Understanding the Mechanism and Ascertaining the Sorption Sites for the Highly Efficient and Selective Sorption of U(VI) and Th(IV) by Graphene/Zeolitic Imidazolate Framework-90 Composites: An Experimental and Theoretical Approach

Efficient removal of radionuclides from aqueous solutions is of utmost importance for public and environmental safety. The development of efficient adsorbents and a deeper understanding of the adsorption mechanism of radionuclides are crucial for designing superior adsorbents. In view of this, the present investigation deals with the understanding of the highly efficient sorption of U(VI) and Th(IV) from an aqueous acidic feed using a multilayer graphene/Zeolitic Imidazolate Framework-90 (ZIF-90G) composite. This is the first ever report on utilization of positron annihilation spectroscopy in order to ascertain the sites of sorption in the three-dimensional pore network of ZIF-90G. The interaction of <i>ortho</i>-positronium with U(VI) in the pore network revealed that the sorption proceeded throughout the pore network with preferential adsorption at the aperture site. The exothermic spontaneous sorption of U(VI) and Th(IV) was found to be maximum in the aqueous feed with pH ranging from 3-4, predominantly following a Langmuir isotherm with pseudo-second-order rate kinetics. In the XPS spectra, the appearance of peaks at ∼392.4 eV (U 4f_5/2) and ∼381.9 eV (U 4f_7/2) for the U(VI) sorbed sample and peaks at ∼344.01 eV (Th 4f_5/2) and ∼334.81 eV (Th 4f_7/2) for the Th(IV) sorbed sample evidenced the effective sorption, while the red shift in the O 1s spectrum (∼531.7 eV and ∼534.4 eV for ZIF-90G) indicated the coordination of metal ions through chelation of carboxaldehyde functionalities. Elemental mapping using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy also evidenced the uniform sorption of the U(VI) and Th(IV). Density functional theory and X-ray absorption spectroscopy have been employed for the further understanding of the mechanism at the molecular level.