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TRANSFORMATION OF CHLOROBENZENE AND 4-
CHLOROPHENOL IN GROUNDWATER BY
ELECTRO-FENTON AND SONO-ELECTRO-FENTON
REACTIONS
A Dissertation Presented
By
Roya Nazari
to
The Department of Civil and Environmental Engineering
in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in the field of
Civil and Environmental Engineering
Northeastern University
Boston, Massachusetts
December 2017
ii
ABSTRACT
The study investigates degradation of CB in groundwater by palladium (Pd)-catalyzed EF
reaction in both batch and plug flow column reactors. In both reactors, Pd-catalyzed EF
was initiated via in-situ electrochemical formation of hydrogen peroxide supported by Pd
catalyst. Depending on the reactor, different types of catalysts were used: Pd on alumina
powder, Pd on alumina pellets (Pd/Al2O3), and palladized polyacrylic acid (PAA)
polyviniledene fluoride (PVDF) membrane (Pd-PVDF/PAA) where each form of support
contained the same amount of Pd. In a mixed batch reactor under 120 mA and 10 ppm
Fe(II), 2 g/L Pd/Al2O3 power and an initial pH of 3, CB degradation followed a first-order
decay rate leading to 96% removal within 60 minutes. Under the same conditions, but using
an innovative, rotating Pd-PVDF/PAA disk (27.8 mg/L immobilized Pd), 88% of CB was
removed.
In the column experiment, 71% of CB was degraded in the presence of 10 ppm
Fe(II), and 2 g/L Pd/Al2O3 pellets under 60 mA. Most of the contaminant removal occurred
within the Pd vicinity via electrochemically induced oxidation. The results show that the
EF reaction can be achieved under flow, without external pH adjustment and external H2O2
addition and can be further optimized and applied as a practical groundwater treatment
method.
iii
In the second part of the study, sono-electro-Fenton, SEF, was evaluated for
degradation of 4-CP in an aqueous solution. SEF ability to degrade 4-CP was compared
with the performance of each process (EF and sonolysis) individually. Initial pH, current
intensity, background electrolyte, Fe(II) concentration, Pd/Al2O3 catalyst dose, pulsed
ultrasound frequencies and sonifier amplitude were optimized in a two electrode (Ti/mixed
metal oxide) batch system. More than 90% of 200 mg L-1 4-CP concentration was removed
within 300 minutes in the presence of 80 mg L-1 Fe(II), 200 mA, 1 g L-1 Pd/Al2O3 catalyst
(10 mg Pd) and initial pH of 3. With ultrasound radiation under 70% amplitude and 1:10
ON/OFF ratio, the removal rate of 4-CP increased to 98% within the first 120 min. 4-CP
degradation efficiency was increased in the order: Electro-Fenton < Ultrasound < Sono-
electro-Fenton processes by 83%, 90%, and 100%, respectively.
iv
ACKNOWLEDGEMENT
It is gives me a great pleasure to thank the many people who made this dissertation
possible. I would not be able to finish my Ph.D. without receiving support from my
excellent advisor, committee members and my lovely family.
First and foremost I offer my sincerest gratitude to my Ph.D. supervisor, Professor
Akram Alshawabkeh, who has supported me throughout my dissertation and graduate
education. He has been a true role model to me with his patience, kindness, sympathy, and
knowledge. I attribute the level of my Ph.D. degree to his encouragement and effort and
without him this dissertation, would not have been completed. One simply could not wish
for a better supervisor. I would also like to thank my committee members, Dr. Philip
Larese-Casanova, Dr. Loretta A. Fernandez and Dr. Ljiljana Rajic for their technical
comments and help on this dissertation.
I would like to appreciate Dr. Ljiljana Rajic for guiding my research as well as for
being there for me not only as a great teacher but also as a wonderful friend and helping
me.
I am grateful to many people who have helped me all these years, administrative
personal and faculty at the Department of Civil and Environmental Engineering, especially
Mr. Michael MacNeil, Mr. Kurt Braun for their technical support to fabricate the
experimental set-up for this research, and many thanks to my colleague in the PROTECT
v
research team, PROTECT training core, Dr. Thomas Sheahan, Ms. Anne Magrath, Ms.
Melanie Smith, and Ms. Kristin Hicks, deserve special mention.
I would like to appreciate special thank Kim Timberly, Ashkan Ghanbarzadeh, and
Lura Slowinski for their amazing support and help in my writing and editing part.
I would also want to appreciate my wonderful friend Hiva Hosseini for all of her
support and all the good things she gave me for helping me achieve my goals in life during
all these years being hundreds of miles away from home.
I am heartily thankful to my entire extended family for providing a loving
environment for me. My brothers; Dr. Ramin Nazari for the memories of my childhood
would have been a dark night if it were not for a brother like you – the sun that lit up my
life, Dr. Reza Nazari for being the most significant character and help during my educations
and many thanks for giving your little sister big bundles of advice and support which helped
her take the little steps towards big goals in her life, Dr. Rouzbeh Nazari for all of his
endless support, help, many thanks that you stood up tall to defend me and walked with
your head high to set a perfect example for me, my lovely sister in laws Dr. Leeda, Dr.
Tannaz, and Dr. Maryam for their love and being a part of my life and my most amazing
and lovely nephews, Amir, Ali, Ryan, and Kian joon, could not ask more.
Lastly, and most importantly, I wish to thank, my parents separately who well
raised me, supported me, taught me, and loved me unconditional. And unfortunately, I am
extremely sad for not having my dad on my side in my special day of my life to show him
how grateful I am now to say he was the best role model and amazing dad ever anyone
could have ever asked for. And I wish he was here to support his only daughter in the most
important day of her life.
vi
To my mom who is the most reason behind of all my graduate education, there is
not enough words to describe my appreciation for her except saying that you are my whole
world and I appreciate you for everything that you have been doing for me since you gave
me a birth. Now I dedicate this dissertation to them.
This work was supported by the National Institute of Environmental Health
Sciences (NIEHS, Grant No P42ES017198). The content is solely the responsibility of
the authors and does not necessarily represent the official views of the National Institutes
of Health.
vii
TABLE OF CONTENTS
Chapter 1 Introduction ..................................................................................................1
Overview ........................................................................................................................................ 1
Objective of Research .................................................................................................................... 8
Organization of Thesis ................................................................................................................. 11
2 Chapter 2 Literature Review ...................................................................................12
Chlorobenzenes and their chemical and physical properties ...................................................... 12
4-Chlorophenols and their chemical and physical properties ..................................................... 15
Remediation Technologies........................................................................................................... 23
2.3.1 Advanced oxidation processes ................................................................................................ 23
2.3.2 Fenton process and electro-Fenton process ........................................................................... 27
2.3.2.1 Fenton process ............................................................................................................... 27
2.3.2.2 Electrochemical Advanced Oxidation Processes (EAOPs) based on Fenton’s reaction .. 30
2.3.3 Ultrasound and Sono-electro-Fenton Process ......................................................................... 33
2.3.3.1 Sound, Ultrasound, Cavitation and Sono-electro-Fenton ............................................... 33
Electrochemical remediation of groundwater ............................................................................. 48
2.4.1 Chlorinated Solvents Remediation .......................................................................................... 48
2.4.2 Electrochemical Oxidation ....................................................................................................... 49
2.4.3 Electrochemical Reduction ...................................................................................................... 50
3 Chapter 3 Material and methods .............................................................................52
Introduction ................................................................................................................................. 52
Treatment of chlorobenzene in simulated groundwater using Palladium-Catalytic electro-Fenton’s reaction....................................................................................................................................... 53
3.2.1 Materials .................................................................................................................................. 53
3.2.2 Analysis .................................................................................................................................... 55
3.2.3 Experimental Setup for a Batch Reactor ................................................................................. 56
3.2.4 Experimental Setup for a Column Reactor .............................................................................. 60
Treatment of 4-chlorophenol in aqueous solution by Sono-electro-Fenton reactions ............... 64
3.3.1 Materials .................................................................................................................................. 65
3.3.2 Experimental Setup ................................................................................................................. 65
3.3.3 Analysis .................................................................................................................................... 69
viii
3.3.4 Instrument ............................................................................................................................... 70
4 Chapter 4 Chlorobenzene removal by Palladium-Catalytic electro-Fenton’s
reaction..............................................................................................................................72
Introduction ................................................................................................................................. 72
Batch Experimental Setup ............................................................................................................ 72
4.2.1 Membrane characterization .................................................................................................... 72
4.2.2 Influence of pH on CB removal ................................................................................................ 75
4.2.3 Influence of Fe(II) concentrations on CB removal ................................................................... 77
4.2.4 Influence of Pd catalyst dose and form on CB removal ........................................................... 79
4.2.5 Influence of current intensity on CB removal .......................................................................... 84
Column Experimental Setups ....................................................................................................... 87
4.3.2 Influence of current intensity and flow rate on CB removal ................................................... 91
4-Chlorophenol Degradation in Aqueous Solution by Sono-electro-Fenton Reaction ................ 95
4.4.2 Results of Batch Experimental Setup’s: Electro-Fenton Optimization .................................... 95
4.4.2.1 Influence of different Fe2+ concentrations ..................................................................... 95
4.4.2.2 Pd catalyst ....................................................................................................................... 99
4.4.2.3 Current intensity ........................................................................................................... 102
4.4.2.4 Background electrolyte ................................................................................................. 105
4.4.3 Sono-Electro-Fenton (SEF) ..................................................................................................... 107
4.4.4 Comparison EF, Ultrasound, and SEF .................................................................................... 110
4.4.5 Oxidation mechanism ............................................................................................................ 113
5 Chapter 5 Conclusions ............................................................................................115
Summary .................................................................................................................................... 115
Conclusions ................................................................................................................................ 115
REFERENCES ............................................................................................................................................ 120
ix
LIST OF TABLES
Table 1. Chemical and physical properties of CB (USEPA 1995) ................................... 16
Table 2. Categories of 19 various CPs (Jeffrey and Koplan 1999) .................................. 17
Table 3. Chemical Identity of Chlorophenol Compoundsa ............................................... 18
Table 4. Physical and Chemical Properties of Chlorophenol compoundsa ....................... 20
Table 5. Classification of conventional AOP's ................................................................. 25
Table 6. Relative oxidation power of some oxidizing species (Ullmann’s. 1991) ........... 26
Table 7. Different types of electrochemical Fenton reactions, with the Fenton reagent
produced shown in boldface. ............................................................................................ 32
Table 8. Most recent studies on SEF Method ......................................................... 43
Table 9. Batch test experiments ........................................................................................ 59
Table 10. Column experiments test design ....................................................................... 63
Table 11. EF test experiments design ............................................................................... 69
Table 12. Batch tests results .............................................................................................. 86
Table 13. Column tests results .......................................................................................... 94
Table 14. Batch tests results .............................................................................................. 98
x
LIST OF FIGURES
Figure 1: General Structure of CB (Lenker, Harclerode et al. 2014) ............................... 14
Figure 2. Chemical structure of 4-CP ............................................................................... 23
Figure 3. Ultrasound range diagram ................................................................................. 35
Figure 4. Diagram cycles of rarefaction and compression ............................................... 36
Figure 5. Cavitation bubble growth and collapse ............................................................. 38
Figure 6. Cavitational zone ............................................................................................... 39
Figure 7. Batch reactor with: a) Pd/Al2O3 powder, b) Pd membrane (Pd-PVDF/PAA), c)
electrodes (Ti/MMO mesh, Iron electrodes) and (d) Pd membrane set up along with MMO
electrodes .......................................................................................................................... 58
Figure 8. a) A schematic of three electrode column b) Actual column setup ................... 61
Figure 9. A schematic of three electrode column a) column reactor, a) mixed metal oxide
(MMO, b) Pd pellets, c) glass beads, and d) pump ........................................................... 62
Figure 10. a) Schematic batch setup b) Actual batch setup .............................................. 68
Figure 11. SEM images and EDS spectra Pd nanoparticles in functionalized membrane. a)
Top surface bare PVDF membrane, b) top surface Pd-PVDF/PAA membrane, c) FIB cross-
section cut of Pd-PVDF/PAA membrane, d) EDS mapping of top surface of Pd-
PVDF/PAA ....................................................................................................................... 74
Figure 12. TEM images and EDS spectra Pd nanoparticles in membrane. a) Pd
nanoparticles in FIB cross-section lamella, b) SAED pattern corresponds approximately to
xi
(111) of Pd0, c) Pd nanoparticle size distribution, d) EDS of Pd nanoparticles (presence of
Cu from sample tip) .......................................................................................................... 75
Figure 13. Degradation profile of CB at different initial pH values (Conditions: Fe(II): 10
ppm, current intensity: 60 mA, Pd: 20 mg/L, Na2SO4: 10 mM, different pH and CB: 10
mM)................................................................................................................................... 77
Figure 14. a) Effect of Fe(II) concentration on CB concentration decay, and b) Fe
concentration versus CB removal efficiency (Conditions: different Fe(II) concentration,
current intensity: 60 mA, Pd: 20 mg/L, Na2SO4:10 mM, pH=3 and CB: 10 mM) ........... 79
Figure 15. a) Degradation profiles of CB using different Pd/Al2O3 doses, and b) correlation
between Pd dosage and CB removal efficiency (Conditions: Fe(II): 10 ppm, current
intensity: 60 mA, Na2SO4:10 mM, pH=3 and CB: 10 mM) ............................................. 80
Figure 16. a) Comparison of Pd/Al2O3 performance with Pd membrane, and b) H2O2
production during the course of treatment (Conditions: Fe(II): 10 ppm (no Fe(II) added for
H2O2 production measurement), current intensity: 60 mA, Na2SO4:10 mM, pH=3 and CB:
10 mM).............................................................................................................................. 83
Figure 17. a) Degradation profile of CB in different current intensity values, and b)
correlation between removal efficiency and applied current intensity (Conditions: Fe(II):
10 ppm, Pd: 20 mg/L, Na2SO4:10 mM, pH=3 and CB: 10 mM) ...................................... 85
Figure 18. Effect of Pd catalysts presence on degradation of CB (Conditions: Fe(II): 10
ppm, different Pd dosage current intensity: 60 mA, Na2SO4: 10 mM, Q: 2 ml min−1, and
CB: 10 mM) ...................................................................................................................... 89
xii
Figure 19. Effect of Fe(II) concentration on degradation of CB (Conditions: different Fe(II)
concentrations, current intensity: 60 mA, Pd: 10 mg/L, Na2SO4:10 mM, Q:2 ml min−1, and
CB:10mM) ........................................................................................................................ 90
Figure 20. Effect of different current intensity on degradation of CB (Conditions: Fe(II):
10 ppm, different current intensities, Pd: 10 mg/L, Na2SO4: 10 mM, Q:2 ml min−1, and CB:
10 mM).............................................................................................................................. 92
Figure 21. Effect of different flow rate on degradation of CB (Conditions: Fe(II): 10 ppm,
current intensity: 60 mA, Pd: 10 mg/L, Na2SO4: 10 mM, different flow rates, and CB: 10
mM)................................................................................................................................... 93
Figure 22. a) Effect of Fe2+ concentration on 4-CP decay, and b) effect of iron anode on 4-
CP decay (Conditions: different Fe(II) conc., current intensity: 200 mA, Pd/Al2O3: 1 g,
Na2SO4:10 mM, pH=3 and 4-CP: 200 ppm) ..................................................................... 97
Figure 23. a) Degradation profiles of 4-CP using different Pd/Al2O3 doses, b) degradation
profiles of 4-CP using different Pd/Al2O3 doses on TOC, and c) degradation profiles of 4-
CP using different types of Pd (Conditions: Fe(II): 80 ppm, current intensity: 200 mA,
different Pd/Al2O3 conc., Na2SO4:10 mM, pH=3 and 4-CP: 200 ppm) ......................... 102
Figure 24. a) Degradation profile of 4-CP in different current intensity values (Conditions:
Fe(II): 80 ppm, different current intensity, Pd/Al2O3: 1 g, Na2SO4: 10 mM, pH=3 and 4-
CP: 200 ppm) .................................................................................................................. 103
Figure 25. Degradation profile of 4-CP in different background electrolytes (Conditions:
Fe(II): 80 ppm, current intensity: 200 mA, Pd/Al2O3: 1 g, different background, pH=3 and
4-CP: 200 ppm) ............................................................................................................... 107
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Figure 26. a) Degradation profile of 4-CP over time with different amplitudes and ON/OFF
ratios, b) TOC over time with different amplitudes and ON/OFF ratios and c) temperature
over time with different amplitudes and ON/OFF ratios (Conditions: Fe(II): 80 ppm,
current intensity: 200 mA, Pd/Al2O3: 1 g, Na2SO4:10 mM, pH= 3, 4-CP: 200 ppm and
different amplitude) ........................................................................................................ 110
Figure 27. a) Effect of EF, Ultrasound & SEF on 4-CP degradation, b) effect of EF,
Ultrasound (US) & SEF on phenol degradation, and c) effect of EF, Ultrasound (US) &
SEF on H2O2 production ................................................................................................. 113
Figure 28. Degradation profile of 4-CP over time with different concentration of Tert-butyl
(Conditions: Fe(II): 80 ppm, current intensity: 200 mA, Pd/Al2O3: 1 g, Na2SO4=10 mM,
pH=3 and 4-CP: 200 ppm) .............................................................................................. 114
1
Chapter 1 Introduction
Overview
Groundwater makes up about 30% of the freshwater on Earth, and has been a stable
source of water for humans for thousands of years, in both rural areas and large cities.
Approximately 40% of all drinking water comes from groundwater, about 97% of the rural
population relies on groundwater as a drinking water source, and about 30 to 40% of the
water used for agriculture comes from groundwater (Tomašević and Gašić 2015). Due to
several groundwater pollution sources, groundwater remediation is vital. It is easy to
discern if surface waters have become polluted, but groundwater sources are more difficult
to determine. Organic contaminants have been found in surface water and groundwater
supplies (Ghaly, Härtel et al. 2001).
Groundwater is sensitive to contamination due to improper waste disposal and is
considered a serious problem in the areas where the population relies on groundwater as a
main source of drinking water. Since contaminated groundwater can cause adverse effects
on human health and wellness of the ecosystems, advances in groundwater remediation
technologies are of great importance. In situ electrochemical transformation is an example
of the efficient and robust remediation technologies to clean groundwater contaminations
due to their ability to manipulate redox conditions to transform pollutions into non-toxic
forms.
