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

• Ferrate(VI) has a potential to oxidize bromide to bromate at an acidic condition and without phosphate.
• Lower pH, higher Fe(VI) dose and higher Br− concentrations favor the bromate production.
• More H2O2 is observed at a higher phosphate concentration during Fe(VI) oxidation of bromide.
• Phosphate inhibits the bromate formation, probably because H2O2 can reduce HOBr to Br−.
• The formation of bromo-organic disinfection byproducts is of minor concern during Fe(VI) oxidation in natural waters.

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.

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