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

Photolysis of ClO by UVC radiation occurs in several drinking water treatment scenarios (e.g., pre-oxidation by ClO with post-UVC disinfection or a multi-barrier disinfection system comprising ClO and UVC disinfection in sequence). However, whether micropollutants are degraded and undesired byproducts are formed during the co-exposure of ClO 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 ClO and UVC radiation in the synthetic drinking water under the environmentally relevant conditions (UV dose of 207 mJ cm, ClO dose of 1.35 mg L, and pH of 7.0). Trimethoprim and iopromide were predominantly degraded by ClO 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 ClO photolysis, respectively. The yields of total organic chlorine (12.5 µg L from 1.0 mg C L of NOM) and chlorate (0.14 mg L From 1.35 mg L of ClO) during the co-exposure were low. However, the yield of chlorite was high (0.76 mg L from 1.35 mg L of ClO), which requires attention and control.