Freezing-enhanced reduction of chromate by nitrite

- Reduction of Cr(VI) by NO2− was significantly enhanced during freezing.
- NO2− was primarily oxidized to NO3− in ice.
- Enhanced Cr(VI) reduction in ice is ascribed to the freeze concentration effect.
- Reduction of Cr(VI) was enhanced by freezing under various conditions.
- Freezing-enhanced reduction of Cr(VI) in the electroplating wastewater was observed.

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 = 20 μM-2 mM 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.

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