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).