Phytoremediation: An Overview

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Environ. Sci. & Engg. Vol. 11: Soil Pollution and Phytoremediation

6 Phytoremediation: An Overview S.B. ESKANDER1 AND H.M. SALEH1*

ABSTRACT

Phytoremediation refers to the natural ability of certain plants called hyperaccumulators to bioextract, bioaccumulate, biostabilize, biovolatilize, biodegrade, or render harmless contaminants in soils, water, or even the atmosphere. Phytoremediation is applied wherever the soil, sediment, static surface water, and groundwater environment has become polluted or is suffering ongoing chronic pollution. Cleanup objectives for phytotechnologies can be contaminant, removal and destruction, control and containment, or all. Contaminants such as metals, pesticides, solvents, explosives, radiocontaminants and crude oil and its derivatives, have been mitigated in phytoremediation projects worldwide. Phytoremediation seeks to protect human health and the environment from risks associated with hazardous waste sites. International and National governments should encourage development of innovative technologies such as phytoremediation to more efficiently clean up contaminated sites. Key words: Phy toremediation, Bioextrac tion, Bioaccumulation, Biostabilization, Bio volatilization , Biodegradation, Environment, Soil, Groundwater, Solvents, Explosives, Radiocontaminants, Crude oil, Pesticides, Metals INTRODUCTION

Toxic pollution causes immense harm to humans, especially children. Health impacts include physical and mental disabilities, reduced IQ, organ dysfunction, neurological disorders, cancers, reduced life expectancy and in 1

Radioisotope Department, Nuclear Research Center, Atomic Energy Authority, Dokki 12311, Giza, Egypt * Corresponding author: E-mail: [email protected]

Phytoremediation: An Overview

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some cases, death. These pollutants exacerbate other health concerns by weakening the body’s immune system, rendering it more susceptible to diseases. An initial exposure to toxic pollution can be the undocumented cause of later illnesses, including respiratory infections, tuberculosis, gastrointestinal disorders, and maternal health problems. In addition, while most toxic pollution is localized, some pollutants are trans-boundary and end up in food chains in oceans and distant countries[1]. Phytoremediation, known as one of environmental biotechnology, is the name given to a set of technologies that use plants to clean contaminated sites. Phytoremediation process is mainly characterized by low cost, the plants can be easily monitored, the possibility of recovery and reuse the valuable bioaccumulated metals. It is the least harmful method for treating contaminates because it uses naturally occurring organism and preserve the natural state of the ecosystems, besides it can be used where even the other conventional techniques cannot work. Phytoremediation can be accomplished where the spiked problem is located and hence eliminated the need to transport huge quantities of the contaminated wastes off-site and the potential threats to health and the environment that can be arises during such transportation[2]. Phytoremediation is both an in-situ and ex-situ remediation technology. In-situ: contaminant reduction and transport mitigation, not for source control. Many cases of phytoremediation is a polishing technology. Ex-situ: like wastewater irrigation and solid waste remediation plants. The efficiency of pollution removal by phytoremediation can be greatly enhanced by selection of the appropriate plant species. The selection may depend on the nature of contaminant to be remediated, its geographic location, the microclimate, hydrological conditions, soil properties, and accumulation capacity of the plant species. Prior to running through the decision plants, a basic question that needs to be answered is simply whether any species exist that can survive the contaminated site conditions[3]. Phytoremediation is an eco-friendly method for removal of pollutants, which can be relied upon as a sustainable technology, if implemented under optimum conditions of plant growth[4]. Typical organic contaminants (“organics”) such as petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides, and explosive compounds can be remediated using phytotechnologies. Typical inorganic contaminants (“inorganics”) that can be addressed include salts (salinity), heavy metals, metalloids, and radioactive materials. There are major mechanisms associated with phytoremediation: phytosequestration, rhizofiltration, phytohydraulics, phytoextraction, phytodegradation, phytostimulation, phytostablization and phytovolatilization, Fig. 1[3,5].

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The plant-bacteria interactions increase the degradation of hazardous organic compounds in soil. Plants and bacteria can form specific associations in which the plant provides the bacteria with a specific carbon source that induces the bacteria to reduce the phytotoxicity of the contaminated soil. Alternatively, plants and bacteria can form nonspecific associations in which normal plant processes stimulate the microbial community, which in the course of normal metabolic activity degrades contaminants in soil. Plants can provide carbon substrates and nutrients, as well as increase contaminant solubility. These biochemical mechanisms increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants or reduce the phytotoxicity of the contaminated soil[6].

Fig. 1: Phytoremediation of soils contaminated with metals and metalloids (Source: www.intechopen.com)

Do the Plants Become Contaminated in this Process? Due to the growth of vegetation, the mass of plant material will increase with time. Depending on the type of phytoremediation, the biomass that must be removed from the active system will vary. Relatively permanent long-term systems that rely on the establishment of mature vegetation (e.g., poplar trees or grass for rhizodegradation) will not require periodic planned removal of the biomass. In all phytoremediation systems, however, some biomass such as dead or diseased plants, fallen leaves, fallen limbs, or pruned material might have to be removed occasionally to maintain good operation of the system. These uncontaminated plant materials will need to be harvested, stored, and disposed of as necessary. It will be important to confirm that the plant material does not contain any hazardous substances.


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After this confirmation, the material could be composted or worked into the soil on site. If that is not possible, off-site disposal will be required. According to the reported researches, there is little or no accumulation of volatile contaminants in plant roots, wood, stems, leaves, or fruit. Plants may accumulate metals or other toxic materials that reach contaminated levels, but several mechanisms exist that often limit the uptake and/or persistence of nonessential compounds in the plant. The plant material generated may need to be collected and treated as if it is a hazardous waste until appropriate testing for contaminant accumulation can be conducted. Even if the plant waste is not classified as a hazardous waste, it may be sufficiently contaminated to require special handling requirements. If contaminant concentrations in plant tissues do exceed regulatory limits, the cut plant material or litter will need to be treated as a hazardous waste[3]. The secondary waste, (i.e. the spiked plant), originated from the phytoremediation process of radioactive waste streams is treated as solid radwastes. This secondary waste should be collected and treated as hazard component and, therefore, it is subjected to solidification process[7–10].

Phytoremediation Definitions Phytoremediation The application of plant-controlled interactions with groundwater and organic and inorganic molecules at contaminated sites to achieve site-specific remedial goals[11]. Phytoremediation Phytoremediation is the use of green plants to remove pollutants from the environment or render them harmless. ‘Green’ technology uses plants to ‘vacuum’ heavy metals from the soil through the roots. While acting as vacuum cleaners, the unique plants must be able to tolerate and survive high levels of heavy metals in soils[12]. Phytoremediation Phytoremediation uses plants to clean up pollution in the environment. Plants can help clean up many kinds of pollution including metals, pesticides, explosives, and oil. The plants also help prevent wind, rain, and groundwater from carrying pollution away from sites to other areas. Phytoremediation works best at sites with low to medium amounts of pollution. Plants remove harmful chemicals from the ground when their roots take in water and nutrients from polluted soil, streams, and groundwater. Once inside the plant, chemicals can be stored in the roots, stems, or leaves; changed into

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less harmful chemicals within the plant; or changed into gases that are released into the air as the plant transpires (breathes)[13]. Phytoremediation Phytoremediation is the direct use of green plants and their associated microorganisms to stabilize or reduce contamination in soils, sludges, sediments, surface water, or ground water. Sites with low concentrations of contaminants over large cleanup areas and at shallow depths present especially favorable conditions for phytoremediation[14]. Phytoremediation Phytoremediation is the use of vegetation for in situ treatment of contaminated soils, sediments, and water. It is best applied at sites with shallow contamination of organic, nutrient, or metal pollutants. Phytoremediation is an emerging technology for contaminated sites that is attractive due to its low cost and versatility[15]. Phytoremediation Remediation of contaminated soil in situ using vegetation. Phytoremediation is carried out by growing plants that hyperaccumulate metals in the contaminated soil[16]. PHYTOREMEDIATION PROCESSES

Phytoextraction Phytoextraction is the uptake/absorption and translocation of contaminants by plant roots into the above ground portions of the plants (shoots) that can be harvested and burned gaining energy and recycling the metal from the ash, Fig. 2[17–21]. Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass (organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or fifty. In general, this process has been tried more often for extracting heavy metals than for organics. At the time of disposal, contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. ‘Mining with plants’, or phytomining, is also being experimented with.


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Fig. 2: Phytoextraction mechanisms[3].

A hyperaccumulator is a plant species capable of accumulating 100 times more metal than a common non-accumulating plant[22]. For a plant to be classified as a hyperaccumulator, it must be able to accumulate at least 1,000 mg/kg (dry weight) of a specific metal or metalloid (for some metals or metalloids, the concentration must be 10,000 mg/kg)[23]. The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest, a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.

The Two Versions of Phytoextraction • Natural hyper-accumulation: Where plants naturally take up the contaminants in soil unassisted. • Induced or assisted hyper-accumulation: Where a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals. Specific plant species can absorb and hyperaccumulating metal contaminants and/or excess nutrients in harvestable root and shoot tissue, from the growth substrate through phytoextraction process. This is for metals, metalloids, radionuclides, nonmetals, and organics contaminants in soils, sediments, and sludges medium[24, 25]. A relatively small group of hyperaccumulator plants is capable of sequestering heavy metals in their shoot tissues at high concentrations.

