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Free Phytoremediation papers
In recent years it has become clear that some environmental chemicals can cause

risks to the developing embryo and fetus. Evaluating the developmental toxicity

of environmental chemicals is now a prominent public health concern. The

suspected association between TCE and congenital cardiac malformations warrants

special attention because TCE is a common drinking water contaminant that is

detected in water supplies throughout the U.S. and the world. There is a lot of

concern about the clean up of toxic pollutants from the environment. Traditional

methods for cleaning up contaminated sites such as dig and haul, pump and treat,

soil venting, air sparging and others are generally harmful to habitats. Some

methods strip the soil of vital nutrients and microorganisms, so nothing can

grow on the site, even if it has been decontaminated. Typically these mechanical

methods are also very expensive. Most of the remediation technologies that are

currently in use are very expensive, relatively inefficient and generate a lot

of waste, to be disposed of. Cleaning up contamination: Phytoremediation is a

novel, efficient, environmentally friendly, low-cost technology, which uses

plants and trees to clean up soil and water contaminated with heavy metals

and/or organic contaminants such as solvents, crude oil, polyaromatic

hydrocarbons and other toxic compounds from contaminated environments. This

technology is useful for soil and water remediation. Mechanisms:

Phytoremediation uses one basic concept: the plant takes the pollutant through

the roots. The pollutant can be stored in the plant (phytoextraction), volatized

by the plant (phytovolatization), metabolized by the plant (phytodegradation),

or any combination of the above. Phytoextraction is the uptake and storage of

pollutants in the plants stem or leaves. Some plants, called hyperaccumulators,

draw pollutants through the roots. After the pollutants accumulate in the stem

and leaves the plants are harvested. Then plants can be either burned or sold.

Even if the plants cannot be used, incineration and disposal of the plants is

still cheaper than traditional remediation methods. As a comparison, it is

estimated a site containing 5000 tons of contaminated soil will produce only

20-30 tons of ash (Black, 1995). This method is particularly useful when

remediating metals. Some metals are also being recycled from the ash.

Phytovolatization is the uptake and vaporization of pollutants by a plant. This

mechanism takes a solid or liquid contaminant and transforms it to an airborne

vapor. The vapor can either be the pure pollutant, or the plant can metabolize

the pollutant before it is vaporized, as in the case of mercury, lead and

selenium (Boyajian and Carriera, 1997; Black, 1995; Wantanbe, 1997).

Phytodegradation is plants metabolizing pollutants. After the contaminant has

been drawn into the plant, it assimilates into plant tissue, where the plant

then degrades the pollutant. This metabolization by plant-derived enzymes such

as nitrosedictase, laccase, dehalogenase, and nitrilase assimilates into plant

tissue, where the plant then degrades the pollutant. This metabolization by

plant-derived enzymes such as nitroredictase, laccase, dehalogenase, and

nitrilase, has yet to be fully documented, but has been demonstrated in field

studies (Boyajian and Carriera, 1997). The daughter compounds can be either

volatized or stored in the plant. If the daughter compounds are relatively

benign, the plants can still be used in traditional applications. The most

effective current phytoremediation sites in practice combine these three

mechanisms to clean up a site. For example, poplar trees can accumulate, degrade

and volatize the pollutants in the remediation of organics. Techniques:

Phytoremediation is more than just planting and letting the foliage grow; the

site must be engineered to prevent erosion and flooding and maximize pollutant

uptake. There are 3 main planting techniques for phytoremediation. 1.Growing

plants on the land, like crops. This technique is most useful when the

contaminant is within the plant root zone, typically 3 - 6 feet (Ecological

Engineering, 1997), or the tree root zone, typically 10-15 feet. 2.Growing

plants in water (aquaculture). Water from deeper aquifers can be pumped out of

the ground and circulated through a “reactor” of plants and then used in an

application where it is returned to the earth (e.g. irrigation) 3.Growing trees

on the land and constructing wells through which tree roots can grow. This

method can remediate deeper aquifers in-situ. The wells provide an artery for

tree roots to grow toward the water and form a root system in the capillary

fringe. Determining which plant to use: The majority of current research in the

phytoremediation field revolves around determining which plant works most

efficiently in a given application. Not all plant species will metabolize,

volatize, and/or accumulate pollutants in the same manner. The goal is to

ascertain which plants are most effective at remediating a given pollutant.

