the mechanism of the surfactant-aided soil washing system for hydrophobic and partial hydrophobic...

The Science of the Total Environment 307(2003) 83–92

0048-9697/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0048-9697Ž02.00461-8

The mechanism of the surfactant-aided soil washing system forhydrophobic and partial hydrophobic organics

W. Chu*, K.H. Chan

Department of Civil and Structural Engineering, Research Center for Urban Environmental Technology and Management,The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China

Abstract

A surfactant-aided soil washing mechanism is proposed in this study by examining nine common organiccontaminants, which were divided into two groups, hydrophobic compounds and partial hydrophobic compounds,depending on the respective soil partitioning of contaminants,K . The presence of a free non-aqueous phase liquidoc

in the soil washing system is critical to determine the soil washing performance curves. A mathematical model isproposed to describe soil-washing performance at various surfactant concentrations. The resulting slopes and interceptsfrom the model for the performance prediction are linearly related toK . In addition, a transition zone between theoc

hydrophobic and partial hydrophobic compounds was observed, and has been used in verifying the proposedmechanism successfully by overdosing and underdosing the contaminants in the system.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: cmc; Hydrophobic; NAPL; Soil-washing; Surfactant; Partition

1. Introduction

The contamination of hazardous organic pollut-ants in soils or sediments is an environmentalconcern. Organic compounds such as aromaticcompounds, polyaromatic hydrocarbons(PAHs),herbicides and pesticides, are of special interestbecause they are commonly detected in the envi-ronment and may strongly sorb onto soil or beretained in the saturated zone underground. Surfac-tant-aided ground water remediation and soil wash-ing are technologies for enhancing the removal oforganic contaminants(Abdul and Gibson, 1991;Yeom et al., 1995), cleaning hydrophobic organic

*Corresponding author. Tel.:q852-2766-6075; fax:q852-2334-6389.

E-mail address: [email protected](W. Chu).

compounds(HOCs) contaminated soil or sediment(Jafvert et al., 1995; Chu et al., 1998). Surfactantsare particularly attractive for such applications asthey potentially have low toxicity and favorablebiodegradability in the environment than manyorganic-solvent based systems. However, guidancein selecting surfactants to be evaluated in ex situsoil washing is important for the soil remediationindustry(Deshpande et al., 1999). The surfactantyHOCs mixtures extracted from soils and sedimentscan be treated by physicochemical processes suchas the UV-induced photolysis(Chu and Jafvert,1994; Chu, 1999).Solubilization of compounds by surfactants is

generally initiated at thecmc and is proportionalto the surfactant concentration above the criticalmicelle concentration(cmc) (Edwards et al.,

84 W. Chu, K.H. Chan / The Science of the Total Environment 307 (2003) 83–92

1991). Solubilization and lowering of the surfaceand interface tension are thought to be the mainreasons for a facilitated transport of contaminantsadsorbed on solid phases to the surfactant contain-ing aqueous phase(Abdul et al., 1992; Gotlieb etal., 1993; Edwards et al., 1994; Zheng and Obbard,2002). Wayt and Wilson(1989) reported that theconcentration of the hydrophobic contaminants(such as 1,4-DCB, naphthalene, and biphenyl)solubilized in surfactant solutions is a linear func-tion of the total surfactant concentration providingthecmc is reached for in situ soil washing. Similarresults are also reported by Diallo et al.(1994)for HOCs. Kile and Chiou(1989) proposed amodel to account for solubility enhancement dueto both micelles and monomers. They suggestedthat solubility correlated better to the length of thehydrocarbon chain and accessibility to the innercore than to micellar size. The magnitude ofsolubilization of HOCs by surfactant micelles isobserved in the order of non-ionic, cationic, andanionic for similar non-polar chain lengths. Zhengand Obbard(2002) found that maximum sorptionof surfactant onto soil could be used for theestimation of surfactant effectivecmc in the soilyaqueous system, and this provides valuable infor-mation for the application of in situ surfac-tant-enhanced soil bioremediation and ex situ sur-factant soil washing.Various types of partition coefficients, such as

octanol–water partition coefficients(K ), K andow oc

water solubility, have been used to facilitate theprediction of pollutant concentration in differentphases in the environment(Chu and Chan, 2000).Jafvert(1991) correlated the values of a micelle–water partition coefficient(K ) with K for am ow

series of PAH compounds in dodecylsulfatemicelles and found a near-unity slope and near-zero intercept, which suggests the similarity ofsolvation energies for these compounds inmicelles. Jafvert and Heath(1991) proposed asemi-empirical equation which relates theK tom

the K and surfactant structural properties ofow

various non-ionic surfactants. The surfactant prop-erties are defined by the numbers of hydrophobiccarbons (aromatic or aliphatic, straight orbranched, reduced carbons) and hydrophilic car-bons(sorbitan carbons, or ethoxy groups).

