Cobalt molybdate

Toxicity to soil macroorganisms except arthropods

Description of key information

NOEC (24 weeks) = 342 mg cobalt molybdenum oxide/kg soil dw  (Eisenia fetida) for growth (read-across from cobalt chloride hexahydrate)
LC50 (14 d) > 1000 mg cobalt molybdenum oxide/kg soil dw (Eisenia fetida) for mortality

Key value for chemical safety assessment

Long-term EC10, LC10 or NOEC for soil macroorganisms:

342 mg/kg soil dw

Additional information

One study is available on the short-term toxicity of cobalt molybdenum oxide to soil macroorganisms. In this study, earthworms (Eisenia fetida) were exposed to the substance in artificial soil at 1000 mg/kg soil dw in a limit test (He, 2012). The test was conducted according to national guidelines and OECD 207. No effects were observed in any of the treatments and as a result, an LC50 (14 d) of > 1000 mg/kg soil dw was obtained.

No data on long-term toxicity to soil macroorganisms are available for cobalt molybdenum oxide. However, there are reliable data available for different structurally analogue substances.

The environmental fate pathways and ecotoxicity effects assessments for cobalt metal and cobalt compounds as well as for molybdenum metal and molybdenum compounds is based on the observation that adverse effects to aquatic, soil- and sediment-dwelling organisms are a consequence of exposure to the bioavailable ion, released by the parent compound. The result of this assumption is that the ecotoxicology will be similar for all soluble cobalt and molybdenum substances used in the ecotoxicity tests. Therefore, data from soluble cobalt and molybdenum substances are used in the derivation of ecotoxicological and environmental fate endpoints, based on the cobalt ion and molybdenum ion, respectively.

Cobalt

Data on macroorganism (non-arthopod) toxicity tests resulting in high quality EC50 values (expressed as Co) for Eisenia fetida and a nematode species (n=4) are summarised in the WHO CICAD (2006).

No EC50 values are available for any of the three studies on Eisenia fetida. The results range from a NOEC(growth, 8 weeks) of 30 mg Co/kg soil dry weight, tested as unspecified cobalt salt (with significant differences to the control from 300 mg/kg soil dw) to 77% mortality at 4720 mg Co/kg soil dry weight, tested as cobalt chloride (hydration not specified) after 10 weeks exposure, with a recalculated value of between 111 and 17,529 mg cobalt molybdenum oxide/kg soil dw (WHO CICAD, 2006).

The key study for effects on growth reports a NOEC of 92 mg Co/kg soil dry weight after 24 weeks exposure to cobalt chloride hexahydrate (Neuhauser et al., 1984), with the recalculated value of 342 mg cobalt molybdenum oxide/kg soil dw.

Another study on the nematode species Caenorhabditis elegans reports 24 hour LC40 values for total cobalt of 1274 mg/L and for the free ion at 1210 mg/L (Hartenstein et al., 1981; WHO CICAD, 2006).

References: World Health Organization (2006). Concise International Chemical Assessment Document 69. COBALT AND INORGANIC COBALT COMPOUNDS.

Molybdenum

The invertebrate toxicity assays (excluding arthropods) resulted in 20 individual high quality NOEC/EC10 values (for 2 different invertebrates; Enchytraeus crypticus and Eisenia andrei) selected for assessment purposes (van Gestel et al., 2009).

The selected NOEC/EC10 values range from 7,88 mg added Mo/kg for E. andrei in soil 6 to 1661 mg added Mo/kg for E.crypticus in soil 7. All data are based on added measured Mo concentrations in dry weight soil.

For E. crypticus, unbounded NOEC values were observed in soils 1 and 9 (EC10> 2719 and 2816 mg added Mo/kg dw soil respectively).

For the aged soils (soils 4,5 and 6), the comparison of Mo in freshly spiked and 11 -month aged soils show that long-term equilibration of Mo in soils generally decreases its toxicity to invertebrates. Only in one case (out of 6 cases) an effect at a lower dose was observed in the aged soil (E. crypticus in soil 6). The results of the toxicity in aged soils are used to determine a Leaching/Ageing factor to correct for the effect of spiking on the toxicity of Mo in soil organisms.

Conclusion
As the effect values derived from analogue cobalt compounds are considerably lower than those derived from analogue molybdenum substances, it can be reasoned that the cobalt ion will account for the effects in ecotoxicological testing. Hence, it was concluded to put forward the most sensitive and reliable results derived from analogue cobalt compounds for assessment purposes, and recalculate them for CoMoO4.