ENVIRONMENTAL ENGINEERING SCIENCEVolume 23, Number 6, 2006© Mary Ann Liebert, Inc.

Volatile Organic Compounds Emission from ContaminatedSoil During Surfactant Washing

Huan-Ping Chao,1 Jiunn-Fwu Lee,2,* Lain-Chuen Juang,3 Ching-Her Kuo,2and Gurusamy Annadurai2

1Department of Bioenvironmental EngineeringChung Yuan Christian University

Chung-Li, 320, Taiwan, ROC2Graduate Institute of Environmental Engineering

National Central UniversityChung-Li, 320, Taiwan, ROC

3Department of Environmental EngineeringVan-Nung Institute of Technology

Chung-Li, 320, Taiwan, ROC

ABSTRACT

The emission characteristics of aromatic and aliphatic volatile organic compounds (VOCs) produced bytwo kinds of surfactants, nonionic Triton X-100 (TX-100) and cationic domiphen bromide (DB), wash-ing high and low soil organic matter (SOM) soils were evaluated with two mass-balance equations. Theinfluential factors on volatilization mass were primarily related to the affinity of the VOCs to the soilsand to the surfactant solutions. The results show that the VOC emissions are a function of Koc, the sur-factant properties and the Sw and P of the VOC. Nonionic surfactants could more effectively wash theVOCs away from the soil, leading to relatively higher emissions. When the surfactant concentrations ex-ceeded the critical micelle concentrations (CMCs), VOC partitioning into the surfactant sorbed on the soilsurface to reduce the VOC emissions. Whether the VOC can freely dissolve in the solution is the keypoint in determining the volatilization parameters. The emission of dissolved VOC is dependent on KOL

and Sw. The P is the major parameter governing the emission of VOCs in an insoluble state. The solutionproperties can strongly affect the emissions of low Sw compounds due to solubility enhancement effects.The main effects on higher Sw compound emissions were the amounts of the surfactants adsorbing on thesoil surface.

Key words: surfactant; washing soil; volatilization; VOC; CMC; cosolute effect

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*Corresponding author: Graduate Institute of Environmental Engineering, National Central University, Chung-Li, 320, Tai-wan, ROC. Phone: �886-3-4227151-34658, Fax: �886-3-4226742; E-mail: jflee@ncuen.ncu.edu.tw

NOMENCLATURE

The following symbols are used in this paper:

A � Interfacial contact area between air and water (m2)AR � Empirical constant

C � VOC concentration (mg L�1) in equilibriumCL � Concentration of chemical in the solution at time

t (mgL�1)D � Diffusion coefficient (m s�2)H � Henry’s Law constant (dimensionless)

Hc � Apparent H with organic chemicals in the sur-factant solution

Kd � Partitioning coefficient in soil-water system(mg�1L)

Koc � Partitioning coefficient normalized as soil organiccarbon content(mg�1L)

Kom � Partitioning coefficient normalized as soil organicmatter content(mg�1L)

KOL � Overall mass transfer coefficient (m s�1)K s � Mass transfer coefficient of the pure chemical (m

s�1)L � Liquid depth (m)M � Molecular weight

Me � Chemical volatilized amount (mg) Mo � Spiked chemical amount (mg) Ms � Supersaturated chemical amount (mg)

P � Saturated vapor pressure (kPa), 1 atm � 101.325kPa

Q � Mass flux (mg m�2 s�1)Sw � Water solubility (mgL�1)T � Temperature (K)

Vw � Volume of solution (liter)Vg � Volume of headspace in soil washing facilityfoc � Organic carbon content (%) for soilm � Soil weight (g)t � Reaction time (s)x � Amount of VOC partitioning onto soil (mg)� � Evaporation coefficient (dimensionless)� � Volatilization ratio (Me/Mo)

INTRODUCTION

THE PROBLEM of soil pollution has been widely con-sidered in recent years, and how to clean up con-

taminated soil has become an important issue. Amongthe remediation soil methods, contaminated soils washedwith surfactants is a common approach (Abdul and Gib-son, 1991; Dma et al., 1993; Griffiths, 1995; Cheah etal., 1998; Deshpande et al., 1999). Several investigatorshave discussed the adsorption and desorption of organic

contaminants in the surfactant–soil-water system (Sunand Boyd, 1993; Chiou, and Kile, 1994; Edwards et al.,1994; Sheng et al., 1998; Kopinke et al., 2001; Huang etal., 2005) but have neglected the volatile organic com-pound (VOC) emissions in the washing processes. Theemitted VOCs might affect the evaluation of removalVOCs efficiency, and might have an impact on humanhealth.

