Ion diffusion in compacted sodium and calcium bentonites

The diffusivities of Sr2+ and Cs+ in compacted Erbsloeh Ca-bentonite and Mx-80 Na-bentonite have been determined by various tracer techniques. Whereas the diffusivity of Sr2+ was found to be (1,5(2))x10-11m2xsec-1 in both bentonites the diffusivity of Cs+ was found to be (7(2))x10-13 and (2,5(5))x10-12m2xsec-1 in erbsloeh and Mx-80 respectively. A comparison of the diffusivities of Sr2+ and Cs+ with the diffusivities of Cl- and I- earlier obtained shows that high Ksub (d)-values are not necessarily an indication of immobilization i.e. a simple pore diffusion model with retardation of the diffusing ions by sorption processes is not valid for Sr2+ and Cs+. Water ratio (density) and homogenization time were not found to have any measurable effect on the diffusivity of the cations. (Author)