remediation of heavy metal contaminated soils: an overview of site remediation techniques

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Remediation of Heavy MetalContaminated Soils: An Overview of SiteRemediation TechniquesAna P. G. C. Marques a , António O. S. S. Rangel a & Paula M. L.Castro aa Escola Superior de Biotecnologia, Universidade CatólicaPortuguesa, Rua Dr. António Bernardino de Almeida, Porto, PortugalVersion of record first published: 13 Apr 2011.

To cite this article: Ana P. G. C. Marques , António O. S. S. Rangel & Paula M. L. Castro (2011):Remediation of Heavy Metal Contaminated Soils: An Overview of Site Remediation Techniques, CriticalReviews in Environmental Science and Technology, 41:10, 879-914

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Critical Reviews in Environmental Science and Technology, 41:879–914, 2011Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643380903299517

Remediation of Heavy Metal ContaminatedSoils: An Overview of Site Remediation

Techniques

ANA P. G. C. MARQUES, ANTONIO O. S. S. RANGEL,and PAULA M. L. CASTRO

Escola Superior de Biotecnologia, Universidade Catolica Portuguesa,Rua Dr. Antonio Bernardino de Almeida, Porto, Portugal

At the interface between the atmosphere and the earth’s crust andbeing the substrate for natural and agricultural ecosystems, thesoil is open to inputs of heavy metals from many sources. Pollutionof the biosphere with toxic metals has accelerated dramaticallysince the beginning of the industrial revolution. In response to agrowing need to address environmental contamination, many re-mediation technologies have been developed to treat contaminatedsoil, mainly mechanically or physicochemically based remediationmethods, but more recently thermal and biological technologiesseem to call for the attention of the scientific community, remedia-tion project engineers and the general public. These techniques aswell as their application and viability for the remediation of heavymetal contaminated soils is discussed.

KEY WORDS: soil pollution, heavy metals, remediation

INTRODUCTION

Pollutants may be introduced into the environment as a result of accidents,spills, and leaks from storage sites or industrial facilities (Riser-Roberts, 1992).Among pollutants, heavy metals are a group of much concern due to theirimmutable nature. The term heavy metal, although not easily defined, iswidely recognized and used. It is commonly adopted as a group name for

Address correspondence to Paula M. L. Castro, Escola Superior de Biotecnologia, Univer-sidade Catolica Portuguesa, Rua Dr. Antonio Bernardino de Almeida, 4200-072 Porto, Portugal.E-mail: [email protected]

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880 A. P. G. C. Marques et al.

the metals and metalloids, which are associated with pollution and toxicity,but also includes some elements that are essential for living organisms at lowconcentration (Alloway, 1990). The existing classification is based on atomicdensity (>5 g cm−3) but it includes a very disparate group of elements(Adriano, 2001). Heavy metals commonly found in the heart crust includeFe, Pb, Hg, As, Cr, Cd, Ni, Zn, and Cd (Peters, 1999).

Government, industry, and the public now recognize the potential dan-gers that heavy metals pose to human health and the environment (Khanet al., 2004). The danger of toxic metals is aggravated by their almost in-definite persistence in the environment (Garbisu and Alkorta, 2001). Heavymetals cannot be destroyed biologically but can only be transformed fromone oxidation stage or organic complex to another. As a consequence ofthe alteration of its oxidation state, the metal may become either more watersoluble (easily removable by leaching), inherently less toxic, less water solu-ble (so that it precipitates and then becomes less bioavailable), or volatilizedand removed from the polluted area (Garbisu and Alkorta, 1997).

The soil is open to inputs of heavy metals from many sources (Alloway,1990). Metal ions can be retained in soil by adsorption, precipitation, andcomplexation: in the adsorption process, charged solute ions are attracted tothe charged soil surface by electrostatic attraction or through the formation ofspecific bonds; the complexation mechanism involves the formation of bothorganic and inorganic complexes between metals and a range of solutesin soils (Adriano et al., 2004); precipitation appears to be the predominantprocess of metal immobilization in alkaline soils in the presence of anionssuch as sulfate, carbonate, hydroxide, and phosphate, especially when theconcentration of the metal ion is high (Adriano, 2001).

Pollution of the biosphere with toxic metals has accelerated dramat-ically since the beginning of the industrial revolution (Nriagu, 1979). Theprimary sources of this pollution are the burning of fossil fuels, the min-ing and smelting of metalliferous ores, metallurgical industries, municipalwastes, fertilizers, pesticides, and sewage (Alloway, 1990; Kabata-Pendiasand Pendias, 1989). In addition to sites contaminated by human activity, nat-ural mineral deposits containing particularly large quantities of heavy metalsare present in many regions of the globe (Memon et al., 2001). Heavy metalsoccur naturally in soils, usually at relatively low concentrations, as a result ofthe weathering and other pedogenic processes acting on the rock fragmentson which the soils develop (parent material; Alloway, 1990). The heavymetal concentrations inherited from the soil parent material are modified bypedogenic and biogeochemical processes, by natural inputs such as dustparticles derived from soil, rocks, and volcanic ash and, most importantly,by anthropogenic inputs (i.e., pollution).

Although some metals are essential for life, providing essential cofactorsfor metalloproteins and enzymes, at high concentrations they can act in adeleterious manner by blocking essential functional groups, displacing other

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Remediation of Heavy Metal Contaminated Soils 881

metal ions, or modifying the active conformation of biological molecules(Collins and Stotzky, 1989). Heavy metals may also stimulate the formationof free radicals and reactive oxygen species, which may result in oxidativestress (Dietz et al., 1999). In addition, they are toxic for both higher organ-isms and microorganisms. Many of the heavy metals are toxic even at verylow concentrations. In fact, some metals affect directly various biochemi-cal and physiological processes causing reduction in growth, inhibition ofphotosynthesis and respiration, and degeneration of main cell organelles(Vangronsveld and Clijsters, 1994).

In response to a growing need to address environmental contamination,many remediation technologies have been developed to treat soil contami-nated by heavy metals, including in situ and ex situ methods (Riser-Roberts,1998); most of these techniques fall into two major categories: immobilizationand extraction. Immobilization involves the fixation of heavy metals, therebypreventing their migration; extraction procedures employ a combination ofphysical, chemical, thermal, and biological methods for the actual removalof heavy metals from soils.

The aim of this work is to review the main physical, chemical, ther-mal, and biological technologies available for soil remediation, focusing onthe processes, advantages, and disadvantages brought by their application.The methods approached include washing, flushing, solidification, stabiliza-tion, thermal desorption, encapsulation, electrokinetics, chemical oxidation,bioremediation, phytoremediation, and vapor extraction. Since most remedi-ation technologies are site-specific, the selection of appropriate technologiesis often difficult, but is a crucial step in the success of the remediation of aheavy metal contaminated site (Khan et al., 2004).

TECHNOLOGIES FOR HEAVY METAL REMEDIATION FROM SOIL

Soil Washing

Soil washing refers to an ex situ technique that employs physical or chemicalprocedures to extract contaminants from a previous excavated soil. Thewashing process separates the fine clay and silt portion of the soil fromthe sand and gravel part (Khan et al., 2004). Because metals and othercontaminants tend to bind and sorb to smaller soil particles, separating thisfraction from the larger one allows a reduction of the volume of soil actuallycontaminated (Riser-Roberts, 1998). This smaller volume of contaminatedsoil can then be treated by other methods or can be disposed according tothe regulations of the country in state. A scheme of a soil washing procedureis described in Figure 1.

