Assessing the speciation and the biogeochemical processes affecting the mobility of selenium from a geological repository of radioactive wastes to the biosphere
Résumé
In the scope of the studies carried out on the geological disposal of radioactive wastes, it is essential to assess the migration behaviour of long lived radionuclides as a function of the physicochemical conditions (pH, redox potential, temperature, pressure, ...), the nature of the host rocks and the chemical composition of the underground waters. In this study, we have considered the case of selenium which is an element of importance because of its long lived under its 79Se isotope. A review is first performed on the behaviour of selenium from available studies in natural systems in considering particularly the inorganic Se forms. Later on, these results are transposed to the physicochemical conditions occurring in the vicinity of a geological repository. The most stable forms in this context would be Se(-II), Se(0) and Se(IV). Several parameters can govern the mobility of these Se species such as the pH, the potential of the water and the presence of solid phases containing iron, manganese or aluminium (oxy)hydroxides on which Se(IV) can be sorbed. Selenide and selenite mobility can also be retarded by precipitation reactions. This is particularly true for Se(-II) in presence of iron(II) and sulfides. The hypothetical presence of microorganisms is also considered because of its importance on the fate of the Se species.