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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Study period:
From September 02, 2009 to February 22, 2010
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Deviations:
no
Qualifier:
according to
Guideline:
other: EPA OPPTS 835.1230 (Batch Equilibrium )
Deviations:
no
GLP compliance:
yes
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
yes
Details on study design: HPLC method:
Samples of aqueous supernatants, soils, and extracts not directly needed for analysis were immediately stored at < -5 °C. Samples were typically assayed by LSC and HPLC-RAD on the day of collection from the test system.

The test system consisted of individually capped glass test tubes containing the soil and test solution. An appropriate soil-to-solution ratio for use in the definitive studies was determined in preliminary experiments. During equilibration, the test system was continuously agitated using a mechanical shaker to keep the soil in suspension. All experiments were conducted in duplicate (unless otherwise noted) at a temperature of 20 ± 2 °C in the dark. The temperature of the test system was continuously monitored and recorded. The temperature range during the experimental period was 18.5 °C to 20.8 °C.

LIQUID SCINTILLATION COUNTING (LSC)

Liquid scintillation counting analysis was performed on samples to quantify 14Cradioactivity. LSC analysis was performed using a Beckman LS6000IC (Beckman, Fullerton, CA) Liquid Scintillation Counter. Liquid samples (i.e., water phase and soil extracts) were subsampled by volume in duplicate or triplicate. Ultima Gold™ liquid scintillation cocktail (10 mL) was added to each sample. Solid samples were combusted prior to LSC analysis. Samples were counted for 2 minutes each. Counts per minutes (cpm) were converted to disintegrations per minute (dpm) using a quench curve stored in the instrument microprocessor. The instrument was operated in the background-subtract mode, which automatically subtracted the background value entered into the system from samples being analyzed.

COMBUSTION ANALYSIS

Solid subsamples (soil) were combusted in a Harvey Model OX-500 Biological Oxidizer for 2 minutes. The gaseous effluent from the oxidizer was collected directly into Harvey™ oxidizer LSC cocktail (12 mL). Combustion efficiency was determined with each analysis by combusting triplicate subsamples of control soil spiked with a known amount of 14C-Uracil stock standard. An identical amount of [14C] was added to duplicate vials of the LSC cocktail. Combustion efficiency was calculated as the ratio of the mean dpm in the spiked samples to the mean dpm in the cocktail spikes. Combustion efficiencies were 95.0% or greater. Sample dpm were corrected for combustion efficiency.


HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

HPLC analyses conducted on samples involved radioactivity detection using an in-line radioactivity detector. Radioactivity was detected in the effluent using a 500-μL liquid scintillation cell. The effluent was passed through the UV detector and then through the radioactivity flow detector. Radioactivity in the effluent was detected and quantified with a radioactive flow detector using Ultima-FLO M scintillation cocktail. Percentages of radioactivity in the separated components were quantified by integrating the peaks using the TotalChrom version 6.3.1.00504 software (Perkin Elmer Instruments, Norwalk, CT) or Radiomatic FLO-ONE® for Windows, Version 3.65 software (Perkin Elmer, CT). For selected analyses, recovery of radioactivity from the HPLC column was determined. The amount of radioactivity injected onto the column was determined by LSC analysis of aliquots of the sample. The amount of radioactivity eluting from the column was determined by collecting the eluate from the run followed by LSC analysis. Recovery of radioactivity was calculated as the ratio of the radioactivity collected to the radioactivity injected. The recoveries ranged from 90 to 110%.

Analytical monitoring:
yes
Details on sampling:
DEFINITIVE STUDY – ADSORPTION/DESORPTION ISOTHERMS

Preparation of the Dosing Solutions

The [14C] test substance acetonitrile stock solution was used to prepare individual dosing solutions with target concentrations of 0.030, 0.150, 0.300, 1.500 and 3.000 μg/mL in 60 mL of 0.01 M CaCl2 solution. After allowing the acetonitrile to evaporate under a stream of nitrogen, the [14C] test substance was reconstituted in 60 mL of 0.01 M CaCl2 solution. Each jar was sonicated for ca. 1 minute and vortex mixed. Triplicate aliquots (100 μL) were assayed in duplicate to determine the final concentration of each dosing solution.

The 0.3 μg/mL rate dose solution was co-injected with reference standard and analyzed by HPLC with radiochemical and UV-detection to verify test substance purity and identity after all test systems were treated. The 0.3 μg/mL rate dose solution was chosen because the concentration was intermediate between the other four and the desired injection volume of 950 μL resulted in 153,230 dpm for HPLC-RAD assay. The analysis showed [14C] test substance purity at 95.5%. The pH of the [14C] test substance treatment solutions used for dosing was measured before adding it to the soils. The pH ranged from 6.15 to 6.77 across all five dosing solutions.