Physical separation concentrates metal contaminants into a smaller vol-ume of soil by exploiting differences in certain physical characteristics be-tween the metal bearing particles and soil particles (size, density, magnetism,

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soilscreening

soilwashing

contaminatedsoil

contaminatedsludges

cleansoil

rejectsrejects

wastewaterwastewatertreatment

treatedwater

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emissionstreatment

treatedairemissions

surfactants water

FIGURE 1. Diagram of a soil washing process (adapted from FRTR, 2002).

and hydrophobic surface properties). Physical separation techniques are ap-plicable mainly on particulate forms of metals: discrete or metal bearingparticles (Dermont et al., 2008). The efficiency of the method depends onseveral soil properties, namely particle size distribution, particulate shape,clay, moisture and humic contents, heterogeneity of soil matrix, magneticproperties, difference in density between soil matrix and metal contaminants,and hydrophobic properties of particle surface (U.S. Environmental Protec-tion Agency [USEPA], 1995; Williford and Bricka, 2000). The treatment by soilwashing via physical separation is unfeasible when (a) the metal contam-inants are strongly bound to soil particles, (b) high variability of chemicalforms of metals is present, (c) the difference in density of surface prop-erties between metal bearing particles and soil matrix are not significant,(d) the soil contains silt/clay content in excess of 30–50%, (e) the metalsare present in all particle size fractions of contaminated soil, (f) the soilpresents organic compounds with high viscosity, (f) the soil contains highhumic content, and (g) a treatment of the sorbed forms of the metals isto be accomplished. Different types of physical separation methods for soilwashing can be used, such as mechanical screening (size exclusion througha physical barrier), hydrodynamic classification (separates the particles bydifference of settling velocity), gravity concentration (separates based on thedensity of particles), froth flotation (separation based on the hydrophobicproperties of the surface of particles), magnetic separation (mineral particlesare separated according to their different magnetic susceptibilities), electro-static separation (separation based on electrical conductivity properties), andattrition scrubbing (mechanical particle-to-particle scrubbing to remove coat-ing of particle surface; Dermont et al., 2008). In order to render this typeof treatment cost-effective, the volume of soil to be treated should be large,

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because big and expensive equipments are often required, which constitutesa disadvantage. However, it has the advantage of being an established tech-nique, allowing organic and inorganic treatment in a sole unit with a greatreduction in the final volume of soil for further treatment (USEPA, 1995).

In chemical soil washing, soil is scrubbed mechanically with water andwash-improving additives in adequate reactors. Additives applied can bemainly of five types: acids (to extract metals by ion exchange and dissolutionof soil components), salts and high concentration chloride solutions at lowpH (combines the acid action with the formation of metal chlorocomplexesto extract metals from soils), chelating agents (solubilize metals through com-plexation), surfactants (target desorption of metals from soil interface), andreducing or oxidizing agents (used to enhance metal solubilization througha valence change). The choice of the agent to be used in the separationprocedure depends on the metal type, concentration, and speciation, as wellas on soil characteristics. The efficiency of the chemical separation dependson the metal contamination characteristics, dosage, and chemistry of appliedagent and processing conditions, but mainly on soil geochemistry and frac-tioning of the metal in the soil (Dermont et al., 2008). The fractions moreamenable to metal removal by this procedure are those exchangeable, thenthose associated with carbonates, and finally the fraction associated with re-ducible Fe-Mn oxides of soils (Peters, 1999). One of the main disadvantagesof this type of procedure is that the processed soil may become inappropri-ate for revegetation and on site disposal because the physicochemical andmicrobiological properties have been affected. Additionally, the presence ofcertain chemical agents in the wash fluid can complicate water recycling andtreatment, thus increasing the cost of the overall process. However, chem-ically based soil washing can become attractive if the chemicals employedare recycled, detoxified or not hazardous, which, together with the abilityto recover and recycle the metals, constitutes one of the most importantadvantages of the method (Dermont et al., 2008).

For chemical soil washing, several reagents can be applied to composethe washing solution, such as inorganic or organic acids, chelating agents andbiosurfactants, or combinations of those (Mulligan et al., 1999). Hydrochloricacid was used in the washing of Pb (Isoyama and Wada, 2007), As, Cu, andZn (Moutsatsou et al., 2006) contaminated soils with consequent reductionsin metal concentration in the soil solution. Successful cases of applicationof physical soil washing have also been reported, namely the remediationof Hg, As, Pb, Cr, Cu, Ni, and Zn contaminated soils (Dermont et al., 2008;Table 1).

In fact, soil washing by either chemical or physical separation techniquesis a separation method and not really a treatment process, which is its maindisadvantage. Because soil washing does not immobilize contaminants, theresulting soil must be disposed of carefully and washing water needs to betreated. Additionally, the procedure implies the removal of the soil. However,

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soil washing is generally cost-effective because it allows a reduction on theamount of material that requires treatment using another technique (Khanet al., 2004).

Soil Flushing

Soil flushing is an in situ technology that floods contaminated soils with anextraction fluid composed of water with or without further additives, gener-ally metal chelating agents for metal-contaminated soil flushing. The effect ofthese agents consists on increasing the water solubility of the contaminatingspecies, so that they can be easily removed from the solid matrix (Svab et al.,2008). The fluid is added via an injection or infiltration process—via surfaceflooding, sprinklers, leach fields, basin infiltration systems, surface trenches,and horizontal and vertical drains (Mulligan et al., 2001)—which moves themetals to a selected area in the soil where they are captured and pumpedthrough a groundwater extraction well (Khan et al., 2004). Recovered extrac-tion fluids, and pumped groundwater, are subsequently treated to meet thecountry-appropriate standards before being released into the environment(Otterpohl, 2002). The pumped solutions can be reutilized into the flushingprocess (Mulligan et al., 2001). A diagram of the flushing process is shownon Figure 2.

The posttreatment of recovery fluids is one of the most evident disad-vantages of this technique, as it may render the full process of treatment ex-pensive. However, as soil flushing is a remediation method that is performedin situ, the costs of excavation and handling of a contaminated soil are in-existent, rendering the technique cost-reasonable. Further disadvantages arethe dependence on the permeability of the soil to be treated—soils withlow permeability are difficult to treat as the flushing solution cannot movethough the soil and make contact with the disseminated contaminants—andthe need of hydraulic control to avoid the movement of contaminants off-site (Johnston et al., 2002). Therefore, understanding the chemistry of the

FIGURE 2. Diagram of soil flushing (adapted from FRTR, 2002; Mulligan et al., 2001).

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binding of the contaminant and the hydrogeology of the site are crucial.Additionally, remediation times are usually lengthy because of the slownessof the diffusion processes (Khan et al., 2004).

Soil flushing has been reported as a clean-up method in several studies(Di Palma and Medici, 2002; Tsang et al., 2007). with reported recoveries ofthe metal from the resulting extracting solution up to 92% through furtherprecipitation (Di Palma et al., 2003; Table 1).

Solidification

Solidification refers to the physical-based remediation process that encapsu-lates the soils in a monolithic solid of high structural integrity (Khan et al.,2004) and can be performed either in situ or ex situ.

A possible method of solidification is vitrification, which uses a powerfulsource of energy—via electrical, plasma, or thermal processes—to melt soilat extremely high temperatures (1600–2000C), volatilizing volatile metals(e.g., mercury) and immobilizing nonvolatile metals into a chemically inertglass product (Khan et al., 2004), which is strong, durable, and resistant toleaching (Dermatas and Meng, 2003). A diagram of the vitrification processis represented in Figure 3. If performed ex situ, the obtained glasses canthus be disposed into landfills or used for roads and pavements, among oth-ers. An additional advantage of this method is that it can handle wastes ofdifferent origins, compositions, and forms—therefore a well designed vitrifi-cation plant can be flexible enough to treat wastes of various types (Colomboet al., 2003). Nevertheless, toxic gases can be formed during the process andleaching can also occur, which can be pointed as serious disadvantages ofthis remediation technology (Mulligan et al., 2001). The high energetic con-sumption that incurs in elevated cost for the vitrification process can also bepointed out as a serious disadvantage of this method (Colombo et al., 2003).

FIGURE 3. Scheme of an in situ vitrification technique (Mulligan et al., 2001).