The volatilization of VOCs from water is estimatedgenerally using the two-film theory (Liss and Slater,1974). It is assumed that there is a transition layer throughwhich chemicals pass by molecular diffusion bound theinterface between the liquid film and the gas film. Forhigh H (�0.1, dimensionless) solutes, the gas resistanceis negligible and the volatilization flux Q can be ex-pressed as (Mackay and Leinonen, 1975)

Q � KOLCL (1)

In Equation (1), Smith et al. (1980) presented the KOL

values of the high H compounds could be obtained ac-cording to D values of the reference compound oxygenunder the same environmental conditions. However,Equation (1) is no longer effective for the volatilizationof the pure VOCs. An applicable equation has been pre-sented as follows (Chiou et al., 1980).

Q � �� �12 P (2)

The � values approach a constant for different VOCs un-der a given environmental condition.

When VOCs are emitted from a soil-water system, theinfluential force might consist of the affinity of the VOCsin relation to solution, soil, and air. If VOCs exist in asoil and water system without surfactants, the VOCs onlypartitioned into the soil organic matter (SOM). The dis-tribution coefficient Kd is expressed as follows:

x/m � KdC (3)

For natural soils with the different SOM content, the Kd

can be normalized as Koc (Chiou et al., 1983).

Koc � Kd/foc (4)

In Equation (3), the Kd values increase as the SOM in-creases or VOC aqueous solubility decreases. However,the Koc values of a given VOC for the soils with the var-ious SOM approach a constant. For the interaction ofVOC between air and water, H is a good parameter todetermine the trend of the VOC partitioning into the gasphase or liquid phase. A simple approach to estimate Hvalue of the VOC is the ratio of P to Sw.

To combine the above-mentioned result, the empiricalequations for volatilization flux of organic compounds in

M�2�RT

924 CHAO ET AL.

soil–water system were defined as (Woodrow et al., 1997,2001)

Q � (5)

Q � (6)

Equations (5) and (6) offer a simple approach based onthe affinity of VOCs to water and soil on estimating thevolatilization of soluble and insoluble VOCs, respec-tively. It is difficult to predict the AR values that varywith environmental conditions.

However, this approach cannot be directly applied tothe VOC emissions during the surfactant washing con-taminated soil because the presence of the surfactant inthe soil–water system can enhance the apparent VOC sol-ubility or the amount of the VOC uptake on the soils,which complicate this interaction. The Koc and Sw of thedifferent VOCs vary with the properties of soils, surfac-tants, and VOCs. Thus, the apparent volatilization massneeds to be further determined experimentally. For agiven environmental condition, the volatilization flux ex-pected is a function of SOM, surfactant properties, andphysicochemical properties of the selected VOCs.

According to the realistic condition for the organic com-pounds contaminating the soils, the organic contaminantsin the soil-water system may be present in either the com-plete soluble or the insoluble state. The volatilization char-acteristics of the VOCs are primarily dictated by the sol-uble state of the VOCs. In this study, we selected the VOCsas the target compounds that could be commonly foundfrom the petroleum. Two kinds of surfactants to wash thecontaminated soils with the different SOM were applied

P � AR�

KocSw

H � AR�

Koc

to observe the volatilization characteristics of the selectedVOCs. The effects of the parameters including Koc, Sw, P,and H on volatilization of VOCs in the complete solubleor the insoluble state were also discussed. The obtained re-sult will be a good reference point for emission of the dif-ferent VOCs in the surfactant-soil-water system, when thesurfactants are applied to wash the contaminated soils.

MATERIALS AND METHODS

The objective of this study was to observe the volatiliza-tion characteristics of the VOCs in the system of the sim-ulated surfactant washing the contaminated soil. The VOCvolatilization determination was divided into three parts. The first is the VOC volatilization from the surfac-tant-soil-water system. The changes in the volatilizationmass of the VOCs for the different surfactants and soilswere discussed. The second is the pure chemicalvolatilization within the given period. The obtained resultwill be a reference of the volatilization mass for the VOCsin the insoluble state. The third is the VOC volatilizationfrom water without the surfactant and soil. By examiningKOL values of the VOCs under a specific environmentalcondition, the volatilization mass of the VOCs from wa-ter could be determined. All of the experiments conductedin the laboratory are described below.

