Abstract:

Phytoremediation, the use of plants to remediate contaminated soils, sediments and groundwater, may be one of many feasible technologies for MTBE cleanup.  Research efforts are still in the early stages, but preliminary results suggest that 1) plants have the ability to take up and transpire (phytovolatilize) MTBE; 2) a stand of mature trees can attenuate an MTBE groundwater plume as it passes beneath the stand, and 3) a stand of mature trees can hydraulically control a migrating MTBE groundwater plume.  Additional research efforts are needed to more accurately determine the rates and extents of plant uptake, metabolism, and volatilization of MTBE. However, these early findings suggest that phytoremediation may be a feasible alternative for MTBE plume control. 

Phytoremediation

Phytoremediation is inexpensive relative to conventional remedial methods and is ideal for large sites with moderate levels of contamination.  This in situ technology also enjoys high public approval due to its limited environmental impact and high aesthetic value. Although phytoremediation (especially for the treatment of MTBE in groundwater) is a relatively new area, results from case studies have produced promising results.  Several processes are involved in phytoremediation:

Enhanced Biodegradation in the Rhizosphere

The biodegradation of many organic chemical pollutants may be enhanced in the plant root zone (the rhizosphere) (Crowley et al., 1997; Marschner, 1995; Anderson et al., 1993; Shimp et al., 1993; Rovira and Davey, 1971).  Within the rhizosphere, the continuous deposition of plant-derived carbon containing compounds (root exudates) supports an abundance of metabolically active bacteria and fungi that may enhance contaminant degradation.  Further, the organic acids, phenolics, sugars, and amino acids exuded by plant roots have the potential to be stimulated MTBE co-metabolism. 

Stabilization

Many types of organic chemical contaminants are lipophilic and have a high affinity for the hydrophobic surfaces on organic matter.  Such compounds may bind to living or dead plant root tissue and become immobilized.  This process has been termed “phyto-stabilization.”  Such residues are thought to be less bioavailable, less mobile, and less toxic than the free species.  However, the tendency of MTBE to bind to organic matter is expected to be relatively low based upon its partition coefficients.

Plant Uptake

Organic contaminants in the soil water phase may reach the root surface by mass flow, penetrate the root, enter the xylem, and be transported in the transpiration stream.  Once in the transpiration stream, chemicals may react with or partition into plant tissues, be metabolized (detoxified) by plant enzymes, or escape by gaseous diffusion through stomata in leaves. 

Root uptake can vary with the organic matter and water content of the soil, as well as plant characteristics such as type of root system and lipid content (Paterson et al., 1994).  Plant species vary in root lipid content and anatomy, so uptake of organic contaminants is species-dependent.  Root uptake of organic chemical compounds is also governed by environmental conditions.  Climatic conditions determine plant transpiration rates, which in turn control rates of water movement to the root surface and in the xylem.  Physicochemical properties that may influence plant uptake of organics include water solubility, vapor pressure, Henry’s Law constant (KH), and hydrophobicity, measured by the logarithm of the chemical’s octanol-water partitioning coefficient (log Kow) (Simonich and Hites, 1995). 

The transpiration stream concentration factor (TSCF) has been developed to predict the behavior of a chemical compound in plants, and is based upon its log Kow (Russell and Shorrocks, 1959; Shone and Wood, 1972; Briggs et al, 1982; Hsu et al., 1990; Burken and Schnoor, 1998).  The TSCF relates the concentration of the compound in the transpiration stream (xylem) to the exposure concentration:

Note that the exposure concentration refers to the concentration of the chemical compound in the soil solution, not its concentration in the soil solids.  A TSCF of 1.0 indicates unrestricted passive uptake of the compound into the plant.  TSCF values lower than 1.0 indicate exclusion (restricted passive uptake) of the compound by the plant, while TSCF values greater than 1.0 infer active uptake.  Plant uptake of MTBE has been observed in laboratory and field experiments and MTBE has been identified in transpiration condensate, indicating that plant uptake may be a factor in MTBE phytoremediation.  University of Colorado researchers have recently reported TSCF values for MTBE of around 1.0 in hybrid poplar trees.

