Cobalt molybdate

Toxicity to microorganisms

Description of key information

IC50 (9h) = 185.7 mg cobalt molybdenum oxide/L (Tetrahymena pyriformis) for growth inhibition (read-across from cobalt chloride)

Key value for chemical safety assessment

EC50 for microorganisms:

185.7 mg/L

Additional information

No data on toxicity to microorganisms 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 single-species toxicity tests resulting in high quality L(I)C50 values (expressed as Co) for microorganisms are summarised in the WHO CICAD, 2006 (see attached table).

In the key study, the effects of cobalt chloride on the protozoan Tetrahymena pyiformis in a 9 hour flask test were investigated (Sauvant et al., 1995). The IC50 (9h) for growth inhibition was 50 mg Co/L, resulting in a recalculated value of 185.7 mg cobalt molybdenum oxide/L.

Another study on the ciliate Spirostomum ambiguum showed a LC50 (24h) value of 11.8 mg Co/L using cobalt nitrate as salt, equivalent to a value of 43.8 mg cobalt molybdenum oxide/L (WHO CICAD, 2006).

 

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

Molybdenum

Data on the toxicity tests performed with aquatic bacteria and protozoa, reported as EC₅₀, EC₁₀ and NOEC values are summarized below. It can be noted that there are insufficient useful data for aquatic micro-organisms to apply statistical extrapolation. From the extracted results, two test results were considered relevant for assessment purposes for micro-organisms in STPs: the activated sludge test with pure MoO₃(HRC, 1994) and the activated sludge test with sodium molybdate (Stearns, 1985). As the second study resulted in the highest, unbounded NOEC (i.e. > 950 mg/L), the value taken from the HRC (1994) study is put forward.

The lowest reliable observed EC10-value for respiration (inhibition of respiration after a 3h incubation period) using activated sludge was 216.5 mg Mo/L (HRC, 1994d). The test was conducted with molybdenum trioxide (MoO3) as test substance.

Bacteria – accepted studies Test species/ inoculum Test compound Test medium Exp. time Endpoint End parameter Result (mg Mo/L) Reference
Domestic activated sludge population	MoO3, technical	activated sludge + artificial sewage	3 hour	EC50	Inhibition of respiration	1926	Huntington Research Center, 1994c
Domestic activated sludge population	MoO3pure	activated sludge + artificial sewage	3 hour	EC50 EC10	Inhibition of respiration	546 216.5 (169.1-277.3)	Huntington Research Center, 1994d
Domestic activated sludge population	Sodium molybdate	activated sludge	30 min	NOEC	Oxygen utilization BOD-removal	> 950	Stearns, 1985
Domestic activated sludge population	Sodium molybdate	activated sludge	30 min	NOEC	Sludge dewatering	> 950	Stearns, 1985

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.