Electrochemical Treatment of Recalcitrant Waste: a Study of Chlorophenols and - Nitroaromatic Compounds
ELECTROCHEMICAL TREATMENT OF RECALCITRANT WASTE: A STUDY OF CHLOROPHENOLS AND - NITROAROMATIC COMPOUNDS
A Thesis
Presented to
The faculty of Graduate Studies
of
The University of Guelph
JAMES D. RODGERS
In partial fùlfillment of requiremnets
for the degree of
Doctor of Philosophy
August, 2000
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Electrochemical Treatment of Recalcitrant Waste: A Study of Chlorophenols and Nitroaromatic Compounds
James D. Rodgers Advisor
University of Guelph, 2000 Dr. N.S Bunce
Electrochemical treatment was investigated as a treatment option for the oxidation of chlorophenols and the reduction of nitroaromatics. Chlorophenols are used in the manufacturing of pesticides and herbicides and are added to many formulations to prevent microbial growth. Nitroaromatics are associated with the explosives industry and have caused widespread contamination as a result of their use and mmufacturing, Both compounds pose environmental threats as a result of their recalcitrance to chexnical or biological oxidation.
In the case of chlorinated phenols, we explored the problem of anode fouling which has previously hampered electrolytic treatment of phenolic compounds. Linear sweep voltamrnograms at a Pt anode were exarnined for phenols dsering in the extent of chlotination. Passivation increased in parallel with the uncompensated resistmce of the solution and occurred only at potentiais at which water is oxidized, suggesting that the formation of the oligomer film involves attack of hydroxyi radicals on electrochemically oxidized substrate. Chronopotentiometry was used to deduce the oxidation potential of the chlorophenols (-1.3 V vs- saturated calomel electrode) and the number of electrons involved in oxidation; monochlorophenol and trichlorophenol required four electrons while pentachlorophenol required only two. During electrolysis, relative reactivities of congeners were anode-dependent, which we interpreted as different mechanisms of oxïdation: direct electron transfer oxidation at Pb02 and hydroxyl radical attack at Sn02 and 11-0~.
Voltarnrnetry of nitroaromatics (2,4-dinitrotoluene, 2,6- dinitrotoluene, 2,4,6- trinitrotoluene) at pH 2 and at a mercury electrode was consistent with Iiterature values
(e.g. 2,4,6- trinitrotoluene; -0.18 V, -0.35 V, -0.48 V vs. saturated calomel electrode) suggesting electrolysis as a viable alternative to current techniques such as granula. activated carbon and advanced oxidation processes. dinitrotoluene was reduced at several cathodes with the most promising result at Ni-plated Ni wire were at 0.1 rnA with current efficiencies > 80%.
Subsequent treatment of the reduction products (aminotoluenes) was shown to be possible with enzymatic catalysis, air sparging and anodic oxidation. The latter alternative was the most successfùl in terms of efficiency and simplicity. For Karina and Alexandra ACKNOWLEDGEMENTS
A major work Like this could not have been completed without a lot of help fiom a number of hdividuals. Thank you ail.
The author would like to thank Eltech Corporation for supplying the doped Sn02 electrode and for NSERC for its industrial grant, which was responsible for the fiinding of the project.
The support staff in Chemistry were amazingiy talented individuals, with Steve
Sefied, Teny White, Uwe Oehier all helping me overcome technological obstacles.
I would like to my advisory cornmittee for their input Prof. Lipkowski for his electrochemical assistance and Dr. Barry MacDougall for serving as extemal referee.
Thanks to Dr. Perry Martos (OMAFRA) and Dr. Kirk Green (McMaster) for their use of their respective LC/MS7sand training on the equipment.
1 would like to thank my lab mates (Pete, Michelle, Naomï, Christine, John, Brie
Monika) and fnends both in Engineering (Sandra, Shelly, Laurie, Mke, Tej) and
Chemistry (Grzegorz, Ian, Mike, Steve, Ai Chen) for their support. Without fiends graduate school would have been a lonely existence. A special thanks to Alex for the cornputer help, the coffee breaks and the pool games. Kim, Don thanks for the great
Saturday nights and neighbor support.
1 would like to thank Professor Richard Zytner for dowing me to pursue this project in the engineering department and for giving me guidance both about what an engineering thesis should be and about engineering Me. Thanks to Isobel Heathcote for her continual career support and mentoring at the administration and engineering levels. I've appreciate you taking me under your wing, I've learnt a lot fiom you about personnel dynamics, public administration and leadership.
A very special thanks to Voiteck Pr. Wojcieh Jedral) whose assistance in electrochernistry was invaluable. In the lab he taught me that anything could be built and the real meaning of'pateince is a virtue". However Our relationship went beyond that. Our daity tea break was incorporated with lessons in politics, history, the world, Me and how to view the world f?om a squirrels eyes.
Professor Nigel Bunce for being the first scientist to teach me the true meaning of science and the transfer of knowledge through publication. 1 also wanted to thank him for fostering our relationship beyond science, 1 really enjoyed our though provoking discussions on He, politics, science, technology and everything else under the sun. You have been a major innuence in my life.
1wanted to thank my father for creating my interest in learning and for a thirst for - the written word. I also wanted hun to how I appreciate his lessons in patience and for teaching me the importance of balance within life. As for my mother it was her who gave me confidence in myself to chase my dreamq as her belief in me extended to levels that only motherhood would aliow,
The most special thaaks to my dear wife Lisa who's support was endless and unwavering. I'U never forget al1 the sacrifices you made and the effort you put into the girls lïves while I was workuig late at the lab. It takes a rare individual to put their career and live on hold to support another individuai mentally, emotionaiiy and financidy. Thank you SP for dowing me to pursue and obtain rny dreams.
llI.S. TABLE OF CONTENTS i) Dedication ...... i .. ii) Acknowledgement s ...... u u)S.. List of Figures...... -.x iv) List of Tables ...... xv v) Nomenclature ...... xvi
Part 1 - Introduction
1. Electrochemical Technology for Waste Remediation ...... 1
1.1, Recalcitrant waste ...... 1 1.2. Physiochemical Destruction Techniques ...... 3 1-3. Electrochemical remediation ...... 4 1.3.1 Advantages of electrochemical remediation ...... 7 1-3 -2.Disadvantages of electrochemical remediation ...... 8 1 -3-3 Engineering parameters ...... 9 2 -3-4 Cost effectiveness ...... 1O 1.3.5 Application ...... 14 1 -4 Objective ...... 15
Part II - Chlorinated Phenots
2 Chlorophenol Review
2.1 Chlorinated PhenoIs ...... -20 2.1.1 History ...... -20 2.1 -2 Toxicity ...... -21 2.1 -3 Treatment ...... -22 2.2 Methods of Remediation ...... -22 2.2. 1 Incineration ...... -23 2.2.2 Granular Activated Carbon ...... -24 2.2.3 Photolysis ...... 25 2-2-4Advanced ûxidation Processes ...... -26 2.2.4.1 Heterogeneous Photolysis ...... -27 2.2.4.2 Hydrogen PeroxideNltraviolet Light ...... -29 2.2.4.3 Ozonation ...... -30 2 .2.4.4 Supercritical Orcidation: ...... -31 2.2.5 Microbial Degradation ...... 32 2.3 Conclusion ...... -34
3 . Volt amrnetric Studies of Chlorophenols
3 .1 Introduction ...... *...... -42 3.1.1 Linear Sweep Voltammetry ...... -42 3.1.2 Polarography...... 45 3 -1-3 Chronopotentiometry ...... -47 3 -2 Literature Review ...... -50 3.2.1 Chlorinated Organics ...... 52 3 -3 Experiment al ...... 53 3 -4 Results /Discussion ...... 34 3.4.1 L.S.V...... 54 3 .4.1.1 Oxidation Potentials ...... -58 3 .4.2. Substrate...... 59 3.4.2.1 Low Concentration of Substrate ...... -59 3.4.3 Concentration (4-MCP) ...... -63
3.4.4 pH ...... , ...... 67 3 .4.5 TCP compared to 4-MCP ...... ,,...... -70 3 -5 Chronopotentiometry...... 79 3 -5-1 Oxidation Mechanism...... -79 3 S.2 Uncompensated Resisîance ...... -83
4 . Electrolysis of Chlorophenols ...... -94
4-1Introduction...... -94 4.2 Literature Review ...... -94 4.2.1 Electrolysis ...... -94 4.2.1.1 Anode Material ...... 94 4.2.1 -2 Operating Conditions ...... -95 4.2.1.3 Rate Constant ...... 96 4.2.2. Phenol ...... 97 4.2.2.1 Omdation.. * ...... -97 4.2.2 -2 Oxidation Mechanism ...... -99 4.2.2.3 SnOz Mechanism...... 100 4.2.3 Chlorophenols ...... 101 4.2.3 -1Electrochemical Reduction ...... -101 4.2.3 -2Electrochemical Oxidation ...... -101 4.3 Experimental ...... 105 4.3.1 Bulk Oxidation ...... 105 4.3 .2. High Performance Liquid Chromatography ...... -108 4.3 -2Substrate ...... -108 4.4 Resdts ...... 109 4.4.1 pH...... 109 4.4.2 Anode Matenal ...... 109 4.4.2.1 PbO;?...... 109 4.4.2.2 DSA Anodes ...... 110 . . 4.4-3 Substrate reactww...... 116 4.4.4 Fouhg ...... 118 4.4.5 Products ...... 119 4.4.5.1 LSV of Products ...... 122 4.4.6 Total Organic Carbon...... 122 4 .4.7 Current Efficiency ...... 123 4.4.8 Operatuig Costs ...... -...... 124
Part III .Nitroaromatics
5 . Treatment Methods for the Remediation of Nïtroaromatic Explosives .... 130 5 . 1 Introduction ...... *...... *...... - ...... 130 5.1.1 Sites...... 133
5 -2 Treatment ...... - *...... *...... *...*. 134 5 .2.1 Granular Activated Carbon...... - ...... 134 5 .2.2 Advanced Oxidation Processes ...... - ...... 136 5 .2.3 Hydrothermal ...... - ...... 140 5 .2.4 Reduction ...... - ...... 140 . . 5 .2.5 Bioremediation...... - ...... 141 5.2.5.1 Aerobic ...... 142 5 .2.5-2 Anaerobic ...... - ...... 143 5.2.5.3 Inorganics ...... -...... 145 5.2.5.4 Biodegradation Products ...... -...... 146 . 5 .2.5-5 Field Apphcation ...... - ...... 146 5 .2.5-6 Phytoremediation ...... - ...... 148
6 . Voltammetry of Nitroaromatics...... -161
6-1 Introduction ...... - ...*.*-...... -161 6.2 Literature Review ...... - ...... 161 6.2.1 Nitroaromatic Reduction ...... - ...... -161 6.2.1. I Nitroaromatics ...... - ...... 161 6.2.2.2 Reduction of DNT, TNT ...... - ...... 166 6.3 ResuIts ...... 170 6.9 The reduction of nitroarornatics with SMDE at pH 5 (1 mV s-')...... 1 76 6-10 LSV; Reduction of 0.4 mM 2-6-DNT at different cathode materiai (pH 2) vs. NHE ...... 177 7-1 Reduction of 2rnM 2,6-DNT as a finction of electrode materid ( 10 mA/cm2)...... -195 7-2 The reduction of dzerent nitroaromatics (2.5 mM, pH 2) at a Nï-plated cathode (-1 -00V vs. S.C.E.)...... 198 7-3A Reduction of 2,4,6-TNT (2.5 mM, pH 2) fitted to a zero order relationship as a hction of potential (V vs. SCEJ at a Ni-plated cathode (IL2 values reported in brackets of graph) ...... 198 7.4 Electrolysis of 2.5 mM 2,4 DNT as a Fmction of Potentiai (V vs. SCE) ...... 200 7.5 Mass Spectrum (GCMS) showing the fragmentation pattern of 2A4NT (dz 152) (top) and cornparison with the iibrary standard (bottom) ...... -202 7-6Caliiration of 2,6-D AT standard using SPME (GCMS) and monitoring the peak area of m/z 122 ...... -.203 7.7 LCMS (ACPI) spectnun at various dzratios showing the parent ions of the reduction products of 2,6-DNT ...... -204 7.8 Selection of 2,6-DNT reaction products...... -205 7.9A Tandem Mass Spectrum (Probe-ACPI) showing the daughter products of the positive ionizaîion of 56-DAT (m/z 123) standard...... -206 7.9B Tandem Mass Spectrum (ACPI) showing the daughter products of the positive ionization of 2,6-DAT (ml2 123) standard...... -207 7.10 An ES1 MS probe mass spectmm at various m/z ratios showing the parent ions of the reduction products of 2,4- DNT (top hottom are DNT, middle is blank) ...... 209 7.1 1 An ES1 LC/MS mass spectmm at various m/z ratios showing the parent ions of the reduction products of 2,4- Dm...... 210 7.12 An ES1 MS probe mass spectmm at various dzratios showing the parent ions of the reduction products of 2,4,6-TNT (top is TNT, bottom is blank) ...... 21 1 7.13A Mass Balance of a 2.2 mM 2,6-DNT solution (pH 2) that has undergone electrolytic reduction (0.9V vs SCE, cathode) ...... -2 13 7,13B Reduction of a 2.5 mM 2,4-DNT solution at different current densities and the mass balance after 2 hours ...... ,,214 7.14 Mass Balance of a 1.9 m.2,4,6-TNT solution (pH 2) that has undergone eIectrolytic reduction (-0.9V vs SCE, cathode) 7.15 Disappearance of 2.2 mM 2,6-DAT at pH 2, with respect to type of orridation treatment ...... -218 7.16 Schematic of an individual process for the removal of nitroaromatics in wastewater ...... -222 LIST OF TABLES
1.1 A cornparison of the molar cost (in US cents) of different materials ...... 11 3.1 The effect of phenol ch.io~aû0non peak curent (O. 1Vk) ...... 60 3 -2 Summary of chronopotentiometric curves for chlorophefiols ...... 73 3 -3 Wdation potzntids of chlorinated phenois with respect to amount of chlorination and pH of solution ...... 77 3.4 The change in i, rL2as a function of nirrent density at dierent chlorophenol (1 mM) and pH...... 78 4.1 Effëct of experimental parameters on the oxidation of phenol (Pm)...... -...... -.... *.--**--% 4.2 Pseudo first-order rate constants (and 2) for phenols on different anode material, ...... -114 4.3 Current efficiencies for the oxidation of 4-chlorophenol at three anodes as a function of current density...... 123 6.1 Cornparison of nitroarornatic potential peak values a? a DME £kom different researchers under different conditions (vs. SCE)...... 169 6.2 Calculated E values for 99.9% removal of 2,6-DNT at dserent elecîrode materials ...... -180 7.1 The electrochemical efficiency (%) associated with the reduction of a 2.5 mM 2,4-DNT solution after 1 hou of electrolysis with a Ni plated NI cathode...... 196 7.2 Removai efficïencies of DAT congeners with HRP peroxidases...... -21 9 7.3 Removal efficiencies of DAT wngeners with Type 1 HRP peroxidases...... -220 ii. NOMENCLATURE
AHR Attack Hydroxyl Radical
AOP Advanced Oxidation Processes
APCI Atmospheric Pressure Chemical Ionkition
CE Counter Electrode
COD Chemical Oxygen Demand
CI Chernical lonization
DAT Diarninot oluene
DCP Dichlorophenol
DDT 17171 -Tricbioro-2,2-Bis-(p-chlorophenyi) ethane
DME Dropping Mercury Electrode
DNT Dinitrotoluene
DPP Differentid Pulse Polarography
DSA Dimensionally Stable Anode
ES1 Electron Spray Ionization
ET Electron Tder
Gcm Gas Chromatography / Mass Spectrornetry
HCB Hexachlorobenzene
HPLC High Performance Liquid Chromatography
Lcm Liquid Chromatography / Mass Spectrome~
LSV Linear S weep Voltamrnetry
Mm Monochlorophenol
MSMS Tandem rnass Spectrornetry Pa3 PoIychiorinated Biphenyl
PCP Pentachiorophenol
PKL P henol
RE Reference Electrode
RT Retention Time
SCE Saturated Calomel Electrode
SMDE Sessile Mercury Dropping Electrode
SPME Solid Phase Microextraction
SEM Scannuig Electron Microscopy
TOC Total Urganic Carbon
TCP Tricfilorophenol
TNT Trinitrotoluene
WE Working Electrode
XPS X-ray Photoelectron S pectroscopy CHAPTER ONE
ELECTROCWEMICAL TECHNOLOGY FOR WASTE RErnDIATION
Biologkal systems for waste treatment are popdar because they are easy to operate and often have hi& efficiencies, in which case they are cost effective. Most cities in the developed world p+ their municipal wastewater in treatment plants that incorporate at least one biological process. This can range fiom natural attenuation, to a secondary treatment asing activated sludge, to a tertiary treatment including options such as a polishing pond, land farming or biomass fuel recovery "'.
Besides municipal wastewater plants, biological treatment is used extensively in industrial applications, treatrnent of landfill leachate, and for in-situ bioremediation of contaminated sites Treatment can occur aerobically or anaerobically, and can be effected via a variety of technological methods, including bioreactors, biofilters, biocurtains, stabilization ponds and activated sludge, or in-situ treatment aith natural, non-indigenous, or bioengineered organisms (4!
Legal requirements demand that a certain percentage of the chernical oxygen demand (COD) must be removed (75% Germany, 85% Switzerland) by the biological treatment. A waste stream is termed ccrecalcitrant7'if it is resistant to biological treatment, because the chernical compounds present are chemicaliy and metabolically unreactive 1 andlor because they are toxic to the microorganisms '? Toxicity in pdcular, can be a major problem as it can effectively shut down a biological treatment plant-
Substances that are unreactive biologically also tend to be meactive chemicaily.
Aromatic rings and halogens are structural features that generally lower chernical reactivity and increase the refiactory character of a molecule. The famiIy of polychlorhated organics includes some of the most recalcitrant compounds. Specific exampies include the wood presemative pentachlorophenol (PCP), fbgicides such as hexachlorobenzene WCB) and insecticides such as DDT. Polychlorinated biphenyls
(PCBs) are among the most well-lmown compounds in this class; for decades they were used as insulators and dielectric fluids in electncal equipment, as plasticizers and as Iubricants '='.
Concern for the environment by scientists and society has grown steadily since
1962 when Rachel Carson published "Silent Spnng" '? This is especially tnie for the environmental impact associated with halogenated compounds. This accounts for their presence on the European community's CCEnvirunmentalPt-iority LisP' and the EPA's
"List of Priority Pollutants". Each list contains approximately 130 compounds, of which
70% can be classified as refiactory chlo~atedcompounds '879'.
The destruction of these compounds represents an ongoing need as they have been produced and used in industrial applications throughout the twentieth century. Many of them have entered the environment in large quantities and have been documented to have caused adverse eEects to environmental biota "O). Their low reactivity causes them to be environmentaily persistent, resulting in detection anywhere these products were 2 manufactureci, used or stored. Furthemore, their physical properties contribute to their toxicity and distribution; many are semi-volatiles, which dow thern to be transported atmosphericaily, while their lipophilic character results in bioaccumulation.
1.2 PEYSIOCHEMICAL DESTRUCTION TECHNIQUES FOR lXl3FR4CTORY
COMPOUNDS
Both destructive and non-destructive methods of treating refiactory organic compounds are available. Non-destructive techniques include adsorption (Le. activated carbon) and extraction (Le. liquid/liquid) (? These methods do not change the chemical nature of the contaminant; instead, they concentrate the substance on to a sorber't or into a solvent for Mer processing. Sorption on to granular activated carbon (GAC) is a common approach, however, the spent GAC may then itself have to be disposed of as a hazardous materiai.
Chemical treatment methods are usuaüy oxidative (hydrogenation, a reductive technique, is used less comtnonly). Oxidative methods include incineration and Advanced
Oxidation Processes (AOP3s) such as ozonation , ultraviolet irradiation (11) ,
Whydrogen peroxide, UV/ozone, UV/Ti02 '12) and gamma irradiation of H$& "". W done causes bond rupture in suitably absorbing substrates, while the other AOP's fiction by the generation of OH radicals. These are very effective at initiating the oxidation of the target compounds by abstraction at C-H bonds or addition at unsaturated and aromatic centres. Like photolysis and radioloysis, dtrasound '"LS' has been investigated as an experimental AOP. A principal disadvantage of AOP's is poor energy efficiency, especidy in the production and utilization of photons. Many AOP's include the use of reactive and dangerous chernicals (03, Ha), or the use of high temperatures and pressures. UV lamps are costIy; they are a replaceable item, and are prone to fouling, which hinders light transmissioe They also tend to be application-specific, meaning that considerable development work is needed for each application.
Incineration is highly effective in destruction, but has tremendous energy requirements (especially for dilute wastewaters) as it completely destroys (mineralkation) organics to CO2 and Hz0 "? Other disadvantages include high capital cost, lack of mobility of the equipment, and treatment of combustion byproducts. Nitrogen oxides
(NO,) are formed as an inevitable accompanirnent to combustion in air; in the case of incinerating chlorinated compounds, there is also the possibility of producing smd amounts of diolons and fiirans if the combustion temperatures are insutFciently high "?
These problems account for the very poor public image of incineration technology (18'.
Despite these drawbacks, incineration remains as one of only two methods of legal treatment for aqueous PCBYsin Canada 'Lg'.
Electrolytic reactions employ electrical energy to dnve an otherwise davourable chernicd process. In most cases the chernical reactions of interest take place at an electrode surface. Electrochemical methods inevitably consist of two components: reduction, which occurs at the negative electrode (cathode), and oxidation, which takes place at the positive electrode (anode). Therefore, electrolysis can be applied to contaminants that are capable of either oxidation or reduction; provided that one of these reactions is possible, electrochemical technologies have the advantage that the electrode processes involve only the removal (oxidation) or addition (reduction) of electrons. No addition of chernicals is in p~ciplerequired, thus making electrolysis a very 'cgreen'7 technology.
Legislators and industry favour waste treatment without the use of added chernicals. This would probably also lead to the acceptance of electrolysis by the general public. The US EPA @O' has classitied a similar method, celectro-migration77as the only in- situ method that can be applied to low penneability clays and silty soils. This is important as these conditions represent halfof the American bazardous waste sites.
Electrochemical treatment can include direct or indirect methods, with the latter includiig rneta.Uk redox couples "", or electrogeneration of strong oxidants '-'.
Examples of metallic redox couples have been ce3"/ce4' and CO'~/CO~'while indirect oxidants have included CIO' and H202-
Electrochemical technology became wel established industrially in the last century and individuals who attempted to capitalize om its success included Albert Einstein's father who was an electricd engineer and designed and built parts for electrochernical apparatus
'24'. Some of the technologies used today include the production of aluminum and chlorine.
Another famous application is the Monsanto adiponitrile process, discovered in 1960 by
Manuel Baker @). This is the reductive couplimg of acrylonitrile to form adiponitrile which is an important precursor in nylon production. Despite these successes, the use of electrochemical technology in environmental engineering is relatively new. While there have been select uses with this technique to treat inorganics such as chromium there have been more iimited trials with organics. Specific examples of substrates include; ammonia ", urea R8)7 cyanides cL'7 ethanol 'O), EDTA "', dye wastes 02' and methanol 03'. Alverez-Gallegos and Pletcher '4' have been very active in tbis resezrch area and have reported on the treatment of phenol, catechol cresol, quinone, hydroquinone, aniline, oxalic acid and amaranth red dye. Lin and Chen "'have
researched the decoloration and TOC removal of textile waste. Hofseth and Chapman '@ at the University of Wisconsin removed cyanide ion fiom wastewater through anodic oxidation. A recent innovative approach is Do and Yeh's destruction of formaldehyde using electrogenerated hydrogen peroxide.
The electrochemical treatment of phenols has only experienced limited success, attempts at phenol pilot plant treatment facilities by both Dabrowski and Chettie in the 70s and 80s never reached industrial scde due to 1ow reaction rates or poor efficiencies. While these inefficiencies were partially overcome by higher temperahires, subsequent oxidation of toxic intermediates remained as a major problem.
1.3.1 Advantages of Electrochemical Remediation
Electrochemical methods for waste control have several inherent advantages "" includhg wide application, as they can be applied either reductively and/or oxidatively.
Reduction can be very effective at dechiorinating a compound as seen below for an aromatic compound. In contrast, the first step in the oxidation of an aromatic involves the removal of
one or more electrons from an organic compound, the ring remaining intact, and the
introduction of a quinone or hydroxyl group. As recalcitrance can be eliminated by the
removal of one or more hdogens or by the introduction of oxygenated fùnctionalities,
both of these processes have the potential to increase biological atîack. Other advantages
include mobiiity and operation on a small scaie, low to moderate capital cost (which is
partidy a function of its operation at atmospheric pressure and ambient temperature), rate
controlied by current, and ease of automation. There is litde or no need for hazardous
chernicals which elirninate their purchasing and disposal costs.
Electrochemical technologies offer the prospect of relatively simple equipment
(and the ability to be modula in design), environmentai fiendliness, and the possibility of higher power efficiencies in cornparison with thermal treatment facilites or photolysis. An
effective electrochemical system will use a minimal amount of energy to transfonn refkactory compounds and their by-products into biodegradable forms. This fkont-end approach results in Iower energy demands and thus can offer considerable monetary saving for industry and local governments. 1.3.2 Disadvantages of Electrochemicai Remediation
A signiscant drawback to electrochemicd remediation is the occurrence of
parasitic reactions such as electrolysis of water and the subsequent gas evolution (02, H2)-
If this occurs in a divided ce11 then changes in pH occur, this can lead to substrate
precipitation or corrosion of ceil components.
The electrolysis (oxidation) of certain organic compounds can be subject to
electrode fouiing, in which deposition of material on the electrode surface may preclude
mass transfer of the pollutant matenal to the electrode. Electrodes able to operate at high
potentials or efficiencies in laboratory experiments may not be practical for industrial use
as they may be expensive (Au, Pt) or be environmentally unacceptable (Hg).
The passage of electricity through a solution requires that the solution be
conductive. Unfortunately, not all waste steams will have sufEicient conductance and the addition of an electrolyte may be necessary. Failure to do this will cause drops in eEciency through ohmic heating.
The problem of "economies of scaie" has prevented e1ectrochemical bench top technology undergoing scaIe up to be used in industrial processes. However as the cost
(purchase, safety and disposal) of using hazardous materials climbs, the development of electrochemical technology to produce chernical reactions or dispose of waste will becorne more competitive econornically
Earlier studies have shown the possibility of eIectrochemical techniques producing toxic by-products with certain substrates. For example, one of the byproducts of 4-
8 chlorophenol oxidation is p-benzoquinone (LDso orally in rats 130 mgkg), which is more toxic than chlorophenol (ID50 670 (ortho, para), 570 (meta) @kg) (*'As such, mass balance with respect to operating conditions and electrode material wu be important engineering parameters.
1.3.3 Engineering Parameters
Electrolysis can occur in a batch system or in a flow cell where the target compounds are continuously flowed past the surface of the electrode. In an unseparated cell (tlow or batch), the target compounds can be exposed to either the cathode or anode and hence both reduction and oxidation. Therefore in order to study single reactions most researchers have chosen to use divided cells. Regardiess of the configuration, the efficiency of the electrochemical processes wiU depend on the relationship between mass transfer of the substrate and electron transfer £kom the electrode or mediator.
For aqueous systems it is important that the anode material has a high over-voltage for oxygen in order to minimize the generation of O2 by electrolysis of water. The overpotential is the merence between the potentid that is thermodynamically required for a reaction to proceed and that actually required at a specific electrode. Typically, the greater the overvoltage of the electrode, the greater the usefid potentid range of the electrode. It is important to chose electrodes carefblly as current efficiencies and the formation of toxic by-products can be influenced by the type (planar, three dimensional) and class of electrodes (Le. Dimensionally Stable Anodes @SA) vs. metallic). 1.3.4 Cost Effmtiveness
In order for an electrochemical remediation system to be accepted by industry, it must be cost effective compared to existing technologies. This is determined by a number of factors, most importantly, initial capital required and operating costs. In this technology electrical consumption will be one of the main opera~gcosts (the impact of electrode cost will be a hction of Methe and material). Table 1.1 shows how electrochemicd treatment has great viability, as electricity is a very cheap commodity. Molar cost ($US)
Electron &WH., 3 SV) 0.66
Hydrogen Peroxide 3 .Se
Sodium 7.5#
Sodium borohydride
Stannous chloride
Table 1.1 A cornparison of the molar cost (in US cents) of different oxiduing
and reducing agents '41).
