2-ethylhexanoic acid, iron salt

Endpoint summary

Description of key information

The fate of 2-ethylhexanoate, iron salt in the environment is most accurately evaluated by separately assessing the fate of its moieties: iron cations and 2-ethylhexanoate anions. Data available for the iron cation and the 2-ethylhexanoate anion indicate that abiotic degradation in respective compartments does not contribute significantly to their fate in the environment. While biodegradation is not relevant for iron cations, 2-ethylhexanoate is readily biodegradable.

 

Iron (Fe2+, Fe3+)

Iron is the fourth most abundant element and second most abundant metal in the Earth’s crust (FOREGS). Thus, iron is prevalent in all environmental compartments.

Abiotic degradation including hydrolysis or phototransformation in water, soil or air, is not relevant for inorganic substances including iron in its different oxidation states. Abiotic degradation is irrelevant for inorganic substances that are assessed on an elemental basis.

Biotic degradation is not relevant for metals and metal compounds. Iron as an element is not considered to be (bio)degradable irrespective of its oxidation state.

Transport and distribution: Iron species are rapidly removed from solution as insoluble precipitate at oxic conditions and moderate pH. With an reported log Kp (suspended matter-water) of 2.34 and log Kp (sediment-water) of 4.997 L/kg, iron is highly immobile under environmental conditions.

 

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

Read-Across:

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on the solubility of 2-ethylhexanoic acid, iron salt in water, a complete dissociation of 2-ethylhexanoic acid, iron salt resulting in iron 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.

However, iron species are rapidly removed from environmental solutions as insoluble precipitate at oxic conditions and moderate pH whereas 2-ethylhexanoic acid is rather soluble. Thus, iron is relatively immobile under most environmental conditions, mainly due to the very low solubility of iron (III) hydroxide in its various forms (Salminen, 2005). Under environmental relevant conditions, ferric iron (3 +) oxide-hydroxide forms dominate and concentrations of free Fe3+ ions are low. In addition, dissolved oxygen oxidises free Fe2+ ions in near neutral solutions and ferric (3 +) hydroxides precipitate. Dissolved iron precipitates rapidly with the concentrations of soluble iron ions mainly being controlled by pH, redox conditions as well as the abundance of complexing agents.

A metal-ligand complexation constant of 2-ethylhexanoic acid, iron salt could not be identified. According to the Irving-Williams series, stability constants formed by divalent first-row transition metal ions generally increase to a maximum stability of copper (Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II)). Therefore, stabilities of monodentate ligand complexes formed with iron are in general expected to be low when directly comparing to other transition metals. Consequently, based on its solubility in water, 2-ethylhexanoic acid, iron salt is expected to dissociate into its moieties Fe cations and 2-ethylhexanoate.

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

 

Salminen, et al. 2005. FOREGS. Geochemical Atlas of Europe, Part 1, Background information, methodology and maps.