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Phototransformation in soil

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Reference
Endpoint:
phototransformation in soil
Type of information:
experimental study
Adequacy of study:
key study
Study period:
1995-12-08 - 1996-04-24
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
Remarks:
U.S. EPA Good Laboratory Practice Standards (40 CFR Part 160)
Specific details on test material used for the study:
Purity analyses were performed by proton nuclear magnetic resonance spectroscopy (NMR) on the actual [14C]ferbam test substance used for dosing.
Radiolabelling:
yes
Analytical monitoring:
not specified
Analytical method:
other: reverse-phase HPLC, TLC
Details on sampling:
Duplicate irradiated and dark control dishes were sampled following 1,2, 3, 7 and 15 days of incubation. One set of duplicate dishes were sampled on Day 0 and therefore not placed in the photolysis apparatus. Upon removal of incubated samples from the chambers (irradiated and dark control), soil samples were extracted immediately.
At each sampling interval, appropriate irradiated and dark control samples were removed from the chambers. Soil was scraped from each dish and transferred to a 50-ml Teflon® centrifuge tube using glass funnels. The dish was rinsed with 6 ml of chloroform that was then added to the tube. The tube was shaken on a wrist-action shaker for 30 minutes, then centrifuged for approximately 10 minutes at 10,000 rpm. The extract was decanted into a foil-covered, silylated 22-ml vial. The procedure was repeated with an additional 6 ml of chloroform. The second chloroform extract was pooled with the first. The extract was filtered through a Whatman No. 1 filter (Maidstone, England) into a graduated cylinder and the final volume adjusted to 12 ml either by rinsing the extracted soil (Day 0 only) with additional chloroform or by addition of chloroform directly to the extract (referred to as Extract 1). Aliquots were taken for radioassay (3 x 100 uL) and HPLC analysis. The remaining extract was stored in a silylated, foil-covered 22-ml vial under freezer conditions (-20°C).
Extracted soil was subjected to further extraction with 10 ml of chloroform:methanol (1:1, v:v) for 30 minutes using a wrist-action shaker (referred to as Extract 2). The chloroform:methanol was first used to rinse the photolysis dish and graduated cylinder (used for measuring the first extract). After shaking and centrifugation, Extract 2 was filtered through the same filter used for Extract 1 into a graduated cylinder and the final volume adjusted to 10 ml by addition of chloroform:methanol. Day 0, Extract 2 was not filtered due to a lack of particulate matter. The extract was transferred to a silylated, foil-covered 22-ml vial. Aliquots were taken for radioassay (3 x 100 uL) and HPLC analysis. The remaining extract was stored in a silylated, foil-covered 22-ml vial under freezer conditions (approximately -20°C).
Post extracted soil was either air-dried immediately or stored under freezer conditions until removed for air drying for subsequent combustion. Post-extracted soils were air dried with the aid of a fan, mixed, subsampled (3 x 0.10 g) and combusted to determine soil bound radiocarbon. Filters used for filtering extracts were cut into small pieces and the total filter combusted.
Trap Solution Maintenance and Sampling
Trap solutions were changed periodically and at each sampling interval during the study. Fresh traps were installed containing either 50 or 70 mis of methanolic KOH, 50 mis of ethanol and either 30 or 50 mis of ethylene glycol. Since the initial methanolic KOH trap decreased in volume during continuous flushing, with condensation of the methanol primarily in the ethylene glycol trap, volumes of the methanolic KOH and ethylene glycol traps were adjusted to either accommodate the time between sample intervals, or to maximize the time between trap changes.
Trapped radiocarbon was found primarily in the methanolic KOH solutions with the initial trap containing most of the radiocarbon. In addition to radiocarbon trapped in Meriden, CT). Output signal s from the detectors were acquired and processed on a personal computer running FLO-ONE for Windows, version 3.1. Data acquisition occurred at 10 samplings per minute during the HPLC run.
Details on soil:
Sandy loam was obtained by PTRL East, Inc. from Fayette County, Kentucky.
The soil was passed through a 2-mm sieve then allowed to air dry. Individual slurries were prepared in each test dish by adding 3.0 ml of sterile (autoclaved, 121°C, 15 psi, 30 minutes) HPLC-grade water to 3.1 g of sandy loam. Dishes were allowed to air dry leaving a uniform layer approximately 0.5 mm in thickness. Sterilized (autoclaved) HPLC-grade water, 351 JLLI, was added to each soil dish just prior to dosing to adjust the moisture to 75% of field capacity at 0.33 bar.
Physicochemical Characteristics of Soil:
Parameter Results / Units
pH 6.8
Texture class: Sandy Loam
Sand 67 %
Silt 23 %
Clay 10 %
Organic Matter 1.9 %
Cation Exchange
Capacity 5.5 meq M / 100 g
Field Capacity (at 0.33bar) 15.1 %
Bulk Density (g/cm3) 1.24
Soil was collected from 0-6'' horizon
Collection Site: Fayette County, Kentucky
USDA Soil classification: Sandy Loam
