The enhanced adsorption of cadmium on hydrous aluminium(III) hydroxide by ethylenediaminetetraacetate

The adsorption of ca. millimolar concentrations of cadmium(II) on freshly precipitated aluminium(III) hydroxide is enhanced, at pH values of below 8, in the presence of ethylenediaminetetraacetate, EDTA, via stabilisation of the adsorbed cation. At levels where the hydroxide phase is in large excess to the cadmium(II), the addition of stoichiometrically equivalent concentrations of EDTA will enhance its adsorption to approximately 95%. In the presence of EDTA, cadmium adsorption increases with pH despite the fact that 99.9% of the dissolved cadmium is present as an anionic cadmium(II) EDTA complex. The adsorption observed has been modelled using the measured values of the solubility of aluminium hydroxide and its adsorption of EDTA, the normal and the EDTA-enhanced cationic binding of cadmium(II) and the accepted equilibrium constants for EDTA complexation of cadmium(II) and aluminium(III). The structures of the dissolved and adsorbed complexes have been inferred from XAFS and Al²⁷ NMR spectroscopy.