Environ. Sci. Techno/. 1995, 29, 3015-3021
Micellar Solubilization of Polynuclear Aromatic Hydrocarbons In Coal Tar-Contaminated Soils ICK TAE YEOM, MRIGANKA M. GHOSH,” C H R I S D . COX, A N D K E V I N G . ROBINSON Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996-201 0
Solubilization of PAHs from a coal tar-contaminated soil obtained from a manufactured gas plant (MGP) site was evaluated using nonionic polyoxyethylene surfactants at dosages greater than cmc. Up to 25% of Soxhlet-extractable PAHs could be solubilized a t surfactant loadings of 0.3 g/g of soil in 16 days in completely stirred batch reactors. Longer periods were required to reach equilibrium at higher surfactant dosages. Raoult’s law satisfactorily described the partitioning of constituent PAHs between the weathered coal tar and the micellar solution. An equilibrium model was developed to predict the solubilization of PAHs from coal tar-contaminated soils for given properties of the soil, surfactant, and component PAHs. The model predicted solubilization of constituent PAHs reasonably well a t low surfactant dosages. At extremely high surfactant dosages, the model failed to reliably predict solubilization. Presumably, mass transfer limitations prevented attainment of equilibrium during the duration (380 h) of solubilization experiments.
Introduction Current interest in surfactants, particularly their application in in situ flushing of contaminated aquifers or ex situ washing of contaminated soils, stems from the ability of these chemicals to partition hydrophobic organic compounds (HOCs) into their micellar core (1). Surfactant processes designed to mobilize trapped ganglia of nonaqueous phase liquids (NAPLs) in the subsurface by lowering capillary forces have also been studied at great length ( 2 , 3 ) .PAHs are characterized by their low aqueous solubility and hydrophobicity. Thus, these compounds bind strongly to the soil matrix, primarily to the soil organic matter. Remediation of soils contaminated with polynuclear aromatic hydrocarbons (PAHs) and other HOCs may benefit from surfactant-based technologies involving nonionic polyoxyethylenes (POEs) ( 4 , 5 ) . By selecting POEs that are not toxic to microorganisms, it may also be possible to enhance bioremediation of PAH-contaminated soils (6). A great deal of laboratory studies have been done on surfactant-mediated solubilization of HOCs from artificially
0013-936X/9510929-3015509.00/0
G 1995 American Chemical Society
contaminated soils. However, results on the solubilization of PAHs from tar-contaminated soils, as encountered in manufactured gas plant (MGP) sites, are sparse. The reported mobilization of HOGS from artificially contaminated soils appears to be unrealistically high compared to that determinedin weathered, contaminated soils. To begin with, weathered soils contain mostly PAHs, of which only a few containing up to five benzene rings can be isolated by chromatographic methods (7). Methylene chloride used in the Soxhlet extraction procedure is able to extract 100% of PAHs in contaminated soils containing six benzene rings or less. The remainder of the organic contaminant referred to as asphaltene is practically insoluble in water. Most of the unextractable organic matter are at once hard to solubilize and are extremely resistant to biodegradation. Therefore, they may not pose a serious environmental threat. The solubilization of individual PAHs in MGP soils is comparable to that of individual components in coal tar (7-10). Since cleanup standards are most often specific for individual compounds, knowing the aqueous solubility of individual contaminants in a multicomponent system is of considerable interest to the regulatory agencies. The purpose of this paper is to present results of a study of micellar solubilization of individual PAHs by nonionic POEs from an MGP soil weathered