University of Delaware Environmental Soil Chemistry Members In The News

Chemical Engineering Progress: July, 1997, p.30.

Immobilizing Metal Soil Contaminants Modeled

New data, based on molecular-scale studies of different metals in soils, may help environmental engineers immobilize these contaminants more effectively, University of Delaware researchers say.

Dr. Donald A. Sparks notes that at the soil's surface, industrial metals including nickel, copper, chromium, cobalt, and zinc -- but not lead -- form mixed metal compounds whose mobility is severely diminished in the environment. "We have been able to precisely identify the chemical structure of these mixed metal compounds, or precipitates, on various soil/mineral surfaces. They form quickly, in some cases in only 15 minutes, and they seem to be quite resistant to degradation. We believe these complexes could be an important mechanism for metal sequestration -- preventing them from leaching into surrounding soil or groundwater."

He believes the strategy may prove useful for trapping many metal cations. Smaller cations such as nickel can promote the degradation of aluminum found in soil minerals. That's good news, he adds, because it suggests 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. For example, EDTA -- a strong "chelator that leaches onto targeted substances like a pair of claws -- removes 96% of nickel from precipitates on the surface of pyrophyllite, a clay mineral found in soils," Sparks notes.

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 says. Consequently, he notes, it should be possible to enhance the formation of these surface precipitates by simply liming the soil.

On the other hand, lead, the researchers say, is characterized by larger cations -- approximately twice the size of nickel -- and do not "fit" into the molecular matrix of the precipitates.

"This shows why certain metals do not migrate in soils. Clearly," Sparks adds, "researchers studying the fate of metals in soil will not need to revise their previous calculations because traditional models have not reflected the formation of these precipitates."

To track the reaction of different metals with soils, clay minerals, and metal oxides, the researched used x-ray absorption fine structure spectroscopy. Electrons, traveling inside this instrument at a speed near the speed of light emit intense x-rays that produce photoelectrons when they hit the sample. By analyzing the energy of these photoelectrons, the researchers were able to identify different chemical fingerprints in the samples.

Funding for the research was provided by the DuPont Co., the State of Delaware, the U. S. Department of Agriculture and the U. S. Geological Survey.

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