Fig. 4a and b plot the single component batch adsorptions of Au(III) and Cu(II) ions on silk sericin and chitosan, respectively.
The experiments were performed at room temperature and pH 2.5.
The silk sericin adsorbed 1.0mmolg−1 (i.e., 0.20 g g−1) of gold and none of the copper ions. It has comparable adsorption capacity and selectivity as the aminopropylated MCM-41 adsorbent.
The chitosan appears to adsorb large quantities of gold (ca. 3.3mmolg−1 or 0.65 g g−1) similar to the report by Donia et al.
However, the rapid process and observed change in solution coloration suggest that metal reduction and micro-precipitation are important contributing factors in the observed gold removal by chitosan.
Fig. 4b shows that chitosan, unlike silk sericin, also adsorbed copper (i.e., 0.35mmolg−1). The Freundlich adsorption model provides a good description of adsorption on heterogeneous surfaces. It was used to fit the adsorption data on both silk sericin and chitosan to reflect the diversity and complexity of adsorption sites on these biosorbents.
Gold adsorption on silk sericin had a calculated KF and bF values of respectively 0.87 and 0.37 as compared to 315 and 1.5 for chitosan.
Thus, indicating that chitosan has higher adsorption capacity and stronger affinity for gold than the silk sericin.
The pH was monitored throughout the adsorption process and fluctuates between pH 2.4 and 2.55.