It’s been known for years that plants can take up and store trace amounts of hazardous metals like nickel, arsenic and cadmium from contaminated soil in a process called phytoremediation.
Until recently, however, scientists assumed that plants weren’t capable of taking up a class of toxic chemicals known as persistent organic pollutants, which include carcinogenic substances such as the polychlorinated biphenyls (PCBs) used in electrical transformers and the widely banned pesticide DDT. Because plants feed by taking up water-borne nutrients — and nutrient-mimicking metals — through their roots, experts figured that water-repelling PCB molecules lacked the critical transport mechanism to carry them into the plant.
Barbara Zeeb, a biologist and Canada Research Chair in Biotechnologies and the Environment at the Royal Military College of Canada in Kingston, Ont., is disproving that assumption. During a soil-cleanup project at an abandoned military site in Northern Ontario, Zeeb discovered that certain grasslike plants called sedges were actively taking up PCBs. And Zeeb recalled from the scientific literature that pumpkins could also take up PCBs, so she began testing this herself.
Some pumpkin varieties take up chemicals better than others, and Zeeb has found that most of the substances end up sequestered not in the pumpkin but in the stem near the root. As a result, she prunes the plant’s flowers to prevent pumpkin growth and cause the plant to become thicker and bushier. Judicious pruning, which encourages secondary root growth, more than doubles the contaminant uptake of each plant.
Zeeb is experimenting in Ontario with the power of pumpkins at test sites in Etobicoke and Lindsay, growing the plants in 250-square-metre plots fenced off from people, animals and birds. Fully grown plants are harvested and composted on-site. Composting reduces the volume of plant material and contains a concentrated volume of PCBs within organic matter that can be easily transported to incinerators or other safe-disposal facilities.
Using pumpkins to remove PCBs and other harmful chemicals in soil is a relatively cheap process and leaves the soil intact. Still, this doesn’t remove all the contaminants, and because plants take months to grow, it’s not suitable for areas where contaminants must be removed quickly.
Zeeb’s work could help in cleaning up the hundreds of brownfields across Canada — sites of former gas stations and industrial facilities contaminated with petroleum products and chemicals — that pose a threat to human health and the environment. Municipalities and builders alike are eager to convert these sites into revenue-generating properties, but the cleanup and insurance costs can be prohibitive. In certain instances, phytoremediation may lower those costs and make development feasible.
The tests Zeeb is conducting suggest that pumpkin phytoextraction may help clean up some temperate-zone brownfields to an industrial, a commercial or even a residential standard. They might also be used for parkland.
Zeeb is starting to experiment with other plants, such as New England aster and lady’s thumb. Preliminary tests suggest that per unit area, these plants are even more efficient than pumpkins at taking up PCBs.
“At optimal densities,” says Zeeb, “certain plants have the potential to do a lot for you.”