Bacteria clean up metal waste, then serve as catalysts.

Synopsis by Evan Beach, Dec 09, 2010

Gauthier, D, LS Sobjerg, KM Jensen, AT Lindhart, M Bunge, K Finster, RL Meyer and T Skrydstrup. 2010. Environmentally benign recovery and reactivation of palladium from industrial waste by using gram-negative bacteria. ChemSusChem 3:1036-1039.

A group of Danish scientists has developed a method to recycle valuable metals that would ordinarily have to be mined and refined before ending up in chemists’ hands. Their discovery means that the metals could be sourced instead from electronic waste or polluted water and soil.

The researchers used two species of bacteria and added hydrogen gas to recover the waste metals – palladium, platinum and rhodium – in a cheaper and more efficient way than conventional processes. Interest in using microbes to remove metals from waste is growing among scientists who are searching for the best methods.

This is the first time that researchers report that they can remove these platinum group metals from industrially contaminated water without altering the bacteria or diluting the liquid. Remarkably, the bacteria could remove up to 100 percent of the palladium from the polluted water.

Mining, industrial activities and manufacturing release these specific metals into the environment, where they can contaminate soil and water. All three of the metals examined are widely used in automotive, chemical, glass, electrical, medical and jewelry applications.

The microbes used in the study are naturally tolerant of metals. One species can be found in typical soils, and the other is more commonly found in industrial areas, near mines and metal factories.

The bacteria bind and absorb metal ions dissolved in water. Hydrogen gas can also remove metal from the water. Metal uptake and recovery are enhanced when the two are combined.

The contaminated water used in the study contained a mixture of eight different metals and was deep orange colored. Hydrogen gas and bacteria with and without added palladium were added to test tube samples.

The liquid cleared after 24 hours, indicating the metals had been removed. The bacteria were most selective for palladium – the recovery rates were 96-100 percent, compared to 70-74 percent for platinum and 55-57 percent for rhodium.

After recovering the bacteria, researchers asked what could be done with the metal-rich material. They went a step further and found a productive use. They showed that the microbes could drive a common chemical reaction that uses palladium to connect two hydrocarbon building blocks, a method often used in synthesizing pharmaceuticals. The conversion rates were 50-100 percent. The effectiveness was higher when the bacteria were pretreated with a small amount of pure palladium before exposure to the wastewater.

Further experiments will be aimed at understanding how the metals compete for the absorption sites on the bacterial surface, and thus, produce treatment methods that select for specific metals. In turn, the selective, one-metal binding could result in more active catalysts to be used in conventional processes.