Phytoremediation

Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants.[1][2] It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronomic techniques to either contain, remove or render toxic environmental contaminants harmless".[3] The term is an amalgam of the Greek phyto (plant) and Latin remedium (restoring balance). Although attractive for its low environmental impact, phytoremediation has not been demonstrated to redress any significant environmental challenge to the extent that contaminated space has been reclaimed.

Phytoremediation is proposed as a cost-effective plant-based approach of environmental remediation that takes advantage of the ability of plants to:

  • (i) concentrate and help remove elements (mainly metal ions) and compounds from the environment.[4][5]
  • (ii) to detoxify various compounds without causing additional pollution.[6]

The concentrating effect results from the ability of certain plants called hyperaccumulators to collect and concentrate chemicals. The remediation effect is quite different. Toxic heavy metals cannot be degraded, but organic pollutants can be, and are generally the major targets for phytoremediation. Several field trials support the feasibility of using plants for environmental cleanup.[7]

Processes

A range of processes mediated by plants or algae are tested in treating environmental problems.:

Phytoextraction

See also: Hyperaccumulator § Phytoremediation

Phytoextraction (or phytoaccumulation or phytosequestration) aims to exploit the ability of some plants or algae to absorb certain contaminants from soil or water. In principle, the plant could be harvested, thus used for the mining of metals. The roots take up substances from the soil or water and concentrate them above ground in the plant biomass[5] Organisms that can uptake high amounts of contaminants are called hyperaccumulators.[9] Phytoextraction can also be performed by plants (e.g. Populus and Salix) that take up low levels of pollutants, but due to their high growth rate and biomass production, could in principle remove a considerable amount of contaminants from the soil.[10] Typically, phytoextraction is proposed for extraction heavy metals.[11] Few or none of these concepts have been commercialized.

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.[12]

Mining of these extracted metals through phytomining is a conceivable way of recovering the material.[13] Hyperaccumulating plants are often metallophyte. Induced or assisted phytoextraction is a process where a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily.[14] While such additives can increase metal uptake by plants, they can also lead to large amounts of available metals in the soil beyond what the plants are able to translocate, causing potential leaching into the subsoil or groundwater.[14]

Phytostabilization

Phytostabilization lowers the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil.[15] It focuses on the long term stabilization and containment of the pollutant. Unlike phytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable. Stabilization results in reduced erosion, runoff, leaching, in addition to reducing the bioavailability of the contaminant.[11] An example application of phytostabilization is using a vegetative cap to stabilize and contain mine tailings.[16] Some soil amendments decrease radiosource mobility – while at some concentrations the same amendments will increase mobility.[17][18] Vidal et al. 2000 find the root mats of meadow grasses are effective at demobilising radiosource materials especially with certain combinations of other agricultural practices.[17][18] Vidal also find that the particular grass mix makes a significant difference.[17][18]

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.[19] This process works best with organic contaminants like herbicides, trichloroethylene, and methyl tert-butyl ether.[11]

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.

In the ideal case, plants and microorganisms detoxify pollutants.[20] The term phytotransformation represents a change in chemical structure without complete breakdown of the compound. The term "Green Liver" is used to describe phytotransformation,[21] as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant).[22][23]

Trinitrotoluene phytotransformation has been extensively researched and a transformation pathway has been proposed.[24]

Phytostimulation

Phytostimulation (or rhizodegradation) is the enhancement of soil microbial activity for the degradation of organic contaminants, typically by organisms that associate with roots.[19] This process occurs within the rhizosphere, which is the layer of soil that surrounds the roots.[19] Plants release carbohydrates and acids that stimulate microorganism activity which results in the biodegradation of the organic contaminants.[25] This means that the microorganisms are able to digest and break down the toxic substances into harmless form.[19] The production of leaf litter and organic matter from plants can stablize microbial soil biosiversity and reduce further loss of microogranisms from contaminents.[26] Phytostimulation has been shown to be effective in degrading petroleum hydrocarbons, PCBs, and PAHs.[11] Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort.[27]

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.[19] This is the most studied form of phytovolatilization, where volatilization occurs at the stem and leaves of the plant, however indirect phytovolatilization occurs when contaminants are volatilized from the root zone.[28] Selenium (Se) and Mercury (Hg) are often removed from soil through phytovolatilization.[15] Poplar trees are one of the most successful plants for removing VOCs through this process due to its high transpiration rate.[11]

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.[19] This process is often used to clean up contaminated groundwater through planting directly in the contaminated site or through removing the contaminated water and providing it to these plants in an off-site location.[19] In either case though, typically plants are first grown in a greenhouse under precise conditions.[29]

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.[30]

Phytodesalination

Phytodesalination uses halophytes (plants adapted to saline soil) to extract salt from the soil to improve its fertility.[5]

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.[31] 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 polyamines, allow the plants to tolerate concentrations of pollutants 500 times higher than untreated plants, and to absorb more pollutants.

Hyperaccumulators and biotic interactions

Main article: Hyperaccumulator

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).[32] 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 mycorrhizae, pollen and seed dispersal), commensalism, and biofilm.[33][34][35]

Tables of hyperaccumulators

Phytoscreening

As plants can translocate and accumulate particular types of contaminants, plants can be used as biosensors of subsurface contamination, thereby allowing investigators to delineate contaminant plumes quickly.[36][37] Chlorinated solvents, such as trichloroethylene, have been observed in tree trunks at concentrations related to groundwater concentrations.[38] 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.[39] Phytoscreening may lead to more optimized site investigations and reduce contaminated site cleanup costs.

See also

References

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Bibliography

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