The Use of Phytoremediation to Combat ...

Vol. 72 | No. 3 | Mar 2016

International Scientific Researches Journal

The Use of Phytoremediation to Combat Contamination of Soil Ecosystems M. Saber 1, H. Abouziena 2*, E. Hoballah 1, Wafaa M. Haggag 3, Fatma Abd-Elzaher 1, Soad El-Ashry 4 and A. M. Zaghloul 4 1

Agricultural Microbiology Department, National Research Centre, Cairo, Egypt, 12622 Botany Department, National Research Centre, Cairo, Egypt, 12622 3 Plant Pathology Department, National Research Centre, Cairo, Egypt, 12622 4 Soil and Water Use Department, National Research Centre, Cairo, Egypt, 12622 2

*Corresponding author, Email: [email protected]

Abstract Phytoremediation is one of the most recent developments in biotechnology valid in remediating contaminated soil ecosystems. The concept of successive bioremediation protocols had been recently highlighted that involves furnishing the contaminated soil with certain remediative amendments followed by biofortification with varied micro-organisms. The released PTE and decomposed organic toxins are thereafter removed from the soil using phytoremediation techniques. The current article covers some phytoremedation processes such as phytoextraction, phytodegradation, phytostimulation, phytostabilization, phytovolatilization, phytorestoration, phytotransformation and phytoremediation of multiinorganic contaminants. Keywords: hyperacumulator plants, phytoextraction, phytodegradation, phytostimulation, phytostabilization, phytovolatilization, phytorestoration, phytotransformation 2. Introduction Phytoremediation refers to the use of plants to remove contaminants from soil and groundwater, or to assist in the degradation of contaminants to a less toxic form. It is a technology bearing great potential, as it is an environment friendly, green technology that is cost effective and energetically inexpensive. Before phytoremediation, both benefits and disadvantages should be accessed to determine whether this type of remediation is the most appropriate for the task. Phytoremediation had many advantages compared to other remediation techniques such as soil extraction, incineration, inorganic treatment, and land filling. It could be used to decontaminate vast areas, could be carried out with little environmental disturbance and is applicable to a broad range of contaminants. There are many studies [1, 2, 3, 4, 5], reported that using certain plants in the phytoremedation with or without soil amendments had a

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significant role in removing the PTEs and Pathogens and some reports concluded that after the phytoremedation process it can growing the sensitive plants to PTEs without any adverse impact on the final products [6, 7] and these processes are economic [8]. Using some integrated measures with corn and sunflower plants to cleanup soil irrigated with sewage effluent from certain heavy metals. However, there are some reports showed there are some risks in using the contaminated soil after the phytoremedation for growing the economic crops [9, 10]. The current article aims at exploring the concepts of phytoremediation biotechnology and its application in bio-remediating contaminated soil ecosystem.

2.1.

Phytoremediation

Phytoremediation is surrounded with some substantial limitations. The first limitation is the solubility of the contaminants in the soil, which determines the possibility of phytoremediation. In the case of PTEs, only PTEs found as free PTEs ions, soil soluble PTEs complexes, or PTEs adsorbed to inorganic soil constituents at ion exchange sites are readily available for uptake by the plants [11]. On the other hand, PTEs that are bound to soil organic matter, precipitated (oxides, hydroxides, carbonates), or embedded in the structure of silicate minerals are not available to the plants. The second limitation is that in situ remediation often takes many years to accomplish compared to other traditional decontamination approaches to substantially restore a polluted area [12]. Many phytoremediation plans had multi-year timetables, but since most soils in need of remediation had been contaminated for many years, hence bioremediation plan does not seem excessive (Table 1). Phytoremediation is the name given to a set of technologies that use different plants as a containment, destruction, or an extraction technique. The phytoremediation as a remediation technology that had been receiving attention lately as the results from field trials indicate a cost savings compared to conventional treatments [13]. While [14] stated that phytoremediation is a long-term, complex biological process. There is not one single measure of the efficiency of a process such as phytoremediation and therefore many aspects must be considered separately, and in conjunction, to reach a conclusion. Efficiency is an arbitrary term unless related to an end point. Unless the end use is specified then the efficiency of the process could not be calculated. The main measurements of efficiency, for example, might be the rate of degradation of organics, or the stabilization, extraction, or volatilization of PTEs, the duration of the remediation process or the cost of the process. In the case of contamination with organic

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inorganic the degradation products and metabolites must be considered as they might be more toxic than the original inorganic. Table 1. Phytoremediation applications and demonstrations in the field. Application

Plants

Contaminants

Performance

Phytoextraction demonstration 200 fl x 300 ft plot Brownfield location

Indian mustard (Brassica juncea)

Pb

Pb cleaned -up to below action level in one season SITE program

Phytostabilization demonstration one acre test plot abandoned smeller, barren land

Poplars populus spp.

