The rare earth elements (REEs) are a series of 17 elements consisting of the 14 lanthanide elements, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Th), and ytterbium (Yb), as well as scandium (Sc) and yttrium (Y).1 The d-block element, lutetium (Lu) is also included in the lanthanide series and classified as a REE.
The lanthanide elements, typically defined by the gradual filling of the 4f electron shell across the series, are generally most stable in their 3+ oxidation state, although for some elements, 2+ and 4+ are also common.2 Many of these elements are vital components in much of our modern-day technology, found in smart phones, TV screens and more. They are also essential in green energy technology, such as wind turbines, solar panels and rechargeable batteries used in electric vehicles.1
The extraction and separation of these elements has long been a challenge due to the similar chemical and physical properties of neighbouring elements, as well as their abundance in the earth’s crust. Despite their name, these elements are quite commonly found within the earth’s crust but at such low concentrations that the mining of these elements requires the processing of large quantities of material and produces large volumes of waste.3
The US Department of Energy, the European Commission, and the British Geological Society have all recognised the REEs as critical elements for the future.
For most of their uses, REEs are needed as single pure elements so the separation processes are of high importance. Traditionally, pyrometallurgical and hydrometallurgical methods have been used for these extractions. However, pyrometallurgical methods are energy intensive and result in poor separations, while hydrometallurgical methods often require many separation steps with large amounts of volatile organic solvents and the production of harmful acidic waste.4
Due to their technological importance, as well as their high supply risk and the difficulty involved in processing these elements, the REEs are frequently listed on reports of critical elements, or element risk lists. In fact, the US Department of Energy, the European Commission, and the British Geological Society have all recognised the REEs as critical elements for the future.5-7 These lists account for the production and recycling rate, as well as the governance of the elements (i.e. production rates and the reserve size countries have of certain elements).
With the growing demand for the REEs, there is an increasing interest in creating and testing greener methods for their separation. In chemistry, “green” methods refer to those which follow some or all of the 12 Principles of Green Chemistry which help the method become safer and more environmentally friendly. Some of the principles most relevant to the separation methods discussed in this article include the reduction or prevention of waste, use of safer chemicals, design of methods which are energy efficient, use of chemicals which are derived from renewable feedstocks (such as plant-based sources) and the prevention of pollution.8
In REE separations, green methods may be designed by avoiding the need for large amounts of solvents, particularly harmful organic solvents, and using extractants which are easily recycled, and which originate from renewable and natural sources. In this article, a small selection of green separation processes which are being explored for the separation of the rare earth elements will be discussed. The discussion will include a brief overview of the use of ionic liquids, biosorbents, and supercritical liquids in REE separations.
Ionic liquids
Ionic liquids (ILs) are defined as being pure salts with a melting point below 100°C. However, it is typically those which are liquid at room temperature (aptly named room temperature ionic liquids) that are of more interest and use.9 ILs have been praised as green alternatives to harmful organic solvents due to their unique properties including their low volatility, low flammability, negligible vapour pressure as well as their recyclability. It should be noted that while these qualities are common in ILs, they are not characteristic of all ILs, with properties varying greatly depending on the cation and anion making up the IL, and the way these interact with each other.10
ILs can be designed and created for specific applications through the careful selection and functionalisation of the ions present in the IL.9 Functionalisation can be used to alter the properties of ILs such as their viscosity, melting point and hydrophobicity. This offers many advantages and allows for a wider range of potential applications of these unique liquids, including uses as catalysts,11 electrolytes,12 corrosion inhibitors13 and as solvents and extractants in separation methods.14
For the separation of REEs using IL extractants, liquid-liquid extractions are one of the most common methods used. Liquid-liquid extractions are often simple to carry out, involving the mixing or shaking of an aqueous REE-containing layer and the organic layer containing the IL. In some cases, it is possible to use a neat IL, but an organic solvent is often required to improve the ease of mixing of the two layers due to the viscous nature of many ILs.