The electrical energy consumed in an electrolytic reaction is given by equation 1.1 where:
E = Electrical energy consumeci (J) q = Charge passed [= ment (i) x tirne (t)] V = AppIied voltage 0.
Electrochemical processes are governed thermodynarnically by the Nernst- equation, which relates the potentid required to drive a particular reaction to the concentrations of the reactants and products of the electrolytic radon-
where:
E = Experimental cell potedal (V) EO = "Standard" ce11 potential (V) n = Number of electrons F = Faraday constant (C mol -') Q = Chemicai reaction quotient (acevity).
Consider the reduction of a compound-
Ox + e- -, Red-
The "Q tenn" for this reaction is [ped-]/[Ox], so that: This tells us that E becomes more negative (requiring a larger cell potential) the larger the conversion of [Ox] to [ped-1.
The theoretid potentids predicted by the Nenist equation neglect other factors that lower the eIectricai efficiency, including overvoltages for the release of gases at the electrodes, resistance firom junction potentials/membranes and ohmic drops (IR.) due to the resistance of the solution. This last factor is the greatest variable with respect io losses in efficiency. We see its effects when we sum up all sources of potential (equation 1.4).
where:
E, = Anodic % cell potential E, = Cathode ?hcell potential V,, = Electrode overvoltage IR = Uncompensated resistance
Not ail of the charge passed through the ceII will be used in a productive manner.
The actuai charge efficiency shown by equation 1.5 will be the number of coulombs
(represented by q) needed in theory for destruction, divided by the total number of cou1ornbs passed through the ceU- In order for efficiency to be calculateci the number of electrons required to produce the products must be known- % Charge Efficiency - 9 q -
Typically, current efficiencies are only calculated with the use of the reaction occurring at a single electrode- However, it is possible to use the power more effectively if both the cathode and anode are used to electrochemicaily mo- a target material. This can occur via two Merent routes: the fist is the use of two different process strearns where one is oxidized and the other is reduced. This has been achieved by workers at
BASF in Gennany who have developed a paired electrosynthesis of phthalide and t- butylbenzaldehyde (42! The second is the use of a "twice through flow cep; where a waste stream is passed through one ceU and then through the other. This type of ceil may be applicable to treat those compounds for which a single reduction or oxidation step does not elhinate the toxicity or recalcitrant problem.
In order for electrolysis to be practical and effective it must meet the following criteria; transformation of recalcitrant material to a nontoxic biodegradable for-energy efficient, minimal electrode fouling, and the use of inexpensive electrode materials.
Furthemore, the system shodd be easily and economically tramferable fiom the laboratory to pilot plant scaie. The technology would probably increase its marketability if it was transportable and able to treat remote sites such as unsecured landfills, groundwater contaminated with pesticide residues and tainted industrial sites. Finally, this process once Mydeveloped must be able to compete on a cost basis with current remediation technologies.
The main objective of the thesis is to investigate electrochemical approaches for the degradation of seiected refkactory organics. In this thesis, we consider elec~ochemicai parameters for the remediation of chlorophenols and nitroaromatics.
The fïrst part of this thesis concems a study that was undertaken tu increase our knowledge of the electrochemical oxidation of chiorinated phenols. Previous work in this area had shown that oxidation of chlo~ated phenols is generally accompanied by electrode fouling. The specinc objectives were ta obtain experimental information about the nature of anode fouling by chlorinated phenols, to compare structure vs. reactivity for phenols differing in the extent of chlorination, mdto relate the efficiency of oxidation to the mechanism of oxidation at di£Ferent electroae types. The latter two objectives were achieved by monitoring the disappearance of various chlorophenols at difFerent anodes during electrolysis. A summary of the results compared substrates at each anode and compared structure vs. reactivity arnongst the dièrent anodes. Fouling was monitored by performing repeated electrolysis with the same anode.
The second project was to evduate the electrochemical reduction of nitroaromatic compounds, which are encountered as wastes and leachates from the explosives industry.
As very little work has been done in this area, it was Our objective to evaluate the use of electrochemical technology by first explorhg electrochemical parameters through - 15 voltammetry and flow-through electrolysis, then to transform nitro compounds, detennine the identities and yields of products and explore elimination strategies of toxic products.
Products were identified using mass spectrometry, mass balances were compiled with the use of standard curves obtained fiom high performance liquid chromatography (HPLC).
Toxic products were polymerized using various oxidation techniques including enzymatic catalysis and anodic electrolysis. 1 Gary N.F.,Biology of W&ewafer Tireutment, Oxford University Press 1989.
2 Corminellis C., Pro. Electrochem. Soc., 1994,94,75.
3 Stukki S., Kotz R, Carcer B-,Suter W., J. Appl. Electrochenz-, 1991,2 1,99.
4 Wentz CA, ccH~mhsW&e Mimugement ", McGraw Hill, N.Y. U. S.A, 1995.
5 Ibrahim M.S., Ali H- I., Taylor K-E., Biswas N., Bewtra J.K., CSCMSCE
Emiromnental Engineering Conference, Edmonton Alberta, Canada, July 1997, 1503.
6 Anderson P., Perso~eldiscussion, Concordia University.
7 Carson R, Silent Spring, 1962, Houghton MïHin Co. Boston, Ma.
8 Schmal D., van Erkel J., van Duin P.J., National Research Council, ChemE Symposium
Series 98.
9 Reutergardh, L., Res. Corn. Rec., 1996, 16, 1-4,36 1.
10 Amdur M.Q., Douii J., Naassen CD.; Casarett's & Doul17s Tolocology 4& ed.,
Mcgraw Hill Inc., New York, 1991, Chp. 26.
11 Lepine F., Milot S., Brochu F., 1992,s 14.
12 SbE. M., Senthurchelvan R., Monoz J., Basak S., Rajeshwar K., J. Electrochem.
Soc., 1996, 143, 5, 1562.
13 Faribourz T., M.A.Sc. ïhesis, Dept. Chem. Eng., Univa of Toronto, 1995. 14 Kuss, I., CIC conference, St. Johns, NFLD, 1996.
15 Trabelsi F., Ait-Lyzaid H, Râtsimba B ., WiehAM., Delmas H., Fabre PL., Berlan
J., Chem. Eng. Sei., 1996, 51, 1857.
17 Hynn TF., A Posiron Pqer of the Amencan Councii on Science and Heaith: Public
Health Concerns About EMonmental Poiychlorinated Biphenyls (PCBs).
http://www.acsh.orglpublications/repo~
18 Smith De Sucre V.; Watkinson A P., CmJ. Chem. Eng., 1981,59, 52.
20 Trombly J., Envi'ron. Sci. Technol., 1994,28, 289A6-
21 Kaba, L., Hitchens D., Brockis J.O., J. Electrochem Soc. 1990, 145, 1341.
22 Matsue T., Fujihira M., Osa T., J.Electrochem. Soc. 1981, 128, 2565.
23 Oiuran M.A, Pinson J., J. Phys. Chern., 1995,99, 13948.
24 http ://mnv.humboldt 1.corn/-gralsto/einsteWeinstein-hd
25 Dady DS., J. Eiectrochem Soc. 1984, 13 1,43SC.
26 Scott K., Proc. Elecîrochern. Soc. 1995, 19,5 1.
27 Al-Haddad A, Abdo M.S.E., World Congress III of Chem. Eng.,Tokyo Japan, 1986.
28 Levina C.D. Electrokhirnja, 1975,11,1644. 29 Hofseth CS., Chapman T. W., J. Elec~ochem.Soc., 1999, 146, 199.
30. Abdo MSE., Al-Enezi G-A, Haddad-Al A, JI Emt Sci- & Hlth. A, 1986,21,487.
3 1. Pakalapati S., Popav EN-, White RE., J. Electrochem- Soc., 1996, 143, 1636.
32 Naumczyk J., Szpyrkowicz L. Water Sci. Tech. 1996, Z 1, 17.
33. Ai-Enezi G-A,J. EmSci & Ulth A, 1990,25,67.
34 Atverez-Gallegos A, Pletcher D., Electrochim Acta, 1999,44,2483.
35 Lin S.H., Chen ML, Waer Res., 1997,868.
36 Hosfeth CS-, Chaprnan T.W., J. Electrochem Soc., 1999, 146, 199.
37 Do J.S., Yeh W.C., J. Appf- Electrochem, 1998,28,703.
38 Rajashekharam MY., Jaganathan R, Chaudhari RV., Chem. Eng. Sci., 1998, 53, 4,
787-805.
39 http://www.icpet.nrc.ca/project/ele~-e.html.
40 Merck Index 8'hedition, Stecher P .G ed. Merck & Co. Rahway, N. J., 1968, USA
4 1 http://www.electrosynthesis.com/site.html
42 13~International Forum in the Chernical lndustry, Clearwater Beach, Florida, 1999. CHAPTER 2
CHLOROPHENOL REVIEW
2.1 CHLORINATED PHENOLS
2.1-1 History
The family of chlorinated phenols comprises nuleteen congeners, ranging fiom monochlorinateci phenols (2-MCP, 3-MCP or 4-MCP) to the fùlly substituted pentachlorophenol (PCP). Technical grades of chlorophenols are usually made either by the stepwise chlorination of phenol at elevated temperatures or the caustic hydrolysis of chlorobenzenes in ethylene glycol; as a result, techical grade chlorophenols are ofien mixtures of several congeners (').
Pollution by phenol (the parent compound) cornes from oil refineries, coke plants, plastic manufactures and as a byproduct in the chemical industry- Chlorophenols are used in the manufacturing of pesticides and herbicides and have been added to products such as adhesives, oils, (i.e. petrochemical drilling oil) and textiles to inhibit microbial growth
'? Their toxicity to many different organisms has also led to their use as bactericides, algaecides, mollusicides, acaricides and fungicides.
PCP has been used as a wood preservative since the 1930's, to treat fiesh cut logs and lumber against sap-stain, fingi and mould O). Creoçote, which was used to treat telephone poles and railway ties, is a mixture of cresol (methyl phenol), tar and PCP (4). Chlorophenols are normally released into the environment f?om leachîng of lumber and agricultural run off. Industrial discharges corne fiom sawmills or planer mills and incineration of wood wastes. Chlorination of municipal or industrial wastewaters that are already contaminated with phenol, (Le. bleaching in the paper industry) can also lead to discharges of chlorophenols (? Poor handling and containment practices in the forest produas indusûy have resulted in serious contamination of aquifers and soils (? The magnitude of the problem cm be seen when production numbers are considered: in 1980 the worldwide production of PCP was in excess of 50,000 metric tons (" while in the
USA, 300 metric tons of PCP (1990) were released directly into the enWonment just fiom home and garden use @).
Chlorophenols are toxic ('), possibly carcinogenic ('O), environmentally persistent and tend to bioaccumulate in food chains (bioconcentration factors >1000) ('? Trace amounts of chiorinated phenols have been found in potable water (12', river water (13), commercial beverages (14), and even orange rinds (? Their presence in drinking water has another adverse effect in that taste or odour contamination are detectable at low ppb levels (odour threshold for 2-MCP is 2 ppb and for 2,4,6 TCP is > 1000 ppb) (L6*17).Under the safe drinhg water act (SDWA), the US-EPA has set a MCL (maximum contaminant level) for PCP of 1 pg L-' and a MCLG (maximum contaminant level goal) of zero ('?
Algae appear to be some of the most sensitive organisms to chlorophenol exposure (19). PCP levels as little as 1 pg L-' can significantly inhibit growth. The toxic effects of chlorophenols are not limited to unicellular species; fish larvae were shown to have LCso values (PCP)as low as 0.01 mg L-' (20'.
For humans and mammals, chlorophenols cm be readily adsorbed through the skin and gastrointestential tract, with greater uptake rates for the lower chlorinated compounds. Chlorophenols can accumulate in the liver, kidney, brain, muscle and fat tissues (Z? Excretion of chlorophenols is through urine after giucuronation or sulfonation
2.1.3 Treatment
The treatment of industrial effluents containing organics such as chlorinated phenols is a difficult problem. In the case of phenol, discharge IimÏts are < 20 ppb, yet it is difficult to extract phenol fiom indusaial wastewater when its concentration is lower than 4000 ppm "2). This makes recycling/recovery impractical and so the phenolic waste must be treated.
Biological treatment (eg. activated sludge) is normally the cheapest and simplest technology to remove waste organics. However, chlorinated phenols are usually recalcitrant and must be treated dinerently (? In a US-EPA survey, 8 out of 14 publicly owned treatment plants could not remove any PCP that was loaded into the system. Of those plants that did have some removal most of it was atlnbuted to adsorption on solids
N. Two separate reports showed that wastewaters containing > 200 ppm of 2-MCP could not be effect ive1y treated at biological treatment facilities (25~6) If a recalcitrant compound such as chlorophenol cannot treated by a conventional biological method, an alt emate treatment technology will be directed towards modfiing its chernical structure to make it more biodegradable, less toxic, or towards complete
oxidization to COÎ
Tradit ionally, wood preserving plants have eit her recycled chlorophenols [through gram separation, filtration (hay, sand or wood particulates)] or treated @y incineration or acclimatized microbiological systems (Le. stabilization ponds)]. In the pesticide industry chlorophenols are fi-equently not recycled and instead are often treated by specially engineered biological systems such as aerated lagoons or trickling filters '28).
2.2.1 Incineration
Combustion of organics was once the principal method of destruction for chiorophenols, especially in the case of PCP, which was often fomulated with a carrier oïl. Although incineration can achieve a high destruction and removal efficiency, concern exists about the formation of dioxins and fhans, especially during uncontrolled accidental bunllng (Z910). hother drawback to the incineration of halogenated compounds is corrosion and emission problems due to the release of HCl during combustion. Although improvements in incineration technology treatments such as temperature controls (combustion > 900 OC) can virtually eliminate PCDD formation, treatment has shifted away from incineration for a number of reasons including cosf
negative public opinion and better rewvery methods which lead to more dilute
wastewater- It is uncertain ifthis trend will continue as American incinerator capacity has
very recently been facing a surplus; changes in Canadian legislation regarding cross
border hazardous waste shipments have led to a four fold decrease in incineration costs.
Other methods for phenol treatment that are being used or are in the development
stage include activated carbon adsorption, solvent extraction and biological, chernical or
electrochemical oxidation n1"22334! Chemical oxidants such as permanganate and
hypochlorite are often applied when wastewater concentrations are too high or variable
for acclimatized biological treatment
2.2.2 Granular Activated Carbon
Granular activated carbon (GAC) is a non-specific adsorbent that can be used to treat both contaminated ground water and prccess waters. Comrnercially available GAC is normally obtained fiom materials such as coconut shells, wood, or coal and is characterised by high surface area (600-2000 mZg-') and well defined microporous structures (average pore opening 1.5 p) Water is pumped through coIums or canisters packed with GAC, to which the contaminants adsorb. The performance of the
GAC bed depends on the type of carbon, particle size and pore size, as well as on extemal parameters such as hydraulic loading and operating temperature.
The lifetime of the bed is affected by the presence of oil and grease, which fou1 the bed, and by contaminant loading, realizing that each bed has a finite adsorption capacity. A common strategr is ta place two beds in series, with the fist being taken out of service when break-through accurs. Removal of suspended solids is an important pretreatment in order to avoid excessive pressure drop across the bed, necessitating back- ffushing or removal of the bed. Silicone-impregnated carbon has been found to improve loading efficiency and to extend the useful life of the bed (3? The use of other organic substrates for manufacturïng GAC (such as straw and rubber f?om discarded automobile tires) has been evaluated. This has tremendous advantage over traditional manufacturing materiais: straw is a cheap raw maiterial and is in great supply while the abundance of used tires in landfilis is an environmental problem. Streat el al (38) compared GAC denved nom these materials to coal, coconut shell and wood. Sorption rates and Freundlich type isotherms for phenol and 4-MCP showed that there was very little dBerence amongst the source of GAC matenal.
Options for the spent caabon normally include disposa1 in a secure hazardous landfill, incineration, or thermal regeneration, which involves partial (545%) oxidation of the GAC, during which the contaniinants are desorbed and bumed. However, while spent
GAC nom most organics can be recovered, GAC that is contaminated with chlorinated organics cmoniy be treated at certain regeneration sites.
2.2.3 Photolysis
Chlorophenols are degraded photochemically by UVC radiation 09'. The photoreactor must be manufacturcd f7om quartz and is surrounded by W lights (i-e. high or low pressure mercury lamps). The pathway for photolysis of 2,4-dichlorophenol (in water) gives successively 4-chlorocatechol, 1,2,4-benzenetriol and a mixture of
25 poIyquinoid humic acids. (4? Photolysis of other chlorophenol congeners bas led to products including resorcinols, quinones, phenol and biphenyïs '41).
Boule et al (39) found that for dichlorophenols, chlorines in the ortho subsitution were Iess easily removed than those in either the para or meta positions. One drawback of photocheMca1 oxidation is that the formation of polychlorodibenzodioxins (PCDD) has been observed (42).
Lipczynska and Bolton (43) studied the mechanism of photolysis of 4-MCP in aqueous solution by flash photolysis. The predomhant product was hydroquinone (with
4-chlorocatechol as a minor product), which was further oxidised to benzoquinone and 2- hydroxy-p-benzoquinone.
2.2.4 Advanced Oxidation Processes
Advanced oxidation processes (AOPs) employ reactive oxidizing agents, including hydrogen peroxide or ozone, with or without the addition of catalysts or photolysis. Many of these technologies generate hydroxyl radicals [a strong oxidant; EO
= 2.3 VI, addition of which to aromatic rings initiates ring opening and ultimate mineralization to CO2 and H20. AOP7s can be placed into four categories (43). In homogenous photolysis, UV radiation is combined with an additive (e-g., H202or 03)to generate OH radicals. Heterogeneous photolysis invoives the use of a semiconductor in the presence of UV light to generate OH radicals. Homogenous dark oxidation employs an oxidant (-02 or 03) without light, and includes methods such as Fenton's reagent
(H20Z+ ~e~"),ozone a. high pH, supercritical water oxidation and ozonehydrogen peroxide. Radiolysis (which cm be camied out homogenously or heterogenously)
employs a source of ionizing radiation to cleave water radiolytically, with the formation
of a variety of radicals (OH*,Hm, hydrated electrons).
Turbidity and the presence of other light absorbers are limitations to UV0
(ultraviolet oxidation) technology because they block transmission of UV radiation-
Metal oxides and grease cm fou1 quartz sleeves through which UV radiation must pas,
and therefore prevent transmission of the W light. Carbonate and hurnic substances can
limit eEciency by acting as radical scavengers "?
Treatment methods for chlorophenols by AOP have included hy drogen peroxide
(with or without the presence of ~e") (45), ozonation (46), wet oxidation " and
photocatalytic oxidation (homogenous photolysis) M.
2-2.4.1 Heterogeneous Photolysis
Considerable work has recently been done towards developing photolysis (UV radiation + semiconductor) into a remediation technology for chlorophenols (4950). h this case, a serniconductor with a small band gap, rather than the organic material absorbs the
UV photons. This causes promotion of electrons into the conduction band leaving
"positive holes" in the semiconductor. Holes competitively recombine with electrons or oxidize water to hydroxyl radicals, which can react with substrate either at the semiconductor surface or fiee in solution ('l).
h+ + Hz0 i H' + OH. In the presence of oxygen the promoted electrons may yield superolade radical
anions, which themselves may react chemicaily with chlorophenols "". Although these
semiconductor-assisted photolyses are inefficient due to competing electron-hole
recombination, the process has attracted much attention because it employs near W
radiation VA), which allows the use of low-cost near UV larnpg or even naturd
sunlight. Also, competing light absorption by other substances is less of a problem in the
UVA region compared with the WCregion emitted by low-pressure mercury lamps.
Two semiconductors that can be used to enhance photodecomposition of
chlorophenols are zinc oxide and titanium dioxide. Barberi et al (53) showed using
summer sunlight in the presence of TiOz (2 mg L-') that PCP7s (12 mg L-') half-life was
only 8 min (pH 3).
Staf3ord et al e4) examined the photodegradation of 4MCP in a Ti02 slurry. They found that increasing the TiOz concentration had little effect on the rate of 4-MCP disappearance but did increase the rate of mineralization. However, the concentration of
4-chlorocatechol (degradation product) decreased with increasing TiOz. In a later paper they used results fkom this work dong with the work of several other researchers to develop a photodegradation mode1 ("? The model incorporated reaction rates fiom processes such as radicals in the aqueous phase, sudêce adsorption, electron scavenging by oxygen and electrodhole recombination. These were correlated to physical parameters including light intensity, TiOz suspension and mixing. Results fYom the model confirmed that the photocatalytic reactions are a combination of aqueous and surface hydroxyl radical reactions. The contribution of each was related to the light intensity and Ti02 concentration. However, rates of forming intermediate and mineralization were much 28 more dependent on Ti02 concentrations. In terrns of kinetics, the reaction foilowed a zero
order relationship with respect to 4-MCP.
2.2.4.2 Hydrogen Peroxide/Ultraviolet Light
Photolytic hydrogen peroxide oxidation (H20m has been used in certain industries such as the paper industry to destroy recalcitrant organics '? The
decomposition of H202 to OH is brought about by UV light (eq. 2-1) and is followed by a
series of subsequent reactions that occur amongst the hydroxyl radical, hydrogen
peroxide and hydroperoxyl radicals (eq. 2-2 - eq. 2-5). Disadvantages of this
technique include the need for Hz02 and the expense associated with W lamp
replacement.
(eq. 2-1) OH. + OH. + HzO2 (eq. 2-1) OH- + Hz02 + HO2. + H20 (eq. 2-3)
(es-2-41 (eq. 2-5) (eq- 2-6)
Li demonstrated the effectiveness of combining Ha2with UV by assessing the detection capabilities of each technique separately and combined. He used 2-MCP as the target substrate and found that direct photolysis (high pressure Hg, 53.1 W cm-2)was
able to degrade 23% of the initial concentration after 90 min; &O2 (144 ml of a 35% solution) was only able to degrade < 5% for the same time period, while combining the two methods degraded 85 %.
Glaze et al a9) studied (H20min combination with ozone (&02/tTV/03).
They found that the destructive capabilities of (&OÎIW/o3) was more than additive of the techniques performed sepaïately [(H*OîRn3 + Op].
2.2.4.3 Ozonation
Ozone appears to cause degradation of chlorophenols via two different mechanisrns, either through reactions involving ozone directly or indirectly through the formation of hydroxyl radicals (60361). Reactions with ozone tend to be selective and proceed via a polar or concerted mechanism, without displacing the halogen.
Determination of which mechanism occurs depends on process conditions but also depends on the organic substrate. Since both 03 and OH. are electrophilic, the rate of degradation by either mechanism increases at high pH, when the chlorophenols are in
Boncz et al (62) showed that the rates of ozone-assisted destruction of 2-MCP and
4-MCP were independent of temperature between 287 and 3 18 K, and that the addition of
W illumination only increased the rate of degradation above pH 10.
Disadvantages of ozonation include the need for on-site generation and protection of personnel fiom this toxic material. The capital costs associated with ozone generation wsts are very high for small waste streams. Both ozone and hydrogen peroxide are expensive; treatment costs can be prohibitive unless a waste Stream that contains the chlorophenols can be selectively targeted for treatment.
2.2.4.4 Supercritical water oxidation:
Above the critical point (Tc > 647.3 K, P, 217.6 atm) water becomes relatively non-polar due to reduced hydrogen bonding, and becomes miscible with organics.
Supercritical water oxidation (SCWO) exploits the high temperature, low surface tension, high diffusivity, and low viscosity of the supercritical fluid to effect rapid mineralization of otherwise recalcitrant organics (63). Various metailic catalysts such as Cr203, M.02,
CeO, CuOlZnO and ZnClz have been added to SCWO reactors in order to enhance oxidation rates and minimise residence times (64,65,9
Lin and Wang '67' were able to increase the decomposition of 2-MCP (1 500 rn&) frorn 29.7 % with no catalyst (673 OK, 238 atm, residence the 5 min), to 88.8% with a
CuO/ZSM-88 catalyst and 97.7% when K' was added in the presence of the catalyst.
They theorised that the presence of K' in solution facilitates the abstraction of the Cl atom fiom the chlorophenol. They argued that precipitation of KCl occurs extremely fast as a result of the lower solubility of inorganic salts at high temperaturelpressures.
One problem with SCWO is that some of the initial chlorophenol can be transformed into higher chlorinated phenols fiom a Cl reinsertion route. Lin and Wang also showed that the addition of zeolites with smaller pore sizes excluded the larger multi-chlonnated phenols and therefore reduced this problem. Using a zeolite that had a minimum pore size of 7.75 A reduced the formation of PCP fiom 2-MCP oxidation fiom 159 pg per gram of 2-MCP to zero and 2,4,6-TCP formation fkom 7056 pg per gram of
2-MCP to zero.
The principal drawbacks to SCWO are that it is very energy intensive and has high capital costs because of the high temperature and high pressures employed.
2.2.5 Microbial degradation
Micro-organisms have been used for decades for the treatment of municipal wastes, and more recently for the &ment of industrial waste ('? As mentioned in chapter one, its advantages uiclude low cost and ease of operation (although these are not always achievable in practice), and public acceptance. Bioremediation most often employs aerobic conditions, in order to mineralize organic compounds to COz and H20.
In general, aerobic bioreactors achieve higher throughput, with fewer problems of sludge formation and by-product off-odours, than anaerobic systems.
While chiorophenols are generaily believed to be recalcitrant, certain bacteria and hngi (69) are capable of degrading them as their primary carbon source under anaerobic conditions. (70) Gram negative bacterial strains including Pseudomonas '71), Arthobacter
"), FIàvobactemirn and Azotobacter (74) and the gram positive ~odoccus(75~7q have al1 been shown to degrade chlorophenols. The rate of degradation decreases with increasing chlorination of the phenolic ring with meta chlorinated phenols usually being more resistant to degradation than other substitution patterns m78).
In commercial practice, up to 900/0 of chlorophenol waste is treated biologically with the remaining undergoing GAC. However, carefùl acclimatization of the microbial consortium is necessary, and the reactor is still very sensitive to shock loading ('?
Among bioremediation technologies, biofilm and fluidised bed reactors have been shown to be more efficient at PCP removal than activated sludge (20!
Bacterial degradation is also sensitive to inhibition by other compounds and especially metal ions. For example, Kuo and Genthner showed that chlorophenol degradation was completely inhibited by 2 ppm Cd 0,5ppm Cr (VI), or 2 ppm Cu @).
In contrast their results also showed that the addition of 2 ppm of Hg (II)enhanced biodegradation b y 42%.
The microbial degradation pat hway is often via a dioxygenase-catalysed ortho- cleavage of chlorocatechols @? For PCP degradation, products include pentachloroanisoie, pentachiorophenyl acetate, tetrachlorophenols and tetrachlorodihydroxybenzenes('O).
Cassidy et al used Pseudomonas Sp. UG 30 for the bioremediation of PCP.
Optimum conditions involved the addition of phosphate and encapsulation of organisms in K-carrageenan. A follow-up study under similar conditions was designed to monitor toxicity as bioremediation progressed @?Red blood ce11 lysis, ~icrotox~and SOS chromotest@ indicated a reduced toxicity with loss of starting PCP, but earthworm survival and seed germination were not enhanced as bioremediation progressed, showing that the metabolism products remained toxic with respect to these end points.
