Extraction of heavy metal ions sorbed on clays

The ability of seventeen different chemical solutions to displace heavy metal ions (Pb, Zn, Cu, Cd), pre-adsorbed on clay (kaolinite, illite and montmorillonite) at either pH 5 or 7, has been examined and the relative efficiency of each extractant ascertained. Of the reagents used, only EDTA (0.001 M, pH 7) quantitatively released all four ions from the three clays; oxalic acid (0.1 M, pH 3.3), totally displaced at least three ions from each clay. Other reagents, for example ammonium oxalate (0.1 M), ammonium nitrate (0.01 M), nitric acid (0.1 M) and sodium citrate (0.01 M) effectively displaced one or more heavy metal ions from individual clays. Near quantitative displacement by an excess of Na (0.1 M) or Ca (0.05 M) ions was observed only on montmorillonite. Pre-adsorption at pH 7 was accompanied by precipitation of excess metal ion, and the extraction efficiency in these systems was determined by the ability of the reagent to both dissolve hydrous oxide species and displace sorbed metal ions. The implications of the results with respect to the nature of the adsorption process and relevance to environmental systems have been considered.