review of in situ remediation technologies for lead, zinc, and cadmium in soil

35

REMEDIATION Summer 2004

This article presents a review of in situ technologies for the remediation of soils contaminated with

lead, zinc, and/or cadmium. The objective of this review is to assess the developmental status of

the available in situ technologies and provide a general summary of typical applications and limi-

tations of these technologies. The literature review identified seven in situ remediation technolo-

gies—solidification/stabilization, vitrification, electrokinetic remediation, soil flushing, phytoextrac-

tion, phytostabilization, and chemical stabilization. These technologies were considered for their

ability to meet a specific set of remediation objectives under a range of conditions. Each of these

technologies has both strengths and weaknesses for addressing particular remedial situations dis-

cussed in the article for each of the technologies. A general summary of which technologies are

most applicable to common remedial scenarios is also provided. © 2004 Wiley Periodicals, Inc.

INTRODUCTION

This review focuses on soils contaminated with lead, zinc, and cadmium and the in situ re-medial strategies available for treatment of these metals. A primary source of lead, zinc,and cadmium contamination in soils is the mining and smelting of ores containing thesemetals. Mining-related wastes, such as waste rock and tailings, represent potential sourcesof metals that can be redistributed to the surrounding environment by aerial and fluvialtransport. In addition, aerial deposition of smelter emissions has led to widespread con-tamination of surface soils at various locations throughout the world. Other industrialsources, such as foundries, refineries, and pesticide, paint, and battery manufacturers, arealso known to be significant sources of metal contaminants to soils.

Lead, zinc, and cadmium in soils are of concern when they are present at sufficientconcentrations to adversely affect human health and the environment. In some cases, soilsare so contaminated that they no longer support a functioning ecosystem. Lead is of spe-cific concern due to its relative abundance at contaminated sites and its known potential tocause adverse health effects in children (Davies & Wixson, 1988). As a result, much of theresearch in evaluating lead contamination has focused on reducing the exposure of humansto lead in soils. Zinc is primarily an ecological risk, because it is known to adversely affectaquatic receptors and can be phytotoxic at high concentrations (United StatesEnvironmental Protection Agency [US EPA], 2003). Cadmium in soil represents a direct-contact risk to both human and ecological receptors due to its relatively high toxicity andplant uptake (Agency for Toxic Substances and Disease Registry [ATSDR], 1999).

Given the widespread distribution of lead, zinc, and cadmium in soil due to human ac-tivities, and the potential human and ecological risks posed by these metals, it is desirable to

© 2004 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.20011

Todd A. Martin

Michael V. Ruby

Review of In Situ RemediationTechnologies for Lead, Zinc, andCadmium in Soil

develop cost-effective remediation strategies for these metals.At this time, there are rela-tively few fully developed, in situ methods available for remediation of metals in soils, andoften, remediation at a given site consists of traditional alternatives such as excavation (fortreatment/disposal) or containment. However, excavation of metal-contaminated soils maybe impracticable due to the excessive cost involved, the magnitude (area, depth, volume) ofthe soil contamination, and the degree of disruption incurred at the site. Containment alter-natives, such as soil caps, are often inconsistent with the desired end use for the site andmay be viewed negatively by the regulatory community and the public.

This review was compiled in an effort to assess the general extent of scientificknowledge regarding in situ remediation technologies for lead, zinc, and cadmium insoils. A literature search was conducted to identify potentially viable in situ remediationtechnologies for sites where lead, zinc, and/or cadmium in soil are the principal con-taminants of concern. Applicable literature was identified through searches of computer-ized databases, Internet searches, and review of gray literature and unpublished litera-ture acquired from professional colleagues.The search resulted in over 400 documents(articles, publications, and reports), which were reviewed and synthesized to summarizethe status and applications of in situ technologies for remediation of lead, zinc, and cad-mium in soils.This article is a synopsis of that investigation.

In Situ Remediation Objectives

As with any remediation, the overall objective of an in situ remediation approach is tocreate a final solution that is protective of human health and the environment. Metalcontamination of soils can represent a threat to human and ecological health through di-rect contact, can be a source for metal transport to critical resources (e.g., groundwaterand surfacewater bodies), and can hinder the ability of the soil to support vegetation anda functioning ecosystem. As a result, in situ remediation of metal-contaminated soils typ-ically concerns three general remediation objectives: 1) reduce metal leaching, 2) reducemetal bioavailability to human and ecological receptors, and 3) reestablish vegetation.

Reduce Metal Leaching

At many sites, there is a potential for metals to be leached from soils by infiltratingwater, spreading the contamination and potentially affecting groundwater and otherdowngradient resources.The success of an in situ remedy in reducing the leachability ofa metal contaminant in soils is typically evaluated through laboratory leaching tests—themost commonly used in the United States are the toxicity characteristic leaching proce-dure (TCLP) and the synthetic precipitation leaching procedure (SPLP; US EPA,1997a).The SPLP is designed to evaluate contaminant release in response to infiltrationof meteoric water and is thus typically more appropriate for the evaluation of in situremedies than the more aggressive TCLP, which is designed to simulate the fluids that atreated waste might encounter in a landfill. In various European countries, leaching testshave been developed to assess the constituents that can be leached from solid waste forregulatory purposes. Leaching tests that are analogous to the U.S. tests described aboveare widely available in Europe.

