Chlorpyrifos

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

GLP compliance:

yes

Type of method:

batch equilibrium method

Media:

soil

Specific details on test material used for the study:

Lot#: F0571-39a
Radiochemical purity: >99%
Specific activity: 25.2 mCi/mmol

Radiolabelling:

yes

Test temperature:

20 ± 1°C

Analytical monitoring:

yes

Details on sampling:

A nominal 0.5 µg/rnl, dosing solution was prepared by transferring 160 µL, of the stock solution, into 130 mL of 0.01 M CaCh in a boiling flask. The dose solution was swirled and sonicated for 30 minutes. Triplicate 0.1-mL aliquots of the dosing solutions were analyzed by LSC before and after test sample dosing to determine the concentration. A nominal 0.5 µg/mL, dosing solution was prepared by transferring 27 µL, of the stock solution,14C-test substance, into 20 mL of 0.01 M CaCl2 in a boiling flask. 5 mL of acetonitrile was added to the dose solution to achieve a ratio of 1:4 v: v organic: aqueous. The dose solution was swirled and sonicated for 10 minutes. The dose solution was swirled and sonicated for 10 minutes. A 10.0-mL volume of each dosing solution was transferred into duplicate 15-mL borosilicate glass culture tubes and duplicate silanized 15-mL borosilicate glass culture tubes. A 25.0-mL volume of the dosing solution was transferred into two, 50-mL Teflon centrifuge tubes. The tubes were capped, shaken by hand, vortexed, wrapped with foil, and placed on the shaker in the environmental chamber set at 20°C. After 20 hours of equilibration, the tubes were removed from the shaker and chamber. Triplicate 0.1-mL aliquots were analyzed by LSC.

Matrix no.:

#1

Matrix type:

other: Light Clay (Soil M549)

% Clay:

40

% Silt:

16

% Sand:

44

% Org. carbon:

3.5

pH:

7.4

CEC:

39.9 meq/100 g soil d.w.

Bulk density (g/cm³):

1.22

Matrix no.:

#2

Matrix type:

other: Loamy sand (Soil M579)

% Clay:

4

% Silt:

4

% Sand:

92

% Org. carbon:

1.5

pH:

6.6

CEC:

8.6 meq/100 g soil d.w.

Bulk density (g/cm³):

1.32

Matrix no.:

#3

Matrix type:

other: Light Clay (Soil M584)

% Clay:

27

% Silt:

26

% Sand:

47

% Org. carbon:

1

pH:

6.1

CEC:

15.1 meq/100 g soil d.w.

Bulk density (g/cm³):

1.15

Matrix no.:

#4

Matrix type:

other: Sandy Clay Loam (Soil M585)

% Clay:

21

% Silt:

8

% Sand:

71

% Org. carbon:

1.6

pH:

7.3

CEC:

21.3 meq/100 g soil d.w.

Bulk density (g/cm³):

1.14

Matrix no.:

#5

Matrix type:

other: Sandy Loam (Soil M601)

% Clay:

5

% Silt:

18

% Sand:

77

% Org. carbon:

4.3

pH:

5.5

CEC:

28 meq/100 g soil d.w.

Bulk density (g/cm³):

1.03

Details on matrix:

COLLECTION AND STORAGE
- Geographic location: Soil M549 was collected from Faringdon, UK. Soil M579 was collected from Bedfordshire, UK (also known as Cuckney). Soil M584 was collected from Charentilly, France. Soil M585 was collected from Marcham, UK. Soil M601 was collected from Coleraine, Northern Ireland.
- Soil preparation: Aliquots of soil [0.48 g dry weight] were weighed into 60 tared, 15-mL silanized borosilicate glass culture tubes (12 tubes per soil). A69, 42, 4, and 54-µL aliquot of water was added to each aliquot of soil M549, M584, M585, and M601, respectively, to bring the moisture content to 75% FMC at 1/3 bar. Water was not added to the M579 soil aliquots because the soil was already above the 75% field moisture capacity level. In addition, four, 15-mL silanized borosilicate glass culture tubes were tared. These tubes were the control tubes and received no soil. All of the tubes were capped and stored in the environmental chamber a minimum of 24 hours prior to dose. Prior to initiation, the total tube weights were measured.

