Carman, Eric P. & Crossman, Tom L. "Phytoremediation" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 9 Phytoremediation Eric P. Carman and Tom L. Crossman CONTENTS Introduction Phytoremediation Applications Types of Vegetation Currently Used in Phytoremediation Benefits and Limitations to Phytoremediation Phytoextraction/Phytovolatilization Phytoextraction Case Histories Phytoextraction and Accumulation of Lead, Magic Marker Site, Trenton, New Jersey Phytoextraction and Degradation of TCE, Controlled Field Study, Washington Phytostabilization Phytostabilization Case History Whitewood Creek Site, South Dakota Enhanced Rhizosphere Degradation Enhanced Rhizosphere Degradation Case Histories Craney Island Fuel Terminal, Virginia Active Industrial Facility, Wisconsin Operation and Maintenance Confirmatory Soil Samples Rhizofiltration Rhizofiltration Case History Rhizofiltration, Milan Army Ammunition Plant, Milan, Tennessee Hydraulic Containment Hydraulic Containment Case Histories ©2001 CRC Press LLC Gasoline Station, Ohio Wood Preservative Site, Tennessee Hydraulic Gradients Water Balance and Flow Model Operation and Maintenance Laboratory Results Alternative Covers (Phyto-Covers) Benefits of a Phyto-Cover Case Histories for Phyto-Covers Municipal and Industrial Landfill, Tennessee Construction of Phyto-Cover Lakeside Reclamation Landfill, Beaverton, Oregon Phytoremediation Engineering Considerations The Future of Phytoremediation Further References References INTRODUCTION Phytoremediation is a diverse and emerging technology that uses green plants to cleanup contaminated environmental media. As phytoremediation has been increasingly recognized, the technology has been applied both in situ and ex situ to contaminated soil, sediment, sludge, groundwater, surface water, and wastewater. In addition, the natural evapotranspiration process of vegetation has been recognized and harnessed as an alternative cover method to reduce landfill infiltration. Although it is now being increasingly applied for environmental mediation, phytoremediation is not a new technology. The Roman civilization reportedly used eucalyptus trees to dewater saturated soils more than two thousand years ago. The excess water use by some plants, namely phreatophytic (waterloving) trees, has been long recognized as a nuisance in the agricultural industry, particularly in more arid regions. Water levels next to cottonwood and willow trees (two common phreato- phytes) in the southwestern United States are known to drop several feet during growing seasons. The principles of phytoremediation which are currently gaining acceptance for contaminant remediation have been reported in the scientific literature only since the late 1970s or early 1980s. The research, development, and application of this technology increased dramatically in the late 1980s and early 1990s because it is low cost and versatile, and in some cases has better public support as a method to cleanup contaminated media. Phytoremediation was first implemented and reported as an environmental cleanup technology for agricultural contaminants such as excess plant nutrients (nitrate, ammonia, and phosphate) and pesticides (Briggs, Bromilow, and Bromilow 1982), although the principles of phytoremediation have been applied in the wastewater industry for many years. USEPA has recently esti- mated that there are currently more than 100 sites around the world where phytore- mediation is being implemented as a remedial technology. ©2001 CRC Press LLC Phytoremediation Applications The USEPA has identified six broad applications of phytoremediation (USEPA 1998). These applications and their definitions include: • Phytoextraction/Phytovolatilization—The uptake and translocation of organics and inorganics from the soil into the roots and above-ground portions of the plant. Organics that are extracted are subsequently either degraded within or volatilized from the plant tissue. Inorganics that are extracted accumulate and/or are methy- lated and volatilized from the plant tissue. • Phytostabilization—The use of plants to immobilize organic and inorganic con- stituents in the soil and groundwater through adsorption and accumulation by roots, adsorption onto roots, or precipitation within the rhizosphere. Phytostabilization also includes site revegetation which reduces windblown dust and direct contact with contaminants. • Enhanced Rhizosphere Degradation—The breakdown of organic constituents in the soil through microbial activity that is enhanced by processes within the rhizo- sphere. • Rhizofiltration—The absorption, adsorption, or precipitation of contaminants