To pluck three golden apples from the garden of the Hesperides, Hercules had to hold the sky on his shoulders instead of Atlas. The pioneers of agromining, on the other hand, hope to have to go through much less effort to harvest a nice crop of nickel or cadmium from a field of wildflowers.
Phrased this way, the whole thing sounds a bit like a matter of alchemy. But in reality, agromining or phytomining is a strand of study and experimentation that already has a recognized tradition of more than 30 years and is now experiencing a peak of interest because of its potential, both in the recovery of critical resources and in the restoration of contaminated soils.
It all hinges on the ability of plants to absorb and store varying amounts of metal from the soil. Some have taken this ability to high levels of specialization, going so far as to accumulate elements such as nickel, cobalt, arsenic or selenium up to a hundred or even a thousand times more than normal. These are the so-called “hyperaccumulator” plants, and several scientists around the world are now working to harness this super-power and succeed in extracting critical elements from the earth without digging.
It takes a flower to make nickel
An expanse of tiny yellow wildflowers in a mountainous area between Albania and Greece: what to the untrained eye might look like a common glimpse of Mediterranean scrub is actually a nickel mine, or rather a nickel farm. “The mountains in this area of Europe,” explains Mirko Salinitro, a researcher at the Department of Biological, Geological and Environmental Sciences at the University of Bologna, to Renewable Matter, “are full of serpentine, a mineral that contains large amounts of nickel. And so, the soils derived from such rocks are also naturally rich in this metal, which makes them not very fertile and practically useless from an agricultural point of view, but definitely interesting for experimenting with agromining.” In fact, the area is among those chosen for the European Life-Agromine project, which conducted field trials between 2016 and 2021 in five countries (in addition to Greece and Albania, also Austria, France, and Spain), each with specific climatic and soil conditions.
“Hyperaccumulators, however,” Salinitro continues, “are spread somewhat worldwide, from tropical and equatorial areas to northern Europe and North America. They have evolved in very difficult and selective habitats, with little other vegetation to provide food for herbivores and insects. It is therefore believed that hyperaccumulation of metals is a trait developed by these species as a defense strategy against animals.” An example of this defensive tactic, well known to mountain experts, is Astragalus bisulcatus, commonly known as silver-leafed milkvetch, a delicate purple clustered flower that drives insane the cattle that unluckily happen to graze it and kills sheep through selenium poisoning.
Other famous hyperaccumulators are the Noccaea caerulescens, which extracts zinc and cadmium from the earth, Haumaniastrum robertii, a devourer of cobalt and copper, Biscutella montanina, which loves titanium, and two types of ferns, Pteris vittata, which absorbs arsenic, mercury and lead, and Dicranopteris linearis, a friend of rare earths. “However, there are so many species,” Salinitro points out, “for example, there are as many as 750 known nickel hyperaccumulators. Of all of them, the most representative and studied is certainly the Alyssum murale or Odontarrhena chalcidica, whose yellow flower has given no small amount of satisfaction in field studies in Albania: according to data from the French University of Lorraine, leader of the Life-Agromine project, its cultivation has in fact been “optimized to produce up to 10 tons of dry biomass per hectare, from which up to 150 kg of nickel can be extracted.” A yield comparable to that of the mine, since on average the extracted ore contains 1 or 2 percent nickel.