Phytostabilization demonstration one acre test plot mine wastes

Poplars populus spp.

As, Cd

95% of trees died. Inclement weather, deer browse, toxicity caused die-off

Phytoextraction pilot mine Wastes

Thlaspi caerulesccns

Zn, Cd

Uptake is rapid but difficult to decontaminate soil

Phytotransformation created wetland and surrounding soil

Pondweed Coontail Arrowroot Hybrid poplars

TNT, RDX

Just beginning

Phytotransformation (groundwater and soil) petroinorganic wastes 4 acre site

Hybird Poplar

BTEX, TPH

Only In Second Year SITE Program

Pb, Zn. Cd 50% survival after 3 years site was Cones, > 20.000 successfully revegetated ppm for Pb and Zn

There was a consensus that defined and internationally adopted guidelines for parameters such as monitoring and analytical tools and methods etc. are needed. They added that these were sorry for international comparability of research and remediation and also efficiency measures. Accepted and tested models would also be needed both for the terrestrial and aquatic environments. It was accepted that phytoremediation is not a remediation technology which could be applied to all contaminated soils, or even the majority of them. The process was slow and could be PTEs specific. Phytoremediation was suggested as a viable technique when the following parameters are satisfied, the site is of low economic value, time constraints do not apply, a low cost solution is required, the main PTEs contamination was with only one or two PTEs and confined to the surface layers, the labile pool is the most toxic form of the PTEs, PTEs and other contaminants are not found at phytotoxic levels, and when there is an infrastructure present to safely treat and dispose of the contaminated biomass which might be produced. These parameters apply to phytoremediation as a stand-alone technology. It was not overlooked that coupling phytoremediation with other techniques such as physical and/or inorganic treatment could also be viable in many cases. He added that phytoremeddiation by

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rotation of crops with certain accumulators or hyperaccumulators is a very real option. Phytoremediation of farmland had met resistance in several countries where it was tried due to farmers disliking growing a crop simply to throw it away, or preferring the use of inorganic stabilizers. Many farmers demand to be paid to grow accumulator crops on their land. The wide use of accumulating and hyperaccumulating crops also faces another pitfall and that is the issue of the introduction of exotic species to an area or country. There are several aspects of phytoremediation; the most important ones are phytoextraction, phytodegradation, Phytostimulation, phytostabilization, phytovolatization, phytorestoration, phytotransfrmation. Another limitation is the availability of the contaminants in question to the plants [15]. Areas where contamination is less than 5 m in depth are the best-suited sites for phytoremediation. The key factors of phytoremediation critical success and conditions are given in Table (2). 2.1.1. Phytoextraction The use of plants to remove contaminants from soil and concentrate them in aboveground plant tissue is known as phytoextraction. At the beginning, it was primarily employed to recover PTEs and PTEloids however, it is now applicable to different types of contaminants [16]. Recovery of PTEs from vegetation had centered on incineration and recovery from ash, or wet extraction techniques. Even if it is not practicable to recover the PTEs from plant biomass or ash, they will have been concentrated into a much smaller volume for ultimate disposal. Plants with high growth rates (>3 tons dry matter/hectare-year) and with the ability to tolerate high PTEs concentrations in harvestable parts of the plants (>1,000 mg/kg) are needed for practicable treatment. Phytoextraction is divided into two types; continuous and induced. Continuous phytoextraction, which uses of plants that accumulate particularly high levels of contaminants throughout their lifetime (hyperaccumulators) and induced phytoextraction approaches which enhance contaminant accumulation at a single time point by addition of accelerants or chelators to the soil. 2.1.2. Phytodegradation In phytodegredation, organic contaminants are converted by plant internal or secreted enzymes into compounds with reduced toxicity [12]. For instance, the major soil contaminant trichloroethylene (TCE) was found to be taken up by hybrid poplar trees, Populus deltoides x nigra, which breakdown the contaminant into its metabolic components. Root exudates from Datura innoxia and Lycopersicon peruvianum containing peroxidase, laccase, and nitrilase 13

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could degrade soil contaminants and nitroreductase and laccase together could breakdown TNT, RDX, and HMX. The plants were able to incorporate the broken ring structures into new plant material or organic soil components that are thought to be non-hazardous. Table 2. Summary of phytoremediation critical success factors and conditions.

