The term "phytotechnologies" broadly refers to the use of plants to address contamination in the environment, including soil, groundwater, surface water, sediment, and waste streams such as leachate, acid mine drainage, and wastewater (Pivetz, 2001)1. Phytotechnologies are considered a green technology (ITRC, 2009; Mench et al., 2010). They are generally less energy-intensive, and less costly and require less operation and maintenance than more active treatment methods, such as excavation or pump and treat (Henry et al., 2013; Pivetz et al., 2001). Plant-based technologies also tend to be an aesthetically pleasing treatment alternative (U.S. EPA, 2004; ITRC, 2009).
Plant-based technologies can be applied to help achieve a remedial objective of contaminant remediation2, containment, or both (ITRC, 2009). Due to the limited reach of plant roots, phytotechnologies are best-suited for addressing large areas of shallow contamination, although some phreatophytes and hybrid approaches have proven effective for treating aquifers at greater depths (USGS 2007; Gestler et al., 2019). Because high levels of contaminants may be toxic to plants and inhibit growth, phytotechnologies are best applied to low and moderate levels of contamination, used with other treatment methods, or applied as a final polishing step (U.S. EPA, 2006). Because plants make take several years to establish and grow, phytotechnologies are not generally selected to address acute risks to human or ecological receptors (Tangahu, et al., 2011).
Phytotechnologies are typically applied in situ (Conesa et al. 2012; Mench et al. 2010) although ex situ applications such as hydroponic systems and constructed treatment wetlands (U.S. EPA, 2004; Horst, et al., 2020; Pittarello et al., 2017; Ali et al., 2020) are available. Plant-based technologies can address a range of organic contaminants including petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides, and explosive compounds, as well as inorganic contaminants such as heavy metals, metalloids, radioactive materials, and salts (ITRC, 2009). In addition, there is ongoing research into the potential of phytotechnologies to treat per- and polyfluoralkyl substances (PFAS) and other emerging contaminants (Huff, et al., 2020; Huff, 2019; Gobelius, et al., 2017; Huang S. and P.R. Jaffe, 2019; Wang et al., 2020).
Many site-specific features, such as the soil, climate, availability of suitable plant species, associated rhizosphere microbes, and types and levels of contaminants, factor into the selection and performance of phytotechnology systems. Systems are designed, installed, and operated to accommodate a site's unique characteristics (ITRC 2009). For example, the suitability of a plant species is determined based on its ability to grow and survive given the soil, contaminants, climate, USDA hardiness zone, altitude, and water availability of the site. In addition, the plant must be able to take up, extract, or treat the contaminants of concern. Other factors-growth rate, habit (perennial, annual, biennial, deciduous, evergreen), form (grass, flowering plant, shrub, tree), rooting depth, disease/pest resistances, tolerances, and the potential to introduce non-native and/or invasive species-must be considered as well (Pivetz, 2001; ITRC 2009).
Six major mechanisms are employed by phytotechnologies to remove, destroy, transfer, stabilize, or contain contaminants:
Typical plants used in phytoextraction (e.g., Indian mustard, Alpine pennycress, sunflowers, ferns, grasses) are most effective in the top one foot of soil because of their shallow root systems and generally slow growth (Mahar et al. 2016). Plants that accumulate contaminants may require periodic harvesting and proper disposal to avoid recontamination of soil when the plants die or drop their leaves (Pivetz, 2001; Suman et al., 2018). Though with the use of plants known as hyperaccumulators (e.g. Sebartia acuminata, alpine pennycress), there is less need for frequent harvesting because these plants are able to withstand the elevated concentrations of contaminants (less phytotoxicity) by accumulating them in their above-ground parts (Suman et al. 2018).
Phytoextraction typically applies to inorganic contaminants, such as metals, metalloids, and radionuclides (Suman et al., 2018), because organic contaminants are more likely to be transformed rather than accumulated within the plant tissue. Testing suggests that phytoextraction can even prove useful for extracting valuable metals for reuse (Corzo Remigio, et al., 2020; Yang, et al., 2017; Anderson, 2013). However, scientists have identified plants that are capable of extracting chemicals, such as chlordane, 2,2-bis(p-chlorophenyl), and 1,1-dichloroethene (1,1-DCE), and storing them in their roots, leaves, and fruits (U.S. EPA 2006).
