Environ. Sci. Technol. 1993, 27, 1340-1346
Sorption of Nonionic Organic Compounds in Soil-Water Systems Containing Petroleum Sulfonate-Oil Surfactants Shaobai Sun+ and Stephen A. Boyd‘
Department of Crop and Soil Sciences, Mlchigan State Unlversity, East Lansing, Michigan 48824 The effects of petroleum sulfonate-oil (PSO) surfactants (commercial petronates) on sorption of representative nonionic organic contaminants naphthalene, phenanthrene, and 2,2’,4,4‘,5,5‘-hexachlorobiphenyl(2,2’,4,4‘,5,5’PCB) in an Oshtemo (B) soil-water system are presented in this report. In the range of petronate equilibrium aqueous concentrations from 0 to 170 mg L-I, we have observed a slight increase of the soil-water distribution coefficient ( K )for naphthalene, a moderate decrease of K for phenanthrene, and a large (202-fold)decrease of K for 2,2’,4,4’,5,5’-PCB. Both aqueous and soil-sorbed PSO surfactant act as partition phases for NOCs. Solute partition coefficients between water and the PSO emulsions in the solution and soil-sorbed form, Kern and Ksem, respectively, were determined. The Kern values were consistently but only slightly larger (1.1-3 times) than Ksem,demonstrating that sorbed PSO is nearly as effective as aqueous-phase PSO emulsion as a partition phase for phenanthrene and 2,2’,4,4’,5,5’-PCB. The ratios of Kaem to KO, for phenanthrene and 2,2’,4,4’,5,5’-PCB were consistently about four, indicating that on a unit mass basis sorbed PSO is about four times more effective as a sorptive phase for these contaminants than natural soil organic matter. We have developed and evaluated a model that can predict accurately the apparent soil-water distribution coefficient of a nonionic organic compound a t different petronate concentrations, by knowing the intrinsic distribution coefficient of the solute in the surfactant-free system and the soil-water distribution of the surfactant itself. These results suggest the utility of petronate surfactants for substantially increasing the aqueous-phase concentrations of poorly water-soluble organic contaminants present in soils.
Introduction The apparent water solubilities of nonionic organic contaminants (NOCs) can be substantially increased in the presence of conventional nonionic (e.g.,Triton, Brij), cationic (e.g., cetyltrimethylammonium bromide), and anionic (e.g., sodium dodecyl sulfate) surfactants (1-3). The extent of solubility enhancement was observed to be much greater above the critical micelle concentration (CMC) of these surfactants than below it. Solubility enhancement was attributed to the partitioning of NOCs into surfactant micelles which behaved as a microscopic pseudosolvent phase that is compositionally similar to a bulk solvent phase (I). The solute solubility enhancement by micelle-forming surfactants was mathematically described as (1) S*/S = 1+ X,,K,, + X,&,, where, S* is the apparent solute solubility at the total
* Corresponding author. t
Current address: Departmentof Plant and Soil Science,Montana
State University, Bozeman, MT 59717. 1540
Envlron. Scl. Technol., Vol. 27, No. 7, 1993
stoichiometric surfactant concentration of X , S is the intrinsic solute solubility in pure water, X,, and X,, are the fractional concentrations of surfactant as monomers and micelles, respectively, and Km, and Km, are the surfactant monomer-water and surfactant micelle-water partition coefficients of the solute, respectively (I). The experimentally derived micelle-water partition coefficients (Kmc) were similar to the corresponding octanol-water partition coefficienta (Kow).Solubility enhancement below the CMC was greater for nonionic surfactants than for ionic Surfactants. In general, however, K m n values were at least 1.5-2 log units less than Kmc values, demonstrating that surfactant micelles were far more effective than surfactant monomers in enhancing solubility. The extent of solubility enhancement of a very poorly water-soluble compound(e.g.,p,p’-DDT,S= 5.5pgL-9wasmuchgreater than a relatively more water-soluble compound (e.g.,1,2,3trichlorobenzene, S = 18mg L-l). The apparent solubility of p,p’-DDT was increased from 5.5 bg L-I in the absence of surfactant to 1300 pg L-I at a Triton TX-100 concentration of 300 mg L-I. The water solubility enhancement of p,p’-DDT above the CMC was linearly related to the surfactant concentration (I). An alternative to conventional surfactants are the petroleum sulfonate-oil surfactants (PSO, or