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Environmental fate & pathways

Phototransformation in water

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Reference
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Study type:
direct photolysis
Principles of method if other than guideline:
Substance was exposed to sunlight at a concentration of about 1 ppm in sterile pure water solution with 10% acetonitrile as cosolvent. A dark control was performed to estimate the approximate stability of the chemical in water. The losses of the substance in the dark control may have been due to oxidation or hydrolysis.
GLP compliance:
not specified
Light source:
sunlight
Details on light source:
midday in August
The experimental tubes were placed in a location free of excessive reflections from walls and windowns and without morning or afternoon shadows.
Dark controls:
yes
% Degr.:
50
Sampling time:
3.1 h
Test condition:
midday

The UV absorption spectrum of Thiofide shows that it absorbs strongly into the solar spectral region. The measurements for the sunlight photolysis of a 0.9-ppm sample presented a photolysis rate constant of 6.2*10-5 /sec. The determined half-life of Thiofide was 3 hours at midday. In the dark control a half-life of approximately 19 hours was obtained at 24°C.

In addition to the major peak, three front-eluting peaks were present in the test solution before photolysis began. As photolysis proceeded, the area of the front three peaks increased. No other photolysis products were detected by HPLC. The HPLC traces for the dark control are similar to those obtained during the sunlight photolysis experiment.

Conclusions:
The screening experiment for the sunlight direct photolysis of MBTS in pure water gave a first-order rate constant of 6.2*10-5sec-1at midday in August, which corresponds to a half-life of 3 hours.Three transformation products were detected by HPLC but not identified.
Executive summary:

The screening experiment for the sunlight direct photolysis of MBTS in pure water gave a first-order rate constant of 6.2*10-5sec-1at midday in August, which corresponds to a half-life of 3 hours (Monsanto, 1980). In both the photolysis experiment and the dark control, three transformation products were detected by HPLC but not identified (Monsanto, 1980).

Description of key information

The screening experiment for the sunlight direct photolysis of MBTS in pure water gave a first-order rate constant of 6.2*10-5sec-1at midday in August, which corresponds to a half-life of 3 hours (Monsanto, 1980). In both the photolysis experiment and the dark control, three transformation products were detected by HPLC but not identified (Monsanto, 1980).

Key value for chemical safety assessment

Half-life in water:
3 h

Additional information

The value mentioned above refers to the top millimetres of a water body in summer, because of factors like cloudiness, shadowing effects of vegetation, absorption and scattering of light by suspended solids etc. the actual environmental lifetime is substantially higher.