Tackling the root causes
Melanie Brown looks at an innovative bioremediation technique from the US, which employs a hybrid strain of fungus to clean up soil contaminated with heavy metalsBioremediation is commonly used to remove toxic organic compounds from contaminated soils. However, the inability of bioremediation processes to remove heavy metals from soils has limited their application.
Many contaminated soils contain metals in addition to a range of toxic organic compounds and these sites have proved difficult to clean up. Dig-and-dump is the most common approach to remediating such soils, but this only relocates the problem.
However, a joint venture company, Phytobials LLC, formed by researchers at Cornell University in the US, the University of Surrey and the University of Naples, Italy, is combining plants with a fungus to remove metals and toxic organics simultaneously from contaminated soils, without the need to excavate soil.
The technology combines the use of metal-accumulating plants with a hybrid fungus, Trichoderma harzianum T22, which has been used in agriculture for several years as a growth promoter and biological control agent.
The fungus develops a protective colony around the plant roots, feeding on root exudates and aggressively fighting off any organism that threatens its food supply. The symbiotic fungus increases plant root depth and density and fosters the formation of fine roots, stimulating the uptake of nutrients and minerals required for plant growth.
Cornell University's Gary Harman patented the T22 strain as a bio-control agent, but he has since demonstrated that relatively insoluble minerals, such as manganese oxide, zinc and iron oxide are solubilised by the fungus through chelation and reduction of the metals.
Professor Jim Lynch at Surrey University has discovered that strain T22 can degrade cyanide in soils. "Plants cannot normally grow in cyanide-polluted soils, but if you add T22 to the seed, they are able to thrive because the fungus degrades the cyanide and protects the plant," he explains. Matteo Lorito of the University of Naples has found that T22 can degrade phenolic compounds in olive oil processing wastes.
Native plants can be selected for particular sites, as the T22 fungus will grow with the roots of any plant species, including trees. In the US, phytoremediation companies are offering plant varieties that have been optimised for metal uptake, such as ferns and cottonwood trees.
A metal accumulating fern - Edenfern - developed by Edenspace Systems, a US phytoremediation company, can accumulate up to 16,030mg/kg of arsenic in above-ground fronds in soils containing concentrations of arsenic at 100mg/kg. Most existing phytoremediation processes rely on the addition of surfactants or chelating agents to solubilise metals. The use of T22 eliminates the need for additives.
As it is already used in agriculture, approval to use the fungal strain for remediation should not present a problem. Strain T22 is registered as a bio-pesticide with the US EPA and is listed for use in organic agriculture.
A trial combining the T22 fungus with Edenfern is currently underway to remove arsenic, lead and mercury from a site contaminated with inorganic pesticides in Geneva, New York.
A full-scale clean up of a six-acre site is also planned for the 2004 growing season. Dan Berler, CEO and president of Phytobials, explains that "arsenic concentrations of 100ppm can be remediated in as little as one growing season - 3-5 months - although higher concentrations will take longer".
The technology appears to have exciting prospects for combining the removal of metals and toxic organics and is likely to be most effective at treating large sites with relatively low levels of contamination.
Although trials are being carried out in the US, the technology will benefit UK developers. The cost of excavating and transporting contaminated soil to landfill (dig-and-dump) in the UK and the US typically varies between £60 and £350 per tonne. Phytobials' phytoremediation treatment costs anestimated £4-£12/tonne, allowing a potential 80% saving or greater compared to conventional methods.
Phytobials sees its biggest market in the clean-up of agricultural land for commercial or residential projects, although it will be targeting brownfield sites, mining and landfill operations as well. It is also planning to apply the technology to the treatment of industrial wastewaters, such as olive oil waste streams contaminated with polyphenols. The company plans to establish a UK operation next year.