Plants hold solution for toxic soils
Throughout the world thousands of sites are polluted from mining, agricultural spraying, and manufacturing. Handling the polluted soil has traditionally been done by digging it up and putting it in containers, never to be used again. Is there a better way? There sure is: plants!
Plants reach into soil to pull up nutrients and water, and scientists have discovered that some plants can also pull up toxic materials. Poplars, willows, mustard, sorghum grass, sunflowers, and duckweed, to name a few, have this special ability. These plants have been used to help clean up soil contaminated with atomic waste, lead, zinc, mercury, and other chemicals in places such as Chernobyl and Fukushima, and contaminated ground worldwide.
The term for this is phytoremediation, phyto meaning plant, and remediation meaning to fix or remedy. While not a recent finding, phytoremediation is becoming better known as we look for cost-effective ways of handling toxic sites. While other ways used to clean up soils polluted with heavy metals can cost $1 million per acre, phytoremediation can cost in the tens of thousands of dollars. The process may take several years and the cleaned area is confined to the soil surface, the root zone, and the area inhabited by the microbes that help the plants deal with the toxicity. Offsetting that, however, is the aesthetic value of pretty plants quietly doing their work.
Not all plants can be used this way; those that can tolerate high concentrations of heavy metals are called hyperaccumulators, a category that includes over 500 flowering plants. Brassica juncea (mustard) accumulates copper, selenium, and nickel, Arabidopsis halleri (rock cress) accumulates zinc and cadmium, and Lemna gibba (duck weed) accumulates arsenic. Scientists are working to improve this capacity by breeding plants that are more effective at taking up toxic materials.
What can be done with the plants after they have done their work? They can’t be plowed back into the soil, consumed by humans or animals, or put into a landfill, but they can be burned. Work is also being done on the extraction of metals from these hyperaccumulators. This process, which is called phytomining, involves the smelting of metals from the plants. The metals in the plant tissues could then be harvested as a commercial product by the mining industry. Researchers are also working on genetically engineered super-hyperaccumulator plants to achieve toxic “mining” with increased speed and volume. With proper guidelines and regulations from the EPA, phytoremediation may play a growing role in the push to a cleaner, greener world.