Phytoremediation Essay

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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,...

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