petronates as a commercial name), which also result in substantial solubility enhancement of NOCs (4). The petronates are mixtures of petroleum sulfonates (5)and free mineral oils. Properties that distinguish these surfactants from conventional homogeneous surfactants are that (a) they form stable emulsions instead of micelles in aqueous solution and (b) they do not have a distinct CMC. Importantly, they enhance the water solubility of NOCs in a linear manner proportional to the PSO concentration from nearly zero to hundreds of milligrams Liter1 (4). In this sense, they are distinct from conventional homogeneous surfactants which only manifest substantial solubility enhancement above their CMC. The linear solubility enhancement effects of petronates were expressed by Kile et al. (4) as
+
S*/S = 1 Xe,Ke,
(2) where S* and S are the apparent and intrinsic solute solubilities, respectively, X e m is the fractional PSO emulsion concentration on a water-free basis (Table I), and K,, is the emulsion-water partition coefficient of the solute. Generally, the Kernvalue of a PSO was found to be greater than or equal to the K,, value of conventional surfactants (I). The absence of a certain CMC and the linear solubility enhancement starting at nearly zero PSO concentration give petronates certain potential advantages over conventional surfactants, one of which is that petronates appear to be much more effective at low concentrations (sub-CMC, e.g.,lower than 100mg L-l) for enhancing solubility. In addition, because they do not form micelles, petronates may have greater biocompatibility with biodegradative bacteria and, hence, be useful 0013-936X/93/0927-1340$04.00/0
0 1993 American Chemlcal Society
-~
Table I. Composition and Properties of Commercial Petronates (Data from Witco Chemical Co., New York, NY) sulfonate, % water, % mineral oil, % inorganic salts, % molecular weight
Petronate L
Petronate HL
Petronate CR
61-63 4-5 33.0 0.5 415-430
61-63 4-5 32.5 0.5 440-470
61-63 4-5 32.5 0.5 490-510
in biorestoration schemes where low solubility may limit biodegradation of target NOCs. Deleterious micellemembrane interactions between the conventional surfactant micelles and the phenanthrene degrading bacteria have been reported recently (6),thwarting efforts to use surfactants to enhance the bioavailability and, hence, biodegradation of phenanthrene in soil. As such, petronates may be useful in the development of biorestoration technologies for contaminated soils. The purpose of this study was to quantitatively assess the effect of petronate surfactants on the soil-water distribution of several NOCs. Although the effect of petronates on contaminant solubility in surfactant-water systems has been established (41, a soil-surfactant-water system is more complicated than a surfactant-water system, based on the following considerations. (a) The solute will partition into the natural soil organic matter from water and exhibit an intrinsic distribution coefficient, K . (b) PSO itself may be sorbed by soil and establish an equilibrium distribution pattern. (c) The sorbed PSO may act as a microscopic partitioning phase and compete with the natural soil organic matter for the sorbed NOCs, analogous to the behavior of residual petroleum and PCB oils in soils (7, 8). (d) The PSO left in the bulk solution will form an emulsion that will enhance the solute solubility in the aqueous phase. These factors would change the intrinsic distribution of NOCs in soil-water systems with addition of the surfactant and, hence, render an apparent distribution coefficient, K*, whichmay besubstantially different from theintrinsic K. However, depending on the net effect of (c) and (d) described above, it is unclear a priori whether there will be an increase or decrease of the K value. The objective of this study was to quantitatively assess the sorption of NOCs in a soil-surfactant-water system. Our results show a slight increase in the soil-water distribution coefficient for naphthalene, a moderate decrease for phenanthrene (from 13.3 to approximately 61,and a dramatic decrease for 2,2’,4,4’,5,5-hexachlorobiphenyl (from 478 to approximately 2.5) in the presence of petronates a t an equilibrium aqueous concentration below 170 mg L-l. A method for predicting the apparent soil-water distribution coefficients of NOCs in the soilsurfactant-water systems, using only a few measured or estimated parameters, is presented.