When contaminants are released to the environment, some forms of the contaminants
infiltrate through the soil, some evaporate in the air, some get trapped by the top soil and
2
the vadose zone, and some migrate vertically by gravity through cracks and permeable soils
(NRC 2005, Cheng, Zeng et al. 2016). Common causes of groundwater and soil
contamination are dense non-aqueous phase liquids (DNAPLs) which are the defined as
the chemicals that are heavier than water; once released into the environment, they remain
in surface and groundwater as a separate phase liquid and cannot dissolve easily in water.
Chlorobenzenes, chloromethane, carbon tetrachloride (CCl4), trichloroethylene (TCE) and
tetrachloroethylene (PCE) are the most common DNAPLs.
Many organic compounds are considered toxic and harmful to human and ecosystems,
even when present at very low concentrations. They cannot be removed either by
conventional physical separation methods or by biological processes due to the recalcitrant
nature of the contaminants present (Detomaso, Lopez et al. 2003). Among those,
chlorinated benzenes and chlorinated phenols are of great concerns due to serious threats
to both human’s health and environment, and have been the focus on this study.
Chlorinated benzenes, such as CB and 1,2-dichlorobenzene (DCB) are widely used as
chemical intermediates and solvents across industries. In addition, chlorobenzene is used
as a raw material for synthesis of triphenylphosphine (catalyst for organic synthesis)
phenylsilane and thiophenol (intermediate for pesticides and pharmaceuticals). The
extensive use of CBs has led to their widespread release into environment during
manufacture in the production of other chemicals or during the disposal of chlorinated
benzenes at hazardous waste sites and incinerators (Guerin 2008, Zhang, Leng et al. 2011).
Chlorinated benzene are hydrophobic, chemically stable in nature, toxic, colorless
liquid, and highly volatile. Due to their high toxicity, low biodegradation properties, and
3
persistence presence of these chemicals in the environment is of great concern.
Furthermore, sorbed chlorinated benzenes may act as a long term source of groundwater
pollutions. Through the food chain, they can accumulate in the human body which can
cause cancer and mutagenesis. It can also damage the central nervous system and produce
anesthetic effects (Ziagova and Liakopoulou-Kyriakides 2007, Liu, Zhao et al. 2009, Liu,
Chen et al. 2011).
The degradation of 4-chlorophenol (4-CP) in groundwater was studied as second
subject of this dissertation. Chlorophenol compounds and their derivatives have become
significant contaminants in the environment during the past five decades. Their general
management, disposal and treatment are considered an important problem in the
environment and health sectors (Weber, Gaus et al. 2008). Due to the formation of
electrophilic metabolites, chlorophenol transformation leads to an increase in the toxicity
of intermediate compounds which may bind and cause damage to DNA or to gene products
(Michałowicz and Duda 2007).
Chlorophenols (CP) are synthetic organic compounds that are introduced into the
environment as a result of chemical and pharmaceutical activities (Jensen 1996, Czaplicka
2004, Michałowicz and Duda 2007, Igbinosa, Odjadjare et al. 2013), or through the
production use and degradation of numerous pesticides, like chlorinated cyclohexane
(Abhilash and Singh 2008) and chlorobenzene (Balcke, Wegener et al. 2008). Once
released to the environment, they are subject to physical, chemical, and biological
transformations. The primary processes governing their fate and transport are sorption,
volatilization, and degradation (Health and Services 1999).
4
Halogenated aromatic compounds are more poisonous and more difficult to treat by
ordinary biodegradation compared to aromatic compounds. To design successful treatment
systems for groundwater, treatability data are obtained by conducting laboratory studies
using polluted groundwater. Bench and pilot-scale treatability studies are valuable for
understanding the processes. Advance Oxidation Processes (AOPs), which involve
chemical, photochemical or electrochemical techniques were discussed by Glaze et al. in
1987 and other researchers, and are considered an effective method for water and
groundwater treatment (Glaze 1987, Ghaly, Härtel et al. 2001, Vázquez, Mughari et al.
2008, Tomašević and Gašić 2015).
AOPs have shown great potential and have proven to be very effective for the removal
of toxic and/ or biorefractory organic (and sometimes inorganic) contaminants from
aqueous solution (such as pesticides, artificial sweeteners, pharmaceuticals and personal
care products, and coloring matters) by oxidation through reactions with OH• (Calza,
Sakkas et al. 2013, Janin, Goetz et al. 2013, El-Ghenymy, Centellas et al. 2014, Rodrigo,
Oturan et al. 2014, Wang, Guo et al. 2014). AOPs are considered very efficient for
aromatic molecules degradation due to the electrophilic aromatic substitution of OH•,
which then leads to the opening of the aromatic ring. In order to reduce the toxic organic
compounds concentration that are recalcitrant to biological wastewater treatments, AOPs
have been successfully used as pretreatment methods (Detomaso, Lopez et al. 2003,
Stasinakis 2008, Cheng, Zeng et al. 2016).
In the presence of one or more primary oxidants (such as, ozone, hydrogen, peroxide,
oxygen) and/or energy sources (like ultraviolet light) or catalysts (such as titanium
5
dioxide), OH• will be produced, in a process which can be applied in water. Almost any
compound present in the water matrix can be oxidized by OH•, which can control the
diffusion reaction speed. AOPs, when applied with the most efficient conditions, have been
shown to reduce the concentration of a range of contaminants in water from several
hundred ppm to less than 5 ppb.
AOPs are based on in situ generation of highly powerful chemical oxidants such as
hydroxyl radicals (OH•, E=2.33 V or E°=2.80 V/SHE) (Evgenidou, Konstantinou et al.
2007, Salazar, Sirés et al. 2013), making them the second strongest oxidizing agents after
fluorine (Mousset, Oturan et al. 2014a). OH• have recently emerged as an important class
of technologies for accelerating the oxidation and destruction of a wide range of organic
contaminants in polluted water and soil, due to being reactive electrophiles (electron
preferring) that react rapidly and non-selectively with nearly all electron-rich organic
compounds (Brillas, Sirés et al. 2009, Mousset, Oturan et al. 2014b). Contaminants can be
broken and converted into small inorganic molecules when OH• is formed, and the radicals
react unselectively. In fact, a large number of organic chemicals influenced by OH• attack
and convert them to less complex and less harmful intermediate products.
High rates of pollutant oxidation, flexibility concerning water quality variations, and
the small dimensions of equipment, are the main advantages of the AOP method. For
example, AOPs can decrease 100 ppm pollutant’s concentration to less than 5 ppb and
break it down to COD and TOC, which earned it the credit of “water treatment processes
of the 21st century” (Naddeo, Rizzo et al. 2011, Shokri 2017). On the other hand, high
6
treatment costs and special safety requirements due to the use of very reactive chemicals
(ozone, hydrogen peroxide, etc), and high-energy sources (UV lamps, electron beams, and
radioactive sources) are the main disadvantages of these methods (Kochany and Bolton
1992, Goi 2005).
Among AOPs, the electrochemical advanced oxidation processes (EAOPs), and in
particular, "Electro-Fenton (EF)" and "Sono-electro-Fenton (SEF)", have demonstrated
good prospective reduction of pollution. Coupled EF and ultrasound radiation (sonolysis)
produce strong oxidizing agents such as hydroxyl radicals and have been of interest for
removal of chlorinated compounds from water. Through sequential oxidation and
reduction reaction, these processes have the potential to treat different types of pollution
from groundwater. Complete degradation of a broad range of harmful pollutants can be
achieved through electrochemical techniques before they reach the receiving aquatic
environment. These techniques are well known as “green technology” methods as few or
no chemicals are needed to facilitate water and groundwater treatment (Särkkä, Bhatnagar
et al. 2015).
Electro-Fenton (EF) and ultrasound radiation (sonolysis) have been an effective
method for transformation of chlorinated solvents in groundwater due to the production of
strong oxidizing agents such as OH•. Electrochemical treatments are recognized to be
environmentally-friendly as they can be performed in-situ without external chemical
additions (Alshawabkeh and Sarahney 2005, Alshawabkeh 2009, Martínez-Huitle and
Brillas 2009), which lowers the cost of remediation. The EF method allows better control
7
of H2O2 and OH• generation, accelerates the production rate of OH• compared to traditional
Fenton’s method, and supports reduction of Fe3+ to Fe2+ at the cathode (Yuan, Gou et al.
2013b).
Sonolysis has also demonstrated a great potential for degradation of contaminants.
Ultrasonic radiation causes the formation of small bubbles with extreme local temperature
and pressure that subsequently collapse and result in water decomposition to hydrogen
atoms and OH•. Application of an ultrasonic field is also a promising process for decreasing
the gas bubbles production on the surface of the electrode and the void fraction inside the
electrolyte bulk. Acoustic waves will oscillate the adhered gas bubbles at the surface of the
electrode, leading to the removal of gas bubbles from the surface of the electrode through
the wave’s vibration (Ellenberger, Van Baten et al. 2003, Wang and Chen 2009). The
oxidation of compounds occurs via reaction with OH• produced by cavitational collapse,
which denotes ultrasound power. The presence of OH• and/or extreme temperatures and
pressure leads to contaminant degradation via oxidation and/or thermolysis (Nagata,
Nakagawa et al. 2000). The combination of ultrasound radiation with EF has resulted in
an increase of contaminant removal compared to using each method separately (Trabelsi,
Ait-Lyazidi et al. 1996, Yasman, Bulatov et al. 2004, Oturan, Sirés et al. 2008).
8
Objective of Research
The main objective of this study was to investigate the removal of chlorinated
aromatic contaminants such as CB and 4-CP from groundwater using electro-Fenton (EF)
reaction supported by Pd catalyst in the presence and the absence of sonolysis (sono-
electro-Fenton or SEF) processes. A two-electrode system with manually adjusted pH
values was used for batch tests, and three sequential electrodes (one anode and two
cathodes) were used in the electrochemical column for automatic pH regulation by current
manipulation. The study investigates the use of different Pd catalyst supports and modes
of use such as Pd immobilized on polyacrylic acid (PAA) polyviniledene fluoride (PVDF)
membrane (Pd-PVDF/PAA) as static disk and an innovative rotating disk. The Pd-
PVDF/PAA is a promising catalysts support that is easily applied and manipulated
comparing to Pd catalyst powder which requires water filtration as an additional treatment
step after the contaminant removal. Pd catalyst on different support materials was used to
support H2O2 production rate by catalytic combination of electro generated H2 and O2 on
Pd surface in both batch and column reactors (H2O2 was further activated to OH radicals
via reaction with Fe(II). Sonolysis was introduced to EF to enhance the degradation of
contaminant by facilitating production of hydroxyl radicals, increasing mass transfer and
supporting degradation in cavitation bubbles. To do so, the following tasks were identified:
1. CB was used as target compound.
2. Current intensities: control, 40, 60, 120 mA were used with two or three
electrodes setup.
3. Effect of different range of pH (3, 4, and 6) were studied.
9
4. Effect of different concentration and types of Pd for example Pd immobilized
on polyacrylic acid (PAA) polyviniledene fluoride (PVDF) membrane was used
as a static disk (Pd-PVDF/PAA), Pd/Al2O3 powder and Pd/Al2O3 pellet were
used.
5. Effect of different ferrous ion concentrations of 0, 2, 4, and 10 were considered.
6. Production of hydroxyl radicals and hydrogen peroxide were measured.
7. Effect of different types of background electrolyte such as Na2SO4 and
NaHCO3.
8. For column experiment, flowrates of 2, 5, 10 ml/min were used.
Sonolysis was introduced to EF to enhance the degradation of contaminant by
facilitate production of hydroxyl radicals and increasing mass transfer. To optimized SEF
following tasks were performed:
1. 4-CP was used as another target compounds.
2. Control, 60, 120, 200 mA current intensities were used.
3. Different Pd types such as Pd/Al2O3 and Pd/C were compared.
4. Two different types of electrodes such mixed metal oxide (MMO) and iron
electrode were investigated.
5. Different sonication’s amplitude values of 10, 20, 30, 50, and 70 percent were
studied.
6. ON/OFF ratio time values of 0.1 and 0.2 were carried out.
7. Different background electrolyte were studied like, NaNO3, Na2SO4, NaHCO3.
10
8. Tert-butyl were used as hydroxyl radicals’ scavenger for oxidation
mechanism.
9. The effect of EF, ultrasound and SEF on removal efficiency of contaminants
were compared together
11
Organization of Thesis
The organization of this dissertation is as follows. Chapter 1 includes a brief
introduction of the groundwater contamination and treatment methods, and introduces the
objectives of this research. Chapter 2 provides literature review of chlorobenzene (CB) and
4-chlorophenol (4-CP), especially, their physical and chemical properties, their influences
on the environment. Subsequently, an overview of electrochemical remediation
technologies, AOPs for water purification, Fenton and electro-Fenton process,
electrochemical advanced oxidation processes (EAOPs) based on Fenton’s reaction,
ultrasound and Sono-electro-Fenton Process, sound, ultrasound, cavitation and Sono-
electro-Fenton, electrochemical remediation technologies in groundwater, introduction to
electrochemical remediation technologies for chlorinated solvents, and electrochemical
oxidation and reduction. Chapter 3 describes introduction, experimental methods,
procedures, analytical methods, sampling processes and equipment for both CB and 4-CP.
The experimental apparatus is explained in details and the chemicals and instruments are
listed. Chapter 4 presents the results and discussion for treating of CB in simulated
groundwater using Palladium-catalytic electro-Fenton’s reaction, also on removal
efficiency of 4-CP in aqueous solution by Sono-electro-Fenton reactions. Variables such
as temperature, flow rate, catalyst type, and current densities have been investigated and
summarized. Chapter 5 provides the summary and conclusions.
12
2 Chapter 2 Literature Review
Groundwater is considered as one of vital sources of water supply and frequently
serves as a major source of drinking water. Groundwater quality can be affected by natural
processes and different types of human activities (or man-made products like gasoline and
chemicals) which release contaminants into environment (e.g., chemical industry,
agriculture). In addition, degraded surface water system, loss of water supply, poor
drinking water quality, and high cleanup costs are results of groundwater contamination.
Groundwater pollution can pose great threat to human health and the ecosystems and lead
to loss of water supply.
Advanced oxidation processes (AOPs), based on generation of strong oxidants such
as hydroxyl radicals (•OH), are effective in degrading and transforming organic
contaminants in groundwater to nontoxic byproducts. Electrochemically-induced AOPs
allow for in situ generation of redox conditions, easy manipulation and control of the
groundwater chemistry, cost-effectiveness and sustainability by using alternative power
sources.
Chlorobenzenes and their chemical and physical
properties
Chlorobenzene (CB) is produced commercially by the chlorination of benzene in the
presence of a catalyst (e.g., ferric chloride, aluminum chloride, or stannic chloride). The
13
water solubility of CB is 0.5 g l−1; some of it will dissolve in water but it can easily
evaporate into air. CB is a flammable, colorless, aromatic, chlorinated solvent with an
almond-like odor that is generally used as an intermediate in the synthesis of various
pesticide formulations, degreasers, dyes, and solvents. It also is used as an intermediate in
phenol and DDT production. Furthermore, in production of ortho- and para-
nitrochlorobenzenes which are used as intermediates in the manufacture of rubber
chemicals, agricultural chemicals, and antioxidants, CB is used as a chemical intermediate
(USEPA 1995).
CB compounds do not occur naturally in the environment. Extensive use of CB in
industry and agriculture has caused contamination in soil and groundwater (Jiade, Yu et al.
2008, Moreira, Amorim et al. 2012). Once CB is released to the environment and absorbed
by the organisms, it can bio-accumulate through the food chain. Exposure to CB may cause
cancer, teratogenesis, mutagenesis and damage to the nervous system (Zhang, Leng et al.
2011). It persists several months in soil, 3.5 days in air (as fugitive emissions from
industrial or/and agricultural waste), and less than 1 day in water.
The general structure of all different CB is shown in Figure 1 while chemical and
physical properties can be seen respectively in Table 1 (Denecker 2009). Due to CB’s poor
water solubility, their concentration is rather low in the aqueous phase, while CB
concentrations can be high in polluted soils and sediments. The concentration of CB was
detected in several U.S cities at []1≤5.6 ppb, in surface water in 9.6% of the unspecified
1 []= Concentration
14
“large number” of samples, and in groundwater in 0.1% of 945 wells tested at two sites
(USEPA 1995, Hayward 1998); therefore it has been identified as a priority pollutant by
the US Environmental Protection Agency (EPA) with a maximum contaminant level
(MCL) of 100 μg/L (US EPA). In most developed countries, the production, use and
discharge of CB are subject to regulation due to their persistence, toxicity, and
bioaccumulation potential (Adebusoye, Picardal et al. 2007).
Figure 1. General Structure of CB (Lenker, Harclerode et al. 2014)
Cl
ClCl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
ClCl
Cl
Cl Cl
ClCl
Cl
Cl
Cl
Monochlorobenzene 1,2-dichlorobenzene 1,3-dichlorobenzene
1,4-dichlorobenzene 1,2,3-trichlorobenzene 1,2,4-trichlorobenzene
1,3,5-trichlorobenzene
Cl
Cl
Cl
Cl Cl
Cl
Cl
Cl Cl
Cl
Cl
Cl
Cl Cl
Cl
1,2,3,4-tetrachlorobenzene 1,2,3,5-tetrachlorobenzene
1,2,4,5-tetrachlorobenzene Pentachlorobenzene Hexachlorobenzene
15
Concerns over environmental impacts of CB and other chlorinated solvents have led
to the development of different methods to transform these compounds into inert and less
toxic chemicals. These methods include catalytic hydro-dechlorination (Lee, Jou et al.
2009, Liu, Zhao et al. 2009, Lee, Jou et al. 2010, Liu, Chen et al. 2011, Pagano, Volpe et
al. 2011), incineration (Veriansyah and Jae-Duck 2007, Lee, Jou et al. 2010, Liu, Chen et
al. 2011, Pagano, Volpe et al. 2011), biodegradation (Ziagova and Liakopoulou-Kyriakides
2007, Liu, Chen et al. 2011), and adsorption (Liu, Chen et al. 2011). In addition, advanced
oxidation processes such as ultrasonic oxidation (Stavarache, Nishimura et al. 2003, Liu,
Zhao et al. 2009), ozone oxidation (Babuponnusami and Muthukumar 2012), sonolysis
(Jiang, Yan et al. 2009, Liu, Chen et al. 2011), H2O2 oxidation (Liu, Zhao et al. 2009),
photo-catalytic oxidation (Tahiri, Ichou et al. 1998, Liu, Zhao et al. 2009, Liu, Chen et al.
