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The Use of Plants for the Removal of Toxic Metals 

from Contaminated Soil

Mitch M. Lasat 

American Association for the Advancement of Science 

Environmental Science and Engineering Fellow
NOTICE

This publication was developed under Grant No. CX 824823 awarded by the U.S.

Environmental Protection Agency. It has not been formally reviewed by EPA. The views

expressed in this document are solely those of Mitch Lasat and EPA does not endorse any

products or commercial services mentioned in this publication.

 
Abstract

Phytoremediation is an emerging technology that employs the use of higher plants for the

cleanup of contaminated environments. Fundamental and applied research have unequivocally

demonstrated that selected plant species possess the genetic potential to remove, degrade,

metabolize, or immobilize a wide range of contaminants. Despite this tremendous potential,

phytoremediation is yet to become a commercial technology. Progress in the field is precluded by

limited knowledge of basic plant remedial mechanisms. In addition, the effect of agronomic

practices on these mechanisms is poorly understood. Another limitation lies within the very

biological nature of this novel approach. For example, potential for phytoremediation depends

upon the interaction among soil, contaminants, microbes, and plants. This complex interaction,

affected by a variety of factors, such as climatic conditions, soil properties, and site hydrogeology, argues against generalization, and in favor of site-specific phytoremediating practices.

Thus, an understanding of the basic plant mechanisms, and the effect of agronomic practices on

plant/soil/contaminant interaction would allow practitioners to optimize phytoremediation by

customizing the process to site specific conditions. 

Remediation of metal contaminated soil faces a particular challenge. Unlike organic

contaminants, metals cannot be degraded. Commonly, decontamination of metal-contaminated

soils requires the removal of toxic metals. Recently, phytoextraction, the use of plants to extract

toxic metals from contaminated soils, has emerged as a cost-effective, environment-friendly

cleanup alternative. In this paper, we review the processes and mechanisms that allow plants to

remove metals from contaminated soils and discuss the effects of agronomic practices on these

processes.
TABLE OF CONTENTS

I. INTRODUCTION 

Background

Advantages and disadvantages of phytoremediation

Markets for phytoremediation

II. TOXIC METALS IN SOIL

Sources of contamination 

Risk assessment

Total and bioavailable soil fractions

Effect of soil properties on metal bioavailability

III. PHYTOREMEDIATING PLANTS

Why do plants take up toxic metals?

What is a hyperaccumulator species?

How do plants tolerate high metal concentration in soil?

Mechanisms of metals uptake into root and translocation to shoot

Plant mechanisms for metal detoxification

Plant limitations 

Improving phytoremediating plants 

IV. PLANT-METAL INTERACTION IN THE RHIZOSPHERE

Metal bioavailability for uptake into roots

Effect of soil microorganisms on metal uptake

Effect of root exudates on metal uptake

V. OPTIMIZATION OF METAL PHYTOEXTRACTION WITH AGRONOMIC

 PRACTICES

Plant selection

Soil fertilization and conditioning

Enhancing metal bioavailability with synthetic chelators

Sowing

Crop rotation

Crop maintenance: pest control and irrigation

Handling and disposal of contaminated waste

Cost and time projections

Research needs

VI. LITERATURE CITED 
5

I. INTRODUCTION

Background

The concept of using plants to clean up contaminated environments is not new. About

300 years ago, plants were proposed for use in the treatment of wastewater (Hartman,1975). At

the end of the 19th century, Thlaspi caerulescens and Viola calaminaria were the first plant

species documented to accumulate high levels of metals in leaves (Baumann,1885). In 1935,

Byers reported that plants of the genus Astragalus were capable of accumulating up to 0.6 %

selenium in dry shoot biomass. One decade later, Minguzzi and Vergnano (1948) identified

plants able to accumulate up to 1% Ni in shoots. More recently, Rascio, (1977) reported

tolerance and high Zn accumulation in shoots of Thlaspi caerulescens. Despite subsequent

reports claiming identification of Co, Cu, and Mn hyperaccumulators, the existence of plants

hyperaccumulating metals other than Cd, Ni, Se and Zn has been questioned and requires

additional confirmation (Salt et al., 1995). The idea of using plants to extract metals from

contaminated soil was reintroduced and developed by Utsunamyia (1980) and Chaney (1983),

and the first field trial on Zn and Cd phytoextraction was conducted in 1991 (Baker et al.). In the

last decade, extensive research has been conducted to investigate the biology of metal

phytoextraction. Despite significant success, our understanding of the plant mechanisms that

allow metal extraction is still emerging. In addition, relevant applied aspects, such as the effect

of agronomic practices on metal removal by plants are largely unknown. It is conceivable that

maturation of phytoextraction into a commercial technology will ultimately depend on the

elucidation of plant mechanisms and application of adequate agronomic practices. Natural

occurrence of plant species capable of accumulating extraordinarily high metal levels makes the

investigation of this process particularly interesting. 

Advantages and disadvantages of phytoremediation

Metal-contaminated soils are notoriously hard to remediate. Current technologies resort to

soil excavation and either landfilling or soil washing followed by physical or chemical separation

of the contaminants. The cost of soil remediation is highly variable and depends on the

contaminants of concern, soil properties, and site conditions. Cost estimates associated with the

use of several technologies for the cleanup of metal-contaminated soil are shown in Table 1.
6

Table 1. Cost of soil treatment (Glass, 1999a)

 

Treatment Cost ($/ton) Additional factors/expenses 

Vitrification 75-425 Long-term monitoring

Landfilling 100-500 Transport/excavation/monitoring

Chemical treatment 100-500 Recycling of contaminants 

Electrokinetics 20-200 Monitoring 

Phytoextraction 5- 40 Monitoring 

Cleaning of metal-contaminated soils via conventional engineering methods can be

prohibitively expensive (Salt et al., 1995). The costs estimated for the remediation of sites

contaminated with heavy metals, and heavy metals mixed with organic compounds are shown in

Table 2. 

Table 2. Projected five-year costs for remediation of sites contaminated with toxic metals

only, and mixtures of toxic metals and organics (U.S. EPA, 1993).

