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EFFECT OF SCALE DURING ELECTROCHEMICAL
DEGRADATION OF NAPHTHALENE AND SALICYLIC ACID

presented by

DONG GEUN LEE

has been accepted towards fulfillment
of the requirements for the

MS. degree in Civil Engineering

 

 

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5/08 KlProj/AccsxPres/ClRC/DateDue.indd

EFFECT OF SCALE DURING ELECTROCHEMICAL DEGRADATION
OF NAPHTHALEN E AND SALICYLIC ACID

By

Dong Geun Lee

A THESIS

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

MASTER OF SCIENCE

Civil Engineering

2008

ABSTRACT

EFFECT OF SCALE DURING ELECTROCHEMICAL DEGRADATION
OF NAPHTHALENE AND SALICYLIC ACID

By

Dong Geun Lee

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds that
consists of two or more benzene rings and they are highly recalcitrant molecules due to
their hydrophobicity and low solubility in water. At the laboratory scale, electrochemical
degradation of naphthalene and salicylic acid was investigated to evaluate the effect of
size of electrochemical cell (1 L versus 3.5 L), AC versus DC current, current density (1
mA/cm2 and 3 mA/cmz), and aqueous versus sandy soil media. The tests were carried out
in undivided cells immersed in water baths to control the temperature rise.

The key results of this study are: (1) for a given current density, the rate of
degradation of naphthalene or salicylic acid was independent of the size of the cell; (2) the
energy consumption per unit volume of the electrolyte was 2 to 4 fold greater for the
larger cell; (3) while rate of degradation was greater for DC compared to AC for equal
current densities, the increase in the rate of degradation was much higher for AC than DC
when the electrolyte was continuously stirred; and (4) organic acids can find soil particles
containing oxides near the cathode region as ideal sites for adsorption where the
degradation is slower when DC is used. However, AC will not allow such adsorption and

may be more effective in degradation across the entire sample volume.

ACKNOWLEDGEMENT

This project has been partially funded by 2007 I/UCRC—MTP fellowship and
Grant No. BBS-0402772 from the National Science Foundation (NSF). This thesis has not
been reviewed by the NSF.

I would like to thank all those individuals who helped me in with the research
experiments related to my masters thesis. I am grateful to my major advisor Dr. Milind V.
Khire who helped me whenever I was in need. I am also thankful to my committee
members Dr. Irene Xagoraraki and Dr. Hui Li for their valuable time and support. I also
acknowledge the help extended by my lab mates Dr. Emmanuel Pepprah, Moumita
Mukherjee, Ramil Mijares, and Carolyn Hardt during the research experiments.

I would like to thank my wife Mrs. Misuk Lee who has been my side to support
me. I am also thankful to my parents and parents-in-law for their sincere love and belief in
me. Finally, I would like to thank my friends back in the Republic of Korea and here in
the US. for their constant encouragement and support.

I express my gratitude to Republic of Korea Army for giving me an opportunity
and financial support to obtain Masters Degree at Michigan State University. Specially, I

would like to thank to LTC, Yongjae Lee, and Moonkyoung Kim for their encouragement.

iii

TABLE OF CONTENTS

LIST OF TABLES .......................................................................................................... vii
LIST OF FIGURES ....................................................................................................... viii
CHAPTER 1: INTRODUCTION ..................................................................................... 1
1.1 OCCURRENCE OF PAHs IN WATER ........................................................................ 1
1.2 EXISTING REMEDIATION TECHNIQUES .............................................................. 1
1.2.1 Advanced Oxidation Processes (AOPs) ................................................................ 2
1.2.2 Permeable Reactive Barriers (PRBs) .................................................................... 2
1.2.3 Membrane Processes ............................................................................................. 2
1.2.4 Electrokinetic Remediation ................................................................................... 3
1.2.5 Granular Activated Carbon (GAC) Adsorption .................................................... 3

1.3 OBJECTIVES ................................................................................................................ 3
1.3.1 Key Challenges ..................................................................................................... 3
1.3.2 Target Contaminants ............................................................................................. 4
1.3.3 Key Objectives ...................................................................................................... 5
CHAPTER 2: ELECTOCHEMICAL DEGRADATION .............................................. 7
2.1 DEGRADATION MECHANISM ................................................................................. 7
2.1.1 Direct Electrolysis ................................................................................................. 7
2.1.2 Indirect Electrolysis .............................................................................................. 8

2.2 FACTORS AFFECTING ELECTROCHEMICAL DEGRADATION ........................ 8
2.2.1 Electrodes .............................................................................................................. 9
2.2.2 Supporting Electrolyte ........................................................................................ 10
2.2.2.1 Sodium Chloride (NaCl) ......................................................................... 10

2.2.2.2 Sodium Sulfate (NaZSO4) ........................................................................ 10

2.2.3 Current Type ....................................................................................................... 11
2.2.4 Current Density ................................................................................................... 12

2.3 INDEX OF EFFICIENCY ........................................................................................... 12
2.3.1 Instantaneous Current Efficiency (ICE) .............................................................. 12
2.3.2 Electric Energy .................................................................................................... 13
2.3.3 Pollutant Breakdown Rate per Charge ................................................................ 13
2.3.4 Electrical Efficiency (Long Order Reduction) .................................................... 13
2.4 MASS TRANSFER REACTION: THE NERNST-PLANCK EQUATION ................ 14

iv

CHAPTER 3: EXPERIMENTAL METHODOLOGY ................................................ 16

3.1 EXPERIMENTAL SETUP .......................................................................................... 17
3.1.1 Reaction Chamber ............................................................................................... 17
3.1.1.1 Titanium Electrode .................................................................................. 18

3.1.1.2 Cell Volume (1 L Cell vs. 3.5 L Cell) ..................................................... 19

3.1.2 Electrical Equipment ........................................................................................... 25
3.1.3 Spiked Medium Preparations .............................................................................. 26
3.1.3.1 Naphthalene in Aqueous Solution .......................................................... 26

3.1.3.2 Salicylic Acid in Aqueous Solution ........................................................ 26

3.1.3.3 Sand Spiked with Naphthalene or Salicylic Acid ................................... 26

3.1.4 Stirring Apparatus ............................................................................................... 28

3.2 TESTING, SAMPLING AND ANALYZING TECHNIQUES .................................. 29
3.2.1 Sampling Technique ........................................................................................... 29
3.2.1.1 Sampling Frequency ............................................................................... 29

3.2.1.2 Sampling Method .................................................................................... 29

3.2.2 HPLC Analysis ................................................................................................... 31
3.2.3 Duplicate Sampling ............................................................................................. 32

3.3 MONITORED EXPERIMENTAL PARAMETERS .................................................. 33
3.4 RECYCLING SOILS AND ELECTRODES .............................................................. 35
3.4.1 Soil Recycling ..................................................................................................... 35
3.4.2 Electrode Recycling ............................................................................................ 37
CHAPTER 4: RESULT AND DISCUSSION ................................................................ 38
4.1 AQUEOUS PHASE EXPERIMENTS ........................................................................ 38
4.1.1 Naphthalene ........................................................................................................ 39

I 4.1.1.1 Effect of Current Density on Naphthalene Degradation ......................... 39
4.1.1.2 Effect of Current Type (AC vs. DC) on Naphthalene Degradation ........ 41

4.1.1.3 Effect of Size of Reaction Cells on Naphthalene Degradation ............... 41

4.1.1.4 Degradation Kinetics .............................................................................. 42

4.1.1.5 Cumulative Electrical Energy Consumption .......................................... 44

4.1.1.6 Measured Initial and Final Test Parameters ............................................ 46

4.1.2 Salicylic Acid ...................................................................................................... 49
4.1.2.1 Effect of Current Density on Salicylic Acid Degradation n ................... 49

4.1.2.2 Effect of Current Type (AC vs. DC) on Salicylic Acid Degradation ..... 49

4.1.2.3 Effect of Scale of Cells on Salicylic Acid Degradation .......................... 51

4.1.2.4 Degradation Kinetics .............................................................................. 51

4.1.2.5 Cumulative Electrical Energy Consumption .......................................... 53

4.1.3 Effect of Stirring During Electrochemical Degradation of Salicylic Acid ......... 54
4.1.3.1 Degradation Kinetics .............................................................................. 54

4.1.3.2 Effect of Stirring on Degradation Rate of Salicylic Acid ....................... 56

4.1.3.3 Cumulative Energy Consumption ........................................................... 58

4.1.3.4 Measured Initial and Final Test Parameters ............................................ 59

4.2 SANDY SOIL PHASE EXPERIMENTS .................................................................... 61
4.2.1 Naphthalene ........................................................................................................ 61
4.2.1.1 Rate of Degradation ................................................................................ 61

4.2.1.1 Cumulative Electrical Energy Consumption .......................................... 63

4.2.2 Salicylic Acid ...................................................................................................... 64

4.3 ANALYTICAL MODELING USING NERNST-PLANCK EQUATION ................. 71
4.3.] Key Assumptions ................................................................................................ 71
4.3.2 Naphthalene ........................................................................................................ 72
4.3.3 Salicylic Acid ...................................................................................................... 74
CHAPTER 5: SUMMARY AND CONCLUSIONS ..................................................... 76
REFERENCES ................................................................................................................. 78

vi

LIST OF TABLES

Table 1.1: Properties of naphthalene and salicylic acid ....................................................... 6
Table 3.1: Dimensions of electrodes, glass beaker, and Teflon cap ................................ 22
Table 3.2: Physical properties of l L and 3.5 L cells ........................................................ 24
Table 3.3: Currents densities and equivalent currents for 1 L to 3.5 L cells ..................... 24
Table 3.4: Preparation of standard solutions for naphthalene and salicylic acid ............... 32
Table 4.1: Summary of aqueous phase experiments .......................................................... 38

Table 4.2: Degradation rate constant (k) and the corresponding half-life (II/2) for
naphthalene tests in aqueous solution (time period = 24 hours) ........................................ 43

Table 4.3: Comparison of electrical energy consumption costs for 1 L and 3.5 L cells....45

Table 4.4: Summary of key physical parameters for naphthalene tests in aqueous
solution ..................................................................................................................... 48

Table 4.5: Degradation rate constant (k) and the corresponding half-life (ti/2) for salicylic
acid tests in aqueous solution (time period = 24 hours) .................................................... 52

Table 4.6: Degradation rate constant (k) and the corresponding half-life (ti/2) for salicylic
acid tests in stirring aqueous solution (time period = 24 hours) ........................................ 55

Table 4.7: Summary of key parameters for salicylic acid tests in aqueous solution
measured at initial and final time elapsed .......................................................................... 60

Table 4.8: Summary of experimental variables for sandy soil phase experiments ............ 61

Table 4.9: Summary of key parameters for salicylic acid tests in sandy soil measured at
initial and final time elapsed .............................................................................................. 7O

vii

LIST OF FIGURES

Figure 2.]: Scheme of pollutant removal pathways in electrochemical oxidation .............. 7
Figure 3.1: Schematic of testing procedures ...................................................................... 16
Figure 3.2: Representation of experimental setup ............................................................. 17
Figure 3.3: Picture of a reaction chamber for the 3.5 L cell .............................................. 18
Figure 3.4: Schematic of the titanium electrodes, assembled cell, and Teflon cap ........... 21

Figure 3.5: Pictures of cap attached to electrodes (top) and the two cells having 1 L and

3.5 L volume (bottom) ....................................................................................................... 23
Figure 3.6: Pictures of electrical equipment used in the setup: power supply (top),
function generator (middle), and digital oscilloscope (bottom) ......................................... 25
Figure 3.7: Pictures of materials used in spiked medium preparations ............................. 27

Figure 3.8: Pictures of stirrer with 3.5 L cell (top), stirrer (bottom, left), and magnetic bars

(bottom, right) .................................................................................................................... 28
Figure 3.9: Pictures of syringes equipped with needles of different lengths (top) and
amber vials with caps used for HPLC analyses (bottom) .................................................. 30
Figure 3.10: Pictures of HPLC instrument ........................................................................ 31
Figure 3.11: Pictures of measuring devices ....................................................................... 34
Figure 3.12: Pictures of procedure followed for cleaning and recycling used sand .......... 36

Figure 3.13: Picture while polishing an electrode to clean the fouled electrode surface...37

Figure 4.1: Normalized concentration of naphthalene in aqueous phase
experiments for DC (a); and AC (f = 0.1 Hz) (b) performed at jeqvof 1 and 3 mA/cm2
in 1 L and 3.5 L cells ......................................................................................................... 40

Figure 4.2: Degradation rate constant (k) and the corresponding half-life (II/2) for
naphthalene tests in aqueous solution (time period = 24 hours) ........................................ 43