for about 50 years. In particular, existinglinear partitioning theories are extended to develop a model to predict the partitioning of individual PAHs from multicomponent tar-contaminated soils into a micellar surfactant solution. Based on model predictions, estimates are made of the extent of PAH release from tarcontaminated soils at different soil:surfactant ratios. Micellar Solubilization of Soil-Bound PAHs in Multicomponent Systems. Surfactants are amphipathic molecules, and hence they are fairly soluble in water and are also capable of interacting with HOCs. Hydrophobic compounds partition into the micellar pseudophase of the surfactant, thus enhancing the aqueous solubility of HOCs. However, depending on its hydrophobicity, as measured by the octanol-water partition coefficient KO,,a HOC can exhibit an enhancement of its aqueous solubility in the presence of surfactant monomers. Kile and Chiou (11) likened such enhancement to partition-like interaction of the nonpolar content of a dilute surfactant solution with highly hydrophobic molecules. The solubility of HOCs in the micellar pseudophases is many times higher than that in the dilute monomeric surfactant solutions. The main objective of the present study is to develop a model to predict the micellar solubilization of individual PAHs from soils contaminated with coal tar, a multicomponent mixture. The transfer of a component iis envisioned to occur from tar to water and then to micelle. For convenience, the subscript “soilltar”is used to denote the tar-soil composite phase. Raoult’s law is assumed to describe partitioning of individual components between tar and surfactant solution. The tar-micelle partitioning coefficients are normalized for organic matter content of the MGP soil (fom = 0.75) to obtain soil/tar-micelle partitioning coefficients. A closed-system mass balance for an individual PAH i in such a soil mixed in a micellar surfactant solution can be written as:
VOL. 29, NO. 12, 1995 i ENVIRONMENTAL SCIENCE &TECHNOLOGY
3015
q'soi1itar.i
i
Smic
= 4soilitat.i 1 +
where psoilitar,i and qsoll/tar,iare the initial and equilibrium concentrations of i on the soil (mollg), Smic is the concentration of micelles in solution (g/L),W is the mass of soil per unit volume of solution (g/L),Ksoil/tar-aq,i is the soil/ tar-water (g/L) partitioning coefficient, and Ksoil/tar-mic,i is the soilltar-micelle (gig) partitioning coefficient for component i. Ksoil,tar.mic,f is a derived equilibrium constant. In eq 1,the enhancement of solubility due to the association of PAHs with surfactant monomers is ignored. In light of the operational definition of the cmc, it is likely that immature micellar forms with small aggregation numbers (dimer, trimer, etc.) may be present even below the cmc and that much of the enhanced solubility observed in this region, especially for very hydrophobic compounds, is due to partitioning into these micelles (1,11). At the surfactant dosages used in this work, at least several times the cmc, the contribution of surfactant monomers to solubility is deemed negligible. The defining characteristic of the contaminated soil used in this investigation is the high organic matter content (75% by mass). Given the origin of the vast majority of this organic matter, partitioning of individual compounds from the contaminated soil may be expected to be similar to that from coal tar; the mineral fraction of the soil is assumed to be inert. With these assumptions, the aqueous concentration of an individual PAH (Caq,Jin equilibrium with tar may be estimated by assuming that the coal tar mixture behaves ideally, that is, Raoult's law is valid (8, 9):
where C,,,,, is the concentration of i in the micellar pseudophase based on the total solution volume. C,,,,, can be expressed in terms of the micelle-water partitioning coefficient K, as defined by Jafvert et al. (1): Cmic,,
