A Review on HeavyMetals (as, Pb, And Hg) Uptake By

Hindawi Publishing CorporationInternational Journal of Chemical EngineeringVolume 2011, Article ID 939161, 31 pagesdoi:10.1155/2011/939161

Review Article

A Review on Heavy Metals (As, Pb, and Hg) Uptake byPlants through Phytoremediation

Bieby Voijant Tangahu,1 Siti Rozaimah Sheikh Abdullah,2 Hassan Basri,1

Mushrifah Idris,3 Nurina Anuar,2 and Muhammad Mukhlisin1

1 Department of Civil and Structural Engineering, Faculty of Engineering and Built Environment,Universiti Kebangsaan Malaysia, 43600 Bangin, Malaysia

2 Department of Chemical Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia,43600 Bangin, Malaysia

3 Tasik Chini Reasearch Centre, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangin, Malaysia

Correspondence should be addressed to Bieby Voijant Tangahu, [email protected]

Received 17 March 2011; Accepted 3 June 2011

Academic Editor: Hans-Jorg Bart

Copyright © 2011 Bieby Voijant Tangahu et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Heavy metals are among the most important sorts of contaminant in the environment. Several methods already used to cleanup the environment from these kinds of contaminants, but most of them are costly and difficult to get optimum results.Currently, phytoremediation is an effective and affordable technological solution used to extract or remove inactive metals andmetal pollutants from contaminated soil and water. This technology is environmental friendly and potentially cost effective. Thispaper aims to compile some information about heavy metals of arsenic, lead, and mercury (As, Pb, and Hg) sources, effectsand their treatment. It also reviews deeply about phytoremediation technology, including the heavy metal uptake mechanismsand several research studies associated about the topics. Additionally, it describes several sources and the effects of As, Pb, andHg on the environment, the advantages of this kind of technology for reducing them, and also heavy metal uptake mechanismsin phytoremediation technology as well as the factors affecting the uptake mechanisms. Some recommended plants which arecommonly used in phytoremediation and their capability to reduce the contaminant are also reported.

1. Introduction

Heavy metals are among the contaminants in the envi-ronment. Beside the natural activities, almost all humanactivities also have potential contribution to produce heavymetals as side effects. Migration of these contaminants intononcontaminated areas as dust or leachates through the soiland spreading of heavy metals containing sewage sludge are afew examples of events contributing towards contaminationof the ecosystems [1].

Several methods are already being used to clean up theenvironment from these kinds of contaminants, but most ofthem are costly and far away from their optimum perfor-mance. The chemical technologies generate large volumetricsludge and increase the costs [2]; chemical and thermalmethods are both technically difficult and expensive that allof these methods can also degrade the valuable component

of soils [3]. Conventionally, remediation of heavy-metal-contaminated soils involves either onsite management orexcavation and subsequent disposal to a landfill site. Thismethod of disposal solely shifts the contamination problemelsewhere along with the hazards associated with transporta-tion of contaminated soil and migration of contaminantsfrom landfill into an adjacent environment. Soil washingfor removing contaminated soil is an alternative way toexcavation and disposal to landfill. This method is very costyand produces a residue rich in heavy metals, which willrequire further treatment. Moreover, these physio-chemicaltechnologies used for soil remediation render the land usageas a medium for plant growth, as they remove all biologicalactivities [1].

Recent concerns regarding the environmental contami-nation have initiated the development of appropriate tech-nologies to assess the presence and mobility of metals in

2 International Journal of Chemical Engineering

soil [4], water, and wastewater. Presently, phytoremedia-tion has become an effective and affordable technologicalsolution used to extract or remove inactive metals andmetal pollutants from contaminated soil. Phytoremediationis the use of plants to clean up a contamination fromsoils, sediments, and water. This technology is environmentalfriendly and potentially costeffective. Plants with exceptionalmetal-accumulating capacity are known as hyperaccumu-lator plants [5]. Phytoremediation takes the advantage ofthe unique and selective uptake capabilities of plant rootsystems, together with the translocation, bioaccumulation,and contaminant degradation abilities of the entire plantbody [3].

Many species of plants have been successful in absorbingcontaminants such as lead, cadmium, chromium, arsenic,and various radionuclides from soils. One of phytoremedia-tion categories, phytoextraction, can be used to remove heavymetals from soil using its ability to uptake metals which areessential for plant growth (Fe, Mn, Zn, Cu, Mg, Mo, and Ni).Some metals with unknown biological function (Cd, Cr, Pb,Co, Ag, Se, Hg) can also be accumulated [5].

The objectives of this paper are to discuss the potentialof phytoremediation technique on treating heavy metal-contaminated side, to provide a brief view about heavymetals uptake mechanisms by plant, to give some descriptionabout the performance of several types of plants to uptakeheavy metals and to describe about the fate of heavy metalsin plant tissue, especially on arsenic (As), lead (Pb), andmercury (Hg). This study is related to a research project thataims to identify potential plants in tropical country such asMalaysia which can uptake heavy metal contaminants frompetrochemical wastewater.

2. Heavy Metals: Sources andEffect in the Environment

Heavy metals are conventionally defined as elements withmetallic properties and an atomic number >20. The mostcommon heavy metal contaminants are Cd, Cr, Cu, Hg, Pb,and Zn. Metals are natural components in soil [6]. Some ofthese metals are micronutrients necessary for plant growth,such as Zn, Cu, Mn, Ni, and Co, while others have unknownbiological function, such as Cd, Pb, and Hg [1].

Metal pollution has harmful effect on biological sys-tems and does not undergo biodegradation. Toxic heavymetals such as Pb, Co, Cd can be differentiated fromother pollutants, since they cannot be biodegraded but canbe accumulated in living organisms, thus causing variousdiseases and disorders even in relatively lower concentrations[7]. Heavy metals, with soil residence times of thousands ofyears, pose numerous health dangers to higher organisms.They are also known to have effect on plant growth, groundcover and have a negative impact on soil microflora [8]. It iswell known that heavy metals cannot be chemically degradedand need to be physically removed or be transformed intonontoxic compounds [1].

2.1. Arsenic (As). Arsenic (atomic number 33) is a silver-greybrittle crystalline solid with atomic weight of 74.9, specific

gravity 5.73, melting point 817◦C (at 28 atm), boiling point613◦C, and vapor pressure 1 mm Hg at 372◦C [9]. Arsenicis a semimetallic element with the chemical symbol “As”.Arsenic is odorless and tasteless. Arsenic can combine withother elements to form inorganic and organic arsenicals[10]. In the environment, arsenic is combined with oxygen,chlorine, and sulfur to form inorganic arsenic compounds.Inorganic arsenic compounds are mainly used to preservewood. Organic arsenic compounds are used as pesticides,primarily on cotton plants [11].

Arsenic exists in the −3, 0, +3, and +5 valence oxidationstates [9], and in a variety of chemical forms in natural watersand sediments [12]. Environmental forms include arseniousacids (H3AsO3, H3AsO3, H3AsO3

2−), arsenic acids (H3AsO4,H3AsO4

−, H3AsO42−), arsenites, arsenates, methylarsenic

acid, dimethylarsinic acid, and arsine. Two most commonforms in natural waters arsenite (AsO3

3−) and inorganicarsenate (AsO4

3−), referred as As3+ and As5+ [9]. From boththe biological and the toxicological points of view, arseniccompounds can be classified into three major groups. Thesegroups are inorganic arsenic compounds, organic arseniccompounds, and arsine gas [13].

It is a hard acid and preferentially complexes with oxidesand nitrogen. Trivalent arsenites predominate in moderatelyreducing anaerobic environments such as groundwater [9].The most common trivalent inorganic arsenic compoundsare arsenic trioxide, sodium arsenite, and arsenic trichloride[13]. Trivalent (+3) arsenates include As(OH)3, As(OH)4

−,AsO2OH2−, and AsO3

3− [9]. Arsenite (As(OH)3, As3+) ispredominant in reduced redox potential conditions [12].

Arsenic is one of the contaminants found in the envi-ronment which is notoriously toxic to man and other livingorganisms [14]. It is a highly toxic element that existsin various species, and the toxicity of arsenic dependson its species. The pH, redox conditions, surroundingmineral composition, and microbial activities affect the form(inorganic or organic) and the oxidation state of arsenic.It is generally accepted that the inorganic species, arsenite[As3+] and arsenate [As5+], are the predominant species inmost environments, although the organic ones might also bepresent [15].

In general, inorganic compounds of arsenic are regardedas more highly toxic than most organic forms which are lesstoxic [10, 14, 16, 17]. The trivalent compounds (arsenites)are more toxic than the pentavalent compounds (arsenates)[16, 17]. It has been reported that As3+ is 4 to 10 timesmore soluble in water than As5+. However, the trivalentmethylated arsenic species have been found to be more toxicthan inorganic arsenic because they are more efficient atcausing DNA breakdown [17]. Although As5+ tends to beless toxic compared to of As3+, it is thermodynamically morestable due to it predominates under normal conditions andbecomes the cause of major contaminant in ground water[14]. Arsenate which is in the pentavalent state (As5+) is alsoconsidered to be toxic and carcinogenic to human [18].

2.2. Lead (Pb). Lead (Pb), with atomic number 82, atomicweight 207.19, and a specific gravity of 11.34, is a bluishor silvery-grey metal with a melting point of 327.5◦C and a

International Journal of Chemical Engineering 3

boiling point at atmospheric pressure of 1740◦C. It has fournaturally occurring isotopes with atomic weights 208, 206,207 and 204 (in decreasing order of abundance). Despitethe fact that lead has four electrons on its valence shell, itstypical oxidation state is +2 rather than +4, since only two ofthe four electrons ionize easily. Apart from nitrate, chlorate,and chloride, most of the inorganic salts of lead2+ have poorsolubility in water [19]. Lead (Pb) exists in many forms inthe natural sources throughout the world and is now one ofthe most widely and evenly distributed trace metals. Soil andplants can be contaminated by lead from car exhaust, dust,and gases from various industrial sources.

Pb2+ was found to be acute toxic to human beings whenpresent in high amounts. Since Pb2+ is not biodegradable,once soil has become contaminated, it remains a long-termsource of Pb2+ exposure. Metal pollution has a harmful effecton biological systems and does not undergo biodegradation[7].

Soil can be contaminated with Pb from several othersources such as industrial sites, from leaded fuels, old leadplumbing pipes, or even old orchard sites in productionwhere lead arsenate is used. Lead accumulates in the upper8 inches of the soil and is highly immobile. Contamination islong-term. Without remedial action, high soil lead levels willnever return to normal [20].

In the environment, lead is known to be toxic to plants,animals, and microorganisms. Effects are generally limited toespecially contaminated areas [21]. Pb contamination in theenvironment exists as an insoluble form, and the toxic metalspose serious human health problem, namely, brain damageand retardation [5].

2.3. Mercury (Hg). Mercury is a naturally occurring metalthat is present in several forms. Metallic mercury is shiny,silver-white, odorless liquid. Mercury combines with otherelements, such as chlorine, sulfur, or oxygen, to form inor-ganic mercury compounds or salts, which are usually whitepowders or crystals. Mercury also combines with carbon tomake organic mercury compounds [22]. Mercury, which hasthe lowest melting point (−39◦C) of all the pure metals,is the only pure metal that is liquid at room temperature.However, due to its several physical and chemical advantagessuch as its low boiling point (357◦C) and easy vaporization,mercury is still an important material in many industrialproducts [23]. As any other metal, mercury could occur inthe soil in various forms. It dissolves as free ion or solublecomplex and is nonspecifically adsorbed by binding mainlydue to the electrostatic forces, chelated, and precipitated assulphide, carbonate, hydroxide, and phosphate. There arethree soluble forms of Hg in the soil environment. The mostreduced is Hg0 metal with the other two forms being ionicof mercurous ion Hg2

2+ and mercuric ion Hg2+, in oxidizingconditions especially at low pH. Hg+ ion is not stable underenvironmental conditions since it dismutates into Hg0 andHg2+. A second potential route for the conversion of mercuryin the soil is methylation to methyl or dimethyl mercury byanaerobic bacteria [24].

Mercury is a persistent environmental pollutant withbioaccumulation ability in fish, animals, and human beings

[23]. Mercury salts and organomercury compounds areamong the most poisonous substances in our environment.The mechanism and extent of toxicity depend strongly on thetype of compound and the redox state of mercury [25].

Environmental contamination due to mercury is causedby several industries, petrochemicals, minings, painting, andalso by agricultural sources such as fertilizer and fungicidalsprays [26]. Some of the more common sources of mercuryfound throughout the environment include but may notbe limited to the household bleach, acid, and causticchemicals (e.g., battery acid, household lye, muriatic acid(hydrochloric acid), sodium hydroxide, and sulfuric acid),instrumentation containing mercury (e.g., medical instru-ments, thermometers, barometers, and manometers), dentalamalgam (fillings), latex paint (manufactured prior to 1990),batteries, electric lighting (fluorescent lamps, incandescentwire filaments, mercury vapor lamps, ultraviolet lamps),pesticides, pharmaceuticals (e.g., nasal sprays, cosmetics,contact lens products), household detergents and cleaners,laboratory chemicals, inks and paper coatings, lubricationoils, wiring devices and switches, and textiles. Thoughmercury use in many of the above items being produced nowis restricted or banned, there are still some existing, olderproducts in use [22].

