Membrane Behavior of Compacted Clay Liners

The containment function of clay barriers used for waste containment applications (e.g., landfills) can be enhanced if such clays exhibit membrane behavior or the ability to restrict the migration of solutes (e.g., contaminants). In this regard, compacted specimens of a locally available natural clay known as Nelson Farm Clay (NFC), as well as NFC amended with 5% (dry weight) sodium bentonite, were evaluated for hydraulic conductivity, $k$ , and the potential for membrane behavior. The membrane efficiencies of specimens of both soils compacted such that $k$ was less than ${10}^{-7}\text{cm}/\text{s}$ were measured by establishing steady salt (KCl) concentration differences, $-\Delta C_o$ , ranging from 3.9 to 47 mM across the specimens in a flexible-wall cell under closed-system boundary conditions. The measured membrane efficiency for the unamended NFC was negligible (i.e., $\le 1.4\mathrm{\%}$ ), even though the $k$ was suitably low (i.e., $k<{10}^{-7}\text{cm}/\text{s}$ ). In contrast, compacted specimens of the bentonite amended NFC exhibited not only lower $k$ but also significant membrane behavior, with membrane efficiencies as high as 97.3% for $-\Delta C_o$ of 3.9-mM KCl. The results suggest that natural clays typically suitable for use as compacted clay liners (CCLs) are not likely to behave as semipermeable membranes unless the clay is amended with bentonite or the clay is inherently rich in high swelling clay minerals (e.g., sodium smectite). The potential benefit resulting from membrane behavior in a CCL constructed with the bentonite amended NFC is illustrated analytically in terms of liquid flux.