Figure 1: Different ligands used for REEs adsorption in chelating
ion-exchange resins or grafted onto solid supports.
The most common functional groups used in ion-exchange and adsorption of
REEs are carboxyl, found in both cation-exchangers (Bezzina et al.,
2018; Shu et al., 2007; Xiong et al., 2011; Xiong, 2008) and various
functionalized adsorbents (Figure 1) (Ahmed, Lee, et al., 2019; Callura
et al., 2018; Noack et al., 2016; Polido Legaria, Rocha, et al., 2017;
Ravi, Zhang, et al., 2018). The mechanism of REEs uptake by
carboxyl-based adsorbents can be ion-exchange process in
cation-exchangers or surface complexation in case of a chelating
carboxyl-based surface ligand (e.g., complexation of REEs with
diethylenetriaminepentaacetic acid (DTPA) functionalized adsorbent
(Noack et al., 2016)). Many commercially available carboxyl-based cation
exchange resins have been used for REEs extraction (Bezzina et al.,
2018; Shu et al., 2007; Xiong et al., 2011; Xiong, 2008). Since carboxyl
acid is a weak acid, its behavior is highly dependent on pH.
Sulfonic functional groups are the active component of the strongly
acidic cation-exchange resins. The affinity of the counter-ion for the
ion-exchanger depends on its charge and hydration radius (Harris &
Lucy, 2015). REEs exist as triply charged positive ions in acidic
aqueous solutions and hence have a high affinity to negatively charged
sulfonic ions and can be separated from singly and doubly charged metal
cations using interaction with sulfonic anion (Fritz & Garralda, 1963;
Korkisch et al., 1967; Page et al., 2019; Strelow, 1960; Strelow et al.,
1965). However, due to similar radii of adjacent Ln(III), selectivity
for a particular Ln(III) is limited with sulfonic-acid cation-exchangers
(Boyd, 1978; Strelow, 1960; Strelow et al., 1965; Strelow & Bothma,
1964). The sulfonic group is a strong acid; hence its behavior is mostly
independent of pH.
Phosphorus-containing functional groups used for REEs sorption are
organophosphoric acid, phosphonic acid (-PO(OH)2), and
phosphonic ester PO(OR)2 based derivatives (Egawa et
al., 1994; Ihara et al., 2001; Miklishanskii et al., 1968; Page et al.,
2017). The ion-exchange resin containing phosphonic acid-based groups
are considered weakly acidic cation-exchange resins similar to
carboxyl-based ion-exchange resins and are dependent on pH.
Additionally, P-based chelating ligands have been used as functional
groups in different chelating resin (Callura et al., 2019; Hérès et al.,
2018) and functionalized adsorbents (Figure 1) (Artiushenko, Kostenko,
et al., 2020; Bertelsen et al., 2019; Callura et al., 2018; de Decker et
al., 2016; Kavun et al., 2021; Noack et al., 2016; Ravi, Lee, et al.,
2018; Z. . Wang et al., 2002; X. Zheng et al., 2020).
Most common among N-containing functional groups is amine-based
compounds. The REE sorption mechanism for amino-based sorbent is
anion-exchange process. These amino-based sorbents can be used to
separate Th (IV) and U(IV), which form strong anionic compounds with
nitrate and phosphate (Jackson, 1954). Among REEs, Ce(IV) can form
anionic nitrates, which can be separated from other REEs using
amino-based resins or adsorbents (Zhu & Chen, 2011). Moreover, N-based
ligands are used for surface complexation of REEs in chelating resin
(Niu et al., 2021; Page et al., 2019) and other adsorbents (Figure 1)
(de Decker et al., 2016; Lee et al., 2019; Ramasamy, Puhakka, Repo, Ben
Hammouda, et al., 2018; Ramasamy, Repo, et al., 2017).
In naturally occurring oxidic adsorbents such as ferric hydroxide or
silica oxide, the surface OH groups undergo protonation-deprotonation
and produce surface charges which lead to electrostatic attraction of
cations and anions present in the solution. They can electrostatically
attract REEs cations resulting in the adsorption of REEs.