Figure 4.3: Cumulative normalized electric energy consumed during electrochemical

degradation of naphthalene in aqueous phase experiments for DC and AC (f = 0.1 Hz)
performed at few of 1 mA/cm2 in 1 L and 3.5 L cells ........................................................ 45

viii

Figure 4. 4. Normalized concentration of salicylic acid in aqueous2 phase
experiments for DC (a) and AC (f= 0.1 Hz) (b) performed at jeqv of 1 and 3 mA/cm2 in the
1Land3.5Lcells .............................................................................................................. 50

Figure 4.5: Variation of k observed for salicylic acid tests in aqueous solution ............... 52

Figure 4.6: Normalized cumulative electrical energy consumed during electrochemical
degradation of salicylic acid in aqueous phase experiments for jeq“ of 1 and 3 mA/cm2 in
1 L and 3.5 L cells .............................................................................................................. 53

Figure 4.7: Variations of k observed for salicylic acid tests in aqueous solution with
stirred or without stirred ..................................................................................................... 55

Figure 4.8: Normalized concentration of salicylic acid in aqzueous phase experiments for
DC (a) and AC (f: 0.1 Hz) (b) performed at jeqv 3 mA/cm 1n 1 L and 3. 5 L cells when
stirred or unstirred .............................................................................................................. 57

Figure 4. 9: Cumulative electrical energy consumed during electrochemical degradation of
salicylic acid 1n aqueous phase experiments for DC and AC (f= O. 1 Hz) performed at jeqv
= 3 mA/cm2 in l L and 3. 5 L cells when stirred or unstirred ............................................ 58

Figure 4.10: Normalized concentration of naphthalene2 for aqueous phase and sandy soil
phase experiments for DC performed at jeqv 1 mA/cm2 (212 mA) for 3. 5 L cell ........... 62

Figure 4.11: Cumulative normalized electrical energy consumed during degradation of
naphthalene for2 aqueous phase and sandy soil phase experiments with DC performed at
jeqv =1mA/cm2 for3.5Lcell ............................................................................................ 63

Figure 4.12: Normalized concentration of salicylic acid for aqueous phase and sandy soil
phase experiments for DC performed at jqu- 1 mA/cm2 for 1 L and 3. 5 L cells ............. 64

Figure 4.13: Cumulative normalized electrical energy consumed during electrochemical
degradation of salicylic acid 1n aqueous phase and sandy soil phase experiments for DC
performed at jeqv of 1 mA/cm2 in 1 L and 3. 5 L cells ........................................................ 65

Figure 4.14: Normalized concentration of salicylic acid 1n sandy soil phase experiments
for DC (a) and AC (f= 0.1 Hz) (b) performed at jeqv—I mA/cm2 in 1 L and 3. 5 L cells. .67

Figure 4.15: Normalized concentration of salicylic acid across the cell in
sandy soil phase experiments for DC performed at jeqv = 1 mA/cm2 (212 mA)
for 3.5 L cell ............................................................................................. 68

Figure 4.16: Experimental and predicted concentration rations of naphthalene for jeqv = 1
and 3 mA/cm2 in 1 L and 3. 5 L cells for DC application .................................................. 73

ix

Figure 4.17: Experimental and predicted concentration ratios of salicylic acid for jeqv = 1
mA/cm2 (a) and 3 mA/cm2 (b) in 1 L and 3.5 L cells ........................................................ 75

LIST OF ABBREVIATIONS

AC = Alternating Current

ATSDR = Agency for Toxic Substance and Disease Registry
AV = Alternating Voltage

DC = Direct Current

D1 = De-ionized

D0 = Dissolved Oxygen

EIA = Energy Information Administration

HPLC = High Performance Liquid Chromatography
LCR = Inductance, Capacitance, Resistance

MCL = Maximum Contaminant Level

MCLG = Maximum Contaminant Level Goal

MDL = Method Detection Limit

OH ° = Hydroxyl Radical

PAHs = Polycyclic Aromatic Hydrocarbons

PRBs = Permeable Reactive Barriers

RMS = Root Mean Square

US EPA = the United States Environmental Protection Agency

xi

LIST OF SYMBOLS

C = Concentration with time

C0 = Initial concentration

D = Diffusion coefficient

E = Electrical potential

f = AC frequency

F = Faraday’s constant (= 96,487 C/mol)
j = Current density

jeqv, = Equivalent current density

J = Mass flux

k = Pseudo-first-order degradation rate constant

1,) = Peak current

[mm = Root-mean-square current

R = Molar gas constant (= 8.314 kJ/mol-K)
T = Temperature
11/2 = Half-life

v = Linear velocity of solution

Vp = Peak voltage

Vrms = Root-mean-square voltage

2 = Charge on a reacting species

Z = Impedance

xii

CHAPTER ONE
INTRODUCTION

1.1 OCCURRENCE OF PAHs IN WATER

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic compounds that
consist of two or more fused benzene rings and they are highly recalcitrant molecules that
can persist in the environment due to their hydrophobicity and low solubility in water
(Bamforth and Singleton 2005). According to the United States Environmental Protection
Agency (USEPA 2006), PAHs similar to benzo(a)pyrene potentially cause health effects
from acute exposure at levels above the maximum contaminant level (MCL) such as red
blood cell damage leading to anemia and suppressed immune system. MCL is used as the
drinking water standard in the US. MCL and maximum contaminant level goal (MCLG)
of benzo(a)pyrene are 0 and 0.2 pg/L, respectively. This shows that extremely low
concentration of PAHs can cause fatal diseases (USEPA 2006). It has been observed that
that benzo(a)pyrene has the potential to cause cancer from lifetime exposure at levels

above the MCL (USEPA 2006).

1.2 EXISTING REMEDIATION TECHNIQUES

Several techniques for remediation of contaminants including PAHs in water, soil,
or sediments at the laboratory scale and at pilot or field scale have been investigated.

These technologies are briefly described below.

1.2.1 Advanced Oxidation Processes (AOPs)

AOPs are based on the chemical, photochemical, and photo-catalytic production
of hydroxyl radicals (OH'), which act as strong oxidant agents able to react with
organics yielding dehydrogenated or hydroxylated derivatives. This radical is the main
oxidizing agent of organic material, causing its mineralization, in other words, its
conversion to C02, water, and inorganic ions (Louhichi et al. 2006). This method has
major advantages, relatively high oxidation efficiency, fast reaction rate, and easy

operation.

1.2.2 Permeable Reactive Barriers (PRBs)

A permeable reactive barrier (PRB) is defined as an in situ method for the
remediation of contaminated groundwater. The PRB concept consists of reactive media
such as zero valent iron, placed in the subsurface in the form of a vertical curtain through
which the plume of contaminated groundwater moves and is treated (US EPA 1998). A

PRB is a trap for contaminants where the barrier acts as a passive treatment system.

1.2.3 Membrane Processes

Membrane processes are modern physicochemical separation techniques that use
differences in permeability (of water constituents) as a separation mechanism. Four types
of pressure-driven membranes are currently used in municipal water treatment:
microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse-osmosis (RO)

(Crittenden et al. 2005).

1.2.4 Electrokinetic Remediation

Electrokinetic remediation process typically removes heavy metals, anions, and
polar organic contaminants from low permeability soil, mud, sludge, and marine dredging,
using electrochemical and electrokinetic processes of desorption and removal. In other
words, this in situ soil processing technology is primarily a separation and removal

method for extracting contaminants by the means of electromigration.

1.2.5 Granular Activated Carbon (GAC) Adsorption

Granular activated carbon (GAC) adsorption is frequently used for the removal of
hazardous organic pollutants from groundwater or wastewater. This technology achieves
a rapid removal of the organic pollutants and retains them on the GAC surface (Cafiizares

et al. 2004).

1.3 OBJECTIVES

1.3.1 Key Challenges

Electrochemical degradation processes run at very high efficiency and operate
essentially under the same conditions for a wide variety of waste (Grimm et al. 1998).
Many researchers have reported lab-scale electrochemical degradation of organic
compounds (Alshawabkeh and Saraheny 2005). However, there is lack of research on
effects of scale on the rate of electrochemical degradation and power consumption. It
would be vital to have such date if lab-scale data is to be used for the design of pilot-scale

tests.

1.3.2 Target Contaminants

Two chemical compounds, naphthalene and salicylic acid, were selected to
investigate in this study. Naphthalene, one of the model PAH compounds exists in soil,
sediment, groundwater, and the atmosphere, and is naturally present in fossil fuels such
as petroleum and coal. In addition, most naphthalene consumption (60%) is through its
use as an intermediary in the production of phthalate plasticizers, resins, phthaleins, dyes,
pharmaceuticals, and insect repellents. About 5% of naphthalene is released into water,
primarily through coal tar production and distillation processes (ASTDR 1995). In other
words, the widespread use and production of naphthalene in the United States is directly
connected with the releases through various media and, furthermore, this inflow of
naphthalene to the surface or the subsurface (e.g. to groundwater) is potentially
threatening human health. According to Alshawabkeh and Sarahney (2005) and Goel er
al. (2003), naphthalene has been used as a target contaminant because it has chemical and
physical properties similar to other PAHs except that it is more soluble in water. This
property of naphthalene makes its use attractive for lab-scale research.

Salicylic acid (2-hydroxybenzoic acid) is the key metabolite of various analgesics
(e.g. aspirin) and is a common component in sewage effluent. Exposure of salicylic acid
to human body can cause impacts to the central nervous system and the acid-base
imbalance in the body, resulting in delirium and tremor, and dermatitis for the long and
short-term (CDC 1997).

While the key objective of this study was to study the effect of scale on
electrochemical degradation of naphthalene (a PAH), salicylic acid was also used to

make up for these disadvantages of naphthalene: relatively low solubility, comparing

(0.0031g/100 mL for naphthalene versus 0.2 g/lOO mL for salicylic acid); and its
volatility as a function of temperature. In addition, salicylic acid dissociates into charged
ions when mixed with water. The key physical and chemical properties of naphthalene

and salicylic acid are presented in Table 1.1.

1.3.3 Key Objectives

The objectives of this study were to:

I Investigate the effect of current type and current density on electrochemical
degradation of naphthalene and salicylic acid in an aqueous solution and sandy
soil;

II Investigate the effect of scale of electrochemical cell on the rate of degradation
and electrical power consumption; and

' Predict the change in concentrations during the test using the Nemst-Planck mass

transport equation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Partition Coefficient

 

Table 1.1: Properties of naphthalene8 and salicylic acidb
Naphthalene Salicylic Acid
IUPAC Name Naphthalene 2-Hydroxybenzoic acid
CAS No. 91-20-3 69-72-7
ICSC No. 0667 0563
EINECS No. 202-049-5 200-712-3
Chemical Formula CmHa C7H603
Chemical Structure OH
Molecular Mass 128.18 138.1
(g/mole)
A earance White solid in various forms, Colorless crystalline powder
pp with characteristic odor or needle-shaped crystals
Melting Point
81.2 159
(°C)
8 'l' P ' t
°' "(1.90) °'" 218 211 (at 2666 Pa)
Flash Point
1 7
(o C) 80 5
Vapor Pressure 0 o
11 (at 25 C) 114 (at 20 C)
(Pa)
Density
1.16 °
Solubility in Water 0
(g/100mL) 0.0031 0.2 (at 20 C)
Octanol/ Water 3.3 22

 

Note: “CDC (2005), b CDC (1997)

2.1 DEGRADATION MECHANISM

CHAPTER TWO

ELECTROCHEMICAL DEGRADATION

During electrochemical degradation, the organic chemicals are destroyed or

converted by either direct or indirect oxidation processes (Pepprah and Khire 2008). The

schematic of these processes is illustrated in Figure 2.1 and discussed in the following

sections.

Electron v

Anode

—. i

 

Pollutants

Destroyed
pollutants

Ial Direct Oxidation Process

Pollutants \

Oxidant

 

*— Electrode

Pollutants
Oxidation
In the bulk
Destroyed
pollutants

I bl Indirect Oxidation Process

Figure 2.1: Schematic of pollutant removal pathways during electrochemical oxidation

(Chiang et al. 1994)

2.1.1 Direct Electrolysis

During direct anodic oxidation [Figure 2.1 (a)], the chemicals are initially

adsorbed on the surface of the anode where they are degraded by the anodic electron

transfer reaction. Direct electrolysis methods include: anodic and cathodic processes.

