Electrochemical oxidation of several chlorophenols on diamond electrodes Part I. Reaction mechanism

The electrochemical oxidation of 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol aqueous wastes using boron-doped diamond electrodes was studied. This treatment led to complete mineralization of the wastes regardless of the operating conditions. A simple mechanistic model is consistent with the voltammetric and electrolysis results. According to this model, the electrochemical treatment of chlorophenol aqueous wastes involves the anodic and cathodic release of chlorine followed by the formation of non-chlorinated aromatic intermediates. Subsequent cleavage of the aromatic ring gives rise to non-chlorinated carboxylic acids. Chlorine atoms arising from the hydrodehalogenation of the chlorophenols are converted into more oxidized molecules at the anode. These molecules react with unsaturated C₄ carboxylic acid to finally yield trichloroacetic acid through a haloform reaction. The non-chlorinated organic acids are ultimately oxidized to carbon dioxide and the trichloroacetic acid into carbon dioxide and volatile organo-chlorinated molecules. Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment of chlorophenols.     

Electrochemical treatment of 2,4-dinitrophenol aqueous wastes using boron-doped diamond anodes

The electrochemical oxidation of 2,4-dinitrophenol (2,4-DNP) aqueous wastes has been studied using both, bulk electrolysis and voltammetric techniques. To carry out the bulk electrolysis, a bench-scale plant with a single compartment electrochemical flow cell was used. Boron-doped diamond (BDD) materials were used as the anode and stainless steel (AISI 304) as the cathode. According to the obtained results, a simple mechanistic model has been proposed. The oxidation of 2,4-DNP leads to the appearance of phenol and quinonic compounds and to the release of the nitro groups from the aromatic ring, in a first step. In a second step, these organics are transformed into carboxylic acids (mainly maleic and oxalic acid). The process ends with the formation of carbon dioxide (CO₂). The effects of the waste characteristics (composition and pH) and of the operation parameters of the process (temperature and current density) have also been studied in this work. The complete removal of the organic compounds contained in the waste has been obtained in all essays.     

Veratric acid treatment by anodic oxidation with BDD anode

BACKGROUND: Veratric acid (VA, 3,4‐dimethoxy‐benzoic acid) is representative of the polyphenolic type compounds present in olive mill wastewater (OMW). Given the bactericide factor, the inhibitor character and the anti bacteriological activity of this compound, traditional biological digestion cannot be applied and therefore new technologies, such as electrochemical oxidation using a boron‐doped diamond (BDD) anode have to be considered to avoid its accumulation in the environment. RESULTS: The electrochemical oxidation of aqueous solutions containing 1 mmol L^?1 VA has been investigated using a filter‐press reactor with a BDD anode during galvanostatic electrolysis. The influence of several operating parameters, such as applied current density, temperature, flow‐rate and supporting electrolyte concentration and type has been investigated. The experimental results showed that under the experimental conditions used the oxidation of VA was under mass‐transfer control and VA was completely degraded by the reaction with hydroxyl radicals electrogenerated at the BDD surface. The chemical oxygen demand (COD) decay kinetic followed a pseudo‐first‐order reaction and the apparent rate constant increased with flow rate and temperature. Under optimal experimental conditions of flow‐rate (300 L h^?1), temperature (35 °C) and current density (10 mA cm^?2), 99.5% of COD was removed during 2 h electrolysis, with 16.4 kWh m^?3 energy consumption. CONCLUSIONS: This study suggests that anodic oxidation with a BDD electrode is an excellent method for the treatment of effluents contaminated with VA and related polyphenols. Copyright © 2011 Society of Chemical Industry     

Treatment of Fenton‐refractory olive oil mill wastes by electrochemical oxidation with boron‐doped diamond anodes

