Soil contamination can be one of the most difficult decontamination operations to
deal with. The objective is to remove contaminants without causing further damage to
the soil. So how do scientists and engineers accomplish such a task?
Carbonated Water Using trichloroethylene to wash soil by vapor
extraction has always been an effective means of cleaning the soil. The problem has
been the five year time period required to remove the trichloroethylene containing the
contaminants. According to Hazama Corporation in Tokyo, washing the soil with water
containing 100-1000 ppm carbon dioxide reduces the time period required to remove the
trichloroethylene by one-third. The dissolved carbon dioxide accelerates permeation
of the water through the soil by osmotic pressure. Trichloroethylene dissolves 4-10
times faster in carbonated water than in still water.
Bacteria Dinitrotoluene (DNT) is a carcinogenic
decomposition product of trinitrotoluene (TNT) and is also produced during polyurethane
production. Researchers at the Illinois Institute of Technology have engineered a
bacteria that can reduce DNT concentrations from 0.5 mg to 0.05 mg per gram of soil very
efficiently. The Burkholderia bacteria is combined with bacterial (Vitreoscilla)
hemoglobin to produce the DNT eaters. The new bacteria can function in extreme
oxygen-deficient conditions as well. To be useful in the polyurethane industry, the
bacteria must be able to eliminate DNT from aqueous solutions as well. Tests show
that the new bacteria are 10 times more efficient than standard Burkholderia in
solution tests as they reduced concentrations from 200 mg/L to 1 mg/L.
Citric Acid Metal contamination in soil has always been a large
concern. The U.S. Department of Energy is tackling the problem with the following
recipe: citric acid, naturally occurring soil bacteria, and a pinch of sunlight.
Preliminary tests indicate that the recipe is able to remove 90% of toxic metals (plus
uranium) from soil samples. The materials that can be removed (which include
cadmium, lead, zinc, copper, uranium, thorium, plutonium, cobalt, cesium, and strontium)
can be separated out and recovered.
First the soil is washed with citric acid which binds to the
contaminants. The citric acid is them removed and treated with bacteria to degrade
the contaminants-citric acid materials. Finally, the citric acid is exposed to
sunlight which releases any uranium in the acid so that it can be recovered. The
citric acid can then be re-used as well. Thusfar, the process has removed more than
95% of lead and other toxic metals from municipal solid waste incinerator ash, lead from
soil contaminated with lead paint, arsenci from wood ash residues, and cadmium, copper,
lead, and zinc from electric arc furnace dust.
References:
1. Chemical Engineering Progress, American Institute of Chemical Engineers,
February 1999, p. 16
2. Chemical Engineering, McGraw-Hill Company, December 1998, p. 23
