Comparison of Extractants for Removal of Lead, Zinc, andPhenanthrene from Manufactured Gas Plant Field Soil

Kranti Maturi1; Amid P. Khodadoust2; and Krishna R. Reddy, M.ASCE3

Abstract: Polycyclic aromatic hydrocarbons �PAHs� and heavy metals found at former manufactured gas plant �MGP� sites are of majorenvironmental concern. An investigation is made into evaluating the capability of various extractants including surfactants, cosolvents,chelating agents, acids, and cyclodextrins for removing phenanthrene, lead, and zinc from a contaminated MGP soil. The determinationof the most promising extracting solutions is critical for the development of effective treatment technologies such as soil washing/flushingand electrokinetic remediation. MGP contaminated soil was silty sand with 11% organic matter containing higher levels of phenanthreneand two heavy metals �lead and zinc�. Several batch tests were conducted using the soil with different extracting solutions at variousconcentrations to enhance the removal efficiency and to optimize the concentration of each extractant. The test results showed that bothsurfactants �Igepal CA-720 and Tween 80� at a wide range of concentrations were effective for the removal of phenantherene from thesoil. Only selective cosolvents and cyclodextrins �n-butylamine and HPCD� at higher concentrations were found to be effective in thesolubilization of phenanthrene. Selective acids �citric acid and phosphoric acid� were found to be effective in the removal of heavy metals,while chelating agents �EDTA and DTPA� resulted in a moderate heavy metal removal. Overall, none of the selected extractants removedboth heavy metals and PAHs; therefore, sequential extraction schemes using the selected extractants were investigated. The sequential useof 5% Tween 80 followed by 1 M citric acid was found to remove over 90% phenanthrene and over 70% lead and nickel each from theMGP soil.

DOI: 10.1061/�ASCE�1090-025X�2008�12:4�230�

CE Database subject headings: Extraction procedures; Soil pollution; Lead; Zinc; Acids; Surface-active agents.

Introduction

In the United States, many former gas manufacturing plant�MGP� sites have been found to be highly contaminated by avariety of polycyclic aromatic hydrocarbons �PAHs� as well asheavy metals �USEPA 1997�. These contaminants are of environ-mental concern because many PAHs and heavy metals are poten-tial carcinogens. To obviate these problems, it is necessary toundertake some remedial action. Although there are several reme-diation techniques available to remediate PAH-contaminatedsites, very few technologies are available for the remediation ofboth PAH and heavy metal-contaminated sites. The conventionalin situ remediation methods for PAHs include bioremediation,thermal treatments, stabilization or solidification processes, andsoil flushing �Roote 1998; USEPA 2000; Sharma and Reddy2004�. Technologies available for remediating heavy metal con-

taminated soils can be divided mainly into two groups, namely,immobilization methods and separation/concentration methods. Inthe first type of remediation, heavy metals are immobilized,thereby nullifying the leaching of contaminants into the ground-water. Containment, in situ and ex situ solidification and stabili-zation, in situ and ex situ vitrification fall under this category. Thesecond type of remediation deals with separating the heavy metalsfrom the soils or reducing the volume of contaminated soil. Thereduced volume of the soil may be disposed of in the landfills,and separated contaminants may be treated on site or off site withsuitable treatment methods. In low permeability or heterogeneoussoils, methods such as solidification/stabilization and bioremedia-tion fail because it is hard to introduce the appropriate reagents ornutrients into the ground due to the low permeability of the soils.Therefore, the potential in situ treatment technologies available totreat metal-contaminated clays are limited to in situ vitrification,phytoremediation, and electrokinetic remediation.

Many remediation technologies are ineffective for the reme-diation of soils when the PAHs and heavy metals coexist. Previ-ous research at the University of Illinois at Chicago �UIC� hasshown that electrokinetic remediation has the potential to removeboth the heavy metals and PAHs from contaminated soils. How-ever, in this previous research, electrokinetic remediation technol-ogy was applied either to the heavy metal contaminated soils�Reddy et al. 1997� or to the PAH contaminated soils �Reddy andSaichek 2003�. USEPA �1997� reported that 41% of the nationalpriority list �Superfund� sites contain both heavy metals andPAHs. Therefore, a single capable technology, such as electroki-netic remediation, can save both money and time. However, thistechnology is highly dependent on the type of extracting solutionused. Because of the different nature of the heavy metals and the

1Graduate Research Assistant, Dept. of Civil and MaterialsEngineering, Univ. of Illinois at Chicago, 842 West Taylor St., Chicago,IL 60607.

