Tuning extracellular cytochromes for enhanced metal recovery and nanoparticle formation

Platinum group metals (PGMs) are exceptionally rare, high value metals that have important roles in electronics and industrial goods. One-quarter of all manufactured goods either contain a platinum group metal, or require a platinum group metal during the production process. The PGMs are extremely costly to produce, with the majority of PGM mining production in South Africa and Russia. The high price and low abundance of PGMs in natural environments means that recycling from waste electronic and industrial devices is a potentially economically viable mechanism to return valuable materials back into usage as part of a circular economy.
Bacteria have the potential to recover PGMs from waste streams, as they can transform metals into different states using electrons that are released by the bacteria during metabolism. Adding electrons to a metal is a process known as reduction and changes the properties of the metal, causing it to aggregate into a solid mass known as a nanoparticle. Nanoparticles can be used as catalysts in important industrial reactions and are a valuable product in themselves.
Often metal reduction processes happen inside the bacterium, which limits the size of the nanoparticle and can harm the cell, limiting its ability to survive. However, some bacteria can reduce metals on the surface of the cell, through a process known as Extracellular Electron Transfer (EET). This is adventitious as it makes the nanoparticles easier to harvest while not interfering with the internal metabolism of the cell or limiting the size of the nanoparticle.
The bacterial family known as Shewanella are used for studies on PGM reduction because their surfaces are coated with proteins known as cytochromes, which makes them highly efficient at EET and metal reduction. The cytochromes that coat Shewanella can be grouped into four different clades, and these four groups have shown varying affinities for different metals suggesting that the overall specificity of Shewanella for different metals can be tuned depending on the types of cytochrome expressed on the cell surface.
In this project we aim to extensively characterise the different interactions between these cell surface cytochromes and PGMs, specifically the metals iridium, platinum and palladium. These high value metals are present at low concentrations in waste effluent produced during the recycling of electronic devices. Our proposal aims to identify how soluble PGM interact with the different cytochromes (Objective 1), and understand how these interactions lead to the formation of nanoparticles in different waste streams (Objective 2). We will also use these findings to maximise PGM recovery from industrial waste streams (Objective 3).
In Objective 1 we will determine where and how these precious metals associate to the different cytochrome. This will be achieved by first measuring the rate of electron exchange between cytochrome and PGMs at different metal concentrations. Objective 2 will use techniques developed in our laboratory to study these cell surface cytochromes. A light sensitive chemical bound to the cytochrome provides a continuous supply of electrons into the cytochrome. This will be used to study the different stages of PGM reduction on the cytochrome surface and study for the first time the initial steps of nanoparticle formation. We will also use synthetic membrane systems called vesicles to reduce the cytochromes and use these to explore the mechanism of formation of larger PGM nanoparticles. Finally in objective 3 we will use Shewanella cells optimised for enhanced cytochrome expression to improve the reduction and recovery of specific PGMs in different metal mixtures.
These research objectives will show how Shewanella cytochromes can be used to capture different PGMs, and provide routes for further research around improving specificity as well as engineering systems for use in recovering metals from different waste streams.

Platinum group metals (PGMs) are precious metals that have high technological importance and are limited in amount and supply. Their efficient recovery from waste streams, where they are present in low concentrations, is an unsolved problem facing the goal of a circular economy. Some bacteria, such as Shewanella sp., are a potential solution for waste stream metal extraction as they can directly reduce soluble metal ions using a range of different redox active outer membrane cytochromes (OMCs) on the surface of the cell. This reaction precipitates the metal into nanoparticles on the cell surface, although currently the precise interaction between cytochrome and nanoparticle is unclear. In this project we propose to study the ability of these OMCs to reduce and precipitate soluble PGMs as nanoparticles. In particular, we will focus on the interactions between reduced OMCs and the PGMs iridium, platinum and palladium that are often present in industrial waste streams. We will initially study the interaction between soluble PGMs and cytochromes. This will involve measuring the formation of PGM-OMC complex after reaction of reduced OMC with oxidised PGMs. We will also solve the structures of PGM bound cytochromes using X-ray crystallography. Once the affinity and mechanism of interaction between specific PGMs and OMC is realised, we will investigate how PGM nanoparticles can be formed with OMC as the nucleation centre. This will use technologies developed in our laboratory including photoactive labelled OMCs, and synthetic vesicles that function as OMC reductases. As our understanding of OMC/PMG interactions progresses, we will obtain industrial electronic waste samples from our project partners and extract PGMs from these using our optimised Shewanella strains. Consquently, this research will both improve understanding of the mechanisms of biological nanoparticle formation, and enable us to directly apply it to optimise metal recovery from industrial waste streams.