Mineralization of herbicide mecoprop by photoelectro-Fenton with UVA and solar light

The photoelectro-Fenton degradation of 2.5 l of solutions containing herbicide mecoprop (2-(4-chloro-2-methylphenoxy)-propionic acid) up to 0.64 g l−1, 0.05 M Na2SO4 and 0.5 mM Fe2+ of pH 3.0 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 either a UVA or solar photoreactor. Electrolyses performed in batch at low constant current density yield overall mineralization in both methods, although the process is much faster and less expensive with solar light. Under these conditions, the degradation rate and efficiency increase strongly with rising mecoprop content. The kinetics for the herbicide decay follows a pseudo-first-order reaction. Mecoprop reacts rapidly with hydroxyl radical (OH) produced from Fenton's reaction between Fe2+ and H2O2 electrogenerated at the cathode to yield 4-chloro-o-cresol, 2-methylhydroquinone and 2-methyl-p-benzoquinone as primary reaction products. Fe3+-oxalato and Fe3+-acetato complexes are detected as the most persistent final products. Overall mineralization is attained because Fe3+-oxalato complexes are efficiently photodecomposed by solar irradiation, whereas Fe3+-acetato complexes are slowly destroyed by OH formed at the BDD anode from water oxidation. The initial chlorine is released as Cl−, which is slowly oxidized to Cl2 on BDD. Photoelectro-Fenton with solar light appears to be a viable method to remove chlorophenoxy herbicides in wastewaters at industrial scale.