Suthersan, Suthan S. “Phytoremediation” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 5 Phytoremediation CONTENTS 5.1 Introduction 5.2 Chemicals in the Soil–Plant System 5.2.1 Metals 5.2.2 Organics 5.3 Types of Phytoremediation 5.3.1 Phytoaccumulation 5.3.2 Phytodegradation 5.3.3 Phytostabilization 5.3.4 Phytovolatilization 5.3.5 Rhizodegradation 5.3.6 Rhizofiltration 5.3.7 Phytoremediation for Groundwater Containment 5.3.8 Phytoremediation of Dredged Sediments 5.4 Phytoremediation Design 5.4.1 Contaminant Levels 5.4.2 Plant Selection 5.4.3 Treatability 5.4.4 Irrigation, Agronomic Inputs, and Maintenance 5.4.5 Groundwater Capture Zone and Transpiration Rate References … many accepted agricultural techniques for cultivating, harvesting, and pro- cessing plants have now been adapted for phytoremediation. Overall, the appli- cation of phytoremediation is being driven by its technical and economic advan- tages over conventional approaches … .phytoremediation’s future is not a scientific issue, but rather a “scientific sociology” issue…. ©2001 CRC Press LLC 5.1 INTRODUCTION Phytoremediation is defined as “the engineered use of plants in situ and ex situ for environmental remediation.” The technology involves removing or degrading organic and inorganic contaminants and metals from soil and water. The processes include all plant-influenced biological, chemical, and physical processes that aid in the uptake, sequestration, degradation, and metabolism of contaminants, either by plants or by the free living organisms that constitute a plant’s rhizosphere. Phytore- mediation takes advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant storage and degradation capabilities of the entire plant body. The concept of using plants to alter the environment has been around since plants were first used to drain swamps. What is new within the context of this new technology called phytoremediation is the systematic, scientific investigation of how plants can be used to decontaminate soil and water. 1 Interest in phytoremediation has been growing in the U.S. during the past few years with potential applicaton of this technology at a wide range of sites contaminated with heavy metals, pesticides, explosives, and solvents. The potential benefits of phytoremediation seem to be as numerous as the problems it might address. One reason this technology is gaining attention is because it is potentially cheaper than conventional treatment approaches for contaminated soils and traditional pump and treat systems for contaminated groundwater, such as incineration or soil washing. Another attraction of this technology is that it may leave topsoil in usable condition, keeping soil fertility and structure intact while reducing contamination levels at the same time. Phytoremediation is well suited for applications in low permeability soils, where most currently used technologies have a low degree of feasibility or success, as well as in combination with more conven- tional remediation technologies. The main advantages of phytoremediation are the low capital costs, aesthetically pleasing technique, minimization of leaching of contaminants, and soil stabilization. The operational cost of phytoremediation is also substantially less than that of conven- tional treatments and involves mainly fertilization and watering for maintenance of plant growth. In the case of heavy metals remediation, additional operational costs include harvesting, disposal of contaminated plant mass, and repeating the plant growth cycle. It should be emphasized that there is more to phytoremediation than merely putting plants in the ground and letting them do the work. Phytoremediation also has its drawbacks, which even its ardent champions are quick to acknowledge. First of all, it is a time-consuming process that can take several growing seasons to clean a site. Vegetation that absorbs toxic heavy metals will have to be harvested and managed as a waste. This vegetation containing high concentrations of toxic metals and organics may also pose a risk to wildlife. The shutdown of plant activity during winter months and the seasonal variation of plant metabolic activity is a drawback for application of this technology in colder climates. Other limitations of phytore- mediation are that contaminants present below rooting depth will not be treated or extracted and that the plant or tree may not be able to grow in soils at heavily contaminated sites due to plant toxicity. ©2001 CRC Press LLC Phytoremediation as a technology is still in its early stages. While many scien- tists, engineers, and regulators are optimistic that it will eventually be used to clean up organic and metallic contaminants, at least two or three more years of field tests and analyses are necessary to validate the initial, small-scale field tests. 