In this work, the electrochemical oxidation of an actual industrial waste with conductive diamond anodes has been studied. The wastewater is the effluent of a wastewater treatment plant consisting of a Fenton reactor followed by a settler and a sand filter, in which the wastes generated in an olive oil mill are treated. These wastes contain a residual chemical oxygen demand of nearly 700 mg dm^?3 which cannot be further oxidized with the Fenton process. The electrolyses were carried out under galvanostatic conditions, using a bench‐scale plant equipped with a single‐compartment electrochemical flow cell. Boron‐doped diamond (BDD) and stainless steel (AISI 304) were use as anode and cathode of the cell, respectively. The complete mineralization of the waste was obtained with high current efficiencies limited only by mass transport processes. This confirms that besides the hydroxyl radical‐mediated oxidation that occurs in the Fenton process, the electrochemical oxidation with conductive diamond electrodes combines other important oxidation processes such as direct electro‐oxidation on the BDD surface and oxidation mediated by other electrochemically formed compounds generated in this electrode. Copyright © 2006 Society of Chemical Industry     

Electrochemical oxidation of benzoic acid at boron-doped diamond electrodes

The electrochemical oxidation of benzoic acid (BA) has been studied on boron-doped diamond electrodes on acid medium by cyclic voltammetry and bulk electrolysis. The results showed that in the potential region where the supporting electrolyte is stable, reactions occur, resulting in the loss of activity due to electrode fouling. Electrolysis at high anodic potentials in the region of electrolyte decomposition causes complex oxidation reactions that lead to incineration of BA. There is no indication of electrode fouling. Salicylic acid, 2,5-hydroxybenzoic acid and hydroquinone have been detected at trace level by HPLC as soluble products during the electrolysis of BA.     

Electrochemical Oxidation of Benzoic Acid Derivatives on Boron Doped Diamond: Voltammetric Study and Galvanostatic Electrolyses

The electrochemical oxidation of aqueous wastes polluted with benzoic and salicylic acids and phthalic anhydride on boron‐doped diamond electrodes has been studied. The complete mineralization of the organic waste has been obtained in each case regardless of the nature of the compound, composition of the solution, and current density. Different voltammetric behaviors between benzoic acid, salicylic acid, and phthalic anhydride were obtained in the voltammetric study, but no differences in the electrochemical oxidation of the three compounds seems to exist in the bulk electrolyses study. The total mineralization of the three compounds at different current densities confirms that the oxidation must occur directly on the electrode surface and/or by hydroxyl radicals generated by decomposition of water and/or by other oxidants electro‐generated from the sulfate oxidation.     

Biological and electrochemical oxidation of naphthalenesulfonates

A biofilm airlift suspension (BAS) reactor and an undivided flow cell equipped with a boron‐doped diamond (BDD) anode and a stainless‐steel cathode were used to investigate the effects of varying operating conditions on process performance in the biological and electrochemical oxidation of a mixture of naphthalenesulfonates contained in the infiltration water of a contaminated industrial site. The experiments were aimed at evaluating the feasibility of process integration and the criteria for optimization (i.e. how to maximize degradation efficiency with minimum energy consumption) in combined biological and electrochemical oxidation of scarcely biodegradable compounds. Because of high reactor biomass concentration and long biomass retention time, the BAS reactor achieved a high degradation capacity (up to 6.8 kg COD m^?3 d^?1). On the other hand, owing to the recalcitrant character of some of the aromatic sulfonates in the leachate, the overall degradation efficiency did not exceed 70% based on COD measurements. All naphthalene‐mono‐ and ‐disulfonates (except naphthalene‐1,5‐disulfonate) were completely degraded in the BAS reactor, whereas more complex molecules (e.g. naphthalenetrisulfonates) were more recalcitrant to biological oxidation. These compounds were completely mineralized by electrochemical oxidation using a boron‐doped diamond anode. The energy consumption and the time required for the complete mineralization of the infiltration water decreased from 80 kWh m^?3 and 4 h to 61 kWh m^?3 and 3 h for the oxidation of raw and biologically pretreated leachate, respectively. Copyright © 2005 Society of Chemical Industry     

Electrochemical decomplexing and oxidation of organic (chelating) additives in effluents from surface treatment and metal finishing

The electrochemical decomplexing and oxidation of two frequently used complexing agents in surface treatment and metal finishing—EDTA (ethylenediaminetetraacetic acid) and NTA (nitrilotriacetic acid)—and of organic non‐complexing additives used in nickel‐plating baths were the subject of this study. Using a Ti–RuO₂ electrode, a partial indirect oxidation by in‐situ electrochemical generation of chlorine compounds could be achieved for EDTA and NTA. At a boron‐doped diamond (BDD) electrode however, complete decomplexing and high COD (Chemical Oxygen Demand) and TOC (Total Organic Carbon) (up to 95%) removal occurred at an average current density of 2 A dm^?2. It is shown that direct electrochemical oxidation at a BDD electrode resulted in lower energy consumption and higher treatment rates than indirect oxidation at a Ti–RuO₂ electrode. Decomplexing at the BDD electrode occurred at high current efficiencies ranging from 71% to 95% with decomplexing rates in the order of 3.13 mmol (Ah)^?1 and 5.02 mmol (Ah)^?1 for EDTA and NTA respectively. COD removal rates obtained were 0.090 g (Ah)^?1 for EDTA, 0.100 g (Ah)^?1 for NTA and 0.205 g (Ah)^?1 for the nickel‐plating additives. Electrochemical decomplexing and oxidation of common chelating agents can render the subsequent metal precipitation and biological wastewater treatment of surface treatment and metal finishing effluents more efficient. Copyright © 2003 Society of Chemical Industry     

Electrochemical oxidation of 1,4‐dioxane at boron‐doped diamond electrode

BACKGROUND: The electrochemical oxidation of 1,4‐dioxane at a boron doped diamond (BDD) surface on a niobium substrate anode was studied because (i) 1,4 dioxane is a resistant contaminant in waste‐waters and ground‐waters which needs to be removed/oxidized and (ii) most of the currently applied techniques for removal/oxidation require chemicals. RESULTS: Results show that in the potential region supporting electrolyte stability 1,4‐dioxane can be oxidized directly. Adhesive products, which cause electrode fouling, are also formed during oxidation in this potential region. The BDD anode can be restored to its initial activity by simple anodic treatment in the potential region of electrolyte decomposition. In this region, oxidation reactions leading to complete oxidation of 1,4‐dioxane, can take place due to electro‐generated hydroxyl radicals. Therefore, dioxane can only be effectively oxidized at these potentials. The effect of current density on the oxidation of 1,4‐dioxane has been investigated. The experimental results have also been compared with a theoretical chemical oxygen demand (COD)–instantaneous current efficiency (ICE) model. At a current density above 32 mA cm^?2, the oxidation process is completely controlled by mass transfer and no intermediates are formed. 92% of the COD can be removed with a total consumption of 7 Ah L^?1. CONCLUSIONS: Results show that dioxane can be effectively and completely oxidized at a BDD anode. Copyright © 2010 Society of Chemical Industry     

Electrochemical incineration of chloromethylphenoxy herbicides in acid medium by anodic oxidation with boron-doped diamond electrode

The electrochemical degradation of saturated solutions of herbicides 4-chloro-2-methylphenoxyacetic acid, 2-(4-chlorophenoxy)-2-methylpropionic acid and 2-(4-chloro-2-methylphenoxy)propionic acid in 1 M HClO₄ on a boron-doped diamond (BDD) thin film anode has been studied by chronoamperometry, cyclic voltammetry and bulk electrolysis. At low anodic potentials polymeric products are formed causing the fouling and deactivation of BDD. This is reactivated at high potentials when water decomposes producing hydroxyl radical as strong oxidant of organics. Electrolyses in a batch recirculation system at constant current density ≥8 mA cm⁻² yielded overall decontamination of all saturated solution. The effect of current density and herbicide concentration on the degradation rate of each compound, the specific charge required for its total mineralization and instantaneous current efficiency have been investigated. Experimental results have been compared with those predicted by a theoretical model based on a fast anodic oxidation of initial herbicides, showing that at 30 mA cm⁻² their degradation processes are completely controlled by mass transfer. Kinetic analysis of the change of herbicide concentration with time during electrolysis, determined by high-performance liquid chromatography, revealed that all compounds follow a pseudo first-order reaction. Aromatic intermediates and generated carboxylic acids have been identified using this technique and a general pathway for the electrochemical incineration of all herbicides on BDD is proposed.     