Chemicals

The selected VOCs consist of the relatively higher Sw

aromatic compounds and lower Sw aliphatic ones, whichare commonly found in gasoline-contaminated sites. Theinterested physicochemical properties are listed in Table1. All of the VOCs were purchased from the Fluka Co.

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Table 1. Physicochemical properties of selected compounds (25°C).

aSw aH Density aPCompounds M (mg/L) dimensionless (g/L) (kPa) blogKoc

Benzene 78.11 1780 0.223 0.88 12.7 1.44Toulene 92.1 515 0.273 0.87 3.8 2.49Ethylbenzene 106.17 152 0.356 0.87 1.27 2.27p-Xylene 106.17 198 0.252 0.86 1.17 2.23o-Xylene 106.17 175 0.215 0.88 0.882 2.32n-Pentane 72.15 36 51.3 0.63 68.4 2.65n-Hexane 86.17 13 58.9 0.66 20.2 2.95n-Heptane 100.20 2.4 92.2 0.68 6.11 3.61n-Octane 114.23 0.70 130 0.70 1.88 4.04n-Nonane 128.26 0.070 300 0.72 0.571 4.81n-Decane 142.28 0.0090 433 0.73 0.175 5.49

aObtained from Mackay and Shiu (1981): bCalculated with logKom � 0.729logSw � 0.001; Koc � 1.742Kom (Chou et al., 1983)

Japan, and had high purities �98%. Two types of sur-factants are nonionic TX-100 and cationic DB suppliedby the Riedel de Haën Company, Germany. The molec-ular structures and CMCs for the surfactants are given inTable 2. The tested soils obtained from Taiwan possessthe obvious difference in the surface area and the SOMcontent. The soil characteristics are shown in Table 3.

Preparation of contaminated soil

Two types of natural soils were air dried, ground, andsieved to obtain particles �2.0 mm in size, then employedto evaluate the VOC emissions after the different runs.The 50-g soils were first put into a 250-mL glass cone-shaped beaker. Stock solutions with 1,000-mg/L mixturesfor each VOC in methanol, calculated by the product ofthe density and the volume, were prepared. One- or 3-mg aliquot amounts of each VOC were spiked into thebeaker, which was sealed immediately by Teflon-linedsepta and then mixed with a roller for 24 h to producethe contaminated soil samples.

Volatilization of VOCs from surfactant-soil-watersystem

Surfactant solutions (200 mL) containing 100 mg/L[below critical micelle concentration (CMC)] or 1,000mg/L (above CMC) were injected into the beakers andthe Teflon caps were replaced with the activated carbonadsorbent (SKC Inc., Japan Cat. No. 575-001), as shownin Fig. 1. The headspace volume was maintained at ap-proximately 30 mL so as to reduce the amount of theVOC accumulating in the gas phase. In order to be wellmixed, the beaker was placed on a shaker (Tungtec In-struments Co., Ltd, Taiwan, BT-25D) and shaken at 120

rpm under 25°C for 24 h. After the run, the activated car-bon adsorbent was transferred to a vial and extracted with20 mL of carbon disulfide (with purity �99%, benzenewas less than 0.1 ppm) purchased from Tedia Co. (USA)The extractives were directly analyzed by gas chro-matography (GC). Each experiment was replicated andthe data averaged. When the bias of repeated experimentsexceeded 15%, the triplicate repetitions would proceed.A blank experiment was carried out in every run to ex-amine the recoveries. For the aromatic compound recov-eries for the all the blank experiments ranged from 85 to95%. For the aliphatic compounds, this recovery couldnot be held due to existence of the nonaqueous liquidphase (NAPL).

Volatilization of pure chemicals

Individual VOC was added into the above-mentionedbeaker without the activated carbon adsorbent until thesame solution volume as the surfactant washing experi-ment was reached. The beaker was shaken under the samecondition for 1.0 h. Selecting 1.0 h as the experimentaltime is because the volatilization mass of the high volatilecompounds were constrained. The volatilization mass ofthe selected VOCs was determined by weight loss.

