Elucidating the selective adsorption of heavy rare earth by phosphate modified silica: The cooperation of pore confinement and surface structure regulating

Abstract

Rare earths are considered strategic metals, with heavy rare earth elements of greater importance due to their supply and utilization field. However, efficient separation of heavy rare earth elements is a great challenge as the similar physicochemical properties between light and heavy rare earths. In this study, the highly efficient adsorbents with confinement effect are synthesized by the modification of MCM-41 materials with dimethyl clodronate (NP2) and methylene dichlorophosphate (NDP2). The adsorbents were characterized by XRD, TEM, SEM-EDS, N2 adsorption–desorption, FTIR, TG, 13C NMR, and XPS. It was discovered that the pore size of MCM-41 affected markedly the adsorption selectivity, and the cooperation of the organic adsorption center and pore confinement of MCM-41 support enabled the selective adsorption of heavy rare earth ion. The adsorbent modified by NP2 exhibited high adsorption selectivity toward heavy rare earth ions (Lu3+) with the increase in pore size of MCM-41. In contrast, the expanded pore of MCM-41 enhanced the selectivity of light rare earth ions for the NDP2 modified adsorbents. The distribution coefficient of Lu3+ adsorption over NP2-MCM-41(III) adsorbent reached 20,132 mL g−1 and the separation factor of Lu to La reached 82.17. DFT calculations revealed that a vital driving force for the selective adsorption of Lu was the gap of binding energy between Lu and La complexes with the surface NP2/NDP2 groups. The synergy of pore confinement and organic functional groups in adsorbents in this paper offers a valuable strategy for designing efficient materials in the separation of rare earth elements.