Terrestrial plants are generally insensitive to the harmfuleffects of mercury compounds; however, mercury is knownto affect photosynthesis and oxidative metabolism by inter-fering with electron transport in chloroplasts and mitochon-dria. Mercury also inhibits the activity of aquaporins andreduces plant water uptake [27].

Mercury and its compounds are cumulative toxins and insmall quantities are hazardous to human health. The majoreffects of mercury poisoning manifest as neurological andrenal disturbances as it can easily pass the blood-brain barrierand has effect on the brain [26].

3. Phytoremediation Technology

Phytoremediation techniques have been briefly depictedin many literatures or articles. The generic term “phy-toremediation” consists of the Greek prefix phyto (plant),attached to the Latin root remedium (to correct or removean evil) [28, 29]. Some definitions on phytoremediationthat have been described by several researchers are listed inTable 1.

Generally, according to the above researchers, phytore-mediation is defined as an emerging technology usingselected plants to clean up the contaminated environmentfrom hazardous contaminant to improve the environmentquality. Figure 1 depicts the uptake mechanisms of bothorganics and inorganics contaminants through phytore-mediation technology. For organics, it involves phytosta-bilization, rhizodegradation, rhizofiltration, phytodegrada-tion, and phytovolatilization. These mechanisms related toorganic contaminant property are not able to be absorbedinto the plant tissue. For inorganics, mechanisms which canbe involved are phytostabilization, rhizofiltration, phytoac-cumulation and phytovolatilization.


Table 1: Definition of phytoremediation.

No. Researchers Definition of phytoremediation

(1) [30] The use of plants to improve degraded environments

(2) [31]The use of plants, including trees and grasses, to remove, destroy or sequester hazardous contaminants frommedia such as air, water, and soil

(3) [24]The use of plants to remediate toxic chemicals found in contaminated soil, sludge, sediment, ground water,surface water, and wastewater

(4) [32]An emerging technology using specially selected and engineered metal accumulating plants forenvironmental cleanup

(5) [33] The use of vascular plants to remove pollutants from the environment or to render them harmless

(6) [3]

The engineered use of green plant to remove, contain, or render harmless such environmental contaminantsas heavy metals, trace elements, organic compounds, and radioactive compounds in soil or water. Thisdefinition includes all plant-influenced biological, chemical, and physical processes that aid in the uptake,sequestration, degradation, and metabolism of contaminants, either by plants or by the free-living organismsthat constitute the plant rhizosphere

(7) [29]Phytoremediation is the name given to a set of technologies that use different plants as a containment,destruction, or an extraction technique. Phytoremediation is an emerging technology that uses variousplants to degrade, extract, contain, or immobilize contaminants from soil and water

(8) [34]Phytoremediation in general implies the use of plants (in combination with their associatedmicroorganisms) to remove, degrade, or stabilize contaminants

Organiccontaminants

Medium Inorganiccontaminants

Remediatedcontaminant

Phytovolatilization Atmosphere Phytovolatilization

Remediatedcontaminant

Phytodegradation Plant PhytoaccumulationPhytoextraction

RhizofiltrationRhizofiltration

Rhizodegradation

Phytostabilization

SoilPhytostabilization

Contaminated media

Figure 1: Uptake mechanisms on phytoremediation technology. Source: [35].

Based on Figure 1, some certain essential processesinvolved in phytoremediation technology [29, 31] are phy-tostabilization and phytoextraction for inorganic contami-nants, and phytotransformation/phytodegradation, rhizofil-tration, and rhizodegradation for organic contaminants.

The root plants exudates to stabilize, demobilize andbind the contaminants in the soil matrix, thereby reducingtheir bioavailability. These all are called as phytostabilizationprocess. Certain plant species have used to immobilizecontaminants in the soil and ground water through absorp-tion and accumulation by roots, adsorption onto roots,or precipitation within the root zone. This process is fororganics and metals contaminants in soils, sediments, andsludges medium [29, 31].

Specific plant species can absorb and hyperaccumulatemetal contaminants and/or excess nutrients in harvestableroot and shoot tissue, from the growth substrate through

phytoextraction process. This is for metals, metalloids,radionuclides, nonmetals, and organics contaminants insoils, sediments, and sludges medium [29, 31].

Phytovolatilization process is the plants ability toabsorb and subsequently volatilize the contaminant intothe atmosphere. This process is for metal contaminantsin groundwater, soils, sediments, and sludges medium.Since phytotransformation/phytodegradation process is thebreakdown of contaminants taken up by plants throughmetabolic processes within the plant or the breakdown ofcontaminants externally to the plant through the effect ofcompounds produced by the plants. This process is forcomplex organic molecules that are degraded into simplermolecule contaminants in soils, sediments, sludges, andgroundwater medium [29, 31].

Plant roots take up metal contaminants and/or excessnutrients from growth substrates through rhizofiltration


(=root) process, the adsorption, or, precipitation onto plantroots or absorption into the roots of contaminants that arein solution surrounding the root zone. This process is formetals, excess nutrients, and radionuclide contaminants ingroundwater, surface water, and wastewater medium [29,31].

The breakdown of contaminants in the soil throughmicrobial activity that is enhanced by the presence of theroot zone is called rhizodegradation. This process usesmicroorganisms to consume and digest organic substancesfor nutrition and energy. Natural substances released bythe plant roots, sugars, alcohols, and acids, contain organiccarbon that provides food for soil microorganisms andestablish a dense root mass that takes up large quantities ofwater. This process is for organic substance contaminants insoil medium [29, 31].

4. Mechanisms of Heavy Metal Uptake by Plant

Contaminant uptake by plants and its mechanisms havebeen being explored by several researchers. It could beused to optimize the factors to improve the performanceof plant uptake. According to Sinha et al. [36], the plantsact both as “accumulators” and “excluders”. Accumulatorssurvive despite concentrating contaminants in their aerialtissues. They biodegrade or biotransform the contaminantsinto inert forms in their tissues. The excluders restrictcontaminant uptake into their biomass.

Plants have evolved highly specific and very efficientmechanisms to obtain essential micronutrients from theenvironment, even when present at low ppm levels. Plantroots, aided by plant-produced chelating agents and plant-induced pH changes and redox reactions, are able tosolubilize and take up micronutrients from very low levelsin the soil, even from nearly insoluble precipitates. Plantshave also evolved highly specific mechanisms to translocateand store micronutrients. These same mechanisms arealso involved in the uptake, translocation, and storage oftoxic elements, whose chemical properties simulate those ofessential elements. Thus, micronutrient uptake mechanismsare of great interest to phytoremediation [37].

The range of known transport mechanisms or specializedproteins embedded in the plant cell plasma membraneinvolved in ion uptake and translocation include (1) pro-ton pumps (′′-ATPases that consume energy and generateelectrochemical gradients), (2) co- and antitransporters(proteins that use the electrochemical gradients generatedby ′′-ATPases to drive the active uptake of ions), and (3)channels (proteins that facilitate the transport of ions intothe cell). Each transport mechanism is likely to take up arange of ions. A basic problem is the interaction of ionicspecies during uptake of various heavy metal contaminants.After uptake by roots, translocation into shoots is desirablebecause the harvest of root biomass is generally not feasible.Little is known regarding the forms in which metal ions aretransported from the roots to the shoots [37].

Plant uptake-translocation mechanisms are likely to beclosely regulated. Plants generally do not accumulate traceelements beyond near-term metabolic needs. And these

requirements are small ranging from 10 to 15 ppm of mosttrace elements suffice for most needs [37]. The exceptionsare “hyperaccumulator” plants, which can take up toxicmetal ions at levels in the thousands of ppm. Anotherissue is the form in which toxic metal ions are stored inplants, particularly in hyperaccumulating plants, and howthese plants avoid metal toxicity. Multiple mechanisms areinvolved. Storage in the vacuole appears to be a major one[37].

Water, evaporating from plant leaves, serves as a pump toabsorb nutrients and other soil substances into plant roots.This process, termed evapotranspiration, is responsible formoving contamination into the plant shoots as well. Sincecontamination is translocated from roots to the shoots,which are harvested, contamination is removed while leavingthe original soil undisturbed. Some plants that are usedin phytoextraction strategies are termed “hyperaccumula-tors.” They are plants that achieve a shoot-to-root metal-concentration ratio greater than one. Nonaccumulatingplants typically have a shoot-to-root ratio considerablyless than one. Ideally, hyperaccumulators should thrive intoxic environments, require little maintenance and producehigh biomass, although few plants perfectly fulfill theserequirements [38].

Metal accumulating plant species can concentrate heavymetals like Cd, Zn, Co, Mn, Ni, and Pb up to 100 or1000 times those taken up by nonaccumulator (excluder)plants. In most cases, microorganisms bacteria and fungi,living in the rhizosphere closely associated with plants, maycontribute to mobilize metal ions, increasing the bioavailablefraction. Their role in eliminating organic contaminantsis even more significant than that in case of inorganiccompounds [39, 40].

Heavy metal uptake by plant through phytoremediationtechnologies is using these mechanisms of phytoextraction,phytostabilisation, rhizofiltration, and phytovolatilization asshown in Figure 2.

4.1. Phytoextraction. Phytoextraction is the uptake/absorp-tion and translocation of contaminants by plant roots intothe above ground portions of the plants (shoots) that can beharvested and burned gaining energy and recycling the metalfrom the ash [28, 39–42].

4.2. Phytostabilisation. Phytostabilisation is the use of certainplant species to immobilize the contaminants in the soil andgroundwater through absorption and accumulation in planttissues, adsorption onto roots, or precipitation within theroot zone preventing their migration in soil, as well as theirmovement by erosion and deflation [28, 39–42].

4.3. Rhizofiltration. Rhizofiltration is the adsorption orprecipitation onto plant roots or absorption into andsequesterization in the roots of contaminants that are insolution surrounding the root zone by constructed wetlandfor cleaning up communal wastewater [28, 39–42].

4.4. Phytovolatilization. Phytovolatilization is the uptake andtranspiration of a contaminant by a plant, with release ofthe contaminant or a modified form of the contaminant to


Phytoaccumulation/phytoextraction

Phytovolatilization

Phytodegradation

Rhizodegradation

Contaminants uptake

Phytostabilization

Figure 2: The mechanisms of heavy metals uptake by plant through phytoremediation technology.

the atmosphere from the plant. Phytovolatilization occurs asgrowing trees and other plants take up water along with thecontaminants. Some of these contaminants can pass throughthe plants to the leaves and volatilize into the atmosphere atcomparatively low concentrations [28, 39–42].

Plants also perform an important secondary role in phys-ically stabilizing the soil with their root system, preventingerosion, protecting the soil surface, and reducing the impactof rain. At the same time, plant roots release nutrientsthat sustain a rich microbial community in the rhizosphere.Bacterial community composition in the rhizosphere isaffected by complex interactions between soil type, plantspecies, and root zone location. Microbial populations aregenerally higher in the rhizosphere than in the root-freesoil. This is due to a symbiotic relationship between soilmicroorganisms and plants. This symbiotic relationship canenhance some bioremediation processes. Plant roots alsomay provide surfaces for sorption or precipitation of metalcontaminants [27].

In phytoremediation, the root zone is of special interest.The contaminants can be absorbed by the root to be subse-quently stored or metabolised by the plant. Degradation ofcontaminants in the soil by plant enzymes exuded from theroots is another phytoremediation mechanism [43].

For many contaminants, passive uptake via micropores inthe root cell walls may be a major route into the root, wheredegradation can take place [3].

5. Factors Affecting the Uptake Mechanisms

There are several factors which can affect the uptakemechanism of heavy metals, as shown in Figure 3. By havingknowledge about these factors, the uptake performance byplant can be greatly improved.

5.1. The Plant Species. Plants species or varieties arescreened, and those with superior remediation propertiesare selected [31]. The uptake of a compound is affectedby plant species characteristic [44]. The success of thephytoextraction technique depends upon the identificationof suitable plant species that hyperaccumulate heavy metalsand produce large amounts of biomass using established cropproduction and management practices [24].

5.2. Properties of Medium. Agronomical practices are devel-oped to enhance remediation (pH adjustment, addition ofchelators, fertilizers) [31]. For example, the amount of leadabsorbed by plants is affected by the pH, organic matter, and


Plantspecies

Propertiesof medium

Root zone

Environmentalcondition

Chemicalproperties

of thecontaminant

Bioavailabilityof themetal

Chelatingagentadded

Uptakemechanisms

Figure 3: Factors which are affecting the uptake mechanisms ofheavy metals.

the phosphorus content of the soil. To reduce lead uptake byplants, the pH of the soil is adjusted with lime to a level of 6.5to 7.0 [20].

