.2.a,b). This is in stark contrast to the very small
quantities of carbaryl and 1-naphthol that could be extracted with methanol from

whole clay (Arroyo, et al., 2004).

Scanning electron microscopy (SEM)

The >2 um particles (sediment without carbonates removed and named
“heavy fraction” in our previous study, Arroyo et al., 2004) from Method B were
examined to determine particle morphologies and semi-quantitative elemental
compositions. Figure 2.3 shows x-ray maps of the >2 um fraction for 0, Fe, Na,
Mg, AI, Si, K, Ca, and Ti. Note the presence of a Ca-rich but Si-poor particle in
the lower left that is probably a carbonate mineral. Figure 2.4 shows a
photomicrograph of the light fraction (Arroyo et al., 2004), a sample much like
clay-cent. but containing only the pale yellow-colored material. As described in
Arroyo et al. (2004), this version of purified smectite yields a suspension pH of
8.2 and causes carbaryl to hydrolyze to 1-naphthol. The SEM image (Figure 2.4)

looks like a pure smectite due to its closed fine texture and honeycombed

55

arrangement of particles, presenting no visual evidence of solid phase
carbonates. Also EDS spectra for whole clay show the presence of Ca and Ti
(Figure 2.5.a), but neither were detected in EDS spectra of the light fraction
(Figure 2.5.b). Thus, solid-phase carbonates can be present and chemically
active, despite not being apparent in SEM images (Figure 2.4), and undetected

by x-ray diffraction (Arroyo et al., 2004) or by EDS analysis (Figure 2.5.b).

56

DISCUSSION

Since 1972 the Clay Minerals Society (CMS) has provided a
common set of well characterized reference clays for research purposes through
the Source Clays Project (Costanzo, 2001). The reference clays facilitate
comparisons of data obtained from different studies and laboratories. However,
different sample purification/preparation methods may undercut efforts to ensure
the use of common standard materials in studies utilizing clay minerals, and in
allowing comparison of results from different studies. It is probably most
common in the fields of clay and soil science to purify clay mineral samples by
fractionation based on particle size, although this is not performed by any
uniform method. In some instances, commercially available clays, including the
CMS reference clays, are used as received without purification. The variety of
clay preparation methods utilized in the past demonstrates the necessity of
developing a standard method for purifying reference clays prior to their use in
surface chemistry studies. Such a method would be useful to clay scientists and
those in related fields such as soil and environmental sciences that commonly
use commercially available clay mineral samples.

It has been demonstrated that smectite clays may potentially contribute to
the degradation of certain carbamate pesticides (Wei, et al., 2001) and other
pesticides (Pusino, et al., 2000). In our previous study (Arroyo et al., 2004) we
observed both the apparent sorption and hydrolysis of carbaryl in slurries of the

reference smectite K-SWy-2. The extent of carbaryl disappearance from solution

57

and appearance of its hydrolytic product 1-naphthol in solution varied depending
on the clay preparation and purification method. We reported that several
different methodological approaches used would not reliably removed carbonate
impurities, and that such carbonate impurities may influence or even control
sorption and degradation processes. We concluded that even trace quantities of
carbonates in K-SWy—2 could, through dissolution, cause alkaline pH
manifesting the alkaline hydrolysis of carbaryl. Such hydrolytic activity, present
even in partially purified clays (e.g. after overnight gravity sedimentation), could
be mistakenly attributed to the clay mineral itself or to some other reactive
mineral component. Other reference clays obtained from natural deposits may
contain even greater amounts of carbonate impurities (e.g., Hectorite SHCa-1),
where commonly used purification approaches based on sedimentation or low-
speed centrifugation would be even less proficient at removing carbonates. In
fact, when we collected the <2 um particle size of the hectorite SHCa-1 clay by
gravity sedimentation and low-speed centrifugation methods, both caused
alkaline pH after stirring the clay in water for a few minutes. It therefore seems
apparent that particulate carbonate impurities as well as carbonate cementing
materials should be removed by selective acid dissolution, along with other
soluble salts (e.g., gypsum and metal oxides present as coatings), which
potentially may be reactive or cause chemically reactive conditions.

Particle size separations are commonly recommended prior to
mineralogical analyses of mineral components present in soils and other

geosolids. Other methods are used to remove carbonates, organic matter and

58

. .

 

free iron oxides from soil samples (Jackson, 1979; Kunze and Dixon, 1986; Gee,
and Bauder, 1986; Moore and Reynolds, 1989). The need for use of such
treatments when dealing with soil samples is widely recognized. Specimen
clays or reference clays, like soils, are natural materials that may require
pretreatment to remove impurities. In a previous study (Arroyo et al., 2004) the
use of the carbamate pesticide carbaryl, which is subject to alkaline hydrolysis,
provided an excellent case-in-point that reference clays should be chemically
treated to remove carbonates, which might othenrvise dissolve and cause
alkaline conditions leading to unexpected chemical reactivity. To achieve this,
the method of Kunze and Dixon (1986) was modified to avoid subjecting the clay
to high temperatures or to large volumes of buffer at low pH, yet to ensure that

reference clays be free of carbonates and other soluble impurities. Dissolution of
carbonates in clay suspensions, and removal of 002 (g), was reliable achieved

by constant stirring in an open beaker accompanied by the slow addition of Na-
acetate acid buffer (pH 5). The clay suspensions titrated to stable pH of 6.8 in
this manner minimized the potential for silicate dissolution.

In developing and evaluating methods for preparing carbonate-free clay

sized materials, it was noted that a clay-water mixing period of 24 hours was
required to ensure the hydration and dispersion of clay particles. Although, Na+-
saturation provides a high degree of hydration and gives effective dispersion of
the clay, we did not add Na+ during the initial clay-water mixing period in order to

prevent flocculation of the clay during the acetate buffer titration (Moore and

59

Reynolds, 1989). The initial clay-water ratio of 1:20 suggested by (Costanzo,
2001) was appropriate.