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Vitrification has been successfully used to remediate sediments contaminatedwith Be (Bhat et al., 2002), as described in Table 1.

Other solidification techniques have been reported and can be usedto treat contaminated soils. Bituminization is one of these methods; in thisprocess, excavated soils are embedded in molten bitumen and encapsulatedwhen the bitumen cools. The process combines heated bitumen and a con-centrate of the soil material, usually in slurry form, in a heated extrudercontaining screws that mix the bitumen and waste. Water is evaporated fromthe mixture to about 0.5% moisture. The final product is a homogenousmixture of extruded solids and bitumen (Federal Remediation TechnologiesRoundtable [FRTR], 2002).

Other solidification methods, involving the addition of cement to thecontaminated soil, are documented but not far tested. According to Mulliganet al. (2001), cement was successfully applied to contaminated soil to reduceleachability of As, Ba, Cd, Cr, Pb, Hg, Se, and Ag in pilot-scale studies; Bhatet al. (2002) also described the application of cement to Be-contaminatedsoils with positive results concerning g reduction of leaching (Table 1).Processes such as modified sulfur cement (MSC) are amongst these possi-ble methods of solidification. MSC is a commercially available thermoplasticmaterial; it is easily melted (127–149C) and then mixed with the contam-inated soils to form a homogenous molten slurry, which is discharged tosuitable containers for cooling, storage, and disposal. A variety of commonmixing devices such as paddle mixers and pug mills can be used. The rela-tively low temperatures used limit emissions of sulfur dioxide and hydrogensulfide to allowable threshold values. Another process, the polyethylene ex-trusion method, involves the mixing of polyethylene binders and dry soilusing a heated cylinder containing a mixing/transport screw. The heated,homogenous mixture exits the cylinder through an output die into a mould,where it cools and solidifies. Polyethylene’s properties produce a very sta-ble, solidified product. Cementitious waste forms have been used also byapplying sulfur polymer cement (SPC) to stabilize soils with high loadingsof volatile toxic metals; SPC is a sulfur composite material with a meltingpoint of 110–120C that resists attack by most acids and salts. Studies haveshown that the compound has a very long life and its strength greatly in-creases within the first few years after forming. The advantages of SPC isthat it has a greater soil-to-agent ratio and is less permeable than concrete,and it has the ability to be remelted and reformed (Hamby, 1996). Finally,the Pozzolan/Portland cement process can be applied to contaminated soilsconsisting primarily of silicates from pozzolanic-based materials like fly ash,kiln dust, pumice, or blast furnace slag, and cement-based materials such asPortland cement. These materials chemically react with water to form a solidcementious matrix that improves the handling and physical characteristics ofthe soil. They also raise the pH of the water, which may help precipitate andimmobilize some heavy metal contaminants (FRTR, 2002). As an example

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of the application of this type of treatment, the successful pilot-scale projectconsisting on the addition of mineral compounds for Pozzolan productionfor metal dilution and fixation has been included in the reports of Mulliganet al. (2001; Table 1).

If not completed properly, these solidification processes may result ina significant increase in contaminant volume, which represents one of therisks of the technology. As other disadvantages it should be pointed that thedepth of the contamination may limit these procedures and that long termmonitoring is often necessary to ensure that the contaminants are indeedimmobilized (Khan et al., 2004).

Stabilization

Stabilization (or immobilization) is a remediation technology that allows thereduction of the hazard posed by the contaminated soil by converting themetals into less soluble, immobile, and less toxic forms (Khan et al., 2004).Most metals occur naturally at varying concentrations and in varying chemicalforms, and chemical form of the original metal may vary from solid metal(less mobile) to aqueous solution of a salt (more mobile). Chemical formsare interchangeable depending upon the soil conditions and history, and thestabilization process relies on this interchangeability, as shown in Figure 4,in order to displace the equilibrium to the immobilization of the metals byincreasing the soil ability to bound metals and thus decrease its availability.

The application of this technique mostly relies on the fundamental un-derstanding of natural geochemical processes, governing the speciation, mi-gration, and bioavailability of metals in the soil (Raicevic et al., 2005) andgenerally involves the addition of amendments to the soil. The use of in-expensive additives such as minerals (apatite, zeolite, or clay) or waste by-products (steel shot, beringite, iron-rich biosolids) can dramatically decrease

FIGURE 4. Scheme of a stabilization process (adapted from Danish EPA, 2003).

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the cost of remediation of a polluted soil (Raicevic et al., 2005). The pro-cess can be performed in situ or ex situ; if performed in situ mixing thesoil in place and a means to deliver the reagent into the soil mixing zoneare necessary (Nyer, 1996). If performed in situ the costs of stabilizationare significantly reduced. In fact, using inexpensive reactive amendmentsis considered as a simple and cost-effective method for the remediation ofmetal-contaminated soils when these contaminated matrices are difficult orcostly to remove to be treated ex situ (Raicevic et al., 2005). However, costsassociated with the necessary long-term monitoring should also be consid-ered. This method does not allow the removal of the contaminants fromthe soil, but only their immobilization; additionally, if performed in situ,the depth of the contaminants may limit the procedure (Khan et al., 2004).Furthermore, organic contaminants that can not be immobilized can furthermigrate through the soil.

Stabilization is by far one of the techniques most frequently applied inpilot-scale studies to treat metal-contaminated soils. Bes and Mench (2008)assessed the potential of several organic and phosphate amendments for thestabilization of a Cu-contaminated soils and found that the application ofactivated carbon and zerovalent iron grit (single or in combination) signif-icantly reduced Cu concentration in soil solution. Reagent grade stabilizerssuch as CaHPO4 and CaCO3 have been used with success by Wang et al.(2001) in the remediation of a multimetal-contaminated soil via stabilization.Mahabadi et al. (2007) reported that the application of clinoptitolite has re-duced Cd leaching from a contaminated soil. Kumpiene et al. (2008) assessedthe stabilization of a Pb- and Cu-contaminated soil with coal fly ash for atwo-year period and observed that the amount of metals leached decreasedby up to 99%. Further details of these studies are described in Table 1.

Thermal Desorption

Thermal desorption is a remediation technology in which contaminated soilis excavated, screened, and heated to release volatile components from it. Itinvolves heating the contaminated soil to temperatures of 100–600C in anappropriate chamber so that those contaminants with boiling points in thatrange are vaporized and consequently separated from the soil (Khan et al.,2004). A diagram of a general process of thermal desorption is presented inFigure 5.

Three types of thermal desorption are available and briefly describedas the following (a): direct-fired thermal desorption, in which fire is applieddirectly on the surface of contaminated media. The main purpose of the fireis to desorb contaminants from the soil though some contaminants may bethermally oxidized; (b) indirect-fired thermal desorption, in which a direct-fired rotary dryer heats an air stream which, by direct contact, desorbs waterand organic contaminants from the soil; and (iii) indirect-heated thermal

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FIGURE 5. Diagram of a thermal desorption process (adapted from FRTR, 2002).

desorption, in which an externally fired rotary dryer volatilizes the waterand organics from the contaminated media into an inert carrier gas stream.The carrier gas is later treated to remove or recover the contaminants.

Based on the operating temperature of the desorber, thermal desorp-tion processes can be categorized into two groups: high-temperature thermaldesorption (HTTD) and low-temperature thermal desorption (LTTD). HTTDis a full-scale technology in which wastes are heated to 320–560C and is fre-quently used in combination with solidification and stabilization, dependingon site-specific conditions; in LTTD, soil is heated to between 90 and 320C(FRTR, 2002).

