Metal hyperaccumulation in plants - Biodiversity prospecting for phytoremediation technology
Narasimha Vara Prasad*
Maria de Oliveira Freitas
Keywords: agricultural crops, aquatic macrophytes, biodiversity, Brassicace, cell cultures, hyperaccumulators, metals, ornamentals, remediation, tree crops, vegetable crops.
All compartments of the environment viz., air, water and soil are polluted by a variety of inorganic and organic pollutants. Noosphere is man dominated ecosytem responsible for the release of a variery of pollutants (anthropogenic). Phytosphere (green belts) act as sinks for these pollutants. Phytoremediation, an emerging tool of biogeotechnology with cutting edge applications for sustainable development. The fundamental disciplines of Biogeotechnology include i) Geology + Environmental biology = Environmental geology; ii) Biotechnology + Geology = Biogeotechnology, iii) Environmental biology + Biotechnology = Environmental biotechnology. iv) Environmental geology + Biogeotechnology + Environmental Biotechnology = Environmental Biogeotechnology.
Phytoremediation can be defined as the use of plants, including trees and grasses, to remove, destory or sequester hazardous contaminants from media such as air, water and soil (Terry and Bañuelos, 2000). This strategy is not only growing as science; but also emerging as a potential industry (Glass, 1999). Phytoremediation uses living green plants for in situ, or in place, risk reduction for contaminated soil, sludges, sediments, and ground water, through contaminant removal, degradation, or containment. Growing and, in some cases, harvesting plants on a contaminated site as a remediation method is an aesthetically pleasing, solar-energy driven, passive technique that can be used to clean up sites with shallow, low to moderate levels of contamination. This technique can be used along with or, in some cases, in place of mechanical cleanup methods. Phytoremediation can be used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydrocarbons, and landfill leachates.
Certain essential processes involved in Phytoremediation:
i)Phytostabilization: This involves the use of plants especially roots and/or plant exudates to stabilize, demobilize and bind the contaminants in the soil matrix, thereby reducing their bioavailability. This approach is suitable for both organic and metal contaminated soils. Several plant species including common agricultural and horticultural crops have shown potential in phytostabilization. Certain plant species have been used to immobilize contaminants in the soil and ground water through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants (rhizosphere). This process reduces the mobility of the contaminant and prevents migration to the ground water or air, and it reduces bioavailability for entry into the food chain. This technique can be used to reestablish a vegetative cover at sites where natural vegetation is lacking due to high metals concentrations in surface soils or physical disturbances to surficial materials. Metal tolerant species can be used to restore vegetation to the sites, thereby decreasing the potential migration of contamination through wind erosion and transport of exposed surface soils and leaching of soil contamination to ground water.
ii)Phytoextraction: It involves specific plant species which can absorb and hyperaccumulate metal contaminants and/or excess nutrients in harvestable root and shoot tissue, from the growth substrate (soil). This approach is suitable for removing most metals (such as Pb, Cd, Ni, Cu, Cr, V) and excess nutrients (such as NH4NO3) from contaminated soils. Examples of plants used are Thlaspi sp., Brassica sp., Alyssum sp.(Brassicaceae), and Pelargonium sp (Geraniaceae). It consists of i) planting a speices that tends to accumulate and store, transpire or degrade the target contaminant, ii) letting the crop grow and iii) harvesting it. Low-cost, phytoremediation is most often used when a large area contains low-level contamination close to the soil surface.
iii) Phytovolatilization: This uses the plants ability to absorb and subsequently volatilize the contaminant into the atmosphere. This approach is suitable for remediating metals such as Hg and Se from contaminated soils. Examples of plant species used are transgenic Nicotiana and Brassica plants containing bacterial genes.
iv) Phytotransformation: It is the breakdown of contaminants taken up by plants through metabolic processes within the plant, or the breakdown of contaminants external to the plant through the effect of compounds (such as enzymes) produced by the plants. Pollutants (complex organic molecules) are degraded into simpler molecules and are incorporated into the plant tissues to help the plant grow faster. Plants contain enzymes, complex chemical substances (proteins), that cause rapid chemical reactions to occur. Some enzymes breakdown and convert ammunition wastes, others degrade chlorinated solvents such as trichloroethylene (TCE), and others degrade herbicides and potentially toxic cyanide.
v) Rhizofiltration (=root): This utilizes plant roots to take up and sequester metal contaminants and/or excess nutrients from aqueous growth substrates (waste water streams, nutrient-recycling systems). This approach is suitable for remediating most metals (i.e. Pb, Cd, Ni, Cu, Cr, V), excess nutrients (such as NH4NO3) and radionuclide (such as U, Cs, Sr) contaminated water. Examples of plants used are the species of Helianthus sp., Brassica, Populus, Pelargonium, Lemna, and Thlaspi.