The fractionation process to obtain the <2 um particles and the
purification of SWy-2 clay by Na-acetate buffer by Method B was simpler and
easier than by Method A. Using Method A to remove carbonate impurities from
the SWy-2 clay suspension by titration with Na-acetate buffer, prior to particle

size separation was laborious and required a considerable amount of time (one
working day). Carbonates dissolved by the Na+-acetate buffer caused an

increase in suspension viscosity. The addition of water to decrease the ionic
strength caused a large increase in volume of clay suspension and consequently
an increase in the time spent for centrifuging. It was also more difficult to obtain
a stable pH of 6.8 when titration of the clay with Na-acetate buffer was the initial
purification step, i.e., in Method A. In Method B, after the initial clay-water
mixing period, the clay-sized particles are separated by low-speed centrifugation
(~6009). During this step, some clay-sized particles were removed along with
the coarser particles. Therefore, the centrifuge pellet was redispersed in water
to resuspend and capture more clay-sized particles. Carbonate impurities
present in the resultant fractionated SWy—2 clay suspension were easy to
eliminate by titration with Na-acetate buffer, i.e., much less buffer was required
and pH in the titrated sample was more stable. When preparing larger amounts
of clays, the practical advantages of Method B (more stable pH and much less
volume enhancement) are particularly attractive. The washing process to

eliminate soluble salts and the homoionic cation saturation is similar in

60

complexity in both methods A and B. Likewise, similar yields of carbonate-free
clay sized materials are afforded by Methods A and B. Both methods achieved
removal of carbonates and eliminated the hydrolytic activity associated with the

smectite SWy-2. Method B is recommended due to its convenience.

61

Carbaryl and naphthol in solution (% of moles added)

 

   

 

         

 

 

 

 

100 i a) whole clay d) carbonate-free clay,
‘ MethOdA
80
60 -.
40 ‘
20 ‘
100 ‘ .
b) clay-sed. 9) camgztgfree clay,
80
60
40
20 ‘
0 I .. -__—|——.——|——|
100 ‘ c) clay-cent. 0 20 40 60 80 100
80 I Time (h)
60
40 ' + amend
20 + 1-naphthol
+ sum
0 W

 

0 20 40 60 80 100
Time (h)
Figure 2.1. Sorption and hydrolysis of carbaryl by clay K-SWy-2:

(a) whole clay; (b) clay-sed.; (c) clay-cent; (d) carbonate-
free clay, Method A; (e) carbonate-free clay, Method B.

62

 

150
Methanol extraction, Method A

125 —
ii i

100- ii 4 l i

75 ~ i

 

 

 

 

 

Methanol extractable carbaryl (mole % of that sorbed by the clay)

 

 

50 ~

150 a . . . .
Methanol extraction, Method B

125 1
100 . ii § § %

75 1 i

50 -

O 2 O 4 0 6 0 8 O 1 O 0
Time (h)

Figure 2.2. Amounts of methanol extractable carbaryl from carbonate—
free K-SWy-2: (a) Method A and (b) Method B.

63

 

Figure 2.3. X-ray map of the heavy fraction. Elements as 0, Fe, Na, Mg, Al, Si,
K, Ca, and Ti, were identified. The brightness of pixel in x-ray map is directly
proportional to the quantity of element present.

64

 

" (Arroyo et al.,

“light fraction

2

SEM photomicrographs of the SWy-

Figure 2.4

in a closed fine texture showing a honeycombed

which occurs
arrangement of particles.

2004)

65

 

 

 

 

 

 

 

 

 

 

 

 

 

800. (3 AIl-Sii K A)
c
o .
u Ca Ie
. l
s K ‘
Na Ca TI

 

 

 

 

 

 

 

 

 

 

800. ("3 Al§i K _ B)
t M
0
11 Fe
II
t
S
K
K Fe Fe
all . W .

 

keV

Figure 2.5. EDS spectra of K-SWy-2: (a) whole clay, with elemental
composition of O (50.1%), M9 (0.6%), Al (4.9%), Si (35.6%), K (2.7%), Fe
(3.6%), Na (0.7%), Ca (1.2%), and Ti (0.5%), and (b) “light fraction” with
elemental composition of O (51.5%), M9 (1.7%), Al (11.5%), Si (28.3%), K
(3.5%), Fe (3.4%). While qualitative, this analysis would result in a unit cell

formula 0f K0.34(SI3.88AI0.12)(AI1.51Fea+0.23M90.26)O10 H20-

66

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Aguer, J. P.; Hermosin, M. C.; Calderon, M. J.; Cornejo, J. Fenuron sorption on
homoionic natural and modified smectites. J. Environ. Sci. Health 2000,
B35 (3), 279—296.

Arroyo, L. J.; Hui, L.; Teppen, B. J.; Johnston, C. T.; Boyd, S. A. Hydrolysis of
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Chem, in press.

Boyd, S. A.; Sheng, G. Y.; Teppen, B. J.; Johnston, C. J. Mechanisms for the
adsorption of substituted nitrobenzenes by smectite clays. Environ. Sci.
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Costanzo, P.M. Baseline studies of The Clay Minerals Society Source Clays:
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Doner, H. E.; Lynn, W. C. Carbonate, Halide, Sulfate, and Sulfide Minerals. In
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Forteza, M.; Galan, E.; Cornejo, J. Interaction of dexametasone and
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Fukuto, T. R. Organophosphorus and carbamate esters: The anticholinesterase
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Gee, G. W.; Bauder, J. W. Particle size analysis. In Methods of Soil Analysis,
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Grim, R. E.; Guven, N. Bentonites—Geology, Mineralogy, Properties, and Uses;
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Haderlein, S. B. and Schwarzenbach, R. P. Adsorption of substituted

nitrobenzenes and nitrophenols to mineral surfaces. Environ. Sci. Tech.
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67

Haderlein, S. B.; Weissmahr, K. W.; Schwarzenbach, R. P. Specific adsorption
of nitroaromatic explosives and pesticides to clay minerals. Environ. Sci.
Tech.1996,30,612—622.

lsaacson, P. J.; Sawhney, B. L. Sorption and transformation of phenols on clay
surfaces: effect of exchangeable cations. Clay Min. 1983, 18, 253—265.

