Abstract:
Current regulation established for disposal of solid waste (e.g., municipal solid waste, and coal combustion residuals) requires disposal facilities to include a composite liner system consisting of a geomembrane overlying a 0.6-m-thick clay liner. Geosynthetic clay liners (GCLs) provide a sustainable alternative to use of in lieu of a conventional clay liner, due to benefits of GCLs having low hydraulic conductivity (< 1 x 10-10 m/s), thin to save air space, and easy to install. GCL consists of one layer of sodium bentonite sandwiched between two layers of geotextile that can be woven or non-woven. These GCLs can be modified through addition of polymerized bentonite, called bentonite polymer composite (BPC), to improve the chemical compatibility of GCLs against aggressive landfill leachates. Due to biodegradation of waste materials, each landfill has an associated temperature gradient that describes an increase in temperature with an increase in depth of the landfill causing leachate within to become hotter with depth of the landfill (Yeşiller, N., and Hanson, J. L. 2003). So far, limited study has investigated the effect of elevated temperatures on the hydraulic conductivity of BPC GCLs
Four commercial GCLs were evaluated in this study, including three BPC GCLs and one Na-B GCLs. One BPC GCL consists of bentonite modified with cross-linked polymer and the other two BPC GCLs have liner polymer additives. Hydraulic conductivity tests were conducted with DI water, 20 mM CaCl2 and 50 mM CaCl2 solutions at temperatures of 20 oC, 35 oC, and 60 oC. Swell index tests (ASTM D5890) were conducted to analyze the swelling of BPC compared to Na-B at elevated temperatures. Viscosity tests were conducted to analyze rheology of BPC exposed to DI water, 20 mM CaCl2 and 50 mM CaCl2 solutions at elevated temperatures.
Both linear polymers produced an increase in hydraulic conductivity permeated with 50 mM CaCl2 at elevated temperatures, however, BPC GCLs exposed to high temperatures show increase in swell index that can potentially affect hydraulic conductivity results. Additionally, viscosity of permeant solution decreases allowing more polymer to be flushed causing an increase in hydraulic conductivity. The cross link GCL showed little to no change in hydraulic conductivity with change in temperature, however, the elevated temperature test showed a reduction in overall polymer eluted. These observations indicated that the hydrogel swell allowed for less polymer to be eluted during permeation because of the larger size clogging polymer hydrogels between bentonite granules.
