title: Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways.
abstract: The UV/chlorine process, which forms several reactive species including hydroxyl radicals (HO) and reactive chlorine species (RCS) to degrade contaminants, is being considered to be an advanced oxidation process. This study investigated the kinetics and mechanism of the degradation of trimethoprim (TMP) by the UV/chlorine process. The degradation of TMP was much faster by UV/chlorine compared to UV/H2O2. The degradation followed pseudo first-order kinetics, and the rate constant (k') increased linearly as the chlorine dosage increased from 20 μM to 200 μM and decreased as pH rose from 6.1 to 8.8. k' was not affected by chloride and bicarbonate but decreased by 50% in the presence of 1-mg/L NOM. The contribution of RCS, including Cl, Cl2-and ClO, to the degradation removal rate was much higher than that of HO and increased from 67% to 87% with increasing pH from 6.1 to 8.8 under the experimental condition. The increasing contribution of RCS to the degradation with increasing pH was attributable to the increase in the ClO concentration. Kinetic modeling and radical scavenging tests verified that ClO mainly attacked the trimethoxybenzyl moiety of TMP. RCS reacted with TMP much faster than HOCl/OCl-to form chlorinated products (i.e., m/z 325) and chlorinated disinfection byproducts such as chloroform, chloral hydrate, dichloroacetonitrile and trichloronitromethane. The hydroxylation and demethylation of m/z 325 driven by HO generated m/z 327 and m/z 341. Meanwhile, reactions of m/z 325 with HO and RCS/HOCl/OCl-generated dichlorinated and hydroxylated products (i.e., m/z 377). All the chlorinated products could be further depleted to produce products with less degree of halogenation in the UV/chlorine process, compared to dark chlorination. The acute toxicity to Vibrio fischeri by UV/chlorine was lower than chlorination at the same removal rate of TMP. This study demonstrated the importance of RCS, in particular, ClO, in the degradation of micropollutants in the UV/chlorine process.
authors.affiliation: Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China.
authors.forename: Zihao
authors.initials: Z
authors.lastname: Wu
authors.affiliation: Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. Electronic address: fangjy3@mail.sysu.edu.cn.
authors.forename: Jingyun
authors.initials: J
authors.lastname: Fang
authors.affiliation: Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China; Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Yingying
authors.initials: Y
authors.lastname: Xiang
authors.affiliation: Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
authors.forename: Chii
authors.initials: C
authors.lastname: Shang
authors.affiliation: School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, 310018, China.
authors.forename: Xuchun
authors.initials: X
authors.lastname: Li
authors.affiliation: Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China.
authors.forename: Fangang
authors.initials: F
authors.lastname: Meng
authors.affiliation: Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Yang
mesh_terms: D002713:Chlorine; D006861:Hydrogen Peroxide; D007700:Kinetics; D010084:Oxidation-Reduction; D014295:Trimethoprim; D014466:Ultraviolet Rays; D014874:Water Pollutants, Chemical; D018508:Water Purification
chemical_list: D014874:Water Pollutants, Chemical; D002713:Chlorine; D014295:Trimethoprim; D006861:Hydrogen Peroxide
keywords: Advanced oxidation process (AOP); ClO radical; Pharmaceuticals and personal care products (PPCPs); Reactive chlorine species; The UV/chlorine process; Water treatment