Polymer Elution and Hydraulic Conductivity of Bentonite–Polymer Composite Geosynthetic Clay Liners

Abstract

Hydraulic conductivity tests were conducted on two commercially available bentonite–polymer composite (BPC) geosynthetic clay liners (GCLs) containing the same dry blend of sodium bentonite (NaB) and polymer sandwiched between two nonwoven geotextiles (GTs) bound by needlepunching: a bentonite–polymer GCL (BP GCL) and bentonite–polymer GCL with silt film (BPS GCL). The GCLs were essentially identical except that the BPS GCL contained a woven slit-film GT to constrain polymer elution. The slit-film GT was located inside the GCL adjacent to the downstream nonwoven GT. The test liquids were deionized (DI) water and 20–500 mM CaCl2 representing dilute to high-concentration permeant solutions. Effluent from the hydraulic conductivity tests was analyzed for total organic carbon (TOC) to quantify polymer elution. Scanning electron microscopy (SEM) was used to capture how the solution chemistry affected the fabric of the BPC and how hydraulic conductivity was related to pore-scale features in the fabric. Both BPC GCLs were approximately one to four orders of magnitude less permeable than a conventional NaB GCL permeated with the same solutions. Polymer elution data and SEM images of polymer structure suggest that polymer retention in the pore space and pore clogging by polymer hydrogel are responsible for the lower hydraulic conductivity of BPC GCLs. The BPS GCL had less polymer elution and hydraulic conductivity three orders of magnitude lower than the BP GCLs when permeated with 50 and 100 mM CaCl2 solutions because the slit-film GT reduced polymer elution from the bentonite. A conceptual framework is proposed relating solution chemistry, pore-scale fabric, and hydraulic conductivity of dry-blended BPC GCLs.