2Associate Professor, Dept. of Civil and Materials Engineering, Univ.of Illinois at Chicago, 842 West Taylor St., Chicago, IL 60607.

3Professor, Dept. of Civil and Materials Engineering, Univ. of Illinoisat Chicago, 842 West Taylor St., Chicago, IL 60607 �correspondingauthor�. E-mail: kreddy@uic.edu

Note. Discussion open until March 1, 2009. Separate discussions mustbe submitted for individual papers. The manuscript for this paper wassubmitted for review and possible publication on October 3, 2005;approved on December 3, 2007. This paper is part of the PracticePeriodical of Hazardous, Toxic, and Radioactive Waste Management,Vol. 12, No. 4, October 1, 2008. ©ASCE, ISSN 1090-025X/2008/4-230–238/$25.00.

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PAHs, promising extracting solutions must be selected that canremove both the contaminant groups simultaneously.

In this study, various extracting solutions were evaluated toidentify the most promising extractants that could be used in re-mediation technologies, such as soil-washing/flushing and elec-trokinetic remediation. Several batch tests were performed usingdifferent types of extracting solutions such as surfactants, cosol-vents, chelating agents, cyclodextrins, and acids in order to elu-cidate their affinity to solubilize/desorb PAHs and heavy metalsfrom a contaminated MGP field soil.

Materials and Methods

Soil

In this study, a MGP field contaminated soil was used for theextraction experiments. The properties of the field soil are sum-marized in Table 1. The field soil was basically organic silty sandcontaminated with PAHs and heavy metals. The concentrations ofthese contaminants are listed in Tables 2 and 3 for PAHs andheavy metals, respectively. Specifically, the field soil was foundcontaminated with 193 mg /kg of phenanthrene, 776 mg /kg of

zinc, and 1,477 mg /kg of lead. The effectiveness of different ex-tractants was evaluated based on these specific contaminants.

Extractants

For the present study, five different classes of extracting solutions,namely, nonionic surfactants, cosolvents, chelating agents, cyclo-dextrins, and acids, were selected.

Surfactants are shown to be particularly attractive extractantsfor PAHs �Patterson et al. 1999; Edwards et al. 1994; Yeom et al.1996�. Nonionic surfactants, Tween 80 �Aldrich� and Igepal CA-720 �Aldrich�, were selected based on the previous research at theUIC �Saichek 2002�. The properties such as critical micelle con-centration �CMC� and the hydrophile-lipophile balance number�HLB� were taken into consideration during the selection. Each ofthe surfactants was employed at four different concentrations,namely, 0.5, 1, 3, and 5% in water.

Based on the literature, n-butylamine �99.5%, Acros Organics�and tetrahydrofuran �THF� �99.5%, Acros Organics� were se-lected among the cosolvents and were employed at 5, 10, 15, and20% concentrations. The selected cosolvents were shown to bevery effective in increasing the solubility of organics such asPAHs �Peters and Luthy 1993�. For a PAH such as phenanthrene,cosolvents increase contaminant solubility by more than five or-ders of magnitude �Li et al. 2000�.

Chelating agents, ethylene diamine tetraacetic acid �EDTA��Dissolvine�, and diethylene triamine pentaacetic acid �DTPA��Dissolvine� were chosen and each of these chelating agents wasinvestigated at 0.01, 0.05, 0.1, and 0.2 M concentrations. Thesechelating agents possess proven efficiency for heavy metals�Reddy and Chinthamreddy 2000; Neale et al. 1997; Tuin andTels 1990�.

Hydroxypropyl-�-cyclodextrin �HPCD� �98%, Acros Organ-ics� and �-cyclodextrin hydrate ��-CD hydrate� �99%, Acros Or-ganics� were selected among the cyclodextrins. HPCD wasinvestigated at concentrations of 1, 3, 5, and 10% in water,whereas �-CD hydrate was investigated at concentrations of 0.05,0.1, 0.5, and 1% in water. These cyclodextrins were investigatedby a number of researchers for the remediation of contaminatedsoils �Brusseau et al. 1997; Wang and Brusseau 1993; Ko et al.1999; McCray et al. 2000�. Low concentrations of �-CD hydratewere selected because of its low solubility in water.