Single bacterial strains are rarely useful in bioremediation of chlorophenols because a single organisrn cannot degrade all congeners (84! For example, 2,4,6-TCP can be partially dechlorinated to 4-MCP by anaerobic micro-organisms, where the process is
33 halted because 4-MCP is not degraded any Mer(8"86787? Bae et al solved this problem in a laboratory setting by using a mixed strain mixture of TCP (Pseudomonas) and 4-MCP (Arthrobacfer)degrading organisms. For industrial application, the use of acclimatised microbial consortia is the only practical approach-
From this chapter, is it clear that although many alternatives for chlorophenol treatment have been propose& only biotreatment and GAC have been used commercially.
There still remains the need for a reliable, cheap, safe and alternative teclmology.
Electrochemicd oxidation (as discussed in chapter one) is one conceivable candidate for such a technology. Discussion of the limited number of laboratory studies of this approach wiil follow in chapters three and four (89,901 . 1 Cochrane W.P., Lanouette M., SinaJ., J. Assoc. AdChem., 1983,66,804.
2 ChlorophenoZs other than Pentachlorophenol. Environmentai Health Criteria, 93,
World Health Organization, Geneva, 1989, pg 46-49.
3 Chlorophenols oiher than Pentachlorophenol. Environmental Health Criteria, 93,
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2883. CHAPTER THIWE
VOLTAMM-ETRY STUDIES OF CHLOROPHIENOLS
3.1 INTRODUCTION
Electrochemical potentials (E) can be determined using electroanalytical techniques, which can be active or passive depending on the net change in concentration at the electrode surface. Voltammetry belongs to the former technique while potentiometry is an example of the latter.
This chapter d use voltarnrnetnc methods to determine the potential at which electrooxidation of chlorophenols is possible. Voltammetric measurernents are usually performed with a potentistat, using a three-electrode system: a working electrode (WE) where the reaction of interest occurs, a reference electrode (EE) and a counter electrode
(CE) to complete the electrochemical ceil. Current flows between the CE and WE and is negligible at the reference electrode, which is used to masure the potential vs. WE. The electrodes are intercomected by operationai ampKers to compensate for po~arizationof the counter electrode during operation"'.
3 - 1.1 Linear Sweep Volt ammetry
In linear sweep voltammetry @SV) the current flowing through the cd is measured whiie the potential at the working electrode increases linearly with tirne. The
42 total current response consists of two wntriiutions: non-fàradaic and faradaic. Although voltammetry (and electrolysis) are kuietic processes they can only proceed when conditions are thermodynamicaliy favourable (AG is negative). As the potential is swept during the LSV the current wiil be non-faradaic until a hi& enough potentiai is reached to reduce/oxïdize the substrate, whereupon there will be a comesponding increase in current.
The totai current will then increase untii the mass transfer fiom the solution becomes rate iimiting wîth respect to charge trander at the .daceof the electrode. The plot is initially flat (non-faradaic current), the current inmeases as EOis approached and then reaches a maximum as the reaction bewmes mass transfer limiteci (diagram 3.1; shows the effect of i as a fiuiction of r and E with a typical LSV response curve is shown bottom right). The maximum current is known as the limiting current (equation 3.1) '2'. Figure 3.1 Representation of a portion of the i-t-E surface for a Nernstian reaction (top), linear potential sweep or ramp and resulting i-E curve (bottom) (Bard p.214)
where: ilM = Limiting current (A cmdZ) n = Number of electrons exchanged F = Faraday constant (C mol-') Dj = Diffusion coefficient of species '5" (cm2S-') Cj = Concentration in bulk solution (mol cm-') Cj[o,~= Concentration at the electrode surface (mol cm-3) 6, = Nernst diffiision layer thickness (cm) The voltarnmetric technique of polarography was invented by a Czechoslovakian scientist Jaroslav Heyrovsky nearly 70 years ago. Heyrovsky noticed that if a linear potentiai was swept across a dropping rnercury cathode under nadconvection the '5-E" scan was reproducible and was related to the concentration and iden%ties of analytes in the water. The subsequent impact of this technique in analytical analysis was responsible for his receipt of the Nobel prize for chemistry in 1959 "'.
Besides a renewable surface there are other rasons for mercury's advantage as an electrode material. It is chemicdy inert and has a high overprotential for hydrogen evolution, allowing it to be used at high negative potentids. fiterference hm the reduction of oxygen cmbe a problem, however this can be eliminated with purging with nitrogen or argon prior to analysis. Another drawback to this technique is that mercury undergoes oxidation easily and therefore has Iimited use under positive potenh'al '4'.
At a dropping mercury electrode @ME), current measurrements are made at successive mercury drops fiom a capillary tube, which act as the cathode- In modem instruments, an interna1 hammer is used to dislodge the drop at the emd of its Lifetlme and a new drop is fomed by opening a solenoid valve '? This replaces the problems associated with drop growth in classicd gravity fed instruments.
The total current passing through the ceil consists of residual current (i,), faradaic
(dfision) current (id) and capacitance current ci). The diffusion cment is proportional to concentration of the target anaiyte "? Where:
id = Diffusion current D = Diffusion coefficient (nitrobenzene 8.28 x 10~cm2 S-') n = Number of electrons/mole aaafyte C = Analyte concentration (mol cm-3). t = Hg drop lifetime (s)
The capacitance current is related to the charging of the electrode/electroIyte interface and contributes to the total current when ushg a kear ramp potentid at a DME.
It can be related as:
Where:
i, = Capacitance current O, = Charge density at electrode surface (C cm-2) t = Hg drop iifetime (s)
To rninimize this eEect and increase concentration sensitivity, several methods have been developed. In Tast polargraphy, the current is only sampled near the end of a drop's lifetime. Several pulse excitation methods have also been developed, these include normal pulse voltarnmetry, square wave pulse and dserential pulse voltammetry (DPP).
In DPP, a smd amplitude pulse is superhposed over a voltage ramp and the current is sampled twice per drop, once just before a potentid puise is applied (E, the base potentid
46 for that &op) and once just before the drop fds (E +- fi). The capacitance current is therefore smaller and can be expressed as the merence of the mûasurements ".
(eq. 3.4)
This approach also dows for an increase in faradaic current, as the concentration of the target analyte within the boundary layer has not been not decreased in the sweep '".
The plot of (Ai) versus applied potential results in a curve in the shape of a peak and not a wave. This is important as it can increase the resolution between analytes having Werent reduction potentials. Furthemore, the combination of increased fàradaic and decreased capacitance current can increase the sensitivity of the method by several orders of magnitude compared with the classicd method.
In chronopotentiometry a constant current is applied (i,,~ or i.) to the working electrode and the resultïng potential is measured as a fùnction of the. The initial plateau
(Fi)represents the potentiat required to oxidize (in our system) the substrate having the
Iowest EO(the flat part of the curve der the time period during which the anode has charged). The potential wiil remain at the ondation potential of the substrate untii the concentration at the electrode surface tends to zero whereupon the potential will increase (EOZ) so as to oxldize the next most easily oxidizable species in solution (diagram 32).
Chronopotentiometry
- * i
8 fi E0 \ i! 8 Transition time 1 * , \ charging j 8 t
Time (sec)
Figure 3.2 Theoretical example of chronopotentiometry
The tune required to complete the oxidation of the substrate in the vicinity of the rnicroelectrode (Le. until surface concentration becomes zero) is defined as the transition the z. The theoreticai relationship between z and i, is given by the Sand equation
(equation 3.5) "'. (eq. 3.5)
where
z = Transition tirne (s) n = Number of electrons F = Faraday constant (C mol-') 2 1 Dj = Diffusion coefficient '7 (cm s ) Cj = Bulk concentration '7 (mol cm-') i, = Current (A cd)
Chronopotentiometry can dso be employed to measure the uncompensated
resistance (Ru) which is the resistance between the working electrode and the capillary
tip of the reference electrode. Ru depends on the electrode resistance, and changes
when there is deposition of material on the electrode surface (the method will be
discussed in section 3.3).
In order to design a practical electrolysis systern for an industrial application, an important parameter to consider is the potential (E); since unWre current density, which can be modified via electrodes properties or mass transfer with turbulence promoters, it is a hed thermodynamic value. The objective of the work in this chapter was to use voltamrnetry and chronotpotentiometry to determine the oxidatior. potential of chlorophenols as a fiinction of electrode material. Electrochemicd remediation of phenols has been attempted, to compensate for the inabiiity of bioreactors to treat them- The extensive use of phenols in industry makes this an important issue and since they are relatively easy to oxidize, electrochemicai remediation may be an effective option ('O). The present limitation to this method is electrode deactivation and consequent low current efficiencies ('? The deactivation of anode materials is believed to involve the deposition of oligomers "'13' nom the attack of phenoxy radicals on unreacted phenols. This fouls the electrode surface and impedes mas transfer. A brief summary of some of the oxïdation work that has been performed follows.
Preliminary research involved electrolysis at lead, graphite and platinized titanium
anodes "". OBde-based anodes were found to be less prone to fouling than pure metals.
Macdougall et al (ln demonstrated that phenols oxidized faster at Pt02 electrodes than at
Pt with less tendency to deactivation. More recent studies have involved the use of other metailic oxides (MOs).For example, Scott"" reported higher rates of oxidation at Pb02 than at graphite, Pt or Ni. ComnineUis and Pulgarin 'ln found that the oxidation rate of a
20 mM solution of phenol was the same using either Pt or Sn02 at a current density of 20
crn-2 (18)
Dimensionally stable anodes @SA@) (19', are a special class of metal oxides prepared by thermal deposition of a thlayer (a few pm) of metal oxide such as SnOz,
IrOz, and RuOa alone or in combination, (eg. SnO2/'ïa2O5) on a base metal such as Ti, Zr,
Ta, or Nb '2021PP24). Advantages of DSAs include efficient charge transport, low cost and high chernicaVelectrochemical stability towards oxidative corrosion (z2?ComnineUis 50 showed that phenol was oxidized five times faster at a Sb-doped SnO2/Ti anode than at Pt, cm with a corresponding increase in the loss of total organic carbon (TOC). An important factor was that the over-potential for O2 evolution was 600 mV more positive on Sn02/Ti than at Pt, a result confirmed by Kotz (28' who deposited Sn02 on Ti by sprayhg a solution of SnCL + SbCG in MeOH.
It is believed that DSA/MO, cari oxidize organïcs via two different catalytic processes* both of which postulate electrochemical oxidation of the surface metal oxide to a higher oxidation state w&+&The fist mechanism involves the rnetal oxide reacting with a water molecule to fonn hydroxyl radicals (an electrophilic species@))which are adsorbe4 at Ieast initidy, at the sdace of the electrode ~o,.(oH*~)]before oxidizing the target substrate, competitively with disproportion to O*. However the hydroxyl radicals may also
&se into the bullc solution. The second (Mech-2) is that hydroxyl radicals fkom
[MO..(OH*~)]interact with the oxygen in the anode forming the higher oxîde ml]. this can then oladise the substrate directly, retuming the metal oxide to the lower oxidation state "O! Comnineliïs "'' has hdicated that DSAs based on Sn02 are more iikely to undergo Mech-1 while Ir02-based DSAs are more likely to undergo Mech-2.
Regardless of the mechanism, the electrode matenôl must be capable of eiectron trander, but yet have a hi& Oz overvoltage. Step 1
MO, + Hz0 - MOX(OH'A) + El? + e-
Mech-l
MOX(OH**) + t --+ MO^ + AIQH
MOX(OH*~&)+ Ar --) MOx + OH* -b MOx + ArOH
Mech-2 MO~OH'~~)- MOs+i +W + e- MOX+I+ Ar MOx+ArOH
Compared to conventional electrodes where electron transfer to or fiom the substrate occurs at the electrode surface, these differences in mechanism have led researchers to the suggestionG2'that metallic oxide electrodes could probably be used for the anodic treatment of organic pollutants-
3.2-1 Chlorinateci Organic
Research on the electrochernicai degradation of refhztory chlorinated organics is in its idànq. According to Macdougall G3' this is because earlier work showed that chlorinated aromatics omdized with low efficiencies and also behaved as deactivation agents. Published reports 'UZ5Zm suggest that chlorophenols are more ditFcult to oxidize than phenol itself, although exact oxidation potentials have not yet been reporteci. 3.3 EXPERIMENTAL
Studies began with platinum (Pt) as an electrode matenal because of its stability, overpotential and ruggedness. Although Pt is expensive for industrial applications, its properties are extremely suitable for laboratory stuclies. From this baseline we could make cornparisons with metallic oxides and DSAs.
Voltmetry and chronopotentiometxy were performed in unstirred bulk solutions with a platinum bulb (0.3 cm2) as the working electrode. A platinum sheet (3 cm2) was used as the counter electrode and an extemal saturated AdAgCl electrode as the reference. The reference electrode was comected to the cell by a salt bridge filled with 0.1
M NazSOJ solution, which was also used as an electrolyte in all of the rneasurements.
Experiments were performed with an EG&G Princeton Applied Research Potentiostat
(model 273) operated by M.270 software and for purposes of consistency aü potentiak are reported vs. a standard hydrogen electrode (SHE). Solutions were in equilibrium with air and had a pH near 6. Experiments were conducted in a 30 mL cylindrical glass flask and changes in pH were made by the addition of a few drops of 2 M NaOH to the solution
(there was no attempt to maintain ionic strength).
This set-up was also used to determine the uncompensated resistance obtained fiom the chronopotentiometric measurement. Determination of this value was obtained indirectly fiom the current intempt technique (Potentistat model 273, instruction manual chapter 4) which relates it to the ratio of uncompensated voltage (VJapplied current. For purposes of this thesis, Vu will be defined as the IR drop between the working and reference electrodes. The determination was lirnited by the precision in 53 meamring the uncompensated voltage measmernent (+ 1 mV in our system). Therefore
Ru could be measured only with current densities above 0.1 mA cm-2.
Substrates were phenol, 2-clitorophenol (2-MCP), 3-chlorophenol (3-MCP), 4- chlorophenol (4-MCP), 2,4-dichlorophenol (DCP), 2,4,5-tnchlorophenol (TCP) and pentachlorophenol (PCP), as its sodium salt which, unWce PCP, is fkely soluble in water.
Chernids were obtained from Aidrich (3-MCP, 4MCP, PCP), Sigma (TCP), Acros
(DCP) and Fisher Scient& (2-MCP, hydroquinone, benzoquinone). AU experiments were run at ieast twice with most run more thes than that.
3.4.1 LSV
The possibility of using linear sweep voltammetry to determine oxidation potentials of chlorophenols was explored using 1 mM CMCP as a substrate. The sweep rates ranged fiom 0.01 to 1 V/s over the potential range 0.4 to 2.2 V.
Instead of the expected sigrnodial voltmetric waves, it was observed that the anodic current increased and then decreased with increasing potential (Figure 3 -3). Typicd of irreversible reactions, the position of the voltammetric cuve was strongly dependent on the scan rate. Accordingly, at higher sweep rates, the voltamme~cpeak shifted to more positive potentials.
The peak amplitudes also increased with sweep rate. The variation of scan rate followed the Randles-Sev~ikequation (3"" (equation 3 -3) in which the dependence of the 54 peak ment (i,) on the square root of the scan rate (vm)(inset Figure 3 -3) is linear for a
dBbsion controlled process.
where:
Jp = Current densis. (A cm-*) n =Numuer of electrons exchanged F = Faraday constant (C mol-') R = Gas constant (J mol-' R') T = Absolute temperature (K) D = Difnision coefficient (cm2 s-') v = Scan rate (Wdt) (V s-') Co, = Concentration of material in bullc phase (mol cm;i)
Figure 3.3 fùrther shows that simdtaneously with the displacement of the 4-MCP oxidation peak, the onset of oxidation of electrolyte (water) moved to more positive potentials at low sweep rates, increasing fiom 1.6 V at 1.O V/s to > 1-8 V at 0.0 1 V/s.
Evev cuve presented in Figure 3 -3was recorded as the fïrst run after cleaning the
Pt electrode in a flame, Second runs on the same electrode showed no peak current.
Solvent washing (methmol, chloroform, THF, methylene chioride) of the electrode did not restore electrode activity. However, the high temperature ~ornthe flame was able to burn off any organic deposit and thus was capable of renewing the electrode surface. This behaviour is consistent with the electrode deposit being due to oligomeric oxidation
55 products rather than to starting matenal or monomeric products. This regeneration problem Lunited our voltarnmetric midies, to the extent that only Pt could be used as the anode. Active surfaces of most other electrode materials (especially the metal oxide coatings of DSA) are incompatible with bumhg off a deposit in a flame.
The formation of oligomeric products would also explain the results in Figure 3 -3.
At high scan rates oligomer formation is iimited as a result of a srnaiier time interval to the transition tirne. The shift of O2 evolution to more positive potentials would dso be consistent with the build-up of oligomers at the electrode surface causuig inhibition of charge transfer and an increase in the resistance at the electrode surface.
These effects ïndicate that deactivation is a mass transfer related phenornenon that depends on the amount of substmte that has been oxidized- Therefore, it is reasonable that higher scan rates produce larger oxidation currents, because f'aster sweep rates allow a more positive potential to be achieved prior to the anode losing its electrochemical activity.
Our conclusion that the electrode was being deactivateci by the deposition of oligomeric films of the oxidation products fkom phenotic substrates on the electrode surface is consistent with previous observations during phenol oxidation "9740741742!
3.4.1. 1 Oaidation Potentials
The shapes of the cuves in Figure 3.3 and their dependence on scan rate prevented the determination of the chlorophenol oxidation potentials. However, the observations of the oxidation peak of 4-MCP in the range from 0.70 to 1.25 V was important because it predicts that oxidation will begin at potentials less positive than the eiectrolysis of water. We conclude that chlorophenol electrooxidation has the prospect of being energy efficient if suitable experimental conditions can be found. From the analfical perspective, there is a trade-off between greater sensitivity
@eak amplitude) at faer scan rates, and better separation of the oxidation peak due to oxidation of substrate and that due to the electrolyte at low scan rates. The subsequent figures were al1 recordeci under compromise conditions of 0.1 V/s.
3.4.2 Substrate
3.4.2-1 Low Concentration of Substrate
Figure 3.4 compares the L.S. voltammagrams of 1 m.solutions of several chlorinated phenols at pH 6. The congeners have slightly different oxidation peak potentials: phenol and 4-chlorophenol, +1.2 V; dichlorophenol and trichlorophenol,
41.1 V; pentachlorophenol 4-1.O V. Both the dichlorophenol and the tnchlorophenol show two sequential oxidation processes with a small oxidation peak at 40.8 V foliowed by the larger peak discussed eariier. However, since these peak potentials are infiuenced by anode deactivation they can not be related to EOor Ern values and we can only Say that the oxidation of ali these substrates at a Pt anode commences at potentials near +l V vs.
SHE.
Although the observed peak potentials are similar, there was a trend to smaller peak amplitudes with increased chIorination (Table 3.1). As current is proportional to the amount of substrate oxidized, we interpret this trend as more facile electrode deactivation with greater chlorination. However it is also possible that that increased chlorination of the phenol may interfere with difihion fiom bulk solution to the electrode surface, or of the electron transfer £rom the electrode to the substrate,
Table 3.1. The effect of pbenol chlorination on peak current (O.lV/s)-
Another observation is that the LSV response of the 4-MCP is unique: 1) a much
more pronounced decrease in current after the peak potentiai, 2) a shift to higher potentid
for the oxidation of the water. We hterpret these as indications that the deactivation ofthe
electrode is more prevdent with 4-MCP as a substrate,
When chlorinated phenols are compared in a basic solution (0.1 M NaOH) the
dEerence in the oxidation potentials becomes narrower pess than 0.15 V (0.80 - 0.95 V)]
(1 mM not show 10 rnM Figure 3.9, and there is a shift in the onset of oxidation to lower values (Figure 3.5 vs. Figure 3.4), consistent with the easier orcidation of chlorophendate anions compared with chlorophenols. The similarities of the LSV's arnongst the chlorinated phenols suggests that any of them could be used as a mode1 substrate for fùrther voltarnrnetric studies. As 4-MCP appears to be the compound most likely to causing fouling, based on figure 3 -4, we have chosen it for fùture studies-
3.4.3 Concentration (4-MCP)
Figure 3.6 shows LSV's for various concentrations of 4-MCP at pH 6- Below I
mM the voltamrnagrams show a peak current at t1.2 V with an approximately linear
relationship between iw and concentration. Above 1 mM the peak current shifts to less
positive potentials and Eel,M1, increases. This is consistent with deactivation dependig
on the total amount of substrate that has been oxidized, so that as the concentration increases the onset of deactivation occurs earlier in the voltammetric sweep. This is a
simcant detail as it demonstrates that elecprode deactivation will be less at low substrate concentration; this will be important when parameters for electrolysis are developed in chapter 4.
Experirnents were next carried out at lower and higher pH. Fiwe 3.7 shows results for (0.1-10 mM) at pH 2, where concentrations ImM show i- approxirnately proportional to concentration while those above >1 rnM show iw shift to lower potentials. Regardless of concentration, there was no change in Ee~cctroIYtc.The shift does not occur even though electrode deactivation is present.
In Figure 3-8, the same conditions are again repeated (0.1 - 10 mM, O. Z Vh), except this tirne at pH 12, where once more the deactivation (with respect to E shift) was found to increase together with concentration However, for pH 12 (highest concentration of phenolates) the derived current is closer to being linear with respect to concentration (up to 3 mM) than at either pH 2 or 6.
Voltammograms were obtained for 1 mM 4-MCP over the pH range 2-12 (Figure
3.9A). Increases in pH (between pH 2 and 9) caused progressive shifts in the onset of oxidation towards lower potentids, consistent with easier olodation of the phenolate than the neutrd phenol '43'. Zuman et a1 have shown for nitrobenzene that plotthg the peak potential as a fùnction of pH results in a straight Luie at values below the pK, and a straight horizontai line above if with the intersection of the two lines at the pK. Our resdts for 4-
MCP for the -1 -2 V peak (Figure 3.9B) resemble this closely except that we did not see a constant at pH > pKa. Possibly this was due to cornpethg formation of PtOz at the electrode surface in this pH range'45'. The pK, (10) fiom our graph compares weil with literature values for 4-chlorophenol of 9.18.
Remembe~gthat the oxidation of the chlorophenol (ArOH) to the radical compound (ArO-) is accompanied with the loss of a protog we can determine the number of electrons involved in the reaction per decade (or 1 pH unit). Therefore considering the slope of the curve between pH 2 and 10 we get:
Since the theoretical slope is 59 mV per e-, it can be deduced that there is 1.48e- associated with each H'.
For pH's between 9 and 10 the voltammograms show two distinct oxidation peaks that are 0.3 V apart and therefore we assign the two peaks as oxidation of the two conjugates. This observation probably indicates that both species are oxidized faster at the
anode than equilibration can occur between the conjugates.
The dependence of oxidation potential with respect to pH is important to understand; as during electrolysis the pH of an un-bufEered solution (oxidized solution) will decrease with time (oxidation of water generates H') and therefore under amperostatic conditions, the potential wiil have to increase to maintain %.
Figure 3-9B Variation of Anodic Peak Potential with pH
3-4.5 TCP compared to 4-MCP
Figure 3.10 shows the eEkt of pH on the LSV's of 1 m.TCP- The results are rather similar to those for 4-MCP. Oxidation potentid decreases wÏîh pH and at the pK,
(pH 8.4) there are two peaks of almost equal magnitude.
Figure 3-11 shows LSV's for TCP as a fùnction of concentration at pH 8.4 and shows that current is almost linear with concentration, However, at the higher concentration (10 mM) the second oxidation peak (1.1 V) disappears. This is probably related to deactivation of the electrode occurring fiom the initial oxidation. These two effects (linearity and deactivation only at high concentration) indicate that the lower peak potential associated with multichlorinated phenols (Figure 3.4 and Figure 3.5) is not a result of increased deactivation but instead either related to a lower rate of desion to the electrode surface or slower rate of electron transfer (see page 46).
3.5 CHRONOPOTENIOMETRY
The deactivation of electrodes seen with L.S. voltammetry did not aliow us to determine oxïdation potentials of chlorophenols. Therefore we used chronopotentiometry as an alternative method to determine these oxidation potentials.
Chronopotentiometry results (i, = 0.033 to 1 mA cm-2) for a 1 mM solution of 4-
MCP, at pH 2 are shown in Figure 3.12. Based on the theoretical exarnple in Figure 3.2, the objective was to determine E'I, the oxidation potential of the fkst (most easily oxidized) substrate, which in this case is the chlorophenol. At the lower polarization currents (0.033-0.2 mA cme2) the oxidation potential occurs at +l.l vs. SHE. At higher curent densities deviations fiom this value occur, as the depletion of substrate is so fast that oxidation potentials can not be read fiom the graph. We associate the '%umps" in the chronopotentiometric curves at higher concentration with the onset of deactivation.
Exarnples of the curves are shown in Figures 3.12 (4-MCP), 3.13 (TCP) and 3.14
(PCP) respectively, with a summary of the chronopotentiomeûic results found in Table
3 -2. 1 pH 1 Conc. 1 Substrate Comments
EO=+i .i V, pronounced humps
EO=+l. 1 V, more pronounced humps
EO=+0.7 & 0.9 V, Iarge peaks der''z"
EO=+1 .2.V, "T" not clearly dehed
EO=+1.2 Y, "T" not clearly deked
EO=+0.8 V, high E fier "T" IpH> 12 10 mM TCP EO=+0.8 V, high E after 'Y' PI rnM PCP
Table 3.2 Summary of chronopotentiometric curves for chlorophenols
In Figure 3.13 (TCP, pH 7) we notice that the transition tirne is not clearly defined, which may ùidicate a second oxidation process. The transition time is even more blurred at pH 9 where at 0.1 rnA it take more than 100 seconds to reach the transition time.
EeImmIWfor the PCP curve reacts differently than with the other chlorophenols, as it increases in paraiiel with current density.
The data for MCP, TCP, and PCP at different pH (Table 33) show that they al1 behaved in a qualitatively similar fashion, with oxidation potentials close to + 1 V and that there is only moderate dependence on the extent of chlorination. This compares to Legros et al who found that an unsubstituted phenol at pH 7 had an oxidation potential of 0.97 V using photo-induced electron tramfer. It also shows that the phenolates are generally oxidized at less positive potentials than the corresponding phenols.
Conditions E, TCP E, PCP I E=4-MCP I Acidic form
Basic form
Table 3.3 Oxidation potentiais of chlorinated phenois *th respccfe Ç.t& amount of chlorination and pa of soiution (Note: pK. values are 9.9 for 4-MCP, 7.1 for TCP, and 4.9 for PCP)
In chronopotentiometry the product of " i, s cc is used to determine if the electroactive species is adsorbed at the dace of the electrode '4? Since the transition the decreases with an increase in current, an increqse in the above product must indicate greater adsorption at the surface of the electrode. mwever, under al1 experimental conditions (r~ardkssof pK concentration or substrate), cc i, r cc was near constant hdicating deactivation depends on total charge passed. Current Density m~/cm-~
Substrate
PCP,10 m.
Table 3.4 io T versus current density and pH for different chlorophenols
3.5.1 Oxidation Mechanism
Knowing the transition time z, which was read off the graphs accordig to the method show by Galus '48' it was possible to apply the Sand equation to determine '51': the number of electrons involved in the oxidation process. For systems that show
"7.mcomplicated" behaviour, meaning no deposition of matenal at the electrode, a plot of tKVS. i,," gives a line with a slope of 2i, lx" n D' FcO.Udiortunately, in our system ,this plot was non-linear- An alternative plot of (26 .çX hKD% F CO) VS. io-' (assuming D = 5 x
79 104 cm2 s-') '49' gives a straight line pardel to the i, axis (for an electrochemical reaction governed by diffùsion alone).
Figure 3.15 shows these data for 4-MCP under various conditions of concentration and pH. In the presence of electrode fouiing, the number of electrons is the extrapolation on the ordinate as &-' going to zero (or & going to m)- In the absence of anode deactivation, these plots would lie pardel to the abscissa. This behaviour was followed closely for 1 mM 4-MCP at pH 12, and shows a 4e process. Under other conditions the plots tend to 4e as L' goes to zero. The physical explmation is that at high i, there is less time for deactivation to occur, prior to the transition the being reached and the potential increasing to the next level. At lower pH and/or higher concentrations n = 4 is also the limiting behaviour as b-l- zero. The chronopotentiomeaic behaviour of TCP was similar to that of 4-MCP, and dso showed a limiting 4electron oxidation. By contrast PCP showed a 2-electron oxidation.
