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Environmental fate & pathways

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Description of key information

Weight of evidence approach

In the assessment of the environmental fate and pathways of naphthenic acids, zinc salts (neutral and basic), a weight of evidence approach from data for the metal cation and the organic anion is followed. This strategy is based upon the assumption that upon release to the environment and dissolution in aqueous media, naphthenic acids, zinc salts will completely dissociate and only be present in its dissociated form, i.e. as zinc cation and naphthenate anion.


Upon dissolution in water, it is indeed predicted that metal carboxylates dissociate completely into the metal cation and the organic anion at environmentally relevant conditions. No information is available on the stability constants of naphthenic acids, zinc salts, but predictions of stability of other zinc carboxylates (Zn propionate, Zn valerate, Zn isovalerate and Zn benzoate) in a standard ISO 6341 medium (2 mM CaCl2, 0.5 mM MgSO4, 0.77 mM NaHCO3 and 0.077 mM KCl, pH 6 and 8) clearly show that monodentate ligands such as carboxylic acids have no potential for complexing zinc ions in solution (Visual minteq. Version 3.0, update of 18 October 2012. http://www2.lwr.kth.se/English/OurSoftware/vminteq/index.html).


Upon dissolution and dissociation of naphthenic acids, zinc salts into Zn2+ and the naphthenate anion, both constituent ions will each show its proper (bio)degradation, bioaccumulation and partitioning behaviour in the environment, as reported for the corresponding metal ion and carboxylic acid (Zn2+ and naphthenic acid). The environmental fate and behaviour for the metal and organic moieties is predicted to be clearly different from each other, resulting in a different relative distribution over the environmental compartments (water, air, sediment and soil).

Additional information

Because naphthenic acid is a mixture of many different isomers of cycloalkyl carboxylic acids, physicochemical and fate properties vary according to the proportions of the individual compounds in their composition. Therefore in many cases it is not possible to set a specific endpoint value as results will vary dependent on the hydrocarbon component make-up of the complex mixture. Variable values are available from publications on naphthenic acids. Some endpoints are also addressed using the EPISuite computer models (US EPA, 2009).

Hydrolysis:

The chemical components of naphthenic acids are hydrocarbons that are not subject to hydrolysis because they lack functional groups that hydrolyse.

Biodegradation:

Because naphthenic acid is a mixture of many different isomers of cycloalkyl carboxylic acids the rate of biodegradation strongly varies depending upon the composition of the complex mixture. Available data on both model and commercially available naphthenic acids however show that biodegradation occurs. Furthermore the data suggest that the microbial degradation is the predominant factor in the reduction of aquatic acute toxicity of naphthenic acids. A number of relevant scientific journal articles and QSAR estimations show that naphthenic acid is inherently biodegradable.

Bioaccumulation:

A range of 3.2 to 56.2 for the BCF was obtained by QSAR tool. A literature reference reported a BCF of 2 based on one C-13 component of a naphthenic acid mixture. The Japanese METI-NITI database reports a range of BCF between 1.6 and 27 for sodium naphthenate. Overall, the available data suggest low or no bioaccumulation potential.

Adsorption:

Based on the analysis results (IUCLID section 1.4) of a number of representative naphthenic acids samples, a list of potential chemical components was compiled. The criteria for choosing these specific structures were the occurrence of C-numbers, the number of rings and their relative share in the analysed samples. The log Koc of these structures was estimated with the equation of Di Toro et al. (1985). The log Koc range is 2.02-6.59.

Janfada et al. (2006) reported Kd values for the naphthenic acids from oil sands, between 1.3 and 17.8 L/kg at 4°C, which corresponds to calculated values of Log Koc of 1.85 to 2.82, corresponding to the lower end of the estimated values (2.02-6.59).