Copper ion affects oxidant decay and combined aspartic acid transformation during chlorination in water pipes: Differentiated action on the yield of trihalomethanes and haloacetonitriles.

The chlorination of extracellular polymeric substances (EPS) secreted by biofilm often induces the formation of high-toxic disinfection byproducts (DBPs) in drinking water distribution systems. The protein components in EPS are the main precursors of DBPs, which mostly exist in the form of combined amino acids. The paper aimed to study the action of a pipe corrosion product (Cu) on the formation of DBPs (trihalomethanes, THMs; haloacetonitriles, HANs) with aspartic acid tetrapeptide (TAsp) as a precursor. Cu mainly promoted the reaction of oxidants with TAsp (i.e., TAsp-induced decay) to produce DBPs, rather than self-decay of oxidants to generate BrO and ClO. Cu increased THMs yield, but decreased HANs yield due to the catalytic hydrolysis. Cu was more prone to promote the reaction of TAsp with HOCl than with HOBr, leading to a DBPs shift from brominated to chlorinated species. The chemical characterizations of Cu-TAsp complexations demonstrate that Cu combined with TAsp at the N and O sites in both amine and amide groups, and the intermediate identification suggests that Cu enhanced the stepwise chlorination process by promoting the substitution of chlorine and the breakage of CC bonds. The effect of Cu on THMs yield changed from promoting to inhibiting with the increase of pH, while that on HANs yield was inhibiting regardless of pH variation. Additionally, the impact of Cu on the formation of DBPs was also affected by Cu dose, Cl/C ratio and Br concentration. This study helps to understand the formation of EPS-derived DBPs in water pipes, and provides reference for formulating control strategies during biofilm outbreaks.