| Although compacted soil liners are designed to minimize advective transport of leachate, a significant quantity of contamination can be transported through liners due to diffusion. Due to the inability of landfill liners to impede solute transport, enhancement of the sorption capacity of compacted soil liners has been considered as a means to improve liner performance. One method of enhancing pollutant sorption in compacted clay liners is to amend liners with materials such as organobentonites, shale or activated carbon that are capable of strongly sorbing organic pollutants. Laboratory sorption, permeability and mechanical testing was performed on four different materials: hexadecyltrimethyl-ammonium-bentonite (HDTMA-bentonite), benzyltriethylammonium-bentonite (BTEA-bentonite), shale, and granular activated carbon in order to evaluate their potential for use as sorptive amendments in compacted liners. A genetic algorithm optimization model was developed and used to design a liner containing sorptive amendments that minimized the costs of the system while also minimizing the amount of organic solute transport through the liner. Results from the laboratory sorption experiments indicated that granular activated carbon exhibited the highest sorptive capacity for all three solutes (benzene, trichloroethylene and 1,2-dichlorobenzene), followed by BTEA-bentonite, HDTMA-bentonite and shale. Permeability testing results indicate that specimens composed of a mixture of conventional bentonite, sand and either 3% or 9% (by weight) sorptive amendment had a hydraulic conductivity less than or equal to the regulatory requirement of 1 x 10-7 cm/s, with the exception of the specimen amended with 3% GAC, which had a measured conductivity value of 2 x 10-7 cm/s. Results from the optimization analysis indicate that the inclusion of sorptive amendments as a component in compacted soil liners can effectively minimize the transport of organic contaminants through the liner without violating regulatory permeability requirements. When aqueous transport was considered as a cost in the liner design optimization scenario, the resulting liner was composed of at least one lift containing a sorptive amendment. Shale and activated carbon were predominantly chosen for inclusion in the liner over the two organobentonites. |