2011), and Fenton’s reaction (Jiade, Yu et al. 2008) were also investigated for CB
degradation.
4-Chlorophenols and their chemical and physical
properties
A group of chemicals that are produced by adding chlorine to phenol are called
chlorophenols (CPs). Phenol is an aromatic chemical compound derived from benzene.
The chemical identity information of chlorophenols and the physical and chemical
properties of chlorphenols (except 2-chlorophenol) are shown in Table 3 and Table 4,
respectively (Jeffrey and Koplan 1999). Mono (one) chlorophenols, di (two)
chlorophenols, tri (three) chlorophenols, tetra (four) chlorophenols, and penta (five)
16
chlorophenols are five basic types of CPs. In general, there are 19 different CPs, of which
eighteen are solids and one, 2-chlorophenol, is a liquid at room temperature (Jeffrey and
Koplan 1999); they all can be seen in the following Table 2.
Table 1. Chemical and physical properties of CB (USEPA 1995)
Parameter CB
Chemical Formula C6H5Cl
Synonyms
Monochlorobenzene;
Benzene chloride;
Phenylchloride; MCB;
Chlorobenzonl
Molecular Weight 112.56 g/mol
Physical State Colorless Liquid
Boiling Point 132 °C
Melting Point −45.6 °C
Vapor Pressure 11.7 mm Hg at 20 °C
Density/Specific Gravity 1.107 at 20 °C
Odor threshold 1 to 8 mg/m3
Solubility in water 0.5 g l−1 in water at 20 °C
Solubility in other solvents soluble in most organic solvents
Flammability Limit 1.8%-9.6%
17
Table 2. Categories of 19 various CPs (Jeffrey and Koplan 1999)
Chlorophenols Categories
Monochlorophenol
2-chlorophenol
3-chlorophenol
4-chlorophenol
Dichlorophenol
2,3-Dichlorophenol
2,4-Dichlorophenol
2,5-Dichlorophenol
Trichlorophenol
2,3,4-Trichlorophenol
2,3,5-Trichlorophenol
2,3,6-Trichlorophenol
2,4,5-Trichlorophenol
2,4,6-Trichlorophenol
3,4,5-Trichlorophenol
Tetrachlorophenol
2,3,4,5-Tetrachlorophenol
2,3,4,6-Tetrachlorophenol
2,3,5,6-Tetrachlorophenol
Tetrachlorophenol _
18
Table 3. Chemical Identity of Chlorophenol Compoundsa
19
Table 3. Chemical Identity of Chlorophenol Compoundsa (continued)
20
Table 4. Physical and Chemical Properties of Chlorophenol compoundsa
21
Table 4. Physical and Chemical Properties of Chlorophenol compoundsa (continued)
22
4-Chlorophenol (4-CP) chemical structure is shown in Figure 2. 4-CP is a toxic,
hardly bio-degradable substance that is widely used in herbicides, pesticides, and
disinfectants and is released into water by oil, pulp, paper and pharmaceuticals industries.
4-CP is identified as a priority toxic pollutant by the US EPA (Hayward 1998, EPA 2002)
and the European Union (EU) (EEC 1990), with a value of 0.5 mg/l as upper permissible
limit in publicly supplied waters (Elghniji, Hentati et al. 2012).
Due to the high toxicity and persistence of CPs in the environment, various treatment
methods have been investigated for the transformation of 4-CP into more inert and less
toxic chemicals (Hamdaoui and Naffrechoux 2008). Electrochemical incineration (Balcke,
Wegener et al. 2008), anodic oxidation (Abhilash and Singh 2008, Liu, Zhao et al. 2009),
photolytic oxidation (Weber, Gaus et al. 2008), biological processes (Jeffrey and Koplan
1999), ultrasound (Nagata, Nakagawa et al. 2000, Guerin 2008, Liu, Chen et al. 2011),
electro-Fenton (EF) (Abhilash and Singh 2008), and peroxi-coagulation processes
(Abhilash and Singh 2008) have been studied as methods to degrade CPs.
Electrochemical treatments are recognized to be the most environmentally-friendly
methods as they can be performed in-situ without external chemical additions
(Alshawabkeh and Sarahney 2005, Alshawabkeh 2009, Martínez-Huitle and Brillas 2009),
which lowers the cost of remediation. Contaminants can be electrochemically transformed
via direct or indirect oxidation and/or reduction mechanisms (Gent, Wani et al. 2012, Mao,
Ciblak et al. 2012, Yuan, Gou et al. 2013b).
23
Figure 2. Chemical structure of 4-CP
Remediation Technologies
A number of technologies have been proposed and implemented for groundwater
remediation. Conventional approaches, such as pump and treat and in-situ air sparging, are
the most common. Monitored natural attenuation, bioremediation, and phytoremediation
are the most common biotechnologies. We focus here on technologies relevant to removal
of CB and 4-CP from groundwater.
2.3.1 Advanced oxidation processes
Degradation of several chemicals released into the aquatic environment is very
difficult because they are toxic and partly biodegradable. In order to remove organic
contaminants, either by degrading them into less harmful compounds or causing their
complete mineralization, effective methods need to be developed. Advanced oxidation
processes (AOP) that involve the in situ generation of highly potent chemical oxidants such
as the hydroxyl radical (OH•) have been considered to effectively degrade pollutants and
toxic chemicals from water (Glaze 1987, Goi 2005).
Cl
OH
24
Research on advanced oxidation processes (AOPs) has attracted more attention
(Andreozzi, Caprio et al. 1999, Herrmann, Guillard et al. 1999, Tarr 2003, Laine and
Cheng 2007, Zaviska, Drogui et al. 2009), because these technologies have shown great
potential to efficiently oxidize most organic pollutants and mineralize to inorganic carbon
(CO2). In addition, these technologies can accelerate the oxidation and destruction of a
wide range of persistent organic contaminants in polluted water. They are also considered
promising, efficient and environmentally friendly methods. Among AOPs, the
electrochemical advanced oxidation processes (EAOPs), and in particular, "electro-
Fenton" and "Sono-electro-Fenton", have demonstrated good prospective reduction of
pollution (Martinez-Huitle and Ferro 2006, Feng 2014).
Under this broad definition of AOP, many systems are qualified. The combination of
strong oxidants (e.g. O3 and H2O2), catalysts (e.g. transition metal ions or photocatalyst),
and irradiation (e.g. ultraviolet (UV), ultrasound (US)) was used by most of these systems.
Table 5 shows the classification of conventional AOP’s based on the source used for the
generation of hydroxyl radicals (Babuponnusami and Muthukumar 2014). Surface and
groundwater treatment, odors and volatile organic compound degradation, phytosanitary
and pharmaceutical products, water disinfection, etc. are some of the studies which have
been used AOPs techniques.
25
Table 5. Classification of conventional AOP's
Type of process Examples
Homogeneous Fenton based processes
Fenton: H2O2 + Fe2+
Fenton like: H2O2 + Fe3+ /mn+
Sono-Fenton: US/ H2O2 + Fe2+
Photo-Fenton: UV/ H2O2 + Fe2+
Electro-Fenton
Sono-electro-Fenton
Photo-electro-Fenton
Sono-photo-Fenton
O3 based processes
O3
O3 + UV
O3 + H2O2
O3 + UV + H2O2
Heterogeneous H2O2 + Fe2+ /Fe3+ /mn+ -solid
TiO2/ZnO/CdS + UV
H2O2 + Fe0/Fe (nano-zero valent iron)
H2O2 + immobilized nano-zero valent iron
Radical entities such as OH• is the production of these techniques. High rates of
pollutant oxidation, flexibility concerning variations of water quality, and the small
dimensions of the equipment are considered major benefits of these methods. On the other
hand, high treatment costs and special safety requirements are considered vital
disadvantages due to the use of very reactive chemicals and high-energy sources (Kochany
26
and Bolton 1992). A combination of an oxygen and hydrogen atom possessing an unpaired
electron (single electron) on its external orbital is described as a hydroxyl radical (OH•).
This molecule is a highly active and capable of reacting with organic molecules (such
as aromatic and aliphatic), as well as inorganic and bacterial compounds (Zaviska, Drogui
et al. 2009). Furthermore, OH• are strong and non-selective oxidants that can act very
quickly with most organic compounds Table 6. Fluorine gas is the only oxidant species
that is not used in the treatment of water and it is the only one with higher electronegative
oxidation potential.
Table 6. Relative oxidation power of some oxidizing species (Ullmann’s. 1991)
Oxidation species Oxidation potential, E° [V]
Fluorine 3.06
Hydroxyl radical 2.80
Nascent (Singlet) oxygen 2.42
Ozone 2.07
Hydrogen peroxide 1.77
Perhydroxyl radical 1.70
Hypochlorous Acid 1.49
Chlorine 1.36
27
2.3.2 Fenton process and electro-Fenton process
2.3.2.1 Fenton process
In the nineteenth century Fenton’s methodology was studied by Henry J. Fenton,
when he observed that Fe(II) could activate H2O2 to oxidize tartaric acid (Fenton H 1894).
Active oxygen species resulted from the combination of H2O2 and Fe(II) ions (called the
Fenton’s reagent) that can oxidize organic and inorganic compounds when they are present
in water, soil, and groundwater. H2O2 is inexpensive, safe, and easy to handle, and is not
considered a threat for the environment since it readily breaks down to water and oxygen,
unlike bulk oxidants. Meanwhile, iron is inexpensive, safe, and environmentally friendly
as well. Although Fenton chemistry has been used in industry on a small scale (Eisenhauer
1964), research on applications for waste treatment started only around 1990 in academic
laboratories.
This method received researchers’ attention for water and soil treatment (Merli,
Petrucci et al. 2003, Ikehata and El-Din 2006, Bautista, Mohedano et al. 2008). In
biological chemistry, the chemistry of natural water, and the treatment of hazardous wastes,
Fenton and other related reactions have also shown great potential in degrading
contaminants in the hundred years since (Joseph J. Pignatello et al 2006). Joseph J.
Pignatello et al. also reported that the Fenton and related reactions are viewed as potentially
convenient and economical ways to produce oxidizing species for removing chemical
waste. In 1934 Haber and Weiss reported that the Fenton reaction produces OH• as an
28
active oxidant, which is considered the strongest oxidant (E° = 2.73 V) (Haber and Weiss
1934).
O3/H2O2, H2O2/UV, and O3/UV are the most frequently applied of non-Fenton AOPs
categories. The need for pH control and the problem of sludge generation limits Fenton-
based AOPs for wastewater treatment systems. In order to find the optimum reagent
conditions, the majority of the studies and assessments of compound transformation and
mineralization rates (of actual, or simulated, industrial waste streams) are bench-scale
treatable.
Fenton's reaction is an effective method for transformation of chlorinated solvents in
groundwater (Kavitha and Palanivelu 2004). When the optimum pH range of contaminated
aqueous is about 2.8-3, Fenton method can be efficiently applied. In Fenton's reaction, the
transformation of hydrogen peroxide (H2O2) in the presence of Fe(II) generates highly
reactive hydroxyl radicals (OH•) (Eq. 1).
Eq. 1: Fe2+ + H2O2 → Fe3+ + OH• + OH− k = 76 L mol−1s−1
Ferrous ion then regenerates (Eq. 2), due to the reduction by hydrogen peroxide. In
fact, the catalysts is either generated (Eq. 2) (Walling 1975, Joseph J. Pignatello et al 2006,
Bautista, Mohedano et al. 2008) or form from the reaction of Fe3+ with intermediate
organic radicals ((Eq. 3)-(Eq. 4))
Eq. 2: Fe3+ + H2O2 → Fe2+ + HOO• + H+ k = 0.01 L mol−1s−1
Eq. 3: RH + OH• → R• + H2O
Eq. 4: R• + Fe3+ → R+ + Fe2+
29
However, several competitive reactions can also happen (Eq. 5-Eq. 8), which negatively
influence the oxidation process (Bautista, Mohedano et al. 2008):
Eq. 5: Fe2+ + OH• → Fe3+ + OH− k = 3.2 × 108 L mol−1s−1
Eq .6: H2O2 + OH• → HOO• + H2O k = 2.7 × 107 L mol−1s−1
Eq. 7: HO2 + OH• → O2 + H2O
Eq. 8: OH• + OH• → O2 + H2O2
Fenton reactions have some advantages for wastewater, water and groundwater. For
example, they are easy to handle, inexpensive, and do not require energy input (Bautista,
Mohedano et al. 2008, Oturan and Aaron 2014). Furthermore, they have been used to
remove different toxic organic contaminants from water such as phenol (Kavitha and
Palanivelu 2004, Zazo, Casas et al. 2005), and dyes (Muruganandham and Swaminathan
2004, Sun, Li et al. 2009).
On the other hand, Fenton reactions also have some disadvantages. First, a high
concentration of Fe(II) concentration is required which is expensive to treat wastewater,
water, and groundwater treatment and requires a large amount of chemicals and manpower
(Chou, Huang et al. 1999, Nidheesh and Gandhimathi 2012). Second, the amount of Fe(II)
ions that are regenerated is less than what they consumed more rapidly. Third, the range of
pH used for Fenton reaction (2-3) limits the Fenton reaction at higher pH ranges because
iron ions will be precipitated (Tarr 2003, Nidheesh and Gandhimathi 2012). Finally, as
previously mentioned, the transport and storing of H2O2 are expensive. Therefore,
30
electrochemical advance oxidation processes (EAOPs) based on Fenton’s reaction were
proposed to address some of these difficulties.
2.3.2.2 Electrochemical Advanced Oxidation Processes
(EAOPs) based on Fenton’s reaction
Degradation of organic and inorganic impurities from fresh water, drinking water,
wastewater and groundwater occurs through different electrochemical treatments such as
electrochemical flotation, electrochemical coagulation, electrochemical reduction,
electrodeposition and electro-oxidation, all of which have received attention. In order to
treat various wastewaters, disinfect drinking water or enhance the remediation of polluted
soils, electrochemical techniques have been applied extensively, especially electro-
oxidation treatment. The main use of this method is to remove aromatic compounds,
pesticides, industrial contaminants, pharmaceutical waste and other organics (Yuan, Gou
et al. 2013b, Rajic, Fallahpour et al. 2016, Rajic, Nazari et al. 2016).
Electrochemical (EC) processes have received attention for the removal of
contaminants from water, wastewater, and groundwater. For the purposes of environmental
safety, versatility, high efficiency, and the possibility of automation EC technology was
combined with AOPs to produce a large variety of EAOPs. Electrochemical peroxidation
process, Fered-Fenton process (or EF-Fere process), and the electro-Fenton process are
three classifications of EAOPs based on Fenton’s reaction.
31
Electro-Fenton methods broadly include electrochemical reactions that are used to
produce in situ one or both of the reagents for the Fenton reaction (Yuan, Fan et al. 2011,
Mao, Ciblak et al. 2012, Yuan, Mao et al. 2012, Yuan, Chen et al. 2013, Yuan, Gou et al.
2013b). Cell potential, solution conditions and the nature of the electrodes are the
characteristics that the reagent (s) produced depend on. Depending on Fenton’s reagent
electro-Fenton reactions are classified into four groups as follows:
1. Using a sacrificial anode and an oxygen spargin cathode, hydrogen peroxide and
ferrous ion are electro-generated respectively through reactions.
2. Ferrous ion is generated from sacrificial anode while hydrogen peroxide is added
externally to the solution.
3. Using an oxygen sparging cathode hydrogen peroxide is produced while ferrous
ion is added externally to the solutions.
4. Using Fenton reagent in an electrolytic cell and through the ferric ions reduction
on the cathode, hydroxyl radical and ferrous ion are produced and regenerated
respectively.
For this dissertation, in order to study electro-Fenton reaction, type three is used to
perform all the experiments because it is less expensive, safer, and more easily produced.
Table 7 shows and defines more than different types of electro-Fenton reactions (Joseph
J. Pignatello et al 2006).
Type 4 is considered the most promising electro-Fenton mode in which ferric ion
is reduced to ferrous ion at the cathode.
32
Table 7. Different types of electrochemical Fenton reactions, with the Fenton reagent
produced shown in boldface.
Type Anode Reaction Cathode reaction Reagent introduced externally
1 Fe° → Fe2+ + 2 e- 2H2O + 2e− → H2 + 2OH− H2O2
2 Fe° → Fe2+ + 2 e- O2 + 2H+ + 2e− → H2O2 __
Fe3+ + e- → Fe2+
3 2H2O → 4H+ + O2 + 2e− O2 + 2H+ + 2e− → H2O2 Fe2+
4 2H2O → 4H+ + O2 + 2e− Fe3+ + e- → Fe2+ H2O2
In the conventional electro-Fenton’s (EF) reaction (Eq. 9), for Pd-catalytic electro-
Fenton system, H2O2 is produced electrochemically by the reaction of O2 and H2 produced
in the anode and cathode, respectively (Eq. 10). The H2O2 then, in the presence of Fe(II)
ion, generates hydroxyl radicals (OH•) (Eq. 11). Following the reaction described in Eq.
11, Fe(III) reduces at the cathode (Eq. 12). Electro-Fenton’s reaction has been an effective
method for transformation of chlorinated solvents in groundwater due to its intrinsic Fe(II)
content.
Eq. 9: O2 + 2H+ + 2e− → H2O2
Eq. 10: 𝐻2 + 𝑂2𝑃𝑑→ 𝐻2O2
Eq. 11: H2O2 + Fe2+ + H+ → Fe3+ + OH• + H2O
Eq. 12: Fe3+ + e- → Fe2+
33
Eq. 13: Fe3+ + H2O2 → Fe2+ + HO•2 + H+
Electro-Fenton has some advantages over the Fenton process that can be
described as follows (Martínez-Huitle and Brillas 2009):
1. The production of H2O2 through electro-Fenton reaction is much safer and easier to
form. Therefore, there is no need to add H2O2 to the solution externally because it can
be formed by the reduction of dissolved oxygen on the surface of the cathode.
