Modelling sorption edge and isotherm data for the uptake of Am(III), Np(V), Pa(V) and U(VI) on montmorillonite

Full text of publication follows: 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 and some sorption isotherm measurements have been performed for the actinides Am(III), Np(V), Pa(V) and U(VI) on purified and conditioned SWy-1 montmorillonite under anoxic, carbonate free conditions in 1:1 and 2:1 background electrolytes. Where literature data exist, these are compared and contrasted with the current results. No reliable literature data could be found for the uptake of Pa(V) on montmorillonite. Eu(III) is often take as a chemical analogue for the trivalent actinides and the sorption edges for Am(III) and Eu(III) and sorption isotherm data determined under the same conditions are compared. The 2 site proto-lysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) quasi mechanistic sorption model, used in a number of previous investigations [1, 2], has been applied to model the in-house and literature data in terms of mono-dentate surface species. Proto-lysis constants and site capacity values had the same fixed values as in the other studies. For the four actinides the modelling data are expressed in terms of binding constants for the inner sphere surface complexes and selectivity coefficients for outer sphere sorbed species. [1] Bradbury M. H. and Baeyens B. (1997) A mechanistic description of Ni and Zn sorption on Na-montmorillonite. Part II: Modelling. J. Cont. Hydrol. 27, 223-248. [2] Bradbury, M.H. and Baeyens, B. (2005) Modelling the sorption of Mn(II), Co(II), Ni(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and predictions of surface binding constants for some selected heavy metals and actinides. Geochim. Cosmochim. Acta (In Press). (authors)