Environ. Sci. Technol. 1989, 23, 978-984
Carter, W. P. L.; Atkinson, R.; Winer, A. M.; Pitts, J. N., Jr. Znt. J. Chem. Kinet. 1982, 14, 1071-1103. Pitts, J. N., Jr. Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Tuazon, E. C. Chemical Consequences of Air Quality Standards and of Control Implementation Programs. California Air Resources Board Contract No. A1-030-32, 1983.
Carter, W. P. L.; Atkinson, R.; Long, W. D.; Parker, L. N.; Dodd, M. C. Effects of Methanol Fuel Substitution on Multiday Air Pollution Episodes. California Air Resources Board Contract No. A3-125-32, 1986. Killus, J. P.; Whitten, G. Z. Znt. J . Chem. Kinet. 1981,13, 1101-1103.
Leone, J. A.; Flagan, R. C.; Grosjean, D.; Seinfeld,J. H. Znt. J . Chem. Kinet. 1985, 17, 177-216. Kelly, N. A. J . Air. Pollut. Control Assoc. 1985,35,27-34. Glasson, W. A. The Teflon Bag Irradiation Facility: Physical and Chemical Characterization. General Motors Research Laboratories Publication GMR-6250, 1988. Wu, C. H.; Niki, H. Environ. Sci. Technol. 1975,9,46-52. Demerjian, K. L.; Schere, K. L.; Peterson, J. J. Adv. Enuiron. Sci. Technol. 1980, 10, 369-459. Joseph, D. W.; Spicer, C. W. Anal. Chem. 1978, 50, 1400-1403. Atkinson, R.; Lloyd, A. C. J . Phys. Chem. Ref. Data 1984, 13, 315-444. Demore, W. B.; Margitan, J. J.; Molina, M. J.; Watson, R.
T.; Golden,D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling. JPL Publication No. 85-37, 1985.
Baulch, D. L.; Cox, R. A.; Crutzen, P. J.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1982, 11, 327-496. Baulch, D. L.; Cox, R. A,; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J.; Watson, R. T. J . Phys. Chem. Ref. Data 1984,13, 1259-1380. Dunker, A. M. J. Chem. Phys. 1984,81, 2385-2393. Sakamaki, F.; Hatakeyama, S.;Akimoto, H. Znt. J. Chem. Kinet. 1983, 15, 1013-1029. Pitts, J. N., Jr.; Sanhueza, E.; Atkinson, R.; Carter, W. P. L.; Winer, A. M.; Harris, G. W.; Plum, C . N. Znt. J . Chem. Kinet. 1984, 16, 919-939. Gill, P. E.; Murray, W.; Wright, M. H. Practical Optimization; Academic Press: New York, 1981. Hindmarsh, A. C. ACM Signum Newsletter 1980,15,10-11. Dunker, A. M., unpublished work. Beck, J. V.; Arnold, K. J. Parameter Estimation in Engineering and Science; John Wiley: New York, 1977. Carter, W. P. L.; Lurmann, F. W.; Atkinson, R.; Lloyd, A. C. Development and Testing of a Surrogate Species Chemical Reaction Mechanism. Environmental Protection Agency Contract No. 68-02-4104, 1986. Carter, W. P. L.; Atkinson, R. Environ. Sci. Technol. 1987, 21, 670-679.
Akimoto, H.; Takagi, H.; Sakamaki, F. Znt. J . Chem. Kinet. 1987, 19, 539-551. (34) Dunker, A. M. Proceedings, Workshop on Evaluation/
Documentation of Chemical Mechanisms. Environmental Protection Agency, 60019-87-024, 1987.
Received for review June 30, 1988. Accepted April 19, 1989.
