Methomyl

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: U.S. EPA Subdivision N 163-1

Deviations:

no

GLP compliance:

yes

Type of method:

batch equilibrium method

Media:

soil

Specific details on test material used for the study:

Radio labelled test substance:
Methomyl
Position of radiolabel: *[1-14C]
Specific activity: 42.96 μCi/mg (95400 dpm/μg)
Batch number: 555
Radiochemical purity: >95.0%

Non-radiolabelled test substance:
Methomyl
Batch number: DPX-X1179-357
Purity: >99.0%

Radiolabelling:

yes

Test temperature:

20°C

Analytical monitoring:

yes

Details on sampling:

For samples of all soil types prepared at the highest test substance concentration the supernatants from the single vessels removed after the adsorption phase and the duplicate vessels removed after the second desorption phase of the main experiment were analysed by TLC (solvent system 1) to establish the proportion of test substance in these samples. Additionally, the supernatants from the Speyer 2.2 samples equilibrated for 8 hours were analysed by TLC. The soil remaining after all the required adsorption and desorption phases had been carried out on each sample of soil from the highest test substance concentration tested was extracted. Therefore, for each soil type one sample was extracted after the adsorption phase and the remaining two samples were extracted after the desorption phase. Additionally, samples of Speyer 2.2 soil that had been equilibrated for 8 hours were extracted. Representative pools (25% by volume) were made of the soil extracts. Samples of the pools were analysed directly by TLC (solvent system 1) with no concentration. The soil residues remaining after extraction was complete were allowed to air-dry. Their total weights were measured and triplicate portions (ca. 0.4-0.5 g) were taken for combustion and radioassay by LSC.

Matrix no.:

#1

Matrix type:

sandy loam

% Clay:

5.2

% Silt:

24.4

% Sand:

70.4

% Org. carbon:

2.09

pH:

6.2

CEC:

9.66 meq/100 g soil d.w.

Bulk density (g/cm³):

1.11

Matrix no.:

#2

Matrix type:

loam

% Clay:

12.8

% Silt:

42.8

% Sand:

44.4

% Org. carbon:

0.99

pH:

5.1

CEC:

4.41 meq/100 g soil d.w.

Bulk density (g/cm³):

1.3

Matrix no.:

#3

Matrix type:

silty clay loam

% Clay:

36.8

% Silt:

48.8

% Sand:

14.4

% Org. carbon:

2.27

pH:

6

CEC:

29.5 meq/100 g soil d.w.

Bulk density (g/cm³):

1.19

Matrix no.:

#4

Matrix type:

sandy loam

% Clay:

10.8

% Silt:

36.8

% Sand:

52.4

% Org. carbon:

1.05

pH:

8.1

CEC:

5.61 meq/100 g soil d.w.

Bulk density (g/cm³):

1.23

Matrix no.:

#5

Matrix type:

loam

% Clay:

17.2

% Silt:

38.4

% Sand:

44.4

% Org. carbon:

1.1

pH:

6.6

CEC:

9.97 meq/100 g soil d.w.

Bulk density (g/cm³):

1.31

Details on matrix:

Five test soils covering a wide range of agricultural soil types were used, namely:
Matrix no. 1: Speyer 2.2: Sandy loam, Speyer Institute, Germany
Matrix no. 2: Mattapex: Loam, Rock Hall, Maryland, USA
Matrix no. 3: Drummer: Silty clay loam, Rochelle, Illinois, USA
Matrix no. 4: Nambsheim: Sandy loam, Nambsheim, France
Matrix no. 5: Flakkebjerg: Loam, Flakkebjerg, Denmark
- All soils were passed through a 2-mm sieve and allowed to air dry at ambient temperature. The water content of each soil was determined by drying subsamples at 105°C to a constant weight using an Electronic Moisture Analyser. Soils were stored at 4°C prior to use.

Details on test conditions:

