Electroreductive Remediation of Halogenated ... - ACS Publications

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Electroreductive Remediation of Halogenated Environmental Pollutants Erin T. Martin,† Caitlyn M. McGuire,† Mohammad S. Mubarak,‡ and Dennis G. Peters*,† †

Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States Department of Chemistry, The University of Jordan, Amman 11942, Jordan

‡

ABSTRACT: Electrochemical reduction of halogenated organic compounds is gaining increasing attention as a strategy for the remediation of environmental pollutants. We begin this review by discussing key components (cells, electrodes, solvents, and electrolytes) in the design of a procedure for degrading a targeted pollutant, and we describe and contrast some experimental techniques used to explore and characterize the electrochemical behavior of that pollutant. Then, we describe how to probe various mechanistic features of the pertinent electrochemistry (including stepwise versus concerted carbon−halogen bond cleavage, identification of reaction intermediates, and elucidation of mechanisms). Knowing this information is vital to the successful development of a remediation procedure. Next, we outline techniques, instrumentation, and cell designs involved in scaling up a benchtop experiment to an industrial-scale system. Finally, the last and major part of this review is directed toward surveying electrochemical studies of various categories of halogenated pollutants (chlorofluorocarbons; disinfection byproducts; pesticides, fungicides, and bactericides; and flame retardants) and looking forward to future developments.

CONTENTS 1. Introduction 2. Beginning an Electrochemical Remediation Process 2.1. Electrodes, Solvents, and Electrolytes 2.2. Cells for Benchtop Voltammetry and Bulk Electrolysis 2.2.1. Cyclic Voltammetry 2.2.2. Controlled-Potential Bulk Electrolysis 2.2.3. Direct versus Catalyzed Bulk Electrolysis 2.2.4. Constant-Current Bulk Electrolysis 3. Mechanistic Aspects of Carbon−Halogen Bond Cleavage 3.1. Transfer Coefficient (α); Stepwise versus Concerted Carbon−Halogen Bond Cleavage; Other Heterogeneous and Homogeneous Processes 3.2. Trapping and Identifying Intermediates (Radicals and Carbanions) 3.3. Hydrodynamic Voltammetry to Identify and Characterize Intermediates 3.4. Correlating Product Distributions with Mechanisms 3.5. Stripping Analysis 3.6. Surface Phenomena 4. Techniques for Electrochemical Remediation: From Benchtop to Industrial Scale 4.1. Instrumentation 4.2. Electrochemical Cell Designs 4.2.1. Volume and Shape of the Cell 4.2.2. Number of Electrodes

© XXXX American Chemical Society

4.2.3. Number of Compartments: Divided versus Undivided Cells 4.2.4. Modes of Mass Transport 4.2.5. Temperature Control 4.3. Large-Scale Bulk Electrolysis 4.3.1. Batch Reactors 4.3.2. Flow-Through Cells 4.3.3. Custom-Made Cells 4.3.4. Separation, Identification, and Quantitation of Products 5. Halogenated Environmental Pollutants 5.1. Indirect or Catalytic Cleavage of Carbon− Halogen Bonds 5.1.1. Nickel Complexes 5.1.2. Cobalt Complexes 5.1.3. Contrasting the Catalytic Behavior of Nickel(I) and Cobalt(I) Salens 5.2. Electrochemical Studies of Halogenated Pollutants 5.2.1. Chlorofluorocarbons (CFCs) 5.2.2. Disinfection Byproducts (DBPs) 5.2.3. Pesticides, Fungicides, and Bactericides 5.2.4. Flame Retardants 6. Future Prospects Author Information Corresponding Author ORCID Notes Biographies

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G H H H I I I J J J J

J K K K K L L L M N N O O P P P R V W Y Y Y Y Y

Received: August 9, 2016

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DOI: 10.1021/acs.chemrev.6b00531 Chem. Rev. XXXX, XXX, XXX−XXX

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the potential of an electrode. On the other hand, the development of any successful and cost-effective electrochemical process requires monumental attention to the complexity of processes that ordinarily involve both electron transfer and accompanying chemical events. One must choose appropriate electrode materials, solvents, and supporting electrolytes, along with designing, engineering, and refining large and efficient electrochemical cells (reactors) and performing the often laborious tasks of product isolation and purification, recovery and recycling of solvents and electrolytes, and cleaning and preparing the reactors for reuse. In a real situation, the nature of the pollutant (and its matrix) dictates the choices of solvent, supporting electrolyte, electrode material, and procedure. As an example, a nonaqueous solvent is needed for the electroreductive remediation of dichlorodiphenyltrichloroethane (DDT), because of solubility considerations. In addition, this process can be accomplished by direct reduction at a carbon or silver cathode in a divided cell, or it can be performed with an electrogenerated complex such as nickel(I) salen. On the other hand, for pesticides in water, electrooxidative remediation can be carried out by means of an electro-Fenton process.4 Alternatively, halogenated pollutants in aquatic environments can be extracted into an appropriate organic solvent and recovered as pure liquids or solids, which can then be subjected to electrochemical reduction. In the sections of this review that follow, we offer comments and references to the literature pertaining to (a) choices in parameters (electrodes, solvents, and electrolytes), plus the virtues of direct versus mediated electron-transfer processes that govern and affect the electrochemical behavior of a particular compound; (b) comparisons between voltammetry and controlled-potential (bulk) electrolysis as methodologies to characterize and predict the electrochemical behavior of a chosen substrate on the way to development of a remediation procedure; (c) essential and practical information about cells and instrumentation; (d) separation, identification, and quantitation of electrolysis products; (e) delineation of mechanistic pictures involved in the electrochemistry of a halogenated compound; (f) summaries of the literature concerning the electrochemistry of various families of halogenated pollutants as well as other halogenated organic compounds; and (g) some conclusions and suggestions about future prospects and new directions.

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1. INTRODUCTION In recent years, there has been heightened interest in the application of electrochemical techniques for the study and possible remediation of environmental pollutants, especially halogenated species, many of which have been sequestered in massive stockpiles; for example, as mentioned later, the global amount of stockpiled chlorofluorocarbons (CFCs) totaled 2.25 megatons in 2009. Accordingly, our primary interest in this review is to describe strategies for removing stockpiles of halogenated organic pollutants from the environment. In addition, many of these pollutants are found in comparatively low concentrations in the aquatic environment or dispersed in small amounts in the atmosphere. Reviews dealing with this topic have included discussions of the electrodegradation of halogenated compounds throughout the environment. Rondinini and Vertova1 contributed a chapter on the reduction of halogenated organic species to a book that places some emphasis on the electrochemical treatment of wastewater. A short work2 pertaining to both direct and catalytic reduction of halogenated environmental pollutants was published in 2014. In a review by Panizza and Cerisola,3 the major focus was on various anode materials for the direct oxidation of organic pollutants and on mediated oxidation by electrogenerated species such as silver(II), cerium(IV), cobalt(III), ozone, and persulfate. Brillas, Sirés, and Oturan4 discussed the electro-Fenton process as well as related strategies for the degradation of various organic compounds, including some halogenated herbicides and insecticides. Electrochemically assisted remediation of a substantial list of pesticides in soils and water was the subject of a recent review by Rodrigo, Oturan, and Oturan.5 In an earlier article by Jüttner and co-workers,6 general problems associated with environmental pollution in industry were discussed, including the destruction of a number of halogenated environmental pollutants such as chlorobenzene, hexachlorobenzene, lindane, and 1,2-dichloroethane. In the fall of 2014, a joint international meeting sponsored by both The Electrochemical Society (ECS) and the Mexican Society of Electrochemistry (SMEQ) included a one-day symposium entitled “Electrochemical Treatments for Organic Pollutant Degradation in Water and Soils”; several of the oral and poster presentations pertained specifically to halogenated substances. At this moment, there are some rivals to electrochemistry as possible methodologies for the remediation of halogenated environmental pollutants. With a few references to the recent literature, these include biological or microbial,7−9 chemical,10−13 photochemical,14,15 thermal,16−18 and mechanochemical19 strategiesand all of these options, including electrochemistry, appear to have advantages and drawbacks. Microbial degradation has a tendency to be time-consuming and expensive and to afford incomplete dehalogenation. Chemical methods of dehalogenation can involve the use of large volumes of solvents that might be harsh or toxic. Thermal decomposition or incineration of halogenated compounds is wasteful of energy and can produce carbon dioxide and hydrogen halides as byproducts. Electrochemistry is not without its suite of advantages and challenges. Optimally, lower consumption of energy and time is possible, and desired products might be attainable because of the selectivity offered by precise control of

2. BEGINNING AN ELECTROCHEMICAL REMEDIATION PROCESS For many years, as a university-based research group, our laboratory has focused on benchtop electrochemical studies that pertain to electrochemical reduction of halogenated organic compounds, including some well-known environmental pollutants. Among our interests are (a) questions about the number of stages of reduction that these compounds undergo; (b) identification and quantitation of products that arise from these electron-transfer processes; (c) mechanisms by which these products arise; and (d) how the identities of variables such as choice of cathode material, solvent/electrolyte, and temperature influence the overall outcome of an electrochemical process. If any benchtop experiments are to lead to a successful and practical large-scale process for remediation of an environmental pollutant, a thorough knowledge of the just-mentioned set of objectives (and probably some other initially unforeseen variables) is paramount. In the early sections of this review, we attempt to provide (with references to the literature) some overviews of what is needed to advance benchtop observations to a macroscale process. B


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Investigators and students unfamiliar with the vast body of literature on electrochemistry might benefit from the names of numerous journals that appear as part of the extensive list of references at the end of this review. In addition, by providing the names of all investigators who have made and continue to make important contributions to this field, the list of references serves another purpose as well. 2.1. Electrodes, Solvents, and Electrolytes

When starting a study of the electrochemical behavior of a targeted chemical species, be it a compound or an ion, one is immediately confronted with decisions as to choice(s) of electrodes, solvents, and electrolytes. For a beginner, two excellent books contain helpful information about these three topics,20,21 although one must still rely on good judgment and prior experience. A review chapter by Mann22 is a valuable source of material about nonaqueous solvents and their purification as well as supporting electrolytes and reference electrodes appropriate for those systems. For anodic processes, the common and classic choices for electrodes are carbon, gold, and platinum; however, borondoped diamond, iron, lead, steel, and titanium have been employed. For cathodic reactions, the most familiar options are carbon, mercury, copper, steel, tin, and several of the noble metals. However, the literature of electrochemistry is becoming increasingly rich in the use of numerous other electrode materials. Nowadays, the candidate list of possible electrodes has been greatly expanded by the advent of new nanomaterials23 and other composites. Thus, as dictated by a particular application, there is great need for innovation and creativity in the selection and design of new working electrodes for both anodic and cathodic processes. Despite its cost, silver has become exceedingly popular in the past two decades as a cathode for the reduction of halogenated organic compounds, owing to its propensity to “electrocatalyze” reductive cleavage of carbon−halogen bonds; this topic has been well documented in the recent literature.24−29 A striking example, arising from the use of two different cathode materials (glassy carbon and silver) to investigate the same system, was encountered in our laboratory during a study30 by means of cyclic voltammetry of the direct reduction of the flame retardant 1, 2,5,6,9,10-hexabromocyclododecane (HBCD) in dimethylformamide (DMF) containing tetramethylammonium tetrafluoroborate (TMABF4) as the supporting electrolyte. Shown in Figure 1 is a pair of cyclic voltammograms, recorded under identical conditions, for the reduction of HBCD (purchased as a mixture of three enantiomeric pairs of diastereomers) at glassy carbon and silver cathodes. Note the substantial difference in the cathodic peak potential (Epc) for the irreversible reduction of HBCD at silver (Epc = −0.43 V) and at glassy carbon (Epc = −1.33 V); clearly, reductive cleavage of the carbon−bromine bonds of HBCD is much easier at silver than at glassy carbon. However, the reduction of functional groups other than carbon−halogen bonds might be unaffected by the choice of a cathode material. It is interesting that only one prominent cathodic peak is seen with either electrode, and that bulk electrolyses of HBCD at a potential slightly more negative than each of these cathodic peaks cause complete debromination of the flame retardant to afford enantiomers of 1,5,9-cyclododecatriene almost exclusively. To choose a solvent and a supporting electrolyte for a particular application, several criteria must be considered: (a) chemical compatibility with the substrate(s) of interest; (b) cost,

Figure 1. Cyclic voltammograms recorded at a scan rate of 100 mV s−1 for direct reduction of a solution containing 5.0 mM 1,2,5,6,9,10hexabromocyclododecane (HBCD) in oxygen-free dimethylformamide (DMF)/0.10 M tetramethylammonium tetrafluoroborate (TMABF4) at (A) glassy carbon and (B) silver cathodes (each a planar, circular electrode with a geometric area of 0.071 cm2). Each scan started and ended at approximately 0 V with respect to a reference electrode consisting of a cadmium-saturated mercury amalgam in contact with DMF saturated with both cadmium chloride and sodium chloride; this electrode has a potential of −0.76 V vs SCE at 25 °C.31−33

stability, inertness, toxicity, ease of purification, and dryness (if the presence of adventitious water is a liability); and (c) the solvent/electrolyte medium must be conductive and must exhibit a wide “potential window” (thereby permitting significantly positive and negative potentials to be accessed) to ensure that all of the electrochemistry of a target compound can be explored (with this last issue being closely linked to the identity of the working electrode). Often, the best approach is to put together a system that appears to satisfy each of these criteria and then to conduct a preliminary electrochemical survey of that system by means of cyclic voltammetry. 2.2. Cells for Benchtop Voltammetry and Bulk Electrolysis

For benchtop experiments, our laboratory employs voltammetric techniques (primarily cyclic voltammetry) and controlledpotential (bulk) electrolysis. These two methodologies provide important and complementary insights into the identities and characteristics of electron-transfer processes as well as accompanying chemical reactions. As discussed in the following subsections, each technique requires electrochemical cells that are designed and used differently. It is not possible to overestimate the importance of good cell design. 2.2.1. Cyclic Voltammetry. Cyclic voltammetry is done on a short time scale; for example, the complete scan depicted by curve A in Figure 1 took just 48 s at a scan rate of 100 mV s−1. However, for the investigation of exceptionally fast electrontransfer events,34 it is possible with modern instrumentation to carry out cyclic voltammetric studies at scan rates approaching 106 V s−1, which necessitates the construction and use of ultramicroelectrodes and sophisticated instrumentation.35,36 Information that can be obtained from a cyclic voltammogram includes (a) how many stages of reduction or oxidation a compound undergoes; (b) the potential at which each stage of reduction or oxidation occurs; (c) the number of electrons involved in each electron-transfer event; (d) whether a particular electron-transfer process is reversible or irreversible; (e) whether a reactant, intermediate, or product species might be adsorbed C


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drop working electrode.38 Jacketed cells for experiments below and above room temperature as well as cells for experiments in small volumes (∼5 mL) of solutions containing rare or expensive compounds have been constructed. Descriptions of common electrode configurations and other cell designs for cyclic voltammetry can be found in various references cited throughout this review. Cyclic voltammetry is well suited to the discovery and identification of oxidation−reduction processes for compounds of environmental interest. If changes in the redox state or structure of a species can be induced by one or more electrontransfer reactions, evidence of these events can be revealed by means of cyclic voltammetry. As the potential of the working electrode is scanned over an appropriate range, one or more electrons are transferred to or from the compound of interest, and the resulting current flow is recorded in the form of a voltammogram. Because diffusion controls the flux of compound to the electrode surface, an electron-transfer event manifests itself as a peak, such as those shown in Figure 1. For a compound that can undergo a series of electron-transfer events, each at a separate potential, a voltammogram can exhibit peaks that correspond to each individual step in the overall process of reduction or oxidation. An example of such a system was encountered in our laboratory during an investigation38 of the direct and catalytic reduction of hexachlorobenzene (a fungicide prohibited from use in 1966) to yield less harmful benzene at a glassy carbon cathode. As depicted in Figure 3, sequential

onto the electrode surface; and (f) mechanistic information about chemical reactions that precede or follow electron transfer. Most important is the fact that cyclic voltammetry is carried out in a cell (sometimes mounted on a vibration-free stand) in which the solvent/electrolyte containing an electroactive species is unstirred (motionless), so that diffusion is the sole means of mass transport. Cyclic voltammetry is usually performed with a one-compartment, three-electrode cell, in which a potential is applied between the working and reference electrodes and the corresponding current between the working and auxiliary (counter) electrodes is recorded. A plot of current versus applied potential (i.e., a voltammogram) yields information about the electrochemical system. Linear-scan voltammetry is the most fundamental technique, where the applied potential is scanned in just one direction, either toward more positive (anodic) potentials or toward more negative (cathodic) potentials, the latter being the essence of classic polarography. More commonly, cyclic voltammetry, as it is practiced today, involves both a forward scan and a return scan, so that the electrochemistry of the parent compound and that of any product(s) or intermediate(s) can be revealed. Today, all of the required, computer-controlled instrumentation and data-acquisition systems that are needed to perform cyclic voltammetry over a wide range of scan rates can be obtained from commercial sources that are often identified in the experimental sections of published reports. Shown in Figure 2 is a photograph of a one-compartment glass cell for cyclic voltammetry that was designed and fabricated in

Figure 3. Cyclic voltammograms recorded at a scan rate of 100 mV s−1 for the direct reduction of separate solutions containing (A) 5.0 mM hexachlorobenzene and (B) 5.0 mM pentachlorobenzene in oxygen-free dimethylformamide (DMF)/0.050 M tetramethylammonium tetrafluoroborate (TMABF4) at a glassy carbon disk electrode with a geometric area of 0.071 cm2.38 Each scan went from −0.2 to −2.3 to −0.2 V. Information about the reference electrode is given in the caption for Figure 1. Reprinted with permission from ref 38. Copyright 2007 Elsevier B.V.

