Temperature effect on selenium oxyanions retention onto anatase

In the context of nuclear waste management, long-term safety assessments have shown that selenium-79, released from the solid waste matrix, could be one of the major isotopes contributing to the global radioactivity potentially reaching 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. Thus, it is of great importance to characterize the relevant processes occurring at mineral-water interfaces.
Heat emitted by high level and long-lived radioactive waste is well-known to increase the temperature at the vicinity of the waste disposal site for at least 10,000 years. Such a thermal effect raises the question how the retention of selenium is influenced at elevated temperatures. However, so far, only a few sorption studies, which were performed at higher temperatures with single oxides like goethite [1,2], ferromanganese nodules [3] and TiO2 nanoparticles [4], are available. They showed a lowering of Se sorption with increasing temperature. However, no information and insights about mechanisms involved at higher temperatures were provided.
The present study focuses on the impact of temperature on the sorption of selenium oxyanions, i.e. selenium(VI) and selenium(IV), onto pure anatase (TiO2). Because of its abundance in soils and its well-known crystal structure, anatase represents an ideal model system for the study of sorption behavior of Se onto transition metal oxide phases. This will also lead to a completion of thermodynamic databases used for safety assessments of water contamination.
To get a better understanding of involved sorption mechanisms, a combined approach of both macroscopic and microscopic techniques was applied. To avoid an activation of anatase photocatalytic properties, all tubes were covered by aluminum foil. Batch experiments results performed at room temperature showed that anatase has a higher affinity towards selenium(IV) compared to selenium(VI), which is in agreement with former studies on iron, aluminum and titanium oxides [6,7,8]. Selenium(VI) and selenium(IV) sorption onto anatase were strongly dependent on the pH of the suspension. Sorption of both oxyanions was at a maximum in the acidic pH range and decreased when the pH became more alkaline. Sorption of selenium(VI) onto anatase was dependent on the ionic strength of the suspension, while no influence could be noticed for selenium(IV). No reduction of Se oxyanions at both homogenous and heterogeneous levels was noticed during HG-AAS and XPS measurements. Electrophoresis measurements have also been performed during this work. No shift of the isoelectric point of anatase (pHIEP) upon selenium(VI) sorption was observed. On the contrary, selenium(IV) sorption clearly shifted the pHIEP of anatase to lower pH values. Based on EXAFS and ATR FT-IR spectroscopic observations, we concluded that selenium(VI) is sorbed onto anatase as outer-sphere surface complexes, while the sorption of selenium(IV) proceeds via the formation of inner-sphere complexes, at room temperature.
Furthermore, batch sorption experiments of selenium(VI) and selenium(IV) onto anatase at different temperatures ranging from 25 to 60°C have been performed in NaCl. As shown in Fig.1, the influence of the pH on the sorption of selenium(VI) onto anatase shows a similar general tendency, i.e. a decrease of the sorption with increasing pH. However, the sorption capacity of anatase towards selenium(VI) is lowered at higher temperatures. The thermodynamic parameters, i.e. ΔRH°, ΔRS° and ΔRG° for Se sorption onto anatase were determined from the temperature dependence sorption data using the van´t Hoff equation and the exothermic/endothermic and spontaneous sorption characteristics were discussed.
Finally, in situ ATR FT-IR measurements have been performed using an experimental design which allows data collection at elevated temperatures up to 60°C. In the IR spectrum of selenium(VI) sorbed onto anatase obtained at room temperature, the asymmetric v3(Se-O) stretching mode located at 880 cm−1 was significantly shifted to higher wavenumbers compared to the ν3 mode of the free SeO42− species in solution observed at 867 cm−1, indicating the formation of outer-sphere complexes on the anatase surface. At higher temperatures, ATR FT-IR measurements evidenced a decrease of selenium(VI) sorption onto anatase (Fig. 2), in agreement with batch experiments investigations. Additionally, a small blue shift (885 cm−1) of the asymmetric v3(Se-O) stretching mode was noticed when the temperature was increased. However, no significant changes on the sorbed selenium(VI) surface complexes appeared at higher temperatures.