Engineers Find New Ways to Decontaminate Soil

August 24, 1998
Contact: Leila Belkora (312) 996-3457; [email protected]

ENGINEERS FIND NEW WAYS TO DECONTAMINATE SOIL

Engineers at the University of Illinois at Chicago are experimenting with promising new methods to remove metal and organic contaminants from soil without excavating the site. By fine-tuning a common technique known as electrokinetics, adapting it to the chemistry of particular pollutants and soils, they have achieved 90 percent removal rates for metals. They also demonstrate that electrokinetics, previously limited to extracting metals, can be made to work on organic contaminants in low-permeability soils. The problem of soil contamination is acute, and current techniques don't work in all situations, said Krishna Reddy, assistant professor of civil engineering and one of several engineers pursuing soil remediation at UIC. The Environmental Protection Agency estimates it must clean up over 200,000 sites around the country at a total cost of about $187 billion in 1996 dollars. The sites include former dumping grounds used by the electroplating industries, underground storage tanks at gasoline stations, weapons laboratories and civilian sites. "Industries and other polluters used to dump waste liquids on the ground before we had laws against those practices," said Reddy. "Now we must remediate these sites, or the pollutants will get into the ground water and cause problems." Electrokinetics works on the principle that opposite electric charges attract. Engineers at contaminated sites, which are typically about football-field size, drill an array of wells into the soil, place electrodes or wires inside, and connect the electrodes to a small direct-current source. This sets up an electric field between pairs of electrodes, pulling positively charged pollutants toward the negative electrode, or cathode, and negatively charged pollutants toward the positive electrode, or anode. It may take years for pollutants to migrate through the soil, but eventually they concentrate around the wells and can be pumped out or excavated from a small area.

"Electrokinetics has advantages over other approaches," said Reddy, who worked as a professional engineer before coming to UIC. "You're not excavating and disposing of the soil on a large scale, so it's safe, and you don't disturb the site operations." In practice, however, he encountered limitations to the technique. "All the research was done under idealized conditions, with one type of clay soil, and one contaminant, lead," he said. In his own research on metal contaminants such as cadmium, chromium and nickel, Reddy discovered that soil chemistry and synergistic effects between different elements of the mix of contaminants may limit the usefulness of the technique. For example, positive and negative ions composing the pollutants migrate at different rates through the soil, causing changes in soil acidity across the site. In turn, the changes in soil acidity may cause contaminants to condense out of the water that carries them along, like salt crystals forming along a string in a beaker of brine, so that contaminants do not reach the electrodes.

"I asked, what are the mechanisms that prevent us from totally removing contaminants, and how can we counter those hindering mechanisms," said Reddy.

To better remove metals, Reddy devised an approach using so-called purging solutions. Engineers would apply water-soluble chemicals around the cathodes to correct the soil acidity problem, draw pollutants out, then, perhaps, repeat the procedure on the anode, until all types of contaminants are cleared out. In ongoing research, he seeks to optimize the approach, so that engineers in charge of site remediation can select the most cost-effective method based on the soil type, purging solution concentrations and amount of time and energy required.

An Li, assistant professor of environmental and occupational health sciences at UIC, is working with Reddy on the problem of remediating soils contaminated with organic products such as polycyclic aromatic hydrocarbons, known as PAHs, or polychlorinated biphenyls, known as PCBs.

"It's hard enough to remove metals, but for organics, a further problem is that they don't dissolve in water," and so can't be carried through the soil in standard electrokinetics, said Li.

The trick in this case, she said, is to add something to the water that will enhance the solubility of the organic material. The substance added is called a co-solvent, because water is the primary solvent.

If engineers choose the appropriate co-solvent, "the organic material will dissolve in the co-solvent, then will get carried along with the water," said Li.

"We expect that combining electrokinetics and use of a co-solvent will efficiently remediate low-permeability soils, like clay, that are contaminated with organic material," she said.

Both Li and Reddy will present their research groups' results at the annual meeting of the American Chemical Society in Boston, Aug. 23-28.

-- UIC --