Role of Fe in extractive decontamination of Pb-polluted soils

The role of Fe in Pb solubilization by ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) from a highly contaminated (21% Pb by weight) battery reclamation site soil was investigated using batch extraction and sequential fractionation experiments. Based on relative complexing ability, soluble Fe(III) should inhibit Pb recovery using chelating agents, particularly under acidic conditions. Despite a large pool of available Fe (6.7% of soil by weight) and a marked increase in soluble Fe(III) with decreasing pH, Pb solubilization was nearly pH invariant. Since only 12% Fe(T) compared to 86% of Pb(T) was solubilized by 4 x 10^-2 M EDTA at pH 6, Pb and Fe apparently exist in differentially reactive forms within the soil. A nine-step chemical fractionation procedure confirmed that 80% of the soil Pb was present in non-detrital (soluble, weakly sorbed) forms in contrast to 94% of Fe existing in detrital (oxide occluded, residual) fractions. The kinetically slow dissolution of indigenous Fe oxides compared to anthropogenically-added Pb makes unnecessary strongly alkaline conditions to reduce competitive chelation by Fe in soil decontamination processes. Although generally regarded as important in metal fixation, the Fe oxides retained <1% of the total soil Pb.