Impact of Several Water-Miscible Organic Solvents on Sorption of

Environ. Sci. Technol. 1996, 30, 1533-1539

Impact of Several Water-Miscible Organic Solvents on Sorption of Benzoic Acid by Soil

For hydrophobic organic chemicals (HOCs), solubility and sorption have been well characterized in several mixedsolvent systems. Solubility of HOCs has been adequately described assuming a log-linear increase in HOC solubility as a function of the volume fraction of organic cosolvent (1, 2). Based on the inverse relationship between solubility and sorption, the following log-linear cosolvency model was derived for sorption of HOCs by soils (3, 4):

L I N D A S . L E E * ,† A N D P. SURESH C. RAO‡ Department of Agronomy, Purdue University, 1150 Lilly Hall, West Lafayette, Indiana 46907-1150, and Soil and Water Science Department, University of Florida, 2169 McCarty Hall, Gainesville, Florida 32611

Sorption of benzoic acid by a surface soil was measured from several binary mixtures of water and various organic cosolvents spanning a wide range in solvent properties. For all solvents investigated, the addition to an aqueous solution resulted in an increase in solubility and an alkaline shift in the conditional ionization constant (pKac) of benzoic acid. Sorption data were assessed using a cosolvency model that incorporated speciation of the organic acid as determined by the pKac and soil-solution pH. The model provided reasonable predictions of the sorption trends observed from acetone/water, acetonitrile/ water, and 1,4-dioxane/water solutions. However, enhanced sorption observed from DMSO/water solutions was not well described by the cosolvency model similar to what was previously observed for the sorption of carboxylic acids from methanol/water solutions. The relative importance of cosolvent properties and various solvent-specific mechanisms is discussed. Hydrogen bonding along with preferential solvation are hypothesized as the primary mechanisms responsible for the observed deviations from the model.

Introduction Codisposal of contaminants and mixing of contaminant plumes from different sources can result in environmental contamination problems at waste disposal sites consisting of various classes of organic chemicals in complex solvent mixtures. Therefore, understanding the behavior of both nonpolar and ionogenic organic chemicals in systems other than dilute aqueous solutions is necessary. This need is further warranted by the development of alcohol-based fuels and implementation of cosolvents for in-situ flushing or ex-situ washing as remediation schemes. * To whom correspondence should be addressed; telephone: 317494-8612; fax: 317-496-2926; e-mail address: [email protected]. edu. † Purdue University. ‡ University of Florida.

0013-936X/96/0930-1533$12.00/0

 1996 American Chemical Society

log Km ) log Kw - Rσfc

(1)

where K is the sorption coefficient (mL/g) with superscripts m, c, and w referring to mixed solvent, pure cosolvent, and water, respectively; fc is the volume fraction cosolvent; σ is the cosolvency power; and R is an empirical factor that represents the average deviation observed between the sorption-fc relationship and solubilization. For a given solvent, values for σ increase with increasing HOC hydrophobicity, e.g, octanol-water partition coefficients (Kow). The R term has been attributed to solvent/sorbent interactions, and assumed to be constant for a given cosolvent. Equation 1 has been successfully employed to describe sorption of HOCs spanning a wide range in hydrophobicity and from several mixed-solvent systems (5-7). For hydrophobic, ionizable organic compounds (HIOCs), predicting sorption and solubility in mixed solvents is confounded by additional solution-phase phenomena not relevant to HOCs. Sorption of HIOCs is dependent on the formation of neutral and ionized species, as determined by soil-solution pH and the solute’s acid dissociation constant (pKa), both of which are impacted by the addition of an organic cosolvent. Several researchers have demonstrated the pH dependence of the sorption of organic acids from aqueous solutions (8-12). Lee et al. (8) showed that the pH dependence of pentachlorophenol (PCP) sorption by several soils from aqueous solution can be described by

K ) Kw,nφn + Kw,i(1 - φn)

