Phytoremediation of Environmental Pollutants: An Eco-Sustainable Green Technology to Environmental Management
Introduction
A pollution-free environment is a significant challenge of the 21st century, with traditional remediation methods often being costly and environmentally damaging To address this issue, bioremediation and phytoremediation have emerged as effective, low-cost, and eco-sustainable alternatives Phytoremediation utilizes green plants and their associated microbiota to remediate both organic and inorganic environmental pollutants Organic contaminants include substances like trichloroethylene (TCE), trinitrotoluene (TNT), and benzene, while inorganic pollutants consist of essential and nonessential elements found in the Earth’s crust, such as nitrates, phosphates, and various trace metals.
V, Cd, Co, Se, Hg, F, Pb, As and W; and radionuclides such as 238 U, 137 Cs and
90Sr Environmental pollutants, whether organic or inorganic, severely affect human health and environments (Bridge 2004).
Hyperaccumulators are specialized plants that can absorb and concentrate toxic metals from contaminated soils, making them crucial for environmental cleanup Identifying suitable hyperaccumulator species is essential for effective phytoremediation, particularly in developing nations like India, where funding for remediation efforts is limited This eco-friendly approach not only helps remove pollutants but can also generate income through phytomining, where extracted metals serve as valuable bio ore, and biomass can be utilized for energy production The successful implementation of phytoremediation and phytomining can lead to economically viable metal products and improved land for agriculture and habitation Ongoing research continues to explore the economic benefits of these technologies, with several plant species now recognized as effective for phytoremediation.
This chapter explores various phytoremediation techniques, including phytoextraction, phytodegradation, rhizofiltration, phytostabilization, and phytovolatilization The integration of these methods presents the most effective strategy for remediating contaminated environments Additionally, the strategic use of plant growth-promoting rhizobacteria (PGPRs) significantly enhances the efficacy of these phytoremediation technologies.
Phytoremediation of Environmental Pollutants useful and eco friendly techniques that is currently considered a useful pro cess in phytoremediation.
The rising interest in molecular genetics has enhanced our comprehension of how plants tolerate heavy metals, leading to the development of transgenic plants with improved heavy metal resistance Additionally, genetic engineering advancements that modify traits such as metal uptake, transport, accumulation, and tolerance in plants present limitless opportunities for phytoremediation.
Phytoremediation and Associated Phytotechnologies
Phytoremediation, a concept that emerged in the 1980s, refers to the ability of certain plant species to accumulate high levels of toxic metals in their tissues By the early 1990s, various technologies were developed to utilize these plants for soil and water decontamination, leading to the term 'phytoremediation' being introduced in scientific literature around 1993 Over time, this definition expanded to encompass 'phytotechnologies,' which include a range of methods for pollutant remediation through stabilization, volatilization, metabolism (including rhizosphere degradation), and accumulation and sequestration For a detailed exploration of phytotechnologies, refer to McCutcheon and Schnoor (2003).
Phytoremediation is an eco-friendly and non-invasive green technology that effectively treats environmental pollutants using plants and their associated microbiota This innovative method facilitates the uptake, sequestration, detoxification, or volatilization of both organic and inorganic pollutants from various substrates, including soil, water, and air.
However, the application of phytoremediation technology has been reviewed by many researchers (Table 1.1).
The primary goal of this technique is to extract pollutants from contaminated environments by utilizing plant tissues This approach enables the growth of plants in both artificial (constructed wetlands) and natural settings, allowing for the effective removal of pollutants over a designated growth period It facilitates either the immobilization (binding/containment) or degradation (detoxification) of harmful substances, promoting environmental restoration.
Phytotechnologies encompass a range of techniques that utilize plants to meet environmental objectives by extracting, degrading, containing, or immobilizing pollutants in various mediums such as soil, groundwater, and surface water These methods effectively remediate a diverse array of contaminants.
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(See color insert.) Concept of phytoremediation technologies.
