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Environmental fate & pathways

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Description of key information

The fate of strontium 2-ethylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its moieties: strontium cations and 2-ethylhexanoate anions.

 

In the assessment of environmental fate and behaviour of strontium 2-ethylhexanoate, data available for the strontium cation and the 2-ethylhexanoate anion indicate that abiotic degradation in respective compartments does not contribute significantly to its fate in the environment. Whereas biodegradation is not relevant strontium, 2-ethylhexanoate is readily biodegradable.

 

Strontium

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including strontium ions. In general, (abiotic) degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for metals and metal compounds. Strontium as an element is not considered to be (bio)degradable.

Transport and distribution: Available Kd values indicate a low potential of strontium to partition into sediment or soil. Regarding the partitioning of strontium in the water column, stream water/sediment partition coefficients range from 38.64 L/kg to > 10000 L/kg. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, a European median log Kp value of 2.96 is derived for sediment-water partitioning. In addition, a mean log Kp value of 1.81 (solids-water in soil) was derived from five different studies.

 

2-ethylhexanoic acid

Abiotic degradation may affect the environmental fate of 2-ethylhexanoic acid since it is prone to slow degradation by photochemical processes. Hydrolysis, however, is not expected to be an important fate path.

Biotic degradation: 2-ethylhexanoate is readily biodegradable. Based on the biodegradation in water, biodegradation in soil and sediment is also expected.

Bioaccumulation: 2-ethylhexanoate has a low potential for bioaccumulation (logPow = 2.96)

Transport and distribution: According to predictions of the Level III fugacity model of EPI Suite (v4.11) for the partitioning between air, soil, sediment and water in an evaluative environment assuming steady-state but not equilibrium conditions, 2-ethylhexanoate will preferentially partition into water and has a low potential for volatilisation. A significant adsorption to solid phases is not expected.

Additional information

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of strontium 2-ethylhexanoate in water, a complete dissociation of strontium 2-ethylhexanoate resulting in strontium and 2-ethylhexanoate 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 2-ethylhexanoate 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 2-ethylhexanoate 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 and benzoic acid, ranging from 0.1 to 0.47, 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

KML is the metal-ligand formation constant, KHL is 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 2-ethylhexanoic acid of 4.72 results in:

log KML= 0.160 * 4.72 + 0.069

log KML= 0.824 (estimated strontium-ethylhexanoate formation constant).

Thus, in the assessment of environmental fate and pathways of strontium 2-ethylhexanoate, read-across to the assessment entities 2-ethylhexanoate and soluble strontium substances is applied since the individual ions of strontium 2-ethylhexanoate determine its environmental fate. Since strontium ions and 2-ethylhexanoate ions behave differently in the environment, regarding their fate and toxicity, a separate assessment of each assessment entity is performed. Please refer to the data as submitted for each individual assessment entity. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for strontium 2-ethylhexanoate.