Jackson, M. L. Mineral fractionation for soils. In Soil Chemical Analysis-
Advanced Course. Second ed.; Jackson, M. L., Ed.; Madison, WI, 1979:
pp 100—166.

Kunze, G. W. and Dixon, J. B. Pretreatment for mineralogical analysis. In
Methods of Soil Analysis, Part 1. Physical and Mineralogical Methods.
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Society of America: Madison, WI, 1986: pp 91—100.

Larson, R. A.; Hufnal, M. Jr. Oxidative polymerization of dissolved phenols by
soluble and insoluble inorganic species. Limnol. Oceanography 1980, 25
(3), 505—512.

Li, 2.; Williams, C.; Kniola, K. Removal of anionic contaminants using
surfactant-modified palygorskite and sepiolite. Clays Clay Min. 2003, 51
(4),445—451.

Loeppert, R. H.; Suarez, D. L. Carbonate and Gypsum. In Methods of Soil
Analysis, Part 3. Chemical Methods. Second ed.; Sparks, D., Ed.;
American Society of Agronomy, Soil Science Society of America:
Madison, WI, 1996: pp 437—473.

Mermut, A. R.; Cano, A. F. Baseline studies of The Clay Minerals Society
Source Clays: Chemical analyses of major elements. Clays Clay Min.
2001, 49 (5), 381—386.

Mingelgrin, U.; Saltzman, S.; Yaron, B. A possible model for the surface-
induced hydrolysis of organophosphorus pesticides on Kaolinite clays.
Soil Sci. Soc. Am. J., 1977, 42, 519—523.

Moore, D. M.; Reynolds, R. C. Jr. Sample Preparation Techniques for Clay
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Minerals. Oxford University Press: Oxford, U.K.,1989: pp 179—201.

Mortland, M. M.; Halloran, L. J. Polymerization of Aromatic Molecules on
Smectite. Soil Sci. Soc. Am. J. 1976, 40, 367—370.

Pusino, A.; Gessa, C.; Kozlowski H. Catalytic hydrolysis of quinalphos on
homoionic clays. Pest. Sci. 1988, 24, 1-8.

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Pusino, A.; Gelsomino, A.; Gessa, C. Adsorption mechanisms of
imazamethabenz-methyl on homoionic montmorillonites. Clays Clay Min.
1995,43 (3), 346—352.

Pusino, A.; Braschi, l.; Gessa, C. Adsorption and degradation of triasulfuron on
homoionic montmorillonites. Clays Clay Min. 2000, 48 (1 ), 19—25.

Pusino, A.; Gelsomino, A.; Fiori, M.G.; Gessa, C. Adsorption of two
quinolinecarboxylic acid herbicides on homoionic montmorillonites. Clays
Clay Min. 2003, 51 (2), 143-149.

Sawhney, B. L.; Kozloski, R. K.; lsaacson, P. J.; Gent, M. P. N. Polymerization
of 2,6-dimethylphenol on smectite surfaces. Clays Clay Min. 1984, 32 (2),
108—114.

Sheng, G. Y.; Johnston, C. T.; Teppen, B. J.; Boyd, S. A. Potential contributions
of smectite clays and organic matter to pesticide retention in soils. J. Ag.
Food Chem. 2001, 49 (6), 2899-2907.

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molecules in the interlayer of montmorillonites studied by resonance
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Strathmann, T. J.; Stone, A. T. Reduction of the carbamate pesticides oxamyl
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2461—2469.

Strathmann, T. J.; Stone, A. T. Reduction of the pesticides oxamyl and
methomyl by Fe": effect of pH and inorganic ligands. Environ. Sci. Tech.
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Vogt, C.; Lauterjung, J.; Fischer, R. X. Investigation of the clay fraction (<2 pm)
of the Clay Minerals Society Reference Clays. Clays Clay Min. 2002, 50
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2232.

Weissmahr, K. W.; Haderlein, S. B.; Schwarzenbach, R. P. In situ spectroscopic

investigations of adsorption mechanisms of nitroaromatic compounds at
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69

Weissmahr, K. W.; Hildenbrand, M.; Schwarzenbach, R. P.; Haderlein, S. B.
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70

 

CHAPTER 3

REDUCTION OF STRUCTURAL FE+3 IN SMECTITE COUPLED TO
OXIDATION OF 1-NAPHTHOL

ABSTRACT

Sorption, oxidation and transformation of 1-naphthol by K-smectite (K-
SWy-2) were studied using batch sorption isotherms, Fourier-transform infrared
spectroscopy (FTIR), and X-ray diffraction (XRD). Three preparations of the
reference smectite clay (SWy-2) were evaluated: (a) whole clay (with naturally
occurring carbonate impurities), (b) the <2 um fraction of SWy-2, extracted to
remove carbonate impurities, and (c) the carbonate-free <2 um fraction of SWy-
2 amended with calcite. For the whole clay and carbonate-free clay amended
with calcite, >80% of added 1-naphthol disappeared from aqueous solution
within 24 h, corresponding to a sorbed concentration of 22 mg/9 of clay. In the
carbonate-free clay only 35% of the added 1-naphthol disappeared from solution
after 24 h. For all clay preparations studied only small amounts of sorbed 1-
naphthol (<1%) could be recovered by methanol extraction of the clay. XRD
patterns suggested that 1-naphthol was intercalated in the smectite, but were
not conclusive because 1-naphthol sorption of 1.5—2.8 mg/g of clay had
relatively minor effects on XRD patterns. FTIR spectra of sorbed 1-naphthol-
clay complexes demonstrated structural Fe3+ reduction. FTIR spectra also
showed evidence of the transformation of 1-naphthol. It appears that reduction

of structural iron may be coupled to oxidation of 1-naphthol. Further

71

transformation of oxidized 1-naphthol, eg. by coupling reactions, is indicated by

a darkening of color of the clay and the inability to extract sorbed 1-naphthol.

