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Environmental fate & pathways

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Additional information

Based on the considerations described in the attached document “Read-across substantiation C5-6 branched alkylmethyl-ethers” (incl. the supporting references Tuppurainen et al., 2007 and Niska et al., 2008), it can be concluded that the available information of TAME can be used to predict the environmental fate and pathways of the substance ‘C5-6 branched alkylmethyl-ethers’ with sufficient certainty.

Based on the physical-chemical properties of TAME (and the properties of other structurally related aliphatic ethers), TAME is not expected to significantly hydrolyse in natural waters under environmentally relevant pH conditions (pH 4-10) (Prager, 1992).

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/s 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 of TAME in water is not available. However, because of structural reasons, TAME is not expected to photolyse directly, or photooxidise 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 constant used in the assessment are:

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

4.62E-03 d-1

Biodegradation in aerated sediment

2.31E-03 d-1

Biodegradation in soil

2.31E-03 d-1

 

The organic carbon-water partitioning coefficient (Koc) of 80 l/kg was derived based on the available data for the consituents of C5-6 branched alkylmethyl-ethers. No bioaccumulation tests are available, but based on the estimated log Kow (below 3) bioaccumulation is not expected (see attachment for details).

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 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.