Delftia and E-Waste: Literature Findings

Our community of annotators had an interesting challenge these last few weeks. We asked you to scour scientific literature and discuss how Delftia and its ability to biomineralize gold could be applied to recover gold from e-waste.

Thanks to our volunteer annotators, light has been shed on Delftia‘s ability to improve e-waste recycling. Here’s what we found:

Authors: Lauren Ramilo, Daiza Norman, Rabeya Tahir, Huma Hashmi

Learn more about the Delftia Hypothes.is Group — Join us on Hypothes.is
*You must join the Delftia Hypothes.is Group to see Annotation links

E-waste Impacts

Love them or hate them, electronics are a huge part of our everyday lives. We’re living in the digital age where society is dependent on these electronics for communication, information seeking, recordkeeping, cloud computing, and more. Because of this, we are producing and disposing massive amounts of electronic waste.

Annually, 44.7 million tonnes of electronic waste are generated. Of this, 40 million tonnes are discarded in a landfill, burned, illegally traded, or treated in a substandard way. Source Annotation

E-waste makes up 2% of solid waste streams, but 70% of hazardous waste in landfills. Source Annotation

Three-quarters of the environmental impacts from mobile phones is due to gold and palladium. Source Annotation

E-waste recycling is not a common practice. This is, in part, due to the lack of efficient and targeted methods. Consequently, massive amounts of waste accumulate in landfills and leach harmful and toxic compounds into the soil and water. Incineration releases pollutants and massive amounts of CO2 into the air. 

Current Methods

Some of our readings looked into the current infrastructure of e-waste recycling, to see where and how Delftia could lend a hand. Metals are the prime target for recycling, and there are currently three different methods of recovering metals from e-waste. 

Three techniques used to recover metals from electronic waste. Made with BioRender.com

Pyrometallurgy

The most popular way to extract metals is pyrometallurgy. It is cost-effective, but is energy-intensive and harmful to the environment (Annotation). Waste is mechanically broken down and separated by magnetic and electrostatic properties. Materials are melted down, and the liquid separates into layers. The metal-containing fraction can then be isolated and further refined. This process requires reliance on fossil fuels and creates volatile byproducts. Also unattractive is the fact that pyrometallurgy can not target and refine specific materials. 

Hydrometallurgy

Hydrometallurgy is a less environmentally intensive method to extract metal than pyrometallurgy, and can also target metals specifically to separate them into high-purity products. Waste is broken down and physically separated, and then treated with a lixiviant (a liquid used to extract desired metals from a medium). Lixiviants are usually strong acids such as cyanide that leach, or extract, metals. Leach liquor, the output, can be refined by applying electric voltage to attract specific metals to the charged surfaces. This process is called electrowinning. 

Biohydrometallurgy

We’ve saved the best for last! Biohydrometallurgy is a sustainable and efficient way to recycle methods in e-waste! Same as before, the waste processed and separated. Then, microorganisms are used to recycle metal in ways that are analogous to natural environmental cycling. 

First, iron and sulfide oxidizing microorganisms are used. Oxidation behavior frees protons from iron or sulfur. These protons attack metals (Cu, Ni, Al, Pb, Cd) and convert solid metals to dissolved metal ions. Then, cyanide-producing microorganisms take the stage. Secreted HCN dissolves precious metals (gold and silver) by forming a water soluble metal-cyanide complex. 

Current methods separate and refine the dissolved metal ions through chemical reactions and electrowinning. We think that another metal-associated bacteria, Delftia acidovorans, better suited to transform gold ions in solution into solid gold. Implementing bacteria into this final step of metal recovery could improve efficiency, cost-effectiveness, and reduce environmental impacts.

The Final Step: Delftia

Delftia acidovorans is an environmental bacterium that secretes a peptide called delftibactin. D. acidovorans is commonly found in places with heavy metals, and it uses delftibactin to keep toxic metal ions away from the cell surroundings. When delftibactin finds these poisonous molecules, it uses a chemical reaction to convert the dissolved ions into solid metal. As far as we know, this survival strategy is entirely unique to Delftia species. Why do we care? Delftibactin is particularly suited for gold recovery. 

Delftibactin is a short chain of amino acids with polyketide characteristics. It binds gold at the (yellow) active site.

Delftibactin is a nonribosomal peptide (NRP), a type of secreted secondary metabolite. This poses a challenge for using Delftia for gold recovery because it is only produced under stressful conditions. However, bacteria must be happy and healthy to produce lots of biomolecules for long periods of time. We have to “hack” delftibactin regulation somehow to make gold recycling a reality. 

Nonribosomal peptides are proteins that are transcribed using enzymes, instead of ribosomes.

Secondary metabolites are molecules that not necessary for reproduction or survival, but confer survival advantages under stressful conditions. 

The research team that won iGEM 2013, team Heidelberg, tried to find a way to make delftibactin-driven gold recovery possible. They inserted the delftibactin gene, called the del cluster, into E. coli for controlled and abundant delftibactin expression. They decided to use E. coli because D. acidovorans grows slow—3 days in ACM, a DIY medium (Annotation1 Annotation2).

In our lab, we use a rich medium called peptic soy broth that can grow cultures overnight. We haven’t observed gold precipitation though, so ACM medium is the current preferred way to culture precipitating D. acidovorans. 

Unfortunately, they found that the delH gene product was toxic to E coli. The delH gene product is a part of the delftibactin peptide, along with delG and delE. Delftibactin can’t precipitate gold without it, but E. coli can’t survive with it. To scale up to industrial requirements, we need to tweak this system to make E. coli tolerant to delH, or come up with an entirely new plan altogether. 

The iGEM team also had trouble inserting the del cluster into E. coli. It’s so big (59,000 base pairs) that it took three plasmids to insert. This has made it challenging to produce recombinant organisms with the del cluster. 

Our Thoughts

We think that it would be possible to insert a regulated promoter into the del cluster. Inserting a gene that lets us control delftibactin expression would make it possible to establish a healthy and happy culture of D. acidovorans. After the culture has been established the inserted promoter can upregulate delftibactin expression to create lots of gold-precipitating delftibactin. Turning off delftibactin gives the culture a chance re-establish its health before turning expression back on. Lather, rinse, repeat!

We are also curious about whether gold-precipitation would be most efficient using biofilms, cell cultures, or isolated delftibactin. Research has observed gold precipitation from each, but no one has compared gold precipitation amongst them. 

What do you think about the e-waste crisis? Can we find a solution within Delftia?

Thanks to the Delftia Hypothes.is group! Join the community to annotate for our next topic: Delftia as a pathogen. 

All annotations tagged e-waste within the Delftia Hypothes.is group are listed below. Annotations that are explicitly referenced are also cited with the text.