Solubilization of nitrotoluenes in micellar nonionic surfactant solutions

Solubilization of nitrotoluenes in micellar nonionic surfactant solutions

Chemosphere 68 (2007) 1961–1967 www.elsevier.com/locate/chemosphere Solubilization of nitrotoluenes in micellar nonionic surfactant solutions Dianne ...

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Chemosphere 68 (2007) 1961–1967 www.elsevier.com/locate/chemosphere

Solubilization of nitrotoluenes in micellar nonionic surfactant solutions Dianne J. Luning Prak

*

US Naval Academy, 572M Holloway Road, Annapolis, MD 21402, USA Received 21 August 2006; received in revised form 14 February 2007; accepted 14 February 2007 Available online 28 March 2007

Abstract A key factor in selecting surfactants to enhance chemical or biological transformation or physical removal of an organic pollutant from contaminated soil is knowledge of the pollutant’s solubility behavior in the surfactant solution. This study investigated the influence of nonionic surfactant structure on the solubility of 4-nitrotoluene (NT), 2,3-dinitrotoluene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, and 2,4,6-trinitrotoluene (TNT) at room temperature. For a series of alkyl phenol ethoxylates (Tergitol NP-8 to NP-40), decreasing the ethoxylate chain length increased the solubility of these nitrotoluenes by a factor of two or less in 10 g l1 surfactant solutions, but did not significantly change their molar solubilization ratios (MSR, e.g. 0.02 for TNT) or their micelle–water partition coefficients (Km, e.g. 3.4 for TNT). For Tergitol NP-8 solutions ranging from 1.0 to 12.4 g l1, no enhancement in NT solubility was found, suggesting that the cloud point was reached. The MSRs for Tween 80 were higher than those of Tween 20 and the MSRs of Brij-58 were higher than those for Brij-35. When comparing solutes, NT had the highest solubility and MSR (0.28–0.41), while TNT had the lowest solubility and MSR (0.02–0.03). A linear relationship between Km values and octanol–water partition coefficients based on Triton X-100 predicted the log Km values within 0.5 of their measured values. A linear solvation free energy correlation for Km suggested the importance of solute volume and effective hydrogen bond basicity in the partitioning process while implying that the nitrotoluenes are solubilized in a polar portion of the micelle. Published by Elsevier Ltd. Keywords: Dinitrotoluene; Trinitrotoluene; Solubility; Molar solubilization ratio; Micelle-water partition coefficient

1. Introduction Soil, groundwater, and surface water have become contaminated with nitrotoluenes by the production and use of explosive materials. These compounds pose a potential threat to human health, due to their carcinogenic nature (NIOSH, 1985), and may become more of a concern as military bases close and return to public use. The impact of nitrotoluene pollution depends on the capacity of the system to chemically or biologically transform these pollutants into less harmful species or on the speed at which they can be removed through engineering technologies. All these process are affected by the pollutant’s solubility in water. Physical, chemical, and biological transformations of nitro*

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toluenes have been studied in aqueous systems (Mabey et al., 1983; Simmons and Zepp, 1986; Rodgers and Bunce, 2001; Esteve-Nunez et al., 2001; Johnson and Spain, 2003). Standard removal techniques, such as pump-and-treat operations, are most successful for compounds with high aqueous solubilities. For compounds that dissolve to a small extent, one way of enhancing the solubility is to add surfactants at sufficiently high concentrations to form micelles and solubilize the organic compounds. Pilot-scale studies have shown that with careful design and implementation, the use of surfactants in conjunction with pumpand-treat operations can effectively remove low solubility compounds (Abriola et al., 2005; Ramsburg et al., 2005). Surfactants have also been found to enhance biological transformation (Hodgson et al., 2000; Boopathy, 2002) and chemical photolysis (Larson et al., 2000; Diehl et al., 2002) of 2,4,6-trinitrotoluene (TNT). No study, however,

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has assessed how surfactant structure influences the solubility of TNT and other nitrotoluenes. Solubility enhancements have been quantified using the molar solubilization ratio (MSR) and the micelle–water partition coefficient (Km). The MSR, which is the ratio of the mol of solute solubilized to the mol of surfactant present as micelles, can be obtained from the slope of the solubility curve above the critical micelle concentration (CMC). The Km, which represents the distribution of solute between surfactant micelles and the aqueous phase, is given as Km ¼

Xm ; Xa

ð1Þ

where Xm is the mol fraction of the solute in the micellar phase and Xa is the mol fraction of the solute in the micelle-free aqueous phase. The value of Xm can be calculated from the MSR: Xm ¼

