Synthesis of carboxyl-introduced chitosan with C₂ amine groups protected and its use in copper (II) removal

A route for the formation of carboxyl-introduced chitosan (CI-CS) with C₂ amine groups protected (CIAP-CS) was investigated to improve copper (II) adsorption. First, the C₂ amine groups of the chitosan (CS) were protected via a Schiff-base reaction by benzaldehyde. Then the product was obtained by the introduction of pyromellitic dianhydride to the C₆ hydroxyl groups on CS via epichlorohydrin. The last product was obtained by removing the Schiff base with dilute hydrochloride solution. CI-CS without C₂ amine groups protected was directly synthesized as well. The adsorbents were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The adsorption properties for copper (II) were investigated. FTIR spectroscopy and XPS clearly showed that most of the amine groups in CS were converted to -N = CH₂ groups after the benzaldehyde treatment and that no cross-linking reactions with CS were involved; the HCl treatment after the cross-linking reaction effectively released nitrogen atoms protected into the form of the primary amine again. The results confirm that the CIAP-CS cross-linked with the new method had significantly greater adsorption capacities than the CI-CS cross-linked directly with CS. Mechanism study revealed that the increased adsorption performance is attributed to the large number of primary carboxyl and amine groups available on the surfaces of the CIAP-CS. The adsorption mechanism is based on ion exchange and chelating action, and the adsorption process is mainly chemisorption.