2-methoxy-2-methylbutane

Endpoint summary

Additional information

Based on the structural and physical-chemical properties of TAME, hydrolysis is not expected to be an important environmental fate process since this compound lacks functional groups that hydrolyse under environmental conditions (Toxnet, 2014).

According to existing data, the degradation half-life of TAME in the air is 2.55-3.07 days depending on environmental conditions (predominantly OH-radical concentration). Using a degradation rate constant of 5.22 -12 cm3/molecule/2 and an OH-radical concentration of 5E05 radicals/cm3 a half-life of 3.07 days is calculated. This half-life for degradation in air will be used in further assessment.

Data on experimental determination of photolysis in water is not available. However, because of structural reasons, TAME is not expected to photolyze directly, or photooxidize significantly via reactions with photochemically produced hydroxyl radicals in water.

Two closed bottle tests (OECD 301D) are available (Hazleton Europe, 1995; Slovnaft VÚRUP, a.s., 2005a). The percentage of biodegradation observed is ca. 5% after 7 days in both studies. However, certain adapted micro-organisms are capable of degrading TAME (e. g. Kharoune et al., 2002). These studies show that at least some microbial species are capable to degrade TAME and to use it even as their sole carbon source. It may be concluded that TAME is inherently biodegradable under certain conditions in the aquatic aerobic environment. However, the non-standard test data available indicate that TAME degradation might not fulfil the test criteria (OECD 302) to be classified “inherently biodegradable”. In contrast, adapted sewage sludge is able to rapidly degrade TAME.

Therefore, in the further assessment a distinction will be made between non-adapted municipal STPs which will be classified as “Inherently biodegradable, not fulfilling criteria” and adapted industrial STPs where there are continuous releases of TAME which will be classified as “Readily biodegradable”. For these adapted STPs the Monod kinetics are used for the degradation of TAME in the STP instead of the more simplified first-order kinetics.

In anaerobic, static sediment/water microcosms, TAME does not biodegrade (Suflita and Mormile, 1993; Mormile et al., 1994; Somsamak et al., 2001).

Based on the few studies available it should be concluded that rapid and reliable biodegradation of TAME in soil can not be assumed in any normal environmental conditions indicating very slow degradation in soil (Jensen and Arvin, 1990; Mormile et al., 1994; Zenker et al., 1999). The biodegradability of TAME in soil in aerobic and anaerobic conditions seems to be very slow and favourable conditions for degradation are difficult to attain.

The rate constants used in the assessment are listed in the following table:

 

Degradation for hydrolysis	0 d-1
Degradation for photolysis	0 d-1
Degradation in air	0.226 d-1
Degradation in a non-adapted STP	0 d-1
Degradation in an adapted STP	Monod kinetics (default values)
Biodegradation in water	0 d-1
Biodegradation in aerated sediment	2.31E-03 d-1
Biodegradation in soil	2.31E-03 d-1

 

No bioaccumulation tests are available, but based on the low octanol-water partition coefficient of 1.55, bioaccumulation is not expected.

The organic carbon-water partitioning coefficient (Koc) calculated from the octanol-water partition coefficient (log Kow = 1.55) using the equation from the Technical Guidance Document (2003) (predominantly hydrophobics) is 22.7 l/kg (log value = 1.36). This predicted value is used in the assessment.

Using a Level I fugacity model, the theoretical distribution of TAME based on physico-chemical properties between four environmental compartments at equilibrium can be calculated. The results in indicate that 95.7% is distributed to the atmosphere and that volatilisation may be expected from water and soil and adsorption to particulate matter is poor.