Phytoremediation
combines the Greek word "phyton", (plant), with the Latin word "remediare",
(to remedy) to describe a system whereby certain plants, working together with
soil organisms, can transform contaminants into harmless and often, valuable forms.
This practice is increasingly used to remediate sites contaminated with heavy
metals and toxic organic compounds.
Planning, engineering and design
with the ecological paradigm as our template is the work of Sustainable Strategies.
For example, the ecological paradigm reveals how to safely utilize all of the
polluting components and water of human and animal wastewater to ultimately grow
plants that have economic value.
We use the term Wastewater Garden to
describe our phytoremediation and evapo-transpiration approach to effluent management
problems. The objective is to drain pretreated wastewater into an appropriately
engineered gardens or forests of phreatophytes: plants known for fast growth and
high water usage rates. These plants and their microbially-active rhizosphere
will transform pollutants, including the nutrient nitrogen, into valuable biomass
and use up the remaining water via evaporation and transpiration.
Phytoremediation
takes advantage of plants' nutrient utilization processes to take in water and
nutrients through roots, transpire water through leaves, and act as a transformation
system to metabolize organic compounds, such as oil and pesticides. Or they may
absorb and bioaccumulate toxic trace elements including the heavy metals, lead,
cadmium, and selenium. In some cases, plants contain 1,000 times more metal than
the soil in which they grow. Heavy metals are closely related to the elements
plants use for growth. "In many cases, the plants cannot tell the difference"
says Ilya Raskin, professor of plant sciences in the Center for Agricultural Molecular
Biology at Rutgers University.
Phytoremediation is an affordable technology
that is most useful when contaminants are within the root zone of the plants (top
three to six feet). For sites with contamination spread over a large area, phytoremediation
may be the only economically feasible technology. The process is relatively inexpensive
because it uses the same equipment and supplies used in agriculture.
Soil microorganisms can degrade organic contaminants. This is called bioremediation
and has been used for many years both as an in-situ process and in land farming
operations with soil removed from sites.
Dr. Raskin also demonstrated
the utility of certain varieties of mustard plants in removing such metals as
chromium, lead, cadmium and zinc from contaminated soil and used hydroponic plant
cultures to remove toxic metals from aqueous waste streams.
Plants can
accelerate bioremediation in surface soils by their ability to stimulate soil
microorganisms through the release of nutrients from and the transport of oxygen
to their roots. The zone of soil closely associated with the plant root, the rhizosphere,
has much higher numbers of metabolically active microorganisms than unplanted
soil. The rhizosphere is a zone of increased microbial activity and biomass at
the root-soil interface that is under the interface of the plant roots. It is
this symbiotic relationship between soil microbes that is responsible for the
accelerated degradation of soil contaminants.
The interaction between
plants and microbial communities in the rhizosphere is complex and has evolved
to the mutual benefit of both organisms. Plants sustain large microbial populations
in the rhizosphere by secreting substances such as carbohydrates and amino acids
through root cells and by sloughing root epidermal cells. Also, root cells secrete
mucigel, a gelatinous substance that is a lubricant for root penetration through
the soil during growth. Using this supply of nutrients, soil microorganisms proliferate
to form the plant rhizosphere.
In addition to this rhizosphere effect,
plants themselves are able to passively take up a wide range of organic wastes
from soil through their roots. One of the more important roles of soil microorganisms
is the decomposition of organic residues with the release of plant nutrient elements
such as carbon, nitrogen, potassium, phosphate and sulfur. A significant amount
of the CO2 in the atmosphere is utilized for organic matter synthesis primarily
through photosynthesis. This transformation of carbon dioxide and the subsequent
sequestering of the carbon as root biomass contributes to balancing the effect
of burning fossil fuels on global warming and cooling.
Compounds are
frequently transformed in the plant tissue into less toxic forms or sequestered
and concentrated so they can be removed (harvested) with the plant. For example,
mustard greens were used to remove 45% of the excess lead from a yard in Boston
to ensure the safety of children who play there. The sequestered lead was carefully
removed and safely disposed of. Besides mustard greens, pumpkin vines were used
to clean up an old Magic Marker factory site in Trenton, New Jersey. Hydroponically
grown sunflowers were used to absorb radioactive metals near the Chernobyl nuclear
site in the Ukraine as well as a uranium plant in Ohio. The mustard's hyper-accumulation
results in much less material for disposal. The composting of plant material can
be another highly efficient stage in the breakdown of contaminants removed from
the soil.
