Relationship Between Membrane Efficiency and Solute Diffusion Through a Dense, Prehydrated Geosynthetic Clay Liner

Geosynthetic Clay Liners (GCLs) are manufactured flat sheets composed of a layer of bentonite sandwiched between two geotextiles. GCLs are used as containment material in environmental and hydraulic applications (Koerner 1998). Previous studies have shown that hydraulic conductivity and diffusion coefficient of bentonite tend to increase when in contact with chemical solutions, making this material less efficient. The purpose of this study was to investigate the relationship between the membrane efficiency coefficient and effective salt-diffusion coefficient of a dense, prehydrated geosynthetic clay liner (DPH-GCL) in the presence of monovalent salt (KCl) solutions. It was also aimed to compare the DPH-GCLs to conventional GCLs based on the aforementioned properties. The relationship between membrane efficiency and effective salt-diffusion coefficient determined for DPH-GCLs was found to be accurately approximated by linear fit as in the case of conventional GCLs tested in previous studies. The membrane efficiency coefficients and effective salt-diffusion coefficients, Ds*, were determined for 5-mm-thick DPH-GCL specimens subjected to five different KCl solutions (source concentration, Cot = 8.7, 20, 47, 80 and 160 mM) in rigid-wall diffusion cells under no-flow conditions. The source KCl solutions and de-ionized water (DIW) were circulated across the top and bottom specimen boundaries, respectively, in a closed system and values of membrane efficiency coefficients were determined based on the differential pressure induced across the specimens due to prevention of chemico-osmotic liquid flux. The Ds* values were calculated using the measured concentrations and electrical conductivities of the solutions exiting top and bottom boundaries. The DPH-GCL specimens exhibited the same trends of decreasing membrane efficiency coefficient with increasing Ds* as conventional (non-prehydrated) granular GCL specimens tested in previous studies. However, the DPH-GCL specimens exhibited higher membrane efficiency and lower Ds* values per given source concentrations relative to conventional GCL specimens tested at the same source concentrations. These findings are consistent with the lower hydraulic conductivities, k, measured for the DPH-GCL specimens and are attributed primarily to the higher dry density of the DPH-GCL specimens (~1.2 Mg/m3) relative to the conventional GCL specimens (~0.4 Mg/m3), although differences in bentonite texture (i.e., powdered bentonite in the DPH-GCL versus granular bentonite in the conventional GCL) and polymer treatment of the bentonite in the DPH-GCL (via the prehydration solution) also may have contributed to higher membrane efficiency and lower Ds* for the DPH-GCL

Membrane Efficiency and Diffusive Tortuosity of a Dense Prehydrated Geosynthetic Clay Liner

In this study, a series of membrane/diffusion tests were conducted on specimens of a dense prehydrated geosynthetic clay liner (DPH GCL) subjected to KCl solutions (source concentration, Co = 8.7-160 mM) in rigid-wall cells. The source KCl solutions and de-ionized water (DIW) were circulated across the top and bottom specimen boundaries, respectively, and membrane efficiency coefficients, w, were measured based on the differential pressures induced across the specimens due to prevention of liquid flux. Also, effective salt-diffusion coefficients, Ds*, and apparent tortuosity factors, ta, were determined for each specimen based on the steady-state diffusive Cl- flux measured at the exit (bottom) boundary. The DPH GCL specimens exhibited higher w and lower Ds* (and, likewise, lower ta) relative to conventional (granular, non-prehydrated) GCL specimens tested under similar conditions. The results were consistent with the lower hydraulic conductivities, k, measured for the DPH GCL specimens and are attributed primarily to the higher bentonite dry densities in the DPH GCL specimens (1.1 Mg/m3) relative to the conventional GCL specimens (~0.4 Mg/m3), although differences in bentonite texture (powdered versus granular), bentonite type (treated versus untreated), and testing apparatus (rigid wall versus flexible wall) may have been contributing factors. Both the DPH GCL and the conventional GCL exhibited similar trends of decreasing ta with increasing w. The ta values are considered to be a function of both a matrix tortuosity factor, tm, that accounts for the geometry of the interconnected pores, and a restrictive tortuosity factor, tr, that accounts for solute exclusion due to membrane behavior. Whereas the DPH GCL exhibited a lower tm relative to the conventional GCL, both GCLs exhibited similar trends of decreasing tr with increasing w. The relationship between tr and w for both GCLs is reasonably well represented by tr = 1 – w, an expression that has been proposed for clay membranes in previous theoretical and experimental studies