Seven different acids, namely, lactic acid �Acros Organics�,n-butyric acid �99+%, Acros Organics�, propionic acid �99%,Acros Organics�, oxalic acid �98%, Acros Organics�, citric acid�99%, Acros Organics�, phosphoric acid �85+%, Acros Organics�,and acetic acid �100.4%, Fisher�, were investigated at 1 M con-centration. These acids were selected based on their potential forremoval of heavy metals from soils �Neale et al. 1997; Reddy and

Table 1. Properties of MGP Field Soil �ASTM 1996�

Particle size distribution�%� �ASTM D422�

Gravel 0

Sand 84

Fines 16

Specific gravity�ASTM D854�

2.63

Hydraulic conductivity�cm/s� �ASTM D2424�

1.6�10−4

Organic content �%� �ASTMD2974�

11.1

pH �ASTM D4972� 7.05–7.14

USCS classification�ASTM D2487�

Organic silty sand,SM

Table 2. PAHs in MGP Field Soil

PAH Concentration �mg/kg�

Naphthalene 6.03

Acenaphthene 37.7

Fluorene 39

Acenaphthylene 263

Phenanthrene 193

Anthracene 32.8

Benzo�a�anthracene 4.81

Chrysene 8.65

Pyrene 40.2

Fluoranthene 58

Benzo�b�fluoranthene 1.71

Benzo�k�fluoranthene 2.28

Benzo�a�pyrene 0.419

Dibenzo�a,h�anthracene 0.124

Indeno�1,2,3-cd�pyrene 0.345

Benzo�g,h,I�perylene 0.286

Total PAHs 427.98

Table 3. Heavy Metals in MGP Field Soil

Metal Concentration �mg/kg�

Arsenic 3.81

Cadmium 8.39

Chromium 19.5

Copper 263

Lead 1,477

Nickel 18.6

Silver 1.48

Mercury 9.31

Zinc 776

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Chinthamreddy 2000�. The same concentration of 1 M was se-lected for all the acids to compare the relative performance ofdifferent acids and to eliminate the acids that are ineffective infurther testing.

Batch Extractions

The batch extraction experiments were conducted using a soil towater ratio of 1:5, or specifically 5 g of the soil were combinedwith 25 mL of solution. First, 5 g of contaminated soil wereweighed and placed in a 40 mL glass vial. Next, 25 mL of theextracting solution were added, and the vial was sealed with aTeflon screw-type top. Each vial was then shaken by hand forabout a minute to ensure the soil was fully saturated with solu-tion, and the vials were shaken in a rotary shaker table at 250 rpmfor 24 h. After the shaking was completed, the solution was de-canted into a clean polycarbonate centrifuge tube, and the soil-solution mixture was centrifuged at 4,000 rpm for 28 min. Thesupernatant was then filtered through a glass funnel holding aWhatman GF/C glass fiber filter �1.2 �m particle retention� toremove any floating particles or debris, and the effluent from thefunnel was collected into a clean 40 mL glass vial. In the case ofsequential extractions, the first stage of batch experiments was thesame as the single extraction. After the supernatant was collectedfrom the first stage, 25 mL of the appropriate extracting solutionwas added to the residual soil and the same extraction procedurewas repeated. Once the supernatant was observed to be clear ofany suspended particles, the concentration of selected contami-nants, phenanthrene, lead, and zinc was determined. Batch extrac-tions were performed in duplicates to ensure reproducibility of theresults.

Phenanthrene Analysis

To determine phenanthrene concentration in supernatant, a liquid-liquid extraction procedure was followed. It consists of placing1 mL of the contaminated supernatant in a conical flask using asyringe. The syringe was thoroughly rinsed with ethanol beforeand after use. The sample was then diluted in the ratio of 1:10with water. The conical flask was shaken thoroughly before trans-ferring the diluted sample into a test tube. Next, 2-fluorobiphenyl�96%, Aldrich� was added. After that, 2 mL of methylene chloridewas added into the test tube. The test tube was hand shaken atleast for 5 min. The two phases, methylene chloride phase and theaqueous phase, were then allowed to separate. Approximately1 to 2 mL of the methylene chloride phase was taken using asyringe into a 2 mL autosampler vial. The sample was then run onthe gas chromatograph �GC�. For quality control, all the sampleswere run in duplicates.

