Electrochemical degradation of 2,4,5-trichlorophenoxyacetic acid in aqueous medium by peroxi-coagulation. Effect of pH and UV light

Acidic aqueous solutions with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) concentrations up to 270 ppm in the pH range 2.0-6.0 at 35 °C can be rapidly degraded by peroxi-coagulation using an Fe anode and an O₂-diffusion cathode. 2,4,5-T and its products are then efficiently oxidized with OH radicals produced from Fenton's reaction between Fe²⁺ and H₂O₂ generated by the electrodes. Under pH regulation at low currents, more than 90% of organics are destroyed at pH 3.0, although its optimum pH is 4.0. Higher degradations are reached without pH regulation. Solutions with 2,4,5-T concentration ≤200 ppm are more rapidly depolluted under UV irradiation, because of the production of more OH from photo-Fenton reaction. Coagulation of products with the Fe(OH)₃ precipitate formed predominates when pH is regulated to 2.0 and 3.0 in the absence of UV light, whereas parallel mineralization is favored without pH regulation and under UV irradiation. The 2,4,5-T decay follows a pseudo first-order reaction, with the same rate constant in the presence and absence of UV light. 2,4,5-trichlorophenol, 2,5-dichlorohydroquinone, 4,6-dichlororesorcinol and 2,5-dihydroxy-p-benzoquinone have been identified and quantified as aromatic intermediates by reverse-phase chromatography. Chloride ions are released from the oxidation of these chlorinated products. The evolution of generated carboxylic acids, such as glycolic, glyoxylic, formic, malic, maleic, fumaric and oxalic, has been followed by ion-exclusion chromatography. The great stability of oxalic acid and its complexes with Fe³⁺ at pH regulated to 3.0 can explain that concentrated solutions can not be completely degraded.