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Feasibility study of surfactant use for remediation oforganic and metal contaminated soilsChih Huang a , John E. Van Benschoten a , Tara C. Healy b & Michael E. Ryan ca Civil Engineering Department , State University of New York , Buffalo, NY, 14260b GE Corporate Research and Development , Schenectady, NY, 12301c Chemlcal Engineering Department , State University of New York , Buffalo, NY, 14260Published online: 02 Dec 2008.

To cite this article: Chih Huang , John E. Van Benschoten , Tara C. Healy & Michael E. Ryan (1997) Feasibility study ofsurfactant use for remediation of organic and metal contaminated soils, Journal of Soil Contamination, 6:5, 537-556

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Journal of Soil Contamination, 6(5):537-556 (1997)

Feasibility Study of SurfactantUse for Remediation of

Organic and Metal Contaminated Soils

Chih Huang,1 John E. VanBenschoten,1 Tara C. Healy,2 andMichael E. Ryan3

1Civil Engineering Department, State Universityof New York at Buffalo, Buffalo, NY 14260;2GE Corporate Research and Development,Schenectady, NY 12301; 3Chemlcal EngineeringDepartment, State University of New York atBuffalo, Buffalo, NY 14260.

In this Investigation, four surfactants wereexamined in a laboratory study that includedtesting for naphthalene solubilization ca-pacity, suríactant sorption to soil, and treat-ability for a sandy soil artificially contami-nated with lead and naphthalene. Of interestwas an examination of surfactant perfor-mance characteristics under the acidic con-ditions that may be required for metal re-moval. Although pH is recognized as acritical factor in metal sorption to soils, itdid not significantly impact the solubiliza-tion ability of any of the surfactants fornaphthalene. The sorption of nonionic sur-factants to the soil was not affected by pH,while sorption of anionic surfactants in-creased as the pH decreased. Althoughnonionic surfactants showed better solubil-ity enhancement for naphthalene than an-ionic surfactants, the latter may enhancelead desorption from the soil, presumablydue to interactions between the anionichead groups of the surfactant and the Pb2+

ion. A mass balance model was used tointerpret experimental data for naphtha-lene solubilization. The results of the studysuggest that remediation of metal-organiccontaminated soils by an acidic solution(for metal removal) containing surfactants(for organic removal) may be feasible.

KEY WORDS: surfactant, lead, naphthalene, soil remediation.

1058-8337/97/$.50© 1997 by AEHS

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INTRODUCTION

Í LJ OLYCYCLIC aromatic hydrocarbons (PAHs) are strongly adsorbed to- X - soils and sediments and are a concern in the environment and for humanhealth (Karickhoff, 1984; Edward et ai, 1991). PAHs are frequently found athazardous-waste sites (Riley and Zachara, 1992), as are combinations of PAHs andmetals. For example, in a survey by the Department of Energy, fuel hydrocarbons(i.e., HOCs) and metals were the major binary contaminants at 19 sites (out of 91)and 6 facilities (out of 21) and were found at all of the sites and facilitiesinvestigated (Riley and Zachara, 1992). From a remediation perspective, removingPAHs and metals from soils and sediments is difficult due to the differences in theirphysical-chemical properties. PAHs are bound to soils by nonspecific hydrophobicmechanisms, whereas metal ions are bound to surface sites via specific chemicalinteractions and also weaker electrostatic forces. Consequently, it is usually diffi-cult to establish a remediation process that is effective for both of these types ofbinding mechanisms.

The use of surfactants for solubility enhancement of PAHs has attracted recentresearch interest in remediation of soils and sediments (e.g., Edward et al., 1991;Edward et al, 1994; Nayyar et al., 1994; Palmer et al, 1992). Surfactants are ableto form micelle pseudo phases that have a hydrophobic core that can attract PAHs.Therefore, addition of surfactants can increase the apparent solubility of PAHs insolution and can enhance the removal of sorbed PAHs on soils. A surfactant willnot form micelles until the surfactant concentration is at or above a thresholdconcentration, which is defined as the critical micelle concentration (CMC). TheCMC is a function of the structure of the surfactant, the temperature of thesurfactant solution, the concentration of added electrolytes, and the concentrationof solubilizates (Harwell, 1992). At sub-CMC levels, little solubility enhancementis observed, while large amounts of PAHs can be solubilized at surfactant concen-trations greater than the CMC (Nayyar et ai, 1994).

