Phase-Dependent Thermodynamics and Kinetics of Sulfate Radical Oxidation of Metazachlor Herbicide A Theoretical Study

The sulfate radical anion (SO ₄ ^●– )-initiated oxidation of the herbicide metazachlor (MTZ) was investigated in aqueous and gaseous phases 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. Three oxidation mechanisms, including abstraction (Abs), addition (Add), and single electron transfer (SET), were explored to elucidate mechanisms and kinetics. Most oxidation reactions were thermodynamically favorable and spontaneous in both phases. The overall rate constant at 298.15 K was significantly higher in the gas phase (1.51 × 10 ¹³ M ^− 1 s ^− 1 ) compared to water (5.06 × 10 ¹⁰ M ^− 1 s ^− 1 ). In water, degradation was non-selective, with SET dominating the kinetics (5.76 × 10 ⁹ M ^− 1 s ^− 1 ) having 11.39% of the branching ratio (Γ). In contrast, gas-phase degradation was highly selective, with the Abs-H24 pathway showing the highest rate (1.08 × 10 ¹³ M ^− 1 s ^− 1 , 71.76% of Γ). Temperature-dependent analysis revealed that reaction rates increased with temperature in water (283 to 323 K) reducing MTZ lifetimes from microseconds to hours, but decreased in the gas phase (253 to 323 K). Notably, abstraction reactions involving methyl and methylene groups predominantly followed a proton-coupled electron transfer mechanism. These findings highlight the efficacy and potential of SO ₄ ^●– based advanced oxidation processes for MTZ removal across different phases.