The sorption and diffusion of 133Ba in crushed and intact granitic rocks from the Olkiluoto and Grimsel in-situ test sites

Abstract

The distribution coefficients of barium on Olkiluoto pegmatitic granite and veined gneiss, Grimsel granodiorite and their main minerals (quartz, plagioclase, potassium feldspar and biotite) were obtained by batch sorption experiments carried out as a function of the concentration of barium. The distribution coefficients on biotite and veined gneiss were modelled with the PHREEQC computing program. In addition, the diffusion of barium into rock cubes was also studied both experimentally and computationally and the rock cubes from the diffusion experiments were studied with autoradiography and scanning electron microscopy. The results showed that the distribution coefficients of barium were largest on biotite and the sorption as a function of barium concentration on all the minerals followed a similar trend. The distribution coefficients of barium on veined gneiss, pegmatitic granite and granodiorite followed the distribution coefficient trend of their main minerals and the distribution coefficients were largest on granodiorite due to the low salinity of the groundwater simulant. The distribution coefficients of barium on veined gneiss were of the same magnitude as on granodiorite because of the high biotite content of veined gneiss and those on pegmatitic granite were a magnitude smaller. It was also discovered that the distribution coefficients in saline water were considerably smaller than the results obtained in low salinity water, which suggests that competing ions play a significant role in the sorption of barium. Finally, the concentration decrease of barium in the solution in the diffusion experiments was largest in granodiorite which can be explained with both the porous structure of all the minerals of granodiorite leading to high accessible surface area and with the sorption properties of barium. It was discovered with autoradiography that the barium was mainly sorbed on the dark minerals of the rocks, but, additionally, barium sorption on plagioclase was also concluded to be significant. In addition, the sorbed barium could be seen with scanning electron microscopy on the biotite and calcite veins in the fissures of the rock in granodiorite and veined gneiss.