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 center dot), 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 center dot and Cl-center dot impacts, but enhanced Cl-2(center dot-) effect on micropollutants reactive toward Cl-2(center dot-). The presence of 30 mu M bromide led to an 82% decrease in the specific pseudo-first-order rate constants (k') by HO center dot (k(HO center dot)'), 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 center dot 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 k(KO center dot)' and 12% similar to 100% decreases of k' by reactive halogen species (RHS) and RNS (k(RHS) + (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 center dot and RCS, but enhanced that of CO3 center dot- for micropollutants reactive toward CO3 center dot- The presence of 1 mg/L NOM scavenged over half the amount of HO center dot, 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.

Automated summary

The study found that different components in water have varying effects on radical generation and degradation of micropollutants in the UV/NH2Cl process. Chloride weakens the impacts of HO· and Cl·, bromide leads to a decrease in HO· and RCS, alkalinity enhances the degradation of micropollutants, while natural organic matter inhibits radical activity.