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Water solubility

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Endpoint:
transformation / dissolution of metals and inorganic metal compounds
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Remarks:
Well documented, scientifically sound study that determined the dissolution of the test substance according to the UN Guidance on transformation/dissolution (T/D) of metals and metal compounds (UN GHS Annex 10).
Qualifier:
according to
Guideline:
other: UN Guidance on transformation/dissolution (T/D) of metals and metal compounds (UN GHS Annex 10)
Principles of method if other than guideline:
Standardized amounts of powdered solids are added to specified simulated aquatic media and amounts of the relevant soluble metal ions are measured after defined timepoints.
GLP compliance:
not specified
Type of method:
other: Guidance on transformation/dissolution (T/D) of metals and metal compounds (UN GHS Annex 10)
Key result
Type of test:
screening transformation/dissolution test - sparingly soluble metal compounds
Mean dissolved conc.:
ca. 868 µg/L
Element analysed:
Dissolved tungsten
Loading of aqueous phase:
100 mg/L
Incubation duration:
24 h
Test conditions:
21°C; pH=6.0
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 1 258 µg/L
Element analysed:
Tungstate
Loading of aqueous phase:
100 mg/L
Incubation duration:
24 h
Test conditions:
21°C; pH=6
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 2 198 µg/L
Element analysed:
Dissolved tungsten
Loading of aqueous phase:
100 mg/L
Incubation duration:
24 h
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 2 912 µg/L
Element analysed:
Tungstate
Loading of aqueous phase:
100 mg/L
Incubation duration:
24 h
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 988 µg/L
Element analysed:
Dissolved tungsten
Loading of aqueous phase:
10 mg/L
Incubation duration:
7 d
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 1 339 µg/L
Element analysed:
Tungstate
Loading of aqueous phase:
10 mg/L
Incubation duration:
7 d
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 16 509 µg/L
Element analysed:
Dissolved tungsten
Loading of aqueous phase:
100 mg/L
Incubation duration:
7 d
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 22 050 µg/L
Element analysed:
Tungstate
Loading of aqueous phase:
100 mg/L
Incubation duration:
7 d
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 188 mg/L
Element analysed:
Dissolved tungsten
Loading of aqueous phase:
1 mg/L
Incubation duration:
28 d
Test conditions:
21°C; pH=8.5
Key result
Type of test:
full transformation/dissolution test - metals and sparingly soluble metal compounds
Mean dissolved conc.:
ca. 260 µg/L
Element analysed:
Tungstate
Loading of aqueous phase:
1 mg/L
Incubation duration:
28 d
Test conditions:
21°C; pH=8.5
Details on results:
Only the most relevant data for classifictaion and risk assessment are presented in the sections above, but all results are presented in this text box.

Definitive seven-and 28-day tests at pH 6:
For the 1 mg/L loadings, the net average concentrations of W and WO4-- anion increase linearly with time, and attain levels of 130 ug/L and 180 at 672 hr. The attendant 672-hr y value of 0.723 indicates that most of the W existed as WO4-- anion. These concentrations represent about 13% W dissolution.
For the higher loadings, increases that are more or less linear with time were also observed, with the W and WO4-- anion concentrations at 168 hr attaining 513 ug/L and 767 ug/L, respectively, for the 10 mg/L loadings, and 3,029 ug/L and 4,428 ug/L, respectively, for the 100 mg/L loadings. For the 10 and 100 mg/L loadings, the total dissolved W concentrations correspond to 5% and 3% W metal dissolution, respectively, which, taken with the 3% dissolution for the 1 mg/L loading at 168 hr, suggest that W reactivity is not much affected by loading. For undetermined reasons, the 0.669 and 0.684 y values are lower than anticipated.

