Long-Term Field-Based Evidence for Chemical Pathways Reducing Lead Bioaccessibility in Phosphorus-Amended Urban Soils

Soil lead (Pb) is a major source of human lead exposure, especially for children. Phosphorus (P)-based amendments can transform Pb into stable Pb–phosphate minerals, reducing its bioaccessibility and the risk of human exposure. However, field-based studies in moderately contaminated urban soils are lacking. This study aimed to address this knowledge gap by evaluating the effectiveness of in situ P-based amendments in mitigating Pb exposure risks in three moderately to highly contaminated urban vacant lot sites in Kansas City, MO, with average total Pb concentrations ranging from 387 to 727 mg kg−1, under real field conditions. The study evaluated changes in soil chemical properties, Pb bioaccessibility, and Pb speciation following amendment applications to assess whether shifts in Pb speciation were associated with reductions in Pb bioaccessibility. Treatments included inorganic (commercial fertilizers) and organic (Class A biosolids) P sources, along with an untreated control. Phosphorus-based treatments significantly reduced Pb bioaccessibility over time, with maximum reductions of 34.1%, 52.3%, and 53.2% across the three study sites compared with the levels of the untreated control, with effects persisting for the duration tested (24 months). Bioaccessible Pb showed a positive relationship with soil pH and a negative relationship with resin-extractable P, indicating that increased P availability and slight soil acidification favored Pb immobilization into the more stable Pb-P phases, confirmed by XANES analysis, with pyromorphite comprising up to 34% of total Pb. These results suggest that P-based amendments effectively reduce bioaccessible Pb through precipitation and organic complexation mechanisms.