Barium 3,5,5-trimethylhexanoate

Endpoint summary

Description of key information

The fate of barium 3,5,5-trimetylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its constituents barium and trimetylhexanoate. In the assessment of environmental fate and behaviour of barium 3,5,5 -trimethylhexanoate data available for the barium cation and the trimethylhexanoate anion indicate that biotic degradation in respective compartments do not contribute significantly to its fate in the environment.

Barium does not contain hydrolysable groups. Further, biotic degradation is not relevant for inorganic substances such as barium. The coefficient for partitioning of barium between particulate matter and water (Kpsusp) of 5,217 L/Kg was derived for EU waters whereas the Kp for the distribution between sediment and water (Kpsed) was estimated with 3,478 L/kg. For soil, a solid-water partitioning coefficient of 60.3 L/kg was determined experimentally.

 

Trimethylhexanoic acid (and its structural analogue neodecanoic acid):

Abiotic degradation is not expected to significantly affect the environmental fate of trimethyl hexanoic acid and neodecanoic acid since the acids are lacking hydrolysable functional groups. Further, neodecanoic acid 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. However, based on the QSAR prediction with BIOWIN (v4.10), 3,5,5-trimethylhexanoic acid may be considered as readily biodegradable.

Transport and distribution: The estimated Koc of neodecanoic acid is 121 and may be sensitive to pH. Based on the outcome of the Molecular Connectivity Index model of KOCWIN (v 2.00), the logKoc of trimethyl hexanoate is 1.48 (Koc = 30.4 L/kg). The vapor pressure of trimethylhexanoic acid and neodecanoic acid is very low, i.e. 6.14 Pa and 0.65 Pa, respectively suggesting a limited volatilization from soil. Henry’s Law constant for trimethylhexanoic acid and neodecanoic acid is calculated with 0.40 and 0.54 Pa-m3/mole at 25 °C, respectively, indicating that volatilization from water is not expected to occur at a rapid rate, but may occur. Trimethyl hexanoic acid and neodecanoic acid are weak organic acids with estimated dissociation constants (pKa) of 5.23 and 4.69, respectively. Consequently, trimethyl hexanoic acid and 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

The fate and toxicity of barium 3,5,5 -trimethylhexanoate in the environment is most accurately evaluated by separately assessing the fate of its constituents barium and trimethylhexanoate.

Metal carboxylates are substances consisting of a metal cation and a carboxylic acid anion. Based on its water solubility, barium 3,5,5 -trimethylhexanoate is expected to dissociate completely under environmental conditions resulting in barium and trimethylhexanoate ions.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 barium 3,5,5-trimetylhexanoate could not be identified. Data for barium appear to be generally limited. However, barium tends to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of barium 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 et al. (2007) of monodentate binding of barium to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as trimethylhexanoate anions are not expected to bind strongly with barium. 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 slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of 3,5,5-trimethylhexanoic acid of 5.23 results in:

log KML= 0.186 * 5.23 – 0.171

log KML= 0.80 (estimated barium-trimethylhexanoate formation constant).

 

Thus, it may reasonably be assumed that based on the estimated barium- trimethylhexanoate formation constant, the respective behaviour of the dissociated barium cations and trimethylhexanoate anions in the environment determine the fate of barium 3,5,5-trimethylhexanoate upon dissolution with regard to (bio)degradation, bioaccumulation and partitioning, resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently its ecotoxicological potential.

 

In the assessment of environmental fate and pathways of barium 3,5,5-trimethylhexanoate, read-across to the assessment entities soluble barium substances and 3,5,5-trimetylhexanoic acid (and its structural analogue neodecanoic acid) is applied since the ions of barium 3,5,5-trimethylhexanoate determine its environmental fate. Since barium cations and trimethylhexanoate 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 order to evaluate the environmental fate of barium 3,5,5-trimethylhexanoate, information on the assessment entities barium cations and trimethylhexanoate anions were considered. For a documentation and justification of that approach, please refer to the separate document attached to section 13, namely Read Across Assessment Report for barium 3,5,5-trimethylhexanoate.

 

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.