Callovo-Oxfordian clay (COx) is under investigation as a host rock formation for long term nuclear waste repository in France. For that purpose, safety assessment models must account for radionuclides' interaction with repository materials over time- and distance- scales spanning many order of magnitude. The clayey component of COx consists mostly of illite, smectite and interstratified illite/smectite (I/S) minerals, and the radionuclide sorption on COx is usually predicted considering an �additivity rule�, e.g., averaging between their behavior with respect to illite and smectite separately.Indeed, experimental studies of cation exchange indicate that for some cations (e.g. Ca2+, Cs+, Ni2+), I/S behaves more like a mixture of illite and smectite for the following reasons: (i) metal ion sorption on I/S can be quantitatively explained by two sub-models describing the sorption by cation exchange on illite and smectite; (ii) the measured value of I/S cation exchange capacity (CEC) is very close to the value calculated by the CECs of illite and smectite according to their ratio in I/S. However, this conclusion cannot be further verified because the cation exchange selectivity coefficients for illite and smectite are similar. Therefore, more detailed understanding and description of the adsorption and transport properties of radionuclides in I/S clay becomes a pressing issue.Molecular dynamics (MD) computer simulations are applied here to better understand the interaction of Cs+ with the basal surfaces of illite, smectite and I/S. A new structural model of I/S is developed according to the atomic composition of the ISCz1 type I/S. The cation sorption properties for the three minerals, such as preferable sorption sites and their surface distributions, most stable adsorption distances, free energies of adsorption, are compared. The potential of mean force (PMF) calculations are performed to obtain the equilibrium constants for quantitative comparison with available experimental data.

Cation exchange on interstratified illite/smectite minerals: Experimental study and molecular dynamics simulations

248th National Meeting (2014)

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