# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 1 Introduction  
Rare earth elements (REEs), especially when considering lanthanides, are known for their similar chemical properties. They all exist commonly in a REE\({}^{3+}\)-oxidation state, but few of them can also have oxidation states of either REE\({}^{2+}\) or REE\({}^{4+}\)[1]. All REEs have partially filled 4f-orbitals, giving them their unique properties due to the phenomenon known as lanthanide contraction. This unique property of the electron shell configuration affects their chemical bonding, further intensifying their chemical similarities [2]. These minor differences between individual lanthanides, scandium, and yttrium result in significant difficulty in the complete separation of these elements. Thus, either highly advanced chemical processes and/or many process cycles are needed to achieve high purities. Different methods can be used to fractionate lanthanide mixtures to pure single metal fractions or smaller sets of REE (e.g., adjacent REE pairs such as La/Ce, Sm/Eu, and Ce/Pr). These methods include precipitation [3], solvent extraction (SX) [4], ion exchange (IX), and ion chromatography (IC) [5, 6]. All these methods have their advantages and disadvantages. Precipitation of REEs is usually performed with a ligand that produces poorly soluble solids, such as oxalate [7]. These precipitation methods usually require precise control of the pH due to small differences in the formation energies of individual REE complexes [8]. The greatest advantage of direct precipitation is an instant solid product, which can easily be turned into oxides. For solvent extraction, the overall separation is not technically difficult, but its downside is its many consecutive steps [9]. In addition, solvent extraction operationsrequire large-scale use of chemicals (acids, organic solvents, and other chemicals) even though they are usually recyclable [10]. In addition to conventional organophosphorus extractants, new \(\alpha\)-aminophosphorous extractants have been developed for REE separation by solvent extraction [11]. The development of solvent extraction separation of REEs in academic literature also deals with ionic liquids [12]. A new kind of separation material has been developed to achieve highly selective REE sorption and separation [13; 14; 15; 16]  
In the Manhattan project, the IX methods, for example with cation or anion ion exchange, were developed to remove REEs and other valuable elements from the radioactive material [17]. Before IX-based methods, separation was based on the crystallization of pure REE components, which is very time consuming. The IX methods significantly reduced the required time and work [18]. Chromatographic separation has become one of the viable methods for REE separation in addition to solvent extraction. Now, with modern HPLC (High Performance Liquid Chromatography) equipment and materials, the complete separation of REEs from their mixture can be achieved in a single column step [19]. However, the scalability of these kinds of separation systems is limited; the superior separation efficiency of the HPLC in pilot scale or larger is cost intensive. Conventional preparative columns that operate with completely different parameters (pressure, column dimensions, and particle and/or pore size of separation material) are unable to reach a similar level of separation efficiency. The large variety of chromatographic methods are based on the affinities and interactions between the species to be separated, the stationary phase, and the eluent medium [20]. One of the chromatographic methods is high-speed counter-current chromatography (HSCCC) [21]. Chromatographic separation of REEs can be based on the separation of complexed REEs [22]. One type of complex chromatography is ligand assisted chromatography [23]. Complexation of REEs enhances minor differences in the chemistry of the elements. The complex agent may or may not be chelating if they result in a complex with a charge. The largest difference is that chelating agents tend to form more stable complexes than non-chelating ones.