Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 2. Describing PCB Extraction with Simple Diffusion Models

A simple diffusion model is fitted to selective SFE profiles obtained in Part 1 for native (not spiked) PCBs in historically contaminated sediments and soils. The model takes two types of geometry into consideration, a spherical one (Hot Ball, HB) and a planar one (Finite Slab, FS) and is extended to describe the influence of increasing temperature during the extraction. In this work, soil/sediment organic matter (SOM) is treated as a polymer-like organic material (POM). Consequently, diffusion is considered to be an activated process where the diffusion coefficients depend on the temperature according to the Arrhenius equation. When using one type of POM in the model, the characteristic “temperature humps” obtained in the extraction profiles are simulated relatively well, but at least three types of POM are needed for excellent fits to experimental data. The final model, consisting of three different POMs with spherical geometry, was fitted to all matrixes to estimate diffusion coefficients and activation energies. These data correlated with relevant literature values obtained from long-term water desorption experiments, indicating that it might be possible to replace these experiments with rapid SFE to characterize soils and sediments and to predict the rate of release of pollutants under field conditions.