Thiophanate-methyl

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Study period:

from 2012-02-01 to 2015-03-02

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)

Version / remarks:

2000

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

yes

Remarks:

labelled in phenyl ring

Test temperature:

20.2 ± 0.4 °C

Analytical monitoring:

yes

Remarks:

Liquid Scintillation Counting (LSC)

Details on sampling:

- Concentrations:
Advanced test
Soil 1, 2 and 4 0.0630 0.0199 0.0063 0.0020 0.0006
Soil 3 0.0504 0.0159 0.0050 0.0016 0.0005

Matrix no.:

#1

Matrix type:

clay

% Clay:

41

% Silt:

36.8

% Sand:

22.2

% Org. carbon:

1.64

pH:

7.1

CEC:

23.7 meq/100 g soil d.w.

Bulk density (g/cm³):

1.37

Matrix no.:

#2

Matrix type:

loamy sand

% Clay:

6.8

% Silt:

12.6

% Sand:

80.6

% Org. carbon:

1.87

pH:

5.5

CEC:

9.9 meq/100 g soil d.w.

Bulk density (g/cm³):

1.26

Matrix no.:

#3

Matrix type:

sandy loam

% Clay:

8.9

% Silt:

28.7

% Sand:

62.5

% Org. carbon:

0.99

pH:

6.7

CEC:

10 meq/100 g soil d.w.

Bulk density (g/cm³):

1.29

Matrix no.:

#4

Matrix type:

loam

% Clay:

26.9

% Silt:

40.3

% Sand:

32.8

% Org. carbon:

2.42

pH:

7.1

CEC:

29.3 meq/100 g soil d.w.

Bulk density (g/cm³):

1.31

Details on matrix:

COLLECTION AND STORAGE
- Collection procedures: Sampling and handling of the soils was performed under consideration of ISO 10381-6. The sampling sites had not been treated with pesticides or fertilisers for at least 5 years. The plant cover was removed, if needed, and the soil sampled from the top 20 cm soil layer. All four soils were sterilised by gamma-irradiation (31.1 - 32.0 kGy).
- Geographic location:
Matrix 1: Speyer 6S, Batch F6S3411(Siebeldingen)
Matrix 2: Speyer 2.2, Batch F2.22811(Hanhofen)
Matrix 3: Speyer 2.3, Batch F2.31411(Offenbach)
Matrix 4: Speyer 2.4, Batch F2.40412 (Leimersheim)
- Soil preparation and storage: The soils, air-dried at ambient temperature, were passed through a 2-mm sieve. Disaggregation was performed with minimal force, so that the original texture of the soil changed as little as possible. The soils were stored at IES Ltd., in a closed container at room temperature until use. The soil was homogenised and the moisture content determined by heating three soil aliquots in an infrared drier until there was no significant change in weight. Thereafter, the soil samples were pre-equilibrated with about 90 % of the targeted volume of the aqueous phase (0.01 M CaCl2) by shaking for at least over night at 20 ± 2 °C prior to application of the test item.
- Moisture content:
Soil 1 2 3 4
Moisture content 2.54 0.75 0.78 2.39
in g per 100 g dry soil

Details on test conditions:

Adsorption
All test tubes (sealed Teflon centrifuge tubes) were shaken on an end-over-end shaker (approximately 30 revolutions per minute) in a temperature controlled room. The agitation devices kept the soils in suspension during shaking. At each interval, samples were taken and centrifuged for 10 minutes at approximately 1640 G. Thereafter, subsamples of the supernatants were submitted for radioactivity content measurements by LSC and HPLC analysis to determine the test item concentration.
The concentration of the test item in the aqueous phase (Ce) and the amount of test item adsorbed onto soil particles (x/m; x: amount of test item adsorbed, m: mass of dry soil) were calculated based on the results of the radio-assays and HPLC analyses.

Mass balance of radioactivity
Mass balance determinations were performed for selected samples directly after the adsorption step. After sampling, the supernatants were removed and the soils were extracted three times using acetonitrile/water (4:1; v/v). The extracts resulting from the same tube were combined and analysed by LSC and HPLC. The extracted soils were air-dried and aliquots combusted to determine the amount of non-extractable radioactivity.

