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e Pergamon Waf. Sci. Tech. Vol. 34, No. 7-8, pp. 327-334,1996.Copyright © 1996 fAWQ. Published by Elsevier Science Ltd

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EFFECT OF ANIONIC AND NONIONICSURFACTANTS ON SORPTION ANDMICELLAR SOLUBILIZATION OFMONOCYCLIC AROMATIC COMPOUNDS

Ruey-an Doong, Wen-gang Lei, Tsu-feng Chen,Chen-yeu Lee, Jee-hua Chen and Wen-heui Chang

Department ofNuclear Science, National Tsing Hua University, Hsinchu 30043,Taiwan

ABSTRACT

The effect of anionic and nonionic surfactants on the sorption and micellar solubilization of monocyclicaromatic compounds in soil-free and soil-water systems was investigated at 25°C to examine thefeasibility of in situ remediation. Benzene, chlorobenzene and styrene (BCS) were selected as the targetcompounds due to their suspected carcinogenic and mutagenic properties. Sodium dodecyl sulfate(SDS) and Triton X-I 00 were used to represent the anionic and nonionic surfactants, respectively. Theaddition of Triton X-lOa had little effect on the micellar solubilization of BCS. However, thesolubilization of aromatic compounds increased significantly with the increase of SDS concentration. A20% to 43% enhancement of the solubilization in SDS-amended systems was demonstrated. Theadsorption isotherms ofBCS with Triton X-lOa can conveniently be fitted by Langmuirian expression.However, multilayer adsorption of chlorobenzene and styrene was observed in SDS-amended systems.The values ofmaximum adsorption capacity ranged from 323 to 736 J.lg/g. Also, the effect ofTriton X•100 on maximum adsorption capacity was greater than that of SDS. Moreover, a correlation betweenthe maximum sorption capacity and partition coefficient was established. The results of this studydemonstrate that surfactants can be effectively used as chemical amendments to minimize the volatilitiesof monocyclic aromatic compounds and enhance sorption and solubilization in soil environmentscontaminated by proper selection of surfactant type and concentration.Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS

monocyclic aromatic compounds; sorption; micellar solubilization; sodium dodecyl sulfate; Triton X•100; critical micelle concentration; surfactant; in situ remediation.

INTRODUCTION

Volatile aromatic compounds, such as benzene, toluene, chlorobenzene, and styrene are the most oftenfound contaminants in soil environments. Owing to their widespread usage as organicsolvents/degreasers and improper disposal, these compounds have become ubiquitous environmental

327

328 R.-A. DOONG el al.

contaminants (Barbaro, 1992). The immiscibility with water and high volatility of these compoun~sresult in their occurrence in the environment as non-aqueous phase liquids (NAPLs) and hazardous aIrpollutants (HAPs). Due to their suspected carcinogenic and mutagenic properties, an understanding .ofthe fate of these compounds in soil environments is sought so that successfully apply remedIaltechniques can be developed.

Bioremediation is one of the innovative and low-cost technologies that stimulates the naturalbiodegradation of aromatic hydrocarbons in soil environments by providing supplemental substrateand/or mineral ions to the indigenous microflora. Research has demonstrated that aromatichydrocarbons can be degraded under various electron acceptor conditions (Barbaro et al., 1992;Chaudhuri and Wiesmann, 1995). However, evidence has accumulated that the bioavailability of thexenobiotics could be diminished due to sorption, volatilization or abiotic decomposition (Alexander,1995). The addition of surfactants to environments contaminated with hydrophobic compounds is apossible means of increasing the bioavailability of these compounds and to facilitate their biodegradation(Shiau et aI., 1994; Aronstein and Paterek, 1995). It has been demonstrated that the addition ofsurfactants can enhance biodegradation efficiency when the surfactant concentration is above a criticalmicelle concentration (CMC). On the other hand, the inhibition of nonionic surfactants onbiodegradation at concentrations above CMC was also reported (Laha and Luthy, 1991; Roch andAlexander, 1995). This may be due to the difference of the sorption kinetics and micellar solubilizationof surfactants. Valsaraj and Thibodeaux (1989) evaluated the partition coefficients for sodium dodecylsulfate and 11 hydrophobic nonpolar organics and observed a correlation between the hydrophobicity ofthe contaminant and the partition coefficient. A similar correlation was also established by Edward etaI. (1991) who used nonionic surfactant to enhance the solubilzation of polynuclear aromatichydrocarbons (PARs). However, the effect of the type of surfactant on the sorption and micellarsolubilization of monocyclic aromatic compounds is not well known.