In a query about the desorption ability of sur-factants on HOCs from soils and sediments, Jafvertet al.(1994) examined in detail the distribution ofnon-polar compounds between surfactant micellesand soil particles. Abu-Zreig et al.(1999) sug-gested that non-ionic surfactants are better choicesthan anionic surfactants in washing performanceto decrease the portion of HOCs sorpted in soilparticles. Chu and So(2001) developed a two-stage model to described the surfactant-aided soilwashing performance on HOCs contaminated soilremediation.Previous work has focused on the micellar

solubilization of individual compounds or partic-ular groups of HOCs, whereas in real situationscontaminated soil may contain mixtures of HOCsand partial hydrophobic organics(PHOC, com-pounds with semi-hydrophobic andyor semi-hydro-philic characteristics). In addition, information onsurfactant-aided washing mechanisms and mathe-maticalyphysical relationships regarding theremoval efficiency or washing performance bysurfactant micelles on organic contaminated soil isvery limited. The objectives of this research wereto examine the characteristics of the ex situ soilwashing process for HOC and PHOC, and to finda simple model that can be used in the predictionof surfactant-aided washing performance for bothHOC and PHOC from contaminated soils.

2. Methodologies

2.1. Materials

All chemicals were used as received from thesupplier. Non-ionic surfactant Brij 35 with concen-tration of 0.02 molyl was prepared as the stocksolution. Probe compounds including DDD, DDT,pyrene, hexachlorobenzene(HCB), 2,4,6-tetrach-lorophenol (2,4,6-TCP), naphthalene, atrazine(ATZ), 1-Naphthylamine, and 2,4-dichlorophen-oxyacetic acid(2,4-D). HPLC graded acetone,acetonitrile and chloroform were used as solvents.The soil used for this study was uncontaminatedtopsoil(medium sandy texture) and collected fromTai Mo Shan in Hong Kong. Coarse soil particleslarger than 2 mm are oversize for the pretreatmentrequirement and ineffective for soil washing

85W. Chu, K.H. Chan / The Science of the Total Environment 307 (2003) 83–92

Table 1Properties of selected HOCs and PHOCs

Chemicals Initial Koc Solubility Kow MW Wavelengthconcentration (lykg) (molyl) (gymol) (l, nm)(molyl)

DDD 5.30E-05 7.70Eq05 3.12E-07 1.58Eq06 320 230DDT 2.09E-05 2.43Eq05 1.96E-08 1.55Eq06 254 283Pyrene 1.84E-05 3.80Eq04 6.53E-07 7.59Eq04 202 240Hexachlorobenzene(HCB) 3.51E-05 3.90Eq03 2.00E-08 1.70Eq05 285 217(overdose) 6.23E-02

2,4,6-Tetrachlorophenol 2.95E-02 2.00Eq03 4.05E-03 7.41Eq03 197 280(2,4,6-TCP)

Naphthalene 1.30E-03 1.30Eq03 2.47E-04 2.76Eq03 128 220Atrazine(ATZ) 3.07E-04 1.63Eq02 1.53E-04 2.12Eq02 216 2211-Naphthylamine 1.22E-01 6.10Eq01 1.64E-02 1.17Eq02 143 247(underdose) 2.33E-02

2,4-Dichlorophenoxyacetic 1.51E-02 1.96Eq01 2.80E-03 6.46Eq02 221 280acid (2,4-D)

Remarks: cited from Chu and Chan(2000).

(Noyes, 1994). Thus, the soil sample was air-driedand screened through a US Standard no. 10 mesh(2 mm) sieve to remove the coarse fragments. Thesoil was stored in a 1058C oven for drying andreserved for later use. The average fractions oforganic carbon(f ) in the soil samples wereoc

determined to be 0.457% by a Shimadzu TotalOrganic Carbon Analyzer(model TOC-5000A).Organic carbon was determined by total carbon(dry combustion) less inorganic carbonates(gaspurge of acidified suspensions).

