Cobalt molybdate

Endpoint summary

Additional information

No data on the aquatic ecotoxicity are available for the test substance cobalt molybdenum oxide, except for one short-term toxicity test with freshwater fish conducted according to national guidelines and OECD 203 on Chinese rare minnows. In this static limit test,no mortality or any other effects were observed in any of the treatments, hence an LC50 (96 h) of > 100 mg/L was reported, equivalent to 22.16 mg/L (measured concentrations as derived from analytical measurements).

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

Information taken from WHO CICAD (2006):

A 96-h EC50 for cobalt based on growth of the freshwater green alga Chlorella vulgaris was reported as 0.56 mg/L, whereas EC50s for aquatic vascular plants were 0.14 and 0.24 mg/L. The 5-day EC50 for cobalt based on growth of the marine diatom Ditylum brightwellii was 0.3 mg/L. For freshwater invertebrates, acute LC50s (24–96 h) range from 1.1 to 239 mg/L. Several studies on Daphnia magna reproduction were reported, with a 21-day EC50 at 0.01 mg/L and a 28-day NOEC of 0.003 mg/L; however, later studies found 21-day NOECs ranging from 0.03 to 0.05 mg/L for varying levels of calcium carbonate.

The lowest reported NOEC for aquatic organisms was for the water flea Ceriodaphnia dubia in a 7-day test, at <0.003 mg/L. The most sensitive marine invertebrates were lobster larvae, with 96-h LC50s ranging from 4.5 to 22.7 mg/L. 96-hour LC50s for freshwater fish range from 1.4 to 333 mg/L. A 16-day NOEC based on survival was reported at 0.06 mg/L. Test results for marine fish suggest that at least the species tested are relatively insensitive to cobalt, with 96-h LC50s ranging from 52.5 to >1000 mg/L.Ca2+ competition and dissolved organic matter complexation were the most important factors preventing Co2+ from binding at the gills in natural water tests. However, the effect of Ca2+ ions on the uptake and potential toxicity of cobalt occurs at very low Ca2+ concentrations, probably lower than those used in any of the reported toxicity tests.

Under most environmental conditions, including both fresh water and marine water, much of cobalt is dissolved either as cobalt carbonate or as Co2+ ions. However, the actual bioavailability appears to depend on the water chemistry and particularly the concentration of Ca2+ ions and dissolved organic matter complexation. It is suggested that there might be an effect of water hardness on aquatic toxicity (WHO CICAD, 2006).

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

Molybdenum

Acute freshwater data

For fish, the lowest reliable data point for ammonium molybdate was put forward for hazard assessment purposes:

- 96h-LC₅₀ of 237 mg Mo/L for the fish O. mykiss (HRC, 1994); Mo-salt was (NH₄)₂Mo₂O₇.

For invertebrates (test organism: Daphnia magna) the lowest reliable data point for ammonium molybdate was put forward for hazard assessment purposes:

- 48h-LC₅₀of 79 mg Mo/L for the cladoceran D. magna (HRC, 1994); Mo-salt was (NH₄)₂Mo₂O_7.

For algae, data that were generated with sodium molybdate were considered. For the most sensitive strain of P. subcapitata that was used in reliable study reports, 4 acute data points were availabe. The geometric mean of those 4 data points was 333.1 mg/L. This value is considered as a reliable acute algal toxicity reference value for hazard assessment purposes (e.g. classification).

Chronic freshwater data

A Species Sensitivity Distribution (SSD) has been developed for the assessment of molybdenum in the freshwater compartment, using the reliable species-specific chronic toxicity effect levels that have been identified. An overview of these species-specific data is given below. All toxicity tests were performed using sodium molybdate as test substance.

 

Overview of most sensitive species-specific K1-NOEC/EC₁₀-values for molybdenum in the freshwater environment

Species	Trophic level	NOEC/EC10-value (mg Mo/L)	Reference studies
Oncorhynchus mykiss	Fish	43.2	Parametrix, 2008
Pimephales promelas	Fish	60.2	Parametrix, 2007; GEI, 2009
Ceriodaphnia dubia	Crustacea	63.0	De Schamphelaere et al, 2008; GEI, 2009
Pseudokirchneriella subcapitata -CIMM strain	Algae	74.3	Rodriguez, 2008; De Schamphelaere and Janssen, 2008
Daphnia magna	Crustacea	89.5	De Schamphelaere et al, 2008 Rodriguez, 2007 ; GEI, 2009
Xenopus laevis	Amphibia	115.9	De Schamphelaere et al, 2008
Chironomus riparius	Insecta	121.4	De Schamphelaere et al, 2008
Brachionus calyciflorus	Rotifera	193.6	De Schamphelaere et al, 2008
Lymnaea stagnalis	Gastropoda	221.3	De Schamphelaere et al, 2008
Lemna minor	Aquatic plant	241.5	De Schamphelaere et al, 2008

 

 

No-effect levels for dissolved molybdenum were situated between 43.2 mg Mo/L and 241.5 mg Mo/L, i.e. a difference of a factor of 5.6 between the most and least sensitive species. Fish appear to be the most sensitive trophic level, representing the two lowest no-effect concentrations, followed by the algal species P. subcapitata and the two cladocerans D. magna and C. dubia.  The lowest sensitivity was found with duckweed (L. minor).

