Review Article Heavy Metal Polluted Soils: Effect on …downloads.hindawi.com/journals/aess/2014/752708.pdfReview Article Heavy Metal Polluted Soils: Effect on Plants and Bioremediation

Review ArticleHeavy Metal Polluted Soils Effect on Plantsand Bioremediation Methods

G U Chibuike1 and S C Obiora2

1 Department of Soil Science University of Nigeria Nsukka Nigeria2 Department of Geology University of Nigeria Nsukka Nigeria

Correspondence should be addressed to G U Chibuike chibuikegyahoocom

Received 13 June 2014 Revised 2 August 2014 Accepted 2 August 2014 Published 12 August 2014

Academic Editor Yongchao Liang

Copyright copy 2014 G U Chibuike and S C Obiora This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Soils polluted with heavy metals have become common across the globe due to increase in geologic and anthropogenic activitiesPlants growing on these soils show a reduction in growth performance and yield Bioremediation is an effective method oftreating heavy metal polluted soils It is a widely accepted method that is mostly carried out in situ hence it is suitable forthe establishmentreestablishment of crops on treated soils Microorganisms and plants employ different mechanisms for thebioremediation of polluted soils Using plants for the treatment of polluted soils is a more common approach in the bioremediationof heavy metal polluted soils Combining both microorganisms and plants is an approach to bioremediation that ensures a moreefficient clean-up of heavy metal polluted soils However success of this approach largely depends on the species of organismsinvolved in the process

1 Introduction

Although heavy metals are naturally present in the soilgeologic and anthropogenic activities increase the concen-tration of these elements to amounts that are harmful toboth plants and animals Some of these activities includemining and smelting of metals burning of fossil fuels useof fertilizers and pesticides in agriculture production ofbatteries and other metal products in industries sewagesludge and municipal waste disposal [1ndash3]

Growth reduction as a result of changes in physiologicaland biochemical processes in plants growing on heavy metalpolluted soils has been recorded [4ndash6] Continued declinein plant growth reduces yield which eventually leads to foodinsecurityTherefore the remediation of heavymetal pollutedsoils cannot be overemphasized

Various methods of remediating metal polluted soilsexist they range from physical and chemical methods tobiological methods Most physical and chemical methods(such as encapsulation solidification stabilization electroki-netics vitrification vapour extraction and soil washing andflushing) are expensive and do not make the soil suitable forplant growth [7] Biological approach (bioremediation) on

the other hand encourages the establishmentreestablishmentof plants on polluted soils It is an environmentally friendlyapproach because it is achieved via natural processes Biore-mediation is also an economical remediation techniquecompared with other remediation techniques This paperdiscusses the nature and properties of soils polluted withheavy metals Plant growth and performance on these soilswere examined Biological approaches employed for theremediation of heavy metal polluted soils were equallyhighlighted

2 Heavy Metal Polluted Soils

Heavy metals are elements that exhibit metallic propertiessuch as ductility malleability conductivity cation stabilityand ligand specificity They are characterized by relativelyhigh density and high relative atomic weight with an atomicnumber greater than 20 [2] Some heavy metals such asCo Cu Fe Mn Mo Ni V and Zn are required in minutequantities by organismsHowever excessive amounts of theseelements can become harmful to organisms Other heavymetals such as Pb Cd Hg and As (a metalloid but generally

Hindawi Publishing CorporationApplied and Environmental Soil ScienceVolume 2014 Article ID 752708 12 pageshttpdxdoiorg1011552014752708

2 Applied and Environmental Soil Science

referred to as a heavy metal) do not have any beneficial effecton organisms and are thus regarded as the ldquomain threatsrdquosince they are very harmful to both plants and animals

Metals exist either as separate entities or in combina-tion with other soil components These components mayinclude exchangeable ions sorbed on the surfaces of inorganicsolids nonexchangeable ions and insoluble inorganic metalcompounds such as carbonates and phosphates solublemetal compound or free metal ions in the soil solutionmetal complex of organic materials and metals attachedto silicate minerals [7] Metals bound to silicate mineralsrepresent the background soil metal concentration and theydo not cause contaminationpollution problems comparedwith metals that exist as separate entities or those present inhigh concentration in the other 4 components [8]

Soil properties affect metal availability in diverse waysHarter [9] reported that soil pH is the major factor affectingmetal availability in soil Availability of Cd and Zn to theroots of Thlaspi caerulescens decreased with increases in soilpH [10] Organic matter and hydrous ferric oxide have beenshown to decrease heavy metal availability through immobi-lization of these metals [11] Significant positive correlationshave also been recorded between heavy metals and somesoil physical properties such as moisture content and waterholding capacity [12]

Other factors that affect the metal availability in soilinclude the density and type of charge in soil colloids thedegree of complexation with ligands and the soilrsquos relativesurface area [7 13] The large interface and specific surfaceareas provided by soil colloids help in controlling the concen-tration of heavy metals in natural soils In addition solubleconcentrations of metals in polluted soils may be reducedby soil particles with high specific surface area though thismay bemetal specific [7] For instanceMcbride andMartınez[14] reported that addition of amendment consisting ofhydroxides with high reactive surface area decreased thesolubility of As Cd Cu Mo and Pb while the solubility ofNi and Zn was not changed Soil aeration microbial activityand mineral composition have also been shown to influenceheavy metal availability in soils [15]

Conversely heavy metals may modify soil propertiesespecially soil biological properties [16] Monitoring changesin soil microbiological and biochemical properties after con-tamination can be used to evaluate the intensity of soil pol-lution because these methods are more sensitive and resultscan be obtained at a faster rate compared with monitoringsoil physical and chemical properties [17]Heavymetals affectthe number diversity and activities of soil microorganismsThe toxicity of these metals on microorganisms depends on anumber of factors such as soil temperature pH claymineralsorganic matter inorganic anions and cations and chemicalforms of the metal [16 18 19]

There are discrepancies in studies comparing the effectof heavy metals on soil biological properties While someresearchers have recorded negative effect of heavy metals onsoil biological properties [16 17 20] others have reportedno relationship between high heavy metal concentrationsand some soil (micro)biological properties [21] Some ofthe inconsistencies may arise because some of these studies

were conducted under laboratory conditions using artificiallycontaminated soils while others were carried out using soilsfrom areas that are actually polluted in the field Regardlessof the origin of the soils used in these experiments the factthat the effect of heavy metals on soil biological propertiesneeds to be studied inmore detail in order to fully understandthe effect of these metals on the soil ecosystem remainsFurther it is advisable to use a wide range ofmethods (such asmicrobial biomass C and N mineralization respiration andenzymatic activities) when studying effect of metals on soilbiological properties rather than focusing on a single methodsince results obtained fromuse of differentmethods would bemore comprehensive and conclusive

The presence of one heavy metal may affect the avail-ability of another in the soil and hence plant In otherwords antagonistic and synergistic behaviours exist amongheavy metals Salgare and Acharekar [22] reported that theinhibitory effect of Mn on the total amount of mineralized Cwas antagonized by the presence of Cd Similarly Cu and Znas well as Ni and Cd have been reported to compete for thesame membrane carriers in plants [23] In contrast Cu wasreported to increase the toxicity of Zn in spring barley [24]This implies that the interrelationship between heavy metalsis quite complex thus more research is needed in this areaDifferent species of the same metal may also interact withone another Abedin et al [25] reported that the presence ofarsenite strongly suppressed the uptake of arsenate by riceplants growing on a polluted soil

3 Effect of Heavy Metal PollutedSoil on Plant Growth

The heavy metals that are available for plant uptake are thosethat are present as soluble components in the soil solutionor those that are easily solubilized by root exudates [26]Although plants require certain heavymetals for their growthand upkeep excessive amounts of these metals can becometoxic to plants The ability of plants to accumulate essentialmetals equally enables them to acquire other nonessentialmetals [27] Asmetals cannot be broken down when concen-trations within the plant exceed optimal levels they adverselyaffect the plant both directly and indirectly

Some of the direct toxic effects caused by high metalconcentration include inhibition of cytoplasmic enzymes anddamage to cell structures due to oxidative stress [28 29] Anexample of indirect toxic effect is the replacement of essentialnutrients at cation exchange sites of plants [30] Furtherthe negative influence heavy metals have on the growth andactivities of soilmicroorganismsmay also indirectly affect thegrowth of plants For instance a reduction in the number ofbeneficial soil microorganisms due to high metal concentra-tion may lead to decrease in organic matter decompositionleading to a decline in soil nutrients Enzyme activities usefulfor plant metabolism may also be hampered due to heavymetal interference with activities of soil microorganismsThese toxic effects (both direct and indirect) lead to a declinein plant growth which sometimes results in the death of plant[31]

