PHYTO4METAL: Development of an integrated phytomining system for the recovery of metal nanoparticles from mine waste

The ever-growing build-up of mine waste across the planet constitutes an opportunity for the recovery of residual metals of technological significance. Whilst exploiting these low-grade resources via conventional techniques is impractical, a plant-based technique known as phytomining offers an environment-friendly and cost-effective alternative. Some plants, dubbed hyperaccumulators, can uptake extraordinary amounts of metals into their harvestable parts. What's more, under certain conditions some plants have the ability to accumulate metals like gold, silver, and copper and reduce them inside their living tissue to nanoparticles (NPs) with unique, beneficial properties. Since NPs have a myriad of biomedical, agricultural, environmental, and technological applications, there has been increased interest in biogenic NPs due to advantages over chemically synthesized NPs. In this light, producing NPs via phytomining heralds great promise as it would represent a low-cost, eco-friendly, and carbon-neutral strategy to farm nanoparticles from residues.
This project brings together the expertise from Scotland's Rural College (SRUC), the University of York, the University of Edinburgh, and the University of Nottingham, along with critical industrial knowledge from Scotgold Resources and Promethean Particles. We propose the use of UK native plants for the exploitation of the Cononish gold mine tailings, and the retrieval of the biosynthesized NPs through enzymatic processing. We will conduct a comprehensive array of glasshouse and field experiments to enhance the process through several green approaches, including the use of beneficial microorganisms and environment-friendly compounds to promote plant growth and metal uptake. The harvested plant biomass will undergo advanced enzymatic processing for NP recovery. With the aim of fostering the circular bioeconomy, we will determine the potential of the waste biomass for production of value-added bioproducts. Following thorough characterisation, a preliminary evaluation of the recovered NPs potential will be carried out. The project will be complemented with a life cycle assessment and techno-economic analysis for determining its environmental impacts and hot-spots identification.
The project will generate unprecedented knowledge about using phytomining to produce NPs from metal-rich wastes. The proposed work will help elucidating numerous processes related to metal uptake and biosynthesis of NPs in living plants. The research will facilitate a first-time assessment of biogenic NPs, which will help inferring their potential in the industrial, agricultural, environmental, and biomedical sectors. Additionally, this proposal will enable the development of new enzyme technology to support the R&D sector, and the design of specific high-value bioproducts from the waste biomass for commercial-scale production. This project will be a critical steppingstone to the development of an important biotechnology-based solution for recovering technology-relevant metals. Indeed, its strategic value is highlighted by the participation of two key players in the UK mining and nanotechnology industry.

The strategic significance of some elements found in sub-economic ores and soils has enabled the advancement of phytomining. Moreover, several studies have demonstrated the biosynthesis of Ag, Au, and Cu nanoparticles (NPs) in planta. Although Au can be amply found in mine wastes, field-scale development of Au phytomining has been hampered by environmental concerns related to the use of chelating agents. These chemicals, typically CN-based, increase Au bioavailability but their reduced biodegradability, leaching risk, and toxicity have drawbacks. However, research suggests that this obstacle could be surmounted through the inoculation with PGPB and the application of eco-friendly compounds (e.g., glycine, also a precursor for microbial HCN production). It should be noted that the low-concentrations, high-biodegradability, and targeted rhizospheric release observed in biological cyanidation negates the detrimental effects of the chemical approach used in hydrometallurgy.
This project combines phytomining with enzymatic processing for NP recovery. The feasibility of this approach will be significantly greater than that of conventional phytomining, with a key factor being the value of the NP-form of the metal, rather than bulk metal salts. Additionally, this proposal is fully grounded on green principles, using a carbon negative plant-based system, and prioritising the use of beneficial microorganisms and environment-friendly compounds to promote plant growth and metal uptake. Likewise, the project advocates for a circular system, recovering metals from mining waste and converting them into high value NPs of significant industrial potential. Furthermore, biomass waste will be used to produce value-added products. The project will generate invaluable knowledge about in vivo biosynthesis of NPs by plants, related techno-economic aspects and environmental impacts, novel enzymatic technology, and biomass valorisation, which can be readily transferred to the industry.