Tungsten dioxide

Endpoint summary

Description of key information

No information on the bioconcentration or accumulation in aquatic or terrestrial organisms was found for tungsten dioxide

Aquatic Bioaccumulation:

Bioconcentration is the tendency of materials to concentrate directly from water in a living organism over time. There is no testing performed according to standard methodology in the published literature regarding bioconcentration of tungsten compounds in general or tungsten metal specifically, in aquatic organisms. However, in a static renewal, toxicity test onPoecilia reticulatetesting sodium tungstate, Strigul et al (2010) measured tungsten uptake in 5 fish- 2 controls and 3 exposed to 7.5 g/L (nominal sodium tungstate concentration). The fish from the test group had died within the first 24 hours of exposure. The BCF was calculated as the ratio of tungsten concentration in fish tissue (in mg W per kg wet or dry) to tungsten concentration in water (in mg/L). BCF was calculated on both wet and dry weight of fish. Wet weight BCF for the test substance was calculated as 0.29 +/- 0.94 L/kg. Dry weight BCF for the test substance was calculated as 1.57 +/- 0.5 L/kg. These BCFs are low, indicating little to no immediate accumulation even at toxic exposure levels.

The most prevalent bioavailable form of tungsten is the soluble tungstate ion. However, because tungsten has a significant affinity for adsorption onto soils and stream or river sediments, levels in proximal natural waters are relatively much lower than the surrounding sediment and soil (see section 4.2.1 for more information). The extent to which tungsten compounds would release bioavailable tungstate ions into the aquatic environment is furthermore dependent on many factors including dissolved organic carbon (DOC), pH, and water hardness (Bednar et al, 2009). These data indicate that more alkaline waters will potentially possess much higher levels of bioavailable tungsten when exposed to the same amounts of tungsten than more acidic waters. A test performed using tungsten metal powder, according to the Transformation/Dissolution Protocol (UN GHS, 2007) showed that, under simulated natural conditions, after seven days, and at a loading rate of 100 mg/L, approximately 16509µg/L of tungsten ion is released at a pH of 8.5 (CANMET-MMSL, 2010). Thus, even at a relatively high pH, the magnitude of release would relatively low at environmentally relevant loadings. Furthermore, the median calculated tungsten partition coefficient for water-sediment of 140000 L/kg (Salminen (Ed) et al, 2005) indicates that upon reaching the water compartment, much tungsten is removed via adsorption to the sediment. Overall, it is unlikely that substantial exposure, and consequent uptake, would result from environmentally-relevant loadings.

Another important concern for the bioaccumulation/bioconcentration of metals is methylation. Methylation of metals (ie mercury) can allow metals to passively cross membranes and accumulate without homeostatic regulation. There is currently no evidence of methylated species of tungsten in the natural environment.

It is also important to consider active uptake of bioavailable tungsten. According to Adams and Chapman (2007) “Most metal species that form in aquatic solutions are hydrophilic and do not permeate the membranes (typically gills) by passive diffusion….uptake of metals is dependent on the presence of transport systems that provide biological gateways for the metals to cross the membrane.” Therefore, most metals enter organisms through active transport via transport proteins specific to that particular metal, as occurs with essential metals. Though tungsten is a non-essential metal, it is possible for metals such as tungsten, which mimic essential metals such as molybdenum, to be taken up. This has been demonstrated in studies examining chicks and rats fed sodium tungstate supplemented diets, which have demonstrated that tungsten may act as a competitive inhibitor of molybdenum uptake (Higgins et al, 1956). This phenomenon has not been studied in aquatic organisms; however, organisms such as fish have metabolic mechanisms to eliminate metals that are taken up or even to acclimate to metal exposure by decreasing metal uptake (McDonald and Wood, 1993 in Adams and Chapman, 2007).

Terrestrial Bioaccumulation:

Relatively low bioaccumulation of tungsten is observed in sunflower leaves at soil concentrations of 3900 mg W/kg soil, with calculated concentration factors plateauing at approximately 0.05 (Johnson et al, 2009). Tungsten concentrations factors calculated for ryegrass were higher and ranged from 56.1-0.202 (Strigul et al, 2005). However, it should be noted that background levels of tungsten in the collected soils used for testing were not determined prior to testing. Tungsten concentrations measured in earthworm tissue ranged from 1.52-193.2 mg/kg wet weight in soils with tungsten concentrations of 10-10000 mg/kg soil, respectively (non-aged soil) (Strigul et al, 2005). Additionally, tungsten concentrations of 10 and 10000 mg/kg soil yielded earthworm tissue concentrations of 3.45 and 25.9 mg/kg wet weight, respectively (Strigul et al, 2005). Using these paired concentration data the BCFs for earthworms in non-aged soils ranged 0.152-0.019 and BCFs for aged soils ranged 0.345-0.00259. However, it should be noted that background levels of tungsten in the collected soils used for testing were not determined prior to testing. Tungsten is not expected to bioaccumulate in terrestrial organisms.

Additional information

No aquatic or terrestrial bioaccumulation data of sufficient quality were available specifically on tungsten dioxide (target substance). However, aquatic and terrestrial bioaccumulation data is available on sodium tungstate and tungsten metal (source substances), respectively, which are used for read-across.