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ELECTROCHEMICAL DEGRADATION OF
METHYL TERT—BUTYL ETHER (MTBE) USING
ALTERNATING AND DIRECT CURRENTS

presented by

CHEON YONG SEO

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MS degree in CIVIL ENGINEERING

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5I08 KzlProlecd-PrWCIRCIDatODm. hdd

ELECTROCHEMICAL DEGRADATION OF METHYL TERT-BUTYL ETHER
(MTBE) USING ALTERNATING AND DIRECT CURRENTS

By

Cheon Yong Seo

A THESIS

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

MASTER OF SCIENCE
Civil Engineering

2009

ABSTRACT

ELECTROCHEMICAL DEGRADATION OF METHYL TERT-BUTYL ETHER
(MTBE) USING ALTERNATING AND DIRECT CURRENT S

By

Cheon Yong Seo

Electrochemical degradation of methyl tert-butyl ether (MTBE) was investigated
to evaluate the effect of current type and density, electrode material, deionized (DI) water
and tap water as electrolytes and initial concentration of MTBE. MTBE is an organic
compound, which is highly soluble in water and is common ground water (GW)
contaminant that is typically released to the GW from gasoline spills. MTBE dissolved in
DI water at initial concentrations equal to 25, 250 and 2,500 mg/L was subjected to
alternating and direct currents (AC and DC) at current density ranging from 0.5 to 2
mA/cmz. The volume of the electrochemical cell was 1 L and titanium, graphite, and
boron doped diamond electrodes were tested. Sodium sulfate (NaZSO4) was used as the
supporting electrolyte during tests at a concentration of 300 mg/L. The key results are:
(1) both AC and DC resulted in electrochemical degradation of MTBE but the rate of
degradation for DC was greater than that for AC at an equivalent current density; (2) rate
of degradation increased as the current density increased for DC as well as for AC; (3)
rate of degradation defined as change in normalized concentration decreased as the initial
concentration of MTBE was increased; (4) rate of degradation for the graphite electrode
was the least among the three materials tested; and (5) Nemst-Planck equation was able
to accurately model the decrease in the MTBE concentration for low initial

concentrations. At higher initial concentrations, the model results were not consistent.

ACKNOWLEDGEMENT

I would like to thank all individuals who helped me in the research. I am grateful
to my advisor Dr. Milind V. Khire who helped me whenever I was in need. I am also
thankful to my committee members Drs. Hui Li and Scott C. Barton for their valuable
time and support. I am also thankful to my colleague Ramil Mijares. Finally, I would like
to thank Michael Becker who works in the Center for Coatings and Laser Applications,
Fraunhofer USA, for providing me the boron doped diamond (BDD) electrodes for this

research.

I would like to thank my wife Mrs. Mijung Kwon who supported me throughout
the study. I am also thankful to my parents for their prayers and sincere love and my
parents-in-law for their belief. I would like to thank pastors and all deacons and friends of
New Hope Baptist Church for their constant support and encouragement. Finally, I am

also thankful to my friends of republic of Korea for their support.

I express my gratitude to the 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 Colonel, Yongjae Lee; LTC; Moonkyoung Kim; and

Major, Chunghee Kim for their encouragement.

iii

TABLE OF CONTENTS

LIST OF TABLES ............................................................................... vi
LIST OF FIGURES ............................................................................... vii
LIST OF ABBREVIATIONS .................................................................... xi
LIST OF SYMBOLS ......................................................................... xii
CHAPTER 1: INTRODUCTION ................................................................. 1
1.1 MTBE AS GROUNDWATER CONTAMINENT .......................................... 1
1.2 BACKGROUND .................................................................................. 2
1.2.1 Degradation Mechanism ................................................................ 2

1.2.2 Electrical Efficiency ................................................................... 4

1.2.3 Current Type ........................................................................... 5

1.2.3.1 Direct Current (DC) ......................................................... 5

1.2.3.2 Alternating Current (AC) ................................................... 6

1.2.4 Current Density ........................................................................ 7

1.2.5 Mass Transfer Process: The Nemst-Planck Equation ............................ 7

1.3 CONVENTIONAL REMEDIATION TECHINQUES ..................................... 8
1.3.1 Air Sparging ........................................................................... 8

1.3.2 Chemical Oxidation Processes ....................................................... 9

1.3.3 Natural Attenuation ................................................................... 9

1.3.4 Bloremediation9

1.4 OBJECTIVES ................................................................................. 10
1.4.1 Key Challenges ..................................................................... 10

1.4.2 Key Objectives ................................................................ 10
CHAPTER 2: MATERIALS ............................................................. 1 1
2.1 CHEMICALS .................................................................................... 11
2.2 ELECTRODES .................................................................................. 11
2.2.1 Titanium ............................................................................... 11

2.2.2 Graphite ............................................................................... 12

2.2.3 Boron Doped Diamond (BDD) ..................................................... 14

2.3 SUPPORTWG ELECTROLYTE ............................................................ 14
CHAPTER 3: EXPERIMENTAL METHODOLGY ....................................... 16
3.1 EXPERIMENTAL SETUP ..................................................................... 17
3.1.1 Electrolytic Reactor .................................................................. 17

3.1.2 Electrical Equipment ................................................................. 17

3.1.3 MTBE in Aqueous Solution ......................................................... 18

iv

3.2 TESTING, SAMPLING AND ANALYSIS .................................................. 18

3.2.1 Testing ................................................................................. 18
3.2.2 Sampling .......................................... . ..................................... 21
3.2.3 Gas Chromatography (GC) Analyzing ............................................ 21
3.3 MON IT ORED PARAMETERS .............................................................. 23
3.4 DECONTAMINATION AND CLEANING ................................................. 26
3.4.1 Electrode ............................................................................ .26
3.4.2 Micro needle, Beaker and Miscellanea ........................................... .26
CHAPTER 4: RESULTS AND DISCUSSION ................................................ 29
4.1 EXPERIMENTAL RESULTS ............................................................. 29
4.1.1 CONTRIL CELL ...................................................................... 29
4.1.2 EFFECT OF CURRENT DENSITY AND TYPE ............................... 32
4.1.3 EFFECT OF ELECTRODE MATERIAL .......................................... 33
4.1.4 EFFECT OF INITIAL CONCENTRATION OF MTBE ....................... 36
4.1.5 EFFECT OF DI WATER VS TAP WATER ..................................... 38
4.1.6 DEGRADATION KINETICS ...................................................... 44
4.1.7 MEASURED INITIAL AND FINAL PARAMETERS ........................ .47
4.2 ELECTRICAL ENERGY CONSUMPTION ............................................... 48
4.2.1 EFFECT OF CURRENT DENSITY AND TYPE ................................ 53
4.2.2 EFFECT OF ELECTRODE MATERIAL ......................................... 53
4.2.3 EFFECT OF INITIAL CONCENTRATION OF MTBE ....................... 54
4.2.4 EFFECT OF DI VS TAP WATER AS ELECTROLYTES ..................... 54
4.3 ANALYTICAL MODELLING USING NERST-PLANCK EQUATION ............. 60
CHAPTER 5: SUMNIARY AND CONCLUSIONS ......................................... 65
REFERENCES ...................................................................................... 67

Table 1.1:

Table 2.1:

Table 3.1:

Table 3.2:

Table 4.1:

Table 4.2:

Table 4.3:

Table 4.4:

Table 4.5:

LIST OF TABLES

Key physical properties of MTBE ................................................ 3
Dimensions of titanium, graphite and BDD electrodes ...................... 12

Volumes of MTBE and DI water used for preparation standard

solutions .......................................................................... 22
Current densities and equivalent currents ..................................... 22
Summary of experiments carried out ........................................ 30
Effect of current densities at 25, 250 and 2,500 mg/L ............................... 31
Degradation of rate constant (k) and half-life of MTBE ..................... 49
Summary of voltage, pH, and temperature of electrolytic cell .............. 52
Electrical energy consumption during the tests. .............................. 55

vi

Figure 2.1:
Figure 2.2:
Figure 2.3:

Figure 3.1:
Figure 3.2:
Figure 3.3:
Figure 3.4:

Figure 3.5:
Figure 3.6:
Figure 3.7:
Figure 3.8:

Figure 3.9:

Figure 3.10:

Figure 3.11:

Figure 4.1:

Figure 4.2:

LIST OF FIGURES

Photograph and schematic diagram of titanium electrodes attached to the

top cap .............................................................................. 13
Photograph and schematic diagram of graphite electrodes attached to the
top cap ........................................................................... 13
Photograph of Boron Doped Diamond (BDD) electrodes attached to the
top cap .............................................................................. 15
Outline of experimental procedures ............................................ 16
Experimental setup ................................................................ 19
Reaction chamber placed inside water bath .................................. 19
Photographs of electrical equipment used in the study .................. 20

Photographs: (a) aluminum cap; (b) silicon septa and combined cap;
(0) micro needle (2 mL capacity); and (d) capped vials ................... 24

Photograph of setup to clean the sampling equipment ...................... 24

Photographs of: (a) carrier gas (H2, Air and He); (b) head space sampler;
(c) GC (Gas Chromatography); and ((1) connected computer for data
access. .............................................................................. 25

Schematic diagram of carrier gas (H2, Air and He), head space sampler,
GC, and connected computer .................................................... 25

Photographs: (a) pH meter; (b) Thermometer; (c) LCR meter; and
(d) Oscilloscope .................................................................. 27

Procedures followed for cleaning titanium electrodes ....................... 27

Photographs of decontamination and cleaning of micro needle,
Beakers, and sampling vials ...................................................... 28

Normalized Concentration (Ct/C0) for MTBE 25 mg/L with titanium
electrodes .......................................................................... 34

Normalized Concentration (Ct/Co) for MTBE 250 mg/L with titanium
electrodes .......................................................................... 34

vii

Figure 4.3:

Figure 4.4:

Figure 4.5:

Figure 4.6:

Figure 4.7:

Figure 4.8:

Figure 4.9:

Figure 4.10:

Figure 4.11:

Figure 4.12:

Figure 4.13:

Figure 4.14:

Figure 4.15:

Figure 4.16:

Normalized Concentration (Ct/Co) for MT BE 2,500 mg/L with titanium
Electrodes: (a) AC; and (b) DC electrolysis .......................... 35

Normalized concentration of MTBE with graphite, BDD and titanium
electrodes during DC electrolysis ............................................. 37

The electrode surface of graphite, BDD and titanium electrodes before
(upper) and after (below) electrolysis ........................................ 37

Normalized concentration of MTBE solution for 250 mg/L with
BDD and titanium electrodes during AC electrolysis. ....................... 39

Effect of initial concentration of MTBE solution at AC density of
lmA/sz for Co = 25, 250 and 2,500 mg/L with titanium electrodes. . ...39

Effect of initial concentration of MTBE solution at AC density of
1 mA/cm2 for Co = 25, 250 and 2,500 mg/L with titanium electrodes. ...40

Effect of initial concentration of MTBE solution at DC density of
0.5 mA/cm2 for C0 = 25, 250 and 2,500 mg/L with titanium electrodes...40

Effect of initial concentration of MTBE solution at DC density of
1 mA/cm2 for Co = 25, 250 and 2,500 mg/L with titanium electrodes. ...41

Cumulative MTBE mass converted at AC density of 1 mA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes .......................... 41

Cumulative MTBE mass converted at AC density of 2 mA/cm2 for Co =
25, 250 and 2,500 mg/L with titanium electrodes ............................. 42

Cumulative MTBE mass converted at DC density of 0.5 mA/cm2 for 25,
250 and 2,500 mg/L with titanium electrodes ................................. 42

