Electrochemical Dechlorination Research Articles

Electrocatalytic properties of transition metals toward reductive dechlorination of polychloroethanes

Abstract Chlorinated volatile organic compounds (VOCs) such as polychloromethanes and polychloroethanes (PCEs) are among the most ubiquitous pollutants in the environment. Reductive electrochemical dechlorination at catalytic cathodes is one of most promising methods of abatement of these pollutants. In this study, the electrochemical reduction of several PCEs has been investigated in DMF + 0.1 M (C3H7)4NBF4 at Ag, Au, Pd, Pt, Cu, Fe, Ni, Pb, Zn and glassy carbon (GC), the last used as a reference system with negligible catalytic properties. The reduction potentials of all investigated PCEs are remarkably influenced by the nature of the electrode material and the chemical structure of the compound. In any case, Ag, Cu, and Au show powerful electrocatalytic activities. Based on voltammetric investigations on all PCEs and electrolyses of 1,1,1-trichloroethane and 1,1,1,2-tetrachloroethane in different conditions, the reduction mechanism is shown to be strongly dependent on the nature of the PCE. Geminal PCEs undergo a sequential hydrodechlorination mechanism in which one chlorine atom is lost in each reduction step until a completely dechlorinated ethane is obtained. In contrast, reduction of vicinal PCEs involves removal of two chlorine atoms in an overall 2e− process resulting in the formation of the corresponding (chloro)ethylene, which may be further reduced at more negative potentials.

Optimization of electrochemical dechlorination of trichloroethylene in reducing electrolytes

Electrochemical dechlorination of trichloroethylene (TCE) in aqueous solution is investigated in a closed, liquid-recirculation system. The anodic reaction of cast iron generates ferrous species, creating a chemically reducing electrolyte (negative ORP value). The reduction of TCE on the cathode surface is enhanced under this reducing electrolyte because of the absence of electron competition. In the presence of the iron anode, the performances of different cathodes are compared in a recirculated electrolysis system. The copper foam shows superior capability for dechlorination of aqueous TCE. Electrolysis by cast iron anode and copper foam cathode is further optimized though a multivariable experimental design and analysis. The conductivity of the electrolyte is identified as an important factor for both final elimination efficiency (FEE) of TCE and specific energy consumption. The copper foam electrode exhibits high TCE elimination efficiency in a wide range of initial TCE concentration. Under coulostatic conditions, the optimal conditions to achieve the highest FEE are 9.525mm thick copper foam electrode, 40mA current and 0.042molL−1 Na2SO4. This novel electrolysis system is proposed to remediate groundwater contaminated by chlorinated organic solvents, or as an improved iron electrocoagulation process capable of treating the wastewater co-contaminated with chlorinated compounds.

Studies on Electrochemical Dechlorination Reaction of 2,4-Dichlorobenzoic Acid on Pd/Ti Electrode

Abstract: Pd/Ti electrode was prepared on Ti substrate electrode in hydrochloric acid containing PdCl2 using cyclic voltammetry. The electrocatalytic performance toward electrochemical reduction reactivity of 2,4-dichlorobenzoic acid (2,4-DCBA) on Cu, Ag, Ti and Pd/Ti was investigated. Compared with Cu, Ag and Ti electrodes, Pd/Ti electrode exhibited a high electrochemical activity for the reduction process of 2,4-DCBA in NaOH solution. In situ FTIR was applied to analyze the electrochemical dechlorination mechanism of 2,4-DCBA systematically. The results showed that 2,4-DCBA was electroreduced to its free radical ion after receiving an electron. Then the 2,4-DCBA free radical took off two chloride ions and produced the benzoic acid free radical. After receiving another two electrons and two protons, the final product benzoic acid was obtained.

Redox Control for Electrochemical Dechlorination of Trichloroethylene in Bicarbonate Aqueous Media

The role of iron anode on electrochemical dechlorination of aqueous trichloroethylene (TCE) is evaluated using batch mixed-electrolyte experiments. A significantly higher dechlorination rate, up to 99%, is reported when iron anode and copper foam cathodes are used. In contrast to the oxygen-releasing inert anode, the cast iron anode generates ferrous species, which regulate the electrolyte to a reducing condition (low ORP value) and favor the reduction of TCE. The main products of TCE electrochemical reduction on copper foam cathode include ethene and ethane. The ratio of these two hydrocarbons gases varied with the electrolyte ORP condition and current density as more ethane gas generates at more reducing electrolyte condition and at higher current condition. A pseudofirst-order model is used to describe the degradation of TCE; the first-order rate constant (k) increases with the current applied but exhibits a negative relation with initial concentration. Depending on the current, electrolysis by iron anode causes a reduction in the ORP and an increase in the pH of the mixed electrolyte. Enhanced reaction rates in this investigation indicate that the electrochemical reduction using copper foam and iron anode may be a promising process for remediation of groundwater contaminated with chlorinated organic compounds.

