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Biodegradation is not relevant for metals and metal compounds. Thus, biodegradation is not relevant for strontium.Neo-decanoic acid is not readily biodegradable (11% biodegradation in 28 days) based on a standard OECD test. Thus, data available for the strontium cation and the neodecanoate anion indicate that biotic degradation in respective compartments do not contribute significantly to the fate of strontium neodecanoate in the environment.

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.

Therefore, in the assessment of the biodegradation properties of strontium neodecanoate, read-across to neodecanoic acid (CAS 26896-20-8; EC 248-093-9) and soluble strontium substances is applied since the ions of strontium neodecanoate determine its biodegradability.

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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