Extractant immobilized materials
Extractants immobilized materials (EIMs) are a class of adsorbents in
which the active component or extractant used to bind the target analyte
separating it from its original medium is physically immobilized onto a
solid support (Hidayah & Abidin, 2017). The active component or the
extractant is a compound capable of ionic or complex interaction with
the target metal ions. The active component is attached to the solid
support’s surface by physical adsorption, or it resides in a thin
solvent layer covering the support surface, or it can be part of the
bulk solvent filling the pores of the solid support. In literature,
solvent impregnated resins (SIRs) is commonly used to identify adsorbent
obtained when solvent extractant such as N,N-dimethylformamide (DMF),
N,N,N′,N′‐Tetraoctyl diglycolamide (TODGA), and Cyanex-923 are infused
onto solid support like silica or resin (Sun et al. 2009; Kabay et al.
2010; Zhu and Chen 2011). Another similar term, supported ionic liquid
(SIL), describes adsorbents where ionic liquids are infused into the
solid support (Bao et al., 2016; Zhu et al., 2012).
The separation processes involving EIMs have been termed as supported
liquid extraction (SLE) or solid-supported liquid-liquid extraction
(SLLE) (Florek et al., 2016; Hidayah & Abidin, 2017; Hu et al., 2018).
Supported liquid extraction (SLE) is analogous to traditional
liquid-liquid extraction (LLE), where target species are separated based
on their relative solubilities in two different immiscible liquids,
usually water and an organic solvent containing the extractant but
instead of shaking the two immiscible phases together, the extractant is
supported on a solid support and put in a column (or cartridge). The
REEs containing solution is passed through, REEs ion gets adsorbed, and
then eluent is used to desorb the adsorbed REEs species. In SLE, it is
possible to achieve higher recovery due to the increase in surface area
of interaction between the extractant and the target cations and
increasing the diffusion rate through control of the porous structure of
the support material. Other advantages over traditional LLE are less use
of organic compounds, waste reduction, and lack of third phase formation
(Florek et al., 2016; Hidayah & Abidin, 2017; Kabay et al., 2010).
The synthesis of EIMs (SIRs and SILs) involves modification of already
synthesized polymers or other solid supports like silica or activated
carbon using dry or wet methods. In the dry method, the active component
is impregnated into support in excess using a volatile solvent then the
excess amount is evaporated by heating, thus leaving the active
component physically adsorbed to the support surface. In the wet method,
support is impregnated with a solution of active component in a
non-polar or weakly polar solvent followed by repeated washing with
water, leaving a layer of the non-polar solution of the active component
(Bao et al., 2016; Ehrlich & Lisichkin, 2017). Based on the types of
solid support used for preparing, these adsorbents can be broadly
divided into organic polymer-based support and inorganic supports.
Organic polymer-based
supports
Many researchers have used organic polymers as the solid support for
solvent impregnated resins (SIRs) used for REEs uptake from solution.
Relevant examples of polymer based SIRs include N,N,N′N′
tetraoctyldiglycolamide (TODGA) and N,N,N′N′
tetrakis‐2‐ethylhexyldiglycolamide (TEHDGA) impregnated Amberchrom CG‐71
for REEs separation; tributyl phosphate infused Styrene–divinylbenzene
to copolymer separate Am and Eu (Louis & Duyckaerts, 1984, 1985);
2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC88A) infused
Amberlite XAD-7 for separation of Gd and Y mixtures (WAKUI et al.,
1988); 8-Quinolinol (oxine) and 2-(2-(5
chloropyridylazo)-5-dimethylamino)-phenol (5ClDMPAP) immobilized
Amberlite XAD-4 and XAD-7 each for preconcentration of Ce, La, and Pr
(Masi & Olsina, 1993); Amberlite XAD-7 impregnated with binary mix of
cobalt dicarbollide and dibutyl-N,N-diethylcarbamoylmethyl phosphonate
(DBDECMP) for Eu sorption (Svoboda et al., 1997); bis(2,4,4-trimethyl
pentyl) phosphinic acid (Cyanex 272) (Wang et al., 1998),
1-hexyl-4-ethyloctyl hydrogen isopropylphosphonic acid (HEOPA) (Wang et
al., 2002) , and bis(2,4,4-trimethylpentyl) monothiophosphinic acid
(CL302, HL) (Jia et al., 2004) infused resin for HREEs separation.
Additional examples include PC88A infused styrene–divinyl benzene
copolymer for Gd(III) and Tb(III) chromatographic separation (PARK et
al., 2005), Octacarboxymethyl-C-methylcalix 4-resorcinarene impregnated
Amberlite XAD-16 for La(III), Ce(III), and Y(III) preconcentration (Gok
et al., 2007); XAD-4 infused with tri-n-octylmethylammonium chloride
(Aliquat-336) in benzene for Ln(III) and Gd(III) (Elsofany, 2008);
microporous polymer infused with phosphorus podands bearing two
Ph2P(O)CH2C(O)NH for REEs (Turanov et
al., 2008); Cyanex272- 2-ethylhexyl phosphoric acid-2-ethylhexyl ester
(P507) impregnated resin for HREEs (Liao et al., 2010); TOPS-99 infused
Amberlite XAD-4 to extracted Tb (Kumar et al., 2011),
β-glycerophosphate impregnated Amberlite XAD 7 for La(III) (Gabor et
al., 2016); di-(2-ethylhexyl)phosphinic acid (P227) impregnated XAD-7HP
for HREEs (Yang et al., 2020); and HDEHP impregnated XAD-7 for REEs
(Sert et al., 2021).
