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

Monochloramine (NHCl) is applied upstream of reverse osmosis (RO) membranes for biofouling control. Residual NHCl can undergo UV photolysis downstream, generating reactive species for an AOP to occur. At the bench-scale, NHCl 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/NHCl at the bench scale to study their reactivity with reactive chlorine species (RCS) and reactive nitrogen species (RNS). With methanol acting as a scavenger of OH radicals, the performance of UV/NHCl was compared to UV/HO and UV/HOCl. In UV/HO, dioxane was severely inhibited at 1-2 mg/L HO and comparable at 5 mg/L to UV/NHCl. When ammonium sulfate ((NH)SO) was used as the ammonia source over ammonium chloride (NHCl), the overall degradation of micropollutants was higher and caffeine was exclusively degraded. At 1-2 mg/L NHCl, dioxane degraded by 16.2-17.8% and 2.92-5.29% from (NH)SO and NHCl respectively while caffeine degraded by 7.45-9.61% with NHCl ((NH)SO), but not degrade with NHCl (NHCl). The higher concentration of chloride ions from NHCl significantly influenced the speciation of generated radicals and impacted micropollutant degradation. This suggests that the reactivity of more selective RCS (Cl, ClO, ClOH) and RNS (NH, NO, NO, etc.) varies with micropollutants of interest. The presence of higher chloride concentration from the ammonia source inhibited the generation of OH radicals with OH consumed by RNS to form NO (μg/L levels), showing the impact of the choice of ammonia source and the water matrix on UV/NHCl performance.