1,2 Issues like soil characteristics and length of the growing season will need to be taken into account and scientists must also determine what sites are most amenable to phy- toremediation. Other issues such as the potential impact on wildlife remain to be fully explored. Simultaneously, researchers working in the lab are trying to better understand the processes behind phytoremediation to possibly improve its perfor- mance during cleanup applications. This chapter will not do justice to this technology by claiming that it will cover the rapidly progressing state of the science and also describe how these scientific advances are being applied in the field for efficient remediation. Instead it will serve as a brief state of the science summary that will allow the reader to understand the current status of the technology and its applications, as well as activities of the research community to further enhance this technology. 5.2 CHEMICALS IN THE SOIL–PLANT SYSTEM 5.2.1 Metals Elements occur in the soil in a variety of forms more or less available for uptake by plants. Many of the contaminants of concern at waste sites are metals or metal- loids. Availability is determined by characteristics of the elements, such as behavior of the ion as a Lewis acid (electron acceptor) which determines the predominant type of strength of bond created (ionic or covalent) and, therefore, the mobility of the metal in the soil environment. Soil characteristics (e.g., pH, clay and organic matter content and type, and moisture content) also determine availability to plants by controlling speciation of the element, temporary immobilization by particle surfaces (adsorption-desorption processes), precipitation reactions, and availability in soil solution. The most general sinks for metals are iron and manganese oxides and organic matter. Although particulate soil organic matter serves to immobilize metals, soluble organic matter may act to keep metals in solution in a form absorbed and translocated by plants. Metal fractionation or sequential extraction schemes — such as toxicity charac- teristic leaching procedure (TCLP) — sometimes are used to describe metal behavior in soils. Most metals interact with the inorganic and organic matter that is present in the root-soil environment. Potential forms of metals include those dissolved in the soil solution, adsorbed to the vegetation’s root system, adsorbed to insoluble organic matter, bonded to ion exchange sites on inorganic soil constituents, precip- itated or coprecipitated as solids, and attached to or inside the soil biomass. The final control on availability of metals and metalloids in soil to plants is the selective absorption from soil solution by the root. Metals may be bound to exterior exchange sites on the root and not actually taken up. They may enter the root passively in organic or inorganic complexes with the mass flow of water or actively ©2001 CRC Press LLC by way of metabolically controlled membrane transport systems often meant to take up a nutrient which the “contaminant” metal mimics. At different soil solute con- centrations, metals may be absorbed by both processes. Absorption mechanisms and quantity absorbed are influenced by plant species (and cultivar), growth stage, physiological state, and the presence of other elements. Once in the plant, a metal can be sequestered in the roots in vacuoles or in association with cell walls and organelles, or translocated to above ground parts in xylem as organic or inorganic complexes. Location and forms of metals in plants, as well as their toxic effects, depend on plant species, growth stage, physiological state, and presence of other metals. Mechanisms of toxicity of metals tend to be dependent on the nature of the reactivity of the metal itself and its availability in the soil and soil solution media. They may alter or inhibit enzyme activity, interfere with deoxyribonucleic acid (DNA) synthesis or electron transport, or block uptake of essential elements. 2 Avail- ability in response to toxic levels of metals by different plants is due to a number of defenses. These include exclusion from the root, translocation in nontoxic form, sequestering in nontoxic form, sequestering in nontoxic form in the root or other plant parts, and formation of unusable complexes containing metals that may oth- erwise be inserted into biomolecules instead of the proper element (e.g., As replacing P). 5.2.2Organics Organic compounds of environmental concern include nonionic compounds (such as PAHs, chlorinated benzenes, polychlorinated biphenyls (PCBs), BTEX compounds, and many pesticides), ionizable compounds (chlorophenols, carboxylic acids, surfactants, and amines), and weakly hydrophobic volatile organic compounds (trichloroethene). For the nonionic compounds, sorption in soil is mainly a function of degree of hydrophobicity and amount of sorbent hydrophobic phase (i.e., soil organic matter). Sorption of the compound by soil organic matter is reversible. The activities of these compounds in soil can be predicted by the organic matter-water coefficient, K om , as estimated by the octanol-water coefficient, K ow . 