OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

The electrochemical oxidation of phenol in an aqueous solution is a complex transformation involving several transfer steps of oxygen atoms and electrons. Transfer of the oxygen atom occurs through the intermediary of hydroxyl radicals adsorbed on the active sites of the anode. Galvanostatic electrolyses of phenol (10.5 to 105 mmol dm3) in aqueous solution at pH 2 on a Ta/Pb02 anode were followed by high-pressure liquid chromatography and by analysis of the total organic carbon. Hydroquinone, catechol, 1,4-benzoquinone (1,4-BQ), maleic and fumaric acids, and carbon dioxide are the main products. The nonidentified products consist mainly of polymers. Study of the influence of temperature shows that the rate consumption of phenol initially at 21 mmol dm3 is mass transport limited. CO2 is immediately formed following the 1,4-BQ-maleic acid pathway involving 20 faradays and forming 4 mol of CO2 and/or the 1,4-BQ-intermediary in C2 pathway at 16 faradays with formation of 2 mol of CO2. The faradaic yield values show that a phenol molecule adsorbed on a catalytic site undergoes a succession of oxidation steps involving, on average, five electrons without desorption of the intermediate products. This number of electrons varies according to the operating conditions (temperature, anodic current density, initial phenol concentration, hydrodynamic conditions, etc.). The mean faradaic yield decreases during electrolysis; it can reach 70% at the beginning of electrolysis of a 21 mmol dm3 phenol solution for an anodic current density of 100 mA cm2. The phenol conversion into insoluble polymers increases as a function of its initial concentration and the anodic current density but it does not exceed 10%.

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The electrooxidation of aqueous solutions containing 5mM of o-, m- and p-cresol at pH 4.0 has been investigated using a flow filter-press reactor with a boron-doped diamond (BDD) under galvanostatic electrolysis. All cresols are degraded at similar rate up to attaining overall mineralization. Comparable treatment of the m-cresol effluent on PbO(2) leads to partial electrochemical incineration. However, this pollutant is more rapidly removed with PbO(2) than with BDD. The decay kinetics of all cresols follows a pseudo-first-order reaction. Aromatic intermediates such as 2-methylhydroquinone and 2-methyl-p-benzoquinone and carboxylic acids such as maleic, fumaric, pyruvic, malonic, tartronic, glycolic, glyoxylic, acetic, oxalic and formic, have been identified and followed during the m-cresol treatment by chromatographic techniques. From these oxidation by-products, a plausible reaction sequence for m-cresol mineralization on both anodes is proposed. The energy consumption for the corresponding electrochemical process is also calculated.

Maleic acid (MA) is one of the main intermediates formed during mineralization, by electrooxidation, of aromatic compounds contained in aqueous wastes. This work investigates oxidation of maleic acid with or without the presence of oxalic acid (OA) and formic acid (FA) in aqueous solution by using boron-doped diamond (BDD) electrodes. OA and FA are the main products formed in MA electrooxidation. Voltammetric studies conducted with a BDD electrode of small surface (0.196 cm 2 ) show that MA oxidation takes place at a potential very close to that of the discharge of water. But, under potentiostatic conditions and at concentrations higher than 0.001 M, adsorption of MA blocks its own oxidation. Oxalic and formic acids are oxidized before the discharge of water. Again, the presence of maleic acid blocks the oxidation of formic and oxalic acids. Galvanostatic electrolyses of aqueous solutions of MA, OA, FA and mixtures of theses acids were conducted on a BDD electrode. Electrolyses were controlled by measurements of Total Organic Carbon, Chemical Oxygen Demand and by Liquid Chromatography. Results showed that MA was totally mineralized; FA and OA were very low concentration intermediaries. Electrolyses of solutions containing MA, initially in the presence of OA or FA, showed that the OA was oxidized at the same rate as the MA, whereas the FA oxidation began only when the MA had completely disappeared. These results suggest that OA oxidizes by a mass transport limited process coupled with a direct electron transfer with the anode. Under galvanostatic conditions, maleic acid and formic acid are probably oxidized via OH • radicals generated by water discharge.

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This paper deals with the treatment of aqueous phenol solutions using an electrochemical technique. Phenol can be partly eliminated from aqueous solution by electrochemically initiated polymerisation. Galvanostatic electrolyses of phenol solutions at concentration up to 0.1 mol dm -3 were carried out on a Ta/PbO 2 anode. The polymers formed are insoluble in acidic medium but soluble in alkaline. These polymers were filtered and then dissolved in aqueous solution of sodium hydroxide (1 mol dm -3 ). The polymers formed were quantified by total organic carbon (TOC) measurement. It was found that the conversion of phenol into polymers increases as a function of initial concentration, anodic current density, temperature, and solution pH. The percentage of phenol polymerised can reach 15%.