Dichloramine Hydrolysis in Membrane Desalination Permeate: Mechanistic Insights and Implications for Oxidative Capacity in Potable Reuse Applications.

Dichloramine (NHCl) naturally exists in reverse osmosis (RO) permeate due to its application as an antifouling chemical in membrane-based potable reuse treatment. This study investigated mechanisms of background NHCl hydrolysis associated with the generation of oxidative radical species in RO permeate, established a kinetic model to predict the oxidative capacity, and examined its removal efficiency on trace organic contaminants in potable reuse. Results showed that NHCl hydrolysis generated transient peroxynitrite (ONOO) and subsequently dissociated into hydroxyl radical (HO). The maximal HO exposure was observed at an RO permeate pH of 8.4, higher than that from typical ultraviolet (UV)-based advanced oxidation processes. The HO exposure during NHCl hydrolysis also peaked at a NHCl-to-NHCl molar ratio of 1:1. The oxidative capacity rapidly degraded 1,4-dioxane, carbamazepine, atenolol, and sulfamethoxazole in RO permeate. Furthermore, background elevated carbonate in fresh RO permeate can convert HO to carbonate radical (CO). Aeration of the RO permeate removed total carbonate, significantly increased HO exposure, and enhanced the degradation kinetics of trace organic contaminants. The kinetic model of NHCl hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.