Distributed-Rate Model Analysis of 1,2-Dichlorobenzene Batch Sorption and Desorption Rates for Five Natural Sorbents

Rate-limited sorption and desorption strongly influence the fate, transport, and remediation of organic pollutants in subsurface environments. In this study, the rates of sorption and desorption were quantified for 1,2-dichlorobenzene to and from five natural sorbents using a batch methodology. Solute/sorbent contact times of 3, 7, 14, 49, and 99 d were studied for the desorption rate experiments. The sorption and desorption data were simulated with a distributed-rate model that used the Γ probability density function to generate the distribution of first-order rate coefficients. Ninety-five percent confidence intervals on the optimal model parameters were developed and translated into 95% confidence intervals on the various rate-coefficient distributions. Development of the confidence intervals on the rate coefficients facilitated a statistically rigorous evaluation of whether the rate of desorption was equal to the rate of sorption for the different sorbents. For three of the five sorbents studied, the rates of desorption were significantly slower than the rates of uptake for all solute/sorbent contact times studied. For the remaining two sorbents, the rates of desorption were significantly slower than the rates of uptake for solute/sorbent contact times greater than 2−3 d. For contact times greater than 2 d, a significant fraction of the 1,2-dichlorobenzene appeared to be resistant to desorption. However, the rate of desorption and the magnitude of the resistant fraction were independent of contact time for all but one sorbent. The rate observations for this study were consistent with an intraorganic matter diffusion mechanism.