Details on matrix:
The soils

The test soils were ordered from Agvise Laboratories (Northwood, ND) and delivered directly to the Test Facility. The test soils were air-dried, passed through a 2-mm sieve, and mixed thoroughly before use.

The soils were designated Sandy Loam RMN-PF, Sandy Loam MSL-PF, Loam HCB-PF, Sand RC-PF and Clay MT-CL-PF. Soil moisture content was determined by weighing three subsamples of sieved soil into pre-weighed aluminum pans. The subsamples were dried 20 hours in an oven at approximately 110 °C, cooled and weighed.

The percent moisture content of the sieved soil was determined as follows:

% Moisture = [((wet weight-dry weight) / dry weight)] x 100
The dry weight equivalent was determined by the equation
Dry Weight Equivalent = ((% soil moisture/100) x target dry weight) + target dry weight

The soils were kept in loosely sealed plastic buckets in an incubator set at 4 ± 2 °C after the soil moisture content was determined. In addition to the microbial biomass carbon analyses performed by Agvise, aerobic microbial population determinations were conducted on the five test soils prior to use in the adsorption/desorption study. The microbial activity of each soil was determined by total plate count analysis of bacteria and fungi. The test soils were determined to be microbially active.

Apparatus
A mechanical shaker (New Brunswick Scientific, Gyratory Shaker Model G76) was used to agitate the test samples. The mechanical device was operated at a speed sufficient to keep the soil in suspension with the 0.01 M CaCl2 solution.
Sample tubes were centrifuged at a speed of ca. 3000 rpm using either a Sorvall RC-5 super speed refrigerated centrifuge or a Beckman TG-6 tabletop centrifuge. Samples were held in a walk-in environmental chamber held at 20 ± 2 °C in the dark. The temperature was continuously monitored and recorded.
Details on test conditions:
Correction for Metabolism

The adsorption and desorption calculations were adjusted to correct for metabolism that occurred during the experimental procedures. The concentrations of test substance in the adsorption and desorption supernatants in μg/mL were determined by multiplying the concentration of radioactivity in each phase by the percentages of test substance in representative HPLC-RAD profiles of the supernatants from the corresponding soils. The concentrations of test substance in the soil in μg/g were determined by multiplying the concentration of radioactivity in the soil by the percentages of the test substance in representative HPLC-RAD profiles of the extracts from the corresponding soils. The concentration of test substance at each timepoint was based on the sum of individual components, therefore the data do not reflect differences in adsorption among test substance components. Adsorption and desorption coefficients corrected for metabolism were calculated from the corrected aqueous and soil phase corrected concentrations. For the corrected adsorption supernatant concentrations, the percent of profile from the 0.3 μg/mL level was applied to all concentrations in a given soil. The concentration of test substance in the adsorption supernatants of both the screening and definitive studies were calculated as follows: % test substance in 0.3 μg/mL ads supernatant profile x conc. rad. in ads supernatant (μg/mL) = conc. test substance in ads supernatant (μg/mL) where, the percent of test substance from replicate 1 was multiplied by the concentration of radioactivity in replicate 1. The same calculation was done for replicate 2.
Key result
Type:
Kd
Value:
151.1 other: mL/g
Matrix:
Sandy Loam (RMN-PF)
% Org. carbon:
2.32
Key result
Type:
Kd
Value:
244.2 other: mL/g
Matrix:
Sandy Loam (MSL-PF)
% Org. carbon:
1.86
Key result
Type:
Kd
Value:
313.5 other: mL/g
Matrix:
Loam (HCB-PF)
% Org. carbon:
3.71
Key result
Type:
Kd
Value:
74.4 other: mL/g
Matrix:
Sand (RC-PF)
% Org. carbon:
0.81
Key result
Type:
Kd
Value:
64.9 other: mL/g
Matrix:
Clay (MT-CL-PF)
% Org. carbon:
0.87
Details on results (HPLC method):
LIQUID SCINTILLATION COUNTING (LSC)
The solubility of the test substance in 0.01 M CaCl2 solution at a nominal concentration of 6 μg/mL was verified over a 24 hour period. The prepared test solution gave a concentration of 6.25 μg/mL in 0.01M CaCl2 as determined by LSC. Three tubes containing the test solution were maintained in the dark at 20 ± 2 °C for 24 hours. The average recovery of radioactivity from the three tubes was equivalent to 6.12 μg/mL (97.9%), demonstrating that the test substance would be soluble at 0.3 μg/mL under the test conditions.

HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)
The radiochemical purity of the [14C] test substance was determined using reverse phase HPLC-RAD to be 97.2% prior to sample dosing in the preliminary studies. The radiochemical purity was reassessed prior to sample dosing in the adsorption kinetics and adsorption/desorption isotherm studies. The [14C] test substance purity was found to be 94.7% and 95.5%, respectively. Although the purity of the test material had decreased slightly during freezer storage, it was judged to be sufficiently pure to proceed with the studies. The identity of the [14C] test substance was confirmed by HPLC co-injection with an authentic standard of nonradiolabeled test substance prior to sample dosing. The individual C5-C9 peaks of the test substance are highly diagnostic for test substance.
Adsorption and desorption constants:
The values for the Freundlich adsorption isotherm parameters, KF(ads), ranged from 74.77 mL/g in the MT-CL-PF soil to 276.18 mL/g in the HCB-PF soil.
The values for the Freundlich desorption isotherm parameters, KF(des), ranged from 55.37 mL/g in the MT-CL-PF soil to 326.44 mL/g in the HCB-PF soil.
Recovery of test material:
Recovery of radioactivity was calculated as the ratio of the radioactivity collected to the radioactivity injected. The recoveries ranged from 90 to 110%.
% Adsorption:
91.1
Remarks on result:
other: Sandy Loam (RMN-PF)
% Adsorption:
93.4
Remarks on result:
other: Sandy Loam (MSL-PF)
% Adsorption:
94.1
Remarks on result:
other: Loam (HCB-PF)
% Adsorption:
87.6
Remarks on result:
other: Sand (RC-PF)
% Adsorption:
85.5
Remarks on result:
other: Clay (MT-CL-PF)
% Desorption:
6.8
Remarks on result:
other: Sandy Loam (RMN-PF)
% Desorption:
5.4
Remarks on result:
other: Sandy Loam (MSL-PF)
% Desorption:
6.9
Remarks on result:
other: Loam (HCB-PF)
% Desorption:
9.7
Remarks on result:
other: Sand (RC-PF)
% Desorption:
18.5
Remarks on result:
other: Clay (MT-CL-PF)
Transformation products:
no
Details on results (Batch equilibrium method):
DEFINITIVE STUDY - ADSORPTION/DESORPTION ISOTHERMS
The adsorption/desorption isotherm experiment was conducted in duplicate using all five test soils at a 1:20 soil-to-solution ratio and [14C] test substance concentrations of 0.003, 0.015, 0.03, 0.15, and 0.3 μg/mL in 0.01 M CaCl2. The adsorption aqueous supernatants and the two aqueous desorption supernatants were analyzed for radioactivity after each was subjected to 6 hours of equilibration.

Adsorption Results
The percent of radioactivity adsorbed (A) during the adsorption interval was calculated for all five soils at all five test solution concentrations (0.003, 0.015, 0.030, 0.150 and 0.300 μg/mL).

Desorption Results
The percent of radioactivity desorbed from the soil during two desorption intervals, DT, was calculated for all five soils at each of the five test solution concentrations

Isotherm Results
The values for the Freundlich adsorption and desorption isotherm parameters, KF, were derived from the linear form of the Freundlich equation.
The Freundlich adsorption isotherm parameters, KF(ads), were corrected for the organic matter and organic carbon content for each soil to calculate the soil sorption coefficients, KFom(ads) and KFoc(ads). The KFom(ads) values ranged from 4037 mL/g in the RMN-PF soil to 6823 mL/g in the MSL-PF soil while the KFoc(ads) values ranged from 6959 mL/g in the RMN-PF soil to 11762 mL/g in the MSL-PF soil.

The values for the Freundlich desorption isotherm parameters, KF(des), ranged from 55.37 mL/g in the MT-CL-PF soil to 326.44 mL/g in the HCB-PF soil. The values for 1/n(des) ranged from 0.8565 in the RC-PF soil to 1.0368 in the HCB-PF soil. The Freundlich desorption isotherm parameters, KF(des), were corrected for the organic matter and organic carbon content for each soil to calculate the soil sorption coefficients, KFom(des) and KFoc(des). The KFom(des) values ranged from 3692 mL/g in the MT-CL-PF soil to 9780 mL/g in the MSL-PF soil while the KFoc(des) values ranged from 6349 mL/g in the MT-CL-PF soil to 16861 mL/g in the MSL-PF soil.
Statistics:
The coefficient of determination values (r2) for the analyses ranged from 0.9958 to 0.9989 for the adsorption phase indicating the Freundlich equation accurately predicts the adsorption of radioactivity over the concentration range studied.