Phytoremediation Process

Critical success Factors/ Design considerations

Conditions for optimum likelihood of success

Basis

Moderately hydrophobic organics taken-up

Data needs*

Vegetation

Toxicity, fate

Trees, grasses

Phytotransformation

Uptake by plant; bound residue or metabolism volatilization required

Non-toxic concentrations

Rhizosphere Bioremediation

Degradation by microbes; dense root system needed

Compounds amenable to aerobic biodegradation

Dense roots sorbs chemicals and enhance microbial degradation

Toxicity, fate

Trees, grasses, legumes

Phytostabilization

Hydraulic control, soil stabilization immobilization

Vigorously growing roots; hydrophobic or immobile chemicals

Roots hold soil and water, immobilize metals

Toxicity, fate

Trees, grasses legumes

Phytostabilization

Plant productivity accumulation in harvestable portion of plant

>3 tons dry matter/ acre-yr; > 1,000 mg/kg metals lightly contaminated soil near to clean-up standard

Vigorous plants growth provides acceptable uptake rate high ability to accumulate contaminants desirable

Toxicity, fate

Terrestrial plants or aquatic emergent plants for sediments

Rhizofiltration

Sorption/ filtration by roots, water in contact with roots, hydraulic detention time

Plant densities 200-1000 g/m2, hydraulic detention time of several days

Roots sorb and immobilize contaminants

Toxicity, fate

Aquatic emergent of submerged plants

2.1.3. Phytostimulation Phytostimulation referred to as enhanced rhizosphere biodegradation or plant-assisted bioremediation/degradation, is the breakdown of organic contaminants in the soil via enhanced microbial activity in the plant root zone (rhizosphere). Microbial activity might be stimulated in the rhizosphere in several ways, compounds, such as sugars, carbohydrates, amino acids, acetates, and enzymes exuded by the roots enrich

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indigenous microbe populations, root systems bring oxygen to the rhizosphere hence ensures aerobic transformations, fine-root biomass increases available organic carbon, mycorrhizae fungi which grow within the rhizosphere could degrade organic contaminants that could not be transformed solely by bacteria because of unique enzymatic pathways, and the habitat for increased microbial populations and activity is enhanced by plants. Table 3. Contaminants applicable to phytoremediation technology. Technology

Target Contaminants

Phytoextraction

PTEs - Cd, Cu, Ni, Pb, Zn

Phytostabilization

PTEs - As, Cd, Cr, Cu, Pb, Se, Zn Hydrophobic organics - DDT, Dieldrin,

Phytostimulation

Organics - aromatics, PAH, pesticides

Phytotransformation

Ammunition wastes - RDX, TNT Aromatics - BTEX Chlorinated Aliphatic - TCE Herbicides - Atrazine, Alachlor Hydrocarbons - TPH

Biodegradation

COD Organics - BTEX NAPL Pesticides Solvents

Dioxins, Furans, PAH, PCB, PCP

Five enzyme systems in soils had been investigated, dehalogenase which is important in dechlorination reactions of chlorinated hydrocarbons; nitroreductase which is required in the first step of nitroaromatic degradation; peroxidase which is important in oxidation reactions, laccase which breaks aromatic ring structures of organic compounds and nitrilase which is important in oxidation reactions. Phytostimulation is useful in removing organic contaminants, such as pesticides, aromatics, and polynuclear aromatic hydrocarbons (PAHs), from soil. Locations at which phytostimulation is to be implemented should had low levels of contaminations in shallow areas. High levels of contaminants could be toxic to plants. Degradation by microorganisms and dense root systems are needed for a successful design. Toxicity and fate of contaminants need to be evaluated and understood prior to implementing this technology. Vegetation might include trees, grasses, and legumes. 2.1.4. Phytostabilization Phytostabilization involves the reduction of the mobility of PTEs in soil. Immobilization of PTEs could be accomplished by decreasing wind-blown dust, minimizing soil erosion, and reducing contaminant solubility or bioavailability to the food chain. The

