Oxidation of metazachlor herbicide by ozone in the gas and aqueous phases: mechanistic, kinetics, and ecotoxicity studies.

The atmospheric and aqueous ozonolysis of metazachlor (MTZ) is investigated using high-level quantum chemical and kinetic calculations (M06-2X/6-311 +  + G(3df,3pd)//M06-2X/6-31 + G(d,p) level of theory). The ozone (O3)-initiated degradation pathways of MTZ under three different mechanisms, namely cycloaddition, oxygen-addition, and single electron transfer (SET), are explored in the temperature range of 283-333 K and 1 atm pressure. As a result, the cycloaddition reaction at the C16C18 double bond of the benzene ring of MTZ is found to be the most dominant channel in the atmosphere with the standard Gibbs free energy of reaction (ΔrG0g) of - 129.13 kJ mol-1 and the highest branching ratio of 95.18%. In the aqueous phase, the main reaction channel turns into the SET mechanism, which owns the lowest Gibbs free energy of activation (ΔG#aq) of 73.8 kJ mol-1 and contributes 87.8% to the ktotal. Over the temperature range of 283-333 K, the total rate constant (ktotal) significantly increases from 8.42 to 5.82 × 101 M-1 s-1 in the atmosphere and from 4.10 × 102 to 2.40 × 104 M-1 s-1 in the aqueous environment. Remarkably, the ecotoxicity assessment shows that MTZ may be harmful to fish and chronically harmful to daphnia. In contrast, its main ozonolysis products exhibit no acute or chronic toxicity or mutagenic effects.