Estlmatlng the Effects of Dispersed Organic Polymers on the Sorption of Contaminants by Natural Solids. 1. A Predictive Thermodynamic Humic Substance-Organic Solute Interaction Model Yu-Ping Chin”
Ralph M. Parsons Laboratory, The Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Walter J. Weber, Jr. The Environmental Engineering and Water Resources Program, The University of Michigan, Ann Arbor, Michigan 48 109 ~~~~
The binding of hydrophobic organic compounds to humic polymers commonly dispersed in water has been shown to alter the fate and transport of organic contaminants. In an effort to better characterize and quantify the effects of such binding reactions a modified FloryHuggins model was applied in conjunction with solute aqueous activity coefficient data to estimate the association of organic contaminants to humic and other organic polymers in the aqueous phase. The solubility parameters of “humic-like” organic “surrogates” having well-defined physicochemical properties were used to calibrate the model. Predictions based on model calibrations to methyl salicylate agreed well with experimental values for the binding of 14 target compounds, exhibiting a wide range of properties, to commercial humic acid substrates. Attempts to estimate the association of several hydrophobic contaminants to polar organic polymers found in natural surface waters, using polymaleic acid as a fulvic acid surrogate, were less successful. Model predictions and experimental evidence both show a lower binding affinity of the target species studied to dispersed organic substrates present in natural aquatic systems than to commercial humic substances.
Introduction Natural aqueous samples that will pass through a 978
Environ. Sci. Technol., Vol. 23, No. 8, 1989
0.45-pm fiiter contain a broad spectrum of organic material that range from small dissolved polymers to discrete microparticulates. These “dispersed organic polymers” are comprised of humic/ fulvic acids and other heterogeneous macromolecules and are known to affect both the behavior and bioavailability of hydrophobic organic compounds in the aquatic environment (1-5). They interact with and may alter ultimately the treatability, fate, and transport of hazardous substances. A concerted research effort has been mounted in recent years to study and measure the degree of organic contaminant binding to dispersed organic polymers at equilibrium (6-12) and the kinetics of such reactions (13,14). It has been hypothesized that binding is analogous to partitioning, and that a target solute leaves the aqueous phase to interact with the polymer at specific “hydrophobic centers” within the macromolecular matrix (15). Several investigators (8,14) have invoked observed correlations between equilibrium polymer binding constants and the hydrophobicity of the target compound quantified either by aqueous solubility or by octanol/water partition coefficients. While such correlations demonstrate the diversity of the impact of humic substances, most of them provide little insight on specific thermodynamic mechanisms that control molecular interactions within the polymer phase. Chiou and co-workers (11,12) were among the first to
0013-936X/89/0923-0978$01.50/0
0 1989 American Chemical Society
note that humic polymers associated with natural solids generally constitute a less efficient phase for hydrophobic compounds than does a more nonpolar substance such as octanol, and that substantial differences in molecular size between a solute and a host polymer may bring about large deviations from Raoult’s law for such systems (16). They were able to quantify nonideal molecular interactions for such systems using the Flory-Huggins equation. This approach is conceptually realistic, since it takes into account deviations from the ideal free energy of mixing caused by disparities in molecular size between the target compound and humic polymer and changes in the heats of interactions as predicted by the Regular Solution model. The work presented in this two-part series applies the Flory-Huggins concept to development of a predictive and conceptual hydrophobic organic compound-humic polymer association model. This type of approach suggests the possibility of estimating equilibrium binding polymerpollutant binding constants from known molecular properties, a highly desirable objective with respect to understanding fate and transport processes. It is presumed in this model development that dispersed humic polymers will interact strongly and specifically with both water and the target compounds, and modifications of the original equation derived by Chiou and co-workers (16) are therefore made to account for specific molecular interactions (both polar and inductive forces). Estimated equilibrium binding constants are compared to experimental values for four hydrophobic compounds measured by the dialysis technique as well as to binding constants reported in the literature for other substances.
