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Phytoremediation. S.C.Santra. Department of Environmental Science University of Kalyani Kalyani, Nadia Email: scsantra@yahoo.com. Phytoremediation:.
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Phytoremediation S.C.Santra Department of Environmental Science University of Kalyani Kalyani, Nadia Email: scsantra@yahoo.com
Phytoremediation: Application of biological processes for decontaminating the contaminated or polluted sites is a challenging task because heavy metals cannot be degraded and hence permit in the soil. In order to clean up the contaminated sites, heavy metals should be extracted and concentrated by an appropriate technique for proper disposal in designated secure landfill sites. The established conventional techniques (viz. thermal processes, physical separation, electrochemical methods, washing, stabilization etc.) for clean up of metal contaminated soil are generally too expensive and often harmful to soil microbial diversity. Plant mediated decontamination/detoxification processes are commonly referred to as phytoremediation. It has been proposed as an alternative method to remove pollutants from air, soil, and water or to render pollutants harmless and does not affect soil biological activity, structure and fertility (Prasad, 2003, 2004). There are several types of phytoremediation technologies currently adopted as successful clean up process, both in contaminated soils and water. (Schulz, and Beck, 2002; Prasad et al 2010).
Phytoextraction – Reduction of metal concentration in the soil by cultivating plants with a high capacity for metal accumulation in the shoots; • Rhizofiltration – Adsorption or precipitation of metals onto roots or absorption by the roots of metal tolerant aquatic plants; • Phytostabilization – Immobilization of metals in soils by adsorption onto roots or precipitation in the rhizosphere; • Hydraulic control – Absorption of large amounts of water by fast growing plants and thus prevent expansion of contaminants into adjacent uncontaminated areas; • Rhizodegradation – Decomposition of organic pollutants or biotransformation of metals by rhizospheric organisms. • Phyto volatilization – Detoxify soil metal contaminant by bio-methylation processes. • Phyto degradation – Uptake of contaminants and the subsequent transformation, mineralization or metabilisation by the plant itself through various internal enzymatic reactions and metabolic processes (Prasad et al 2010).
Phytosequestration – Phytochemical complexation in the root zone, leading to the precipitation or immobilization of target contaminants in the root, such complexes and there by stored in the vacuolar space of root cells. Transport proteins are also present that facilitate transfer of contaminants between cells (Prasad, 2011). • Phytoattenuation – Production of plant biomass as sink of toxic metals at contaminated sites, where conventional agriculture is affected by the presence of elevated amounts of plant available trace elements, causing economic losses and endanger or diminish food and feed quality and safety (Meers et al 2005, 2010). • Biofortification of phytofortification – It is the process of increasing the bioavailable concentrations of essential nutrients in edible portions of food crops through agronomic intervention or genetic selection; essential nutrients like trace elements, vitamins and metabolites which are accumulated or synthesized during the growth and development of selected plants can be used as food or feed (healthy diet) in many areas (Kralova & Masarovicova, 2006). • Metal Nanoparticles – The natural production of metal nanoparticles viz. TiO2, ZNo, Al2O3, AgNO3 etc. in plants of contaminated sites are often offered resistance to plants against invasion of disease and pests or render better stress adaptability to salt or water (li & Xing, 2007; Havenkamp & Karshall, 2009).
Cadmium phytoremediation: The main anthropogenic pathway through which Cd enters the water bodies is via wastes and waste waters from industrial processes such as electroplating, plastic manufacturing, metallurgical processes and industries of pigments and Cd/Ni batteries Cadmium exists in wastewaters in many forms including soluble, insoluble, inorganic, metal organic, reduced, oxidized, free metal, precipitated adsorbed and complexed forms. Watanabe et al. (2009) grew selected species of family Amaranthaceae viz. Amaranthus tricolor, and observed the higher Cd-accumulating properties in plants. Later similar hyper accumulation properties of Cd were reported in Brassica juncea (Family Cruciferae), Chrysanthemum indicum (Family Asteraceae) and some other plants, using EDTA chelator, the hyper accumulation of Cd – can be enhanced many fold n vitro & in vivo.