Quantification of non-extractable radioactivity
The extracted soil samples were air dried and homogenised. Aliquots were added to ceramic sample boats and combusted at 900 °C in a stream of nitrogen and oxygen gas. The combustion products were passed through a series of catalysts at 680 °C. All determinations were done in triplicate. Evolved CO2 was absorbed in 10 mL OxySolve C-400. The absorption solution was then mixed with 10 mL of IRGA SAFE Plus and the radioactivity quantified by LSC. Aliquots of calibration standard solutions were combusted at the beginning of each series. The yield of combustion of calibration standards solution was approximately 100 %.

Type:

other: KF

Value:

1.421 other: mL/g

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 1

% Org. carbon:

1.64

Type:

other: KF

Value:

1.485 other: mL/g

pH:

5.5

Temp.:

20.4 °C

Matrix:

Matrix 2

% Org. carbon:

1.87

Type:

other: KF

Value:

0.877 other: mL/g

pH:

6.7

Temp.:

20.4 °C

Matrix:

Matrix 3

% Org. carbon:

0.99

Type:

other: KF

Value:

1.302 other: mL/g

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 4

% Org. carbon:

2.42

Type:

Koc

Value:

134 L/kg

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 1

% Org. carbon:

1.64

Type:

Koc

Value:

91 L/kg

pH:

5.5

Temp.:

20.4 °C

Matrix:

Matrix 2

% Org. carbon:

1.87

Type:

Koc

Value:

106 L/kg

pH:

6.7

Temp.:

20.4 °C

Matrix:

Matrix 3

% Org. carbon:

0.99

Type:

Koc

Value:

65 L/kg

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 4

% Org. carbon:

2.42

Key result

Type:

Koc

Value:

99 L/kg

Remarks on result:

other: mean

Type:

other: KFoc

Value:

87 other: mL/g

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 1

% Org. carbon:

1.64

Type:

other: KFoc

Value:

79 other: mL/g

pH:

5.5

Temp.:

20.4 °C

Matrix:

Matrix 2

% Org. carbon:

1.87

Type:

other: KFoc

Value:

89 other: mL/g

pH:

6.7

Temp.:

20.4 °C

Matrix:

Matrix 3

% Org. carbon:

0.99

Type:

other: KFoc

Value:

54 other: mL/g

pH:

7.1

Temp.:

20.4 °C

Matrix:

Matrix 4

% Org. carbon:

2.42

Key result

Type:

other: KFoc

Value:

77 other: mL/g

Temp.:

20.4 °C

Remarks on result:

other: mean

Sample no.:

#1

Duration:

2 h

% Adsorption:

95.5

Remarks on result:

other: Soil 1: mean value of two replicates from the aqueous phase and the extracts

Sample no.:

#2

Duration:

2 h

% Adsorption:

104.3

Remarks on result:

other: Soil 2: mean value of two replicates from the aqueous phase and the extracts

Sample no.:

#3

Duration:

2 h

% Adsorption:

95.1

Remarks on result:

other: Soil 3: mean value of two replicates from the aqueous phase and the extracts

Sample no.:

#4

Duration:

2 h

% Adsorption:

100.2

Remarks on result:

other: Soil 4: mean value of two replicates from the aqueous phase and the extracts

Transformation products:

not measured

Preliminary test 1

After 24 hours of shaking samples with a soil-to-solution ratio of 1:25, between 76.9 and 77.6 % of the applied radioactivity was present in the aqueous phase. At a soil-to-solution ratio of 1:5, the corresponding values ranged from 41.0 to 43.9 % of applied radioactivity. No adsorption of radioactivity on the Teflon tubes was observed. The experiment was conducted at a temperature of 20.8 ± 0.6 °C.

