Naphthenic acids

Endpoint summary

Additional information

For the key short-term studies of fish, daphnia and algae, the same naphthenic sample is used. These samples are a good representative for the naphthenic acids of the current registration dossier because of comparable specifications of the acidic fraction, which is the main fraction. Specifications of the non-acidic part of the naphthenic acids used for the ecotoxicity testing are not reported.

For both naphthenic acids, the predominant number of saturated rings is 0 -3. The predominant C-range of the naphthenic acids in this dossier is C8-C20 (cut-off is ≥5% of the 90^thpercentile), while in the ecotoxicity tests, the range was C6-C16 (cut-off unknown). In general, the higher the C-number, the lower the solubility and log Kow (IUCLID section 4.7) and hence the bioavailability for aquatic organisms. Substances in the higher C-range may become too insoluble to exert ecotoxicity. Hence the naphthenic acids used for the ecotoxicity testing can be considered to be conservative enough regarding ecotoxicity of naphthenic acid in the current dossier.

Table 1 presents the predominant approximate C-range and ring distribution for the naphthenic acids of the ecotoxicity key studies and of the current dossier.

NA specifications	NA used in key studies on ecotoxicity	NA in current registration   dossier
Predominant C-range	C6-C16	C8-C20
Approximate saturated ring distribution	0 rings 24% 1 ring 39% 2 rings 30% 3 rings 5% 4 rings 2%	0 rings 16% 1 ring 32% 2 rings 24% 3 rings 10% 4 rings 3%

Table 1: Specifications of naphthenic acids (NA) used for ecotoxicity testing and NA of the current registration dossier

The effect of the water accommodated fraction (WAF) of naphthenic acids (NA)on the aquatic organismsPseudokirchneriella subcapitata,Daphnia magna and Pimephales promelas were measured. The test protocols were according to OECD guidelines and the tests were performed according to GLP principles.The test results can be considered as reliable.

For algae a static set up was used, for daphnia and fish a semi static set up was used with medium renewal every 24 hours. Analytical results are available for all 3 studies. Effect values are expressed both in loading rate and in measured dissolved concentration. 

A long-term daphnia study was performed following the OECD guideline No 211. No effects were seen up to a concentration of 4.1 mg/L.

Although, no guidelines were followed four long-term fish studies (literature data) were available. No effects were seen up to a concentration of 1 mg/L. Based on the available information (no long-term effects for aquatic invertebrates and no effects seen in the supporting studies for long-term fish), the long-term toxicity test to fish can be waived.Also data on microbial toxicity (Vibrio fisheri) are reported In literature for NA surrogates. In the microtox assay the EC50 for cyclohexane carboxylic acid was 0.07 mM ( 13.0 +/- 1.6 mg/L).

From biodegradation tests it can also be concluded that NA are not toxic to microbial communities.

Enrichment cultures, obtained from an active tailings settling pond, using commercially available NAs as the sole carbon source, resulted in the isolation of a co-culture containingPseudomonas putidaandPseudomonas fluorescens. These microorganisms are not affected by the presence of naphthenic acid.

Also in other biodegradation tests using naphthenic enrichement cultures show the survival ofPseudomonas stutzeriandAlcaligens denitrificans. And in yet another enriched cultureAcinetobacter calcoaceticusand a member of thePseudomonas fluorescensgroup were demonstrated. From these tests it can be concluded that microbial populations are able to survive naphthenic acid exposure.