This post is based on this article from New Scientist by Michael Allen.
Dear Readers, for many years it’s been known that plants are useful for bioremediation: some species of brassica guzzle up metals such as nickel from the soil, cleaning it in the process, and lichens are also known to help clean up pollutants. It’s thought that plants do this because the metals are toxic, and might therefore help to protect them against insect predators. Such plants are known as hyperaccumulators because they store so much of the element.
However, when Anthony van der Ent, a plant-hunter based at the University of Queensland in Australia, found a shrub called Phyllanthus rufuschaneyi at a park ranger’s station in Malaysian Borneo, he noticed that it oozed a bright blue-green sap. Upon analysis, it turned out that the sap contained 25% nickel by weight.
Nickel is an essential ingredient in products such as computers and smart phones, but will become even more important with the advent of the rechargeable batteries in electric cars. The metal is also needed for wind turbines. It’s estimated that for electric cars alone, the amount of nickel needed will double, to 256,000 tonnes, by 2025. But the normal method of getting the metal is by strip-mining, one of the most environmentally devastating extraction methods: it creates defoliation, soil erosion and pollutant run-off which contaminates sea water and rivers. One of the leading world nickel producers is the tiny island of New Caledonia.
So, would it be possible to grow hyperaccumulating plants so that the nickel could be extracted from them, rather than despoiling the environment? One problem is that the plants don’t grow just anywhere: the metals in the soil are found in areas which had a lot of tectonic activity which meant that instead of just sinking, the elements were raised to the surface. Such soil is known as ‘ultramafic’.
Having found his plant, Anthony van der Ent set about creating the world’s ‘first tropical metal farm’ in Sabah in Borneo. He and his colleagues are growing Phyllanthus ruruschaneyi: every year the shrub is coppiced, the stems and leaves are pulped, and the nickel is extracted. In 2019 they reported a yield of 250 kilograms per hectare, currently worth almost $4000.
A long-time collaborator of van der Ent’s, Guillaume Echevarria of the University of Lorraine in France, also wanted to see what was possible, but using a tropical plant didn’t seem the right way to go. Instead, he used a different hyperaccumulator (not specified in the article but probably an Alyssum species). He has chosen some plots on ultramafic soil in Albania, and the plant is sowed and harvested by local farmers. The plant is then transported to France and burned to produce nickel-rich ash, from which the metal is extracted. The energy yielded by the burning is used as a heat source for nearby buildings, so Echevarria considers that the whole project comes in as carbon-neutral.
Although the results are not as promising as in Borneo, the plant still yields about 200 kilograms per hectare which, at around $3000 at today’s prices still makes this a viable business. For comparison, a hectare’s worth of wheat in the UK can be sold for about $2100.
While Van der Ent thinks that the whole project could be scaled up in areas where there are ultramafic soils, such as Indonesia, Echevarria is more cautious, and I have to say that I would be worried about large scale ‘phytomining’ too. Many areas of the world which are otherwise suitable for growing hyperaccumulators are also biodiversity hotspots and protected areas, and having seen the palm oil plantations in Sabah, the last thing the world needs is more hectares of monocultures. However, there are some areas, particularly in Greece, Albania and Bulgaria, where farms are being abandoned because the soil is so poor for other agricultural applications, and at least growing plants could help to stabilise and revegetate such areas, whilst providing the farmers with some extra income. Echevarria thinks that phytomining could provide a few percent of the global nickel requirements, which is not to be sniffed at.
It’s not just nickel either. Plants that hyperaccumulate arsenic, cobalt, manganese, zinc and rare earths have been discovered. Marie-Odile Simonnot, also at the University of Lorraine, has been assessing Dicranopteris dichotoma, a fern that grows naturally on spoil heaps near rare earth mines in China’s Jiangxi province.
It seems to be possible to harvest about 300 kilograms of mixed rare earth metals per hectare, including lanthanum, cerium, prasedoymium and neodymium from this plant, and Simonnot is working with Chinese scientists to run trials at old mining sites. This seems like a win-win to me, as the plant seems to grow in landscapes that are already environmentally devastated, and which could only be improved by a bit of native plant cover.
Nowadays, though, Van der Ent is no longer trudging through the jungles of Borneo. Instead, he is hunting through the herbariums of the world’s museums with a handheld X-Ray flourescence spectroscope. This gives an instant read-out of the elements that a specimen contains, and hundreds of new hyperaccumulators have been found in this way. Who knows what other secrets the plant kingdom contains? Let’s hope that this time we are able to work with nature to make the most of them, rather than against her.
Photo One by Anthony van der Ent from https://www.newscientist.com/article/mg24933160-900-plants-that-suck-metals-from-the-soil-can-be-farmed-to-make-our-tech/
Photo Two by Gunnar Ries, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons