Mechanistic and kinetic understanding of micropollutant degradation by the UV/NH2Cl process in simulated drinking water

The UV/monochloramine (UV/NH₂Cl) process has attracted increasing attention in water treatment, in which hydroxyl radicals (HO•), reactive chlorine species (RCS) and reactive nitrogen species (RNS) are produced. This study investigated the effects of water matrices including halides, natural organic matter (NOM), alkalinity and pH, on the degradation kinetic of a variety of micropollutants and radical chemistry in the UV/NH₂Cl process. The presence of chloride blunted HO• and Cl• impacts, but enhanced Cl₂•– effect on micropollutants reactive toward Cl₂•–. The presence of 30 μM bromide led to an 82% decrease in the specific pseudo-first-order rate constants (k′) by HO• (kHO•′), and significantly diminished RCS efficacy. Reactive bromine species (RBS) were formed in the presence of bromide, while the contribution could not compensate for the decrease of HO• and RCS due to their lower reactivity toward micropollutants. Iodide rapidly transformed to HOI via reacting with NH₂Cl, which resulted in a 59% decrease of kHO•′ and 12% ∼ 100% decreases of k′ by reactive halogen species (RHS) and RNS (kRHS ₊ RNS′) for most micropollutants. Nevertheless, k′ of phenolic compounds, such as paracetamol, bisphenol A and salbutamol, increased in the presence of iodide by 78%, 360% and 130%, respectively, due to the roles of HOI and reactive iodine species (RIS). Bicarbonate decreased the contributions of HO• and RCS, but enhanced that of CO₃•⁻ for micropollutants reactive toward CO₃•⁻. The presence of 1 mg/L NOM scavenged over half the amount of HO•, and also consumed RCS and RNS, resulting in significantly decreased removal of micropollutants. High pH value witnessed enhanced degradation for those micropollutants reactive toward RCS and RNS through deprotonation. The degradation of most micropollutants was inhibited in real drinking water and in the coexistence of halides. This study provides a better understanding of radical chemistry in the UV/NH₂Cl process under a practical water treatment condition.