The secret lies in choosing a type of plant that has an unusual appetite for the metal you are interested in. Most plants cannot grow on soil that contains high concentrations of heavy metals, such as nickel, cadmium or zinc. But certain plants thrive under such conditions, gorging on these poisonous metals. Termed "hyperaccumulators", they can be found around the world, growing in areas where the soil is rich in natural deposits of heavy metal ores. By cultivating appropriate plants on these metal-rich soils it is possible to harvest the crop and - with an appropriate (and commercially confidential) treatment - to recover the metal.
The process, which has been given the name "phytomining", is the latest technology to spin off from one of the most rapidly advancing fields of plant science - phytoremediation, or the use of green plants to clean up contaminated land. Dr Baker, who is based in the Department of Animal and Plant Sciences at the University of Sheffield and was one of the pioneers of the discipline, was originally interested in plants that could tolerate hostile soil conditions, such as those found in slag heaps. In 1988, he wrote an article in New Scientist magazine suggesting that hyperaccumulators might be more than a simple ecological curiosity - they could possibly be harnessed to clean up land contaminated by poisonous metals. "That article caused an enormous amount of interest, and what was essentially a throwaway remark seems to have spawned a whole new field of science," he says.
The contamination of land with heavy metals has been a problem for many years. Large numbers of "brownfield" sites in urban areas cannot be developed because of the legacy of pollution left by previous industrial activity. Heavy metals also find their way into the environment by other routes. Cadmium, for example, can accumulate in agricultural land simply by the repeated application of phosphate fertilisers. Cleaning up such land is expensive. The main options are to either dump the soil at a designated hazardous waste landfill, or treat it - on or off site - by a variety of means from chemical or physical extraction of the contaminant to high temperature "vitrification" (where the metal is fused with the silica in the soil). Such extreme treatments invariably result in a sterile end product, devoid of organic matter. For these reasons, the use of a "benign" technology such as phytoremediation has obvious attractions - despite some significant limitations.
In the early Nineties, Dr Baker teamed up with one of his former research students, Steve McGrath, to carry out a series of studies to test the feasibility of phytoremediation technology. At the Institute of Arable Crops Research at Rothamsted, Hertfordshire, there were a series of plots of land with varying degrees of low-level metal contamination. These had been used some years earlier to test the possibility of spreading sewage sludge (which had inadvertently been collected from works receiving industrial effluent) on land as a source of nutrients. It left a cocktail of heavy metals behind in the top layers of the soil. The level of contamination was not extreme, although higher than the current European Community guidelines for certain of the metals.
"We selected plants from the Brassica family, known for their powers of metal accumulation," says Dr Baker. "One called thlaspi caerulescens grows wild in various parts of north-west Europe, including Britain and France. Species of alyssum from Mediterranean Europe were also included. We planted it under agronomic conditions - sowing it close together, and fertilising and irrigating the crop.
"The crops did remarkably well and we grew them into a second season - even the plants from Greece got through the winter without too many problems. The results were very encouraging, with zinc, cadmium and nickel being taken up in substantially higher amounts by these plants than in the non-accumulator varieties which we had planted alongside."
The scientists were able to calculate that it would need between nine and 13 generations of growth to bring the metal concentrations down to an acceptable level whereas a non-accumulating plant crop would take two millennia to achieve the same results. Since then, scores of research groups have sprung up across the world investigating the phytoremediation process.
At Rutgers University in New Jersey, for example, Professor Ilya Raskin has been using Indian Mustard, another member of the brassica family, as a sponge for toxic metals. This plant does not hyperaccumulate dramatically, but it grows prolifically and it is possible to obtain several croppings in one season. So while it is less efficient at sucking up metal than other plants, its rapid growth compensates. The Rutgers group has set up the world's first commercial company to use the technology, Phytotech Inc, which has successfully cleaned up lead from a number of small sites in the US.
Disposing of the metal-laden plant material once it has been harvested remains an important issue. "One way is to let the plant dry out then crop it as hay," says Dr Baker. "You can then incinerate it at a low temperature to obtain ash containing the metal in its oxide form. This increases its concentration by a factor of around 10, and we have found that you can get between 10 and 20 per cent nickel in the ash. This is, in effect, a "bio-ore" and, depending on the value of the metal, it could even be economically feasible to recover it."
A venture capital company in America hired Dr Baker, together with colleagues from the US Department of Agriculture and the University of Maryland, to develop the idea. The company has grown another species of alyssum from nickel-rich sites in the Mediterranean and Middle East, and it has produced metallic nickel from the crop. The source is so low-grade that conventional mining for it would not be feasible.
Dr Baker has travelled the world identifying metal hyperaccumulators. So far there are around 430 in the collection, ranging from small herbs to shrubs and trees. "I have found plants on mine spoil heaps which contain up to 5 per cent zinc - they are virtually galvanised."
Research is now concentrating mainly on the physiology and molecular biology of metal accumulation by these plants: how do they hyperaccumulate and can they be engineered to become more efficient or faster growing?
The answers to some of these questions are beginning to emerge. The Sheffield researchers have shown that roots of the thlaspi plants appear actively to forage for heavy metals. By growing the plant in a transparent container, called a rhizobox, it is possible to create zones in the soil that have high concentrations of a particular metal. The researchers have clearly demonstrated that the plant's roots make a beeline for these metal-rich areas and cluster around and within them.
Precisely why plants would want to accumulate heavy metals is not entirely clear. "There is evidence that high concentrations of these metals in the plant tissues could offer protection against smaller herbivores such as insect larvae and also against fungal attack," says Dr Baker. "After all, many commercial fungicides do contain heavy metals."
The mechanism by which the metals are soaked up remains poorly understood. It is clear that around the tips of the roots there is a lot of biological activity, and it is thought that the plant does exude substances that can somehow "trap" the metal ions to package them in a form suitable for absorption across the root's walls.
Professor Andrew Smith's group at the Department of Plant Sciences at Oxford University, has identified an amino acid, histidine, as being responsible for binding nickel inside the root cells of alyssum. This appears to "inactivate" the metal, rendering it non-toxic to the plant. Once absorbed and inactivated, the metal ions are transported through the plant's vascular system and into cells, where they are combined with simple organic acids and deposited in a compartment called the vacuole.
"The pieces of the jigsaw are gradually coming together, but a lot of basic research remains to be done," says Dr Baker.
The ultimate aim would be to develop - either through genetic engineering or conventional plant breeding techniques - "designer" plants that could be tailored to soak up any given contaminant.Reuse content