Acar and Alshawabkeh (1996) suggested the following chemical reactions at the anode

and at the cathode, if the electrolytes are separated using a protonic membrane to prevent

mixing but allow charge flow:

Anode: 2H2 —+ 02 + 411* + 4e'; 15" = +1229 v (2.1)

Cathode: 2.1120 + 2e' —) H2 + ZOH’; E" = -0.828 V (2.2)

2.1.2 Indirect Electrolysis

During indirect anodic oxidation [Figure 2.1 (b)], strong oxidants such as
hypochlorite/ chlorine, ozone, or hydrogen peroxide are electrochemically generated. The
pollutants are then degraded by the oxidation reaction with these strong oxidants. Among
the oxidants, generation of hypochlorite is relatively common because a majority of the
effluents contain chloride (Rajkumar et al. 2005). The chemical oxidation/reduction

reactions of chlorine and hypochlorite are presented below:

Anode: 2Cl' —-> Cl2(g) + 2e'; E" = -I.3583 V (2.3)
Cathode: 21120 + 26 —> H2 + 2011'; E’ = -O.828 v (2.4)
Bulk Solution: C12 + H20 —> HOCI + 11* + CI' (2.5)
HOCI —> H” + ocr (2.6)

2.2 FACTORS AFFECTING ELECTROCHEMICAL DEGRADATION

Effect of concentration of electrolyte and electrode material, surface area, and size
of electrodes, pH, and type of supporting electrolyte on the electrochemical degradation

of pollutants have been presented in various studies (Wu et al. 2007). In this study, effect

of the size of the electrochemical cell for AC and DC application at various current

densities was evaluated.

2.2.1 Electrode

Various types of electrodes have been used in the electrochemical degradation

investigations and are introduced as follows.

Titanium metal having dimensions: 5.0x 3.5 x 0.1 cm, which was polished roughly
(Wu et al. 2007 and Pepprah and Khire 2008);

Titanium core with mixed metal coating having dimensions: 10.2x 1.3x0.12 cm
(Alshawabkeh and Sarahney 2005);

Stainless steel plate as cathode and titanium mesh with a mixed metal oxide
coating as anode having dimensions: 3.18x6.35 x0.15 cm for both electrodes and
distance between electrodes equal to 1.4 cm (Goel et al. 2003);

Boron doped diamond electrode (BDD) as anode and a stainless steel plate as
cathode, with an immersed area of 20 cm2 (Carvalho et al. 2007);

BDD anode, stainless steel cathode, and disks for both electrodes with a
geometrical area of 50 cm2 each and an inter-electrode gap of 1cm (Panizza et al.
2008); and

Diamond-based material as anode, stainless steel as cathode, having circular
shape (100 mm diameter) for both electrodes with geometric area of 78 cm2 for

each electrode and an electrode gap of 9 .mm (Canizares et a1. 2005).

2.2.2 Supporting Electrolyte

2.2.2.1 Sodium Chloride (NaCl)

Alshawabkeh and Sarahney (2005) used sodium chloride (NaCl) as a supporting
electrolyte for electrochemical degradation of naphthalene in aqueous solution and
showed that generating C12 (9 enhances the degradation of naphthalene in the solution. It
is due to the formation of hypochloric acid. In addition, Panizza et al. (2005) have
reported that the presence of hypochlorite, formed from the active chlorine
electrochemically generated on the anode surface, must be related due to the oxidation of
chloride ions when NaCl was used as a supporting electrolyte (Equation 2.3 to 2.6).
However, post-treatment for the occurrence of chlorine should be considered as well as
its efficiency in electrochemical remediation of organic compounds. Pepprah and Khire
(2008) have reported faster decay and fouling of 99% pure titanium electrodes when

NaCl instead of Na2804 was used as the supporting electrolyte.

2.2.2.2 Sodium Sulfate (NazSO4)

Louhichi er al. (2006) used 1 M sodium sulfate (Nast4) as the supporting
electrolyte in the electrochemical oxidation of benzoic acid on boron-doped diamond in
aqueous solution. Furthermore, Feng and Li (2003) employed 0.25 M Na2$O4 as the
supporting electrolyte to investigate the electro-catalytic oxidation of phenol (100 ppm)
on titanium plated electrode with several metal-oxide electrodes, such as Sb-Sn-Rqu-
Gd, Sb-Sn-Rqu, and Rqu, in aqueous solution. Pepprah and Khire (2008) reported a
lower degradation rate when Na2804 was used, compared to when NaCl was used as the

supporting electrolyte. The key reason was for higher degradation rate when NaCl was

10

used as the supporting electrolyte was attributed to the formation of hypochlorite which

acts as a strong oxidant.

2.2.3 Current Type

Chin and Cheng (1985) explored alternating voltage (AV) waveforms (sinusoidal,
square, and triangular waves) where the frequency ranged from 20 to 6,000 Hz, and to
determine the effect of AC on the oxidation of phenol using platinum electrodes in an
aqueous H2304 supporting electrolyte. The electrolyte consisted of 0.01 to 0.1 M phenol
in l M H280; and platinum rotating disk having surface area of 12.5 cm2 was immersed
in the electrolyte contained in a l L Pyrex glass jar (undivided cell). The rate of
conversion of phenol increased with increasing magnitude of the AC voltage and
decreased with increasing AC frequency. Electric energy consumption was observed to
be smaller than during DC electrolysis.

Nakamura et al. (2005) proposed the mechanism of decomposition of organic
compounds based on selective redox reactions with various radicals generated by AC
electrolysis that allows both oxidation and reduction in the same cell between adjacent
electrodes. For this study, trichlorobenzene (TCB) solution (0.66 uM) containing NaOH
or NaCl (20 mM) was selected as the supporting electrolyte. In addition, titanium plated
electrodes having dimensions equal to 35 x 175 x 1 mm, the distance between neighboring

electrodes was 25 mm, and contact area with electrolyte was 56 cm2. The different

radicals such as hydrogen atom (H’) and hydroxyl radical (OH'), generated by AC

electrolysis, react with TCB, while having different pathways of decomposition.

ll

2.2.4 Current Density

Sarahney and Alshawabkeh (2007) investigated the effect of current density on
electrolytic transformation of benzene for groundwater remediation. By using 1, 5, and
10 mA DC that were converted to the current densities of 1.8, 9.0, and 18.2 mA/L, it was
possible to indicate the amount of electric charge applied per unit volume of electrolyte

and show that degradation rate is dependent on the current density.

2.3 INDEX OF EFFICIENCY

There are various indices used by researchers to evaluate the effect of various
parameters related to the applied current. Based on the energy efficiency, the fate of the
technology for potential field application can be assessed. These efficiency indices are as

follows.

2.3.1 Instantaneous Current Efficiency (ICE)

The instantaneous current efficiency (ICE) of the electrolysis was calculated using

the following relation introduced by Comninellis and Pulgarin (1991):

= (COD, - CODHA,

ICE <%)
81At

 

)FV x 100 (2.7)

where (COD), and (COD),+A, are the chemical oxygen demands at time t and t+At

(g/L); respectively; I is the current (A); F is the Faraday constant; V is the volume of

electrolyte (L); and 8 is the equivalent mass of oxygen (g/eq).

12

2.3.2 Electrical Energy

As expressed in Eq. 2.8, electrical energy is consumed when current flows. This
parameters expressed in the units of joule (J), watt (W), and watt-hour (Whr) are directly
related with the operational cost and thus, will be a significant factor to determine both

the applicability and efficiency of electrochemical degradation.

Power = Current x Voltage (2.8)

2.3.3 Pollutant Breakdown Rate per Charge

According to Alshawabkeh and Sarahney (2005), pollutant breakdown rate as a
function of current density can be expressed as shown in Eq. 2.9:
AC / At

rate = . 2.9
J ( )

 

where “rate” is the degradation rate of the pollutant (mg/L-hr); AC is the decrease in

concentration (mg/L); At is the duration (hr); and j is the current density (mA/L or

mA/cmz).

2.3.4 Electrical Efficiency (Long Order Reduction)

EE/0= m (2.10)

Vxlog(C, /Cf)

 

where EE/O is electrical efficiency as per long order reduction (kWh/m3); P is lamp
power output, (kW); t is irradiation time (hours); V is reactor volume (m3); and C,- and Cf

is initial and final concentration (mg/L), respectively.

13

2.4 MASS TRANSFER PROCESS: THE NERNST-PLANCK EQUATION

Mass transfer is the migration of mass from one location in solution to another
and arises either from differences in electrical or chemical potential at the two locations
or due to the mechanical movement (e.g. stirring or mixing) of the solution (Bard et al.

2001). The modes of mass transfer are defined by:

II Migration: movement of a charged body under the influence of an electric
field (i.e. gradient of electrical potential);

' Diffusion: movement of a species under the influence of a gradient of
chemical potential (i.e. a concentration gradient); and

I Convection: stirring or hydrodynamic transport. Fluid flow is divided by
natural convection (convection caused by density gradient) and forced

convection.

Equation 2.1] is the Nemst-Planck equation that simulates mass transfer due to

migration, diffusion, and convection.

 

J(X)=-D,X()

6C1(x)_IIZFDCI—a———:(x)+Cvx() (2.11)
x

RT
where .l,-(x) is the total flux of specie 1' (mol 5'1 cm'z) at distance x from the electrode
surface; D,- is the diffusion coefficient (cm2 s"); a C,(x)/6x is the concentration gradient
at distance x; 5¢(x)/0x is the potential gradient; 2,1 and C) are the charge (dimensionless)
and concentration (mol cm'3) of species 1'; respectively; and v(x) is the velocity (cm s")

with which a volume element in solution moves along the axis.

14

The Hayduk-Laudie relationship (1974) (Eq. 2.12) was used to estimate the

diffusion coefficient for naphthalene and salicylic acid.

 

_13.26x10‘5
19- ,1, —0.539 (2.12)
77' xV
where D is molecular diffusion coefficient (cm2 s"); 77 is the solution viscosity in

centipoises (10'2 g/cm-s) at the temperature of interest; and V is the molar volume of the

chemical (cm3/mol).

15

CHAPTER THREE
EXPERIMENTAL METHODOLOGY

The overall experimental procedure is outlined in Figure 3.]. Each step is

described in detail in the following sections.

 

 

Name Description
Spiked a. Prepare 20 mg/L of naphthalene or salicylic
Solution acid.
Preparation b. Add 5 mg/L of Na2804 as supporting
electrolyte.

 

 

 

G

 

 

Reaction
Chamber
Preparation

 

 

a. Prepare reaction vessel containing
the pollutant sealed with Teflon cap.

b. Put reaction vessel in water bath.

 

@

 

 

 

 

 

 

 

 

 

 

 

 

 

Electrical a. Apply given current to the cell for given
Equipment time period.
Setup b. Measure some parameters such as
temperature, current, and voltage.
Sampling a. Take samples at specific elapsed times.
Ana? sis b. Achieve the quantified concentrations
V with the aid of an HPLC
c. Obtain the quantified concentrations from
the analysis.
Cleaning up a. Polish the electrodes used in the test.
Rec 8‘2“" b. Wash sand and put it into oven over 48
y 9 hours at temperature of 80 °C.

 

 

 

Figure 3.1: Schematic of testing procedure

16

 

 

 

 

 

3.1 EXPERIMENTAL SETUP
The experimental setup, as depicted in Figure 3.2, consists of reaction chamber,
electrical equipment, and analytical instruments. Each constituent is specified and

described below.

 

< Electrical Equipment >

Function Generator

    

 

< Reaction Chamber >

 

 

 

Figure 3.2: Representation of experimental setup

3.1.1 Reaction Chamber

The electrochemical degradation of naphthalene and salicylic acid was carried out
in a single compartment (i.e. undivided cell) made of Pyrex glass beakers having 1 L or 4
L volumes, separately. The cell was capped with a Teflon cap. These materials (glass and
Teflon) were selected to reduce or minimize sorption of contaminants onto to the walls of

the reaction vessel as well as the cap. A water bath was used to prevent the cell from

Joule heating and, the room temperature was maintained at around 22 0C. The picture of

the reaction chamber immersed in the water bath is shown in Figure 3.3.

Teflon Cap

4 L Beaker

Titanium
Electrode

 

Figure 3.3: Picture of a reaction chamber for the 3.5 L Cell

3.1 .1.1 Titanium Electrode

Titanium plate electrodes were used for this study. The titanium electrode used in
this study were not coated. Uncoatcd titanium is cheaper than gold, platinum, or titanium
coated with mixed metal oxides. Furthermore, titanium offers resistance to corrosion in
wide range of aggressive conditions. In addition, titanium anodes are more stable than
nickel, lead, zinc or mercury (Titanium Information Group 2002). Also, it is well known
that titanium is easily reduced from Ti4+ to Ti3+ and acts as a charge carrier when an
electric field is applied (Chen et al. 2003). For these reasons, it was thought that titanium

is better a choice of electrode for this project. The purity of the titanium used was about

18

99%. For a smaller cell, each titanium electrode was 12 cm x 5 cm and the resulting
area was 60 cm2. However, area of the electrode immersed in the electrolyte was about
55 cm2 and the spacing between adjacent electrodes was maintained at 8 cm. The
dimensions of titanium electrodes for the two cells are presented in Figure 3.4 and Table
3.1. In addition to the plate, titanium screws and nuts were used as the current collector
on both anode and cathode to minimize the reactivity of the current collectors and to

reduce the possibility of corrosion due to a material difference.

