NO₃^- Promotes Nitrogen-Containing Disinfection Byproduct Formation in Corroded Iron Drinking Water Pipes

Nitrogen-containing disinfection byproducts (N-DBPs) are highly toxic DBPs in drinking water. Though, under normal conditions, NO₃^- could not directly participate in disinfection reactions to generate N-DBPs, here, we first found that NO₃^- could promote the formation of N-DBPs in corroded iron drinking water pipes. The coexistence of corrosion produced Fe(II) and iron oxides is a critical condition for the transformation of N species; meanwhile, most of the newly generated N-DBPs had aromatic fractions. The Fe-O-C bond formed between iron corrosion products and natural organic matter promoted electron transfer for the N transformation with pyrrolic N as the intermediate N species. Density functional calculation confirmed that the coexistence of Fe(II) and iron oxides effectively reduced the Gibbs free energy for NO₃^- reduction. ΔG of the key rate-determining step from NO* to NOH* decreased from 1.55 eV on FeOOH to 1.35 eV on Fe(II)+FeOOH. In addition, the large decrease of cell viability of the water samples from 74.3% to 45.4% further confirmed the formation of highly toxic N-DBPs. Thus, in a drinking water distribution system with corroded iron pipes, the low toxic NO₃^- may increase toxicity risks via N-DBP formation.