Screening test results:

 

Average Adsorption Coefficients from Screening Study

Soil

Kd (mL/g)

Kom (mL/g)

Koc (mL/g)

Sandy Loam (RMN-PF)

125.4

3135

5405

Sandy Loam (MSL-PF)

186.3

5822

10016

Loam (HCB-PF)

229.6

3588

6189

Sand (RC-PF)

70.8

5055

8737

Clay (MT-CL-PF)

60.8

4050

6983

 

Validity criteria fulfilled:
yes
Conclusions:
Under the study conditions, the average adsorption coefficients from the definitive test, Kd(ads), Kom(ads), and Koc(ads), ranged from 119.2 to 317.5, 4962 to 10417, and 8554 to 17959, respectively, across all five soils and all five concentrations. The average desorption coefficients, Kd(des), Kom(des), and Koc(des), ranged from 87.5 to 338.1, 4133 to 13598, and 7125 to 23444, respectively. The values for the Freundlich adsorption isotherm parameters, KF, KFom, KFoc, and 1/n were derived from the linear form of the Freundlich equation. Average values for each of these parameters across all five soils and all five concentrations ranged as follows: KF(ads)– 74.77 to 276.18, KFom(ads)– 4037 to 6823, KFoc(ads)– 6959 to 11762, and 1/n(ads)– 0.8819 to 0.9784. The average Freundlich desorption isotherm parameters ranged as follows: KF(des)– 55.37 to 326.44, KFom(des)– 3692 to 9780, KFoc(des)– 6349 to 16861, and 1/n(des)– 0.8565 to 1.0368. The results from the isotherm experiments indicate that there is a positive correlation of the adsorption and desorption of the test substance with organic matter and organic carbon in the soil.
Executive summary:

A study was conducted to determine adsorption and desorption properties of [14C] labeled test substance using a batch equilibrium method according to OECD Guideline 106 and EPA OPPTS Method 835.1230, in compliance with GLP. The adsorption and desorption of [14C] test substance was examined on five representative U.S. agricultural soils with varying percent organic carbon ranging from 0.81 to 3.71% and pH values (in 0.01 M CaCl2) ranging from 6.4 to 8.2. In the definitive test, one adsorption experiment and two desorption experiments were performed at 20±2°C using the batch equilibration method on the soils with five concentrations covering two orders of magnitude (0.003 μg/mL – 0.3 μg/mL) of the test substance in 0.01 M calcium chloride at a soil-to-solution ratio of 1:20. The equilibration time for all five soils was determined to be 6 hours to minimize the effect of metabolism on the results. The average percentage of radioactivity adsorbed during the adsorption interval (A) ranged from 85.5 to 94.1% across all five soils at all five test solution concentrations. The average percent of radioactivity desorbed from the soil during two desorption intervals (DT) ranged from 5.4 to 18.5% across all five soils at all five test solution concentrations. Analysis of selected aqueous adsorption and desorption supernatants by HPLC-RAD showed that partial metabolism of the test substance occurred during the equilibration periods amounting to ca. 3 to 5% of the applied dose in the adsorption supernatants and ca. 1 to 5% in the desorption supernatants. In each case, the profiles also showed significant decreases in the C8 and C9 components due to selective adsorption of those components. Analysis of selected soil sample extracts by HPLC-RAD gave percent of profile values for test substance ranging from 76.1 to 94.3%, but with ratios for the components of test substance similar to those of the starting parent material. Values from the supernatant and extract profiles were used to correct the adsorption coefficients for metabolism. No additional corrections were made for the selective adsorption of components in the supernatants. The corrected adsorption coefficient values were higher than the corresponding uncorrected values in all five soils. The corrected desorption coefficient values were higher than the corresponding uncorrected values for four of the five soils, while values were slightly lower for the MT-CL-PF soil. However, because the coefficients calculated from the data not corrected for metabolism are more conservative, and the differences between the uncorrected and metabolism-corrected coefficients are relatively small, the uncorrected values are considered to be the definitive values for the study. Under the study conditions, the average adsorption coefficients from the definitive test, Kd(ads), Kom(ads), and Koc(ads), ranged from 119.2 to 317.5, 4962 to 10417, and 8554 to 17959, respectively, across all five soils and all five concentrations. The average desorption coefficients, Kd(des), Kom(des), andKoc(des), ranged from 87.5 to 338.1, 4133 to 13598, and 7125 to 23444, respectively. The values for the Freundlich adsorption isotherm parameters, KF, KFom, KFoc, and 1/n were derived from the linear form of the Freundlich equation. Average values for each of these parameters across all five soils and all five concentrations ranged as follows: KF(ads)– 74.77 to 276.18, KFom(ads)– 4037 to 6823, KFoc(ads)– 6959 to 11762, and 1/n(ads)– 0.8819 to 0.9784. The average Freundlich desorption isotherm parameters ranged as follows: KF(des)– 55.37 to 326.44, KFom(des)– 3692 to 9780, KFoc(des)– 6349 to 16861, and 1/n(des)– 0.8565 to 1.0368. The results from the isotherm experiments indicate that there is a positive correlation of the adsorption and desorption of the test substance with organic matter and organic carbon in the soil (Walsh, 2010).