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Current progresses on understanding cellular/molecular mechanisms of metal tolerance/hyperaccumulation by plants are basis to explain the phytoaccumulation mechanism. The major processes involved in hyperaccumulation, of trace metals for example, from the soil to the shoots by hyperaccumulators include: (a) Bioactivation of metals in the rhizosphere through root-microbe interaction; (b) Enhanced uptake by metal transporters in the plasma membranes; (c) Detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; and (d) Sequestration of metals into the vacuole by tonoplast-located transporters[26]. A hyperaccumulator or a combination of these plants is selected and planted at a particular site based on the type of metals present and certain site conditions. After the growth period of the plants, they are harvested and either incinerated or composted to recycle the metals. This procedure may be repeated if necessary, to bring soil contaminant levels down to permissible limits. If plants are incinerated, ash is disposed of in a hazardous waste landfill. The volume of ash will be less than 10% of the volume that is created, if the contaminated soil itself were dug up for treatment. Metals such as nickel, zinc and copper are the best candidates for removal by phytoextraction because it has been shown that they are preferred by a majority of the plants (approximately 400 species) that uptake and absorb unusually large amounts of these metals[27]. Advantages The main advantage of phytoextraction is environmental friendliness. Traditional methods that are used for cleaning up heavy metal-contaminated soil, disrupt soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to soil quality. Another benefit of phytoextraction is that it is less expensive than any other clean-up process. In some cases, the contaminant can be recycled from the contaminated plant biomass. Disadvantages As this process is controlled by plants, it takes more time than anthropogenic soil clean-up methods. The use of hyperaccumulator species is limited by slow growth, shallow root system, and small biomass production. In addition,


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the plant biomass must also be harvested and disposed of properly, complying with standards[28]. The method is also usually limited to metals and other inorganic compounds in soil or sediment[29].

Phytostabilization Phytostabilization focuses on the long term stabilization and containment of the pollutant. The plant’s presence can immobilize the pollutants by adsorption or accumulation or precipitation, and provide a zone around the roots where the pollutant can precipitate and stabilize. Unlike phytoextraction, phytostabilization focuses mainly on sequestering pollutants in soil near the roots but not in plant tissues. Pollutants become less bioavailable, and consequently livestock, wildlife, and human exposure is reduced as preventing their migration in soil, as well as their movement by erosion and deflation. An example application of this sort is using a vegetative cap to stabilize and contain mine-tailings[19–21,30]. This is also referred to as in-place inactivation. It is used for the remediation of soil, sediment, and sludge. It is the use of certain plant species to immobilize contaminants in the soil and ground water through absorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants (rhizosphere). This process decreases the mobility of the contaminant and prevents migration to the ground water and it reduces bio-availability of metal into the food chain. This technique can also be used to reestablish vegetation cover at sites where natural vegetation fails to survive due to high metals concentrations in surface soils or physical disturbances to surface materials. Metal-tolerant species is used to restore vegetation at contaminated sites, thereby decreasing the potential migration of pollutants through wind erosion and transport of exposed surface soils and leaching of soil contamination to ground water. Phytostabilization can occur through the sorption, precipitation, or metal valence reduction, Fig. (3)[31]. The physical presence of the plants may also reduce contaminant mobility by reducing the potential for water and wind erosion[32]. Advantage of this method is the changes that the presence of the plant induces in soil chemistry and environment. These changes in soil chemistry may induce adsorption of contaminants onto the plant roots or soil or cause metals precipitation onto the plant root. Phytostabilization has been successful in addressing metals and other inorganic contaminants in soil and sediments. Some of the advantages associated with this technology are that the disposal of hazardous material/biomass is not required and it is very effective when rapid immobilization is needed to preserve ground and surface waters. The presence of plants also reduces soil erosion and decreases the amount of water

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Fig. 3: Phytostabilization (Source: https://vetiverendonesia.lileswordpress.com/2012/ 01/skimatik.fit.endoensia.jpg)

available in the system. However, this clean-up technology has several major disadvantages including: Contaminant remaining in soil, application of extensive fertilization or soil amendments, mandatory monitoring is required, and the stabilization of the contaminants may be primarily due to the soil amendments[33].

Phytotransformation In the case of organic pollutants, such as pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants, such as Cannas, render these substances non-toxic by their metabolism[34]. In other cases, microorganisms living in association with plant roots may metabolize these substances in soil or water. These complex and recalcitrant compounds cannot be broken down to basic molecules (water, carbon-dioxide, etc.) by plant molecules, and, hence, the term Phytotransformation represents a change in chemical structure without complete breakdown of the compound. The term “Green Liver” is used to describe Phytotransformation[35], as plants behave analogously to the human liver when dealing with these xenobiotic compounds (foreign compound/pollutant)[36]. After uptake of the xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding functional groups such as hydroxyl groups (–OH). This is known as Phase I metabolism, similar to the way that the human liver increases the polarity of drugs and foreign compounds (drug metabolism). Whereas in the human liver enzymes such as cytochrome P450s are responsible for the initial reactions, in plants enzymes such as peroxidases, phenoloxidases, esterases and nitroreductases carry out the same role[34]. In the second stage of


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Phytotransformation, known as Phase II metabolism, plant biomolecules such as glucose and amino acids are added to the polarized xenobiotic to further increase the polarity (known as conjugation). This is again similar to the processes occurring in the human liver where glucuronidation (addition of glucose molecules by the UGT class of enzymes, e.g. UGT1A1) and glutathione addition reactions occur on reactive centers of the xenobiotic. Phase I and II reactions serve to increase the polarity and reduce the toxicity of the compounds, although many exceptions to the rule are seen. The increased polarity also allows for easy transport of the xenobiotic along aqueous channels. In the final stage of Phytotransformation (Phase III metabolism), a sequestration of the xenobiotic occurs within the plant. The xenobiotics polymerize in a lignin-like manner and develop a complex structure that is sequestered in the plant. This ensures that the xenobiotic is safely stored, and does not affect the functioning of the plant. However, preliminary studies have shown that these plants can be toxic to small animals (such as snails), and, hence, plants involved in Phytotransformation may need to be maintained in a closed enclosure. Hence, the plants reduce toxicity (with exceptions) and sequester the xenobiotics in Phytotransformation. Trinitrotoluene phytotransformation has been extensively researched and a transformation pathway has been proposed[37].

Fig. 4: Phytotransformation (Source: www.greenandgarden.org/images/photo//ecosys/ pltdepollunte/Phytotransformation)

Rhizofiltration This process is very similar to phytoextraction in that it removes contaminants by trapping them into harvestable plant biomass. Both phytoextraction and rhizofiltration follow the same basic path to remediation. First, plants are put in contact with the contamination. They absorb contaminants through their root systems and store them in root biomass and/or transport them up into the stems and/or leaves. The plants continue to absorb contaminants until they are harvested. The plants are then replaced to continue the growth/harvest cycle until satisfactory levels of contaminant are achieved. Both processes are also aimed more toward concentrating and precipitating heavy metals than organic contaminants. The major difference between rhizofiltration and phytoextraction is that

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rhizofiltration is used for treatment in aquatic environments, while phytoextraction deals with soil remediation. Rhizofiltration may be applicable to the treatment of surface water and groundwater, industrial and residential effluents, downwashes from power lines, storm waters, acid mine drainage, agricultural runoffs, diluted sludges, and radionuclide-contaminated solutions. Plants suitable for rhizofiltration applications can efficiently remove toxic metals from a solution using rapidgrowth root systems. Various terrestrial plant species have been found to effectively remove toxic metals such as Cu2+, Cd2+, Cr6+, Ni2+, Pb2+, and Zn2+ from aqueous solutions[38]. Advantages Rhizofiltration is a treatment method that may be conducted in situ, with plants being grown directly in the contaminated water body. This allows for a relatively inexpensive procedure with low capital costs. Operation costs are also low but depend on the type of contaminant. This treatment method is also aesthetically pleasing and results in a decrease of water infiltration and leaching of contaminants[39]. After harvesting, the crop may be converted to biofuel briquette, a substitute for fossil fuel[40]. Disadvantages This treatment method has its limits. Any contaminant that is below the rooting depth will not be extracted. The plants used may not be able to grow in highly contaminated areas. Most importantly, it can take years to reach regulatory levels. This results in long-term maintenance (Fig. 5). Also, most contaminated sites are polluted with many different kinds of contaminants. There can be a combination of metals and organics, in which treatment through rhizofiltration will not suffice[39]. Plants grown on polluted water and soils become a potential threat to human and animal health, and therefore, careful attention must be paid to the harvesting process and only non-fodder crop should be chosen for the rhizofiltration remediation method[40].

Phytostimulation[3] Phytostimulation, sometimes called rhizodegradation, rhizosphere biodegradation, or plant assisted bioremediation/degradation, is the enhanced breakdown of a contaminant by increasing the bioactivity using the plant rhizosphere environment, Fig. 6, to stimulate the microbial populations. This enhanced bioactivity represents the primary means through which organic contaminants can be remediated, including into harmless products that can be converted into a source of food and energy for the plants or soil organisms[41].


