Breakpoint electrochlorination in ammonia removal: Unveiling the impact of convective mass transfer

Breakpoint chlorination, the point at which ammonia is completely oxidized by chlorine to nitrogen gas, may occur during electrochemical water treatment due to the simultaneous abundance of inorganic nitrogen species and chloride ions in many water matrices. Nevertheless, little is known about the difference between the chemical breakpoint chlorination and electrochemical ammonia abatement as well as the impact of the electrode-electrolyte interface that drives the breakpoint electrochlorination. This study investigates the influence of the interface on ammonia oxidation by comparing indirect breakpoint electrochlorination with the chemical approach and by examining the impact of varying convective mass transfer on breakpoint electrochlorination. Our results revealed that, under identical conditions and bulk pH, breakpoint electrochlorination releases much lower residual chlorine species in the bulk solution before ammonia is oxidized, as compared to chemical breakpoint chlorination. It was observed that lower convective mass transfer not only accelerates ammonia removal but also increases the chlorine evolution reaction. Results from a closed divided cell experiment confirmed that chlorine evolution is enhanced under lower convective mass transfer, which suggests a relevant role of species distribution within electrode-electrolyte interface. We hypothesize that this effect may be due to a more acidic local pH under lower mass transfer conditions, which favors chlorine evolution over oxygen evolution reaction. These findings provide insights into the fundamental differences of chemical breakpoint chlorination and indirect breakpoint electrochlorination. The results can guide operating strategies for electrochemical water treatment that can potentially reduce energy consumption by lowering flow speeds, while achieving higher chlorine yield and faster ammonia removal.