Cobalt molybdate

Toxicity to aquatic algae and cyanobacteria

Description of key information

LC50 (96h) = 2.08 mg cobalt molybdenum oxide/L (Chlorella vulgaris) (read-across from cobalt chloride)
LC50 (96h) = 1.11 mg cobalt molybdenum oxide/L (Ditylum brightwellii) (read-across from cobalt chloride)
NOEC (21d) = 2.23 mg cobalt molybdenum oxide/L (Chlorella vulgaris) (read-across from cobalt nitrate)

Key value for chemical safety assessment

EC50 for freshwater algae:

2.08 mg/L

EC50 for marine water algae:

1.11 mg/L

EC10 or NOEC for freshwater algae:

2.23 mg/L

Additional information

No data on toxicity to aquatic algae and cyanobacteria 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 NOEC/L(E)C10 values (expressed as Co) for aquatic algae and cyanobacteria (n=8) are summarised in the WHO CICAD, 2006 (see attached table).

Acute and chronic data for different algae species in fresh- and saltwater were extracted and used in the effects assessment. The EC50 values for freshwater algae range from 0.56 mg Co/L for Chlorella vulgaris (Rachlin and Grosso, 1993) to 10.8 mg Co/L for Spirulina platensis (WHO CICAD, 2006). Two EC50 values were available for marine species with a similar range of 0.3 mg Co/L for Ditylum brightwellii and 10.2 mg Co/L for Nitzschia closterium (WHO CICAD, 2006). The 21-day NOEC and LOEC values for freshwater algae ranged from 0.6 to 1.6 mg Co/L (WHO CICAD, 2006).

These findings result in recalculated values from 2.08 to 40.1 mg cobalt molybdenum oxide/L for acute freshwater endpoints, from 1.11 to 37.9 mg cobalt molybdenum oxide/L for acute marine endpoints and from 2.23 to 5.94 mg cobalt molybdenum oxide/L for chronic freshwater endpoints.Further results for aquatic algae are available and comprised in the attached table.

In the key study, the effects of cobalt chloride on Chlorella vulgaris were investigated in a 96 -h test according to methods comparable to guidelines (Rachlin and Grosso, 1993). As a result, an EC50 of 0.56 mg Co/L was obtained, equivalent to 2.08 mg cobalt molybdenum oxide/L.

 

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

Molybdenum

Freshwater:

Acute toxicity

The reliable ErC₅₀-values that were determined by De Schamphelaere et al. (2008) for the green algae P. subcapitata were situated between 295 and 1568.9 mg Mo/L. Rodriguez (2008) reported reliable ErC₅₀ values that ranged from 289.2 to >419.9 mg/L for the same species. The acute data confirmed the findings of the chronic data (see further), i.e. that the CIMM algal strain was the most sensitive strain. The geometric mean of the 4 CIMM-data points is 333.1 mg/L and is considered as a reliable acute toxicity value for hazard assessment purposes (e.g. classification).

No reliable EC₅₀-values were reported for any other freshwater algal species.

	72h-ErC50(mg Mo/L)
	P. subcapitataCIMM	P. subcapitataUGhent
OECDCIMM, tested at CIMM	289.2 (254.1 – 340.1)	362.9 (316.8 – 415.9)
OECDUGhent, tested at CIMM	369.3 (297.2 – 495.6)	> 419.9
OECDCIMM, tested at UGhent	390.9 (312.4 -489.2)	1568.9 (1431.2 – 1719.8)
OECDUGhent, tested at UGhent	295.0 (247.8 – 351.3)	1094.5 (1019.6 – 1174.8)
Mean: Geomean:	336.1 333.1

Chronic toxicity

For Pseudokirchneriella subcapitata, a 72h-ErC₁₀of 156 mg Mo/L was reported by ECTX (2007a) but this value was based on nominal exposure concentrations.

