Sorption reversibility kinetics in the ternary system radionuclide–bentonite colloids/nanoparticles–granite fracture filling material

The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, ²³²Th(IV), ²⁴²Pu(IV) and ²⁴³Am(III) are clearly colloid associated in contrast to ²³³U(VI), ²³⁷Np(V) and ⁹⁹Tc(VII). Concentrations of colloid-borne ²³²Th(IV), ²⁴²Pu(IV) and ²⁴³Am(III) decrease after ∼100 h showing desorption from bentonite colloids while ²³³U(VI) and ⁹⁹Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. ²³²Th(IV) and ²⁴²Pu(IV) data yield a slower dissociation from colloids compared to ²⁴³Am(III) indicating stronger RN–colloid interaction. In the case of ²³⁷Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).