Micropollutant abatement and byproduct formation during the co-exposure of chlorine dioxide (ClO2) and UVC radiation.

Photolysis of ClO2 by UVC radiation occurs in several drinking water treatment scenarios (e.g., pre-oxidation by ClO2 with post-UVC disinfection or a multi-barrier disinfection system comprising ClO2 and UVC disinfection in sequence). However, whether micropollutants are degraded and undesired byproducts are formed during the co-exposure of ClO2 and UVC radiation remain unclear. This study demonstrated that four micropollutants (trimethoprim, iopromide, caffeine, and ciprofloxacin) were degraded by 14.4-100.0% during the co-exposure of ClO2 and UVC radiation in the synthetic drinking water under the environmentally relevant conditions (UV dose of 207 mJ cm-2, ClO2 dose of 1.35 mg L-1, and pH of 7.0). Trimethoprim and iopromide were predominantly degraded by ClO2 oxidation and direct UVC photolysis, respectively. Caffeine and ciprofloxacin were predominantly degraded by the radicals (HO• and Cl•) and the in-situ formed free chlorine from ClO2 photolysis, respectively. The yields of total organic chlorine (12.5 µg L-1 from 1.0 mg C L-1 of NOM) and chlorate (0.14 mg L-1 From 1.35 mg L-1 of ClO2) during the co-exposure were low. However, the yield of chlorite was high (0.76 mg L-1 from 1.35 mg L-1 of ClO2), which requires attention and control.