## 3 Results and Discussion
### Sorption Properties
#### 3.2.6 Application to Ore Raffinate
About 38.8% of the initial aluminum content remained in the outlet of the Dowex 50X8 column, and the residual concentration reaches about 5 g Al L\({}^{-1}\). A new precipitation step was processed at pH 5 to remove the aluminum (to limit its competition effect in the next sorption step while using MC-UR/S). After this step, the uranium concentration was barely changed, while the residual concentrations of the REEs remained close to 211 mg REE L\({}^{-1}\), 57 mg Nd L\({}^{-1}\) and 32.1 mg Ce L\({}^{-1}\)). Figure S20 summarizes the distribution of the main relevant elements in the different compartments of the pre-treatment leachates. Rare earth elements are mainly accumulated in the Dowex 50X8 sulfonic resin (71.5-85.0%), which also binds lead (21.2%), while the relative fractions of the other metals are systematically lower than 7.2%. Iron is mainly present in the precipitate collected at pH 4 (i.e., 94.5%). The precipitation at pH 5 recovers 59.3% of Al(III), 39.4% of Cu(II), 17.7% of Mn(II), and 12.0% of Ca(II). The quaternary ammonium salt resin Amberlite IRA-400 recovers 81.1% of U(VI) (14.8% remains in the raffinate, to be treated with MC-UR/S), and up to 20.6% of Si(IV) is bound to the resin, while for the other metal ions, the relative fractions remain below 10.6%. In fact, the raffinate contains appreciable fractions of Ni(II) (80.4%), Si(IV) (i.e., 66.7%), Pb(II) (i.e., 53.8%), Ca(II) (i.e., 50.9%), and Cu(II) (i.e., 40.0%), and 14.8% U(VI) (referring to the amounts present in the leachate). The raffinate contains limited values of REEs (between 2.4 and 7.5%), and the residual concentrations are limited to 32.1 mg Ce L\({}^{-1}\) (i.e., 0.242 mmol Ce L\({}^{-1}\)) and 57.4 mg Nd L\({}^{-1}\) (i.e., 0.398 mmol Nd L\({}^{-1}\)). These concentration ranges are compatible with the conditions for the effective application of MC-UR/S. The treatment of the raffinate solution was performed at different pH values. Figure S21 shows that for all the elements, the sorption capacity increases with the pH of the solution. Under the most favorable conditions (i.e., pH\({}_{\text{eq}}\): 4.87), the recovery of metal ions from the raffinate reaches 59.4% for Ce(III) and 38.7% for Nd(III) (60.4% for the global REE index). Uranyl was also co-extracted (SE: 33.6%) together with Fe(III) (SE: 32.7%). Figure S22 reports the log\({}_{10}\) plot of the distribution ratio vs. pH\({}_{\text{eq}}\). The rare earth elements show higher relative distribution ratios than the other metal ions or metalloids. The sensitivity to pH is demonstrated by the slopes of the plots: the highest sensitivities to pH are reported for Ce(III) > U(VI) > Pb(II) > Fe(III) > Nd(III) > Ca(II) \(\approx\) Ni(II) > Al(III) \(\approx\) Si(IV) > Mn(II). There is no direct correlation between this ranking and the intrinsic physicochemical properties of the target metals. The effects of the differential concentration levels cannot apply to the relationships established above for the equimolar multi-component solutions (in Section 3.2.4). The comparison of the molar distribution of the main elements in the raffinate (Figure S23a) and in the residue of the sorption using MC-UR/S at the optimum pH (Figure S23b) shows that, for most of the target elements, the molar fraction increases with the sorption. The extent depends on the metal or metalloid, according to the following series:  
\[\text{Pb(II)}>\text{Ce(III)}\approx\text{Fe(III)}>\text{U(VI)}>>\text{Mn(II)}> \text{Al(III)}\approx\text{Ni(II)}>>\text{Nd(III)}>>\text{Ca(II)}\approx\text{Si(IV)}.\]  
This scale can be used to evaluate the potential of the sorbent to selectively enrich some metals on the sorbent. Figure S24 compares the plots of the selectivity coefficients \(\text{SC}_{\text{Ce/metal}}\) and \(\text{SC}_{\text{Nd/metal}}\) at different pH values. The sorbent has a marked preference for Ce(III) against the heavy metals. This preference is more marked than that for Nd(III).