No abstract

Acidic solutions of the herbicide 4-chlorophenoxyacetic acid ͑4-CPA͒ 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 Fe 2ϩ and H 2 O 2 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 Ͻ400 ppm at pH 3.0, operating at 35°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 у 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.

The mineralization of aqueous solutions of the herbicide 4-chloro-2-methylphenoxyacetic acid ͑MCPA͒ in 0.05 M Na 2 SO 4 with adjusted pH 2.0-6.0 using H 2 SO 4 has been studied by an UV-assisted cathodic Fenton treatment such as photoelectro-Fenton at low current and at 35°C, using an undivided cell with a Pt anode and an O 2 -diffusion cathode. This method allows the complete depollution of solutions of pH 3.0 with an herbicide concentration of up to 380 mg L Ϫ1 and 1 mM Fe 2ϩ . This is feasible by the high concentration of oxidizing hydroxyl radicals produced in the medium by the Fenton's reaction between added Fe 2ϩ and H 2 O 2 electrogenerated by the cathode, along with photolysis of some products. Comparative treatment by a general cathodic Fenton method such as electro-Fenton only yields 70-75% of mineralization due to the formation of hardly oxidizable products. MCPA is much more slowly degraded by anodic oxidation in the absence and presence of electrogenerated H 2 O 2 . The herbicide decay always follows a pseudo first-order reaction. Aromatic products such as 4-chloro-o-cresol, methylhydroquinone, and methyl-pbenzoquinone have been followed by reverse-phase chromatography. Chloride ions are formed from oxidation of chlorinated products. Generated carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric, and oxalic, have been detected by ion-exclusion chromatography. These products are destroyed by electro-Fenton, except oxalic acid that forms stable complexes with Fe 3ϩ . The efficient photodecarboxylation of such complexes by UV light explains the complete mineralization of MCPA by photoelectro-Fenton.