Figure 2. Photograph of a one-compartment, three-electrode cell for cyclic voltammetry. The size and construction of this cell are described in the literature.37

our laboratory more than 30 years ago.37 This cell has close relatives that can be purchased from companies specializing in electrochemical instrumentation and supplies. Over the years, we have modified our original design to accommodate different kinds of working electrodes; for example, the working (central) electrode shown in Figure 2 is a glassy carbon disk (constructed by press-fitting a short length of commercially available glassy carbon rod into the bottom of a machined Teflon tube). In addition, the same cell has been used with a hanging mercury

reduction of each carbon−chlorine bond results in a cyclic voltammogram with six well-resolved cathodic peaks, with reductive dehalogenation becoming more difficult as each aryl carbon−chlorine bond is broken; similar behavior was seen for the five-step reduction of pentachlorobenzene. No anodic peaks are seen in Figure 3, because cleavage of a carbon−chlorine bond is completely irreversible; however, if the potential had been D


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2.2.2. Controlled-Potential Bulk Electrolysis. A controlled-potential (bulk) electrolysis, which represents a small step toward the eventual development of a large-scale method of electrochemical remediation, takes a much longer time than cyclic voltammetry. Depending on the nature of the chemical system and the cell design, a bulk electrolysis might require only 30−60 min, but benchtop electrolyses that last for several hours or even longer are not uncommon. Unlike cyclic voltammetry, controlled-potential electrolyses require highly efficient stirring of the solution (even ultrasonication) so that mass transport of the electroactive species to the electrode surface is high, which shortens the electrolysis time and minimizes the occurrence of perhaps unwanted side reactions (follow-up chemistry) involving intermediates and products. Among the most important goals of a bulk electrolysis are (a) controlling the potential of the working electrode to select a desired electron-transfer process and to drive it to completion; (b) separating, identifying, and quantitating the electrolysis products in ways that account for all of the starting compound; (c) probing and testing mechanisms for the formation of various products through the addition of chemical trapping agents for intermediates (radicals, carbanions, or carbocations); and (d) determining whether the results of a bulk electrolysis are in accord with earlier cyclic voltammetry studies. Only by knowing as much as possible about the products and mechanistic features of an electrochemical process can one hope to exploit it to full advantage for a practical remediation procedure. To begin a bulk electrolysis, one must first choose the potential at which to hold the working electrode (cathode or anode) versus a reference electrode to effect only the desired electron-transfer process. This potential is ordinarily selected from a cyclic voltammogram that was previously recorded for the same system (solvent, electrolyte, and electroactive species). For a bulk reduction, the potential of the working cathode should typically be at least 100 mV more negative than the cathodic peak potential (Epc) observed from a cyclic voltammogram (to maximize the current and to complete the electrolysis expeditiously); the opposite scenario applies for an anodic process.41 These criteria apply with ease to situations where the previously recorded “reference cyclic voltammogram” exhibits only a single stage of reduction or oxidation. However, if a cyclic voltammogram for reduction or oxidation of the chosen compound exhibits two or more cathodic or anodic peaks (and if these peaks are too close together in their potentials), there is a danger that more than a single electron-transfer event will occur and that the selectivity in the choice of the desired process will be compromised or lost. Indeed, because a bulk electrolysis invariably requires the passage of substantial current, the working cathode or anode will seldom be a truly equipotential surface, in which case it might become very challenging, if not impossible, to carry out a “true” controlled-potential electrolysis. This feature of bulk electrolysis should always be borne in mind. At the start of an electrolysis of a halogenated organic compound (or any other substrate), a known quantity of an electroinactive internal standard (e.g., n-decane or n-hexadecane) should be introduced into the solution to be electrolyzed. This internal standard must be properly chosen and have excellent and distinct characteristics for the separation and quantitation [usually by a chromatographic method such as gas chromatography (GC) or gas chromatography−mass spectrometry (GC− MS)] of an eventual mixture of electrolysis products. At the end of the reduction, the catholyte (containing products and internal standard) can be partitioned between diethyl ether and water,

scanned far enough in the positive direction at the end of the cathodic scan, an anodic peak for the conversion of liberated chloride ions to chlorine would have appeared. In addition to the measurement of redox potentials, determining the reversibility of an electron-transfer process is important and meaningful with respect to mechanistic features of the system of interest. With cyclic voltammetry, reversibility can easily be assessed by the observation of a return peak on the reverse scan. For a truly reversible system that begins with a reduction, the ratio of anodic to cathodic peak currents (Ipa/Ipc) will be 1, and the peaks themselves should be separated by (59 mV)/n, where n is the number of electrons involved in the process of interest; however, variations of the peak separation often occur experimentally.39 Another clear indication of reversibility is that the value of the peak potential for the forward potential scan is independent of the scan rate. Even simple linear-scan voltammetry can be used to determine reversibility through the latter criterion. In practical terms, electrochemical processes done to accomplish environmental remediation of halogenated organic compounds are inherently irreversible because of the cleavage of carbon−halogen bonds, a process that does render these pollutants less harmful. Such a scenario was already mentioned above for the direct reductions of hexachlorobenzene and pentachlorobenzene. Sometimes, during the electrochemical reduction of a halogenated pollutant, a new intermediate is generated that can be further reduced. During a recent study40 of the reduction of the legacy pesticide 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) or DDT at silver cathodes, our laboratory found that an intermediate (1,1-diphenylethylene, 1), arising from the electrolysis of DDT, can itself undergo final irreversible reduction of its carbon−carbon double bond to afford 1,1′ethylidenebisbenzene (2):

In Figure 4, which shows a complete cyclic voltammogram for the irreversible reduction of DDT at a silver electrode, the peak

Figure 4. Cyclic voltammogram recorded at 100 mV s−1 for direct reduction of 5.0 mM DDT at a silver disk cathode (geometric area of 0.071 cm2) in oxygen-free dimethylformamide (DMF)/0.050 M tetramethylammonium tetrafluoroborate (TMABF4). The scan went from ca. 0 to −2.15 to 0 V. Information about the reference electrode is given in the caption for Figure 1.

seen at −1.80 V is associated with the conversion of 1 to 2. However, the most compelling discovery of this investigation was that, at a silver gauze cathode, DDT can be completely dechlorinated when an exhaustive electrolysis is conducted at a potential of −1.62 V (corresponding to the third cathodic peak in Figure 4). E


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gram (perhaps revealing the appearance of a new electroactive species). For other applications involving the consumption or formation of a colored species (intermediate), we fused a quartz spectrophotometer cell to the bottom of the electrolysis cell so that ultraviolet−visible spectra could be acquired in situ.44 Electrochemical−electron paramagnetic resonance (EC−EPR) experiments provide another technique for the identification of intermediates or products of an electrochemical process.45 For readers seeking other sources of information about cyclic voltammetry and controlled-potential electrolysis, we suggest the books by Kissinger and Heineman,20 Bard and Faulkner,46 and Wang.47 2.2.3. Direct versus Catalyzed Bulk Electrolysis. When a bulk electrolysis is carried out, another important consideration pertains to the strategy employed to cleave reductively the carbon−halogen bond(s) of a particular pollutant. Is it possible to carry out direct reduction of one or more carbon−halogen bonds, or must the desired bond cleavage(s) be accomplished with the aid of a catalyst that is electrogenerated in situ? To address this question, knowledge of the ease or difficulty of cleaving a particular carbon−halogen bond is helpful. Bond enthalpies (in kJ mol−1) for the four relevant carbon−halogen bonds are as follows: CI, 213; CBr, 285; CCl, 327; and CF, 566.48 These data indicate that carbon−iodine and carbon−bromine bonds are easiest to reduce and that a carbon− fluorine bond is most difficult to cleave. Depending especially on the identity of the working electrode, carbon−chlorine bonds might be reducible directly, whereas direct electrochemical cleavage of a carbon−fluorine bond has not been observed. This topic will be revisited in more detail later, when we discuss the electrochemical behavior of specific classes of halogenated pollutants. If preliminary cyclic voltammetric experiments show no evidence for direct cleavage of a targeted carbon− halogen bond, it might be possible to electrogenerate in situ an active catalyst that will promote reduction of the desired species at a potential where the carbon−halogen bond is not directly reducible. However, using a catalyst, electrogenerated in situ, can affect the mechanisms involved in the formation of products. In a recent publication49 from our laboratory, criteria for the selection and behavior of an electrogenerated catalyst, exemplified by [[2,2′-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]N,N′,O,O′]nickelate(I), better known as nickel(I) salen (3), were discussed as a way to achieve more facile and complete reduction of halogenated compounds.

after which the ether phase is dried and concentrated. If selected properly, the internal standard will not be lost during these operations, so the ether extract containing the electrolysis products and the internal standard can be analyzed to identify and quantitate each product in absolute terms. Ideally, the sum of the absolute yields of the various products should match the amount of starting material used at the beginning of the electrolysis. A publication from our laboratory describes this procedure for the quantitation of products (which can be volatile if a sealed electrolysis cell is employed).42 Figure 5 shows a two-compartment cell (with the lower, cathode compartment having a capacity of 20−30 mL) that can

Figure 5. Photograph of a two-compartment cell that can be used to carry out a benchtop controlled-potential (bulk) electrolysis.43 Reprinted with permission from ref 43. Copyright 2005 The Electrochemical Society.

be used for benchtop controlled-potential (bulk) electrolyses. A full description of the construction of this cell appears in a publication43 from our laboratory. Using this cell, we have carried out bulk electrolyses with reticulated vitreous carbon cathodes with surface areas of 200 cm2 and with silver mesh cathodes having areas ranging from 20 to 40 cm2. For every bulk electrolysis, separation of the cathode and anode compartments by an ion-conductive membrane is mandatory to minimize, as much as possible, the transport of electrolysis products from one compartment to the other; if this is not done with some success, products formed at the cathode will migrate to the anode and undergo oxidation, and vice versa. Another design feature of this cell is that the upper anode compartment is positioned symmetrically above the cathode so that the latter is as close as possible to being an equipotential surface. In the older electrochemical literature, numerous cells for bulk electrolysis were fashioned after the classic H-cell from polarography, a practice ensuring that the working electrode was definitely not an equipotential surface. Although one usually monitors the progress of a controlledpotential electrolysis by recording (monitoring) the current− time curve (a first-order exponential decay), from which one can calculate the quantity of electricity passed and thus the coulometric n value for an electron-transfer process, the cell in Figure 5 included a provision for the use of cyclic voltammetry to monitor the progress of the electrolysis. Accordingly, one could periodically interrupt an electrolysis to record a cyclic voltammo-

On the basis of the preceding paragraph, it is evident that many halogenated pollutants cannot be reduced directly at common cathodes, but might be reducible indirectly with the aid of an electrogenerated catalyst. Therefore, the choice of a homogeneous-phase catalyst that is electrogenerated in situ is a vital one. For example, as discussed in the previously cited reference,49 the catalytic reduction of ethyl 2-bromo-3-(3,4-dimethoxyphenyl)3-(prop-2-yn-1-yloxy)propanoate by electrogenerated nickel(I) salen would have been less successful because the potentials for reduction of the substrate and of nickel(II) salen (the catalyst precursor) are so close to each other that both catalytic and direct F


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reduction would occur simultaneously. Thus, the electrolysis might have proceeded slowly, and undesired products might have been formed. Therefore, the catalyst precursor chosen for this research was nickel(II) tetramethylcyclam, because it is more than 500 mV easier to reduce than the substrate, thereby ensuring only a catalytic process. This important topic was delineated in detail in a series of articles by Andrieux, DumasBouchiat, and Savéant.50−54 Using an electrogenerated catalyst for a controlled-potential electrolysis necessitates an extra experimental step if the coulometric n value for reduction of the halogenated compound is to be determined. Specifically, the procatalyst (or catalyst precursor) must be reduced completely before the substrate is added to the electrochemical cell and the electrolysis is resumed, so that the quantity of electricity and the n value associated with reduction of that substrate can be unambiguously obtained. 2.2.4. Constant-Current Bulk Electrolysis. Another form of bulk electrolysis, namely, constant-current electrolysis, deserves brief mention at this point. This method involves passage of a chosen constant current between just two electrodes, a working electrode and an auxiliary (counter) electrode, in either a one- or two-compartment cell, the choice being dictated by what “side reaction” takes place at the auxiliary electrode. Such a strategy allows for the use of simple instrumentation: All that is needed is an adjustable direct-current power supply, which makes this approach, at least in principle, more easily adaptable to large-scale electrochemical remediation. However, this constant-current technique suffers from at least three possible disadvantages. First, the current efficiency for the desired electron-transfer process diminishes continually throughout the electrolysis, as starting material is consumed. Second, if a onecompartment cell is utilized, the process occurring at the auxiliary electrode can interfere chemically with the desired reaction at the working electrode. Third, consumption of starting material eventually forces the potential of the working electrode to shift in a direction that will sustain the preselected constant current; for example, if one is reducing a halogenated organic pollutant, the potential of the cathode will shift eventually to a more negative value, causing product species or even the solvent/electrolyte to undergo coreduction. To overcome these problems, one might design a programmable constant-current source, but if this device decreases the current systematically, the electrolysis time will be prolonged.

