Search

Boron-doped diamond anodes and stainless-steel cathodes were employed in an electrochemical flow cell and evaluated for inactivation efficacy on the plant pathogen Rhizoctonia solani J.G. Kühn [Thanatephorus cucumeris (A.B. Frank) Donk] in hydroponic fertigation water. The electrochemical system showed promise as an electrochemical advanced oxidation process (EAOP) in that significant reductions in R. solani (AG-8) were achieved; however, to achieve complete inactivation the system required supplemental chloride (20 mg L−1), while using 9.09 mA cm−2. The chloride allowed for low levels of free chlorine (≤0.17 mg L−1) that were more active in the bulk solution, complementing the electrode surface EAOP reactions. Perchlorate production was an initial concern but was found to be negligible under the conditions tested. Small but significant increases in nitrate, ammonium, and sulphate were observed following treatment. These increases are hypothesized to originate from the degradation of proteins and amino acids released during pathogen cell disruption.

The capture and re-use of greenhouse fertigation water is an efficient use of fertilizer and limited water resources, although the practice is not without risk. Plant pathogens and chemical contaminants can build up over successive capture and re-use cycles; if not properly managed they can lead to reduced productivity or crop loss. There are numerous established and emerging water treatment technologies available to treat fertigation water. Electrochemical processes are emerging as effective means for controlling pathogens via in situ regenerative hypochlorination; a process that is demonstrated here to achieve pathogen control in fertigation solutions without leading to the accumulation of potentially phytotoxic free chlorine residuals associated with other chlorination processes. An electrochemical flow cell (EFC) outfitted with ruthenium dioxide (RuO2) dimensionally stable anodes (DSA) was characterized and evaluated for free chlorine production and Rhizoctonia solani inactivation in both irrigation and fertigation solutions. Pathogen inactivation was achieved at low current densities and short residence or cell contact times. Effluent free chlorine concentrations were significantly lower than commonly reported phytotoxic threshold values (approximately 2.5 mg/L) when fertilizer (containing ammonium) was present in the test solution; an effect attributable to reactions associated with breakpoint chlorination, including chloramine formation, as well as the presence of other oxidizable compounds in the fertilizer. Chloride concentrations were stable under the test conditions suggesting that the EFC was operating as a regenerative in situ electrochemical hypochlorination system. No significant changes to macronutrient concentrations were found following passage through the EFC.

We have studied electrochemical advanced oxidation of three single substrates (oxalic acid, maleic acid, and 2-naphthol) and real wastes (primary sewage, automotive wastewater, and composting leachate), at boron doped diamond, at which anode organic substrates undergo electrochemical mineralization by reactive hydroxyl species. Experimental data obtained under batch conditions at laboratory scale were fitted to three kinetic models under various conditions of current density and substrate concentration. The optimum kinetic regime of linear loss of substrate with time at constant current density can be achieved for almost all substrates to high conversion, except under extreme conditions of low substrate concentration and high current density. Proposals are made for optimizing this kinetic regime to the treatment of real wastes under flow conditions.

We investigated the deposition of silver, copper, and lead from the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] under potentiostatic conditions in the presence of water. This was part of a larger project involving the extraction of metal ions from mining waste into an ionic liquid followed by electrodeposition so that the ionic liquid could be recycled. All three elements were deposited in metallic form by electrolysis in the ionic liquid, and the process was enhanced rather than hindered by the presence of water. The deposited metals did not adhere strongly to the cathode of the electrochemical cell, especially when Ebonex® was used as the cathode. The deposition of silver showed little temperature dependence, and at temperatures close to ambient, the ionic liquid was not adversely affected. The deposits of copper and lead gradually re-dissolved after electrodeposition, suggesting that chemical re-oxidation of these metals by air is more facile in the ionic liquid than in water. Copper showed strong evidence of formation of a Cu+ species upon reduction of Cu2+ (not seen in water); lead (Pb2+) showed evidence of a time-dependent complexation with the anions of the ionic liquid.

