SURFACTANT-ENHANCED SOLUBILITY AND MOBILITY OF POLYCYCLIC AROMATIC HYDROCARBONS IN SOIL
X. Sun and R.K. Puri
Environmental Trace Substances Laboratory, University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409-0530, Phone: 573-341-6613, FAX: 573-341-6605
The role of some selected nonionic, anionic and cationic surfactants was investigated in solubilizing and mobilizing polycyclic aromatic hydrocarbons (PAHs) from soil. The data from the batch experiment showed that Brij 30 (a nonionic surfactant) started transporting the PAHs from soil to water at concentrations well below its apparent critical micelle concentration (ACMC). At its high concentrations, however, Brij 30 transported more PAHs to the aqueous phase. Thus, it showed a great potential in remediation of PAH-contaminated soils. The tested anionic and cationic surfactants did not show the solubilization effect until the concentrations reached their ACMCs. The experiment showed that the decomposition of the surfactants was more significant than that of the PAHs with the passage of time. A considerable portion of the solubilized PAHs was either re-adsorbed by the soil particles or was hanging in the mobile phase after 170 days, depending on the nature and concentration of the individual surfactants. The data showed that the solubilized portion of the PAHs became more persistent in the soil-water system, and its transport is proportional to the concentration and nature of the surfactants studied.
Keywords: polycyclic aromatic hydrocarbons, surfactants, fate and transport, mobilization, solubilization
Toxic chemicals once released into the environment become a pervasive environmental problem. Remediation of these chemicals has proven to be a very difficult task. Many government agencies and private organizations are investing significant financial resources into the development of new technologies for their remediation.
Most recently, one of the promising technologies is the use of surfactants to mobilize and solubilize hydrophobic xenobiotics from the surface or subsurface contamination.
Previous studies in our laboratory have shown that surfactants influenced the transport of dioxin in the soil (Puri, et al., 1989). Later on, we also observed that surfactants enhanced the mobility of organophosphates in soil (Sun, et al.,1994).
A study designed specifically to evaluate the solubility and mobility of polycyclic aromatic hydrocarbons (PAHs) in the soil, under the influence of ionic and nonionic surfactants, is currently underway in our laboratory and comprises the present paper.
Polycyclic aromatic hydrocarbons are among the most notorious environmental pollutants. They are hydrophobic and readily sorbed onto soil. Their toxicity varies depending upon their chemical structure. High molecular-weight PAHs are more persistent and toxic. Very little data are available on the fate and transport of these compounds, particularly the compounds with more than three to four benzene rings. So, it is considered desirable to study the fate and transport of the PAHs under the influence of surfactants.
A survey of the literature revealed that studies on hydrophobic organic compounds including PAHs (Kile and Chiou, 1989; Edwards, et al., 1991) were conducted to observe the role of surfactants in remediation of contaminated soils. Consequently, these studies were focused on high concentrations of the surfactants (more than ten times higher than their critical micelle concentrations, CMCs) to enhance the mobility of hydrophobic organic compounds from the soils. Obviously, the large amount of surfactant residue left in the soil causes problems for post-treatment.
So, a systematic study was designed to investigate the effect of various surfactants in a wide concentration range on the solubility and mobility of a large number of polycyclic aromatic hydrocarbons in soils. The project is comprised of the following:
Determining the apparent critical micelle concentration (ACMC) of the selected surfactants under the experimental conditions.
Investigating solubilization effect of the surfactants and correlating the solubilization effect and persistence data to the molecular weight of the PAHs.
Monitoring the fate of the PAHs and surfactants in the soil-water system with the passage of time.
Materials and Methods
The soil was collected locally from Columbia, Missouri. It was air dried and passed through a 60-mesh sieve prior to use. The soil was characterized as follows by the Soil Testing Laboratory, University of Missouri-Columbia: 2% organic matter, 2% sand, 62% silt, 34% clay, and pH 5.9. The surfactants were purchased from Aldrich Chemical Company, Inc. and used without further purification. All of the solvents were OPTIMA grade of Fisher Scientific.