(2)

where the fraction of neutral species (φn) is

φn ) (1 + 10pH-pKa)-1

(3)

and subscripts n and i refer to neutral and ionized species, respectively. For HIOCs with log Kow > 1.5 for the neutral species, solubility increases with increasing fc, similar to that observed for HOCs; thus, a decrease in sorption with cosolvent addition may be expected. However, organic solvents with low dielectric constants (e.g., methanol, acetone, dimethylsulfoxide) cause an alkaline shift in the pKa of an organic acid relative to water, resulting in an increase in the fraction of neutral species. Lee et al. (13) incorporated cosolvent-enhanced solubility and cosolventinduced speciation by combining eqs 1-3:

Km ) Kw,nφnβn + Kw,iφiβi

(4)

where

βn ) 10-Rnσnfc

βi ) 10-Riσifc

(5)

The shape and direction of the resulting cosolvency curve

VOL. 30, NO. 5, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1

List of Selected Chemical and Physical Properties of Solvents parameter

acetone

acetonitrile

DMSO

methanol

1,4-dioxane

water

mol wt boiling point (°C)a log Kow density (g/mL) (20 °C)c Hildebrand solubility parameter (MPa-1/2)d surface tension (10-5 N/cm)a HBDe HBAf refractive index (20 °C)g dielectric constant (25 °C)g dipole moment (D)g σnh σn* h,i pKa,s (benzoic acid)k Rn

58.1 56.1 -0.24b 0.791 20.2 26.3 0 27 1.36 20.6 2.7 3.4

41.0 81.6 -0.34b 0.790 24.3 29.6 0 18 1.34 35.9 3.54 3.1 3.3 20.7 0.71

78.1 189 -2.03b 1.101 24.5 43.5 0 28 1.48 46.4 4.05 3.01 3.08 8.7 0.99

88.2 64.5 -0.66c 0.796 29.6 24 25 49 1.33 32.7 1.71 2.08

88.2 101.3 -0.42c 1.03 20.5 36.2 0 46 1.422 2.21 0.45 3.3j

18.0 100

8.96 0.86

NAl 0.76

18.2 0.63

1.00 47.9 73.0 111 111 1.33 78.3 1.77 4.22

a Dean (32). b Taft et al. (33). c Verschueren (34). d Yalkowsky (24). e Hydrogen bond donor densities (no. of donatable hydrogens × density/MW). Hydrogen bond acceptor densities (no. of nonbonding electron pair × density/MW) (24). g Reichardt (26). h Estimated by a log-linear regression of solubility versus volume fraction cosolvent (fc ) 0-0.6) with a force fit through the aqueous solubility. i Solution matrix contained 0.01 N CaCl2. j Estimated by adding the difference observed between σ values for anthracene in DMSO/water and 1,4-dioxane/water solutions (35) to the σ value n for benzoic acid in DMSO/water solutions. k Ludwig et al. (36). l NA, not available. f

will depend on the relative magnitudes of the cosolventinduced shift in the acid dissociation constant (pKa), the cosolvent-enhanced solubility, and the relative differences in the hydrophobicities of the ionized and neutral species. Lee et al. (13) and Lee (14) measured sorption of a few substituted phenols and several substituted carboxylic acids from methanol/water solutions (0.01 N CaCl2 matrix) varying in acidity and hydrophobicity. The log Kow values for the compounds (neutral species) in these studies ranged from 2 to 5 for the phenols and from 2 to 3.5 for the carboxylic acids; the addition of methanol enhanced the solubility of all these compounds. Sorption or retardation of the substituted phenols decreased with the addition of methanol, which is consistent with the cosolvent-induced shifts in pKa and the relative differences in the hydrophobicities of the ionized and neutral species incorporated in eq 4. However, for the carboxylic acids investigated, sorption or retardation generally increased upon the addition of methanol in a manner not predicted by eq 4. Soerens and Sabatini (15) reported a similar increase in the sorption of an acidic fluorescent dye Rhodamine WT by subsurface materials from methanol/water and acetone/ water solutions. Comparatively, carboxylate groups increase acidity more than a phenolic group (e.g., the pKa of benzoic acid is 4.22 versus 10 for phenol), and carboxylate groups have a greater propensity for hydrogen bonding. In the present study, benzoic acid was used as a representative probe for investigating if the previously observed enhanced sorption of carboxylic acids was specific to methanol. Sorption of benzoic acid by a surface soil was measured from binary solutions of water and several water-miscible organic cosolvents spanning a range of solvent properties.

