R. Paz

Effect of the Operating Conditions on the Oxidation Mechanisms in Conductive-Diamond Electrolyses

The electrochemical window of conductive-diamond electrodes is large enough to produce hydroxyl radicals with high efficiency, and this species seems to be directly involved in the oxidation mechanisms that occur on diamond surfaces. To verify the role of these radicals in the conductive-diamond electrolyses of organic and inorganic aqueous solutions, several experiments were carried using two well-known processes, which were selected as models: the electrosynthesis of peroxodiphosphate and the treatment of phenolic wastes in sulfate supporting media. Results show that there is an abrupt change in the efficiencies and product conversions of the processes with the anodic potential, although there is no change in the nature of the reaction intermediates or final products. Working at high anodic potentials led to very efficient processes in both cases. The changes observed have been interpreted in terms of the oxidation mechanisms involved in the process, taking into account the contribution of hydroxyl radicals for higher potentials and the formation of stable oxidants through the oxidation of the electrolyte by these radicals.

Electrochemical Oxidation of Wastewaters Polluted with Aromatics and Heterocyclic Compounds A Comparison with Other AOPs

In this work, the conductive-diamond electrochemical oxidation of several aromatics (phenol, 4-chorophenol, 2-naphtol) and heterocyclic compounds (2,6-dimethyl-pyridine, thiophene) were studied. The results obtained are compared with those obtained in the oxidation with Fenton reagent and with ozone at pH 12. Significant differences are observed among the three technologies and between the range of pollutant concentration studied [10 2 - 2 X 10 3 mg dm -3 of chemical oxygen demand (COD)]. For weakly loaded synthetic wastes (COD around 10 2 mg dm -3 ) the three technologies are able to diminish the COD significantly. The dose of oxidant required to obtain the maximum removal increases importantly in the series Fenton oxidation < ozonation < electrolysis. At high pollutant concentrations (COD around 2 X 10 3 mg dm -3 ) results obtained change abruptly. The electrolytic technology reduces the COD almost completely and the total organic carbon (TOC) of all the wastes, and ozonation becomes less effective in the COD removal. Even for some compounds, the Fenton oxidation overcomes the COD removals obtained by ozonation, although the TOC removal is still very low as compared with that obtained by ozonation and especially by electrochemical treatment. The differences between technologies in the dose of oxidant required become negligible in the COD removal percentages attained. The results have been used to guess some insights about the oxidation mechanisms in this electrolytic process.