# Chromatographic Separation of Rare Earth Elements as MGDA Complexes on Anion Exchange Resins
## 2 Materials and Methods
### Synthetic Feed Solutions  
The chemicals used in this study were HCl (CAS: 7647-01-0) (VWR Chemicals, Briare, France, 36%, GPR Rectapur), HNO\({}_{3}\) (CAS: 7697-37-2) (Merck, Darmstadt, Germany, 65%, Analysis purity), H\({}_{2}\)SO\({}_{4}\) (CAS: 7664-93-9) (VWR Chemicals, Briare, France, 96%, GPR Rectapur), MGDA (CAS: 16446-16-2), (Honeywell Fluka, Seelze, Germany, \(\geq\)90%), LaCl\({}_{3}\)-7 H\({}_{2}\)O (CAS: 10025-84-0) (Fluka Chemika, Seelze, Germany, Puriss. p.a. ), Nd\({}_{2}\)O\({}_{3}\) (CAS: 1313-97-9) (Sigma-Aldrich, Taufkirchen, Germany, 99.9%), and Eu\({}_{2}\)O\({}_{3}\) (CAS: 1308-96-9) (Sigma-Aldrich, Taufkirchen, Germany, 99.9%)  
The feed simulates the real MGDA-REE product that was previously obtained from authentic industrial phosphogypsum waste by eluting REE-loaded resin with the alkaline MGDA solution [32]. REEs in the loaded resin originated from an authentic industrial phosphogypsum waste that was treated with a cross-current ion exchange resin-in-leach process. The feed solutions were prepared by dissolving solid rare earth chemicals. Solids were dissolved by concentrated HCl, then diluted with water. Although selected REE solids are also soluble in other minerals acids (HNO\({}_{3}\) and H\({}_{2}\)SO\({}_{4}\)), HCl was selected because it has the fewest unfavorable interactions with REEs and SBA, (i.e., a low affinity towards the resin and REE in a complex formation) [33; 34]. The main negative effect of HCl is its corrosive effect on the used equipment. HNO\({}_{3}\) affects the resin matrix (nitration) and chelating MGDA ligand negatively (oxidizing) and H\({}_{2}\)SO\({}_{4}\) was rejected due to its double anion formation (HSO\({}_{4}\)\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\)) and strong affinity towards both type I and type II SBA resins. The total REE concentration of the prepared stock solution was 0.3 M, and it was used for preparing all the feed solutions. The pH adjustments were done either with HCl or NaOH, if needed. The ratio of individual REEs in stock solution was 1:1:1 (\(\pm\)5%), as it allows to detect selectivity and affinity differences more easily. The composition and properties of the feed solutions are shown in Table 3. Solutions were prepared by mixing REE stock solution (0.3 M) with solid MGDA-Na3 salt. For most samples, the REE:MGDA stoichiometric ratio was 1:1 since their dominant complex structure is 1:1 [32], and complexation is estimated to be very strong at this ratio (i.e., the majority of the REE and MGDA are in complexes). In addition, the 1:1 ratio of REE:MGDA does not allow resin to transform to the MGDA-form since nearly all of it is in REE-MGDA complexes. To study the effect of excess MGDA in the sample, a feed with a 1:4 REE-MGDA stoichiometric ratio was also prepared. The pH of the samples was adjusted to be slightly alkaline to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Resin** & **Dowex 1X8** & **IRA-402** & **IRA-410** \\ \hline Functional group & N\({}^{+}\)(CH\({}_{3}\))\({}_{3}\) & N\({}^{+}\)(CH\({}_{3}\))\({}_{3}\) & N\({}^{+}\)(CH\({}_{3}\))\({}_{2}\)C\({}_{2}\)H\({}_{4}\)OH \\ Matrix & PS-DVB & PS-DVB & PS-DVB \\ Structure & gel & gel & gel \\ DVB Content, wt-\% & 8 & 6 & 8 \\ Particle size, mm & 0.6–0.75 & 0.6–0.75 & 0.6–0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Physical and chemical properties of the resins used in this study. Figure 1: Functional groups of (**a**) Dowex 1X8 and IRA-402; (**b**) IRA-410; (**c**) MGDA-Na\({}_{3}\). ensure the complex formation between the ligand and REE since the MGDA is an acid with pKa values pKa1 = 9.9; pKa2 = 2.6, and pKa3 = 1.5 [35]. Regardless of the high pH of feed solutions, the REEs are not in the form of hydroxide precipitate in the solution. Chelation with MGDA alters the solution chemistry of REE cations and they exist as water soluble MGDA-REE complexes within the pH range of 9 to 12.50. Deviations in feed concentrations between each other are due to the pH adjustments of the feed.