In this study, the effect of surfactant concentration on the sorption and micellar solubilization ofmonocyclic aromatic compounds in soil-free and soil-water systems was evaluated to determine thefeasibility of in situ bioremediation. Benzene, chlorobenzene and styrene (BCS) were selected as thetarget compounds. These compounds are promulgated by the U.S. EPA as priority pollutants and posea threat to human health and environments. The effect of surfactant type was also examined toelucidate the potential performance of different surfactants in enhancing contaminant solubilizationand/or mobilization. Sodium dodecyl sulfate (SDS) and Triton x-loa were used to represent anionicand nonionic surfactants, respectively. Cationic surfactant was not selected because of its expectedstrong complexation with soil matrix.

MATERIALS AND METHODS

Benzene (99.5%, GC grade) was purchased from Janssen Co., Belgium. Chlorobenzene (99.5%, GCgrade), styrene (99%, GC grade), and the surfactant, SDS (C~(C~)llS03Na) and Triton X-lOa(C~C(C~)2C~C(C~)2C6HiOC~)nOH, n=9-10) were obtained from Merck Co., Darmstadt,Germany. The surfactants were used as received from the supplier without further purification. Thecharacteristics of the Triton x-loa and SDS are described in Table 1. The hydrophilic-lipophilicbalance (HLB) and aggregation number values were obtained from literature (Kile and Chiou, 1989;Shiau et aI., 1994; Yeom, et aI., 1995). The CMCs for SDS and Triton X-lOa were determined byconductivity and turbidity measurements, respectively.

A swelling clay soil containing montmorillonite and trace kaolinite obtained from Jang-Yuan (Taitung,Taiwan) was used in this study. It consists 6.4% sand, 35.1% silt and 58.5% clay. The organic mattercontent (fam> of 0.0535 was determined by combustion at 800 0c. The pH value and bulk density were

8.37 and 1.82 glcm3, respectively.

Monocyclic aromatic compounds 329

Batch experiments were conducted to obtained isotherms by placing a constant ratio of soil material tosurfactant solution (5g/I00mL) in a 160-mL serum bottle. Bottles were sealed with a Teflon-linedrubber septa and aluminum crimp caps (Wheaton Co., N.J.) after the aqueous solution was added.Various volumes of the stock solutions of BCS were delivered into the batches through the septum bygas tight syringes to give the final concentrations of I mgIL. The samples were mixed gently andequilibrated at 25°C for 3 to 4 days. The initial Triton X-IOO concentrations were 54 161 268 749, , , ,and 1248 mgIL. The initial SDS concentrations were 500, 1000,2000,3000 and 4000 mgIL. The totalvolume ofthe liquid phase was 100 mL, resulting in 60 mL for headspace analysis.

TABLE I. Characteristics of surfactants used in this study

surfactant chemical formula M.W. HLB CMC aggregation(g) (mgIL) number

Triton X-IOO Cgll17C6114«()C2114)lO()1I 625 13.5 268 100-155

SDS - +C12~5()S()3 Na 288 40 2300 71

The critical micelle concentrations (CMC) of Triton X-IOO and SDS were determined by turbidity andconductivity measurement, respectively. Surfactant solutions of varying concentration were made withsurfactant stock and deionized water. Multiple testing of each surfactant solution was performed toensure consistent readings. The concentration of surfactants in the liquid phase was determined bycolorimetric measurement and the concentration of surfactants in the solid phase was determined bymass balance method.

The concentrations of aromatic compounds in the headspace of the test bottles were monitored bydrawing 50 flL of gas phase with a 100 flL gas-tight syringe and injecting the mixture into a Perkin•Elmer autosystem gas chromatograph equipped with a flame ionization detector (Fill) and an integrator.A 30-m DB-624 fused-silica megabore capillary column (0.53mm i.d., 3.0 flm film thickness, J&WScientific Inc., Folsom, CA.) was used for separating BCS. The column temperature was kept at 50°Cand programmed to 120°C at a rate of 5°C/min with carrier gas (N2) flow rate of 3.3 mL/min (linearvelocity of25 cm/sec). The temperatures of the injector and Fill were 200°C and 250 °C, respectively.The analytical error from the instrument was controlled within 20%.