3.1.1.2 Cell Volume (1 L Cell vs. 3.5 L Cell)

As mentioned in Chapter 1, one of the key objectives was to explore the effect of
cell scale under same conditions except for the size of electrodes, the spacing between the
two neighboring electrodes, and the resulting electrolyte volume participated in
electrochemical reactions. To develop this, the Pyrex glass beakers having of 1 L and 4 L
volume were selected and the electrolyte solution or other media was filled to l L (or
1,000 cm3) and 3.5 L (or 3,500 cm3), respectively. In this study, the former and the latter
will be designated by l L cell and 3.5 L cell, respectively. These cells having different
sizes, as depicted in Figure 3.5, can be characterized by physical and electrical
parameters. The contact areas with electrolyte for the 1 L and 3.5 L cells are 55 and 178
cm2, respectively (Table 4.1). Based on this data and the spacing between the electrodes
(8 cm for 1 L cell, 9.5 cm for 3.5 L cell), the volume (areaxspacing) participated in the
electrochemical reactions was obtained and the resulting volumes for each cell were 440
and 1,691 cm3, respectively. This study started from the measures of degradation in the l
L cell so that a volumetric ratio of volume of the 3.5 L cell to the l L cell (~3.854) could

be used to determine the equivalent current density applied for the 3.5 L cell. To maintain

l9

the current density of 1 mA/cm2 for the 1 L cell, the magnitude of the current was 55 mA.
To maintain the current density of l mA/cm2 for the 3.5 L cell, the magnitude of the
current was 212 mA (~ 55 mA X 3.85). In a similar manner, applied current for various

jeqv can be computed and the computations are presented in Table 3.2 and 3.3.

20

 

mac couch Ea duo 3383mm £38820 52:3: 2: .«o ossEonom ”Ym Rama

=oo .5333. EU coco... ouoboofi

Q
LSEBQ 8Q

 

 

 

 

 

 

 

 

2

Table 3.1: Dimensions of electrodes, glass beaker, and Teflon cap

 

 

 

 

fl 1L Cell 3.5L Cell Unit
1. Electrode ..
Length (H a) 11 19.2 cm
Width (W) 5 10.2 cm
Thickness (B) 1.63 1.55 mm
Diameter of Titanium Screw (D 50,”) 0.5 0.5 cm
2. Glass Beaker
Depth of solution (H 1) 12.5 19.5 cm
Immersed depth of electrode (H 2) 11 17.5 cm
Diameter of Bottom (0 beaker) 11 16 cm
3. Teflon Cap
Diameter of Cap (D c,,,,,) 13 19.5 cm
Diameter of temperature hole (D temp.) 0.61 1.3 cm
Spacing between Electrodes (S) 8 9.5 cm

 

 

 

 

22

 

:11.

Figure 3.5: Pictures of cap attached to electrodes (top) and the two cells having 1 L and
3.5 L volume (bottom).

23

Table 3.2: Physical properties of l L and 3.5 L cells

 

 

Spacing Area Treated Volume
Cell Type (s, cm) 4A, cm’) (V, cm?)
1 L Cell 8 55 440
3.5 L Cell 9.5 178 1691

 

 

Table 3.3: Currents densities and equivalent currents for l L to 3.5 L cells

 

Current aspects for reaction cells

 

Current applied to electrodes ( i, mA)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cell Type j 9W = 1 mA/cm2 j eqv, =2 mA/cm2 j em) = 3 mA/cm2
1 L Cell 55 110 165
3.5 L Cell 212 424 636
Actual Current Density ( j am“ mAIcmz)
Cell Type j W, = 1 mA/cm2 1'qu- =2 mA/cm2 j ,qv, = 3 mA/cm2
1 L Cell 1 2 3
3.5 L Ce" 1.19 2.38 3.57
Actual Current Density ( hm“ mAIL)
Cell Type j W, = 1 mA/cm2 j m =2 mA/cm2 j em, = 3 mAlcm2
1 L Cell 125.00 250.00 375.00
3.5 L Ce" 125.37 250.74 376.11

 

 

 

24

3.1.2 Electrical Equipment

The electrical equipment consisted of power supply, function generator, and
oscilloscope (Figure 3.6). A 0 to 2 MHz sweep function generator was used to generate
square wave form of AC. A square wave form was selected to equalize between root-
mean-square voltage (VW) and peak voltage (VP). Theoretically, a square wave form
stays for half a cycle and shifts suddenly from negative to positive or vice versa. Thus,
current with square wave form AC delivers greater power, compared to a sinusoidal or a

triangular‘wave form of AC.

 

Figure 3.6: Pictures of electrical equipment used in the setup: power supply (top),
function generator (middle), and digital oscilloscope (bottom)

25

The power supply consisted of a bipolar operational power supply/amplifier
(Kepco) capable of producing up to 200 V AC/DC and 1 A AC/DC output or a maximum
electrical power of 200 W. Values of electrical parameters such as voltage, current, and

AC frequency were monitored by the aid of an oscilloscope (Agilent, 54621A).
3.1.3 Spiked Medium Preparations

3.1.3.1 Naphthalene in Aqueous Solution

Aqueous solutions, spiked with naphthalene, were prepared by adding
naphthalene (EMD Chemicals Inc., 98% purity) crystals to deionized (DI) water in a
Pyrex glass bottle and leaving it in a suspension for more than 72 hours. This was
followed by the addition of supporting electrolyte to increase electrical conductivity. In
this study, anhydrous Na2SO4 was used as the supporting electrolyte. The corresponding
concentration of NaZSO4 was 500 mg/L (~ 3.5 mM) and this was based upon the MCLG

of sulfate regulated by US EPA.

3.1.3.2 Salicylic Acid in Aqueous Solution

20 mg of salicylic acid purchased from EMD Chemicals Inc. (99% purity) was
added to a 1 L Pyrex glass bottle containing DI water in order to have concentration of 20
mg/L. The addition of the supporting electrolyte follows the same procedure as that used

for preparing the naphthalene solution.

3.1.3.3 Sand Spiked with Naphthalene or Salicylic Acid

For experiments with sandy soil spiked with naphthalene or salicylic acid, the

solution prepared in steps described above was poured in 1,000 cm3 (or 3,500 cm3) of dry

26

Ottawa sand in 1 L (or 4 L) Pyrex glass beaker until full saturation of the sand was
reached. It was presumed that the spiked sand was fully saturated when the surface of the

solution rose above the sand surface.

 

Figure 3.7: Pictures of materials used in spiked medium preparations

27

3.1.4 Stirring Apparatus

A magnetic stirrer was used to continuously stir the solutions in reaction vessels
(Fig. 3.8). The rate of the rotating magnetic bar was maintained at 1,200 rpm for both 1 L
and 3.5 L cells. Depending upon the size of the reaction cell, magnetic bar having an

appropriate length was used.

Q .

It

I

(T ..

Q __,
a; .
I an

i

 

3.5 L Cell

Figure 3.8: Pictures of stirrer with 3.5 L cell (top), stirrer (bottom, left), and magnetic
bars (bottom, right)

28

3.2 TESTING, SAMPLING, AND ANALYZING TECHNIQUES

3.2.1 Sampling Technique

3.2.1.1 Sampling Frequency

During tests, samples were taken for high performance liquid chromatography
(HPLC) analyses at time intervals equal to 0, 1, 2, 4, 8, and 24 hours for salicylic acid

tests and 0, l, 2, 4, 8, 24, and 48 hours for naphthalene tests.

3.2.1.2 Sampling Method

While the electrical current was applied, the test was not paused during sampling
and samples were collected using syringes (Hamilton Co.) equipped with spiral needles
(Popper & Sons, Inc.) having length appropriate for the size of the cell. After each
sampling event, it was injected into amber vials (Supelco Co.) capped with a membrane
(Supelco Co.), and then directly kept in a refrigerator for HPLC analyses. Needles with
different lengths for different cells were determined and customized in order to take
samples in the center of the volume that participated in the electrochemical degradation
between the two electrodes. Syringes were cleaned with acetone and DI water before any
sampling event. Pictures of syringes equipped with needles are shown in Figure 3.9. In
aqueous phase tests, only one sample was taken in the center between electrodes. Pepprah
and Khire (2008) have shown that the solution is well-mixed within the treated volume
between electrodes and observed negligible difference in concentrations across the cell.
Unlike aqueous phase tests, samples were collected three times at different three
positions in the soil phase tests: one in the center between the electrodes, and one each in

the vicinity of the two electrodes.

29

.. 5. . .,.
li’fl,‘ “a“, of in are
' a ~

,. 3.1!

3.5 L Cell

.61.

 

  

_ . ' -~. ' l. . '» in.) .. “:1 ’m:'hh“‘-“‘~’ 'r ‘

Figure 3.9: Pictures of syringes equipped with needles of different lengths (top) and
amber vials with caps used for HPLC analyses (bottom)

30

3.2.2 High Performance Liquid Chromatography (HPLC) Analysis

 

Figure 3.10: Pictures of HPLC instrument

Concentrations of naphthalene and salicylic acid were measured using high
performance liquid chromatography (HPLC). A Perkin-Elmer® system equipped with a
tunable UV detector (detection at 254 nm), a Waters C-18 PAH, 250 mmx4.6 mmx 5 pm
column, an automated gradient controller, and deionized (DI) water/ acetonitrile mobile
phase. The flow rate of the mobile phase was 1.2 mL/min and the column was flushed
with 10:90 (v/v) acetonitrile/ DI water for 5 minutes followed by 60:40 (v/v) acetonitrile/
DI water for 10 minutes and by 100:0 (v/v) acetonitrile/ DI water for 5 minutes. This

gradient was used for the analysis of naphthalene and salicylic acid in order to obtain

31

appropriate separation of the degradation product peaks on the chromatograms during the

tests. These instruments can be seen in Figure 3.10.

Table 3.4: Preparation of standard solutions for naphthalene and salicylic acid

 

 

Volume of 1000 mgIL Volume of Concentration in
of Stock Solution Acetonitrile' Acetonitrile Solution
(11L) (mL) (mg/L)
10 4 2.49
30 4 7.44
50 4 12.35
70 4 17.2
100 4 24.39
150 4 36.14

 

 

 

* Note: DI water was used to make standard solutions for salicylic acid.

As shown in Table 3.4 above, standard solutions were prepared to bracket the
expected concentration level of the analytes. The identification of naphthalene and
salicylic acid was achieved by comparison of retention times to those of the respective
standards. R2 values in the resulting calibration curves were observed by the range of

0.998 to 0.999.

3.2.3 Duplicate Sampling

Most of the experiments were repeated to confirm changes in concentrations of
naphthalene and salicylic acid existing in aqueous or sandy soil phase at a given elapsed
time. When an experiment was repeated, total four analyses were conducted for each

sampling round by HPLC because each sample is analyzed twice by the HPLC.

32

3.3 MONITORED EXPERIMENTAL PARAMETERS

Distinct from taking samples from the cells and obtaining concentrations of
naphthalene and salicylic acid, other experimental parameters were also monitored. These
parameters included temperature, pH, electrical conductivity, and standard redox
potential monitored at given initial and final time elapsed. A portable dissolved oxygen/
pH meter, equipped with a Platinum Series pH electrode was used to measure pH of the
solution. A portable electrical conductivity meter (Oakton, Con 5 acon series) and a
microprocessor thermometer (Omega, model HH21) were used to measure electrical
conductivity and temperature within the cell, respectively. These measuring devices are

displayed in Figure 3.11.

33

 

        

a‘fl

Figure 3.11: Pictures of measuring devices

.1 --

34

3.4 RECYCLING SOILS AND ELECTRODES

3.4.1 Soil Recycling

Soil washing is a water-based process that uses a combination of particle size
separation and aqueous chemical separation to reduce contaminant concentrations in soil
(US EPA 2007). Soil washing is effective on homogeneous, relatively simple
contaminant mixtures in terms of its applicability and effectiveness. For the sandy soil
phase experiments, Ottawa sand was used as a single media spiked with naphthalene or
salicylic acid and hence, this method could be suitable for contaminant removal and soil
recycling.

