By Sally Pobojewski
News and Information Services
Adding finely ground shale to barrier or “slurry” walls appears to be an inexpensive and effective way to retard the spread of toxic contaminants from landfills and hazardous waste sites, according to U-M researchers.
“Natural shale could prove to be an ideal material for use in slurry walls and clay landfill liners,” said Richard W. Gullick, graduate student in civil and environmental engineering.
“Shale may improve the effectiveness of slurry walls by preventing toxic substances from diffusing through the wall and contaminating ground water. Because it is a natural material, it should have excellent long-term viability and should contain no harmful trace metals or toxic substances. It also is inexpensive, readily available and relatively easy to work with.”
Gullick presented results of his experiments at the American Chemical Society (ACS) meeting in San Diego earlier this month. The research is the topic of Gullick’s dissertation for a Ph.D. degree in environmental engineering.
The Environmental Protection Agency sometimes requires the construction of slurry walls around hazardous waste sites to prevent water from flowing out of the site and contaminating ground water with common toxic organic substances, such as trichlorobenzene (TCB) and trichloroethylene (TCE).
To build a slurry wall, engineers dig a trench about one yard wide and often 30 or more feet deep around the waste site, Gullick explained. Excavated soil from the trench is mixed with bentonite—a special type of clay that expands in water and essentially plugs the pores of the soil. This soil-bentonite mixture is then placed back in the trench.
Like plugs in a leaky dike, slurry walls typically do an excellent job of blocking the flow of ground water and ground water-borne contaminants from landfills, according to Gullick. But they do little to stop toxic chemicals from diffusing through the wall to aquifers on the other side.
“Diffusion is a very slow process, but over a period of years, it can create significant pollution problems around landfills,” Gullick said. “Depending on the toxic substance and local soil conditions, up to 10 percent of the total concentration of a landfill contaminant can diffuse through the slurry wall within about 15 years, and up to 50 percent within 50 years. Once outside the landfill, it is often easy for hazardous substances to get into nearby drinking water supplies.”
Nothing can stop diffusion, but engineers can slow the process considerably by adding materials they call “sorbents” to the slurry wall. Sorbents attract and hold molecules of toxic substances “like a fly stuck to fly paper,” according to Gullick.
Gullick is now testing what engineers and scientists call the “sorptive capacity” of shales against those of three varieties of commercially available synthetic organo-clays, which have been proposed for use as sorbents in landfill barriers. Organo-clays are prepared in laboratories through chemical modification of natural clays to improve their ability to trap contaminants.
One of the organo-clays Gullick tested—trimethylphenyl ammonium bentonite (TMPA-bentonite)—proved to be most effective at trapping three common landfill contaminants—TCB, TCE and methyl isobutyl ketone. Pulverized natural shale obtained from a deposit in northern Ohio proved to be more effective than the other two types of organo-clays tested, and was nearly as effective as TMPA-bentonite.
“TMPA-bentonite was the best sorbent, but at a price of approximately one dollar per pound, it’s not a feasible choice for large-scale operations such as slurry walls,” Gullick said. “Shale is much less expensive. Even including the expense of transporting, grinding and mixing shale with bentonite and backfill soil during construction of a slurry wall, the costs should be minimal.”
Even though current EPA design criteria for slurry wall construction do not require the addition of sorbents to retard the diffusion process, Gullick believes environmental engineers will still be interested in adding shale.
Gullick’s research is funded by the National Institutes of Environmental and Health Sciences. Others involved in the research project are Walter J. Weber Jr., the Earnest Boyce Distinguished Professor of Civil and Environmental Engineering, and Mark A. Schlautman, post-doctoral fellow.