Chromatographic analysis of the PAHs was carried out on a GC/MS system - HP 5972/MSD hooked up to GC 5890, equipped with a DB-5, 30 m, 0.25 mm ID capillary column. Surface tension was measured with a Cenco-DuNoüy interfacial tensiometer. The concentration of sodium dodecylsulfate was measured spectrophotometrically by following the EPA method using a Bausch & Lomb Spectronic 20 spectrophotometer.
Measurement of Critical Micelle Concentration from Water Solutions and Soil Suspensions
The critical micelle concentration values in water solution for most commercial surfactants are available in the literature (Mukerjee and Mysels, 1971), and the results from an individual laboratory may vary slightly depending upon the method and conditions of the laboratory. In a soil-water system, the apparent critical micelle concentration (ACMC) of the surfactants in the soil suspension was no longer the same as CMC. Thus, the ACMC was determined in our laboratory which reflects the behavior of a surfactant more closely with the presence of soil. The ACMC values in this experiment were measured by the surface tension method.
A series of solutions with concentrations ranging from below to above the literature CMC value was made for each of the three surfactants. 30 ml of each solution were agitated with 1.00 g of soil in a 40-ml centrifuge tube. The tubes were shaken continuously for 24 hours and then centrifuged for 30 minutes at 15,000 rpm. Surface tension was measured from each of the centrifuge supernatants. The ACMC was determined from the plot of surface tension vs. the initial surfactant concentration. Surface tension from pure deionized water solutions of each surfactant was measured to determine the true CMC values. A control experiment was also conducted.
Surfactant Solubilization of Soil-Sorbed PAHs
The experiment was designed to correlate the response of desorbed PAH to surfactant concentrations. Surfactant solutions were made with concentrations lower and higher than the predetermined ACMC. 30 ml of each solution were pipetted into a 40-ml centrifuge tube, which contained 1.00 g of PAH spiked soil. The tubes were shaken on a reciprocating shaker for 24 hours to allow the PAHs to reach equilibrium between soil and water, and then centrifuged at 15,000 rpm for 30 minutes. An aliquot of 25.0 ml of each supernatant was transferred to a separatory flask for liquid-liquid extraction. The extracts were cleaned and analyzed for the PAHs.
Effect of Surfactants vs. Time
One solution of each surfactant was made with the concentration twice as high as its ACMC. 360 ml of this solution was agitated with 12 g of soil spiked with a mixture of the PAHs in a 500-ml amber bottle. The bottles were set on a shaker for continuous agitation. An aliquot of 20.0 ml of the aqueous phase was sampled for analysis of the PAHs at desired time intervals. The experiment lasted six months. The analyses of surfactants from the aqueous phase were carried out simultaneously.
Analysis of the PAHs from Soil Suspension and Soil
The analysis of the PAHs was carried out according to the USEPA Test Method 8270. A high speed centrifuge (15,000 rpm) was used to separate the suspended fine soil particles from the water media. The resulting supernatant (essentially a surfactant water solution) was thoroughly clear. One PAH blank, surfactant blank, and duplicate sample were processed with each batch of the samples for quality assurance/control purposes. The soil samples were extracted with organic solvent by the shaking method. The extracts from the supernatant and soil were cleaned by Florisil and silica gel columns. The final analysis was carried out on a high resolution GC/low resolution MS system.
RESULTS AND DISCUSSION
Surfactant molecules exist as amphiphilic monomers in a dilute solution. These monomers reduce the interfacial tension by linking the two surfaces of the different media. The interfacial tension decreases proportionally with the increase of the monomer concentration in a dilute solution. When all the interfaces are covered by a monolayer of the surfactant and the solution is saturated with the monomers, the excess surfactant molecules start to aggregate and form a pseudophase with micellae. This saturation concentration is defined as critical micelle concentration (CMC). The CMC is a characteristic constant of a surfactant, and it marks the beginning of solubilization by trapping the hydrophobic molecules of a solute in the cores of the micellae. Therefore, the CMC has been frequently used as a reference point in measuring the surfactant effect on mass transport of xenobiotic. It is, therefore, necessary to determine the CMC of the surfactants with the specific soil-water combination.
