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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:
23 July 1992 - 18 September 1992
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil)
Deviations:
no
Qualifier:
according to guideline
Guideline:
other: Agriculture Canada: Environmental Chemistry and Fate. Guidelines for registration of pesticides in Canada, 15 July 1987. 6.2 Laboratory Studies, A. Physicochemical Properties and Processes of Parent Compound and Major Transformation Products
Deviations:
no
GLP compliance:
yes
Specific details on test material used for the study:
Details on properties of test surrogate or analogue material (migrated information):
Not applicable.
Radiolabelling:
yes
Analytical monitoring:
yes
Analytical method:
other: TLC... (see attached file)
Details on sampling:
- Sampling intervals of soil samples: 0, 1, 2, 4, 6, 8 and 24 hours
- Sampling method: Soil samples extracted twice in methanol/deionized water (80 + 20) by shaking for 2 minutes followed by centrifuging at 2500 rpm for 4 minutes. For the last extraction the sample was additionally treated for 2 minutes in an ultrasonic bath before centrifuging.
- Sampling method for CO2: CO2 gas trapped in 2N NaOH and analysed as 14CO2 by liquid scintillation spectrometry. For extracted soil samples, samples combusted in oxygen stream at 800ºC with copper oxide catalyst. The 14CO2 liberated was absorbed in a Vigreux absorption trap containing 2N NaOH and analysed by liquid scintillation spectrometry.
- Sampling methods for the volatile compounds: Volatile components were collected using a moist air flow through system. The outlet air was passed through ethylene glycol and two times 2N NaOH.
- Other observation, if: Characterisation of soil was performed by AGROLAB AG, CH-6030, Ebikon/KU (Switzerland. Micro Biomas, Max waterholding capacity (MWC) and Field Capacity (FC) were determined in the test facilities of Ciba-Geigy AG, CH-4000, Basle.
Details on soil:
COLLECTION AND STORAGE
- Geographic location: Stein soil stock collected on 27 July 1992
- Storage and transport conditions: Soil kept vital in laboratory by cultivating grass
- Soil preparation (e.g.: 2 mm sieved; air dried etc.): Aqueous slurry of soil to glass plate to form 1 mm thickness and dried.

PROPERTIES
- Soil classification system and year: Analysed 3 October 1991.
- Soil texture: Silty loam
- % sand: 29.5
- % silt: 56.9
- % clay: 13.6
- pH: 7.8
- Organic carbon (%): 2.3
- CEC (mmol/100 g): 14.0
- Water holding capacity: 52.6 g/100g dry soil
- Field Capacity: 40.8 g/100g dry soil
- Initial microbial biomass/microbial population (unit): 49 mg C/100g soil

PREPARATION OF SOIL
- Condition of soil: Soil kept vital by cultivating grass for four weeks prior to testing.

Light source:
Xenon lamp
Light spectrum: wavelength in nm:
>= 300 - <= 800
Relative light intensity:
>= 2.08 - <= 2.35
Details on light source:
- Emission wavelength spectrum: 300 to 800 nm
- Filters used and their purpose: Filter used to cut off light < 290 nm
- Light intensity at sample and area irradiated: Run 1: 25.21 ± 3.6 W/m2. Run 3: 22.25 ± 2.9 W/m2. Run 3: 22.66 ± 2.81 W/m2.
- Relative light intensity based on intensity of sunlight: 2.08 to 2.35
- Duration of light/darkness: Continuous for 2, 4, 6, 8 and 24 hours
Details on test conditions:
TEST SYSTEM
- Type, material and volume of test apparatus or thin layers: Stainless steel water cooled tank with quart lid and flow through system to collected volatile components.
- Application procedure:
- Volume of test solution used/treatment: 30 µL of 1000 mg 14C-CGA 185072/L in acetone. Treated surface 28 cm2, penetration depth 0.1 mm.
- Method of application: 10 droplets evenly over soil surface
- Co-solvent evaporated: yes
- Details of traps for volatile, if any: Ethylene glycol and 2 x 2N sodium hydroxide
- Indication of test material adsorbing to the walls of test apparatus: No details

PREPARATION OF THIN-LAYER PLATES: Silica gel 60 F254, layer thickness 0.25 mm

REPLICATION
- No. of replicates (dark): 1
- No. of replicates (irradiated): 1

MAINTENANCE OF TEST CONDITIONS SPECIFIED UNDER "DURATION"
- Temperature maintenance method: 25 ± 2.3ºC
- Moisture maintenance method: Flow of pre-wetted air at 5ml/min
Duration:
24 h
Temp.:
25 °C
Reference substance:
no
Dark controls:
yes
Computational methods:
Experimental data were analysed assuming first order kinetics according to the equation: -dc/dt = kc
DT = In 2/k and DT90 = In10/k
Preliminary study:
Run 1 - rangefinding (with soil directly from storehouse):
- DT50 13.0 hours
- DT90 43.1 hours
- K-rate 0.0534 hours
Test performance:
Run 3 - test according to guideline (soil activated four weeks in the laboratory by cultivating grass):
- DT50: 9.7 hours
- DT90 32.4 hours
- K-rate 0.0712 hours

Run 2 - control = without exposure to light (soil activated for two weeks in the laboratory by cultivating grass):
- DT50 5.9 hours
- DT90 19.6 hours
- K-rate 0.01174 hours

