General Radial Diffusion Model for Heterogeneous Sorbents

Sorption kinetics modeling can be improved by accounting for sorbent heterogeneity. This paper demonstrates how heterogeneity can be accommodated with a composite sorbent particle whose shape is described by a general radial geometry. Batch experiments involving sorption and desorption of anthracene from paraffin spheres are used to test the model. The performance of the model is also demonstrated with simulations based on Borden soil. Experiments and modeling demonstrated that sorption by a heterogeneous mix of sorbent particle sizes can be predicted better using a general composite radial geometry than as an equivalent sphere geometry. However, predicting the composite geometry (shape factor and radius) from sorption rate data is difficult due to the nonuniqueness of sorption rate curves that evolve from higher shape factor geometries. Use of models such as ours that better describe sorption sites at long diffusion path lengths may improve predictions when slow, long-term desorption is an important factor.