Although it is mainly applied for contamination with hydrocarbons,thermal desorption can also be used in the remediation of soils contami-nated with volatile metals, such as mercury (Khan et al., 2004). According toWeyand et al. (1994), thermal desorption has been applied with success formercury recovery obtaining metallic mercury at 99% purity (Table 2). Gen-erally, the mercury exists in the soil only in the form of elemental state oras mercury (II) compounds, such as HgS, HgO, and HgCO3. When temper-ature reaches 600–800C, these mercury compounds can be converted intogaseous mercury (Chang and Yen, 2006). The vaporized contaminants arecollected and need then to be treated by further means. This appears to beone of the main disadvantages of this method, similarly to the technologiesdescribed previously, as the treatment of the resulting contaminated gaseouseffluent results in additional costs. Furthermore, being an ex situ technol-ogy, it involves excavation and handling of the soils, which also representsadditional costs. However, and when compared to other remediation tech-nologies, it has higher desorption efficiency (up to 99%) if soils with lowmoisture are used, as the presence of water reduces the efficiency of themethod and is insensitive to the concentration of the target contaminants(s)in the soil.

As seen in Table 2, few studies have assessed the efficiency of thermaldesorption to remediate Hg-contaminated soils, indicating that within middlerange temperatures (540–650C) a decrease of the concentration of mercuryto a level below 2 mg/kg dry soil was obtained and that it was possible toreclaim the mercury with a purity of up to 99% (Chang and Yen, 2006).

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TA

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Soil decontaminated by this method retains its physical properties, whichconstitutes an important advantage. Unless the soil is heated to the higherend of the LTTD temperature range, organic components in the soil are notdamaged, which enables treated soil to retain the ability to support furtherbiological activity (FRTR, 2002).

Encapsulation

Encapsulation is an in situ remediation method in which the soil is physicallyisolated and contained. The disturbed soils are isolated by a low permeabil-ity protection media, such as caps, curtains, or walls, designed to limit theinfiltration of precipitation and consequently prevent leaching and furtherdispersion of the metals throughout the soil and to the groundwater (Khanet al., 2004). Vertical barriers reduce the movement of groundwater throughthe contaminated soil; horizontal barriers within the soil are helpful in avoid-ing downward movement of contaminants (Mulligan et al., 2001). Encapsu-lation can also be used to contain soil while other treatment is applied orto create a land surface that can support vegetation or be used for otherpurposes (FRTR, 2002).

Encapsulation is one of the most common forms of remediation be-cause it is generally less expensive than other technologies and effectivelymanages the human and ecological risks associated with a remediation site.The design of an encapsulation project is site specific and depends on theintended functions of the system. A diagram representing a putative encap-sulation system is shown in Figure 6. Systems can range from a one-layercap of vegetated soil to a complex multilayer system of soils and geosynthet-ics. In general, less complex systems are required in dry climates and more

FIGURE 6. Scheme of an encapsulation process (adapted from FRTR, 2002).

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complex systems are required in wet climates. The materials used in the con-struction of encapsulation systems include low- and high-permeability soilsand low-permeability geosynthetic products. The low-permeability materialsdivert water and prevent its passage into the contaminated soil, whereas thehigh-permeability materials carry water away that percolates into the cap.Other materials may be used to increase stability of the system. The mostcritical components of an encapsulation system are the barrier layer andthe drainage layer. The barrier layer can be low-permeability soil (clay) orgeosynthetic clay liners (GCLs). A flexible geomembrane liner is placed ontop of the barrier layer. The list of polymers commonly used is lengthy,including polyvinyl chloride (PVC), polyethylenes of various densities, re-inforced chlorosulfonated polyethylene (CSPE-R), polypropylene, ethyleneinterpolymer alloy (EIA), and many newcomers. Soils used as barrier materi-als generally are clays that are compacted. A composite barrier uses both soiland a geomembrane, taking advantage of the properties of each material.The geomembrane is essentially impermeable, but if it develops a leak, thesoil component prevents significant leakage into the underlying soil (FRTR,2002).

Anderson and Mitchell (2003) successfully applied encapsulation usingsilica barriers in the treatment of an Hg-contaminated site, preventing leach-ing and migration of the contaminants through the soil and to groundwater.It should be kept in mind however, that, similarly to stabilization, encapsula-tion does not allow the removal of the contaminants from the soil, which canbe referred as a serious disadvantage of these technologies. Additionally, theefficiency of the process of encapsulation decreases with time and dependson the characteristics of the site and the depth of contamination, which areresponsible for the costs in the implementation of the method (Khan et al.,2004).

Electrokinetics

Electrokinetics is an electrolytic process that involves passing a low-intensityelectric current between a cathode and an anode imbedded in the contami-nated soil. Ions and small charge particles, as well as water, are transportedbetween the electrodes, anions moving toward the cathode and cations mov-ing toward the anode. Buffer solutions are used to maintain the pH at theelectrodes (Mulligan et al., 2001).

The main goal of electrokinetic remediation is to promote the migrationof subsurface contaminants in an imposed electric field via electroosmosis,electromigration, and electrophoresis. These three phenomena can be sum-marized as the following: (a) electroosmosis is the movement of soil mois-ture or groundwater from the anode to the cathode of an electrolytic cell,(b) electromigration is the transport of ions and ion complexes to the elec-trode of opposite charge, and (c) electrophoresis is the transport of charged

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particles or colloids under the influence of an electric field; contaminantsbound to mobile particulate matter can be transported in this manner. Thephenomena occur when the soil is charged with low-voltage direct current.The process may be enhanced through the use of surfactants or reagentsto increase the contaminant removal rates at the electrodes (Virkutyte et al.,2002). Additionally, other nonionic contaminants can also be removed asthey are transported due to the flow caused by electroosmosis. When theremediation process is over, extraction and removal of heavy metal con-taminants are accomplished by electroplating at the electrode, precipitationor coprecipitation at the electrode, pumping water near the electrode, orcomplexing with ion exchange resins. Adsorption onto the electrode mayalso be feasible, as some ionic species change their valence near the elec-trode, depending on the soil pH., making them more likely to adsorb (VanCauwenberghe, 1997). A scheme of the electrokinetics process is shown inFigure 7. Large objects and rocks that may be in the soil can cause obstacles

FIGURE 7. Diagram of an electrokinetics procedure (adapted from FRTR, 2002; Mulliganet al., 2001).

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and interfere in the procedure (Acar and Gale, 1995), which represents oneof the main disadvantages of the method. As an advantage, it can be statedthat electrokinetics can be used either in situ or ex situ.

During the electrokinetics process, the equilibrium at the soil surfaceis modified by the acid and basic fronts, the first generated by oxidationof water occurring at the anode, which advances toward the cathode, andthe second caused by the reduction of water at the cathode, which movestoward the anode. The basic front may cause precipitation of heavy metalsas hydroxides, decreasing the effectiveness of the process (Mascia et al.,2007). In order to prevent these problems, several enhanced processes havebeen developed, which are based on the control of pH near the cathode byaddition of organic solutions (Puppala et al., 1997; Zhou et al., 2004).

Two approaches can be taken during electrokinetic remediation: en-hanced removal and treatment without removal. Enhanced removal is widelyused on remediation of metal-contaminated soils and is achieved by elec-trokinetic transport of contaminants toward the polarized electrodes to con-centrate the contaminants for subsequent removal and ex situ treatment.Removal of contaminants at the electrode may be accomplished by sev-eral means: electroplating at the electrode, precipitation or coprecipitationat the electrode, pumping of water near the electrode, or complexing withion exchange resins. Treatment without removal is achieved by electroos-motic transport of contaminants through zones placed between electrodes.The polarity of the electrodes is reversed periodically, which reverses thedirection of the contaminants back and forth through treatment zones—thefrequency with which electrode polarity is reversed is determined by the rateof transport of contaminants through the soil. This late approach however ismore used on in situ remediation of soils contaminated with organic species(FRTR, 2002).

