Consideration on modeling of Nb sorption onto clay minerals

Abstract

Sorption distribution coefficient (Kd) of niobium-94 on minerals are an important parameter in safety assessment of intermediate-depth disposal of waste from core internals etc. The Kd of Nb on clay minerals in Ca(ClO4)(2) solutions were, however, not successfully modeled in a previous study. The high distribution coefficients of Nb on illite in Ca(ClO4)(2) solutions were successfully reproduced by taking Ca-Nb-OH surface species into account. Solubility of Nb was studied in Ca(ClO4)(2) solutions and the results were reproduced by taking an aqueous Ca-Nb-OH complex species, CaNb(OH)(6)(+), into account in addition to previously reported Nb(OH)(6) and Nb(OH)(7)(2)(-). Based on this aqueous speciation model, the Ca-Nb-OH surface species responsible for the sorption of Nb on illite in Ca(ClO4)(2) solutions was presumed to be X_OCaNb(OH)(6). Although uncertainties exist in the speciation of aqueous Ca-Nb-OH species, the result of this study proposed a possible mechanism for high distribution coefficient of Nb on illite in Ca(ClO4)(2) solutions. The mechanism includes Ca-Nb-OH complex formation in aqueous, solid and surface phases.