Cumulative MTBE mass converted at DC density of l mA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes ............................. 43

Effect of electrolyte on the rate of degradation of MTBE for Co = 25mg/L
at DC density = 0.5 mA/cm2 with titanium electrodes ...................... 46

Effect of electrolyte on the rate of degradation of MTBE for C0 = 2,500
mg/L at DC density = 0.5 mA/cm2 with titanium electrodes ............... 46

viii

Figure 4.17:

Figure 4.18:

Figure 4.19:

Figure 4.20:

Figure 4.21:

Figure 4.22:

Figure 4.23:

Figure 4.24:

Figure 4.25:

Figure 4.26:

Figure 4.27:

Figure 4.28:

Photographs of titanium electrodes surface after a 24-hr tests when
Co = 25mg/L for MTBE in Tap water ........................................ 47

Photographs of titanium electrodes surface after a 24-hr tests when
Co = 2,500 mg/L for MTBE in adding 300 mg of Na2804 to
DI water 1L ........................................................................ 47

Estimated k (hr'l) for (a) 25, (b) 250 and (c) 2,500 mg/L of MTBE
solution at AC and DC during 24 hr electrolysis with titanium
electrodes......... .................................................................. 50

Effect of electrode material on K during DC electrolysis ................... 51

Cumulative electrical energy consumption (kJ/L) at AC — 2 mA/cmz,
DC — 1 mA/cmz, DC — 0.5 mA/cm2 and AC — 1 mA/cm2 with titanium
electrodes .......................................................................... 57

Cumulative mass converted per unit electrical energy consumed for

Co = 250 mg/L during AC and DC electrolysis using
titanium electrodes ............................................................... 57

Cumulative electrical energy consumption (kJ/L) at titanium, graphite and
BDD electrodes ................................................................... 59

Cumulative electrical energy consumption (kJ/L) at AC — l mA/cm2 with
titanium electrodes ................................................................ 59

Cumulative mass converted per unit electrical energy consumed for AC

electrolysis at current density = 1 mA/cm2 for C0 = 25, 250 and 2,500
mg/L using titanium electrodes ................................................. 60

Cumulative electrical energy consumption for Tap versus DI water as
electrolyte for 25 mg/L using polished and unpolished
titanium electrodes ............................................................... 60

Cumulative electrical energy consumption for C0 = 2,500 mg/L using
polished titanium electrodes in DI water ...................................... 61

Experimental and predicted normalized concentrations of MTBE for

C0 = 25 mg/L for DC electrolysis using current densities equal to 0.5 and
1 mA/cm2 ........................................................................... 65

ix

Figure 4.29:

Figure 4.30:

Experimental and predicted normalized concentrations of MTBE for
C0 = 250 mg/L for DC electrolysis using current densities equal to 0.5 and
1 mA/cm2 .......................................................................... 65
Experimental and predicted normalized concentrations of MT BE for Co =
2,500 mg/L for DC electrolysis using current densities equal to 0.5 and l
mA/cm2 ............................................................................ 66

LIST OF ABBREVIATIONS

AC = Alternating Current

AOP = Advanced Oxidation Process
BCF = Bioaccumulation Factor
BDD = Boron Doped Diamond

BTEX = Benzene, Toluene, Ethyl benzene and Xylenes

DC = Direct Current

D1 = De-ionized

EFOA = the European Fuel Oxygenates Association
EIA = Energy Information Administration

GAC = Granular Activated Carbon

GC = Gas Chromatography

MCLG = Maximum Contaminant Level Goal

MS = Mass Spectrometry

MTBE = Methyl tert-Butyl Ether

TBA = Tertiary Butyl Alcohol

TCB = Trichlorobenzenes

US EPA = the United States Environmental Protection Agency

xi

LIST OF SYNIBOLS

A = Current
Ct = Concentration with time
Cf = Final concentration

Co = Initial concentration

C = Coulomb

D =Diffusion coefficient

F = Faraday’s constant (96,487 C/mol)
J = Mass flux

Koc = Soil-water partitioning coefficient
R = Gas Constant (8.314 kJ/mol.K)

T = Temperature

V = Voltage

W = Power

f = AC frequency

j = Current density

jeqv = Equivalent current density

k = Pseudo-first-order degradation rate constant

tm = Half-life
v = Velocity of solution
2 = Net charge of carrier

Q=ohm

’7 = Viscosity of solution

xii

CHAPTER 1

INTRODUCTION

1.1 MTBE AS GROUNDWATER CONT ANIINANT

Methyl tertiary-butyl ether (MTBE) is a chemical compound that is manufactured by
the chemical reaction of methanol and isobutylene. MTBE was produced in relatively
large quantities (over 200,000 barrels per day) in the US. in 1999 and is almost
exclusively used as a fuel additive in motor gasoline. It is a group of chemicals
commonly known as "oxygenates" because it raises the oxygen content of gasoline. At
room temperature, MTBE is volatile, flammable and colorless liquid that dissolves rather
easily in water. MTBE could improve fuel combustion and reduce carbon monoxide but
it is a risk to human health as a possible carcinogen and is classified by the United States
Environmental Protection Agency (USEPA) (Song Hong et al., 2007) as a toxic chemical.
MT BE has relatively high solubility (~ 42 g/L), low soil-water partitioning coefficient
(Koc = 11; logm KOC = 1.04), and relatively low retardation factors when compared with
other gasoline additive such as benzene (EFOA, 2002) (Table 1.1). Therefore, MTBE is
highly mobile, and is relatively difficult to remove. USEPA drinking water advisory
(USEPA, 2000) recommended concentration of MTBE not to exceed 20 to 40 ug/L.
More than 40 US. states developed drinking water/groundwater standards with action
levels that range from 6.4 to 240 ug/L. While there are thousands of MTBE impacted

groundwater sites in the US, a spill in Monica, CA is one of the major spills where

MTBE concentration in groundwater ranged from 116 to 824 rig/L (US Water news,

1996).

1.2 BACKGROUND
1.2.1 Degradation Mechanism

Organic compounds can be degraded and transformed by electrochemical
degradation (Pepprah and Khire, 2008; Pepprah, 2007; Alshawabkeh et al., 2005).
Electrochemical degradation of organic compounds in aqueous phase occurs due to direct
oxidation and/or indirect oxidation or reduction. During the indirect oxidation processes,
the agents that oxidize organic compounds are generated. Previous research has reported
hydrogen peroxide (Brillas et al., 1995), metal mediators (Farmer et al., 1992),
hypochlorite (Rajkumar et al., 2005), ozone (Stucki et al., 1987) and hydroxyl radicals
(Yangqing et al., 2007) as strong reactive agents during electrochemical degradation.
Equations 1.1 and 1.2 show breakdown of water at anode and cathode and resulting

formation of oxygen at anode and hydrogen at cathode (Acar and Alshawabkeh, 1996).
Anode: 2H2 —> 02 + 4H+ + 4e' (1.1)
Cathode: 2H20 + 2e- —> H2 + 2OH° (1.2)

During electrochemical conversion, hydroxyl radicals are also formed at the anode

(Equation 1.3 and 1.4).
H20+e' —> H' +OH' (1.3)

OH" —-> OH. + e‘ (1.4)

Table 1.1: Key physical properties of MTBE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

IUPAC Name 2-Methoxy-2-
Identifiers (International Union of Pure and Applied Chemistry) methylpropane
CAS No. 1634-04-4
Molecular Formula C5H120
Molecular Weight (g/mol) 88.15
DensiWcm3) 0.7404
Melting Point ~109°C / -l64°F/ 164K
Boling Point 55.2'C/ 131°F / 328K
Physical Flash Point -10°C/ 14°F / 263K
an‘? Auto ignition 425°C / 797°F/ 698K
Chemcal v Pr kP 33 4
Properties apor essure( a) .
Water Solubility 42 g/L
Partitioning Coefficient n-octanol/water .
(Koc) (log10) 1.04 (estimated) .
Henry's Constant (Pa/m3/mol) 65 .4
. . 1.6 (estimated) /
Bioaccumulation factor (BCF) 15 (measured for fish)
CH3
Structure H3C C 0 CH3 CH3'O'C(CH3)3
I
CH3

 

 

 

Note: 1. Source: European Fuel Oxygenates Association (EFOA)

2. MTBE at 25°C and 100 kPa

Hydroxyl radicals are strong oxidizing agents that can breakdown many organic

compounds including MTBE.

1.2.2 Electrical Efficiency

Electrical efficiency is one of the considerations for operation of electrochemical
systems. Among other parameters, electrical power usage is important consideration for
electrochemical systems. The electrical power consumption can be computed using Eq.
1.5.

Power (W) = Current (A) X Voltage (V) (1.5)

Electrical efficiency (Eq. 1.6) is another important parameter which relates degradation

of the pollutant to the power usage (Alshawabkeh and Sarahney, 2005).

PXt
Vxlog(C0 /Cf) (1'6)

 

EE/0=

where EE/O is electrical efficiency (kWh/m3); P is power usage (kW); t is operational
time (hours); V is electrochemical reactor volume (m3); and C, and Cf are initial and final
concentration of the pollutants, respectively.

Pollutant breakdown rate is the ratio of rate of change in the concentration (i.e.,
degradation) of the pollutant to current density (Alshawabkeh and Sarahney, 2005), (Eq.

1.7) is as follow:

_AC/At
1'

77k (1.7)

where 71k is the efficiency of pollutant breakdown (mg/C); AC is change in pollutant

concentration (mg/L); At is electrolysis time (seconds); and j is the current density (A/L).
1.2.3 Current Type
1.2.3.1 Direct Current (DC)

Direct Current (DC) is a form of electrical current which does not change the
directions of its flow. Usually constant DC means a DC which has zero frequency. A
majority of published work on electrochemical breakdown of organic compounds has
been focused on the use of direct current (DC). Wu and Lin (2007) used DC electrolysis
for degradation of MTBE using iridium electrodes with supporting electrolytes that
included sulfuric acid (H2804), hydrochloric acid (HCl), and nitric acid (HNO3). They
used initial concentration of MTBE equal to 20 mg/L. Removal rate of MTBE ranged
from 27% to 92% after electrolysis for 3 hr for various values of input voltage, which
ranged from 0.5 to 3.0 V for the various electrolytes. Degradation rate of MTBE
increased as a function of applied voltage.

While the use of DC in the lab-scale studies have demonstrated acceptable rates
of breakdown of many organic chemicals, use of DC for field application poses many
challenges (Khire and Pepprah 2008). Khire and Pepprah (2008) and Lee (2008) have
indicated that power usage during DC is relatively high because the resistance of the
electrolyte increases as the time of application of DC increases. Increased resistance
requires proportionately greater voltage to maintain the current which, among other
parameters, dictates the rate of breakdown of organic molecules in the solution. In
addition, application of DC results in decrease in the pH near the anode and increase in

the pH near the cathode. This results in relatively rapid fouling of the electrode which
5

 

causes decrease in the rate of breakdown and affects the physical integrity of the
electrode.
1.2.3.2 Alternating Current (AC)

Alternating Current (AC) is an electrical current which changes its direction
periodically. AC wave form can have well defined shapes such as sinusoidal, triangular,
square, or it can follow an arbitrary shape. While the frequency of sinusoidal AC
available in north American power grid is 60 Hz, AC function generators typically can
produce AC at frequencies ranging from 0.1 Hz to 1 MHz. Relatively few published
studies where AC has been used for electrochemical breakdown of organic compounds
exist. Nakamura et al (2005) presents degradation of Trichlorobenzene (TCB) using AC
electrolysis in aqueous solution. Decomposition rate of TCB was 29.6% after electrolysis
for 0.5 hr due to hydrogen and hydroxyl radicals produced during AC electrolysis.