that are in solution surrounding the roots. • Hydraulic Containment—The use of plants, especially phreatophytes, to control the migration and flow of porewater, shallow groundwater, and contaminants dis- solved in the groundwater. • Alternative Covers (Phyto-Covers)—The use of vegetation as a long-term, self- containing cap growing in and/or over waste in a landfill. Phytoremediation can be an effective technology to address both organic and inorganic constituents. Plants remediate organic compounds through several mech- anisms. Organics can be taken up directly from the rhizosphere (defined as a zone of increased microbial activity at the root-soil interface that is under the influence of the plant root) and either metabolized by the plant, accumulated in the plant tissue, or transpired through the leaves (Schnoor et al. 1995 and Newman et al. 1999). These mechanisms are vital in the applications of phytoextraction, phytosta- bilization, and enhanced rhizosphere degradation. Figure 1 represents mass flow through a woody plant species. Water and nutrients are taken up by the plant and carbon dioxide, oxygen, water, and photosynthates are released to the environment. In the case of phreatophytes, such as trees from the willow and poplar genus ( Salix and Populus ), the volume of water taken up by a single tree can be from several gallons to several thousand gallons of available water per day. The processes occurring within the rhizosphere are integral to phytoremediation. Plants supply oxygen to the soil and release exudates, which include sugars, alcohols, amino acids, and enzymes. The exudates and enzymes enhance microbial growth and the growth of mycorrhizal fungi. The overall effect of the plant-microbe growth is an increase in microbial biomass by up to an order of magnitude or more, compared with microbial populations in the bulk soil. The microbes and mycorrhizal fungi ©2001 CRC Press LLC subsequently promote degradation and co-metabolism of organics (Schnoor et al. 1995). Organics are also taken up directly by plants and either accumulated, metabo- lized, or transpired through the leaf tissue. The fate of organics and inorganics in the rhizosphere, and the corresponding tendency of these constituents to be taken up by plants, can be predicted using the logarithm of the octanol-water partition coefficient (Kow) of the particular constituent. This relationship was reported by Briggs et al. (1982) and is commonly known as Brigg’s Law. Table 1 illustrates fate of organics using Briggs Law. Direct uptake of organics is an efficient process to remove moderately hydro- phobic constituents, with a log Kow ranging from 0.5 to 3. Organics within this range include many of the volatile organic compounds (VOCs) including benzene, toluene, ethylbenzene, xylene, chlorinated solvents (such as trichloroethylene [TCE]), and aliphatics. Generally, constituents with log Kow less than 0.5 are too water soluble to be taken up into roots, and constituents with a log Kow greater than 3.0 are bound too tightly to the soil particles or roots to be taken up into the plant. Figure 1 Mass flow through a woody plant. Table 1 Organic Fate Predictions Using Briggs’ Law log K ow Mechanisms <1.0 Possible Uptake & Transformation 1.0 - 3.5 Uptake, Transformation, Volatilization >3.5 Rhizosphere bioremediation or Phytostabilization * There are exceptions (1,4-dioxane) due to Brigg’s emphasis on agricultural organics (pesticides, herbicides) not on soil, groundwater contaminants (BTEX, TCE, etc.) encoun- tered in environmental remediation ©2001 CRC Press LLC Examples of organic compounds with a log Kow less than 0.5 include methyl tertiary butyl ether (MTBE), and 1,4-dioxane. Constituents with a log Kow greater than 3.0 include most polycylclic aromatic hydrocarbons (PAHs). It is also possible to predict the concentration of a contaminant that will absorb into the roots using the Root Concentration Factor (RCF). If the Kow is known, the RCF can be used to predict the ratio of the concentration in the roots, to the concentration in the external solution (Figure 2). However, Briggs Law is only generalized, and as research in the field of phy- toremediation increases, more constituents are likely to be found susceptible to treatment. Recent hydroponic studies at the University of Iowa suggest that 1,4- dioxane, a commonly detected solvent and suspected carcinogen with a log Kow of 0.27, is taken up and volatilized by the hybrid poplar Populus deltoides x nigra , DN34 (Kelley et al. 1999). In addition, recent laboratory tests and research hold promise that MTBE may also be susceptible to phytoremediation (Newman et al. 1998). Metals have posed a considerable challenge to remediation by conventional technologies, which are generally expensive ex-situ processes that involve removal and transportation to cleanup soil. Exposure pathways from sites that are contami- nated with metals include direct contact with the waste materials or soil/sediment contaminated by the metals, inhalation of windblown dust or particulate matter, and exposure to groundwater or surface water that has leached the metals. Remediation of metals-contaminated sites can include three possible changes in the chemical characteristics of the metal or the medium in which the metal is present. The concentration of the metal can be reduced by direct removal, the hazardous nature Figure 2 Root concentration factor (RCF) = (concentration in roots/concentration in external solution). ©2001 CRC Press LLC of the metal can be reduced without removing any of the metal (for example through an in situ method like solidification or vitirification), or the metal bioavailability can be reduced. The applications of phytoremediation that directly address metals and other inorganics include phytoextraction, phytostabilization, and rhizofiltration. Phytosta- bilization involves covering a site with vegetation, thereby reducing erosion, enhanc- ing soil nutrients, and eliminating direct contact and transport off-site of metals containing media such as wind and water. The effectiveness of phytostablization in limiting direct contact, and transport by wind and water of metals, was demonstrated at two Superfund sites in the Midwest (Pierzynski et al. 1994). Phytostabilization can also refer to the use of rhizosphere processes to tightly bind metals to soil within the rhizosphere, or to the root tissue itself. Exudates released in the rhizosphere can increase the soil pH up to 1.5 pH units and increase soil oxygen content, having a significant effect on the redox conditions of the soil and promoting oxidation, reducing mobility and bioavailability of metals (Azadpour and Matthews 1996). Phytoextraction is the use of vegetation to uptake and accumulate inorganics into plant tissue, both from the soil and from metals dissolved in pore water or shallow groundwater. Plants that accumulate high concentrations of metals are known as hyperaccumulators. Certain plant tissue and tree sap may contain up to 3 percent zinc and 25 percent nickel by dry weight, without apparent harm to the plant. Certain metals, including selenium and mercury can also be taken up, meth- ylated, and volatilized (Meagher et al. 1998 and Banuelos et al. 1998) Types of Vegetation Currently Used in Phytoremediation As the technology of phytoremediation expands, the types of plants identified for applications of phytoremediation for organic and inorganic compounds has expanded. Early efforts were focused on utilization of hybrid poplar trees, fast growing phreatophytic trees which have a well-documented physiology and genetic characteristics from their use in the pulp and paper industry and fuel from biomass research. Currently numerous types of vegetation, including trees and grasses, have been applied in phytoremediation to address VOCs, PAHs, radionuclides, pesticides, and herbicides. In addition, geobotanical exploration has revealed many more metal hyperaccumulators than were previously identified. Approximately 400 plant taxa are now known for Cd, Co, Cu, Pb, Ni, Se, and Zn hyperaccumulation (Flathman and Lanza 1998). Benefits and Limitations to Phytoremediation Phytoremediation is becoming recognized as a cost effective remedial method to address contaminated sites and landfills. Advantages to phytoremediation are its low capital cost, generally about one third to one fifth of the cost of more conven- tional technologies. In addition, this technology tends to have low costs for ongoing operation and maintenance (O&M), although it should not be construed as mainte- nance free. The combination of effectiveness, low cost, and low O&M make phy- ©2001 CRC Press LLC toremediation attractive for non-point source contamination, such as nitrates and pesticides in agricultural settings and parking lot runoff in urban areas. Phytoreme- diation also minimizes wind and water erosion and minimizes the production of undesirable waste by-products. Some plant species can also reduce the net infiltration of surface water, which minimizes the potential for leaching of contaminants into groundwater. Phreatophytes can take up large volumes of available water, and can be used to capture shallow groundwater (less than about 20 feet below land surface), in a manner analogous to