In the creation of ILs for REE separations, organophosphorus extractants currently used in industrial separation methods can be used. For example, 2-ethylhexyl phosphonic acid mono(2-ethylhexyl) ester (P507) is widely used in China to achieve high purity REEs.15 This success has seen it used in a range of ILs for the same purpose. For example, Shen et al. tested the IL [N1888][P507] (Fig. 1) in a sulfuric acid medium, achieving separations of trivalent Tm, Yb, Lu and Y from the lighter La, Gd and Tb elements.16 Xu et al. also achieved promising results using the same IL mixed with a trialkyl phosphine oxide mixture (TRPO), resulting in good selectivity and extraction efficiencies for some of the heavy REEs including Yb and Lu which achieved extraction efficiencies greater than 96%.17 These results, and many more, show the potential ILs have in REE separations, although difficulty is often faced with the separation of neighbouring REEs.
ILs can be designed and created for specific applications through the careful selection and functionalisation of the ions present in the IL.9.
While liquid-liquid extractions appear to be the most common method for REE separations using ILs, the immobilisation of ILs on solid supports to create supported ionic liquids (SILs) is also popular in separations. SILs offer some advantages over those used in liquid-liquid extractions. For example, SILs often maintain the properties of the unsupported IL which are important for separations, while also reducing the leaching of the IL into the aqueous phase and requiring smaller volumes of IL to complete extractions.18 In some cases, the addition of a support has been reported to improve metal separation.19
Common supports used to create SILs include polymer supports, silica supports and metal organic frameworks, with different separation methods applied. In a column chromatographic method aiming to separate the REEs from other metals present in NdFeB and SmCo magnets, Avdibegovic and Binnesmans used a SIL phase, betainium sulfonyl(tri-fluoromethanesulfonylimide) poly(styrene-co-divinylbenzene).20 This method achieved extraction recoveries of 82% and 90% for Nd and Sm respectively, as well as high selectivities towards the trivalent lanthanide elements over the divalent iron and copper also present in the spent magnet leachate.
Biosorbents
Biosorbents offer another opportunity for green separations of the REEs. These are naturally occurring sorbents, which are derived from a range of natural sources including plants, fungi, algae and animals.21 Typically considered for the removal of heavy metals and other contaminants from wastewater, biosorption refers to processes which remove metal ions through passive binding to biomass materials in aqueous solutions.22 With the trivalent lanthanides generally behaving as strong Lewis acids, they are found to bind to groups with basic properties. For example, Ln(III) ions can strongly bind to the electron donating oxygen groups present in many biosorbents, and may bind weakly to P and S ion donor groups.21
In recent years, biosorption methods have been developed for the separation of the REEs from nuclear and electronic waste. These methods offer advantages of being affordable, simple and environmentally friendly, and often result in high extraction efficiencies and ease of recycling.21 These sorbents are also generally widely available and can be considered green due to their natural origin, as well as largely being biodegradable. Furthermore, much like ionic liquids, biosorbents can also be immobilised to offer similar advantages to SILs.
Extraction and separation using biosorbents typically involve a variety of processes including absorption, adsorption, ion exchange, complexation, chelation, electrostatic interactions and precipitation.22 These occur between the REE ion in solution, and the functional groups found bonded to the cell walls of bio sorbents. In fact, one of the advantages of the use of biosorbents for separations are the functional groups which are naturally found bonded to cell walls, some of which are shown in Fig. 2.21
One of the drawbacks often observed in biosorbents for metal separations is their low selectivity towards metals, owing to the range of functional groups which are capable of extracting a large range of metals.21 However, similar to ILs, biosorbents are able to be modified and functionalised to improve their separation ability.23 In some cases, modification is required to stabilise the biosorbent to ensure it can withstand the conditions used throughout separation experiments. In other cases, biosorbents are modified to improve separation performance and ease of removal following the extraction step.
These methods offer advantages of being affordable, simple and environmentally friendly, and often result in high extraction efficiencies and ease of recycling.
Many examples of biosorbent modification can be found, particularly those involving the modification of chitosan. Chitosan (poly-β-(1→4)-2-amino-2-deoxy-d-glucose) is a common biosorbent as it is abundant, inexpensive, biodegradable, and has low toxicity.23 Due to the nitrogen and hydroxyl groups found in the structure, it is also a good contender for the separation of heavy metals, including the REEs, as it acts an effective chelating agent. However, modification is often required to improve the stability of chitosan in acidic solutions.
