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Adsorption / desorption

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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: USA Environmental Protection Agency, Pesticide Assessment Guidelines, Subdivision N, Chemistry: Environmental Fate, Series 163-1, Leaching and Adsorption/Desorption Studies
Deviations:
no
GLP compliance:
yes
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
yes
Test temperature:
20 ± 2°C
Analytical monitoring:
yes
Details on sampling:
- Concentrations: 0.05, 0.01, 0.2, 1.0, 2.0 µg/mL
- Sample storage before analysis: -20 ± 5°C
Matrix no.:
#1
Matrix type:
sandy loam
% Clay:
6
% Silt:
21
% Sand:
73
pH:
6
CEC:
3.4 meq/100 g soil d.w.
Matrix no.:
#2
Matrix type:
silty clay loam
% Clay:
36
% Silt:
52
% Sand:
12
pH:
6.2
CEC:
28.3 meq/100 g soil d.w.
Matrix no.:
#3
Matrix type:
sandy loam
% Clay:
17
% Silt:
13
% Sand:
70
pH:
8.5
CEC:
12.5 meq/100 g soil d.w.
Matrix no.:
#4
Matrix type:
sandy loam
% Clay:
19
% Silt:
23
% Sand:
58
pH:
7.4
CEC:
14.4 meq/100 g soil d.w.
Matrix no.:
#5
Matrix type:
sand
% Clay:
4
% Silt:
4
% Sand:
92
pH:
5.7
CEC:
1.8 meq/100 g soil d.w.
Matrix no.:
#6
Matrix type:
sandy clay loam
% Clay:
21
% Silt:
25
% Sand:
54
pH:
7.5
CEC:
13.3 meq/100 g soil d.w.
Details on matrix:
COLLECTION AND STORAGE
The soils used in this study were collected between February 1993 and January 1996. The locations of the sites sampled are shown in Table 1. The soils were air dried, then passed through a sieve (2 mm mesh). In order to minimize degradation of test substance during the study period a sample of each soil (approximately 500 g) was gamma irradiated prior to use. The dose received was between 25 and 40 kGy, which is sufficient to eliminate any living organisms within the soil. The moisture contents of the air dried soils were determined by oven-drying a sample (approximately 20 g) of each soil at approximately 1000C for 17 hours. Prior to 14C-test substance treatment, each soil sample (5 g equivalent to oven dry weight) was equilibrated overnight in 0.01 M aqueous CaCl2 (approximately 19 mL) by continuous mixing on an end-over-end shaker at approximately 1320 revolutions per hour (rph) at 20 ± 2°C. Slurries were equilibrated in glass centrifuge tubes with ground glass stoppers. All aqueous CaCl2 solutions used in this study were sterilised by autoclaving prior to use. The use of 0.01 M CaCl2 solution prevented dispersion of the clay colloids and simulated the salt concentration normally found in soils. Samples were also generated as treatment controls (containing no soil) to determine if the chemical adsorbed to the glass tubes, and soil blanks (containing only soil and aqueous CaCl2 solution) to be used for background determinations in liquid scintillation counting (LSC).

