Organoclays as Variably Permeable Reactive Barrier Media to Manage NAPLs in Ground Water

Abstract

Three commercial organoclays were evaluated as media for a variably permeable reactive barrier (VPRB) to manage coal-tar creosote, a non-aqueous-phase liquid (NAPL) consisting primarily of polynuclear aromatic hydrocarbons (PAHs). Organoclays and organoclay-sand mixtures having at least 25% organoclay had NAPL conductivities of less than 10-10 m/s, whereas hydraulic conductivities to water were on the order of 0.001 m/s. Water migration was negligible in organoclays solvated with NAPL, but PAHs were released to the aqueous phase when the solvated clay contacted deionized water. Sorption isotherms for aqueous-phase PAHs are convex upward within the range of concentrations of environmental interest, and greater sorption occurs when the aqueous phase contains multiple PAHs. In column tests, breakthrough occurred for only naphthalene (lowest Kow) and for only one organoclay (lowest organic carbon content) for tests on 100% organoclay and organoclay-sand mixtures conducted for 208–276 pore volumes of flow (PVF). Breakthrough predicted with solute transport analyses conducted using batch isotherm parameters as input occurred later than observed in the column test because isotherm models fit to the batch test data overpredicted sorption at low concentrations. The findings indicate that organoclays should be effective media for VPRBs to manage creosote NAPL and associated PAHs dissolved in groundwater. They block NAPL advection, readily permit the flow of water with dissolved PAHs, and sorb dissolved PAHs from water flowing through the clays.