Degradation of metribuzin in the UV/chlorine process: kinetic model, degradation by-products and transformation pathways

Abstract

This study novelly reported the degradation kinetics and the correlated formation of disinfection by-products (DBPs) of metribuzin, a widely used herbicide that contains nitrogen, in the UV/chlorine process. The influencing factors, including chlorine dosage, pH, and the presence of chloride (Cl⁻) and natural organic matter (NOM, simulated using humic acid (HA)) in water for metribuzin degradation were evaluated. The pseudo-first-order rate constant (k_obs) of metribuzin during UV/chlorination was calculated to be 0.0986 min⁻¹, which was 6.90 times higher than those of UV-alone and 1.53 times greater than that of chlorine-alone. The k_obs increased with increasing chlorine dosage and decreasing pH value. Cl⁻ could promote the degradation of metribuzin, while the content of metribuzin changed non-monotonically with HA concentration. Through the establishment of the kinetic model, the k_obs of HO• with metribuzin was calculated to be 5.6 × 10⁹ M⁻¹ s⁻¹, and HO• contributed to 20% of metribuzine degradation. The possible degradation pathways of metribuzin in the UV/chlorine process were proposed. In the sequential chlorine disinfection process, trichloromethane (TCM) was the dominant DBP in non-bromine water, while the content of tribromomethane (TBM) raised with increasing molar ratio of bromine to chlorine, which has to be carefully controlled in drinking water treatment plants to provide safe drinking water to the public.