Transport number of sodium ions in water-saturated, compacted Na-montmorillonite

Diffusion of radionuclides in bentonite buffer material has been studied by many researchers. The nuclides in a form of ion diffuse in the bentonite satisfying electro-neutrality. It is important to examine major charge carriers as a possible counter ion for understanding the diffusion behavior in the bentonite. In this study, transport numbers of Na+ ions, which are the major exchangeable cations in the sodium montmorillonite, which is the major mineral of bentonite, were experimentally determined as a function of dry density by a moving boundary method, using electromigration and electro-osmosis in the montmorillonite. The transport numbers were determined based on the electromigration of the Na+ ions, because the contribution of the electro-osmosis to the migration of the cations is considered to be negligible. The effect of pore water ionic strength in the montmorillonite on the transport number was also examined. It was found that the transport numbers did not depend on the dry density and were nearly equal to 1. Therefore, the Na+ ions behave as the predominant charge carrier in the montmorillonite. When salinity was increased up to 0.5 M, the transport numbers decreased down to 0.8. This suggests that the major part of the charge was carried by the Na+ ions but the contribution of other ions to the charge carrier increased gradually with increase of the ionic strength.