Simultaneous removal of chlorite and contaminants of emerging concern under UV photolysis: Hydroxyl radicals vs. chlorate formation.

It is well known that using chlorine dioxide (ClO) as a disinfectant inevitably produces a common disinfection byproducts chlorite (ClO). In this study, we found that UV photolysis after ClO disinfection can effectively eliminate both ClO and contaminants of emerging concern (CECs). However, the kinetic mechanisms of UV/ClO process destructing CECs, as well as transformation of ClO in UV/ClO system are not clear yet. Therefore, we systematically investigated the UV/ClO system to assist us appropriately design this process under optimal operational conditions. In this work, we first investigated the impact of water matrix conditions (i.e., pH, bicarbonate and natural organic matter (NOM)) and ClO dosage on the UV/ClO process. We found that bicarbonate and NOM have inhibition effects, while lower pH and higher ClO dosage have enhancement effects. Besides, hydroxyl radical (HO) and reactive chlorine species (RCS) are generated from UV/ClO system, and RCS are main contributors to CBZ degradation. Then we proposed a possible degradation pathway of CBZ based on the determined products from experiments. Additionally, we found that photolysis of ClO resulted in the generation of chloride (Cl) and chlorate (ClO). As the ClO dosage increases, the yield of ClO increased while that of Cl decreased. Finally, we elucidated the second order rate constant of the target organic compound with HO• has a strong correlation with the formation of ClO.