Retention of selenium oxyanions at the water-mineral interface in the context of nuclear waste repositories

The radioactive isotope Selenium-79 is a fission product found in nuclear waste. Due to its long half-life of 3.27 ∙ 105 years and its high mobility, it is expected to be one of the isotopes contributing significantly to the potential radiation dose according to safety assessments of nuclear waste underground repositories. A detailed knowledge of the mobility and bioavailability of selenium in its different oxidation states is therefore of great importance for a safe disposal of radioactive waste.
Adsorption onto mineral surfaces of both the engineered and geological barrier is a major process controlling the retention of the water-soluble selenium oxyanions, selenate (SeVIO42 ) and selenite (SeIVO32 ). 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 radioactive waste. These parameters include inter alia the presence of different background salts and elevated temperatures.
In this study, a combination of macroscopic sorption experiments, electrophoretic mobility measurements and in-situ ATR FT-IR spectroscopy was used to study the interaction of SeVIO42 and SeIVO32 with aged γ-Al2O3 in the presence of NaCl and MgCl2 and at elevated temperatures up to 60 °C. In this context, γ-Al2O3 can be considered as model mineral phase for more complex rock and backfill materials associated with a nuclear waste repository.
It could be shown that the retention of both Se(VI) and Se(IV) is affected by all three investigated factors pH, ionic strengths of the solution, and temperature. The increase of each parameter results in a decrease of sorption, with the retention of Se(IV) generally being higher than the one of Se(VI). In-situ ATR FT-IR spectroscopy and electrophoretic mobility measurements evidenced the formation of an outer-sphere surface complex of Se(VI) on γ-Al2O3. Concerning Se(IV), a mixture of inner-sphere and outer-sphere surface complexes could be derived.
Any sorption on mineral surfaces is dominated by the surface charge of the mineral. The impact of salinity and temperature on the variable surface charge of γ−Al2O3 was evaluated by zeta potential measurements using laser Doppler electrophoresis. The isoelectric point (pHIEP) of γ-Al2O3 is located at pH 9.6 with a positively charged surface at lower pH values and a negatively charged surface for higher pH values. Increasing the amount of NaCl in the solution (up to I = 1 M) reduces the zeta potential for both the acidic and alkaline pH range. However, in the alkaline range the decrease of the zeta potential is more pronounced. In the presence of 0.1 M MgCl2, the surface charge of γ-Al2O3 becomes positive throughout the studied pH range (3-11). Above pH 10, a sharp potential decrease occurs due to Mg(OH)2 precipitation. The increase of temperature shifts the pHIEP to lower values and decreases the zeta potential in the acidic range.
These changes in the surface properties of the γ-Al2O3 are consistent to the changes in the sorption behaviour of selenate and selenite.
These results indicate that for geochemical modeling and long-term safety assessments concerning selenium-containing waste it is crucial to include the impact of temperature and ionic strengths effects.