## 3 Results
### Sorption Tests  
#### 3.2.1 Effect of pH  
The pH of the solution influences both the speciation of metal ions (including their ionic charge) and the surface charge of the sorbent. These characteristics directly influence the attraction vs. repulsion mechanisms in the interaction of metal ions with sorbent surface. The sorption properties were carried out in the range pH\({}_{0}\): 3-6.5. The determination of pH\({}_{\text{PZC}}\) (Section 3.1.3.) showed that the sorbent is positively charged in this pH range. The protonation of reactive groups increases with the acidity of the solution; the repulsion effect of protonated sorbent surface with metal cations is thus decreasing when the pH increases. This can explain the general trend observed for the three metal ions: the sorption efficiency increases with equilibrium pH (Figure 4), especially between pH 3 and pH 5-6, above which the sorption efficiency tends to stabilize. Figure S3 shows the speciation diagram of the three metal ions (under the experimental conditions used for the study of pH effect, taking into account metal salt, acid and base used for pH control); the speciation diagram was calculated using Visual Minteq [51]. Nickel is present in its free form in the whole range of pH; other species represent less than 0.5%. Zinc behaves similarly to Ni(II); between pH 3 and 6.5, Zn\({}^{2+}\) exceeds 99.5%, while other species are negligible except ZnCl\({}^{+}\) (about 2.1% at pH 2) and ZnOH\({}^{+}\) (less than 0.9% at pH 7). These metal cations are bound by complexation; the strong competition of protons limits the sorption, which progressively increases with pH. Actually, for the two metal ions, the sorption capacity begins to significantly increase above pH\({}_{\text{eq}}\) 4.5 (pH\({}_{0}\): 3.7-4.7). The case of Hg(II) in Figure 4 differs slightly because a strong increase (almost linear) in metal sorption is observed even at pH\({}_{0}\) 3.5 (pH\({}_{\text{eq}}\) 3.7). Actually, the speciation diagram shows that mercury is mainly present as a neutral species (mainly HgCl\({}_{2}\), up to pH -6, completed above with HgClOH and Hg(OH)\({}_{2}\)), with the exception of about 10% of HgCl\({}_{3}\)\({}^{-}\) anionic species at pH 2. Anionic species can be readily bound onto protonated reactive groups, though at high acidity the counter ions of the acid (herein chloride ions) may strongly compete with HgCl\({}_{3}\)\({}^{-}\) for binding. At higher pH values, the repulsive effect of protonated amine groups reducing the sorption of Ni(II) and Zn(II) does not inhibit the binding of HgCl\({}_{2}\) (and other neutral species). Consequently, the density of counter anions bound onto  
Figure 4: Effect of pH on the sorption efficiency for Ni(II), Zn(II), and Hg(II) using G@MChs (C\({}_{0}\): 0.3 mmol L\({}^{-1}\); sorbent dose, SD: 1 g L\({}^{-1}\); T: 21 \(\pm\) 1 \({}^{\circ}\)C; time: 480 min; agitation speed: 150 rpm). the reactive groups decreases; therefore, the reactive groups become more available for metal complexation. Sorption processes may change the pH of the solution does to proton binding or release by the sorbent. The speciation of the metal ions (which may change during the sorption by displacement of the equilibrium of complexation) may also affect the final pH of the solution. The comparison of initial and equilibrium pH may be helpful for understanding the mechanisms involved in metal binding. Figure S4 shows these comparisons. Whatever the metal considered, the final pH is systematically increased during the sorption and the \(\Delta\)pH may reach up to 1-1.5 pH unit (especially for nickel). Actually, the pH variations globally follow the trend: Hg(II) \(<\) Zn(II) \(<\) Ni(II), inversely with the ranking in sorption efficiency Hg(II) [\(\sim\)86%] \(>\) Zn(II) [\(\sim\)71%] \(>\) Ni(II) [\(\sim\)55%]. The sorption of mercury consumes less protons than the binding of other divalent cations. These results offer a first indication as to the preference of the sorbent for metal ions according the series: Hg(II) \(>\) Zn(II) \(>\) Ni(II).