Pepprah (2007), Pepprah and Khire (2008), and Khire and Pepprah (2008) have
compared the use of DC and AC for electrochemical breakdown of naphthalene, pyrene,
phenanthrene, and salicylic acid in aqueous solutions. For a given current density, these
studies concluded that the rate of breakdown of the organic compounds was greater for
DC compared to AC. For AC, the rate of breakdown decreased as the frequency of the
AC was increased from 0.1 Hz to 1,000 Hz. However, the extent of electrode fouling and
electrode mass loss was greater for DC compared to when AC was applied.

Lee (2008) evaluated the power consumption for AC versus DC when
naphthalene was degraded in electrochemical cells having volumes equal to 1 L and 3.5 L.
Lee (2008) concluded that the power consumption per unit volume of the electrochemical

cell was about 2 to 5 greater when DC was used. Lee (2008) also found that as long as the

6

current density (defined as total current divided by the total area of the electrodes that
faces each other) was constant, the size of the cell did not influence the rate of

degradation of naphthalene in aqueous solution.

1.2.4 Current Density
Pepprah (2007), Pepprah and Khire (2008), Khire and Pepprah (2008), and Lee

(2008) studied decomposition of naphthalene with AC and DC at current densities
ranging from 0.5 to 6 mA/cmz. The degradation rate increased when current density

was increased during AC as well as DC electrolysis.

1.2.5 Mass Transfer Process: The Nemst-Planck Equation

Mass transfer of charged species (i.e. ions) in solution subjected to electrical
current involves migration, diffusion and convection (Bard et al., 2001). Migration is the
movement of a charged specie due to electrical potential difference. Diffusion is
movement of chemical (i.e., contaminant in this study) under concentration gradient. Last,
convection is from stirring or hydrodynamic movement. Equation 1.8 is the Nemst-
Planck equation, which presents mass transfer due to migration, diffusion and convection

between the two electrodes subjected to DC.

8C,(x) _ ZtF DiCi Mil... C,V(x) (1.8)

ax RT ax

 

J; (X) = "D.’ (X)

Where Ji(x)is the flux of contaminant 1' (mol 3'1 cm'z) at distance x from the electrode

8C1. (x)

surface; Dis the diffusion coefficient (cm'2 s' ); ax is the convection gradient at

7

distance x from the electrode surface; 2 is valency of the net charge carrier

(dimensionless); F is the number of coulombs of charge per mole of ion (C); Ci is the

concentration of contaminant i (mol cm'3); R and T are gas constants (j mol'l K") and

absolute temperature (K), respectively; is the potential gradient; v(x) is the

a¢<x>
8x
velocity (cm s") of hydrodynamic flow between electrodes; and D is the diffusion

coefficient of the specie. D of MTBE was estimated from the Hayduk-Laudie equation

(1974) as shown in Eq. 1.9:

_ 13.26x10‘5
- _ 0.589 (19)
771.14 XV

D

 

where D is diffusion coefficient (cm.2 5'1); nis the solution viscosity (10-2 g cm'1 s") at

specific temperature; and V is the molar volume of contaminant (cm3 mol'l).

1.3 CONVENTIONAL REMEDIATION TECHNIQUES
1.3.1 Air Sparging

Air sparging involves pumping air in the shallow plume of organic contaminants
that are present in the ground water that are volatile. The air bubbles created by the
pumped air allow the dissolved volatile organic compounds to volatilize at a faster rate.
The vapors are removed by vacuum applied using an extraction system.

Many sites contaminated with benzene, toluene, ethyl benzene and xylenes
(BTEX) compounds in shallow ground water have been successfully remediated by air

sparging (Suthersan, 1997; USEPA, 1996). However, this technology is not effective for

8

dense non-aqueous phase liquids such as the chlorinated solvents and also MT BE which
has relatively low liquid-gas partitioning coefficient.
1.3.2 Chemical Oxidation Processes

The chemical Oxidation Processes are used to convert contaminant to end
products, which are non toxic. Typically, the degradation or conversion occurs due to
hydroxyl radicals which are produced. Hydroxyl radicals are strong oxidizing agents. A
few examples of this application include the use of Fenton’s reagent, hydrogen peroxide,

and ozonation.

1.3.3 Natural Attenuation

Natural Attenuation involves several processes: (1) adsorption to aquifer materials
leading to contaminant retardation; (2) dilution of contaminants through advection,
dispersion and diffusion; and (3) volatilization. However, half-life period of MTBE is 0.1
year. Hence, it will require relatively long for MTBE to degrade using natural

degradation (Hert et al., 1999)

1.3.4 Bioremediation

Bioremediation uses microorganisms to degrade or immobilize contaminants. In
order to optimize the rate of degradation or immobilization, several options are required.
These options include injection of oxygen or electron acceptor, nutrients, and growth
simulating materials (National Academy Press, 1993). In addition, temperature and pH

have effect on decomposition efficiency (LaGrega et al., 1994).

1.4 OBJECTIVES
1.4.1 Key Challenges

MTBE has relatively high solubility and low adsorption ability. Hence, air
sparging or granular activated carbon adsorption does not work well (Sutherland et al.,
2004; Shih et al., 2003). Advanced oxidation methods do work. However, they are
relatively expensive. (Hseih et al., 2004; Watt, 1998; Anderson, 1994; Wagler and
Malley, 1994).

Electrochemical methods have a potential for source zone remediation.
Electrochemical methods are yet not proven to be technically sound and cheap. Hence,
additional research is needed to explore the use of AC versus DC and electrode materials
on the efficiency of degradation of MTBE. Published studies have tested various
materials for electrodes including: platinum (Ernst and Knoll, 2001); nickel (Wu, 2007);
and iridium (Wu and Lin, 2007). These electrodes are expensive and undergo physical

decay. There is lack of data on efficiency of electrochemical techniques.

1.4.2 Key Objectives
The key objectives of this study are to investigate:
1. Effect of current types and density on the degradation rate;
2. The effect of electrode material on the rate of electrochemical degradation of
MTBE in aqueous solution;
3. The effect of initial concentration of MTBE on the rate of degradation; and
4. The effect of DI water versus MSU tap water on the rate of degradation and

energy consumptions.

10

CHAPTER 2

MATERIALS

2.1 CHEMICALS

The chemicals used for this project included MTBE (target contaminant), acetone
(cleaning solvent), sodium sulfate (NaZSO4) which was the supporting electrolyte, and

deionized (DI) water which was the primary medium in which MTBE and Na2SO4 were
dissolved.

MTBE was purchased from Sigma-Aldrich Company (Serial No. 306975).
Acetone was purchased from J.T. Baker Company (Serial No. 9002-03). Acetone was
99.8% pure. DI water is obtained from the lab DI water system which produces DI water
that has an electrical conductivity that exceeds 1 M. All chemicals were environmental

grade.

2.2 ELECTRODES
Three material types for the electrodes were evaluated in this study. These
materials included titanium, graphite, and boron doped diamond (BDD). BDD is also

referred to as diamler.

2.2.1 Titanium
The titanium electrodes used in this study were uncoated titanium. This material
was selected because it is cheaper than platinum, nickel, and mixed metal electrodes used

in published electrochemical studies. Furthermore, it is corrosion-resistant, lustrous and
11

strong so can be potentially used for long-terrn field use (Chen et a1, 2003). Zaggout and
Hallway (2008) used titanium electrodes for degradation of o-nitro phenol and observed

relatively high removal rate. In this study, titanium electrodes were 12 cm high (H) x 5

cm wide (W) 0.1 cm thick (area ~ 60 cmz). Immersed area of the electrodes in 1 L

aqueous solution was about 55 cm2. Photographs of the titanium electrodes are presented

in Figure 2.1 and dimensions of all electrodes are summarized in Table 2.1.

2.2.2 Graphite

Graphite is one of the allotropes of carbon and is an electrical conductor. Graphite
is relatively cheap. Sathish and Viswanath (2005) used graphite electrodes for
degradation of phenol in aqueous solution and reported 85% degradation within 30 hrs
using DC at a 5 V. The dimensions of the graphite electrodes used in this project were

12.8 cm (H) by 5.3 cm (W) by 0.6 cm (immersed area ~ 68 cmz). A photograph of the

electrodes and their dimensions are presented in Figure 2.2 and Table 2.1, respectively.

Table 2.1: Dimensions of titanium, graphite and BDD electrodes

 

 

 

 

 

. , Immersed

Electrode Shape of Dtlameter 166.11g“! or Wldth or Area of the tWO Thickness

Type Electrode 0 screw lameter Diameter Electrode (B, cm)

(Dscrew) (H, cm) (W, cm) 2
(cm )

Titanium Rectangle 0.5 12 5 55 0.1
Graphite Rectangle 0.5 12.8 5.3 68 0.6

BDD Circle 0.5 7.4 35.9 0.1

 

 

 

 

 

 

12

DEF!“

 

 

 

 

 

 

 

Figure 2.1 Photograph and schematic diagram of titanium electrodes attached to the top
cap

 

Figure 2.2: Photograph of graphite electrodes attached to the top cap

13

2.2.3 Boron Doped Diamond (BDD)

Similar to graphite, diamond is also an allotrope of carbon. However, diamond
electrodes, while relatively expensive, are stable. In this study, silicon wafer was coated
with diamond and boron at the Center for Coatings and Laser Applications, Fraunhofer
USA. There are many published studies where BDD electrodes have been used for

electrochemical research. Ammar et al. (2006) studied degradation of indigo dye at BDD
electrodes using DC applied at a current density of 100 mA/cmz. Oliveira et al. (2007)
and Zhao et al. (2008) degraded benzene and phenol using BDD with DC applied at a
current density of 2.5 V and 20 mA/cmz, respectively. In this study, circular BDD
electrodes were fabricated. The total and immersed areas of the electrodes were 43 cm2
and 35.9 cmz, respectively. A photograph of the electrodes is presented in Figure 2.3 and

dimensions are listed in Table 2.1.

2.3 SUPPORTING ELECTROLYTE

Sodium sulfate (NaZSO4) was used as the supporting electrolyte in this study. Lee
et al. (2003) selected Na2804 as the electrolyte for decontamination of radioactive metal

waste. Pepprah and Khire (2008) compared the use of Na2804 and NaCl as supporting

electrolytes during electrochemical degradation of naphthalene and concluded that NaCl
caused rapid decay of uncoated titanium electrodes due to the formation of hypochloric

acid.

14

 

Figure 2.3: Photograph of Boron Doped Diamond (BDD) electrodes attached to the top
cap

CHAPTER 3

EXPERIMENTAL METHODOLOGY

An outline of the experimental procedure is presented in Figure 3.1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Steps Description
. . Prepare for 25, 250 and 2,500 mg/L
3011111011. of MT BE solution
Preparanon . Add 300 mg of Nazsoa as
supporting electrolyte
Reaction . Add the spiked solution to the
Chamber reaction vessel .
, . Place reaction vessel in water bath
Preparation
Electrical . Apply the pre-deterrnined
Equipment electrical current to the cell
Setup . Measure temperature, current
and voltage at specrfic time
Sampling . Collect liquid samples at specific
d Anal sis "mes
a“ Y . Run GC to analyze MTBE
concentration
. Wash and polish the Ti and
Graphite electrodes
Cleaning . Wash the BDD electrodes, all

beakers and vials using acetone
and DI water

 

 

Figure 3.1: Outline of experimental procedure

16

 

 

 

 

3.1 EXPERIMENTAL SETUP

The Experimental setup consisted of electrochemical reactor and electrical

equipment as displayed in Figure 3.2.

