Selective recovery and separation of rare earth elements by organophosphorus modified MIL-101(Cr)

Development of state-of-the-art selective adsorbent materials for recovery of rare earth elements (REEs) is essential for their sustainable usage. In this study, a metal-organic framework (MOF), MIL-101(Cr), was synthesized and post-synthetically modified with optimised loading of the organophosphorus compounds tributyl phosphate (TBP), bis(2-ethylhexyl) hydrogen phosphate (D2EHPA, HDEHP) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex®-272). The materials were characterized and their adsorption efficiency towards Nd³⁺, Gd³⁺ and Er³⁺ from aqueous solutions was investigated. The MOF derivatives demonstrated an increase in adsorption capacity for Er³⁺ at optimal pH 5.5 in the order of MIL-101-T50 (37.2 mg g⁻¹) < MIL-101-C50 (48.9 mg g⁻¹) < MIL-101-H50 (57.5 mg g⁻¹). The exceptional selectivity of the materials for Er³⁺ against transition metal ions was over 90%, and up to 95% in the mixtures with rare earth ions. MIL-101-C50 and MIL-101-H50 demonstrated better chemical stability than MIL-101-T50 over 3 adsorption−desorption cycles. The adsorption mechanism was described by the formation of coordinative complexes between the functional groups of modifiers and Er³⁺ ions.