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A research team at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has managed to purify water containing uranium using a special kind of bacteria known as magnetotactic bacteria.
In a paper published in the Journal of Hazardous Materials, the scientists explain that the name of the bacteria derives from their ability to react to magnetic fields. They can accumulate dissolved heavy metals in their cell walls. These research findings also shed new light on the interaction between uranium and bioligands.
“Our experiments are geared towards potential industrial applications in the field of microbiological remediation of water, especially when it is contaminated with heavy metals of the type you find in mine drainage water in the old uranium mines,” Evelyn Krawczyk-Bärsch, lead author of the study, said in a media statement.
Krawczyk-Bärsch explained that because they exhibit a feature that differentiates them from other bacteria, magnetotactic bacteria form nanoscopic magnetic crystals within the cell. They are arranged like a row of beads and so perfectly formed that humans would currently be unable to reproduce them synthetically. Each individual magnetic crystal is embedded in a protective membrane.
Together, the crystals and membrane form the so-called magnetosome which the bacteria use to align themselves with the earth’s magnetic field and orientate themselves in their habitat. It also makes them suitable for simple separation processes.
Magnetotactic bacteria can be found in almost any aqueous environment from freshwater to saltwater, including environments with very few nutrients.
They can also survive at neutral pH values, even in aqueous solutions containing higher concentrations of uranium. Over a wide pH range, they bind the uranium almost exclusively in their cell walls—an excellent basis for dealing with the conditions found in water associated with mining. None of the uranium penetrates into the interior of the cell in the process, nor is it bound by the magnetosome.
According to the paper, it was already known that different types of bacteria could bind heavy metals in their cell walls despite being quite differently structured. In the case of magnetotactic bacteria, the cell walls are formed of a peptidoglycan layer, a macromolecule composed of sugars and amino acids which is the main component of the cell walls of many bacteria. Such a layer is only four nanometers thick.
The cell walls of magnetotactic bacteria are surrounded by an external membrane composed of sugars and fat-like components: potential docking sites for uranium.
“Our results show that in magnetotactic bacteria peptidoglycan plays the main role in absorbing uranium. This knowledge is new and unexpected in this type of bacteria,” Krawczyk-Bärsch said.
Her team even managed to identify three specific uranium peptidoglycan species and confirm their findings with reference samples.
“It’s conceivable this could be done on a large scale by carrying out the treatment right in the surface water or by pumping water from underground mines and directing it to pilot treatment plants,” she noted.
In her view, using magnetotactic bacteria could be an effective alternative to expensive, conventional chemical treatments—because these bacteria are undemanding in terms of upkeep; implementing other biomass-based solutions, by contrast, regularly fails due to the costs involved in increased nutrient and energy requirements.
Krawczyk-Bärsch pointed out that another detail has sparked the researchers’ interest in these bacteria: their proteins can stabilize divalent and trivalent iron so that the magnetite stored in the magnetosomes can be synthesized.
“So, we are really asking ourselves how these microorganisms interact with radionuclides in various oxidation states. In particular, we are thinking of plutonium,” co-author Johannes Raff said.