This process is specifically described below and illustrated in Figure 3.12:

0 Step 1: clean the container using acetonitrile and DI water.

0 Step 2: dump used sand into the container after the test is finished.

0 Step 3: fill DI water in the container.

0 Step 4: stir the “contaminated” sand and solution by the means of a steel rod
gently and repeat this step for 4 times.

0 Step 5: transfer washed sand to an aluminum tray.

0 Step 6: put the tray containing sand and water into a hot air oven maintained at a

temperature of 80 0C for over 48 hours.

In order to verify the effectiveness of this method, the HPLC analysis was
followed by taking samples after each washing event covering Step 3 and 4 and the

spiked chemicals were not detected in the washed Ottawa sand.

35

 

Figure 3.12: Pictures of procedure followed for cleaning and recycling used sand

36

3.4.2 Electrode Recycling

The surface texture of the electrode and how clean the surface is plays an
important role in the rate of reactions that occur at the electrode surface (Pepprah 2007).
Instead of using new electrode for each test, in order to lower the cost of the project,
electrodes were re-used by cleaning and polishing after each test.

The picture of how the electrode was polished is presented in Fig. 3.13. A metal
brush grinder operated pneumatically was used to clean the surface until the original

electrode surface was restored.

 

Figure 3.13: Picture while polishing an electrode to clean the fouled electrode surface

37

CHAPTER 4
RESULTS AND DISCUSSION

The results of the electrochemical degradation experiments carried on spiked
solutions of naphthalene and salicylic acid in an aqueous solution and in saturated sandy

soil are presented in this chapter.

4.1 AQUEOUS PHASE EXPERIMENTS

The aqueous phase experiments carried out are summarized in Table 4.1

Table 4.1: Summary aqueous phase experiments

Currant Density
Equivalent
Current Applied Current
Supporting Current Cumnt

NazSO4 Control
N02804 AC
M2804 AC
Nn2804 AC
M2804 DC
DC

Solution Control
Solution Control
Solution AC
Solution DC
Solution DC
SquIIon DC

 

38

4.1.1 Naphthalene

Figure 4.1 shows the average normalized concentration (C/Co) of naphthalene
versus elapsed time for the test cells where DC and AC (peak) current densities of 1 and
3 mA/cm2 was applied. Data for control cell is also presented for which no current was
passed. All experiments were carried out with NaZSO4 as the supporting electrolyte. As
presented in Chapter 3, equivalent current density (jeqv) of l mA/cm2 converts to 55 and
212 mA current applied to the 1 L and 3.5 L cells, respectively. jeq, of 3 mA/cm2 converts
to 165 and 636 mA current applied to the 1 L and 3.5 L cells, respectively. The error bars
shown in all plots presented in this chapter represent the maximum and minimum values

of C/Co obtained from all tests, including the replicate tests.

4.1.1.1 Effect of Current Density on Naphthalene Degradation

When DC having jeqv of 1 mA/cm2 was passed through the 1 L and 3.5 L cells,
about 60% decrease in naphthalene concentration was observed within 24 hours.
Whereas, when DC having jeq, of 3 mA/cm2 was passed, about 75% reduction in the
concentration of naphthalene was observed within 8 hours for the 1 L cell. When DC
having jeqv of 3 mA/cm2 was applied to the 3.5 L cell, heating was excessive. Hence, due
to significant volatilization potential of naphthalene, the experiment was terminated.
Instead, similar test was done with salicylic acid and the results are presented in
subsequent sections of this chapter. The control cell showed relatively small loss in
naphthalene, which is believed to be due to volatilization of naphthalene that can occur at

room temperature.

39

(a) DC 8. control

' I' j Tr I I I I V I fi" V U I V V V f1 f. '

 

1.2

  
   
  
 

 

ENconmiw‘
:-Co-18~ 22mglL _
g -Supporting Electrolyte: Na “30 .
E - Line: Bold (3. 5L cell), Thin (12L cell)

Normalized Naphthalene Concentration (C/Co)
i

 

 

 

 

Elapsed Time (hrs)

(b) AC (r= o. 1 Hz) a. control

I I I' I I' T I V I I I I! W V

 

. .
08 . a 2 I
......................
. I . .
. .
. , oqv. .
. .
.
03 ................................... . ..... _
. l a l l

0.4

 

' é ' .. '2 .
-Co- 1a~ ~22 mgIL 13w. 3mAIcm' ............ ..
~ ~Supportlng Electrolyte: Na “80 g g \93 -

- Linel: Bold (3.5L cell) Thin (1L cell); § .
o a A n a 1 j J L A a A a

0 10 2.0 30 40 50

h
h
i-
0.2 -

Normalized Naphthalene Concentration (C/Co)

 

 

 

Elapsed Time (hrs)

Figure 4.1: Normalized concentration of naphthalene in aqueous phase experiments for
DC (a); and AC (f= 0. 1 Hz) (b) performed at 1qu of 1 and 3 mA/cm2 in 1 L and 3. 5 L
cells

40

All other parameters being constant, for higher current density (jeqv, = 3 mA/cmz),
the observed degradation rate was greater than that for the lower current density (1'qu = l
mA/cmz) regardless of the size of the cell. This finding is consistent with the results of
Alshawabkeh and Sarahney (2005) and Pepprah and Khire (2008). For these tests, it was
presumed that the electrode fouling did not happen at the electrodes for the time duration
of 48 hours or it occurred at the same rate for various current densities. However, higher
current densities would not guarantee a high current efficiency. Rodgers et al. (1999)
reported that the current efficiency was consistently higher at low current densities
because the positive polarization of the anode increases with current density and hence a

greater fraction of the applied current is wasted in the electrolysis of water.

4.1.1.2 Effect of Current Type (AC vs. DC) on Naphthalene Degradation

In the DC application for jeqv equal to l mA/cm2 in the 1 L and 3.5 L cells, almost
90% reduction in naphthalene concentration was observed within 24 hours in both cells.
However, only 45% reduction in the concentration was observed when AC (f = 0.1 Hz)
having an equivalent current density equal to l mA/cm2 was applied. Pepprah and Khire
(2008) have attributed this due to the reversal in current direction during AC causing
delay in mass transfer of naphthalene to the anode where it is oxidized. This results in a
slower rate of degradation even if both electrodes (referred to as instantaneous anodes)

participate in the oxidation when AC is used.

4.1.1.3 Effect of Size of Reaction Cells on Naphthalene Degradation

While the current passed through the l L and 3.5 L cells was different (55 mA for

l L cell and 212 mA for 3.5 L cell for jeq. equal to 1 mA/cmz), the shape and the rate of

41

degradation are very close for the two cells having different sizes for both AC and DC
applications (compare bold and thin lines in Fig. 4.1). However, the rate of degradation
was slightly greater for the 3.5 L cell. These results indicate that the size of the cell plays
a relatively minor role for controlling the rate of degradation as long as the equivalent

current densities are the same.

4.1.1.4 Degradation Kinetics

The degradation rate of naphthalene in aqueous solution during the
electrochemical test could be described as a pseudo-first-order reaction (decay) that

follows Equation 4.1.

dC
—=—k C
dt [ ] (4.1)

where dC/dt is the rate of change of concentration with time; C is the concentration of

naphthalene (mg/L); and k is the pseudo-first-order degradation rate constant (hr 'l).

ln(2) 0.693
tl/2= k = k

 

(4.2)

A plot of ln[C/Co] yields k based on Eq. 4.2. The observed pseudo-first-order rate
constant (k), the R2 value, and the corresponding half-life (ti/2) obtained from Equation
4.2 for the initial 24 hours for all experiments plotted in Fig. 4.1 are presented in Table
4.2 and plotted in Fig. 4.2.

Table 4.2 and Fig. 4.2 show that k increases and m; decreases when the equivalent

current density is increased from 1 to 3 mA/cm2 for the l L and 3.5 L cells. Table 4.2 also

42

shows that the size of the cell has minor influence on the k and rm values for a given

current density.

Table 4.2: Degradation rate constant (k) and the corresponding half-life (rm) for
naphthalene tests in aqueous solution (time period = 24 hours)

 

 

 

 

 

 

 

 

  

 

 

 

1L cell 3.5L cell
Current jaw. k t 172 k t 1/2
Type (mA/cmz) (hr'l) R’ (hr) (hr‘) R2 (hr)
DC 1 0.117 0.997 5.9 0.114 0.997 6.1
3 0.126 0.939 5.5 - -
AC 1 0.023 0.978 29.9 0.024 0.964 29.0
(f = 0.1112) 3 0.042 0.993 16.5 - -
go.6b....,.r.r, ...r.
E --Co-18~22mgIL 1Loe|| ~
‘5 j -Supporting Electrolyte: Naaso4 3-5 L99" 2
g 0.5 ~ -
r: " .
O " .i
0 at
3 4 Z. .‘
é o. _ ,
c .
O P .i
‘3 ’ 1
1, 0.3 - _
e r 1
O " u
8 . .
3 0.2 :- _
1‘3 ; AC (r- 0.1 Hz) DC ‘
“E 0'1 L , AC(f 01112“:
'g - AC(f=0.1Hz) 7 H ' )3
5 o E ear/a W

 

 

 

      

] 8 1 mAlem2 1 I 3 mAIcm2 j I 1 mAIcm2 j =- 3 mA/cmz

Testing Condition

Figure 4.2: Estimated k from electrochemical degradation of naphthalene in aqueous

solution

43

4.1.1.5 Cumulative Electrical Energy Consumption

Figure 4.3 shows the cumulative electrical energy consumed during the
electrochemical degradation of naphthalene for the AC and DC tests when jem. of 1
mA/cm2 was applied. During the application of DC, about 835 and 1,110 kaL of
electrical energy was consumed in the l L and 3.5 L cells during the test duration of 48
hours, respectively. When AC was used (f = 0.1 Hz), about 210 and 320 kJ/L of electrical
energy was consumed in the 1 L and 3.5 L cells during the test duration of 48 hours,
respectively. While the power consumption for the AC tests was less than that for the DC
tests, AC resulted in about 65% reduction in the concentration and DC resulted in about
95% reduction. The normalized power consumption for the 3.5 L cell was about 30% and
50% more than the 1 L cell for DC and AC application, respectively. The power
consumption for the larger cell was more because the resistance (DC application) and
impedance (AC application) of the larger cell was greater than the smaller cell. Hence, it
required greater current which resulted in greater power consumption.

The 2008 cost for electricity generation is about U.S.$ 0.065/kW-hr (source:
Energy Information Administration). Table 4.3 presents the electricity cost (based on:
electricity generation only) for the 1 L and 3.5 L experiments carried out in this project.
The volume of liquid that was between the two electrodes in each of the two cells (440
cm3 for the 1 L cell and 1,691 cm3 for the 3.5 L cell) was used for the cost calculation.
This volume was believed to have participated in the electrochemical reactions. Table 4.3
shows that the electricity consumption cost increased five times when the cell size

increased by about four times.

44

 

 

 

 

1200 . .
Q [We 1mA/cm2: 55 mA (1L Cell), 212 mA (3 5L Cell) / :
2 -Co= 18~ 22 mg/L ; oc -
§ 1000 :- Supporting ElectrolytezNaZSO‘ """"" j
g i- Line Bold (3 5L cell) Thin (1L cell) / -
in '- _Point: Diamond Control Circle DC S uare AC '

5 800 ( .......... )’ ......... (m).':...q ....... () ............ p
o t : DC .
>9 " - I
9 _ :1
g 600 .. .............. -
m h d
.2 i i .
‘8' l .......... i ._ '
m 4°” , ' ' 2' .
'43 . AC (f=01 Hz) .
""'" 200 — ---------------- .. .................................................... -
g - ; AC (f 0.1 Hz) .
o ' s 1 I ‘
' ,/.' 3 AC f=60H —121-

o ‘ui fig”!_ 1 a n n .L n n g n n .2): a
O 10 20 30 40 50

Elapsed Time (hrs)

Figure 4. 3: Cumulative normalized electric energy consumed during electrochemical
degradation of naphthalene in aqueous phase experiments for DC and AC (f= 0. 1 Hz)
performed at jeqv of 1 mA/cm2 in 1 L and 3. 5 L cells

Table 4.3: Com arison of electric energy consumption costs for l L and 3.5 L cells

 

 

 

 

Equivalent Cumulative Electric Energy
Ac Current ( at t = 48 hrs )
”6335“” (22:11, 13:; ...... .... .1... .3215, £28171":
AC 0.1 1 1 L 211.35 92995.65 25.85 1.66 5.8
3.5 L 317.95 537661.89 149.47 9.72
DC 0 1 1 L 836.01 367844.40 102.26 6.65 5.1
3.5 L 1107.58 187291667 520.67 33.84

 

 

 

 

 

 

 

 

 

Note: The end-use prices of electricity in industrial sector are 6.5 cents per kilowatt-hour.
The value of 0.000278 was used to convert the units from kilo joule to kilowatt-hour.