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Fig. 5: Sunflowers used for rhizofiltration (Source: http//www,jpece.org/WRKY/ Brachy/WRKY/WRKY/IMG/ Rhizofiltration.JPG)

Fig. 6: Phytostimulation process (Source: http://imageslidesharecdn.com)

Specifically, the contaminants themselves may be analogs to the phytochemical naturally exuded by the plant and fortuitously metabolized as a substitute to the primary carbon source. Alternatively, the exuded phytochemicals may be co-metabolites to organisms that are able to breakdown the contaminants as the primary metabolite. In this case, the contaminant is still metabolized (i.e., biodegraded) but at a slower rate or through a less efficient metabolic pathway than when the cometabolite is present. Similarly, the specific proteins and enzymes, or analogs to those produced by the soil organism needed to breakdown the contaminant, may be produced and exuded by the plant itself. In general, a symbiotic relationship evolves between plants and soil microbes in the rhizosphere. Plants provide nutrients necessary for the microbes to thrive, while the microbes provide a healthier soil environment where plant roots can grow. Specifically, plants loosen soil and transport oxygen and water into the rhizosphere. Furthermore, plants exude specific phytochemicals (sugars, alcohols, carbohydrates, etc.) that are primary sources of food (carbon) for the specific soil organisms that aid in providing the healthier soil environment. Alternatively, the exuded phytochemical

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may be an allelopathic agent meant to suppress other plants from growing in the same soil. In return for exporting these phytochemicals, plants are protected from competition, soil pathogens, toxins, and other chemicals that are naturally present or would otherwise be growing in the soil environment. Microbial populations can be several of orders of magnitude higher in a vegetated soil compared to an unvegetated soil. Cyanobacteria are commonly for phytostimulation of crops due to their nitrogen fixing ability. Cyanobacteria has the capability to release phytohormones in the rhizosphere from where the plant root may absorb these hormones and help in phytostimulation process[42-43].

Phytovolatilization Phytovolatilization is the volatilization of contaminants from the plant either from the leaf stomata or from plant stems[44], as shown in Fig. 7. Chemical characteristics such as the Henry’s constant and vapor pressure dictate the ability of organic contaminants to volatilize. In some cases, a breakdown product derived from the rhizodegradation and/or phytodegradation of the parent contaminant along the transpiration pathway may be the phytovolatilized constituent. This effect was studied for the uptake and phytovolatilization of trichloroethene (TCE) or its breakdown products in poplars[45]. Similarly, certain inorganic constituents such as mercury may be volatilized as well.

Fig. 7: Phytovolatilization (Source: www.maire-villentaneuse.Fr.)

Specifically, tobacco plants have been modified to be able to take up the highly toxic methyl-mercury, alter the chemical speciation, and phytovolatilize relatively safe levels of the less toxic elemental mercury into the atmosphere[46] . Once volatilized, many chemicals that are recalcitrant in the subsurface environment react rapidly in the atmosphere with hydroxyl radicals, an oxidant formed during the photochemical cycle.


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Phytovolatilization also involves contaminants being taken up into the body of the plant, but then the contaminant, a volatile form thereof, or a volatile degradation product is transpired with water vapor from leaves[29]. Phytovolatilization may also entail the diffusion of contaminants from the stems or other plant parts that the contaminant travels through before reaching the leaves[28].

Phytodegradation[3] Depending on factors such as the concentration and composition, plant species, and soil conditions, contaminants may be able to pass through the rhizosphere only partially or negligibly impeded by phytosequestration and/ or rhizodegradation. In this case, the contaminant may then be subject to biological processes occurring within the plant itself, assuming it is dissolved in the transpiration stream and can be phytoextracted. Specifically, phytodegradation, also called “phytotransformation,” refers to the uptake of contaminants with the subsequent breakdown, mineralization, or metabolization by the plant itself through various internal enzymatic reactions and metabolic processes. Fig. 8 depicts these mechanisms.

Fig. 8: Phytodegradation mechanisms. (A) plant enzymatic activity, (B) photosynthetic oxidation

Plants catalyze several internal reactions by producing enzymes with various activities and functions specifically, oxygenases have been identified in plants that are able to address hydrocarbons such as aliphatic and aromatic compounds. Phytodegradation has been observed to remediate some organic contaminants, such as chlorinated solvents, herbicides, and munitions, and it can address contaminants in soil, sediment, or groundwater. Similarly, nitroreductases are produced in some plants that can reduce and breakdown energetic compounds such as the explosives trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), and 1,3,5,7-tetranitro1,3,5,7-tetrazocane (HMX). Although not known to be naturally produced

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in plants, dehalogenase-like activity has also been identified and isolated that can remove halogen subgroups from compounds such as chlorinated solvents. Many of these plant enzymes may even be able to metabolize or mineralize these chemicals completely to carbon dioxide and water. In addition, research has shown that the endophytic symbiotic bacteria Methylobacterium populum that lives within poplar can mineralize RDX and HMX. In addition, the oxidation and reduction cycle operating during photosynthesis offer additional contaminant breakdown potential. Stronger oxidants and reductants are produced in the plant system (from +1.1 V to –1.3 V) than are commonly available in biodegradation processes (from +0.5 V to –0.3 V). Specifically, the redox potential for aerobic reactions with dissolved oxygen as the electron acceptor range +0.25 V and higher, possibly up to +0.5 V, while other electron acceptors (nitrate, iron-III, Mn, sulfate) range from +0.25 V down to –0.2 V. Below this redox potential, perhaps to –0.3 V, methanogenesis may occur. Therefore, organic chemicals (electron donors) in the transpiration stream reaching the photosynthetic centers of a plant are potentially subject to these strong redox conditions as well. This effect has been observed for RDX. Phytodegradation involves the degradation of complex organic molecules to simple molecules or the incorporation of these molecules into plant tissues. Generally, when the phytodegradation mechanism is at work, contaminants are broken down after they have been taken up by the plant. As with phytoextraction and phytovolatilization, plant uptake occurs only when the contaminants’ solubility and hydrophobicity fall into a certain acceptable range[47]. Phytoremediation includes a range of plant-based remediation techniques, as stated, such as phytoextraction, phytostabilization, phytoimmobilization, rhizofiltration, phytovolatilization and others focused on reduction of the environment pollution level. At a phytoremediation site, combinations of the phytoremediation processes may occur simultaneously or in sequence for a particular contaminant, or different processes may act on different contaminants or at different exposure concentrations. The success of phytoremediation at a given site cannot always be attributed to just one of these mechanisms because a combination of mechanisms may be at work. In addition to this, phytoremediation is easy to implement and maintain, does not require the use of expensive equipment or highly specialized personnel and is environmentally friendly and aesthetically pleasing to the public.

Hydraulic Control Hydraulic control (or hydraulic plume control) is one of the phytoremediation processes and based on the use of vegetation to influence the movement of


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ground water and soil water, through the uptake and consumption of large volumes of water. Hydraulic control may influence and potentially contain movement of a ground water plume, reduce or prevent infiltration and leaching, and induce upward flow of water from the water table through the vadose zone. Vegetation water uptake and transpiration rates are important for hydraulic control and remediation of ground water. Water uptake and the transpiration rate depend on the species, age, mass, size, leaf surface area, and growth stage of the vegetation. They also are affected by climatic factors, such as temperature, precipitation, humidity, insolation, and wind velocity, and will vary seasonally[48]. Plants can act as hydraulic pumps when their roots reach down toward the water table and establish a dense root mass that takes up large quantities of water. Poplar trees, for example, can transpire between 50 and 300 gallons of water per day out of the ground. The water consumption by the plants decreases the tendency of surface contaminants to move towards groundwater and into drinking water. The use of plants to rapidly uptake large volumes of water to contain or control the migration of subsurface water is known as hydraulic control. There are several applications that use plants for this purpose, such as riparian corridors/buffer strips and vegetative caps Fig. 9.

Fig. 9: Hydraulic plume control (Source: Shama, R. (2014). Bioremediation. Credit Seminar I-ENS- 691-June)

Factors Affecting the Phytoremediation Mechanisms There are several factors which can affect the phytoremediation mechanisms of pollutants, as shown in Fig. 10. The efficiency of the techniques regarding the type of pollution depends on numerous environmental factors. Some of them are described in this chapter. Proper plant growth is not possible without access to water, which determines transport of numerous substances and compounds important in the life of plants. Stress associated with water availability leads to disruption

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of water potential gradients, loss of turgor, disruption of membrane integrity, and denaturation of proteins. Soil being a very complex medium is the most important environmental factor in the growth and development of plant life. Many of the soil components and parameters have an essential influence on the effectiveness of the phytoremediation process. Metals are not degraded by chemical or microbial processes and in consequence are accumulated in soils and aquatic sediments. During phytoremediation, plants may transport trace elements and bind them in their cell walls, chelate them in the soil in inactive forms using secreted organic compounds, or complex them in their tissue after transporting them into specialized cells and cell compartments. Availability for plants of trace elements contaminating the environment depends on the pH value, a very important factor in the form of their occurrence. Rhizospheric microorganisms (mainly bacteria and mycorrhizal fungi) may significantly increase the bioavailability of trace metals in soil, and significantly affect phytoremediation, as described in detail in this chapter. An important factor in trace elements’ availability in the soil environment, apart from bacteria, is the effect of fungi-organisms closely involved in the carbon cycle in nature. The increasing salinity of soil worldwide reduces growth of plants because it is a stress factor strongly influencing plants. Moreover, earthworms have a direct impact on the cycle and metabolism of nutrients, which has a significant effect on the physical, chemical, and biological properties of the soil and as a consequence influence the techniques[49].

Fig. 10: Factors affecting the phytoremediation mechanisms (Source: www.intecopen. com)


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By having knowledge about these factors, the uptake performance by plant can be greatly improved. Properties such as moisture contents, aeration, nutrient status, pH of rhizosphere and temperature as well as wastes characteristics, like their concentrations and molecular structure of the contaminants, the presence of inhibitors and co-metabotic substrates and the degree of contaminants sequestration which often leads to serious bioavailability limitations especially in aged soil.