Generally, surfactants can be divided into three categories based on their hydro-philic head groups: nonionic, anionic, and cationic. In soil remediation applica-tions, loss of surfactant is a major concern as it not only reduces the effectivenessof the surfactant but also increases the operational cost by higher dose require-ments. Sorption of surfactants depends on the nature of the soil surface and the pHin the system (Harwell, 1992; Scamehorn et al, 1982). Anionic surfactants usuallyhave a low susceptibility for sorption because most soil surfaces are negativelycharged (Harwell, 1992). However, higher adsorption of anionic surfactant to soilusually is observed at lower pH because the solid/liquid interface is more posi-tively charged at low pH (Harwell, 1992; Scamehorn et al., 1982). In contrast,cationic surfactant losses often are significant, making their use for remediationless favorable compared with anionic and nonionic surfactants.

For removal of sorbed metals in soils, the techniques employed generallyinvolve bringing the soil into contact with an aqueous solution (Peters and Shem,

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1992; Mañanan, 1990). Pickering (1986) identified four approaches that can mo-bilize sorbed metals on soils: (1) change in acidity, (2) change in ionic strength, (3)change in the oxidation/reduction potential, and (4) complex formation. Underacidic conditions, sorbed metals can readily be mobilized and extracted from soils.For example, Tuin and Tels (1990) found satisfactory removal of four metals fromsoil with 0.1 N HC1 in a bench scale study. In a study of seven Pb-contaminatedsoils, Van Benschoten et al. (1997) showed that a so-called non-detrital Pb fractionwas readily removed at acidic pH, leaving a residual or detrital fraction that washighly immobile. Complexation with metals can also increase recovery of metalson soils. This approach has led to the use of chelating agents in soil washings (e.g.,Peters and Shem, 1992). While numerous studies on chelating reagent applicationsfor soil washing have been conducted, relatively little is known about metalinteractions with surfactants. Hessling et al. (1989) reported that a 0.5% anionicsurfactant solution in soil washing enhanced Pb removal from contaminated soilsat battery-recycling facilities. Based on their findings, the authors suggested thatsurfactants offer good potential as soil washing additives for enhancing the re-moval of lead. The characteristic of anionic surfactants complexing and associatingwith metal cations has been applied to enhance the removal of metal ions (Scamehornet al., 1989; Simmons et al, 1992) as well as to simultaneously remove organicsand divalent metal cations from water (Dunn et ai, 1989). The process is knownas micellar-enhanced ultrafiltration (MEUF). Dunn et al. (1989) showed that ananionic surfactant solubilized organic solutes (phenol or o-cresol) within thehydrophobic cores of the micelles, while divalent metals (Zn2+ and Ni2+) werebound on the anionic exterior micelle surfaces.

Considering the simultaneous removal of PAHs and metals from soils usingsurfactant solutions in soil washing, it is likely that acidic pH conditions may berequired for effective metal removal. Thus, the effect of acidic conditions on thesolubilization ability of surfactants and potential losses via sorption or precipita-tion need to be examined. In this paper, laboratory study results for solubilizationof naphthalene at pH 8.3 and pH 1, the effect of pH on sorption of surfactants ontosoils, and treatability tests using artificially contaminated soils for several surfac-tants are presented.

MATERIALS AND ANALYTICAL METHODS

Materials

The soil used in this study was a Minoa sand that was well characterized andpreviously had been used in metal adsorption studies (Reed et al., 1995). The soilwas presieved with a 2-mm (no. 10) sieve to remove coarse sand and stone and thenwas air dried and stored in secured plastic buckets at room temperature. Thecharacteristics of the soil are listed in Table 1. Naphthalene was selected as a

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TABLE 1Characteristics of the Minoa Sandy Soil

Parameter

PHCation exchange capacity (meq/100 g)Organic matter (%)Coarse sand (%)Fine sand (%)Silt/clay (%)pHzpc

Value

5.5'7.6 ±0.4»1.19 ±0.45

4.193.62.4

5.56"

From Gariand, 1994.Based on acid/base titration at 0.01 N, 0.1 N, and 1.0 N ofsodium nitrate.

representative PAH because it is a common soil and sediment contaminant. Severalphysical/chemical characteristics of naphthalene are listed in Table 2. The naph-thalene (Fluka Chemical Co.) was >97% in purity and used as received. Leadnitrate (Pb(NO3)2) was used for making Pb solutions to contaminate the soil. ThePb(NO3)2 was purchased from Fisher Scientific and used as received.

Four surfactants were used in the study. Straight-chain diphenyl oxide disulfonates(DPDSs) were selected as representative anionic surfactants. Two surfactants ofthis family were used, Dowfax CIO and Dowfax 8390 (Dow Chemical). Dowfax8390 is also known as Dowfax C16. These are both DPDS anionic surfactants buthave different alkyl chains length (a 10 carbon chain for CIO and a 16 carbon chainfor C16). The sulfonated surfactants were used for various reasons, including lowsorption onto soil, stability in a low pH environment, and biodegradability (DowChemical Co., 1991). Triton X-100 (Union Carbide Chemical) was selected as amodel ethoxylated nonionic surfactant. A quaternary ammonium surfactant, EmcolCC-9 (Witco Co., 1990), was chosen as the model cationic surfactant. Emcol CC-9 was selected based on previous research pertaining to ex situ soil remediationconcerning enhanced desorption of polar and nonpolar organic compounds (Rajput

TABLE 2Physical/Chemical Properties of Naphthalene

Property Values Ref.

Molecular weight 128Molecular surface area (Â2) 155.8 Valsaraj and Thibodeaux (1989)Solubility in water (mg/1) 31.7 Karickhoff (1981)K^, 2.3 x 103 Karickhoff et al. (1979)

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et al, 1994). The characteristics of these surfactants are listed in Table 3, and themolecular structures of the surfactants are shown in Figure 1.

The procedure for soil contamination with naphthalene followed the methodused by Liu and Roy (1992). Five hundred grams of Minoa Sand were placed ina 2-1 beaker with 500 ml of HPLC-grade methanol (99.9% in purity, MallinckrodtChemical) containing 100 mg of naphthalene. The soil solvent solution was mixedslowly using an overhead laboratory stirrer for 30 min and the methanol wasallowed to evaporate over a period of 10 d. Soil was used after additional dryingfor 3 d. The contamination level was determined by simple extraction usingacetone as an extractant, yielding an initial naphthalene contamination level of 58.8mg/kg.

Minoa sand spiked with Pb in a study by Garland (1994) was used in this study.In that study, the Minoa sand was packed in a column and was spiked with leadusing a 100-mg/l Pb solution of Pb(NO3)2 at an ionic strength of 0.05 N NaNO3.The contaminated soil was stored at room temperature for about 1 year prior to use.The Pb content of the soil was determined by acid digestion (Method 3050, EPA,1986) and atomic absorbance spectrophotometry (AAS). The contamination levelwas 600 mg Pb2+/Kg soil. Details of the Pb contamination procedure may be foundin Garland (1994).

Analytical Methods

Surfactants and naphthalene concentrations were measured using high-perfor-mance liquid chromatography (HPLC) with a UV-VIS spectrophotometric detec-tor (Shimadzu, SPD-6AV). The optimum UV wavelengths were determined fromthe literature (Liu and Roy, 1992; Rouse et al, 1993; Rajput et al, 1994; Sun etal., 1995) and confirmed from absorbance scans using a UV diode array spectro-photometer (HP 8340A). The optimal wavelengths used were 254 nra for anionicand cationic surfactants and naphthalene, and 276 nm for the nonionic surfactant.An Envirosepp C-l 8 reverse phase column (250 x 4.6 mm, Phenomenex) was used

TABLE 3Characteristics of the Surfactants Used in the Study

Property

Average molecular weightActive ingredient (%)CMC(mM)

• From Rouse et al., 1993.b From Edward et al., 1991.c From Witco Chemical Co.

Dowfax C10

55545.56.3»

Dowfax C16

64335.76.3»

Triton X-100

624>990.17"

Emcol CC-9

550>9912.3C

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~1

SOfla SOJfa

(A) Dowfax C-10 and C-16

CH,— C —CH2 Ç (OCHCHJjfiH

ß) Triton X-100

Cfl, CP CH,

lo ICH, N CH2 CH, (CHJCHO) H

Cfls

(C)EmcolCC-9

FIGURE 1

Gênerai molecular structures of the surfactants used in the study.

for separation. The mobile phase was a combination of acetonitrile and nanopurewater at a ratio of 70:30 and was filtered and degassed prior to use. Isocraticconditions were used throughout the study. The method used for quality assuranceand quality control for the analytical procedures generally followed the EPAguideline described in SW-846, Method 8000 (EPA, 1986). QA/QC results aredescribed in detail elsewhere (Healy, 1995).

An atomic absorbance spectrophotometer (Perkin Elmer Series 500) was usedfor measuring the lead concentrations in the samples. The procedure followed thestandard method for heavy metal determination using atomic absorption (APHA etai, 1989).

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EXPERIMENTAL

Solubilization Experiments

For each surfactant, experiments were conducted to determine the concentration ofPAH solubilized at basic and acidic pH conditions. All experiments were carriedout at room temperature (25°C ± 2°C). Stock surfactant solutions at either pH 1 orpH 8.3 were prepared, and dilutions were made from stock solutions to obtain theappropriate range of surfactant concentrations. These pH values were chosen torepresent extremes in pH that may be observed in soil remediation systems. Formetal contaminated soils, it has been shown that washing at pH as low as pH 1 maybe required to meet cleanup standards (Van Benschoten et al., 1997). Sodiumbicarbonate (0.1 N) and hydrochloric acid (0.1 N) solutions were used for pHadjustment. Diluted surfactant solutions were placed in 40-ml amber glass vialsand 40 mg of naphthalene were added to each vial. Samples were sealed with open-top screw caps and Teflon®-backed septa and were tumbled with an end-over-endtumbler (VPE-110, Browing) for 24 h. Previous research has shown that 24 h wassufficient for surfactant and PAH to reach equilibrium (Rouse et al., 1993). Afterequilibration, samples were filtered with a 0.2 \Lm Teflon® filter (Millipore) toseparate the solid-phase PAH from dissolved PAH (PAH dissolved in water plusPAH solubilized within the micelles). The filtrate was analyzed for PAH andsurfactant concentrations using HPLC.

Surfactant Sorption Edge Experiments

Twelve 20-ml vials were each filled with surfactant solution and 2 g of soil(solid:liquid ratio =1:10). The ionic strength was controlled at 0.1M using NaNO3.The pH was adjusted initially by varying amount of 1.0 N NaOH or 1.0 N HNO3.The amount of acid/ base added was minimized so the total volume of the samplewas not affected significantly by the addition (maximum 100 p.1 added). The vialswere tumbled with an end-over-end tumbler for 24 h. The samples then werefiltered with a 0.2-u.m Teflon® filter and the filtrate was analyzed for the surfactantconcentration. The reported pH value for each sample was measured after 24 htumbling and ranged from pH 1.5 to 9.5.

Naphthalene Treatability Experiments

All experiments were carried out at pH 1 or pH 8.3 at room temperature (25°C ±2°C). Surfactant solutions were made to obtain the following range of surfactantconcentrations: (1) Dowfax C10, 0 to 45 mA/ (0 to 25 g/1); (2) Dowfax C16, 0 to39 mM (0 to 25 g/I); and (3) Triton X-100, 0 to 16 xaM (0 to 10 g/1). Note that the

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differences in molar concentrations were a result of equal mass concentrations forCIO and C16. Two grams of naphthalene contaminated soil were placed in 20-mlglass vials with Teflon®-lined phenolic caps. To each vial, 20 ml of pH-adjustedsurfactant solution was added (solid:liquid ratio = 1:10). Samples were sealed andtumbled on an end-over-end tumbler for 24 h. After tumbling, samples wereallowed to settle and filtered with a 0.2-(xm Teflon® filter. The filtrate then wasanalyzed for naphthalene and surfactant concentrations by HPLC.

Lead Desorption Experiments

One gram Pb-contaminated soil samples were placed in 40-ml glass vials withopen-top screw caps and Teflon-backed septa. To each vial, 25 ml of surfactantsolution were added (solid:liquid ratio = 1:25). Due to limited quantities of Pbcontaminated soil, the solid/liquid ratio was decreased somewhat compared withprevious studies. The ionic strength of the solution was controlled at 0.1 M usingNaNO3. The pH of each vial was adjusted by adding a small amount of 1.0 N HNO3

or 1.0 N NaOH so the pH of the samples covered a range of pH from 2 to 8. Thesamples then were tumbled for 24 h with an end-over-end tumbler. The sampleswere filtered using glass fiber filters (GF/C, Whatman) to separate soil from theliquid. The filtrates were analyzed for Pb concentrations by AAS.

