Environ. Sci. Technol. 1994, 28, 1003-1009
Thermodynamics of Organic Chemical Partition in Soils. 3. Nonlinear Partition from Water-Miscible Cosolvent Solutions Frank C. Spurlock’ and James W. Biggar Hydrologic Sciences, Department of Land, Air, and Water Resources, University of California, Davis, Davis, California 956 16 Solubility and partitioning behavior of four substituted phenylureas were investigated in various aqueous methanol (MeOH) and aqueous dimethyl sulfoxide (DMSO) solutions. The solubility data for these compounds were reasonably well described by a log-linear relationship between mole fractional solubility and volume fraction cosolvent. Sorption was evaluated in terms of a thermodynamic nonlinear partition model, and solute sorbedphase activity coefficients y1were found to be independent of solution-phase composition in solutions containing up to 60 % volume fraction MeOH. For the systems examined, it is shown that solution-phase effects are the primary determinant for soil partitioning in the MeOH cosolvent systems and that solvent-humic polymer effects are inconsequential. In contrast, increasing solution volume fraction DMSO was associated with decreasing y1 and increasing dissolved solution-phase organic matter for volume fraction DMSO >40%. The DMSO data suggest that increasing volume fraction DMSO results in swelling of the humic Dhase.
Introduction
A number of papers have evaluated the partitioning of organic chemicals between water-miscible cosolvent solutions and soils or sediments (1-12). In part, these studies have been conducted in recognition that the solution phase near hazardous waste disposal sites or contaminated aquifers may consist of complex mixtures of solvents. Cosolvent solutions have also been suggested as experimental tools for estimating difficult to measure aqueous soil partition coefficients of highly hydrophobic compounds (2, 7) and for evaluating sorption kinetic phenomena (8). However, a number of questions remain unanswered regarding the effect of cosolvent on the partitioning of organic compounds in soil. The thermodynamic basis for the observed log-linear decrease in nonionic organic sorbate partitioning with increasing solution-phase volume fraction cosolvent is not fully understood. A related question concerns the influence of solution-phase cosolvent-sorbent interactions on sorption.
of organic compounds in aqueous solution (13-15) and mixed solvents (16,17).They introduced the log-linear equation to describe the commonly observed exponential increase in solubility with increasing volume fraction cosolvent: ln(%)=ajc
XO , and Xwo are mole fractional solubilities in mixed solvent (water cosolvent) and aqueous solutions, respectively, f , is volume fraction miscible cosolvent in solution, and uc is known as the solubilizing power of the cosolvent for the solute (18). The parameter uc varies with the cosolvent and the solute under consideration. Across a family of chemically similar compounds, values of u, in a particular cosolvent generally increase with compound hydrophobicity. In essence, those compounds that are less polar (i.e., poorly soluble in water) display the greatest relative increase in solubility with miscible cosolvent addition. Attempts to understand the physical significance of o, have focused on nonpolar and polar contributions to molecular surface area and their respective molecular scale interfacial surface tension parameters (16, 17). Practically speaking, many parameters (i.e., polar and nonpolar surface areas of molecules, the respective interfacial tensions, and curvature correction factors) required to calculate ac are generally unavailable or are poorly defined on a molecular scale. In this paper, we consider Q, as an empirical constant. Experimentally, uc is most easily estimated by determining compound solubility in aqueous solution and in neat cosolvent. From eq 1, a, is approximately given by
+
a,
= In X : - In X &
The activity coefficient (Raoult’s law convention) of a poorly soluble nondissociating solute in a solution i may be estimated by (13) (3)
yielding
Theory Rao et al. (1) have proposed a generalization of hydrophobic partition theory to characterize the sorption and the transport of organic compounds in aqueous cosolvent systems. Their solvophobic sorption model was originally applied to systems containing solution-phase mixtures of water and water-miscible cosolvents. Subsequent work has extended the model to include partially miscible cosolvent/water mixtures (4,5). The solvophobic model is derived by combining the log-linear solubility model with the hydrophobic sorption model. Log-Linear Solubility Model. Yalkowsky and coworkers have studied solubility phenomena for a variety 0013-936X/94/0928-1003$04.50/0