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addition of soil amendments, such as organic matter, phosphates, alkalizing agents, and biosolids could decrease solubility of PTEs in soil and minimize leaching to groundwater. The mobility of contaminants was reduced by the accumulation of contaminants by plant roots, absorption onto roots, or precipitation within the root zone. In some instances, hydraulic control to prevent leachate migration could be achieved because of the large quantity of water transpired by plants. 2.1.5. Phytovolatilization The release of volatile contaminants to the atmosphere via plant transpiration is called phytovolatilization. Although transfer of contaminants to the atmosphere might not achieve the goal of complete remediation, phytovolatilization might be desirable in that prolonged soil exposure and the risk of groundwater contamination are reduced. In this process, the soluble contaminants are taken up with water by the roots, transported to the leaves, and volatized into the atmosphere through the stomata. The best example of this is the volatilization of mercury by conversion to the elemental form in transgenic Arabidopsis and yellow poplars containing bacterial mercuric reductase merA [17]. In a study where the movement of volatile organics was monitored by Fourier transform infrared spectrometry (FT-IR) in hybrid poplars (Populus deltoides x nigra, Tamarix parviflora (saltcedar) and Medicago sativa (alfalfa), chlorinated hydrocarbons were found to move readily through the plants, but less polar compounds like gasoline constituents did not [18]. Selenium is a special case of PTEs that is taken up by plants and volatilized following conversion to dimethylse-lenide by microorganisms and algae [19]. 2.1.6. Phytorestoration Phytorestoration involves the complete remediation of contaminated soils to fully functioning soils. In particular, this subdivision of phytoremediation uses plants that are native to the particular area, in an attempt to return the land to its natural state. An examination of phytorestoration compared to the other forms of phytoremediation brings to light an important issue, what degree of decontamination do phytoremediation projects aim to achieve? There is a vast difference between removing just enough soil contaminants to reach legally defined levels of compliance, remediating soils to a level at which they could be used again, and completely restoring land from its contaminated state to an environmentally uncontaminated state. The objective of many phytoremediation projects is to restore the land to a legally acceptable level of contamination. A combination of phytoremediation approaches could be use for more effective environmental restoration. For example, a remediation system could include plants that hyperaccumulate PTEs and plants that stimulate the activity of microorganisms that specialize in organic contaminant degradation.

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2.1.7. Phytotransformation Phytotransformation is the breakdown of organic contaminants sequestered by plants via metabolic processes within the plant, or the effect of compounds, such as enzymes, produced by the plant. The organic contaminants are degraded into simpler compounds that are integrated with plant tissue, which in turn, foster plant growth. Remediation of a soil by phytotransformation is dependent on direct uptake of contaminants from the media and accumulation in the vegetation. Direct uptake of inorganic into plant tissue via the root system is dependent on uptake efficiency, transpiration rate, and concentration of the inorganic in soil water. Uptake efficiency depends on inorganic speciation, physical/inorganic properties, and plant characteristics, whereas transpiration rate depends on plant type, leaf area, nutrients, soil moisture, temperature, wind conditions, and relative humidity. Two processes of remediation could occur after the organic compound had been translocated by the plant, storage of the inorganic and its fragments into the plant via lignification and complete conversion to carbon dioxide and water. Successful implementation of phytotransformation requires that the transformed compounds that accumulate within the plant be non-toxic or significantly less toxic than the parent compounds. In some applications, phytotransformation might be use in concert with other remediation technologies or as a polishing treatment. This technology usually requires more than one growing season to be efficient. Soil must be less than 3 ft in depth and groundwater within 10 ft of the surface. Contaminants might still enter the food chain through animals or insects that eat plant material. Soil amendments might be required, including chelating agents to facilitate plant uptake by breaking bonds binding contaminants to soil particles. 2.2.

Phytoremediation of multi-inorganic contaminants

As far as the PTEs contaminants are considered, plants showed a promising role for phytoextraction (uptake and recovery of contaminants into aboveground biomass) which is an emerging technology that should be consider for remediation of contaminated soils irrigated with sewage soils because of its cost effectiveness, versatility, esthetic advantages, and longterm applicability. It is well known that Cd, Ni, Zn, As, Se, and Cu are readily bioavailable PTEs. Co, Mn and Fe are consider moderately bioavailable PTEs. Pb, Cr, and U are not very bioavailable, although the addition of EDTA, to soil (0.5 to 10 µg EDTA/kg) could improve the bioavailability of Pb. Effective extraction of PTEs by hyperaccumulators is limited to shallow soil depths of up to 24 inches. If contamination is at substantially greater depths (e.g., 6 to 10 feet), deep-rooted poplar trees could be use, however, there is concern about leaf litter and associated toxic residues. Despite having amiable PTE-accumulating characteristics, 17