The GC used was an Agilent Model 6890 GC equipped with aflame ionization detector �FID�. The injection volume was 1 �L,and it was injected via an auto injector at an inlet temperature of250°C. The column used on the GC was a J&W Scientific �Fol-som, CA� DB-5, 30 m�0.32 mm�25 �m. The carrier gas wasnitrogen at 172 kPa constant pressure. The oven ramp was startedat 100°C and increased to 250°C at 18°C /min for 1.5 min andthen held at 250°C until the end of the run time. The instrumentwas calibrated for phenanthrene �98%, Aldrich� using2-flourobiphenyl as an external standard. The calibration rangewas from 1 to 40 mg /L. The extraction efficiency was calculatedbased on a surrogate concentration obtained from the GC.

Lead and Zinc Analysis

The supernatant from the batch tests was directly analyzed usingan atomic absorption spectrophotometer �AAS� �Model Video 22�to determine the concentration of lead and zinc in accordancewith the USEPA methods 7420 and 7950, respectively �USEPA1986�.

Results and Discussion

For each extractant, the removal efficiency was calculated as theratio of the mass of the contaminant extracted from the soil �de-termined from contaminant concentration in extracting solution�to the initial mass of the contaminant in the soil and multiplied by100. The pH of the soil-solution mixtures were measured after thesamples were shaken until the equilibrium time was reached.

Removal with Surfactants

The pH values of all the Igepal CA-720 and Tween 80 samples atdifferent concentrations were in the range of 7.0 and 7.5. Differ-ent surfactant solutions differ in their affinity towards thesolubilization/desorption behavior of PAHs from the soil due tothe difference in their solution properties, such as CMC and theHLB number. Recent research carried out at the UIC had shownthat two nonionic surfactants solutions, Igepal CA-720 and Tween80, have comparable efficiency to desorb and solubilize phenan-threne �Saichek 2002�. It was observed that the removal ofphenanthrene using surfactants, Igepal CA-720 and Tween 80gradually increased as the concentration increased up to a concen-tration of 3% and thereafter remained constant as shown in Fig.1�a�. A complete removal of phenanthrene was observed at 3 and5% concentrations. At higher concentrations, a larger number ofmicelles is formed resulting in the increased solubilization ofphenanthrene from the soil. It was observed that Tween 80showed better removal efficiency over Igepal CA-720. Tween 80has a lower CMC and a higher HLB value than Igepal CA-720,thus, Tween 80 micelles formed at a lower surfactant concentra-tion. Therefore, the better performance of Tween 80 when com-pared to Igepal CA-720 in the removal of phenanthrene isattributed to enhanced solubilization of phenanthrene due to theformation of more stable micelles.

The data from Fig. 1�b� show that both the surfactants werenot effective in the removal of zinc. For both Igepal CA-720 andTween 80, the removal efficiency was less than 0.5% at all theconcentrations. It can be observed from Fig. 1�c� that the surfac-tants were not that effective in the removal of lead. The removalefficiency was less than 0.5% at all the concentrations with boththe surfactants. In the case of heavy metals, surfactants werenot able to solubilize the heavy metals because of the inorganicnature of the heavy metals and nonionic nature of the selectedsurfactants.

Removal with Chelating Agents

The pH values of both the chelating agents were in the range of8.20 and 11.19. The pH values increased with increasing chelat-ing agent concentration. Generally, EDTA had lower pH valuesthan DTPA at all the corresponding concentrations. Both chelat-ing agents were found to be ineffective in the removal of phenan-threne with removal efficiency less than 4% at all theconcentrations as shown in Fig. 2�a�. Neither of the two chelating

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agents could solubilize nor desorb phenanthrene from the soil asthey are ligands. These chelating agents could not form any stablecomplexes with phenanthrene because of their very chemicalstructure and affinity towards charged metals.

The zinc removal efficiency of EDTA increased with increas-ing concentration. It was in the range of 8 and 12%, as shown inFig. 2�b�. The removal efficiency of DTPA increased with increas-ing concentration, ranging from 10 to 12%. The pH values in-

creased with the increasing concentration. The lead removalefficiency of EDTA was almost the same at all the concentrationsexcept at 0.01 M concentration where it showed 13 to 20% re-moval as shown in Fig. 2�c�. The removal efficiency of DTPA wasalmost the same at all the concentrations, ranging from 18 to23%.

