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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
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
guideline study with acceptable restrictions
Qualifier:
according to
Guideline:
EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil)
GLP compliance:
yes
Radiolabelling:
yes
Analytical monitoring:
yes
Remarks:
TLC, HPLC, LSC
Analytical method:
high-performance liquid chromatography
Details on sampling:
Duplicate light exposed and dark samples, including volatile traps, were collected for analysis at 0, 24, 48, 72, 191,5, 479 hours after treatment (0, 1, 2, 3, 8 and 20 days).
Details on soil:
- soil type: Sable silty clay loam, obtained from Monmouth, IL, by Monsanto Company. This was the same type of soil used in the aerobic soil metabolism study.
- characteristics: 7% sand, 62% silt, 31% clay, 3.5% organic matter, pH: 5.8, field capacity (%) (1/3 bar): 29.5
- preparation: Soil was initially passed through a 2 mm sieve. Water was added to produce an aqueous slurry resulting in a thin even layer on the petri dish. Soil was then air dried. 686 µL of water was added just prior to application to achieve 75 % field capacity.
Light source:
sunlight
Light spectrum: wavelength in nm:
>= 250 - <= 700
Details on light source:
- light source: natural sunlight, Richmond, California
- exposure start date: April 17,1990
- exposure termination date: May 7, 1990
- latitude: 37.45°N
- longitude: 122.26°W
Details on test conditions:
- level of fortification: 0.41 lbs/acre based on soil surface area.
- light source: natural sunlight
- temperature range: light samples: 10.3 to 36.8°C (average 22.8+-3 .1°C), dark samples: 10.7 to 36.4°C (average 21.6+-1.6°C)
- dark control: Prepared and exposed concurrently in an identical apparatus with tight exposed samples, but were covered to prevent irradiation.
- sample intervals: Day 0, 1, 2, 3, 8 and Day 20
- volatile trapping: 1 Ethylene Glycol trap, 2 NaOH (10%) traps, continuous air flow.
- extraction methods: initial extraction: acetonitrile (5 mL), acetonitrile/water (1 :1, v/v) (5 mL), water (5 mL) (extracts were combined for analysis), additional base extraction (selected samples): 0.5N NaOH (3 d) 80°C.
- extraction efficiency: initial extraction: 78.9+-13.2% (light samples), 93.7+-5.2% (dark samples)
- analytical methods: Liquid Scintillation Counting, HPLC, Thin Layer Chromatography
- storage conditions: <0°C in freezer
- storage stability: Some samples were re-analyzed after 30 days of storage and no change in chromatographic distribution was noted
Duration:
20 d
% Moisture:
75
Temp.:
22.8 °C
Initial conc. measured:
29.3 mg/kg soil d.w.
Reference substance:
not specified
Dark controls:
yes
Preliminary study:
Pilot exposures of [14C]MON 13900 indicated Mat the half-life would be less than 10 days thus the exposure times were planned to incorporate at least two half-lives rather than 30 days.
DT50:
8.8 d
Test condition:
exposure to natural sunlight
DT50:
1 435 d
Test condition:
in the dark
Transformation products:
yes
No.:
#1
No.:
#2
Details on results:
The photodegradation half-life of [14C]MON 13900 was calculated to be 8.8 days (211.3 hours) from a first-order plot. MON 13900 degraded slowly in the dark with an estimated half-life of 1435 days.

Average material balance and prodcut distribution expressed as a percent of applied radiocarbon

Day 20  % MON 13900  % MON 13900 oxazolidine acid  % unknowns  % Non-Extracted  % CO2  Total recovery
 Light  42.3  5.6  33.8  7.9  3.3  92.9
 Dark  94.4  -  0.2  7.8  0.7

 103.1

This value is the sum of unidentified peaks detected in the organic and base extracts and in ethylene glycol traps (none exceeding 11.4% with evidence of more than one componen in this quantitated peak). MON 13900 degraded to several polar products which eluted at the solvent front in reverse phase HPLC. Analysis of base extracts of the soil using a Bio-Rad LC column for organic acids separatet at least 3 unknown peaks with early elution times. HPLC recoveries for the base extracts were consistently low (60 -70%) for both methods indicating the possibility of additional unknowns which were retained the columns.

