Like most biosorbents, silk sericin and chitosan contain numerous functional groups that can interact and adsorb metal ions.
This is confirmed by the FTIR and XPS analyses of the two biosorbents (cf. Figs. 2, 7 and 8). This gives most biosorbents their high adsorption capacity, but may result in poor selectivity. The zeta potential plots of silk sericin and chitosan showed that both biosorbents have
positively charged surface groups at pH< 4 from protonation of surface functional moieties.
The positively charged surface groups would have higher affinity for the adsorption of AuCl4
− anion than the metal cations (i.e., Co2+, Cu2+, Ni2+, Zn2+) resulting in excellent
adsorption selectivity.
The XPS data in Fig. 7 strongly suggest that gold are adsorbed on the amide groups of the silk sericin, while copper and the other metal cations appeared to have poor affinity for the amide groups of the chitosan and were not adsorbed.
On the other hand, gold and copper are adsorbed by the amino groups of chitosan via charge-interactions and complexation, respectively.
This clearly explains why more coppers are adsorbed at higher pH and why chitosan adsorbed copper more than silver.