Introduction to Phytoremediation - CLU-IN

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Scientists from Rutgers University and Phytotech, Inc., have conducted laboratory, greenhouse, and field pilot-scale rhizofiltration studies. Phytotech tested a pilot-scale rhizofiltration system in a greenhouse at a DOE uraniumprocessing facility in Ashtabula, Ohio (Dushenkov et al. 1997). This engineered ex situ system used sunflowers to remove uranium from contaminated groundwater and/or process water. Phytotech also conducted a small-scale field test of rhizofiltration to remove radionuclides from a small pond near the Chernobyl reactor, Ukraine, using sunflowers floating on a raft. The use of constructed wetlands for wastewater treatment and/or acid mine drainage is a related technology that has a significant history of research and application. Ex situ rhizofiltration in engineered systems might be the phytoremediation technology that most often uses traditional engineering methods. 3.2.10 System Cost The cost of removing radionuclides from water by using sunflowers has been estimated to be $2 to $6 per thousand gallons of water. 3.2.11 Selected References Dushenkov, V., P. B. A. Nanda Kumar, H. Motto, and I. Raskin. 1995. Rhizofiltration: The Use of Plants to Remove Heavy Metals from Aqueous Streams. Environ. Sci. Technol. 29:1239-1245. This study examined metals removal by roots of a variety of plant species. It provides bioaccumulation coefficents and discusses the mechanisms of uptake. The study focuses on lead, but also provides information on other metals. Dushenkov, S., D. Vasudev, Y. Kapulnik, D. Gleba, D. Fleisher, K. C. Ting, and B. Ensley. 1997. Removal of Uranium from Water Using Terrestrial Plants. Environ. Sci. Technol. 31(12):3468-3474. This research included growth chamber, greenhouse, and field-scale studies for remediation of uranium-contaminated water. Continuous operation and optimization of an ex-situ system were examined. 3.3 Phytostabilization 3.3.1 Definition/Mechanism Phytostabilization is defined as (1) immobilization of a contaminant in soil through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants, and (2) the use of plants and plant roots to prevent contaminant migration via wind and water erosion, leaching, and soil dispersion. Phytostabilization occurs through root-zone microbiology and chemistry, and/or alteration of the soil environment or contaminant chemistry. Soil pH may be changed by plant root exudates or through the production of CO 2 . Phytostabilization can change metal solubility and mobility 21 or impact the dissociation of organic compounds. The plantaffected soil environment can convert metals from a soluble to an insoluble oxidation state (Salt et al. 1995). Phytostabilization can occur through sorption, precipitation, complexation, or metal valence reduction (EPA 1997a). Plants can also be used to reduce the erosion of metalcontaminated soil. The term phytolignification has been used to refer to a form of phytostabilization in which organic compounds are incorporated into plant lignin (Cunningham et al. 1995b). Compounds can also be incorporated into humic material in soils in a process likely related to phytostabilization in its use of plant material. 3.3.2 Media Phytostabilization is used in the treatment of soil, sediments, and sludges. 3.3.3 Advantages Phytostabilization has the following advantages: • Soil removal is unnecessary. • It has a lower cost and is less disruptive than other more-vigorous soil remedial technologies. • Revegetation enhances ecosystem restoration. • Disposal of hazardous materials or biomass is not required. 3.3.4 Disadvantages Phytostabilization has the following disadvantages: • The contaminants remain in place. The vegetation and soil may require long-term maintenance to prevent rerelease of the contaminants and future leaching. • Vegetation may require extensive fertilization or soil modification using amendments. • Plant uptake of metals and translocation to the aboveground portion must be avoided. • The root zone, root exudates, contaminants, and soil amendments must be monitored to prevent an increase in metal solubility and leaching. • Phytostabilization might be considered to only be an interim measure. • Contaminant stabilization might be due primarily to the effects of soil amendments, with plants only contributing to stabilization by decreasing the amount of water moving through the soil and by physically stabilizing the soil against erosion. 3.3.5 Applicable Contaminants/ Concentrations Phytostabilization has not generally been examined in terms of organic contaminants. The following is a discus-
