Natural and Enhanced Remediation Systems - Chapter 5 pdf

Suthersan, Suthan S. “Phytoremediation”

Natural and Enhanced Remediation Systems

Edited by Suthan S. Suthersan
Boca Raton: CRC Press LLC, 2001
©2001 CRC Press LLC

CHAPTER

5
Phytoremediation

CONTENTS

5.1 Introduction
5.2 Chemicals in the Soil–Plant System
5.2.1



Metals
5.2.2



Organics
5.3 Types of Phytoremediation
5.3.1 Phytoaccumulation
5.3.2 Phytodegradation

5.3.3 Phytostabilization
5.3.4 Phytovolatilization
5.3.5 Rhizodegradation
5.3.6 Rhizofiltration
5.3.7 Phytoremediation for Groundwater Containment
5.3.8 Phytoremediation of Dredged Sediments
5.4 Phytoremediation Design
5.4.1 Contaminant Levels
5.4.2 Plant Selection
5.4.3 Treatability
5.4.4 Irrigation, Agronomic Inputs, and Maintenance
5.4.5 Groundwater Capture Zone and Transpiration Rate
References

… many accepted agricultural techniques for cultivating, harvesting, and pro-
cessing plants have now been adapted for phytoremediation. Overall, the appli-
cation of phytoremediation is being driven by its technical and economic advan-
tages over conventional approaches … .phytoremediation’s future is not a
scientific issue, but rather a “scientific sociology” issue….

©2001 CRC Press LLC

5.1



INTRODUCTION

Phytoremediation is defined as “the engineered use of plants

in situ

and

ex situ

for environmental remediation.” The technology involves removing or degrading
organic and inorganic contaminants and metals from soil and water. The processes
include all plant-influenced biological, chemical, and physical processes that aid in
the uptake, sequestration, degradation, and metabolism of contaminants, either by
plants or by the free living organisms that constitute a plant’s rhizosphere. Phytore-
mediation takes advantage of the unique and selective uptake capabilities of plant
root systems, together with the translocation, bioaccumulation, and contaminant
storage and degradation capabilities of the entire plant body.
The concept of using plants to alter the environment has been around since plants
were first used to drain swamps. What is new within the context of this new
technology called phytoremediation is the systematic, scientific investigation of how
plants can be used to decontaminate soil and water.

1

Interest in phytoremediation
has been growing in the U.S. during the past few years with potential applicaton of
this technology at a wide range of sites contaminated with heavy metals, pesticides,
explosives, and solvents.
The potential benefits of phytoremediation seem to be as numerous as the
problems it might address. One reason this technology is gaining attention is because
it is potentially cheaper than conventional treatment approaches for contaminated
soils and traditional pump and treat systems for contaminated groundwater, such as
incineration or soil washing. Another attraction of this technology is that it may

leave topsoil in usable condition, keeping soil fertility and structure intact while
reducing contamination levels at the same time. Phytoremediation is well suited for
applications in low permeability soils, where most currently used technologies have
a low degree of feasibility or success, as well as in combination with more conven-
tional remediation technologies.
The main advantages of phytoremediation are the low capital costs, aesthetically
pleasing technique, minimization of leaching of contaminants, and soil stabilization.
The operational cost of phytoremediation is also substantially less than that of conven-
tional treatments and involves mainly fertilization and watering for maintenance of plant
growth. In the case of heavy metals remediation, additional operational costs include
harvesting, disposal of contaminated plant mass, and repeating the plant growth cycle.
It should be emphasized that there is more to phytoremediation than merely
putting plants in the ground and letting them do the work. Phytoremediation also
has its drawbacks, which even its ardent champions are quick to acknowledge. First
of all, it is a time-consuming process that can take several growing seasons to clean
a site. Vegetation that absorbs toxic heavy metals will have to be harvested and
managed as a waste. This vegetation containing high concentrations of toxic metals
and organics may also pose a risk to wildlife. The shutdown of plant activity during
winter months and the seasonal variation of plant metabolic activity is a drawback
for application of this technology in colder climates. Other limitations of phytore-
mediation are that contaminants present below rooting depth will not be treated or
extracted and that the plant or tree may not be able to grow in soils at heavily
contaminated sites due to plant toxicity.

©2001 CRC Press LLC

Phytoremediation as a technology is still in its early stages. While many scien-
tists, engineers, and regulators are optimistic that it will eventually be used to clean
up organic and metallic contaminants, at least two or three more years of field tests
and analyses are necessary to validate the initial, small-scale field tests.

1,2

Issues like
soil characteristics and length of the growing season will need to be taken into
account and scientists must also determine what sites are most amenable to phy-
toremediation. Other issues such as the potential impact on wildlife remain to be
fully explored. Simultaneously, researchers working in the lab are trying to better
understand the processes behind phytoremediation to possibly improve its perfor-
mance during cleanup applications.
This chapter will not do justice to this technology by claiming that it will cover
the rapidly progressing state of the science and also describe how these scientific
advances are being applied in the field for efficient remediation. Instead it will serve
as a brief state of the science summary that will allow the reader to understand the
current status of the technology and its applications, as well as activities of the
research community to further enhance this technology.

5.2 CHEMICALS IN THE SOIL–PLANT SYSTEM
5.2.1 Metals

Elements occur in the soil in a variety of forms more or less available for uptake
by plants. Many of the contaminants of concern at waste sites are metals or metal-
loids. Availability is determined by characteristics of the elements, such as behavior
of the ion as a Lewis acid (electron acceptor) which determines the predominant
type of strength of bond created (ionic or covalent) and, therefore, the mobility of
the metal in the soil environment. Soil characteristics (e.g., pH, clay and organic
matter content and type, and moisture content) also determine availability to plants
by controlling speciation of the element, temporary immobilization by particle
surfaces (adsorption-desorption processes), precipitation reactions, and availability
in soil solution. The most general sinks for metals are iron and manganese oxides

and organic matter. Although particulate soil organic matter serves to immobilize
metals, soluble organic matter may act to keep metals in solution in a form absorbed
and translocated by plants.
Metal fractionation or sequential extraction schemes — such as toxicity charac-
teristic leaching procedure (TCLP) — sometimes are used to describe metal behavior
in soils. Most metals interact with the inorganic and organic matter that is present
in the root-soil environment. Potential forms of metals include those dissolved in
the soil solution, adsorbed to the vegetation’s root system, adsorbed to insoluble
organic matter, bonded to ion exchange sites on inorganic soil constituents, precip-
itated or coprecipitated as solids, and attached to or inside the soil biomass.
The final control on availability of metals and metalloids in soil to plants is the
selective absorption from soil solution by the root. Metals may be bound to exterior
exchange sites on the root and not actually taken up. They may enter the root
passively in organic or inorganic complexes with the mass flow of water or actively

