Phytoremediation is the use of green plants to remove pollutants from the environment or render them harmless. Current engineering-based technolo-gies used to clean up soils — like the removal of contaminated topsoil for storage in landfills — are very costly and dramatically disturb the landscape. But the “green” technology of using plants to take up heavy metals and radioisotopes can, in certain situations, provide a more economical approach and one that is less disruptive as well.
Certain plant species — known as metal hyperaccumulators — have the ability to extract elements from the soil and concentrate them in the easily har-vested plant stems, shoots, and leaves. These plant tissues can be collected, reduced in volume, and stored for later use. In addition, of course, while acting as vacuum cleaners these unique plants must also be able to tolerate and survive high levels of heavy metals in soils — like zinc, cadmium, and nickel.
We are at the early stages of identifying and learning about the transport and tolerance mechanisms of these plants. One for instance, Thlaspi caer-ulescens, or alpine pennycress, is a small, weedy member of the broccoli and cabbage family and thrives on soils having high levels of zinc, nickel, cobalt and cadmium. Researchers have been studying the underlying mechanisms that enable T. caerulescens to accumulate excessive amounts of heavy metals.
It apparently possesses genes that regulate the amount of metals taken up from the soil by its roots and deposited at other locations within the plant. These genes govern processes that can increase the solubility of metals in the soil surrounding the roots as well as the transport proteins that move metals into root cells. From there, the metals enter the plant›s vascular system for further transport to other parts of the plant and are ultimately depos-ited in leaf cells.
Thlaspi accumulates these metals in its shoots at astoundingly high levels. Where a typical plant may accumulate about 100 parts per million (ppm) zinc and 1 ppm cadmium, and would be poisoned with as little as 1,000 ppm of zinc or 20 to 50 ppm of cadmium, Thlaspi can accumulate up to 30,000 ppm zinc and 1,500 ppm cadmium in its shoots, while exhibiting few or no toxicity symptoms.
Whereas in normal plants zinc transporter genes, for instance, appear to be regulated by the zinc levels in the plant, in Thiaspi some sort of mutation has enabled these genes to stay maximally active at all times, independent of zinc levels until they are raised to very high concentrations.
Fortunately, zinc, nickel, cobalt and cadmium are metals that can be economically extracted from the shoots of Thiaspi, providing a viable process for removing these metals as soil contaminants. The crop would be grown as hay, and the plants cut and baled after they’d taken in enough minerals. Then they’d be burned and the ash sold as ore. Ashes of alpine pennycress grown on a high-zinc soil in Pennsylvania yielded 30 to 40 percent zinc — which is as high as high-grade ore.
Phytoremediation and Radioactivity
Phytoremediation has also shown promise as a means of dealing with radioactive elements. For soil contaminated with uranium, studies have found that adding the organic acid citrate to soils greatly increases both the solubility of uranium and its bioavailability for plant uptake and translocation. Cit-rate does this by binding to insoluble uranium in the soil. With the citrate treatment, shoots of test plants increased their uranium concentration to over 2,000 ppm — 100 times higher than the control plants.
The radioactive element cesium-137, with a half-life of 32.2 years, was released during the cold war by aboveground nuclear testing. Large land areas are now polluted with radiocesium and the costs of cleaning up just Department of Energy sites in the United States with engineering technology has been estimated at over $300 billion.
Fortunately, phytoremediation is an attractive alternative to these approaches. Studies showed that the primary limitation to removing cesium from soils with plants was its bioavailability. The form of the element made it unavailable to the plants for uptake. But now it appears that the ammonium ion is effective in dissolving cesium-137 in soils. This treatment increases the availability of cesium-137 for root uptake and significantly stimulates radioac-tive cesium accumulation in plant shoots.
And one plant species, a pigweed called Amaranthus retroflexus, has been found to be highly effective in removing radiocesium from soil. Researchers were able to remove 3 percent of the total amount in just one 3-month growing season. With plantings of two or three crops annually, the plant could clean up a contaminated site in less than 15 years.
Aluminum Tolerance in Acid Soils
Aluminum is the third most abundant element in the Earth’s crust and is a major component of clays in soil. At neutral or alkaline pH values, aluminum is not a problem for plants. In acid soils, however, a form of aluminum — Al+3 — is solubilized into a soil solution that is quite toxic to plant roots. Well over half the world’s 8 billion acres of arable land suffer from some degree of aluminum toxicity, including 86 million acres in the US.
Studies have found that, in aluminum toxicity, the root tip is the key site of injury, leading to inhibited root growth, a stunted root system, and reduced yields or crop failures from decreased uptake of water and nutrients. In some plants, however, aluminum triggers the release of protective organic ac-ids from the root tip into adjacent soil. When released, these acids form a complex with the toxic aluminum, preventing the metal’s entry into the root. Wheat and corn, for instance, tolerate aluminum by excluding the metal from the root tip.
Researchers looking for better aluminum tolerance are studying Arabidopsis thaliana, or thale cress, a diminutive, weedy member of the mustard fami-ly. Mutants have been found which are quite aluminum tolerant and those mutations are being analyzed for genes conferring aluminum tolerance. It should then be possible to improve the tolerance of relatively aluminum-sensitive crop species, such as barley, or to further enhance the tolerance of existing aluminum-tolerant germplasm.