Enhanced sensitivity of fluorescence-based Fe(ii) detection by freezing.

The first example of combining the fluorescent probe-based freeze concentration effect with N-oxide chemistry is reported for the highly sensitive and selective detection of ferrous ion (Fe(ii)). Interestingly, our preliminary results demonstrated that the fluorescence intensity of Fe(ii) was markedly enhanced upon freezing, and the location of Fe(ii) in the freezing state was visualized by confocal microscopy using a cryostage.

Nitrite-Induced Activation of Iodate into Molecular Iodine in Frozen Solution.

A new mechanism for the abiotic production of molecular iodine (I2) from iodate (IO3-), which is the most abundant iodine species, in dark conditions was identified and investigated. The production of I2 in aqueous solution containing IO3- and nitrite (NO2-) at 25 °C was negligible. However, the redox chemical reaction between IO3- and NO2- rapidly proceeded in frozen solution at -20 °C, which resulted in the production of I2, I-, and NO3-. The rapid redox chemical reaction between IO3- and NO2- in frozen solution is ascribed to the accumulation of IO3-, NO2-, and protons in the liquid regions between ice crystals during freezing (freeze concentration effect). This freeze concentration effect was verified by confocal Raman microscopy for the solute concentration and UV-visible absorption spectroscopy with cresol red (acid-base indicator) for the proton concentration. The freezing-induced production of I2 in the presence of IO3- and NO2- was observed under various conditions, which suggests this abiotic process for I2 production is not restricted to a specific region and occurs in many cold regions. NO2--induced activation of IO3- to I2 in frozen solution may help explain why the measured values of iodine are larger than the modeled values in some polar areas.

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.

Accelerated redox reaction between chromate and phenolic pollutants during freezing.

The redox reaction between 4-chlorophenol (4-CP) and chromate (Cr(VI)) (i.e., the simultaneous oxidation of 4-CP by Cr(VI) and reduction of Cr(VI) by 4-CP) in ice (i.e., at -20°C) was compared with the corresponding reaction in water (i.e., at 25°C). The redox conversion of 4-CP/Cr(VI), which was negligible in water, was significantly accelerated in ice. This accelerated redox conversion of 4-CP/Cr(VI) in ice is ascribed to the freeze concentration effect occurring during freezing, which excludes solutes (i.e., 4-CP and Cr(VI)) and protons from the ice crystals and subsequently concentrates them in the liquid brine. The concentrations of Cr(VI) and protons in the liquid brine were confirmed by measuring the optical image and the UV-vis absorption spectra of cresol red (CR) as a pH indicator of frozen solution. The redox conversion of 4-CP/Cr(VI) was observed in water when the concentrations of 4-CP/protons or Cr(VI)/protons increased by 100/1000-fold. These results corroborate the freeze concentration effect as the reason for the accelerated redox conversion of 4-CP/Cr(VI) in ice. The redox conversion of various phenolic pollutants/Cr(VI) and 4-CP/Cr(VI) in real wastewater was successfully achieved in ice, which verifies the environmental relevance and importance of freezing-accelerated redox conversion of phenolic pollutants/Cr(VI) in cold regions.