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Phytoremediation: An ecological solution to organic chemical contamination
Ecological Engineering 18 (2002) 647– 658
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Phytoremediation: An ecological solution to organic chemical contamination Sridhar Susarla a, Victor F. Medina b, Steven C. McCutcheon c,* a GeoSyntec Consultants, 1100 Lake Hearn Dr., N.E., Suite 200, Atlanta, GA 30342, USA Department of Ci6il and En6ironmental Engineering, Washington State Uni6ersity, 2710 Uni6ersity Dri6e, Richland, WA 99352, USA c United States En6ironmental Protection Agency, National Exposure Research Laboratory, 960 College Station Road, Athens, GA 30605, USA b
Received 30 October 1998; received in revised form 8 February 1999; accepted 23 June 1999
Abstract Phytoremediation is a promising new technology that uses plants to degrade, assimilate, metabolize, or detoxify metals, hydrocarbons, pesticides, and chlorinated solvents. In this review, in situ, in vivo and in vitro methods of application are described for remediation of these compounds. Phytoaccumulation, phytoextraction, phytostabilization, phytotransformation, phytovolatilization and rhizodegradation are discussed and the role of enzymes in transforming organic chemicals in plants is presented. The advantages and constraints of phytoremediation are provided. Our conclusions is that phytoremediation prescriptions must be site-specific; however, these applications have the potential for providing the most cost-effective and resource-conservative approach for remediating sites contaminated with a variety of hazardous chemicals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chlorinated solvents; Pesticides; Plants; Phytodegradation; Phytoremediation; Rhizofiltration; TNT
1. Introduction Advances in science and technology have enabled humans to exploit natural resources to a great extent, generating unprecedented disturbances in global elemental cycles. The relatively recent introduction of anthropogenic toxic chemicals, and the massive relocation of natural materials to different environmental compartments * Corresponding author. E-mail address: [email protected] (S.C. McCutcheon).
(soils, ground water, and atmosphere), has resulted in severe pressure on the self-cleansing capacity of recipient ecosystems. Consequently, accumulated pollutants are of concern relative to both human and ecosystem exposure and potential impact. Currently, efforts are underway in many countries to control the release of contaminants (Schnoor et al., 1995) and to accelerate the breakdown of existing contaminants by appropriate remediation techniques. For example, existing ex-situ methods for remediation of contaminated ground waters include extraction and treatment by activated carbon adsorption, microbes or air
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stripping. On the other hand, in situ methods involve stimulation of anaerobic and aerobic microbial activities in the aquifer. All of these technologies involve relatively high capital expenditure and manpower as well as long term operating costs. Hence, efforts are underway to develop more cost-effective approaches to treat large volumes of contaminated natural resources such as soil, ground water and wetlands. Phytoremediation is an emerging technology that utilizes plants and then the associated rhizosphere microorganisms to remove, transform, or contain toxic chemicals located in soils, sediments, ground water, surface water, and even the atmosphere. Currently, phytoremediation is used for treating many classes of contaminants including petroleum hydrocarbons, chlorinated solvents, pesticides, explosives, heavy metals and radionuclides, and landfill leachates. According to a recent report (Best et al., 1997), approximately 80% of the polluted groundwaters are within 20 m of the surface. This suggests that a significant number of sites are potentially suitable for low cost phytoremediation applications. In recent years, a number of articles have addressed the role of plants in remediating contaminated soils and ground waters (Paterson et al., 1990; Shimp et al., 1993; Schnoor et al., 1995; Simonich and Hites, 1995; Watanabe, 1997). Chang and Corapcioglu (1998) describe how plants promote by various processes the remediation of a wide range of chemicals at toxic waste sites. These processes include: (1) modifying the physical and chemical properties of contaminated soils; (2) releasing root exudates, thereby increasing organic carbon; (3) improving aeration by releasing oxygen directly to the root zone, as well as increasing the porosity of the upper soil zones; (4) intercepting and retarding the movement of chemicals; (5) effecting co-metabolic microbial and plant enzymatic transformations of recalcitrant chemicals; and (6) decreasing vertical and lateral migration of pollutants to ground water by extracting available water and reversing the hydraulic gradient. In many remediation projects, phytoremediation is seen as a final polishing step following the initial treatment of the high-level contamination.
However, when contaminants are in low concentration, phytoremediation alone may be the most economical and effective remediation strategy (Jones, 1991). Many sites with less toxic contaminants are suitable for phytoremediation as a longterm solution to the problem. Some advantages and constraints of phytoremediation are summarized in Table 1. This review article focuses on recent advances made in applications of phytoremediation to sites contaminated with chlorinated solvents, explosives, and pesticides. Additionally, the different processes involved in phytoremediation and the role of certain enzymes is discussed.
2. Methods of application
2.1. In situ phytoremediation In situ phytoremediation involves placement of live plants in contaminated surface water, soil or sediment that is contaminated, or in soil or sediment that is in contact with contaminated ground water for the purpose of remediation. In this approach, the contaminated material is not reTable 1 Advantages and constraints of phytoremediation Advantages
Constraints
In situ
Limited to shallow ground water, soils, and sediments High concentrations of hazardous materials can be toxic to plants and animals that consume the plants Mass transfer limitations associated as with other biotreatments Slower than mechanical treatments Only effective for moderately hydrophobic compounds Toxicity and bioavailability of biodegradation products is not known Contaminants may be mobilized into the ground water Influenced by soil and climate conditions of the site
Passive
Solar driven
Costs 10–20% of mechanical treatments Faster than natural attenuation High public acceptance
Fewer air and water emissions Conserves natural resources
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moved prior to phytoremediation. If the phyto mechanism consists of only uptake and accumulation as opposed to transformation of a contaminant, the plants may be harvested and removed from the site after remediation for disposal or recovery of the contaminants. A requirement of the in-situ approach is that the contaminant must be physically accessible to the roots. This approach generally is the least expensive phytoremediation strategy.
2.2. In-6i6o phytoremediation with relocated contaminants For sites where the contaminant is not accessible to the plants, such as contaminants in deep aquifers, an alternate method of applying phytoremediation is possible. In this approach the contaminant is extracted using mechanical means, then it is transferred to a temporary treatment area where it can be exposed to plants selected for optimal phytoremediation. After treatment, the cleansed water or soil can be returned to its original location and the plants may be harvested for disposal if necessary. This approach generally would be more expensive than the more passive method described above. Treatment could occur either at the site of contamination or at another location.
2.3. In 6itro phytoremediation In the first two approaches, live plants are used for phytoremediation. A third method of applying phytoremediation is via components of live plants, such as extracted enzymes. In theory, this approach could be applied in situ under some situations, e.g. applying plant extracts to a contaminated pond or wetland, or through use of on enzyme impregnated porous barrier in a contaminated ground water plume. More likely, this approach could also be applied to contaminated material that has been relocated to a temporary treatment area, as described in Section 2.2. Theoretically, this approach would be the most expensive method of phytoremediation because of the costs of preparing/extracting the plant enzymes; however, in some plants, such as tarragon,
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(Artemisia dracunculas var satiya), exudates are released under stress that could result in reduced production costs. One important factor to consider for this approach is the length of time the enzymes remain active for breakdown of contaminants.
3. Mechanisms of phytoremediation There are numerous mechanisms by which plants may remediate contaminated sites. In some cases, the transformation takes place by live plants as described in Section 2.1 and Section 2.2. Some of the chemicals that can be treated by these mechanisms are listed in Table 2. Plant exudates or enzymes that are responsible for the breakdown of compounds are summarized in Table 3 and Table 4. Some of the factors affecting chemical uptake and distribution within living plants include: (1) physical and chemical properties of the compound (e.g. water solubility, vapor pressure, molecular weight, and octanol-water partition coefficient, Kow); (2) environmental characteristics (e.g. temperature, pH, organic matter, and soil moisture content); (3) plant characteristics (e.g. type of root system, and type of enzymes). Some of the mechanisms used by plants to facilitate remediation include: phytoextraction, phytopumping, phytostabilization, phytotransformation/degradation, phytovolatilization, and rhizodegradation, which are described in the following sections.
