Naphthenic acids, zinc salts

Biodegradation in water: screening tests

Description of key information

In water, zinc naphthenate is expected to dissociate to zinc ions and naphthenic acids anions. As an inorganic metal, the zinc ion will not undergo biodegradation, however, the acid component may be biodegraded. Naphthenic acids mixtures and model compounds for naphthenic acids mixtures have been investigated for their biodegradability. The experimental data show that naphthenic acids mixtures are at least inherently biodegradable with certain naphthenic acids and naphthenic acids model compounds being readily biodegradable. QSAR predictions have also been provided for representative structures which bracket the range of potential constituents of zinc naphthenate. Most representative structures are readily biodegradable, though the structures with three rings (C15, 3 rings; C20, 3 rings) are predicted to be not readily biodegradable. None of the compounds meet the screening criteria for P in the PBT assessment.

Key value for chemical safety assessment

Biodegradation in water:

inherently biodegradable

Type of water:

freshwater

Additional information

Zinc naphthenate consists of zinc salts of naphthenic acids and in the aquatic environment is expected to dissociate into zinc cation and naphthenic acid components. As an inorganic metal, the zinc ion will not undergo biodegradation, however, the acid component may be biodegraded. Therefore, only data on the organic (naphthenic acid) component are presented.

 

BIOWIN

Zinc naphthenate is a UVCB containing a large number of constituents, each of which could potentially have properties varying from other constituents in the salt. Therefore, QSAR predictions have been provided for representative structures which bracket the range of potential constituents present in zinc naphthenate with regard to the molecular weight and number of cyclic groups. The biodegradation potential of the representative structures of zinc naphthenate were estimated using a QSAR model (BIOWIN v 4.10 in EPISUITE v 4.11, US EPA 2010).

 

As the zinc cation of the salts is an inorganic metal, the structures which have been modelled reflect only the organic (naphthenic acid) components of the substance. Most representative structures are readily biodegradable, though the structures with three rings (C15, 3 rings; C20, 3 rings) are predicted to be not readily biodegradable. According to the screening criteria for P in the PBT assessment, none of the compounds meet the screening criteria for P, as all of the modelled structures have a BIOWIN 3 prediction of >2.75.

 

Clemente et al (2004)

Clemente et al (2004) investigated the biodegradability of naphthenic acids using a method similar to OECD 310 (CO2 in sealed vessels, headspace test), with inoculum obtained from process affected waters (adapted microorganisms) and enriched culture media. The CO2 results showed 60% degradation was reached after 17 days (Merichem sample) to 22 days (Kodak naphthenic acid, sodium salt), while the analytical results showed concentrations of naphthenic acids decreased to a level approaching 0 % by day 14. Stearic acid and palmitic acid were identified as decomposition products, which themselves are readily biodegradable.

 

The study is not sufficient to conclude the substance is readily biodegradable but the data show that naphthenic acids can be degraded by adapted microorganisms.

 

Herman et al (1994)

Herman et al (1994) conducted four studies on naphthenic acids: (1) evaluating mineralization of naphthenic acids sodium salts (NAS) and oil sands tailings extracts of naphthenic acids (TEX); (2) evaluating mineralization of four model naphthenic acid compounds, cyclohexane carboxylic acid (CCA), cyclohexane pentanoic acid (CPA), 2-methyl-1-cyclohexane carboxylic acid (2MCCA), and trans-4-pentylcyclohexane carboxylic acid (4PCCA); (3) gas chromatographic analysis of NAS and TEX biodegradation; and (4) respirometry measurements of cyclohexane pentanoic acid, NAS, and TEX in tailings microcosms.

 

The first experiment showed degradation of NAS and TEX over 20-30 days reached 20-48% using adapted microorganisms. The second experiment showed 6-67% degradation in 24 hours for the model naphthenic acid compounds with adapted microorganisms. The third experiment showed a reduction in the overlapping peaks of the GC spectra within 4 days for the NAS adapted culture, indicating degradation (not quantified), while analysis of the samples in the TEX adapted cultures did not result in a noticeable size change in spectra peaks, despite evident mineralisation. The fourth experiment showed the addition of naphthenic acid compounds, NAS and TEX to tailing pond water (TPW) resulted in increased microbial activity (shown through increased CO2 production), with microbial activity increasing further with N and P added to the media. Analysis after 35 days showed naphthenic acid compound concentrations below the level of detection in two out of the three microcosms tested, with a 10-fold reduction in the third microcosm, and all three N and P amended microcosms.

 

The study is not sufficient to conclude the substance is readily biodegradable but the data show that naphthenic acids are can be degraded by adapted organisms.

 

Herman et al (1993)

Herman et al (1993) conducted four studies on the biodegradation of specific naphthenic acid (cycloalkane carboxylic acids) compounds: (1) biodegradation of naphthenic acid in tailing ponds water (TPW, adapted microorganisms) through analysis of four compounds; (2) biodegradation of two naphthenic acid compounds in TPW microcosms; (3) mineralisation of carboxylated cycloalkanes by TPW bacteria and carboxylate-degrading bacteria enriched with TPW; (4) mineralisation of radiolabelled hexadecane by TPW.

 

The first experiment showed that degradation of naphthenic acid compounds required N and P nutrients to be added to the tailing ponds water (TPW) media, with 100% degradation achieved after 16 days for two compounds (cyclopentane carboxylic acid (CCP) and cyclohexane carboxylic acid (CCH)), 51% degradation after 26 days (and 100% degradation after 40 days) for the third compound (2-methyl-1-cyclohexane carboxylic acid (2MCCH)) and no degradation observed after 40 days for the fourth compound (1-methyl-1-cyclohexane carboxylic acid (1MCCH)). No degradation was observed for any of the compounds after 40 days using TPW without additional nutrients. This was supported by the second experiment, which showed complete degradation of CCP in TPW microcosms within 1 week, with no degradation of 1MCCH after 6 weeks but, when N and P were then added to the media, complete degradation was observed between weeks 6 and 9. The third experiment showed one colony type isolated from TPW used CCP and CCH as its sole carbon source, with complete degradation of the compounds within 2 weeks, and that mixed bacteria cultures, enriched with TPW, degraded 1MCCH by 100% and 2MCCH by 67% after 14 days but only when supplemented with yeast extract. The fourth experiment showed 50% mineralisation of hexadecane within oil sands tailings by day 16, with a plateau

maintained through day 40.

 

The study is not sufficient to conclude the substance is readily biodegradable but the data show that naphthenic acids can be degraded by adapted organisms.

 

Conclusion

Zinc naphthenate is a mixture of many different constituents and the rate of biodegradation can vary between the compounds present in the UVCB. Clemente et al (2004) and Herman et al (1993 and 1994) show that naphthenic acids can be degraded by adapted microorganisms. The QSAR estimated biodegradation potential of the representative structures of zinc naphthenate show that most representative structures are readily biodegradable and, although some are predicted as not readily biodegradable, none of the modelled structures meet the screening criteria for P in the PBT assessment.