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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. 

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 ammonium metungstate, 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, approximately 65,467 µg/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 (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). 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)

Additional information

Due to the formation of a common, soluble tungsten ion (tungstate) at a similar rate between ammonium metatungstate (target substance) and sodium tungstate (source substance), these substances are expected to have a similar bioaccumulation potential. In addition, due to a much higher water solubility of ammonium metatungstate (source substance) compared to sodium tungstate (source substance), the resulting bioaccumulation potential of ammonium metatunsgstate compared to sodium tungstate is expected to be significantly hgher, so the usage of sodium tungstate data for read-across is appropriate to adequately capture the bioaccumulation potential range of ammonium metatungstate (target substance) in the environment. For more details, refer to the read-across category approach description in the Category section of this IUCLID submission or Annex 3 of the CSR.