5.3. The Root Zone. The Root Zone is of special interestin phytoremediation. It can absorb contaminants and storeor metabolize it inside the plant tissue. Degradation ofcontaminants in the soil by plant enzymes exuded from theroots is another phytoremediation mechanism. A morpho-logical adaptation to drought stress is an increase in rootdiameter and reduced root elongation as a response to lesspermeability of the dried soil [43].

5.4. Vegetative Uptake. Vegetative Uptake is affected by theenvironmental conditions [44]. The temperature affectsgrowth substances and consequently root length. Rootstructure under field conditions differs from that undergreenhouse condition [43]. The success of phytoremediation,more specifically phytoextraction, depends on a contami-nant-specific hyperaccumulator [45]. Understanding massbalance analyses and the metabolic fate of pollutants in plantsare the keys to proving the applicability of phytoremediation[46].

Metal uptake by plants depends on the bioavailability ofthe metal in the water phase, which in turn depends on theretention time of the metal, as well as the interaction withother elements and substances in the water. Furthermore,when metals have been bound to the soil, the pH, redoxpotential, and organic matter content will all affect the ten-dency of the metal to exist in ionic and plant-available form.Plants will affect the soil through their ability to lower the pHand oxygenate the sediment, which affects the availability ofthe metals [47], increasing the bioavailability of heavy metals

by the addition of biodegradable physicochemical factors,such as chelating agents and micronutrients [34].

5.5. Addition of Chelating Agent. The increase of the uptakeof heavy metals by the energy crops can be influencedby increasing the bioavailability of heavy metals throughaddition of biodegradable physicochemical factors such aschelating agents, and micronutrients, and also by stimulatingthe heavy-metal-uptake capacity of the microbial commu-nity in and around the plant. This faster uptake of heavymetals will result in shorter and, therefore, less expensiveremediation periods. However, with the use of syntheticchelating agents, the risk of increased leaching must betaken into account [34]. The use of chelating agents inheavy-metal-contaminated soils could promote leaching ofthe contaminants into the soil. Since the bioavailability ofheavy metals in soils decreases above pH 5.5–6, the use of achelating agent is warranted, and may be required, in alkalinesoils. It was found that exposing plants to EDTA for a longerperiod (2 weeks) could improve metal translocation in planttissue as well as the overall phytoextraction performance.The application of a synthetic chelating agent (EDTA) at5 mmol/kg yielded positive results [8]. Plant roots exudeorganic acids such as citrate and oxalate, which affect thebioavailability of metals. In chelate-assisted phytoremedi-ation, synthetic chelating agents such as NTA and EDTAare added to enhance the phytoextraction of soil-pollutingheavy metals. The presence of a ligand affects the biouptakeof heavy metals through the formation of metal-ligandcomplexes and changes the potential to leach metals belowthe root zone [48].

6. Effectiveness of Heavy MetalsUptake by Plants

Several studies have described the performance of heavymetals uptake by plants. It is reported that phytoreme-diation technology is an alternative to treat heavy-metal-contaminated side which will be more admitted in order toremediate the environment. Table 2 lists some research doneto remediate heavy metals from contaminated soil, whileTable 3 lists some research conducted to remediate themfrom contaminated water and wastewater.

Based on the collected data from the phytoremediationresearch listed in Tables 2 and 3, the accumulation of heavymetals As, Pb, and Hg in plant tissue is summarized inrespective, Figures 4, 5, and 6.

According to Figure 4, the highest accumulation of As inplant tissue (the researchers have not detailed which part itis, but it might be the whole plant) occurs in Pteris vittataL. species. It can reach more than 0.7 mg As/g dry weightof plant. In plant root, the highest accumulation of As is inPopulus nigra, which can reach more than 0.2 mg As/g dryweight of plant root.

As can be seen in Figure 5, several plants could accumu-late Pb in their tissue of more than 50 mg/g dry weight ofplant. Among those species are species of Brassica campestrisL, Brassica carinata A. Br., Brassica juncea (L.) Czern. and


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HA

sO4·7

H2O

),th

em

ixtu

rew

hic

hco

nta

ined

50m

g/kg

ofA

s(w

etw

eigh

t)

Leer

sia

oryz

oide

s(r

ice-

cut

gras

s)—

terr

estr

ialp

lan

t

Th

ein

crea

sein

plan

tsi

zeis

mat

ched

bya

decr

ease

insh

oot

arse

nic

con

cen

trat

ion

.T

he

data

show

that

12,1

3,an

d13

mg/

m2

ofar

sen

icw

ere

abso

rbed

byth

esh

oots

at6,

10,a

nd

16w

eeks

,res

pec

tive

ly.S

ince

the

SRQ

and

PE

Cs

alle

xhib

itth

esa

me

dow

nwar

dtr

end

afte

r6

wk,

itis

sugg

este

dth

atpe

riod

icm

owin

gof

Leer

sia

oryz

oide

sgr

own

for

phyt

oext

ract

ion

purp

oses

onco

nta

min

ated

lan

dco

uld

mai

nta

inth

eh

igh

arse

nic

upt

ake

at6

wee

k.

(2)

[33]

Labo

rato

ry(p

otex

peri

men

t)(9

0da

ys)

Fly

ash

and

soil

mix

ture

s

Pb

asle

adn

itra

te,Z

nas

zin

csu

lfat

e,N

ias

nic

kel

sulf

ate,

Mn

asm

anga

nes

ech

lori

de,a

nd

Cu

asco

pper

sulf

ate

(100

0pp

mco

nce

ntr

atio

nea

ch(S

pike

d))

Scir

pus

litto

ralis

—se

mia

quat

ic

Th

em

etal

con

ten

tra

tios

BO

/soi

l(B

/S)

wer

eh

igh

erth

ansh

oot/

soil

rati

os(T

/S)

for

allt

he

met

als,

the

hig

hes

tbe

ing

for

Ni.

Met

alra

tios

BO

/wat

er(B

/W)

wer

eal

soh

igh

erth

ansh

oot/

wat

er(T

/W)

rati

os,b

ut

the

B/W

rati

ow

asm

axim

um

for

Zn

.All

the

met

als

exce

ptN

ish

owed

neg

ativ

eco

rrel

atio

nw

ith

nit

roge

nbu

tth

eyw

ere

alln

onsi

gnifi

can

t.H

owev

er,P

upt

ake

show

edpo

siti

veco

rrel

atio

ns

wit

hal

lth

em

etal

s,an

dal

lwer

esi

gnifi

can

tat

1%co

nfi

den

celim

it.

(3)

[49]

Fiel

dst

udy

(90

days

)So

il(a

gric

ult

ura

llan

dar

ea)

(Cu

,Cd,

Cr,

Zn

,Fe,

Ni,

Mn

,an

dP

b)

Wh

eat

(Tri

ticu

mae

stiv

umL.

)—te

rres

tria

lIn

dian

mu

star

d(B

rass

ica

cam

pest

ris

L.)—

terr

estr

ial

An

alys

esof

efflu

ents

and

soil

sam

ples

hav

esh

own

hig

hm

etal

con

ten

tth

anth

epe

rmis

sibl

elim

itex

cept

Pb.

An

alys

esof

plan

tsa

mpl

esh

ave

indi

cate

dth

em

axim

um

accu

mu

lati

onof

Fefo

llow

edby

Mn

and

Zn

inro

ot>

shoo

t>le

aves>

seed

s.M

axim

um

incr

ease

inph

otos

ynth

etic

pigm

ent

was

obse

rved

betw

een

30an

d60

days

wh

ilepr

otei

nco

nte

nt

was

fou

nd

max

imu

mbe

twee

n60

and

90da

ysof

grow

thpe

riod

inbo

thpl

ants

.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(4)

[5]

Labo

rato

ry(6

5da

ys)

Phy

toex

trac

tion

(soi

l)P

bby

usi

ng

stan

dard

Pb

solu

tion

s(7

5m

gP

b/1

kgso

il)

Cre

epin

gzi

nn

ia(A

lter

nant

hera

phyl

oxer

oide

s)—

aqu

atic

Mos

sro

se(S

anvi

talia

proc

umbe

ns)—

terr

estr

ial

Alli

gato

rw

eed

(Por

tula

cagr

andi

flora

)—aq

uat

ic

Alt

erna

nthe

raph

ylox

eroi

des

show

sth

eh

igh

est

lead

con

ten

tin

its

tiss

ues

.Th

ism

igh

tbe

cau

sed

byit

form

ing

lon

gst

olon

s,a

mas

sive

fibr

ous

root

syst

em,

and

larg

esu

rfac

ear

eaw

hic

hbe

nefi

tsth

eac

cum

ula

tion

ofle

ad.E

ffici

ency

proc

ess

30–8

0%.

(5)

[34]

Lite

ratu

rere

view

Soil

Cd,

Cr,

Cu

,Ni,

Pb,

and

Zn

Bra

ssic

aju

ncea

(In

dian

mu

star

d),B

rass

ica

rapa

(fiel

dm

ust

ard)

,an

dB

rass

ica

napu

s(r

ape)

—te

rres

tria

l

Bra

ssic

ara

paex

hib

ited

the

hig

hes

taffi

nit

yfo

rac

cum

ula

tin

gC

dan

dP

bfr

omth

eso

il,ei

ther

wit

h/w

ith

out

addi

tion

alu

seof

mob

ilizi

ng

soil

amen

dmen

ts.T

wo

Bra

ssic

asp

ecie

s(B

rass

ica

napu

san

dR

apha

nus

sati

vus)

wer

em

oder

atel

yto

lera

nt

wh

engr

own

ona

mu

lti-

met

alco

nta

min

ated

soil.

Th

edi

stri

buti

onof

hea

vym

etal

sin

the

orga

ns

ofcr

ops

decr

ease

din

the

follo

win

gor

der:

leav

es>

stem

s>ro

ots>

fru

itsh

ell>

seed

s.

(6)

[50]

Lab

orat

ory—

pot

exp

erim

ent

(12

days

)

Agr

opea

tan

dh

alf

stre

ngt

hH

oagl

and

solu

tion

Ars

enic

(As)

asof

sodi

um

(met

a-)

arse

nit

e(5

0u

M,1

50u

Man

d30

0u

M)

Bra

ssic

aju

ncea

var.

Var

un

aan

dP

usa

Bol

d—te

rres

tria

l

Incr

ease

/dec

reas

eof

anti

oxid

ant

enzy

mes

acti

viti

essh

owed

not

mu

chch

ange

sat

the

give

nco

nce

ntr

atio

ns.

Th

eda

tapr

esen

ted

indi

cate

sth

edi

ffer

enti

alre

spon

ses

inbo

thth

eva

riet

ies

and

also

that

the

incr

ease

dto

lera

nce

inP.

Bol

dm

aybe

due

toth

ede

fen

sive

role

ofan

tiox

idan

ten

zym

es,i

ndu

ctio

nof

MA

PK

,an

du

preg

ula

tion

ofP

CS

tran

scri

ptw

hic

his

resp

onsi

ble

for

the

prod

uct

ion

ofm

etal

-bin

din

gpe

ptid

es.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(7)

[51]

Fiel

dst

udy

(tri

als

toex

trac

th

eavy

met

als

from

two

con

tam

inat

edso

ils,o

ne

calc

areo

us

(5ye

ars)

and

one

acid

ic(2

year

s))

Phy

toex

trac

tion

(soi

l)C

dan

dZ

nW

illow

(Sal

ixvi

min

alis

)—te

rres

tria

l

Salix

had

perf

orm

edbe

tter

onth

eac

idic

soil

beca

use

ofla

rger

biom

ass

prod

uct

ion

and

hig

her

met

alco

nce

ntr

atio

ns

insh

oots

.Add

itio

nof

elem

enta

lsu

lphu

rto

the

soil

did

not

yiel

dan

yad

diti

onal

ben

efit

inth

elo

ng

term

,bu

tap

plic

atio

nof

anFe

chel

ate

impr

oved

the

biom

ass

prod

uct

ion

.Cd

and

Zn

con

cen

trat

ion

sw

ere

sign

ifica

ntl

yh

igh

erin

leav

esth

anst

ems.

On

both

soils

,con

cen

trat

ion

insh

oots

decr

ease

dw

ith

tim

e.

(8)

[52]

Labo

rato

ry(2

6da

ys)

Slu

dge-

amen

ded

soils

Cd

and

Zn

Rap

hanu

ssa

tivu

sL.