0 1994 American Chemical Soclety
a,
= In yw - In y,
(4)
In eq 3, ASf is the entropy of fusion of the crystalline solute (which may be estimated from structure; ref 19), T is the ambient temperature, and T, is the solute melting point. Equation 3 can also be applied to solutes that are liquid at temperature T by adopting the convention that T , = T for these compounds. The yjin eq 4 refers to the activity coefficient of the solute in solution i, neat aqueous (W), or neat cosolvent (c), respectively. Solvophobio Model. Theoretically, for a pure hydrophobic linear partition process in which sorbed-phase activity coefficients are constant, the organic carbonEnvlron. Scl. Technol., Vol. 28, No. 6, 1994
1003
normalized partition coefficient Koc is proportional to the aqueous-phase activity coefficient yw among chemically similar sorbates (20): KO,
a
Yw
(5)
In reality, sorbate-sorbent interactions vary for different members of a family of sorbates (21). A practical relationship for correlating sorbate solubility and Koc within a family of similar sorbates comes from eqs 3 and 5:
where a and /3 are regression-fitted parameters whose values vary among chemical families. Rao et al. ( 1 )have supposed that the aqueous solubilitypartition coefficient relationship across a chemical family also applies to a single compound in systems of varying solvent composition. Using this insight into the contribution of solution-phase solubility into the partition process, these workers substitute the log-linear solubility relationship (eq 11, obtaining the solvophobic model ln(Km/Kw)= - a c f C
(7)
where the subscripts m and W refer to mixed and aqueous solvents, respectively. The parameter a is likely to vary among different chemical families, and eq 7 has been applied using both linear partition coefficients (7, 9, I I ) , and mass-based Freundlich coefficients (2, 6) k~ (mLN pg'-N g-1). From a theoretical point of view, several shortcomings of the solvophobic model as commonly applied and currently formulated are evident. (1) The application of the solvophobic model using Freundlich nonlinear partition coefficients is questionable. The common Freundlich nonlinear partition coefficient kF is essentially empirical. As such, conventional Freundlich parameters are not particularly useful for comparative studies. (2) Although a linear In-ln relationship generally exists between KOCvalues for a family of linearly sorbing compounds and aqueous activity coefficients defining the slope parameter a, such relationships are variable and of questionable validity for nonlinearly sorbing compounds (21).
(3) The solvophobic model has often been applied using mass-based linear partition coefficients. From a thermodynamic standpoint, comparisons of partitioning should account for solution composition on a molar basis (11). (4) The solvophobic model considers sorbate-solvent interactions only. If it is assumed that a gel model is applicable for humic substances (22),the possibility arises that solvent composition may have an effect on swelling of the soil organic matter fraction. Rao et al. have previously suggested the need to address this question (1, 2, 6). The latter point has been the topic of discussion in the literature. Fu and Luthy (11)observed a smaller decrease than expected in sorption arising from methanol cosolvent addition. They postulate a swelling of the organic matter arising from the presence of cosolvent. The swelling is thought to partially offset the decrease in sorption by rendering more organic matter available for sorption. Nkedi-Kizza et al. (6) observed a decrease in sorption 1004
Envlron. Sci. Technol., Vol. 28, No. 6, 1994
nonequilibrium (i.e., increase in sorption kinetic rates) in column cosolvent studies with methanol, attributing the effect to organic matter swelling. Brusseau et al. (8) postulated a similar mechanism, but recognized that the conclusion is speculative without additional evidence. In contrast, Walters and Guiseppi-Elie (7) observed no anomalous effects that could be attributed to organic matter swelling and no effect on sorption behavior because of long-term exposure of the sorbent to methanol solvent. Wood et al. (9) observed no effect due to long-term exposure of soil to methanol cosolvent, whereas results of short-term exposure were equivocal. Nkedi-Kizza et al. (IO) suggested that solubility effects are dominant in effecting sorption kinetics and that organic matter swelling effects, if present, are of much less consequence. The resolution of this question is necessary to fully understand the influence of organic solvent on the transport and partition of chemicals in complex solutions.