Some of the Applications of Phytoremediation
Mechanism Pollutant Media Plant Status Reference
Phytoextraction Zn, Cd and As Soil Datura stramonium and Chenopodium murale
Phytodegradation As Soil Cassia fistula Applied Preeti et al (2011) Phytostabilisation Mn Soil Chondrila juncea and
Chenopodium botrys Soil Cheraghi et al
(2011) Phytoextraction 137 Cs Soil Catharanthus roseus Applied Fulekar et al
(2010) Phytoextraction Cr Soil Anogeissus latifolia Applied Mathur et al (2010) Phytodegradation Zn and
Field demo Mukhopadhyay and Maiti (2010)
Cd Soil Jatropha curcas L Applied Mangkoedihardjo and Surahmaida (2008)
Phytostabilisation Cd Soil Sunflower Applied Zadeh et al (2008)Phytodegradation U Soil Brassica juncea Field demo Huhle et al (2008)
Phytoremediation of Environmental Pollutants using several different mechanisms dependent on the application, although not all mechanisms are applicable to all pollutants or all matrices.
Phytotechnologies offer a promising solution for environmental remediation by effectively cleaning moderate to low levels of specific elemental and organic pollutants across extensive areas Additionally, they can maintain sites by addressing residual pollution post-cleanup, serve as a buffer against potential waste releases, and support voluntary cleanup initiatives.
(5) facilitate nonpoint source pollution control and (6) offer an effective form of monitored natural attenuation (McCutcheon and Schnoor 2003).
Several types of phytoremediation can be defined as follows.
Phytoextraction, also known as phytoaccumulation, phytoabsorption, and phytosequestration, utilizes pollutant-accumulating plants to absorb and concentrate harmful substances, such as metals and organics, from the soil into their above-ground parts This eco-friendly technique contrasts with traditional methods by producing a smaller mass of waste for disposal or recycling, making it a more sustainable option Currently being evaluated in a Superfund Innovative Technology Evaluation (SITE) demonstration, phytoextraction also offers potential for pollutant recovery and recycling Additionally, this low-impact technology helps reduce soil erosion and leaching by covering the soil with plant growth.
It involves (1) cultivation of the suitable plant/crop species on the polluted site, (2) removal of harvestable plant parts containing metal from the site and
Post-harvest treatments such as composting, compacting, and thermal treatments are essential for minimizing biomass volume and weight These methods facilitate the safe disposal of hazardous waste and enable the recycling process to recover valuable metals.
Phytoextraction can be categorized into two types: continuous phytoextraction and induced phytoextraction Continuous phytoextraction involves the use of hyperaccumulator plants that absorb high levels of toxic pollutants throughout their lifespan, while induced phytoextraction enhances the accumulation of toxins at a specific time by adding accelerants or chelators to the soil After a growth period, these plants are harvested and either incinerated or composted to recycle the accumulated metals, with the process repeated as needed to reduce soil pollutant levels to acceptable limits Incineration results in ash that must be disposed of in a hazardous waste landfill, though its volume is significantly less than that of the original polluted soil Additionally, metals can be recovered through phytomining, a technique typically employed for precious metals.
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Metals like copper (Cu), nickel (Ni), and zinc (Zn) are ideal for phytoextraction, as many of the roughly 400 identified plant species that can absorb significant metal quantities show a strong affinity for these particular metals.
The efficiency of phytoextraction is primarily constrained by soil metal phytoavailability and the translocation of metals to the above-ground parts of plants To enhance these factors, various authors, including Blaylock and Huang (2000), have proposed and tested the application of soil amendments One notable agent is ethylene diamine tetraacetic acid (EDTA), which has been utilized in agriculture since the 1950s as an additive in micronutrient fertilizers.
Recent experiments have demonstrated that EDTA significantly enhances the phytoextraction capabilities of Eleusine indica, indicating its potential as a metal excluder and its ability to extract Pb from contaminated soils Additionally, EDTA facilitates the mobilization and accumulation of various soil pollutants, including Zn, Cd, Ni, Cu, Cr, and Pb, in plants like Brassica juncea and Helianthus annuus While other chelators such as CDTA, DTPA, EGTA, EDDHA, and NTA have also been evaluated for their effectiveness in increasing metal accumulation in plants, there are concerns regarding the environmental risks of using certain chelators due to the high water solubility of some chelator-toxin complexes, which may lead to groundwater contamination When assessing a plant's potential for phytoextraction, two key factors must be considered: bioconcentration, which measures the pollutant concentration in the plant relative to the soil, and biomass production, which is essential for making phytoextraction a commercially viable remediation strategy.
