Strontium neodecanoate

Endpoint summary

Description of key information

In the assessment of enviromental fate and behaviour of strontium neodecanoate, data available for the strontium cation and the neodecanoate anion indicate that abiotic and biotic degradation in respective compartments do not contribute significantly to its fate in the environment.

Furthermore, a low potential to bioaccumulate or biomagnify at different trophic levels can be concluded for strontium neodecanoate based on the data available for strontium and neodecanoate.

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.

The analysis by Carbonaro & Di Toro (2007) suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms of carboxylic functional groups:

log KML= αO* log KHL+ βO; where

KMLis the metal-ligand formation constant, KHLis the corresponding proton–ligand formation constant, and αO and βO are termed the Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of neodecanoic acid of 4.69 results in:

log KML= 0.160 * 4.69 + 0.069

log KML= 0.82 (estimated strontium-neodecanoate formation constant).

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.

Thus, in the assessment of enviromental fate and pathways 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 eniviromental fate.

 

Environmental fate properties of strontium

Abiotic degradation: Abiotic degradation including hydrolysis or phototransformation in water, soil or air, are not relevant for the environmental fate of inorganic substances including strontium.

Biotic degradation: (Bio)degradation is not relevant for metals and metal compounds. Strontium is not considered to be (bio)degradable.

Transport and distribution: The partitioning coefficient for the sediment compartment of 915 L/kg (log Kd = 2.96) represents the median of 757 paired stream water and sediment samples taken on a grid across Europe within the FOREGS monitoring program (Salminen et al. 2005).The soil-water partitioning coeficient of 65 L/kg (log Kd = 1.81) represents median of different literature values (Bunzl and Schimmack, 1989; Kamei-Ishikawa et al., 2008; Twinning et al., 2004;Wang and Staunton, 2005; Sheppard and Thibault,1990).

Bioaccumulation: Several studies on the bioaccumulation in freshwater and seawater species are available for strontium and strontium compounds. Since strontium behaves very similar to calcium, strontium enriches in thebones or rather hard tissues of animals(ATSDR, Public Health Statement: Strontium; 2004). Respectively higher bioaccumulation factors (BAF) were measured in hard tissue offish and invertebratescompared to BAFs of soft tissues. However, BCFs of hard tissues of freshwater organisms are relatively low and even lower for saltwater species.Therefore, it can be considered that strontium has a low potential to bioaccumulate in aquatic organisms. Supportive information on bioaccumulation in terrestrial organisms (plants) indicates that strontium bioaccumulation in terrestrial plants is also low.

Environmental fate properties of neodecanoic acid

Abiotic degradation: Indirect photochemical degradation of neo-decanoic acid (NDA) as mediated by OH-attack is estimated to have a half-life of 1.2 days or 13.9 hours based on a 12 -hour sunlight day. A 12-hour day half-life value normalizes degradation to a standard day light period during which hydroxyl radicals needed for photolysis are generated in the atmosphere. Although NDA has the potential to degrade rapidly by OH-attack, multimedia distribution modeling indicates NDA is predicted to partition negligibly (0.1%) to the air compartment. Although NDA has a relatively short atmospheric oxidation half-life (13.9 hours), this process is unlikely to contribute significantly to the loss of NDA from the environment.

Direct photochemical degradation in water occurs through the absorbance of solar radiation by a chemical substance. If the absorbed energy is high enough, then, in the resultant excited state, the chemical may undergo a transformation. A prerequisite for direct photodegradation is the ability of one or more bonds within a molecule to absorb ultraviolet (UV) /visible light in the 290 to 750 nm range. Light wavelengths longer than 750 nm do not contain sufficient energy to break chemical bonds, and wavelengths below 290 nm are shielded from the earth by the stratospheric ozone layer. An approach to assessing the potential for NDA to undergo direct photochemical degradation is to assume that degradation will occur in proportion to the amount of light wavelengths >290 nm absorbed by NDA molecules. NDA does not absorb light within a range of 290 to 750 nm. Therefore, direct photolysis will not contribute to the degradation of NDA in the aquatic environment.

Biotic degradation: Neo-decanoic acid is not readily biodegradable (11% biodegradation in 28 d) based on results from a standard OECD ready biodegradation test. Studies are not available to assess the biodegradability of neo-decanoic acid under simulated conditions or in soil, but given the limited biodegradation in water, biodegradation under simulated conditions, or in soil is not expected to occur to a great extent.

Transport and distribution: The estimated neo-decanoic acid Koc is 121. The Koc of neo-decanoic acid may be sensitive to pH. Neo-decanoic acid vapor pressure is very low, 0.65 Pa, which suggests limited volatilization from the terrestrial compartment. In comparison, Henry’s Law constant for neo-decanoic acid is calculated as 0.54 Pa-m3/mole at 25 degrees C, which indicates that volatilization from water is not expected to occur at a rapid rate, but may occur. Neodecanoic acid is a weak organic acid with an estimated dissociation constant (pKa) of 4.7. Consequently, neodecanoic acid, at neutral pH, which is typical of most natural surface waters, is expected to dissociate to the ionised form and therefore, remain largely in water.

Bioaccumulation: A bioconcentration study with neo-decanoic acid (CAS #26896 -20 -8) was conducted with the rainbow trout, Oncorhynchus mykiss. The calculated mean bioconcentration factor (BCF) after 14 days of exposure was < 225 L/kg wet fish weight, based on the detection limit for neodecanoic acid. Therefore, based on the BCF data, neodecanoic acid exhibits a low potential to bioaccumulate in aquatic ecosystems Based on experimental data in fish, neo-decanoic acid is not expected to bioaccumulate in terrestrial species.

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.