Phytoremediation

by Kelly E. Belz

Fall, 1997

Table of Contents

Introduction:

Phytoremediation is the use of plants and trees to clean up contaminated soil and water. This technology is currently in its infancy, and more research needs to be done before it is widely accepted as a remediation technique. However, the future is promising. Currently, the majority of research is concentrated on determining the best plant for the job, quantifying the mechanisms by which the plants convert pollutants, and determining which contaminants are amenable to phytoremediation. Polluted sites are being studied, and phytoremediation looks promising for a variety of contaminants. This technology is useful for soil and water remediaton, however, this discussion primarily focuses on groundwater phytoremediation.

Mechanisms:

Phytoremediation uses one basic concept: the plant takes the pollutant through the roots. The pollutant can be stored in the plant (phytoextraction), volatized by the plant (phytovolatization), metabolized by the plant (phytodegradation), or any combination of the above.

Phytoextraction is the uptake and storage of pollutants in the plants stem or leaves. Some plants, called hyperaccumulators, draw pollutants through the roots. After the pollutants accumulate in the stem and leaves the plants are harvested. Then plants can be either burned or sold. Even if the plants cannot be used, incineration and disposal of the plants is still cheaper than traditional remediation methods. As a comparison, it is estimated a site containing 5000 tons of contaminated soil will produce only 20-30 tons of ash (Black, 1995). This method is particularly useful when remediating metals.


Click on the image to bring up an animation of accumulation in a plant.

Phytovolatization is the uptake and vaporization of pollutants by a plant. This mechanism takes a solid or liquid contaminant and transforms it to an airborne vapor. The vapor can either be the pure pollutant, or the pollutant can be metabolized by the plant before it is vaporized, as in the case of mercury, lead and selenium (Boyajian and Carriera, 1997; Black, 1995; Wantanbe, 1997).


Click on the image to bring up an animation of volatization in a plant.

Phytodegradation is plants metabolizing pollutants. After the contaminant has been drawn into the plant, it assimilates into plant tissue, where the plant then degrades the pollutant. This metabolization by plant-derived enzymes such as nitroredictase, laccase, dehalogenase, and nitrilase, has yet to be fully documented, but has been demonstrated in field studies (Boyajian and Carriera, 1997). The daughter compounds can be either volatized or stored in the plant. If the daughter compounds are relatively benign, the plants can still be used in traditional applications. If the daughter compounds are less harmful than the parent compound, but not benign, then the plants can be burned or used in alternate applications. This is illustrated below in Figure 1, a proposed degradation mechanism for atrazine (Burken and Schnoor, 1997).


Figure 1. Mechanism (proposed) for atrazine degradation by poplar trees. HA - hydroxyatrazine. DEA - de-ethylatrazine. DEHA - de-ethylhydroxyatrazine. DIA - de-isopropylatrazine. DIHA de-isopropylhydroxyatrazine. DDA - dide-alkylatedatrazine. HDA - hydroxylated de-alkylated products. (Burken and Schnoor, 1997)

The most effective current phytoremediation sites in practice combine these three mechanisms to clean up a site. For example, poplar trees can accumulate, degrade and volatize the pollutants in the remediation of organics.

Techniques:

Phytoremediation is more than just planting and letting the foliage grow; the site must be engineered to prevent erosion and flooding and maximize pollutant uptake. There are 3 main planting techniques for phytoremediation.

1. Growing plants on the land, like crops. This technique is most useful when the contaminant is within the plant root zone, typically 3 - 6 feet (Ecological Engineering, 1997), or the tree root zone, typically 10-15 feet (T. Crossman, personal communication, November 18, 1997).
2. Growing plants in water (aquaculture). Water from deeper aquifers can be pumped out of the ground and circulated through a "reactor" of plants and then used in an application where it is returned to the earth (e.g. irrigation).
3. Growing trees on the land and constructing wells through which tree roots can grow. This method can remediate deeper aquifers in-situ. The wells provide an artery for tree roots to grow toward the water and form a root system in the capillary fringe. This is illustrated in Figure 2 (M. Wagner, personal communications, September, 1997).
4. 
   
   Figure 2. Illustration of remediation of deeper wells in-situ.

Determining which plant to use:

The majority of current research in the phytoremediation field revolves around determining which plant works most efficiently in a given application. Not all plant species will metabolize, volatize, and / or accumulate pollutants in the same manner. The goal is to ascertain which plants are most effective at remediating a given pollutant.

Research has yielded some general guidelines for groundwater phytoremediation plants. The plant must grow quickly and consume large quantities of water in a short time. A good plant would also be able to remediate more than one pollutant because pollution rarely occurs as a single compound. Poplars and cottonwoods are being studied extensively because they can used as much as 30 and 350 gallons of water per day, and they can remediate a wide variety of organic compounds, including LNAPL's.

Phytoremediation has been shown to work on metals and moderately hydrophobic compounds such as BTEX compounds, chlorinated solvents, ammunition wastes, and nitrogen compounds. Table 1 shows a partial listing of plants and which pollutants they are capable of remediating. Table 2 shows a partial listing of current remediation projects to give the reader an idea of remediation possibilities.

