Phytoremediation

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Some heavy metals such as copper and zinc are removed from the soil by moving up into the plant roots. Phytoextraction diagram.svg
Some heavy metals such as copper and zinc are removed from the soil by moving up into the plant roots.

Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. [1] 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". [2] The term is an amalgam of the Greek phyto (plant) and Latin remedium (restoring balance). Although attractive for its cost, phytoremediation has not been demonstrated to redress any significant environmental challenge to the extent that contaminated space has been reclaimed.

Contents

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. [3] The concentrating effect results from the ability of certain plants called hyperaccumulators to bioaccumulate 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 confirmed the feasibility of using plants for environmental cleanup. [4]

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.[ citation needed ]

Phytoremediation could in principle be a more cost effective solution. [5] Phytoremediation may be applied to polluted soil or static water environment. This technology has been increasingly investigated and employed at sites with soils contaminated heavy metals like with cadmium, lead, aluminum, arsenic and antimony. [6] These metals can cause oxidative stress in plants, destroy cell membrane integrity, interfere with nutrient uptake, inhibit photosynthesis and decrease plant chlorophyll. [7]

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.[ citation needed ] Contaminants such as metals, pesticides, solvents, explosives, [8] and crude oil and its derivatives, have been mitigated in phytoremediation projects worldwide. Many plants such as mustard plants, 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. [9] Even cultivars within the same species have varying abilities to accumulate pollutants. [9]

Advantages and limitations

Advantages

Limitations

Processes

Phytoremediation process Phytoremediation Process.svg
Phytoremediation process

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

Mechanisms involved in hydrocarbon phytoremediation Mechanisms involved in phytoremediation.png
Mechanisms involved in hydrocarbon phytoremediation

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 [10] Organisms that can uptake high amounts of contaminants are called hyperaccumulators . [13] 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. [14] 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. [15] At the time of disposal, contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the soil is remediated. [ citation needed ]

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. [16]

Mining of these extracted metals through phytomining is a conceivable way of recovering the material. [17] 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. [18] 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. [18]

Examples of plants that are known to accumulate the following contaminants:

Phytostabilization

Phytostabilization reduces the mobility of substances in the environment, for example, by limiting the leaching of substances from the soil. [9] It focuses on the long term stabilization and containment of the pollutant. The plant immobilizes the pollutants by binding them to soil particles making them less available for plant or human uptake.[ citation needed ] Unlike phytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable, resulting in reduced exposure. The plants can also excrete a substance that produces a chemical reaction, converting the heavy metal pollutant into a less toxic form. [10] Stabilization results in reduced erosion, runoff, leaching, in addition to reducing the bioavailability of the contaminant. [15] An example application of phytostabilization is using a vegetative cap to stabilize and contain mine tailings. [27] Some soil amendments decrease radiosource mobility – while at some concentrations the same amendments will increase mobility. [28] [29] 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. [28] [29] Vidal also find that the particular grass mix makes a significant difference. [28] [29]

Phytodegradation

The roots secrete enzymes that degrade (breakdown) organic pollutants in the soil. Phytoremediation by Degradation.svg
The roots secrete enzymes that degrade (breakdown) organic pollutants in the soil.

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. [30] This process works best with organic contaminants like herbicides, trichloroethylene, and methyl tert-butyl ether. [15]

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. [31] In other cases, microorganisms living in association with plant roots may metabolize these substances in soil or water. These complex and recalcitrant compounds cannot be broken down to basic molecules (water, carbon-dioxide, etc.) by plant molecules, and, hence, the term phytotransformation represents a change in chemical structure without complete breakdown of the compound. The term "Green Liver" is used to describe phytotransformation, [32] as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant). [33] [34] After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding functional groups such as hydroxyl groups (-OH).[ citation needed ]

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. [31]

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.[ citation needed ]

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.[ citation needed ]

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.[ citation needed ]

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. [35]

Phytostimulation

Phytostimulation (or rhizodegradation) is the enhancement of soil microbial activity for the degradation of organic contaminants, typically by organisms that associate with roots. [30] This process occurs within the rhizosphere, which is the layer of soil that surrounds the roots. [30] Plants release carbohydrates and acids that stimulate microorganism activity which results in the biodegradation of the organic contaminants. [36] This means that the microorganisms are able to digest and break down the toxic substances into harmless form. [30] Phytostimulation has been shown to be effective in degrading petroleum hydrocarbons, PCBs, and PAHs. [15] Phytostimulation can also involve aquatic plants supporting active populations of microbial degraders, as in the stimulation of atrazine degradation by hornwort. [37]

Phytovolatilization

Contaminates are then broken down and the fragments are then subsequently transformed and volatilized into the atmosphere. Phytoremediation by Phytovolatilization.svg
Contaminates are then broken down and the fragments are then subsequently transformed and volatilized into the atmosphere.

