Determining the fate of lead (Pb) and phosphorus (P) in alkaline Pb-polluted soils amended with P and acidified using...

The effect of acidification on lead (Pb) and phosphorus (P) speciation in alkaline Pb-polluted soils that are amended with P to stabilize Pb is still unclear. It was studied in three alkaline Pb-polluted soils containing specific amounts of Soil Organic Matter (SOM), using multiple synchrotron-based techniques, i.e. bulk X-ray Absorption Fine Structure (XAFS) spectroscopy at Pb LIII- and P K-edges, micro-X-ray Fluorescence (μ-XRF), and micro-X-ray Diffraction (μ-XRD). These techniques provided unambiguous evidences that the formation of pyromorphite, i.e. the desired Pb stabilized chemical form, was severely limited in the acidified soil samples amended with fish bones or phosphoric acid (H₃PO₄). Most Pb present in the H₃PO₄-amended soil samples did not convert to pyromorphite due to Pb and P leaching and PbSO_4(s) formation. In contrast, most Pb present in the fish bone-amended soil samples was unaffected by acidification and did not convert to pyromorphite as it was inaccessible to soil solution or retained by SOM, similarly to P. Additionally, Pb-SOM association increased with increasing SOM content. Results had important implications on the applicability of the P-based method to stabilize Pb within the first centimeters below surface of Pb-polluted alkaline soils, which potentially represent the most hazardous part of these soils.