Phytotransformation, also referred to as phytodegradation, is the breakdown of organic contaminants sequestered by plants via: (1) metabolic processes within the plant; or (2) the effect of compounds, such as enzymes, produced by the plant. The organic contaminants are degraded into simpler compounds that are integrated with plant tissue, which in turn, foster plant growth . Remediation of a site by phytotransformation is dependent on direct uptake of contaminants from the media and accumulation in the vegetation.
Direct uptake of chemicals into plant tissue via the root system is dependent on uptake efficiency, transpiration rate, and concentration of the chemical in soil water . Uptake efficiency depends on chemical speciation, physical/chemical properties, and plant characteristics, whereas transpiration rate depends on plant type, leaf area, nutrients, soil moisture, temperature, wind conditions, and relative humidity. Two processes of remediation can occur after the organic compound has been translocated by the plant: (1) storage of the chemical and its fragments into the plant via lignification; and (2) complete conversion to carbon dioxide and water .
The release of volatile contaminants to the atmosphere via plant transpiration, called phytovolatilization, is a form of phytotransformation. Although transfer of contaminants to the atmosphere may not achieve the goal of complete remediation, phytovolatilization may be desirable in that prolonged soil exposure and the risk of groundwater contamination are reduced.
Phytotransformation can be employed to remediate sites contaminated with organic compounds. Certain enzymes produced by plants are able to breakdown and convert chlorinated solvents (e.g., trichloroethylene), ammunition wastes, and herbicides . This technology also can be used to remove contaminants from petrochemical sites and storage areas, fuel spills, landfill leachates, and agricultural chemicals . Successful implementation of this technology requires that the transformed compounds that accumulate within the plant be non-toxic or significantly less toxic than the parent compounds. In some applications, phytotransformation may be used in concert with other remediation technologies or as a polishing treatment.
This technology usually requires more than one growing season to be efficient. Soil must be less than 3 ft in depth and groundwater within 10 ft of the surface. Contaminants may still enter the food chain through animals or insects that eat plant material. Soil amendments may be required, including chelating agents to facilitate plant uptake by breaking bonds binding contaminants to soil particles .
Several laboratory and field studies have been performed. It has been demonstrated that trichloroethylene (TCE) is transformed to trichloroethanol, trichloroacetic acid, and dichloroacetic acid by hybrid poplar trees, with partial mineralization of TCE to CO2 being observed . Another study revealed that the pesticide atrazine was transformed to ammeline with no mineralization to CO2 .
Demonstrations to investigate the remediation of heavy metals and organic contaminants have been performed or are on-going. Performance data from phytotransformation field investigations are summarized in Table 1.
Table 1. Performance data of phytotransformation field demonstrations .
1 acre plot - groundwater capture
TCE, PCA (1,1,2,2,-TCE)
NA (second year demonstration project)
Abeerdeen, MD (J-field site)
4 acre plot - groundwater capture
NA (second year SITE project)
Carswell AFB, Ft. Worth, TX
army ammunition plant - engineered wetland
Elodeia, Bullrush, Canary Grass
created wetland and surrounding soil
Pondweed, Coontail, Arrowroot, Hybrid Poplars
NA (early stages)
4 acre site - petrochemical wastes (soil and groundwater)
NA (second year SITE program)
wood preservative wastes
NA (second year SITE program)
agricultural runoff and co-op sites
90% reduction of NO3 atrazine reductions in groundwater
Martell/Clarence/ Amana, IA
NA: not available
SITE: Superfund Innovative Technology Evaluations (by EPA)
BTEX: benzene, toluene, ethylbenzene, xylene
TPH: total petroleum hydrocarbons
PAH: polycyclic aromatic hydrocarbon
Toxicity and fate of contaminants need to be evaluated and understood. The bound residue or metabolism of the tree or grass used also must be considered as a critical success factor for this technology .
Estimated cost for a phytotransformation-based system using hybrid poplar trees is described in Table 2 . The figures given in Table 2 represent a five-year operation to remediate nitrate-contaminated groundwater. The cost for a conventional pump and treat system with reverse osmosis ($660,000) is over two and one-half times the cost of the phytotransformation system .
Table 2. Five-year cost of phytotransformation using hybrid poplar trees
to treat nitrate-contaminated groundwater.
Design and Implementation
Five-year Monitoring Travel and Administration
Data from: Gatliff, E.G., 1996, Phytoremediation, Groundwater
Monitoring Review, Winter 1996.
Status of Technology
Numerous laboratory and greenhouse studies have been performed, but very few full-scale remediation operations exist . Phytotransformation is currently being tested (demonstration-scale) on explosives-contaminated groundwater (TNT and RDX) at Milan Army Ammunition Plant in Tennessee by the U.S. Army Corps of Engineers - Waterways Experimental Station. Refer to Table 1 for more details of on-going demonstrations. The Environmental Security Technology Certification Program project is testing the ability of trees used for phytotransformation to degrade TCE and hydrazine in aquifers .
1. EPA, 1998, A Citizen's Guide to Phytoremediation, U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response, EPA 542-F-98-011, August.
2. Schnoor, J.L., 1997, Phytoremediation, Technology Overview Report, Ground-Water Remediation Technologies Analysis Center, Series E, Vol. 1, October.
3. Miller, R., 1996, Phytoremediation, Technology Overview Report, Ground-Water Remediatoin Technologies Analysis Center, Series O, Vol. 3, October.
4. Newman, L.A., S.E. Strand, N. Choe, J. Duffy, G. 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, 31 (4), pp. 1062-1067.
5. Burken, J.G. and J.L Schnoor, 1997, Uptake and Metabolism of Atrazine by Poplar Trees, Environmental Science and Technology, 31 (5), pp. 1399-1406.
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