Rapid urea decomposition via bromination for ultrapure water production: New insights into pH-dependent reaction kinetics and mechanisms compared to chlorination

Abstract

Urea, a persistent organic contaminant, poses significant challenges in ultrapure water production due to its resistance to conventional treatment processes. This study comprehensively investigated the reaction kinetics and mechanisms of urea bromination, with a comparative analysis of urea chlorination. Bromination proved significantly faster than chlorination, with optimal urea removal occurring at pH 10.5 for bromination and pH 6.0 for chlorination. The initial halogenation of urea to mono-halogenated urea was identified as a key step in facilitating total urea decomposition. Successive halogenation decreased the pKa of halogenated ureas, enhancing their deprotonation and promoting further transformation via hydrolysis. Acidic conditions favored the first halogenation step, while neutral to alkaline conditions facilitated multi-halogenation and subsequent mineralization. Complete urea decomposition required three bromine molecules at pH ≥ 6, whereas four to eight bromine molecules at pH ≤ 5, with bromine consumption increasing under acidic conditions. Treatment efficiency improved with higher oxidant-to-urea molar ratios but declined as initial urea concentrations decreased. Elevated oxidant dosages were necessary for rapid urea removal at ppb levels, typical for ultrapure water production. Urea reactions with bromine and chlorine produced halogenated ureas, which subsequently underwent hydrolysis and/or further halogen attack, forming haloamines, NH4+, NO2−, NO3−, N2, N2O and CO2. This study presents the first detailed comparison of pH-dependent kinetic and mechanistic differences between urea bromination and chlorination, revealing the distinct and superior efficiency of bromination at µg/L levels and new insights into mineralization pathways. The findings highlight the potential of free bromine as a more reactive and cost-effective alternative for water treatment. © 2025 Elsevier B.V.