Soil viability was evaluated prior to the incubation period by enumerating the total colony forming units (CFU) of aerobic bacteria, actinomycetes and fungi. Aerobic bacteria were enumerated on trypticase soy agar (TSA), actinomycetes on actinomycetes isolation agar (AIA) and fungi on potato dextrose agar (PDA).
Soil samples were serially diluted in sterile water from 10E-1 to 10E-3. Aliquots (1-ml) from the 10E-1 to 10-6 dilutions were then plated in duplicate on trypticase soy agar and actinomycetes isolation agar. Aliquots (1-ml) from the 10"1 to 10"3 dilutions were plated in duplicate on potato dextrose agar. The culture dishes were incubated at approximately 35 °C for 1 day to detect aerobic bacteria and 2 days to detect actinomycetes and at approximately 25°C for 2 days to detect fungi.
Light source:
Xenon lamp
Light spectrum: wavelength in nm:
>= 250 - <= 750
Details on light source:
The temperature-controlled chambers were specially designed stainless steel units (11 1/4" x 4 5/8" interior dimensions for the irradiated chamber, 13" x 2 5/8" for the dark control chamber) covered with either a quartz plate (light exposed) or a Pyrex® glass plate covered with black rubber to prevent exposure to light (dark control). Each chamber was equipped with an exterior jacket through which ethylene glycol circulated. Coolant for the irradiated temperature-controlled chamber was temperature maintained and circulated using a Neslab Coolflow Refrigerated Recirculator (Model HX-300, Neslab Instruments, Inc., Portsmouth, NH). The coolant for the dark control chamber was temperature maintained and circulated using a Lauda Constant Temperature Circulator (Model RMT-20, Brinkman Instruments, Inc., Westburg, NY). Temperatures were monitored at intervals of 10 seconds, then averaged and recorded at 10-minute intervals throughout the study using a Campbell Scientific Model 21X Datalogger and a thermistor type temperature probe (Model 107B, Campbell Scientific, Inc., Logan, UT). A modified probe was attached with epoxy resin beneath the soil surface in a control Petri dish positioned in the irradiated soil chamber. A second temperature probe was attached with epoxy resin directly to the surface of the dark control soil chamber.
The Petri dishes containing treated soil layers to be irradiated were placed in the temperature-controlled soil chambers and these, in turn, placed in the irradiation chamber which contained two xenon lamps (1500 W each) filtered through a borosilicate glass cutoff sleeve for removal of light with wavelengths less than 290 nm. The dishes were oriented at 30° with respect to the xenon lamps and irradiated continuously for 15 days, equivalent to 30 days natural irradiation. Ambient air was drawn through a Gelman bacterial air vent (Model No. 4210, Gelman Sciences, Ann Arbor, MI) at approximately 50 to 70 ml per minute, then bubbled through a humidifying flask containing HPLC-grade water. The humidified air was continuously drawn through the soil chambers into a series of traps (dispersion tubes) containing methanolic IN KOH (two traps), ethanol and ethylene glycol for collection of volatile compounds. Separate sets of traps were used for the irradiated and dark control chambers. Traps were filled with approximately 50 ml of trapping solution at study initiation.
Intensity of the xenon lamps was measured using an International Light Radiometer (Model EL 1700, International Light, Inc., Newburyport, MA) equipped with a Model SUD 400 photodetector enclosed in a quartz casing (1/8" thick) located within the irradiation chamber. Intensity measurements were monitored at intervals of 10 seconds and recorded at 10-minute intervals with the Campbell Scientific Model 21X Datalogger. Intensity measurements were used to confirm the output of the xenon lamps during the course of the study and to allow calculation of a total intensity for the study period.
The spectral distribution of emitted light from the xenon lamps was determined prior to the beginning and at the end of the study to assess any effects of lamp aging during the course of the study. Spectral intensity was measured at each 10-nm increment from power supply and an IL 1700 radiometer. These instruments were obtained from International Light, Inc. (Newburyport, MA). These measurements present monochromatic intensity at each specified wavelength as opposed to the integrated intensity in the 10-nm window.
Details on test conditions:
Preparation of Test Substance and Dosing of the Test System
A solution of [14C]ferbam (specific activity of 59.96 mCi/mmole, 144.14 uCi/mg) was prepared in chloroform-d3 and assayed for identity and purity by proton NMR. Subsequently, the contents of the tube were nitrogen evaporated and solubilized in acetonitrile. A 50-uI Hamilton syringe was used to evenly disperse 50 ul of this solution over the surface of each soil-layered dish containing 3.1 g of sandy loam. The concentration of [14C]ferbam was determined from radioassay of diluted aliquots of the treatment solution (50 uL in 10 ml of acetonitrile, 100-uL aliquots radioassayed) taken prior to dosing (119,859 dpm/100 uL) (pre-dose), following dosing of dishes designated for irradiation (123,012 dpm/100 uL) (mid-dose) and following dosing of dishes designated as dark controls (121,308 dpm/100 uL (post-dose). The average concentration of [14C]ferbam was 121,393 dpm/100 uL, equivalent to a dose rate of 12.2 ppm.