RESULTS AND DISCUSSION

Naphthalene solubilization results for all four surfactants at two pH values areshown in Figure 2. For all the surfactants tested, pH did not significantly affect thesolubilization capability for naphthalene, suggesting that solubilization should beunaffected by variations in pH needed for metal removal. Using a poly(sodiumstyrenesulfonate-co-2-vinylnaphthalene) copolymer for solubilizing perylene,Nowakowska et al. (1989) showed that maximum solubilization occurred at acidic(pH < 4) and alkaline pH (pH > 10), where the micelle core is the most compact.A more compact conformation of micelle core was thought to lead to a morehydrophobic environment, and consequently favored solubilization (Hurter et ai,1995). The similarity in solubilization capacity at pH = 1 and pH = 8 of thesurfactants examined in this study suggests that solubilization of naphthalene in thepH 1 to 8 range may be relatively constant, although additional testing at interme-diate pH values should be conducted.

To quantify the solubilization effectiveness of surfactants, two approaches wereused. The first is the molar solubilization ratio (MSR), which is defined as thenumber of moles of organic compound solubilized per mole of surfactant added(Attwood and Florence, 1983). The MSR for a particular surfactant can be calcu-lated as follows (Edwards et al, 1991):

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10

2 -

0

•ä-g1guo

CJ

ene

•a

apht

h

8 -

6 -

4 -

VTO•A•0*

A

C10LatpH=lC10LatpH=8.3C16atpH=lC16atpH=8.3Triton at pH=lTriton at pH= 8.3CC-9atpH=lCC-9atpH=8.3

•

o

O

O

•V

o o

20r

40

i

60i

80 100

Surfactant Concentration (mM)

FIGURE 2

Solubilizafíon of naphthalene for various surfactants at pH = 1 and 8.3.

S -S )3 PAH.«tic ^PAH.cmc)

where SPAHmic i s t h e t o t a l apparent solubility of a PAH (mM) at a particular

surfactant concentration above the CMC, SPAHcmc is the apparent solubility of aPAH (mM) at the CMC, and the Csurf is the particular surfactant concentration

OThe second approach used was the micelle-phase to aqueous-phase partition

coefficient (K^, which is defined as the mole ratio of the PAH compound in the

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micellar pseudophase, X,,,, to the mole fraction of the PAH compound in theaqueous pseudophase, Xa. The K„, can be calculated by the following formula(Edwards et ai, 1991):

Y Ç VA o °PGAH,cmc Yw

where Vw is the molar volume of water (1.8 x 1Û"2 cm3/mmol). The calculated MSRand Kn, values to naphthalene for respective surfactants are listed in Table 4.

As expected, Triton X-100 showed better solubilization of naphthalene (highestMSR and Km among the surfactants tested) than CIO, C16, and CC-9. This can beattributed to the more hydrophobic core of the nonionic surfactant (Triton X-100)due to the absence of electrostatic charge on the exterior of the micelle. The resultsalso showed C16 was a better solubilizer (higher MSR and K^ for naphthalenethan CIO. This indicates that a surfactant with a longer hydrophobic chain exhibitsa higher capacity for accommodating PAH in the micellar hydrophobic core. Thisphenomenon has been reported by several researchers (e.g., Harwell, 1992) andwas attributed to larger hydrophobic core volumes within micelles for a surfactantwith a longer hydrophobic tail. The CC-9 (cationic) showed low solubilizationcapability for naphthalene compared with the other surfactants. The possiblereasons for the poor solubilization capability of CC-9 could be the lower molecularweight and more branching of the hydrocarbon tail. Generally, solubilizationincreases with increasing molecular weight (Tadros, 1984) and less hydrocarbontail branching Baraket et ai (1983). Also, the high CMC of CC-9 further decreasesthe amount of micelles compared with other surfactants in the study at a givensurfactant concentration. The CC-9 surfactant was therefore excluded from furtherstudy.