The sirnilarity among the chlorophenols (excluding PCP) suggests that they may undergo correspondmg mechanisms and reactions. From the literature, the oxidation of 4-
MCP is typically associated with the products of benzoquinone ('*) and 4chlorocatechol
(51)
The latter would be more consistent with these results as 1) the oxidation of 4-
MCP to 4-chlorocatechol requires 4 electrons, 2) while oxidation to benzoquinone is only a two electron process, uniess the Mer oxidation of the chlorine radical is also considered, whereby the process would be a 3-electron process. However benzoquinone corresponds better with the experimental pH data (3.4.4), which demonstrated a -1Se- 80 change per H+;as it has a 1 Se- per H' (with chlorine oxidation) while chiorocatechol has a le- per W. There also remains the possibility that oxidation is neither of these reactions but instead results in the formation of a dimer. 3.15 Plot of the apparent number of electrons for oxidation of 4-MCP(derived from experimental conditions applied to the Sand equation) as a function of concentration and pH (o=lmM, pH 12; O = 1 mM, pH2 ;?k = 1 3.5.2 Uncompensated Resistance
Using a current intempt technique we were able to record the uncompensated resistance during chronopotentiometric measurements. This recorded the ratio of uncompensated voltage&,, (Ru) as a fünction of time. Because we codd measure the uncompensated voltage to a precision of only j= 1 mV, the measurement of Ru was feasible only when i,, > 0.1 m.cm-2. Measuring the increase in Vu allowed for the dculation of
Ru for 10 mM 4-MCP; the high concentration was chosen in order to give a Iarge deviation f?om the Sand equation As seen in Figure 3.16, Ru uicreases fiom 15 to -70 Cl after the transition time at pH 2, providing excellent evidence for the electrode surfàce becomuig more resistive. This observation was interpreted as being consistent with oligomer formation on the electrode.
While the transition time is reached slightly faster at pH 7 (Figure 3.17) the increase in resistance is not abrupt but increases consistently over a much longer time period. This observation is consistent with slower formation of the oligomer. Moreover, the observation of a smailer-than-expected transition time at low current densities irnphes that deactivation of the electrode is a co-operative process that involves not only the oxidation of the chIorophenol but the oxidation of water. This wouid be consistent with the production of the passivating film on the anode involving attack of hydroq4 radicals fiom water on the initial oxidation products of the phenol. 3.16 Uncompensated resistance (Ru) as a function of time, and polnrizntion current (I,,) during oxidation (WE =
platinum bulb) of 10 mM 4-MCP,pH 2
Tirne, sec
For TCP at pH 7, (Figure 3.18) the increase in resistance is fiom 40 to > 600 Q
and for TCP at 0.1 M NaOH the resistance increases fiom 35 to 100 Sa (Figure 3.19)
however for this latter cwe, resistance reaches a maximum and then decreases. For PCP
(Figure 3.20) the increase in resistance is fiom 40 to 350 8. A detail to be noted fkom
Figure 3.16 is that Ru decreases as i, increases; a possible explanation of this phenomenon
is removal of the ohgomer film by Meroxidation, but we have no direct evidence for
this. At high pH however, Ru and i, were observed to increase in pardel.
Closer examination of the change in resistance with the demonstrates that there is no increase until there is solvent oxidation @&O). This is consistent with oligomer formation being a two-step process, which involves both direct oxidation of the phenol and attack on the oxidized product by (e-g.) hydroxyl radicals. Again it is possible that 4-MCP may be oxidized to either chlorocatechol or quinone, both of which can act as a radical trap for hydroxyl radicds, leading eventually to oligomeric products depositing on the anode and inhibithg electron transfer. The increase in Ru after the transition time was sirniiar for 4-
MCP and TCP, but was considerably larger for PCP. Wethe formation of oxygen bubbles on the surface of the electrode will also contribute to an increslse in resistance this phenomenon will be sporatic and not consistent as seen in the results. Removal of the film may be caused by chemical oxygen evolution or dislodgement by oxygen bubbles. The second explanation seems more likely, because in pH 7 and 0.1 M NaOH (Figure 3.20) resistance and current increase in paraliel; it is difncult to see why physical removal would be dxerent at pH 2 and pH 15whereas the oxidation processes might be quite Merent.
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5 1 Polcaro AM-, Palmas S ., Ind Eng. Chem Res-, 1997,3 6, 179 1. CHAPTER FOUR:
ELECTROLYSIS OF CHLOROPHENOLS
4.1 INTRODUCTION
The objectives of this study were to examine: 1) the reacfivty of daererît chlorophenol congeners during electrolysis at different electrode rnaterials, 2) to determine how voltammetry results could be applied to minïmize fouling and maximize efficiency.
4.2.1 Electrolysis
4.2.1.1 Anode Material
Anode material is one of the rnost important variables associated with electrolysis, as it cm influence selectivity, conversion and final products '? For example, a hydroxyl mediateci electro-oxidation reaction will only proceed if the anode material has a high enough potential to discharge the radical f?om the surface of the electrode
The performance of anodes of the same matenal can be affected by roughness, impunties and cornpositional homogeneity. For example a DSA that contains a mixhire of two different oxides (Le. SnOz and Ru03 can have large changes in surface area
(depending on the mixture and method of preparation) ". Another example is the B-Pb02 anode fiom Asea Brown Bovery (ABB), whkh preferentially generates 01over 02du~g the oxidation of water '4! This has two benefits shce the osdation occurs at a higher potentiai (1 -5 1 V vs. 1-23 V) the electrode operates at a greater overpotential and second because the ozone that is generated is capable of performïng as an oxidan+
Kotz '3 used Merent anodes to oxidize benzoic acid and found rates in the following order: doped-Sn02 (Ti base) » Pb02 > Pt. Sharifan and Kirk '' found electrode materiais to yield rates of phenol oxidation in the following order: doped Sn02 >
Pb02 > carbon > or Pt.
4.2.1.2 Operating Conditions
Engineering parameters such as celi design, flow rate and pH can inûuence electrolysis efficiency in terms of the rate of mass transfer or charge transfer. Stucki " found that when oxidizing phenol at Sn02 anodes there was little merence in phenol disappearance if the electrolysis cells were divided or not. Undivided cells are desirable from the engineering point of view as they are shpler and cheaper to construct. In contrast, when an undivided cell constructed with Pt electrodes was used, the electrochemical efficiency decreased. This was a result of redox cycling within the ceil, which was not observed at the SnOz electrodes. Stucki explained this difference in terms of the high overpotential of Sn02 electrodes for the electrolysis of Ha; this leads to reaction products that can not be reduced at the cathode. Reversibility is undesirable as it causes parasitic curent consumption. 4.2.1.3 Rate constant
De Sucre found that on Pb@, the percentage of phenol (LOO m@) oxidizd increased wRh ment den* and increased with decreasing concentration, flow rate, and pH (? A summary of these results is presented in Table 4.1 -
Conditions 1 % Phenol Oxidized 1 % Phenol Oxidized
Concentration (der 30 min) 5% (1 100 mg L-') 85% (100 mgL?)
Flow rate (singie pas) 40% (O. 1 L d') 5% (1.0 L muil)
Table 4.1 Effect of experimental parameters on the oHdation of phenol (PbOt)
Al-Enezi's "' oxidation of methauol and Abdo's et al "O' oxidation of ethyl aicohol with PbOz are in agreement with De Sucre's oxidation of phenol where the percent conversion was iduenced by the concentration of substrate and appiied current. Al-Enezï determineci the process to be a second order reaction related to these two experirnental conditions (eq. 4.1).
(eq. 4-1)
The rate constant 'Y is an empirical value that depends on the initial concentration and the current. 4-2.2. Phenol
4.2.2.1 Oxidation
The oxidation reactions of organics are thermodynamically favourable in the presence of oxygen but kinetically slow udess a catalyst is used. When an aromatic is subject to oxidation (either through electrolysis or other means) the process usually involves the introduction to the ring of hydroxyl groups, This is then foilowed by ring opening to form acids and, if the process is dowed to continue, shorter and shorter chahs und complete minerakation occurs and only CO2 remains. When phenol is oxïdized
(Figure 4.1) it has been shown to generate products such as biphenols, hydroxy-aromatics
(e-g. catechol, dihydroxyphenols) and organic acids (e-g. maleic, oxalic, formic) '"'.
Scott wâs able to idente benzoquinone and rnaleic acid as products when a Pb& anode was used to oxïdize phenol. Smith de Sucre" who also used Pb@, found that that the total organic carbon (TOC) was not lowered (as Cais fomed TOC will decrease) because the products oxiclized at a slower rate than phenol. In contrast at Sn% Comninellis et al ''" found that the toxic intermediates (Le. hydroquinone, catechol and benzoquinone) were virtually elimùiated and that the aliphatic acids were rapidly oxidised. phenoxy Radical 1 1 -e-
Figure 4.1 The Degradation Pathway of Phenol via Oxidation
Further oxidation ofthe bernoquinones results in the formation of organic acids and COî. There are merences in opinion as to the oxidation mechanism of phenol, however the prevailing opinion is that it is an ECE (electrochemical - chemical - electrochemical) process '13). One theory is that the initial product is a cation radical [C&T&@-] which is hydroxylated by OH radicals formed during cataiytic oxidation of water. Flesar and
Ploszynska '14) suggest uistead that the oxidation mechanism must be a surface reaction and that (OH-)& attacks the phenol to form a dihydroxy radical POC&OK-1. Their evidence is the rates of oxidation of benzene and phenol at Pb02 to hydroquinone were similar, even though their oxidation potentials are separated by - 1 V.
Regardless of the initial oxidation products that are formed (see discussion in chapter 3), they are susceptible to attack from phenoq radicals, resulMg in polymerization (through intermediate coupling). This is believed to be responsible for fouIing and blocking of the electrode surface. It has been found that for mhïmkhg electrode fouhg Pb02 is more efficient than glassy carbon, which in turn is better thsn nickel or platinum. In agreement with some the findings in chapter three, Gattrell proposes that the fouling problem can be minimised by either decreasing the concentration or Lifetime of radicals. He suggests parameters that can be used to achieve the former include lower substrate concentrations and mlluminng current density, while lowering pH or inmeashg ceil voltage are suggestions for the latter.
4.2.2.3 Sn02 Mechanism
To diffèredate between mech 1 and mech 2 (section 3.2), Stuckï used the reversible redox couple F~(CN)~~&which bas a single electron transfer- He estimated rnass trader by measuring the limiting current at various concentrations and flow rates- From this he calcuiated the lirniting mrrent required for complete oxidation (to Ca)of phenol on Pt and Pb& (which he had ernpiricaily detennined as a l7-electron tram&&). Results showed that the removal rate for phenol (mg A-' h-') for the Sna anode exceeded the maximum rate calcuiated fiorn the mastransfîer experiment. There appeared to be no threshold regarding a limiting current, even at a value twice that predicted by the electron transf'ér experirnental resdts. He believes that this information (and that oxldation was independent of flow), shows that the mechanism of oxidation can not be an electrochemical charge trdrat the electrode interfàce (as charge transfer must be mass transfer limiteci). This conciusion is in agreement with other research conducteci by Wabner and Granbow ''' who studied the oxidation of organics at Pb%. Thar results Ied thern to the theory that the electrode produces reactive hydroxyl radicals that produce a homogenous oxidation reaction in the buik phase of the electrolyte. However, Stucki et al disagree with this theory on the basis that hydroxyl radical reactions would also be &sion controlled. Instead thqr beliwe that the cation radical
[C&OH'-] is generated at the electrode SUTfàce, and then reacts with oxygen or other organics in the buk solution However they did not offer experimental evidence in support of this theory.
4.2.3 Chlorophenols
4.2.3-1 EIectrochemical Reduction
Schmal 'lmshidied electrochemical rernediation of PCP through reduction
(dehalogenation) using a high surface area carbodgraphite electrode, however after 90% removal he only obtained 2% current efficiency because of cornpethg Hm evolution.
Direct anodic oxidation of chiorophenols~17718~has been postulated to involve initial removal of one electron, to give a chlorophenol radical cation which readily deprotonates.
Subsequent reactions include fùrther oxidation to a benzoquinone derivative (Figure 4. l), and subsequent ring-opening reactions (19,201
Gattrell and Macdougall were able to decrease a 300 pprn PCP solution to less than 1 pprn using a carbon felt electrode at a current of 0.1 mA. However, no chlorines were removed fiom the PCP substrate and it was unclear as to what exactly the products were. They concluded that oltidation could not be used to remediate PCP in solution but might be able to concentrate the substrate on the anode surface and therefore act as a
101 complementary method to physical carbon loading. They were able to enhance PCP
Ioading on the carbon felt fkom 0.26 mg/g to 23.4 mg/g (mg PCP/g carbon felt electrode) using electrochemical oxidation. Electrochemical Ioading was also shown to enhance physical loading for 2,4,6-TCP, 2,4-DCP, p-cresol and phenol. In a Iater paper they used I4c labeiling to show that they could also rernove upwards of 90% of the PCP products that were adsorbed to the carbon during oxidatioa This consisted of using cathodic polarization to remove the buik (74%) and the remaining 16% was removed with various washing steps with solvents such as ethanol, and toluene.
GattreLl and Kirk oxidised phenol at a glassy carbon electrode and found that the rate of oxidation increased at higher temperature or higher current. They estimated resistance values for their anodes fiom IR values and currents. They then related increases in resistance ro corrosion of the eiectrütc: ~"mts.Xow-ever, t%ey &G îiüt pïûvide my physicd evidence of this corrosion and so the increase in resistance could easily be attributed to deposition of polymeric material on the carbon (which Gattreil and Kirk did observe). Their data demonstrated that increases in current correspondingly increased resistance at the electrodes, while temperature was inversely correlated with resistance.
They also fond that increasing the potential (1.7 V to 2.3 V) did not drastically change the rate of degradation but did change the products (towards less polymers as descnbed in chapter three) as confirmed by increases in quinones and organic acid concentrations.
Regardless of temperature they found that a higher fiaction of insoluble compounds were formed with increased concentrations of substrate. Brillas and Sadeda used 4-MCP as a mode1 substrate to compare indirect electrooxïdation methods, including electro-Fenton (EF), photoelectron-Fenton VEF) and peroxi-coagulation (PC)against direct Hz02 oxidation. For the Fenton readons they used a Pt anode and a carbon polytetrafluoroethylene PTE) cathode to generate hydroxyl radicals- They then added 1.0 mM FeS04 (EF) or 1.0 mM FeS04 in the presence of UV light (PEF). For the PC reaction, they used a Fe anode which yields excess I?e2+such that
Fe(OliQî precipitation occurs. This has the advantage of being able to remove the substrate by reaction with hydroxyl radicais and coagulation. However the anode is sacficial and cornpetes with the reaction for electrons. They concludecl that of the different techniques, PEF was the most efficient at reducing TOC levels.
Polcaro and Palmas oxidized 2-MCP at a carbon felt anode and found 2- chlorobenzoquinone as the first isolable intermediate (for 2,6-DCP, 2,6- dichiorobenzoquinone was the first intermediate). Further products included aliphatic acids (maleic, oxalic) and CO2 @'.Beattïe et al attempted unsuccessfully to use a DSA
(Ruoz) to oxidize PCP at acidic and akaline pHs. The maximum degradation they achieved was in alkaline conditions and was only 10.5% &er 65 h at 0.6 V and 60 OC.
Gattreli and Ma~dougall'~'oxidized PCP on platinum foil and used attenuated reflectance
Sared spectroscopy to determine the presence of a dimer; zi,3//il ,4,5,6 -pentachloro4 pentachIorophenoxy-23-cyclohexanedienone on the surface- From these resuIts they proposed the following mechanism: Pentachlorophenolate ion CI
Pentachl orophenoxy radical
O' O' cl $ Dimerization step (insolubie product) Cl - CI CI Cl Cl Cl
Figure 4.2 The Dimerization Pathway of Pentachlorophenol The objective of the work, discussed in this chapter, was to study the efficiency and oxidation rates of several chlorophenoIs at difFerent anode materials
4.3.1 Bulk Oxidation
Galvanostatic eiectrolyses were c-ed out using a flow-through, sandwich type ce11 constructed fiom ~lexi~las@pieces with dimensions of90 x 15 x 45 mm. The centre of each piece was bored out to provide anode and cathode compartments having 2.5-mL volumes. A ~afïon@417 membrane separated the cell compartments (Figure 4.3A).
Anolyte and catholyte solutions were delivered fkom reservoirs, each of which consisted of a 60 mL glass vesse1 closed by a rubber top. To facilitate removal of gas bubbles the direction of flow through the cell was upwards. Continuous circulation was achieved with a peristaltic pump, and was held constant at 7 mL min? LF'L-T~~O~@was attached to the pump head; ~eflon@tubing was used for connections between the pump and the reservoirs (Figure 4-3B).
Sodium sulfate (0.05 - 0.1 M) prepared wîth Millipore ~illi-Q@water was used as the supporting eIectroIyte, except in experiments where the solution was buffered, in which case it was disodium hydrogen phosphate, adjusted to pH 6.7 by the addition of phosphoric acid. Sodium dateand sodium phosphate were chosen because they do not contain chlonde ions and because sodium, and suIfate have hi& permissible Ievels in the environment. The cell was based on a three electrode system with anodes (10-30 cm2) constructed fiom Pb02 or class-1' DSA materials (IrOtITi doped Sn02)- The latter consisted of either low (plates) or high (cross-hatch) surface area. The doped Sn02was comrnercialiy obtained from ~ltech@Corporation (specifïcation EC420). The Pb02 electrodes were either cut fiom a new automotive battery, or were prepared in our laboratory by anodic oxidation of solid Pb in 30% H2S04 at 10 mA cm-*. The counter electrode was either Pt or Ti (10 cm2).
Anode potential was measured using an extemal Ag/AgCI reference electrode which was connected to the electrode cornpartment by Tygon tubing containing electrolyte solution. Electrolyses were carried out amperostatically or potenti~tatidy~ using a Pine RDE4 potentiostat. AU potentials are reported against a standard hydrogen electrode (SHE). Electrochemical parameters were also constantly monitored with externd multimeters, the potential at the working electrode (anode) ranged fiom 1.4 to
2.2 V depending on the counter electrode and the experimental conditions. % e
IN OUT m a
Figure 4.3A Diagram of Experimental Flow CeU
A - ANODE C - CA?-WODE R - REFEREN- M- nAEblE3RAN-E P -PW E - ELECTROL- S - SAMPLE
Figure 4.3B Experimental Set-up 4.3.2. High Performance Liquid Chromatography (HPLC)
Andysis was performed with a waters@HPLC, consisting of a model 440 W absorbame detector at 254 nm connected to a model 501 pump with a flow rate ranging from 0.5-1.0 dmin. The detector response was compiled and was processed wah
~eaksim~le@software. Separation was achieved with a reverse phase Cl8 (25 cm x 4.6 mm) stainless steel colurnn. Mobile phases used were; 60 MeOH:40 H20 mixture for lower chlorinated phenols; an 80 MeOH:20 H20 mumire for TCP, and a 90 CH3CN:IO
Hz0 for PCP. A 150 a sample was injected via a lXheodynea injector contahing a 20 pL sample loop. AU solvents used were HPLC grade and were obtained fiom Fisher
Scientific. ALI chlorophenols were detectable at a minimum concentration of <10" M.
4.3.2 Substrate
In the &st senes of experiments, the sarne seven phenols as studied in chapter 3
(unsubstituted to pentachloro-) were oxidized at the different anodes at a constant current.
From the literature data and Our own voltarnmetrîc studies, we realized that electrode fouling wodd be a significant practical issue. Using IO mM solutions, anodes typically lost all their catalytic activity derseverai kinetic runs, and could not be reactivated. When the chiorophenol concentration was Iowered to 2.0 mM loss of activity ody occurred over the long term, which was monitored by using benchrnarks. This was accomplished by repeatïng the earliest studied chlorophenol (usually 4-MCP) at the end of a series to dernonstrate that its rate of oxidation had not changed. The pH of the anolyte decreased during electrolysis fiom 6 to 2, on account of the competing oxidation of water, udess the solution was buffered-
EIectrolyses with PCP were canied out in buffered solution (pH 6.7 - 1.42g
Nal&P04: 0.459 Na&P04, 1000 mL H20,ionic strength 0.0014 M) because this substrate is insoluble at low pH. In other electrolyses, the pH of the solution in the reservoir was re-adjusted to -7 after the removal of each aliquot for HPLC analysis by the addition of 3 M NaOH. There was no detectable merence in reaction rates between the chlorophenol solutions that were buffered and solutions whose pH was adjusted with
NaOH,
4.4.2 Anode Material
Figure 4.4 shows the behaviour of 1 mM solutions of the different chlorophenols at a commercial PbOz anode (17 cm2; 5 mNcm2). Phenoi reacted substantially faster than its chlorinated analogs, but no obvious structure-reactivity trend was evident at this 109 electrode: the order of reactivity was Pa> PCP, > 2- and 4-MCP > 3-MCP, 2,4-DCP and 2,4,5-TCP. The disappearance of the parent compound (0.26 mol phenol removecl/ mol e) during the the first 30 minutes compared well with De Sucre's Pb02 anode plate experirnents (0.18 mol phenol removed/mol e-).
4.4.2.2 DSA Anodes
For the rate of disappearance of the W2anodes (Figure 4.3, the three MCPs were similar to each other, aithough they were slower than at PbOz, with about 70% of the original concentration remaining after 3-5 hours. Furthermore, there was very little merence between them and the unchlorinated phenol, which now had the slowest disappearance. The substrates with more than I chlorine substituent reacted quite differently, DCP had oniy 50% remaining at the end of the experiment whiie TCP had less than 20% and PCP was undetectable after 2.5 h. (we had anticipated that PCP would be the slowest degraded not the fastest) O 'd-
At the doped Sn02 anode (13 cm2; 6.5 mA cm-2) (Figure 4.6) the rate of electrolysis generaliy increased with chlorination of the reactant- @dation rates were higher than at h02, for most substrates, and the reactions foUowed fïrst order kinetic behaviour much more closely. At the doped Sn02, unlike at Hl2, phenol oxidised faster than TCP and was undetectable after 3 h; PCP, which again disappeared the fastest, was undetectable after 2 h. Control experiments at zero current showed no losses > IO%, due to adsorption or volatilisation over a 4 hour period for any anode material. Using the benchmark technique with 4-MCP it was demonstrated that the Ir02 anode was the least prone to deactivation.
Anode comparisons in the literature are limited to phenol as the substrate; Stucki et al "found that phenol oxidised 8 times faster at Sn02 than at Pb02 at a current density of 5 mA cm-2, adat similar rates at Pt and SnOz (20 mA cm2). However, comparisons are difficdt to evaluate because oxidation rate wili depend on the method of preparing the eiectrodes and current density. Research with Sn02 has claimed that doping with I 4% Sb could change the concentration of charge carriers by 300 times at a potential of 1.5 V, although no details were given about the Sn02method of preparing the electrodes '28! Nor was any information given about whether Stucki et al's results were affected by electrode fouling.
Table 4.2 summarizes the pseudo-first order kinetics at ail three anodes. The Sn02 anode had the kinetics which were most closely la order, with aU seven compounds having regressions greater than 0.993. On the assumption of pseudo-fist order kinetics, entries with a poor 2 generally exhibited slow reaction rates that were closer to zero order
in substrate.
i PHL 0.97 0-94 4.2 1.O0 0.72 1-00
2-MCP 0.27 0-97 5 -5 1-00 0.48 0-99 3-MCP 2.0 0.85 0.1 1 0-99 0.21 0-99 4-MCP 0-79 0.96 0.47 0.97 0.40 0-99 2,4-DCP 0-27 0.97 0.41 0.97 1.16 0.99
2,4,5-TCP 0.83 0.98 O. 13 0.97 1.17 1.O0
PCP 3 -9 0.99 0.98 0.99 1.18 1.O0
Table 4.2 Pseudo First-Order Rate Constants (and 2) for Phenols on different Anode Material
Our own results have shown the preparation of Pb02 can greatly affect its reactivity; the rate of disappearance of 4MCP was 2.7 times faster at a Pb02 anode that was prepared in our laboratory compared with the commercial material, usïng the same
(nominal) current density The potential required to initiate oxidation was also found to be
200 mV higher for the commercial material-
4.4.3 Substrate Reactivity
Comparing the same substrate at different anodes, the reactivity of phenol was in the order Pb& > Sn02 > IrOz; for monochlorophenols it was Pb& - Sn& > Ir&, while the more heady chlorinated phenols followed the order SnOz > IrO2 > PbOz.
Consequently, the performance of an anode matenal cannot be evaluated by testing only a single substance.
The difference in substrate reactivity order between the different anode materials indicates that they probably involve different mechanisms, with the Pb02 anode causing oxidation by a mechanism different fiom the two DSA@ anodes. For Pb02, at whicb phenol reacts faster than the chlorinated phenols, the results are broadly compat8.de with a conventional electromidation process, involving electron transfer £tom the substrate to the electrode (see section 3.2). This mechanism may or may not be relevant to electrolyses at
DSAs, because as discussed in Chapter 3, they can oxidize organics via two different processes; (Mech-1) where it is hydroxyl radicals adsorbed at the electrode or in solution that oxidize the target substrate, or (Mech-2) in which the substrate is oladized directly at the electrode surface ". However, the obse-ed order of reactivïty (with highly chlorinated phenols the most reactive) is incompatible with the rate-detennining step being the attack of OH on the phenolic substrate. This reactivity order could be compatible with increasingly easy expulsion of Cl' fiom a cyclohexadienyl intermediate. It would also be consistent with oxidation by M0,I but we have no frame of reference to predict what factors would influence ease of oxidaiion in the latter case, At this stage, we cannot exclude the possibility that the merences in oxïdation rate with substrate at the DSAs may involve both electron transfer and attack of hydroxyl radicals. Eleceon tramder in a DSA d probably proceed via a hole-transfer mechanism m, which is slower than conventional electron transfer, md therefore may require a longer residence tune of the chlorophenol at the anode surface. As au n-type semi-conductor such as Sn02 has been shown to have low hole densities 'O), doping is used to increase the hole concentration-
We know for a semiconductive electrode under anodic conditions that the boles will move towards the positive surface '". Kotz used X-ray photoelectron spectroscopy
(XPS) to observe this effect with doped Sn02. It is believed that the holes react with adsorbed water molecules to form (OH)& which then reacts with organic compounds or recombines to forrn H202and Oz.
Phenol adsorption was shown to increase on the Sn02 anode du~geIectrolysis as a response to the Sn4bond becoming hydrated with anodic polarkation (13'. This was attributed to a donor-acceptor mechanism between oxygen groups on the anode surface and K electrons of the benzene ring "? This donation of the electrons to the anode surface is also compatible with physical organic studies on chlo~atedaromatics, which show that the chlorine substituent is weakly deactivating in processes that lead to a buiid-up of
positive charge on the aromatic ring (a' = 0.O3 5) @". This is consistent with the failure to find any striking trend in oxidation potentials among the chlorinateci phenols in our voltammetric measurements on Pt. It has also been shown that the lower solubility associated with higldy chlorinated aromatics enhances adsorption on the eIectrode surface
(34)
Repeated use eventually fouled all of the electrodes. Attempts were made to remove the insoluble compounds f?om the electrode surface and regenerate its catalytic properties. The effectiveness of the method afker 3 hours of treatrnent by perfomiing an electrolysis is shown in Figure 4.7. These include washing with a surfactant (Tween 80), combined SUrfactant/sonication washing and reductiodoxidation cychg (fiorn +1 V to -1
V), results are compared to the poisoned electrode prior to treatment (poisoned electrode). The addition of a sdactant with sonication for several hours did appear to increase the oxidation rate; unfortunately the improvement remained rather srnail. Tirrie (hr) 1 -Risunecl 6ectrode -m- Surfactant Washhg -SurfsctanVSanication tRedox Cycihg
Figure 4.7 Attempts at Cleaning Fouled Electrode
4.4.5 Products
HPLC and Gas Chrornatographyhlass Spectrometry (GCMS) both support the
formation of benzoquinone as a product fiom 4-MCP. In temof reference, two electrons
are required to convert a phenol group to a hydroquinone (or equivalent olcidation state).
