Surface functional groups or
ligands
The active components of an ion-exchange resin or adsorbent that
interacts and binds the aqueous REEs species are the functional groups
present on the surface. These functional groups can be active molecules
of the polymeric ion exchange resins or ligands grafted on the solid
support in functionalized adsorbents (shown in figure 1) or the existing
functional groups of a naturally occurring surface, such as -O/-OH on
the surface of a mineral oxide/hydroxide. They are involved in the
uptake of REEs from solution through electrostatic interaction or
surface complexation. The different functional groups can be classified
based on the heteroatom bonded to the adsorbent surface (Yang et al.,
2019). The common groups are O-containing-functional groups (e.g., -OH,
-COOH, -C=O, and -C-O), S-containing functional groups (e.g.,
-SO3H, C-S, C=S, and S=O), P-containing functional
groups (e.g., -PO2H2,
-PO(OH)2), and N-containing functional groups (e.g.,
-NH2, -NH, -C=N and -C-N) (Figure 1). Additionally,
ligands containing multiple types of functional groups are common,
especially in chelating resins and functionalized adsorbents (Figure 1),
resulting in wide variations in REE sorption chemistry (Hérès et al.,
2018; Page et al., 2017; Trochimczuk & Alexandratos, 1994).
Polyfunctional ligands can provide additional affinities (Alexandratos
& Smith, 2004a). Also, the same functional group can have a higher or
lower affinity for REEs based on its side group (Martell & Hancock,
1996; Zhu & Alexandratos, 2015).