Selective Immobilization of Highly Valent Radionuclides by Carboxyl Functionalized Mesoporous Silica Microspheres: Batch, XPS, and EXAFS Analyses

The removal of radionuclides on natural inorganic materials is a pursuing issue for many nuclear-related processes requiring remediation of radioactive wastewater. However, the current natural adsorbents with low cost and great biocompatibility suffer from limitations in removal efficiency, regeneration capability, or/and operation conditions. Herein, silicon dioxide (SiO2) as the major ingredient of clay minerals was modified by carboxyl groups to improve its adsorption affinity for highly valent radionuclides. The batch experiments for U(VI) capture showed that the carboxyl-modified mesoporous silica (SiO2-COOH) microspheres had fast sorption velocity and were exceptionally capable in efficiently sequestering U(VI) under various relevant interferences. The resulting SiO2-COOH possessed great potential for controlled loading of highly valent radionuclides in a selective adsorption order of U(VI) > Th(IV) > trivalent ions > divalent ions > Cs(I). The XPS and EXAFS analyses further confirmed that the interaction mechanism between SiO2-COOH and U(VI) was mainly attributed to the inner-sphere complexation with one or two oxygen atoms shared between the UO22+ and the carboxyl ligand. Carboxyl group modification of natural inorganic materials provides a general and powerful approach to eliminate highly valent radionuclides in various wastewater systems.