Applied and Environmental Soil Science 3

The effect of heavy metal toxicity on the growth of plantsvaries according to the particular heavy metal involved inthe process Table 1 shows a summary of the toxic effectsof specific metals on growth biochemistry and physiologyof various plants For metals such as Pb Cd Hg and Aswhich do not play any beneficial role in plant growth adverseeffects have been recorded at very low concentrations ofthese metals in the growth medium Kibra [32] recordedsignificant reduction in height of rice plants growing on asoil contaminatedwith 1mgHgkg Reduced tiller and panicleformation also occurred at this concentration of Hg in thesoil For Cd reduction in shoot and root growth in wheatplants occurred when Cd in the soil solution was as lowas 5mgL [33] Most of the reduction in growth parametersof plants growing on polluted soils can be attributed toreduced photosynthetic activities plant mineral nutritionand reduced activity of some enzymes [34]

For other metals which are beneficial to plants ldquosmallrdquoconcentrations of these metals in the soil could actuallyimprove plant growth and development However at higherconcentrations of these metals reductions in plant growthhave been recorded For instance Jayakumar et al [42]reported that at 50mgCokg there was an increase in nutri-ent content of tomato plants comparedwith the control Con-versely at 100mgCokg to 250mgCokg reductions in plantnutrient content were recorded Similarly increase in plantgrowth nutrient content biochemical content and antiox-idant enzyme activities (catalase) was observed in radishand mung bean at 50mgCokg soil concentration whilereductions were recorded at 100mgCokg to 250mgCokgsoil concentration [43 44] Improvements in growth andphysiology of cluster beans have also been reported at Zn con-centration of 25mgL of the soil solution On the other handgrowth reduction and adverse effect on the plantrsquos physiologystarted when the soil solution contained 50mgZnL [67]

It is worth mentioning that in most real life situa-tions (such as disposal of sewage sludge and metal miningwastes) where soil may be polluted with more than oneheavy metal both antagonistic and synergistic relationshipsbetweenheavymetalsmay affect plantmetal toxicityNichollsand Mal [70] reported that the combination of Pb andCu at both high concentration (1000mgkg each) and lowconcentration (500mgkg) resulted in a rapid and completedeath of the leaves and stem of Lythrum salicariaThe authorsreported that there was no synergistic interaction betweenthese heavy metals probably because the concentrations usedin the experiment were too high for interactive relationshipto be observed between the metals Another study [71]examined the effect of 6 heavy metals (Cd Cr Co Mnand Pb) on the growth of maize The result showed thatthe presence of these metals in soil reduced the growthand protein content of maize The toxicity of these metalsoccurred in the following order Cd gt Co gtHg gtMn gt Pb gtCr It was also observed in this study that the combined effectof 2 or more heavy metals was only as harmful as the effectof the most toxic heavy metal The researcher attributed thisresult to the antagonistic relationship which exists betweenheavy metals

It is important to note that certain plants are able totolerate high concentration of heavy metals in their envi-ronment Baker [72] reported that these plants are able totolerate these metals via 3 mechanisms namely (i) exclusionrestriction of metal transport and maintenance of a constantmetal concentration in the shoot over a wide range ofsoil concentrations (ii) inclusion metal concentrations inthe shoot reflecting those in the soil solution through alinear relationship and (iii) bioaccumulation accumulationofmetals in the shoot and roots of plants at both low and highsoil concentrations

4 Bioremediation of HeavyMetal Polluted Soils

Bioremediation is the use of organisms (microorganismsandor plants) for the treatment of polluted soils It is a widelyaccepted method of soil remediation because it is perceivedto occur via natural processes It is equally a cost effectivemethod of soil remediation Blaylock et al [73] reported50 to 65 saving when bioremediation was used for thetreatment of 1 acre of Pb polluted soil compared with thecase when a conventional method (excavation and landfill)was used for the same purpose Although bioremediationis a nondisruptive method of soil remediation it is usuallytime consuming and its use for the treatment of heavymetal polluted soils is sometimes affected by the climatic andgeological conditions of the site to be remediated [74]

Heavy metals cannot be degraded during bioremediationbut can only be transformed from one organic complex oroxidation state to another Due to a change in their oxidationstate heavy metals can be transformed to become either lesstoxic easily volatilized more water soluble (and thus can beremoved through leaching) less water soluble (which allowsthem to precipitate and become easily removed from theenvironment) or less bioavailable [75 76]

Bioremediation of heavy metals can be achieved via theuse of microorganisms plants or the combination of bothorganisms

41 Using Microbes for Remediation of Heavy Metal PollutedSoils Several microorganisms especially bacteria (Bacillussubtilis Pseudomonas putida and Enterobacter cloacae) havebeen successfully used for the reduction of Cr (VI) to theless toxic Cr (III) [77ndash80] B subtilis has also been reportedto reduce nonmetallic elements For instance Garbisu et al[81] recorded that B subtilis reduced the selenite to the lesstoxic elemental Se FurtherB cereus andB thuringiensis havebeen shown to increase extraction of Cd and Zn from Cd-rich soil and soil polluted with effluent from metal industry[82] It is assumed that the production of siderophore (Fecomplexing molecules) by bacteria may have facilitated theextraction of these metals from the soil this is because heavymetals have been reported to simulate the production ofsiderophore and this consequently affects their bioavailability[83] For instance siderophore production by Azotobactervinelandii was increased in the presence of Zn (II) [84]

4 Applied and Environmental Soil Science

Table 1 Effect of heavy metal toxicity on plants

Heavy metal Plant Toxic effect on plant Reference

AsRice (Oryza sativa) Reduction in seed germination decrease in seedling

height reduced leaf area and dry matter production [35 36]

Tomato (Lycopersiconesculentum) Reduced fruit yield decrease in leaf fresh weight [37]

Canola (Brassica napus) Stunted growth chlorosis wilting [38]

CdWheat (Triticum sp) Reduction in seed germination decrease in plant

nutrient content reduced shoot and root length [33 39]

Garlic (Allium sativum) Reduced shoot growth Cd accumulation [40]Maize (Zea mays) Reduced shoot growth inhibition of root growth [41]

Co

Tomato (Lycopersiconesculentum) Reduction in plant nutrient content [42]

Mung bean (Vignaradiata)

Reduction in antioxidant enzyme activities decrease inplant sugar starch amino acids and protein content [43]

Radish (Raphanussativus)

Reduction in shoot length root length and total leafarea decrease in chlorophyll content reduction in plantnutrient content and antioxidant enzyme activitydecrease in plant sugar amino acid and protein content

[44]

Cr

Wheat (Triticum sp) Reduced shoot and root growth [45 46]Tomato (Lycopersiconesculentum) Decrease in plant nutrient acquisition [47 48]

Onion (Allium cepa) Inhibition of germination process reduction of plantbiomass [49]

Cu

Bean (Phaseolusvulgaris)

Accumulation of Cu in plant roots root malformationand reduction [50]

Black bindweed(Polygonum convolvulus) Plant mortality reduced biomass and seed production [51]

Rhodes grass (Chlorisgayana) Root growth reduction [52]

Hg Rice (Oryza sativa)Decrease in plant height reduced tiller and panicleformation yield reduction bioaccumulation in shootand root of seedlings

[32 53]

Tomato (Lycopersiconesculentum)

Reduction in germination percentage reduced plantheight reduction in flowering and fruit weightchlorosis

[54]

Mn

Broad bean (Vicia faba) Mn accumulation shoot and root reduction in shootand root length chlorosis [55]

Spearmint (Menthaspicata)

Decrease in chlorophyll a and carotenoid contentaccumulation of Mn in plant roots [56]

Pea (Pisum sativum)Reduction in chlorophylls a and b content reduction inrelative growth rate reduced photosynthetic O2evolution activity and photosystem II activity

[57]

Tomato (Lycopersiconesculentum)

Slower plant growth decrease in chlorophyllconcentration [58]

Ni

Pigeon pea (Cajanuscajan)

Decrease in chlorophyll content and stomatalconductance decreased enzyme activity which affectedCalvin cycle and CO2 fixation

[59]

Rye grass (Loliumperenne)

Reduction in plant nutrient acquisition decrease inshoot yield chlorosis [60]

Wheat (Triticum sp) Reduction in plant nutrient acquisition [61 62]Rice (Oryza sativa) Inhibition of root growth [63]

Applied and Environmental Soil Science 5

Table 1 Continued

Heavy metal Plant Toxic effect on plant Reference

Pb

Maize (Zea mays)Reduction in germination percentage suppressedgrowth reduced plant biomass decrease in plantprotein content

[64]

Portia tree (Thespesiapopulnea)

