Kinetic and competitive effects of sorption on multi-element migration through crushed granite and biotite gneiss in Ca-HCO3-SO4 type groundwater.

ABSTRACT

Crystalline rock is used as the host rock for the disposal of high-level radioactive waste. Two cationic elements (Cs(I) and Ni(II)) and three anionic elements (Se(IV/VI), Mo(VI), and U(VI)) were selected to comprehensively evaluate the sorption behaviors of these radionuclides on crystalline granite and biotite gneiss. The anionic elements showed weak sorption (log Kd (L·kg-1) < 1) and little competition effect, while the cationic elements (log Kd (L·kg-1) = 2-3) showed clear competition (18-98% in Kd values) even at low concentrations. Analysis by pseudo-second-order kinetics showed that Cs(I) sorbed at similar rates on both rocks (20% faster on biotite gneiss), but Ni(II) sorbed 190% faster on biotite gneiss than on granite. That is why the retardation factors for Cs(I) and Ni(II) were reversed in the biotite gneiss column compared to their distribution coefficients. Therefore, the sorption kinetics cannot be neglected in groundwater systems with high flow rates. In the desorption column test, the retardation followed the order of the distribution coefficient. The desorption column test revealed that the distribution coefficient determines the strength of sorption on crystalline rocks.