- Type, size and further details on reaction vessel: Adsorption and desorption experiments were conducted in 55 mL borosilicate glass vessels (150 × 29 mm o.d.) with ground glass stoppers. The vessels were shaken at a constant rate on an orbital shaker. The shaker was kept in darkness in a controlled temperature incubator at 20 - 22°C. To separate soil and solution, samples were centrifuged at 2500 rpm for 20 minutes.
- Amount of soil: The equivalent of 20 g oven dry weight of each soil was added to each test vessel for the main experiment to produce a final soil-to-solution ratio of 1:1.
- Preparation of the treatment solutions: Five treatment solutions were prepared in 0.01 M aqueous calcium chloride at nominal concentrations of 0.1, 0.02, 0.005, 0.002 and 0.001 mg/mL. For preparing the 0.1 mg/mL solution, an accurately measured weight of non-radiolabelled test substance was mixed with a known volume of [14C]test substance stock solution. The resulting solution was evaporated to dryness under a stream of nitrogen at room temperature and the solid radiodiluted [14C]test substance was redissolved in a known volume of 0.01 M aqueous calcium chloride solution to form the treatment solution. The remaining treatment solutions were prepared using [14C]test substance without radiodilution. For these solutions known volumes of the [14C]test substance stock solution were concentrated to dryness under a stream of nitrogen at room temperature and the solid [14C]test substance was redissolved in a known volume of 0.01 M aqueous calcium chloride. Triplicate (0.1 mL) aliquots of each of the treatment solutions were taken for LSC to determine solution concentrations.
- Number of reaction vessels/concentration: For each soil type 13 pre-weighed tubes were prepared, each containing 20 g dry weight equivalent of soil (to produce a final soil-to-solution ratio of 1:1). An additional set of tubes was prepared for the Speyer 2.2 soil type to provide for a sample set equilibrated for a shorter period of time (8 hours). These samples were to be analysed in the event of poor recovery of radioactivity from the samples equilibrated for 24 hours. For each soil type and test substance concentration (except the highest concentration) the test was run in duplicate. The highest test substance concentration in each soil was run in triplicate to allow for an assessment of test substance degradation in the soil after the adsorption phase. In addition, for each soil type untreated solution/soil mixtures were set up to provide radioassay blanks.
- Aqueous calcium chloride solution 0.01 M (approximately 19 mL, such that the total volume of water after test substance application (ca. 1 mL) in each tube was approximately 20 mL) was mixed with the air-dried soil. The soil in the tubes was pre-equilibrated by shaking for approximately 18 hours at ca. 20 - 22°C in darkness. After pre-equilibration the soil slurries were treated with their respective application solutions (1 mL) to achieve nominal concentrations of the test substance of 5, 1, 0.25, 0.1 and 0.05 μg/mL. The blank samples were treated with 1 mL of aqueous calcium chloride solution (0.01 M).
- Pilot experiments: Three pilot experiments were carried out.
1. The optimum soil-to-solution ratio for the adsorption of the test substance was determined.
2. The volume of supernatant which could be removed after equilibration and centrifugation using the soil-to-solution ratio as determined in 1 (above) was measured.
3. The time for the test substance to reach equilibrium with each soil type was determined.

Duration:

8 h

Temp.:

>= 20 - <= 22 °C

Remarks:

For Speyer 2.2 samples at all the concentrations (0.05-5.0 µg/mL)

Duration:

24 h

Temp.:

>= 20 - <= 22 °C

Remarks:

For Mattapex, Drummer, Nambsheim and Flakkebjerg samples at all the concentrations (0.05-5.0 µg/mL)

Duration:

24 h

Temp.:

20 °C

Remarks:

Desorbed twice for Mattapex, Drummer, Nambsheim and Flakkebjerg samples at all the concentrations (0.05-5.0 µg/mL)

Computational methods:

- Adsorption coefficients (Kd): concentration of the test substance in the soil phase at adsorption equilibrium/ concentration of the test substance in the aqueous phase at adsorption equilibrium
- Adsorption coefficient per organic carbon (Koc): adsorption coefficient/% organic carbon * 100

Key result

Sample No.:

#1

Type:

Koc

Value:

16.4 other: mL/g

Temp.:

20 °C

Matrix:

Sandy loam

% Org. carbon:

2.09

Key result

Sample No.:

#2

Type:

Koc

Value:

23.8 other: mL/g

Temp.:

20 °C

Matrix:

Loam

% Org. carbon:

0.99

Key result

Sample No.:

#3

Type:

Koc

Value:

50.1 other: mL/g

Temp.:

20 °C

Matrix:

Silt clay loam

% Org. carbon:

2.27

Key result

Sample No.:

#4

Type:

Koc

Value:

17.3 other: mL/g

Temp.:

20 °C

Matrix:

Sandy loam

% Org. carbon:

1.05

Key result

Sample No.:

#5

Type:

Koc

Value:

50.8 other: mL/g

Temp.:

20 °C

Matrix:

Loam

% Org. carbon:

1.1

Key result

Sample No.:

#1

Type:

Kd

Value:

0.342 other: mL/g

Temp.:

20 °C

Matrix:

Sandy loam

% Org. carbon:

2.09

Key result

Sample No.:

#2

Type:

Kd

Value:

0.236 other: mL/g

Temp.:

20 °C

Matrix:

Loam

% Org. carbon:

0.99

Key result

Sample No.:

#3

Type:

Kd

Value:

1.136 other: mL/g

Temp.:

20 °C

Matrix:

Silt clay loam

% Org. carbon:

2.27

Key result

Sample No.:

#4

Type:

Kd

Value:

0.182 other: mL/g

Temp.:

20 °C

Matrix:

Sandy loam

% Org. carbon:

1.05

Key result

Sample No.:

#5

Type:

Kd

Value:

0.559 other: mL/g

Temp.:

20 °C

Matrix:

Loam

% Org. carbon:

1.1

Adsorption and desorption constants:

The Freundlich adsorption constants (KF) ranged from 0.145 to 0.809, showing that the test substance was weakly adsorbed to the range of soils tested. Apparent desorption constants (Kdes) calculated after the second desorption gave values in the range 2.3-16.9 mL/g.

