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Originally published November 7 2015

Switchgrass naturally removes toxic PCB pollution from soil

by David Gutierrez, staff writer

(NaturalNews) A grass native to the North American prairies can remove nearly 50 percent of all PCB contamination from soil in just six months, according to a study conducted by researchers from the University of Iowa and published in the journal Ecological Engineering.

The grass was most effective when paired with a bacterial species already known to remove PCBs from the soil. The grass also encouraged the bacteria to act more effectively.

"One surprising finding was that the presence of the switchgrass seemed to promote the survival and the activity of the added (aerobic PCB-oxidizing microorganism) LB400 bacteria," corresponding author Tim Mattes said.

The study was funded by the National Institute of Environmental Health Sciences Superfund Research Program and the University of Iowa.

Potent results in six months

Polychlorinated biphenyls, more popularly known as PCBs, are highly toxic chemicals that were once widely used for a variety of industrial functions. They are classified as persistent organic pollutants, meaning that they resist being broken down and tend to accumulate in the bodies of living creatures. Thus, even though the U.S. Congress banned the chemicals in 1979, PCB contamination is still widespread. For example, residents are advised not to eat too many fish from many regions of the Great Lakes, due to lingering and severe PCB contamination from industrial manufacturing in the 1950s and 1960s.

To test how effective switchgrass (Panicum virgatum) - once a dominant species on the North American prairie - could be at removing PCBs, the researchers first took clean soil and contaminated it with PCBs at concentrations that are regularly found in U.S. soils, but that are dangerous to humans and other living creatures. They then sealed the soil in tubs and aged them for two months at 25 degrees Celsius (77 degrees Fahrenheit), so that the PCBs could fully leach into the soil and better approximate real-world conditions.

This soil was then placed into plastic containers (about 5.5 pounds per container), which were then planted with switchgrass seeds. For comparison, another container was filled with the contaminated soil and left unplanted, while another was filled with clean soil and planted with switchgrass seeds. All the soils were analyzed after 12 and 24 weeks.

After 24 weeks, the PCB levels in the switchgrass-planted containers had dropped by 40 percent, compared with only a 30 percent drop in the unplanted control container. When the soil was treated with a PCB-oxidizing microorganism (Burkholderia strain LB400) prior to planting, PCB levels had fallen 47 percent by 24 weeks. The researchers also found that the bacteria survived better in switchgrass-planted containers than in bare soil.

"Normally, we think that if we can get plants to grow in degraded lands (so-called brownfields) that the proper 'bugs' will grow in the root zone to degrade the contaminants," researcher Jerry Schnoor said. "What's new in this story is that we can actually help the process along by adding the proper bugs (LB400) to the root zone at the time of planting and beyond."

Plants vs. pollution

The study opens up exciting new possibilities in the field of phytoremediation, which consists of using plants and their companion microorganisms to remove soil contamination. Phytoremediation includes mechanisms such as breaking down toxic chemicals, expelling chemicals as less toxic gases, or making contaminants less bioavailable. It can also include phytomining, in which plants take up heavy metals for the soil, which are then reclaimed and used for industrial purposes.

"The possibility of synergistic interactions between the switchgrass and the bioaugmented PCB-degrading bacteria suggests that employing both plants and bacteria in PCB remediation strategies holds promise for enhanced removal of these recalcitrant compounds from contaminated sites," Mattes said.


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