Desorption rates of two PCB congeners from suspended sediments---II. model simulation

A two-compartment diffusion model has been developed and applied to simulate the desorption kinetics of two PCB congeners [2,5,2′,5′-tetrachlorobiphenyl (PCB-52) and 2,4,5,2′,4′,5′-hexachlorobiphenyl (PCB-153)] from suspended aquatic sediments (experimental results presented in the preceding paper). Simulation results suggest that (1) both non-equilibrium sorption and non-uniform particle size distribution of the natural sediments used in this study may have contributed to the observed two-distinct-rate desorption behavior of PCBs; and (2) that the two-compartment diffusion model, which assumes that one fraction of PCBs in solid phase reaches an instantaneous equilibrium with the surrounding aqueous phase while the other fraction follows intraparticle diffusion, fits the data far better than the single retarded pore diffusion model. Simulation results further suggest that increasing adsorption equilibration time (aging) will generally decrease the instantaneous equilibrium fraction and the effective pore diffusion coefficient.