3.1. Phytoextraction/phytoaccumulation Phytoextraction is the removal of a contaminant from the soil, ground water or surface water by live plants. Phytoaccumulation occurs when the contaminant taken up by the plant is not degraded rapidly or completely, resulting in an accumulation in the plant. Certain plants hyperaccumulate metals (e.g. nickel, zinc, copper, chromium), and radionuclides. Heavy metal hyperaccumulation is defined as accumulation of more than 0.1% by dry weight in plant tissue (0.01% for cadmium. Hyperaccumulation of more common elements such as iron and manganese is defined as more
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650 Table 2 Phytoremediation mechanisms Type
Chemicals Treated
Phytoaccumulation/ phytoextraction,
Cadmium, chromium, lead, nickel, zinc and other heavy metals, selenium, radionuclides; BTEX (benzene, ethyl benzene, toluene and xylenes), pentachlorophenol, short-chained aliphatic compounds, and other organic compounds Munitions (DNT, HMX, nitrobenzene, nitroethane, nitromethane, nitrotoluene, picric acid, RDX, TNT), atrazine; chlorinated solvents (chloroform, carbon tetrachloride, hexachloroethane, tetrachloroethene, trichloroethene, dichloroethene, vinyl chloride, trichloroethanol, dichloroethanol, trichloroacetic acid, dichloroacetic acid, monochloroacetic acid, tetrachloromethane, trichloromethane), DDT; dichloroethene; methyl bromide; tetrabromoethene; tetrachloroethane; other chlorine and phosphorus based pesticides; polychlorinated biphenols, other phenols, and nitriles Proven for heavy metals in mine tailings ponds and expected for phenols and chlorinated solvents (tetrachloromethane and trichloromethane) Polycyclicaromatic hydrocarbons; BTEX (benzene, ethylbenzene, toluene, and xylenes); other petroleum hydrocarbons; atrazine; alachlor; polychlorinated biphenyl (PCB); tetrachloroethane, trichloroethane and other organic compounds Chlorinated solvents (tetrachloroethane, trichloromethane and tetrachloromethane); mercury and selenium Heavy metals, organic chemicals; and radionuclides
Phytodegradation/ phytotransformation
Phytostabilization Phytostimulation
Phytovolatilization Rhizofiltration
than 1% of the element by dry weight in plant tissue (0.01% for Cadmium). In the process of hyperaccumulating contaminants (Dushenkov et al., 1995), some plants can remediate the contaminated soils to acceptable levels. Some plants can grow in contaminated areas and tolerate hyperaccumulation of metals and other contaminants such as perchlorate (Susarla et al., 1999). Other plants may die or experience severe stress under conditions of hyperaccumulation. Less tolerant plants can still be used in areas of contamination then harvested and disposed after these plants have hyperaccumulated the contaminant to their maximum extent. Such crops can be replanted, if necessary, to complete this remediation. Another option is to recover the contaminant after harvesting the plants (Dushenkov et al., 1995; Nanda Kumar et al., 1995; Moffat, 1995; Kelley and Guerin, 1995; Cornish et al., 1995; Wang et al., 1995; Ban˜ uelos et al., 1998). If the remediation goal is to harvest after these plants hyperaccumulate a contaminant, then it is desirable for the selected plants to be able to translocate the contaminant from the root into above ground tissue, such as shoots and leaves (Nellessen and Fletcher, 1993). If the contaminant remains in the roots, harvesting for disposal or recovery may be more difficult.
3.2. Phytopumping and water balance control Phytopumping is another mechanism that can be used to remove or minimize migration of contaminants. In this case, plants are used as organic ‘pumps’ to pull-in large volumes of the contaminated water as part of the transpiration process. The result is reduced migration of the contaminant in ground water, in addition to potential uptake. Plants that are capable of removing large amounts of water from the soil are best for this purpose. For example, the willow tree (Salix spp.) may use up to 200 liters of water per day (Gatliff, 1994). Plants having these characteristics may provide an inexpensive alternative to mechanical pump and treat systems for contaminated ground water in shallow aquifers (Gatliff, 1994; Licht, 1995).
3.3. Phytostabilization Phytostabilization is another mechanism that can be used to minimize migration of contaminants in soils. This process takes advantage of plant roots ability to alter soil environment conditions, such as pH and soil moisture content. Many root exudates cause metals to precipitate,
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thus reducing bioavailability. One advantage of this strategy over phytoaccumulation is the disposal of the metal-laden plant material is not required. By choosing and maintaining an appropriate cover of plant species, coupled with appropriate soil amendments, it may be possible to stabilize certain contaminants (particularly metals) in the soil (Cunningham et al., 1995), and reduce the interaction of these contaminants with associated biota.
3.4. Phytotransformation/phytodegradation A contaminant can be eliminated via phytodegradation or phytotransformation by plant enzymes or enzyme co-factors (Dec and Bollag, 1994; Strand et al., 1995). Dec and Bollag (1994) describe plants that can degrade aromatic rings in the absence of microorganisms. Polychlorinated biphenyls (PCBs) have been metabolized by sterile plant tissues. Phenols have been degraded by plants such as horseradish, potato (Solanum tuberosum), and white radish (Raphanus sati6us) that contains peroxidase (Dec and Bollag, 1994; Roper et al., 1996). Poplar trees (Populus spp.) are capable of transforming trichloroethylene in soil and ground water (Strand et al., 1995; Newman et al., 1997). Enzymes of particular interest for phytoremediation include: (1) dehalogenase (transformation of chlorinated compounds); (2) peroxidase (transformation of phenolic compounds); (3) nitroreductase (trans-
formation of explosives and other nitrated compounds); (4) nitrilase (transformation of cyanated aromatic compounds); and (5) phosphatase (transformation of organophosphate pesticides).
3.5. Phyto6olatilization Phytovolatilization is a mechanism by which plants convert a contaminant into a volatile form, thereby removing the contaminant from the soil or water (Terry et al., 1995) at a contaminated site. For example, plants, possibly in association with microorganisms, can convert selenium to dimethyl selenide. Dimethyl selenide is a less toxic, volatile form of selenium. Phytovolatilization may be a useful, inexpensive means of removing selenium from sites contaminated with high concentration selenium wastes. Similarly, some transgenic plants (e.g. Arabidopsis thaliana) have converted organic and inorganic mercury salts to the volatile, elemental form (Watanabe, 1997).