3 Absorption onto colloidal organic matter in solution may alter the availability of these nonionic compounds. Ionizable compounds contain anionic or cationic moieties or both within their structure. These charged structures interact with organic and inorganic charged surfaces in the soil in a variety of reversible reactions. The extent and nature of the associations with charged surfaces depends on characteristics of the organic com- pound, solution pH and ionic strength, and mineral composition of the soil partic- ulates. Organic compounds may be degraded by microorganisms in the soil to metabolites with greater or lesser toxicity. Very stable compounds, like highly chlo- rinated PCBs, may persist in essentially unaltered form for many years. Plant roots are not discriminating in uptake of small organic molecules (molec- ular weight less than 500) except on the basis of polarity. 1-4 More water-soluble molecules pass through the root epidermis and translocate throughout the plant. The less soluble compounds (like many polycyclic aromatic hydrocarbons) seem to have limited entry into the plant and minimal translocation once inside. Highly lipophilic ©2001 CRC Press LLC compounds, such as PCBs, move into the plant root via the symplastic route (from cell to cell, as opposed to between cells) and are translocated within the plant. Within a plant the contaminant may be adsorbed on a cell surface or accumulated in the cell. Many contaminants become bound on the root surface and are not translocated. Not all organic compounds are equally accessible to plant roots in the soil environment. The inherent ability of the roots to take up organic compounds can be described by the hydrophobicity (or lipophilicity) of the target compounds. This parameter is often expressed as the log of the octanol-water partioning coefficient, K ow . Direct uptake of organics by plants is a surprisingly efficient removal mechanism for moderately hydrophobic organic compounds. There are some differences between the roots of different plants and under different soil conditions, but, gen- erally, the higher a compound’s log K ow , the greater the root uptake. Hydrophobicity also implies an equal propensity to partition into soil organic matter and onto soil surfaces. Root absorption may become difficult with heavily textured soils and soils with high native organic matter. There are several reported values available in the literature regarding the optimum log K ow value for a compound to be a good candidate for phytoremediation (as an example, log K ow = 0.5–3.0; log K ow = 1.5–4.0). 2,13 It has also been reported that compounds that are quite water soluble (log K ow < 0.5) are not sufficiently sorbed to the roots or actively transported through plant membranes. From an engineering point of view, a tree could be thought of as a shell of living tissue encasing an elaborate and massive chromatography column of twigs, branches, trunk, and roots. The analogous resin in this system is wood, the vascular tissue of the tree, and this “resin” is replenished each year by normal growth. Wood is composed of thousands of hollow tubes, like the bed of a hollow fiber chromatog- raphy column, with transpirational water serving as the moving phase. The hollow tubes are actually dead cells, whose death is carefully programmed by the tree to produce a water conducting tissue, which also functions in mechanical support. A complex, cross-linked, polymeric matrix of cellulose, pectins, and proteins embed- ded in lignin forms the walls of the tubes. The cell wall matrix is chemically inert, insoluble in the majority of solvents, and stable across a wide range of pH. Once an organic chemical is taken up, a plant can store (sequestration) the chemical and its fragments in new plant structures via lignification, or it can vola- tilize, metabolize, or mineralize the chemical all the way to carbon dioxide, water, and chlorides. Detoxification mechanisms may transform the parent chemical to nonphytotoxic metabolites, including lignin, that are stored in various places in plant cells. Many of these metabolic capacities tend to be enzymatically and chemically similar to those processes that occur in mammalian livers; one report has equated plants to” green livers” due to similarities of detoxification processes. Different plants exhibit different metabolic capacities. This is evident during the application of herbicides to weeds and crops alike. The vast majority of herbicidal compounds have been selected so that the crop species are capable of metabolizing the pesticide to nontoxic compounds, whereas the weed species either lack this capacity or perform it at too slow a rate. The result is the death of the weed species without the metabolic capacity to rid itself of the toxin. ©2001 CRC Press LLC The shear volume and porous structure of a tree’s wood provide an enormous surface area for exchange or biochemical reactions. Some researchers are attempting to augment the inherent metabolic capacity of plants by incorporating bacterial, fungal, insect, and even mammalian genes into the plant genome. 