K, = -
(7)
C a q ,i s m i c
Combining eqs 2, 3 , 6, and 7 and converting KFol~.lnli , to units of gram per gram gives: (8)
where MWsurfis the molecular weight of the surfactant. At the relatively high surfactant concentrations used in this study, the mass of solute in the aqueous phase is negligible compared to that in the micellar pseudophase (Le.,Caq,rotal I Cmic,J.Under these conditions, the second term in eq 1 can be ignored. Rearranging, eq 1 becomes:
Using eq 1 and defining micelle loading (SL' 1 as the mass of micelle applied to unit mass of soil/tar, the fractional solubilization of i in a closed system at equilibrium with a given micelle concentration is given by: % solubilization =
(1 - qsoil/tar,i)
=
qOsoil/rar.i
where xct,!is the mole fraction of i in the tar phase and CIL is the aqueous solubility of the pure subcooled liquid i at the temperature of the solution. Since most pure PAHs are solids at ambient temperatures, CILcannot be measured directly. It can be obtained using the solubility of pure solid PAH (CIS) and the fugacity ratio of the pure compound in subcooled liquid and solid states: Cll- = ( f l / f s ) p u r e CIS (12). Further, qsoI~,, can be expressed in terms of the composition of the coal tar: (3)
where F,, is the mass fraction of coal tar in the soil and MW,, is the weight-average molecular weight of the coal tar. Using eqs 2 and 3, an expression for the soil/tarwater partitioning coefficient for i (K,,,~,t,r.a,,j)can be obtained: (4)
Lee et al. (9) proposed a similar relationship for the partitioning of PAH from coal tar into water: Ktar-water.1
-- 1
(5)
SL' (Ksoilitar-mic,;
+ SL')
x 100 (10)
where SL' = SL - (cmc/W) - qsurf.SL is the surfactant loading per unit mass of soil/tar (g/g) and qsurtis the mass of surfactant sorbed to soil/tar (g/g). In describing the partitioning behavior of individual components in a multicomponent system, it was assumed that the activity coefficients in the aqueous, coal tar, and micellar phases were independent of phase composition and that the total concentration of solutes in the micellar phase was small in comparison to the surfactant concentration (molar ratio of solute/surfactant= 0.03-0.06 in this study). It allows one to predict the solubilization of PAHs given the mass loadings of individual contaminants and gross coal tar on the soil, the coal tar molecular weight, the surfactant application rate, the surfactant properties (MW, cmc), the surfactant sorption isotherm, the aqueous solubility of pure subcooled PAH, and the micelle-water partitioning coefficient. The micelle-water partition coefficient K, bears a loglinear relationship with KO, , the octanol-water partition coefficient, for many HOCs of environmental interest ( 2 , 13). Typically, the slope of such a plot is close to unity. Therefore, K, in eq 8 can be replaced by K,,,,a known characteristic of the PAH:
C,LVm.r
where Vm,iis the molar volume of i. Similarly, Ksoll/tar.mlc,j is defined as: 3016
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1 2 . 1 9 9 5
const (11)
TABLE 1
Characteristics of Polyoxyethyliene Surfactants trade name Triton X-100 Brij 35 Tween 80 a
chemical POE(10) octylphenol POE(23) lauryl ether POE(20) sorbitan monooleate
cmc (M)
structurea CsPEio C12E23 CiaS&o
1.7 9.2 1.2
10-4 10-5 10-5
MW
HLBb
aggr no.c
625 1198 1310
13.5 16.9 15.0
100- 155 40 58
C represents alkyl chain length (CHI), P represents a phenol ring (CeHe), E represents an ethoxylate group (C2H10), and Ssrepresents a sorbitan
ring.
Hydrophile-lipophile
balance (percent mass of E/5)(23). cAverage number of surfactant molecules per micelle (24).
In the absence of any surfactant, eq 11 reduces to eq 4. In summary, the micellar solubilization of PAHs depends on their chemical characteristics (Kow and CL),those of the soil (F,,, MW,, , and MW,,,f), and the molecular weight of the surfactant.
