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Phytoremediation of Cd-Contaminated Soil and Water
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Phytoremediation of Cd-Contaminated Soil and Water Nadeem Iqbal*, Malik Tahir Hayat*, Bibi Saima Zeb*, Zaigham Abbas†, Toqeer Ahmed‡ *
Department of Environmental Sciences, COMSATS, Abbottabad, Pakistan Ministry of Climate Change, Islamabad, Pakistan ‡ Centre for Climate Research and Development (CCRD), COMSATS, Islamabad, Pakistan †
Abstract Phytoremediation is the best technique among other conventional methods to treat contaminated sites. It is an inexpensive, environmentally safe technique that can remove cadmium (Cd) from contaminated soil and water through different processes, such as phytoextraction, phytosequestration, and phytostimulation. The use of hyperaccumulator plants can enhance the removal of Cd from the polluted site and result in productive land. These plants accumulate heavy metals such as Cd into vacuoles of root cells and in the aerial parts of the plants, i.e., the shoot and leaf. Different mechanisms have been studied by which Cd is accumulated into the plants. There are different factors that can increase the removal of Cd through plants, such as microbial-assisted phytoremediation, the addition of chelating agents, and decapitation. The efficiency of phytoremediation can be enhanced by lowering the pH of the contaminated site, which ultimately helps in more removal of Cd from contaminated water and soil. There are several advantages of adopting this efficient and more productive technique. It can be the best option, or be used as an alternative to other traditional methods. This innovative and promising technique enhances the removal of Cd from contaminated sites, and converts these barren sites into productive lands. Keywords: Phytoremediation, Cadmium, Phytoextraction, Phytosequestration, Microbe-assisted phytoremediation, Soil and water contamination
1 INTRODUCTION Contamination of soil and water is a major problem causing drastic effects on all living organisms, and ultimately on humans. Soil and water pollution is caused due to many natural and anthropogenic sources. Natural sources include forest fires, volcanic eruptions, radioactivity, and weathering of rocks. Anthropogenic sources include consumption of fossil fuels, incineration of solid waste, and industrial processes such as smelting of ore, mining, Cadmium Toxicity and Tolerance in Plants https://doi.org/10.1016/B978-0-12-814864-8.00021-8
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and production of fuel and energy. The world population is growing rapidly, and the availability of nonrenewable resources is gradually decreasing. It is time to turn our attention toward renewable or regenerative resources to meet the needs of the growing population. Both natural and anthropogenic activities have badly affected the balanced ecosystem. Therefore, it is necessary to recover contaminated soil and water (Pavaloaia et al., 2015). Many toxic organic and inorganic compounds from various sources are being released into the environment, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and heavy metals, including cadmium (Cd), zinc (Zn), lead (Pb), chromium (Cr), and nickel (Ni). These heavy metals can cause disease in animals and humans. Direct and indirect sources of these heavy metals result in air, water, and soil pollution. The transformations of these heavy metals result in many economic, public, and environmental consequences. Heavy metals are not degraded by microbes, and they are persistent in the environment. Heavy metals are mostly carcinogenic in nature, and accumulate in the organs of organisms (plants and animals). These metals have various applications in consumer products, pulp and paper engineering works, the leather industry, materials used in photography, plastic stabilizers, batteries, and fertilizers (Ali et al., 2013). Cd has toxicological effects on humans and other living organisms. If a small amount of Cd is exposed for a short time, it can be harmful, and result in adverse health effects (Abu-El-Halawa and Zabin, 2017). Cd contamination in cultivated soils comes from several sources, including phosphate fertilizers, compost, manure, and sewage sludge. Cd causes serious threats to animals and humans by being taken up by plants, accumulating in them, and exceeding harmful levels (Cheng et al., 2017). Cd binds at active sites of enzymes, and is selected by competing with other essential nutrients, and then causes toxicity to the host cells of exposed organisms. Therefore, different, effective ways are needed to treat Cd-contaminated soils to mitigate its risk. Different techniques have been used to treat contaminated soil and water. Phytoremediation is a promising technique that is cost effective and environmentally safe. This technique has the potential to accumulate heavy metals, such as Cd, in plants from soil (Cheng et al., 2017). The cost analysis of phytoremediation was carried out, and it was found that this technique is more cost effective than other treatment techniques, such as ion exchange, solvent extraction, adsorption, oxidationreduction, and reverse osmosis. Additionally, phytoremediation improves the soil quality and soil biological processes (Wan et al., 2016).
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2 PHYTOREMEDIATION Phytoremediation is a type of bioremediation in which plants are used to degrade or immobilize contaminants in soil or water to mitigate the effects of toxic pollutants. This word is from the Greek language, and it refers to restoring contaminated sites through plants. Phytoremediation is an economical and environmentally safe technique in which contaminants are extracted from soil. Phytoremediation involves various processes, which are described as follows ( Jiang et al., 2015). Phytoremediation technology uses plants to remediate or treat contaminated soil and water to make them harmless. This technique has attracted the attention of stakeholders because it has useful applications in the remediation of soil and water, and improves the quality of land and water by managing waste. The contamination of these heavy metals has severe health risks and drastic impacts on human health. Heavy metals, including Cd, are mutagenic, teratogenic, and carcinogenic to human beings. These metals affect the endocrine and nervous systems, especially in children. Several physio-chemical methods have been used to treat Cd-contaminated soils, but traditional methods are too expensive to implement on a large scale. Such methods need more processing, and result in a change in the soil properties. Therefore, phytoremediation can be applied at a large scale with sustainable costs (Sharma and Pandey, 2014). Phytoremediation exploits herbs, shrubs, and woody species because they have the potential to remove or accumulate different environmental pollutants, such as heavy metals, including Cd in soil and water. The technique can also make the pollutants harmless. Phytoremediation also mitigates the risk of dispersion of contaminants, and is helpful in decontaminating soil and water. Several factors affect the mechanism and efficacy of phytoremediation, such as bioavailability, soil characteristics, plant species, and the nature of the contaminants. Plants showing more efficient removal of heavy metal are called hyperaccumulators. These plants have the capacity to remove or accumulate large amounts of heavy metals, but have less biomass production. Biomass production of plants affects their extraction efficiency, because more biomass production will remove more heavy metal, but it will also need more harvests for the plants’ removal. However, the quantity of total harvests will represent the total cost of the complete operation, including incineration and disposal (Sreelal and Jayanthi, 2017).