Test System and Sample Identification
The test system consisted of dishes containing 3.1 g of sandy loam each treated with [14C]ferbam housed in chambers capable of being irradiated or maintained in darkness. Each dish was identified by PTRL East, Inc. project number, sample identification, personnel initials and date of application.
Duration:
15 d
Temp.:
25 °C
Initial conc. measured:
12.2 ppm
Reference substance:
not specified
Dark controls:
yes
Computational methods:
Statistical Methods
Means, standard deviations, standard errors and linear regression were the only statistical analyses performed on the data.
Preliminary study:
not specified
Test performance:
not specified
Remarks:
see 'Details on results'
Remarks on result:
other: see 'Details on results'
Remarks on result:
other: see 'Details on results'
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Details on results:
Justification of the Test System
A continuous irradiation regime was used to maximize photolytic degradation relative to hydrolysis and/or microbial degradation on soil layers.
Purity of r14C1Ferbam
As a result of the instability of ferbam in either reverse- or normal-phase HPLC or TLC chromatographic systems, purity of [14C]ferbam was established by NMR spectroscopy prior to application to soil. The actual contents of the NMR tube containing [14C]ferbam were subsequently used for preparation of the dosing solution. The NMR spectra of the [14C]ferbam used is presented in Figure 5. Non-radiolabelled reference substances were analyzed qualitatively by HPLC prior to initiation of the study and following the last sampling interval for determination of stability at the test site.
Exposure of Test Solution to Xenon Light
Irradiated and dark control samples were placed in the soil chambers (irradiated samples under xenon lamps 24 hours per day) for 15 days (March 19, 1996 -April 3, 1996), equivalent to a 30-day natural sunlight exposure. Duplicate samples were removed for analysis at each sample interval. The mean incubation temperature was 25.1 ± 0.0°C (mean ± standard error) for irradiated samples and 24.9 ± 0.0°C for dark controls. The average light intensity per day for the entire study was 5,551 ± 12 uW/cm2 (mean ± standard error). Light energy, expressed as W*min/cm2/day, was 8.0 ± 0.0 W*min/cm2/day (mean ± standard error). For the 15-day study, cumulative light energy was 119.9 W*min/cm2.
Material Balance of f14C1Ferbam Throughout the Study Period
Total radiocarbon recoveries (mean ± standard deviation) for irradiated and dark control samples were 98.1 ± 2.6% and 97.7 ± 2.7%, respectively.
Degradation of l"14C1Ferbam
The most notable differences in degradation of ferbam between irradiated and dark control samples were the extent of formation of volatile products and difference in soil-bound radiocarbon following incubation. Following 15 days, volatile products accounted for 38.5% of the applied radiocarbon for irradiated samples compared to 15.8% for samples maintained in darkness. Conversely, bound radiocarbon was lower for irradiated samples following 15 days of incubation (41.3%, mean of replicates) versus dark controls (58.9%). Extractable radiocarbon, although decreasing throughout the incubation period, was comparable between irradiated and dark control samples. At Day 15, extractable radiocarbon accounted for 17.2 and 20.7% of the applied radiocarbon for irradiated and dark control samples (mean of replicates), respectively. Since extractable radiocarbon was comparable both in amount and composition, the higher volatile product formation for irradiated samples indicated decomposition of soil-bound radiocarbon. Quantitative assessment of the photolytic contribution to overall degradation could be performed only by comparison of rates of formation of volatile products. The rate of formation of volatile products (captured in trap solutions), equivalent to the rate of degradation of total radiocarbon to volatile products, was 0.0314/days (r2 = 0.9921) for irradiated samples and 0.0101/days (r2 = 0.8199) for dark controls. Respective half-lives for formation of volatile products were 22.1 and 68.8 days. The rate of formation of volatile products attributed to photolysis (rate for irradiated samples - rate for dark controls) was 0.0213/days (equivalent to a photolytic half-life contribution of 32.6 days).
Quantitative Characterization of [14C[Ferbam and Its Dégradâtes
Ferbam degraded rapidly on soil with formation of TMTD and volatile products. HPLC radiochromatograms of extracts of Day 1 and subsequent intervals did not show any chromatographic indication of the presence of ferbam in incubated solutions, i.e., no broad early-eluting region on HPLC or extraneous peaks characteristic of ferbam on-column degradation. Day 0 samples, injected without fortification with TMTD, showed the presence of an additional peak eluting beyond the retention time of TMTD and low column recoveries (70 - 75%). Upon fortification and re-injection, only the TMTD peak was present. TMTD remained the only extractable product throughout the incubation period. Minor unknown degradates were also present in both irradiated and dark control samples. The maximum amount of any single degrádate in any sample was 1.2% of the applied radiocarbon (Day 0, Replicate A).
Degradation to volatile products was observed under both irradiated and dark conditions although to a greater extent under irradiated conditions. Following one day of incubation, trapped volátiles accounted for 5.6% of the applied radiocarbon for irradiated samples and 4.2% for dark controls. Following 15 days of exposure, volatile products accounted for 38.5% of the applied radiocarbon for irradiated samples versus 15.8% for dark controls. Barium chloride precipitation of the methanolic KOH trap solutions and water rinses of the traps indicated that both CS2 and CO2 were present. In addition to greater formation of volatile products under irradiated conditions, the relative amounts of 14C-CS2 and 14C-CO2 were different for irradiated and dark control samples. Relative amounts of 14C-CS2 and 14C-CO2 were comparable throughout the study for dark controls. Following 15 days of incubation, 14C-CS2 accounted for 8.0% of the applied radiocarbon in dark controls versus 7.6% 14C-CO2 (approximately 1.05:1) whereas for irradiated samples, 14CS2 accounted for 22.7% versus 15.7%14CC>2 (approximately 1.45:1). Degrádate profiles for irradiated and dark control samples are presented in Figures 12 and 13.