The efficiency of EDTA and DTPA systems was found almost

Fig. 1. Removal of: �a� phenanthrene; �b� zinc; and �c� lead usingsurfactants Fig. 2. Removal of: �a� phenanthrene; �b� zinc; and �c� lead using

chelating agents

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to be the same. The difference in the removal of lead and zincusing both the chelating agents was not significant. The poor re-moval of both the heavy metals might be because of the presenceof other heavy metals in the soil that might have affected thestability of the lead-chelate and zinc-chelate complexes. It is as-sumed that chelating agents might have formed complexes withother metals that were not analyzed in this investigation. In addi-tion, the relatively high pH conditions may have favored the met-als to exist as precipitates.

Removal with Cosolvents

The pH values in n-butylamine were in the range of 11.78 and12.10 whereas in THF, they were in the range of 7.66 and 7.88.The phenanthrene removal efficiency of n-butylamine system de-pends on the concentration, and it increases with an increase inconcentration as shown in Fig. 3�a�. A removal efficiency of 2%was achieved with 5% n-butylamine and it increased to 100%when 20% n-butylamine is employed. The change in removalefficiency was gradual up to 15% n-butylamine but was drasticbetween 15 and 20% n-butylamine. The cosolvent, n-butylamineat 20% concentration had the highest removal efficiency and thismay be because of the availability of more concentrated solutionfor greater solubility of phenanthrene. Hydrophobic organic com-pounds �HOCs� exhibit low concentrations in the aqueous phase�Cw�, the Freundlich isotherm equation, Cs=K�Cw�n, is often lin-ear, which means that n=1 �Li et al. 2000�. Under these condi-tions, the Freundlich constant is Kd, and it describes thedistribution ratio of the HOC, or the concentration of contaminantin the sorbed phase divided by its concentration of contaminant inthe aqueous phase �Schwarzenbach et al. 1993�.

The Kd is a complex parameter that depends on a number offactors that are related to the soil properties and the chemicalspecies in the soil and solution �Schwarzenbach et al. 1993�. Theeffect on Kd caused by using cosolvents can be estimated bydividing the apparent distribution ratio in a water-cosolvent mix-ture �Kd�� by the distribution ratio in pure water �Kd� by using

log�Kd�

Kd� = − ���fc

where �, �, and �=parameters representing the molecular inter-actions between the soil and cosolvent, water and cosolvent, andcontaminant and cosolvent, respectively; and fc=volume fractionof the cosolvent in the solution �Li et al. 2000�. Generally, thegreater the product of ���, the greater will be the removal by thecosolvent. For a PAH such as phenanthrene and for the cosolventsn-butylamine and THF, � values i.e., contaminant-cosolvent mo-lecular interactions are high. THF was not found to be effective inthe removal of phenanthrene. The removal efficiency at all theconcentrations was less than 10% as shown in Fig. 3�a�. Litera-ture shows that higher concentration cosolvents are necessary toproduce significant changes in the solubilization of organic con-taminants �Staples and Geiselmann 1988; Augustijn et al., 1994�.The high pH values of n-butylamine might have also contributedto the greater removal of phenanthrene. Overall, n-butylaminewas found effective as compared to THF in the removal ofphenanthrene because of the water-cosolvent interactions that fa-vored the solubilization of phenanthrene.

The removal of zinc using n-butylamine was found to be in-significant, less than 6% removal efficiency, was ascertained at allthe tested cosolvent concentrations, while the removal efficiencyof THF was almost zero as shown in Fig. 3�b�. The lead removal

efficiency of n-butylamine was less than 0.6% at all the testedcosolvent concentrations, while the removal efficiency of THFwas almost zero as shown in Fig. 3�c�. The poor removal of heavymetals using cosolvents was because of their organic nature thatwas not able to solubilize heavy metals in the absence of suitablebinding sites. In the case of n-butylamine, the poor removal canalso be attributed to the high pH values that could precipitate theheavy metals in the soil itself.