Possible chemical methods for the treatment of ammonia in the air of livestock holding facilities, with particular focus on poultry production, are reviewed in the context of eliminating ammonia by oxidation to elemental nitrogen. Gas phase catalytic oxidation processes are incompatible with the needs of the poultry industry on grounds of both capital cost and energy intensiveness. Most chemical oxidants convert ammonia principally to nitrate rather than N2. So-called advanced oxidation processes are unsuited to ammonia oxidation because the hydroxyl radicals that characterize these oxidations react poorly with both NH3 and NH4+. One promising option is electrochemical oxidation, which does not require the purchase of stoichiometric amounts of chemical oxidants. Among possible electrochemical methods, we favour electrochemical hypochlorination, whereby the denitrification of ammonia to elemental nitrogen is mediated by hypochlorous acid, which is formed reversibly from chloride ion. This technique is compatible with currently available scrubbing technology, with the modification of using acidic brine as the scrubbing solution. Because electrochemical hypochlorination can be applied without costly and complicated pH adjustment of the scrubbed solution with chemical additives, it constitutes an example of best available technology.

Ammonia in poultry barns produced by microbial action on the birds’ excreta can be removed by scrubbing into aqueous solution at pH ∼4. However, disposal of the resulting solution remains a problem. In this work, ammonia was oxidized electrochemically in the presence of unreactive electrolytes (NaClO4, Na2SO4), but the conditions were not compatible with treatment inside or outside a poultry barn (high pH, closed electrochemical reactor, and high ammonia concentration). Efficient denitrification is possible without pH adjustment of the scrubbed solution when chloride ion is also present in the scrubbing solution. This reaction is based on electrochemical hypochlorination, which is similar to breakpoint chlorination for the chemical elimination of ammonia. This work confirms a recent mechanistic proposal that efficient denitrification at pH ∼3 is the result of concomitant oxidation of water and acidification at the anode, but shows in addition that the mechanisms of both chemical and electrochemical hypoch...

The electrochemical oxidation of ammonia has been studied in the most detail in alkaline solution at platinized platinum. Almost all work supports the essence of a mechanism first proposed by Gerischer and Mauerer (1970) [19] , in which elemental nitrogen is formed at mildly positive potentials with near quantitative current efficiency through dimerization of partly dehydrogenated ammonia molecules NH x (ads). The major intermediate, NH 2 (ads), is formed at Pt(1 0 0) domains on the metal surface, where it dimerizes to hydrazine, and rapidly oxidizes to N 2 . At somewhat more positive potentials, the formation of adsorbed nitrogen atoms poisons the anode, and nitrogen evolution ceases. At potentials where water is oxidized, the Pt anode is modified by a surface oxide; under these conditions, nitrogen evolution is accompanied by nitrogen oxides and oxyanions. Similar mechanisms are most probably followed on other noble metals and their alloys, although there is less experimental information. In the past decade there has been preliminary study of other anode materials, such as Ni/Ni(OH) 2 , Ti/IrO 2 , and boron-doped diamond, with a view to finding inexpensive and long-lasting anodes for ammonia oxidation, but so far, little is known about the mechanism of oxidation at these materials.