Theory and Background The binding of a hydrophobic organic compound with a separate polymer phase dispersed in an aqueous system can be quantified by a simple linear equilibrium relationship analogous to partitioning: Kb
=
(1)
cp/ce
where C, is the amount of solute bound per unit mass of polymer, and C, is the “free” solute in the aqueous phase. The binding constant defined in terms of the activity coefficient for the solute in each phase is In (Kb)= In (yiw)- In
(yip)
+ In (V,)
- In
(V,) (2)
where yiwand yip are, respectively, the activity coefficients for the solute in the aqueous phase and polymer, and V, and V, are the polymer and water molar volumes. The activity of an organic solute, ai, in a polymer phase can be estimated by the Flory-Huggins equation (16-19): In (ai), = In di
+ dp(l - Vi/Vp) + xdp2
(3)
where r#q. and dP are the respective solute and polymer volume fractions, Vi is the molar volume of the target compound, and x is the Flory parameter. The activity of the solute is also defined as the product of its activity coefficient and its mole fraction solubility, Xi, and eq 3 becomes In (yiXi)p = In 4i +
- Vi/Vp)
+ xdp2
(4)
The mole fraction of the organic solute within the polymer phase is defined as
(XJP= ni/np
(5)
where ni and np are the number of moles of solute and polymer, respectively. The solute volume fraction, di, can be approximated by
di = ni(Vi)/n,(V,)
(6)
Substitution of eq 5 and 6 into 4 yields In (Ti), = In (Vi) - In (Vp) +
- Vi/Vp) + ~ 4 : (7)
Because the molar volume of the organic polymer phase is much greater than the solute molar volume (104-106 vs lo2),the ratio Vi/V, can be neglected (16). The volume fraction of the polymer, +p, also approaches a constant value of unity for solutes at infinite dilution. Equation 7 thus simplifies to In
(yip)
= In (Vi) - In (V,)
+1+x
(8)
Combination of eq 2 and 8 and conversion to the common logarithm yields an expression similar to the one derived by Chiou et al. (16):
1% (Kb) = log (yi”) + log (V,/Vi) - log p - (1 + ~ ) / 2 . 3 0 3(9) where the term p is the density of the polymer added to ultimately express Kb in terms of volume per unit mass. Chiou and co-workers (16) observed that p for humic polymers is approximately 1.2 based upon values reported for polymers similar in structure to humic substances. The value x quantifies the “compatibility” of the target compound with it’s host polymer and is composed of enthalpy (Xh) and entropy (x,) contribution terms:
x
= Xh + x s
(10)
The entropy term is determined empirically, while Xh can be estimated by utilizing regular solution models such as the Scatchard-Hildebrand equation (16,20). It is readily evident that accurate estimations of x are central for purposes of predicting binding constants. The Flory parameter remains relatively constant for dilute solutions, although it may be slightly dependent upon the molecular weight of the polymer (20). Studies have shown that there exists a critical value of the Flory parameter xc where
xc = 0.5(1 + l/m1/2)2
(11)
where m is equal to V /Vi. Because the term m is usually very large, xc generailly has a value approximating 0.5. When the term x is less than xc, the solute becomes miscible with the polymer phase over the entire mole fraction concentration range from 0 to 1. For values of x greater than 0.5, less compatible molecular interactions may take place. The entropic contribution, xs, may vary from 0 (molecules of approximately equal molar volume) to 0.5 (molecules that are highly dissimilar in size). Chiou and co-workers (16) used an entropic Flory parameter value of 0.25 for humics associated with soils, while Blanks and Prausnitz (21) determined xs to be 0.34 for polar polymers. The later value was selected for purposes of predicting Kb because organic macromolecules present in natural waters are polar by nature. A range of xS values from 0.1 to 0.5 (x,) is evaluated here to test the model’s sensitivity to the entropic contribution parameter. Hydrophobic organic contaminants are highly insoluble in water and as a consequence would not be entirely miscible with polar polymers. Many naturally occurring polymers present in natural aquatic systems contain functional groups that can interact appreciably with water (primarily through hydrogen bonding). Under these circumstances, a nonpolar target compound would be subject to both dispersive molecular interactions and nonsymmetrical induction forces. Hence, the enthalpic contribution to the Flory parameter may be significant, and x would exceed xc for most interactions between hydroEnviron. Sci. Technol., Vol. 23, No. 8 , 1989