3.6. Rhizodegradation Rhizodegradation is a biological treatment of a contaminant by enhanced bacterial and fungal activity in the rhizosphere of certain vascular plants. The rhizosphere is a zone of increased microbial density and acitivity at the root/surface, and was described originally for legumes by Lorenz Hiltner in 1904 (Curl and Truelove, 1986). Plants and microorganisms often have symbiotic
Table 3 Plant enzymes that have a role in transforming organic compounds Enzyme
Plants known to produce enzymatic activity
Application
Dehalogenase
Hybrid poplar (Populus spp.), algae (various spp.), parrot feather (Myriophyllum aquaticum) Stonewort (Nitella spp.), parrot-feather (Myriophyllum aquaticum) Willow (Salix spp.) Hybrid poplar (Populus spp.), Stonewort (Nitella spp.), parrot feather (Myriophyllum aquaticum)
Dehalogenates chlorinated solvents
Laccase Nitrilase Nitroreductase
Peroxidase Phosphatase
Horseradish (Armoracia rusticana P. Gaertner, Meyer & Scherb) Giant duckweed (Spirodela polyrhiza)
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Cleaves aromatic ring after TNT is reduced to triaminotoluene Cleaves cyanide groups from aromatic rings Reduces nitro groups on explosives and other nitroaromatic compounds, and removes nitrogen from rings structures Degradation of phenols (mainly used in wastewater treatment) Cleaves phosphate groups from large organophosphate pesticides
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Table 4 Plant species used in phytoremediation of organic compounds Plant species
Contaminant
Reference
Barley (Hordeum 6ulgare L. cv. Klages) Forage grasses
Hexachlorobenzene, PCBs, pentachlorobenzene, trichlorobenzene Chlorinated benzoic acids
Parrot feather Hybrid poplar
Tetrachloroethane (PCE), Trichloroethane (TCE), TNT Atrazine, nitrobenzene, TCE, TNT
Prairie grass Soyabean (Glycine max [L.] Merr. Cv. Fiskby v) Eurasian watermilfoil (Myriophyllum spicatum) Waterweed (Eichhornia crassipes)
2-chlorobenzoic acid Bromacil, nitrobenzene, phenol
McFarlane et al., 1987 Siciliano and Germida, 1998 Best et al., 1997 Burken and Schnoor, 1997 Topp et al., 1989 Fletcher et al., 1990
TNT
Hughes et al., 1997
Pentachlorophenol, PCE, TCE
Roy and Hanninen, 1994
relationships making the root zone or rhizosphere an area of very active microbial activity (Anderson et al., 1993, 1994; Schwab et al., 1995; Jordahl et al., 1997; Siciliano and Germida, 1998). Plants can moderate the geochemical environment in the rhizosphere, providing ideal conditions for bacteria and fungi to grow and degrade organic contaminants. Plant litter and root exudates provide nutrients such as nitrate and phosphate that reduce or eliminate the need for costly fertilizer additives. Plant roots penetrate the soil, providing zones of aeration and stimulate aerobic biodegradation. Many plant molecules released by root die back and exudatation resemble common contaminants chemically and can be used as co-substrates. For example, phenolic substances released by plants have been found to stimulate the growth of PCBdegrading bacteria (Fletcher et al., 1993; Donnelly and Fletcher 1994a; Fletcher and Hegde, 1995; Fletcher et al., 1995). Recent studies have described enhanced degradation of pentachlorophenol in the rhizosphere of wheat grass (Agropyron cristatum) (Ferro et al., 1994), increased initial mineralization of surfactants in soil-plant cores (Knabel and Vestal, 1992), and enhanced degradation of TCE in soils collected from the rhizospheres. Anderson et al., (1993) provides a review of microbial degradation in the rhizosphere. Thus, current research suggests the interaction between
plants and soil microbes may be an important factor influencing biological remediation of contaminated soils.
3.7. Combined mechanisms In many cases, phytoremediation involves combinations of the mechanisms described above. For example, phytoextraction and phytovolatilization have been credited with the removal of excess selenium in soil (Cornish et al., 1995). It is likely that both processes occur simultaneously. The treatment of TCE in ground water using poplar trees requires extraction of the ground water by the plant (phytoextraction) that will also degrade TCE (phytodegradation) within the plant. Another example is degradation of PCB’s by plant cells (Fletcher et al., 1987), as well as by microorganisms stimulated by plants (Fletcher et al., 1993; Donnelly and Fletcher, 1994b; Fletcher and Hegde, 1995; Fletcher et al., 1995), creating the opportunity to combine phytodegradation and rhizodegradation. 4. Applications of phytoremediation Selected examples of the application of phytoremediation for hazardous chemicals using the previously described mechanisms are listed in Table 5.
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4.1. Munitions Numerous researchers have documented the ability of plants to take-up and transform trinitrotoluene (TNT) from soil and water (Palazzo and Leggett, 1986; Folsom et al., 1998; Cataldo et al., 1989; Young, 1995; Mueller et al., 1995; Schnoor et al., 1995; Medina and McCutcheon, 1996; Best et al., 1997; Medina et al., 1998). Axenic studies have shown that plants are capable of transforming TNT without microbial contribution (Vanderford et al., 1997; Hughes et al., 1997). However, very little accumulation of TNT has been found in plant material (Hughes et al., 1997; Vanderford et al., 1997). The rates of phytotransformation compare favorably to those of published microbial studies, and there appears to be no lag phases in plant based TNT remediation (Medina and McCutcheon, 1996). Hughes et al., (1997) provided the first confirmation that plants have the intrinsic capacity to transform TNT. In their experiments, axenic parrot feather (Myriophyllum spicatum) was used to demonstrate transformation and to determine mass balances using radiolabeled TNT. The results showed that complete mineralization of the TNT was not observed, but that no TNT or 2-amino-4,6-dinitrotoluene or 4-amino-2,6-dinitrotoluene were indentified in extracellular media or tissue extracts. However, several unidentified metabolites were observed in the plant. The unknown characteristics of these transformation products are of concern in the potential use of phytoremediation for sites contaminated with TNT. Vanderford et al. (1997) reported that TNT accumulated primarily in the roots of ax-
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enic parrot feather (Myriophyllum aquaticum). As incubation time increased, the plant-bound radiolabelled fraction, consisting largely of unidentified transformation products, became increasingly more difficult to extract. Another concern is that some species selected for laboratory-scale testing that performed satisfactorily, such as Eurasian water-milfoil, are aggressive non-native species (Godfrey and Wooten, 1981). Establishment and spread of many of these nonnative plant species have resulted in considerable economic burden and environmental degradation by displacing valuable native plant and animal species. Pilot studies have been conducted to determine the treatment performance of TNT-contaminated ground water by constructed wetlands. In order to model the disappearance of TNT and the associated breakdown, a pseudo-first order, non-reversible reaction, finite difference model was used with batch-scale experiments to determine disappearance kinetics for individual species (Medina and McCutcheon, 1996). The results of the model suggested that reasonably sized wetlands could treat a waste stream with an influent TNT concentration of 2.25 mg l − 1 at flow rates ranging from 40 to 20 000 l min − 1. Continuous, flowthrough pilot systems treating 0.001 to 0.01 g l − 1 solutions of TNT resulted in rapid removal of TNT at a retention time of 12 days (Medina et al., 1998). However, aminodinitrotoluene (ADNT), a transformation product, was identified in the effluent of the reactor. Increasing the residence time to 76 days decreased but did not eliminate the effluent concentration of ADNT. No TNT was found in extractions from the plant
Table 5 Examples of phytoremediation test sites (EPA, 1996) Location
Application
Contaminants
Medium
Plant
Ogden, UT
Phytoextraction
PAHs
Portsmouth, VA Milan, TN Aberdeen, MD
Rhizofiltration, phytodegradation Phytodegradation Organic pumping phytovolatilization rhizofiltration
PAHs TNT TCE, PCE
Soil, groundwater Soil Groundwater Groundwater
Alfalfa (Medicago sati6a), hybrid poplar trees Grasses, clover (Trifolium spp.) Duckweed, parrot feather Hybrid poplar trees
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tissue. However, small concentrations (up to 0.7490.133 mg/kg) of ADNT, as well as some trinitrobenzene (up to 0.0879 0.031 mg/kg), and a small concentration of 3,5-dinitroaniline (up to 0.0809 0.020 mg/kg) were found in plant tissues. Thompson et al. (1998a) investigated the use of hybrid poplar trees (Populus deltoides × nigra DN34) to treat soil contaminated with TNT. In soil, TNT uptake was governed by its ability to binding to soil particles (Thompson et al., 1998b). When the plant accumulated small amounts of TNT, which remained primarily in the roots, the contaminant was transformed during a 10-day experiment. A small amount of TNT was translocated to the leaves during the same period of time. However, in aqueous experiments more TNT was taken-up by the same plant. The accumulated TNT was transformed to 2-amino-4,6-dinitrotoluene, and 4-amino-2,6-dinitrotoluene, and into unidentifiable, insoluble, polar compounds that were bound in the plant tissue. The TNT also was found to have measurable toxic effects on hybrid poplar trees (Thompson et al., 1998b). However, removal and degradation rates were rapid enough to justify using poplar trees as an experimental treatment technology for TNT based on biomass production. Much less work has been done on the treatment of other munitions, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraocine (HMX). However, preliminary results suggest that treatment of these compounds via plants is not as successful as phytoremediation of TNT (Best et al., 1997; Medina et al., 1998; Sikora et al., 1998). Results of at least one study suggest phytodegradation of nitroglycerin can be accomplished (Goel et al., 1997). In conclusion, phytoremediation of TNT is promising due to rapid removal coupled with extensive transformation rather than phytoaccumulation. Unfortunately, the exact toxicity of the transformation byproducts is not understood. Furthermore, the formation and temporary accumulation of amino dinitrotoluenes needs investigation. Finally, more work is needed to investigate the treatment of other munitions, particularly RDX and HMX.
