Surface-functionalized
adsorbents
In this class of adsorbents, the active component or the extractant is
chemically immobilized onto the surface of the solid support through
chemical bonds. The active component is usually a complexing ligand
containing specific functional groups that can selectively bind to the
target REE. These adsorbents have been used for solid-phase extraction
(SPE) of various elements. Solid-phase extraction (SPE) is similar in
the process as the solid-liquid extraction (SLE); however, the
extractant or the ligand is chemically immobilized onto the surface in
SPE as opposed to physical immobilization in the case of SLEs (Florek et
al., 2016; Hidayah & Abidin, 2017; Hu et al., 2018).
A typical ligand functionalized adsorbent involves solid support (e.g.,
silica, polymer beads, or carbon nanomaterials), coupling agent (e.g.,
silane), ligand containing a specific functional group (i.e., binding
unit), and spacer between the coupling agent and the ligand (Pallavicini
et al., 2014). Since the ligand is chemically bound to the surface, it
is more robust than EIMs, provides better control of surface ligand
distribution, and can work under extreme solution conditions. The
surface-functionalized adsorbents used for REEs adsorption can be
classified based on the type of solid support. Silica-based and
carbon-based supports have been the primary focus for solid-phase
extraction (SPE) of REEs; however, magnetic nanomaterials, metal-organic
frameworks (MOFs), and other composite supports are gaining attention
due to their various advantages.
Silica-based supports
Silica-based supports are popular
for functionalized adsorbents used in SPE as they fulfill the
requirements of good solid support: 1) They can have a large specific
surface area (silica gel: usually 100-750 m2/g
(Bhatnagar & Sillanpää, 2010), mesoporous silica >1000
m2/g (Meynen et al., 2009)), 2) are robust and
reusable, 3) are easy to modify with functional groups, 4) have the
possibility of shape control for various uses, and 5) show negligible
swelling (Florek et al., 2016; Hu et al., 2018). Limiting factors
affecting the use of silica as solid support are its tendency to lose
the integrity of its porous structure under strong acidic conditions (El
Mourabit et al., 2012; Florek et al., 2016) and its high dissolution
rate in alkaline solution (Croissant et al., 2017; Crundwell, 2017).
Silica gel is a commonly used silica-based support. The other type of
silica support is nano-porous silica material, further classified as
macroporous (pore size>50 nm), mesoporous (pore size 2-50
nm), microporous (pore size< 2 nm), and hierarchically porous
materials, which contain multiple levels of above porosities (Sun et
al., 2016; Wan & Zhao, 2007). Mesoporous silica nanoparticles are
gaining widespread attention in various fields due to uniform and
tunable pore size, presence of internal and external pores, controlled
morphology, and easy functionalization (Hu et al., 2018; Mehmood et al.,
2017; Narayan et al., 2018).
Silica gel
Silica gel is the porous amorphous form of silicon dioxide
(SiO2), composed of interconnected networks of
microscopic pores. It can have a pore size of 2-25 nm. The surface
silanol groups present in silica gel are used for attaching
organofunctional alkoxysilane through the silanization process. The
organofunctional alkoxysilane is used for attaching functional groups in
a process known as grafting. The most commonly used silane are
amino-propyl alkoxy silanes (3-aminopropyl triethoxysilane (APTES) and
3-aminopropyl trimethoxysilane (APTMS)) as they allow easy attachment of
amino-poly(carboxylic acid) and other functional groups (Noack et al.,
2016; Ramasamy, Khan, et al., 2017). Both the silica gel and silica gel
with attached organo-silanes (e.g., 3-aminopropyl silica gel) are
available commercially under various brands and have been functionalized
further for adsorption of REEs (Asadollahzadeh et al., 2020; Callura et
al., 2018; Noack et al., 2016; Ogata et al., 2014, 2015a; Ramasamy,
Khan, et al., 2017; Ramasamy, Puhakka, et al., 2017).
Instead of surface modification of already prepared silica gel or
3-aminopropyl silica gel, co-condensation of tetra-alkoxy silane
Si(OR)4 (generally, R = Me or Et) with silane coupling
agent (RO)3Si(CH2)3X
(where R = Me or Et, X = complexing ligand group) have been used to
obtain the required adsorbent (El-Nahhal & El-Ashgar, 2007).
