Determining PCB sorption desorption behavior on sediments using selective supercritical fluid ertraction. 1. Describing PCB extraction with simple diffusion models

A simple diffusion model is fitted to selective SFE profiles obtained in Pa rt 1 for native (not spiked) PCBs in historically contaminated sediments an d soils. The model takes two types of geometry into consideration, a spheri cal one (Hot Ball, HE) and a planar one (Finite Slab, FS) and is extended t o describe the influence of increasing temperature during the extraction. I n this work, soil/ sediment organic matter (SOM) is treated as a polymerlik e organic material (POM). Consequently, diffusion is considered to be an ac tivated process where the diffusion coefficients depend on the temperature according to the Arrhenius equation. When using one type of POM in the mode l, the characteristic "temperature humps" obtained in the extraction profil es are simulated relatively well, but at least three types of POM are neede d for excellent fits to experimental data. The final model, consisting of t hree different POMs with spherical geometry, was fitted to all matrixes to estimate diffusion coefficients and activation energies. These data correla ted with relevant literature values obtained from long-term wafer desorptio n experiments, indicating that it might be possible to replace these experi ments with rapid SFE to characterize soils and sediments and to predict the rate of release of pollutants under field conditions.