Diffusion of tritiated water, 137Cs+, and 125I− in compacted Ca-montmorillonite: Experimental and modeling approaches

Mechanistic understanding and predictive modeling of radionuclide diffusion in Na- and Ca-montmorillonite are essential to evaluate the long-term evolution of the bentonite barrier and their impact on radionuclide migration during geological disposal of radioactive wastes . Thus, the diffusion behavior of 137 Cs + , 125 I − , and tritiated water (HTO) in compacted Ca-montmorillonite was investigated as a function of porewater salinity and dry density via both experiments and models. The effective diffusion coefficient ( D e ) followed in the order of 137 Cs + > HTO > 125 I − . The D e of 137 Cs + decreased with increasing salinity, whereas the dependence of D e of 125 I − on salinity was uncertain. The cation excess and anion exclusion effects for Ca-montmorillonite were lower than those for Na-montmorillonite. The integrated sorption and diffusion (ISD) model, assuming the homogeneous pore structure and the electrical double layer (EDL) theory for 2:1 electrolyte (CaCl 2 ), could account for the observed trends for D e in Ca-montmorillonite. The lower dependence of D e on the porewater salinity in Ca-montmorillonite was caused by the reduction of the EDL thickness for divalent cations (Ca 2+ ) in comparison with that for monovalent cations (Na + ). The multipore model could improve the fit for D e of 125 I − at low salinity due to the reduction of interlayer pore volumes and anion exclusion effect, however, the disparity at higher densities was considerably larger. From these results, cation diffusion for compacted Ca-montmorillonite could be mainly explained by the electrostatic interactions in the homogeneous pore model; in contrast, anion diffusion was sensitive to both electrostatic interactions and heterogeneous pore structures. The proposed ISD model is an effective tool to evaluate the radionuclide diffusion and sorption behavior in both compacted Ca-montmorillonite and Na-montmorillonite.