Sorption of nickel and europium on biotite

This work focuses on the sorption of Ni(II) and Eu(III) on selected rock types in the area of the planned deep repository for spent fuel at Olkiluoto. The specific objective is to interpret from experimental data the chemical reactions governing sorption by molecular modelling of mineral surfaces and by well-established thermodynamic modelling. The motivation for the selection of mineral phases to be studied is given in detail elsewhere (Olin et al., 2006). Biotite was the mineral chosen for the experiments; two different types of biotite were used, Olkiluoto and Luumäki. In this report, experimental and modelling studies, including characterisation and acid-base titration of biotite and the sorption of nickel and europium on biotite, are described. The characterisation of biotite samples showed that they differ according to the origin, that they were not pure biotite, but included other mineral phases, too, and that the initial cation-exchange complex showed also other cations than potassium alone. The titration experiments were carried out on both Olkiluoto and Luumäki biotite samples, but only the results for the Luumäki samples were possible to fit into a 1-pK surface complexation model. The sorption experiments with nickel and europium on both biotite types were successfully carried out, as was modelling, but the models optimised for the two ionic strengths, 0.05 M and 0.5 M, consisted of different values for the cation-exchange selectivity constants. Consequently, it might be more logical to optimise different models for the Olkiluoto and Luumäki samples. The sorption modelling was supported by molecular-level calculations to obtain better optimised models and model parameters. Molecular modelling techniques were utilised in order to obtain an insight into biotite surface reactivity on the molecular level. It was possible to compare differences between potassium and nickel on mineral surfaces. However, the model created was not spatially large enough (contained too few atoms) to be used for the substitution of europium on three potassium sites.