When large plants such as willows, poplars and bamboo are
used, the idea is to move as much water through them as possible so that they
take up as much of the contaminants as possible. In 1991 the Miami Conservancy
District Aquifer Update, No. 1.1 reported that a single willow tree can, on a
hot summer day, transpire over 19 cubic meters of water (5,000 gallons)!, One
hectare of a herbaceous plant like saltwater cord grass evapotranspires up to
80 cubic meters (21,000 gallons) of water per day. Once the heavy metals are absorbed,
they are sequestered in the plants' leaves and/or roots. Any organic compounds
that are absorbed are metabolized.
Absorption of large amounts of nutrients
by plants (and only a small amount of plant toxins that might be harmful to them,)
is the key factor. Plants generally absorb large amounts of elements they need
for growth and only small amounts of toxic elements that could harm them. Therefore,
phytoremediation is a cost-effective alternative to conventional remediation methods.
Cleaning the top 15 centimeters (six inches) of contaminated soil with phytoremediation
costs an estimated $2,500 to $15,000 per hectare (2.5 acres), compared to $7,500
to $20,000 per hectare for on-site microbial remediation. If the soil is moved,
the costs escalate, but phytoremediation costs are still far below those of traditional
remediation methods, such as stripping the contaminants from the soil using physical,
chemical or thermal processes according to Dr. Scott Cunningham, a scientist at
Dupont Central Research for Environmental Biotechnology.
Plants are effective
at remediating soils contaminated with organic chemical wastes, such as solvents,
petrochemicals, wood preservatives, explosives and pesticides. The conventional
technology for soil cleanup is to remove the soil and isolate it in a hazardous
waste landfill or incinerate it.
"Phytoremediation", says Dr.
Ray Hinchman, botanist and plant physiologist at Argonne National Laboratory,
is "an in-situ approach," not reliant on the transport of contaminated
material to other sites. Organic contaminants are, in many cases, completely destroyed
(converted to CO2 and H2O) rather than simply immobilized or stored.
Salt-tolerant plants, called halophytes, have reduced the salt levels in soils
by 65% in only two years in one project involving brine-damaged land from run-off
from oil and gas production in Oklahoma. After the salt was reduced, the halophytes
died and native grasses, which failed to thrive when too much salt entered the
soil, naturally returned, replacing the salt-converting plants.
The establishment
of vegetation on a site also reduces soil erosion by wind and water, which helps
to prevent the spread of contaminants and reduces exposure of humans and animals.
Classes of organic compounds that are more rapidly degraded in rhizosphere
soil than in unplanted soil include:
· Total petroleum hydrocarbons;
polycyclic aromatic hydrocarbons
· Chlorinated pesticides (PCP, 2,4-D)
· Other chlorinated compounds (PCBs, TCE)
· Explosives (TNT,
DNT)
· Organophosphate insecticides (diazanon and parathion)
·
Surfactants (detergents)
· Nutrients (N,P,K) and organic compounds
Some plants used for phytoremediation are:
· Alfalfa (symbiotic
with hydrocarbon-degrading bacteria)
· Arabidopsis (carries a bacterial
gene that transforms mercury into a gaseous state)
· Bamboo family (accumulates
silica in it's stalk and nitrogen as crude protein in it's leaves)
·
Bladder campion (accumulates zinc and copper)
· Brassica juncea (Indian
mustard greens) (accumulates selenium, sulfur, lead, chromium, cadmi um, nickel,
zinc, and copper)
· Buxaceae (boxwood) and Euphorbiaceae (a succulent)
(accumulates nickel)
· Compositae family (symbiotic with Arthrobacter
bacteria, accumulates cesium and strontium)
· Ordinary tomato and alpine
pennycress (accumulates lead, zinc and cadmium)
· Poplar (used in the
absorption of the pesticide, atrazine)
