Bio-Fishing for Rare Earth Recycling
Selective Separation of Minerals By Phage-bound Peptides
- Fig. 1: Illustration of the Biokollekt concept.
- Fig. 2: Examining the number of binding phage clones on the LAP target material. The phage clone binding is x-times better than that of the phage clone Pep3. The data was generated from at least six independent experiments.
- Fig. 3: Examining the specific phage bindings, which present the Pep7 peptide on the phage coat, to a mixture of LAP and ZnS (A-C) and mixture of LAP and SiO2 (D-F). The green arrows indicate the LAP particles; the blue arrows indicated the ZnS particles; and the red arrows indicate the SiO2 particles. The light microscopic images show all existing particles in the image section (A, D). LAP particles always glow green (B, E) when excited with a FITC fluorescence filter. Mineral particles were 5-15 µm in size; mineral-bound phage were made visible by the phage-specific antibodies and glow red (C, F) in the TRITC filter.
Rare earth elements (REE) are a group of seventeen elements consisting of scandium, yttrium as well as what are known as lanthanides. These elements are found only in a few regions worldwide in quantities worth mining. REEs are considered key components in the high-tech industry and are utilized, for example, in wind turbines, smartphones and energy-saving lamps.
Their recycling from end-of-life electronic products is very inefficient compared to the recycling of precious metals. As yet, only yttrium (31%), europium (38%), terbium (22%) and praseodymium (10%) are recycled in quantities worth noting .
Common end-of-life products include energy saving lamps, which contain fluorescent powder that is rich in rare earths. This fluorescent powder is currently collected separately so that it can be separated according to its valuable components in the future within new recycling processes.
In figure 1, a new recycling process of this type based on highly selective biomolecules is shown. These biomolecules are identified using phage surface display (PSD) technology, for which the inventor, George P. Smith, was awarded the 2018 Nobel Prize for chemistry . They bind very selectively to their carrier material (Fig. 1., Stage 1). Material-specific biomolecules are to be chemically or biologically produced in the future and coupled with a carrier material (Fig. 1, Stage 2). Depending on the carrier, the bio-collectors can then find use in different separation processes, such as in flotation (Fig. 1, Stage 4). There, the highly specific peptides can serve as a collector that binds the target material from a material mixture (Fig. 1, Stage 3). The next stage is to remove the loaded bio-collectors from the mixture and recover the target material separately from the bio-collector (Fig. 1, Stage 5).
The first stage could be successfully demonstrated using PSD technology. This technology is based on using a commercially available bacteriophage library. It contains viruses that use bacteria as host organisms and which differ from each other in their appearance by the additional presence of a certain peptide sequence per phage.
Peptides are short protein fragments that bind to their target material. Phage libraries contain up to 2x109 different phage types, which vary in their peptides. The search for the perfectly suited peptide for the target material can be compared to a keychain (bacteriophage library), with which the right key (peptide) is sought for the lock (target material). To locate the perfectly suited peptide for the target material lanthanum phosphate, doped with cerium and terbium (LaPO4: Ce3+, Tb3+, LAP), pure lanthanum phosphate (LAP) was mixed with one such library. Much of the contained phage could not bind to the LAP and were washed away. A small portion of the phage, however, showed an increased affinity for the LAP. These phage were then chemically separated from the material and amplified. In the subsequent stage, the amplified bacteriophage, which now all possess a higher affinity for LAP, were mixed with LAP as a phage library. By using rinsing solutions with a modified composition, the loosely bound phage were washed away. The more strongly bound phage could in turn be chemically separated and amplified, and then be used again in a total of three to five such selection rounds called biopanning. Following these selection rounds, up to sixty individual phage clones were analyzed for the genetically encoded peptide sequence. These peptides differ in their amino acid composition and are located on the phage coat. Phage clones with frequently occurring peptide motives and those with interesting amino acid compositions were amplified, and each phage was individually examined for its capacity to bind to LAP. It is in this manner that the researchers specifically identified peptides for the different fluorescent powder components. The peptides Pep1 and Pep7 were characterized as particularly good LAP binders [3,4]. In comparison to the wild-type bacteriophage, which presents no additional peptides on its coat, the peptide-expressing phage Pep1 (371x better) and Pep7 (102x better) bind with high affinity to the target material LAP (Fig. 2).
Phage, which present the peptide Pep7 on their coats, were added to a mixture of LAP and ZnS (Fig. 3, A-C) and a mixture of LAP and SiO2 (Fig. 3, D-F). After their interaction with the material mixture, bound phage were detected through fluorescent microscopy using fluorescent antibodies. It was found that the phage-coupled peptide Pep7 has little or no affinity for other materials such as ZnS or silicate, while simultaneously binding LAP with high affinity and specificity.
Developing bio-based separation processes
Working with bacteriophage is laborious, and reliable production of additional peptides on the phage coat is not always guaranteed. For this reason, all future work stages will be based on peptide use, free of their phage bodies. Such peptides are chemically synthesized or produced from bacteria with the help of the heterologous expression method. Through modification using different functional groups, the peptides can be coupled to a carrier material (Fig. 1, Stage 2). Bound to a carrier material, such peptides can be used in suitable separation processes, such as foam flotation, as specific mineral collectors. They can selectively separate the target material from the material mixture (Fig. 1, Stage 4). The carrier material-peptide complexes are then to release the bound target material again and are to be used in new separation rounds (Fig. 1, Stage 5).
The price of the peptides and carriers as well as the selectivity of the method ultimately determine the economic efficiency. Producing peptides in small quantities is very expensive. If, however, they are produced on a scale of tons, then these processes are certainly competitive with existing mineral processing methods. Furthermore, suitable biotechnological production methods can help in reducing prices.
The identification of phage-bound peptides, which specifically bind to the fluorescent powder LAP, but hardly to other materials in the fluorescent powder, showed that the idea of selective separation of minerals by peptides is in principle feasible. Using the phage surface display method also for other economically interesting materials is the focus of future work. The planned separation processes must simultaneously be developed and optimized.
This project was supported by a Marie Curie International Outgoing Fellowship, part of the EU 7th Framework Programme for Research and Technological Development.
Franziska L. Lederer
Helmholtz-Zentrum Dresden-Rossendorf, Helmholtz Institute Freiberg for Resource Technology
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