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3 WHY DO WE NEED PHYTOREMEDIATION? Phytoremediation is an environmentally safe approach that has the potential to use the capacity of plants to accumulate contaminants from the environment, and to degrade or accumulate these toxic pollutants. The accumulation in certain plants leverages their natural ability to render harmless pollutants in soil or water. With the knowledge of molecular and physiological changes, the technology of phytoremediation has been improved, and has emerged as an innovative technique. In the phytoremediation technique, more harmful heavy metals and toxic organic contaminants are targeted. Different types of engineering works and biological strategies have been developed to improve the process of phytoremediation. The feasibility of various plants has been confirmed using phytoremediation for environmental cleanup purposes (Ali et al., 2013).
4 PHYTOREMEDIATION AS AN ENVIRONMENTALLY FRIENDLY TECHNIQUE Phytoremediation is a cost effective and environmentally friendly technology that is considered a green, or sustainable, remediation method. In this technique, plants (hyperaccumulators) are used to treat polluted sites (water or soil) to remove harmful contaminants. Phytoremediation is a promising technique that provides the best alternative to other conventional methods. It has been reported that more than 500 plant species are well recognized as accumulators of Cd and other heavy metals from polluted sites (Kramer, 2010; Hemen, 2011; Yu et al., 2013). Plants are considered the primary acceptors of metals such as Cd. Therefore, plants remediate such contaminated sites to treat xenobiotic metals through phytoremediation, which is an effective and technique for removing pollutants from soil or water. It does not disrupt the environment and it is economically feasible. It has the ability to treat the polluted site to remove more than one contaminant. It is easy, and can be implemented in situ and ex situ. Due to the use of plants, it is pleasing aesthetically (Ma et al., 2005).
5 PROCESSES OF PHYTOREMEDIATION The following sections describe the different remediation processes, including how contaminants are accumulated and removed.
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5.1 Phytosequestration Phytosequestration is also known as phytostabilization. In this process, chemicals or pollutants are absorbed or adsorbed on the surface of roots. Toxic pollutants are immobilized or sequestrated in the vacuoles of roots in contaminated soil or water. Contaminants are absorbed by the roots of plants and immobilized in the rhizosphere. It decreases the mobility of pollutants and their bioavailability in the food chain. This technique also prevents the movement of contaminants into the ground water. Contaminated sites are being treated using metal-tolerant species of plants. The heavy metals can include Cd, Cr, As, Cd, or Zn (Zhao et al., 2016). This technique initiates changes in the composition of the soil chemistry, providing the surface of the roots with the facility to absorb the metals (Yang et al., 2016). However, this method cannot remove pollutants from treated soil until the plants are eradicated.
5.2 Phytoextraction This process is also called phytoaccumulation. Contaminants are taken up by the plants and stored in the tissues of the stems or leaves. In this process, pollutants are degraded and removed from the environment. It is a useful technique for removing heavy metals from the contaminated soil or water. Through the process of incineration of these plants, these heavy metals can be recovered for reuse, which is referred to as phytomining (Eissa, 2017). Roots absorb pollutants from the soil and transport them to the shoots and leaves, where they are accumulated. Plants that have a high capacity to remove pollutants and produce a large biomass are ideal for this technique. The hyperaccumulator species has the ability to store pollutants, but exhibits less biomass production. Those species that produce more biomass, but perform less removal of heavy metals can be selected and used for this purpose. This process ends with harvesting of the plants, followed by their incineration, and landfilling (Mahar et al., 2016). This technique is environmentally friendly, among other advantages. It poses no damage on the landscape. It protects the ecosystem and is a primary phytoremediation technique. It is inexpensive, and the most commercially acceptable technique (Shiri et al., 2015). Additionally, the rhizosphere has a low bioavailability of metal, and a low absorption rate of heavy metal from the roots. Metals are retarded within the roots (Fiorentino et al., 2013). Research has been conducted to elaborate on the capability of plants to remove or accumulate the heavy metals from contaminated soil and water.
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Specific research has also been conducted to explain the performance of the herbaceous plant Aruddox donax in Cd contaminated soil. A 20% concentration of Cd in leaves and 30% in the rhizome was accumulated, which was more efficient than the control group (Van Oosten and Maggio, 2014).
5.3 Phytostimulation The activity of rhizobacteria is enhanced by the addition of mycorrhizalhelping bacteria. The removal of toxic heavy metals is increased by the addition of microbes in the roots of plants. This process is also known as rhizosphere remediation. It is a technique in which different compounds are released from the roots of plants, and they enhance microbial activity. It is a low-cost technology for removing Cd, heavy metals, and other organic compounds ( Jia et al., 2016). There are several reasons for phytostimulation to be used for rhizoremediation. A particular niche is provided by rhizosphere-to-soil microorganisms. Exudates released by roots provide nutrients and enhance microbial growth. A habitat is provided by the rhizosphere for the soil microbes. These microorganisms, in return, cause the degradation, or biotransformation, of many organic and inorganic compounds, such as heavy metals. Another method is the addition of microorganisms in the soil. These resistant microbial strains can cause the breakdown of organic compounds and the accumulation of toxic heavy metals such as Cd (Yanai et al., 2006).
