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Biodegradation in water: screening tests

Description of key information

The two tests to estimate biodegradation were conducted using related, but different, methods.  The two methods did not yield the same result, but the results were consistent in that they showed C-4000 degraded significantly under both sets of test conditions [~98% in 11 days or ~50% in 29 days or ~98% in 11 days].  ].  The difference in the results is likely related to the physical and chemical properties of C-4000 as they impact each method.  Regarding physical properties, C-4000 has a low vapour pressure and low water solubility and therefore Henry’s Law can have an impact on a test run using a purge system where volatilization may occur.  We also observe rapid hydrolysis of C-4000 in the aquatic compartment, which means degradation can occur by a combination of hydrolysis and degradation.
The guideline documentation indicates the 301D test is likely more suitable for measuring degradation of poorly water soluble substances because the application rate of the test substance is lower and the system is not purged.  In this test we observe rapid degradation of C-4000, up to 98% ThOD.  This rapid degradation is supported by the fact that the hydrolytic degradation products also show a very high biodegradation in tests that measure ThOD.  
The OECD 301B test uses a higher loading of C-4000, and the system is purged during the test to capture the CO2 generated.  In this test one sees a rapid degradation early, 40% ThCO2 in 14 days, which then plateaus and only increases another 10% ThCO2 in the next 14 days to the measured total of 50%.  This may be the result of several factors.  With a low water solubility and low vapor pressure, C-4000 could undergo volatilization according to Henry’s Law during the course of the test.  Also, rapid hydrolysis will produce the smaller hydrolytic fragments which might also volatilize into the purge stream.    Lastly, some of the C-4000 could be used to support cellular growth in the medium.  Most importantly, the loss of material b...

Additional information

Based on weight of evidence, the substance is not expected to meet the persistent (P) criteria in a PBT/vPvB assessment. C-4000 is expected to be readily biodegradable or to quickly biodegrade in the environment and is therefore unlikely to constitute a potential longterm and/or delayed danger to the aquatic environment. C-4000 should not be classified as R53 according to the Directive 67/548/EEC or Chronic category 4 under the Regulation 1272/2008.