Citric Acid Process Cleans Polluted Soil And Incinerator Ash

A new process based on natural ingredients and developed at the DOE's Brookhaven National Laboratory (Upton, NY), cleans polluted soil and waste ash without damaging the environment. The patented process, which uses simple citric acid, naturally occurring soil bacteria and sunlight, extracts metal contaminants from soil and wastes and converts them to a concentrated, stable form.

In tests, the 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. The process may also 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," says 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 by "washing" the soil with liquid citric acid. The acid binds to the metals and radionuclides, carrying the contaminants 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 as other chemical agents do.

After the citric acid washes through the soil, it is recovered and mixed with naturally occurring soil bacteria. The bacteria degrade most of the metal-citrate complexes. The metals can then be easily removed from the liquid, allowing them to be recycled.

The bacteria do not degrade the uranyl citrate complex, however. To degrade this complex, the mixture is exposed to sunlight, which breaks down the bond between the acid and uranium. This allows it to be separated in a concentrated form. Unlike other processes, there is no secondary waste stream—the citric acid can be re-used for multiple processing steps.

Francis and his colleague Cleveland Dodge from Brookhaven's department of applied science described the process in the December 1998 issue of Environmental Science & Technology. In their paper, the researchers report that the process removed 99% of the uranium from soil and sludge taken from real-world polluted sites. In a related study, the process has been shown to remove more than 95% 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.

Brookhaven creates and operates major facilities available to industrial, university and government personnel for basic and applied research in the physical, biomedical, energy and environmental sciences. The laboratory is operated by Brookhaven Science Assoc., a not-for-profit research management company, under contract with the DOE.

For more information, e-mail mrowe@bnl.gov or call 516-344-5056.