Functional ligands grafted on the silica gel for REEs adsorption are
4-acylpyrazolone Schiff base (Amarasekara et al., 2009), diglycolamic
acid (Ogata et al., 2014, 2015a, 2015b, 2016),
1-(-2-pyridylazo) naphthol (PAN)
(Ramasamy, Puhakka, Repo, et al., 2018; Ramasamy, Repo, et al., 2017),
acetylacetone (Ramasamy, Repo, et al., 2017), phosphonoacetic acid
(PAA), N, N-bis(phosphonomethyl) glycine (BPG),
diethylenetriaminepentaacetic dianhydride (DTPADA) (Callura et al.,
2018; Noack et al., 2016), diethylenetriaminepentaacetic acid (DTPA)
(Noack et al., 2016), diglycolamide (Zhe Liu et al., 2019)
N-Benzoyl-N-phenylhydroxylamine (BPHA) (Artiushenko, Ávila, et al.,
2020), amino-di(methylene-phosphonic) acid
(Artiushenko, Kostenko, et al.,
2020), N, N-dioctyldiglycolic acid (DODGA)(Li et al., 2020),
polyhexamethylene guanidine and Arsenazo I or Arsenazo III (Losev et
al., 2020), and bis(ethylhexyl)amido diethylenetriaminepentaacetic acid
(Hovey et al., 2021).
The diglycolamic Acid -functionalized silica gel showed selectivity for
HREEs with the highest adsorption capacity of 0.148 mmol/g (or 24.4
mg/g) for Ho(III) (Ogata et al., 2015a). Another adsorbent
PAN-functionalized silica gel showed capacities of 82.72, 75.5, and
62.92 mg/g for La(III), Sc(III), and Y(III), respectively (Ramasamy,
Puhakka, Repo, et al., 2018; Ramasamy, Repo, et al., 2017). The BPHA
functionalized SiO2 with adsorption capacities
(qm) of 6.7 and 8.3 mg/g for Eu(III) and Tb(III),
respectively, was selective for HREEs with Kd values of
3500 ml/g for Lu(III) and Yb(III) each (legend 21, Figure 2)
(Artiushenko, Ávila, et al., 2020).
Ordered mesoporous silicas
Ordered mesoporous materials have
pore sizes of 2-50 nm with ordered arrangements of pores. Common ordered
mesoporous silica materials are available as groups, defined as MCM
(Mobil Composition of Matter) (e.g., MCM-41 (2d hexagonal, p6m), MCM-48
(cubic Ia3d), MCM-50 (lamellar)) (Beck et al., 1992; Florek et al.,
2016; D. Kumar et al., 2001), SBA (Santa Barbara Amorphous), KIT (Korean
Advanced Institute of Science and Technology), and COK (Centre for
Research Chemistry and Catalysis). Within each mentioned group, there
are different types of OMS with different pore symmetry, pore sizes, and
pore volume (Beck et al., 1992; Florek et al., 2016; Hu et al., 2018;
Huo et al., 1996; Trewyn et al., 2007).
Similar to surface functionalization of silica gel, the silica
mesoporous can be functionalized through the reaction of organo-silanes
with surface silanol group (grafting) or by co-condensation with
tetra-alkoxy silane Si(OR)4 (generally, R = Me or Et)
and tri-alkoxyorganosilanes (R’O)3SiRX (X = ligand) in
the presence of a surfactant template (one-pot synthesis) (Florek et
al., 2016; Hoffmann et al., 2006; Hu et al., 2018).
The abundance of silanol groups on the surface of silica (1-2 Si-OH per
nm2 on average (Ide et al., 2013)) allows for facile
and efficient functionalization through grafting. The limitation of
grafting manifests in the case of small pore size or narrow entrance of
support and/or with a bulky size of the functional group. In such cases,
the entrance may have a higher number of functional groups, or some
smaller pores may lack functional groups due to pore blocking, resulting
in an overall non-homogenous distribution (Hoffmann et al., 2006). The
limitation of pore-blocking is not faced in co-condensation synthesis,
and organic units are more homogeneously distributed than the grafting
process (Hoffmann et al., 2006). However, in co-condensation, the
concentration of ligands in the pore-wall tends to be lesser than the
starting (R’O)3SiRX in the reaction mixture.
Additionally, the degree of mesoscopic order of the OMS decreases in
co-condensation with increasing concentration of
(R’O)3SiRX in the reaction mixture, which leads to
disordered products (Hoffmann et al., 2006). The increasing concertation
of (R’O)3SiRX in the reaction mix favors
homocondensation over co-condensation, thus reducing the homogeneity and
mesoscopic order of the OMS. Another challenge with one-pot syntheses of
OMS is removing the surfactant template after co-condensation without
destroying the surface ligands (Hoffmann et al., 2006).
Mesoporous KIT-6 has been functionalized with ethylenediaminetetraacetic
acid (EDTA) (Ravi, Zhang, et al., 2018), phenylenedioxy diamide (PDDA)
(Hu et al., 2019), bidentate phthaloyl diamide (PA) (Hu et al., 2017),
diglycolamide-based (Florek et al., 2014, 2015, 2020),
furan-2,4-diamido-propyltriethoxysilane (Florek et al., 2015, 2020), and
3,6-dioxaoctanedioic acid (Florek et al., 2015, 2020) for REEs
adsorption. Similarly, MCM-41 has been functionalized with titanium(IV)
alkylphosphate (Wenzhong Zhang, Avdibegović, et al., 2017),
diglycolylamide (Juère et al., 2016), and iminodiacetamide (Fryxell et
al., 2011) and SBA15 has been functionalized with diglycolamide (DGA)
(Juère et al., 2016), benzene-1,3,5-triamido-tetraphosphonic acid
(BTATPA) (Ravi, Lee, et al.,
2018), phosphoric acid (Zheng et
al., 2020), and 1,4-phthaloyl diamido-propyltriethoxysilane (Ryu et al.,
2021) for REEs adsorption. In case of unmodified SBA-15 and KIT-6,
uptake of Sc(III) (KIT-6: qm = 1.0 mg/g (legend 36,
Figure 2), SBA-15: qm = 1.1 mg/g (legend 34, Figure 2))
was achieved through adsorption to accessible surface silanols (Giret et
al., 2018).
