Concurrent removal and accumulation of heavy metals by the three aquatic macrophytes
Introduction
Contamination of the aquatic environment by the heavy metals has become a serious concern in the developing world. Heavy metals unlike organic pollutants are the persistent in nature, therefore, tends to accumulate in the different components of the environment (Chandra et al., 1997). These metals are released from a variety of sources such as mining, urban sewage, smelters, tanneries, textile industry and chemical industry. These heavy metals are highly toxic to the aquatic plants and animals as well as do not vanish easily from the environment. Their treatment usually requires removal through some technology. The technologies used for their treatment are reverse-osmosis, ion-exchange, electrodialysis, adsorption, etc. Most of these technologies are quite costly, energy intensive and metal specific. Contrary to this phytoremediation, i.e. removal of pollutants by the use of plants offers a promising technology for heavy metal removal from waste water (Singh et al., 1996, Miretzky et al., 2004). Aquatic macrophytes have great potential to accumulate heavy metals inside their plant body. These plants can accumulate heavy metals 100,000 times greater than in the associated water (Mishra et al., in press). Therefore, these macrophytes have been used for heavy metal removal from a variety of sources (Axtell et al., 2003, Maine et al., 2001, Miretzky et al., 2004, Hassan et al., 2007, Mishra et al., 2008). The aquatic macrophytes are thought to remove metals by three patterns (a) metals are restricted from entering the plant and attaches to the cell wall (b) metals are accumulated in the root, but translocation to the shoot is constrained (c) hyperaccumulation, metals are concentrated in the plant parts. The hyper accumulative capacities of the aquatic macrophytes are beneficial for the removal of heavy metals. Most of the studies regarding the phytoremediation of heavy metals are confined to a few plants and metals. Still, there are a few studies available regarding the removal efficiencies of different plant groups and removal of several heavy metals at a time. Therefore, present study was performed to harness the metal removal capacities of selected macrophytes and to determine the suitability of these plants for their large scale utilization.
Section snippets
Experimental plants
Three aquatic macrophytes Pistia stratiotes L. (water lettuce), Spirodela polyrrhiza W. Koch (duckweed) and Eichhornia crassipes (Water hyacinth), were selected to asses their heavy metal removal capacities for five heavy metals (Cu, Cd, Cr, Zn and Fe) from water under laboratory conditions. These macrophytes are perennial aquatic weed spread all over the world, considered noxious and extremely invasive for freshwater environments. These species carry out their entire life cycle as
Heavy metal removal from metal solution
Chemical composition of the water used for the removal experiment was slightly alkaline with moderately high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). All the parameters were maintained within the same range except the heavy metal concentration which was 1, 2 and 5 mg l−1 in three different experimental sets. The ambient air and water temperature was maintained between 25 and 27 °C. Results revealed increasing trend of removal with the increasing incubation period. Analysis
Conclusions
Three aquatic macrophytes P. stratiotes, S. polyrrhiza and E. crassipes were tested for concurrent removal of five heavy metals Fe, Cu, Zn, Cr and Cd. The macrophytes proved highly effective in the uptake of these heavy metals at the three concentrations, i.e. 1.0, 2.0 and 5.0 mg l−1. These plants have removed the metals successfully without production of toxicity. The high correlation between the final metal concentration in the water and the metal concentration in macrophytes indicates that
Acknowledgement
Authors are thankful to the Council of Scientific and Industrial Research, New Delhi, for financial assistance.
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