Diarsenic trioxide

Endpoint summary

Description of key information

Biodegradation

Biodegradation is not relevant for metals and metal compounds since they are inorganic and not biodegradable. Biotic processes may alter the speciation form of an element, but it will not eliminate the element from the environment by degradation or transformation. The chemical safety assessment of inorganic arsenic substances is based on elemental arsenic concentrations (pooling all speciation forms together) in the environment, which is considered a worst-case approach.

 

Bioaccumulation

The assessment of bioaccumulation of As in the aquatic and terrestrial organisms is based on field data on total As concentrations in organisms and their environment. This ensures that As burdens in biota are in equilibrium with As concentrations in their environment because organisms have been exposed during their whole life to this environment.

Bioaccumulation factors (BAF) for aquatic organisms vary across organisms, environmental setting (marine, estuarine, freshwater) and across arsenic concentrations in fresh- and saltwater. Biomagnification of arsenic in the food chain does not occur because BAF values tend to decrease with increasing trophic level and the largest bioaccumulation was observed for primary consumers (mainly data for zooplankton). There is no indication for difference in bioaccumulation of arsenic between fish and other organisms from higher trophic levels. Species mean BAF values for forage and predatory fish range from 0.4 to 1093 L/kg wet weight for 64 freshwater species and from 17.5 to 1511 L/kg wet weight for 27 marine species. BAF values for fish significantly decrease with increasing arsenic water concentrations and differ between the freshwater and marine environment. The median of freshwater and saltwater fish species mean BAF values is 14 L/kg wet weight and 619 L/kg wet weight, respectively. These values are selected for the chemical safety assessment.

Comparable statistics for bioaccumulation in plants, earthworms and small mammals, i.e. a multi-species biota-to-soil accumulation factor (BSAF) of 0.0375, 0.22 and 0.0038, respectively, indicate that plants, earthworms and small mammals accumulate arsenic to levels much lower than those measured in the associated soils. The BSAF of 0.22 (dimensionless, dry weight based) for earthworms was selected for the chemical safety assessment.

 

Adsorption / desorption

There is substantial reliable information available for adsorption/desorption of arsenic, reporting partitioning coefficients (Kp values, i.e. ratio of As concentration in solid phase over dissolved As concentration in solution phase) for soil, sediments and suspended matter. All the information available for sediment and suspended matter is based on paired monitoring data of elemental As concentrations in sediment or suspended matter and water. The reliable information for Kp values in soil is based on batch adsorption or desorption experiments with added As(III) and As(V) salts.

Soil: Reliable Kp values for As(III) and As(V) salts were identified for 52 soils with varying properties (pH: 3.9-8.8; organic carbon: 0.2-21.7%; clay: 1-68%). Log Kp values range between 0.11 and 4.06 L/kg dry weight, with a median of 2.50 L/kg dry weight. No significant differences were observed in Kp values between As salts added to the soils and Kp values were not strongly correlated with soil properties.The median log Kp of 2.50 L/kg dry weight from results for the 52 soils was selected for the partitioning of As between solids and water in soil for the chemical safety assessment.

Freshwater sediment: Log Kp values for As in sediment of 7 river systems vary between 2.63 and 4.03 L/kg dry weight. The median log Kp value of 3.68 L/kg dry weight for these 7 rivers was selected for the partitioning of As between solids and water in freshwater sediment for the chemical safety assessment.

Marine sediment: Two reliable studies were identified for Kp values of As in marine sediments, reporting Kp data for 2 marine systems. Average log Kp values for each location are 4.09 and 2.17 L/kg.The median log Kp value of 3.13 L/kg dry weight for these 2 marine systems was selected for the partitioning of As between solids and water in marine sediment for the chemical safety assessment.

Freshwater suspended mater: Log Kp values for As in suspended matter for 13 freshwater systems vary between 2.86 and 4.97 L/kg dry weight. The median log Kp value of 4.10 L/kg dry weight for these 13 freshwater systems was selected for the partitioning of As between solids and water in freshwater suspended matter for the chemical safety assessment.

Marine suspended mater: Log Kp values for As in suspended matter for 6 marine systems vary between 3.04 and 4.47 L/kg dry weight. The median log Kp value of 3.88 L/kg dry weight for these 6 marine systems was selected for the partitioning of As between solids and water in marine suspended matter for the chemical safety assessment.

Additional information

Read-across approach

For the assessment of the environmental fate and behaviour of diarsenic trioxide, a read-across approach based on all available data for inorganic arsenic compounds is applied. Reliable environmental fate and behaviour results selected are based on experiments with tri- and pentavalent As substances (sodium dioxoarsenate, disodium hydrogenarsenate, potassium dihydrogenarsenate) and monitoring data of elemental As concentration in biota and the environment.

This grouping of arsenic substances for estimating their environmental properties is based on the hypothesis that the common inorganic As moiety is the common driver for the fate 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 the environmental behaviour (bioaccumulation, adsorption, etc.), 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).

Therefore, a read-across approach from reliable environmental fate and behaviour results from experiments with tri- and pentavalent As substances or based on monitoring data of elemental As concentration in biota and the environment 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