MSR MSR þ 1

ð2Þ

and Xa may be estimated as X a ¼ C o;cmc V w ;

ð3Þ

where Co,cmc is the solute concentration at the CMC, and Vw is the molar volume of water (0.01805 M1 at 25 °C). Often times, Co,cmc is estimated as the compound’s solubility in water (Pennell et al., 1997; Luning Prak and Pritchard, 2002). The influence of nonionic surfactant structure on the extent of solubilization has been found to vary with solute. A surfactant’s hydrophobicity is often represented by hydrophile–lipophile balance (HLB), which decreases as the surfactant becomes more nonpolar. For polar solutes which can be solubilized among the polar chains as well as in the hydrophobic core of the micelle, no consistent trend in MSR has been found. Xia and Hu (1986) found the ethylbenzene MSR values increased as ethoxylate chain length of polyethylene glycol n-dodecyl alcohols increased from 6 to 8. Diallo et al. (1994) reported MSR values for toluene and xylene that increased and then decreased as the polar portion of dodecyl alcohol ethoxylate (DAE) increased. Tokiwa (1968) found that the MSR values for Yellow OB (1-o-tolyl-azo-2-naphthylamine) did not vary as the oxyethylene chain length in dodecyl polyoxyethylene ether was increased from 7 to 20. Since nitrotoluenes are polar compounds, the influence of surfactant structure on their solubility can not be easily predicted. The goal of this work was to investigate how nonionic surfactant structure influences the solubility of nitroaromatic compounds. This information can be used to screen surfactants for use in chemical photolysis, biological degradation, or pump and treat systems. 2. Materials and methods Each surfactant type has its limitations when applied to soil systems. Cationic surfactants sorb to negatively

charged silica surfaces, anionic surfactants often require cosolutes or cosurfactants, and nonionic surfactants sorb to clay particles (Fountain, 1998). For pump and treat operations, the loss of nonionic surfactants to clay is minimized by the clay’s low hydraulic conductivity (Abriola et al., 2005). Nonionic surfactants were selected for study because they form micelles at lower concentrations and have larger solubilizing capacities than do ionic surfactants (Rosen, 2004), are commercially available, and have been studied with other solutes. To assess the influence of surfactant structure on the solubility of nitroaromatic compounds, three classes of nonionic surfactants were tested: polyoxyethylenated straight-chain alcohols (Brij-35, Brij-58, Aldrich Chemical, standard grade), polyoxyethylenate sorbitol esters (Tween 20, Tween 80, Aldrich Chemical, standard grade) and alkylphenol ethoxylates [Tergitol NP-8 (<0.3% water), NP-10 (<0.3% water), NP-13 (<0.3% water), NP-15 (<0.5% water), and NP-40 (<1.0% water), Dow Chemical]. Although alkylphenol ethoxylates have been used in field studies (Fountain, 1997), they are unlikely candidates for aquifer remediation because their degradation products are toxic and persistent in the environment (Servos, 1999). These surfactants do provide a way to systematically investigate surfactant structure. Straightchained alcohols are biodegradable, and Tween-80 has been approved for use in pilot-scale demonstrations because it is a food grade additive and is biodegradable (Ramsburg et al., 2005). All surfactants were used as received from the supplier and prepared using de-ionized water purified by a Millipore Milli-Q Plus water system. Relevant properties of these surfactants are given in Table 1. Five solid nitroaromatic compounds, 4-nitrotoluene (NT, 99% pure, Aldrich), 2,3-dinitrotoluene (2,3-DNT, 99% pure, Aldrich), 2,4-dinitrotoluene (2,4-DNT; 97% pure, Aldrich), 2,6-dinitrotoluene (2,6-DNT; 98% pure, Aldrich), and TNT (>95% pure, Eastman Chemical) were used in these studies (Table 2) as representatives of explosives or intermediates in the production of explosives, polyurethanes, and toluene diisocyanate. Batch mixing experiments were conducted in 15-ml borosilicate glass vials (Pierce, Rockford, IL) sealed with TeflonÒ screw caps. Acetone (Fisher Scientific) solutions containing the nitroaromatic compound were added to each vial, and the acetone evaporated. Each vial contained more mass than needed to reach equilibrium. Five ml of surfactant solution at concentrations ranging from 1.0 to 15 g l1 were added to each vial. Duplicate vials were run for each concentration, and five or more concentrations were tested for each solute. For each solute, at least three vials were also run using Milli-Q water to determine aqueous solubility. The vials were shaken at on a Labquake tube shaker (Barnstead International) at room temperature for 48 h or more to reach equilibrium as in Luning Prak and O’Sullivan (2006). Resampling of various vials after a greater contact time produced the same measured concentrations. After shaking, the solid was allowed to settle before an aqueous sample was taken and analyzed

