We take a closer look at 5 of best plants for phytoremediation. Brassicaceae species are really useful to accumulate certain metals while. Phytoremediation /ˌfaɪtəʊrɪˌmiːdɪˈeɪʃən refers to the technologies that use living plants . Processes. Phytoremediation process. A range of processes mediated by plants or algae are useful in treating environmental problems. Phytoremediation entails the use of plants to mitigate the effects of some type of Plants that have been employed and are useful in parts of Appalachia include: .
Phytoremediation? Useful Which are Plants in
A basic problem is the interaction of ionic species during uptake of various heavy metal contaminants. After uptake by roots, translocation into shoots is desirable because the harvest of root biomass is generally not feasible. Little is known regarding the forms in which metal ions are transported from the roots to the shoots [ 37 ].
Plant uptake-translocation mechanisms are likely to be closely regulated. Plants generally do not accumulate trace elements beyond near-term metabolic needs. Another issue is the form in which toxic metal ions are stored in plants, particularly in hyperaccumulating plants, and how these plants avoid metal toxicity.
Multiple mechanisms are involved. Storage in the vacuole appears to be a major one [ 37 ]. Water, evaporating from plant leaves, serves as a pump to absorb nutrients and other soil substances into plant roots. This process, termed evapotranspiration, is responsible for moving contamination into the plant shoots as well. Since contamination is translocated from roots to the shoots, which are harvested, contamination is removed while leaving the original soil undisturbed.
Nonaccumulating plants typically have a shoot-to-root ratio considerably less than one. Ideally, hyperaccumulators should thrive in toxic environments, require little maintenance and produce high biomass, although few plants perfectly fulfill these requirements [ 38 ].
Metal accumulating plant species can concentrate heavy metals like Cd, Zn, Co, Mn, Ni, and Pb up to or times those taken up by nonaccumulator excluder plants. In most cases, microorganisms bacteria and fungi, living in the rhizosphere closely associated with plants, may contribute to mobilize metal ions, increasing the bioavailable fraction. Their role in eliminating organic contaminants is even more significant than that in case of inorganic compounds [ 39 , 40 ].
Heavy metal uptake by plant through phytoremediation technologies is using these mechanisms of phytoextraction, phytostabilisation, rhizofiltration, and phytovolatilization as shown in Figure 2. Phytostabilisation is the use of certain plant species to immobilize the contaminants in the soil and groundwater through absorption and accumulation in plant tissues, adsorption onto roots, or precipitation within the root zone preventing their migration in soil, as well as their movement by erosion and deflation [ 28 , 39 — 42 ].
Rhizofiltration is the adsorption or precipitation onto plant roots or absorption into and sequesterization in the roots of contaminants that are in solution surrounding the root zone by constructed wetland for cleaning up communal wastewater [ 28 , 39 — 42 ].
Phytovolatilization is the uptake and transpiration of a contaminant by a plant, with release of the contaminant or a modified form of the contaminant to the atmosphere from the plant.
Phytovolatilization occurs as growing trees and other plants take up water along with the contaminants. Some of these contaminants can pass through the plants to the leaves and volatilize into the atmosphere at comparatively low concentrations [ 28 , 39 — 42 ]. Plants also perform an important secondary role in physically stabilizing the soil with their root system, preventing erosion, protecting the soil surface, and reducing the impact of rain.
At the same time, plant roots release nutrients that sustain a rich microbial community in the rhizosphere. Bacterial community composition in the rhizosphere is affected by complex interactions between soil type, plant species, and root zone location. Microbial populations are generally higher in the rhizosphere than in the root-free soil. This is due to a symbiotic relationship between soil microorganisms and plants.
This symbiotic relationship can enhance some bioremediation processes. Plant roots also may provide surfaces for sorption or precipitation of metal contaminants [ 27 ].
In phytoremediation, the root zone is of special interest. The contaminants can be absorbed by the root to be subsequently stored or metabolised by the plant. Degradation of contaminants in the soil by plant enzymes exuded from the roots is another phytoremediation mechanism [ 43 ].