The Plant Species Plants species or varieties are screened, and those with superior remediation properties are selected[25]. The uptake of a compound is affected by plant species characteristic[50]. The success of the phytoextraction technique depends upon the identification of suitable plant species that hyperaccumulating heavy metals and produce large amounts of biomass using established crop production and management practices[51]. Phytoremediation takes the advantage of the unique and selective uptake capabilities of plant root systems, together with the translocation, bioaccumulation, and contaminant degradation abilities of the entire plant body[52]. The ability to tolerate large concentrations of heavy metals is a rare phenomenon in the plant kingdom as a whole, but is widespread in particular plant groups: some hyperaccumulating or metal-tolerant species have been investigated for several years. Their ability to accumulate high concentrations of metals was observed for both essential nutrients, such as copper (Cu), iron (Fe), zinc (Zn) and selenium (Se), as well as non-essential metals, such as cadmium (Cd), mercury (Hg), lead (Pb), aluminum (Al) and arsenic (AS). Metal concentrations in the shoots of accumulating plants can be 100–1,000 fold higher than in non-accumulating plants[53].

Properties of Medium Agronomical practices are developed to enhance remediation (pH adjustment, addition of chelators, fertilizers)[25]. For example, the amount of lead absorbed by plants is affected by the pH, organic matter, and the phosphorus content of the soil. To reduce lead uptake by plants, the pH of the soil is adjusted with lime to a level of 6.5 to 7.0[54].

The Root Zone The Root zone is of special interest in phytoremediation. It can absorb contaminants and store or metabolize it inside the plant tissue. Degradation

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of contaminants in the soil by plant enzymes exuded from the roots is another phytoremediation mechanism. A morphological adaptation to drought stress is an increase in root diameter and reduced root elongation as a response to less permeability of the dried soil[55].

Vegetative Uptake Vegetative Uptake is affected by the environmental conditions. The temperature affects growth substances and consequently root length. Root structure under field conditions differs from that under greenhouse condition [55] . The success of phytoremediation, more specifically phytoextraction, depends on a contaminant specific hyperaccumulator[56]. Understanding mass balance analyses and the metabolic fate of pollutants in plants are the keys to proving the applicability of phytoremediation[57]. Metal uptake by plants depends on the bioavailability of the metal in the water phase, which in turn depends on the retention time of the metal, as well as the interaction with other elements and substances in the water. Furthermore, when metals have been bound to the soil, the pH, redox potential, and organic matter content will all affect the tendency of the metal to exist in ionic and plant-available form. Plants will affect the soil through their ability to lower the pH and oxygenate the sediment, which affects the availability of the metals[58], increasing the bioavailability of heavy metals by the addition of biodegradable physicochemical factors, such as chelating agents and micronutrients[59].

Addition of Chelating Agent The increase of the uptake of heavy metals by the energy crops can be influenced by increasing the bioavailability of heavy metals through addition of biodegradable physicochemical factors such as chelating agents, and micronutrients, and also by stimulating the heavy-metal-uptake capacity of the microbial community in and around the plant. This faster uptake of heavy metals will result in shorter and, therefore, less expensive remediation periods. However, with the use of synthetic chelating agents, the risk of increased leaching must be taken into account[59]. The use of chelating agents in heavy-metal-contaminated soils could promote leaching of the contaminants into the soil. Since the bioavailability of heavy metals in soils decreases above pH 5.5–6, the use of a chelating agent is warranted, and may be required, in alkaline soils. It was found that exposing plants to ethylene diamine tetra acetic acid (EDTA) for a longer period (2 weeks) could improve metal translocation in plant tissue as well as the overall phytoextraction performance. The application of a synthetic chelating agent (e.g. EDTA) at 5 mmol/kg yielded positive results[60].


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Plant roots can exude organic acids such as citrate and oxalate, which affect the bioavailability of metals. In chelate-assisted phytoremediation, synthetic chelating agents such as NTA and EDTA are added to enhance the phytoextraction of soil-polluting heavy metals. The presence of a ligand affects the biouptake of heavy metals through the formation of metal-ligand complexes and changes the potential to leach metals below the root zone[61].

Phytoremediation of Petroleum Hydrocarbon It is estimated that between 1.7 and 8.8 million metric tons of crude oil are released into the world’s water every year, of which more than 90% is directly related to human activities including deliberate waste disposal. Marine oil spills, particularly large-scale spill accidents, have received great attention due to their catastrophic damage to the environment. For example, the spill of 37,000 metric tons (11 million gallons) of North Slope crude oil into Prince William Sound, Alaska, from the Exxon Valdez in 1989 led to the mortality of thousands of seabirds and marine mammals, a significant reduction in population of many intertidal and subtidal organisms and many long-term environmental impacts. An even more devastating spill occurred recently due to the explosion of the Transocean Deepwater Horizon rig on 20th April, 2010 killing 11 people and led to the British Petroleum (BP) oil spill that threatens coastal Louisiana, Gulf Coast fisheries, Gulf of Mexico ecosystems and perhaps the East Coast, as the spill reaches the loop current, Fig. 11. The British Petroleum oil spill has now obtained the dubious distinction of being the worst oil spill in United States history, surpassing the damage done by the Exxon Valdez tanker. Unlike the Exxon Valdez tragedy, in which a tanker held a finite capacity of oil, British Petroleum’s rig is tapped into an underwater oil well and could pump more oil into the ocean indefinitely until the leak is plugged. About $2.65 billion have been spent on cleanup[62].

Fig. 11: The 2010 Gulf of Mexico oil spill caused “immense environmental damage along the Gulf. (Source: http:// articles.latimes.com/2014)

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Environmental pollution arising from oil spill is a multi-facet problem presently ravaging oil producing communities all over the globe; it causes loss of species diversity, loss of habitat, destruction of breeding grounds of aquatic organisms and sometimes death of organisms including man[63].

Mechanisms for the Phytoremediation of Petroleum Hydrocarbons There are 3 primary mechanisms by which plants and micro-organisms remediate petroleum contaminated soil and ground water. These mechanisms include: Degradation, Containment and Phytovolatilization[64].

The Role of Plants in Degradations Direct degradation There is paucity of information on the direct degradation of petroleum hydrocarbon by plant. It has been reported that corn seedlings, tea and poplar shoots were able to metabolize methane into various acids. The ability to assimilate n-alkanes and liberate 14CO2 was identified in leaves and roots of both whole and cut plants. The general pathway of conversion for alkanes in plants was generalized as: n-alkane  Primary alcohols  Fatty acids  Acetyl-COA  Various compounds Indirect degradation[64–66] In contrast to the limited information available on the direct degradation of petroleum hydrocarbon by plants, there is a considerable body of information available regarding the indirect roles played by plants in the degradation of petroleum hydrocarbons. These include: • The supply of root exudates that cause the rhizosphere effect and enhanced co-metabolic degradation. • The release of root-associated enzymes capable of transforming organic pollutants. The physical and chemical effects of plants and theirs root system on soil conditions.

Root Exudates Root exudates are the link between plants and microbes that leads to the rhizosphere effect. The type and quantity of root exudate are dependent on


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plant species and the stage of plant development. For example, Hegde and Fletcher[67], found that the release of total phenolics by the roots of red mulberry (Morus rubra L.) increased continuously over the life of the plant with a massive release at the end of the season accompanying leaf senescence. The type of root exudate is also likely to be site and time specific. Site and time factors include variables such as soil types, nutrient levels, pH, water availability, temperature, oxygen status, light intensity and atmospheric carbon dioxide concentration- all of which significantly affect the type and quantity of root exudates.

Co-metabolism Co-metabolism is the process by which a compound that cannot support microbial growth on its own can be modified or degraded when another growth-supporting substrate is present. Organic molecules, including plant exudates, can provide energy to support population of microbes that co-metabolize petroleum hydrocarbons. For example, it had reported that plant exudates may have served as cometabolites during the biodegradation of (14C) Pyrene in the rhizosphere of crested wheatgrass.

Plant Enzymes Involved in Phytoremediation The release of enzymes from roots is yet another indirect role that plants play in the degradation of petroleum hydrocarbons. These enzymes are capable of transforming organic contaminants by catalyzing chemical reaction in soil. Plenty of plant enzymes were identified as the causative agents in the transformation of contaminants mixed with sediment and soil. Isolated enzymes systems included dehalogenase, nitroreductase, peroxidase, laccase and nitrilase. These findings suggest that plant enzymes may have significant spatial effects extending beyond the plant itself and temporal effects continuing after the plant has died.

The Role of Micro-organisms in Degradation Bioremediation is the use of micro-organisms to destroy or immobilize organic contaminants in the absence of plant. It is important to look at the role of micro-organisms in the degradation of petroleum hydrocarbons in the presence of plants - a mechanism of Phytoremediation. Issues to be considered include the types of micro-organisms involved in phytoremediation, reasons for microbial degradation, differences in degradation by various micro-organisms, characteristics of microbial communities involved in degradation, and the role of micro-organisms play in reducing phytotoxicity to plants.

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Generally, degradation occurs as result of these organisms using the organic contaminants for growth and reproduction. The organic contaminants provide the micro-organisms with the carbon and electron used by the organism to obtain energy.

Containment Containment can be direct or indirect. Direct containment involves the accumulation of contaminants within the plants, adsorption of contaminants onto roots and binding of contaminants in the rhizosphere through enzymatic activities. Containment involves using plants to reduce or eliminate the bioavailability of contaminants to other biota. Contaminants are not necessarily degraded when they are contained. Indirect containment involves plants supplying enzymes that bind contaminants into soil organic matter (or humus) in a process called Humification and by increasing soil organic matter content, which allows for humification.

