Quantification of contaminant sorption-desorption time scales from batch experiments.

The ability to predict rates of contaminant sorption and desorption in the environment is essential in order to determine contaminant bioavailability, predict contaminant fate and transport, and assess risk. In this paper, we present a new method to determine sorption-desorption time scales from the temporal moments of batch experimental data. Here, the term time scale has a precise meaning: Time scales are defined in terms of the parameters of kinetic sorption models. The method can be implemented with either a diffusion-based model (t(diff) = a2/15D) or a linear-driving-force model (t(LDF) = I/k) for sorption kinetics and can be implemented with either a discrete or a continuous distribution of rate parameters. Three advantages to the new method are that the time scales t(diff) or t(LDF) can be calculated directly without best-fitting the experimental data, the calculated sorption-desorption time scales are not dependent on an arbitrarily chosen distribution (e.g., the commonly used gamma or lognormal distributions), and the time scales implied by the analysis are consistent with the time scale of the actual experiment. We apply the method to previously reported experiments of 1,2-dichlorobenzene (DCB) sorption onto four natural sorbents. Comparing the newly calculated sorption-desorption time scales to those reported previously indicates a different order for the four sorbents with regard to DCB sorption rate. Further applications and limitations of the method are discussed.