Geothermal brines associated with natural gas extraction present an environmental problem because of the need to avoid the escape of toxic and odorous hydrogen sulfide. In this work, carbon-based anodes (including graphite, granulated activated carbon (GAC), and industrial coke) were used in the electrochemical oxidation of sulfide ion in synthetic geothermal brines in both batch and flow cells, with a view to remediating this troublesome contaminant. Experiments were carried out in alkaline solution (to prevent volatilisation of H2S), in the absence or presence of added chloride ion and naphthenic acids (NAs). The product spread was variable due to the large number of competing reactions, including formation of polysulfide, oxidation to sulfate (either directly or via hypochlorination), sacrificial anode oxidation combined with sulfide trapping, and Kolbe oxidation of NAs combined with sulfide trapping. From the technological perspective, the product distribution is immaterial, because sour brines contain such high concentrations of inorganic and naphthenate salts that reinjection of the treated brine will always be required. The most efficient systems, on the basis of charge per mol sulfide remediated, employed packed bed reactors with ground coke or GAC anodes. In these reactors, the disappearance of sulfide ion was promoted, as expected, at low flow rate and high current. However, the packed beds were limited to low applied current, in order to avoid compromising the electrical conductivity of the anode bed by the production of gas (O2). Les saumures geothermiques associees a l'extraction du gaz naturel presentent un probleme environnemental a cause du besoin d'eviter l'echappement de sulfures d'hydrogene toxiques et malodorants. Dans cette etude, des anodes au carbone (y compris graphite, charbon actif en granules et coke industriel) ont ete utilisees pour l'oxydation electrochimique de l'ion sulfure dans des saumures geothermiques synthetiques, a la fois en lots et en cuves a flux continu, avec l'objectif d'eliminer ce contaminant ennuyeux. Des experimentations ont ete effectuees dans une solution alcaline (pour prevenir la volatilisation de H2S), en l'absence ou presence d'ions chlorures ajoutes et d'acides naphtheniques. La repartition du produit etait variable en raison du grand nombre de reactions concurrentes, y compris la formation of polysulfide, l'oxydation en sulfates (soit directement, soit par hypochloration), l'oxydation de l'anode sacrificielle combinee a un piegeage du sulfure et une oxydation Kolbe d'acides naphtheniques combinee a un piegeage du sulfure. Dans cette perspective technologique, la distribution du produit est immaterielle car les saumures acides contiennent de telles fortes concentrations de sels inorganiques et naphthenates qu'une reinjection de la saumure traitee sera toujours requise. Les systemes les plus efficients, sur la base d'une charge par mole de sulfure eliminee, employaient des reacteurs a lit fixe avec anodes a coke ou a carbon actif en granules. Dans ces reacteurs, la disparition de l'ion sulfure etait promue et attendue a un faible debit et fort courant. Cependant, les lits fixes etaient limites a un faible courant afin d'eviter de compromettre la conductivite electrique du lit anodique par la production de gaz (O2). © 2011 Canadian Society for Chemical Engineering

Positive polarization of a graphite anode in aqueous solution functionalizes the surface and releases soluble organic carbon to the solution concurrent with the electrolysis of water. Mineralization of the anode occurs at more positive potentials, and can be explained as a repetitive sequence involving functionalization, oxidation to carboxyl, and Kolbe decarboxylation, without recourse to hydroxyl radicals. Other lines of evidence against the intermediacy of hydroxyl radicals include the resistance of p-benzoquinone towards oxidation at graphite – i.e., graphite does not function as an inactive anode towards the oxidation of added substrates. A direct electron transfer mechanism operates for substrates that are oxidizable in the range of water stability, such as acetaminophen and sulfide ion. In the potential range of oxygen evolution we propose that graphite behaves as a modified active anode, at which the oxygen atom to be transferred to an oxidizable substrate first becomes bonded to the previously functionalized surface.

The Magneli phase Ti4O7 (Ebonex®) was used as both anode and cathode in the electrochemical oxidation of sulfide ion in alkaline solution in the absence and presence of chloride and naphthenate ions. Ebonex anodes gradually lost their activity through the formation of an over-oxidized surface layer, but their activity could be maintained by periodic polarity reversal. In the context of the current paradigm for the mechanistic behaviour of oxide-based anodes, Ti4O7 has properties that combine those of “inactive” anodes (formation of hydroxyl radicals) and “active” anodes (formation of a higher oxide at the surface), with the exception that the higher oxide in the case of Ti4O7 is TiO2, which is incapable of substrate oxidation. Sulfate is the major oxidation product, especially in the presence of chloride, an ubiquitous component of sour brines, via mediated electro-oxidation to hypochlorite. Unlike at boron-doped diamond anodes, at which sulfide is oxidized with near-quantitative current efficiency, significant parasitic oxidation of water to O2 occurs at Ebonex, and oxidation of sulfide requires ~16 F mol–1, corresponding to a 50% current efficiency.

