Sodium selenate

Endpoint summary

Additional information

Added risk approach

Selenium is naturally present in all environmental compartments. The median ambient background concentrations in agricultural soil and grazing land are 0.35 and 0.40 mg Se/kg, respectively (Vercaigne et al., 2010). Background Se concentrations were only measured in a few soils used for the terrestrial ecotoxicity tests and vary between 0.23 and 1.5 mg Se/kg dw (Somogyi et al., 2007; Wilke, 1989; Cartes et al., 2005; Soltanpour and workman, 1980; Liu et al., 2016). Because these background concentrations are significant compared to the NOEC and EC10 values for effects of inorganic selenium substances (selenite or selenate) to terrestrial organisms, the added risk approach is employed as a pragmatic solution. All NOEC and EC10 values are therefore based on added selenium concentrations, without taking into account the natural background in the soil. 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”).

Summary toxicity data

The available ecotoxicity results for the effect of selenium on terrestrial organisms are all based on either Na₂SeO₃ or Na₂SeO₄. All data reported are based on nominal added or background corrected measured Se concentrations in soil. A clear difference in toxicity was observed between selenite and selenate, with selenate showing significantly higher toxicity to invertebrates (Somogyi et al. 2007 and 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, results for sodium selenite are used for the assessment of inorganic tetravalent Se substances and results for sodium selenate are selected for the assessment of inorganic hexavalent Se substances.

The data available do not allow conclusions on the effect of soil properties (pH, organic carbon content, etc.) on the toxicity of selenite or selenate to terrestrial organisms. Therefore, all reliable toxicity data, expressed on an added concentration basis, were grouped for either selenite or selenate. The table below presents an overview of the reliable toxicity data selected for hazard assessment of selenate to terrestrial organisms.

Reliable chronic toxicity data are available for the long-term effect of selenate on 14 terrestrial species or microbial endpoints covering the 3 trophic levels (7 terrestrial plants, 4 invertebrates and 3 microbial endpoints). When several data were available for one species, the lowest reliable NOEC value was selected for the derivation of the PNEC. Selected reliable NOEC values range between 0.39 mg added Se/kg soil dry weight for shoot yield of Medicago sativa and ≥20 mg added Se/kg dry weight for microbial respiration processes in soil.

Test organism	Taxonomic group	Endpoint	Effect parameter	Value (mg added Se/kg)	Reference
Eisenia fetida	Lumbricidae (annelida)	reproduction	NOEC	1.7	Checkai et al., 2004
Enchytraeus albidus	Enchytraeidae (annelida)	reproduction	NOEC	3.4	Somogyi et al., 2012
Enchytraeus crypticus	Enchytraeidae (annelida)	reproduction	NOEC	2.2	Checkai et al., 2004
Folsomia candida	Isotomidae (arthropoda)	reproduction	NOEC	2.35	Checkai et al., 2004
Beta vulgaris	Amaranthaceae(eudicotyledon)	shoot yield	NOEC	≥1.5	Wan et al., 1988
Hordeum vulgare	Poaceae(monocotyledon)	shoot yield	NOEC	≥1.5	Wan et al., 1988
Lollium perenne	Poaceae(monocotyledon)	yield	NOEC	2.0	Cartes et al., 2005
Medicago sativa	Fabaceae(eudicotyledon)	shoot yield	NOEC	0.39	Soltanpour and Workman, 1980
Prunus	Rosaceae(eudicotyledon)	Twig length	NOEC	1.0	Pezzarossa et al., 2009
Lycopersicon escultentum	Solanaceae(eudicotyledon)	shoot yield	NOEC	≥1.5	Wan et al., 1988
Triticum aestivum	Poaceae(monocotyledon)	shoot yield	NOEC	2.0	Lyons et al., 2005
Natural soil microbial community	Bacteria	microbial biomass	NOEC	4.2	Wilke, 1988
Natural soil microbial community	Bacteria	microbial N transformation (nitrification)	NOEC	≥5.9	Wilke, 1989
Natural soil microbial community	Bacteria	microbial C transformation (respiration)	NOEC	≥20	Chander and Joergensen, 2007