Gallium arsenide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

5.6 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

4.7 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.46 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

70.5 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

35.7 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

2.9 mg/kg soil dw

Assessment factor:

2

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

1 mg/kg food

Assessment factor:

30

Additional information

Gallium arsenide is usually produced by crystallisation in the form of a massive block, from which wafers (disks) are then produced. Only for experimental testing of intrinsic physico-chemical, toxicological and ecotoxicological properties, a GaAs powder sample has been prepared by the REACH registrant. This powder, which is not marketed, was produced by breaking/crushing and grinding of wafers and its particle size is characterised by the following parameters: D10 = 41.1 μm, D50 = 299.4 μm and D90 = 816 μm. This material has been used in a standard water solubility test (OECD 105), as well as in ecotoxicological tests (OECD 201,202,203).

Gallium arsenide is a metallic compound and as other metallic compounds practically not soluble in water. In an oxygen-free atmosphere a solubility of 13 μg GaAs/l (total As concentration) was observed. Under these conditions all dissolved As was found as As(III). Therefore one can assume that not GaAs itself was dissolved but the oxidized surface layer of the particles.

 

In the presence of oxygen and after a longer period of time, i.e. several days, higher solubilities seem to occur. These higher amounts of Gallium and Arsenic in the medium are nearest a consequence of an oxidizing process. Therefore, the standard concept of an equilibrium/saturation water solubility is not applicable to this substance.

 

Read-across approach from Arsenic

Only few water accommodated fraction (WAF) based acute ecotoxicitydata are available for the poorly soluble substance gallium arsenide (GaAs). For metals and poorly soluble metal compounds, WAF testing should not be used and ecotoxicity information should be derived with tests on a soluble metal salt and differences in solubility addressed by results from transformation/dissolution tests on the poorly soluble compound. Upon dissolution, GaAs yields both soluble gallium and arsenic ions. Because the available toxicity data show that As ions are more toxic in the environment compared to Ga ions, the ecotoxicity of GaAs is predicted based on read across from GaAs to soluble inorganic As compounds. In order to still account for the potential contribution of Ga ions to toxicity of GaAs in the environment, the toxicity results for As ions are not corrected for their abundance in GaAs, which means that the toxicity of Ga is considered similar as As. This is a worst-case scenario based on the available toxicity data for soluble Ga and As compounds.

For further justification of the read-across approach between GaAs and arsenic, see also the justification document attached in IUCLID section 13).

 

For the ecotoxicity assessment of arsenic (metal), a read-across approach based on all available data for inorganic arsenic compounds is applied. Reliable ecotoxicity data are based on experiments with trivalent (diarsenic trioxide and sodium arsenite) and pentavalent (disodium hydrogenarsenate, sodium metaarsenate, sodium arsenate and diarsenic pentaoxide) arsenic substances.

This grouping of arsenic substances for estimating their ecotoxicological properties is based on the hypothesis that the common inorganic As moiety is the common driver for the ecotoxicological properties of the substances covered and that the specific environmental conditions predominantly affect speciation and toxicity of arsenic substances and not the inorganic arsenic source. For most of the metal-containing substances, it is the metal ion that becomes available upon contact with and dissolution in water and that is predominantly of concern. This assumption holds when i) differences in solubility of different As compounds do not affect ecotoxicity, and ii) there are no important differences in the speciation of inorganic arsenic substances in the environment among the substances tested.

Arsenic is present naturally in the aquatic and terrestrial environments from weathering and erosion of rock and soil. Because of its reactivity and mobility, however, arsenic can cycle extensively through both biotic and abiotic components of local aquatic and terrestrial systems, where it can undergo a variety of chemical and biochemical transformations. Three major modes of (bio)transformation of arsenic species have been found to occur in the environment: redox transformation between arsenite and arsenate, the reduction and methylation of arsenic, and the biosynthesis of organoarsenic compounds (WHO, 2001).