2.2. Methods

Surface tension was used to quantify thecmc ofpure surfactant(CMC ) and thecmc of surfac-surf

tant-soil solution(CMC ). The surfactant solu-soil

tion-to-soil ratio was designated as 6:1(vyw) soas to reach the optimal washing performance(Joshiand Lee, 1996). Various surfactant concentrationswere made for measuring the surface tension ofCMC . For the surface tension ofCMC , asurf soil

series of surfactant solutions with different concen-trations were shaken with designated ratio ofweight of soil together with 18 mg of mercuricchloride by rotary shaker for more than 24 h(Liuet al., 1991), in which the mercuric chloride wasused to inhibit bacterial degradation of the surfac-tant (Liu et al., 1992). Then, the surfactant–soil

solutions were centrifuged at 8000 r.p.m. for over1 h, and surface tension was determined from thesupernatants by CSC-Du Nouy Tensiometer.¨A series of batch tests were employed to deter-

mine the partitioning of organics in a surfactant-aided soil washing system for nine designatedchemicals. The properties of selected chemicalsare summarized in Table 1, sorted according totheir K and solubility. The nine chemicals wereoc

divided into two groups, where DDD, DDT,pyrene, HCB, and 2,4,6-TCP were assigned as thegroup 1 compounds(HOC with K values higheroc

than 1500); conversely naphthalene, ATZ, 1-naphthylamine, and 2,4-D were group 2(PHOCwith K values lower than 1500). The initial dosesoc

of HOC and PHOC used in this study were twoto three times their respective water solubility. Thestock solution of each chemical was prepared ineither acetone or chloroform. Then, a predeter-mined amount of stock solution was transferred bymicro-volume syringe and spread in six 30-mlPyrex tubes. The tubes were air-dried so that thechemical could be evenly coated on the tube walls.Various amounts of surfactant solution were addedto each tube, by referring to theCMC , andsoil

diluted with distilledydeionized water to an appro-priate bulk solution concentration. The designatedamount of soil was mixed with the solution ineach tube, and the tubes were sealed with Teflon-


Fig. 1. Sorption of Brij 35 onto soil.

lined screw caps and shaken in a rotary shaker at25 8C (room temperature) for 24 h. After equilib-rium was reached, the samples were centrifuged at8000 r.p.m. for 1 h. The supernatants were filteredby 90-mm filter and analyzed by HPLC to quantifythe chemical in the liquid phase. The HPLC,comprising a pump, a reverse-phase column(Res-tek pinnacle octyl amine 5mm, 0.46=25 cm),and a WATER� 486 UVyVIS detector, was used.The mobile phase was 85% acetonitrile with 15%water for all chemicals. All batch tests were carriedout in duplicate.

3. Results and discussion

3.1. Sorption of non-ionic surfactant onto soil

Sorption of surfactant onto soil may result insurfactant loss and reduced performance for thesolubilization of organics(Liu et al., 1992). Inthis study, this effect was quantified by plottingthe surface tension curves of a pure surfactantsystem and a surfactant–soil system vs. the loga-rithm of the surfactant concentration(Chu and So,2001) (Fig. 1). The inflection points of both curvesrepresent the critical micellar concentration(cmc)of the systems, in which the bulk solutions aresaturated with surfactant monomers. The theoreti-cal cmc of Brij 35 in water (CMC ) obtainedsurf

from this test was 1.61=10 molyl, and for they4

surfactant–soil system, itsCMC increased tosoil

1.00=10 molyl. Assuming both systems havey3

the same amount of surfactant monomers uponreachingcmc, the difference betweenCMC andsurf

CMC indicates that 84%(or 8.39=10 molyly4soil

by mass balance) of surfactant input was adsorbed(or lost) to the soil. In this study, it has beenassumed that any additional surfactant added tothe soilysurfactant system beyond 1.00=10y3

molyl will be present in the micellar form, and themaximum surfactant loss to the soil will be fixedat 8.39=10 molyl.y4