Materials and Methods Sorbent. The sorbent used in this study was a surface horizon sample of Webster soil from Iowa (fine loamy mixed mesic, Typic Haplaquoll) collected in spring 1991 containing 30%, 43%, and 27% sand, silt, and clay (predominately montmorillonite), respectively. Soil organic carbon (OC) content was 3.12%, and the soil-solution pH in 0.01 N CaCl2 was 5.8. The soil was air-dried and passed through a 2 mm sieve prior to use.

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Chemicals. The water-miscible organic solvents used in this study are listed in Table 1. Solvents were purchased from either J. T. Baker (high purity, HPLC grade) or Aldrich Chemical Co. (purity >99%) and were used without further preparation. Uniformly 14C ring-labeled benzoic acid was purchased from Sigma Chemical Co. with a specific activity of 13.3 mCi/mmol and a reported purity of >98%. Unlabeled benzoic acid was purchased from Fisher Scientific (purity >99%). Determination of Ionization Constants. Ionization constants for benzoic acid in various cosolvent/water solutions were determined by potentiometric titration. Solutions with 20, 40, 60, and 80% cosolvent volumes were prepared and degassed prior to use. Cosolvent/water solutions containing 0.01 M benzoic acid were titrated in duplicate with 0.1 M NaOH while being stirred continuously. Solution pH and temperature were measured continuously using a Radiometer titrating system (PHM84 Research pH meter; TTT80 titrator; ABU80 autoburett) employing a glass pH electrode (Radiometer G2040B), a calomel reference electrode (Radiometer K4040), and a Radiometer resistance thermometer. The pH meter was calibrated using aqueous buffers. The apparent pH values (pHapp) obtained during the titrations in mixed solvents were corrected using an empirical procedure described by Van Uitert and Haas (16). This method essentially consists of calibrating the glasscalomel pH electrode system with solutions of strong acid using the pH scale derived in aqueous buffer solutions. In our studies, the difference between the pH of 10-3 M hydrochloric acid measured in the mixed solvent and water was used as the correction factor. The ionization constants were then calculated from the titration curve as described by Albert and Sergeant (17) using the corrected pH values. Equilibrium Sorption Isotherms. Sorption of benzoic acid by Webster soil was measured from different solvent/ water solutions using the batch-equilibration method described by Rao et al. (18). The vials used for this study were 5 mL (1 dram) screw-cap borosilicate glass autosampler vials with Teflon-lined septa inserts. Amber vials were employed to minimize photolysis. Soil mass to solution volume ratios ranged from 1:2 to 1:1 to achieve sorption of

50% (( 20%) of the chemical added. Initial solution concentrations (Ci) added to the soils ranged from 0 to 40 mg/L for all solutes in a 0.01 N CaCl2 matrix. All sorption isotherms were measured at room temperature (T ) 23 ( 2 °C). Each isotherm was based on benzoic acid sorption measured in duplicate at four initial concentrations. For each isotherm, control blanks (no benzoic acid present) containing the solvent with and without soil were run to obtain an appropriate background count in assays for radioactivity. Chromatographic analysis was also performed on the controls blanks to ensure that no benzoic acid was initially present in the soil matrix. Samples in cosolvent/water solutions were equilibrated by rotating for 16-24 h. Samples in aqueous solutions (fc ) 0) were equilibrated for slightly less than 2 h. In previous studies, values for Kd appeared constant between 1 and 2 h; however, loss of benzoic acid from a spiked soil-solution matrix was observed after 4 h (13) presumably via biodegradation. Following equilibration, the solution and solid phases were separated by centrifuging the soil samples at approximately 300 RCF (relative centrifugal force) using a Sorvall RT6000 centrifuge. Aliquots (0.5 mL) of the supernatant and stock solutions were then mixed with 10 mL of scintillation cocktail (Scinti-Verse II) and assayed for 14C radioactivity using a Searle Delta 300 liquid scintillation counter. Solution concentrations at equilibrium (Ce, µg/mL) were determined from measured radioactivity, whereas sorbed concentrations (Se, µg/g) were estimated by difference in initial and equilibrium solution concentrations: Se ) (Ci Ce)(V/M), where V is the solution volume (mL) and M is the soil mass (g). The pH of the supernatant and/or the resuspended soil samples was measured using a Corning Model 130 pH meter and a Fisher Scientific or Orion combination microelectrode (AgCl-saturated 3 M KCl filling solution) following equilibration and analysis of the sample. For the high mass to volume ratio samples (1:1), the volume of the remaining supernatant was insufficient for a pH measurement; therefore, additional replicates were run and sacrificed for pH meaurements only.