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Table 1 Selected properties of nonionic surfactants Trade name

Average molecular formula

Molar mass (g mol1)

HLBa

CMCb (mg l1)

Nac

Tergitol NP-8 Tergitol NP-10 Tergitol NP-13 Tergitol NP-15 Tergitol NP-40 Tween 20 Tween 80 Brij-35 Brij-58

C9H19(C6H4)O(CH2CH2O)8H C9H19(C6H4)O(CH2CH2O)10H C9H19(C6H4)O(CH2CH2O)13H C9H19(C6H4)O(CH2CH2O)15H C9H19(C6H4)O(CH2CH2O)40H C12H34O2C6H10O4(CH2CH2O)20 C18H34O2C6H10O4(CH2CH2O)20 C12H25(CH2CH2O)23OH C16H33(CH2CH2O)20OH

573 683 793 881 1983 1228 1310 1198 1124

12.6 13.2 13.9 15.0 17.8 16.7 15.0 16.9 15.7

NA 57 NA 97 461 44–58 33–45 74 4.2

474 216 138 100 26 NA 110 40 NA

a

Hydrophile–lipophile balance, Dow Chemical for Tergitols, Aldrich for others. Critical micelle concentration, for Tergitols (Pennell et al., 1997), others (Luning Prak and Pritchard, 2002). c Aggregation number, for Tergitols and Tween 80 (Pennell et al., 1997); for Brij-35 (Rosen, 2004); C16H33(CH2CH2O)21OH (aggregation number of 70) is similar to Brij-58. b

Table 2 Selected properties of nitroaromatic compounds Organic solute

Molecular formula

Molecular weight (g mol1)

Aqueous solubility (mg l1)

log Kowa

Parameters for solvation equationc E

S

A

B

V

NT 2,3-DNT 2,6-DNT 2,4-DNT TNT

C6H4(CH3)NO2 C6H3(CH3)(NO2)2 C6H3(CH3)(NO2)2 C6H3(CH3)(NO2)2 C6H2(CH3)(NO2)3

137.1 182.14 182.14 182.14 227.15

345a (20 °C) N/A 208a (25 °C) 150 ± 8b (20 °C) 94 ± 3b (20 °C)

2.37, 2.42 2.0 1.89 1.98 1.84, 1.86, 2.00

0.87 1.15 1.15 1.15 1.43

1.11 1.58 1.60 1.70 2.23

0 0 0 0 0

0.28 0.49 0.45 0.40 0.61

1.0315 1.2057 1.2057 1.2057 1.3799

N/A – not available. a Verschueren (2001). b Luning Prak and O’Sullivan (2006). c PharmaAlgorithms (2006).

following the procedures in Luning Prak and O’Sullivan (2006). For several surfactant/solute pairs, the mass of the solid was doubled (producing a larger volume) to determine if surfactant sorption to the solid was a problem. It was found that the amount of the solute solubilized was the same for the differing masses of solid, suggesting no problem with sorption. 3. Results and discussion For each combination of surfactant and nitroaromatic solute, the nitroaromatic compound concentration in surfactant solution increased linearly with surfactant concentration except for NT in Tergitol NP-8. The mixing of Tergitol NP-8 and NT formed emulsions, which separated into a clear phase above a dense cloudy phase. Samples taken from the clear phase of all six Tergitol NP-8 concentrations tested contained approximately the same concentration of NT, 340 mg l1, which is close to the aqueous solubility. Repeating this experiment yielded the same results. This behavior is consistent with the surfactant reaching its cloud point. The cloud point is the temperature at which the solution separates into two phases, a surfactant-rich micellar phase and an almost micelle-free dilute solution of the nonionic surfactant at a concentration equal to its CMC (Rosen, 2004). It would be expected that the