3.2. Proposed surfactant-aided soil washing mech-anism for HOC and PHOC

Many researchers(Liu et al., 1991; Edwards etal., 1994; Iglesias-Jimenez et al., 1996; Chu and´So, 2001) have examined the partitioning of sol-utes in different phases of the surfactant–soilsystem. Theoretically, the distribution of HOCbetween the soil and the solution phase(wPx qw

wPx ) can be described by an overall distributionmic

coefficient,K ,d

w xP soilK s (1)d w x w xP q Pw mic

where wPx is the pollutant concentration(molysoil

l) in the soil phase,wPx is the pollutant concen-w

tration (molyl) in water, andwPx is the pollutantmic

concentration (molyl) dissolved in surfactantmicelles. For the purpose of examining the soilwashing performance, Eq.(1) can be rearrangedby taking a reciprocal,

w x w xP Pw mic1s q (2)

w x w xK P Pd soil soil

where 1yK is the performance indicator of soild

washing in terms of the fraction of pollutant beingsolubilized in the liquid phase over the pollutantremained in the soil phase after the soil washingprocess. For HOCs, the first term on the right handside in Eq.(2) was often neglected in practice,becausewPx is limited to the water solubility ofw

the pollutant, and it is normally small comparedto wPx . However, it is found that the free non-mic


Fig. 2. Soil washing performance curve(1yK ) for HOCs atd

various surfactant concentrations.

Fig. 3. Soil washing performance curve(1yK ) for PHOCs atd

various surfactant concentrations.

aqueous phase liquid, NAPL, which does notchemically associated with soil, coexist in thesubsurface before the remediation, especially inthe cases of accidental spills with heavy pollution.In this case, the NAPL of the pollutant likelybecomes another major source(or the third phaseother than the contaminant in the soil and liquidphases) interfering with the soil washing perform-ance(or partitioning of the pollutant) (Zemaneket al., 1997; Pennell and Abriola, 1998). If theNAPL is present in the contaminated subsurfaceor ground water after a reasonable contact time, itcan be assumed that all the sorption sites of thesoil have been saturated. The residual NAPL inthe system after the remediation therefore willmainly exist in the aqueous phase and be detectedin the soil washing effluent. So for a more generalexpression of washing performance, 1yK couldd

be rewritten to include all possible sources ofpollutant, as indicated in Eq.(3):

w x w x w xP q P q Pw mic NAPL1s (3)

w xK Pd soil

wherewPx is the pollutant in NAPL form thatNAPL

exists in the liquid phase. Since the numerator ofEq. (3) is possible to quantify in the laboratory at

one time; Eq.(3) can therefore be modified to asimpler format:

w xP liq1s (4)

w xK Pd soil

where wPx is the total concentration of pollutantliq

in the liquid phase(i.e. summation of wPx ,w

wPx and wPx ). Together with the finding ofmic NAPL

CMC from the previous discussion, six Brij 35soil

doses higher thanCMC (3.33=10 , 6.67y3soil

=10 , 1.00=10 , 1.33=10 , 1.67=10 andy3 y2 y2 y2

2.00=10 molyl) were therefore used in the soily2

washing process to ensure the presence of surfac-tant micelles. The resulting soil washing perform-ances are shown in Figs. 2 and 3, where Fig. 2 isfor group 1 compounds(HOC), and Fig. 3 is forgroup 2 compounds(PHOC). It is interesting tonote that the performance curves of the HOCswere exponentially increased with the surfactantconcentration(Fig. 2); however, straight lines wereobserved for the PHOCs(Fig. 3). To justify thedifferent patterns of performance curves for thetwo groups of compounds, a new soil washingmechanism is proposed. It is stated that no matterwhether for HOCs or PHOCs, there are two mainsources of HOC(also PHOC) in the soil system


Fig. 4. Modeling the soil washing performance of HOCs byEq. (7).

before the washing is initiated. One is the fixedcompound that is physically or chemicallyadsorbed or bonded in the soil media, and anotheris the free NAPL that resides in the liquid phase(Boyd and Sun, 1990).For HOCs, during the soil washing process the

washing mechanism can be divided into three typesdepending on the concentration of surfactantmicelles.

i. As the surfactant concentration is relativelylow to extract the HOCs, due to the hydropho-bic properties of the HOC, the fixed HOCs thatoriginally sorbed in the soil have higher affinityto be retained in soil particles, whereas the‘free’ NAPL in the liquid phase is more easilytrapped in the hydrophobic cores of the surfac-tant micelles. Under this circumstance, onlyinternal relocation of HOCs fromwPx toNAPL

wPx occur (i.e. the terms ofwPx and wPxmic liq soil

are unaffected, see Eq.(4)), and no effectivesoil washing really occurs. Such phenomenacan be justified by some of the flattened per-formance curves at low surfactant micelle con-centrations, as shown in Fig. 2.

