Phytoremediation
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 compound s 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.