sion of metals and metal concentrations, with implications for phytostabilization: • Arsenic: As (as arsenate) might be taken up by plants because it is similar to the plant nutrient phosphate, although poplar leaves in a field study did not accumulate significant amounts of As (Pierzynski et al. 1994). Poplars were grown in soil containing an average of 1250 mg/kg As (Pierzynski et al. 1994). • Cadmium: Cd might be taken up by plants because it is similar to the plant nutrients Ca, Zn, although poplar leaves in a field study did not accumulate significant amounts of Cd (Pierzynski et al. 1994). Poplars were grown in soil containing an average of 9.4 mg/kg Cd. Plants were grown in mine waste containing up to 160 mg/kg Cd (Pierzynski et al. 1994). • Chromium: Indian mustard (Brassica juncea) might be able to reduce Cr 6+ to Cr 3+ . • Copper: Mine wastes containing copper were stabilized by grasses (Salt et al. 1995). • Mercury: Mercury might be one of the leading candidates for the phytostabilization of metals, although additional study is required (EPA 1997b). • Lead: Pb in leachate was 22 g/mL in soil containing Indian mustard (Brassica juncea) compared to 740 µg/ mL in soil without plants (Salt et al. 1995). Mine wastes containing lead were stabilized by grasses (Salt et al. 1995). 625 µg/g Pb was used in a sand-Perlite mixture that supported Indian mustard (Brassica juncea) (Salt et al. 1995). Soil with 1660 mg/kg Pb had less than 50% plant cover. Plants in soil with 323 mg/kg Pb exhibited heavy chlorosis. Plants were grown in mine waste containing up to 4500 mg/kg (Pierzynski et al. 1994). • Zinc: Mine wastes containing zinc were stabilized by grasses (Salt et al. 1995). Soil with 4230 mg/kg Zn had less than 50% plant cover. Plants in soil with 676 mg/kg Zn exhibited heavy chlorosis. Plants were grown in mine waste containing up to 43,750 mg/kg Zn (Pierzynski et al. 1994). 3.3.6 Root Depth The root zone is the primary area affecting chemicallymoderated immobilization or root precipitation. Plants can be selected for their root depth; for example, poplars can be used for remediation of soil to a depth of 5 to 10 feet. The impact of the roots may extend deeper into the soil, depending on the transport of root exudates to lower soil depths. 3.3.7 Plants Metal-tolerant plants are required for heavy-metal-contaminated soils. Brassica juncea has been shown to reduce leaching of metals from soil by over 98% (Raskin et al. 1994). 22 The following grasses have been used to reduce metals leaching (Salt et al. 1995): • Colonial bentgrass (Agrostis tenuis cv Goginan) for acid lead and zinc mine wastes. • Colonial bentgrass (Agrostis tenuis cv Parys) for copper mine wastes. • Red fescue (Festuca rubra cv Merlin) for calcareous lead and zinc mine wastes. Native and tame grasses and leguminous forbs including big bluestem (Andropogon gerardi Vit.), tall fescue (Festuca arundinacea Schreb.), and soybean [Glycine max (L.) Merr.] were studied to determine their effectiveness in remediating mine wastes (Pierzynski et al. 1994). In addition, hybrid poplars were evaluated in a field study at a Superfund site to determine their metal tolerance (Pierzynski et al. 1994). 3.3.8 Site Considerations Plants used will require long-term maintenance if sitespecific constraints prohibit reversal of the stabilization process. 