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- Description -

Extracting lanthanides from technological waste

Introduction

Earth's resources are finite, and none more so than its elements. Whereas molecules can be converted via chemical reactions (e.g. nitrogen fixation by the Haber process), elements themselves are not practicably interconvertible. Therefore, it is not possible to engineer synthesis routes for rare elements, so they must instead be scavenged and recycled carefully.

Lanthanides are a series of rare earth elements (REEs) that have significant value in electronics, magnets, superconductors, lasers, and more [1]. Despite this, recycling efforts are extremely limited, because individual electronics contain only small amounts of lanthanides, which results in low yields from intensive chemical processes.

[Periodic Table]
Figure 1: Periodic table, lanthanides highlighted

Lanthanide bioaccumulation has not previously been thoroughly explored, because the role of lanthanides in biology has until recently remained poorly characterised. With the recent advances in the field, we now have sufficient biological tools to generate a bioaccumulation strategy for these elements, that has novel principles applicable to other bioaccumulation efforts.

The Problem

Less than 10% of neodymium, a lanthanide, is currently being recycled, highlighting the urgent need to improve recycling efforts from electronic waste [2]. Lanthanides are only used in very small quantities, which makes it very difficult to retrieve these valuable elements from technological waste. As a result, we are depleting our supply of these rare metals [3].

Traditionally, lanthanides are mined from monazite and bastnäsite ores, but these are scarce minerals and require extensive and harsh chemical pathways to extract lanthanides [4]. Furthermore, the mining of lanthanides raises significant social concerns due to harsh working conditions, as well as environmental challenges as we learned through consulting an expert in sustainable development. Our project aims to address and mitigate these issues.

Although there exist pyrometallurgical and hydrometallurgical methods to recycle lanthanides, these methods are energy intensive and produce large amounts of chemical waste [5]. Therefore, there exists a need to recycle lanthanides in a sustainable manner, which is where our project comes into play.

Our Solution

We are developing a strain of bacteria that scavenges for lanthanide ions and swims towards a point for collection through an engineered chemotactic system.

Methylobacterium extorquens (Mex) has natural uptake and sensing mechanisms for lanthanides [6], which we are exploiting and optimising to make Mex an ideal scavenger.

Our engineering approach takes three angles:

  1. Engineer foreign chemotaxis into Mex
    We will engineer in an E. coli MCP that endows Mex with a foreign chemotaxis towards a cheap, accessible chemoattractant, which is expressed in the presence of lanthanides.
  2. Optimise uptake of lanthanides
    By using in silico models, we will determine the optimal rate of transcription of the lanthanide transporter lutH for maximum rate of growth and Ln scavenging.
  3. Mitigate the stress of extra Ln uptake
    Lanthanides can be harmful to cells at high concentrations. Since we are engineering Mex to uptake higher [Ln] than wild-type Mex, we are upregulating a cytosol-localised variant of the lanthanide binding protein LanM. This should sequester excess cellular lanthanides, thereby reducing the stress they cause.

[Chemotaxis]
Figure 2: Ln-activated chemotaxis. Chemoattractant in red
[Uptake in wild-type bacterium] [Uptake in engineered bacterium]
Figure 3: Wild-type vs Engineered uptake

Why this Project?

During our project decision brainstorming, we quickly realised that we shared an interest as a group in the efficient and sustainable use of the Earth's resources.

We found a very promising line of research into the recently characterised lanthanide binding protein lanmodulin (LanM). The papers surrounding this protein discuss lanthanide purification by a proteinaceous affinity-based method, which has been done by the 2021 Calgary iGEM team. We want to take this further and engineer a whole-cell based method for lanthanide scavenging and extraction.

Most bioaccumulation projects focus on immobilised bacteria, requiring systems in place to slowly feed through the substances containing accumulants. Here, we present a bacteria that can accumulate lanthanides in any solution in which it can grow and swim, thus reducing the need for specialist equipment.

[Gas vesicles]
Figure 4: Uptake-activated gas vesicles

Other groups and iGEM teams have used dynamically moving bacteria, such as the 2009 Groningen team with arsenic, and the 2018 Warwick team with lead. These teams upregulated synthesis of gas vesicles upon accumulation of these metals, which makes the bacteria float for easy removal.

Our project takes a step further. The principles behind our project can be adapted for other elements and molecules. In theory, any number of elements or molecules can be scavenged for and enriched, provided that the strains of bacteria used have been either engineered to have, or naturally have, non-overlapping uptake systems and chemotaxis. Therefore, additional projects for other elements or compounds that utilise our uptake-sensing-chemotaxis system can be used in tandem with our bacteria.

Conclusion

Our project reduces both the need for mining lanthanides across the globe, and the reliance of the UK on the import of lanthanide-containing minerals, for which deposits are not found in the UK. We manage this by making the lanthanide recycling process cheaper and more efficient than otherwise would be possible. Additionally, our project is designed to be easily integratable into existing recycling plants, as it acts as a preceding step to current methods.

Whilst engaging with professionals in the recycling industry to explore the implementation of our project, we were pleasantly surprised to find that the industry professionals expressed interest in the project's sustainable approach to metal recycling and its cost-effectiveness, as they were interested in the lower energy requirement compared to traditional processes.

References

^[1] Goodenough, Kathryn M., Frances Wall, and David Merriman. "The rare earth elements: demand, global resources, and challenges for resourcing future generations." Natural Resources Research 27 (2018): 201-216.

^[2] www.rsc.org. “Neodymium - Element Information, Properties and Uses | Periodic Table,” n.d. https://www.rsc.org/periodic-table/element/60/neodymium

^[3] Tesfaye, Fiseha, Hong Peng, and Mingming Zhang. "Advances in the circular economy of lanthanides." Jom 73.1 (2021): 16-18.

^[4] Bailey, Gwendolyn, et al. "Review and new life cycle assessment for rare earth production from bastnäsite, ion adsorption clays and lateritic monazite." Resources, Conservation and Recycling 155 (2020): 104675.

^[5] Periyapperuma, Kalani, Laura Sanchez-Cupido, Jennifer M. Pringle, and Cristina Pozo-Gonzalo. “Analysis of Sustainable Methods to Recover Neodymium.” Sustainable Chemistry 2, no. 3 (September 17, 2021): 550–63. https://doi.org/10.3390/suschem2030030.

^[6] Vu, Huong N., et al. "Lanthanide-dependent regulation of methanol oxidation systems in Methylobacterium extorquens AM1 and their contribution to methanol growth." Journal of Bacteriology 198.8 (2016): 1250-1259.