6 PHYTOREMEDIATION MECHANISMS FOR Cd-CONTAMINATED SOIL/WATER Contaminated soils can be treated in a number of ways to reduce Cd toxicity. The common methods are use of landfills, incineration, washing, leaching, vitrification, electroreclamation, and excavation. These methods are expensive and need capital investment. These techniques are nonbiological processes, and cause disruption of the surrounding environment (Gao et al., 2012). Usually, plants absorb Cd metal through the roots from the soil, which serves as a bioavailable pool of Cd metal, and nutrients required by plants as well. Different factors affect the bioavailability of Cd metal in soil. These factors can be organic matter, root exudates, pH, competitive cations, and microbial biomass (Sarwar et al., 2010). Once a Cd metal is taken up by plants, it can either be stored or accumulated in root tissues, or translocated to upper parts of plants through apoplastic or symplastic routes. Hyperaccumulators have vital role in Cd
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metabolism: to keep it away from the other metabolic activities in order to hinder its deleterious effects on plants. Vacuoles can accumulate Cd metal in the shoots. There are five main steps involved in phytoextraction; Cd mobility in the rhizospheric zone, plant roots’ uptake of Cd metal, the transfer or translocation of Cd ions to upper plant roots, plant tissues sequestration of Cd, and tolerance of Cd metal (Ali et al., 2013). If a plant is more tolerant to a specific metal, it will accumulate a huge portion of that metal inside its tissues, and undergo minimum adverse impacts. The potential of a plant to tolerate Cd metal depends upon various mechanisms. These mechanisms can be Cd metal binding to the cell wall, movement of Cd ions into vacuoles through active transport, or binding of proteins with metal chelates (Memon and Schr€ oder, 2009).
7 USE OF PLANTS AND THEIR BIOCHEMICAL PROCESSES IN PHYTOREMEDIATION Different essential nutrients are required by the plants, which are absorbed by the roots. These nutrients are N, P, K, Ca, S, Mg, Cl, Fe, Zn, Cu, B, Mn, Mo, and B. These nutrients are transported to the upper parts of plants, or the cells of roots through active transport or passive transport. Transport proteins are also associated with cell membranes. These nutrients are transported within the plants through two ways; apoplast and symplast (Lu et al., 2011). Certain plant species (Thlaspi caerulescens) showed tolerance against Cd and Zn due to their metal chilate formation. Such plants reduce the cytosolic concentration of the metals, and store them in the vacuoles. Willows exhibit the potential to uptake different metals such as Cd, Cu, Zn, Fe, and Pb. As compared with other herbaceous plants, willows possess deeper root systems, and can decontaminate a large zone of contaminated soil (Xiao et al., 2015).
8 MOBILITY/AVAILABILITY OF Cd Total concentrations of metals in contaminated soil cannot be predicted on the basis of their behavior. The toxicity and uptake of most of the metals is dependent on specific metals and free ions in the soil. Except for this general observation, free metal can also be available to plants, and is more active than the ionic form. Mostly, the concentrations of metals soluble in water are considered available forms to plants (Abe et al., 2008). Equilibrium
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partitioning is used to describe the metal distribution between pore water and the solid phase of soil. The total distributed or dissolved concentration of metal does not necessarily correspond to the amount that is available to plants. Complex ions, ion pairs, microparticulates, or polymers can decrease free ionic species in a solution. Speciation is the process in which the identification and quantification of various metals is carried out in a sample. Different models are present in order to calculate the metal speciation in water, soil, or sediments, i.e., the free ion activity (FIAM) model and wandermere humic aqueous model (WHAM) (Fang et al., 2012). Cd has better ability to become available for plant uptake as compared with other heavy metals, such as Cu, Ni, Pb, Zn, and Mn. Therefore, high Cd uptake by plants results from low concentrations of Cd in the soil. It has already been established that Cd is a nonessential trace element for the growth of plants (Peng et al., 2009).
9 FACTORS AFFECTING THE SOLUBILITY/BIOAVAILABILITY OF Cd IN SOIL There are a number of factors that affect Cd availability in plants. One of them is pH, which affects the concentrations of soluble Cd in plants. At low pH, the solubility of Cd metal increases, and decreases at higher pH. In accordance with some research, at pH 7, all metal ions are removed from the solution as hydroxo species, which are being adsorbed on the soil’s surface. The buffering capacity of the soil for an acidic surrounding can also affect the metal mobility in soil. Moreover, the leaching of these metals can increase, due to high pH. However, Cd adsorption was found at 3.7 and 4 pH values. At pH 6–7, precipitation of the insoluble solids becomes essential (Zhang et al., 2010). It has been reported that the decrease in redox potential increased the solubility of Cd in soil. The mechanism that is behind that fact is the dissolution of Fe-Mn oxyhydroxides, which release the absorbed metals under reducing conditions. According to some scientists, the availability of Cd to plants is decreased in water logging conditions because of CdS formation. This will also result in Zn solubility in water. Many studies have shown that soil’s organic matter also affects the Cd uptake by plants in a negative association (Arnold and McDonald, 1999).
