Phytotechnology is broadly defined as the use of vegetation to address contaminants in soil, sediment, surface water, and groundwater. Cleanup objectives for phytotechnologies can be contaminant removal and destruction, control and containment, or both. Phytoremediation (i.e., contaminant removal and destruction) is a phytotechnology subset (ITRC 2009). A layman's discussion of plant-based remediation can be found in A Citizen's Guide to Phytoremediation
, which is also available in a Spanish translation.
While phytotechnologies generally are applied in situ, ex situ applications (e.g., hydroponics systems) are possible. Typical organic contaminants, such as petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides, and explosive compounds, can be addressed using plant-based methods. Phytotechnologies also can be applied to typical inorganic contaminants, such as heavy metals, metalloids, radioactive materials, and salts (ITRC 2009).
Six major plant mechanisms enable phytotechnologies to remove, destroy, transfer, stabilize, or contain contaminants:
- Phytoextraction
Phytoextraction involves contaminant uptake by plant roots, with subsequent accumulation in plant tissue. Plants that accumulate contaminants may require periodic harvesting and proper disposal to avoid recontaminating the soil when the plants die or drop their leaves. Phytoextraction typically is used to address inorganic contaminants, such as metals, metalloids, and radionuclides. Organic contaminants are more likely to be transformed rather than accumulated within the plant tissue. Successful field applications of phytoextraction to take up metals have been limited; however, promising research is underway for using phytoextraction on mercury and persistent organic pollutants (USEPA 2006). Additional information on phytoextraction of metals is available in the following resource:
Phytoremediation and Hyperaccumulator Plants
W.A. Peer, I.R. Baxter, E.L. Richards, J.L. Freeman, and A.S. Murphy.
Chapter 7 in Molecular Biology of Metal Homeostasis and Detoxification. Springer, Berlin. ISBN-10: 3-540-22175-1, p 299-340, 2006
The major focus of this chapter is phytoextraction of heavy metals-arsenic, cadmium, chromium, copper, mercury, nickel, lead, selenium, and zinc.
Pesticides classified as persistent organic pollutants resist biodegradation and can remain in the environment for decades. Scientists have identified plants that are capable of extracting chemicals, such as chlordane and 2,2-bis(p-chlorophenyl)1,1-dichloroethene (p,p'-DDE), and storing them in their roots, leaves, and fruits (USEPA 2006).
Plants used in phytoextraction (e.g., Indian mustard, Alpine pennycress, sunflowers, ferns, grasses) typically are effective only in the top one foot of soil because of their shallow root systems and generally slow growth. Researchers are working on genetic modifications that increase the survivability of plants that hyperaccumulate toxic contaminants (USEPA 2006).
- Phytodegradation
Like phytoextraction, phytodegradation involves the uptake of contaminants; however, metabolic processes within the plant subsequently break down the contaminants. Phytodegradation also encompasses the breakdown of contaminants in the soil through the effects of enzymes and other compounds produced by plant tissues other than the roots (USEPA 2006).
Phytodegradation is applicable to organic contaminants. Their uptake is affected by their hydrophobicity, solubility, and polarity; moderately hydrophobic and polar compounds are more likely to be taken up after sorbing to plant roots. Chlorinated solvents, herbicides, insecticides, pentachlorophenol (PCP), polychlorinated biphenyls (PCBs), and munitions constituents have phytodegradation potential (USEPA 2006).
- Phytovolatilization
Phytovolatilization is the uptake of a contaminant into a plant and its subsequent transpiration to the atmosphere, or the transformation or phytodegradation of the contaminant with subsequent transpiration of the transformation or degradation products to the atmosphere. Phytovolatilization generally is applied to groundwater but also can be applied to soluble soil contaminants (USEPA 2006).
Transformation or degradation of the contaminant within the plant can create a less toxic product that is transpired; however, degradation of some contaminants, like trichloroethene (TCE), may produce even more toxic products (e.g., vinyl chloride). Once in the atmosphere, these products may be degraded more effectively by sunlight (photodegradation) than they would be by the plant (phytodegradation), but the potential advantages and disadvantages of phytovolatilization must be assessed on a site-specific basis (USEPA 2006).
Phytovolatilization has been applied to both organic and inorganic (e.g., selenium, mercury, arsenic) contaminants, but it must be reiterated that simply volatilizing a contaminant may not be an acceptable alternative (USEPA 2006).
- Rhizodegradation
The rhizosphere is the zone of soil influenced by plant roots. Essentially, rhizodegradation is "plant-assisted bioremediation" in that the root zone enhances microbial activity, thus increasing the breakdown of organic contaminants (such as petroleum hydrocarbons, PAHs, pesticides, BTEX, chlorinated solvents, PCP, PCBs, and surfactants) in the soil. The rhizosphere extends only about 1 mm from each root. The presence of plant roots moderates soil moisture and increases soil aeration, making conditions more favorable to bioremediation. The production of root exudates, such as sugars, amino acids, and other compounds, stimulates the population growth and activity of native microbes. Root exudates also may serve as food for the microbes, which can result in cometabolism of contaminants as degradation of exudates occurs. Because the microbes consume nutrients, the plants in a rhizodegradation plot often require additional fertilization (USEPA 2006).
Rhizodegradation actually breaks down contaminants; thus, plant harvesting and disposal is not necessary. In some instances, complete mineralization of the contaminant can occur. The success of this technique is site-specific, however, and laboratory microcosms may not reflect the microbial conditions encountered in the field. Petroleum hydrocarbons have been degraded successfully in the rhizosphere, but degradation of aged hydrocarbons has been shown to be more problematic (USEPA 2006).