3.1.1 Electrolytic Reactor

The electrochemical cell consisted of 1 L Pyrex beaker, Teflon cap, and
electrodes. Pyrex beaker and Teflon cap minimize sorption and volatilization of MT BE.
A water bath was used to minimize temperature increase and fluctuation. The room
temperature was maintained between 20 to 22 °C. Figure 3.3 shows the reaction chamber

placed inside a water bath.

3.1.2 Electrical Equipment

The electrical equipment consisted of: DC power supply/AC amplifier, function
generator, and oscilloscope (Figure 3.4). The function generator was able to generate
triangular, sinusoidal, or square wave AC signals having 0 to 2 MHz frequency. Square
wave AC having 0.1 Hz frequency was selected because Pepprah (2007) has shown that
when AC is used, lower the frequency, greater the rate of degradation rate due to more
time given for the reactions to occur at each instantaneous anode and cathode. The power
supply used for this project was manufactured by Kepco, model BOP 200W. It consisted
of bipolar operational amplifier and power supply with maximum 200 V and 1 A of
AC/DC. Oscilloscope, which is Agilent model No. 54621A, was used to measure the

current passing through the reaction chamber and the voltage applied to the chamber. It

17

has two channels to measure voltage. The Kepco amplifier has a special setup that allows
measurement of the current supplied by the amplifier in the form of an equipment voltage

signal.

3.1.3 MTBE in Aqueous Solution
MTBE in aqueous solution was prepared by adding 300 mg of N a2804 (in crystal
form, EMD chemicals Inc.) to 1 L of DI water and 0.0337, 0.337 and 3.37 mL of MTBE

in liquid form to the solution to prepare 25, 250 and 2500 mg/L, respectively of MTBE

spiked test solutions. Volume of MTBE was converted to weight using the density of
MTBE which is equal to 0.74 g/cm3. 300 mg of N a2804 was selected based on Maximum
Contaminant Level Goal (MCLG) (1990, 55 FR 30370), which EPA has proposed for

Nast4.

3.2 TESTING, SAMPLING AND ANALYSIS
3.2.1 Testing

In order to measure the concentration of MT BE, six standard solution samples
were prepared as Gas Chromatograph (GC) standards. The identification of MTBE and
quantification of its concentration was achieved by comparison of retention times and
area under the chromatograms for standard solutions. R-square values in calibration

curves obtained for the standard solution ranged from 0.9399 to 0.999. When 2,500 mg/L
MTBE solution was analyzed, all samples were diluted to 2/3rd or 1/3rd to keep the
measurement within the most accurate range of the GC.

18

Function
Generator

Digital Oscilloscope

\

 

Figure 3.2: Experimental setup

 

Figure 3.3: Reaction chamber placed inside water bath

19

Function Generator

 

 

Oscilloscope

Figure 3.4: Photographs of electrical equipment used in the study

20

Table 3.1 presents volumes of specific concentration of MTBE solution and DI water for

making standard solutions for 25, 250 and 2,500 mg/L of MTBE. Current density tested
in this project ranged from 0.5 to 2 mA/cm2 for DC and 1 to 5 mA/cm2 for AC. Table 3.2

shows the current that corresponds to the specific current density. Testing time was fixed

at 24 hr for all tests.

3.2.2 Sampling

Samples of test solution were collected at 0, 1, 2, 4, 8, and 24 hours after the test
began. Micro syringes (2 mL, capacity) manufactured by Hamilton Company were used
to collect the samples from the center of the cell, halfway between the electrodes. The
sample was stored in a glass GC vial which was capped with silicon Teflon and 20 mm
standard seals (Grace Davison Discovery Deerfield) and stored in refrigerator until it was
analyzed using GC within 24 hours. Photographs of the vials and the micro syringe are
shown in Figure 3.5. The sampling syringe was decontaminated and cleaned after every
sampling by using acetone and DI water. Figure 3.6 shows a photograph of the setup used

to clean the sampling equipment. The chemical current was not stopped during sampling.

3.2.3 Gas Chromatography (GC) Analysis
GC manufactured by Perkin Elmer (model EK-3441) was used to analyze the

concentrations of MTBE in the test solution. The GC was connected to a head space
sampler (Perkin Elmer, HS-40). The GC run was set up for MTBE. The specific details

related to the setup are as follows:

21

Table 3.1: Volumes of MTBE and DI water used for preparation of standard solutions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Target Concentration -) 0 5 10 15 20 25
mgr.)
Volume of 25 mg/L of
MTBE solution (mL) 0.0 0.4 0.8 1.2 1.6 2.0
Volume of DI
water (mL) 2.0 1.6 1.2 0.8 0.4 0.0
E244..- _.:-. —. - - - . - it are? 1:13 I . _~ ”:4. . -_..~ “.13.“ . ». - .. 7 _ ~31. ;;.;;!§
'3‘ r." 1'"??? 3:1 "WE: aselt -: -2. '"~ 4+— a: e ‘- .‘ -..« -_:-—, a
Target Concentration -) 0 50 100 150 200 250
(mg/L)
Volume of 250 mg/L of
MTBE solution (mL) 0.0 0.4 0.8 1.2 1.6 2.0
Volume of D1
....,. 1 Amt“ ("ll”) __ , . 2.0 1.6 1.2 0.8 0.4 - r 0.0
Target Concentration -) O 500 1,000 1,500 2,000 2,500
(mg/L)
Volume of 2,500 mg/L of
MTBE solution (mL) 0.0 0.4 0.8 1.2 1.6 2.0
Volume of DI
water (mL) 2.0 1.6 1.2 0.8 0.4 0.0

 

 

 

 

 

 

Table 3.2: Current densities and equivalent currents

 

 

 

 

 

Measured Current (mA) Corresponding the Current Density
EleCtrOde jeqv. = 2 jeqv. = 2 jeqv. = 2 jeqv. = 2 jeqv. = 2
0.5 mA/cm 1 mA/cm 2 mA/cm 4 mA/cm 5 mA/cm
Titanium 55 1 10 220 440 550
Graphite 68 136 272 544 680
BDD 35 70 140 280 350

 

 

 

 

 

 

22

1. GC cycle time: 10 min
2. Heating time per vial: 30 min
3. Pressurizing time per vial: 2 min
4. Injection and withdrawal time of needle to vials: 0.2 min, respectively
5. Sampler, needle and transfer temperature: 80°C, 120°C and 120°C, respectively
6. Helium, hydrogen and air gas cylinder controlled pressure: 525 kPa (77 psi), 340
kPa (50 psi) and 500 kPa (72 psi), respectively.
Figure 3.7 shows a photograph of the GC setup. Fig. 3.8 shows a schematic of the

connections used for the GC setup.

3.3 MONITORED PARAMETERS

During the test, the parameters that were monitored included pH, temperature,
voltage, and electrical resistance, R (for DC) and impedance, Z (for AC). Temperature
and voltage were measured at the sampling time, which was 0, 1, 2, 4, 8, and 24 hours
using microprocessor thermometer (OE Omega) and digital oscilloscope, respectively.
Electrical resistance and pH were monitored just before the test started and just after the
test ended using LCR ESR meter (BK Precision) and pH meter (IQ. scientific
instruments), respectively. The resistance was also calculated from the voltage readings
that were taken throughout the test and the fixed current which was maintained

throughout the test. These measurement devices are shown in Figure 3.9.

23

(e)

 

Figure 3.5: Photographs: (a) aluminum cap; (b) silicon septa and combined cap;
(c) micro needle (2 mL capacity); and (d) capped vial

 

Figure 3.6: Photograph of setup to clean the sampling equipment

24

 

Figure 3.7: Photographs: (a) carrier gas (H2, Air and He); (b) head space sampler;
(c) GC; and ((1) connected computer for data access

Peak Area
/

l

I
If
Samples Injector _ij
Control Pressure

.\ 4a Computer
t, ’I_
l I Ail-x1

 

 

 

 

 

 

A,

f .‘ .

x

 

 

\
\
\

\

 

 

\.
Detector

 

Carrier Gas (Air, H2 and He)

Figure 3.8: Schematic of carrier gas (H2, Air and He), head space sampler, GC, and
connected computer

25

3.4 DECONTAMINATION AND CLEANING
3.4.1 Electrode

Cleaning electrodes was critical for many reasons including deposits on the
electrode surface have effect on the reaction rate that results in the degradation of the
target chemical and the electrodes were reused. The used titanium electrodes were
polished using a pneumatic metal brush scrubber followed by cleaning it using detergent
(Alcox) and rinsing with DI water. The process of cleaning titanium electrodes is
depicted in Figure 3.10. Graphite electrodes were cleaned by scrubbing with a sand paper

followed by cleaning and rinsing with the detergent and DI water.

3.4.2 Microneedle, Beaker and Miscellanea

Microneedle was washed using one part acetone and 2 parts DI water. A
photograph of how the micro needle was cleaned is presented in Figure 3.11. Beaker and
vials were washed using detergent (Alcox) and rinsed using DI water and oven dried for
24 hours. Immersed components of all measuring devices also were cleaned using the

same procedure as discussed above after every sampling/monitoring event.

26

 

 

Figure 3.9: Photographs: (a) pH meter; (b) Thermometer; (c) LCR meter; and
(d) Oscilloscope

        

 

LL_j

Used electrode Scrubbing Washing Drying Recycled electrode

Figure 3.10: Procedure followed for cleaning titanium electrodes

27

      

DI water washing for 2 times

 

Cleaning beakers and vials

Figure 3.11: Photographs of decontamination and cleaning of micro needle, beakers, and
sampling vials

28

CHAPTER 4

RESULTS AND DISCUSSION

The effect of current type and current density, electrode materials, use of tap
water versus DI water, and initial concentration of MTBE on the rate of degradation of
MTBE in an aqueous solution was evaluated from total 37, 24-hr experiments. Table 4.1

summarizes the list of experiments carried out.

4.1 EXPERIMENTAL RESULTS

Figures 4.1 to 4.3 show normalized MTBE concentration (Ct/Co) versus elapsed
time during electrolysis with titanium electrodes where DC and AC at current densities
equal to 0.5, 1, 2, 4 mA/cm2 were applied with initial concentration of MT BE equal to 25,
250 and 2,500 mg/L. The error bars shown in all graphs are maximum and minimum

values of the ordinate measured from duplicate tests. Table 4.2 presents the final

concentration versus initial concentration ratio (Cf/C0) for all current densities, current

types, and initial concentrations.

4.1.1 Control Cell

Control tests were carried out for each of the initial concentrations using an identical
setup as that where current was applied. However, for the control tests, no current was
passed. When current passes through an electrochemical cell, hydrogen and oxygen gases

bubble out at the cathode and anode, respectively.