Preliminary test 1 was performed despite a test item purity of less than 95%, in order to pre-select suitable soil-to-solution ratios, by only considering the distribution of radioactivity between the aqueous phase and soil. No further chromatographic analyses were performed. The results showed that the percentage of radioactivity adsorbed to soils increased with increasing amount of soil. Based on the insufficient purity of the test item, it was decided to repeat the preliminary test with purified test substance and at the same time investigate the adsorption kinetics at a ratio of 1:12.5. This soil/solution ratio was selected since the replicates exhibited the lowest variability in the measured radioactivity.

 

Preliminary test 2 / screening test 1

After 24 hours of shaking at a soil-to-solution ratio of 1:25, between 75.5 and 77.7 % of the applied radioactivity was present in the aqueous phase. At a soil-to-solution ratio of 1:5, the corresponding values ranged from 43.7 to 49.8 % of applied radioactivity. No adsorption of radioactivity on the Teflon tubes was observed as could be displayed by the constant level of radioactivity over time. These results confirmed the first preliminary test.

The screening test results at the soil-to-solution ratio of 1:12.5 showed that no adsorption equilibrium was reached in all four soils. After 2 hours of adsorption, between 80.5 % and 88.8 % of applied radioactivity was present in the aqueous phase of soils 1 to 4. After 48 hours of adsorption, the corresponding values ranged from 55.2 % to 66.0 % of applied radioactivity. The amount of radioactivity recovered in the control samples remained constant.

HPLC analysis of the aqueous phase showed a degradation of the test item in soil samples as well as in control samples. The test item was not stable despite the use of sterilised soil. The experiments were conducted at a temperature of 20.4 ± 0.2 °C.

 

Screening test 2

An adsorption kinetic test was performed for all four soils at a soil-to-solution ratio of 1:12.5 and mass balances were established for each sampling interval. For all four soils, the amount of radioactivity in the aqueous phase decreased over lime, while the radioactivity in the soil extracts increased. The amount of non-extractable radioactivity increased over time from 0.4 - 1.2 % of AR after 2 hours to 6.1 - 7.9 % AR after 48 hours of adsorption.

HPLC analysis of the aqueous phase and soil extracts showed a continuous degradation of the test item in soil samples. However, the test item concentration was determined in both phases, enabling to calculate the distribution of the test item and Koc values.

	Soil
Parameter	1	2	3	4	Mean
Kd [mL/g]	2.14	1.51	0.95	1.92	1.63
Koc [mL/g]	131	81	96	79	97

The pH of the 0.01 M CaCl₂ solution before contact with the soils was 6 8. The pH of the aqueous phases after contact with soils 1 to 4 were 7.9, 5.9. 7.6 and 8.3, respectively. A hydrolytic degradation at the observed pH values could not be excluded and therefore a stabilisation of the solutions (before analyses) could be achieved by acidification (since the test item is more stable under acidic conditions). The experiment was conducted at a temperature of 20.4 ± 0.2 °C.

The 2-hour interval seemed to be adequate to determine the K_oc for the test item, since degradation of the lest item was still low in both phases. However, adsorption to soil was relatively low. Therefore, it was concluded to perform another screening test at a higher soil-to-solution ratio.

 

Screening test 3

Mass balances were established after 2 hours of adsorption at a soil-to-solution ratio of 1:5. Compared to the corresponding sampling interval with a soil-to-solution ratio of 1:12.5, less radioactivity was present in the aqueous phase, while an increase in extractable and non-extractable radioactivity was observed.

	Soil
Parameter	1	2	3	4	Mean
Kd [mL/g]	1.83	1.84	0.94	1.76	1.59
Koc [mL/g]	111	98	95	73	94

The experiment was conducted at a temperature of 20.3 ± 0.3 °C.

A higher amount of adsorption was achieved while the K_oc values were in the same order as for the lower soil-to-solution ratio. For soil 3, the amount adsorbed was still considered too low (the value K_d multiplied by the soil-to-solution ratio was below 0.3), and further soil-to-solution ratios were tested.

 

Preliminary test 3

For soil 3, additional mass balances were established after 2 hours of adsorption at a soil-to-solution ratio of 1:2 and 1:3. In combination with the corresponding sampling interval at a soil-to-solution ratio of 1:12.5 and 1:5, the results showed a clear increase in the adsorption of radioactivity with increasing soil-to-solution ratio. However, an increase in the soil-to-solution ratio also resulted in an increased degradation of the test item. The experiment was conducted at a temperature of 20.5 ± 0.3 °C.

