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On the basis of ecotoxicity studies with the substance itself and based on aquatic toxicity data of its moieties, i.e. strontium and neodecanoic acid, strontium neodecanoate appears to have a low potential for short- and long-term toxicity to aquatic organisms.

Reliable acute aquatic toxicity data of strontium neodecanoate are available from GLP-conform guideline studies for algae, daphnia, fish and microorganisms. Respective EC/LC50 values are well above respective test limits of 100 mg/L, i.e., 96-h LC50 for fish (Danio rerio): >299.7 mg/L, 48-h EC50 for immobilisation of Daphnia magna: >299.7 mg/L, 72-h EC50 for algal growth rate (Pseudokirchneriella subcapitata): >250 mg/L, 3-h EC50 for the respiration inhibition of activated sludge: >871.3 mg/L. Therefore, strontium neodecanoate has a low potential for acute toxicity to aquatic organisms.

Reliable chronic aquatic toxicity data of strontium neodecanoate are available from GLP-conform guideline studies for algae and microorganisms. Respective EC10 values are well above 100 mg/L, i.e., 72-h EC10 for algal growth rate (Pseudokirchneriella subcapitata): 129 mg/L, 3-h EC10 for the respiration inhibition of activated sludge: >871.3 mg/L. Thus, strontium neodecanoate has a low potential for chronic toxicity to aquatic algae and microorganisms.

Additional information

Read-across approach

Metal carboxylates are substances consisting of a metal and a carboxylic acid. Based on the solubility of strontium neodecanoate in water (8.47 g dissolved Sr/L at pH 8.4 corresponding to 45.07 g strontium neodecanoate/L), a complete dissociation of strontium neodecanoate resulting in strontium and neodecanoate ions may be assumed under environmental conditions. The respective dissociation is reversible and the ratio of the salt /dissociated ions is dependent on the metal-ligand dissociation constant of the salt, the composition of the solution and its pH.

A metal-ligand complexation constant of strontium neodecanoate could not be identified. Data for alkaline earth metals appear to be generally limited. However, alkaline earth metals tend to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of alkaline earth metals is typically described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions. Based on an analysis by Carbonaro & Di Toro (2007) of monodentate binding of strontium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as neodecanoate are not expected to bind strongly with strontium. Accordingly, protons will always out-compete strontium ions for complexation of monodentate ligands given equal activities of free strontium and hydrogen ions. The metal-ligand formation constants (log KML) of strontium with other carboxylic acids, i.e. acetic, propanoic and butanoic acid, ranging from 0.78 to 0.89, further point to a low strength of the monodentate bond between carboxyl groups and strontium.

Thus, it may reasonably be assumed that based on the estimated strontium-neodecanoate formation constant, the respective behaviour of the dissociated strontium cations and neodecanoate anions in the environment determine the fate of strontium neodecanoate upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently its ecotoxicological potential (or the lack thereof).

In the assessment of the aquatic toxicity of strontium neodecanoate, substance-specific data are considered. Further, read-across to data for neodecanoic acid (CAS 26896-20-8; EC 248-093-9) and soluble strontium substances is applied since the ions of strontium neodecanoate determine the enviromental fate and toxicity.

Strontium

Reliable acute aquatic toxicity data of strontium are available for daphnia and fish. The 48-h LC50 value for immobilisation of Daphnia magna amounts to 125 mg strontium/L. The acute toxicity of strontium as measured by mortality of striped bass (Morone saxatilis) was evaluated in saltwater resulting in a 96-h LC50 of >92.8 mg strontium/L. Further supporting acute toxicity data for freshwater invertebrates and fresh- and saltwater fish as well as freshwater algae toxicity data indicate that respective EC/LC50 values are well above 50 mg/L and the majority are above the test limit of 100 mg/L. Thus, strontium appears to have a low potential for acute toxicity to freshwater algae and invertebrates as well as fresh- and saltwater fish.

Reliable chronic aquatic toxicity data of strontium are available for daphnia. The respective 21-d NOEC for reproduction of Daphnia magna amounts to 21 mg strontium/L. Supporting chronic toxicity data for freshwater invertebrates and algae indicate, however, that respective long-term EC values are well above 1 mg/L. Thus, strontium appears to have a low potential for long-term toxicity to freshwater algae and invertebrates.

Neodecanoic acid

For algae, daphinia and fish reliable acute toxicity data are available. Respective EC/LC50 values are above the respective test limits of 100 mg/L, i.e. 96 -h LL50 for fish (Oncorhynchus mykiss): >100 mg neodecanoic acid/L, 48 -h EL50 for Daphina magna: >1000 mg neodecanoic acid/L, 72 -h EL50 for algal growth rate (Pseudokirchneriella subcapitata): >100 mg neodecanoic acid/L. Therefore, neodecanoic acid has a low potential for acute toxicity to aquatic organisms.

Reliable chronic toxicity data are available for fish and daphnia. Furthermore, reliable supporting information is available for microorganisms. Respective 30-d ChrVs for fish and Daphnia are estimated by US EPA with 1.6 mg 2-ethyl-2,5-dimethylhexanoic acid/L. Reliable supportive information are available for fish, daphnia and microorganisms, i.e. 14-d NOEC for fish (Oncorhynchus mykiss): >2.22 mg 2-ethyl-2,5-dimethylhexanoic acid/L, 21-d NOEC for immobilisation of Daphnia magna: 4.78 mg Neoheptanoic acid/L, 4-h NOEC for the nitrification rate of Nitrosomonas sp.: >200 mg 2-ethyl-2,5-dimethylhexanoic acid/L. Thus, based on read-across, neodecanoic acid is assumed to have a low potential for chronic toxicity to aquatic organisms.

Reference:  

Carbonaro RF & Di Toro DM (2007) Linear free energy relationships for metal–ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms. Geochimica et Cosmochimica Acta 71: 3958–3968.

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