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 UO₂ ²⁺ 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.