Reduction in number of leaves and leaf area reducedplant height decrease in plant biomass [65]

Oat (Avena sativa) Inhibition of enzyme activity which affected CO2fixation [66]

Zn

Cluster bean (Cyamopsistetragonoloba)

Reduction in germination percentage reduced plantheight and biomass decrease in chlorophyllcarotenoid sugar starch and amino acid content

[67]

Pea (Pisum sativum)Reduction in chlorophyll content alteration instructure of chloroplast reduction in photosystem IIactivity reduced plant growth

[68]

Rye grass (Loliumperenne)

Accumulation of Zn in plant leaves growth reductiondecrease in plant nutrient content reduced efficiency ofphotosynthetic energy conversion

[69]

Hence heavy metals influence the activities of siderophore-producing bacteria which in turn increases mobility andextraction of these metals in soil

Bioremediation can also occur indirectly via bioprecip-itation by sulphate reducing bacteria (Desulfovibrio desulfu-ricans) which converts sulphate to hydrogen sulphate whichsubsequently reacts with heavy metals such as Cd and Zn toform insoluble forms of these metal sulphides [85]

Most of the abovemicrobe assisted remediation is carriedout ex situ However a very important in situmicrobe assistedremediation is the microbial reduction of soluble mercuricions Hg (II) to volatile metallic mercury and Hg (0) carriedout by mercury resistant bacteria [86] The reduced Hg (0)can easily volatilize out of the environment and subsequentlybe diluted in the atmosphere [87]

Genetic engineering can be adopted in microbe assistedremediation of heavy metal polluted soils For instance Vallset al [88] reported that genetically engineered Ralstoniaeutropha can be used to sequester metals (such as Cd) inpolluted soils This is made possible by the introductionof metallothionein (cysteine rich metal binding protein)from mouse on the cell surface on this organism Althoughthe sequestered metals remain in the soil they are madeless bioavailable and hence less harmful The controversiessurrounding geneticallymodified organisms [89] and the factthat the heavy metal remains in the soil are major limitationsto this approach to bioremediation

Making the soil favourable for soil microbes is onestrategy employed in bioremediation of polluted soils Thisprocess known as biostimulation involves the addition ofnutrients in the form of manure or other organic amend-ments which serve as C source formicroorganisms present inthe soil The added nutrients increase the growth and activ-ities of microorganisms involved in the remediation processand thus this increases the efficiency of bioremediation

Although biostimulation is usually employed for thebiodegradation of organic pollutants [90] it can equallybe used for the remediation of heavy metal polluted soils

Since heavy metals cannot be biodegraded biostimulationcan indirectly enhance remediation of heavy metal pollutedsoil through alteration of soil pH It is well known that theaddition of organic materials reduces the pH of the soil[91] this subsequently increases the solubility and hencebioavailability of heavy metals which can then be easilyextracted from the soil [92]

Biochar is one organic material that is currently beingexploited for its potential in the management of heavy metalpolluted soils Namgay et al [93] recorded a reduction inthe availability of heavy metals when the polluted soil wasamended with biochar this in turn reduced plant absorptionof the metalsThe ability of biochar to increase soil pH unlikemost other organic amendments [94] may have increasedsorption of these metals thus reducing their bioavailabilityfor plant uptake It is important to note that since the char-acteristics of biochar vary widely depending on its methodof production and the feedstock used in its productionthe effect different biochar amendments will have on theavailability of heavy metals in soil will also differ Furthermore research is needed in order to understand the effectof biochar on soil microorganisms and how the interactionbetween biochar and soil microbes influences remediation ofheavy metal polluted soils because such studies are rare inliterature

42 Using Plants for Remediation of Heavy Metal PollutedSoils Phytoremediation is an aspect of bioremediation thatuses plants for the treatment of polluted soils It is suitablewhen the pollutants cover a wide area and when they arewithin the root zone of the plant [76] Phytoremediationof heavy metal polluted soils can be achieved via differentmechanisms These mechanisms include phytoextractionphytostabilization and phytovolatilization

421 Phytoextraction This is the most common form ofphytoremediation It involves accumulation of heavy metals

6 Applied and Environmental Soil Science

in the roots and shoots of phytoremediation plants Theseplants are later harvested and incinerated Plants used forphytoextraction usually possess the following characteristicsrapid growth rate high biomass extensive root system andability to tolerate high amounts of heavy metals This abilityto tolerate high concentration of heavy metals by these plantsmay lead to metal accumulation in the harvestable part thismay be problematic through contamination of the food chain[7]

There are two approaches to phytoextraction dependingon the characteristics of the plants involved in the processThe first approach involves the use of natural hyperaccu-mulators that is plants with very high metal-accumulatingability while the second approach involves the use of highbiomass plants whose ability to accumulate metals is inducedby the use of chelates that is soil amendments with metalmobilizing capacity [95]

Hyperaccumulators accumulate 10 to 500 times moremetals than ordinary plant [96] hence they are very suitablefor phytoremediation An important characteristic whichmakes hyperaccumulation possible is the tolerance of theseplants to increasing concentrations of these metals (hyper-tolerance) This could be a result of exclusion of thesemetals from the plants or by compartmentalization of thesemetal ions that is the metals are retained in the vacuolarcompartments or cell walls and thus do not have access tocellular sites where vital functions such as respiration and celldivision take place [76 96]

Generally a plant can be called a hyperaccumulator if itmeets the following criteria (i) the concentration of metalin the shoot must be higher than 01 for Al As Co CrCu Ni and Se higher than 001 for Cd and higher than10 for Zn [97] (ii) the ratio of shoot to root concentrationmust be consistently higher than 1 [98] this indicates thecapability to transport metals from roots to shoot and theexistence of hypertolerance ability [7] (iii) the ratio of shootto root concentration must be higher than 1 this indicatesthe degree of plant metal uptake [7 98] Reeves and Baker[99] reported some examples of plants which have the abilityto accumulate large amounts of heavy metals and hence canbe used in remediation studies Some of these plants includeHaumaniastrum robertii (Cohyperaccumulator)Aeollanthussubacaulis (Cu hyperaccumulator) Maytenus bureaviana(Mn hyperaccumulator)Minuartia verna and Agrostis tenuis(Pb hyperaccumulators) Dichapetalum gelonioides Thlaspitatrense and Thlaspi caerulescens (Zn hyperaccumulators)Psycotria vanhermanni and Streptanthus polygaloides (Nihyperaccumulators) Lecythis ollaria (Se hyperaccumulator)Pteris vittata is an example of a hyperaccumulator thatcan be used for the remediation of soils polluted with As[100] Some plants have the ability to accumulate morethan one metal For instance Yang et al [101] observedthat the Zn hyperaccumulator Sedum alfredii can equallyhyperaccumulate Cd

The possibility of contaminating the food chain throughthe use of hyperaccumulators is a major limitation in phy-toextraction However many species of the Brassicaceaefamily which are known to be hyperaccumulators of heavymetals contain high amounts of thiocyanates which make

themunpalatable to animals thus this reduces the availabilityof these metals in the food chain [102]

Most hyperaccumulators are generally slow growers withlow plant biomass this reduces the efficiency of the remedi-ation process [103] Thus in order to increase the efficiencyof phytoextraction plants with high growth rate as wellas high biomass (eg maize sorghum and alfalfa) aresometimes used together with metal chelating substances forsoil remediation exercise It is important to note that somehyperaccumulators such as certain species within theBrassicagenus (Brassica napus Brassica juncea and Brassica rapa) arefast growers with high biomass [104]

In most cases plants absorb metals that are readily avail-able in the soil solution Although some metals are presentin soluble forms for plant uptake others occur as insolubleprecipitate and are thus unavailable for plant uptakeAdditionof chelating substances prevents precipitation and metalsorption via the formation of metal chelate complexes thissubsequently increases the bioavailability of these metals [7]Further the addition of chelates to the soil can transportmore metals into the soil solution through the dissolutionof precipitated compounds and desorption of sorbed species[13] Certain chelates are also able to translocate heavy metalinto the shoots of plants [73]

Marques et al [7] documented examples of syntheticchelates which have successfully been used to extractheavy metals from polluted soils Some of these chelatesinclude EDTA (ethylenediaminetetraacetic acid) EDDS(SS-ethylenediamine disuccinic acid) CDTA (trans-12-diaminocyclohexane-NNN1015840N1015840-tetraacetic acid) EDDHA(ethylenediamine-di-o-hydroxyphenylacetic acid) DTPA(diethylenetriaminepentaacetic acid) and HEDTA (N-hydroxyethylenediaminetriacetic acid) EDTA is a syntheticchelate that is widely used not only because it is the leastexpensive compared with other synthetic chelates [105] butalso because it has a high ability to successfully improve plantmetal uptake [106ndash108] Organic chelates such as citric acidand malic acid can also be used to improve phytoextractionof heavy metals from polluted soils [109]

