Sorption mechanism of U(VI) on a reference montmorillonite: Binding to the internal and external surfaces

Batch type experiments of U(VI) sorption on a reference montmorillonite (SWy-2) were carried out over wide ranges of pH, ionic strength, and total U(VI) concentration. The influences of these factors on the sorption behavior of U(VI) were analyzed to gain a macroscopic understanding of the sorption mechanism. The sorption of U(VI) on montmorillonite showed a distinct dependency on ionic strength. When it was low (0.01 or 0.001M), almost all of the total U(VI) was sorbed over the whole pH range studied, therefore, the dependency on pH was not clear. But the sorption of U(VI) on montmorillonite showed a sorption pH edge in the high ionic strength condition (0.1 M), like those on other clay minerals, kaolinite and chlorite. A mechanistic model was established by considering the mineral structure of montmorillonite together with our previous EPR result, which successfully explained the U(VI) sorption on montmorillonite over the whole range of experimental conditions. The model describes the U(VI) sorption on montmorillonite as simultaneous and competitive reactions of ion exchange and surface complexation, whose relative contribution to the total sorption depends on pH and ionic strength. At low ionic strength and low pH conditions, ion exchange was the dominant mechanism for U(VI) sorption on montmorillonite. At high ionic strength and high pH conditions, surface complexation was the dominant mechanism. At intermediate pH and ionic strength conditions, the two mechanisms contributed to the total U(VI) sorption to varying degrees.