micelle-free phase would contain a concentration similar to aqueous solubility, which was found here. Without an organic solute, Tergitol NP-8 has a cloud point of 43 °C (Dow Chemical, 2004), which is higher than the temperature used in this experiment. With tetrachloroethylene, Pennell et al. (1997) found that the cloud point of Tergitol NP-8 dropped to below room temperature. Other researchers have found that polar solutes can also decrease the cloud point of nonionic surfactants (Rosen, 2004). Aqueous solubility measurements for the nitrotoluenes are consistent with literature values (Table 3). For NT, 2,4-DNT, and TNT, the aqueous solubilities match the literature values (Table 2). For 2,6-DNT, the aqueous solubility of 151 mg l1 at 20 °C is less than the 208 mg l1 reported at 25 °C, which is expected because solubility increases with increasing temperature. Luning Prak and O’Sullivan (2006) found that a 10 °C temperature change increased the aqueous solubility of 2,4-DNT by 85 mg l1. The amount of nitrotoluenes that was solubilized in the surfactant solution decreased as the surfactant became more polar (higher HLB), as is illustrated for 10 g l1 surfactant solutions in Fig. 1. This means that more mass of the more polar surfactants would be needed to achieve the same solubilization results. For the 10 g l1 solutions, the enhancement in solubility was at most five times the aqueous solubility, as shown for 2,3-DNT Tergitol NP-8.

D.J. Luning Prak / Chemosphere 68 (2007) 1961–1967

NT 2,4 DNT 2,6 DNT 2,3 DNT TNT

1000

-1

Concentration (mg l )

The errors for the MSR and log Km represent the 95% confidence interval. The value is less than 0.03 for log Km unless reported. The error in aqueous solubility is the standard deviation.

3.77 ± 0.10 3.71 3.76 ± 0.05 3.67 3.67 3.75 3.73 ± 0.07 3.80 ± 0.07 0.377 ± 0.062 0.315 ± 0.013 0.367 ± 0.029 0.277 ± 0.006 0.281 ± 0.006 0.359 ± 0.009 0.328 ± 0.038 0.408 ± 0.048

1200

a

MSR 0.022 ± 0.001 0.024 ± 0.001 0.021 ± 0.001 0.023 ± 0.001 0.023 ± 0.003 0.016 ± 0.001 0.025 ± 0.001 0.022 ± 0.002 0.028 ± 0.001 MSR 0.114 ± 0.003 0.117 ± 0.002 0.121 ± 0.007 0.113 ± 0.002 0.117 ± 0.004 0.122 ± 0.001 0.150 ± 0.001 0.101 ± 0.008 0.134 ± 0.005 MSR 0.087 ± 0.002 0.092 ± 0.004 0.090 ± 0.003 0.087 ± 0.002 0.081 ± 0.002 0.097 ± 0.003 0.117 ± 0.001 0.083 ± 0.006 0.118 ± 0.003 MSR 0.159 ± 0.004 0.163 ± 0.003 0.160 ± 0.006 0.151 ± 0.003 0.169 ± 0.007 0.173 ± 0.007 0.213 ± 0.004 0.143 ± 0.008 0.179 ± 0.003 log Km MSR

log Km 4.01 4.01 4.01 3.99 4.03 4.04 4.11 3.97 4.05 ± 0.04

20 °C 150 ± 11 19 °C 137 ± 11 21 °C 353 ± 23

Temperature Aqueous solubility (mg l1) Surfactant Tergitol NP-8 Tergitol NP-10 Tergitol NP-13 Tergitol NP-15 Tergitol NP-40 Brij 35 Brij 58 Tween 20 Tween 80

2,3-DNT 4-NT

Table 3 Aqueous solubilities, molar solubilization ratios and micelle-water partition coefficientsa

2,4-DNT

log Km 3.73 3.75 3.74 3.73 3.70 3.77 3.85 3.71 ± 0.05 3.85

19 °C 151 ± 4

2,6-DNT

log Km 3.84 3.85 3.86 3.83 3.85 3.86 3.94 3.79 ± 0.05 3.90

20 °C 97 ± 3

2,4,6-TNT

log Km 3.46 3.49 3.44 3.46 3.46 ± 0.08 3.32 ± 0.05 3.50 3.44 ± 0.06 3.55

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800

600

400

200

0 12

13

14

15

16

17

18

Surfactant HLB

Fig. 1. Solubility of nitrotoluenes in 10 g l1 of all surfactants tested. The solubility of NT in Tergitol NP-8 (340 mg l1) is not shown since it is not solubilized in the micelles.