72

INTRODUCTION

Carbaryl (1-naphthyl N-methylcarbamate) is one of the most commonly
used carbamate insecticides (EPA, 1987; USDA, 1994—2001). In 1997,
estimated carbaryl usage in the United States was 50000 kilograms (Barcelo
and Hennion, 1997) which increased in to 1.4 million kilograms in 2001 (USDA,
1994—2001). The Mississippi River Basin Study (May to September, 1991)
conducted by Larson et al. (1996) found carbaryl in surface water at ~0.010 ug/L
(detection limit of 0.002 ug/L) at seven of eight sampling sites. Carbaryl is
subject to alkaline hydrolysis in aqueous media resulting in the formation of 1-
naphthol. Rajagopal et al. (1984) reported that more than 50% of the soil-
applied carbaryl accumulated as 1-naphthol. 1-Naphthol has been detected in
California groundwater at concentrations up to 610 ug/L (Barbash and Resek,
1996; Barcelo and Hennion, 1997). It was reported that 1-naphthol was more
toxic to soil bacteria, mollusks, invertebrate insects, and three species of marine
fish compared with carbaryl (Bollag and Liu, 1971; Mount and Oehme, 1981;
Day, 1990).

Recent studies demonstrated that under environmentally relevant
conditions clay minerals provide binding sites for certain aqueous phase
pesticides and organic contaminants including triazines, nitroaromatics, ureas
and carbamates (Laird et al., 1992; Hinedi, et al., 1993; Haderlein et al., 1996;
Boyd et al., 2001; Sheng et al., 2001; Li et al., 2003; Li et al., 2004a, Arroyo, et

al., 2004a). It has been demonstrated that phenol and phenolic compounds

73

react with clay minerals and form high molecular-weight products (Mortland and
Halloran, 1976, Larson and Hufnal, 1980, Stone and Morgan, 1984; Some et al.,
1986; Ukrainczyk and McBride, 1992). Final products of these redox reactions
depended on pH, exchangeable cations on the clay and availability of ortho and
para positions of the phenolic compounds (Ukrainczyk and McBride, 1992; del
Castillo et al., 1996; Mortland and Halloran, 1976; Larson and Hufnal, 1980;
Sawhney et al., 1984; Kung and McBride, 1988. Wang et al. (1978) and Wang
and Huang (1989) showed that humic and fulvic acid-like products were formed
by oxidative polymerization of naturally occurring phenolic compounds (e.g.,
gallic acid, protocatechuic acid, and pyrogallol) by clays.

Recent studies have implicated clay minerals in the degradation of
several pesticides including carbosulfan, carbofuran, aldicarb, pirimicarb,
chlorprofan and triasulfuron (Wei et al., 2001; Pusino et al., 2000). Arroyo et al.
(2004a) studied sorption and hydrolysis of carbaryl by reference smectite clay
SWy-2 and demonstrated that carbaryl was hydrolyzed to 1-naphthol in aqueous
slurries of K-SWy-2. Hydrolysis of carbaryl was attributed to alkaline conditions
caused by dissolution of carbonate impurities present in the reference smectite.
Darkening of the clay was also observed, suggesting some chemical
transformation of sorbed carbaryl or 1—naphthol. When 14C-Iabeled carbaryl was
added to slurries of SWy-2 most of' the carbaryl removed from solution was
unextractable from the clay by methanol. In contrast, when carbaryl was added

to slurries of SWy-2 which had been treated to remove carbonates, no

74

hydrolysis to 1-naphthol was observed and 100% of the clay-sorbed pesticide

was methanol extractable (Arroyo et al., 2004b).

75

OBJECTIVE

The objective of this study was to evaluate the reactions of 1-naphthol in
aqueous slurries of the reference smectite clay SWy-2. Sorption and
transformation of 1-naphthol by SWy-2 was studied using two different clay
preparations, i.e., unfractionated clay and the clay-sized (<2 um) fraction of
SWy-2 which was treated to remove carbonate impurities. Sorption and
transformations of 1-naphthol in slurries of homoionic K-SWy—2 was evaluated
using batch sorption isotherms, Fourier-transform infrared spectroscopy (FTIR)

and X-ray diffraction (XRD).

76

MATERIALS AND METHODS

The reference clay, SWy—2, used in the study was obtained from the
Source Clays Repository of the Clay Minerals Society (Columbia, MO). Milli-Q
deionized (d.i.) water (Millipore Corp.) was used for all sample preparations. Two
separate procedures were used to prepare homoionic K-SWy—2 (whole clay and
carbonate-free clay). First, the reference clay, as received, was suspended in
water, then K+ saturated using 0.1 M KCI solutions; this material is referred to as
whole clay. Second, SWy—2 was suspended in water, subjected to low-speed
centrifugation to isolate the <2 um particles, titrated to pH 6.8 with sodium
acetate buffer (pH 5.0) to remove carbonate impurities, and subjected to K"-
saturation (as above). This clay sample is referred to as carbonate-free clay.
After K*-saturation, clays were washed with d.i. water until free of chloride as
indicated by AgNOg. Samples were quick-frozen, freeze-dried and stored.
Additional details regarding clay preparation procedures can be found in Arroyo
etaL(2004b)

1-Naphthol (>99% purity), obtained from Sigma-Aldrich (St. Louis, MO),
was used to prepare standard solutions (0.1 M KCI at pH 3.0 adjusted with HCI,
protected from light) for subsequent HPLC analysis of experimental samples.
Solutions of 30 ug/mL of 1-naphthol (in 0.1 M KCI solution at pH 6.5, protected
from light) were prepared and used immediately for sorption/transformations

experiments.

