## 3 Results
### Sorption Tests
#### 3.2.4. Sorption Isotherms  
The sorption isotherms report the distribution of the adsorbate between liquid and solid phases, at fixed temperature, at a wide range of concentrations (Figure 7). The profiles for Ni(II) and Zn(II) are very close, indicating their similarity in sorption mechanism. The sorption capacity progressively increases with residual concentration, before reaching an apparent plateau; the maximum sorption capacities are close to 0.45 mmol g\({}^{-1}\). The sorption of mercury shows a different type of curve: the isotherm begins with steep initial slope that reflects a quasi-irreversible behavior (almost rectangular isotherm); the saturation plateau (more marked than for the other metal ions) is reached at Ceq \(\sim\) 1 mmol Hg g\({}^{-1}\). The maximum sorption capacity is significantly lower than for Ni(II) and Zn(II) (i.e., 0.33 mmol Hg g\({}^{-1}\)). The Langmuir, Freundlich and Sips equations were tested for modeling the isotherms (Table 3). The saturation plateau appearing in the figure suggests that the Langmuir or the Sips equations better fit the experimental profile than the Freundlich equation (which is a power-type function). This is confirmed by the statistical parameters in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline
**Model** & **Parameter** & **Ni(II)** & **Zn(II)** & **Hg(II)** \\ \hline Experimental & qeq,exp. (mmol g\({}^{-1}\)) & 0.070 & 0.097 & 0.216 \\ \hline \multirow{3}{*}{PFORE} & qeq,1 (mmol g\({}^{-1}\)) & 0.070 & 0.097 & 0.214 \\  & k\({}_{1}\times 10^{2}\) (min\({}^{-1}\)) & 6.79 & 6.23 & 9.25 \\  & R\({}^{2}\) & 0.980 & 0.977 & 0.980 \\  & AIC & \(-114\) & \(-110\) & \(-86\) \\ \hline \multirow{3}{*}{PSORE} & qeq,2 (mmol g\({}^{-1}\)) & 0.081 & 0.115 & 0.246 \\  & k\({}_{2}\) (L mmol\({}^{-1}\) min\({}^{-1}\)) & 0.985 & 0.608 & 0.445 \\  & R\({}^{2}\) & 0.980 & 0.975 & 0.976 \\  & AIC & \(-111\) & \(-100\) & \(-82\) \\ \hline \multirow{3}{*}{RIDE} & D\({}_{\rm e}\times 10^{12}\) (m\({}^{2}\) min\({}^{-1}\)) & 6.38 & 6.17 & 4.44 \\  & R\({}^{2}\) & 0.978 & 0.971 & 0.970 \\ \cline{1-1}  & AIC & \(-122\) & \(-108\) & \(-79\) \\ \hline \end{tabular}
\end{table} 
Table 2: Ni(II), Zn(II) and Hg(II) uptake kinetics using G@MChs for multi-component solutions—parameters of the models. Figure 7: Sorption isotherms for Ni(II), Zn(II) and Hg(II) using G@MChs for single-component solutions—modeling with the Langmuir equation (C\({}_{0}\): 0.3 mmol L\({}^{-1}\); pH\({}_{0}\): 5.5; pH\({}_{\rm eq}\): 5.75 (Ni), 5.80 (Zn), and 6.01 (Hg); sorbent dose, SD: 1 g L\({}^{-1}\); T: 21 \(\pm\)1 \({}^{\circ}\)C; time: 60 min; agitation speed: 150 rpm). The Langmuir equation best fits the experimental profile; except for Hg(II), where the Sips equation better fits the curve (but the corresponding affinity (infinite) has no physical significance). The Langmuir fits are presented as solid lines in Figure 7. The sorption capacities at saturation of the monolayer (q\({}_{\rm m,L}\)) are consistent with the experimental maximum sorption capacities. The greater affinity of the sorbent for Hg(II), already apparent from the initial steep slope, is confirmed by the values of the affinity coefficient (i.e., b\({}_{\rm L}\)): Hg(II) [18.5] >> Zn(II) [6] > Ni(II) [4.9]. The Langmuir equation is based on the sorption of the adsorbate as a monolayer, without interactions between sorbed molecules, and with homogeneous energy of sorption at the surface of the sorbent. Figure S10 shows the fitting of experimental profiles using the Sips equation (and the parameters summarized in Table 3); in the case of Hg(II), the Sips equation overestimates the sorption capacity in the highest curvature section of the curve (contrary to the Langmuir equation, which underestimates these values), making the isotherm rectangular. Table 4 summarizes the sorption properties of a series of chitosan-based sorbents investigated for the binding of Ni(II), Zn(II), and Hg(II). Apparently, the sorption properties of G@MChs are significantly lower than these alternative materials.