Volatilization of VOCs from water

Except for benzene, the VOC concentrations in the so-lutions were prepared with 50% Sw. The initial concen-tration for benzene was limited to 500 mg/L. Because thevolatilization process is regarded as a first-order reaction,the differences in the initial concentration should have anegligible influence on the estimation of k. The solutionswere shaken for 1.0 h to make the mixing complete. Then,

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Table 2. Molecular weights and structures of selected commercial surfactants.

Cationic surfactant Nonionic surfactantCharacteristics (DB) (TX-100)

Molecular structure CH3(CH2)11N(CH3)2(CH2CH2OC6H5)Br C8H17C6H4O(CH2CH2O)9.5HMolecular weight 414 g/mol 624 g/molCMC 730 mg/L 130 mg/L

Table 3. Characteristics of tested soils.

Surface area foma CEC

Soil source (m2/g) (%) (Cmol/kg)

Lu-Tsu soil (LTS) 05.21 01.80 05.0Sha-Mao Mountain soil (SMM) 57.20 11.00 440

aPercentage of organic matter in the soil.

the solution volume about 250 mL added into the beakerto examine k via sampling for the given intervals underthe same environmental conditions. Because the Sw val-ues of the selected aliphatic compounds are lower thanthose of the aromatic ones, the experiments need to beconducted separately. For aromatic compounds, 1-mLaliquots of the solution were sampled at 10-min intervaland extracted with 2-mL of carbon disulfide (Tedia Co.).The extracted solution was put into a brown vial andshaken on a reciprocating shaker for 3 h. After being keptat a standstill about 30 min, the extracted solution wasanalyzed using the Hewlett-Packard 5890A gas chro-matograph. For the aliphatic compounds, a 5 mL sampleat a 10-min interval was directly injected to a Purge &Trap Concentration or (Tekmar, USA, LSC3100) con-necting a GC to determine the volatilization rate constantk. As the k values of the VOCs are obtained, the KOL val-ues under the same condition could be calculated as follows.

KOL � L k (7)

In this study, the liquid depth (L) is about 6.5 cm.

Analysis condition

The GC analysis was performed on a Model 6890AHewlett Packard equipped with FID. The VOCs wereseparated on the capillary column (J & W DB-502) with105 � 5.3 mm i.d. and 3 �m film thickness. The columnwas held at 40°C for 1 min and then increased at 5°Cmin�1 to 100°C, and finally increased at 8°C min�1 to240°C equilibrated for 2 min.

RESULTS AND DISCUSSION

In Equation (1), it could be found that the volatilizationflux of VOCs in the dilute solution is mainly dictated bythe soluble VOC concentration (CL). The volatilization fluxof the pure chemicals in Equation (2) correlated with theirP. We assume the volatilization process of VOCs in thesurfactant-soil-water system is the VOCs release from thecontaminated soils to surfactant solution and then directlyemit into air. The result could be discussed based on thesoluble and insoluble VOCs. For the soluble VOCs, whenthe VOCs emit from a known container, the VOCs in steadyand thermodynamic equilibrium states can correspond tothe following mass balance equation.

Mo � Kd � CL � m � CL � Vw � CL � Hc

� Vg � KOL CL A t (8)

By contrast, the VOC volatilization flux (term KOL CL

A t) mainly correlate to the KOL, the Kd and the CL, if theother parameters do not change. The apparent CL valuesdepend on the level of the VOCs partitioning into theSOM.

However, frequently another condition is that the or-ganic contaminants exceed their Sw to form NAPL. Thevolatilization flux of the pure chemicals is obviouslyhigher than that of the organic compounds dissolved inwater (Lee et al., 2004b). A similar mass balance equa-tion for the insoluble VOCs can be written as

Mo � Kd � Sw � m � Sw � Vw � M

� P � Vg/(RT) � Ms � Ks P A t/(RT) (9)

According to the above-mentioned assumption, a majorvolatilization flux resulted from the insoluble VOCs, andthat CL was maintained in Sw until the NAPL phase dis-appeared. In Equations (8) and (9), the terms KOL CL At and Ks P A t/(RT) can be experimentally determined forthe absence of the surfactants and soils. The volatiliza-tion mass of the pure VOCs and VOC in the solution with50% Sw for 1 h are listed in Table 4. From this table, thevolatilization flux of the VOCs is far higher than that ofthe VOCs dissolved in water, which agree with the ear-lier assumption. There are very obvious differences in thevolatilization fluxes of the different VOCs. Especially foraliphatic compounds, the volatilization fluxes dramati-cally decrease as the M increases.