Materials and Methods A coal tar-contaminated soil from an MGP site in NewYork
was used in this study. The soil was homogeneouslymixed using a bromide tracer. To ensure proper mixing, randomly collected samples of mixed soil were analyzed for bromide using a bromide electrode (Model 94-35, Orion Co.) (14). The fraction of particles in the size range of 74-500pm was used in this study. It had a bulk density of 1.72 g/cm3,an organic matter content of 75% = 0.751, and an organic carbon content of 62% (foe = 0.621, both determined by combustion. While HCl/HF pretreatment (15) was used for determining organic matter, no pretreatment was done for determining total organic carbon (Leco Model CR12 carbon analyzer). Three nonionic POEs (Triton X-100, Tween 80, and Brij 351, all obtained from Aldrich Chemical Co.,were used in the study. The characteristics of the these surfactants are listed in Table 1. 3H-Labeled Triton X-100 was obtained from DuPont Co.; reagent-grade PAHsphenanthrene, anthracene, pyrene, and benzo [alpyrene-were from Sigma Chemical Co.; phenylisocyanate was from Fluka Chemical Co.; and methylene chloride was from Baker Co. Extraction of PAHs from MGP Soil. The PAHs in the soil were determined by Soxhlet extraction (Soxtec HT 6, Itecator Co., Sweden) for 6 hwith boiling (110 "C) methylene chloride (MC) followed by rinsing for 1 h. Thirty milliliters of MC/g of soil was used for extraction. The remaining MC residue in the extract was evaporated by air purging, and the extract was reconstituted with 5 mL of fresh MC. No additional PAHs could be extracted by sequential Soxhlet extractions. One hundred microliters of the extract dissolved in MC was diluted with 10 mL of methanol (1:lOO dilution) and analyzed for PAHs using HPLC. A reversephase C-18 column (Vydac Co. Model 201 TP 5p) was used, and PAHs were measured by U V absorbance (Waters Co. Model 486 HPLC) at 254 nm. The solvent flow (2 mL/min) was initially 50% water and 50% acetonitrile for 2 min and varied linearly to 100% acetonitrile for the next 14 min. After pumping 100% acetonitrile for 8 min, the solvent flow returned to the initial condition for. 4 min. In most cases 2OpL of sample was injected by WISP autosampler (Waters Co. Model 712). By comparing their observed retention times with those of standard compounds (PAH calibration mixture, Restek Co.), it was possible to identlfy 16 PAHs in the extract (Table 2). While the soil had 75% (by mass) of organic matter, only 1.5%of this amount could be extracted with methylene chloride. The 16 identifiable PAHs accounted for only30% of the extracted mass. In other words,
worn
TABLE 2
Concentration of PAHs in MGP Soil
2219 p w n e naphthalene 2-bromonaphthalene 256 benzo[alanthracene 312 chrysene acenaphthylene 244 benzo[plfluoranthene acenaphthene 112 benzo[alpyrene fluorene 333 dibenz[a,hlanthracene phenanthrene 106 benzo[ghiJperylene anthracene 41 indeno[l,2,3-cdlpyrene fluoranthene -
170 67 143 41 43 26 34 32
the 16 PAHs constituted only 0.45% of the soil organic matter. Surfactant Solubilization of Reagent-GradePAH. Solubilization of four individual PAHs (phenanthrene, anthracene, pyrene, and benzo[alpyrene) in surfactant solutions were determined in single-component systems using reagent-grade PAHs as follows. Two grams of each PAH (0.5 g for anthracene) was dissolved in 20 mL of acetone. An appropriate volume (0.5 mL for anthracene and 0.2 mL for other PAHs) of the solution was added to a series of 8-mL vials such that the PAH in each vial was 2-3 times higher than the amount that could be dissolved by the highest concentration (30 g/L) of the surfactant tested. Having evaporated the acetone, 4 mL of surfactant solutions of various concentrations was added to the vials. Given that the amount of PAH remaining in the vial was significantly higher than what could be solubilized by the highest concentration of surfactant used, the loss of PAH due to evaporation was inconsequential. The vials were sealed with Teflon-coated septa and open-port screw caps and mechanically shaken for 5 days. Preliminary studies with phenanthrene and 6 g/L TritonX- 100 solution showed that equilibrium could be reached within 2-3 days. A portion of the sample was withdrawn into a 5-mL glass syringe and expressed through a 0.2 pm aluminum filter (Aerodisc 13, Whatman Co.) to remove undissolved PAH crystals. The soluble PAH concentration was determined by HPLC analysis of the filtrate. Surfactant Solubilization