For many contaminants, passive uptake via micropores in the root cell walls may be a major route into the root, where degradation can take place [ 3 ]. There are several factors which can affect the uptake mechanism of heavy metals, as shown in Figure 3. By having knowledge about these factors, the uptake performance by plant can be greatly improved.
Plants species or varieties are screened, and those with superior remediation properties are selected [ 31 ].
The uptake of a compound is affected by plant species characteristic [ 44 ]. The success of the phytoextraction technique depends upon the identification of suitable plant species that hyperaccumulate heavy metals and produce large amounts of biomass using established crop production and management practices [ 24 ].
Agronomical practices are developed to enhance remediation pH adjustment, addition of chelators, fertilizers [ 31 ]. For example, the amount of lead absorbed by plants is affected by the pH, organic matter, and the phosphorus content of the soil. To reduce lead uptake by plants, the pH of the soil is adjusted with lime to a level of 6. The Root Zone is of special interest in phytoremediation.
It can absorb contaminants and store or metabolize it inside the plant tissue. Degradation of contaminants in the soil by plant enzymes exuded from the roots is another phytoremediation mechanism. A morphological adaptation to drought stress is an increase in root diameter and reduced root elongation as a response to less permeability of the dried soil [ 43 ].
Vegetative Uptake is affected by the environmental conditions [ 44 ]. The temperature affects growth substances and consequently root length. Root structure under field conditions differs from that under greenhouse condition [ 43 ]. The success of phytoremediation, more specifically phytoextraction, depends on a contaminant-specific hyperaccumulator [ 45 ].
Understanding mass balance analyses and the metabolic fate of pollutants in plants are the keys to proving the applicability of phytoremediation [ 46 ]. Metal uptake by plants depends on the bioavailability of the metal in the water phase, which in turn depends on the retention time of the metal, as well as the interaction with other elements and substances in the water. Furthermore, when metals have been bound to the soil, the pH, redox potential, and organic matter content will all affect the tendency of the metal to exist in ionic and plant-available form.
Plants will affect the soil through their ability to lower the pH and oxygenate the sediment, which affects the availability of the metals [ 47 ], increasing the bioavailability of heavy metals by the addition of biodegradable physicochemical factors, such as chelating agents and micronutrients [ 34 ]. The increase of the uptake of heavy metals by the energy crops can be influenced by increasing the bioavailability of heavy metals through addition of biodegradable physicochemical factors such as chelating agents, and micronutrients, and also by stimulating the heavy-metal-uptake capacity of the microbial community in and around the plant.
This faster uptake of heavy metals will result in shorter and, therefore, less expensive remediation periods. However, with the use of synthetic chelating agents, the risk of increased leaching must be taken into account [ 34 ].
The use of chelating agents in heavy-metal-contaminated soils could promote leaching of the contaminants into the soil. Since the bioavailability of heavy metals in soils decreases above pH 5. It was found that exposing plants to EDTA for a longer period 2 weeks could improve metal translocation in plant tissue as well as the overall phytoextraction performance.
Plant roots exude organic acids such as citrate and oxalate, which affect the bioavailability of metals. In chelate-assisted phytoremediation, synthetic chelating agents such as NTA and EDTA are added to enhance the phytoextraction of soil-polluting heavy metals.
The presence of a ligand affects the biouptake of heavy metals through the formation of metal-ligand complexes and changes the potential to leach metals below the root zone [ 48 ]. Several studies have described the performance of heavy metals uptake by plants. It is reported that phytoremediation technology is an alternative to treat heavy-metal-contaminated side which will be more admitted in order to remediate the environment. Table 2 lists some research done to remediate heavy metals from contaminated soil, while Table 3 lists some research conducted to remediate them from contaminated water and wastewater.
Based on the collected data from the phytoremediation research listed in Tables 2 and 3 , the accumulation of heavy metals As, Pb, and Hg in plant tissue is summarized in respective, Figures 4 , 5 , and 6. According to Figure 4 , the highest accumulation of As in plant tissue the researchers have not detailed which part it is, but it might be the whole plant occurs in Pteris vittata L. It can reach more than 0.
