What could be greener than using plants to clean up polluted soil? Phytoremediation, the use of green plants to repair unhealthy soil, has sparked the curiosity of scientists and plant lovers for decades. Plants can do things that people can’t, like pull pollutants out of soil while leaving the soil in place.
Why worry about leaving soil in place? Can’t we just remove it and replace it?
Even soil with contaminants in it is still a living, breathing ecosystem special to the landscape where it evolved. And, the amount of soil on earth is limited. It takes several hundred years to form one inch of topsoil. In addition, throwing away tons of topsoil creates a lot of waste! If we can pull the contaminants out, the soil can continue supporting life. We minimize the waste we create during cleanup. This process of using plants to pull contaminants out of soil is known as phytoextraction.
When we want to pull contaminants out of soil, we turn to superhero plants known as “hyperaccumulators.” These special plants take up pollutants and store the toxins in their leaves. About 450 hyperaccumulators are known to exist. They can accumulate arsenic, nickel, cadmium, and other metals.
Imagine if I could survive having 10,000 times the lethal dose of arsenic in my body. That’s what one arsenic hyperaccumulator, the brake fern (Pteris vittata), can do. This fern is found living in ancient mining sites in China. These locations contain large amounts of arsenic, lead, copper, and zinc. When arsenic is stored in the fern leaves, we can easily harvest this biomass to remove pollution from soil. Of course, the metals are so concentrated in the biomass that it becomes hazardous waste. But the biomass waste from these plants is smaller than if we had removed all the soil.
Phytoextraction has some limitations. Scientists are working to determine how this great plant capability can be practical for human use. For example, even though the brake fern can remove large amounts of arsenic, it would still take decades to fully clean up moderately polluted soil. While the 5-foot tall brake fern is large for a fern, even more biomass is needed to speed up remediation.
Researchers, including me, are looking at other ways to increase arsenic uptake in the brake fern. This might include supplying the fern with fertilizers. Or we might partner the fern with symbiotic fungi or bacteria to make the arsenic in the soil more available for fern roots to take up.
Phytoextraction is also limited to the root zone, but this is not necessarily a deal breaker. Much contamination, like fallout from metal smelters or dust blowing from mining operations, is on the soil surface. Finally, beware the invasive plant: hyperaccumulators can be tough survivalists. We wouldn’t want to plant a hyperaccumulator, only to have it get loose and take over an ecosystem.
A nickel hyperaccumulator (Alyssum murale) did just that. It was planted in southern Oregon as a commercial venture to extract nickel from soil. However, it wasn’t managed properly and is now an invasive, noxious weed in the botanically-rich area.1
Beyond hyperaccumulators, other more common plants can help remediate soil. Poplar trees (Populus ssp.) can absorb and help degrade a variety of organic carbon pollutants like TNT and other explosives2. Poplars release compounds from their roots that break down chlorinated solvents, and can host microbes in their roots that help break down other compounds. These fast-growing trees can also be planted to absorb landfill chemicals. Crucially, poplar trees are much larger than the brake fern, tipping the biomass balance towards faster cleanup.
From fancy ferns to common poplars, phytoremediation offers great potential to sustainably clean up and preserve precious soil for future generations. Because plants are living beings, developing these plant-based technologies for practical use takes time and effort. Most research to date involves short term, controlled experiments. That means that scientists must do large scale pilot projects to launch phytoremediation towards commercial remediation use. Think about old, polluted industrial sites in your area. Clean up now would likely involve excavators and dump trucks full of hazardous waste. Let’s work towards a greener future where remediation professionals carefully tend fields of green plants!
Answered by Sarick Matzen, University of California, Berkeley
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- Amsberry et al. 2008. Pest Risk Assessment of Alyssum murale and corsicum. Oregon Dept. of Agriculture/USFS. 1-26.
- Susarla et al. 2002. Phytoremediation: An ecological solution to organic chemical contamination. Ecological Engineering 18:647–658