Naphthenic acids, lithium salts

Biodegradation in water: screening tests

Description of key information

Naphthenic acids mixtures and model compounds for naphthenic acids mixtures have been tested for their biodegradability.  The tests have shown that naphthenic acids mixtures are at least inherently biodegradable with certain naphthenic acids and naphthenic acids model compounds being readily biodegradable. QSAR results for representative structures of lithium naphthenate support the ready and inherent biodegradation potential of the substance.

Key value for chemical safety assessment

Biodegradation in water:

inherently biodegradable

Additional information

In water, lithium naphthenate is expected to dissociate to lithium ions and naphthenate acids. As an inorganic metal, the lithium ion will not undergo biodegradation, however, the acid component may be biodegraded.

BIOWIN

The biodegradation potential of the representative structures of lithium naphthenate were estimated using a QSAR model (BIOWIN v 4.10 in EPISUITE v 4.11, US EPA 2010). Most representative structures are readily biodegradable (Lithium salt of C8, 0 rings; Lithium salt of C8, 1 ring; Lithium salt of C20, 0 rings; Lithium salt of C20, 1 ring; Lithium salt of C20, 2 rings; C12 linear; C22 linear), though some are predicted as not readily biodegradable (Lithium salt of C11, 2 rings; Lithium salt of C15, 3 rings; Lithium salt of C20, 3 rings). The constituents containing a higher proportion of cyclic carbon are less likely to be predicted as readily biodegradable but none of the prediction results for the structures meet the screening criteria for P or vP in the PBT assessment.

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 are can be degraded by adapted organisms.

Harlan (2013)

Lithium behenate, a lithium salt of a C22 straight chain monocarboxylic acid, is a structural analogue for the representative constituent “Acidic fraction, lithium salt of C20, 0 rings”. The data for lithium behenate have been read across to lithium naphthenate, which supports that some structures within lithium naphthenate are readily biodegradable. Lithium behenate reached 97% degradation after 28 days, meeting the 10 day window, and therefore is considered to be readily biodegradable. The ready biodegradability of lithium behenate was determined in a GLP-compliant CO2 evolution study following OECD guideline 301B (Harlan 2012). The study is considered relevant and reliable for use for this endpoint. Lithium behenate is not considered to be inhibitory to sewage treatment micro-organisms.

Gerike and Fischer (1979) and Kim et al (2001)

Adipic acid, a C6 straight chain dicarboxylic acid, is a structural analogue for the representative constituent “Acidic fraction, lithium salt of C8, 0 rings”. The data for adipic acid have been read across to lithium naphthenate, which supports that some structures within lithium naphthenate are readily biodegradable.

The OECD has published a risk assessment under the high production volume program which considers the lithium salts of dicarboxylic acids (C6 - C10) as part of a larger aliphatic acid category (CoCAM 2014). This risk assessment covers 78 member substances consisting of C4-C22 aliphatic acids (also called fatty acids) and their salts. The CoCAM report (2014) concludes that 'the weight of evidence indicates that the aliphatics acid category members are readily biodegradable'. They share a common degradation pathway in which they are degraded to acetyl-Co A or other key metabolites in all living systems. Differences in metabolism or biodegradation of even or odd numbered carbon chain compounds are not expected (CoCAM 2014).

Gerike and Fischer (1979) investigated the ready biodegradation of adipic acid using studies following OECD guideline 301B (CO2 Evolution test), 301C (Modified MITI test (I)), 301D (Closed Bottle Test), 301E (Modified OECD Screening Test) and the inherent biodegradation following OECD guideline 302B (Zahn-Wellens/EMPA Test). The studies follow methods subsequently adopted as standard OECD test methods, however, limited detail on the specific methodologies is reported. The ready biodegradation results showed 83-100% degradation of adipic acid after 28 days. The inherent biodegradation results showed 100% DOC removal after 4 days.

Kim et al (2001) tested the ready biodegradability of adipic acid following ASTM D 5209-91 (Modified Sturm Test). The biodegradation rate is > 70% after 10 days and >80% after 30 days based on CO2 evolution.

Conclusion

Lithium 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. Harlan (2013) shows that lithium behenate, a structural analogue for the representative constituent “Acidic fraction, lithium salt of C20, 0 rings”, is readily biodegradable. Gerike and Fischer (1979) and Kim et al (2001) show that adipic acid, a structural analogue for the representative constituent "Acidic fraction, lithium salt of C8, 0 rings", is readily biodegradable. The QSAR estimated biodegradation potential of the representative structures of lithium naphthenate show that most representative structures are readily biodegradable, though some are predicted as not readily biodegradable, and none of the prediction results for the structures meet the screening criteria for P or vP in the PBT assessment.