Plant Metabolism

Following plant uptake, organic chemical contaminants may be metabolically transformed.  Plants have evolved compound-specific detoxification (metabolic) pathways.  Generally,  plants enzymatically oxidize, reduce, or hydrolyze organic chemical contaminants.  The products of these transformation reactions are covalently attached (conjugated) to water-soluble moieties and the conjugated compounds are removed from the cytoplasm, either by transport into vacuoles or by conversion into insoluble, frequently covalent complexes with the cell wall (a process called “lignification”) (Sandermann, 1992).   Certain reports in the literature suggest that mineralization of organic contaminants within plant tissues may be possible.  However, mineralization would be extremely difficult to measure in whole plant systems because the CO2 resulting from mineralization would be assimilated by the plant during photosynthesis.  Addition of radiolabeled organic contaminants to non-photosynthetic plants cell cultures may be a useful way to determine if plants are capable of mineralizing the contaminant.  University of Washington researchers reported that hybrid poplar cell cultures mineralized 0.03% of the [14C] MTBE added to 14CO2 (Newman et al., 1999).  However, the small amount of 14CO2 recovered may have resulted from mineralization of impurities in the [14C] preparation rather than from the mineralization of the [14C] MTBE itself.  While MTBE mineralization may be enhanced within the microbe-rich plant rhizosphere, it is unclear whether plants themselves are capable of MTBE mineralization.

Phytovolatilization

When volatile organic compounds are taken up and translocated to plant leaves, they may volatilize through stomata, the tiny pores in the leaf which are the sites of gas exchange (mainly O2 and CO2) with the atmosphere.  Volatilization has been observed in laboratory studies with the organic chemical contaminants trichloroethylene (Gordon et al., 1997; Newman et al., 1997; Burken and Schnoor, 1998) and BTEX (Burken and Schnoor, 1998), as well as with MTBE (see case studies below).  In the atmosphere, MTBE reacts with hydroxyl radicals and has a half-life on the order of days (USEPA, 1993).

Hydraulic Control

A stand of freely transpiring trees planted into the saturated zone can potentially serve as a hydraulic barrier to groundwater (and dissolved MTBE) migration.  Depending on site-specific conditions, rapid groundwater use by the trees can result in a zone of capture in which all of the groundwater within a certain thickness of the saturated zone is used via transpiration.  Creation of a zone of capture results in the mass flow of water and contaminants from the saturated zone into the vadose zone, where aerobic biodegradation processes may be enhanced.  In addition, the upward flux of water generated by rapidly transpiring trees may reduce the tendency for plume diving and increase residence time of contaminants beneath the tree stand.  This phytoremediation mechanism may be especially important for control and enhanced biodegradation of MTBE groundwater plumes.

Phytoremediation of MTBE: Case Studies

Researchers at Kansas State University (Zhang et al.) conducted studies in which alfalfa plants growing in soil or unplanted soil controls were dosed with solutions of MTBE.  The experimental systems were channels (110 cm long x 60 cm deep x 10 cm wide) through which flowed MTBE-containing water.  MTBE concentrations in the effluent water and gaseous MTBE flux measured at the soil surface were monitored throughout the experiment.  Concentrations of MTBE in the effluent from the planted channels were reportedly lower than those for the unplanted channels.  However, the flux of MTBE gas from the soil surface was higher from the planted channels than from the unplanted.  This effect was attributed to the larger upward mass flow of water resulting from evapotranspiration in the planted channels and it was concluded that MTBE was dissipated more quickly in the planted channels than in the unplanted channels.  In a subsequent study, (Zhang et al., 2000) the researchers added two strains of MTBE degrading bacteria (Rhodococcus and Arthrobacter) to the soil.  In this experiment, the flux of MTBE gas from the surface soil was lower from the vegetated channels spiked with bacteria than from the control channels (vegetated, but without introduced bacteria).  Concentrations of MTBE in water recovered from plant tissue ranged from 2.7% to 18% of the influent concentration, and approximately 31% of the MTBE added was reportedly ‘lost’ from the vegetated channels due to the presence of plants and microorganisms.  The extent to which MTBE was biodegraded in each channel system was not determined.  However, microcosm experiments were conducted with soils from the channel systems, and MTBE was removed at a rate of up to 5 mg per Kg soil per day.

