Heterogeneous Cross-Linking of Chitosan Gel Beads: Kinetics, Modeling, and Influence on Cadmium Ion Adsorption Capacity

Abstract

Chitosan, a linear biopolymer of glucosamine residues, selectively adsorbs transition-metal ions such as cadmium from dilute solution. In order to process chitosan into a more durable form, a 5 wt % chitosan solution was cast into spherical gel beads of 3 mm diameter and then reacted with glutaraldehyde at free amine sites to form imine cross-links between linear chitosan chains. The rate processes of the heterogeneous cross-linking reaction and the effect of cross-linking on the cadmium ion adsorption capacity were determined. The cross-linking reaction was complete within 48 h at 27 °C, and the final extent of cross-linking ranged from 0.07 to 2.40 mol of glutaraldehyde consumed/mol of amine. Heterogeneous cross-linking was modeled as a shrinking core process where the molecular diffusion of glutaraldehyde through the cross-linked shell of the gel bead limited the overall rate of glutaraldehyde consumption. The effective diffusion coefficient of glutaraldehyde through the cross-linked layer was 4.7 × 10^-8 cm²/s. The saturation adsorption capacity of cadmium ions on the cross-linked gel beads exponentially decreased from 250 to 100 mg of Cd/g of chitosan as the extent of cross-linking increased from 0 to 1.3 mol of glutaraldehyde consumed/mol of amine. At higher extents of cross-linking, the saturation adsorption capacity remained at 100 mg of Cd/g of chitosan. Highly porous chitosan beads formed by freeze-drying of cross-linked gel beads had the same cadmium ion adsorption capacity as the cross-linked gel beads over the same extents of cross-linking.