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

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 C max, and the first order rate constant, k. Unfortunately, the C max 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 C max not lumped with the reaction rate constant, k, so it is possible to obtain unique estimates of J and C max 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.