Sorption of U(VI) on MX-80 bentonite, illite, shale and limestone in Na-Ca-Cl - Ca - Cl saline solutions

Uranium (U) has been identified as an element of interest for the safety assessment of a deep geological repository for used nuclear fuel. In this study, the sorption of U(VI) was studied in Na-Ca-Cl solutions at ionic strengths = 0.1-6 mol/kgw (m) in a CO2 free environment at pH(m) (molal H+ concentration where pH(m) = -log m(H+)) = 4-9 for MX-80 bentonite, illite and Queenston shale and at pH(m) = 5-9 for limestone. U(VI) sorption on MX-80 bentonite increased with pH(m) from pH(m) = 4 to 6, then decreased with pH(m) until pH(m) = 7, and then increased again with pH(m) to pH(m) = 9. U(VI) sorption on illite increased with pH(m) reaching a maximum at pH(m) = 7, and then decreased with further increases in pH(m). The sorption behavior of U(VI) on shale was similar to that of illite, but the extent of decrease in the sorption coefficient (R-d) value with pH(m) was slightly more pronounced for the shale than observed for sorption on illite at pH(m) > 7. U(VI) sorption on limestone increased with pH(m) up to pH(m) = 8 and then seemed to be constant at pH(m) = 8-9. U(VI) sorption on all four solids was independent of ionic strength (0.1-6.0 m). The 2 site protolysis non-electrostatic surface complexation and cation exchange model successfully simulated the sorption of U(VI) onto MX-80 and illite, and the optimized values of surface complexation constants were estimated.