A conventional plot of surface tension vs. surfactant concentration for Brij 30 in deionized water and soil suspension is shown in Figure 1. The critical micelle concentration is determined from the sharp turning point of the curves. The figure reads the CMC value as 18 mg/L in the deionized water solution, which is equal to the literature values at 25 mC with reasonable variation (Mukerjee and Mysels, 1971). The measured CMC from soil suspension is termed as apparent critical micelle concentration (ACMC) since it is only the initial dose (not the actual concentration) of the surfactant. Apparently, the ACMC depicts the behavior of a surfactant more closely in a soil-water system. The ACMC value determined from Figure 1 is 460 mg/L.
The value of the ACMC for Brij 30 is more than one order of magnitude higher than that of the CMC. There are several factors, such as soil sorption, solubilized organic mater, ion strength, and suspended soil particles, that could cause the CMC to increase in soil suspension. One of the major factors is sorption of surfactant molecules onto the great surface of the soil particles. Our observations were similar to those published by several authors (Liu, et al., 1992; DiToro, et al., 1990; Urano, et al., 1984; Valoras, et al., 1969; Yediler, et al., 1991) who have conducted studies to evaluate sorption of surfactant onto soil and sediment. Jafvert and Heath (Jafvert and Heath, 1991)) found that the predominant mechanism of surfactant loss to most natural soils and sediments was precipitation of dodecylsulfate with solubilized calcium ions. Microbial activities are also responsible for the loss of the surfactant from the aqueous phase. However, the biodegradation in this case should be minor due to the short equilibrium time. The soil was not autoclaved to limit or exclude the microbial activities because the object of this study was to simulate the actual conditions in the environment as much as possible. Apparently, the soil/water ratio also affects the actual ACMC value for a specific system. The ratio of 1:30 (g/ml) has been maintained throughout this entire study to avoid inconsistence.
Similar ACMC measurements were also carried out for sodium dodecylsulfate and dodecyltrimethylammonium bromide. The results, together with the literature values for comparison, are presented in Table 1. The differences between the ACMC and CMC for these two surfactants are less significant than that for Brij 30. In these cases, the loss of the surfactant due to soil sorption or calcium precipitation is minor owing to their high CMC values.
The transport of the PAHs from soil to water was studied with surfactant concentrations lower and higher than the ACMC. Figure 2 displays the solubilization results of Brij 30 and the data of the curves at 100 and 500 mg/L of the surfactant are tabulated in Table 2. The figure shows a steady increase of all PAHs in the aqueous phase over a wide concentration range of the surfactant. For most of the PAHs, 40% of the totally spiked amount was solubilized at about 2000 mg/L of the surfactant. The upright curves can be reasonably extrapolated to a higher percentage of the solubilized PAHs with a higher surfactant concentration. The efficient solubilization indicates a great potential of the surfactant in remediation of PAH-contaminated soil. Also present in the graph is the linear fit of the experimental data of surface tension measurement from the same solutions to indicate the value of the ACMC. The curves in Figure 2 show the solubilization effect well before the ACMC of the nonionic surfactant. A considerable amount of every compound (about 10% for the majority) is transported from soil to water in the sub-ACMC range of the surfactant concentration.
The enhancement factor in Table 2, defined as the ratio of the solubilized PAH concentration at 500 mg/L over that at 100 mg/L of the surfactant, ranges from 5 to 47, with an increasing tendency toward the compounds with high molecular weight. The difference of the factor values between small and large molecules of the PAHs is not caused by the selective solubilization of the surfactant, but by the initial dissolvability of the compounds. In the Brij 30 solution of 500 mg/L, where the surfactant has taken effect, the solubilized amount (in percentage) of all the compounds are confined in a narrow range. The micellar effect of the surfactant has no obvious discrimination against individual PAHs. However, in the solution of 100 mg/L Brij 30, where the surfactant has no effect yet, the solubility of the PAHs varies greatly depending on their molecular structure. For instance, the initial concentration of naphthalene is about five times higher than that of benzo(ghi)perylene.
Results of a similar experiment for sodium dodecylsulfate (SDS, anionic surfactant) and dodecyltrimethylammonium bromide (DTAB, cationic surfactant) are shown in Figures 3 and 4, respectively. The curves in the two graphs share the same pattern: linear and slope close to zero before the ACMC, greatest slope at the ACMC and sigmoid after the ACMC. Unlike Brij 30, the anionic and cationic surfactants essentially have no solubilization effect until the concentration is high enough to form micelle in the soil suspension. The sharp turning of the curves at the ACMC indicates that the micellar pseudophase extracts the PAHs from the soil very efficiently. To recapitulate this pivotal point, the aqueous PAHs concentrations in the narrow supra-ACMC range of the surfactants (1800-2700 mg/L for SDS and 6000-9000 mg/L for DTAB) are calculated and presented in Table 3.
