HETEROGENEOUS SORPTION PROCESSES IN SUBSURFACE SYSTEMS .2. DIFFUSION MODELING APPROACHES

The pore diffusion modeling approach is often used to model the sorption of organic chemicals by natural sorbents, implicitly assuming that the physical and sorptive properties of the sorbent are homogeneous. But application of such approaches to systems with significant variations in particle size and sorption equilibrium properties may be inappropriate; most natural systems have such variations. A general multiple-particle class pore diffusion model that accounts for variations in physical and sorptive properties was developed and used to interpret the results of a batch sorption rate study. Results indicated that multiple-particle class models provide a more accurate representation of long-term sorption rate data than traditional single-particle class approaches, the common approach of adding an instantaneous equilibrium fraction to single-particle class models leads to model parameters that are sensitive to the range over which data is collected, and tortuosity factors obtained with multiple-particle class modeling approaches were consistent with the range of values expected based upon mechanistic reasoning.