Ion-exchange resins

Ion-exchange in the context of solid-phase extraction involves a reversible interchange of ions between a solid phase (or ion-exchanger) and the solution through electrostatic sorption and desorption to and from the ion-exchange material. The ion-exchangers are classified as cation or anion-exchanger based on the mobile exchangeable ions (i.e., counter-ions). Acidic cation-exchangers, introduced in 1944 by cross-linking polystyrene with 6-8% divinylbenzene, have cations as counterions and attract and bind cations, while anion exchangers, introduced later in 1948, can attract and bind anions from solution. Based on the physical form of the ion-exchanger, ion-exchangers can be classified as membranes, fibers, hydrogels, or resins. Ion-exchange resins (IERs), which represent the major class of ion-exchangers, are insoluble polymers with active-ion groups, commercially available as spherical beads or particles, and have been used to separate REEs mainly in chromatographic setup (Ehrlich & Lisichkin, 2017).

Cation-exchange resins

Strongly acidic cation-exchange resins: Strongly acidic cation-exchange resins are formed by copolymerization of polystyrene and divinylbenzene and have sulfonic (SO3-) groups introduced to the benzene rings. There are many sulfonic (SO3-) group-based resins available under various trade names (e.g., Amberlite IR-120, Dowex 50WX), having different grain sizes, pore diameters, sorption capacities, degrees of cross-linking, and other characteristics (Ehrlich & Lisichkin, 2017). Sulfonic acid-based cation-exchangers have been used to separate REEs but are not very selective for a specific REE over the other.
In cation-exchange resins (CERs), the strength of cation binding depends on the charge and radius of the hydrated ion (Harris & Lucy, 2015). The electrochemical attraction increases with decreasing ionic radius for ions with the same charge. Within Ln(III), the ionic radius decreases with the increasing atomic number from La(III) (ionic radius: 103 pm) to Lu(III) (ionic radius: 86.1 pm), and thus the binding strength to cation-exchanger increases with atomic number. However, the use of strongly acidic cation-exchange resins for the separation of individual REEs in mineral acids, such as HCl, H2SO4, and HNO3 is not selective due to similar ionic radius of adjacent Ln(III) (Boyd, 1978; Strelow, 1960; Strelow et al., 1965; Strelow & Bothma, 1964). The similar ionic radius of adjacent Ln(III) results in similar electrostatic attraction as evident by similar equilibrium constants for an ion-exchange reaction involving a strongly acidic cation-exchanger (3a). The ratio of the ion-exchange equilibrium constants of adjacent Ln(III) for reaction similar to (3a) does not exceed 1.1 (Boyd, 1978).
Ce3+ + LnR3 = CeR3 + Ln3+ (3a)
This limits the use of strongly acidic cation-exchange resins in individual REE separation from a solution containing a mix of different REEs. Still, these resins can be used for the separation of Ln(III) from other cations of lower or higher ionic charge (Fritz & Garralda, 1963; Korkisch et al., 1967; Page et al., 2019; Strelow, 1960; Strelow et al., 1965).
The separation of individual REEs increased with the use of complexing ligands as eluents for strongly acidic cation-exchanger (Boyd, 1978; B. Chen et al., 2017; Spedding et al., 1947, 1954; Strelow & Victor, 1990). The REE ions form complexes with the ligand. These REE-ligand complexes have less affinity for the exchanger than corresponding REE ions, which results in easier desorption of REE-ligand complex and a difference in relative REE desorption based on the stability of individual REE-ligand complex. With the same ligand, the relative concentration of different REE-ligand complexes depends on the ratio of their REE-ligand complex’s stability constants. Since the ratio of stability constants of REE-ligand complexes (Karraker, 1961; Mackey et al., 1960; Schoeb, 1965; Suzuki et al., 1980; Wheelwright et al., 1953; Wood, 1993) of neighbor REEs is significantly higher than the ratio of ion-exchange constants (Boyd, 1978), better separation between REE is obtained during elution (Chen et al., 2017). The stability of REE-ligand complexes increases with the increasing atomic number for most complexing ligands, which leads to an order of elution from Lu(III) to Ln(III)) with complexing ligand as eluent (Karraker, 1961; Mackey et al., 1960; Schoeb, 1965; Suzuki et al., 1980; Wheelwright et al., 1953; Wood, 1993) The different complexing ligands used as eluent for REEs chromatographic separation using strong acidic cation-exchangers are discussed later in the chapter (section 8.2).
The sulfonic cation-exchangers with complexing ligand eluents have been mainly used in the chromatographic separation of REEs (Ehrlich & Lisichkin, 2017). However, alternatively, in a batch adsorption process, Khawassek et al. (2019) used macroreticular sulfonic acid resin Dowex 50X8 to adsorb REEs from effluents solution collected from the hydrometallurgical process for El-Erediya mineralization. The maximum total REE uptake from leachate solution was 80 mg REEs/g. Similarly, strong acid resin Amberlite 200C Na and Amberlite 200C H showed high sorption efficiency (more than 99.8%) for La(III), Ce(III), and Nd(III) (Kołodyńska et al., 2019). Page et al. (2019) used strong acidic cation exchange resin, Lewatit MonoPlus SP 112, for La(III), Sm(III), and Er(III) adsorption in sulfate and chloride medium in the presence of competitive ions, such as Fe(III) and Al(III), and reported maximum adsorption capacity of 42 mg/g for Sm(III). The equilibrium constants for the ion-exchange reactions and the REEs selectivities were higher in the chloride medium than the sulfate medium. The ion-exchange equilibrium constants for competitive ions, Fe(III) and Al(III), were significantly lower in both media. A macro-porous strongly acidic CER, SQS-6, adsorbed 97.6% La(III) and 90.36% Nd(III) in 4.0 M H3PO4 media with equilibrium capacities of 13.8 and 12.70 mg/g (in 8 M H3PO4), respectively (Abu Elgoud et al., 2019).