Increasing the activity and stability of chemi-deposited palladium catalysts on nickel foam substrate by electrochemical deposition of a middle coating of silver

The objective of this study was to enhance the activity and stability of the chemi-deposited palladium/nickel foam (Pd/Ni foam) electrode used in the electrochemical dechlorination process. The electrochemical deposition of silver onto a nickel foam substrate (Ag/Ni foam) was applied before the chemical deposition of palladium. The physicochemical properties of the resultant material (Pd/Ag/Ni foam) were characterized by X-ray diffraction and scanning electron microscopy. The results showed that the silver coating was in the form of spherical apophyses, and the palladium was dispersed finely on the surface of the Ag/Ni foam. Dichloroacetic acid was selected as an indicator to evaluate the activity and stability of the palladium catalyst. Pd/Ag/Ni foam electrode consistently had relatively higher dechlorination rate and current efficiency (CE) in the current density range of 1.5–10.5 mA cm −2 compared to the Pd/Ni foam electrode. Quantitative analysis of the main intermediates and final products involving monochloroacetic acid, acetic acid and the chloride ion further demonstrated the enhancement of electrochemical dechlorination by the presence of silver. Moreover, after five cycles of dechlorination, the CE for the Pd/Ni foam electrode decreased dramatically from 67% to 13%, whereas that for the Pd/Ag/Ni foam electrode was decreased by much less, from 78% to 62%, and the palladium loss was clearly reduced by the addition of a silver coating between the Ni foam and the Pd outer layer.

Electrochemical dechlorination of 2,4-dichlorophenol in aqueous solution on palladium-loaded meshed titanium electrode

Electrochemical dechlorination of 2,4-dichlorophenol (2,4-DCP) in aqueous solution was investigated on palladium-loaded meshed titanium (Pd/Ti) electrode at ambient temperature. Pd/Ti electrode was prepared with an electrodepositing method. Scanning Electron Microscope (SEM) micrographs show that Pd microparticles uniformly disperse on the meshed Ti supporting electrode with spheroidal structure. Dechlorination experimental results indicate that, in aqueous solution, with the current efficiency of 24.3%, the removal efficiency of 100 mg/L 2,4-DCP on Pd/Ti electrode was 93.2% under the conditions of the dechlorination current of 5 mA and dechlorination time of 90 min. The effect of initial 2,4-DCP concentration was also investigated.

Electrochemical reduction of propazine: NMR evidence for dechlorination and reduction of the triazine ring

Propazine (2-chloro-4,6-diisopropylamino-1,3,5-triazine) is a triazine herbicide used to control broadleaf weeds and annual grasses during the production of milo grain sorghum. This compound provides post-emergent protection by interfering with photosynthetic electron transport of target weeds. Tolerant crops are able to degrade the applied herbicide to nontoxic metabolites. Propazine is electrochemically active under acidic conditions. Electrochemical reduction pathways for this herbicide have been proposed based on differential pulse polarography (DPP) studies. Theoretical deconvolutions of the experimental polarograms were consistent with a 2-electron cleavage of the C–Cl bond via a mechanism involving a protonation step in between the two electron transfers. The resulting intermediate was then reduced by an irreversible 2-electron reduction of the ring to produce the final product. We report supporting nuclear magnetic resonance (NMR) evidence for the electrochemical dechlorination of propazine and reduction of the triazine ring using controlled potential electrolysis (CPE) and a mercury pool electrode under acidic conditions.

Complete electrochemical dechlorination of chlorobenzenes in the presence of various arene mediators

Electrochemical dechlorination of chlorobenzenes in the presence of various arene mediators such as naphthalene, biphenyl, phenanthrene, anthracene, and pyrene, was studied. The amount of mediator required was able to be reduced to 0.01 equiv. for all mediators except for anthracene, with the complete dechlorination of mono-, 1,3- di- and 1,2,4- trichlorobenzene still achieved. This catalytic amount of mediator plays an important role in accelerating the dechlorination through the rapid formation of radical anions prior to reduction of the chlorobenzenes.

