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).