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Environmental fate & pathways

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Read-across approach

The chemical safety assessment of zinc selenite is based on the elemental Zn and Se concentrations and read-across to Zn and Se substances. Because ZnSeO3 releases almost similar amounts of Zn and Se upon dissolution (100 mg ZnSeO3 releases 34 mg Zn and 41 mg Se), and a comparison of PNEC values for Zn and Se learns that Se is the most critical element (see IUCLID section 6), the chemical safety assessment for ZnSeO3 will be based on read-across to Se only. Therefore, only information of environmental fate and behaviour of selenite is discussed in the chemical safety assessment.

In the assessment of the environmental fate and behaviour of selenite, a read-across approach is followed based on all relevant and reliable information available for 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 behaviour (adsorption, bioaccumulation etc.), and

ii) after emission to the environment, the various Se compounds do not show differences in speciation of selenium in the environment.

The reliable data selected for the environmental fate and behaviour of selenium are all based on either monitoring data of prevailing elemental selenium concentrations in water, soil, sediment, suspended matter and organisms or on experimental results with H2SeO3, Na2SeO3, Na2SeO4 and seleno-methionine.

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, for soil, a significant difference in adsorption of selenite (SeO32-) and selenate (SeO42-) 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).

Therefore only data for selenite are selected for all terrestrial endpoints, while all data based on monitoring data or on soluble Se substances are used in a read-across approach for bioaccumulation and adsorption/desorption in sediments and suspended matter. Results for environmental fate and behaviour are all expressed based on elemental selenium concentrations.

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:

Summary data for environmental fate and behaviour of zinc selenite


There is no requirement for data on (bio)degradability of zinc selenite because these are not relevant for inorganic substances. The chemical safety assessment of zinc selenite will be based on elemental Zn and Se concentrations, regardless of their (pH-dependent) speciation in the environment and therefore, (a)biotic degradation processes in the environment are considered not relevant. Although the speciation of Zn and Se in the environment may change, elemental Zn and Se will not be eliminated by (bio)degradation reactions. This elemental-based assessment (pooling all speciation forms together) can be considered as a worst-case assumption for the chemical assessment.


Reliable data for distribution of Se between solid and liquid phase in soil, sediment and suspended matter are available for Na2SeO3 and Na2SeO4 (soil) or based on monitoring data of elemental selenium in the environment (sediment and suspended matter). All results for sediment and suspended matter were used in a read-across approach for selenite. For soil, a significant difference in sorption was observed between selenite and selenate, and therefore data were not combined and the 50th percentile of the Kp soil values for selenite was selected for the chemical safety assessment of zinc selenite. No assessment was made for Zn.


Since direct and indirect exposure in the aquatic compartment is considered likely, bioaccumulation information is required according to Annex IX under REACH. There is substantial information available on bioconcentration and bioaccumulation of Se in freshwater organisms (fish, aquatic invertebrates, aquatic plants and cyanobacteria, tadpoles) and on trophic transfer of Se from primary producers (algae) to primary consumers (aquatic invertebrates) and from primary consumers or secondary consumers to secondary or tertiary consumers (fish). Reliable information has been obtained from field studies (ambient Se) as well as laboratory experiments exposing organisms to waterborne/and or dietary Se added as Na2SeO3, Na2SeO4, H2SeO3 or seleno-methionine. The identified information was combined and used in a read-across approach for Se compounds.

In contrast to Se, there is no potential for bioaccumulation of Zn.