Assessment of the biodegradability of mineral oil and synthetic ester base stocks, using CO2 ultimate biodegradability tests and CEC‐L‐33‐T‐82

Abstract

AbstractThe EPA (Gledhill) Shake Flask test, OECD Modified Sturm test, and CEC (CEC‐L‐33‐T‐82) Biodegradability test have been used to assess the degradability of mineral oil, natural ester, and synthetic ester base stocks. The Shake and Sturm tests are ultimate biodegradability tests which determine the percentage of test material carbon converted to CO2 over a 28‐day period; the CEC test determines, at 21 days, the disappearance of the carbon‐hydrogen stretch in Freon extracts of the test medium. In all three tests, the test material is added to a mineral salts medium, the medium is inoculated with mixed liquor from a municipal waste water treatment plant and test flasks are shaken. The Shake Flask and CEC tests were able to show expected differences in degradability between paraffinic and naphthenic mineral oils. A poor correlation was found between ultimate biodegradability test results and results from the CEC test when synthetic esters were studied. Dissolved organic carbon (DOC) analysis of the test medium indicated that degradation of some synthetic esters led to the accumulation of water‐soluble metabolites which are not extractable into Freon, as required for quantification by the CEC test. Thus, while the CO2 tests detected differences in the degradability of these test materials, the CEC test did not. The CEC test can, therefore, overestimate degradability of some synthetic esters, leading to the erroneous conclusion that these materials would meet OECD Ready Biodegradability criteria. Biodegradability results for the EPA, OECD, and CEC tests are presented and results compared.