of Soil-Bound PAH. The method for the solubilization of PAHs with surfactant is described below. Surfactant solutions of various concentrations were prepared in 0.05% HgC12 solution using deionized (DI)water. Twenty five milliliters of surfactant solutions was added t o 2.5 g of MGP soil in 25-mL glass centrifuge tubes. The tubes were sealedwithTeflon-coated septa and shaken by an end-on-end shaker. The head space in the tube was only 1.0-1.5 mL. At given time intervals, soil suspensions were centrifuged at 8000g for 40 min, and an aliquot ofthe supernatant,,withdrawn into a 5-mL glass syringe and expressed through a 0.2-pm aluminum filter to remove fine particles. The filtrates from duplicate samples VOL. 29, NO. 12, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
3017
were analyzed for P W s by HPLC. To minimize PAH loss due to adsorption to the filter assembly, samples for HPLC analysis were collected after discarding the first 3-4 mL of the filtrate, especially from samples in the low surfactant concentration range ('0.2 g/L). A maximum PAH loss of 3-4% was observed (measured as total U V absorbance) when no surfactantwas added. With surfactant added ( 2 3 x cmc), losses were not significant. The temporal release of PAHs were estimated from aqueous PAHs concentrations. Surfactant Sorption to Soil. Batch experiments were conducted to obtain isotherms for the sorption of surfactants to the MGP soil. To determine sorption of Triton X- 100,test sorbate solutions were prepared by adding [3H]Triton X-100 (activity = 14 000-16 000 dpm/mL) to nonlabeled surfactant solutions. In 25-mL glass centrifuge tubes, 2.5 g of soil and 25 mL of surfactantsolutions (0.05% HgC12)were mechanically mixed for 2 weeks. At selected times, samples of the soil-surfactant solution were centrifuged at 8000gfor 40 min, and surfactant concentration in the supernatant was radiochemically measured using a liquid scintillation counter (Beckman Model LSSOOOTD). In this study, the equilibration time was operationally defined as the time when aqueous surfactant concentration in the reactor became relatively constant. Several methods were tested to measure Brij 35 and Tween 80 in solution. Organic compounds released from the soil during surfactant adsorption caused serious interferences in colorimetric measurements, the CTAS method (16)and the PPAS method (17 ) . Derivatization-HPLC method was used to analyze both surfactants using a CISreverse-phase column (18).A complexant, phenyl isocyanate, was used to produce a UVactive derivative upon reaction with the ethoxylate group. At concentrations higher than 0.6 glL, the method produced acceptable results (SD .C 0.04 for Brij 35). However, at low concentrations ( ~ 0 . g/L), 6 the accuracyofmeasurement was unacceptable, especially for Brij 35. To determine concentrations less than 0.6 g/L, surface tension measurements were used (Fisher Tensiomat Model 21). Prior to the measurement, the samples were appropriately diluted with 0.05%HgC12solution such that the find concentration was about one-tenth of the cmc. A log-linear standard calibration curve was prepared using standard solutions (19).
Results and Discussion Soil-Surfactant Partitioning of PAHs from MGP Soil. The equilibrium partitioning of PAHs between the soilltar and water was obtained in a short period of time, less than 3 h, as reported by Lee et al. (9). However, with surfactant solutions, equilibration took much longer. Figure 1shows the solubilization of phenanthrene by three nonionic surfactants. Release of phenanthrene increased with surfactant concentration, but longer time was required to reach equilibrium at higher concentration. At a dosage of 6 g/L, more than 250 h were required to obtain equilibrium. When the dosage was increased to 30 g/L, even after 380 h equilibrium was not attained. Seemingly,low difhsivities of constituent PAHs in the weathered coal tar matrix imposed mass transfer limitation on solubilization. Nonequilibrium effects become more pronounced as the fractional solubilization of PAHs increases because solubilized PAH must be accessed from deeper within the soill coal-tar matrix. Of the three surfactants, Tween 80 was the most efficient in solubilizing phenanthrene. Similar solubilization data for other PAHs were obtained. In 3018
E N V I R O N M E N T A L SCIENCE &TECHNOLOGY / VOL. 29, NO. 12. 1995
10
4
-1
2
o b 0
"
,e' 50
"
'
" "
100
"
150
"
'
" "
200
'e'
250
"
'
300
"
'
'
350
1 .