This level of solubility enhancement is low compared with solubility enhancements used in field trials of surfactant flushing. Solubility enhancements of 50 times (tetrachloroethylene (PCE) in a 2% nonylphenol ethoxylate solution), 100 times (carbon tetrachloride in a 1% Tergitol 15 solution), and over 200 times (PCE in a 6% Tween 80 solution) have been used in field tests (Fountain, 1997; Ramsburg et al., 2005). While the low nitrotoluene solubility values in surfactant solutions may limit their use in soil flushing, they may still be useful in applications such as bioremediation or photolysis. To better understand the solubilization behavior, the slopes of the solubility curves, which represent the MSR, were determined using least-squares linear regression. MSR values (with their 95% confidence intervals, most of which are based on 12 data points) are given in Table 3, along with the log Km values calculated using Eqs. (1)–(3). The assumption that Co,cmc is aqueous solubility in Eq. (3) was verified for a subset of surfactants and solutes. Of the nine surfactants, Tween 80 and Brij-58 tend to have the largest MSR and Km values. Other researchers have also found that the Km values for Tween 80 were higher than those of Tween 20 (Jafvert et al., 1994; Luning Prak and Pritchard, 2002). Within the series of Tergitol surfactants, no consistent trend is found in the MSR values or the log Km for any of the nitrotoluenes. In many cases, the confidence intervals overlap signifying no difference in the MSR values. These results are consistent with results found by Tokiwa (1968) for the solute Yellow OB. In his work, the MSR values did not change when the dye was solubilized in dodecyl polyoxyethylene ethers with oxyethylene chain lengths varying from 7 to 20. His measurements showed that as the surfactant became more nonpolar, more monomers joined to form a micelle (larger aggregation number) and each micelle contained a larger amount of dye. For a given number of monomers (surfactant concentration), fewer micelles were formed, but their larger capacity enabled the MSR to remain constant. In the present study, the aggregation number of the Tergitols (Table 1) also

D.J. Luning Prak / Chemosphere 68 (2007) 1961–1967

increases as the ethoxylate chain length decreases, causing fewer micelles to be formed. Each micelle must accommodate more solute for the MSRs to be same. The number of nitro groups on the toluene influenced the solubility and partitioning of the solute into the micelle. For each surfactant except Tergitol NP-8, a greater mass of NT was solubilized than those of dinitrotoluenes, and the least mass solubilized was that of TNT (Fig. 1). The MSR values decreased as the number of nitro groups increased for all surfactants tested, but no consistent trend was found in the log Km values (Table 3). Previous researchers have found linear relationships exist between log Km and the octanol–water partition coefficient, log Kow (Valsaraj and Thibodeauz, 1989; Edwards et al., 1991; Pennell et al., 1997). Within the nitrotoluenes, no consistent pattern is found between log Km and log Kow, but the very small differences between the log Kow values makes comparison difficult. Correlations have been developed for sodium dodecyl sulfate (SDS) (Valsaraj and Thibodeauz, 1989), Triton X-100 (Edwards et al., 1991) and DAE with an average ethoxylate chain length of 10.3 (Pennell et al., 1997) for a larger range of Kow values. Comparison of nitrotoluene data with the values predicted by these correlations shows that the Triton X-100 (octyl phenol ethoxylate EO 9-10) correlation predicts the log Km values for the nitrotoluenes studied within 0.5 of their measured values, while the correlation for SDS and DAE underestimate the log Km values (Fig. 2). Others researchers have developed correlations for log Km using the linear solvation free energy relationship (Quina et al., 1995; Abraham et al., 1997): log K m ¼ c þ rE þ sS þ aA þ bB þ vV :

ð4Þ

Here c, r, s, a, b, and v are used to fit the equation; E is an excess molar refraction; S is the solute dipolarity/polarizability; A is the solute overall or effective hydrogen-bond acidity; B is the solute overall or effective hydrogen-bond basicity; and V is the McGowan characteristic volume

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(cm3 mol1/100). These solute descriptors are found in Table 2 for the nitrotoluene compounds tested. Since only five solutes are present for each surfactant in the current study, individual correlations for each surfactant generated using Minitab Statistical Software (Minitab, Inc., 2000) for Eq. (4) are poor with correlation coefficients less than 0.36. Using all the data together, the following correlation was developed (R2 = 0.752): log K m ¼ 13:1 þ 24:3V  4:96S  9:67B:

