Oxalate extraction of Pb and Zn from polluted soils: Solubility limitations

Oxalate (Ox) was used to extract Pb and Zn from industrially contaminated soils. Although Ox effectively releases metals bound by hydrous oxide soil components, it forms insoluble salts with some heavy metals unlike conventional extractants (e.g., EDTA). The insolubility of PbOx(s) (K_sp=2.74 × 10^-11) precluded the use of Ox as a single-step extradant even for soils mildly contaminated with Pb. The usefulness of Ox as a Zn extradant, however, depends on the level of soil contamination. A Zn solubility model, based on published equilibrium constants, was developed to assess Ox suitability as a function of system conditions. Precipitation of ZnOx(z) hindered Zn recovery under acidic conditions where formation of soluble oxalato complexes was small. For pH < 3, the presence of 1 M Ox actually reduced Zn release compared to simple acid washing. Although Ox displaces oxide-bound metals and thus is potentially useful in soil washing, solubility limitations must be defined for effective remediation of metal-laden soils.