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currently available hyperaccumulators lack suitable biomass production, physiological adaptability to varying climatic conditions, and adaptability to current agronomic techniques. Research on specific plant species determined that some plants concentrated PTEs of up to several percent of their dried shoot biomass. These plants (designated as hyperaccumulators) had toxic element levels in the leaf and stalk biomass of about 100 times those of nonaccumulator plants in the same soil and in some cases more than a thousand times. The direct uptake of inorganic contaminants into the plant through roots depends on the uptake efficiency, transpiration rate, and concentration of inorganic contaminant in soil water [20]. Uptake efficiency, in turn, depends on physical-inorganic properties, inorganic speciation, and the plant itself. Transpiration is a key variable that determines the rate of inorganic contaminant uptake for a given phytoremediation design; it depends on the plant type, leaf area, nutrients, soil moisture, temperature, wind conditions, and relative humidity. Contamination with PTEs represents one of the most pressing threats to soil resources and added that phytoremediation could be potentially used to remediate PTE-contaminated soils [21]. Their study evaluated the potential of 36 plants (17 species) growing on a contaminated soil in North Florida. Plants and the associated soil samples were collected and analyzed for total PTEs concentrations. While total soil lead, cupper and zinc concentrations varied from 90 to 4100, 20 to 990, and 195 to 2200 mg kg−1, those in the plants ranged from 2.0 to 1183, 6.0 to 460, and 17 to 598 mg kg−1, respectively. None of the plants were suitable for phytoextraction because no hyperaccumulator was identified. However, plants with a high bioconcentration factor (BCF, PTEs concentration ratio of plant roots to soil) and low translocation factor (TF, PTEs concentration ratio of plant shoots to roots) had the potential for phytostabilization. Among the plants, Phyla nodiflora was the most efficient in accumulating cupper and zinc in its shoots (TF=12 and 6.3), while Gentiana pennelliana was most suitable for phytostabilization of soils contaminated with lead, cupper and zinc (BCF=11, 22 and 2.6). Plant uptake of the three PTEs was highly correlated, whereas translocation of lead was negatively correlated with cupper and zinc though translocation of cupper and zinc were correlated. They showed that native plant species growing on contaminated soils might had the potential for phytoremediation. Arsenic a general rule, readily bioavailable PTEs for plant uptake include cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately bioavailable PTEs are cobalt, manganese, and iron; while lead, chromium, and uranium are not very bioavailable. Lead could be made greatly more bioavailable by the addition of EDTA to soils. Lead, chromium, and uranium could be removed by binding to soils and root mass via rhizofiltration. Soluble PTEs could enter into the root symplast by crossing the plasma membrane of the root endodermal cells or they could enter the root apoplast through the space between cells. If the PTEs are translocated to aerial tissues, then it must enter the xylem. To enter the xylem, solutes must cross the casparian strip, a waxy coating which is impermeable to solutes, unless they pass through the cells of 18

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the endodermis probably through the action of a membrane pump or channel. Once loaded into the xylem, the flow of the xylem sap will transport the PTEs to the leaves, where it must be loaded into the cells of the leaf, again crossing a membrane. Once in the shoot or leaf tissues, PTEs could be stored in various cell types, depending on the species and the form of the PTEs, since it could be converted into less toxic forms to the plant through inorganic conversion or complexation. The PTEs could be sequestered in several subcellular compartments (cell wall, cytosol, and vacuole) or volatilized through the stomata. While many examples in both literature and patents propose the potential of different plants to remove PTEs from soils, [22] explained how the bioconcentration factor of many of these plants is not conducive to actual phytoremediation. The bioconcentration factor is the ratio of the plants shoot PTEs concentration to the soil PTEs concentration, which could be interpreted as the ability of a plant to take up the PTEs and transport it to its shoots. While most plants had a bioconcentration factor for PTEs and PTEloids of less than one, a much greater value is required for phytoremediation. Even if one assumed a high biomass production of fifty tones per feddan per crop, a bioconcentration factor of greater than ten is required to reduce soil PTEs by half in less than ten crops. Thirty tones per feddan per crop is possible for many agricultural crops, and the bioconcentration factor would need to be twenty or greater to reduce soil PTEs by half in less than ten crops. Many of these soils had been contaminated for more than ten years as such a ten years remediation plan does not seem excessive. They added that successful implementation of phytoextraction depends on bioavailability of the contaminant in the environmental matrix; root uptake; internal translocation of the plant; and plant tolerance. Plant productivity (i.e., amount of dry matter that is harvestable each season) and the accumulation factor (ratio of PTEs in plant tissue to that in the soil) are important design parameter. The rate of PTEs phytoextraction is governed by both soil and plant characteristics [23]. Most effort had focused on identifying appropriate plants for phytoextraction, but the benefits from this effort will be marginal unless the PTEs are in phytoavailable forms in the rhizosphere. The concentration of a PTE in the rhizosphere could be estimated using solute transfer models that incorporate the PTEs concentration in the bulk soil solution, the buffer power of the soil, diffusion coefficient for the PTEs, water movement, root size and morphology, and the rate of entry of PTEs into the roots. Such models could be used to identify constraints to efficient phytoextraction (whether plant or soil) and to determine whether commercial phytoextraction is feasible. Phytoextraction could provide an effective in situ technique for removing PTEs from contaminated soils [24]. They studied the basic potential of phytoextraction of Brassica napus (canola) and Raphanus sativus (radish) grown on a multi-PTE contaminated soil in a pot

