Sulfate radical anion-based degradation of metazachlor herbicide in the water and gas: A theoretical study

Metazachlor (MTZ) herbicide oxidation initiated by sulfate radical anion (SO4●-) in water and gas was investigated using the density functional theory (DFT) at the M06-2X/6-311++G(3df,3pd)//M06-2X/6-31+G(d,p) level of theory. Mechanisms and kinetics of MTZ oxidation via three oxidation mechanisms were investigated, including abstraction (Abs), addition (Add), and single electron transfer (SET). Results show that most oxidation reactions are favorable and spontaneous in both phases. The overall rate constants at 298.15K in water is 5.06 × 1010 M-1 s-1 whereas the one in gas is many times higher, 1.51 × 1013 M-1 s-1. Notably, the degradation in water is non-selected with the fastest reaction being SET with 5.76 × 109 M-1 s-1 and 11.39% of the kapp and Г, respectively. On the contrary, the one in gas almost occurs via Abs reaction with the fastest one being Abs-H24 with kapp and Г values being 1.08 × 1013 M-1 s-1 and 71.76%. In addition, the influence of temperature on the degradation kinetics is evaluated. Results show that the degradation in water increases as a function of temperature (283 to 323 K), while the drawback trend is found in the gas phase from 253 to 323 K. Diving into the chemical nature of the hydrogen abstraction processes, it is noteworthy that the most predominant Abs reactions at the methyl and methylene groups occur via the proton-coupled electron transfer (PCET) mechanism. Overall, the SO4●--based degradation is an effective and potential method for removing MTZ herbicide.