45

4.1.1.6 Measured Initial and Final Test Parameters

Table 4.4 summarizes the measured initial and final values of temperature, pH,

standard redox potential, and electrical conductivity of the solutions in both the control

and the test cells. These parameters are discussed below.

Temperature

A water bath was used to eliminate the degradation or volatilization of chemical
compounds from Joule heating due to the electricity. All cells (including the
control cell) were placed in a water bath. Heat generated in the test cells due to
Joule heating was removed by the water bath and hence temperature within the
cells stayed in the range of 19 to 23 0C. At the final measurement (t = 48 hr),
temperatures observed in the 3.5 L cell was higher than in the l L cell due to the
application of approximately four-times greater current. The temperature in the
3.5 L cell was similarly controlled with the l L cell by using a relatively large

water bath where the 4 L glass beaker was immersed to the top.

pH

The initial pH values ranges from 5.6 to 6.5. The final pH values measured after
the test period (t = 48 hr) ranged from 3.8 to 4.5 in the AC test cells and from 6.5
to 11.5 in the DC test cells, respectively. From the water electrolysis, the
oxidation of water molecules produces I~l+ at the anode while the reduction of
water molecules generates OH’ ions at the cathode. In addition, formation of
intermediate byproducts containing acidic substances may cause possible source

of H’r ions in the solution. For the AC tests, full degradations of naphthalene were

46

not reached at the final elapsed time. Consequently, I-F ions from the oxidation of
water molecule was probably more prominent in the AC tests and made the pH
values decrease. In contrast, the DC tests show an increase in pH values as the test
was completed (48 hr). In other words, indicating that the reduction of water

molecules was more prominent as the test progressed.

Redox Potential
The standard redox potential values measured at the beginning of the experiment
for all AC test cell solutions were generally higher than those at the final elapsed

time.

Electrical Conductivity
The electrical conductivity values measured at the initial elapsed time are similar
to those measured at the final elapsed time except for the DC tests. For the DC

tests, there was a slight increase in the electrical conductivity.

47

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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48

4.1.2 Salicylic Acid

Figure 4.4 shows the average normalized concentration (C/Co) of salicylic acid
versus elapsed time for the experiments where salicylic acid was used as the target
chemical for the experiments with l L and 3.5 L cells. The current densities used for AC
and DC applications were 1 and 3 mA/cm2 which is consistent with the naphthalene
experiments. NaZSO4 was used as the supporting electrolyte similar to the naphthalene

experiments.

4.1.2.] Effect of Current Density on Salicylic Acid Degradation

When the 1 L and 3.5 L cells were subjected to jeqv of l mA/cm2 and 3 mA/cm2 in
DC application, the rate of degradation for the 3.5 L cell was more for higher current
density application (Fig. 4.4a). However, for the 1 L cell, the increase in current density
resulted in a greater increase in the rate of degradation only during the first 8 hours of the
experiment. This is because the concentration reduced by 80% in the first hour and being

a first order decay, the rate slowed down after 8 hrs.

4.1.2.2 Effect of Current Type (AC vs. DC) on Salicylic Acid Degradation

The I L and 3.5 L cells were subjected to jeqv of 3 mA/cm2 (636 mA) with DC and
AC (f = 0.1 Hz, square wave) applications. The rate of degradation for the DC experiment
was much higher (about 3 fold) than that when AC was used. This finding is consistent
with the results of Pepprah and Khire (2008) and the results for naphthalene presented in

previous sections.

49

(a) DC 8. control

 

1.2 I r I I FI r I I I I I I I ! I r 1 I I I I I I

Normalized Salicylic Acid Concentration (C/Co)

 

 

n u
I I I I I I L J I I L I L I

0 5 10 16 20 25

 

Elapsed Time (hrs)

 

(b) AC (f = 0.1 Hz) & control

1.2 III I l I I I Ij I I r T r I I I I I I I

 

   

Normalized Salicylic Acid Concentration (C/Co)

 

 

 

I3mAlcm2
04 - ................ ............ ................. ................. ............
j ; -Co-19~'21mg/L j
03 L ....... . ...... ................. f..-Supportlng ElectrolytezflaZSO4 ..
Z -Line:Bold (3.5L cell), Thin (1L cell) 1
0H...1....i....i....i....*
o 5 1o 15 20 25

Elapsed Time (hrs)

Figure 4.4: Normalized concentration of salicylic acid in aqueous phase experiments for
DC (a) and AC (f = 0.1 Hz) (b) performed at jeqvcf l and 3 mA/cm2 in the l L and 3.5 L
cells

50

4. l .2.3 Effect of Scale of Cells on Salicylic Acid Degradation

For the DC application when jeqv, Equal to 1 and 3 mA/cm2 were applied, the rates
of degradation for the 1 L cell was higher compared to the 3.5 L cell during the first 8
hours of the experiments. However, the rates were about the same after 8 hrs. For the AC
application, for jeqv. equal to 3 mA/cmz, the rate of degradation for both cells were about
the same. While the current densities were the same for these cells, the ratio of surface
area of the electrode in contact with the electrolytes to the volume of the electrolyte was
higher for the I L cell. All reactions primarily occur at the electrode surface. This may be
the reason why in DC tests the smaller cell showed faster rate of degradation. During AC
tests, the surface area ratio does not dominate because the mass transfer to the electrodes

is slowed due to the constant change in the direction of the current.

4.1.2.4 Degradation Kinetics

The plots of ln(C/C0) versus time yield the pseudo-first—order reaction (decay)
similar to that for naphthalene tests discussed previously. The degradation rate constant
(k) of salicylic acid and the corresponding half-life (1)/2) in the electrochemical
application could be calculated using Equations 4.1 and 4.2, respectively. The values of k,

R2, and 11/2 obtained for the tests are summarized in Table 4.5 and plotted in Fig. 4.5.

51

Table 4.5: Degradation rate constant (k) and the corresponding half-life (mg) for salicylic
acid tests in aqueous solution (time period = 24 hours)

 

 

 

 

 

 

 

 

    

 

      

1L cell 3.5L cell
Current j oqv. k t m k t 1/2
Type (mA/cmz) 3hr“) R2 (hr) hr"1 R2 hr
DC 1 0.148 0.966 4.7 0.122 0.995 5.7
3 0.216 0.998 3.2 0.167 0.996 4.1
AC 3 0.033 0.989 20.9 0.031 0.990 22.4
E 0.6 I I l l I I I l l I I I I I I r I I I I l I I I I I I I I I -
7- --Co=19~21mg/L % 1L -
v . . cell .
*5 _ - Supporting Electrolyte : Na SO I 3 5 L cell ,
:3 0.5 .. 2 ‘ ' -
W b I
C
o r .
o - I
d) ' .
‘5 0.4 - -
a: ' .
C ' .
o - .
a: - -
3 0.3 - _
E ' '
U) : :
8 I I
‘ 0.2 ..
O
2 E :
2 - .
a F u
'3 0-1 . .
b - I
= . .
3'1 ' -1
O. 0

DC, 1“” =1 mA./cm2 DC, jaw = 3 mA/cm2 AC, jm = 3 mAlcm

Testing Condition

2

Figure 4.5: Variation of k observed for salicylic acid tests in aqueous solution

52

4.1.2.5 Cumulative Electrical Energy Consumption

The normalized cumulative electric energy (energy per unit volume of the
electrolyte) consumed during the degradation of salicylic acid in aqueous solution is
presented in Figure 4.6. For all current densities, the normalized electrical energy
consumption for the 3.5 L cell was much greater than that for the l L cell. These results

are similar to those when naphthalene was used as the target contaminant.

 

350° ' I I I I I 1 I I I I I ! I I I I l I I
- Co=19~"21 mglL : I

3000 -Supporting Electrolyte: Na 2SO

- Line: Bold (3. 5L cell), Thin (1 L cell)
- Point: Square (DC), Circle (AC) '

 

Cumulative Electric Energy Consumed (kJIL)

 

 

 

Elapsed Time (hrs)

Figure 4. 6: Normalized cumulative electrical energy consumed during electrochemical
degradation of salicylic acid in aqueous phase experiments for 1qu of l and 3 mA/cm2 in
1L and 3. 5 L cells

53

4.1.3 Effect of Stirring During Electrochemical Degradation of Salicylic Acid

According to the Nemst-Planck equation (Eq. 2.11), mechanical stirring of
electrolyte solution increases the mass flux of the target specie towards the electrodes.
The increase in the flux will aid in increasing the rate of degradation (Pepprah 2007). It is
due to convection. The effect of stirring was investigated because such data could be
potentially used for sites where groundwater flow velocities can aid in transporting the
chemical specie to the electrode(s) or where stirring can be used to decrease the current

density to achieve an acceptable rate of degradation.

4.1 3.] Degradation Kinetics

To evaluate the effect of stirring, the pseudo-first-order reaction (decay) equation
was applied to the In[C/Co] versus time plots for all tests where the electrolyte was stirred
or not stirred. The resulting degradation rate constants (k) and the resulting half-lives
(ti/2) for both cells are summarized in Table 4.6 and plotted in Fig. 4.7. For the 1 L cell, at
jag... of 1 mA/cm2 when DC was used, it yields k equal to 0.216 hr'l when the electrolyte
was not stirred. When the electrolyte was stirred, k for the same cell increased to 0.458
hr". It is more than two-fold increase. For the 3.5 L cell with DC, the increase in k was

about three-fold and about four-fold for AC.

54

Table 4.6: Degradation rate constant (k) and the corresponding half-life (II/2) for salicylic
acid tests in stirring aqueous solution (time period = 24 hours)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1L cell 3.5L cell
Current joqv. k t 112 k t 10
Type (mA/cmz) (hr'I) F"?z (hr) (hr'1) R2 (hr) Remark
0c 3 0.216 0.998 3.2 0.167 0.996 4.1 '
3 0.458 0.970 1.5 0.562 0.976 1.2 stirred
AC 3 0.033 0.989 20.9 0.031 0.990 22.4
(f = MHZ) 3 - - - 0.137 0.978 5.0 stirred
E 0.6 I I I I l I I I I I r I I I T I I I r
E ~Co=19~21mglL ll . -
g, . . [E Unstlrred .
g _- Supporting Electrolyte . NaZSO4 I Stlrred ‘
3 0.5 - .1
2 - «
3 : l 1
3 0 4 ' i
a e - -
0: '- -
C " .
.2 ' -
‘6 ' ‘
'U 0.3 - .1
E: l' 1
a 1 E -
8 0.2 - _
E I [E I
2 _ . .
E ' 1
kg 0.1 " j
'o - .
3 3 1 L cell 3.5 L cell [g 3.5 1. cell ;
& o I I I I I I J I I I I I I I I I I I I

C, = ' = =
D 1”“ 3 DC, jw. 3 AC, j.“ 3
Testing Condition

Figure 4.7: Variations of k observed for salicylic acid tests in aqueous solution with
stirred or without stirred

55

4.1.3.2 Effect of Stirring on Degradation Rate of Salicylic Acid

Figure 4.8 shows the normalized concentration (C/Co) of salicylic acid during
tests when the electrolyte was continuously stirred using a magnetic stirrer. Both AC and
DC applications were tested at jeqv, Equal to 3 mA/cmz. The ratio of C/Co with stirring to

C/Co without stirring ranged from 3.2 for DC to 5 for AC at 8 hours of elapsed time.

Pepprah and Khire (2008) have shown that Hydroxyl radicals (OH °) which are produced
at the anode (or instantaneous anode for AC) are extremely potent oxidizing agents with a
relatively short life, which is able to oxidize organic compounds by hydrogen abstraction
reaction or by redox reaction as shown in Equation 4.3 and 4.4, respectively (Oturan
2000)

RH+OH° —->R° +H20 (4.3)

OH' +RX—+Rx+' +OH‘ (4.4)

In DC electrolysis, a cathode acts as a passive electrode, whereas in AC
electrolysis, both electrodes participate in the reactions, switching their role as an anode
and a cathode to each other. Hydroxyl radicals that act as a strong oxidant are produced
in both electrodes when AC is used as a current mode and thus, stirring solutions
contaminated with salicylic acid has a stronger effect in the mode of AC in the
electrochemical degradation because both electrodes are able to react with the chemical

more efficiently.