Th

isst

udy

has

show

nth

atcl

ear

evid

ence

ofas

ludg

e-dr

iven

plat

eau

resp

onse

inm

etal

upt

ake

bypl

ants

will

only

beob

tain

edw

hen

stu

dies

hav

efo

un

da

good

hype

rbol

icre

lati

onsh

ipbe

twee

nso

ilso

luti

onm

etal

con

cen

trat

ion

wit

hin

crea

sin

gsl

udg

eap

plic

atio

nra

tean

dca

nlin

kth

isto

apl

atea

ure

spon

sein

plan

tu

ptak

eof

met

als.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

Labo

rato

ry—

lysi

met

erpo

t(M

arch

1995

–Sep

tem

ber

1995

)So

il

Zn

asZ

nSO

4(5

0,1,

500,

2,00

0µ

g/g

(ppm

)Z

n.

and

2,00

0µ

g/g

(ppm

),an

d0µ

g/g

(ppm

)(c

ontr

ol)

rece

ived

nu

trie

nt

only

)

Hyb

rid

popl

ar(P

opul

ussp

.)—

terr

estr

ial

At

leve

lsof

zin

cab

ove

1,00

0µ

g/g

(ppm

)in

nu

trie

nt

adde

d,le

ach

ate

leve

lsw

ere

alw

ays

belo

w10

0µ

g/g

(ppm

)in

sam

ples

asth

ezi

nc

addi

tion

;th

ese

leve

lsin

crea

sed

the

follo

win

gda

yan

dth

ende

crea

sed

shar

ply

the

seco

nd

day

afte

rth

ezi

nc

addi

tion

,to

con

cen

trat

ion

sle

ssth

an10

0µ

g/g

(ppm

).T

he

zin

cco

nce

ntr

atio

nst

eadi

lyde

crea

sed

asth

epl

ants

appa

ren

tly

reab

sorb

edth

ezi

nc

asth

en

utr

ien

tw

ascy

cled

thro

ugh

the

pots

onsu

bseq

uen

tda

ys.T

he

root

tiss

ues

show

edm

uch

hig

her

con

cen

trat

ion

sof

accu

mu

late

dan

dse

ques

tere

dm

etal

than

did

the

abov

egr

oun

dpa

rts.

(9)

[3]

Labo

rato

ry(A

pril

1996

,2

mon

ths)

Soil

Zn

(160

µg/

gZ

n,

600µ

g/g

Zn

,an

d0µ

g/g

Zn

(con

trol

))

Eas

tern

gam

agra

ss(T

rips

acum

dact

yloi

des)

—te

rres

tria

l

Leac

hat

ean

alys

esfo

rzi

nc

indi

cate

that

init

ially

plan

tssu

bjec

ted

tobo

thle

vels

ofzi

nc

wer

ere

mov

ing

up

to70

%of

the

zin

cfr

omth

ele

ach

ate.

Th

epl

ants

rece

ivin

g16

0µ

g/g

Zn

had

grow

nco

nsi

dera

bly

and

wer

eal

mos

tth

esa

me

size

asth

eco

ntr

ols

(no

zin

c),b

ut

som

eof

the

mat

ure

leaf

blad

esw

ere

rolle

d;th

em

ean

zin

cre

mov

alra

tefo

rth

ese

plan

tsw

as50

%of

the

zin

cin

the

leac

hat

e.T

he

plan

tsre

ceiv

ing

600µ

g/g

Zn

wer

esm

alle

rth

anth

eco

ntr

ols,

thei

rco

lor

was

ada

rker

gree

n,m

ost

ofth

em

atu

rele

afbl

ades

wer

ero

lled,

and

the

mea

nzi

nc

rem

oval

rate

was

abou

t30

%of

the

zin

cin

the

leac

hat

e.

Soil

Pb

and

As

(up

to10

00µ

g/g

Pb

and

up

to20

0µ

g/g

As)

Hyb

rid

will

ow(S

alix

sp.)

and

hybr

idpo

plar

(Pop

ulus

sp.)

—te

rres

tria

l

Th

ew

illow

sw

ere

able

tore

mov

eap

prox

imat

ely

9.5%

ofth

eav

aila

ble

lead

and

abou

t1%

ofth

eto

tala

rsen

icfr

omth

eco

nta

min

ated

soil.

Th

ele

ssm

atu

rep

opla

rsre

mov

edab

out

1%of

the

avai

labl

ele

adan

d0.

1%of

the

tota

lars

enic

from

the

sam

eso

il.In

the

san

dex

peri

men

t,th

ew

illow

sto

oku

pab

out

40%

ofth

ead

min

iste

red

lead

and

30to

40%

ofth

ead

min

iste

red

arse

nic

.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(10)

[53]

Fiel

d(1

976–

2001

)(S

oil)

Non

esse

nti

al(C

d,N

i,P

b)an

des

sen

tial

hea

vym

etal

s(C

u,F

e,M

n,Z

n).

Th

ete

tras

odiu

msa

ltof

ED

TAw

asap

plie

dat

rate

sof

0,0.

5,1,

2g

ED

TAsa

lt/k

gsu

rfac

e(2

5cm

dept

h)

soil

Sun

flow

er(H

elia

nthu

san

nuus

L.)

and

Hyb

rid

popl

ar(P

opul

usde

ltoi

des

Mar

sh.x

P.ni

gra

L.)—

terr

estr

ial

For

sun

flow

er,t

he

1.0

g/kg

rate

ofch

elat

ead

diti

onre

sult

edin

max

imal

rem

oval

ofth

eth

ree

non

esse

nti

alh

eavy

met

als

(Cd,

Ni,

Pb)

.Upt

ake

ofth

ees

sen

tial

hea

vym

etal

sby

sun

flow

erw

aslit

tle

affec

ted

byth

eE

DTA

.Th

ele

aves

ofsu

nfl

ower

grow

nw

ith

1.0

gE

DTA

Na 4·2

H2O

/kg

soil

accu

mu

late

dm

ore

Cd,

Ni,

and

Pb

than

leav

esof

sun

flow

ergr

own

wit

hou

tth

eE

DTA

salt

.Rem

oval

ofth

en

on-e

ssen

tial

hea

vym

etal

sby

sun

flow

erw

asgr

eate

rat

the

hig

her

plan

tde

nsi

tyco

mpa

red

toth

elo

wer

one.

(11)

[54]

Labo

rato

ry

18di

ffer

ent

phyt

orem

edat

ion

trea

tmen

ts.I

.par

cel:

min

ew

aste

wit

hou

tfl

yas

h.C

ontr

olan

du

ntr

eate

dpl

ot.3

test

plan

ts.I

I.m

ine

was

te+

fly

ash

wit

hou

tlim

ing.

Con

trol

and

un

trea

ted

plot

.3te

stpl

ants

.III

.m

ine

was

te+

fly

ash

+lim

ing.

Con

trol

and

un

trea

ted

plot

.3te

stpl

ants

.

As,

Cd,

Mo,

Pb,

Zn

(soi

l)an

dA

s,C

d,P

b,N

i,Z

n(w

ater

)

Gra

sses

(mix

ture

ofse

lect

edsp

ecie

s),s

orgh

um

(Sor

ghum

bico

lor

L.)

and

Suda

ngr

ass

(Sor

ghum

suda

nens

e)—

terr

estr

ial

Th

ech

emic

alri

sks

ofth

eG

yon

gyos

oros

zisp

oils

wer

eas

sess

ed.T

he

maj

orco

nta

min

ants

ofth

ew

aste

min

ew

ere

iden

tifi

ed:P

b,Z

n,C

d,A

s.T

he

con

cept

ofth

ein

tegr

ated

phyt

orem

edia

tion

was

succ

essf

ully

appl

ied

tove

geta

teG

yon

gyos

oros

zisp

oil.

Th

ebi

omas

spr

odu

ctio

nw

asdi

ffer

ent,

depe

ndi

ng

onth

ete

chn

olog

yva

rian

t.T

he

hig

hes

tbi

omas

spr

odu

ctio

nw

asac

hie

ved,

wh

enm

ult

ileve

lrev

ital

izat

ion

was

also

appl

ied.

Th

ein

tegr

ated

phyt

orem

edia

tion

trea

tmen

tsn

oton

lypr

odu

ced

hig

hbi

omas

s,bu

tal

sode

crea

sed

the

hea

vym

etal

con

ten

tin

the

plan

ts.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(12)

[55]

Fiel

d(1

995–

1997

)So

ilN

i,C

u,C

d,Z

nW

illow

(Sal

ixsp

p.)—

terr

estr

ial

On

egr

oup

ofw

illow

had

rela

tive

lylo

wN

ian

dC

uin

the

bark

and

hig

hC

dan

dZ

nin

the

woo

d,w

ith

ago

odsu

rviv

alra

tean

dbi

omas

spr

odu

ctio

n.T

he

seco

nd

grou

pof

will

owh

adre

lati

vely

hig

hN

ian

dC

uin

the

bark

and

low

Cd

and

Zn

inth

ew

ood

and

per

form

edpo

orly

inte

rms

ofsu

rviv

alan

dbi

omas

spr

odu

ctio

n.

(13)

[8]

Labo

rato

ry(1

5M

ayan

d25

Sept

embe

r20

02)

Phy

toex

trac

tion

(soi

l)C

u,P

b,Z

n

Fesc

ue

(Fes

tuca

arun

dina

cea

Sch

reb.

),In

dian

mu

star

d(B

rass

ica

junc

ea(L

.)C

zern

.),a

nd

will

ow(S

alix

vim

inal

isL.

)—te

rres

tria

l

Th

eu

seof

the

free

acid

form

ofE

DTA

and

exp

osu

reti

me

ofon

eto

two

wee

ksbe

fore

har

vest

ing

incr

ease

dth

eco

nce

ntr

atio

nof

met

als

tran

sloc

ated

topl

ant

tiss

ues

.It

isfo

un

dn

osi

gnifi

can

tdi

ffer

ence

inh

eavy

met

alco

nce

ntr

atio

ns

inh

igh

eran

dlo

wer

soil

hor

izon

sbe

twee

nE

DTA

trea

ted

and

un

trea

ted

soils

.Exp

osin

gpl

ants

toE

DTA

for

alo

nge

rpe

riod

(2w

eeks

)co

uld

impr

ove

met

altr

ansl

ocat

ion

inpl

ant

tiss

ue

asw

ella

sth

eov

eral

lphy

toex

trac

tion

per

form

ance

.

(14)

[24]

Fiel

dex

per

imen

t(3

year

s)P

hyto

extr

acti

on(s

oil

con

ten

tw

ith

Hg)

Hg

(mea

nH

gco

nte

nt

ofth

eso

ilw

as29

.17µ

g/g

for

the

0–10

cmh

oriz

onan

d20

.32µ

g/g

for

10–4

0cm

hor

izon

wit

hle

ssth

an2%

ofth

eto

tal

Hg

bein

gbi

oava

ilabl

e)

Th

ree

agri

cult

ure

crop

plan

ts:T

riti

cum

aest

ivum

(wh

eat)

—te

rres

tria

lH

orde

umvu

lgar

e(b

arle

y)—

terr

estr

ial

Lupi

nus

lute

us(y

ello

wlu

pin

)—te

rres

tria

l

Th

ede

crea

seof

mea

nH

gco

nce

ntr

atio

nfr

om29

.17µ

gg–

1at

0–10

cmh

oriz

onto

20.3

2µ

gg–

1at

10–4

0cm

hor

izon

dem

onst

rate

dth

ean

thro

pog

enic

orig

inof

the

mer

cury

inth

eso

il.P

relim

inar

yre

sult

ssh

owth

atal

lcro

psex

trac

ted

mer

cury

,wit

hH

gpl

ant

con

cen

trat

ion

reac

hin

gu

pto

0.47

9µ

gg–

1in

wh

eat.

Th

em

ercu

ryco

nce

ntr

atio

nin

the

plan

tsac

cou

nte

dfo

rle

ssth

an3%

ofm

ercu

ryco

nce

ntr

atio

nin

the

soil.

Th

eH

gco

nce

ntr

atio

ns

inth

epl

ants

wer

esi

mila

ror

even

hig

her

than

that

ofth

ebi

oava

ilabl

eH

gin

the

soils

.Mer

cury

extr

acti

onyi

elds

reac

hed

up

to71

9m

g/h

afo

rba

rley

.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(15)

[56]

Pot

exp

erim

ent

(20

wee

ks)

Soil

from

was

tede

posi

tsof

the

lead

smel

ter

Pb

Agr

osti

sca

pilla

ris—

terr

estr

ial

Inoc

ula

tion

wit

hin

dige

nou

sor

non

indi

gen

ous

AM

Fin

this

expe

rim

ent

did

not

decr

ease

Pb

upt

ake

byth

eh

ost

inco

mpa

riso

nw

ith

non

myc

orrh

izal

plan

tsgr

own

inco

nta

min

ated

soil.

Itca

nbe

con

clu

ded

that

13m

onth

sof

subc

ult

uri

ng

inan

iner

tsu

bstr

ate

did

not

affec

tde

velo

pmen

tof

G.i

ntra

radi

ces

PH

5is

olat

edfr

omth

ew

aste

depo

sits

ofa

Pb

smel

ter

inco

nta

min

ated

soil

ofit

sor

igin

.T

he

inte

ract

ion

ofth

efu

ngu

sw

ith

the

hos

tpl

ant

was

chan

ged:

the

abili

tyof

the

linea

gecu

ltu

red

wit

hou

tH

Mto

supp

ort

plan

tgr

owth

inP

b-co

nta

min

ated

soil

was

decr

ease

d,w

hile

tran

sloc

atio

nof

Pb

from

plan

tro

ots

tosh

oots

incr

ease

d.

