Cryosphere plays an important role in the global circulation of various chemical substances through the chemical processes that occur therein. Metal ions are adsorbed on adsorbents, such as ferric (oxy)hydroxides (FeOxH) and iron manganese oxides, and can be accumulated in the frozen phases. The metal ions can be supplied from the cryosphere to the hydrosphere in the bioavailable form by the desorption of metal ions during ice melting. Therefore, quantitative evaluation of the adsorption of metal ions in the frozen state is important to understand their exchange between the frozen and solution phases. Here, we propose in situ synchrotron X-ray fluorescence (XRF) as an appropriate method to quantitatively measure the adsorption of metal ions on FeOxH in a frozen solution. When NaCl solution freezes, a freeze-concentrated solution (FCS) is separated from ice. XRF imaging confirms that the first-row transition metal ions, that is, Mn2+, Co2+, Cu2+, and Zn2+, are enriched with FeOxH in the FCS. While in the presence of tris(hydroxymethyl)aminomethane (tris), Cu2+ forms a stable tris-complex and does not adsorb on FeOxH, other metal ions are partially adsorbed on FeOxH. Therefore, Cu2+ is always dissolved in the FCS, but other metal ions are distributed in both FCS and FeOxH. The linear correlations between the normalized XRF intensity of Cu2+ and those of other metal ions dissolved in the FCS are confirmed. Also, the normalized XRF intensity of Fe in FeOxH is proportional to those of the adsorbed metal ions. Thus, the XRF signals of Cu and Fe are considered to represent the other metal ions contained in the FCS and those adsorbed on FeOxH, respectively. The multivariate regression analysis provides the relative contributions of these two different states of the metal ion in frozen samples. From the regression coefficients, the adsorption ratio of the metal ion in the frozen phase can be estimated. The time change in the adsorption ratio is also successfully evaluated using this method.
Iron governs biological activities in the natural environment as an essential element. Although the concentrations of dissolved iron species are primarily regulated by the solubility of iron minerals, there are additional pathways of the supply of dissolved iron species from solids. Redox processes play important roles in the dissolution of iron species from minerals because ferrous ions are more soluble than ferric ions. In addition, recent studies have indicated that freezing enhances the dissolution of iron species from oxides in the presence of acids or ligands. Thus, although freeze-specific phenomena may play essential roles in the determination of this process of fundamental and environmental importance, the details have not been elucidated. In the present paper, the redox behaviors of major iron oxides, that is, magnetite, maghemite, and hematite, are studied using cyclic voltammetry in frozen acidic media. The results indicate that freezing facilitates the leaching of Fe2+ from magnetite but hinders the reduction of ferric ions in the oxide crystals. The latter is commonly observed for all iron oxides studied here. This hindrance is caused by the freeze enrichment of Fe2+ at the interface between the iron oxide and freeze-concentrated solution and by the surface adsorption of Fe2+.
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