77

A batch equilibration method was used to evaluate sorption of 1-naphthol
by K-SWy—2 clay. Five mL of the 30 ug/mL 1-naphthol solution (described
above) were pipetted into 7.4 mL borosilicate amber glass vials containing 60
mg of clay which were sealed with Teflon-lined caps. Vials, in replicates of
three, were mixed briefly using a vortex mixer, then mechanically rotated
continuously at room temperature (23 t 1 °C) for intervals of 2, 4, 8, 12, 16, 24,
48, 72, and 96 hours. Vials were centrifuged at ~35009 for 20 minutes to
separate solid and liquid phases and the latter analyzed for 1-naphthol by HPLC.
The centrifuged clay pellets were extracted with methanol, and the extracts
analyzed by HPLC for 1-naphthol. Amounts of 1-naphthol extracted from the
clay pellets were obtained by subtracting the mass of 1-naphthol present in
residual water from the mass of 1-naphthol extracted by methanol. Two kinetics
experiments were performed to evaluate the effect of alkalinity produced by the
dissolution of carbonate impurities present in SWy-2. First, 1-naphthol was
added to a calcite suspension, and second, to a carbonate-free clay suspension
amended with calcite. In these experiments 5.0 mL of the 30 ug/mL 1-naphthol
solution were pipetted into 7.4 mL borosilicate amber glass vials containing 1.3
mg of calcium carbonate (with and without clay). Vials, in replicates of three,
were mixed briefly using a vortex mixer and then mechanically rotated
continuously at room temperature (23 :l: 1 °C). The 1-naphthol concentration
was determined after equilibration times of 2, 4, 8, 12, 24, 48, 72, 96 hours.

Sampling and analyses of 1-naphthol were as described above.

78

The HPLC system consisted of a Perkin-Elmer 250 pump connected to a
UV—vis detector (230 nm) and a 150 x 4.6 mm Supelcosil-C18 column (5 pm
particle size and 120 A pore size, Supelco). Chromatographic data were
acquired using Turbochrom software 6.1. The mobile phase was an isocratic
mixture of 71% methanol and 29% water adjusted to pH 3.2 with HCI. The
injection volume was 25 uL and the flow rate was 1.0 mL/minute.

Basal spacings of K-SWy-2 clays with and without sorbed 1-naphthol
were determined by XRD analysis. Suspensions of K-SWy—2 were dropped onto
glass slides and allowed to air-dry at ambient conditions to obtain oriented clay
films. XRD patterns of oriented films were also analyzed after exposure for 48

hours to 1% relative humidity (RH) and after 48 h exposure to 100% RH at 25
°C. The XRD patterns were recorded using Cu-K-ot radiation and an XRD

system consisting of a Philips 3100 X-ray generator (Philips Electronic
Instrument, lnc., Mahwah, NJ), Philips 3425 wide range goniometer fitted with a
9-compensating slit, 3 0.2 mm receiving slit, a diffracted-beam graphite
monochromator, and PW1877 automated powder diffraction (Philips Electronics)

control software. Diffraction patterns were measured from 4 to 11° 29, in steps

of 002° 29, at 2 s/step.

FTIR experiments were performed to evaluate the potential
transformations of 1-naphthol sorbed by K-SWy—2. Batch equilibrations of 1-
naphthol in aqueous suspension of the whole clay and carbonate-free clay were
set up as described above. Experiments were stopped after 48 and 144 hours,

and the whole clay- and carbonate-free clay-1-naphthol suspensions were used

79

to prepare self-supporting films for FTIR analysis. Films were prepared in
triplicate by passing a volume of clay-1-naphthol suspension containing 20 mg of
clay through a 0.45 pm Supor—450 hydrophilic polyethersulfone membrane (47
mm diameter) on a Millipore filtration system. The resulting clay deposit on the
filter was protected from light, allowed to air-dry overnight, and then removed
from the filter by using a metal spatula (Johnston et al., 2002). Infrared spectra
were obtained on a Perkin-Elmer GX2000 FTIR spectrometer equipped with
deuterated triglycine (DTGS) and mercury-cadmium-telluride (MCT) detectors,
an internal wire grid IR polarizer, a KBr beam splitter, and a sample
compartment purged with dry air. The unapodized resolution for the FTIR
spectra was 2.0 cm", and a total of 64 scans were collected for each spectrum.
GRAMS/32 (Galactic Software) program was used to analyze and plot spectra.

Post processing of the FTIR spectra was restricted to baseline correction.

80

RESULTS

Transformation of 1-naphthol in aqueous suspension containing the
whole clay was indicated by a color change of the clay from off-white to gray,
similar to the observations reported in our previous study with carbaryl (Arroyo et
al., 2004a) and by Wang et al. (1978). 1-Naphthol added to the carbonate-free
clay slurry amended with calcite also developed a gray color, which intensified
over time. In contrast, color change was not observed when 1-naphthol was
added to the carbonate-free clay suspension or to the aqueous suspension
containing only calcite. Thus, it was apparent that that combination of smectite
and calcite was necessary to produce the 1-naphthol transformations indicated
by the color change of the clay.

Analysis by HPLC of the aqueous phase from slurries of whole K-SWy-2
and carbonate-free clay amended with calcite clearly demonstrated the
disappearance of 1-naphthol from solution (Figure 3.1 ). In whole clay slurries, ~
80% of 1-naphthol initially added had disappeared from solution after 24 hours,
corresponding to. ~20 mg/g of clay. ln slurries of carbonate-free clay amended
with calcite, ~95% of 1-naphthol had disappeared from solution after 24 hours,
corresponding to ~24 mg/g of clay. After 96 hours, 1-naphthol had completely
disappeared from solution in both clay slurries. Disappearance of 1-naphthol
from solution in carbonate-free clay slurries (not amended with calcite) occurred
to a much lesser extent. Approximately 35% of the initially added 1-naphthol

had disappeared from solution within 24 hours, and ~45% after 96 hours.

81

Disappearance of 1-naphthol in the calcite suspension was ~2% after 24 hours,
and ~20% at 48 hours. The disappearance of 1-naphthol was very similar in
slurries of whole SWy-2 that naturally contain carbonate impurities (e.g. calcite,
dolomite) and the SWy-2 that had been extracted to remove these naturally
occurring carbonates then amended with calcite. It appeared that
disappearance of 1-naphthol from solution was strongly favored by alkaline pH
(produced by the dissolution of carbonates) and the presence of SWy-2.

