Unraveling the multiple roles of VUV mediated hydroxyl radical in VUV/UV/chlorine process: Kinetic simulation, mechanistic consideration and byproducts formation

The application of VUV/UV/chlorine process for abatement of three representative organic contaminants, including naproxen (NPX), carbamazepine (CBZ) and iopamidol (IPM) was systematically investigated in this study with focus on the kinetic simulation, oxidation mechanisms and byproducts formation. The results demonstrated that VUV/UV/chlorine process was of outstanding efficiency with fluence-based pseudo-first-order rate constants of selected contaminants increased by 53.5–141% and 52.3–67.9% compared with UV/chlorine and VUV/UV processes, respectively. A chemical kinetic model was successfully developed to predict chlorine decay, steady-state concentrations of hydroxyl radical (•OH) and elimination of NPX, CBZ and IPM in VUV/UV/chlorine process. The modeling results suggested the VUV induced •OH could unexpectedly improve the concentrations of reactive chlorine species compared with UV/chlorine. In addition, effects of chlorine dosage, pH, natural organic matter, bicarbonate and chloride on the specific contributions of UV, •OH, chlorine radical (Cl•) and dichlorine radical (Cl2•−) to the degradation of selected contaminants were also evaluated and simulated in VUV/UV/chlorine process. The reactive sites and transformation pathways of NPX, CBZ and IPM were proposed via Fukui function analysis and UPLC-Q-TOF-MS identification. Meanwhile, the oxidation feasibility of NPX, CBZ and IPM by •OH and Cl• via single electron transfer mechanism was clarified based on theoretical thermodynamics calculation. Compared with UV/chlorine, VUV/UV/chlorine enhanced the generation of chlorate and bromate but retarded the formation of disinfection byproducts. This study first clarified the radical chemistry, contaminants transformation and byproducts formation in VUV/UV/chlorine process.