400
Time, hr
FIGURE 1. Solubilization of phenanthrene from MGP soil by surfactants.
general, solubilization of coal tar components in surfactant solution can occur by two mechanisms: emulsification and micelle solubilization. Emulsification of coal tar, though sparse in the literature, could result in a massive solubilization of coal tar either in the form of submicron particles (microemulsion) or larger particles (macroemulsion). In both cases, the coal tar is solubilized as a bulk phase in the emulsified particle. The extent of emulsification depends on the hydrophile-lipophile balances (HLB)of the coal tar and that of the surfactant. For best emulsification, the HLB numbers of the NAPL and the surfactant should be the same. In this study, emulsification of coal tar was never observed; ultracentrifugation of the supernatant of soilsurfactant solution did not reveal separation of an emulsion phase. The semisolid organic residue remaining on the MGP soil after the methylene chloride extraction may not be emulsified by any surfactant. Therefore, only micellar solubilization was assumed to contribute to enhanced aqueous solubilities of the soil-bound PAHs. Model Prediction of Micellar Solubilization. In applying the model, most thermodynamic data for the constituent PAHs were obtained from published sources or measured in the laboratory. Though the weight-average molecular weight of coal tar can be determined using methods such as vapor phase osmometry, an average molecular weight for the tar-like contaminants in a weathered MGP soil may be extremely difficult to measure. The molecular weight of coal tar has been reported to range from 230 to 1600 ( 7 ) . In many abandoned MGP sites, a large fraction of the lighter coal tar constituents had been removed by decades of weathering since the soils were originally contaminated. Most of the coal tar components in the MGP soil used in the present study could not be determined and were assumed to be insoluble polymerized hydrocarbons. Avalue of 1600 was selected as the average molecular weight of the tar residues in the MGP soil used in the present study. In analyzing the results, no attempt was made to optimize the molecular weight of the coal tar to obtain a better fit of the predictive equations to the data. At ambient temperatures, most pure PAHs exist as solids, but coal tar mixtures are liquid. Whether the coal tar coating
TABLE 3
Solubilization of PAHs from MGP Soil in Aqueous Suspension PAH napthalene acenaphthene fluorene phenanthrene anthracene pyrene benzo[alpyrene
128.18 154.20 166.23 178.24 178.24 202.26 252.32
2.42e-4 2.47e-5 1.19e-5 7.25e-6 3.93e-7 6.93e-7 1.51e-8
3.53 5.05 7.94 5.65 77.5 19.8 32.3
8.54e-4 1.25e-4 9.46e-5 4.10e-5 3.05e-5 1.37e-5 4.88e-7
2219 244 112 333 106 170 43
1.10e-5 5.75e-7 1.60e-7 1.44e-7 3.06e-8
3.16e-5 4.22e-7 1.44e-7 1.64e-7 3.88e-8 2.46e-8 1.78e-10
1.58 2.76 4.21 13.0 19.5
0.55 3.75 4.68 11.4 15.4 34.2 960
a Fugacity ratios (25). Detection limits: naphthalene (30pg/L), acenaphthene (50 pg/L), fluorene (6@g/L),phenanthrene (3 pg/L). anthracene (1.5 pg/L), pyrene (10 pg/L), and benzo[alpyrene (5 pg/L). Estimated using eq 2. Estimated using eq 4.