29

Table 4.1: Summary of experiments carried out

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initial , Electrode Current Current Total Number
Concentration Material Type densrty 2 Current of Tests
(mg/L) '1 (mA/cm ) (mA)
0 Tm. AC 1 / 2 110/ 220 2
1 “1‘“ DC 0.5/l 55/110 2
23/520? Titanium Control - ' 3
AC 1 110 l
25 Titanium 220 1
DC 0.5 55 2
1 1 10 l
1 Titanium DC 0-5 55 1
25 Unpolished DC 05 55 1
Titanium
1 1 10 1
AC
Titanium 2 220 2
DC 0.5 55 2
l 1 10 l
250 AC 5 696 l
Graphite 0'5 58 2
Dc 1 174 1
2.5 348 l
BDD AC 1 7O 1
BDD DC 1 70 2
1 l 10 2
AC 2 220 1
Titanium 4 440 1
2500 0.5 55 2
DC 1 1 10 1
2 220 l
U°P°l‘.Sh°° DC 0.5 55 1
Titanium

 

 

 

 

 

 

Total number of tests : 37

Notes: 1. Electrolyte prepared using MSU tap water. For all other tests,
DI waster used
2. AC Frequency was 0.1 Hz for all AC tests

30

Table 4.2: Effect of current densities at C0 = 25, 250 and 2,500 mg/L

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Initial .
Type Concentration (mg/L) Jqu' 2 Cl/Co
of MTBE (“A/cm )

0.5 0.02

25 l 0.03

0.5 0.33

DC 250 1 0.1
0.5 0.72

2,500 1 0.51

2 0.12

1 0.3

25 2 0.14

1 0.64

AC 250 2 0.56
l 0.82

2,500 2 0.68

4 0.7

 

 

 

 

Hence, these gases can remove the volatile organic chemical (e.g., MTBE) via sparging
or aeration. For the current densities used in this study, the amount of oxygen and
hydrogen produced at the electrodes ranged from 0.45 l/d to 3.6 Ud based on estimates
using the Faraday’s equation (Eq. 4.1)

: NH 20r02
n F (4.1)

where j is current density (mA/cmz); n is number of electrons; F is faraday constant

(C/mol); and N is gas flux (mol/cm2.s).
Goel et al (2003) evaluated the effect of aeration by sparging 1.6 L/d of nitrogen
gas in a cell containing 10 mg/L of naphthalene. The authors reported insignificant

31

 

mi

stripping of naphthalene due to the aeration. Drogos and Diaz (2000) evaluated removal
of MTBE (concentration ~ 10 mg/L) added to gasoline in a large-scale lab model (~ 500
m3) filled with saturated sand by pumping air at a rate of about 1.9 x105 IJd for five days
and 7.8x105 L/d for 10 additional days. After 20 days of continuous pumping of air,
MTBE removed from the tank was about 4% while toluene and xylene removals were
about 11% and 14%, respectively. The authors attributed the relatively low removal of

MTBE to its relatively low Henry’s Law liquid/gas partitioning coefficient equal to 5.87

x 10'4 atm-m3/g-mole. The Henry’s Law liquid/gas partitioning coefficient for

naphthalene (4.5 x 10'3 atm-m3/g-mole) is about one order greater than MTBE and Goel

et al. (2003) observed negligible removal of naphthalene at the rate of aeration which is
similar to the rate of gas production in the experiments carried out in this project. Hence,
the control cell in this study did not include aeration. Removal of MTBE due to aeration

is expected to be relatively small.

4.1.2 Effect of Current Density and Current Type

Figures 4.1 to 4.3 and Table 4.2 show that as the current density increases, the
rate of degradation also increases. However, for a fixed current density and initial
concentration, the rate of degradation for DC was greater than that for AC. Pepprah and
Khire (2008) and Alshawabkeh and Sarahney (2005) also reported similar results during
electrolysis of naphthalene in an aqueous phase. Greater the current density, higher the
rate of production of hydroxyl radicals (Panizza and Cerisola 2009). Because hydroxyl

radicals are strong oxidizing agents, they react with MTBE and breakdown MTBE.

32

n:-

Pepprah and Khire (2008) report the reason for lower degradation rate for AC
compared to DC is because during an AC electrolysis, the current direction is reversed at
time period equal to the reciprocal of the AC frequency (equal to 5 sec when f = 0.1 Hz)
and this results in : (1) delay in mass transfer of the organic chemical to the electrode
where it is degraded; and (2) both electrodes act as instantaneous anode and cathode
where oxidation and reduction reactions occur, respectively, but some of these reactions
are reversed when the electrode changes from anode to cathode or vice versa. While the
rate of degradation for AC was lower than that for DC, Pepprah and Khire (2008) report
that the electrode decay in the presence of NaCl as the supporting electrolyte for AC was

relatively small compared to that for DC.

4.1.3 Effect of Electrode Material

Three electrode materials were tested: (1) uncoated 99% purity titanium; (2) solid
graphite; and (3) BDD. The initial concentration of MTBE and the current density (DC)
were fixed at 250 mg/L and 1 mA/cmz, respectively. Fig. 4.4 shows the normalized
concentration versus time for the three types of electrodes tested in the 1 L cells when DC
was applied. The rate of degradation of MTBE when titanium or BDD electrodes were
used was significantly greater than that for graphite (~ 13%). Titanium and BDD showed
very similar rates of degradation (about 82 to 89%). These results indicate that the rate of
degradation is a function of electrode material. It may be because the rate of production

of hydroxyl radicals is a function of the electrode material.

33

 

..L
'0

CO = 25 mg/L
Frequency: 0.1 Hz for AC

_L

9
m

,“AC-1mAmm2

- 2 mA/cm2

9
A

DC-O.5mA/cm2

-1mA/cm2

Normailized Concentration of MTBE (Ct / Co)
9 o
N O)

 

O

0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.1: Normalized concentration (Ct/Co) for MTBE for C0 = 25 mg/L with titanium
electrodes

—l
b

 

Co = 250'mg/L
Frequency: 0.1 Hz for AC

2 _ a ---------------
”K; Control—,‘fig

T“‘£L\\

 

_A

 

i

‘AC - 1 mA/cm2

N\
\ ::::::::T‘AC-2mAmm2T‘e

\

/

 

 

P
4;

>14

\\DC—1mAmm
O - i i
o 5 10 15 20 25
Elapsed Time (hrs)

 

 

9
m

 

Normailized Concentration of MTBE (01/ Co)
0
0')

 

 

 

 

 

Figure 4.2: Normalized concentration (Ct/Co) for MTBE for C0 = 250 mg/L with
titanium electrodes
34

0.6

0.4

0.2

Normailized Concentration of MTBE (01/ CO)

0.8

0.6

0.4

0.2

Normailized Concentration of MTBE (C t / Co)

 

 

 

 

 

 

 

       
  

 

 

 

 

 

1 : %
700:2,500 mg/L """"""""""""""""""" _
EAC, Frequency: 0.1 Hz
i l l i l
0 5 10 15 20 25
Elapsed Time (hrs)
1 I l !
"""""""""""" Control—e“
i l i l l
0 5 10 15 20 25

Elapsed Time (hrs)

35

Figure 4.3: Normalized concentration (Ct/Co) for MTBE for C0 = 2,500 mg/L with
titanium electrodes: (a) AC; and (b) DC

 

Because hydroxyl radicals are primarily responsible for degradation of organic molecules
such as MT BE (Pepprah 2007), the rate of degradation varies for various electrode
materials. Graphite electrodes most likely produced the least amount of hydroxyl radials.
Hence, while graphite is relatively cheap, it is not as effective in electrochemical

degradation of MTBE.

After the tests were completed, the electrode surfaces were observed and noted.
Titanium electrodes had deposits whereas BDD and graphite electrodes looked unaltered.

Figure 4.5 shows the surface of each of the electrodes before and after electrolysis.

Fig. 4.6 shows the normalized concentration versus time for titanium and BDD
when AC having current density equal to lmA/cmzwas applied. The rates of degradation
for both electrodes were about the same. However, these rates were significantly less than
those for DC. This finding is consistent with the AC and DC comparisons presented in

Figs. 4.1 to 4.3.

4.1.4 Effect of Initial Concentration of MTBE

Figs 4.7, 4.8, 4.9 and 4.10 show the normalized concentration versus time for
experiments where the initial concentration of MTBE was 25 mg/L, 250 mg/L, and 2,500
mg/L. Greater the initial concentration, lower was the rate of degradation. However, the
cumulative mass of MTBE degraded increased with increase in the initial concentration
of MTBE (Figs. 4.11 to 4.14). Wang et al. (2009) also reported similar results for

degradation of 4-chlorophenol during electrolysis.

36

 

 

 

 

 

   

Boo
: \
Titanium\ """"""""""""""""" ‘

 

 

 

02 a- DC. jqu_ =1 mA/cm2

Normailized Concentration of MTBE (Ct / CO)

 

 

 

 

0 i 1 i
0 5 10 15 20 25
Elapsed Tlrne (hrs)

 

Figure 4.4: Normalized concentration of MTBE with graphite, BDD and titanium
electrodes during DC Electrolysis

I.”
G

      

Graphite Boron Doped Diamond (BDD) Titanium

Figure 4.5: The electrode surface of graphite, BDD and titanium before (upper) and after
(below) electrolysis

37

Assuming all other parameters are fixed, the rate of production of hydroxyl radicals is a
function of the current density (Pepprah 2005; Yanqing et al., 2007). Hence, while the
initial concentration of MTBE was increased, the rate of production of hydroxyl radicals
was unchanged. The cumulative mass of MTBE degraded or converted increased (Figs.
4.11 to 4.14) as the initial concentration was increased because for a given current density,
more MTBE molecules were available in the solution for the hydroxyl radicals to react

with.

4.1.5 Effect of DI Water vs. Tap Water

Most experiments in this study were carried out using DI water as the electrolyte
with electrodes having polished surfaces to maintain the surface properties of the
electrodes consistent throughout the experimental program. The effect of using of tap
water as the electrolyte and potential electrode fouling during electrolysis were evaluated
by carrying out these tests where: (1) the electrolyte was tap water from the lab; and (2)
the titanium electrode surfaces from a previous test with tap water were not polished to
simulate the effect of electrode fouling. Initial concentrations equal to 25 mg/L and 2,500
mg/L and current density for DC equal to 0.5 mA/cm2 were used for these tests. The
results are presented in Figs. 4.15 and 4.16. The results show that the use of tap water
instead of DI water as the electrolyte and by not polishing the electrode surface after a

test had insignificant impact on the rate of degradation.