In plant root, the highest accumulation of As is in Populus nigra , which can reach more than 0. Among those species are species of Brassica campestris L , Brassica carinata A. Figure 6 shows that accumulated Hg in Brassica juncea L. Phytoremediation techniques may also be more publicly acceptable, aesthetically pleasing, and less disruptive than the current techniques of physical and chemical process [ 38 ]. Advantages of this technology are its effectiveness in contaminant reduction, low-cost, being applicable for wide range of contaminants, and in overall it is an environmental friendly method.
Figure 7 simplifies some advantages of phytoremediation technology. The major advantages of the heavy metal adsorption technology by biomass are its effectiveness in reducing the concentration of heavy metal ions to very low levels and the use of inexpensive biosorbent materials [ 2 ].
Phytoremediation as possibly the cleanest and cheapest technology can be employed in the remediation of selected hazardous sites [ 29 ]. Phytoremediation encompasses a number of different methods that can lead to contaminant degradation [ 24 ].
Phytoremediation is a low-cost option and inexpensive approach for remediating environmental media, particularly suited to large sites that have relatively low levels of contamination [ 34 ]. This technology has been receiving attention lately as an innovative, cost-effective alternative to the more established treatment methods used at hazardous waste sites [ 29 ]. Phytoremediation potentially offers unique, low cost solutions to many currently problems of soil contamination [ 32 , 75 ].
It is cost-effective for large volumes of water having low concentrations of contaminants and for large areas having low to moderately contaminated surface soils [ 46 ]. It is applicable to a wide range of toxic metals and radionuclides [ 32 ] and also useful for treating a broad range of environmental contaminants, including organic and inorganic contaminants [ 46 ].
Phytoremediation is regarded as a new approach for the cleanup of contaminated soils, water, and ambient air [ 34 ]. Phytoremediation research can also contribute to the improvement of poor soils such as those with high aluminum or salt levels [ 75 ]. It is applicable to a range of toxic metals and radionuclides, minimal environmental disturbance, elimination of secondary air or water-borne wastes, and public acceptance [ 32 ].
Phytoextraction is considered as an environmentaly friendly method to remove metals from contaminated soils in situ. This method can be used in much larger-scale clean-up operations and has been applied for other heavy metals [ 76 ].
It is an esthetically pleasing, solar-energy-driven cleanup technology and there is minimal environmental disruption and in situ treatment preserves topsoil. In Situ applications decrease the amount of soil disturbance compared to conventional methods. It can be performed with minimal environmental disturbance with topsoil left in a usable condition and may be reclaimed for agricultural use. The organic pollutants may be degraded to CO 2 and H 2 O, removing environmental toxicity [ 46 ].
Phytoremediation can be an alternative to the much harsher remediation technologies of incineration, thermal vaporization, solvent washing, or other soil washing techniques, which essentially destroy the biological component of the soil and can drastically alter its chemical and physical characteristics as well as creating a relatively nonviable solid waste.
Phytoremediation actually benefits the soil, leaving an improved, functional soil ecosystem at costs estimated at approximately one-tenth of those currently adopted technologies [ 3 ]. It is the most ecological cleanup technology for contaminated soils and is also known as a green technology. Another advantage of phytoremediation is the generation of a recyclable metal-rich plant residue [ 32 ].
Phytoremediation could be a viable option to decontaminate heavy-metal-polluted soils, particularly when the biomass produced during the phytoremediation process could be economically valorized in the form of bioenergy.
The use of metal-accumulating bioenergy crops might be suitable for this purpose. If soils, contaminated with heavy metals, are phytoremediated with oil crops, biodiesel production from the resulting plant oil could be a viable option to generate bioenergy [ 34 ]. In large-scale applications, the potential energy stored can be utilized to generate thermal energy [ 46 ]. The success of the phytoextraction technique depends upon the identification of suitable plant species that can hyperaccumulate heavy metals and produce large amounts of biomass using established crop production and management practices [ 24 ].
On the other hand, there are certain limitations to phytoremediation system Figure 8. Among them are being time-consuming method, the amount of produced biomass, the root depth, soil chemistry and the level of contamination, the age of plant, the contaminant concentration, the impacts of contaminated vegetation, and climatic condition.
Phytoremediation can be a time-consuming process, and it may take at least several growing seasons to clean up a site.
