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

Abstract

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 x 10⁻⁶ x T⁵͘͘ ⁹⁵³ x exp[929.0 (J mol⁻¹)/(RT)] (M⁻¹ s⁻¹), showing a rate constant of 6.09 x 10⁹ м⁻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.