Cement prison for old radioactive waste
to set a trap for it in the ground. The researchers are betting that the same chemical similarity that draws strontium-90 to bone will bind it into a microbe-made calcite “prison.”
Calcium is an essential ingredient for calcite. Fortunately, there is no shortage of calcium in the ground at most sites with old radioactive contamination.
Much of the groundwater in the arid American West contains so much dissolved calcium that crusty deposits of the mineral often clog water pipes near the DOE’s Washington and Idaho sites.
Getting that calcium to turn into calcite on location and on demand requires molasses, urea and a little help from microbes. First, the microbes chow down on molasses and multiply, swelling their numbers by tenfold or more. A second course of urea, a cheap nitrogen fertilizer, prompts part of the expanded microbe population to generate calcite. They do so by using a protein called urease, to convert urea and water to carbonate and ammonium.
The carbonate then joins with calcium in the ground and groundwater to form calcite.
Ammonium, on the other hand, performs the critical function of scrubbing strontium-90 ions off solid surfaces. The contaminant tends to cling to dirt and rock and resists efforts to extract it from soil. But ammonium, like strontium-90, carries a positive charge, which helps it boot strontium-90 off the surfaces of solids.
The research team has screened the Washington and Idaho sites to see if microbially generated calcite might offer a solution at contaminated sites in the West. Fujita and her colleagues describe the results of testing samples from the Washington site in a September 2010 paper in the journal Environmental Science & Technology. In addition to containing enough calcium and the right chemical conditions to support long-lived calcite formation, the water and soil at that site contained plenty of bacteria that could make urease and calcite — as many as several thousand such microbes in a drop of water or a gram of sediment.
Earlier studies of molasses and urea injections into the groundwater near INL showed that the microbes there will make calcite, too. Indeed, many places around the world contain all the ingredients to make microbial calcite. For example, urea and microbes have been used to make sustainable bricks from sand in Abu Dhabi, and have been proposed as a way to repair stone surfaces in Europe.
Before researchers can use the technique to clean up contamination
