Compatibility of geosynthetic clay liners with acidic solutions
thesisposted on 28.02.2017, 04:10 by Liu, Yang
Geosynthetic clay liners (GCLs) are widely used as hydraulic barriers because of their sealing capacity in contact with water. However, with the increasing interest in their use in mining applications, the compatibility issues of GCLs to mining leachates are of some concern. This research work has been undertaken to enhance our understanding of compatibility of GCLs with strong acidic leachates, as for example, which may occur in mining leachates. Three types of bentonites were selected for this research program. Tests included index tests such as Atterberg limits, swell index, fluid loss and suspension rheology under various concentrations of sulfuric acid solutions. The results indicate that all index parameters degraded when the materials were exposed to acidic solutions. The degradation of these indices would indicate a high probability for the degradation of hydraulic performance of GCLs to occur based on established correlations between hydraulic conductivity and these indices. The mineralogical analyses on bentonite components of GCLs before and after reaction with sulfuric acid solutions were conducted using X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. The dissolution of bentonite, loss of water from smectite interlayers, and the collapse of smectite layers were observed using these methods, these observations explained in part the degradation of the engineering indices and hydraulic performance of bentonites or GCLs. A series of flexible-wall permeameter tests were conducted to directly evaluate the hydraulic performance of GCLs when subjected to different concentrations of sulfuric acid solutions. Effective stress and prehydration condition, both of which can affect the hydraulic performance, were also considered. The hydraulic conductivity was observed to increase with increasing sulfuric acid concentration, but specimens prehydrated with water had lower resulting hydraulic conductivity to sulfuric acid leachates than non-prehydrated specimens. Higher applied effective stress tended to decrease the hydraulic conductivity presumably by compressing the specimens thereby resulting in smaller hydraulically active pores. Three powdered polymers, having either negative, positive or neutral charges, were used in this study to determine whether the hydraulic performance of bentonites under acidic conditions could be improved. Any improvements achieved were only observed under deionized water and low sulfuric acid concentration (≤0.015 M). Generally, no significant differences in test parameters caused by the polymer types (cationic, non-ionic and anionic) were observed under DI water and low sulfuric acid concentration (≤ 0.015 M). However, at higher acid concentrations, these polymers tended to lose effectiveness due probably to pH induced changes in polymer conformation, from presumably elongation to aggregate, at the clay surface. Sulfuric acid is detrimental to the hydraulic performance of GCLs, even if GCLs are prehydrated with good quality water. With better knowledge of how GCLs behave under strong acidic conditions, the geotechnical engineering community will be better equipped to approach and resolve potential problems involving interaction between GCLs and acidic solutions.