Two one-dimensional TLC systems were used to confirm the HPLC characterization of TMTD in Extracts 1 and 2 of both irradiated and dark control samples Degradation to volatile products was observed under both irradiated and dark conditions although to a greater extent under irradiated conditions. Following one day of incubation, trapped volátiles accounted for 5.6% of the applied radiocarbon for irradiated samples and 4.2% for dark controls. Following 15 days of exposure, volatile products accounted for 38.5% of the applied radiocarbon for irradiated samples versus 15.8% for dark controls. Barium chloride precipitation of the methanolic KOH trap solutions and water rinses of the traps indicated that both CS2 and CO2 were present. Distribution of radiocarbon in methanolic KOH trap solutions is presented in Table XI. In addition to greater formation of volatile products under irradiated conditions, the relative amounts of 14CS2 and 14C02 were different for irradiated and dark control samples. Relative amounts of 14CS2 and 14C02 were comparable throughout the study for dark controls. Following 15 days of incubation, 14CS2 accounted for 8.0% of the applied radiocarbon in dark controls versus 7.6% 14C02 (approximately 1.05:1) whereas for irradiated samples, 14CS2 accounted for 22.7% versus 15.7%14CC>2 (approximately 1.45:1).
Two one-dimensional TLC systems were used to confirm the HPLC characterization of TMTD in Extracts 1 and 2 of both irradiated and dark control samples of Day 15, Replicates A and B. Integration summaries derived from scraping radioactive spots from the plates, solubilization of radiocarbon and subsequent radioassay by LSC are presented in Appendix 2. Plate recoveries, obtained by summing the radiocarbon in each sample's lane of the TLC plates, were <90% for irradiated samples (72.9 - 84.9%). The loss of radiocarbon was attributed to the volatility of TMTD. Thus, relative amounts of radiocarbon in each spot or zone, presented in the integration summaries, may understate the amount of TMTD present and overstate the relative amounts of other products. TLC confirmed the presence of TMTD as the major component in both irradiated and dark control samples. In the toluenermethanol system, considerably greater degradation was observed than with chloroform:acetonitrile:methanol. Some origin-bound radiocarbon was observed as well as one additional spot in significant amount chromatographing slightly below TMTD. Although this spot cochromatographs with TMTM, the presence of TMTM was excluded by HPLC analysis; TMTM was adequately separated on HPLC from TMTD and other available standards to provide unequivocal chromatography if present. In summary, TLC provided a qualitative confirmation of the HPLC characterization of TMTD. Although the presence of other degradation products cannot be excluded, it is suspected that the other products observed in the toluene:methanol TLC system arose from degradation of TMTD by sample matrix upon development in this TLC system.
In summary, the only extractable product following application of ferbam to soil by HPLC analysis was TMTD. TLC qualitatively confirmed the HPLC characterization of TMTD as the major component of soil extracts. Relative amounts of TMTD extractable from soil were comparable for irradiated and dark control samples. A photolytic contribution to degradation was observed in the amount and composition of volatile products and extractability of ferbam dégradâtes. Volatile product formation was greater under irradiated conditions with relatively greater amounts of 14C-CS2 versus 14C-CO2. Soil-bound radiocarbon was lower for irradiated samples indicating that volatile products were formed from radiocarbon bound to soil rather than extractable TMTD.
Sample Storage Stability
Extraction of soil with chloroform and chloroform:methanol was performed on the same day as sample collection. HPLC analyses were performed as soon as possible after sample collection. Samples were maintained under freezer conditions for any interim storage. With the exception of Day 0 and 3, all chloroform extracts (Extract 1) were injected on the same day as sampling. Day 0 chloroform extracts were injected on the day of sampling (no TMTD fortification) and two days later (fortified with TMTD). Day 3 chloroform extracts were injected following two days of storage. Likewise, the Day 3 chloroform:methanol extracts (Extract 2) were analyzed following two days of storage. Chlorofornrmethanol extracts of Day 1 (dark controls only) and Day 7 (one dark control replicate) were re-injected following 14 and 8 days of storage to improve column recoveries. Day 0 chloroform:methanol extracts were injected following one day of storage (without TMTD fortification) and re-injected following 15 days of storage (with TMTD fortification). With the exception of Day 0 samples where the peak eluting after TMTD disappeared upon fortification with TMTD, all samples showed the same degrádate profile following storage as the initial analysis.
Results with reference substance:
not specified
Validity criteria fulfilled:
yes
Conclusions:
Microbially active sandy loam, treated with [14C]ferbam at 12.2 ppm, was irradiated for 15 days in artificial (xenon) light. Samples maintained under the same conditions except in darkness served as controls. The only extractable product following application of ferbam to soil by HPLC analysis was tetramethylthiuram disulfide (TMTD). TLC qualitatively confirmed the presence of TMTD as the major component of soil extracts arising from ferbam degradation. Relative amounts of TMTD extractable from soil were comparable for irradiated and dark control samples. A photolytic contribution to degradation was observed in the amount and composition of volatile products and extractability of ferbam dégradâtes. Volatile product formation was greater under irradiated conditions with the half-lives for formation of volatile products at 22.1 days and 68.8 days for irradiated and dark control samples, respectively. In addition, irradiated conditions yielded relatively greater amounts of14C-CS2 versus 14C-CO2. Soil-bound radiocarbon was lower for irradiated samples indicating that volatile products were formed from radiocarbon bound to soil rather than extractable TMTD. The results of this study indicate that photolysis would contribute to overall dissipation of ferbam from soil.
Executive summary:

This study was designed and conducted according to the U.S. EPA Pesticide Assessment Guidelines, Subdivision N, Series 161-3, to establish the significance of photolysis on soil surfaces in artificial light (xenon) as a route of degradation for ferbam and to quantitate any degradation products formed.

Ferbam was sufficiently unstable in either reverse- or normal-phase high performance liquid chromatography (HPLC) or thin layer chromatography (TLC) to preclude these methods for determination of purity of the test substance. As a result of this instability, the purity of [14C]ferbam was determined in a chloroform-d3 solution by proton nuclear magnetic resonance spectroscopy (NMR). Aliquots of a [14C]ferbam solution in acetonitrile (50 ul) were applied to moist sandy loam films prepared in Pyrex® petri dishes (3.1 g/dish). A treatment rate of 12.2 ppm was determined from radioassay of aliquots of the treatment solution taken prior to dosing (pre-dose), following dosing of dishes designated for irradiation (mid-dose) and following dosing of dishes designated as dark controls (post-dose). Two dishes were chosen as Day 0, Replicates A and B, and were sampled immediately following dosing of all dishes. Of the remaining dishes, four were designated for each sampling interval (two as irradiated (light exposed) samples and two as dark control samples). Irradiated samples were subjected to continuous irradiation with light from a xenon bulb for 15 days. Each set of samples (irradiated and dark controls) were continuously purged with humidified air under negative pressure with the air being passed through a series of volatile collection traps. Traps consisted of two methanolic KOH traps (IN KOH), one ethanol trap and one ethylene glycol trap in series. Temperatures (mean ± standard error) of irradiated and dark control samples were maintained at 25.1 ± 0.0°C and at 24.9 ± 0.0°C, respectively. Sampling was performed following 1, 2, 3, 7 and 15 days of incubation. At sampling, soil was removed from the dishes and extracted with chloroform followed by extraction with chloroformrmethanol. Quantitative analysis of soil extracts was determined by reverse-phase high performance liquid chromatography (HPLC). Degradate identity was confirmed by thin layer chromatography (TLC).