In addition to the TCLP and SPLP tests, there are a variety of leaching/extractionprocedures that are used to assess metal solubility (and thus mobility) under variousconditions (US EPA, 1991).The simplest of these procedures quantifies the “water

Review of In Situ Remediation Technologies

© 2004 Wiley Periodicals, Inc.36

As with any remediation,the overall objective ofan in situ remediationapproach is to create afinal solution that is pro-tective of human healthand the environment.

extractable” fraction of metal in the soil, which is a measure of the metal mass that ishighly mobile and readily bio-/phytoavailable. Another common laboratory procedure issequential extraction testing, which provides an indication of the form in which themetal mass is present in the soil matrix.

Reduce Metal Bioavailability

Metals in contaminated soils are often present in chemical forms that exhibit varying de-grees of bioavailability to human and ecological receptors. As a result, there has beengreat interest (particularly for lead-contaminated soils) in in situ remedial strategies thatrender metals less bioavailable, such that the metal-contaminated soil no longer repre-sents an unacceptable exposure risk.The bioavailability of metals in soils to ecological re-ceptors is often established by directly measuring uptake of metals into the receptor ofinterest (e.g., plants or soil invertebrates). However, indirect methods, such as measure-ment of soil pore-water concentrations or diffusive gradients in thin films (Davison et al.,2000) are gaining acceptance as viable methods to estimate soil metal concentrations thatare available for biological uptake.The bioavailability of a metal in soil to humans is gen-erally estimated using either in vivo or in vitro methods. In vivo testing involves measure-ment of metal uptake in a living organism, such as a rat, swine, or human volunteer. Invitro methods, such as the physiologically based extraction test (PBET; Ruby et al.,1999), involve laboratory chemical extractions that are designed to mimic the physiologyand chemistry of the in vivo system and determine the fraction of a metal that would beavailable for absorption. In vivo measures of bioavailability are more readily accepted bythe regulatory community but are more time-consuming and costly to obtain.

A variety of in vitro testing methods have been developed in addition to the PBET,all based on the principle of measuring the fraction of a metal that will be biologicallyavailable. Oomen et al. (2002) describe five methods developed in Europe, including thesimple bioaccessibility extraction test (SBET) developed by the British GeologicalSociety; the German DIN model developed by the Ruhr-Universität Bochum; theNetherlands’ static and dynamic in vitro digestion (RIVM and TIM) models; andBelgium’s Simulator of the Human Intestinal Microbial Ecosystems of Infants (SHIME)procedure. In vitro testing methods are slowly gaining acceptance as a result of metal-specific validation studies for these methods.The regulatory authorities will accept insitu remediation strategies that center on reducing metal bioavailability only if reducedbioavailability is equated with reduced risk, and if the bioavailability reductions aredemonstrated to be long-term.

Reestablish Vegetation

Some metal-contaminated soils do not support plant growth due to the phytotoxicity ofthe metals and/or other plant stresses posed by co-contaminants in the soils (e.g., lowpH, high salinity).The lack of vegetative cover leaves the soils exposed to erosional pro-cesses and direct-contact exposures and inhibits the ability of the soils to support a func-tional ecosystem. As a result, many in situ remediation strategies are designed to facilitatethe development of a vegetative cover over the metal-containing soils.The effectiveness ofa remediation strategy for revegetation of metal-contaminated soils is evaluated throughthe quantitative assessment of the plant species’s health (e.g., plant mass, coverage, andrate of growth, indicators of metal-related stresses). Measuring ecosystem health typically


© 2004 Wiley Periodicals, Inc. 37

A variety of in vitro testingmethods have been devel-oped in addition to thePBET, all based on theprinciple of measuring thefraction of a metal that willbe biologically available.

involves quantifying the diversity, health, and abundance of plant and biological species.An important consideration is the potential for metal accumulation in plant tissue, whichcan represent a pathway for transport of the metal to higher trophic levels. Evaluation ofrevegetation efforts for metal-contaminated soils typically involves quantifying the “phy-toavailable” fraction of metals in soils.

In Situ Remediation Technologies

In situ remediation technologies for metal-contaminated soils are based on three generalremedial strategies: isolation, removal, and stabilization (Exhibit 1). For the purposes ofthis review, isolation technologies are considered to be those that are implemented to re-duce contaminant availability by reducing the exposed surface area, reducing the soil per-meability, and/or reducing the contaminant solubility. Removal technologies are consid-ered to be those that are employed in situ to remove metals from a contaminated soilmatrix through the use of physical and/or chemical processes. Stabilization technologiesinclude those that involve the use of chemical amendment(s) and/or plants to reduce theleachability and/or bioavailability of metals in contaminated soils. Seven in situ remedia-tion technologies were identified during the literature review: solidification/stabilization,vitrification, electrokinetic remediation, soil flushing, phytoextraction, phytostabilization,and chemical stabilization. Physical containment, though technically an in situ technology,was not considered in this review because it does not involve treatment of the soils.