4.2. Chlorinated sol6ents Chlorinated solvents, such as TCE and PCE, are major contaminants of the soil and ground water in the US. They are used in many forms, including as anesthetics by the medical community, degreasers by industry and the military, solvents by the dry cleaning industry and other forms by the general public. Past widespread use and indiscriminate disposal make chlorinated solvents the most common pollutants in the country. Phytoremediation has been applied successfully to treat sites contaminated with chlorinated solvents. The initial data on TCE degradation in plant–rhizosphere–soil systems provided a strong incentive to explore the general interactions between the soil, plant, root and associated microbes, in addition to the variables that may influence biodegradation of waste chemicals in surface and near-surface soils. Later work by Anderson and Walton (1995) indicated that TCE is oxidized more readily in the rhizosphere of certain types of plants, such as pine (Pinus spp.) or legumes, as compared with unvegetated soil. However, the mineralization of TCE was limited. Uptake of TCE by plant roots was correlated with water uptake, but total uptake was small and translocation insignificant. Others have reported that plants also take up TCE vapor volatilized from the soil (Newman et al., 1997). Strand et al. (1995) demonstrated that hybrid poplar trees have the capability to assimilate and degrade the chlorinated solvent TCE to the aerobic degradation products 2,2,2-trichloroethanol, trichloroacetic acid, and dichloroacetic acid. In that study, dichloroacetic acid detected in leaf tissues was the most prominent metabolite. In similar studies, Schnabel et al. (1997) investigated the uptake and transformation of TCE by edible garden plants. Carrots (Daucus carota 6ar. sati6a), spinach (Spinacia oleracea), and tomatoes (Lycopersicon esculentum) were exposed to [14C] radiolabeled and unlabeled TCE. Most of the TCE (74 to 95%) was volatilized through the plants. The remaining TCE was adsorbed to soil particles. Very small, non-extractable amounts (1 to 2%) of the [14C]-label were found in the plant tissue. This suggested that the TCE was taken-up, transformed, and bound to the plant material.
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Newman et al. (1997) were first to report degradation of TCE into several known oxidized metabolites in hybrid poplar plant tissues, and demonstrated the potential for the use of poplars for in situ remediation of TCE. In their study, axenic poplar cell cultures were used to eliminate any aspects of microbial degradation. Recently, the uptake and transformation of TCE, PCE, and other chlorinated solvents were examined in a number of aquatic plants including waterweed (Elodea canadensis), parrot feather (M. aquaticum) and giant duckweed (Spirodela polyrhiza). Rennels (1995) also reported that these aquatic plants take-up TCE and PCE, then break the chemicals down to a number of compounds. These results support the potential usefulness of aquatic plants in the removal of chlorinated contaminants from contaminated ground and surface waters. Although the studies referenced above made significant contributions to the understanding of phytotransformation of chlorinated solvents, additional research is necessary to evaluate the long-term field performance of these plants, as well as the effects on animals that may use these plants.
4.3. Other aromatic compounds Chlorinated aromatics, including pesticides, such as atrazine and 1,1,1-trichloro-2,2-bis(pchlorophenyl)ethane (DDT), are common contaminants in soils, surface and ground waters. Phytoremediation has been effective in treating these types of contaminants. For example, the uptake and translocation of phenol, nitrobenzene and bromocil were related directly to transpiration rate in mature soybean plants (McFarlane et al., 1987). Recently, the use of minced horseradish roots has been proposed for the decontamination of surface waters polluted with chlorinated phenols (Roper et al., 1996). In sealed-batch experiments using waterweed, p,p%-DDT and o,p%-DDT were degraded (Garrison et al., 1996) with 1,1dichloro - 2 - (o - chlorophenyl) - 2 - (p-chlorophenyl)ethane (o,p =-DDD) and 1,1-dichloro-2-(pchlorophenyl)-2-(p-chlorophenyl)ethane (p,p = DDD) appearing as metabolite products. No
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accumulation of DDD was observed during the course of the experiments. Burken and Schnoor (1997) used poplar trees for uptake and metabolism of the pesticide atrazine. Results indicted that poplar trees can take-up, hydrolyze, and dealkylate atrazine to less toxic metabolites. In this study, the parent compound atrazine, and [14C]-radiolabeled metabolites were separated and identified for the first time. Transformation occurred in roots, stems, and leaves. These findings suggest that hybrid poplar trees have potential for phytoremediation of sites contaminated with atrazine.
5. Conclusions
5.1. Constraints of using plants for remediation of hazardous wastes Despite the diversity of potential options, phytoremediation is in its infancy. The majority of the research has been conducted in laboratories under relatively controlled conditions for short periods of time. More extensive research under field conditions for longer durations is required for a better understanding of the potential role of phytoremediation. A limiting factor of phytoremediation treatment includes the fact that a specific phytoremediation ‘prescription’ cannot be applied to every site with a certain chemical contaminant because different site-specific conditions (e.g. soil and climate) may not be suitable for the target plant. Plants also interact with and are affected by other living organisms such as insects, pests and pathogens, and exposure of plants to contaminants and related stresses can make the phyto treatment more susceptible to these other agents, ultimately influencing the outcome of phytoremediation attempts. Additionally, phytoremediation generally is restricted to sites where the concentration levels of contaminants are not toxic to the plants proposed for remediation. Finally, the contaminants must be accessible to the tissue responsible for uptake (e.g. root system) in plants. As a result, in situ phytoremediation using live plants is restricted to sites conducive to growth of the selected plant with the contaminant located
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within the potential root zone of the selected plant.
5.2. Ad6antages of using plants for phytoremediation of organic chemical contaminated sites Despite constraints referenced above, plants have many features that result in a high potential for environmental cleanup. Energy costs and expenses are reduced and natural resources are conserved because plants use solar energy. Plants are adapted to a wide range of environmental conditions and are capable of modifying conditions of the environment to some extent. The unique enzyme and protein systems of some plant species appear to be beneficial for phytoremediation. Additionally, since plants lack the ability to move, many plants have developed unique biochemical systems for nutrient acquisition, detoxification, and controlling local geochemical conditions. Some plants that grow well in nutrient poor soils may have useful mechanisms for removing and transforming contaminants that resemble certain nutrients. Infiltration is a primary pathway in contaminant migration to ground water, and plants play an important role in regulating water content in soil. Plant roots aerate the soil, which may stimulate microbial activity in the soil. Root exudates may be a nutrient source for microorganisms, since the rhizosphere generally contains significantly higher numbers and more active microorganisms than similar soils without plants. Thus, plants can contribute in many ways to enhance biodegradation in the soil. Phytoremediation provides an aesthetically pleasing alternative to structural remediation and decontamination technologies. As a result of these advantages, phytoremediation has considerable potential for environmental restoration of contaminated sites.
Acknowledgements Sydney Bacchus provided information on aquatic plants. The strategic Environmental Research and Development Program (Project c720 managed by the U.S. Army Waterways Experi-
ment Station), U.S. Air Force Human Systems Centre, U.S. Air Force Restoration Division, and U.S. Navy Southern Command provided support during the writing of this review.