The only extractable product following application of ferbam to soil by HPLC analysis was TMTD. TLC qualitatively confirmed the presence of TMTD as the major component of soil extracts arising from ferbam degradation. Relative amounts of TMTD extractable from soil were comparable for irradiated and dark control samples. However, a photolytic contribution to degradation was observed in the amount and composition of volatile products and extractability of ferbam dégradâtes. Volatile product formation was greater under irradiated conditions with the half-lives for formation of volatile products at 22.1 days and 68.8 days for irradiated and dark control samples, respectively. In addition, irradiated conditions yielded relatively greater amounts of 14C-CS2 versus 14C-CO2. Soil-bound radiocarbon was lower for irradiated samples indicating that volatile products were formed from radiocarbon bound to soil rather than extractable TMTD. The results of this study indicate that photolysis would contribute to overall dissipation of ferbam from soil.

Total radiocarbon recovery (mean ± standard deviation) for irradiated samples was 98.1 ± 2.6%. For dark control samples, radiocarbon recovery from the test system was 97.7 ± 2.7%.

Description of key information

This study was designed and conducted according to GLP and the U.S. EPA Pesticide Assessment Guidelines, Subdivision N, Series 161-3, to establish the significance of photolysis on soil surfaces in artificial light (xenon) as a route of degradation for ferbam and to quantitate any degradation products formed. The results of this study indicate that photolysis would contribute to overall dissipation of ferbam from soil.

Key value for chemical safety assessment

Additional information

Key: Nixon, 1996

This study was designed and conducted according to the U.S. EPA Pesticide Assessment Guidelines, Subdivision N, Series 161-3, to establish the significance of photolysis on soil surfaces in artificial light (xenon) as a route of degradation for ferbam and to quantitate any degradation products formed.

