Final Report: Uptake of BTEX Compounds by Hybrid Poplar Trees in Hazardous Waste Remediation

EPA Grant Number: R825549C060
Subproject: this is subproject number 060 , established and managed by the Center Director under grant R825549
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: HSRC (1989) - Great Plains/Rocky Mountain HSRC
Center Director: Erickson, Larry E.
Title: Uptake of BTEX Compounds by Hybrid Poplar Trees in Hazardous Waste Remediation
Investigators: Schnoor, J. L.
Institution: University of Iowa
EPA Project Officer: Hahn, Intaek
Project Period: May 18, 1997 through May 17, 2000
Project Amount: Refer to main center abstract for funding details.
RFA: Hazardous Substance Research Centers - HSRC (1989) RFA Text |  Recipients Lists
Research Category: Organic Chemical Contamination of Soil/Water , Land and Waste Management


The goal of this research is to determine the feasibility and efficacy of vegetative bioremediation at sites contaminated with the BETX compounds. Poplar trees, Populus spp., will be utilized because of their resiliency in the presence of high levels of BEIX pollutants and because of the potential advantages of growth rates and ease of implementation. The proposed research is a laboratory study to be conducted in bioreactors at the University of Iowa's Hazardous Substance Research Center and at our field site at Amana, Iowa The hypothesis is that the poplar trees will decrease the concentration of the BE1X chemicals at highly polluted sites through a variety of processes. The specific goals and objectives are:

1. Determine the capacity for uptake and translocation of BEIX compounds by poplar trees while also analyzing metabolites within the tissues of the poplar trees.

2. Formulate quantitative Structure Activity Relationships (SARs) to determine which chemicals are best treated with vegetative remediation.

3. Quantify the effects of sorption and volatilization of chemicals in a vegetative remediation treatment site.

4. Evaluate the impact of root exudates and oxygen transfer on the microbial degradation/mineralization.

Summary/Accomplishments (Outputs/Outcomes):

The rationale for the research is that vegetative remediation has become a promising, inexpensive, publicly accepted, and innovative technique for cleaning contaminated hazardous waste sites. Vegetative remediation is best suited for sites of shallow contamination that is in the zone of impact for deep-rooted poplar trees. The uptake by plants of pollutants, a major component of vegetative remediation, is related to the octanol/water partition coefficient of the contaminant in question and is greatest for compounds with log (Kow) between 1.0 and 3.0. BETX contamination is ideally suited for vegetative remediation. Being LNAPL contaminants, the BETX compounds are often located near the surface at hazardous waste sites. These compounds also have log (Kow) values of 2.1 to 3.1. BETX contamination is also ubiquitous in today's environment and many of these sites are located at rural and abandoned sites where little money is available for the more expensive traditional remediation techniques.

Burken and Schnoor (1998) demonstrated that a wide variety of organic chemicals are taken-up and translocated in hybrid poplars using a series of radio-labeled chemicals in the laboratory. They showed a similar relationship to that of Briggs and Bromilow with a maximum Transpiration Stream concentration Factor (TSCF) related to the log Kow of the compound in the range of 1.5-4.0. Compounds more hydrophobic than log Kow = 4.0 are bound too tightly to soils and roots to be taken-up into the vascular plant. Furthermore, compounds with log Kow < 1.5 are generally rejected at the plant membrane - they are not lipophilic enough to bind onto the lipid bilayer of the membrane and enter the plant, and passive (diffusive) transport is not appreciable in these systems.

Uptake, volatilization, and translocation of the following organic compounds by hybrid poplar trees was investigated using the 14C - labeled compounds:

1. Benzene
2. Trichloroethylene (TCE)
3. Toluene
4. Ethylbenzene
5. m-Xylene
6. 1,2,4-Trichlorobenzene (TCB)
7. Pentachlorphenol (PCP)
8. Nitrobenzene
9. Phenol
10. Aniline
11. Atrazine

Octanol-water partition coefficients were the best predictor of plant uptake that we found. It is in relatively good agreement with previous works by Briggs and Bromilow. It indicates that moderately hydrophobic chemicals (log Kow = 1.5-4.0) are the best candidates for phytoremediation if uptake, transformation, or volatilization is the desired end-point. However, some hydrophobic organics (log Kow > 4.0) are also good candidates for rhizosphere biodegradation (if the plants release exudates that provide a carbon source or a structural analog to induce enzymes in microbial populations in the rhizosphere). In addition, hydrophilic compounds (log Kow < 1.5) can sometimes be rapidly taken-up by deep-rooted plants from groundwater plumes, but this was beyond the scope of this project.