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J.W., Heilman, P.E., Gordon, M.P., 1995. Removal of trichloroethylene from aquifers using trees. In: Vidic, R.D., Pohland, F.G. (Eds.), Innovative Technologies for Site Remediation and Hazardous Waste Management, Proceedings of the National Conference of the Environmental Engineering Division of the American Society of Civil Engineers, Pittsburgh, PA, New York, NY. Terry, N., Zayed, A., Pilon-Smits, E., 1995. Bioremediation of selenium by plant volatilization. In: Teddar, D.W. (Ed.), Emerging Technologies in Hazardous Waste Management VIII. 1995 Extended Abstracts for the Special Symposium. American Chemical Society, Industrial and Engineering Division, Atlanta, GA. Thompson, P.L., Ramer, L.A., Schnoor, J.L., 1998a. Uptake and transformation of TNT by hybrid poplar trees. Environ. Sci. Technol. 32, 975 – 980. Thompson, P.L., Ramer, P., Guffey, A.P., Schnoor, J.L., 1998b. Decreased transpiration in poplar trees exposed to 2,4,6-trinitrotoluene. Environ. Toxicol. Chem. 17, 902 – 906. Topp, E., Scheunert, I., Korte, F., 1989. Kinetics of the uptake of 14C-labeled chlorinated benznes from soil by plants. Ecotoxicol. Environ. Safety 17, 157 –166. Vanderford, M., Shanks, J.V., Hughes, J.B., 1997. Phytotransformation of trinitrotoluene and distribution of metabolic products in Myriophyllum aquaticum. Biotechnol. Lett. 19, 277 – 280. Wang, T.C., Weissman, J.C., Ramesh, G., Varadarajan, R., Benemann, J.R., 1995. Bioremoval of toxic elements with aquatic plants and algae. In: Hinchee, R.E., Means, J.L., Burris, D.R. (Eds.), Bioremediation of Inorganics. Battelle Press, Columbus, OH. Watanabe, M.E., 1997. Phytoremediation on the brink of commercialization. Environ. Sci. Technol. 31, 182A – 186A. Young, D.G., 1995. The biochemical remediation of TNT contaminated soil. Ph.D. Dissertation, Auburn University, AL.
www.elsevier.com/locate/ecoleng
Phytoremediation: An ecological solution to organic chemical contamination Sridhar Susarla a, Victor F. Medina b, Steven C. McCutcheon c,* a GeoSyntec Consultants, 1100 Lake Hearn Dr., N.E., Suite 200, Atlanta, GA 30342, USA Department of Ci6il and En6ironmental Engineering, Washington State Uni6ersity, 2710 Uni6ersity Dri6e, Richland, WA 99352, USA c United States En6ironmental Protection Agency, National Exposure Research Laboratory, 960 College Station Road, Athens, GA 30605, USA b
Received 30 October 1998; received in revised form 8 February 1999; accepted 23 June 1999
Abstract Phytoremediation is a promising new technology that uses plants to degrade, assimilate, metabolize, or detoxify metals, hydrocarbons, pesticides, and chlorinated solvents. In this review, in situ, in vivo and in vitro methods of application are described for remediation of these compounds. Phytoaccumulation, phytoextraction, phytostabilization, phytotransformation, phytovolatilization and rhizodegradation are discussed and the role of enzymes in transforming organic chemicals in plants is presented. The advantages and constraints of phytoremediation are provided. Our conclusions is that phytoremediation prescriptions must be site-specific; however, these applications have the potential for providing the most cost-effective and resource-conservative approach for remediating sites contaminated with a variety of hazardous chemicals. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Chlorinated solvents; Pesticides; Plants; Phytodegradation; Phytoremediation; Rhizofiltration; TNT
1. Introduction Advances in science and technology have enabled humans to exploit natural resources to a great extent, generating unprecedented disturbances in global elemental cycles. The relatively recent introduction of anthropogenic toxic chemicals, and the massive relocation of natural materials to different environmental compartments * Corresponding author. E-mail address: [email protected] (S.C. McCutcheon).
(soils, ground water, and atmosphere), has resulted in severe pressure on the self-cleansing capacity of recipient ecosystems. Consequently, accumulated pollutants are of concern relative to both human and ecosystem exposure and potential impact. Currently, efforts are underway in many countries to control the release of contaminants (Schnoor et al., 1995) and to accelerate the breakdown of existing contaminants by appropriate remediation techniques. For example, existing ex-situ methods for remediation of contaminated ground waters include extraction and treatment by activated carbon adsorption, microbes or air
0925-8574/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 8 5 7 4 ( 0 2 ) 0 0 0 2 6 - 5
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stripping. On the other hand, in situ methods involve stimulation of anaerobic and aerobic microbial activities in the aquifer. All of these technologies involve relatively high capital expenditure and manpower as well as long term operating costs. Hence, efforts are underway to develop more cost-effective approaches to treat large volumes of contaminated natural resources such as soil, ground water and wetlands. Phytoremediation is an emerging technology that utilizes plants and then the associated rhizosphere microorganisms to remove, transform, or contain toxic chemicals located in soils, sediments, ground water, surface water, and even the atmosphere. Currently, phytoremediation is used for treating many classes of contaminants including petroleum hydrocarbons, chlorinated solvents, pesticides, explosives, heavy metals and radionuclides, and landfill leachates. According to a recent report (Best et al., 1997), approximately 80% of the polluted groundwaters are within 20 m of the surface. This suggests that a significant number of sites are potentially suitable for low cost phytoremediation applications. In recent years, a number of articles have addressed the role of plants in remediating contaminated soils and ground waters (Paterson et al., 1990; Shimp et al., 1993; Schnoor et al., 1995; Simonich and Hites, 1995; Watanabe, 1997). Chang and Corapcioglu (1998) describe how plants promote by various processes the remediation of a wide range of chemicals at toxic waste sites. These processes include: (1) modifying the physical and chemical properties of contaminated soils; (2) releasing root exudates, thereby increasing organic carbon; (3) improving aeration by releasing oxygen directly to the root zone, as well as increasing the porosity of the upper soil zones; (4) intercepting and retarding the movement of chemicals; (5) effecting co-metabolic microbial and plant enzymatic transformations of recalcitrant chemicals; and (6) decreasing vertical and lateral migration of pollutants to ground water by extracting available water and reversing the hydraulic gradient. In many remediation projects, phytoremediation is seen as a final polishing step following the initial treatment of the high-level contamination.
However, when contaminants are in low concentration, phytoremediation alone may be the most economical and effective remediation strategy (Jones, 1991). Many sites with less toxic contaminants are suitable for phytoremediation as a longterm solution to the problem. Some advantages and constraints of phytoremediation are summarized in Table 1. This review article focuses on recent advances made in applications of phytoremediation to sites contaminated with chlorinated solvents, explosives, and pesticides. Additionally, the different processes involved in phytoremediation and the role of certain enzymes is discussed.
2. Methods of application
2.1. In situ phytoremediation In situ phytoremediation involves placement of live plants in contaminated surface water, soil or sediment that is contaminated, or in soil or sediment that is in contact with contaminated ground water for the purpose of remediation. In this approach, the contaminated material is not reTable 1 Advantages and constraints of phytoremediation Advantages
Constraints
In situ
Limited to shallow ground water, soils, and sediments High concentrations of hazardous materials can be toxic to plants and animals that consume the plants Mass transfer limitations associated as with other biotreatments Slower than mechanical treatments Only effective for moderately hydrophobic compounds Toxicity and bioavailability of biodegradation products is not known Contaminants may be mobilized into the ground water Influenced by soil and climate conditions of the site
Passive
Solar driven
Costs 10–20% of mechanical treatments Faster than natural attenuation High public acceptance
Fewer air and water emissions Conserves natural resources
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moved prior to phytoremediation. If the phyto mechanism consists of only uptake and accumulation as opposed to transformation of a contaminant, the plants may be harvested and removed from the site after remediation for disposal or recovery of the contaminants. A requirement of the in-situ approach is that the contaminant must be physically accessible to the roots. This approach generally is the least expensive phytoremediation strategy.
2.2. In-6i6o phytoremediation with relocated contaminants For sites where the contaminant is not accessible to the plants, such as contaminants in deep aquifers, an alternate method of applying phytoremediation is possible. In this approach the contaminant is extracted using mechanical means, then it is transferred to a temporary treatment area where it can be exposed to plants selected for optimal phytoremediation. After treatment, the cleansed water or soil can be returned to its original location and the plants may be harvested for disposal if necessary. This approach generally would be more expensive than the more passive method described above. Treatment could occur either at the site of contamination or at another location.
2.3. In 6itro phytoremediation In the first two approaches, live plants are used for phytoremediation. A third method of applying phytoremediation is via components of live plants, such as extracted enzymes. In theory, this approach could be applied in situ under some situations, e.g. applying plant extracts to a contaminated pond or wetland, or through use of on enzyme impregnated porous barrier in a contaminated ground water plume. More likely, this approach could also be applied to contaminated material that has been relocated to a temporary treatment area, as described in Section 2.2. Theoretically, this approach would be the most expensive method of phytoremediation because of the costs of preparing/extracting the plant enzymes; however, in some plants, such as tarragon,
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(Artemisia dracunculas var satiya), exudates are released under stress that could result in reduced production costs. One important factor to consider for this approach is the length of time the enzymes remain active for breakdown of contaminants.