Figure 6. Cyclic voltammograms recorded at four different scan rates (50, 100, 250, and 500 mV s−1) for the direct reduction of a 5.0 mM solution of 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) at a silver disk cathode (geometric area of 0.071 cm2) in oxygen-free dimethylformamide (DMF) containing 0.050 M tetra-n-butylammonium tetrafluoroborate (TBABF4). Each scan went from ca. 0 to −2.2 to 0 V. Information about the reference electrode is given in the caption for Figure 1. Reprinted with permission from ref 55. Copyright 2016 The Electrochemical Society.

trichloroethane-1,1-diyl)bis(methoxybenzene); this compound, known more familiarly as methoxychlor, served temporarily as a replacement for DDT.55 As the scan rate is increased, the respective cathodic peak currents increase as expected; correspondingly, the various cathodic peak potentials (Epc) shift toward more negative values. In principle, sequential reductive cleavage of the three aliphatic carbon−chlorine bonds of this compound could proceed by stepwise or concerted mechanisms, the distinction being that a stepwise process involves a short-lived radical-anion intermediate: RX + e− ⇌ [RX•−] → R• + X−

RX + e− → R• + X−

(stepwise)

(concerted)

In an important monograph, Savéant56 described how to make this distinction on the basis of the value of the parameter α, called the transfer coefficient, as well as from the experimentally observed variation of cathodic peak potential (Epc) as a function of cyclic voltammetric scan rate (v). In simplest terms, as described by Bard and Faulkner,46 α is a measure of the symmetry of the energy barrier for the electron-transfer process, which reveals whether the transition state more closely resembles the starting material or the product species. Of further relevance is a publication by Isse and co-workers,27 who compared the cyclic voltammetric characteristics for the reductions of various organic halides at both glassy carbon and silver cathodes. From experimentally acquired data (Epc versus v), these investigators obtained values for ∂Epc/(∂ log v), α, and the width of the voltammetric peak, so that each of the halogenated organic species could be characterized as undergoing concerted or stepwise reduction. Using the same approach, our laboratory concluded that reductive cleavage of each carbon−chlorine bond of methoxychlor is a concerted process. Although there have been efforts to correlate values of α with whether reductive cleavage of a carbon−halogen bond is a concerted or stepwise event, there are enough cases where the correlation is less than perfect that caution must always be exercised. A different strategy for the acquisition of transfer coefficients (values of α) involves the construction of Tafel plots, which

3. MECHANISTIC ASPECTS OF CARBON−HALOGEN BOND CLEAVAGE This section is intended to survey various strategies employed to investigate mechanistic features of processes associated with the reductive cleavage of carbon−halogen bonds that might be encountered in the electrochemical remediation of a halogenated organic pollutant: (a) correlating the transfer coefficient (α) with the occurrence of stepwise or concerted carbon−halogen bond cleavage; (b) trapping and identifying intermediates (radicals and carbanions); (c) performing hydrodynamic voltammetry to identify and characterize intermediates; (d) correlating product distributions with mechanisms; (e) performing stripping analysis; and (f) analyzing surface phenomena. 3.1. Transfer Coefficient (α); Stepwise versus Concerted Carbon−Halogen Bond Cleavage; Other Heterogeneous and Homogeneous Processes

Shown in Figure 6 is a family of cyclic voltammograms recorded at four different scan rates for the reduction of 4,4′-(2,2,2G


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consist of plots of current versus overpotential;46 however, this approach seems to have waned in popularity. Rates of heterogeneous electron-transfer processes are dependent on (a) the concentrations of species at the surface of the electrode (as governed by diffusional and/or convective mass transfer); (b) chemical reactions that take place before or after electron transfer; and (c) the potential, area, and identity of the electrode. Such phenomena have been treated in the literature. A seminal article by Nicholson and Shain39 focused on how to interpret cyclic voltammograms for systems in which a solution-phase reversible or irreversible chemical reaction is coupled with an electron-transfer event. In a subsequent publication, Nicholson57 described how cyclic voltammetry can be used to determine the standard rate constant for a heterogeneous electron-transfer reaction. In later research, Klingler and Kochi58 expanded the latter work to measure rate constants for the irreversible reductions of a number of alkylmetal compounds at a platinum cathode in acetonitrile containing tetraethylammonium perchlorate.

In a study60 of the reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) at silver cathodes in organic and organic/ aqueous media, deuterium oxide (D2O) was added to the system for some experiments designed to detect the intermediacy of carbanionic species. In particular, 1,2-dichloro-1,1,2-trifluoroethane (or HCFC-123a), one of the products arising from the electrolytic reduction of CFC-113, was examined by means of GC−MS after an electrolysis in a 50:50 mixture of dimethylformamide and deuterium oxide; the mass spectrum revealed that HCFC-123a was monodeuterated, a finding that indicated the presence of a carbanion intermediate. Cells have been designed for simultaneous electrochemical and nuclear magnetic resonance (NMR) experiments that permit real-time monitoring of the formation and follow-up reactions of intermediates.61 One trapping method for unstable radical intermediates involves their reaction with a nitrone to form long-lived species that can be conveniently studied with the aid of electron paramagnetic resonance (EPR) spectroscopy.62 Radicals have been trapped and observed by means of electrochemiluminescence.63 Anions can be trapped through their reaction in situ with carbon dioxide, which leads to the formation of identifiable carboxylates, a process that has attracted attention for the sequestration of carbon dioxide.64−66 When various tetraalkylammonium salts (except for those with a tetramethylammonium cation) are employed as supporting electrolytes in organic solvents, electrogenerated bases can induce a classic Hofmann elimination, which produces the corresponding trialkyl amine, a species readily detected by means of GC or GC−MS, which indicates the presence of carbanionic intermediates. In our laboratory, the use of deuterated reagents to test for the possible intermediacy of radicals or carbanions has become a routine and recommended practice for elucidating mechanisms for the electrochemical reductions of halogenated organic compounds.

3.2. Trapping and Identifying Intermediates (Radicals and Carbanions)

Depending on the identity of a halogenated environmental pollutant, as well as the nature of the solvent/electrolyte system and the electrode material, radicals or carbanions can be generated as intermediates during the reductive cleavage of carbon−halogen bonds. Trapping and identifying these species is vital to elucidating the mechanistic features of the reactions that ultimately lead to the final electrolysis products. In this section, we describe in some detail two examples of studies conducted in our laboratory, where it was of interest to demonstrate the intermediacy of either a radical or a carbanion. Then, some additional approaches to the identification of intermediates are mentioned. In an investigation of the reduction of lindane (4) at silver cathodes in both organic and organic/aqueous media,59 we found that benzene was the predominant product. Thus, one could imagine a mechanistic scenario in which a carbon−chlorine bond undergoes a two-electron cleavage, ejecting a chloride ion and leaving behind a carbanion that intramolecularly displaces an adjacent chloride ion with formation of a carbon−carbon double bond. On the basis of this picture, a total of six electrons would be injected into each molecule of lindane to form benzene along with six chloride ions. Experimentally, we usually observed coulometric n values between 5 and 6 (depending on the medium), which suggested that one of the carbon−chlorine bonds of 4 (or of an intermediate) might undergo just a oneelectron cleavage to afford a radical intermediate. Therefore, bulk electrolyses of 4 were conducted in fully deuterated dimethylformamide (DMF-d7), and gas chromatography−mass spectrometry (GC−MS) was employed to identify the products. Two prominent signals were seen for m/z 218.95 and 220.95, which were attributed to C6H5D35Cl4 and C6H5D35Cl337Cl, respectively, leading us to conclude that one-electron cleavage of a carbon−chlorine bond to afford a radical intermediate is likely.

3.3. Hydrodynamic Voltammetry to Identify and Characterize Intermediates

Hydrodynamic voltammetry consists of a family of electrochemical methods in which the electrode system itself moves (rotates) with respect to the solution in which the electrode is immersed.46,67,68 Most well-known hydrodynamic methods utilize a rotating disk electrode (RDE) or a rotating ring-disk electrode (RRDE). Many companies that market electrochemical instrumentation (including potentiostats, constantcurrent supplies, and cells) offer a remarkable selection of electrode systems and instrumentation that are designed specifically for hydrodynamic voltammetry. For mechanistic studies, the RRDE is especially useful. The potentials of the individual disk and ring electrodes can be individually controlled (kept constant or swept) with the aid of a bipotentiostat. Thus, one can initiate a particular electrontransfer event at the disk, and the intermediate(s) or product(s) of that process are swept radially toward the ring at which they can be detected, monitored, and even identified through the electrochemical “signature” of some species of interest. In our laboratory, a home-built glassy carbon rotating ring-disk electrode was employed to investigate the base-catalyzed isomerization of 1-phenyl-1-hexyne to 1-phenyl-1,2-hexadiene in dimethylformamide containing tetra-n-butylammonium perchlorate.69 3.4. Correlating Product Distributions with Mechanisms

Brief mention should be made here of an important set of publications from the laboratory of Savéant and co-workers who showed how product distributions arising from preparative-scale H


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benzyl chloride at silver cathodes, provided compelling evidence based on a combination of electrochemistry, electrochemical surface-enhanced Raman spectroscopy (EC-SERS), and density functional theory (DFT) for the presence of adsorbed benzyl chloride, benzyl radicals, and benzyl carbanions. In the last of these publications,84 the authors recognized that benzyl chloride undergoes a concerted electron-transfer dechlorination, in contrast to what was reported in the preceding two articles, and that adsorption of benzyl chloride and its reduction products (benzyl radical and, perhaps, chloride anion) takes place, according to a previous investigation by Isse et al.85 These findings allowed a distinction to be made between the occurrence of concerted and stepwise carbon−chlorine bond scission, events that are favored thermodynamically and kinetically, respectively. Simonet and colleagues86 employed scanning electron microscopy (SEM) to detect n-butyl iodide adhered to the surface of a silver electrode; such a method is valuable for the examination of stable adsorbed films but does require disassembly of the electrochemical cell. Other examples of the use of a spectroscopic technique coupled with electrochemistry to investigate surface phenomena can be found in the articles by ́ pez.88 Cao et al.87 and by Rodriguez-Ló In recent (unpublished) work in our laboratory, the electroreductions of 1-iododecane and 1-bromodecane at silver cathodes in dimethylformamide containing 0.050 M tetramethylammonium perchlorate were examined with the aid of EC-SERS and DFT. Evidence was obtained for the adsorption of each of these alkyl halides onto silver. One limitation of EC-SERS is that signals from multiple adsorbates can overlap each other in the same spectrum. However, there is a complementary in situ technique known as the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) that can probe for the presence of both an adsorbed alkyl halide and an intermediate arising from its reduction and that deserves future attention for electroreduction of halogenated compounds.89−93 So far, however, this strategy has not been applied to electrochemical studies in nonaqueous solvents.

electrolyses could be utilized to discern various mechanistic schemes for the formation of those products.70−77 3.5. Stripping Analysis

Electrochemical reduction of a halogenated organic pollutant releases free halide ions into the solvent/supporting electrolyte. For example, if dibromochloromethane (CHBr2Cl) is electrolyzed at a silver cathode in water containing 0.010 M tetraethylammonium benzoate, 2 equiv of bromide ion and 1 equiv of chloride ion are released in an overall six-electron process: CHBr2Cl + 3H+ + 6e− → CH4 + 2Br − + Cl−

One can determine the concentration of CHBr2Cl (or a different trihalomethane) by itself in a water sample by using the classic method of standard addition in conjunction with the technique of cathodic stripping analysis.46,78,79 If, under carefully controlled conditions, an originally clean silver electrode is anodized in a solution of volume Vunknown that results from prior reduction of CHBr2Cl, a silver halide film will be deposited onto the silver anode. Then, the silver halide-coated electrode can be cathodically polarized to determine the quantity of electricity (Q1) associated with the reduction of the silver halide back to elemental silver. Next, a small known volume (Vknown) of a sodium chloride solution of known concentration (Cknown) is added to the original halide-containing sample arising from the original reduction of CHBr2Cl, and the quantity of electricity (Q2), now greater than Q1, that is associated with reduction of the new silver halide film is measured. Finally, the concentration of dibromochloromethane (CCHBr2Cl) in the original water sample can be calculated from the equation CCHBr2Cl =

Q 1(C known)(Vknown) 3[Q 2(Vunknown + Vknown) − Q 1(Vunknown)]

where the factor of 3 indicates that three halide ions are released from each molecule of CHBr2Cl that is reduced. The results of such experiments can be used to confirm the suspected stoichiometry of the cathodic process for reduction of CHBr2Cl. This method of stripping analysis has been applied to the determination of several other trihalomethanes that comprise one family of disinfection byproducts that arise from the chlorination of drinking water.80 This strategy should be worthwhile for electrochemical studies of other halogenated organic pollutants.

4. TECHNIQUES FOR ELECTROCHEMICAL REMEDIATION: FROM BENCHTOP TO INDUSTRIAL SCALE Although our laboratory is not involved in the development and testing of large electrochemical reactors that could be employed for the bulk remediation of halogenated organic pollutants, the short sections below are intended to make readers aware of various factors that come into play for these purposes. As overviews, the several versions of Organic Electrochemistry, edited originally by Baizer and later by others, are notable because they address topics pertaining to industrial-scale electrochemistry.94−96 Chapters in a recent book edited by Pletcher, Tian, and Williams97 discuss many practical aspects of large-scale organic electrochemistry. As described earlier, the two principal and most popular techniques that can be utilized for benchtop electrochemistry are (a) cyclic voltammetry and (b) controlled-potential (bulk) electrolysis, although some bulk electrolyses can be accomplished with constant current. Properly conceived and executed, these methodologies can lead to the eventual development of industrial-scale electrochemical reactors for environmental remediation. Electrode materials were discussed in an earlier section of this review; however, to be of use, electrodes must have a computer interface to permit potential and current to be

3.6. Surface Phenomena

Although difficult to characterize, surface phenomena such as the adsorption of starting materials, intermediates, and products can play a pivotal role for mechanisms involved in the reduction of halogenated organic compounds. In cyclic voltammetric studies, adsorption can manifest itself by the appearance of pre- or postpeaks in cyclic voltammograms;81 however, complex chemical systems often exhibit small shoulder peaks or merged peaks that can be mistaken as evidence for adsorption. In a recent investigation of the electrochemical behavior of several families of alkyl monohalides at silver cathodes in dimethylformamide,29 the cyclic voltammograms for the reductions of 1-iododecane, 2-iodohexane, and tert-butyl iodide each exhibited normal-looking, diffusion-controlled peaks together with anomalously sharp cathodic peaks suggestive of adsorption phenomena; plots of cathodic peak current versus scan rate supported this picture. Previously, three articles by Amatore and co-workers,82−84 dealing with the reduction of I


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working electrode and an auxiliary (counter) electrode, as the potential of the working electrode is scanned (or fixed) with respect to the reference electrode. If the current flowing through the electrochemical cell is very small (probably true for many applications involving the detection and measurement of a pollutant but certainly not for a bulk electrolysis), one can use a two-electrode cell, in which the reference and auxiliary electrodes are one and the same. In every design, the reference electrode must be placed very near the surface of the working electrode to minimize the voltage drop between these two electrodes and to ensure that the potential of the working electrode is reliably measured and controlled. Ideally, the auxiliary electrode should be positioned so that a uniform current distribution is achieved across the surface of the working electrode. Three-dimensional electrodes, such as carbon felt101,102 or packed beds, are ideally used in large cells containing low concentrations of substrate or mediators (including electro-Fenton processes). An especially interesting electrode configuration is known as the “Swiss-roll” in which sheets of anode and cathode material are rolled into a three-dimensional spiral.103 In larger reactors, multiple anodes and cathodes can be used in either a monopolar or bipolar configuration to amplify the current. Bipolar systems have the advantage that only the first and last electrodes need to be connected to the power supply. We encourage the reader to consult some of the previously cited references3,4,94−100 for additional information pertaining to the number of electrodes, cell designs, and operating procedures employed for industrialscale applications. 4.2.3. Number of Compartments: Divided versus Undivided Cells. In an electrochemical cell, oxidation and reduction take place, respectively, at the anode and cathode, either of which can serve as the working electrode. In many cases, products generated at the working electrode migrate into the auxiliary-electrode compartment and subsequently participate in undesirable reactions. This problem can be largely eliminated if a low-resistance sintered-glass disk or some other permeable membrane, for example, porous diaphragms or cationic (Nafion) membranes, is placed between the cathode and anode compartments to create a divided cell. More information about separating the anode and cathode compartments can be found elsewhere.3,4,94−100 Usually, a sintered-glass disk is backed by an ionpermeable gel formed from a hot mixture of solvent, electrolyte, and methyl cellulose (or agar) that is allowed to cool and set in place. Divided cells are more costly and difficult to construct than undivided cells, and they tend to suffer from electrode and membrane damage over time. In cases where use of a divided cell is not feasible, a sacrificial auxiliary anode can be placed, along with the working cathode, in an undivided cell. Of course, corrosion (dissolution) of this sacrificial electrode does occur; therefore, inexpensive metals such as aluminum, magnesium, and iron are common choices, but there can be situations in which the cation formed from oxidation of the sacrificial anode can play a role in the chemistry that takes place. Industrial electrodeposition of metal, through reduction of metal ions found in wastewater, is routinely performed in this manner in large electrolytic cells.104 Degradation of 2,4,5-trichlorophenoxyacetic acid (5), an undesirable herbicide, by means of an electro-Fenton oxidation carried out in an undivided cell was reported by Boye and coworkers.105

controlled and monitored and to store these data for subsequent analysis. 4.1. Instrumentation

