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An interview (22 March 2017) by Holly Squire (@HollyEsquire) for The Anthill (a podcast for The Conversation), highlights the work we are doing here at CNAP to understand how palladium group metals (PGMs) are taken up by plants.
Platinum group metals are rare elements that are important in an increasing number of biotechnological applications. However, with demand spiralling upwards, these metals are becoming increasingly dispersed and diluted into the environment. Calculations suggest that global reserves of PGMs will be depleted in the next 5-50 years [1]. Of perhaps even more concern is that PGMs are vulnerable to geopolitically-controlled supply restrictions, and in many applications, lack suitable substitutes [2]. These metals also have relatively high cradle-to-grave environmental impacts per kilogram [2] and it is therefore essential that these metals are utilised and recycled responsibly, and not lost into the environment.
Phytomining, the use of plants to extract metals from the environment, is not new, but the costs of growing, harvesting and transporting the metal-rich plant biomass, in addition to the costs of smelting to the base metal, have been prohibitive to the development of metal phytoextraction. Our research [3,4], in collaboration with Prof James Clark and Dr Andrew Hunt at the University of York's Green Chemistry Centre for Excellence, has developed methods which add value to PGM-rich plant biomass as a critical step towards viable phytomining.
Using hydroponic studies, we have shown that the PGM palladium is taken up by plants and deposited as metal nanoparticles. Following a low-energy pyrolysis step, this metal-rich plant biomass can be used directly as a catalyst comparable with commercially available Pd/C catalysts. Life cycle analysis conducted by Prof Thomas Graedel and Dr Nedal Nassar at Yale University suggests that this process is not only environmentally friendlier than current catalyst production methods, but also adds value beyond that of simply extracting the bulk metal making it potentially economically viable [4]. But how do our laboratory-based studies using Arabidopsis thaliana (Arabidopsis), a tiny, weedy plant that is wonderful for molecular genetics but unsuitable for phytomining, compare with a real-world environment? In collaboration with Dr John Meech and Dr Scott Dunbar, at the Norman B. Keevil Institute of Mining Engineering, University of British Columbia, and Dr Chris Anderson at Massey University, New Zealand, we have been able to test plants on palladium mine wastes. These wastes still contain significant levels of palladium, but extraction is uneconomic using existing mining techniques.
Using mustard, and the biomass crop species miscanthus (Miscanthus giganteus; kindly supplied by our collaborators Mike Cooper at Miscanthus Nurseries Ltd and David Stone at Agrikinetics Ltd) and willow (Salix sp.), our studies show that yes, these species are able to withstand and grow on these waste materials, which contain levels of metals such as nickel that are toxic to many plant species. However, palladium uptake is much lower than in our controlled laboratory experiments [4]. To develop mechanisms to increase the concentration of palladium in the plant, we need to understand the fundamental genetics behind palladium uptake, and for that we are going back to Arabidopsis…
While the ultimate aim of this research is to develop plants that can be used to extract these metals from waste sources, such as mine wastes, there are several additional benefits that may even outweigh phytomining. In recent years, a number of dams holding mine wastes have collapsed, with devastating consequences (such as the collapse of a Samarco-owned tailings dam, Brazilian 2015, and the Mount Polley mine disaster in British Columbia, 2014). Introducing plants, with their penetrating roots and canopies, could help to stabilize these areas, reduce levels of toxic contaminants in rainwater run-off and restore lost ecology to these environmentally-damaged areas.
For further information see our collaborative website Phytocat.
Doroshenko A. et al. (2018) Using in vivo nickel to direct the pyrolysis of hyperaccumulator plant biomass. Green Chemistry DOI: 10.1039/c8gc03015d. Follow link here.
Harumain, Z. A. et al. Toward Financially Viable Phytoextraction and Production of Plant-Based Palladium Catalysts. Environ Sci Technol 51, 2992-3000, doi:10.1021/acs.est.6b04821 (2017). Access here.
Hunt, A. J., et al. The importance of elemental sustainability and critical element recovery. Green Chemistry 17, 1949-1950, doi:10.1039/C5GC90019K (2015). Access here.
Graedel, T. E. et al. Criticality of metals and metalloids. Proc Natl Acad Sci U S A 112, 4257-4262, doi:10.1073/pnas.1500415112 (2015). Access here.
Parker, H. L. et al. Supported palladium nanoparticles synthesized by living plants as a catalyst for suzuki-miyaura reactions. PLoS One 9, e87192, doi:10.1371/journal.pone.0087192 (2014). Access here
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