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Using plants to “micro-mine” metals

Certain plants have the ability to extract metal from the soil. A pilot facility in Lorraine is set to extract nickel from hyperaccumulating plants grown in Albania. The extraction process has been granted an international patent.

Alyssum murale, hyperaccumulator plant of  nickel  Albania.. © INRA
By Pascale Mollier, translated by Daniel McKinnon
Updated on 05/30/2013
Published on 05/02/2013

Plants that can mine ore? They are known as phytomining plants – specific plants that grow in metal-rich soils and that are able to absorb metals at rates thousands of times higher than other plants. These plants are able to extract metals even when very little metal is present in the soil. In such situations, traditional mining techniques are inefficient. Compared to normal mining, phytomining’s impact on the land, and on the environment in general, is emphatically less.

Interview with Jean-Louis Morel of the Joint Research Unit for Soils and the Environment in Nancy.

What is phytomining?

J-L. Morel: Actually, we prefer the term “agromining”, since it is more inclusive than phytomining. It is important to understand that this new type of mining is not limited to the plant itself but encompasses the entire soil–plant–ore agrosystem. The agrosystem shapes both the plant’s extraction of the metal and the equally important process to recover metal accumulated in the plant.

Our most developed project is for agromining nickel. Together with several international partners, we have developed a new process (1) that has been granted an international patent for recovering nickel from plants using hydrometallurgy to obtain highly valuable nickel salt. The pilot project in Nancy (2) is currently treating one tonne of biomass from Albania, where field trials have been underway for five years in soil naturally rich in nickel. This year, we are also putting in place a project in Homécourt, France (3) to use hyperaccumulating plants on contaminated soils and industrial wastes.

Why Albania?

J-L. M.: The story began in 1988, before the fall of the dictatorship, with the visit of two Albanian students to France. In Albania, there are “ultramafic” soils that naturally contain a hundred times more nickel than normal soils and are also rich in iron, cobalt and magnesium. Following this initial collaboration, we carried out two studies investigating nickel hyperaccumulating plants and conducted field trials. Alyssum murale, the plant selected for the studies, commonly grows in ultramafic soils in Albania. It can accumulate as much as 100 kg of nickel per hectare using normal agricultural techniques using amounts of fertilizers in line with local practices. Alyssum murale is of particular interest to Albanian farmers who, following post-dictatorship land redistribution, found themselves owning small ultramafic plots poorly suited for crops. Making their living more through tourism than agriculture, these farmers could join together and capitalise on nickel to finally make the most of their land. However, agromining infrastructure must first be built entirely from the ground up.

How can agromining be used in France?

J-L. M.: All aspects of the pilot system will be studied. A market study is underway to determine economic viability, and the system’s environmental effects will be assessed. A life-cycle assessment (LCA) is envisaged as a part of the “Agronick” project (“agromining” and “nickel”) submitted to the French National Research Agency (ANR) in 2013. Ultramafic soils are not very common in France except in small pockets in regions like the Vosges, or over larger areas in mountainous regions such as Corsica. Consequently, nickel extraction will focus more on land contaminated by industrial activity, thereby killing two birds with one stone: cleaning polluted soil and producing nickel. LORVER, a large project launched in 2012 and financed by the Lorraine Region, seeks to remediate abandoned or underused lands such as brownfield sites, contaminated soils, sludge and waste. The use of agromining plants that extract metals such a nickel or cadmium will develop if there is a cost-effective extraction process, as is the case for nickel. Other plants will also be tested for their fibre- and energy-production potential. As a result, industrial brownfields will no longer be seen as a hindrance but as a resource. 

What is the outlook for the future?

J-L. M.: There are prospects for exploiting other metals, in particular very high value elements, used in new technologies, such as lithium, indium and neodymium, 90% of the world’s reserves of which are in China. These strategic metals are at the heart of the Resources 21 laboratory of excellence (Labex) that we are a part of.

Another possible application would be the use of plants to extract gold. Studies to this end were even carried out twenty years ago. The difficulty is that gold is not easily absorbed and required chemicals added to the soil to make it more soluble. As a result, the process held limited appeal. But a number of challenges have been set and agromining’s potential is huge.

(1) At the Scientific Interest Group on Industrial Wasteland (GISFI) research station, http://www.gisfi.fr 
(2) The project brings together a number of partners: LSE (INRA-UL), LRGP (CNRS-UL) and LEM (CNRS-UL) in France; the Agricultural University of Tirana in Albania; and INRS-ETE in Canada.
(3) Nickel produces strong alloys that are widely used in equipment and in aeronautics. Nickel is also used to make coins, electrodes, cells and batteries, while nickel salt is used in surface treatments.

References

- Bani A., Echevarria G., Sulce S., Morel J.L., Mullai A. 2007. In-situ phytoextraction of Ni by a native population of Alyssum murale on an ultramafic site (Albania). Plant & Soil, 293:79-89.

- Barbaroux R., Plasari E., Mercier G., Simonnot M.O., Morel J.L., Blais J.F., 2012. A new process for nickel ammonium disulfate production from ash of the hyperaccumulating plant Alyssum murale. The Science of the Total Environment, 423:111–119.

- Patent: Production of a crystallized nickel salt from hyperaccumulator plants – Canada n°2.731.457 4 February 2011 – N/Ref: 00 5199-0025. Inventors: Mercier G., Barbaroux R., Plasari E., Blais J.F., Simonnot M.O., Morel J.L. International patent filed on 6/2/2012, PCT/CA2012/050059, INRS-ETE, Université de Lorraine, INRA, CNRS.

Unneighbourly plants

It seems that phytoextraction confers a selective advantage. Phytoextracting plants are adapted to grow where others cannot, in metal-rich soils too toxic for most plants.

Phytoextraction is only one part of phytoremediation, which includes:

- phytostabilisation, limiting the leaching of heavy metals into water or the erosion of contaminated soil (red fescue, poplar, etc.);

- phytodegradation, for organic pollutants that do not break down naturally, such as polycyclic aromatic hydrocarbons (PAH) and chlorinated solvents. Particularly effective for phytodegradation is alfalfa due to its highly developed root system and, in particular, its symbiotic bacteria;

- phytovolatilisation, for certain organic pollutants with low molecular weight, such as trichloroethylene, or certain minerals such as selenium, which are absorbed into plant sap and then released into the atmosphere through transpiration from plant leaves.