A chemical-free approach for effective abatement of acetochlor in drinking water

The extensive use of the herbicide acetochlor has led to its widespread detection in water sources, resulting in its recent inclusion as a regulated contaminant in China's updated Drinking Water Quality Standards (GB5749-2022). This study investigates a chemical-free approach using far-UVC (222 nm) radiation for the sustainable control of acetochlor in drinking water. Acetochlor exhibited substantially enhanced direct photolysis under UV₂₂₂, with a fluence-based decay rate constant (1.06 × 10⁻² cm² mJ⁻¹) approximately 13.33-fold higher than that under UV₂₅₄ (7.95 × 10⁻⁴ cm² mJ⁻¹) in buffer, and the direct UV₂₂₂ photolysis of acetochlor in real surface water and tap water followed pseudo-first-order kinetics, with degradation rate constants enhanced by 28.34-fold and 19.11-fold, respectively, compared to conventional UV₂₅₄ treatment. The apparent quantum yields for acetochlor photolysis at 222 nm were determined as 0.36 molecule einstein^-1. While nitrate exhibited a negligible effect under UV₂₅₄, it promoted acetochlor degradation under UV₂₂₂. Residual disinfectants (chlorine, monochloramine, and chlorine dioxide) in tap water enhanced acetochlor degradation by UV₂₅₄ but had minimal impact on the UV₂₂₂ process. By combining experiments with kinetic modeling, we quantified the radical concentrations generated from disinfectant photolysis at 222 nm and 254 nm, enabling a direct comparison of radical generation potential and micropollutant degradation efficiency between the two wavelengths. The formation profiles of trihalomethanes (THMs) and haloacetic acids (HAAs) during UV₂₂₂ treatment in the presence of residual disinfectants were distinct from those observed with UV₂₅₄ treatment, with or without post-chlorination. These findings provide critical insights into the photochemical behavior of acetochlor and highlight the potential of far-UVC as an effective, chemical-free technology for controlling persistent herbicides, offering a promising strategy for compliance with increasingly stringent water quality standards.