The effect of equilibration time on desorption rate measurements with chlorinated alkenes and aquifer particles

Abstract

AbstractSorption and desorption phenomena affect the uptake and release of contaminants from aquifer solids, and can play an important role in determining the duration of an aquifer remediation effort. Independent estimates of desorption rates are useful in evaluating various cleanup strategies (e.g., pump‐and‐treat, in situ biorestoration). In this study, two type of aquifer solids, a weakly sorbing sand (Borden) and a relatively strongly sorbing solids fraction (Moffett), were equilibrated for periods of one day and greater than 190 days with tetrachloroethane (PCE) and trichloroethene (TCE), respectively. The samples were subjected to an intermittent purging technique, and the resulting contaminant release curves were analyzed using a radial pore diffusion model which incorporated the effects of equilibrium partitioning. Failure to account for the insufficient equilibrium conditions led to a moderate underestimation of the effective pore diffusion coefficient value (Dp) for the Borden under‐equilibrated sample (one day), and a substantial overestimation of the Dp value for the Moffett under‐equilibrated system. A second set of simulations, was used to find the Dp value that best fit the observed release data while accounting for the nonequilibrium initial conditions. Given these adjustments, the Borden/PCE under‐equilibrated system provided approximately the same desorption rate value (effective pore diffusion coefficient, Dp) as its long‐term sample. For the Moffett/TCE under‐equilibrated sample, accounting for the insufficient equilibration time led to a significantly greater Dp value than was obtained for the long‐term sample. The results for the two solute/sorbent systems are discussed with respect to the pore diffusion mechanism, and the prospect of short‐term parameter estimation methods.