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Environmental fate & pathways

Endpoint summary

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Additional information

MTBE is not readily biodegradable in the aquatic environment according to the standardised aerobic ready-biodegradation tests (Hüls AG, 1991a; RBM, 1996a; Slovnaft VÚRUP, a.s., 2005a). However, high degradation rates have been observed in non-standard tests using special types of inoculum, pure cultures and mixed cultures (Shell, 1993; Salanitro et al., 1994; Steffan et al., 1997; Cano et al., 1998; Kharoune et al., 2001; 2002). These studies show that at least some microbial species are capable of degrading MTBE and to use it even as their sole carbon source. It could be concluded that MTBE is inherently biodegradable under certain conditions in the aquatic aerobic environment. However, the non-standard test data available indicate that MTBE degradation might not fulfil the test criteria (of OECD 302). In contrast, it also shows that adapted sewage sludge is able to rapidly degrade MTBE.

It could be assumed that where there are continuous releases of MTBE to a STP, such as for large production and processing sites, sewage sludge will have become adapted to the substance and in these cases, the substance could be considered as readily biodegradable. For professional and consumer releases and releases on the regional scale, where adaptation may not occur, the non-standard test data available indicate that MTBE degradation might not fulfil the test criteria (of OECD 302), and a slower degradation categorisation as “inherently biodegradable, not fulfilling criteria” could be considered.

This approach is consistent with the conclusion from the EU Risk Assessment Report for MTBE, which concludes “MTBE is inherently biodegradable under certain conditions in aquatic aerobic environment” (European Commission, 2002).

However, owing to the lack of ready biodegradation seen in the available standard screening tests, and as mandated by ECHA via a substance evaluation decision, a conservative approach to the available data means that the conclusion ‘under test conditions no biodegradation observed’ has been used for exposure assessment purposes.

MTBE has not been shown to biodegrade in anaerobic, static sediment/water microcosms (Suflita et al., 1993; Mormile et al., 1994). However, under mixed aerobic/anaerobic conditions, biodegradation has been shown to be a significant removal process of MTBE in sediment (Bradley et al., 1999).

Several studies are available for degradation of MTBE in soil. The results are conflicting. In a study in which soil was polluted with gasoline containing MTBE it was shown that aerobic biodegradation was observed after the spill (Yuan, 2006). This behaviour of MTBE was also observed by Borden et al. (1997). However, other studies concluded that rapid and reliable biodegradation of MTBE in soil cannot be assumed under any normal environmental conditions (both aerobic and anaerobic), indicating very slow degradation in soil (Yeh and Novak, 1994; Allard et al., 1996; Reisinger et al., 2000). As the study by Yuan (2006) was better in design and reporting than the other studies mentioned, the worst-case half-life of 101.6 days in soil from this study is used in the assessment.

 

The rate constant used in the assessment are:

Degradation in a non-adapted STP

0 d-1

Degradation in an adapted STP

Monod kinetics (default values)

Biodegradation in water

4.62E-03 d-1

Biodegradation in aerated sediment

2.31E-03 d-1

Biodegradation in soil

1E-03 d-1

  Monitoring data support the paramaters used in the modelling, as discussed in Chapter 9 on Exposure Assessment.