2. Due to continuous regeneration of Fe(II) at the cathode and the on-site production of
H2O2, a higher degradation rate of the organic contaminant is obtained.
2.3.3 Ultrasound and Sono-electro-Fenton Process
2.3.3.1 Sound, Ultrasound, Cavitation and Sono-electro-
Fenton
Sonochemistry has received researchers’ attention as the chemical applications of
ultrasound. The processes behind sonochemistry have been known since the late 1800s”
and in the 1980’s the renaissance of sonochemistry happened (Tomašević and Gašić 2015).
Sonolysis has shown great potential in degrading organic effluent into less toxic
compounds and is also proven to be an effective method for removing the contaminants
(Kotronarou, Mills et al. 1991, Petrier, Micolle et al. 1992, Petrier, Lamy et al. 1994,
Serpone, Terzian et al. 1994, Ghaly, Härtel et al. 2001). Homogeneous sonochemistry of
liquids, heterogeneous sonochemistry of liquid-liquid or liquid-solid systems, and
sonocatalysis are the three zones where ultrasounds’ chemical effect happens.
34
The waves of compression and expansion (or rarefaction) passing through gases,
liquids or solids are considered a sound’s definition. The frequencies from the range of
Hertz2 to 16 KHz3 are the waves that can be heard directly through human ears. Although
these frequencies and low frequency radio waves are similar, sound is different from radio
or other electromagnetic radiation such as infrared and ultraviolet. For example,
electromagnetic radiation can easily pass through a vacuum without difficulty while,
sound cannot because the compression and expansion waves of sound must be contained
in some form of matter.
Cyclic sound pressure (expansion and compression) with a frequency greater than
20 KHz is well known as an ultrasound. Some characteristics of ultrasound processes
include not requiring addition of oxidants or catalyst, not producing additional waste
streams as compared to adsorption or ozonation processes, and remaining unaffected by
the toxicity and low biodegradability of compounds (NRC 2005, Cheng, Zeng et al. 2016).
A diagram of ultrasound range with various applications at different frequencies is shown
in Figure 3.
Ultrasounds are divided into three categories based on the frequency as follows:
power ultrasound (20-100 KHz), high frequency ultrasound (100 KHz–1 MHz), and
diagnostic ultrasound (1–500 MHz). The audible range for acoustic waves is between 20
Hz and 20 KHz, through hearing differs with the individual and the age (Tomašević and
Gašić 2015); ranges from 20 to 100 KHz, 1 to 10 MHz, and 20 Hz down to 0.001 Hz are
2 Hertz unit is compression or expansion cycles per second 3 kilohertz which is defined as thousands of cycles per second
35
used in chemically important systems, animal navigation and communication, and
seismology (Detomaso, Lopez et al. 2003) and medical application (Czaplicka 2004)
respectively.
Figure 3. Ultrasound range diagram
When ultrasound irradiation is applied to liquid, it produces waves through
mechanical vibration that contain expansion (rarefaction) and compression phases
(Cheng, Zeng et al. 2016). The compression (high molecular density and pressure) and
rarefaction cycles (low molecular density and pressure) are shown in Figure 4
(Michałowicz and Duda 2007). Positive pressures are exerted on a liquid by compression
cycles of ultrasound waves that push the molecules of liquid together, while negative
pressures are exerted on a liquid by rarefaction cycles that pull the molecules of liquid
apart from each other.
36
Figure 4. Diagram cycles of rarefaction and compression
Sonochemistry is principally based on acoustic cavitation which includes the
formation, growth, and implosive collapse of bubbles in liquid. Cavitation can only occur
in liquid systems; during the ultrasonic irradiation of solids, or solid-gas systems,
chemical reactions do not occur. Cavitation bubbles are formed in the rarefaction phase,
when large amounts of negative pressures are applied to the liquid (Balcke, Wegener et
al. 2008). The main factor in growing cavitation bubbles is the intensity of sound used
throughout both positive and negative pressure cycles. High intensity ultrasound causes
such rapid expansion that the bubbles created during the negative pressure cycle are
unable to reduce in size during the successive positive pressure cycle.
Low intensity ultrasound produces bubbles that fluctuate in size; these bubbles
grow and shrink in time with the expansion and compression cycles. Frequency plays an
important role in both cavitation bubble size and efficiency of energy absorption. During
this time the cavity grows and absorbs energy more efficiently. When a cavity grows too
rapidly it absorbs energy at too slow a rate and thus it will implode (Abhilash and Singh
37
2008). Therefore, an unusual environment can be caused by the cavitational collapse for a
chemical reaction in terms of large amounts of local pressures and temperatures (Weber,
Gaus et al. 2008).
Both cavitation bubble formation and growth is shown in Figure 5 Stable and
transient cavitation are two different types of cavitation phenomena that occur in the liquid
once a bubble is formed. After a bubble is created, it expands until it reaches a critical size
known as resonance size, which depends on the applied frequency of the sound field.
Rectified diffusion or coalescence are two possible phenomena that might take place, once
bubbles reach their resonance size. Therefore, within a single acoustic cycle or over a
small amount of cycles, the bubble may become unstable and collapse, which is known as
transient cavitation. The other possibility is that the bubble oscillates for many cycles at,
or near, the linear resonance size. This is termed stable cavitation (Ghaly, Härtel et al.
2001, NRC 2005).
38
Figure 5. Cavitation bubble growth and collapse
Figure 6 shows three popular zones that are associated with cavitational bubbles
that can be explained as follows (Jeffrey and Koplan 1999, Tomašević and Gašić 2015):
1) Hot spot or thermolytic center, which is defined as the core of the bubbles. During the
final collapse of cavitation, the pressure and temperature of this core reaches 500 atm
and 5000 K, respectively. The phenomena of this region take place in the gas phase.
2) Interfacial region, the zone between cavitational bubble and bulk liquid that happens in
aqueous phase.
3) The bulk region.
39
Figure 6. Cavitational zone
Chemical (indirect) and physical (direct) mechanisms are two different types of
actions that can be applied by ultrasounds in aqueous medium. At high frequency, the
indirect action can be realized; water and dioxygen molecules undergo homolytic
fragmentation and yield •OH, HO2•, and •O radicals (Riesz, Berdahl et al. 1985, Zaviska,
Drogui et al. 2009). The formation by ultrasound of cavitation bubbles is considered
sonication, which is well known as the direct action. Sonochemistry usually deals with
reactions in liquid component. Once ultrasound is applied inside the liquid, it causes small
bubbles to form. These bubbles subsequently collapse and result in extreme local
temperatures and pressures, which can cause water to decompose to hydrogen atoms and
hydroxyl radicals. The presence of hydroxyl radicals and/or extreme temperatures and
40
pressures leads to contaminant degradation via oxidation or thermolysis (Nagata,
Nakagawa et al. 2000).
In other words, different types of reactive species will be produced by the sonolysis
of water molecules and thermal dissociation of oxygen molecules such as OH•, H•, O•, and
hydroperoxyl radicals (OOH•). The production of these reactive species follows the
following reactions, with ‘)))’ denoting ultrasound power (Eqs. 14–Eq. 26) (Pagano, Volpe
et al. 2011). Through OH• and H•, water sonolysis generates H2O2 and H2 gas. Although
oxygen improves sonochemical activities, its presence is not essential for water sonolysis
as the sonochemical oxidation and reduction process can proceed in the presence of any
gas. However, the presence of oxygen could scavenge the H• (thus suppressing the
recombination of OH• and H•) to form OOH•, which acts as an oxidizing agents (Adewuyi
2001).
Eq. 14: H2O + ))) → OH• + H•
Eq. 15: O2 + ))) → 2O•
Eq. 16: OH• + O• → OOH•
Eq. 17: O• + H2O• → 2OH•
Eq. 18: H• + O2 → OOH•
Eq. 19: OH• + H• → H2O•
Eq. 20: 2OH• → H2O + O•
Eq. 21: OOH• + OH•→ O2 + H2O
Eq. 22: 2OH• → H2O2
41
Eq. 23: 2OOH• → H2O2 + O2
Eq. 24: H• + H2O2 → OH• + H2O
Eq. 25: OH• + H2O2 → OOH• + H2O
Eq. 26: 2H• → H2
The ultrasound method is not an efficient method for water treatment because the
number of OH• produced through ultrasound is not sufficient for degradation. Therefore,
for a better removal efficiency, ultrasound radiation has been combined with other AOPs
(such as UV (Naffrechoux, Chanoux et al. 2000, Anju, Jyothi et al. 2012, Rokhina,
Makarova et al. 2013), ozonization (Janin, Goetz et al. 2013) or photocatalysis (Stasinakis
2008, Elghniji, Hentati et al. 2012, Calza, Sakkas et al. 2013)), and it has been shown that
these coupled processes increase contaminant removal efficiency, compared to using each
method separately. Naffrechoux et al. 2000 showed a significant increase in the removal
rate of phenol and chemical oxygen demand (COD) with simultaneous use of sonolysis
and ultraviolet radiation, (Naffrechoux, Chanoux et al. 2000).
Electro-Fenton (EF) reactions have been combined with ultrasound radiation to
remove a wide range of the contaminants from aqueous solutions. This combination is
known as Sono-electro-Fenton (SEF) reaction, and its high performance relies on the great
oxidation power of hydroxyl radicals (OH•) from Fenton reactions. The improvement of
degradation/destruction of contaminated organic compounds in water and groundwater and
of the reduction of the sonochemical treatment time is also a result of SEF techniques.
42
Another benefit of this method is the rapid and a very promising technique for removal of
organic pollutants.
Trabelsi et al. 1996 used 500 KHz ultrasound radiation along with EF (current
density=68 Am-2) and showed that a total degradation of phenol within 20 minutes with no
production of toxic intermediates is possible (Trabelsi, Ait-Lyazidi et al. 1996). Yasman et
al. used ultrasonic radiation (20 KHz) along with EF mechanism to treat 2,4-
dichlorophenxyacetic acid (2,4-D) and its derivative 2,4-dichlorophenol (2,4-DCP). They
accomplished almost 50% oxidation of 2,4-D solution (300 ppm) in only 60 seconds, while
complete removal was achieved after 10 minutes (Yasman, Bulatov et al. 2004).
In another study, Mehmet et al. used an undivided electrolytic cell with a Pt anode and
a 3-dimensional carbon-felt cathode to carry out EF and SEF oxidation for three
contaminants, 2,4-dichlorophenoxyacetic acid (2,4-D) and 4,6-dinitro-o-cresol (DNOC),
and the synthetic azo dye azobenzene (AB). It was observed that synergistic effect between
EF and ultrasound provides a higher degradation rate than that provided by the two
techniques separately for 2,4-D and DNOC, but not for AB. Similar results have been
obtained at low and high frequencies which suggests that the main mechanism in the
oxidation process is Fenton reaction, not the effects of sonication. Readily oxidizable
compounds such as AB can rapidly become degraded by EF process, so it is difficult to
observe improvements made by the addition of ultrasound radiation to the system (Oturan,
Sirés et al. 2008). Some of the most important studies on Sono-electro-Fenton technique
are listed in Table 8.
43
Table 8. Most recent studies on SEF Method
Authors Waste/Organic
used Reactor
Current
Density Frequency Remarks Year
Yasman et al. 2,4-D4
2,4-DCP5 Batch
10-100
mA/cm2 20KHZ
SEF process showed a promising result in full
degradation of chloroorganic compounds at a shorter time. The optimum concentration of Fe(II) ions was
found to be 2 mM.
2004
Yasman et al. 2,4-D
2,4-DCP Batch
10-100
mA/cm2 20KHZ
The bimetallic catalyst appears to be energetically and
economically superior to the Pd such as (black) Pd and
Pd–Fe powder which used in the sono-electro-catalytic technique. The result showed that bimetallic Pd/Fe
catalyst appears to be superior to the pure Pd catalyst.
2005
Oturan et al. DNOC6
2,4-D Batch
45
mA/cm2
28 &
460KHZ
SEF process was performed at both low and high frequency which resulted in more removal efficiency at
low frequency. It is due to (i) the enhanced mass transfer
rate of both reactants (Fe3+ and O2) towards the cathode for the electrochemical generation of Fenton’s
reagent (ii) the additional generation of OH by
sonolysis, and (iii) pyrolysis of organics due to cavitation generated by ultrasound irradiation.
Kinetic studies shows that, degradation of DNOC, 2,4-
D, AB follow pseudo-first order kinetics.
2008
Zhao et al. Phenol &
phthalic acid Batch
20
mA/cm2 33KHZ
Results show that ultrasound (US) has remarkable
influence on electrochemical (EC) oxidation of the two pollutants including degradation efficiency, EC
oxidation energy consumption, mass transport and
electrochemical reaction. The removal efficiency
enhancement and phenol’s EC oxidation energy
consumption are more obvious in the present of US.
2009
4 2,4-dichlorophenoxyacetic acid 5 2,4-dichlorophenol 6 4,6-dinitro-o-cresol
44
Authors Waste/Organic
used Reactor
Current
Density Frequency Remarks Year
Zhihui et al. Dye:
Rhodamine B
Batch
with air
1.2 V to 8
V7 22KHZ
Oxidation parameters such as applied potentials, ultrasound power, initial pH of the solution, and initial
concentration of Rhodamine B (RhB) were studied and
optimized. Maximum treatment efficiency was reached when potential, ultrasound power initial pH and initial
RhB concentrations had a value of 8.0 V, 400 W, 7.8
and 5ppm.
2010
Saez et al. 2,4-D8
2,4-DCP9 Batch
10-100
mA/cm2 20KHZ
At a frequency of 20 kHz, the lower ultrasound intensity
provides a smaller influence of the ultrasound field, but
for ranges of higher ultrasound intensities, the increase of the ultrasound intensity does not exhibit a better
performance.
The energetic consumption with sono-electrochemical treatment is lower than that presented by sono-chemical
treatment, due to the fact that the treatment time is
significantly reduced. Sono-electrochemical ultrasound strategies and/or flow sono-electrochemical reactors
should provide economically viable treatments.
2010
Lie et al. 2,4-D
2,4-DCP Batch
10-100
mA/cm2 20KHZ
Comparative experiments were performed to
demonstrate the effect of ultrasonic irradiation applied
in the EF process. The positive effect of the ultrasonic irradiation on the electro generation of H2O2 was
evidenced by the increasing hydrogen peroxide
production rate and the reduction of the time to a maximum value for the H2O2 concentration.
It was concluded that low frequency ultrasonic
irradiation has a positive effect on the degradation of the
dye effluent when combined with the electro-Fenton
process.
2010
7 Different potentials were applied to electrode 8 2,4-dichlorophenoxyacetic acid 9 2,4-dichlorophenol
Table 8. Most recent studies on SEF Method (continued)
45
Authors Waste/Organic
used Reactor
Current
Density Frequency Remarks Year
Esclapez et al. DNOC10
2,4-D Batch
45
mA/cm2
28 &
460KHZ
In this work, the electrochemical route was improved
using an ultrasound field obtaining higher degradation efficiencies (from 17 to 26%) when the process was
developed using a flow sono-electrochemical reactor. 2010
Saez et al. Phenol &
phthalic acid Batch
20
mA/cm2 33KHZ
In this work, complete dechlorination was achieved in
pure water solutions containing perchloroethylene and
its degradation by-products when no background electrolyte was added.
The obtained performance parameters for these sono-
electrolysis experiments at low electrical conductivities are enhanced compared with those observed when
background electrolyte is added. However, the
energetic costs are considerable from an economic point of view.
2011
Bringas et al. Diuron11 Batch 60
mA/cm2 20 KHZ
In the presence of ultrasound irradiation, the mineralization kinetics are 43% faster than silent
conditions.
The removal efficiency of diuron (herbicide) is favored at acid or neutral pH values.
2011
10 4,6-dinitro-o-cresol 11 (3-(3,4 dichlorophenyl)-1,1 dimethylurea)
Table 8. Most recent studies on SEF Method (continued)
46
Authors Waste/Organic
used Reactor
Current
Density Frequency Remarks Year
Martinez et al. Azure B dye Batch
0.5, 0.7 &
0.9 V
23KHZ
Dye degradation follows first-order kinetics in all the
experiments. In the following order, the first-order rate constant decreased: Sono-electro-Fenton > Fenton >
sonolysis. Both solution pH and Fe(II) initial
concentration influence on the rate constant; highest removal efficiency was observed between pH values of
2.6 and 3.
2012
Somayajula et
al.
Reactive Red
195 Batch
1-5
mA/cm2 20 KHZ
The removal efficiency of KCl and NaCl is higher than Na2CO3 and Na2SO4 which result in the in-situ
generation of hypochlorite ion. By increasing ultrasonic
power, the decolorization efficiency decreases.
2012
Babuponnusai
&
Muthukumar
Phenol Batch 1-16
mA/cm2 34 KHZ
Kinetic studies show that, the reaction order follows pseudo-first order kinetics for all four processes; their
optimum conditions are 0.0067, 0.0286, 0.0683 and
0.0934 min−1 for Fenton, electro-Fenton, sono-electro-Fenton and photo-electro-Fenton processes
respectively.
Higher phenol removal efficiency and COD removal were obtained at the following optimum conditions:
electrode distance of 5 cm, 4 mg/L Fe, initial pH of 3
and 12 mA/cm2 current density.
2012
Ren et al. Phenol Batch 30 Volts 850 KHZ
Phenol removal efficiency follows pseudo-first order
kinetics. The amount of phenol degradation is
obtained by sodium sulfate is more than sodium hydroxide and sulfuric acid; which resulted in more
energy efficient at higher electrolyte concentration
(4.26 g/L Na2SO4) and at higher electric voltages (30 V).
2013
Table 8. Most recent studies on SEF Method (continued)
47
Authors Waste/Organic
used Reactor
Current
Density Frequency Remarks Year
Ren et al.
Triclosan Batch 10 Volts 850 KHZ
Triclosan removal efficiency follows pseudo-first order
kinetics. Higher removal efficiency of triclosan was achieved at lower pH due to its predominately
hydrophobic molecular structure. The energy efficiency
and the absolute degraded mass of triclosan increases with increasing initial triclosan concentration. Lower
resistance and higher conductivity of the solution is
obtained through an increase in electrolyte concentration.
