Synthesis of a chitosan derivative recognizing planar metal ion and its selective adsorption equilibria of copper(II) over iron(III)

In order to improve the selectivity of copper(II)/iron(III), crosslinked N-(2-pyridylmethyl)chitosan (PMC) with lower affinity for iron(III) was synthesized by crosslinking the Schiff's base formed in order to prevent amino groups to be metal adsorption sites from a attack of (chloromethyl)oxirane. The final chitosan derivative, PMC, was obtained by reducing the imine moiety of the Schiff's base with sodium borohydride. By comparing the selectivity of PMC for copper(II)/iron(III) with those of the crosslinked copper-complexed chitosan (CLC), it was found that the order of the selectivity towards copper(Il)/iron(III) was reversed. Copper(II) was selectively adsorbed on PMC at a lower pH by 4 units compared with on CLC from 1 mol dm(-3) aqueous ammonium nitrate solution. PMC selectively adsorbed also nickel(II) and palladium(II) at a lower pH region, while other metal ions such as cadmium(II), zinc(II), cobalt(II) which form an octahedral type of complexes were adsorbed at almost the same pH region as around 3. This suggests that PMC selectively recognized the metal ions that form a planar type of complexes. Only copper(II) or palladium(II) was selectively adsorbed on PMC even from the mixture containing 50-fold iron(III) or copper(II), respectively. It was found by examining the adsorption stoichiometrics that copper(II) was adsorbed as a chelate complex accompanied by two nitrate anions as counter-ions. (C) 1998 Elsevier Science B.V. All rights reserved.