97s
phobic organic compounds and water-soluble polymers. The presence of specific molecular interactions between the solute and the polymer further complicates matters by severely limiting the applicability of the ScatchardHildebrand equation, which assumes that all molecular interactions are weak and dispersive (20). Blanks and Prausnitz (21), however, were able to provide a means of evaluating a priori the excess enthalpic contribution parameter by applying a modified regular solution equation: Xh = (Vi/Rn?[AlZI (12) where AI2is an interchange energy density. This parameter can be estimated through the use of an expanded regular solution theory treatment that takes into account dispersive, polar, and inductive interactions: where 1c, is an induction energy term that quantifies permanent dipole-induced dipole molecular interactions, hi is the solute solubility parameter, and A, and 7 , are the respective nonpolar (London/van der Waal forces) and polar (Keesom and hydrogen bonding interactions) polymer solubility parameters (21). This particular approach recognizes the importance of dispersive, polar, and inductive interactions. The parameter rc/ can be estimated by using several well-established correlations and is a function of the polar solubility parameter (20). The heterogeneous nature of humic substances precludes any attempt to determine their polar and nonpolar solubility parameters through the use of conventional techniques (Le., heat of vaporization, refractive index, Small’s molar attraction constants). Chiou and co-workers (16) determined a value of 13 for the overall solubility parameter of organic polymers associated with soils. Karickhoff (22) postulated that the solubility parameter of sedimentary humic substances may be around 10 because dichloromethane (6 = 9.7) was found to be the most effective solvent for extracting di(2-ethylhexyl) phthalate from natural sediments. Curtis and co-workers (3) used an average of these two values, 6 = 11.5, for their prediction of soil KO,values. An alternative approach to elucidating the solubility parameters of humic substances is to identify certain characterizable organic compounds and substitute their physicochemical properties into the Flory-Huggins model as ”surrogates”. While humic and fulvic acids are complex macromolecules, they are nonetheless polymers comprised of several repetitive molecular building components. The manner in which these individual molecular components are linked together dictates the ultimate structure and form of a particular humic polymer. Hildebrand and coworkers (20) suggested that the solubility parameters of a polymer are similar to its major subunit. Thus, it may be plausible to characterize the properties of a humic polymer if some knowledge of its basic component structure is known. Based upon the several hypothesized humic substance configurations, methyl salicylate was chosen in this work as a surrogate polymer subunit compound. This substance possesses all of the important functional groups that are characteristic of humic and fulvic acids. It has an overall solubility parameter of 10.6, with a polar contribution term of 7.16, and a nonpolar solubility parameter value of 7.8 (23). Two other substances that are distinctively “humic-like” in structure with definable physicochemical properties are polymaleic acid (PMA) and lignin. With respect to the former, several investigators (24,25) have shown through degradative and spectroscopic studies that PMA is similar in structure to fulvic acids. Lignin is the recalcitrant aromatic polymer found in woody plants 980
Environ. Sci. Technol., Vol. 23, No. 8, 1989
and is believed to be a precursor to both terrestrial and aquatic humic materials (26).