Arsenic can exist in four valency states in the environment: -3, 0, +3 and +5. Under strongly reducing conditions, elemental arsenic (As(0)) and arsine (As(-III)) can exist, however, the most common forms of As in the environment are the inorganic oxyions of arsenite As(III) and arsenate As(V) (Mahimairaja et al., 2005; Van Herwijnen et al., 2015; WHO, 2001). In oxygenated environments, the thermodynamically more stable arsenate is generally the predominant form, while arsenite is formed under anaerobic/moderately reducing conditions (Environment Canada, 1993; WHO, 2001). The relationship between arsenate and arsenite in soil and water systems is influenced by several factors, most importantly redox potential (E_h), pH, presence of chemical oxidizing agents such as iron and manganese oxyhydroxides (Environment Canada, 1993) as well as microbial action (Environment Agency, 2009). Overall, results presented in literature support the assumption that the predominant arsenic species in oxidizing environments is the thermodynamically stable form, i.e. arsenate (Le et al., 2000; Pongratz, 1998; Van Herwijnen et al., 2015; Environment Agency, 2009; WHO, 2001). Arsenite is present in amounts exceeding those of arsenate only in reduced, oxygen-free micro- and macro-environments (Kim et al., 2001; Sorg, 2013). Eh-pH diagrams of the system As-O-H confirm these findings: The prevailing arsenic compounds under environmentally relevant conditions are dihydrogen arsenate (H₂AsO₄[-]) and hydrogen arsenate (HAsO₄[2-]), as well as, under moderately to strongly reducing conditions, arsenous acid (As(OH)₃) (Takeno, 2005).

When arsenic is deposited directly into aerobic surface waters, it forms As(III) species, i.e. arsenite. As explained above, arsenite is thermodynamically unstable in most environments, and therefore tends to oxidize to dissolved As(V) species, i.e. arsenates. This oxidation can be accelerated by oxidizing agents such as manganese and iron oxyhydroxides which are fairly abundant in natural environments or by the action of certain bacteria. Some As(III) and As(V) species can interchange oxidation states depending on Eh, pH and biological processes. The ratio between oxidized and reduced species appears to be significantly influenced by the presence of iron and manganese oxides. However, the predominant arsenic species in oxidizing environments is the thermodynamically stable form, i.e. arsenate.

Information about the redox speciation of arsenic compounds during the various tests was not available, but the reliable data for the various endpoints do not appear to differ significantly between the different arsenic substances tested. Thus, all reliable ecotoxicity data for inorganic arsenic substances were considered. For the aquatic environment, only results based on measured dissolved arsenic concentrations are considered. For soil and sediment compartments, the assessment is based on results expressed as total As concentrations from tests with soluble As substances.

For the ecotoxicity assessment of metals in different environmental compartments (aquatic, soil and sediment), it is typically assumed that the toxicity is not controlled by the total concentration of a metal, but rather by the bioavailable form in the respective medium. Regarding metals, this bioavailable form is typically accepted to be the free metal-ion or the oxy-anion in solution. In the absence of speciation data and as conservative assessment, it was assumed that i) all dissolved arsenic is bioavailable when dissolved concentrations are provided, and that ii) in the absence of information about dissolved levels, all of the applied arsenic is dissolved and potentially bioavailable.

Reliable ecotoxicity results selected for read-across from different arsenic substances are based on tri- and pentavalent As substances (diarsenic trioxide, sodium arsenite, diarsenic pentaoxide, disodium hydrogenarsenate, sodium metaarsenate and sodium arsenate). The counter-ion (Na⁺) is abundant in natural environments and has a low toxicity profile and therefore is not expected to cause any toxic effect at the concentration tested. Thus, the hazard assessment based on dissolved arsenic levels is considered conservative. Therefore, a read-across approach from reliable ecotoxicity data from experiments with inorganic tri- and pentavalent As substances is justified.

For further information on the read-across approach, see also the read-across justification document (attached in IUCLID section 13).

 

References

Environment Canada, 1993. Canadian Environmental Protection Act Priority Substances List Assessment Report: Arsenic and Its Compounds

 

Environment Agency, 2009. Soil Guideline Values for inorganic arsenic in soil. Science Report SC050021/ arsenic SGV. Bristol: Environment Agency

 

Kim, M.-J., Nriagu, J., Haack, S., 2001. Arsenic species and chemistry in groundwater of southeast Michigan. Environmental Pollution 120: 379-390

 

Le, X. C.; Yalcin, S., Ma, M., 2000. Speciation of Submicrogram per Liter Levels of Arsenic in Water: On-Site Species Separation Integrated with Sample Collection. Environ. Sci. Technol. 34: 2342-2347

 

Mahimairaja, S., Bolan, N.S., Adriano, D.C., Robinson, B., 2005. Arsenic Contamination and its Risk Management in Complex Environmental Setting. Adv. Agron. 86: 1-82

 

Pongratz, R., 1998. Arsenic speciation in environmental samples of contaminated soil. The Science of the Total Environment 224: 133-141.