38

 

1.2 g , , .

o)

C o = 250 mg/L
rc' Frequency: 0.1. Hz for AC-

 

 
   

——D

Normailized Concentration of MTBE (01/ C

 

 

 

0.4 ~§ ------------------ ----------------- --------------- —
. _ 2 E E :
0.2 ~;"°‘C"eqv.'1 "'Ncm ----------------- i ---------------- —
0 l l l l l
o 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.6: Normalized concentration of MTBE solution for C0 = 250 mg/L with BDD
and titanium electrodes during AC electrolysis

 

1.2 i ! ! I

5 5 . AC -1 mA/om2
............. Frequency 0.1 Hz

 

Normailized Concentration of MTBE (Ct / Co)

 

 

 

0 l l l l l
0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.7: Effect of initial concentration of MTBE at AC density of lmA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes

39

 

 

1.2 I 1 I I ‘

AC - 2 mA/om2
Frequency: 0.1 Hz

 

Normailized Concentration of MTBE (Ct / Co)

 

 

 

0 5 1o 15 2o 25
Elapsed Time (hrs)

Figure 4.8: Effect of initial concentration of MT BE at AC density of 2 mA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes

 

1 .2 ! ! IT ! I
' ' ' ‘ DC - 0.5 mA/cm2

Normailized Concentration of MTBE (C 1 / Co)

 

 

 

0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.9: Effect of initial concentration of MTBE at DC density of 0.5 mA/cm2 for CO
= 25, 250 and 2,500 mg/L with titanium electrodes

40

 

1.2 i l j l .
i ' I ' DC - 1 mA/cm2

 

 

Normailized Concentration of MTBE (C t / Co)

 

 

Elapsed Time (hrs)

Figure 4.10: Effect of initial concentration of MTBE at DC density of 1 mA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes

 

1400 I l I I

_ AC-1 mA/cm2
1200 t- ---------------- g ------------------ ------ Frequency: 0.1 Hz
was ................. ————————————————— ---------------- ............... —

Cumulative MTBE Mass Converted (mg)

 

 

 

 

 

Elapsed Time (hrs)

Figure 4.11: Cumulative MTBE mass converted at AC density of 1 mA/cm2 for C0 = 25,
250 and 2,500 mg/L with titanium electrodes
41

 

5 5 , AC-2mA/cm2
1200 _ ................ g ------------------ ------ Frequency:0.1HZ

  

Cumulative MTBE Mass Converted (mg)

 

 

 

 

 

600 .................................................................................... _.
400 —- -------------------------------- i --------------------------------------------------- .—
200 ------“ ---------------- -1
. . __250mg/L_____e
0 1 Q ‘ —=.=25mg/l7——'A
O 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.12: Cumulative MTBE mass converted at AC density of 2 mA/cm2 for C0 = 25,
250 and 2,500 mg/L with titanium electrodes

 

1400 I 1 1 i

g : DC-O.5 mA/cm2
1200 — ---------------- g ------------------ ---------------- -
1000 — ---------------- ----------------- --------------- -

Cumulative MTBE Mass Converted (mg)

 

 

 

 

 

0 5 10 15 2o 25
Elapsed Time (hrs)

Figure 4.13: Cumulative MTBE mass converted at DC density of 0.5 mA/cm2 for C0 =
25, 250 and 2,500 mg/L with titanium electrodes

42

 

1 400 I 1 f
1 200

1000

is 01 on
8 8 8

Cumulative MTBE Mass Converted (mg)
N
8

 

 

 

 

 

o 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.14: Cumulative MT BE mass converted at DC density of lmA/cm2 for C0 = 25,
250 and 2,500 mg/L With titanium electrodes

43

Pepprah and Khire (2008) achieved significantly lower removal rate of

phenathrene and pyrene when they used unpolished titanium electrodes with AC density
equal to 18.5 mA/cmz. Greater current density may result in greater electrode fouling

which impacts the rate of degradation. While there was no significant difference in the
rate of degradation, the applied voltage increased more rapidly when tap water and
unpolished electrodes were used. Electrical conductivities of DI water, the tap water, and
300 mg of Na2S04 dissolved in 1 L of DI water were 0.9, 754, and 866 1.18, respectively.
Hence, more rapid increase in the voltage during the tests when tap water and unpolished
electrodes were used was most likely due to deposit formation or fouling of the electrodes
that resulted in a rapid increase in the resistance of the cell. In order to maintain the
current density, the constant current amplifier increased the voltage proportionate to the
resistance. White precipitates were observed after the test when tap water was used as the

electrolyte (Figs. 4.17 and 4.18).

Final voltage of 2,500 mg/L and 25 mg/L of MT BE solution in DI water was 45
and 48 V, respectively. For unpolished electrode for C0 = 2,500 mg/L, 25 mg/L (tap

water) and 25 mg/L (DI water), the final voltages were 52.5, 67 and 83 V.

4.1.6 Degradation Kinetics

The degradation rate of MTBE solution during electrolysis can be described as a

pseudo-first-order decay reaction, which is presented in Eq. 4.2:

dC
E——k[Cl (4.2)

44

DC - 0.5 mA/cm2 in DI water

DC - 0.5 mA/cm2 in Tapwater-

DC - 0.5 mA/cm2
electrodes
in Tapwater

Normailized Concentration of MTBE (C t / Co)

 

0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.15: Effect of electrolyte on the rate of degradation of MTBE for Co = 25 mg/L
at DC density = 0.5 mA/cm2 with titanium electrodes

 

l
Co = 2,500 mg/L

 

 

..u

. Control-

0.8 _ ___________ /;\\‘x I
: DC-O.5mA/cm 3 fl

(unpolished electrode)

 

 

 

 

 

 

 

 

 

 

 

Normailized Concentration of MTBE (Ct/ Co)

 

 

 

 

 

 

 

0'6 j V DC - 0.5 mA/cm2 """"
(clean electrode)
0.4 ----- -
0.2 4 ----------
0 l
0 5 1O 15 20 25

Elapsed Time (hrs)

Figure 4.16: Effect of electrolyte on the rate of degradation of MTBE for Co = 2,500

mg/L at DC density = 0.5 mA/cm2 with titanium electrodes
45

 

Clean electrode
(Front and Back sides)

    

 

 

 

 

 

Anode electrode Cathode electrode
after 24-hr test after 24-hr test
(Front and Back sides) (Front and Back sides)

aritiitl‘tiith-mi.

    

Figure 4.17: Photographs of titanium electrode surface after a 24-hr test when Co = 25

mg/L for MTBE in Tap water

 

Clean electrode
(Front and Back sides)

(Front and Back sides)

       

Anode electrode
after 24-hr test

 

(Front an

Cathode electrode
after 24-hr test
(1 Back sides)

 

 

n13" in“).

1,;
l

:éx
:5

     

   

. 3.3:;
,

Figure 4.18: Photographs of titanium electrode surface after a 24-hr test when C0 = 2,500
mg/L for MT BE in 300 mg of Na2S04 dissolved in l L of DI water

46

where C is the concentration of MTBE (mg/L); dC/dt is the rate of change of contaminant
concentration; and k is the pseudo-first—order rate constant (T'l). Half life of MTBE is

estimated from Equation 4.3

_ 0.693

’1/2 -7 ' (4.3)

The values of pseudo-first-order rate constant for each test were calculated by making a
ln[Ct/Co] versus time plot and measuring the slope of the line. Initial concentration of

MTBE, current type and current density, electrolyte, and electrode materials influenced
the k values (Table 4.3; Figs 4.19 and 4.20). Fig. 4.19 shows that the degradation rate
constant increases as the: (1) current density increases; (2) DC is used rather than AC;
and (3) initial concentration is lowered. The rate constant was highest for titanium

electrodes (0.09/hr) and was smallest for graphite (0.005/hr) (Fig. 4.20).

4.1.7 Measured Initial and Final Parameters
Table 4.4 summarizes measured initial and final (after the 24-hr test) parameters
during the tests. These parameters were: Voltage (V), Resistance (R) or Irripedance (Z),
pH, and Temperature (°C). During a majority of the experiments, the applied voltage
increased continuously during the electrolysis because the resistance of the cell increased.
Voltage went up more rapidly at greater current densities especially when DC was used.
For the DC tests, the initial resistance of the electrochemical cell ranged from 116

Q to 161 Q and the final resistance ranged from 122 Q to 191 O. For the AC tests, the

47

 

initial impedance of the electrochemical cell ranged from 146 Q to 188 Q and the final
impedance ranged from 131 Q to 172 O.

For the DC tests, initial pH ranged from 6 to 7.4 and final pH ranged from 6.4 to 8.1. For
AC tests, initial pH ranged from 6 to 7.3 and final pH ranged from 5.5 to 8.4. Wu (2007)
also observed increase in pH when DC was used during electrolysis of MT BE using
nickel electrodes. Pepprah and Khire (2008) reported increase in pH during DC and a
decrease during AC electrolyses. Thus, oxidation of water molecule was prominent
during AC and reduction of water was more dominant during DC electrolysis.

Room temperature during the tests ranged from 19 ~ 24 °C. However, during a
specific test, the room temperature usually fluctuated within 2 °C. The temperature of the
electrochemical cell followed the room temperature because it was immersed in a
relatively large water bath. However, when the current densities were 2 mA/cm2 and 4
mA/cmz, temperature of the cell increased to 29 to 32 °C due to joule heating. This
increase in temperature will have effect on volatilization and diffusion coefficient of
MTBE. As temperature increases, the rate of volatilization and diffusion coefficient
increased too. Hence, the control sample was placed in the same water bath adjacent to

the cell that was subjected to electricity to experience the elevated temperature.

4.2 ELECTRICAL ENERGY CONSUMPTION

Table 4.5 summarizes the total electrical energy consumed during the 24-hr
electrolysis for all tests. The electrical energy in k] and kWh was calculated from

equation (1.3) and kWh/L was computed from equation (1.4).

48

Table 4.3: Degradation of rate constant (k) and half—life of MTBE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

, .1qu Immersed k
( 3L) C;rrent ( IA) (mA/ 2::3936 Electrode Constant R2 t1/2
m ype 111 cm2) n s Area (cmz) (hr-1) (hr)
AC 110 1.0 55 0.049 0.98 14.3
220 2.0 . . 55 0.079 0.98 8.7
Titanium
55 0.5 55 0.155 0.96 4.5
25 110 1.0 55 0.152 0.99 4.6
DC 55 0.5 .T‘mmum 55 0.102 0.95 6.8
In tapwater
55 0.5 U“.p°“.5h°d 55 0.141 0.97 4.9
Titanlfln
110 1.0 , _ 55 0.016 0.98 43.0
Titanium
AC 220 2.0 55 0.025 0.98 28.2
680 5.0 Graphite 68 0.005 0.32 144.4
55 0.5 , . 55 0.040 0.95 17.2
Titanium
250 110 1.0 55 0.096 0.99 7.3
70 1.0 BDD 35 0.023 0.74 30.1
DC 70 1.0 35 0.075 0.97 9.2
58 0.5 68 0.003 0.77 256.7
136 1.0 Graphite 68 0.007 0.63 101.9
170 2.5 68 0.01 1 0.99 64.2
1 10 1.0 55 0.008 0.90 91.2
AC 220 2.0 55 0.017 0.87 39.8
440 4.0 mm 55 0.018 0.55 37.7
2 500 55 0.5 ' um 55 0.007 0.95 105.0
’ l 10 1.0 55 0.030 0.80 22.9
220 2.0 55 0.085 0.98 8.1
DC .
55 0.5 U“P°“.Sh°d 55 0.013 0.98 53.3
Titanium

 

 

 

 

 

 

 

 

 

Note: 1. Unless specified, all tests were carried out using DI water
2. AC Frequency = 0.1 Hz for all AC tests

49

Degradation Rate Constant, k (hr-1)

Degradation Rate Constant, k (hr—1)

 

  
 
 
 
 
 
  

 

 

1mA/cm2

 

 

2 mA/cm2

  

0.5 mA/cm2

 

r l . . .
I i D C
71856;; as}; 571' """"""""" "

- Eletrolysis for 24 hr

1 mA/cm2

lll'lllllll'lllllll

  

 

 

 

(b) Co = 250
Eletrolys

mg/L
is for 24 hr

............

 

 

 

 

 

 

 

1mA/cm2

 

 

2 mA/cm2

50

0.5 mA/cm2

 

  

1mA/cm2

I I

lllllllllllllllllll

 

 

 

o_14 ; ————————————— - --------------------------------------------------------- —‘
— (0) Co = 2500 mg/L -
- Eletrolysis for 24 hr -

0.12 r -------------

0.1 -_ ............. ....

