July, 1997, p.8. Remediation In Review One of the ironies of modern life in Western nations is the need for more and more scientists and engineers to clean up the mess created by our breathtaking technological success. Efforts to restore poisoned soil and water systems increasingly involve collaborative approaches from geochemists, environmental engineers, and biologists, working on cross-disciplinary projects. In April, at the annual meeting of the American Chemical Society (ACS) in San Francisco environmental scientists discussed the results of some current remediation efforts. Immobilizing Metals University of Delaware researchers have been conducting molecular-scale studies to determine the behavior of various metals in soils. At the soil's surface, key industrial metals -- including nickel, copper, chromium, cobalt, and zinc -- form mixed metal compounds that are considerably less mobile in the natural environment. Researchers from Delaware have been able to precisely identify the chemical structures of these mixed metal compounds, or precipitates, on various soil/mineral surfaces, reported Donald Sparks, chairman of the Department of Plant and Soil Sciences. He presented the researchers' findings at the symposium, "Kinetics and Mechanisms of Reactions at the Mineral-Water Interface," held at the ACS annual meeting. "They [the metal compounds] form quickly -- in some cases, in just 15 minutes -- and they seem to be quite resistant to degradation," said Sparks. "We believe these complexes could be an important mechanism for metal sequestration, to prevent them from leaching into surrounding soil or groundwater." The strategy could prove useful for trapping many metal "cations," positively charged ions in terrestrial and aquatic systems, added researcher Kirk G. Scheckel, an associate of Sparks. Smaller cations, such as nickel, can promote the degradation of aluminum found in soil minerals. The native aluminum then complexes with nickel to form a "mixed cation hydroxide phase," rapidly accumulating and changing to create a kind of blanket, said Scheckel. Lead, however, is characterized by larger cations, which don't fit into the molecular matrix of the precipitates. These results suggest a way to immobilize metals within surface precipitates. Contaminants might also be removed more easily from surface precipitates, using traditional cleanup techniques such as soil washing. Metals such as nickel form mixed metal compounds at neutral and slightly alkaline conditions and at relatively low metal concentrations on the soil's surface, Sparks also reported. Consequently, it should be possible to enhance the formation of these surface precipitates by simply liming the soil. Noel Scrivner, a principal division consultant at the DuPont Company, says that Sparks' research team has made a "significant contribution" to environmental science. "The ...... researchers have provided, for the first time, a firm scientific explanation for why certain metals don't seem to migrate in soils," says Scrivner. "It's a whole new class of phenomena...." "Traditional models have not reflected the formation of these precipitates," noted Sparks, | |
