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.