Complete removal of the insecticide azinphos-methyl from water by the electro-Fenton method - A kinetic and mechanistic study

The removal of organophosphorous insecticide azinphos-methyl (AZPM) from water has been investigated by the electro-Fenton method which produces hydroxyl radicals electrocatalytically. The reaction between these radicals and AZPM has led to the oxidation of AZPM up to carbon dioxide and inorganic ions indicating its complete removal from water. The oxidation kinetics was fitted to pseudo-first order reaction and absolute rate constant of the second order reaction kinetic was determined as (6.82 +/- 0.18) x 10(9) M-1 s(-1) by using competitive kinetics method. The oxidation of AZPM by hydroxyl radicals leads to the formation of different intermediates species; such as aromatic derivatives, short-chain carboxylic acids and inorganic ions as end products. The identification and quantification of these intermediates were deeply investigated by HPLC, GC-MS and ion chromatography analyses. Based on the identified intermediates, a general oxidation mechanism was proposed. Mineralization ability of the process was also tested by using aqueous AZPM solutions and its commercial formulation, Gusathion M WP 25 (GMWP25). Mineralization efficiency was followed by the chemical oxygen demand (COD) analysis of treated solutions. The overall mineralization of synthetic AZPM solution and that of commercial formulation at the end of treatment highlights the outstanding mineralization power of the electro-Fenton process. (C) 2012 Elsevier Ltd. All rights reserved.