Experimental Section
Soil Sorption Isotherms. Sorption isotherms were determined for the following systems in aqueous methanol (MeOH) and aqueous dimethyl sulfoxide (DMSO) solutions: fenuron on soil c ( ~ M ~ O=H 0, 0.1, 0.2, and 0.3), monuron on soil b (fMeOH = 0, 0.1,0.2,0.4), diuron on soil b ( ~ M ~ O=H 0, 0.2,0.4, 0.61, and diuron on soil c ( ~ D M S O= 0, 0.2, 0.4, 0.6, 0.7). Following Acree (23) and others (4, 5, 111, volume fraction solution compositions are operationally defined here assuming volume additivity. Selected properties of the compounds and also details on the preparation, source, and texture of soils b (3.4% OC) and c (4.9 % QC) have been reported in previous publications (21,24). Procedures for the batch isotherm measurements have also been previously reported in detail (24). Briefly, soil and solution (containing 0.01 N CaClz supporting electrolyte) were introduced into 40-mL glass vials with PTFE-lined caps containing neat sorbate. The vials were agitated in an isothermal shaker bath (24 "C), over equilibration times of approximately 80-96 h. After phase separation by centrifugation, the supernatant was analyzed via HPLC, and the sorbed-phase compound was calculated by the difference. Selected vials were extracted in their entirety to verify mass balance. Aqueous Solubilities. Aqueous solubilities for fenuron and monuron were determined by adding excess solid compound to aqueous solution and heating the mixture to approximately 80 "C to facilitate dissolution. Each flask, containing excess solid compound, was then allowed to equilibrate for a minimum of 3 days a t 23 "C in an isothermal shaker bath. Duplicate aliquots of the saturated solutions were then centrifuged and filtered through a nonsorbing (as determined in our lab) 0.45-pm poly(vinylidene fluoride) membrane filter before analysis. For the compounds of lesser solubility (diuron and neburon), a generator column was used for solubility determinations. Clean 100-pm glass beads were added to a concentrated solution of the particular compound in acetone, and the solvent was removed under vacuum. The coated beads were packed into a 10 cm X 0.75 cm i.d. J-shaped glass column stoppered at both ends with glass wool, and distilled water (23-25 " C ) was then passed through the column at a low rate (-0.25 mL/min). The first 15mL of effluent was discarded, and triplicate samples were taken for analysis.
Table 1. Solubility and Crystal Energy Data at 24 OC fc
0.0
0.2
0.4
0.6
0.8
1.0
UC
R2 a
4.97 6.43 7.60 11.86
0.993 0.994 0.987 0.984
10.17
0.975
1.Logarithm Mole Fraction Solubility
methanol fenuron monuron diuron neburon DMSO diuron
-3.77 -5.52 -6.39
-11.50
-4.90 -6.98 -8.16 -9.09
-6.10
-3.03 -4.44 -5.67 -3.26
-9.91
-8.03
-5.26
-2.51
-7.76 -10.60 -12.74 -14.90
-7.08 -9.76 -11.66 -13.57
-6.10 -8.50 -10.16
-12.74
-11.31
2. Estimated Crystal Energies = ASf(Tm- T)/RT (24 "C)
fenuron monuron diuron neburon a
melting point datab ("C)
2.44 3.33 3.04 2.62
133 174 159 103
R2 for regression of In X,o/ln Xwo on fe From ref 21.