- Phytosequestration
Phytosequestration, also referred to as phytostabilization, is a mechanism that immobilizes contaminants—mainly metals—within the root zone, limiting their migration. Immobilization of contaminants can result from adsorption of metals to plant roots, formation of metal complexes, precipitation of metal ions (e.g., due to a change in pH), or a change to a less toxic redox state. Phytosequestration can occur when plants alter the chemical and microbial makeup of the soil (e.g., through the production of exudates or carbon dioxide), which impacts the fate and transport of the soil metals. (USEPA 2006) Although transport proteins within the plant facilitate the transfer of contaminants between cells, plant cells contain a compartment called the vacuole that acts, in part, as a storage and waste receptacle for the plant. The vacuoles of root cells can sequester contaminants, preventing further translocation to the xylem (ITRC 2009).
Because contaminants are retained in the soil, phytosequestration does not require plant harvesting and disposal; however, evaluation of the system is necessary to verify that translocation of contaminants into the plant tissue is not occurring. Due to the continuing presence of contaminants in the root zone, plant health must be monitored and maintained to ensure system integrity and prevent future release of contaminants. Phytosequestration also can be used to prevent migration of soil contaminants with wind and water erosion, soil dispersion, and leaching (USEPA 2006).
- Phytohydraulics
Phytohydraulics is the ability of vegetation to evapotranspire sources of surface water and groundwater. Water interception capacity by the aboveground canopy and subsequent evapotranspiration through the root system can limit vertical migration of water from the surface downward. The horizontal migration of groundwater can be controlled or contained using deep-rooted species, such as prairie plants and trees, to intercept, take up, and transpire the water. Trees classified as phreatophytes are deep-rooted, high-transpiring, water-loving organisms that send their roots into regions of high moisture and can survive in conditions of temporary saturation. Typical phreatophytes include species such as cottonwoods, poplars, and willows (ITRC 2009).
Additional information specific to uses of plants for soil and groundwater cleanup, and constructed wetlands is available in the Federal Remediation Technologies Roundtable's Remediation Technologies Screening Matrix and Reference Guide.
ITRC (Interstate Technology & Regulatory Council). 2009. Phytotechnology
Technical and Regulatory Guidance and Decision Trees, Revised. Phyto-3
USEPA. 2006. In Situ Treatment Technologies for Contaminated Soil: Engineering Forum Issue Paper. EPA 542-F-06-013.
Introduction to Phytoremediation
EPA 600-R-99-107, 2000
This introduction is designed to help site regulators, owners, neighbors, and managers to evaluate the applicability of phytoremediation to a site. This document is a compilation of research and remediation work that defines terms and provides a framework to understand phytoremediation applications. See also the 2001 summary of this report.
Phytoremediation
D. Tsao (ed.).
Springer, New York. ISBN: 978-3-540-43385-9, 206 pp, 2003
This volume contains discussions of soil/plant microbe interactions in phytoremediation, a field assessment of the effect of plants on petroleum degradation in soil, phyoextraction of heavy metals from soil, hydraulic control of groundwater using deep-rooted tree systems, and vegetative covers for waste containment. Table of Contents
Phytoremediation: Transformation and Control of Contaminants
S.C. McCutcheon and J.L. Schnoor.
J. Wiley, New York. ISBN: 9780471273042, 987 pp, 2003
This comprehensive book details phytoremediation at all levels, from basic molecular and biochemical processes to practical considerations in field applications. The first of 7 sections contains detailed descriptions of all fields of phytoremediation and their state of development. Plant/contaminant interactions are discussed in the Section 2, covering mechanisms by which contaminants are degraded, plant tolerance, root architecture, and determination of tree water use. Sections 3, 4, and 5 present the degradation mechanisms, volatilization, and hydraulic control of different contaminant classes. Section 6 covers practical aspects of implementing phytotechnology. Section 7 outlines some of the latest advances of phytoremediation with discussions of atmospheric pollutants, MTBE, cyanide, and perchlorate. The text also describes plant and microbe database tools that can be used to assist selection of suitable organisms and offers case studies of phytoremediation in the field.
Table of Contents
Phytotechnology Project Profiles Database
Over 165 projects encompassing international, completed, and ongoing phytotechnology applications have been found in the literature and documented in this database. Each profile contains information about relevant site background, the types of contaminants treated, type of vegetation used, phytotechnology mechanisms, planting date, project size, location, cost, monitoring and performance results, as well as points of contact and references.
Phytotechnology Technical and Regulatory Guidance and Decision Trees, Revised
Interstate Technology & Regulatory Council (ITRC) Phytotechnologies Team.
PHYTO-3, 187 pp, 2009
This document is an update to Phytoremediation Decision Tree (PHYTO-1 1999) and Phytotechnology Technical and Regulatory Guidance Document (PHYTO-2 2001) and replaces the previous documents entirely. It merges the concepts of both previous documents and includes new and practical information on the process and protocol for selecting and applying various phytotechnologies as remedial alternatives. The technical descriptions of phytotechnologies in this document concentrate on the functioning mechanisms: phytosequestration, rhizodegradation, phytohydraulics, phytoextraction, phytodegradation, and phytovolatilization. Decision trees (Remedy Selection, Groundwater, Soil/Sediment, and Riparian Zone) help guide the user through the application of phytotechnologies to a remediation project.
