Exploring Br^-'s roles on non-brominated NDMA formation during ozonation: Reactive oxygen species contribution and...

Bromide ions (Br^-) affected non-brominated nitroso-dimethylamine (NDMA) formation during ozonation, but the mechanism is still unclear. 1,1,1',1'-tetramethyl-4,4'-(methylene-di-p-phenylene) di-semicarbazide (TMDS) was chosen to further probe this problem. The results indicated that low levels of Br^- (≤20 μM) enhanced NDMA from 3.27 to 7.56 μg/L, while its amount slightly dropped to 6.22 μg/L raising Br^- to 100 μM. It was experimentally verified that intermediates 1,1-dimethylsemicarbazide (DMSC) and 1,1-dimethylhydrazine (UDMH) played important roles on promoting NDMA generation, whose contribution rates were 40.2% and 32.2%, respectively. The brominated substances with higher NDMA molar yields were detected. ∙OH reduced NDMA formation without Br^-, while it played promotion role with Br^-; the corresponding contribution rates were - 26.9% and 29.2%, respectively. No matter with or without Br^-, both ∙O₂^- and ^lO₂ brought a boost to NDMA formation, their contribution ratios were 34.9% and 58.1% without Br^-, while raised significantly to 64.6% and 81.5% when Br^- existed. Br^- not only facilitated NDMA formation, but also benefited the degradation of TMDS. Based on the calculation results and intermediates detected, the influence mechanisms of Br^- were proposed. The results would provide theoretical basis and technical guarantee for treating NDMA precursors and bromide co-existing water in the future.