Efficient and selective separation of U(VI) and Th(IV) from rare earths using functionalized hierarchically mesoporous silica

The separation and recovery of U(VI) and Th(IV) from rare earths are very important to avoid environmental pollution. A functional fibrous hierarchically bimodal mesoporous silica (F-SiO2-DP) as an high efficient adsorbent for selective adsorption of uranium and thorium, which had hierarchically bimodal mesoporous structure and specific surface area of ca. 474.1 m2 g−1, was prepared by post-modification method using 2,9-diamide-1,10-phenanthroline. The synthesized adsorbent was characterized by HR-TEM, SEM, FT-IR, N2 adsorption–desorption, TGA techniques, XRD and NMR spectroscopy. The effects of pH, contact time, initial U(VI) and Th(IV) concentration, temperature and the presence of competitive metal ions on U(VI) and Th(IV) adsorption behavior were studied in detail. The hierarchically bimodal mesoporous structure endows F-SiO2-DP with faster adsorption rate, which can reach equilibrium within 10 min, as well as higher adsorption capacities for U(VI) and Th(IV) compared with unimodal mesoporous silica like MCM-41 functionalized with 2,9-diamide-1,10-phenanthroline. The maximum adsorption capacities reach 416 mg g−1 for U(VI) at pH 5.0 and 277 mg g−1 for Th(IV) at pH 3.8, which are much higher than most of previously reported mesoporous silica-based adsorbents. The selectivity coefficient of U(VI) and Th(IV) over Ce(III), Nd(III), Sm(III), Eu(III), Gd(III), Y(III) and Yb(III) is all larger than 9 at pH 3.8, indicating that F-SiO2-DP possesses a good simultaneously selective adsorption performance for U(VI) and Th(IV). This work demonstrates that F-SiO2-DP with hierarchically bimodal mesoporous structure has promising application in adsorption and separation of actinides from lanthanides, which is crucial to the environmental protection.