Novel modification of chitosan hydrogel beads for improved properties as an adsorbent

Chitosan, a natural polysaccharide, has increasingly been studied as an adsorbent for heavy metal removal in recent years. The amine groups in chitosan are generally known to be the main adsorption sites for heavy metal ions. However, due to the instability of chitosan in acidic solutions, chemical crosslinking has often been used to enhance the acidic resistance of the material. Since most of the chemical crosslinking agents are prone to act on the amine groups instead of the hydroxyl groups in chitosan, some of the amine groups will be consumed in the crosslinking reaction, and therefore, the adsorption capacity of the adsorbent reduced. In this study, chitosan hydrogel beads were modified by formaldehyde solution to shelter the amine groups before the crosslinking reaction with ethylene glycol diglycidyl ether (EDGE), and then the amine groups of the modified chitosan hydrogel beads after the crosslinking were released by a strong acid treatment. Fourier Transform Infrared (FTIR) Spectroscopy and X-ray photoelectron spectroscopy (XPS) revealed that the crosslinking reaction took place with the hydroxyl groups in chitosan when the amine groups were sheltered. Batch adsorption tests for copper ion removal showed that the chitosan hydrogel beads through the novel crosslinking reaction had very good adsorption capacities in a wide solution pH range. Both the primary amine groups and the –N= groups were found to interact with copper ions in the adsorption.