Run 4 - test with non-vital (dry) soil
- DT50 101.1 hours
- DT90 335.8 hours
- K-rate 0.0069 hours
DT50:
9.7 h
Test condition:
Test according to guideline (soil activated four weeks in the laboratory by cultivating grass):
DT50:
5.9 h
Test condition:
Control = without exposure to light (soil activated for two weeks in the laboratory by cultivating grass):
DT50:
101.1 h
Test condition:
Test with non-vital (dry) soil
DT50:
2.14 d
Test condition:
Calculated value under Florida summer sunlight
Transformation products:
yes
No.:
#1
No.:
#2
Details on results:
HALF-LIFE

TEST CONDITIONS
- Moisture, temperature, and other experimental conditions maintained throughout the study: Yes

MAJOR TRANSFORMATION PRODUCTS (distinguish between dark and irradiated samples)
- Range of maximum concentrations in % of the applied amount and day(s) of incubation when observed: See Tabls below
- Range of maximum concentrations in % of the applied amount at end of study period: See Tables below

MINOR TRANSFORMATION PRODUCTS (distinguish between dark and irradiated samples)
- Range of maximum concentrations in % of the applied amount and day(s) of incubation when observed: See Tables below
- Range of maximum concentrations in % of the applied amount at end of study period: See Tables below

TOTAL UNIDENTIFIED RADIOACTIVITY (RANGE) OF APPLIED AMOUNT (distinguish between dark and irradiated samples):

EXTRACTABLE RESIDUES(distinguish between dark and irradiated samples)
- % of applied amount at day 0: See Tables below
- % of applied amount at end of study period: See Tables below

NON-EXTRACTABLE RESIDUES(distinguish between dark and irradiated samples)
- % of applied amount at day 0: See Tables below
- % of applied amount at end of study period: See Tables below

VOLATILIZATION (distinguish between dark and irradiated samples)
- % of the applied radioactivity present as volatile organics at end of study: See Tables below

TRANSFORMATION PATHWAY
- Description of biotransformation pathway: Degradation by soil microorganisms
- Figure attached: No

Results with reference substance:
Not applicable

Table 5: Soil Surface Photolysis - Test Run 2: Control (dark)
Time (hours) CGA185072 CGA153433 Bound Residues Volatiles Recovery
0 95.66 0.00 3.04 0.00 98.70
1 90.10 0.00 13.81 0.02 103.93
2 70.63 3.87 20.21 0.03 94.74
4 56.26 6.81 33.68 0.05 96.80
6 47.95 9.50 42.54 0.09 100.08
8 38.64 9.59 46.93 0.14 95.30
24 10.17 14.49 67.66 0.18 92.50
Table 6: Soil Surface Photolysis - Test Run 3: According to Guidelines
Time (hours) CGA185072 CGA153433 Bound Residues Volatiles Recovery
0 95.17 0.00 3.65 0.00 98.82
1 87.97 0.00 12.86 0.00 100.83
2 80.37 0.00 15.42 0.00 95.79
4 65.82 6.00 25.96 0.02 97.80
6 56.19 6.11 31.16 0.06 93.52
8 48.68 6.65 35.44 0.09 90.86
24 32.92 9.82 52.15 0.15 95.01
Table 7: Soil Surface Photolysis - Test Run 4: Dry Soil
Control Dark
Time (hours) CGA 185072 CGA 153433 Bound Residues Volatiles Recovery CGA 185072 CGA 153433 Bound Residues Recovery
0 92.10 n.d.. 3.24 0.00 95.34
2 87.78 n.d. 6.41 0.03 94.22
4 87.76 n.d. 8.12 0.05 95.93
6 87.76 n.d. 8.12 0.05 95.93
8 82.99 n.d. 10.05 0.23 97.84
24 80.16 n.d. 14.58 0.82 95.57 90.74 n.d. 3.96 94.70
Validity criteria fulfilled:
not specified
Conclusions:
The half life (DT50) for the photolysis of cloquintocet-mexyl on a vital silty loam soil was determined to be 9.7 hours. The equivalent DT50 for degradation of the substance in vital soil under Florida summer sunlight conditions was calculated to be 2.14 days. Under the test conditions used the dissipation of cloquintocet-mexyl was primarily by degradation by soil microorganisms.
Executive summary:

In a GLP study compliant to relevant guidelines to determine the soil photolysis of cloquintocet-mexyl under laboratory conditions, the half life (DT50) for photolysis on a vital silty loam soil was determined to be 9.7 hours. For the dry soil the DT50 was 101.1 hours. The equivalent DT50 for degradation of the substance in vital soil under Florida summer sunlight conditions was calculated to be 2.14 days. Under the test conditions used the dissipation of cloquintocet-mexyl was primarily by degradation by soil microorganisms. In viable soils the main degradation products were (5-chloroquinolin-8-yl)oxy]acetic acid and carbon dioxide.

Description of key information

A study of photodegradation in soil was conducted, according to EPA guideline. This study reported a DT50 of 9.7 hours on silty loam soil, under laboratory conditions. The equivalent value for use in the chemical safety assessment is the DT50 under summer sunlight conditions of 2.1 days (latitude 30°N).

Key value for chemical safety assessment

Half-life in soil:
2.1 d

Additional information