One major disadvantage of using chelates in phytoex-traction is the possible contamination of groundwater vialeaching of these heavy metals [110] This is because of theincreased availability of heavy metals in the soil solutionwhen these chelates are used In addition when chelates(especially synthetic chelates) are used in high concentra-tions they can become toxic to plants and soil microbes[106] In general solubilityavailability of heavy metals forplant uptake and suitability of a site for phytoextraction areadditional factors that should be considered (in addition tosuitability of plants) before using phytoextraction for soilremediation [26]

422 Phytostabilization Phytostabilization involves usingplants to immobilize metals thus reducing their bioavailabil-ity via erosion and leaching It is mostly used when phy-toextraction is not desirable or even possible [98] Marqueset al [7] argued that this form of phytoremediation is bestapplied when the soil is so heavily polluted so that using

Applied and Environmental Soil Science 7

plants for metal extraction would take a long time to beachieved and thus would not be adequate Jadia and Fulekar[111] on the other hand showed that the growth of plants(used for phytostabilization) was adversely affected when theconcentration of heavy metal in the soil was high

Phytostabilization of heavy metals takes place as a resultof precipitation sorption metal valence reduction or com-plexation [29] The efficiency of phytostabilization dependson the plant and soil amendment used Plants help in stabi-lizing the soil through their root systems thus they preventerosion Plant root systems equally prevent leaching viareduction of water percolation through the soil In additionplants prevent manrsquos direct contact with pollutants and theyequally provide surfaces for metal precipitation and sorption[112]

Based on the above factors it is important that appropri-ate plants are selected for phytostabilization of heavy metalsPlants used for phytostabilization should have the followingcharacteristics dense rooting system ability to tolerate soilconditions ease of establishment and maintenance underfield conditions rapid growth to provide adequate groundcoverage and longevity and ability to self-propagate

Soil amendments used in phytostabilization help to inac-tivate heavymetals thus they prevent plantmetal uptake andreduce biological activity [7] Organic materials are mostlyused as soil amendments in phytostabilization Marques et al[113] showed that Zn percolation through the soil reduced by80 after application of manure or compost to polluted soilson which Solanum nigrum was grown

Other amendments that can be used for phytostabiliza-tion include phosphates lime biosolids and litter [114] Thebest soil amendments are those that are easy to handle safeto workers who apply them easy to produce and inexpensiveand most importantly are not toxic to plants [113] Most ofthe times organic amendments are used because of their lowcost and the other benefits they provide such as provision ofnutrients for plant growth and improvement of soil physicalproperties [7]

In general phytostabilization is very useful when rapidimmobilization of heavy metals is needed to prevent ground-water pollution However because the pollutants remain inthe soil constant monitoring of the environment is requiredand this may become a problem

423 Phytovolatilization In this form of phytoremediationplants are used to take up pollutants from the soil these pollu-tants are transformed into volatile forms and are subsequentlytranspired into the atmosphere [115] Phytovolatilization ismostly used for the remediation of soils polluted with HgThe toxic form of Hg (mercuric ion) is transformed into theless toxic form (elemental Hg)The problemwith this processis that the new product formed that is elemental Hg maybe redeposited into lakes and rivers after being recycled byprecipitation this in turn repeats the process of methyl-Hgproduction by anaerobic bacteria [115]

Raskin and Ensley [116] reported the absence of plantspecies with Hg hyperaccumulating properties Therefore

genetic engineered plants are mostly used in phytovolatiliza-tion Examples of transgenic plants which have been usedfor phytovolatilization of Hg polluted soils are Nicotianatabacum Arabidopsis thaliana and Liriodendron tulipifera[117 118] These plants are usually genetically modifiedto include gene for mercuric reductase that is merAOrganomercurial lyase (merB) is another bacterial gene usedfor the detoxification of methyl-Hg Both merA and merBcan be inserted into plants used to detoxify methyl-Hgto elemental Hg [119] Use of plants modified with merAand merB is not acceptable from a regulatory perspective[119] However plants altered with merB are more acceptablebecause the gene prevents the introduction ofmethyl-Hg intothe food chain [120]

Phytovolatilization can also be employed for the reme-diation of soils polluted with Se [7] This involves theassimilation of inorganic Se into organic selenoamino acids(selenocysteine and selenomethionine) Selenomethionine isfurther biomethylated to dimethylselenide which is lost inthe atmosphere via volatilization [121] Plants which havesuccessfully been used for phytovolatilization of soils pollutedwith Se are Brassica juncea and Brassica napus [122]

43 Combining Plants and Microbes for the Remediationof Heavy Metal Polluted Soils The combined use of bothmicroorganisms and plants for the remediation of pollutedsoils results in a faster and more efficient clean-up of thepolluted site [123] Mycorrhizal fungi have been used inseveral remediation studies involving heavy metals and theresults obtained show that mycorrhizae employ differentmechanisms for the remediation of heavy metal pollutedsoils For instance while some studies have shown enhancedphytoextraction through the accumulation of heavy metalsin plants [124ndash126] others reported enhanced phytostabi-lization through metal immobilization and a reduced metalconcentration in plants [127 128]

In general the benefits derived from mycorrhizalassociationsmdashwhich range from increased nutrient andwater acquisition to the provision of a stable soil for plantgrowth and increase in plant resistance to diseases [129ndash131]mdashare believed to aid the survival of plants growing inpolluted soils and thus help in the vegetationrevegetation ofremediated soils [132] It is important to note that mycorrhizadoes not always assist in the remediation of heavy metalpolluted soils [133 134] and this may be attributed tothe species of mycorrhizal fungi and the concentrationof heavy metals [7 132] Studies have also shown thatactivities of mycorrhizal fungi may be inhibited by heavymetals [135 136] In addition Weissenhorn and Leyval[137] reported that certain species of mycorrhizal fungi(arbuscular mycorrhizal fungi) can be more sensitive topollutants compared to plants

Other microorganisms apart from mycorrhizal fungihave also been used in conjunction with plants for theremediation of heavy metal polluted soils Most of thesemicrobes are the plant growth-promoting rhizobacteria(PGPR) that are usually found in the rhizosphere ThesePGPR stimulate plant growth via several mechanisms such as

8 Applied and Environmental Soil Science

production of phytohormones and supply of nutrients [138]production of siderophores and other chelating agents [139]specific enzyme activity and N fixation [140] and reduc-tion in ethylene production which encourages root growth[141]

In general PGPR have been used in phytoremediationstudies to reduce plant stress associated with heavy metalpolluted soils [142] Enhanced accumulation of heavy metalssuch as Cd and Ni by hyperaccumulators (Brassica junceaand Brassica napus) has been observed when the plants wereinoculated with Bacillus sp [143 144] On the other handMadhaiyan et al [145] reported increased plant growth dueto a reduction in the accumulation of Cd and Ni in theshoot and root tissues of tomato plant when it was inoculatedwith Methylobacterium oryzae and Burkholderia spp Thusthis indicates that the mechanisms employed by PGPR inthe phytoremediation of heavy metal polluted soils may bedependent on the species of PGRP and plant involved in theprocess Although studies involving both the use of myc-orrhizal fungi and PGPR are uncommon Vivas et al [146]reported that PGPR (Brevibacillus sp) increased mycorrhizalefficiency which in turn decreased metal accumulation andincreased the growth of white clover growing on a heavymetal (Zn) polluted soil

5 Conclusion

Plants growing on heavy metal polluted soils show a reduc-tion in growth due to changes in their physiological andbiochemical activities This is especially true when the heavymetal involved does not play any beneficial role towards thegrowth and development of plants Bioremediation can beeffectively used for the treatment of heavy metal pollutedsoil It is most appropriate when the remediated site is usedfor crop production because it is a nondisruptive method ofsoil remediation Using plants for bioremediation (phytore-mediation) is a more common approach to bioremediationof heavy metal compared with the use of microorganismsPlants employ different mechanisms in the remediation ofheavy metal polluted soils Phytoextraction is the mostcommon method of phytoremediation used for treatment ofheavy metal polluted soils It ensures the complete removalof the pollutant Combining both plants andmicroorganismsin bioremediation increases the efficiency of this method ofremediation Both mycorrhizal fungi and other PGPR havebeen successfully incorporated in various phytoremediationprogrammes The success of the combined use of theseorganisms depends on the species of microbe and plantsinvolved and to some extent on the concentration of the heavymetal in soil

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] B J Alloway Heavy Metal in Soils John Wiley amp Sons NewYork NY USA 1990

