INFLUENCE OF PREFERENTIAL FLOW ON PERCHLOROETHYLENE REDUCTION IN A SURFACTANT MODIFIED ZEOLITE/ZERO-VALENT IRON PERMEABLE REACTIVE BARRIER

We conducted a pilot-scale test to evaluate the performance of a permeable reactive barrier filled with foamed surfactant modified zeolite/zero-valent iron (SMZ/ZVI) pellets for chromate and perchloroethylene (PCE) reduction. Based on our laboratory column results (using crushed SMZ/ZVI pellets), we predicted that the 1-m-wide SMZ/ZVI barrier would reduce input PCE concentrations by at least four orders of magnitude. However, during the pilot-scale test the barrier only reduced input PCE concentrations by two orders of magnitude. We suspected that the large size of the SMZ/ZVI pellets (25-mm cubes) created preferential flow paths and limited mass transfer of PCE to the reactive pellet interiors. In addition, the reactivity of the bulk-produced SMZ/ZVI pellets may have been compromised due to less than optimum surfactant loading and/or elevated pH in the barrier caused by release of base from the glass foam substrate. A dual-permeability dual-porosity transport model was used to examine the influence of preferential flow and pellet reactivity (in terms of sorption constant and surface reaction rate coefficient) on PCE reduction efficiency. Modeling results showed that reducing the pellet dimensions by a factor of two would significantly increase the mass transfer of PCE between the preferential flow paths and the pellet interiors, leading to an order of magnitude improvement in PCE reduction. The model also predicted that doubling the sorption constant or the surface reaction rate coefficient would increase PCE reduction by another order of magnitude.