Membrane and diffusion behavior of a compacted sand-bentonite mixture for hydraulic and chemical containment applications
Date
2016
Authors
Meier, Amara Joy, author
Shackelford, Charles D., advisor
Bareither, Christopher A., committee member
Butters, Gregory L., committee member
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Abstract
Due to the ability of sodium bentonite (Na-bentonite) to exhibit high swell, low hydraulic conductivity, k (≤ 10-10 m/s), and semipermeable membrane behavior when exposed to water and dilute chemical solutions, Na-bentonite is commonly used as a component for engineered barriers (e.g., geosynthetic clay liners (GCLs)), or as an engineered barrier (e.g., compacted Na-bentonite buffers) used to contain solid wastes and liquid contaminants. Compacted sand-bentonite (SB) mixtures typically comprising 5 to 20 % Na-bentonite (by dry weight) are commonly used as alternatives to compacted clay liners (CCLs) for containment of solid wastes and liquid contaminants when a suitable natural clay source is not readily or economically available. In addition, membrane behavior, or the ability of a porous material to selectively restrict the passage of dissolved chemical species (solutes), has been shown to exist in many of these bentonite-based barriers, including GCLs, bentonite amended natural clays used for CCLs, and soil-bentonite backfills for in situ vertical cutoff walls. However, compacted SB mixtures suitable for use as engineered hydraulic and chemical containment barriers previously have not been evaluated for membrane behavior. As a result of these considerations, the purpose of this study was to evaluate simultaneously the membrane and diffusion behavior of a SB mixture that would be suitable for use as an engineered barrier for hydraulic and chemical containment applications. Accordingly, membrane tests were conducted on duplicate specimens of a compacted SB mixture comprising 15 % bentonite that was shown to exhibit sufficiently low k (≤ 2.7 x 10-11 m/s) to be suitable for use as a hydraulic and chemical containment barrier. In addition, the simultaneous diffusion of the principal salt species evaluated in the study, viz., Cl- and K+, was evaluated for one of the specimens. The results indicated that both specimens exhibited virtually the same magnitude of membrane behavior, with measured values of the membrane efficiency coefficients, ω, ranging from 0.395±0.053 to 0.063±0.012 when exposed to KCl solutions with source concentrations, Cot, ranging from 5 mM KCl to 80 mM KCl, respectively. In addition, the diffusion of both Cl- and K+ was found to be restricted relative to the case in which the specimen would not exhibit membrane behavior (i.e., ω = 0). Despite the imposition of chemical conditions in the tests that were more complex than those imposed previously, the measured values of ω and the effective diffusion coefficients, D*, for Cl- were in good agreement with those reported in the literature for other bentonite-based engineered barriers when exposed to similar or the same types of salts and salt concentrations. Thus, this study provides the first results to illustrate that a compacted SB mixture that is suitable for use as a hydraulic and chemical containment barrier behaves as a semipermeable membrane that can restrict aqueous-phase diffusion of chemical species to an extent that the chemical containment function of the barrier is improved.
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Subject
compacted sand-bentonite liners
membrane behavior
bentonite
waste containment
diffusion