Polychlorinated biphenyls (PCBs) are persistent environmental contaminants that have a range of toxic effects, including skin lesions, lowered immune response, liver disease and probably cancer. Most countries banned their use some 20 years ago and their production was prohibited by the 2001 Stockholm Convention on Persistent Organic Pollutants, but they remain in worrying concentrations in the environment.
PCBs were used in industry as coolants and insulating fluids and also as plasticisers or stabilisers in a wide range of products — including even surgical implants. Their presence in the soil has prevented the reuse of many brownfield sites that once housed such industries, as well as many landfill sites.
In the US, large areas of agricultural land have become contaminated through the use of PCB-laden sewage sludge as a field fertiliser. And because PCBs leach from the soil into groundwater, many US lakes and rivers are contaminated, with consumers warned to avoid eating fish caught in them.
The UK has been relatively free of problems, except at Groes-faen in South Wales. In the 1960s and 1970s, a disused quarry on the edge of this Cardiff commuter village was unlawfully used as a dump for waste from a Monsanto chemical works in Newport, where PCBs were manufactured. The quarry remains one of the most contaminated sites in Britain, containing at least seven PCBs and about 60 other toxic chemicals.
So how do you clear contaminated land? There are three established ways of destroying PCBs — physical, chemical and microbial. Physical methods such as incineration, irradiation and pyrolysis are not practical for recovering large tracts of land, and nor are the various chemical methods.
Recent work has centred on micro-organisms that decompose PCBs, either by using them as a carbon source or by replacing chlorine with hydrogen on the biphenyl skeleton. However, there are problems. First, microbes that show promise in the laboratory may be of little use in a natural system because they may decompose other sources of carbon in preference to PCBs. Secondly, the microbes may be highly selective, decomposing some PCBs but not others. Thirdly, they tend to be slow-acting.
But new hope has arisen from the University of Iowa, where researchers have been studying switchgrass (Panicum virgatum), also known as tall prairie-grass. Once common across North America’s prairies, switchgrass is being reintroduced for soil conservation, since its height makes it an effective barrier against wind erosion and its deep root system anchors the soil in place, preventing erosion from flooding and run-off.
In laboratory tests of contaminated soil, the researchers found that switchgrass removed 40 per cent of PCB pollution in 24 weeks. This was boosted to 47 per cent by adding a PCB-oxidising bacterium, Burkholderia xenovorans LB400. A bonus was that the presence of switchgrass seemed to help the microbe’s survival in the soil. And the researchers believe that the plant’s deep root system could mop up PCBs well below ground level.
So switchgrass could be a natural, environmentally friendly answer to both soil erosion and soil pollution.