Diglycolylamide (DGA) was used as a common ligand for three different
OMS solid supports, SBA-15, MCM-41, and SBA-16, to investigate the
effect of silica support pore network structure (2D vs. 3D), pore shape
(cylindrical vs. cage-like), and pore size on the adsorption of REE from
synthetic samples of REEs and REEs with Al, Fe, Th and U ions (shown as
legend 9-13, Figure 2) (Juère et al., 2016). The 2D hexagonal structure
(SBA-15) has the advantage over the 3D cage-like structure of SBA-16
since the cage-like pore structure and narrow connectivity made SBA-16
more prone to pore blocking. The pore size of SBA-15 was tuned by
varying aging temperatures during synthesis. The highest capacity of
extraction was observed for SBA-15(80)–DGA (pore size, 5.2 nm), which
was attributed to the presence of certain confinement of the targeted
ions by SBA-15 material with a smaller pore (5-8 nm) (Juère et al.,
2016). Functionalization of DGA onto MCM-41 (3.2 nm, 2D pore) resulted
in the obstruction of the entrance of pore during grafting due to
smaller pore size hence had lower extraction capacity (Juère et al.,
2016).
The ligand immobilization on the surface of solid support can lead to
reduced flexibility, which results in a more stable bite angle (angle
formed by chelate ligands) and yields enhanced selectivity towards
particular REE cations (Hu et al., 2018). The ligand DGA was
functionalized onto KIT-6 by anchoring on both ends (KIT-6-N-DGA-1) or
single-end (KIT-6-N-DGA-2) (Florek et al., 2014). KIT-6-N-DGA-1 had a
higher extraction capacity for REEs (especially MREEs) than
KIT-6-N-DGA-2 (ligand 1-4, Figure 2). The higher extraction capacity of
KIT-6-N-DGA-1 was attributed to increased rigidity in the DGA ligand
anchored on the surface at both ends, which increased the size-specific
cavity (Hu et al., 2018). The presence of competitive ions did not
affect Kd values (ligand 3-4, Figure 2). The separation
factor between Eu (III) and Th(IV) was 6.3, and between Eu(III) and
U(IV) was 3.4 for KIT-6-N-DGA-1 (Florek et al., 2014).
The effect of different bite angles of ligands on the selectivity of
REEs was studied using DGA, 3,6-dioxaoctanediamidopropyl (DOODA), and
furan-2,4-diamidopropyl (FDGA) functionalized KIT-6 (Florek et al.,
2015, 2020). The DOODA ligand has a smaller bite angle than DGA (Figure
3); hence showed a preference for smaller Ln(III) (Ho-Lu, legend 4-5,
Figure 2) (Florek et al., 2015; Hu et al., 2018). Hu et al. (2017a)
grafted phthaloyl diamide (PA) bearing different bite angles onto KIT-6
by varying the position (ortho, meta, and para) of the amide groups. The
KIT-6-1,2-PA (ortho-position) with a smaller bite angle showed higher
affinity for heavy Ln(III), which have smaller ionic radii (legend 14,
Figure 2), whereas the KIT-6-1,3-PA showed selectivity for middle-size
Ln(III) due to its larger bite angle (legend 15, Figure 2). KIT-6-1,4-PA
did not show selectivity for a particular lanthanide (legend 16, Figure
2) due to the absence of any synergist action of the moieties (Hu et
al., 2017, 2018). In another study, the bite angle of phenylenedioxy
diamide (PDDA) functionalized modified KIT-6 was varied by using PDDA
with amide at different positions (Hu et al., 2019). . Results analogous
to KIT-6-PA systems were obtained as adsorbent KIT-6-1,2-PDDA having the
smallest bite angle (Figure 3) showed high affinity for heavy
lanthanides (Tm(III), Yb(III), and (Lu(III)) with smallest ionic radii
(legend 22, Figure 2), adsorbent KIT-6-1,3-PDDA with larger bite angle
showed higher selectivity for light lanthanides (La(III), Ce(III), and
Pr(III)) with larger ionic radii (legend 23, Figure 2). KIT-6-1,4 PDDA
did not show selectivity (legend 24, Figure 2) and behaved similarly to
KIT-6-1,4-PA.