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

Uranium has been identified as an element of interest for the safety assessment of a deep geological repository (DGR) for used nuclear fuel. This paper studied the sorption of tetravalent U(IV) onto MX-80 bentonite, illite, shale and limestone in ionic strength 0.1 – 6 mol/kgw (m) Na-Ca-Cl solutions at pH m = 4 – 10 (for MX-80, illite, shale) and pH m = 5 – 10 (for limestone) under reducing conditions. Sorption of U(IV) on MX-80 increased with pH m until pH m = 8 and then reached a sorption plateau. Sorption of U(IV) on illite and shale increased from pH m = 4 to 5 and then reached a sorption plateau. Sorption of U(IV) on limestone increased with pH m up to pH m = 7 and then steeply decreased with pH m . Sorption of U(IV) on all four solids was not influenced by the ionic strength of Na-Ca-Cl solutions. The sorption of U(IV) onto MX-80 and illite in Na-Ca-Cl solutions was successfully simulated by a 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model using three inner-sphere surface complexation reactions. Values of surface complexation constants (log K 0 ) were optimized.