Advantages and Disadvantages to Phytoremediation:

Advantages:

1. Aesthetically pleasing. Click here to see a web site which has pictures of phytoremediation sites.
2. Solar driven.
3. Works with metals and slightly hydrophobic compounds, including many organics.
4. Can stimulate bioremediation in the soil closely associated with the plant root. Plants can stimulate microorganisms through the release of nutrients and the transport of oxygen to their roots.
5. Relatively inexpensive - phytoremediation can cost as little as $10 - $100 per cubic yard whereas metal washing can cost $30 - $300 per cubic yard (Wantanbe, 1997).
6. Even if the plants are contaminated and unusable, the resulting ash is approximately 20-30 tons per 5000 tons soil (Black, 1997).
7. Having ground cover on property reduces exposure risk to the community (i.e. lead).
8. Planting vegetation on a site also reduces erosion by wind and water
9. Can leave usable topsoil intact
10. .

Disadvantages:

1. Can take many growing seasons to clean up a site.
2. Plants have short roots. They can clean up soil or groundwater near the surface in-situ, typically 3 - 6 feet (Ecological Engineering, 1997), but cannot remediate deep aquifers without further design work.
3. Trees have longer roots and can clean up slightly deeper contamination than plants, typically 10-15 feet (T. Crossman, personal communication, November 18, 1997), but cannot remediate deep aquifers without further design work (see Figure 2).
4. Trees roots grow in the capillary fringe, but do not extend deep in to the aquifer. This makes remediating DNAPL's in situ with plants and trees not recommended.
5. Plants that absorb toxic materials may contaminant the food chain.
6. Volatization of compounds can transform a groundwater pollution problem to an air pollution problem.
7. Returning the water to the earth after aquaculture must be permitted.
8. Less efficient for hydrophobic contaminants, which bind tightly to soil.

Case Studies:

At the Naval Air Station Joint Reserve Base Fort Worth, phytoremediation is being used to clean up trichloroethylene (TCE) from a shallow, thin aerobic aquifer. Cottonwoods are being used, and after 1 year, the trees are beginning to show signs of taking the TCE out of the aquifer. (Betts, 1997)

At the Iowa Army Ammunitions Plant, phytoremediation is being used as a polishing treatment for exposive-contaminated soil and groundwater. The demonstration, which ended in March, used native aquatic plant and hybrid poplars to remediate the site where an estimated 1-5% of the original pollutants still remain. A full scale project is estimated to reduce the contamination by an order of magnitude (Betts, 1997).

After using phytoremediation on a site contaminated with hydrocarbons, a site was granted closure by the Alabama Department of Environmental Management. The site involved about 1500 cubic yards of soil, and began with approximately 70% of the baseline samples containing over 100 ppm of total petroleum hydrocarbon (TPH). After 1 year of vegetative cover, approximately 83% of the samples contained less than 10 ppm TPH (T. Crossman, personal communication, November 18, 1997). This site was managed by Geraghty and Miller.

References:

1. Betts, K. (1997). Phytoremediation project taking up TCE. Environmental Science and Technology, Volume 31, No. 8. p. 347A.
2. Black, H (1995). Absorbing possibilities: Phytoremediation. Environmental Health Perspective. Volume 103, Number 12, December 1995.
3. Boyajian, G. and L. H. Carriera (1997). Phytoremediation: A clean transition from laboratory to marketplace. Nature Biotechnology. Volume 15, February, 1997, p. 127-128.
4. Burken, J.G., and J. L. Schnoor. Uptake and Metabolism of Atrazine by Poplar Trees. Environmental Science and Technology, Volume 31, No. 5. p. 1399 - 1405.
5. Chaney, R. (1995). Metal-scavenging plants to cleanse the soil. Agricultural Research, USDA-ARS, November 1995, P. 4-9.
6. Ecological Engineering (1997). What is Phytoremediation and Why are we Offering this Service to Our Clients. http://www.ecological-engineering.com/phytorem.html, September 9, 1997.
7. Goel, A., G. Kumar, G. F. Payne, and S. K. Dube (1997) Plant cell biodegradation of a xenobiotic nitrate ester, nitroglycerine. Nature Biotechnology. Volume 15, February, 1997, p. 174 - 177.
8. Newman, L. S. E. Strand, N. Choe, J. Duffy, C. Ekuan, M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M. P. Gordon (1997). Uptake and Biotransformation of Trichloroethylene by Hybrid Poplars. Environmental Science and Technology, Volume 31, No. 4. p. 1062 - 1067.
9. Salt, D. E., I. J. Pickering, R. C. Prince, D. Gleba, S. Dushenkov, R. D. Smith, and I. Raskin (1997). Metal Accumulation by Aquacultured Seedlings of Indian Mustard. Environmental Science and Technology, Volume 31, No. 6. p. 1636 - 1644.
10. Thompson, L. (1997). Exciting Environmental Technologies. http://www.betterworld.com/BWZ/9608/cover.htm#Phytoremediation, September 9, 1997.
11. United States Environmental Protection Agency (1996). A Citizen's Guide to Phytoremediation. http://clu-in.com/citguige/phyto.htm, September 9, 1997.
12. United States Environmental Protection Agency Technology Innovation Office (1996). Workshop on phytoremediation of organic contaminants. http://clu-in.com/phytomin.htm, September 10, 1997.
13. Wagner, M (1997).
14. Wantanabe, M (1997). Phytoremediatoin on the Brink of Commercialization. Environmental Science and Technology, Volume 31, No. 4. p. 182A-186A.
15. Wright, A. G. and Roe, A (1997). Focus on Environment. http:/www.enr.com/NEW/waste.htm. September 9, 1997.

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Student author: Kelly E. Belz
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998