Calcium molybdate

Endpoint summary

Description of key information

Read-across statement

Apart from a few exceptions, ecotoxicological data were generated with disodium molybdate as the test substance. Sodium molybdate is a readily soluble substance, and yields molybdate ions upon dissolution.

For metals it is generally assumed that toxicity is controlled by the dissolved metal ion. The speciation of molybdenum in aqueous media as a function of pH and molybdenum concentration has been thoroughly investigated and reported upon in open literature. Under physiological conditions (pH > 6.5) the sole molybdenum(VI) species is the molybdate anion, MoO42- (Cruywagen, 2000; Cruywagen et al, 2002). 

For the Mo substance in this dossier, UV-spectra of aqueous solutions of this substance indeed demonstrated that the only dissolved molybdenum species, originating directly from the substance, is molybdate.

It should be noted that all effect levels in this section of the dossier are expressed as molybdenum (and not molybdate).

An overview of all the aquatic toxicity data that were identified for molybdenum is given in the Background Document "Environmental effects assessment of molybdenum", which is attached in the technical dossier in IUCLID Section 13.

 

 

Summary of relevant acute ecotoxicity data

Relevant information on standard fish species, invertebrates and algae were retained for classification purposes when tests were in line with accepted standard testing guidelines. An overview of the critical data is given hereunder: 

For fish, the lowest reliable data point for molybdenum, added to the test medium as sodium molybdate, was a 96h-LC50 of 609.1 mg Mo/L for the fish P. promelas (GEI, 2009). This value is put forward for hazard assessment purposes.

For invertebrates, the following lowest reliable data point for sodium molybdate (test organism: D. magna) was a 48h-LC50 of 131 mg Mo/L (HRC, 1994), and this value was put forward for hazard assessment purposes:

For algae there were four reliable acute data points, representing a sensitive strain of P.subcapitata, that were available for hazard assessment purposes. The geometric mean value of those four data points was 333.1 mg Mo/L and is considered as a reliable acute toxicity value for hazard assessment purposes. No reliable EC50-values were reported for other relevant algal species.

 

Summary of chronic freshwater data

Relevant and reliable chronic no-effects values were identified for twelve aquatic freshwater species: Brachionus calyciflorus, Ceriodaphnia dubia, Chironomus riparius, Daphnia magna, Hyalella azteca, Lemna minor, Lymnaea stagnalis, Oncorhynchus mykiss, Pimephales promelas, Pseudokirchneriella subcapitata (CIMM strain), Salmo trutta and Xenopus laevis. These organisms represent nine different taxonomic groups, and all toxicity tests were performed using sodium molybdate as the test substance.

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. Standard test species of the three trophic levels that are considered relevant for classification purposes appear to be the most sensitive species, representing five of the six lowest no-effect concentrations. The least sensitive species of the data set was duckweed (L. minor).

The data were used for the construction of a Species Sensitivity Distribution (SSD) from which the median 5th percentile was derived. This value represents the HC5,50% 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 12 data points. The outcome of this analysis allows the derivation of the HC5,50% with 5%-95% confidence interval, and this value should be used for PNEC-derivation (i. e., PNECaquatic= HC5,50%/ Assessment Factor).    

The identified long-term data for standard species (and trophic levels) that are considered for classification purposes, showed that no effects were observed at the classification threshold of 1 mg/L.

 

Summary of chronic marine data

Relevant and reliable chronic no-effects values were identified for twelve aquatic marine species: Acartia tonsa, Americamysis bahia, Ceramium tenuicorne, Crassostrea gigas, Cyprinodon variegatus, Dendraster excentricus, Dunaliella tertiolecta, Menidia beryllina, Mytilus edulis, Phaeodactylus tricornutum, Strongylocentrotus purpuratus. These organisms represent nine different taxonomic groups. With exception of the test with the mussel M.edulis, all toxicity tests were performed using sodium molybdate as the test substance.

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). 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. 

Unbounded NOEC-values have been identified in a number of reliable studies where no adverse effects were observed. Some of the assessed test organisms represented taxonomic groups that were previously not considered in the marine SSD.