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experiment. Chlorophyll contents and gas exchanges were measured during the experiment; the PTEs phytoextraction efficiency of canola and radish were also determined and the phytoextraction coefficient for each PTE was calculated. Data indicated that both species are moderately tolerant to PTEs and that radish was more so than canola. These species showed relatively low phytoremediation potential of multi-contaminated soils. They could possibly be used with success in marginally contaminated soils where their growth would not be impaired and the extraction of PTEs could be maintained at satisfying levels. Concentrations of lead, zinc, copper and cadmium accumulated by 12 emergent-rooted wetland plant species including different populations of Leersia hexandra, Juncus effusus and Equisetum ramosisti were investigated [25]. Their results showed that PTEs accumulation by wetland plants differed among species, populations and tissues. Populations grew in substrata with elevated PTEs contained significant higher PTEs in plants. PTEs accumulated by wetland plants were mostly distribute in root tissues, suggesting that an exclusion strategy for PTEs tolerance widely exists in them. That some species/populations could accumulate relatively high PTEs concentrations (far above the toxic concentration to plants) in their shoots indicates that internal detoxification PTEs tolerance mechanism(s) were also included. They added that the factors affecting PTEs accumulation by wetland plants include PTEs concentrations, pH, and nutrient status in substrata. Mostly concentrations of lead and cupper in both aboveground and underground tissues of the plants were significantly positively related to their total and/or DTPA-extractable fractions in substrata while negatively to soil nitrogen and phosphorus, respectively. In the case of PTEs, chelators like EDTA assist in mobilization and subsequent accumulation of soil contaminants such as lead, cadmium, chromium, copper, nickel, and zinc in Indian mustard Brassica juncea and sunflower Helianthus anuus [16]. The ability of other PTEs chelators such as CDTA, DTPA, EGTA, EDDHA, and NTA to enhance PTEs accumulation had also been assessed in various plant species. However, there might be risks associated with using certain chelators considering the high water solubility of some chelatortoxin complexes which could result in movement of the complexes to deeper soil layers. Phytoremdiation is the most likely technique for marginally contaminated agricultural soils, where phytoextraction could be used as a polishing technology [27]. They added that an alternative and more useful practical approach in many situations currently would be to give more attention to crops selected for phytoexclusion, i.e., selecting crops that do not translocated high concentrations of PTEs to edible parts. Soils of urban and industrial areas provide a large-scale opportunity to use phytoremediation, but the focus here should be on the more realistic possibilities of risk-managed phytostabilization and monitored natural attenuation. An additional focus on biomass energy, improved biodiversity, watershed

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management, soil protection, carbon sequestration, and improved soil health is required for the justification and advancement of phytotechnologies. The biomass species had relatively low PTEs uptake and hyperaccumulator species had low biomass yields [14]. There is potential for plant breeding and genetic modification to create species with high accumulating capabilities and also high biomass. There is also the possibility of engineering plants that are easy to grow but do not run wild in new habitats. Further research, and perhaps using genetic modification both the efficiency and the viability of phytoremediation as a competitive remediation technology would be much improved. Phytostabilisation was more viable at present than phytoextraction in soils of very high PTEs due to the very large time scales involved. The plant biomass and PTEs shoot accumulation were key factors for efficient phytoextraction [28]. They found that inorganic mutagenesis improved the phytoextraction potential of sunflowers towards cadmium, zinc and lead. The best sunflower mutants showed either higher PTEs accumulation in shoots or enhanced PTEs accumulation in roots, suggesting to transgenic hyperaccumulator plants were frequently use to remove PTEs from terrestrial as well as aquatic ecosystems [29]. The technique makes use of the intrinsic capacity of plants to accumulate PTEs and transport them to shoots, ability to form phytochelatins in roots and sequester the PTEs ions. Harboring the genes, that are considered as signatures for the tolerance and hyperaccumulation from identified hyperaccumulator plant species into the transgenic plants, provided a platform to develop the technology with the help of genetic engineering. This would result in transgenics that might had large biomass and fast growth a quality essential for removal of PTEs from soil quickly and in large quantities. Despite so much of a potential, the progress in the field of developing transgenic phytoremediator plant species is rather slow. This could be attributed to the lack of our understanding of complex interactions in the soil and indigenous mechanisms in the plants that allow PTEs translocation, accumulation and removal from a soil. They reviewed the work carried out in the field of PTEs phytoremediation from contaminated soil and concluded that there is an urgent need to assesses the current status of technology and to evaluate its future prospects with emphasis on a combinatorial approach. They added that with the current high costs of soil remediation, it is important to develop and refine innovative low-cost methods for cleaning the environment. Advances in soil remediation continue to lead to a better understanding of the many processes by which plants could have a positive impact on the decontamination of the environment. Their realization had provided impetus to studies in an emerging field of interest, which employed certain plants possessing the natural ability to take up PTEs for an inexpensive means of environmental cleanup. Their method was referred to as plant-assisted remediation or phytoremediation, and it also had the benefit of contributing to site restoration when remedial action is ongoing.