The oxidation of the sulfide ion occurs efficiently in batch cells at massive coke electrodes. At all currents studied, the products included a low yield of elemental sulfur, which deposited on the anode; the yields of sulfate were also low, except at the highest current. The remaining products were soluble organosulfur species, indicating that the coke anodes acted sacrificially. The reaction displayed unusual kinetic behavior with respect to the disappearance of sulfide: a two-stage reaction was observed in which the loss of sulfide was faster in the early stages of reaction, while elemental sulfur deposited on the anode. A subsequent slower current-controlled reaction was associated with the formation of the remaining products.

This work was undertaken to distinguish four pathways for the electrochemical oxidation of acetaminophen as a model organic substrate: (i) direct electron transfer from the substrate to the anode, (ii) reaction of the substrate with HO• at boron-doped diamond anodes, (iii) non-radical (two-electron) oxidation of the substrate at Ti/RuO2 anodes, and (iv) electrochemical hypochlorination if Cl– is present. Pathway (i) was isolated as a slow reaction when boron-doped diamond (BDD) was used as the anode in the range of water stability, whereas in the corresponding reaction with Ti/RuO2 only pathway (iii) could be detected. Pathway (ii) predominated for BDD in the potential range of water oxidation, and was the only mechanism leading to mineralization of the substrate. Comparison between chemical hypochlorination and electrochemical oxidation at Ti/RuO2 in the presence of chloride ion indicated that the latter process principally involves mediated hypochlorination. Oxidation at boron-doped diamond anodes in the presence of chloride was the most complex mechanistically, with competition between hypochlorination and the electrochemical “advanced oxidation process”; this led to the formation of chlorinated byproducts. The observation of mineralization under these conditions demonstrated cross-over between reaction pathways (ii) and (iv), even though hypochlorination appeared to be the initial pathway for loss of acetaminophen. The presence of chloride ion did not significantly retard mineralization of acetaminophen in the initial stages of oxidation, but significantly increased the energy requirement for complete mineralization. The results are discussed in the context of the use of electrochemical oxidation in waste management.

Sulfamethoxazole (SMX) was used as a model substrate for electrochemical oxidation at a boron-doped diamond anode in the presence of chloride ion, which is present in many waste streams. In the absence of chloride, oxidation of SMX involves mineralization, an electrochemical advanced oxidation process (EAOP) that is initiated by attack of anode-derived hydroxyl radicals. The rate of disappearance of SMX increased monotonically upon addition of chloride ion but without inhibiting mineralization in the early stages of oxidation. This demonstrated that electrochemical hypochlorination (EH) and EAOP are not mutually exclusive reaction pathways; products of EH can undergo EAOP and vice versa. Persistent chlorinated byproducts were formed in the presence of chloride ion, indicating that chloride is a significant detriment to the success of EAOP. No mineralization was observed upon chemical hypochlorination of SMX with sodium hypochlorite.

Ebonex® is a conductive and corrosion-resistant ceramic with the approximate composition Ti4O7. Anodic and/or cathodic polarization of a pair of Ebonex electrodes changed their surface composition, as shown by the development of a potential difference between them. In consequence, the activity of an Ebonex anode with respect to oxidation of an organic substrate depends on its past history. The anodic oxidation of p-nitrosodimethylaniline, which has been used as a model compound for the detection and quantitation of hydroxyl radicals, was studied in order to determine whether hydroxyl radicals are produced upon anodic polarization of Ebonex. The results were ambiguous, because direct oxidation of this substrate and oxidation of water to hydroxyl radicals occur at similar potentials. p-Benzoquinone (BQ) was found to be a more satisfactory mechanistic probe because it is resistant to direct oxidation. The rates of both disappearance and overall mineralization of BQ at Ebonex were intermediate between the corresponding rates at boron-doped diamond (BDD) and Ti/IrO2–Ta2O5 anodes, which promote one-electron and two-electron oxidations respectively. However, it is not yet clear whether mineralization is initiated by hydroxyl radicals formed in lower yield than at ‘active’ materials such as BDD, or whether oxidation involves less reactive intermediates such as HO2 radicals.

