Aggregated germanium saponite: Removal and retention of polymeric thorium and uranium complexes

The present study concerns the removal of thorium and uranium from aqueous solution, and particularly the effect of clay particle size and morphology on the adsorption of polynuclear actinide complexes by synthetic saponite nanoscale powders. Saponites were synthesized from a reactant mixture containing aluminum nitrate, magnesium nitrate, and either sodium silicate or sodium germanate. The saponite powders were analyzed by TEM, XRD, SAXS, N2-adsorption-desorption isotherms and FT-IR. Ge-saponite contained randomly stacked platelets ≈ 100 nm long and 80 nm wide, while Si-saponite showed roughly spherical agglomerates ≈ 60–80 nm in size. Small-angle X-ray scattering measurements indicated a hierarchical pore network among the plates and their agglomerates. Thorium polymers composed of fractal aggregates of branched filaments were characterized by synchrotron radiation-SAXS. The distribution coefficient (Kd mL/g) values for thorium (20 μg/mL) at pH 4 were: Ge-saponite (105) ≈ Si-saponite (105) > raw saponite (104) ≈ raw saponite (washed) (104). The Kd values for uranium (20 μg/mL) at pH 5.5 were: Ge-saponite (103) ≈ raw saponite (washed) (103) ≈ Si-saponite (103) ≈ raw saponite (103). The difference in uranyl adsorption performance observed between the two nanoparticle forms of saponite was attributed to a larger pore width for Ge-saponite (3.5 nm) compared with Si-saponite (0.8 nm). The results have implications for mineral ore tailings containing thorium(IV) and, by analogy, Pu(IV) in a clay buffer environment within a geological repository for high-level nuclear waste. [Display omitted] •Nanostructured silicon- and germanium- saponite powders are synthesized.•Thorium polymers less than 20 nm (radius of gyration) are fractal aggregates of branched filaments.•Kd (mL/g) for thorium: Ge-saponite (105) ≈ Si-saponite (105) > raw saponite (104).