2014
Chen &
Huang
DNT12
TNT13
Batch
with O2
inflow
Electrode
potential
:3-6 V
120 KHZ
Sono-electro-Fenton oxidation was verified to be an
effective method for oxidative degradation of DNTs
and 2,4,6-TNT. At the optimal conditions of 6V electrode potential, 30 T, 150 mL/min O2, 0.1 pH,
and 150 mg/L Fe (II), nearly complete mineralization
of nitrotolurne was achieved.
2014
12 Dinitrotoluene 13 2,4,6-trinitrotoluene
Table 8. Most recent studies on SEF Method (continued)
48
Electrochemical remediation of groundwater
2.4.1 Chlorinated Solvents Remediation
Industrial activities have led to a release of many toxic chemical to the environment.
The components released are contained in heavy metals and other organic pollutants such
as 4-chlorophenol and chlorobenzene which can get into the environment either due to
accidental spills or due to improper management. The result of these spills has led to
millions of contaminated sites all over the world and has endangered the lives of human
and other living organisms.
Electrochemical remediation technologies are fast becoming the leading techniques
in as far as effective remediation of pollution is concerned. They are important approaches
for remediation of pollution because of their efficiency. The electrochemical remediation
technologies differ widely from the traditional systems of electrokinetics. The first
difference is the input of electricity required as the remediation technologies require very
low input energy in their performance. Other differences include ease of operation and the
ability to remediate mixed contaminants including chlorinated solvents. Other advantages
include the ability to treat a range of contaminants by subsequent reduction and oxidation
reactions. Furthermore, the process avoids the injection of chemicals into the in which it
uses the electrochemical remediation as the clean reagents and the reactant being the
electron. Finally, it is a cost effective alternative. Various electrodes can also be used to
enhance the performance of contaminant transformation. An important desirable feature is
that the rate of the electrochemical reactions occurring at the electrodes can be adjusted by
49
controlling the electrical potential between the electrodes. This makes it possible to
regulate the chemistry of the groundwater, thus achieving the desired degradation of the
targeted contaminants. Transformation will occur by different mechanisms which are
discussed below.
2.4.2 Electrochemical Oxidation
Electrochemical oxidation is an effective method for the treatment of toxicity in the
groundwater. This technique is reliable for the on-site treatment of complex and highly
volatile organic compounds and oxidative degradation of herbicides. The concept involves
the electrochemical oxidation process as a source of generating strong oxidants and is
similar to chemical destruction. The versatility, environmental compatibility, energy
efficiency and amenability to automation are some benefits of this technique (Yuan, Chen
et al. 2013, Yuan, Gou et al. 2013b, Li, Shi et al. 2014). In other words, electrochemical
oxidation is distinctive by three features. First, electrochemical oxidation is a process that
is more versatile in the treatment area of water. In addition, pathogen removal as well as
biological and pharmaceutical residues, and organic micropollutants removal for instance
heavy metals like chromium and arsenic from water and the pesticides are considered the
second features. The other feature is that electrochemical oxidation can complement other
conventional technologies in the treatment of polluted water (Woisetschläger, Humpl et al.
2013).
There are several studies that concentrated on contaminant removal such as TCE
by using electrochemical oxidation (Yuan, Fan et al. 2011, Yuan, Mao et al. 2012, Yuan,
50
Chen et al. 2013). The performance of new EF process was investigated by Yuan et al.
2013 for degrading contaminants of emerging concerns in aqueous solutions (Yuan, Gou
et al. 2013b). Furthermore, in order to automatically produce localized acidic condition in
the reaction zone and neutral effluent after treatment, a three electrode electrolytic system
was developed and investigated by Yuan’s group (Yuan, Chen et al. 2013).
Direct oxidation and indirect oxidation are the two distinct forms of electrochemical
oxidation. Direct oxidation involves the direct change of the electrons between the
contaminants and the anode surface where no other substance is involved. While indirect
electrochemical oxidation occurs through a process in which there is no direct exchange of
electrons with the surface of the anode but as a result of remediation of the electro active
species which shuttle between the organic commands and the electrodes (Panizza and
Cerisola 2009, Lenker, Harclerode et al. 2014, Favara, Tunks et al. 2016). In order to
avoid the minor problems of electrode fouling and /or corrosion, the indirect
electrochemical approach is more favorable and effective than the direct one.
2.4.3 Electrochemical Reduction
Even though the oxidative electrochemical methods are broadly studied and used
for the complete degradation of compounds, the treatment of electro-reductive technologies
received many attention due to potentially result to in the partial recovering/recycling of
chemicals or to production of value-added substances (Cabot, Segarra et al. 2004).
Electrochemical reduction is directed by the level of charge transfer to surface, transport
51
of the target chemical to the surface of cathode, or a combination of the two (He, Ela et al.
2004).
The electrolysis of water at the surface of the cathode results in the development of
hydrogen through electrochemical hydrogen adsorption in which the hydrogen atom is
chemically absorbed on the active site of the surface electrode (Rajic, Fallahpour et al.
2016). Thus, the cleavage of electroreductive of the carbon-halogen bond results in total
dehalogenated goods with C-H formulation or single, multiple, triple C-C bonds according
to the prevailing coupling or elimination reactions (Rondinini and Vertova 2010).
Eq. 27: H2O + e−→ H+ + OH− (Atomic hydrogen formation)
Eq. 28: H• + H• → H2 (Hydrogen evolution)
Eq. 29: 2H• + RCl → RH + H+ + Cl− (Hydrochlorination)
However, the majority of the literature reviewed has concentrated on the cathodic
discussion of chlorinated aliphatic hydrocarbons toward the corresponding dehalogenated
hydrocarbons (Mao, Ciblak et al. 2011, Mao, Ciblak et al. 2012). Direct electron transfer
(which take place heterogeneously through the transfer of electrons between electrodes of
solid-state and targeted chemical species) (Gent, Wani et al. 2012) and indirect electron
transfer (which happens through hydrogen atom generation on the surface of the cathode)
are two mechanisms of reductive electrochemical (Wang and Farrell 2003).
52
3 Chapter 3 Material and methods
Introduction
This chapter explains the experimental procedures followed to investigate and
evaluate the application of EF for chlorobenzene (CB) degradation and investigate the
influences of three advanced oxidation processes on removal efficiency of 4-chlorophenol
(4-CP) in groundwater. This chapter also explains the treatment efficiency that was
investigated under different Fe(II) concentrations, initial pH values and current intensities
as well as Pd catalyst dosages and forms, among which we tested the performance of a
functionalized polyacrylic acid (PAA)/polyviniledene fluoride (PVDF) membrane with
Pd0 nanoparticles (no iron). The CB degradation in a three-electrode column with
automatic pH regulation was also evaluated and optimized relative to same conditions
including flow rate. Furthermore, the chemicals/reagents and materials used during the
study are described in this chapter. In addition, the experimental procedures and analytical
methods utilized during the experiments are also explained in detail.
There were two phases for conducting the experiments: (a) effect of electro-Fenton
(EF) reaction on CB (section 4.2 and Section 4.3) (b) effect of electro-Fenton reaction,
ultrasound and sono-electro-Fenton (SEF) reactions on 4-CP removal efficiency
(section4.4). In this study, two different experimental set-up were designed. The
electrochemical degradation of CB in simulated groundwater was tested by Pd-catalyzed
electro-Fenton’s reaction in both batch and column (flow-through) reactors.
53
Treatment of chlorobenzene in simulated groundwater
using Palladium-Catalytic electro-Fenton’s reaction
Objective:
In this study, we evaluate the effect of palladium form, as a powder, pellets, and as
a membrane, in the application of EF for CB degradation in both batch and column (flow-
through) reactors. CB was chosen due to its high toxicity, low biodegradability and
accumulation potential in soil and water, and because it is considered a model molecule of
dioxins-like chemicals. The electrochemical degradation of CB in simulated groundwater
was tested by Pd-catalyzed electro-Fenton’s reaction in a two-electrode batch reactor. The
treatment efficiency was investigated under different Fe2+ concentrations, initial pH values
and current as well as Pd catalyst loading and forms, among which we tested the
performance of a functionalized polyacrylic acid (PAA)/polyviniledene fluoride (PVDF)
membrane with Pd0 nanoparticles (no iron). The CB degradation in a three-electrode
column with automatic pH regulation was also evaluated and optimized relative to same
conditions including flow rate.
3.2.1 Materials
All chemicals used in this study were analytical grade. Chlorobenzene (99+%),
palladium on alumina pellets (Pd/Al2O3, 0.5% wt. Pd with average particle size of 3.2 mm
and total surface area of 90 m2) and Pd/Al2O3 powder (1% wt. Pd on Alumina powder with
average particle size of 40 µ and total surface area of 150 m2) were purchased from Alfa
54
Aesar. Sodium sulfate anhydrous (Na2SO4, 99%), sulfuric acid (98%), sodium bicarbonate
(NaHCO3, 99-100%), acetonitrile (99.8+%) and methanol (99.9%) were purchased from
Fisher Scientific. Ferrous sulfate (FeSO4•7H2O, 99-104.5%) was purchased from Baker
Analyzed. All solutions were prepared in deionized water (18.2 mΩ.cm), from a Millipore
Milli-Q system.
For membrane functionalization: acrylic acid (AA, 99%), palladium (II) nitrate
hydrate (Pd(NO3)2), sodium borohydride (NaBH4, 99.99%) (Sigma-Aldrich, St. Louis, MO,
USA); ammonium persulfate (APS, (NH4)2S2O8) (EM Science for Merck KGaA,
Darmstadt, Germany); sodium hydroxide (NaOH) solution, sodium chloride (NaCl), (Fisher
Scientific, Fair Lawn, NJ, USA); isopropyl alcohol (IPA, 99.9%) and N,N′-
methylenebis(acrylamide) (MBA > 99%) (Acros, New Jersey, NJ, USA); hydrophilized
PVDF microfiltration membranes (average pore size: 0.50 μm, thickness: 125 μm, diameter:
4.7 cm) (Nanostone, Oceanside, CA, USA). The surface area for calculations was based on
the top surface area of the membrane (17.35 cm2).
CB-contaminated groundwater was prepared by mixing CB saturated stock solution
in background electrolyte in presence of different concentrations of Fe(II) ions. In all tests,
initial CB concentration was set to 8 ppm. 10 mM sodium sulfate solution was used as
background electrolyte for all batch tests while sodium sulfate or sodium bicarbonate
solution were used for column tests. Sodium bicarbonate was chosen as background
electrolyte for column experiments to assess the impact of its buffering capacity in the
55
process. Sulfuric acid was used to adjust the initial pH of the solution. All tests were
performed at room temperature.
3.2.2 Analysis
At defined time intervals, samples were collected from each of the sampling ports
(Figure 7 and Figure 8) and immediately mixed with 1 ml of methanol. CB was measured
by a 1200 Infinity Series HPLC (Agilent) equipped with a 1260 DAD detector and a
Thermo ODS Hypersil C18 column (4.6 × 50 mm) with a 5 µm particle size. The mobile
phase was a mixture of acetonitrile and water (60:40, v/v) with a 1 ml min-1 flow rate. The
detection wavelength was 210 nm. Samples were also taken for measurement of dissolved
and total Pd in the solution after the treatment by inductively coupled plasma–mass
spectrometry (ICP-MS) (Bruker aurora M90).
To observe the Pd nanoparticles (Pd NPs) morphology and analyze their elemental
composition within the membrane, a scanning electron microscopy (SEM) (Hitachi S4300)
was used. A focus ion beam (FIB) (Helios NanoLab 660) coupled with an SEM and an
energy dispersive X-ray spectrometry (EDS) detector, and a transmission electron
microscopy (TEM) (JEOL 2010F), also coupled with EDS, were also used.
56
3.2.3 Experimental Setup for a Batch Reactor
A one-liter acrylic cell (Figure 7a, b) was used as a batch electrochemical cell for
testing CB degradation. Two titanium based mixed metal oxide (Ti/MMO, IrO2/Ta2O5
coating on titanium (Figure 7c) mesh type, 3N international, USA) meshes with
dimensions of 85 mm ×15 mm ×1.8 mm (length × width × thickness) were used as anode
and cathode with a cathode-anode spacing of 4 cm. In both batch reactors, sampling port
was located 2.6 cm from the bottom of the reactor.
After adding the solution, specific mass of palladium on alumina (1% wt. Pd on
Alumina powder with average particle size of 40 µ and total surface area of 150 m2) was
added to the solution with a stirring rate of 70 rpm. Experiments were also performed in a
system shown in Figure 7b, where Pd immobilized on polyacrylic acid (PAA)
polyviniledene fluoride (PVDF) membrane was used as a static disk (Pd-PVDF/PAA)
mounted in Teflon holder as well as connected to the rotor (set at 70 rpm). The PVDF
membrane was functionalized with PAA by in situ polymerization of acrylic acid
(Detomaso, Lopez et al. 2003, Michałowicz and Duda 2007), followed by a double ion
exchange NaCl/ Pd(NO3)2 on the carboxylic groups of the PAA. The Pd-PVDF/PAA
membrane was prepared by reducing the Pd (II) from the ion exchange using NaBH4,
creating Pd NPs. Based on the ICP-MS analysis, the amount of Pd in Pd-PVDF/PAA is 1.6
mg/cm2 (27.8 mg) and there was no detectable leaching of Pd from both Pd/Al2O3 and Pd-
PVDF/PAA catalysts. Adsorption of CB on Pd/Al2O3 powder and Pd-PVDF/PAA was
found to be negligible. All tested parameters are summarized in Table 9.
57
The experiments were conducted under constant current (to provide constant rates
of electrolysis) by a power source (Agilent E3612A). The current efficiency (∅) was
calculated using Faraday’s law (Eq. 30):
Eq. 30: ∅ = 𝑉∗𝐶∗𝑧𝑒∗𝐹∗100
𝐼𝑎𝑝𝑝𝑙𝑖𝑒𝑑∗𝑡
where V is reactor volume (L), C is CB removed in the reactor (mol/L), ze is number of
electrons involved in the reaction of one mole of CB, F is Faraday’s constant with a value
of 96485 C mol-1, Iapplied is the current applied to the reactor (A), and t is experiment
duration (s).
58
Figure 7. Batch reactor with: a) Pd/Al2O3 powder, b) Pd membrane (Pd-PVDF/PAA), c)
electrodes (Ti/MMO mesh, Iron electrodes) and (d) Pd membrane set up along with MMO
electrodes
(c) (d)
59
Table 9. Batch test experiments
Fe(II) (mg/L) Pd (mg/L) Initial pH Current (mA)
10 20 3 60
10 20 4 60
10 20 6 60
0 20 3 60
2 20 3 60
4 20 3 60
10 20 3 60
10 0 3 60
10 5 3 60
10 10 3 60
10 20 3 60
10 20 3 40
10 20 3 60
10 20 3 120
10 Pd-PDVF/PAA (static
disk with stirring) 27.8
mg/L
3 120
10 Pd-PDVF/PAA
(rotating disk)
27.8 mg/L
3 120
60
3.2.4 Experimental Setup for a Column Reactor
Column tests were performed in a vertical acrylic tube (Figure 8)with a 3.175 cm
inner diameter and 30 cm in length. Three MMO mesh electrodes (
Figure 9a) were installed in a sequence as Anode, Cathode 1 and Cathode 2. The
anode was placed downstream (below both cathodes) to generate acidic conditions and
minimize Fe(III) precipitation. The current was split into two thirds passing through
Cathode 1 and one third passing through Cathode 2 to maintain acidic conditions in the
catalyst vicinity. A summary of the parameters tested for the column experiments are listed
in Table 10.
Pd/Al2O3 pellets (
Figure 9b) (Pd/Al2O3, 0.5% wt. Pd with average particle size of 3.2 mm and total
surface area of 90 m2) and Pd-PVDF/PAA (when used) (1.6 mg Pd/cm2) were placed on
Cathode 1. Adsorption of CB on Pd/Al2O3 pellets and glass beads (
Figure 9c) was insignificant. Based on the ICP-MS analysis there was no detectable
leaching of Pd from wither Pd/Al2O3 or Pd-PVDF/PAA. The column was packed with 4-
mm glass beads with total porosity of 0.65, excluding the space between the electrodes.
The total and pore volume of the column were 245 mL and 160 mL, respectively. To make
sure the column is operating in steady state conditions, measurements were performed after
160 min of operation. Flow rate of 2 mL/min was maintained by a peristaltic pump (Cole
Parmer, Masterflex C/L).
61
Figure 8. a) A schematic of three electrode column b) Actual column setup
Column inner diameter 3.175 cm Column Length: 30 cm Sampling ports distances from bottom P6: 27.9 cm P5: 19.7 cm P4: 14.3 cm P3: 13.3 cm P2: 10.2 cm P1: 7.6 cm
Electrode distances from bottom: Anode: 9.0 cm Cathode 1: 11.3 cm Cathode 2:14.0 cm b)
a)
62
Figure 9. A schematic of three electrode column a) column reactor, a) mixed metal oxide
(MMO, b) Pd pellets, c) glass beads, and d) pump
c)
b) a)
d)
63
Table 10. Column experiments test design
Background
electrolyte
Pd (mg/L) Fe(II) (mg/L) Current Intensity (mA) Flow rate (mL/min)
0 10 10 60(40-20) 2
NaHCO3 (1 mM) 10 10 60(40-20) 2
NaHCO3 (5 mM) 10 10 60(40-20) 2
Na2SO4 (10 mM) 0 10 60(40-20) 2
Na2SO4 (10 mM) 5 10 60(40-20) 2
Na2SO4 (10 mM) 10 10 60(40-20) 2
Na2SO4 (10 mM) 10 0 60(40-20) 2
Na2SO4 (10 mM) 10 5 60(40-20) 2
Na2SO4 (10 mM) 10 10 60(40-20) 2
Na2SO4 (10 mM) 10 10 30(20-10) 2
Na2SO4 (10 mM) 10 10 60(40-20) 2
Na2SO4 (10 mM) 10 10 120(80-40) 2
Na2SO4 (10 mM) 10 10 60(40-20) 2
Na2SO4 (10 mM) 10 10 60(40-20) 5
Na2SO4 (10 mM) 10 10 60(40-20) 10
Na2SO4 (10 mM) Pd-PVDF/PAA
27.8 mg
10
60(40-20)
10
64
Treatment of 4-chlorophenol in aqueous solution by Sono-
electro-Fenton reactions
Objective:
In this study, the byproducts of 4-CP were not investigated, with the exception of
phenol. Studies have evaluated the main decomposition intermediates during the
photocatalytic 4-CP removal (Mousset, Oturan et al. 2014a) and the influences of the Ag
content on removal of 4-CP along with the types and amounts of byproducts that were
produced during the reaction (Mousset, Oturan et al. 2014b). On the other hand, phenol
removal has been investigated through different AOPs such as sonochemical (Petrier,
Lamy et al. 1994), O3, O3/H2O2, UV, UV/O3, UV/H2O2, O3/ UV/H2O2, Fe2+/H2O2 (Särkkä,
Bhatnagar et al. 2015), photosonochemical (Yuan, Fan et al. 2011), electrochemical
degradation (Yuan, Chen et al. 2013).