Electrokinetics processes can present different apparatus and designs.In the cation-selective membrane procedure, which occurs under alkalinemedium, heavy metals are likely to be adsorbed onto the soil particles andform insoluble precipitates. Removal of heavy metals using surfactant-coatedceramic casings is also possible: in order to control the hydraulic flux ofwater in the treated soil, porous ceramic castings are used, generally in theanode. In the Lasagna process there is a creation of several permeable treat-ment zones in close proximity through the whole soil matrix and applica-tion of an electric current in order to transport contaminants into the zonescreated (Virkutyte et al., 2002). The Electro-Klean electrical separation isanother electrokinetics process, whichuses two electrodes to apply the elec-tric field directly into the contaminated soil mass. Further processes includeelectrochemical geooxidation, which involves the application of an electri-cal current to probes driven into the ground. The applied current createsfavorable conditions for redox reactions, which lead to the immobilization

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of inorganic contaminants in the soil between the electrode locations (VanCauwenberghe, 1997).

To dissolve metallic hydroxides and carbonates formed or for othercompounds absorbed by soil it is necessary to create an acid medium. How-ever this acidification has disadvantages, as it favors the release of the heavymetal contaminants into the solution phase resulting in a low process ef-ficiency, which seems to be a disadvantage of the method. Acid additionleads to high soil acidification whose consequences cannot be estimated.However, achieving these acidic conditions might be difficult when the soilbuffering capacity is high. Additionally, the process is quite time consum-ing and highly dependent on charge density from the clay particles surface,cation type and concentration, organic matter and carbonates presence, andsoil’s pH (Virkutyte et al., 2002).

Electrokinetics has been evaluated in bench- and field-scale studies.Gent et al. (2004) showed that electrokinetic treatments could significantlyreduce the levels of Cr and Cd in a metal-contaminated soil. Reddy andChinthamreddy (1999) observed successful results in the bench-scale elec-troremediation of a soil contaminated with Ni, Cd, and Cr. Further details onthe previously mentioned studies are described in Table 2.

Chemical Oxidation

Chemical oxidation is a remediation technology based on the redox reactionswith contaminants. This method involves the percolation of inorganic andorganic reagents to reduce metals to their lowest valence state and to formstable organometallic complexes. Consequently, the treated residue becomesless soluble over time and thus has less leaching probability. A diagram of achemical oxidation treatment is presented in Figure 8. Although not far doc-umented for metals, oxidation technologies are part of the many treatmentalternatives that have the capacity to reduce the toxicity, and sometimes thevolume, of contaminants in soil. The oxidizing compounds that are addedto the system should be easy to incorporate into the selected environmentalmedia under treatment. Chlorine dioxide, hydrogen peroxide, and potassiumpermanganate are frequently used as oxidizing agents either for organic orinorganic contaminants (Hamby, 1996). Sodium polythiocarbonate has alsobeen used successfully for the conversion of heavy metals, such as Cd, intostable, nontoxic forms (Mulligan et al., 2001).

The low cost of this procedure appears as one of the main advantagesof chemical oxidation as a soil remediation technology. Nevertheless, theprocess is not cost-effective for high-contaminant concentrations because ofthe large amounts of oxidizing agent required, and oil and grease in themedia should be minimized to optimize process efficiency (FRTR, 2002).

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FIGURE 8. Scheme of a chemical oxidation treatment (adapted from ARS, 2008).

Bioremediation

Bioremediation refer to the general use of microbiota to degrade hazardousmaterials into innocuous materials. The use of biological systems is of par-ticular interest (Tan et al., 1994). At metal-contaminated sites, biologicalattenuation and stabilization of heavy metals by biological processes mayoccur. In fact, microbiological processes are important in determining metalmobility and have potential application in bioremediation of metal pollu-tion (Gadd, 2004). Remediation of metal-contaminated soil using microbeshas been studied, especially in the last few years. A wide variety of fungi,algae and bacteria are already in use as tools for heavy metal remediation(Gadd, 1992; Volesky and Holan, 1995). Depending on the degree of theintervention, bioremediation is generally considered as natural attenuation(little or no human action), biostimulation (addition of nutrients as well aselectron donors or acceptors to promote the growth or metabolism of certainmicroorganisms), or bioaugmentation (the addition of natural or engineeredmicroorganisms with the desired capabilities; Lorenzo, 2008).

Recent studies have indicated that most of the transition between metalspeciation forms are controlled by microbial behavior (Hall and Puhlmann,2004; Hall et al., 2005) and that metal mobilizing bacteria can be easilyenriched from most type of soils (Bock and Bosecker, 1997; Gomez andBosecker, 1997) and can be effectively used for immobilization of heavy

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metals. Many of the organisms—known as dissimilatory metal reducers—thatcatalyze such reactions use the metals as terminal electron acceptors in anaer-obic respiration (Lovley and Coates, 1997). The microbial reduction of Cr (VI)to Cr (III) has been one of the used forms of heavy metal bioremediation:diverse heterotrophic microorganisms can carry out anaerobically or aero-bically this reaction (Lovley, 1993; Wang and Shen, 1995). It has also beendemonstrated that microbial reduction of the highly soluble oxidized form ofSe (III) to Se is a natural mechanism that can be used for the remediation ofSe-contaminated soils via immobilization. In addition to reductively precipi-tating some metals, dissimilatory metal reducers can solubilize other metallicelements—microbial alteration of the redox state of either the contaminantsor the Fe (III) and Mn (IV) oxides, which bind most heavy metals in thesoil, can make metals more soluble (Lovley and Coates, 1997)—making therecovery of the contaminants easier, in a process termed bioleaching. Dur-ing this process, a highly oxidizing environment coupled with very low pHlevels brings about the dissolution of heavy metals and also the digestion oforganic matter (Wong et al., 2002). A typical bioremediation process usingbioleaching is described in Figure 9. As an example, bioleaching of metalsulfide ores has been studied using acidotermophilic bacteria with success

FIGURE 9. Scheme of a bioremediation treatment (adapted from White et al., 1998).

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(Umrania, 2006), with the highly potential isolates presenting maximum ad-sorptions of Pb, Zn, Ni, and Cu of 35, 34, 29, and 97% respectively, five daysafter inoculation. Bioleaching using these types of bacteria was also testedfor soils contaminated by tannery effluents with positive results (NareshKu-mar and Nagendran, 2008). Some microorganisms, including Geospirillumarsenophilus (Ahmann et al., 1994), Geospirillum barnseii (Laverman et al.,1995), and Chrysiogenes arsenatis (Macy et al., 1996; Table 3), have alsobeen reported as having metal reductive mechanisms for arsenic, reducingAs (V) to the more soluble As (III), promoting its leaching from arsenic-contaminated soils (Lovley and Coates, 1997).

Dissimilatory metal reducers can also volatilize some metals. In fact,conversion of metals to their volatile derivatives (methyl and hydride deriva-tives) by organisms is a well-known phenomenon in nature (Craig, 2002).Microorganisms that reduce Hg (II) to volatile elemental Hg as a mechanismof mercury resistance naturally contribute to the volatilization of the metalfrom contaminated soils (Saouter et al., 1995). Bacteria-mediated volatiliza-tion was also reported by Meyer et al. (2007) for As, Se, and Sb from acontaminated alluvial soil.

However, metal sorption by microorganisms can also be used as a re-mediation method using microbes. Although inorganic soil constituents aremore often considered to be the main soil components that sequester metals,some types of microbial biomass actually have a significant capacity for met-als adsorption (Berthelin et al., 1995). Biosorption is the process by whichmetals are sorbed or complexed to either living or dead biomass (Voleskyand Holan, 1995) or even precipitated in bacterial exudates (Appanna et al.,1996), rendering the method as a potentially important process for the con-centration of metals in soils.