Results pKa Measurements. When measuring pH of a mixedsolvent solution using an aqueous-based reference, it is necessary to account for differences in the liquid junction potential and in the standard potential of the glass electrode that exists between mixed solvents and aqueous solutions. There are several theoretical approaches for assessing these differences; however, experimental determinations of the parameters needed is difficult and often unreliable (19). In order to minimize these difficulties and associated errors, the apparent or measured operational pH values were converted to -log [H+] or pcH using an empirical procedure, originally described by Van Uitert and Haas (16) for dioxane/ water solutions. If the acid is completely dissociated, cH in these reference solutions is known and a correction factor (pcH - pHapp) can be estimated. Literature data suggest that this assumption is indeed reasonable when the solvent dielectric constant () is greater than 39 (16, 20, 21). This condition is met in all binary solutions examined here, except for 80/20 v/v acetone/water solutions and greater than 60% volume fraction dioxane. The correction factors used in the adjustment of pHapp for each binary solvent solution are summarized in Figure 1A. For all cosolvent/ water solutions except those with DMSO, pHapp was lower

FIGURE 1. (A) Correction factors (pcH - pHapp) used to calculate pKac from the titration curve; (B) pKac values for benzoic acid in each binary solvent solutions as a function of volume fraction cosolvent (fc).

than pcH. In DMSO/water solutions, the opposite trends were observed, i.e., pHapp was greater than pcH. For all solvents, the absolute value of (pcH - pHapp) increased with increasing fc and remained within 1 pH unit in the cosolvent range investigated (0 e fc e 0.8). The ionization constants that result from standardizing from pHapp to pcH values are concentration-based or classical ionization constants (Kac). The pKac values measured for benzoic acid in the cosolvent/water solutions as a function of volume fraction cosolvent (fc) are shown in Figure 1B. An alkaline shift in pKac with increasing fc is noted for all four organic solvents as would be expected based on the Born equation (22), i.e., an inverse relationship between dielectric constant () and pKa. Also shown in Figure 1B are pKa values for benzoic acid in DMSO/water solutions measured by Fiordiponti et al. (23) where the activity coefficients for all species in the mixed solvents were estimated using the Debye-Huckel equation. Good agreement suggests the approach used in this study is more than adequate for assessing trends in pKa values induced by cosolvents. Solubility. All of the organic cosolvents used in this study will increase the solubility of benzoic acid relative to water. Solubility data available from Yalkowsky (24) for benzoic acid in all binary solutions except 1,4-dioxane/ water solutions are shown in Figure 2. The substantial departures from log-linearity observed at high cosolvent contents (fc > 0.7) are due to the high solute concentrations (105 mg/L). The log-linear cosolvency model assumes negligible solute-solute interactions and that the volume fractions of water and organic cosolvent sum are one; both assumptions are invalid at high solute concentrations.


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FIGURE 2. Solubility of benzoic acid in binary mixtures of water and several organic cosolvents as a function of volume fraction cosolvent (fc). Sm and Sw are solubilities in the mixed solvent and water, respectively.