ii. As the micellar concentration in the systemprogressively increases, the remaining NAPLin the liquid phase is reduced(Vigon andRubin, 1989; Fountain et al., 1991; Pope andWade, 1995; Pennell et al., 1996; Chevalier etal., 2000), which allows a better chance for thefixed HOC to make contact with the unoccu-pied micelles, therefore the termswPx andmic

wPx in Eq. (3) start to rise and fall, respec-soil

tively. This will move the performance curvesupward with constant slopes.

iii. As the surfactant concentration is at a relativehigh level, NAPL has been completely dis-solved in the micellar phase, and the fixedHOC in the soil turns out to be the only sourcebeing extracted by surfactant micelles, whichresults in a rapid reduction ofwPx , a suddensoil

rise of the wPx ywPx curve, and a muchliq soil

steeper slope.

For PHOCs, their soil washing performancecurves are basically proportionate to the surfactantconcentration,wSx, as shown in Fig. 3. Since thelinearship betweenwSx and wPx was well estab-liq

lished previously in the surfactant-enhanced solu-bility for organics without the involvement of soilmedia (Kile and Chiou, 1989), this implies thatthe fixed PHOCs sorbed in soil and free PHOCsin the NAPL form (if any) should carry similaraffinity to the micellar core; this assumption isphysically possible, owing to the semi-hydropho-bic properties of PHOCs. Thus, the soil washingcurves of the PHOCs are mainly shaped by thereduction ofwPx together with the increment ofsoil

wPx (Eq. (4)), which result in linear curves inliq

Fig. 4. The increase ofwPx is mainly due to theliq

rise of the termwPx in Eq. (3), which wasmic

originally contributed by the extraction of fixedPHOCs from soil and the internal relocation ofPHOCs fromwPx , even though the latter doesNAPL

not give useful washing performance in the system.

3.3. Proposed mathematical model

According to the observation in Figs. 2 and 3,a mathematical model was proposed to correlatethe soil washing performance curves for bothHOCs and PHOCs:

axysbe (5)

By taking a natural logarithm on both sides, itcan be linearized as

lnysaxqlnb (6)


Fig. 5. Modeling the soil washing performance of PHOCs byEq. (7).

Fig. 6. Correlation betweena and lnK .oc

Table 2Summary of the plots on ln(1yK ) against surfactant concentrations(Figs. 4 and 5)d

Chemicals a lnb R2

DDD 161.43 0.5543 0.9532DDT 182.32 0.6217 0.9612Pyrene 288.86 y0.3062 0.9845Hexachlorobenzene(HCB)-original 328.65 y3.3658 0.97252,4,6-Tetrachlorophenol(2,4,6-TCP) 365.93 y3.9537 0.9909Naphthalene 25.07 1.0108 0.9852Atrazine(ATZ) 80.04 0.7152 0.96111-Naphthylamine-original 120.61 y2.5490 0.97002,4-Dichlorophenoxyacetic acid(2,4-D) 82.95 y0.4740 0.9688

Thus, for the surfactant-aided soil washing sys-tem, the equation can be rewritten as

1w xln sa S qlnb (7)

Kd

where wSx is the surfactant concentration in thesystem, anda and lnb are the slope andy-intercept, respectively. Based on Eq.(7), the datawere plotted in Figs. 4 and 5, in which very highR (0.9532–0.9909) was observed for all cases,2

suggesting that the proposed model is useful forpredicting works in practice.(The regression dataare summarized in Table 2.)In addition, the slopesa and counter-calculated

interceptsb were cross-analyzed with their respec-tive K of each HOC and PHOC as shown inoc

Figs. 6 and 7, where a linear correlation wasfound. Two distinct regions were identified forboth a and b: one was recognized at higherKoc

levels for HOCs; and another at lowerK levelsoc

for PHOCs. In addition, there is a gap betweenthe two regression lines, defined as the ‘transitionzone’, which will be discussed later.