It is relatively straightforward to assemble home-built, benchtop electronic circuitry necessary for electrochemical measurements, particularly with a series of operational amplifiers. An adder circuit is commonly used as the basic electronic system to supply complicated waveforms (synthesized by addition of simpler signals). A voltage follower and a current follower allow for the monitoring of potential and current at the working electrode.20,46 Once this instrumentation is available, it is easy to transfer output data to a computer. Our laboratory utilizes data acquisition (DAQ) cards, which can be interfaced with most computer systems, to export experimental data into a text file for easy analysis with the aid of a computer program. On the other hand, there are many potentiostat/galvanostats available on the market. For many years, our laboratory has consistently used instrumentation available from the Princeton Applied Research Corporation; however, there are many other sources worthy of consideration, including Atlas-Sollich, BioLogic, Bioanalytical Systems Inc., CH Instruments Inc., Gamry Instruments Inc., Ivium Technologies BV, Metrohm Autolab BV, NuVant Systems Inc., and Stanford Research Systems. Most of these suppliers have an ever-increasing selection of electrochemical cells and electrodes for both cyclic voltammetry and controlled-potential electrolysis, as well as more specialized techniques such as hydrodynamic voltammetry with rotating disk and ring-disk electrodes. It is important to recognize that some instruments are more suited for certain experiments than others, so electrochemical instrumentation must be compatible with the aim and scale of the intended investigation. For example, one will need to anticipate the voltage and currents expected in a particular set of experiments; a potentiostat designed to deliver nanoampere currents very precisely is unlikely to provide the high currents required for the bulk electrolysis of large amounts of material. In addition, some applications might require a bipotentiostat or a high-frequency generator. 4.2. Electrochemical Cell Designs

The design of an electrochemical cell must be undertaken with a variety of factors in mind: (a) volume to be contained, (b) shape, (c) number of electrodes (and the structure of each), (d) number of compartments (and means for their separation, if necessary), (e) mode of mass transport, (f) temperature control, and (g) electrical connections to interface with the instrument. Brief descriptions of each of these topics follow; further reading from other sources is recommended.98−100 4.2.1. Volume and Shape of the Cell. Cell volume is largely dictated by the goal of the electrochemical experiment. For small-scale cyclic voltammetric detection of a pollutant, a very small cell volume (even at the microchannel scale) is ideal to conserve sample. However, in a bulk electrolysis intended for the degradation of a large chemical stockpile or for the treatment of an effluent waste, a much larger reactor is desired that might be limited by economic concerns. Similarly, the shape of a cell is often determined by the application and by the desired area of the working electrode. To convert an electroactive pollutant into a more environmentally friendly product, a cell must be large enough to accommodate a working electrode of large surface area and to allow for the efficient mass transport of the pollutant to the surface of that electrode. 4.2.2. Number of Electrodes. For most systems, a threeelectrode geometry is preferred. Current is passed between a J


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stirring of the solution, currents for a benchtop bulk electrolysis can initially range from 10 to 50 mA (if not higher) before undergoing a normal exponential decay. Thus, the temperature (Joule heating) of the solvent/electrolyte can rise significantly during the early stages of an electrolysis, which demands some form of temperature control.

4.2.4. Modes of Mass Transport. Transporting electroactive material to the surface of the working electrode is a challenge in cell engineering: the higher the rate of mass transport of electroactive species to the surface of an electrode, the larger the current and the less time needed to complete an electrolysis. Of course, the required presence of a sufficiently high concentration of supporting electrolyte increases the conductivity of the solvent medium while minimizing the transport of ionic species other than the supporting electrolyte to and from an electrode surface through electrical migration. Ultimately, the only significant mode of mass transport of electroactive species to an electrode during an electrolysis is diffusion. Even if a solution in contact with an electrode is vigorously stirred, there is always a thin, hydrodynamically immobile layer of liquid (the so-called Nernst diffusion layer) next to the electrode, and it is diffusion alone through this diffusion layer that brings the electroactive substance to the electrode surface where electron transfer can occur. However, in any bulk electrolysis, it is essential to introduce forced convection to diminish the thickness of the Nernst diffusion layer, thereby maximizing both the flux of fresh substrate to the surface of the working electrode surface and the resulting current due to the electron-transfer process. Efficient stirring can be accomplished in a number of ways. It can be achieved by continuous rotation of the solution or the electrode. In many benchtop experiments, a magnetic stirbar suffices, whereas industrial-scale reactors often are equipped with a rotating paddle within the reaction vessel. Rotating cylindrical electrodes are a creative way to increase solution convection.106 Other examples include the development of the Eco-Cell,107 a cell used to recover silver,108 and a cell designed to remove zinc ions,109 and the principles involved in these applications could be extended to the electrochemical remediation of environmental pollutants. As will be mentioned later, there is a class of largescale reactors that utilizes pumps to circulate solution in a flowthrough manner. Banks and Compton110 reviewed the use of ultrasonic horns to enhance mass transport in electrochemical studies. Sonoelectrochemistry has shown great promise in several published reports,111−113 and at least in one case, it has already been scaled up to a prepilot reactor.114 A recent review highlights additional applications of sonoelectrochemistry as an emerging technology for pollutant degradation.115 4.2.5. Temperature Control. At times it might be desirable, if not essential, to conduct electrolyses at different temperatures to enhance product selectivity or to deal with volatile substrates or products. In our laboratory, for example, in an exploratory study of the nickel(I) salen-catalyzed reduction of 1,2dichlorotetrafluoroethane (CFC-114) in dimethylformamide containing 0.10 M tetramethylammonium tetrafluoroborate, the volatility of the freon (boiling point = 3.5 °C) mandated that cyclic voltammetry studies be conducted at −38 °C in a small one-piece cell with a glass jacket through which an appropriate coolant was pumped continuously. Depending on the concentration of a targeted substrate, the number of electrons involved in the reduction or oxidation of that substrate, the area of the working electrode, and the rate of

4.3. Large-Scale Bulk Electrolysis

Although there is much interest in environmental electrochemistry, the scaling-up of laboratory experiments to the level of a practical process is an area where much work needs to be done. If the eventual goal is to electrolyze a large amount of a material, small-scale “pilot-plant” experiments must be conducted on the benchtop with ordinary cells and equipment; in this way, operating parameters can be tested, modified, and optimized before “large-plant” tests are undertaken. Arguably, the most famous and successful large-scale electrolytic process is the electrohydrodimerization of acrylonitrile to afford adiponitrile (a building block of nylon); this and other examples from industry provide environmental electrochemists with appropriate models for scaled-up processes. Earlier reviews describe some large-scale electrochemical reactors already used for environmental remediation;116,117 the second of these focuses particularly on industrial heavy-metal effluents, but there is still much room for future developments. In industrial electrochemistry, large-scale reactors are often categorized as being of a “batch” or “continuous” design. Further classification is possible on the basis of the extent of solvent/ electrolyte recycling, the flow pattern around electrodes (if rods are in series or parallel, or if solution flows through a porous three-dimensional electrode), and whether electrical connections are monopolar or bipolar. Other issues for plant reactors are the overall cost of the process, possibilities for recycling impurities and side products, concerns about corrosion, electrode stability over time, and selectivity versus yield of the product. Although it is beyond the scope of this review to present equations that describe each cell configuration that is in use, much consideration has gone into the modeling of the flow rate, kinetics, thermal and potential distributions, current flow, and mass transport in several common bulk-cell geometries; further reading is recommended for specific reactors of interest.96−100,117 4.3.1. Batch Reactors. Batch reactors operate by the complete electrolysis of a small amount of material, followed by removal of the products, whereupon the process is repeated for the next batch. Some industrial applications implement an electrolyte recirculation system to conserve solvent and electrolyte materials. Additionally, the flow contributes to mass transport. However, it can be difficult to extract the product(s) completely from the solvent/electrolyte; thus, over the course of time, contaminants can accumulate. Scaled-up batch-mode treatment of gray water containing personal care and household products revealed that phenolic rings were transformed on anodes; however, organohalogen concentrations were significantly increased during oxidation in this reactor. Although these results do not recommend oxidation as a treatment, possible solutions are proposed.117 Another batch reactor was designed with a volume capacity of 1.3 L with concentric electrodes of different lengths arranged in such way as to promote a spiral flow pattern, which improved the performance to achieve 89% degradation of the pesticide metribuzin (6).118 When a mercury lamp was positioned near the anode, the combined photochemical−electrochemical K


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An interesting cell was described in which a flowing solution is directed through a concentric anode, wrapped around a cylindrical cathode; with this setup, carbon tetrachloride was reduced at porous copper.127 Cordeiro and co-workers128 designed a system that can provide laminar or turbulent flow through a recirculating reactor around a boron-doped diamond− titanium anode; under optimal anode conditions, the insecticide profenofos (8) was degraded to the extent of 96.5% after 120 min of electrolysis.

process resulted in the degradation of metribuzin to an extent of 95%.

We suggest that these concepts for the operation of a batch reactor can be easily applied to the remediation of a halogenated pollutant. Thus, the targeted pollutant to be dehalogenated can be continuously electrolyzed in an appropriate batch reactor, with the resulting product(s) separated or extracted from the medium. 4.3.2. Flow-Through Cells. Batch reactors have the disadvantage of having a limited capacity, even when equipped with a recirculation system. For very high-throughput bulk electrolyses, a true flow-through cell is advantageous, as for the treatment of effluents. For long, porous (three-dimensional) electrodes, choosing the appropriate length is crucial; an electrode that is too short might not provide enough surface to convert all of the reactive species effectively, whereas an electrode that is too long might not be an equipotential surface such that poor selectivity is achieved. In many cases, flow-through cells do indeed result in more complete reactions than do single-pass or batch reactors. A filter-press-type cell is an interesting configuration in which plate-like anode and cathode chambers are parallel to each other, separated by gaskets and, in many cases, a charged, porous barrier. These cells are typically connected to a pump so that a pollutant-containing solution can be flushed into and out of the active chamber for a single pass or for multiple passes.119,120 Another recent development in the area of flow-through cells reported by Basha and co-workers121 involved testing the optimum reactor configuration to degrade dye industry effluents; it was found that recirculation is superior to a single-pass bath and that a recycle system of operation performs even better. Scialdone and colleagues122 conducted preparative-scale reductions of benzyl chlorides at silver electrodes in the presence of carbon dioxide both in an undivided glass tank cell and in a continuous recirculation system equipped with a filter press undivided microflow cell. Whereas the recirculation system allowed for faster mass transport and afforded higher faradaic efficiency, a loss in selectivity was noted. Additionally, scale-up at the silver cathode yielded results similar to those of smaller bench-scale reductions without degeneration of the silver. A recirculation flow plant was equipped with Pt/air-diffusion or boron-doped diamond electrodes to treat aqueous solutions of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D, 7) through electrochemical oxidation or electro-Fenton processes.123 In a related investigation,124 boron-doped diamond anodes were employed to treat secondary effluent from a municipal sewage treatment plant containing pharmaceuticals and pesticides. A homemade flow cell, designed by Fontmorin and co-workers, has been used for the oxidation of 2,4-D at graphite felt.125,126

Recently, a flow-through cell was developed by Ma and colleagues129 for the degradation of pollutant-containing effluents; a unique design allows these effluents to be directed either through the cathode or perpendicularly across the cathode. 4.3.3. Custom-Made Cells. Several specific environmental problems have been addressed through the design of novel electrochemical reactors. For example, different cell configurations were tested for the oxidation of synthetic wastewater containing 2,4-D at boron-doped diamond anodes; energy efficiency improved with an increase in the number of stacked cells.130 A recent study demonstrated that stacking 10 detachable electrolytic cells in a fashion similar to a single large flow-through cell results in a much higher efficiency for removal of a pollutant than a single batch reactor.131 A similar continuous multicell reactor with a volume of 8.4 L was successfully used to treat effluents from a textile plant.132 Arredondo and colleagues133 described a unique reactor containing a rotating cathode cylinder with six stationary anode plates. Rotation provides unique mass transport; as described, the cell has a capacity of 500 mL but can be enlarged for future applications. Another growing area of research is the coupling of electrochemical and photochemical processes. For example, Malpass et al.134 investigated the photoassisted degradation of pesticides in a single-compartment electrochemical cell. An azo dye was completely mineralized by Tantis and co-workers,135 who employed glass coated with a nanoparticulate titania film that functioned both as an anode and as a window to transmit light from a xenon lamp. Zhu and co-workers136 developed an apparatus that couples a solar cell to an electrochemical reactor to promote the photoreduction of heavy-metal cations in wastewaters. Interesting examples of sonoelectrochemical cells were described by Yasman and co-workers.111,112 Anode and cathode cylinders were suspended in an undivided jacketed cell, along with a transducer connected to a frequency generator. An electro-Fenton process was conducted with iron particles to oxidize chloroorganic pesticides. Esclapez and co-workers114 designed both a large batch reactor and a large flow-through reactor with ports for an ultrasonic horn for sonoelectrolyses of large amounts of material. 4.3.4. Separation, Identification, and Quantitation of Products. After completion of a bulk electrolysis, it is desirable to separate the products of interest from the solvent/electrolyte to identify and quantitate the former species. Often, the solvent can be distilled and recycled for use in future electrolyses. When solid products are formed, they can be recovered from the L


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solvent/electrolyte by ordinary filtration. Solution-soluble products can be isolated by means of distillation, crystallization, or extraction. Further separation of product mixtures can be accomplished with the aid of preparative-scale gas chromatography (GC), high-performance liquid chromatography (HPLC), or even one of the various forms of column chromatography. Depending on column conditions and the nature of the product(s), a derivatization procedure can be implemented to achieve better separation or easier detection. Once products have been initially separated and purified, they can be characterized and identified with the aid of NMR spectroscopy or crystallography techniques. In addition, GC and HPLC can be employed in tandem with high-resolution mass spectrometry to assist identification. After a preliminary product identification is made, the chromatographic response and mass spectrum of that species can be compared with those of a chemically pure standard that has been purchased or synthesized. Depending on the characteristics of a particular product, its yield can be determined by means of appropriate calibration curves that are based on UV−vis spectroscopy, GC, and HPLC. Solids can be quantitated by means of straightforward gravimetry. Whatever method is used to quantitate a product, it is crucial to have an appropriate internal standard or reference compound, so that the yield of each product is absolute in terms of the amount of starting compound that has been electrolyzed. The old practice of recording a gas or liquid chromatogram and measuring relative peak areas to obtain relative yields cannot be recommended. After many years of research with halogenated compounds that frequently afford volatile products, our laboratory has come to prefer an analytical strategy that emphasizes the use of a gas chromatograph with a capillary column and a flame-ionization detector (FID) to quantitate solution-phase and volatile products; the peak area of each separated product is compared with that of a carefully chosen internal standard, and an experimentally determined gas chromatographic response factor is used to calculate the absolute yield of each product.42 Of course, a proper capillary column must be chosen for each system of interest. For many environmental applications, we begin with a capillary column that has a stationary phase consisting of 5% phenylpolysiloxane and 95% dimethylpolysiloxane, because it is ideally suited for semivolatile organic compounds and alkyl halides, including most common pesticides. Halogenated compounds of higher molecular mass, such as polychlorinated biphenyls (PCBs), can require a polar column consisting of 100% dimethylpolysiloxane. For more volatile compounds of lower molecular weight, various porous layer open tubular (PLOT) columns are available on the market, each coated with a specialized adsorbant. Using a variety of PLOT columns in our laboratory, we have been able to separate chlorofluorocarbons from their dechlorinated analogues137,138 and light hydrocarbons.139 At present, we are working to separate CO2 (arguably the most widely dispersed environmental pollutant) from its reduction product CO. For many common pollutants, detection methods have been published by the U.S. Environmental Protection Agency. Furthermore, many manufacturers of instrumentation for gas chromatography publish column-phase applications guides. Using these resources, one should be able to formulate an analytical strategy that is specific to the needs of any project.

