Here, the contaminated soil is treated on- or offsite, and then returned to its original site. Conventional ex situ methods applied to remediate polluted soils include excavation, detoxification, and/or destruction of the contaminant physically or chemically, meaning that the contaminant undergoes stabilization, solidification, immobilization, incineration or destruction.
These methods perform remediation without the need to excavate the contaminated site. Reed et al. defined in situ remediation technologies as the destruction or transformation of the contaminant, its immobilization to reduce bioavailability, and the separation of the contaminant from the soil (Reed et al. 1992). In situ techniques are often preferable to ex situ techniques due to their low cost and reduced impact on the ecosystem. Conventional ex-situ techniques involve excavating the soil contaminated with heavy metals and burying it in a landfill site (McNeil and Waring 1992; Smith 1993). However, offsite burial merely shifts the contamination problem elsewhere (Smith 1993), and leads to hazards associated with the transport of contaminated soil (Williams 1988). Diluting the heavy metal content up to a safe level by importing clean soil and mixing it with the contaminated soil may be an alternative for onsite management (Musgrove 1991). Onsite containment and barriers also provide an alternative; these involve covering the soil with inert material (Body et al. 1988). Immobilizing the heavy metals through the application of inorganic contaminants can also be used as a remedial method for heavy metal contaminated soils (Mench et al. 1994). This involves complexing the contaminants or increasing the soil pH by liming (Alloway and Jackson 1991), since increasing the pH decreases the solubilities of heavy metals such as Cd, Cu, Ni, and Zn in the soil. However, most of these conventional remediation technologies are very costly to implement and cause further disturbance to the already damaged ecology (Mench et al. 1994; Alloway and Jackson 1991).
Plant-based bioremediation technologies have been collectively termed "phy-toremediation," and involve the use of green plants and their associated microbes for the in situ treatment of contaminated soil and groundwater (Sadowsky 1999). Metal-accumulating plants were first used to remove heavy metals from the soil in 1983, but this concept has actually been implemented for the past 300 years (Henry 2000). This technology can be applied to both organic and inorganic pollutants found in the soil (solid substrates) as well as water (liquid substrates) and the air (Salt et al. 1998; Raskin et al. 1994). The application of physicochemical techniques for soil remediation renders the land useless for plant growth, as they remove all of the biological components, such as useful microbes (nitrogen-fixing bacteria), mycorrhiza, fungi, and fauna during the process of decontamination (Burns et al. 1996). Finally, conventional methods of bioremediation can cost from $10 to $1,000 m-3, while the cost of phytoextraction costs is estimated to be as low as $0.05 m-3 (Cunningham et al. 1997).
Phytoremediation consists of five main processes, as described below.
Rhizofiltration involves using the plants (terrestrial or aquatic) to absorb, concentrate, and precipitate low-concentration contaminants from aqueous sources in their roots. This process can be applied to partially treated industrial discharges, agricultural runoff, or acid mine drainage. It can be used for heavy metals like lead, cadmium, copper, nickel, zinc, and chromium, which are primarily held within the roots (Chaudhry et al. 1998; USEPA 2000). The advantages of this process are that it can be applied in situ or ex situ, and that species other than hyperaccumulators can also be used. Plants like sunflower, mustard, tobacco, rye, spinach, and corn have been studied for their ability to remove lead from effluent, with sunflower showing the greatest ability. Mustard has proven to be very effective at removing lead at a wide range of concentrations (4-500 mg L-1) (Raskin and Ensley 2000). The technology has also been applied to water from a field contaminated with uranium. Uranium concentrations of 21-874 |g L-1 were treated; these concentrations were reduced to < 20 |g L-1 before the water was discharged into the environment (Dushenkov et al. 1997).
This process is generally used for the remediation of soil, sediment, and sludges (USEPA 2000; Mueller et al. 1999), and it depends on the ability of plant roots to limit the mobility and bioavailability of the contaminant in the soil. This can occur through sorption, precipitation, complexation, or metal valence reduction. The primary purpose of the plants is to decrease the amount of water percolating through the soil matrix, which reduces the formation of hazardous leachate and prevents soil erosion and thus the redistribution of the toxic metal to other sites. A dense root system stabilizes the soil and prevents erosion (Beti and Cunningham 1993). Phytostabilization is considered a very effective process whenever rapid immobilization is required to preserve ground and surface waters, and the disposal of biomass is not required in this process. However, its major disadvantage is that the contaminant remains in the soil and so needs regular monitoring.
This is the best phytoremediation process. It involves first removing the contamination from the soil and then isolating it without harming the soil structure and fertility. It is also referred as phytoaccumulation (USEPA 2000). As the plants absorb, concentrate, and precipitate toxic metals and radionuclides from contaminated soils into their biomass, it is best suited for the remediation of diffusely polluted areas, where pollutants occur at relatively low concentrations (Rulkens et al. 1998). Several approaches have been used, but the two main phytoextraction methods are (1) chelate-assisted phytoextraction or induced phytoextraction, where artificial chelates are added to the soil to increase the mobility and uptake of metal the contaminant; (2) continuous phytoextraction, where the removal of the metal depends on the natural ability of the plant to remediate the soil; only the number of plant growth repetitions is controlled (Salt et al. 1995, 1997). In order to make this technology feasible, the plants must draw up large concentrations of heavy metals into their roots, translocate the heavy metals to their biomass on the surface, and produce large quantities of plant biomass. The heavy metal removed by the plant can be recycled from the contaminated plant biomass (Brooks et al. 1998). Factors such as plant growth rate (vigor), element selectivity, resistance to disease, and method of harvesting are also important (Cunningham and Ow 1996; Baker et al. 1994). However, slow growth, a shallow root system, low biomass production, and difficult final disposal of the extracted metal all limit the use of a particular plant species as a hyperaccumulator (Brooks 1994).
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