Fig. 3. Removal of: �a� phenanthrene; �b� zinc; and �c� lead usingcosolvents

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Removal with Cyclodextrins

The pH of both the cyclodextrins was in the range of 7.2 and7.65. The pH of �-CD hydrate system decreased with the increas-ing concentration. The removal efficiency gradually increasedwith increasing HPCD concentration. The removal efficiency ofHPCD was found to be 18% at a concentration of 1% and itgradually increased to 96% at a concentration of 10% as shown inFig. 4�a�. The removal efficiencies were found to be 1, 2, 5, and

10% at 0.05, 0.1, 0.5, and 1% �-CD hydrate solutions, respec-tively. The removal efficiency of �-CD hydrate system was lessthan 10% at all the concentrations. The �-CD hydrate system wasfound to be ineffective in the removal of phenanthrene. HPCDperformed better when compared to �-CD at all the concentra-tions due to enhanced solubilization of phenanthrene by forming amore stable complex with the cavity in the HPCD due to thepresence of the additional hydropropyl group in the HPCD.

Fig. 4�b� shows that cyclodextrins are ineffective in removingzinc from the soil. A removal efficiency of less than 0.5% wasobserved for all the tested concentration ranges. Cyclodextrinswere also found to be ineffective in the removal of lead as shownin Fig. 4�c�. A removal efficiency of less than 0.5% was observed.The poor removal of lead and zinc with cyclodextrins may be dueto the inability of incorporating the heavy metals into their cavi-ties because of their size differences. The other possible reasonfor the poor removal of zinc and lead is that cyclodextrins mighthave formed inclusion complexes with other heavy metals presentin the soil.

Removal with Acids

Lactic acid, n-butyric acid, propionic acid, oxalic acid, citric acid,phosphoric acid, and acetic acid were the seven different acidsselected for the extraction testing. It was observed that all acidswere ineffective in the removal of phenanthrene. The removalefficiency was less than 2% for all the acids as shown in Fig. 5�a�.These results show that solubility of phenanthrene is not en-hanced by any of the selected acids.

Organic acids were effective in the removal of zinc. The re-moval efficiencies of phosphoric acid, citric acid, lactic acid, andacetic acid were 65, 64, 54, and 47%, respectively, as shown inFig. 5�b�. Organic acids were also effective in the removal of leadwith the removal efficiencies of phosphoric acid, citric acid, andlactic acid at 68, 67, and 24%, respectively, as shown in Fig. 5�c�.A significant removal of heavy metals using the acids is attributedto the low pH �less than 3.5� conditions at which metals existmostly in the aqueous phase. The chelating ability of citric acidmay also have contributed to the removal of metals; however, atlow pH conditions, the stability of citrate-metal complexes is gen-erally low.

Sequential Removal of Phenanthrene, Zinc, and Lead

On the basis of the single extraction batch test results, it wasconcluded that extractants have different affinity and selectivitytowards the removal of lead, zinc, and phenanthrene from the soil.The most effective extractants for heavy metals �lead or zinc�,were acids �phosphoric acid and citric acid�. For both metals, theremoval with chelating agents �EDTA and DTPA� was signifi-cantly lower than the removal with acids. The most effective ex-tractants for phenanthrene were surfactants �Tween 80 and IgepalCA-720�. In order to effectively remove both metals and phenan-threne from the soil, several combinations of individual extract-ants, which showed effective extraction of either metals orphenanthrene, were selected to sequentially extract both contami-nants. The combinations of extractants selected for sequential ex-tractions are listed in Table 4.

The data from Fig. 6�a� show the extraction of phenanthreneusing various sequential combinations of extractants. The highestremoval of phenanthrene �greater than 90%� was obtained withthe following combinations of extractants: �1� 3% Igepal CA 720followed by 1 M phosphoric acid, �2� 5% Igepal CA 720 followed

Fig. 4. Removal of: �a� phenanthrene; �b� zinc; and �c� lead usingcyclodextrins

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by 1 M citric acid, �3� 5% Tween 80 followed by 1 M citric acid,�4� 5% Tween 80 followed by 1 M phosphoric acid, �5� 5%Tween 80 followed by 0.2 M EDTA, �6� 1 M citric acid followedby 3% Tween 80, and �7� 0.2 M EDTA followed by 5% Tween80. The other sequential extraction schemes removed less than90% removal of phenanthrene from the soil. In all sequentialextractions, phenanthrene was removed during the use of surfac-

tants as a result of formation of micelles, which enhanced phenan-threne solubilization. When the surfactants were used in the firststep, most of the phenanthrene was removed. When acid or achelating agent was used during the second step, a small amountof additional phenanthrene was removed, and this removal is at-tributed to the presence of residual surfactant in the soil from thefirst step. The differences observed in removal efficiencies be-tween the single extraction and the first or second step in sequen-tial extraction for the same extractant may be attributed toheterogeneous distribution of phenanthrene in the soil.

