Desorption rates of two PCB congeners from suspended sediments---I. experimental results

Desorption of 2,5,2′,5′-tetrachlorobiphenyl (PCB-52) and 2,4,5,2′,4′,5′-hexachlorobiphenyl (PCB-153)from suspended sediment particles has been studied in a gas stripping reactor. Physical properties of the sediment particles such as particle size distribution, pore size distribution, surface area and organic carbon content were independently measured. Experimental results indicated that PCB desorption can be characterized by: (1) two-stage behavior (i.e., an initial rapid desorption followed by a prolonged slower desorption); (2) a dependence on Kow; (3) an independence of the first stage and a dependence of the second stage on particle size; and (4) a dependence on equilibration time (aging effect). The equilibration time tested ranged from one month to three years. The aging effect observed in this study suggests that release rates of PCBs in natural systems are likely much lower than those observed in short-term laboratory experiments, indicating not only that a kinetic model should be used in many aquatic system models, but also that kinetic constants obtained in short-term laboratory experiments may not be directly applicable to the desorption process in natural systems.