RESULTS AND DISCUSSION

Determination of CMe. Measurements of CMCs were perfoI1l)ed with solutions of surfactant indeionized water. Figure I shows the CMC values of the surfactants used in this study. The CMCvalues were obtained from a plot of the turbidity or conductivity versus the surfactant concentration.The CMC value of SDS obtained in this study was 2300 mgIL, which is similar to the reported value(Kile and Chiou, 1989). This suggests that conductivity measurement is a valid method for determiningthe CMC of ionic surfactants. 1I0wever, a higher CMC value of Triton X-IOO than reported previouslywas obtained. The CMC values determined by surface tension measurement ranged from 107 to 195mgIL were well documented (Kile and Chiou, 1989; Edwards et al., 1992). A value of 268 mgIL forTriton X-IOO by turbidity measurement was obtained in this study. It should be noted that the CMC ofa surfactant varies to some extent with the solution properties being measured and the evaluationmethod applied. Since Triton X-IOO is a nonhomogeneous surfactant, the successive mecellization ofthe heterogeneous monomers at different stoichiometric concentrations of the surfactant results in a

330 R.-A. DOONG et al.

broadening of the monomer-micelle transition zone. Therefore, it is reasonable that the CMC value mayvary with different measurement methods.

6 A CM:C

j

5

t2 4

6

f 3

~2

M ~ ~ ~ M ~ M ~ MTriton X-I 00 COR:entration (mM)

800

B

6 600

~

~

:f 400

1l0

U200

00

CM:C

5 10 1580s concentration (mM)

20

Figure I. The CMC values of Triton X-I 00 and SDS determined by turbidity and conductivitymeasurements, respectively.

Solubilization ofRes in soil-free systems. Batch experiments concerning the solubilization of BCS byTriton X-IOO or SDS surfactant in the soil-free systems were conducted at 25 °C. Figure 2 illustratesthe solubilization of BCS in liquid phase at various concentrations of Triton X-I 00. The enhancementof the solubilization of BCS by Triton X-I 00 at surfactant concentrations below CMC shows a linearbut relatively insignificant relationship. Less than 10% enhancement of Triton X-IOO on the BCSsolubilization was observed. At concentrations exceeding the CMC, the solubilization ofBCS increasedslightly. It has been demonstrated that the solubilization of hydrophobic compounds, such as PAHs andDDT can be significantly enhanced when the surfactant concentrations exceed the CMC (Kile andChiou, 1991; Yeom et al., 1995). However, a small solubility enhancement effect of Triton X-IOO wasdemonstrated in the present study. This is likely due to the relatively high water solubilities and lowpartition coefficients (Kow) of BCS (Table 2). In general, the extent to which a solute will concentrate

on a micelle can be related to the octanol-water partition coefficient of the solute. The larger the Kow of

a solute the greater will be its tendency to concentrate inside the micelle. Moroi et al. (1983) alsoreported that no enhancement of organic solute solubility was observed in a surfactant solution when awater soluble solute, N-alkylphenothiazine, was used as the target compound. This clearly shows thatnonionic surfactant is not a suitable chemical for enhancing the solubility of monocyclic aromaticcompounds.

TABLE 2. Basic physicochemical properties of benzene, cWorobenzene and styrene

vapor Henry's lawchemical density solubility pressure constant

(g/cm3) (mg/l) (mmHg) (atm'm3/mole)

benzene 0.88 1780 95.2 5.59xI0-3

chlorobenzene 1.11 466 11.7 3.72xlO-3

styrene 0.91 300 24.5 2,05xI0-3

83330776

Unlike Triton X-I 00, the solubilization of aromatic compounds by SDS surfactant increased linearlywith the increasing concentration. As depicted in Figure 3, solubilization enhancements of aromatichydrocarbons of 20% to 43% with SDS were measured. This difference is likely due to the degree ofhydration with the dissolved surfactant molecules. As shown in Table I, Triton X-IOO has 10wer:m..B

Monocyclic aromatic compounds

and CMC values than SDS due to the large numbers of ethoxyl groups. The lfl.,B is an indicator of therelative affinity of a surfactant to partition between water and oil phases. A higher value of lfl.,B, asfound in SDS, indicates surfactant preference for the water phase while a lower lfl.,B value indicatespreference for the oil phase. Because of the relatively high water solubility of monocyclic aromaticcompounds, the hydration effect of Triton X-IOO with the nonionic polar group should be lesssignificant than that of SDS and thus interfering with the molecular interaction of the nonpolarhydrocarbon group with BCS.

