Bacteria instead of hammer drills
Now that ore mining has dwindled in Germany, many mines only open up their galleries and shafts to visitors. Scientists on the team headed up by BRAIN staff members Dr Esther Gabor and Dr Yvonne Tiffert explored underground mines on specially guided tours.
“We were led to untouched places where hardly any contamination from the outside where to be expected,” Gabor reports. The team took samples at many former mines – the silver and vitriol mine in nearby Schriesheim; Rammelsberg in the Harz region, a World Heritage site; the Haslach silver mine in the Black Forest, and a mercury mine in Obermoschel in Rhineland-Palatinate. At their lab in Zwingenberg, the BRAIN scientists also examined a 700-metre-long core sample drilled from a deposit in Storkwitz, Saxony, as well as other rocks rich in metal, in the hopes of finding signs of life. The results were overwhelming.
We isolated 250 strains of bacteria from the core sample alone – all of them were very undemanding bacteria that feed on residues of enclosed organic substances and only divide every few hundred to one thousand years.
In all, the campaign provided a pool of 2,000 different bacterial strains that have adapted to a metal-rich environment.
In view of the growing scarcity of metal raw materials, BRAIN is particularly interested in finding out how the microorganisms living in deep rock layers interact with metals in their environment, how they access essential elements and break down toxic ones. Some microbial strategies might help industry to safeguard its supply of valuable metals.
In the core sample from Storkwitz, for example, the BRAIN scientists discovered bacteria of the Bacillus and Pseudomonas genera that have a strong preference for scandium. Scandium is a technically relevant light metal that belongs to the rare earth family. It makes aluminium alloys stronger and is used in laser crystals, stadium lamps and data storage media. Scandium is mainly mined in Russia, whereas most of the other rare earth elements come from China. The metals are washed out of the ground using strong acids. The wastewater, sludge and often radioactive tailings produced during this process pollute the environment.
A ton of printed circuit boards from computers contains about 500 grams of gold. The aim is to unlock this treasure trove, not by means of problematic chemicals in Asia or Africa, but right here in Germany using nature’s own tools.
Can bacteria make the extraction of high-tech metals more efficient and environmentally friendly? Does biotechnology even offer a way out of dependency on raw materials from countries such as China? The discovery of bacteria with an affinity for scandium raises hopes in this respect. They selectively bind dissolved scandium even where there is an excess of other rare earth elements or mass metals like copper, iron and calcium. BRAIN is now developing a simple flow-through process that separates scandium from leach solutions, wastewater and other types of solution. This technology can be transferred to other metals, too. While Gabor and her colleagues continue to search for organisms that bind other rare earth metals, they have already isolated bacteria that bind gallium, indium, palladium, silver and gold.
The BRAIN scientists are also working on biotechnological processes to extract metals from solids. Bacteria are to be used to extract residues from waste incineration and old electrical equipment as well as from ores. Incineration bottom ash offers a similar gold content to gold ores, and e-waste even contains several hundred times that content. A ton of printed circuit boards from computers contains about 500 grams of gold. The aim is to unlock this treasure trove, not by means of problematic chemicals in Asia or Africa, but right here in Germany using nature’s own tools.
Bioflotation: foam with a “golden lining”
Some bacteria have a weakness for gold. They attach to gold-containing particles and give them a water-repellent surface. “That makes them ideal for recycling waste that contains precious metals,” explains BRAIN engineer Benedikt Hoffmann, who is developing a separation technique called bioflotation. For this, an aqueous suspension of the ground starting material is mixed with inactivated bacteria and stirred while adding air. The particles coated with gold by the cells rise to the surface with the air bubbles, forming a foam, and thus separate themselves from the useless remaining material. The precious metal can be melted out of the skimmed-off foam.
Bioflotation is to be scaled up in cooperation with a metal extraction industry supplier. BRAIN has installed a 200-litre fermenter to cultivate the cells. At present, Hoffmann is optimising the nutrient medium in order to produce the bacteria efficiently and cost-effectively. This is no easy task, given that wild-type strains are used. In a study for the German Federal Environment Agency, scientists at BRAIN have already shown that gold-containing particles can be separated from incineration bottom ash by bioflotation.
Bleaching : gentle soil washing
About 15 percent of copper extracted worldwide is dissolved from the ore using bacteria. Bioleaching is more environmentally friendly and efficient than conventional leaching using strong acids. “The microorganisms attach to the rock and produce acids precisely where they are needed,” explains BRAIN Project Manager, Dr Esther Gabor. However, the process comes up against its limits in connection with starting material that contains lime, such as copper slate.
On the one hand, the lime neutralises the acids, and on the other, the sulphur bacteria used for bioleaching only flourish in an acid medium. BRAIN therefore proposes a two-stage process. First, the ore is treated with alkalitolerant bacteria that dissolve the lime. Only then are the sulphur bacteria added to initiate bioleaching proper. Among the alkali-tolerant microorganisms, BRAIN scientists have also discovered some that dissolve the precious metals. They enable bio-leaching of alkaline ash, sludge and other residues from which the precious metals have so far not been recoverable. BRAIN is currently setting up a joint venture with companies from the recycling sector.
Biosorption: perfect scandium binders
Rare earth metals can be found in electric cars, windpower systems and many other high-tech applications. BRAIN has shown that biotechnology can help safeguard supplies of metal raw materials, using the light rare earth element scandium as an example. In a screening process, over 10,000 different strains of bacteria were incubated in a leach solution containing all 16 stable rare earth elements. Analysis of the bacteria’s interaction with metal showed that certain strains only bind scandium.
BRAIN is now developing a biosorption process using the best scandium binders to isolate the light metal from leach solutions, wastewaters and other liquids. Since scandium binds equally to the surface of living and dead cells, biosorption also works with inactivated bacteria. These are granulated or immobilised on plastic pellets and filled into a column. When scandium-containing solutions flow through the system, the rare earth metal adheres to the cell surfaces. It can be rinsed from the cells using ethylenediaminetetraacetic acid (EDTA) or, in the case of the bacterial granules, recovered by incineration.