Conclusions:
The calculated half-life of photolysis for MON 13900 was 8.8 days using a linear first order kinetic model. Minimal degradation of MON 13900 occurred on the dark control soils during the study period, with an estimated half-life of 1435 days.
Executive summary:

This study was carried out according to EPA Pesticide Assessment Guidelines, Subdivision N, Section 161 -3 to fulfill the data requirements for Photodegradation on Soil. The purpose of the study was to determine the reaction kinetics of photolysis of [14C]MON 13900 on Sable silty clay loam soil surfaces and if possible to characterize any significant photoproducts.

[14C]MON 13900 was exposed to natural sunlight on viable Sable silty clay loam soil for 20 days, concurrency with dark control samples. Samples were maintained in temperature controlled chambers with a constant humidified air flow and trapping of volatiles. The average temperatures throughout the study period were 22.8 +-3.1°C and 21.6 +-1.6 °C in the light and dark soil chambers, respectively. The nominal test substance concentration was 0.4 lbs/acre (based on soil surface area = 29.3 mg/kg soil d.w.). MON 13900 degraded rapidly on irradiated soils to a mixture of photoproducts (oxazoladine acid and several polar products which were readily extracted with acetonitrile and water), none of which exceeded 8.1 % of applied radiocarbon. 14CO2 represented 3.3% of applied radiocarbon at the end of the study. Soil-bound radiocarbon increased with time in the light exposed soil and required additional extraction with 0.5 N NaOH to be released. The calculated half-life of photolysis was 8.8 days using a linear first order kinetic model. Minimal degradation of MON 13900 occurred on the dark control soils during the study period, with an estimated half-life of 1435 days. All extracts were analysed by HPLC and selected samples were examined by TLC. Radiocarbon recoveries averaged 97.1+-5.3% and 98.7 +-5.3% for light exposed and dark control sample sets, respectively, throughout the study.

Description of key information

This study was carried out according to EPA Pesticide Assessment Guidelines, Subdivision N, Section 161 -3 to fulfill the data requirements for Photodegradation on Soil. The purpose of the study was to determine the reaction kinetics of photolysis of [14C]MON 13900 on Sable silty clay loam soil surfaces and if possible to characterize any significant photoproducts.

[14C]MON 13900 was exposed to natural sunlight on viable Sable silty clay loam soil for 20 days, concurrency with dark control samples. Samples were maintained in temperature controlled chambers with a constant humidified air flow and trapping of volatiles. The average temperatures throughout the study period were 22.8 +-3.1°C and 21.6 +-1.6 °C in the light and dark soil chambers, respectively. The nominal test substance concentration was 0.4 lbs/acre (based on soil surface area = 29.3 mg/kg soil d.w.). MON 13900 degraded rapidly on irradiated soils to a mixture of photoproducts (oxazoladine acid and several polar products which were readily extracted with acetonitrile and water), none of which exceeded 8.1 % of applied radiocarbon. 14CO2 represented 3.3% of applied radiocarbon at the end of the study. Soil-bound radiocarbon increased with time in the light exposed soil and required additional extraction with 0.5 N NaOH to be released. The calculated half-life of photolysis was 8.8 days using a linear first order kinetic model. Minimal degradation of MON 13900 occurred on the dark control soils during the study period, with an estimated half-life of 1435 days. All extracts were analysed by HPLC and selected samples were examined by TLC. Radiocarbon recoveries averaged 97.1+-5.3% and 98.7 +-5.3% for light exposed and dark control sample sets, respectively, throughout the study.

Key value for chemical safety assessment

Half-life in soil:
8.8 d

Additional information