While most people associate bacteria with disease, many can be useful, not only for human health but also for protecting the environment. Biochemists from Masaryk University’s Faculty of Science, together with their Austrian colleagues, want to use bacteria to help solve the problem of metals that accumulate in fly ash and slag during waste incineration and go on to harm the environment.
According to scientists, with the help of bacteria and the appropriate recycling biotechnology, metals can now be removed from the end products of waste incineration. What is more, these rare raw materials can then be put back into production, thus supporting the country’s self-sufficiency in minerals.
The researchers will be targeting metals that accumulate at high concentrations in slag and fly ash generated during waste incineration, which is then usually dumped in landfill sites in the countryside or used in construction. Not only are the metals contained in the slag and fly ash an unused resource, they can be very harmful for the environment.
Bacteria will play a crucial role at two stages in the proposed procedure, which scientists are now verifying thanks to a new international project. First, bacteria in the bioreactor produce sulphuric acid and surfactants. The sulphuric acid and other metabolites produced are applied to another bioreactor containing the slag or fly ash, where they are leached by the acid. This leaching is further promoted by acid-forming bacteria, some of which are additionally able to oxidise iron.
In describing the advantages of the procedure, Jiří Kučera from the Department of Biochemistry (Faculty of Science, Masaryk University) said that “Bacterial production of sulphuric acid is more environmentally friendly and cheaper than its chemical production. In addition, surfactants are produced that contribute to the dissolution of metals. Thanks to this, it is possible to convert precious metals found in the slag and fly ash into a solution called leachate”. He explained that the process of bioleaching metals from mineral ores has been known to scientists for several decades and is mainly used in other countries for copper mining.
An innovative benefit of the technique being proposed here is the ability to recover dissolved metals from the solution so that they can then be reused. And this is where bacteria once again enter the equation, though not the acid-forming bacteria used in the first phase. These are naturally occurring fermentation bacteria, which can be found, for example, in the lower layers of sludge at wastewater treatment plants.
Jiří Kučera went on to say that “the metal-containing leachate is then applied to a microbial fuel cell, which relies on the principle of decomposition of organic matter (preferably waste) by bacteria in an anaerobic environment around the anode. This metabolic process produces electrons and protons. Electrons flow to the cathode and form an electric current, while the protons pass through a membrane toward the cathode, where they combine with oxygen to form water. Pure metals, such as zinc and copper, are then obtained at the cathode”.
According to scientists, while the amount of electricity produced during this process is not so crucial, the recovery of metals contained in slag and fly ash is a very positive finding. “Our research results so far show that, using this method, we can extract up to 100 percent of some metals contained in the waste material. In other words, clean ash and slag with no metals. What is more, we can then reuse the precious metals obtained”, added Jiří Kučera.
The effectiveness of the proposed procedure is now being tested by scientists in collaboration with partners from the Universität für Bodenkultur in Tulln, Austria, who undertake applied research into bioleaching optimisation in bioreactors, and the K1-MET GmbH metallurgical centre in Linz, where bioelectrochemical processes in microbial fuel cells are being tested.
“Our common task now is to transfer optimised laboratory-scale bioprocesses to a larger pilot plant, and to ensure that the bacteria can adapt to increased concentrations of metals”, Jiří Kučera explained. Further results will be presented by the scientists involved over the summer.
Within the project, evaluation of the economic and environmental aspects of the proposed procedure is the task of the Vysočina Energy Agency, which, as a project partner, also ensures access to the waste management sector. The project has received funding of almost 20 million crowns from the European Regional Development Fund under the Interreg V-A Austria-Czech Republic cross-border cooperation program, of which almost a third went to Masaryk University.
Translated by Kevin Roche.
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