Unease Over Tungsten

AS THE FILAMENT in an incandescent light bulb, tungsten can illuminate a room, but it hasn't been spotlighted as an environmental contaminant in the same way that lead or mercury has. In fact, in the mid-1990s, believing that tungsten is relatively insoluble in water and nontoxic, the Army replaced the lead core in military bullets with tungsten alloys through its Green Ammunition Program. Similar bullets became available to hunters soon after as states began to ban lead ammunition to protect birds from lead poisoning.

Recent studies, however, indicate that under certain environmental conditions, some forms of tungsten can move readily through soil, leach into groundwater, and induce greater biological effects than previously known. These findings do not definitively raise a red flag about tungsten, but they have spurred more study on the metal's effects because of the increased use of tungsten in military ammunition and in civilian applications ranging from tools to tire studs.

"At present, nobody knows whether or not tungsten will become a big concern ecologically because only limited findings have been accumulated," says Yutaka Tajima, an M.D./Ph.D. who studies the biochemistry and toxicology of heteropolyanions, including those of tungsten, at Gunma University, in Japan.

Although recommended occupational exposure limits exist, tungsten is not regulated in drinking water or as an environmental contaminant in the U.S. and Western Europe. Tungsten was regulated in the former Soviet Union, and the regulations still apply to the now-independent states. In the past couple of years, the Defense Department and the Environmental Protection Agency each has listed tungsten as an emerging contaminant of concern.

The metal's toxicity is relatively low compared with, for example, mercury's or lead's, but "tungsten anions are never inert to living things," Tajima says. Chronic exposure to tungsten species by, for example, ingesting tainted water or foods, even at a very low concentration, is probably a more important issue than acute toxicity, he says.

Tungsten has the highest melting point among metals. Dense alloys such as tungsten carbide are sought after for welding, metal cutting, drilling, aerospace applications, and jewelry. Tungsten is also a good conductor of electricity, which makes it desirable as filaments for light bulbs.

Also known as wolfram, tungsten occurs naturally in mineral forms such as wolframite [(FeMn)WO₄] or scheelite (CaWO₄), but typically not as a pure metal. As rocks and soil weather, soluble and insoluble tungsten dusts drift into waterways. Water effluent from mining and manufacturing processes or landfills may also distribute soluble forms of tungsten.

IN SOIL, tungsten metal oxidizes to the tungstate anion (WO₄^2–). Previous toxicology studies indicate that tungsten would be stable in the environment. Although thermodynamically stable under most environmental conditions, the tungstate ion does have biological effects. For example, tungsten may substitute for molybdenum in certain enzymes, inactivating the enzymes.

Furthermore, the tungstate anion polymerizes to polytungstates, says Nikolay S. Strigul, an ecologist at the Center for Environmental Systems at Stevens Institute of Technology, in Hoboken, N.J. And because knowledge of tungsten's toxicity and its biological and environmental effects is based on studies of monotungstates, little is known about how polytungstates behave in the environment, he adds.

Polytungstates comprise a range of chemical species, explains Anthony J. Bednar, a geochemist at the Environmental Laboratory of the U.S. Army Engineer Research & Development Center (ERDC) in Vicksburg, Miss. Examples include the heptamer (7 W and 24 O atoms) and the dodecamer, a complex better known as metatungstate (12 W and 40 O). Heteropolytungstates, which also incorporate phosphorus or silicon, have been found in the environment as well, he adds.

Polymerization of tungstates is not analogous to the way organic polymers form or tetrahedral phosphate or silicate units couple to yield a range of polymers, notes Michael T. Pope, a professor emeritus of chemistry at Georgetown University who studies polyoxometalates. Polytungstates are discrete entities with specific and characteristic structures that do not necessarily contain identifiable monomer units, he says.

In the environment, Bednar explains, the geochemistry of the soil will ultimately influence what species—monomeric or a wide range of polymeric moieties—will form. Monomeric tungstates tend to form under alkaline soil conditions; polymeric species emerge under acidic conditions. Bednar says each species will have different properties in soil, such as binding, and these properties, along with conditions such as pH, ultimately affect the species' mobility and bioavailability.

Bednar and colleagues have done both laboratory studies and fieldwork at military sites such as Army training ranges over the past four years. They are identifying various tungsten species found in soil and developing methods to quantify them in environmental samples.

The researchers use liquid chromatography and inductively-coupled plasma mass spectrometry (ICP-MS) to determine submicrogram levels of tungsten, molybdenum, and phosphorus oxyanions in groundwater and soil extracts. Using model field soil, they also determine partition coefficients for various monotungstate, polytungstate, and heteropolytungstate compounds. They have found that the mobility of the mono- and polytungstate compounds decrease with time as they become more firmly bound to the soil particles.

In one of two upcoming papers in the journal Land Contamination & Reclamation, Bednar and colleagues report a method to separate and quantify monomeric and polymeric tungsten species by size exclusion chromatography (SEC) interfaced to ICP-MS. Then they use direct infusion electrospray MS to identify the polymeric species. For the second paper, they use SEC to analyze extracts from soil and sunflower plants (Helianthus annuus L.). They find that plant root tissue bioaccumulates roughly twice the amount of tungsten in the soil, but the leaves and stems accumulate less.

Strigul and colleagues have also demonstrated that when tungsten metal oxidizes in soil, the soil becomes acidified, which results in poor growth of plants and soil microbes. Changes in soil pH are well known to alter the availability of plant nutrients.

Both Bednar and Strigul have studied organisms exposed to tungstate species.

In a study published in the journal Environmental Toxicology & Chemistry (2006, 25, 763), Bednar and colleagues have found that sodium tungstate is less lethal than lead to earthworms (Eisenia fetida) but completely inhibits the invertebrate's ability to reproduce. (Lead's biggest effect was lethality.)

And an upcoming paper by Strigul and colleagues in the journal Desalination indicates that polytungstates may be more than an order of magnitude more acutely toxic than monotungstates to different organisms. He and his coworkers find that more guppies, tiny aquatic invertebrates known as Daphnia, redworms, and freshwater algae die when exposed to sodium metatungstate (Na₆H₂W₁₂O₄₀) than to sodium tungstate (Na₂WO₄).

"Polytungstates are apparently much more toxic than monotungstates," Strigul says. In light of these new findings, "a revision of our current knowledge of tungsten toxicological and environmental effects is necessary," he adds.

RESEARCHERS have yet to elucidate how tungsten causes ph ysiological stress to flora or fauna, although they have a few ideas. Strigul and his collaborators hypothesize that the observed difference in animal toxicity between the monotungstates and polytungstates may be explained by the tungstate species' different abilities to penetrate biological membranes as a result of anionic charge distribution in solution.

David R. Johnson, a toxicologist in the Environmental Laboratory at ERDC who has collaborated with Bednar, has been examining the effects of tungsten on phosphate-dependent pathways in animal cells. These pathways are important for normal cellular function, such as energy production and cell-signaling interactions. Disrupting these pathways can lead to potentially detrimental and toxic effects.

Polytungstates may play a role in plant toxicity by disrupting the production of energy-transporting adenosine triphosphate (ATP) and signaling pathways. Moreover, the researchers say, the biological activity of tungsten hinges on its physiochemical form and on whether or not it undergoes further reactions inside the organism. For example, tungstate may polymerize with phosphate in a plant or animal, deplete intracellular stores of phosphate, and disrupt phosphorylation reactions involved in ATP synthesis and cellular signaling.

The verdict is still out on tungsten in the environment and more study is necessary. However, Strigul suggests that because plants have already been shown to take up tungstates, phytoremediation technologies may help if cleanup is deemed necessary.