Oxidation of carboxylic acids at boron-doped diamond electrodes for wastewater treatment

Thin boron-doped diamond films have been prepared by HF CVD (hot filament chemical vapour deposition technique) on conductive p-Si substrate (Si/Diamond). The morphology of these Si/diamond electrodes has been investigated by SEM and Raman spectroscopy. The electrochemical behaviour of the Si/diamond electrodes in 1 M H₂SO₄ and in 1 M H₂SO₄ + carboxylic acids has been investigated by cyclic voltammetry. Finally, the electrochemical oxidation of some simple carboxylic acids (acetic, formic, oxalic) has been investigated by bulk electrolysis. These acids can be oxidized at Si/diamond anodes to CO₂, in the potential region of water and/or the supporting electrolyte decomposition, with high current efficiency.     

Evaluation of boron doped diamond electrodes for organic electrosynthesis on a preparative scale⋆

We have investigated some anodic and cathodic transformations using boron doped diamond (BDD) electrodes. The oxidation of a propargylic alcohol as well as the aromatic side chain oxidation in water as electrolyte did not␣yield the desired products in high yield and selectivity and led mainly to the formation of CO₂ due to␣electrochemical incineration of the starting material. With methanol as electrolyte, however, the reactivity of BDD electrodes is similar to graphite in most anodic methoxylation reactions, but the inactive behaviour of BDD electrodes leads to a different reaction pathway possibly involving methoxyl radicals with charge transfer from the electrolyte. It has been found that at BDD anodes benzylic single and double bonds can be split yielding aromatic acetals and esters. With phenanthrenes as starting material, o,o′-disubstituted biaryls were obtained. So the use of BDD electrodes provides an efficient and environmentally friendly access to this interesting class of compounds. The high H₂ overpotential of BDD cathodes enables smooth and selective reduction of functional groups like oximes. Due to the high chemical and mechanical stability of the diamond layer of today’s electrodes, electrode lifetime as well as reproducibility of the electrosyntheses has improved markedly. Aqueous basic conditions, however, must be avoided for BDD anodes. These conditions result in degradation of the diamond surface. ^⋆Dedicated to Professor G. Kreysa on the occasion of his 60th birthday.     

Electrochemical oxidation of Basic Blue 3 dye using a diamond anode: evaluation of colour,COD and toxicity removal

BACKGROUND: Textile industries generate considerable amounts of waste‐water, which may contain strong colour, suspended particles, salts, high pH and high chemical oxygen demand (COD) concentration. The disposal of these coloured wastewaters poses a major problem for the industry as well as a threat to the environment. In this study, electrochemical oxidation of Basic Blue 3 (BB3) dye was studied in a bipolar trickle tower (BTT) reactor using Raschig ring shaped boron‐doped diamond (BDD) electrodes in recirculated batch mode. The effects of current density, temperature, flow rate, sodium sulfate concentration (Na₂SO₄) as supporting electrolyte, and initial dye concentration were investigated. RESULTS: The best experimental conditions obtained were as follows: current density 0.875 mA cm^?2, temperature 30 °C, flow rate 109.5 mL min^?1, Na₂SO₄ concentration 0.01 mol L^?1. Under these conditions, 99% colour and 86.7% COD removal were achieved. Toxicity tests were also performed on BB3 solutions under the best experimental conditions. CONCLUSION: Based on these results, the BDD anode was found to be very successful for the simultaneous degradation of BB3 and removal of COD. Additional toxicity test results also showed that electrochemical treatment using a BDD Raschig ring anode in a BTT reactor is an effective way of reducing toxicity as well as removing colour and COD. Copyright © 2010 Society of Chemical Industry     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.     

