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Heavy Metal Farming

The sap exuding from Pycnandra acuminata in New Caledonia

The sap exuding from Pycnandra acuminata in New Caledonia contains up to 25% nickel.

By Antony van der Ent

Special plants called hyperaccumulators can extract valuable metals from mineralised soils, yielding metallic crops that are more valuable than food grown in soils that are unsuitable for normal agriculture.

Hyperaccumulators are rare plants that have the unusual ability to concentrate heavy metals, like nickel, into their living shoots. These plants can contain more than 3% nickel in their leaves and up to 25% nickel in their sap, making them truly metallic plants. Unlike the silvery shine of the metal, nickel compounds typically have a bright green colour, and hence the sap of the extreme hyperaccumulators is literally green from nickel. Some of these plants can grow quite big, so a 20-metre high hyperaccumulator tree can contain more than 5 kg of pure nickel.

To most plants, heavy metals such as zinc, copper and nickel are toxic when present in the soil in high concentrations. Therefore, the majority of plants have evolved highly efficient mechanisms to regulate their uptake and keep these metals out of their living shoots. But the behaviour of hyperaccumulator plants is the exact opposite.

So far about 450 hyperaccumulators have been discovered around the world. The greatest numbers have been found in Cuba and New Caledonia, but two species are also known from Australia, one from Queensland (Stackhousia tryonii) and the other from Western Australia (Hybanthus floribundus). My work in Malaysia and Indonesia focuses on discovering more of these plants, and trying to understand how they are able to take up and concentrate metals in their shoots.

Phytomining

The idea to grow hyperaccumulator plants and harvest metals as a new kind of “bio-ore” originates from the early 1980s. Since then, field trials around the world have demonstrated that 100–200 kg of nickel metal can be produced per hectare per year. As nickel is currently worth about $15/kg, such crops are worth more than any food crop.

Nickel is a very versatile metal. It is mainly used to make stainless steel and coins, and for electroplating a thin layer of nickel onto iron objects to make them resistant to corrosion.

In a phytomining operation, hyperaccumulator crops are cultivated in fields and their biomass harvested periodically. In the case of annual species this means harvesting the whole plant to produce hay, while for perennial species, such as woody shrubs, it means regular pruning to produce the biomass.

Once harvested, the biomass is usually dried and then burnt to create ash. The burning reduces the weight of the biomass significantly and concentrates the nickel to between 10% and 20% in the ash. This ash can then be further processed to extract pure nickel metal or to make nickel chemicals that may be used, for example, in electroplating.

Where Could Phytomining Take Place?

Hyperaccumulator plants can only extract nickel from soil that already contains nickel. Fortunately these “ultramafic” soils are widespread and cover large areas of the world. While ultramafic soils are naturally enriched in nickel, they are also very nutrient-poor and therefore unsuitable for most food crops.

Ultramafic soils exist where the Earth’s upper mantle is exposed to the surface along major fault lines. Although the concentrations of nickel are generally low in these soils (0.1–1%), they form one of the major sources of nickel for the mining industry. Thick layers of ultramafic soils occur in places like Brazil, Cuba, New Caledonia, Indonesia and the Philippines, where they are strip-mined for nickel.

However, this is a very costly process because nickel is difficult to extract and huge acid-leaching installations are required. Also, conventional nickel mining targets only the very highest grades in soils that contain more than 1% nickel.

In contrast, hyperaccumulator plants can extract nickel from as low as 0.1% in the soil yet still achieve more than 2% in their dried biomass. Therefore, phytomining could take place in the enormous areas of ultramafic soils where the nickel grade is too low for conventional mining.

However, phytomining is a finite resource. At some point all the nickel will be extracted from the soil. At depths accessible by plants, ultramafic soil contains up to 40 metric tons of nickel per hectare. Therefore, a nickel yield of 100 kg/ha/year could hypothetically be phytomined for 400 years.

It is not so simple, though, as not all the nickel in the soil can be extracted by the plants, but we estimate that phytomining can last at least several decades. And after phytomining has taken place, the soil will be substantially more fertile than before and therefore more suitable for the production of food crops.

In many tropical areas, much land on ultramafic soils has been cleared or has been degraded by forest fires. Such landscapes are especially suitable for phytomining. In a typical operation, farmers or farming cooperatives could grow metal crops on their land and sell the harvested biomass to a processing business that can extract the nickel.

Unfortunately, despite all the scientific evidence, no large-scale commercial phytomining operations are active today because the mining industry is hesitant to embrace radically new technologies to extract metals. However, it is important to emphasise that phytomining will not compete with conventional mining operations as it is far more economically viable to extract metals from high-grade deposits using mechanical and chemical methods. Instead, phytomining’s niche would be low-grade surface deposits found in ultramafic soils and as part of the rehabilitation strategy after strip-mining has already taken place.

In Sabah, Malaysia, I am currently conducting research into nickel hyperaccumulator plants with Sabah Parks (a local government-affiliated organisation). Part of this work involves setting up fields to cultivate selected species at scale to demonstrate the viability of phytomining under tropical conditions. We are also trying to understand how these plants extract the nickel from the soil, and how it is then transported to the shoots of the plant. This knowledge has the potential to then improve these characteristics and develop the optimal strains of “metal crops”.

The results so far are highly encouraging. Over the past few years we have discovered more than 20 new nickel hyperaccumulators, of which two have just the right characteristics as a metal crop as they are fast-growing, easy to propagate and very efficient in accumulating nickel.

Antony van der Ent works at The University of Queensland in Australia and Université de Lorraine in France on nickel hyperaccumulator plants.