[2] I Raskin P B A N Kumar S Dushenkov and D E SaltldquoBioconcentration of heavy metals by plantsrdquo Current Opinionin Biotechnology vol 5 no 3 pp 285ndash290 1994

[3] Z Shen X Li C Wang H Chen and H Chua ldquoLead phyto-extraction from contaminated soil with high-biomass plantspeciesrdquo Journal of Environmental Quality vol 31 no 6 pp1893ndash1900 2002

[4] J Chatterjee and C Chatterjee ldquoPhytotoxicity of cobaltchromium and copper in cauliflowerrdquo Environmental Pollutionvol 109 no 1 pp 69ndash74 2000

[5] I Oncel Y Keles and A S Ustun ldquoInteractive effects oftemperature and heavy metal stress on the growth and somebiochemical compounds in wheat seedlingsrdquo EnvironmentalPollution vol 107 no 3 pp 315ndash320 2000

[6] S Oancea N Foca and A Airinei ldquoEffects of heavy metals onplant growth and photosynthetic activityrdquo Analele Stiintifice aleUniversitatii ldquoAL I CUZA1 IASI Tomul I s Biofizica Fizicamedicala si Fizica mediului pp 107ndash110 2005

[7] A P G C Marques A O S S Rangel and P M L CastroldquoRemediation of heavy metal contaminated soils phytoreme-diation as a potentially promising clean-up technologyrdquoCriticalReviews in Environmental Science and Technology vol 39 no 8pp 622ndash654 2009

[8] L Ramos L M Hernandez and M J Gonzalez ldquoSequentialfractionation of copper lead cadmium and zinc in soils from ornear Donana National Parkrdquo Journal of Environmental Qualityvol 23 no 1 pp 50ndash57 1994

[9] R D Harter ldquoEffect of soil pH on adsorption of lead copperzinc and nickelrdquo Soil Science Society of America Journal vol 47no 1 pp 47ndash51 1983

[10] A S Wang J S Angle R L Chaney T A Delorme and RD Reeves ldquoSoil pH effects on uptake of Cd and Zn by Thlaspicaerulescensrdquo Plant and Soil vol 281 no 1-2 pp 325ndash337 2006

[11] L Yi Y Hong D Wang and Y Zhu ldquoDetermination of freeheavy metal ion concentrations in soils around a cadmium richzinc depositrdquo Geochemical Journal vol 41 no 4 pp 235ndash2402007

[12] M S Rakesh Sharma and N S Raju ldquoCorrelation of heavymetal contamination with soil properties of industrial areasof Mysore Karnataka India by cluster analysisrdquo InternationalResearch Journal of Environment Sciences vol 2 no 10 pp 22ndash27 2013

[13] W A Norvell ldquoComparison of chelating agents as extractantsfor metals in diverse soil materialsrdquo Soil Science Society ofAmerica Journal vol 48 no 6 pp 1285ndash1292 1984

[14] M B Mcbride and C E Martınez ldquoCopper phytotoxicity in acontaminated soil remediation tests with adsorptive materialsrdquoEnvironmental Science and Technology vol 34 no 20 pp 4386ndash4391 2000

[15] M L Magnuson C A Kelty and K C Kelty ldquoTrace metalloading on water-borne soil and dust particles characterizedthrough the use of Split-flow thin-cell fractionationrdquo AnalyticalChemistry vol 73 no 14 pp 3492ndash3496 2001

[16] M Friedlova ldquoThe influence of heavy metals on soil biologicaland chemical propertiesrdquo Soil and Water Research vol 5 no 1pp 21ndash27 2010

[17] P Nannipieri L Badalucco L Landi and G PietramellaraldquoMeasurement in assessing the risk of chemicals to the soil

Applied and Environmental Soil Science 9

ecosystemrdquo in Ecotoxicology Responses Biomarkers and RiskAssessment J T Zelikoff Ed pp 507ndash534 OECD WorkshopSOS Publ Fair Haven NY USA 1997

[18] E Baath ldquoEffects of heavy metals in soil on microbial processesand populations (a review)rdquoWater Air amp Soil Pollution vol 47no 3-4 pp 335ndash379 1989

[19] K E Giller EWitter and S PMcgrath ldquoToxicity of heavymet-als to microorganisms and microbial processes in agriculturalsoilsrdquo Soil Biology and Biochemistry vol 30 no 10-11 pp 1389ndash1414 1998

[20] M Smejkalova O Mikanova and L Boruvka ldquoEffects of heavymetal concentrations on biological activity of soils microorgan-ismsrdquo Plant Soil and Environment vol 49 pp 321ndash326 2003

[21] S Castaldi F A Rutigliano and A Virzo de Santo ldquoSuitabilityof soil microbial parameters as indicators of heavy metalpollutionrdquoWater Air amp Soil Pollution vol 158 no 1 pp 21ndash352004

[22] S A Salgare and C Acharekar ldquoEffect of industrial pollutionon growth and content of certain weedsrdquo Journal for NatureConservation vol 4 pp 1ndash6 1992

[23] D T Clarkson and U Luttge ldquoMineral nutrition divalentcations transport and compartmentationrdquo Progress in Botanyvol 51 pp 93ndash112 1989

[24] Y Luo and D L Rimmer ldquoZinc-copper interaction affectingplant growth on a metal-contaminated soilrdquo EnvironmentalPollution vol 88 no 1 pp 79ndash83 1995

[25] M J Abedin J Feldmann and A A Meharg ldquoUptake kineticsof arsenic species in rice plantsrdquo Plant Physiology vol 128 no3 pp 1120ndash1128 2002

[26] M J Blaylock and J W Huang ldquoPhytoextraction of metalsrdquo inPhytoremediation of Toxic Metals Using Plants to Clean up theEnvironment I Raskin and B D Ensley Eds pp 53ndash70 WileyNew York NY USA 2000

[27] R Djingova and I Kuleff ldquoInstrumental techniques for traceanalysisrdquo in Trace Elements Their Distribution and Effects inthe Environment J P Vernet Ed Elsevier London UK 2000

[28] F Assche and H Clijsters ldquoEffects of metals on enzyme activityin plantsrdquo Plant Cell and Environment vol 24 pp 1ndash15 1990

[29] C D Jadia and M H Fulekar ldquoPhytoremediation of heavymetals recent techniquesrdquoAfrican Journal of Biotechnology vol8 no 6 pp 921ndash928 2009

[30] L Taiz and E Zeiger Plant Physiology Sinauer AssociatesSunderland Mass USA 2002

[31] A Schaller and T Diez ldquoPlant specific aspects of heavy metaluptake and comparison with quality standards for food andforage cropsrdquo in Der Einfluszlig von festen Abfallen auf BodenPflanzen D Sauerbeck and S Lubben Eds pp 92ndash125 KFAJulich Germany 1991 (German)

[32] M G Kibra ldquoEffects of mercury on some growth parametersof rice (Oryza sativa L)rdquo Soil amp Environment vol 27 no 1 pp23ndash28 2008

[33] I Ahmad M J Akhtar Z A Zahir and A Jamil ldquoEffect ofcadmium on seed germination and seedling growth of fourwheat (Triticum aestivum L) cultivarsrdquo Pakistan Journal ofBotany vol 44 no 5 pp 1569ndash1574 2012

[34] A Kabata-Pendias Trace Elements in Soils and Plants CRCPress Boca Raton Fla USA 3rd edition 2001

[35] A R Marin S R Pezeshki P H Masscheleyn and H SChoi ldquoEffect of dimethylarsinic acid (DMAA) on growth tissuearsenic and photosynthesis of rice plantsrdquo Journal of PlantNutrition vol 16 no 5 pp 865ndash880 1993

[36] M J Abedin J Cotter-Howells and A A Meharg ldquoArsenicuptake and accumulation in rice (Oryza sativa L) irrigated withcontaminated waterrdquo Plant and Soil vol 240 no 2 pp 311ndash3192002

[37] A C Barrachina F B Carbonell and J M Beneyto ldquoArsenicuptake distribution and accumulation in tomato plants effectof arsenite on plant growth and yieldrdquo Journal of PlantNutritionvol 18 no 6 pp 1237ndash1250 1995

[38] M S Cox P F Bell and J L Kovar ldquoDifferential tolerance ofcanola to arsenic when grown hydroponically or in soilrdquo Journalof Plant Nutrition vol 19 no 12 pp 1599ndash1610 1996

[39] M S Yourtchi and H R Bayat ldquoEffect of cadmium toxicity ongrowth cadmium accumulation and macronutrient content ofdurum wheat (Dena CV)rdquo International Journal of Agricultureand Crop Sciences vol 6 no 15 pp 1099ndash1103 2013

