A range of common weeds could be used to extract polychlorinated biphenyls (PCBs) from contaminated soil, Canadian research has shown.
The plants concentrated PCBs in their shoots and could be harvested for disposal, cutting the need to expensively remove and incinerate contaminated soil.
PCBs were used in applications including flame retardants and insulating fluids for transformers. Around 70,000 tonnes were made in the UK up to the late 1970s.
For instance Monsanto’s ‘Aroclor’ products sold in the UK and Canada contained PCB mixtures. All PCBs have now been banned as they are bioaccumulative, persistent and toxic.
Scientists at Queen’s University in Ontario studied 27 weed species growing naturally at two contaminated sites in the province. One site, a former transformer manufacturing facility is contaminated with Aroclor at an average concentration of 31 micrograms per gram of soil.
At the second site, a chemical company had used PCB-containing waste oil leaving soil contaminated with 5μg/g PCBs.
Weeds including redshank, ragweed, vetch and red clover all performed as well as or better than pumpkin, already noted for its ability to extract contaminants from soil. The weeds extracted between 1.5 and 110 milligrams of PCBs over each square metre.
In comparison with food crops like pumpkin, weeds might offer other benefits. They tend to be more hardy and tolerant of poor environmental conditions. They are easy and cheap to grow and propagate, and there is no risk of them being accidentally eaten.
The UK Environment Agency has set a soil guideline value of 8 nanograms per gram limit for residential soils, covering 29 substances including dioxin-like PCBs (ENDS Report 418, p 37).