Introduction to Phytoremediation - CLU-IN

dia, contaminated groundwater upgradient of the plants
may flow into the area of influence of the plants. Contaminated
water may also be pumped and watered on
plants.
3.6.7 Plants
Plants used for phytovolatilization include:
• University of Washington researchers have extensively
studied the use of poplars in the phytoremediation of
chlorinated solvents. In these studies, transformation
of TCE was found to occur within the trees (Newman
et al. 1997a).
• Alfalfa (Medicago sativa) has been studied by Kansas
State University researchers for its role in the
phytovolatilization of TCE.
• Black locust species were studied for use in
remediating TCE in groundwater (Newman et al.
1997b).
• Indian mustard (Brassica juncea) and canola (Brassica
napus) have been used in the phytovolatilization
of Se. Selenium (as selenate) was converted to lesstoxic
dimethyl selenite gas and released to the atmosphere
(Adler 1996). Kenaf (Hibiscus cannabinus L.
cv. Indian) and tall fescue (Festuca arundinacea
Schreb cv. Alta) have also been used to take up Se,
but to a lesser degree than canola (Bañuelos et al.
1997b).
• A weed from the mustard family (Arabidopsis thaliana)
genetically modified to include a gene for mercuric reductase
converted mercuric salts to metallic mercury
and released it to the atmosphere (Meagher and Rugh
1996).
3.6.8 Site Considerations
Because phytovolatilization involves the transfer of contaminants
to the atmosphere, the impact of this contaminant
transfer on the ecosystem and on human health needs
to be addressed.
3.6.8.1 Soil Conditions
For significant transpiration to occur, the soil must be
able to transmit sufficient water to the plant.
3.6.8.2 Ground and Surface Water
Groundwater must be within the influence of the plant
(usually a tree) roots.
3.6.8.3 Climatic Conditions
Climatic factors such as temperature, precipitation, humidity,
insolation, and wind velocity can affect transpiration
rates.
3.6.9 Current Status
Several research groups are performing active laboratory
and field studies of TCE phytovolatilization and other
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chlorinated solvents. A SITE demonstration project has
been started at the Carswell Site, Fort Worth, TX using
poplars to phytoremediate TCE-contaminated groundwater
and to examine the possible fate of the TCE, including
volatilization.
A significant amount of research, including field testing
and application, has been conducted on selenium volatilization.
3.6.10 System Costs
Cost information is being collected as part of the SITE
demonstration project at the Carswell Site.
3.6.11 Selected References
Bañuelos, G. S., H. A. Ajwa, N. Terry, and S. Downey.
1997a. Abstract: Phytoremediation of Selenium-Laden Effluent.
Fourth International In Situ and On-Site
Bioremediation Symposium, April 28 - May 1, 1997, New
Orleans, LA. 3:303.
This abstract summarizes the methods used in field investigations
of the use of Brassica napus (canola) to
remediate water contaminated with selenium. These field
studies included an investigation of the volatilization of
selenium by the plants.
Bañuelos, G. S., H. A. Ajwa, B. Mackey, L. L. Wu, C.
Cook, S. Akohoue, and S. Zambrzuski. 1997b. Evaluation
of Different Plant Species Used for Phytoremediation of
High Soil Selenium. J. Environ. Qual. 26:639-646.
This evaluation discusses three plant species (canola,
kenaf, and tall fescue) grown in seleniferous soil under
greenhouse conditions. Total soil selenium was
significantly reduced by each species. A partial mass
balance indicated that some selenium was lost by a
mechanism that was not measured. Selenium volatilization
was hypothesized as the cause of the decrease
in soil concentration.
Meagher, R. B., and C. Rugh. 1996. Abstract: Phytoremediation
of Mercury Pollution Using a Modified Bacterial
Mercuric Ion Reductase Gene. International Phytoremediation
Conference, May 8-10, 1996, Arlington, VA. International
Business Communications, Southborough, MA.
This abstract describes transgenic plants developed
to reduce mercuric ion to metallic mercury, which was
then volatilized, and additional plants developed to
process methyl mercury to metallic mercury.
Newman, L. A., S. E. Strand, N. Choe, J. Duffy, G. Ekuan,
M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M.
P. Gordon. 1997a. Uptake and Biotransformation of Trichloroethylene
by Hybrid Poplars. Environ. Sci. Technol.
31:1062-1067.
Whole trees were exposed to 50 ppm TCE and bags
were placed around leaves. Analysis of the entrapped
air indicated that TCE was transpired from the trees.