PROPERTIES
- Soil classification system: USDA method
- Moisture holding capacity 1/3 bar (% moisture content): 9.3 (ERTC), 22.7 (Champaign), 17.1 (Kenny Hill and 18 Acres), 3.1 (Lilly Field), 17.4 (Hyde Farm)
- Moisture holding capacity 15 bar (% moisture content): 3.2 (ERTC), 13.5 (Champaign), 9.9 (Kenny Hill), 10.4 (18 Acres), 1.1 (Lilly Field), 8.1 (Hyde Farm)
- Organic matter (%): 1.0 (ERTC), 3.7 (Champaign), 5.1 (Kenny Hill), 3.1 (18 Acres), 0.5 (Lilly Field), 3.0 (Hyde Farm)
Details on test conditions:
TEST SYSTEM
- Type, size and further details on reaction vessel: Glass centrifuge tubes (50 mL) with ground glass stoppers
- Amount of soil: 5 g of soil (dry weight equivalent)
- Number of reaction vessels/concentration: 4
- Radiochemical content Measuring equipment: Liquid scintillation counting
- Method of preparation of test solution: The appropriate volume of stock 14C-test substance solution was added to aqueous CaCl2 solution (0.01 M) in volumentric flasks, such that when aliquots (1 mL) of the prepared treatment solutions were added to the soil:water slurries the required concentration of 14C-test substance in the aqueous phase would be achieved (i.e., 0.05, 0.1, and 0.2 µg/mL). For rates 1.0 and 2.0 µg/mL, the equivalent volume of 14C-test substance stock solution was added to the volumentric flask as for the 0.2 µg/mL rate but in addition, unlabelled test substance stock solution was added to produce the required concentration of the test substance. The quantity of 14C-test substance applied to each slurry was checked by LSC and found to be withing 7% of that intended. Once treated, all the soil:water slurries were replaced on the end-over-end shakers and allowed to reach equilibrium.
Replicated sampling of the treatment solutins demonstrated that they were homogeneous.
Duration:
24 h
Initial conc. measured:
>= 0.05 - <= 2 other: µg/mL
Temp.:
20 °C
Remarks:
The initial aqueous phase solution concentrations were 0.05, 0.1, 0.2, 1.0 and 2.0 µg/mL
Duration:
21 h
Temp.:
20 °C
Remarks:
Duration is >21 h
Key result
Sample No.:
#1
Type:
Koc
Value:
940 dimensionless
pH:
6
Temp.:
20 °C
Remarks on result:
other: ERTC (sandy loam)
Key result
Sample No.:
#2
Type:
Koc
Value:
1 100 dimensionless
pH:
6.2
Temp.:
20 °C
Remarks on result:
other: Champaign (silty clay loam)
Key result
Sample No.:
#3
Type:
Koc
Value:
790 dimensionless
pH:
8.5
Temp.:
20 °C
Remarks on result:
other: Kenny Hill (sandy loam)
Key result
Sample No.:
#4
Type:
Koc
Value:
920 dimensionless
pH:
7.4
Temp.:
20 °C
Remarks on result:
other: 18 Acres (sandy loam)
Key result
Sample No.:
#5
Type:
Koc
Value:
1 200 dimensionless
pH:
5.7
Temp.:
20 °C
Remarks on result:
other: Lilly field (sand)
Key result
Sample No.:
#6
Type:
Koc
Value:
840 dimensionless
pH:
7.5
Temp.:
20 °C
Remarks on result:
other: Hyde Farm (sandy clay loam)
Key result
Sample No.:
#1
Type:
Kd
Value:
5.4 dimensionless
pH:
6
Temp.:
20 °C
% Org. carbon:
1
Remarks on result:
other: ERTC (sandy loam)
Key result
Sample No.:
#2
Type:
Kd
Value:
24 dimensionless
pH:
6.2
Temp.:
20 °C
% Org. carbon:
3.7
Remarks on result:
other: Champaign (silty clay loam)
Key result
Sample No.:
#3
Type:
Kd
Value:
23 dimensionless
pH:
8.5
Temp.:
20 °C
% Org. carbon:
5.1
Remarks on result:
other: Kenny Hill (sandy loam)
Key result
Sample No.:
#4
Type:
Kd
Value:
17 dimensionless
pH:
7.4
Temp.:
20 °C
% Org. carbon:
3.1
Remarks on result:
other: 18 Acres (sandy loam)
Key result
Sample No.:
#5
Type:
Kd
Value:
3.5 dimensionless
pH:
5.7
Temp.:
20 °C
% Org. carbon:
0.5
Remarks on result:
other: Lilly Field (sand)
Key result
Sample No.:
#6
Type:
Kd
Value:
15 dimensionless
pH:
7.5
Temp.:
20 °C
% Org. carbon:
3
Remarks on result:
other: Hyde Farm (sandy clay loam)
Key result
Sample No.:
#1
Type:
other: K'oc
Value:
870 dimensionless
pH:
6
Temp.:
20 °C
Remarks on result:
other: ERTC (sandy loam)
Key result
Sample No.:
#2
Type:
other: K'oc
Value:
990 dimensionless
pH:
6.2
Temp.:
20 °C
Remarks on result:
other: Champaign (silty clay loam)
Key result
Sample No.:
#3
Type:
other: K'oc
Value:
750 dimensionless
pH:
8.5
Temp.:
20 °C
Remarks on result:
other: Kenny Hill (sandy loam)
Key result
Sample No.:
#4
Type:
other: K'oc
Value:
820 dimensionless
pH:
7.4
Temp.:
20 °C
Remarks on result:
other: 18 Acres (sandy loam)
Key result
Sample No.:
#5
Type:
other: K'oc
Value:
1 200 dimensionless
pH:
5.7
Temp.:
20 °C
Remarks on result:
other: Lilly Field (sand)
Key result
Sample No.:
#6
Type:
other: K'oc
Value:
760 dimensionless
pH:
7.5
Temp.:
20 °C
Remarks on result:
other: Hyde Farm (sandy clay loam)
Adsorption and desorption constants:
Test substance was moderately to strongly adsorbed in the six soils. Average adsorption partition coefficients (Kd values) ranged from 3.5 to 24. Freundlich adsorption coefficients (K') were similar ranging from 3.6 to 22. The Freundlich equation showed a good fit to the data in all soils with r² values equal to 1. 1/n values ranged from 0.96 to 1.01, demonstrated almost completely linear adsorption across the concentration range tested. These data are consistent with the very similar average Kd and K' values determined.

