Enhanced containment of hypersaline leachates with cyclic organic carbonate modified bentonite
2017-02-21T02:59:18Z (GMT) by
Geosynthetic clay liners (GCLs) are widely used hydraulic barriers in many landfill applications. The favourable geotechnical characteristics (e.g. high swelling and low permeability) of these liners originate from the properties of their bentonite component. As such, GCLs have become the leading material in waste management and disposal facilities. However, the hydraulic performance of GCLs is expected to degrade when subjected to leachates having high ionic strength (I) due to osmotically induced water loss. In this study the cyclic organic carbonate (COC) glycerol carbonate (GC) is proposed for modification of bentonites to improve their barrier performance to saline leachates. Previously propylene carbonate (PC), another COC, was studied for the same purpose. Based on the results gained in this research, PC has limitations in providing a long-term solution for issues related to containment of hypersaline (ionic strength > 1 M) leachates, especially if divalent ions are presented in the leachate. On the other hand, glycerol carbonate has advantageous properties over PC (e.g., higher dielectric – 110 for GC compared to 65 for PC) and the resulting stronger interaction with bentonites overcomes most of these limitations. Under the scope of my study modifications of bentonite using both GC and PC were studied and their relative efficacies in providing saline resistance were compared. The mineralogical properties of the COC-bentonite complexes and potential effects that salt solutions may have on them were studied with XRD and FTIR techniques. Based on the outcomes of these experiments, GC-bentonite had stronger resistance against high concentration NaCl and CaCl₂ solutions and retained more COC compared to PC-bentonite. The hydraulic performance of untreated and COC modified bentonites in NaCl and CaCl₂ solutions with different ionic strengths were tested primarily with swell index (SI), fluid loss (FL) and solution retention capacity (SRC) tests. It was found that while PC-bentonites performed well, the selection of PC may not always be appropriate for containment of hypersaline leachates. Better hydraulic barrier performance was observed for GC-bentonites at high ionic strengths. Furthermore, the performance of GC-bentonite was tested and compared to untreated bentonite in flexible-wall permeameters. From these triaxial hydraulic conductivity tests it was concluded that the GC modified bentonite maintained low hydraulic conductivity in I = 3 M CaCl₂ solution (kTri ≈ 9.0 × 10⁻¹² m/s if the sample was directly contacted with the salt solution and kTri ≈ 8.0 × 10⁻¹² m/s for a specimen initially prehydrated in distilled water) over a long period of time (~20 weeks). In addition to the promising test results reported herein, GC also has the advantage that it can be produced using a cost effective, green chemical method from relatively inexpensive reactants. The significantly greater chemical stability of GC in bentonite complexes compared to PC, provides the opportunity to use it as a bentonite modifier for hypersaline and calcium-enriched applications, where currently no such materials are known. While the research reported herein indicates a strong potential for the successful containment of hypersaline leachates using GC-modified bentonites, it also indicates areas where further work may be required.