Phytodegradation typically applies to organic contaminants (Yadav et al. 2018). Uptake is affected by the contaminant's hydrophobicity, solubility, and polarity. Moderately hydrophobic and polar compounds are more likely to be taken up after they have sorbed to plant roots. Chlorinated solvents, herbicides, insecticides, polyaromatic hydrocarbons (PAHs), pentachlorophenol (PCP), polychlorinated biphenyls (PCBs), and munitions constituents can potentially be phytodegraded (U.S. EPA 2006; RTDF 2005; Dixit, et al., 2011).
Phytovolatilization typically is used to treat groundwater but also can treat soluble contaminants in soil. It is applicable to the treatment of volatile organic compounds, although phytovolatilization of inorganic (e.g., selenium, mercury, arsenic) contaminants is possible (Limmer, et al., 2016; Wang, et al., 2012). Simply transferring a contaminant to the air by transpiration may not be an acceptable alternative, and it is preferable that phytodegradation or transformation of contaminants to less toxic compounds occurs prior to transpiration. Note, however, that degradation of some contaminants such as TCE may produce even more toxic compounds (e.g., vinyl chloride) to be transpired (U.S. EPA 2006; RTDF 2005).
Once in the atmosphere, sunlight can photodegrade the transpired compounds. The potential advantages and disadvantages of phytovolatilization must be assessed on a site-specific basis (U.S. EPA 2006; RTDF 2005).
Rhizodegradation is considered "plant-assisted bioremediation." The presence of plant roots moderates soil moisture and increases soil aeration, making conditions more favorable to microbial habitats and thus, bioremediation. Root exudates (substances secreted by the plant) stimulate microbial communities or induce specific enzymes in organisms to enhance rhizodegradation (Dzantor, 2007). The rhizosphere is small, extending about one millimeter from the roots. At any given time, the percentage of soil in the rhizosphere is small and it can take time for new root growth to reach contaminated areas (Pivetz, 2001).
Rhizodegradation may treat soil contaminants amenable to bioremediation, such as petroleum hydrocarbons (Hoang, et al., 2021), PAHs, pesticides, BTEX, chlorinated solvents, PCP, PCBs, and surfactants. Rhizodegradation breaks down contaminants in the soil and, in some cases, completely mineralizes them; thus, plant harvesting and disposal is not necessary. The success of this technique is site-specific, however, and laboratory microcosms may not reflect the microbial conditions encountered in the field.
Phytosequestration limits the migration of contaminants, mainly metals, through 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. Conditions that promote phytosequestration occur when plants alter the chemical and microbial makeup of the soil (e.g., through the production of exudates [fluids, generally sugars and amino acids, emitted by roots] or carbon dioxide), which impacts the fate and transport of the soil metals (U.S. EPA 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, RTDF 2005).
Because phytosequestration retains contaminants in the soil, plant harvesting and disposal are not required; 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 (U.S. EPA 2006).
Plant canopies intercept some precipitation. The water is held on leaves, stems, and branches where it evaporates before reaching the ground and groundwater and mobilizing contaminants. Plants also transpire water taken in through the root system, which limits downward migration of precipitation and groundwater. 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 water. Trees classified as phreatophytes - deep-rooted, high-transpiring, water-loving organisms - are best for implementing phytohydraulics. Phreatophytes 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; RTDF 2005).
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Website for the International Phytotechnology Society, a nonprofit, worldwide professional society, includes news, projects and upcoming conferences and events.
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The commonly used term "phytoremediation" refers to remediation mechanisms (i.e., those that remove or reduce the toxicity of contaminants). ↩
The commonly used term "phytoremediation" refers to remediation mechanisms (i.e., those that remove or reduce the toxicity of contaminants). ↩