0.08 L -------------

Degradation Rate Constant. k (hr-1)

O

O

O)
lillliilrii

 

 

 

 

 

 

 

 

 

 

I"

0

Figure 4.19: Estimated k (hr'l) for MTBE C0 = 25 mg/L (a); 250 mg/L (b); and 2,500
mg/L (c) using AC and DC during 24 hr electrolysis with titanium electrodes

 

0.16 . I r
L A

. . :

0.14 — ---------------------------- a ---------------------------------------------------------- —

E Co = 250 mg/L :

TE 0.12 __‘EIetI’OIy5|S for 24:hr _____________________________________________________ _
x . : _.
E 0.1 :---DC,] —1mA/cm2 ----------------------------------------------- -:
7;; . eqv 4
C - .1
8 - .
2 0.08 _- ---------------------------- -:
m - -1
cc _ .
.5 0.06 — ---------------------------- .5
15 - a
‘D ' .
g - .
g 0.04 _— ---------------------------- —_
o - -
0.02 — ---------------------------- —

o _

 

 

 

 

 

 

Graphite BDD Titanium

Figure 4.20: Effect of electrode material on k during DC electrolysis
51

888 O< :8 88 NE mo 83 55:35 O< .m
888828 8828:0983 .N
.88: 8883 HQ .888 850 :8 com 8883 m8 392 was: 8238a 850.585 A 8qu

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8.8 8.2 8.8 88 882 8.82 8.8 8.2 Neaeea. no 8 on

8.8 8.8 88 88 8.82 8.8_ 88: 8.8 8.8 88

8.8 8.8 8.8 88 8.5 8.82 8.8 8.8 on c: on

8.8 8.8 88 88 8.5 88: 8.8 8.8 ease; no 8 88.8
8.8 8.8 8.8 88 882 8.88 8.82 8.8 . . 8. 83.

8.8 8.8 888 88 28: 282 8.8 8.8 8.8 88 u<

m8 8.8 88 :8 8.5 8.88 8.8 8.8 3 o:

8.8 8.8 88 :8 888 888 8.2 8.: 3 8 on

8.8 8.8 88 88 888 8.88 8.: 8.2 mam 8.8 8 9..

8.2 8.8 :8 8.8 888 888 8.2 8.2 8:880 88 8 on

8.8 8.8 88 88 8.8 8.88 2.8 8.8 . 88 88 o< 88
8.8 8.8 88 88 888 8.8_ 8.8 8.8 3 8: on

#8 8.8 8.8 88 882 8.8: 8.3. 8.8 52:58 no 8

8.8 8.8 8: 88 882 882 8.8 8.8 . . 8.8 88 9..

8.8 <8 88 88 882 88: 8.8 8.8 8.8 o:

8 8.8 88 8.8 2.8.8 8.82 8.8 2.2 85258:. no 8 on 88
8 8.8 8.8 88 8.88 8.5 8.8 8.2 no 8 on .8
8.8 8.8 88 88 8.88 882 8.8 8.8 8.8 8: on

2: 8.8 8.8 88 8.88 8.5 8.8 8.8 5258 no 8

8.8 8.8 88 88 8.8: 8.82 8.8 8.8 8.8 88 8
8.8 8.8 28 88 882 8.88 8.8 8.8 A: o: o<

.88 .53 .88 .85.: .88 .855 as... .83 geese... $588.58 08 38.5
688582858. :8 68 8:889:— 98 85.3, 8888.8 .88 22:: 3.. co

 

 

 

 

 

 

 

 

 

=88 ova—oboe? of Co 238388 new In 883.3 ho EEEsm 31838 .H.

52

The electrical energy cost was assumed equal to 6.867 cents/kWh as presented in Energy

Information Administration (2008).

4.2.1 Effect of Current Density and Type

Figure 4.21 presents the cumulative energy consumed per unit volume of the
electrolyte during AC and DC electrolysis carried out for Co = 250 mg/L. For a given
current density, DC consumed more electrical energy compared to AC during the 24-hr
testing period. However, the rate of degradation as well as the MTBE mass degraded or
converted by DC was greater than that by AC for a given current density during the 24-hr
testing period. Hence, the cumulative MT BE mass converted during electrolysis was
normalized with respect to the cumulative electrical energy (Fig. 4.22). Fig. 4.22 shows
that DC was more energy efficient for degrading MT BE because it resulted in greater

mass converted per unit energy consumed.

4.2.2 Effect of Electrode Material

Figure 4.23 presents the cumulative energy consumed per unit volume of the

electrolyte during DC electrolysis carried out using titanium, graphite, and BDD
electrodes for MTBE Co = 250 mg/L and current density = 1 mA/cmz. While the rate of

MTBE degradation was highest for titanium, BDD consumed the least total electrical

energy.

53

4.2.3 Effect of Initial Concentration of MTBE

Figure 4.24 presents the cumulative electrical energy consumed per unit volume

of the electrolyte during AC electrolysis carried out for Co = O, 25, 250, and 2,500 mg/L
at current density = 1 mA/cmz. Fig. 4.24 shows that as Co increases, the total electrical

energy consumption during the 24-hr period decreases. Comparison of initial and final
voltages measured during the tests (Fig. 4.24) indicates that the final voltage decreased as
the initial concentration was increased. This may be because greater initial concentration
of MTBE resulted in greater concentration of ions formed as intermediate byproducts of
degradation of MTBE. Greater mass of ions reduced the impedance of the electrolyte and
hence the energy consumption was less.

Fig. 4.25 shows the cumulative MTBE mass converted during electrolysis
normalized with respect to the cumulative electrical energy. It can be observed from Fig.
4.25 that degradation of MT BE was more energy efficient at higher initial concentrations

of MTBE because it resulted in greater MTBE mass converted per unit energy consumed.

4.2.4 Effect of D1 vs. Tap Water as Electrolytes

Figure 4.26 presents the cumulative electrical energy consumed per unit volume

of the electrolyte during DC electrolysis carried out for Co = 25 mg/L at current density =
0.5 mA/cm2 using tap water and DI water as the electrolytes for polished and unpolished

titanium electrodes. The polished electrode in DI water consumed the least amount of
electrical energy during the 24-hr test duration. Unpolished electrode in tap water

consumed the most electrical energy.

54

 

Table 4.5 Electrical energy consumption during the tests

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

AC . . C0“
C0 / l Jeqv Electrode 24hr kWh/m3 kWh (USD)
(mg/L) DC (mA) (m A/cmz) materials (kJ/L) per
24hr
AC 1 10 1 .0 Titanium 321 30 13 22
25 AC 220 2.0 Titanium 1105 104 46 75
DC 55 0.5 Titanium 172 16 7 l 1
DC 1 10 1.0 Titanium 579 54 24 39
1 DC 55 0.5 Titanium 226 21 9 15
25 DC 55 0.5 Titaniumz 281 26 11 19
AC 1 10 1.0 Titanium 277 26 1 1 19
AC 220 2.0 Titanium 1056 100 44 72
AC 680 5.0 Graphite 3488 330 145 239
DC 55 0.5 Titanium 161 15 6 11
250 DC 110 1.0 Titanium 523 49 21 35
DC 70 1.0 BDD 95 14 4 6
DC 68 0.5 Graphite 67 6 3 4
DC 136 1.0 Graphite 282 26 1 l 19
DC 170 2.5 Graphite 973 92 4O 66
AC 1 10 1.0 Titanium 207 19 8 14
AC 220 2.0 Titanium 803 76 33 55
AC 440 4.0 Titanium 3419 323 142 234
2 500 DC 55 0.50 Titanium 141 13 5 9
’ DC 1 10 1.0 Titanium 552 52 23 37
DC 220 2.0 Titanium 1848 175 77 126
DC 55 0.5 Titaniumz 197 18 8 13
Note: 1. Electrolyte prepared using MSU tap water. For all other tests, DI water used.

2. Unpolished electrode.

3. AC Frequency is 0.1 Hz for all AC tests

55

1200

1000

800

600

400

200

Cumulative Electrical Energy Consumed (kJ/L)

0

 

 

 
   
  
    

 

 

 

 

 

? l l i
g co = 250 mg/L%
Initial / Final Voltage (Volts) j
2. AC - 2 mA/cm2: 49.0/61.0 ------------------ ‘ -------
2 AC - 2 mA/cm2
DC -1 mA/cm :28.0/70.0 1
__ oc - 0.5 mA/cmZ: 18.0/44.0 ............................ ________________ _
AC - 1 mNCmZ: 18.5 / 30.0 /E
_ ” DC - 1 irriA/cm2 """" fl
............... ---»~---»~~;~---»-DC-0.5mA/cm2~---—
- I ' v-‘EJ
. AC - 1 mA/cm2
I l i
o 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.21: Cumulative electrical energy consumption (kJ/L) at AC — 2 mA/cmz, DC — 1
mA/cmz, DC — 0.5 mA/cm2 and AC — 1 mA/cm2 with titanium electrodes

93’ Consumed (mg/Ki)
'oo —. iv '4;

P
0:

Cumulative Mass Converted versus
.0
.h

Cumulative Electrical Ener

.0
M

O

 

 

 

 

 

Co = 250 mg/L
DC - 0.5 mA/cm2
- 1 mA/cm2—’A
2
- 2 mA/cm —(—D
0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.22: Cumulative mass converted per unit electrical energy consumed for Co =
250 mg/L during AC and DC electrolysis using titanium electrodes

56

 

 

 

 

 

   

   

 

 

 

 

3 1200 W 1 l
\ t
3, Co=250 mg/L
'8 1000 -------- ‘ ------------ —
E
3
in
S
0 8m . ......................... __
3
3:; DC, jeqv. = 1 mA/cm2 .
E 600 mlnitiaI/Final Voltage (volts)
8 Titanium: 28.0 / 70.0
'5 Graphite: 20.0 / 17.0 3 :
g 400 — BDD:14.0/15.0 -------------- 2' -------------- g --------------- ~
Lu i Titanium ;
E 200 ' """""""" g """"""""" yGraphite”""‘g """""""" ‘
3 : 1 :
1 E ..‘4/9

o 0:: v l I

0 5 10 15 20 25

Elapsed Time (hrs)

Figure 4.23: Cumulative electrical energy consumption (kJ/L) for titanium, graphite and
BDD electrodes

 

500 T l |

AC - 1 mA/cm2
Frequency: 0.1 Hz
400 i

 

 

Initial / Final Voltage (Volts)
0 mg/L: 29.5 / 46.6 g
25 mg/L: 28.0 / 48.0 0 "(Q/L /A

300 ‘" 25o mg/L:18.5/30.0 ;/
2,500 mg/L:28.5/2a.o /25.mg/L/o
? -/

 

 

 

 

 

 

 

Cumulative Electrical Energy Consumed (kJ/L)

 

 

Elapsed Time (hrs)

Figure 4.24: Cumulative electrical energy consumption (kJ/L) at AC -1 mA/cm2 with
titanium electrodes

57

 

AC - 1 mA/cm2
Frequency: 01 Hz

 

 

 

 

 

 

Cumulative Mass Converted versus
Cumulative Electrical Energy Consumed (mg/KJ)

 

Elapsed Time (hrs)

Figure 4.25: Cumulative mass converted per unit electrical energy consumed for AC

electrolysis at current density = 1 mA/cm2 for Co = 25, 250, and 2,500 mg/L using
titanium electrodes

 

350 I I ”F lVlt l(V Its) '

nitia ina 0 age 0 ' _

Unpolished electrode in Tap water: 13.1 I83.0 Co " 25 mg/L

300 . Clean electrode in Tap water: 12.5 / 67.5'-~"'---~; ---------------- -*
Clean electrode in DI water. 18.0 / 48.0 '

  
  

250 — ---------------- ----- DCvO;5§mA/cm-2 ....... ............. _
(unpolished electrode in tap water) :

 
 

 

Cumulative Electrical Energy Consumed (kJ/L)

 

 

2m .. ........... 5 : . . .......... i ............. .4
150 — ----------- z ‘ '. --------------- E -------------- —
100 i 3 ------- f -------------- --------------- —
50 .. .. ........... [DC-0.5 [TIA/C1712 --._
' ' (clean electorde in DI water)
0 _ | I i
0 5 10 15 20 25
Elapsed Time (hrs)

Figure 4.26: Cumulative electrical energy consumption for tap water versus DI water as
electrolyte for Co = 25 mg/L using polished and unpolished titanium electrodes

58

 

350

I i I

!
.. 9 = C = 2,500 mg/L
Initial/Final Voltage (Volts) o ‘

 

 

 

 

 

 

3
\
3, .
g 300 —'Unpolishedelectrode:18.0/33.0 ------- ---------------- -
E Clean electrode : 15.6 /52.5 i 1
3 2 : I
‘0 _ ..... L ............... _
S 250 3 ; DC-0.5 mA/cm2 g
(i ' (unpolished electrode) 5
9 200 — ---------------------------------------------------- E - ------------- -4
<1)
C
LIJ
'5 150 — ------------------------------------------------------------------------------------ —
E
8
m 100 — ------------------------------------------------------------------------------- —
a)
.2 I - 2
E 50 ................ DC - 0.5 mA/cm ............ a
:E’ ' (clean electrode)
8 02 i i i
0 5 1O 15 20 25
Elapsed Time (hrs)

Figure 4.27: Cumulative electrical energy consumption for C0 = 2,500 mg/L using

polished and unpolished titanium electrodes in DI water

59

 

 

The rate of increase in the voltage as the experiment progressed indicates that when
unpolished electrodes were used in tap water, the fouling of the electrode surface resulted
in a more rapid increase in the resistance of the cell which resulted in greater power
consumption. Similar trend in energy consumption was observed when the initial

concentration of MTBE was 2,500 mg/L (Fig. 4.27).