Description of key information

A study was conducted to determine adsorption and desorption properties of [14C]labeled test substance using a batch equilibrium method according to OECD Guideline 106 and EPA OPPTS Method 835.1230, in compliance with GLP.

The adsorption and desorption of [14C] test substance was examined on five representative U.S. agricultural soils with varying percent organic carbon ranging from 0.81 to 3.71% and pH values (in 0.01 M CaCl2) ranging from 6.4 to 8.2. In the definitive test, one adsorption experiment and two desorption experiments were performed at 20±2°C using the batch equilibration method on the soils with five concentrations covering two orders of magnitude (0.003 μg/mL – 0.3 μg/mL) of the test substance in 0.01 M calcium chloride at a soil-to-solution ratio of 1:20. The equilibration time for all five soils was determined to be 6 hours to minimize the effect of metabolism on the results. The average percentage of radioactivity adsorbed during the adsorption interval (A) ranged from 85.5 to 94.1% across all five soils at all five test solution concentrations. The average percent of radioactivity desorbed from the soil during two desorption intervals (DT) ranged from 5.4 to 18.5% across all five soils at all five test solution concentrations. Analysis of selected aqueous adsorption and desorption supernatants by HPLC-RAD showed that partial metabolism of the test substance occurred during the equilibration periods amounting to ca. 3 to 5% of the applied dose in the adsorption supernatants and ca. 1 to 5% in the desorption supernatants. In each case, the profiles also showed significant decreases in the C8 and C9 components due to selective adsorption of those components. Analysis of selected soil sample extracts by HPLC-RAD gave percent of profile values for test substance ranging from 76.1 to 94.3%, but with ratios for the components of test substance similar to those of the starting parent material. Values from the supernatant and extract profiles were used to correct the adsorption coefficients for metabolism. No additional corrections were made for the selective adsorption of components in the supernatants. The corrected adsorption coefficient values were higher than the corresponding uncorrected values in all five soils. The corrected desorption coefficient values were higher than the corresponding uncorrected values for four of the five soils, while values were slightly lower for the MT-CL-PF soil. However, because the coefficients calculated from the data not corrected for metabolism are more conservative, and the differences between the uncorrected and metabolism-corrected coefficients are relatively small, the uncorrected values are considered to be the definitive values for the study. Under the study conditions, the average adsorption coefficients from the definitive test, Kd(ads), Kom(ads), and Koc(ads), ranged from 119.2 to 317.5, 4962 to 10417, and 8554 to 17959, respectively, across all five soils and all five concentrations. The average desorption coefficients, Kd(des), Kom(des), andKoc(des), ranged from 87.5 to 338.1, 4133 to 13598, and 7125 to 23444, respectively. The values for the Freundlich adsorption isotherm parameters, KF, KFom, KFoc, and 1/n were derived from the linear form of the Freundlich equation. Average values for each of these parameters across all five soils and all five concentrations ranged as follows: KF(ads)– 74.77 to 276.18, KFom(ads)– 4037 to 6823, KFoc(ads)– 6959 to 11762, and 1/n(ads)– 0.8819 to 0.9784. The average Freundlich desorption isotherm parameters ranged as follows: KF(des)– 55.37 to 326.44, KFom(des)– 3692 to 9780, KFoc(des)– 6349 to 16861, and 1/n(des)– 0.8565 to 1.0368. The results from the isotherm experiments indicate that there is a positive correlation of the adsorption and desorption of the test substance with organic matter and organic carbon in the soil (Walsh, 2010).

Key value for chemical safety assessment

Koc at 20 °C:
7 466

Additional information