The appropriateness of an in situ remediation technology for a given site depends onthe remediation target or objective, the technology status, and site-specific considera-tions. Each of the seven remediation technologies has both strengths and weaknesses foraddressing certain remediation targets (i.e., reduced leaching, reduced bioavailability,revegetation) and site conditions (e.g., site end use, metal concentrations, soil proper-ties, extent of contamination). Exhibit 2 provides a general assessment of each of the



Exhibit 1. Summary of in situ remediation technologies for Pb, Zn, and Cd in soils

seven technologies in terms of categories that are often significant to the selection of aremediation strategy at a contaminated site.This assessment represents a qualitative eval-uation by the authors based on the overall findings of the literature review. Although theauthors have attempted to minimize bias, it is recognized that such an assessment hassome inherent subjectivity.

It should be noted that the majority of in situ remediation technologies for metals insoils are still in the developmental stage, and site-specific testing is required prior to im-plementation at a given site. However, the costs associated with these additional effortsmay be warranted by the advantages offered by in situ remediation alternatives over con-ventional approaches.The following sections describe each of the seven in situ remediationtechnologies identified in this review for remediation of lead, zinc, and cadmium in soils.

Solidification/Stabilization

Solidification/stabilization (S/S) is the most common approach used to immobilizemetals in soils and wastes (Evanko & Dzombak, 1997), although in situ applications aresomewhat rare.The process involves the addition of binding agents to the soil matrix toimmobilize the metal contaminant within the soil matrix through a combination ofchemical reaction, encapsulation, and reduced permeability/surface area (Evanko &Dzombak, 1997).The mechanisms by which S/S technology operates are well devel-oped and understood. As a result, it is not surprising that there are few recent researchefforts investigating these technologies (US EPA, 1997b). Cement-based and pozzolanmaterials (e.g., fly ash) are commonly used as S/S binders for metal remediation—with the predominant immobilization mechanism for metals being precipitation of hy-droxides, carbonates, and silicates (Evanko & Dzombak, 1997; US EPA, 2000a).The



Exhibit 2. Summary of the development status and site considerations of in situ remediation tech-

nologies for Pb, Zn, and Cd in soils

permeability and porosity of the soils are often reduced, which limits the potential ex-posure of the metals in the soil to infiltrating water.

The primary limitation to in situ application of S/S technology is the ability toachieve sufficient mixing of the binding agent throughout the contaminated zone (USEPA, 1997b). Solidification/stabilization binders can be mixed with soils in situ throughthe use of conventional earth-moving equipment, vertical auger mixing, or injectiongrouting. Evaluation of alternative methods is a focus of more recent research and mayoffer more effective means of achieving in situ mixing.

In situ S/S can be used for a wide range of metal concentrations and to depths as greatas 100 ft through the use of vertical auguring techniques (US EPA, 1997b). In general, insitu S/S is most applicable to contamination that occurs over a small to medium-size area.Use of S/S technology for widespread contamination (e.g., soils affected by smelter stackfallout) is typically cost-prohibitive. In situ S/S technologies can result in a significant in-crease in the soil volume and can dramatically alter the physical and chemical nature of thesoil (Federal Remediation Technologies Roundtable [FRTR], 2001). As a result, S/S treat-ment can hinder the ability of the soils to support vegetation, without the addition of aclean soil cover, and can often limit the potential end uses for a site.

In situ S/S is commercially available and has been demonstrated at several sites andunder the US EPA’s Superfund Innovative Technologies Evaluation (SITE) program (USEPA, 1989).The available information suggests that in situ S/S has been largely successfulat achieving the desired remedial endpoint of reducing metal mobility. However, pub-lished data on the performance of in situ S/S are not widely available, and concerns re-main regarding the uniformity of treatment and long-term stability of the treated soils.

In Situ Vitrification

In situ vitrification (ISV) involves the application of electric current to melt soil at hightemperatures (1,600 to 2,000 °C)—causing the soil to form a stable, glass-like matrix inwhich most inorganic contaminants are immobilized. Organic contaminants, if present, aretypically volatilized or pyrolyzed during the treatment. Due to its high cost, ISV is mostapplicable to soils containing high levels of mixed organic and inorganic contaminants thatcannot be otherwise treated cost-effectively or safely. In general, ISV is applicable for sitescontaining nonvolatile metals at levels that do not exceed their glass solubilities (e.g., � 25wt%) (Evanko & Dzombak, 1997; US EPA, 2000a).The effectiveness of ISV for treatmentof lead, cadmium, and zinc in soils will depend on the difficulty associated with maintain-ing the metals within the melt and the ability to control volatile emissions.The contami-nated soils must also contain sufficient glass-forming materials (US EPA, 2000a).