Materials and Experimental Methods
CommercialPetronatesand Their Properties. The petronates used in this study were obtained from Witco Chemical Corp., New York, NY. The commercial petronates are mixtures which contain petroleum sulfonates (greater than 60%) and free mineral oils. Table I
Table 11. Properties of 2,2f,4,4f,5,5f-Hexachlorobiphenyl (2,2‘,4,df,5,5‘-PCB),Phenanthrene, and Naphthalene
s, structure
MW
mg/L
log
log
Kom Kow
phenanthrene
178.22 1.6’
3.48* 4.46‘
naphthalene
128.12 31.7’
2.5od 3.4SC
*
a Ref 15, S at 15 O C . Ref 16. Ref 17, S at 25 “C. Ref 18. e Ref 19, S at 25 “C.
summarizes the composition of Petronates L, HL, and CR, which differ primarily in the apparent molecular weight. Solutes. Naphthalene, phenanthrene, and 2,2’,4,4’,5,5’hexachlorobiphenyl(2,2’,4,4’,5,5’-PCB) were studied. They represent a series of decreasing water solubilities in the order listed here. Table I1 lists the chemical structures, molecular weights, water solubilities, log KO, (KO,,, is the soil organic matter normalized NOC sorption coefficient), and log KO,values of these three compounds. 14C-Labeled naphthalene, phenanthrene, and 2,2’,4,4’,5,5/-PCB(Sigma Chemical Co., St. Louis, MO),which had radiochemical purities of greater than 99%, were used. The Soil. The sorbent used in the study was an Oshtemo soil (B horizon), which contains 0.10% organic carbon (ca.0.17%soil organic matter), 89% sand, 5% silt, and 6 % clay. It was air-dried and sieved ( 0.99). This suggests that the uptake in this system involves predominantly a partitioning mechanism (11,13). When no petronate was added, the soil-water distribution coefficients ( K , corresponding to the slope of the linear isotherms) for naphthalene, phenanthrene, and 2,2’,4,4’,5,5’PCB were 1.50,13.3, and 478, respectively, in inverse order of their solubilities,as expected. As clearlyshown in Figure 2, the sorption of phenanthrene and especially2,2‘,4,4’,5,5’PCB is diminished substantially when petronates are added to the soil-water system. Table I11 lists the apparent soil-water distribution coefficients (K*)of naphthalene, phenanthrene, and 2,2’,4,4’,5,5’-PCB at different levels of added petronate. In a range of 0-200 mg L-1 of Petronate L added, the K* of naphthalene, which has the highest S (31.7 mg L-l) of
16
9 "
0.6
I
44
P
PCB at o mgn Petonate L
14
e
%'
12
10
g 6
f B
5
l5
z m
9
8 4
6
4 150
b
t
Naphthalene
100
1
0 0
0.6
0.3
0
0.9
Equll. Conc. of Phenanthrene in Solution, m@l
Table 111. Measured Apparent Soil-Water Distribution Coefficients (K*) of Naphthalene, Phenanthrene, and 2,2',4,4',5,S'-PCB at Different Petronate Equilibrium Concentrations petronate added, mgiL 10
Calculated Petronate Petronate L 0.00 6.40 Petronate HL 0.00 5.10 Petronate CR 0.00 5.03 K* Petronate L 1.50 Petronate L Petronate HL Petronate CR
13.3 13.3 13.3
PetronateL 478 PetronateHL 478 PetronateCR 478
20
40
80
120
160
200
Equilibrium Concentration, mg/L 14.03 30.23 64.14 99.03 134.48 170.29 11.75 26.36 57.77 90.58 124.19 158.33 11.53 25.79 56.49 88.61 121.55 155.05 of Naphthalene 2.01 1.54 1.73 2.08 2.07
K* of Phenanthrene 11.5 9.36 12.6 10.8 12.7 9.38 K* of 2,2',4,4',5,5'-PCB 28.3 15.1 8.01 4.93 27.7 13.9 7.50 4.96 23.5 13.8 9.20 5.06
40
60
80
100
120
140
160
180
Equil. Conc. of Patronate in Solution, mg.4
Flgure2. Typicalsorption isothermsofphenanthreneand2,2',4,4',5,5'FCB in the studied soil-water system with or without Petronate L added.
