Elucidating Solvation Effects in Reactive Halogen Species-Initiated Ammonium Oxidation.

Breakpoint chlorination is widely used for ammonium (NH4+) removal in water treatment, yet often requires excess chlorine and is hindered under acidic conditions. The underlying mechanisms of the proton-induced suppression remain elusive. This study demonstrates that NH4+ hydration significantly impacts reactive halogen species (RHS)-initiated oxidation. Our experimental and theoretical analyses revealed that the unique solvation shell of NH4+ features strong hydrogen bonding, which creates a protective barrier that limits oxidant access particularly at acidic pH values. HClO/ClO- was less effective in interacting NH4+ due to the strong solvation shell surrounding NH4+. In contrast, HBrO/BrO- with a larger ion radius and lower charge density can disrupt the solvation layer, resulting in superior oxidation efficiency. The addition of methanol effectively alleviated NH4+ solvation and lowered the energy barrier associated with the rate-determining desolvation step, and thus markedly enhancing its reactivity. Electrochemical experiments involving in situ RHS generation validated superior NH4+-N removal performance in NaBr-based systems compared to NaCl. Moreover, the NaBr-involved electrochemical approach enabled simultaneous NH4+ oxidation and Cu recovery from [Cu(NH3)4]2+-containing synthetic wastewater. This study highlights the important role of solvation effects on NH4+ oxidation and offers valuable insights for developing efficient strategies for NH4+-N removal in water treatment.