3. Mechanisms of phytoremediation There are numerous mechanisms by which plants may remediate contaminated sites. In some cases, the transformation takes place by live plants as described in Section 2.1 and Section 2.2. Some of the chemicals that can be treated by these mechanisms are listed in Table 2. Plant exudates or enzymes that are responsible for the breakdown of compounds are summarized in Table 3 and Table 4. Some of the factors affecting chemical uptake and distribution within living plants include: (1) physical and chemical properties of the compound (e.g. water solubility, vapor pressure, molecular weight, and octanol-water partition coefficient, Kow); (2) environmental characteristics (e.g. temperature, pH, organic matter, and soil moisture content); (3) plant characteristics (e.g. type of root system, and type of enzymes). Some of the mechanisms used by plants to facilitate remediation include: phytoextraction, phytopumping, phytostabilization, phytotransformation/degradation, phytovolatilization, and rhizodegradation, which are described in the following sections.
3.1. Phytoextraction/phytoaccumulation Phytoextraction is the removal of a contaminant from the soil, ground water or surface water by live plants. Phytoaccumulation occurs when the contaminant taken up by the plant is not degraded rapidly or completely, resulting in an accumulation in the plant. Certain plants hyperaccumulate metals (e.g. nickel, zinc, copper, chromium), and radionuclides. Heavy metal hyperaccumulation is defined as accumulation of more than 0.1% by dry weight in plant tissue (0.01% for cadmium. Hyperaccumulation of more common elements such as iron and manganese is defined as more
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650 Table 2 Phytoremediation mechanisms Type
Chemicals Treated
Phytoaccumulation/ phytoextraction,
Cadmium, chromium, lead, nickel, zinc and other heavy metals, selenium, radionuclides; BTEX (benzene, ethyl benzene, toluene and xylenes), pentachlorophenol, short-chained aliphatic compounds, and other organic compounds Munitions (DNT, HMX, nitrobenzene, nitroethane, nitromethane, nitrotoluene, picric acid, RDX, TNT), atrazine; chlorinated solvents (chloroform, carbon tetrachloride, hexachloroethane, tetrachloroethene, trichloroethene, dichloroethene, vinyl chloride, trichloroethanol, dichloroethanol, trichloroacetic acid, dichloroacetic acid, monochloroacetic acid, tetrachloromethane, trichloromethane), DDT; dichloroethene; methyl bromide; tetrabromoethene; tetrachloroethane; other chlorine and phosphorus based pesticides; polychlorinated biphenols, other phenols, and nitriles Proven for heavy metals in mine tailings ponds and expected for phenols and chlorinated solvents (tetrachloromethane and trichloromethane) Polycyclicaromatic hydrocarbons; BTEX (benzene, ethylbenzene, toluene, and xylenes); other petroleum hydrocarbons; atrazine; alachlor; polychlorinated biphenyl (PCB); tetrachloroethane, trichloroethane and other organic compounds Chlorinated solvents (tetrachloroethane, trichloromethane and tetrachloromethane); mercury and selenium Heavy metals, organic chemicals; and radionuclides
Phytodegradation/ phytotransformation
Phytostabilization Phytostimulation
Phytovolatilization Rhizofiltration
than 1% of the element by dry weight in plant tissue (0.01% for Cadmium). In the process of hyperaccumulating contaminants (Dushenkov et al., 1995), some plants can remediate the contaminated soils to acceptable levels. Some plants can grow in contaminated areas and tolerate hyperaccumulation of metals and other contaminants such as perchlorate (Susarla et al., 1999). Other plants may die or experience severe stress under conditions of hyperaccumulation. Less tolerant plants can still be used in areas of contamination then harvested and disposed after these plants have hyperaccumulated the contaminant to their maximum extent. Such crops can be replanted, if necessary, to complete this remediation. Another option is to recover the contaminant after harvesting the plants (Dushenkov et al., 1995; Nanda Kumar et al., 1995; Moffat, 1995; Kelley and Guerin, 1995; Cornish et al., 1995; Wang et al., 1995; Ban˜ uelos et al., 1998). If the remediation goal is to harvest after these plants hyperaccumulate a contaminant, then it is desirable for the selected plants to be able to translocate the contaminant from the root into above ground tissue, such as shoots and leaves (Nellessen and Fletcher, 1993). If the contaminant remains in the roots, harvesting for disposal or recovery may be more difficult.
3.2. Phytopumping and water balance control Phytopumping is another mechanism that can be used to remove or minimize migration of contaminants. In this case, plants are used as organic ‘pumps’ to pull-in large volumes of the contaminated water as part of the transpiration process. The result is reduced migration of the contaminant in ground water, in addition to potential uptake. Plants that are capable of removing large amounts of water from the soil are best for this purpose. For example, the willow tree (Salix spp.) may use up to 200 liters of water per day (Gatliff, 1994). Plants having these characteristics may provide an inexpensive alternative to mechanical pump and treat systems for contaminated ground water in shallow aquifers (Gatliff, 1994; Licht, 1995).
3.3. Phytostabilization Phytostabilization is another mechanism that can be used to minimize migration of contaminants in soils. This process takes advantage of plant roots ability to alter soil environment conditions, such as pH and soil moisture content. Many root exudates cause metals to precipitate,
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thus reducing bioavailability. One advantage of this strategy over phytoaccumulation is the disposal of the metal-laden plant material is not required. By choosing and maintaining an appropriate cover of plant species, coupled with appropriate soil amendments, it may be possible to stabilize certain contaminants (particularly metals) in the soil (Cunningham et al., 1995), and reduce the interaction of these contaminants with associated biota.
3.4. Phytotransformation/phytodegradation A contaminant can be eliminated via phytodegradation or phytotransformation by plant enzymes or enzyme co-factors (Dec and Bollag, 1994; Strand et al., 1995). Dec and Bollag (1994) describe plants that can degrade aromatic rings in the absence of microorganisms. Polychlorinated biphenyls (PCBs) have been metabolized by sterile plant tissues. Phenols have been degraded by plants such as horseradish, potato (Solanum tuberosum), and white radish (Raphanus sati6us) that contains peroxidase (Dec and Bollag, 1994; Roper et al., 1996). Poplar trees (Populus spp.) are capable of transforming trichloroethylene in soil and ground water (Strand et al., 1995; Newman et al., 1997). Enzymes of particular interest for phytoremediation include: (1) dehalogenase (transformation of chlorinated compounds); (2) peroxidase (transformation of phenolic compounds); (3) nitroreductase (trans-
formation of explosives and other nitrated compounds); (4) nitrilase (transformation of cyanated aromatic compounds); and (5) phosphatase (transformation of organophosphate pesticides).
3.5. Phyto6olatilization Phytovolatilization is a mechanism by which plants convert a contaminant into a volatile form, thereby removing the contaminant from the soil or water (Terry et al., 1995) at a contaminated site. For example, plants, possibly in association with microorganisms, can convert selenium to dimethyl selenide. Dimethyl selenide is a less toxic, volatile form of selenium. Phytovolatilization may be a useful, inexpensive means of removing selenium from sites contaminated with high concentration selenium wastes. Similarly, some transgenic plants (e.g. Arabidopsis thaliana) have converted organic and inorganic mercury salts to the volatile, elemental form (Watanabe, 1997).
