Hydroxyl radical-initiated decomposition of metazachlor herbicide in the gaseous and aqueous phases: Mechanism, kinetics, and toxicity evaluation.

The oxidation of widely-used herbicide metazachlor (MTZ) by hydroxyl radical (HO•) in the gas and the aqueous phases was investigated in terms of mechanistic and kinetic behaviors using the M06-2X/6-311++G (3df, 3pd)//M06-2X/6-31 + G (d,p) level of theory over the temperature range 250-400 K. The formal hydrogen transfer, HO•-addition, and single electron transfer mechanisms were considered. The overall rate constants in the gas phase range from 8.40 × 1010 to 8.31 × 109 M-1 s-1 at the temperature from 250 to 400 K, respectively, while the ones in the aqueous phase are close to diffusion-controlled rates, with diffusion-corrected rate constants being 1.31 × 109 to 1.27 × 109 M-1 s-1. The formal hydrogen transfer mechanism is the most dominant in the gas phase, whereas the HO•-addition is the most favorable in the aqueous phase. The H-abstraction at two methyl groups and the HO•-addition to C11 and C12 atoms (pyrazole ring), C16 and C18 atoms (benzyl ring) are significant. The short lifetime in the environment, equal to only 4.16 h, requires more attention to this herbicide compound, whereas its lifetime in the aqueous condition varies sharply from half second to several thousand days depending on the HO• concentration. The ecotoxicity estimation of MTZ and its principal transformation products to aquatic organisms suggests that they are harmful or toxic substances. Moreover, the MTZ is a developmental toxicant and mutagenicity-positive, while its decomposed products are developmental toxicants with no mutagenic toxicity. Their bioaccumulation in aquatic organisms is negligible.