Solar photoelectro-Fenton degradation of cresols using a flow reactor with a boron-doped diamond anode

The solar photoelectro-Fenton degradation of 2.5 l of acidic solutions containing o-cresol, m-cresol and p-cresol up to ca. 1 g l−1, 0.05 M Na2SO4 and Fe2+ as catalyst has been studied using a flow plant with a one-compartment filter-press electrolytic reactor with a boron-doped diamond (BDD) anode and an O2-diffusion cathode, both of 20-cm2 area, coupled to a solar photoreactor. In this environmentally friendly indirect electrooxidation method pollutants are mainly oxidized by hydroxyl radical formed at the anode surface from water oxidation and in the medium from Fenton's reaction between Fe2+ and cathodically electrogenerated H2O2, giving rise to complexes of Fe(III) with final carboxylic acids that are rapidly photodecomposed by UVA light supplied by solar irradiation. Electrolyses performed in batch using 1.0 mM Fe2+ at pH 3.0 and 50 mA cm−2 yield complete mineralization of all cresols up to ca. 0.5 g l−1. The effects of current density, solution pH and concentrations of Fe2+ and cresols on the degradation rate, efficiency and energy costs of the solar photoelectro-Fenton process have been examined. The decay kinetics for all cresols follows a pseudo-first-order reaction. Initial hydroxylation of o-cresol and m-cresol gives 2-methyl-p-benzoquinone via 2-methylhydroquinone, whereas dihydroxylation of p-cresol leads to 5-methyl-2-hydroxy-p-benzoquinone. These aromatic intermediates are rapidly converted into a mixture of carboxylic acids, being oxalic and acetic acids the most persistent final products. Overall mineralization is attained by the efficient photodecarboxylation of Fe(III)-oxalate complexes. Solar photoelectro-Fenton with a BDD anode appears to be a viable method to remove cresols in wastewaters at industrial scale.