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, ω, 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, τa, 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 ω and lower Ds∗ (and, likewise, lower τa) 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 τa with increasing ω. The τa values are considered to be a function of both a matrix tortuosity factor, τm, that accounts for the geometry of the interconnected pores, and a restrictive tortuosity factor, τr, that accounts for solute exclusion due to membrane behavior. Whereas the DPH GCL exhibited a lower τm relative to the conventional GCL, both GCLs exhibited similar trends of decreasing τr with increasing ω. The relationship between τr and ω for both GCLs is reasonably well represented by τr = 1 – ω, an expression that has been proposed for clay membranes in previous theoretical and experimental studies.