Carboxylic acid-based cation-exchange resins: Carboxylic acid-based cation-exchange resins can form complexes with REEs in addition to electrostatic interaction (Arnold & Hing, 1967; Kazantsev et al., 1974). The carboxylic functionalized cation-exchangers involve copolymerization of methacrylic acid or acrylic, or acrylonitrile, generally, with divinylbenzene. The type of carboxylic group on cation-exchanger affects the adsorption of REEs; for example, polyacrylic acid resin (Amberlite XE 89) showed lower selectivity for lanthanide than the polymethacrylic acid resin (Amberlite IRC 50) (Arnold & Hing, 1967).
Many macro-porous carboxylic cation-exchange resins such as D113, D155, D152, D151, and SQD-85 have been used for REEs adsorption in acetate medium. The macro-porous carboxylic acid resin D113 was used for adsorption of La (III) (qm = 273.3 mg/g (Shu et al., 2007)), Dy(III) (qm = 292.7 mg/g (Wang & Gao, 2007)), Nd(III) (qm = 232.56 mg/g (Xiong et al., 2011)) and Er(III) (qm = 250 mg/g (Xiong et al., 2009)). The separation coefficients for La (III) adsorption onto D113 were 2.29, 3.64, 4.27, and 0.627 against Ce(III), Gd(III), Er(III), and Y(III), respectively (Shu et al., 2007). With another carboxylic acid resin D152, the maximum adsorption capacities at pH 6.70 for different REEs were between 238 mg/g (for Er(III)) and 510 mg/g (for Sm (III)) (Xiong et al., 2008). Similarly, D155 was used for Ce(III) (qm= 294 mg/g) and Gd(III) (qm = 283 mg/g) (Xiong, 2008), SQD-85 for Yb(III) (qm = 347.6 mg/g) (Xiong et al., 2011), D151 for Ce(III) (qm = 392 mg/g) (Yao, 2010), and a gel-type weak acid resin (110) for Yb(III) (qm = 265.8 mg/g) (Zheng & Xiong, 2011). Commercial resin Amberlite IRC86 (functional group: carboxylic acid) and Purolite S910 (functional group = amidoxime) were used in various buffer solutions (i.e., malic acid, formic acid, acetic acid, alanine, and lactic acid) for adsorption of REE in the presence of competitive Fe(III) and Al(III) ions (Bezzina et al., 2018). The use of a carboxylate buffered system allowed easy separation of REE from Fe(III) and Al(III) ions. The carboxylic resin IRC86 showed higher capacity for HREEs in acetic acid media at pH ∼4.4, showing preference order of Y(III) > Er(III) > Sm(III) > La(III).
The limitation of using carboxylic cation-exchange resins is that these resins operate in the weak acidic pH range (pH>3.5), posing additional challenges due to hydrolysis of REE ions and occurrence of other competitive cations such as Fe(III) (Ehrlich & Lisichkin, 2017; Kazantsev et al., 1974; Wood, 1993).
P-containing cation-exchange resins: The phosphorus-based ligand containing resins are considered part of the family of the weakly acidic cation-exchangers. Among different phosphorus-containing functional groups, phosphinic showed higher selectivity than phosphonic, with both showing higher selectivity than their respective oxidized derivatives (Egawa et al., 1994). Their Ln(III) distribution coefficient increased with increasing atomic number in general but with a plateau from Sm(III) to Ho(III). Macroreticular methylenephosphonic acid resin showed affinity series of Fe (III) ~ U(VI) ~ Mo(VI) > Bi(III) > AI(III) > Gd(III) > La(III) ~ V(V) > Pb(II) > Cd(II) > Cu(II) ≈ Ca(II) ≈ Ba(II) ≈ Zn(II) > Mg(II) ≈ Co(II) ≈ Ni(II) (Jyo et al., 1997); whereas macroreticular phosphonic acid resin had affinity series of Mo(VI) ≈ Fe(III) ≈ U(VI) > Bi(III) ≈ Lu(III) > Al(III) ≈ Gd(III) > La(III) > Cr(III) > Pb(II) > Mn(II) ≈ Cd(II) ≈ Cu(II) > Ca(II) ≈ Co(II) ≈ Zn(II) ≈ Ba(II) ≈ Sr(II) > Ni(II) ≈ Mg(II) (Ihara et al., 2001).
Many different P-based cation-exchange resins used for REE adsorption involve different side-group with phosphorous. The P-based resin containing P(R)(O)OH (R = Et, Bu) and –P(O)(OR)OH (R = Et, Pr, Bu) were used for chromatographic separation of La, Pm, and Eu with increasing Kd values with increase in atomic number (Miklishanskii et al., 1968). Among different adsorbents with phosphorus-containing ligands, the strength of metal ion binding is affected by electron density on the oxygen atom, the spacer connecting the phosphorous-containing ligand to the polymer matrix, and the hydrophilicity of its environment (Spiro D. Alexandratos & Hussain, 1998; Spiro D. Alexandratos & Zhu, 2005, 2008, 2015; Andrzej W. Trochimczuk & Alexandratos, 1994; X. Zhu & Alexandratos, 2014, 2015) The affinity for REEs increased with the introduction of hydroxyl or polyol into the spacer due to their stronger interaction with REEs (Alexandratos & Zhu, 2008).
Adsorbents containing di- and polyfunctional groups in addition to P-based groups have been studied for REE separation (Alexandratos & Natesan, 1999; Alexandratos & Hussain, 1995; Alexandratos & Smith, 2004b; Shumilova et al., 2012; Trochimczuk, 2000). The addition of a second functional group can enhance the behavior of the adsorbent, as in phosphorylated adsorbents, where the introduction of highly hydrophilic sulfonic acid eliminated intra-ligand hydrogen bonding, which reduced intra-ligand cooperation, and increased the accessibility of P-containing groups (Alexandratos & Smith, 2004b). Additionally, the sulfonic acid itself has an affinity for REE, which can increase REE adsorption (Ehrlich & Lisichkin, 2017). Bonding sulfonic acid groups to the phosphinated polystyrene matrix increased the Eu (III) distribution coefficient from 61.8 to 220.5 in a 1 N HNO3 solution (Alexandratos & Hussain, 1995). Hérès et al. (2018) used Monophos and Diphonix resin, which had sulfonic groups together with alkylphosphonic groups for adsorption of REEs. The extraction efficiency of Monophos and Diphonix resin increased with increasing ionic radius of REEs.