Concentrations of sorbed 1-naphthol that could be methanol extracted
from any K-SWy-2 samples were minimal (<1%). Arroyo et al. (2004a)
demonstrated that ~61% of 14C activity, added as 14C-ring labeled carbaryl to
whole SWy-2, was nonextractable by methanol. In that study, recovery of
sorbed carbaryl by methanol extraction of carbonate-free clays was ~100%.

The XRD patterns for air-dried films of whole clay, carbonate-free clay,
and these two clays after treatment with 1-naphthol for 120 h are shown in
Figure 3.2. In both cases the clay basal spacings increased (to ~11.2 A, Table
3.1) implying some intercalation of 1-naphthol and/or its transformation products
between smectite layers. Similar expansions of the interlayer distances were
noted for these same clay films that were equilibrated at 1% relative humidity
(RH) (Table 3.1). The shapes and widths of the XRD patterns for both untreated
clays (Figure 3.2) imply an interstratified mixture of clay layers with ~10 and 12.5
A dam-spacings, which are both quite reasonable for K-saturated smectites at
low humidity (MacEwan et al., 1980). When 1-naphthol was added to the

carbonate-free clay, there was a modest increase in the number of ~12.5 A

82

layers in this mix, shifting the overall XRD peak to a slightly larger dam-spacing
(Figure 3.2b). This is consistent with our previous observations of intercalated
aromatic compounds (Li et al., 2004b), in which the dam-spacings of air-dried K-
smectite clay films increase slowly toward ~12.5 A as the aromatic compound
(i.e. 1,3-dinitrobenzene) loading increases from 0—34 mg/g of clay. Here, the 1-
naphthol loading rate is only 1.4 mg/g of clay (Figure 3.1 b), so the XRD data are
consistent with our hypothesis of weak interlayer sorption of 1-naphthol in the
carbonate-free system.

Interactions between 1-naphthol and the whole clay seem to produce
transformation products, as described below, and the XRD data (Figure 3.2a)
also indicate differences in the interlayer distances induced by sorption of 1-
naphthol. The maximum intensity of the XRD pattern for the 1-naphthol-whole-
clay systems (air-dried, or air-dried then equilibrated at ambient condition or at
1% RH) still occurs near 11.2 A, indicating again that most dam-spacings are
either ~10 or 12.5 A (Figure 3.2a). However, the peak is much broader than that
for the carbonate-free clay (Figure 3.2b). implying that many ~15 A up to even
~18 A dew-spacings are present in the K-smectite film of the whole clay after
exposure to 1-naphthol. These larger dom-spacings may indicate that some
transformation products of 1-naphthol in the whole clay are nonplanar and serve
to prop the layers open. Alternatively, Ca2+ ions from dissolved carbonates
occupy some interlayer cation-exchange sites and cause those interlayers to

remain more swollen upon air-drying.

83

To investigate the effect of 1-naphthol sorption on interlayer expansion by
water, the air-dried clay films were exposed to 100% RH. Under these
conditions the clay control not treated with 1-naphthol expanded to 15.55 A, and
the clay treated with 1-naphthol to 15.39 A (Table 1). Note that other studies
(Sheng et al., 2002; Li et al., 2004b) have shown that intercalated aromatic
compounds inhibit K-smectite swelling at 100% humidity. In those studies,
organic solute (e.g. 1,3-dinitrobenzene) sorption >8.4 mg/g of clay was sufficient
to retain ~12.5 A dent-spacings even after exposure to 100% RH, with d001-
spacings steadily increasing toward 15.5 A as organic solute loading rates
decreased (Li et al., 2004b). Thus, swelling of our clay-1-naphthol complex at 1-
naphthol loading of 1.4 mg/g to a slightly lower dam-spacing (compared to the
K*-saturated clay alone) upon exposure to 100% RH is again consistent with our
hypothesis that 1-naphthol is intercalated in K-smectite.

FTIR absorbance spectra of 1-naphthol sorbed to the carbonate-
containing SWy—2 and to carbonate-free SWy—2 clay are compared in Figure 3.3
in the 4000 to 400 cm'1 region. An expanded region of these spectra in the 2200
to 1350 cm'1 region are shown (baseline corrected) in Figure 3.4. This spectral
region contains the most prominent bands from the sorbed 1-naphthol and its
transformation products. As shown in Figure 3.4, the relative intensity of the
FTIR bands from the sorbed organic species are low compared to the strong
clay bands. This is consistent with the fact that a relatively small amount of 1-
naphthol was added initially (<4 mg/g of clay). The spectra of the whole clay K-

SWy—2 (carbonate-containing clay, represented by the dotted lines) has

84

additional band intensity present as a broad spectral component under the 1701,
1636, and 1531 cm'1 bands. This ‘new’ spectral features present in the
carbonate-containing clay is attributed to the transformation product(s) of 1-
naphthol. The FTIR spectra are consistent with our macroscopic evidence (e.g.
unextractability of sorbed 1-naphthol, color change of clay) showing that a new '
spectral component is present in the whole clay. Unfortunately, the spectral
fingerprint of the sorbed species is characterized by broad, overlapping bands
that do not provide detailed clues as to the identification of the sorbed species.
Similar broad, featureless bands have been observed in other chemisorption
studies of other aromatic compounds on smectites. In a FTIR study of benzene
chemisorption on Cu-smectite, for example, FTIR spectra of the benzene
polymerized on the clay surface showed similar broad features in this spectral
region (Hinedi et al., 1993).

FTIR spectra of the OH bending and stretching regions of the carbonate-
containing and carbonate-free K-SWy-2 smectite samples are shown in Figures
3.5 and 3.6, respectively. SWy—2 has three bands at 920, 885, and 845 cm'1
which are assigned to the structural OH-bending vibrations of AIAIOH,
AIFe3+OH, and AlMgOH groups, respectively (Farmer, 1974). The spectra were
run in triplicate and there are a total of six spectra overlaid in Figure 3.5. There
is a consistent reduction in intensity of the AlFe3+OH bending mode of the
carbonate-containing K-SWy—2 smectite relative to the intensity of this band in
the carbonate-free clay. There has been renewed interest in this spectral region

of smectites, and quantitative analyses of these three bands have been used to

85

 

predict the extent and isomorphic substitution in smectites (Vantelon et al.,
2001). As shown, there was virtually no change in the intensities of the AIAIOH
and AlMgOH bands (Figure 3.6). This result provides surprisingly clear evidence
that structural Fe3+ is being reduced during the 1-naphthol transformation.
Reduction of structural Fe3+ may be coupled via electron transfer to oxidation of
sorbed 1-naphthol.

