Accelerated Reduction of Bromate in Frozen Solution

Bromate is a common disinfection byproduct formed during ozonation. Reducing bromate into bromide can remove this toxic pollutant, however, not many studies have been done for its environmental fate. In this work, we demonstrate a new transformation pathway that bromate can be efficiently reduced to bromide in frozen solution in the presence of organic reductants like humic substances (HS). The results showed that bromate in frozen solution could be removed by 30–40% in dark condition and 80–90% in irradiation condition (λ > 300 nm) in 24 h, while around 1% bromate was reduced in aqueous solution. The bromate reduction by HS induced a partial oxidation of HS, which was confirmed by X-ray photoelectron spectroscopic analysis of the HS sample recovered from the frozen solution. Photoluminescence analysis of HS revealed that the fluorescence quenching by bromate was observed only with very high concentration of bromate (0.1–0.2 M) in aqueous solution whereas the quenching effect in frozen solution was seen with much lower bromate concentration (5–100 μM). The highly enhanced removal of bromate in ice is ascribed to the freeze concentration effect that bromate and HS are concentrated by orders of magnitude to accelerate the bimolecular transformation in the ice grain boundary region. Freezing process in cold environments would provide a unique chemical mechanism for the removal of persistent bromate.