Ensuring the overall combustion of herbicide metribuzin by electrochemical advanced oxidation processes. Study of operation variables, kinetics and degradation routes.

Highlights • Metribuzin solutions treated by AO-H 2 O 2 , EF and PEF with a BDD/air-diffusion cell. • Two kinetic stages in EF and PEF: uncomplexed and Fe(III)-complexed herbicide. • Overall mineralization by all methods is ensured at current densities ≥100 mA cm−2. • PEF as the best method: total combustion of 0.523 mM metribuzin at pH 3.0 in 420 min. • Heteroaromatic, dimeric and aliphatic by-products degraded by BDD(OH) and OH. Abstract This article reports the electrochemical degradation of the herbicide metribuzin (MTZ) in sulfate medium by advanced oxidation processes like anodic oxidation with electrogenerated H 2 O 2 (AO-H 2 O 2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). A boron-doped diamond (BDD) anode was combined with an air-diffusion cathode with ability to produce H 2 O 2. Unprecedented overall combustion was feasible by all methods at a constant current density (j) ≥100.0 mA cm−2. The total organic carbon (TOC) removal achieved by AO-H 2 O 2 was independent from pH within the range 3.0–9.0, whereas the oscillatory dependence of the pseudo-first-order MTZ decay rate constant with this variable was ascribed to adsorption on the BDD surface. In EF and PEF at pH 3.0, 0.50 mM Fe2+ was determined as optimum catalyst content and the MTZ removal showed two consecutive pseudo-first-order kinetic stages. These were related to the fast reaction of the target molecule with OH formed from Fenton's reaction, followed by a slower attack of physisorbed BDD(OH) onto Fe(III)-MTZ complexes. The effect of j and MTZ content on decay kinetics and TOC removal was examined. PEF was the best treatment due to the decomposition of photoactive intermediates by UVA radiation, yielding total mineralization of a 0.523 mM herbicide solution after 420 min of electrolysis at 100.0 mA cm−2. A thorough reaction pathway for MTZ degradation is proposed from the sixteen heteroaromatic by-products and three aliphatic molecules identified by GC–MS and LC-MS/MS. Oxalic and oxamic acids were detected as final carboxylic acids by ion-exclusion HPLC. [ABSTRACT FROM AUTHOR]