Electrochemical reductive dechlorination of carbon tetrachloride on nanostructured Pd thin films

The nanostructured Pd thin films prepared via cyclic voltammetric deposition method are proved to be a promising electrocatalyst for electrochemical reductive dechlorination of carbon tetrachloride (CT). The use of as-prepared Pd thin films as the working electrode material provides a possibility to separately study the role of various forms of hydrogen in the dechlorination reactions. Electrochemical characterization and gas chromatography analysis clearly indicate for the first time that the adsorbed hydrogen has excellent ability to remove CT from acidic solutions through the surface reaction with the chemisorbed CT molecules, which is of fundamental importance to have a better understanding of the reaction mechanism of electrochemical dechlorination.

Synthesis of Reactive Nano-Fe/Pd Bimetallic System-Impregnated Activated Carbon for the Simultaneous Adsorption and Dechlorination of PCBs

Synthesis and use of reactive metal particles have shown significant environmental implications for the remediation of groundwater and sediment contaminated with chlorinated compounds. Herein, we have developed an effective strategy, employing a series of innovative granular activated carbon (GAC) composites incorporated with iron/palladium (Fe/Pd) bimetallic nanoparticles. The physical adsorption of polychlorinated biphenyls (PCBs) to GAC and their electrochemical dechlorination by Fe/Pd bimetal on the GAC could be simultaneously achieved. The novel synthesis approaches and targets to achieve in this study are (i) introduction of mesoporous GAC for Fe/Pd bimetal placement, (ii) in situ formation and incorporation of Fe particles in the GAC pores, (iii) modification of Fe surface with a discontinuous layer of noble metal Pd, and (iv) nanoscaling of Fe and Pd particles. Fe was imbedded in 7−40 nm mesoporous GAC via an incipient wetness impregnation method employing Fe(NO3)3. Heat-treated GAC/Fe (as Fe2O3) ...

Electrochemical dechlorination of chloroform in neutral aqueous solution on palladium/foam-nickel and palladium/polymeric pyrrole film/foam-nickel electrodes

Electrochemical dechlorination of chloroform in neutral aqueous solution was investigated using palladium-loaded electrodes at ambient temperature. Palladium/foam-nickel (Pd/foam-Ni) and palladium/polymeric pyrrole film/foam-nickel (Pd/PPy/foam-Ni) composite electrodes which provided catalytic surface for reductive dechlorination of chloroform in aqueous solution were prepared using an electrodepositing method. Scanning electron microscope (SEM) micrographs showed that polymeric pyrrole film modified the electrode-surface characteristics and resulted in the uniform dispersion of needle-shaped palladium particles on foam-Ni supporting electrode. The experimental results of dechlorination indicated that the removal efficiency of chloroform and current efficiency in neutral aqueous solution on Pd/PPy/foam-Ni electrode could be up to 36.8% and 33.0% at dechlorination current of 0.1 mA and dechlorination time of 180 min, which is much higher than that of Pd/foam-Ni electrode.

Self-regeneration of activated carbon modified with palladium catalyst for electrochemical dechlorination

Catalyst regeneration and the retention of high catalytic activity are still the critical issues in environmental application. A novel fluidized gas–liquid–solid electrochemical reactor was developed to simultaneously remove chlorinated pollutants and in situ regenerate the spent catalyst. Activated carbon modified with palladium catalyst (AC–Pd) was prepared for electrochemical dechlorination. For the 4-chlorophenol wastewater of initial concentration 200 mg L −1, the removal efficiency could nearly reach 100% in less than 30 min. Catalytic activity of AC–Pd catalyst was preserved effectively even in consecutive cycling run without special regeneration. OH radicals, generated by electrochemical reaction, played a critical role in self-regeneration of AC–Pd. High catalytic activity of spent AC–Pd catalyst provided an attractive alternative in wastewater treatment.

