Natural and Enhanced Remediation Systems - Chapter 5 pdf

pro-5.1 INTRODUCTIONPhytoremediation is defined as “the engineered use of plants in situ and ex situ for environmental remediation.” The technology involves removing or degradingorganic

Suthersan, Suthan S “Phytoremediation”

Natural and Enhanced Remediation Systems

Edited by Suthan S SuthersanBoca Raton: CRC Press LLC, 2001

5.2.2 Organics5.3 Types of Phytoremediation 5.3.1 Phytoaccumulation5.3.2 Phytodegradation5.3.3 Phytostabilization5.3.4 Phytovolatilization5.3.5 Rhizodegradation5.3.6 Rhizofiltration5.3.7 Phytoremediation for Groundwater Containment5.3.8 Phytoremediation of Dredged Sediments5.4 Phytoremediation Design

5.4.1 Contaminant Levels 5.4.2 Plant Selection5.4.3 Treatability5.4.4 Irrigation, Agronomic Inputs, and Maintenance5.4.5 Groundwater Capture Zone and Transpiration Rate References

… many accepted agricultural techniques for cultivating, harvesting, and 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….

pro-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 degradingorganic and inorganic contaminants and metals from soil and water The processesinclude all plant-influenced biological, chemical, and physical processes that aid inthe uptake, sequestration, degradation, and metabolism of contaminants, either byplants or by the free living organisms that constitute a plant’s rhizosphere Phytore-mediation takes advantage of the unique and selective uptake capabilities of plantroot systems, together with the translocation, bioaccumulation, and contaminantstorage and degradation capabilities of the entire plant body

The concept of using plants to alter the environment has been around since plantswere first used to drain swamps What is new within the context of this newtechnology called phytoremediation is the systematic, scientific investigation of howplants can be used to decontaminate soil and water.1 Interest in phytoremediationhas been growing in the U.S during the past few years with potential applicaton ofthis 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 theproblems it might address One reason this technology is gaining attention is because

it is potentially cheaper than conventional treatment approaches for contaminatedsoils and traditional pump and treat systems for contaminated groundwater, such asincineration or soil washing Another attraction of this technology is that it mayleave topsoil in usable condition, keeping soil fertility and structure intact whilereducing contamination levels at the same time Phytoremediation is well suited forapplications in low permeability soils, where most currently used technologies have

a low degree of feasibility or success, as well as in combination with more tional remediation technologies

conven-The main advantages of phytoremediation are the low capital costs, aestheticallypleasing 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 plantgrowth In the case of heavy metals remediation, additional operational costs includeharvesting, disposal of contaminated plant mass, and repeating the plant growth cycle

It should be emphasized that there is more to phytoremediation than merelyputting plants in the ground and letting them do the work Phytoremediation alsohas 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 andmanaged as a waste This vegetation containing high concentrations of toxic metalsand organics may also pose a risk to wildlife The shutdown of plant activity duringwinter months and the seasonal variation of plant metabolic activity is a drawbackfor application of this technology in colder climates Other limitations of phytore-mediation are that contaminants present below rooting depth will not be treated orextracted and that the plant or tree may not be able to grow in soils at heavilycontaminated sites due to plant toxicity

Phytoremediation as a technology is still in its early stages While many tists, engineers, and regulators are optimistic that it will eventually be used to clean

scien-up organic and metallic contaminants, at least two or three more years of field testsand analyses are necessary to validate the initial, small-scale field tests.1,2 Issues likesoil characteristics and length of the growing season will need to be taken intoaccount and scientists must also determine what sites are most amenable to phy-toremediation Other issues such as the potential impact on wildlife remain to befully explored Simultaneously, researchers working in the lab are trying to betterunderstand 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 coverthe rapidly progressing state of the science and also describe how these scientificadvances 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 thecurrent status of the technology and its applications, as well as activities of theresearch 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 loids Availability is determined by characteristics of the elements, such as behavior

metal-of the ion as a Lewis acid (electron acceptor) which determines the predominanttype of strength of bond created (ionic or covalent) and, therefore, the mobility ofthe metal in the soil environment Soil characteristics (e.g., pH, clay and organicmatter content and type, and moisture content) also determine availability to plants

by controlling speciation of the element, temporary immobilization by particlesurfaces (adsorption-desorption processes), precipitation reactions, and availability

in soil solution The most general sinks for metals are iron and manganese oxidesand organic matter Although particulate soil organic matter serves to immobilizemetals, soluble organic matter may act to keep metals in solution in a form absorbedand translocated by plants

