Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

79Se, a long-lived (t1/2 ~ 3.27 × 105 years) and radiotoxic fission product, was identified by safety performance assessments to be one of the most contributing isotopes to the total radioactivity that could be potentially released to the biosphere. Selenium has a quite complex speciation, with four main oxidation states, depending on both the pH and the redox potential of the surrounding environment. Sorption of selenium oxyanions onto mineral surfaces (iron, alumina, titanium oxides and clays) has been extensively investigated based on batch experiments and spectroscopic studies. However, the sorption processes of selenium oxyanions onto maghemite was not explicitly studied before and thus the potential ability of maghemite to significantly retard selenium migration in the environment is therefore still unknown. Maghemite was identified as a corrosion product of steel waste canisters and iron archaeological analogues, and is also a ubiquitous mineral in the environment.
In this work, we investigated in detail the sorption of selenium(VI) and selenium(IV) onto maghemite (γ-Fe2O3), for the first time. The effect of pH and ionic strength was studied by batch experiments. Sorption of both oxyanions onto maghemite was found to decrease with increasing pH. An increase of the ionic strength (from 0.01 M to 0.1 M) impacted the sorption of selenium(VI), while it had no significant effect for selenium(IV) uptake. Electrophoretic mobility measurements revealed that the isoelectric point (pHIEP) of maghemite was shifted to lower pH values upon selenium(IV) sorption, while it was not significantly modified upon selenium(VI) sorption. By combining EXAFS and in situ ATR FT-IR spectroscopic measurements, the formation of inner-sphere complexes during selenium(IV) sorption onto maghemite was observed. Selenium(VI) sorption proceeded via the formation of predominant outer-sphere complexes (together with a small amount of inner-sphere complexes).