In the case of SDS, the solubility enhancement factor (SEF)-concentration of the PAHs at supra-ACMC divided by the concentration of the PAHs at ACMC-hardly shows any relationship with the molecular weight of the compounds. The average value of the SEF is 35 when the absolute concentration of SDS increases by 900 mg/L. Note that the curves in this concentration range (from about 2000 to 3000 mg/L) have the greatest slope. In another words, the PAHs are most responsive to the surfactant concentration in this range. The SEF would be much smaller if the concentration increases, for example, from 1000 to 2000 mg/L.
The cationic surfactant shows greater SEF values than SDS in Table 3. The absolute increase of the surfactant concentration is also greater (comparing 3000 mg/L for DTAB with 900 mg/L for SDS). The factor values in the last column of Table 3 show an equivocal increase trend toward the high molecular weight PAHs. As discussed in the case of Brij 30, the tendency again is caused by the difference of the initial dissolvability and soil sorption among the compounds other than the surfactant solubilization effect. In fact, the two surfactants have very similar effect in transporting a PAH from soil to water at concentrations of 1.5 times higher than their ACMCs. The table shows that the concentrations of a PAH compound in the solution of 2700 mg/L sodium dodecylsulfate are almost the same as in that of 9000 mg/L dodecyltrimethylammonium bromide.
The sigmoid shape of the curves in the supra-ACMC portion in Figures 3 and 4 indicates the emergence of a maximum surfactant-assisted solubility of the PAHs in the experimental range of the surfactant concentration. The most affected compounds, naphthalene and acenaphthylene, show (in Figure 3) about 85% of the total adsorbed amount being transported from soil to water. The majority of the compounds are below 70%. Based on the consideration of the high concentration and toxicity, it is concluded that sodium dodecylsulfate and dodecyltrimethylammonium bromide are not suitable for remediation purpose.
The micellar solutions of all three tested surfactants have shown significant effect on transporting the PAHs from soil to water. The PAH molecules that merged into the micellar pseudophase become mobile with water. The chance for the compounds to spread with rainfalls and contaminate the groundwater increases greatly. How far these chemicals can travel with water is determined by their chemical stability and their resident time in the mobile phase. Thus, a profile of the solubilized PAHs in the aqueous phase as a function of time is necessary to understand the mobility of the PAHs with the surfactant. Figure 5 shows such a profile for the nonionic surfactant Brij 30. The surfactant solution chosen for this experiment is twice as concentrated as its ACMC.
In the solution of 1000 mg/L Brij 30, shown in Figure 5, the aqueous PAH concentrations decrease rapidly at the beginning and slow down after about 40 days. Most of the compounds reduce to half in 30 days. In 170 days, the concentrations of benzo(a)pyrene, dibenz(ah)anthracene, and benzo(ghi)perylene fall below the detection limit of the analytical method. The aqueous concentrations of the PAHs at the first and last days of this experiment, as well as the decline factor, are listed in Table 4. The decline factor in the last column of the table, defined as the lost portion of the PAHs from the aqueous phase in percentage, indicates that 85 to 100 percent of the dissolved compounds are gone from the aqueous phase in 170 days.
What was the fate of the PAHs in the aqueous phase? Were they decomposed? Or were the surfactants degraded and the PAHs simply returned to the soil? Two experiments were designed to clarify these fundamental questions. One was to measure the surface tension of the soil suspensions to track the effective concentrations of the surfactants. Another was to determine the total amount of the PAHs remaining in the system at the end of the experiment. However, the causes for the degradation of the PAHs and surfactants, which was beyond the scope of this study, will not be identified by these experiments and, therefore, will not be discussed.