Electrochemical Dechlorination of Chlorobenzene in Acetonitrile with Various Water Concentrations

Electrochemical Dechlorination of Chlorobenzene in Acetonitrile with Various Water Concentrations

Electrochemical dechlorination of chlorobenzene with a mediator on various metal electrodes

Electrochemical dechlorination of chlorobenzene with a mediator on various metal electrodes

Electrochemical dechlorination of chlorobenzene with a mediator on various metal electrodes

Dechlorination of chlorobenzene has been studied in acetonitrile on metal electrodes (Pt, Cd, Sn, In, Pb, Zn, Ag) using naphthalene as a mediator. Voltammograms of the metal electrodes, except Pt, give redox peaks at −2.94 V (Ag∣Ag +) in the solution containing 10 mM of naphthalene. The cathodic peak at −2.94 V is enhanced with an increase of the concentration of chlorobenzene, evidence that chlorobenzene is also reduced at this potential. The current density of the enhanced cathodic peak depends on the metals in the following order: CdSn < InPb < Zn < Ag. Chlorobenzene has been electrolyzed on Zn and Ag and Pt electrodes at −2.94 V (Ag∣Ag +) in 0.1 M tetraethyl ammonium perchlorate (TEAP) + acetonitrile containing 10 mM of chlorobenzene and naphthalene. Main product of the electrolysis is benzene. The partial current density of benzene also depends on the metals as follows: PtZn < Ag.

Electrochemical Dechlorination of 1,2,4-Trichlorobenzene Using a Reticulated Vitreous Carbon Electrode

Stepwise dechlorination of 1,2,4-trichlorobenzene was observed at a glassy carbon electrode in dimethylformamide containing 0.1 M tetraethylammonium perchlorate. Especially, dechlorination to dichlorobenzene and further to monochlorobenzene or benzene was successfully demonstrated with a porous reticulated vitreous carbon electrode. Electrochemical dechlorination of polychlorobenzenes employing a flow cell with a reticulated vitreous carbon working electrode is also described. Preliminary experiments with a flow cell showed that dechlorination of trichlorobenzene to dichlorobenzene was partially completed while dechlorination to benzene or monochlorobenzene was not successful, suggesting that a flow rate and electrolysis time should be further optimized for the complete electrolysis.

Electrochemical Dechlorination of Chlorinated Hydrocarbons – Electrochemical Reduction of Chloroform in Acetonitrile/Water Mixtures at High Current Density

Abstract Electrochemical reduction of chloroform was studied in 0.1 M (1 M = 1 mol dm−3) tetraethyl ammonium perchlorate solutions of acetonitrile–water mixture using various metal electrodes. High water concentration (1 M) promoted the electrolysis with the products mainly CH4 and CH2Cl2. The partial current density of CH4 formation amounted to 0.3 A cm−2 at an Ag electrode.

Electrochemical Reduction of Tetrachloromethane. Electrolytic Conversion to Chloroform

The feasibility of electrolytic removal of tetrachloromethane from industrial effluents has been investigated. A new method based on the electrochemical reductive dechlorination of CCl4 yielding chloroform is described. The main goal was not only to remove CCl4 but also to utilize the process for obtaining chloroform, which can be industrially reused. GC-MS analysis of the electrolysed samples showed that chloroform is the only product. Voltammetric experiments were made in order to select experimental conditions of the electrolysis. Using energetic and economic criteria, ethanol-water (1 : 4) and LiCl were found to be the optimum solvent and supporting electrolyte tested. No great differences were found while working at different pH values. Chronoamperometric and voltammetric experiments with convolution analysis showed low kf0 and α values for the reaction. A new differential pulse voltammetric peak deconvolution method was developed for an easier and faster analysis of the electrolysis products. Electrolysis experiments were carried out using both a bulk reactor and a through-flow cell. Thus, three different kinds of galvanostatic electrolyses were carried out. Under all conditions, CCl4 conversions ranging from 60 to 75% and good current efficiencies were obtained.

