Bromide Catalyzes the Transformation of Nitrite by Sulfate Radical Oxidation

Bromide (Br-) and nitrite (NO2 (-)) can be transformed to brominated and nitrophenolicbyproducts, respectively, in sulfate radical (SO4 (& BULL;) (-))-based oxidation processes, which has raisedgrowing concern. In the present work, we examined the byproducts inthe heat-activated persulfate oxidation system with the copresenceof Br- and NO2 (-), findingthat Br- acted as a catalyst to accelerate the NO2 (-) transformation and nitrophenolic byproductformation. Nitryl bromide (NO2Br) was the key intermediatein this process. It was generated by the reaction between NO2 (-) and free bromine in situ formedas the product of the oxidation of Br- by SO4 (& BULL;) (-). As a strong nitratingagent, NO2Br reacted with natural organic matter directly,resulting in nitrophenolic byproducts. Alternatively, it reacted withexcess NO2 (-) to form N2O4, which hydrolyzed to NO3 (-) ultimately.Both pathways were accompanied by releasing Br-.There was little Br- loss in the earlier reactionphase. Only after NO2 (-) was completelytransformed, brominated byproduct formation occurred. Thus, the transformationsof Br- and NO2 (-) areinterweaved in SR-AOPs, which facilitates the formation of nitrophenolicbyproducts but inhibits brominated byproducts. These new insightsunderscore the potential hazardous effects when SO4 (& BULL;) (-) oxidation is applied under realisticenvironmental conditions. Br- catalyzes the transformation of NO2 (-) to NO3 (-) andnitrophenolic byproducts in sulfate radical oxidation processes.