5. HALOGENATED ENVIRONMENTAL POLLUTANTS In this section, the major section of this review, we initially provide some historical background to the broad field pertaining to the electrochemical reduction of halogenated organic compounds and then describe what has been done electrochemically in an effort to remediate some specific classes of halogenated environmental pollutants. Electrochemical reduction of halogenated organic compounds has been studied extensively, and numerous reviews140−147 have appeared since 1980. These reviews, along with their extensive bibliographies, clearly reveal that the reductive cleavage of carbon−halogen bonds in environmental pollutants is a complicated process that is affected by at least four parameters: (a) the structural features of the molecule of interest, (b) the identity of the cathode material, (c) the particular solvent/ electrolyte combination that is employed, and (d) the potential of the cathode itself. For the purposes of this discussion, we consider the electrochemical reduction of a simple alkyl monohalide (RX), the generally accepted mechanism for which is depicted in Figure 7. Transfer of a single electron from the

Figure 7. Electron-transfer and chemical reactions involved in the direct electrochemical reduction of an alkyl monohalide (RX).

cathode to RX could produce a transient radical anion (RX•−). Depending on the nature of RX•− and the cathode material, the radical anion could (a) exist just long enough for a single vibration of the RX bond to occur before loss of X− (the stepwise process, reactions 1 and 2 in Figure 7) or (b) lose X− in the very act of electron transfer (the concerted process, reaction 3 in Figure 7). Recent publications by Isse and co-workers148−150 have addressed this issue for a variety of halogenated organic compounds. Interplay among the four parameters listed in the preceding paragraph can result in the formation of alkyl radicals (R•) or carbanions (R−) as intermediates (compare reactions 2 and 3 M


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Olivero157 described the use of nickel and palladium species as catalysts for the reduction of halogenated organic compounds. However, the two most prominent catalysts for the reduction of halogenated organic compounds, including environmental pollutants, have been electrogenerated low-valent nickel and cobalt species. 5.1.1. Nickel Complexes. Of particular interest in our laboratory is electrogenerated nickel(I) salen (3), which was introduced earlier in this review. This species has been used as a catalyst for the reductive intramolecular cyclizations of a variety of olefinic and acetylenic halides,157−163 as well as for the reduction of α,ω-dihaloalkanes.164 In the presence of carbon dioxide, nickel(I) salen was shown to promote the reductive electrocarboxylation of benzylic halides165 and arylethyl chlorides.166 In the presence of dioxygen, light, and water, electrogenerated nickel(I) salen is involved in the conversion of alkyl monohalides to aldehydes and ketones.167−169 In addition, nickel(I) salen has been used to promote ring-expansion reactions of some 1-haloalkyl-2-oxocycloalkanecarboxylates.170 In the arena of environmental electrochemistry, our laboratory has investigated the nickel(I) salen-catalyzed reductions of 1,1,2trichloro-1,2,2-trifluoroethane (CFC-113),137 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT), 1 7 1 1,2,5,6,9,10-hexabromocyclododecane (a flame retardant),172 and 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(methoxybenzene) (methoxychlor).173 Nickel(I) salen exhibits several interesting behavioral features as a catalyst. These include its tendency toward alkylation of one or both imino (CN) bonds,174 its ability to exhibit both metaland ligand-centered one-electron reductions,175 and its behavior when structural variations of the ligand are introduced.176 Various electrogenerated nickel(I) tetraazamacrocycles with ligands such as cyclam, tetramethylcyclam, and hexamethylcyclam can catalyze the reduction of halogenated organic compounds. Parent nickel(II) analogues of these compounds are easier to reduce than nickel(II) salen and, therefore, should be catalytically active toward carbon−halogen bonds that are more easily reducible than those identified in the preceding two paragraphs. Tuning the electrochemistry of the catalyst precursor to the electrochemistry of targeted substrates is crucial for success; this topic was discussed in some detail in a recent publication49 from our laboratory. Although the use of electrogenerated nickel(I) tetraazamacrocycles for environmental remediation appears to have been explored only slightly at best, it is an area that merits attention. Accordingly, we cite in the paragraphs below some more recent publications pertaining to organic halide systems that have been reduced with the aid of these nickel(I) complexes. These references describe representative systems, and they identify key contributors to this area of research. Electrogenerated nickel(I) cyclam and nickel(I) tetramethyl cyclam have been employed to carry out a variety of reductive cyclization reactions involving allyl 2-halophenyl ethers,177 propargyl and allyl bromoesters,178−181 and N-allyl-α-haloamides.182 Espenson and co-workers used these catalysts to reduce alkyl halides183 and some α,ω-dihaloalkanes.184,185 With this same class of catalysts, Ozaki et al. accomplished a number of interesting electrosyntheses: (a) intramolecular cyclization of nallylic and n-propargylic α-bromoamides and of o-bromoacryloylanilides to give five-membered lactams;186 (b) stereoselective addition of n-, sec-, and tert-butyl radicals to αmethylenebutyrolactones;187 (c) reduction of 2-bromo- and 2-

with reaction 4 in Figure 7). There might be instances when reduction of R• is easier than that of RX, and there might be circumstances that lead to both intermediates (R• and R−). Once R• arises, it can engage in several processes that include radical coupling or disproportionation (reaction 5), as well as abstraction of a hydrogen atom from a molecule of the solvent (SH, reaction 6). In reaction 7, R− abstracts a proton from the solvent (e.g., acetonitrile or an alcohol) to afford RH, whereas in reaction 8, the electrogenerated organic carbanion attacks a molecule of unreduced starting material (RX) to afford a dimer (R2). Alternatively, as shown in reaction 9, the carbanion (R−) can be protonated by the medium (including the solvent, supporting electrolyte, and adventitious water in the system) to form a hydrocarbon (RH) and the conjugate base (B−) of the proton donor (HB). Another viable process (reaction 10, E2 elimination) involves attack of a base (B−) on unreduced starting material (RX) to afford an alkene R(H) and a free halide ion (X−). Finally, as depicted in reaction 11 (SN2 process), the starting compound (RX) can be converted to RB through interaction with B−. When elements such as mercury, lead, and tin are employed as cathodes, the electrogenerated alkyl radicals can interact strongly with the electrode to enhance the reduction of the alkyl monohalide and to afford organometallic compounds. In addition, results have shown that tertiary alkyl halides are easier to reduce than secondary alkyl halides, which are, in turn, easier to reduce than primary alkyl halides. Moreover, as indicated earlier in this review, the ease of reductive cleavage of a carbon− halogen bond depends on the identity of the halogen atom: (a) organic iodides are much easier to reduce than bromides, (b) chlorides can be so difficult to reduce that they often appear to undergo no direct reduction, and (c) direct reduction of an alkyl monofluoride has not been observed. 5.1. Indirect or Catalytic Cleavage of Carbon−Halogen Bonds

As discussed near the beginning of this review, the direct electrolytic scission of a carbon−halogen bond is an irreversible process. Consequently, the potential needed to reduce a carbon− halogen bond cathodically at an electrode is usually more negative than the thermodynamically reversible potential, where the latter is experimentally accessible with the aid of a procedure, known as homogeneous redox catalysis, developed by Andrieux and co-workers.151−153 Fortunately, as indicated earlier, a carbon−halogen bond can be cleaved chemically with an electron-transfer mediator (catalyst) that is electrogenerated at a potential significantly more positive than that required for direct cleavage of the carbon−halogen bond. One of the earliest applications of this approach was carried out by Lund and coworkers,154 who electrogenerated the radical anion of chrysene as a catalyst for the reduction of bromobenzene. A reference147 to other applications involving the catalytic reduction of halogenated organic compounds by electrogenerated aromatic radical anions was cited earlier. In more recent years, electrogenerated, low-valent transitionmetal complexes have attracted attention as mediators for the catalytic reduction of halogenated organic compounds. Nédélec, Périchon, and Troupel155 reviewed organic electroreductive coupling reactions that make use of transition-metal complexes as catalysts, and a subsequent article by Durandetti and Périchon156 discussed nickel-catalyzed coupling reactions involving the reactions of aryl, heteroaryl, and vinyl halides with activated alkyl halides. In addition, Duñach, Medeiros, and N


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Figure 8. Structures, shorthand designations, and names for various chlorofluorocarbons (CFCs) and their degradation products.

sections, electrogenerated nickel(I) salen and cobalt(I) salen have both played prominent roles as catalysts for the reductions of a large palette of halogen-containing compounds, including environmental pollutants. Structurally, cobalt(I) salen resembles nickel(I) salen, which was depicted earlier in this review (see section 2.2.3). However, as catalysts for the reduction of halogenated organic compounds, these two species behave quite differently. When electrogenerated nickel(I) salen, [Ni(I)L]− (where, throughout this discussion, L denotes the salen ligand), interacts with a molecule of a substrate, such as 1-iodooctane (C8H17I), one electron is transferred from the active catalyst to the substrate to form a transient radical anion and to regenerate nickel(II) salen:

iodo-1,6-dienes to give bicyclo[3.1.0]-, 3-azabicyclo[3.1.0]-, and 3-oxabicyclo[3.1.0]hexane derivatives;188 and (d) preparation of pyrrolopyridines and pyrrolopyrrole derivatives through reductive cycloaddition of 1-(2-iodoethyl)pyrrole to activated olefins or cyclization of 1-(ω-iodoalkyl)pyrroles.189 Gómez and colleagues190 converted a nickel(II) tetraazamacrocyclic complex to its zerovalent state to reduce unsaturated o-haloaryl and ohalobenzyl ethers. Electrogenerated nickel(I) cyclam was also used by Pelletier et al.191 to synthesize dihydrobenzo[b]thiophenes from o-haloaryl allyl thioethers and by Nunnecke and Voss 192 to dehalogenate chlorinated benzenes and dibenzofurans. Electrogenerated nickel(I) tetrapyrrole complexes have been shown to catalyze the reductions of dichloromethane and methyl iodide,193 alkyl halides,194 and aryl halides.195 The catalytic reduction of trans-1,2-dibromocyclohexane to cyclohexene by electrogenerated nickel(I) porphyrin complexes was studied by Lexa et al.196 5.1.2. Cobalt Complexes. Publications describing cobalt(I) salen-mediated reductions of simple alkyl halides and dihalides are numerous,197−201 and there has been a comparable number of studies involving aromatic halides.197,202−207 Rusling and coworkers investigated the use of several electrogenerated cobalt(I) complexes [cobalt(I) salen, vitamin B 12s, and cobalt(I) phthalocyanine] as catalysts, both in homogeneous phases and in bicontinuous microemulsions, for the reduction of a number of compounds, including 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene) (DDT).208−212 In our laboratory, electrogenerated cobalt(I) salen has been employed for the catalytic reductions of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113),213 DDT,2 1,1,1-trichloro-2,2,2-trifluoroethane (CFC113a),214 and hexa- and pentachlorobenzene.38 5.1.3. Contrasting the Catalytic Behavior of Nickel(I) and Cobalt(I) Salens. As revealed in the two preceding

[Ni(I)L]− + C8H17I → [Ni(II)L] + C8H17I•−

In turn, the radical anion undoubtedly survives only long enough for one vibration of the carbon−iodine bond before splitting into an n-octyl radical and an iodide ion: C8H17I•− → C8H17• + I−

Then, the n-octyl radical can capture a hydrogen atom from the solvent (SH) to form n-octane: C8H17• + SH → C8H18 + S•

However, the main fate of the n-octyl radical is to undergo coupling to afford n-hexadecane, along with a small amount of radical disproportionation to give n-octane and 1-octene: 2 C8H17• → C16H34

2 C8H17• → C8H18 + C8H16 O


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propellants.218 In addition, these same products might find a place as starting materials for the synthesis of polymers and biologically active compounds. Decades ago, it was discovered that the separate reductions of CFC-113 and CFC-114 at a mercury cathode in an ethanol/ water medium lead mainly to the formation of CFC-1113 and HFC-1114, respectively.219 However, under the same conditions, attempts to electrodegrade CFC-11 and CFC-12 led to the production of HFC-41 and HFC-32, respectively, in moderate yields at best. Subsequently, using different solvents and cathode materials, Smirnov and co-workers220 found that CFC-113 is converted into a mixture of CFC-1113 (78%) and HFC-1123 (10%). On the other hand, dechlorination of CFC113 at a zinc-coated copper cathode in an aqueous medium was found to be less efficient.221 Electrochemical reduction of several CFCs at various cathodes in aqueous hexamethylphosphoramide containing tetra-n-butylammonium tetrafluoroborate was investigated by Tezuka and Iwasaki.222 They reported that the principal products obtained from electroreductions of CFC-112, CFC-112a, and CFC-113 are the corresponding olefins. However, HCFC-123 was the major product seen when CFC113a was reduced at mercury; when platinum and carbon cathodes were used, the olefin was obtained. Savall and co-workers223,224 used a rotating zinc cathode in an aqueous medium in an effort to dechlorinate CFC-113. In a series of five articles, Cabot and co-workers225−229 described their investigations of the electrodegradation of CFC-113 and CFC11 at a lead or copper cathode by employing a palladium-based hydrogen diffusion anode and a one-compartment cell to lower energy costs. Hydrophobized cathodes composed of acetylene black containing polytetrafluoroethylene were used to promote the reduction of CFC-113 to CFC-1113 in an aqueous medium.230 Sonoyama and co-workers231−234 employed gasdiffusion electrodes to investigate the electrochemical behaviors of CFC-12 and CFC-13, whereas Kyriacou and co-workers235−238 studied the effects of various cathode materials (silver, gold, copper, platinum, and nickel) on the reduction of CFC-12. The effects of the medium and cathode material (glassy carbon, platinum, and silver) on the electrochemical reduction of CFC113 were examined by Titov and colleagues.239−241 The direct electrochemical reduction of CFC-113 at silver and glassy carbon electrodes in propylene carbonate, CH3CN, and DMF, each containing tetramethylammonium tetrafluoroborate, was explored recently in our laboratory.137,138 In addition, reductions of CFC-113 and CFC-113a catalyzed by electrogenerated cobalt(I) salen or nickel(I) salen have been reported.2,137,214 5.2.2. Disinfection Byproducts (DBPs). An environmentally important class of halogenated pollutants is disinfection byproducts (DBPs). Among a huge number of compounds that are classified as DBPs, the two most prominent families of pollutants, which find themselves listed on annual water quality reports for both large and small communities, are trihalomethanes (THMs) and haloacetic acids (HAAs). Richardson242 published an excellent review that discusses this topic. Disinfection byproducts are pervasive contaminants and pollutants that pose a threat to human lives and need to be degraded and removed from wastewater effluents and drinking water sources. These DBPs are formed in drinking water, primarily through the reaction of disinfectants, notably chlorine, with natural organic matter (humic acids) along with bromide and iodide ions in water. As reported by Ghernaout and Ghernaout, chlorination was patented by Lieds in 1898 and still is the most important method for the disinfection of water.243

