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Some Like It Toxic

Beyond Science

Certain plants are capable of extracting heavy metals from contaminated soil while at the same time supplying important raw materials. This process is known as phytomining, and it is considered to be both sustainable and forward-looking.

The father of modern botany, Carl Linnaeus, favored the twinflower (Linnaea borealis), whereas the founder of evolutionary theory, Charles Darwin, adored orchids. Plant expert Antony van der Ent, on the other hand, prefers Phyllanthus rufuschaneyi, because “it is the most successful metal-absorbing plant we know”, says the biogeochemist who is based at The University of Queensland in Australia.

Plants hoover up the poisons
Phyllanthus rufuschaneyi belongs to a genus of herbaceous plants with leaves that are either spiral-shaped or arranged in two rows, and whose 900 species grow predominantly in the tropical and subtropical regions of our planet. It was discovered in Malaysia in 2013, near Kinabalu National Park in the state of Sabah. Its special feature: growing in nickel-rich soil, the plant is able to absorb this heavy metal, which is toxic at high concentrations, and store it – thus engaging in phytomining. “A sustainable method of mining metals, with a low impact on the environment”, says van der Ent. “Phyllanthus rufuschaneyi, by the way, was named after the researcher Dr. Rufus Chaney who is considered one of the scientists who discovered phytomining as early as 40 years ago.”

Soils all over the world contain metals such as lead, nickel or germanium. Their presence can sometimes be traced back to the formation process, as the source material of these soils is the rock that slowly disintegrated through erosion and weathering, and which has been stripped down to its components. The largest offender when it comes to contamination, however, is man. Fine toxic dust from industry and traffic is deposited in the environment to the same extent that pesticides and fertilizers are used in agriculture. The consequence: the soil is no longer able to filter water, bind carbon or serve as home to living things. Foods are increasingly contaminated; for example, rice is contaminated by arsenic. Scientists worldwide are searching for options for removing heavy metals from the soil.

Phytomining for metal recovery
And this is where phytomining comes in. As a hyperaccumulator plant, Phyllanthus rufuschaneyi thrives in soil with high concentrations of metal ions which the plant takes up through its roots. It does not fall ill from the overdose; in fact, it stores the heavy metals in special hollow spaces in the outer layer of its leaves – far away from the chlorophyll inside the leaf, which is crucial for photosynthesis. If the plants which accumulate the toxins are harvested on a regular basis, the toxic metals can then be removed from the ground. A close relative of Phyllanthus rufuschaneyi, Arabidopsis halleri, for example, grows in a region of the Sauerland in Germany which had been subject to lead mining since Roman times. This area harbors some of the most contaminated soils in Europe, and the plant with the white flowers is capable of taking up and storing lead, zinc und cadmium in its leaves. This unique faculty helps it render the soil of former industrial sites serviceable once again.

A gentle alternative for the environment
These super-plants with their vacuuming-like qualities may also be planted in a more targeted fashion in order to actually mine valuable metals. Nickel, for example, is used during the dyeing process in glass factories. While it can be mined by industrial methods, traditional nickel mining uses immense amounts of water while leaching toxins into the environment. In these cases, phytomining would definitely present a gentler alternative.

Antony van der Ent is conducting studies on the Pacific island of New Caledonia – the place which harbors the greatest nickel reserves on Earth. These were mercilessly exploited for decades, and now hyperaccumulators are employed in order to detoxify the soil. He and his team founded a metal farm in Malaysia in order to demonstrate “that phytomining really works”. While the mining industry is showing a strong interest in phytomining, thus far, the plants have not been used across large areas of land for the purpose of extracting heavy metals. “At this time, we are waiting for investment from the industry to establish metal farming globally.”

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