Rhizofiltration utilizes both terrestrial and aquatic plants to absorb and concentrate pollutants from low-concentration aqueous sources, effectively cleaning polluted surface waters and wastewaters, including industrial discharges and agricultural runoff This process involves the adsorption or precipitation of metals onto plant roots or absorption by metal-tolerant aquatic plants While similar to phytoextraction, rhizofiltration primarily focuses on the remediation of contaminated groundwater rather than polluted soils Its key advantage lies in the ability to harness the natural filtering capabilities of plants to address environmental pollution.
Phytoremediation of Environmental Pollutants terrestrial and aquatic plants for either in situ or ex situ applications and
(2) the pollutants do not have to be translocated to the shoots Thus, species other than hyperaccumulators may be used It remediates metals such as As,
Plants used for rhizofiltration must exhibit metal resistance and possess a high absorption surface while tolerating low oxygen levels Ideal candidates should generate substantial root biomass, accumulate and tolerate significant amounts of target metals, and be easy to handle with low maintenance costs, minimizing secondary waste disposal Terrestrial plants are particularly suitable for this process due to their extensive, fibrous root systems that provide large surface areas for effective metal adsorption.
Pteris vittata, or Chinese brake fern, is recognized as the first hyperaccumulator of heavy metals Various studies have highlighted several aquatic plants capable of extracting heavy metals from water, including Indian mustard (Brassica juncea) and sunflower (Helianthus annuus), which are particularly effective in this process Indian mustard, in particular, has shown significant efficacy in removing cadmium (Cd) from contaminated water sources.
Sunflower and Indian mustard are effective in absorbing heavy metals from hydroponic solutions, with sunflower specifically taking up lead (Pb) and uranium (U), while Indian mustard can remove a wide range of Pb concentrations, from 4 to 500 mg/L (Dushenkov et al 1995; Dushenkov et al 1997; Raskin and Ensley 2000).
Mechanism of Metal Hyperaccumulation in Plants
The process of metal hyperaccumulation in plants is accomplished in several steps (Figure 1.2).
Solubilisation of the metal from the soil matrix
Most metals in soil exist in insoluble forms, making them unavailable for plant uptake To address this issue, plants utilize two primary methods: rhizosphere acidification via plasma membrane proton pumps and the secretion of ligands that chelate metals These evolutionary adaptations help plants solubilize essential metals from the soil; however, soils with high levels of toxic metals can release both essential and harmful metals into solution.
Soluble metals can enter plant roots via two mechanisms: through the symplast by crossing the plasma membrane of endodermal cells, or through the apoplast by moving between cells While solutes can travel upward via apoplastic flow, the more efficient route is through the xylem, the plant's vascular system To access the xylem, solutes must navigate the Casparian strip, a waxy barrier that requires them to pass through endodermal cells Thus, for metals to enter the xylem, they must cross a membrane, likely facilitated by cellular processes.
Recent advancements in biodegradation and bioremediation of industrial waste highlight the role of membrane pumps and channels in the transport of toxic metals These toxic metals often enter plants through these transport mechanisms, which are originally designed for essential elements Excluder plants have adapted by increasing their specificity for essential nutrients or by actively expelling toxic metals, thus ensuring their survival in contaminated environments (Hall 2002).
After solutes are introduced into the xylem, the xylem sap transports these metals to the leaves, where they are subsequently absorbed into the leaf cells.
Crop processed As and stored in the landfills that does not pose risks to the environment
Cr(VI)-Cr(III) Phytotransformation
Xylem Pericycle Endodermis Symplastic pathway
Phytotechnologies involve various mechanisms for managing metal contaminants in the environment These include phytoextraction, which transfers arsenic (As) from soil to the plant's aerial parts, such as leaves and stems; phytotransformation, which converts chromium (Cr(VI)) from soil into a less toxic form, chromium (Cr(III)), within the plant; phytostabilization, which immobilizes metal contaminants in the soil; and phytovolatilization, which releases mercury (Hg) from the soil into the atmosphere.
Phytoremediation of Environmental Pollutants the leaf, again crossing a membrane The cell types in which the metals are deposited vary between hyperaccumulator species.
Metal toxicity can be mitigated through chemical conversion or complexation, leading to less harmful forms The varying oxidation states of toxic elements significantly influence their uptake, transport, sequestration, and toxicity in plants Additionally, the chelation of toxins by endogenous plant compounds can similarly affect these properties Notably, many chelators utilize thiol groups as ligands, highlighting the importance of sulfur biosynthetic pathways for hyperaccumulator functionality and potential phytoremediation strategies.