5.3 TYPES OF PHYTOREMEDIATION A review of where pyhtoremediation fits into the scheme of hazardous waste remediation enables us to differentiate the various types and mechanisms of phy- toremediation (Figure 5.1). The scientific understanding of plant, soil and rhizo- sphere biochemistry, and contaminant fate and transport must be contrasted with field and pilot studies that represent the current proof of concepts. The technology is summarized below as those approaches ready for application, promising treatments expected to be tested soon, and concepts of phytoremediation requiring intensive development. Finally, the intrinsic strengths of phytoremediation as a technology and the future potential of this technology must be reviewed for regulatory accep- tance in terms of hazardous waste remediation. 1,2 Phytoremediation approaches can be summarized as follows based on current understanding of the technology: •Phytoaccumulation, phytoextraction, hyperaccumulation •Phytodegradation or phytotransformation •Phytostabilization •Phytovolatilization • Rhizodegradation, phytostimulation, or plant assisted bioremediation • Rhizofiltration or contaminant uptake Optimal performance of the technology is an important key to phytoremedia- tion’s ability to gain wider acceptance as a presumptive remediation technique. With Figure 5.1 Potential contaminant fates during phytoremediation in the soil–plant–atmosphere continuum. Mechanisms for Organics Mechanisms for Inorganics Atmosphere Contaminant in the air Plant Contaminant in the plant Soil Contaminant in the root-zone (Rhizosphere) Phytovolatilization Phytodegradation Rhizodegradation Rhizofiltration Phytostabilization Impacted Media Impacted Media Phytostabilization Rhyzofiltration Phytoaccumulation Phytovolatilization Remediated Contaminant ©2001 CRC Press LLC the possible exception of some of the above mechanisms that are already widely studied and understood, all of phytoremediation’s major applications require further basic and applied research in order to optimize field performance. Significant research and development should be carried out to 1) obtain a better understanding of mechanisms of uptake, transport, and accumulation of contaminants; 2) improve collection and genetic evaluation of hyperaccumulating plants; and 3) obtain a better understanding of interactions in the rhizosphere interactions among plant roots, microbes, and other biota. Short of true regulatory reform, phytoremediation’s ability to make further inroads will depend on how quickly federal, state, and local regulators become convinced of the technology’s efficacy. While not involved in every decision making process, the public is sometimes a key constituency as well. One can expect public interest groups to be more concerned about efficacy and safety issues than cost or other economic factors. However, phytoremediation seems to be faring well with the general public and, according to many practitioners, has already proven popular with neighbors and other interested parties at field remediation sites. 5.3.1Phytoaccumulation Remediation of contaminated soils using nonfood crops, called phytoaccumula- tion, has attracted a great deal of interest in recent years. Also called phytoextraction, phytoaccumulation, refers to the uptake and translocation of metal contaminants in the soil by plant roots into the above ground portions of plants. 2 Certain plants, called hyperaccumulators, absorb unusually large amounts of metals in comparison to other plants and the ambient metals concentration (Table 5.1). Phytoaccumulators or phytoextractors must have a high accumulation factor, that is, a high uptake of metals from the soil. The uptake should be metal specific, which diminishes the risk of impoverishing the soil of nutrient elements. The property of having a high specific uptake must be genetically stable. Since the removal of metals from the soil is actually achieved through the harvest, it is necessary that the plant have a high transport of the metal(s) from the roots to the shoots to be effective during remediation applications. In addition, a high biomass production of the Table 5.1The Number of Taxonomic Groups of Hyperaccumulators Varies According to Which Metal is Hyperaccumulated 2 Metal Number of Taxonomic Groups of Hyper Accumulators Ni>300 Co26 Cu24 Zn18 Mn8 Pb5 Cd1 ©2001 CRC Press LLC phytoaccumulator is needed for high removal of metals per unit area. It is also an advantage if biomass production is of economic interest. Hyperaccumulators have been preferred during phytoaccumulation applications because they take up very large amounts of a specific metal. They are often endemic and of a specific population (genotypes/clones) of a species. 5 However, these plants seldom have high biomass production and may also have low competitive ability in less polluted areas, probably because the plant uses its energy to tolerate such high levels of metals in the tissue instead of growth. Hyperaccumulators can accumulate ≥ 0.01% of Cd, ≥ 0.1% of Cu, or ≥ 1.0% Zn in leaf dry mass and may have the metal evenly distributed throughout the plant. 