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.