RO permeate DOM alteration and emerging oxidation byproduct formation in the UV/NHCl₂ and UV/NH₂Cl processes in the presence of bromide

In potable reuse facilities employing the UV/chloramine process for micropollutant abatement in reverse osmosis (RO) permeate, dichloramine (NHCl2) often co-exists with monochloramine (NH2Cl) due to the rejection of NH4+ by RO and the acidic pH in the permeate. Additionally, bromide may exist in the RO permeate when the system is fed with saline wastewater effluent. In such a scenario, interactions between bromide, NH2Cl/NHCl2, and reactive species in the UV/chloramine process can alter the radical speciation toward reactive bromine species (RBS), subsequently affecting byproduct formation. In this study, the alteration of the elemental composition of RO permeate dissolved organic matter (rpDOM) and the formation of emerging halogenated oxidation byproducts (OBPs) in the UV/NH2Cl and UV/NHCl2 processes were investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). In the UV/NHCl2 process, compounds in rpDOM detected only in the presence of bromide exhibited higher H/Cw (1.55 vs.1.33) and lower O/Cw (0.38 vs. 0.52), double-bond equivalent (DBEw) (5.24 vs. 6.41), and aromaticity index (AImod,w) (0.13 vs. 0.29) compared to those detected solely in the absence of bromide. A total of 52 brominated OBPs were generated in the UV/NHCl2 process in the presence of bromide, with some chlorinated OBPs transforming into their brominated analogs. In the UV/NH2Cl process, the presence of bromide induced similar alterations in rpDOM and OBP formation as observed in the UV/NHCl2 process. The specific transformation pathways were further elucidated by using 4-aminophenol as a representative model compound of rpDOM. In the presence of bromide, its transformation was initiated through oxidation, nitration/nitrosation, and hydroxylation, and the participation of RBS and haloamines subsequently led to the formation of a total of 12 brominated byproducts in the UV/NHCl2 process. The findings unravel the alterations in the elemental composition of rpDOM and the formation of emerging halogenated OBPs by the UV/chloramine process in bromide-containing system for the first time, which are crucial for controlling the formation of halogenated OBPs in RO-based water reuse scenarios.