conventional pump and treat systems. In addition, the technology can greatly improve soil conditions by increasing soil organic carbon, enhancing microbial and fungal populations, and humifying metals and recalcitrant organics by complexing the metals with soil organics (Schwab and Banks 1994). Phytoremediation has been accepted by the public, since it is environmentally compatible and can improve the long-term aesthetics of a site. Phytoremediation can be used as a single treatment technology, or it can be coupled with more aggressive conventional technologies. For example, contaminated soils from a site can be excavated and treated in engineered phytoremediation treatment units (EPTUs), rather than thermally treated or taken off-site and disposed of in a landfill. Contaminated groundwater can also be pumped from a site using conventional methods, and then used to irrigate trees or grasses, rather than treated using con- ventional technologies (e.g., air stripping or bioreactor). At a landfill in Oregon, the City of Beaverton uses effluent from the publicly owned treatment works (POTW) as irrigation water for hybrid poplars which have been planted as an alternative cover to their city landfill. Furthermore, these trees are periodically harvested and sold to a nearby paper mill for a net profit for the landfill (Madison, Licht, and Ricks 1991). Phytoremediation can also be integrated with landscape design practices, so that the remediation system is an attractive addition to the property. Despite the benefits of phytoremediation, there are disadvantages to the technol- ogy that make it unsuitable or undesirable for some environmental applications. Phytoremediation is a long-term remedial technology at most sites, with treatment times on the order of several years. In addition, the technology can be directly implemented only where the contaminants are present at depths within about 20 feet of the land surface. If vegetation is used for the purpose of extracting groundwater, the contaminants must be located within a few feet of the water table surface. Plants have adapted to grow in some of the most inhospitable conditions known to exist. However, phytoremediation will not be successful if soil conditions or contaminant characteristics/concentrations prove to be phytotoxic. In addition, some types of vegetation, while suitable for phytoremediation, may not be desirable or acceptable in certain applications. Phytoremediation of metals poses special considerations that can make its use impracticable at the current time. For example, the consequences of transferring contamination from soil or groundwater into plants that can enter the food chain must be considered, particularly for heavy metals such as lead and cadmium because of their known human health aspects (Mench et al. 1994). Research has focussed on improving the efficiency by which plants can uptake metals by introducing synthetic complexing agents. Although the addition of the agents can enhance root uptake, the complexing agents can also increase downward mobility of the metals ©2001 CRC Press LLC away from the rhizosphere, such that contamination spreads and poses a new threat to groundwater. As the technology matures, some of these limitations may be over- come, and other limitations will undoubtedly be identified. There is considerable overlap between several of the phytoremediation applica- tions, and in many cases the distinction between the applications becomes blurred. Furthermore, most phytoremediation projects will generally harness more than one of these six broad applications to achieve site remediation. A more detailed descrip- tion of each of the six USEPA-described applications of phytoremediation and several case histories are presented below. PHYTOEXTRACTION/PHYTOVOLATILIZATION Phytoextraction refers to the uptake and translocation of contaminants into the roots and above-ground portions of plants. Phytovolatilization refers to the gaseous discharge of methylated inorganic or organic compounds from the plant tissue. Phytoextraction/phytovolatilization in particular is an example of how the distinction between phytoremediation applications can overlap. For example, phytoextraction has been used to describe the uptake and translocation and accumulation of inorganic compounds by plants, specifically metals or radionuclides. However, some organic compounds (e.g., TCE) can also be extracted from the subsurface, and subsequently degraded within or volatilized from the plant tissues. For the purpose of this chapter, we have grouped phytoextraction/phytovolatilization of inorganics and organics together, and we