Phytoremediation: Past Promises and Future Practices

Plant-Microbe Interactions

Microbial Biosystems: New Frontiers

Proceedings of the 8

International Symposium on Microbial Ecology

Bell CR, Brylinsky M, Johnson-Green P (eds)

Atlantic Canada Society for Microbial Ecology, Halifax, Canada, 1999.

Phytoremediation: past promises and future practises

M. J. Sadowsky

Department of Soil, Water, and Climate, University of Minnesota, St. Paul, Minnesota, 55108,

USA

ABSTRACT

Plant-based bioremediation technologies have received recent attention as strategies to

clean-up contaminated soils and water. These strategies have collectively been termed

phytoremediation and refer to the use of green plants and their associated microbiota for

the in situ treatment of soil, sediment, and ground water. Biologically based remediation

strategies, including phytoremediation, have been estimated to be four to 1000 times

cheaper, on a per volume basis, than current non-biological technologies. Compounds

targeted for phytoremediation strategies include heavy metals, chlorinated solvents,

polycyclic aromatic hydrocarbons, polychlorinated biphenyls, pesticides, munitions and

radionuclides. While some of these contaminants are more readily degraded or detoxified

than others, plants or their attendant rhizosphere microbes have been shown in several

instances to transform these compounds to some degree. The main types of

phytoremediation strategies used include the stimulation of non-specific and specific

authochthonous and zymogenous rhizosphere microorganisms (both bacteria and fungi) for

the accelerated biodegradation of herbicide and solvent contaminants, the use of

"hyperaccumulating" plants for remediation of soils contaminated with metals, and the use

of plants to transform soluble contaminants to less soluble or less toxic forms. The ultimate

goal of all phytoremediation technologies is to either remove the contaminant from the

affected area, a process termed phytodecontamination, or to stabilize the contaminant to

prevent movement or toxicological affects. Below ground phytodecontamination processes

are thought to chiefly rely on rhizosphere degradation activity (either plant enzyme-or

microbiologically-driven) to transform hazardous waste materials. Future biotechnological

strategies for enhancing phytoremediation include enlarging root mass to increase

adsorption area, using Agrobacterium rhizogenes, the direct genetic engineering of plants

for altered biodegradation potential, and the genetic engineering of rhizosphere

microorganisms. However, while phytoremediation processes hold great promise as means

to remediate contaminated soils and water, there are advantages and disadvantages

associated with these strategies that must be carefully considered. Whereas attractions of

phytoremediation processes include cost effectiveness and non-invasiveness, they require

relatively long periods of time, often require the disposal of toxic vegetation, are

ineffective at remediating sites containing pollutants located deep into the soil profile, do

not work on all contaminants, are sensitive to contaminant types and concentrations, may

end up producing secondary metabolites which are more toxic than parent compounds, and

in many instances don’t remove environmentally significant quantities of pollutants.

Remediation of soils, water, and sediments contaminated with organic and inorganic

pollutants is of major importance and concern. It has been estimated that it will require

Plant-Microbe Interactions

over $20 billion annually to clean-up contaminated sites in the United States and Europe

[5]. However, estimates of the costs of remediating contaminated soil and water vary

widely, depending on: (1) the location of the contaminant; (2)the chemical, physical and

biological properties of the contaminant; (3) whether the contaminated soil contains more

than one type of pollutant; (4) the degree of remediation desired; (5) subsequent disposal

requirements; and (6) the techniques used. For example

in situ

remediation techniques

have been estimated to cost $10-100/m

ex-situ

processes $30-$300/m

, and

in situ

soil

vitrification processes over $1,000/m

[15]. On the other hand, biologically based

remediation technologies, including phytoremediation, have been estimated to be 4 to1000

times cheaper, on a per volume basis, than current non-biological techniques [14].

Consequently, the lower cost of phytoremediation makes it an attractive alternative over

other existing technologies, and in many instances, cost will be the driving force behind

adoption of plant-based remediation on a large scale.

Biologically-based remediation strategies (bioremediation) have received much recent

attention as means to clean-up contaminated soils and water. Phytoremediation,

collectively referring to all plant-based remediation strategies, uses green plants to

remediate contaminated sites. Several features make phytoremediation an attractive

alternative to most currently practiced

in situ

and

ex situ

techniques. These include low

capital cost, relatively minor on-going maintenance costs, non-invasiveness, easy start-up,

high public acceptance, regulatory agency acceptance, and the techniques provide a

pleasant appearance to the landscape [5].

In the last several decades, phytoremediation strategies have been examined as a means

to clean-up a number of hazardous organic and inorganic pollutants, including: heavy

metals [11,28,38], chlorinated solvents [22,43], agrochemicals [1,24,27]; polycyclic

aromatic hydrocarbons [2,33], polychlorinated biphenyls [7,18], munitions [39] and

radionuclides [20]. Those soluble organic and inorganic contaminants, whhich move into

plant roots or the rhizosphere by the processes of mass flow or diffusion, appear to be the

most amenable to phytoremediation technologies [14,15,39]. In several instances, plants

and/or their attendant rhizosphere microbes have been shown to transform these

compounds to some degree [13,41,43]. Diverse plant species show great promise as

phytoremediation agents. These plants include: grasses, legumes, trees and several other

monocots and dicots [9,11,14,19,38,39]. Several different species of aquatic plants also

appear to be useful for phytoremediating contaminated surface water [34].

Phytoremediation technologies can be directed to above or below ground contaminants

and either remove pollutants from the affected area (phytodecontamination) or stabilize

them to prevent off-site movement (phytosequestration or phytostabilization). These later

techniques are useful for contaminants having low biodegradation potential or those which

rapidly move into the soil profile. Below ground phytostabilization processes involve the

sequestration of contaminants into soil particles, cell wall lignins, or into the soil humus

fraction [14] and reduce the bioavailability of contaminants [38]. Below ground

phytodecontamination processes, on the other hand, often rely on rhizosphere degradation

activity (either plant enzyme- or microbiologically-driven) to transform hazardous waste

materials (see below). In addition, several of these processes can occur

ex planta

planta

Ex planta

phytoremediation processes refer to those driven by the action of plant-

or microbially-derived soil enzymes [39] or by plant-associated microorganisms

[1,4,12,22,26,35,37]. While not yet used on field scale levels, enzymes responsible for

Plant-Microbe Interactions

planta

soil enzyme biodegradation (dehalogenases, nitroreductases, nitrilases, peroxidases,

and laccases) have been investigated in some detail [5,39]. Cell free enzyme systems,

whether added to the soil or excreted from plant roots, may hold particular promise in

environments that are adverse for the growth and persistence of microorganisms [4].

The in

planta

phytoremediation processes require that the pollutant is taken up into the plant.

Pollutants are taken up by roots, and either sequestered or translocated to shoots and leaves

[17]. Plants usually uptake organic compounds in the aqueous phase, by diffusion or mass

flow processes, although in some instances vapor phase transport can occur [14]. For many

organic compounds, root uptake has been shown to be proportional to

, the n-

octanol/water partition coefficient [8,15]. Organic and inorganic compounds can be

transported to other portions of the plant apoplastically or symplastically [38]. Ultimately,

the compound is either metabolized within the plant by conjugation to glutathione [21],

sequestered, or transpired from the plant.

Sequestration of pollutants within plants is the basis for phytoextraction of soils and

water contaminated with heavy metals [28,32]. Metals targeted for this type of

phytoremediation process include Cd, Pb, Zn, Cu, Cr, Ni, Se, and Hg [see 11,14].

Phytoextraction, using "hyperaccumulating" plants is proving to be one of the most

effective phytoremediation methods to clean-up metal contaminated soils and water [3].

Several plant species, including

Thlaspi caerulescens

have been shown to accumulate very

high levels of Zn and Cd from soils [3].

Brassica juncea

has also been found to be an

excellent accumulator plant for metals in soils, such as Cd, Cr, Ni, Zn, and Cu [28,38] and

several plant species have been shown to accumulate Pb [15,19,25]. Plants, such as

Eichhornia crassipes

Hydrocotyle umbellata

Lemna minor

Scirpus lacustris

Phragmites

karka

Bacopa monnieri

, and

Azolla pinnata

and are also effective at removing metals

from aquatic systems [see 10,38]. Plant shoots and roots containing metals are

subsequently harvested and treated as hazardous waste or the metals are recovered as ore.

Ex planta

phytoremediation can also occur via the degradative activity of rhizosphere

microorganisms. The rhizosphere is operationally defined as the "soil-root interfacial area"

and relatively large numbers of diverse species of microorganisms live in association with

plant roots [16]. The word "rhizosphere", first introduced by Hiltner in 1904 [23] to

describe the interaction between bacteria and the roots of legumes, has been operationally

defined to mean many things to many researchers [29]. The rhizosphere consists of the

endorhizosphere (various cell layers of the root), ectorhizosphere (the immediate soil area

surrounding the root) and the rhizoplane (the root surface) [29]. Microorganisms colonize

and live within these areas and the degree of intimacy at which a microorganism interacts

with a root varies in proportion to the distance from the root surface. The closer a microbe

is to the root surface, the more its growth and behavior is influenced by plant-released

materials [36]. The intimacy of the association between soil microorganisms and plant

roots is determined, in part, by the types and concentrations of compounds exuded by roots

[35]. Root exudations are thought to have a stimulatory affect on rhizosphere microbes,

which in turn, are purported to accelerate biodegradation in the rhizosphere

[1,2,13,22,30,33,41]. While there has been much discussion on the usefulness of this

technology, the reader is cautioned to examine published studies carefully, since in many

instances reported enhanced biodegradation is relatively small, environmentally

insignificant, or occurs at rates not suitable for field use. Moreover, inconsistent results

have been reported from several studies, using various plants and microorganisms and

Overview

Phytoremediation: Past Promises and Future Practices

Phytoremediation explores the use of plants and their associated microorganisms to remediate contaminated soils and water. Authored by M. J. Sadowsky, this work delves into the effectiveness and cost-efficiency of plant-based bioremediation strategies compared to traditional methods. It discusses various contaminants, including heavy metals and organic pollutants, and outlines the mechanisms through which plants can detoxify or stabilize these substances. The document also highlights the potential advancements in phytoremediation technologies and the challenges that remain. This resource is valuable for environmental scientists, ecologists, and students studying bioremediation techniques. Key Points Explores the effectiveness of phytoremediation for heavy metals and organic pollutants…

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FAQs

What is phytoremediation and how does it work?

Phytoremediation is a bioremediation technology that utilizes green plants and their associated microorganisms to clean up contaminated soils and water. It involves processes such as phytodecontamination, where plants absorb and detoxify pollutants, and phytostabilization, which prevents contaminants from migrating. Different plant species can target various contaminants, including heavy metals and organic compounds, through their root systems and rhizosphere interactions.

What types of contaminants can be addressed through phytoremediation?

Phytoremediation can effectively address a range of contaminants, including heavy metals like cadmium, lead, and mercury, as well as organic pollutants such as chlorinated solvents and pesticides. The document outlines how specific plant species, including hyperaccumulators, can absorb and concentrate these harmful substances from the soil or water, making them easier to manage and remove.

What are the advantages of using phytoremediation over traditional methods?

Phytoremediation offers several advantages over traditional remediation methods, including lower costs, non-invasiveness, and the ability to enhance the aesthetic value of contaminated sites. It is estimated to be four to 1000 times cheaper than conventional techniques on a per volume basis. Additionally, phytoremediation can improve soil health and restore ecosystems while effectively removing or stabilizing pollutants.

What challenges does phytoremediation face?

Despite its potential, phytoremediation faces several challenges, including the time required for effective remediation and its limitations with deep soil contamination. Some contaminants may not be effectively removed or could transform into more toxic forms during the process. Moreover, the disposal of toxic vegetation after remediation can pose additional environmental concerns.

How do rhizosphere microorganisms contribute to phytoremediation?

Rhizosphere microorganisms play a crucial role in enhancing phytoremediation by interacting with plant roots and aiding in the degradation of contaminants. These microorganisms can break down pollutants through enzymatic processes, thereby increasing the overall effectiveness of phytoremediation. The document discusses how root exudates from plants stimulate microbial activity, leading to improved biodegradation rates.

What future advancements are expected in phytoremediation technology?

Future advancements in phytoremediation technology may include genetic engineering of plants to enhance their ability to absorb and detoxify contaminants. Researchers are exploring methods to increase root mass and improve the efficiency of rhizosphere microorganisms. These innovations aim to make phytoremediation a more viable option for large-scale environmental cleanup efforts.