ð5Þ

Since E and V were both closely correlated, the software would not fit an equation with both parameters. Parameter A was not included because all the values are zero. As was found for the surfactant Brij-35, the magnitude and sign of the coefficients are consistent with the process of transferring the solute from water to the micelle pseudophase: the large positive value of v means that creating a cavity for the solute to occupy is easier in the micelle than in water and the large negative value for b implies that bulk water is a better hydrogen donor than the solubilization sites in the micelle (Quina et al., 1995). The small negative value for s suggests that based on polarity, the solutes would prefer water, but the difference between the polarity of the water and the solubilization site is a small factor in determining the solute partitioning. This would imply that the solubilization site is among the polyoxyethylene chains and not within the hydrophobic core of the micelle. This result is consistent with the commonly held notion that polar compounds reside in the more polar portion of the micelle (Rosen, 2004). Previously developed correlations are also available that relate Km as defined by Eq. (1) to surfactant structure and Kow (Pennell et al., 1997). Pennell et al. (1997) modified Jafvert et al.’s (1994) correlation for DDT, polycyclic aromatic hydrocarbons, and trichlorobenzene K m ¼ K ow ð1:65y 1  0:30y 2 Þ

ð6Þ

and generated a correlation for linear alkanes 9

7 6

log Km

K m ¼ K ow ð24:2y 1  7:6y 2 Þ:

Triton X-100 [Edwards et al., 1991] Triton X-100: log Km = 0.791 log Kow + 1.976 SDS [Valsaraj and Thibodaux, 1989] SDS: log Km = 0.849 log Kow + 1.09 DAE [Diallo et al., 1994] DAE: log Km = 1.265 log Kow + 0.334 this study

8

5 4 3 2 1 0 0

1

2

3

4

5

6

7

log Kow Fig. 2. Relationship between Km and Kow for Triton X-100, SDS, and DAE EO = 10.3.

ð7Þ

In both correlations, y1 is the number of hydrophobic carbons (e.g. aromatic or aliphatic, straight or branched, reduced carbons) and y2 is the number of hydrophilic groups (e.g. sorbitan carbons or ethoxy groups) on the surfactant. As shown in Fig. 3a, the empirical correlation given in Eq. (6) underestimates the Km values for the all the nitroaromatic solutes except nitrotoluene, while the prediction based on Eq. (7) overestimates the Km (Fig. 3b) except for the dinitrotoluenes with some surfactants. These results are not surprising because these correlations were developed for compounds whose solubility increases with surfactant hydrophobicity, which was not found for nitrotoluenes. In summary, this study examined how nonionic surfactant structure influences the solubility of nitrotoluenes.

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Acknowledgments

10.0

Predicted log Km

9.0 8.0 7.0 6.0

Jafvert et al.,1994 Edwards et al., 1991 Kile and Chiou, 1989 Km = Kow (1.65 y1 - 0.30 y2) NT 2,4-DNT 2,6-DNT 2,3-DNT TNT

This work was funded in part by a grant from the Naval Academy Research Council and startup funding from a General Electric Fellowship. References

5.0 4.0 3.0

Predicted log Km

9.0 8.0 7.0

Pennell et al.,1997 Km = Kow (24.2 y1 - 7.6 y ) 2 NT 2,4 - DNT 2,6 - DNT 2,3 - DNT TNT

6.0 5.0 4.0 3.0 2.0 2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Experimental log Km Fig. 3. Measured and predicted values of log Km based on a correlation (a) developed by Jafvert et al. (1994) and modified by Pennell et al. (1997) for several hydrophobic compounds and (b) developed by Pennell et al. (1997) for dodecane, tetrachloroethylene, and dichlorobenzene. See also Jafvert et al. (1994), Edwards et al. (1991) and Kile and Chiou (1989).

The solubility of nitrotoluenes increased as the polarity of the surfactant decreased (below the cloud point) indicating that more mass of the more polar surfactant would be needed to achieve the same solubilization results. This study also found that for the Tergitol surfactants, MSR and Km values for nitrotoluenes did not vary in a systematic way with surfactant structure. When comparing solutes, the MSR values were highest for nitrotoluene (highest Kow) and lowest for TNT (lowest Kow), but did not vary in a systematic way for the dinitrotoluenes. A correlation between Km and Kow based on Triton X-100 was able to predict the Km values of the nitrotoluenes, but correlations that included surfactant structure were not able to predict all the results. A linear solvation free energy correlation was generated whose coefficients suggest that solute volume and the difference between the basicity of the solubilization site and water are important, while the small impact of polarity/polarizability suggests that nitrotoluenes are solubilized in the more polar portion of the micelle. Understanding the solubility behavior of nitrotoluenes in surfactant solutions provides useful information for selecting a surfactant for use in chemical photolysis, bioremediation, pump-and-treat operations or other remediation technologies.

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