On Receipt 7/26/95
RESEARCHERS FIND WAY TO IMMOBILIZE LEAD IN SOIL
COLUMBUS, Ohio -- Researchers at Ohio State University have found a way to immobilize lead-contaminated soil by covering affected areas with finely ground phosphate rocks.
Results show that phosphate rocks reduce the amount of water-soluble lead in contaminated soils by 57 to 100 percent.
"We have found a way to change the nature of lead so that it is insoluble and stable. Even though it remains in the soil, it won't leach into the groundwater, be taken up by crops or be released into the bloodstream if it is ingested," said Terry Logan, professor of natural resources and director of the Environmental Science Graduate Program at Ohio State.
The technology, which has been patented by Ohio State and the Environmental Protection Agency, can also be used to treat soil contaminated with other heavy metals, such as zinc, aluminum, cadmium and possibly uranium. But lead poses the biggest health risk, Logan said.
Lead is highly toxic and, if ingested, can cause serious
damage to the brain, kidneys and nervous system. Young children are especially susceptible to lead poisoning.
Since 1910, American consumers have used more than 11 million tons of lead in the forms of paint and gasoline. Logan and three other researchers began working on the project about five years ago with a grant from the EPA. Their research is now supported by the Florida Institute for Phosphate Rock Research.
The work first looked at the effectiveness of hydroxyapatite, a synthetic compound made from phosphate rocks, in immobilizing lead in soil. Phosphate rocks are primarily mined in Florida and North Carolina.
"What was surprising was how quickly the hydroxyapatite worked to immobilize the lead," Logan said. "It worked within a matter of minutes to just a few days."
When applied to the contaminated soil, hydroxyapatite breaks down into calcium and phosphate. The phosphate combines with the lead to form lead phosphate, which is insoluble and stable. As long as there is an excess amount of phosphate, the compound will not break down and the lead will not be absorbed by crops or seep into groundwater, Logan said.
"Another important thing we found was that it worked regardless of what the source of the lead was, including soluble forms of lead, mineral forms, or even leaky batteries," he said.
The researchers next tried using phosphate rocks directly on the contaminated soil and found that it worked just as well and was cheaper to use than hydroxyapatite. "Phosphate rocks are abundant and easy to mine," Logan said. "It makes more sense to use the phosphate rocks when large areas need to be treated."
Lead-contaminated soil is covered with ground phosphate rocks on a ratio of two parts phosphate to one part lead. To be effective, the amount of lead in the contaminated soil must be known in advance. In most cases, it can be applied just like a fertilizer over the soil surface.
"Since phosphorous is a fertilizer, we were concerned that crops growing in the soil would take out the phosphorous and the lead would be soluble and able to move into the groundwater and be absorbed by the crops," Logan said. "But as long as there is an excess of phosphorous, this won't be a problem."
In most cases, affected areas would only need one application of the phosphate rocks. One exception might be in areas that have extremely high levels of lead, Logan said.
But for alkaline soils, such as those found in the western United States, a liquid phosphate, such as the type found in most garden supply stores, might work better, Logan said.
Using phosphate rocks to treat lead-contaminated soil is different from conventional technologies because it focuses on managing the lead where it is. Other treatments seek to remove the lead from the soil, which can rob the soil of important nutrients, Logan said.
"We took a different approach and focused on managing the lead where it was, which is much more cost-effective and eliminates the need to store the contaminated soil in a landfill or to incinerate it," Logan said.
"Using this technology will cost hundreds of dollars to treat an acre of contaminated soil compared to thousands or tens of thousands of dollars to treat with any other technology."
While the research could prove useful to businesses who must clean up contaminated soil around a plant that uses lead, Logan predicts the technology could have uses in urban neighborhoods.
"A combination of leaded paint and gasoline has caused the soils in some urban areas to be very high in lead," he said. "We envision using our treatment and then covering the surface with a couple inches of clean soil and then planting vegetation."
The researchers plan to begin treatment trials in both urban and rural areas in Ohio later this year. The researchers are also working on a project with the Department of Energy to see if phosphate rocks could be used to immobilize uranium-contaminated soils at the Fernald uranium processing plant near Cincinnati.
"The key to this technology is that it doesn't use another synthetic, man-made chemical," Logan said. "It uses a natural product that we know and understand well to treat a very serious problem in a cost-effective manner."
Other researchers on the project include Samuel Traina, associate professor of natural resources at Ohio State; James Ryan with the EPA; and Qi Ying Ma, at the University of Florida.
The research was published in a recent issue of Environmental Science and Technology.
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Contact: Terry Logan, (614) 292-9043
Written by Kelli Whitlock, (614) 292-9475