Thermodynamic modelling of Selenium in environmental conditions: traps, pitfalls and perspectives

Any safety assessment of nuclear waste disposal concepts requires comprehensive and consistent thermodynamic data for the respective reactive transport modelling. This includes sorption, ion exchange or surface precipitation as major retardation processes, as well as a correct description of the aqueous chemistry including redox processes.
Selenium (with the isotope Se-79 being an important fission product) can occur in oxidation states varying between +VI and –II, the latter also including several polynuclear species. Most often negatively charged species are formed rendering them extraordinarily mobile in groundwater systems. Namely for Se solubility in highly saline solutions and for Se sorption onto minerals several competing thermodynamic datasets are published to predict the behavior of selenium under environmental conditions. However, there are still critical data gaps. For example, recent findings like the selenite dimerization have to be parameterized and included into thermodynamic data collections. In addition, for the selenide chemistry, solubility data are missing as well as ion-ion interaction parameters for the calculation in brines.
Another aspect of the reactive transport of selenium is the retardation of mainly selenate and selenite via sorption onto mineral phases. For these processes, recent spectroscopic investigations in combination with batch experiments have enabled deeper insights into the sorption chemistry of selenium. Here, the various data received must be processed into consistent data sets.
Several approaches to close critical selenium data gaps will be presented. These are essential steps towards a consistent and quality approved thermodynamic data set that can be included into databases needed for the geochemical and reactive transport calculations, namely THEREDA (http://www.thereda.de) and RES³T (http://www.hzdr.de/res3t).