Transfer of Petroleum Hydrocarbons to the Atmosphere (Phytovolatilization) The natural ability of a plant to volatilize a contaminant that has been taken up through its roots can be exploited as a natural air-stripping pump system. Phytovolatilization is most applicable to those contaminants that are treated by conventional air-stripping, i.e., contaminants with a Henry’s constant KH >10 atm m3 water/(m–3 air), such as BTEX, TCE, vinyl chloride and carbon tetrachloride. Chemicals with KH >10 atm m3 water/(m–3 air) such as phenol and PCP are not suitable for the air-stripping mechanism because of their relatively low volatility. Removal of petroleum hydrocarbons from soil in phytoremediation is often attributed to microorganisms living in the rhizosphere under the influence of plant roots. Synergistic cooperation of plant roots and soil microorganisms promotes the degradation of persistent organic contaminants in phytoremediation. Microbial communities in planted soils are greater and more active than unplanted soils[68]. Microorganisms in the rhizosphere benefit from the root exudates and plants, in turn, from the metabolic detoxification of potentially toxic compounds brought about by microbial communities. Additionally, microbial populations benefit the plant through recycling and solubilization of mineral nutrients as well as by supplying vitamins, amino acids, auxins, cytokinins, and gibberellins, which stimulate plant growth. Tall fescue and maize are well-known and easy to access plant species in many countries. The use of vegetation as a feasible remediation approach for soils contaminated with petroleum hydrocarbons may become attractive in developing countries because it is inexpensive and requires minimum maintenance and little management[69].


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The ability of Glycemic max to reduce the level of crude oil in oil polluted soil can help to restore polluted soils back for agricultural use. The high acceptability of G. max due to its high nutritional value, high adaptability and ease of propagation will make it an easy tool for remediation of soil contaminated with crude oil[70]. The decomposition of used motor oil in soil as influenced by plant treatment was monitored in a green house study. Soil contaminated with used motor oil (1.5% w/w) was seeded with soybean (Glycine max), Fig. 12; green bean (Phaseolus vulgaris); sunflower (Helianthus annuus)/Indian mustard (Brassica juncea); mixed grasses / maize (Zea mays); and mixed clover (red clover, Trifolium pratense/ladino clover, Trifolium repens) and incubated.

Fig. 12: Leaves of soybean grown on used oil-contaminated soil, showing chlorosis and necrosis[71].

Reduction of hydrocarbons in soil is enhanced in the presence of plants, but the mechanisms by which plants enhance hydrocarbon removal are not completely understood. The uptake of hydrocarbons from contaminated soils by plants (phytoaccumulation) is limited by the high lipophilicity of hydrocarbons and there is little data available on plant-induced sequestration (phytostabilization) of hydrocarbons in soil. The mechanism responsible for the phytoremediation of contaminated soil is thought to be an increase in microbial activity. Supporting this hypothesis, the population levels of contaminant-degrading bacteria and the potential of soil to degrade contaminants typically increases during phytoremediation. It is hypothesized that there may be two mechanisms by which plants increase catabolic activity in the rhizosphere or the bulk soil. Increases in catabolic activity may result from an enhancement of general microbial activity. For example, plants sometimes increase catabolic activity in rhizosphere soil independent of contaminants. This suggests that the enhancement of catabolic activity may simply be the result of microbial activity increasing due to the release of plant lysates and root exudates by what is commonly termed the rhizosphere effect. Additionally, increases in catabolic activity may result from the proliferation of specific microbial

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groups as the microbial community size increases due to the rhizosphere effect. This is the reasoning behind the use of microbial inoculants that stimulate phytoremediation and also alter the diversity of the root-associated community. In this scenario, the microbial community has little inherent ability to degrade hydrocarbons, and it requires either the selective enrichment or addition of specific microbial species before significant remediation activity is observed. A present challenge in phytoremediation research is to identify the appropriate plant species that can beneficially alter microbial diversity for a specific soil contamination scenario or, alternatively, that is susceptible to manipulation by the appropriate bacterial inoculant[72]. Although a large number of studies concerning phytoremediation of organic pollutants like petroleum hydrocarbons exist in the literature deals with various and serious kinds of oil pollution, yet, only a few applications in such an important field have been carried out.

Role of Phytoremediation in Radioactive Waste Treatment Radiation exists all around us, and radioactive materials provide numerous benefits to humans and society, and play a significant role in daily life. Applications for radiation include scientific, medical, agricultural, industrial and energy generation. Therefore, it is inevitable that such diverse activities lead to radioactive waste being generated. The International Atomic Energy Agency[73] has defined radioactive waste as any material that contains levels of radioactive emitting particles greater than those deemed safe by national and international standards, and for which no use is envisaged. For example, hazardous radioactive waste is generated at every stage of the uranium nuclear fuel cycle. It is unfortunate that at present no country has established permanent repositories and storage facilities for the most dangerous high-level nuclear wastes from nuclear power plants, Fig. 13[73].

Fig. 13: Worldwide radioactive waste inventory (based on 2008 data). Waste material in storage or stored (left), or disposed or assigned for disposal (right). Values are in (m3 × 105)[73].


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Radioactive waste can be solid, liquid or gas generated from a diverse group of operations and activities (uranium mining and nuclear power) and accidents (spills and reactor meltdown). No matter of its origin, radioactivity poses a risk to human health, the environment and has the potential to disrupt ecosystems. The nuclear accident at Chernobyl, Ukraine alone has been calculated to have increased the risk of cancer to humans by 0.1%[74]. Additionally, a recent report from Fukushima, Japan has already detected elevated levels of radioactive caesium in woody plants so soon after the accident[75]. Radioactive caesium (Cs), strontium (Sr), uranium (U) and plutonium (Pu) are the main radioactive isotopes present in the environment as a consequence of nuclear activities, and are the radionuclides of most concern for environmental and health concern. Some of the radioactive waste products have military potential; e.g. depleted uranium is used in munitions, and spent nuclear fuel from reactors contain weapons-usable plutonium. Nuclear waste on the one hand can be short-lived radiation, which is generally of little concern because it disappears quickly by natural radioactive decay, and/or weak radioactive waste with intensities comparable to natural background radiation. However high-level and medium-level long-lived radioactive nuclear waste is more problematic and safe disposal of this waste is necessary. Most of the nuclear waste of concern is produced in nuclear power plants, and much of it is in temporary storage. In particular, the concern is for spent fuel elements recently removed from reactors, not yet reprocessed and held in cooling ponds. In addition, there is an alarming accumulation of separate radioactive material cast in glass or ceramics, encased in stainless steel containers and held in dry storage at deposits all over the world[76]. Radioactive waste needs to be managed in a safe and secure manner, and it must be isolated from people and the environment for as long enough a period as the waste remains dangerous. Phytoremediation of radioactive waste is a process that uses plants to remove, transfer, or immobilize radionuclides from the contaminated soil, sediment, sludge, or water, and it is a useful method for treating large scale low-level radionuclide contamination. Phytoremediation has not been used so far in commercial or industrial decontamination of radioactive sites; however, it has been successfully tested, and some of these advances in radioactive remediation are reported[77]. The important factors influencing the selection of natural plant to remediate radioactive waste, including the characteristics of radioactive waste, the vegetation plant species and vegetation community composition in the radioactive waste deposited area, the concentration of a target radionuclide in the plant, the biomass of the plant, and the concentration of

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a target radionuclide in the radioactive, are analyzed, and the criteria based on the phytoremediation factor (PF) have been proposed for the selection of natural plant to remediate radioactive waste. In July 2009, 15 dominant plant species belonging to 9 families were collected from the uranium mill tailings impoundment in South China[78]. The concentrations of uranium and thorium in the samples of plant species and tailings were determined. Among the plant samples collected, Cyperus iria accumulated the highest concentration of Uranium (U) in its shoot which reached 36.41 g g–1 (Air dried or oven dried weight basis of samples (DW)). Juncellus serotinus accumulated the highest concentration of Thorium (Th) in its root which reached 3.661 g g–1 (DW). In September 2009, a wide survey was conducted in the uranium mill tailings impoundment in South China[79]. Thirty-five plant species were collected, and the concentrations of uranium in the samples were determined. J. serotinus accumulated the highest concentration of U in its stem which reached 1.52 mg g –1 (Ash weight basis of samples (AW)). Furthermore, K. brevifolia, C. difformis, M. cordata, Geranium carolinianum, E. annuus, P. nervosa, C. iria, and A. fatua accumulated relatively high concentrations of U in their aerial parts. In September 2011, an extensive survey was conducted in the uranium mill tailings impoundment in South China[80]. Thirty-three dominant plant species belonging to 16 families were collected, and the activities of 226Radon (Ra) in the samples were determined. There was great variation in the activities of 226Ra in the tissues of different plant species. The average activities of 226Ra in terms of AW for seeds, leaves, stalks, and roots were 30.99, 13.34, 5.772, and 4.515 Bq g –1, respectively. The high activities of 226Ra were found in the leaves of P. multifida (150.6 Bq g–1 of AW), in the leaves of P. aquilinum (122.2 Bq g–1 of AW), in the leaves of D. scottii (105.7 Bq g–1 of AW), and in the seed of P. fugax (105.5 Bq g–1 of AW). In contrast, the activity of 226Ra was found below the detection limit in the stalk and root of Ixeris chinensis and in the stalk of S. nigrum. Although the hyperaccumulators for U, Th, and 226Ra have not been defined so far, Baker and Brooks[81], have proposed a two criteria approach for defining the metal hyperaccumulator. First, the concentration of an element accumulated in an organism can be higher than that in the soil. Second, the amount of an element accumulated in an organism can be ten times greater than that in other organisms investigated. Based on this approach, C. iria and J. serotinus satisfied the criteria for a hyperaccumulator for U.P. multifida, P. aquilinum, and D. scottii satisfied the criteria for a hyperaccumulator for 226Ra. Although the high concentration of a target radionuclide in the plant species has been found in our investigation, all the experiments were carried out on contaminated areas with different histories, different contents of nutrients and organic


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matter; the areas were situated in different vegetation climates; and the plant species growing naturally on these areas were also quite different[79]. The nuclear fallout from the tsunami forced nearly 80,000 people to evacuate their homes, not knowing if or when they may return. The 30 miles surrounding the Fukushima Daiichi nuclear plant has been left contaminated and relatively barren. Even more disturbing, reports of radioactive rice, beef, vegetables, milk, seafood, and even tea have been found more than 60 miles away from the site, outside the mandatory evacuation zone.

