Estimation of the Adsorption Coefficient on Soil and Sewage Sludge (OECD 121)
Estimation of the Adsorption Coefficient (Koc) on Soil and Sewage Sludge using HPLC (OECD 121)
An important parameter in the sorption behaviour of substances is the adsorption coefficient (Koc). This is defined as the ratio between the concentration of the substance in the soil/sludge and the concentration of the substance in the aqueous phase at adsorption equilibrium. The adsorption coefficient normalised to the organic content of the soil Koc is a useful indicator of the binding capacity of a chemical on organic matter of soil and sewage sludge and allows comparisons to be made between different chemicals.
The experimental method uses High Performance Liquid Chromatography (HPLC) for the estimation of the adsorption coefficient (Koc) in soil and sewage sludge. While passing through the column in the mobile phase the test substance interacts with the stationary phase. As a result of partitioning between mobile and stationary phases the test substance is retarded. The dual-composition of the stationary phase having polar and non-polar sites allows for the interaction of polar and non-polar groups of a molecule in a similar way as is the case for organic matter in soil or sewage sludge matrices. This enables the relationship between the retention time on the column and the adsorption coefficient on organic matter to be established.