Solubilities in Mixed Solvents. Solubilities of the four compounds were determined in aqueous MeOH solutions, and for diuron alone in aqueous DMSO solutions. Volume fraction cosolventf, in these mixed solvent systems was 0.2,0.4,0.6,0.8, and 1.0. Solubilities were determined in the various cosolvent solutions similar to the aqueous samples with the following modifications. The solutions were not heated as the 1,l-dimethylureas were reported (25) to undergo solvolysis with alcohols at elevated temperature, being converted to the corresponding Nphenyl carbamates. Also, equilibration times were extended to 7 days for the cosolvent solutions. Some cosolvent solution-compound pairs presented analytical problems due to the extremely high solubilities (up to mole fractional solubility = 0.05). The higher concentration solutions were sampled by weighing approximately 1.5 g of filtrate (saturated solution) into a tared vial and were subsequently diluted by the addition of 35.0 mL of MeOH. The resultant sample was analyzed by HPLC after further dilution, and the concentration of the initial diluted solution was determined. An iterative procedure was then used to calculate the mole fraction solute in the original undiluted saturated sample. Soil Supernatant Solution uv Absorbance. To assess the relative amount of soluble organic matter present in supernatant solutions of varying composition, 3 g of soil c were equilibrated with aqueous MeOH and aqueous DMSO solutions (7 ml) for 48 h. The suspensions, in 40mL borosilicate vials with PTFE-lined septa, were shaken during the equilibration period. Before reading UV absorbance (415 nm), the solutions were centrifuged and filtered through 1-pm glass fiber filters. Supernatant absorbances were read against blank solutions of the same solvent composition using a Bausch and Lomb Spectronic 21 spectrophotometer. Results and Discussion
Solubility and Cosolvency. Experimentally determined mole fractional solubilities of diuron in aqueous dimethyl sulfoxide solutions and fenuron, monuron, diuron, and neburon in aqueous MeOH solutions are given in Table 1, and Figure 1illustrates the log-linear solubility profiles of the four substituted phenylureas in aqueous MeOH solutions. Deviations from the log-linear model
12
10 c
8
3
6
& ?$
e 4 2 0
0
0.2
0.4
0.6
0.8
1
Volume Fraction Methanol Figure 1. Log-linear mole fractional solubility profilesfor four substituted Fenuron. phenylurea herbicides in aqueous methanol solutions. (0) (0) Monuron. (A)Diuron. (ST) Neburon.
for these compounds are comparable to other log-linear plots reported in the literature for moderately polar compounds (18). The aqueous solubilities are within 10% of reported l i t e W ~ r evalues (25) and range Over more than 3 orders of magnitude. Soil Partitioning. Figure 2 is a plot of mole-based Freundlich sorption parameters according to the solvoPhobic model (eq 7). Table 2 reports values obtained for the mole-based Freundlich coefficients KFX [(mol of (mol o f s ~ l u t i o n(g ) ~of OC)-'l, N, the observed slopes (Figure 21, and estimated values of a based on observed sloPe/a for each case. The Freundlich KFXand Nvalues were obtained by linear regression of In S (sorbed concentration, mol/g of OC) on In X (mol fraction sorbate in solution) for each set of isotherm data (21).Although a log-linear dependence of the relative Freundlich sorption coefficients on volume fraction cosolvent is observed, the estimated a values are variable, Walters and Guiseppi-Elie (7)report a = 0.8 described sorption of 2,3,7,8-TCDD to soil from aqueous MeOH solutions. Fu and Luthy (11)report a values in the range of 0.47-0.57 for sorption to soil of naphthalene, naphthol, quinoline, and 3,5-dichloroaniline. From literature data, they calculate a = 0.67 and 1.1for anthracene sorption Envlron. Scl. Technol., Vol. 28. No. 6, 1994
1005
Table 3. General Partition Model Isotherm Parameters, 90% Confidence Limits (in Parentheses), and Flory Standard-State Interaction Parameters x* no.
system fenuron, soil c tl
monuron, soil b
-2
0
0.2
0.6
0.4
0.8
diuron, soil b
f, (Volume Fraction Cosolvent) Figure 2. Observed log-linear relationship between relative molebased Freundlichpartition coefficientsand f, for various systems studed. Subscripts M and W refer to mixed and aqueous solvents, respectively; Fen, Mon, and Di refer to fenuron, monuron, and diuron; Me and DMSO refer to methanol and dimethyl sulfoxide; b and c refer to soils. (0) Di/Me/b. (0) FenlMelc. (*) DiIDMSOlc. (A) MonlMelb.
