Attenuation of dilute aromatic hydrocarbon transport by a block copolymer in a compacted vertisol

Abstract

Municipal solid waste landfills in the United States are built with a composite bottom liner consisting of a flexible membrane liner of high-density polyethylene overlying a compacted soil liner. Hydrocarbons have been shown to pass through the flexible membrane liner by diffusion. Flexible membrane liners often have flaws allowing direct contact between the leachate and the compacted soil liner. The transmission of hydrocarbons to the compacted soil liner presents a threat to groundwater supplies. The study was performed to determine if the modification of a compacted soil liner with a thermoplastic elastomer block copolymer could successfully sequester benzene, toluene, ethylbenzene, and xylenes and meet the United States Environmental Protection Agency's saturated hydraulic conductivity requirement of 1x10⁻⁷ cm sec⁻¹. Compacted Ships clay modified with 0, 1, 3, 5, and 10% weight of a thermoplastic elastomer block copolymer was tested for saturated hydraulic conductivity using 10.2 cm fixed wall permeameters. The compacted Ships clay met the United States Environmental Protection Agency's mandated saturated hydraulic conductivity of 10⁻⁷ cm sec⁻¹ at polymer contents of 3% (wt) polymer or less. The presence of dissolved aromatic hydrocarbons had no effect on the saturated hydraulic conductivity. The ability of the polymer to attenuate the transport of dilute aromatic hydrocarbons was tested by permeating the compacted soil/polymer treatments with a 0.01N CaSO4 solution contaminated with benzene, toluene, ethylbenzene, and xylenes. Leachate from permeameters packed with soil containing more than 1% (wt) polymer had BTEX concentrations below the drinking water standard for 3 or more pore volumes. The findings of this research were applied to a hypothetical compacted soil liner constructed with Ships clay modified to include 3% (wt) polymer and having a saturated hydraulic conductivity of 4.23 x 10⁻⁸ cm sec⁻¹. It was assumed that the soil liner was in direct contact with landfill leachate. The hypothetical liner would protect the groundwater from contamination above the maximum contamination limit for drinking water by benzene for 350 years, toluene for 140 years, and ethylbenzene for 260 years.