EVIDENCE FOR THE NEOFORMATION OF CLAYS UPON SORPTION OF CO(II) AND NI(II) ON SILICATES

The adsorption of metal ions onto phyllosilicates has been classically interpreted in terms of formation of isolated or multinuclear, surfac e complexes or surface precipitates bridged to mineral surface. In the present note it is shown that in several Co(II) and Ni(II) sorption s tudies performed on silicates and silica at low temperature (25-degree s-C), surface clusters of the sorbate metal possess a clay-like local structure therefore pointing to the precipitation of a new hydrous sil icate. This interpretation is based on (1) metal-metal and metal-Si di stances and (2) coordination numbers derived from EXAFS spectroscopy. This neoformation of clays during sorption experiments is thought to o ccur through the interaction of the metal ion with dissolved silicic a cid, the sorbent acting as a source of silicic acid and eventually as a template.