SAN FRANCISCO, CA--New information, based on molecular-scale studies of different metals in soils, may help environmental engineers immobilize these contaminants more effectively, University of Delaware researchers reported April 14 during the American Chemical Society meeting.
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 UD'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," says Sparks, an invited ACS speaker who co-organized a prestigious geochemistry symposium. "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, pronounced CAT-eye-uns) 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, is being presented at the ACS 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 onto 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 UD 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 class of phenomena, and the research has been done so well that it's absolutely definitive. I suspect it's going to put UD's Department of Plant and Soil Sciences on the map."
Clearly, Sparks says, researchers studying the fate of metals in soil will now need to revise their previous calculations. "Traditional models have not reflected the formation of these precipitates," he explains.
To track the reaction of different metals with soils, clay minerals and metal oxides, Sparks' research team uses x-ray absorption fine structure (XAFS) spectroscopy at Brookhaven National Laboratory. Electrons, traveling inside the tube-like XAFS device at a speed almost as fast as light, emit intense x-rays. When the x-rays hit a sample, they produce photoelectrons. By analyzing the energy of these photoelectrons, researchers can identify different chemical fingerprints in a sample. Atomic force microscopy (AFM) then provides a spatial map of precipitates on a soil surface, Sparks says.
Sparks and Tim Grundl of the University of Wisconsin are co-organizers of a four-day ACS symposium on the "Kinetics and Mechanisms of Reactions at the Mineral-Water Interface." Beginning April 14 at 8:00 a.m. Pacific Time, the symposium will feature 22 invited speakers and roughly 40 contributed papers from around the world. The speakers represent a broad range of disciplines--from soil chemistry, geology and geochemistry to civil and environmental engineering.
Research by Sparks and his students is supported by the DuPont Co., the State of Delaware, the U.S. Department of Agriculture and the U.S. Geological Survey. A book will be published based on the symposium proceedings.
ACS presentation information - Donald L. Sparks:
San Francisco Hilton, Imperial B, Ballroom Level
Monday, April 14, 1997, 8:00 a.m. (Pacific Time)