Development and Assessment of a Rate Equation for Chemical Transformations in Reactive Porous Media

Abstract

With increasing interest in reactive porous media for groundwater remediation, such as granular iron, kinetic rate equations based on the Langmuir-Hinshelwood (L-H) assumptions have proven useful. Three parameters describe L-H kinetics: the two Langmuir sorption parameters, J and Cmax, and the first order rate constant, k. Unfortunately, the Cmax and k are lumped in the L-H equation, making it impossible to estimate their individual magnitudes. A re-examination of the theory underlying the L-H rate equation showed that L-H kinetics are not necessarily appropriate for packed reactive porous media experiments in columns or in the field. A more general rate equation was derived by accounting for changes in sorbed concentrations over time. The equation contains the Langmuir sorption parameter Cmax not lumped with the reaction rate constant, k, so it is possible to obtain unique estimates of J and Cmax and the rate constant, k. A sensitivity analysis suggested that this separation of variables can be achieved over a finite range of conditions applicable to granular iron media. The equation was demonstrated to be applicable, and the separation of variables possible, using the reduction of 4-chloronitrobenzene with Connelly granular iron as a test case.