Microbially mediated functionalised magnetic nanoparticles from acid mine drainage

Pollution caused by acid mine drainage (AMD) is an enormous ecological problem which is second only to climate change in terms of global risk. AMD is an acidic solution containing high concentrations of toxic metals as a direct result of mining and other industrial processes.

There is a considerable opportunity to combine remediation strategies of AMD, aimed at cleaning up contaminated areas, with the recovery of valuable metals with high economic value. AMD typically contains a range of metals (Fe, Co, Zn, Cu etc.) that are present at low concentrations but remain an untapped resource, especially when the total volumes are taken into consideration. Recovering specific metals from AMD poses a significant challenge, of which technological solutions are currently deficient. However, a major untapped opportunity lies in the high concentration of iron which could be used to produce magnetic nanoparticles through microbiological processes. This proposal will develop a route through which functionalised, high purity, biogenic magnetic nanoparticles can be produced at scale. The global market for nanoparticles is expanding rapidly and is already worth $25 billion, with magnetic nanoparticles an especially important type that can be used for remediation of drinking water, in agriculture, medical therapies, or catalysis amongst others. Nevertheless, current approaches to producing magnetic nanoparticles rely on high cost, unsustainable linear multi-step processes. In contrast, bacteria can be exploited to produce magnetic nanoparticles with tuneable properties under ambient conditions. By turning to bacteria as "mini-factories" we can develop a circular approach where waste, such as AMD, can be used as a feed stock to produce a highly sought after commodity.

This project will take advantage of two types of bacteria which are able to metabolise iron and are ubiquitous in the environment. We will use magnetotactic bacteria, which produce intracellular grains of magnetic nanoparticles via biologically controlled mineralisation. These bacteria use the magnetic grains for navigation and due to the presence of specific channels responsible for the uptake of iron, tend to produce magnetic nanoparticles with extremely high purity. The magnetic nanoparticles produced via this pathway are functionalised with organic coatings that enhance their reactive properties. One draw back of this method though is the relatively low concentration of nanoparticles which are produced. Consequently, we will also exploit iron reducing bacteria, which produce extracellular grains of magnetic nanoparticles via biologically induced mineralisation. These nanoparticles tend to exhibit lower purity than from magnetotactic bacteria but can be produced at much larger scale. By developing two methods of producing magnetic nanoparticles from AMD, this project offers unique opportunities to tackle both scaling and purity issues, whilst simultaneously delivering a high value product which meets the goals of a circular economy.

Metalliferous acid mine drainage (AMD) will be used as an iron substrate required for iron metabolising bacteria to produce magnetic nanoparticles (MNP). Two types of bacteria will be used: 1) Iron(III) reducing Geobacter sulfurreducens, which uses iron(III) as an electron acceptor and produces extracellular MNP via biologically induced mineralisation; 2) Magnetotactic bacteria Magnetospirillum gryphiswaldense which produces intracellular grains of MNP within vesicles via biologically controlled mineralisation. Due to the biological nature of the synthesis, both types of nanoparticles will also be associated with organic molecules (e.g., negatively organic molecules with a variety of different functional groups including quinones, carboxyls, carbonyls, hydroxyls, etc. which can bind to the surfaces of iron minerals) which functionalise the surface of the MNP and enhance reactive properties. In comparison, magnetic nanoparticles produced via chemical or other abiotic pathways need to be coated in an additional step to achieve the same effect.

This project will use AMD from different field sites including County (Cornwall) and Dyffryn Adits (Anglesey), which are the most prominent metalliferous AMD environmental discharges in the UK. By using AMD we are overcoming the unsustainable issue of linear multi-step processing whereby the starting iron substrate for biological nanoparticle synthesis needs to be ultimately derived from an iron salt produced from an ore. The use of environmentally relevant AMD also means transforming a problem material and turning it into a resource. Ultimately, this alternative green synthesis fits firmly into a future circular economy approach to producing MNP which could be used for a broad range of applications.