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The plants were selected in bioremediation procceses according to the needs of the application and the contaminants of concern [16]. Grasses are often planted in tandem with trees as the primary remediation method. They provide a tremendous amount of fine roots in the face soil which is effective at binding and transforming hydrophobic contaminants such as TPH, BTEX, and PAHs. In phytoextraction, one is seeking to concentrate them in the aboveground portion of the biomass, and to harvest and recover PTEs from the biomass, if practicable. Plants used to date in phytoextraction remedies include sunflowers and Indian mustard plants for lead; Thlaspi spp for zinc, cadmium and nickel, Sunflowers and aquatic plants for radionuclides (Table 4). Table 4. Phytoremediation applications and demonstrations in the field. Contaminants Application Phytoextraction plot Brownfield location

Performance

Plants Indian mustard Brassica juncea

Pb

Pb cleaned -up to below action level in one season SITE program

Phylovolatilization agricultural soils

Brassica sp.

Se

Selenium is partly taken-up and volatilized, but difficult to decontaminate soil

Phytotransformation engineered wetland

Elodeia Bullrush Canary Grass

TNT, RDO

> 90% removal

Phytotransformation created wetland and surrounding soil

Pondweed Coontail Arrowroot Hybrid poplars

TNT, RDX

Just beginning

Phytotransformation of groundwater and soil

Hybird Poplar

BTEX, TPH

Only in second year site program

In a pot culture experiment, grew five different species of Brassica (Brassica juncea, Brassica campestris, Brassica carinata, Brassica napus, and Brassica nigra) for screening possible accumulators of PTE, viz. zinc, cupper, nickel, and lead [30]. The plants were grown to maturity in a soil irrigated with sewage effluent for more than two decades in West Delhi, India. The soil analyses showed enhanced accumulation of zinc, cupper, nickel, and lead in this sewage-irrigated soil. Among all species, B. carinata showed the highest concentration (mg/kg) as well as uptake (g/pot) of nickel and lead at maturity. Although B. campestris showed a higher concentration of zinc in its shoots (stem plus leaf), B. carinata extracted the largest amount of this PTE due to greater biomass production. On the other hand, B. juncea extracted the largest amounts of copper from the soil. In general, the highest concentration and uptake of PTEs was observed in shoots compared to roots or seeds of the different 22