Surfactant sorption edges are illustrated in Figures 3 through 5 for CIO, C16,and X-100, respectively. The amount of anionic surfactant sorbed onto the soilincreased as pH decreased. As suggested by Scamehorn et al. (1982), at constantmonomer concentration, the adsorption of anionic surfactants on mineral oxidesincreases with decreasing pH due to the increased electrostatic attraction between

TABLE 4Calculated MSR and K,,, Values to

Naphthalene from the Surfactant Solubilization Test

Values

MSRlog Kra

Dowfax C10

PH = 1

0.0664.14

pH = 8

0.0744.18

Dowfax C16

pH = 1

0.1454.45

p H = 8

0.1944.56

Triton

pH = 1

0.2554.65

X-100

pH = 8

0.2264.61

Emcol

PH = 1

0.0032.83

CC-9

pH = 8

0.0022.74

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oen

•

•s1

CO

o

I

10

9 -

8 -

7 -

6 -

5 -

4 -

3 -

2 -

1 -

D•A

TO

C¡nit=8.62mM0^=2.53111*1C init=1.31mMC init=1.20mMCinit= °-6 4 " ^

D

D DD

o ^•i

I

3I

4i

7i

8

pH

FIGURE 3

Sorption edge for Dowfax C10 anionic surfactant.

30

o 2 5 -'5

20 -

"S•S 15oen

^ io Ho

I 5H

C in¡t=4.45mM

Cb; t=2.19mM

C init=1.13mM

9 10

u • i i 1 I I ~ I I

1 2 3 4 5 6 7 8 9 10

pH

FIGURE 4

Sorption edge for Dowfax C16 anionic surfactant.

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5 4 -oon

"Óo

•Bin

13<4

I

3 -

2 -

1 -

D•A

Cinit"c inifCinif

= 0.29= 0.18= 0.10

mMmM

mM

DD D

o -1

AAA

I

2

I

3\4

I

5

1

6

I

7I T

8. 9 10

PH

FIGURE 5

Sorption edge for Triton X-100 nonionic surfactant.

adsórbate and the soil surface. Based on soil titration data, the pH2pc= 5.6 for theMinoa soil (Table 1), suggesting a positively charged surface at pH < pHzpc. Forthe anionic surfactant, increased adsorption at pH lower than pHzpc is apparent(Figures 3 and 4). It also is plausible that specific adsorption between the anionichead groups and soil hydrous oxide surfaces may occur, as anionic surfactantsorption edges are similar to those of common solution anions. The existence ofbilayer adsorption for anionic surfactants has been reported previously (Tadros,1974; Scamehom et ai, 1982) with maximum adsorption observed at low pH. Itwas suggested that sorption was limited mainly by steric hindrance and not thesurface activity (Scamehorn et al., 1982). At pH lower than about 2.5, there is asudden decrease in sorption with decreasing pH for the anionic surfactants tested(Figures 3 and 4). This phenomenon is consistent with results previously reportedby other researchers (Scamehom et ai, 1982). One possible explanation is thealteration of ionic strength in the system. Although experiments were conductedwith 0.1 M NaNO3 as background electrolyte, low pH samples probably were athigher ionic strength due to addition of acid and base. As mentioned previously,ionic strength is one of the factors that affects the CMC of ionic surfactants. At highionic strength, the CMC decreases and the effect is usually more significant forionic surfactants than for nonionic surfactants (Myers, 1992). The observed de-

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pression of the CMC is due to a reduction in the electrostatic repulsion betweenhead groups and, therefore, a smaller contribution of those groups to the freeenergy opposing micellization (Myers, 1992). Thus, the CMC should decrease asa result of the higher ionic strength at low pH and decrease the monomer concen-tration. Because sorption of surfactant is mainly due to monomer, the lower themonomer concentration the lower the sorption would be (Myers, 1992). Anotherexplanation for decreased sorption at the lowest pH values is that the sulfonatefunctional moieties on the anionic surfactants may become protonated and reducethe electrostatic attraction to the soil surface. The reported pKa of sulfonic acid is1.8 (Bailar et al., 1973), which corresponds approximately to the pH where achange in anionic surfactant sorption occurred.

In contrast to the anionic surfactants, sorption of Triton X-100 was relativelyinsensitive to pH (Figure 5), and the amount adsorbed was less than the anionicsurfactants at pH < pHzpc (i.e., pH < 6). This phenomenon could be attributed notonly to the neutral hydrophilic head of Triton X-100 but also to a much lower CMC(one order of magnitude) than the anionic surfactants tested.