The cost-effective and eco-friendly processes involved in bioremedia-tion are widely accepted, inclusively by the general public, which representsone of the advantages of these biotechniques. Additional benefits include thenoninvasive character of the technique, especially if performed in situ, theresidues for the treatment are usually harmless products and the fact that it iscost-effective. As main disadvantages, extended treatment time, monitoringdifficulties, bioavailability limitations, and the susceptibility to environmentalconstrains should be regarded. In fact, biological processes are often highlyspecific. Important site factors required for success include the presence ofmetabolically capable microbial populations, suitable environmental growthconditions, and appropriate levels of nutrients and contaminants. Moreover,it is difficult to extrapolate from bench and pilot-scale studies to full-scalefield operations, although research is needed to develop and engineer biore-mediation technologies that are appropriate for sites with complex mixturesof contaminants that are not evenly dispersed in the environment. Despitethe research efforts made in the last years in this field, regulatory uncertaintyremains regarding acceptable performance criteria for bioremediation.

Dow

nloa

ded

by [

Uni

vers

ity o

f T

enne

ssee

, Kno

xvill

e] a

t 07:

01 2

8 A

pril

2013

TA

BLE

3.

Outc

om

esofso

ilre

med

iatio

nusi

ng

bio

logi

cally

bas

edtrea

tmen

tm

ethods

Met

hod

Ref

eren

ceRes

ults

Scal

e

Bio

rem

edia

tion

Um

raia

,20

06So

ilsin

am

etal

pro

cess

ing

area

,co

nta

min

ated

with

Cu,Zn,N

i,an

dPb

(conce

ntrat

ions

ofup

to11

00,64

0,32

5,an

d35

0m

gkg

−1,re

spec

tivel

y),w

ere

use

dto

test

the

bio

rem

edia

tion

abili

ties

ofac

idote

rmophili

cau

totrophs.

The

sele

cted

bac

teria

solu

bili

zed

up

to34

%ofZn,29

.4%

N,

35.2

6%Pb,an

d97

.5%

ofCu.

Labora

tory

study

Nar

eshK

um

aran

dN

agen

dra

n,20

08Conta

min

ated

soil

(9.1

mg

Cd

kg−1

,11

810

mg

Cr

kg−1

,96

mg

Cu

kg−1

,an

d23

8m

gZn

kg−1

)co

llect

edfrom

anin

dust

rial

ized

regi

on

was

use

din

abio

rem

edia

tion

exper

imen

tw

ithA

cid

oth

ioba

cillu

sth

ioox

ida

ns.

This

stra

inw

asab

leto

solu

bili

zeth

em

etal

s,pre

sentin

gso

lubili

zatio

nef

fici

enci

esof88

,93

,92

,an

d97

%,fo

rCr,

Cd,Cu,an

dZn,re

spec

tivel

y.

Labora

tory

study

Ahm

ann

etal

.,19

94La

verm

anet

al.,

1995

Mac

yet

al.,

1996

Com

ple

tedis

appea

rance

ofAs

(V)

and

corr

esponden

tst

oic

hio

met

ric

appea

rance

ofA

s(I

II)

inAs-

conta

min

ated

med

ia(u

pto

10m

M)

was

obta

ined

inth

epre

sence

ofse

lect

edbac

terial

isola

tes.

Labora

tory

studie

s

Mey

eret

al.,

2007

Bio

vola

tiliz

atio

nofan

allu

vial

soil

conta

inin

g8.

9m

gAs

kg−1

,1.

3m

gSb

kg−1

,an

d1

mg

Sekg

−1w

assu

cces

sfully

achie

ved

usi

ng

anae

robic

albac

terial

stra

ins,

with

the

leve

lsof

vola

tiliz

edm

etal

sre

achin

gup

to12

0-fo

ldhig

her

than

bef

ore

the

applic

atio

nofth

eis

ola

tes.

Phyt

ore

med

iatio

nChen

etal

.,20

04H

elia

nth

us

an

nu

us

and

Bra

ssic

aju

nce

a(a

mong

oth

ersp

ecie

s)w

ere

grow

nin

soil

artifi

cial

lyco

nta

min

ated

with

800

mg

Pb

kg−1

and

ED

TA

was

applie

das

atrea

tmen

tin

sele

cted

pots

.Pla

nts

grow

ing

inED

TA

trea

ted

soils

pre

sente

dan

incr

ease

ofm

ore

than

31-

and

96-fold

,re

spec

tivel

y,ofth

em

etal

upta

kein

the

shoots

(fro

mac

cum

ula

tions

ofci

rca

57to

1800

mg

Pb

kg−1

for

H.

an

nu

san

dfr

om

circ

a30

to29

00m

gPb

kg−1

for

B.ju

nce

a)

Labora

tory

study

Grc

man

etal

.,20

03The

applic

atio

nof10

mm

olkg

−1ED

TA

and

ED

DS

toa

conta

min

ated

soil

nea

ra

form

ersm

eltin

gpla

nt(1

100

mg

Pb

kg−1

,80

0m

gZn

kg−1

,an

d5.

5m

gCd

kg−1

)ca

use

dth

eco

nce

ntrat

ions

ofPb

inth

ele

aves

ofB

rass

ica

rapa

toin

crea

seci

rca

94-an

d10

2-fo

ld,

resp

ectiv

ely,

rela

tive

toth

eco

ntrol.

The

sam

edose

ofED

TA

incr

ease

dth

eco

nce

ntrat

ion

of

Zn

and

Cd

4.3-

and

3.8-

fold

and

ofED

DS

4.7-

and

3.5-

fold

,re

spec

tivel

y.

Labora

tory

study

Mar

ques

etal

.,20

08a

Ach

elat

e-as

sist

edphyt

oex

trac

tion

appro

ach

usi

ng

Sola

nu

mn

igru

man

dth

ech

elat

ing

agen

tsED

TA

or

ED

DS

(ata

rate

of0.

5g

kg−1

)w

asas

sess

edfo

rpla

nts

grow

ing

inZn-c

onta

min

ated

soil

(up

to96

4m

gkg

−1).

The

Zn

conce

ntrat

ions

inw

ater

extrac

tsofth

eso

ilsco

llect

edat

the

time

ofhar

vest

wer

ein

crea

sed

inso

ilsw

ithad

ded

ED

TA

or

ED

DS

by

up

tofo

ur-

and

thre

efold

,re

spec

tivel

y,an

dco

nse

quen

tlyS.

nig

rum

accu

mula

ted

leve

lsup

to23

1,12

4,an

d10

4%hig

her

inth

ele

aves

,st

ems,

and

roots

(473

5,82

67,an

d79

48m

gZn

kg−1

),re

spec

tivel

y.

Labora

tory

study

Luo

etal

.,20

06U

sing

potex

per

imen

ts,th

eef

fect

ofth

eco

mbin

edap

plic

atio

nofED

TA

and

ED

DS

on

the

upta

keofPb

by

Zea

ma

ysfrom

conta

min

ated

soil

(250

0m

gkg

−1)

was

studie

d.The

tota

lphyt

oex

trac

tion

ofPb

reac

hed

1.7

mg

kg−1

soil,

with

accu

mula

tion

leve

lsin

the

roots

incr

easi

ng

from

10m

gPb

kg−1

inth

eco

ntrolto

up

to56

9m

gPb

kg−1

intrea

ted

pots

.The

com

bin

edap

plic

atio

nofED

TA

and

ED

DS

also

sign

ifica

ntly

incr

ease

dth

etran

sloca

tion

ofPb

from

the

roots

toth

esh

oots

(fro

m0.

03in

the

controlto

up

to0.

64).

Labora

tory

study

(Con

tin

ued

onn

ext

page

)

901

Dow

nloa

ded

by [

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ity o

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ssee

, Kno

xvill

e] a

t 07:

01 2

8 A

pril

2013

TA

BLE

3.

Outc

om

esofso

ilre

med

iatio

nusi

ng

bio

logi

cally

bas

edtrea

tmen

tm

ethods

(Con

tin

ued

)

Met

hod

Ref

eren

ceRes

ults

Scal

e

Heg

goet

al.,

1990

Gly

cin

em

ax

pla

nts

wer

egr

ow

nin

met

al-c

onta

min

ated

soil

(272

0m

gZn

kg−1

and

35.3

mg

Cd

kg−1

).The

inocu

latio

nw

ithA

MF

reduce

dth

eZn

and

Cd

accu

mula

tions

from

1020

to78

0m

gZn

kg−1

and

26.2

to19

.2m

gCd

kg−1

.