Metal fractionation or sequential extraction schemes — such as toxicity teristic leaching procedure (TCLP) — sometimes are used to describe metal behavior

charac-in soils Most metals charac-interact with the charac-inorganic and organic matter that is present

in the root-soil environment Potential forms of metals include those dissolved inthe soil solution, adsorbed to the vegetation’s root system, adsorbed to insolubleorganic 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 theselective absorption from soil solution by the root Metals may be bound to exteriorexchange sites on the root and not actually taken up They may enter the rootpassively in organic or inorganic complexes with the mass flow of water or actively

by way of metabolically controlled membrane transport systems often meant to take

up a nutrient which the “contaminant” metal mimics At different soil solute centrations, metals may be absorbed by both processes Absorption mechanisms andquantity absorbed are influenced by plant species (and cultivar), growth stage,physiological state, and the presence of other elements

con-Once in the plant, a metal can be sequestered in the roots in vacuoles or inassociation with cell walls and organelles, or translocated to above ground parts inxylem as organic or inorganic complexes Location and forms of metals in plants,

as well as their toxic effects, depend on plant species, growth stage, physiologicalstate, and presence of other metals

Mechanisms of toxicity of metals tend to be dependent on the nature of thereactivity 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 otherplant parts, and formation of unusable complexes containing metals that may oth-erwise be inserted into biomolecules instead of the proper element (e.g., Asreplacing P)

5.2.2 Organics

Organic compounds of environmental concern include nonionic compounds(such as PAHs, chlorinated benzenes, polychlorinated biphenyls (PCBs), BTEXcompounds, and many pesticides), ionizable compounds (chlorophenols, carboxylicacids, 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., soilorganic matter) Sorption of the compound by soil organic matter is reversible Theactivities of these compounds in soil can be predicted by the organic matter-watercoefficient, Kom, as estimated by the octanol-water coefficient, Kow.3 Absorption ontocolloidal organic matter in solution may alter the availability of these nonioniccompounds Ionizable compounds contain anionic or cationic moieties or both withintheir structure These charged structures interact with organic and inorganic chargedsurfaces in the soil in a variety of reversible reactions The extent and nature of theassociations 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 tometabolites 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 ular weight less than 500) except on the basis of polarity.1-4 More water-solublemolecules pass through the root epidermis and translocate throughout the plant Theless soluble compounds (like many polycyclic aromatic hydrocarbons) seem to havelimited entry into the plant and minimal translocation once inside Highly lipophilic

(molec-compounds, such as PCBs, move into the plant root via the symplastic route (fromcell 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 thecell Many contaminants become bound on the root surface and are not translocated.Not all organic compounds are equally accessible to plant roots in the soilenvironment The inherent ability of the roots to take up organic compounds can bedescribed by the hydrophobicity (or lipophilicity) of the target compounds Thisparameter is often expressed as the log of the octanol-water partioning coefficient,

Kow Direct uptake of organics by plants is a surprisingly efficient removal mechanismfor moderately hydrophobic organic compounds There are some differencesbetween the roots of different plants and under different soil conditions, but, gen-erally, the higher a compound’s log Kow, the greater the root uptake

Hydrophobicity also implies an equal propensity to partition into soil organicmatter and onto soil surfaces Root absorption may become difficult with heavilytextured soils and soils with high native organic matter There are several reportedvalues available in the literature regarding the optimum log Kow value for a compound

to be a good candidate for phytoremediation (as an example, log Kow = 0.5–3.0; log

Kow = 1.5–4.0).2,13 It has also been reported that compounds that are quite watersoluble (log Kow < 0.5) are not sufficiently sorbed to the roots or actively transportedthrough plant membranes

From an engineering point of view, a tree could be thought of as a shell of livingtissue encasing an elaborate and massive chromatography column of twigs, branches,trunk, and roots The analogous resin in this system is wood, the vascular tissue ofthe tree, and this “resin” is replenished each year by normal growth Wood iscomposed of thousands of hollow tubes, like the bed of a hollow fiber chromatog-raphy column, with transpirational water serving as the moving phase The hollowtubes are actually dead cells, whose death is carefully programmed by the tree toproduce a water conducting tissue, which also functions in mechanical support Acomplex, 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) thechemical 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 tononphytotoxic metabolites, including lignin, that are stored in various places in plantcells Many of these metabolic capacities tend to be enzymatically and chemicallysimilar to those processes that occur in mammalian livers; one report has equatedplants to” green livers” due to similarities of detoxification processes

