Selective separation of rare earth elements (REEs) from solutions of mixed heavy and light metals by solid adsorbents is an important challenge in the fields of water treatment and metal recovery. The main challenge is water instability of many adsorbents, specifically metal–organic frameworks (MOFs), and their low selectivity. Grafting particular organophosphorus compounds (OPCs) on the MIL-101(Cr) MOF can provide both stability and selectivity. When the tributyl phosphate (TBP), bis(2-ethylhexyl) hydrogen phosphate (D2EHPA or HDEHP) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex®-272) OPCs are grafted and applied to mixed-metal aqueous solutions containing Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Nd³⁺, Gd³⁺ and Er³⁺, MIL-101(Cr) offers high selectivity towards the Nd³⁺, Gd³⁺ and Er³⁺ REEs (with stronger affinity towards Er³⁺). However, the underlying chemistry is unknown and the factors leading to the selectivity remain poorly understood. To uncover the key molecular-level factors, we performed state-of-the-art computational simulations using a combination of high-level density functional theory (DFT), semi-empirical calculations, and configurational sampling of the metal ion-MOF binding modes in aqueous solutions. Our simulation study reproduced the available experimental results, in addition to determining the contributing intermolecular interactions, uptake modes and the most significant structural features for improving selectivity towards the REEs. Therefore, our most important result is rationalization of the mechanism of REE separation by OPC-grafted MOFs using quantum mechanical and electrostatic principles. The results provide guidelines for synthesis of OPC-grafted MIL-101(Cr) structures with enhanced selectivity and stability. Moreover, an efficient computational framework is proposed to facilitate comprehensive modeling of similar systems.

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.