Labora

tory

study

Mar

ques

etal

.,20

06M

arques

etal

.,20

07The

inocu

latio

nof

Sola

nu

mn

igru

mgr

ow

ing

inar

tifici

ally

conta

min

ated

soil

(up

to10

00m

gZn

kg−1

dry

sand)

and

inm

etal

-conta

min

ated

soil

pre

ceed

ing

from

afo

rmer

indust

rial

ized

site

(426

mg

Zn

kg−1

)w

ithG

lom

us

cla

roid

eum

or

Glo

mu

sin

tra

rad

ices

incr

ease

dZn

accu

mula

tion

by

up

to83

or

49%

,re

spec

tivel

y.In

pla

nts

pre

sentin

gno

visu

alto

xici

tysi

gns,

accu

mula

tions

wer

eup

to38

10,32

40,an

d14

50m

gZn

kg−1

inth

ero

ots

,st

ems,

and

leav

es,

resp

ectiv

ely.

Labora

tory

studie

s

Ree

dan

dG

lick,

2005

Bra

ssic

an

apu

spla

nts

grow

ing

inCu-c

onta

min

ated

soil

(100

0m

gkg

−1)

show

edre

duce

dac

cum

ula

tion

when

inocu

late

dw

ithP

seu

dom

ona

sa

sple

nii

(fro

m55

0m

gkg

−1in

the

control

to50

0m

gkg

−1).

Labora

tory

study

Ree

det

al.,

2005

Ph

ragm

ites

au

stra

lis

pla

nts

,gr

ow

ing

ina

met

al-c

onta

min

ated

soil

inocu

late

dw

ithbac

teria

from

Pse

ud

omon

as

stra

ins

show

edup

to40

%in

crea

sed

seed

ling.

Labora

tory

study

Abou-S

han

abet

al.,

2003

When

Sph

ingo

mon

as

ma

crog

olta

bid

us,

Mic

roba

cter

ium

ara

bin

oga

lact

an

olyt

icu

m,an

dM

icro

bact

eriu

mli

quef

aci

ens

wer

ead

ded

tosu

rfac

e-st

erili

zed

seed

sof

Aly

ssu

mm

ura

legr

ow

nin

conta

min

ated

soil

(439

0m

gN

ikg

−1),

they

incr

ease

dN

iupta

kein

toth

esh

ootby

17%

(S.m

acr

ogol

tabi

du

s),24

%(M

.li

quef

aci

ens)

,an

d32

.4%

(M.a

rabi

nog

ala

cta

nol

ytic

um

),co

mpar

edw

ithunin

ocu

late

dco

ntrols

(circa

8500

mg

Nikg

−1).

Labora

tory

study

Idris

etal

.,20

04In

this

study

the

rhiz

osp

her

ean

dsh

oot-as

soci

ated

(endophyt

ic)

bac

teria

colo

niz

ing

Th

lasp

igo

esin

gen

sew

ere

char

acte

rize

d.Rhiz

osp

her

eis

ola

tes

bel

onge

dm

ainly

toth

ege

ner

aM

eth

ylob

act

eriu

m,

Rh

odoc

occu

s,an

dO

kiba

cter

ium

,w

her

eas

the

maj

ority

ofen

dophyt

essh

ow

edhig

hle

vels

ofsi

mila

rity

toM

eth

ylob

act

eriu

mm

esop

hil

icu

m.A

dditi

onal

ly,

Sph

ingo

mon

as

sp.w

ere

abundan

t.Is

ola

tes

wer

ere

sist

antto

Nico

nce

ntrat

ions

bet

wee

n5

and

12m

M.

Labora

tory

study

Chen

etal

.,20

04H

elia

nth

us

an

nu

us

and

Bra

ssic

aju

nce

a(a

mong

oth

ersp

ecie

s)w

ere

grow

nin

soil

artifi

cial

lyco

nta

min

ated

with

800

mg

Pb

kg−1

and

ED

TA

was

applie

das

atrea

tmen

tin

sele

cted

pots

.Pla

nts

grow

ing

inED

TA

trea

ted

soils

pre

sente

dan

incr

ease

ofm

ore

than

31-an

d96

-fold

,re

spec

tivel

y,ofth

em

etal

upta

kein

the

shoots

(fro

mac

cum

ula

tions

ofci

rca

57to

1800

mg

Pb

kg−1

for

H.

an

nu

san

dfr

om

circ

a30

to29

00m

gPb

kg−1

for

B.ju

nce

a).

Labora

tory

study

Mar

ques

etal

.,20

08b

The

esta

blis

hm

entof

Sola

nu

mn

igru

min

aso

ilco

nta

min

ated

with

Zn

due

toth

epas

tin

dust

rial

activ

ityin

the

area

(up

to96

4m

gkg

−1)

com

bin

edw

ithth

eap

plic

atio

nofm

anure

led

up

to80

%re

duct

ion

inth

eam

ountofZn

leac

hed

thro

ugh

the

soil;

the

additi

on

ofth

ead

diti

veal

sopro

mote

da

reduct

ion

ofth

em

etal

accu

mula

ted

inth

etis

sues

for

am

inim

um

leve

lof12

25m

gkg

−1in

the

roots

,66

8m

gkg

−1in

the

stem

s,an

d19

1m

gkg

−1in

the

leav

es(c

om

par

edw

ithup

to74

65m

gkg

−1in

the

roots

,54

46m

gkg

−1in

the

stem

s,an

d20

82m

gkg

−1in

the

leav

esw

ithoutm

anure

additi

on).

Labora

tory

study

902

Dow

nloa

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ity o

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, Kno

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pril

2013


Phytoremediation

Phytoremediation is a remediation technology that uses plants to treat thecontaminated matrix, extracting, degrading or immobilizing the contami-nants. The plants used in heavy metal phytoremediation should be chosenbased on their capacities to tolerate and bioaccumulate particular contami-nants, but their growth rate and biomass production as well as the depth oftheir root zone are also important characteristics to be considered (Meagher,2000).

Early research indicates that phytoremediation is a promising clean-upsolution for a wide variety of contaminated sites, although it has its limi-tations. Many of the limitations and advantages of phytoremediation are adirect result of the biological aspect of this type of treatment system (Singhet al., 2003). Plant-based remediation technologies can function with min-imal maintenance after its establishment, and consequently this techniquepresents low costs. In fact, as the involved biological processes are fun-damentally solar driven, phytoremediation is on average tenfold cheaperthan engineering-based remediation methods, such as soil excavation, soilwashing or burning, or pump-and-treat systems (Glass, 1999). The fact thatphytoremediation can be carried out in situ also contributes to its cost-effectiveness and to the reduction of the exposure of the polluted substrateto the environment and human beings in general (Pilon-Smits, 2005). How-ever, the method also presents some disadvantages. The use of phytoreme-diation is limited by the climatic and geological conditions of the site to becleaned, such as temperature, altitude, soil type, and the accessibility foragricultural equipment (Salt and Kramer, 2000; Schmoger et al., 2000). Otherproblems may arise, such as the accumulation of contaminants in wood thatcan be used as fuel or the further release of the pollutants to the soil again,caused by the senescence of contaminated leaves during litter fall (Maceket al., 2000; Schmoger et al., 2000). Nevertheless, the main disadvantage ofthis method is that the contaminants must be available to the plant throughits root system, so the contamination should be quite shallow (Pilon-Smits,2005).

Various phytoremediation strategies are possible (Salt et al., 1998), withdifferent phytotechnologies profiting from different plant properties (Pilon-Smits, 2005). Concerning metal contamination of soils specifically, the maintreatment streamlines are phytovolatilization, phytoextraction, and phytosta-bilization, which are adequately represented in the diagram of the phytore-mediation processes shown in Figure 10.