Electrolysis of liquid hog manure under direct current achieved amelioration of odour, in terms of both odour quality and odour intensity, and simultaneously reduced the population of odour-causing bacteria by two orders of magnitude. A comparison of anode materials indicated that hydroxyl radical-forming anodes (boron-doped diamond and Ebonex) were the most effective, but unfortunately these materials are not yet available commercially in large size format. Dimensionally stable anodes composed of Ti/IrO2 were found to be satisfactory; lead-based anodes leached unacceptable quantities of inorganic lead into the treated manure, and graphite anodes tended to disintegrate on long-term use. The mechanism of action of the bactericidal effect involves a combination of toxicity by O2 that is released at the anode and hypochlorination due to the chloride ion that is present in manure. The proposed technology was shown to be successful at the 27 L scale in an on-farm demonstration. L'electrolyse du purin de porc liquide avec un courant continu a permis d'ameliorer l'odeur, tant en qualite qu'en intensite, et simultanement, de reduire la population de bacteries a l'origine de l'odeur de deux ordres de grandeur. Une comparaison des materiaux anodiques indique que les anodes formant des radicaux libres d'hydroxyle (diamant ameliore au bore et Ebonex) sont les plus efficaces, mais malheureusement ces materiaux ne sont pas encore disponibles en grand quantite dans le commerce. On a trouve que des anodes dimensionnellement stables composees de Ti/IrO2 etaient satisfaisantes, tandis que les anodes a base de plomb ont donne des quantites inacceptables de plomb lixivie non organique dans le purin traite et les anodes graphites tendent a se desintegrer avec une utilisation prolongee. Le mecanisme d'action de l'effet bactericide implique une combinaison de toxicite par l'O2 qui est libere a l'anode et par l'hypochloruration a cause de l'ion de chlorure present dans le purin. On montre que la technologie proposee est un succes dans une demonstration a l'echelle de 27 L dans une ferme

The forthcoming introduction of lower standards for arsenic in drinking water requires new technologies for arsenic removal. We report the development of an electrochemical unit for remediating domestic water supplies for homes without municipally treated water. Electrolysis in a two-anode system provides oxidants to convert As(III) to As(V) in situ, and a sacrificial anode to deliver iron into solution. Conditioning tanks after each electrolysis step ensure completion of the chemical reactions. At the pH of domestic water, As(V) co-precipitates with Fe(OH)3; subsequent filtration leaves

Pharmaceutical residues in the aquatic environment represent an emerging environmental problem, because many pharmaceuticals are refractory towards conventional waste water treatment. This study focussed on the oxidation of the sulfonamide antibiotic sulfamethoxazole (SMX) at a boron-doped diamond anode, at which reactive hydroxyl radicals are formed. Electrochemical oxidation led to mineralization with high current efficiency, but without the formation of known toxic products of partial oxidation. A “mixed” kinetic order with respect to substrate concentration was observed; the kinetics could be shifted in the direction of either diffusion control (first order in SMX) or current control (zero order in SMX) by adjusting the substrate concentration and current density. Alternatively, the electrooxidation could be described by a model, applicable to a wide range of reaction conditions, in which the kinetic orders with respect to current and initial substrate concentration were approximately 0.4 and 0.5, respectively.

The goal of the present research was the direct conversion of sulfide, an important contaminant in geothermal brines, to sulfate, whose discharge limits are much less stringent than those for sulfide. By the use of a novel anode material boron-doped diamond (BDD), we achieved near-quantitative electrochemical conversion of sulfide ions to sulfate with current efficiency of 90%. Kinetically, the reaction is first order in current density and zero-order in sulfide concentration. The current efficiency becomes essentially quantitative in the presence of chloride ion; under these conditions the reaction is chloride-mediated, at least in part, through the electrochemical formation of hypochlorite ion. Control experiments showed that hypochlorite oxidizes sulfide to sulfate quantitatively under the same conditions.