The appearance and subsequent disappearance of a yellow colour in the reaction vesse1 also suggests this pathway.
Peaks fiom HPLC results showed that DCP, the three MCP's and PHL all yielded two major products. For 4-MCP one of these peaks was confirmed through GCMS as benzoquinone, the second peak had an HPLC retention the (RT) sùnilar to that of a maleic acid standard. Each of the phenols af5orde.d severd other minor peaks seen with
HPLC. It is possible that these peaks are either different dihydroxy products (however there were no peaks with RT's matching that of catechol) or other products nom the ring breakup (Le. oxalic acid). Chlorobenzoquinone (RT of 5.2 minutes) was found in some samples but it was benzoquinone (RT of 4.8 minutes) which was seen to accumulate and persist with thefor most samples. Using potentiometric determination (Am), chloride ion was not detected arnong the products. This was expected, as a DSA anode would oxidise any Liberated &onde to Cl2 under electrolysis conditions. Figure 4.8 GCMSDetermination of beiiuoquinone as a pmduct Mm4-MCP dter 5 Ys of ondation at an SnOt anode (10 mA cm4) SampLe extmcted into dichIoromethane pCLOr to inj &ion 4.4.5.1 LSV of Products
Hydroquinone, benzoquinone and mdeic acid were subjected to L.S. voltmetry
in acetonitrile solution to determine their oxidation potentials. Acetonitrile was used as a
solvent as it does not have the interference of oxygen evolution and therefore can be used across a wider anodic range. Hydroquinone was oxidized at lower potentials than 4-MCP which is consistent with previous reports = 0.70 V) O? Benzoquinone and maleic
acid were subjected to voltammetry and did not appear to be oxidized even at potentials as high as 3.2 V vs. Sm.
4.4.6 TOTAL ORGANIC CARBON
Analysis of the electrolyzed solutions for TOC indicated that some fiaction of the substrate always proceeds to COz, that is, the TOC progressively decreased with the extent of electrolysis. We found that between 4040% of the TOC fkorn 4-MCP was removed in the first two hours of electrolysis with the DSA electrodes, however alrnost no
TOC was removed with the Pb02 anode. This mers firom DeSucre who removed 30% of
TOC f?om a 100 mgL phenol solution using an Pb02 anode with a current density of 850
&cm2.
Although the voltammetric result s suggest that benzoquinone should not react fùrther under our conditions, owing to its extremely positive oxidation potential, quinones are lcnown to be efficient trapping agents for f?ee radicals 0738'. This fact, dong with the
TOC decrease, would support the indirect hydroxyl radical mechanism (Mech-1) for
DSA's as discussed in Section 3.2.
122 4.4.7 Current EfEciency
The current efficiency at each anode material was calcdated at diEerent current densities using 1 mM 4-MCP as the substrate (see Table 4.3). It was assumed for the purpose of calculation that oxidation wouid be a 4-electron process, as determined by chronopotentiometry in chapter 3 (see section 3 -5.1 and Figure 3.15)- Eficiencies were calculated based on the total charge passed and were corrected for volume changes due to withdrawd of sample aliquots. Current efficiencies were calculated for the period when the loss of substrate is approximately linear with tirne: the first 1 h of oxidation at 5 mA
and 0.6 m. and the first 3 h of oxidation at 0.06 rnA cm-2. The values in brackets show the approximate potential of the anode vs. S.H.E. as measured £kom the celi potential during electrolysis.
Anode 5 mA cm-' 0.6 mA cm-" 0.06 m.cm-'
3 m2 1.8% [1.7V] 18.5% [l-3VJ 53.6% [1.2v
Pb02 2.1%[Lw 21.3% [1.3VJ 76.9% [l.lv
Sn02 2.8% [2.2v 28.6% [1 .m 61,4% C1.3V-J
Table 4.3 Current efficiencies for the oxidation of 4-chlorophenoI at three anodes as a function of current density A substantiai increase in current efficiency occurs at all three elecîrodes as the current density decreases. This is to be expected, because under amperostatic conditions, the positive polarkation of the anode must inevitably increase in parallel with increasing current density, therefore more of the charges passed will be diverted to the unproductive reaction of oxygen evolution fiom water.
Al1 anodes exhibited current efficiencies > 50% at low current density, a promishg result in tems of our ultimate goal of developing an electrochemical oxidation as a method of remediation technology for chlorophenols. The progressive loss of TOC from the soiution during electrolysis suggests that the current efficiency is actually higher than this (as CO* evolution requires more than the 4e- assumed in the calculation): for example, during the fkst 2 h of electrolysis at a SnOz anode, 60% of the 4-MCP reacted and TOC decreased by 40%. Since low current densities are ofien accompanied by low potentials, this according to the voltammetry results in chapter 3, indicates that electrode deactivation will be minimized-
4.4.8 Operating Costs
The cost of electricity for the electrochemical oxidation of chlorophenols was considered, using the foilowing simple analysis. At a cost of 6 $ per kWh, electrons and the cell has a potential dinerence of 8 V, a 4-electron oxidation with a 50% current efficiency, would cost $2.40 per mole of chlorophenol treated. As depicted below: using
kW=kJs*' or kWh=3600kJ
AG (J mol -') = n (mol) F (C mol-') E 01) x 4
We detennine
AG = (96,485 C/m0l)(8 V) = 771,880 J/mol
Therefore, each kWh
passes 0.2 mol of electrons through the ce&
If we require 4 electrons mol -'of compound treated
-1 -2 moles are treated for every kWh
At a cost of 0.06%(kWh)-', and at 50 % efficiency
chlorophenols are treated at a cost of- $0.14 mol-'
This would be the equivalent of treating 1 m3 of a 1 m.solution and can be
compared to $170 m3 (1994 estimate) for incineration @" or $2 m3 (1994 estimate) for
W/oxidation (please note these as overall costs not electncal costs) (? Using this assumption any solution under 0.2 mM would be more cost effective using electrolysis
(excluding capital and electrode costs). Kotz " calculated the energy consumption as 50 kW per kg of COD for an Sn02 anode at 35% efficiency and ceIl voltage 4.0 V. He concludeci that electrochemical oxidation might be economically cornpetitive with wet oxidation and incineration at hi&
COD concentrations and with adsorption or precipitation at low concentrations.
It is important to remember that while incineration has a fked energy costs for burning, electrochemical treatment is dependent on the electrical efficiency, which wili decrease with concentration. This cost estimate is aiso low enough for a wide margin of error for changes in efficiency and operatUig/maintenance (including electrode costs). 1 Simond O., Schaiier V., Cornnineliïs Ch., Electrochimica Acta, 1997,42,2009.
3 Chu D., Gilman S ., J. Electrochem. Suc., 1% 143, 1685.
5 Kotz S., J. A& Electrochem., 1991,21,99.
6 Sharifan H., Ki& D.W., J Electrochem. Soc. 1986, 133,921.
7 Stucki. S, AppL Electrochem., 1991,21,99.
8 Smith De Sucre V., Cm. J. Ch.Eng. 1981,59, 52.
9 Al-Enezi G-A,J. Env. Sci. Healh A, 25,1998,67.
10 Abdo M.S.E., Al-Enezi GA, Haddad-Al A, J. Environ Sci. HeaZth, A21,1986,487.
11 LeBang U., Ebert K., Hory K., Gaiia U., Schneider H., Sep. Sci. TechnoII, 1995,30,
1883.
12 Comninellis Ch., Pulgarin C., J. A& EZecfrocktem.> &993,23, 108. 13 Nison A., Rolan A,Parker V.D. J. Ch.Soc. Perkin. Tram 2 13. t ,2937. 14 Flezar B., Electrochim. Atm 1985,3 0,31. 15 Wabner D., Granbow W., JI ElecfroanaZ. Ch,1985, 195,95.
16 Schmal D.; van Erkel J.; van Duin P. I., Be Ihstitution of Chernical Engineers
Symposium Series, no 98 (2?lectrochemicaZ Engineering), 1986,28 1-29 1.
17 Gattrell M., MacDougail B., Approaches tu Elecfrochernical Treafment of Chforinated
Orgmics, NRC, Insitute for Environmental Chemistry, 1994. 1-2 1
18 Bdas E., Sauleda R., J. Electrochem. Soc., 1988, 145,759.
19 Trabelsi F., Ait-Lyzaid H., Ratsimba B., Wfielm AM., Deimas H., Fabre P.L., Berlan
11, J. Chern. Eng. Sci., 1996,51, 1857.
20 Cornninellis Ch.; Vercesi, G.P., J. AppL Electrochem., 1991, 21, 3 3 5.
2 1 Gattrell M., MacDougall B., ne10th Intl. Fomm on Elecfrospthesis in the Chernical
Indiis~,Cleansrater Beach, Florida, Nov, 1996.
22 Gattreli M., MacDougali B., Proc. of S'positm on Water Purification by
PhotocafaZyfryfrc,Photoelectrochemical and Electrochemical Processes, Electrochem
Soc., 1994, 99.
23 Gattreil M., Kirk W., CmJ. Chem. Eng., 1990,68,997.
24 Brillas E., Sauleda R, J. Electrochem Soc. 1998, 145,759.
25 Polcaro AM., Palmas S., Ind Eng. Chem. Res., 1997,36, 1971.
26 Beattie J. K., De mark J.A-, Kennedy B.J., Aust. J. Chem., 1994,47, 1859. 128 27 Stuki S., Kotz R,Carcer B., Suter W. J. Appl- Electrochem., 1991,21,99.
28 Kotz R., Stucki S., Carcer B., J. AppL Electrochern., 1991, 21, 14.
29 Vijh AK., Elect?ochemistry of Metals and Semicondi~cfors,Marcel Dekker, 1973. p.
30 Finklea HO., Studies in Physièal and Theoretical Chemisny; Vol 5, Semicon~hctor
Electrds, Elsevier, 1988, pp. 203-23 9.
3 1 Tibor E., Kinetics of Elecfrode Processes, Adam EUger, London 1972, pp. 38 1.
32 Coughlui RW., Ena R S., Tan RN., J. Colloid Sci., 1968,28,3 86.
33 March J., Advanced Orgmic Chemistry, McGraw Hill, New York, 1977, pp. 253.
34 Streat M., Patrick J.W., Camporro P.M-, Watt Res. 1995,29,467.
3 5 Puschaver S., Chem. Ind. (Zondon) 1975, 19,236
36 CRC Nmdbook of Chemistry and Physics, 65th Edition. p. D-155, 1984.
37 March J., Advanced Orgmic Chernisty, McGraw Hill, New York, 1977, pp.620.
3 8 Carey F.A, Orgm~icChernistry, McGraw Hill, New York, 1987.Chp 26.
39 Schmai D.; van Erkel J.; van Duin P. J., The Institution of Chernical Engilleers
Symposium Series, no 98 (ElecfrochemicalEngineer+n@, 1986,28 1-29 1.
40 Trach RJ., U7frmîolet Rernediation, GWRTAC, Pittsburgh, Pa, 1996. p. 1-49.
129 CHAPTER Fm:
TREATMENT METHODS USED FOR THE REMEDIATION
OF NITROAROMATIC EXPLOSIVES
Environmental contamination by nitro compounds is associated principdy with the
explosives industry, which developed in the lgLbcentury following the discovery by Med
Nobel that kieselguhr could'be used for the stabilization of nitroglycerine. AU modem
explosives are oxygen-containing organic compounds that have the potential for self-
oxïdation to smaU gaseous molecules (m, and CO*). For this reason, many
expiosives are polynitroaromatic compounds 'l'; one of the best known is 2,4,6-
trinitrotoluene (2,4,6-TNT), which for many years dominated the explosive industry,
because its low melting point (80 OC) allowed it to be cast safely by pouring fhm a steam-
heated melt. Others include 1,3,5-trinitrobenzenene (TNB), dinitrotoluene (2'4-DNT, 2'6-
DNT), dinitrobenzene PNB) and 2'4'6-trinitrophen01 (picric acid). Some of these
compounds also have other industriai uses; e.g, the dinitrotoluene isomers are used as
intermediates in the manufacture of polyurethane foams O).
Environmental contamination by nitro compounds is a problem because of the scale on which explosives have been manufactured, used, and tested. This problem is of particular concem to those countries that were involved in the second World War- In 2945
Germany's and the United State's production were 2.36 x 103 tondmonth, and 1.95 103 tondmonth respectively "'. ûther countries that have explosives contamination include (but not limited to) Canada, Australia, Spain and England '4!
In the United States, the Department of Defense @oD) continues to rernain affected because of the open detonation and buniing of explosives at army depots, evduation facilities, artillery impact ranges, and ordnance disposal sites "*? Contamination dso occurs during manufacturing, which in the case of 2,4,6-TNT requires large amounts of water for purification '? Primary aqueous wastes are known as red water, which is associated specifically with the manufacture of 2,4,6-TNT (but may contain up to 30 other nitroaromatics "), and pi& water, which origuiates with loading, packing or assembluig, and which often contains high concentrations of other nitroaromatic explosives O). The
United States stopped 2,4,6-TNT production in the mid-1980~~but environmental contamination has continued as a result of demilitarization, including '%ashout operations" which involve removing the &ses fiom munitions and removing the explosive charge by spraying jets of hot water into the casings. The wastewater from this process is then placed in Iagoons or sedimentation basins,
Nitroaromatic explosives pose environmental rïsks to humans and other anirnals;
2,4,6-TNT is on the list of US EPA pnority pollutants because it is a hown mutagen and because it can cause pancytopeuïa as a result of bone marrow failure Furthemore,
Mutatox and green alga bioassays have confïmed it to be the most toxk of the nîtroaromatic explosives ''O! Dinitrotoluenes can induce Iiver cancer in rats, with the 2,6 isomer much more active in initiating hepatocellular foci than the 2,4 isomer 'l? These compounds exhibit lethai toxicity towards the guppy (Poecilia reticuIzzta), with Wday LCso values of 12.5 mg L-' for 2,4-DNT and 18 mg L-' for 2,6-DNT "". However, these values do not adequately reflect the overail toxïcity of a contaminated water body, because environmental transformation products can be equdy or more toxic as the parent nitroaromatic explosive. For example, chemical or biological reduction of nitrocompounds leads eventudy to amines, which are toxic in their own ri@+ as also are intermediate reduction products, notably aryIhydroxyllamines, and condensed products such as azoxy- and azo-compounds (13! Arylhydroxytarnines are implicated as carcinogenic intemediates as a result of ~treniumions formed by enzymatic oxidation "?
The remediation of these waste streams is difficult because of the environmentd persistence of aromatic nitrocompounds on account of the5 resistance to oxidation
(chemicd or biologicai) or hydrolysis (14). This stability is a result of the electron withdrawing nitro groups and electron deficiency of the R-ring system of nitroarenes 'ln. In particular, 2,4,6-TNT's low mobility and reactivity make it very commonly encountered in both soi1 and water ('?
Remediation strategies must be considered on a site-by-site basis, because most sites contain multiple contaminants, and a single technology may not be applicable to aü contaminants. For example, toxicity to rnicroorganisms at high concentrations makes bioremediation of nitroaromatics of limited applicability; in other cases the treatment process may produce recalcitrant reaction byproducts fkom the original contaminant "?
Chernical treatments may be energy intensive (e-g.,rotary kiln incinerator), or ineffective at low concentrations, or may cause other environmental problems such as NOx emissions. 5.1.1 Sites
The US DoD has identïfïed more than 1000 sites with explosives contamination, for example Ft. Wmgate mydepot in Gallup, NM, Navalo arrny depot in Flagsta AZ, and Lousiana army ammunition depot in Shreveport, LA "? At least 95% of these sites studied were contaminated with 2,4,6-TNT and 87% exceed permissible groundwater contaminant levels Many sites need immediate cleanup because pnor to the 19807s, waste was oflen dumped in unlined pits, and has now contaminated both soil and groundwater,
An example of a clean-up strategy is the long-term remediation plan that has been developed for the Joliet army ammunition plant located in Illinois (20', where more than 4 billion pounds (2 x 1O' tonnes) of munitions were produced fiom the early 1940's until the
Iate 1970's- Groundwater, sediments, soii and surface water are contaminated; this is of concern because over 2,000 people live within three miles of the site. Contamination of two nearby creeks was caused by routine discharge of process and spill waters into drainage ditches, by seepage from unlined ash piles, and fiom a leak in the hed storage
Iagoon. Remediation to date has consisted of removing 7 dongallons of water to a secure disposal facility, sludge removal and capping contaminated areas with clay.
However, this treatment strategy is expensive, labour-intense and not generally capable of achieving the legislated soil and water limits for 2,4,6-TNT, of 33 mgkg and O. 1 ug/L
respectiveiy @". Many commonly used remediation technologies in this field are separation
processes rather than destruction processes. They include resin adsorption, surfactant
complexing, Liquid-liquid extraction, ultrafiltration and reverse osmosis "), all of which
remove the contaminants fiom soi1 or water, and concentrate thern into another medium for further treatment or for landfill disposal. These methods, however, do not mod* the
nitroaromatic expIosives to non-hazardous rnaterials-
5.2- 1 Granular Activated Carbon
Granular activated carbon (for review- see chapter two) is most effective as a means of polishing wastewater, as its high affinity for nitroarornatics will rernove even low concentrations, and because low concentrations extend the Me of the bed. It is used at almost ali ammunition plants to treat wastes as they are produced, except for the case of red water, for which it is unsuited. Once a bed is taken out of service, the GAC is classified as a K045 hazardous waste @PA) and must be tùrther treated ". The use of GAC to remove nitroglycerine renders the spent bed shock-sensitive and it must be treated as an explosion hazard. Options for the K045-GAC include disposal in a secure hazardous landnll, incineration, or regeneration.
GAC can also be combined with bioremediaton technology, for example the use of a three step GAC bioreactor in which the spent GAC column, loaded with nitroaromatic explosives, is taken off-line, inoculated with explosive-degradhg organisms and nutrients, followed by heating to 55 OC until aerobic transformation is complete "". The column is then washed, cooled, and set to adsorb again. Problems include the need for specialized organisms, the requirement to reacclllnate the column after each batch and the need for multiple columns; while one is off-line for regeneration, another must stay on heto adsorb compounds, and a thkd is needed in case breakthrough occurs.
Another example of a combined technology is a fluidized bed bioreactor (FBB) Ca' based on a GAC medium, GAC is an excellent material for the fluidized bed as it minimizes shock loads and offers a good growth medium. During operation, the FBB is heated to 35'~ and recycle hes circulate wastewater, buffer, oxygen, nutrients and a primaty carbon source through the bed "? A microbial biofïh on the GAC creates an anaerobic environment for reducing the nitroaromatic explosives to amines; this step is followed by an extemal aerobic process (activated sludge or rotating biological contactors) for complete mineralization. Advantages of the fluidized bed include efficient mass transfer and the prevention of the formation of thick biofilms (flow causes shea~g).Laboratory scale operations have shown successful removd of2,4,6-TNT and TNB to below detection lïmits "? Problems wïth this technology include retention of untreated nitrocompounds on the GAC, incomplete degradation of 2,4,6-TNT, and the requirement for added nutrients.
The concept of a combined GAC/microbiological process is an interestkg one, but it has a fimdamental problem As an ex-situ technique that combines two expensive treatments
(GAC and anaerobic treatment) it wiu probably never be able to compete with other treatment methods. 5.2.2 Advanced Oxidation Processes
Photolysis of hydrogen peroxide (UVO/H202)(for review see chapter two) was first applied to wastewater treatment by Koubek and subsequently for remediating nitroaromatic explosives R4! Photolysis is usualiy performed with low-pressure, high power lamps (6 to 60 kW) that have p~cipalem-ssion at 254 nm. The Iow molar extinction coefficient of hydrogen peroxide necessitates the use of high power Iamps and is a source of serious inefficiency. Lower power lamps are used in conjunction with ozone
whose extinction coefficient at 254 nm is much higher than that of hydrogen peroxide; against this, an ozone-generating system must be provided on-site, whereas hydrogen peroxide, a cornmodity chemical, can be shipped in. While a UV/03 system uses Iess energy, the gas generation associated with O3 may increase mass transfer of volatiles and semi-volatiles partitioning to air
tTVO/&02 has been used in batch or continuous modes to treat a wide range of
nitroaromatic explosive-contaminated water, including process waters and groundwater.
Destruction efficiencies of 99.9% have been achieved for destroying 2,4,6-TNT and 1,3,5-
TNB '2? Solarchem has successfuuy used W/&û2, marketed under the name ItayoxPDto
treat nitroglycerine for the US Navy Although UV0 is an effective (destructive)
treatment and has been proven at several test sites, it still lags far behind GAC in
irnplementation Gn.
WhiIe the use of UV0/H2O2is effective in elirninating 2,4,6-TNT fiom solution, it does not necessarily ameliorate toxicity. The chemical degradation of 2,4,6-TNT proceeds through a senes of steps resulting in the formation of trinitrobenzene -, which was
136 responsible for maintaining the toxicity of the solution as assessed using a 96 hour rainbow
trout LCSo- In order to elùninate the TNB the solution must be exposed to much higher
W doses. Solarchem has had some success with an UVfozone (Rayoxm-O)to overcome
this problem when treating 2,4,6-TNT contamuiated groundwater. However, the reaction
mechanism is not entirely clear as both ozone and H202fùnction mainly to generate OH-
radicals.
In dark photolysis (for review see chapter two) the use of Fenton's reagent is
common; this uses iron sdts instead of UV to activate the H202:
Hfiz + Fe2' -- HO + HO- + Fe3' H202+ ~e~~ - HO= + H++ ~e~*
Li et al "" studied the use of Fenton7s reagent, alone or in combination with W
(254 nm), in the rernediation of aqueous wastes that contain nitroaromatic explosives.
They found that the order of oxidation rates of a nitrotoluene solution was UVEenton at
pH 3.0 > dark-Fenton at pH 3.0 > UV/Fenton at pH 6.1 > UV/H202 > H202 only. The
UV/Fenton system was able to degrade 2,4,6-TNT (0 -31 rnM) wih1 hour, compared to 8 hours for a dark-Fenton system. When the experiment was nin continuously for 24 hr, the
UWenton system was able to minerfie 95% of the 2,4,6-TNT, whde the dark-Fenton was only able to mineralize 45%. Li et al @'also found that the reaction rate depended on the number and position of the nitro groups on the arornatic ring. Regardless of the experimental conditions (pH,
UV or no UV) the rate constants were approxïmately 30 times higher for 2-nitrotoluene than 2,4,6-TNT and 15 times higher for 4-nitrotoluene over 2,4,6-TNT. Similarly, 2- nitrotoluene was consistenly oxidized faster than 4-nitrotoluene, and 2,4-DNT was oladized faster than 2,6-DNT.
TiOz-assisted photolysis has been applied to explosives remediation by SchmeIling and Gray who used W radiation to mineralize 2,4,6-TNT. They used a wavelength of
340 nm with an aqueous particulate Ti02 slurry and were able to mineralize more than 90% of a 0.2 mM solution. The nitrogen hmthe 2,4,6-TNT was converted to ammonium ion
(35%) and nitrate ion (55%). In a later paper they showed that the disappearance of dissolved organic carbon was slower when a positive bias was applied to the semiconductor, confirming that the overall reaction mechanism must involve both reductive and oxïdative steps 'la.
Ozonation technologies can be employed with or without UV photolysis and with or without Hz02. The high capital cost of the ozonation unit makes ozonation technologies best suited to large-scde applications. In the case of nitroarornatics, ozonation alone is rather inefficient as OW production is low and because the nitro groups deactivate the substrate towards electrophilic substitution by ozone. Beltran et al "O' found that the optimum conditions for this mode of ozonation of nitrobemene and 2,6-DNT occurred at temperatures < 30 OC and pH 7-9. Ozonation was alrnost completely quenched in the presence of hydroxyl radical scavengers, indicating that attack of OH. on the substrate is the principal reaction pathway. In later work these authors used a peroxone (W2+ 03) system with or without UVC photolysis, in order to promote radical formation- These additional treatments increased the rate of destruction of 2,6-IdNT but not that of nitrobenzene "? Under ail conditions the p~cipalozonation products were nitrophenols fkom nitrobenzene and dinitrobenzaldehydes Eom DNT.
Aminodinitrotoluenes (ADNTs) are commonly encountered contaminants associated with 2,4,6-TNT. Spanngord et al "') oxidized 2-amino4,6-dinitrotolene and 4- amino-2,6 dinitrotoluene separately and together with hy drolcyl radical (derived fkom
Fenton's reaction) and ozone in order to evaluate the effectiveness of a peroxone system.
They found that although the rate constants for oxïdation are four orders of magnitude larger for hydrolryl radicals than ozone, it is ozone which is the dominant oxidant for
ADNTs. The chernical efficiency was increased f?om 3.6% (10 ppm Hz02 / 7 pprn 03) to
13.7% when O3was used alone. In a later paper they showed that ADNT undergoes a 1,3- dipolar cycloaddition with O3 leading to the formation of glyoxlic and pyruvic acids and
NO; and NO3- as products "3). Since hydroxyl radicals have been shown to play a significant role in 2,4,6-TNT chernistry and ozone is important in the degradation of
ADNT, it is important to continue to pursue research in areas which combine these two techniques.
Regardless of the UV0 technique, costs will depend on both the identity and the concentration of the contaminants, as weil as the degree of destruction required. If pretreatment to remove turbidity is necessary, this will raise costs. There is no clear-cut cost advantage to either GAC or UV(H2Oz or 03) for treating nitroaromatic expiosive- contamùiated water, GAC costs ranged fiom $0.03 - $3.00 per m3 while UV0 was in the range of $0.32 to $1.70 per m3 -'. However it is fair to say that at lower contaminant concentrations in wastewater, GAC tends to be the more cost-effective technique.
5.2.3 Hydrothermal
Hydrothermal chemistry uses high temperatures and pressures in the presence of oxygen or Hz02 to mheralize organics. These technologies include wet air oxidation and supercritical water oxidation. In the NitRem system, for example, &ûz and ammonium carbonate are added to contaminated water and then the muchire is subjected to near- critical conditions (350 OC, 3500 PSI). This system was evaluated at the Radford army ammunition plant for the destruction of DNT and 2,4,6-TNT wastes, using a 0.40 GPM pilot plant with a 16 min residence tune O". DNT concentrations were reduced fi-om
128,000 ppb to 5 ppb (99.996% removal); simultaneously the chemicd oxygen demand of the solution was reduced fiom 350 to 65 mg/L and nitrate was reduced f?om 243 to 4.4 mg/L. Molecula. nieogen, water and carbon dioxide were formed as products. Problems with this technique include energy intensiveness and NOx emissions; previous problems with corrosion of the pressure vessels were avoided by using a dual shell reactor O".
5-2.4 Reduction
Aromatic nitro compounds are easily reduced to the corresponding anilines in acidic solution, using cat alytic hydrogenation. Although noble metals such as palladium are used in laboratory applications, they are too costly for waste treatment moreover, it has been found that in certain instances the catalyst dissolved in the presence of nitroarornatic explosives "? Materials such as C, Fe, and &O3 can be combined with Pd to overcome these problems ""? Other catdytic materials include WdZeoliteY, NdSiOî and Raney Ni
"" Hydmgenation in itself is not a compiete remediation method because the anilines are themselves toxic and must be treated Mer, for example by oxidative polyrnerization which causes them to precipitate, or by biological transformation.
Malyala .ad Chaudhari ("' hydrogenated 2,4-DNT using 10% Ni supported on
Zeolite Y. The reaction tended to zero kinetic order at high organic and & concentration, indicating that the reaction was not mass transfer limited; instead, the adsorbed 2,4-DNT was proposed to react with dissociatively-adsorbed hydrogen in the rate-limiting step. By contrast, Pd-catalysis is proposed to involve hydrogen transfer fkom an intermediate Pd hydride. Rajashekhararn et al '42' hydrogenated 2,4-DNT with a similar catalyst, ushg a trickle bed reactor at 45-SOC, and developed a mode1 to predict hydrogenation rates at difrent particle sizes and gas/liquid velocities.
