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

Dichloramine (NHCl 2 ) 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 2 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 2 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 2 hydrolysis also peaked at a NH 2 Cl-to-NHCl 2 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 3 •- ). 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 2 hydrolysis predicted well the degradation of contaminants in RO permeate. This study provides new mechanistic insights into NHCl 2 hydrolysis that contributes to the oxidative degradation of trace organic contaminants in potable reuse systems.