10 FACTORS INCREASING Cd UPTAKE BY PLANTS There are several ways to increase the uptake of Cd in the plants from soil.
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10.1 Microbe-Assisted Phytoremediation Several bacteria (rhizobacteria) have been successfully applied to plants, resulting in plant growth. These bacteria assisted in decreasing Cd toxicity and increased plant growth in Cd contaminated soils. These plant-growthpromoting bacteria transform Cd into a soluble and available form to plants by changing their chemical properties. These microbes use different binding compounds, such as siderphores, organic acids, biosurfactants, and exopolymers (Ullah et al., 2015). Another promising technique is microbe-assisted phytoremediation. The deficiency of nutrients in the soil can inhibit the growth of bacteria, and then inhabiting bacteria are activated by adding nutrients (bioaugmentation). Therefore, microbes are biostimulated with nutrients to increase microbial growth to remove organic and heavy metal pollutants (Rojjanateeranaj et al., 2017). Different types of crops (maize and soybean) have been used to treat Cd contaminated soil, which was assisted by the bacterial community. These crops were selected due to rapid growth, large biomass, and good energy production (Chen et al., 2017).
10.2 Decapitation Increased Phytoremediation The decapitation method is practiced in agronomy, in which the number of branches is increased by enhancing apical dominance. It has been reported that removing or cutting of apical buds increased shoot growth in various plants. It is well known that Cd accumulates in the shoots of hyperaccumulators because it is a predominant part of the plants. Decapitation increased the process of transpiration, biomass production, and accumulation of Cd (Liu et al., 2017).
10.3 Effect of pH on Cd Availability in Plants Soil properties are also affected by the availability of Cd in soil. Cd mobilization is caused by texture, nutrient levels, organic matter, and pH. It has been reported that the bioavailability of Cd to plants is greatly influenced by pH. An inverse relationship was found between pH and Cd uptake in general. The uptake of Cd in plants increases with a decrease in pH. When the pH of soil decreases, the sesquioxides and variable charged clays release Cd in the soil. The solubility of Cd compounds (CdCO3 and Cd3(PO4)2 also increases with decreasing pH. Therefore, the bioavailability of Cd to plants can be enhanced by increasing rhizospheric acidification, and it can be an efficient technique to increase the efficacy of phytoremediation, and lessens the remediation time period (Yanai et al., 2006).
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10.4 Enhancement of Phytoremediation Using Mobilized Chelating Agents To obtain more efficient removal of Cd, different chelating agents are used. Ethylenediaminetetraacetic acid (EDTA) has application in detergents and other cleaning products, the paper and pulp industries, and water treatment. EDTA can be used as a chelating agent because it has a high affinity to combine with heavy metals such as Cd, and forms metal and EDTA complex compounds. It is also being applied to remove different metals from contaminated soil and water. This agent can enhance the availability of Cd-bound EDTA to plants and increase the translocation of these metals from the roots to the shoots of plants. Studies have shown that the addition of EDTA in soil (12 mmol kg 1) enhanced the availability of Cd up to 233%. Therefore, the addition of chelate ligands to contaminated soils can increase the uptake of Cd to plants, and the removal of Cd from the environment (Eissa, 2017).
11 ADVANTAGES OF PHYTOREMEDIATION Phytoremediation is a cost-effective technique. It is inexpensive due to that fact that very little labor is required to monitor plants. It does not interfere with the ecosystem, and adds an aesthetic value to the treated land through plant cover. Different heavy metals can be recovered and reused for several purposes in industries. It is an environmentally safe process, because it poses no potentially harmful impacts on the environment. This technique uses naturally occurring living organisms (Tahir et al., 2016).
12 DISADVANTAGES OF PHYTOREMEDIATION Phytoremediation is a slow process, compared with other engineering techniques. It can take years to treat slightly polluted soil or water (Han et al., 2018). It is confined to the surface area and the depth of roots of plants. The contaminated area in soil is next to the roots of plants. It requires more time for plants’ biomass production, and exhibits slow growth. It is impossible to prevent the complete leaching of pollutants into ground water. The toxicity caused by contaminants also affects the survival of plants. Phytoremediation also requires the safe disposal of contaminated plant material. The accumulation of toxic metals from soil in plants is passed through the food chain, and ultimately affects animals and humans.
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13 CONCLUSION AND FUTURE PERSPECTIVES Cd contamination in soil and water is a major concern to health and the environment because of its adverse impacts on food chain contamination, and other connected health risks. In this case, the technique of phytoremediation can be a helpful tool to tackle environmental pollution. This is an emerging and innovative method. This technique is economically feasible and more effective than other physical and chemical techniques. Hyperaccumulators can be efficiently used to remove large concentrations of Cd and other heavy metals from contaminated soil and water. Other conventional methods are expensive and a significant amount of labor is required. Phytoremediation is a slow process, yielding less biomass production above ground. Other approaches, such as use of microbes, decapitation, and low pH maintenance are also useful in combination with phytoremediation to decontaminate Cd-polluted soil and water. The phytoremediation technique requires further research to improve the abilities of plants in order to increase their effectiveness and efficiency. Phytoremediation can serve as a reliable alternative to traditional methods because it is time-saving and more costeffective. Additionally, its harvests can be utilized for biofuel production. The incineration of metal-tolerant plants can yield valuable metals, which can be reused in industry. Further research should be done on transgenic plants to determine all of the environmental and economic benefits. It provides a better option compared with other conventional techniques. More knowledge of this technique is required in order to increase its efficiency and potential in remediating Cd-contaminated sites.
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FURTHER READING Kavamura, V.N., Esposito, E., 2010. Biotechnological strategies applied to the decontamination of soils polluted with heavy metals. Biotechnol. Adv. 28, 61–69. Zhang, H., Zheng, L.C., Yi, X.Y., 2009. Remediation of soil co-contaminated with pyrene and cadmium by growing maize (Zea mays L.). Int. J. Environ. Sci. Technol. 6, 249–258.
