Complexation with ferrous ions accelerate the decay of N,N-dichloramines in corroded iron drinking water pipes

This study systematically investigates the behaviors and mechanisms of N,N- dichloramine decomposition in new iron pipes (NIP) and corroded iron pipes (CIP). The half-life of N,N -dichloramine in the CIP is orders of magnitude shorter than in the NIP (0.46 min vs. 35.5 h), accompanied by substantial formation of ammonia (0.27 mg/L as NH 3 -N). We demonstrate that the Fe 2+ - N,N- dichloramine complex formed in CIP is a critical factor in catalyzing the decay of N,N -dichloramine and promoting ammonia formation. Density functional calculations confirm that the coordination bond formed between Fe 2+ and N,N -dichloramine effectively lowers the energy barrier for N,N -dichloramine decomposition via the nitrile pathway, thereby promoting ammonia formation. Furthermore, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) provides molecular-level evidence for the Fe 2+ - N,N -dichloramine complex-catalyzed deamination of CHNO compounds to CHO species. These findings suggest that metal-induced decomposition of organic chloramines could trigger unforeseen ammonia release, underscoring the need to account for disinfection strategies in iron-based DWDS.