Comparison of First-Order Sorption Kinetics Using Concept of Two-Site Sorption Model

We compared two different approaches available in the literature for first-order sorption kinetics, which constitutes the slow time-dependent part of the two-site sorption model commonly used for solute transport. One approach assumes that sorbed concentrations in the Type-1 site (S(1)) and the Type-2 site (S(2)) are directly associated with a solute concentration (C) in liquid phase by forming two-site sorption with a parallel interface; the other assumes that S(2) is associated with only S(1) at the Type-1 site, resulting in two-site sorption with a serial interface. Application of different kinetics to a batch system where solute mass is conserved resulted in an identical type of liquid and sorbed concentration versus time equation in the form of exponential decay. However, because of the different structure and components used to define the kinetic part (S(2)), some parameters estimated from observed kinetic sorption data deviated significantly from one another. For instance, the fraction (f) of the Type-1 site varied as much as twofold, which led to different transport behavior. This indicates that choice of the kinetic part in the two-site sorption model becomes important when attempting appropriate transport modeling on reactive solutes.