Also shown in Figure 2 is the solubility of benzoic acid in DMSO/water and acetonitrile/water solutions containing 0.01 N CaCl2. The presence of the elcetrolyte matrix had only a small effect on the relative solubility of benzoic acid (Sm/Sw). The cosolvency power (σ) increased by only 2% for DMSO and 6% for acetonitrile, thus σ* ≈ σ where the superscript * refers to a 0.01 N CaCl2 matrix (see Table 1). Assuming similar results would be obtained for the other solvent/water systems, the σ values calculated from pure solvent/water solutions (see Table 1) were used as estimates for solvent/water solutions containing a 0.01 N CaCl2 matrix. pH of Soil Suspensions in Mixed Solvents. The success of eq 4 in describing the sorption of organic acids is predicated on the ability to measure (or define) the ionization constant and pH in the solutions of interest. When considering the measurement of pH in mixed-solvent soil suspensions, problems in addition to those previously discussed for pH measurements in mixed solvents arise. It has long been recognized that the ambiguity of measuring the pH in aqueous soil solutions, and even more so in soil suspensions, is due to the inability to accurately determine liquid junction potential differences between standard buffer solutions and soil solutions. Even with this ambiguity, the error in the measured pH resulting from differences in the liquid junction potentials is usually assumed to be within 0.2 pH units for an aqueous soil solution or dilute soil-suspension given a background electrolyte concentration of approximately 0.01 N (25). Similar assumptions were made in our studies with mixed solvent soil solutions. Apparent pH values were corrected using the procedures previously described for determining pKac in mixed solvents, except the pH of 0.001 N HCl was measured in water and mixed-solvent solutions with the electrolyte matrix used in the sorption studies (i.e., 0.01 N CaCl2). These correction factors were within 0.05 pH unit of those estimated for Radiometer electrodes in the absence of an electrolyte matrix. The corrected soil-solution pH values (noted as pH*) are shown in Figure 3A as a function of cosolvent content (fc). Changes in pH* with fc were relatively small (a maximum of 0.3 pH unit) in the presence of all solvents, except in DMSO/water solutions with fc > 0.6. Corrected pH* values in 80% DMSO/water was about 1 pH unit larger than that in water (fc ) 0). Overall, the small changes in pH* and the large alkaline shift in pKac (Figure 1B) result

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FIGURE 3. For binary mixtures of water and several organic cosolvents as a function of volume fraction cosolvent (fc) equilibrated with Webster soil, (A) measured trends in soil-solution pH*; (B) estimated fraction of benzoic acid existing as a neutral species.

in the neutral fraction (φn) of benzoic acid increasing with increasing fc (Figure 3B). Batch Equilibration Studies. To investigate if specific solvent properties were responsible for the methanolenhanced sorption of caboxylic acids observed by Lee et. al (13), benzoic acid sorption by Webster surface soil was measured in binary solutions of water and several organic solvents spanning a wide range of solvent properties. Representative isotherms along with linear regressions for sorption of benzoic acid in several solvent/water mixtures are shown in Figure 4. Equation 4 was derived to predict the sorption of organic acids by incorporating cosolvency phenomena in terms of solubility and cosolvent-induced speciation effects. Failure of eq 4 to predict the enhanced sorption of benzoic acid in methanol/water solutions was attributed to neglect of specific solvent-sorbent interactions (13). Of the parameters needed in eq 4, pKac and pH* values were measured as described previously in this study. Values for Kw,n (4.6 L/kg) and Kw,i (0.1 L/kg) were estimated from isotherm data collected at pH 3.4 (φn ≈ 0.87) and 5.8 (φi ≈ 0.97), respectively. The cosolvency power of the solvents on neutral benzoic acid (σn) was estimated from the solubility data shown in Figure 2 by a log-linear regression from fc ) 0 to 0.6 with log Sw fixed as the y-intercept (See Table 1). Solubility data at higher fc values were not included in the regression for reasons previously stated. The impact of all cosolvents on benzoate solubility was assumed to be zero (i.e., σi ) 0, thus Riσi ) 0). This assumption was based on previous observations by Lee et al. (13) on benzoate solubility in methanol/water solutions (fc < 0.5). Values