Different plants exhibit different metabolic capacities This is evident during theapplication of herbicides to weeds and crops alike The vast majority of herbicidalcompounds have been selected so that the crop species are capable of metabolizingthe pesticide to nontoxic compounds, whereas the weed species either lack thiscapacity or perform it at too slow a rate The result is the death of the weed specieswithout the metabolic capacity to rid itself of the toxin

The shear volume and porous structure of a tree’s wood provide an enormoussurface 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 wasteremediation 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 withfield and pilot studies that represent the current proof of concepts The technology

is summarized below as those approaches ready for application, promising treatmentsexpected to be tested soon, and concepts of phytoremediation requiring intensivedevelopment Finally, the intrinsic strengths of phytoremediation as a technologyand 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 currentunderstanding 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 tion’s ability to gain wider acceptance as a presumptive remediation technique With

phytoremedia-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

Phytostabilization Rhyzofiltration Phytoaccumulation Phytovolatilization

Remediated Contaminant

the possible exception of some of the above mechanisms that are already widelystudied and understood, all of phytoremediation’s major applications require furtherbasic and applied research in order to optimize field performance Significantresearch and development should be carried out to 1) obtain a better understanding

of mechanisms of uptake, transport, and accumulation of contaminants; 2) improvecollection and genetic evaluation of hyperaccumulating plants; and 3) obtain a betterunderstanding of interactions in the rhizosphere interactions among plant roots,microbes, and other biota

Short of true regulatory reform, phytoremediation’s ability to make furtherinroads will depend on how quickly federal, state, and local regulators becomeconvinced of the technology’s efficacy While not involved in every decision makingprocess, the public is sometimes a key constituency as well One can expect publicinterest groups to be more concerned about efficacy and safety issues than cost orother economic factors However, phytoremediation seems to be faring well withthe general public and, according to many practitioners, has already proven popularwith neighbors and other interested parties at field remediation sites

5.3.1 Phytoaccumulation

Remediation of contaminated soils using nonfood crops, called tion, has attracted a great deal of interest in recent years Also called phytoextraction,phytoaccumulation, refers to the uptake and translocation of metal contaminants inthe soil by plant roots into the above ground portions of plants.2 Certain plants,called hyperaccumulators, absorb unusually large amounts of metals in comparison

phytoaccumula-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, whichdiminishes the risk of impoverishing the soil of nutrient elements The property ofhaving a high specific uptake must be genetically stable Since the removal of metalsfrom the soil is actually achieved through the harvest, it is necessary that the planthave a high transport of the metal(s) from the roots to the shoots to be effectiveduring remediation applications In addition, a high biomass production of the

Table 5.1 The Number of Taxonomic

Groups of Hyperaccumulators Varies According to Which Metal

phytoaccumulator is needed for high removal of metals per unit area It is also anadvantage if biomass production is of economic interest.

Hyperaccumulators have been preferred during phytoaccumulation applicationsbecause they take up very large amounts of a specific metal They are often endemicand of a specific population (genotypes/clones) of a species.5 However, these plantsseldom have high biomass production and may also have low competitive ability inless polluted areas, probably because the plant uses its energy to tolerate such highlevels 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 metalevenly distributed throughout the plant.6

There are also high accumulators that accumulate somewhat lower metal centrations than hyperaccumulators but much more than “normal” plants Theyusually 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 accumulationeither in the roots or in the shoots These plants are selected and planted at a sitebased on the type of metals present and other site conditions After they have beenallowed to grow for several weeks or months, they are harvested

con-Landfilling, incineration, and composting are options to dispose of or recyclethe metals, although this depends upon the results of TCLP and cost Planting andharvesting of plants may be repeated as necessary to bring soil contaminant levelsdown to allowable limits A plan may be required to deal with the plant biomasswaste 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 methodand 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 andstalk biomass (LSB) about 100 times more than nonaccumulator plants growing inthe same soil, with some species and metal combinations exceeding conventionalplant levels by a factor of more than 1000.8

Many hyperaccumulator plants, which are nonwoody (not a tree), have beenidentified as having the capacity to accumulate metals Thlaspi caerulascens wasfound to accumulate Zn up to 2000–4000 mg/kg.9 The Indian mustard plant Brassica

significant amounts of lead.10 One planting of mustard in a hectare of contaminatedland was found to soak up two metric tons of lead If three plantings could besqueezed in per year, six tons of lead per hectare can be extracted Both hempdogbane (Apocynum sp.) and common ragweed also have been observed to

accumulate significant levels of lead Aeollanthus subcaulis var lineris and Papsalum