ISV has been demonstrated to depths of 20 ft and is not considered to be cost-effec-tive at depths of less than 6 ft (US EPA, 2000a).The presence of buried metal or under-ground structures/utilities within 20 ft of the melt zone may preclude the use of ISV(US EPA, 2000a). ISV may not be appropriate for soils with a slope of > 5 percent, be-cause the melted soils may flow (US EPA, 2000a). Due to the high cost, ISV is generallynot appropriate for sites with widespread contamination. Furthermore, because ISVconverts the soil to a glass block, the resulting material is unlikely to support plantgrowth, and the end use for the site may be limited.

US EPA (2000a) reports that vitrification (presumably both in situ and ex situ) hasbeen operated at a large scale at least ten times and has been demonstrated in more than



. . . published data on theperformance of in situ S/Sare not widely available,and concerns remainregarding the uniformity oftreatment and long-termstability of the treated soils.

150 tests of varying scale on a broad range of soils and sludges. However, applications ofISV for treatment of metals in soils are limited, and the literature search revealed onlyone site where ISV has been applied for the remediation of lead, cadmium, or zinc insoil. At this site, ISV was applied for treatment of soils containing both organic (pesti-cides, dioxins) and inorganic (lead, mercury, and zinc) contaminants.

Electrokinetic Remediation

Electrokinetic remediation (ER) involves the installation of electrodes into the subsur-face and application of low-intensity direct current through the soil to stimulate electro-chemical and electrokinetic processes that desorb metals from the soil matrix and mobi-lize them toward the electrodes for removal or treatment (FRTR, 2001).The processresults in the generation of an acid front at the anode that eventually moves from theanode to the cathode, enhancing desorption/dissolution of metal contaminants from thesoil matrix (US EPA, 1997b).The process is typically applied to concentrate the con-taminants at or near the electrode, such that they can be removed via excavation, elec-troplating at the electrode, precipitation at the electrode, pumping of water near theelectrode, or complexing with ion exchange resins (FRTR, 2001).

The effectiveness of ER technologies is a function of the contaminant concentrationand type and the soil type, structure, and chemistry. Metal-removal efficiency can be lim-ited by the degree of solubilization and desorption of the metal contaminant that is achiev-able (Cauwenberghe, 1997).The addition of conditioning fluids, such as ethylenediaminetetraacetic acid (EDTA), ammonia, sodium acetate, and water, has been shown to improvemetal recoveries by increasing the fraction of metals in solution (Clifford et al., 1993;Mohamed, 1996). However, addition of conditioning fluids has the potential to mobilize themetals and spread the contamination to underlying soils and groundwater. ER can be ap-plied to a wide range of contaminant concentrations and depths but is generally not cost-ef-fective for surface contamination or for sites with widespread metal contamination.Thetreatment process can significantly alter the soil properties, particularly when conditioningreagents are used, and thus can potentially inhibit the soil’s ability to support plant growth.

Although laboratory bench tests have demonstrated that ER has the potential to beeffective for a variety of soil matrices, including clay, sand, and silt (Clifford et al., 1993;Hamed et al., 1991; Reed et al., 1996; Suèr et al., 2003;Wong et al., 1997), ER is mostapplicable to saturated soils with low groundwater flow rates and moderate to low per-meability (Evanko & Dzombak, 1997).The use of ER is limited in highly conductive soilmatrices (FRTR, 2001), because these matrices will require higher energy inputs to ef-fect remediation. Features common to contaminated soil systems, such as the presenceof iron oxide–rich zones or lenses of alkaline mineral, can limit the effectiveness of ERin field-scale applications and lead to incomplete treatment (Marceau & Broquet, 1999;Reddy & Chinthamreddy, 1999; Reddy & Parupudi, 1997;Yeung et al., 1996).

ER is a fairly well-developed technology (Cauwenberghe, 1997; Evanko &Dzombak, 1997; US EPA, 1997b) and is offered by several vendors. Several bench-scalestudies have suggested that ER is a potentially effective means of remediating lead-, cad-mium-, and zinc-contaminated soils. However, bench-scale tests represent idealized con-ditions and are not necessarily representative of ER performance at the field scale. ERwas evaluated as part of the EPA SITE program, in which a pilot-scale study demon-strated it to be effective at removing lead from kaolinite soils and kaolinite and sand



The effectiveness of ERtechnologies is a functionof the contaminant con-centration and type andthe soil type, structure,and chemistry.

mixtures spiked with lead nitrate, when very close electrode spacing was used (2.3 ft)(US EPA, 1995, 1997b).The literature search identified six sites where ER has been im-plemented on a pilot or full scale in the United States or Europe for remediation oflead, cadmium, and/or zinc in soils. At five of these sites, ER was reported to be effec-tive at reducing the soil concentrations of lead, and at one of the sites, cadmium andzinc. However, much of the information obtained appears to have been vendor-pro-vided, and the available data are insufficient to allow for a critical evaluation.The sixthsite—Point Mugu, California—found that ER was ineffective during a pilot-scale evalu-ation for removal of cadmium and chromium from a sandy soil, despite the fact that pre-liminary laboratory bench-testing results had been favorable (US EPA, 2000b).