Phytoremediation of Cd-Contaminated Soil and Water Nadeem Iqbal*, Malik Tahir Hayat*, Bibi Saima Zeb*, Zaigham Abbas†, Toqeer Ahmed‡ *
Department of Environmental Sciences, COMSATS, Abbottabad, Pakistan Ministry of Climate Change, Islamabad, Pakistan ‡ Centre for Climate Research and Development (CCRD), COMSATS, Islamabad, Pakistan †
Abstract Phytoremediation is the best technique among other conventional methods to treat contaminated sites. It is an inexpensive, environmentally safe technique that can remove cadmium (Cd) from contaminated soil and water through different processes, such as phytoextraction, phytosequestration, and phytostimulation. The use of hyperaccumulator plants can enhance the removal of Cd from the polluted site and result in productive land. These plants accumulate heavy metals such as Cd into vacuoles of root cells and in the aerial parts of the plants, i.e., the shoot and leaf. Different mechanisms have been studied by which Cd is accumulated into the plants. There are different factors that can increase the removal of Cd through plants, such as microbial-assisted phytoremediation, the addition of chelating agents, and decapitation. The efficiency of phytoremediation can be enhanced by lowering the pH of the contaminated site, which ultimately helps in more removal of Cd from contaminated water and soil. There are several advantages of adopting this efficient and more productive technique. It can be the best option, or be used as an alternative to other traditional methods. This innovative and promising technique enhances the removal of Cd from contaminated sites, and converts these barren sites into productive lands. Keywords: Phytoremediation, Cadmium, Phytoextraction, Phytosequestration, Microbe-assisted phytoremediation, Soil and water contamination
1 INTRODUCTION Contamination of soil and water is a major problem causing drastic effects on all living organisms, and ultimately on humans. Soil and water pollution is caused due to many natural and anthropogenic sources. Natural sources include forest fires, volcanic eruptions, radioactivity, and weathering of rocks. Anthropogenic sources include consumption of fossil fuels, incineration of solid waste, and industrial processes such as smelting of ore, mining, Cadmium Toxicity and Tolerance in Plants https://doi.org/10.1016/B978-0-12-814864-8.00021-8
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and production of fuel and energy. The world population is growing rapidly, and the availability of nonrenewable resources is gradually decreasing. It is time to turn our attention toward renewable or regenerative resources to meet the needs of the growing population. Both natural and anthropogenic activities have badly affected the balanced ecosystem. Therefore, it is necessary to recover contaminated soil and water (Pavaloaia et al., 2015). Many toxic organic and inorganic compounds from various sources are being released into the environment, such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and heavy metals, including cadmium (Cd), zinc (Zn), lead (Pb), chromium (Cr), and nickel (Ni). These heavy metals can cause disease in animals and humans. Direct and indirect sources of these heavy metals result in air, water, and soil pollution. The transformations of these heavy metals result in many economic, public, and environmental consequences. Heavy metals are not degraded by microbes, and they are persistent in the environment. Heavy metals are mostly carcinogenic in nature, and accumulate in the organs of organisms (plants and animals). These metals have various applications in consumer products, pulp and paper engineering works, the leather industry, materials used in photography, plastic stabilizers, batteries, and fertilizers (Ali et al., 2013). Cd has toxicological effects on humans and other living organisms. If a small amount of Cd is exposed for a short time, it can be harmful, and result in adverse health effects (Abu-El-Halawa and Zabin, 2017). Cd contamination in cultivated soils comes from several sources, including phosphate fertilizers, compost, manure, and sewage sludge. Cd causes serious threats to animals and humans by being taken up by plants, accumulating in them, and exceeding harmful levels (Cheng et al., 2017). Cd binds at active sites of enzymes, and is selected by competing with other essential nutrients, and then causes toxicity to the host cells of exposed organisms. Therefore, different, effective ways are needed to treat Cd-contaminated soils to mitigate its risk. Different techniques have been used to treat contaminated soil and water. Phytoremediation is a promising technique that is cost effective and environmentally safe. This technique has the potential to accumulate heavy metals, such as Cd, in plants from soil (Cheng et al., 2017). The cost analysis of phytoremediation was carried out, and it was found that this technique is more cost effective than other treatment techniques, such as ion exchange, solvent extraction, adsorption, oxidationreduction, and reverse osmosis. Additionally, phytoremediation improves the soil quality and soil biological processes (Wan et al., 2016).
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2 PHYTOREMEDIATION Phytoremediation is a type of bioremediation in which plants are used to degrade or immobilize contaminants in soil or water to mitigate the effects of toxic pollutants. This word is from the Greek language, and it refers to restoring contaminated sites through plants. Phytoremediation is an economical and environmentally safe technique in which contaminants are extracted from soil. Phytoremediation involves various processes, which are described as follows ( Jiang et al., 2015). Phytoremediation technology uses plants to remediate or treat contaminated soil and water to make them harmless. This technique has attracted the attention of stakeholders because it has useful applications in the remediation of soil and water, and improves the quality of land and water by managing waste. The contamination of these heavy metals has severe health risks and drastic impacts on human health. Heavy metals, including Cd, are mutagenic, teratogenic, and carcinogenic to human beings. These metals affect the endocrine and nervous systems, especially in children. Several physio-chemical methods have been used to treat Cd-contaminated soils, but traditional methods are too expensive to implement on a large scale. Such methods need more processing, and result in a change in the soil properties. Therefore, phytoremediation can be applied at a large scale with sustainable costs (Sharma and Pandey, 2014). Phytoremediation exploits herbs, shrubs, and woody species because they have the potential to remove or accumulate different environmental pollutants, such as heavy metals, including Cd in soil and water. The technique can also make the pollutants harmless. Phytoremediation also mitigates the risk of dispersion of contaminants, and is helpful in decontaminating soil and water. Several factors affect the mechanism and efficacy of phytoremediation, such as bioavailability, soil characteristics, plant species, and the nature of the contaminants. Plants showing more efficient removal of heavy metal are called hyperaccumulators. These plants have the capacity to remove or accumulate large amounts of heavy metals, but have less biomass production. Biomass production of plants affects their extraction efficiency, because more biomass production will remove more heavy metal, but it will also need more harvests for the plants’ removal. However, the quantity of total harvests will represent the total cost of the complete operation, including incineration and disposal (Sreelal and Jayanthi, 2017).
