Oxidation of W-Metal Shot in Soils and Plant Response to Tungstate.

Tungsten (W) is becoming a popular metal replacing lead in small-caliber and shotgun ammunition because it is considered to be significantly less toxic than lead in the environment. However, questions have been raised about the environmental risks associated with tungsten and its interaction with molybdenum in soil. As of today, limited knowledge exists about soil properties controlling dissolution of W-metal and W-metal alloys, fate and transport of W in soil, and effects of tungstate (WO₄^2-) on plant uptake and development. We examined the oxidation of W-metal alloy pellets in two soils using synchrotron-based microprobe on beam line 2-3 and bulk XAFS on beam line 11-2 at Stanford Synchrotron Radiation Laboratory. We found preliminary evidence that W-metal pellets incubated in low Fe-oxide soil under aerobic conditions rapidly oxidize to WO₄^2- and can produce more than 2000 mg kg^-1 of plant available WO₄^2- in the local soil environment. Conversely, soils high in Fe-oxides adsorbed large amounts of WO₄^2- thus rendering it less available for plant uptake. Plant trials using lettuce (Lactuca sativa) and alfalfa (Medicago sativa) were performed to evaluate increasing W concentrations (0 to 2000 ppm) on seed germination, initial root length development, and plant uptake. No difference in seed germination was observed, however, a reduction in sprout length was observed starting at 500 ppm W. Tungsten was readily taken up into the leaves of both plants, ranging as high as 900 mg kg^-1 (alfalfa) and 400 mg kg^-1 (lettuce). Microprobe XAS images of alfalfa root nodules show concentrated areas of W within the root nodules, possibly disrupting or replacing Mo, essential for nitrogen fixation in alfalfa. Use of W-metal in shotgun ammunition could lead to elevated WO₄^2- concentrations in soils and plants. Work is ongoing to evaluate the effects of WO₄^2- on MoO₄^2- uptake by plants.