Callovian-Oxfordian clay (COx), a sedimentary hard clay rock from the East Paris basin in France, is under investigation for long term nuclear waste repositories. It is a main concern and pressing need to predict the uptake of radionuclide on COx under “in situ” conditions.
Most of the radioactive species undergo retention processes via ion exchange and/or innersphere/ outer-sphere surface complexation on clays and oxides. Parameters are available in the literature to describe such processes, and they are generally successfully used to describe sorption reactions occurring in simple laboratory model systems from batch-type experiments measured in the dispersed state. The use of these models for predicting the uptake of radionuclide on natural complexes systems (e.g. Cox) remains a subject of discussion for two reasons:
(1) the applicability of the additivity rule at two levels, i.e. (i) from pure illite and smectite phases to interstratified illite/smectite (I/S), an important phase in COx, and (ii) from the individual components to the COx
(2) the transposition of model from dispersed systems to consolidated/compacted systems (if compacted effect exists)
Such questions have been addressed for two cations, Cs(I) and Ni(II), from sorption data measured at both macroscopic (batch-type experiments) and molecular (EXAFS, XPS) levels.
Cs(I) sorption is dominated by a cation exchange, while for Ni(II) both cation exchange and surface complexation reactions are important.
For Cs(I), the sorption is dominated by the clay fraction of COx. It can be described by the additivity rule with published models available for illite and montmorillonite, respectively.
The model transposition from dispersed o consolidated/compacted systems works. In the case of Ni(II), the agreement is less good. Ni(II) sorption on COx is obviously underestimated when only the clay fraction is considered. The carbonate phases seem important to be considered as well. Furthermore, the additivity rule considering I/S as a mixture of illite and smectite fails to explain the sorption data on the enriched clay fraction. A better description of Ni(II)/I/S interaction is therefore needed. In addition to batch experiment and spectroscopic techniques, molecular dynamic simulations (MDS) are used to help to understand the difference which can exist between I/S and illite and smectite phases. This approach is first applied in conditions where the the cation exchange mechanism controls Ni(II) sorption.