The objective of this study is to investigate oxidation of 4-CP by Pd-catalyzed EF
process coupled with sonolysis using pulsed ultrasound frequencies. While the benefits of
ultrasound pulse by boron-doped diamond electrodes were reported (Yuan, Gou et al.
2013b), the application of pulse ultrasound frequencies along with Pd-catalyzed EF
reaction has not been reported. In this research, we examine the application of ultrasound
frequencies at ON/OFF ratio of 0.1 (ultrasound was ON:5.9 sec. and OFF:59 sec.) and 0.2
(ultrasound was ON:11.9 sec. and OFF:59 sec.). The performance of EF under different
initial pH, Fe2+ concentration, palladium (Pd) catalyst concentration, background
electrolytes and current intensities was also tested. The SEF tests were conducted under
65
optimum conditions and contaminant removal by SEF process was then compared with
both EF and sonolysis.
3.3.1 Materials
4-chlorophenol (C6H5ClO, 99+ %) and palladium catalyst (Pd/Al2O3, 1% Pd on
alumina powder, with a specific surface area of 150 m2g-1) were purchased from Acros
and Alfa Aesar, respectively. Phenol (C6H5OH, 89.6%), sodium sulfate anhydrous
(Na2SO4, 99%), sulfuric acid (H2SO4, 98%), sodium bicarbonate (NaHCO3, 99-100%),
acetonitrile (99.8+ %), sodium hydroxide (NaOH, 96%), and Acetic Acid (Glacial, HPLC
grade) were acquired from Fisher Scientific. Ferrous sulfate (FeSO4.7H2O, pro analysis)
was obtained from J.T. Baker Analyzed. Palladium catalyst (1% on carbon 4 to 8 mesh),
Potassium-hydrogen phthalate (C8H5KO4), HPLC grade water and methanol were bought
from Sigma-Aldrich. The syringe filters with 0.22 μm and 0.45 μm pore sizes were
purchased from Millex. Titanium sulfate (TiSO4, 65%) was obtained from GFS
Chemicals. All solutions were prepared in de-ionized water (18.2 mΩ.cm), obtained from
a Millipore Milli-Q system.
3.3.2 Experimental Setup
As shown in Figure 10, a one liter cylindrical acrylic cell with an 11.4 cm inner
diameter and a 10 cm height was used as batch reactor. Two Ti based mixed metal oxide
meshes (Ti/MMO, IrO2/Ta2O5 coating on titanium mesh type, 3N international, USA) with
3.6 cm in diameter and 1.8 mm thick and surface area of 11.8 cm2 were used as both anode
66
and cathode. The distance between electrodes was 6 cm. The synthetic contaminated
groundwater was prepared by adding 4-CP to reach final concentration of 200 ppm to the
background electrolyte (10 mM Na2SO4, NaHCO3 or NaNO3) containing different Fe2+
concentrations. Alternative to addition of ferrous sulfate as Fe2+ source, cast iron anode
with dimensions of 85×15×1.8 mm (length × width × thickness) was used to produce Fe2+
in situ (current calculated based on Faraday’s law, assuming the charge transfer between
electrode surface and electrolyte is 100% faradaic process). The current was applied
through rheostat, which allows the current split between Ti/MMO and iron anodes.
The Fe2+ production was tested for the optimized EF conditions: continuous during
3 h, supplied during first 30 minutes of treatment and after 30 minutes of treatment. The
current applied to iron anode was calculated to supply total of 80 ppm Fe2+. The use of an
iron anode in the electrochemical cell allowed for the generation of a wide range of Fe(II)
concentrations in situ, making possible high concentrations of Fe(II) in the cell which
overcome the limitations of the lower concentrations of Fe(II) naturally present in
groundwater, which rarely exceed 50 ppm.
The conditions used were 18 mA of current for the iron electrode on 300 min, 20
mA of current for the iron electrode off only in the first 30 min, and 144 mA of current for
the iron electrode on only for the first 30 min, all of which were calculated based on
Faraday’s law. The iron anode’s ON/OFF periods were applied based on the H2O2
production; the generation reaches the maximum values after 30 min.
67
Sulfuric acid and sodium hydroxide were used to adjust pH of the synthetic
groundwater. After adding the synthetic groundwater, defined Fe2+ and Pd/Al2O3 catalyst
dose, cell was sealed and the solution was stirred at a rate of 180 rpm using a magnetic
stirrer. As shown in Table 11, the influence of different current intensities on EF efficiency
and SEF efficiency towards 4-CP transformation were tested.
During SEF and sonolysis tests, a sonifier (20 KHz Branson Ultrasonics Co.) with
a 7.7 cm titanium horn was placed between the electrodes to provide different ultrasound
frequencies. As we applied pulsed ultrasound, different ON/OFF ratios were tested and
optimized. The SEF tests were conducted under 80 mg L-1 Fe2+, 1 g L-1 Pd/Al2O3 powder,
initial pH of 3 and 200 mA current intensity, with the amplitudes (%) of: 10, 30, 50, and
70 using ON/OFF pulses with ratio 0.1, and the amplitudes (%) of: 10, 20, and 30 using
ON/OFF pulses with ratio 0.2. All tests were performed at the initial temperature of 25±3
°C degree and it was monitored over the time (the temperature was not constant during the
experiments).
68
a)
b)
Ultrasound
horn
Electricit
y
Stirring plate
Solution Ultrasound
Figure 10. a) Schematic batch setup b) Actual batch setup
69
Table 11. EF test experiments design
Fe(II) Conc. (mg L-1) Pd/Al2O3 (g L-1) Initial pH Current (mA)
19 1 3 200
40 1 3 200
80 1 3 200
160 1 3 200
80 1 3 Control
80 1 3 60
80 1 3 120
80 1 3 200
80 0 3 200
80 0.5 3 200
80 1 3 200
80 1 3 200
80 1 4 200
80 1 5 200
3.3.3 Analysis
At defined time intervals 2 ml of solution was sampled from the sampling port
(located 2.4 cm from the bottom of the reactor, Figure 10) and was filtered through a 0.22
70
μm pore size syringe filter. 4-CP and phenol was then measured by a 1200 Infinity Series
HPLC (Agilent) equipped with a 1260 DAD detector and a Thermo ODS Hypersil C18
column (4.6 × 50 mm) with a 5 µm particle size. Mobile phase was a mixture of methanol,
water and glacial acetic acid (49:49:2) with a 1 mL min-1 flow rate. Detection wavelength
was 254 nm. The retention time for phenol was 2.5 min and for 4-CP it was 4.34 min
(Ellenberger, Van Baten et al. 2003). Total organic carbon (TOC) measurements were
performed by a TOC analyzer TOC-L CPH-CPN (Shimadzu, Japan) after samples filtration
through 0.45 μm pore size filters (Millipore) and acidification (pH≤2) with concentrated
HCl. The 4-CP removal efficiency was calculated by Eq. 31:
Eq. 31: Removal Efficiency (%) =C0−Ct
C0∗ 100
Where C0 is the initial concentration of 4-CP (mg L-1) and Ct is 4-CP concentration at a
defined time during treatment (mg L-1).
3.3.4 Instrument
In order to run the experiments, some instrument were used which includes as the
following:
High-Performance Liquid Chromatography (HPLC): Separating, identifying, and
quantifying each component in a mixture was achieved with this instrument.
71
Total Organic Carbon (TOC): this instrument measures the carbon dioxide (CO2)
formed when organic carbon and inorganic carbon, is oxidized and acidified respectively.
Ultraviolet-Visible Spectrophotometer: in order to measure the amount of hydrogen
peroxide (H2O2) produced during the experiments, this instrument were used. In fact, the
intensity of light passing through a sample was measured.
Ion chromatography (IC): for analyzing water chemistry IC is usually used which
measures anions concentrations like chloride, sulfate, fluoride, nitrate, and nitrite; it also
measures cations concentration such as potassium, sodium, lithium, calcium, ammonium,
and magnesium in the parts-per-billion (ppb) range. In addition, it measures organic acids
concentrations.
72
4 Chapter 4 Chlorobenzene removal by Palladium-
Catalytic electro-Fenton’s reaction
Introduction
This chapter describes the results of two series experiments and shows their specific
laboratory design in the following order:
1) Characterization and regeneration of Pd/Al2O3 catalyst along a two electrodes and
a three electrodes for chlorobenzene remediation in batch and column reactor
respectively.
2) Degradation of 4-Chlorophenol in Aqueous Media using combination of Electro-
Fenton and ultrasound reaction.
Batch Experimental Setup
4.2.1 Membrane characterization
The membrane after functionalization with PAA and subsequent Pd nanoparticle
synthesis shows an almost complete covering of the porous surface (Figure 11b) compared
with the bare PVDF membrane (Figure 11a). In Figure 11b is evident that the membrane
is functionalized with PAA polymer (smooth surface) but in addition it has a very large
distribution of Pd NPs. The cross-section of the membrane shows that these Pd NPs are
distributed in depth with a very dense distribution, see Figure 11c. The depth of the Pd
73
NPs distribution goes up to 10 μm (not shown). From the EDS spectra of the top surface
of the Pd-PVDF/PAA membrane, the amount of Pd is even higher than the characteristic
peak of fluorine of the PVDF, implying that the PAA layer with Pd NPs is covering most
of the membrane surface. The atomic ratio Pd/Na from the reduction with NaBH4 is 2/3
which is greater than theoretical value of 1 Pd per 2 Na in each carboxylic group of the
PAA.
Due to the smaller sizes of the Pd NPs, TEM was required to analyze the size and
the nature of Pd NPs. Aggregated Pd NPs exhibit asymmetrical shapes, but the base
particles of Pd (crystal phase) are shown with particle sizes between 2 and 5 nm (Figure
12a). The Pd NPs size distribution was determined by image analysis (2D metrics)
(Hernandez, Lei et al. 2015). The characteristic particle size of 2.21±0.06 nm is based on
the median of the gamma distribution (Figure 12c) (Vaz and Fortes 1988), which was
fitted with P-values larger than the statistics Kolmogorov-Smirnov (D = 0.029, P > 0.250)
and the Cramer-von Mises (W-Sq = 0.056, P > 0.250). Pd NPs could be identified by EDS
and a selected area electron diffraction (SAED) (Figure 12b-d). The SAED ring pattern
coincided to Miller index of Pd0 with face centered cubic structure (111, d-spacing = 0.224
nm) (Mejías, Serra-Muns et al. 2009, Navaladian, Viswanathan et al. 2009). The EDS
spectra also confirm the presence of Pd (Figure 12d).
74
Figure 11. SEM images and EDS spectra Pd nanoparticles in functionalized membrane. a)
Top surface bare PVDF membrane, b) top surface Pd-PVDF/PAA membrane, c) FIB cross-
section cut of Pd-PVDF/PAA membrane, d) EDS mapping of top surface of Pd-
PVDF/PAA
75
Figure 12. TEM images and EDS spectra Pd nanoparticles in membrane. a) Pd
nanoparticles in FIB cross-section lamella, b) SAED pattern corresponds approximately to
(111) of Pd0, c) Pd nanoparticle size distribution, d) EDS of Pd nanoparticles (presence of
Cu from sample tip)
4.2.2 Influence of pH on CB removal
According to Figure 13, for pH = 3.0 and pH = 4.0 with R2 of 0.984 and 0.995,
respectively, the CB degradation rates follow the first-order reaction model:
Eq. 32: −𝑑[𝐶]
𝑑𝑡= 𝑘[𝐶]
76
Where C is the concentration of CB (mol/L or mg/L), t is the time (min) k is the first-order
reaction constant (min-1). The first-order rate constant increased about 50% (from 0.022 to
0.032 min-1).
The Fenton based oxidation is pH dependent; at pH values of 6 generation of H2O2
is limited (Choudhary, Samanta et al. 2007, Yuan, Fan et al. 2011) and most Fe(III) ions
precipitates with OH-, this could explain why the reaction seems to be stopped after 10 min
and the following decay could be associated with some Fe regeneration according to Eq.
12 and 13. With the pH decrease, OH• generation rate and CB removal increase as a result
of more intensive H2O2 generation and higher dissolved Fe(III) concentrations available
for regeneration to Fe(II). The control experiments indicate that Pd catalysts (Pd –
PVDF/PAA and Pd powder) are stable under lower pH values since no Pd was measured
in solution after the treatment (both dissolved and total).
77
Figure 13. Degradation profile of CB at different initial pH values (Conditions: Fe(II): 10
ppm, current intensity: 60 mA, Pd: 20 mg/L, Na2SO4: 10 mM, different pH and CB: 10
mM)
4.2.3 Influence of Fe(II) concentrations on CB removal
The presence of Fe also influences the CB degradation and follows a first-order
reaction rate model (Eq. 7). The first-order reaction constants go from 0.002 to 0.032 min-
1, as shown in Figure 14a. In the absence of Fe(II) CB removal was limited (15%) due to
electro-induced reduction via hydrodechlorination mechanism, which take place at the
cathode. The CB decay rate increased from 0.002 min-1 in the presence of 2 mg/L Fe(II) to
0.032 min-1 in the presence of 10 mg/L Fe(II). As presented in Figure 14b, the correlation
is linear for the range of 0-10 ppm Fe(II) (R2=0.98, k=6.54 min-1). This proves that higher
Fe(II) concentrations lead to an increase in OH• concentration and, therefore, more efficient
78
CB removal, which is in accordance with previous studies (Yuan, Chen et al. 2013). All
further tests were conducted with 10 ppm of Fe(II).
The similar results for pH (Figure 13) and Fe (Figure 14a) imply a direct relationship
between the first-order rate constants and the change in pH and/or Fe concentrations: [H+]
and [Fe2+] are related to the free radical production (Eq. 11).
79
Figure 14. a) Effect of Fe(II) concentration on CB concentration decay, and b) Fe
concentration versus CB removal efficiency (Conditions: different Fe(II) concentration,
current intensity: 60 mA, Pd: 20 mg/L, Na2SO4:10 mM, pH=3 and CB: 10 mM)
4.2.4 Influence of Pd catalyst dose and form on CB
removal
Figure 15a shows CB concentration decay over time in the presence of different
Pd/Al2O3 catalyst concentrations. Under identical conditions, an increase in Pd/Al2O3
concentration from 0 to 2.0 g/L (0 to 20 mg total Pd), increased CB removal from 48% to
84%, (C/C0 decreased from 52% to 16%) and the First-order rate constant for CB removal
increased from 0.012 min-1 in the absence of Pd/Al2O3 to 0.032 min-1 in the presence of 2
g/L Pd/Al2O3 (20 mg/L Pd) (Table 12). In the absence of Pd, 48% of CB was removed due
to (a) reaction with H2O2 produced via two electron reduction of anodic oxygen at the
cathode, and/or (b) hydrodechlorination at the cathode. In Pd/Al2O3 tests, CB removal
increase from 48% in absence of catalyst to 68% in presence of 0.5 g/L of Pd/Al2O3 (5
mg/L Pd), less improvement in removal efficiency is observed with higher Pd/Al2O3
dosages. Addition of Pd/Al2O3 enhances the formation of H2O2, due to the high ability of
Pd to capture hydrogen within its lattice as well as the high surface area available for the
reaction. Figure 15b, the correlation is linear for the range of 0-1 g/L Pd (R2=0.84, k=16.56
min-1).
80
Figure 15. a) Degradation profiles of CB using different Pd/Al2O3 doses, and b) correlation
between Pd dosage and CB removal efficiency (Conditions: Fe(II): 10 ppm, current
intensity: 60 mA, Na2SO4:10 mM, pH=3 and CB: 10 mM)
81
As shown in Figure 16a, the CB removal after 60 minutes of treatment increases
from 84% in the presence of Pd/Al2O3 to 18.51% decrease with static Pd membrane and
88% when Pd-PVDF/PAA was used in the rotation mode. The reaction follows the first-
order kinetics, with k = 0.037 min-1, in comparison to the reactions supported by Pd powder
which follow First-order reaction (Table 12). The surface area-normalized reaction rate
constant (kSA = 0.0059 L·m-2·min-1) is calculated from the specific surface area of the Pd
NPs (aS = 225.68 m2/g), obtained from the characteristic particle size, and the Pd loading
into the membrane (ρM = 27.8 mg/L), see (Eq. 33.)
Eq. 33: 𝑘 = 𝑘𝑆𝐴 ∙ 𝑎𝑆 ∙ 𝜌𝑀
The performance of the static PD-PVDF/PAA membrane was limited although the
solution was stirred under the same speed due to the limited mass transfer to Pd sites within
the membrane. Rotating Pd membrane was based on the concept similar to the cage paddle
for the solid phase extraction developed by (Shao, MacNeil et al. 2016). The rotation of
the membrane disk enhances mass transfer and improves the availability of reactive
species. Similar concept has been also applied for a rotating disk, ring-disk or cylinder
electrode used in the pre-concentration step which has been shown to greatly enhances the
mass-transfer efficiency and ensures more reproducible mass-transfer conditions than
stirring of the solution (Lee, Pyun et al. 2008).
