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

The UV/monochloramine (UV/NHCl) 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/NHCl process. The presence of chloride blunted HO and Cl impacts, but enhanced Cl effect on micropollutants reactive toward Cl. The presence of 30 μM bromide led to an 82% decrease in the specific pseudo-first-order rate constants (k') by HO (k'), 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 NHCl, which resulted in a 59% decrease of k' and 12% ∼ 100% decreases of k' by reactive halogen species (RHS) and RNS (k') 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 CO for micropollutants reactive toward CO. 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/NHCl process under a practical water treatment condition.