[40] W Jiang D Liu andWHou ldquoHyperaccumulation of cadm iumby roots bulbs and shoots of garlicrdquoBioresource Technology vol76 no 1 pp 9ndash13 2001

[41] M Wang J Zou X Duan W Jiang and D Liu ldquoCadmiumaccumulation and its effects onmetal uptake inmaize (ZeamaysL)rdquo Bioresource Technology vol 98 no 1 pp 82ndash88 2007

[42] K Jayakumar M Rajesh L Baskaran and P VijayarenganldquoChanges in nutritional metabolism of tomato (Lycopersiconesculantum Mill) plants exposed to increasing concentrationof cobalt chloriderdquo International Journal of Food Nutrition andSafety vol 4 no 2 pp 62ndash69 2013

[43] K Jayakumar C A Jaleel and M M Azooz ldquoPhytochemicalchanges in green gram (Vigna radiata) under cobalt stressrdquoGlobal Journal of Molecular Sciences vol 3 no 2 pp 46ndash492008

[44] K Jayakumar C A Jaleel and P Vijayarengan ldquoChanges ingrowth biochemical constituents and antioxidant potentialsin radish (Raphanus sativus L) under cobalt stressrdquo TurkishJournal of Biology vol 31 no 3 pp 127ndash136 2007

[45] D C Sharma and C P Sharma ldquoChromium uptake and itseffects on growth and biological yield of wheatrdquoCereal ResearchCommunications vol 21 no 4 pp 317ndash322 1993

[46] S K Panda and H K Patra ldquoNitrate and ammonium ionseffect on the chromium toxicity in developing wheat seedlingsrdquoProceedings of the National Academy of Sciences India vol 70pp 75ndash80 2000

[47] R Moral J Navarro Pedreno I Gomez and J Mataix ldquoEffectsof chromium on the nutrient element content and morphologyof tomatordquo Journal of Plant Nutrition vol 18 no 4 pp 815ndash8221995

[48] R Moral I Gomez J N Pedreno and J Mataix ldquoAbsorptionof Cr and effects on micronutrient content in tomato plant(Lycopersicum esculentumM)rdquoAgrochimica vol 40 no 2-3 pp132ndash138 1996

[49] N Nematshahi M Lahouti and A Ganjeali ldquoAccumulation ofchromium and its effect on growth of (Allium cepa cv Hybrid)rdquoEuropean Journal of Experimental Biology vol 2 no 4 pp 969ndash974 2012

[50] C M Cook A Kostidou E Vardaka and T Lanaras ldquoEffectsof copper on the growth photosynthesis and nutrient concen-trations of Phaseolus plantsrdquo Photosynthetica vol 34 no 2 pp179ndash193 1997

[51] C Kjaeligr and N Elmegaard ldquoEffects of copper sulfate onblack bindweed (Polygonum convolvulus L)rdquo Ecotoxicology andEnvironmental Safety vol 33 no 2 pp 110ndash117 1996

10 Applied and Environmental Soil Science

[52] A R Sheldon and N W Menzies ldquoThe effect of copper toxicityon the growth and root morphology of Rhodes grass (Chlorisgayana Knuth) in resin buffered solution culturerdquo Plant andSoil vol 278 no 1-2 pp 341ndash349 2005

[53] X Du Y-G ZhuW-J Liu and X-S Zhao ldquoUptake of mercury(Hg) by seedlings of rice (Oryza sativa L) grown in solutionculture and interactions with arsenate uptakerdquo Environmentaland Experimental Botany vol 54 no 1 pp 1ndash7 2005

[54] C H C Shekar D Sammaiah T Shasthree and K J ReddyldquoEffect of mercury on tomato growth and yield attributesrdquoInternational Journal of Pharma and Bio Sciences vol 2 no 2pp B358ndashB364 2011

[55] S K Arya and B K Roy ldquoManganese induced changesin growth chlorophyll content and antioxidants activity inseedlings of broad bean (Vicia faba L)rdquo Journal of Environmen-tal Biology vol 32 no 6 pp 707ndash711 2011

[56] Z Asrar R A Khavari-Nejad and H Heidari ldquoExcess man-ganese effects on pigments ofMentha spicata at flowering stagerdquoArchives of Agronomy and Soil Science vol 51 no 1 pp 101ndash1072005

[57] SDonchevaKGeorgievaVVassileva Z StoyanovaN Popovand G Ignatov ldquoEffects of succinate on manganese toxicity inpea plantsrdquo Journal of Plant Nutrition vol 28 no 1 pp 47ndash622005

[58] M Shenker O E Plessner and E Tel-Or ldquoManganese nutri-tion effects on tomato growth chlorophyll concentration andsuperoxide dismutase activityrdquo Journal of Plant Physiology vol161 no 2 pp 197ndash202 2004

[59] I S Sheoran H R Singal and R Singh ldquoEffect of cadmiumand nickel on photosynthesis and the enzymes of the photosyn-thetic carbon reduction cycle in pigeonpea (Cajanus cajan L)rdquoPhotosynthesis Research vol 23 no 3 pp 345ndash351 1990

[60] B Y Khalid and J Tinsley ldquoSome effects of nickel toxicity onrye grassrdquo Plant and Soil vol 55 no 1 pp 139ndash144 1980

[61] T Pandolfini R Gabbrielli and C Comparini ldquoNickel toxicityand peroxidase activity in seedlings of Triticum aestivum LrdquoPlant Cell and Environment vol 15 no 6 pp 719ndash725 1992

[62] V S Barsukova and O I Gamzikova ldquoEffects of nickel surpluson the element content in wheat varieties contrasting in Niresistancerdquo Agrokhimiya vol 1 pp 80ndash85 1999

[63] Y-C Lin and C-H Kao ldquoNickel toxicity of rice seedlingsCell wall peroxidase lignin and NiSO

4-inhibited root growthrdquo

Crop Environment Bioinformatics vol 2 pp 131ndash136 2005[64] A Hussain N Abbas F Arshad et al ldquoEffects of diverse doses

of lead (Pb) on different growth attributes of Zea mays LrdquoAgricultural Sciences vol 4 no 5 pp 262ndash265 2013

[65] M Kabir M Z Iqbal andM Shafiq ldquoEffects of lead on seedlinggrowth of Thespesia populnea Lrdquo Advances in EnvironmentalBiology vol 3 no 2 pp 184ndash190 2009

[66] M Moustakas T Lanaras L Symeonidis and S KarataglisldquoGrowth and some photosynthetic characteristics of field grownAvena sativa under copper and lead stressrdquo Photosynthetica vol30 no 3 pp 389ndash396 1994

[67] R Manivasagaperumal S Balamurugan G Thiyagarajan andJ Sekar ldquoEffect of zinc on germination seedling growth andbiochemical content of cluster bean (Cyamopsis tetragonoloba(L) Taub)rdquo Current Botany vol 2 no 5 pp 11ndash15 2011

[68] S Doncheva Z Stoynova and V Velikova ldquoInfluence of succi-nate on zinc toxicity of pea plantsrdquo Journal of Plant Nutritionvol 24 no 6 pp 789ndash804 2001

[69] M Bonnet O Camares and P Veisseire ldquoEffects of zincand influence of Acremonium lolii on growth parameterschlorophyll a fluorescence and antioxidant enzyme activities ofryegrass (Lolium perenne L cv Apollo)rdquo Journal of ExperimentalBotany vol 51 no 346 pp 945ndash953 2000

[70] A M Nicholls and T K Mal ldquoEffects of lead and copperexposure on growth of an invasive weed Lythrum salicaria L(Purple Loosestrife)rdquoOhio Journal of Science vol 103 no 5 pp129ndash133 2003

[71] A Ghani ldquoToxic effects of heavy metals on plant growth andmetal accumulation in maize (Zea mays L)rdquo Iranian Journal ofToxicology vol 3 no 3 pp 325ndash334 2010

[72] A J M Baker ldquoAccumulators and excluders strategies in theresponse of plants to heavy metalsrdquo Journal of Plant Nutritionvol 3 pp 643ndash654 1981

[73] M J Blaylock D E Salt S Dushenkov et al ldquoEnhanced accu-mulation of Pb in Indian mustard by soil-applied chelatingagentsrdquo Environmental Science and Technology vol 31 no 3 pp860ndash865 1997

[74] M E V Schmoger M Oven and E Grill ldquoDetoxification ofarsenic by phytochelatins in plantsrdquo Plant Physiology vol 122no 3 pp 793ndash801 2000

[75] C Garbisu and I Alkorta ldquoBioremediation principles andfuturerdquo Journal of Clean Technology Environmental Toxicologyand Occupational Medicine vol 6 no 4 pp 351ndash366 1997