 

Sector Metals Only Metals and Organics

 ---------------------$ million------------------------

Superfund12,400 10,400

RCRA23,000 12,800

DOD3 400 2,400

DOE4 900 6,500

State5 200 800

Private6 200 2,500

Total 7,100 35,400 

1

 Sites ranked on the National Priorities List

2

 Sites requiring corrective action under the provisions of Resource Conservation and

 Recovery Act RCRA

3

 Department of Defense

4

 Department of Energy

5

 State-funded contaminated sites

6

 Private-funded contaminated sites 

Because of the high cost, there is a need for less-expensive cleanup technologies.

Phytoremediation is emerging as a cost-effective alternative. Several analyses have demonstrated

that the cost of metal phytoextraction is only a fraction of that associated with conventional

engineering technologies (Table 1). In addition, because it remediates the soil in situ,
7

phytoremediation avoids dramatic landscape disruption, and preserves the ecosystem. Despite

these advantages, several disadvantages and constraints restrict the applicability of

phytoextraction (Table 3).

Table 3. Major factors limiting the success and applicability of phytoextraction

Plant-based biological limitation Regulatory limitations Other limitations

1) Low plant tolerance

2) Lack of contaminant

translocation from root to

shoot

3) Small size of remediating

plants

1) Lack of cost and performance

data

2) Regulators unfamiliarity with

the technology

3) Disposal of contaminated

plant waste

4) Risk of food chain

contamination

1) Contaminant beneath root

zone

2) Lengthy process

3) Contaminant in biologically

unavailable form

4) Lack of remediating plant

species

Markets for phytoremediation

A comprehensive analysis of phytoremediation markets was published by Glass (1999a;

1999b). The author indicated that the estimated 1999 phytoremediation markets was two fold

greater than 1998 estimates. This growth has been attributed to an increased number of

companies offering services, particularly companies in the consulting engineering sector, and to

growing acceptance of the technology. An estimate of 1999 U.S. phytoremediation markets

related to a variety of contaminated media and contaminants of concern is shown in Table 4. 

Table 4. Estimated 1999 U.S. phytoremediation markets (Glass, 1999b)

 

Organics in groundwater $ 7-12 million

Landfill leachate $ 5-8 million

Organics in soil $ 5-7 million

Metals in soil $ 4.5-6 million

Inorganics in wastewater $ 2-4 million 

Inorganics in groundwater $ 2-3 million

Organics in wastewater $ 1-2 million

Metals in groundwater $ 1-2 million

Radionuclides $ 0.5-1 million

Metals in wastewater $ 0.1-0.2 million

Other $ 1.9-3.8 million

Total $ 30 -49 million 
8

Current estimates of 1999, and 2000 revenues were slightly lower than what had been

previously projected, largely due to slower commercialization of the technology for the cleanup

of metal- and radionuclide-contaminated sites (Glass, 1999b).

The second largest market for phytoremediation was identified in Europe, although

European market was estimated to be 10-fold smaller than the U.S. market (Glass 1999b). 

II. TOXIC METALS IN SOIL

Sources of contamination

Heavy metals are conventionally defined as elements with metallic properties (ductility,

conductivity, stability as cations, ligand specificity, etc.) and atomic number >20. The most

common heavy metal contaminants are: Cd, Cr, Cu, Hg, Pb, and Zn. Metals are natural

components in soil. Contamination, however, has resulted from industrial activities, such as

mining and smelting of metalliferous ores, electroplating, gas exhaust, energy and fuel

production, fertilizer and pesticide application, and generation of municipal waste (KabataPendias and Pendias,1989). Soil concentration range and regulatory limits for several major

metal contaminants are shown in Table 5. 

Table 5. Soil concentration ranges and regulatory guidelines for some toxic metals

Metal

Soil concentration rangea

(mg kg-1)

Regulatory limitsb

(mg kg-1)

Pb

Cd

Cr

Hg

Zn

1.00-6,900

0.10-345

0.05-3,950

<0.01-1,800

150.00-5,000

600

100

100

270

1,500

a) Riley et al., 1992

b) Nonresidential direct contact soil cleanup criteria (NJDEP, 1996)

High levels of metals in soil can be phytotoxic. Poor plant growth and soil cover caused

by metal toxicity can lead to metal mobilization in runoff water and subsequent deposition into

nearby bodies of water. Furthermore, bare soil is more susceptible to wind erosion and spreading

of contamination by airborne dust. In such situations, the immediate goal of remediation is to

reclaim the site by establishing a vegetative cover to minimize soil erosion and pollution spread. 
9

Risk assessment

Soil remediation is needed to eliminate risk to humans or the environment from toxic

metals. Human disease has resulted from Cd (Nogawa et al., 1987; Kobayashi 1978; Cai et al.,

1990), Se (Yang et al., 1983), and Pb in soil (Chaney et al., 1999). Livestock and wildlife have

suffered from Se poisoning (Rosenfeld and Beath, 1964; Ohlendorf et al., 1986). In addition, soil

contamination with Zn, Ni and Cu caused by mine wastes and smelters is known to be phytotoxic

to sensitive plants (Chaney et al., 1999). One of the greatest concerns for human health is caused

by Pb contamination. Exposure to Pb can occur through multiple pathways, including inhalation

of air and ingestion of Pb in food, water, soil or dust. Excessive Pb exposure can cause seizures,

mental retardation and behavioral disorders. The danger of Pb is aggravated by low

environmental mobility even under high precipitations. 

Total and bioavailable soil fractions

In soil, metals are associated with several fractions: (1) in soil solution, as free metal ions

and soluble metal complexes, (2) adsorbed to inorganic soil constituents at ion exchange sites,

(3) bound to soil organic matter, (4) precipitated such as oxides, hydroxides, carbonates, and (5)

embedded in structure of the silicate minerals. Soil sequential extractions are employed to isolate

and quantify metals associated with different fractions (Tessier et al., 1979). 

