Hydraulic Conductivity and Cation Exchange in Non-prehydrated And Prehydrated Bentonite Permeated with Weak Inorganic Salt Solutions

Abstract

Sets of replicate hydraulic conductivity tests were conducted using 100 mM KCl and 20 and 40 mM CaCl₂ solutions to evaluate how changes in hydraulic conductivity are related to changes in the exchange complex and physical properties (water content and free swell) of prehydrated and non-prehydrated bentonite used for geosynthetic clay liners (GCLs). Essentially no change in hydraulic conductivity and water content (or void ratio) occurred during tests with the 100 mM KCl solution even though K⁺ was replacing Na⁺ on the exchange complex. In contrast, for the CaCl₂ solutions (20 mM and 40 mM), the hydraulic conductivity increased and the free swell and water content decreased as exchange of Ca²⁺ for Na⁺ occurred. Faster changes in hydraulic conductivity and the exchange complex occurred in the tests with the 40 mM CaCl₂ solution and the non-prehydrated bentonite (i.e. the hydraulic conductivity changed more rapidly when exchange occurred more rapidly). Even though exchange of Ca²⁺ for Na⁺ was essentially complete at the end of testing, the hydraulic conductivity obtained with the 20 mM CaCl₂ solution was less and the water content greater than obtained with the 40 mM CaCl₂ solution (2.6 × 10⁻⁸ cm/s vs. 6.7 × 10⁻⁸ cm/s, 122% vs. 111%, and 3.2 vs. 2.9). Similarly, the prehydrated bentonite had lower hydraulic conductivity (1.9 × 10⁻⁸ cm/s vs. 6.7 × 10⁻⁸ cm/s) and greater water content (167% vs. 111%) than the non-prehydrated bentonite at the end of testing, even though Ca-for-Na exchange was essentially complete.