In divided flow-through cells, reductive electrolysis of Orange II (1) was first order in both substrate and applied current and gave the products of reductive cleavage at the azo linkage in essentially quantitative yield. Oxidative electrolysis was also pseudo-first-order and led to mineralization. At a boron-doped diamond anode, the rate of disappearance of 1 closely tracked the loss of total organic carbon from solution. In undivided cells oxidation and reduction occurred simultaneously; under acidic conditions the reduction products (anilines) were rather persistent because they were present as ammonium ions, but under alkaline conditions the anilines were mineralized along with the starting material. When chloride ion was present in the supporting electrolyte, electrochemical oxidation afforded hypochlorite, and the disappearance of 1 proceeded by way of hypochlorination.

Oxidation of acetaminophen at boron-doped diamond (BDD) and at Ti/SnO2 anodes in a plug-flow divided electrochemical reactor led to electrochemical combustion, whereas at Ti/IrO2 benzoquinone was the exclusive product except at very long electrolysis times. The difference is explicable in terms of the different mechanisms of oxidation: direct oxidation at the anode for Ti/IrO2 vs. indirect oxidation involving electrogenerated hydroxyl radicals at BDD and Ti/SnO2. At BDD, at which the efficiency of degradation of acetaminophen was greatest, the rate of electrolysis at constant concentration was linearly dependent on the current, and at constant current linearly dependent on the concentration. Current efficiencies for mineralization up to 26% were achieved without optimization of the cell design.

Electrolysis was investigated as a method for the amelioration of odor from the supernatant phase of centrifuged liquid hog manure. Methods examined included the use of dual anodes to provide both oxidizing intermediates from water electrolysis and a sacrificial input of iron into solution to remove organic constituents of hog manure by coagulation. The most promising approaches used a “dimensionally stable anode” composed of tin dioxide coated on a base of titanium, and the newly developed material boron-doped diamond, which has an exceptional stability to both oxidizing and reducing electrolytic conditions. These anodes promote the formation of hydroxyl radicals which initiate oxidation of organic constituents of the manure.

At only ppm concentrations, arsenic is incompatible with the deposition of pure nickel at the cathode of a nickel electrorefining bath. We report an electrochemical method for the removal of arsenic. At a Ti/IrO2 anode, chloride ion in the bath is oxidized to HOCl, which effects the chemical oxidation of As(III) to As(V). HOCl also reoxidizes any As(III) that might be formed by back-reduction at the pure nickel cathode, preventing cathodic reduction of As(III) to elemental arsenic or to the toxic gas arsine, AsH3. If, alternatively, a pure copper cathode is employed, the cathode potential can be controlled to electroseparate copper from nickel during the remediation of the bath, while achieving complete oxidation of As(III). After pH adjustment to ∼4, arsenic is removed with added Fe(III) as a chemisorbed arsenate−ferrihydrite precipitate, leaving the bath essentially free of both iron and arsenic.

Acid mine drainage (AMD), which is caused by the biological oxidation of sulfidic materials, frequently contains arsenic in the form of arsenite, As(III), and/or arsenate, As(V), along with much higher concentrations of dissolved iron. The present work is directed toward the removal of arsenic from synthetic AMD by raising the pH of the solution by electrochemical reduction of H+ to elemental hydrogen and coprecipitation of arsenic with iron(III) hydroxide, following aeration of the catholyte. Electrolysis was carried out at constant current using two-compartment cells separated with a cation exchange membrane. Four different AMD model systems were studied: Fe(III)/As(V), Fe(III)/As(III), Fe(II)/As(V), and Fe(II)/As(III) with the initial concentrations for Fe(III) 260 mg/L, Fe(II) 300 mg/L, As(V), and As(III) 8 mg/L. Essentially quantitative removal of arsenic and iron was achieved in all four systems, and the results were independent of whether the pH was adjusted electrochemically or by the addition of NaOH. Current efficiencies were approximately 85% when the pH of the effluent was 4-7. Residual concentrations of arsenic were close to the drinking water standard proposed by the World Health Organization (10 microg/L), far below the mine waste effluent standard (500 microg/L).

