Tuning extracellular cytochromes for enhanced metal recovery and nanoparticle formation

铂族金属(PGM)是一种特别稀有的高价值金属，在电子产品和工业产品中具有重要作用。四分之一的制成品要么含有铂族金属，要么在生产过程中需要铂族金属。PGM的生产成本极高，PGM的大部分开采都在南非和俄罗斯。自然环境中的高价格和低丰度的PGM意味着，从废弃的电子和工业设备中回收是一种潜在的经济可行的机制，可以将有价值的材料作为循环经济的一部分重新投入使用。细菌有可能从废流中回收PGMS，因为它们可以利用细菌在新陈代谢过程中释放的电子将金属转化为不同的状态。向金属中添加电子是一个被称为还原的过程，它会改变金属的性质，使其聚集成一种被称为纳米颗粒的固体质量。纳米粒子在重要的工业反应中可以用作催化剂，本身就是一种有价值的产品。通常，金属还原过程发生在细菌内部，这限制了纳米颗粒的大小，可能会损害细胞，限制其生存能力。然而，一些细菌可以通过一种被称为细胞外电子转移(EET)的过程来还原细胞表面的金属。这是不确定的，因为它使纳米颗粒更容易获得，同时不干扰细胞的内部新陈代谢或限制纳米颗粒的大小。被称为希瓦氏菌的细菌家族被用于PGM还原的研究，因为它们的表面覆盖着被称为细胞色素的蛋白质，这使得它们在EET和金属还原方面非常有效。覆盖Shewanella的细胞色素可以分为四个不同的分支，这四个组对不同的金属表现出不同的亲和力，这表明Shewanella对不同金属的总体特异性可以根据细胞表面表达的细胞色素的类型进行调节。在这个项目中，我们的目标是广泛地描述这些细胞表面细胞色素与PGMS之间的不同相互作用，特别是金属Ir、铂和钯。这些高价值金属以低浓度存在于电子设备回收过程中产生的废水中。我们的建议旨在确定可溶性PGM如何与不同的细胞色素相互作用(目标1)，并了解这些相互作用如何导致不同废流中纳米颗粒的形成(目标2)。我们还将利用这些发现，最大限度地从工业废流中回收PGM(目标3)。在目标1中，我们将确定这些贵金属在哪里以及如何与不同的细胞色素相关联。这将通过首先测量不同金属浓度下细胞色素和PGMS之间的电子交换速率来实现。目标2将使用我们实验室开发的技术来研究这些细胞表面细胞色素。一种与细胞色素结合的光敏化学物质为细胞色素提供持续的电子供应。这将被用来研究细胞色素表面PGM还原的不同阶段，并首次研究纳米颗粒形成的初始步骤。我们还将使用被称为囊泡的合成膜系统来减少细胞色素，并使用这些系统来探索更大的PGM纳米颗粒的形成机制。最后，在目标3中，我们将使用为增强细胞色素表达而优化的Shewanella细胞，以提高不同金属混合物中特定PGMS的还原和回收。这些研究目标将展示如何使用希瓦纳氏菌细胞色素来捕获不同的PGM，并为围绕提高特异性以及用于从不同废流中回收金属的工程系统的进一步研究提供途径。

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.