will present two case histories that highlight applications for both types of compounds. Although both inorganics and organics can be extracted by plants, the fate of the compounds once extracted by the plant are very different. Inorganics, such as metals, tend to accumulate in the roots and shoots and phytoextraction of metals capitalizes on the tendency of some metals to relocate from soil or water to plant tissue. When in plants, the metals can be more cost effectively disposed of than in soil, sediment, or groundwater. Inorganics can also be methylated and volatilized from leaf tissue. Meagher et al. (1998) have shown that engineered plant species containing bacterial genes allowed plants to convert root extracted ionic mercury and methyl mercury to metallic mercury. The metallic mercury is then volatilized from the plants at rates which are below those that would cause airborne mercury hazard. The relative tendency of plants to uptake, immobilize, or exclude metals is highly contaminant specific and soil specific. Soil factors that influence the tendencies include: • Soil pH—increases in soil pH generally reduce the solubility of metals and the uptake of plants • Cation exchange capacity (CEC)—increases in CEC of soil reduces plant uptake • Organic matter—inorganic forms of metals are generally taken up more readily than organic forms ©2001 CRC Press LLC • Natural and synthetic complexing agents—the presence of complexing agents such as ethylene-diaminetetra-acetate (EDTA) and diethylene-triaminepenta-acetic acid (DPTA) generally increases the solubility of metals, making them more available to roots and more likely to be taken up and accumulated in a plant Plants that grow in environments with high concentrations of metals can either adapt to accumulate the metals, or exclude, or avoid the metals. Hyperaccumulators avoid the toxic effects of metals, such as clorosis, necrosis, disruption of chlorophyll synthesis, alteration in water balance, and stunted growth by binding the metals to cell walls, pumping metal ions into vacuoles, or complexing heavy metals by organic acids (Azadpour and Matthews 1996, and Pierzynski et al. 1994). Excluder plant species may absorb heavy metals, but restrict their transport to the shoots of the plants. This type of heavy metal tolerance does not prevent uptake of heavy metals, but restricts translocation, and detoxification of the metals takes place in the roots. Mechanisms for excluder detoxification include immobilization of heavy metals on cell walls, exudation of chelate ligands, or formation of a redox or pH barrier at the plasma membrane (Taylor 1987). Organics, once extracted by a plant, tend to be broken down and metabolized, or volatilized from the leaf tissue. Whether or not organics are extracted by the plant is generally dictated by Brigg’s Law, which was discussed previously in this chapter. Regardless, the reader needs to be aware that most phytoremediation sites incorporate more than one of the six applications. Even if the main design application for the plants is something other than extraction/volatilization, these processes may also be occurring during the remediation. Phytoextraction Case Histories Phytoextraction and Accumulation of Lead, Magic Marker Site, Trenton, New Jersey This Brownfield site located in Trenton, New Jersey has been the focus of a Superfund innovative technology evaluation (SITE) demonstration project that addresses lead contaminated surface soils in a residential/commercial part of the city. Contamination of the Magic Marker site resulted from various manufacturing processes, including lead-acid battery production between 1947 and 1987. The site soils consist of gravelly sand and miscellaneous debris, and site inves- tigations identified lead in the upper 0.61 meters (2 feet) of soils that exceed the regulatory limit of 400 mg/kg. Lead contamination, ranging from 200 to 1,800 mg/kg, exhibited considerable variation across the site. The demonstration project evaluated a total of three crops grown in a 9.1 x 17.4 meter (30 x 57 foot) plot and compared the results to a 9.1 x 12.2 meter (30 x 40 foot) control plot. Two crops of Brassica junacea (Indian Mustard) plants were grown for a 6 week period and harvested over the spring and summer of 1997. One crop of sunflower plants was grown in the summer of 1998. Harvested plant tissue samples were collected to evaluate the amount of lead uptake in each crop, and soil samples were collected to evaluate the change in lead concentrations in the root zone. EDTA and other amend- [...]