# Rare Earth Elements Uptake by Synthetic Polymeric and Cellulose-Based Materials: A Review
## 2 Chemistry of REEs  
The chemical properties of REEs may be divided in two categories: geochemical and chemical. Chemical properties allow the prediction of the behavior of REEs, and their understanding should be considered in the design of REE compounds and materials. In the case of REE recovery, these properties should be considered for the development of adsorbents or ion exchange materials. REEs are considered reactive elements; they are as reactive as group II elements from the periodic table and are more reactive than transition metals [2]. The most stable oxidation state of REEs is 3+, and in aqueous media, they are present as [Ln(H\({}_{2}\)O)\({}_{9}\)]\({}^{3+}\) and [Ln(H\({}_{2}\)O)\({}_{8}\)]\({}^{3+}\) (Ln: lanthanide), whose coordination numbers depend on the REEs' ionic radii [2]. REE complexes show between 6 and 12 ligands per metallic center, which are influenced by steric effects rather than crystal field splitting and  
Figure 1: Abundance of REEs and semiprecious and precious metals in the earth’s upper crust. The graph was tabulated using information from [1].  
a preference for hard donor ligands, such as oxygen [2; 4; 5]. Furthermore, REEs' nitrogen compounds, such as amino and pyridine complexes, do not have enough stability due to the basification of aqueous media, which are produced by the equilibria of nitrogenated compounds acting as weak Bransted-Lowry bases. The preference of REEs for OH\({}^{-}\), which is a harder donor ligand than nitrogen from amino compounds, causes precipitation as Ln(OH)\({}_{3}\) instead of complexation with nitrogenated ligands in aqueous media [2]. REEs' ionic compounds show fast ligand exchange in aqueous media [2]. Although REEs have \(f\) orbitals, those orbitals are shielded by \(5s\) and \(5p\) orbitals, and they do not participate directly in bonding [2; 5]. REEs are subdivided into light REEs and heavy REEs, where light REEs refer to La, Ce, Pr, Nd, Sm, and Eu, and heavy REEs refer to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, together with Sc and Y [3; 5]. Although this classification could be considered quite controversial because it initially seems focused on REEs' atomic weight, Sc and Y are the lightest REE elements. Nevertheless, Sc and Y are considered heavy REEs because they share some chemical properties with heavier REEs due to lanthanide contraction of ionic radii [2; 3; 4; 5]. The lanthanide contraction of ionic radii is caused by poor \(f\)-orbital screening, pushing electrons closer to the atomic nucleus and leading to similar physical and chemical properties of REEs [2; 3; 5]. As previously mentioned, Sc could be excluded from REEs due to its chemical properties, which differ enough from other REEs [2; 3; 4; 5] (Figure 2).  
The geochemical properties of REEs refer to the behavior of the elements in the earth's crust, and their understanding is crucial for understanding REE distribution, mining, and mobility in the environment. The geochemical behavior of REEs follows all chemical properties already mentioned. REEs are not found as native elements, and they are always part of minerals and ones [4; 5]. REE minerals and ores are primarily formed by magmatic and hydrothermal processes [4; 5]. These processes allow the accumulation of REEs in carbonatites, alkaline, and peralkaline igneous rocks [4; 5]. REEs are lithophile (rock-loving) elements, occurring as oxides, silicates, phosphates, and oxyhydroxide salts [3; 4; 5]. REEs can substitute other cations of similar radius, which change the structure of minerals [3; 4; 5]. Erosion, rock weathering of primary sources and other mineral deposits, together with mine exploitation, contributes to the mobility of REEs in the environment and the formation of secondary REE deposits [4; 5].