The final stage of metal accumulation in plants involves sequestering metals away from cellular processes, primarily occurring in the vacuole, where metals must be transported across the vacuolar membrane Alternatively, metals may accumulate in the cell wall, interacting with negatively charged sites that attract polyvalent cations Additionally, selenium can be volatilized through the stomata, further impacting metal accumulation dynamics.
Metallothioneins are low molecular weight proteins rich in cysteine, synthesized on ribosomes based on mRNA instructions They are classified into four categories, with class I metallothioneins found in mammalian cells and class II in yeast In plants, at least seven genes encode these proteins, as observed in Arabidopsis thaliana Additionally, the metallothionein gene in Pisum sativum has been studied for its roles and implications.
In a study involving A thaliana, the expression of the metallothionein gene (PsMTA) led to a significant increase in copper accumulation in the roots of transformed plants compared to control plants Additionally, metallothionein proteins AtMT2a and AtMT3 were fused with fluorescent proteins and introduced into the guard cells of Vicia faba These metallothioneins effectively protected chloroplasts in the guard cells from degradation caused by cadmium (Cd) exposure by decreasing reactive oxygen species levels The findings indicated that cadmium remains bound to metallothioneins in the cytoplasm, rather than being sequestered in vacuoles as it is when detoxified by phytochelatins (PCs) (Lee et al 2004).
Transporters play a crucial role in the exclusion of toxic metal ions by facilitating their transport into the apoplastic space and vacuole, where their harmful effects are minimized (Tong et al 2004) In studies involving overexpressing lines subjected to lethal concentrations of Zn or Cd, it was observed that these metals were translocated more extensively to the shoot, while root metal levels remained relatively constant This suggests that the roots' metal uptake compensates for the increased translocation to the shoot (Verret et al 2004) The vacuole is regarded as a key component in this process.
Recent advancements in biodegradation and bioremediation of industrial waste highlight the role of metal storage in yeast and plant cells, particularly through phytochelation and metal complexes being transported into vacuoles Notably, YCF1 from Saccharomyces cerevisiae is recognized as a prominent vacuolar transporter, functioning as a Mg ATP-energized glutathione S-conjugate transporter Other significant transporter proteins include the A thaliana antiporter CAX2, which plays a crucial role in ion transport, and LCT1, a versatile transporter for various metals such as Ca²⁺, Cd²⁺, Na⁺, and K⁺ Additionally, the heavy metal ATPase TcHMA4 from Thlaspi caerulescens and a unique family of cysteine-rich membrane proteins that confer cadmium resistance in A thaliana further exemplify the diversity of mechanisms involved in metal transport and detoxification AtMRP3, an ABC transporter, also contributes to these processes, underscoring the complexity of metal management in plants.
Plant Response to Environmental Pollutants
Plants have three basic strategies for growth in metal contaminated soil (Raskin et al 1994):
Certain plant species effectively limit metal uptake in their aerial parts by maintaining low and stable metal concentrations across varying soil metal levels They achieve this primarily by restricting metal absorption in their roots, which involves modifying membrane permeability, enhancing the metal binding capacity of cell walls, or increasing the secretion of chelating substances.
Certain plant species are capable of accumulating metals in their aerial tissues, which often mirrors the metal concentration present in the soil These plants can tolerate high metal levels by producing intracellular metal-binding compounds known as chelators or by changing their metal compartmentalization patterns, storing metals in less sensitive parts of their structure.
Hyperaccumulator plants have the remarkable ability to concentrate metals in their aerial parts, achieving levels significantly higher than those found in the soil These plants can absorb substantial amounts of contaminants through their roots, shoots, and leaves, making them essential for bioremediation efforts.
Hyperaccumulators for Phytoremediation
Successful phytoremediation depends on those plants (woody or herbaceous) that can accumulate desired levels of heavy metal concentration in their shoots
Hyperaccumulators are a rare group of plants, constituting less than 0.2% of angiosperms, that can absorb heavy metals at concentrations 100 to 1,000 times greater than normal without showing visible symptoms For effective phytoremediation, ideal plants must possess several key traits: the capacity to hyperaccumulate heavy metals, rapid growth rates, tolerance to high salt concentrations and varying pH levels, substantial biomass, and ease of harvest, along with efficient uptake and translocation of metals to their aerial parts The main criteria for identifying hyperaccumulator plants are outlined in Table 1.2.
