Degradation of six typical pesticides in water by VUV/UV/chlorine process: Evaluation of the synergistic effect

Abstract

Vacuum ultraviolet/ultraviolet/chlorine (VUV/UV/chlorine) is considered a novel advanced oxidation process (AOP), but little is known about its kinetics for pollutant degradation in water treatment. This study investigated the degradation of six typical pesticides, namely dimethoate (DMT), atrazine (ATZ), prometon (PMT), propoxur (PPX), bromacil (BRM) and propachlor (PPC), by VUV/UV/chlorine. The results show that all pesticides were rapidly degraded by VUV/UV/chlorine with a high removal efficiency of over 95% after 60 s. The pesticide degradation fitted well with pseudo-first-order reaction kinetics and a significant synergistic effect was observed during the VUV/UV/chlorine process. The synergistic factor (FV/U/Cl) for DMT, ATZ, PMT, PPX, BRM and PPC were determined to be 1.75, 1.70, 2.06, 1.57, 2.84 and 1.61, respectively, indicating a synergistic improvement of 57%–184% for all pesticides. As hydroxyl radical (HO•) transformed into reactive chlorine species (RCSs), the contribution ratio of RCSs for the pesticide degradation was much higher than that of HO• in the VUV/UV/chlorine process, thus causing the synergistic effect. Solution pH ranging from 5.0 to 10.0 had various influence on the pesticide degradation by VUV/UV/chlorine. As initial concentration of free chlorine increased from 0 to 0.25 mM, the apparent rate constants of the pesticides kept on increasing while the FV/U/Cl first increased and reached the highest value, and decreased afterwards. The formation of nitrite was significantly inhibited during the degradation of all pesticides by VUV/UV/chlorine. It suggests that VUV/UV/chlorine is a promising AOP for the pesticide degradation in water treatment.