FIGURE 4. Representative isotherms benzoic acid sorption by Webster soil from (A) acetone/water; (B) acetonitrile/water; (C) DMSO/water; and (D) 1,4-dioxane/water solutions.

for Rn were estimated using the following relationship derived by Kimble and Chin (7):

R)1-

( )( )

( )

σsom fc,som σsom )1Kc σ fc σ

(6)

where σsom is the cosolvency power in the cosolventmodified soil organic mater (SOM), fc,som is the volume fraction cosolvent in SOM, and Kc is the partition coefficient of the cosolvent between water and SOM. Equation 6 was derived by assuming that the solubility of an organic compound in cosolvent-modified SOM is analogous to the solute’s solubility in cosolvent/water solutions. The cosolvent’s Kow was used as an approximation for Kc (see Table 1). A value of 0.64 was estimated for the ratio σsom/σ using using Rnσn ) 1.79 determined by Lee (14) for the sorption of neutral benzoic acid by Webster soil from methanol/ water solutions and the Kow for methanol and σn value for benzoic acid in methanol/water solutions reported in Table 1. The Rn values for all other cosolvents were then estimated using their respective Kow values from Table 1 and the σsom/σ value estimated from methanol/water solutions as just described. The linear distributions coefficients (Kd, mL/g) estimated from the batch-equilibration studies with benzoic acid are plotted as a function of fc in Figure 5 along with predictions from eq 4. For benzoic acid sorption from acetone/water, acetonitrile/water, and dioxane/water solutions, eq 4 did a reasonable job in predicting the observed trends. However, eq 4 failed to predict benzoic acid sorption from DMSO/water solutions (Figure 5c) similar to what was previously observed by Lee et al. (13) from methanol/water solutions (also shown in Figure 5C). These findings support the hypothesis that specific solvent-sorbent interactions, not considered in the derivation of eq 4, were responsible for the failure of eq 4 in describing the enhanced sorption of carboxylic acids previously observed (13).

Discussion Sorption of benzoic acid from methanol/water and DMSO/ water solutions could not be adequately predicted by simply accounting for enhanced solubility of the neutral species and changes in speciation as determined by pKac and pH*. This could imply that the same mechanisms are controlling benzoic acid sorption from methanol and DMSO. Alternately, sorption could be driven by two completely different mechanisms but leading to similar macroscopic results. In search for a similar mechanism, solvent properties summarized by Reichardt (26) (see Table 1 for a partial listing) were reviewed to find trends in a pure solvent property consistent with the sorption phenomena observed between the different solvent/water systems. Acetone, acetonitrile, DMSO, and 1,4-dioxane are all dipolar aprotic solvents whereas methanol is an amphiprotic solvent. DMSO is the aprotic solvent with the highest polarizability (as reflected in a high refractive index) and a high dielectric constant (s); but similar polarizability is not evident for methanol. Hydrogen bond donor and acceptor numbers can be used to assess hydrogen bonding characteristics. Pure solvent data indicate that only methanol acts as a hydrogen bond donor; whereas, methanol, DMSO, and acetone have a large ability to accept hydrogen bonds. However, spectroscopic studies (Raman and infrared spectroscopy) indicate that, in the presence of water, DMSO acquires hydrogen-donating characteristics by forming DMSO‚(H2O)2 complexes (27, 28). Thus, the ability to donate hydrogen bonds may link the macroscopic sorption phenomena observed for the sorption of benzoic acid from solutions with methanol and DMSO. A similar hypothesis may explain several previous findings by Lee et al. (13, 29) involving both the nature of the solute and the role of electrolyte composition and concentration. In previous studies, the sorption of both substituted phenols and carboxylic acids was investigated