In a typical experiment performed in our laboratory,175 a solution containing 20 mM 1-iodooctane and 2.0 mM nickel(II) salen in dimethylformamide containing 0.10 M tetramethylammonium tetrafluoroborate was electrolyzed at a potential to generate nickel(I) salen, which catalytically and quantitatively reduced 1iodooctane to afford a product distribution consisting of nhexadecane (88%), n-octane (7%), and 1-octene (1%). These findings are in agreement with the generation of an n-octyl radical intermediate and in accord with known solution-phase ratios of radical coupling to radical disproportionation.215 The one-electron reduction of cobalt(II) salen to cobalt(I) salen is approximately 400 mV easier than the conversion of nickel(II) salen to nickel(I) salen. In the presence of an alkyl halide such as 1-iodooctane, cobalt(I) salen reacts quickly with this substrate by a process known as oxidative addition to form a cobalt(III) intermediate, H17C8Co(III)L, with the iodide ion being displaced into solution: [Co(I)L]− + C8H17I → H17C8Co(III)L + I−

This resulting alkylcobalt(III) salen undergoes subsequent oneelectron reduction at a potential approximately 600 mV more negative than that needed to electrogenerate cobalt(I) salen from the original cobalt(II) salen: H17C8Co(III)L + e− → [H17C8Co(II)L]−

Then, the product, [H17C8Co(II)L]−, decomposes into [Co(I)L]− and an octyl radical [H17C8Co(II)L]− → [Co(I)L]− + C8H17•

and the octyl radical behaves in the same way as when it arises through the nickel(I) salen-catalyzed reduction of C8H17I. An early example of the reduction of an alkyl halide (benzyl chloride) by electrogenerated cobalt(I) salen was described by Isse and co-workers.216 5.2. Electrochemical Studies of Halogenated Pollutants

5.2.1. Chlorofluorocarbons (CFCs). These compounds constitute a class of environmental pollutants that can have adverse effects on the ozone layer.217 Cleavage of one chlorine atom from a single molecule of a chlorofluorocarbon can destroy as many as 100,000 molecules of ozone. The names, structural formulas, and abbreviations of the most familiar CFCs are presented in Figure 8. In 1986, the Montreal Protocol on Substances that Deplete the Ozone Layer was introduced to mandate the elimination of CFCs. According to Rondinini and Vertova,1 the global amount of “banked” CFCs totaled 2.25 megatons in 2009, of which approximately 45% was CFC-11 and 45% was CFC-12, with CFC-113 apparently comprising a majority among other CFCs. Remediation of these compounds entails removal of the chlorine atoms, a process that can be accomplished by electrochemical degradation or high-temperature incineration. The electroreduction of CFCs has been extensively studied, in contrast to traditional incineration. An electrochemical protocol can entail milder reaction conditions, lower energy and operating costs, and the possibility of better product selectivity. Furthermore, secondary pollutants (carbon dioxide and hydrogen halides) that arise from the process of incineration can be precluded. Another advantage of the electrochemical remediation of CFCs is that the products are fully dechlorinated hydrofluorocarbons (HFCs) and fluorocarbons (FCs), species that could serve as more environmentally benign refrigerants and P


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Fiori and co-workers257 discovered that chloroform and dichloromethane can be completely reduced at an activated silver cathode in a one-to-one mixture of water and acetonitrile, whereas incomplete reduction occurs in pure acetonitrile. Radjenović et al.258 demonstrated that a resin-impregnated graphite cathode can be used for the electroreductive degradation of trihalomethanes. In our study of the measurement of THMs in drinking water,80 bromoform was reduced quantitatively at a silver cathode in less than 15 min in an aqueous medium; a coulometric n value of 6.34 ± 0.16 confirmed that scission of each carbon−bromine bond involves two electrons. Three publications by Sonoyama and co-workers259−261 described the use of flow systems that incorporate metalimpregnated carbon-fiber electrodes to remove trihalomethanes from water. In the first of these studies, chloroform was completely reduced at zinc- and silver-impregnated fibers in an aqueous medium; on the other hand, under the same conditions, dichloromethane was only partially degraded. In the second report, it was shown that dichloromethane can be completely decomposed at a copper-powder electrode. In the third article, complete dehalogenation of chloroform, bromodichloromethane, dibromochloromethane, and bromoform was accomplished at a spiral platinum wire-activated carbon cathode impregnated with silver in a flow-through cell, whereas degradation of dichloromethane was incomplete. Haloacetic Acids (HAAs). Six decades ago, remediation of bromoacetic and chloroacetic acids was first investigated electrochemically at mercury cathodes in aqueous media.262,263 In more recent research, Korshin and Jensen264 found that, using a copper cathode, bromoacetic acids are easier to reduce than their chlorinated analogues. Alatorre Ordaz and co-workers265 examined the reduction of dichloro- and trichloroacetic acids at electrodes whose surfaces had been modified with films of either (a) didodecyldimethylammonium bromide imbedded with hemin or (b) electropolymerized cobalt porphyrin and cobalt salen. Altamar et al.266 used a gold electrode modified with a polymeric nickel(II)-containing species for the electroreduction of mono-, di-, and trichloroacetic acid. Esclapez et al.114 employed sonoelectrochemistry to treat water polluted with trichloroacetic acid. Several chloroacetic acids were electrocatalytically reduced at activated silver cathodes in aqueous media by Xu et al..267 Jüttner and co-workers6 demonstrated that dichloroacetic acid can be converted to chloroacetic acid at a graphite cathode in the presence of lead(II), copper(II), or gold(III) ions. Scialdone and colleagues268 used macrofluidic and microfluidic cells for the removal of chloroacetic acid from water. In recent work,269 the dechlorination of trichloroacetic acid in an alkaline medium was carried out at a porous thin foam of electrodeposited silver−palladium. Detection and Determination of THMs and HAAs. The electroreduction of THMs at different cathodes depends on the identity of the compound of interest and, of course, on the solvent/supporting electrolyte system or matrix. Thus, it would be fruitful to develop electrochemical sensors to detect the presence of DBPs and to measure their concentrations in the environment. Toward that end, polymer-coated gold electrodes were designed270 to detect chloroform in aqueous media; other DBPs could be monitored in a similar fashion. Using porphyrinmodified electrodes, Dobson and co-workers271 found that the limit of detection for haloalkanes is 5 μM. Wiyaratn and colleagues272−274 designed an electrode plated with a zinc metal−polytetrafluoroethylene (PTFE) composite for the

Unfortunately, chlorination has significant shortcomings. One of these is that disinfection byproducts such as trihalomethanes and haloacetic acids arise from the reaction of chlorine with organic matter dissolved or suspended in water. It has been estimated that the number of DBPs arising from the chlorination of water can exceed 700;243 however, the total number of halogenated compounds arising from chlorination could be as high as 1400. In 2009, Richardson and Postigo244 summarized information dealing with regulated DBPs (trihalomethanes, haloacetic acids, bromate, and chlorite) in addition to emerging (not yet regulated) DBPs. Brominated DBPs are generally more carcinogenic than their chlorinated analogues, and there is evidence that iodinated compounds are even more toxic than their brominated counterparts.245 Brominated and iodinated DBPs include compounds such as iodoacids, bromonitromethanes, and iodotrihalomethanes. Trihalomethanes (THMs) and haloacetic acids (HAAs), two prominent families of environmental pollutants, comprise the majority of DBPs. Furthermore, trihalomethanes constitute a class of DBPs that includes CHCl2Br, CHClBr2, CHBr3, and CHCl3; in the groundwater supply of the United States,246 chloroform is the most frequently detected volatile pollutant. Determination of these THMs in water can be achieved electrochemically with the aid of stripping analysis; details can be found in a recent publication from our laboratory. 80 Members of the group of HAAs include ClH 2 CO 2 H, Cl 2 HCO 2 H, Cl 3 CO 2 H, BrH 2 CO 2 H, and Br2HCO2H. Trihalomethanes and haloacetic acids are both widely dispersed pollutants that are toxic, mutagenic, carcinogenic, and teratogenic.247−250 In the following sections, we describe the electrochemical detection and remediation of THMs and HAAs at a variety of cathode materials. Trihalomethanes (THMs). The electroreduction of chloroform at a mercury drop cathode in water, methanol, and dimethyl sulfoxide (DMSO) solvents was investigated with the aid of square-wave and cyclic voltammetry.251 Only a single cathodic peak was seen for each solvent; however, a higher sensitivity and a lower detection limit were realized when DMSO was employed as the solvent for the analytical determination of chloroform. On the other hand, chloroform undergoes a two-electron reduction to afford dichloromethane. For the same experimental conditions, bromoform is reduced in a stepwise fashion, with each stage corresponding to scission of a carbon−bromine bond, eventually leading to the production of methane. The electroreductions of chloroform and dichloromethane at different cathodes have been reported; interestingly, palladium and silver electrodes were found to be most promising.252,253 Isse and co-workers27,254 compared the potentials for the reduction of chloroform at silver and glassy carbon electrodes in dimethylformamide, reporting that chloroform is 600 mV easier to reduce at silver. Rondinini and Vertova255 employed silver and silver alloys in different solvents for the electroreduction of chloroform and dichloromethane; a controlled-potential electrolysis of chloroform at silver in a CH3CN/H2O medium led to the formation of chloromethane and methane. In dimethylformamide, the electroreduction of chlorinated methanes is easier at silver than at carbon, as well as in the presence of acetic acid.256 Recently, we employed silver cathodes to measure reduction potentials for several trihalomethanes such as bromoform, chloroform, bromodichloromethane, and dibromochloromethane, along with their less-halogenated products in an aqueous medium.80 Q


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Organophosphates. Among the first pesticides to appear, organophosphates, of which some are chlorinated, are significant environmental pollutants, principally because of their overuse in agriculture and their neuroactivity. Therefore, the detection and monitoring of these compounds in the environment is important. Only a limited number of reports on the electrochemistry of these species has been published. Subbalakshmamma and Reddy280 employed several techniques (ac and dc polarography, cyclic voltammetry, millicoulometry, and controlled-potential electrolysis) to investigate the electrochemical reductions of organophosphorous pesticides such as phosphamidon (9) and dichlorvos (10); they found that these compounds undergo a single two-electron transfer attributable to the reduction of the carbon−carbon double bond to afford the saturated analogues of these compounds, whereas reduction of these organophosphates does not affect the phosphate moiety.

measurement of organohalides in aqueous media at detection limits close to 0.1 nM. In subsequent determinations of bromoform and chloroform in drinking water, detection limits of 3.0 μg L−1 (12 nM) and 6.0 μg L−1 (50 nM), respectively, were achieved by means of stripping analysis with silver.80 Altamar et al.266 devised a gold electrode coated with a film of polynickel(II)-tetrasulfonated phthalocyanine that they used for the determination, by differential pulse voltammetry, of various chloroacetic acids in aqueous alkaline solutions. A molecularly imprinted polymer was prepared by Suedee and co-workers275 and incorporated into an electrochemical sensor for the detection and determination of trichloroacetic acid. If a water sample containing HAAs is passed through a conducting-polymer-membrane electrode, these pollutants can be measured by the changes in current associated with their electrochemical reduction.276 However, this procedure can suceed only if a chromatographic separation precedes the measurement step. To determine the concentrations of mono-, di-, and trichloroacetic acids in a CH3CN/H2O medium by means of current measurements, Wei et al.277 devised a carbon electrode surface-modified with C60-[dimethyl-(β-cyclodextrin)]2 encased in Nafion. An electrode was also designed for the square-wave voltammetric detection of trichloroacetic acid;278 this electrosensor consists of silver nanoparticles coated onto multiwalled carbon nanotubes. Electrochemical Behavior of Other DBPs. After water is disinfected by treatment with chlorine, it appears that THMs and HAAs constitute only approximately one-fourth of the halogenated compounds quantitated so far. Thus, an enormous list of other DBPs must fall outside these two classes of pollutants, such as halogenated aldehydes, amides, furanones, ketones, nitriles, nitromethanes, pyrroles, and quinones. Furthermore, the previously identified THMs and HAAs in drinking water do not include any iodinated compounds. Approximately 70% of DBPs are still unknown. Some of these species could be pharmaceuticals, personal care products, estrogen, pesticides, and surfactants,244,245 but literature citations dealing with these materials are not a significant part of this review. However, the electrochemistry of some of these species has not been completely overlooked. For example, using a resinimpregnated graphite cathode, Radjenović and co-workers258 successfully achieved the electrochemical reduction and degradation of haloacetonitriles, halopropanones, chloral hydrate, and trichloronitromethane. 5.2.3. Pesticides, Fungicides, and Bactericides. The following “official definition” of a pesticide was formulated by the U.S. Environmental Protection Agency: a pesticide is “any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest”.279 In this category, we include so-called fungicides and bactericides. Many of these compounds pose significant health problems that require the elimination of these substances from the environment. This section begins with a short overview of the electrochemistry of organophosphate pesticides. Next, we focus on the electrochemical behavior of dicarboximides. A larger portion of this section is aimed at halogenated compounds that are toxic and persistent in the environment. Then, the electrochemical remediation of triazines and benzonitriles is described. Finally, we discuss pyrethroids that mimic natural insect hormones. An excellent review dealing with the removal of pesticides from soils and water by means of electrochemical technologies was recently published.5

Madhavi et al.281 investigated the electroreduction of organophosphorous pesticides such as tetrachlorvinphos (11), an insecticide employed to eliminate infestations of fleas and ticks. Under the experimental conditions used, this compound underwent reduction to its saturated analogue. It would be interesting to reexamine the reduction of each of the preceding three compounds (9−11) at a silver cathode to establish whether the electrocatalytic ability of such cathodes to facilitate carbon− halogen bond cleavage could alter this scenario. Dicarboximide Pesticides. A number of studies has been carried out to characterize the electrochemical degradation of dicarboximides. Although dicarboximides are effective against many fungi, investigations have revealed other adverse effects such as endocrine disruption and antiandrogenic and reproductive developmental effects.282 All environmentally important dicarboximides have a 3,5-dichlorophenylimide group connected to a five-membered heterocyclic ring. A brief survey of the electrochemical reduction of vinclozoline (12) is available in an article by Will.283 Employing a number of R


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Figure 9. Structures of DDT and its degradation products.

electrochemical techniques, Sreedhar and co-workers284 investigated the behavior of iprodione (13); reduction of this compound appears as a single four-electron process that corresponds to the conversion of the two carbonyl moieties into hydroxyl groups. In two related investigations, Pospiš́ il et al.285,286 observed two stages for the reduction of vinclozoline (12), iprodione (13), and procymidone (14) in CH3CN/ tetrabutylammonium hexafluorophosphate (TBAPF6) at a mercury cathode and then studied the kinetics of the reduction of vinclozine (12). Electrolyses of 12−14 at a potential corresponding to the first cathodic step for each compound indicated loss of the heterocyclic ring along with scission of one or both carbon−chlorine bonds. Electrolysis of 12 leads to

In subsequent research, Hromadová and co-workers287 observed that the electroreductions of 12−14 at a mercury cathode in DMSO/TBAPF6 involve cleavage of the carbon− chlorine bonds and formation of host−guest complexes of

products related to reduction of the OCO moiety of the heterocyclic ring. S


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cyclodextrin. Sreedhar and colleagues288,289 investigated the reduction of 12 at both mercury and platinum cathodes over a wide pH range to gain insight into the mechanism. From an investigation of the reduction of 14 in a mixture of Britton− Robinson buffer (pH 2.0−6.0) with DMF, Sreedhar and coworkers290 concluded that each carbonyl moiety undergoes a two-electron reduction. Halogenated Organic Pesticides. Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), and organochlorine insecticides have been recognized as serious environmental pollutants. All of these compounds are extremely resistant to biodegradation. A number of different methods including chemical and biological degradation has been developed for the decontamination of polluted environmental samples. However, chemical methods are often too complicated or require the use of dangerous chemicals, whereas smooth biological degradation takes place only with compounds of lower chlorine content. Electrochemical methods, on the other hand, have been found to be very effective for the dechlorination of PCDDs,291 PCDFs,291 and PCBs.292 Gassmann et al.293 dechlorinated a number of organochlorine insecticides, including mirex (15), kepone (16), aldrin (17), chlordane (18), and alodan (19), by electrochemical reduction at a lead cathode in methanol. One of the first studies of this class of compounds was reported by Cisak.294 Halogenated organic pesticides are extremely toxic, because they accumulate throughout the environment. Both direct and catalytic reductions of these pollutants offer the possibility for the emergence of selective techniques to transform these compounds into harmless and perhaps useful products. In the following sections of this review, we consider some classes of chlorinated pesticides that have been subjects of electrochemical studies.