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 treescan be used for phytoextraction of heavy metals These trees can accumulate theheavy metals by sequestration However, there are concerns specifically for treesthat include leaf litter and associated toxic residues being blown off site This concernmay be tested in the laboratory to see whether uptake and translocation of the metalsinto the leaves exceed standards

Hyperaccumulators have metal accumulating characteristics that are desirable,but lack the biomass production, adaptation to current agronomic techniques, andphysiological adaptations to climatic conditions required at many contaminated sites

It has been reported that harvesting at different seasons in a year had pronounceddifferences in accumulation levels In the future, genetic manipulation techniquesmay provide better hyperaccumulator species The success of phytoextractiondepends on the use of an integrated approach to soil and plant management: thedisciplines of soil chemistry, soil fertility, agronomy, plant physiology, and plantgenetic engineering are currently being used to increase the rate and efficiency ofheavy metal phytoextraction

Chelates have been used not only to enhance metal uptake but also to avoidmetal toxicity Metal accumulator plants have been studied extensively for organo-metallic complexes It has been suggested that there is a relationship between metaltolerance and carboxylic acids Organo-metallic complexes increase the translocationand tolerance of plants to the toxic effects of metals For example, in Sebertia

phytotoxic Ni from root systems to the leaves until leaf fall.5,6 It has also beensuggested that in copper (Cu) and cobalt (Co) accumulator plants, Co existed as anoxalate complex within the leaf The formation of Zn–citrate complexes in Zn-tolerant plants was the reason for high levels of organic acid accumulation Reportshave indicated that histidine was responsible for accumulation, tolerance, and trans-port to shoots in nonaccumulating and hyperaccumulating (Ni) plant species.11 In

majority of Zn in the roots was coordinated with histidine, whereas organic acidswere 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 (C2O4)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 tometal contaminated soils will significantly increase the metal concentrations in plantshoots and roots 5 However, the application of these chelates during a full scaleremediation application has to be carefully controlled; if not, the increased solubility

of the metal chelates formed could drive these contaminants to migrate furtherdownward by leaching when plant uptake rates are not adequate Controlling the

pH and conditioning the soils for optimum pH is an important factor when dealingwith metals-contaminated soils

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 forease of harvesting After harvesting, a proper, regulartorily acceptable biomassprocessing step or disposal methods should be implemented

5.3.2 Phytodegradation

Phytodegradation, also called phytotransformation, is the breakdown of inants taken up by plants through metabolic processes within the plant, or thebreakdown 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 endproducts are rereleased to the environment depending on the contaminant and plantspecies (phytovolatilization) (Figure 5.3)

contam-Plants synthesize a large number of enzymes as a result of primary and secondarymetabolism and can quickly uptake and metabolize organic contaminants to lesstoxic 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.

growth conditions resulting from contamination of the soils Plant-formed enzymesthat are useful for phytodegradation are nitroreductases (for munitions and pesti-cides); dehalogenases (for chlorinated solvents and pesticides); phosphatases (forpesticides); peroxidases (for phenols); laccases (for aromatic amines); cytochromeP-450 (for pesticides and chlorinated solvents); nitrilase (for herbicides).

Plant transformation pathways can be of many different types and obviouslydepend on plant species and tissue type In simplistic terms, these pathways can becategorized as reduction, oxidation, conjugation, and sequestration The “green livermodel” has been proposed to describe the metabolic pathways of herbicides, pesti-cides, explosives, and other nitroaromatic compounds Contaminant degradation byplant-formed enzymes can occur in an environment free of microorganisms (forexample, an environment in which the microorganisms have been killed by highcontaminant levels) Thus, phytodegradation potentially could occur in soils wherebiodegradation cannot

The current state of science in phytodegradation (phytotransformation) is marized below:1,2

sum-• 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.