©2001 CRC Press LLC

by way of metabolically controlled membrane transport systems often meant to take
up a nutrient which the “contaminant” metal mimics. At different soil solute con-
centrations, metals may be absorbed by both processes. Absorption mechanisms and
quantity absorbed are influenced by plant species (and cultivar), growth stage,
physiological state, and the presence of other elements.
Once in the plant, a metal can be sequestered in the roots in vacuoles or in
association with cell walls and organelles, or translocated to above ground parts in
xylem as organic or inorganic complexes. Location and forms of metals in plants,
as well as their toxic effects, depend on plant species, growth stage, physiological
state, and presence of other metals.
Mechanisms of toxicity of metals tend to be dependent on the nature of the
reactivity of the metal itself and its availability in the soil and soil solution media.
They may alter or inhibit enzyme activity, interfere with deoxyribonucleic acid

(DNA) synthesis or electron transport, or block uptake of essential elements.

2

Avail-
ability in response to toxic levels of metals by different plants is due to a number
of defenses. These include exclusion from the root, translocation in nontoxic form,
sequestering in nontoxic form, sequestering in nontoxic form in the root or other
plant parts, and formation of unusable complexes containing metals that may oth-
erwise be inserted into biomolecules instead of the proper element (e.g., As
replacing P).

5.2.2Organics

Organic compounds of environmental concern include nonionic compounds
(such as PAHs, chlorinated benzenes, polychlorinated biphenyls (PCBs), BTEX
compounds, and many pesticides), ionizable compounds (chlorophenols, carboxylic
acids, surfactants, and amines), and weakly hydrophobic volatile organic compounds
(trichloroethene). For the nonionic compounds, sorption in soil is mainly a function
of degree of hydrophobicity and amount of sorbent hydrophobic phase (i.e., soil
organic matter). Sorption of the compound by soil organic matter is reversible. The
activities of these compounds in soil can be predicted by the organic matter-water
coefficient, K

om

, as estimated by the octanol-water coefficient, K

ow

.

3

Absorption onto
colloidal organic matter in solution may alter the availability of these nonionic
compounds. Ionizable compounds contain anionic or cationic moieties or both within
their structure. These charged structures interact with organic and inorganic charged
surfaces in the soil in a variety of reversible reactions. The extent and nature of the
associations with charged surfaces depends on characteristics of the organic com-
pound, solution pH and ionic strength, and mineral composition of the soil partic-
ulates. Organic compounds may be degraded by microorganisms in the soil to
metabolites with greater or lesser toxicity. Very stable compounds, like highly chlo-
rinated PCBs, may persist in essentially unaltered form for many years.
Plant roots are not discriminating in uptake of small organic molecules (molec-
ular weight less than 500) except on the basis of polarity.

1-4

More water-soluble
molecules pass through the root epidermis and translocate throughout the plant. The
less soluble compounds (like many polycyclic aromatic hydrocarbons) seem to have
limited entry into the plant and minimal translocation once inside. Highly lipophilic

©2001 CRC Press LLC

compounds, such as PCBs, move into the plant root via the symplastic route (from
cell to cell, as opposed to between cells) and are translocated within the plant. Within
a plant the contaminant may be adsorbed on a cell surface or accumulated in the
cell. Many contaminants become bound on the root surface and are not translocated.

Not all organic compounds are equally accessible to plant roots in the soil
environment. The inherent ability of the roots to take up organic compounds can be
described by the hydrophobicity (or lipophilicity) of the target compounds. This
parameter is often expressed as the log of the octanol-water partioning coefficient,
K

ow

. Direct uptake of organics by plants is a surprisingly efficient removal mechanism
for moderately hydrophobic organic compounds. There are some differences
between the roots of different plants and under different soil conditions, but, gen-
erally, the higher a compound’s log K

ow

, the greater the root uptake.
Hydrophobicity also implies an equal propensity to partition into soil organic
matter and onto soil surfaces. Root absorption may become difficult with heavily
textured soils and soils with high native organic matter. There are several reported
values available in the literature regarding the optimum log K

ow

value for a compound
to be a good candidate for phytoremediation (as an example, log K

ow

= 0.5–3.0; log
K

ow

= 1.5–4.0).

2,13

It has also been reported that compounds that are quite water
soluble (log K

ow

< 0.5) are not sufficiently sorbed to the roots or actively transported
through plant membranes.
From an engineering point of view, a tree could be thought of as a shell of living
tissue encasing an elaborate and massive chromatography column of twigs, branches,
trunk, and roots. The analogous resin in this system is wood, the vascular tissue of
the tree, and this “resin” is replenished each year by normal growth. Wood is
composed of thousands of hollow tubes, like the bed of a hollow fiber chromatog-
raphy column, with transpirational water serving as the moving phase. The hollow
tubes are actually dead cells, whose death is carefully programmed by the tree to
produce a water conducting tissue, which also functions in mechanical support. A
complex, cross-linked, polymeric matrix of cellulose, pectins, and proteins embed-
ded in lignin forms the walls of the tubes. The cell wall matrix is chemically inert,
insoluble in the majority of solvents, and stable across a wide range of pH.
Once an organic chemical is taken up, a plant can store (sequestration) the
chemical and its fragments in new plant structures via lignification, or it can vola-
tilize, metabolize, or mineralize the chemical all the way to carbon dioxide, water,
and chlorides. Detoxification mechanisms may transform the parent chemical to
nonphytotoxic metabolites, including lignin, that are stored in various places in plant

cells. Many of these metabolic capacities tend to be enzymatically and chemically
similar to those processes that occur in mammalian livers; one report has equated
plants to” green livers” due to similarities of detoxification processes.
Different plants exhibit different metabolic capacities. This is evident during the
application of herbicides to weeds and crops alike. The vast majority of herbicidal
compounds have been selected so that the crop species are capable of metabolizing
the pesticide to nontoxic compounds, whereas the weed species either lack this
capacity or perform it at too slow a rate. The result is the death of the weed species
without the metabolic capacity to rid itself of the toxin.

©2001 CRC Press LLC

The shear volume and porous structure of a tree’s wood provide an enormous
surface area for exchange or biochemical reactions. Some researchers are attempting
to augment the inherent metabolic capacity of plants by incorporating bacterial,
fungal, insect, and even mammalian genes into the plant genome.