The intermediates formed from those organic and inorganic contaminants may be cytotoxic to plants [ 46 ]. Phytoremediation is also limited by the growth rate of the plants. More time may be required to phytoremediate a site as compared with other more traditional cleanup technologies. Excavation and disposal or incineration takes weeks to months to accomplish, while phytoextraction or degradation may need several years.
Therefore, for sites that pose acute risks for human and other ecological receptors, phytoremediation may not be the remediation technique of choice [ 29 , 46 ]. Phytoremediation might be best suited for remote areas where human contact is limited or where soil contamination does not require an immediate response [ 38 ]. One of the unresolved issues is the tradeoff between toxic element accumulation and productivity.
This is often an acceptable rate of contaminant removal, allowing site remediation over a few years to a couple of decades, particularly where the concentration of the contaminant can be lowered sufficiently to meet regulatory criteria.
The success of phytoremediation may be limited by factors such as growing time, climate, root depth, soil chemistry, and level of contamination [ 38 ]. Root contact is a primary limitation on phytoremediation applicability. Remediation with plants requires that contaminants be in contact with the root zone of the plants. Either the plants must be able to extend roots to the contaminants, or the contaminated media must be moved to be within range of the plants [ 29 ]. Restricted to sites with shallow contamination within rooting zone of remediative plants, ground surface at the site may have to be modified to prevent flooding or erosion [ 46 ].
Age greatly affects the physiological activity of a plant, especially its roots. Generally, roots of a young plant display greater ability to absorb ions than do those of an old plant when they are similar in size. It is important to use healthy young plants for more efficient plant removal. However, this does not rule out the use of larger older plants whose larger size may compensate for their lower physiological activity as compared to smaller younger plants [ 45 ].
High concentrations of contaminants may inhibit plant growth and, thus, may limit application on some sites or some parts of sites.
This phytotoxicity could lead to a remedial approach in which high-concentration waste is handled with expensive ex situ techniques that quickly reduce acute risk, while in situ phytoremediation is used over a longer period of time to clean the high volumes of lower contaminant concentrations [ 29 ]. A major limitation in the phytoremediation of toxic elements is the maximal level that can be accumulated by plants. Restricted to sites with low contaminant concentrations, the treatment is generally limited to soils at one meter from the surface and groundwater within a few meters of the surface with soil amendments may be required [ 46 ].
Some ecological exposure may occur whenever plants are used to interact with contaminants from the soil. The fate of the metals in the biomass is a concern.
Although some forms of phytoremediation involve accumulation of metals and require handling of plant material embedded with metals, most plants do not accumulate significant levels of organic contaminants.
While metal-accumulating plants will need to be harvested and either recycled or disposed of in compliance with applicable regulations, most phytoremediative plants do not require further treatment or disposal [ 29 ].
Harvested plant biomass from phytoextraction may be classified as a hazardous waste, hence, disposal should be proper. Climatic or hydrologic conditions may restrict the rate of growth of plants that can be utilized. Introduction of nonnative species may affect biodiversity [ 46 ]. Heavy metals uptake, by plants using phytoremediation technology, seems to be a prosperous way to remediate heavy-metals-contaminated environment.
It has some advantages compared with other commonly used conventional technologies. Several factors must be considered in order to accomplish a high performance of remediation result. The most important factor is a suitable plant species which can be used to uptake the contaminant. Even the phytoremediation technique seems to be one of the best alternative, it also has some limitations. Prolong research needs to be conducted to minimize this limitation in order to apply this technique effectively.
International Journal of Chemical Engineering. In Arabidopsis , Indian mustard, and tobacco plants, improved metal tolerance was achieved through the over-expression of enzymes that induce the formation of phytochelatins 4, 5, and 6. Plants naturally tolerant to heavy metals have also been used as a source of genes for phytoremediation. Transgenic Arabidopsis plants expressing a selenocysteine methyltransferase SMTA gene from the selenium hyperaccumulator Astralagus bisulcatus contain eight times more selenium in their biomass when grown on selenite compared to non-transgenic controls.
