Optimal Design of a Compacted Soil Liner Containing Sorptive Amendments

Although compacted soil liners are designed to minimize advective transport of leachate, a significant quantity of contamination can be transported through liners by diffusion. Due to the inability of landfill liners to impede diffusive transport, enhancement of the sorption capacity of compacted soil liners has been considered as a means to improve liner performance. To mitigate organic contamination, one method of enhancing pollutant sorption is to amend liners with materials such as organobentonites, shale, or activated carbon that are capable of strongly sorbing organic pollutants. In this study, laboratory sorption and permeability testing were performed on hexadecyltrimethylammonium (HDTMA) bentonite, benzyltriethylammonium (BTEA) bentonite, shale, and granular activated carbon in order to evaluate their potential for use as a sorptive amendment in compacted liners. A genetic algorithm optimization model was then 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 laboratory sorption experiments indicated that granular activated carbon exhibited the highest sorptive capacity for all three solutes tested (benzene, TCE, 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% amendment by weight had a hydraulic conductivity less than or equal to the regulatory requirement of 1 x 10^–7 cm/s, with the exception of 2.3 x 10^–7 cm/s measured for 3% granular activated carbon. 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. Shale and activated carbon were predominantly chosen for inclusion over the two organobentonites, a result that can be entirely attributed to the relatively high cost of the organobentonite amendments.