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Environmental fate & pathways

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No data on the behavior of tungsten carbide in the environment are available. However, data for sodium tungstate and tungsten metal are expected to adequately capture the range of mobility of tungsten carbide in the environment.

Hydrolysis is not a relevant process for tungsten substances, as in principle, water acts as an oxidising agent with tungsten. Tungsten is a stable element and cannot degrade chemically, then the process of degradation is not a relevant fate pathway for tungsten substances. Furthermore, the process of biotic or abiotic biodegradation is not a relevant fate pathway for inorganic metal elements such as tungsten.  Environmental data for tungsten metal and sodium tungstate are presented in the adsorption/desorption section. The soluble species (tungstate, WO42-) released are expected to be similar for each of the tungsten substances 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. For more details refer to the attached description of the read-across category approach and/or Annex 3 in the CSR.  

The adsorption/desorption is highly dependent on the characteristics of the soil system in question. For example, soil sorption coefficients of tungsten metal and sodium tungstate are found to increase with decreasing pH (Bednar et al, 2008). Additionally, soil-tungsten systems may take up to approximately 3-4 months to reach equilibrium (Griggs et al, 2009 and Bednar et al, 2008).

Soil sorption coefficients measured for sodium tungstate ranged from 16.6 to 863 (Bednar et al, 2008 and Griggs et al, 2009).

Other partitioning coefficients for tungsten in the environment were estimated using paired sampling data. Tungsten partition coefficients for water-sediment were derived using paired environmental monitoring samples of tungsten in water and sediment collected in various parts of the EU, resulting in a median calculated Kd of 140000 (Salminen R (Ed.) et al, 2005).

The following partitioning coefficients were statistically derived based on studies using appropriate methodology:

  • Kd soils (Griggs et al, 2009 and Bednar et al, 2008):
    • 10thpercentile: 44 L/kg
    • Median: 174 L/kg
    • 90thpercentile: 692 L/kg
  • Kd sediment (Salminen R (Ed.) et al, 2005):
    • 10thpercentile: 28395 L/kg
    • Median: 140000 L/kg
    • 90thpercentile: 700000 L/kg

Volatilization is not expected to be a significant pathway for tungsten carbide, based on the low vapor pressure and ionic or insoluble state of most tungsten compounds in the environment.

No data on volatilisation are available for tungsten carbide. However, most tungsten compounds are expected to exist as ions or insoluble solids in the environment. Therefore, volatilization from moist soil and water surfaces is not expected to be an important fate process (HSDB, 2008). Tungsten is also not expected to volatilise from dry soil surfaces based upon its ionic character and extremely low vapor pressure at environmentally relevant temperatures (1 x 10-10 Pa at 1700°C (Lassner et al, 2005) and 8.15 x10-8 Pa at 2000°C (Lassner and Schubert, 1999).

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