Stearic acid

Biodegradation in water: screening tests

Description of key information

Readily biodegradable

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

The ready biodegradability of stearic acid (C18, saturated) is assessed in a weight of evidence approach based on the results of the substance on its own and on the results of the closely related read across substance oleic acid (C18, 1 double bond,9-Octadecenoic acid, (Z)-) as well as on QSAR calculations.

Stearic acid, saturated was tested by Bogers (1989) for ready biodegradability according to OECD 301B and GLP. At concentrations of 10 and 20 mg/L the determined degradation values were 72% and 71%, respectively at test termination (28 d). The pass criterion for ready biodegradability (60% degradation reached within 10 days once exceeded 10% degradation) was barely missed. However, the sampling interval was not as narrow as recommended by the OECD guideline (sampling every second day, followed by sampling every fifth day) which might have led to the barely missing of the 10-day window.
The failure of the 10-day window in biodegradation tests due to the low water solubility/bioavailability as well as to the inappropriate sampling intervals were already recognized and discussed within the framework of the SIDS Initial Assessment Report for the Category “Aliphatic Acids” (OECD, 2009) and judged not to preclude the ready biodegradability of the fatty acids.

The analogue substance oleic acid, unsaturated (9-Octadecenoic acid, (Z)-) was also assessed for ready biodegradability.
Coenen (1991) conducted a GLP study according to OECD 301B. After 28 days 93% and 75% of oleic acid were biodegraded at concentrations of 10 mg/L and 20 mg/L, respectively und thus passed the 60% degradation level. Furthermore, at 10 mg/L test substance the 10 day window was met. According to the criteria for ready biodegradation oleic acid (9-Octadecenoic acid, (Z)-) is readily biodegradable. Since the reference substance itself failed the pass criterion for validity (60% degradation was not reached within 14 d), the study should had been repeated.

Reliable results of the QSAR model BIOWIN v.4.10 (EPI Suite, 2010) predicts ready biodegradability for both substances. This method is based on the application of Bayesian analysis to ready biodegradation data for chemicals, derived collectively from all six OECD301 test methods plus OECD310.

Overall, the substance stearic acid is regarded as readily biodegradable. This judgment is consistent with the hazard assessment presented in the OECD SIDS (2009) for the category “Aliphatic Acids Category” where aliphatic fatty acids with a carbon chain length in the range of C8 – C22 were judged to be readily biodegradable.