For phytoextraction to occur, contaminants must be bioavailable (ready to be absorbed by

roots). Bioavailability depends on metal solubility in soil solution. Only metals associated with

fractions 1 and 2 (above) are readily available for plant uptake. Some metals, such as Zn and Cd,

occur primarily in exchangeable, readily bioavailable form. Others, such as Pb, occur as soil

precipitate, a significantly less bioavailable form.

Effect of soil properties on metal bioavailability

The chemistry of metal interaction with soil matrix is central to the phytoremediation

concept. In general, sorption to soil particles reduces the activity of metals in the system. Thus,

the higher the cation exchange capacity (CEC) of the soil, the greater the sorption and

immobilization of the metals. In acidic soils, metal desorption from soil binding sites into

solution is stimulated due to H+competition for binding sites. Soil pH affects not only metal

bioavailabilty, but also the very process of metal uptake into roots. This effect appears to be

metal specific. For example, in T. caerulescens, Zn uptake in roots showed a small pH

dependence, whereas uptake of Mn and Cd was more dependent on soil acidity (Brown et al.,

1995a).

 
10

III. PHYTOREMEDIATING PLANTS

Why do plants take up toxic metals?

To grow and complete the life cycle, plants must acquire not only macronutrients (N, P,

K, S, Ca, and Mg), but also essential micronutrients such as Fe, Zn, Mn, Ni, Cu, and Mo. Plants

have evolved highly specific mechanisms to take up, translocate, and store these nutrients. For

example, metal movement across biological membranes is mediated by proteins with transport

functions. In addition, sensitive mechanisms maintain intracellular concentration of metal ions

within the physiological range. In general, the uptake mechanism is selective, plants

preferentially acquiring some ions over others. Ion uptake selectivity depends upon the structure

and properties of membrane transporters. These characteristics allow transporters to recognize,

bind and mediate the trans-membrane transport of specific ions. For example, some transporters

mediate the transport of divalent cations, but do not recognize mono- or trivalent ions. 

Many metals such as Zn, Mn, Ni and Cu are essential micronutrients. In common

nonaccumulator plants, accumulation of these micronutrients does not exceed their metabolic

needs (<10ppm). In contrast, metal hyperaccumulator plants can accumulate exceptionally high

amounts of metals (in the thousands of ppm). Since metal accumulation is ultimately an energy

consuming process, one would wonder what evolutionary advantage does metal

hyperaccumulation give to these species? Recent studies have shown that metal accumulation in

the foliage may allow hyperaccumulator species to evade predators including caterpillars, fungi

and bacteria (Boyd and Martens, 1994; Pollard and Baker, 1997).

Hyperaccumulator plants do not only accumulate high levels of essential micronutrients,

but can also absorb significant amounts of nonessential metals, such as Cd. The mechanism of

Cd accumulation has not been elucidated. It is possible that the uptake of this metal in roots is via

a system involved in the transport of another essential divalent micronutrient, possibly Zn2+.

Cadmium is a chemical analogue of the latter, and plants may not be able to differentiate between

the two ions (Chaney et al., 1994). 

What is a hyperaccumulator species?

Interest in phytoremediation has grown significantly following the identification of metal 

hyperaccumulator plant species. Hyperaccumulators are conventionally defined as species

capable of accumulating metals at levels 100-fold greater than those typically measured in

common nonaccumulator plants. Thus, a hyperaccumulator will concentrate more than: 10 ppm

Hg; 100 ppm Cd; 1,000 ppm Co, Cr, Cu, and Pb; 10,000 ppm Ni and Zn. To date, approximately

400 plant species from at least 45 plant families have been reported to hyperaccumulate metals. 

Most hyperaccumulators bioconcentrate Ni, about 30 absorb either Co, Cu, and/or Zn, even
11

fewer species accumulate Mn and Cd, and there are no known natural Pb-hyperaccumulators

(Reeves and Baker, 1999). Several hyperaccumulators and their bioaccumulation potential are

listed in Table 6.

Table 6. Several metal hyperaccumulator species and their bioaccumulation potential

 

Plant species Metal Leaf content (ppm) Reference 

Thlaspi caerulescens Zn:Cd 39,600:1,800 Reeves&Brooks(1983);Baker&Walker(1990) 

Ipomea alpina Cu 12,300 Baker&Walker (1990)

Haumaniastrum robertii Co 10,200 Brooks (1977)

Astragalus racemosus Se 14,900 Beath et al. (1937) 

Sebertia acuminata Ni 25% by wt dried sap Jaffre et al. (1976) 

 

Possibly, the best-known metal hyperaccumulator is Thlaspi caerulescens (alpine

pennycress). While most plants show toxicity symptoms at Zn accumulation of about 100 ppm,

T. caerulescens was shown to accumulate up to 26,000 ppm without showing any injury (Brown

et al., 1995b). Possibly, hyperaccumulator plants may have a higher requirement for metals such

as Zn than non-accumulator species (Hajar, 1997). In support of this, many hyperaccumulators,

including T. caerulescens, have been shown to colonize metal-rich soils such as calamine soil

(soil enriched in Pb, Zn, and Cd). Because of this ability, considerable efforts have been directed

to identify hyperaccumulator plants endemic to metal rich soils (Baker and Proctor, 1990).

How do plants tolerate high metal concentration in soil?

Ecological studies have revealed the existence of specific plant communities, endemic

floras, which have adapted on soils contaminated with elevated levels of Zn Cu, and Ni.

Different ecotypes of the same species may occur in areas uncontaminated by metals. To plants

endemic to metal-contaminated soils, metal tolerance is an indispensable property. In

comparison, in related populations inhabiting uncontaminated areas, a continuous gradation

between ecotypes with high and low tolerance usually occurs. Plants evolved several effective

mechanisms for tolerating high concentrations of metals in soil. In some species, tolerance is

achieved by preventing toxic metals uptake into root cells. These plants, coined excluders, have

little potential for metal extraction. Such an excluder is “Merlin,” a commercial variety of red

fescue (Festuca rubra), used to stabilize erosion-susceptible metal-contaminated soils. A second

group of plants, accumulators, does not prevent metals from entering the root. Accumulator

species have evolved specific mechanisms for detoxifying high metal levels accumulated in the

cells. These mechanisms allow bioaccumulation of extremely high concentration of metals. In
12

addition, a third group of plants, termed indicators, shows poor control over metal uptake and

transport processes. In these plants, the extent of metal accumulation reflects metal

concentration in the rhizospheric soil. Indicator species have been used for mine prospecting to

find new ore bodies (Raskin et al, 1994).