5.3 TYPES OF PHYTOREMEDIATION

A review of where pyhtoremediation fits into the scheme of hazardous waste
remediation enables us to differentiate the various types and mechanisms of phy-
toremediation (Figure 5.1). The scientific understanding of plant, soil and rhizo-
sphere biochemistry, and contaminant fate and transport must be contrasted with
field and pilot studies that represent the current proof of concepts. The technology
is summarized below as those approaches ready for application, promising treatments
expected to be tested soon, and concepts of phytoremediation requiring intensive
development. Finally, the intrinsic strengths of phytoremediation as a technology
and the future potential of this technology must be reviewed for regulatory accep-
tance in terms of hazardous waste remediation.

1,2

Phytoremediation approaches can be summarized as follows based on current
understanding of the technology:

•Phytoaccumulation, phytoextraction, hyperaccumulation
•Phytodegradation or phytotransformation
•Phytostabilization
•Phytovolatilization
• Rhizodegradation, phytostimulation, or plant assisted bioremediation
• Rhizofiltration or contaminant uptake

Optimal performance of the technology is an important key to phytoremedia-
tion’s ability to gain wider acceptance as a presumptive remediation technique. With

Figure 5.1

Potential contaminant fates during phytoremediation in the soil–plant–atmosphere
continuum.
Mechanisms
for Organics
Mechanisms
for Inorganics
Atmosphere
Contaminant
in the air
Plant
Contaminant
in the plant
Soil

Contaminant
in the root-zone
(Rhizosphere)
Phytovolatilization
Phytodegradation
Rhizodegradation
Rhizofiltration
Phytostabilization
Impacted Media Impacted Media
Phytostabilization
Rhyzofiltration
Phytoaccumulation
Phytovolatilization
Remediated
Contaminant

©2001 CRC Press LLC

the possible exception of some of the above mechanisms that are already widely
studied and understood, all of phytoremediation’s major applications require further
basic and applied research in order to optimize field performance. Significant
research and development should be carried out to 1) obtain a better understanding
of mechanisms of uptake, transport, and accumulation of contaminants; 2) improve
collection and genetic evaluation of hyperaccumulating plants; and 3) obtain a better
understanding of interactions in the rhizosphere interactions among plant roots,
microbes, and other biota.
Short of true regulatory reform, phytoremediation’s ability to make further
inroads will depend on how quickly federal, state, and local regulators become
convinced of the technology’s efficacy. While not involved in every decision making
process, the public is sometimes a key constituency as well. One can expect public

interest groups to be more concerned about efficacy and safety issues than cost or
other economic factors. However, phytoremediation seems to be faring well with
the general public and, according to many practitioners, has already proven popular
with neighbors and other interested parties at field remediation sites.

5.3.1Phytoaccumulation

Remediation of contaminated soils using nonfood crops, called phytoaccumula-
tion, has attracted a great deal of interest in recent years. Also called phytoextraction,
phytoaccumulation, refers to the uptake and translocation of metal contaminants in
the soil by plant roots into the above ground portions of plants.

2

Certain plants,
called hyperaccumulators, absorb unusually large amounts of metals in comparison
to other plants and the ambient metals concentration (Table 5.1).
Phytoaccumulators or phytoextractors must have a high accumulation factor, that
is, a high uptake of metals from the soil. The uptake should be metal specific, which
diminishes the risk of impoverishing the soil of nutrient elements. The property of
having a high specific uptake must be genetically stable. Since the removal of metals
from the soil is actually achieved through the harvest, it is necessary that the plant
have a high transport of the metal(s) from the roots to the shoots to be effective
during remediation applications. In addition, a high biomass production of the

Table 5.1The Number of Taxonomic
Groups of Hyperaccumulators
Varies According to Which Metal

is Hyperaccumulated

2

Metal
Number of Taxonomic Groups
of Hyper Accumulators

Ni>300
Co26
Cu24
Zn18
Mn8
Pb5
Cd1

©2001 CRC Press LLC

phytoaccumulator is needed for high removal of metals per unit area. It is also an
advantage if biomass production is of economic interest.
Hyperaccumulators have been preferred during phytoaccumulation applications
because they take up very large amounts of a specific metal. They are often endemic
and of a specific population (genotypes/clones) of a species.

5

However, these plants
seldom have high biomass production and may also have low competitive ability in
less polluted areas, probably because the plant uses its energy to tolerate such high
levels of metals in the tissue instead of growth. Hyperaccumulators can accumulate

≥

0.01% of Cd,

≥

0.1% of Cu, or

≥

1.0% Zn in leaf dry mass and may have the metal
evenly distributed throughout the plant.

6

There are also high accumulators that accumulate somewhat lower metal con-
centrations than hyperaccumulators but much more than “normal” plants. They
usually have high biomass production. In these plants, there is no uniform distribu-
tion of metal throughout the plant, and thus the plant might have high accumulation
either in the roots or in the shoots. These plants are selected and planted at a site
based on the type of metals present and other site conditions. After they have been
allowed to grow for several weeks or months, they are harvested.
Landfilling, incineration, and composting are options to dispose of or recycle
the metals, although this depends upon the results of TCLP and cost. Planting and
harvesting of plants may be repeated as necessary to bring soil contaminant levels
down to allowable limits. A plan may be required to deal with the plant biomass
waste. Testing of plant tissue, leaves, roots, etc., will determine if the plant tissue
is a hazardous waste. Regulators will play a role in determining the testing method
and requirements for the ultimate disposal of the plant waste.
The state of science in phytoaccumulation is as follows:

7

• Botanical prospecting dating to the 1950s in the former USSR and U.S. is available
to practitioners.
•Over 400 species of hyperaccumulators worldwide have been cataloged.
•Field test kits for metal hyperaccumulation have been developed.
• Uptake and segregation processes using cation pumps, ion transporters, Ca blocks,
metal chelating exudates and transporters, phytochelatin peptides, and metallothio-
neins have been evaluated and continuous research is being performed to develop
further understanding.

The hyperaccumulator plants can contain toxic element levels in the leaf and
stalk biomass (LSB) about 100 times more than nonaccumulator plants growing in
the same soil, with some species and metal combinations exceeding conventional
plant levels by a factor of more than 1000.

8

Many hyperaccumulator plants, which are nonwoody (not a tree), have been
identified as having the capacity to accumulate metals.

Thlaspi caerulascens

was
found to accumulate Zn up to 2000–4000 mg/kg.

9

The Indian mustard plant

Brassica
juncea

, grown throughout the world for its oil seed, was found to accumulate
significant amounts of lead.

