Oxidation pathways of malachite green by Fe3+-catalyzed electro-Fenton process

A very detailed scheme for the Fe3+-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe3+ its catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with it graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe2+ and H2O2 from quick cathodic reduction of Fe3+ and dissolved O-2 (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical (center dot OH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k(LMG) = 0.244 min(-1) was obtained for total destruction of malachite green by action of center dot OH at 200 mA, requiring 22 min for total decoloration of the solution. lit the same experimental conditions, overall mineralization was reached ill 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based oil the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of center dot OH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid. whereas N-demethylation of some of them produces formic acid its well. Oxalic acid and its Fe2+ complexes, as well its formic acid, can be slowly but totally mineralized by center dot OH. (C) 2008 Elsevier B.V. All rights reserved.