56

(a) DC & control

I I I I

 

1.2

I T T I I j I I I I I I I I I I I I I

1m = 3 mAIcmz : 155 mA (1L cell), 535 mA (3.5 L cell)

     
   
   

--------- Control -—-—-O

 

- Co= 19 ~ 21 mgIL

- Supporting Electrolyte : NaZSO4

- Line : Bold (3.5L cell), Thin (1L cell)
Plain (Stirred), Dashed (Unstirred)

  

Normalized Salicylic Acid Concentration (C/Co)

LJIIIIIIIIIIIIIII+IIIII

 

 

 

 

Elapsed Time (hrs)

(b) Ac 0 = 0.1 Hz)& control

I I I I I I II I I I I I I I I I I I I I I r I I

m = 3 mA/cm’ : 155 mA (1L cell), 535 mA (3.5 L cell)

 

1.2

    
   
  
  

 

1 - “44 - Control ._._._..
0.5
0.6 Unstlrredfin
- 19~21 IL .“'---.
Co = mg ‘2}

- Supporting Electrolyte : llIaZSO4

ILIIIILlIIIIIkIlIIIIIII

Normalized Salicylic Acid Concentration (C/Co)

 

 

 

0.4 -

. - Line : Bold (3.5L cell), Thin (1L cell)

' Plain (Stirred). Dashed (Unstirred)
0.2 -

- Stirred
o'.l.41....lrz,.lm

0 5 10 15 20 25

Elapsed Time (hrs)

Figure 4.8: Normalized concentration of salicylic acid in a ueous phase experiments for
DC (a) and AC (f = 0.1 Hz) (b) performed at jeqv.= 3 mA/cm in l L and 3.5 L cells when
stirred or unstirred

57

4.1.3.3 Cumulative Electrical Energy Consumption

Line : Bold (3.5L cell), Thin (1 L cell) / Plain (Stirred), Dashed (Unstirred)
Point : Diamond (Control), Circle (DC), Square (AC)

40m 7 I I I I l I I I I I I I I I I I I I I l I II I I
1W= 3 mA/cm’: 535mA (3.5L Cell)

 

3500
g: = 130:121E'lgglti-olyte - Na so Dc’ Stirred a"
3000 pm 9 ' 2 4 . -

I
I
I

2500 * \

DC, Unstirred

IIIIlIIII

IlLIII

Cumulative Electrical Energy Consumed (kJIL)

 

 

2000 '5
150° AC (0.1Hz), Stirred E
'.I
1000 .O"" r 1
500 ' AC (0.1Hz), Unstirred 1
0 r 7:? . l . . . . l . . . . l . . . . l l

0 5 10 15 20 25

Elapsed Time (hrs)

Figure 4.9: Cumulative electrical energy consumed during electrochemical degradation of
salicylic acid in aqueous phase experiments for DC and AC (f: 0.1 Hz) performed at jeqv,
= 3 mA/cm2 in 1 L and 3.5 L cells when stirred or unstirred

Figure 4.9 shows the cumulative electrical energy consumed during the
electrochemical degradation of salicylic acid when the electrolyte was stirred or unstirred.
Focusing on the electrical energy measured at the final elapsed time (24 hours), the gap

between tests with a stirrer and without a stirrer is approximately 100 kJ per unit volume

(one liter) regardless of AC or DC current modes. This difference is relatively small

58

4.1.3.4 Measured Initial and Final Test Parameters

Table 4.7 summarizes the measured initial and final values of key parameters -
temperature, pH, standard redox potential, and electrical conductivity of the solution in

the control and test cells.

59

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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60

4.2 SANDY SOIL PHASE EXPERIMENTS

Table 4.8 outlines the summary of experimental variables used in Ottawa sand

spiked with naphthalene or salicylic acid solution.

Table 4.8: Summary of experimental variables for sandy soil phase experiments

 

 

Comgund 7urn T7ll|3
Naphthalene Sand
Naphthalene: Sand
Naphthalene 1 Sand
:‘a'ml 1e!"-
Salicylic Acid

1' .' '
~( .‘ L

’11

    
     
   
 
 
 

 

 

 

 

Equivalent
AC Current
Supporting Current risqucllyy Density;
777Elect|7e77 7T7ge (Hz) 7mAlcm2
Control
AC 0.1
DC 0

v 4 ‘i‘! I l‘ M‘
4.5%: u.-.) NW" “:1.-

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Applied 73""9m Number
Current ensltyz
7 mA/cmZ) mA/7L Stirrln Tests
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1:”; $777 1: ,‘f m7". _7
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Totsl number of tests: 9

Current Density

 

4.2.1 Naphthalene

4.2.1.1 Rate of Degradation

Figure 4.10 shows the normalized concentration (C/Co) of naphthalene observed

for experiments with 3.5 L cell with or without Ottawa sand, performed using DC

application (fem. =

l mA/cmz). The results from control experiments with and without

sand (aqueous solution) are also presented in Fig. 4.10. For the tests with sandy media,

samples were collected from the center of the cell as well as from points close to the

anode and cathode to evaluate the spatial changes in concentration under the influence of

applied current. For samples without sand, the concentration of naphthalene at the center

of the cell was always less than when sand was present. For the sand sample, near anode,

61

due to oxidation occurring at the anode surface, the concentration of naphthalene was
always less than that at the center or near the cathode. Thus, the presence of sandy soil
slows down the rate of migration of naphthalene molecules to the anode where they are

converted.

- Current Type : DC 8. Control ' CO = 17 "’ 21 mglL
_ Cell Volume : 375 L - Supporting Electrolyte . Naaso4

- Line : Plain (Sand), Dashed (Solution)

 

 

 

 

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8 " .1
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S L _ 1
2 0.4 I g -
I- \ q
E )- ‘\‘ r d
R - s. Sand (Anode) <
E 0.2 7" ‘\‘ '1
O 1- = 2 ‘s
z 7 [m7 1mAlcm (212 mA) Tin-“Solution. 7
o n n s n I L L n n J ; L J n l 1 1 1 n. fi-1-:-.‘ I
0 10 20 30 4O 50

Elapsed Time (hrs)

Figure 4.10: Normalized concentration of naphthalene for aqueous phase and sandy soil
phase experiments for DC performed at jeqv,= l mA/cm2 (212 mA) for 3.5 L cell

62

4.2.1.2 Cumulative Electrical Energy Consumption

Compared with the aqueous phase experiment, the one with soil phase showed a
nearly 50% increase in the cumulative electrical energy consumed during the 48 hour
experiment as illustrated in Figure 4.11. The primary reason for increase in the electrical
energy consumption in the presence of sand is increase in resistance due to the presence
of sand displacing the electrolyte. Hence, for a given voltage, the experiments with sand
required greater voltage. A greater amount of electrical energy was also used for Joule

heating when sand was present.

- Current Type : DC 8. Control - Co = 17 ~ 21 mglL
- Cell Volume : 3.5 L - Supporting Electrolyte : NaZSOl
- Line : Plain (Sand), Dashed (Solution)

 

  

 

 

 

2000 I I I I I I I I I I I I I I l I I I I ' I I I I

1': 2 ‘
3 - [We 1mAlcm (212 mA) -
'u ' ‘
0 - .1
S 1500 - ..
0

C " q
0 _ 7
‘g _ Sand .
a ' x"
.“ 1000 - 7" e
E : Solution j
'5 L ‘

0 u
2 _ ,
l.l..l

g 500 - -
'3: l -
a , .
3

E .
3

U .

o I I l I I I J. l I I I I l I I .I I l I I I I
0 10 20 30 40 50

Elapsed Time (hrs)

Figure 4.11: Cumulative normalized electrical energy consumed during degradation of
naphthalene for aqueous phase and sandy soil phase experiments with DC performed at
jeq. = 1 mA/cm2 for 3.5 L cell

63

4.2.2 Salicylic Acid

As shown in Figure 4.12, about 40% and 60% degradation was recorded at the
elapsed time of 8 hours in the 3.5 L cell when DC was applied for aqueous phase and soil
phases, respectively. About, 65% and 80% degradation was observed in the l L cell. For
both types of medium: aqueous solution and sandy soils, the control cell showed

negligible loss of mass of salicylic acid.

- Co =19 ~ 21 mgIL
_ Current Type : DC - Supporting Electrolyte : Nazso‘
- Line : Bold (3.5L cell), Thin (1L cell) I Plain (Sand), Dashed (Solution)

 

 

 

 

 

 

 

1.2 ..........fi.,.. fi,....
3 1m: 1mAlcm’: 55 mA(1L Cell), 212 mA(3.5L Cell) .
o ,. '
7E 1 9‘ = ’ ’ ’ Control <>"
3 _ V \- (Sand or Solution) '
(I
8 0.8 - , \‘ '-
C I- “ I II
o I \ \‘

g . , ,‘ 3.5 L cell (Sand)
".3 0.6 - I \\ x -
< P \ \ 'l
g - fl \\ ‘\ 3.5 L cell (Solution) 4
a 0.4 - — ,‘ ~.. -
m l- \ \ \ .‘.~ J
E :1Lcell(Sand) i “‘xf‘u. j
g 0.2 l- ‘l:l'~.~ “ \‘~\:‘~. -
g . 1Lce|l (Solution) ~..

0 I. n n L 1 l n l 1 Pl l I 1 1 1 l n I .‘

o 5 1o 15 20 25

Elapsed Time (hrs)

Figure 4.12: Normalized concentration of salicylic acid for aqueous phase and sandy soil
phase experiments for DC performed at jar).= l mA/cm2 for 1 L and 3.5 L cells

64

The overall degradation of salicylic acid when sandy soil was present was slower
than when only aqueous solution was tested.

Figure 4.13 shows the cumulative electrical energy consumed during the
electrochemical degradation of salicylic acid in aqueous phase and sandy soil phase
experiments. The figure shows the effect of not only the size of the cell but also of the
media in the cell. The electrical energy consumed is lower when sandy phase is absent

and the size of the cell increases the energy consumption.

-Co=19~21mglL

- S ortin Electrol e : Na 80
- Current Type : DC upp g yt 2 4

- Line : Bold (3.5L cell), Thin (1L cell) I Plain (Sand), Dashed (Solution)

 

 

 

 

 

A 1200 I I I I l I I I I I I I I r l I I I I rII I I I
.J " q
2 : jm= 1mAlcm2 : 55 mA (1L Cell), 212 mA (3.5L Cell) :
3 1000 - .
E " d
3 . .
in

C ' d
8 800 - -
> I q
g r 3.5 L cell (Sand) '
ii 600 - -
g i 1 L cell (Sand) '
L3 400 - -
LIJ .. .
.g I. "a" , 4:] ‘
a zoo - ,.--' , , - p -
E - .' , , - , ' ’ k 3.5 L cell (Solution)-
3 . 'D’ , - - ' .
0 . , , - — r ’ 1 L cell (Solution) .

0 3 1 I l . . . I4 . . . . I . 1 . . 1 . . . .
o 5 1o 15 20 25
Elapsed Time (hrs)

Figure 4.13: Cumulative normalized electrical energy consumed during electrochemical
degradation of salicylic acid in aqueous phase and sandy soil phase experiments for DC
performed at jeqv, of 1 mA/cm2 in l L and 3.5 L cells

65

Similar to the naphthalene tests in Ottawa sand, samples were collected at these
three points: in the vicinity of the anode and cathode, and at the center point between the
electrodes. The results are presented in Fig. 4.14 in the form of normalized concentrations
(C/Co) of salicylic acid versus elapsed time for DC and AC applications. Fig. 4.l4(a)
shows these results for the DC tests where the normalized concentrations were least at the
cathode and progressively increased towards the center and cathode. At cathode, the
normalized concentration of salicylic acid reached about 4 as the experiment progressed.
The normalized concentration was always less than one when AC was used with salicylic
acid (Fig. 4.14b) or when naphthalene was tested with DC (Fig. 4.10).

The concentration gain at the cathode can be explained by adsorption of salicylic
acid on the sand particles in the vicinity of the cathode (-ve electrode). Xu et al. (2007)
investigated the adsorption of salicylic acid on two variable charge soils (a Rhodic
F erralsol and a Haplic Acrisol). At an initial concentration of 1.0 mM, the adsorption of
salicylic acid by the Haplic Acrisol andRhodic Ferralsol was 9.2 and 13.3 mmol kg",
respectively. Iron and aluminum oxides in variable charge soils are key adsorbents for
anions, and the greater the content of the oxides in a soil, the larger the amount of the
organic acid adsorbed. The presence of oxidants in the solution mixed with DI water and
Ottawa sand was analyzed for Ca2+. Ca2+ concentration was about 1.39 mg/L. Thus, the
sand around cathode provided sites where the organic acid was adsorbed during the DC
application. Fig. 4.14 (a) shows that the amount of the adsorption at the cathode in the 3.5
L cell is larger than in the 1 L cell. It supports that the soil particles surrounding a larger
surface area of the electrode (the 3.5 L cell) provide more adsorption sites for salicylic

acid than those on a smaller surface area.