10

One planting of mustard in a hectare of contaminated
land was found to soak up two metric tons of lead. If three plantings could be
squeezed in per year, six tons of lead per hectare can be extracted. Both hemp
dogbane (

Apocynum

sp.) and common ragweed also have been observed to

©2001 CRC Press LLC

accumulate significant levels of lead.

Aeollanthus subcaulis

var

lineris

and

Papsalum
notatus

are other hyperaccumulator plants known to accumulate Cu and Cs, respec-
tively. Hyperaccumulator plants can address contamination in shallow soils only, up
to 24 inches in depth. If contamination is deeper, 6–10 feet, deep-rooted poplar trees
can be used for phytoextraction of heavy metals. These trees can accumulate the
heavy metals by sequestration. However, there are concerns specifically for trees
that include leaf litter and associated toxic residues being blown off site. This concern
may be tested in the laboratory to see whether uptake and translocation of the metals
into the leaves exceed standards.
Hyperaccumulators have metal accumulating characteristics that are desirable,
but lack the biomass production, adaptation to current agronomic techniques, and
physiological adaptations to climatic conditions required at many contaminated sites.
It has been reported that harvesting at different seasons in a year had pronounced
differences in accumulation levels. In the future, genetic manipulation techniques
may provide better hyperaccumulator species. The success of phytoextraction
depends on the use of an integrated approach to soil and plant management: the
disciplines of soil chemistry, soil fertility, agronomy, plant physiology, and plant
genetic engineering are currently being used to increase the rate and efficiency of
heavy metal phytoextraction.
Chelates have been used not only to enhance metal uptake but also to avoid
metal toxicity. Metal accumulator plants have been studied extensively for organo-
metallic complexes. It has been suggested that there is a relationship between metal
tolerance and carboxylic acids. Organo-metallic complexes increase the translocation
and tolerance of plants to the toxic effects of metals. For example, in

Sebertia
acuminata

citrate seems to be a detoxifying agent as well as an agent in transporting
phytotoxic Ni from root systems to the leaves until leaf fall.

5,6

It has also been
suggested that in copper (Cu) and cobalt (Co) accumulator plants, Co existed as an
oxalate complex within the leaf. The formation of Zn–citrate complexes in Zn-
tolerant plants was the reason for high levels of organic acid accumulation. Reports
have indicated that histidine was responsible for accumulation, tolerance, and trans-
port to shoots in nonaccumulating and hyperaccumulating (Ni) plant species.

11

In

Thlaspi

, a Zn hyperaccumulator plant species, it has been determined that the
majority of Zn in the roots was coordinated with histidine, whereas organic acids
were involved in xylem transport and Zn storage in the shoots. Similarly in a Cr-
accumulating plant,

Leptospermum scoparium , it was found that soluble Cr in leaf
tissue was present as the trioxalatochromium (III) ion, [Cr (C

2

O

4

)

3

]

3–
. The function
of the Cr-organic acid complex was to reduce the cytoplasmic toxicity of Cr.

5

Adding ethylenediaminetetraacetic (EDTA) acid, citric acid, or oxalic acid to
metal contaminated soils will significantly increase the metal concentrations in plant
shoots and roots.
5

However, the application of these chelates during a full scale
remediation application has to be carefully controlled; if not, the increased solubility
of the metal chelates formed could drive these contaminants to migrate further
downward by leaching when plant uptake rates are not adequate. Controlling the
pH and conditioning the soils for optimum pH is an important factor when dealing
with metals-contaminated soils.

©2001 CRC Press LLC

The schematic of the process involved in heavy metal phytoextraction is shown
in Figure 5.2. Translocation from the root to the shoot must occur efficiently for

ease of harvesting. After harvesting, a proper, regulartorily acceptable biomass
processing step or disposal methods should be implemented.

5.3.2 Phytodegradation

Phytodegradation, also called phytotransformation, is the breakdown of contam-
inants taken up by plants through metabolic processes within the plant, or the
breakdown of contaminants external to the plant through the effect of compounds
(such as enzymes) produced by the plants. Pollutants are degraded, used as nutrients,
and incorporated into the plant tissues. In some cases metabolic intermediate or end
products are rereleased to the environment depending on the contaminant and plant
species (phytovolatilization) (Figure 5.3).
Plants synthesize a large number of enzymes as a result of primary and secondary
metabolism and can quickly uptake and metabolize organic contaminants to less
toxic compounds. Plant enzyme systems can be constitutive or induced and can play
a role in solar driven transformations and plant adaptation and/or tolerance to adverse

Figure 5.2

Process schematic describing the various processes during phytoaccumulation of
heavy metals.

©2001 CRC Press LLC

growth conditions resulting from contamination of the soils. Plant-formed enzymes
that are useful for phytodegradation are nitroreductases (for munitions and pesti-
cides); dehalogenases (for chlorinated solvents and pesticides); phosphatases (for
pesticides); peroxidases (for phenols); laccases (for aromatic amines); cytochrome
P-450 (for pesticides and chlorinated solvents); nitrilase (for herbicides).
Plant transformation pathways can be of many different types and obviously

depend on plant species and tissue type. In simplistic terms, these pathways can be
categorized as reduction, oxidation, conjugation, and sequestration. The “green liver
model” has been proposed to describe the metabolic pathways of herbicides, pesti-
cides, explosives, and other nitroaromatic compounds. Contaminant degradation by
plant-formed enzymes can occur in an environment free of microorganisms (for
example, an environment in which the microorganisms have been killed by high
contaminant levels). Thus, phytodegradation potentially could occur in soils where
biodegradation cannot.
The current state of science in phytodegradation (phytotransformation) is sum-
marized below:

1,2

•Plant-formed enzymes that degrade organic contaminants have been isolated and
metabolic pathways can be predicted.
•Phytodegradation can be used for the treatment of soil, sediments, sludges, and
groundwater depending on contaminant type and concentrations.

Figure 5.3

Phytodegradation and phytovolatilization mechanisms associated with some other
mechanisms essential for plant life.
O
2
2
CO
Photosynthesis
2
+O
Phloem

Photosynthates
Xylem
H O, Nutrients
2
2
H O Transpiration and
Volatilization of VOCs
Dark Respiration
O
2
2
CO , H O
Lignification,
Metabolites
Sequestration
CO , H O
2
2
O
Root Respiration
Exudation
O , CH COOH, C H OH
Cometabolism
2
3
4
5
Contaminant
CO , H O, Cl
Mineralization

22
Contaminent
Uptake
2
H O, Nutients, O
Transpiration
2
Phytodegradation
- Metabolism within the plant
- Production of enzymes which

help to catalyze degradation

©2001 CRC Press LLC
• Mass balance and pathway analyses studies have been conducted to prove com-
plete degradation; potential toxicity of intermediate compounds also can be pre-
dicted.
•Differentiation between degradation by plant enzymes, rhizosphere microorgan-
isms, and other breakdown processes is being performed.
•Development of engineered solutions based on the use of monocultures vs. mul-
ticultures found in wetlands and terrestrial communities is being further investi-
gated.
•Organic contaminants are the main category of contaminants with the highest
potential of phytodegradation. Inorganic nutrients are also consumed through plant
uptake and metabolism. Phytodegradation outside the plant does not depend on
log K

ow

and plant uptake.

