Uptake of Selenium Oxyanions by δ-alumina at elevated Temperatures

One major process controlling the mobility and bioavailability of selenium, a long-lived fission product in nuclear waste, is the adsorption onto mineral surfaces of both the engineered and geological barrier. In this context, it is important to understand to what extent this sorption is influenced particularly by characteristic parameters as expected in deep underground repositories for high level and long-lived radioactive waste. These parameters include inter alia elevated temperatures originating from heat generating waste and natural ground heat. For the investigation of the sorption processes, δ−Al2O3 was chosen because it is omnipresent in the environment and it represents a model oxide for more complex aluminosilicates.
In the present study, a combination of macroscopic sorption experiments, electrophoretic mobility and in-situ ATR FT-IR spectroscopy measurements was used to study the interaction of selenate and selenite with aged δ-Al2O3 at different temperatures between 25°C and 60°C. From in-situ ATR FT-IR spectra, a change in the symmetry of the aqueous tetrahedral selenate anion can be derived evidencing the formation of a surface complex on δ-Al2O3. From batch experiments, we observe a decreasing sorption of both selenate and selenite upon increasing temperature.
The isoelectric point (pHIEP) of δ-Al2O3 was located at pH 9.6 at 25°C. At higher temperatures, the pHIEP was shifted towards lower pH with a value of 8.6 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 and selenite sorption at higher temperatures could be assigned to a change in the surface properties of δ−Al2O3.
This effect may significantly increase the mobility of these Se−species and must be taken into account in future safety assessments of nuclear waste repositories.