What's great about gallium the metal

16.03.2021 11:18

Biological recycling of high-tech waste: an innovative biotechnological process dissolves gallium from industrial wastewater

Anne-Kristin Jentzsch Communication and media
Helmholtz Center Dresden-Rossendorf

Gallium is a rare metal, but it is widely used in the high-tech industry. This stark contrast makes recycling indispensable. However, previous methods are costly and chemically polluted. Biotechnological approaches therefore use peptides, as they are able to bind metallic particles, minerals and metal ions in an environmentally friendly way and to differentiate them from one another in a targeted manner. Scientists from the Helmholtz Institute Freiberg for Resource Technology (HIF) at the Helmholtz Center Dresden-Rossendorf (HZDR) have now shown that a peptide-based material can be used to extract gallium from production waste water from the semiconductor industry.

Our high-tech world would be inconceivable without rare metals, because they are found in many devices and systems that surround us. Many of these metals are of course only found in small quantities, and they are difficult to break down, which also leads to high costs. This is why recycling plays an even more important role. By recovering the rare metals from industrial wastewater, slag or devices that are no longer in use, they can be returned to the material cycle. HIF researchers around Dr. Katrin Pollmann have demonstrated that selective biosorption, i.e. the ability of certain microorganisms, such as bacteria, yeast, fungi or algae, to enrich themselves with metals or metal ions, is a suitable process for the recovery of gallium from industrial wastewater.

This high-tech metal is found in light-emitting diodes and solar cells, for example, it serves as a coating for mirrors and is used in medicine for tumor diagnostics. It is mainly used in the semiconductor industry. In conjunction with arsenic, it is used in the production of wafers. Most of the industrially used gallium is lost during manufacture. A high recycling rate from the processing industry is all the more important, as they represent a much richer and more important source for secondary gallium production.

Bio-recycling - ecological alternative to classic separation processes
Previous recycling processes for the production of gallium are often based on chemical electrolysis processes. The very energy-intensive electrolysis produces water, which leads to the dilution of valuable substances and the increase in salinity of the wastewater. The situation is different with so-called biocomposites, they are recyclable and can be used again and again in separation processes. This innovative recycling method includes processes that use biological systems to extract metal from ores or waste materials. The release of metals can take place directly through the metabolism of microorganisms or indirectly through their metabolic products.

Biosorption is one of these bio-recycling processes. “The biomass binds certain ions or other molecules in an aqueous solution or concentrates them. Biosorption is not dependent on metabolic activity and does not require the supply of nutrients. This enables their use even in highly toxic environments. Biosorption is therefore an environmentally friendly alternative to recovering metals from industrial wastewater, leaching solutions or mine water, ”explains Katrin Pollmann, head of the biotechnology department at the HIF. The challenge, however, is to find the appropriate biomolecules.

Peptides as a fishing rod
The Freiberg scientists are working on specialized biosorption materials that are able to specifically recognize individual elements, bind them selectively and thus remove them from solutions. They use peptides, small proteins made up of amino acids. “Peptides are small and therefore robust and are therefore particularly suitable. There are 20 naturally occurring amino acids that have different properties and can be linked together in a peptide chain and freely combined. This creates biomolecules that are able to sit on every conceivable surface and recognize them very specifically. The right peptides are selected using the phage surface display method, ”describes HIF scientist Dr. Nora Schönberger the procedure.

The focus of this method is on bacteriophages, i.e. viruses that specialize in infecting bacteria. The element-specific peptides are firmly anchored on a carrier material with certain properties. The resulting biocomposites “fish” the target substances from a complex mixture of materials due to the unique peptide structures. Peptides thus enable selective metal recovery.

Biotechnological approach with gallium-binding peptides
In the joint project “EcoGaIN”, in which the extraction of gallium from production waste water from the semiconductor industry was investigated, Schönberger worked intensively on the selection of the gallium-binding peptides. “We used chromatopanning - a technique that selects peptides that bind to a specific target. With the help of this separation process, we determined the special peptides that capture the gallium ions from the process wastewater, "explains Schönberger and continues:" For the biosorption studies, I packed a peptide-based filter material in a column and built a kind of gallium filter, through which the sewage could run. The material is well suited if it retains the gallium in the filter material as efficiently as possible, while the other contaminants that are contained in the process wastewater, i.e. above all arsenic, simply flow away. Then the gallium bound to the filter material must be able to be separated again from the column in order to recover the gallium. "

One of the five tested peptide filter materials was shown in these experiments to be particularly suitable for efficiently recovering gallium. “The use of biological systems in resource technology complements the repertoire of conventional metallurgical processes. It currently fills a methodical niche by making metals accessible that cannot be efficiently exploited with classical metallurgical approaches, ”says Pollmann. However, this method has to be further developed for industrial use, since the chemical synthesis of the peptides is too expensive for economic use in resource technology and is not yet sufficiently environmentally friendly. Furthermore, it is important to optimize the peptides in such a way that a better metal binding capacity is achieved and thus a more efficient use is possible.

Scientific contact:

Dr. Katrin Pollmann | Dr. Nora Schönberger
Helmholtz Institute Freiberg for Resource Technology at the HZDR
Tel .: +49 351 260-2946
Email: [email protected] | [email protected]

Original publication:

N. Schönberger, C. Taylor, M. Schrader, B. Drobot, S. Matys, F. Lederer, K. Pollmann, Gallium-binding peptides as a tool for the sustainable treatment of industrial waste streams, in Journal of Hazardous Materials ( DOI: 10.1016 / j.jhazmat.2021.125366)

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