Chlorine–nickel interactions in gas phase catalytic hydrodechlorination: catalyst deactivation and the nature of reactive hydrogen

The gas phase hydrodechlorination of chlorobenzene and 3-chlorophenol (where 473 K⩽T⩽573 K) has been studied using a 1.5% w/w Ni/SiO2 catalyst which was also employed to promote the hydrogenation of benzene, cyclohexene and phenol. In the former two instances the catalyst was 100% selective in removing the chlorine substituent, leaving the aromatic ring intact. While the dechlorination of chlorobenzene readily attained steady state with no appreciable deactivation, the turnover of 3-chlorophenol to phenol was characterised by both a short and a long term loss of activity. Chlorine coverage of the catalyst surface under reaction conditions was probed indirectly by monitoring, via pH changes in an aqueous NaOH trap, HCl desorption after completion of the catalytic step. Contacting the catalyst with the chlorinated reactants was found to severely limit and, depending on the degree of contact, completely inhibit aromatic ring reduction although a high level of hydrodechlorination activity was maintained. Hydrogen temperature programmed desorption (TPD) reveals the existence of three forms of surface hydrogen which are tentatively assigned as: (i) hydrogen bound to the surface nickel; (ii) hydrogen at the nickel/silica interface; (iii) spillover hydrogen on the silica support. The effect of chlorine–nickel interactions on the resultant TPD profiles is presented and discussed. The (assigned) spillover hydrogen appears to be hydrogenolytic in nature and is responsible for promoting hydrodechlorination while the hydrogen that is taken to be chemisorbed on, and remains associated with, the surface nickel metal participates in aromatic hydrogenation. Hydrodechlorination proceeds via an electrophilic mechanism, possibly involving spillover hydronium ions. The experimental catalytic data are adequately represented by a kinetic model involving non-competitive adsorption between hydrogen and the chloroaromatic, where incoming chloroaromatic must displace the HCl that remains on the surface after the dechlorination step. Kinetic parameters extracted from the model reveal that chlorophenol has a higher affinity than chlorobenzene for the catalyst surface but the stronger interaction leads to a greater displacement of electron density at the metal site and this ultimately leads to catalyst deactivation. IntroductionThe reductive dehalogenation of organic halides is not only important as a synthetic rout but is now gaining increasing significance as a potential methodology for treating toxic halogenated waste In the latter application, the organic halide is converted to the corresponding hydrocarbon and the HCl that is produced can be readily separated while the hydrocarbon is recycled as a means of waste minimisation. Thermal hydrodechlorination processes only proceed to an appreciable degree at temperatures in excess of 973 K5 but the presence of a catalyst lowers the operating temperature significantly The catalytic hydrodechlorination of chlorobenzene and chlorophenol(s), known environmental hazards promoted using a solid silica supported nickel catalyst is considered in this paper. The catalytic hydrodechlorination of chlorobenzene(s) has been reported in both the ga–11 and liqui–14 phases using palladium–16 platinum rhodiu and nicke based catalysts. The treatment of chlorophenols has, by comparison, received less attention but catalytic data are available for the liquid phase reaction over carbon supported palladiu and gas phase transformations over nickel system as well as the electrochemical dechlorination on palladized electrodes Liquid phase hydrodehalogenations can proceed in the presence of both molecular hydrogen (at pressures up to 50 atm) and hydrogen donors such as metal hydrides, formic acid and its salts and alcohols The mechanism of C–Cl bond hydrogenolysis in heterogeneous systems is still far from understood and a number of kinetic models have been propose–11,13,15 to account for the observed catalytic trends. Moreover, the reaction has been viewed in terms of both electrophili and nucleophili substitution and attempts have been made to identify the possible reactant–catalyst interactions A marked drop in dechlorination activity with reaction time has been reported for supported palladium rhodiu and bulk nickel catalyst while time invariant reaction profiles have been generated for Ni/Al2O3 (ref. 8) and a Pt/zeolite The drop in dechlorination activity has been linked in one instance to a loss of the supported active phas and also to a surface poisoning due to the formation of stable surface chloride species The study of catalytic dechlorination is still in a formative stage and the published studies are, in essence, a compilation of rate data which characterise individual systems while such issues as the nature of the reactive adsorbed species, the catalytically active site(s) and the source of catalyst deactivation (when it occurs) are still not established. In this paper we provide kinetic data for the gas phase (molecular) hydrogen treatment of chlorobenzene and 3-chlorophenol and, where feasible, compare our results with the above cited reports. The effect of prolonged exposure of the nickel/silica catalyst to concentrated chlorinated aromatic gas streams is examined, particularly in terms of the changes to the nature of the surface reactive hydrogen.

Electrochemical reductive dehalogenation of chlorobenzenes

Electrochemical dechlorination of 1,2,3,5-tetrachlorobenzene in methanol and chlorobenzene in dimethylsulfoxide with tetraalkylammonium salts as supporting electrolytes was carried out. The extent of dechlorination depends significantly on the electrode composition.