IMMOBILIZATION OF LEAD ON SMALL ARMS FIRING RANGES VIA PHOSPHATE AMENDMENT

Small arms firing ranges can serve as point sources for lead pollution into associated watersheds as most bullets are made of copper jacketed elemental lead. Upon impact, these jackets shatter releasing lead fragments that react with the environment to form lead oxides and carbonates. These lead compounds are bioavailable and can be transported off range posing environmental health risks. Efforts to prevent lead migration have included adding phosphate amendments to soil as lead phosphates typically have very low solubility. Previous studies have found conflicting results: in some cases, the newly formed lead phosphates have retarded lead movement but in others, lead mobility increased. In this study, we sought to characterize how differences in the way lead phosphate is precipitated affect lead mobility. Flow through experiments were conducted in six inch long, one inch diameter plastic columns hand packed with clean silica sand and saturated with a 1 mM NaCl solution. Lead oxide, lead nitrate, hydroxyapatite and sodium phosphate served as reactants for lead phosphate precipitation. The reactants were mixed together in the four possible combinations and each solution was applied as a slug to the top of a sand column. The solutions were subsequently carried through the sand by the 1mM NaCl solution. Approximately 300 mls of effluent were collected in 20 ml batches, each batch was measured for pH and then analyzed for total and dissolved lead using atomic absorbance and phosphate using ion chromatography. After the experiment, the sand column was dismantled and sand from the top, middle, and bottom was digested and analyzed for lead. PO4 was not found in the effluent when P was added as hydroxyapatite, but was in the effluent when Na2HPO4 was mixed with PbO. Dissolved lead was present in the effluent when ionic lead was combined with hydroxyapatite implying that the reaction between ionic Pb and solid P is either energetically unfavorable or kinetically too slow to form a lead-phosphate compound. No lead was found in the effluents from runs containing PO4 ions. Sand digestions revealed that in cases where no lead was found in the effluent, the top of the sand column had the highest Pb concentrations suggesting limited Pb mobility under these conditions.