Hydrotherrnal and reduction treatrnent of nitroaromatics seems to ody be of modest practicality, they are ex-situ batch process, which are expensive to operate and do not provide complete destruction.
5.2.5 Bioremediation
As already mentioned, nitro compounds are resistant to oxidation and exhibit toxicity to the microorganisms, but can be easily reduced, since both aerobic and anaerobic organisms contain non-specific nitroreductase enzymes. Research has focused on isolathg bacteria capable of degrading nitroaromatic explosives; these include both gram positive
and negative bacteria and thermophilic organisms (55-65 OC) (43,W
5.2.5.1 Aero bic
An imporîant strategy is to promote the growth of naturally ocnirïing consortia that cmdegrade the target contaminants, rather than to select or engineer specsc strains, because engineered straks tend to lack survivability in the field, whatever their promise under laboratory conditions. Lendenrnann et al '455'found that single aero-bic bacterial strains were unable to degrade multiple DNT isomers, whereas a mixed consortium was successfid in this respect; using a fluidized-bed bionlm reactor, they achieved removal efficiencies of >
94% at a Ioading rate of 36-600 mg nf2 d-'.
Because of the recalcitrance of nitroaromatic explosives towards oxïdation, a
comrnon approach is to stimulate their metaboiism through the use of an organic co-
substrate ''? Simple aromatics such as benzene and toluene did not enhance
biodegradation. However, the addition of glucose or citrïc acid at 3040 mg/L increased
the rate of metabolism of 30 pprn 2,4,6-TNT fiom 9% over nine days, without a co-
substrate, to complete degradation within 24 hours and in 72 hours for a 100 ppm
solution. Boopathy et al '4n used an aerobic/anoxic soil slurry reactor with mi>ang and air
dfision to remediate soil contaminated with 2,4,6-TNT and 2,4-DNT using 0-3%
molasses as a CO-substratewith 2,3-butanediol as the major product. Radiolabelkg of
2,4,6-TNT showed 23% degradation, 27% conversion to biomass, 8% adsorption to soil
and the rest accounted for as metabolites. The degradation of 2,4,6-TNT is complicated
by its transformation to amines under anaerobic and aerobic conditions. These amuies (including trïaminotoluene) often undergo transformation to azo, azoxy-, acetyl-, and
polymer derivatives and as such are prevented fiom undergoing rnineralizabon '48'. This
has been a major problem in developing an enective method for TNT bioremediation (4?
Residence tirne in a soi1 sluny reactor can be minimized by the addition of a food grade surfactant, this has the dual ability of increasing bioavailability and provide a carbon
co-substrate source. Boopathy and Manning (49' used Tween 80 to enhance the
minerakation of 2,4,6-TNT in a soi1 stuny (35 days fiom 45 days) however degradation
products were still present after 60 days. When 0.3% molasses was added to the reactor as a CO-substratethe products were undetectable after 35 days.
5.2.5.2 Anaerobic
Kwon and Yen '"' used strict anaerobic conditions to degrade 2,4,6-TNT through mixed spe&c reductases obtained fiom a digested sewage. The four main metabolites were 2-hydroxylamino-4,6-dinitrotoluene, 4-hydroxylamino-2,6-dinitrotoluene, and the fùrther reduced 2-amino4,6-DNT and 4-amino-2,6-DNT, of which 2-amino-4,6-DNT was the major product. They also found that the various reducing bacteria (sulfate, nitro and methanogenic) were able to effect mineralization through stepwise deamination fouowed by ring cleavage. While this is an important development, the reaction only proceeds at low efficiencies and more work is need to optimize this process.
Knimholz et al "') compared the degradation rates of 2,4,6-TNT by sulfate, nitro and methanogenic reducing organisms under bioslurry conditions. The sulfate reducers were slightly more active than the nitrate reducers while the rnethanogenic reducers were approxïmately six times more active. The pathway for degradation in al1 cases was reduction to 2-amUi0-4~6-DNTthen 2,4-diamino-6-nitrotoluene, which was firther reduced by methanogenic and suwate reducing bacteria, but not by nitrate-reducing bacteria- In contrast, 2,4- and 2,6-diaminotoluene were degraded by nitrate reducing bacteria but not by the sulfate or methanogenic strains under the same conditions.
Lenke et al "" used an anaerobic sludge reactor to treat a mixture of compounds
(2,4-DNT and 2,4,6-TNT) with sucrose as a CO-substrate. The reduced products were observed to bind irreversibly to the soil, and were mineralized subsequently under aerobic conditions following dewatering. The combined treatment removed > 99% of contaminants, with no evidence of residuai toxicity.
However, in achiai in-situ situations it is impossible to separate oxidation and reduction processes. For example, Bruns-Nage1 et al '*"found that the major product fiom
2,4,6-TNT at a former ammunition plant in Gerrnany was 2-amino-4,6-diaitrobenzoicacid, in which one nitro group had been reduced and the methyl group had been oxidized. The authors were unable to determine whether the mechanism was chemical, photochemicai, biological or some combination.
In a laboratory study using microorganisrns that were indigenous to an explosives- contaminated site, 28 days of aerobic treatment showed that 67% of "C labeled 2,6-DNT was unchanged, 14% was reduced to 2-arnino-6-nitrotoluene, and 8% was rnineralized to
CO*. With labelled 2,4-DNT, 20% rernained unchanged, 22% was reduced to 4-amïno-2- nitrotoluene, 6% to 2-afnino-4-nitrotoluene and 28% was mineratized '54). It has been shown that nitroaromatics can undergo a combineci aerobic and anaerobic process, in which the reduced products such as 2-hydroxylamino4,6- dinitrotoluene, 4-hydroylamino-2,6-dinitrotoluene, 2,4,6-triaminotoluene and azoxy compounds interact with soi1 fiactions '". Under these circurnstances Achtnich et al found that 98% of the products (measured as "c)were bound irreversibly to soil components such as humic and fülvic acids ? Afker binding to humic acids the arornatic amines are transfomed to tertiary amines and amides. The amide can then undergo hydrolysis resulting in ammonia formation
Inorganic electron donors may also be used as CO-substratesfor the anaerobic biodegradation of nitrotoluenes. Sade (O mM - 4.2 mM) in the presence of different minerais (0.42 rnM -0.84 mM) was used in this capacity by Cheng et al to assist the ariaerobic reduction of 2,4-DNT at 35 OC. They found that the rate and extent of reduction were dependent on the suEde concentration and that suifide preferentiaily reduced the 2- nitro group compared to the 4-nitro group in a ratio of 2: 1. Addition of nickel, iron or cobalt &de minerds enhanced the degradation rate of 2,4-DNT but instead drove the reduction to 4-amino-2-nitrotoluene. Manganese and copper sulfides also enhanced the selectivity of the products (to 4-amino-2-nitrotoluene) but did not increase the rate of reduction, whereas calcium, zinc and magnesium inhibited 2,4-DNT reduction. Heavy metals such as iron lead, and zinc were found by Roberts et al to have Little eKect on the rate of anaerobic biodegradation of 2,4,6-TNT in soil slurry reactors. However, the removal of 2,4-diamino-6-nitrotoluene or 4-amino-2,6-DNT was affected by some metais- 145 Iron had no effect, lead and zinc delayed the degradation of 2,4-diamine-6-nitrotoluene while the presence of copper (lowest concentration used was 4 mg/g soil) completely prevented the removal of 2,4-diamino-6-nitrotoluene but not 4-amino-2,6-DNT- However, at copper concentrations above 8 mg/g soil, the removal of any compounds was prevented, presumabIy because the copper was toxic to soil microorganisms,
5.2.5.4 Biodegradation Products
In any remediation program it is vital to determine whether the biological degradation products themselves are toxic- Dodard et al "O' used the ~icroto>Passay to study the degradation products of DNT isomers. This assay employs the naturally luminescent marine bacterhm Vibrio fischeri; substances that are toxic to this organism inhibit its light production in a dose-dependent mamer- In the case of 2,4-DNT the monoamines showed greater toxicity than the parent compound but the diamine was less toxic, whereas for 2,6-DNT ali products were less toxic. Honeycutt et al '"' studied the cytotoxicity of 2,4,6-TNT degradation products towards rat hepatoma H4IIE cells and
Chinese hamster ovary ceils, and found that of five degradation products studied, only 4- amino-2,6-d'îtrotoluene was less cytotoxic than 2,4,6-TNT itself. The implication of greater toxicity among the metabotites was demonstrated by Roberts et al '59), who found that spiking a 2,4,6-TNT containhg solution with eit her 4-amino-2,6-DNT or 2,4-diamino-
6-nitrotoluene inhibited 2,4,6-TNT degradation. 5.25-5 Field Application
Bioremediation can be carried out in-situ or ex-sihi, Ex-situ methods include windrow composting, in which the waste is placed in long pires and then mixed using conventional agrkultural equipment. The contaminated soiI is screened to remove large rocks and debris, followed by mixïng with organic materials such as manure, straw or alfalfa as a bulking agent and carbon source. The windrow piles are tumed regularly to control heat transfer and aeration, and monitored for rnoisture content, oxygen level, pH and temperature- Wmdrow composting was used at the Umatilla anny depot in Oregon (a
Superfùnd site), where it achieved removal rates of 2,4,6-TNT, up to 99.7%, using a contartinated/regular soil mixture (30:70). Aiter 40 days of operatio- 80% of samples demonstrated non-detect levels of nitro compounds &er remediation. '60'.
Static pile composting involves aerating extracted soil in piles using fans or
blowers. In the biosiurry method (dso cded vesse1 cornposthg), a soil-water slurry is
mechanicaiiy agitated and aerated in the bioreactor. Bioslurrying does not require the
addition of carbon sources, and ofien offers a faster and more complete degradation than
other biologicai processes Against this, the technique requires more expensive
equiprnent, including a means of agitation, and greater process control. Research has been
carried out into the use of large rotating drums to rninimize the energy requirements of
mixîng @*! BiosIurry was s~ccessfkliyused at the Joliet army munition plant and at the
Iowa army ammunition plant where molasses was added as a CO-substrateand intermittent
aeration was used to cycle between aerobic and anaerobic conditions. This process was
able to successfLUy degrade 99 +% of 2,4,6-TNT, '63'. The US Army Environmental Center has estunated costs of several remediation processes for volumes of contaminated soil > 10,000 m3. W~ndrowcomposting was least expensive, with overall costs ranghg fiom $250 to $300/ton, bioslurry costs were found to range from $230 to $270/ton with dl prices including the costs of excavation and compost disposal '5? Both processes were much cheaper than incineration ($740/ton). Both composting and bioslurry are ex-situ methods, which are therefore more expensive than in- situ ones such as natural attenuation and phytoremediation. A disadvantage of composting is that it requires large amounts of additives or buking agents (straw and animal feed) and as such only a srnall fraction of the total volume composted is contaminated soil.
5.2.5.5 Phytoremediation
Phytoremediation involves the use of green plants to remediate contaminated soil or water '? This technique shows promise as it is an inexpensive, low maintenance technology that can usually tolerate higher concentrations of contaminants than microorganisms. While there are already 500 constnicted treatment wetlands in Europe and 600 in North Amerka, its use is still in development and is not universally accepted by regdatory bodies. Phytoremediation can be accomplished by several means; in phytoextraction contaminants are accurnulated by bioconcentrating them in the harvestable zones of the plant, in contrast phytostabiliion reduces the bioavailability of the pollutants by binding them in plant tissues. In phytodegradation the enzymatic systems and microorganisms associated with the plants degrade the toxic compounds, while phytovolatilization uses plants to voltatilize pollutants. The root system can also play an active role in remediation, rhizonltration refers to the adsorption and absorption of pollutants via this route '?
Plants can uptake chernicals fiorn the vapour, liquid and solid phases, however, the movement of the organics within the plant usually occurs through the liquid phase.
Chernicals most likely to be taken up are those with octanol water partition coefficients
(Log &) that are between 0.5 to 3.0 (Nitrotoluene - Log K, 2.37, 2,4-DNT - Log K,
1.98) "? Uptake efficiency can depend on several factors including pH, pKa, organic composition of the soit its water content and plant physiology There has been some success in using synthetic (Triton X-100) or naîurally produced biosurfactants
(rhamnoiipids) to enhance the water solubility and hence uptake of contaminants '? Once the compounds are taken up by the plant they can be degraded, stored (majority of which usually remains in root system), volatilized or mineralized.
In phytodegradation, the secretions of plants (especialiy enzymes) can help degradation of contarninants, enzymes produced include laccase, mono/di-oxygenases and nitro reductase enzymes. These enzymes cm be found in a variety of living species including spinach, funW yellow nutsedge, bush beans, bacteria, garden vegetables and buttermilk '68p69). For nitroaromatics the enzymes responsible for degradation were shown to include nitroreductases and laccases ('O1.
Schnoor et al were able to show that the broken ring structures were either used for new plant matenal or organic detritus. Secretion of sugars, alcohols and acids can also support the growth of rhizosphenc bacteria around the root system, whose levels can exkt at levels of two to four orders of magnitude compared to populations in the surroundmg bulk sol Besides acting as building blocks for the bacteria, the secretions can be cornetabolites for the rhizospexic enzymes. The rhizosperic bacteria/fùngi can enhance degradation of contaminants by humidScation of the organics or secreting enzymes (e-g- peroxidases) "? An accelerated removal of TNT nom an active rhizosperic zone has been obsenred with the use of prairie gras
Scheidemann et a! explored the use of eleven plant species during an eight week growing season to degrade soi1 contaminated with various levels of 2,4,6-TNT. Extraction of the plant roots showed that at low 2,4,6-TNT concentrations (10 mg/L) Medicago safiva had the highest level of nitroaromatic uptake, while at high levels of soi1 contamination (500 mg/L) only one species Phaseolus wdgms was able to grow. The majority (> 95%) of the nitroaromatic was detected as amino products and not the parent
TNT. However it was unclear if this transformation occurred prior to or after uptake ")-
Most 2,4,6-TNT phytoremediation studies have focussed on the reductive processes and have shown the formation of 2-arnino-4,6-DNT, 4-amino-2,6-DNT amino which may be preceded by conjugates of these compounds (='. The oxïdation products, which include transformation of the ring rnethyl group and hydroxylation are a hction of mixed function oxïdases, have been largely unstudied. These compounds have been shown to include 2-amino46-dinitrobenzoic acid, 2,4-dinitro-6-hyroxylberizyl alcohol, 2-N- acetoxyamino-4,6-dinitrobenzaledehyde, 2,4-dinitro-6-hydroxytoluene and two binuclear metabolites. The importance of determining the oxidation products with respect to the overall toxicity and metabolic pathways of Zh6-TNT phytoremediation is demonstrated by the fact that they were shown to account for 41 -5% of ail products ? Sens et al n4' used 14c-radiolabeling to study the uptake and metabolism of ZA6-
TNT in Tririnun aestiwm (wheat). Uptake resulted in the partitioning of 43% 14c in the cytoplasm and 57% in the ceil wail. Within the cytoplasm the TNT metabolites consisted of three unpolar and 10 polar compounds, the majority of which couid not be identified. In the cell wall 27% of the 14c was found bound to the lignin fiaction and 5% to the pectin fiaction. Studies with bushbean had a shuwed a sùnilar distribution however, identified products consisted of 10 unpoïar metabolites and a large number of polar metabolites.
Phytodegradation has been proposed as a remediation method for nitroaromatic explosives that may be able to compete with GAC adsorption. In a pilot study, the
National Defense Center for Environmental Excellence (NDCEE) constructed a 9,600 gallon indoor wetland-lagoon, using the several aquatic plants: M~ophyylumupaticzïm and Elodea. The contaminated waste ffowed through the wetiand (which acts as an equaliz;ition vessel) where the residence time was forty days- Du~gtfüs time the contaminants were transformed into cornplex iignùi-like compounds, and showed no bioaccumulation in the plant tissues. The authors compared the Life cycle costs (over 15 years with no inflation) of this system to a GAC-FBB and found for the treatment of 4 m3 of waste the wetland scenarïo was slightly more cost effective ($84.55 aquatic plants;
$87.63 GAC-FBB).
A full-scale project at the Iowa Army ammunition plant ''' involved the use of a constructed wetland. This was used as a polishing treatment for an upstream remediation technique, which was only able to rernove 95-99% of the original explosives (2,4,6-TNT).
Plants used for remdation consisted of three aquatic species; pondweed, arrowroot and coontail and one non-aqueous species (poplar trees), which were planted around the edge of the wetiand. The created wetlands were able to degrade up to 0.019 mg/L TNT per day-
Although plants are effective remediators due to their large amount of bimmass, their abilities (per biomass unit) are Iess than bacteria. In order to maximize the strengths of both techniques French et al 06', used a transgenic technique to add a gene within tobacco plants that expressed a denimg enzyme (pentaerythritol reductase). When grown in
Liquid medium containing 2,4,6-TNT, wild type tobacco seedlings were unable to resist a concentration of 0.025 mM, whereas the transgenic Iine was able to be grown norrnally at
0.05 M. Importantly, the transgenic plants were able to denitrate glycerol trimitrate, producing dinitrate and mononitrate. This ability is of particular sign.ï£icance because ot her enzymes capable of degrading TNT give only partial reduction of nitro groups, yielding compounds that are toxïc and resistant to additional degradation.
Phytorernediation, like bioremediation, suffers fiom unpredictable chate variations. However, since substrates are degraded by the products fiom rnulticellular organisms it is much more diflicult to understood and predict behaviour and the to completion ". In a technique such as phytoremediation, it is important to detedethe short and long term fate and toxicity of any metabolites. This is a very important point, as in a recent study most of the transformation products of TNT with Myriophyll~m~icaturn were unidentifïed.
Phytoremediation's mggedness with respect to physical conditions anzf reslstance to higher levels of contaminants can offer a trempdous potentiat over micro organisms, hewever, major issues with the technique remah ta be addressed. nlese kclude a better understanding of the specïficity of genes, their isolation and recombination into species which have other desirable characteristics such as growth rate, root mass and hardiness.
This technology shodd move away f?om trial and error plant analysis to a more mechanistic approach including mass balance, metabolites and pathways such that fùture processes can be modeled and stredined. One drawback is that a portion of the uptaken compounds and their metabolites have been shown to partition into the upper parts of the plants including edible fractions (e-g. leaves, produce) and seeds '74', clearly the impact of this on other species must be cleary identifïed. However it's believed that phytoremediation will have an important role in remediation/reclamation, as a polishing step for sites which have undergone more ngorous treatments.
Electrochemical remediation of explosive wastes is a treatment option that offers the possibility of high energy efficiency and relatively simple equipment "". To date there has been only one report of the use of this approach with nitroaromatics (7979'.In this report the nitroaromatics were reduced at the cathode, but there was no discussion of products, mass balance or optirnization. In this next chapter the electrolysis of 2,4-DNT, 2,6-DNT and 2,4,6-TNT is explored, addressing some of these details previously unstudied. -- -
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DAAHO 1=9 1CO73 8 1992. CHAPTER 6
6.1 INTRODUCTION
The material presented in this chapter is related to understanding the mechanistic pathways of nitroarornatic reduction and the associated thermodynamic/kinetic conditions. The latter will be examined by voltammetry, to determine at what potentials the cathodic reaction proceeds.
6.2 LITERATURE REVIEW
6.2.1 Nitroaromatic Reduction
6.2.1.1 Nitroaromatics
It has been known for decades that organic nitrocompounds can be reduced electrochemically ('"). Nïtrobenzene was among the first organic compounds to be studied by polarography (for a review of polarography see section 3.1.2) When nitrobenzene undergoes reduction in aqueous solutions at pH 0-12 it demonstrates a singie irreversible 4-electron wave (at low pH, the wave may be increased to 6 electrons).
In linear sweep polarography, Ela is more negative at higher pHs (- -0.2 vs. SCE at tow pl3, to - 0.75 V vs. SCE at high pH) (4! In order to determine the mechaaism of reduction, researchers have used radical detection techniques such as electron spin resonance @SR) and electrochemical methods such as ring disk electrodes @DE) to detect intermediates
C5.6)
The reduction takes place in several steps within a process that is both electrochemical and chernical- Nitrobenzene is first reduced to a radid anion (E), followed by rapid protonation (C), reduction of the resulting radical (El7&en formation of Ntrosobenzene (77g.80). Nitrosobenzene is reduced more easily than nitrobenzene and thus it is immediately Mer reduced at the surface of the electmde forming phenylhydroxylamine (PHA), which can be isolated under acidic or neutral solutions, however under very strongly acidic conditions or at high temperature> rearrangement occurs to form p-aminophenol.
Laviron and Roullier ('O) have represented the mechanism as an e-,H~, e- followed by an e-, H', e-, H' sequence (see Figure 6.l). Figure 6.1 The reduction of nitrobenzene to phenylhydroxylamine
At low pH, phenylhydroxylamine is Merreduced to aniline, which is apparent by a second (although less well defined) 2-electron wave at more negative potentids.
However in the absence of surfactants or organics the nurnber of electrons in the first wave varies between 4-5, but the sum of the two waves always remains fixed at 6. A possible reason for this dichotomy is two competing reactions, with one being the 4- electron reduction of the nitrobenzene to protonated phenylhydroxylamine (Pm) (Arrn20E3) foiiowed by its 2-electron reduction at a more negative potentïai (- -0.7 V
vs. SCE) to aniline (Il). The other mechanism is a coupling of nitrosobenzene with PHA
to azoxybenzene which is merreduced to hydrazobenzene and then to aniline (12!
When the PHA is unprotonated (ArNHOH) it cm act as a nucleophile in solution, therefore the ability of the wmpound to act as a nucleophile decreases at Iow pH (13).
Azoxybenzene
Hydrazobenzene
Figure 6.2 Dimeric reductioo products of nitrobenzene This second wave dïminishes with increasing pH and disappears at pH 2 5 ''" when the reduction will only be 4e- overall. As acids are usuaUy more easily reduced than their conjugate bases, the pH effect is probably related to PHA's pKa Evidence to support this is given by Bergman and James who determineci the pKa to be 3.2 "".
Reduction products can be afkcted by the presence of other nucleophiles. For example, durhg the rduction of phenylhydroxylamine, the Armintermediate can be attacked by nucleophiies that include chloride and water. Sokolskii et al found that in the presence of 4 M HC1 a mixture ofp-, o-chloroaniline and p-aminophenol was formed in a
3: 1: 1 ratio fiom nitrobenzene. However in sulfuric acid the hydrogen sulfate ion was too weak a nucleophile to compete with water so the oniy product fomed was p- arninophenol('*).
The course of the reduction is different in aprotic solvents such as acetonitde and
DMF; nitroaromatics are reduced to their corresponding anions in a one electron reversible step and at more negative potentials to the dianion. If protic impwities are present, Ureversible reduction to phenylhydroxylamine occurs in a 3-electron step, otherwise the anions are stable (16,'?
EaIcohol is present in an aqueous solution its adsorption to the electrode surface simulates an aprotic environment, hindering reduction. Evidence for this includes that the interference effect depends on the ease of adsorption of the alcohol (MeOH < EtOH c isopropanol). The behaviour of the nitrobenzene can be electrode dependent, with CV's for some metais displaying reduction to PHA, oxidation to nitrosobenzene and on the second sweep, reduction of nitrosobenzene to aniline. Glassy carbon has shown a similar pathway, however accompanied with a substantial shift in potential, but the nitroaromatics were not reduced at platïnum '? In a study in 15% methanol, gold, and silver electrodes showed two waves (1 and 3-electron); however a chrome-nickel steel electrode produced only a single 4-electron wave (19).
At cathode materials such as Raney Ni, Devarda Cu and Raney Co, amuies are formed in high yields in neutral and acidic solutions "". Since pH control is not necessary in these reactions it has been suggested that the amine formation is via electrolytic hydrogenolysis and not reduction (21).
Holleck et al (22) examined the reduction ofp-nitrotoluene (15% methanol, 0.1 M
NaOH) at different electrode materials and found reduction potentials to be within O. IO V of -0.715 V vs. SCE for gold, silver and mercury; reduction at platinum was not observed.
6.2.2.2 Reduction of DNT, TNT
Dinitroaromatics reduce in a similar manner to mononitroaromatics, but in a stepwise manner, with two distinct 4-electron waves. Complete reduction of one nitro group to the amine occurs prior to reduction of the next. A general rule appears to be that it is more difficult to reduce a nitro group when it is ortho to a methyl group. This is mostly likely due to steric hindrance, because the methyl group twists the nitro group out of planarity (? In the reduction of 2,3-dinitrotoluene, the fist step is the formation of 2- nitro-3-hydroxylaminotoluene while the reduction of 3,4-dinitrotoluene produces a mixture of 3-hydroxylamino4nitrotolueneand 4-hydroxylamino-3-nitrotoluene.
As with nitrobemene, more negative potentiais cause fùrther reduction to the amines (24*25). This is a result of each amino group stabilizing the compound (ortho, para) or making it less electrophilic (meta) and therefore more difficult to be redüced. This can also be observed in TNT, with the reduction of 2,4,6-TNT to 2,4,6-triamixiotoluene, where the most difficult step is the reduction of 2,4-diamino-6-nitrotoluene(2?
DifEerent electrolytic conditions in the reduction of dinitrotoluene can lead to a number of products including amines, hydroxyfamines, aminophenols, azoxytoluenes, azotoluenes, hydrazotoluene and benzidine; examples of the monomenc products are shown in Figure 6.3 (27).
Polarography has also been used for the analysis of samples containing TNT,
RDX and HMX, In 1963, the US Naval ordnance test station evaluated the use of polarographic methods to replace infkared (IR) spectromeûk analysis in the detection of explosive materials in warheads 08). In lithium chloride solutions LSV's of TNT showed a peak potential (i,) with a maximum at 4-66V. In Nmo3 - borax electrolyte, i, was observed at a more positive potential (-0.55 V) and was not as distinct- Other electrolytes such as acetate buffers and HC1 did not exhibit sufficiently defined LSV's for accurate analysis. The TNTLiC1 system had Merreduction waves at more negative potentials due to ArNHOH reduction but the -0.66 V wave had an i, more than 2.5 times greater than any of the others. The polargrams for TNT could be resolved fiom mixtures with other nitroaromatics even at 5050 ratios. m+m2 -1 -0 V (SCE), 20" C
+0.05 V (SCE), 80 OC . 10 N H2S04 - EtOH (1 :2) HO
-0.25 V (SCE), 80 OC 5 NH2S04 -EtOH(l:I)
Figure 6.3 Electrolysis products of 2,6-dinitrotoluene at diflerent potentials, temperatures and solvent mixtures.
The reduction of polynitroaromatics has been studied by other researchers such as
Hetman '29) and Pearson 0°) under different conditions than Whitnack A sumrnary of the results is shown below in Table 6.1, Substrate ~hitnack"'~" etm man^^^' ~earson~~~'
TNT -0.66,-0-83,-2-03,-1,47V
L t " 25 % acetone, 75% 0-1 M LiCl. 30 ml pyridine, 7 ml 1 M KN03, 35 ml 2 M N&No3,28 ml Hfl, pH 7.6
5% ethanol, 95% 0.01 M potassium hydrogen phthalate - El,pH 2.5.
Table 6.1 Cornparison of nitroaromatic potential peak values at a DME from different researchers under different conditions (vs. SCE)
Despite many fundamental studies of the electrochemistry of aromatic nitrocompounds, there seems to be no mention in the fiterature of adapting this concept to waste remediation, However, we can see fkom Table 6.1 that the reduction potentials are very specific to experimental conditions and therefore it was necessary to carry out our voltammetry experiments prior to electrolysis.
The formation of aniline requires weil-defined conditions of acid concentration, cathode material, potential and in certain circumstances stirring 0? These limitations will be important to consider in Chapter 7 when we explore electrolysis. Substrates were nitrotoluene (NT), 2,4-dinitrotoluene (2,4-DNT), 2,6- dinitrotoluene (2,6-DNT), and 2,4,6-trinitrotoluene (TNT). Chernicals were obtained f?om Aldrich (2,4-DNT, 2,6-DNT), BDH (NT), and Chemservice (TNT), Chemicals were used as is, however purities of al1 compounds were verified with HPLC to be greater than
98%.