Arsenic phytoremediation: In lower part of gangetic delta & Brahmaputra basin, groundwater arsenic contamination is a natural geogenic processes. However, in many parts of Asia, arsenical products have been widely used in agriculture and industrial practices viz. pesticides, fertilizers, wood preservatives, smelter wastes and coal fly ash, which are of great environmental concern. The Chinese brake fern Pteris vittata is a major arsenic hyperaccumulating plants which can accumulate 23 g kg-1 of arsenic in its fronds. Similarlarly, other species Pteris longifolia, Pteris umbrosa and Pityrogramma calomelanos are also known to be hyperaccumualtors. In presence of available phosphate, the uptake of As- by the plants appears to be higher. The root associated VAM fungi in ferns also helps in hyperaccumulation of arsenic.
Mercury phytoremediation: The mercury contamination in environment is primarily anthropogenic, more precisely, industrial sources viz. coal thermal plant, iron and steel industry, chloralkali plants, battery industry and so on being the mail sources. Bioremediation by microbes in mercury contaminated site detoxification is quite established. Bacteria and several higher plants have properties to make phytovolatilization of mercury at contaminated sites.
Chromium phytoremediation: Chromium is the chief heavy metal contaminant found in the tannery effluent and chromite mine areas. Phytoremediation appears to be one of the major thrust areas in chromium contaminated land detoxification. Selected chromium tolerant plants root zone in association with VAM fungi, helps in hyperaccumulation of chromium on site. A number of tree species helps in phytoextraction of chromium at contaminated sites.
Cyanide phytoremediation: Cyanide primarily found in waste water of steel plants, gold mine areas. Selected water plants like water hyacinth (Eichhornia crassipes) and bacteria are often used for phytoremediation/ bioremediation purpose.
Constructed Wetland for Removal of Metal in Aquatic Environment
BOD/ Carbon Pathways Legend DM Dissolved Matter PM Particular Matter # Denotes dissolved or in solution Se Sedimentation Re Resuspension of bed particulates Bu Biofilm Uptake Rs Respiration by algal biofilm and Ao Aerobic decomposition phytoplankton An Anaerobic decomposition Fb Bio film fall and deposition Fm Microphyte litter Di Dilution of O at water/air interface
contd… Source: Glass 1988, McGutcheon and Schnoor 2003, Prasad 2004, 2007, 2012, Prasad and Strzalka, 2002.
Phytoremediation and Genetic Engineering: Understanding the physiology and biochemistry of metal accumulation in plants is important for several reasons. The main implications are that this knowledge allows the identification of agronomic practices capable of optimizing the potential for phytoremediation and permits the identification and isolation of gene responsible for the expression of the hyperaccumulating phenotypes. Thus ideal plant for the phytoremediation of any metal must have a substantial capacity for metal uptake, bioaccumulation and stability as well as durability to reduce the length of treatment as for as possible and practicable. It then follows that there is a promising alternative in the development of transgenic plants with enhanced properties of metal uptake and translocation, bioaccumulation potential and higher tolerance to toxicity. Such heightened metal bioaccumulation and tolerance could be mainly achieved by normally over expression the natural or modified genes encoding antioxidant enzymes. Several researchers have reported to date rather encouraging results using plants genetically engineered with increased cadmium tolerance and uptake for phytoremediation purposes. However, a majority of these genetically manipulated plants for phytoremediation have only been tested under strict laboratory conditions and a very scanty few have been analysed for their phytoremediation potential at field scale. Metal hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate and detoxify metals, including cadmium and other metal or metalloids hence constitute an essential pool of material for genetic modification for targeted enhancements in phytoremediation potential (Fulekar et al 2009).
Genetic engineering modifications of the physiological and molecular mechanism of plants, cadmium uptake and tolerance have also been successfully achieved and these show promose in opening new avenues for enhancing the overall efficiency of cadmium phytoremediation (Eapen and D’souza, 2005). The possibilities for genetic engineering plants for phytoremediation is shown in the Fig. Fig. : Possibilities for genetic engineering in phytoremediation
A number of genetically modified (transgenic) plants have been generated and tested in recent studies and they have demonstrated the merits of genetic engineering in enhancing the tolerance, uptake and /or bioaccumulation of Cadmium. Wojas et al. (2009) have demonstrated in a study first of its kind that the hetergenous expression of Arabidopsis MRP7 in tobacco (Nicotiana tabacum var. xanthi L.) could modify cadmium accumulation, distribution and tolerance.
UNUSUAL ACCUMULATION OF METALIC ELEMENTS /METALLOID BY PLANTS Sources: Brown H.J.M. (1966), H eevit E.J. and Smith, T.A. (1975)