According to Equations (8) and (9), the spiked VOCamounts were thought as reference amounts (Mo). Theapparent volatilization amounts of the VOCs in the sur-factant–soil–water system were indicated as Me. Thevolatilization ratio coefficient � can be defined as

� � � � (11)Me�Mo

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Activated carbonadsorbent

Teflon capVent

Figure 1. Adsorption equipment for studying VOC emis-sions.

It can be expected that the � values are less than 1, be-cause the surfactants and soils in the solution can inhibitthe VOC emissions. The Me value is equal to the KOL CL

A t term in Equation (8) and to the Ks PAt term in Equa-tion (9). To compare the difference in � values of VOCsin the different surfactant-soil-water systems, thevolatilization characteristics of the VOCs affected by thesurfactants and the soils can be understood.

VOC emissions were the result of competition betweenthe system energy and the affinity of the solutes to thesolutions (Chao et al., 2000). The former parameters arerelative to the temperature and the stirring. The latter pa-rameters have been collected in Equations (5) and (6). Inthis case, the stirring rate was maintained at 120 rpm dur-ing the washing process in order to facilitate the VOCmixing well in the soil–surfactant solution. In addition,the environmental conditions, such as temperature,spiked VOC amounts, and the container volume havebeen effectively controlled. In other words, Koc, Sw, P,and H of VOCs are major factors to determine thevolatilization mass according to Equations (8) and (9).

Among these parameters, the Koc values of a givenVOC regarding to the different soils are thought as a con-stant that is a function of Sw. Theoretically, the SOM con-tents do not affected the VOC emissions. However, theVOCs are difficultly washed from the high SOM soil (Leeet al., 2004c). This might cause a relatively lowervolatilization flux. In addition, the surfactant properties,whether cat- or nonionic, and the surfactant concentra-tion, whether below or above CMCs, also affect the ap-parent Koc (Lee et al., 2000, 2004c). Figures 2 and 3 il-lustrate the total volatilization mass of the differentspiked VOCs in the tested soil-water-surfactant systems.The results of the volatilization mass indicated that theVOCs emitted from the surfactant–LTS were slightlyfaster than those from surfactant–SMM. In terms of the

surfactant concentrations, the total volatilization masswas in the following order: TX-100 below CMC � TX-100 above CMC � DB below CMC � DB above CMC.It was assumed that volatilization process is that theVOCs are washed from the soil and then emit from thesolution to the gas phase. The results demonstrate theSOM content might weakly affect the emissions of theVOCs. The degree of surfactant washing VOCs awayfrom the soil would be one of the considerable factors.However, variations between the different soils wererarely obvious. This phenomenon also represented thatVOC emissions might depend more strongly on the prop-erties of the surfactants and the Sw of the VOCs.

It is well known that a cationic surfactant can stronglyadsorbed on the soil surface through ion exchange(Goloub et al., 1996, Huang et al., 2005). The cationicsurfactant generates a poor effectiveness to wash the or-ganic compounds away the soils (Chu and Chan, 2003).As the properties and concentration of the surfactants aretaken into account, the nonionic surfactant produces a

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Table 4. Volatilization mass of the pure VOCs and the VOCs from the dilutesolution (25°C), t � 1 h, unit :mg.

Compounds Ks P A t/(RT) KOL CL A t

Benzene 13.40 0.8300Toluene 03.87 0.3800Ethylbenzene 01.24 0.1100p-Xylene 01.15 0.1400o-Xylene 00.83 0.1300n-Pentane 66.30 0.0430n-Hexane 17.3 0.0110n-Heptane 05.52 0.0018n-Octane 03.03 0.0006n-Nonane 01.39 5.20E-05n-Decane 00.78 741E-06

Figure 2. Total volatilization mass when spiking 3 mg of eachVOC.

more effective surfactant washing to cause the highervolatilization flux. Thus, the higher volatilization masswas found in the TX-100-soil solution. On the other hand,if the surfactants form micelles in the solution, the com-plicate explanation needs to be considered further. Forthe relatively lower Sw compounds, the micelle offers ahydrophobic environment to attract organic compoundreleasing from the soils to the solution (Lee et al., 2004c).Also, the micelles of surfactants inhibit the volatilizationof these lower Sw compounds (Chao et al., 2000; Lee etal., 2004a). For the relatively higher Sw compounds, theadsorbed surfactants are thought as the enhanced SOM.This increases the ability of the higher Sw compoundspartitioning into the SOM, but the micelles in the solu-tion hardly reduce their volatilization flux. If the surfac-tant further adsorbed on the soil surface to form admi-celles, onto which VOCs can partition (Xu and Boyd,1995). This would significantly reduce the volatilizationflux. As a result, the volatilization mass for the 1000mg/L surfactant solution is lower than that for the 100mg/L one.