6.3-1 Voltammetry
6.3.1.1 Polarography
Reduction potentials were determined using a dropping mercury electrode @ME) using a &idiometer@polarographic andyzer with a platinurn bulb as a counter electrode and a SCE as the reference electrode. Solutions were purged with nitrogen and experimental conditions for diEerentia1 pulse polarography @PP) consisted of the following;
Purge time = 1000 s Initial potential = O mV Final potential = -2200 mV Step duration = 0-4 s Step amplitude = 1 mV Pulse duration = 40 ms Pulse amplitude = 25 mV
Results for the reduction of selected nitroaromatics using DPP are shown versus
SCE, in Figure 6.4. As observed previously (Table 6-1) that the number of peaks corresponds to the number of nitro groups wîthin the target substrate. As TNT has three nitro groups the compound is quite electrophilic and the first reduction potential is observed at -0.18 V vs. SCE and the second peak is slightly more negative at 4.35 V- In
cornparison, the fist peak for DNT (which was run under acidic conditions) has shifted to more positive potential as per the discussion in the literature review. Mer the second
peak, there is another peak which is poorly defineci and represents the reduction to the
amine. The relationship with respect to concentration/current was also obtained for each
substrate and was liiear over an order of magnitude (not shown). The number of electrons involved for each reaction can be calcuiated fTom a cornparison with the reduction of ferrocyanide wkch is a weil known 1-electron reactioa In Figure 6.4 ferrocyanide is the broader single peak and has a peak height of 1.15 CLA_ From this we determine the number of electrons for the nitroaromatics as [(NT - 4-36)> (2,6-DNT - 3-4,
4-9, (2,4,6-TNT - 3.49,3.49,3.94)]- Figure 6.4 DPP scans of NT (0.489 mM, pH 4.5), 2,6-DNT (0.35 mM, pH 2) and 2,4,6-TNT (1.1 mM, pH 4.5) and 1.4 mM Ferrocyanide (FECN) in 0.1 M Hg04 reported with potentials vs. SCE.
6.3.1.2 Microelestrodes
As a dropping mercury electrode can not be used as an electrode in indu-, we reaiized it was also necessary to evaiuate more practical electrodes. In order to bridge the
DME results to industrial type electrodes we first evaluated a sessile mercury drop
(SMDE) as the WE, this is similar to DME except the electrode sufiace is static and is therefore not refkeshed (Figure 6.5). The sessile electrode consists of a shaped glass tube at which on end sits the mercury &op, which has been dispensed with a syringe. Figure 6.5 Schematic of a sessile mercury drop electrode (SMDE)
Voltammetry &SV) was performed with an EG&G Princeton Applied Research
iostat (mode1 273) using a sirnilar set-up as that used in chapter 2 @uk solution,
CE = platinum sheet (3 cm2), RE = saturated Ag/AgC1 electrode, purging was with argon
for 5 minutes.
6-3.1.3 Scan rate
The LSV of the nitroaromatics were recorded using 0.1 M -PO4 buffer
adjusted to pH 2 with H#04 as an electrolyte and the effects of scan rate are presented
below in Figure 6.6 and show how peak current is proportional to sweep rate and therefore there is no electrode fouling. Figure 6.6 Reduction of 3-25mM 2,4 DNT at different scan rates
6.3.2 Effect of pH
Further experiments evaluated different substrates in a phosphate buffer at pH 2 and pH 5. These are show in Figures 6.9 and 6.10 respectivefy. Similarly to the results presented in Table 6-1 Iower pHs require a less negative poteda1 for reduction
[(nitrobenzene -200 vs. -450 mV), (2'6-DNT -180 vs. -230 mV) and (TNT -100 vs. -
200 mV) vs. SCE].
These are in good agreement with Pearson's (acidic conditions) values in TabIe
6.1. Therefore we have confirmeci the potentials at which to conduct electrolysis. O -100 -200 -300 -400 -500 400 -700 800 -900 -1000 -1100 mV vs SCE
Figure 6.8 The reductioa of oitroaromatics with SMDE at pH 2 (1 mV s")
Figure 6.9 The reduction of nitroaromatics with SMDE at pK 5 (1 mV s-l) This agreement wïth Literature values was also found at pH 7 where a - 0.1 V
more negative shifi of the peaks was typically seen (not shown). The reproducibility of
vo1tammograms with the same electrode der several mns dernonstratecl that there was
no fouling,
6.3-2.1 Electrode Material
Several cathodes that are regularly used in industry were assessed with 2,6-DNT
as a substrate; they included iron (Fe), stainiess steel (SS), reticulated carbon (RTC), lead
(Pb), cadmium (Cd) and nickel (Ni). Results are presented in Figure 6.10. Fe and RTC
did not exhibit any peaks, the RTC also exhibited a very large current (12 mA at -1000
V) with no apparent response fkom the 2,6-DNT reduction. The results show that Pb and
Ni appear to be the two best materials for the reduction as they have the highest wave
heights indicating the reduction of 2,6-DNT. The Ni has a slightly higher current
response (9.65 vs. 6.53 mA rnM1)The Pb also appears to derEom some sort to fouling as the peak height reaches a maximum and then decreases although this phenomenon was reversible- Figure 6.10 LSV; Reduction of 0.4 mM 2,6-DNT at Merent cathode material
(pH 2) vs. NHE
6.4 PRACTICAL CONSIDERATIONS
Electrochemical processes are govemed thermodynamicaily by the Nernst equation, which relates the potential required to drive a parhcular reaction to the concentrations of the reactants and products of the electrolytic reaction.
In this equation, E is the expenmental ce11 potential, EO is the standard cell potential and Q is the chemicai reaction quotient. At 2S°C, the term @Th* h(Q) has the value (0.0592h) logio(Q). We consider the overall reduction of a mononitrotoluene or mononitrobenzene
compound to the correspondhg amine at the surface of the electrode
(eq. 6.4)
The "Q tem" for this reaction will be [A~NH~]I([A~NO~]~~),so that:
E = EO-0.0 1xlog([Ar~]/([ArNO2]} - 0.059xpHx716 (eq. 6.5)
This tells us that E becomes more negative (requiring a larger cell potential) the larger the conversion of ArNOz to ArNH3 and the higher the pH. Maintainhg a low pH should not be dEcult as explosives leachates are by their nature acidic. The high concentration of H' ions will assist the electrolytic process as they can provide supporting electrolyte for the electrolysis. An important consideration is that the Nernst equation is defined for reversible reactions and that nitroaromatic reduction is irreversible. However, the irreversibility will only introduce a small over potential term into eq. 6.5.
For electrochemical reduction occurring at pH 2 and with 99.9% conversion of
ArNOz, these values can be substituted into eq. 6.5 giving
(eq. 6.6). Using the values determioed in Figure 6.10 E can be calculatecl for the different electrodes (Table 6.2) (assuming no overvoltage required). It is important to note that this refers only to the cathode reaction; the potential of the anode reaction must be added to obtain the overall ce11 voltage. Clearly, the electrochemical conditions required for an effective treatment are within a practical range. E (Fig 6.10) vs. N.H.E. E (99.9% Removal )
Table 6.2 Calculated E values for 99.9% removal of 2,6-DNT at different electrode materials
In practice, the efficiency of the ce11 will be constrained by limitations in mass transfer to the electrodes. In addition, the theoretical potentials predicted by the Nernst equation neglect other factors that alter the electrical eficiency, including the electrode overvoltage and ohrnic drops due to the resistance of the solution. One side reaction that will inevitably occur and which we observed is the electrochemical reduction of hydrogen-
(eq. 6.6) Although, a large overvoltage is normally to suppress the reaction of hydrogen, in this case it may be an advantage as electrocatalytic hydrogenation is an alternative mechanism for the reduction of nitroaromatic compounds. Z Shitaka M., T'ans. Far- Soc., 1925,21,42.
2 Abel E. Von., 2.-F. Elecktroch. Bd., 1906,36,681.
3 Cyr A, Huot P., Belot G., Lessard J., J. Appl. Electrochem., 1990,20,527.
4 Demis S.F., Powell A S-, Astle M_J,, J. Amer. Chem. Soc., 1949,71, 1484.
5 Geske D.H,, Makia H., J. Amer. Chem,Soc., 1960,82,2671.
6 Pietîe L-H, Ludwig P., Adams RN., J. Amer. Ch.Soc., 1961,83,3909.
7 Page J.E., Smith T.W., Waller J.G., J. Phys. COM. Chem., 1949,53,545.
8 Strassner J.E., Delahay P., J. Amer. Chem. Soc., 1952,74,6232.
9 Pearson J., Trans. Far- Soc., 1948,44,683
10 Laviron J., Rouilier R, J. Electronal. Chem,, 1990,288, 165.
11 Davidson L., Quinn Y., Steele. D.F., Platimcm Metals. Rev-, 1998,42,90.
12 Lund H., Baizer M.M., Organic Electrochemistry, 3d Marcel Dekker Inc., 1991, New
York p.411419.
13 Ohrnori M., Takagi M., Bull. Ch.Soc. Jpm, 1977, 50,773.
14 Stocesova A, CoZZ. Czech. Chem. Comm., 1949, 14,615.
15 Bergman I., James J-C., Trans. Fmad;?y Soc., 1949,48, 1956.
182 16 Geske DR., Ragle J.L., Barnbanek M-A, Balch AL., J. Amer. Ch.Soc., 1964, 86,
987,
17 Mohamrnad M., Dost N.P., Qureshi R, Salim M., Khan AY., Electrochem., 1988,4,
287.
1 8 Creighton H., Hales R, US patent 2,458,895, through Chem. Abst., 1949,43,2 1 O4A.
19 Rubenstein I., J. ElectroanaZ. Ch.,1985, 183,379.
20 Belot G., Desjardins S., Lessard L, Tetrahedron. Letr., 19% 25, 5347.
21 Cote B.J., Despres R., Labreque J., Lamothe J., Chapuzet M., Lessard J., J:
Electroanal. Chern,, 1993,219,3 55.
22 Holleck L., Kastening A, Vogt H, Elecfrochimica Acta., 1963,8,225.
23 Fry AJ., S'fhetic Organic Electrochemisfry, Harper & Row Publishers, New York,
1977, p 189-198,
24 Tallec A, Am. Chim., 1968, 164,3.
25 Tallec A, Ann. Chim., 1969,4,67.
26 Preuss A., Fimple J., Diekert G., Arch. Microbiol., 1992, 159, 345.
27 Tallec A., Ann. Chim., 1969, 14,67.
28 Whitnack G. C.,Anal, Chem., 1963,35,970.
29 Hetman J. S., AdChem., 1960,32, 1699. 183 30 Pearson J., The Reduction of Niirocompot(n& at the Dropping Merctlry-Cafhode,
Aberdeen University Press, 1947, p. 683-697.
3 1 Chapuzet J.M., Labrecque R, Lavioe L., Martel E., Lessard J., J. Chim. Phys., 1996,
93,601. ELECTROLYSIS OF NITROAROMATICS
7.1 INTRODUCTION
In this work, a novel rernediation technology for nitroaromatics was explored.
Electrochemical treatment consisted of a two-siep process, where in the fïrst part
compounds were reduced, products identified and mass balances determineci. The second
process compared methods for the polymerization of the reduced products. This was via
oxidation either electrochemicaily or enzymatically. The electrochemical oxidation has a
great benefit compared to enzymatic oxidation, as the anode reaction is complementary to
the cathodic reaction and wodd lead to more efficient use of electrical energy.
7.2-1 Reduction
In chapter five, the difficulties associated with the treatment of nitroaromatics
were documented. This results in persistent contamination of many sites including
manufadng plants, army depots artillery impact ranges, and ordnance disposal sites.
Chapter six demonstrated the ease with which the irreversib le electrochemical
reduction of nitroaromatics to anilines occurs. Unfortunately, the anilines are at least equally toxic as the parent nitroaromatic. Aromatic amines are well known chernical carcinogens, aniline is a hematopoietic poison that can result in methemoglobinemia, and
chronic exposure can cause sarcoma (?
Increasing the toxicity of a waste product is an unsatisfactory resuit for any
remediation technology. However, anilines are readiIy oxidizable, not back to
nitroaromatics, but to insoluble oligomeric materials. This suggested that reduction
followed by partial oxidation has the ability to completely remove poilutants and
eliminate toxicity concerns.
Strategies for conducting the oxidation include the possibility of tramferring the
reduced waste Stream fiom the cathode to the anode cornpartment. The attraction of this
option is that dl electrolytic processes involve oxidation at the anode to complement
reduction at the cathode. In the absence of harnessing the anode reaction usefùlly, a
sacrificial reaction such as oxidation of water to 02must OCCUT- It would clearly be more
eEcient to harness the anode reaction to oxidize the aniline product. This two-flow ce11
(or paired electrosynthesis) approach is ideal for efficiency but the availability of suitable
systems has limited its use in industry (see example in section 1.3.4.). For our purposes the practicality of this idea was best studied with each system (reduction, oxidation) as a
separate entity-
Tiiere is a possible drawback to the foregoing approach, in that the precipitated oligomer may cause electrode fouling. As we saw in chapter 3, fouling has the effect of impeding mass transfer and increasing the resistance at the electrode, thereby raising the potential needed to maintain current levels. One option is the use of DSAs to produce hydroxyl radicals (see section 3.2), this rnay promote oligomer formation in solution, and limit electrode fouling-
7.2.2-2 Peroxidase
Enzymes are biological catalysts that can enhance the rate of chemicai reactions-
They have many advantages over chemical catalysts including a high degree of specificity, operaîion at low temperaturdpressure, and a high reaction velocity. An alternative to the anodic oxidation is to use an enzyme-catalyzed oxidation, such as a peroxidasehydrogen peroxide system that would operate ex-situ of the electrochemical cell. These enzymes are typically stable over a temperature range of 5-55 OC and a pH of
6-9 (2*3! This latter condition may be a disadvantage since the nitroaromatic waste is often found in an acidic environment.
The enzymes used can either be isolated fiom host organisms or the intact organisms can be used. Isolation has several advantages including greater specificity, standardisecl activity, easier storagehandling and no dependence on growth rates.
Isolated horseradish peroxidase (HRE') enzyme has been the system most studied, but other examples include the polyphenol oxidase eqme f?om mushrooms and the micro- organism coprinus macrorhinrs (43.
HRP (also called hydrogen-peroxidase orcidoreductase) is an enzyme isolated fiom the horseradish root systern. It has a molecuiar mass of approximately 40,000 and contains a single protoporphyrin DC heme group. In the presence of hydrogen peroxide it
187 catalyses many reactions hcluding hydroxylations, N-demethylations, and sulfoxidations especially with phenols and aromatic amines as substrates (?
The process is a one-electron step, which involves a change in the oxidation state of the iron atom at the centre of the catalytic site of the enzyme. The sequence of events is usually depicted as follows (?
In this catalytic cycle the peroxidase enzyme is first oxidised by hydrogen peroxide to the active intermediate Ei (eq 7.1). When a substrate such as an aromatic
(AH4 enters the catalytic centre of E; it is oxîdised and releases a fiee radical into solution, with the enzyme converted to the Eii state (eq 7.2). This intermediate is capable of oxidizing a second substrate whereupon the enzyme is rehtmed to its initial state (E)
(eq 7.3). This reaction (without the catalyst) can be summarized as the following:
The f?ee radicals reIeased f?om the enzyme combine in the bulk solution to form dimers. These dimers have a lower solubility than the monomers and usually precipitate out of solution. If precipitation does not occur the fkee radid may be attacked by other monomers, resulting in even larger polymers The advantage of this technique compared to the two-flow ce11 is that solids would be precipitated outside the electrolytic ceII, thereby avoiding electrode fouling-
7.3.1 Substrates
Compounds used in this chapter were the same as those used in chapter 6 (2,4-
DNT, 56-DNT and TNT) and also included the nitroaromatic reduction products. The reduction products of the DNT isomers were obtained fiom Aldrich and wnsisted of 2- amino-4-nitrotoluene (2A4NT), 4-amino-2-nitrotoluene (4A2NT), 2,4-diaminotoluene
(2,4-DAT), and 56-diaminotoluene (2,6-DAT). The TNT reduction products were obtained fiom Chemservice and included two monoaminodinitro congeners [2-amino-
4,6-dinitrotoluene (2A4,6DNT), 4-amino-2,6-dinitrotoIuene (4A2,6DNT)] and two diaminomononitro congeners [2,4-diamino-6-nitrotoLuene (2,4DAN), 2,6-diamino-4- nitrotoluene (2,6DA4NT)], The firliy reduced 2,4,6-triaminotoluene (TAT) was not cornrnercially available but was graciously donated by Dr Halawari fiom the National
Research Council's Biotechnology Group- The hydroxylamines are not cornrnercially available due to their ease of oxidation and therefore short sheif-life.
7.3-2 Electrolysis
Electrolyses were carried out using a flow-through cell similar to that described in chapter 4. Cathodes were typically 10 cm2 in size and were constructed from solid Pb,
189 Ni-plated Ni wire (0.5 mm diameter), Raney nickel, Pt, or retictiiated carbon @TC). Pb and Ni were obtained from Aldrich and the RTC was obtained Erom ERG@Materials. The
Ni-plated cathode was prepared by eIectrodepositing a 1-4 M NïSO&JiC12so1ution at 15 rnA cm-' for two hours onto Ni wire. The Raney Ni was prepared by moistening a 4:l mixture of Raney Ni with nickel powder and compressing the mixture at 10,000 psi
(using an IR press) followed by extraction in NaOH for 1 h as described by Lessard and coworkers (99'0). The counter electrodes were either Pt or LrOfli (10 cm2). The supporting electrolyte was adjusted to pH 2 to promote aniline formation and was 0.05-0-1 M disodium hydrogen phosphate. For the oxidation expenments the anode was Ir02 with Pb as the cathode. Electrode potentials were measured vn an extemal Ag/AgCl reference electrode.
7.3.3 Chromatography
7.3.3.1 HPLC
CathoIyte and anolyte (oxidation only) aliquots were anaiyzed with an HPLC system similar to that described in chapter 4. Detection was with a W detector (h=254 nm), flow rates ranged kom 0.5-1.0 mL min-' and the composition of the mobile phase varied fiom 40:60 to 7O:3 0 (vfv) MeOH: sodium phosphate beer (pH 2).
7.3.3.2 Mass Spectrometry
Mass spectrometry (MS) is an analytical technique that is usually combined with a separation method such as gas chromatography (GC) or HPLC. The difference with this technique over other detection systems is its highly diagnostic nature for compound
identification. This is a result of the detector's ability to select analytes depending on
their rnass to charge ratio (dz).
In MS analysis, it is necessary to ionize samples prior to detection- The choice of
ionization technique is one of the most important factors in detennining the output (mass
spectrum) and sensitivity of the MS. There are severai different rnethods for achieving
ionization however, two of the most common are electron impact (EI) and chemical
ionization (CI) (APCI = atmospheric pressure chernical ionization) ('? The ion source
can either be a hard source or a soft source. Hard sources irnpart high enough energy to cause fiagmentation of the analyte. This results in mass spectra consisting of the ionized anal yte (parent ion) and several lower molecular weight ments(daughter ions). A soft ion source uses a lower energy source, which results in the parent peak and few if any fragmentation peaks-
The type ofionkation can also lead to different ions, for example in CI (or APCI) a proton transfer results in the parent ion with a mass of one more than the anaiyte @f+I), whiie a hydnde transfer results in a parent ion with a rnass of one less than the analyte
(M-1). For ES1 (electron spray ionization) the (M+l) and (M-1) ions are created as a result of the potential applied across a filament or a capillary tube.
Tandem mass spectrometry (MS/MS) is often used for more elaborate identification and consists oftwo mass analyzers in series. The fist usually consists of a sofi source producing the parent ion, which is then filtered and introduced to the second
MS. This second one usually consists of a collision chamber for fiagmentation, followed by detection of the fkagments in a second mass filter.
191 A 5890 Series II HewIett Packard Gas Chromatograph (GC) wiîh a HP 5971 series quadrapole MS detector was used for identification of the intermediates. Separation was achieved with a 30 meter DBSMS column (Ld- 0.25 mm) with helium as a carrier gas. Temperatures of the injector and the detector were 225 OC and 250 OC respectively.
Oven temperature was initially set at 60 OC, ramped at 7 "Chin for 15 minutes, 15
"C/min for 9 minutes and was then held constant at 300 OC for 6 minutes-
Samples fiom electrolysis were either injected with a syringe or desorbed fiom a solid phase microextraction fiber (SPME) ("). For those samples injected, sampie volumes were 2 & derextraction into ethyl acetate or hexane.
SPME consists of extracting the organics onto nised silica fibres coated with a stationary phase, which in our case was a 100 pm polydimethylsiloxane (PDMS) coating, obtained fiom Supleco and conditioned at 200 OC prior to use. For the protection of the fiber and to ensure that products containing NH2 groups were not protonated, sample solutions were adjusted to pH 6-8 with KOH prior to extraction. Extraction consisted of immersing the fiber in a 2 mL aliquot of the aqueous sample for 30 minutes while stUnng with a rnagnetic stir bar. The fiber was then placed into the injector for 3 minutes to ensure complete desorption.
The products of electrolysis were also analysed with a liquid chromatograph coupled to a mass spectrometer (LCMS) whose use was supe~sedby the support staff at Laboratory Services Division WSD-0MAE;RA) or McMaster University. Anal y sis consisted of probe MS or separation with a 10 cm Clg COIUI~in a mobile phase of 40
H20:60 CH3CN at a fIow rate of 0-7&min. For those samples undergoing MS andysis, electrdysis was carried out with ammonium acetate as an electrolyte, since ~a+ions fiom sodium phosphate interfered- Sarnples were nin in (Mf1) and (M-1) modes and then subjected to MS/MS for positive identification.
7.3.4 Peroxidase Oxidation
Enzymatic oxidation was performed in a 25 mL batch reactor containhg bufSer solution, peroxidase, hydrogen peroxide and a nitroaromatic reduction product
Horseradish peroxidases (HRP) were obtained fiom Sigma and consisted of a Type T (EC
1.1 1.1.7, RZ 1-3) and a Type II @C 1.1 1.1.7, RZ 1-9) with activities of 220 and 240 units mg-'respectively. One unit of activity is defined as the number of pmoles of hydrogen
peroxide converted per minute at pH 7.4 at 25 O.
The reaction mixture was prepared by the addition of 10 mL of a 0-2 g/L (-50 unitslml) HRP solution prepared in 0.05 M sodium phosphate buffer (pH 7) and 1 mL of a 36 mM 2,6-DAT or 2,4-DAT solution prepared with the same baer (2,4,6-TNT products were assayed at 5 rnM). In order to prevent the inactivation of the HRP it is important to maintain the Hf12 and substrate at equirnolar concentrations, therefore we began the reaction with the addition of 1 mL of a 36 rnM solution of HzOz (or 5mM for
2,4,6-TNT products). This resulted in a final concentration of 3 mM 2,6-DAT and 3 mM
HzO2. Aliqouts were taken at specific time intervals, filtered through a 0.45 pm filter and then a few drops of concentrated sulphuric acid were added to stop the reaction.
193 7.4 RESIJLTS AND DXSCUSSION
7.4.1 Electrolysis (nitroaromatic reduction)
7.4.1.1 Cathode Material
Small-scde electrolysis (10 mA cm-2) of 22,-DNT was camied out at different cathodes. Based on previous work involving the reduction of hexachlorobenzene ("), it was anticipated that Pb would be a suitable electrode material. However it is important to remember in electrolysis that the morphology of the electrode surface, as well as its identity, are important characteristics. Figure 7.1 shows the results, with a reactivity order of Ni-plated wire > Pb > Raney Ni > Reticulated Carbon > Pt (Pt and RTC are plotted with a linear fit, al1 others are exponential). Although Pt is usually an effective electrode, its poor performance is consistent with the voltammetry results in chapter 6 (section
6.2.1.1). The inability of the Raney Ni to reduce DNT is probably a result of the Raney
Ni not being able to chemisorb the hydrogen at such low negative potentials, thereby preventing H' going to Hz (14'. O 0.5 1 1.5 2 2.5 Time (h)
Figure 7.1 Reduction of 2mM 2,6-DNT as a function of electrode material (10 m~crn-~)
7.4-1.2 Current Effrciency
As in chapter 4 current efficiency was defhed as the ratio of electrons used in the
transformation of the analyte to the total number of electrons passed during the sarne time
interval. If this transformation is considered as the reduction of a single nitro group to the
amine, then thïs would be a six-electron process.
Using this critenon the highest efficiencies obtained for 2,6-DNT at the lowest
current density were studied (0.1 mA ranging from
plated Ni wire cathode. WhiIe the Ni plated Ni wire cathode has a high efficiency at this
1ow current, removal times are too slow, making product study impracticable- Therefore the efficiency of other current density values was explored which for 2,4-DNT at the Ni-
plated Ni-wire are reported in Table 7-1. The high efficiency at the Ni wire is
195 encouraging for Merresearch, as it is welI above the 20% efkiency required to compete with the raw material costs for other treatment methods (see Table 1-1).
However, it is important to remember this is onIy a partial cost oftreatrnent and does not include other costs such as electrodes,
The increase in efficiency with a decrease in current is a result of the positive polarkation of the electrode decreasing in parallel with ciirrent density, and Iess of the charges passed causulg electrolysis of water- This is an important consideration for nitroaromatics as the low potentials required for electrolysis [(-0.10, -0.20, -0.3 1 V vs.
SCE for 2,4,6-TNT), (-0-18, -0-30 vs. SCE for 2,4-Dm), (-0.24, -0.40 vs, SCE for 2,6-
DNT)] also allow for low current densities, which in turn enable high current efficiencies to be achieved,
Current % Efficiency
Table 7.1 The electrochemical efficiency (%) associated with the reduction of a 2.5 mM 2,4-DNT solution after 1 hour of electrolysis with a Ni plated Ni cathode. As the Ni-plated cathode had the highest current efficiency for 2,6-DNT, we used it potentiostatically with the other nitrotoluenes in the series- A potential of -1-00 V vs
SCE- was chosen to ensure that it was negative enough to ensure reduction of the substrates (see section 6-3-11.Figure 7.2 shows that at high potentials there is very little difference in the removai rates between the substrates, possible indication of diffiision limitation-
If we examine the rate of 2,4,6-TNT reduction with respect to potential, we see that there is indeed an effect. Attempts to determine the kinetic order of the reaction are summarized in Figure 7.3A and 7.3B for various reductions with 2,4,6-TNT. If the reaction is a zero order process then it is charge transfer limited and a plot of concentra#ion with time should be linear (Figure 7.3A). However, if the reaction is lP order then the process is mass transfer limited and a plot of ln(concentration) with time should be Iinear (Figure 7.3B). The resdts are incondusive, however at putentials < -0.50
V it is apparent that the zero order fit is much closer while above this value the opposite is true. This suggests that below a certain threshold the process is charge transfer Iimited and at high enough potential it becornes mass transfer limited.
However the Iower potentials are more effective in removai in terms of efficiency
(41% at -0-25V compared to 13% at -1.2 V). Potential was observed to have no effect with the 2,4 and 2,6-DNTs (Figure 7.4) O 0 -5 1 1.5 2 2 -5 Tim e (h)
Figure 7.2 The reduction of different nitroaromatics (2.5 mM, pH 2) at a Ni- pIated cathode (-1.00 V vs. S.C.E.). 2,4-DNT: A,-- ;2,6-DNT: X , -- ; TNT,, -.
