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.