USING PHOSPHATE TO IMMOBILIZE LEAD ON SMALL ARMS FIRING RANGES

Copper jacketed lead bullets are ubiquitous on small arms firing ranges. The jackets on these bullets shatter upon impact with the ground releasing lead fragments to the environment. These elemental lead fragments react with soil compounds, water and gases to form various lead oxides and carbonates. As these lead compounds migrate into associated ground and surface waters, they pose a significant health threat to humans and surrounding ecosystems. A potential solution to this problem is adding phosphate to the soil to prevent lead migration as lead phosphates typically have very low solubilities. Previous studies, however, have shown varied results with phosphate both retarding and enhancing lead movement. Here we investigate how different forms of lead and phosphate affect lead's mobility. We conducted flow through experiments in six and twelve inch long, one inch diameter plastic columns hand packed with clean silica sand and saturated with unbuffered (1 mM NaCl, pH 5.65) and buffered (1 mM NaCl, 1x10-5 mM Na2CO3, pH 7.8) eluent solutions. We created four different starting lead suspensions using combinations of lead oxide or lead nitrate and hydroxyapatite or sodium phosphate. Five ml of each lead suspension was introduced as a slug to the sand column top. Eluent solutions were passed through the columns by gravity at flowrates of about 5 mL/minute. Approximately 300 – 400 total mls of effluent were collected in 20 ml batches from the bottom of the column. The pH of each batch was recorded and the effluent was analyzed for total phosphate using ion chromatography and total and dissolved lead using atomic absorbance. Interestingly, particulate lead that was formed when ionic lead was combined with hydroxyapatite had a greater mobility than all other tested forms of lead. This mobility was unaffected by the tested buffer and pH. Phosphate was found in the effluent when Na2HPO4 was mixed with both sources of lead but not when hydroxyapatite was used as the phosphate source. The buffer had no effect on phosphate mobility. Lead was not found in the effluent from runs containing phosphate ions suggesting that the lead – ionic phosphate reactions are kinetically rapid.