Hydraulic conductivity of two geosynthetic clay liners permeated with a hyperalkaline solution

GCLs containing powdered Na-bentonite treated with different dosages of a proprietary additive intended to reduce the impacts of chemical interactions were permeated with three solutions: a hyperalkaline solution (1 M NaOH and 1.3 mM CsCl) having similar pH to aluminum refining leachate, a 1.3 mM CsCl solution (no NaOH), and DI water. For a given permeant solution, the hydraulic conductivity of both GCLs was similar. Thus, the higher additive dosage had no measureable impact on hydraulic conductivity. Hydraulic conductivity of both GCLs decreased by a factor of approximately 1.5–1.8 during permeation with CsCl in response to osmotic swelling induced by the low ionic strength of the CsCl solution entering the pore space. In contrast, permeation with the NaOH–CsCl solution caused the hydraulic conductivity of both GCLs to increase modestly (<50 times the hydraulic conductivity to DI water), and then level out (or decrease slightly) as a result of reduced osmotic swelling in the interlayer combined with dissolution of the mineral. For the tests conducted with CsCl solution, nearly all of the Cs was adsorbed by the bentonite. In contrast, Cs broke through readily when the NaOH–CsCl solution was used as the permeant solution. Permeation with the NaOH–CsCl solution also increased the sodicity of the bentonite by replacing bound K, Ca, and Mg on the mineral surface.