(16)

[38]

Fiel

dan

dgr

een

hou

seex

peri

men

ts

Phy

toex

trac

tion

(As-

and

Pb-

con

tam

inat

edso

il)

Ars

enic

(As)

and

lead

(Pb)

Ch

ines

eB

rake

Fern

s(P

teri

svi

ttat

a)—

terr

estr

ial

Indi

anM

ust

ard

(Bra

ssic

aju

ncea

)—te

rres

tria

l

Itap

pea

rsth

atE

DTA

isn

eces

sary

for

Pb

extr

acti

ondu

eto

the

low

soil

Pb

bioa

vaila

bilit

y.So

ilam

endm

ents

like

ED

TAar

en

eces

sary

beca

use

they

mob

ilize

soil

Pb,

mak

ing

itav

aila

ble

topl

ant

root

s.It

may

not

bead

visa

ble

toap

ply

ED

TAin

the

envi

ron

men

t,be

cau

seE

DTA

mob

ilize

sm

etal

s,w

hic

hm

ayle

ach

into

surr

oun

din

gpr

oper

tyof

grou

ndw

ater

.Th

epr

esen

ceof

oth

erm

etal

sth

atco

mpe

tefo

rE

DTA

may

incr

ease

the

amou

nt

ofE

DTA

requ

ired

for

Pb

rem

edia

tion

.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(17)

[27]

Labo

rato

ryex

peri

men

t,u

sin

gch

ambe

r(6

wee

ks)

Phy

tost

abili

zati

on(m

ercu

ry-c

onta

min

ated

soil

use

din

this

exp

erim

ents

was

obta

ined

from

ach

emic

alfa

ctor

ylo

cate

din

the

sou

thea

stpa

rtof

Pola

nd,

wh

ich

has

been

inop

erat

ion

for

over

50ye

ars)

Hg

Spec

ies

Fest

uca

rubr

a(r

edfe

scu

e)—

terr

estr

ialP

oapr

aten

sis

(mea

dow

gras

s)—

terr

estr

ial

Arm

orac

iala

path

ifol

ia(h

orse

radi

sh)—

terr

estr

ial

Hel

iant

hus

tube

rosu

s(J

eru

sale

msu

nfl

ower

)—te

rres

tria

lS.

vim

inal

is(w

illow

)—te

rres

tria

l

Th

eh

igh

est

con

cen

trat

ion

sof

mer

cury

wer

efo

un

dat

the

root

s,bu

ttr

ansl

ocat

ion

toth

eae

rial

part

also

occu

rred

.Mos

tof

the

plan

tsp

ecie

ste

sted

disp

laye

dgo

odgr

owth

onm

ercu

ryco

nta

min

ated

soil

and

sust

ain

eda

rich

mic

robi

alpo

pula

tion

inth

erh

izos

pher

e.A

nin

vers

eco

rrel

atio

nbe

twee

nth

en

um

ber

ofsu

lfu

ram

ino

acid

deco

mpo

sin

gba

cter

ia,a

nd

root

mer

cury

con

ten

tw

asob

serv

ed.

Th

ese

resu

lts

indi

cate

the

pote

nti

alfo

ru

sin

gso

me

spec

ies

ofpl

ants

totr

eat

mer

cury

-con

tam

inat

edso

ilth

rou

ghst

abili

zati

onra

ther

than

extr

acti

on.

(18)

[57]

Fiel

d(J

uly

and

Oct

ober

)P

hyto

extr

acti

onan

dph

ytos

tabi

lisat

ion

(soi

l)Z

n,C

u,C

ran

dC

d

Two

popl

arcl

ones

(Pop

ulus

delt

oide

sx

max

imow

iczi

i-cl

one

Eri

dan

oan

dP.

xeu

ram

eric

ana-

clon

eI-

214)

—te

rres

tria

l

Leaf

,ste

m,r

oot

and

woo

dycu

ttin

gbi

omas

ses

oftr

eate

dpl

ants

wer

esi

gnifi

can

tly

grea

ter

than

thos

ein

the

con

trol

sin

both

clon

es,e

xcep

tfo

rst

embi

omas

sat

the

begi

nn

ing

ofO

ctob

er.

Am

ong

the

fou

rh

eavy

met

als

(Zn

,Cu

,C

r,an

dC

d),o

nly

Zn

,Cu

,an

dC

rco

nce

ntr

atio

ns

inpl

ants

diff

ered

con

sist

entl

ybe

twee

ncl

ones

orso

iltr

eatm

ents

,wh

ileC

dle

vels

wer

eal

way

sbe

low

the

dete

ctio

nlim

its.

(19)

[58]

Fiel

dst

udy

and

labo

rato

ryex

per

imen

t(2

002-

2003

(fiel

dst

udy

),3

mon

ths

for

labo

rato

ryex

peri

men

t)

Soil

Fe,Z

n,P

b,C

u,N

i,C

r,M

nB

rach

ythe

cium

popu

leum

Th

ere

sult

sob

tain

edfr

omth

isst

udy

onB

.pop

uleu

mle

adto

the

infe

ren

ceth

atph

ysio

logi

cal/

bio-

chem

ical

anal

ysis

ofep

iphy

tic

bryo

phyt

esca

nse

rve

asco

st-e

ffec

tive

indi

cato

rs/m

onit

ors

for

the

envi

ron

men

talq

ual

ity

ofan

yar

ea,a

nd

onth

eba

sis

ofth

isin

form

atio

nap

prop

riat

est

eps

can

beta

ken

toim

prov

eth

eai

rqu

alit

yof

anar

ea.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(20)

[59]

Pot

expe

rim

ent

and

fiel

dtr

ial(

2004

-200

5fo

rpo

tex

per

imen

t,an

d20

05fi

eld

tria

l)

Phy

toex

trac

tion

and

phyt

osta

biliz

atio

n(s

oil)

As,

Co,

Cu

,Pb,

and

Zn

Th

ree

popl

arsp

ecie

s(P

opul

usal

ba,P

opul

usni

gra,

Popu

lus

trem

ula)

and

Salix

alba

—te

rres

tria

l

Trac

eel

emen

tco

nce

ntr

atio

ns

wer

em

uch

hig

her

inro

ots

than

inab

ove-

grou

nd

tiss

ues

,wit

hpa

rtic

ula

rly

hig

hco

nce

ntr

atio

ns

infi

ne

root

s.T

he

hig

hes

tac

cum

ula

tion

sw

ere

mea

sure

din

P.ni

gra

and

S.al

ba.I

nw

ood,

the

hig

hes

tco

nce

ntr

atio

ns

ofC

uan

dZ

nw

ere

inS.

alba

.Sal

ixal

bafo

liage

con

tain

edh

igh

est

con

cen

trat

ion

sof

As,

Cu

,Pb,

and

Zn

;lea

fZ

nco

nce

ntr

atio

nex

ceed

edth

ose

ofw

ood

byal

mos

t6

tim

es.T

he

over

allr

emov

alof

trac

eel

emen

tsw

ason

lysi

gnifi

can

tly

hig

her

inP.

alba

than

inS.

alba

;P.a

lba.

(21)

[60]

Pot

expe

rim

ent

and

fiel

dtr

ial(

2ye

ars

(200

4-20

05)

for

pot

expe

rim

ent

and

fiel

dtr

ialo

nM

ay–S

epte

mbe

r20

05)

Phy

toex

trac

tion

and

phyt

osta

biliz

atio

n(s

oil

(Pyr

ite

ore

con

tain

sm

ain

lypy

rite

(FeS

2),

less

eram

oun

tsof

chal

copy

rite

(Cu

FeS 2

),sp

hal

erit

e(Z

nS)

,m

agn

etit

e(F

e 3O

4),

and

vari

ous

trac

eel

emen

ts))

As,

Co,

Cu

,Pb

and

Zn

P.al

baL

.(w

hit

epo

plar

)—te

rres

tria

lP.

nigr

aL.

(bla

ckpo

plar

)—te

rres

tria

lP.

trem

ula

L.(E

uro

pea

nas

pen

)—te

rres

tria

lSa

lixal

baL

.(w

hit

ew

illow

)—te

rres

tria

l

Th

ere

sult

show

nth

ates

tabl

ish

men

tof

Popu

lus

and

Salix

spec

ies

atth

esi

teis

ach

ieva

ble

thro

ugh

ripp

ing

ofth

esu

rfac

e,m

inim

alti

llage

,som

em

ixin

gof

the

was

tes

wit

him

port

edso

il,ir

riga

tion

and

fert

ilise

rs.P

oten

tial

ly,t

he

elev

ated

con

cen

trat

ion

sof

Pb,

As

and

oth

erel

emen

tsco

uld

bele

ach

edfr

omth

ere

med

iate

dw

aste

sto

war

dsgr

oun

dwat

eror

oth

erre

cept

ors,

and

thes

efl

uxe

sco

uld

also

bein

flu

ence

dby

soil

amen

dmen

ts,

chan

ges

inth

erh

izos

pher

eor

both

.Im

mob

ilisa

tion

oftr

ace

elem

ents

inbo

thco

arse

and

fin

ero

ots

may

redu

cele

ach

ing,

part

icu

larl

yof

Cu

and

Zn

but

also

As

and

Pb.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(22)

[61]

Gre

enh

ouse

Phy

toex

trac

tion

and

phyt

osta

biliz

atio

n(s

oil)

Six

sedi

men

t-de

rive

dso

ilsw

ith

incr

easi

ng

fiel

dC

dle

vels

(0.9

–41.

4m

g/kg

)

Two

will

owcl

ones

(Sal

ixfr

agili

s“B

elgi

sch

Roo

d”an

dSa

lixvi

min

alis

“Aag

e”)—

terr

estr

ial

No

grow

thin

hib

itio

nw

asob

serv

edfo

rbo

thcl

ones

for

any

ofth

etr

eatm

ents

.D

ryw

eigh

tro

otbi

omas

san

dto

tals

hoo

tle

ngt

hw

ere

sign

ifica

ntl

ylo

wer

for

S.vi

min

alis

com

pare

dto

S.fr

agili

sfo

ral

ltr

eatm

ents

.Will

owfo

liar

Cd

con

cen

trat

ion

sw

ere

stro

ngl

yco

rrel

ated

wit

hso

ilan

dso

ilw

ater

Cd

con

cen

trat

ion

s.B

oth

clon

esex

hib

ited

hig

hac

cum

ula

tion

leve

lsof

Cd

and

Zn

inab

ove

grou

nd

plan

tpa

rts.

Cu

,Cr,

Pb,

Fe,

Mn

,an

dN

iwer

efo

un

dm

ain

lyin

the

root

s.B

ioco

nce

ntr

atio

nfa

ctor

sof

Cd

and

Zn

inth

ele

aves

wer

eth

eh

igh

est

for

the

trea

tmen

tsw

ith

the

low

est

soil

Cd

and

Zn

con

cen

trat

ion

.

(23)

[62]

Labo

rato

ryan

dfi

eld

Rh

izob

oxex

peri

men

tw

asu

sed

toin

vest

igat

eth

esh

ort-

term

effec

tof

will

owro

ots

onm

etal

avai

labi

lity

inox

ican

dan

oxic

sedi

men

t.Lo

nge

r-te

rmeff

ects

wer

eas

sess

edin

afi

eld

tria

l(s

oil)

Cd,

Zn

,Cu

,an

dP

bW

illow

(Sal

ixsp

p.)—

terr

estr

ial

Th

erh

izob

oxtr

ials

how

edth

atC

d,Z

n,

and

Cu

extr

acta

bilit

yin

the

rhiz

osph

ere

incr

ease

dw

hile

the

oppo

site

was

obse

rved

for

Pb.

Th

efi

eld

tria

lsh

owed

that

Cu

and

Pb,

but

not

Cd,

wer

em

ore

avai

labl

ein

the

root

zon

eaf

ter

wat

eran

dam

mon

ium

acet

ate

(pH

7)ex

trac

tion

com

pare

dw

ith

the

bulk

sedi

men

t.Se

dim

ent

inth

ero

otzo

ne

was

bett

erst

ruct

ure

dan

dag

greg

ated

and

thu

sm

ore

per

mea

ble

for

dow

nwar

dw

ater

flow

s,ca

usi

ng

leac

hin

gof

afr

acti

onof

the

met

als

and

sign

ifica

ntl

ylo

wer

tota

lco

nte

nts

ofC

d,C

u,a

nd

Pb.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(24)

[63]

Pot

expe

rim

ent

Phy

toex

trac

tion

(soi

l)

As

(as

Na 2

HA

sO4),

Cd

(as

CdC

l 2),

Pb

(as

Pb(

CH

3C

OO

) 2),

and

Zn

(as

Zn

(CH

3C

OO

) 2)

(100

mg

As/

kg,4

0m

gC

d/kg

,200

0m

gP

b/kg

,an

d20

00m

gZ

n/k

g)

Salix

spp.

—te

rres

tria

l

Alt

hou

ghA

san

dC

du

ptak

esl

igh

tly

incr

ease

din

Such

dol-

Zn

soil

com

pare

dto

Such

dol-

Pb

soil,

the

elem

ent

rem

oval

from

soil

was

sign

ifica

ntl

yh

igh

erin

Such

dol-

Pb

soil

due

toa

sign

ifica

nt

redu

ctio

nof

abov

egro

un

dbi

omas

syi

eld

inSu

chdo

l-Z

nso

il.T

he

yiel

dre

duct

ion

decr

ease

dth

eu

ptak

eof

plan

t-av

aila

ble

elem

ents

bybi

omas

s;th

us

hig

her

plan

t-av

aila

ble

port

ion

sof

As

and

Cd

wer

efo

un

din

Such

dol-

Zn

soil.