Different group compound functions possess differentmolecular structures and physicochemical properties toexhibit disparate volatilization characteristics (Chao etal., 2005). The obtained results should be discussed re-spectively. As mentioned earlier, the SOM content onVOC emissions has been thought to have little impor-tance. Accordingly, an LTS sample was selected as thetarget soil to examine the volatilization mass differencebetween the aromatic and the aliphatic VOCs. Figure 4presents the � values of the aromatic and aliphatic groupswith lower and higher spiked amounts produced from thesurfactant–LTS system. Higher � values were found fromthe aromatic group under all conditions. A possible rea-son could account for the result. The tested aliphatic com-pounds possess a relatively lower Sw, resulting in obvi-

ous solubility enhancement based on the cosolute concept. Although less-soluble VOCs in the surfactant-soil-water system would be attracted by the surfactant inthe solution to release into the solution, the volatilizationof the released VOCs might be inhibited by the surfac-tant. The lower Sw VOCs have been demonstrated to ex-hibit the more obvious volatilization reduction (Lee etal., 2004a). When the solution properties were thoughtas a mainly influential factor, the obtained result is rea-sonable.

Comparing the more high-contaminated soil (3 mgeach of the VOCs) with the less-contaminated soil (1 mgeach of the VOCs) under the same surfactant conditions,the differences between the aromatic group and thealiphatic group are comparable. For the aromatic com-pounds, the � values for high and low contaminated levelwere similar, but the aliphatic compounds indicate obvi-ous difference. This might be a result of the less solublealiphatic compounds reaching the NAPL phase. Accord-ing to Table 4, the volatilization fluxes of the insolublecompounds are far higher than those for the soluble com-pounds. Thus, whether form NAPL or not is a key pointin determining the volatilization fluxes of the VOCs dur-ing the surfactant washing process.

In order to clearly differentiate insoluble compoundsfrom soluble ones, the � values for the respective VOCwith different spiked VOC amounts in the surfactant–SMMsystem are graphically shown in Fig. 5 based on the Sw.From Table 4, it can be found the volatilization mass of theVOCs are correlated to their M. For aromatic compounds,the volatilization mass are the following order: benzene �toluene � ethylbenzene � p-xylene � o-xylene. The ali-phatic compounds indicate the following order: n-pen-tane � n-hexane � n-heptane � n-octane � n-nonane �n-decane. The above orders are also inversely propor-tional to their Sw. As expected in Fig. 5, the � valuesshould increase approximately as the Sw increases, if theVOCs are classified on the basis the molecular structures.

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Figure 3. Total volatilization mass when spiking 1 mg of eachVOC.

Figure 4. � values of aromatic and aliphatic compounds withlow (L:1 mg each VOC) and high (H: 3 mg each VOC) spikedamount in the LTS system with different surfactant concentra-tions.

Especially, the highest � value among the aliphatic com-pounds is located in a2 and b2, that is, n-heptane. The re-sult violates the measured volatilization mass as shownin Table 4. A possible reason is that the VOCs formNAPL. To examine changes in the � values regarding tothe Sw, it can be found that the n-hexane (log Sw � 1.11)is the key compound. The point for n-hexane on the less-contaminated soil is located in the valley, but the pointat the same site on the high-contaminated soil representsn-pentane. This is ascribed to that the n-hexane in thehigh-contaminated soil forms the NAPL. Regardless ofthe solubility enhancement and soil adsorption, 3 mg ofn-hexane in a 200-mL solution has exceeded the Sw of n-hexane (13 mg/L) in pure water, and therefore, to assumethat presence of n-hexane in the NAPL state is reason-able. With n-hexane continuously volatilized from the so-lution, the amount of insoluble n-hexane is gradually de-pleted until it disappears. In region b3 to b4, the � values

have in the following order benzene � toluene � p-xy-lene � o-xylene � ethylbenzene � n-pentane � n-hexane, which is consistent with their Sw values. Ac-cording to the traditional two-films theory, the estimationof KOL considering M and H should be in another ordern-pentane � n-hexane � benzene � toluene � ethyl-benzene � p-xylene � o-xylene. It is apparent that thecosolute effects changed the sequence. These results in-dicated that the VOC volatilization flux has a close rela-tionship to the cosolute effect, except that the VOC con-centration forms the NAPL in surfactant solutions. Byextension, compounds in a surfactant–soil–water systemwith a low Sw and a high Koc value are difficult tovolatilize.

