Adsorption behaviors of some metal ions on chitosan modified with EDTA-type ligand

Abstract

Ethylenediaminetetraacetic acid-type chitosan (EDTA-type chitosan) was synthesized to examine its adsorption behavior for several base metal ions as well as a series of rare earth metals from dilute sulfuric acid solution. The EDTA type functional group was incorporated onto chitosan nearly quantitatively. Unlike adsorption with original chitosan, that with EDTA-type chitosan takes place at a lower pH than that with diethylenetriamine-pentaacetic acid-type chitosan (DTPA-type chitosan); for example, copper(II)was adsorped at pH=0 on EDTA-type chitosan but at pH=4 on original chitosan. The selectivity series for the adsorption of base metals was as follows: Cu²⁺ ≅ MoO₂²⁺ > Ni²⁺ > VO²⁺ > Zn²⁺ ≥ Co²⁺ > Al³⁺. The plots of the distribution ratio of trivalent rare earths against pH lie on straight lines with a slope of 3, suggesting that adsorption takes place by a cation-exchange mechanism. The apparent equilibrium constants for adsorption on EDTA-type chitosan were evaluated as well as those on DTPA-type chitosan. The sequence of the apparent equilibrium constants with a series of rare earths was roughly consistent with that of the stability constants in the case of EDTA-type chitosan as well as with DTPA-type chitosan. Consequently, EDTA and DTPA are still functional after immobilization on chitosan. The EDTA-type chitosan seems to exhibit promise for the mutual separation of adjacent rare earths except for middle rare earths.