Soil Flushing

Soil flushing involves the use of water or another suitable solution to extract contami-nants from the soil matrix (FRTR, 2001).The extraction fluid is passed through the soilmatrix via infiltration or injection techniques and then is recovered and treated to re-move the contaminant. If possible, the extraction fluid is recycled—a key factor in de-termining whether soil flushing will be cost-effective for a site (US EPA, 1997b). Due toreagent demands and associated costs, soil flushing is generally most applicable to siteswith soils containing moderate to low levels of metal contaminants and a moderate tolow aerial extent of contamination.The depth of application of the technology is limitedonly by the ability to reliably recover the flushing agents.

The metal-removal efficiency during soil flushing depends on the degree of contactachieved between the extraction fluid and the contaminated soil matrix, the solubility ofthe metal in the extraction fluid, and the tendency for the metal to sorb to the soil ma-trix as the metal-laden extraction fluid migrates to the water extraction point. Soilflushing is most appropriate for relatively homogeneous and permeable soils (US EPA,1997b). Heterogeneities common to the subsurface can lead to channeling of extractionfluids and, thus, incomplete treatment. Due to the complexity of identifying an appro-priate extraction fluid, soil flushing is most appropriate for sites where a single metalneeds to be removed. Another factor that limits the application of soil flushing is the un-certainty in estimating the completeness with which the applied flushing reagents willbe captured, and the associated risk of spreading the contamination to underlying soilsand groundwater. Furthermore, the flushing solution can substantially alter the soilproperties and limit the soil’s ability to support plant growth.

A large body of literature is available describing research related to soil flushing forremoval of lead, cadmium, and zinc from soils.The majority of the research involvessimple bench-scale testing of potential reagents that can be used to create an effectiveextraction fluid.These tests represent idealized conditions and do not consider the diffi-culty of applying the technology under field conditions. EDTA is the most commonly in-vestigated reagent for enhancing the removal of lead, cadmium, and/or zinc from soils,and several researchers have demonstrated its potential effectiveness (Doong et al.,1998; Elliott & Herzig, 1999; Price et al., 1998;Wasay et al., 1998, 2001). A range ofother reagents, including diethylenetriamine pentaacetic acid (DPTA), citrate, oxalate,and cyclodextrin, has been evaluated, with varying levels of success (Brusseau et al.,1997; Chen et al., 1995; Doong et al., 1998; Francis & Dodge, 1998; Mulligan et al.,1999a, b; Peters, 1999; Price et al., 1998; US EPA, 1997b;Wasay et al., 1998, 2001).



Due to the complexity ofidentifying an appropriateextraction fluid, soil flush-ing is most appropriate forsites where a single metalneeds to be removed.

Although laboratory testing has shown that soil flushing can effectively remove met-als from soils, the technology has seen limited application at the field scale—reflectingthe complexity of successfully applying the technology under real-world conditions. USEPA reported in 1997 that soil-flushing technologies have been selected as the preferredremedy for remediation of metals at seven Superfund sites but were only operational attwo of them (US EPA, 1997b). One of these sites, the Lipari Landfill in New Jersey, in-volves injection of a reagent to flush contaminants, including lead, from the landfill, fol-lowed by extraction of leachate.

Phytoextraction

Phytoextraction involves the use of successive crops of plants that tolerate and accumu-late heavy metals by translocating the metals from soil to shoot tissues, allowing themetals to be removed from the soil through harvesting and, potentially, recovered bysmelting of the plant tissue (Brown et al., 1994a, b).The potential cost benefit of phy-toremediation for metal-contaminated soils has stimulated a large amount of recent re-search—the majority of which focuses on two general categories: identification of po-tential hyperaccumulators (plant species that can accumulate high levels of metals) andevaluation of the use of high-biomass crops in conjunction with chelating agents (to sol-ubilize soil metals to increase plant uptake) for metal removal.The ideal plant for phy-toextraction should grow on metal-bearing soil, exhibit high biomass and growth rates,accumulate and tolerate high concentrations of metal in shoots, have the ability to accu-mulate several metals, and exhibit resistance to diseases and pests (Salt et al., 1998;Watanabe, 1997).

Phytoextraction is limited to the rooting depths of the metal-accumulating plantsand is most effective for sites with widespread, surficial contamination. Phytoextractionis typically limited to sites with low- to mid-range levels of metal contamination anddoes not substantially alter the end use of the site following treatment. In general, phy-toextraction results in a reduction in the most available and mobile fraction of metals insoils, and residual metals present in the soils after treatment, therefore, can represent alesser threat to the environment.