•Axenic plant tissue cultures of the aquatic plant

Myriophyllum

and the periwinkle

Catharanthus

are being used for elucidating plant transformation pathways.

The aquatic plant parrot feather (

Myrioplillum aquaticum

) and the algae

Nitella

have been used for the degradation of TNT. The nitroreductase enzyme has also
been identified in other algae, ferns, monocots, dicots, and trees.
Degradation of TCE has been detected in hybrid poplars and in poplar cell
cultures, resulting in production of metabolites and in complete mineralization of a
small portion of the applied TCE.

12,14

Poplars have been used to remove atrazine
and inorganic nutrients.

2

Black willow (

Salix nigra

), yellow poplar (

Liriodendron
tulipifera

), bald cypress (

Taxodium diskchum

), river birch (

Betula nigra

), cherry bark
oak (

Quercus falcata

), and live oak (

Quercus viginiana

) have been known to support
degradation of herbicides.

13

One recent study demonstrated that poplar trees, which
possess cytochrome P-450s analogous to the oxygenases responsible for transfor-
mation of compounds such as TCE in the mammalian liver, exposed to 100 mg/L
of TCE did uptake and chemically alter this contaminant. TCE and its metabolites
were found in the roots and tissue of the study trees, but not in control trees or in
the soil used for potting the trees. In a subsequent study, poplar seedlings exposed
to

14

C-labeled TCE were found to generate

14

C-labeled carbon dioxide. Intermediate
compounds generated during oxidation are thought to be 2,2,2-trichloroethanol, and
di- and trichloroacetic acid. Similar studies have shown positive results for toluene
and benzene.
A recent study using parrot feather showed positive results for phytotransforma-
tion of perchlorate at concentrations of up to 20 ppm.

22
Based on the results of these
experiments and ecological knowledge of parrot feather, this species is an excellent
candidate for future research on

in situ

phytoremediation of contaminated water
bodies. Parrot feather also is a good candidate for phytoremediation of contaminated
groundwater temporarily held in artificial ponds.

5.3.3 Phytostabilization

Phytostabilization is the use of certain plant species to immobilize contaminants
in the soil and groundwater through absorption and accumulation by roots, adsorp-
tion onto roots, or precipitation within the root zone and physical stabilization of
soils. It is also used as a means to stabilize contaminated soil by decreasing wind

©2001 CRC Press LLC

and water erosion and to decrease water infiltration and the subsequent leaching of
contaminants. This process reduces the mobility of the contaminant and prevents
migration to the groundwater or air. This technique can be used to re-establish a
vegetative cover at sites where natural vegetation is lacking due to high metal
concentrations. Metal-tolerant species may be used to restore vegetation to such
sites, thereby decreasing the potential migration of contamination through wind
erosion, transport of exposed surface soils, and leaching of soil contamination to
groundwater.
Implementation of phytostabilization involves reduction in the mobility of heavy
metals and high molecular weight organics by minimizing soil erodibility, decreasing
the potential for wind blown dust, and reduction in contaminant solubility by the
addition of soil amendments. Containment using plants either binds the contaminants
to the soil, renders them nonavailable, or essentially immobilizes them by removing
the means of transport.
Erosion leads to the concentration of heavy metals because of selective sorting
and deposition of different size fractions of the soil. Eroded material is often trans-
ported over long distances, thus selectively extending the effects of contamination

and increasing the risk to the environment. Erosion can, therefore, cause the build
up of concentrations of normally nontoxic contaminants to toxic levels at locations
where transported material is deposited.
Planting of vegetation at contaminated sites, particularly abandoned strip mining
sites, will significantly reduce the erodibility of the soils by water and wind; density
of vegetation will effectively hold the soil and provide a stable cover against erosion.
An excellent example of phytostabilization is everyone’s family garden where plants
help to minimize erosion and enhance the stability of the soil.
Another element of phytostabilization is to supplement the system with a variety
of alkalizing agents, phosphates, organic matter, and biosolids to render the metals
insoluble and unavailable to leaching. Materials with a calcareous character or a
high pH, such as lime and gypsum, can be added to influence the acidity. Specific
binding conditions can be influenced by adding concentrated Fe, Mn or Al com-
pounds. To maintain or raise the organic matter content in the soils, various materials
such as humus or peat materials, manure, or mulch can be added.
This chemical alteration should be quickly followed by establishing a plant cover
and maximizing plant growth. The amendments sequester the metals into the soil matrix
and plants keep the stabilized matrix in place, minimizing wind and water erosion.

5.3.4 Phytovolatilization

Phytovolatilization is the uptake and transpiration of a contaminant by a plant,
with release of the contaminant or a modified form of the contaminant to the
atmosphere from the plant. Phytovolatilization occurs as growing trees and other
plants take up water and organic and inorganic contaminants. Some of these con-
taminants can pass through the plants to the leaves and volatilize into the atmosphere
at comparatively low concentrations (Figure 5.3). Many organic compounds tran-
spired by a plant are subject to phytodegradation.

©2001 CRC Press LLC

Thus far, phytovolatilization has mainly been applied to groundwater contami-
nation. However, the potential exists for application to soil, sediments, and other
contamination and needs some careful applications.

2

The state of science with respect
to phytovolatization can be summarized as follows:

2,17

• Contaminants could be transformed to less toxic forms (e.g., elemental Hg and
dimethyl selenite gas).
• The contaminant or a hazardous metabolite might accumulate in vegetation.
•Significant reductions of TCE, TCA, and carbon tetrachloride have been achieved
in experimental studies.
• Poplars, alfalfa (

Medicago sativa

), and black locust species have been studied to
evaluate phytovolatilization.
• Indian mustard and canola have been used in phytovolatilization studies of Se.

2

Selenium (as selenate) was converted to less toxic dimethyl selenite gas and
released to the atmosphere. Kenaf and tall fescue have also been used to take up
Se, but to a lesser degree than canola.

•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.