Materials and Methods Solute-Polymer Binding Studies. Equilibrium constants quantifying the interaction between organic solutes and polymeric suspensions were measured by use of a semipermeablemembrane dialysis technique. The dialysis method involves measurement of diffusive transport across a membrane that restricts the migration of molecules greater than 1000 daltons (the polymer), while passing target solute freely. Four target compounds, p-dichlorobenzene, 1,2,4-trichlorobenzene, cis-chlordane, and 2,5,2’-PCB, were chosen on the basis of their toxicity and physicochemical properties (solubilities that span 3 orders of magnitude, and volatility), as well as ease of quantitative analysis. Aldrich humic acid (Aldrich Chemical Co.) was selected as the target polymer. While commercial humic acids are not entirely representative of organic polymers found in natural aquatic systems (8,12), they have been extensively studied (8, 10, 121, and a large database of pollutant binding constants exists, which was used for verifying the expanded Flory-Huggins model results. Stock solutions were made by adding specified amounts of Aldrich humic acid to distilled water and raising the pH to 11 with 1.0 N NaOH to facilitate dissolution. After 1 h of agitation, the pH was readjusted to 7 with 0.1 N HC1 and the solution filtered through two prewashed glass fiber filters (Gelman Science). Working concentrations of the humic acids were prepared by diluting the stock solutions with 0.1 M NaHC03 buffered water. The organic solutes were introduced into 150-mL Hypo-Vials (C. P. Pierce Inc.) at different concentrations by using a volatile carrier solvent (acetone). The solvent phase was volatilized and 100 mL of buffered water (0.1 M NaHC03) containing 10 mg of reagent-grade NaN3 (to inhibit microbial activity) was added to each reactor. The ionic strength and pH of the solution were determined to be 0.1 and 8.0, respectively. Spectra-Por 6 tubular dialysis material (Spectrum Medical) was triple washed in Milli-Q water (Millipore Corp.) to remove any preservatives and other associated impurities and cut into sections long enough to hold 5 mL of liquid. One end of the tubing was tied off with acetone-washed cotton thread and the bag filled with either an aqueous suspension of polymer (Aldrich humic acid solution) or buffered water (control). The other end was subsequently secured in the same manner, placed within each reactor, capped with a Teflon-lined aluminum seal, and allowed to stand for 120 h at 22 “C. A series of controls (no DOP present) was also run for material balance purposes. Studies by Carter and Suffet (7) indicated that equilibrium for these types of experiments was generally attained in 3 days. A 5-day-run period was used in the present work to ensure equilibrium. At the end of this period, the dialysis bags were carefully opened and 4 mL of sample was pipeted into 4 mL of hexane. Similarly, 25 mL of the buffered solution was removed and extracted with 5 mL of hexane. Assays of the amount of compound present in both the polymer suspension and the buffered solution were performed by electron capture gas chromatography. Dialysis experiments with volatile target compounds were performed in a manner similar to that outlined above, except that they incorporated a no-headspace analysis. The polymer suspensions were secured in each dialysis tube and placed in 150-mL Hypo-Vial reactors. The bottles were subsequently filled to capacity with sodium bicarbonate buffered water and spiked with 10 mg of sodium azide. An aliquot of stock solution was drawn and
i500
9
Table I. Octanol/Water Partition Coefficients and Binding Constants for Selected Organic Solutes and Aldrich or Fluka Humic Acid Polymers
compound
0
0
10
20
2.5.2-PCB Chlordane
30
cw (Ugm Flgure 1. Binding of 2,5,2'-PCB and a-chlordane to an Aklrich humic acid substrate as determined by the dialysis method.
introduced into each dialysis reactor, immediately sealed, and allowed to stand for 120 h. The methods used to assay the target compounds followed the procedures outlined above.
