Selenite Retention by Iron Oxy-Hydroxides: Role of Adsorption, Reduction and Dissolution/Co-Precipitation

Goethite and magnetite are naturally occurring iron oxides with very different properties in terms of microstructure, surface area and water solubility. In addition, the presence of FeII in magnetite can be of importance in the retention of redox-sensitive radionuclides if reduction reactions are triggered. The study of the radionuclide retention made in parallel with both oxides is of great help to get insight on the underlying mechanisms.
Selenite (SeO32-) sorption onto goethite and magnetite was investigated by batch experiments and XAS techniques to evaluate the role played by adsorption, reduction and dissolution/co-precipitation processes. Sorption was studied as a function of the pH (2-10), ionic strength, solid to liquid ratio and Se concentration (1•10-10 – 10-3 M), both under oxic and anoxic conditions.
Sorption onto both oxides was independent on the ionic strength and decreased when the pH increased but the shape of the sorption edges was not totally comparable.
Under oxic conditions, the results obtained for goethite were very similar to those previously obtained by other authors, and all the goethite data could be simulated considering the formation of inner-sphere complexes between selenite and the iron oxide surface sites (SOH). By contrast, the sorption edges of selenite on magnetite presented features that could not be fit considering the same simple model.
XANES analysis performed on samples obtained at different pH, discarded that these “anomalies” were the consequence of selenite reduction at the magnetite surface or of precipitation of metallic selenium.
To explain the experimental sorption results in magnetite it was necessary to account for the oxide dissolution at acidic pH, the formation of iron-selenite aqueous species and their complexation with SOH sites, apart from the formation of inner-sphere complexes between selenite and the iron oxide surface sites. In addition, at higher selenite concentration (>1•10-4 M) and pH< 5-6 the precipitation of ferric selenite was observed to be the dominating retention process, in agreement with EXAFS analyses.
The sorption behaviour on magnetite was similar under oxic and anoxic conditions. With goethite, increased sorption at very acidic pH was observed under anoxic conditions. This fact can be explained by the increase of goethite solubility when the pCO2 decreases, as reported in the literature.
These results evidenced the importance of dissolution and co-precipitation processes in the iron oxides/selenite systems.