Cleavage of alkenes by anodic oxidation

Oxidative cleavage of olefinic double bonds to carboxylic acids, aldehydes or ketones is one of the important reactions in organic synthesis. Ozonolysis is mostly applied for this purpose, however, high costs for safety precautions in technical scale conversions demand alternatives. For this purpose different electrochemical methods are investigated. In the direct oxidation of cyclohexene at a platinum or graphite anode no cleavage occurs, but substituted and rearranged products are obtained. At the boron doped diamond electrode (BDDE) aliphatic olefins with high oxidation potential are not converted. Electrochemical ozonolysis by oxidation of water to ozone at the lead dioxide electrode leads to carboxylic acids as cleavage products in high material yield but low current yield. Anodic bromo-formyloxylation followed by an anodic cleavage provides a two step conversion of cyclohexene to hexane-1,6-dial derivatives. Thereby anodic discharge of bromide in formic acid leads to (2-bromocyclohexyl)-formate, which is converted to cyclohexane-1,2-diol and the major part of potassium bromide is recovered for the next cycle. This electrochemical conversion appears to be an attractive alternative to chemical oxidations with oxygen and catalysts or with hydrogen peroxide. The diol is cleaved in high yield to hexane-1,6-dial or its acetal either directly or indirectly with periodate as mediator.     

Electrochemical bromination and oxidation of alkyl aromatic compounds by two-phase electrolysis

A simple, regioselective, environmentally clean and economical method for the preparation of side chain/ ring brominated aromatic compounds is reported in 70–98% yield by an electrochemical method using two phase electrolysis technique. Electrochemical reactions were carried out using aqueous 25–50 wt% sodium bromide containing catalytic amount (5 wt%) of hydrobromic acid as an aqueous phase and chloroform containing alkyl aromatic compounds as an organic phase, at a temperature of 0–30 °C in an undivided cell. The same two-phase electrolytic system can be used for the oxidation of benzylic alcohols to the corresponding benzaldehydes in 80–94% yield without over oxidation to carboxylic acids. The advantage of this very mild procedure is a room temperature reaction used with an undivided cell. Excellent conversions are observed. After completion of alcohol oxidation the electrolyte can be reused for a number of times, demonstrating “spent reagent” free electro organic reaction as an attractive one. In the case of side chain/ring bromination of alkyl aromatic compounds, the electrolyte can be reused after making up the concentration of the electrolyte with 47 wt% HBr solution. In some cases homogeneous electrolysis is applied, where the two-phase electrolysis did not work. Styrene epoxidation and α-bromination of ketones underwent homogeneous electrolysis at room temperature without any catalyst. The reaction was performed in CH₃CN-water (3: 2) using equimolar amount of NaBr as an electrolyte to get 68% of styrene epoxide. Use of an ionic liquid 1-butyl 3-methyl imidazolium bromide (Bmim) Br, instead of NaBr improved the yield and current efficiency of styrene epoxide to 86%.     

Purification of Wet‐Process Phosphoric Acid by Hydrogen Peroxide Oxidation,Activated Carbon Adsorption and Electrooxidation

In this work, three technologies are studied for the purification of phosphoric acid produced by the wet process: chemical oxidation with hydrogen peroxide, adsorption onto activated carbon, and electrochemical oxidation by boron‐doped diamond anodes. The treatment of wet‐process phosphoric acid by chemical oxidation with H₂O₂ as oxidizing agent can remove 75 % of the initial TOC as maximum, indicating that this wet‐process phosphoric acid contains an important amount of organics that cannot be oxidized by hydrogen peroxide under the operation conditions used. High temperatures and hydrogen peroxide/TOC ratios close to 150 g H₂O₂/g TOC allow obtaining the best chemical oxidation results. The adsorption onto activated carbon can remove between 40 and 60 % of the initial TOC as maximum. Adsorption times of 2 hours and activated carbon/WPA ratios close to 12 g AC/Kg WTP assure both steady state and maximum adsorption of organics. The electrochemical process is the only technique by which complete mineralization of WPA organics can be achieved. Operating at 60 mA cm^–2 and at room temperature, high current efficiencies are achieved which only seem to decrease by mass transport limitations.     

