77Se-NMR spectroscopic investigations on aqueous selenium speciation at higher temperatures and in the presence of divalent metal ions

The radioactive isotope selenium-79 is a long-lived fission product found in nuclear waste. Due to its half-life of 327,000 years, it is expected to be one of the important isotopes contributing to the potential radiation dose of nuclear waste underground repositories. High level and long-lived radioactive waste increases the temperature in the vicinity of the waste disposal site for at least 10,000 years. Thus, it is important to understand to what extent this temperature increase influences the aqueous speciation of the selenium itself and, hence, its sorption behaviour onto mineral phases.
With respect to the redox properties of selenium with its four main oxidation states (-II, ±0, +IV, +VI), the speciation may become quite complex depending on the prevailing environmental conditions.
The spin ½ nucleus of the inactive isotope Se-77 is well suited to be directly observed by NMR spectroscopy. This method provides a valuable tool for the discrimination of oxidation and protonation states of selenium compounds because of the high structural sensitivity of the selenium nucleus to its electronic environment. Thus, changes in speciation due to elevated temperatures or interactions with metal ions can be investigated.
The spectra clearly demonstrate that the aqueous speciation of Se(VI) is not changed within the investigated temperature range (20 °C – 60 °C). However, the interaction of selenium oxyanions with Ca2+ and Mg2+ can be tracked by changes in the selenium chemical shifts. These findings serve as reference data for further investigations addressing the mobility and the sorption behaviour in the presence of ubiquitous metal ions.