Stabilizing lead bullets in shooting range soil by phosphate-based surface coating

Soil lead (Pb) is well known as a threat to human health and ecosystem. Although relatively insoluble, lead bullets in shooting range soil can be readily released into soluble forms through natural weathering processes and thus pose significant human and environmental risks. In this study, laboratory experiments were conducted to investigate if the Pb bullets in shooting range soil can be stabilized through surface coating of phosphate-based materials. Results indicated that FePO4 or AlPO4 coatings, insoluble metal phosphates, have been successfully formed on the surface of the Pb bullets. The EPA Toxicity Characteristic Leaching Procedure (TCLP) test showed that FePO4 or AlPO4 surface coating would effectively reduce the Pb solubility or leachability of the bullets. The surface coating under pH of <5.5 for 7 days could achieve 92-100% reduction, with 85-98% by FePO4 coating and 77-98% by AlPO4 coating as compared with the non-coating. Leachable Pb concentration in the contaminated shooting range soil was reduced by 85-98% or 77-98% as a result of the FePO4 or AlPO4 solution treatment. This study demonstrated that the FePO4 or AlPO4-based surface coating on lead bullets can effectively inhibit the Pb weathering and significantly reduce the Pb release from soil through in situ chemical stabilization, which could be potentially applicable as a cost-effective and environmental-sound technology for the remediation of Pb-contaminated shooting range soil.