 

Sorg, T., 2013. Arsenic Species in the Ground Water. Presented at AWWA Inorganics Workshop Sacramento, CA, February 05-06, 2013

 

Van Herwijnen, R., Postma, J., Keijzers, R., 2015. Update of ecological risk limits for arsenic in soil. National Institute for Public Health and the Environment. Bilthoven: RIVM

 

World Health Organization (WHO), 2001. Environmental Health Criteria 224: Arsenic and Arsenic Compounds (2nd edn.). International Programme on Chemical Safety (IPCS). Geneva: WHO

 

Conclusion on classification

Ecotoxicity data for GaAs are available from a powdered form only. The marketed massive form (wafers = disks; falling under the definition of articles under CLP- and REACH-regulation) is expected to have negligible water solubility. Therefore, the marketed massive form does not need to be classified and labelled. Tests have been performed with a powder form, which is not marketed (Particle size: D10=41.1, D50=299.4, D90=816.0 µm (total range: 4 – 2000 µm)). This does not represent the manufactured substance, which is a massive form. For powders a higher dissolution (or reaction) rate than for the massive form can be expected leading to a more stringent classification. Thus, the results are considered a worst case for powdered material.

Lowest EC50 (Daphnia magna, 48 h) = 47.9 % WAF of 100 mg/L GaAs in its powder form.

Short term toxicity tests with fish and algae revealed no effects up to 100% WAF of 100 mg/L GaAs in its powder form.

The classification shall be based on the smallest particle size marketed. For GaAs the powder form is not marketed. GaAs in its massive form is considered for classification.

GaAs massive

A transformation/dissolution test with the massive GaAs form shows maximum dissolved concentrations of 0.146 mg As/L and 0.134 mg Ga/L at a loading of 100 mg GaAs/L and 7 days equilibarion at pH 8 (maximum dissolution). Both concentrations are well below the acute exotox reference values (ERV) for As and Ga (1.5 mg As/L and 14.65 mg Ga/L), showing no need for an acute classification of massive GaAs.. Dissolved concentrations after 28 day equilibration for a loading rate of 1 mg/L at pH 8 are 0.027 mg As/L and 0.023 mg Ga/L. These values are again well below the corresponding exotox reference values for chronic toxicity of As and Ga (0.234 mg As/L and 0.465 mg Ga/L). Therefore, it is concluded that there is no need for a classification of GaAs in its massive form as hazardous to the aquatic environment because water solubility of the massive form, i.e. the marketed wafers, is negligible and well below the corresponding ERV values for As and Ga.

No bioaccumulation potential is expected for GaAs or its free ions. Because there is no bioaccumulation potential the safety net classification does not apply for GaAs massive.

GaAs dust/ powder:

According to the available ecotoxicity data (48-h EC50 of 47.9 mg/L for Daphnia magna) and table 4.1.0 b.iii of CLP-regulation (ammended by regulation (EC) 286/2011), GaAs powder should be classified as Aquatic Chronic 3, Hazard statement: H412 (Harmful to aquatic life with long lasting effects). A transformation/dissolution test with GaAs powder however shows maximum dissolved concentrations of 0.368 mg As/L and 0.337 mg Ga/L at a loading of 1 mg GaAs/L and 28 days equilibarion at pH 8 (maximum dissolution). The concentration for As is above the corresponding exotox reference value (ERV) for chronic effects of As (0.234 mg As/L), while the concentration for Ga is below the ERV for chronic effects of Ga (0.465 mg Ga/L). According to table 4.1.0 b.i of the CLP-regulation, it is therefore concluded that the (non-marketed) powder form of GaAs has to be classified as: Aquatic Chronic 2, Hazard statement: H411 (Toxic to aquatic life with long lasting effects).