Recovery of test material:

Recoveries of radioactivity from samples set aside for the determination of the material balance on completion of adsorption were in the range 84.2-92.7% of the amount applied.
Recoveries of radioactivity after adsorption and two desorption equilibrations (each of 24 hours duration) ranged from 88.1-94.4% applied radioactivity. The Drummer and Flakkebjerg soil types had recoveries of <90% (88.1 and 89.6% respectively).

Concentration of test substance at end of desorption equilibration period:

TLC analysis of soil extracts and aqueous solution showed that over the incubation period (72 hours- one 24-hour adsorption phase followed by two 24-hour desorption phases) low amounts of degradation of the test substance were observed. The total desorbed (DT) ranged from 1 - 30% of that adsorbed in the four soil types tested.

Duration:

24 h

Remarks on result:

other: The mass balance lay in the range 90.8-107.7% for Mattapex, Drummer, Nambsheim and Flakkebjerg samples

Transformation products:

no

Details on results (Batch equilibrium method):

PRELIMINARY TEST
- Pilot experiments: The results for these pilot experiments suggested that a 24-hour equilibration time was appropriate for the main experiment. However, because of the observed inconsistency between the various experiments conducted with the Speyer 2.2 soil type an additional replicate set of Speyer 2.2 samples was prepared for the main experiment. This sample set was to be equilibrated for 8 hours only (adsorption phase) with no subsequent desorptions.
The stability of the test substance in solution and in soil extracts was measured by chromatographic analysis (TLC) of samples of supernatant solution and soil extracts from the samples equilibrated for 24 hours (and 8 hours for Speyer 2.2). The proportion of the test substance (as a % sample radioactivity) lay in the range 90.8-95.7% for both supernatants and soil extracts demonstrating that the test substance was stable in the test system for 24 hours.

MAIN TEST: PERFORMANCE
- Test material stability during adsorption/desorption phase: Yes
- Experimental conditions maintained throughout the study: Yes

Validity criteria fulfilled:

yes

Conclusions:

The test substance was weakly adsorbed to all five soils tested. The test substance that was adsorbed was poorly desorbed with mean totals (DT) of 1-30% of that adsorbed, desorbed. This possibly reflects the potential of the test substance binding preferentially to soil at limited sites through chemical processes.

Executive summary:

The soil leaching potential of carbon-14 labelled test substance was studied using the adsorption/desorption batch equilibration method in accordance with OECD 106 and US EPA Pesticide Assessment Guidelines Subdivision N 163-1.

Five test soils covering a wide range of agricultural soil types were used, namely:

Speyer 2.2: Sandy loam, Speyer Institute, Germany

Mattapex: Loam, Rock Hall, Maryland, USA

Drummer: Silty clay loam, Rochelle, Illinois, USA

Nambsheim: Sandy loam, Nambsheim, France

Flakkebjerg: Loam, Flakkebjerg, Denmark

Initial test substance solution concentrations in the range 0.05-5.0 µg/mL in 0.01 M aqueous calcium chloride were studied. For the adsorption phase four of the five soil types and solutions were equilibrated for 24 hours at 20°C in the dark using a soil-to-solution ratio of 1:1. One soil (Speyer 2.2) was equilibrated for 8 hours only as pilot studies indicated test substance was not stable in this soil type for 24 hours. Soil samples at the highest concentration for all soil types, except Speyer 2.2, were then desorbed twice with fresh calcium chloride solutions for 24 hours each time.

Aqueous solutions and soil extracts were analyzed by TLC to determine the actual concentrations of the test substance in soil extracts and solutions during the equilibration phases.

The Freundlich adsorption isotherms obtained from a linear regression using the log-transformed concentrations showed good linearity. The slopes of the Freundlich isotherms (1/n) were in the range 0.82-0.89.

The Freundlich adsorption constants (KF) ranged from 0.145 to 0.809, showing that the test substance was weakly adsorbed to the range of soils tested. Adsorption constants normalized for soil organic carbon content (KFoc) appear to correlate reasonably well, suggesting that soil organic carbon content was a significant factor in determining the extent of adsorption of the test substance. KFocvalues ranged from 13.3-42.8.

The percentage of test substance desorbed after the first and second desorption’s (D1and D2) was low. The total desorbed (DT) ranged from 1-30% of that adsorbed in the four soil types tested. A possible correlation between the amount of soil organic matter and the proportion of desorption was observed. The soils with a high organic matter content showed less desorption than soils with a low organic matter content. Apparent desorption constants (Kdes) calculated after the second desorption gave values in the range 2.3-16.9 mL/g.

The test substance that was adsorbed was poorly desorbed with mean totals (DT) of 1-30% of that adsorbed, desorbed. This possibly reflects the potential of the test substance to bind strongly at limited sites to soil through chemical processes.

Key value for chemical safety assessment

Koc at 20 °C:

16.4