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The electrochemical oxidation of aqueous wastes containing 4-nitrophenol (4-NP) using a borondoped diamond thin-film electrode has been studied. Within the parameter ranges used (150-4000 mg of 4-NP dm -3 , pH 2-12, 30-60 mA cm -2 , 25-60 °C), the complete treatment of the organic waste was achieved. The maximum current efficiencies were obtained under kinetic control. On the basis of the results of voltammetric and galvanostatic electrolysis studies, a simple mechanistic model was proposed. The first stage in the treatment of 4-NP-containing aqueous wastes is the release of the nitro group from the aromatic ring. As a consequence, phenol or quinones are formed. These organic compounds are oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. On the cathode, the reduction of the 4-NP to 4-aminophenol takes place. In alkaline media, this compound can be polymerized and transformed into a dark brown solid.

In this work it has been studied the electrochemical production of different oxidants with conductivediamond anodes. This technology was found to allow the production of stable oxidants that can not be easily synthesized using other electrodes or by other more usual techniques. Thus, it has been found that monoperoxophosphoric acid, and peroxodiphosphate, peroxodisulphate and ferrate salts can be easily produced by this technology, when the right operation conditions are used. In contrast, the production of pure hypochlorite is not favoured, because this technology leads to the formation of a mixture of hypochlorite with other oxoanions of chlorine in a higher oxidation state, particularly chlorates and perchlorates, which seems to be the final product in these electrolyses. The production of perbromates salts has also been pointed, although in this case only preliminary results are shown. In every case, it has been observed two marked trends in the process efficiency as function of the current density applied. This fact can be explained in terms of the contribution of hydroxyl radicals in the oxidation mechanisms that occur on diamond surfaces. Results obtained in this work also allow explaining the influence of the electrolyte salt on the efficiency of the electrochemical oxidation of wastewaters.

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