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.