In phytovolatilization metals taken up by the roots pass through theplants to the leaves and are volatized through stomata where gas exchangeoccurs (Vroblesky et al., 1999). As examples, Se can be volatilized by Brassicajuncea from soils (Banuelos et al., 1993; Banuelos and Meek, 1990) andmercury in its elemental form is also easily volatilized, as it is liquid at room

Dow

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2013


FIGURE 10. Representative diagram of the different types of phytoremediation of metal-contaminated soils (Marques et al., 2009).

temperature (Fox and Walsh, 1982); however, mercury volatilization does notappear to be a significant process in plants but rather in selected bacteria(Rugh et al., 1996).

Concerning phytoextraction (or phytoaccumulation) plant roots take upmetals and store them in the harvestable tissues (Kumar et al., 1995). Theideal plant to be used in phytoextraction should have the following charac-teristics: (a) be tolerant to high levels of the metal, (b) have a profuse rootsystem, (c) have a rapid growth rate, (d) have the potential to produce ahigh biomass in the field, and (e) accumulate high levels of the metal in theharvestable parts, as generally the harvestable portion of most plants is lim-ited to the above-ground parts (although the roots of some crops may alsobe harvestable). Two different approaches have been generally proposed forthe phytoextration of heavy metals. One of them is the use of natural hy-peraccumulator plants with exceptional metal-accumulating capacities. Themajor mechanism in these plant species appears to be compartmentalizationof metal ions (i.e., sequestration in the vacuole or cell walls), thus excludingthem from cellular sites where processes such as cell division and respirationoccur (Chaney et al., 1997). For example, one of the best known Zn hyper-accumulators is Thlaspi caerulescens. Although most plants exhibit toxicitysymptoms at Zn concentrations of about 100 mg kg−1, T. caerulescens wasshown to accumulate up to 26000 mg kg−1 without showing any damage tothe plant (Brown et al., 1995). In addition, this species extracted up to 22%of soil-exchangeable Cd from a contaminated site (Gerard et al., 2000). Theutilization of high biomass plants with a chelate-enhanced method of phy-toextraction (Salt et al., 1998) is another possible phytoextraction strategy.The addition of chelating agents and the consequent formation of metal-chelate complexes prevents precipitation and sorption of the metals in the

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soil, thereby maintaining their availability in soil solution for plant uptake(Norwell, 1984). Sometimes the application of certain chelates to the soilmay even increase the translocation of heavy metals into the shoots (Blayloket al., 1997). As seen in Table 3, many reports indicate EDTA and EDDS(SS-ethylenediaminedissucinic acid) as being able of successfully improvingheavy metal uptake by plants (Chen et al., 2004; Grcman et al., 2003; Luoet al., 2005; Luo et al., 2006; Marques et al., 2008a).

Phytostabilization considers the use of metal-tolerant plant species toimmobilize heavy metals belowground, decreasing metal mobility and re-ducing the likelihood of metals entering into the food chain (Wong, 2003).There are thus two major components in the phytostabilization process: theplant itself and the amendments added to the system. Plants play an impor-tant role in phytostabilization, not only by protecting the soil surface fromhuman contact and rain impact with a dense canopy, but also by physicallystabilizing the soil with dense root systems to prevent erosion and minimizecontaminants leaching through the soil (Berti and Cunningham, 2000) andalso providing surfaces for sorption or precipitation of metal contaminants(Laperche et al., 1997). Soil amendments can be applied and assume a roleof great importance by helping to inactivate metal contaminants, preventingplant uptake, decreasing biological activity, and reducing the percolation ofmetals (Ruttens et al., 2006). Marques et al. (2008b) showed that the soleapplication of organic matter amendments, such as manure or compost, tometal-contaminated soil led to a significant reduction in the amount of Znleached through the soil; in combination with the plant species Solanumnigrum, the reduction in metal percolation ascended up to 80%.

Metal uptake by plants can be influenced by soil microorganisms that as-sociate with the plant roots to form the rhizosphere community (Shilev et al.,2001). As an example, arbuscular mycorrhizal fungi (AMF) have shown toenhance plant tolerance the presence of high levels of heavy metals (Leyvalet al., 2002). As seen in Table 3, when the host is exposed to metal stress,the role of AMF in the plant stress response is variable, with some studiesindicating reduced metal concentrations in plants due to mycorrhizal colo-nization (Heggo et al., 1990), whereas others indicate enhanced metal uptakeand accumulation in plants due to AMF colonization (Marques et al., 2006;Marques et al., 2007). Plant growth promoting bacteria (PGPR) communitiesin the rhizosphere can also be an important tool in the decontamination ofmetal-contaminated soils through plant use. Plant growth promoting bacteriahave shown to reduce plant stress at metal exposure, as seen by the exam-ples shown in Table 3: Phragmites australis and Brassica napus have shownincreased seedling and reduced accumulation of Cu when inoculated withPseudomonas strains (Reed and Glick, 2005; Reed et al., 2005). Nevertheless,other studies have indicated that the presence of selected PGPR promoted theincrease in the availability of heavy metals in soil, consequently enhancingmetal accumulation by plants, as reported for the uptake of Ni by Alyssum

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FIGURE 11. Diagram of a soil vapor extraction installation (adapted from USEPA, 2006).

murale and Thlaspi goesingense (Abou-Shanab et al., 2003; Idris et al., 2004;Table 3).

Soil Vapor Extraction

Soil vapor extraction (SVE), also known as soil venting or vacuum extrac-tion, is an accepted, recognized, and cost-effective technology for remedi-ating unsaturated soils contaminated with volatile and semivolatile organiccontaminants (Halmemies et al., 2003), and is pointed by some researchersas a possible method for the remediation of soils contaminated with volatilemetals (Virkutyte et al., 2002)—however, no reports on the application ofthe technique in lab or field trials are available. SVE involves the installationof wells in the area of soil contamination and vacuum is applied throughthe wells in order to aid evaporation of the volatile constituents, which arethen withdrawn through extraction wells (Khan et al., 2004). A scheme of aSVE treatment installation is shown in Figure 11. Extracted vapors are thendirected for posterior treatment, a step that represents, together with thelarge energy requirements, the main disadvantages of the technique, as thisrepresents added costs. Nevertheless, the short treatment time is an obviousadvantage of SVE (USEPA, 1995).

CONCLUSIONS

Although numerous techniques are available for soil remediation, many ofthem are only applicable to soils contaminated with organic compounds. Thetechnologies listed in this work are those that have shown to be practically

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applied, either in lab, pilot, or field studies, or even in full-scale applications.It should, however, be remembered that even though these technologies areavailable for the treatment of metal-contaminated sites, the selection of thetechnology to apply to a particular case depends on contaminant and sitecharacteristics, regulatory requirements, costs, and time constraints. In fact,no single technology is adequate for all metal contamination types and all theconditions existing at all the environmental disturbed sites. Attention shouldbe given to the site conditions, as well as to the contaminant characteristics,levels, and sources. The impact of the application of the remedial measureson the site as well as its fauna and flora, should also not be disregarded.

It should also be kept in mind that treatment processes can be combina-tions of two or more technologies for more effective remediation of the heavymetal contaminated site in stake. Biological, physical, thermal, and chemicaltechnologies may be used in conjunction to reduce the contamination to asafe and acceptable level.

Therefore, the successful treatment of a heavy metal contaminated sitedepends on proper selection, design, and adjustment of the remediationtechnology or technologies operations based on the properties of the con-taminants and soils and on the performance of the system.

ACKNOWLEDGMENTS

This work was supported by Fundacao para a Ciencia e a Tecnologia andFundo Social Europeu (III Quadro Comunitario de apoio) a research grantof Ana Marques (SFRH/BPD/34585/2007).

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remediation of heavy metal contaminated soils: an overview of site remediation techniques

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