TABLE 4
Solubilization of PAHs from MGP Soil in Surfactant Solutions log Km(M-l)
Triton X-100
PAH
Triton X-100
Brij 35
Tween 80
phenanthrene anthracene pyrene benzo[alpyrene
4.188 4.173 4.780 5.996
4.280 4.261 4.790 6.059
4.503 4.412 5.044 6.238
a
expt 0.367 0.447 0.383 0.810
i 0.04 i 0.03 i 0.07 f 0.04
Brii 35
Tween 80
esta
expt
ests
expt
ests
0.410 0.645 0.347 0.638
0.436 f 0.06 0.587 f 0.09 0.490 f 0.10 1.014 f 0.24
0.721 1.011 0.681 1.007
0.339 i 0.03 0.507 f 0.03 0.372 f 0.05 0.817 f 0.17
0.407 0.777 0.396 0.765
Estimated using eq 8.
MGP soils should be considered a solid, liquid, or a mixture thereof cannot be determined by visual inspection. Regardless of the phases present, the aqueous solubility of PAH components in organic mixtures can be described by one of two limiting cases: Raoult's law (eq 2) or the aqueous solubility of the pure PAH solid (Cis) (20,21). The former applies to multicomponent single-phase liquids and interacting solid-phase mixtures; the latter applies to pure solid-phase crystals or noninteracting solid-phase mixtures. A comparison of experimentally measured values of Caq,I with predicted ones using the two limiting cases is given in Table 3 and in Figure 2. Clearly, Raoult's law adequately describes Caq,i of PAHs in the coal tar-contaminated soil used in the present study. These results are in agreement with those reported by Lane and Loehr (8). A comparison of experimental and predicted (eq 4)values Of Ksoil/tar-aq are shown in Table 3. A regression analysis of the experimental values of Ksoilitar.aq for the PAHs yielded the relationship: log Ksoilirar-aq = -0.83 log d - 3.13 using MWct = 1600 g mol-' and Fct = 0.75 for the untreated MGP soil. The corresponding relationship obtained using Raoult's law is logKsoilitar-aq= - l o g e - 3.30. ThevaluesofK, andKsoilitar. mic calculated from experimental data for pure PAH crystals and MGP soil, respectively, are reported for different surfactant-PAH combinations in Table 4. Values of Ksoilitar-mic predicted using eq 8 are also listed in Table 4. Though some of the predictedvalues ofKsoil/tar.mic fell outside the range of the experimental error, overall the predictions were satisfactory. It should be noted that Raoult's law may not describe the partitioning of organic solutes between soil organic matter and water (22). Solubilization of four PAHs from MGP soil using Triton X-100 is shown in Figure 3. The correspondence between experimental results and predicted values (eq 9) are satisfactory. Similar correspondence between predicted and observed solubilization was obtained for Brij 35 and Tween 80. Solubilization was over predicted for some data points and underpredicted for others. Therefore, it is
le-3
5 le-4 P
0 .-~
le-5
a
le-6 D *
.*
w"
le-7
1e-8 le4
2 Acenaphthene
I
B
0
le-4
/
E
le-9
Te-8
le-7
le-6
le-5
le-4
le-3
Measured Caq, M
FIGURE 2. Comparison of measured aqueous solubilities of PAHs from MGP soil with (A) Raoult's law estimates (eq 2) and with (6) solubilities of crystalline PAHs.
unlikely that a different value of MW,,would substantially improve the overall accuracy of the estimates. The sensitivity of eq 9 to MWctfor the solubilization of pyrene using Triton X-100 is shown in Figure 4. No single value of MWCtresults in accurate predictions of Caq,total over the range of the data, suggesting that the system may not have reached complete equilibrium during the 380-h experiment, especially at the higher surfactant dosages. Values of MW,, VOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCiENCE & TECHNOLOGY
3019
a=234CO4'
/ .
: 001
l
,
Triton X-I00 S0,l 100 giL
,&
m
-
Phenanthrene Anthracene Pyrene Benzo(a)pyrene
i
I
0 001
1
0 000
-
* Model Experiment (Eq 91
0 002
0 004
z
- -
3 In
Triton X-100 soil 100giL Equilibration Time
n _ c ..reo "-0 l , L
2.