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. [30] 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. [38] Selenium (Se) and Mercury (Hg) are often removed from soil through phytovolatilization. [9] Poplar trees are one of the most successful plants for removing VOCs through this process due to its high transpiration rate. [15]

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. [30] 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. [30] In either case though, typically plants are first grown in a greenhouse under precise conditions. [39]

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. [40]

Phytodesalination

Phytodesalination uses halophytes (plants adapted to saline soil) to extract salt from the soil to improve its fertility. [10]

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. [41] 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.[ citation needed ]

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). [42] 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. [43] [44] [45]

Tables of hyperaccumulators

Phytoscreening

As plants are able to translocate and accumulate particular types of contaminants, plants can be used as biosensors of subsurface contamination, thereby allowing investigators to quickly delineate contaminant plumes. [46] [47] Chlorinated solvents, such as trichloroethylene, have been observed in tree trunks at concentrations related to groundwater concentrations. [48] 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. [49] Phytoscreening may lead to more optimized site investigations and reduce contaminated site cleanup costs.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Toxic heavy metal</span> Category of substances

A toxic heavy metal is a common but misleading term for a metallic element noted for its potential toxicity. Not all heavy metals are toxic and some toxic metals are not heavy. Elements often discussed as toxic include cadmium, mercury and lead, all of which appear in the World Health Organization's list of 10 chemicals of major public concern. Other examples include chromium and nickel, thallium, bismuth, arsenic, antimony and tin.

Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance faster than it can be lost or eliminated by catabolism and excretion. Thus, the longer the biological half-life of a toxic substance, the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. Bioaccumulation, for example in fish, can be predicted by models. Hypothesis for molecular size cutoff criteria for use as bioaccumulation potential indicators are not supported by data. Biotransformation can strongly modify bioaccumulation of chemicals in an organism.

<span class="mw-page-title-main">Environmental remediation</span> Removal of pollution from soil, groundwater etc.

Environmental remediation is the cleanup of hazardous substances dealing with the removal, treatment and containment of pollution or contaminants from environmental media such as soil, groundwater, sediment. Remediation may be required by regulations before development of land revitalization projects. Developers who agree to voluntary cleanup may be offered incentives under state or municipal programs like New York State's Brownfield Cleanup Program. If remediation is done by removal the waste materials are simply transported off-site for disposal at another location. The waste material can also be contained by physical barriers like slurry walls. The use of slurry walls is well-established in the construction industry. The application of (low) pressure grouting, used to mitigate soil liquefaction risks in San Francisco and other earthquake zones, has achieved mixed results in field tests to create barriers, and site-specific results depend upon many variable conditions that can greatly impact outcomes.

<span class="mw-page-title-main">Bioremediation</span> Process used to treat contaminated media such as water and soil

Bioremediation broadly refers to any process wherein a biological system, living or dead, is employed for removing environmental pollutants from air, water, soil, flue gasses, industrial effluents etc., in natural or artificial settings. The natural ability of organisms to adsorb, accumulate, and degrade common and emerging pollutants has attracted the use of biological resources in treatment of contaminated environment. In comparison to conventional physicochemical treatment methods bioremediation may offer advantages as it aims to be sustainable, eco-friendly, cheap, and scalable.

Biological augmentation is the addition of archaea or bacterial cultures required to speed up the rate of degradation of a contaminant. Organisms that originate from contaminated areas may already be able to break down waste, but perhaps inefficiently and slowly.

<span class="mw-page-title-main">Mycoremediation</span> Process of using fungi to degrade or sequester contaminants in the environment

Mycoremediation is a form of bioremediation in which fungi-based remediation methods are used to decontaminate the environment. Fungi have been proven to be a cheap, effective and environmentally sound way for removing a wide array of contaminants from damaged environments or wastewater. These contaminants include heavy metals, organic pollutants, textile dyes, leather tanning chemicals and wastewater, petroleum fuels, polycyclic aromatic hydrocarbons, pharmaceuticals and personal care products, pesticides and herbicides in land, fresh water, and marine environments.

Cometabolism is defined as the simultaneous degradation of two compounds, in which the degradation of the second compound depends on the presence of the first compound. This is in contrast to simultaneous catabolism, where each substrate is catabolized concomitantly by different enzymes. Cometabolism occurs when an enzyme produced by an organism to catalyze the degradation of its growth-substrate to derive energy and carbon from it is also capable of degrading additional compounds. The fortuitous degradation of these additional compounds does not support the growth of the bacteria, and some of these compounds can even be toxic in certain concentrations to the bacteria.

<span class="mw-page-title-main">Soil contamination</span> Pollution of land by human-made chemicals or other alteration

Soil contamination, soil pollution, or land pollution as a part of land degradation is caused by the presence of xenobiotic (human-made) chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons, solvents, pesticides, lead, and other heavy metals. Contamination is correlated with the degree of industrialization and intensity of chemical substance. The concern over soil contamination stems primarily from health risks, from direct contact with the contaminated soil, vapour from the contaminants, or from secondary contamination of water supplies within and underlying the soil. Mapping of contaminated soil sites and the resulting clean ups are time-consuming and expensive tasks, and require expertise in geology, hydrology, chemistry, computer modelling, and GIS in Environmental Contamination, as well as an appreciation of the history of industrial chemistry.