The surface tension data measured from the Brij 30 solutions over the time period of 170 days are shown in Figure 6. The figure is presented in the same manner as Figure 1 for CMC determination. Note that the dose of Brij 30 in the graph is the initial concentration of the surfactant. The actual concentration at that time is unknown, so these curves are not linear in nature and cannot be used to determine the ACMC. The surface tension of each solution increases with time and approaches the value of the pure deionized water. The values of surface tension are normally inappropriate to quantitate the surfactant concentration, but in this case they should at least indicate the relative concentrations of the surfactant at different times. For instance, the solution with the initial concentration of the ACMC (400 mg/L) lost approximately all the surfactant monomers by the 70th day, as indicated by the surface tension value of pure water at the time.
The experiment with the dose of 1000 mg/L of Brij 30 is of most concern, since the persistence data for the surfactant (Figure 5) were measured directly from this solution. There was no surface tension increase within 40 days for this solution (Figure 6). The constant surface tension does not mean that the solution was stable over this time period. It is a micellar solution and the surface tension is basically not responsive to the surfactant concentration in the supra-ACMC region. The stable surface tension does suggest, however, that the concentration of this solution sustains higher than the ACMC in the initial 40 days. We can further conclude that the effective concentration of the surfactant in this micellar solution decreases from the original 1000 mg/L to about 500 mg/L (the ACMC value) in this time period. Figure 2 has shown that most of the PAH solubility curves have the greatest slope in this range (from 500 to 1000 mg/L), which indicates that the aqueous PAHs should decrease dramatically with the decay of the surfactant. The rapid decrease of the aqueous PAH concentration in a short time (40 days) explains the steep portion of the curves in Figure 5 at the beginning of the experiment.
In the rest of the period, the surface tension of the 1000 mg/L solution increases from the CMC value to about 35 dyne/cm. It clearly indicates a sub-CMC system in this time period. As shown in Figure 2, the concentration of the dissolved PAHs should decrease moderately with the decrease of surfactant concentration compared with the supra-CMC period. Moreover, the time it takes to reduce the surface tension (over four months) is much longer than the supra-CMC period. This explains why Figure 5 has a smaller slope for all the curves from 40 to 170 days.
The surface tension data have correlated the loss of the PAHs from the aqueous phase to the decay of surfactant micellae and/or monomers. If this is true, the lost portion of the PAHs must have returned to the soil. To confirm this and to examine the significance of the degradation of the PAHs, the total amount of the PAHs in soil and water were analyzed.
The PAH recoveries from both soil and the Brij 30 solution at the 170th day are shown in Table 5. It is interesting to note that the recovered amount of the PAHs increases with the molecular weight of the compounds. The concentration of dibenzo(ah)anthracene (MW=278) is three times more of acenaphthylene (MW=152) remained in the system. It indicates that under the experimental conditions, the high molecular weight PAHs are significantly more persistent in the system than the low molecular weight ones. Almost half of the spiked amount of the high molecular weight PAHs can still be recovered from the soil-water system after 170 days.
With the recovery data in hand, we are now able to determine the causes for losing the PAHs from the aqueous phase. The data in the last column of Table 5 show the unrecovered amount of each PAH, which is lost presumably due to degradation. The data in the last columns of Tables 4 and 5 indicate that the loss of the low molecular weight PAHs from the aqueous phase is predominantly caused by the decomposition of the compounds themselves. For instance, 85% of acenaphthylene disappeared from the aqueous phase (in Table 4) while 85% of the spiked amount is found decomposed at the end of 170 days (in Table 5). The influence of the surfactant decay to these light PAHs is very minor, if any. The data, however, display a different scenario for the high molecular weight PAHs. The decay of the surfactants becomes responsible to the loss of the PAHs from the aqueous phase. The solubilized portion of the PAHs disappeared from the aqueous phase by almost 100% (in Table 4), but they are decomposed by less than 60% (in Table 5). Therefore, significant amounts of these compounds returned from water to soil due to the decay of the surfactant.
The similar persistence experiments are also carried out for the anionic and cationic surfactants. The experiments with both surfactants, sodium dodecylsulfate and dodecyltrimethylammonium bromide, show a moderate loss of the solubilized PAHs over the experiment period (Figures 7 and 8). The surface tension data indicate that the aqueous surfactant concentration of these two solutions with initial 3000 mg/L of SDS and 10000 mg/L DTAB never fell below their ACMCs in six months. Therefore, the loss of the PAHs from the aqueous systems is mainly due to the decomposition of the compounds. The surfactant decay plays a minor role in this case.
