Degradation of paraquat herbicide by electrochemical advanced oxidation methods

Various electrochemical advanced oxidation processes (EAOPs) including anodic oxidation (AO), electro-Fenton (EF) and photoelectro-Fenton (PEF) methods were applied to study the degradation of paraquat (1,1'-dimethyl-4,41-bipyridinium dichloride), a highly toxic and widely used herbicide, in aqueous acidic solution at pH 3.0. After initial optimization studies, EAOPs were compared to classic Fenton treatment (CFT) for their effectiveness at mineralizing the herbicide solution. Chemical oxygen demand (COD), parent compound and degradation intermediates were analyzed over the course of the treatment time. Results indicate that AO and CFT were the least efficient methods, yielding less than 64% and 51% of COD abatement, respectively. CFT exhibited the fastest initial removal rate, due to the rapid generation of (OH)-O-center dot during the first few minutes of treatment. On the other hand, PEF and EF displayed the most efficient treatments (yielding 97% and 94% COD decay, respectively), with PEF being slightly more efficient due to additional effect of UV-irradiation. Paraquat decay kinetics during EAOPs treatments were found to obey the pseudo-first order model very well and the absolute second-order rate constant with hydroxyl radicals was determined as (k(PQ) = 2.55 x 10(9) M-1 s(-1)). Three aromatic degradation intermediates were formed and subsequently degraded, although not always completely, leading to short-chain carboxylic acids (oxalic, formic, acetic and succinic) and nitrate ions. It was finally concluded that only PEF and EF are sufficiently powerful and effective detoxification processes towards paraquat herbicide. (C) 2009 Elsevier B.V. All rights reserved.