PROPERTIES
- Soil classification system: International textural class (hydrometer method) and all other soil properties are given under "Matrix properties"

Details on test conditions:

TEST CONDITIONS
- Buffer:
- pH: 6.1 - 7.4
- Suspended solids concentration:

TEST SYSTEM
- Type, size and further details on reaction vessel: 15-mL silanized borosilicate glass culture tubes (12 tubes per soil).
- Amount of soil/sediment/sludge and water per treatment (if simulation test): Aliquots of soil [0.48 g dry weight] were weighed into 60 tared, 15-mL silanized borosilicate glass culture tubes (12 tubes per soil). A 69,42,4, and 54-µL aliquot of water was added to each aliquot of soil M549, M584, M585, and M601, respectively, to bring the moisture content to 75% field moisture capacity (FMC) at 1/3 bar. Water was not added to the M579 soil aliquots because the soil was already above the 75% field moisture capacity level. In addition two, 15-mL silanized borosilicate glass culture tubes were tared
- Number of reaction vessels/concentration: 2
- Method of preparation of test solution: A 0.933-mL volume of the test substance stock solution was transferred to a l-L boiling flask containing a 700-mL volume of 0.01 M CaCI2, thus bringing the nominal test substance concentration to 0.5 µg/mL. The solution was sonicated for approximately 15 minutes. Triplicate 0.1-mL aliquots of the dosing solution were analyzed by LSC before, during, and after dosing the sample tubes.

Sample No.:

#1

Initial conc. measured:

0.5 other: µg/mL

pH:

8.12

Temp.:

20 °C

Remarks:

The equilibration time was set at 4 hours for soil M549.

Sample No.:

#2

Initial conc. measured:

0.5 other: µg/mL

pH:

7

Temp.:

20 °C

Remarks:

The equilibration time was set at 4 hours for soil M579.

Sample No.:

#3

Initial conc. measured:

0.5 other: µg/mL

pH:

7.31

Temp.:

20 °C

Remarks:

The equilibration time was set at 8 hours for soil M584.

Sample No.:

#4

Initial conc. measured:

0.2 other: µg/mL

pH:

7.66

Temp.:

20 °C

Remarks:

The equilibration time was set at 4 hours for soil M585.

Sample No.:

#5

Initial conc. measured:

0.2 other: µg/mL

pH:

6.86

Temp.:

20 °C

Remarks:

The equilibration time was set at 8 hours for soil M601.

Sample no.:

#1

Conc. of adsorbed test mat.:

0.5 other: The desorption kinetics test was conducted at a 0.5 µg/mL, nominal test substance.

pH:

6.64

Temp.:

20 °C

Remarks:

The equilibrium time was set at 4 hours for soil M549.

Sample no.:

#2

Conc. of adsorbed test mat.:

0.5 other: The desorption kinetics test was conducted at a 0.5 µg/mL, nominal test substance.

pH:

5.32

Temp.:

20 °C

Remarks:

The equilibration time was set at 4 hours for soil M579.

Sample no.:

#3

Conc. of adsorbed test mat.:

0.5 other: The desorption kinetics test was conducted at a 0.5 µg/mL, nominal test substance.

pH:

5.27

Temp.:

20 °C

Remarks:

The equilibration time was set at 8 hours for soil M584.

Sample no.:

#4

Conc. of adsorbed test mat.:

0.5 other: The desorption kinetics test was conducted at a 0.5 µg/mL, nominal test substance.

pH:

6.27

Temp.:

20 °C

Remarks:

The equilibration time was set at 4 hours for soil M585.

Sample no.:

#5

Conc. of adsorbed test mat.:

0.5 other: The desorption kinetics test was conducted at a 0.5 µg/mL, nominal test substance.

pH:

5.16

Temp.:

20 °C

Remarks:

The equilibration time was set at 8 hours for soil M601.

Computational methods:

- Freundlich adsorption and desorption coefficients: The Freundlich constants and n were determined for each soil type from the slope (1/n) and y intercept of the corresponding linear equation. The upper and lower 95% confidence ranges were also calculated from the respective range values.

Key result

Sample No.:

#1

Type:

Kd

Value:

98.8 other: mL/g

pH:

7.4

Temp.:

20 °C

Matrix:

#1

% Org. carbon:

3.1

Key result

Sample No.:

#2

Type:

Kd

Value:

116 other: mL/g

pH:

6.6

Temp.:

20 °C

Matrix:

#2

% Org. carbon:

1.5

Key result

Sample No.:

#3

Type:

Kd

Value:

55.2 other: mL/g

pH:

6.1

Temp.:

20 °C

Matrix:

#3

% Org. carbon:

1

Key result

Sample No.:

#4

Type:

Kd

Value:

67.9 other: mL/g

pH:

7.3

Temp.:

20 °C

Matrix:

#4

% Org. carbon:

1.6

Key result

Sample No.:

#5

Type:

Kd

Value:

295 other: mL/g

pH:

5.5

Temp.:

20 °C

Matrix:

#5

% Org. carbon:

4.3

Key result

Sample No.:

#1

Type:

Koc

Value:

2 785 other: mL/g

pH:

7.4

Temp.:

20 °C

Matrix:

#1

% Org. carbon:

3.1

Key result

Sample No.:

#2

Type:

Koc

Value:

7 965 other: mL/g

pH:

6.6

Temp.:

20 °C

Matrix:

#2

% Org. carbon:

1.5

Key result

Sample No.:

#3

Type:

Koc

Value:

5 582 other: mL/g

pH:

6.1

Temp.:

20 °C

Matrix:

#3

% Org. carbon:

1

Key result

Sample No.:

#4

Type:

Koc

Value:

4 323 other: mL/g

pH:

7.3

Temp.:

20 °C

Matrix:

#4

% Org. carbon:

1.6

Key result

Sample No.:

#5

Type:

Koc

Value:

6 851 other: mL/g

pH:

5.5

Temp.:

20 °C

Matrix:

#5

% Org. carbon:

4.3

Adsorption and desorption constants:

Refer Table 1 & 2

Recovery of test material:

The dosing solution analysis showed a mean percent of 98.7% recovered activity as 14C-test substance, while the control showed 95.1% recovered activity as 14C-test substance. Analysis of the adsorption phase supernatants for samples showed percent recoveries ranging from 66.4% to 97.4% recovered activity as 14C-test substance. The remainder of the recovered activity was 14C-TCP, as shown by direct comparison of retention time. Analysis of the soil extracts showed percent recoveries ranging from 85.1 to 98.8% recovered activity as 14C-test substance. Column recoveries for the isotherm HPLC analysis ranged from 90.0 to 105.8%.

Concentration of test substance at end of adsorption equilibration period:

The results for equilibration times showed that the mean percent adsorbed values for soils M549, M579, and M585 at the 4-hour time point were 63.7, 79.6, and 68.9%, respectively, and 70.8 and 92.0% at the 8-hour time point for soils M584 and M601. Only soil M601 was not within the ideal 20-80 percent adsorbed range at the tested soil:water ratio.

Concentration of test substance at end of desorption equilibration period:

The results for equilibration times showed that the mean percent desorbed values at the 4-hour time point for soils M549, M579, and M585, and the 8-hour time point for soils M584 and M601 were 18.2, 13.0, 26.8, 27.8, and 5.30%, respectively.

Sample no.:

#1

Duration:

4 h

Remarks on result:

other: The mean 14C-mass balance for M549 was 119.3%

Sample no.:

#2

Duration:

4 h

Remarks on result:

other: The mean 14C-mass balance for M579 was 103.7%

Sample no.:

#3

Duration:

8 h

Remarks on result:

other: The mean 14C-mass balance for M584 was 105.6%

Sample no.:

#4

Duration:

4 h

Remarks on result:

other: The mean 14C-mass balance for M585 was 104.5%

Sample no.:

#5

Duration:

8 h

Remarks on result:

other: The mean 14C-mass balance for M601 was 108.3%

Sample no.:

#1

Remarks on result:

other: Mass balance (greater than 90%) was successfully determined at these time points (4 and 8 h) for all soils.

Transformation products:

yes

Remarks:

The primary degradate of test substance was TCP.

Details on results (Batch equilibrium method):

MAIN TEST: PERFORMANCE
- Test material stability during adsorption/desorption phase: The stability of the test substance during the test was established by analyzing the dosing solution, 48-hour control supernatants, adsorption supernatants (4 hour for soils M549, and M585; 8 hour for soils M579, M584, and M601), and filtered soil extracts (4 hour for soils M549, M579, and M585; 8 hour for soils M579 and M601). The HPLC fractionation histograms show consistency in peak shape and retention time of the test substance in the concentrated soil extracts. 92% of 14C-chlorpyrifos was recovered from the control. It was found that degradation occurred in the presence of soil, 4-hour adsorption supernatants for soils M549, M579, and M584, and in the 8-hour adsorption supernatant for soils M584 and M601. The percent of 14C-chlorpyrifos recovered from the adsorption supernatants was between 51.1 and 89.0%. The remainder of 14C activity recovered was consistent with 14C-TCP. The degradation was minimized with the use of the shorter equilibration times. Mass balance
(greater than 90%) was successfully determined at these time points for all soils during desorption phase.
- Experimental conditions maintained throughout the study: Yes


TRANSFORMATION PRODUCTS
- TLC analysis confirmed the presence of test substance in all test samples. In addition, the degradate was confirmed to be TCP. Due to volatility of the test substance, plate recoveries for most samples were well below 90% and served only as a qualitative confirmation for the identity of test substance and its primary degradate, TCP.

