## 3 Results and Discussion
### Sorption Properties
#### 3.2.5 Metal Desorption and Sorbent Recycling  
As the sorption of cerium is favored at pH 5, acidic solutions are logically good candidates for reversing the binding of the metal from loaded sorbents. This is confirmed by a scan of the literature on cerium sorption, as 0.2 M HCl solutions were successfully used for Ce(III) desorption from metal-bound biosorbents [26; 79; 80]. [PERSON] et al. [28] used 0.05 M HCl solutions in the case of brown algal biomass. [PERSON] et al. [48] used a combination of HCl and CaCl\({}_{2}\) (0.5 M/0.2 M CaCl\({}_{2}\) solution) for the elution of REEs from algal/PEI beads (calcium chloride contributes to the stabilizing of the alginate network). [PERSON] et al. [31] used zero-valent iron biochar composites for the sorption of REEs, and they found that molar acidic solutions successfully desorb rare earths with a preference for Ce(III) \(>\) Nd(III) \(>\) La(III), with HNO\({}_{3}\) being more efficient (82-95%) than HCl (80-93%) and H\({}_{2}\)SO\({}_{4}\) (77-86%). [PERSON] et al. [81], using another type of magnetic chitosan derivative,  
Figure 5: Selectivity tests for Ce(III) sorption using the MC-UR (**a**) and MC-UR/S (**b**) sorbents and the multicomponent equimolar solutions—effect of the pH\({}_{\text{eq}}\) on SC\({}_{\text{Ce/metal}}\) (SD: 1 g L\({}^{-1}\); C\({}_{0}\): \(\approx\)1 mmol L\({}^{-1}\); time: 10 h; v: 210 rpm; T: 22 \(\pm\) 1 \({}^{\circ}\)C).  
showed that even with a 0.5 HCl solution, the release of iron does not exceed 1.1%, meaning that the magnetite remains relatively stable. Herein, the HCl concentration was set to 0.2 M to limit the possible leakage of the iron and degradation of the magnetite. Figure S18 compares the Ce(III) desorption kinetics of MC, MC-UR, and MC-UR/S. The metal is completely desorbed within 20-40 min, and the required time increases according to the order of: MC-UR/S < MC < MC-UR. This is slightly faster than the proper uptake kinetics.  
After demonstrating the effectiveness of the elution, the recycling of the sorbent was evaluated over five successive cycles of sorption and desorption (Table 1). Comparing the sorption and desorption efficiencies, we can see that the cerium is systematically, completely eluted, meaning that the metal is not accumulated during the sorption lifecycle. On the other hand, the sorption efficiency progressively decreases. However, the loss in the metal recovery remains lower than 9% over the fifth cycle for the MC sorbent. The stability is even better in the case of the functionalized adsorbents; the loss in cerium removal is diminished by 3.9% and 3.1% for MC-UR and MC-UR/S, respectively. The functionalization enhances the stability of the sorption performance.  
The weak loss in the sorption efficiency is apparently not associated with the progressive saturation of the sorbent, since desorption is complete. Therefore, this may be due to physicochemical changes derived from the sorbent processing. The successive incidences of contact with pH 5 and the 0.2 M HCl solutions may cause some alterations in the sorbents. The FTIR spectra of the sorbents after metal desorption (in the fifth cycle) were compared with the spectra of the original materials but also with their spectra after being exposed to the 0.2 M HCl solution (to isolate the respective contributions of the desorption and pH change, see Section SB in the Supplementary Materials and Figure 1).