Effect of Ammonium on the Hydraulic Conductivity of Geosynthetic Clay Liners and Measurement in the Exchange Complex of Bentonite
Setz, Melissa Carolyn
Benson, Craig H.
MetadataShow full item record
Geosynthetic clay liners (GCLs) are hydraulic barriers that consist of a thin layer (<10 mm) of sodium bentonite clay encased by geotextiles or geomembranes held together by needle punching or glue. Sodium bentonite, mainly comprised of the mineral montmorillonite, has characteristically high swell and low hydraulic conductivity making it an ideal material to use in waste containment scenarios, including municipal solid waste (MSW) landfills, to decrease the release of contaminants into the environment. Cation exchange of bound sodium for ammonium (NH4+) in nitrogen rich environments, such as municipal solid waste (MSW) containment facilities, can occur over time potentially affecting a GCL?s hydraulic performance. This study investigated an alternative method to ASTM D 7503, a standard procedure for determining the exchange complex of clay minerals to allow for the measurement of bound NH4+ via a lithium bromide (LiBr) extraction method. This method was used to determine the bound NH4+ in bentonite from GCLs that were permeated with MSW bioreactor leachates having up to 0.05 M NH4+. Hydraulic conductivity tests were also conducted with GCLs using varying concentrations of NH4+ (0.005, 0.05, 0.1, 0.5, and 1.0 M) to understand the effect of NH4+ on the hydraulic conductivity of GCLs. Hydraulic conductivity results represent a snapshot in time as chemical equilibrium (influent NH4+ = effluent NH4+) has not been achieved in all tests and therefore may not reflect the long-term equilibrium condition. The current hydraulic conductivity of a GCL relative to that of DI water is 0.9 for 0.005 M NH4+, 1.6 to 0.05 M NH4+, and 3.3 for 0.1 M NH4+. Currently, solutions with typical NH4+ concentrations for MSW bioreactor leachates (? 0.05 M) do not significantly alter the hydraulic conductivity of conventional Na-bentonite GCLs (< 1.6 times). Low hydraulic conductivity was observed in a mock NH4-bentonite GCL (2.9x10-11 m/s) that indicates that NH4+ in the exchange complex of bentonite has the ability to swell osmotically, and could be related to the low hydraulic conductivity observed in the < 0.1 M NH4+ hydraulic conductivity tests. The hydraulic conductivity of GCLs permeated with 0.5 and 1.0 M NH4+ solutions (e.g. increased ionic strength) are four orders of magnitude greater than that of the GCL permeated with DI water (1.8x10-11 m/s), indicating that solutions containing > 0.5 M NH4+ can greatly increase the hydraulic conductivity of conventional GCLs. In a separate study, GCLs containing bentonite with polymer additives and conventional Na-bentonite were studied by permeating GCLs with inorganic salt solutions of varying ionic strength (I) (1.5, 1.0, 0.5, 0.2, 0.05, and 0.01 M) and ratio of monovalent to divalent cations (RMD) (5, 0.5, 0.05, and 0.005 M1/2) to determine effects on hydraulic conductivity, ion exchange and swelling. The purpose of this study is to determine the thresholds of I and RMD at which the hydraulic conductivity of polymer modified bentonite and convention Na-bentonite GCLs increases appreciably. Iso-hydraulic conductivity lines in the I-RMD space will be created to determine the effectiveness of the polymer modified bentonite in GCLs for various environmental conditions. Three GCLs products were tested and include a bentonite polymer nanocomposite (BPN) GCL, a dry polymer mix bentonite GCL, and a conventional untreated Na-bentonite GCL.