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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:
not reported
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Remarks:
The study report contains some missing information due to the poor quality of printing/copying and so cannot be considered reliable without restrictions.
Qualifier:
according to guideline
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
Relative light intensity:
600 - 650
Duration:
15 d
Key result
DT50:
37.45 d
Test condition:
continuous irradiation
Transformation products:
yes
No.:
#1

Table 1: Average degradation results (as percentage of applied radioactivity)

 Time (day)  Propargite  TPBC (glycol ethyer)
 Irradiated 
 0  89.44  4.25
 2  81.64  6.57
 5  76.78  8.45
 9  70.52  10.08
 12  73.60  12.16
 15  58.62  20.34
 Dark controls      
 0  91.37  3.46
 2  81.93  5.31
 5  86.97  6.77
 9  81.23  5.24
 12  86.10  5.24
 15  75.37  15.55

Mass balance = 86.2 to 105.1 %

Validity criteria fulfilled:
not specified
Conclusions:
Under the conditions of the test, propargite degrades moderately slowly under photolytic conditions in soil (DT50 37.5 days continuous irradiation) for form TBPC. This pathway also occurred in the dark controls, indicating that the route of photolytic degradation is the same as for aerobic degradation under dark conditions.
Executive summary:

Omite® was found to have a half-life of 899 hours (37.45 days) relative to the dark control under continuous irradiation at 25 ºC on the surface of sandy loam soil. This is equivalent to 1798 hours or 74.9 days of 12 hour photocycle. The rate constant relative to the dark control is calculated to be -6.3 x 10-4/day. One major photoproduct was detected and identified by cochromatography as being Omite glycol ether. Soil binding played a significant role in the breakdown of 14C-Omite on soil; some of the degradation observed in this study is not attributable to photodegradation. Roughly 6 % of material applied to photolysis samples was considered unextractable after 15 days; this figure was closed to 4 % for dark control samples.

Description of key information

Half-life in soil was determined to be 899 hours; study conducted in accordance with EPA Guideline, Subdivision N, 161-3; Korpalski (1990)

Key value for chemical safety assessment

Half-life in soil:
899 h

Additional information

Omite®was found to have a half-life of 899 hours (37.45 days) relative to the dark control under continuous irradiation at 25 ºC on the surface of sandy loam soil. This is equivalent to 1798 hours or 74.9 days of 12 hour photocycle. The rate constant relative to the dark control is calculated to be -6.3 x 10-4/day. One major photoproduct was detected and identified by cochromatography as being Omite glycol ether. Soil binding played a significant role in the breakdown of 14C-Omite on soil; some of the degradation observed in this study is not attributable to photodegradation. Roughly 6 % of material applied to photolysis samples was considered unextractable after 15 days; this figure was closed to 4 % for dark control samples.