## 5 Conclusions  
This work demonstrates that simple procedures for the functionalization of magnetic chitosan microparticles (\(\approx\)6 \(\upmu\)m in size) can be used to successively immobilize amine groups and grafted sulfonate moieties (confirmed by elemental analysis: \(\approx\)4.6 mmol N g\({}^{-1}\), and 1 mmol S g\({}^{-1}\), when relevant). The FTIR analysis confirms the contributions of different reactive groups to the binding of the metal (amine/amide and hydroxyl/carbonyl from the chitosan and urea, and S-based groups after the sulfonation of the sorbent). The magnetite nanoparticles (5-7 nm) embedded in the polymer matrix confers superparamagnetic properties on the composite (MC-UR/S). The sorption capacity at pH\({}_{0}\) 5 doubled for the aminated sorbent and tripled after dual functionalization (amination and sulfonation). The improvement in the sorption performance is not correlated with the density of the reactive groups, and this enhancement also results from the synergetic effects of the bi-functionalization. The fast kinetics (though partially controlled by the resistance to intraparticle diffusion) enable the reaching of the equilibrium within 60 min of contact, and the kinetic profiles are effectively described by the pseudo-first-order rate equation. The acidic solutions (0.2 M HCl) are highly efficient in the cerium elution from the metal-loaded sorbents. Despite the changes observed in the FTIR spectra after the metal desorption and sorbent recycling (in the fifth cycle), the comparisons of the desorption efficiency (which remained close to 100%) and sorption efficiency (which lost only 3-4% for the functionalized sorbent vs. 11% for the raw MC) confirm the outstanding stability of the functionalized sorbents. The study of the pH effect on the metal sorption using the equimolar multi-component solutions shows that the functionalized sorbents have a marked preference for rare earth elements against heavy metals in mild acidic solutions. The sorption performances are correlated with the intrinsic physicochemical characteristics of the metals; the sorption capacities for the grouped metals (according to their ionic charge) are correlated with their ionic index as an indication of the preference for ionic interaction rather than covalent bonding. The acidic leachate of an Egyptian ore generates leachates containing high concentrations of Al, Fe, and U, REEs that can be pre-treated using resins for the recovery of U (Amberlite IRA-400) and REEs (Dowex 50X 8) and for the removal of excessive amounts of Al and Fe (by precipitation). However, the effluent still contains appreciable quantities of REEs. In order to valorize this effluent, MC-UR/S was successfully applied for the sustainable recovery of REEs. We observed that the sorbent has a great selectivity against heavy metals, especially at a pHeq of around 4.2 (iron being the least efficiently separated within this series of competitor ions). The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/molecules27217562/s1](https://www.mdpi.com/article/10.3390/molecules27217562/s1)]([https://www.mdpi.com/article/10.3390/molecules27217562/s1](https://www.mdpi.com/article/10.3390/molecules27217562/s1)), References [23; 27; 29; 33; 34; 47; 48; 51; 52; 68; 74; 75; 76; 77; 78; 79; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 90; 91; 92; 94; 96; 97; 98; 99; 11; 11; 11; 12; 13; 14; 15; 16; 17; 18; 19; 101; 19; 11; 13; 14; 16; 18; 19; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135; 136; 137; 138] are cited in the supplementary materials. Table S1. Examples of functionalized sorbents for rare earth recovery. Table S2a. FTIR spectra of the sorbents (MC-UR, MC-UR/S) before (raw) and after Ce(III) sorption (+Ce(III)) and after the fifth desorption (5 th Des.). Table S2b. FTIR spectra of the sorbents (MC-UR and MC-UR/S) before (raw) and after Ce(III) sorption (+Ce(III)) and after conditioning at pH 5. Table S2c.