Lead immobilization using phosphoric acid in a smelter-contaminated urban soil

Transformation of soil lead (Pb) to pyromorphite, a lead phosphate, may be a cost-effective remedial strategy for immobilizing soil Pb and reducing Pb bioavailability, Soil treatment using phosphoric acid (H3PO4) was assessed for its efficacy to reduce Pb solubility and bioaccessibility. Soil contai ning 4360 mg of Pb kg(-1), collected from a smelter-contaminated site in Jo plin, MO, was reacted with 1250, 2500, 5000, and 10 000 mg of P kg(-1) as H 3PO4. The reaction was followed by measurements of Pb bioaccessibility, sol ubility products, and microprobe analyses. Soluble Pb concentration in the soil decreased with increasing H3PO4 addition. Adding 10 000 mg of P kg(-1) reduced bioaccessible Pb by 60%. The logarithm of bioaccessible Pb decreas ed as a linear function of increasing H3PO4 addition with an R-2 of 0.989. A higher soil/solution ratio was required to extract bioaccessible Pb after the treatment. Microprobe analyses showed that the Pb particles contained P and Cl after the reaction, and the spectra generated by the wavelength-di spersive spectrometer were similar to those of synthetic chloropyromorphite . Lead solubility in the P-treated soil was less than predicted for hydroxy pyromorphite [Pb-5(PO4)(3)OH] and greater than predicted for chloropyromorp hite [Pb-5(PO4)(3)Cl]. The P treatment caused similar to 23% redistribution of soil Pb from the clay and silt size fractions to the sand fraction. Soi l treatment with H3PO4 resulted in the formation of a compound similar to c hloropyromorphite and reduced bioaccessibility of soil Pb, which may have a potential as an in situ technique for Pb-contaminated soil remediation.