Although the steady state removal increase is insignificant, the removal rate in first
30 minutes of treatment increased by 44% in the Pd membrane presence. The results
82
presented in Figure 16b indicate that the Pd membrane supports H2O2 formation through
reaction between oxygen and hydrogen throughout the course of treatment. Some studies
propose that palladium nanoparticles with a high number of Pd atoms with a low degree of
coordination are active and selective catalysts for hydrogen peroxide (Campos‐Martin,
Blanco‐Brieva et al. 2006). The achieved removal efficiency with Pd/Al2O3 and Pd-
PVDF/PAA are similar but the main advantage of the Pd-PVDF/PAA use is easy
application and manipulation in contrast with the Pd/Al2O3 powder use, which requires
filtration and removal as additional step after water treatment.
83
Figure 16. a) Comparison of Pd/Al2O3 performance with Pd membrane, and b) H2O2
production during the course of treatment (Conditions: Fe(II): 10 ppm (no Fe(II) added for
H2O2 production measurement), current intensity: 60 mA, Na2SO4:10 mM, pH=3 and CB:
10 mM)
84
4.2.5 Influence of current intensity on CB removal
Figure 17a shows different current intensities versus CB removal efficiency and
changes in CB concentration over time in the presence of different current intensities. CB
degradation via electro-Fenton reaction depends on current intensity; increasing current
intensity increases the rate of water electrolysis and, therefore, production of oxygen and
hydrogen needed for H2O2 generation. Increasing the current from 0 (control) mA to 60
mA increased CB removal efficiency from 1.15% to 84%, but further current increase (120
mA) did not improve the removal rate (Table 11). The CB decay rate increases from 0.0007
min-1 to 0.052 min-1 when current increases from 0 (control) mA to 120 mA. Current
efficiency was calculated by Eq. 6 and it was estimated to be 85.34, 68.45 and 37.34% for
40, 60 and 120 mA respectively.
Although an increase in current intensity results an increase in H2 and O2 gas
production, the formation of bigger gas bubbles lowers the gas mass transfer, dissolution
in the electrolyte and consequently availability to Pd catalyst. The preliminary results show
that an increase in stirring rate might enhances the dissolved oxygen mass transfer and the
production of H2O2 even under higher current intensities (doubling the rate of stirring
increases the concentration four times) but those would promote the volatilization of CB
in the headspace rather than degradation. As shown in Figure 17b, the correlation is linear
for the range of 0-120 mA current intensity (R2=0.78, k=0.743 min-1).
85
Figure 17. a) Degradation profile of CB in different current intensity values, and b)
correlation between removal efficiency and applied current intensity (Conditions: Fe(II):
10 ppm, Pd: 20 mg/L, Na2SO4:10 mM, pH=3 and CB: 10 mM)
86
Table 12. Batch tests results
Variable Removal
efficiency
Degradation rate constant
(min-1)
R2
Initial pH (current intensity=60 mA, Pd dose=2 g/L, Fe(II)=10 ppm)
pH=3 84% 0.032 0.98
pH=4 71% 0.022 0.99
pH=6 22% 0.003 0.75
Fe concentration (pH=3, current intensity=60 mA, Pd dose=2 g/L)
0 ppm Fe(II) 15% 0.002 0.89
2 ppm Fe(II) 38% 0.009 0.97
4 ppm Fe(II) 57% 0.013 0.99
10 ppm Fe(II) 84% 0.032 0.98
Pd dose (pH=3, current intensity=60 mA, Fe(II)=10 ppm)
Pd = 0 mg/L 48% 0.012 0.98
Pd = 5mg/L 68% 0.018 0.99
Pd = 10 mg/L 77% 0.024 0.98
Pd = 20 mg/L 84% 0.032 0.98
Current intensity (pH=3, Pd dose=2 g/L, Fe(II)=10 ppm)
Current = Control 1.15% 0.0007 0.95
Current = 40mA 64% 0.018 0.95
Current = 60mA 84% 0.032 0.98
Current = 120mA 95.8% 0.052 0.94
Pd form (pH=3, current intensity=60 mA, Fe(II)=10 ppm)
Pd-PVDF/PAA
(rotating)
88% 0.0144 0.89
87
Column Experimental Setups
4.3.1. Influence of Pd catalyst, flow rate and Fe(II) on CB
removal
Different Pd/Al2O3 catalyst doses and Fe(II) concentrations were evaluated for
CB removal in the three-electrode column (Figure 18 and
Figure 19, respectively). Figures present the removal efficiencies in the steady-state
(after 120 minutes of treatment) at different zones within the reactor. As expected, Pd
catalyst presence enhances the CB removal: in the absence and presence of 1 g and 2 g of
Pd/Al2O3 pellets (5 and 10 mg/L Pd), the CB removal was 29%, 60% and 71%, respectively
(Figure 18 and Table 13). This is in accordance with results reported by Yuan et al. 2013
where 36% of TCE removed in the absence of Pd catalyst increased to 68% in the presence
of 2 g Pd/Al2O3 (Yuan, Chen et al. 2013). In the absence of Pd catalysts, CB removal
efficiency was 10% after anode, 17% after Cathode 1 and 29% in the effluent. The overall
removal mechanism under these conditions is impacted by direct electrooxidation at the
anode, Fenton reaction supported by H2O2 generated via two electron reduction of anodic
oxygen, and/or hydrodechlorination of CB at the cathode surface.
Figure 18 shows that the highest rate of CB degradation takes place in the
cathode zone (in the Cathode 1 vicinity), which promotes in situ H2O2 generation.
However, as shown in Figure 18, the presence of Pd catalyst caused a significant
increase in the CB degradation in both anode and cathode zones. The degradation in
anode zone is supported by direct electro oxidation at the anode but also by the H2O2 and
OH radical diffusion from cathode zone that occurs under the applied flow rate (2
88
mL/min). In support to the proposed mechanism, CB decay in anodic zone reached 15%,
18% and 20% in the absence and presence of 5 mg/L and 10 mg/L of Fe (II) (
Figure 19). The impact of Fe(II) concentration on CB removal in anodic region
indicates that H2O2 diffusion contributes to the removal and that the degradation
mechanism in this zone relies on Fenton reaction. Further, in the absence and presence of
5 mg/L and 10 mg/L of Fe (II), CB removal efficiency in the effluent was 35%, 55% and
71%, respectively (
Figure 19a and Table 13). As shown in
Figure 19b, the correlation between removal efficiency and the Fe(II) concentration
is linear (R2=0.99, k=3.6 min-1). These results are in accordance with batch tests, where
higher Fe(II) dosage led to higher removal efficiencies. The more significant increase in
the removal is observed in cathode zone than anodic compartment, which is in accordance
with H2O2 generation in Cathode 1 vicinity. The performance of Pd-PVDF/PAA membrane
was insufficient due to limited max flux of dissolved gases towards membrane surface.
This is caused by small pore size of the membrane material that prevented gas bubbles
passage thus limited the active surface area of the Pd-PVDF/PAA.
pH value in the Pd vicinity remains < 4 (as needed for Fenton reaction), while
maintaining a neutral effluent pH even in the presence of a buffer (NaHCO3). This is
supported by the current splitting between two cathodes as reported by Yuan et al. 2013.
89
Figure 18. Effect of Pd catalysts presence on degradation of CB (Conditions: Fe(II): 10
ppm, different Pd dosage current intensity: 60 mA, Na2SO4: 10 mM, Q: 2 ml min−1, and
CB: 10 mM)
90
Figure 19. Effect of Fe(II) concentration on degradation of CB (Conditions: different
Fe(II) concentrations, current intensity: 60 mA, Pd: 10 mg/L, Na2SO4:10 mM, Q:2
mlmin−1, and CB:10mM)
91
4.3.2 Influence of current intensity and flow rate on CB
removal
Increasing the current from 0 (control) mA to 60 mA increased removal efficiency
in the effluent from 2.15% to 71% while 120 mA cause a decrease in the degradation
(Figure 20a and Table 13). An increase from 0 mA to 60 mA enhanced the production of
H2 and O2 at cathodes and anode and consequently H2O2 generation. Higher current leads
to excess formation of H2 and O2 bubbles that are entrapped within electrodes vicinity and
cause a decrease in EF reaction efficiency as well as electric conductivity. Figure 20b
shows that current intensity of 60 (40-20) mA yields the highest removal efficiency.
Current efficiency was estimated to be 13.8%, 8.8% and 3.9% for 30, 60 and 120 mA
respectively.
92
Figure 20. Effect of different current intensity on degradation of CB (Conditions: Fe(II):
10 ppm, different current intensities, Pd: 10 mg/L, Na2SO4: 10 mM, Q:2 ml min−1, and CB:
10 mM)
The performance of the column setup was tested under flow rates of 2, 5 and 10
mL/min. The removal rates under flow of 2, 5 and 10 mL/min were 71%, 46% and 33%,
respectively. As presented in Figure 21a and Table 13, lower flow rates promote CB
degradation due to an increase in contact time of reactive species and support CB oxidation
mechanism. Figure 21a shows that the degradation in both reactive zone is significantly
affected by the flow. And flow rate increases, the CB mass fluxes range from 16 µg/min to
40 µg/min and 80 µg/min. However, flow increase decreases the retention times in
following order: 80 minutes, 32 minutes, and 16 minutes, which adversely affects the
oxidation of CB but also H2O2 generation and OH radical production. The correlation
between the removal efficiency and applied flow rate is linear at each sampling port
(Figure 21b). In anodic zone the R2=0.97 and k=1.46 min-1, in the cathode 1 the R2=0.99
93
and k=4.04 min-1, and in cathode 2 zone the R2=0.99 and k=4.15 min-1. This proves that
lower flow rate resulted in more efficient CB removal.
Figure 21. Effect of different flow rate on degradation of CB (Conditions: Fe(II): 10 ppm,
current intensity: 60 mA, Pd: 10 mg/L, Na2SO4: 10 mM, different flow rates, and CB: 10
mM)
94
Table 13. Column tests results
Variable Removal efficiency
0 mM NaHCO3 -
1 mM NaHCO3 -
5 mM NaHCO3 -
Pd = 0 mg/L 29%
Pd = 5 mg/L 60%
Pd = 10 mg/L 71%
0 ppm Fe(II) 35%
5 ppm Fe(II) 55%
10 ppm Fe(II) 71%
Current=Control 2.15%
Current=30(20-10) 56%
Current=60(40-20) 71%
Current=120(80-40) 62%
Flow rate= 2 mL/min 71%
Flow rate= 5 mL/min 46%
Flow rate= 10 mL/min 33%
Pd-PVDF/PAA= 53.3 mg/L Pd 50.18%
95
4-Chlorophenol Degradation in Aqueous Solution by
Sono-electro-Fenton Reaction
4.4.2 Results of Batch Experimental Setup’s: Electro-
Fenton Optimization
4.4.2.1 Influence of different Fe2+ concentrations
The impact of initial Fe2+ concentration on 4-CP removal was examined under a
wide range of concentrations: 20, 40 and 80 mg L-1 and the 4-CP decay over time is
presented at Figure 22a. Preliminary tests showed that pH=3 provides the higher removal
efficiency, which is in accordance with other studies (Wang and Chen 2009, Yuan, Chen
et al. 2013). The degradation of 4-CP via electro-Fenton reaction follows Zero-order
kinetics indicating that degradation is limited by the availability of the reactive agent
hydroxyl radicals (Table 14). The 4-CP degradation rate increased from 0.0004 min-1 in
the absence of Fe2+ to 0.0043 min-1 in the presence of 80 mg L-1 Fe2+. In the absence of
Fe2+ only 11% of 4-CP was removed via: (i) indirect hydrodechlorination at the Ti/MMO
cathode, and/or (ii) Pd-catalyzed reduction processes (Yuan, Chen et al. 2013). The results
show that an increase in Fe2+ increases the 4-CP degradation rate as it increases OH•
concentration reactions.
In order to evaluate the mechanism of 4-CP removal, EF was performed in the
presence of two different doses of tert-butyl (OH• radical scavenger) (Eq. 8) (Igbinosa,
Odjadjare et al. 2013). Results show significant changes in 4-CP removal after 60 minutes
of treatment in the absence and presence of radical scavenger; the 4-CP Zero-order decay
96
rate in the absence and presence of tert-butyl are 0.0041 min-1 and 0.0016 min-1
respectively. Also, the changes in degradation rate during 60 minutes of treatment in the
presence of tert-butyl were negligible. This indicates that OH• radicals are the primary
reactive species responsible for 4-CP degradation (approximately 75%). Therefore, it can
be concluded that OH• radicals are the main reason for this degradation (Igbinosa,
Odjadjare et al. 2013).
Eq. 34: (CH3)3COH + OH• → (CH3)2•CH2COH + H2O
In order to continuously supply the system Fe2+ and delay the precipitation of
ferrous ion to ferric ion, an iron electrode was used instead of externally adding Fe2+
(Shokri 2017). In addition, the capability to maintain in situ Fe2+ sources are of great
importance for the treatment of groundwater with low natural Fe2+ content. Iron anode has
multiple advantages. Fe2+ is continuously released from the sacrificial iron anode. By
manipulating the current intensity, the electrolytic production of Fe2+ can be controlled and
the utilization of both Fe2+ and oxidants can be enhanced by the controlled release of Fe2+.
By reversing the polarity of iron electrode, the generation of Fe2+ can be prevented or
suppressed (Trabelsi, Ait-Lyazidi et al. 1996). In addition, Iron anode is considered to be a
less energy demanding reaction (Yasman, Bulatov et al. 2004). Figure 22b shows the
comparison of the system’s performance for 4-CP removal in the presence of iron anode
and external Fe2+ addition. The results indicate that the EF system is most efficient (100%
removal) when iron anode was ON for the first 30 min of experiment; because Fe2+ is
continuously produced in situ by anodic corrosion, by applying a positive current using an
97
iron anode. In fact, the generation of ferrous species by iron anode can serve either as
electron donors for the contaminants reduction or as adsorbents for the contaminants
immobilization. On the other hand, due to the cathodic protection effect Fe2+ production
can be suppressed or prevented (Yuan, Fan et al. 2011, Rajic, Fallahpour et al. 2015).
Figure 22. a) Effect of Fe2+ concentration on 4-CP decay, and b) effect of iron anode on
4-CP decay (Conditions: different Fe(II) conc., current intensity: 200 mA, Pd/Al2O3: 1 g,
Na2SO4:10 mM, pH=3 and 4-CP: 200 ppm)
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Table 14. Batch tests results
Changing parameter Removal efficiency Zero-order decay rate constant
min-1 R2
0 ppm Fe(II) 11%
0.0004 0.99
19 ppm Fe(II) 65%
0.0028 0.99
40 ppm Fe(II) 79%
0.0031 0.97
80 ppm Fe(II) 100%
0.0043 0.98
Current = Control 3.23%
0.0001 0.33
Current = 60mA 44%
0.0021 0.98
Current =120mA 77%
0.0034 0.98
Current = 200mA 100%
0.0043 0.98
Pd= 0 g/l 24%
0.0012 0.96
Pd= 0.5 g/l 64%
0.0028 0.99
Pd= 1 g/l 100%
0.0043 0.98
pH=3 100%
0.0043 0.98
pH=4 86%
0.0035 0.98
pH=5 62%
0.0027 0.99
10 mM Na2SO4 100%
0.0043 0.98
10 mM NaHCO3 65%
0.0029 0.99
10 mM NaNO3 67% 0.0025 0.95
99
4.4.2.2 Pd catalyst
The amount and type of Pd catalyst influences the rate of H2O2 production. Figure
23a shows 4-CP concentration profile over time in the presence of different Pd/Al2O3
catalyst dosages. Under the same conditions, an increase in Pd/Al2O3 dosage from 0 to 1.0
g L-1 increased 4-CP removal from 24% in 5 hours to 100% in less than 4 hours. The Zero-
order rate constant for 4-CP removal increased from 0.0012 min-1 in the absence of
Pd/Al2O3 to 0.0043 min-1 in the presence of 1 g L-1 Pd. Decay of 4-CP in the absence of Pd
indicates that processes on the Ti/MMO electrodes contribute to 4-CP degradation but the
rate and overall removal efficiency is low; the H2O2 electrogeneration can occur via two
electron oxygen reduction but the amount is approximately 30% of the amount produced
in Pd presence. The addition of a catalyst significantly increased the removal which is due
to the production of higher H2O2 concentrations and, consequently, production of OH•. The
correlation between Pd/Al2O3 dosage and H2O2 production has been previously proven
(Yuan, Chen et al. 2013).
The influence of catalyst dose was also valuated based on TOC removal in addition
to 4-CP transformation and decay (Figure 23b). The decay in TOC during the treatment
indicates the total mineralization of parent compound (4-CP) and its oxidation byproducts.
Similarly, to 4-CP decay, higher Pd dosages increase TOC removal rates. However, overall
TOC removal is limited (% with 1 g Pd L-1) indicating that 4-CP transforms into other
dissolved organic compounds (e.g., phenol) within the first six hours of treatment.
However, up to 85% of TOC was removed after prolonged duration of treatment (10
100
hours), since hydroxyl radicals are continuously generated during EF, causing total
mineralization of 4-CP and its byproducts.
In addition to Pd dosage, we also tested the influence of Pd support type on the
overall degradation efficiency. Although Pd/Al2O3 is a commercial catalyst extensively
investigated for catalytic oxidation of volatile organic carbons (VOCs), we tested Pd on
active carbon (Pd/C) as alternative catalyst support. As shown in Figure 23c both catalysts
support the 4-CP removal, however, the transformation pathways significantly differ.
Based on the control experiments (without current), 59.71% of 4-CP was removed from
the solution in 240 min when Pd/C was applied while 4-CP concentration decay was only
3.23% when Pd/Al2O3 was used. Absorption rate of Pd/AL2O3 is lower than Pd/C, which
is in accordance with other studies (Wang and Chen 2009). This indicates that Pd/C
supports 4-CP sorption over EF reaction and, although removal rate and efficiency is
significant, Pd/C is not suitable catalyst for 4-CP degradation via electro-Fenton reaction.
Because of the intrinsic properties of Al, such as its low standard reduction potential, high
abundance, high reactivity, stability, and inexpensiveness, Pd/Al2O3 was used as a catalysts
type in all experiments.
