Phytoremediation of Soil Contaminated With Copper and Zinc from Pig Waste

Copper (Cu) and Zinc (Zn) are two elements of great concern due to their potential toxicity to plants and/or animals. Most pig farmers in Malaysia fed two to three folds of the recommended Cu and Zn to their animals. Only about 15% of the dietary Cu and Zn were apparently digested while the remaining 85% were excreted mainly via the feces. It is estimated that 390 tons of Cu and 303 tons of Zn are being excreted annually by approximately two million pigs in Malaysia. Appropriate management of the pig waste is, therefore, essential to ensure that these heavy metals do not pollute the environment. Phytoremediation, the use of plants to restore polluted sites, has recently become a popular alternative to the traditional techniques of cleaning up polluted soils. Five locally available plants, namely Kenaf (Hibiscus cannabinus L.), Typha (Typha spp.), Vetiver (Vetiveria zizanioides), Canna (Canna x generalis), Cyathula (Altemanthera Ficoidea cv. "Songuinea") were shown to exhibit their ability to accumulate Cu and Zn from the soil applied with pig manure. Roots were the main site for Cu and Zn accumulation followed by stems and leaves. On the average, Cu and Zn concentrations in all parts of Cyathula were the highest. However, because of its low biomass, accumulation of Cu and Zn per plant basis for Cyathula was significantly lower than the other plants, particularly Canna and Typha. Since, heavy metal tolerance in plants is often species and metal specific, different plants can better accumulate heavy metals in the different plant tissues or cell organelles. Examination of heavy metal accumulation at cellular level is thus important to understand the tolerance mechanism of the metal-accumulating plants. Ultrastructural investigation of the root-tips of Kenaf, Canna and Cyathula indicated that Cu and Zn mainly deposited in the cell wall, cytoplasm and nucleus. In leaves, the main sites of deposition were chloroplast and nucleus, followed by cell wall, cytoplasm and mitochondria. Vacuole had the lowest Cu and Zn deposition. High concentration of Cu and Zn deposition resulted in changes in root cell structure, including cell wall deformation and vacuolization. The above effects were, however, not consistent among the three plant species studied. For examples, cell wall deformations were observed in root cells of Canna, but not Kenaf and Cyathula while strong vacuolization of root cells was observed only in Cyathula. Cell wall deformation and vacuolization in cells could be symptoms of toxicity and/or self defense mechanism to prevent toxicity of heavy metals to the plant tissues. There was no observable change in the leaf cell structures of these plants under different Cu and Zn treatments, presumably because of the low level of Cu and Zn accumulation in the leaf cells were below the toxicity level.

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