Origin of the difference in the distribution behavior of tellurium and selenium in a soil–water system

Abstract

The distribution behavior of tellurium (Te) between soil and water in a synthetic soil–water system was studied coupled with the speciation of Te both in soil and water phases by using X-ray absorption fine structure (XAFS) spectroscopy and a high-performance liquid chromatography connected to an ICP-MS (HPLC-ICP-MS), respectively. The results were compared with a similar data set for Se, which was simultaneously obtained in this study. The oxidation states and host phases of Te and Se in the soil samples were given by XAFS, while the oxidation states in water were given by HPLC-ICP-MS. It was found that both Te and Se in soil are mainly associated with Fe(III) hydroxides under oxic conditions. From the EXAFS analyses, the outer-sphere complex is important for the Se(VI) sorbed on Fe(III) hydroxides in soils, while Se(IV), Te(IV), and Te(VI) form inner-sphere complexes. Under reducing condition, it was found that Te(0) and Se(0) species were formed and that Se was more readily reduced to Se(0) than Te, as is predicted from their Eh–pH diagrams. The reduction process from hexavalent to zerovalent species was different between Se and Te, that is, the direct reduction from Se(VI) to Se(0) was observed for Se, while Te was reduced stepwise from Te(VI) to Te(0) via Te(IV). In terms of the distribution between soil and water, Se distribution to water was much higher than that of Te under wide redox conditions. For Se, selenate is the predominant species in water even under reducing condition due to the much higher solubility of Se(VI) than Se(IV). Furthermore, a much smaller distribution of Te in water was primarily due to the larger affinities of Te(IV) and Te(VI) to Fe(III) hydroxides than Se(VI), which originates from the formation of the inner-sphere complexes of Te(IV) and Te(VI) to Fe(III) hydroxides.