Fig. 14: Millions of sunflowers soak up nuclear radioactivity in Fukushima. (Source: http:inhatat.com.08/22/11)

At least 8 million sunflowers and 200,000 other plants have been distributed and their seeds to be planted all over Fukushima. The plants are known to soak up toxins from the soil, and patches of sunflowers are now growing between buildings, in backyards, alongside the nuclear plant, and anywhere else they will possibly fit[82]. The phytoremediation efficiency of live floating plant, water hyacinth (Eichhornia crassipes), towards the effluents contaminated with 137Cs and/ or 60Co was studied on bench scale experiments. The main idea of this work is using undesirable species, water hyacinth (Eichhornia crassipes), Fig. 15, in purification of radiocontaminated aqueous solutions has been receiving much attention. The controlling factors such as radioactivity concentration, pH values, the amount of biomass and the light were studied. The uptake rate of radiocesium from the simulated waste solution is inversely proportional to the initial activity content and directly proportional to the increase in mass of plant and sunlight exposure. A spiked solution of pH  4.9 was found to be the suitable medium for the treatment process. The uptake efficiency of 137Cs present with 60Co in mixed solution was higher than if it was present separately. On the contrary, uptake of 60Co is affected negatively by the presence of 137Cs in their mixed solution. Sunlight is the most required factor for the plant vitality and radiation resistance. The results of the present study indicated that water hyacinth may be a potential candidate plant of high concentration ratios (CR) for phytoremediation of radionuclides such as 137Cs and 60Co[83].

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Fig. 15: Water hyacinth(Eichhornia crassipes) (Source: http://medplants.blogspot.com/ 20/2/08eichhotnia-rassipeswaterhycinth.html)

Lemna species have many unique properties ideal for remediation process among them, they are fast grown and primary production, have high bioaccumulation capacity and a good phytoremediation agent, Fig. 16. Lemna gibba (duck weed) is a free floating aquatic plant from family Lemnaceae. They commonly grow in stagnant or slow flowing water throughout tropical and temperate zones.

Fig. 16: September population bloom of Lemna gibba in Twin Oaks Valley, San Marcos (San Diego County (Source: http://waynesword,palomar.edu/imglegi.htm)

Some parameters that supposed to affect the biosorption efficiency of Lemna gibba for the both radionuclides Cs-137 and Co-60 (e.g: contact time, radioactivity content, pH of the simulated waste solution, the biomass of the plant added,) were assessed systematically[84]. Lemna gibba represents a promising biosorbent that candidate to bioseparate and bioaccumulate two risky gamma emitters, namely, cesium137 and cobalt-60 from low and intermediate radioactive waste solutions generated from peaceful applications of nuclear technology in the daily life. The proposed batch-wise phytoremediation process reached maximum within approximately 24 hours at pH=6.9 and under laboratory conditions (e.g.: room temperature, shed light and atmospheric pressure). The uptake values of the two radionuclides from their waste streams by Lemna gibba, under the state conditions, were 1213 Bq/gm and 872 Bq/gm for Co-60 and Cs-137


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respectively. The process is environmentally non-destructive and costeffective. Therefore, the destruction or harvesting of Lemna gibba biomass in surface water should be avoided where it can be applied as phytoremediation agent for various radioactive contaminants as well as other hazardous materials. Besides the low running cost advantage, the process is very simple and, therefore, can be smoothly applied at semifield and field levels. Efficiency of Veronica anagallis-aquatica plants for bioaccumulation/ biostabilization of 137Cs and 60Co from radwastes solution simulates was evaluated on batch-wise laboratory scale experiments. The plants were grown on spiked solutions having four different initial activity concentrations of 137Cs and 60Co. Additional factors such pH values, the amount of biomass and the light exposure that assumed to affect the treatment process were studied systematically. The uptake values of 137Cs from the simulated waste solution are inversely proportional to the initial activity contents and directly proportional to increase in the plant mass and sunlight exposure. Bioaccumulation of 60Co is independent on its initial activity concentrations. The spiked solution of pH  4.9 was found to be the suitable medium for the treatment process. The process was carried in vivo laboratory scale experiments and is considered as a promising technique for bioaccumulation and biostabilization of radiocontaminants from low and intermediate level liquid wastes. The study suggested that Veronica anagallis-aquatica could be used as a potential candidate plant for phytoremediation for the radioactive waste especially those contaminated with radiocesium and/or radiocobalt[85].

Is Phytoremediation Safe? Phytoremediation is a low-risk and attractive cleanup method. Fences and other barriers are constructed to keep wildlife from feeding on contaminated plants. In certain instances, plants may release chemical vapors into the air in the phytovolatilization process. When this occurs, workers sample the air to make sure the plants are not releasing harmful amounts of vapors.

Benefits of Phytoremediation Numerous benefits of phytoremediation have been established or hypothesized: • Phytoremediation can be less invasive and destructive than other technologies. • Studies have indicated that implementing phytoremediation may result in a cost savings of 50 to 80 percent over traditional technologies.

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• Phytoremediation may provide habitat to animals, promote biodiversity, and help speed the restoration of ecosystems that were previously disrupted by human activity at a site. • Phytoremediation installations can improve the aesthetics of brownfields or other contaminated sites. • Phytoremediation may promote better air or water quality in the vicinity of the site. • Vegetation may help reduce erosion by wind or water. • Planted trees may also provide shade to buildings, helping to decrease energy consumption. The plants can be easily monitored: • It is potentially the least harmful method because it uses naturally occurring organisms and preserves the environment in a more natural state.

Limitations of Phytoremediation Phytoremediation is not universally appropriate or successful; some important limitations must be noted: • Extremely high contaminant concentrations may not allow plants to grow or survive; phytoremediation is likely to be more effective or reasonable for lower concentrations of contaminants • For remediation to be successful, contamination must generally be shallow enough that plant roots can reach the contaminants, or contamination must be brought to the plant • Phytoextraction techniques can cause contaminants to accumulate in plant tissues, which could cause ecological exposure issues and thus form part of the food chain • Phytovolatilization may remove contaminants from the subsurface, but might then cause increased airborne exposure • If non-native species are selected for phytoremediation, the consequences of introducing them to the ecosystem may be unknown or unexpected • The time required to achieve the remedial goals may be longer with phytoremediation than with other treatment technologies (e.g., Bioremediation). Phytoremediation can require several growing seasons for a tree stand to be established and for contaminant concentrations to be reduced • With plant-based systems of remediation, it is not possible to completely prevent the leaching of contaminants into the groundwater (without the complete removal of the contaminated ground, which in itself does not resolve the problem of contamination)


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Market Potential and Cost of Phytoremediation A recent published report on phytoremediation‘s current and future market potential reports that phytoremediation gains in market acceptance in recent years[86]. Acceptance of phytoremediation is based on several factors: including natural attenuation approaches, and its potential versatility to treat a diverse range of hazardous materials in different contaminated media, such as soil, ground water, waste water and landfill leachate. The 1998, USA market of $ 16.5 million to $ 29.5 million was estimated to grow to $ 55 to 103 million by the year 2000 and to $ 214 million by the year 2005. Currently the vast majority of phytoremediation market is in the United States. However, international market are forming, and significant growth should occur in the very near future. Cost comparisons of phytoremediation to the other remediation technologies have recently become available also. According to a recent survey the consensus cost of phytoremediation has been estimated at $ 25 to $ 100 per ton of soil treatment and at $ 2.27 to 22.7 per litre for treatment of aqueous waste water. In both cases, the remediation of organic contaminants can be expected to fall at the lower ranges and the remediation of heavy metal to fall at the higher ends[87].

Future Requirements for Phytoremediation Since last decade phytoremediation has gained acceptance as a technology and has been acknowledged as an important area of research. Phytoremediation’s basic processes are still largely not clear and hence require further basic and applied research to optimize its field performance. Information collected from basic research at physiological, biochemical and genetic level in plants will be helpful in understanding the processes of passive adsorption, active uptake, translocation, accumulation and chelation mechanisms. Research aimed at better understanding of the interactive roles among plants roots and microbes will help scientists to utilize their integrative capacity for soil decontamination. Further, genetic evaluation of hyperaccumulators growing in metal contaminated soil and associated microbes would provide the researchers with a gene pool to be used in genetic manipulation of other non-accumulators and production of transgenics. CONCLUSIONS

Phytoremediation is a general term used to clean up contaminants using plants, or remediate sites by removing pollutants from soil and water. Plants can break down or degrade organic pollutants or contain and stabilize metal

