Fatty acids, C18-24, zinc salts

Endpoint summary

Description of key information

Environmental fate and pathway studies with substance Fatty acids, C18-24, zinc salts are not available. In the assessment of substance Fatty acids, C18-24, zinc salts, read-across to analogue substances and/or the assessment entities soluble zinc substances and fatty acids is applied since the ions of substance Fatty acids, C18-24, zinc salts determine its fate and toxicity in the environment. For details on the environmental fate and pathways of the individual moieties, please refer to the respective assessment entities. In brief, metal partition coefficients for distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) are the only parameter relevant for the fate of zinc in the environment and biotic degradation is the only fate process expected to influence fate of fatty acids in the environment. For both moieties, bioconcentration/bioaccumulation is considered not relevant.

Additional information

Metal carboxylates are salts consisting of metal cation and carboxylic acid anion. Based on the solubility of substance Fatty acids, C18-24, zinc salts in water, a complete dissociation upon dissolution (even if low based on conservative water solubility limit of 0.63 mg/L) resulting in zinc and fatty acids, C18-24 ions 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. However, under environmental conditions, a reunion of the dissociated ions is highly unlikely and it may reasonable be assumed that the respective behaviour of the dissociated zinc cations and fatty acids anions in the environment determine the fate of substance Fatty acids, C18-24, zinc salts 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 the (eco)toxicological potential.

In general terms, the stability constant of a metal complex can be calculated as follows: K = [ML] / [M][L], where K is the stability constant (expressed as a logarithm); M is the amount of metal ion such as Zn2+ ion, and L is the amount of a ligand such as fatty acids, C18-24 anions. The total concentration of metal CM can be computed with specialized computation programs. The basic equation CM = [M] + [ML] with [ML] = K [M] [L] becomes CM = [M] (1 + K [L]); hence [M] = CM / (1 + K [L]) shows that the concentration of the metal depends on the stability constant of the complex and free concentration of the ligand which is dependent upon corresponding pK and pH values. However, a metal-ligand complexation constant of Fatty acids, C18-24, zinc salts or a single salt of the UVCB e.g. zinc didocosanoate could not be identified. Data for zinc appear to be generally limited. However, zinc ions tend to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of zinc 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 zinc to negatively-charged oxygen donor atoms, including carboxylic functional groups, monodentate ligands such as docosanoate anions are not expected to bind strongly with zinc. Accordingly, protons will always out-compete zinc ions for complexation of monodentate ligands given equal activities of free zinc and hydrogen ions. The metal-ligand formation constants (log KML) of zinc with other carboxylic acids, i.e. acetic and benzoic acid, ranging from 0.56 to 1.59 (Bunting & Thong, 1969), further point to a low strength of the monodentate bond between carboxyl groups and zinc. 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 docosanoic acid of 4.95 results in:

log KML = 0.301 * 4.95 + 0.015

log KML = 1.51 (estimated metal-ligand formation constant for zinc-docosanoate, representing the main single salt of the UVCB Fatty acids, C18-24, zinc salts).

Thus, it may reasonably be assumed that based on the stability constant for zinc didocosanoate, the respective behaviour of the dissociated zinc cations and fatty acids, C18-24 anions under physiological conditions and in the environment determine the fate of Fatty acids, C18-24, zinc salts 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 (eco)toxicological potential.

Therefore, in the assessment of the (eco)toxicity of Fatty acids, C18-24, zinc salts, a read-across to data for fatty acids, C18-24 and soluble zinc substances is applied since only the ions of zinc and fatty acids, C18-24 are available in the environment and systemically and determine the (eco)toxicological potential.

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 concept Category approach for substance Fatty acids, C18-24, zinc salts".

FATTY ACIDS, C18-24, ZINC SALTS:

Only the ions of substance Fatty acids, C18-24, zinc salts determine its fate and toxicity in the environment. In brief, metal partition coefficients for distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) are the only parameter relevant for the fate of zinc in the environment and biotic degradation is the only fate process expected to influence fate of fatty acids in the environment. For both moieties, bioconcentration/bioaccumulation is considered not relevant.

ZINC:

According to Annex IX of REACH Regulation, information on hydrolysis is not required for inorganics.

Biodegradation is not applicable to metals/inorganic substances. Tests are not to be conducted if the substance is inorganic (Column 2 of Annex VII of REACH regulation).

However, for water, information is available on the removal of metals from the water column (given under 5.6.). The removal from the water column was modeled referring to the EUSES model parameters and different conditions of pH. Zinc is removed by > 70% under the reference conditions for the EU regional waters (EUSES) (see section 5.6.: "removal from the water column"). And the transport and distribution over the different environmental compartments e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) is described and quantified by the metal partition coefficients between these different fractions. Information on these partition coefficients is given under 5.4 and 5.6.

Zinc is an essential element which is actively regulated by organisms, so bioconcentration/bioaccumulation is not considered relevant for all inorganic zinc substances.

In sum, metal partition coefficients for distribution between different fractions e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...) are the only parameter relevant for the fate of zinc in the environment

FATTY ACIDS, C18-24:

Abiotic degradation: Abiotic degradation in the aquatic environment is not relevant for saturated fatty acids like fatty acids, C18-24, since they lack hydrolysable functional groups and do not absorb light within a range of 290 to 700 nm, the range in which photolysis occurs.

Biotic degradation: Fatty acids are readily biodegradable.

Bioaccumulation: As fatty acids represent a significant part of the normal daily diet of e.g. fish and are rapidly degraded, they are not expected to accumulate in the environment.

Transport and distribution: QSAR estimates let assume a high affinity of fatty acids to soil and a low mobility therein. However, fatty acids are also biodegradable by soil microorganisms and thus not expected to accumulate in soil.

In sum, based on available information, biotic degradation is the only fate process expected to influence fate of fatty acids in the environment. Bioconcentration/bioaccumulation is considered not relevant.