In the 1970s, scientists discovered an astonishing plant growing on a small island in the Pacific Ocean. This plant, named Pycnandra acuminata, native to New Caledonia, exudes a vibrant turquoise-colored sap when its bark is cut. Research has revealed that the sap contains up to 25% nickel, and scientists hypothesize this might be a natural defense mechanism against insect encroachment.
This tree currently exists only in a remaining patch of ancient tropical rainforest. Compared to common plants, it thrives in soil with high concentrations of heavy metals, absorbing and accumulating these metalsāoften toxic to most plantsāinto its trunk, leaves, and even seeds. Its tissues, rich in nickel compounds, give the sap its unique color.
Ongoing research has identified more hyperaccumulator plants that can concentrate metals. Dr. Atony van der Ent, from the University of Queensland, has been studying these plants for some time. He notes that these slow-growing plants might take decades to flower and bear fruit, while modern mining activities, logging, and forest fires pose severe threats to their survival.
Scientists are also exploring how this “green sap tree” evolved the capacity to highly concentrate nickel, adapting to such harsh natural environments, currently believed to not be a result of human interference. They think that hyperaccumulator plants have gradually evolved this ability among different populations over millions of years, meaning these plants are native to metal-rich soils.
At the same time, hyperaccumulator plants have shown great potential in remediating soils polluted with metals or organic compounds. In these plants, metals are mainly concentrated in the leaves, while the roots contain significantly less. Scientists believe this suggests a mechanism where the plant transports metals from the roots to the leaves, thereby protecting the roots from metal toxicity. This is an effective defense strategy against herbivores and harmful insects or pathogens. In this way, hyperaccumulator plants can concentrate toxic compounds from the environment and decompose them, repairing the environment without causing secondary pollution.
It’s well known that many plants have been studied and utilized to absorb heavy metal pollutants from the soil, such as cadmium and lead. These plants can absorb metals like copper and zinc from the soil and transport them to their roots and leaves, helping to remove these harmful substances from the soil. However, while plants can absorb these heavy metals, they are unable to decompose them. Therefore, care must be taken in subsequent processing to ensure these plants are not used as food or consumer products like cosmetics.
Thanks to their hyperaccumulation ability, plants have found a new useāas phytominers. The process of plant mining is straightforward: harvest the plants when they mature, then dry, incinerate them, and recover the target metal through processing.
In the state of Nevada, USA, the United States Bureau of Mines (now abolished) conducted the first experimental phytomining using a hyperaccumulator plant that can enrich nickel. The experiment showed that in soils containing only 0.35% nickel, phytomining could produce 100 kilograms of nickel per hectare, with a value of about 10,000 yuan, not inferior in economic benefits when compared to other crops.
Dr. van der Ent proposed in his research that since metal-rich soils are detrimental to the yield improvement of traditional grain crops like wheat or rice, phytomining could become a viable alternative planting method that could also bring better economic benefits to the local economy. He believes, “Phytomining is not intended to replace the planting of grain crops, but as a transitional planting solution, after which it can switch back to planting grain crops once soil conditions are improved.”
For today’s society undergoing a transformation to new energy sources, phytomining technology brings new hope. With the booming development of electric vehicles and other green technologies, the demand for metals such as cobalt and nickel is increasingāall of which are within the range that hyperaccumulator plants can extract. Compared to the metal content in the soil, phytomining can achieve a higher concentration and has fewer impurities. The environmental damage caused by phytomining is much less than that of traditional mining techniques, and it also provides the added value of remediating metal-contaminated soils, presenting a very promising development prospect.
Even though there have been many successful experimental cases, scientists still believe that phytomining technology is difficult to commercialize, requiring large-scale trial operations to evaluate potential risks and to comprehensively analyze its profit potential. Van der Ent also emphasized that only plants with a certain proportion of nickel in their leaf dry matter have commercial planting significance. “Not all plants are suitable for mining purposes. To make it a viable solution, we need to increase both the overall yield of plants and the enrichment ratio of metals.” However, the low yield of these hyperaccumulator plants, their stringent environmental requirements, and slow growth rate are all factors limiting their commercialization. Researchers are currently seeking solutions to these issues to push these plant miners onto the official commercial path.
Scientists have identified a series of special plants that have hyperaccumulating properties and can accumulate specific minerals from the soil. Among them, a plant known as mustard (Brassica juncea), with its unique ability, has become a focal point of research; it is capable of accumulating precious metals like gold.
Mustard is not only popular in cooking, but it also attracts the attention of scientists for its ability to extract heavy metals from the soil. By planting mustard, we could not only obtain delicious condiments but also explore an eco-friendly and potentially economically valuable method of mineral extraction. In searching for a gold extraction technology that is adaptable and cost-effective, mustard may pave a new path.
Due to this characteristic of mustard, it can be used as an eco-friendly technology for waste material treatment in mining fields, thus collecting rare metals that are difficult to extract in conventional ways. This technique of using plants to extract metals from the soil is called phytomining or “Phytomining”, and it may offer us a new approach to uncovering the treasures of the Earth.
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