A Novel Amine-Shielded Surface Cross-Linking of Chitosan Hydrogel Beads for Enhanced Metal Adsorption Performance

A new amine-shielded cross-linking method of the chitosan hydrogel beads (CHBs) was investigated to improve the metal adsorption performance of the cross-linked beads. The CHBs were reacted with formaldehyde before the beads were cross-linked with EGDE, and, finally, the amine groups of cross-linked CHBs were released by HCl treatment. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy clearly showed that most of the amine groups in chitosan were converted to the −N CH 2 groups after the formaldehyde treatment and they were not involved in the cross-linking reaction with EGDE, and the HCl treatment after the cross-linking reaction effectively released these shielded nitrogen atoms into the form of the primary amine again. Copper ion adsorption experiments confirmed that the CHBs cross-linked with the new method had significantly greater adsorption capacities than the beads cross-linked directly with EGDE. Mechanism study revealed that the increased adsorption performance was attributed to the large number of primary amine groups available on the surfaces of the cross-linked beads from the new cross-linking method.