331

Triton X-IOO

~•

• benzene* chIorobe:nzene~ styrene

10

9

8'0'~ 7'-"§

"+=! 6

~ 5~

4 •tf)

] 3

2 *1 ~

00

•

CMC

~200 400 600 800 1000 1200 1400

Surfuctant concentration (mWL)

Figure 2. The solubilization ofBCS in liquid phase at various concentrations ofTriton X-IOO

50

SDS system

*40'0'~'-"§

30"+=!

~~ •

20 * •tf)

]}IE ~ •

• benzene10 ~ • * chIorobe:nzene

CMC D. styrene

~0

1000 2000 3000 40000Surfuctant concentration (mWL)

Figure 3. The solubilization ofBCS in liquid phase at various concentrations ofSDS.

332 R.-A. DOONG et al.

Solubilization and sorption of BCS in soil-water systems. The batch sorption isotherm of BCS wasconducted in soil-water systems at 25 °C to evaluate the effect of surfactant on the sorption of BCS.Figure 4 summarizes the sorption isotherms of BCS in the presence of Triton X-lOa. The adsorptionisotherms ofBCS with Triton X-lOa can conveniently be fit by Langmuirian expression. The maximumsorption capacity can be reached at the CMC. However, the adsorption isotherms of chlorobenzeneand styrene in SDS-amended systems showed a significant increase in apparent adsorption atconcentrations above the CMC (Figure 5). It has been suggested that the increase in adsorption abovethe CMC is due to multilayer adsorption. At low surfactant concentrations, surfactant monomers arelikely to lie parallel to the soil surface and begin to form micelle-like structures called hemimicelle (yVestand Harwell, 1992). As the surfactant concentration increases, the hydrophobic portion of thesurfactant may be displaced from the mineral surface, allowing for lateral interactions between adjacenthydrophobic groups of sorbed monomers. Above the CMC, the sorbed surfactant phase can exist aseither a monolayer or a bilayer, subsequently increasing the adsorption capability.

800 Triton X-loo l~

~

!'-.-/ 600 I I~

·B X 1-

i~

400 :Zit

Iu •

i 1• • benzene•200 lIE ch1orobenzeneCZl

CMC b. styrene

0 ~0 200 400 600 800 1000 1200 1400

Surfactant concentration (rngIL)

Figure 4. The sorption isotherms ofBCS in the presence of Triton X-lOa.

The sorption isotherm for BCS in a surfactant-amended system can essentially be determined by theLangmuir equation:

(1)

Where q is the amount of organic compound sorbed per unit weight of sorbent, qm represents the

maximum sorption capacity, KL is the Langmurian coefficient equal to the rate of adsorption divided by

the rate of desorption, and Ceq is the concentration of organic compounds in the aqueous phase at

equilibrium. These values along with those of qm and KL are listed in Table 3. The maximum sorption

capaci~ ran~ed from 323 to 736 ~g/g. Significant sorption of Triton X-lOa is demonstrated bycompanng WIth SDS. Also, the qm was increased as the hydrophobicity of the aromatic compounds

increased (styrene> chlorobenzene > benzene). A correlation between the hydrophobicity of the

Monocyclic aromatic compounds 333

con~aminant and the partition coefficient has been demonstrated when surfactant is amended intoenVIronments ~o enhance the solubilization of hydrophobic compounds (Edward et al, 1992; Shiau et al.,1994).. In this study, a similar relationship between the maximum sorption capacity and partitioncoeffi.c~ent. was also demo~strated,. indicating that application of surfactant to enhance the sorption andsolublhzatIOn ofmonocychc orgaruc compounds is feasible.

TABLE 3. Sorption ofBCS on SDS and Triton X-100 systems.

surfactants benzene chlorobenzene styrene

qrnax KL qrnax KL qrnax KL(~g/g) (L/g) (~g/g) (L/g) (~g/g) (L/g)

Triton X-100 400 28 610 113 621 19.5SDS 323 4 500 19.2 763 0.36

700

600SDS system I

,.-.... i~5oo

I'--"s::

°1 400I

~ 300 :I..

I:0 Ic.> ., ~

i .. T200 -.L benzene•0

tZl lIE chlorobenzene100 CMC A styrene

0~

0 1000 2000 3000 4000Smfdctant concentration (rngIL)

Figure 5. The sorption isotherms ofBCS in the presence of SDS.