The most extensively studied hyperaccumulators include Thlaspi sp., Arabidopsis sp., and Sedum alfredii sp., all of which belong to the Brassicaceae and Alyssum families Research by Baker et al has significantly contributed to our understanding of these plants.
(2000) found many species that can be classified as hyperaccumulators based on their capacity to tolerate toxic concentration as given in Table 1.3.
Criteria for Metal Accumulation in Plants
Accumulating ability Zhou and Song (2004)
Tolerance ability Sun et al (2009)
Removal efficiency (RE) Soleimani et al (2010)
Bioconcentration factor (BCF) Yoon et al (2006)
Bioaccumulation coefficient (BAC) McGrath and Zhao (2003)
Transfer factor (TF) Liu et al (2010)
Some Metal Hyperaccumulator Plant Species
Biden spilosa Cd Sun et al (2009)
Brassica junceae Ni and Cr Saraswat and Rai (2009)
Thlaspi caerluescen Cd, Zn and Pb Banasova et al (2008)
Solanum nigrum L Cd Sun et al (2008)
Sedum alferedii Cd Sun et al (2007)
Helicotylenchus indicus Pb Sekara et al (2005)
Pistia stratiotes Zn, Pb, Ni, Hg, Cu, Cd and Cr Odjegba and Fasidi (2004)
Pityrogramma calomelanos As Dembitsky and Rezanka (2003)
Thordisa villosa Cu Rajakaruna and Bohm (2002)
Croton bonplandianus Cu Rajakaruna and Bohm (2002)
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Approximately 420 species from around 45 plant families are recognized as metal hyperaccumulators While new species are still being identified through field collections, only a limited number have undergone laboratory testing to verify their hyperaccumulating capabilities A significant challenge with most hyperaccumulators is their low biomass and growth rate Researchers believe that the optimal approach is to transfer the desirable traits of hyperaccumulators into high biomass plants, necessitating a deeper understanding of how these plants tolerate and accumulate elevated heavy metal concentrations.
Plant Growth–Promoting Rhizobacteria in Environmental
Rhizobacteria, or plant growth-promoting rhizobacteria (PGPR), are microorganisms found in the rhizosphere of plants Notable species include Pseudomonas, Azospirillum, and Azotobacter, which play a crucial role in enhancing plant growth and health.
Klebsiella, Enterobacter, Alcaligenes, Arthrobacter, Burkholderia, Bacillus and Serratia, have been reported to enhance plant growth (Kloepper et al 1989;
Plant growth-promoting rhizobacteria (PGPR) have been utilized in agriculture and forestry to enhance plant yield, growth, and disease tolerance (Glick et al., 1995; Joseph et al., 2007) Recent studies highlight their essential role in environmental remediation, particularly in alleviating plant stress under challenging conditions such as flooding, high temperatures, and acidity (Lucy et al., 2004) Metal-resistant plant growth-promoting bacteria (PGPB) act as effective bioinoculants for plants in metal-stressed soils, helping to sequester metals (Kloepper et al., 1989) The adverse effects of heavy metals absorbed from the environment can be mitigated through the application of PGP bacteria or mycorrhizal fungi (Joseph et al., 2007) Soil microbes, including PGPR, phosphate-solubilizing bacteria, mycorrhizal helping bacteria (MHB), and arbuscular mycorrhizal fungi (AMF), play a crucial role in phytoremediation of trace metal-contaminated soils (Kloepper et al., 1989) PGPR comprise a diverse array of free-living soil bacteria that enhance host plant growth and help alleviate the toxic impacts of heavy metals (Belimov et al., 2004) High metal concentrations can be detrimental even to metal-tolerant plants, often leading to iron deficiency and chlorosis in various species grown in contaminated soils (Wallace et al., 1992).
Phytoremediation is enhanced by the use of microbial iron siderophore complexes, which help plants overcome iron deficiency that inhibits chloroplast development and chlorophyll biosynthesis Providing plants with siderophore-producing bacteria can prevent chlorosis in the presence of heavy metals like nickel, lead, and zinc, promoting growth and root establishment Once seedlings are established, these bacteria assist in iron acquisition for optimal growth However, the effectiveness of plant growth-promoting rhizobacteria (PGPR) is limited to slightly and moderately polluted sites, as their tolerance to heavy metal concentrations is crucial Additionally, the diversity of PGPR populations among plants can vary based on root-exuded organic compounds and the type and concentration of heavy metals present in the soil.
