Sodium selenate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

6.38 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

PNEC freshwater (intermittent releases):

6.38 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

4.09 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

10 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

19.7 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

12.6 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:

0.47 mg/kg soil dw

Assessment factor:

3

Extrapolation method:

sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

2.39 mg/kg food

Assessment factor:

1

Additional information

In the assessment of the ecotoxicity of sodium selenate, a read-across approach is followed based on all relevant and reliable information available for inorganic Se compounds. This grouping of selenium compounds for estimating their properties is based on the assumption that properties are likely to be similar or follow a similar pattern as a result of the presence of the common selenium ion.

This assumption can be considered valid when

i) differences in solubility among Se compounds do not affect the results for ecotoxicity (i.e. toxicity occurs below the solubility limit),

ii) ecotoxicity is only affected by the selenium-ion and not by the counter ions, and

iii) after emission to the environment, the various Se compounds do not show differences in speciation of selenium in the environment or differences in speciation do not affect toxicity.

In order to correct for differences in solubility among Se compounds, all reliable data on ecotoxicity selenium to aquatic organisms were selected based on measured dissolved selenium concentrations. It is indeed assumed that toxicity is not controlled by the total concentration of an element, but by the bioavailable form. No evidence is available on the bioavailable form of selenium, but as a conservative approximation, it can be assumed that the total soluble selenium pool is bioavailable. For soils, data were only available for the soluble sodium selenite and sodium selenate salts (solubility > 1g/L) and therefore no effect of solubility on the toxicity was expected.

The reliable ecotoxicity results were all mainly derived for sodium selenite (Na₂SeO₃) and sodium selenate (Na₂SeO₄), but some reliable data are also available for H₂SeO₃, SeO₂, seleno-methionine and seleno-cysteine. There is no concern on the effect of the counter-ions (Na⁺) in the concentration ranges tested.

The data for organic Se compounds (Se-methionine and Se-cysteine) were not taken into account for the assessment of direct effects of selenite to aquatic or terrestrial organisms because there is some concern on different biochemical behaviour of this selenium containing amino acid compared to inorganic Se compounds. However, because inorganic Se can be transformed into this organic form in the environment, data for seleno-methionine and seleno-cysteine are included for the assessment of secondary poisoning (through PNECoral and bioconcentration factors).

For aquatic organisms, the comparison in toxicity among the various inorganic Se substances (H₂SeO₃, Na₂SeO₃, SeO₂or Na₂SeO₄) did not yield consistent or significant differences. Therefore, results for all these substances were used in a read-across approach. In contrast, for soils a clear difference in toxicity was observed between selenite and selenate, with selenate showing significantly higher toxicity to terrestrial invertebrates (Somogyi et al. 2007 & 2012) and plants (Cartes et al., 2005; Carlson et al., 1991). This is consistent with the lower adsorption and resulting higher bioavailability of selenate in soil compared to selenite. Therefore, only the available reliable results for toxicity of selenate to terrestrial organisms (plants, invertebrates and micro-organisms) are taken into account for the hazard assessment of sodium selenate in soils.

Selenium is chemically related to sulphur and can exist in a multitude of different oxidation states from -2 to +6 and in both organic and inorganic forms. Under conditions commonly found in oxic fresh waters (i.e., pH between 5 and 9; redox potential [Eh] between 0.5 and 1 V), the hexavalent oxidation state is predicted to be the most prevalent (Takeno, 2005). However, tetravalent selenium also exists under some conditions (low pH, low redox potential).

No information is available on the speciation of the selenium compounds of interest upon dissolution in water and on the redox speciation of the selenium compounds during the various tests available. Some measured data were found on speciation of selenium in the environment. These results confirm that hexavalent Se dominates in most surface waters, while elemental Se and organic Se species dominate in sediments (Zhang and Moore, 1996; Van Derveer and Canton, 1997). Based on limited information available, the environmental conditions are expected to largely control the (redox) speciation of selenium upon dissolution in water, regardless of the Se compound added. However, as mentioned above, a significant difference in adsorption of selenite (SeO₃^2-) and selenate (SeO₄^2-) to soil was observed, with lower adsorption for selenate (median log Kp of 0.87 L/kg dry weight) compared to selenite (median log Kp of 1.73 L/kg dry weight).

In conclusion, all available reliable data for inorganic selenium compounds were used in a read-across approach for aquatic toxicity, except for toxicity to aquatic microorganisms. Only data for selenate were selected for toxicity to aquatic microorganisms and terrestrial organisms (plants, invertebrates and micro-organisms). For toxicity to above-ground organisms (birds, mammals and reptiles) and fish via diet all Se compounds were taken into account, including Se containing amino acids seleno-methionine and seleno-cysteine. All results for ecotoxicity of Se are expressed based on elemental selenium concentrations.

See also read-across justification document attached to IUCLID section 13.