[76] C Garbisu and I Alkorta ldquoBasic concepts on heavy metal soilbioremediationrdquo The European Journal of Mineral Processingand Environmental Protection vol 3 no 1 pp 58ndash66 2003

[77] P Wang T Mori K Komori M Sasatsu K Toda and HOhtake ldquoIsolation and characterization of an Enterobacter cloa-cae strain that reduces hexavalent chromium under anaerobicconditionsrdquo Applied and Environmental Microbiology vol 55no 7 pp 1665ndash1669 1989

[78] Y Ishibashi C Cervantes and S Silver ldquoChromium reductionin Pseudomonas putidardquo Applied and Environmental Microbiol-ogy vol 56 no 7 pp 2268ndash2270 1990

[79] C Garbisu M J Llama and J L Serra ldquoEffect of heavy metalson chromate reduction by Bacillus subtilisrdquo Journal of Generaland Applied Microbiology vol 43 no 6 pp 369ndash371 1997

[80] C Garbisu I Alkorta M J Llama and J L Serra ldquoAerobicchromate reduction by Bacillus subtilisrdquo Biodegradation vol 9no 2 pp 133ndash141 1998

[81] C Garbisu S GonzalezW-H Yang et al ldquoPhysiological mech-anisms regulating the conversion of selenite to elementalselenium by Bacillus subtilisrdquo BioFactors vol 5 no 1 pp 29ndash371995

[82] R Ajaz Haja Mohideena V Thirumalai Arasuc K R Naray-ananb and M I Zahir Hussaind ldquoBioremediation of heavymetal contaminated soil by the exigobacterium and accumula-tion of Cd Ni Zn and Cu from soil environmentrdquo InternationalJournal of Biological Technology vol 1 no 2 pp 94ndash101 2010

[83] D van der Lelie P Corbisier L Diels et al ldquoThe role of bacte riain the phytoremediation of heavy metalsrdquo in Phytoremediationof Contaminated Soil andWater N Terry and E Banuelos Edspp 265ndash281 G Lewis Boca Raton Fla USA 1999

[84] M Huyer and W J Page ldquoZn2+ increases siderophore pro-duction in Azotobacter vinelandiirdquo Applied and EnvironmentalMicrobiology vol 54 no 11 pp 2625ndash2631 1988

[85] C White A K Sharman and G M Gadd ldquoAn integratedmicrobial process for the bioremediation of soil contaminatedwith toxic metalsrdquo Nature Biotechnology vol 16 no 6 pp 572ndash575 1998

Applied and Environmental Soil Science 11

[86] J L Hobman and N L Brown ldquobacterial mercury-resistancegenesrdquoMetal ions in biological systems vol 34 pp 527ndash568 1997

[87] D R Lovley and J R Lloyd ldquoMicrobes with a mettle for biore-mediationrdquo Nature Biotechnology vol 18 no 6 pp 600ndash6012000

[88] M Valls S Atrian V de Lorenzo and L A Fernandez ldquoEngi-neering amousemetallothionein on the cell surface ofRalstoniaeutropha CH34 for immobilization of heavy metals in soilrdquoNature Biotechnology vol 18 no 6 pp 661ndash665 2000

[89] M Urgun-Demirtas B Stark and K Pagilla ldquoUse of geneticallyengineered microorganisms (GEMs) for the bioremediation ofcontaminantsrdquo Critical Reviews in Biotechnology vol 26 no 3pp 145ndash164 2006

[90] O P Abioye ldquoBiological remediation of hydrocarbon and heavymetals contaminated soilrdquo in Soil Contamination S PascucciEd InTech Vienna Austria 2011

[91] A McCauley C Jones and J Jacobsen ldquoSoil pH and organicmatterrdquo in Nutrient Management Module vol 8 Montana StateUniversity Extension Bozeman Mont USA 2009

[92] A Karaca ldquoEffect of organic wastes on the extractability ofcadmium copper nickel and zinc in soilrdquo Geoderma vol 122no 2ndash4 pp 297ndash303 2004

[93] T Namgay B Singh and B P Singh ldquoInfluence of biocharapplication to soil on the availability of As Cd Cu Pb andZn tomaize (Zeamays L)rdquo Soil Research vol 48 no 6-7 pp 638ndash6472010

[94] J M Novak W J Busscher D L Laird M Ahmedna D WWatts and M A S Niandou ldquoImpact of biochar amendmenton fertility of a southeastern coastal plain soilrdquo Soil Science vol174 no 2 pp 105ndash112 2009

[95] D E Salt R D Smith and I Raskin ldquoPhytoremediationrdquoAnnual Review of Plant Biology vol 49 pp 643ndash668 1998

[96] R L Chaney M Malik Y M Li et al ldquoPhytoremediation ofsoil metalsrdquo Current Opinion in Biotechnology vol 8 no 3 pp279ndash284 1997

[97] A J M Baker and R R Brooks ldquoTerrestrial higher plantswhich hyperaccumulate metallic elements a review of theirdistribution ecology and phytochemistryrdquo Biorecovery vol 1pp 81ndash126 1989

[98] S P McGrath and F Zhao ldquoPhytoextraction of metals and met-alloids from contaminated soilsrdquoCurrentOpinion in Biotechnol-ogy vol 14 no 3 pp 277ndash282 2003

[99] R D Reeves and A J M Baker ldquoMetal-accumulating plantsrdquoin Phytoremediation of Toxic Metals Using Plants to Clean Upthe Environment I Raskin and B D Ensley Eds pp 193ndash229Wiley New York NY USA 2000

[100] L Q Ma K M Komar C Tu W Zhang Y Cai and ED Kenelley ldquoA fern that hyperaccumulates arsenicmdasha hardyversatile fast-growing plant helps to remove arsenic fromcontaminated soilsrdquo Nature vol 409 p 579 2001

[101] X E Yang X X Long H B Ye Z L He D V Calvert and P JStoffella ldquoCadmium tolerance and hyperaccumulation in a newZn-hyperaccumulating plant species (Sedum alfredii Hance)rdquoPlant and Soil vol 259 no 1-2 pp 181ndash189 2004

[102] F Navari-Izzo and M F Quartacci ldquoPhytoremediation ofmetalsrdquoMinerva Biotecnologica vol 13 no 2 pp 73ndash83 2001

[103] LVanGinneken EMeers RGuisson et al ldquoPhytoremediationfor heavy metal-contaminated soils combined with bioenergyproductionrdquo Journal of Environmental Engineering and Land-scape Management vol 15 no 4 pp 227ndash236 2007

[104] S D Ebbs and L V Kochian ldquoToxicity of zinc and copper toBrassica species implications for phytoremediationrdquo Journal ofEnvironmental Quality vol 26 no 3 pp 776ndash781 1997

[105] R L Chaney S L Brown L Yin-Ming et al ldquoProgress in riskassessment for soil metals and in-situ remediation and phyto-extraction of metals from hazardous contaminated soilsrdquo inProceedings of the US EPArsquos Conference Phytoremediation Stateof the Science Conference Boston Mass USA 2000

[106] Y Chen X Li and Z Shen ldquoLeaching and uptake of heavymetals by ten different species of plants during an EDTA-assisted phytoextraction processrdquo Chemosphere vol 57 no 3pp 187ndash196 2004

[107] H Lai and Z Chen ldquoThe EDTA effect on phytoextraction ofsingle and combined metals-contaminated soils using rainbowpink (Dianthus chinensis)rdquo Chemosphere vol 60 no 8 pp1062ndash1071 2005

[108] S C Wu K C Cheung Y M Luo andM HWong ldquoEffects ofinoculation of plant growth-promoting rhizobacteria on metaluptake by Brassica junceardquo Environmental Pollution vol 140no 1 pp 124ndash135 2006

[109] K K Chiu Z H Ye and M H Wong ldquoGrowth of Vetiveriazizanioides and Phragmities australis on PbZn and Cu minetailings amended with manure compost and sewage sludge agreenhouse studyrdquoBioresource Technology vol 97 no 1 pp 158ndash170 2006

[110] E Lombi F J Zhao S J Dunham and S P McGrath ldquoPhyto-remediation of heavy metal-contaminated soils Natural hyper-accumulation versus chemically enhanced phytoextractionrdquoJournal of Environmental Quality vol 30 no 6 pp 1919ndash19262001

[111] C D Jadia and M H Fulekar ldquoPhytotoxicity and remediationof heavy metals by fibrous root grass (sorghum)rdquo Journal ofApplied Biosciences vol 10 pp 491ndash499 2008