Mechanisms of metals uptake into roots and translocation to shoots

Because of their charge, metal ions cannot move freely across the cellular membranes,

which are lipophilic structures. Therefore, ion transport into cells must be mediated by membrane

proteins with transport functions, generically known as transporters. Transmembrane transporters

possess an extracellular binding domain to which the ions attach just before the transport, and a

transmembrane structure which connects extracelluar and intracellular media. The binding

domain is receptive only to specific ions and is responsible for transporter specificity. The

transmembrane structure facilitates the transfer of bound ions from extracellular space through

the hydrophobic environment of the membrane into the cell. These transporters are characterized

by certain kinetic parameters, such as transport capacity (Vmax) and affinity for ion (Km). Vmax

measures the maximum rate of ion transport across the cellular membranes. Km measures

transporter affinity for a specific ion and represents the ion concentration in the external solution

at which the transport rate equals Vmax/2. A low Km value, high affinity, indicates that high levels

of ions are transported into the cells even at low external ion concentration. By studying kinetic

parameters, Km and Vmax, plant biologists gain insights to specificity and selectivity of the

transport system. 

It is important to note that of the total amount of ions associated with the root, only a part

is absorbed into cells. A significant ion fraction is physically adsorbed at the extracellular

negatively charged sites (COO-) of the root cell walls. The cell wall-bound fraction cannot be

translocated to the shoots and, therefore, cannot be removed by harvesting shoot biomass

(phytoextraction). Thus, it is possible that a plant exhibiting significant metal accumulation into

the root, to express a limited capacity for phytoextraction. For example, many plants accumulate

Pb in roots, but Pb translocation to shoot is very low. In support of this, Blaylock and Huang

(1999) concluded that the limiting step for Pb phytoextraction is the long distance translocation

from roots to shoots. 

Binding to the cell wall is not the only plant mechanism responsible for metal

immobilization into roots and subsequent inhibition of ion translocation to the shoot. Metals can

also be complexed and sequestered in cellular structures (e.g., vacuole) becoming unavailable for

translocation to the shoot (Lasat et al., 1998). In addition, some plants, coined excluders, possess

specialized mechanisms to restrict metal uptake into roots. However, the concept of metal

exclusion is not well understood (Peterson, 1983). 
13

Zn

Zn Zn

Zn Zn

Pb 

Pb 

Pb 

Pb 

Cd

Cd

Cd Cd

Cd

Cd

Ni

Ni

Ni

Ni

Ni

Zn

PbCd

Pb

Pb Cd

Cd

Zn

Pb

PbCd

Zn Vacuole

Root Cells

Xylem

To

Leaves

1 2 3 4

Figure 1. Metal uptake and accumulation in plants

1. A metal fraction is sorbed at root surface

2. Bioavailable metal moves across cellular membrane into root cells 

3. A fraction of the metal absorbed into roots is immobilized in the vacuole 

4. Intracellular mobile metal crosses cellular membranes into root vascular tissue

(xylem)

5. Metal is translocated from the root to aerial tissues (stems and leaves)

Cd

Pb

Cd

5

Uptake of metals into root cells, the point of entry into living tissues, is a step of major

importance for the process of phytoextraction. However, for phytoextraction to occur metals

must also be transported from the root to the shoot. Movement of metal-containing sap from the

root to the shoot, termed translocation, is primarily controlled by two processes: root pressure

and leaf transpiration. Following translocation to leaves, metals can be reabsorbed from the sap

into leaf cells. A schematic representation of metal transport processes that take place in roots

and shoots is shown in Figure 1. 
14

Plant mechanisms for metal detoxification

Although micronutrients such as Zn, Mn, Ni and Cu are essential for plant growth and

development, high intracellular concentrations of these ions can be toxic. To deal with this

potential stress, common nonaccumulator plants have evolved several mechanisms to control the

homeostasis of intracellular ions. Such mechanisms include regulation of ion influx (stimulation

of transporter activity at low intracellular ion supply, and inhibition at high concentrations), and

extrusion of intracellular ions back into the external solution. Metal hyperaccumulator species,

capable of taking up metals in the thousands of ppm, possess additional detoxification

mechanisms. For example, research has shown that in T. goesingense, a Ni hyperacccumulator,

high tolerance was due to Ni complexation by histidine which rendered the metal inactive

(Krämer et al., 1997; Krämer et al., 1996). Sequestration in the vacuole has been suggested to be

responsible for Zn tolerance in the shoots of the Zn-hyperaccumulator T. caerulescens (Lasat et

al., 1996; Lasat et al., 1998). Several mechanisms have been proposed to account for Zn

inactivation in the vacuole including precipitation as Zn-phytate (Van Steveninck et al., 1990),

and binding to low molecular weight organic acids (Mathys, 1977; Tolrà et al., 1996; Salt et al.,

1999). Complexation to low molecular weight organic compounds (<10 kD) was also shown to

play a role in tolerance to Ni (Lee et al., 1977). Cadmium, a potentially toxic metal, has been

shown to accumulate in plants, where it is detoxified by binding to phytochelatins (Wagner 1984;

Steffens, 1990; Cobbett and Goldsbrough, 1999), a family of thiol (SH)- rich peptides

(Rauser,1990). Metallothioneins (MT), identified in numerous animals and more recently, in

plants and bacteria (Kägi, 1991), are also compounds (proteins) with heavy metal-binding

properties (Tomsett et al., 1992). 

Plant limitations

When the concept of phytoextraction was reintroduced (approximately two decades ago),

engineering calculations suggested that a successful plant-based decontamination of even

moderately contaminated soils would require crops able to concentrate metals in excess of 1-2%.

