Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants.
Phytoremediation is proposed as a cost-effective plant-based approach of environmental remediation that takes advantage of the ability of plants to concentrate elements and compounds from the environment and to detoxify various compounds without causing additional pollution.
Background
Soil remediation is an expensive and complicated process. Traditional methods involve removal of the contaminated soil followed by treatment and return of the treated soil.
Phytoremediation could in principle be a more cost effective solution.[
] These metals can cause oxidative stress in plants, destroy cell membrane integrity, interfere with plant nutrition uptake, inhibit photosynthesis and decrease plant chlorophyll.
Phytoremediation has been used successfully in the restoration of abandoned metal mine workings, and sites where polychlorinated biphenyls have been dumped during manufacture and mitigation of ongoing coal mine discharges reducing the impact of contaminants in soils, water, or air. Contaminants such as metals, pesticides, solvents, explosives,[ Phytoremediation of soils using Ralstonia eutropha, Pseudomonas tolaasi, Burkholderia fungorum reported by Sofie Thijs ] and crude oil and its derivatives, have been mitigated in phytoremediation projects worldwide. Many plants such as , alpine pennycress, hemp, and pigweed have proven to be successful at hyperaccumulating contaminants at toxic waste sites.
Not all plants are able to accumulate heavy metals or organics pollutants due to differences in the physiology of the plant.
Advantages and limitations
Advantages
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the cost of the phytoremediation is lower than that of traditional processes both in situ and ex situ
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the possibility of the recovery and re-use of valuable metals (by companies specializing in "phytomining")
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it preserves the topsoil, maintaining the fertility of the soil
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Increase soil health, yield, and plant phytochemicals
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the use of plants also reduces erosion and metal leaching in the soil
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Noise, smell and visual disruption are usually less than with alternative methods. The Galmeivegetation of hyperaccumulator plants is even protected by environmental legislation in many areas where it occurs.
Limitations
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phytoremediation is limited to the surface area and depth occupied by the roots.
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with plant-based systems of remediation, it is not possible to completely prevent the leaching of contaminants into the groundwater (without the complete removal of the contaminated ground, which in itself does not resolve the problem of contamination)
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the survival of the plants is affected by the toxicity of the contaminated land and the general condition of the soil
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bio-accumulation of contaminants, especially metals, into the plants can affect consumer products like food and cosmetics, and requires the safe disposal of the affected plant material
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when taking up heavy metals, sometimes the metal is bound to the soil organic matter, which makes it unavailable for the plant to extract
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some plants are too hard to cultivate or too slow growing to make them viable for phytoremediation despite their status as hyperacumulators. Genetic engineering may improve desirable properties in target species but is controversial in some countries.
Processes
A range of processes mediated by plants or algae are tested in treating environmental problems.:
Phytoextraction
Phytoextraction (or
phytoaccumulation or
phytosequestration) exploits the ability of plants or algae to remove contaminants from soil or water into harvestable plant biomass. It is also used for the mining of metals such as copper(II) compounds. The roots take up substances from the soil or water and concentrate them above ground in the plant biomass
Organisms that can uptake high amounts of contaminants are called
hyperaccumulators.
Phytoextraction can also be performed by plants (e.g.
Populus and
Salix) that take up lower levels of pollutants, but due to their high growth rate and biomass production, may remove a considerable amount of contaminants from the soil.
[Guidi Nissim W., Palm E., Mancuso S., Azzarello E. (2018) "Trace element phytoextraction from contaminated soil: a case study under Mediterranean climate"
]
/ref> Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or so. Typically, phytoextraction is used for heavy metals or other inorganics.
Of course many pollutants kill plants, so phytoremediation is not a panacea. For example, chromium is toxic to most higher plants at concentrations above 100 μM·kg−1 dry weight.
Mining of these extracted metals through phytomining is a conceivable way of recovering the material.
Examples of plants that are known to accumulate the following contaminants:
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Arsenic, using the sunflower ( Helianthus annuus),
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Cadmium, using willow ( Salix viminalis), which as a phytoextractor of cadmium (Cd), zinc (Zn), and copper (Cu).
[.]
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Cadmium and zinc, using alpine pennycress ( Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be poison to many plants. Specifically, pennycress leaves accumulate up to 380 mg/kg Cd.
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Chromium is toxic to most plants.
[ However, tomatoes ( Solanum lycopersicum) show some promise.]
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Lead, using Indian mustard ( Brassica juncea), ragweed ( Ambrosia artemisiifolia), hemp dogbane ( Apocynum cannabinum), or poplar trees, which sequester lead in their biomass.
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Salt-tolerant (moderately halophyte) barley and/or sugar beets are commonly used for the extraction of sodium chloride (common salt) to reclaim fields that were previously flooded by sea water.
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Caesium and Strontium were removed from a pond using after the Chernobyl accident.
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Mercury, selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have been removed from soils by containing for bacterial enzymes.
Phytostabilization
Phytostabilization reduces the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil.
Phytodegradation
Phytodegradation (also called phytotransformation) uses plants or microorganisms to degrade organic pollutants in the soil or within the body of the plant. The organic compounds are broken down by enzymes that the plant roots secrete and these molecules are then taken up by the plant and released through transpiration.
Phytotransformation results in the chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation), or immobilization (phytostabilization). In the case of organic pollutants, such as pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants, such as Cannas, render these substances non-toxic by their metabolism.[.] as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant).
