OECD TG 106: Adsorption-Desorption using a batch equilibrium method

The test is aimed to estimate the adsorption/desorption behaviour of a test substance on different soils types. Known concentrations of the test substance in 0.01 M CaCl2 solutions are added, by defined volumes,  to soil samples pre-equilibrated in the same medium. The mixture is agitated for a proper time. The soil suspensions are then separated by centrifugation and the aqueous phase is analysed for the test substance content. Using indirect method, the amount of test substance adsorbed on the soil sample is calculated as the difference between the initial amount of test substance and the amount remaining at the end of the test in the aqueous solution. Using direct method, usually recommended where the difference in the test substance concentration in solution can't be accurately determined, the amount of the test substance adsorbed can be directly determined by soil analysis. 2 methods are available to carry out the analyses of the aqueous phase:
- the parallel method in which samples with the same soil/solution ratio are prepared, as many as the time intervals at which it is desired to study the adsorption kinetics
-  the serial method in which a small aliquot of the aqueous phase is taken at the defined time intervals (immediately analysed for the test substance determination) and the test goes on with the original mixture.



STUDY DESIGN

Preliminary test (Tier 1) with 1 concentration of the the test substance and 2 soil types
Selection of optimal soil/solution ratio and determination of the equilibration time
Adsorption on vessel surface check
Stability check in the test time scale
2 replicates for soil type/soil to solution ratio/contact time

Adsorption kinetics (Tier 2) with 1 concentration of the the test substance and 5 soil types
Determination of distribution coefficient (kd) and adsorption coefficient (KOC)
2 replicates for soil type (optimal soil/solution ratio) for each suitable contact time

Desorption kinetics (Tier 3) with 1 concentration of the the test substance and 5 soil types
Determination of apparent desorption coefficient (Kdes)
2 replicates for soil type (optimal soil/solution ratio) for each suitable contact time

Adsorption isotherms (Tier 3) with 5 concentrations of the the test substance and 5 soil types
Influence of the test substance concentration on the extent of adsorption process
2 replicates for soil type (optimal soil/solution ratio) at the adsorption equilibrium

Desorption isotherms (Tier 3) with 5 concentrations of the the test substance and 5 soil types
Influence of the test substance concentration on the extent of desorption process
2 replicates for soil type (optimal soil/solution ratio) at the desorption equilibrium
                                                                                                                                                                                                                                                                                                          Analyses performed using a method validated according to SANCO/825/00 rev. 8.1 and SANCO/3029/99 rev. 4 guidance documents.

ENDPOINTS
Adsorption/desorption distribution coefficients: Kd, kOC, Kdes
Freundlich Adsorption/desorption coefficients: KFads, KFdes , KFOCads, KFOCdes
Study includes GLP management and reporting.

REFERENCES AND GUIDELINES
OECD Guideline for Testing of Chemicals, No.106 (21st January 2000) - Adsorption - Desorption Using a Batch Equilibrium Method.
SANCO/825/00 rev. 8.1 (16/11/2010) - Guidance document on pesticide residue analytical methods.
SANCO/3029/99 rev. 4 (11/07/2000) - Residues: guidance for generating and reporting methods of analysis in support of pre-registration data requirements for Annex II (Part A, section 4) and Annex III (Part A, section 5) of Directive 91/414.

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Alberto Sala
E-mail: asala@biotecnologiebt.it
Phone: +39 0371 4662 - Ext. 710
Valerio Frittella
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Phone: +39 075 895 0045 - Ext. 246

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