Table 2. Freundlich Isotherma and Solvophobic Parameters for Aqueous and Cosolvent Sorption Experiments
fenuron, soil c
monouron, soil b
diuron, soil b
diuron, soil cb
fMeOH
0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.4 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.7
I~KFx -1.72 -1.83 -2.20 -2.52 -0.71 -0.90 -1.43 -2.20 1.30 -0.34 -1.49 -2.71 0.76 -0.05 -1.10 -1.80 -2.39
N
slope
aEatimated
0.84 0.87 0.87 0.87 0.82 0.85 0.84 0.88 0.88 0.83 0.84 0.85 0.82 0.87 0.89 0.94 0.98
2.78
0.56
3.87
0.60
6.60
0.87
4.46
0.44
Freundlich parameters determined by regression of In S (molig of OC) on In Xsolution (mol fraction sorbate in solution). Based on ~ D M S Oinstead of fMeOH.
from aqueous MeOH and aqueous acetone solutions, respectively. For their study, Fu and Luthy (11)speculate that solvent-induced soil OC swelling increases accessibility of the solute to organic carbon, resulting in a smaller decrease in sorption than predicted by the solvophobic model (Le., low a values). The interpretation of solventsorbent effects based on CY is unclear given the variable nature of this parameter. The situation is further obscured for the ureas due to isotherm nonlinearity, i.e., a is not well defined for nonlinear sorption. Essentially, the problem is to determine whether the reduction in sorption due to cosolvent addition is commensurate with the increase in sorbate solution-phase activity coefficient, and if not, to determine if swelling of the organic matter matrix contributes to the effect of cosolvent on sorption. The general partition model (24,26)explicitly considers both solution-phase and sorbed-phase activity coefficients for Freundlich nonlinear isotherms (of which linear isotherms are a special case) and organic carbon-based 1006
Environ. Sci. Technol., Vol. 28, No. 6, 1994
hyl*
l / ( m + 1)
0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.4 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.7
1.66 (0.19) 1.61 (0.37) 1.68 (0.22) 1.58 (0.30) 2.07 (0.12) 2.10 (0.43) 2.21 (0.49) 2.09 (0.74) 2.55 (0.30) 2.81 (0.08) 2.77 (0.18) 2.36 (0.19) 2.49 (0.14) 2.50 (0.22) 2.47 (0.13) 1.79 (0.34) 1.22 (0.26)
0.84 (0.02) 0.87 (0.06) 0.87 (0.02) 0.87 (0.03) 0.82 (0.07) 0.84 (0.07) 0.84 (0.07) 0.87 (0.10) 0.87 (0.06) 0.83 (0.01) 0.84(0.04) 0.85 (0.03) 0.82 (0.02) 0.87 (0.04) 0.89 (0.01) 0.94 (0.05) 0.98 (0.03)
Based on ~ D M S Oinstead
Of
R2 obs. x* 0.998 0.993 0.999 0.998 0.999 0.984 0.980 0.971 0.988 0.999 0.997 0.998 0.998 0.997 0.999 0.996 0.998
12 8 8 8 16 10 10 10 12
8 8 8 12 8 8 8 8
1.29 1.26 1.31 1.24 1.57 1.58 1.66 1.58 1.91. 2.11 2.08 1.78 1.87 1.87 1.85 1.37 1.04
Of fMeOH.
sorption. The relationship between volume fraction sorbed-phase concentration and solution-phase concentration is
observed system
diuron, soil cn
fMeOH
(8) where X is solute mole fraction in solution, ys is the solution-phase activity coefficient of the solute, is the concentration (volume fraction) of solute in the (amorphous) polymeric humic phase, m is the degree of nonlinearity (= 0 for linear isotherms), and y1* is the sorbate standard-state equilibrium activity coefficient in the humic phase. y1* is the sorbate activity coefficient (Raoult’s law convention) for the theoretical limiting case of (amorphous) humic polymer in equilibrium with the pure liquid sorbate. Alternately, sorbed-phase interactions under standard-state equilibrium conditions are described by the Flory interaction parameter x*. Low values of x* correspond to more favorable sorbate-sorbent interactions compared with sorbate-sorbate interactions in the standard state. The method for determining ys,y1*,m, and x* from experimental cosolvent data is straightforward and completely analogous to the aqueous case previously presented (24). Results obtained for these cosolvent studies are presented in Table 3. The data (Table 3) show that, within 90% confidence limits, In y1* for fenuron, monuron, and diuron is unaffected by the presence of MeOH up to fMeOH 0.3, 0.4, and 0.6, respectively. In addition, within 90% confidence limits the parameter l / ( m + 1) shows no variation with MeOH concentration. Sorbed-phase activity coefficients y1 are calculated as a function of 41 from m and y1* (26),i.e., the urea y1 is essentially independent of solution-phase composition over the experimental ranges of ~ M ~ O HIt. may be inferred that the steric and electronic environment in the vicinity of sorbate molecules within the sorbed phase remains constant, Le., organic matter swelling is not a significant factor in these experiments. Therefore, the observed reduction in sorption that accompanied MeOH cosolvent addition was a result of solution-phase effects and is described solely by variation of the sorbate solution-phase activity coefficient.