2

• Groundwater must be within the influence of plant (usually a tree) roots and soil
must be able to transmit sufficient water to the plant.
•Climatic factors such as temperature, precipitation, humidity, solar radiation, and
wind velocity can affect transpiration rates and thus the rate of phytovolatilization.
• Improved methods for measuring phytovolatilization, diurnal and seasonal varia-
tions, and precipitation vs. groundwater use need to be developed.
•Significant research needs to be focused on modeling impacts of vegetation such
as transpiration stream concentration factors, canopy effects, and root concentra-
tion factors.

5.3.5Rhizodegradation

Rhizodegradation (also called phytostimulation, rhizosphere biodegradation,
enhanced rhizosphere biodegradation, or plant-assisted bioremediation/degradation)
is the breakdown of contaminants in the soil through microbial activity enhanced
by the presence of the rhizosphere (Figure 5.4). Microorganisms (yeast, fungi, and/or
bacteria) consume and degrade or transform organic substances for use as nutrient
substances. Certain microorganisms can degrade organic substances such as fuels
or solvents that are hazardous to humans and ecoreceptors and convert them into
harmless products through biodegradation. Natural substances released by plant roots

— such as sugars, alcohols, and acids — contain organic carbons that act as nutrient
sources for soil microorganisms; these additional nutrients stimulate their activity.
Rhizodegradation is aided by the way plants loosen the soil and transport oxygen
and water to the area. Plants also enhance biodegradation by other mechanisms such
as breaking apart clods and transporting atmospheric oxygen to the root zone.
Soil adjacent to the root contains increased microbial numbers and populations.

15

It is common knowledge that the number of bacteria in the rhizosphere is as much
as 20 times that normally found in nonrhizosphere soil (Figure 5.4). Short gram
negative rods (specifically

Pseudomonas, Flavobacterium,

and

Alcaligens

) are most

©2001 CRC Press LLC

commonly found in the rhizosphere.

15

The increased microbial numbers are primarily
due to the presence of plant exudates and sloughed tissue that serve as sources of
energy, carbon, and other growth factors. The products excreted by plants include

amino acids, carboxylic acids, carbohydrates, nucleic acid derivatives, growth
factors, and enzymes. The activity of microorganisms in the root zone stimulates
root exudation further stimulating microbial activity.

16

Several studies have evaluated the effect of plants and the associated rhizosphere
on the fate of petroleum contaminants.

2,4,15

For the most part, the presence of plants
enhanced the degradation of contaminants. Also, in studies using

14

C-labeled con-
taminants in closed plant chambers, mineralization was greater in rhizosphere soils
than in unvegetated soils, indicating that the bioavailability of the contaminant was
higher in the rhizosphere.

15

Studies using deep rooted prairie grasses to remediate soils contaminated with
PAH suggest that the roots of these perennial grasses may be more effective at
stimulating the rhizosphere microflora due to their fibrous nature. Fibrous roots offer
more root surface area for microbial colonization than other roots and result in a
larger microbial population in the contaminated soil. Big bluestem (

Andropogon

gerardii

), indian grass (

Sorghastrum nutans

), switch grass (

Panicum virgatum

),
Canada wild rye (

Elymus canadensis

), little bluestem (

Schizachyrium scoparius

),
side oats grama (

Bouteloua curtipendula

), western wheatgrass (

Agropyron smithic

),
and blue grama (

Bouteloua gracilis

) are some of the species known to enhance
degradation of petroleum compounds. Crested wheatgrass (

Agropyron desertorum

)
is known to degrade PCP contaminated soils.

15

Alfalfa (

Meticago sativa

), fescue
(

Festuca anundinacea

), big bluestem (

Andropogon gerardii

), and sudan grass

Figure 5.4

Rhizodegradation and associated processes in the root zone.
Enhanced rhizosphere biodegradation
- Supply of nutrients, cometabolites
- Transport and retention of water
- Aeration
Root respiration
Sloughing
Enzymes
dehalogenase
nitroductase
Uptake
Root intrusion
Soil dessication

©2001 CRC Press LLC

(

Sorghum vulgare sudanense

) are known to enhance the degradation of PAH com-
pounds in the rhizosphere. The degradation rates among various PAHs studied
correlated with the water solubility of the compound with the more soluble com-
pound, showing the highest degradation.
Cometabolic transformation of chlorinated solvents and other compounds also
has been reported in the literature.

2

Wherever significant cometabolic transforma-

tions took place, the following enzyme systems were present: dehalogenase, nitrore-
ductase, peroxidase, laccase, nitrylase, and oxygenase.
The rhizosphere is often divided into two general areas: the inner rhizosphere
at the very root surface and the outer rhizosphere embracing the immediately adjacent
soil. The microbial population is larger in the inner zone where biochemical inter-
actions are most pronounced and root exudates are concentrated. In addition to plant
exudates, the rapid decay of fine-root biomass can also become an important addition
of organic carbon to soils. A recent report considers some strategies for engineering
plants to improve bioremediation in the root zone. One of the simpler approaches
is to make use of the organism

Agrobacterium rhizogenes

to induce a state called
“hairy root disease.” Depending on virulence of the strain used, the extent of root
production is variable, but generally, infection leads to a significant enhancement of
rooting without obvious detrimental effects on the host plant. Increased root mass
has the apparent advantage of increasing the surface area available for microbial
colonization. Root exudation may be increased in proportion to increase in root area.
Such rhizosphere enhancements could improve bioremediation potential of the plant-
microbial system. It is suggested that when water is not freely available in unlimited
quantities, increased root mass could lead to greater water uptake, and hence greater
contaminant mobilization and potential degradation.
Different plant species often establish somewhat different subterranean floras
(Figure 5.5). The differences are attributed to variations in rooting habits, tissue
composition, and excretion products of the plant. The primary root population is

Figure 5.5

Examples of different root depths.

Alfalfa 4-6 ft.
Grasses 2 ft.
Indian
Mustard 1 ft.
Poplar Trees 15 ft.

©2001 CRC Press LLC

determined by the habitat created by the plant; the secondary flora, however, depends
upon the activities of the initial population. The age of the plant also alters the
microbial population in the rhizosphere. Roots also harbor mycorrhizae fungi, which
metabolize organic contaminants. These fungi, growing in symbiotic association
with the plant, have unique enzymatic pathways, similar to white rot fungus enzymes
that help to degrade organics that could not be transformed solely by bacteria.
In summary, plants provide exudates that offer an excellent habitat for increased
microbial populations and pump oxygen to roots, a process ensuring aerobic trans-
formations near the root that otherwise may not occur in bulk soil. Due to the
presence of certain primary substrates in the exudate system, anaerobic cometabolic
transformations may also take place in the rhizosphere. Typical microbial population
in the rhizosphere comprise: 5

¥

10

6

bacteria, 9

¥

10

5

actinomycetes, and 2

¥

10

3

fungi per gram of air dried soil.
The state of science in phytodegradation can be summarized as follows:

• Contaminant degradation can be achieved

in situ

, which is the biggest advantage.
•Translocation of the contaminant to the plant or atmosphere is less likely than
with other phytoremediation techniques since degradation takes place at the source
of contamination.
• There are low installation and maintenance cost(s) since no harvesting and disposal
are required.
•Various microorganism species and enzymes have been isolated which degrade
different contaminants.
• Analytical methods to better quantify treatment efficiency and success are
improving.