A number of new research studies carried out in relation to the effects of PGPR on plant growth and/or heavy metal concentration in polluted soil are given in Table 1.4.
1.6.1 Plant Growth–Promoting Rhizobacteria in Terrestrial Plants
The alteration of rhizospheric microbial communities significantly influences the uptake of essential elements like Mn +2 and Fe +3, enhancing phytoremediation efficiency Research by Hasnain and Sabri demonstrated that inoculating Triticum aestivum seeds with specific Pseudomonas strains improved seedling growth in varying Pb concentrations compared to uninoculated controls However, the safety of using Plant Growth-Promoting Rhizobacteria (PGPR) for phytoremediation is crucial; for instance, while Burkholderia cepacia poses health risks due to its multidrug resistance, it also enhances phytoremediation efficiency.
1.6.2 Plant Growth–Promoting Rhizobacteria in Aquatic Plants
Aquatic plants have recently gained approval for use in remediation efforts, particularly through methods such as rhizofiltration, phytofiltration, and constructed wetlands Research by Zurayk et al (2001), Bennicelli et al (2004), and Abou Shanab et al (2008) highlights the growing interest in these phytoremediation techniques.
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Recent research highlights the positive effects of plant growth-promoting rhizobacteria (PGPR) such as Bradyrhizobium sp., Pseudomonas sp., and Ochrobactrum cytisi on plants growing in heavy metal-contaminated soils These beneficial bacteria enhance plant resilience and growth, mitigating the detrimental impacts of heavy metal toxicity.
Lupinus luteus Pb, Cu and Cd Decr eased the metal accumulation; however , plant biomass incr eased Dary et al (2010) Bacillus subtilis , Bacillus cer eus , Pseudomonas aeruginosa , Flavobacterium sp.
Orychophragmus violaceus has been shown to enhance shoot biomass and zinc accumulation, as noted by He et al (2010) Pseudomonas aeruginosa significantly increased the uptake of chromium and lead in maize by factors of 5.4 and 3.4, respectively, according to Braud et al (2009) Additionally, Ralstonia metallidurans improved chromium accumulation in maize shoots by a factor of 5.2 Achromobacter xylosoxidans strain Ax10 notably enhanced the root and shoot length, as well as the fresh and dry weight of Brassica juncea, leading to significantly improved copper uptake compared to control plants.
Research has shown that various bacterial strains can enhance plant growth and metal accumulation For instance, Ma et al (2009) found that *Microbacterium sp G16* and *Pseudomonas fluorescens G10* increased root elongation and total lead (Pb) accumulation in rape seedlings compared to control plants Similarly, Sheng et al (2008) demonstrated that *Pseudomonas aeruginosa* reduced cadmium (Cd) accumulation in black gram plants while promoting extensive rooting and overall growth Additionally, Ganesan (2008) reported that *Burkholderia sp J62* significantly boosted the biomass of maize and tomato plants, with Pb and Cd content in plant tissues increasing by 38% to 192% and 5% to 191%, respectively.
Jiang et al (2008) demonstrated that the use of Bradyrhizobium sp RM8 significantly enhanced plant growth while simultaneously reducing the uptake of heavy metals, such as Ni and Zn, in plants, as supported by Wani et al (2007).
Phytoremediation has emerged as a promising solution to combat increasing water pollution, particularly through the study of wetland hyperaccumulator species However, research on the influence of rhizospheric or rhizoplanic bacteria on metal uptake in aquatic plants remains limited Notably, So et al (2003) found that copper (Cu²⁺) and zinc (Zn²⁺) resistant bacteria isolated from water hyacinths (Eichhornia crassipes) significantly enhanced the plant's Cu²⁺ removal capacity Similarly, Xiong et al (2008) demonstrated that rhizospheric bacteria associated with the terrestrial plant Sedum alfredii provided protection against heavy metal toxicity in aqueous environments The root surfaces of terrestrial plants host approximately 10⁷ bacteria per cm², indicating a vital relationship between plants and their microbial communities in mitigating environmental pollutants.