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contaminants by acting as filters or traps. Phytoremediation involves growing plants in a contaminated matrix, for a required growth period to remove contaminants from the matrix or facilitate immobilization or degradation (detoxification) of the pollutants. The plants can be subsequently harvested, processed and disposed off in an environmentally sound manner. A phytoremediation system capitalizes on the synergistic relationships among plants, microorganisms, water and soil that have evolved naturally in wetlands and upland sites over millions of years. In the biological sequences that transform contaminants to neutral compounds, plants contribute inherent enzymatic and uptake processes that can recycle or sequester the organic molecules they encounter. Plants act as hosts to aerobic and anaerobic microorganisms, supplying them with both physical habitat and chemical building blocks. Plant roots and shoots increase microbial activity in their direct environment by providing colonies additional surface area, increasing readily-degradable carbon substrates by organic exudates and leachates and by decomposition of part of their mass and creating temporally and spatially varying oxygen regimes. Physically, plants slow the movement of contaminants in soil (reduced run-off), increase evapotranspiration and adsorbs compounds through their roots. Once a wetland or upland phytoremediation system is in place, its biological components would be naturally self-sustaining (due to photosynthesis). Phytoremediation can be applied in terrestrial and aquatic environments. It can be used as a preparative or finishing step for other clean-up technologies. Plants are aesthetically pleasing and these systems are relatively self-sustaining and cost-effective. Plants have evolved a great diversity of genetic adaptations to handle the accumulated pollutants that occur in the environment. Growing, and in some cases harvesting the plants on a contaminated site as a remediation method is a passive technique that can be used to clean up sites with shallow, low to moderate levels of contamination. Phytoremediation can be used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydrocarbons and landfill leachates. It can also be used in river basin management through the hydraulic control of contaminants. Phytoremediation is a potential remediation strategy that can be used to decontaminate soils contaminated with inorganic pollutants. Research related to this relatively new technology needs to be promoted and emphasized and expanded in developing countries since it is low cost. Phytoremediation has been perceived to be a more environmentally-friendly “green” and low tech alternative to more active and intrusive remedial methods. In most cases phytoremediation risks are small compared to the risks of doing nothing or the financial and engineering risks of ‘dig and haul.’


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REFERENCES [1] (2010). Blacksmith Institute’s report. World´s Worst Pollution Problems Report Top Six Toxic Threats. Produced in collaboration with Green Cross Switzerland; p. 76. [2] Sen, R. and Chakrabarti, S. (2009). Biotechnology–applications to environmental remediation in resource exploitation. Current Science, 97(6): 25. [3] Interstate Technology & Regulatory Council (ITRC). (2009). Phytotechnology Technical and Regulatory Guidance and Decision Trees, Revised, PHYTO-3, Washington D.C. [4] Swarnalatha, K. and Radhakrishnan, B. (2015) Studies on removal of Zinc and Chromium from aqueous solutions using water Hyacinth. Pollution, 1(2): 193–202. [5] (1999). U.S. Environmental protection agency, Phytoremediation Resource Guide, EPA 542-B-99-003. [6] Siciliano, S.D. and Germida, J.J. (1988). Mechanisms of phytoremediation: Biochemical and ecological interactions between plants and bacteria Environmental Reviews, 6(1): 65–79. [7] Hamdy, K.H. (2012) Treatment and conditioning of hazardous and radioactive wastes. M. of Science, Faculty of Science, Cairo Uni. [8] Saleh, H.M. and Eskander, S.B. (2012). Using Portland cement for encapsulation of Epipremnum aureum generated from phytoremediation process of liquid radioactive wastes. International Journal of Chemical and Environmental Engineering Systems, 3(2): 1–8. [9] Saleh, H.M. (2014). Stability of cemented dried water hyacinth used for biosorption of radionuclides under various circumstances. Journal of Nuclear Materials, 446(13): 124-133. [10] Bayoumi, T.A. (2013). Cemented solidified biomass waste form under drastic climatic conditions. Isotope and Radiation Research, 43(3): 577–588. [11] Landmeyer, J.E. (2011). Introduction to phytoremediation of contaminated groundwater, p. 431. [12] Becker, H. (2000). Phytoremediation—using plants to clean up soils, Agricultural Research, 48(6). [13] (2001). U.S. Environmental Protection Agency.” A citizen’s guide to phytoremediation”, U.S. Environmental Protection Agency, Technology Innovation Office. [14] (2011). U.S. Environmental Protection Agency. [15] Schnoor, J.L. (1997). Phytoremediation ground-water remediation technologies analysis center. Technology Evaluation Report TE-98-01. [16] (1997). National Research Council. Innovations in ground water and soil cleanup— From concept to commercialization”, Washington, D.C., National Academies Press, p. 310. [17] Resaee, A., Derayat, J., Mortazavi, Yamini, S.B.Y. and Jafarzadeh, M.T. (2005). Removal of Mercury from chlor-alkali industry wastewater using Acetobacter xylinum cellulose. American Journal of Environmental Sciences, 1(2): 102–105. [18] Salido, A.L., Hasty, K.L., Lim, J.M. and Butcher, D.J. (2003). Phytoremediation of arsenic and lead in contaminated soil using Chinese Brake ferns (Pteris vittata) and Indian mustard (Brassica juncea). International Journal of Phytoremediation, 5(2): 89–103. [19] Erdei, L., Mezôsi, G., Mécs, I., Vass, I., Fôglein, F. and Bulik, L. (2005). Phytoremediation as a program for decontamination of heavy-metal polluted environment. In: Proceedings of the 8th Hungarian Congress on Plant Physiology and the 6th Hungarian Conference on Photosynthesis. [20] Erdei, L., Mezôsi, G., Mécs, I., Vass, I., Fôglein, F. and Bulik, L. (2005). Phytoremediation as a program for decontamination of heavy-metal polluted environment. Acta Biologica Szegediensis, 49(1-2): 75–76.

158


[21] (2005). U.S. Environmental Protection Agency, Use of Field-Scale Phytotechnology, for Chlorinated Solvents, Metals, Explosives, and Propellants, and Pesticides Phytotechnology Mechanisms. Solid Waste and Emergency Response (5102G), EPA 542-R-05-002. [22] Etim, E.E. (2012). Phytoremediation and its mechanisms: A review. International Journal of Environment and Bioenergy, 2(3): 120–136. [23] Baker, A.J.M., Brooks, R.R. and Reeves, R.D. (1988). Growing for gold and copper and Zinc. New Science, 177: 44–48. [24] (2000). U. S. Environmental Protection Agency, “Introduction to Phytoremediation,” National Risk Management Research Laboratory, EPA/600/R-99/107. [25] Prasad, M.N.V. and De Oliveira Freitas, H.M. (2003). Metal hyperaccumulation in plants-biodiversity prospecting for phytoremediation technology. Electronic Journal of Biotechnology, 6(3): 110–146. [26] Yang, X., Feng, Y., He, Z. and Stoffella, P.J. (2005). Molecular mechanisms of heavy metal hyperaccumulation and phytoremediation. J. Trace Elem. Med. Biol., 18(4): 339–53. [27] (2002). Phytoremediation: An Environmentally sound technology for pollution prevention, control and remediation – An Introductory guide for decision makers. Freshwater Management Series No.2, United Nations Environment Programme, Division of Technology, Industry, and Economics. UNEP. [28] Raskin, I. and Ensley, B.D. (2000). Recent developments for in situ treatment of metal contaminated soils. In: Phytoremediation of Toxic Metals: Using Plants to Clean Up the Environment. John Wiley & Sons Inc., New York. [29] (2000). United States environmental protection agency “A citizen’s guide to phytoremediation”. EPA 542-F-98-011, p. 6. [30] Mendez, M.O. and Maier, R.M. (2008). Phytostablization of mine tailings in arid and semiarid environments-An emerging remediation technology. Environ Health Perspect, 116(3): 278–83. [31] Sharma, P. and Pandey, S. (2014). Status of phytoremediation in world scenario. International Journal of Environmental Bioremediation and Biodegradation, 2(4): 178–191. [32] (2000b). U. S. Environmental Protection Agency, “Introduction to Phytoremediation,” Office of Research and Development. EPA/600/R-99/107. [33] Zhang, H., Zheng, L.C. and Yi, X.Y. (2009). Remediation of soil co-contaminated with pyrene and cadmium by growing maize (Zea mays L.). Int. J. Environ. Sci. Tech., 6: 249–258. [34] Kvesitadze, G., Khatisashvili, G., Sadunishvili, T. and Ramsden, J.J. (2006). Biochemical mechanisms of detoxification in higher plants. Basis of Phytoremediation Springer, Berlin, Heidelberg. [35] Sanderman, H. (1994). Higher plant metabolism of xenobiotics: The 3 green liver3 concept. Pharmacogenetics, 4: 225–241. [36] Burken, J.G. (2004). Uptake and metabolism of organic compounds: Green-liver model. In: McCutcheon, S.C. and Schnoor, J.L., Phytoremediation: Transformation and Control of Contaminants, A Wiley-Interscience Series of Texts and Monographs, Hoboken, NJ: John Wiley, p. 59. [37] Subramanian, M., Oliver, D.J. and Shanks, J.V. (2006). TNT phytotransformation pathway characteristics in arabidopsis: Role of aromatic hydroxylamines. Biotechnol. Prog., 22(1): 208–216. [38] Dushenkov, V., Kumar, N.P.B.A., Motto, H. and Raskin, I. (1995). Rhizofiltration: The use of plants to remove heavy metals from aqueous streams. Environmental Science and Technology, 29(5): 1239-1245. [39] Suthersan, S.S. (1997). Remediation engineering: Design concepts. CRC P. [40] Verma, P., George, K.V., Singh, H.V., Singh, S.K., Juwarkar, A. nad Singh, R.N. (2006). Modeling rhizofiltration: Heavy-metal uptake by plant roots. Environmental Modeling and Assessment, 11: 387–394.