Accumulation of such high levels of heavy metals is highly toxic and would certainly kill the

common nonaccumulator plant. However, in hyperaccumulator species, such concentrations are

attainable. Nevertheless, the extent of metal removal is ultimately limited by plant ability to

extract and tolerate only a finite amount of metals. On a dry weight basis, this threshold is around

3% for Zn and Ni, and considerably less for more toxic metals, such as Cd and Pb. The other

biological parameter which limits the potential for metal phytoextraction is biomass production.

With highly productive species, the potential for biomass production is about 100 tons fresh

weight/hectare. The values of these parameters limit the annual removal potential to a maximum

of 400 kg metal/ha/yr. It should be mentioned, however, that most metal hyperaccumulators are
15

slow growing and produce little biomass. These characteristics severely limit the use of

hyperaccumulator plants for environment cleanup.

Improving phytoremediating plants

It has been suggested that phytoremediation would rapidly become commercially

available if metal removal properties of hyperaccumulator plants, such as T. caerulescens, could

be transferred to high-biomass producing species, such as Indian mustard (Brassica juncea) or

maize (Zea mays) (Brown et al., 1995b). Biotechnology has already been successfully employed

to manipulate metal uptake and tolerance properties in several species. For example, in tobacco

(Nicotiana tabacum) increased metal tolerance has been obtained by expressing the mammalian

metallothionein, metal binding proteins, genes (Lefebvre et al., 1987; Maiti et al., 1991).

Possibly, the most spectacular application of biotechnology for environmental restoration

has been the bioengineering of plants capable of volatilizing mercury from soil contaminated

with methyl-mercury. Methyl-mercury, a strong neurotoxic agents, is biosynthesized in Hgcontaminated soils. To detoxify this toxin, transgenic plants (Arabidopsis and tobacco) were

engineered to express bacterial genes merB and merA. In these modified plants, merB catalyzes

the protonolysis of the carbon-mercury bond with the generation of Hg2+, a less mobile mercury

species. Subsequently, MerA converts Hg(II) to Hg (0) a less toxic, volatile element which is

released into the atmosphere (Rugh et al., 1996; Heaton et al., 1988). Although regulatory

concerns restrict the use of plants modified with merA and merB , this research illustrates the

tremendous potential of biotechnology for environment restoration. In an effort to address

regulatory concerns related to phytovolatilization of mercury, Bizili et al. (1999) demonstrated

that plants engineered to express MerBpe (an organomercurial lyase under the control of a plant

promoter) may be used to degrade methyl-mercury and subsequently remove ionic mercury via

extraction. Despite recent advances in biotechnology, little is known about the genetics of metal

hyperaccumulation in plants. Particularly, the heredity of relevant plant mechanisms, such as

metal transport and storage (Lasat et al., 2000) and metal tolerance (Ortiz et al., 1992; Ortiz et al.,

1995) must be better understood. Recently, Chaney et al. (1999) proposed the use of traditional

breeding approaches for improving metal hyperaccumulator species and possibly incorporating

significant traits such as metal tolerance and uptake characteristics into high-biomass-producing

plants. Partial success has been reported in the literature. For example, in an effort to correct for

small size of hyperaccumulator plants, Brewer et al. (1997) generated somatic hybrids between T.

caerulescens (a Zn hyperaccumulator) and Brassica napus (canola) followed by hybrid selection

for Zn tolerance. High biomass hybrids with superior Zn tolerance were recovered. These authors

have also advocated a coordinated effort to collect and preserve germplasm of accumulator

species.
16

 IV. PLANT-METAL INTERACTION IN THE RHIZOSPHERE

Metal bioavailability for uptake into roots

A major factor limiting metal uptake into roots is slow transport from soil particles to root

surfaces (Nye and Tinker, 1977; Barber, 1984). With the possible exception of volatile mercury,

for all other metals, this transport takes place in soil solution. In soil, metal solubility is restricted

due to adsorption to soil particles. Some of the soil binding sites are not particularly selective.

For example, they bind Cd as strong as Ca. Nonspecific binding occurs at clay cation exchange

sites and carboxylic groups associated with soil organic matter. Other sites are more selective and

bind Cd stronger than Ca. For example, most clay particles are covered with a thin layer of

hydrous Fe, Mn, and Al oxides. These selective sites maintain Cd activity in the soil solution at

low levels (Chaney, 1988). Lead, a major contaminant, is notorious for the lack of soil mobility,

primarily due to metal precipitation as insoluble phosphates, carbonates and (hydr)oxides

(Blaylock and Huang, 1999). Thus, increasing metal solubility in the soil is an important

prerequisite to enhance the potential for Pb phytoextraction. This subject is detailed in the next

section. 

Two mechanisms are responsible for metal transport from the bulk soil to plant roots: 1)

convection or mass flow, and 2) diffusion (Corey et al., 1981; Barber, 1984). Due to convection,

soluble metal ions move from soil solids to root surface. From the rhizosphere, water is absorbed

by roots to replace water transpired by leaves. Water uptake from rhizosphere creates a hydraulic

gradient directed from the bulk soil to the root surface. Some ions are absorbed by roots faster

than the rate of supply via mass flow. Thus, a depleted zone is created in soil immediately

adjacent to the root. This generates a concentration gradient directed from the bulk soil solution

and soil particles holding the adsorbed elements, to the solution in contact with the root surface.

This concentration gradient drives the diffusion of ions toward the depleted layer surrounding the

roots.