Sorption (binding) of chemicals to hybrid poplar roots was investigated initially by Joel Burken, and further work was performed by Sybil Lang. Sorption of substituted benzenes and other compounds were determined using root concentration factors (RCF). The RCF is simply the slope from a linear sorption isotherm that exists between the poplar roots and bulk solution.

As with the TSCF, estimates of chemicals sorption to poplar roots in phytoremediation efforts can be determined with the concentration in bulk solution and the RCF of a particular chemical. The data reported compares the sorption of chemicals to hydroponically grown roots to the sorption of poplar roots grown under field conditions.

We noticed that aromatics with -OH, -NH2, and -NO2 substitution groups were tightly bound to root tissues and not extractable (bound residues). Even though many of these chemicals are quite hydrophilic (log Kow , 1.5), they were tightly bound and likely transformed. It was reasoned that the nitro-substituent reacts with membrane-bound proteins such as nitroreductase at the membrane. A corresponding hydroxyamino-compound or amine is formed. Potentially, the amine group then binds covalently with glutathione, and the chemical is bonded in cell vacuoles or to cell wall materials in a lignification reaction. Likewise, amine-substituents undergo similar reactions. We do not have spectroscopic evidence of these reactions, but the exact mechanism is a question for future research.

The main conclusion is that the Briggs Root Concentration Factor (RCF) is somewhat misleading - there are classes of compounds (e.g. nitroaromatics, phenols, and aromatic amines) which are tightly bound to roots and not translocated. These chemical move to the root surface not based on physical chemistry (log Kow), but rather on their biochemical reactivity. Some chemicals such as aniline, nitrobenzene, catechol, and chlorobenzene were much more strongly bound to roots than would have been predicted by the Briggs RCF relationship. Furthermore, sorption isotherm experiments showed these binding reactions to be irreversible - it is a biochemical bonding and not hydrophobic partitioning behavior.

The importance of oxygen transport in phytoremediation efforts has been investigated recently. Plant roots need an aerobic environment to survive, the lack of soil gas oxygen is commonly the inhibiting factor in whether roots will penetrate a contaminated zone. For these reasons, several physical and chemical methods for passively supplying oxygen to the root zone in contaminated soils were examined in the greenhouse using site soils.

The use of aeration tubes in the upper soil and the introduction of oxygen below the smear soil did not significantly improve plant growth or root development. However, the pots with ORC (oxygen release compound) in filters above the smear zone soil yielded much higher root densities for the portions of soil near the location of the oxygen amendment. The root density in the top six inches of the smear zone soils was about 8 mg/cm3 in control pots, and similar root densities were achieved just above the root zone when ORC was amended into the soils. These results suggest that chemical addition of oxygen or direct contact of a smear zone with ambient concentrations of oxygen (ORC amendments) can increase significantly the root density of poplar trees. The use of passive oxygen addition appeared to have little affect on root density. This information should be used to design future field experiments that are more realistic for the engineered environment.

The technology from this HSRC project has been transferred to a 20-acre site in Charleston, West Virginia (Cabin Creek). The site is a former refinery site of the Unocal Corporation contaminated with BTEX and total petroleum hydrocarbons (TPH). Professor Schnoor serves as the chief consultant to Unocal Corporation and Dames and Moore consulting engineers on the project. The site was planted as a full-scale phytoremediation effort in April, 1999. The State of West Virginia has approved the Corrective Action Plan. Results from the HSRC project have proved to be important in the phytoremediation design, estimation of the fate of contaminants, and treatability study. Identical hybrid poplars (Imperial Carolina, DN 34) were planted to fully utilize the fate and transport results obtained in the HSRC research. Groundwater monitoring will be performed quarterly to evaluate efficiency of the treatment. More than 70 percent of the trees survived the first season and grew to 6 - 8 feet in height. The main problems encountered at the site were drought and the excessive growth of weeds competing with the trees for water and nutrients. Quarterly monitoring reports have been acceptable to the state so far.