It is the adsorption or precipitation onto plant roots or absorption into the roots of contaminants that are in solution surrounding the root zone. Rhizofiltration is similar to phytoextraction, but the plants are used primarily to address contaminated ground water rather than soil. The plants to be used for cleanup are raised in greenhouses with their roots in water rather than in soil. To acclimate the plants once a large root system has been developed, contaminated water is collected from a waste site and brought to the plants where it is substituted for their water source. As the roots become saturated with contaminants, they are harvested. For example, sunflowers were used successfully to remove radioactive contaminants from pond water in a test at Chernobyl, Ukraine. Treating Organic Contaminants Organic contaminants (those that contain carbon and hydrogen atoms) are common environmental pollutants. There are several ways plants can be used for the phytoremediation of these contaminants: phytodegradation, rhizodegradation, and phytovolatilization. Phytodegradation, also called phytotransformation, is the breakdown of contaminants taken up by plants through metabolic processes within the plant, or the breakdown of contaminants external to the plant through the effect of compounds (such as enzymes) produced by the plants. Pollutants (complex organic molecules) of soil contamination to ground water. Treating Organic Contaminants (those that contain carbon and hydrogen atoms) are common environmental pollutants.
vi) Rhizodegradation: It is the breakdown of contaminants in the soil through microbial activity that is enhanced by the presence of the root zone (the rhizosphere) and is a much slower process than phytodegradation. Microorganisms (yeast, fungi, or bacteria) consume and digest organic substances for nutrition and energy. Certain microorganisms can digest organic substances such as fuels or solvents that are hazardous to humans and break them down into harmless products in a process called biodegradation. Natural substances released by the plant roots sugars, alcohols, and acids contain organic carbon that provides food for soil microor and establish a dense root mass that takes up large quantities of water. Poplar trees, for example, can transpire between 50 and 300 gallons of water per day out of the ground. The water consumption by the plants decreases the tendency of surface contaminants to move towards ground water and into drinking water. The use of plants to rapidly uptake large volumes of water to contain or control the migration of subsurface water is known as hydraulic control. There are several applications that use plants for this purpose, such as riparian corridors/ buffer strips and vegetative caps. This approach is suitable for remediating trinitrotoluene (TNT), polyaromatic hydrocarbons (PAH), petroleum hydrocarbons from contaminated soils.
vii) Hydraulic barriers, vegetative caps and constructed wetlands: Thease can also be included in the overall classification of the phytoremediation approaches. However, results obtained so far with various phytoremediation approaches show that phytoextraction, rhizofiltration and phytostabilization methods hold more promises as successful commercial technologies.
Phytoremediation applications stated above would fall into two broad categories: in situ or ex situ. In situ bioremediation treats the contaminated soil or groundwater in the location in which it was found. Ex situ bioremediation processes require excavation of contaminated soil or pumping of groundwater before they can be treated.
Contaminated soil is combined with water and other additives in a large tank called a "bioreactor" and mixed to keep the microorganisms which are already present in the soil in contact with the contaminants in the soil. Nutrients and oxygen are added, and conditions in the bioreactor are controlled to create the optimum environment for the microorganisms to degrade the contaminants. Upon completion of the treatment, the water is removed from the solids, which are disposed of or treated further if they still contain pollutants. Slurry phase biological treatment can be a relatively rapid process compared to other biological treatment processes, particularly for contaminated clays. The success of the process is highly dependent on the specific soil and chemical properties of the contaminated material. This technology is particularly useful where rapid remediation is a high priority. Plants that hyperaccumulate heavy metals, metalloids, organics and radio nulcides could potentially recontaminate the environment if not treated or processed appropriately (Prasad and Freitas, 1999; Prasad, 2001). The following approaches would enhance the efficiency of phytoremediation: i) Plants species or varieties are screened and those with superior remediation properties are selected, ii) Agronomical practices are developed to enhance remediation (pH adjustment, addition of chelators, fertilizers and iii) Biotechnological approach to enhance the phytoremediation capacity of plants (Kramer and Chardonnens, 1991; Pilon-Smits and Pilon, 2000).
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