Field assessment of treatment efficacy by three methods of phosphoric acid application in lead-contaminated urban soil

In situ soil treatment using phosphoric acid (H3PO4) may be an effective remedial technology for immobilizing soil Pb and reducing Pb risk to human health and ecosystem. The treatment efficacy of three H3PO4 application methods was assessed in a smelter-contaminated urban soil located in the Jasper County Superfund Site, Missouri. Soil, with an average of 3529 mg Pb kg 1 and in the 2- by 4-m plot size, was treated with H3PO4 at a rate of 10 g P kg 1 in four replicates by each of three methods: rototilling; surface application; pressure injection. Three soil cores, 2.5-cm diameter and 30-cm long, were taken from each plot before and 90 days after treatment and analyzed for soluble P, bioaccessible Pb and solid-Pb speciation. Applications of H3PO4 induced the heterogeneity of soluble P in soil, with the highest concentrations in the surface. Three application methods mixed the H3PO4 more effectively in the horizontals than the verticals of treated soil zone. The H3PO4 applications significantly reduced Pb bioaccessibility in the soil, which was influenced by the concentrations of soil soluble P and solid-Pb species. The risk reductions of soil Pb were achieved by formation of pyromorphites or pyromorphite-like minerals. The rototilling appears to be the most effective treatment method in context of the homogeneity of soluble P and the reduction of Pb bioaccessibility in treated soil.