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Environmental fate & pathways

Phototransformation in soil

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Reference
Endpoint:
phototransformation in soil
Type of information:
experimental study
Adequacy of study:
key study
Study period:
21 June 1988 to 08 February 1990
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil)
Deviations:
no
GLP compliance:
yes
Radiolabelling:
yes
Analytical monitoring:
yes
Analytical method:
high-performance liquid chromatography
Light source:
Xenon lamp
Light spectrum: wavelength in nm:
> 290
Duration:
30 d
Temp.:
25 °C
Initial conc. measured:
2 other: µg/cm2 soil
Key result
DT50:
50.4 d
Test condition:
Based on volatilisation in irradiated test system
Key result
DT50:
4.7 d
Test condition:
Based on volatilisation in dark test system
Transformation products:
not specified

The study samples showed an exponential decrease in parent from the soil with increasing time. This decrease in parent compound from the study soil was attributed to volatilisation and not to photodegradation. Rate constants were determined as:

Irradiated test system: 0.0138 days-1

Dark test system: 0.146 days-1

While no specific studies were conducted to evaluate the difference in the volatility observed between the dark and the irradiated systems, it may be due to condensation of volatilised test material coming in contact with the cooled inner surface of the exposed test chamber. In order to maintain the soil surface temperature within an acceptable range it is necessary to cool the interior surface by circulating cooling water around the inner wall of the test chamber. The volatilisation rate of any compound coming in contact with this cooled surface could be expected to be less than that observed for another system without water cooling.

The mean mass accountability ranged from 100-84 % with a mean % recovery of 92.4 % for the exposed system. The loss of activity was attributed to volatile components in the test system. The result is not entirely unexpected due to the volatile nature of the test material. Results of the two additional day-30 exposed samples gave mass balances of less than 60 % for each sample point, further indicating the volatile nature of the test material and the inability to effectively trap the compound. All efforts to minimise the losses were ineffective in maintaining the mass balance of the day-30 samples above 84.3 %. The overall mass balance for all sample points, however, was above 92 %.

Validity criteria fulfilled:
not specified
Conclusions:
Under the conditions of the test, half-lifes of the test material were determined to be 50.4 days (irradiated test system) and 4.7 days (dark test system). The test material is volatile enough so that volatilisation accounts of most of the disappearance from the soil surface. Photodegradation was not detected and the loss of the test material from the soil surface can be attributed almost entirely to volatilisation of the test material.
Executive summary:

In a GLP compliant photolysis study conducted in accordance with standardised guideline EPA Guideline 161-3, the photodegradation of the test material at the soil surface was determined. Under the conditions of the test, half-lifes of the test material were determined to be 50.4 days (irradiated test system) and 4.7 days (dark test system). The test material is volatile enough so that volatilisation accounts of most of the disappearance from the soil surface. Photodegradation was not detected and the loss of the test material from the soil surface can be attributed almost entirely to volatilisation of the test material.

Description of key information

The DT50 value of the test material was determined to be 50.4 days (irridated test system) and 4.7 days (dark test system) according to a study performed in line with EPA 161-3.

Key value for chemical safety assessment

Additional information

See 'Environmental Fate and Pathways' endpoint summary.