Further support of this proposed electron transfer mechanism is observed
in the v(OH) region of the structural OH group (Figure 3.6). The band at 3624
cm'1 corresponds to the v(OH) mode of the structural OH group of K-SWy-2
smectite. Because of the isomorphous substitution, there are several types of
structural OH groups, similar to the OH bending region, corresponding to
AIAIOH, AlFe3"OH, AIMgOH groups. What is observed in the 1-naphthol FT IR
spectra is a composite band that reflects the overall contribution of each band.
In a recent study of dioctahedral smectites, the v(OH) band of the structural OH
groups of different dioctahedral smectites was decomposed into individual
components of AIOHAI (3630), AlMgOH (3601), AlFe3+OH (3595), FeOHFe
(3572) (Zviagina et al., 2004). Of particular interest to this study is that the Fe3+
containing components have the lowest position, relative to the AIAIOH and
AIMgOH bands. Consequently, reduction of structural Fe?” would cause a
decrease in the lower energy portion of this band and the 3595 and 3572 cm‘1
components were lost due to reduction. The FTIR spectra are consistent with
this hypothesis. The v(OH) band of the whole clay (carbonate-containing clay)

has lower intensity in the lower energy portion of this spectrum precisely in the

86

area where the AlFe3+OH and FeOHFe components occur. Furthermore, these
spectral results are strongly correlated and supportive of the changes observed
in the OH bending region. The FTIR spectra provided direct evidence that
structural Fe3+ was involved in the transformation of 1-naphthol on the whole

clay (carbonate-containing clay).

87

DISCUSSION

Batch sorption experiments, XRD and FTIR in spectral analysis have
produced insights into the reactions of 1-naphthol with smectite clay. It is clear
that upon treatment with 1-naphthol octahedral Fe3+ in K-SWy-2 was reduced to
Fe2+ (Figures 3.5 and 3.6). Furthermore, reduction of structural Fe3” appears to
be accompanied by oxidation of 1-naphthol and the formation of dark-colored
products. Alkaline pH conditions emanating form the dissolution of carbonate
impurities in SWy-2 are likely to promote proton extraction from the hydroxyl
groups of 1-naphthol, followed by electron transfer to structural Fe3" in SWy—2. It
is plausible that reactive aryloxy radicals produced from 1-naphthol in this
fashion underwent coupling reactions on the clay surface. The presence of
bound residues in the whole clay, which resist extraction by water and methanol,
is consistent with the formation of higher molecular weight products. This type of
oxidative-coupling reaction is thought to be involved in humic substance

formation from polyphenols in soils (Stevenson, 1981). Our results suggest a

role for smectite clays in these processes.

88

 

120
100 ~ r

60 - Whole K-SWy-Z
pH=88

40-
20*

o 4
120 I I I l l I l J l l

100 ~ c) . d)
80 1 Calcite amended - MK

60 - gargogngte-free K-SWy-2 _ Calcite
' pH=95
40 1

20‘

- Carbonate-free K-SWy-2
pH=68

 

 

1-Naphthol in solution (% of ug added)

 

 

 

 

 

 

0 20 40 60 80 1000 20 40 60 80 100

Time (h)

Figure 3.1. Disappearance of 1-naphthol from aqueous solution by: (a) whole
clay (carbonates-containing); (b) carbonate-free clay; (c) carbonate-free clay,
amended with calcite; (d) disappearance of 1-naphthol in a 0.01% calcite
solution.

89

 

 

   
 

 

 

 

 

 

 

a) WhOIG K'SWY'Z b) carbonate-free K-SWy-2
11.24 A,
1-naphthol-clay, 11.19 A
120 h 1-na phthol-clay,
120 h l
l 10.70 A, /
5% clay %. 10.83 A
5 control 5 carbonate-free
E g clay control
2.5 7.5 12.5 2.5 7.5 12.5
Angle (2 Theta) Angle (2 Theta)

Figure 3.2. X-ray diffraction patterns of oriented air-dried films of 1-naphthol
and clay K-SWy-2: (a) whole clay (control) and 1-naphthol-whoIe-clay

complex at 120h; (b) carbonate-free clay (control) and 1-naphthol-carbonate-
free-clay complex at 120h.

90

.......

 

 

 

   
 

v(OH) structural v(Si-O) of SWy-2__—_. I
3 - OH groups smectlte “2 é)" V
2 9
6 °° l
2.5 « 9% j I
=6 a I
m 5 B l
g 2 . Internal modes ‘1’ 5 I]
3 VIC”) (e.g., v(C-C)) of o a j
g 1 5 sorbed sorbed1-napthol j i .
g ' “ water and transformation 1,, ----- ,'

 

I, 1 products V.

 

 

 

 

3500 3000 2500 2000 1500 1000 500

Wavenumber (cm“)

Figure 3.3. FTIR spectra of 1-naphthol sorbed to the carbonate-containing
SWy-2 and to carbonate-free SWy-2 clay in the 4000 to 400 cm'1 region,
containing the most prominent bands from the sorbed 1-naphthol and its
transformation products.

91

 

.08-

.06 -

 
 
 
  

absorbance
'o
A
l

b
N
1

 

 

 

2'200 5100 2000 11900 1000 1'700 1'600 1'500 1'400
Wavenun'ber cm1

Figure 3.4. An expanded region in the 2200 to 1350 cm"1 of the FTIR
spectra of whole K-SWy—2 (carbonate-containing clay, represented by the
dotted lines) shows additional band intensity present as a broad spectral
component under the 1701, 1636 and 1531 cm'1 bands, which is attributed

to the transformation product(s) of 1-naphthol.

