Freezing-induced bromate reduction through iodide and its implications

Freezing, which is the naturally facile process in the cold climate regions, has been extensively investigated as a non-contamination and effective cost method in the environmental treatment. The reactive halogens chemistry has a huge impact on the global environment, especially polar regions. Here, we elucidated the generation of iodine (I₂), tri-iodide (I₃^-), and bromide (Br^-) through the bromate (BrO₃^-) reduction by iodide (I^-) in the unfrozen solution of ice while it did not take place in aqueous solution. This appreciably enhanced transformation was attributed majorly to the freeze concentration effect of BrO₃^-, I^-, and protons (H⁺) in the liquid boundary of ice. The ice grain boundary regions created as well as the consumption of BrO₃^- in the BrO₃^-/I^-/freezing systemin those regions during freezing were visualized with the confocal Raman microscope. pH decrease (the accumulation of H⁺) during freezing was measured quantitatively by the UV-Vis absorption spectra of cresol red (as the acid-base indicator). Also, the freeze concentration effect of I^- on the BrO₃^- transformation was verified in the differently experimental conditions of pH and/ or I^- concentration. The study on the acceleration of BrO₃^-/I^-/freezing system provides not only an unknown production pathway of bromine and iodine speciation in the polar environment but also the environmentally friendly insight into BrO₃^- treatment (known as the disinfection byproduct during ozonation in water treatment).