Anodic oxidation, electro-Fenton and photoelectro-Fenton treatments of 2,4,5-trichlorophenoxyacetic acid

Abstract

The mineralization of acidic aqueous solutions of the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in 0.05 M Na 2SO 4+H 2SO 4 at 35 °C has been studied by different electrochemical methods. Photoelectro-Fenton treatment yields a fast and complete depollution of solutions with 2,4,5-T concentrations up to about saturation within the pH range 2.0–4.0, operating at low current and with 1 mM Fe 2+. This method is carried out in an undivided cell with a Pt anode and an O 2-diffusion cathode under UV irradiation, and its great oxidative ability is due to the fast reaction of organics with the high concentration of oxidizing hydroxyl radicals produced in the medium from Fenton's reaction between added Fe 2+ and H 2O 2 electrogenerated by the cathode. Comparative treatment by an electro-Fenton reaction only leads to 60–65% of mineralization because of the formation of hardly oxidable products with hydroxyl radical. 2,4,5-T is very poorly mineralized by anodic oxidation in the absence and presence of electrogenerated H 2O 2. The herbicide decay always follows a pseudo first-order reaction. 2,4,5-Trichlorophenol, 2,5-dichlorohydroquinone, 4,6-dichlororesorcinol and 2,5-dihydroxy- p-benzoquinone have been identified and quantified as aromatic products by reverse-phase chromatography. Generated carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric and oxalic have been detected and followed by ion-exclusion chromatography. All these products are destroyed by electro-Fenton treatment, except oxalic acid which forms stable complexes with Fe 3+. These complexes are rapidly photodecarboxylated by UV light in the photoelectro-Fenton treatment.