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species. Among the Brassica spp., B. carinata cv. DLSC1 emerged as the most promising, showing greater uptake of zinc, nickel, and lead, while B. juncea cv. Pusa Bold showed the highest uptake of cupper. The B. napus also showed promise, as it ranked second with respect to total uptake of lead, zinc, and nickel, and third for cupper. Total uptake of PTEs by Brassica spp. correlated negatively with available as well as the total soil PTE concentrations. Among the root parameters, root length emerged as the powerful parameter to dictate the uptake of PTEs by Brassica spp. They concluded that probably for the first time, B. carinata was reported as a promising phytoextractor for Zinc, Ni, and Pb, which performed better than B. juncea. The practical capability of five common crop plants, i.e. maize (Zea mights), sunflower (Helianthus annuus), canola (Brassica napus), barley (Hordeum vulgare) and white lupine (Lupinus albus) for their absorption and accumulation of lead, zinc, and cupper in six contaminated soil samples taken from pasture and arable soils around an old lead-zinc -mine in Spain were studied [31]. With the exception of the highest polluted sample, soil total PTEs concentration did not influence significantly biomass yields of each crop for the different growth substrates. The order found for the total PTEs accumulation rate (TMAR) in the crops was Zn>>Pb > Cu, with maize reaching the highest PTEs concentrations. Lead root concentrations were markedly higher than those of shoots for all the crops, while zinc and cupper were translocated to shoots more efficiently. Concentrations of PTEs extracted by EDTA and BCR sequential extraction were well correlated, in general, with both root PTEs content and TMAR. CaCl2-extracted zinc was well correlated with root concentrations, TMAR and, in some cases, with shoot contents. There are very few practical demonstrations of the phytoextraction of PTEs and PTEloids from soils and sediments beyond small-scale and short-term trials [32]. The two approaches used had been based on using hyperaccumulator species, such as Thlaspi caerulescens (lead, zinc, cadmium, nickel), Alyssum spp. (nickel, cupper), and Pteris vittata (arsenic) or fast-growing plants, such as Salix and Populus spp. that accumulate aboveaverage concentrations of only a smaller number of the more mobile trace elements (cadmium, zinc, boron). There is a high risk in marketing either approach as a technology or stand-alone solution to clean up contaminated soil. They added that there are particular uncertainties over the longer-term effectiveness of phytoextraction and associated environmental issues. Marginally contaminated agricultural soils provide the most likely land use where phytoextraction could be used as a polishing technology. An alternative and more useful practical approach in many situations currently would be to give more attention to crops selected for phytoexclusion, selecting crops that do not translocate high concentrations of PTEs to edible parts. They added that the wider practical applications of phytoremediation are too often overlooked. There was huge scope for cross-cutting other environmental agenda, with synergies that involve the recovery and provision of services from degraded landscapes 23

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and contaminated soils. An additional focus on biomass energy, improved biodiversity, watershed management, soil protection, carbon sequestration, and improved soil health is required for the justification and advancement of phytotechnologies. The plant biomass and PTEs shoot accumulation are key factors for efficient phytoextraction [33]. They found that inorganic mutagenesis improved the phytoextraction potential of sunflowers to cadmium, zinc and lead. The main goal of their present study was to assess the stability of sunflower mutants with improved biomass and PTEs accumulation properties in the 3rd and 4th generations. As compared to control plants, the best mutants showed the following improvement of PTEs extraction: Cd 3-5-fold, Zinc 4-5-fold, and Pb 35-fold. The best sunflower mutants showed either higher PTEs accumulation in shoots or enhanced PTEs accumulation in roots, suggesting to improved phytoextraction or rhizofiltration efficiency, respectively. Soil turnover and dilution methods was used to reduce the total concentration of PTEs in soil, but this technique might be not suitable for shallow soil depths less than 60 cm [34]. A 1.3-ha area contaminated by multiple PTEs (arsenic, chromium, nickel, cupper, and zinc) located in central Taiwan was selected for this large-area phytoremediation experiment. According to the Taiwan Environmental Protection Administration project contract, insitu selection experiments were conducted to select 12 potential species from 33 tested species for further large area experiment. After in-situ planting of 33 species of plants in the contaminated soil for 33 d, bougainvillea and cockscomb showed yellow-colored leaves and withered as the result of the toxicity of PTEs. Herbaceous plants could accumulate higher concentration of PTEs and had higher bioconcentration factor in relative to woody plants. Three weighting models of growth condition and the PTE-accumulated concentration of plants growing in the soil were evaluated and compared. There are very few practical demonstrations of the phytoextraction of PTEs and PTEloids from soils beyond small-scale and short-term trials [35]. They used two approaches based on using hyperaccumulator species, such as Thlaspi caerulescens (lead, zinc, cadmium, nickel), Alyssum spp. (nickel, cobalt), and Pteris vittata (arsenic) or fast-growing plants, such as Salix and Populus spp. that accumulate above-average concentrations of only a smaller number of the more mobile trace elements (cadmium, zinc, boron). Until they had advanced much more along the pathway of genetic isolation and transfer of hyperaccumulator traits into productive plants, there was a high risk in marketing either approach as a technology or standalone solution to clean up contaminated soil. There were particular uncertainties over the longer-term effectiveness of phytoextraction and associated environmental issues. Marginally contaminated agricultural soils provided the most likely land use where phytoextraction could be used as a polishing technology. An alternative and more useful practical approach in many situations currently 24

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would be to give more attention to crops selected for phytoexclusion, selecting crops that do not translocate high concentrations of PTEs to edible parts. They argued that the wider practical applications of phytoremediation are too often overlooked. There was huge scope for cross-cutting other environmental agenda, with synergies that involve the recovery and provision of services from degraded landscapes and contaminated soils. An additional focus on biomass energy, improved biodiversity, watershed management, soil protection, carbon sequestration, and improved soil health was required for the justification and advancement of phytotechnologies. 3. References [1]

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