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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Tripropylene glycol was found readily biodegradable after 28 days under aerobic conditions.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Four reliable screening tests (GLP compliant guideline studies) and a non-GLP result from the MITI database are available which show different results on the biodegradability of the substance. In the OECD Guideline 301 F test (Ready Biodegradability: Manometric Respirometry Test) with sludge from a municipal sewage treatment plant, 81.9% biodegradation was found after 28 days determined as O2 consumption and 60.1% determined as CO2 removal (West et al., 2007). This test is a better simulation of realistic conditions than the other screening tests and will therefore be further used as key study in the assessment. In the present study, 55.3 ± 1.9 % biodegradation based on O2 consumption was observed in the 10-day window, indicating that the 10-day window criterium of 60% ThOD removal for ready biodegradability was just not met. A previous study has shown that the various isomers and homologues typically comprising technical surfactant mixtures can be biodegraded preferentially and sequentially in the ready biodegradability test (Richter and Steber, 2001). In such cases, the extent of degradation occurring over the 10-d window represents the average rate of degradation for all components of the mixture and has little relevance for individual components. For this reason, a recent revision to Section 3: Degradation and Accumulation of the OECD guidelines has excluded the 10-d window criterion for ready biodegradability tests involving substances that occur as mixtures of isomers and/or homologues in their purest commercial form (OECD, 2006). In the present study, markedly different rates of biodegradation were observed among the discrete propylene glycol oligomers having one to four oxypropylene repeating units. Kawai (1987) also showed that biodegradation of dipropylene glycol involves preferential oxidation of the structural isomers having secondary alcohol groups. Thus, different rates of biodegradation could be expected for the distinct homologues and isomers comprising the propylene glycol substances, and the 10-d window criterion should not be applied in evaluating their ready biodegradability.

It is therefore concluded that tripropylene glycol can be regarded as ready biodegradable.

Under the conditions of other screening tests (OECD301B, OECD301C and OECD301D with municipal inoculum) the test substance was not ready biodegradable (Shell Research, 1993, MITI, 1995, The Dow Chemical Company, 1991). However, in an OECD301D test with industrial inoculum, 69% of the theoretical oxygen demand was consumed after 28 days in reaction mixtures containing the industrial inoculum. These data suggest that some adaptation of the microorganisms may be required for biodegradation. Significant biodegradation was found in an OECD306 test with natural seawater after 64 days (West et al, 2007). Therefore tripropylene glycol is not expected to persist in water or marine environments. In the present study, 55.3 ± 1.9 % biodegradation based on O2 consumption was observed in the 10-day window, indicating that the 10-day window criterium of 60% ThOD removal for ready biodegradability was just not met. A previous study has shown that the various isomers and homologues typically comprising technical surfactant mixtures can be biodegraded preferentially and sequentially in the ready biodegradability test (Richter and Steber, 2001). In such cases, the extent of degradation occurring over the 10-d window represents the average rate of degradation for all components of the mixture and has little relevance for individual components. For this reason, a recent revision to Section 3: Degradation and Accumulation of the OECD guidelines has excluded the 10-d window criterion for ready biodegradability tests involving substances that occur as mixtures of isomers and/or homologues in their purest commercial form (OECD, 2006). In the present study, markedly different rates of biodegradation were observed among the discrete propylene glycol oligomers having one to four oxypropylene repeating units. Kawai (1987) also showed that biodegradation of dipropylene glycol involves preferential oxidation of the structural isomers having secondary alcohol groups. Thus, different rates of biodegradation could be expected for the distinct homologues and isomers comprising the propylene glycol substances, and the 10-d window criterion should not be applied in evaluating their ready biodegradability. It is therefore concluded that DPG can be regarded as ready biodegradable. Kawai F. 1987. The biochemistry of degradation of polyethers. CRC Crit Rev Biotechnol 6:273–307. Organization for Economic Cooperation and Development. 2006. Revised introduction to the OECD Guidelines for Testing of Chemicals, Section 3: Biodegradation and Bioaccumulation, Part 1—Principles and Strategies Related to the Testing of Degradation of Organic Chemicals. 2006 Revision. Paris, France. Richterich K, Steber J. 2001. The time-window: An inadequate criterion for the ready biodegradability assessment of technical surfactants. Chemosphere 44:1649–1654.