Sorption of selenium oxyanions onto hematite

The 79Se isotope, which is a long-lived (t1/2 ~ 3.27 × 105 years[1]) and radiotoxic fission product found in spent nuclear fuels, is of high importance in the context of geological disposal facilities. Safety calculations assessments have shown it 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. The concentration, the bioavailability, the mobility, the distribution and the oxidation state of selenium in the environment are greatly influenced by the pH, nature of mineral sorbent as well as potential redox reactions at mineral surfaces. Among the mechanisms which enable selenium retardation and reduces its migration, adsorption processes onto solid surfaces (iron, alumina, titanium oxides) has been extensively investigated at room temperature [2-4].
Our study focuses on selenium(VI) and selenium(IV) sorption onto hematite (α-Fe2O3), which was so far not thoroughly characterized yet. By means of EXAFS and ATR FT-IR spectroscopic studies, it was observed that selenium(VI) forms purely monodentate inner-sphere complexes onto hematite, but the study was only performed at pD 3.5 [5]. To our knowledge, the only spectroscopic characterization of the binary selenium(IV)/hematite system concluded the formation of bidentate bridging inner-sphere complexes onto single hematite crystal using x-ray standing wave (XSW), but the measurements were only performed at pH 4.0 [6]. Hematite was chosen because it is a ubiquitous iron oxide mineral present in the environment. In addition, it is an iron phase often found in rocks and soils in the vicinity of underground repositories [7]).
At the macroscopic level, the effect of pH and ionic strength was studied by means of batch experiments. Sorption of both oxyanions 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 the selenium(IV) uptake was found to be not significantly affected. Electrophoretic mobility measurements revealed that selenium(IV) sorption shifted the isoelectric point (pHIEP) of hematite to lower pH values, while the pHIEP was not significantly modified upon selenium(VI) sorption. At the molecular level, in situ ATR FT-IR measurements revealed the formation of inner-sphere complexes during selenium(IV) sorption onto hematite, while the sorption of selenium(VI) proceeded via the formation of outer-sphere complexes. Complementary information about the Se reactivity at the hematite surface is provided by EXAFS spectroscopy.
High level and long-lived radioactive wastes are well-known to increase the temperature at the vicinity of the waste disposal site. Such a thermal effect raises the question how the retention of selenium is influenced at elevated temperatures. By means of batch sorption experiments, electrophoretic mobility measurements and in situ ATR FT-IR spectroscopic studies, information and insights about mechanisms involved at higher temperatures (from 25 °C to 60 °C) are provided.

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