Determining PCB sorption desorption behavior on sediments using selective supercritical fluid ertraction. 1. Desorption from historically contaminated samples

Supercritical fluid extraction (SFE) conditions using pure CO2 from 40 to 1 50 degrees C were developed that allow sequential extraction of PCBs associ ated with "rapidly desorbing, moderately, slowly, and very slowly desorbing " sites on historically contaminated sediments and soil. Five samples with individual PCB concentrations ranging from a few to similar to 100 000 ng/g all showed PCBs associated with each type of site, but the fraction of PCB s associated with rapidly desorbing or slowly desorbing sites varied greatl y from sample to sample. Although some small differences in the desorption behavior of individual PCBs was observed from a particular sample, the diff erences among samples were much greater demonstrating that the sample matri x and not the PCB identity was most important in controlling desorption beh avior. When all five samples were compared, simple considerations concernin g sample particle size, water content, organic content, and PCB concentrati on could not predict desorption behavior, demonstrating that PCB desorption processes depend on multiple sample characteristics. Comparison of the qua ntity vs distribution of PCBs on the fast to very slow sites indicates that association with slowly desorbing sites is not limited by the number of si tes, but rather that PCBs on historically contaminated samples tend to dist ribute among the various sites available.