Journal Articles on this Report : 6 Displayed | Download in RIS Format

Other subproject views: All 11 publications 7 publications in selected types All 6 journal articles
Other center views: All 904 publications 230 publications in selected types All 182 journal articles
Type Citation Sub Project Document Sources
Journal Article Burken JG, Schnoor JL. Uptake and metabolism of atrazine by poplar trees. Environmental Science & Technology 1997;31(5):1399-1406. R825549C060 (Final)
not available
Journal Article Burken JG, Schnoor JL. Predictive relationships for uptake of organic contaminants by hybrid poplar trees. Environmental Science & Technology 1998;32(21):3379-3385. R825549C060 (Final)
not available
Journal Article Burken JG, Schnoor JL. Distribution and volatilization of organic compounds following uptake by hybrid poplar trees. International Journal of Phytoremediation 1999;1(2):139-151. R825549C060 (Final)
not available
Journal Article Burken JG, Schnoor JL. Phytoremediation Success Leads to Growth. HazTech Transfer 1999;7:4-5. R825549C060 (Final)
not available
Journal Article Jordahl JL, Foster L, Schnoor JL, Alvarez PJJ. Effect of hybrid poplar trees on microbial populations important to hazardous waste bioremediation. Environmental Toxicology and Chemistry 1997;16(6):1318-1321. R825549C060 (Final)
not available
Journal Article Schnoor JL. Vegetation Remediation of Volatile and Semi-Volatile Organics. PBI Bulletin 1998:6-9. R825549C060 (Final)
not available

Supplemental Keywords:

vegetative remediation, poplar trees, BTEX, soil, plants, Scientific Discipline, Toxics, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Contaminated Sediments, Environmental Chemistry, Geochemistry, Analytical Chemistry, Fate & Transport, Bioremediation, Ecology and Ecosystems, 33/50, fate and transport, plant-based remediation, migration, contaminant transport, Toluene, biodegradation, BTEX migration, contaminated sediment, Poplar trees, Xylenes, adsorption, BTEX, benzene, bioremediation of soils, biotechnology, contaminants in soil, chemical kinetics, phytoremediation

Relevant Websites: Exit

Progress and Final Reports:

Original Abstract
  • 1997
  • 1998

  • Main Center Abstract and Reports:

    R825549    HSRC (1989) - Great Plains/Rocky Mountain HSRC

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R825549C006 Fate of Trichloroethylene (TCE) in Plant/Soil Systems
    R825549C007 Experimental Study of Stabilization/Solidification of Hazardous Wastes
    R825549C008 Modeling Dissolved Oxygen, Nitrate and Pesticide Contamination in the Subsurface Environment
    R825549C009 Vadose Zone Decontamination by Air Venting
    R825549C010 Thermochemical Treatment of Hazardous Wastes
    R825549C011 Development, Characterization and Evaluation of Adsorbent Regeneration Processes for Treament of Hazardous Waste
    R825549C012 Computer Method to Estimate Safe Level Water Quality Concentrations for Organic Chemicals
    R825549C013 Removal of Nitrogenous Pesticides from Rural Well-Water Supplies by Enzymatic Ozonation Process
    R825549C014 The Characterization and Treatment of Hazardous Materials from Metal/Mineral Processing Wastes
    R825549C015 Adsorption of Hazardous Substances onto Soil Constituents
    R825549C016 Reclamation of Metal and Mining Contaminated Superfund Sites using Sewage Sludge/Fly Ash Amendment
    R825549C017 Metal Recovery and Reuse Using an Integrated Vermiculite Ion Exchange - Acid Recovery System
    R825549C018 Removal of Heavy Metals from Hazardous Wastes by Protein Complexation for their Ultimate Recovery and Reuse
    R825549C019 Development of In-situ Biodegradation Technology
    R825549C020 Migration and Biodegradation of Pentachlorophenol in Soil Environment
    R825549C021 Deep-Rooted Poplar Trees as an Innovative Treatment Technology for Pesticide and Toxic Organics Removal from Soil and Groundwater
    R825549C022 In-situ Soil and Aquifer Decontaminaiton using Hydrogen Peroxide and Fenton's Reagent
    R825549C023 Simulation of Three-Dimensional Transport of Hazardous Chemicals in Heterogeneous Soil Cores Using X-ray Computed Tomography
    R825549C024 The Response of Natural Groundwater Bacteria to Groundwater Contamination by Gasoline in a Karst Region
    R825549C025 An Electrochemical Method for Acid Mine Drainage Remediation and Metals Recovery
    R825549C026 Sulfide Size and Morphology Identificaiton for Remediation of Acid Producing Mine Wastes
    R825549C027 Heavy Metals Removal from Dilute Aqueous Solutions using Biopolymers
    R825549C028 Neutron Activation Analysis for Heavy Metal Contaminants in the Environment
    R825549C029 Reducing Heavy Metal Availability to Perennial Grasses and Row-Crops Grown on Contaminated Soils and Mine Spoils
    R825549C030 Alachlor and Atrazine Losses from Runoff and Erosion in the Blue River Basin
    R825549C031 Biodetoxification of Mixed Solid and Hazardous Wastes by Staged Anaerobic Fermentation Conducted at Separate Redox and pH Environments
    R825549C032 Time Dependent Movement of Dioxin and Related Compounds in Soil
    R825549C033 Impact of Soil Microflora on Revegetation Efforts in Southeast Kansas
    R825549C034 Modeling the use of Plants in Remediation of Soil and Groundwater Contaminated by Hazardous Organic Substances
    R825549C035 Development of Electrochemical Processes for Improved Treatment of Lead Wastes
    R825549C036 Innovative Treatment and Bank Stabilization of Metals-Contaminated Soils and Tailings along Whitewood Creek, South Dakota
    R825549C037 Formation and Transformation of Pesticide Degradation Products Under Various Electron Acceptor Conditions
    R825549C038 The Effect of Redox Conditions on Transformations of Carbon Tetrachloride
    R825549C039 Remediation of Soil Contaminated with an Organic Phase
    R825549C040 Intelligent Process Design and Control for the Minimization of Waste Production and Treatment of Hazardous Waste
    R825549C041 Heavy Metals Removal from Contaminated Water Solutions
    R825549C042 Metals Soil Pollution and Vegetative Remediation
    R825549C043 Fate and Transport of Munitions Residues in Contaminated Soil
    R825549C044 The Role of Metallic Iron in the Biotransformation of Chlorinated Xenobiotics
    R825549C045 Use of Vegetation to Enhance Bioremediation of Surface Soils Contaminated with Pesticide Wastes
    R825549C046 Fate and Transport of Heavy Metals and Radionuclides in Soil: The Impacts of Vegetation
    R825549C047 Vegetative Interceptor Zones for Containment of Heavy Metal Pollutants
    R825549C048 Acid-Producing Metalliferous Waste Reclamation by Material Reprocessing and Vegetative Stabilization
    R825549C049 Laboratory and Field Evaluation of Upward Mobilization and Photodegradation of Polychlorinated Dibenzo-P-Dioxins and Furans in Soil
    R825549C050 Evaluation of Biosparging Performance and Process Fundamentals for Site Remediation
    R825549C051 Field Scale Bioremediation: Relationship of Parent Compound Disappearance to Humification, Mineralization, Leaching, Volatilization of Transformaiton Intermediates
    R825549C052 Chelating Extraction of Heavy Metals from Contaminated Soils
    R825549C053 Application of Anaerobic and Multiple-Electron-Acceptor Bioremediation to Chlorinated Aliphatic Subsurface Contamination
    R825549C054 Application of PGNAA Remote Sensing Methods to Real-Time, Non-Intrusive Determination of Contaminant Profiles in Soils
    R825549C055 Design and Development of an Innovative Industrial Scale Process to Economically Treat Waste Zinc Residues
    R825549C056 Remediation of Soils Contaminated with Wood-Treatment Chemicals (PCP and Creosote)
    R825549C057 Effects of Surfactants on the Bioavailability and Biodegradation of Contaminants in Soils
    R825549C058 Contaminant Binding to the Humin Fraction of Soil Organic Matter
    R825549C059 Identifying Ground-Water Threats from Improperly Abandoned Boreholes
    R825549C060 Uptake of BTEX Compounds by Hybrid Poplar Trees in Hazardous Waste Remediation
    R825549C061 Biofilm Barriers for Waste Containment
    R825549C062 Plant Assisted Remediation of Soil and Groundwater Contaminated by Hazardous Organic Substances: Experimental and Modeling Studies
    R825549C063 Extension of Laboratory Validated Treatment and Remediation Technologies to Field Problems in Aquifer Soil and Water Contamination by Organic Waste Chemicals