# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 3 Results and Discussion
### Selection of the Resin and the Mobile Phase
#### 3.1.2 Mobile Phase  
Dilute solutions of strong mineral acids were used as the mobile phase in the elution of MGDA-REE complexes for two reasons. Firstly, the acidic eluent weakens the stability and changes the total net charge of the MGDA-REE complexes by protonating at least one of the carboxylic acid groups of the chelating ligand (MGDA). The protonation of functional groups in the resin is not considered. The stability of, for example, the IRA-410 resin, is good for the whole pH range from 0 to 14. Therefore, operating the separation dilute acids does not result in protonation of the functional groups. Secondly, the different anions from the acid compete with the MGDA-REE complex for the functional group of the resin. In this way, the pH and choice of acid could be used to enhance the separation of the REEs with a simple mobile phase.  
Common mineral acids used in this type of separation are H\({}_{2}\)SO\({}_{4}\), HNO\({}_{3}\), and HCl. Preliminary tests revealed that no separation of REEs is achieved when sulfuric acid is used as the mobile phase, even in small concentrations (10 mM). This stems from the extreme affinity of sulfate and bisulfate towards both type I and type II SBA resins in acidic pH. The use of sulfuric acid results in a persistent sulfate form in the resin, requiring large volumes of suitable salts to convert the resin back to either their Cl\({}^{-}\) or NO\({}_{3}\)\({}^{-}\) form [38]. Therefore, sulfuric acid is an unsuitable mobile phase due to its strong interaction with all types of the resin phase.  
The general trend of affinity towards SBA resins is SO\({}_{4}\)\({}^{2-}\) > NO\({}_{3}\)\({}^{-}\) > Cl\({}^{-}\). There is a notable difference in the affinities of NO\({}_{3}\)\({}^{-}\) and Cl\({}^{-}\) to Type I and Type II resins when OH\({}^{-}\) is used as the reference. For Type I resins, the selectivity coefficients are in the following order: \(K_{OH^{-}}^{Cl^{-}}\) = 22 and \(K_{OH^{-}}^{NO^{-}_{3}}\) = 65 for Type I resins, and \(K_{OH^{-}}^{Cl^{-}}\) = 2.2 and \(K_{OH^{-}}^{NO^{-}_{3}}\) = 8 for Type II resins [34]. The Type I resin is less selective to NO3- than the Type II resin (\(K_{Cl^{-}}^{NO^{-}_{3}}\) = 2.95 vs. \(K_{Cl^{-}}^{NO^{-}_{3}}\) = 3.49). It is expected that these affinity differences would cause variations in the elution behavior of different REE-MGDA complexes with different mineral acids.  
Disqualification of H\({}_{2}\)SO\({}_{4}\) as a mobile phase leaves us with two options: HNO\({}_{3}\) and HCl. Figure 4 shows that HNO\({}_{3}\) results in slightly lower retention, but the separation of peaks is not that great. Respectively, with HCl, the retention of metals increases, and the profiles are broader. Generally, using HNO\({}_{3}\) with an anion exchange resin as the mobile phase is not recommended since it might undergo nitration, oxidizing the resin  
Figure 3: Elution profiles of (**a**) La; (**b**) Nd; and (**c**) Eu on IRA-402 and IRA-410. (T = 298 \(\pm\) 2 K; flowrate = 2.98 BV/h; feed pulse = 0.025 BV, Eluent = 10 mM HNO\({}_{3}\); Feed solution = #2).  
matrix. Therefore, HCl is chosen to be the most suitable eluent for REE. This minimizes the interaction between the MGDA-REE complex and anions present in the mobile phase. The formation constant of, for example, an NdCl\({}_{\mathrm{x}}\) complex in Cl-medium is low compared to the formation constants of the MGDA-REE complex at an alkaline pH. At 25 \({}^{\circ}\)C, \(\beta_{1}\) and \(\beta_{2}\) for NdCl\({}^{2+}\) and NdCl\({}_{2}\)\({}^{+}\) are (0.28) and (no value), respectively. Based on the formation constants of REE-Cl\({}_{\mathrm{x}}\) (\(x\geq 1\)) complexes, these complexes are not significantly present in dilute chloride solutions [33].