The Power of Plants, Using Nature to Mine

One South African researcher is looking into the use of hemp to clean the soil around abandoned and polluted gold mining areas. The pollution is the result of acid mine drainage and concentrations of runoff heavy metals that contaminate the soils surrounding old mine workings. 

Tiago Campbell, a masters student in environmental science at the University of the Witwatersrand, is studying the potential to remediate lands impacted by 130+ years of mining in areas near Johannesburg. 

Campbell’s research confirms hemp as a “heavy metal hyper-accumulator” in comparison to other plants studied for their phytoremediation potential such as Indian mustard, water hyacinth, alfalfa and sunflower. He said nearly 1,000 cannabis plants put into soil collected from the polluted lands grew normally in lab tests.

The hyper-accumulator attributes of hemp were earlier tested in cleanup efforts in the 1990s when Ukraine’s Institute of Bast Crops researched the plant’s ability to absorb heavy metals such as lead, nickel, cadmium, zinc, and chromium in the Chernobyl nuclear fall-out zone.

The modern origin of using nature to help clean man-made messes come from the US military. When US armed forces were fighting in Korea during the 1950s, scientist Howard Worne researched the degradation of uniforms in the moist and humid climate. 

During his research, he discovered a microorganism that broke down fabrics previously thought to be non-biodegradable. Worne went on to isolate an organism capable of degrading phenols, a common and highly toxic pollutant in industrial wastewater.

From microbes to algae, this research has advanced to look at a variety of plants from around the world. The hemp plant is not alone as a ‘hyperaccumulator’ organism with the unusual ability to absorb and concentrate metals from soil at high levels. 

The American Environmental Protection Agency (EPA) has estimated the cost of phytoremediation as a technique for removing hazardous heavy metals from soil ranges from 20-50% of outlays required for conventional methods that employ physical, chemical or thermal technologies.

In 2000, the US Department of Agriculture found that ash from pennycress grown on high-zinc soil in Pennsylvania yielded 30% to 40% zinc. According to the BBC, around 450 species of nickel-absorbing plants have been documented worldwide in places such as Indonesia, Cuba, southern Europe, New Caledonia and Malaysia. Some with up to 16% nickel in their sap which colours the sap of the plant blue. 

These types of grades raise the possibility of mining the plants or “phytomining” for “bio-ore” after cleaning polluted soil. The University of Queensland’s Sustainable Minerals Institute (SMI) researcher Dr Philip Nkrumah is developing phytomining technology at the Centre for Mined Land Rehabilitation.

In an interview with Australia’s Stockhead website, the professor says mine tailings are “some of the largest untapped resources globally.” Many tailings facilities not only have toxic elements in them but also valuable minerals that are too expensive or difficult to mine at low grades using conventional methods. 

Tapping into this waste through phytomining could create additional revenue streams for mining companies, while offering solutions to offset the environmental impacts of mining waste.

“Some species of plants can contain up to 1% cobalt or 4% nickel in their shoots, translating to more than 25% metal in their ash or ‘bio-ore’,” says Dr Nkrumah. The high purity of bio-sourced metals makes them especially suited for applications like lithium-ion batteries, he continued.

Currently, only ore that consists of at least 1% nickel can be mined in the traditional way. But according to another University of Queensland professor, Anthony van der Ent, hyper-accumulator plants can achieve high levels of nickel accumulation in a soil that comprises 0.1% nickel.

According to a study conducted by van der Ent in Malaysia, “metal farms” can produce between 170 to 280 pounds per acre annually. At today’s prices, a farmer could earn ~$3,800 per acre of nickel which according to van der Ent is competitive with some of the best-performing agricultural crops on fertile soils with similar operating costs. 

The research and the large-scale applicability of this biotechnology is still early, but like all great modern solutions, it may take a university student growing cannabis to find solutions for the next generation.