UPTON, NY -- Some of the most polluted soil and waste ash in the world may have met its match with a new process based on natural ingredients and developed at the U.S. Department of Energy's Brookhaven National Laboratory.
The patented process, which uses simple citric acid, naturally occurring soil bacteria and sunlight, is described in the current edition of Environmental Science & Technology by two scientists from BNL's Department of Applied Science. It extracts metal contaminants from soil and wastes and then converts them to a concentrated and stable form.
The researchers report that their process removed nearly all the toxic metals and uranium from soil taken from polluted sites in Ohio and Tennessee. It also successfully cleaned incinerator ash from a municipal solid waste plant. In addition, it may be useful for other wastes and sludges.
"The resulting cleaned soil is much less hazardous and costly to dispose of, and can even be re-used," said A. J. Francis, one of the co-authors and co-inventors of the process. "Since the process separates the metals from the radioactive elements, the problem of mixed toxic-radioactive waste disposal is solved, the amount of waste is diminished greatly, and it's possible to reclaim the metals for a beneficial use."
Among the metals that can be separated from soil and ash using the process are cadmium, lead, zinc and copper. It can also remove radioactive elements, or radionuclides, such as uranium, thorium, plutonium, cobalt, cesium and strontium.
The process starts with a "washing" of the soil with liquid citric acid, the same acid found in oranges and lemons. The acid binds to the metals and radionuclides, carrying them with it when it washes through the soil. This forms compounds called metal citrates and, in the case of uranium uranyl citrate.
The citric acid is much less destructive on the soil than other methods, and does not linger in the soil after treatment like other chemical agents do.
After the citric acid washes through the soil, it is recovered, and naturally occurring soil bacteria are added to the liquid. The bacteria degrade several metal citrate complexes, but not the uranyl citrate complex. The metals can be easily removed from the liquid, allowing them to be recycled.
Meanwhile, the uranium-citric acid mixture goes through one more step: exposure to sunlight. The light breaks down the bond between the acid and uranium, allowing it to be separated in a concentrated form. Unlike other processes, there is no secondary waste stream -- the citric acid can even be re-used.
In their paper, Francis and his colleague Cleveland Dodge report that the process removed 99 percent of the uranium from soil and sludge taken from real-world polluted sites.
And, in a related study, this process has been shown to remove more than 95 percent of lead and other toxic metals from municipal solid waste incinerator ash; lead from soils contaminated with lead paint; arsenic from wood ash residues; and cadmium, copper, lead and zinc from electric arc furnace dust.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences, and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
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Environmental Science & Technology