6 There are also high accumulators that accumulate somewhat lower metal con- centrations than hyperaccumulators but much more than “normal” plants. They usually have high biomass production. In these plants, there is no uniform distribu- tion of metal throughout the plant, and thus the plant might have high accumulation either in the roots or in the shoots. These plants are selected and planted at a site based on the type of metals present and other site conditions. After they have been allowed to grow for several weeks or months, they are harvested. Landfilling, incineration, and composting are options to dispose of or recycle the metals, although this depends upon the results of TCLP and cost. Planting and harvesting of plants may be repeated as necessary to bring soil contaminant levels down to allowable limits. A plan may be required to deal with the plant biomass waste. Testing of plant tissue, leaves, roots, etc., will determine if the plant tissue is a hazardous waste. Regulators will play a role in determining the testing method and requirements for the ultimate disposal of the plant waste. The state of science in phytoaccumulation is as follows: 7 • Botanical prospecting dating to the 1950s in the former USSR and U.S. is available to practitioners. •Over 400 species of hyperaccumulators worldwide have been cataloged. •Field test kits for metal hyperaccumulation have been developed. • Uptake and segregation processes using cation pumps, ion transporters, Ca blocks, metal chelating exudates and transporters, phytochelatin peptides, and metallothio- neins have been evaluated and continuous research is being performed to develop further understanding. The hyperaccumulator plants can contain toxic element levels in the leaf and stalk biomass (LSB) about 100 times more than nonaccumulator plants growing in the same soil, with some species and metal combinations exceeding conventional plant levels by a factor of more than 1000. 8 Many hyperaccumulator plants, which are nonwoody (not a tree), have been identified as having the capacity to accumulate metals. Thlaspi caerulascens was found to accumulate Zn up to 2000–4000 mg/kg. 9 The Indian mustard plant Brassica juncea , grown throughout the world for its oil seed, was found to accumulate significant amounts of lead. 10 One planting of mustard in a hectare of contaminated land was found to soak up two metric tons of lead. If three plantings could be squeezed in per year, six tons of lead per hectare can be extracted. Both hemp dogbane ( Apocynum sp.) and common ragweed also have been observed to ©2001 CRC Press LLC accumulate significant levels of lead. Aeollanthus subcaulis var lineris and Papsalum notatus are other hyperaccumulator plants known to accumulate Cu and Cs, respec- tively. Hyperaccumulator plants can address contamination in shallow soils only, up to 24 inches in depth. If contamination is deeper, 6–10 feet, deep-rooted poplar trees can be used for phytoextraction of heavy metals. These trees can accumulate the heavy metals by sequestration. However, there are concerns specifically for trees that include leaf litter and associated toxic residues being blown off site. This concern may be tested in the laboratory to see whether uptake and translocation of the metals into the leaves exceed standards. Hyperaccumulators have metal accumulating characteristics that are desirable, but lack the biomass production, adaptation to current agronomic techniques, and physiological adaptations to climatic conditions required at many contaminated sites. It has been reported that harvesting at different seasons in a year had pronounced differences in accumulation levels. In the future, genetic manipulation techniques may provide better hyperaccumulator species. The success of phytoextraction depends on the use of an integrated approach to soil and plant management: the disciplines of soil chemistry, soil fertility, agronomy, plant physiology, and plant genetic engineering are currently being used to increase the rate and efficiency of heavy metal phytoextraction. Chelates have been used not only to enhance metal uptake but also to avoid metal toxicity. Metal accumulator plants have been studied extensively for organo- metallic complexes. It has been suggested that there is a relationship between metal tolerance and carboxylic acids. Organo-metallic complexes increase the translocation and tolerance of plants to the toxic effects of metals. For example, in Sebertia acuminata citrate seems to be a detoxifying agent as well as an agent in transporting phytotoxic Ni from root systems to the leaves until leaf fall. 5,6 It has also been suggested that in copper (Cu) and cobalt (Co) accumulator plants, Co existed as an oxalate complex within the leaf. The formation of Zn–citrate complexes in Zn- tolerant plants was the reason for high levels of organic acid accumulation. Reports have indicated that histidine was responsible for accumulation, tolerance, and trans- port to shoots in nonaccumulating and hyperaccumulating (Ni) plant species. 11 In Thlaspi , a Zn hyperaccumulator plant species, it has been determined that the majority of Zn in the roots was coordinated with histidine, whereas organic acids were involved in xylem transport and Zn storage in the shoots. Similarly in a Cr- accumulating plant, Leptospermum scoparium , it was found that soluble Cr in leaf tissue was present as the trioxalatochromium (III) ion, [Cr (C 2 O 4 ) 3 ] 3– . The function of the Cr-organic acid complex was to reduce the cytoplasmic toxicity of Cr. 5 Adding ethylenediaminetetraacetic (EDTA) acid, citric acid, or oxalic acid to metal contaminated soils will significantly increase the metal concentrations in plant shoots and roots. 