Mineralization of desmetryne by electrochemical advanced oxidation processes using a boron-doped diamond anode and an oxygen-diffusion cathode

The mineralization of acidic aqueous solutions of the herbicide desmetryne has been studied by electrochemical advanced oxidation processes (EAOPs) such as anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with UVA light. Electrolyses were conducted in an open and cylindrical cell with a boron-doped diamond (BDD) anode and an O2-diffusion cathode for H2O2 generation. The main oxidizing species are OH radicals formed at the BDD surface in all treatments and in the bulk from Fenton’s reaction between added Fe2+ and electrogenerated H2O2 in EF and PEF. A poor mineralization was attained using AO-H2O2 by the slow oxidation of persistent by-products with OH at the BDD surface. The synergistic action of OH in the bulk enhanced the degradation rate in EF, although almost total mineralization was only achieved in PEF due to the additional OH generation and photolysis of intermediates by UVA irradiation. The effect of current, pH and herbicide concentration on the mineralization degree and mineralization current efficiency of each EAOP was examined. Desmetryne decay always followed a pseudo first-order kinetics, being more rapidly destroyed in the sequence AO-H2O2 < EF < PEF. In all EAOPs, ammeline and cyanuric acid were identified as persistent heteroaromatic by-products and oxamic and formic acids were detected as generated carboxylic acids. The generation of cyanuric acid mainly by oxidation with OH at the BDD surface is the predominant path for desmetryne degradation. The initial nitrogen of desmetryne yielded NO3- ion in low proportion and NH4+ ion in much lesser extent, suggesting that its major part was lost as volatile N-derivatives.

► Desmetryne can be almost totally mineralized by photoelectro-Fenton with a BDD anode.
► Poorer mineralization is reached in absence of Fe2+ and by electro-Fenton in the dark.
► Ammeline and cyanuric acid are formed as persistent heteroaromatic by-products.
► The generation of cyanuric acid at BDD anode is the main oxidative route of desmetryne.
► Oxamic and formic acids are generated as carboxylic acids in all methods.