Sorption studies of actinides/lanthanides onto clay minerals under saline conditions

Actinide retention at clay mineral surfaces is a well established process, which has been intensely investigated in the context of nuclear waste disposal (e.g. Bradbury and Baeyens, 2009). Up to now, however, no mechanistic sorption model exists that reliably describe actinide uptake under ionic strength conditions in groundwater upto high salinity (~ 5 mol/kg). Such conditions are to be expected e.g. in the vicinity of a repository in rock salt formations, in the Jurassic and lower Cretaceous clay rock layers in Northern Germany (Brewitz, 1980) or in sedimentary rocks in Canada (Fritz and Frape, 1982), all identified as potential host rocks for high level nuclear waste disposal. Within the present study lanthanide/actinide ion (Eu(III), Cm(III), U(VI)) interactions with montmorillonite and illite in aqueous solution (I ≤ 4 mol/L NaCl, CaCl2 and MgCl2) in the absence of carbonate are investigated. Batch sorption and time-resolved laser fluorescence spectroscopy (TRLFS) experiments are carried out for a given ionic strength at fixed metal concentration and constant solid to liquid ratio for 3 < pHc < 12 (for MgCl2 solutions up to pHc = 9). ... mehrHighly saline conditions have a surprisingly small impact on KD - values in the NaCl system (Fig. 1, left). A significant decrease in sorption extent is observed in solutions with elevated Mg/CaCl2 concentrations (Fig. 1, right), notably for the trivalent cations. Nonetheless, KD values remain high (log KD ≥ 4.5) for all systems in the pHc range 7 - 11. TRLFS studies using Cm(III) as a fluorescent probe do not display any in the metal ions first coordination sphere at a given pHc if ionic strength is increased. As a consequence, we do not expect any change in the surface speciation of Cm(III) at elevated ionic strength compared to a previous study (Rabung et al., 2005). The same surface species are assumed and uptake data are described using the non-electrostatic 2SPNE/SC CE approach (Bradbury and Baeyens, 2002). The impact of elevated ionic strength on the activities of solutes and of water is taken into account by applying the Pitzer approach.