• Zhang Y.Q., Moore J.N. (1996) Selenium Fractionation and Speciation in a Wetland System. Environmental Science & Technology 30:2613-2619.
• Van Derveer W.D., Canton S.P. (1997) Selenium Sediment Toxicity Thresholds and Derivation of Water Quality Criteria for Freshwater Biota of Western Streams. Environmental Toxicology and Chemistry 16:1260-1268.
• Takeno N. 2005. Atlas of Eh–pH diagrams. Intercomparison of thermodynamic databases. Geological Survey of Japan Open File Report No. 419. Tokyo (JP): National Institute of Advanced Industrial Science and Technology, Research Center for Deep Geological Environments. 285 p. Available from: http://www.gsj.jp/GDB/openfile/files/no0419/openfile419e.pdf

Added risk approach

Selenium is a natural element and therefore naturally present in all environmental compartments. Median background concentrations in pristine waters, agricultural soil and grazing land in Europe are 0.32 µg Se/L, 0.35 and 0.40 mg Se/kg, respectively (Vercaigne et al., 2010). Background Se concentrations are significant compared to the PNEC values for both freshwater and soil and therefore, the added risk approach is preferred. All NOEC and EC10 values are based on added concentrations, without taking into account the natural selenium background. In essence this added risk assessment approach assumes that species are fully adapted to the natural background concentration and therefore that only the anthropogenic added fraction should be regulated or controlled (Appendix R.7.13-2 of the REACH guidance on “Environmental risk assessment for metals and metal compounds”). For essential elements, like Se, this assumption is most plausible. Although the added risk approach acknowledge that negative effects from the bioavailable fraction of the background concentration on some organisms in the ecosystem may occur, or that organisms may even have become acclimated/adapted to it, from an environmental policy point of view, such effects may be ignored and may even be regarded as desirable, since these effects may in theory lead to an increase in ecosystem differentiation or biodiversity (Crommentuijn et al, 1997). Another argument for the added risk approach is the extremely narrow range between dietary essentiality and toxicity for selenium. Application of assessment factors on total concentrations (including background concentration) may result in total PNEC values that cause deficiency for some species.

• Crommentuijn T, Polder M. and Van de Plassche, E 1997. Maximum permissible concentrations and negligible concentrations for metals, taking background concentrations into account. RIVM, Report 601501001
• Vercaigne, Claeys and Oorts (2010) Exposure assessment of selenium: Measured Se-levels in EU surface water and soil. Report for the Selenium & Tellurium Consortium.

Comparison of background concentrations of Se in the environment with derived PNEC values.

Compartment	Unit	Median background concentration	90^thpercentile of background concentrations	PNECadded
Fresh surface water (unimpacted areas)	µg Se/L	0.32	0.85	2.67 (direct toxicity)0.21 (secondary poisoning)
Marine water	µg Se/L	0.085	no data	1.71
Freshwater sediment	mg Se/kg dw	0.1^a	0.27^a	8.2
Agricultural soil (0 -20 cm)	mg Se/kg dw	0.35	0.59	0.044
Grazing land (0 -10 cm)	mg Se/kg dw	0.40	0.71	0.044

^a: based on the data for surface water and the equilibrium partitioning approach

Conclusion on classification

Reliable acute and chronic toxicity data are available for effect of inorganic selenium compounds on the three trophic levels.

The acute classification is based on the lowest value that was identified for any of the three trofic levels that are taken into account: 

·      The lowest relevant and reliable acute fish LC₅₀ for soluble inorganic selenium substances is 2060 µg Se/L;

·      The lowest relevant reliable acute invertebrate E(L)C₅₀ for soluble inorganic selenium substances is 550 µg Se/L;

·      The lowest relevant reliable algal E(L)C₅₀ for soluble inorganic selenium substances is 240 µg Se/L.

Algae and invertebrates are markedly more senstive to dissolved Se than fish. The ERV_acute for selenium is based on algal data and is 240 µg Se/L, corresponding to 574 µg Na₂SeO₄ /L.

The chronic classification is based on the lowest value that was identified for any of the three trophic levels that are taken into account: 

·      The lowest relevant and reliable chronic fish NOEC for soluble inorganic selenium substances is 10 µg Se/L

·      The lowest relevant and reliable chronic invertebrate NOEC for soluble inorganic selenium substances is 70 µg Se/L

·      The lowest relevant and reliable algal NOEC for selenium soluble inorganic selenium substances is 197 µg Se/L  

Fish were the most sensitive trofic level upon long-term exposure and determined the ERV_chronic for selenium of 10 µg Se/L, corresponding to 23.9 µg Na₂SeO₄ /L.

Taking into account these acute and chronic reference values, the classification for sodium selenate is “Aquatic acute Category 1" with M-factor 1 (0.1 mg/L < ERV_acute ≤ 1 mg/L) and "Aquatic Chronic Category 1” with M-factor 1 (0.01 mg/L < ERV_chronic ≤ 0.1 mg/L for non rapid degradable substance).