Newman and coworkers, at the University of Washington, conducted phytoremediation studies with MTBE.  Hybrid poplar and eucalyptus tree seedlings growing in columns were dosed with [14C] MTBE at a concentration of 5 mg/L.  The experiment ran for two weeks.  For both tree species, approximately 0.4% of the dosed radiolabel was incorporated into plant tissues, with ~ 0.1% in the shoots and ~ 0.3% in the roots.  The extent of phytovolatilization of the radiolabel was 5.1% for poplars and 16.5% for the eucalyptus trees.  However, when the amount of radiolabel volatilized was normalized to leaf dry mass, the two species were similar: The poplar trees volatilized 0.8%/g and the eucalyptus trees volatilized 0.7%/g (Newman, Personal Communication).  Mass balance recoveries of radiolabel were not reported .  These preliminary studies led to a field investigation at the Construction Battalion Base (CBC) at Port Hueneme, CA.  In the field study, transpiration gas and plant tissue samples were collected from eucalyptus trees growing above gasoline-contaminated groundwater.  The samples did not contain detectable levels of MTBE.

At the University of Colorado in Denver, Rubin and Ramaswami (2000) conducted week-long experiments to determine the rate and extent of MTBE uptake, transformation, and volatilization by hybrid poplar saplings.  MTBE was added to the root zones of hybrid poplar saplings growing in individual hydroponic containers at concentrations of 300 and 1600 µg/L.  An airtight seal between the root compartment and the shoots was maintained, and headspace in the root compartment was eliminated.  The results indicated that the poplars removed roughly 30% of the mass of MTBE initially present in the water, at both concentration levels.  Analysis of plant tissue from the high dose indicated MTBE concentrations of approximately 125 µg/Kg in the roots and 60 µg/Kg in the shoots.  Concentrations in plant tissue at the 300 µg/L dose were below detection limits.  Mass balance experiments were also conducted where individual plants were placed in 8-L glass chambers through which air flowed   Effluent air was passed through carbon tubes to trap volatile MTBE.    Recoveries of nearly 100 % were obtained indicating that MTBE was untransformed during transport through the small poplar saplings.  A TSCF of ~ 1 was calculated.   The fate and transport of MTBE in larger trees is now being assessed.   In larger trees, the residence time of MTBE within the tree is expected to be longer, thus increasing the chance for plant transformation (Ramaswami, Personal Communication). 

The California EPA’s State Water Resources Control Board collected transpiration condensate from a row of mature Monterey pine (Pinus radiata) trees which are down gradient from a leaking underground storage tank in northern California (Parfitt et al., 2000).  The concentrations of MTBE and TBA in groundwater decrease by approximately three orders of magnitude as the contaminant plume passes beneath the trees, and detectable levels of MTBE and TBA were recovered in transpiration condensate.  Preliminary data indicated that the trees transpired to the atmosphere 4 to 6 grams of MTBE and TBA daily from the shallow groundwater.  In addition, groundwater elevation measurements indicated that transpirational water use by the stand is causing a measurable draw down in the water table ( the subsurface contains bay muds and silty clay that have relatively low hydraulic conductivities). 

Researchers at the U.S. Geological Survey in South Carolina investigated a dense stand of  live oak (Quercus virginiana) trees growing around an active gasoline station near Beaufort, South Carolina where gasoline leaked from an underground storage tank. The mature (>40 years old) trees have an extensive network of roots that extend down to the shallow aquifer which is contaminated with MTBE and BTEX (Landmeyer et al., 2000).  Cores from trees growing within the gasoline plume contained measurable concentrations of MTBE and BTEX compounds.  These compounds were not detected in cores of trees growing outside the plume area.

Conclusion

The case studies suggest that phytoremediation may be a feasible alternative for MTBE plume management.  Additional research is needed to more accurately determine the rates and extents of plant uptake, metabolism, and volatilization of MTBE.  Hydraulic control of groundwater also may be an effective phytoremediation mechanism.  Rapid transpirational water use trees can result in slowed migration and in some cases complete capture of MTBE groundwater plumes.  Hydraulic effects will vary with site hydrogeology, tree species and planting density.  As the technology evolves and as phytoremediation systems are being installed with increasing frequency, the potential for effective application of this technology continues to grow.

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