The distribution coefficients of barium in the Olkiluoto pegmatite, 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 were modelled with the PHREEQC calculation code. The results of different rocks and minerals were compared with each other and the sorption mechanisms onto different mineral surfaces were evaluated. The sorption results of barium were also compared with sorption results obtained from a previous study for caesium. In addition, the diffusion of barium into the rock cubes was also studied both experimentally and computationally. Furthermore, 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 the concentration of barium on all the minerals followed the same trend. The distribution coefficient results of veined gneiss, pegmatite and granodiorite followed the results of their main minerals. The distribution coefficients were largest on granodiorite which suggests ion exchange between barium and calcium, which is a common element in plagioclase occurring abundantly in granitic rock. It was also discovered that the distribution coefficients in saline water were considerably smaller than the results obtained from previous studies in low salinity water. This suggests that competing ions play a significant role in the sorption of barium.
Finally, the concentration decrease of barium in the diffusion experiments was largest in granodiorite which can be explained with both the porous structure of all the minerals of granodiorite and with the sorption properties of barium. It was discovered with autoradiography that the barium was mainly sorbed in 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 veins in the fracture zones of the rock confirming that the barium was sorbed.
