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Environmental fate & pathways

Phototransformation in air

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Description of key information

Phototransformation in air:

Rate constant for reaction with OH radicals:

Parent substance 6.0 E-13 cm3 / molecule.sec (half-life 26.7 days);

Silanol hydrolysis product, methylsilanetriol, 1.1 E-11 cm3 / molecule.sec (half-life 1.5 days);

Key value for chemical safety assessment

Half-life in air:
26.7 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

No measured data are available for phototransformation in air. 2,4,6,8-Tetramethylscyclotetrasiloxane is hydrolytically unstable (t1/2= 2.2 min at pH 7), therefore, reaction with water vapour, rather than photolysis, is expected to be the primary degradation process in air.

H-D4 and its hydrolysis product methylsilanetriol contain no chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur.

 

The AOPWIN program (v1.92, EPA, 2010) has been used to obtain values of the rate constant kOH for reaction of HD4 and methylsilanetriol with hydroxyl radicals. This prediction method has not been validated to assess applicability to organosilicon substances; therefore, there is uncertainty associated with the calculated values obtained.

The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:

kdegair(d-1) = kOH(cm3/molecule.sec) x OH Concair(molecules/cm3) x 24 x 3600

 

DT50(d) = ln 2/ kdegair(d-1)

 

Where:

kdegair= total rate constant for degradation in air

kOH= rate constant for reaction with hydroxyl radicals

OH Concair= concentration of hydroxyl radicals in air = 5 x 105OH molecules/ cm3

DT50= half-life

 

The concentration of hydroxyl radicals in air of 5 x 105OH molecules/ cm3, and the 24 hour photoperiod, are the values specified in ECHA Guidance on Information requirements and chemical safety assessment, Part R.16 Environmental exposure estimation (R.16, A.16-3.2.2 C) Photochemical reactions in the atmosphere)(ECHA, 2016).

 

The results are given in the table below:

Table: Results of photodegradation in air calculations

Parameter

HD4

Methylsilanetriol

kOH (cm3/ molecule.sec)

6.0 x 10-13

1.1 x 10-11

kdegair(d-1)

0.03

0.45

DT50(days)

26.7

1.5

 

Measured data for reaction with hydroxyl radicals in air are available for some organosilanes. A summary of these measured data is in the table below.

AOPWIN predictions are also presented for comparison with the measured data.

Table: Measured data and AOPWIN predictions for reaction with hydroxyl radicals in air.

Substance

Rate constant for reaction with hydroxyl radicals (kOH(cm3/ molecule. sec))

Half-life (days)

Tetramethylsilane

1.28 x 10-12(Sommerladeet al., 1993)

0.6 x 10-12(AOPWIN)

1.0 x 10-12(Atkinson, 1991)

8.5 x 10-13(Tuazonet al., 2000)

13

27

16

19

Hexamethyldisiloxane

1.19 x 10-12(Sommerladeet al., 1993)

0.9 x 10-12(AOPWIN)

1.4 x 10-12(Atkinson, 1991)

14

18

12

Octamethylcyclotetrasiloxane

1.26 x 10-12(Sommerladeet al., 1993)

1.2 x 10-12(AOPWIN)

1.0 x 10-12(Atkinson, 1991)

13

13

16

Hexamethylcyclotrisiloxane

0.9 x 10-12(AOPWIN)

0.5 x 10-12(Atkinson, 1991)

18

31

Decamethylcyclopentasiloxane

1.5 x 10-12(AOPWIN)

1.6 x 10-12(Atkinson, 1991)

11

10

Dimethylsilanediol

7.2 x 10-12(AOPWIN)

8.1 x 10-13(Tuazonet al., 2000)

2

20

Trimethylsilanol

3.95 x 10-12(Sommerladeet al., 1993)

3.9 x 10-12(AOPWIN)

7.2 x 10-13(Tuazonet al., 2000)

4

4

22

 

The measured values from Sommerlade et al.(1993) and Atkinson (1991) are in sufficient agreement, and correlate well with the predicted values. Indeed, the data from these two studies were used in the training set for the AOPWIN program.

 

The measured values from Tuazon et al.(2000) indicate slightly lower rates of reaction for the silanols compared to the AOPWIN predictions and the measured value from Sommerlade et al.(1993).

 

References:

 

EPA, 2010. US Environmental Protection Agency.AOPWIN program v1.92a (September, 2010)

 

ECHA (2016). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0 February 2016, R.16, A.16-3.2.2 C) Photochemical reactions in the atmosphere

 

Sommerlade, R., Parlar, H., Wrobel, D. and Kochs, P. (1993). Product Analysis and Kinetics of the Gas-Phase Reactions of Selected Organosilicon Compounds with OH Radicals Using a Smog Chamber-Mass Spectrometer System. Environ. Sci. Technol. 1993, 27 (12), 2435-2440.

 

Tuazon E C, Aschmann S M and Atkinson R (2000) Atmospheric Degradation of Volatile Methyl-Silicon Compounds Environmental Science and Technology, Vol. 34, No. 10, 1970-1975

 

Atkinson R. 1991. Kinetics of the Gas-Phase Reactions of a Series of Organosilicon Compounds with OH and NO3 Radicals and O3 at 297 +/- 2 K. Environ. Sci. Technol. 25(5):863-866.