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Phototransformation in air

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Description of key information

Phototransformation in air: Rate constant for reaction with OH radicals:

Hexamethylcyclotrisiloxane: 1.09E-12 cm3 / molecule.sec (half-life 14.73 days);

L3-diol: 7.8E-12 cm3 / molecule.sec (half-life 2.1 days)

DMSD: 8.1 E-13 cm3 / molecule.sec (half-life 20 days).

Key value for chemical safety assessment

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

Additional information

D3 and its hydrolysis products, L3-diol and DMSD, 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.

An experimental relative rates study (Atkinson, 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for this compound. The dominant gas-phase chemical loss process is by reaction with the OH radical, with measured rate constant of 5.2 E-13 cm3molecule-1second-1at 24°C (calculated half-life 31 days, using tropospheric concentration of OH radicals 5 E+05 molecule/cm3over 24-h period (ECHA, 2012)).

A measured OH radical rate constant (kOH­) of 0.91 x 10-12cm3/ molecule.sec was determined by Kim and Xu (2017), based on a relative rate method. A further measurement by Xiao et al.(2015) of 1.84 x 10-12 is slightly higher.

The AOPWIN program (v1.92, EPA 2010) has been used to obtain predicted values of the rate constant kOH for reaction of D3, L3-diol and DMSD with hydroxyl radicals. This prediction method has not been validated to assess applicability to organosilicon substances; therefore, there is a small 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 E+05 OH molecules/ cm3

DT50= half-life

The concentration of hydroxyl radicals in air of 5 E+05 OH 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 (ECHA, 2012).

The results are given in the table below.

Table: Results of photodegradation in air calculations

Parameter

D3

L3-diol

DMSD

kOH(cm3/ molecule.sec)

0.9 E-12

7.8 E-12

7.2 E-12

kdegair(d-1)

0.04

0.33

0.3

DT50(days)

18

2.1

2

 

Measured data for reaction with hydroxyl radicals in air are available for some organosilanes. A summary of these measured data, including the data for D3 discussed above, and data for DMSD, 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)

Hexamethyldisiloxane (HMDS)

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

0.90 x 10-12(AOPWIN)

1.38 x 10-12(Atkinson, 1991)

1.32 x 10-12(Markgraf and Wells, 1997)

1.58 x 10-12(Kim and Xu, 2017)

13.5

17.8

11.6

12.2

10.2

Octamethyltrisiloxane (L3)

1.83 x 10-12(Markgraf and Wells, 1997)

2.15 x 10-12(Kim and Xu, 2017)

1.20 x 10-12(AOPWIN)

8.8

7.5

13.4

Decamethyltetrasiloxane (L4)

2.66 x 10-12(Markgraf and Wells, 1997)

3.37 x 10-12(Kim and Xu, 2017)

1.50 x 10-12(AOPWIN)

6.0

4.8

10.7

Dodecamethylpentasiloxane (L5)

4.03 x 10-12(Kim and Xu, 2017)

1.80 x 10-12(AOPWIN)

4.0

8.9

Hexamethylcyclotrisiloxane (D3)

0.90 x 10-12(AOPWIN)

0.52 x 10-12(Atkinson, 1991)

1.84 x 10-12(Xiaoet al. 2015)

0.91 x 10-12(Kim and Xu, 2017)

17.8

30.9

8.7

17.6

Octamethylcyclotetrasiloxane (D4)

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

1.20 x 10-12(AOPWIN)

1.01 x 10-12(Atkinson, 1991)

1.90 x 10-12(Safronet al. 2015)

2.34 x 10-12(Xiaoet al. 2015)

0.95 x 10-12(Kim and Xu, 2017)

12.7

13.4

15.9

8.4

6.9

16.9

Decamethylcyclopentasiloxane (D5)

1.50 x 10-12(AOPWIN)

1.55 x 10-12(Atkinson, 1991)

2.60 x 10-12(Safronet al. 2015)

2.46 x 10-12(Xiaoet al. 2015)

1.46 x 10-12(Kim and Xu, 2017)

10.7

10.4

6.2

6.5

11.0

Dodecamethylcyclohexasiloxane (D6)

2.44 x 10-12(Kim and Xu, 2017)

2.80 x 10-12(Safronet al. 2015)

1.8 x 10-12(AOPWIN)

6.6

5.7

8.9

Tetramethylsilane

1.28 E-12 (Sommerlade et al., 1993)

0.6 E-12 (AOPWIN)

1.0 E-12 (Atkinson, 1991)

8.5 E-13 (Tuazon, 2000)

13

27

16

19

Dimethylsilanediol (DMSD)

7.2 E-12 (AOPWIN)

8.1 E-13 (Tuazon, 2000)

2

20

Trimethylsilanol (TMS)

3.95 E-12 (Sommerlade et al., 1993)

3.9 E-12 (AOPWIN)

7.2 E-13 (Tuazon, 2000)

4

4

22

 

For the parent substance, D3, the three measured values are in reasonable agreement with each other and with the AOPWIN prediction. The arithmetic mean of the measured values is used in the exposure assessment.

For the silanols, the measured value from Tuazon (2000) for DMSD indicates a slightly lower rates of reaction for the silanols compared to the AOPWIN predictions and the measured value from Sommerlade (1993) for TMS. The measured value for DMSD is used in the exposure assessment as a worst case.

The AOPWIN predicted value is used for the L3-diol; since photodegradation is unlikely to be the primary removal process for L3-diol, the uncertainty in the value is not considered to be significant in terms of the exposure assessment.

References:

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

ECHA (2012). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1 October 2012. R.16.5.4.3. 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.