DDT and Its Degradation Products. There can be no doubt that the most prominent member of this family of environmental hazards is 4,4′-(2,2,2-trichloroethane-1,1-diyl)bis(chlorobenzene), better known as DDT. In the past, DDT was utilized extensively in agriculture and to combat malaria. However, almost every industrialized country has prohibited its distribution and use because it has been associated with cancer and with reproductive and neurophysiological disorders. The polarographic reduction of DDT in a dimethylformamide medium was first investigated more than six decades ago, and the major product was 4,4′-(2,2-dichloroethane-1,1-diyl)bis(chlorobenzene) (DDD).295 Figure 9 depicts structures of DDT and its various degradation products. Electrochemical research has involved the reduction of DDT at different electrodes and in various solvents.296−299 Depending on the choice of conditions, both partial and complete dechlorinations of DDT can be achieved to afford products such as DDD, 4,4′-(2,2-dichloroethene-1,1-diyl)bis(chlorobenzene) (DDE), 4,4′-(2-chloroethene-1,1-diyl)bis(chlorobenzene) (DDMU), and 4,4′(ethene-1,1-diyl)bis(chlorobenzene) (DDNU). Moreover, the electrodegradation of DDT in an ionic liquid was examined.300 Beginning in 2006, our laboratory initiated a series of investigations of the reduction of DDT under a variety of conditions. In a study301 of the electrochemical behavior of DDT at a glassy carbon electrode by means of both cyclic voltammetry and controlled-potential electrolysis, it was concluded that DDT undergoes a four-electron reduction, first to DDD and then to 4,4′-(2-chloroethane-1,1-diyl)bis(chlorobenzene) (DDMS). Cyclic voltammograms for DDD resemble those for DDT, with the exception that the cathodic peak attributable to reduction of the CCl3 moiety to a CHCl2 group is absent. Owing to the comparatively long time required for a bulk electrolysis (in comparison with the time scale of cyclic voltammetry), base-promoted elimination reactions that follow electron transfer led to the formation of other products (DDE, DDMU, and DDNU). In other work2 involving the cobalt(I) salen-catalyzed reduction of DDT, more than 90% of the starting material was converted to DDNU. Recently, DDT was reduced with the aid of electrogenerated nickel(I) salen as a catalyst; electroreduction of DDT afforded DDNU in a three-electron process through radical intermediates. In addition, some of the DDT undergoes total dechlorination to yield 1,1′-diphenylethylene (DPE).171 Finally, our laboratory investigated the use of a silver cathode for the reduction of DDT.40 It was found that reduction of all three of the aliphatic carbon−chlorine bonds of DDT in DMF occurs at less negative potentials at a silver cathode than at a glassy carbon cathode. Even more significant is the fact (mentioned earlier in this review) that the reductive cleavage of the aryl carbon− chlorine bonds can be achieved. Large-scale electrolyses of DDT in DMF containing deliberately added water (in the form of D2O) result in the formation of a fully dechlorinated product, namely, 1,1′-ethylidenebisbenzene (EBB). Additional research was carried out by Zhang et al.,302 who employed cyclic voltammetery, controlled-potential electrolysis, and in situ UV−visible spectroelectrochemistry to monitor the dechlorination of DDT. Utilizing two iron(I) porphyrins, each electrogenerated from its corresponding iron(III) complex in DMF, they discovered that the iron(I) porphyrins efficiently catalyzed electroreduction of DDT to afford DDD, DDE, and DDMU. A mechanistic scheme was proposed to account for the formation of various products. Zhu and colleagues303 T


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benzene.328 Furthermore, other publications have dealt with the bulk electrolysis of chlorinated benzenes329 and the mechanism of electroreduction of hexachlorobenzene.330 Lindane. In 2007, this compound, the structure (4) of which was shown earlier (see section 3.2), was banned by the Stockholm convention because it can damage mammaliam nervous systems. Various cathode materials, such as mercury, copper-coated steel, carbon cloth, zinc-modified carbon cloth, palladium, and ruthenium, have been used for the direct reduction of lindane; the product of this process is benzene.330−338 Matsunaga and Yasuhara328 attained the nearly quantitative dechlorination of lindane at a glassy carbon cathode in dimethylformamide with electrogenerated naphthalene radical anions as the reducing agent. Research centered on developing electrochemical sensors for lindane in aqueous systems has been reported.337,338 The electroreduction of lindane in DMF at a glassy carbon electrode was investigated in our laboratory.339 Cyclic voltammograms, recorded at 100 mV s−1, showed two peaks assigned to the reductions of lindane and chlorobenzene, the latter being a minor product. Bulk reductions at a potential just past the first cathodic peak form benzene (80−93%) and small amounts of chlorobenzene; however, beyond the second cathodic peak, complete dechlorination occurs. In a later study, silver cathodes were employed to reduce lindane in both organic and aqueous/organic media. To account for the formation of benzene as a major product, a combination of one- and twoelectron processes was suggested. Complete dechlorination of lindane was achieved both in DMF and in mixtures of water with DMF, CH3CN, and ethanol; however, chlorobenzene was detected in trace amounts for electrolyses in ethanol and CH3CN.340 Triclosan and Methyl Triclosan. A significant amount of research has been aimed at the electrochemical behavior of 5chloro-2-(2,4-dichlorophenoxy)phenol (triclosan, 20), once a popular pesticide. Reports suggest that triclosan can affect the endocrine system and can cause cancer.341 Furthermore, in October 2016, the Food and Drug Administration of the United States issued a ban on the use of triclosan and other chemicals in hand and body washes. In the development of an analytical method to detect traces of triclosan in toothpastes and mouthwashes, Pemberton and Hart342 employed screen-printed carbon electrodes in a diethanolamine buffer to explore the voltammetric behavior of triclosan. Analogous research, pertaining to the reduction of triclosan at mercury, was carried out by Safavi and co-workers.343 The direct electrochemical reduction of triclosan at carbon-fiber cathodes in aqueous buffers, as well as in methanol and DMF, leads to chlorobenzene and 2-phenoxyphenol.344 In subsequent work, carbon-fiber electrodes were used for the detection of triclosan in urine and serum.345

employed electrogenerated cobalt(I) porphyrins for the reductive dechlorination of DDT. Polychlorinated Benzenes. Polychlorinated benzenes cause numerous health problems, including liver, kidney, and thyroid toxicity, as well as cancer.304 Although benzene is an inevitable final product of the reduction of any chlorinated benzene, and although benzene is toxic, its health effects are less severe than those of any of its chlorinated analogues. In addition, benzene has not been banned from production and use, and it can be destroyed by incineration. A similar argument can be applied to halogenated polyaromatic pollutants. The electrochemical reduction of di-, tri-, and tetrachlorobenzene at glassy carbon cathodes in dimethylformamide, containing tetramethylammonium perchlorate as a supporting electrolyte, was investigated in our laboratory.305 Cyclic voltammograms for the reduction of these compounds show irreversible cathodic peaks for the sequential two-electron reduction of each carbon−chlorine bond and bulk electrolyses revealed that the final product is benzene. At zinc electrodes, the syntheses of mono- and dicarboxylic acids have been achieved through the electrocarboxylation of di-, tri-, tetra-, and hexachlorobenzenes.306,307 In studies by Miyoshi and co-workers308−310 of the reduction of 1,2,3-trichlorobenzene (a persistent organic pollutant) at a variety of sintered noble-metal electrodes, the extent of dechlorination was found to depend on the choice of cathode material; in addition, research was aimed at minimizing the consumption of solvent and the time required for the process. Guena et al.311 investigated the electrochemical reduction of 1,2,3- and 1,2,4-trichlorobenzene at cathode materials such as carbon, iron, lead, mercury, and platinum. The selective conversion of 1,2,3,4-tetrachlorobenzene to 1,2,3-trichlorobenzene at lead or mercury cathodes in ethanol was the basis of a patent by the Dow Chemical Company.312 Platinum, nickel, titanium, lead, copper, carbon cloth, and palladium-modified carbon cloth have been used by the groups of Kulikov and Plekhanov313,314 to effect the stepwise dechlorination of 1,2,3,5tetrachlorobenzene. Superoxide ions (electrogenerated at platinum, gold, and carbon cathodes in oxygen-saturated dimethylformamide) were used by Lee et al.315 and Sugimoto et al.316 to mediate the reduction of hexa-, penta-, and tetrachlorobenzenes; this process involves the loss of a single chloride ion to form the corresponding alcohol. Electrogenerated cobalt(I) salen has been used to catalyze the reductions of hexa- and pentachlorobenzene;38 similar work was carried out by Páramo-Garció et al.,317,318 who reduced hexachlorobenzene either directly at a carbon cathode or catalytically with cobalt(I) salen electrogenerated at carbon in acetonitrile containing tetra-n-butylammonium perchlorate. Simagina and co-workers319,320 used carbon-supported palladium and nickel as catalysts for the reduction of hexachlorobenzene to afford benzene, chlorobenzene, and dichlorobenzenes. Furthermore, the same workers investigated the kinetics of hexachlorobenzene hydrodechlorination at a carbon surface modified with catalysts such as nickel, nickel− palladium, and copper−palladium.321,322 The electroreduction of hexachlorobenzene at lead and mercury cathodes to afford triand dichlorobenzenes in methanol, methanol−water, and aqueous micellar solutions of Triton-SP 175 was the subject of three publications.323−325 Additional articles about the electrochemical reduction of chlorinated benzenes include their degradation in contaminated soil326,327 and electrogenerated naphthalene radical-anion-mediated reduction of hexachloroU


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atrazine at carbon and solid amalgam electrodes was also explored.354 Finally, electrochemical sensors have been developed for the measurement of triazine in the environment.355,356 Halogenated benzonitriles have been employed as herbicides to inhibit electron transfer at the photosystem II receptor. The photodegradation and electroreduction of these compounds have been investigated;357 mechanisms for these events were recently elucidated. Sokolová and co-workers358 electrolyzed ioxynil (26), bromoxynil (27), and chloroxynil (28) in dimethyl sulfoxide; reduction of each compound involved loss of a single halogen moiety.

In our laboratory,346 cyclic voltammograms recorded for the reduction of triclosan at a glassy carbon cathode in dimethylformamide containing tetra-n-butylammonium tetrafluoroborate displayed three irreversible cathodic peaks, each of which was attributed to cleavage of a carbon−chlorine bond. However, bulk electrolyses always gave 5-chloro-2-(4chlorophenoxy)phenol as the major product (49−73%), regardless of the chosen potential. In a subsequent study,347 we sought to improve the electrochemical degradation by working with a slightly different substrate, 2,4-dichloro-1-(4chloro-2-methoxyphenoxy)benzene (methyl triclosan, 21). Surprisingly, the electrochemical degradation of methyl triclosan at a reticulated vitreous carbon cathode affords an unanticipated set of products consisting of 4-chloro-1-(4-chlorophenoxy)-2methoxybenzene, 4-chloro-2-methoxy-1-phenoxybenzene, 1-(4chlorophenoxy)-2-methoxybenzene, 1-methoxy-2-phenoxybenzene, anisole, and phenol. Triazines and Benzonitriles. Although atrazine (22) has been excluded as an herbicide in Europe, it is widely applied in North America. Human exposure to atrazine has been linked to endocrine disruption and cancer, as well as miscarriage and male infertility. Therefore, the possibility that atrazine might be remediated through electrochemical reduction is of interest. The electroreductions of both atrazine and tert-butylazine (23) were investigated at mercury cathodes in aqueous media.348 Apparently, reduction involves expulsion of a chloride ion by a two-electron process. In highly protic media, unethylated products have been detected.

Pyrethroids. These compounds have been widely distributed for agricultural and household purposes, and their structures resemble those of natural pyrethrins. In small doses, these substances pose little threat; however, respiratory problems, neurotoxin effects, and temporary illness can arise from exposure to large amounts.359 In a study by Jehring and co-workers,360 cyclic voltammograms for the reduction of deltamethrin (29) at a mercury cathode in either DMSO or aqueous methanol revealed cathodic peaks indicative of stepwise scission of each carbon− bromine moiety.

Coomber and co-workers361−364 employed cyclic voltammetry and bulk electrolysis at carbon and mercury cathodes in CH3CN to explore the reduction of halogenated pyrethroids. Other chlorinated pyrethroids were examined electrochemically by Sreedhar and co-workers.365,366 Oudou and co-workers367,368 explored the reduction of λ-cyhalothrin, cypermethrin, and deltamethrin at mercury, and an analogous investigation of the reduction of fenvalerate was carried out by Naidu et al.369 5.2.4. Flame Retardants. Flame retardants comprise a family of brominated compounds found in consumer products such as electronics, furniture, plastics, and textiles.370−373 These compounds accumulate throughout the environment, including in humans.373,374 Brominated flame retardants fall into the category of “persistent organic pollutants” (POPs); their production has been either stopped or banned.373 Brominated phenols, polybrominated biphenyls and diphenyl ethers, and hexabromocyclododecanes are the three major families of flame retardants.374 Owing to the facts that debrominated flame retardants constitute a comparatively minor threat to the environment and that the parent compounds contain reducible carbon−bromine bonds, electrochemistry might play a major role in the remediation of these species. Although numerous flame retardants exist, there have been relatively few electrochemical studies of these compounds.