0
20
8.05 9.29 7.78
6.24 7.34 6.87
6.02 6.88 5.98
3.71 3.76 3.36
2.70 2.43 3.01
2.36 2.28 2.86
the three solutes tested, shows a slight increase. In the case of 2,2/,4,4',5,5'-PCB, which has the lowest S (0.001 mg L-l), a 202-fold decrease of the K* (Le., from 478 to 2.36) was observed. For phenanthrene, which has an intermediate S of 1.6 mg L-l, the K* was reduced by a factor of about 2 (ie.,from 13.3 to 6.02). The CMC requirement was not observed in these systems, which is in agreement with the conclusions of Kile et al. (4). That is, a substantial decrease in sorption coefficient was observed even at the lowest petronate equilibrium concentrations. This is one of the unique properties of the petronate surfactants that makes them potentially useful for environmental remediation technologies (e.g., increasing bioavailability). It is particularly noticeable for the 2,2',4,4/,5,5/-PCB case where K* dropped from 478 to 8.01 (a 60-fold decrease) when the equilibrium concentration of Petronate L in solution was only approximately 30 mg L-1. Figure 3 plots the measured K* values as a function of the equilibrium aqueous concentration of petronates (mg
Flgure 3. Apparent soli-water distribution COeffiCientS (K') of naphthalene, phenanthrene, and 2,2',4,4',5,5'-PCB in soli-Petronatewater systems.
L-1) in the system. The 2,2',4,4',5,5'-PCB K* values show a sharp initial decline at low petronate equilibrium concentrations followed by a more gradual decline at higher petronate equilibrium concentrations, whereas the phenanthrene K* values show a more linear decrease. For naphthalene, a slight increase is observed in the petronate concentration range tested. It is demonstrated clearly in Figure 3 that the change in K* values is a function of the intrinsic water solubility (S) of the solute. We attempted to develop a predictive model to describe the NOC sorptive processes involved in a soil-surfactantwater system. The conceptualization of this model is described as follows: (a) In the solution phase, the equilibrium concentration of a NOC is greatly affected by the PSO emulsion and can be described by eq 2, in which K,, is defined as the partition coefficient of NOCs between the emulsified PSO and water, Le., the NOC concentration in the PSO emulsion divided by the NOC concentration in water, at sorption equilibrium, and X e m is the fractional concentration of water-free PSO emulsion in solution (4). (b) The amount of sorbed PSO is approximately from 0.009 % to 0.1 % (relative to soil mass) while the added PSO ranges from 20 to 200 mg L-l. The sorbed PSO may function microscopically as an interactive (sorptive) phase for NOCs and will add a positive effect on the uptake of NOCs from water. This case resembles the PSO emulsion formed in the solution phase. Because it is expected that the sorbed NOC either is associated with the sorbed PSO or partitions into the natural soil organic matter, a variable, Xsem/om, can be defined as the mass of sorbed PSO divided by mass of the natural soil organic matter and serves as a relative measurement of the quantities of the two sorptive materials (ie.,sorbed Envlron. Sci. Technol., Vol. 27, No. 7, 1993 1343
PSO and the natural soil organic matter). A NOC distribution coefficient between sorbed PSO and water, K s e m , can be defined with analogy to Kern. Thus, Kern and K s e m are the NOC partition coefficients between the PSO emulsions, in solution and sorbed form, respectively, and water. (c) The positive contribution of sorbed PSO to uptake of NOCs by soil can be characterized using a sorbed PSO-soil organic matter partition coefficient, Ksemlom, which is defined as the sorbed NOC concentration associated with the sorbed PSO divided by the sorbed NOC concentration in the natural soil organic matter phase. Accordingly, because the sorbed PSO as a sorptive medium for NOCs is expected to be less (or at best equally) effective than the PSO emulsion in solution, K s e m / o m should be equal to Ksemdivided by Komand maximally equal to Kern divided by KO,:
by empirical equations. For instance, Jafvert (3)proposed the followingequation for dodecyl sulfate surfactant micelles: log K,, = 1.04 log KO,- 0.126
Although this equation was developed specifically for dodecyl sulfate surfactant systems and cannot be extrapolated to other surfactants,it does point to the general linearity of these relationships. A similar linear relationship between log Kern and log KO,for petronate systems can be developed using the data of Kile et al. (4): log Kern= 1.07 log KO,- 0.498
This empirical equation was used to estimate
1+ X s e m / o m K s e m / o m (5) 1+ X e m K e m
where C,* and C, are the solute equilibrium concentrations in the soil phase with and without PSO in the system, respectively, and Ce* and Ce are the solute equilibrium concentrations in the aqueous phase with and without PSO in the system, respectively. The numerator of eq 5 accounts for the effect of sorbed PSO, which attempts to increase the K* value, and the denominator accounts for the effect of PSO emulsion in solution, which attempts to decrease K* value. In a system where the sorption of PSOs onto the soil is weak enough to make X , e m / o m negligible or where Xsem/omKsem/om is much smaller than X e m K e m , eq 5 can be reduced to a simpler model, in which the effect of the sorbed PSO on K* has been neglected: K* = K/(1+ XemKem)
(6)
Unfortunately, the Kern, K s e m , and K s e m i o m values are not available for most of the environmentally relevant NOCs. However, the Kern and Ksem/om values can be estimated from earlier work on the effects of petronates on the apparent water solubility of NOCs (4). (a) Knowing the octanol-water partition coefficient (KO,)of the NOC, its micelle-water partition coefficient (Kmc)can be estimated 1344
Environ. Sci. Technoi., Vol. 27, No. 7, 1993
(8)
Kern values for phenanthrene and 2,2’,4,4’,5,5‘-
where Komis the partition coefficient of NOCs between the natural soil organic matter and water. (d) The apparent soil-water distribution coefficient of a NOC (K*) in a soil-PSO-water system should be determined by the intrinsic soil-water distribution coefficient ( K ) in the PSO-free system, as well as the overall effect of the PSO emulsion in solution (which attempts to decreasem and the soil-sorbed PSO (which attempts to increase K ) . Therefore, the apparent distribution coefficient of a given NOC in a soil-PSO-water system may be predicted by
K
(7)
PCB. (b) In this study, the measured K* values (Table I11 and Figure 3) can be analyzed by least-square curve-fitting, using either of the two models (eqs 5 or 6) to obtain the best fitted Kern (and KBemlom) values. Here, X e m and XSem/om can be evaluated from the initial or equilibrium concentrations of the PSO using the Freundlich coefficients (K and n in Figure l),which describe the sorption of PSO by soil. Thus, the curve-fitting can be done setting the equilibrium aqueous concentration of PSO as the independent variable and the apparent soil-water distribution coefficient (K*)of the solute as the dependent variable. Once Kmm/om is determined, KBem can be obtained using eq 4. The log Kern, log Ksem, and KSemlom values, both estimated and determined, are listed in Table IV. The log Kernvalues of 2,2’,4,4’,5,5’-PCB and phenanthrene determined experimentally (frommethod b above) are in good agreement with the values estimated using eq 8 which