Formation of bromate during ferrate(VI) oxidation of bromide in water

Ferrate (VI) is traditionally recognized as a safe oxidant without production of disinfection byproducts (DBPs). However, here we detected probable carcinogenic bromate (BrO₃^-) during ferrate(VI) oxidation of bromide (Br^-)-containing water, and evaluated the effects of pH, ferrate(VI) dose, initial Br^- concentration, and co-existing anions on the BrO₃^- formation. BrO₃^- was produced at a moderately-weakly acidic pH condition and in the absence of phosphate that was commonly applied as a buffer and stabilizing agent in previous ferrate(VI) studies. At pH 5.0, the produced BrO₃^- was increased from 12.5 to 273.8 μg/L with the increasing initial Br^- concentration from 200 to 1000 μg/L at 10 mg/L Fe(VI), corresponding to an increase in the molar conversion ([BrO₃^-]/initial [Br^-]) from 2.3% to 10.3%, in a bicarbonate-buffered solution. As pH increased to 7.0, the BrO₃^- concentration gradually dropped. The BrO₃^- production appeared to be associated with the oxidation by high valence iron species (i.e. Fe(VI), Fe(V) and Fe(IV)). Two key intermediate products (i.e. hypobromous acid/hypobromite (HOBr/OBr^-) and hydrogen peroxide (H₂O₂)) relevant to the bromate formation were identified. The production of HOBr, a requisite intermediate for the ensuing bromate formation, was indirectly validated through identification of bromine-containing trihalomethanes and haloacetic acids during ferrate oxidation in a natural water, though these bromo-organic DBPs produced were insignificant. Furthermore, the inhibition effects of various anions on the formation of BrO₃^- followed chloride < sulfate < silicate < phosphate. More H₂O₂ was detected at higher phosphate concentration. It could reduce HOBr to Br^-, thereby inhibiting the bromate formation.