## 3 Results
### Sorption Tests
#### 3.2.6 Selectivity--Sorption from Multi-Component Solutions  
The evaluation of the sorption of metal ions in complex solutions (involving different metal ions; herein Zn(II), Cd(II), Ni(II), Cu(II), Hg(II) and Pb(II)) is an important criterion in the design of sorbents. Indeed, the selectivity issue is critical for separating the metals in order to enhance their valorization. Sorption tests were performed from multi-component solutions with concentrations ranging between 10 and 200 mg L\({}^{-1}\), at pH 5.5. Figure S13 shows the sorption isotherms obtained for the different metal ions from these multi-component solutions; the sorption isotherms are incomplete, the saturation \begin{table}
\begin{tabular}{l l c c c c c} \hline
**Metal** & **Sorbent** & **pH0** & **T** & **teq** & **qmL** & **bL** & **Ref.** \\ \hline MCSB & - & 298 & 60 & 3.9\(\cdot\) 10\({}^{-4}\) (a) & 1160 & [54] \\ GLU-MCSB & - & 298 & 60 & 7.8\(\cdot\) 10\({}^{-4}\) (a) & 5465 & [54] \\ Amino-thiocarbamate derivative of alginate, carboxymethyl & 6.0 & 298 & 180 & 2.93 & 354 & [55] \\ chitosan and TiO\({}_{2}\) & 7.0 & 298 & 1440 & 0.990 (b) & - & [17] \\ Hydrazinyl amine magnetite-chitosan & 5.0–5.1 & 298 & 1440 & 4.33 & 2.35 & [56] \\ Chitosan-vanillin & 4.0 & 303 & 1440 & 0.324 & 30.6 & [57] \\ Magnetic chitosan hexyl acrylate (Mag-CSg-HA) & 5.5 & 293 & 600 & 2.08 & 9.39 & [58] \\ Anidoximated & - & 298 & 60 & 7.8\(\cdot\) 10\({}^{-4}\) (a) & 5465 & [54] \\ chitosan/acrylamide/acrylonitrile/3-dimethylaminoallyl & 5.0 & 303 & 720 & 3.64 & 0.177 & [59] \\ phosphoric acid & - & 6 & 298 & 360 & 1.85 & 13.5 & [60] \\ Magnetic activated carbon/chitosan beads & 6 & 298 & 240 & 0.375 & 1.67 & [61] \\ PVA/chitosan/ZnO-NH\({}_{2}\) nano-fiber & 6 & 298 & 240 & 0.375 & 1.67 & [61] \\
4-aminobenzzoic acid grafted chitosan & 7 & 298 & 60 & 2.34 & 0.317 & [62] \\ Si/Fe nanostructures -chitosan polymer composites (CF4) & - & 298 & 50 & 1.32 & 21.2 & [63] \\ G@MChs & 5.5 & 293 & 120 & 0.497 & 4.94 & herein \\ \hline EDTA-modified \(\gamma\)-MnO\({}_{2}\)/Chs magnetic nanocomposite & 6 & 298 & 110 & 2.08 & 5.69 & [64] \\ Carboxylate funct. chitosan copolymer (CFCCPeCOOH) & 6 & 313 & 60 & 0.369 & 175.2 & [65] \\ Formaldehyde cross-linked chitosan & 4 & 298 & 50 & 0.0048 & 3633 & [66] \\
2-Aminobenzic acid grafted chitosan & 6 & 298 & 60 & 2.00 & 0.281 & [62] \\ Si/Fe nanostructures-chitosan polymer composites (CF4) & - & 298 & 50 & 1.04 & 14.3 & [63] \\ G@MChs & 5.80 & 293 & 120 & 0.457 & 6.01 & herein \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison of Ni(II), Zn(II) and Hg(II) sorption properties (temperature, T, K; teq, min, qm,L, mmol g\({}^{-1}\); bL, L mmol\({}^{-1}\)) for alternative chitosan-based sorbents. plateau was not reached for some of these metal ions (i.e., Pb(II), Ni(II) and Cu(II)). The sorption capacities can be roughly ranked according the series (compared for Ce\({}_{\rm eq}\sim\)0.4 mmol L\({}^{-1}\)): Hg(II) > Cu(II) > Pb(II) > Ni(II) > Zn(II) > Cd(II). The saturation of the sorbent has not been reached; therefore, the trend should be considered only indicative. The poor efficiency for cadmium may be explained by intermediary and hard base characteristics of the reactive groups (carboxylate and amine groups) on the sorbent, while cadmium is part of the strong acid metals. According to Pearson's rules, G@MChs (classified as intermediary/hard base) has preferential affinity for borderline and hard elements (mercury behavior does not follow this trend because the formation of chloro-mercuric species modulates this effect). The selectivity coefficient is calculated as the ratio of distribution ratios:  
\[SC_{Mc1/Mc2}=\frac{D(Me_{1})}{D(Me_{2})}=\frac{q_{eq,Mc1}\times C_{eq,Mc2}}{q_{ eq,Me2}\times C_{eq,Mc1}} \tag{1}\]  
Cadmium being the metal with the lowest affinity for G@MChs, the selectivity coefficients were determined with reference to this metal in Figure S14 (\(SC_{Cd/Cd}=1\)); the different \(SC_{Metal/Cd}\) values are compared for the different metal concentrations (10-200 mg L\({}^{-1}\)).