In addition to acting as adsorbent for aromatic contaminants, soil can be favorable adsorption sites forsurfactant molecules. Surfactant sorption to soils causes a change in the surfactant micellization patternsuch that aqueous- and gas-phase surfactant is substantially less than the total added. Tsomides et al.(1995) demonstrated that more Triton X-lOO was needed to reach the CMC in sediment microcosmsthan in sediment-free microcosms. Also, Roch and Alexander (1995) showed that the CMC value ofTriton X-100 in a low-salts solution was higher than that in distilled waster. In this study, we foundthat, unlike SDS, more Triton X-I 00 was needed to reach the CMC in soil-water systems than in soil•free systems. It can be postulated that Triton X-I 00 is a nonionic surfactant and can be sorbed ontosoil. The presence of soil serves as an adsorbent for surfactant molecules, thereby raising the number ofsurfactant molecules required to reach CMC.

334 R.-A. DOONG el al.

SUMMARY AND CONCLUSIONS

The results of this study identify that an addition of surfactant is a potential mean of enhancing thesorption and micellar solubilization of monocyclic aromatic compounds. Triton X-IOO does not holdmuch promise for enhancing the solubilization of BCS. Addition of Triton X-IOO has a linear butrelatively insignificant enhancement effect on micellar solubilization at concentrations above the CMC.However, a significant effect on the enhancement of BCS solubilization was obseIVed for SDS due toits anionic moiety and high HLB value. Both nonionic and anionic surfactants can exert a significanteffect on sorption. Also, surfactant and BCS sorption conformed to the Langmuir isotherm and theadsorption capacity was increased as the hydrophobicity of the organic compounds increased. TritonX-I00 has the higher adsorption capacity than SDS due to its relative high maximum sorption capacityand Langmuirian coefficient. This study indicates that the remediation of soil environmentscontaminated with monocyclic aromatic compounds is feasible by proper selection of a suitablesurfactant to enhance the sorption and solubilization as well as increase the bioavailability of thesecontaminants.

ACKNOWLEDGMENTS

The author would like to thank the National Science Council, R. O. C. for financial support of thisinvestigation under Contract No. NSC-84-2113-M007-048ZA.

REFERENCES

Alexander, M. (1995) How toxic are toxic chemicals in soil? Environ. Sci. Techno!., 29, 2713-2717.Aronstein, B. N. and Paterek, 1. R. (1995) Effect of nonionic surfactant on the degradation of glass•

sorbed PCB congeners by integrated chemical-biological treatment. Environ. Toxico!. Chem., 14,749-754.

Barbaro, 1. R., Barker, 1. F., Lemon, L. A. and Mayfiels, C. I. (1992) Biotransformation of BTEXunder anaerobic, denitrifying conditions: field and laboratory obseIVations. J. Contam. Hydro!., 11,245-272

Chaudhuri, B. K. and Wiesmann, U. (1995) Enhanced anaerobic degradation of benzene by enrichmentof microbial culture and optimization of the culture media. App!. Microbio!. Biotechno!., 43, 178•187.

Edwards, D. A., Luthy, R. G. and Liu, Z. (1991) Solubilization of polycyclic aromatic hydrocarbons inmicellar nonionic surfactant solutions. Environ. Sci. Techno!., 25, 127-133.

Laha, S. and Luthy, R. G. (1991) Inhibition of phenanthrene mineralization by nonionic surfactants insoil-water systems. Environ. Sci. Technol., 25, 1920-1930.

Moroi, Y, Morna, H. and Matuura, R. (1983) Solubilization of N-alkylphenothiazine in aqueousanionic surfactant micelles. J. Phys. Chem., 87, 872-876.

Roch, F. and Alexander, M. (1995) Biodegradation of hydrophobic compounds in the presence ofsurfactants. Environ. Toxico!. Chem., 14, 1151-1158.

Shiau, B. 1., Sabatini, D. A. and Harwell, 1. H. Solubilization and microemulsification of chlorinatedsolvents using direct food additive (edible) surfactants, Ground Water, 32, 561-569.

Valsaraj, K. and Thibodeaux, L. 1. (1989) Relationships between micelle-water and octanol-waterpartition constants for hydrophobic organics of environmental interest. Wat. Res., 23,183-189.

Yeom, I. T., Ghosh, M. M., Cox, C. D. and Robinson, K. G. (1995) Micellar solubilization ofpolynuclear aromatic hydrocarbons in coal tar-contaminated soils. Environ. Sci. Techno!., 29,3015-3021.

West, C. C. and Harwell, 1. H. (1992) Surfactants and subsurface remediation. Environ. Sci. Technol.,26, 2324-2330.