Introduction to Phytoremediation - CLU-IN

Whitewood Creek Superfund site, SD, and vegetative
remediation has been proposed as part of the
remediation at the Galena Superfund site in southeastern
KS (Pierzynski et al. 1994).
• The IINERT (In-Place Inactivation and Natural Ecological
Restoration Technologies) Soil-Metals Action
team coordinated by EPA’s Jim Ryan and Dupont’s
Bill Berti under the RTDF program has used plants to
physically stabilize metal-contaminated soil in order to
decrease the off-site movement of contaminants.
• Researchers at Kansas State University and Montana
State University, among others, are actively examining
the use of vegetation in reclaiming sites contaminated
by mining wastes.
3.3.10 System Cost
Cropping system costs have been estimated at $200 to
$10,000 per hectare, equivalent to $0.02 to $1.00 per cubic
meter of soil, based on a 1-meter root depth (Cunningham
et al. 1995b).
3.3.11 Selected References
Azadpour, A., and J. E. Matthews. 1996. Remediation of
Metal-Contaminated Sites Using Plants. Remed. Summer.
6(3):1-19.
This is a literature review of factors that affect metals
uptake by plants. It discusses plant tolerance to heavy
metals and summarizes work done on the use of plants
in soils that contain high levels of metal.
Cunningham, S. D., W. R. Berti, and J. W. Huang. 1995b.
Remediation of Contaminated Soils and Sludges by Green
Plants. pp. 33-54. In R.E. Hinchee, J. L. Means, and D. R.
Burris (eds.), Bioremediation of Inorganics. Battelle Press,
Columbus, OH.
The chemistry of metals is discussed in this paper,
with a focus on lead. The article examines the stabilization
and bioavailability of lead using sequential extractions.
Phytoextraction of metals and
phytoremediation of organic contaminants are also discussed.
Pierzynski, G. M., J. L. Schnoor, M. K. Banks, J. C. Tracy,
L. A. Licht, and L. E. Erickson. 1994. Vegetative Remediation
at Superfund Sites. Mining and Its Environ. Impact
(Royal Soc. Chem. Issues in Environ. Sci. Technol. 1). pp.
49-69.
This paper discusses in detail the chemical and microbiological
aspects of metal-contaminated soils. Two
case studies of the phytoremediation of mine waste
sites are presented along with a modeling discussion
of the fate of heavy metal in vegetated soils.
Salt, D. E., M. Blaylock, P. B. A. Nanda Kumar, V.
Dushenkov, B. D. Ensley, I. Chet, and I. Raskin. 1995. Phy-
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toremediation: A Novel Strategy for the Removal of Toxic
Metals from the Environment Using Plants. Biotechnol.
13:468-474.
This article is an introduction to the use of
phytoremediation technologies for reducing metals contamination.
Field research is presented on the use of
plants to immobilize metals in soils. Bioavailability issues
and mechanisms of plant accumulation are discussed
in detail.
3.4 Rhizodegradation
3.4.1 Definition/Mechanism
Rhizodegradation is the breakdown of an organic contaminant
in soil through microbial activity that is enhanced by
the presence of the root zone (Figure 3-2). Rhizodegradation
is also known as plant-assisted degradation, plant-assisted
bioremediation, plant-aided in situ biodegradation, and enhanced
rhizosphere biodegradation.
Root-zone biodegradation is the mechanism for implementing
rhizodegradation. Root exudates are compounds produced
by plants and released from plant roots. They include
sugars, amino acids, organic acids, fatty acids, sterols,
growth factors, nucleotides, flavanones, enzymes, and other
compounds (Shimp et al. 1993; Schnoor et al. 1995a). The
microbial populations and activity in the rhizosphere can be
increased due to the presence of these exudates, and can
result in increased organic contaminant biodegradation in
the soil. Additionally, the rhizosphere substantially increases
the surface area where active microbial degradation can be
stimulated. Degradation of the exudates can lead to
cometabolism of contaminants in the rhizosphere.
Plant roots can affect soil conditions by increasing soil
aeration and moderating soil moisture content, thereby creating
conditions more favorable for biodegradation by indigenous
microorganisms. Thus, increased biodegradation
could occur even in the absence of root exudates. One study
raised the possibility that transpiration due to alfalfa plants
drew methane from a saturated methanogenic zone up into
the vadose zone where the methane was used by
methanotrophs that cometabolically degraded TCE
(Narayanan et al. 1995).
The chemical and physical effects of the exudates and
any associated increase in microbial populations might
change the soil pH or affect the contaminants in other ways.
3.4.2 Media
3.4.3 Advantages
Rhizodegradation has the following advantages:
• Contaminant destruction occurs in situ.
• Translocation of the compound to the plant or atmosphere
is less likely than with other phytoremediation
technologies since degradation occurs at the source of
the contamination.
• Mineralization of the contaminant can occur.