Research has yielded some general guidelines for groundwater phytoremediation

plants. The plant must grow quickly and consume large quantities of water in a

short time. A good plant would also be able to remediate more than one pollutant

because pollution rarely occurs as a single compound. Poplars and cottonwoods

are being studied extensively because they can used as much as 25 to 350 gallons

of water per day, and they can remediate a wide variety of organic compounds,

including LNAPL’s. Phytoremediation has been shown to work on metals and

moderately hydrophobic compounds such as BTEX compounds, chlorinated solvents,

ammunition wastes, and nitrogen compounds. Yellow poplars are generally favored

by Environmental Scientists for use in phytoremediation at this time. They can

grow up to 15 feet per year and absorb 25 gallons of water a day. They have an

extensive root system, and are resistant to everything from gypsy moths to toxic

wastes. Partial listing of current remediation possibilities. Plant Chemicals

Clean-up numbers Pondweed TNT & RDX 0.016-0.019 mg of TNT / L per day Poplar

Trees Atrazine 91% of the Atrazine taken up in 10 days Poplars Nitrates from

fertilizers From 150 mg/L to 3 mg / L in under 3yrs. Mustard Greens Lead 45% of

the excess was removed Pennycress Zinc & Cadmium 108 lb./acre per year &

1.7 lb./acre per yr. Halophytes Salts reduced the salt levels in the soils by65%

Advantages and Disadvantages to Phytoremediation: Advantages: ( www.rtdf.org/genlatst.htm)

1.Aesthetically pleasing and publicly accepted. 2.Solar driven. 3.Works with

metals and slightly hydrophobic compounds, including many organics. 4.Can

stimulate bioremediation in the soil closely associated with the plant root.

Plants can stimulate microorganisms through the release of nutrients and the

transport of oxygen to their roots. 5.Relatively inexpensive - phytoremediation

can cost as little as $10 - $100 per cubic yard whereas metal washing can cost

$30 - $300 per cubic yard. 6.Even if the plants are contaminated and unusable,

the resulting ash is approximately 20-30 tons per 5000 tons soil (Black, 1997).

7.Having ground cover on property reduces exposure risk to the community (i.e.

lead). 8.Planting vegetation on a site also reduces erosion by wind and water.

9.Can leave usable topsoil intact with minimal environmental disturbance.

10.Generates recyclable metal rich plant residue. 11.Eliminates secondary air or

water-borne wastes. Disadvantages: 1.Can take many growing seasons to clean up a

site. 2.Plants have short roots. They can clean up soil or groundwater near the

surface in-situ, typically 3 - 6 feet (Ecological Engineering, 1997), but cannot

remediate deep aquifers without further design work. 3.Trees have longer roots

and can clean up slightly deeper contamination than plants, typically 10-15

feet, but cannot remediate deep aquifers without further design work . 4.Trees

roots grow in the capillary fringe, but do not extend deep in to the aquifer.

This makes remediating DNAPL’s in situ with plants and trees not recommended.

5.Plants that absorb toxic materials may contaminant the food chain.

6.Volatization of compounds may transform a groundwater pollution problem to an

air pollution problem. 7.Returning the water to the earth after aquaculture must

be permitted. 8.Less efficient for hydrophobic contaminants, which bind tightly

to soil. Case Studies: 1) At the Naval Air Station Joint Reserve Base Fort

Worth, phytoremediation is being used to clean up trichloroethylene (TCE) from a

shallow, thin aerobic aquifer. Cottonwoods are being used, and after 1 year, the

trees are beginning to show signs of taking the TCE out of the aquifer. (Betts,

1997) 2) At the Iowa Army Ammunitions Plant, phytoremediation is being used as a

polishing treatment for explosive-contaminated soil and groundwater. The

demonstration, which ended in March, 1997, used native aquatic plant and hybrid

poplars to remediate the site where an estimated 1-5% of the original pollutants

still remain. A full-scale project is estimated to reduce the contamination by

an order of magnitude (Betts, 1997). 3) After investigating using

phytoremediation on a site contaminated with hydrocarbons, the Alabama

Department of Environmental Management granted a site. The site involved about

1500 cubic yards of soil, and began with approximately 70% of the baseline

samples containing over 100 PPM of total petroleum hydrocarbon (TPH). After 1

year of vegetative cover, approximately 83% of the samples contained less than

10-PPM TPH. 4) Phytoremediation was used at the decommissioned Detroit Forge

plant to clean up approximately 5,800 cubic yards of lead-impacted soil. Two

plantings were completed, the first using sunflowers and the second mustard

plants. Following treatment, analysis indicated soil lead concentrations were

below the target clean-up criteria. The project resulted in an estimated saving

of $1,100,000 over hazardous waste disposal. 5) Water, soil, and trees

transpired gases were monitored to track the fate of TCE. About 2-4% of the TCE

remained in the effluent as compared to 68% in a non-vegetated control group.

The field trial demonstrated that over 95% of TCE were removed by planting trees

and letting them grow. Additional studies showed that the trees did not release

TCE into the air, as no measurable TCE was present in the air immediately

surrounding the leaves (captured in small leaf bags and analyzed) or in the

general atmosphere (using a laser technology that can see TCE in the air in the

tree canopy). CONCLUSION: Phytoremediation is an aesthetically pleasing,

solar-energy driven, and passive technique that can be used at sites with low to

moderate levels of contamination. Phytoremediation is more than just planting

and letting the foliage grow; the site must be engineered to prevent erosion and

flooding and maximize pollutant uptake. Currently, the majority of research is

concentrated on determining the best plant for the job, quantifying the

mechanisms by which the plants convert pollutants, and determining which

contaminants are amenable to phytoremediation. Polluted sites are being studied,

and phytoremediation looks promising for a variety of contaminants.
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