Organic Sn compounds are toxic and known to be formed from inorganic Sn(IV) compounds by microorganisms. On the other hand, inorganic Sn(II) compounds, such as SnCl2, are not recognized as harmful and therefore, they are not controlled by law. However, inorganic Sn(II) compounds may be oxidized to Sn(IV) in environment and resulted in formation of organic Sn compounds. To elucidate this possibility, adsorption behavior of Sn(II) species onto minerals such as Fe2O3 plays crucial role. Therefore, we studied adsorption and oxidation behavior of Sn(II) species on Fe2O3.
Adsorption experiment was performed at 25℃ by adding SnCl2 powder into 1 g/cm3 suspension of Fe2O3. The amount of the SnCl2 powder was adjusted so to the concentration of Sn in the resulted solution be 100 ppm. At adequate interval, aliquots of reacting solution were taken out and filtered with a membrane filter with pore size of 0.20 μm. The Sn concentration of the filtrate was determined by ICP-AES. The chemical states of Sn and Fe in the resulted solid were observed by Sn K-edge XA spectroscopy and 119Sn and 57Fe Mössbauer spectroscopy.
Dissolved Sn(II) species in the reacting solution was gradually adsorbed and all the Sn(II) species was adsorbed onto Fe2O3 in 10 hours. Beside this, Fe concentration in liquid phase increased, suggesting that Fe2O3 was dissolved by the adsorption of Sn(II) species. Sn K-edge XA and 119Sn Mössbauer spectra revealed that the oxidation state of Sn species adsorbed on Fe2O3 was +4 and the Sn atom was coordinated by oxygen atoms. In addition, 57Fe Mössbauer spectrum suggested that there were two kinds of Fe compounds in the resulted solid. These results suggest that electrons transferred from Sn(II) to Fe(III) in Fe2O3 via Sn-O-Fe bond formed by inner-sphere complex at the solid-liquid interface.