-5
50 hr
A 6
-
\
01 0 01
_ A
0 006
-
i'
2
0 008
0 010
1
01
Surfactant in aqueous phase M
10
Surfactant in aqueous phase giL
FIGURE 3. Comparison of model prediction of micellar solubilities of PAHs from MGP soil with measured values.
FIGURE 5. Sorption of Triton X-100 on MGP soil. Surfactant loading on soil gig
10
7--
1
002
0 00
25
s-
,
5 +
,
,
006
004 ~
~
,
,
,
I
~
! A
Model Experiment
1
- -
Benzo(a)pyrene
008 ,
.
,
0 10 I
~
~
~
15L
-mN-
l
l
0
3 1OL
90 80
001 i
.-. 0 000
,
0 002
, ,
,
0 004
,
0 006
0 008
-
- B
-
0010
Surfactant in aqueous phase M
FlGURE4. Effect of molecular weight of coal tar on predicted aqueous solubility (eq 9) of pyrene.
in the range of 1200-2400 adequately describe the data. Estimating Release of PAHs by Surfactant Washing. Using eq 10, one could estimate the extent of release of specific PAH compounds from a contaminated soil. We present only the results of sorption of 3H-labeled Triton X-100 on MGP soil. The soil (100 g/L) was equilibrated with the various surfactant solutions for 50 h. Any surfactant would partition strongly into a soil with such high organic content as the one used in the present study. In a soil washing application, the amount of sorbed surfactant may be irretrievably lost and be of no benefit in mobilizing contaminants. A typical plot of surfactant loading, Q (mg/ g), on soil against the equilibrium aqueous concentration, Clq (g/L), is shown in Figure 5. The sorption process is described by two distinct linear regions, the sub-cmc and the supra-cmc, with the discontinuity occurring at about the cmc (-0.11 g/L) dosage of the surfactant. At cmc, sorption density is about 1.5 mg/g (2.4 x mol/g). At a supra-cmc dosage of 5 g/L, the sorption density is about 6 mg/g (9.6 x mol/g). Conceivably, soil-micelle equilibrium was not reached in 50 h, the duration of the 3020
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL 29, NO. 12, 1995
00
02
04
06
08
10
Surfactant loading on soil gig
FIGURE 6. Comparison of predicted solubilization of PAHs (eq 10) from MGP soil at various surfactant loadings.
batch equilibrium experiments. Most solubilization experiments were conducted at surfactant dosages of 1-6 g / L with some at dosages as high as 30 g/L. Clearly, the loss of surfactant due to sorption is inconsequential at these dosages. In Figure 6A, measured solubilization of four PAHs by washing with Triton X-100 is compared with values predicted by eq 10. The value of qsurfwasdetermined from Figure 5. At relatively low surfactant dosages, solubilization is observed to increase linearly with dosage, as predicted by eq 10. Congruence between the model and data is best at low surfactant dosages. As the removal of PAH increases, micelle-solubilized PAHs must be removed from deeper areas within the soil-tar matrix, and nonequilibrium effects
,
I
imposed by mass transfer limitations may become significant under these conditions. In Figure 6B, predicted solubilization of PAHs over a much wider dosage range of Triton X-100 is plotted. At 0.3 glg loading of surfactant on soil, the model overpredicts solubilization by awide margin. The most plausible explanation for such behavior is the nonequilibrium conditions that exist in the solubilization of PAHs at such high dosages. In fact, equilibrium conditions may never be achieved in weathered soils within a realistic treatment period. Only 55-70% removal of individual PAHs is predicted by the model at dosages as high as 1 g of surfactantlg of soil.
Acknowledgments This study was supported by the U.S. Environmental Protection Agency through Grant R819168010. The administrative support from Dr. Louis Swaby of the U.S. EPA Office of Exploratory Research is gratefully acknowledged. The authors thank Dr. Richard G. Luthy for his helpful comments.
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ES950161G @Abstractpublished in AduanceACSAbstracts, September 1,1995.
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