UV activated monochloramine promotes metribuzin degradation and disinfection by-products formation

Abstract

UV/monochloramine (MC) is a new advanced oxidation process for micro-pollutants degradation. This study investigated the degradation kinetics and mechanism of metribuzin (MBZ) by UV/MC as well as the variations of disinfection by-products (DBPs) formation. The pseudo-first-order rate constant of MBZ degraded by UV/MC (0.084 min−1) was about 3.6 times the sum of those by UV (0.023 min−1) and MC (<0.001 min−1) alone. The accelerated degradation by UV/MC was mainly owed to the formed reactive chlorine species (RCSs) and HO. The degradation rate was pH-dependent, which increased from 0.081 to 0.148 min−1 with pH decreasing from 8.5 to 5.5, due to varied species contribution rates. HO and RCSs played dominant roles under pHs > 7.0 and pHs ≤ 7.0, respectively. The contribution rate of RCSs at pH 5.5 (0.112 min−1) was 5.3 times that at pH 8.5 (0.021 min−1). Enhanced DBPs formation was observed in UV/MC process. Trichloromethane, trichloronitromethane and dichloroacetonitrile were simultaneously formed, and their total concentration with 30 min UV/MC treatment (19.0 μg L−1) was 1.4 times those formed with 1 min UV/MC treatment (13.7 μg L−1). The DBPs formation potential in post-chlorination following 1 min UV/MC treatment (55.7 μg L−1) was 7.6, 13.8 and 2.4 times those formed in chlorination (7.31 μg L−1), chloramination (4.02 μg L−1), and post-chlorination following 30 min UV irradiation (23.6 μg L−1), respectively. With the analysis of transformation by-products, the pathways of MBZ degradation and DBPs formation were proposed. Although UV/MC accelerated MBZ degradation, the enhanced DBPs formation should be eagerly concerned.