Anion-exchange resins

REE ions do not form anions with inorganic ligands thus are poorly adsorbed on the anion-exchange resins (Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012; J Minczewski et al., 1982). Still, Th(IV) and U(VI) can be separated from solution containing REEs using anion-exchange resins (AERs) as they form anion complexes with nitrate and phosphate (Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012). Among REEs, cerium (IV) can form negatively charged complexes with nitrate and can be adsorbed to the AERs, e.g., anion-exchanger containing N-methyl-imidazolium group (Zhu & Chen, 2011).
Anionic REE-ligand complexes can be formed by adding organic compounds (e.g., methanol (Faris & Warton, 1962), ethanol (Hubicki & Olszak, 1998), and propanol (Hubicki & Olszak, 2002)) to nitric acid. These anionic complexes can be separated using a strongly basic anion-exchange resin. Anionic REE-ligand complex electrostatically binds to the positive anion-exchanger. Similarly, complexing ligands such as ethylenediaminetetraacetic acid (EDTA) and cyclohexane-1,2-diaminetetraacetic acid (CDTA) were used to form anionic REE-ligand complex for REEs separation using anion-exchange resins (Hubicka & Hubicki, 1986; Hubicka & Kołodyńska, 2004, 2008; Jerzy Minczewski & Dybczyński, 1962; Wódkiewicz & Dybczyński, 1968). Binary mixtures of Y-Nd, Sm-Tb, La-Nd, and Dy-Er were separated using frontal chromatography with EDTA as eluent using different Dowex anion-exchangers (Hubicka and Hubicki 1986). Strongly basic anion resin Dowex 1 and Dowex 2 were used to separate REE complex with N-hydroxyethylenediaminetriacetic acid (HEDTA) and with iminodiacetic acid (IDA) (Hubicka & Drobek, 1997, 1998, 1999). Similarly, REEs complexes with DCTA of the Ln(dcta)- type were chromatographically separated using different polyacrylate anion-exchangers (Amberlite IRA 68, Amberlite IRA 458, and Amberlite IRA 958) and different polystyrene anion-exchangers (Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 64, Lewatit MonoPlus MP 500, and Lewatit MP 62) (Hubicka & Kołodyńska, 2004, 2008).