Further chemical and physical modifications can also be carried out to achieve better separations and improve selectivity. For example, Roosen and Binnemans created functionalised chitosan polymers with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA).24 The goal of this functionalisation was to decrease the solubility of the chitosan in acidic aqueous solution, as well as improve separation performance, as both EDTA and DTPA have shown good binding abilities with the lanthanide elements. The structures of the EDTA- and DTPA- chitosan are shown in Fig. 3.
While methods of SCF extractions are showing promising results, it appears there is still work to be done to improve their use in REE separations.
Other methods of REE separations using biosorbents have also shown promising results. For example, Dong et al. tested the performance of spores of Bacillus subtilis PS533 and PS4150 for the removal of low concentrations of Tb(III) from wastewater.25 Results showed 94% Tb(III) removed within 30 minutes using PS4150 spores with the amino, hydroxyl, methyl and phosphate functional groups all playing a role in the extraction. High adsorptions of other lanthanide elements were also observed, indicating the potential of the PS4150 spores for the wider applications of rare earth extractions.
In another application of biosorbents for REE separation, Mohammadi et al. tested the biomass, Magnetospirillum magneticum AMB-1, for the separation of La(III) from aqueous solution.26 This bacterium naturally possesses magnetic qualities enabling easy removal of the biosorbent following extraction using an external magnet. This biosorbent achieved adsorption efficiencies as high as 98.7% for Sc, with over 90% of all other REEs adsorbed. While biosorbents appear to show promising results for REE separations, the main drawback appears to be their low selectivity observed in many applications including the examples discussed here.
Supercritical fluids
Supercritical fluid (SCF) extraction utilises fluids which have reached supercritical temperature, that is to say, they have been heated and compressed above their critical temperature and pressure to a point where they possess both liquid- and gas-like properties.27 The use of supercritical fluids for the separation of REEs has been offered as another alternative greener method to traditional separation methods. SCFs offer many advantages including their low viscosity, fast mass transfer and ease of removal of separated elements from the SCF following extraction.28 These properties can help increase the rate of extraction compared to alternative liquid extractants. Carbon dioxide (CO2) is one of the most used SCFs as it is known to be inert, inexpensive, renewable and abundant. Furthermore, it does not require extreme temperature and pressure to reach critical conditions (Tc = 31℃, Pc = 7.38 MPa).29
Due to its structure, supercritical CO2 alone is a poor extractant for polar compounds and metal ions, including the REEs. However, the addition of CO2-soluble complex-forming agents, such as tributyl phosphate (TBP) in nitric acid, can produce good extractions for the REEs as well as simple back-extraction and recyclability of the CO2.29
Much like ionic liquids made for REE separations, SCF extractions often include the addition of common REE organophosphorus extractants to improve separations, with tributyl phosphate being one of the most widely reported. In many methods involving the use of sc-CO2, extraction occurs through the mixing of the sc-CO2 complex with lanthanide oxides. The dissolving of the lanthanide oxides and extraction of the lanthanides into the sc-CO2-TBP phase in nitric acid can be described by Eq. 1 and 2 whereby the Ln3+ ions are extracted through the formation of a TBP complex as described by Yao et al.30:
Following extraction, the removal of the hydrophobic lanthanide complex formed with TBP can be simply achieved through the gasification of CO2 at atmospheric pressure which results in the newly formed complex being precipitated out. Following this step, the CO2 can be easily recycled for further extraction cycles. Methods like this have achieved promising results, with Shimuzu et al. reporting extraction efficiencies as high as 99% for the separation of Y and Eu from waste fluorescent lamps.29 However, in a more recent study considering the recycling of REEs from fluorescent lamps, Zhang et al. discovered that these results may only be viable in synthetic mixtures due to competition between the REEs and Al3+ and Ca2+ ions commonly found in these waste materials.31 Therefore, while methods of SCF extractions are showing promising results, it appears there is still work to be done to improve their use in REE separations.
Conclusions
The rare earth elements are vital components in much of our everyday and green-energy technology, but due to similarities in their physical and chemical properties, the separation of these elements can be challenging to achieve. The development of greener separation methods is important as demand for these elements grow. In this article, green REE separation methods involving the use of ionic liquids, biosorbents and supercritical fluids were briefly discussed to show their potential in the separation of the REEs.
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