Sorption mechanism of Th(IV) at iron oxyhydroxide (IOHO)/water interface: Batch, model and spectroscopic studies

In this study, sorption mechanism of Th(IV) at iron oxyhydroxide (IOHO)/water interface was studied under ambient conditions to explore the retardation effects of IOHO on actinides in the engineering barrier system of the high-level radioactive waste geological repository. The sorption of Th(IV) on IOHO was strongly dependent on pH, and the intra-particle diffusion was the rate-controlling step. The presence of humic acid can enhance the sorption of Th(IV) on IOHO below pH ~ 3.5, while slightly inhibit Th(IV) sorption above pH 3.5 due to the competing effect of humic acid remaining in aqueous phase. The surface complexation model (SCM) confirmed that the bi-dentate complex of (FeO)2Th2 + and mono-dentate complexes of FeOThOH2 +, FeOTh(OH)2+ and FeOTh(OH)30 are controlling the sorption process of Th(IV) on IOHO. Th(IV) sorption on IOHO is favorable to high temperature with respect to the hydrolysis effect of Th(IV). XPS analysis also confirmed the above mono- and bi-dentate surface complexes of Th(IV) on IOHO, which are very sensitive to environmental conditions. Mössbauer spectroscopy showed a decreasing geometry of each components of IOHO after Th(IV) sorption, indicating the chemisorption of Th(IV) on IOHO. Moreover, the contributions from Fe3O4(A) and Fe3O4(B) components to Th(IV) sorption were higher under high temperature and initial Th(IV) concentration in comparison with α-FeOOH and amorphous Fe3O4 components. The mono-dentate FeOThOH2 + and FeOTh(OH)2+ complexes formed at Fe3O4(A) and Fe3O4(B) surfaces, whereas the bi-dentate (FeO)2Th2 + complex was preferring forming at the surface of α-FeOOH and amorphous Fe3O4.