(25)

[64]

Fiel

dsu

rvey

:fro

m12

As-

con

tam

inat

edsi

tes

(Sep

tem

ber

toN

ovem

ber

2003

)

Fiel

dst

udy

:co

nta

min

ated

soil

As

Sam

ples

of24

fern

spec

ies

belo

ngi

ng

to16

gen

era

and

11fa

mili

esas

wel

las

thei

ras

soci

ated

soils

wer

eco

llect

ed—

terr

estr

ial

Pte

ris

mul

tifid

aan

dP.

oshi

men

sis

can

(hyp

er-)

accu

mu

late

As

inth

eir

fron

dsw

ith

hig

hco

nce

ntr

atio

ns.

Tota

lAs

con

cen

trat

ion

sin

soils

asso

ciat

edw

ith

P.m

ulti

fida

and

P.os

him

ensi

sva

ried

from

1262

to47

,235

mg/

kg,b

ut

the

DT

PA-e

xtra

ctab

leA

sco

nce

ntr

atio

ns

wer

ere

lati

vely

low

,wit

ha

max

imu

mof

65m

g/kg

.Alt

hou

ghA

sco

nce

ntr

atio

ns

inth

efr

onds

ofP.

oshi

men

sis

wer

eco

mpa

rati

vely

low

erth

anth

ose

ofP.

mul

tifid

a,it

sh

igh

abov

egr

oun

dbi

omas

sm

akes

itm

ore

suit

able

for

phyt

orem

edia

tin

gA

s-co

nta

min

ated

soils

.


Ta

ble

2:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsn

ame

and

typ

eR

esu

lt

(26)

[65]

Fiel

dsu

rvey

(con

tam

inat

edsi

tesi

nce

1976

;th

esa

mpl

ew

asta

ken

in20

06)

Soil

Cu

,Pb,

Cd,

and

Zn

Paul

owni

fort

unei

(see

m)

Hem

s

Inth

erh

izos

pher

ean

dbu

lkso

ilsof

P.fo

rtun

ei,a

llph

ysic

o-ch

emic

alpr

oper

ties

incr

ease

dw

ith

the

reve

geta

tion

tim

e.T

he

tota

lcon

ten

tsof

Cu

,Pb,

Cd,

and

Zn

also

con

sist

entl

yin

crea

sed

wit

hth

ere

-veg

etat

ion

tim

e;m

oreo

ver,

rhiz

osph

ere

soils

accu

mu

late

dm

ore

hea

vym

etal

sth

anbu

lkso

ilsw

ith

the

reve

geta

tion

tim

e.In

the

rhiz

osph

ere

soils

ofP.

fort

unel

,th

eim

mob

ility

and

bioa

vaila

bilit

yof

hea

vym

etal

sw

ere

enh

ance

d.In

the

rhiz

osph

ere

mic

roen

viro

nm

ent,

pH,O

M,a

nd

EC

wer

eim

port

ant

fact

ors

affec

tin

gth

edi

stri

buti

onof

hea

vym

etal

frac

tion

s.A

mon

gdi

ffer

ent

hea

vym

etal

frac

tion

s,th

eex

chan

geab

lean

dor

gan

ical

lybo

un

dfr

acti

ons

wer

eea

sily

avai

labl

efo

rP.

fort

unei

,bu

tca

rbon

ate,

Fe–M

nox

ide,

and

resi

dual

frac

tion

sw

ere

not

easi

lyav

aila

ble

for

P.fo

rtun

ei.

(27)

[4]

Gre

enh

ouse

pot

exp

erim

ent

(Au

gust

-Sep

tem

ber

2002

)

Soil

was

sam

pled

intw

osi

tes:

con

tam

inat

edso

ilw

asta

ken

nea

rro

adw

ith

hea

vytr

affic

and

clea

nso

ilw

asta

ken

from

park

prot

ecte

dfr

omth

ero

adby

build

ings

Ag,

As,

B,B

a,B

e,B

i,C

a,C

d,C

o,C

r,C

u,F

e,K

,Li,

Mg,

Mn

,Mo,

Na,

Ni,

P,P

b,R

b,S,

Sb,S

e,Sr

,Th

,T

i,T

l,U

,Van

dZ

n

Wh

eat

Trit

icum

vulg

are,

sort

Um

anka

—te

rres

tria

l

Con

cen

trat

ion

sof

Ag,

Cd,

Cu

,Pb,

Sb,a

nd

Zn

inth

ein

itia

lcon

tam

inat

edso

ilw

ere

3–6

tim

esh

igh

erth

anth

ose

inth

ein

itia

lcle

anso

il.In

part

icu

lar,

con

ten

tsof

Cu

,Mo,

Ni,

Pb,

Sban

dZ

nin

root

sof

the

wh

eat

grow

nin

the

con

tam

inat

edso

ilw

ere

hig

her

than

thos

ein

the

root

sof

the

plan

tsgr

own

inth

ecl

ean

soil.

Mor

eove

r,al

lth

eel

emen

tsex

cept

Pb

tran

sfer

red

mor

eea

sily

from

root

sto

leav

es.

(28)

[66]

Fiel

dex

per

imen

t(1

55da

ys(M

ay–N

ovem

ber)

)So

il(a

gric

ult

ura

lsoi

l)C

d,C

r,P

b,A

s,an

dH

gR

ice

(Ory

zasa

tiva

L.)—

terr

estr

ial

Th

ere

sult

ssh

owed

the

rice

grai

nco

nta

ined

sign

ifica

ntl

ylo

wer

amou

nts

offi

vem

etal

sth

anst

raw

and

root

inal

lsam

plin

gsi

tes.

Ric

ero

otac

cum

ula

ted

Cd,

As,

and

Hg

from

the

padd

yso

il.T

he

rice

plan

ttr

ansp

orte

dA

sve

ryw

eakl

y,w

her

eas

Hg

was

tran

spor

ted

mos

tea

sily

into

the

stra

wan

dgr

ain

amon

gst

udi

edh

eavy

met

als.

No.

(7),

(10)

,(12

),(1

8),(

22),

(23)

adap

ted

from

no.

(20)

.Phy

tore

med

iati

onB

iblio

grap

hy,A

nn

otat

edB

iblio

grap

hyon

Phy

tore

med

iati

onpr

epar

edby

Mar

kC

olem

an,B

iolo

gica

lSci

enti

st,U

SDA

Fore

stSe

rvic

eSo

uth

ern

Res

earc

hSt

atio

nan

dR

onal

dS.

Zal

esny

Jr.,

Res

earc

hP

lan

tG

enet

icis

t,U

SDA

Fore

stSe

rvic

eN

orth

Cen

tral

Res

earc

hSt

atio

nM

ay1,

2006

.


Ta

ble

3:P

hyto

rem

edia

tion

stu

dyon

wat

erm

ediu

m(h

ydro

pon

ic).

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(1)

[67]

Fiel

dst

udy

(Oct

ober

–Ju

ly20

05)

Wat

erof

Tasi

kC

hin

iC

d,C

u,a

nd

Pb

Five

aqu

atic

plan

tsp

ecie

s,Le

piro

nia

arti

cula

ta,

Pand

anus

helic

opus

,Sci

rpus

gros

sus,

Cab

omba

furc

ata,

and

Nel

umbo

nuci

fera

—aq

uat

ic

Th

eh

igh

est

con

cen

trat

ion

ofh

eavy

met

als

amon

gth

eaq

uat

icpl

ants

and

plan

tpa

rts

was

fou

nd

inth

ero

ots

ofS.

gros

sus.

Th

eco

nce

ntr

atio

ns

ofC

din

the

leav

esan

dst

ems

ofsu

bmer

ged

aqu

atic

plan

t,C

.fur

cata

,wer

eh

igh

erth

anth

atin

the

leav

esan

dst

ems

ofem

erge

nt

aqu

atic

plan

tan

dfl

oati

ng

leaf

plan

t.T

he

con

cen

trat

ion

ofC

uin

the

stem

ofC

.fur

cata

was

grea

ter

than

that

inth

ele

af,w

hile

the

con

cen

trat

ion

ofC

dw

asm

ore

inth

ele

afth

anth

atin

the

stem

.Th

eh

eavy

met

alco

nte

nts

ofth

eaq

uat

icpl

ants

wer

ein

desc

endi

ng

orde

rof

Pb>

Cu>

Cd.

Th

eh

igh

est

inte

rnal

tran

sloc

atio

nw

asfo

un

din

P.he

licop

us,w

hile

the

low

est

inte

rnal

tran

sloc

atio

nw

asfo

un

din

S.gr

ossu

s.

(2)

[68]

Lab

orat

ory

(pot

expe

rim

ent)

/14

days

Hyd

ropo

nic

As

and

Seas

Na 2

HA

sO4·7

H2O

and

Na 2

SeO

3/0

,0.7

3,2.

5,4.

27,5

.00

mg/

L

Ch

ines

ebr

ake

fern

(Pte

ris

vitt

ata

L.)—

terr

estr

ial

At

low

leve

lsof

Se,A

sen

han

ced

both

Seu

ptak

ean

dth

etr

ansl

ocat

ion

ofSe

from

root

sto

fron

ds.A

th

igh

erle

vels

ofSe

,As

supp

ress

edth

eu

ptak

eof

Se.T

hes

ere

sult

ssu

gges

tth

atA

sse

rves

tobo

thst

imu

late

and

supp

ress

Seu

ptak

e.T

he

resu

ltis

also

inag

reem

ent

wit

hth

ew

ell-

know

nfa

ctth

atSe

isan

elem

ent

wit

hbo

thbe

nefi

cial

and

toxi

cpr

oper

ties

.Th

eeff

ect

can

chan

gefr

ombe

nefi

cial

toto

xic

base

don

the

con

cen

trat

ion

ofSe

inpl

ants

.


Ta

ble

3:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(3)

[47]

Labo

rato

ryan

dFi

eld

stu

dy:w

etla

nd-

pon

dsy

stem

(Lab

orat

ory

scal

e:3

days

cult

ivat

ive

and

84h

ours

expo

sure

)

Fiel

dst

udy

:co

nta

min

ated

soil.

Med

ium

ofla

bora

tory

scal

eex

per

imen

t:L

0.1%

Hoa

glan

dso

luti

on

Fiel

dst

udy

:Zn

,Cu

,Cd,

and

Pb.

Labo

rato

ry:

Zn

Cl 2

,Cu

Cl 2

,CdC

l 2,

and

Pb(

NO

3) 2

(mix

ture

of20

µm

olZ

n,0

.5µ

mol

Cu

,1.5µ

mol

Cd,

and

1.5µ

mol

Pb/

L)

Pota

mog

eton

nata

nsL.

-aqu

atic

Lem

nagi

bba

L.-a

quat

icA

lism

apl

anta

go-a

quat

ica

L.-

aqu

atic

Sagi

ttar

iasa

gitt

ifol

iaL.

-aqu

atic

Junc

useff

usus

L-aq

uat

icLe

mna

min

orL.

-aqu

atic

Elo

dea

cana

dens

isM

ichx

.-aq

uat

icLy

thru

msa

licar

iaL.

-aqu

atic

Pha

lari

sar

undi

nace

aL.

-aqu

atic

Impa

tien

spa

rvifl

ora

DC

.-te

rres

tria

lUrt

ica

dioi

caL.

—te

rres

tria

lFi

lipen

dula

ulm

aria

L.-a

quat

icP.

nata

ns-a

quat

icA

.pl

anta

go-a

quat

ica-

aqu

atic

F.ul

mar

ina-

aqu

atic

Th

eaq

uat

icpl

ants

seem

toh

ave

ah

igh

erm

etal

accu

mu

lati

onca

paci

tyin

shoo

tsth

ante

rres

tria

lpla

nts

.T

his

may

bedu

eto

the

capa

city

ofaq

uat

icpl

ants

tota

keu

pby

shoo

tdi

rect

lyfr

omth

ew

ater

.Wh

ensu

bmer

sed

and

free

-floa

tin

gpl

ants

are

acti

vely

grow

ing

and

accu

mu

lati

ng

met

als

dire

ctly

from

the

wat

er,t

hey

will

fun

ctio

nas

aneff

ecti

vefi

lter

inst

orm

wat

ertr

eatm

ent.

Em

erge

nt

plan

tsin

gen

eral

med

iate

the

bin

din

gof

thes

em

etal

sin

the

sedi

men

t.A

lso,

the

terr

estr

ialp

lan

tsh

ave

the

capa

city

tobi

nd

Cd

and

Zn

toth

eir

root

s,an

d;th

eref

ore,

they

can

med

iate

ago

odst

abili

zati

onof

thes

em

etal

sin

soil.

