GLRPPR Sector Resource: Tungsten Speciation in Firing Range Soils
Tungsten Speciation in Firing Range Soils
Synchrotron-based X-ray absorption spectroscopy (XAS) of select surface soil sam-ples 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 olyoxometallates (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 com-pared 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.
Soil pore waters in equilibrium with contaminated soils during laboratory experiments yielded tungsten concentrations in excess of 5000 mg L?1, considerably in excess of predicted solubility limits of common tungsten minerals. These findings are consistent with field observations whereby tension lysimeters installed in the shallow vadose zone to monitor the soil pore water at the Bravo Range at Camp Edwards had tungsten concentrations as high as 400 mg L?1. The high solubility and limited adsorption of tungsten in these soils is attributed to the formation of POMs such as W12SiO404?, an gamma-Keggin cluster, in soil solutions in addition to other poly-tungstates. Polytungstates are quite soluble and can yield water concentrations of several hundred mg L?1. Although, not detected in groundwater, possibly because of analytical limitations, the presence of polytungstates in the vadose zone pore water at Camp Edwards suggests their presence cannot be completely ruled out. In contrast, the presence of tungstates in groundwater has been confirmed at Camp Edwards.
The weak retention of tungsten, in general to soils and observation of tungsten in soil pore water and groundwater at Camp Edwards attests to tungsten's potential mobility and transport in groundwater. The slow rates of conversion between POMs, polytungstates, and tungstates are likely to affect their solubility and transport considerably. Additionally, the presence of iron oxides such as ferrihydrite and organic matter may limit tungsten mobility. In general, tungsten mobility from highest to lowest proceeds from Tungstate --> Polytungstate --> POM --> Tungsten Oxide --> Metallic Tungsten.
U.S Army Corps of Engineers
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