It is observed by comparing Figures 5, 7, and 8 that the amount of the PAHs lost from the aqueous phases is not the same for the three surfactant systems. The recovery data of the PAHs at the end of the experiment show the same result for the total PAHs. Moreover, it is also observed that the loss of the PAHs is relevant to the surfactant solubilization effect: the more the PAHs are solubilized, the less they are lost. For instance, the 10000 mg/L DTAB solution has the strongest solubilization effect (compared with the solutions of 1000 mg/L Brij 30 and 3000 mg/L SDS), and it holds the compounds in the aqueous phase the longest. The decomposition of the PAHs from this cationic surfactant system is the least. It is most likely that the surfactant micellae provide sheltering for the solubilized PAH molecules. The accessibility of microorganisms and UV radiations to the compounds, therefore, is reduced. The longer the compounds stay in the micellar phase, the less they degrade. The higher recovery of the total PAHs from the DTAB solutions is an evidence.
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Table 1. Critical micelle concentrations in water (CMC) and soil suspension (ACMC) from literature and experiments.
|Literature Values||Experimental Data|
|Surfactant Name||Chemical Formula||CMC
Table 2. Solubilized PAHs (in percentage of the total spiked amount) in Brij 30 solutions.
|Name of PAHs||Solubilized PAHs (%)||Enhancement Factorb|
|CBrij 30=100 mg/L||CBrij 30=500 mg/La|
a Approximate ACMC value of Brij 30.
b The enhancement factor is equal to the ratio of the solubilized PAH concentration at 500 mg/L over
that at 100 mg/L of the surfactant.
Table 3. Concentrations of solubilized PAHs in solutions of anionic and cationic surfactants.
|Sodium Dodecylsulfate (mg/L)||Dodecyltrimethylammonium
a Surfactant concentrations 1.5 times higher than their ACMCs.
b Solubility enhancement factor (SEF) of the PAHs: aqueous PAH concentrations at supra-ACMC divided by that at CMC.
Table 4. Concentrations of Solubilized PAHs in 1000 mg/L of Brij 30.
|Name of PAHs||Solubilized PAHs (mg/L)||DFb|
|Day 1||Day 170a|
a The concentrations of benzo(a)pyrene, dibenzo(ah)anthracene and benzo(ghi)perylene listed in this column are the detection limits of the analytical method rather than the real values. See text for explanation.
b DF (decline factor) = (1 - C170/C1)´100, C1: PAH concentration at day 1. C170: PAH concentration at
Table 5. Amount of the PAHs (mg) recovered from water and soil in the solution of 1000 mg/L Brij 30 after 170 days.
|From Water||From Soil||Total Amounta||Recovered(%)b||Decomposed(%)c|
a The amount of the PAHs initially spiked into the system.
|b Recovery =||Amount from Water + Amount from Soil||× 100|
c Decomposed amount in percentage = 100 - recovery
Figure 1. Surface tension of Brij 30 measured from pure water solutions and soil
suspensions. The CMC and ACMC values are determined from the points where the
slope of the curves changes.
Figure 2. Concentration of solubilized PAHs analyzed from soil suspensions with
different concentrations of Brij 30. The dash curve is surface tension of the solutions
measured simultaneously to indicate the value of the surfactant ACMC in the system.
Figure 3. Concentration of solubilized PAHs analyzed from soil suspensions with
different concentrations of sodium dodecyl sulfate. The dash curve is surface tension of
the solutions measured simultaneously to indicate the value of the surfactant ACMC in the
Figure 4. Concentration of solubilized PAHs analyzed from soil suspensions with different
concentrations of dodecyltrimethylammonium bromide. The dash curve is surface tension
of the solutions measured simultaneously to indicate the value of the surfactant ACMC in
Figure 5. Concentration of the solubilized PAHs as a function of time in the water-soil
system with 1000 mg/L Brij 30.
Figure 6. Surface tension measured with the passage of time from the water-soil system
with different concentrations of Brij 30.
Figure 7. Concentration of the solubilized PAHs as a function of time in the water-soil
system with 3000 mg/L sodium dodecyl sulfate.
Figure 8. Concentration of the solubilized PAHs as a function of time in the water-soil
system with 10000 mg/L dodecyltrimethylammonium bromium.