3.3.8.1 Soil Conditions Phytostabilization might be most appropriate for heavytextured soils and soils with high organic matter content (Cunningham et al. 1995a). Phytostabilization can be performed after more active soil treatment technologies have been tried. “Hot spots” of higher contaminant concentrations can be excavated and treated using other technologies, or landfilled. Soil amendments can also be used to stabilize metals in soils. Amendments should be selected that will maximize the growth of vegetation, which then also helps to phytostabilize the soil (Berti and Cunningham, 1997). 3.3.8.2 Ground and Surface Water Soil water content, which can affect redox conditions in the soil, must be appropriate for plant growth. 3.3.8.3 Climatic Conditions As discussed in Chapter 4, plans for remedial activities must take into account the fact that phytoremediation systems can be severely impacted by weather conditions. 3.3.9 Current Status The following are examples of typical phytostabilization studies: • Land affected by mining activities has been revegetated with potentially useful plants. For example, a stabilizing cover of vegetation was successfully established on metalliferous mine wastes in the United Kingdom (Salt et al. 1995). • Phytostabilization using metal-tolerant grasses is being investigated for large areas of Cd- and Zn-contaminated soils at a Superfund site in Palmerton, PA. Experimental plots of poplars have been studied at the
Page 1 and 2: Introduction to Phytoremediation Na
Page 3 and 4: Foreword The U.S. Environmental Pro
Page 5 and 6: Table of Contents Foreword ........
Page 7 and 8: Figures Figure 2-1. Mechanisms for
Page 9 and 10: Acronyms AAP Army Ammunition Plant
Page 11 and 12: Acknowledgements This Introduction
Page 13 and 14: • Chapter 3 provides a literature
Page 15 and 16: Figure 2-1. Mechanisms for phytorem
Page 17 and 18: Table 2-2. Root Depth for Selected
Page 19 and 20: Table 2-3. Phytoremediation at Supe
Page 21 and 22: egulated by the requirement. The Ma
Page 23 and 24: site. Plants and animals recolonize
Page 25 and 26: This chapter presents a literature
Page 27 and 28: Figure 3-1. Phytoextraction. The fo
Page 29 and 30: In this literature review of resear
Page 31: Rhizofiltration has not been evalua
Page 35 and 36: Figure 3-2. Rhizodegradation. • L
Page 37 and 38: - PCP at 400 to 4100 mg/kg (in soil
Page 39 and 40: This paper introduces the important
Page 41 and 42: - A qualitative study indicated tha
Page 43 and 44: Figure 3-4. Phytovolatilization. 3.
Page 45 and 46: Newman, L. A., C. Bod, N. Choe, R.
Page 47 and 48: 3.8 Vegetative Cover Systems 3.8.1
Page 49 and 50: also be used in the treatment of so
Page 51 and 52: trial habitats are greatly improved
Page 53 and 54: • Achieve objectives - Perform qu
Page 55 and 56: could be more effectively treated t
Page 57 and 58: Table 4-2. (continued) • Sterile/
Page 59 and 60: The following are examples of commo
Page 61 and 62: arrier to block other site activiti
Page 63 and 64: 5.1 Remedial Objectives Chapter 5 R
Page 65 and 66: tems should not be used to infer th
Page 67 and 68: in soil and binding to soil were al
Page 69 and 70: The six case studies presented in t
Page 71 and 72: mum concentration of 640 ppb of 112
Page 73 and 74: a comparison of the performance of
Page 75 and 76: 6.2.6 Contacts Greg Harvey US Air F
Page 77 and 78: Figure 6-4. Sampling Grid Edward Se
Page 79 and 80: gravel drainage layer, and compacte
Page 81 and 82: variety had only a 54% survival rat
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surements are being taken to produc
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Phytoaccumulation: The uptake and c
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Project Name/ Size Primary Media an
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Adler, T. 1996. Botanical Cleanup C
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Ferro, A. M., L. Stacishin, W. J. D
Page 99 and 100:
Newman, L. A., S. E. Strand, D. Dom
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Appendix D Common and Scientific Na
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Scientific Name Listed First Scient
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