Effects of compaction and ionic strength on cation exchange selectivity coefficients of Ca(II), Mg(II), and K(I) for Na(I) in bentonite

Cation exchange selectivity coefficients of bentonite are crucial for estimating cation compositions of bentonite buffer in the geological disposal of radioactive wastes. For confirming the difference in selectivity coefficients of Ca(II), Mg(II), and K(I) for Na(I) between compacted and dispersed bentonite, column and batch experiments were performed under various ionic strengths. Further, as an applied study, a column experiment was performed using coastal groundwater (ionic strength 0.28 M), and the cation compositions of bentonite after the experiment were numerically simulated using selectivity coefficients for the compacted and dispersed states. The experimental results showed that the selectivity coefficients in the compacted state were higher than those in the dispersed state, and they increased slightly with increasing ionic strength in both states. The numerical simulations agree with the experimental results of cation composition after the column experiment when using the selectivity coefficients determined under the compacted state at the ionic strength of 0.25 M. These results suggest that the selectivity coefficients obtained under the compacted state are preferable for predicting the cation composition of compacted bentonite, especially in saline groundwater environments. • Column and batch experiments were conducted to determine selectivities of Ca(II), Mg(II), and K(I) for Na(I) in bentonite. • Selectivities in compacted state were higher than those in dispersed state and increased with increasing ionic strength. • Selectivities in compacted state are preferable for estimating cation compositions in compacted bentonite.