1 Time (h)
Figure 7.3A Reduction of 2,4,6-TNT (2.5 mM, pH 2) fitted to a zero order relationshi as a function of potentid (V vs. S.C.E.) at a Ni-plated cathode (R P values reporteci ia brackets of graph) 1 iïme (h)
Figure 7.3B Reduction of 2,4,6-TNT (2.5 mM, pH 2) titted to a fimt order relationship (ln (concentrstion)) as a function of potential (V vs. S.C.E.) at a Ni-plated cathode (lC2 values reported in brackets of @=ph)
The higher efficiency can be explained by several factors; the lower current
density associated with the lower potentials minirnizes ohmic heating and thereby wasted
energy. Furthemore the higher potentiais may be suficient to reduce the products, this
will lower the overall efficiency, as the reduction of the products will compete with the
nitro group and increase the total number of electrons required. For exarnple, if the
monoaminotoluene is reduced to the diarninotoluene then 12 electrons instead of 6 will be consumed. Figure 7.4 Electrolysis of 2.5 mM 2,4 DNT as a Function of Potential (V vs. SCE)
7.4.1.4 Products
7.4.1.4-1 GCMS results
Up to this point the products of reduction of the nitroaromatic had been seen only as HPLC peaks- It was necessary therefore to confïrm that these peaks represented the amino and hydroxyIamino derivatives.
We first atternpted this using GC/MS, where we began by analysing standards of
2,6-DNT (dz 1 8Z), 2,6-DAT (dz 1 22) and 2A4NT (m.z1 52). Hydroxy lamines are not detected by GCMS as they are not thermally stable (l? Injections were performed using both direct aqueous injection and SPME.
Samples fiom electrolysis of 2,6-DNT had mass spectra matching the RT and fragmentation patterns of 2A4NT (Figure 7.5) and 2,6-DNT but not 2,CDAT. 2A4NT was observed only after 30 minutes of electrolysis consistent with it being an intermediate reduction product-
Attempts to extract the 2,GDAT at neutral pH (7-8) hto ethyl acetate or benzene were unsuccessfùl. However, 2,GDAT was det ected &er spiking the electrolyzed solution. Unfortunately these were limited by DNT's solubility and the correspondhg final DAT concentration was also lirnited, This limitation resulted in a 100% conversion to DAT would be barely above its detection Limit (Figure 7.6).
In order to prevent the autooxidation of 2,6-DAT (and thereby maintain high concentrations) in solution, an antioxidant (butylated hydroxytoluene) was added. While this was successfûl in maintainhg high levels in the standard solutions, it did not ailow detection of 2,6-DAT in electrolysis samples.
There was also some difficulty with the GCIMS spectnim due to a high level of background noise fiom phthalateq which are a common source of contamination ('?It is possible that the phthalates present in Our samples were fiom dissolution of the plastic caps of the sample vials. ------______O___ - Scan 1412 (22.776 min) : H55-TO.D (*) 7p
I 1
Figure 7.5 A Mkss Spectrum (GCIMS) showing the fragmentation pattern of 2A4NT (ml2 152) (top) and cornparison with the iibrary standard (bottom) 0 *O 10.0 20 -0 30 .O 40 -0 DAT Concentration (m M)
Figure 7.6 Calibration of 2,6DAT standard using SPME (GC/MS) and monitoring the peak area of m/z 122
Because we were unable to confirm the identification of 2,6-DAT by GG/MS, we switched to LCJMS as an alternative method. Soon after electrolysis (< 48 h) of 2-6-DNT, samples were taken for LC/MS (APCI) analysis, an (M-1) scan had m/z ratios of 182,
15 1, and 121- The LCIMS (M+l) scan is show in Figure 7.7, depicting integration versus time for different m/z ratios 140, 169, 153, and 123 (each panel represents an individual scan with the m/z value present in the top nght hand corner). From the integration values (located just below the m/z values) it is apparent that the DAT (mlz
123) is present in the largest concentration. A summary of the dzratios (M-1, M+l) that were found are shown in bold below. This would correspond to the reduction pathway discussed in Figure 6.1 with the products and corresponding m/z ratios as follows:
997538 Scan AP146 : Z3WL ~eïghtb-
... Scan AP+ 153
Scan AP+ 123
Figure 7.7 LC/MS (ACPI) spectrum at various m/z ratios showing the parent ions of the reduction products of 56-DNT If the results fkom the (M+1) and (M-1) peaks are examined, it is apparent that they correspond to 2,6-DAT, monoamino-monohydroxylaminotoluene (MAMHAT), mononitro-monohydroxy1aminotoluene (MNMHAT) and monoamino-mononitrotoluene
(MAMNT) respectively (see Figure 7.8).
MAMNT MNMHAT
138 151
140 153
Figure 7.8 A selection of 2,6-DNT reaction products
In order to obtain positive confirmation of the 123 m/z ratio as 2,6-DAT, a 800 ppb standard solution was subjected to tandem mass spectrometry (probe). The standard had daughter products having m/z ratios of 79.0, 106.1 and 108.2 (Figure 7.9A).
Electrolysis samples were then analysed by LCMS to separate the products, and isolate compounds with a m/z of 123 (RT 2.0 minutes). Figure 7.9B depicts integration versus time for the MS spectra which has been selected for a rn/z of 123 in the first MS and then selected for specific daughter products in the second MS. The two panels correspond to the two daughter products of 106 and 108 as seen wÏth the authentic sample. This thereby confirms the presence of 2,6-DAT.
Figure 7.90. A tandem Mas Spectrum (Probe-ACPI) showing the - daughter products of the positive ionization of 2,6-DAT (mfz 123) standard. MRM of 2 Channels AP+ 123.10 > 106.10 1.71 e4 Height
MRM of 2 channets AP+ 123.10 > 10820 7.89e3 Height
Figure 7.9b A tandem Mass Spectrum (ACPI) showing the daughter products of the positive ionkation of Z,6-DAT (mh 123) standard. Instead of LCMS, products nom 2,4-DNT and 2,4,6-TNT were examined by ES1
MS. Figure 7.10 depicts a (M+l) mass probe, (mtensity vs. m/z) with the top and bottom
panels as electrolysis solutions and the rniddle as the blank (electrolyte). Subtracting the
middle panel fiom either top or bottom it is apparent that m/z ratios of 123, 139, 153, and
169 are present, corresponding to the correspondhg products shown earlier with 2,6-
Dm.
Figure 7.11 shows an LCMS analysis (intensity versus time) for the same sample, depictïng the presence of specific m/z ratios (153, 139, 123) and showing a UV diode array analysis (bottom panel). The results f?om this figure are in agreement with the MS probe and fùrther conclude the presence of 2,4-DAT as a reduction product.
For 2,4,6-TNT, electrolyses were can5ed out for extended periods (18 h) pnor to
LCMS (since earlier results indicated only a smdl arnount of triaminotoluene (TAT) is fomed at shorter electrolysis tirnes). Figure 7.12 shows an ES1 (MH) probe with the top panel as electrolysis solutions and the rniddle as the blank (electrolyte). Again, subtracting the b1an.k panel fiorn the sample it is apparent that the m/z ratios (139, 167,
183) are present, which we assign to, TAT, diamino-mononitrotoluene and mononitro- monoamino-monohydroxylaminotoluene. Figure 7.10 An ES1 MS probe mass specmim at various m/z ratios showhg the parent ions of the reduction pmducts of 2,4-DNT (top hottom are DNT, middle is blank) Figure 7.11 An ES1 LCMS mass spectmm at various dzratios showing the parent io~oûf ihe reduction products of 2,4-DNT Figure 7.12 An ES1 MS probe mass spectrum at various m/z ratios showing the parent ions of the reduction products of 2,4,TNT (top is TNT, bottom is blank)
Aliquots of the 2,6 DNT electrolysis (Ni-plated cathode) samples were analysed by EPLC for the disappearance of the starting matenal and to determine the retention times of product(s). Several peaks appeared on the KPLC chromatogram, of which two matched the retention time (RT) of 2,6-DAT and 2A6NT standards fi-oom which we determineci a mass balance- It is suspected that the other peaks are arylhydroxylamhes as
per the DNT reduction pathway shown in Figure 6.1. Consistent with this hypothesis they are never present in large amounts and eventudy disappear alltogether.
Figure 7-13 shows that as expected 2,6-DAT is the major product and the behaviour of 2A6NT is consistent with that of an intermediate reaching a maximum at -1 hour and then declining to zero. Since the mass balance approached 100%, it is apparent that the hydroxylamines only play a minor role. The overd mass balance begins to decrease after 4 hours, and is accompanied by formation of a reddish product in solution-
Since this is similar to the fate of an exposed DAT solution to air, this is consistent with auto-oxidation of the 2-6-DAT occurring in the storage reservoir which was open to atmosphere. Loss of material was also seen by Musolino et al (lq who studied the catalytic hydrogenation of 2,4-DNT with PdK. Following reduction, samples exposed to the atmosphere for 12 h, (concentrations not given) showed a 50% loss of DAT and dimerization of the hydroqlamine product s to 4y4'-dinitro-2,2'-az~xytoluene, 2,4'- dinitro-4,~'-azoxytoluene and 2,2'-dinitro-4,4'-azoxyto1uene (18). O 0.5 1 3 3.5 4 4.5 Time (hr)
I~DNT W~CDDAT~~~: W~AGNTI
Figure 7.13A Mas Balance of a 2.2 mM 2,CDNT solution (pH 2) that has undergone electrolytic reduction (09V vs SCE, Ni cathode)
Quantification of the 2,4-DNT reductioo products was determined by HPLC analysis of sample aliquots with peaks matching the RTs of 2A4NT, 4A2NT and 2,6-
DAT standards. Again DAT was the major product, with the two amino-nitrotoluenes formed in similar amounts. A surnmary of these results at diEerent current densities and afker 2 hours of electrolysis are shown in Figure 7.13B. The presence of a dark reddish precipitate in soiution with time suggested that the 2,4-DNT products aiso underwent autooxidation. Figure 7.13B Reduction of a 2.5 mM 2,4-DNT solution at different current densities and the mass balance after 2 hours.
The course of reduction for 2>4,6-TNT follows a similar trend (Figure 7.14) except that even at long electrolysis times TAT was never more than a minor product. In this case the mass balance fell short of 100%, this may be partially explained by two quality control issues. Firstly the concentration of the standards lacked analytical precision as concentrations fiom the manufacturer were ody reported with either one or two significant figures (0.1 or 1-0 mg/mL). Second impurity peaks were visible on the
HPLC chromatograrns of the standards. The apparent mass balance decreases with he7 which is a sirnilar trend as to that seen with DNT and probably indicates that there is autooxidation of the products. Clearly any fiiture work such as scale-up would have to address this mass balance issue more carefiilly. O 0.5 1 1-5 2 Time (hr) IEITNT Eà2,6DA4NT El2,4DA6NT n2A4,6DNT BTAT 1
Figure 7.14 Mass Balance of a 19 mM 2,4,&TNT solution (pH 2) that has undergone electrolytic reduction (-0.9V vs SCE, Ni cathode)
7.4.2 Oxidation
At this point it is apparent that the dinitro and trinitro toluenes can be successfully reduced to amino compounds (either partially or fùlly). However chapter 5 demonstrated that these arnino compounds are at least equally toxic and therefore will follow a discussion of options to transform them to more labile compounds.
Po ssibilities to partially oxidize and precipitate out the anilines include; air sparging, 'hvice-through" and peroxidasdhydrogen peroxide. In each case, the important parameters will be the percent removal of the anilines and the associated costs. 7.4.2.1 Air Oxidation
Atmosphenc oxidation of anilines was studied by either storing sarnples under nitrogen or leaving them open to the atmosphere- Samples were left for a 24 hour time period and analysed by SPME/GC/MS. It was found that the concentration remaining fiorn those purged with nitrogen (15 minute purge tirne) was only moderately higher than those uncapped and left open to the atmosphere (22.6% vs. 5.00h). However a standard solution of 2,6-DAT which was stored in MeOH and open to the atmosphere, did not change in concentration- It is certainly possible that part of the loss of materiai is via volatilization,
In a separate study, 2,6-DAT solutions were left exposed to air, room temperature and natural light. As per previous observations, a reddish precipitate was formed with time. Samples were taken at various time intervals and filtered to remove the precipitate which was analysed for carbodnitrogen (CN) content. A 2,6-DAT dimer would contain
69% carbon and 23% nitrogen, however the results obtained found a content of 62% and
14% respectively. One possibility that may explain this discrepancy, is the introduction of oxygen into the compound. This would in effect increase the molecular weight of the compound and lower the percent weight contribution of both the C and N. If we assume that oxygen introduction came about as two hydroxyl groups (as per Figure 6-21>then the resulting C/N ratio would be lowered fiom 69/23 @AT) to 64/16- 7.4.2-2 Anodic oxïdation
Anodic removal of 2,6-DAT is compared to other treatments in Figure 7.15.
Focus was on starting matenal disappearance rather than product identification, because we expect the products to be insoluble oligomers.
LSV's of 2,6-DAT with a platinum buIb showed a peak at 0.62 V vs. SCE, fiirthermore any subsequent scans produced only Bat curves- This latter observation is interpreted as the occurrence of fouling at the anode. Using this information electrolysis was conducted at an Ir02 anode at (2-2 rnM pH 2, 0-8V vs. SCE)- The "non-aerated" solution treatments included capping the 2,6-DNT solution and leaving it on the bench.
"Aerated" was also ex-situ of the cell, however for this air was continually sparged through the solution- 'Cathode flow" and "oxidized anode" were both in-situ treatments where the DNT solution was circuiated through the respective compartment and under electrolytic conditions. From this figure, anodic treatment appears to be the most an effective option and is much more efficient than air sparging (70% vs. 30% removai).
The similarity of the rate of removal for the "cathode Bow" and the "aerated" suggests that volatilization plays an important role as there should be Limited air within the cathode compartment.
In terms of the anodic treatment, electrode fouling did not appear to be an issue, as three repeated trials had similar removai rates. However fouling may become more prevalent during scale up as the overail amount of materid exposed to the ekctrode will increase by orders of magnitude. Furthemore, fouling may become an important issue if three-dimensio na1 electrodes are used, Figure 7.1 5 Disappearance of 2.2 rnM 2,6-DAT at pH 2, with respect to type of oxidation treatment
7.4.2.3 Peroxidase
Enzymatic poiymerisation was the other oxidation method îhat was assessed- This
has the advantage of promoting the reaction outside the cell. Two different horseradish
peroxidases were assayed with the diaminotoluene congeners (as per experirnental
section) and the results from a 10 minute reaction are presented in Table 7.2.
The solution becarne coloured as soon as the hydrogen peroxide was added, with
a similar colour to that observed in the auto-oxidation of DAT (only slight a colour
change was seen for diaminomononitrotoluenes or monoaminodinitrotoluenes)- The
amount of the precipitates found fiom 2,6-DAT was much larger than that from 2,4-DAT
and therefore read'iy distinguishabie to the naked eye. These observations are also consistent with the precipitates seen during auto-oxidation. These results for DNT's are similar to those obtained previously for chlorophenols <19! In that case the 2,4-substitution also led to a decrease in reactiviity;
78% loss of 2,4dichlorophenol cornpared with 84-99% loss of uther chlorophenol congeners under sirnilar conditions. However, CO-precipitation of 2,4-dichlorophenol with other chlorophenols improved its removal rate.
When we attempted to oxidize a mixture of the two congeners, the removal of the mhre was sirnilar to the average of the individual congeners. Thus in this case co- precipitation was not advantageous.
Substrate l % removal Oh removal
Mixture of 2,4-DAT, 2,6-DAT 71.8% 71-IYo
Table 7.2 Removal efficiencies of DAT congeners with HRP peroxidases
Solutions of TAT were not analysed due to the limitation in obtaining sufficient quantities, therefore the diaminornoaunitrotoluenes and monoaminodinitrotoluenes cornpounds were assayed instead. Analysis was carried out at 5 ppm as a result of the concentration the standards. Results are presented in Table 7.3 and show the ineffectiveness of HRP in removing the amino products, with the monoamino more difficult to remove than the diarnino. Substrate % removal
2,6-DA-4NT 74.1%
2,4-DA-6NT 55.8%
Table 7.4 Removal efficiencies of DAT congeners with Type 1 HRP peroxidases.
While the HRP has the advantage of precipitation outside the cell, it will add another cost to the overall process. No cost estimates have been calculated for anilines to date but Taylor et al ('O) have calculated this cost per m3 for a 1.0 mM solution of seven different chlorophenol congeners ranging eom $2 for p-chlorophenol to $60 for m- chlorophenol. Costs were based on a 95% removal rate with a HRP dose of 1.2-1.5 molar
- equivalent, based on an enzyme cost of US $0.75/kg at 35% purity. These values do not include H202costs calculated as $0.1 1 per 1-0 mM treated.
Ifa similar cost analysis ($0-06 kWh, 8V, $0.30 mole electrons) to that in chapter
4 is useci, a partial cost estimate for the electrochemical portion of the nitroaromatic treatrnent can be determined. Remembering fiom chapter 6 that the reduction of the nitro group to the amine is a 6-electron process, we can estimate treatment as at 50% current efficiency this equates to $3.60 per mole of nitrotoluene treated, equivalent to $0.65 for
220 treating 1 m3 of a 1 mM solution. The advantage of the anodic oxidation is that there will
be no additionai electrical costs, however if the HRP option is pursued additional costs
will occur.
Chapter 5 outlined the costs for treatments of nitroaromatics including GAC
($0.03 - $3.00 per m3) CTVO ($0.32 to $1.70 per m3) composting/bioslurry ($21 to $32
per m-3) and incineration at $82 me3.
Although it is acknowledged that is difficult to compare overail costs vs. e1eCtrical
costs the opinion is that at this stage the difference between technologies is great enough to pursue merwork towards electro-remediation.
7.6 OVERALL PROCESS
The experimental approach led to an overd treatment process which is similar to that as outlined in Figure 7.16. This demonstrates that after reduction, the reduced waste
Stream is either passed back through the anodic cornpartment or subject to air oxidation outside the cell. Route #1
Route #2
Figure 7.16 Schematic of an iudividual process for the removal of nitroaromatics in wastewater 1 Amdur M.O., Doull J., Kiaassen CD.; Casarett's & Doull's ToxicoIogy 4th ed.,
McGraw Hill Inc., New York, 1991, p. 174-1 75,
2 Klibanov A.M., Alberti B.N., Morris E.D., FeIshin L.M., J. Appl. Biochem., 1980, 2,
414.
3 NiceII J.A., Bewtra JX., Biswas N., Talyor K.E., Wmer Res., 1993,27, 1629.
4 Al-Kassim L., Taylor K., J. Ch.Tech. Biofechnol, 1994,6 1, 179.
5 Atlow SC,Bonadonna-Aparo L., Klibanov M., Biotech. Bioeng, 1984,26, 599.
6 Urrutïgoity M., Souppe J. BiocataZysis, 1989,2, 145.
7 Arnao M.B ., Acosta M., Del Rio J.k, Varon R,Garci-Canovas F., Biochim. Biophy.
Acta., 1990, lO4lY43.
8 Mcell J.A., Al-Kassim, Bewtra J.K., Taylor K.E.,Biodefer. Abs., 1993, 7, 1.
9 Velin-Prikidanovics k, Lessard J., J. Appl. Elecfrochem., 1990,20, 527.
10 BeIot G., Desjardins S ., Lessard J., TepahehLeft., 1984,25,5347.
11 Skoog DA, Holler F. J., Nieman T.A, Principals of Instrumental Adysis, sa ed.
Saunders College Publishing Philadelphia, USA, 1998. p. 501-506.
12 Louch D., Motlagh S., Pawliszyn J., Anal, Chem., 1992,64, 11 87.
13 Merica S.G., Jedral W., Banceu C.E., Lipkowski J., Bunce N.J., Environ. Sci.
Technol., 1998. 32, 1509. 14 Chapuzet JM., Lasia-A, Lessard J., ElectrocataZys~s,edited by Lipkowski J., Ross P.,
1998, Wdey-VCH, New York pp. 155-196.
15 Kosak R, Catalysis of Organic Reactions Ede's Rylander P.N., Greenfield H.
Augustine RL., Marcel Dekker, New York, 1988, p. 177.
16 Pattinson S-J., Wilkins JP. G., Anulyst, 1989, 114,429.
17 Musolino NO., Bonaccorsi MG., Donato L-, Mercadante L.; Ind & Eng Chem.
Res., 1997,36,3619.
18 Musolino MG, Neri G., Milone C., Minico S., Galvagno S., J Chromatogr A.,
1998,818, 123.
19 Al-Kassim L, Taylor KE.., Bewtra JX., Biswas N., Plant Peroxidases: Biochernistry
and Physoloog Welinder KG., Rasmussen S.K., Penel C. Eds, University of
Geneva, 1993, p. 197-200.
20 Taylor KE., Al-Kassim L., Bewtra JX., Biswas N., Talyor J., International
S'posium on Enviromnentul Biotechmlogy, University of Waterloo, July 4-8,
1994. In Environmental Biotechnology: Principles and Applications, M. Moo-
Young, W. A. Anderson, AM. Chakrabarty, eds., Kluwer Academic hblishers,
Dordrecht, 1996, p. 524 - 532. CONCLUSIONS AND FUTURE WORK
8.1 INTRODUCTION
This project studied the rernediation of chlorophenols and nitroaromatics using electrolysis. Specific objectives were to increase knowledge conccrning choice of electrode materid the potential(s) at which the reduction/o>ridationprocess occurs, and product formation. The project also addressed specific issues for each system such as electrode fouling for chlorophenols and Mer treatment of reduction products for nitroaromatics.
With the tum of the 21° century cornes a societal pressure to remediate contaminated sites. Electrochemical treatment of waste is a relatively new concept that can offer an important role in this area as it can combine cost effectiveness and environmental stewardship. It consists of relatively simple equipment, electrodes which can donate or accept electrons and has the possibility of high energy efficiency. 8.3 CHLOROPHENOLS
8.3.1 Voltammetry
The potentials at which oxidation occurs for a number of different chlorophenols was investigated, and the values were found to be similar (-1.1 V vs. SCE). This would
ùidicate that the site of oxidation is with the hydroxyl group and that chiorination or
steric hindrances only play a rninor role. For al1 chlorophenols, oxidation occurred at less
positive potentials in basic solution, when the substrate was present as the phenolate.
Regardless of pH, the nurnber of electrons involved in the oxidation were experimentally
determined to be four for MCP and TCP and two for PCP.
8.3.1.1 Electrode fouling
Electrode poisoning was a serious problern with dl electrode rnaterials. However, by following the resistance at the suface of the electrode with tirne, for a constant applied current, we were able to demonstrate clearly that the poisoning of the polymeric film is a fbnction of water electrolysis. It is postulated that uiis results from attack of hydroxyl radicals on the chlorophenols, which then deposit on the surface of the electrode and increase resistance. This use of Iow potentials was used during electrolysis to overcome electrode poisoning.
Electrolyses was successful in the removal of al1 chlorophenol congeners fiom solution. The doped Sn02 anode was the most successful in oxidizing phenols in terms of
226 efnciency and removal (TCP undetectable afler 3 4 PCP &ter 2 h; 6.5 mA cm-').
However substrate removal rates varied less between congeners with the PbOz electrode.
A wntinuing obstacle to a practicai technology for anodic remediation of chlorinated phenols is the lack of electrode materials that combine high current efficiency with fieedom fkom fouling. This research shows that two opposing factors must be considered- At "directly oxidizingn anodes such as Pt and PbOz- passivation would be inhibited by rninimizing hydroxyl radical production. At DSAs, oxidation is believed to involve hydroxy 1 radical chemi stry, suggesting that less polymer is formed when hydroxyl radicals attack unreacted chlorophenol than pre-oxidized material. In that case, the goal for optimum design of a DSA would be to maximize hydroxyl radical chemistry at the expense of direct oxidation. However, hydroxyl radical formation cannot be increased indefinitely without comprornising current efficiency due to self-quenching of the radicals. Therefore in terms of waste treatment Pb02 may be the most appropriate anode, furthemore it is easier and cheaper to fabricate.
The electrochemical reduction of nitroaromatics bas been studied electrochernically for the past 80 years but has not previously been considered as a remediation method,
8.4.1 Reductive Electrolysis
Experiments included voltammetry for each substrate shidied, product identification and quantitation, selection of cathûde materials, and effect of electrochemical parameters. Mass balance for 2,6 -DNT was 99?h for the est4 hours but only 65% der 2 hours for 2,4y6-TNT. Removal of 2,6-DNT was monitored with RTC,
Pt, Pb, Raney Ni and Ni plated Ni wire cathodes. Efficiencies were highest at the Ni plated Ni cathode (80% at 0.1 mA cm-2).
The reduction products aminotoluenes are more tox5c ththan the nitroaromatics. In order to eliminate this problem several methods for Mertreatment wsre explored.
Polrnerizing the reduction products using enzymatic catalysis (HRP), was found to work moderately well for 56-DAT and 2,4-DAT but not for 2,6-DA-4NT, 2,4-DA-6NT, 4A-
2,6-DNT and 2A-4,6-DNT. Transfemng the reduced waste stream tu the anode cornpartment for partial oxidation was successful. The attraction of this latter option is that al1 electrolytic processes involve oxidation at the anode to wmplement reduction at the cathode.
The significance of the research is the increased knowledge concerning a system that cm be used to treat wastewater to degrade and eliminate refiactory organics as a complete treatment or prior to biological rernediation. The high current efficiency suggests it cm be cost effective (sections 4.4.8 and 7.5). There are also other advantages such as mobility, modularity, ease of implementation and no sludge production. For remote sites the cost merence may be much more significant and more environmentally benign as transportation of dangerous goods such as pi& water can be minimized. In practice, the thermodynamic efficiency of the electrochemical ce11 will be
constrained by limitations in mass transfer to the electrodes. In addition, potentials
predicted by the Nernst equation neglect oîher factors that alter the electrical efficiency,
including the electrode overvoltage and ohmic drops due to the resistance of the solution.
Therefore wilI be have to be determined scale-up.
8.6 FUTURE WORK
The work showed that electrochemical treatment remains a feasible process at least at the laboratory scale. As in al1 work answers to questions lead to fùrther questions the following are proposed as possible precursors.
8.6-1 Chlorophenols
Using the knowledge gained fiom this study, fùrther research should focus on optimizing electrolysis conditions such as electrode material, supporting electrolyte and current density. For the former this might include exploring the performance of other
DSA anodes with variables including which oxide film metals, oxide fiIm thickness, doping the oxide with different metals, doping at different concentrations, doping with metal mixtures and finally, which metal to use as the base material. Elevated temperatures could be explored to see if they enhance reaction rates, while colder temperatures could be used to see if they minimize substrate polymerization Ce11 design could explore increasing mass transfer by varying flow rates and patterns. On a more fbndamental level, analysis of the electrode surface may be able to deduce the structure of the polymer and its thickness. 8.6-2 Nitroaromatics
Research wit h nitroaromatics should be expanded to include nitroalip hat ics (e-g.
HMX, RDX), mixtures and eventually authentic waste saniples. There should be more
work obtaining an overall complete mass balance of the products and whether the
discrepancies fiom 100% in this thesis were a result of oligeromization, volatilization or
some other factor. Further studies of the polyrnerization of the aminotoluene should include charactensation of the final products. If possible results should detennine the
amount of linking that occurs (e-g. dirner vs. trimer vs. oligomer).
The next phase should also examindevaluate a complete two-£low ce11 during operation, such that the cathodic (nitrotoluene) and anodic (aminotoluene) systems in operation simultaneously. This should also explore the optimum method for transfemng f?om one compartment to the other (batch, equalization tank, or other).
Minirnizing the fouling of the anodes will be a key component of any commercial technology and friture work should examine this issue. Expenmeas should explore the advantages/disadvantages of a divided versus an undivided cell. In preparation for scale- up the addition of an aerobic biological system post electrolysis would be beneficial.
8.6-3 Engineering ImplEêations
The fundamentai science of this project has shown that the technology is attainable, and has helped define operating conditions to optimize the process (e-g. potential, electrode material, product formation) and prevent electrode fouling. However, there remain some engineering and design questions to be resolved, In the friture a scde up pilot should be built and should explore issues such as mass tramfer. This should include electrode placement and structure, for example a cornparison of honeycomb, interweaving and 3D electrodes. Mass transfer should also explore linear versus turbulent fiow, flow rate and ce11 residence time. These latter parameters should be cross- referenced with charge transfer parameters including charge density.