Naphthenic acids

Endpoint summary

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. However valuable values are obtained 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 acids are mixtures 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 , QSAR estimations for the acidic fraction and measured or predicted data on key constituents of the hydrocarbon (non-acidic) fraction (Concawe 2010*), show that naphthenic acids are inherently biodegradable.

* CONCAWE. Hazard classification and labelling of petroleum substances in the European Economic Area – 2010

Bioaccumulation:

A range of 3.2 to 56.2 for the BCF was obtained by QSAR tool. A literature reference reported a value of BCF=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. 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 values for the Kd of naphthenic acids from oil sands, between 1.3 and 17.8L/kg at 4°C, which corresponds to calculated values of Log Koc of 1.85 to 2.82, corresponding in turn to the lower end of the estimated values (2.02-6.59).