The effectiveness of phytoextraction depends on numerous factors, including theconcentration and mineralogical form of the metal, the type, strength, and delivery effi-ciency of conditioning fluid (if used), and the growing conditions. Several researchershave found that lead, cadmium, and zinc uptake tends to increase with increasing initialsoil metal concentration (Davis et al., 1995; Dudka et al., 1996; Salt et al., 1995;Xiong, 1997), although Blaylock et al. (1997) found that, due to lead’s relative insolubil-ity, high total lead concentration in the soil does not necessarily result in high lead con-centrations in aboveground plant shoots. Metal chelates that increase the water solubilityof metals have the potential to enhance or induce heavy metal accumulation in a varietyof plant species, such as Indian mustard, corn, and sunflower (Blaylock et al., 1997;Huang et al. 1997).The effectiveness of chelates, such as EDTA, depends on the miner-alogical form of the metal in the contaminated soil system (Elless & Blaylock, 2000).Several researchers have found that soil acidity strongly influences the extent of heavymetal uptake in plants (Brown et al., 1994a, b, 1995; Davis et al., 1995; Li & Chaney,1998). However, the addition of such conditioning fluids has the potential to mobilizethe metal contaminants to underlying soils and groundwater.



Phytoextraction is limitedto the rooting depths ofthe metal-accumulatingplants and is most effectivefor sites with widespread,surficial contamination.

Successful full-scale applications of phytoextraction for metal-contaminated soils arestill somewhat rare. However, numerous projects are underway, and several pilot- orsmall-scale projects have shown positive results. Phytoremediation is considered anemerging technology and it is likely that more and more successful full-scale applicationswill be completed in the near future. Edenspace Systems Corporation (formerlyPhytotech Inc.), a leader in the commercial application of phytoremediation for metal-contaminated soils, has conducted more than 19 phytoremediation projects since 1994,which have produced promising results.

During a phytoextraction field application, Blaylock et al. (1997) found that Indianmustard was capable of removing 180 kg of lead per hectare of soil in a single growingseason, when EDTA and acetic acid amendments were applied to a soil containing 1,200mg of lead per kilogram of soil. In a similar field demonstration funded by theEnvironmental Security Technology Certification Program, phytoextraction using cornin conjunction with EDTA and acetic acid application was relatively ineffective for leadremoval, likely due to low plant yields attributed to the poor agronomic quality of thesoil and, potentially, downward migration of EDTA-metal complexes. Brown et al.(1995) evaluated cadmium and zinc uptake by Thlaspi caerulescens, silene, and lettuce intwo-year-long field studies of soils contaminated by historical sewage sludge applica-tions.The authors concluded that phytoremediation is most likely limited to less con-taminated soils, and remediation is likely to take a fair amount of time.The authors esti-mate an 18-growing-season treatment period for a soil containing zinc at 400 mg/kg.Chen et al. (2000) demonstrated in a field study that vetiver grass could accumulate 218g of Cd/ha at a soil concentration of 0.33 mg of Cd/kg and estimated that four crop-pings with vetiver grass would reduce the cadmium concentration to background levels.Robinson et al. (1998) evaluated cadmium, zinc, and lead uptake by Thlaspi caerulescensin pot trials and in wild populations at a mine waste site in France.The authors esti-mated that the plant could remove 60 kg of Zn/ha and 8.4 kg of Cd/ha per croppingand concluded that these rates were sufficient for effective remediation of cadmium, butwould be inefficient for remediation of zinc.

Chemical Stabilization

In concept, chemical stabilization parallels in situ S/S applications and is thus subject tomany of the same constraints.The primary distinction between chemical stabilization andS/S technologies (for the purposes of this review) is that chemical stabilization technolo-gies involve the use of generally nonconventional chemical amendments to induce specificchemical reactions within the soil matrix that render the metal contaminants inert (i.e.,nontoxic or nonbioavailable) and/or immobile. Unlike many S/S approaches, chemicalstabilization technologies do not target encapsulation of the metal contaminant or a re-duction in the permeability of the treated soil matrix.Typically, chemical stabilizationremedies involve lower amendment addition rates relative to S/S remedies and, thus, donot substantially alter the soil properties (e.g., permeability, volume, structure).

A substantial amount of research has been conducted to evaluate the potential effi-cacy of chemical stabilization technologies for the remediation of lead, cadmium, andzinc in soils.The majority of these studies are bench-scale laboratory investigations—particularly evaluating the effects of phosphate amendments on lead solubility. Phosphatehas long been known to be effective at stabilizing lead, as demonstrated by Nriagu



Phytoremediation is con-sidered an emerging tech-nology and it is likely thatmore and more successfulfull-scale applications willbe completed in the nearfuture.

(1974).The concept is to induce the formation of highly insoluble lead phosphate min-erals that have a low bioavailability and mobility and are stable under a variety of envi-ronmental conditions (Ruby et al., 1994). A large body of research has shown that vari-ous forms of phosphate amendments can be effective at stabilizing lead in soils (Berti &Cunningham, 1997; Boisson et al., 1999a, b; Chen et al., 1997; Cotter-Howells &Caporn, 1996; Hettiarachchi & Pierzynski, 2002; Ma et al., 1993, 1995; Mosby, 2000;Pierzynski & Schwab, 1993; Rabinowitz, 1993;Vangronsveld & Cunningham, 1998;Xenidis et al., 1999; Zhang et al., 1998). Phosphate-based amendments also have thepotential to be effective for cadmium and zinc (Boisson et al., 1999a, b; Chaney et al.,1997; Chlopecka & Adriano, 1997; Hamon et al., 2002; Laperche et al., 1997;Phosphate Induced Metal Stabilization [PIMS], 2000).

