Modelling sorption data for the actinides Am(III), Np(V) and Pa(V) on montmorillonite

The retention characteristics of the bentonite near-field engineered barrier proposed in most of the concepts for the deep geological disposal of high-level waste and spent fuel are an important component in repository performance assessment studies. Montmorillonite generally constitutes 65 to 90 wt. % of the bentonite. Sorption edge measurements have been performed at trace concentrations for the actinides Am(III), Np(V) and Pa(V) on purified and conditioned SWy1 montmorillonite under anoxic, carbonate free conditions. To the best of the author's knowledge, this is the first time a sorption data set has been measured for Pa-233(V) uptake on montmorillonite. In some earlier work, the 2 site protolysis non-electrostatic surface complexation and cation exchange quasi mechanistic sorption model has been used to model the sorption edges measured for 11 metals with valencies between II and VI. From this study a linear correlation between strong site surface complexation constants and the corresponding hydrolysis constants was established: a Linear Free Energy Relation (LFER). The new measurements for Am-241 (III), Np-239(V) and Pa-211(V) fit very well on this previously established LFER. Through the LFER, a chemically consistent picture emerges which explains quantitatively why some metals tend to exhibit very high sorption and others only moderate to weak sorption.