•Field management techniques for nutrients, water, and plant selection are
advancing.
• TPH and PAHs up to hundreds of ppm have been studied in the field with varying
success.

2

•Degradation of various pesticides (atrazine, metolachlor, parathion, diazinon, and
2,4-D, 2,4,5-T herbicides) has been studied, again with mixed results.

2

• TCE, PCP and PCB degradation have also been investigated — again with varying
success.
• More research needs to be done to further elucidate: microbial metabolism in the
rhizosphere, toxicity towards plants, biodiversity in the rhizosphere, biogeochem-
ical optimization in the rhizosphere, and interrelation between biological, chemical
and physical characteristics of the rhizosphere.

The following plants, in addition to the ones discussed previously, have been
used for successful implementation of phytodegradation at field sites:

2

1) red mul-
berry, crabapple, spearmint, and osage orange that are capable of stimulating PCB
degradation; 2) alfalfa, loblolly pine, and soybean for TCE degradation;3) alfalfa
for TCA degradation; and 4) rye, St. Augustine, and white clover for TPH. Growth
of hybrid poplar trees for the application of phytodegradation and rhizodegradation
is shown in Figures 5.6a, b, and c.

©2001 CRC Press LLC
5.3.6 RhizoÞltration
Rhizofiltration is the adsorption or precipitation of contaminants onto plant roots
or the absorption of contaminants into the roots when contaminants are in solution
Figure 5.6a Phytoremediation System, August 6, 1998.
Figure 5.6b Phytoremediation System, September 13, 1999.
©2001 CRC Press LLC
surrounding the root zone. In some applications, the plants are raised in greenhouses
hydroponically (with their roots in water rather than in soil). Once a large root system
has been developed, contaminated water is diverted and brought in contact with the
plants or the plants, are moved and floated in the contaminated water. The plants
are harvested and disposed as the roots become saturated with contaminants. Plant
uptake, concentration and translocation might occur, depending on the contaminant.
Exudates from the plant roots might cause precipitation of some metals. Rhizofil-
tration first results in contaminant containment, in which the contaminants are
immobilized or accumulated on or within the plant; contaminants are then removed
by removing the plant.
Aquatic plants and algae are known to accumulate metals and other toxic ele-
ments from solution.
18
There are large differences in bioremoval rates due to species
and strain differences, cultivation methodology, and process control techniques. In
the past, commercial systems have used immobilized algae biomass for removing
radionuclides and other heavy metals in the aqueous phase.
19
Naturally immobilized, plants such as attached algae and rooted plants, and those
easily separated from suspension, such as filamentus microalgae, macroalgae, and
floating plants, have been found to have high adsorption capacities. In a recent study,
one blue green filamentous alga of the genus Phormidium and one aquatic rooted

plant, water milfoil (Myriophyllum spicatum), exhibited high specific adsorption for
Cd, Zn, Ph, Ni, and Cu.
18
It has been reported that porous beads containing immobilized biological mate-
rials such as sphagnum peat moss can be used for extracting metals dissolved in the
aqueous phase.
20
The beads designated as BIO-FIX beads readily adsorbed Cd, Pb,
and other toxic metals from dilute waters. In one recent study, it was reported that
Figure 5.6c Phytoremediation System, August 22, 2000.
©2001 CRC Press LLC
Saccharomyces cerevisiae yeast biomass, when treated with a hot alkali, exhibited
an increase in its biosorption capacity for heavy metals.
21
It was also reported that
caustic treated yeast immobilized in alginate gel could be reactivated and reused to
remove Cu, Cd, and Zn in a manner similar to the ion exchange resin.
Phytoremediation applications are summarized in Tables 5.2a and b based on
contaminant fate, degradation, extraction, containment type, or a combination of
these applications. In the soil–plant–atmosphere continuum, a specific contaminant
can be remediated at specific points along this continuum by different phytoreme-
diation mechanisms.
Table 5.2a Types of Phytoremediation for Organic Constituents
Type of Phytoremediation Process Involved Contaminant Treated
1. Phytostabilization Plants control pH, soil gases, and
redox conditions in soil to
immobilize contaminants.
HumiÞcation of some organic
compounds is expected.
Expected for phenols,

chlorinated solvents
(tetrachloromethane and
trichloromethane) and
hydrophobic organic
compounds
2. Rhizodegradation
(phytostimulation,
rhizosphere
bioremediation, or
plant-assisted
bioremediation)
Plant exudates, root necrosis, and
other processes provide organic
carbon and nutrients to spur soil
bacteria growth by two or more
orders of magnitude. Exudates
stimulate degradation by
mycorrhizal fungi and microbes.
Live roots can pump oxygen to
aerobes and dead roots may
support anaerobes.
Polyaromatic hydrocarbons,
BTEX, and other
petroleum hydrocarbons,
perchlorate, atrazine,
alachlor, polychlorinated
biphenyl (PCB), and other
organic compounds
3. RhizoÞltration
(contaminant uptake)

Compounds are taken up or
sorbed by roots (or sorbed to
algae and bacteria).
Hydrophobic organic
chemicals
4. Phytodegradation
(phytotransformation)
Aquatic and terrestrial plants take
up, store, and biochemically
degrade selected organic
compounds to harmless
byproducts, products used to
create new plant biomass, or
byproducts that are further
broken down by microbes and
other processes to less harmful
products. Reductive and
oxidative enzymes may be used
in series in different parts of the
plant.
Munitions (TNT, DNT, HMX,
nitrobenzene, picric acid,
nitrotoluene), atrazine,
halogenated compounds
(tetrachloromethane,
trichloromethane,
hexachloroethane, carbon
tetrachloride, TCE,
tetrachloroethane,
dichloroethant), DDT and