A significant concern is that phosphate amendments have the potential to increasethe mobility and plant uptake of arsenic (Boisson et al., 1999a, b).These effects can po-tentially be mitigated by the inclusion of iron (as hydrous ferric oxide) with the phos-phate amendment (Jones, 1997). Another concern is the effect of continual removal ofphosphorus due to plant growth. Hettiarachchi and Pierzynski (2002) concluded thatplant removal of phosphorus could reduce the effectiveness of phosphorus amendmentson lead bioavailability, unless sufficient excess was applied or if the phosphorus wasadded in combination with manganese oxide.

Several non-phosphorus-based amendments have been evaluated to stabilize lead,cadmium, and zinc in acidic soils. Municipal biosolids are a potentially promising chemi-cal amendment due to their widespread availability and low cost. Condor et al. (2001)demonstrated in bench tests that lime-stabilized biosolids were capable of immobilizingzinc in smelter-impacted soils and reduced the ecotoxicity of the soils to earthworms.Lime, a common soil amendment long used in agriculture, induces a rise in soil pH,causing metals to precipitate as oxides and carbonates. Lime is anticipated to be effectiveonly for a relatively short period of time before the pH-buffering capacity is depleted;therefore, repeated applications are often required (Vangronsveld & Cunningham,1998). Lime has been shown to be effective at reducing plant uptake of zinc, but mixedresults have been reported for plant uptake of cadmium (Krebs et al., 1998; Pierzynski& Schwab, 1993). Lime was fairly ineffective for treatment of high-zinc-content soilsfrom the Palmerton Zinc Superfund site, unless the lime was combined with the high-iron biosolids and applied as a phytostabilization strategy (Li & Chaney, 1998).

Amendments that provide sorption sites that have a strong affinity for trace metalshave been shown to effectively stabilize lead, cadmium, and zinc in soils by limiting thesoluble fraction of the metals in the soil matrix. Addition of iron and/or manganese in avariety of forms (e.g., hydrous oxides, steel shot, steel sludge, furnace slag, and zero-valentiron) has been shown to be effective at reducing the leachability, bioaccessibility, andphytoavailability of lead, cadmium, and zinc (Berti & Cunningham, 1997; Chen et al.,2000; Chlopecka & Adriano, 1997; Hettiarachchi & Pierzynski, 2002; Mench et al., 1994;Pierzynski & Schwab, 1993; Sappin-Didier et al., 1997; Shuman, 1997). Zeolites and alu-minosilicates have also been demonstrated to have a high retention capacity for metals andcan be used as stabilizing agents (Boisson et al., 1999a; Chlopecka & Adriano, 1997;Edwards et al., 1999; Garcia-Sanchez et al., 1999; Gworek, 1992; Lothenbach et al.,1997; Miinyev et al., 1990; Rebedda & Lepp, 1994;Vangronsveld & Cunningham, 1998).

Chemical stabilization is a relatively new technology—particularly as a strategy toreduce metal bioavailability in soils—and thus has seen limited application at the field



Lime, a common soilamendment long used inagriculture, induces a risein soil pH, causing metalsto precipitate as oxidesand carbonates.

scale.The literature search revealed five sites where chemical stabilization has been ap-plied at the field scale. In general, the field tests indicate that chemical amendments havethe potential to be effective in stabilizing lead, zinc, and cadmium in soils—thereby re-ducing the mobility and bioavailability of these metals. A major focus of research is thepotential for the amendments to reduce the bioavailability of lead in soils to humans. Atthe Joplin National Priorities List (NPL) site, EPA’s Remedial Technology DevelopmentForum sponsored a field-scale evaluation of the use of phosphate-based soil amendmentsto reduce lead bioavailability. Results indicate that addition of phosphoric acid (0.5 per-cent or 1 percent phosphorus by weight) will reduce lead bioavailability from Joplin soilin rats, juvenile swine, and human volunteers (Graziano et al., 2001; Ryan & Berti,2001). In the most promising results, soil amended with 1 percent phosphoric acid, andweathered for 18 months in the field, reduced lead bioavailability by 69 percent in adulthuman volunteers (Graziano et al., 2001). However, these results appear to be specificto the Joplin site, and different soil types and lead forms at other sites may require dif-ferent amendment types or application strategies.