-6
I
/
t
/
I
/i
s0.61
-81 -6 c
0
6
i1
-lo
-12
-14' -20
'
I
'
-18
-16
,
,
-12
-14
-10
-8
In X
(.li
Y = 0.86X - 1.61 R2 = 0.996 n = 36
1 8
-12
-10
-8
In 1
fMeOH
=0
3
-4
-6
I -2
xT,
0.1
0.4
0
0.2
0.6
0.8
1.o
- vol frac cosolvent
Flgure 4. Supernatant absorbance at 415 nm (1-cm path length) for 7 mL of various cosolvent solutions plus 3 g of soil c; equilibration time, 48 h. (0)DMSO. (A)Methanol. (m) Acetone.
8
-14 -14
0.2
f,
-4 I
-12j
0.0 0.0
* 0.3
Figure 3. (a) In-In plot of volume fraction sorbed-phase concentration 4, vs solution-phase concentration( X ) for fenuron methanol cosolvent sorption data on soil c. (b) In-In plot of volume fraction sorbed-phase concentration 4, vs solution-phase activity (XyJ for fenuron methanol cosolvent sorption data on soil c. The slope corresponds to an average l l ( m 1) over all data; the intercept is the average value for -In yl*.
+
This point is illustrated in Figure 3, aplot of sorbed-phase concentration 41 (volume fraction) vs solution-phase activity ( X y s ) for fenuron on soil c in various aqueous methanolsolutions. The slope and the intercept in Figure 3 correspond to average values of l / ( m + 1)and -In y1* for the sorption of fenuron (soil c, 0 5 fMeOH I 0.3). The MeOH partition data considered here extend from fMeOH = 0 to ~ M ~ O=H0.6. Walters and Guiseppi-Elie (7) investigated sorption of 2,3,7,8-TCDD in MeOH solutions up to fMeOH = 1.0 and found a log-linear relationship between mole-based linear sorption coefficients and fMeOH. If such a relationship holds here, extrapolation for the diuron/MeOH/soil b system yields a diuron Freundlich partition coefficient in 100% MeOH of (=lo-3) in aqueous solvent. This suggests that 100% MeOH should be an effective solvent for extraction of diuron from soil. Calculations show that a 1 g:10 mL of soi1:MeOH slurry at equilibrium should contain >99.9 % diuron in the MeOH phase. In reality, preliminary experiments for extraction of diuron residues clearly show incomplete recoveries (-80-90% for 24-96 h MeOH extraction, even for fairly short compound-soil contact times (1-7 days) before the extraction procedure. In contrast, extraction recoveries
using the aqueous dispersant/ethyl acetate method previously reported (24) were close to 100%. Apparently the desorption step for diuron is subject to kinetic limitations, or some other effect on partitioning beyond sorbatesolution interactions plays a role at high MeOH concentrations. In contrast to the MeOH data, y1* (Table 3) for diuron sorption from aqueous DMSO solutions decreases for fDMS0 > 0.4, suggesting that the solvent may cause swelling of the organic matter matrix. Diuron solubility in the sorbed phase (corrected for crystal energy effects) $I* = l/yl*. The data correspond to an increase in sorption capacity, intuitively consistent with swellingphenomena. The visual evidence of increasing dissolved organic matter (yellowcolored supernatant) with increasing f ~ ~ iss demono strated in Figure 4, a plot of supernatant absorbance (415 nm) vs fc for soil c in acetone, MeOH, and DMSO solutions. On the basis of visual comparison of the 70% DMSO supernatant solutions and aqueous leachate from soil columns, an order of magnitude estimate for soluble organic carbon in the 70% DMSO experiments is
