PETAL: Developing a plant-based platinum group metal recovery system

Platinum group metals, including palladium, are relatively rare and increasingly important in developing technologies as nano-sized metal particles (a human hair is approximately 80,000- 100,000 nanometers wide). Metal nanoparticles (NP) are used as catalysts in chemical synthesis, and in biomedical sciences including biosensing systems, drug delivery and cancer treatments. Palladium is an industrially important metal with applications in catalytic converters, chemical processing, the manufacturing of electrical conductors and pharmaceuticals. However, metals are finite resources, and calculations suggest that, at current consumption rates, global reserves of Pd for example, will last perhaps 100 years. Of perhaps even more concern is that these metals are vulnerable to geopolitically-controlled supply restrictions; with over 99% in South Africa, Russia, Zambia, and the United States. With no appreciable reserves in the UK or Europe, and no suitable substitutes in many technological applications, it is critical that exiting supplies in the UK are recycled. But where have these reserves gone? Historically, PGMs have been discarded as diluted metal waste alongside road verges, or buried in landfill as electronics waste. The resulting mixed-metal pollution is expensive to decontaminate, with few viable technologies able to tease-apart the individual metals, and no financially viable, environmentally sustainable methodologies currently available.
Plants have an exquisite ability to selectively take-up and store metals from the environment, and can be used to scavenge metals from their surroundings, a process called phytomining. While using plants to extract metals from the environment is not new, the costs of growing, harvesting and transporting metal-rich plant biomass, in addition to the cost of smelting to the base metal, have been prohibitive to the development of this technology. At the University of York, we have demonstrated that, following a low-energy microwave step, plant-derived palladium NP-containing biomass can be used directly as effective catalysts. This use adds value to the phytoremediation process.
The purpose of the research is to develop plants that can extract Pd from soils, with future aims to translate this technology into plant species that can be used to recover PGMs from wastes. Our industrial partners Yorkwaste Ltd, will supply sweepings from road verges to test our technology. There are four main objectives of the research:
1. To assess the ability of cyanide-producing plants and bacteria to solubilise the relatively inert metals from the soils so that they can be taken up by plants. To do this, we will use pot-based experiments with Arabidopsis plants grown in pristine soil dosed with Pd and with and without the bacteria, or intercropped with cyanogenic Lotus japonicus.
2. Evaluate the efficacy of Arabidopsis plants expressing azurin-Pd
Azurin is a small (~14kDa) bacterial, copper-containing protein with a characteristic deep-blue colour. A mutant of azurin (azurin-Pd) has been identified that binds Pd and Pt. We will produce Arabidopsis lines expressing azurin-Pd in their shoot tissues.
3. Quantification of Pd-specific peptides Q7 and Pd4 to seed palladium NPs in plant aerial tissues
Researchers have designed peptides (small proteins) that when mixed with solutions of palladium seed the production of NPs. This project will transfer the genes that make these peptides into plants to increase the number, size and shape of palladium NPs in the plant tissues.
4. Testing ability of Pd-rich pyrolysed biomass to catalyse key reactions
We have demonstrated that, following a low-energy pyrolysis using microwaves, plant-derived Au- and Pd NP-containing biomass can be used directly as effective catalysts. This use adds value to the phytoremediation process.

Platinum group metals including palladium (Pd) are relatively rare and essential for emerging technologies, but these finite resources are running out. Remaining reserves are geopolitically controlled and outside the UK and Europe. The metals have been dispersed into roadside verges from vehicle catalytic converter, buried in landfill as electronics waste or lost in other waste streams, and the resulting diluted, mixed-metal pollution is an increasing problem: expensive to decontaminate, with, remarkably, no financially-viable technologies able to tease-apart the individual metals at the scale required.
Plants have an exquisite ability to selectively take-up and store metals from the environment. At the University of York, we have demonstrated that, following a low-energy microwave step, plant-derived palladium nanoparticle (NP)-containing biomass can be used directly as effective catalysts.
However, uptake rates of PGMs are low. This project will assess methods to increase PGM solubilisation and uptake by plants. We will test the ability of cyanogenic plants and bacteria to increase PGM solubilisation and uptake from the rhizosphere. Once inside the plant, we will test the expression of palladium-binding azurin protein, and peptide sequences, Pd4 and Q7, to increase Pd uptake and deposition as NPs. Pyrolysed Pd-rich biomass samples will be tested for their ability to catalyse a range of Suzuki-Miyaura reactions.
The purpose of the research is to develop plants that can extract Pd from soils, with future aims to translate this technology into plant species that can be used to recover PGMs from wastes. Our industrial partner Yorkwaste Ltd, will supply sweepings from road verges to test our technology.