In 1998, it was observed that the bacterial population in aquatic plants decreased to 106 cell/cm², as noted by Fry and Humphrey in 1978 This decline in bacterial numbers can be attributed to various factors, particularly the fluctuations in oxygen levels around the roots of aquatic plants, which may alter the dynamics of phytoremediation across different environments.
Transgenic Approach to Phytoremediation
Genetic engineering technologies can significantly enhance a plant's phytoremediation efficiency, with current transgenic research focusing on the genomics of certain plants and bacteria that can alter or eliminate pollutants (Doty 2008) This research spans various applications, including constructed treatment wetlands, field crops, and tree plantations targeting multiple contaminants However, as of now, there are no known full-scale applications of genetically modified plants for the remediation of polluted sites Promising results from several laboratory and pilot studies utilizing transgenic plants for phytoremediation are summarized in Table 1.5.
Technological Development
Phytoremediation is an innovative approach for addressing various environmental pollutants, yet it faces significant challenges, including insufficient research data on metal mass balance and a lack of economic data, making cost estimation difficult Recently, scientists have classified different metals based on the research status of phytoextraction, their readiness for commercialization, and the regulatory acceptance of phytoremediation methods (Lasat 2000).
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Recent advancements in genetically engineered plants, such as Brassica juncea modified with ATP sulfurylase from Arabidopsis thaliana and Se Cysmethyltransferase from Astragalus bisulcatus, have shown promising applications in phytoremediation These transgenic plants not only target specific pollutants but also offer additional benefits, enhancing their potential for environmental remediation.
Enhancing selenium accumulation, tolerance, and volatilization is crucial for biodiesel production and carbon sequestration Research by Dhankher et al (2012) highlights the potential of hybrid poplar (Populus seiboldii × Populus grandidentata) engineered with the manganese peroxidase (MnP) gene from Trametes versicolor to improve these processes.
The degradation of bisphenol A using biomass can significantly enhance bioenergy production, pulp, charcoal, and carbon sequestration Research by Iimura et al (2007) highlights the use of Populus deltoides engineered with the bacterial mercuric ion reductase (merA) gene, which improves the reduction and resistance to mercuric ions, thereby contributing to effective biomass utilization for sustainable energy and environmental benefits.
Che et al (2003) Populus canescens over expr essing γ glutamylcysteine synthetase Tolerance to Zn str ess Biomass for bioener gy , pulp, char coal, carbon sequestration
Bittsanszkya et al (2005) Hybrid aspen ( Populus tr emula × Populus tr emuloides ) expr essing bacterial nitr or eductase ( pnrA )
Enhanced bioremediation techniques utilizing biomass, such as hybrid poplar (Populus tremula × Populus alba), have shown effectiveness in removing contaminants like TCE, vinyl chloride, CCl4, benzene, and chloroform, while also contributing to bioenergy production, pulp, charcoal, and carbon sequestration (van Dillewijn et al., 2008) Additionally, research by Doty et al (2007) on Populus trichocarpa, which overexpresses γ-glutamylcysteine synthetase, demonstrates increased tolerance to chloracetanilide herbicides, further enhancing its potential for biomass applications in bioenergy and environmental remediation.
Advantages and Disadvantages
Phytoremediation is an eco-friendly process that utilizes specific plants to remove contaminants from polluted environments, contributing to environmental protection and human health through biotechnological advancements This method not only aids in the cleanup of contaminated sites but also has the potential to concentrate and harvest valuable metals found in low concentrations in the soil Despite its benefits, phytoremediation remains a developing technology with limited awareness in the scientific community, which restricts its application in larger contaminated areas The advantages and disadvantages of phytoremediation technology are detailed in Table 1.7.
Recent Research Status, Readiness for Commercialisation and
Regulatory Acceptance of Phytoremediation for Some Metal and
Recent Research Status Commercial Readiness a Regulatory Acceptance b
Source: Adapted from Mukhopadhyay, S., and Maiti, S.K., Applied Ecology and Environmental Research 8, 3, 207–222, 2010. a Rating: 1 – basic research underway; 2 – laboratory stage; 3 – field deploy ment; 4 – under commercialisation. b Y – yes; N – no.