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FIGURE 5. Measured (symbols) and predicted (eq 5; lines) sorption of benzoic acid by Webster soil from (A) acetone/water; (B) acetonitrile/ water; (C) DMSO/water and methanol/water; and (D) 1,4-dioxane/water solutions as a function of volume fraction cosolvent (fc). The electrolyte matrix was 0.01 N CaCl2 for all cosolvent/water solutions.

from methanol/water solutions; however, enhanced sorption with increasing fc was only observed for the carboxylic acids and could not be adequately described by eq 4. Hydrogen bonding potential is greater with carboxylate groups compared to phenolic groups. Lee et al. (13, 29) initially investigated the role of the electrolyte matrix to assess if enhanced sorption of carboxylic acids was due to charged ion-pair formation. The electrolyte matrix in all previous studies and those discussed thus far was 0.01 N CaCl2. Lee et al. (13, 29) measured sorption of benzoic acid from methanol/water solutions containing 0.01 N KCl. A decrease in sorption with increasing fc was observed in the KCl system in contrast to what was previously observed with CaCl2. Although these findings support the potential for the formation and sorption of calcium benzoate [Ca(RCOO-)+], increasing CaCl2 concentrations (0.002-0.05 N) did not appear to further enhance sorption from methanol/water solutions (13). For an ion-pairing phenomenon, both electrolyte composition and concentration would be expected to influence sorption as well as the solvent. Ion-pair formation is promoted by a decrease in the dielectric constant (30). All cosolvents used in the present study have dielectric constants lower than that of water; therefore, in each case, the addition of a solvent would enhance the potential for ion pairing. However, only sorption of benzoic acid from DMSO/water solutions was similar in direction and magnitude to that observed in methanol/water solutions. Hydrogen bonding interactions are also influenced by the cations present on the sorbent. An increase in valence state of a cation, i.e., Ca+2 versus K+, enhances polarization of the molecules in the solvation sphere of a cation and results in stronger hydrogen bonding characteristics (31).

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Hydrogen bonding interactions in mixed solvents can also bring about preferential or selective solvation. Preferential or selective solvation comes about when more than one solvent is present and the composition of the solvent shell around a solute (or sorbent) is different than the composition of the bulk solution. Preferential solvation is a result of differences in the free energy of solvation (∆Gsolv) for a given solute and is induced by both nonspecific and specific solute-solvent interactions. Nonspecific solutesolvent associations are caused by a dielectric enrichment in the solvent shell of ions or dipolar molecules, whereas specific solute-solvent associations result from interactions such as hydrogen bonding or electron pair donating/ accepting (28). While many uncertainties remain concerning the behavior of HIOC sorption by soils from mixed solvents, it is clear that an inverse relationship between solubility and sorption does not exist for all organic acids. This understanding will directly impact the assessment of contaminant mobility from codisposal sites, the choice of tracers for alcohol-based fuels (e.g., alternate fuels), and schemes for the extraction of HIOCs from soils. For example, a lack of awareness of this phenomenon has probably contributed to the inaccurate assessment of “bound residues” and the inapproriate use of organic solvents for efficient soil extractions.

Acknowledgments This research was funded in part by the Subsurface Science Program, Office of Health and Environmental Research, U.S. Department of Energy through Contract DE-AC0676RLO 1830 to Battelle PNL, Richland, WA. Approved for publication as Purdue Agricultural Research Program Journal Series No. 14905 and Florida Agricultural Experi-

ment Station Journal Series No. R-04916. Special thanks are extended to Ms. Vicki Neary, Nicolas Priddy, and Arto Nyman for their technical assistance in conducting the batch equilibration experiments, the determination of ionization constants, and the solubility measurements, respectively, and to Dr. Rodolfo Pinal (Hoffman La Roche) and Dr. John Zachara (Battelle PNL) for helpful discussions.

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Received for review July 11, 1995. Revised manuscript received January 25, 1996. Accepted January 29, 1996.X ES9505107 X

Abstract published in Advance ACS Abstracts, April 1, 1996.


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Impact of Several Water-Miscible Organic Solvents on Sorption of

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