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

The adsorption of ca. millimolar concentrations of cadmium(II) on freshly p recipitated 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 concentr ations of EDTA will enhance its adsorption to approximately 95%. In the pre sence of EDTA, cadmium adsorption increases with pH despite the fact that 9 9.9% of the dissolved cadmium is present as an anionic cadmium(II) EDTA com plex. The adsorption observed has been modelled using the measured values o f the solubility of aluminium hydroxide and its adsorption of EDTA, the nor mal and the EDTA-enhanced cationic binding of cadmium(II) and the accepted equilibrium constants for EDTA complexation of cadmium(II) and aluminium(ii i). The structures of the dissolved and adsorbed complexes have been inferr ed from XAFS and A NMR spectroscopy.