Chelating ion-exchange resins

Chelating ion-exchange resins are equipped with ligands that form complexes with metal ions through their functional groups (Garg et al., 1999). Chelating ion-exchange resins used for REE separation contain iminodiacetate, aminomethylphosphonic, hydroxamine, and diamide (Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012). Many commercially available iminodiacetate based resins such as Amberlite IRC 718, Chelex 100, Dowex A-1 (Mathur & Khopkar, 1985; Schrobilgen & Lang, 1968), Duolite ES466, Lewatit TP 207 (Junior et al., 2021; Niu et al., 2021), and Purolite S930 Plus (Page et al., 2019) have been used for REE separation (Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012).
Iminodiacetic acid resins are also characterized by a very high selectivity for REE over the alkali and alkaline earth metals, often present in high concentrations in alternative REE resources (Ehrlich & Lisichkin, 2017; Koodynska & Hubicki, 2012; Page et al., 2017, 2019). An iminodiacetic acid resins, Purolite S930Plus, showed higher selectivity for REEs in the presence of high Na and Ca concentration than the sulfonic acid resin (SA), sulfonic-phosphonic resin (Purolite S957), and aminophosphonic resins (Purolite S950) (Page et al., 2017). However, the SA resin, Purolite S957, and Purolite S950 were tolerant towards REEs adsorption from acid solutions up to 0.5 M [H+], whereas iminodiacetic acid resins effectively adsorbed REE only at [H+] < 0.001 M. For the sulfonic acid resin, the selectivity was REE ≃ Th > Fe ≃ Al whereas for other resins it was Th ≃ Fe ≫ REE ≃ Al. In another study with Purolite S930Plus, the resin selectively adsorbed La(III) over Mg(II) with a selectivity factor of 3046 in 1 M MgCl2, but the selectivity factor decreased to 453 and 240 in 0.5 M MgCl2 + 0.5 M MgSO4 and 1 M MgSO4 medium, respectively (Page et al., 2019). Another iminodiacetic acid-based resin, TP207, showed adsorption capacity order of La(III) > Eu(III) > Y(III), with capacity of 0.73 mmol/g (or 101.5 mg/g) for La(III) at pH 4.0 (Niu et al., 2021).
The REEs extraction efficiency using different aminophosphonic resins (Tulsion CH-93, Purolite S940, Amberlite IRC-747, Lewatit TP-260) decreased with an increase in Z/IR (atomic number/ionic radius) from Sc up to a plateau of La, Nd, and Gd, with an increase and increased after that with Z/IR from Dy to Yb (Hérès et al., 2018). Concurrently, the extraction efficiency decreased with an increase in ionic radius (IR) in general, with a slight spike at Nd (III) and Pr(III). Another chelating ligand diglycolamic acid-containing polymeric adsorbent showed adsorption capacity (qm) of 0.113 mmol Dy/g (or 18.3 mg/g) and selectively adsorbed REEs at low pH values (Shinozaki et al., 2018). Other chelating ligands ethylenetriaminepentaacetic dianhydride (DTPADA), phos-phonoacetic acid (PAA), and N,N-bis(phosphonomethyl)glycine (BPG) containing polymers had maximum adsorption capacities of 2.9, 5.0, 3.0 mg/g, and respectively for a mixture of three REEs (Nd(III), Gd(III), and Ho(III) (Callura et al., 2019, 2021). N-based chelating resin, Dowex M 4195 with bis(2-pyridylmethyl) amine showed very low sorption (maximum 3.1%) for La(III), Ce(III), Nd(III) (Kołodyńska et al., 2019).