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Future Outlook
Phytoremediation research is currently focused on laboratory studies where plants are grown in hydroponic systems with heavy metal diets, but scientists acknowledge that soil conditions present unique challenges due to the insolubility of most metals To advance commercially viable phytoremediation practices, it is essential to optimize agronomic methods and enhance plant genetics There is a need for further discovery of hyperaccumulators and a deeper understanding of their eco-physiology Key areas for future research include improving plant growth rates, biomass production, metal uptake, translocation, and tolerance through genetic engineering For transgenic plants to gain acceptance in the remediation industry, ongoing field testing and reevaluation of regulatory restrictions are crucial Additionally, selecting and testing multiple hyperaccumulators can significantly improve phytoremediation efficiency, contributing to a cleaner environment.
Advantages and Disadvantages of Phytoremediation
Cheap and aesthetically pleasing (no excavation required) The plant must be able to grow in the polluted media.
Soil stabilization plays a crucial role in minimizing water leaching and the transport of inorganic materials within the soil Plants have the ability to absorb these inorganic substances through their root systems, provided they are soluble in the soil.
Generation of a recyclable metal rich plant residue Time consuming process can take years for pollutant concentrations to reach regulatory levels (long term commitment).
Applicability to a wide range of toxic metals and radionuclides The pollutant must be within or drawn toward the root zones of plants that are actively growing.
Minimal environmental disturbance as compared to conventional remedial methods It must not pose harm to human health or further environmental problems
Removal of secondary air or water borne wastes Climatic conditions are the limiting factor.
Enhanced regulatory and public acceptance Introduction of exotic plant species may affect biodiversity.
Regulatory Considerations
Various federal and state regulatory programs influence site-specific decisions regarding technology use, including the Resource Conservation and Recovery Act (RCRA) for waste management, the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) or 'Superfund' for environmental cleanup standards, and the Clean Air Act (CAA) for regulating hazardous air emissions Additionally, the Toxic Substances Control Act (TSCA) oversees commercial bioremediation plants, while the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) governs pesticide usage The Federal Food, Drug, and Cosmetic Act (FFDCA) regulates phytoremediation plants used as food, alongside statutes enforced by the U.S Department of Agriculture.
Research Needs
1 Further exploration of plants suitable for phytoremediation is required.
2 Continued field demonstration is required to determine the extent of pollutant removal by selected plant species.
3 High resolution microanalyses of hyperaccumulator plants by SEM or TEM are required to determine the discrete site of metal seques tration and bioaccumulation in specific plant organs, tissues, cells and organelles.
4 There is a need to evaluate the procedure for disposal, processing and volume reduction of polluted biomass.
5 Studies on root and other plant biomass decomposition in soil are required to understand the kinetics and cycling of contaminants.
6 There is a need to extend the investigation of the most promising research on phytoremediation, which also includes the following: a Mechanism of pollutant uptake, transport and accumulation in plant tissues b Better understanding of rhizosphere interaction among plant roots, microorganisms and other biota c Role of both natural and artificial metal chelators and their metal complexes, their dynamics and decomposition in rhizosphere and plant tissues
Recent advancements in biodegradation and bioremediation of industrial waste include the development of fertilizers and soil amendments that boost the phytoremediation efficiency of hyperaccumulators Additionally, the creation of transgenic plants has shown promise for the effective removal of environmental pollutants Furthermore, the breeding of genetically modified organisms specifically designed to accumulate and degrade pollutants is paving the way for innovative solutions to environmental challenges.
Concluding Remarks
Environmental pollution is a pressing global issue, and phytoremediation emerges as a promising, eco-friendly cleanup technology that utilizes green plants to restore contaminated sites This approach is particularly beneficial for developing countries like India, where knowledge of effective phytoremediation plants is still limited and funding remains a significant challenge To enhance the effectiveness of this technology, it is essential to allocate financial resources for better understanding the ecology and behavior of plants in polluted environments Rigorous field research is necessary to grasp the movement and fate of pollutants, considering the specific nature of the contaminants and the environmental context Additionally, understanding how plants absorb trace and toxic elements is crucial Efforts should also focus on conserving and establishing more mangrove species and other vegetation to support phytoremediation efforts Furthermore, increasing public awareness and providing clear information about this sustainable green technology is vital for its global acceptance and implementation.
Financial support from the Department of Science & Technology (DST) and the Science and Engineering Research Board (SERB) of the Government of India is gratefully acknowledged for Prof Ram Chandra Additionally, Mr Vineet Kumar, PhD, receives recognition for the Rajeev Gandhi National Fellowship (RGNF) awarded by the University Grant Commission (UGC).
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