(4)

[32]

Labo

rato

ry(1

5da

ys)

Hyd

ropo

nic

Pb

as(P

b(N

O3) 2

)In

dian

mu

star

d(B

rass

ica

junc

eava

r.m

egar

rhiz

a)—

terr

estr

ial

Bra

ssic

aju

ncea

ison

epl

ant

wh

ich

accu

mu

late

sh

igh

leve

lsof

Pb

and

oth

erh

eavy

met

als.

Th

ere

sult

sin

dica

teth

atle

adn

itra

teob

viou

sly

inh

ibit

sth

ero

ot,h

ypoc

otyl

s,an

dsh

oot

grow

thof

Bra

ssic

aju

ncea

atth

eco

nce

ntr

atio

nof

10−3

MP

b2+.

Bra

ssic

aju

ncea

has

the

abili

tyto

accu

mu

late

Pb

prim

arily

init

sro

ots,

tran

spor

t,an

dco

nce

ntr

ate

itin

its

hyp

ocot

yls

and

shoo

tsin

mu

chle

sser

con

cen

trat

ion

s.


Ta

ble

3:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(5)

[30]

Labo

rato

ry(5

days

)P

hyto

filt

rati

on(w

ater

)M

ercu

ryas

HgC

l 2(0

,0.

05,0

.5,1

,2.5

,5,

10m

g/L)

Indi

anm

ust

ard

(Bra

ssic

aju

ncea

)—te

rres

tria

l

Roo

ts-c

once

ntr

ated

Hg

100–

270

tim

es(o

na

dry

wei

ght

basi

s)ab

ove

init

ials

olu

tion

con

cen

trat

ion

s.M

ercu

ryw

asm

ore

toxi

cto

plan

tsat

5an

d10

mg/

L.T

he

plan

tstr

ansl

ocat

edlit

tle

Hg

toth

esh

oots

,w

hic

hac

cou

nte

dfo

rju

st0.

7–2%

ofth

eto

talH

gin

the

plan

ts.M

ost

Hg

vola

tilis

atio

noc

curr

edfr

omth

ero

ots.

Vol

atili

sed

Hg

was

pred

omin

antl

yin

the

Hg(

0)va

pou

rfo

rm.V

olat

ilisa

tion

was

dep

enda

nt

onro

otu

ptak

ean

dab

sorp

tion

ofH

gfr

omth

eam

bien

tso

luti

on.

Effi

cien

cypr

oces

s>

95%

.

(6)

[69]

Labo

rato

ryH

ydro

pon

icA

rsen

ate

(As(

V))

and

dim

ethy

lars

inic

acid

(DM

AA

)

Du

ckw

eed

(Spi

rode

lapo

lyrh

iza

L.)—

aqu

atic

Th

ere

sult

ssh

owth

atn

oton

lyin

tern

aliz

ed,b

ut

also

surf

ace-

adso

rbed

arse

nic

(mos

tly

arse

nat

e)co

ntr

ibu

tes

sign

ifica

ntl

yto

the

tota

lam

oun

tof

arse

nic

upt

ake

inaq

uat

icm

acro

phyt

eS.

poly

rhiz

aL

.Th

ear

sen

icu

ptak

ein

S.po

lyrh

iza

L.o

ccu

rred

thro

ugh

the

phos

phat

eu

ptak

epa

thw

ayas

wel

las

byph

ysic

och

emic

alad

sorp

tion

onFe

plaq

ues

ofpl

ant’s

surf

aces

.T

he

arse

nat

eu

ptak

ein

the

plan

tis

rela

ted

toth

eFe

ion

and

phos

phat

eco

nce

ntr

atio

ns

incu

ltu

rem

ediu

mw

hile

DM

AA

was

not

.


Ta

ble

3:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(7)

[2]

Lab

orat

ory

(th

eco

nta

ctti

mes

of25

–200

min

wer

ese

lect

edfo

rth

em

etal

solu

tion

s(C

o=

1.00

mM

)w

ith

2.0

gbi

omas

s/la

tth

eob

tain

edop

tim

alpH

sfo

rea

chm

etal

ion

from

the

prev

iou

sst

udy

)

Ads

orpt

ion

(wat

er)

Th

eH

g2+,C

r3+,C

r6+

and

Cu

2+st

ock

solu

tion

sw

ere

prep

ared

bydi

ssol

vin

gth

eir

corr

espo

ndi

ng

salt

s,vi

z.H

gCl 2

,CrC

l 3·3

H2O

,K

2C

r 2O

7,C

uC

l 2(a

nal

ytic

algr

ade

from

Mer

ck)

indi

still

edw

ater

(pH

valu

esw

ere

alm

ost

7.0,

5.0,

3.0

and

6.0

for

Hg2+

,Cr3+

,Cr6+

and

Cu

2+,r

esp

ecti

vely

)

Lem

nam

inor

—aq

uat

ic

Th

ep

oten

tiom

etri

cti

trat

ion

can

beu

sefu

lto

stu

dyth

epr

etre

atm

ent

proc

ess

ofbi

omas

s(L

.min

or)

usi

ng

the

acid

ican

dal

kali

agen

ts,t

he

Qm

axan

dK

Lva

lues

tore

mov

eH

g(II

),C

r(II

I),C

r(V

I),a

nd

Cu

(II)

from

the

aqu

eou

sso

luti

onby

the

acti

vate

dL

.min

orat

the

alka

liso

luti

onan

dby

CaC

l 2/M

gCl 2

/NaC

lw

ith

1:1

:1m

olar

rati

ow

ere

hig

her

than

thos

efo

rth

ere

fere

nce

one

atth

esa

me

con

diti

ons,

the

rem

oval

perc

ents

ofm

etal

ion

sby

no.

AC

SL

.m

inor

was

hig

her

than

AC

Son

eat

the

pre-

trea

tmen

tpH

sbe

fore

7.0,

but

itw

ash

igh

erby

AC

Sbi

omas

sth

ann

o.A

CS

one

atth

epr

e-tr

eatm

ent

pHs

afte

r7.

0.

(8)

[70]

Lab

orat

ory

(see

dlin

g2

wee

ksan

dtr

eatm

ent

2w

eeks

)H

ydro

pon

ic

Hg

and

Au

(0,5

0,10

0,an

d20

0u

MH

g(a

sH

g(C

H3C

OO

) 2)

and

0an

d50

uM

Au

(as

KA

uC

l 4)

inhy

drop

onic

s)

Chi

lops

islin

eari

s(C

av.)

swee

t—te

rres

tria

l

Th

eda

tash

owed

that

Au

equ

imol

arto

Hg

redu

ced

the

Hg

toxi

city

.Th

eco

nce

ntr

atio

nof

Au

and

Hg

insh

oots

indi

cate

dth

atC

.lin

eari

sab

sorb

edan

dtr

ansl

ocat

edbo

thA

uan

dH

gat

hig

her

con

cen

trat

ion

s,co

mpa

red

tore

port

edda

ta.T

he

data

show

edth

atth

etr

eatm

ents

prod

uce

dst

ruct

ura

lalt

erat

ion

sin

both

the

vasc

ula

rcy

linde

ran

dth

eco

rtex

.At

the

hig

hes

tco

nce

ntr

atio

n,H

gpr

odu

ced

abr

eakd

own

ofth

esp

ongy

pare

nch

yma.


Ta

ble

3:C

onti

nu

ed.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(9)

[71]

Labo

rato

ry(3

0da

ys)

Phy

toex

trac

tion

(wat

er)

Mer

cury

asH

gSO

4(0

,0.

5an

d2

mg/

L)

Wat

erhy

acin

th(E

icho

rnia

cras

sipe

s)—

aqu

atic

Wat

erle

ttu

ce(P

isti

ast

rati

otes

)—aq

uat

icZ

ebra

rush

(Sci

rpus

tabe

rnae

mon

tani

)—se

mi

aqu

atic

Taro

(Col

ocas

iaes

cule

nta)

—aq

uat

ic

Th

eh

igh

erth

em

ercu

ryco

nce

ntr

atio

n,t

he

grea

ter

the

amou

nt

ofm

ercu

ryre

mov

edby

the

plan

ts.T

he

larg

est

upt

ake

and

accu

mu

lati

onca

pabi

lity

isfo

rw

ater

lett

uce

,fol

low

edby

wat

erhy

acin

th,

taro

and

rush

,res

pect

ivel

y.

(10)

[72]

Lab

orat

ory—

(pot

expe

rim

ent

(10

days

))H

ydro

pon

ic

As

and

Se(0

,150

,or

300

uM

ofar

sen

at(N

a 2H

AsO

4·7

H2O

)in

the

pres

ence

of0,

5or

10u

Mof

sele

nat

(Na 2

SeO

4))

Pte

ris

vita

tta

L.—

terr

estr

ial

App

licat

ion

of5

uM

Seen

han

ced

As

con

cen

trat

ion

byP.

vitt

ata

fron

dsby

7–45

%.A

t5

uM

,Se

acte

das

anan

tiox

idan

t,in

hib

itin

glip

idp

erox

idat

ion

(red

uce

dby

26–4

2%in

the

fron

ds)

via

incr

ease

dle

vels

ofth

iols

and

glu

tath

ion

e(i

ncr

ease

dby

24%

inth

efr

onds

).T

he

resu

lts

sugg

est

that

Seis

eith

eran

anti

oxid

ant,

orit

acti

vate

spl

ant

prot

ecti

vem

ech

anis

ms,

ther

eby

alle

viat

ing

oxid

ativ

est

ress

and

impr

ovin

gar

sen

icu

ptak

ein

P.vi

ttat

a.

International Journal of Chemical Engineering 25T

abl

e3:

Con

tin

ued

.

No.

Res

earc

her

Res

earc

hsc

ale

and

dura

tion

Upt

ake

mec

han

ism

san

dm

edia

(su

bstr

ate)

Con

tam

inan

tor

para

met

eran

dco

nce

ntr

atio

nP

lan

tsR

esu

lt

(11)

[45]

Labo

rato

ry(7

2h

ours

(for

kin

etic

sof

Ars

enic

upt

ake)

,3da

ys(e

ffec

tsof

plan

tde

nsi

ty,P

lan

tre

-use

,an

dpl

ant

age)

,10

days

(gro

un

dwat

erre

med

iati

on))

Th

egr

oun

dwat

erw

asco

llect

edfr

oma

loca

tion

wh

ich

may

hav

ebe

enco

nta

min

ated

from

appl

icat

ion

ofar

sen

ical

her

bici

des

inth

epa

st.

As

(pH

7.0,

tota

lAs

of46

µg/

L,A

s3+of

1.6µ

g/L,

and

tota

lPof

20µ

g/L)

Ch

ines

eB

rake

fern

(Pte

ris

vitt

ata

L.)

plan

ts—

terr

estr

ial

Ch

ines

ebr

ake

fern

was

effici

ent

inta

kin

gu

par

sen

icfr

oma

con

tam

inat

edgr

oun

dwat

eran

dw

asca

pabl

eof

redu

cin

gar

sen

icco

nce

ntr

atio

ns

inth

egr

oun

dwat

er.

On

epl

ant

was

suffi

cien

tto

redu

cear

sen

icin

600

mL

grou

ndw

ater

tobe

low

10µ

g/L

in3

days

.You

ng

fern

plan

tsw

ere

mor

eeff

ecti

vein

arse

nic

rem

oval

than

old

fern

plan

tsof

sim

ilar

size

.Fer

ns

can

bere

use

dto

rem

ove

arse

nic

from

grou

ndw

ater

,bu

tat

asl

ower

rate

give

nth

ein

terv

albe

twee

nex

posu

res

and

nu

trit

ion

alst

atu

s.

(12)

[73]

Labo

rato

ryH

ydro

pon

icC

uan

dN

i

Salix

vim

inal

iscl

ones

and

the

bask

etw

illow

Bla

ckM

aul(

S.tr

iand

ra).

S.bu

rjat

ica

“Ger

man

y”,S

.xda

sycl

ados

,S.c

andi

daan

dS.

spae

thii

—te

rres

tria

l

Th

em

ore

resi

stan

tcl

ones

prod

uce

dm

ore

biom

ass

inth

egl

assh

ouse

and

fiel

dan

dh

adh

igh

erm

etal

con

cen

trat

ion

sin

the

woo

d.T

he

less

resi

stan

tcl

ones

had

grea

ter

con

cen

trat

ion

sof

Cu

and

Nii

nth

eba

rkan

dpr

odu

ced

less

biom

ass

inth

egl

assh

ouse

and

fiel

d.Si

gnifi

can

tre

lati

onsh

ips

wer

efo

un

dbe

twee

nth

ere

spon

seof

the

sam

ecl

ones

grow

nin

the

shor

t-te

rmgl

assh

ouse

hydr

opon

ics

syst

eman

din

the

fiel

d.

(13)

[74]

Labo

rato

ry(1

0da

yscu

ltiv

ate

and

7da

ysex

posu

re)

Nu

trie

nt

solu

tion

As

(0,5

,10,

20,4

0an

d80

uM

)A

zolla

:A.c

arol

inia

naan

dA

.filic

uloi

des—

aqu

atic

Th

eeffl

ux

ofar

sen

ate

was

mu

chh

igh

er(b

yab

out

9-fo

ld)

than

that

ofar

sen

ite.