66

 

 

 

 

Normalized Salicylic Acid Concentration (C/Co)

 

 

 

 

 

 

 

 

 

s :
Q ; - Co = 21 mgIL -
g 5 2' """"""""" """"""" - Supporting Electrolyte : Na 2SO ‘ """ '.
E ' ' - Line : Bold (3.5L cell), Thin (1L cell) 2
I: E 3 E J
c 4 _ ................................................. . ....................................
a - 1
o - .
° 1
'0
E 3 L- .................................................................................. -
0 " .
= - 1
5‘ - .
a '1
«n I
3 .
a 1
g i
z . I
o n I n I 4 1 n a 1 4 4 I 4 l n J n n l n 1 a 4
0 5 10 15 2O 25
Elapsed Time (hrs)

Figure 4.142Normalized concentration of salicylic acid in sandy soil phase experiments
for DC (a) and AC (f= 0.1 Hz) (b) performed at jeqv_=l mA/cm2 in 1 L and 3.5 L cells

67

The adsorption explanation used for DC (Fig. 4.15) can be further strengthened
when concentration data for AC (f = 0.1 Hz) application shown in Fig. 4. 14(b) is studied.
Because the anode and cathodes are being swapped in every AC cycle, the concentration
gain due to adsorption observed at the cathode in the DC test was not present at both
electrodes during the AC test. All concentration ratios were one or below. Therefore, in
summary, presence of Ca2+ as an oxidant and the temporary cation exchange capacity
created by the negatively charged electrode (cathode) in the sand can be attributed for the
adsorption of the organic acid on the cathode resulting in an increase in concentration of

salicylic acid at the cathode during DC test.

 

 

 

 

 

 

 

 

 

6 I I I I I I I I I l I I I I I I I I I I I I I I I I I I I

A r ' ' -
8 :j =1mA/cm’:55 mA(1L cell), 212 mA(3.5L cell) 1
B . "'V' , , .
E 5 p .......................................................... '. .................. . ....... ..
z: : t=1hr T '
“l . - t=24
*3 _ t:4hrs tgs .
q, 4 __ t-8hrs ............................................. ..
g - t=24hrs -
o - . .
o b : I
2 3 '_.-Co= 20~23 mgIL ________________________ j
2 . -Supporting Electrolyte:Nast4 .
é _ -3.5LCe|l 3

h - I
g 2 b.-Dc ‘ .......... t-1 -
fl 1- ' II
‘0 . ' .
U l E .
s . s -
E 1 f """""""""""""""" ‘j
g l- , .. .
z 0 pfil+n n A n I L I 51% “:5:

 

 

 

 

 

 

Cathode
Sampling Point (Anode to Cathode)

Figure 4.15: Normalized concentration of salicylic acid across the cell in sandy soil phase
experiments for DC performed at jag... = 1 mA/cm2 (212 mA) for 3.5 L cell

68

Apart from monitoring the aqueous phase concentrations of salicylic acid over
time, other experimental parameters including temperature, pH, standard redox potential,
electrical conductivity were measured at initial (t = O h) and final (t = 24 h) elapsed times.

Table 4.10 presents the results.

69

 

 

 

 

 

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70

4.3 ANALYTICAL MODELING USING NERNST-PLAN CK EQUATION

It is well understood that electrochemical reactions primarily occur on the surface
or in the vicinity of electrodes and thus, the rate of degradation would be predictable if
reactions taking place on the surface of the electrodes are defined and modeled. The
reactions are represented by direct/indirect oxidation processes and can be related to the
mass transfer to the electrode expressed in the Nernst-Planck Equation (Eq. 2.11). Eq.
2.11 shows the three terms corresponding to the three modes of mass transfer to the
electrodes: diffusion, migration, and convection. If the test cell is not stirred, the third

term (convection) in Equation 2.11 is disregarded and the equation becomes (Eq. 4.5):

 

ac, (x) _ z,F DC: a¢(x)

J. =—D.
,(X) .00 6x RT . ax

(4.5)

Again, for the uncharged reactant, the second term (migration) in Equation 2.11

can be neglected and then the equation becomes (Eq. 4.6):

J I (x) = —D, (x)-a£(—x—)— (4.6)
6x

In this section, the mass flux obtained by the two terms - diffusion and migration -
and the resulting concentration of a pollutant are predicted using Equation 4.5 and 4.6
based on the Nernst-Planck equation. This approach was not applied to the stirred

experiments because the convection velocities were not measured during the tests.

4.3.1 Key Assumptions

The assumptions followed when applying Eq. 4.5 are presented as follows.

71

' Reactants (naphthalene and salicylic acid) are instantaneously converted
(degraded) to products when they come into contact with the electrode (in this
case anode). Hence, concentration versus time (t) at the surface of electrode is
equal to zero at t 2 0 hr. This establishes a concentration gradient that drives
reactants by diffusion to the surface of the electrodes and this condition is
considered as the maximum concentration gradient that may be established in a
given time step;

- Steady-state diffusion is assumed during the time step of 1 hr; and

' Linear mass transfer and electric field exist in the cell between the electrodes.

4.3.2 Naphthalene

The diffusion coefficient (D) was estimated before applying Eq. 4.5 as it is one of
the required input parameters. As introduced in Equation 2.12, the Hayduk-Laudie’s
relationship (1974) was used to estimate the diffusion coefficient (D) of naphthalene.
After interpolating the solution viscosity at a given temperature, using the molar volume
of naphthalene (~ 125.7 cm3 mol'l), and plugging these parameters into Equation 2.12, D
for naphthalene at various temperature conditions ranged from 1.0x 10'5 to 4.0x 10'6 cm2
5". Schwarzenbach et al. (2003) have reported diffusion coefficients of organic solutes
equal to about 3.0x 10'5 cm2 5" for relatively small molecules to 5.0x10'6 cm2 s'I for
those of molar mass near 300 g mol".

The observed and simulated normalized concentrations when both diffusion and
migration are used for simulating the mass flux of naphthalene to the anode are presented

in Figure 4.16.

72

-Co=18~20mglL -DC

 

- Supporting Electrolyte : NaZSO4 - Aqueous Solution
1.2 7 I I I I I I I I I I I t fl I I I I I I IIIII
- Point: Measured C/Co: Circle (1 L cell), Square (3.5L cell) .
- Dashed Line : Predicted C/Co '
- Line : Thin (1L cell), Bold (3.5L cell) _‘
“x z = -O.5
0.8 \

‘\ jaw: 1 mNcmz

I I I I I I

Normalized Naphthalene Concentration (C/Co)

2

.1qufidh... 1...:E
30 40 50

 

 

 

 

Elapsed Time (hrs)

Figure 4.16: Experimental and predicted concentration rations of naphthalene for jeq. = 1
and 3 mA/cm2 in 1 L and 3.5 L cells for DC application

For the simulations using Equation 4.5, the value of net charge (2) was empirically
obtained from the best fit to the measured concentration profile. The best fit was obtained
when 2 was assumed equal to -0.50. Pepprah (2007) also obtained the same 2 for
experiments carried on 1 L cells. While the 2 value should be zero for naphthalene
because the naphthalene molecule is neutral. However, it is hypothesized that the anions
in the supporting electrolyte (anhydrous sodium sulfate) drag naphthalene to the anode.
Fig. 4.16 shows a relatively accurate prediction of concentration changes for the l L and

3.5 L cells.

73

4.3.3 Salicylic Acid

As discussed in section 4.1.2.4, the rate of salicylic acid degradation in the l L
cell was recorded slightly higher than the one in the 3.5 L cell. To further this finding, it
is necessary to simulate the degradation rates and compare between the observed and
predicted concentrations using Equation 4.5 where the key parameters: temperature and
voltage measured in tests were used. For simulations, the net charge (2) and the diffusion

coefficient (D) for salicylic acid were obtained from Pepprah (2007). The value of 2 was

2 1

assumed equal to -0.35 and D ranged from 1.17x10'S to 4.65x 1045 cm s' as a function
of measured temperature.

Figure 4.17 shows the experimental and predicted concentration ratios of salicylic
acid for jeq. Equal to 1 and 3 mA/cm2 for both cells. Overall the predicted concentration
rations are close to the measured values. However, for the l L cell, the predicted values
are underestimated. Whereas those values for the 3.5 L cell are slightly overestimated.
The predicted curves show faster degradation in the 3.5 L which is opposite to what is
observed from the experiments.

The Nernst-Planck equation does not include the effect of size or surface area of
the electrodes. A majority of the reactions (degradation) occur at the surface of the
electrode. The ratio of the surface area of electrodes over the volume of the cell is 55
cmZ/L and 50.9 cmz/L for the l L and the 3.5 L cells, respectively. The ratio is slightly
lower for the 3.5 L cell. This may be the reason why the observed rate of degradation in

the 3.5 L cell was less than the 1 L cell. However, the Nernst-Planck equation does not

simulate the effect of electrode area.

74

(a) jaw = 1 mA/cm’: 55 mA (1L cell), 212 mA (3.5L cell)

 

 

 

 

 

 

 

 

 

1.2 I I I I l I I I I I I I I I I I I I I I If I i I
3 - Co = 20 mgIL - DC
5 - Supporting Electrolyte : Nazso‘ - Aqueous Solution
2 1 '
g ”13% - Square : Observed C/Co '
g : ‘ . - Dashed Line : Predicted C/Co
3 0.8 - ‘sé z = -0.35 -
1: - \ .
8 \ \ \ "
u ' X ‘ 3.5 L cell 1
'5 0.6 " \ j
< .
.2 - .
5. .- ..
=5, 0.4 - -
m - ..
*6 ~ \ ,
.5 ‘ ‘ . ‘
a 0.2 I. y. ‘ x _
E r ‘c.. ‘ ~ - g ‘ ~ 1
2 _ 1Lcell -....____‘ q
0 . . . . 1 L . . . I L . . . 1 . . . +1 . . ‘.
0 5 10 15 20 25
Elapsed Time (hrs)
(b) jm= 3 mAIcm2 : 165 mA (1 L cell), 636 mA (3.5L cell)
102 j I I I I I I I I I I I I I I I I ' fi I I I
g - Co = 20 mgIL - DC
,3 - Supporting Electrolyte : Na 2so 4 - Aqueous Solution
‘5 1 "
.3 ;\ - Square : Observed C/Co '
g E - Dashed Line : Predicted C/Co '
C \ .
3 0.8 11‘ ‘. z = -0.35 ‘
s ' . 1
o ' § ‘\ 3.5 L cell i
:3 0 5 L ‘ \ / .
< ' .
.2 - , .
= 0.4 ' x
'0 L
g .
E 0.2 '
E l-
o .
2 b
o I I
0

 

Elapsed Time (hrs)

Figure 4.17: Experimental and predicted concentration ratios of salicylic acid for jeqv = 1
mA/cm2 (a) and 3 mA/cm2 (b) in l L and 3.5 L cells

75

CHAPTER 5

SUMMARY AND CONCLUSIONS

The key objectives of this study were to evaluate the effect of scale during
electrochemical degradation of naphthalene and salicylic acid in aqueous as well as sandy
soils phases. The effect of scale was evaluated by measuring the rate of degradation and
electrical energy consumption for experiments carried out at using AC and DC at two
current densities for cells having 1 L and 3.5 L volume.

1. Degradation rates increased as the current density was increased for both 1 L and
3.5 L cells.

2. For an equivalent current density expressed in the form of current per unit volume
of the electrolyte. The rate of degradation was virtually independent of the scale
of the cell. However, the electrical energy consumption was two to four folds
more for the larger cell. Thus, larger cell was not as energy efficient as the smaller
cell. Greater resistance of the larger cell resulting in greaterjoule heating and loss
of energy may be the reason for greater energy consumption.

3. Degradation kinetics observed in the aqueous phase experiments could be fitted
by the pseudo-first order decay model when naphthalene and salicylic acid were
used as contaminants.

4. In the presence of sandy soil phase, adsorption of organic acid on the sand
particles near cathode yielded greater than one concentration ratios. This was
attributed to the presence of Ca++ and oxides in the sand and negatively charged

cathode creating temporary cation exchange capacity for the sand particles.

76

5. Stirring significantly enhanced the rate of degradation for both AC and DC.
However, it was more significant for AC. Stirring accelerates the electromigration
of the species to the electrodes (anode for the chemicals studied) where it can be
converted.

6. Nemst-Planck equation simulating migration of the chemical specie to the
electrodes using electromigration and diffusion simulated concentration rations
that agreed relatively well with the measured values. Thus, both diffusion and
electromigration processes contributed to the rate of mass transfer of naphthalene
as well as salicylic acid to the reaction sites (e.g. on the surface of electrodes or in
the vicinity of electrodes) without convection unless the solution was stirred.
However, migration was the dominant mass transfer process when stirring was

not carried out.

77

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