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Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Aquatic bioaccumulation

No information on the bioconcentration or accumulation in aquatic organisms was found for tungsten blue oxide. However, data was available for sodium tungstatewhich was used for reading across.

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 TBO specifically, in aquatic organisms. However, in a static renewal, toxicity test on Poecilia reticulatetesting sodium tungstate, Strigul et al. (2010) measured tungsten uptake in 5 fish-2 controls, 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. 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). The 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 blue oxide than more acidic waters. A test performed using tungsten blue oxide powder, according to the Transformation/Dissolution Protocol (UN GHS, 2007) showed that, under simulated natural conditions, after 24 hours, at a loading rate of 100 mg/L, and at a pH of 8.5, approximately 6.62 mg/L of tungsten ion is released (CANMET-MMSL, 2010). Thus, even at a relatively high pH, the rate and magnitude of release are relatively low. Furthermore, studies have found that adsorption coefficients for tungsten compounds increase over time and system equilibration may not be reached for 3-4 months. Thus, a large fraction of the soluble tungsten would likely be removed from the water column via sorption over time. 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 (i.e., 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 metal 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). However, this phenomenon has not been studied in aquatic organisms. Furthermore, 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

No data on the behavior of tungsten blue oxide in the environment are available. Bioconcentration data for tungsten metal and sodium tungstate are presented in this section. The soluble species released are expected to be similar for each of the compounds, and are thus expected to behave similarly in the environment. However, the amount of soluble species resulting from tungsten metal and sodium tungstate is different, with sodium tungstate being much more soluble. Therefore, data for sodium tungstate and tungsten metal are expected to adequately capture the range of bioavailability of tungsten blue oxide in the environment. For more details, refer to the attached read-across document (see Annex 1).

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, in this study, 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 from 0.345 - 0.00259. Tungsten concentrations measured in earthworm tissue in another study with soil spiked with sodium tungstate (Inouye et al., 2006) ranged from 2.9 - 41.3 mg/kg wet weight in soils with tungsten concentrations of <2 – 4643 mg/kg soil, respectively. These data would indicate concentration factors ranging from 1.45 – 0.008, respectively, with only the lowest tungsten concentration resulting in a BCF of > 1. Therefore, tungsten compounds such as tungsten blue oxide are not expected to bioaccumulate in terrestrial organisms.

Additional information

Due to the similar chemical constituency and structure, the physico-chemical properties would be expected to the the same or sufficiently similar such that application of this read-across is appropriate for this endpoint. For more details, refer to the attached read-across approach document.