As suggested by the preceding paragraph, the electrochemical reductions of simazine (24) and propazine (25) at mercury appear to proceed through a two-electron process.349 The electroreductions of triazines and some diazines were explored in acidic media with the aid of polarography, and a reversible ringopening reaction was considered as part of the mechanistic picture.350 To substantiate various aspects of the mechanism, other investigators351−353 used NMR techniques to analyze postelectrolysis solutions to confirm the presence of a ring-opened intermediate and a dechlorinated product. The reduction of V


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reduction by electrogenerated nickel(I) salen.172 The bulk electrolysis of HBCD at both electrode materials causes complete debromination, affording isomers of 1,5,9-cyclododecatriene and cyclododecane-1,5-dien-9-yne. As a method of remediation, direct reduction at carbon appears to be preferred for high concentrations of HBCD. When the concentration of HBCD is higher than 2 mM, the debromination of HBCD at a silver cathode is not complete. The catalytic reduction of HBCD at concentrations of less than 20 mM by electrogenerated nickel(I) salen can promote full debromination in no longer than 3 h. The direct reduction of HBCD at a carbon electrode in DMF was investigated by Baron et al.384,385 In addition, these researchers examined the reduction of HBCD in the presence of catalyst precursors such as cobalt tetraphenylporphyrin (CoTPP) and cobalamin; they discovered that the catalytic reduction of HBCD in the presence of CoTPP is more than 1 V easier than direct reduction. Romańczyk and co-workers386 used tetraphenylporphyrin as a procatalyst for the reduction of HBCD in dimethylformamide containing tetra-n-butylammonium tetrafluoroborate. Miscellaneous Compounds. In this section of the review, we include halogenated anhydrides and esters, such as tetrabromophthalic anhydride, a flame retardant that can be used in foams, papers, textiles, epoxides, and wool. Even though studies mentioned in the remainder of this section do not involve designing new remediation procedures, the articles below describe the electrochemistry of some interesting compounds. de Luca and colleagues387 used cyclic voltammetry with a glassy carbon cathode in dimethylformamide to examine the electrochemical behaviors of tetrabromo- and tetrachlorophthalic anhydride. Troll and Ollmann388 reduced brominated and chlorinated derivatives of phthalic anhydrides, along with phthalic imides, to electrosynthesize some 1,3-bis(trimethyl)silyloxy-substituted isobenzofurans and isoindoles. The electrochemical reduction of halogenated esters of fumaric acid was investigated by Goulart and colleagues389 at mercury and carbon cathodes in dimethylformamide and in acetonitrile/water mixtures. Electrochemical Sensors for Flame Retardants. A need exists for selective sensors for flame retardants. Of the two examples mentioned here, one is based on a molecularly imprinted polymer (MIP), which is a film prepared in the presence of a target molecule that, after being removed, leaves behind a complementary cavity in the film, thereby enabling a target species to be selectively detected. Chen and co-workers390 developed a specific MIP sensor to detect tetrabromobisphenol A indirectly in natural water samples. A poly(dopamine)-coated, gold nanocluster-functionalized electrochemical immunosensor for detecting and determining brominated flame retardants such as 3-bromobiphenyl has been described by Lin and coworkers.391

Brominated Phenols. Silver electrodes have been used for the dehalogenation of brominated phenols. Using bulk electrolysis and silver cathodes in an acetonitrile medium, Rondinini and coworkers375 examined how the structure of a brominated phenol affects the course of its reduction. Electrolyses of meta- and parabromophenol exhibited no significant difference in the resulting distribution of products. However, when 2,4- and 2,6dibromophenol were reduced, the results revealed that it is easier to expel a para than an ortho bromine. Furthermore, when the reduction of 2,4,6-tribromophenol in pure acetonitrile and in an acetonitrile/water mixture was examined, the degree of debromination was found to be lower in the latter solvent mixture. Controlled-potential and constant-current electrolyses were employed by Fiori and co-workers376 for the reduction of several polybrominated phenols in CH3CN, H2O, and mixtures of the two. These researchers found that the ease of debromination depends on the structure of the brominated phenol. The electroreduction of polyhalogenated phenols was investigated at both polished and surface-roughened silver cathodes, with the latter electrode affording better results.377,378 Polybrominated Biphenyl and Diphenyl Ether. Hexabromo-, octabromo-, and decabromobiphenyl ether are the three most common flame retardants. Amazingly, there have been no publications pertaining to the electrochemical debromination of these three compounds. Using a mercury drop electrode in dimethylformamide containing tetra-n-butylammonium iodide as the supporting electrolyte, Rusling and co-workers379,380 measured reduction potentials for 4,4′-dibromobiphenyl and 3,3′,5,5′-tetrabromobiphenyl. Additionally, a mercury film electrode was employed for the reductions of 4,4′-dibromobiphenyl and 2,2′,5,5′-tetrabromobiphenyl in a bicontinuous microemulsion. Several articles have focused on the electrochemistry of brominated diphenyl ethers. The reduction of 4-bromodiphenyl ether and 4,4′-dibromodiphenyl ether at reticulated vitreous carbon electrodes in a mixture of methanol and water, in contact with a palladium-on-aluminum catalyst, affords diphenyl ether in yields of 48% and 35%, respectively.381 Konstantinov et al.382 used constant current to reduce decabromodiphenyl ether at a platinum-black electrode in tetrahydrofuran; this process afforded a mixture of products ranging from tetra- to monobromodiphenyl ether. In our laboratory,383 we have employed cyclic voltammetry and bulk electrolysis to investigate the electrochemical reduction of decabromodiphenyl ether at both glassy carbon and silver cathodes in dimethylformamide and dimethyl sulfoxide. At both cathodes, the controlled-potential electrolyses afforded diphenyl ether in the highest yield when dimethyl sulfoxide was used as the solvent; however, substantial amounts of compounds ranging from mono- to nonabromodiphenyl ether were found. Furthermore, in controlled-potential electrolyses, approximately 42% unreduced decabromodiphenyl ether was detected when a silver electrode was used and dimethyl sulfoxide was the solvent. However, when dimethylformamide was the solvent, the only brominated products obtained were dibromodiphenyl ethers along with phenol. Hexabromocyclododecane. Recent studies have dealt with the electrochemistry of hexabromocyclododecane (HBCD), the most widely used cyclic aliphatic brominated flame retardant. Our group has examined the direct reduction of HBCD with the aid of cyclic voltammetry and controlled-potential electrolysis at both glassy carbon and silver electrodes30 and its catalytic

6. FUTURE PROSPECTS As revealed earlier, the majority of the research related to the possible electrochemical remediation of halogenated organic pollutants has focused on either direct or mediated reduction of these species. To take this area of research forward, work should be directed at the use of novel and more efficient electrodes.392−398 Future directions might involve the improvement of existing electrochemical techniques for (a) large-scale industrial processes, (b) miniaturization of sensor technology, (c) design and construction of cells for flow-by or flow-through W


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electrochemical cells with gold working and auxiliary electrodes along with a silver/silver chloride reference electrode. A review of paper-based microfluidic devices that can be employed for on-site environmental monitoring in the field was recently published by Meredith and co-workers.402 A compact flow system equipped with a pair of electrochemical transducers in a miniaturized cell was fabricated by Gutiérrez-Capitán et al.403 for the trace analysis of copper(II), catechol, and parathion in aqueous environmental samples. Nouanthavong and colleagues404 developed a rapid and accurate paper-based assay for the colorimetric measurement of organophosphate pesticides, including chlorpyrifos (30); the detection limit for the latter compound was 5.3 ng mL−1.

methods, (d) use of new solvent systems (e.g., ionic liquids and solvent mixtures), and (e) other processes such as reactions of electrogenerated carbanions with carbon dioxide. In addition, near the end of this section, we note that many halogenated organic pollutants have not yet been investigated electrochemically. For the purpose of electrochemical remediation, cathodes should be reasonably cheap and should have high catalytic activity for the reductive cleavage of carbon−halogen bonds. For the electroreduction of α-acetobromoglucose, benzyl bromide, 8-bromooctanol, and halothane, Bellomunno et al.392 employed silver, copper, gold, lead, mercury, bismuth, and tin cathodes; the cyclic voltammetric peak potentials that they measured were consistent with the electrocatalytic activity of each metal. Isse and colleagues24,393 used glassy carbon, silver, copper, and palladium to examine the electroreduction of various organic chlorides. They discovered that the ease of the cathodic process at copper, palladium, and silver was similar to that seen by Bellomunno and co-workers.392 For the reduction of a variety of alkyl iodides, Simonet,394,395 Simonet and co-workers,396 and Bui et al.397 have published exciting work pertaining to the use of smooth palladium as well as palladized platinum, glassy carbon, gold, copper, and nickel. Cathodic peak potentials for reduction of 2iodopropane, 1-iodohexane, and 1-iodo-3-phenylpropane at palladized gold are shifted to more positive values than for pure silver.395 In addition to studies with new families of polycrystalline materials, it should be fruitful to explore how exposed facets of single crystals affect the electroreduction of halogenated pollutants, possibly as an avenue to engineer better cathodes. Ardizzone and co-workers398 investigated the reduction of organic iodides and bromides at both monocrystalline and polycrystalline silver. They found that the catalytic activity increased with the surface roughness of polycrystalline silver; on the other hand, for monocrystalline silver, the electrocatalytic activity was found to increase with atomic density for all investigated organic halides, except for a reversal of this trend for an aryl bromide and bromotoluene. For the electroremediation of halogenated pollutants, nanoparticles have been employed as cathode materials owing to their enormous surface-to-volume ratios. However, the design of appropriate nanoparticles requires knowledge about the size and morphology of the cathode material. In a study of the electroreduction of chloroform, Minguzzi et al.399 achieved more efficient reduction when they used electrosynthesized silver nanoparticles on Vulcan XC72-R pretreated with nitric acid than when they used silver nanocubes obtained with untreated Vulcan XC72-R. To deal with the electrochemical remediation of large amounts of halogenated pollutants, one must design a reactor with the following specifications: (a) a high ratio of electrode area to volume; (b) a high current efficiency, (c) efficient mass transport, and (d) a low cell voltage. A number of reviews have described various reactor and cell designs.116,127,400,401 He and coworkers127 used a two-dimensional reactor with a porous copper foam cathode to dehalogenate aqueous-phase chlorohydrocarbons. They succeeded in dechlorinating carbon tetrachloride with 80% efficiency. Scale-down is a necessity for the design of cheap and practical sensors for detecting and measuring halogenated pollutants. Popovtzer et al.401 invented a biosensor to identify and quantitate pollutants. This bacteria-based sensor consisted of eight small

In comparison with classic techniques in electrochemistry, flow-by and flow-through cells can offer improved efficiency. Lunte and co-workers405 have provided information about flow cells. Although flow-by cells are available commercially, most researchers prefer to fabricate their own cells, as seen in a number of recent publications.406−410 For the electrochemical remediation of halogenated pollutants, an ionic liquid can replace a classic organic solvent/ electrolyte, thereby providing an environmentally friendly medium. Research pertaining to this topic, in the form of both original articles and reviews, is expanding at a rapid pace.239,241,411−426 A typical study is that conducted by Shen and co-workers,422 who examined the reduction of organic bromides catalyzed by electrogenerated cobalt(I) salen in ionic liquids. A topic that deserves further attention is electrocarboxylation. One main advantage of this method is that it leads to both dehalogenation of a pollutant and conversion of that pollutant into useful carboxylic acids. In addition, electrocarboxylation could consume significant amounts of carbon dioxide, which could have a positive impact on the “greenhouse effect”. We suggest reviews by Sánchez-Sánchez and co-workers427,428 that provide details about the electrocarboxylation of organic halides. To improve the product yields and lower the potentials required for the electrocarboxylations of a broad spectrum of halogenated species, solution-phase catalysts44,429,430 and a variety of cathode materials (mercury, platinum, stainless steel, carbon, and silver) have been explored.122,431−439 It is worthwhile to describe several specific examples of this electrocarboxylation process. Heintz and co-workers432 prepared a number of substituted benzoic acids in yields ranging from 65% to 80% through the electroreduction of various substituted chlorobenzenes at stainless-steel cathodes in dimethylformamide saturated with carbon dioxide. In an experimental and theoretical investigation, Scialdone et al.122 used potentiostatic and amperostatic methods to examine the electrocarboxylation of 1-phenyl-1-chloroethane to the corresponding acids in yields of 70−80% at silver cathodes in different organic solvents saturated with carbon dioxide; both bench-scale and continuous batch recirculation systems were employed. In more recent work by Zhang and colleagues,438 substituted bromobenzenes were converted to their corresponding methyl benzoates in yields X


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ranging from 30% to 78% through electroreduction at silver in carbon dioxide-saturated dimethylformamide followed by methylation. Electrogenerated cobalt(I) salen was employed by Folest and co-workers430 to effect the electrocatalytic reductive carboxylation of benzylic and allylic chlorides in a carbon dioxide-saturated tetrahydrofuran/hexamethylphosphoramide solvent mixture. A final and important direction is to continue explorations of the electrochemical reduction of other halogenated environmental pollutants. Although we have touched on the electrochemical reductions of chlorofluorocarbons, pesticides, flame retardants, disinfection byproducts, and other halogenated pollutants, some of these families are too large to permit exhaustive coverage. Furthermore, there are some wellrecognized and historically significant pollutants that are absent from this review, including polychlorinated biphenyls,292,440 dioxins,291,441 chlorinated phenols,442 and chlorobenzoic acids. Thus, there remain many compounds and concepts that need to be investigated much more intensely for the electrochemical remediation of environmental pollutants.

postdoctoral work at the University of Oklahoma between October 1982 and August 1983. He began his teaching career at King Saud University, Abha, Saudi Arabia, as an Assistant Professor in 1983 and then became an Associate Professor in 1992. In January 1993, he returned home to join the teaching staff at the Department of Chemistry of The University of Jordan, and in 1998, he was promoted to the rank of Professor. Dr. Mubarak has held a number of administrative positions at The University of Jordan, including Chairman of the Department of Chemistry and Vice Dean of the Faculty of Science, 2009−2011. In addition, he held the position of Academic Consultant and Vice Dean at Al-Ghad International Medical Sciences College, Riyadh, Saudi Arabia, 2011−2012. He has been an Adjunct Professor of Chemistry at Indiana University, Bloomington, IN, since 2009. In addition to electrochemistry, Professor Mubarak’s research program is broadly based on synthetic organic chemistry (especially the synthesis of compounds with expected biological activity), medicinal chemistry, and drug design and discovery. He is the author and coauthor of more than 150 research articles published in peer-reviewed journals, along with chapters in specialized books. He has supervised 39 theses and dissertations. Dennis G. Peters earned a B.S. degree in chemistry from the California Institute of Technology in 1958 under the tutelage of Professor Ernest H. Swift and a Ph.D. from Harvard University under the direction of Professor James J. Lingane in 1962. He joined the faculty at Indiana University in the latter year and became the Herman T. Briscoe Professor of Chemistry in 1975. He was elected Fellow of The Electrochemical Society in 2007, and from The Electrochemical Society, he received the Henry B. Linford Award for Distinguished Teaching in 2002 and the Manuel M. Baizer Award in Organic Electrochemistry in 2012. He has received 12 teaching awards from Indiana University, as well as the Chemical Manufacturers Association National Catalyst Award (1988), the American Chemical Society Division of Analytical Chemistry Award for Excellence in Teaching (1990), and the James Flack Norris Award for Outstanding Achievement in Teaching (2001). In 2012, he was elected a Fellow of the American Association for the Advancement of Science. He is coauthor of 210 scientific publications and five textbooks on analytical chemistry. His research interests include electroanalytical chemistry, organic electrochemistry (especially halogenated organic compounds), surface-modified electrodes, environmental electrochemistry, catalytic electrochemical processes, and ionic liquids.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Dennis G. Peters: 0000-0002-3716-8355 Notes

The authors declare no competing financial interest. Biographies Erin T. Martin earned a B.S. degree in Chemistry at the University of Missouri-St. Louis in 2013 and is currently pursuing a Ph.D. degree in analytical chemistry at Indiana University under the supervision of Professor Dennis G. Peters. In 2016, she received the Robert and Marjorie Mann research fellowship from Indiana University to continue her research on the use of electrochemical reductions for the detection, degradation, and understanding of environmental pollutants. In addition to studying the behavior of common pollutants such as pesticides and disinfection byproducts, her collaborative efforts have been focused on the practical application of novel electrode materials and the electrochemical behavior of natural components in environmental systems.

ACKNOWLEDGMENTS Appreciation is expressed to our Indiana University colleague, Professor Stephen C. Jacobson, for calling our attention to some references dealing with the development and use of microfluidics platforms for environmental analysis.

Caitlyn M. McGuire earned a B.S. degree in Chemistry at Truman State University in 2012. She completed her Ph.D. in analytical chemistry at Indiana University in 2016, with a dissertation research focus on the application of electrochemical reduction for the detection, degradation, and understanding of chlorinated pollutants. She is a member of The Electrochemical Society and served as President of Indiana University’s Student Chapter in 2015−2016. While at Indiana University, she also studied environmental science and the role of science in policy and legislation. Caitlyn was recently awarded a 2016−2017 American Chemical Society Congressional Fellowship, partnering with the larger American Association for the Advancement of Science (AAAS) Science and Technology Policy Fellowship program under their mandate to “advance science and serve society”. Caitlyn is interested in broad legislative initiatives concerning the environment, chemical regulation, scientific education, and clean energy.

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Mohammad S. Mubarak received his B.Sc. and M.Sc. degrees in chemistry from the University of Jordan in 1976 and 1978, respectively, and obtained his Ph.D. degree from Indiana University, Bloomington, IN, in 1982, under the supervision of Professor Dennis G. Peters. He did Y


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