Statistics:

Statistics utilized in this study included means, standard deviations, least squares linear regression (including 95% confidence intervals), and minimum quantifiable limits. Correlation coefficients were calculated for all linear regressions. Sufficient replication was planned to control bias.

Table 1. Adsorption isotherms:

Soil Type	Average Kd(mL/g)	Average Koc(mL/g)	Correlation (r)	n	KF^ads
M549	98.8±18.4	2785 ± 519	0.9728	1.16	53.2
M579	116±22	7965 ± 1489	0.9931	1.11	76.7
M584	55.2 ± 7.2	5582 ± 731	0.9955	1.03	48.7
M585	67.9 ± 16.8	4323 ± 1073	0.9887	1.11	45.2
M601	295 ± 54	6851 ± 1253	0.9898	1.06	234

K_dand k_oc: Adsorption partition coefficients

n: Regression constant

K_F^ads: Freundlich Isotherm Coefficient

Table 2. Desorption isotherms

Soil Type	Average Kdes(mL/g)	Average Koc^des(mL/g)	Correlation (r)	n	KF^des
M549	127±9	3574 ± 254	0.9852	1.05	97.9
M579	188± 14	12939 ± 940	0.9988	1.03	164
M584	82.3 ± 7.1	8330 ± 732	0.9993	1.06	64.7
M585	87.6 ± 19.2	5581 ± 1223	0.9921	1.1	58.9
M601	506 ± 74	11753 ± 1711	0.9948	1.06	392

K_desand k_oc^des: Desorption partition coefficients

n: Regression constant

K_F^des: Freundlich Isotherm Coefficient

Conclusions:

- The test substance was classified as immobile for soil M549, M579, and M584, and as having a slight mobility potential for soils M585 and M601.
- These soils demonstrated similarities ofn values between the adsorption/desorption phases, which indicated that sorption to soil had a slight dependence on the test chemical concentration.

Executive summary:

The study was conducted according to OECD guideline 106 to evaluate the adsorption-desorption of test substance on five different European soils at 20 ± 1°C using batch equilibrium method.

Based on the results of the preliminary tests for 14C-test substance, the adsorption and desorption equilibration times were determined to be 4 hours for the Tier 3 isotherm tests with soils M549, M579, M585. The adsorption and desorption equilibration times for soils M584 and M601 were determined to be 8 hours in the Tier 3 isotherm tests.

These isotherm tests were conducted at five nominal test substance concentrations for all soils. The nominal concentrations were 0.025, 0.050, 0.1, 0.25, and 0.5 mg/L in 0.01 M CaCI₂, together with a blank (no test substance). A soil:water ratio of 1:25 w/w was used for all soils, except for soil M601 in which a 1:100 w/w ratio was used.

Isotherm adsorption K_d values were 98.8, 116, 55.2, 67.9 and 295 mL/g for soils M549, M579, M584, M585, and M601, respectively. Isotherm adsorption K_oc values were 2785, 7965, 5582, 4323, and 6851 mL/g for soils M549, M579, M584, M585, and M601, respectively. Isotherm desorption K_des values were 127, 188, 82.3, 87.6 and 506 mL/g for soils M549, M579, M584, M585, and M601, respectively. Isotherm desorption K_oc^des values were 3574, 12939, 8330, 5581 and 11753 mL/g for soils M549, M579, M584, M585, and M601, respectively.

Representative HPLC analysis of the dosing solution, soil-less control supernatants, adsorption supernatants, and soil extracts (after adsorption and desorption) showed that >67% of the 14C-activity was recovered as 14C-test substance. The remainder of the 14C-activity was determined to be 14C-TCP. These results were confirmed by TLC. Co-chromatography was performed on representative samples and showed co-elution of the test substance and a radio labelled reference standard of test substance and TCP in the dosing solution, control supernatants, adsorption supernatants, and soil extracts.

Description of key information

Isotherm adsorption K_ocvalues were 2785, 7965, 5582, 4323, and 6851 mL/g for soils M549, M579, M584, M585, and M601, respectively.

Key value for chemical safety assessment

Koc at 20 °C:

7 965