101
102
Figure 23. a) Degradation profiles of 4-CP using different Pd/Al2O3 doses, b) degradation
profiles of 4-CP using different Pd/Al2O3 doses on TOC, and c) degradation profiles of 4-
CP using different types of Pd (Conditions: Fe(II): 80 ppm, current intensity: 200 mA,
different Pd/Al2O3 conc., Na2SO4:10 mM, pH=3 and 4-CP: 200 ppm)
4.4.2.3 Current intensity
Current intensity is an important factor affecting the EF process as it is directly
related to the formation of hydrogen peroxide, the regeneration rate of Fe(II) and
consequently the generation rate of hydroxyl radicals. In order to investigate the effect of
applied current on the oxidation of 4-CP, several experiments were performed with
different current intensities in the range of 0-200 mA at the optimal Fe(II) concentration of
80 mM (Error! Reference source not found.a). The decay rate increases from 0.0001 m
in-1 to 0.0043 min-1 when current density increases from 0 mA to 200mA (current density
of 0 to 16.94 mA/cm2). As Error! Reference source not found.a, shows increasing current i
ntensity accelerates the 4-CP decay because of progressively large production of H2O2 and
103
OH• (Jiade, Yu et al. 2008, Yuan, Fan et al. 2011, Yuan, Chen et al. 2013). Higher currents
lead to more H2O2 generation as a result of more O2 and H2 production, which resulted in
more removal efficiency.
Figure 24. a) Degradation profile of 4-CP in different current intensity values (Conditions:
Fe(II): 80 ppm, different current intensity, Pd/Al2O3: 1 g, Na2SO4: 10 mM, pH=3 and 4-
CP: 200 ppm)
Although current efficiency (calculated based on Faraday’s law) indicates that
utilization of 200 mA is significantly less compared to 120 mA, we conducted all our tests
under 200 mA. This is based on the effects of the current intensity on degradation profile
of phenol, a 4-CP degradation byproduct (Figure 24b). Figure 24b shows that under 200
mA, phenol concentration significantly decays after 120 minutes of treatment, while under
the current of 120 mA similar behavior occurs after 180 minutes. Under 60 mA, phenol
concentration keeps increasing, indicating that OH• was mainly utilized for 4-CP oxidation.
The results indicated that under 200 mA, OH• was partially utilized for phenol oxidation
104
in the first 200 minutes of treatment and completely after 250 minutes since 4-CP achieved
complete degradation. Yet, further analysis is needed to identify all oxidation byproducts.
Figure 24c shows removal efficiency of 4-CP per same charge versus the time which is
45%, 46%, and 38% for 60 mA, 120 mA and 200 mA respectively.
105
Figure 24. b) Degradation profile of phenol in different current intensity values, c) 4-CP
removal efficiency per charge (Conditions: Fe(II): 80 ppm, different current intensity,
Pd/Al2O3 conc.:1 g, Na2SO4:10 mM, pH=3 and 4-CP: 200 ppm)
4.4.2.4 Background electrolyte
Background electrolytes are important factors affecting EF due to the fact that they
improve the solution conductivity but also can either support or hinder the efficiency of
electro-Fenton reaction. Evaluation of impacts of different ions in electrolyte solutions is
of great importance for optimizing the treatment for groundwater with complex
geochemistry. Figure 25 shows the concentration of 4-CP over time in the presence of 10
mM NaNO3, 10 mM Na2SO4 and 10 mM NaHCO3. It can be seen that the least 4-CP
removal efficiency appears in the presence of 10 mM NaHCO3 (k=0.0029 min-1) while the
most efficient is the system containing 10 mM Na2SO4 (k=0.0043 min-1). There are a
couple of different possible effects of inorganic anions on Fenton reaction: (i)
complexation or precipitation of iron species, (ii) scavenging of hydroxyl radicals and
formation of less reactive inorganic radicals, and (iii) oxidation including these inorganic
radicals (Moreira, Amorim et al. 2012).
In EF system, NaHCO3 suppresses the performance since it is not a strong
electrolyte but more importantly because HCO3− acts as a OH• radical scavenger (Goi 2005,
Mousset, Oturan et al. 2014b). Although scavenging leads to formation of carbonate
radical, the reduction potential is less (E=1.5V) than that of hydroxyl radical (E=2.43 V)
meaning that the general oxidative activity in system depletes. Further, in the pH range
used in the study, Fe(CO3) complex is expected to be most kinetically active Fe2+ species
106
for the H2O2 activation (Denecker 2009). As expected, Na2SO4 showed higher effect on 4-
CP degradation compared to both NaHCO3 and NaNO3. It is found that sulfate radicals are
predominant reactive species in acidic solutions (pH<2) and can contribute up to 50% of
total inorganic radicals at pH 3 (Moreira, Amorim et al. 2012). The formation of sulfate
radicals follows Eq. 35 and 36; formed species are less reactive than hydroxyl radicals but
are therefore more stable and longer-lived reactive species. The sulfate ion, even at low
concentrations such as 10 mM, can form complexes with Fe3+, which further suppress the
formation of peroxo complexes and decreases the rate of H2O2 activation (Hayward 1998).
However, this was observed in a system with low iron species concentration (e.g., 1 mM
of Fe3+) and pH<3. We assume that complexation would have less impact due to the rate
of Fe3+ reduction at the cathode, initial Fe2+ concentration used in the experiments and
operation under pH=4.
Eq. 35: H2SO42− + OH• + H2O2 → SO4
• − + H+ + H2O
Eq. 36: HSO4− + OH• → SO4
• − + H2O
107
Figure 25. Degradation profile of 4-CP in different background electrolytes (Conditions:
Fe(II): 80 ppm, current intensity: 200 mA, Pd/Al2O3: 1 g, different background, pH=3 and
4-CP: 200 ppm)
4.4.3 Sono-Electro-Fenton (SEF)
Optimized EF parameters (200 mg L-1 4-CP as an initial concentration, 80 mg L-1
Fe(II), 200 mA of current, 1 g L-1 Pd/Al2O3 catalyst and initial pH of 3) were used to
optimize sonifier’s amplitude and ON/OFF time ratio. Figure 26a shows 4-CP
concentration decay during time in the presence of different ultrasound amplitudes. The
graph shows that larger amplitudes are more effective in the degradation of 4-CP. For
example, under the same ON/OFF ratios, amplitude of 70 increases the removal 1.5 times
compared to 50. Under identical ON/OFF ratios, an increase in amplitude increased 4-CP
removal efficiency. Therefore, at a higher output power, more H2O2 was accumulated, and
increases in hydrogen production leads to more hydroxyl radicals. Once ON/OFF time ratio
was set to a higher value equal to 0.2, 4-CP removal efficiency decreased. While
108
degradation of 4-CP increased from 78% to 100% in 4 hours, C/C0 decreased from 22% to
0% by setting ON/OFF ratio equal to 0.1. In this system, the degradation of 4-CP via sono-
electro-Fenton reaction follows Zero-order kinetics. The degradation kinetics of volatile
organic compounds supported in sonolysis suggests two important pathways: (i) oxidation
by hydroxyl radicals formed via collapse of cavitation bubbles, and (ii) OH• reaction with
the solute absorbed at the bubble interface, in the bulk, and to some extent within the
bubbles (Yuan, Gou et al. 2013b). The volatility, hydrophobicity, and the compound
surface activity influences the choice of reaction pathway.
TOC over time in the presence of different ultrasound amplitudes is shown in
Figure 26b. The graph shows that larger amplitudes result in up to 59% TOC removal,
indicating more sufficient mineralization compared to other amplitudes. In addition,
Figure 26c shows the change in the solution temperature over time with different
amplitudes and ON/OFF ratios, which indicate the temperature of the solution increased
with an increase in the ultrasonic amplitudes from 70% ON/OFF ratios of 0.1 to 30%
ON/OFF ratios of 0.2. Therefore, data analysis shows that the ultrasonic treatment leads to
a rapid increase in solution temperature during the first minutes of sonication; then it
gradually increases because of the balance between the amount of input and output energy.
Based on the literature and experimental data, the heating of the solution in an ultrasonic
field is due to the absorption of acoustic energy, which is partly converted into heat
(Thakore 1990, Margulis 2005, Adebusoye, Picardal et al. 2007).
109
110
Figure 26. a) Degradation profile of 4-CP over time with different amplitudes and ON/OFF
ratios, b) TOC over time with different amplitudes and ON/OFF ratios and c) temperature
over time with different amplitudes and ON/OFF ratios (Conditions: Fe(II): 80 ppm,
current intensity: 200 mA, Pd/Al2O3: 1 g, Na2SO4:10 mM, pH= 3, 4-CP: 200 ppm and
different amplitude)
4.4.4 Comparison EF, Ultrasound, and SEF
Different methods were used to investigate 4-CP removal efficiency. Sonolysis of
the 4-CP was performed using optimized amplitude and ON/OFF ratio from the previous
section. When ultrasound is applied directly to the surface of an electrode, a severe surface
degradation by the electrode material erosion can be provided. Due to the electrode
cleaning, it also induced the activation and enhancement in the performance (Petrier, Lamy
et al. 1994, Yuan, Chen et al. 2013). Figure 27a compares EF, SEF and sonolysis reaction
in terms of contaminant removal. It also shows that SEF process can achieve higher
removal efficiencies; 4-CP degradation efficiency was increased in the following order:
ultrasound<electro-Fenton<sono-electro-Fenton processes by 1.85 %, 83%, and 100%,
111
respectively. In addition, sono-electro-Fenton shows favorable phenol decay profile
(Figure 27b). Sonolytic systems with higher frequency have proven to be effective in
organic compounds removal efficiency (Igbinosa, Odjadjare et al. 2013, Särkkä,
Bhatnagar et al. 2015). Figure 27b also shows that EF results in decreased phenol
concentrations and is capable of degrading less of the byproduct of 4-CP. Therefore, more
production of phenol resulted in more 4-CP degradation. Figure 27c shows the effect of
EF, ultrasound and SEF on H2O2 production. As the graph shows, ultrasound does not
produce enough hydrogen peroxide compared to the EF and SEF methods, which can
conclude that hydrogen peroxide seeks out and consumes radicals produced by cavitation
bubbles, thus reducing the oxidizing capacity of the treatment (Lee, Jou et al. 2009, Lee,
Jou et al. 2010). Furthermore, due to the activating effect of low frequency (20KHz)
ultrasonic vibration, the concentration of H2O2 decreases in the solution during the
sonication, which can be related to the formation of excess electrons; because of gas
molecules, polarization decreases inside the cavitation bubble during the neutralization of
anions adsorbed on the surface of bubble (Adebusoye, Picardal et al. 2007).
112
113
Figure 27. a) Effect of EF, Ultrasound & SEF on 4-CP degradation, b) effect of EF,
Ultrasound (US) & SEF on phenol degradation, and c) effect of EF, Ultrasound (US) &
SEF on H2O2 production
4.4.5 Oxidation mechanism
In order to evaluate that the mechanism of 4-CP removal originates from reaction
with OH•, EF was performed in the presence of two different OH• scavenger (tert-butyl)
doses (Eq. 37) (Hamdaoui and Naffrechoux 2008). Figure 28 shows 4-CP concentration
over time for these tests. Results shows that negligible changes in degradation kinetics
happen in the first 60 minutes, in the presence of hydroxyl radical scavenger tert-butyl,
meaning that OH• radicals are the primary reactive species responsible for 4-CP
degradation. This negligible changes in degradation kinetics suggests two important factors
that the removal first occur at the interface of liquid-gas bubbles where it is oxidized by
hydroxyl radicals. Secondly, the formation of volatile products from tert-butyl degradation
114
that accumulate inside of the bubble can decrease 4-CP’s removal efficiency. Therefore, it
can be concluded that OH• radicals are the main reason of this degradation (Hamdaoui and
Naffrechoux 2008). The 4-CP zero order decay rate in the absence, and presence of 500
ppm and 2000 ppm of tert-butyl are, 0.0041 min-1, 0.0016 min-1, and 0.0017 min-1
respectively.
Eq. 37: (CH3)3COH + OH• → (CH3)2•CH2COH + H2O
Figure 28. Degradation profile of 4-CP over time with different concentration of Tert-butyl
(Conditions: Fe(II): 80 ppm, current intensity: 200 mA, Pd/Al2O3: 1 g, Na2SO4=10 mM,
pH=3 and 4-CP: 200 ppm)
115
5 Chapter 5 Conclusions
Summary
Degradation of chlorinated compounds in water and groundwater via enhanced
electrochemical methods has been investigated. Electrochemical degradation of CB in
groundwater by palladium-catalyzed EF reaction and palladized polyacrylic acid (PAA)
polyviniledene fluoride (PVDF) membrane (Pd-PVDF/PAA) was evaluated in both batch
and plug flow column reactors. The changes in the electrolyte, CB concentration at the
effluent and other chemicals concetration were investigated with the number of interfering
parameters. While a two-electrode batch reactor with manually adjusted pH values was
used for a batch tests, the electrochemical column reactor contained three sequentially
arranged electrodes (one anode and two cathodes) to achieve automatic pH regulation by
current sharing. Then removal efficiency of CB was investigated. Furthermore, removal
of 4-CP by SEF was investigated. EF, where ultrasound radiation are known to produce
hydroxyl radicals that are strong oxidative agents.
Conclusions
The following conclusion can be stated based on the results of all the
experiment which was performed during this study:
• There are significant differences in the degradation of the CBs when the Pd catalyst
was present and absent. Pd catalyzed-EF method can effectively treat CB in both
116
batch and column reactors. In both reactors, palladium-catalyzed and palladized
polyacrylic acid (PAA) polyviniledene fluoride (PVDF) membrane (Pd-
PVDF/PAA) were used. In the batch reactor, an increase in Pd/Al2O3 powder
concentration from 0 to 2.0 g/L (0 to 20 mg total Pd), increased CB removal from
48% to 84%. Addition of Pd/Al2O3 enhances the formation of H2O2, due to the high
ability of Pd to capture hydrogen within its lattice as well as the high surface area
available for the reaction.
• While rotating Pd-PVDF/PAA disk generated 88% degradation of CB because the
rotation of the membrane disk enhances mass transfer and improves the availability
of reactive species. On the other hand, in the column experiment, 71% and 50.18%
of CB removal efficiency were achieved in the presence of 2 g/L catalyst in pellet
form (0.5 %Pd, 10 mg/L of Pd) and Pd-PVDF/PAA= 53.3 mg/L Pd respectively.
• The best CB removal efficiency was achieved at a pH of 3 due to an increase in
production of H2O2 and more generation of OH•. At higher pH values these
productions are limited and most Fe(III) ions precipitates with OH-
• Degradation of CB occurs in batch and column reactors under current intensity of
120 mA and 60 mA respectively. Higher current leads to excess formation of H2
and O2 bubbles that are entrapped within electrodes vicinity and cause a decrease
in EF reaction efficiency as well as electric conductivity.
117
• The lower flow rates enhance CB degradation due to an increase in contact time of
reactive species and support CB oxidation mechanism while higher flow-rate
decrease retention time for electrode gradation.
• EF was also performed in the presence of two different concentrations of tert-butyl
(OH• radical scavenger). Results show significant changes in 4-CP removal after
60 minutes of treatment in the absence and presence of radical scavenger. In
addition, the changes in degradation rate during 60 minutes of treatment in the
presence of tert-butyl were negligible. This indicates that OH• radicals are the
primary reactive species responsible for 4-CP degradation.
• When ultrasound is applied directly to the surface of an electrode, a severe surface
degradation by the electrode material erosion can be provided. Due to electrode
cleaning, it also induced the activation and enhancement in the performance.
• Ultrasound does not produce enough hydrogen peroxide compared to the EF and
SEF methods, which can be stated that hydrogen peroxide seeks out and consumes
radicals produced by cavitation bubbles, thus reducing the oxidizing capacity of the
treatment.
• Due to the activating effect of low frequency (20KHz) ultrasonic vibration, the
concentration of H2O2 decreases in the solution during the sonication, which can be
related to the formation of excess electrons; because of gas molecules, polarization
118
decreases inside the cavitation bubble during the neutralization of anions adsorbed
on the surface of bubble (Adebusoye, Picardal et al. 2007).
• Temperature of the solution increased with an increase in the ultrasonic amplitudes
from 70% ON/OFF ratios of 0.1 to 30% ON/OFF ratios of 0.2. Therefore, the
ultrasonic treatment leads to a rapid increase in solution temperature during the first
minutes of sonication; then it gradually increases because of the balance between
the amount of input and output energy.
• Different types of background electrolyte were used in this experiment including
NaNO3, Na2SO4 and NaHCO3 and result indicated that Na2SO4 has higher effect
on removal efficiency of 4-CP. It is found that sulfate radicals are predominant
reactive species in acidic solutions (pH<2) and can contribute up to 50% of total
inorganic radicals at pH 3.
• An iron anode was used to generate continuously Fe2+ ions and delay the
precipitation of ferrous ion to ferric ion. The results indicate that the EF system is
most efficient (100% removal) when iron anode was ON for the first 30 min of
experiment; because Fe2+ is continuously produced in situ by anodic corrosion, by
applying a positive current using an iron anode. In fact, the generation of ferrous
species by iron anode can serve either as electron donors for the contaminants
reduction or as adsorbents for the contaminants immobilization. On the other hand,
due to the cathodic protection effect Fe2+ production can be suppressed or prevented
(Yuan, Fan et al. 2011, Rajic, Fallahpour et al. 2015).
119
• Different types of Pd were used for removal efficiency of 4-CP consisting of Pd/C
and Pd/AL2O3. Based on the control experiments (without current), 59.71% of 4-
CP was removed from the solution in 240 min when Pd/C was applied while 4-CP
concentration decay was only 3.23% when Pd/Al2O3 was used. Absorption rate of
Pd/AL2O3 is lower than Pd/C, which is in accordance with other studies (Wang and
Chen 2009). This indicates that Pd/C supports 4-CP sorption over EF reaction and,
although removal rate and efficiency is significant, Pd/C is not suitable catalyst for
4-CP degradation via electro-Fenton reaction. Because of the intrinsic properties of
Al, such as its low standard reduction potential, high abundance, high reactivity,
stability, and inexpensiveness, Pd/Al2O3 was used as a catalysts type in all
experiments.
• At a higher output power, more H2O2 was accumulated, and increases in hydrogen
production leads to more hydroxyl radicals. There were two different ON/OFF ratio
(=0.1 and =0.2) were set for performing ultrasound and sono-electro-Fenton
technologies. Once ON/OFF time ratio was set to a higher value equal to 0.2, 4-CP
removal efficiency decreased due to an increase in solution temperature.
120
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