Comparison of gene expression profiles between Arabidopsis thaliana and the closely related species A. Mammalian P cytochrome genes have been used to confer herbicide resistance to crop plants, which can be used in herbicide rotation systems designed to delay the evolution of herbicide resistance in weeds, and to reduce the environmental load of agricultural chemicals 5, 6.
Millions of tons of explosives have been released into the environment, with the resulting pollution of vast expanses of land and water resources.
Explosives, and their degradation products, are extremely toxic and corrosive. Landmines affect millions of people, both combatants and civilians, in over 80 countries. Sixty to 70 million active landmines exist throughout the world, and these claim the lives and limbs of 50 people every day, and threaten the livelihood of many more by denying them access to humanitarian aid, agricultural land, and water resources.
Efforts are underway to develop transgenic plants that can be used to warn civilians of the presence of landmines in a field Arabidopsis whose roots change color when they come into contact with degradation products of landmines have been developed.
Scientists are now working to allow the plant to transmit the signal to their leaves, to effect human-readable changes for a practical explosives detection system. Mercury is a highly toxic element found both naturally and as an introduced contaminant in the environment, and is a very serious global environmental problem. Organic mercury organomercurials , the most toxic form to living organisms, is produced when bacteria in the water and soil convert elemental mercury into methylmercury.
Methylmercury is easily absorbed and accumulates at high levels in the food chain. Mercury poisoning affects the immune system, damages the nervous system, and is harmful to developing fetuses. Detoxification of organomercurials has been achieved in transgenic plants by transforming Arabidopsis, tobacco, poplar trees, Indian mustard, and eastern cotton wood with two bacterial genes, merA and merB.
The combined actions of merA and merB transform methylmercury to the volatile elemental form, which is times less toxic, and is released by the plant to the atmosphere at non-toxic concentrations through transpiration. Arsenic occurs naturally in rocks and soil, and is released into underground water. Consumption of contaminated drinking water leads to skin disorders, gangrene, and cancer of the kidneys and bladder.
In addition, high levels of arsenic in agricultural land degrade soils, reduce crop yields, and introduce the pollutant to the food chain 8. Scientists have engineered Arabidopsis plants with arsenic tolerance by introducing two bacterial genes: Double transgenics are not only highly tolerant of arsenic, they also have improved cadmium tolerance, and a six-fold increase in the level of biomass compared to wild-type controls 6. Although the use of biotechnology to develop transgenic plants with improved potential for efficient, clean, cheap, and sustainable bioremediation technologies is very promising, several challenges remain.
Biotech Plants for Bioremediation The Mess Over the last century, global industrialization, war, and natural processes have resulted in the release of large amounts of toxic compounds into the biosphere. Conventional Remediation Strategies Conventional remediation for polluted sites typically involves the physical removal of contaminants, and their disposal in a designated site.
Phytoremediation Phytoremediation, the use of plants to remove or degrade contamination from soils and surface waters, has been proposed as a cheap, sustainable, effective, and environmentally friendly alternative to conventional remediation technologies.
Cadmium, Zinc, Lead, and Selenium Toxic metals affect crop yields, soil biomass, and fertility, and accumulate in the food chain. Herbicides Mammalian P cytochrome genes have been used to confer herbicide resistance to crop plants, which can be used in herbicide rotation systems designed to delay the evolution of herbicide resistance in weeds, and to reduce the environmental load of agricultural chemicals 5, 6.
Explosives Millions of tons of explosives have been released into the environment, with the resulting pollution of vast expanses of land and water resources. Landmines Landmines affect millions of people, both combatants and civilians, in over 80 countries.
Mercury Mercury is a highly toxic element found both naturally and as an introduced contaminant in the environment, and is a very serious global environmental problem. Arsenic Arsenic occurs naturally in rocks and soil, and is released into underground water.
International Journal of Chemical Engineering
Phytoremediation is a good option. Using green plants as weapons, phytoremediation is one of most economical and environment-friendly techniques to target. Plants in phytoremediation work on concentrating specific elements, how do we dispose them the plants would eventually biodegrade and the contaminants. PDF | S.W. Gawronski and others published Plant taxonomy and Plant species useful for phytoremediation need to fulﬁll some requirements.