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Scientists combine chemistry and Bioremediation to Clean Cadmium From Soil

DOE/Brookhaven National Laboratory : 15 January, 2004  (Technical Article)
Scientists at the U.S. Department of Energy's Brookhaven National Laboratory have devised a way to combine chemical treatment with 'pollutant-busting' bacteria to remove cadmium from contaminated soil.
There are two basic ways to deal with soil contaminated with such toxic heavy metals, Vairavamurthy says. One is to mobilize the metals so they can be extracted from the soil and treated. The other is to convert the metals to a stable form that will not migrate into groundwater. To find ways to accomplish one or the other, the Brookhaven team looked for clues from nature.

'Many organisms use thiols, organic sulfhydryl compounds, to detoxify metals in the body,' Vairavamurthy says. Many thiols form soluble complexes with toxic metals, which can then be excreted. 'We thought these might work in soil as well,' he says.

The team was also intrigued by the natural ability of many microbes to detoxify contaminants. 'Bacteria have versatile biochemical mechanisms, which we thought we might be able to exploit intelligently,' Vairavamurthy explains.

In the end, the team has proposed a method that combines both approaches:

First, thiosulfate, an inorganic compound with a sulfhydryl group, is injected into the subsurface soil. Thiosulfate has an affinity for and forms highly soluble complexes with cadmium. Then, when water is pumped out of the soil, the dissolved cadmium thiosulfate should come along for the ride.

The extracted water then can be treated with a new form of bacteria, the Cd-1 strain of Klebsiella planticola, which was recently isolated by former BNL researcher Pramod Sharma from coastal salt marshes on Shelter Island in New York. Through its normal metabolic process, the bacterium converts dissolved cadmium thiosulfate to cadmium sulfide, an insoluble form, which precipitates out of the solution.

Alternatively, the bacteria could also be injected directly into the soil following thiosulfate injection to complete the process in situ. This would leave insoluble cadmium sulfide in place of the original toxic metal. 'Cadmium sulfide is fairly stable and will not move into the groundwater as long as the conditions remain anaerobic,' or without oxygen, Vairavamurthy says.

This in situ technique would potentially be less costly than digging up contaminated soil for treatment elsewhere, and even less costly than the water-pumping method described above. Also, because these bacteria can grow in oxygen-rich as well as oxygen-poor environments, they can be grown in large quantity under normal aerobic conditions in the laboratory, and still survive the oxygen-poor environment below the soil surface. The same technique might also work for treating soil contaminated with some other metals, such as arsenic and cobalt, Vairavamurthy says. This research was funded by the Natural and Accelerated Bioremediation Program of the U.S. Department of Energy.
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