Innovation: Trees that can draw out the poison: Planting may prove to be the best way of cleaning up polluted land. Nuala Moran reports

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COMPANIES responsible for restoring polluted industrial sites may be able to cut clean- up costs by planting trees that absorb poisonous heavy metals. They could then gain commercially by harvesting the wood that shoots from the roots of the trees after they have been cut down.

This approach, known as green remediation, is suggested by scientists at Glasgow University, who are beginning the search for suitable trees by analysing this year's growth of alders, willows and poplars on a site that has been poisoned by decades of heavy industrial use.

The decontamination of soils polluted with heavy metals is one of the most difficult problems in cleaning up derelict land. Existing techniques of chemical and physical extraction or immobilisation in situ, are all expensive and require special equipment and operators. Vitrification, for example, costs about pounds 18,000 for each hectare (2.5 acres). These methods also remove all biological activity from the soil and damage its physical structure.

With funding from Scottish Enterprise, Dr Ian Pulford and his colleagues in Glasgow will also grow trees in soils contaminated with controlled amounts of cadmium, chromium and zinc to measure absorption rates and calculate the time it would take to clean up a given volume of soil. The scientists need to establish where the metals are distributed in the tree. If they end up in the leaves, the scheme would be pointless, because the soil would be recontaminated each autumn.

The Government is currently offering farmers grants in an attempt to revive coppicing - the practice of cutting trees down to root level and then harvesting the wood that shoots from the base - as a fuel for power stations, and cash could also be earnt in this way.

Planting and tending a woodland until it can be coppiced costs about pounds 1,300 a hectare, but as the market is not yet established, it is unclear how much coppiced wood would be worth. Wood from contaminated sites could not be used for fuel, since this would spread the pollution.

Many industrial sites are in built-up areas and Dr Pulford says planting trees that can extract heavy metals will also have the advantage of providing cover to prevent the pollution spreading in water run-off or blowing around in dust.

Another research group, led by Dr Steven McGrath at the government's Arable Crops Research Station at Rothamsted, Hertfordshire, is studying the use of plants that grow in naturally metal-rich soils for remediation. Dr McGrath says some plants accumulate metals at such high levels that they could provide a source of those metals. Genetic engineering to improve the uptake of metal is also on the agenda.

If these methods of green remediation can be proven, Dr Pulford suggests that metal-accumulating plants and trees could be used in combination with bioremediation, in which micro-organisms are added to soil to break down organic pollutants, to clean up polluted sites completely.

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