Molecular signatures of UV/PMS oxidation of bromide-containing secondary effluent: Size reduction of DOM and accumulation of high-risk brominated byproducts.

Ultraviolet/peroxymonosulfate (UV/PMS) is a widely applied advanced oxidation process (AOP) for effective degradation of dissolved organic matter (DOM) in wastewater. However, in bromide-containing secondary effluent (SE-Br), the formation of brominated disinfection byproducts (Br-DBPs) raises concerns due to their heightened biological risks. Herein, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to investigate the molecular-level transformation mechanism of DOM under UV, PMS, and UV/PMS treatments. It was found that PMS and UV/PMS induced substantial DOM degradation, as evidenced by significantly reduced unsaturation (DBE-O) and elevated carbon oxidation state (NOSC) values (p < 0.001). Notably, bromide addition further amplified this trend, resulting in highest H/Cw (1.153) and O/Cw (0.599) values under UV/PMS-Br treatments compared to SE-Br (H/Cw = 1.091, O/Cw = 0.237), together with an increased number of aromatic compounds. In addition, Van Krevelen classification in the numerical fractions revealed an enrichment of tannins (from 10.38% to 14.03%) and aromatic structures (from 12.46% to 13.72%) after PMS-Br treatments, suggesting increased aromaticity and bromination-prone moieties. Consequently, bromide addition markedly elevated AOBr levels (0.354 - 0.376 mg/L in PMS-Br) and promoted Br-DBP formation, with haloacetaldehydes (HALs) being dominant and increasing by 30-fold relative to raw water. To verify the biological risks associated with these identified brominated motifs, the cyto- and geno-toxicity of three representative brominated aromatics was evaluated, including 3-bromo-4,5-dihydroxybenzoic acid (Br-DHBA), bromanilic acid (BrA), and 3-bromo-4-hydroxy-5-methoxybenzyl alcohol (Br-BzOH). Among these, Br-BzOH exhibited the greatest toxic effect. These findings reveal a mechanistic linkage between DOM transformation, Br-DBP speciation and toxicity, providing critical insights for optimizing AOPs to balance pollutant removal and byproduct risk in bromide-rich waters.