The removal of As(III) and As(V) from acidic and basic solutions by electrochemical reduction was studied using a reticulated vitreous carbon cathode and a IrO2/Ti anode in an electrochemical reactor that could be operated divided or undivided. By using a cascade of 7–9 plug flow reactors, residual concentrations of arsenic less than 20 ppb were achieved upon reduction of 100 ppm As(III) in either acidic or alkaline solutions and for 100 ppm As(V) in acidic solution. The reduction of As(V), generally considered electrochemically inactive in alkaline solutions, was proved possible, but was much less efficient. In all cases, the only product of electrochemical reduction was arsine. A moderate improvement in reduction efficiency was achieved under conditions of electrocatalytic hydrogenation using 5% Pd on alumina as catalyst.

The autoxidation of p-t-butyltoluene (TBT) at 80 °C in the liquid phase is carried out with an initial mixture of cobalt(III) and cobalt(II) acetate in an acetic acid solution. The autoxidation kinetics of TBT is appreciably accelerated by electrolysis with a anodic current density of 62.5 A m−2. The electrolysis increases the concentration of cobalt(III) acetate, the actual catalyst of the autoxidation reaction. The end product of oxidation is p-t-butylbenzoic acid (TBBA). p-t-Butylbenzaldehyde (TBBZ) is an intermediary whose concentration passes through a maximum. The oxidation experiments with TBT were performed with total cobalt acetate concentrations ranging from 0.0188 to 0.169 mol dm−3. An increase in total cobalt acetate concentration favours the electrochemical regeneration of CoIII and slightly improves the TBBZ selectivity. The duration of TBT oxidation into TBBA is reduced by a factor of 5 compared with a reaction without electrolysis.

The direct electrochemical oxidation of p -methoxytoluene (MT) in solution in acetic acid on a graphite anode under oxygen-free conditions results in the formation of (MBA) with chemical yields higher than 80%. This paper shows that the same process, operated under oxygen saturation conditions, concurrently leads to the formation of p -methoxybenzaldehyde. The acetate/aldehyde ratio varies from 37 under an oxygen-free atmosphere to 2.3 under oxygen saturation (1 atm, 80°C, 125 A m −2 ). The results of voltammetric studies on the one hand, and those of large-scale electrolyses of MT and MBA on the other hand, permit us to propose a mechanism for the process conducted under oxygen saturation. Accordingly, it is assumed that the benzyl radicals generated on the graphite anode, from MT or MBA in acetic acid, can react directly with the dissolved molecular oxygen. The generated peroxide radicals, ArCH 2 O 2 , undergo chemical reactions leading to the formation of the corresponding aldehyde and alcohol. Under the reaction conditions, most part of the alcohol is electro-oxidized into aldehyde. As the process is run in the absence of complexes of transition metals, the aldehyde obtained is protected from further oxidation.

Liquid phase autoxidation of toluene and p-methoxytoluene in acetic acid has been studied under electrolysis in the presence of cobalt acetate. The latency period disappears when Co3+ is electrogenerated in the medium either before or during autoxidation experiments. The kinetics of the oxidation process is accelerated under electrolysis condition. The effects of temperature, current intensity, Co concentration, oxygen flow rate are investigated. The transition between the kinetic and the mass transfer limitations is determined in the case of p-methoxytoluene.

Acid mine drainage (AMD), caused by biological oxidation of sulfide minerals in the presence of air and water, is a significant environmental problem because of its acidity and the presence of high concentrations of iron and solubilized toxic metal ions. The focus of this review is to consider the prospects for electrochemical technologies for either prevention or remediation of AMD, with physico-chemical technologies mentioned for comparison.