Phytostabilization

Phytostabilization-based remediation strategies involve the use of plants, alone or in con-junction with soil amendments, to stabilize a metal-contaminated soil by limiting metalmobility and bioavailability. In addition, the vegetative cover provides protection againsterosion, reduces exposure to the metal-contaminated soils, and reduces infiltration ofwater and, thus, leaching of metals from the soils. Under ideal circumstances, phytostabi-lization would involve the use of metal-tolerant plants that immobilize/inactivate metals insoils through sorption, precipitation, complexation, or chemical reduction reactions (USEPA, 1997b). Alternatively, high-water-yield plants, such as poplar trees, can also be usedto lower the groundwater table locally and limit contact of shallow contaminated soils withgroundwater (Pierzynski et al., 2000). Unlike phytoextraction, under a phytostabilizationstrategy, plants ideally would not accumulate metals in aboveground plant tissue that po-tentially could be consumed by human or ecological receptors (Vangronsveld &Cunningham, 1998). It should be noted that some consider phytostabilization to be a con-tainment technology that is most applicable as an interim measure (US EPA, 1997b).

In general, phytostabilization is most applicable to sites with widespread surficialcontamination where metals are present in soils at low concentrations.The most com-mon and promising phytostabilization practice is to combine the use of plants and soilamendments to remediate metal-contaminated soils.The amendments act to reduce thebioavailability and phytoavailability of the metals in the soil and/or condition the soil tofacilitate restoration of a vegetative cover on the soils. Chemical amendments may in-clude those described previously for stabilization technologies and/or organic-based soilamendments, such as biosolids compost.The majority of the information derived fromthe literature search pertaining to phytostabilization of metal-contaminated soils con-sisted of descriptions of field trials rather than laboratory studies.This likely reflects thefact that phytostabilization strategies are typically geared toward sites with widespreadmetal contamination, such as mine wastes and soils containing smelter-stack fallout.Also, field studies are generally more appropriate for evaluation of revegetation, becauselarge-scale environmental factors (e.g., climate) can be evaluated.



In general, phytostabiliza-tion is most applicable tosites with widespread sur-ficial contamination wheremetals are present in soilsat low concentrations.

Phytostabilization is a relatively new remediation technology that has emergedprimarily as a result of efforts to revegetate mine spoils and smelter-affected soils, inwhich chemical amendments are often required to establish plant growth. The major-ity of the phytostabilization research has involved application of biosolids, with orwithout additional chemical amendments. Biosolids applications have been shown tobe capable of reducing the bioavailability and phytotoxicity of lead, cadmium, andzinc in contaminated urban soils (Basta et al. 2001; Li & Chaney, 1998). Biosolidshave also been shown to be effective for establishing a healthy vegetative cover formetal-contaminated mine tailings at various sites. Lepp (1998) demonstrated thatamendment of soils with zeolites reduced the water-extractable metal concentrationsand improved plant growth in metal-contaminated grassland soils. Pierzynski et al.(2000) evaluated the ability of poplar trees to remediate lead and zinc contaminationin soils affected by smelting activities in Kansas but found that tree survival rate waspoor—possibly due to poor field conditions (i.e., environmental conditions and pooragronomic practices) and/or zinc toxicity.

SUMMARY

This review article presents a broad summary and evaluation of the in situ remediationtechnologies that are available for soils contaminated with lead, cadmium, and/or zinc.A total of seven remediation technologies were identified, each of which centers on oneof the following remedial strategies—isolation, removal, or stabilization.The technolo-gies included solidification/stabilization, vitrification, electrokinetic remediation, soilflushing, phytoextraction, phytostabilization, and chemical stabilization. Each technologyhas benefits and limitations, depending on the remedial objectives targeted and on site-specific factors (see Exhibit 2). For example, at large-area sites with low to moderatelevels of shallow contamination, a plant-based (phytoextraction or phytostabilization) orchemical amendment strategy likely will be preferred. However, these technologies areoften limited to surficial soils with low to moderate contamination.Thus, for sites withcontamination at depth, or high levels of contamination, a more aggressive remedialtechnology should be selected.

The majority of these in situ technologies are in the developmental stages, and addi-tional research is required to better understand treatment mechanisms to optimizetreatment performance and to gain acceptance by the scientific and regulatory commu-nities. However, several of the technologies discussed above have potential for addressingmetal-contaminated soils in a cost-effective manner.The potential benefits and cost sav-ings offered by these remedial technologies can be substantial relative to conventionalremediation strategies, such as excavation and disposal, and additional research is there-fore warranted to more fully develop and evaluate these technologies.

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Todd A. Martin is a senior engineer at Integral Consulting, with eight years of experience specializing in

environmental engineering, contaminant fate and transport, and remediation alternatives analysis.

Mr. Martin’s background encompasses engineering design and cost analysis, aqueous and soil geochemistry,

hydrogeology, and microbiology. His work has focused on the development of cost-effective alternatives for

site remediation that are protective of human health and the environment and consistent with long-term

site management.

Michael V. Ruby is a principal with Exponent, with 17 years of experience in the fields of environmental

chemistry and toxicology. Over the last 13 years, Mr. Ruby has specialized in the fate and transport of met-

als in the environment, and in research pertaining to human exposures to contaminants in soil.



review of in situ remediation technologies for lead, zinc, and cadmium in soil

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