Abstract
Recently, demands for raw materials like rare earth elements (REEs) have increased considerably due to their high potential applications in modern industry. Additionally, REEs’ similar chemical and physical properties caused their separation to be difficult. Numerous strategies for REEs separation such as precipitation, adsorption and solvent extraction have been applied. However, these strategies have various disadvantages such as low selectivity and purity of desired elements, high cost, vast consumption of chemicals and creation of many pollutions due to remaining large amounts of acidic and alkaline wastes. Membrane separation technology (MST), as an environmentally friendly approach, has recently attracted much attention for the extraction of REEs. The separation of REEs by membranes usually occurs through three mechanisms: (1) complexation of REE ions with extractant that is embedded in the membrane matrix, (2) adsorption of REE ions on the surface created-active sites on the membrane and (3) the rejection of REE ions or REEs complex with organic materials from the membrane. In this review, we investigated the effect of these mechanisms on the selectivity and efficiency of the membrane separation process. Finally, potential directions for future studies were recommended at the end of the review.
Highlights
In recent years, the consumption of scarce raw materials such as rare earth elements (REEs) has exponentially increased due to their fundamental requirements in advanced technologies and industrial applications [1]
The results show that when the trioctyl phosphine oxide (TOPO) was used as an extractant in the 1.8 M H2SO4 stripping phase, the permeation coefficient for Nd3+ and Pr3+ were 0.8627 × 106 m/s and 0.2418 × 106 m/s, respectively, and when TPB was used, the permeation coefficient for Nd3+ and Pr3+ was 0.4043 × 106 m/s and 0.0910 × 106 m/s, which indicate that the separation of Nd3+ from Pr3+ was not acceptable for both extractants
After 30 h, up to 100% of REE ions were transported with just 10% transportation of ferric ions
Summary
The consumption of scarce raw materials such as rare earth elements (REEs) has exponentially increased due to their fundamental requirements in advanced technologies and industrial applications [1]. Membranes 2022, 12, 80 for REEs separation has involved the liquid-liquid extraction method This method is the most used and conventional way to extract REEs on an industrial scale [30], consistently low extraction efficiency, the presence of impurities in the final product, a low contact area and loss of the extractant in the aqueous phase are still present in large-scale processes [31]. To solve these problems, various types of liquid membranes (LMs), such as emulsion liquid membrane (ELM), bulk liquid membrane (BLM) and electrostatic quasiliquid membrane (ESPLM), have been developed to improve the extraction of REEs [32,33,34,35,36,37].
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