Characterization of Fe(III)-saturated montmorillonite and evaluation its sorption behavior for U(VI).

Radioactive waste is usually sealed in steel canisters surrounded by a layer of compacted clay back-fill, and permanent buried in a deep geological repository. Unavoidably, the radionuclide contaminants can be released from repository and then sorbed onto the waste container corrosion products or the Fe-rich minerals. Herein, we characterized the Fe(III)-saturated montmorillonite (Fe(III)-MMT) by using Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET), and found the changes of the surface incorporating Fe(III) and surface micropores. The sorption of U(VI) on Fe(III)-MMT and Na-montmorillonite (Na-MMT) was investigated by batch experiments. The larger surface area and cation exchange capacity, and the existence of Fe(III) (hydr)oxide phases in Fe(III)-MMT contributed greatly to its sorption capacity. In the whole pH range, the sorption of U(VI) on Fe(III)-MMT was higher than on Na-MMT, and the sorption was strongly depended on pH and ionic strength. The sorption isotherms were simulated well by the Langmuir and Freundlich models. The thermodynamic parameters ( ΔH, ΔS and ΔG) calculated from the temperature dependent sorption isotherms indicated that the sorption of U(VI) on Fe(III)-MMT was an endothermic and spontaneous process. The observations suggest that the interactions between U(VI) and Fe(III)-MMT are important in controlling U(VI) retention. The phenomena need to be considered in risk assessment and reactive transport modeling. [ABSTRACT FROM AUTHOR]