Tungsten Speciation in Firing Range Soils

Abstract : Synchrotron-based X-ray absorption spectroscopy (XAS) of select surface soil samples obtained from Camp Edwards, Massachusetts, small arms ranges indicate that little tungsten metal remains in the soil and that is not stable in the natural environment. X-ray absorption near edge structure (XANES) studies indicate rapid oxidation of tungsten metal to form tungsten oxides W(VI), polytungstates, tungstates, and polyoxometallates (POM) in any number of forms. Owing to structural similarities, it is difficult to identify specific species or discriminate between mineral species, although polytungstates and POMs predominate as compared to tungstates in soil. Additionally, this is the first study to identify the presence of tungsten POMs in soil. XANES spectra indicated that, as depth increased, the fraction of soil sorbed tungstate increased and both polytungstate and POM decreased, suggesting POM and polytungstates are more stable in surface soils and likely to persist, whereas tungstate is unstable. Tetrahedral tungstate is unstable in neutral and acidic pH solutions such as are present in Camp Edwards surface soils, resulting in its conversion to a variety of polytungstates. Adsorption of tungstate, although weak, appears to occur on iron oxide surfaces such as ferrihydrite. XAS studies also revealed prevalence of adsorbed polytungstates rather than discrete mineral phases.