 

As mentioned before, these data have been used for the construction of a Species Sensitivity Distribution (SSD) from which the following parameter was derived:

-the median 5^thpercentile, i.e. the HC_5,50%with 5%-95%-confidence interval. The confidence interval is calculated using a Monte Carlo analysis on the log-normal distribution that was fitted through the 10 data points. The outcome of this analysis allows the derivation of the HC_5,50%with 5%-95% confidence interval. The HC_5,50%(± 95% CL) that was associated with this distribution was 38.2 mg Mo/L (95%CL: 18.7 – 57.3 mg Mo/L).  

 

Composition of the test media that were used for the development of the Mo-effects database

Species	Test duration	Endpoint	pH	Hardness^(1) mg/L as CaCO3	mg Ca/L	mg Mg/L
Pimephales promelas	34 d	Biomass	7.5±0.05	98.4	22.3	10.4
Oncorhynchus mykiss	78/84 d	Biomass	7.4±0.1	81.0	18.3	8.6
Pseudokirchneriella subcapitata -CIMM strain	72 h	Growth rate	8.0-8.1	24.1	4.9	2.9
Ceriodaphnia dubia	7 d	Reproduction	7.6-7.9	180	Not specified
Daphnia magna	21 d	Reproduction	7.4-8.2	168 - 250	27.9-80.1	12.2-24.2
Xenopus laevis	4 d	Malformation	7.8	110.7	19.4	15.1
Chironomus riparius	14 d	Growth	6.9-7.1	84.7	14	12.1
Brachionus calyciflorus	48 h	Reproduction	7.5	84.7	14	12.1
Lymnaea stagnalis	28 d	Length	7.8-8.2	140	40.1	9.7
Lemna minor	7 d	Growth rate	6.5-6.7	54.9	9.8	7.4

⁽¹⁾: derived from nominal/measured Ca/mg content

  

Marine freshwater data

A Species Sensitivity Distribution (SSD) has been developed for the assessment of molybdenum in the marine compartment, using the reliable species-specific chronic toxicity effect levels that have been generated in various research studies. An overview of these species-specific data is given in the table below. All toxicity tests were performed using sodium molybdate as test substance.

Overview of most sensitive species-specific K1-NOEC/EC₁₀-values for molybdenum in the marine environment

Species	End parameter	Endpoint	Value (mg Mo/L)	Reference
Mytilus edulis	Embryonal development	48h-EC10	4.4	Morgan et al, 1986
Acartia tonsa	Development	20d-EC10	7.96	Aquasense, 2009
Cyprinodon variegatus	Larval dry weight	28d-EC10	84.1	Parametrix, 2010
Americamysis bahia	Reproduction	28d-NOEC	116	Lehman, 2010
Phaeodactylus tricornutum	Growth rate (cells)	72h-ErC10	170	Aquasense, 2009
Dendraster excentricus	Larval development	48h-EC10	223.6	Parametrix, 2008a
Strongylocentrotus purpuratus	Larval development	48h-EC10	421.3	Parametrix, 2008b
Ceramium tenuicorne	Growth rate (length)	7d-NOEC	641	Le Page et al, 2010
Dunaliella tertiolecta	Growth rate (cells)	72h-ErC10	881	Le Page and Hayfield, 2010
Crassostrea gigas	Larval development	48h-EC10	1174	Aquasense, 2009

No-effect levels for dissolved molybdenum were situated between 4.4 mg Mo/L and 1174 mg Mo/L, i.e., a difference of a factor of 267 between the most and least sensitive species. The mussel M. edulis and the copepod A. tonsa were the most sensitive trophic level. The least sensitive species were the oyster C. gigas and two algal species (microalga D. tertiolecta and macroalga C. tenuicorne).

As mentioned before, these data have been used for the construction of a Species Sensitivity Distribution (SSD) from which the median 5^thpercentile was derived, i.e. the HC_5,50%with 5%-95%-confidence interval. Using the RIVM software package ETX, a Log-Normal Distribution was plotted through this data set. The HC_5,50%(± 95%CL) that was associated with this distribution was 5.74 mg Mo/L (95%CL: 0.58 – 21 mg Mo/L), respectively.