Effect of reduced sulfur group on the formation of CX₃R-type disinfection by-products during chlor(am)ination of reduced sulfur compounds

Previous studies have examined the effects of peptide bond and unsaturated bond on the formation of disinfection by-products (DBPs). However, limited information has been available for the impact of reduced sulfur group on the formation of DBPs. This study investigated the formation of carbonaceous and nitrogenous DBPs (C-DBPs and N-DBPs) with a similar structure of ‘’CX3R” (X = H, Cl, Br or I, R = functional group), including trihalomethanes, haloacetaldehydes, haloketones, haloacetonitriles, haloacetamides and halonitromethanes, during chlor(am)ination of three reduced sulfur compounds (RSCs), such as N-acetylcysteine, glutathione and glutathiol. Results showed that all DBPs except dichloroacetonitrile (DCAN) continuously increased with increasing Cl2 or NH2Cl doses in this study. The chlor(am)ination of three RSCs with lower disinfectant doses (the molar ratio of disinfectant to precursor ≤5) generated low DBPs compare to non-RSCs. More chloroform was observed in alkaline condition, while weak acidic condition was in favor of DCAN and dichloroacetamide formation. The results of frontier electron density calculation reported that the much higher reactivity for Cl2 and NH2Cl toward reduced sulfur group in RSCs protects other groups, which account for the formation of CX3R-type DBPs. This phenomenon has important environmental implications. When RSCs are present in the water, Cl2 or NH2Cl will reacts preferably with them rather than non-RSCs to form RSO3H as the major products. Hence, the trade-offs of between the products generated upon S-chlorination, which account for the formation RSO3H, and alkyl halogenation and N-chlorination, which account for the formation of CX3R-type DBPs, influence the formation of CX3R-type DBPs.