Treatability test results are presented in Figure 6. The trends follow the solubi-lization test results. Triton X-100 exhibited the best removal of sorbed naphtha-

u

g

•g,

KßJ -

90 -

80-

70-

6 0 -

5 0 -

4 0 -

3 0 -

2 0 -I

10 J

0 -

L X/fi / // y /¿/• //+>

i ^

_ C16

/ _ CIO

• a0pH8

A aOpHl• aâpHg

0 Q6 pHl• X100pH8• XlOOpHl

i i

0 5 10 15 20 25 30 35 40

Surfactant Ebse (mM)

FIGURE 6

Treatibility test results for anionic and nonionic surfactants. Lines are model fit forindividual surfactants using both pH 1 and 8 data.

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lene, and C16 was slightly better than CIO. Again, pH did not affect solubilizationfor the respective surfactants. Note that enhancement in solubilization did notoccur at surfactant concentrations below the CMC values of the respective surfac-tants. In the treatability tests, the surfactant loss due to sorption was monitored. Theamount lost was found to be consistent with sorption edge results. For example,0.45 mM of surfactant was lost for the CIO surfactant dose of 25 mM at pH = 1,corresponding to 2.5 mg surfactant/g soil adsorbed. This amount is less than themaximum sorption of CIO of 7 mg/g soil (pH = 2 to 3 data in Figure 3). Also, 3mM of surfactant was lost for the C16 surfactant concentration of 26 mM at pH =1, corresponding to 19 mg surfactant/g soil adsorbed. This amount is less than themaximum sorption of C16 of 28 mg/g soil (pH = 2 data in Figure 4). Bothsurfactants show similar trends to the sorption edge data, that is, the sorption ofanionic surfactants at low pH (pH < 2) is less than the maximum amount sorbed.These results also indicate that surfactant loss due to sorption might not besignificant at pH = 1 for a soil washing process using an anionic surfactant.

Treatability results were also examined to determine if similar solubilizationoccurred in these tests as in the surfactant solubilization experiments (Table 1).Evaluation of surfactant solubilization in the treatability tests is complicated by thefact that it is a three phase system (soil, water, micelles). Thus, plots such as thosein Figure 2 cannot be used to evaluate solubilization, as they are non-linear. As analternative approach, a simple partition model was constructed based on naphtha-lene partitioning between the micelle pseudophase, the aqueous pseudophase, andthe soil. The model is described by three equations:

V = Qsoil

PAHaqMWPAH

K _Xm_PAHcqVw

The first equation describes the mass balance of PAH in the system, where PAHT

is total PAH on the" soil initially and PAH^, PAHmic, and PAHsoiI are PAHconcentrations in aqueous pseudophase, micelle pseudophase, and soil, respec-tively. The second equation describes the partitioning between soil and aqueouspseudophase, where Kp is the partition coefficient for soil/aqueous pseudophase;Qsoil denotes the concentration of PAH on the soil and MWPAH is the molecularweight of the PAH. The last equation describes PAH partitioning between themicelle pseudophase and the aqueous pseudophase, where K,,, is the partitioncoefficient; ST is total surfactant concentration and Ssoi, is surfactant loss due tosorption. The model was fit to experimental data by adjusting the values of Kp and

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to minimize the sum of residuals between measure and fitted concentrations ofsoluble naphthalene (i.e., PAHmic + PAH^). PAH and surfactant concentrations arein mmole/1 and QsOil is in mg/Kg.

Model fitting was conducted for data at pH 1 and 8 individually, as well as acombined data set. The results (Table 5) appear to be in agreement with solubili-zation test data (Table 4) in that differences between pH 1 and pH 8 are minimaland that similar K^ values were observed in the treatability and solubilizationexperiments. Based on these results, it appears that K„, values determined in simplesolubilization experiments can be applied to more complex soil systems. Note alsothat the fitted values of the partition coefficient of for naphthalene between soil andwater (K,,) are relatively constant across the different surfactants. The organicmatter-water partition coefficient, K^, is the partition coefficient for between soiland the aqueous pseudophase, Kp, normalized by organic matter fraction of thesoil. The fitted Kom values are in good agreement with values reported in theliterature. For example, based on a relationship between water solubility and K ^for neutral organic compounds (Karickhoff, 1981), log Kom = 3.1 for naphthalenewas calculated. Given the wide range over which Kom values vary for differentcompounds, coupled with the fact that the relationship established by Karickhoff(1981) was for aquatic sediments, the agreement between the fitted and literatureKom value is quite good. Furthermore, considering that sorption hysteresis often isobserved, it is not surprising to observe somewhat higher Kom values for thedesorption studies conducted in this work compared with the adsorption studies ofKarickhoff (1981).

