Adsorption/Desorption

This Test Guideline is aimed at estimating the adsorption/desorption behaviour of a chemical on different soil types. The goal is to obtain a sorption value which can be used to predict partitioning under a variety of environmental conditions; to this end, equilibrium adsorption coefficients for a chemical on various soils are determined as a function of soil characteristics (organic carbon, clay content, soil texture, and pH). The test comprises three tiers. The tier 1 is the preliminary study, the tier 2 is the screening test (in 5 soils) and the tier 3 is the determination of Freundlich adsorption isotherms or the study of desorption by means of desorption kinetics/Freundlich desorption isotherms, as appropriate. Two methods are possible for analyse: the indirect method and the direct method. The indirect method consists of the adjunction of the test substance to soil samples, the agitation of the mixture for an appropriate time, the analysis of the aqueous phase after centrifugation and the filtration of the soil suspension. The amount of test substance adsorbed on the soil sample is calculated as the difference between the amount of test substance initially present in solution and the amount remaining at the end of the experiment. The direct method is recommended when the difference in the solution concentration of the substance cannot be accurately determined.