Mineral-specific sorption of Cs, Ni, Eu and Am on granodiorite and mica gneiss

The contribution of the University of Helsinki (HU) and VTT, Technical Research Centre of Finland, to FUNMIG WP 1.2 focuses on the sorption of nickel and europium on selected rock types in the area of the planned repository. Mechanistic understanding of the interaction of solutes and rock forming minerals is important because fracture surface minerals may be in a very thin layer or absent in reducing bed rock conditions. The specific objective of the work is to interpret from experimental data and molecular modelling of minerals the chemical reactions governing the sorption of Ni(II) and Eu(III) as well as their sorption kinetics and reversibility using well-established thermodynamic sorption models. In this report, the selection of mineral phases for further sorption experiments is described. The work was concentrated on phyllosilicates (e.g. biotite) and tectosilicates (e.g. plagioclase) typical for the bedrock of coastal regions of the Baltic Sea. Experiments were carried out to identify the important minerals for sorption. The methods used for evaluation of the mica gneiss and granodiorite were optical microscopy for mineral contents of the samples and thin section autoradiography of Ni-63, Cs-134, Eu-152 and Am-241 to identify the minerals on which the radionuclides were mainly sorbed. Biotite was the mineral chosen for the experiments. Modelling techniques were utilised in order to obtain an insight of minerals on the molecular level. The starting point for the modelling studies was the crystallographic structures of eight minerals: biotite, chlorite, cordierite, epidote, K-feldspar, muscovite, plagioclase and sillimanite.