3.6. Rhizodegradation Rhizodegradation is a biological treatment of a contaminant by enhanced bacterial and fungal activity in the rhizosphere of certain vascular plants. The rhizosphere is a zone of increased microbial density and acitivity at the root/surface, and was described originally for legumes by Lorenz Hiltner in 1904 (Curl and Truelove, 1986). Plants and microorganisms often have symbiotic
Table 3 Plant enzymes that have a role in transforming organic compounds Enzyme
Plants known to produce enzymatic activity
Application
Dehalogenase
Hybrid poplar (Populus spp.), algae (various spp.), parrot feather (Myriophyllum aquaticum) Stonewort (Nitella spp.), parrot-feather (Myriophyllum aquaticum) Willow (Salix spp.) Hybrid poplar (Populus spp.), Stonewort (Nitella spp.), parrot feather (Myriophyllum aquaticum)
Dehalogenates chlorinated solvents
Laccase Nitrilase Nitroreductase
Peroxidase Phosphatase
Horseradish (Armoracia rusticana P. Gaertner, Meyer & Scherb) Giant duckweed (Spirodela polyrhiza)
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Cleaves aromatic ring after TNT is reduced to triaminotoluene Cleaves cyanide groups from aromatic rings Reduces nitro groups on explosives and other nitroaromatic compounds, and removes nitrogen from rings structures Degradation of phenols (mainly used in wastewater treatment) Cleaves phosphate groups from large organophosphate pesticides
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Table 4 Plant species used in phytoremediation of organic compounds Plant species
Contaminant
Reference
Barley (Hordeum 6ulgare L. cv. Klages) Forage grasses
Hexachlorobenzene, PCBs, pentachlorobenzene, trichlorobenzene Chlorinated benzoic acids
Parrot feather Hybrid poplar
Tetrachloroethane (PCE), Trichloroethane (TCE), TNT Atrazine, nitrobenzene, TCE, TNT
Prairie grass Soyabean (Glycine max [L.] Merr. Cv. Fiskby v) Eurasian watermilfoil (Myriophyllum spicatum) Waterweed (Eichhornia crassipes)
2-chlorobenzoic acid Bromacil, nitrobenzene, phenol
McFarlane et al., 1987 Siciliano and Germida, 1998 Best et al., 1997 Burken and Schnoor, 1997 Topp et al., 1989 Fletcher et al., 1990
TNT
Hughes et al., 1997
Pentachlorophenol, PCE, TCE
Roy and Hanninen, 1994
relationships making the root zone or rhizosphere an area of very active microbial activity (Anderson et al., 1993, 1994; Schwab et al., 1995; Jordahl et al., 1997; Siciliano and Germida, 1998). Plants can moderate the geochemical environment in the rhizosphere, providing ideal conditions for bacteria and fungi to grow and degrade organic contaminants. Plant litter and root exudates provide nutrients such as nitrate and phosphate that reduce or eliminate the need for costly fertilizer additives. Plant roots penetrate the soil, providing zones of aeration and stimulate aerobic biodegradation. Many plant molecules released by root die back and exudatation resemble common contaminants chemically and can be used as co-substrates. For example, phenolic substances released by plants have been found to stimulate the growth of PCBdegrading bacteria (Fletcher et al., 1993; Donnelly and Fletcher 1994a; Fletcher and Hegde, 1995; Fletcher et al., 1995). Recent studies have described enhanced degradation of pentachlorophenol in the rhizosphere of wheat grass (Agropyron cristatum) (Ferro et al., 1994), increased initial mineralization of surfactants in soil-plant cores (Knabel and Vestal, 1992), and enhanced degradation of TCE in soils collected from the rhizospheres. Anderson et al., (1993) provides a review of microbial degradation in the rhizosphere. Thus, current research suggests the interaction between
plants and soil microbes may be an important factor influencing biological remediation of contaminated soils.
3.7. Combined mechanisms In many cases, phytoremediation involves combinations of the mechanisms described above. For example, phytoextraction and phytovolatilization have been credited with the removal of excess selenium in soil (Cornish et al., 1995). It is likely that both processes occur simultaneously. The treatment of TCE in ground water using poplar trees requires extraction of the ground water by the plant (phytoextraction) that will also degrade TCE (phytodegradation) within the plant. Another example is degradation of PCB’s by plant cells (Fletcher et al., 1987), as well as by microorganisms stimulated by plants (Fletcher et al., 1993; Donnelly and Fletcher, 1994b; Fletcher and Hegde, 1995; Fletcher et al., 1995), creating the opportunity to combine phytodegradation and rhizodegradation. 4. Applications of phytoremediation Selected examples of the application of phytoremediation for hazardous chemicals using the previously described mechanisms are listed in Table 5.
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4.1. Munitions Numerous researchers have documented the ability of plants to take-up and transform trinitrotoluene (TNT) from soil and water (Palazzo and Leggett, 1986; Folsom et al., 1998; Cataldo et al., 1989; Young, 1995; Mueller et al., 1995; Schnoor et al., 1995; Medina and McCutcheon, 1996; Best et al., 1997; Medina et al., 1998). Axenic studies have shown that plants are capable of transforming TNT without microbial contribution (Vanderford et al., 1997; Hughes et al., 1997). However, very little accumulation of TNT has been found in plant material (Hughes et al., 1997; Vanderford et al., 1997). The rates of phytotransformation compare favorably to those of published microbial studies, and there appears to be no lag phases in plant based TNT remediation (Medina and McCutcheon, 1996). Hughes et al., (1997) provided the first confirmation that plants have the intrinsic capacity to transform TNT. In their experiments, axenic parrot feather (Myriophyllum spicatum) was used to demonstrate transformation and to determine mass balances using radiolabeled TNT. The results showed that complete mineralization of the TNT was not observed, but that no TNT or 2-amino-4,6-dinitrotoluene or 4-amino-2,6-dinitrotoluene were indentified in extracellular media or tissue extracts. However, several unidentified metabolites were observed in the plant. The unknown characteristics of these transformation products are of concern in the potential use of phytoremediation for sites contaminated with TNT. Vanderford et al. (1997) reported that TNT accumulated primarily in the roots of ax-
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enic parrot feather (Myriophyllum aquaticum). As incubation time increased, the plant-bound radiolabelled fraction, consisting largely of unidentified transformation products, became increasingly more difficult to extract. Another concern is that some species selected for laboratory-scale testing that performed satisfactorily, such as Eurasian water-milfoil, are aggressive non-native species (Godfrey and Wooten, 1981). Establishment and spread of many of these nonnative plant species have resulted in considerable economic burden and environmental degradation by displacing valuable native plant and animal species. Pilot studies have been conducted to determine the treatment performance of TNT-contaminated ground water by constructed wetlands. In order to model the disappearance of TNT and the associated breakdown, a pseudo-first order, non-reversible reaction, finite difference model was used with batch-scale experiments to determine disappearance kinetics for individual species (Medina and McCutcheon, 1996). The results of the model suggested that reasonably sized wetlands could treat a waste stream with an influent TNT concentration of 2.25 mg l − 1 at flow rates ranging from 40 to 20 000 l min − 1. Continuous, flowthrough pilot systems treating 0.001 to 0.01 g l − 1 solutions of TNT resulted in rapid removal of TNT at a retention time of 12 days (Medina et al., 1998). However, aminodinitrotoluene (ADNT), a transformation product, was identified in the effluent of the reactor. Increasing the residence time to 76 days decreased but did not eliminate the effluent concentration of ADNT. No TNT was found in extractions from the plant
Table 5 Examples of phytoremediation test sites (EPA, 1996) Location
Application
Contaminants
Medium
Plant
Ogden, UT
Phytoextraction
PAHs
Portsmouth, VA Milan, TN Aberdeen, MD
Rhizofiltration, phytodegradation Phytodegradation Organic pumping phytovolatilization rhizofiltration
PAHs TNT TCE, PCE
Soil, groundwater Soil Groundwater Groundwater
Alfalfa (Medicago sati6a), hybrid poplar trees Grasses, clover (Trifolium spp.) Duckweed, parrot feather Hybrid poplar trees
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tissue. However, small concentrations (up to 0.7490.133 mg/kg) of ADNT, as well as some trinitrobenzene (up to 0.0879 0.031 mg/kg), and a small concentration of 3,5-dinitroaniline (up to 0.0809 0.020 mg/kg) were found in plant tissues. Thompson et al. (1998a) investigated the use of hybrid poplar trees (Populus deltoides × nigra DN34) to treat soil contaminated with TNT. In soil, TNT uptake was governed by its ability to binding to soil particles (Thompson et al., 1998b). When the plant accumulated small amounts of TNT, which remained primarily in the roots, the contaminant was transformed during a 10-day experiment. A small amount of TNT was translocated to the leaves during the same period of time. However, in aqueous experiments more TNT was taken-up by the same plant. The accumulated TNT was transformed to 2-amino-4,6-dinitrotoluene, and 4-amino-2,6-dinitrotoluene, and into unidentifiable, insoluble, polar compounds that were bound in the plant tissue. The TNT also was found to have measurable toxic effects on hybrid poplar trees (Thompson et al., 1998b). However, removal and degradation rates were rapid enough to justify using poplar trees as an experimental treatment technology for TNT based on biomass production. Much less work has been done on the treatment of other munitions, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraocine (HMX). However, preliminary results suggest that treatment of these compounds via plants is not as successful as phytoremediation of TNT (Best et al., 1997; Medina et al., 1998; Sikora et al., 1998). Results of at least one study suggest phytodegradation of nitroglycerin can be accomplished (Goel et al., 1997). In conclusion, phytoremediation of TNT is promising due to rapid removal coupled with extensive transformation rather than phytoaccumulation. Unfortunately, the exact toxicity of the transformation byproducts is not understood. Furthermore, the formation and temporary accumulation of amino dinitrotoluenes needs investigation. Finally, more work is needed to investigate the treatment of other munitions, particularly RDX and HMX.