This is known as Phase I metabolism, similar to the way that the human liver increases the polarity of drugs and foreign compounds (drug metabolism). Whereas in the human liver enzymes such as cytochrome P450s are responsible for the initial reactions, in plants enzymes such as peroxidases, phenoloxidases, esterases and nitroreductases carry out the same role.
In the second stage of phytotransformation, known as Phase II metabolism, plant biomolecules such as glucose and amino acids are added to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to the processes occurring in the human liver where glucuronidation (addition of glucose molecules by the UGT class of enzymes, e.g. UGT1A1) and glutathione addition reactions occur on reactive centres of the xenobiotic.
Phase I and II reactions serve to increase the polarity and reduce the toxicity of the compounds, although many exceptions to the rule are seen. The increased polarity also allows for easy transport of the xenobiotic along aqueous channels.
In the final stage of phytotransformation (Phase III metabolism), a sequestration of the xenobiotic occurs within the plant. The xenobiotics polymerize in a lignin-like manner and develop a complex structure that is sequestered in the plant. This ensures that the xenobiotic is safely stored, and does not affect the functioning of the plant. However, preliminary studies have shown that these plants can be toxic to small animals (such as snails), and, hence, plants involved in phytotransformation may need to be maintained in a closed enclosure.
Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics in phytotransformation. Trinitrotoluene phytotransformation has been extensively researched and a transformation pathway has been proposed.[.]
Phytostimulation
Phytostimulation (or rhizodegradation) is the enhancement of soil life activity for the degradation of organic contaminants, typically by organisms that associate with .[.]
Phytovolatilization
Phytovolatilization is the removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile and/or less polluting substances. In this process, contaminants are taken up by the plant and through transpiration, evaporate into the atmosphere.
Rhizofiltration
Rhizofiltration is a process that filters water through a mass of roots to remove toxic substances or excess nutrients. The pollutants remain absorbed in or adsorbed to the roots.
Biological hydraulic containment
Biological hydraulic containment occurs when some plants, like poplars, draw water upwards through the soil into the roots and out through the plant, which decreases the movement of soluble contaminants downwards, deeper into the site and into the groundwater.
Phytodesalination
Phytodesalination uses halophytes (plants adapted to saline soil) to extract salt from the soil to improve its fertility.
Role of genetics
Breeding programs and genetic engineering are powerful methods for enhancing natural phytoremediation capabilities, or for introducing new capabilities into plants. Genes for phytoremediation may originate from a micro-organism or may be transferred from one plant to another variety better adapted to the environmental conditions at the cleanup site. For example, genes encoding a nitroreductase from a bacterium were inserted into tobacco and showed faster removal of TNT and enhanced resistance to the toxic effects of TNT.[.]
Researchers have also discovered a mechanism in plants that allows them to grow even when the pollution concentration in the soil is lethal for non-treated plants. Some natural, biodegradable compounds, such as exogenous , allow the plants to tolerate concentrations of pollutants 500 times higher than untreated plants, and to absorb more pollutants.
Hyperaccumulators and biotic interactions
A plant is said to be a hyperaccumulator if it can concentrate the pollutants in a minimum percentage which varies according to the pollutant involved (for example: more than 1000 mg/kg of dry weight for nickel, copper, cobalt, chromium or lead; or more than 10,000 mg/kg for zinc or manganese).[.] This capacity for accumulation is due to hypertolerance, or phytotolerance: the result of adaptative evolution from the plants to hostile environments through many generations. A number of interactions may be affected by metal hyperaccumulation, including protection, interferences with neighbour plants of different species, mutualism (including , pollen and seed dispersal), commensalism, and biofilm.
Tables of hyperaccumulators
Phytoscreening
As plants can translocate and accumulate particular types of contaminants, plants can be used as of subsurface contamination, thereby allowing investigators to delineate contaminant plumes quickly.[.][.] Chlorinated solvents, such as trichloroethylene, have been observed in tree trunks at concentrations related to groundwater concentrations.[.] To ease field implementation of phytoscreening, standard methods have been developed to extract a section of the tree trunk for later laboratory analysis, often by using an increment borer.
See also
Bibliography
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"Phytoremediation Website" — Includes reviews, conference announcements, lists of companies doing phytoremediation, and bibliographies.
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"An Overview of Phytoremediation of Lead and Mercury" June 6 2000. The Hazardous Waste Clean-Up Information Web Site.
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"Enhanced phytoextraction of arsenic from contaminated soil using sunflower" September 22 2004. U.S. Environmental Protection Agency.
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"Phytoextraction", February 2000. Brookhaven National Laboratory 2000.
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"Phytoextraction of Metals from Contaminated Soil" April 18, 2001. M.M. Lasat
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July 2002. Donald Bren School of Environment Science & Management.
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"Phytoremediation" October 1997. Department of Civil Environmental Engineering.
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"Phytoremediation" June 2001, Todd Zynda.
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"Phytoremediation of Lead in Residential Soils in Dorchester, MA" May, 2002. Amy Donovan Palmer, Boston Public Health Commission.
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"Technology Profile: Phytoextraction" 1997. Environmental Business Association.
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"Ancona V, Barra Caracciolo A, Campanale C, De Caprariis B, Grenni P, Uricchio VF, Borello D, 2019. Gasification Treatment of Poplar Biomass Produced in a Contaminated Area Restored using Plant Assisted Bioremediation. Journal of Environmental Management"
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