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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

An inherent biodegradability study using adapted inoculum was considered the key study.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

Several studies on ready biodegradability of tertiary butyl alcohol have been conducted with results varying from little or no degradation to 99% degradation. An early publication comparing biodegradability of many chemicals in various tests of ready and inherent biodegradability included data on tertiary butanol (source and purity unspecified) showing evidence of DOC removal but not of mineralization (e.g. in a Sturm test 32% DOC removal but 0% CO2 production): it concluded that DOC loss in various tests may have been due, at least in part, to volatilisation (Gerike and Fischer, Ecotoxicology and Environmental Safety 3, 159-173, 1979). Two guideline and GLP-compliant studies of ready biodegradability were considered reliable for inclusion in the registration dossier: an effective absence of mineralisation (2.6 – 5.1% biodegradation as indicated by CO2 production) was reported in an OECD 301B study, while 99% removal of DOC was reported in an EC C.4-A die-away test. Consideration of the properties of tertiary butyl alcohol led to selection of the OECD 301B study as the key indicator for ready biodegradability: its vapour pressure is quite high and QSAR (EPIWIN v4.10) calculation based on molecular structure and physical properties of the substance indicate moderate volatility, despite its miscibility in water (predicted half-life in river water 2.4 days). The C.4-A die-away study used test flasks part filled with medium, loosely closed and shaken for 28 days: noting the possibility of DOC loss by volatilization, Part I of the C.4-A test guideline says “Moderately volatile chemicals may be tested by the DOC Die-Away method if there is sufficient gas space in the test vessels (which should be suitably stoppered). In this case, an abiotic control must be set up to allow for any physical loss”: the reported study did not include such controls

Subsequently, an inherent biodegradability study designed to prevent test material loss from volatilisation and using adapted inoculum was performed. Using the CONCAWE test method (ISO 24593, OECD draft guideline 302D), mineralisation in the presence of an adapted microbial inoculum was monitored by measurement of inorganic carbon (arising via CO2 production) in the headspace of sealed test vessels. Biodegradation began after 7 days and reached a plateau within 28 days: the level of degradation recorded (66% on day 28, mean over days 28-56 62%) demonstrates inherent and ultimate biodegradability under the criteria set in the test method. However REACH guidance considers only two different test methods (OECD 302B and 302C: passmark 70% biodegradation) for assessment of inherent biodegradability and for these methods sets time limits within which an inherent biodegradability passmark must be attained (7 and 14 days) before a degradation rate can be modelled as “inherently biodegradable meeting criteria”. Accepting these stringent criteria, tertiary butyl alcohol is designated “inherently biodegradable not meeting criteria”