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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Reference
Endpoint:
bioaccumulation in aquatic species: fish
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
comparable to guideline study with acceptable restrictions
Justification for type of information:
REPORTING FORMAT FOR THE CATEGORY APPROACH
1. HYPOTHESIS FOR THE ANALOGUE APPROACH: The hypothesis is that properties are likely to be similar or follow a similar pattern because of the presence of a common metal ion, in this case tungstate.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES):
Source: Sodium Tungstate
Target: Tungsten Carbide
3. CATEGORY APPROACH JUSTIFICATION: See Annex 1 in CSR
4. DATA MATRIX: See Annex 1 in CSR
Reason / purpose:
read-across: supporting information
Qualifier:
equivalent or similar to
Guideline:
EPA OPP 72-6 (Aquatic Organism Accumulation Tests)
GLP compliance:
not specified
Remarks:
Research conducted by Center for Environmental Systems, Stevens Institute of Tec hnology, Castle Point on Hudson, Hoboken, NJ 07030, USA
Specific details on test material used for the study:
Sodium tungstate powder was obtained as the hydrate, Na2WO4 2H2O.
Radiolabelling:
no
Vehicle:
no
Test organisms (species):
Poecilia reticulata
Details on test organisms:
Guppies (Poecilia reticulate) were obtained from Ward’s Natural Science. Special attention was paid to provide healthy fish for the test and to reduce possible biases. After arriving, fish were kept for at least 4 weeks in the fish tanks (50 L) under constant temperature (23±1°C) to evaluate whether they were healthy enough to be used in the experiments and to acclimate the fish to the local tap water.
Route of exposure:
aqueous
Test type:
other: Static-Renewal
Water / sediment media type:
natural water: freshwater
Test temperature:
Water temperature was 23.5 ± 0.3°C
pH:
pH between 7.43 and 8.1 (sodium tungstate slightly increased water pH, from 7.4 to 8.02)
Dissolved oxygen:
Dissolved oxygen varied between 3.7 and 5.49 mg/L. Sodium tungstate decreased dissolved oxygen concentration (down to 3.7 mg/L).
Conductivity:
Water conductivity in the range 0.399–5.29 mS/cm.
Details on test conditions:
Ammonia concentration was lower than the detection limit (0.01 mg/L.), nitrates and nitrites did not statistically differ between the control tanks and experimental tanks and the concentrations did not exceed 5 and 0.1 mg/L, respectively. Redox potential between 265 and 291 mV. Sodium tungstate decreased redox potential (from 295 to 265 mV), and wWhile these changes are in agreement with water chemistry theory and somewhat expected, none of them were deemed significant enough to cause fish mortality.
Nominal and measured concentrations:
Animals were exposed to seven distinct concentrations of sodium tungstate and sodium metatungstate (9, 7.5, 5, 3.75, 2.5, 2.5, 0.75 g (Na2WO4 2H2O) L.
Reference substance (positive control):
no
Details on estimation of bioconcentration:
Tungsten uptake was measured in 10 fish used in experiments including 4 fish from control groups and 6 fish that were exposedto tungsten. Dead fish were washed in DI-water, weighed, then dried for 12 h in an oven at 80°C. Dried fish were weighed, chopped and placed in 50 mL digestion vials along with HNO3 (5 mL), HCl (1 mL), H3PO4 (0.5 mL), and H2O2 (3 mL). The vials were open heated in a heating block under the hood for 2 h at 180°C; up to 3 mL H2O2 were added during the digestion to compensate evaporation losses. Subsequently, samples were allowed to cool down, filtered through a Whatman 42 paper, and diluted to 50 mL with DI-water. Finally tungsten concentration in samples was measured by inductively coupled plasmaoptical emission spectroscopy (ICP-OES, wavelength 207.911 nm) calibrated up to 1 mg/mL W with the tungsten catomic absorption standard solution.

Tungsten bioconcentration factors (BCF) were calculated as the ratio between tungsten concentration in fish tissues (expressed in mg of W per kg of wet or dry tissue) to tungsten concentration in water (expressed in mg/L).

Key result
Conc. / dose:
ca. 7.5 g/L
Temp.:
> 23.2 - < 23.8 °C
pH:
7.2
Type:
BCF
Value:
> 0 - < 1.23 L/kg
Basis:
whole body w.w.
Remarks:
0.29 ± 0.94 L/Kg
Time of plateau:
24 h
Calculation basis:
steady state
Key result
Conc. / dose:
ca. 7.5 g/L
Temp.:
> 23.2 - < 23.8 °C
pH:
7.2
Type:
BCF
Value:
> 1.07 - < 2.07 L/kg
Basis:
whole body d.w.
Remarks:
1.57 ± 0.5 L/kg
Time of plateau:
24 h
Calculation basis:
steady state
Details on results:
Fish (3 per each tested chemical) died during the first 24 h after they were exposed to 7.5 g/L of sodium tungstate. The average tungsten concentrations in fish are 1230±390, and 30 ±2 mg of W per kg of fish, for sodium tungstate, and control fish, respectively. BCF for sodium tungstate are 0.29±0.94 and 1.57±0.5 L/kg.where the first coefficient has been calculated using the wet fish weight and the second number is for the dry fish weight.
Validity criteria fulfilled:
yes
Conclusions:
BCF for sodium tungstate are 0.29±0.94 and 1.57±0.5 L/kg.where the first coefficient has been calculated using the wet fish weight and the second number is for the dry fish weight. Sodium tungstate has a low potential for biocncentration in freshwater fish
Executive summary:

No aquatic bioaccumulation are available for tungsten carbide (target substance). However, aquatic bioaccumulation data are available for sodium tungstate (source substance), which were used for reading-across. Due to lower water solubility and lower toxicity for the target substance compared to the source substance, the resulting read across from the source substance to the target substance is appropriate as a conservative estimate of bioaccumulation potential. In addition, read across is appropriate because the classification and labelling is more protective for the source substance than the target substance, the PBT/vPvB profile is the same, and the dose descriptors are, or are expected to be, lower for the source substance. For more details, refer to the read-across category approach included in the Category section of this IUCLID submission and/or as an Annex in the CSR.

Description of key information

No information on the bioconcentration or accumulation in aquatic organisms was found for tungsten carbide. 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 trioxide 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 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 trioxide than more acidic waters. A test performed using tungsten trioxide powder, 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 18150µg/L of tungsten ion is released at a pH of 8.5 (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 and the 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).

Key value for chemical safety assessment

BCF (aquatic species):
0.29 L/kg ww

Additional information