Selective extraction of metal ions associated with humic acids

The ability of a range of electrolyte solutions to release metal ions (Cu, Pb, Cd, Zn) presorbed on two samples of humic acid has been investigated. Though treatment with mineral acid or a chelating agent released a high proportion of the retained metal ion, recoveries were never total. Concentrated salt solutions displaced about 80% of the retained Cd or Zn, and about half of any Cu or Pb held by the organic matter, which indicates that most of the adsorbed metal ion is exchangeable, the extraction efficiency being controlled by competing equilibria. The effect of added clay suspensions was also examined. Analytical procedures for fractionating the total metal content of soils into subgroups have been assessed against the observed extraction behaviour.