92

 

920 (AIAIOH)

  
  
  

885 (AlFe+3OH)
b) Carbonate-free clay

1.2 .

-—L
L

845 (AlMthOl-l)

absorbance

  

a) whole cl

'oo

 

 

 

j I I T r j

940 920 900 880 860 840 820
Wavenurrber cm1

Figure 3.5. FTIR spectra of the structural OH bending region of the (a)
carbonate-containing clay and (b) carbonate-free K-SWy-2 smectite show a
reduction in intensity of the AlFe+3OH bending mode of the carbonate-containing
K-SWy-2 smectite relative to the intensity of this band in carbonate-free clay and
no change in the intensities of the AIAIOH and AIMgOH bands.

93

 

1.1

 

Whole clay

.6 i v 1
3720 3680 3640 3600
Wavenurrber cm1

  

absorbmce

 

 

 

 

I

Figure 3.6. FTIR spectra of the structural OH stretching region of the carbonate-
containing and carbonate—free K-SWy-2 smectite show a composite band that
reflects the overall contribution of AIAIOH, AlFe‘30H, AIMgOH bands. The
v(OH) band of the whole clay (carbonate-containing clay) shows a lower
intensity in the lower energy portion (3595 to 3572 cm") of this spectrum, which
is the area where AlFe+3OH and FeOHFe components occur.

94

Table 3.1. dam-Spacings (A) for K-SWy-Z, and K-SWy-2 Extracted to
Remove Carbonate Impurities. The clay films with and without 1-naphthol
sorption were analyzed by XRD after air-dried, then exposed the air-dried
clay films to 1 and 100% relative humidity (RH).

 

 

 

 

 

Treatment K—SWy-Z Carbonate-free K-SWy-Z
With Without With Without
l-naphthol l-naphthol l-naphthol l-naphthol
Air-dried 11.24 10.7 11.19 10.83
Exposure to 1% RH 11.16 9.99 nd nd
Exposure to 100% RH 15.39 15.55 nd nd

 

nd = not de te mined

95

 

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carbaryl by carbonate impurities in reference clay SWy-2. J. Agric. Food
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Arroyo, L. J.; Hui, L.; Teppen, B.J.; and Boyd, SA. A simple purification method
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99

CHAPTER 4

SUMMARY AND CONCLUSION

This dissertation provides a case-in-point that small amounts of impurities
present in reference clay mineral samples may substantially alter their apparent
sorptive properties or reactivities. The study of the influence of clay preparation
methods on sorption and hydrolysis of carbaryl by reference clay K-SWy—2
demonstrated that common methods used to fractionated and purified reference
clays were not enough to eliminate carbonate impurities. In this study, small
quantities of inorganic carbonates were sufficient to cause substantial alkaline
hydrolysis of carbaryl. Carbaryl was hydrolyzed to 1-naphthol in different
degrees depending on the method used. Methods to purified available
commercial reference clay minerals includes the use of fractionation based on
particle size separation, e.g., the sedimentation and low-speed centrifugation
procedures (Kunze and Dixon, 1986; Gee and Bauder, 1986; Moore and
Reynolds, 1989), but they are not uniformly performed consistently.

This dissertation covers a critical description of the development of a
standard fractionation and partial purification procedure for commercial available
reference clays prior to their use in studies on sorption and reactivity. This study
proved the need to purify reference clays prior to use them in any study of
surface chemistry. The essential steps of such a procedure would include: (1)

separation of the <2 urn clay-sized particles by low speed centrifugation or

100

 

gravity sedimentation. (2) Dissolution of carbonates and soluble salts with
sodium acetate buffer. (3) Washing with water to remove residual soluble salts
and decrease ionic strength. (4) Re-suspension of the (now predominantly Na-)
clay in water. (5) Homocation saturation of the clay. In this dissertation, we

suggest adoption of method B because of its convenience.
The study using [14C]carbaryl provided valuable information on the fate of

the pesticide carbaryl. The use of the radiolabeled compound verified results of
the hydrolysis of carbaryl, obtained by the HPLC/UV and the resistance for
extraction of the sorbed compound.

Analysis of the FTIR spectra about the interaction between the hydrolytic
product 1-naphthol and the reference clay K-SWy—2 confirmed the formation of
new product(s) that may explain its nonextractability. There is also evidence that
structural iron of the reference K-SWy-2 was reduced to Fe2+ and 1-naphthol
was oxidized to dark-colored products, which are weakly intercalated in the clay,
as shown by XRD patterns of clay-1-naphthol complexes. This type of oxidative-
coupling reaction is thought to be involved in humic formation from polyphenols
in soils (Stevenson, 1981 ).

More research is needed on exploring new experiments using carbaryl,
other carbamates and their degradation products with clay homoionized with
higher polarizing and/or transitional cations. The use of better methods for
extraction of clay bound residues and techniques such as GC/MS and HPLC/MS
could give more information about the formation of new products. Studies on

bioavailability of these bound residues could also be of interest.

101

REFERENCES

Gee, G.W.; Bauder, J.W. Particle-Size Analysis. In Methods of Soil Analysis,
Part 1. Physical and Mineralogical Methods, 2nd ed.; Klute, A., Ed.;
American Society of Agronomy, Soil Science Society of America:
Madison, WI, 1986: pp 383—412.

Moore, D. M.; Reynolds, R. C., Jr. X-Ray Diffraction and the Identification and
Analysis of Clay Minerals; Oxford University Press: Oxford, UK, 1989.

Kunze, G. W.; Dixon, J. B. Pretreatment for mineralogical analysis. In Methods
of Soil Analysis, Part 1. Physical and Mineralogical Methods, 2nd ed.;
Klute, A., Ed.; American Society of Agronomy, Soil Science Society of
America: Madison, WI, 1986: pp 91—100.

Stevenson, F. J. Humus Chemistry. In Genesis, Composition, Reactions. Wiley,
New York, 1981; chapters 16—17.

102

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