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3 WHY DO WE NEED PHYTOREMEDIATION? Phytoremediation is an environmentally safe approach that has the potential to use the capacity of plants to accumulate contaminants from the environment, and to degrade or accumulate these toxic pollutants. The accumulation in certain plants leverages their natural ability to render harmless pollutants in soil or water. With the knowledge of molecular and physiological changes, the technology of phytoremediation has been improved, and has emerged as an innovative technique. In the phytoremediation technique, more harmful heavy metals and toxic organic contaminants are targeted. Different types of engineering works and biological strategies have been developed to improve the process of phytoremediation. The feasibility of various plants has been confirmed using phytoremediation for environmental cleanup purposes (Ali et al., 2013).
4 PHYTOREMEDIATION AS AN ENVIRONMENTALLY FRIENDLY TECHNIQUE Phytoremediation is a cost effective and environmentally friendly technology that is considered a green, or sustainable, remediation method. In this technique, plants (hyperaccumulators) are used to treat polluted sites (water or soil) to remove harmful contaminants. Phytoremediation is a promising technique that provides the best alternative to other conventional methods. It has been reported that more than 500 plant species are well recognized as accumulators of Cd and other heavy metals from polluted sites (Kramer, 2010; Hemen, 2011; Yu et al., 2013). Plants are considered the primary acceptors of metals such as Cd. Therefore, plants remediate such contaminated sites to treat xenobiotic metals through phytoremediation, which is an effective and technique for removing pollutants from soil or water. It does not disrupt the environment and it is economically feasible. It has the ability to treat the polluted site to remove more than one contaminant. It is easy, and can be implemented in situ and ex situ. Due to the use of plants, it is pleasing aesthetically (Ma et al., 2005).
5 PROCESSES OF PHYTOREMEDIATION The following sections describe the different remediation processes, including how contaminants are accumulated and removed.
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5.1 Phytosequestration Phytosequestration is also known as phytostabilization. In this process, chemicals or pollutants are absorbed or adsorbed on the surface of roots. Toxic pollutants are immobilized or sequestrated in the vacuoles of roots in contaminated soil or water. Contaminants are absorbed by the roots of plants and immobilized in the rhizosphere. It decreases the mobility of pollutants and their bioavailability in the food chain. This technique also prevents the movement of contaminants into the ground water. Contaminated sites are being treated using metal-tolerant species of plants. The heavy metals can include Cd, Cr, As, Cd, or Zn (Zhao et al., 2016). This technique initiates changes in the composition of the soil chemistry, providing the surface of the roots with the facility to absorb the metals (Yang et al., 2016). However, this method cannot remove pollutants from treated soil until the plants are eradicated.
5.2 Phytoextraction This process is also called phytoaccumulation. Contaminants are taken up by the plants and stored in the tissues of the stems or leaves. In this process, pollutants are degraded and removed from the environment. It is a useful technique for removing heavy metals from the contaminated soil or water. Through the process of incineration of these plants, these heavy metals can be recovered for reuse, which is referred to as phytomining (Eissa, 2017). Roots absorb pollutants from the soil and transport them to the shoots and leaves, where they are accumulated. Plants that have a high capacity to remove pollutants and produce a large biomass are ideal for this technique. The hyperaccumulator species has the ability to store pollutants, but exhibits less biomass production. Those species that produce more biomass, but perform less removal of heavy metals can be selected and used for this purpose. This process ends with harvesting of the plants, followed by their incineration, and landfilling (Mahar et al., 2016). This technique is environmentally friendly, among other advantages. It poses no damage on the landscape. It protects the ecosystem and is a primary phytoremediation technique. It is inexpensive, and the most commercially acceptable technique (Shiri et al., 2015). Additionally, the rhizosphere has a low bioavailability of metal, and a low absorption rate of heavy metal from the roots. Metals are retarded within the roots (Fiorentino et al., 2013). Research has been conducted to elaborate on the capability of plants to remove or accumulate the heavy metals from contaminated soil and water.
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Specific research has also been conducted to explain the performance of the herbaceous plant Aruddox donax in Cd contaminated soil. A 20% concentration of Cd in leaves and 30% in the rhizome was accumulated, which was more efficient than the control group (Van Oosten and Maggio, 2014).
5.3 Phytostimulation The activity of rhizobacteria is enhanced by the addition of mycorrhizalhelping bacteria. The removal of toxic heavy metals is increased by the addition of microbes in the roots of plants. This process is also known as rhizosphere remediation. It is a technique in which different compounds are released from the roots of plants, and they enhance microbial activity. It is a low-cost technology for removing Cd, heavy metals, and other organic compounds ( Jia et al., 2016). There are several reasons for phytostimulation to be used for rhizoremediation. A particular niche is provided by rhizosphere-to-soil microorganisms. Exudates released by roots provide nutrients and enhance microbial growth. A habitat is provided by the rhizosphere for the soil microbes. These microorganisms, in return, cause the degradation, or biotransformation, of many organic and inorganic compounds, such as heavy metals. Another method is the addition of microorganisms in the soil. These resistant microbial strains can cause the breakdown of organic compounds and the accumulation of toxic heavy metals such as Cd (Yanai et al., 2006).