Lee et al. (2009) used trialkylphosphine-based (Cyanex 923), PC88A, and
HDEHP for selective separation of La from Ce, Pr, Nd, and Sm, as well as
from other concomitant metals in chloride medium. HDEHP provided the
best separation with separation factors of 2.57, 3.63, 4.72, and 32.06
for Ce, Pr, Nd, and Sm over La (Lee et al., 2009, 2010b). The same
adsorbent was used, followed by anion exchange and oxalate precipitation
to obtain La2O3 of more than 99.9998%
purity (Lee et al., 2010a). Amberlite XAD-7 coated with
β-glycerophosphate had qm of 33.8 mg/g for La (Gabor et
al., 2016). Mondal et al. (2019) used N,
N,N′,N′-tetrakis-2-ethylhexyldiglycolamide (TEHDGA) impregnated XAD-7
resin to selectively adsorb REEs from coal fly ash solution containing
Fe, Ca, Al, Mg, and Si with Kd of 200-520 ml/g for REEs
and 0.2-2.9 ml/g for impurities. Cyanex 272 impregnated Amberlite XAD-7
resin showed distribution coefficient in order of
Gd > Eu > Sm > Nd > Pr > La
with Kd value for Gd and Eu as 208.1 and 156.4 ml/g
(İnan et al., 2018). The SIR P227 infused XAD-7HP showed selectivity
series of
Fe > Lu > Tm > Zn > Mg > Ca > Ho > Co > Ni > Cu > Al
(B. Yang et al., 2020). Another adsorbent, HDEHP-XAD-7 showed
selectivity series Gd > Eu > Sm >
Nd > Pr > La with separation factors achieved
Gd/La separation factors of >140 (Sert et al., 2021).
The examples SILs with organic polymer include
1-octyl-3-methylimidazolium hexafluorophosphate
(C8mim+PF6-)
containing Cyanex923 immobilized on XAD-7 for REE extraction (Sun et
al., 2008), trialkylmethylammonium sec-nonylphenoxyacetate
([A336][CA-100]) impregnated on Amberlite XAD-7 (Sun et al.,
2009), Trihexyl(tetradecyl) phosphonium mono-(2-ethylhexyl) 2-ethylhexyl
phosphonate ([P66614][EHEHP]) and
trioctylmethylammonium bis(2,4,4-trimethylpentyl) phosphonate
([N1888][BTMPP]) impregnated Amberlite XAD-7
(Zhao et al., 2016), betainium sulfonyl(trifluoromethanesulfonylimide)
poly(styrene-co-divinylbenzene) [Hbet–STFSI–PS–DVB] impregnated
Amberlite XAD-16 (Avdibegović et al., 2017),
1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide
([Bmim][NTf2]) combined with N,N-dioctyldiglycolamic acid
(DODGAA) and impregnated into Amberlite XAD7HP (Friend et al., 2020),
and trihexyl (tetradecyl) phosphine bis (2,4,
4-trimethyl-amyl)–phosphonate (Cyphos IL 104) impregnated polymer
membrane (Wang et al., 2020).
Inorganic supports
Inorganic supports mostly include silica-based polymers. In some cases,
ordered mesoporous silica and carbon nanotubes have been used.
Extractant bis(2- ethylhexyl)
hydrogen phosphate (HDEHP) coated C18- hydrophobized silica was used for
chromatographic separation of lanthanides (Sivaraman et al., 2002);
2-ethylhexylphosphonic acid mono-2-ethylhexyl ester impregnated
silica-based urea–formaldehyde (SiO2/UF) for Eu(III)
and Nd(III) (Naser et al., 2015);
HDEHP impregnated Polyethersulfone
(PES) embedded with polyvinyl alcohol (PVA) and multiwalled carbon
nanotubes (MWCNT) for Y(III) sorption (Yadav et al., 2015); and
tetraoctyldiglycolamide impregnated Carbon inverse opals (C-IOP) for
REEs (Turanov et al., 2015).
Cyanex 272 impregnated mesoporous silica MCM 41 was used for the
extraction of lanthanides. The selectivity between light lanthanide
(La(III)), medium lanthanide (Eu(III)), and heavy lanthanide (Lu(III))
followed the order of Lu(III) > Eu(III) >
La(III) with separation factor of 61, 61 and 3504 for Lu(III)/Eu(III),
Eu(III)/La(III) and Lu(III)/La(III), respectively (Mohammedi et al.,
2019). Silica gel modified with 1-(-2-pyridylazo) naphthol (PAN) and
acetylacetone (Acac) through solvent evaporation process was selective
for Sc(III) against Fe(III) with selectivity factors of 5.79 and 8.95,
respectively at pH 6 (Ramasamy, Puhakka, Repo, & Sillanpää, 2018).
