Oxidation of thiram fungicide by hydroxyl radicals in water: A theoretical study on reaction mechanism and kinetics

The oxidation of thiram (THR) pesticide initiated by HO• radicals in water was investigated using the density functional theory (DFT) approach at the M06‐2X/6‐311++G(3df,3pd)//M06‐2X/6‐31+G(d,p) level of theory. Three oxidation mechanisms were evaluated: radical addition, hydrogen abstraction, and single electron transfer. The results showed that all these reactions are favorable and spontaneous. Among them, the addition reactions at S‐positions have the most prominent rate constants and branching ratios. In addition, the oxidation of THR is temperature‐dependent; the higher the temperature, the faster the oxidation and the higher the removal efficiency. The overall rate constant is k(T) = 7.816 × 10−6 × T5.953 × exp[929.0 (J mol−1)/(RT)] (M−1 s−1), showing a rate constant of 6.09 × 109 m−1 s−1 at 298.15 K which agrees with the previously reported experimental result. In general, thiram can be easily decomposed in a wastewater treatment process with a high concentration of HO•. Still, it may exist in a natural‐like environment with a very low HO• concentration from a few days to several years.