# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 3 Results and Discussion
### Stability of the MGDA-REE Complexes
#### 3.3.2 Presence of Free MGDA in the REE Solution
This indicates that all the excess MGDA and MGDA-REE complexes that have not sorbed to the resin are seen as the first, narrow peak of the chromatogram. These results also indicate that the separation of REEs is based on the MGDA-REE complex; it has been shown to remain intact during the process. Almost the same amount of free MGDA was observed from a 1:4 (metal:ligand) ratio feed as from a 1:1 ratio feed. Excess MGDA is sorbed to the resin, converting the resin to the MGDA form. The REE-MGDA complex with other stoichiometry than 1:1 (M:L) cannot be excluded as an option for excess MGDA, but it is unlikely, and therefore the other M:L ratios (e.g., 1:2) were not investigated. It was seen that either the MGDA form resin or different complex structures affected the retention behavior significantly with a 1:4 (M:L) feed, as seen in Section 3.2.4. Adjusting the pH of the feed was not shown to affect the amount of free MGDA. The major change observed was the change in the MGDA-REE complex retention, as shown in Section 3.2.3. With elevated temperatures, the significant difference in the concentration profile of free MGDA was not seen either. Relative amounts of free MGDA experiments with temperatures of 22 \({}^{\circ}\)C and 50 \({}^{\circ}\)C are shown in Figure S4. Increasing the temperature enhances the sorption of the MGDA-REE complexes but does not affect the amount of free MGDA. Finally, the same MGDA determination was performed for a two-step process. Figure 11 shows the chromatogram for this experiment with a logarithmic presentation of free MGDA. According to previous analyses and hypotheses based on those experiments, the highest amount of MGDA was expected, as would be seen to occur in the first step of the process; this occurred as expected. For the second step, MGDA was at a similar level to the levels in the other experiments. This proposed two-step process was not practical for actual separation, but it can be used to prove the distribution of free MGDA in the system. Since the majority of free MGDA is eluted among the first peak while eluting the feed with water, it can be shown that the propagation of free MGDA is not affected by the HCl. Figure 11: Chromatograms of the two-step process with separate loading and elution steps. In addition to [REE], the logarithmic [MGDA] is shown (T = 298 K \(\pm\) 2 K; flowrate = 2.98 BV/h; feed (#2) = 0.1 BV; eluent = (**a**) deionized water, (**b**) 0.01 M HCl). Although the majority of detectable free MGDA existed in a few fractions at the beginning of the chromatogram, it was also present in other samples. This small amount of MGDA after the frontal peak correlates well with the \(\Sigma\)[REE]. This clear connection in all the experiments can be explained by the equilibrium of the complexation reactions. For chelating reactions between strong ligands and multivalent metal cations, the equilibrium is heavily on the side of the complexes. Even though the complexation phenomenon is strong by nature, a small percentage of the metal cation and MGDA exists in the sample fractions as free species. The amount of free MGDA is presented as a logarithmic value based on relative surface areas obtained from the LC-MS analyses.