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 (BrO 3 - ) 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 BrO 3 - . The results show that the maximum concentration of BrO 3 - 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 BrO 3 - by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO 3 - , and Cl - ) significantly inhibited BrO 3 - 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 BrO 3 - in this process.