Binding of metal cations by N-carboxymethyl chitosans in water

Abstract

The binding of metal cations by N-carboxymethyl chitosan in dilute aqueous solution was studied as a function of charge density, temperature and pH. The techniques employed were absorption and circular dichroism measurements, potentiometric titration, dilatometry and isothermal microcalorimetry. The composition of the polymer significantly affected the binding of Ni(II), Co(II) and Pb(II), and had a small influence on the extent of binding of Cu(II). In any case, the density of glycine residues affected the binding constant with all the counterions studied, thus confirming that this residue is engaged in the complex formation. The whole of the optical, chirooptical and thermodynamic evidence showed that, at least in the case of Cu(II), the ions are bound by both carboxylic and amino groups. The temperature did not affect the extent of binding, at least in the case of Cu(II) and Pb(II) ions. Circular dichroism spectra, however, showed that these counterions are able to interact with NHCOgroups at high temperatures (61°C), very likely owing to the enhanced mobility of the polymer chain. Lowering pH depressed the binding ability of N-carboxymethyl chitosan towards Cu(II) and Pb(II) ions, the effect being much more pronounced in the case of Pb(II). Moreover, a refined method for collecting absorption spectra of the complex between the polymer and Cu(II) or Pb(II) ions, without interference with absorbance by undesired species, was tested and applied. The charge-transfer band exhibited by the N-carboxymethyl chitosan-Cu(II) system, recorded following this method, showed interesting features, on the basis of which the site of the binding of Cu(II) on to N-carboxymethyl chitosan was confirmed.