6 PHYTOREMEDIATION MECHANISMS FOR Cd-CONTAMINATED SOIL/WATER Contaminated soils can be treated in a number of ways to reduce Cd toxicity. The common methods are use of landfills, incineration, washing, leaching, vitrification, electroreclamation, and excavation. These methods are expensive and need capital investment. These techniques are nonbiological processes, and cause disruption of the surrounding environment (Gao et al., 2012). Usually, plants absorb Cd metal through the roots from the soil, which serves as a bioavailable pool of Cd metal, and nutrients required by plants as well. Different factors affect the bioavailability of Cd metal in soil. These factors can be organic matter, root exudates, pH, competitive cations, and microbial biomass (Sarwar et al., 2010). Once a Cd metal is taken up by plants, it can either be stored or accumulated in root tissues, or translocated to upper parts of plants through apoplastic or symplastic routes. Hyperaccumulators have vital role in Cd
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metabolism: to keep it away from the other metabolic activities in order to hinder its deleterious effects on plants. Vacuoles can accumulate Cd metal in the shoots. There are five main steps involved in phytoextraction; Cd mobility in the rhizospheric zone, plant roots’ uptake of Cd metal, the transfer or translocation of Cd ions to upper plant roots, plant tissues sequestration of Cd, and tolerance of Cd metal (Ali et al., 2013). If a plant is more tolerant to a specific metal, it will accumulate a huge portion of that metal inside its tissues, and undergo minimum adverse impacts. The potential of a plant to tolerate Cd metal depends upon various mechanisms. These mechanisms can be Cd metal binding to the cell wall, movement of Cd ions into vacuoles through active transport, or binding of proteins with metal chelates (Memon and Schr€ oder, 2009).
7 USE OF PLANTS AND THEIR BIOCHEMICAL PROCESSES IN PHYTOREMEDIATION Different essential nutrients are required by the plants, which are absorbed by the roots. These nutrients are N, P, K, Ca, S, Mg, Cl, Fe, Zn, Cu, B, Mn, Mo, and B. These nutrients are transported to the upper parts of plants, or the cells of roots through active transport or passive transport. Transport proteins are also associated with cell membranes. These nutrients are transported within the plants through two ways; apoplast and symplast (Lu et al., 2011). Certain plant species (Thlaspi caerulescens) showed tolerance against Cd and Zn due to their metal chilate formation. Such plants reduce the cytosolic concentration of the metals, and store them in the vacuoles. Willows exhibit the potential to uptake different metals such as Cd, Cu, Zn, Fe, and Pb. As compared with other herbaceous plants, willows possess deeper root systems, and can decontaminate a large zone of contaminated soil (Xiao et al., 2015).
8 MOBILITY/AVAILABILITY OF Cd Total concentrations of metals in contaminated soil cannot be predicted on the basis of their behavior. The toxicity and uptake of most of the metals is dependent on specific metals and free ions in the soil. Except for this general observation, free metal can also be available to plants, and is more active than the ionic form. Mostly, the concentrations of metals soluble in water are considered available forms to plants (Abe et al., 2008). Equilibrium
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partitioning is used to describe the metal distribution between pore water and the solid phase of soil. The total distributed or dissolved concentration of metal does not necessarily correspond to the amount that is available to plants. Complex ions, ion pairs, microparticulates, or polymers can decrease free ionic species in a solution. Speciation is the process in which the identification and quantification of various metals is carried out in a sample. Different models are present in order to calculate the metal speciation in water, soil, or sediments, i.e., the free ion activity (FIAM) model and wandermere humic aqueous model (WHAM) (Fang et al., 2012). Cd has better ability to become available for plant uptake as compared with other heavy metals, such as Cu, Ni, Pb, Zn, and Mn. Therefore, high Cd uptake by plants results from low concentrations of Cd in the soil. It has already been established that Cd is a nonessential trace element for the growth of plants (Peng et al., 2009).
9 FACTORS AFFECTING THE SOLUBILITY/BIOAVAILABILITY OF Cd IN SOIL There are a number of factors that affect Cd availability in plants. One of them is pH, which affects the concentrations of soluble Cd in plants. At low pH, the solubility of Cd metal increases, and decreases at higher pH. In accordance with some research, at pH 7, all metal ions are removed from the solution as hydroxo species, which are being adsorbed on the soil’s surface. The buffering capacity of the soil for an acidic surrounding can also affect the metal mobility in soil. Moreover, the leaching of these metals can increase, due to high pH. However, Cd adsorption was found at 3.7 and 4 pH values. At pH 6–7, precipitation of the insoluble solids becomes essential (Zhang et al., 2010). It has been reported that the decrease in redox potential increased the solubility of Cd in soil. The mechanism that is behind that fact is the dissolution of Fe-Mn oxyhydroxides, which release the absorbed metals under reducing conditions. According to some scientists, the availability of Cd to plants is decreased in water logging conditions because of CdS formation. This will also result in Zn solubility in water. Many studies have shown that soil’s organic matter also affects the Cd uptake by plants in a negative association (Arnold and McDonald, 1999).