Plants have evolved specialized mechanisms to increase the concentration of metal ions

in soil solution. For example, at low ion supply, plants may alter the chemical environment of the

rhizosphere to stimulate the desorption of ions from soil solids into solution. Such a mechanism

is rhizosphere acidification due to H+ extrusion from roots (Crowley et al., 1991). Protons

compete and replace metal ions from binding sites, stimulating their desorption from soil solids

into solution. In addition, some plants can regulate metal solubility in the rhizosphere by exuding

a variety of organic compounds from roots. Root exudates complex metal ions keeping them in

solution available for uptake into roots (Romheld and Marschner, 1986).
17

Effect of soil microorganisms on metal uptake

Root growth affects the properties of the rhizospheric soil and stimulates the growth of

the microbial consortium. To illustrate this, research has shown that the population of

microorganisms in the rhizosphere is several orders of magnitude greater than in the surrounding

soil (Anderson, 1997). In turn, rhizospheric microorganisms may interact symbiotically with

roots to enhance the potential for metal uptake. In addition, some microorganisms may excrete

organic compounds which increase bioavailability, and facilitate root absorption of essential

metals, such as Fe (Crowley et al., 1991) and Mn (Barber and Lee, 1974) as well as nonessential

metals, such as Cd (Salt et al., 1995). Soil microorganisms can also directly influence metal

solubility by altering their chemical properties. For example, a strain of Pseudomonas

maltophilia was shown to reduce the mobile and toxic Cr6+ to nontoxic and immobile Cr3+, and

also to minimize environmental mobility of other toxic ions such as Hg2+, Pb2+, and Cd2+ (Blake

et al., 1993; Park et al., 1999). In addition, it has been estimated that microbial reduction of Hg2+

generates a significant fraction of global atmospheric Hg0 emissions (Keating et al., 1977)

Effect of root exudates on metal uptake

Root exudates have an important role in the acquisition of several essential metals. For

example, some grass species can exude from roots a class of organic acids called siderophores

(mugineic and avenic acids), which were shown to significantly enhance the bioavailability of

soil-bound iron (Kanazawa et al., 1995), and possibly zinc (Cakmak 1996a; 1996b). In addition,

root exudates have been shown to be involved in plant tolerance. In support of this, it has been

demonstrated that some plant species tolerate Al in the rhizosphere, by a mechanism involving

exudation of citric and malic acids (Pellet et al., 1995; Larsen et al., 1998). These organic acids

chelate rhizospheric Al3+ which is highly phytotoxic to form a significantly less toxic complex. 

V. OPTIMIZATION OF METAL PHYTOEXTRACTION WITH 

AGRONOMIC PRACTICES 

Plant selection 

The selection of phytoremediating species is possibly the single most important factor

affecting the extent of metal removal. Although, the potential for metal extraction is of primary

importance, other criteria, such as ecosystem protection must be also considered when selecting

remediating plants. As a general rule, native species are preferred to exotic plants which can be
18

invasive and endanger the harmony of the ecosystem. To avoid propagation of weedy species,

crops are in general preferred although some crops may be too palatable and pose a risk to

grazing animals.

The rate of metal removal depends upon the biomass harvested and metal concentration

in harvested biomass. Possibly, one of the most debated controversies in the field refers to the

choice of remediative species; metal hyperaccumulators vs. common nonaccumulator species.

Hyperaccumulator plants have the potential to bioconcentrate high metal levels. However, their

use may be limited by small size and slow growth. In common nonaccumulator species, low

potential for metal bioconcentration is often compensated by the production of significant

biomass (Ebbs et al., 1997). Chaney et al. (1999), analyzed the rate of Zn and Cd removal, and

reached the conclusion that non-accumulator crops will not remove enough metal to support

phytoextraction. Furthermore, these authors argued that at many sites metal contamination is high

enough to cause toxicity to crop species and significant biomass reduction. In support of this,

several maize (one of the most productive crops) inbred lines have been identified which can

accumulate high levels of Cd (Hinesly et al., 1978). However, these lines were susceptible to Zn

toxicity and, therefore, could not be used to cleanup soils at the normal Zn:Cd ratio of 100:1

(Chaney et al., 1999). In addition, when appropriate disposal is an important regulatory concern,

the use of lower biomass producing hyperaccumulator species would be an advantage because

less contaminated biomass will have to be handled. 

For Pb, a major soil contaminant, no hyperaccumulator species has been identified.

However, several species, such as hemp dogbane (Apocynum cannabinum), common ragweed

(Ambrosia artemisiifolia), nodding thistle (Carduus nutans), and Asiatic dayflower (Commelina

communis), were shown to have superior Pb accumulating properties (Berti and Cunningham,

1993). Practices have been developed to increase the potential of common nonaccumulator plants

for Pb phytoextraction. Particularly, the uptake-inducing properties of synthetic chelates open the

possibility of using high biomass producing crops for Pb phytoextraction. Under chelate-induced

conditions, maize (Huang and Cunningham, 1996) and Indian mustard (Blaylock et al., 1997)

have been successfully used to remove Pb from solution culture and contaminated soil,

respectively. 

Physical characteristics of soil contamination are also important for the selection of

remediating plants. For example, for the remediation of surface-contaminated soils, shallowrooted species would be appropriate to use, whereas deep-rooted plants would be the choice for

more profound contamination. 
19

Soil fertilization, and conditioning

Phytoremediation is essentially an agronomic approach and its success depends ultimately

on agronomic practices applied at the site. The importance of employing effective agronomic

practices has been discussed by Chaney et al. (1999). These authors investigated the effect of soil

acidification on Zn and Cd phytoextraction and proposed the use of (NH4)2SO4 as a soil additive

to provide nutrients (N and S) needed for high yield, and to acidify the soil for greater metal

bioavailability. It should be noted that there may be some negative side effects associated with

soil acidification. For example, due to increased solubility some toxic metals may leach into the

groundwater creating an additional environmental risk. Chaney et al. (1999) indicated that

following metal phytoextraction, soil can be limed to elevate the pH near a neutral value, so that

normal farm uses or ecosystem development could resume. However, premature liming may

increase soil capacity for metal binding and restrict the potential for phytoextraction. A similar

effect can be expected following the addition of organic fertilizers. In addition, the raising of pH

may stimulate the formation of metal hydroxy ions, such as ZnOH+ which is more strongly

sorbed to soil solids than the uncomplexed ions.

