In Situ X-ray Fluorescence Evaluation of Metal Ion Adsorption on Ferric Oxyhydroxide in Frozen Solutions

Abstract

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.