Peroxi-coagulation and photoperoxi-coagulation treatments of the herbicide 4-chlorophenoxyacetic acid in aqueous medium using an oxygen-diffusion cathode

Acidic solutions of the herbicide 4-chlorophenoxyacetic acid (4-CPA) are rapidly depolluted by peroxi-coagulation using an Fe anode and an O-2 diffusion cathode, due to the fast production of oxidizing hydroxyl radicals by Fenton's reaction between Fe2+ and H2O2 electrogenerated by such electrodes. A faster degradation is achieved in the photoperoxi-coagulation treatment, where the solutions are also irradiated with UV light, because more hydroxyl radicals are produced from the photo-Fenton reaction. Both methods yield complete depollution of solutions with 4-CPA concentration <400 ppm at pH 3.0, operating at 35 degrees C, low currents, and without pH regulation. When the solution pH is regulated, the methods are more efficient at pH 3.0. Part of the oxidation products are retained in the Fe(OH)(3) precipitate formed. This coagulation process predominates at low herbicide concentrations, high currents, and under pH regulation. However, the products are mainly mineralized for 4-CPA concentration >= 194 ppm without pH regulation. A pseudo first-order reaction is followed by the 4-CPA decay for both methods. Reverse-phase chromatography allows identifying 4-chlorophenol, 4-chlorocatechol, hydroquinone, and p-benzoquine as aromatic intermediates. Oxidation of chlorinated products is accompanied by the release of chloride ions, which are mainly accumulated in the medium. Generated carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric, and oxalic, are followed by ion-exclusion chromatography. A reaction scheme involving all these intermediates is proposed. (C) 2003 The Electrochemical Society.