Effect of pH on permeability of clay-sand mixture containing hydroxy polymers

This study was conducted to determine if the effectiveness of hydroxy-Al and -Fe polymers in stabilizing the hydraulic conductivity of smectite-sand mixture increases with their charge density. The effect of pH 5.5 and 7.5 on the saturated hydraulic conductivity (HC) of Na/Ca-smectite-sand mixtures (5% clay and 95% sand) and in the presence of hydroxy-Al and -Fe polymers was studied by leaching with NaCl-CaCl2 solutions at sodium adsorption ratios (SAR) of 10 and 20 of electrolyte concentrations of 500, 100, 50, 10, 5, 3, 2, and 1 molc m-3 and deionized water. The HC of the mixtures essentially remained unchanged as long as the electrolyte concentration of the leachate was ≥50 molc m-3. A further decrease of the solution concentration produced a drop in HC for both the untreated and treated systems at pH 7.5. No HC reduction occurred at pH 5.5 when hydroxy-Al polymers were present. The HC of both treated and untreated systems at pH 5.5 was higher than that at pH 7.5. The HC reduction was two and three orders of magnitude smaller for the treated systems than for the systems without hydroxy polymers. Hydroxy-Fe polymers also reduced the effect of electrolyte concentration in the leaching solution on the HC of the clay-sand mixture, but to a lesser extent. The effect of pH on the charge density of the hydroxy polymers explained their effectiveness in stabilizing the system at the lower pH. The lower HC of the clay-sand mixture in the presence of hydroxy-Fe polymers suggests that the aggregates with this polymer are weaker than those with hydroxy-Al polymers. This difference in stability is due to the higher point of zero charge (PZC) for the hydroxy-Al polymers.