Elucidating Solvation Effects in Reactive Halogen Species-Initiated Ammonium Oxidation

This is the authors' abstract. We don't add key points for this paper.

Breakpoint chlorination is widely used for ammonium (NH 4 + ) 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 NH 4 + hydration significantly impacts reactive halogen species (RHS)-initiated oxidation. Our experimental and theoretical analyses revealed that the unique solvation shell of NH 4 + 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 NH 4 + due to the strong solvation shell surrounding NH 4 + . 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 NH 4 + 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 NH 4 + -N removal performance in NaBr-based systems compared to NaCl. Moreover, the NaBr-involved electrochemical approach enabled simultaneous NH 4 + oxidation and Cu recovery from [Cu(NH 3 ) 4 ] 2+ -containing synthetic wastewater. This study highlights the important role of solvation effects on NH 4 + oxidation and offers valuable insights for developing efficient strategies for NH 4 + -N removal in water treatment.