As mentioned above, the adding concentration of thesecompounds exceeded their Sw. From Equation (9), wefind that if VOCs present as the insoluble state in the sur-factant-soil-water system, the � values should be depen-dent on their P. The result was significantly proportionalto their M or P, which also corresponded to the pure sub-stance volatilization (Rathbun and Tai, 1984; Lee et al.,2004b). The major volatilization resource for these com-pounds from the NAPL should be recognized; the se-lected compound density is less than water as seen inTable 1. These compounds are directly contacted withthe gas phase. In addition, another interested tendency isthe changes in � values for the organic compounds in100 mg/L DB and 1,000 mg/L TX-100 solutions. For thea1 to a2 and the b1 to b2 regions, the � values of the in-soluble VOCs in 100 mg/L DB solution were higher thanthose in the 1,000 mg/L TX-100 solutions. However, thefreely dissolved VOCs in the a3 to a4 and the b3 to b4 re-gions showed the opposite tendency that is, 1,000 mg/LTX-100 solution more than 100 mg/L DB solution. In theprevious description, the total volatilization amounts ofVOCs in the 1,000 mg/L TX-100 solutions also showedthe relatively higher � values. The reason was attributedto that cationic DB adsorbs on the soil surface to inhibitVOC volatilizations. When the VOCs are further classi-fied according to their Sw, the volatilization characteris-tics can be clarified. In our previous, the characteristicsof the VOCs partitioning into the SOM and sorbed sur-factant is the solution properties affect the emissions ofrelatively less-soluble VOC and properties of the sorbedsurfactants affect relatively higher Sw compounds (Lee etal., 2000, 2004c). Although the analogous CMC leads todifficultly determine the effects of the VOC volatiliza-tion in the DB and TX-100 solutions, a surfactant con-centration above or below the CMC still controls theVOC emissions. The volatilization inhibition of thealiphatic compounds primarily results from the solubil-ity enhancement so as to the obtained � values indicatethat the DB below the CMC is higher than the TX-100

930 CHAO ET AL.

Figure 5. � values dependent on the Sw of the respective com-pounds in an SMM system with different surfactant concentra-tions.

above the CMC. On the contrary, the surfactant concen-tration for the relatively higher solubility aromatic com-pounds exhibited little volatilization inhibition, and thus,the adsorbed DB by the soil indicates a higher volatiliza-tion inhibition. It can be concluded that the major factorsdetermining volatilization are the solution properties forthe low Sw compounds and the soil properties for the highSw ones.

CONCLUSIONS

The volatilization characteristics of VOCs during thesurfactant washing of contaminated soils have been eval-uated in this paper. The results showed that the degreeof VOC desorption, relative to the SOM, the surfactantproperties, and the Sw will dictate the VOC emissions.The SOM is only weakly correlated with the VOC emis-sions. According to the results for the different surfac-tants, the nonionic surfactants washing the contaminatedsoils can produce more emissions than the cationic sur-factants. Cationic surfactants can easily gather on the soilsurface, which enhances VOC partitioning onto the sur-face and reduces the amount of VOC in the solution, aclearly volatilization reduction. If the added surfactantconcentrations exceed the CMC, the VOCs emission inthe surfactant–soil–water system is very substantial in-creased. Besides the Koc values, the cosolute effect willalso determine VOC emissions based on their Sw in acompletely soluble state. On the other hand, the VOCvolatilization ratio � is strongly dependent on the Sw andP when the VOC concentration in the surfactant-soil-wa-ter system exceeds their Sw. The dominant factor deter-mining volatilizations for low Sw compounds are the so-lution properties, while for high Sw compounds are thesoil properties.

ACKNOWLEDGMENTS

Support for this work by the National Science Coun-cil, ROC, under Grant NSC 95-2221-E-033-095 is highlyappreciated.

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