The table listed hereunder gives an overview of the results of an algal exchange and testing program between Ghent University and CIMM (Centro de Investigación Minera y Metalúrgica) with the green algae Pseudokirchneriella subcapitata. Tests that were performed with the algal strain from CIMM resulted in four 72h-ErC₅₀values that were situated between 61.2 and 88.7 mg Mo/L, i.e. only a difference of a factor of 1.4 between the lowest and highest value which can be considered as natural biological variation among ecotoxicity tests that are conducted with the same species and same test substance. These four effect levels also indicate that the origin of the test medium or the test facility had little or no influence on the outcome of the test. Comparable results (62.5 – 99.3 mg Mo/L) were also obtained with the UGhent strain tested at CIMM. The tests conducted with this strain at the University of Ghent, however, produced 72h-ErC₁₀ levels that were ± a factor of 4 to 5 higher that the other six tests, but were in line with the test result that was generated by De Schamphelaere et al. (2008a) at the same testing facility, and using the same strain.

 

	72h-ErC10(mg Mo/L)
	P. subcapitataCIMM	P. subcapitataUGhent
OECDCIMM, tested at CIMM	88.7 (69.2 – 106.0)	62.5 (2.5 – 133.8)
OECDUGhent, tested at CIMM	77.3 (58.1 – 95.5)	99.3 (63.5 – 137.5)
OECDCIMM, tested at UGhent	61.2 (36.5 -102.7)	318.6 (278.9 – 363.9)
OECDUGhent, tested at UGhent	72.6 (48.3 – 109.5)	366.2 (294.4 – 455.5)
Mean: Geomean:	75.0 (CV:11.4%) 74.3	211.7 (CV: 153.0%) 164.0

     

Based on these findings, the following was concluded:

- Similar 72h-ErC₁₀-values were produced by the two testing facilities and using test media from these two facilities, with the P. subcapitata strain originating from CIMM.

- All test results are in line (maximum difference of a factor of 2.5) with the outcome of the accepted test result that was generated by ECTX (2007a).

- Comparison of the research results given in Rodriguez (2008) and De Schamphelaere and Janssen (2008), however, indicates that the CIMM-algal strain may be more sensitive than thestrain. From a conservative point of view it was therefore decided that only the 72h-ErC₁₀values obtained with the former strain should be taken into account for the derivation of a NOEC forP. subcapitata, i.e. excluding the effect levels that were obtained with the UGhent strain. The geometric mean of these four values is proposed as the chronic no-effect level for this species, and is 74.3 mg/L.

It should be noted that the geometric mean of all relevant and reliable data would result in a geometric mean that is higher than the proposed chronic no-effect level of 74.3 mg Mo/L, and this value can therefore be considered as a more conservative value.

Marine:

Acute toxicity

The reliable 72h-ErC₅₀-value determined by Aquasense (2009) for the algae P. tricornutum was 356.9 mg Mo/L. Le Page and Hayfield (2010) reported an unbounded 72h-ErC₅₀ of > 938 mg Mo/L. Both marine EC50-values were higher than the geometric mean value of 333.1 mg Mo/L for the freshwater green alga P. subcapitata which was put forward as an reference value for acute molybdenum toxicity to algae.

Chronic toxicity

For the micro-alga Dunaliella tertiolecta the 72h-EC₁₀ value of 881 mg/L (endpoint: growth rate) is retained for assessment purposes (Le Page and Hayfield, AstraZeneca, 2010). For biomass, the 72h-EC₁₀ was 513 mg/L. Although this value is lower than the EC₁₀ for growth rate, the latter endpoint is considered ecologically more relevant, and is preferred over biomass in the OECD guideline for algal testing. The 72h-NOEC value for both biomass and growth rate was 938 mg/L, which is higher than both EC₁₀values.

For the diatom Phaeodactylum tricornutum the 72h-EC₁₀ value of 169.9 mg/L (endpoint: growth rate) is retained for assessment purposes (Le Page and Hayfield, AstraZeneca, 2010). The 72h-NOEC value for this endpoint was 150 mg/L. The EC₁₀ value, however, is preferred over the NOEC value as the latter is a test design-dependent value. 

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.