# 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  
The majority of the MGDA in the feed was assumed to be in the form of an MGDA-REE complex. This assumption can be justified by the large stability constant of the MGDA-La complex (log K = 11.02; 20 \({}^{\circ}\)C) [44]. To prove complex stabilities and their presence in acidic HCl media after chromatographic separation, the amount of the free, non-complexed form of MGDA was determined from the eluent samples. MGDA--either in the form  
Figure 9: Titration curves of the individual MGDA-REE complexes. The pKa values of the equivalent points are marked on the titration curves. Grey = La (3.82), orange = Nd (3.25), blue = Eu (2.96) (titrant = 10 mM HCl; fixed speed titration; T = 298 \(\pm\) 2 K). of undissociated acid (MGDA-H\({}_{3}\)) or the corresponding sodium salt (MGDA-Na\({}_{3}\))--are considered to be free ligands. Semi-quantitative LC-MS analysis of effluent samples yielded information regarding the MGDA-REE complex stability. A significant amount of free MGDA compared to the overall concentration of MGDA in the feed would indicate a dissociation of the MGDA-REE complex. The amount of free MGDA was analyzed with LC-MS analysis. Figure S3 shows the mass spectrum for a pure MGDA-Na\({}_{3}\) feed. One can see both forms of free MGDA (MGDA-H\({}_{3}\) and MGDA-Na\({}_{3}\)). In addition, one partly fractionated molecule with \(m/z=160\) can be seen; this is the result of decarboxylation of the MGDA. Table 4 shows both experimental and theoretical \(m/z\) values for the MGDA and its decarboxylated form. Experimental \(m/z\) values fit to the theoretical counterparts within an acceptable deviation based on the \(\Delta\)mDa values. The actual samples containing complexed MGDA-REE species were analyzed with similar LC-MS analysis for the free MGDA. The amount of free MGDA was assumed to be minimal because the sample's MGDA-REE ratio was 1:1. Higher concentrations are assumed to be seen in samples with a 4:1 MGDA-REE ratio. Figure 10 displays the \(\sum\)REE concentration from one of the experiments combined with the observed relative amounts of free MGDA in a logarithmic scale. The frontal peak group that contains all three REs in the sample was detected immediately after the injected sample pulse traveled through the void volume of the column (0.4 BV). This narrow part of the chromatogram contains the large majority of the observed free MGDA. The amount of free MGDA is miniscule after the frontal peak group. Both REE and free MGDA concentrations are proportional to each other after the frontal peak group. In fraction #22 (Figure 10), the concentration for MGDA is 10 times smaller than that in fractions #9 and #13 (Figure 10). The concentration of free MGDA based on semi-quantitative analysis shows only \(\mu\)g/L-level concentrations in every sample, while the concentration in the feed was on the mg/L-level. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ion** & **Formula** & **\(m/z\) (Exp.)** & **\(m/z\) (Theor.)** & **\(\Delta\)mDa** \\ \hline
[MGDA + Na]\({}^{+}\) & C\({}_{7}\)H\({}_{11}\)NO\({}_{6}\) + Na & 228.0463 & 228.0479 & \(-\)1.6 \\
[MGDA + H]\({}^{+}\) & C\({}_{7}\)H\({}_{11}\)NO\({}_{6}\) + H & 206.0645 & 206.0659 & \(-\)1.4 \\
[MGDA + H-COOH]\({}^{+}\) & C\({}_{6}\)H\({}_{10}\)NO\({}_{4}\) & 160.0589 & 160.0604 & \(-\)1.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental and theoretical mass-to-charge ratios for measured MGDA forms. Figure 10: Chromatogram of the experiment as \(\sum\)REE with the corresponding logarithmic values of [MGDA]. (flowrate = 2.98 BV/h; feed (#2) = 0.05 BV, eluent = 50 mM HCl; T = 20 \({}^{\circ}\)C). The amount of free MGDA was also measured from different experiments with different conditions. The amount of free MGDA was also measured from other experiments. The trend in all measurements complies with the findings seen in Figure 11. From those results, it can be concluded that practically all the MGDA that was observed in its free form was seen at the beginning of the chromatogram.