Radium sorption on biotite; surface complexation modeling study

Abstract

The sorption of Ra on Olkiluoto biotite in the context of deep geological disposal of spent nuclear fuel was studied with isotherm batch sorption experiments. Ba was used as an analog for Ra in the experiments and modeling studies. A wide concentration range of Ra/Ba was used in the isotherm studies (2.6 x 10(-9) M to 1 x 10(-3) M) in addition to four different Olkiluoto reference groundwaters with salinity types ranging from fresh to saline. Experimental results show that both in the fresh and saline reference groundwaters, the distribution coefficients of Ra tend to decrease in the higher isotherm concentrations of Ba. With one reference groundwater, the distribution coefficients increased with the concentration of Ba due to significant coprecipitation of Ra. With the fresh reference groundwaters, the distribution coefficients of Ra were consistently approximately one order of magnitude lower than in the saline reference groundwater. A PHREEQC multi-site complexation model coupled with an optimization tool in Python was used to interpret the experimental Ra sorption results. Molecular modeling with CASTEP code implemented into Materials Studio was used to update the PHREEQC model with more realistic biotite sorption site density data. It was observed that while the multi-site model predicts the sorption of Ra well in lower isotherm concentrations, auxiliary reactions of Ra disrupt the model in high Ba isotherm concentrations. The experimental and modeled distribution coefficient data of Ra on biotite can be used in the safety case calculations of the deep geological disposal of spent nuclear fuel in Finland and Sweden.