# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 3 Results and Discussion
### Stability of the MGDA-REE Complexes
#### 3.3.1 Titration of the MGDA-REE Complexes  
The differences between the complex stabilities were investigated with a simple titration method. For titration purposes, the REEs (La, Nd, Eu) were complexed with MGDA individually. Initial pHs of the samples were set to 10.5 (>pKa3 = 9.9) when the MGDA forms stable chelate with REE. These samples were then titrated with 0.1 M HCl. The equilibrium point is observed as a gentle slope in the pH/dV curve (Figure 9). The results verify our assumption of the protonation of at least one of the carboxylic acid groups. Values for the first pKa of the MGDA-REE complex can be determined from the equivalent points of the titration curve. The value for the MGDA-La complex is significantly higher than that of the MGDA-Nd and MGDA-Eu complexes. The complex  
Figure 8: The chromatogram of the experiment with a feed solution having a 4:1 MGDA–REE ratio. (resin = IRA–410; T = 298 \(\pm\) 2 K; flowrate = 2.98 BV/h; feed (#4) = 0.025 BV; eluent = 10 mM HCl).  
strengths have an order of La < Nd < Eu, which is in line with the order of the metals in the periodic table; complex strength is inversely proportional to the size of the ion. A decrease in the ionic radius results in more stable complexes [42]. REEs are known for their lanthanide contraction, which results in an atypical size of the ion when the mass of the ion increases [2]. The differences in pKa values between REE-MGDA complexes result in differences in the free energy of adsorption. The La complex with the largest pKa value determined gives a \(\Delta\)G\({}^{\circ}\) (Gibbs free energy of adsorption) of \(-\)21.51 kJ/mol. The values of \(\Delta\)G\({}^{\circ}\) for Nd and Eu are \(-\)18.30 kJ/mol and \(-\)16.67 kJ/mol, respectively. A larger negative free energy of adsorption indicates stronger interaction with the resin. This can be seen in the ratios between La, Nd, and Eu in the front peak. The Eu, which has the weakest interaction with the resin, has a larger share in the front peak compared to the original feed.  
Interactions between negative MGDA-REE complexes and the positively charged SBA resin further modifies the protonation tendency of MGDA. Electrostatic interactions such as London dispersion forces or ion-induced dipole effects could affect the charge distribution within the MGDA-REE complex [43]. This leads to a higher pKa, where the protonation of the first COO\({}^{-}\) occurs. This plausible interaction explains the differences of pH values observed in titration and in the elution process itself.  
Differences in complex stabilities can already be seen when the MGDA-REE loaded column is rinsed with water. Deionized water can elute the MGDA-La complex since its pH is 5.8, which is acidic enough to cause partial protonation and neutralization of the MGDA-La complex, thus liberating it from the resin. Since the third pKa values of the MGDA-Nd and MGDA-Eu complexes are significantly lower than that of the MGDA-La complex, those complexes remain sorbed to the resin during the loading and rinsing of the column with water. This small, steady leakage of the MGDA-La complex during water rinsing can be seen in Figure S2.