Development of cation exchange processes in geosynthetic clay liners After each hydration period, the FIGURE 8 Removal of duct tape from conditioned GCL specimen FIGURE 9 Cutting of GCL specimen and bentonite removal bound cations (BC) and cation exchange capacity (CEC) were determined for the bentonite component of the GCL specimens. BC and CEC also were determined on the virgin GCL that was not subjected to any of the hydration fluids. Soluble cations (SC) and bound cations were determined using the procedure outlined in ASTM D7503. For sampling of bentonite from the GCL specimens for these tests, the bentonite was first dried in a microwave oven to arrest any further cation exchange in a timely manner to maintain representative conditions for the given hydration period. Bentonite specimens were filtered through a 0.45-µm filter and then analyzed for concentrations of major cations (Na, Ca, Mg and K) using inductively coupled plasma/optical emission spectrophotometry (ICP-OES). Tests were conducted using SPECTROBLUE FMT36 (Mahwah, N.J.) following USEPA SW-846 Test Method 6010B (USEPA 2018). CEC was determined using the procedure outlined in ASTM D7503 (Figure 10). FIGURE 10 Laboratory testing of conditioned bentonite specimens ABOVE ICP-OES tests. Photo courtesy of Dong Li RIGHT Swell index tests 16 Geosynthetics | October November 2023 Results The cation exchange capacity of the GCL specimens varied from 70.4 cmol+/kg (average for 2 mM CaCl2 solution) to 70.5 cmol+/kg (average for DI water). The CEC for the virgin GCL was 72 cmol+/kg and the average CEC for the 32 specimens hydrated in the variable solutions was 70.5 cmol+/kg. These values are in line with CEC reported for similar conventional GCLs in previous studies (e.g., Bradshaw and Benson 2014). Variations of bound cations with time and concentration of hydration solution are presented in Figures 11 and 12 with focus on relative fractions and time variability, respectively. The main variations in bound cations were for Na+ and Ca2+ with low variations in the relative bound