Phytomining is a term for collecting metals from a type of plants called hyperaccumulators. Hyperaccumulators are rare and unusual plants that can absorb and accumulate very high concentrations of certain metals from the soil, at levels that would be toxic to most plants. They accumulate the metals in their leaves and stems, and for some of these plants, it is possible to harvest, dry and incinerate this biomass to generate high-grade bio-ore.
Perhaps one of the best known hyperaccumulator plant species is Pycnandra acuminata (previously known as Sebertia acuminata), a rainforest tree that is endemic to New Caledonia. The high concentration of nickel in the plant’s latex gives it a dramatic blue-green colour.
It was first highlighted in the paper ‘Sebertia acuminata: A Hyperaccumulator of Nickel from New Caledonia’ that was published in the journal Science in August 1976. This paper stated that the nickel content of the plant’s latex was 25.74% on a dry weight basis, which the paper’s authors Jaffré et al said is “easily the highest nickel concentration ever found in living material”.
Another paper published in the journal New Phytologist in March 2018, ‘The discovery of nickel hyperaccumulation in the New Caledonian tree Pycnandra acuminata 40 years on’, noted that the discovery of P. acuminata was a major driver of research over the following decades.
The authors wrote: “In its title, the Science article introduced the word ‘hyperaccumulator’ and the term hyperaccumulation was subsequently established [in a 1977 paper] to denote a plant nickel uptake of > 1000 mg kg−1 in dry tissue. The term was later widened to include other elements normally occurring in only trace concentrations in plant tissue.”
Studies of hyperaccumulators
The (now defunct) US Bureau of Mines ran the first phytomining field trials in Nevada in 1994 using the nickel hyperaccumulator species Streptanthus polygaloides. The trial was run on soil with 0.35% nickel, which is well below the economic cut off for conventional mining techniques, and the results suggested that a yield of 100kg of nickel per hectare could be produced via phytomining, which compared well with the average returns from other crops.
The Centre for Mined Land Rehabilitation (CMLR), part of the University of Queensland’s Sustainable Minerals Institute (SMI), launched the Global Hyperaccumulator Database in 2017. It has compiled data on 721 species of hyperaccumulators, representing 52 families and 130 genera, including species that absorb arsenic, cadmium, chromium, copper, cobalt, lead, manganese, nickel, selenium, thallium, zinc and rare earth elements (REEs).
The database was highlighted in the letter ‘A global database for plants that hyperaccumulate metal and metalloid trace elements’ that was published in New Phytologist in November 2017.
The authors Reeves et al said: “Hyperaccumulator plants are of substantial fundamental interest and practical importance. Hyperaccumulators are exceptional models for fundamental science to understand metal regulation including the physiology of metal uptake, transport and sequestration, as well as evolution and adaptation in extreme environments.”
The majority of the hyperaccumulator species listed in the database – 532 of them – are for nickel. This may reflect the fact that worldwide surface exposure of naturally nickel-enriched ultramafic soils cover more than 3% of the earth’s surface. The authors noted that most reported hyperaccumulator plants hyperaccumulate nickel and occur on ultramafic soils that are naturally enriched in nickel and cobalt, as well as manganese in some cases. They highlighted global centres of distribution for nickel hyperaccumulator plants, including the Mediterranean Region and the tropical ultramafic outcrops in Brazil, Cuba, New Caledonia and Southeast Asia.
In August this year, the government of the Australian state of Queensland invested A$1 million (GB£543,400 or US$718,000) into a joint study with the University of Queensland’s SMI that will examine the native plants like selenium weed and a variety of macadamia tree for their phytomining potential. It will also investigate whether the process could be implemented at a large scale and be a sustainable option for mining rare metals and the transition from carbon-fuelled mining.
Professor Peter Erskine, director of CMLR, said: “We’re currently growing plants using metal-rich soil and tailings from around Queensland. Thanks to a previous study conducted by UQ researchers, we know Queensland is home to native plants that have this ability to absorb metal, which are known as hyperaccumulators.
“UQ’s further phytomining research has the potential to unlock a sustainable stream of critical metals, including from mine wastes and tailings, that still hold residual metals of interest. So, in effect, phytomining could turn waste into new resources.”
Farming metals via agromining
Agromining is a term for a variant of phytomining. The chapter ‘The Long Road to Developing Agromining/Phytomining‘ in the book Agromining: Farming for Metals, which is now in its second edition, defines the terms thus:
“While phytomining focuses more on the plant and its potential to extract elements of interest, agromining emphasizes technological processes and their combination (agronomy and metallurgy) to produce commercial compounds. Here… we refer to phytomining for previous research and development activities, and to agromining for more recent ones that consider the entire production of bio-based elements.”
The paper ‘Agromining: Farming for Metals in the Future?’, which was published in the journal Environmental Science & Technology in February 2015, proposed that agromining could create value for local communities during and post-mining. Mining companies should actively support these kinds of projects to improve community sentiment and prosperity. Instead of growing food crops, farmers could grow and periodically harvest ‘metal crops’ using hyperaccumulator plants. They could then harvest the biomass, followed by drying, ashing and processing it to recover target metals.
The paper’s authors, van der Ent et al, described the benefits of agromining on low-productivity agricultural soils, targeting ultramafic areas that are large and relatively flat which have low productivity for food production.
They wrote: “Agromining here would be superior to conventional agricultural production, generating better economic returns to farmers. A co-cropping approach might also be possible: for example, in Greece, olive plantations could be intercropped with Alyssum; and in Malaysia, palm oil estates could be intercropped with Phyllanthus.”
The economics of agromining
Based on field trials using nickel hyperaccumulator species, van der Ent et al said they could expect to harvest 5-10t of dry matter per hectare containing 2% nickel, yielding 100-200kg nickel per hectare.
