Magnesium neodecanoate

Endpoint summary

Description of key information

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

Since magnesium cations and neodecanoate anions behave differently in the environment, including processes such as stability, degradation, transport and distribution, a separate assessment of the environmental fate of each assessment entity is performed.

Please refer to the data as submitted for each individual assessment entity.

 

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

 

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.

 

Neodecanoic acid

Abiotic degradation is not considered to significantly affect the environmental fate of neodecanoic acid since neodecanoic acid is lacking hydrolysable functional groups and does not absorb light within a range of 290 to 750 nm.

Biotic degradation: Neodecanoic 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 neodecanoic 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 and may be sensitive to pH. Neo-decanoic acid vapor pressure is very low, i.e. 0.65 Pa suggesting a limited volatilization from soil. Henry’s Law constant for neo-decanoic acid is calculated with 0.54 Pa-m3/mole at 25 °C indicating 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 4.69. Consequently, neodecanoic acid, at neutral pH, typical of most natural surface waters, is expected to dissociate to the ionised form and therefore to remain largely in water.

Additional information

Read-across
Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of magnesium neodecanoate in water (37.2 g/L at 30 °C), a complete dissociation of magnesium neodecanoate resulting in magnesium cations and neodecanoate anions may be assumed under environmental conditions. The respective dissociation is in principle 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 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 magnesium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as neodecanoate 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 0.82 (Bunting & Thong, 1969), 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 neodecanoic acid of 4.69 results in:

log KML= 0.148 * 4.69 + 0.216

log KML= 0.910 (estimated magnesium-neodecanoate formation constant).

Thus, it may reasonably be assumed that based on the estimated magnesium-neodecanoate formation constant, the respective behaviour of the dissociated magnesium cations and neodecanoate anions in the environment determine the fate of magnesium 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 environmental fate and pathways of magnesium neodecanoate, read-across to the assessment entities neodecanoate and soluble magnesium substances is applied since the individual ions of magnesium neodecanoate determine its environmental fate.Since magnesium ions and neodecanoate 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 neodecanoate.

References:

Bunting JW & Thong KM (1969) Stability constants for some 1:1 metal-carboxylate complexes.Canadian Journal of Chemistry, 48, 1654.

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.