Ferbam was sufficiently unstable in either reverse- or normal-phase high performance liquid chromatography (HPLC) or thin layer chromatography (TLC) to preclude these methods for determination of purity of the test substance. As a result of this instability, the purity of [14C]ferbam was determined in a chloroform-d3 solution by proton nuclear magnetic resonance spectroscopy (NMR). Aliquots of a [14C]ferbam solution in acetonitrile (50 ul) were applied to moist sandy loam films prepared in Pyrex® petri dishes (3.1 g/dish). A treatment rate of 12.2 ppm was determined from radioassay of aliquots of the treatment solution taken prior to dosing (pre-dose), following dosing of dishes designated for irradiation (mid-dose) and following dosing of dishes designated as dark controls (post-dose). Two dishes were chosen as Day 0, Replicates A and B, and were sampled immediately following dosing of all dishes. Of the remaining dishes, four were designated for each sampling interval (two as irradiated (light exposed) samples and two as dark control samples). Irradiated samples were subjected to continuous irradiation with light from a xenon bulb for 15 days. Each set of samples (irradiated and dark controls) were continuously purged with humidified air under negative pressure with the air being passed through a series of volatile collection traps. Traps consisted of two methanolic KOH traps (IN KOH), one ethanol trap and one ethylene glycol trap in series. Temperatures (mean ± standard error) of irradiated and dark control samples were maintained at 25.1 ± 0.0°C and at 24.9 ± 0.0°C, respectively. Sampling was performed following 1, 2, 3, 7 and 15 days of incubation. At sampling, soil was removed from the dishes and extracted with chloroform followed by extraction with chloroformrmethanol. Quantitative analysis of soil extracts was determined by reverse-phase high performance liquid chromatography (HPLC). Degradate identity was confirmed by thin layer chromatography (TLC).

The only extractable product following application of ferbam to soil by HPLC analysis was TMTD. TLC qualitatively confirmed the presence of TMTD as the major component of soil extracts arising from ferbam degradation. Relative amounts of TMTD extractable from soil were comparable for irradiated and dark control samples. However, a photolytic contribution to degradation was observed in the amount and composition of volatile products and extractability of ferbam degradates. Volatile product formation was greater under irradiated conditions with the half-lives for formation of volatile products at 22.1 days and 68.8 days for irradiated and dark control samples, respectively. In addition, irradiated conditions yielded relatively greater amounts of 14C-CS2 versus 14C-CO2. Soil-bound radiocarbon was lower for irradiated samples indicating that volatile products were formed from radiocarbon bound to soil rather than extractable TMTD. The results of this study indicate that photolysis would contribute to overall dissipation of ferbam from soil.

Total radiocarbon recovery (mean ± standard deviation) for irradiated samples was 98.1 ± 2.6%. For dark control samples, radiocarbon recovery from the test system was 97.7 ± 2.7%.

Additional Information: Carpenter, 1987

A photodegradation study was conducted according to EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil) with 14C-Ferbam applied to the surface of soil and exposed to artificial light in an effort to estimate the photolysis rate constant, half-life, the rate of formation of volatiles and identification of major photolysis products for the photolysis reaction.

Study soil samples were dosed at 2005797 DPM/g with 14C-Ferbam and the 14C-activity was monitored over a course of 48 hours. Radioanalysis demonstrated an exponential decrease in 14C-activity bound to the soil and a corresponding increase in the 14C-activity volatilized from the soil. Based on the levels of 14C-residues, an estimated rate constant and half-life were calculated to be: Exposed: k = -0.00871 hoursE-1 , t1/2 = 79.6 hours and Dark: k = -0.00295 hoursE-1 , t1/2 = 235 hours. The rate of formation of_volatiles was +0.0912 hoursE-1 for the exposed samples and +0.107 hoursE-1 for the dark samples. These rates are based on 48 hours continuous exposure of the samples to an Atlas Xenon Arc Light System which is one-half the intensity of sunlight. The percent of the initial dose found in the exposed volatiles after 48 hours was 10.7 percent and 6.52 percent of the initial dose was found in the dark volatiles after 48 hours. One photolysis product was identified as CS2 which was 9.55 of the initial dose in the exposed samples and 5.61 of the initial dose in the dark samples. Recovery of the applied activity averaged 95.3% for all samples.