Extractive decontamination of metal-polluted soils using oxalate

Oxalate (Ox) was investigated as an extractant for decontaminating two metal-polluted soils, one with elevated total zinc (ZnT = 2700 mg kg-1) from the Palmerton, Pennsylvania smelter site and the other from a grossly contaminated (PbT = 210 000 mg kg-1) automobile battery recycling facility in Indiana. Metal retention within the soils was substantially different as shown by sequential fractionation experiments. High Zn removal (>80%) was achieved with 1.0 M Ox when Zn existed predominantly in non-detrital metal fractions. However, Ox was an unsuitable Pb extractant due to the sparing solubility of PbOx(s). Despite the dramatically higher stability of ZnEDTA2- (log K = 16.5) compared to ZnOx (log K = 3.4), Ox released more Zn than EDTA from the Palmerton soil because 40% of ZnT was associated with the oxide fraction. Extract analysis indicated that Ox, but nor EDTA, dissolved soil Fe oxides in the 24 hr extraction period. When contaminating metals are associated with soil oxides, Ox may be a superior extractant to powerful chelants like EDTA. It is essential to establish thoroughly metal solution chemistry and fixation behavior within the soil when extractive decontamination is proposed for site remediation.