University of Delaware Environmental Soil Chemistry Members In The News

Environmental Laboratory Washington Report

May 15, 1997

Vol.8, No. 9

Scientists Learning To Immobilize Metal In Soil

New information, based on molecular-scale studies of different metals in soils, may help environmental engineers immobilize industrial metal contaminants more effectively, University of Delaware researchers reported at the American Chemical Society meeting in San Francisco, Calif., last month.

At the soil's surface, key industrial metals including nickel, copper, chromium, cobalt and zinc, but not lead, form mixed metal compounds that dramatically diminish their mobility in the natural environment, says Donald L. Sparks, distinguished professor and chairperson of the university's Department of Plant and Soil Sciences.

"We have been able to precisely identify the chemical structure of these mixed metal compounds, or precipitates, on various soil/mineral surfaces," said Sparks. "They form quickly, in some cases in just 15 minutes, and they seem to be quite resistant to degradation. We believe these complexes could be an important mechanism for metal sequestration, to prevent them from leaching into surrounding soil or groundwater."

The strategy may prove useful for trapping many metal "cations" (positively charged ions) in terrestrial as well as aquatic systems, explains Kirk G. Scheckel, one of nine graduate students and post-doctoral associates whose work, directed by Sparks, was being presented at the meeting. 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," Scheckel says. "In other words, these metals rapidly accumulate and change, creating a kind of blanket."

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, Scheckel has found that EDTA, a strong chelator that latches into targeted substances like a pair of claws, removes 96 percent of nickel from precipitates on the surface of pyrophyllite, a clay mineral found in soils.

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. Unfortunately, the researchers say, lead is characterized by larger cations, roughly twice the ionic radius of nickel, which don't "fit" into the molecular matrix of the precipitates.

Noel Scrivner, a principal division consultant with the DuPont Co., says Sparks' research team has made a "significant contribution" to environmental science.

"The University of Delaware researchers have provided, for the first time, a firm scientific explanation for why certain metals don't seem to migrate in soils," Scrivner says. "It's a whole new definitive. I suspect it's going to put the university's Department of Plant and Soil Sciences on the map."

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