Definitive seven-and 28-day tests at pH 8.5:
At this pH, the W and WO4-- concentrations at each loading also increased more or less linearly with time, and do not exhibit any tendency towards limiting values. For the 1 mg/L loadings, the net average 672 hr concentrations of W and WO42- were 188 ug/L and 260 ug/L, respectively, and the average value of y was 0.727, indicating that the W metal reacted and formed the WO4-- anion. The 168-hr values for W and WO4-- were 988 ug/L and 1,339 ug/L, respectively, for the 10 mg/L loadings, and for the 100 mg/L loadings, 16,509 ug/L and 22,050 ug/L, respectively. Thus about 16.5% of the total W in the 100 mg/L loadings reacted and, from the 0.741 average value of y, dissolved as WO4--.

The average temperature among all seven- and 28-day tests varied in the range 21.0-21.6 deg C, with 19.9 deg C as the single value below 20.0 deg C, which was not expected to affect the reaction kinetics. With the maximum temperature of 22.0 deg C, essentially all temperatures fell within the 20-23 deg C proposed for the T/DP. The average D.O. readings met or exceeded the >70% saturation criterion.

All target 8.5 pH levels in the W-series for the test substance lay in the range 8.3-8.7, which is the variability proposed from the T/DP. Similarly, all but 21 of the 480 target 6 pHs were less than the proposed upper limit of 6.25, and it was not expected that these variations would have had a significant effect on the T/D kinetics of the tested substances. The pH levels less than the lower limit of 5.85, since the variability appears to have been random, are not expected to have affected the substances’ reaction kinetics.

Conclusions:
Interpretation of results (migrated information): slightly soluble (0.1-100 mg/L)
The test substance was found to be more soluble at a pH of 8.5 than 6. The soluble tungsten values at 1, 10, and 100 mg/L loadings at pH 8.5 were 0.052, 0.988, 16.51 mg/L at 7 days, respectively. The 28 day soluble tungsten value for 1 mg/L loading at pH 8.5 was 0.19 mg/L. The y values at pH 8.5 varied from 0.711 to 0.755, indicating that most of the available tungsten ion was in the form of WO4 --.
Endpoint:
water solubility
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Remarks:
Well-documented study that uses scientifically sound methodology.
Qualifier:
no guideline followed
Principles of method if other than guideline:
The dissolution of munitions-grade tungsten powder alone, and in the presence of other metallic powders simulating a biphasic system (90:10 binary mixtures with Co, Cu, Ni and Fe), under controlled environmental conditions, was conducted to investigate how these metals influence the dissolution process and the resulting effects on environmental conditions. For this study, the term 'dissolution' refers to the chemical reaction taking place and the production of soluble products in the aqueous phase.

In the studies examining tungsten powder alone, the role of dissolved oxygen (DO) on the dissolution process w as investigated by measuring tungsten powder dissolution in the presence and absence of DO.
GLP compliance:
not specified
Type of method:
flask method
Key result
Water solubility:
2.66 other: mg/L day
Conc. based on:
test mat. (dissolved fraction)
Loading of aqueous phase:
500 other: mg of munitions-grade W powder
Incubation duration:
96 h
Temp.:
25 °C
pH:
> 4.65 - < 6.29
Remarks on result:
not determinable because of methodological limitations
Details on results:
In the DO study, an initial rapid dissolution rate of 2.05 mg/L per day was observed for the first six hours (see attached Figure 2); thereafter, the dissolution rate decreased to approximately 0.16 mg/L day. The dissolution rate after the nitrogen was turned off and oxygen was allowed into the system was observed to be one order of magnitude larger than the dissolution rate in the absence of dissolved oxygen (2.66 mg/L day versus 0.16 mg/L day). The initially higher dissolution rate was accompanied by a rapid decrease in the pH of the system, from an initial value of 6.29 to 4.65. It remained relatively steady for the next 90 hours (Day 4) and subsequently began to decline again after the reintroduction of dissolved oxygen to the system.

