Influence of Temperature on the Sorption of Selenate onto Anatase

The radioactive isotope Selenium-79 is a long-lived fission product found in nuclear waste. Due to its long half life of 3.27 ∙ 105 years, it is expected to be one of the isotopes most contributing to the potential radiation dose according to safety assessments of nuclear waste underground repositories. A detailed knowledge of the mobility and bioavailability of selenium is therefore of great importance for a safe disposal of radioactive waste.
One major process controlling selenium mobility and bioavailability is the adsorption onto mineral surfaces of both the geological and engineered barrier. High level and long-lived radioactive waste increase the temperature in the vicinity of the waste disposal site. Thus, it is important to understand to what extent this temperature increase influences the sorption of selenium.
The present study focuses on the impact of temperature on the sorption of selenate (SeO42-) onto anatase (TiO2). Because of its abundance in rocks and its well-known crystal structure, anatase represents an ideal model system for the study of sorption behaviour of Se onto transition metal oxide phases. The sorption of selenate onto anatase at different temperatures (25 °C – 60 °C) was investigated both with batch experiments and ATR FT-IR spectroscopy. In order to explain possible differences in sorption at higher temperatures, the surface of the anatase was investigated.
The sorption of Se(VI) onto anatase as a function of pH was similar at 25 °C and 60 °C, i.e. sorption decreased with increasing pH. However, the sorption capacity of anatase towards Se(VI) was lowered at a higher temperature. Furthermore, the pH value above which no Se(VI) sorption occurs was shifted to lower pH values (from pH 6.5 at 25 °C to pH 5.5 at 60 °C).
The isoelectric point of anatase (pHIEP) was located at pH 6.3 at 25 °C. At higher temperatures, the pHIEP was shifted towards lower pH with a value of 5.5 at 60 °C. In addition, the absolute values of the zeta potential were lowered at higher temperatures. Both findings were in good agreement with the batch experiments.
The observed decrease in selenate sorption at higher temperatures could be assigned to a change in the surface properties of anatase.