The idea of using plants to farm metals can no longer be considered new. Phytomining (as it is technically known) was first talked about in the 1990s, when scientists recognised that some plant species naturally accumulate abnormally high concentrations of certain metals. A good example is nickel…. A normal plant has about 20 mg of nickel per kg of dry mass, but in plants called hyperaccumulators this concentration can be as high as 20,000 mg/kg or 2%. This is more than some laterite ores that are today mined for their metal values.
The nickel hyperaccumulator plant species Berkheya coddii from South Africa that grows to 2 metres tall and has a nickel concentration of almost 2% dry weight.
This metal comes from the soil, and plants that absorb such a large amount of metal represent an innovative way to clean up contaminated land. The technology, when applied for remediation, is known as phytoremediation and is today being successfully used in several parts of the world. Phytomining goes one step further. Here the metal is sufficiently valuable to justify processing to produce a metal product that can be sold. Normally the crop is land-filled or otherwise contained. But the economics of phytomining have always been a struggle and this explains why you don't read about successful and profitable phytomining operations around the world. Fresh, new innovative thinking is needed and I think we are starting to see this happen.
Last week, Dr. Liz Rylott from the Centre for Novel Agricultural Products at the University of York in the UK featured on the Anthill a podcast from The Conversation. About 12 mins into the podcast you can hear Liz start talking about using plants to recover platinum metals from mine waste. This is phytomining, and this is exciting work. Croesus is working with Liz and her colleagues through the Phytocat project, where novel applications for the metals in plants are being actively developed.
A paper recently published in the journal Environmental Science & Technology entitled ‘Toward financially viable phytoextraction and production of plant-based palladium catalysts’ tells the technical story of this cool idea (please read it, I am a co-author). This work recognises that plant biomass is not a normal geological material. Plants are made up of carbon. Rocks are not. And for many metals, carbon is a big problem during a process known as hydro-metallurgy (it steals dissolved gold, for example, in a process known as preg-robbing). This means that we have to think differently. We can’t use the same mindset in designing a process to recover valuable metals from harvested crops that the mining industry uses for ores.
Phytomining research is exciting and with tremendous potential. We are running out of metals, and society has to become more clever in 'reduce, reuse, recycle'. I can’t wait to see phytomining evolve into a mainstream technology.