Oxidation of W-Pellets and Tungstate Adsorption by Soils.

Tungsten (W) is becoming a popular metal replacing lead in shotgun ammunition because it is considered to be significantly less toxic in the environment.  However, there are growing concerns about possible risks associated with elevated tungstate (WO₄^2-) in soils and water. We studied the dissolution of W-shotgun pellets in different soils under oxidizing and reducing conditions. We also studied the reaction of tungstate in soils using batch laboratory adsorption experiments. We found that W-shot rapidly oxidizes in aerobic soils leading to high concentrations (> 2500 mg kg^-1) of water-soluble, plant available WO₄^2-.  Under anaerobic conditions similar to flooded soil sediments, the oxidation of the W-metal was greatly reduced. SEM scans revealed that incubated W-pellet surfaces were significantly more weathered in the aerobic acid soil than either anaerobic incubation or slightly alkaline soil. XANES microprobe imaging of a W-pellet embedded in an aerobic soil shows the diffusion of WO₄^2- away from the oxidizing W-pellet and is highly correlated with Fe-oxide minerals in the soil. Tungstate adsorption increased with decreasing solution pH and was highly correlated to Fe-oxide mineral content of the soils. The constant capacitance model was able to adequately describe WO₄^2- adsorption on the soil samples and was able to predict WO₄^2- adsorption using the surface complexation constants predicted with chemical and physical properties. We expect these results will be applicable to an improved understanding of tungsten biogeochemistry.