GUEST ESSAY
Plants may have a Future in Industrial Cleanups
John R. Pichtel
John Pichtel is Professor of Natural Resources and Environmental
Management at Ball State University in Muncie, Indiana. He is a
certified Hazardous Materials Manager and a Certified
Professional Soil Specialist. He has served as a consultant in field
remediation projects, and has conducted environmental
assessments and remediation research in the United States,
Poland, Finland, and the United Kingdom. He was a Fulbright
Recipient in 1999. His research team is currently conducting
research in the remediation of metal- and hydrocarbon-contaminated soils using
innovative technologies such as bioremediation and phytoremediation. He has
published numerous papers in scientific journals and has published two books
addressing waste management and environmental cleanup.
As human populations grow worldwide, there is an increasing need to address the
problems associated with the generation and disposal of wastes. The waste disposal
dilemma is particularly evident in industrialized societies where the vast majority of
all the material resources produced worldwide are consumed. In many cases in the
past, wastes have been improperly managed, leaving nuisance situations and
sometimes toxic consequences.
According to the U.S. General Accounting Office, a branch of the United
States Congress, the number of contaminated sites in this country may reach as high
as 400,000. Sources of contamination range from the corner gasoline station, to the
1950s-era neighborhood dump which was covered with fill material, to heavily
industrialized facilities covering acres of land that were eventually abandoned. In
many of these sites, significant quantities of heavy metals such as lead, cadmium,
and zinc occur. The estimated cost to clean up metal-contaminated sites alone may
total over $50 billion. Obviously, federal and state governments and property owners
lack the financial resources to address cleanup of all these sites using conventional
methods.
How did we get to this point? Up until the early to mid-1970s there was no
comprehensive body of regulations that called for the coordinated management of
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is a trademark used herein under license.
toxic chemicals during routine use in the workplace, or for their proper disposal after
use. Some disposal decisions were subject to approval by state or county decisions.
Many more were based on what was considered convenient or cost-effective at the
time. By the 1970s, enactment of federal occupational safety laws and the Resource
Conservation and Recovery Act required proper management of hazardous
chemicals; however, we are still left with the legacy of earlier mismanagement of
hazardous wastes.
A variety of innovative technologies such as electrokinetic treatment, soil
washing, and in-situ vitrification have been designed for the remediation (cleanup) of
metal-contaminated soils. However, such methods are very costly and technologyintensive and in some cases may pose detrimental effects to the chemical and
physical properties of the soil under treatment. Also, some of these technologies may
mobilize a particular contaminant and/or render it more hazardous to public health
and the environment. As a result, remediation technologies may be subject to public
opposition due to fears of dispersal of contaminants. Fortunately, new,
environmentally-benign technologies have been tested to treat metal-contaminated
soil.
So, what would you call a machine that removes hazardous heavy metals
from soil and can operate at full capacity for six to eight months continuously, is lowmaintenance, low-cost, produces no air pollution or noise, and runs exclusively on
solar power (it even beautifies industrial areas)? Some call it redroot pigweed, some
ragweed, mustard or corn, some even call it the poplar tree. A range of plant species
including trees, vegetable crops, grasses, and weeds are known to accumulate a
wide range of heavy metals. There are at least 400 known so-called metal
“hyperaccumulating” plants worldwide, and some may serve as an alternative to
more drastic and costly methods of soil treatment. Using green plants to remove soil
metals has been dubbed “phytoextraction.”
For phytoextraction to be both technically feasible and an economically viable
technology, target plants must tolerate, absorb and translocate concentrations of
environmentally important metals in significant concentrations (usually 1% or
greater). Plants also should produce sufficient biomass to allow for efficient
harvesting and disposal of metal-rich tissue. Finally, plants must be responsive to
agricultural practices that allow for repeated planting and harvesting in the
contaminated soils.
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is a trademark used herein under license.
I have been assessing phytoextraction at two sites on the U.S. EPA Superfund list,
with an extremely high content (as much as 10% by weight in some places) of lead
in the soils. Some herbaceous plants such as common ragweed (Ambrosia
artemiisifolia) were found to be very effective in lead uptake; others tolerated the
lead but would not take it up. In some situations, specific chemicals were applied to
the soils to accelerate metal uptake. For example, dilute acids or synthetic agents
such as ethylenediaminetetraacetic acid (EDTA) are known to render metals more
soluble and thus more available for uptake by the root. These studies continue.
To be a safe and effective technology, the metal-enriched plant tissue must
be removed periodically. This biomass can be treated to recover and recycle the
metal; alternatively, the tissue can simply be disposed as hazardous wastes.
The potential for metal extraction is of primary importance in phytoextraction.
However, other criteria such as ecosystem stability must be also considered when
selecting plants for this technology. Native species tend to be preferred to exotic
plants as the latter can be invasive and endanger the integrity of the ecosystem. To
avoid propagation of weedy species, crop plants are typically preferred, although
some may be too palatable and pose a risk to grazing animals.
Phytoextraction can be used in conjunction with other cleanup methods (e.g.,
as a final, polishing step at a partly cleaned site). Much research is on-going in this
innovative application of soil cleanup. To enhance the efficiency of metal
phytoextraction, a range of practical strategies is being investigated. They include
dentification of suitable plant species via screening studies, optimization of
agronomic practices for maximizing biomass production and metal uptake, and
modification of species via conventional breeding or genetic engineering. There have
been limited examples of phytoextraction application in small-scale field tests but the
technique is still in the developmental stage.
Of course, phytoextraction is not practical for metal cleanup in all situations.
For example, a site that poses an immediate public health threat would require a
more rapid and exhaustive remediation technology such as soil excavation or
stabilization with Portland cement.
Plant-based remediation strategies will not work if contaminants occur deep
below the root zone or if the contaminants occur in some difficult to extract form (for
example, as metallic ingots).
There is still much to learn about the physiology of metal-tolerant plants, and
also how to promote their growth and accelerate uptake of metals for eventual
Copyright ©2005 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.
removal. The overall goal of phytoextraction science is to develop a toolbox of plants
that can be matched with cleanup needs at specific sites. A few corporations are now
looking into the phytoextraction market as a possible long-term technology for soil
cleanup. Needless to say, though, the best solution for site cleanup is to manage a
facility correctly in the first place and avoid contamination.
There is plenty of work in various parts of the United States and elsewhere in
the remediation of contaminated soils, sediments and groundwater. For those
students that are interested in pursuing this area I encourage them to study plant
physiology, soil science, microbiology, and, of course, chemistry. A program in civil
engineering would also suit the technical requirements very well. I also recommend
checking with your state or county brownfields commission to gain internship
experience in this interesting and practical area.
Critical Thinking
1. Check your state environmental agency’s website and locate the brownfield site
list for your state. Are there any sites listed which may be suitable for
phytoextraction?
Copyright ©2005 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning
is a trademark used herein under license.