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Environmental fate & pathways

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Both TAME and MTBE are not expected to hydrolyse under environmentally relevant pH conditions (pH 4-10) based on their physical-chemical properties and the properties of other structurally related aliphatic ethers (Lyman et al., 1982; Prager, 1992). Since ETBE is structurally highly related to both substances it can be expected that also ETBE will not significantly hydrolyze in natural waters.

 

According to existing data, degradation half-life of ETBE in the air is 3-12 days depending on environmental conditions (predominantly OH-radical concentration).

Using a degradation rate constant of 3.0E-12 cm3/molecule/s and an OH–radical concentration of 5E05 radicals/cm3 a half-life of 5.35 days is calculated.

 

Direct photolysis will not be an important removal process since aliphatic ethers do not absorb light at wavelengths >290 nm. The UV-spectrum (max at 289 nm) indicates that direct photolysis in water may not occur.

 

ETBE can not be regarded as readily biodegradable in standard test systems (Fayolle et al., 1998; Slovnaft VÚRUP, a.s.). However, certain adapted micro-organisms are capable of degrading ETBE (e.g. Cowan and Park, 1996; Steffan et al., 1997; Kharoune et al., 1998, Kharoune et al., 2001; 2002). Thus, a well adapted industrial STP plant is able to degrade the substance. High degradation rates have been observed in non-standard tests using special types of inoculum, pure cultures and mixed cultures. These studies show that at least some microbial species are capable to degrade ETBE and to use it even as their sole carbon source. It may be concluded that ETBE is inherently biodegradable under certain conditions in aquatic aerobic environment. Although, the non-standard test data available indicate that ETBE degradation might not fulfil the test criteria (OECD 302) to be classified “inherently biodegradable”. In contrast, adpated sewage sludge is able to rapidly degrade ETBE. Therefore, in the further assessment the characterisation of biodegradability in aquatic environment is set at “Readily biodegradable” and the Monod kinetics are used for the degradation of ETBE in the STP instead of the more simplified first-order kinetics as it can be assumed that the sewage treatment plants at industrial site are carrying adapted sludge only. However, for professional and consumer releases and on the regional scale, where adaptation may not be present, it is assumed to be 'inherently biodegradable, not fulfilling criteria.'

 

No biodegradation simulation tests are available. In anaerobic, static sediment/water microcosms, ETBE does not biodegrade (Suflita et al., 1993; Mormile et al., 1994).

 

Several studies are available for degradation of ETBE in soil. The results are conflicting. In a study in which soil was polluted with gasoline containing ETBE it was shown that aerobic biodegradation was observed after the spill (Yuan, 2006). However, other studies concluded that rapid and reliable biodegradation of ETBE in soil can not 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 89.5 days in soil from this study is used in the assessment.

   

The rate constants used in the assessment are:

Degradation for hydrolysis

0 d-1

Degradation for photolysis

0 d-1

Degradation in air

0.130 d-1

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 sediment

2.31E-03 d-1

Biodegradation in soil

4.08E-03 d-1

 

No bioaccumulation tests are available, but based on the low octanol-water partition coefficient of 1.48, bioaccumulation is not expected.

 

The organic carbon-water partitioning coefficient (Koc) calculated from the octanol-water partition coefficient (log Kow = 1.48) using the equation from the Technical Guidance Document (predominantly hydrophobics) is 19.9 l/kg (log value = 1.30). This predicted value is used in the assessment.

Using a Level I fugacity model, the theoretical distribution of ETBE based on physico-chemical properties between four environmental compartments at equilibrium can be calculated. The results in indicate that 96.2% is distributed to the atmosphere and that volatilisation may be expected from water and soil and adsorption to particulate matter is poor.