Anodic oxidation, electro-Fenton and photoelectro-Fenton degradation of cyanazine using a boron-doped diamond anode and an oxygen-diffusion cathode

The degradation of the s-triazinic herbicide cyanazine in 100 ml solutions of pH 3.0 has been comparatively studied by electrochemical advanced oxidation processes (EAOPs) such as anodic oxidation with electrogenerated H2O2 (AO–H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with a 6 W UVA lamp. All the electrolyses were performed in a cell containing a 3 cm2 boron-doped diamond (BDD) anode and a 3 cm2 O2-diffusion cathode able to generate H2O2. Hydroxyl radicals (OH) formed at the BDD surface in all EAOPs and in the bulk from Fenton’s reaction between added Fe2+ and generated H2O2 in EF and PEF, were the main oxidants. The PEF process was more potent than the EF one, allowing attaining an almost total mineralization with 98% total organic carbon decay due to the combined action of the above oxidants with the photolysis of intermediates by UVA light. The lower oxidation power was attained using AO–H2O2 owing to the lower production rate of OH formed at the anode. The effect of current density and herbicide concentration on the degradation behavior of all EAOPs has been examined. The decay kinetics for cyanazine always followed a pseudo-first-order reaction with increasing apparent rate constants in the sequence AO–H2O2 < EF < PEF. Heteroaromatic derivatives such as deisopropylatrazine, desethyldeisopropylatrazine, ammeline and cyanuric acid, as well as generated carboxylic acids such as formic and oxamic, have been quantified by reversed-phase and ion-exclusion HPLC, respectively. Inorganic ions like Cl−, NO3- and NH4+ lost during the degradation processes were detected by ionic chromatography. From these products, a reaction sequence for cyanazine mineralization by all EAOPs has been proposed.

► Almost total mineralization of cyanazine by photoelectro-Fenton with a BDD anode.
► Slower degradation by electro-Fenton and much lower oxidation power of anodic oxidation.
► Ammeline and cyanuric acid as main heteroaromatic products.
► Formic and oxamic acids generated as final carboxylic acids.
► Chloride, ammonium and nitrate ions detected as released inorganic ions.