Lead Bioaccessibility in Soil and Mine Wastes After Immobilization with Phosphate

The immobilization of lead by the reaction with phosphate bearing materials is a promising remediation method for contaminated soils. Low soluble neo-formed lead-phosphate phases similar to chloropyromorphite [Pb₅(PO₄)₃Cl], can control availability and mobility of lead in the environment, and consequently reduce human exposure, if soils are the main contamination pathway. We used three phosphate source materials [NaH₂(PO₄)₃, commercial superphosphate and phosphate rock] to study lead immobilization in soil and mining waste samples. Products were examined after 1, 3 and 6 months of contact. The samples are from a contaminated area by former Pb mining and smelting activities, in southeastern Brazil, where epidemiological studies showed high lead blood levels in local population. The PBET (physiological based extraction test) bioaccessibility test was used to measure changes in the amount of soluble lead after sample treatment. Results show that the most efficient phosphate source was NaH₂(PO₄)₃, which reduced lead solubility to 92% in acidic gastric conditions after the first month of contact. Superphosphate and phosphate rock also diminished Pb solubility, but the effect was more time dependent. None specific Pb–phosphate phases could be identified by XRD in whole treated samples, but the Pb–Ca–P elemental associations, observed on SEM images and EDS spectra of portions of the samples, combined with the reduced solubility, indicate that more insoluble lead phases were formed after the treatment. Based in these results, the in site phosphate application on soils to induce lead immobilization should be considered as a possible alternative to reduce human exposure at the area.