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Phytoremediation : 


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Phytoremediation is an emerging technology which uses plants and their associated rhizospheric microorganisms to remove, degrade, or contain chemical contaminants located in the soil, sediments, groundwater, surface water, and even the atmosphere.

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plants can be used to treat most classes of contaminants, including petroleum hydrocarbons, chlorinated solvents, pesticides, metals, radionuclides, explosives, and excess nutrients.

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Plant species are selected for phytoremediation based on their potential to evapotranspirate groundwater, the degradative enzymes they produce, their growth rates and yield, the depth of their root zone, and their ability to bioaccumulate contaminants

Limitations : 

Limitations Phytoremediation -- work at sites that are well suited for plant growth. means that the concentration of pollutants cannot be toxic to the plants the pollution cannot be so deep in the soils or groundwater that plant roots cannot reach it. As a result, phytoremediation may be a good strategy for sites conducive to plant growth with shallow contamination, it may be a good secondary or tertiary phase in a treatment train for highly polluted sites, or it may not be a viable option for a site.

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phytoremediation covers a broad spectrum of pollutants and treatments, many of these companies focus most of their attention on one niche of the phytoremediation field (e.g., metal extraction from soils, poplar tree buffers, etc.).

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Early research indicates that phytoremediation technology is a promising cleanup solution for a wide variety of pollutants and sites, but it has its limitations. Table 2 summarizes some of the advantages and limitations of phytoremediation. An examination of the table reveals that many of phytoremediation’s limitations and advantages are a direct result of the biological aspect of this type of treatment system. Plant-based remediation systems can function with minimal maintenance once they are established, but they are not always the best solution to a contamination problem. One way to summarize many of phytoremediation’s limitations is that the pollutant must be bioavailable to a plant and its root system. If a pollutant is located in a deep aquifer, then plant roots cannot reach it. If a soil pollutant is tightly bound to the organic portion of a soil, then it may not be available to plants or to microorganisms in the rhizosphere. On the other hand, if a pollutant is too water soluble it will pass by the root system without any uptake.

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lack of performance data length of time involved in a phytoremediation project. Plants can only grow so fast, so obtaining long term performance results is dependent on the rates of plant growth and activity. Also, a number of firms have installed phytoremediation systems at polluted sites owned by private clients, so results from those sites are not publicly available.

Trichloroethylene (TCE) : 

Trichloroethylene (TCE) common contaminant at many of the nation’s hazardous waste sites. can be found at 50% of Superfund National Priority List (NPL) sites with completed Records of Decision (RODs), and it is above action levels in the groundwater of 17% and soils of 16% of RCRA corrective action facilities (USEPA 542-R-96-005).

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TCE pollution became prevalent primarily through its use as an industrial degreasing agent. Other uses of TCE include its use as a solvent for dry cleaning, an anaesthetic for medical and dental use and as an ingredient in paints, inks, cosmetics, disinfectants, and cleaning fluids. Due to widespread TCE contamination, finding innovative ways to clean this pollutant has become a priority in the remediation field

The genus Populus : 

The genus Populus includes a number of species of trees such as poplars, cottonwoods, and aspens. member of the Salicaceae family, which also includes willows 30 species of Populus distributed around the Northern Hemisphere, with eight species indigenous to North America and others that have been introduced Populus sp. have the ability to cross within the genus both in the wild and through controlled breeding, so there are a large number of potential hybrids (Dickmann and Stuart 1983).

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Due to their ability to readily form hybrids, poplars have been crossed by foresters for years in order to maximize growth rates and yield. Hybrid poplars were originally bred and grown as a cash crop for such uses as pulp wood and as a renewable energy source (Poplars and Willows on the WWW), but because of their rapid growth rates and high evapotranspiration rates, they make ideal candidates for phytoremediation.

Advantages of Populus sp in phytoremediation : 

Advantages of Populus sp in phytoremediation Greater than 25 species worldwide Fast growing ( 3 to 5 meters/year) High transpiration rates (100 liters/day optimally for 5 year old tree) Not part of food chain Trees can be used for paper production or as biomass for energy Long lived (25-30 years) Grow easily from cuttings Can be harvested and then regrown from the stump

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The goal of poplar hybridization is to achieve heterosis, which means that the genetic traits of hybrids exceed those of the parents (Dickmann and Stuart 1983). Two species of poplars, Populus deltoides (eastern cottonwood) and Populus trichocarpa (black cottonwood), are commonly crossed for use in phytoremediation. Populus trichocarpa x deltoides, pictured in have leaves that are about four times as large as the leaves of parent plants (ORNL 1996).

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Increasing leaf size increases the potential evapotranspiration rates of these trees due to increased total leaf surface area. Another common cross that has been used in phytoremediation studies is P. deltoides x P. nigra (black poplar). This cross is sometimes referred to as P. x euramericana due to the original distribution of the two species, the black poplar in Europe and the eastern cottonwood in North America. There are many other hybrids of poplars that have been developed, some of which have been or will likely be used in phytoremediation systems

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Populus sp. have been used to treat contaminant problems other than TCE in groundwater. project in Iowa investigated the ability of poplar strips to act as a buffer to protect water bodies from nutrient runoff (Paterson and Schnoor 1993). similar study in Iowa looked at the ability of poplars to buffer triazine pesticide runoff from an agricultural field (Dhileepan et al. 1993). Both studies indicated --poplars could successfully act as a buffer, although they were less effective at buffering nutrients once the trees dropped their leaves in the fall.

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In addition, hybrid poplars are currently being used to act as a hydraulic barrier to contain a plume of gasoline and diesel fuel in the groundwater at a site in Ogden, UT (EPA/540/R-97/502). Another way that poplars have been used is as a component of vegetative caps for landfill facilities (Schnoor et al. 1995). Poplars are also being tested for their ability to phytostabilize metals such as lead, and they have been planted as a part of a constructed wetland design to treat explosives such as TNT and RDX in the soil (Schnoor 1997). There is also a report on the use of poplars to phytoextract zinc from soils (Gatliff 1994)

Populus Phytoremediation Versus Other Treatment Technologies : 

Populus Phytoremediation Versus Other Treatment Technologies phytoremediation would be significantly less expensive than pump and treat due to decreased energy needs. Since poplar phytoremediation systems are primarily used as hydraulic barriers and solar powered pumps, the most closely related engineering technology is a pump and treat system.

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advantage of poplars over pumps--poplars provide their own energy, where pumps often consume large amounts of electricity--could save tremendous amounts of money over the course of a long remediation project. On the other hand, a major disadvantage of poplars is the fact that their pumping rates vary over the course of a year. In addition, poplar systems will only work at sites where the groundwater contamination is within reach of their roots.

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Despite some limitations of poplars when compared to pump and treat, the potential economic advantages of this treatment system are tremendous. A report by the Environmental Security Technology Certification Program (ESTCP) estimates that poplar trees can be used to treat contaminants such as chlorinated solvents and petroleum hydrocarbons at 1,000 DOD cleanup sites around the world (ESTCP). This could save the government hundreds of millions of clean-up dollars.