Enhancing Ferrate Oxidation of Micropollutants via Inducing Fe(V)/Fe(IV) Formation Needs Caution: Increased Conversion of Bromide to Bromate

This study explores the formation of bromate (BrO₃^-) in the copresence of Fe(VI) and bromide (Br^-). It challenges previous beliefs about the role of Fe(VI) as a green oxidant and highlights the crucial role of intermediates Fe(V) and Fe(IV) in the conversion of Br^- to BrO₃^-. The results show that the maximum concentration of BrO₃^- of 48.3 μg/L was obtained at 16 mg/L Br^- and that the contribution of Fe(V)/Fe(IV) to the conversion was positively related to pH. The study suggests that a single-electron transfer from Br^- to Fe(V)/Fe(IV) along with the generation of reactive bromine radicals is the first step of Br^- conversion, followed by the formation of OBr^- which was then oxidized to BrO₃^- by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO₃^-, and Cl^-) significantly inhibited BrO₃^- formation by consuming Fe(V)/Fe(IV) and/or scavenging the reactive bromine species. While investigations proposing to promote Fe(V)/Fe(IV) formation in Fe(VI)-based oxidation to enhance its oxidation capacity have been rapidly accumulated recently, this work called attention to the considerable formation of BrO₃^- in this process.