The role of reactive chlorine and nitrogen species in micropollutant degradation in UV/monochloramine AOP

Abstract

Monochloramine (NH2Cl) is applied upstream of reverse osmosis (RO) membranes for biofouling control. Residual NH2Cl can undergo UV photolysis downstream, generating reactive species for an AOP to occur. NH2Cl is typically generated from combining sodium hypochlorite and ammonium chloride or sulfate. This study investigated the degradation of four compounds of interest – acetaminophen, caffeine, sucralose and 1,4-dioxane – in UV/NH2Cl at the bench scale to study their reactivity with reactive chlorine (RCS) and nitrogen species (RNS). With methanol acting as a scavenger of •OH radicals, the performance of UV/NH2Cl was compared to UV/H2O2 and UV/HOCl. When ammonium sulfate was used as the ammonia source over ammonium chloride, the overall degradation of micropollutants was higher and caffeine was exclusively degraded. The higher concentration of chloride ions, contributed by ammonium chloride for monochloramine formation, significantly influenced the speciation of radicals generated that impacted micropollutant degradation. This suggests that the reactivity of more selective RCS (Cl2•−, •ClO, ClOH•−) and RNS (•NH2, •NO, •NO2, etc.) varies with micropollutants of interest. In presence of higher chloride concentration from the ammonia source, RCS generated •OH radicals and consumed by RNS to form NO3−, showing the impact of the ammonia source and water matrix on UV/NH2Cl performance.