Phosphorus is a major nutrient, and plants respond favorably to the application of P

fertilizer by increasing biomass production. The addition of P fertilizer, however, can also inhibit

the uptake of some major metal contaminants, such as Pb, due to metal precipitation as

pyromorphite and chloro-pyromorphite (Chaney et al., 2000). This underlines the importance of

finding new approaches for P application. Such an alternative may be foliage application. This

method may lead to improvement of plant P status without inhibiting Pb mobility in soil. 

 Enhancing metal bioavailability with synthetic chelators

For some toxic metals such as Pb, a major factor limiting the potential for phytoextraction

is limited solubility and bioavailability for uptake into roots. One way to induce Pb solubility is

to decrease soil pH (McBride, 1994). Following soil acidification, however, mobilized Pb can

leach rapidly below the root zone. In addition, soluble ionic lead has little propensity for uptake

into roots. The use of specific chemicals, synthetic chelates, has been shown to dramatically

stimulate the potential for Pb accumulation in plants. These compounds prevent Pb precipitation

and keep the metal as soluble chelate-Pb complexes available for uptake into roots and transport

within plant. For example, addition of EDTA (ethylene-diamine-tetraacetic acid), at a rate of 10

mmol/kg soil, stimulated Pb accumulation in shoots of maize up to 1.6 % (Blaylock et al., 1997).

In a subsequent study, Indian mustard exposed to Pb and EDTA in hydroponic solution was able

to accumulate more than 1%Pb in dry shoots (Vassil et al., 1998). Another synthetic chelator,

HEDTA (hydroxyethyl- ethylenediamine-triacetic acid) applied at 2.0 g/kg soil contaminated

with 2,500 ppm Pb, increased Pb accumulation in shoots of Indian mustard from 40 ppm to
20

10,600 ppm (Huang and Cunningham, 1996). Accumulation of elevated Pb levels is highly toxic

and can cause plant death. Because of the toxic effects, it is recommended that chelates be

applied only after a maximum amount of plant biomass was produced. Prompt harvesting (within

one week of treatment) is required to minimize the loss of Pb-laden shoots. 

Blaylock et al. (1997), indicated that, in addition to Pb, chelate-assisted phytoextraction is

applicable to other metals. These authors indicated that application of EDTA also stimulated Cd,

Cu, Ni, and Zn phytoaccumulation. Chelate ability to facilitate phytoextraction was shown to be

directly related to its affinity for metals. For example, EGTA (ethylenebis (oxyethylenetrinitrilo)

tetraacetic acid) has a high affinity for Cd2+, but does not bind Zn2+. EDTA, HEDTA, and DTPA

(diethylene-triamine-pentaacetic acid) are selective for Zn. In fact, zinc binding by DTPA is so

strong that plants cannot use Zn from this complex and potentially suffer from Zn deficiency.

Sowing

The extent of metal extraction depends on the amount of plant biomass produced. An

important factor that controls biomass production is plant density (number of plants/m2). Density

affects both yield/plant and yield/ha. In general, higher density tends to minimize yield per plant

and maximize yield per hectare. Density is also likely to affect the pattern of plant growth and

development. For example, at higher stand density, plants will compete more strongly for light.

Thus, more resources (nutrients and energy) may be allocated for plant growth as opposed to

developmental processes (flowering). An extended growth period may be beneficial if plant

metal absorption and accumulation depend upon growth processes. Furthermore, the distance

between plants is likely to affect the architecture of the root system with possible further

implications on metal uptake. However, the effect of this interaction is unknown and awaits

investigation.

Crop rotation

Another agronomic principle, which has been neglected in phytoremediation research, is

crop rotation. Because of the proliferation of weeds, predators, and diseases, which can cause

significant yield reduction, crops, including those used for soil remediation, must be rotated. In

general, crops are rotated less frequently today than 30 years ago. From crop science, it can be

extrapolated that short-term (two to three years) monoculture (the use of the same species in

consecutive seasons), may be acceptable for metal phytoremediation. However, for longer-term

applications, as most metal phytoextraction projects are anticipated, it is unlikely that successful

metal cleanup can be achieved with only one remediative species used exclusively in

monoculture. Plant rotation is even more important when multiple crops per year are projected. 
21

Crop maintenance: pest control and irrigation

Weed control and irrigation are major crop maintenance practices. Weeds can be

controlled by mechanical or chemical methods. Herbicides can be applied before or after the

emergence of phytoremediating species. Application of pre-emergent herbicides ensures good

weed control, quick emergence, and establishment of selected plants. Post-emergent herbicides

control weeds that occur later in the growing season. Because metal uptake into roots depends

on the movement of soil solution from the bulk soil to root surface, maintaining an adequate soil

moisture is important. Depending on the local climate, irrigation may be required to achieve

adequate soil moisture. The volume of water delivered must be carefully considered. This

volume should compensate for losses due to evaporation and transpiration. Excessive water

delivery will not only inflate operational cost, but may also restrict root growth and depress metal

extraction rates. The method of irrigation must also be carefully considered. For example, when

delivered under low pressure directly to the soil, as dripping, losses due to evaporation are kept

to a minimum. In addition, this method will have little effect on air humidity. In contrast, water

delivered under pressure from a nozzle, will elevate air humidity and possibly inhibit leaf

transpiration. Since the movement of metal-containing sap from the root to the shoot depends on

transpiration, transport and rate of metal accumulation in shoots may be affected. Furthermore,

when applied under pressure, water losses due to evaporation are significant and add to the

operational cost.

Handling and disposal of contaminated waste

One concern associated with the application of phytotechnology is handling and disposal

of contaminated plant waste. The need to harvest contaminated biomass and possibly dispose of

it as hazardous waste subject to RCRA standards creates an added cost and represents a potential

drawback to the technology. One option is disposal of contaminated biomass to a regulated

landfill. To decrease handling, processing, and potential landfilling costs, waste volume can be

reduced by thermal, microbial, physical or chemical means. With some metals (Ni, Zn, and Cu)

the value of the reclaimed metal may provide an additional incentive for phytoextraction. Chaney

et al. (1999) proposed incineration of plant biomass to further concentrate the bio-ore. These

authors showed that the value of the metal recovered in the biomass was shown to offset the cost

of the technology. Furthermore, Watanabe (1997) showed that Zn and Cd recovered from a

typically contaminated site could have a resale value of $1,060/ha. 

Cost and time projections

Cost analysis of metal phytoextraction is hampered by a lack of information. In support of

this, to date no metal-contaminated site has been completely remediated with plants. Therefore,
22

available cost data are limited to short-term (two- to three-year-old) field studies. It is doubtful

that these results can be used to accurately estimate the cost of a full-scale project that can last as

long as 15 years. In addition, complexity argues against generic, and in favor of site-specific cost

analysis. Despite these limitations, several authors, have investigated the time-frame and cost of

metal phytoextraction. For example, Brown et al. (1995a) considered a soil contaminated with

400 mg kg-1 Zn, and a desired cleanup level of 40 mg kg-1. These authors used T. caerulescens in

their analysis and assumed a constant rate of uptake of 4,000 mg kg-1, and an annual yield of 10

t/ha. They estimated that it would take 18 growing seasons to remove excess Zn from the soil. In

a subsequent study, the cost of remediating a metal-contaminated soil by conventional

engineering techniques was estimated between $50 and $500 per ton (Cunningham and Ow,

1996). Thus, the price tag of remediating an acre of soil (3-foot-deep contamination), weighing

some 4,500 tons, would be in excess of $ 250,000. These authors estimated that growing a crop

on an acre of land can be accomplished at cost ranging from two to four orders of magnitude less

than current cost for soil excavation and burial. Salt et al. (1995) estimated that using

phytoextraction to cleanup one acre of soil to a depth of 50 cm will cost $60,000-100,000

compared to at least $400,000 for soil excavation and storage alone. 

Research needs

There is a need to optimize the agronomic practices to maximize the cleanup potential of

remediative plants. Since in many instances metal absorption in roots is limited by low solubility

in soil solution, it is important to further investigate the use of chemical amendments to induce

metal bioavailability. Significant results have been obtained in this area. However, there is a need

to find cheaper, environmentally benign chemical compounds with metal chelating properties. 

Research is also needed to identify phytoremediating species capable of being rotated to sustain

the rate of metal extraction. More information is also needed to optimize the time of harvest.

Plants should be harvested when the rate of metal accumulation in plants declines. This will

minimize the duration of each growth cycle and allow more crops to be harvested in a growing

season. The current status of agronomic practices as they apply to metal phytoextraction and

further research needs are assessed in Table 7 .
23

Table 7. Assessment of the current status of agronomic practices as they apply to metal phytoextraction and further

research needs.

 

Agronomic issue

 

Readiness1

 

 Purpose

 

Current status

 

 Research needs

Soil 

 mobilization

 

 1 

 

To control crop pests and condition

surface soil for seed germination.

 

Achieved via rototilling or plowing.

Information available on crop species

can be applied to remediating plants.

 

Research needed to determine whether

plowing may displace soil

contaminants to root inaccessible

depths.

 

Fertilization/ 

Conditioning

 

 3

 

Fertilizers are used to improve soil

nutrient supply and nutrient availability

for uptake into roots. 

 

Conditioners are used to improve soil

aeration and water holding capacity.

 

Soil fertilization has been researched

extensively and a wealth of

information is available in the

literature. However, with the exception

of Pb/P interaction little is known

about the effect of fertilizers/ 

conditioners on the mobility of toxic

metals and their bioavailability in soil. 

 

The effect of fertilizers/conditioners

on the chemistry of soil metals and

plants’ ability to absorb metals must 

be better understood. Of particular 

concern is the application of synthetic

chelates (themselves potentially toxic).

Research should clarify the fate of

these compounds in soil and

groundwater, as well as the effect of

chelate application on metal mobility

and uptake into plants. 

 

 Crop rotation

 

 4

 

To control crop pests such as weeds,

insects and diseases, and to manage

nutrient soil supply.

 

Today crops are rotated less 

frequently than 30 years ago. For

conventional crops, economics favor

the repeated use (rotation) of one or

two profitable species. However, in

phytoremediation, where two or more

crops per year are expected, species

rotation may prove a valuable crop

management tool. Very little is known

about the rotation of phytoremediating

plants.

 

Research must identify metal

accumulator species that can be

rotated within a specific application.
24

 

 Sowing

 

 2

 

The proper amount of seed must be

incorporated in soil to obtain a plant

stand of desired density.

 

A wealth of information exists on

sowing of crop species. This

knowledge can be applied to related

phytoremediating species. For

example, similar techniques would be

employed to sow canola and Indian

mustard.

 

It is not clear how plant density will

affect the rate of metal phytoextraction

and potential for biomass production.

These effects should be investigated.

 

 Irrigation

 

 2

 

To compensate for water loss due to

evaporation from soil and plant

transpiration.

 

A great deal of information exists on

the irrigation of crop species. This

information can be directly applied to 

plants grown for environmental

remediation.

 

The effect of various irrigation

methods on root growth and

expansion, and metal phytoextraction

rate must be better understood. 

 

 Weed control

 

 1

 

To suppress the growth of weeds to the

point that its effect on the rate of metal

extraction is minimal.

 

Mechanical and chemical methods are

the most common approaches.

Biological control (the use of natural

enemies to reduce weed population) is

emerging as an alternative.

 

RPMs must identify the appropriate

weed control strategy (e.g., herbicides

rotation) to prevent the buildup of

weed populations and maximize the

rate of metal phytoextraction.

 

 Harvesting

 

 3

 

Remove plant biomass loaded with metal

contaminants.

 

In general, techniques and equipment

are readily available.

 

Research must be conducted to

determine when to harvest. It is

important to harvest when the rate of

metal extraction starts declining. This

would allow more crops to be obtained

in a season and increase the amount of

metal removed. Appropriate timing

would also maximize the amount of 

biomass removed at harvest.

 

1) Readiness for field deployment: 1) very little research is needed in this area; 2) practice is ready for field deployment although research may improve

its effectiveness; 3) significant research is needed to efficiently apply the practice; 4) practice has not been investigated for metal extraction. 
25

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