In the studies designed to examine the effects of other metals on W dissolution, after 46 days of contact, the W/Ni, W/Co and munitions grade W reactors contained visible amounts of what appeared to be undissolved metal powders, while insoluble products and precipitates were observed in the W/Cu (greenish yellow) and W/Fe (reddish brown) reactors. The variations in dissolved tungsten concentration over time for the different powder mixtures are presented in two time periods: 0–7 days and 7–46 days. The trend of dissolved tungsten concentration during the first week, shown in Figure 3 (attached), suggests that two distinct kinetic phases occur for all the systems. The systems all start off with different rates of dissolution for the first day, although W/Ni and W ammunition grade systems had almost identical slopes. A reduction in the slopes for all the systems then occurs after the first day and they all display a constant linear dissolution trend till the seventh day. The slopes of the two distinct phases for each system are presented in Table below:

Table 1. Slopes of dissolution curves in the first week.

Time  W/Fe  W/Cu  W/Ni  W/Co  W/munitions 
0 -1 day  20.71  14.12  6.51  10.19  6.49 
1 -7 days  17.92  7.58  1.99  2.68  1.40 

In the second period, Day 7 to Day 46 (see attached Figure 4), the tungsten concentrations in the W/Co, W/Ni, and W munitions-grade systems continued to increase slowly but almost linearly throughout the experiment. However, for W/Fe and W/Cu, the dissolved tungsten concentration reached a maximum value and then decreased. The highest concentration was measured after 14 days for W/Cu and after 28 days for W/Fe.

The cause of the decrease in tungsten concentration for the W/Fe system is suspected to be either the precipitation of insoluble tungsten–iron compounds or the adsorption of tungsten by amorphous iron precipitates or a combination of both. Attempts to qualitatively identify the precipitates using X-ray diffraction were unsuccessful. For the W/Cu system, the formation and precipitation of insoluble WO3.H2O and WO3.2H2O (confirmed by X-ray diffraction) at low pH, is responsible for the reduction in the concentration of tungsten in solution.

Conclusions:
The dissolution of tungsten in the presence of dissolved oxygen was observed to be one order of magnitude larger than the dissolution rate in the absence of dissolved oxygen (2.66 mg/L day versus 0.16 mg/L day).

Tungsten is released from tungsten heavy alloys (more appropriately, tungsten composites) regardless of the positions of the alloying metals relative to tungsten in the galvanic series. However, the mechanisms and rates of release are dependent on this positioning. Better understanding of the effects of pH, concentration, and prevailing environmental conditions on the speciation of tungsten is still needed to fully assess potential environmental and health impacts of tungsten releases by linking speciation with observed environmental effects.
Endpoint:
water solubility
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
data from handbook or collection of data
Remarks:
Data from peer-reviewed handbook or collection of data.
Reason / purpose:
reference to same study
Principles of method if other than guideline:
Data in this peer-reviewed publication were taken from many sources, including both compilations and primary literature. Where conflicts were found, the value deemed most reliable was chosen for inclusion in the handbook. Reference sources were not provided for this parameter, nor was the experimental method.
GLP compliance:
not specified
Key result
Water solubility:
3.8 other: ug W x m^-2 x h^-1
Temp.:
38 °C
Details on results:
The reaction with distilled water is very slow at low temperature: the corrosion rate is 3.8 ug W x m^-2 x h^-1 at 38 degrees C. The reaction is accompanied by the formation of a non-protective oxide film, which is in equilibrium with 10^-3 mol/L WO4-2 in solution.

Description of key information

The most appropriate way to quantify tungsten metal solubility is through the Transformation Dissolution data. These values will be passed forward to the risk assessment.

Dissolution of tungsten metal in distilled water is accompanied by the formation of a non-protective oxide film (Lassner and Schubert, 1999). Tungsten metal dissolution was found to take several days to equilibrate (Dermatas et al., 2004), with dissolution slowing after the first 6 hours. The dissolution of tungsten in the presence of dissolved oxygen was observed to be one order of magnitude larger than the dissolution rate in the absence of dissolved oxygen (Ogundipe et al., 2009).  

Key value for chemical safety assessment

Water solubility:
16 509 µg/L
at the temperature of:
25 °C

Additional information

At a loading f 100 mg/L at a pH of 8.5, 16.51 mg W ion/L were determined by the transformation/dissolution study