Freezing-enhanced reduction of chromate by nitrite

The redox reactions between pollutants and chemicals (e.g., pollutant, oxygen, and water) critically affect the fate and potential risk of pollutants, and their rates significantly depend on the environmental media. Although the kinetics and mechanism of various redox reactions in water have been extensively investigated, those in ice have been hardly explored, despite the large areal extent of the cryosphere, which includes permafrost, polar regions, and mid-latitudes during the winter season on Earth. In this study, we investigated the reduction of chromate (Cr(VI)) by nitrite (NO₂^-) in ice (i.e., at -20°C) in comparison with its counterpart in water (i.e., at 25°C). The reduction of Cr(VI) by NO₂^- was limited in water, whereas it was significant in ice with the simultaneous oxidation of NO₂^- to nitrate (NO₃^-). This enhanced Cr(VI) reduction by NO₂^- in ice is most likely due to the freeze concentration effect, that concentrates Cr(VI), NO₂^-, and protons (at acidic conditions) in the liquid brine (the liquid region among solid ice crystals). The increased thermodynamic driving force for the redox reaction between Cr(VI) and NO₂^- by the freeze concentration effect (i.e., the increase in concentrations) enhances the reduction of Cr(VI) by NO₂^-. The freezing-enhanced Cr(VI) reduction by NO₂^- was observed under the conditions of NO₂^- concentration=20μM-2mM and pH=2-4, which are often found in real aquatic systems contaminated by both Cr(VI) and NO₂^-. The reduction kinetics of Cr(VI) in real Cr(VI)-contaminated wastewater (electroplating wastewater) during freezing was significant and comparable to that in the artificial Cr(VI) solution. This result implies that the proposed ice/Cr(VI)/NO₂^- process should be relevant and feasible in real cold environments.