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Environmental fate & pathways

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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 sodium tungstate specifically, in aquatic organisms. However, in a static renewal, toxicity test on Poecilia reticulate testing 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) 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. 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 sodium tungstate than more acidic waters. A test performed using sodium tungstate, according to the Transformation/Dissolution Protocol (UN GHS, 2007) showed that, under simulated natural conditions, after 24 hours, and at a loading rate of 100 mg/L, approximately66935µg/L of tungsten ion is released at a pH of 8.5 (CANMET-MMSL, 2010). However, studies have found that adsorption coefficients increase and speciation profiles change for tungsten compounds over time, and system equilibration may not be reached for 3-4 months. 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). Thus, a large fraction of the soluble tungsten would likely be removed from the water column via sorption soil and sediment 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 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). 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)

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