Diarsenic trioxide

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

7.4 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

6.2 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

80.5 µg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

93.1 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

47.4 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:

3.8 mg/kg soil dw

Assessment factor:

2

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

1.3 mg/kg food

Assessment factor:

30

Additional information

Read-across approach

For the ecotoxicity assessment of diarsenic trioxide, 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 diarsenic trioxide 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 concentrationsare considered. For soil and sediment compartments, the assessment is based on results expressed as total 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

Arsenic and arsenic compounds have a harmonised classification according to Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation) as “Aquatic Acute Category 1” and “Aquatic Chronic Category 1”.

Below, we present the self-classification based on the reliable aquatic ecotoxicity data retrieved. Acute and chronic reference values for environmental classification are based on standard test as laid down in Council Regulation (EC) No 440/2008 on “Test methods pursuant to Regulation (EC) No 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)”.

Relevant information on acute toxicity of arsenic to standard fish species, invertebrates and algae were retained for classification purposes when tests were in line with accepted standard testing guidelines. For fish, the lowest reliable data point for arsenic, added to the test medium as sodium arsenate, was put forward for hazard assessment purposes: 96h-LC₅₀of 12600 µg As/L for the fish P. promelas. For invertebrates, an 48h-LC₅₀ of 1500 µg As/L for sodium arsenate (test organism: D. magna) was put forward for hazard assessment purposes. For algae, two reliable acute data points, representing the most sensitive strain of Chlorella sp. were available for hazard assessment purposes. The lowest value of those two data points was 25200 µg As/L and is considered as a reliable acute toxicity value for hazard assessment purposes. No reliable EC₅₀-values were reported for other relevant algal species. The lowest acute L(E)C₅₀ value, i.e. 48h-LC₅₀of 1500 µg As/L for D. magna, is selected as the acute environmental reference value (ERV) for classification of inorganic arsenic substances.

Chronic toxicity data of arsenic are also available for three trophic levels. The identified long-term data for standard species (and trophic levels) that are considered for classification purposes, revealed a chronic ERV of 234 µg As/L, derived from a 5-day fish early-life stage toxicity test with rainbow trout (O. mykiss). Other data considered for the PNEC derivation were not generated according to standard methods and hence not taken forward for classification purposes.

Acute and chronic reference values of 1.5 mg As/L and 0234 mg As/L are based on dissolved elemental As concentrations. Because arsenic is an inorganic element, there is no potential for degradation and there is not sufficient evidence for removal from the water column. Thus, classification of diarsenic trioxide as long-term hazard is derived according to Regulation (EC) No 1272/2008, Table 4.1.0 (b) for (i) non-rapidly degradable substances for which there are adequate chronic toxicity data available. For the classification of diarsenic trioxide, the acute and chronic reference values based on mg As/L must be corrected for the molecular weight of diarsenic trioxide (75.7% As) resulting in reference values of 1.98 mg As₂O₃/L and 0.309 mg As₂O₃/L, respectively. Based on these reference values, the self-classification for diarsenic trioxide would be “Aquatic Chronic Category2” (acute reference value>1 mg/L and chronic reference value >0.1 mg/L and ≤ 1 mg/L).