The effect of co-precipitation on cadmium(ii) adsorption on hydrous aluminium(iii) hydroxide in the presence of a range of chelates

The adsorption of cadmium(II) on freshly precipitated aluminium(III) hydroxide in the presence of a range of chelates has been investigated. By precipitating the metal, chelate and adsorbent together it is possible to change the pH variation of the metal-complex adsorption from anionic, “ligand-like”, binding to cationic binding. This is a general phenomenon and is explained by the formation of a ternary Al–O–Cd–L surface species. As a consequence of the preparation method, the pH edge is found to shift to lower pH values in the presence of the chelate which gives rise to an apparent increase in adsorption of Cd2+. This increase is, in general, most pronounced at [chelate]/[metal] > 1. Computer modelling shows that the observed trends result from the competition between Al–O–Cd–L and Al–L for the available aluminium(III) binding sites. The enhanced adsorption in the presence of phenylenediaminetetraacetate is anomalous since it is observed at a [chelate]/[metal] ≈ 0.1 and cannot be interpreted by the simple competition model.