EXAFS speciation and phytoavailability of Pb in a contaminated soil amended with compost and gypsum

Due to unregulated uses of lead pellets for hunting purposes in Japan, soils and sediments in some river basins and wetlands have become highly contaminated with Pb. Deterioration of natural vegetation has occurred sporadically in these areas, and therefore revegetation is needed for ecological restoration. The objectives of the present study were to assess the effects of surface applications of compost and gypsum amendments on Pb availability to a watercress plant (Nasturtium officinale W.T. Aiton) and molecular-scale speciation of Pb in soil solid phases. The compost and gypsum amendments significantly decreased dissolved Pb and Sb in pore water. The concentration of Pb in aboveground plant tissues was 190 mg kg− 1 in the control soil and was reduced to < 20 mg kg− 1 in the compost and gypsum-amended soils. The concentration of Sb in plants grown in the control soil was 13 mg kg− 1, whereas that in the soils receiving compost and gypsum decreased below detectable levels. Redox potential was higher in vegetated soils (ave. 349 mV) than in the unvegetated soils (ave. 99 mV) due to oxygen introduced by plant roots. Extended X-ray absorption fine structure (EXAFS) spectroscopy illustrated that Pb occurred as Pb sorbed on birnessite and/or ferrihydrite (Pb–Mn/Fe, ~ 60%) and Pb sorbed on organic matter (Pb-org, ~ 15%), and galena (PbS, ~ 10%) in the vegetated and unvegetated control soils. The compost amendment increased the proportion of Pb-org by 2-fold than in the control soils. The amended soils with plant growth decreased the proportion of Pb–Mn/Fe phases by half of that without plant growth. Galena and anglesite (PbSO4) were not detected in compost-amended soils and even in gypsum-amended soils since a significant soil reduction to anoxic levels did not occur in the entire soil. The present study indicated that, under flooded conditions, surface applications of compost and gypsum amendments reduced plant Pb uptake from the Pb contaminated soil.

Research Highlights
► EXAFS spectroscopy determined the predominance of Pb sorbed on organic matter, ferrihydrite, and birnessite (MnO2) in reducing soils.
► The distribution of these Pb species was altered in rhizosphere.
► Compost and gypsum amendments reduced Pb and Sb uptake by plants.