10 FACTORS INCREASING Cd UPTAKE BY PLANTS There are several ways to increase the uptake of Cd in the plants from soil.
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10.1 Microbe-Assisted Phytoremediation Several bacteria (rhizobacteria) have been successfully applied to plants, resulting in plant growth. These bacteria assisted in decreasing Cd toxicity and increased plant growth in Cd contaminated soils. These plant-growthpromoting bacteria transform Cd into a soluble and available form to plants by changing their chemical properties. These microbes use different binding compounds, such as siderphores, organic acids, biosurfactants, and exopolymers (Ullah et al., 2015). Another promising technique is microbe-assisted phytoremediation. The deficiency of nutrients in the soil can inhibit the growth of bacteria, and then inhabiting bacteria are activated by adding nutrients (bioaugmentation). Therefore, microbes are biostimulated with nutrients to increase microbial growth to remove organic and heavy metal pollutants (Rojjanateeranaj et al., 2017). Different types of crops (maize and soybean) have been used to treat Cd contaminated soil, which was assisted by the bacterial community. These crops were selected due to rapid growth, large biomass, and good energy production (Chen et al., 2017).
10.2 Decapitation Increased Phytoremediation The decapitation method is practiced in agronomy, in which the number of branches is increased by enhancing apical dominance. It has been reported that removing or cutting of apical buds increased shoot growth in various plants. It is well known that Cd accumulates in the shoots of hyperaccumulators because it is a predominant part of the plants. Decapitation increased the process of transpiration, biomass production, and accumulation of Cd (Liu et al., 2017).
10.3 Effect of pH on Cd Availability in Plants Soil properties are also affected by the availability of Cd in soil. Cd mobilization is caused by texture, nutrient levels, organic matter, and pH. It has been reported that the bioavailability of Cd to plants is greatly influenced by pH. An inverse relationship was found between pH and Cd uptake in general. The uptake of Cd in plants increases with a decrease in pH. When the pH of soil decreases, the sesquioxides and variable charged clays release Cd in the soil. The solubility of Cd compounds (CdCO3 and Cd3(PO4)2 also increases with decreasing pH. Therefore, the bioavailability of Cd to plants can be enhanced by increasing rhizospheric acidification, and it can be an efficient technique to increase the efficacy of phytoremediation, and lessens the remediation time period (Yanai et al., 2006).
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10.4 Enhancement of Phytoremediation Using Mobilized Chelating Agents To obtain more efficient removal of Cd, different chelating agents are used. Ethylenediaminetetraacetic acid (EDTA) has application in detergents and other cleaning products, the paper and pulp industries, and water treatment. EDTA can be used as a chelating agent because it has a high affinity to combine with heavy metals such as Cd, and forms metal and EDTA complex compounds. It is also being applied to remove different metals from contaminated soil and water. This agent can enhance the availability of Cd-bound EDTA to plants and increase the translocation of these metals from the roots to the shoots of plants. Studies have shown that the addition of EDTA in soil (12 mmol kg 1) enhanced the availability of Cd up to 233%. Therefore, the addition of chelate ligands to contaminated soils can increase the uptake of Cd to plants, and the removal of Cd from the environment (Eissa, 2017).
11 ADVANTAGES OF PHYTOREMEDIATION Phytoremediation is a cost-effective technique. It is inexpensive due to that fact that very little labor is required to monitor plants. It does not interfere with the ecosystem, and adds an aesthetic value to the treated land through plant cover. Different heavy metals can be recovered and reused for several purposes in industries. It is an environmentally safe process, because it poses no potentially harmful impacts on the environment. This technique uses naturally occurring living organisms (Tahir et al., 2016).
12 DISADVANTAGES OF PHYTOREMEDIATION Phytoremediation is a slow process, compared with other engineering techniques. It can take years to treat slightly polluted soil or water (Han et al., 2018). It is confined to the surface area and the depth of roots of plants. The contaminated area in soil is next to the roots of plants. It requires more time for plants’ biomass production, and exhibits slow growth. It is impossible to prevent the complete leaching of pollutants into ground water. The toxicity caused by contaminants also affects the survival of plants. Phytoremediation also requires the safe disposal of contaminated plant material. The accumulation of toxic metals from soil in plants is passed through the food chain, and ultimately affects animals and humans.
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13 CONCLUSION AND FUTURE PERSPECTIVES Cd contamination in soil and water is a major concern to health and the environment because of its adverse impacts on food chain contamination, and other connected health risks. In this case, the technique of phytoremediation can be a helpful tool to tackle environmental pollution. This is an emerging and innovative method. This technique is economically feasible and more effective than other physical and chemical techniques. Hyperaccumulators can be efficiently used to remove large concentrations of Cd and other heavy metals from contaminated soil and water. Other conventional methods are expensive and a significant amount of labor is required. Phytoremediation is a slow process, yielding less biomass production above ground. Other approaches, such as use of microbes, decapitation, and low pH maintenance are also useful in combination with phytoremediation to decontaminate Cd-polluted soil and water. The phytoremediation technique requires further research to improve the abilities of plants in order to increase their effectiveness and efficiency. Phytoremediation can serve as a reliable alternative to traditional methods because it is time-saving and more costeffective. Additionally, its harvests can be utilized for biofuel production. The incineration of metal-tolerant plants can yield valuable metals, which can be reused in industry. Further research should be done on transgenic plants to determine all of the environmental and economic benefits. It provides a better option compared with other conventional techniques. More knowledge of this technique is required in order to increase its efficiency and potential in remediating Cd-contaminated sites.
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FURTHER READING Kavamura, V.N., Esposito, E., 2010. Biotechnological strategies applied to the decontamination of soils polluted with heavy metals. Biotechnol. Adv. 28, 61–69. Zhang, H., Zheng, L.C., Yi, X.Y., 2009. Remediation of soil co-contaminated with pyrene and cadmium by growing maize (Zea mays L.). Int. J. Environ. Sci. Technol. 6, 249–258.