Chapter 4 - Studies of NeptuniumV Sorption on quartz, Clinoptilolite, Montmorillonite, and α-Alumina

Abstract

Sorption onto minerals present along groundwater flow paths may be an important mechanism for retarding the migration of radionuclides from nuclear waste repositories. 237Np has been identified as a radionuclide of concern with respect to the disposal of high-level nuclear waste (HLW), especially at longer time frames, due to its long half-life, suspected high radiotoxicity, and reportedly low sorption characteristics. Because 237Np is a radionuclide of concern in safety assessments of high-level nuclear waste repositories, sorption of NeptuniumV (Npv) onto quartz, clinoptilolite, montmorillonite, and a-alumina was studied by conducting batch experiments in which solution pH, Npv concentration, ionic strength, PCO2, sorbent surface area, and the solid-mass to solution-volume ratio were varied. The results show that Npv sorption on the four minerals that were studied has similar pH-dependent trends, even though the minerals have different mineralogic and surface charge characteristics. For experiments in equilibrium with atmospheric PCO2, a distinct sorption maximum is observed for all four minerals. The common pattern indicated by the observations of data from this study, and those derived from similar studies on sorption of UVI and other actinides, suggests that modeling approaches that are capable of accounting for changes in solution chemistry are required for successful description and prediction of sorption of Np and other actinides on mineral surfaces over wide ranges of geochemical conditions.