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Environmental fate & pathways

Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Study period:
June 1994 to 22 March 1995
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
other: EPA Guideline No. 163-1
Deviations:
no
GLP compliance:
yes
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
yes
Test temperature:
20 ± 1 °C
Analytical monitoring:
no
Details on sampling:
- Concentrations: 0.04, 0.2 and 1.0 mg/L
- Sampling interval: 2 days
Details on matrix:
See Table 1

COLLECTION AND STORAGE
- Storage conditions: in a cold room until use
- Soil preparation: 2 mm sieved
Details on test conditions:
A preliminary test with the clay soil was performed to determine the minimum agitation time at 20 ± 1 °C needed to ensure that equilibrium was reached under the described test conditions. Equilibrium was reached in 1-2 days and so the incubation time in the definitive adsorption/desorption test was set at two days.

ADSORPTION TEST
A total of 15 vessels per soil type were filled with 2 g soil per vessel (based on dry weight) and the exact wet weight recorded. Each vessel was spiked with 10 mL 14C-dichlobenil in 0.01 M CaCl2 at concentrations of 0, 0.04, 0.2, 1.0 and 4.66 mg/L. Agitation was started immediately and lasted 2 days to achieve the required equilibrium. To separate the phases after the adsorption step, the suspension was centrifuged for 10 minutes at 1000 g. Volume and concentration of 14C-dichlobenil in the water phase was determined by decanting it in a graduated measuring cylinder, recording the volume and determining the radioactivity in triplicate 1 mL aliquots mixed with 5 mL scintillation cocktail and counted by LSC.

DESORPTION TEST
For the first desorption step, 10 mL 0.01 M CaCl2 was added to each vessel after the adsorption step. Agitation was started immediately and lasted 2 days to achieve equilibrium. To separate the phases after the first desorption step the suspension was centrifuged for 10 minutes at 1000 g. The volume was measured in a graduated measuring cylinder and the radioactivity present in the water phase was determined in triplicate by mixing 1 mL with 5 mL Ultima Gold XR and counting by LSC.

The second desorption step was an exact repetition of the first desorption step.

MASS BALANCE
The soil residue after the second desorption step was subjected to combustion analysis (in five fold; 200 mg aliquots) and the dry weight of the soil was determined. The formed 14CO2 was trapped in Carbosorb and counted by LSC. The remaining part of the soil in each vessel was weighed, dried for 3 hours at 110 ºC and weighed again so the moisture content of the soil could be determined.
Computational methods:
The adsorption coefficients (Kd) were calculated from the Freundlich equation:

log (x/m) = log K + 1/n log Ce

where:
x = absorbed test material (µg)
m = dry weight of soil (g)
K = adsorption coefficient
Ce = concentration of the test material remaining in solution after adsorption step (µg/mL)
n = constant

The adsorption coefficients normalised to the organic carbon content of the soil (Koc), were calculated from the organic matter content of the soils.

Koc = (K x 100) / % organic matter
Key result
Type:
Koc
Value:
237 dimensionless
Temp.:
20 °C

The average mass balance at the end of the test ranged from 98.7-113 % (depending on soil type and fortification level). The resulting adsorption coefficients, corrected for the organic matter content of each soil (Koc) ranged from 133 to 322 and demonstrated that dichlobenil can be classified as a compound with low mobility in soil (see Table 2).

Table 2: Adsorption/desorption coefficients of four soils and one hydrosoil

 Soil type  Recovery of [14C]  K  Koc  KdesI  KdesII
 Sandy loam  113.0  10.5  262  12.9  16.4
 Silty loam  106.7  7.4  272  10.0  13.1
 Clay  110.2  9.6  133  11.8  15.1
 Sand  98.7  0.3  195  1.2  n.d.
 Hydrosoil  103.3  4.2  322  6.2  10.0
 Average    237 ± 74      
Validity criteria fulfilled:
not specified
Conclusions:
Under the conditions of the test, the mass balance was considered satisfactory (98.7-113 %). The adsorption coefficients are comparable to the desorption coefficients. The Koc values ranged between 133 and 322 with an average of 237 indicating that the test material has low mobility in soil.
Executive summary:

In a GLP compliant adsorption-desorption study conducted in line with standardised guideline EPA 163-1, the adsorption/desorption of the test material was determined. Under the conditions of the test, the mass balance was considered satisfactory (98.7-113 %). The adsorption coefficients are comparable to the desorption coefficients. The Koc values ranged between 133 and 322 with an average of 237 indicating that the test material has low mobility in soil.

Description of key information

The Koc of the test material was determined to be 237 according to a study performed in line with EPA Guideline 163-1.

Key value for chemical safety assessment

Koc at 20 °C:
237

Additional information

See 'Environmental Fate and Pathways' endpoint summary.