Investigations of electrochemical oxygen transfer reaction on boron-doped diamond electrodes

In this paper, the electrochemical oxygen transfer reaction (EOTR) is studied on boron-doped diamond electrodes using simple C₁ organic compounds (methanol and formic acid). The kinetics of both oxygen evolution (side reaction) and organics oxidation (main reaction) has been investigated using boron-doped diamond microelectrodes-array (BDD MEA). Oxygen evolution, in the high-potential region, takes place with a Tafel slope of 120 mV dec⁻¹ and zero reaction order with respect to H⁺. In the presence of organics, a shift of the polarization curves to lower potentials is observed while the Tafel slopes remain close to 120 mV dec⁻¹. A simplified model of C₁ organics oxidation is proposed. Both water discharge and organics oxidation are assumed to be fast reactions. The slowest step of the studied EOTR is the anodic discharge of hydroxyl radicals to oxygen. Further in this work, electrolysis of formic acid on boron-doped diamond macroelectrode is presented. In order to achieve 100% current efficiency, electrolysis was carried out under programmed current, in which the current density was adjusted to the limiting value.     

Electrochemical mineralization of sodium dodecylbenzenesulfonate at boron doped diamond anodes

Results are reported of the electrochemical oxidation of sodium dodecylbenzenesulfonate (SDBS), a common surfactant, at boron-doped diamond anodes. The measured critical micelle concentration (CMC) for SDBS in water at 24 °C was almost 150 mg dm⁻³, but this decreased to almost 30 mg dm⁻³ in 0.1 M sodium sulfate. Cyclic voltammetry of a boron doped diamond (BDD) electrode in aqueous SDBS solutions exhibited oxidation current densities at very positive potentials; however, solutions of monomers at concentrations <CMC gave rise to higher current densities than in higher concentration solutions that formed micelles. Galvanostatic electrolyses, with samples analyzed for Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD), were performed in an electrolytic flow cell without separator, operating in batch recycle mode, using solutions containing SDBS at initial concentrations of 25 and 250 ppm. SDBS in basic media (pH = 12) exhibited lower TOC removal rates than in acidic or neutral solutions, due to concurrent oxidation of dissolved carbonates at potentials less positive than required for water oxidation, as evident in cyclic voltammograms. Decreasing the [electrolyte]/[surfactant] ratio from 200 to 10 increased TOC removal rates. For solutions containing monomers, TOC removal rates also increased with flow rate in the second part of the electrolysis, corresponding to reaction of smaller, fragmented organic compounds. When COD removal from a solution containing SDBS micelles was mass transport controlled, current efficiencies were constant at ca. 50%, due to dimerisation of hydroxyl radical to H₂O₂ and its oxidation to dioxygen.     

新型多孔Ti/BDD薄膜电极电化学氧化降解低浓度茜素红

运用热丝化学气相沉积（HFCVD）的方法制备了以多孔钛为基体的掺杂硼金刚石（porous Ti/BDD）薄膜电极,并测试了它的主要物理性质,SEM表明金刚石相生长良好并且能均匀地分布在基体表面和孔内,Raman光谱表明电极的金刚石相纯而且质量很高。采用循环伏安法研究了酸性条件下茜素红在多孔Ti/BDD电极上的电氧化行为。通过改变阳极电流密度、支持电解质Na2SO4的浓度来研究茜素红在多孔Ti/BDD电极上的电化学氧化降解的效果影响。结果表明：电流密度40 mA/cm2、支持电解质浓度0.5 mol/L为较理想的工艺参数,总电流效率达到30.2%。在相同条件下,发现多孔Ti/BDD薄膜电极氧化降解茜素红与平板Ti/BDD薄膜电极相比具有更高的电流效率。紫外可见光光谱证实了多孔Ti/BDD电极能够有效地电氧化降解茜素红。