Selenium

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

2.67 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

PNEC freshwater (intermittent releases):

5.5 µg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

2 µg/L

Assessment factor:

3

Extrapolation method:

sensitivity distribution

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1 500 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

8.2 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

6.2 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:

0.044 mg/kg soil dw

Assessment factor:

10

Extrapolation method:

assessment factor

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

1 mg/kg food

Assessment factor:

1

Additional information

General discussion

In the assessment of the ecotoxicity of selenium metal, 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 PNEC_oraland 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 selenite to terrestrial organisms (plants, invertebrates and micro-organisms) are taken into account for the hazard assessment of sodium selenite 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 K_Dof 0.87 L/kg dry weight) compared to selenite (median log K_Dof 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 selenite 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.

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 LC₅₀for soluble inorganic selenium substances is 2060 µg Se/L;
• The lowest relevant reliable acute invertebrate E(L)C₅₀for soluble inorganic selenium substances is 550 µg Se/L;
• The lowest relevant reliable algal E(L)C₅₀for soluble inorganic selenium substances is 240 µg Se/L. Algae and invertebrates are markedly more senstive to dissolved Se than fish. The ERV_acutefor selenium is based on algal data and is 240 µg Se/L.

Algae and invertebrates are markedly more senstive to dissolved Se than fish. The ERV_acutefor selenium is based on algal data and is 240 µg Se/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_chronicfor selenium of 10 µg Se/L.

Taking into account the solubility of Selenium metal there is no need to classify this substance for the environment. Selenium metal, however, has a harmonised classification for the environment (Aq.Chronic Cat.4), and this classification is currently applied.