Figures 7 and 8 present the desorption of Pb from Minoa soil with CIO andTriton X-100, respectively. The addition of the anionic surfactant showed anenhancement for Pb desorption across a wide pH range, with increased amounts ofPb desorbed with increased surfactant concentrations. However, the differences inPb between the two high surfactant concentrations in the test were minor. ForTriton X-100, there was no enhancement of Pb desorption for concentrations up to50 mM. At low pH, the nonionic surfactant actually inhibited the desorption ofsorbed Pb. To verify that this trend was not an artifact due to interference in theanalysis of Pb, measurements of solutions spiked with a constant Pb concentration

TABLESCalculated Km Values to Naphthalene

from the Surfactant Treatability Test by Partition Model

Values

logKm

logK™

Dowfax C10

pH = 1

4.4585.33.86

pH = 8

4.56121

4.01

All

4.4088.43.84

Dowfax C16

pH = 1

4.4854.9

3.66

pH = 8

4.3461.9

3.71

All

4.4258.8

3.69

Triton X-100

pH = 1

4.9579.3

3.82

pH = 8

4.8570.9

3.77

All

4.9274.3

3.79

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100

90 -

80 -

70 -

ö 60 H

g5 50 H"5

JOCL,

40 -

30 -

20 -

10 -

0

C10=98.4mMC10=49.2mMC10=9.8mM

- No surfactant

-*—ï5

pH

i

6

FIGURE 7

Pb desorption using Dowfax C10 anionic surfactant.

(20 mg/1) and several different Triton X-100 concentrations were conducted. Theresults showed no interferences in the measurement of Pb. Elucidation of theinteraction between the nonionic surfactant and sorbed Pb requires further study.

The results of this study indicate that surfactants may function adequately forremediation of soils with both metal and organic contaminants, but that carefulselection of the surfactant is required. For example, the nonionic surfactant testedexhibited a higher solubilization capacity for naphthalene than did the ionic surfac-tants, did not sorb appreciably to soil, but appeared to inhibit removal of Pb. Theanionic surfactants showed higher sorption to soils, particularly as pH decreased.However, interactions between the anionic head groups of these surfactants withPb should aid in the removal of metal from the soil and the sorption of anionicsurfactant should be less significant at extremely low pH. While this study suggeststhat the concept of remediation of co-contaminated soils in a single wash solutionmay be feasible, additional work is needed to better understand the complex

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120

100 -

X 8 0 -"3en

Oeo"S'SS

60 -

<2 40 -

20 -

X-100=50mMX-100=10mMX-100=5mMNo surfactant

4 5

pH

Fè desarption using Triton X-1OO nonionic surfactant.

interactions among soil, metal, organic contaminant, and surfactant. Such anunderstanding is needed to select candidate surfactants and optimize their use forsoil remediation.

CONCLUSIONS

Based on the findings in this study, the following conclusions can be made.

1. The anionic, cationic, and nonionic surfactants used in this study for solu-bilization of naphthalene were unaffected by solution pH. Micelle proper-ties responsible for solubilization did not appear to be impacted even bystrongly acidic pH conditions.

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2. Anionic surfactants adsorb more strongly at pH lower than p H ^ of the soil.At very low pH (pH < 2), a decrease in anionic surfactant was observed. Thesorption of the nonionic surfactant studied generally was unaffected by pH.Sorption of the anionic surfactants may limit their feasibility for remediationapplications.

3. A simple partition model was used to describe the distribution of PAH in asoil-water-surfactant system. Good agreement in the micelle pseudophaseto aqueous pseudophase partition coefficient, K„,, was observed betweenfitted values and experimental values determined in simpler solubilizationexperiments. The results suggest that extrapolation of PAH partitioningdetermined in well-defined laboratory tests to more complex systems maybe possible.

4. Both the nonionic and anionic surfactants studied affected Pb removal fromthe Minoa soil. The anionic C-10 surfactant enhanced Pb removal, while thenonionic surfactant appeared to adversely affect Pb removal. It is likely thatPb reacts with the head group of the anionic surfactant, enhancing Pbdesorption in an analogous manner to that of a soluble complexing ligand.

ACKNOWLEDGMENTS

The authors thank the New York State Center of Hazardous Waste Managementfor the support of this research project.

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