Molecular/Ion-imprinted
polymers
Molecular/ion-imprinted polymers (MIPs/IIPs) are sorbents with specific
recognition sites for a desired molecule or ion (Chen et al., 2016;
Ensing & De Boer, 1999; Fu et al., 2015; Wackerlig & Schirhagl, 2016).
This is achieved by copolymerizing functional monomers and cross-linkers
in the presence of the target molecule/ion that acts as a template
molecule. Then the template ion/molecule is removed chemically, leaving
behind a binding site-specific to the template (Chen et al., 2016; Fu et
al., 2015).
The IIPs show high selectivity for the target REEs. Biju, Gladis, and
Rao (2003) achieved high selectivity for Dy(III) over Y(III), Nd(III),
Lu(III), and La(III) with selectivity factors (SFs) of around 66, 82,
175, and 116, respectively by using Dy(III) ion-imprinted polymer based
on 5,7-dichloroquinoline-8-ol and 4-vinyl pyridine. Similarly, Er(III)
(Kala et al., 2004, 2005; Rao et al., 2006) and Nd(III) (Guo et al.,
2009) ion-imprinted polymers based on 5,7-dichloroquinoline-8-ol and
4-vinyl pyridine were prepared and tested for selectivity over different
lanthanides. In all of these investigations, the selectivity of the
imprinted ion was significantly increased (Guo et al., 2009; Kala et
al., 2004, 2005; Rao et al., 2006). Similarly, several IIPs
(Pr(III)-IIP, Nd(III)-IIP, Sm(III)-IIP, Eu(III)-IIP, and Gd(III)-IIPs)
were synthesized with maximum sorption capacities of 125.3, 126.5,
127.6, 128.2, and 129.1 mg/g for Pr(III), Nd(III), Sm(III), Eu(III), and
Gd(III), respectively (Yusoff et al., 2017). A phosphonic-based
La(III)-ion imprinted polymer showed an adsorption capacity of 62.8 mg/g
for La(III) and a selectivity factor of 54.57 for La(III) over Cu(II)
(Ni et al., 2021).
Dolak et al. (2015) used Nd(III)-IIP to selectively extract Nd(III) in
the presence of Ce(III), La(III), and Eu(III). The selectivity
coefficient for Nd(III) over Ce(III), La(III), Eu(III) were 234.3,
129.6, and 248.4, respectively; however, the maximum adsorption capacity
was only 15.03 mg/g. In different investigations, Gd(III) ion-imprinted
polymers bearing N,N″-bisacetamidostyrene diethylenetriaminetriacetic
acid (DTPA–bisamide) (Garcia et al., 1998; Vigneau et al., 2001) and
N-acetamidostyrene ethylenetriaminetriacetic acid (EDTA–amide) (Vigneau
et al., 2001) exhibited at least 3-fold increase in selectivity than
analogous non-imprinted polymers. The authors noted that DTPA
(nine-coordinate chelating agent) based Gd(III)-IIP showed higher Gd/La
selectivity (maximum value of ~8.3) for Gd(III) in a
binary solution of Gd(III) and La(III) than EDTA (five-coordinate
chelating agent) based Gd(III)-IIP (around 3.0). The same DTPA-based
Gd(III)-IIP was used to separate Gd(III) and other lanthanides and
actinides (Vigneau et al., 2002). Other studies on imprinted polymer
include La(III)-IIP (Mustapa et al., 2016), Ce(III)-IIP (Keçili et al.,
2018; Mustapa et al., 2016), Y(III)-IIPs (Zulfikar et al., 2017),
Nd(III)-IIP (Moussa et al., 2017), ionic imprinted mesoporous bilayer
films (IIBFs) for Dy(III) and Nd(III) adsorption (Zheng et al., 2018),
Gd (III)-imprinted membranes (GIMs) (Cui et al., 2019), and
Gd(III)-imprinted divinylbenzene and methacrylic acid copolymer (Bunina
et al., 2021). The IIPs show a stronger affinity for the imprinted REE
cation compared to other REEs.
Surface ion-imprinted polymers are derivative of IIP technology and
include the formation of IIP on a surface. A surface-IIP was prepared
for L(III) using MCM-41 as support and APTES as the functional monomer
(Qin et al., 2022). The surface-IIP showed an adsorption capacity of
272.2 mg/g. Other examples of surface-IIP are Ce(III)-imprinted polymer
with attapulgite support (Pan et al., 2010), Ce(III)-imprinted polymer
functionalized potassium tetratitanate whisker (Zhang et al., 2010),
Ce(III)-IIP functionalized on SBA-15 (Meng et al., 2014), and Gd(III)
imprinted magnetic ordered mesoporous carbon (OMC) (Patra et al., 2017).
The limitation in the application of IIP stems from the large number of
inaccessible adsorption sites since these sites are embedded inside the
polymer (Canfarotta et al., 2016; Dong et al., 2021; Huang et al.,
2019). Considering that IIP preparations are usually a sophisticated and
chemically expensive process, presence of inaccessible sites is a big
hindrance for widespread industrial application. Additional challenges
arise due to low mass transfer rates (Aravind & Mathew, 2018; Dong et
al., 2021; H. Xu & Guo, 2012). These limitations can be improved with
surface ion-imprinted polymers (Dong et al., 2021), but more research
effort is required in this direction.