Alyssum murale produces 150kg/ha of nickel.

Harvesting metal with plants

Some plants absorb the metal in impres­sive quan­ti­ties. In certain cases, this phytoex­trac­tion can be put to good use.

Plants known as ‘hyper­ac­cu­mu­la­tors’ can tolerate and trans­port metals, storing them just under the epidermis of their leaves. Just a few grams, but enough at field scale to consider “culti­vating ores”, says Guil­laume Echevarria, a specialist in hyper­ac­cu­mu­la­tors from the Univer­sity of Lorraine, France. The researcher is co-founder of start-up busi­ness Econick, which specialises in the phytoex­trac­tion of nickel.

Until now, these plants have mainly been of interest for their ability to decon­t­a­m­i­nate soils polluted with heavy metals like zinc, cadmium, copper, cobalt, lead, thal­lium etc. But some of the metals absorbed can be used. In 2020, Econick signed a contract with the steel­making company Aperam to biosource nickel, which is neces­sary for the produc­tion of stain­less steel.

A symbolic gesture? Not neces­sarily. “‘Agro­mining’ is a very niche industry but all metal econ­o­mists will tell you that the demand for nickel will be such in the coming years that we cannot turn our back on any econom­i­cally sustain­able source,” notes Mr Echevarria. Demand for the metal is on the rise largely due to the expected surge in elec­tric mobility vehi­cles, as battery produc­tion is partic­u­larly nickel-inten­sive. A further chal­lenge is “the enor­mous CO2 cost of metals”. 

A Pycnandra acumi­nata tree from New Cale­donia with a bluish sap colored by nickel ions.

One tonne of nickel on 4 ha

Although it can’t compete with indus­trial extrac­tion, the amount of metal that can be extracted from a field is impres­sive. The shrub phyl­lan­thus rufuschaneyi, culti­vated by Econick, absorbs 250kg/ha of nickel per year. “In theory, it will be possible to meet Aperam’s needs, provided we sow several thou­sand hectares,” says Mr Echevarria. The metal is not a renew­able resource, but is abun­dant in the soil and reveg­e­ta­tion of indus­trial residues (mining tail­ings) will help complete the circle.

“The plants we currently use are used in their natural state but breeding efforts have begun,” says Mr Echevarria. Breeders are focussing on the plant’s behav­iour – in partic­ular its speed of growth, which is more impor­tant than the metal yield. For ‘phyto­mining’ to be prof­itable, peren­nials need to be able to regrow quickly after each harvest: “These species, like phyl­lan­thus rufuschaneyi, have a deli­cate biology and the sowing costs are high. We can’t resow them every year.

Growth of the cadmium and zinc hyper­ac­cu­mu­lator Thlaspi caerulescens on metal-polluted indus­trial soil  and unpol­luted agri­cul­tural soil. (INRAE – SCHWARTZ Christophe)

Zinc derived from plants

In the north of France, Mr Echevarria is also working on decon­t­a­m­i­nating market garden soil, leading to the produc­tion of zinc. This is because some plants, like arabidopsis halleri, hyper­ac­cu­mu­late several metals. One of the projects is located near a former indus­trial site where vegeta­bles system­at­i­cally exceed cadmium stan­dards. “We want to make sure that we don’t cut the vegetable produc­tion line. We are trying to produce healthy vegeta­bles planted between rows of hyper­ac­cu­mu­la­tors that would protect the crops from excess metals. We’re in the exper­i­mental phase.”

As with nickel, extrac­tion relies on burning plants before treating them with chem­i­cals: The ashes are “washed” several times and the deposits collected. The zinc yield is more modest, but the techno-economic analysis demon­strates that, along­side the ecolog­ical benefit it offers, heat cogen­er­a­tion makes produc­tion prof­itable.

The process also has nutri­tional signif­i­cance for humans. “Some species can accu­mu­late up to 2% of their biomass in zinc – in soils where there are normal concen­tra­tions of this metal,” says Mr Echevarria. Zinc is one of the world’s leading human mineral defi­cien­cies. “It could be an inter­esting source, rather than consuming zinc produced from chem­ical substances.” 

Hyper­ac­cu­mu­lating plants: A fasci­nating biology

Mainly crucif­erous vegeta­bles can tolerate toxi­city and have specific storage systems for hyper­ac­cu­mu­lating metals. In nature, these traits provide them with a wealth of advan­tages: Resis­tance to being eaten (low palata­bility) and drought stress (water effi­ciency), as well as natural biochem­ical defences (creation of toxic litter that keeps competi­tors away).

Another advan­tage could be more effi­cient photo­syn­thesis thanks to UV filtering by metals, although this hypoth­esis is yet to be vali­dated. The nickel ion wave­length would, for example, provide a barrier against wave­lengths that reduce the effec­tive­ness of chloro­phyll. This more effi­cient photo­syn­thesis would there­fore compen­sate for the high meta­bolic cost of storing the metal in the leaves.