2

H O, Nutients, O

Phytodegradation

- Metabolism within the plant

- Production of enzymes which

help to catalyze degradation

• Mass balance and pathway analyses studies have been conducted to prove plete degradation; potential toxicity of intermediate compounds also can be pre- dicted.

com-• Differentiation between degradation by plant enzymes, rhizosphere isms, and other breakdown processes is being performed.

microorgan-• Development of engineered solutions based on the use of monocultures vs ticultures found in wetlands and terrestrial communities is being further investi- gated.

mul-• 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 Kow 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 alsobeen identified in other algae, ferns, monocots, dicots, and trees

Degradation of TCE has been detected in hybrid poplars and in poplar cellcultures, resulting in production of metabolites and in complete mineralization of asmall portion of the applied TCE.12,14 Poplars have been used to remove atrazineand inorganic nutrients.2 Black willow (Salix nigra), yellow poplar (Liriodendron tulipifera), bald cypress (Taxodium diskchum), river birch (Betula nigra), cherry barkoak (Quercus falcata), and live oak (Quercus viginiana) have been known to supportdegradation of herbicides.13 One recent study demonstrated that poplar trees, whichpossess 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 metaboliteswere found in the roots and tissue of the study trees, but not in control trees or inthe soil used for potting the trees In a subsequent study, poplar seedlings exposed

to 14C-labeled TCE were found to generate 14C-labeled carbon dioxide Intermediatecompounds generated during oxidation are thought to be 2,2,2-trichloroethanol, anddi- and trichloroacetic acid Similar studies have shown positive results for tolueneand benzene

A recent study using parrot feather showed positive results for tion of perchlorate at concentrations of up to 20 ppm.22 Based on the results of theseexperiments and ecological knowledge of parrot feather, this species is an excellentcandidate for future research on in situ phytoremediation of contaminated waterbodies Parrot feather also is a good candidate for phytoremediation of contaminatedgroundwater temporarily held in artificial ponds

phytotransforma-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, tion onto roots, or precipitation within the root zone and physical stabilization ofsoils It is also used as a means to stabilize contaminated soil by decreasing wind

adsorp-and water erosion adsorp-and to decrease water infiltration adsorp-and the subsequent leaching ofcontaminants This process reduces the mobility of the contaminant and preventsmigration to the groundwater or air This technique can be used to re-establish avegetative cover at sites where natural vegetation is lacking due to high metalconcentrations Metal-tolerant species may be used to restore vegetation to suchsites, thereby decreasing the potential migration of contamination through winderosion, transport of exposed surface soils, and leaching of soil contamination togroundwater.

Implementation of phytostabilization involves reduction in the mobility of heavymetals and high molecular weight organics by minimizing soil erodibility, decreasingthe potential for wind blown dust, and reduction in contaminant solubility by theaddition of soil amendments Containment using plants either binds the contaminants

to the soil, renders them nonavailable, or essentially immobilizes them by removingthe means of transport

Erosion leads to the concentration of heavy metals because of selective sortingand deposition of different size fractions of the soil Eroded material is often trans-ported over long distances, thus selectively extending the effects of contaminationand increasing the risk to the environment Erosion can, therefore, cause the build

up of concentrations of normally nontoxic contaminants to toxic levels at locationswhere transported material is deposited

Planting of vegetation at contaminated sites, particularly abandoned strip miningsites, 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 plantshelp 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 metalsinsoluble and unavailable to leaching Materials with a calcareous character or ahigh pH, such as lime and gypsum, can be added to influence the acidity Specificbinding 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 materialssuch as humus or peat materials, manure, or mulch can be added

This chemical alteration should be quickly followed by establishing a plant coverand maximizing plant growth The amendments sequester the metals into the soil matrixand 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 theatmosphere from the plant Phytovolatilization occurs as growing trees and otherplants 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 spired by a plant are subject to phytodegradation

tran-Thus far, phytovolatilization has mainly been applied to groundwater nation However, the potential exists for application to soil, sediments, and othercontamination and needs some careful applications.2 The state of science with respect

contami-to phycontami-tovolatization 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 tions, and precipitation vs groundwater use need to be developed.

varia-• Significant research needs to be focused on modeling impacts of vegetation such

as transpiration stream concentration factors, canopy effects, and root tion factors.

concentra-5.3.5 Rhizodegradation

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/orbacteria) consume and degrade or transform organic substances for use as nutrientsubstances Certain microorganisms can degrade organic substances such as fuels

or solvents that are hazardous to humans and ecoreceptors and convert them intoharmless products through biodegradation Natural substances released by plant roots

— such as sugars, alcohols, and acids — contain organic carbons that act as nutrientsources for soil microorganisms; these additional nutrients stimulate their activity.Rhizodegradation is aided by the way plants loosen the soil and transport oxygenand 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 gramnegative rods (specifically Pseudomonas, Flavobacterium, and Alcaligens) are most