Levels of heavy metals in some soils are so high that food crops cannot safely be grown in them. The problem is how to use the land economically, and how to clean the soil. The easiest form of remediation is to convert the contaminated land to non-food crops such as flowers and ornamentals. However, this may not be acceptable to farmers. Lime applications may help, by increasing the soil pH and, thus, reducing the uptake of heavy metals by the crop.

Another approach is phytoremediation, planting selected species to take up the heavy metals. Suitable plant species not only tolerate the presence of heavy metals; they also have a strong appetite for them and take them up so they accumulate in the plant. The best plants should have a high uptake, and the ability to grow in contaminated soils. Such species are known as hyperaccumulator plants.

More than 400 different kinds of plants are known to accumulate toxins. Research is now being carried out in many countries in Asia to identify the most promising species.

Plants Used for Phytoremediation

The best plants should have a high uptake, and the ability to grow in contaminated soils.

Goods Plants for Polluted Soils

Eight plants that grow well in fields polluted with heavy metals are: Fireweed (Crassocephalum crepidioides (Benth.) S. Moore), Black nightshade (Solanum nigrum L.), sword fern, Zoysia grass (Zoysia tenuifolia Willd.), rape (Brassica campestris L.), green amaranth (Amaranthus viridus L.), red amaranth (Amaranthus caudatus L.), and edible amaranth (Amaranthusmangostanus L.). These eight plants were studied to see how much heavy metal they removed from polluted soil, when they were grown from seed in polluted fields.

The results indicated that sword fern, Zoysia grass, rape ( Fig. 1), green amaranth ( Fig. 2), red amaranth, and edible amaranth all survived a high concentration of cadmium (4.25 µg/g), chromium (857 µg/g), copper (963 µg/g), nickel (622 µg/g) and zinc (995 µg/g). Furthermore, these plants were able to concentrate the heavy metals from the soil. Zoysia grass contained 80 µg/g of chromium, while green amaranth contained 263 µg/g of copper in the harvested dry matter. The nickel content of green amaranth, red amaranth, and edible amaranth were 245, 252 and 233 µg/g, respectively. Red amaranth, green amaranth, edible amaranth and rape contained as much as > 200 µg/g Zn in the dry matter. Based on their high biomass and ability to accumulate heavy metals, four of the plants - red amaranth, green amaranth, edible amaranth and rape - have the potential to be used as hyperaccumulator plants to clean heavy metals from polluted soils.

How to Use These Plants

Often, contaminated land is derelict. Growing plants to clean up the soil may take some time, but in the meantime, the land will be attractive and colorful.

However, if the land is needed for growing crops, particularly food crops, the hyperaccumulator plants must be harvested and removed. Of course, using their leaves as a green manure or as a mulch will simply return the heavy metals to the soil. Usually, the plants are dried and burnt, and the ash is removed or buried.

Index of Images

Figure 1 Rape (Brassica Campestris L.)

Figure 2 Green Amaranth (Amaranthus Viridis L.)

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