4.3 ANALYTICAL MODELLING USING NERNST-PLAN CK EQUATION
Nemst-Planck equation (Eq. 1.8) can be used to predict migration of charged

specie subjected to electrical gradients in an aqueous solution. The equation calculates

migration under diffusive gradients, electrical gradients, and due to mechanical mixing or

convection. Convection is not considered for unstirred systems. Hence, the equation takes

this form (Eq. 4.4):

 

 

J,- (x) = —D,. (x) 36; (x) _ 41’ 01C.- 3¢(x)
x

RT ax (4'4)

The variables are described in Eq. 1.8. Eq. 4.4 was applied to predict the observed
changes in the MTBE concentrations during the tests carried out in this project.

The key assumptions that were made to use Eq. 4.4 for modeling the experimental data
are as follows.

1. Contaminants react and completely convert (or degrade) to byproducts when they
reach the electrode surface. Therefore, concentration of contaminant at both
reactive electrodes remains zero;

2. A concentration gradient is established that drives reactants by diffusion to the

surface of the electrodes;
60

3. Steady-state diffusion is assumed during the time step of 1 hr used to solve Eq.
4.4; and

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

MTBE diffusion coefficient (D ~ 8.63x10'6cm2/s) was calculated based on Hayduk-
Laudie equation (Eq. 1.9) and also was compared with that presented online at EPA’s
website. The input parameters used for solving Eq. 4.4 are as follow:

1. Diffusion Coefficient, D: 8.63 x 10'6 cm2/s

2. Gas constant, R: 8.31447 J/mol.K

3. Faraday’s Constant, F: 9.65x104 Coulomb

4. Surface area of Titanium electrodes: 55 cm2

5. Distance between the two titanium electrodes: 8 cm; graphite electrodes: 6.5 cm;
and BDD: 4 cm.
MTBE Concentrations measured during the tests and those predicted using the Nemst-

Planck equation (Eq. 4.4.) for initial concentration of MT BE equal to 25 mg/L for DC

electrolysis at 0.5 and l mA/cm2 current densities is presented in Fig. 4.28.

In order to apply Eq. 4.4 to predict the normalized concentration, the value of the
effective charge (z) was empiiically obtained from the best fit to the measured
concentration profile. The best fit shown in Fig. 4.28 was achieved when 2 was assumed
equal to -0.45. MTBE molecule does not have a net charge. However, it is hypothesized
that the ions present in the supporting electrolyte (anhydrous sodium sulfate) as well as

ions of intermediate or final byproducts that are formed during the electrolysis drag

61

MTBE molecules to the anode where it is oxidized. Fig. 4.28 shows that for z = -0.45, the
predicted normalized of concentrations for the two tests carried out at current densities

equal to 0.5 and 1 mA/cm2 are fairly accurate.

Figs. 4.29 and 4.30 present the measured and simulated normalized concentrations
of MT BE for Co = 250 and 2,500 mg/L. The simulated normalized concentrations agree
reasonably well with the measured normalized concentrations for Co = 250 mg/L when
the value of the effective charge is assumed equal to -0.2. For Co = 2,500 mg/L, the best
predicted fit with the measured data is obtained when 2 ranges from -0.06 to -0.12 for
current densities equal to 0.5 to 2 mA/cmz. One of the key trends observed here is that 2
decreases as the initial concentration of MT BE increases. This may be because the rate of
production of hydroxyl radicals responsible for oxidation of MTBE are produced at about
the same rate for a given current density irrespective of the initial concentration of MTBE
in the solution. In addition, one of the assumptions made for simulating normalized
concentration measured during the tests is that MTBE molecules, once they reach the
electrode surface, are instantaneously converted. This assumption may be more realistic
at lower concentrations of MTBE (or relatively high current densities) where the rate at
which hydroxyl radicals are produced at the surface exceeds the stoichiometric demand
of the radicals needed to breakdown the mass of MTBE migrated to the electrodes. This
may be the reason why the 2 values were not consistent when the equation was applied
for the tests carried out at relatively high initial concentration equal to 2,500 mg/L (Fig.

4.30).

62

 

 

 

 

A 1.2 1 I . ,
o - 3 2
0 ' ' 0:25 mg/L]
9: 1H ----------------- ------- -Square and Circle: ObservedC/C i -.
E Q -Dashed:Predictedq/Co
z 03 ...... x .................... .................. g .................. 2.-
‘5 C)“;J s i g s
c : E 3 I 5
8 O\ 5\ 3 z=-O.45; ; g
g 0.6 — ---------- \s-----\ ---------- z ----------------- ----------------- g---
E X \ g - 1 '
8 C?\ )1] 3 . .
c 3 x, ‘1 . 2 3 i
8 0.4 r """"""" : """"""""""" (“DC-0.5 mA/cm -
'O E \ 3 \ \g E I
g . \O E " . . .
:= 0.2 —-----~----? ------------- *"E‘DC-1mA/cm2”‘\“'\"\i ----------------- §---
§ 5 i T ‘ ‘ § 5 \ 1
2 o i 1 1 i‘~$&
4 9 14 19 24
Elapsed Time (hrs)

Figure 4.28: Experimental and predicted normalized concentrations of MT BE for C0 =
25 mg/L for DC electrolysis using current densities equal to 0.5 and 1 mA/cm2

 

 

 

 

A 1.2 i I l l I
O ; I
g . Co =250 mg/L
9: 1 I} }§ -------------------------- ------ - Square and Circle: Observed CIC --1
m $:EL . z=-02 :
E § - Dashed: Predicted q / Co 3
. \ 1 I
5 0.8 — ------------ \>\- ---------- ‘Eik ------------------------------------ 3 ------------------ 3—
3 C) 5‘ 2 E
c : \ 5 00-05 mA/cm i :
.g E \ 3 \ : 3 g
g 0.6 —---~-----~-~: ----------- \~_~-: ----------------- \\ ----------------- g--—
“E . \5 2x 5 .
8 ' DC 1mA/cm \r
c : O ; - ; H\ ;
8 0.4 T """"""" """"""""" '\"\\‘\\:_
t : \
IE", 0.2 _..-........:, """"""""" i """"""""" """"""""" i"§"\ """"" Em“
. , . . \ .
E \g
g :
0 l 1 I 1 l
4 9 14 19 24
Elapsed Time (hrs)

Figure 4.29: Experimental and predicted normalized concentrations of MTBE for C0 =
250 mg/L for DC electrolysis using current densities equal to 0.5 and 1 mA/cm2

63

 

 

 

 

0 5 i C =2,500 mg/L§
9 1 "<'"E'"“‘“'"""'"1"""""“""'J""“""""“". """"""""" 1“
ii ‘3 $8 " ~ +11- — '
l- O 3 DC- 0.5mA/cm2 ,z=-006
2 0.3 ----8\ ..... e —————————— .._ ..................... = .................. s ...... , ---.. _
c . \ ; \ \
5% § \ E DC-1mA/cm2,z=-0.08 .
e 0.6 ~------------. ----------- a <>--g --------------- . \ 5
E ' ’\E ' \ S Q
8 : E\ \ : :\ ‘ I
5 4 _ ............. ......... 5.. -3._
0 °- § g ‘00- 2mA/cm2, z=-0.12 g
8 : ' \ : '
g -Square, Circle and Diamond: Observed QIC x: 5
.6 02 -.é-.\-.\. .......... Eufl
E - Dashed: Predicted Ct/C \ \ 0
o o : ~.
2 O l L I i i
4 9 14 19 24
Elapsed Time (hrs)

Figure 4.30: Experimental and predicted normalized concentrations of MTBE for Co =
2,500 mg/L for DC electrolysis using current densities equal to 0.5 and 1 mA/cmz.

CHAPTER 5

SUMMARY AND CONCLUSIONS

In this lab-scale study, electrochemical degradation of MTBE dissolved in DI
water with Na2804 as the supporting electrolyte was evaluated. The effect of current type
and current density, electrode materials, initial concentration of MTBE in the solution, DI

water versus tap water as electrolyte, and effect of electrode fouling was evaluated by

measuring the concentration of MTBE in a l L cell during 24-hr tests. The current
densities ranged from 0.5 to 4 mA/cmz; current types applied were DC and square-wave

AC (f = 0.1 Hz); and the electrode materials tested were 99% pure titanium, solid

graphite, and boron doped diamond on a silicon wafer. The key findings are as follows:

1. Rate of degradation of MTBE increased when the current density was increased

from 0.5 to 4 mA/cmz;

2. Degradation rates during DC electrolysis were greater than those when AC was

used for a fixed current density;

3. Degradation rate when titanium electrodes were used was about the same as with
BDD electrodes. However, graphite electrodes had the least degradation rate. Visual
observation after the tests indicated deposits on the titanium electrodes. Such

deposits were not seen on the BDD and the solid graphite electrodes;

4. Degradation rates decreased as initial MTBE concentration increased probably

because the rate of production of hydroxyl radicals was not proportionately more
65

for reaction with MTBE and probably additional mass of byproducts also competed
with MTBE for degradation with hydroxyl radicals at higher initial concentrations

of MTBE;

. Degradation rates were similar when the electrolytes were DI water or tap water and
when the electrode surface was polished versus when it was left unpolished for
relatively low current density. However, the consumption of electrical energy was
not the same. Greater energy was consumed for unpolished electrodes used in tap

water. The reason for this is faster fouling of the electrode surface;

. Degradation kinetics of MTBE observed in the lab experiments followed pseudo-

first order decay equation; and

. Nemst-Planck equation was able to relatively accurately predict the concentration
of MTBE during DC electrolysis for lower initial concentrations (25 and 250 mg/L).
The predictions did not consistently match the measured values when the initial
concentration was relatively high (2,500 mg/L). The discrepancy is because the
equation is a transport equation that does not consider the rate of production of
reactive species. The predictions would be more accurate when the concentration of
the specie is relatively low or the applied current is relatively high. However,

additional experiments are needed to confirm this hypothesis.

66

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