“At 2015 prices of US$15 kg−1, and a potential yield of ≥100 kg Ni ha−1, agromining could become a part of a substantial and integrated income stream for ‘metal farmers’ worth more than most food crops,” they wrote. “By means of comparison, a premium rice crop on fertile ‘normal’ soils makes approximately US$850 per ha−1 year in Indonesia, while Ni phytomining on local ultramafic soils has the potential to make US$1000 ha−1 year.”
They also drew attention to the low economic returns of ultramafic soils for producing food crops such as wheat or rice, due to the inherent infertility of the soil. “Under these circumstances, agromining could be a viable alternative generating better economic returns to local communities,” the authors said.
“Therefore, unlike the competition between food crops and biofuels on fertile soils, agromining does not replace food crop production, but is a temporal activity that may improve soil quality sufficiently to allow food crop production after the metal resource has been extracted.”
The authors recommended that phytomining should focus on species that show the highest levels of hyperaccumulation, noting that in practice only those plants with greater than 1% of nickel in foliar dry matter would likely be of commercial relevance.
They added: “Not all are suitable candidates for phytomining, as the utility of a plant species for phytomining is ultimately determined by the annual harvestable biomass… High biomass yield and material hyperaccumulation are both required to make phytomining a commercially viable proposition.”
In addition, plant species native to the area are more likely to be suitable for agromining than introduced species as they are already adapted to the local climate, pests and diseases; this will also avoid the spreading of invasive species into these habitats.
The authors noted that while the commercial returns from an agromining venture will be limited due to the diminishing concentrations of the target metal in the substrate, the time scale for economic agromining may be considerable; in some cases, it could be sustainable over at least 30−60 years.
However, they cautioned that despite numerous successful experiments, commercial phytomining has not yet become a reality. They advised that to build the case for the minerals industry, a large-scale demonstration is needed to identify operational risks and provide ‘real-life’ evidence for the profitability of agromining.
Tackling mine waste
Global mine wastes represent a potential critical metal resource, and phytomining on degraded or mined land could be used as part of a broader rehabilitation strategy. Hyperaccumulator plants could be a valuable resource in rehabilitation of mine sites or other contaminated areas, which has the dual benefit of extracting metals that are in demand, as well as removing them from the environment where they can be toxic.
In the paper ‘Treasure from trash: Mining critical metals from waste and unconventional sources’ that was published in the journal Science of The Total Environment in March 2021, the authors van der Ent et al noted that to meet the future technological demands of our growing global community, new sources of industry critical metals need to be identified.
Extracting minerals from larger, lower grade deposits across most commodities is required to meet these demands, which in turn generates ever increasing amounts of mining waste.
In addition, the authors noted that some of these materials may contain reactive minerals, including sulphides, which require appropriate management to mitigate against environmental impacts, including the formation of acid and metalliferous drainage (AMD).
One of the paper’s authors, Dr Anita Parbhakar-Fox, spoke to The Intelligent Miner earlier this year about her work on developing new ways to characterise mine waste which is definitely worth a read.
In ‘Treasure from Trash’, van der Ent et al explained that “conventional approaches to managing mine waste focus on breaking source-pathway-receptor pollutant linkage chains at the backend rather than the front”. However, they added that “considering mine waste as a potential critical metal resource provides an opportunity to supplement the demand of critical metals”.
The authors noted that in some cases, critical metals were ‘accessory minerals’ to the main target metals at some mines, so they were not recovered during processing as it was not economically or technologically viable at the time. An example of this given in the paper is the Mary Kathleen uranium mine in Queensland, Australia, which closed in 1982 but has rare earth elements in the tailings that could potentially be re-mined.
At other mines, significant amounts of the target metals were not extracted due to dynamic cut-off grades which depended on the metal prices at the time of mining. The paper mentions the Century zinc mine, also in Queensland, that had a mineral resource of 10.2 wt% zinc which was mined. However, the mine tailings contain 3.1 wt% zinc, which is a 77.3Mt resource.
“Exploring mine waste presents an opportunity to conscientiously supplement the demand with the advantage of these materials,” the authors said. “Metal farming is an in situ technique that can be applied to minerals and mining wastes using hyperaccumulator plants to bio-concentrate high levels of metals or metalloids into their shoots and remove them from the substrate, while achieving monetary gain. Indeed, producing critical metals efficiently and sustainably using metal farming techniques to extract nickel, cobalt and thallium appear to be well within reach.
“However, this technology needs industrial research partners to develop appropriately scaled demonstration sites to champion this technology in areas where it is feasible.”
Threat of extinction
There is some urgency to actively engage in hyperaccumulator research. Many hyperaccumulator species are endemic, meaning they are only found in one geographic location. They are vulnerable to threats such as habitat loss which could lead to extinction, and since they grow in areas where the soils are particularly high in certain metals, mining activities can be a major cause of habitat loss.
As Reeves et al (2017) wrote in their letter in New Phytologist: “Timely identification of hyperaccumulator species, along with other metal-tolerant plants, is therefore necessary to preserve them to study their unique physiological mechanisms, and to take advantage of their unique properties.”
Dr Antony van der Ent, senior research fellow at the CMLR who is a named author on many of the papers I referenced in this article, appeared on Gardening Australia in 2019 (see the video below). He said on the programme: “It’s often a race between mining companies wanting to mine a surface outcrop of a metal, and us trying to find these hyperaccumulator plants and try to save some seeds or some plants, that we dig out to keep them basically.”
While hyperaccumulators have several validated applications in the mining industry, they need to be tested on a commercial scale, perhaps via collaboration between researchers and industry partners. Who will be up to the challenge?