Trenfield et al (20156, 2016) reported unbounded NOEC-values of >10 mg Mo/L and >7.0 mg Mo/L for the marine alga Isochrysis galbana and the sea snail Nassarius dorsatus, respectively. Van Dam et al (2016) noted no long-term adverse effects in the acorn barnacle Amphibalanus amphitrite at the highest test concentration of 9.0 mg Mo/L. The same authors also reported on the absence of adverse effects when the hermit crab Coenobita variabilis was exposed to 9.5 mg Mo/L. These unbounded NOECs were not used in an SSD-approach, but the information that was taken from these studies was considered in the overall (uncertainty) assessment of the marine data set , does demonstrate that the marine HC5,50% and resulting PNECmarine for the aquatic compartment are also sufficiently protective for these additional species.

Additional information

 

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
Oncorhynchus mykiss	Fish	43.2	Parametrix, 2008
Hyalella azteca	Crustacea	44.6	Ziese et al, 2016
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 & Janssen, 2008
Daphnia magna	Crustacea	89.5	Rodriguez, 2007 ; De Schamphelaere et al, 2008 ; 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
Salmo trutta	Fish	202.5	Lucas et al, 2017
Lymnaea stagnalis	Gastropoda	221.3	De Schamphelaere et al, 2008
Lemna minor	Aquatic plant	241.5	De Schamphelaere et al, 2008

These data have been used for the construction of a Species Sensitivity Distribution (SSD) from which the median 5th percentile, i.e. the HC5,50% with 5%-95%-confidence interval was derived . The confidence interval is calculated using a Monte Carlo analysis on the log-normal distribution that was fitted through the 12 data points. The calculated HC5,50%-value is used for PNEC-derivation (i.e., PNECaquatic= HC5,50%/ Assessment Factor). The value of this assessment factor depends on the uncertainty analysis that was conducted on this dataset.

An in-depth discussion of the uncertainty analysis is given in the Background Document “Environmental effects assessment of molybdenum”, which is attached in the technical dossier in IUCLID Section 13. Based on this analysis - and taking sufficient conservatism into account - an Assessment Factor of 3 on the HC5,50% is put forward for the derivation of an aquatic PNEC: the use of an AF of 3 results in a PNECaquatic that is below the reported reliable unbounded NOECs and the 95% CL that is associated with the HC5,50%. An assessment factor of 3 is sufficient to cover the uncertainty with regard to the lab-to-field translation of Mo-related effects.

 

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

Species	Trophic level	NOEC/EC10 value	Reference
Mytilus edulis	Bivalve	4.4	Morgan et al, 1986
Acartia tonsa	Copepod	7.96	Aquasense, 2009
Cyprinodon variegatus	Fish	84.1	Parametrix, 2010
Americamysis bahia	Crustacea	116	Lehman, 2010
Menidia beryllina	Fish	139	Dinehart, 2013
Phaeodactylum tricornutum	Diatom	170	Aquasense, 2009
Dendraster excentricus	Echinoderm	233.6	Parametrix, 2008a
Strongylocentrotus purpuratus	Echinoderm	325.8	Parametrix, 2008b
Ceramium tenuicorne	Aquatic plant	641	Le Page et al, 2010
Dunaliella tertiolecta	Algae	881	Le Page & Hayfield, 2010
Crassostrea gigas	Bivalve	1174	Aquasense, 2009

These data have been used for the construction of a Species Sensitivity Distribution (SSD) from which the median 5th percentile was derived, i.e. the HC5,50% with 5%-95%-confidence interval. This value is further used for PNEC-derivation (i.e., PNECmarine = HC5,50% / Assessment Factor). Application of an assessment factor between 1 and 5 on this HC5,50% will result in the final PNECmarine for molybdenum in the marine environment. The value of this assessment factor depends on the uncertainty analysis that is conducted on the chronic marine toxicity dataset.

An in-depth discussion of the uncertainty analysis is given in the Background document “Environmental effects assessment of molybdenum”, which is attached in the technical dossier in IUCLID Section 13. Based on this analysis - and taking sufficient conservatism into account – the same Assessment Factor as the one that is applied for the freshwater compartment (i.e. AF of 3) is put forward for the derivation of an aquatic PNEC for the marine environment. This AF results in a PNEC that is also protective for four additional species for which an unbounded NOEC of >7-10 mg Mo/L was observed.