Electrochemical debromination of the commercial decabromodiphenyl ether flame retardant DE-83TM in partly aqueous tetrahydrofuran (THF) solution gave lower brominated congeners by sequential loss of bromine atoms. Hydrodebromination was most facile for the most heavily brominated congeners. It involves initial electron transfer and proton transfer from water, rather than hydrogen atom abstraction from THF, as shown by experiments with deuterated water. The product distribution from electrolysis involves preferential loss of bromine meta- and para- to the ether linkage, comparable with the products of metabolism of BDE-209 in various organisms. Significantly, the environmentally relevant congeners BDE-47, BDE-99, and BDE-154 were not major products of debromination of BDE-209 by the electron transfer mechanism.

Electrochemical reduction of RDX, hexahydro-1,3,5-trinitro-1,3,5-triazine, a commercial and military explosive, was examined as a possible remediation technology for treating RDX-contaminated groundwater. A cascade of divided flow-through cells was used, with reticulated vitreous carbon cathodes and IrO2/Ti dimensionally stable anodes, initially using acetonitrile/water solutions to increase the solubility of RDX. The major degradation pathway involved reduction of RDX to the corresponding mononitroso compound, followed by ring cleavage to yield formaldehyde and methylenedinitramine. The reaction intermediates underwent further reduction and/or hydrolysis, the net result being the complete transformation of RDX to small molecules. The rate of degradation increased with current density, but the current efficiency was highest at low current densities. The technique was extended successfully both to 100% aqueous solutions of RDX and to an undivided electrochemical cell.

Polybrominated diphenyl ethers (PBDEs) are ubiquitous environmental contaminants due to their use as additive flame-retardants. Conventional catalytic hydrogenolysis in methanol solution and electrocatalytic hydrogenolysis in aqueous methanol were examined as methods for debrominating mono- and di-bromodiphenyl ethers, as well as a commercial penta-PBDE mixture, in each case using palladium on alumina as the catalyst. Electrocatalytic hydrogenolysis employed a divided flow-through batch cell, with reticulated vitreous carbon cathodes and IrO2/Ti dimensionally stable anodes. Both methods gave efficient sequential debromination, with essentially complete removal of bromine from the PBDEs, but the electrocatalytic method was limited by the poor solubility of PBDEs in aqueous methanol.

The demonstration in which a copper penny is converted first to "silver" and then to "gold" involves, as the first step, the deposition of zinc onto copper in hot NaOH solution. The reaction involves an electrochemical cell, with the deposition of zinc in the form of a surface layer of the silver-colored alloy γ-brass, accompanied by evolution of hydrogen at the copper cathode. The cathode potential falls progressively for a series of cells having Zn anode and cathodes varying in composition from Cu to brasses with Cu content down to less than 55% Cu, but H2 production is an independent process which appears to be unaffected. The silvery deposit can also be obtained conveniently by electrolytic deposition from a cell comprising zinc anode, copper cathode, and an electrolyte containing millimolar concentrations of both zinc sulfate and sodium sulfate.

not Available.

not Available.

Environmental Context. Nitramine explosives, including RDX, HMX, and the more newly developed CL-20, are the source of groundwater contamination (‘pinkwater’) especially around military installations. These materials all possess an abundance of nitro (NO2) groups, which, like synthetic organohalogens, render them resistant to biodegradation and thereby allows them to persist in the soil and waters. In this study it was shown that these substances can be indirectly oxidized at a boron-doped diamond electrode to small molecules (carboxylic acids and mineralized nitrogen-containing compounds). Abstract. Electrochemical oxidation at boron-doped diamond (BDD) electrodes was examined as a possible technique for the remediation of water contaminated with nitramine explosives. The advantage of BDD is that it promotes indirect oxidation by electrogenerated active intermediates, such as hydroxyl radicals. For the three explosives RDX, HMX, and CL-20, degradation in both acetonitrile/water mixtures and in water alone was suggested to involve an initial denitration, followed by spontaneous decomposition of the molecules, the net result being the complete transformation of the nitramines to small molecules. Although the rate of degradation increased with current density, the current efficiency was highest at low current densities.