That process, says Banfield, is "what has been envisaged for acid-mine drainage treatment using constructed wetlands which are rich in organic matter. Maybe our work can be used to refine development of these strategies."
The worldwide problem of contamination around mining sites is significant. Acid mine drainage, for example, is a threat to surface and ground water near mines. It occurs when metal-sulfide ores are exposed to air and water and the sulfide is transformed to sulfuric acid. Moreover, metals such as zinc are toxic and can leach into ground water and contaminate wells and other drinking water supplies.
Working with Ken Kemner and colleagues at the Advanced Photon Source at Argonne National Laboratory, the Wisconsin team used the most finely focused high energy X-ray beam in the world to date to show that small but significant quantities of other toxic ions, arsenic and selenium, for example, as well as zinc are extracted from ground water and concentrated in the biofilms.
Another group, led by UW-Madison professor of physics Gelsomina De Stasio, used a novel X-ray microscope to perform a chemical analysis showing that the sulfur compounds found in the biofilms were indeed sulfides.
"The capability of the microscope enabled us to determine that there are a lot of sulfates in the sample, everywhere, whereas localized on the bacterium were sulfide deposits," De Stasio says, "This leads us to think for the first time that these bacteria metabolize sulfates and precipitate sulfides as byproducts."
In essence, then, what Banfield and her group have found is a natural process by which microbes - bacteria from a family known as Desulfobacteriaceae - produce sulfide that scavenges zinc and other toxic metals from the surrounding ground water.