 

Advanced test

After performing various preliminary and screening tests, it was concluded to perform the advanced test at a soil-to-solution ratio of 1:5 for soils 1, 2 and 4, and at a ratio of 1:3 for soil 3 at 2 hours of adsorption.

Radiochemical purity and stability of the test item

The purity of application solution A determined by HPLC analysis for 14C-Thiophanate-methyl was 95.6 %. Degradation of 14C-Thiophanate-methyl after 2 hours of agitation was observed and considered in the evaluation. The experiment was conducted at a temperature of 20.2 ± 0.4 °C.

Mass balance

All samples were extracted after the adsorption step. The pooled extracts were analysed by LSC and aliquots of the extracted soils (residues) were combusted. The recoveries ranged between 99.1 % and 113.1 % of AR. Divergently, the concentration C2 for soil 4 showed a recovery of only 21.7 % AR. It is assumed that the two replicates were treated with insufficient amount of test item, resulting in a lower test item concentration. However, both the aqueous phase and the extract phase were analysed by LSC, which allowed to calculate the corresponding test item concentrations and ultimately the Freundlich adsorption coefficient (K_Foc). The results were appropriate, which is also supported by the high correlation coefficient of the linear Freundlich adsorption isotherm. Recovered radioactivity in the control samples ranged between 90.3 % and 109.7 % of applied radioactivity.

Adsorption isotherms

The percentage of 14C-Thiophanate-methyl in the aqueous phase and soil extracts after 2 hours of adsorption was determined solely for the highest test concentration, since only those samples exhibited measurements above the limit of quantitation for both the aqueous phase and soil extracts. The results are showing that the test item was present in the aqueous phase as well as in soil. The results obtained were evaluated by applying the linear Freundlich equation. The correlation coefficients (r²) for linear regression of the linear form of the Freundlich equation were higher than 0.991.

The comparison of Koc values obtained in screening and preliminary tests under similar test conditions and concentrations with those of the advanced test confirm these results.

Table 1: Adsorption parameters resulting from the Freundlich isotherms

Parameter	Soil 1 Speyer 6S	Soil 2 Speyer 2.2	Soil 3 Speyer 2.3	Soil 4 Speyer 2.4
Type	Clay	Loamy sand	Sandy loam	Loam
pH (CaCl2)	7.1	5.5	6.7	7.1
% OC	1.64	1.87	0.99	2.42
%OM	2.83	3.22	1.71	4.17
KF  [mL/g]	1.421	1.485	0.877	1.316
KFoc [mL/g]	87	79	89	54
KFom [mL/g]	50	46	51	32
1/n	0.89	0.99	0.96	0.97
r^2	0.9911	0.9992	0.9978	0.9999

The Freundlich isotherm coefficient for adsorption K_Foc was 77 ± 16 (mean ± standard deviation). The 1/n values for adsorption ranged between 0.89 and 0.99, indicating that adsorption of the test item was independent of the test item concentration.

Using the McCall Classification scale to assess a chemical's potential mobility in soil (based on its K_Foc), 14C-Thiophanate-methyl can be classified as having a high mobility in the four soils tested.

No desorption test was performed due to the instability of the test item in the test system.

Validity criteria fulfilled:

yes

Conclusions:

The mean adsorption coefficient of the test item in 4 different soils at 20 °C was determined to be 99 L/kg. The Freundlich isotherm constants KF and KFoc were determined in all 4 soil types. The mean KFoc was determined to be 77 ± 16.