was developed from measurements of water solubility enhancement of NOCs by petronates in soil-free water-surfactant systems. This observation indicates that sorption of petronates by soil has not substantially altered the emulsion composition of the aqueous-phase petronate. The Ksem/om values for 2,2’,4,4’,5,5’-PCB and phenanthrene were also determined (Table IV) using method b. They were much smaller in magnitude than Kern and very similar for both phenanthrene and 2,2’,4,4’,5,5’-PCB. In fact, Kgemlom defined in eq 4 is a measurement of the relative affinities of the sorbed PSO phase and the natural soil organic matter phase for the sorbed solute; whereas Kernis a measurement of the relative affinities of the PSO emulsion phase and water for the solute in the aqueous phase. Water is apoor solvent phase for phenanthrene and 2,2’,4,4’,5,5’-PCB, but PSO emulsion provides an excellent microscopicpseudosolvent phase for these solutes. Therefore, the Kern value can be very large for many NOCs, inversely related to their water solubilities. The comparatively smaller values Of KBem/om indicate that both natural soil organic matter and sorbed PSOs function similarly as sorptive phases for NOCs. The measured values of Ksem/om (Table IV), which is the ratio of K s e m to KO,, for both phenanthrene and 2,2‘,4,4’,5,5’PCB were consistently about 4 (8 in one case). This suggests that, on a unit mass basis, sorbed PSO is about
1000
Table IV, Comparison of Solute Partition Coefficients with Aqueous-Phase and Soil-Sorbed Petronate Surfactants log Ken surfactant Petronate L Petronate HL Petronate CR Petronate L Petronate HL Petronate CR a
12
estimateda determinedb 2,2’,4,4’,5,5’-PCB 6.70 6.48 (6.37) 6.70 6.62 (6.48) 6.70 6.79 (6.54) Phenanthrene 4.28 4.42 (3.85) 4.28 4.56 (3.72) 4.28 4.58 (3.86)
6.45 6.42 6.75
4.07 3.82 8.13
1.07 1.58 1.10
4.03 4.12 4.04
3.58 4.33 3.66
2.45 2.75 3.47
Estimated using eq 8. * log Ken was determined using both eq
5 and ea 6 (in Darentheses). Ksen/Konis equal to Kssmlam (eq 4).
four times more effective as a sorptive phase for NOCs than natural soil organic matter. The similarity of Ksem/om for phenanthrene and 2,2’,4,4’,5,5-PCB may be explained by the fact that both K e m and Kom are linearly related to KO, (3,22-14).Thus, although KO,and Komthemselves may change substantially from compound to compound, Ksem/om which is equal to or smaller than the ratio of Kern to KO, (eq 4) would fall in a narrow range. The measured values of Ks,m/om (Table IV) and eq 4 were used to calculate Ksem,which is the partition coefficient of the NOC between the soil-sorbed PSO emulsion phase and water (Table IV). A comparison of K e m and Ksem shows that Kern is consistently but only slightly (1-3 times) larger than Ksem.This demonstrates that sorbed PSO is nearly as effective as aqueous-phase PSO emulsion as a partition phase for phenanthrene and 2,2’,4,4’,5,5‘-PCB. Figure 4 attempts to evaluate the two models (eqs 5 and 6) for their capability to predict soil-water distribution coefficient of NOCs in soil-PSO-water systems. In Figure 4, the predicted curves are compared with the measured K* values of phenanthrene and 2,2’,4,4’,5,5’-PCB in the soil-petronate-water systems. The curves predicted by eq5 use the Ken and Ksem/omvaluesobtained by best fitting eq 5 itself, and the curves predicted by eq 6 use the Kern obtained by best fitting eq 6 itself(Tab1e IV). Thus, Figure 4 demonstrates how well the two models fit the actually measured K* data, i.e., how well they may predict K* values. Generally, both models give satisfactory prediction curves. For phenanthrene, the two models make nearly identical predictions of K* (the norms, Le., the square roots of the sum