... including pruning and fertilization, and application of insecticides has been performed on a monthly basis since 199 7 Water levels after planting in 199 7 and during June 199 9 are shown on Figure 9 and hydraulic gradients across the site are shown on Figure 10 The potentiometric surface map of May 5, 199 7 (one month after the trees were planted), compared to the potentiometric map from June 30, 199 9,... Table 2 Mass Balance for Chlorine in Cells with Hybrid Poplars, Washington Field Study mol of chlorine or chloride ion 199 5 TCE-chlorine lost from water in systema TCE-chlorine recovered from transpirationd TCE-chlorine recovered from oxidative metabolites in plant tissued leaves branches trunk rootsg excess chloride ion in soilb chloride balance recovery efficiency 199 6 199 7b 3-year total loss 2.75 13.4... and/or understory grasses showing worm and/or insect infestation were sprayed with appropriate insecticide (Sevin) during April 199 9 In addition, the area was mowed and trimmed to keep weed growth and water competition to a minimum and fertilizer (N:P:K 10:10:10) was spread along the tree lines in April 199 9 Future O&M activities will include watering (if necessary), continued inspection and fertilization... trench in uence (MW-1 and MW-2) were compared to wells located near the in uence of the phytoremediation and trench (P-2, P-15, and P-16) From June 3 to July 29, 199 8, water levels decreased 1 foot in the wells closer to the trench (Loftis 199 9) Figure 13 presents a cross section of water level elevations across the site from west to east This figure presents data from August 199 8 through June 199 9 and... Specified Implementation costs for Craney Island included 2 years O&M, based on fullscale design presented in Banks et al, 199 9 Active Industrial Facility, Wisconsin During the late 197 0s, a section of below-ground piping transferring No 2 fuel oil from a larger above-ground storage tank (AST) failed at this active industrial facility in Wisconsin, resulting in a subsurface release Remediation was immediately... growing season in 199 9 Operation and Maintenance Site visits were made periodically during the first growing season during 199 6 to monitor the growth of the trees, precipitation at the site, and general health of the trees Precipitation was monitored using data from a local meteorological station and plant tissue samples were collected from trees in the four hot spots during October 199 6 to determine... to 15 August 199 7 c Corrected for presence of reductive dechlorination products d Leaf areas and mass of tree tissues per cell were adjusted by the ratio of tree heights at the end of the respective growing season to that measured at the end of 199 6 e Calculated using average of leaf bag assays in 199 6, 1.5 x 1 0-1 1 mol h-1 cm-2 leaf f No TCE was recovered from the leaf bag assay in 199 7 g Mass of roots... transpiration was estimated to be only 9 percent of the TCE lost from the cells during 199 6 No transpired TCE was detected in tests conducted in 199 7 The results from the rhizosphere study conducted in 199 6 did not indicate the presence of rhizosphere degradation of TCE in soil Chloride concentrations in soil from cells that were planted with the hybrid poplars and dosed with TCE contained higher concentrations... x 1 0-3 0.03 x 1 0-3 0.05 x 1 0-3 0.03 x 1 0-3 ND 0.006 0.005 0.01 0.006 ND 0.002 0.002 0.003 0.002 18 0.008 0.007 0.013 0.008 18 19. 2 70% 27.5 3-year total recovered a Chlorine added in the form of TCE-chlorine was balanced against the amount of TCEchlorine, metabolite-chlorine and free chloride ion recovered from the system Masses given cover the three years that the experiment ran ND, not determined... exhibited yellowing, however, contained lower concentrations of nitrogen than leaves that exhibited normal green color Based on the results from tissue sampling, a fertilization program was initiated beginning in the fall of 199 6 and has continued through 199 9 The program consists of surface applications of a high nitrogen fertilizer in the rows between the trees Subsequent tissue analysis indicates that . Chlorine in Cells with Hybrid Poplars, Washington Field Study mol of chlorine or chloride ion 3-year total loss 3-year total recovered 199 5 199 6 199 7 b TCE-chlorine lost from water in. chloride in soil in one of the TCE exposed cells from 199 5 to 199 7 is presented in Table 2. Included in the table is a summary of the mass of chloride in the TCE which was lost in the cell (i.e., in uent. be only 9 percent of the TCE lost from the cells during 199 6. No transpired TCE was detected in tests conducted in 199 7. The results from the rhizosphere study conducted in 199 6 did not indicate