159

[41] Donnelly, P.K. and Fletcher, J.S. (1994). Potential use of mycorrhizal fungi as bioremediation agents. In: Bioremediation Through Rhizosphere Technology, Anderson, T.A. and Coats, J.R. Eds. Washington, D.C.: American Chemical Society. [42] Beharti, A. (2014). Pphytoremediation: As a degradation of heavy metals. International J. of Eng. and Tech. Res (IJETR), 2(5): 137–139. [43] Singh, A., Kuhad, R.C. and Ward, O.P. (2009). Advances in applied bioremediation. Springer -Verlager. Berlin, Heidelberg. [44] Ma, X. and Burken, J.G. (2002). VOCs fate and partitioning in vegetation: Use of tree cores in groundwater analysis. Environmental Science and Technology, 36(21): 4663– 68. [45] Chappell, J. (1998). Phytoremediation of TCE in groundwater using populus. Status report prepared for the U.S. Environmental Protection Agency Technology Innovation Office. [46] Heaton, C.P., Rugh, C.L.N., Wang, J. and Meagher, R.B. (1998). Phytoremediation of mercury and methylmercury-polluted soils using genetically engineered plants. Journal of Soil Contamination, 7: 497–509. [47] Trap, S., Kohler, A., Larsen, L.C., Zambrano, K.C. and Karlson, U. (2005). Phytotoxicity of fresh and weathered diesel and gasoline to willow and poplar trees. J. Soil Sediments, 1: 71–76. [48] Pivetz, B.E. (2001). Phytoremediation of contaminated soil and ground water at hazardous waste sites” United States Environmental Protection Agency, EPA/540/S01/500,February. [49] Magdziak, Z., G¹secka, M., Goliñski, P. and Mleczek, M. (2015). Phytoremediation and environmental factors, Phytoremediation: Management of Environmental Contaminations, I: 45–55. [50] Burken, J.G. and Schnoor, J.L. (1996). Phytoremediation: Plant uptake of atrazine and role of root exudates. Journal of Environmental Engineering, 122(11) 958–963. [51] Rodriguez, L., Lopez-Bellido, F.J., Carnicer, A., Recreo, F., Tallos, A. and Monteagudo, J.M. (2005). Mercury recovery from soils by phytoremediation, in Book of Environmental Chemistry, pp. 197–204. [52] Tangahu, B.V., Sheikh Abdullah, S.R., Basri, H., Idris, M., Anuar, N. and Mukhlisin, M. (2011). A review on heavy metals (As, Pb, and Hg) uptake by plants through Phytoremediation”. International Journal of Chemical Engineering, Article ID 939161, p. 31. [53] Peuke, A.D. and Rennenberg, H. (2005). Phytoremediation. EMBO Rep., 6(6): 497– 501. [54] Traunfeld, J.H. and Clement, D.L. (2001). Lead in garden soils. Home and Garden,” Maryland Cooperative Extention, University of Maryland. [55] Merkl, N., Schultze-Kraft, R. and Infante, C. (2005). Phytoremediation in the tropics— influence of heavy crude oil on root morphological characteristics of graminoids. Environmental Pollution, 138(1): 86–91. [56] Tu, S., Ma, L.Q., Fayiga, A.O. and Zillioux, E.J. (2004). Phytoremediation of arseniccontaminated groundwater by the arsenic hyperaccumulating fern Pteris vittata L. International Journal of Phytoremediation, 6(1): 35–47. [57] Mwegoha, W.J.S. (2008). The use of phytoremediation technology for abatement soil and groundwater pollution in Tanzania: Opportunities and challenges. Journal of Sustainable Development in Africa, 10(1): 140–156. [58] Fritioff, A. and Greger, M. (2003). Aquatic and terrestrial plant species with potential to re mo ve he avy me tals from sto rmwate r. Internatio nal Jo urnal o f Phytoremediation, 5(3): 211–224. [59] Van Ginneken, L., Meers, E. and Guisson, R. (2007). Phytoremediation for heavy metal-contaminated soils combined with bioenergy production. Journal of Environmental Engineering and Landscape Management, 15(4): 227–236.

160


[60] Roy, S., Labelle, S. and Mehta, P. (2005). Phytoremediation of heavy metal and PAHcontaminated brownfield sites. Plant and Soil, 272(1-2): 277–290. [61] Seuntjens, P., Nowack, B. and Schulin, R. (2004). Root-zone modeling of heavy metal uptake and leaching in the presence of organic ligands. Plant and Soil, 265(1-2): 61– 73. [62] (2010). The Daily Green, The BP Gulf of Mexico oil spill update. http:// www.thedailygreen.com/environmental-news/latest/gulf-of-mexico-oil-spill. [63] Ndimele, P.E. (2008). Evaluation of phyto-remediative properties of water hyacinth (Eichhornia crassipes [Mart.] Solms) and biostimunlants in restoration of oil-polluted wetland in Nigeria Delta. Ph.D., University of Ibadan, Nigeria. [64] Cunningham, S.D. and Ow, D.W. (1996). Promises and prospects of phytoremediation. Plant Physiol., 110: 715-719. [65] Ndimele, P.E. (2010). A review on the phytoremediation of petroleum hydrocarbon. Pakistan Journal of Biological Sciences, 13: 715–722. [66] Frick, C.M., Farrell, R.E. and Germida, J.J. (1999). Assessment of phytoremediation as an in-situ technique for cleaning oil-contaminated sites”. PTAC Petroleum Technology Alliance, Canada, Calgary, p. 88. [67] Hegde, R.S. and Fletcher, J.S. (1996). Influence of plant growth stage and season on the re lease o f root phe nolics by mulberry as re late d to development of phytoremediation technology. Chemosphere, 32: 2471–2479. [68] Mueller, K.E. and Shann, J.R. (2006). PAH dissipation in spiked soil: Impacts of bioavailability, microbial activity, and trees. Chemosphere, 64: 1006–1014. [69] Zand, A.D., Bidhendi, G. N. and Mehrdadi, N. (2010). Phytoremediation of total petroleum hydrocarbons (TPHs) using plant species in Iran. Turk. J. Agric. For., 34: 429–438. [70] Njoku, K.L., Akinola, M.O. and Oboh (2009). Phytoremediation of crude oil contaminated soil: The effect of growth of Glycine max on the physico-chemistry and crude oil contents of soil. Nature and Science, 7(10). [71] Dominguez-Rosado, E. and Pichtel, J. (2004). Phytoremediation of soil contaminated with used motor oil: II. Greenhouse Studies”. Environmental Engineering Science, 21(2). [72] Siciliano, S.D., Germida, J.J., Banks, K. and Greer, C.W. (2003). Changes in microbial community composition and function during a polyaromatic hydrocarbon phytoremediation field trial. Appl. Environ. Microbiol., 69(1): 483–489. [73] (2010). (International Atomic Energy Agency).” Managing Radioactive Waste”. IAEA, Vienna. [74] (2005). International Atomic Energy Agency. Chernobyl’s legacy: Health, environmental and socio-economic impacts, Cherbobyl Forum 2003–2005". IAEA, Vienna. [75] Yoshihara, T., Matsumura, H., Hashida, S.N. and Nagaoka, T. (2013). Radiocesium contamination of 20 wood species and the corresponding gamma-ray dose rates around the canopies 5 months after the Fukushima nuclear power plant accident. J. Environ. Radioactiv., 115: 60–68. [76] Comby, B. (2005). The solutions for nuclear waste. Inter. J. Environ. Stud., 62: 725– 736. [77] De Filippis, L.F. (2015). Role of phytoremediation in radioactive waste treatment. soil remediation and plants prospects and challenges- ACADEMIC PRESS, pp. 207–254. [78] Li, G.Y., Hu, N., Ding, D.X., Zheng, J.F., Liu, Y.L., Wang, Y.D. and Nie, X.Q. (2011). Screening of plant species for phytoremediation of uranium, thorium, barium, nickel, strontium and lead contaminated soils from a uranium mill tailings repository in South China. Bull Environ. Cont. Toxicol., 86: 646–652. [79] Hu, N., Ding, D. and Li, G. (2014). Natural plant selection for radioactive waste remediation radionuclide contamination and remediation through plants. Springer International Publishing, Switzerland, pp. 33–53.


161

[80] Hu, N., Ding, D.X., Li, G.Y., Zheng, J.F., Li, L., Zhao, W.C. and Wang, Y.D. (2014). Vegetation composition and 226Ra uptake by native plant species at a uranium mill tailings impoundment in South China. J. Environ. Radioact., 129: 100–106. [81] Baker, A.J.M. and Brooks, R.R. (1989). Te rrestrial higher plants which hyperaccumulate metallic-A review of their distribution, ecology and phytochemistry. Biorecovery, 1: 81–126. [82] Cotter, M. Millions of sunflowers soak up nuclear radiation in Fukushima”, http:inhatat.com.08/22/11 [83] Saleh, H.M. (2012). Water hyacinth for phytoremediation of radioactive wastes simulate contaminated with cesium and cobalt radionuclides. Nuclear Engineering and Design, 242: 425–432. [84] Eskander, S.B., Nour El-dien, F.A. Hoballa, E.M. and Hamdy, K.H. (2011). Capability of lemna gibba to biosorb cesium-137 and cobalt-60 from simulated hazardous radioactive waste solutions. Journal of Microbiology, Biotechnology and Food Sciences, (2): 148–163. [85] Bayoumi, T.A. (2012). Bioaccumulation of 137Cs and 60Co from radioactive waste streams using Veronica anagallis-aquatica, BioTechnology An Indian Journal, 6(8,9): 282–288. [86] Glass, D.J. (1998). Current market trends in phytoremediation. Intern. J. of Phytorem., 1: 1–8. [87] Lauza, G.R. and Flathhman, P.E. (2001). Phytoremediation technology hazardous and radioactive waste treatment technologies handbook Edited by Oh, C.H. CRC press Amazon, US.

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