Executive summary:

The adsorption characteristics of the test item 14C-Thiophanate-methyl in soil was investigated according to OECD 106. The investigation was conducted using four different soil types, covering a range of soil properties: soil 1 (Speyer 6S, clay), soil 2 (Speyer 2.2, loamy sand), soil 3 (Speyer 2.3, sandy loam) and soil 4 (Speyer 2.4, loam). Different preliminary and screening tests were performed in order to find the optimum parameters for the advanced test. Since the test item was found to be unstable under experimental conditions, concentration in both the aqueous phase and soil extracts were analysed directly. The advanced test of the study was performed in the dark at 20.2 ± 0.4 °C using a standard batch equilibrium method. The samples consisted of 5 g soil in 25 mL (soils 1, 2 and 4) or 15 mL (soil 3) of aqueous 0.01 M CaCl₂ solution. The test item was added to the surface of the supernatants in order to achieve five test concentrations covering two orders of magnitude. An adsorption equilibration time of 2 hours was selected due to degradation of the test item. After 2 hours of shaking, the radioactivity in the aqueous phase and the soil extracts was determined by liquid scintillation counting. Aliquots of selected samples were analysed by high-performance liquid chromatography for quantification of 14C-Thiophanate-methyl. The soil adsorption coefficients, including the Freundlich adsorption constants K_F and K_Foc, were determined. The following K_F constants were determined for Soil 1, 2, 3 and 4 – 1.421, 1.485, 0.877 and 1.316 mL/g, respectively. The K_Foc constants for these soils were determined to be 87, 79, 89 and 54 mL/g, respectively.

In conclusion, the Freundlich isotherm coefficient for adsorption K_Foc was 77 ± 16 (mean ± standard deviation). The 1/n values for adsorption ranged between 0.89 and 0.99, indicating that adsorption of the test item was independent of the test item concentration. Using the McCall Classification scale to assess a chemical's potential mobility in soil (based on its K_Foc), 14C-Thiophanate-methyl can be classified as having a high mobility in the four soils tested. No desorption test was performed due to the instability of the test item in the test system.  

Description of key information

The mean adsorption coefficient of the test item in 4 different soils at 20 °C was determined to be 99 L/kg. The Freundlich isotherm constants K_F and K_Foc were determined in all 4 soil types. The mean K_Foc was determined to be 77 ± 16.

Key value for chemical safety assessment

Koc at 20 °C:

99

Additional information

The adsorption characteristics of the test item 14C-Thiophanate-methyl in soil was investigated according to OECD 106. The investigation was conducted using four different soil types, covering a range of soil properties: soil 1 (Speyer 6S, clay), soil 2 (Speyer 2.2, loamy sand), soil 3 (Speyer 2.3, sandy loam) and soil 4 (Speyer 2.4, loam). Different preliminary and screening tests were performed in order to find the optimum parameters for the advanced test. Since the test item was found to be unstable under experimental conditions, concentration in both the aqueous phase and soil extracts were analysed directly. The advanced test of the study was performed in the dark at 20.2 ± 0.4 °C using a standard batch equilibrium method. The samples consisted of 5 g soil in 25 mL (soils 1, 2 and 4) or 15 mL (soil 3) of aqueous 0.01 M CaCl₂ solution. The test item was added to the surface of the supernatants in order to achieve five test concentrations covering two orders of magnitude. An adsorption equilibration time of 2 hours was selected due to degradation of the test item. After 2 hours of shaking, the radioactivity in the aqueous phase and the soil extracts was determined by liquid scintillation counting. Aliquots of selected samples were analysed by high-performance liquid chromatography for quantification of 14C-Thiophanate-methyl. The soil adsorption coefficients, including the Freundlich adsorption constants K_F and K_Foc, were determined. The following K_F constants were determined for Soil 1, 2, 3 and 4 – 1.421, 1.485, 0.877 and 1.316 mL/g, respectively. The K_Foc constants for these soils were determined to be 87, 79, 89 and 54 mL/g, respectively.

In conclusion, the Freundlich isotherm coefficient for adsorption K_Foc was 77 ± 16 (mean ± standard deviation). The 1/n values for adsorption ranged between 0.89 and 0.99, indicating that adsorption of the test item was independent of the test item concentration. Using the McCall Classification scale to assess a chemical's potential mobility in soil (based on its K_Foc), 14C-Thiophanate-methyl can be classified as having a high mobility in the four soils tested. No desorption test was performed due to the instability of the test item in the test system.