3.4. Verification of soil washing mechanism

The proposed soil washing mechanism can befurther verified viaK , together with the highaoc


Fig. 7. Correlation betweenb and lnK .oc

Fig. 8. Verification of proposed mechanism by increasing ini-tial concentration of HCB.

values of the HOCs and the low ones of thePHOCs. HOCs generally have higherK valuesoc

and low water solubility, such hydrophobic prop-erties resulting in the priority for the pollutants toreside in the soil media. According to our proposedwashing mechanism therefore, thewPx term insoil

the denominator of Eq.(3) is outweighed, and canbe significantly reduced by increasing surfactantconcentration in the surfactant-aided washing. TheHOC is considerably shifted(or partitioned) fromdenominator(wPx ) to numerator(wPx ) in Eq.soil liq

(4), which makes the washing performance curvetoward exponential configuration, and a highera

value results(see Fig. 6).By contrast, PHOCs have lowerK values andoc

higher water solubility than HOCs, so the bondingforce between the absorbed PHOC and the soilmedia is presumably weaker, therefore the estab-lishment of equilibrium amongwPx , wPx andsoil NAPL

wPx should be a rapid and less kinetically limitedmic

procedure during the soil washing process. Asdiscussed earlier, the soil washing of PHOCsdepends on the increase ofwPx and reduction ofliq

wPx ; the presence of NAPL will retard the overallsoil

soil washing performance by supplementing thelost PHOCs(from soil extraction) back into thesoil as another pollution source and relocating thePHOCs from NAPL to micelles(a futile process

in terms of performance). Therefore, smalleravalues(i.e. gentler slopes of linearized soil wash-ing curves) were observed for PHOCs.According to the proposed mechanism, the

NAPL of HOCs or PHOCs plays a critical role indetermining the shapes of performance curves inthe surfactant-aided soil washing process. To fur-ther verify this, it is theoretically possible to screwthe performance curves by overdosing HOC(i.e.increasing the portion of NAPL) to flatten theexponential curve or by underdosing PHOC(i.e.decreasing the portion of NAPL) to make thecurve turn upward. HCB and 1-naphthylaminewere selected as probes from HOCs and PHOCs,respectively, for verification. By overdosing theinitial concentration of HCB in the system, thesoil phase was saturated and NAPL was formed,and a flattened performance curve was observedas indicated in Fig. 8. Alternatively, the underdoseof 1-naphthylamine( just slightly higher than itswater solubility), the original straight line wasturned upward because of the deficiency of 1-naphthylamine NAPL, as shown in Fig. 9.For the b value its physical meaning is the

partition of the contaminant between the liquidand soil phases without the presence of surfactant,i.e. (wPx qwPx )ywPx , which depends onw NAPL soil

both the total mass of contaminant in the system


Fig. 9. Verification of proposed mechanism by decreasing ini-tial concentration of 1-Naphthylamine.

and the partitioning result. To determineb ispractically difficult in the field due to the lowpartitioning characteristics for hydrophobic organ-ics, the unpredictable total mass of pollutants, andthe uncertainty of the equilibrium state. Becausetheb is rather insensitive to the variation of initialHOCyPHOC doses, which can be verified by theprojected intercepts in Figs. 8 and 9, the use ofb

values in Fig. 7, may therefore offer a quick wayto estimate the appropriate surfactant dose and theanticipated washing performance by just knowingtheK of the target contaminant.oc

In addition, the new values ofa for the overdoseand underdose examples moved into the transitionzone(see also Fig. 6), indicating that the transitionzone is the area wherea can be located when aparticularly high or low dose of contaminationtakes place. This property makes the transitionzone a useful indicator to monitor the soil washingprocess in field applications. For example, lowaimplies that the soil is heavily contaminated andmay require extended soil washing, and theincrease ofa during the batchwise or continuousoperation indicates that the process is close tocompletion.

4. Conclusions

The proposed soil washing mechanism andmathematical model can describe the performancecurve of the surfactant-aided soil washing processsuccessfully. The use of HOC and PHOC definedby theK of the contaminant is feasible to explainoc

the existence and importance of NAPL contami-nants in the soil washing system. The presence ofNAPL will retard the overall soil-washing perform-ance. This will be improved by increasing thesurfactant dose of the soil washing system, whichwill not impair the environment since biodegrad-able and low-toxic surfactants are available in themarket. According to the proposed model, a tran-sition zone was defined. This is a useful indicatorto monitor the completeness of soil washing pro-cess in field applications. This provides valuableinformation for the application of ex situ surfac-tant-enhanced soil washing.

Acknowledgments

The work described in this paper was supportedby a grant from the University Research Fund ofthe Hong Kong Polytechnic University(S874).

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the mechanism of the surfactant-aided soil washing system for hydrophobic and partial hydrophobic...

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