The fate of tungsten in the environment has been unexpectedly revisited due impacts to groundwater from military use at Camp Edwards located at the Massachusetts Military Reservation, United States and a cancer cluster identified in Fallon, Nevada, United States. Military small arms training ranges (Figure 1) are vital for preparing troops for combat. These ranges may accumulate substantial quantities of metallic residues that could be an environmental concern. Tungsten metal is used by the military as a replacement for depleted uranium in anti-tank piercing munitions due to its high density. More recently tungsten metal was used as a replacement for lead in the 5.56 mm (M-16 Rifle) small arms ammunition (Figure 2). The 5.56 mm tungsten-core projectile (Figure 3) was developed by the US Army's Green Ammunition Program. Because tungsten was believed to be insoluble in water and non-toxic, the projectile was developed as an environmentally benign replacement for the lead/antimony core used in conventional ammunition. Eighty-five million rounds of tungsten/nylon produced with use initiated in 1999. The US Army halted production in 2003 because of problems with flight instability. The projectile has been redesigned and production could resume except for recent evidence suggesting tungsten in the original projectiles readily dissolves in water and is mobile under certain field conditions. Prior to the previous decade, tungsten’s environmental fate had not been examined with much detail.
Once deposited in metallic form tungsten rapidly oxidizes. During the oxidation process, a hydrogen ion (H+) is released along with the formation of a tungsten oxide, which is unstable in the environment. The stable and soluble forms of tungsten under oxidizing conditions are tungstate (WO42-) or oxide complexes in the +6 oxidation state. Thus, tungsten metal is nearly always found in one of a few mineral tungstates. In contrast to mineral forms, the solution chemistry of tungsten oxide complexes (Tungsten [VI]) is exceedingly complicated.
Tungsten is similar to molybdate by forming a variety of stable polyatomic anions, including H2W12O40-6, HW6O20-3, and W6O20(OH)5-. These tungstate species have well-known thermodynamic properties, with WO42? stable in dilute and basic solutions, while the W12O40-6 and related ions are stable in more concentrated solutions and acidic solutions. Within a narrow range of neutral pH, W6O20(OH)5- is also stable. Complications in tungstate speciation arise because interconversion between these forms is frequently slow, and numerous metastable ions, such as W4O12(OH)4-4, can persist in some solutions.
Tungstate in turn can polymerize, a process favored at lower pH and higher tungsten concentrations, to form polytungstates. If the tungsten mass is high enough and buffering capacity of the soil is low, acidification of the soil can occur during tungsten oxidation and if the soil becomes sufficiently acidic polymerization of the tungstate to a polytungstate may occur. At some sites the soil is sufficiently acidic for polymerization to occur, however where the soil is borderline alkaline, polymerization would not occur on its own. The addition of H+ from oxidation of metallic tungsten may be sufficient in some situations to start the polymerization processes. At sites where soil is sufficiently alkaline and mass of tungsten introduced into the environment is low, polymerization of tungstate would not occur.
The polytungstates can also form a variety of polyoxometallates in solution, which is a class of polytungstates. Polyoxometallates are cluster compounds containing two or more metal atoms. Tungsten polyoxometallates are constructed of WO6 and a central MO4 tetrahedron. One type of POM, known as a ?-Keggin cluster (MW12O40x-) has been intensively (Figure 4). Because of their exceptional size, molecular mass, structure, and chemical reactivity, a variety of other polyoxometallates also are known. Although some evidence suggests such clusters are important in environmental systems, few studies have examined their formation in the natural environment. Furthermore, no thermodynamic data are available to predict the stability of tungsten polyoxometallates in soil systems, nor have they been conclusively identified in soils.
Because tungsten metal rapidly oxidizes and tungsten oxides in turn are unstable tungstate, polytungstate, and polyoxometallates are the key species to consider for environmental mobility. Tungstate readily solubilizes and is transported with water. Although, it can react with soil to form tungstate salts it prefers to remain in solution as an ion. Consequently, tungstate has the potential to be highly mobile and transported from the ground surface to the water table. Once tungstate reaches the water table it will migrate in the direction of ground water flow away from the source area. If soils conditions are favorable for the polymerization of tungstate then a polytungstate species will form and if the rate of polymerization is slower than the precipitation infiltration rate, then tungstates will migrate deeper into the soil. If appropriate conditions are present at depth, tungstate will continue to polymerize to polytungstate as it moves downward. If the rate of polymerization is faster than the infiltration rate, polytungstate will build up in the surface soils and tungstate will not be transported deeper into the soil. Eventually, these polytungstates will be carried deeper into the soil profile. Polytungstates appear to be adsorbed onto soil to a lesser degree than tungstates and thus are more mobile than tungstates. In contrast, polyoxometallates are strongly adsorbed to soil surfaces and is a very large structure, thus it is preferentially retained at the soil surface. The chemical process for the conversion of a polytungstate to a polyoxometallates is not clear.
- US Army Corps of Engineers-Research, Publications, Products
- Center for Disease Control-Cancer Clusters
- Clausen, J. L., M. Ketterer, A. J. Bednar, and M. Koenig. 2009 (In Press). Challenges and successes in using inductively coupled plasma mass spectrometry for measurements of tungsten in environmental water and soil samples. International Journal of Environmental Analytical Chemistry.
- Clausen, J. L. and N. Korte. 2009. Environmental fate of tungsten from military use. The Science of the Total Environment. 407(8):2887-2893