Adsorption of radionuclides on minerals studies illustrating the effect of solid phase selectivity and of mechanisms controlling sorption processes

Currently, extensive research is being done on the geochemistry of Yucca Mountain, Nevada. The purpose of this research is to determine whether this location would be suitable as a permanent high-level radioactive waste repository. Site characterization tests must prove that Yucca Mountains' geology will safely isolate radioactive waste from the environment for at least 10,000 years before approval is granted. In order for this to occur, it is necessary to study the sorptive properties of the host rock, and its selectivity in sorption of solutions containing multiple radionuclides. Validation of this must occur, because in the case of a catastrophic leak, the host rock must have properties that will retard the migration of radionuclides. Columnar experimental techniques were employed using goethite, (a hydrous iron oxide), beidellite (clay mineral), ampersand open-quotes nonscentsclose quotes (a zeolitized volcanic tuff) as sorbents. These sorbents were used to measure the isotherms of an identical binary solution (Ni-Sr) to illustrate the selectivity that occurs in different minerals. In beidellite, the sorption process was ideal, while for open-quotes nonscentsclose quotes there was a strong preference for Sr2+. The sorption process was modeled (Ni-Sr open-quotes Nonscentsclose quotes) using ion exchange theory as the mechanism. In goethite, the sorption of Ni-Sr showed a complete preference for Ni2+ at a pH of 7. In various other systems for goethite. Co-Ni (pH=7) was ideal (no selectivity) where the ratios in the solid and solution phases were relatively equal. Conversely in the case of the Pb-Ni system, the Pb2+ ion predominated completely in the solidphase over Ni2+ at a pH of 5.5. Noting the strong effect of pH on the sorption process in goethite, the selectivity could not necessarily be credited to ion-exchange because of possible exclusion from charged sites at low pH values