The data from Fig. 6�b� show the extraction of zinc usingvarious combinations of extractants. The sequential extractiondata show that the highest removal of zinc �greater than 80%� wasobtained with the following combination of extractants: �1� 1 Mcitric acid followed by 3% Igepal CA-720, and �2� 1 M citric acidfollowed by 5% Tween 80. In both cases, the effective combina-tion consisted of the application of 1 M citric acid in the firstextraction step followed by surfactant solution in the second ex-traction step. Greater than 75% removals were obtained for thesequential extraction: �1� 3% Igepal CA-720 followed by 1 Mcitric acid, �2� 5% Igepal CA-720 followed by 1 M citric acid, �3�3% Tween 80 followed by 1 M citric acid, �4� 5% Tween 80followed by 1 M citric acid, �5� 1 M citric acid followed by 3%Igepal CA-720, and �6� 1 M citric acid followed by 5% Tween80. The other sequential extractions removed less than 70% of thezinc from the soil.

The data from Fig. 6�c� show the extraction of lead usingvarious combinations of extractants. The sequential extractiondata show that the highest removal of lead �greater than 70%� wasobtained when 3% Tween 80 was used in the first step and 1 Mcitric acid in the second step. Greater than 65% removal of leadwas obtained for sequential extractions: �1� 3% Igepal CA-720followed by 1 M citric acid, �2� 5% Igepal CA-720 followed by1 M citric acid, �3� 3% Tween followed by 1 M citric acid, and�4� 5% Tween 80 followed by 1 M citric acid. The other sequen-tial extractions removed less than 65% of the lead from the soil.The schemes in which phosphoric acid was used in the first stageresulted in a negligible or zero removal of lead.

In general, both zinc and lead were removed during the use of

Table 4. Sequential Extraction Experiments

Extraction Step 1 Extraction Step 2

3% Igepal CA-720 1 M citric acid

1 M phosphoric acid

0.2 M EDTA

5% Igepal CA-720 1 M citric acid

1 M phosphoric acid

3% Tween 80 1 M citric acid

1 M phosphoric acid

5% Tween 80 1 M critic acid

1 M phosphoric acid

0.2 M EDTA

1 M citric acid 3% Igepal CA-720

3% Igepal CA-720

3% Tween 80

5% Tween 80

1 M phosphoric acid 3% Igepal CA-720

5% Tween 80

0.2 M EDTA 3% Igepal CA-720

5% Tween 80

Fig. 5. Removal of: �a� phenanthrene; �b� zinc; and �c� lead usingacids

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acids or chelating agents in either the first step or the second stepof sequential extraction schemes. The removal of metals is higherwhen acids are used because of lowering pH to less than 3.5, atwhich metals exist in the aqueous phase. When surfactants wereused following the use of acids or chelating agents, some addi-tional zinc and lead removal was observed, and this additionalzinc and lead removal is attributed to the presence of residualacids or chelating agent in the soil from the first step of theextraction process.

Conclusions

Extraction of PAHs and heavy metals from the MGP soil is highlycontrolled by the type of extracting solution and its affinity to-ward the target contaminant. In the present investigation, differentextracting solutions were found to be effective in removing eithermetals such as zinc and lead or phenanthrene �a representativePAH�. Surfactant solutions were most effective in extractingphenanthrene, while organic acids were most effective in extract-ing lead or zinc. Combinations of surfactant and organic acidswere found to be most effective sequentially aided extracting sys-tems for the removal of both metals and phenanthrene from thesoil. The use of 5% Igepal CA-720 followed by 1 M citric acid orthe use of 5% Tween 80 followed by 1 M citric acid were foundto be consistently effective in simultaneous extraction of zinc,lead, and phenanthrene from the soil. It should be noted thatsite-specific treatability studies, similar to this study, should beperformed to optimize remediation plans, since soil compositionand heterogeneity, contaminant types and concentrations, andother site-specific parameters can affect the removal efficiency.

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