Ammonia Triggers Interfacial Breakpoint Chlorination to Enhance Micropollutant Degradation and Reshapes Chlorinated Byproduct Formation during Electrochlorination

Abstract Electrochlorination is a promising technology for micropollutant degradation in decentralized water treatment. However, ammonia can alter chlorine chemistry. This study investigates the effect of ammonia on micropollutant degradation and chlorinated byproduct formation during electrochlorination using the RuO2/IrO2–Ti anode. Compared to electrochlorination at 10 mM chloride without ammonia, the presence of 1 mM ammonia increased the degradation rate constants of metronidazole and carbamazepine by 2.7- and 4.5-fold, respectively. This enhancement is consistent with interfacial breakpoint chlorination, defined as preferential chlorine–ammonia conversion within an anode-adjacent, chlorine-rich aqueous diffusion layer before complete bulk mixing. During electrochlorination with ammonia, dissolved HO• contributed to micropollutant degradation. Electron paramagnetic resonance provided qualitative evidence of HO• formation, whereas nitrobenzene was used as a probe to quantify its steady-state concentration. The ammonia-induced steady-state HO• concentration reached 2.4 × 10–13 M at 1 mM ammonia, whereas the HO• contribution was negligible without ammonia. Meanwhile, 1–5 mM ammonia reduced the total yields of chlorinated byproducts by only 25–31% during electrochlorination. The limited suppression is consistent with concurrent competition between ammonia and dissolved organic matter for locally generated chlorine within the anode-adjacent region. These findings identify near-anode chlorine–ammonia conversion as a route to reactive-species formation and micropollutant degradation, while showing that chlorinated byproducts remain a concern in ammonia-containing electrochlorination systems.