Popular Article
Popular Kheti Volume -3, Issue-3 (July-September), 2015 Available online at www.popularkheti.info © 2015 popularkheti.info ISSN: 2321-0001
Remediation of Heavy Metal Polluted Soil: Phytoremediation Vis-À-Vis Chemical Immobilization Sovan Debnath1, Prasenjit Ray2* and Rajiv Rakshit3 1 ICAR–Central Institute of Temperate Horticulture, Regional Station- Mukteshwar, Nainital- 263138, Uttarakhand 2 ICAR–National Bureau of Soil Survey and Land Use Planning, Regional Centre, Jorhat-785004, Assam 3 Department of Soil Science and Agricultural Chemistry, Bihar Agricultural University, Sabour, Bhagalpur-813210, Bihar *Email of corresponding author:
[email protected]
Phytoremediation, an emerging cost-effective, non-intrusive, and aesthetically pleasing technology, that uses the remarkable ability of plants to concentrate heavy metals from the contaminated soil and to metabolize them in their tissues, appears very promising for the removal of metals from the polluted soil environment. However, the major problems associated with this approach are low metal removal rates and disposal of the metal accumulating plants. On the other hand, in-situ chemical immobilization is a remediation technique that decreases the concentration of metal contaminants by sorption or precipitation. This technique is also useful in reducing metal transfer from soil to plant to prevent food chain contamination. However, remediation of highly contaminated soil through chemical immobilization may be more expensive in comparison to phytoremediation. Nonetheless, chemical immobilization stands out to be a better practice over phytoremediation for prompt remediation of heavy metal polluted soils. Introduction The accumulation of heavy metals and metalloids in agricultural soils is of increasing concern due to its detrimental effects on soil ecosystems and the potential risks associated with it. Mining and smelting of metal ores, electroplating, energy and fuel production, fertilizer and pesticide applications are the important human activities responsible for the accumulation of heavy metals in soil. Due to their immutable nature, metals are a group of pollutants of much concern. Food chain contamination due to heavy metal pollution in soil is very usual phenomenon. In fact, although several metals are essential for biological systems and must be present within a certain concentration range, at high concentrations, metals can act in a deleterious manner by blocking essential functional groups, displacing other metal ions, or modifying the active conformation of biological molecules. Therefore, excessive intake of such heavy metals due to food chain contamination may lead to several physiological and metabolic disorders in humans. Food chain contamination can be reduced to a great extent, for better human health, through restoration of metal contaminated soil. One of the remediation technologies for the restoration of metal-contaminated soil includes excavation of soil followed by washing and subsequent disposal of treated soil (USEPA, 1991). However, this option of remediation is prohibitively expensive.
Popular Kheti
ISSN: 2321-0001
262
Debnath et al., 2015, Pop. Kheti, 3(3): 262-267
Popular Article
Alternatively, phytoremediation i.e. use of plants for ameliorating metal-contaminated sites, has received considerable attention in recent years. Besides, there has been an increasing interest in the immobilization of metals in soil using range of inorganic and organic materials. Immobilization approach of metals in the soil itself is based on the logic that metals cannot be destroyed biologically (no degradation, change in the nuclear structure of the element occurs) but can be transformed from one oxidation state to another. The present article has focused on the processes and various issues related to phytoremediation and chemical immobilization of metal contaminated soil. 1. Hyperaccumulator and Phytoremediation A hyperaccumulator is a plant capable of growing in soils with very high concentrations of metals, extracting these metals through their roots, and concentrating extremely high levels of metals in their tissues. The hypraccumulators should have the following characteristics: (i) tolerant to high levels of the metal, (ii) accumulate reasonably high levels of the metal, (iii) rapid growth rate, (iv) produce reasonably high biomass in the field, and (v) profuse root system (Garbisu et al., 2002). In general terms, metal concentrations in hyperaccumulators are about 100-1000 fold higher than those found in normal plants growing on soils with background metal concentrations, and about 10-100 fold higher than most other plants growing on metal contaminated soils. Hyperaccumulators are also characterized by a shoot to root metal concentration ratio of >1 i.e., hyperaccumulator plants show a highly efficient transport of metals from roots to shoots. The values chosen as threshold for definition of hyperaccumulator plant species is shown in Table 1. The use of plants to extract, sequester and/or detoxify pollutants, without the need to excavate the contaminant material and dispose of it elsewhere is called phytoremediation. It has been reported to be an effective, nonintrusive, inexpensive, aesthetically pleasing, socially accepted technology to remediate polluted soils. Phytoremediation is widely viewed as the ecologically responsible alternative to the environmentally destructive physical remediation methods currently practiced. Table 1: Threshold level for definition of hyperaccumulator plants Element Average range in Average range in plant Threshold for soilsa tissuesb hyperaccumulatorsc (µg g-1 dry weight) (µg g-1 dry weight) (µg g-1 dry weight) Mercury (Hg)
