Evaluation of Na-Bentonite-Polyacrylte Mixtures to Enhance the Chemical Resistance of Geosynthetic Clay Liners

摘要

Addition of polyacrylate to enhance the chemical resistance of Na-bentonite (NaB) was investigated. To boost the swelling of NaB, and thus boost the mechanism by which NaB derives low hydraulic conductivity, superabsorbent Na-polyacrylate (SAP) was investigated as an additive. Mixtures were tested by direct permeation (i.e., permeation without prehydration with a dilute solution) at low effective stress with 50 mM CaCl_2 or 500 mM CaCl_2. These solutions are known to result in substantially reduced NaB swelling, wider and more direct flow paths, and orders-of-magnitude increases in hydraulic conductivity in NaB. Dry-blended mixtures of granular SAP and NaB did not exhibit improved hydraulic conductivity to 50 mM CaCl_2 or 500 mM CaCl_2 relative to barriers composed of 100% NaB or SAP. Wet-blended mixing of SAP with NaB did not result in improved hydraulic conductivity relative to barriers composed of 100% NaB or SAP when directly permeated with 500 mM CaCl_2, however, a one mass-percent addition of SAP resulted in up to a two orders-of-magnitude reduction in hydraulic conductivity when permeated with 50 mM CaCl_2. The use of non-crosslinked (i.e., non-super absorbent) polyacrylic acid was also investigated by permeation of granular mixtures of NaB and polyacrylic acid with 50 or 500 mM CaCl_2. When the weight-average-molecular weight of the polyacrylic acid additive was less than 1,080,000 g/mol, mass-percentage additions as low as 0.75% (the lowest mass fraction tested) resulted in a greater-than three orders-of-magnitude reduction in hydraulic conductivity to 50 mM CaCl_2. Granular mixing of polyacrylic acid did not improve hydraulic conductivity with 500 mM CaCl_2. Polyacrylic acid is hypothesized to clog hydraulically governing (intergranular) porosity.