Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways

The UV/chlorine process, which forms several reactive species including hydroxyl radicals (HO center dot) 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 mu M to 200 mu M 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-center dot, Cl-2(center dot-) and ClO center dot, to the degradation removal rate was much higher than that of HO center dot 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 center dot concentration. Kinetic modeling and radical scavenging tests verified that ClO center dot 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 center dot generated m/z 327 and m/z 341. Meanwhile, reactions of m/z 325 with HO center dot 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 center dot, in the degradation of micropollutants in the UV/chlorine process. (C) 2016 Elsevier Ltd. All rights reserved.