5 However, the application of these chelates during a full scale remediation application has to be carefully controlled; if not, the increased solubility of the metal chelates formed could drive these contaminants to migrate further downward by leaching when plant uptake rates are not adequate. Controlling the pH and conditioning the soils for optimum pH is an important factor when dealing with metals-contaminated soils. [...]... TCE degradation;3) alfalfa for TCA degradation; and 4) rye, St Augustine, and white clover for TPH Growth of hybrid poplar trees for the application of phytodegradation and rhizodegradation is shown in Figures 5. 6a, b, and c ©2001 CRC Press LLC Figure 5. 6a Phytoremediation System, August 6, 1998 Figure 5. 6b Phytoremediation System, September 13, 1999 5. 3.6 RhizoÞltration Rhizofiltration is the adsorption... concentration factors 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/ or bacteria) consume and degrade or transform... Phreatophytes (like willows, cottonwood, and hybrid poplar), which take up and “process” large volumes of soil water are good candidates for phytoremediation applications specifically for groundwater containment For example, a single willow tree on a hot summer day transpires more than 50 00 gallons of water, and a hybrid poplar can transpire about 50 to 350 gallons per day.23 Phytoremediation of groundwater plumes... Weissman, J S., Ramesh, G., Varadarajan, R., and Benemann, J R., Bioremoval of toxic elements with aquatic plants and algae, in Bioremediation of Inorganics, Hinchee, R E Means, J L., and Burns, D R., Eds., Battelle Press, Columbus, OH, 19 95 19 Feiler, H D and Darnall, D W., Remediation of Groundwater Containing Radionuclides and Heavy Metals using Ion Exchange and the Alga SORD Biosorbent System, Final... mechanisms Table 5. 2a Types of Phytoremediation for Organic Constituents Type of Phytoremediation 1 Phytostabilization 2 Rhizodegradation (phytostimulation, rhizosphere bioremediation, or plant-assisted bioremediation) 3 RhizoÞltration (contaminant uptake) 4 Phytodegradation (phytotransformation) 5 Phytovolatilization ©2001 CRC Press LLC Process Involved Plants control pH, soil gases, and redox conditions... biodegradation - Supply of nutrients, cometabolites - Transport and retention of water - Aeration Soil dessication Root respiration Root intrusion Sloughing Enzymes dehalogenase nitroductase Figure 5. 4 Uptake Rhizodegradation and associated processes in the root zone commonly found in the rhizosphere. 15 The increased microbial numbers are primarily due to the presence of plant exudates and sloughed tissue... draw down the water table below the trees similar to a pump and treat system (Figures 5. 7a and b) Simulations of a proposed design can be carried out based on extent of contamination, hydrogeological data, past precipitation and infiltration records, and evapotranspiration data A big advantage of phytoremediation over conventional pump and treat systems is the ability of the roots to penetrate the microscopic... scope of this chapter REFERENCES 1 McCutcheon, S C., USEPA, personal communications, 1999, 2000 2 USEPA, Introduction to Phytoremediation, EPA/600/R-99/107, Washington D.C., February, 2000 3 Schnoor, J L et al., Phytoremediation of organic and nutrient contaminants, Environ Sci Technol., 29, 1620–1631, 19 95 4 McCutcheon, S C., Phytoremediation of organic compounds: science validation and field testing,... Workshop on Phytoremediation of Organic Wastes, Kovalick, W W and Olexsey, R., Eds., Ft Worth, TX, December, 1996 5 Shahandeh, H and Hossner, L R., Enhancement of Cr (III) phytoaccumulation, Int J Phytoremed., 2, 269–286, 2000 6 Brooks, R R., Plants That Hyperaccumulate Heavy Metals, CAB International, New York, NY, 1998 7 McCutcheon, S C., The science and practice of phytoremediation, in Phytoremediation:... Cornish, J E et al., Phytoremediation of soils contaminated with toxic elements and radionuclides, in Bioremedation of Inorganics, Hinchee, R E et al., Eds., Battelle Press, Columbus, OH, 19 95 9 Brown, S L et al., Zinc and cadmium uptake by hyperaccumulator Thlaspi caerulescens and metal tolerant Silene vulgaris grown on sludge amended soils, Environ Sci Technol., 29, 158 1– 159 0, 19 95 10 Bishop, J E., Pollution . “Phytoremediation” Natural and Enhanced Remediation Systems Edited by Suthan S. Suthersan Boca Raton: CRC Press LLC, 2001 ©2001 CRC Press LLC CHAPTER 5 Phytoremediation CONTENTS 5. 1. Phytovolatilization 5. 3 .5 Rhizodegradation 5. 3.6 Rhizofiltration 5. 3.7 Phytoremediation for Groundwater Containment 5. 3.8 Phytoremediation of Dredged Sediments 5. 4 Phytoremediation Design 5. 4.1 Contaminant. and mechanisms of phy- toremediation (Figure 5. 1). The scientific understanding of plant, soil and rhizo- sphere biochemistry, and contaminant fate and transport must be contrasted with field and