Magnetite nanoparticle was used as a support for coating
di(2-ethylhexyl)phosphonate (HDEHP),
bis(2,4,4-trimethylpentyl)phosphinic acid (CYANEX 272), and
bis(2,4,4-trimethylpentyl)dithiophosphinic acid (CYANEX 301) (Molina et
al., 2019). These coated magnetite nanoparticles had qmof 6.6-8.3 mg/g for La(III), 7.7-8.7 mg/g for Pr(III), and 4.8-8.9 mg/g
for Nd(III). The same extractant, HDEHP was infused into
styrene-divinylbenzene copolymer immobilized silica particles
(HDEHP-SiO2-P) for REEs adsorption (Shu et al., 2017,
2018; Zhang et al., 2019). The adsorption capacity (qm)
of HDEHP-SiO2-P was 38.95 mg/g for Ce(III) and 52.84
mg/g for Gd(III) (Shu et al., 2017). In a different study, the
HDEHP-SiO2-P had qm of 14.1 mg/g for
Sc(III) with Sc(III) selectivity factor of >50 over other
REEs (Zhang et al., 2019).
Inorganic solid supports infused with ionic liquids are mesoporous
silica doped with binary IL mixtures (C8mim+PF6-/Cyphos IL 104 or
C4mim+PF6-/Cyphos IL 104) (Liu et al., 2009), silica gel impregnated
with N-PhenacylPyrNTf2 ionic liquid (Marwani & Alsafrani, 2013), silica
doped with bifunctional ionic liquid trioctylmethylammonium
1-phenyl-3-methyl-4-benzoylpyrazol-5-onate ([A336]+[L]−)
(Turanov et al., 2016), and MCM-41 silica impregnated with
[Hbet–STFSI–PS–DVB] (Avdibegović et al., 2017). Among the SILs,
trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate
(Cyphos) entrapped in silica reached maximum adsorption capacities of
14.7 mg/g for Nd(III) and 19.8 mg/g at pH 4.0 (Mohamed et al., 2017).
Ionic liquid (IL), 1-butyl-3-methylimidazolium bromide
([C4mim]+[Br]−) impregnated
metal-organic framework (MOF) UiO-66 had a maximum Gd(III) adsorption
capacity of 65 mg/g at pH 6 with Gd(III) selectivity in the presence of
other metal ions (Na(I), Ca(II), Ma(II), Al(III), and Fe(III)) (Ahmed,
Adhikary, et al., 2019).
The main limitation of SLEs is the gradual washout of the active
component during operation (Bao et al., 2016). Possible solutions
include the selection of highly hydrophobic and low soluble active
components (Muraviev et al., 1998; Sparfel & Cote, 2004) and/or
thorough washing of weakly adsorbed active components after their
application onto the solid surface (Muraviev, 1998; Muraviev et al.,
1998). Another method to prevent the leaking of the extractant from
adsorbent grain is to coat a layer of polymer on the adsorbent, which
acts as a semipermeable membrane or protective barrier for extractant
while allowing the metal ion diffusion (Bao et al., 2016; Muraviev et
al., 1998; Nishihama et al., 2013; Trochimczuk et al., 2004).
A protective barrier around the SIR was formed using polysulfone in
dimethylformamide (DMF); however, loss of the adsorbed extractant
[bis(2-ethylhexyl) hydrogen phosphate, bis- (2-ethylhexyl) hydrogen
dithiophosphate, or bis(3- propylphenyl) dithiophosphate] still
occurred (Muraviev et al., 1998). Better results were obtained with the
coating of water-soluble polymer poly(vinyl alcohol) (PVA) precipitate
onto SIR (Trochimczuk et al., 2004). The stability of the coating layer
can be increased by cross-linking PVA with vinyl sulfone (Trochimczuk et
al. 2004), boric acid (Yuan et al., 2010), and glutaraldehyde (Nishihama
et al., 2013).
In general, EIMs (SIRs and SILs) can improve the performance of
extractants by avoiding drawbacks of LLE, such as poor contact between
the organic and aqueous phase, requirement of a large amount of
extractant, emulsion formation, and formation of the third phase. The
issue with SLEs is the stability of the active layer, which can wash out
during operation. Moreover, the extractant layer may also be not uniform
throughout the support surface, e.g., capillary forces may accumulate
the extractant in narrow pores, a process enhanced in contact with an
aqueous solution (Shenxu Bao et al., 2016). In addition, the complex
nature of real-life REEs sources such as very acidic solutions can put
extra pressure on these adsorbents, which can limit their industrial
applications.
The adsorbents obtained through ligand functionalization to the surface
of solid support using coupling agents provide an alternative to EIMs.
The surface-functionalized adsorbents can be an alternative with better
control of ligand distribution on the surface and better ligand and
surface bonding and thus better stability, but however, they can be more
resource intensive to produce.