This list covers hyperaccumulators, plant species which accumulate, or are tolerant of radionuclides, hydrocarbons and organic solvents, and inorganic compounds.

<span class="mw-page-title-main">Rhizofiltration</span>

Rhizofiltration is a form of phytoremediation that involves filtering contaminated groundwater, surface water and wastewater through a mass of roots to remove toxic substances or excess nutrients.

<i>Azolla pinnata</i> Species of aquatic plant

Azolla pinnata is a species of fern known by several common names, including mosquitofern, feathered mosquitofern and water velvet. It is native to much of Africa, Asia and parts of Australia. It is an aquatic plant, it is found floating upon the surface of the water. It grows in quiet and slow-moving water bodies because swift currents and waves break up the plant. At maximum growth rate, it can double its biomass in 1.9 days, with most strains attaining such growth within a week under optimal conditions.

Phytoextraction is a subprocess of phytoremediation in which plants remove dangerous elements or compounds from soil or water, most usually heavy metals, metals that have a high density and may be toxic to organisms even at relatively low concentrations. The heavy metals that plants extract are toxic to the plants as well, and the plants used for phytoextraction are known hyperaccumulators that sequester extremely large amounts of heavy metals in their tissues. Phytoextraction can also be performed by plants that uptake lower levels of pollutants, but due to their high growth rate and biomass production, may remove a considerable amount of contaminants from the soil.

Phytotechnology implements solutions to scientific and engineering problems in the form of plants. It is distinct from ecotechnology and biotechnology as these fields encompass the use and study of ecosystems and living beings, respectively. Current study of this field has mostly been directed into contaminate removal (phytoremediation), storage (phytosequestration) and accumulation. Plant-based technologies have become alternatives to traditional cleanup procedures because of their low capital costs, high success rates, low maintenance requirements, end-use value, and aesthetic nature.

Nanoremediation is the use of nanoparticles for environmental remediation. It is being explored to treat ground water, wastewater, soil, sediment, or other contaminated environmental materials. Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States. In Europe, nanoremediation is being investigated by the EC funded NanoRem Project. A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale. During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process or in situ application.

<span class="mw-page-title-main">Pavel Tlustoš</span> Agricultural researcher from the Czech Republic

Pavel Tlustoš is a Czech agricultural chemist. From 2000-10 he was Vice Dean of the Faculty of Agricultural Biology, Food and Natural Resources of the Czech University of Life Sciences Prague (CULS), and 2010-February 2018 he was the Dean of the faculty. Since March 2018 he is again Vice Dean and responsible for international relations the faculty.

Bioremediation of petroleum contaminated environments is a process in which the biological pathways within microorganisms or plants are used to degrade or sequester toxic hydrocarbons, heavy metals, and other volatile organic compounds found within fossil fuels. Oil spills happen frequently at varying degrees along with all aspects of the petroleum supply chain, presenting a complex array of issues for both environmental and public health. While traditional cleanup methods such as chemical or manual containment and removal often result in rapid results, bioremediation is less labor-intensive, expensive, and averts chemical or mechanical damage. The efficiency and effectiveness of bioremediation efforts are based on maintaining ideal conditions, such as pH, RED-OX potential, temperature, moisture, oxygen abundance, nutrient availability, soil composition, and pollutant structure, for the desired organism or biological pathway to facilitate reactions. Three main types of bioremediation used for petroleum spills include microbial remediation, phytoremediation, and mycoremediation. Bioremediation has been implemented in various notable oil spills including the 1989 Exxon Valdez incident where the application of fertilizer on affected shoreline increased rates of biodegradation.

Mycorrhizal amelioration of heavy metals or pollutants is a process by which mycorrhizal fungi in a mutualistic relationship with plants can sequester toxic compounds from the environment, as a form of bioremediation.

<span class="mw-page-title-main">Passive sampling</span>

Passive sampling is an environmental monitoring technique involving the use of a collecting medium, such as a man-made device or biological organism, to accumulate chemical pollutants in the environment over time. This is in contrast to grab sampling, which involves taking a sample directly from the media of interest at one point in time. In passive sampling, average chemical concentrations are calculated over a device's deployment time, which avoids the need to visit a sampling site multiple times to collect multiple representative samples. Currently, passive samplers have been developed and deployed to detect toxic metals, pesticides, pharmaceuticals, radionuclides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other organic compounds in water, while some passive samplers can detect hazardous substances in the air.

<span class="mw-page-title-main">Legacy pollution</span> Pollution caused by historical use of a chemical

Legacy pollution or legacy pollutants are persistent materials in the environment that were created through a polluting industry or process that have polluting effects after the process has finished. Frequently these include persistent organic pollutants, heavy metals or other chemicals residual in the environment long after the industrial or extraction processes that produced them. Often these are chemicals produced by industry and polluted before there was widespread awareness of the toxic effects of the pollutants, and subsequently regulated or banned. Notable legacy pollutants include mercury, PCBs, Dioxins and other chemicals that are widespread health and environmental effects. Sites for legacy pollutants include mining sites, industrial parks, waterways contaminated by industry, and other dump sites.

References

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