Bromide Catalyzes the Transformation of Nitrite by Sulfate Radical Oxidation

Bromide (Br–) and nitrite (NO2–) can be transformed to brominated and nitrophenolic byproducts, respectively, in sulfate radical (SO4·–)-based oxidation processes, which has raised growing concern. In the present work, we examined the byproducts in the heat-activated persulfate oxidation system with the copresence of Br– and NO2–, finding that Br– acted as a catalyst to accelerate the NO2– transformation and nitrophenolic byproduct formation. Nitryl bromide (NO2Br) was the key intermediate in this process. It was generated by the reaction between NO2– and free bromine in situ formed as the product of the oxidation of Br– by SO4·–. As a strong nitrating agent, NO2Br reacted with natural organic matter directly, resulting in nitrophenolic byproducts. Alternatively, it reacted with excess NO2– to form N2O4, which hydrolyzed to NO3– ultimately. Both pathways were accompanied by releasing Br–. There was little Br– loss in the earlier reaction phase. Only after NO2– was completely transformed, brominated byproduct formation occurred. Thus, the transformations of Br– and NO2– are interweaved in SR-AOPs, which facilitates the formation of nitrophenolic byproducts but inhibits brominated byproducts. These new insights underscore the potential hazardous effects when SO4·– oxidation is applied under realistic environmental conditions.