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Environmental fate & pathways

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

 

In the assessment of environmental fate and behaviour of magnesium 2-ethylhexanoate, data available for the magnesium 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 magnesium, 2-ethylhexanoate is readily biodegradable.

 

Magnesium

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including magnesium. 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. Magnesium as an element is not considered to be (bio)degradable.

Transport and distribution: Magnesium is highly mobile under all environmental conditions and occurs in solution as dissociated Mg2+ ions. Regarding the partitioning of magnesium in the water column, stream water/sediment partition coefficients range from 13.05 L/kg to 339,207.75 L/kg. Based on 745 samples, a European median log Kp value of 2.96 is derived for magnesium sediment-water partitioning.

 

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 is also expected to occur to a greater extent.

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

Transport and distribution: Based on modelling, 2-ethylhexanoate will preferentially distribute into water and shows low potential of volatilisation. A significant adsorption to solid phase or sediment is not expected.

Additional information

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of magnesium 2-ethylhexanoate in water (107.4 g/L at 20 °C), a complete dissociation of magnesium 2-ethylhexanoate resulting in magnesium 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 magnesium 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 magnesium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as 2-ethylhexanoate are not expected to bind strongly with magnesium. Accordingly, protons will always out-compete magnesium ions for complexation of monodentate ligands given equal activities of free magnesium and hydrogen ions. The metal-ligand formation constants (log KML) of magnesium 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 magnesium.

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= 0.148 * 4.72 + 0.216

log KML= 0.915 (estimated magnesium-ethylhexanoate formation constant).

Thus, in the assessment of environmental fate and pathways of magnesium 2-ethylhexanoate, read-across to the assessment entities 2-ethylhexanoate and soluble magnesium substances is applied since the individual ions of magnesium 2-ethylhexanoate determine its environmental fate. Since magnesium 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 magnesium 2-ethylhexanoate.