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

Highlights

• MIL-101(Cr) was successfully functionalized with three organophosphorus compounds.

• pH effect, adsorption isotherms, kinetics, selectivity and reusability were studied.

• All modified adsorbents have high affinity towards heavier REEs.

• The functionalized MOFs demonstrated over 90% selectivity towards Er³⁺.

Abstract

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.

Introduction

Rare earth elements (REEs), as critical materials [1], are essential in fields such as the production of high-tech electronic devices and the development of green technologies. To enable a move away from high-grade ores processing with its high energy costs, projected supply shortages and access issues, it is necessary to develop more sustainable methods for recovery and concentration of REEs. Recycling of secondary resources, such as electronic waste, has been attracting considerable interest [2,3] as many of these resources contain valuable elements, for instance, light (La, Nd and Gd) and heavy (Dy, Ho and Er) REEs [4,5]. Moreover, such recycling contributes towards circular economy.

Electronic waste usually contains a small amount (ppm level) of REEs [6] available for further extraction and concentration. The applied hydrometallurgy techniques [2,7], chemical precipitation [8], extraction processes [9,10] and ion exchange [11] possess drawbacks, such as high operating costs, hazardous acidic environments, non-selectivity, and high losses of REEs, especially at low initial concentrations. Clearly, there is a lack of green and cost-efficient methods for effective REE recovery [12].

Adsorption is a feasible alternative for the recovery and separation of rare earth metals due to its environmentally-friendly characteristics, low cost, tuneable selectivity towards REEs, and applicability at low initial concentrations [13]. In recent years, porous adsorbent materials, such as zeolites, silica gel, activated carbon, ion-exchange resins, have been extensively studied and widely applied in a commercial use for broad-ranging wastewater treatments, including recovery of rare earth metals. However, the efficiency of a porous material is highly dependent on its and the adsorbate's nature as well as the aqueous media conditions. For example, the final uptake of Nd³⁺ varies from 7.3 to 232 mg of Nd³⁺ per gram of a material [14]. Although adsorption has many advantages, the reusability of the adsorbents and their selectivity towards REEs in the presence of other metals may be uncertain and differ depending on process conditions.

Metal-organic frameworks (MOFs), a relatively new class of porous adsorbent materials, are constructed from an inorganic part of metal clusters interconnected by an organic part of rigid linkers, resulting in a highly crystalline structure with a large specific surface area, and adjustable volume and porosity [15,16]. These compounds have been successfully used in such applications as heterogeneous catalysis, storage and separation of gases [17], electrochemistry [18] and photocatalysis [19,20]. Their application as adsorbent materials for various compounds, including heavy metal ions, has been studied to some extent [21,22], but only a limited number of publications have investigated adsorption of REEs. Thus, the full potential of this type of porous materials for recovery of REEs is not fully known.

Although MOFs have favourable characteristics, their structural stability in aqueous solutions remains a challenge. MIL-101(Cr) has been found to have remarkable stability during long-term exposure to acidic and alkaline solutions, H₂O₂ and air [23], and it could thus be considered an appropriate candidate for REE recovery. However, the pristine MIL-101(Cr) showed a weak affinity towards REEs, while a post-synthetic modification by various functional groups enhanced the adsorption capacity and selectivity, making it a promising material compared to traditional adsorbents [24].

As typical Lewis acids, REEs have strong affinity to Lewis bases, for instance, phosphorous or various oxygen-based functional groups [25]. Organophosphorus compounds such as TBP, HDEHP and Cyanex-272 (Table S1) are well-known selective acid extractants (Lewis bases) for REEs [[26], [27], [28]]. Several studies have reported the possible functionalization of different substrates [[29], [30], [31], [32]] by organophosphorus extractants. In the study by Shu et al. [32], a HDEHP modified silica-based adsorbent demonstrated relatively high adsorption capacities of 39.6 and 51.4 mg g⁻¹ for Ce³⁺ and Gd³⁺, respectively. In other work, synthesized zirconium organophosphates and phosphorous acid-modified mesoporous SBA-15 showed high uptake of Eu⁺³ (60 mg g⁻¹) [33] and Gd³⁺ (200 mg g⁻¹) [34], respectively. Moreover, it has also been demonstrated that a combination of –POH and –COOH groups on the surface of zirconium-based coordination polymers [35] and MIL-101(Cr)-PMIDA [24] can result in an adsorption capacity for Gd³⁺ of higher than 90 mg g⁻¹.

While some studies were carried out on efficient recovery of REEs using MOFs, the challenge experienced by separation between light and heavy REEs over the past decades remains unresolved [36]. Moreover, there is no studies in which a high separation efficiency between HREEs and LREEs has been achieved. Therefore, in this work, the possibility to develop a stable and selective adsorbent by combining attractive characteristics of MIL-101 and extraction ability of the modifiers was considered.

Furthermore, to the best of the authors’ knowledge, the functionalization of MOFs, specifically MIL-101(Cr), by organophosphorus extractants for recovery of REEs has hitherto not been investigated. In this study, a post-synthetic modification of MIL-101(Cr) was carried out using Cyanex-272, HDEHP and TBP. The synthesized materials were characterized and subsequently tested in aqueous solutions of REEs (Nd³⁺, Gd³⁺ and Er³⁺) to investigate adsorption behaviour, reusability and separation performance for transition metal ions and REEs.