Average Kd values adjusted for the organic carbon content of soil (Koc values) ranged from 790 in Kenny Hill to 1200 in the Lilly Field soil. Corresponding coefficients derived from the Freundlich adsorption coefficient (K'oc values) followed a similar pattern that ranged from 750 in the Kenny Hill soil to 1200 in Lilly Field. Using the McCall Classification scale to assess a chemical's potential mobility in soil, test substance can be classified as having low mobility.

Kd, Koc and K'oc values increased during the single desorption step, indicating that the adsorption of test substance is not entirely reversible. Desorption values ranged from 880 to 1900, thus using the McCall Classification System, further reducing the potential mobility of test substance.
Details on results (Batch equilibrium method):
Recoveries of radioactivity from varius samples assessed ranged from 83-99% and averaged 93% of that nominally applied. These recoveries, therefore, show a good correlation between radiolabelled material applied and that recovered.
Validity criteria fulfilled:
yes
Conclusions:
Kd, Koc, K'oc values increased during the desorption step, indicating that the adsorption of the test substance is not entirely reversible.
Executive summary:

The adsorption and desorption properties of 14C-labelled test substance were studied in six soils of different physicochemical properties: Lilly Field (sand), ERTC (sandy loam), Kenny Hill (sandy loam), Champaign (silty clay loam), Hyde Farm (sandy clay loam) and 18 Acres (sandy loam). The chemical was added to soil:water slurries (5 g soil : 20 mL 0.01 M CaCl2) at five rates of application (0.05, 0.1, 0.2, 1.0 and 2.0 µg/mL). Soils and solutions were pre-sterilized to minimize microbial degradation during equilibration process. The test was conducted according to OECD guideline 106.

Average adsorption partition coefficients (Kd values) ranged from 3.5 in the low clay, low organic matter content Lilly Field soil, to 24 in the much higher clay and organic matter content Champaign soil. Freundlich adsorption coefficients (K') were similar ranging from 3.6 to 22. The Freundlich equation showed a good fit to the data in all soils with r² values always equal to 1.0. 1/n values, which ranged from 0.96 to 1.01, demonstrated almost linear adsorption across the concentration range tested. These data are consistent with the similar average Kd values K' achieved in this study. Average Kd values adjusted for the organic carbon content of soil (Koc values) ranged from 790 in Kenny Hill to 1200 in Lilly Field. Corresponding coefficients derived from the Freundlich adsorption coefficient (K'oc values) followed a similar pattern that ranged from 750 to 1200. Using the McCall Classification scale to assess a chemical's potential mobility in soil, test substance can be classified as having low potential mobility in all the soils studied. Data suggested that the adsorption of test substance was related to the organic matter contents of soil. This was considered reasonable as test substance would be neutral in normal agricultural soils, and would therefore be more strongly attracted to the lipophilic organic material in soil.

During the single desorption step, the adsorption of the test substance was shown to be not entirely reversible, with average increases in Kd values ranging from 14 to 31%, further reducing the chemical's potential mobility in soil.

Description of key information

Average Kd values adjusted for the organic carbon content of soil (Koc values) ranged from 790 in Kenny Hill to 1200 in Lilly Field.

Average K'oc values ranged from 750 in Kenny Hill to 1200 in Lilly Field. The average K'oc value of the six soils was 898.

Kd, Koc, K'oc values increased during the desorption step, indicating that the adsorption of the test substance is not entirely reversible.

Key value for chemical safety assessment

Koc at 20 °C:
898

Additional information

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