[112] V Laperche S J Traina P Gaddam and T J Logan ldquoEffect ofapatite amendments on plant uptake of lead from contaminatedsailrdquo Environmental Science and Technology vol 30 no 10 pp1540ndash1552 1997

[113] A P G C Marques R S Oliveira A O S S Rangel and P ML Castro ldquoApplication ofmanure and compost to contaminatedsoils and its effect on zinc accumulation by Solanum nigruminoculated with arbuscular mycorrhizal fungirdquo EnvironmentalPollution vol 151 no 3 pp 608ndash620 2008

[114] D C Adriano W W Wenzel J Vangronsveld and N SBolan ldquoRole of assisted natural remediation in environmentalcleanuprdquo Geoderma vol 122 no 2ndash4 pp 121ndash142 2004

[115] United States Environmental Protection Agency Electrokineticand Phytoremediation In Situ Treatment of Metal-ContaminatedSoil State-of-the-Practice EPA542R-00XXX EnvironmentalProtection Agency Office of Solid Waste and EmergencyResponse Technology Innovation Office Washington DCUSA 2000

[116] I Raskin and B D Ensley Phytoremediation of Toxic MetalsUsing Plants to Clean Up the Environment John Wiley amp SonsNew York NY USA 2000

[117] C L Rugh J F Senecoff R B Meagher and S A MerkleldquoDevelopment of transgenic yellow poplar formercury phytore-mediationrdquo Nature Biotechnology vol 16 no 10 pp 925ndash9281998

[118] R B Meagher C L Rugh M K Kandasamy G Gragsonand N J Wang ldquoEngineered phytoremediation of mercurypollution in soil and water using bacterial genesrdquo in Phytore-mediation of Contaminated Soil and Water N Terry and G

12 Applied and Environmental Soil Science

Banuelos Eds pp 201ndash219 Lewis Publishers Boca Raton FlaUSA 2000

[119] United States Environmental Protection Agency (USEPA)ldquoIntroduction to phytoremediationrdquo EPA 600R-99107 USEnvironmental Protection Agency Office of Research andDevelopment Cincinnati Ohio USA 2000

[120] R B Meagher ldquoPhytoremediation An Affordable FriendlyTechnology to Restore Marginal Lands in the Twenty-FirstCenturyrdquo 1998 httpwwwlscpsuedunasPanelistsMeagh-er20commenthtml

[121] N Terry A M Zayed M P de Souza and A S Tarun ldquoSele-nium in higher plantsrdquo Annual Review of Plant Biology vol 51pp 401ndash432 2000

[122] G S Banuelos H A Ajwa B Mackey et al ldquoEvaluation ofdifferent plant species used for phytoremediation of high soilseleniumrdquo Journal of Environmental Quality vol 26 no 3 pp639ndash646 1997

[123] NWeyens D van der Lelie S Taghavi L Newman and J Van-gronsveld ldquoExploiting plant-microbe partnerships to improvebiomass production and remediationrdquo Trends in Biotechnologyvol 27 no 10 pp 591ndash598 2009

[124] E J Joner and C Leyval ldquoTime-course of heavy metal uptakein maize and clover as affected by root density and differentmycorrhizal inoculation regimesrdquo Biology and Fertility of Soilsvol 33 no 5 pp 351ndash357 2001

[125] A Jamal N Ayub M Usman and A G Khan ldquoArbuscularmycorrhizal fungi enhance zinc and nickel uptake from con-taminated soil by soybean and lentilrdquo International Journal ofPhytoremediation vol 4 no 3 pp 205ndash221 2002

[126] A P G CMarques R S Oliveira AO S S Rangel and PM LCastro ldquoZinc accumulation in Solanum nigrum is enhanced bydifferent arbuscular mycorrhizal fungirdquo Chemosphere vol 65no 7 pp 1256ndash1263 2006

[127] A Heggo J S Angle and R L Chaney ldquoEffects of vesicular-arbuscular mycorrhizal fungi on heavy metal uptake by soy-beansrdquo Soil Biology amp Biochemistry vol 22 no 6 pp 865ndash8691990

[128] M Janouskova D Pavlıkova andM Vosatka ldquoPotential contri-bution of arbuscularmycorrhiza to cadmium immobilisation insoilrdquo Chemosphere vol 65 no 11 pp 1959ndash1965 2006

[129] L AHarrier andCAWatson ldquoThepotential role of arbuscularmycorrhizal (AM) fungi in the bioprotection of plants againstsoil-borne pathogens in organic andor other sustainable farm-ing systemsrdquo Pest Management Science vol 60 no 2 pp 149ndash157 2004

[130] I M Cardoso and T W Kuyper ldquoMycorrhizas and tropical soilfertilityrdquo Agriculture Ecosystems and Environment vol 116 no1-2 pp 72ndash84 2006

[131] S F Wright V S Green and M A Cavigelli ldquoGlomalin inaggregate size classes from three different farming systemsrdquo Soilamp Tillage Research vol 94 no 2 pp 546ndash549 2007

[132] G U Chibuike ldquoUse of mycorrhiza in soil remediation areviewrdquo Scientific Research and Essays vol 8 no 35 pp 1679ndash1687 2013

[133] G Dıaz C Azcon-Aguilar and M Honrubia ldquoInfluence ofarbuscularmycorrhizae on heavymetal (Zn and Pb) uptake andgrowth of Lygeum spartum and Anthyllis cytisoidesrdquo Plant andSoil vol 180 no 2 pp 241ndash249 1996

[134] E J Joner and C Leyval ldquoUptake of 109Cd by roots and hyphaeof a Glomus mosseaeTrifolium subterraneum mycorrhiza fromsoil amended with high and low concentrations of cadmiumrdquoNew Phytologist vol 135 no 2 pp 353ndash360 1997

[135] C C Chao and Y P Wang ldquoEffects of heavy-metals on theinfection of vesicular arbuscular mycorrhizae and the growthof maizerdquo Journal of the Agricultural Association of China vol152 pp 34ndash45 1990

[136] C Del Val J M Barea and C Azcon-Aguilar ldquoDiversity ofarbuscular mycorrhizal fungus populations in heavy-metal-contaminated soilsrdquo Applied and Environmental Microbiologyvol 65 no 2 pp 718ndash723 1999

[137] IWeissenhorn andC Leyval ldquoSpore germination of arbuscularmycorrhizal fungi in soils differing in heavy metal content andother parametersrdquo European Journal of Soil Biology vol 32 no4 pp 165ndash172 1996

[138] B R Glick D M Karaturovic and P C Newell ldquoA novelprocedure for rapid isolation of plant growth promoting pseu-domonadsrdquo Canadian Journal of Microbiology vol 41 no 6 pp533ndash536 1995

[139] A A Kamnev and D van der Lelie ldquoChemical and biologicalparameters as tools to evaluate and improve heavy metalphytoremediationrdquo Bioscience Reports vol 20 no 4 pp 239ndash258 2000

[140] A G Khan ldquoRole of soil microbes in the rhizospheres of plantsgrowing on trace metal contaminated soils in phytoremedia-tionrdquo Journal of Trace Elements in Medicine and Biology vol 18no 4 pp 355ndash364 2005

[141] B R Glick D M Penrose and J Li ldquoA model for the loweringof plant ethylene concentrations by plant growth-promotingbacteriardquo Journal ofTheoretical Biology vol 190 no 1 pp 63ndash681998

[142] M L E Reed and B R Glick ldquoGrowth of canola (Brassicanapus) in the presence of plant growth-promoting bacteria andeither copper or polycyclic aromatic hydrocarbonsrdquo CanadianJournal of Microbiology vol 51 no 12 pp 1061ndash1069 2005

[143] X Sheng and J Xia ldquoImprovement of rape (Brassica napus)plant growth and cadmium uptake by cadmium-resistant bac-teriardquo Chemosphere vol 64 no 6 pp 1036ndash1042 2006

[144] S Zaidi S Usmani B R Singh and J Musarrat ldquoSignificanceof Bacillus subtilis strain SJ-101 as a bioinoculant for concurrentplant growth promotion and nickel accumulation in Brassicajunceardquo Chemosphere vol 64 no 6 pp 991ndash997 2006

[145] M Madhaiyan S Poonguzhali and S A Torgmin ldquoMetaltoleratingmethylotrophic bacteria reduces nickel and cadmiumtoxicity and promotes plant growth of tomato (Lycopersiconesculentum L)rdquo Chemosphere vol 69 no 2 pp 220ndash228 2007

[146] A Vivas B Biro J M Ruız-Lozano J M Barea and R AzconldquoTwo bacterial strains isolated from a Zn-polluted soil enhanceplant growth and mycorrhizal efficiency under Zn-toxicityrdquoChemosphere vol 62 no 9 pp 1523ndash1533 2006

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