other chlorine and
phosphorus based
pesticides, phenols, and
nitrites
5. Phytovolatilization Volatile organic compounds are
taken up and transpired. Some
recalcitrant organic compounds
are more easily degraded in the
atmosphere (photodegradation).
Chlorinated solvents
(trichloroethane), organic
VOCs, BTEX, MTBE
©2001 CRC Press LLC
5.3.7Phytoremediation for Groundwater Containment
Phytoremediation can be applied for containment of contaminated groundwater
under the right hydrogeologic conditions such as sites with shallow groundwater
depths. In general, favorable economics is one factor in phytoremediation’s favor,
particularly in contrast to the high cost of operation and maintenance of conventional
groundwater treatment systems. Furthermore, the high pumping rates of many deep
rooted trees may make them more efficient at removing water at low permeability sites.
Phreatophytes (like willows, cottonwood, and hybrid poplar), which take up and
“process” large volumes of soil water are good candidates for phytoremediation
applications specifically for groundwater containment. For example, a single willow
tree on a hot summer day transpires more than 5000 gallons of water, and a hybrid
poplar can transpire about 50 to 350 gallons per day.
23
Phytoremediation of groundwater plumes is preferred when the contaminants
are water soluble, leachable organics, and inorganics present at concentrations that
are not phytotoxic. Hydraulic control by plants can occur only within the root zone
or within a depth influenced by roots; the placement depth of roots during planting

can be varied. Root depth, early tree growth, and nutrient uptake were enhanced by
placing poplar tree root balls closer to shallow groundwater during planting.
23
The primary considerations for selecting phytoremediation for hydraulic control
as the method of choice are the depth and concentration of contaminants that affect
plant growth. Soil texture and degree of saturation are also influential factors.
Planning technique and materials can extend the influence of plants through non-
saturated zones to water-bearing layers.
As mentioned earlier, phreatophytes such as poplars are capable of extending
their roots into aerobic water tables. For example, the roots of poplars growing
Table 5.2b Types of Phytoremediation for Inorganic Constituents
Type of PhytoremediationProcess InvolvedContaminant Treated
1.PhytostabilizationPlants control pH, soil gases,
and redox conditions in soil
to immobilize contaminants.
HumiÞcation of some organic
compounds is expected.
Proven for heavy
metals in mine tailing
ponds
2. RhizoÞltration
(contaminant uptake)
Compounds are taken up or
biosorbed by roots (or
sorbed to algae and
bacteria).
Heavy metals and
radionuclides
3. Phytoaccumulation
(phytoextraction or

hyperaccumulation)
Metals and organic chemicals
taken up by the plant with
water, or by cation pumps,
sorption and other
mechanisms.
Nickel, zinc, lead,
chromium, cadmium,
selenium, other heavy
metals radionuclides
4. Phytovolatilization Volatile metals are taken up,
changed in species, and
transpired.
Mercury and selenium
©2001 CRC Press LLC
alongside streams can easily be observed intertwined in the stream bottom. The
degree to which poplar roots would penetrate the saturated zone cannot be easily
estimated. If their access to soil moisture from precipitation is limited, poplars will
draw large amounts of water from the top of saturated aquifer. Evapotranspiration
will draw down the water table below the trees similar to a pump and treat system
(Figures 5.7a and b). Simulations of a proposed design can be carried out based on
extent of contamination, hydrogeological data, past precipitation and infiltration
records, and evapotranspiration data.
A big advantage of phytoremediation over conventional pump and treat systems
is the ability of the roots to penetrate the microscopic scale pores in the soil matrix.
Contaminants adsorbed or trapped in these micropores are impacted minimally or
not at all by the pump and treat system. In the case of phytoremediation, the roots
can penetrate these micropores for contaminant removal.
5.3.8 Phytoremediation of Dredged Sediments
Dredged material is nothing more than displaced topsoil that enters and is

eventually removed from navigable waterways. Contaminant discharges into water-
ways over time result in contamination of bottom sediment. Dredged sediments are
usually stored in confined disposal facilities (CDF).
24
The application of phytoremediation to dredged material presents some chal-
lenges unique to dredged material. Dredged sediments come from an aquatic envi-
ronment and are initially wet and anaerobic after placement in a CDF. Subsequent
drying and oxidation depend on dewatering and management techniques. Drying
and oxidation of surface layers may result in physicochemical changes that may
affect plant establishment and contaminant mobility. Although the surface layer of
Figures 5.7a Placement of root ball with time due to maturation of the tree.
Above
Capillary
Fringe
At Capillary
Fringe
In Capillary Fringe
and Groundwater Table
©2001 CRC Press LLC
dredged sediments in a CDF may be dry and aerobic, deeper layers may remain
anaerobic. Saltwater dredged sediments provide another level of difficulty for veg-
etation and in most cases must be leached to reduce soluble salt levels. Dredged
material management is further complicated by the potential of elevated concentra-
tions of multiple contaminants. The selection of plant species and methods of
establishment will be determined by these factors. Common contaminants present
in dredged sediments are metals, PAHs, polychlorinated phenols, PCBs and other
heavy molecular weight compounds. The current state of knowledge indicates that
phytoremediation of dredged sediments would not be as readily effective as appli-
cation to more heavily contaminated industrial sites.
5.4 PHYTOREMEDIATION DESIGN

The design of a phytoremediation system varies according to contaminants,
conditions at the site, level of cleanup required, and plants used. A thorough site
characterization should provide the needed data to design any type of remediation
system. Clearly, phytoextraction has different design requirements from phytostabi-
lization or rhizodegradation. Nevertheless, it is possible to specify a few design
considerations that are part of most phytoremediation efforts (Figures 5.8a, b, and
c). Site characterization data will provide the information required for the designer
to develop a properly functioning system. The design considerations include con-
taminant levels; plant selection; treatability; irrigation, agronomic inputs (P N, K,
salinity, zinc, etc.), and maintenance; groundwater capture zone and transpiration
rate; and contaminant uptake rate and clean-up time required.
Other factors to be considered during the evaluation, design, and implementation
phases of phytoremediation at a contaminated site are:
Figures 5.7b Predicted groundwater ßow conditions at maturation of tree growth.
Groundwater
flow
Groundwater table elevation contours
Zone of tree plantation
30.0
30.5
31.0
30.0
29.0
28.0
©2001 CRC Press LLC
• Soil Water — The most crucial factor in a plant’s life is water, which links it to
the soil via roots and serves as a vehicle for nutrient transport. Water also controls
the exchange of gases and moderates soil temperature changes. Plant available
water is held in the soil between the field capacity and permanent wilting point.
Plant roots can extract water at lower potentials, depending upon the plant type

and arable environment. Root growth rates are controlled by the presence of
continuing supplies of water to maintain hydrostatic pressure in the elongating
Figure 5.8a Decision tree for phytoremediation in soil.