Speciation and Geochemistry of Tungsten in Soil

In order to limit the dispersive use of lead on small arms firing ranges, 5.56-mm bullets with tungsten-nylon composite material cores have been fired at a number of Army training ranges. Tungsten in bullet residues oxidizes when exposed to the atmosphere, yielding an amorphous tungsten oxide (WO3) coating. This coating rapidly dissolves to yield the tungstate anion (WO4), which can migrate in saturated subsurface and surface water environments. The environmental geochemistry of dissolved tungsten species is complex, and consequently is not well characterized. Tungsten exists in most environmental matrices as the soluble and mobile tungstate anion, which can polymerize with itself and other anions, such as molybdate and phosphate, making determination of tungsten compounds important. Since the geochemical and toxicological properties of these polymer species may vary from monomeric tungstate, determination of tungstate is as critical as determination of total dissolved tungsten concentration. High performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLCICP-MS) has been used to determine tungsten speciation. Specifically, ion exchange chromatographic separations have been employed in investigations of tungsten leached from soil that was aged after amendment with metallic tungsten powder. Deionized water extraction of soil materials with HPLC-ICP-MS separation and detection indicated that up to approximately half of the extracted tungsten was in the form of polymers. Over 15 mg/L of soluble tungsten was measured in the extraction solution of soil aged for approximately 6 months. Further characterization experiments are in progress to elucidate the geochemical properties of the polymer species.