Th

ism

aybe

beca

use

mos

tof

arse

nit

ein

side

the

cells

was

com

plex

edw

ith

thio

lcom

pou

nds

.T

he

hig

hA

s-ac

cum

ula

tin

gA

zolla

(A.c

arol

inia

na)

rele

ased

appr

oxim

atel

ytw

oti

mes

mor

eA

sth

anth

elo

w-A

sac

cum

ula

tin

gA

zolla

(A.fi

licul

oide

s).I

tap

pea

rsth

atth

eam

oun

tof

As

efflu

xw

aspr

opor

tion

alto

the

amou

nt

ofA

sac

cum

ula

tion

inth

etw

ost

rain

sof

Azo

lla.

No.

(12)

adap

ted

from

no.

(20)

.Phy

tore

med

iati

onB

iblio

grap

hy,A

nn

otat

edB

iblio

grap

hyon

Phy

tore

med

iati

onpr

epar

edby

Mar

kC

olem

an,B

iolo

gica

lSci

enti

st,U

SDA

Fore

stSe

rvic

eSo

uth

ern

Res

earc

hSt

atio

nan

dR

onal

dS.

Zal

esny

Jr.,

Res

earc

hP

lan

tG

enet

icis

t,U

SDA

Fore

stSe

rvic

eN

orth

Cen

tral

Res

earc

hSt

atio

n,M

ay1,

2006

.


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5 6 7 8

Plant species

Plant tissue

RootLeave

Ars

enic

(As)

accu

mu

lati

onin

plan

tti

ssu

e(m

g/g

dry

wei

ght)

Azo

llaca

rolin

iana

Azo

llafil

icul

oide

s

Ory

zasa

tiva

L.

Popu

lus

alba

Popu

lus

nigr

a

Popu

lus

trem

ula

Pte

ris

vitt

ata

Salix

alba

Figure 4: As accumulation in plant tissue.

Brassica nigra (L.) Koch that could accumulate more than100 mg Pb/g dry weight.

Figure 6 shows that accumulated Hg in Brassica junceaL. Czern. is much higher than in other species of plants. Itcould reach more than 1 mg Hg/g dry weight of plant, whilethe other plants only accumulate less than 0.2 mg Hg/g dryweight.

7. Advantages of Phytoremediation

Phytoremediation techniques may also be more publiclyacceptable, aesthetically pleasing, and less disruptive thanthe current techniques of physical and chemical process[38]. Advantages of this technology are its effectiveness incontaminant reduction, low-cost, being applicable for widerange of contaminants, and in overall it is an environmentalfriendly method. Figure 7 simplifies some advantages ofphytoremediation technology.

The major advantages of the heavy metal adsorptiontechnology by biomass are its effectiveness in reducing theconcentration of heavy metal ions to very low levels and theuse of inexpensive biosorbent materials [2]. Phytoremedi-ation as possibly the cleanest and cheapest technology canbe employed in the remediation of selected hazardous sites[29]. Phytoremediation encompasses a number of differentmethods that can lead to contaminant degradation [24].

Phytoremediation is a low-cost option and inexpensiveapproach for remediating environmental media, particularly

0

20

40

60

80

100

120

Lead

(Pb)

accu

mu

lati

onin

plan

tti

ssu

e(m

g/g

dry

wei

ght)

Plant species

RootShoot

Leave

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25A

lism

apl

anta

go-a

quat

ica

Alt

erna

nthe

raph

iloxe

roid

esA

mar

anth

ushy

brid

usL.

Am

aran

thus

pani

cula

taL

.B

rass

ica

cam

pest

ris

L.B

rass

ica

cari

nata

A.B

r.B

rass

ica

junc

ea(L

.)C

zern

.B

rass

ica

napu

sL.

Bra

ssic

ani

gra

(L.)

Koc

hB

rass

ica

oler

acea

L.

Filip

endu

laul

mar

iaH

elia

nthu

san

nuus

L.N

icot

iana

taba

cum

L.O

ryza

sati

vaL.

Paul

owni

ufo

rtun

eiPo

pulu

sal

baPo

pulu

sni

gra

Popu

lus

trem

ula

Port

ulac

agr

andi

flora

Pota

mog

eton

nata

nsSa

lixal

baSa

nvit

alia

proc

umbe

nsSo

rghu

mbi

colo

rL.

Trit

icum

aest

ivum

L.Z

eam

ays

L.

Figure 5: Pb accumulation in plant tissue.

suited to large sites that have relatively low levels of contam-ination [34]. This technology has been receiving attentionlately as an innovative, cost-effective alternative to the moreestablished treatment methods used at hazardous waste sites[29]. Phytoremediation potentially offers unique, low costsolutions to many currently problems of soil contamination[32, 75]. It is inexpensive (60–80% or even less costly) thanconventional physicochemical methods, since it does notrequire expensive equipment or highly specialized personnel.It is cost-effective for large volumes of water having lowconcentrations of contaminants and for large areas havinglow to moderately contaminated surface soils [46].

It is applicable to a wide range of toxic metals andradionuclides [32] and also useful for treating a broadrange of environmental contaminants, including organic andinorganic contaminants [46].

Phytoremediation is regarded as a new approach forthe cleanup of contaminated soils, water, and ambient air[34]. Phytoremediation research can also contribute to theimprovement of poor soils such as those with high aluminumor salt levels [75]. It is applicable to a range of toxic met-als and radionuclides, minimal environmental disturbance,elimination of secondary air or water-borne wastes, andpublic acceptance [32]. Phytoextraction is considered as anenvironmentaly friendly method to remove metals from


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Figure 6: Hg accumulation in plant tissue.

contaminated soils in situ. This method can be used in muchlarger-scale clean-up operations and has been applied forother heavy metals [76]. It is an esthetically pleasing, solar-energy-driven cleanup technology and there is minimal envi-ronmental disruption and in situ treatment preserves topsoil.In Situ applications decrease the amount of soil disturbancecompared to conventional methods. It can be performedwith minimal environmental disturbance with topsoil left ina usable condition and may be reclaimed for agricultural use.The organic pollutants may be degraded to CO2 and H2O,removing environmental toxicity [46]. Phytoremediationcan be an alternative to the much harsher remediationtechnologies of incineration, thermal vaporization, solventwashing, or other soil washing techniques, which essentiallydestroy the biological component of the soil and can drasti-cally alter its chemical and physical characteristics as well ascreating a relatively nonviable solid waste. Phytoremediationactually benefits the soil, leaving an improved, functionalsoil ecosystem at costs estimated at approximately one-tenthof those currently adopted technologies [3]. It is the mostecological cleanup technology for contaminated soils and isalso known as a green technology.

Another advantage of phytoremediation is the gener-ation of a recyclable metal-rich plant residue [32]. Phy-toremediation could be a viable option to decontaminateheavy-metal-polluted soils, particularly when the biomassproduced during the phytoremediation process could be

economically valorized in the form of bioenergy. The use ofmetal-accumulating bioenergy crops might be suitable forthis purpose. If soils, contaminated with heavy metals, arephytoremediated with oil crops, biodiesel production fromthe resulting plant oil could be a viable option to generatebioenergy [34]. In large-scale applications, the potentialenergy stored can be utilized to generate thermal energy[46]. The success of the phytoextraction technique dependsupon the identification of suitable plant species that canhyperaccumulate heavy metals and produce large amounts ofbiomass using established crop production and managementpractices [24].

8. Limitations of Phytoremediation Technology

On the other hand, there are certain limitations to phytore-mediation system (Figure 8). Among them are being time-consuming method, the amount of produced biomass, theroot depth, soil chemistry and the level of contamination, theage of plant, the contaminant concentration, the impacts ofcontaminated vegetation, and climatic condition.

Phytoremediation can be a time-consuming process, andit may take at least several growing seasons to clean upa site. The intermediates formed from those organic andinorganic contaminants may be cytotoxic to plants [46].Phytoremediation is also limited by the growth rate ofthe plants. More time may be required to phytoremediatea site as compared with other more traditional cleanuptechnologies. Excavation and disposal or incineration takesweeks to months to accomplish, while phytoextraction ordegradation may need several years. Therefore, for sites thatpose acute risks for human and other ecological receptors,phytoremediation may not be the remediation technique ofchoice [29, 46]. Phytoremediation might be best suited forremote areas where human contact is limited or where soilcontamination does not require an immediate response [38].

Under the best climatic conditions, with irrigationand fertilization, total biomass productivities can approach100 t/ha/y. One of the unresolved issues is the tradeoffbetween toxic element accumulation and productivity. Inpractice, a maximum harvestable biomass yield of 10 to20 t/ha/y would be likely, particularly for heavy metal accu-mulating plants. These values for productivity of biomassand heavy metal content would limit annual toxic ele-ment removal capacity between about 10 and 400 kg/ha/y,depending on the pollutant, plant species, climatic and otherfactors. For a target soil depth of 30 cm (4,000 t/ha), thisamounts to an annual reduction from 2.5 to 100 ppm insoil toxic element levels. This is often an acceptable rateof contaminant removal, allowing site remediation over afew years to a couple of decades, particularly where theconcentration of the contaminant can be lowered sufficientlyto meet regulatory criteria. These values for productivityof biomass and heavy metal content would limit annualtoxic element removal capacity between 10 and 400 kg/ha/y,depending on the pollutant, plant species, climatic and otherfactors [37].

The success of phytoremediation may be limited byfactors such as growing time, climate, root depth, soil


Advantages ofphytoremediation

technology

• Aesthetically pleasing

• The effectiveness in contaminant reduction

• Low cost

• Applicable for wide range of contaminants

• Environmentaly friendly method

• Less disruptive than current techniques

Figure 7: Advantages of phytoremediation technology.

Limitation ofphytoremediation

technology

• The amount of produced biomass

• The root depth

• Soil chemistry

• Level of contamination

• The age of plant

• The contaminant concentration

• The impacts of contaminated vegetation

• Climatic condition

• The time-consuming method

Figure 8: The limitation of phytoremediation technology.

chemistry, and level of contamination [38]. Root contactis a primary limitation on phytoremediation applicability.Remediation with plants requires that contaminants be incontact with the root zone of the plants. Either the plantsmust be able to extend roots to the contaminants, or thecontaminated media must be moved to be within range of theplants [29]. Restricted to sites with shallow contaminationwithin rooting zone of remediative plants, ground surfaceat the site may have to be modified to prevent flooding orerosion [46].

Age greatly affects the physiological activity of a plant,especially its roots. Generally, roots of a young plant displaygreater ability to absorb ions than do those of an old plantwhen they are similar in size. It is important to use healthyyoung plants for more efficient plant removal. However, thisdoes not rule out the use of larger older plants whose largersize may compensate for their lower physiological activity ascompared to smaller younger plants [45].

High concentrations of contaminants may inhibit plantgrowth and, thus, may limit application on some sitesor some parts of sites. This phytotoxicity could lead toa remedial approach in which high-concentration wasteis handled with expensive ex situ techniques that quicklyreduce acute risk, while in situ phytoremediation is usedover a longer period of time to clean the high volumes oflower contaminant concentrations [29]. A major limitationin the phytoremediation of toxic elements is the maximallevel that can be accumulated by plants. Plants with the

highest levels of toxic metal contents, known as “hyperac-cumulators”, generally exhibit, on a dry weight basis, fromabout 2000 ppm (0.2%) for more toxic elements (Cd, Pb) toabove 2% for the less toxic ones (Zn, Ni, Cu) [75]. Restrictedto sites with low contaminant concentrations, the treatmentis generally limited to soils at one meter from the surfaceand groundwater within a few meters of the surface with soilamendments may be required [46].

Some ecological exposure may occur whenever plantsare used to interact with contaminants from the soil. Thefate of the metals in the biomass is a concern. Althoughsome forms of phytoremediation involve accumulation ofmetals and require handling of plant material embeddedwith metals, most plants do not accumulate significantlevels of organic contaminants. While metal-accumulatingplants will need to be harvested and either recycled ordisposed of in compliance with applicable regulations, mostphytoremediative plants do not require further treatment ordisposal [29]. Harvested plant biomass from phytoextractionmay be classified as a hazardous waste, hence, disposal shouldbe proper. Consumption of contaminated plant biomass is acause of concern; contaminants may still enter the food chainthrough animals/insects that eat plant material containingcontaminants [46].

Climatic or hydrologic conditions may restrict the rateof growth of plants that can be utilized. Introduction ofnonnative species may affect biodiversity [46].


9. Conclusions

Heavy metals uptake, by plants using phytoremediation tech-nology, seems to be a prosperous way to remediate heavy-metals-contaminated environment. It has some advantagescompared with other commonly used conventional tech-nologies. Several factors must be considered in order toaccomplish a high performance of remediation result. Themost important factor is a suitable plant species which can beused to uptake the contaminant. Even the phytoremediationtechnique seems to be one of the best alternative, it also hassome limitations. Prolong research needs to be conductedto minimize this limitation in order to apply this techniqueeffectively.

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