14. Electrochemical Incineration of Some Phenolic Compounds and MTBE

Electrochemistry along with the microbial and photochemical approaches is a well established method for the degradation of wastes. It has been frequently stressed, however, that these methods often cannot bring about complete mineralization of several compounds, and to this end various methods broadly classified as AOPs (Advanced Oxidation Processes) provide complementary and alternative means of environment remediation, as outlined in comprehensive recent surveys. These AO systems include ozone, hydrogen peroxide as well as a mixture of them called “Peroxone” which can be activated by Fenton reactions leading to formation of a large amount of OH radicals and, consequently, to a highly oxidizing ­environment. In this work we investigate incineration of some phenolic compounds (4-chlorophenol, 4-nitrophenol, 2,4-dichlorophenoxyacetic acid, trans-3,4-­dihydroxycinnamic acid) and methyl tert-butyl ether (MTBE) using different electrochemical systems involving ozone formation at PbO₂ based anodes: (1) direct electrolysis at constant current; (2) ex-situ use of O₃ and (3) combined use of anodically generated O₃ and H₂O₂ produced at a graphite cathode. According to the results described in this work, what we called direct electrolysis actually turns out to be a mediated process by oxygenated radical intermediates. Subsequently, the active oxygen intermediates can react with the organic pollutants. The homogeneous chemical decomposition by ozone is the basis of the ex-situ methods. It involves the direct reaction of O₃ with the organic substrate and/or a radical chain reaction initiated by radicals formed in the reaction of O₃ with OH⁻ or with HO ₂ ⁻ . The latter pathway is favoured by an increase of the pH. The examination of the results shows that the combined use of anodically generated O₃ and H₂O₂ produced at cathode is the most efficient approach.