Introduction

This chapter briefly covers different technologies involved in the adsorption-based separation of rare earth elements (REE – the 15 lanthanide elements of the periodic table with atomic numbers Z= 51-71, plus scandium Z=21, and yttrium Z=39). The separation technologies include a wide range of materials from ion exchange resins (IERs), metal (hydro)oxides adsorbents, and solvent impregnated resins (SIRs) that have the extractant impregnated into solid support to the recent technologies of surface-functionalized adsorbents with complexing ligand chemically anchored on to the solid support and ion-imprinted polymers with specific recognition sites for the desired REEs.
The adsorption sites in these adsorbents are various surface functional groups or ligands which can adsorb REEs through different mechanisms, i.e., physical adsorption, electrostatic interaction, and/or surface complexation. The adsorbent’s behavior depends on the chemistry and chemical properties of these functional groups or ligands (discussed in section 2). Additionally, the interaction of the surface sites and the REEs is controlled by the chemical properties of REEs, such as charge, ionic radius, coordination number, aqueous speciation, and the ability to form complexes with the ligand.
REEs occurs dominantly in trivalent oxidation state, have variable coordination numbers (CN∼ 8–12 (Cotton & Harrowfield, 2012)), and have similar ionic radii between adjacent REEs (which decreases with increasing atomic number among lanthanides) (Nash, 1993). They are considered hard acid cations and interact strongly with hard anions such as hydroxide, alkoxide, carbonates, and phosphates and have strong complexes with organic ligands containing carboxylates and phosphonates (Johannesson et al., 1995; Noack et al., 2016; Pearson, 1963; Xie et al., 2014). More stable REE-ligand complexes are obtained with multidentate ligands like amino-poly(carboxylic acids) due to the high coordination number of REEs (Noack et al., 2016; Xie et al., 2014). These characteristics of REEs affect the extent, kinetics, and mechanism of the adsorption. In aqueous solutions, the pH of the solution controls the REEs speciation. REEs are predominantly cationic REE(III) in aqueous solution at acidic pH and REEs hydroxides at alkaline pH (Callura, 2018; Ramasamy, Repo, et al., 2017). At high pH, the primary mechanism of REEs uptake can be the surface precipitation of REEs as hydroxides instead of adsorption (Dardenne et al., 2002; Farley et al., 1985; Iftekhar, Ramasamy, et al., 2018; Piasecki & Sverjensky, 2008).
A measure of the adsorbent’s effectiveness for separation and its performance can be obtained by conducting batch or column adsorption experiments. In batch adsorption, the frequently used characteristics are equilibrium adsorption capacity (qe) (eq. 1a), maximum adsorption capacity calculated using Langmuir isotherm (qm) (eq. 1e), % adsorption (eq. 1b), solid-liquid distribution (partitioning) coefficient (Kd) (eq. 1c), and selectivity factor (SF) between element A and B (eq. 1d) in case of competitive adsorption or adsorption from a multi-element solution. These parameters are defined as follows:
\(q_{e}\left(\frac{\text{mg}}{g}\right)=\left(C_{0}-C_{e}\right)*V/m\)(1a)
\(\text{Adsorption}\left(\%\right)=\frac{C_{0}-C_{e}}{C_{0}}*100\,\)(1b)
\(K_{d}\left(\text{ml}/g\right)=\frac{C_{0}-C_{e}}{C_{e}}\frac{V}{m}\)(1c)
\(\text{SF}\left(A/B\right)=\frac{Kd_{A}}{Kd_{B}}\) (1d)
\(q_{e}\left(\frac{\text{mg}}{g}\right)\ =q_{m}\frac{K_{L}C_{e}}{1+K_{L}C_{e}}\ \)(1e)
where Co and Ce are initial and equilibrium concentrations of the adsorbate in the solution in batch adsorption (mg/L), m is the mass of the adsorbent used (g), V is the total volume of the solution (L), q is the adsorption (in mg/g), and KL is Langmuir isotherm constant (L/mg).