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).