of squared residues, are 0.83 and 0.87, respectively). It is noticeable that at very low PSO equilibrium concentration in solution (Le., below about 10 mg L-l), eq 5 predicted a peculiar increase of K* of phenanthrene, because Xsem/omKsem/om is greater than XemKem in this PSO concentration range. However, we did not obtain the data needed to confirm this. In the case of 2,2’,4,4‘,5,5/-PCB, eq 5 appears to make a more accurate prediction of K* (the norm is 0.50) than eq 6 does (the norm is 3.731, especially at higher PSO equilibrium concentration in solution (Le., above 20 mg L-I). This results because eq 6 neglects the role of sorbed PSO (Le., the Xsem/omKsem/om term in eq 5), which functions to increase K*. To test the predictive capability of eq 5, we can generate a theoretical curve for naphthalene in the soil-Petronate L-water system as follows. Here, the log Kern value of
4-c
0
‘,,.\
,
0
. *....
4~, -
I
I
20
60 80 100 120 140 Equil. Conc. of Pefronate L in Solution, mgll 40
160
180
Flgure 4. Comparlson of predicted K’ curves of phenanthrene and 2,2‘,4,4’,5,5‘-PCB uslng eqs 5 (solid Ilnes) and 6 (dotted lines) with actually measured K* values (squares = phenanthrene, triangles = 2,2‘,4,4’,5,5‘-PCB) inthe soil-Petronate L-water system. The K’ values of 2,2’,4,4‘,5,5’-PCB are plotted in a log scale to show the details. Predlcted K* curve (dashed line) and actually measured K’ values (circles)of naphthalene in the soil-Petronate L-water system are plotted as a test of the model (eq 5).
naphthalene is estimated from eq 8 to be 3.20. Because both naphthalene and phenanthrene are polycyclic aromatic hydrocarbon (PAH) compounds, the Ksem/om value of naphthalene is estimated to be the measured Ksemlom value for phenanthrene in the soil-Petronates L-water system (Table IV), Le., 3.58. The predicted curve is plotted in Figure 4, as well as the actually measured apparent soil-water distribution coefficients of naphthalene (Table 111)in this system. Although the predicted values appear to be about 30%greater than the measured ones, the model works well here and also predicts a slight increase of K* that was observed experimentally in the PSO concentration range studied. In fact, the method b described previously would give a best-fit K,, of 3.70 for naphthalene in this system. In this specific case, eq 6, which has a numerator equal to 1 would fail to predict the slight increase of the K* values. In conclusion, these two models (ie., eqs 5 and 6) may be used to predict the apparent soil-water distribution coefficients of NOCs in soil-PSO-water system, by knowing the intrinsic K value of the solute, the soil-water distribution of the PSO, and the Kem (and Ksem/om) of the solute. The intrinsic K can be estimated using the KO, value of the solute and the soil organic matter content. The Kern value can be estimated from KO,, and Kse,/om can either be estimated from Kern and Kom (eq 4) or obtained from experimental data for a particular soilsurfactant-water system suchas thosepresented here (Le., for soil-PSO-water systems, Ksem/om = 4.0). The surfactant equilibrium concentrations in soil and water are obtained from its sorption isotherms. Thus, by simply measuring the surfactant sorption isotherm for a particular soil, accurate predictions of the soil-water distribution coefficient of a NOC at different surfactant concentrations can be obtained. Environ. Scl. Technol., Vol. 27, No. 7, 1993 1345
Acknowledgments This research was supported by the U.S.Environmental Protection Agency Grant R815597,the Institute of Environmental Toxicology of Michigan State University,and the Michigan Agricultural Experiment Station.
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