4.2. Chlorinated sol6ents Chlorinated solvents, such as TCE and PCE, are major contaminants of the soil and ground water in the US. They are used in many forms, including as anesthetics by the medical community, degreasers by industry and the military, solvents by the dry cleaning industry and other forms by the general public. Past widespread use and indiscriminate disposal make chlorinated solvents the most common pollutants in the country. Phytoremediation has been applied successfully to treat sites contaminated with chlorinated solvents. The initial data on TCE degradation in plant–rhizosphere–soil systems provided a strong incentive to explore the general interactions between the soil, plant, root and associated microbes, in addition to the variables that may influence biodegradation of waste chemicals in surface and near-surface soils. Later work by Anderson and Walton (1995) indicated that TCE is oxidized more readily in the rhizosphere of certain types of plants, such as pine (Pinus spp.) or legumes, as compared with unvegetated soil. However, the mineralization of TCE was limited. Uptake of TCE by plant roots was correlated with water uptake, but total uptake was small and translocation insignificant. Others have reported that plants also take up TCE vapor volatilized from the soil (Newman et al., 1997). Strand et al. (1995) demonstrated that hybrid poplar trees have the capability to assimilate and degrade the chlorinated solvent TCE to the aerobic degradation products 2,2,2-trichloroethanol, trichloroacetic acid, and dichloroacetic acid. In that study, dichloroacetic acid detected in leaf tissues was the most prominent metabolite. In similar studies, Schnabel et al. (1997) investigated the uptake and transformation of TCE by edible garden plants. Carrots (Daucus carota 6ar. sati6a), spinach (Spinacia oleracea), and tomatoes (Lycopersicon esculentum) were exposed to [14C] radiolabeled and unlabeled TCE. Most of the TCE (74 to 95%) was volatilized through the plants. The remaining TCE was adsorbed to soil particles. Very small, non-extractable amounts (1 to 2%) of the [14C]-label were found in the plant tissue. This suggested that the TCE was taken-up, transformed, and bound to the plant material.
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Newman et al. (1997) were first to report degradation of TCE into several known oxidized metabolites in hybrid poplar plant tissues, and demonstrated the potential for the use of poplars for in situ remediation of TCE. In their study, axenic poplar cell cultures were used to eliminate any aspects of microbial degradation. Recently, the uptake and transformation of TCE, PCE, and other chlorinated solvents were examined in a number of aquatic plants including waterweed (Elodea canadensis), parrot feather (M. aquaticum) and giant duckweed (Spirodela polyrhiza). Rennels (1995) also reported that these aquatic plants take-up TCE and PCE, then break the chemicals down to a number of compounds. These results support the potential usefulness of aquatic plants in the removal of chlorinated contaminants from contaminated ground and surface waters. Although the studies referenced above made significant contributions to the understanding of phytotransformation of chlorinated solvents, additional research is necessary to evaluate the long-term field performance of these plants, as well as the effects on animals that may use these plants.
4.3. Other aromatic compounds Chlorinated aromatics, including pesticides, such as atrazine and 1,1,1-trichloro-2,2-bis(pchlorophenyl)ethane (DDT), are common contaminants in soils, surface and ground waters. Phytoremediation has been effective in treating these types of contaminants. For example, the uptake and translocation of phenol, nitrobenzene and bromocil were related directly to transpiration rate in mature soybean plants (McFarlane et al., 1987). Recently, the use of minced horseradish roots has been proposed for the decontamination of surface waters polluted with chlorinated phenols (Roper et al., 1996). In sealed-batch experiments using waterweed, p,p%-DDT and o,p%-DDT were degraded (Garrison et al., 1996) with 1,1dichloro - 2 - (o - chlorophenyl) - 2 - (p-chlorophenyl)ethane (o,p =-DDD) and 1,1-dichloro-2-(pchlorophenyl)-2-(p-chlorophenyl)ethane (p,p = DDD) appearing as metabolite products. No
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accumulation of DDD was observed during the course of the experiments. Burken and Schnoor (1997) used poplar trees for uptake and metabolism of the pesticide atrazine. Results indicted that poplar trees can take-up, hydrolyze, and dealkylate atrazine to less toxic metabolites. In this study, the parent compound atrazine, and [14C]-radiolabeled metabolites were separated and identified for the first time. Transformation occurred in roots, stems, and leaves. These findings suggest that hybrid poplar trees have potential for phytoremediation of sites contaminated with atrazine.
5. Conclusions
5.1. Constraints of using plants for remediation of hazardous wastes Despite the diversity of potential options, phytoremediation is in its infancy. The majority of the research has been conducted in laboratories under relatively controlled conditions for short periods of time. More extensive research under field conditions for longer durations is required for a better understanding of the potential role of phytoremediation. A limiting factor of phytoremediation treatment includes the fact that a specific phytoremediation ‘prescription’ cannot be applied to every site with a certain chemical contaminant because different site-specific conditions (e.g. soil and climate) may not be suitable for the target plant. Plants also interact with and are affected by other living organisms such as insects, pests and pathogens, and exposure of plants to contaminants and related stresses can make the phyto treatment more susceptible to these other agents, ultimately influencing the outcome of phytoremediation attempts. Additionally, phytoremediation generally is restricted to sites where the concentration levels of contaminants are not toxic to the plants proposed for remediation. Finally, the contaminants must be accessible to the tissue responsible for uptake (e.g. root system) in plants. As a result, in situ phytoremediation using live plants is restricted to sites conducive to growth of the selected plant with the contaminant located
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within the potential root zone of the selected plant.
5.2. Ad6antages of using plants for phytoremediation of organic chemical contaminated sites Despite constraints referenced above, plants have many features that result in a high potential for environmental cleanup. Energy costs and expenses are reduced and natural resources are conserved because plants use solar energy. Plants are adapted to a wide range of environmental conditions and are capable of modifying conditions of the environment to some extent. The unique enzyme and protein systems of some plant species appear to be beneficial for phytoremediation. Additionally, since plants lack the ability to move, many plants have developed unique biochemical systems for nutrient acquisition, detoxification, and controlling local geochemical conditions. Some plants that grow well in nutrient poor soils may have useful mechanisms for removing and transforming contaminants that resemble certain nutrients. Infiltration is a primary pathway in contaminant migration to ground water, and plants play an important role in regulating water content in soil. Plant roots aerate the soil, which may stimulate microbial activity in the soil. Root exudates may be a nutrient source for microorganisms, since the rhizosphere generally contains significantly higher numbers and more active microorganisms than similar soils without plants. Thus, plants can contribute in many ways to enhance biodegradation in the soil. Phytoremediation provides an aesthetically pleasing alternative to structural remediation and decontamination technologies. As a result of these advantages, phytoremediation has considerable potential for environmental restoration of contaminated sites.
Acknowledgements Sydney Bacchus provided information on aquatic plants. The strategic Environmental Research and Development Program (Project c720 managed by the U.S. Army Waterways Experi-
ment Station), U.S. Air Force Human Systems Centre, U.S. Air Force Restoration Division, and U.S. Navy Southern Command provided support during the writing of this review.
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