Separation of transition metals using inorganic adsorbents modified with chelating ligands

The extraction and separation of transition metals has been investigated using several inorganic adsorbents (silica gel, MCM-41, and aluminum oxide), modified with ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) as chelating ligands. The metal ions in aqueous solution were coordinated successfully by the ligands on the surface of the adsorbents under moderate conditions. Metal-ligand complexes formed on the adsorbents effectively in the region of low pH (1-2) but scarcely formed at pH > 4. Silica gel modified with EDTA (silicaED) was shown to be the most effective adsorbent for the extraction of metals and to form specific metal-EDTA complexes, with stability constants lying in the order Cu2+ > Ni2+ > VO2+ > Zn2+ > CO2+ > Mn2+. The relatively good separation of these six adjacent metals was therefore also carried out using the silicaED. The metals coordinated on the silicaED could be desorbed completely by putting the material into contact with 1 mol/L aqueous HCl solution, such that the silicaED recovered could be reused for further separation.