Results and Discussion Binding of Hydrophobic Compounds to Aldrich Humic Acid. The dialysis technique was applied to determine the binding potential of Aldrich humic acid to 2,5,2'-PCB, a-chlordane, l,CDCB, and 1,2,4-TCB. These compounds were selected because (1)no previous binding data were available for any of these substances and (2) they span a range of solubilities and molar volumes representative of contaminants found in natural aquatic systems. The affinity isotherms for chlordane and 2,5,2'-PCB, presented in Figure 1, are representative, in general form, of those measured for the compounds studied. The term affinity is not meant here to imply any specific type of mechanism (i.e., sorption, solubility enhancement), since it is rather unclear which processes are responsible for the observed behavior. This phenomenon may be analogous to partitioning because the target solutes tested exhibit a stronger preference for the dispersed polymer phase over water, a fact that must relate in one manner or another to hydrophobic effects. The amount of solute bound to the polymer substrate was determined by the relationship
ct - c*(1000 mg/g) c, = X
(14)
where C, is the amount of solute bound per unit weight of polymer expressed in terms of organic carbon (pg/g of OC), C,is the amount of free and bound solute within the dialysis bag (in hg/L), and X is the amount of humic acid present (as mg of OC/L). Isotherms for target solutes were typically linear over the entire range of concentration studied, and a least-squares linear regression analysis was performed to obtain binding constant values. All experimental binding constants were normalized to the organic carbon fraction of the commercial humic substrate, while predicted Kb values were expressed in terms of the organic matter content. McCarthy and Jimenez (14) found that Aldrich humic acid is comprised of approximately 50.2% organic carbon by mass. On this basis, the relationship between Kb and the organic carbon normalized binding constant, Kb,oc,is Kb = 0.502(Kb,0c)
(15)
The normalized commercial humic acid binding constants, Kb,*, for the target compounds and 10 other solutes, for which similar information has been reported in the literature, are summarized in Table I, along with the re-
TCE toluene naphthalene phenanthrenea anthraceneb fluorene biphenyl 2,2',5-PCB 2,2',4-PCB 2,2',5,5'-PCBC a-chlordane ~,~'-DDT~ 1,4-DCB 1,2,4-TCB
log 2.53 2.69 3.38 4.46 4.54 4.18 3.95 6.00 5.67 5.84 6.00 5.98 3.36 3.98
log (Kb,oc)cbsd 2.20e 2.27' 3.04' 4.00 4.21 f 0.11 3.958 3.27h 4.57' 4.84 4.97 f 0.35 4.77' 5.61 f 0.11 2.92' 3.11'
OAverage value from Landrum et al. (5, 6); n = 2. "Average value from Landrum et al. ( 5 , 6 )and McCarthy and Jiminez (14);n = 4. 'Average value from Landrum et al. (5, 6 ) , Hassett and Millicic (13),and Eadie et d. (30); n = 7. dAverage values from Carter and Suffet (7), Landrum et al. (5, 6), and Chiou et al. (12); n = 6. eGarbarini and Lion (10). fMcCarthy and Jimenez (14). BCarter and Suffet (8). hLandrum and co-workers (5). 'This work. Chiou and co-workers (12). kAll K,, values are from Chin et al. (31. 32). J
6 ,
2 5 a 0
Y
8
6
Y
4
0)
9
3
2
log Kow
Figure 2. Correlation between n -octanol/water partition coefficients (Kow)and organic carbon normalized solute-commercial humic acid binding constants (Kb,%).
spective octanol/water partition coefficients for the same compounds. Figure 2 illustrates the relationship between the respective Kb,ocand KO, values. Linear regression analysis of this relationship yields log (Kb,oc)= 0.82 log (KO,)+ 0.1923 r = 0.96; n = 14 (16) The relatively high degree of correlation supports a general hypothesis that the binding constant is strongly dependent upon the hydrophobicity of the compound (14). Inspection of the data in Table I indicates that the observed experimental binding constants are consistently 0.2-1.5 log units lower than their respective octanol/water partition coefficients, and that these differences appear to increase with increasing hydrophobicity. This seems to imply that solute interactions within the octanol phase and the dispersed polymer are different, which is consistent with findings reported previously for sorption by natural solids. Chiou and co-workers (16) noted that humic substances can absorb a significant amount of water, while octanol can hold only -5% water by weight. They concluded from this that humic polymers are more polar than octanol and thus comprise thermodynamically less favorable partitioning phases for nonpolar organic solutes. A similar partitioning phenomenon has been documented by Leo and co-workers (27) for organic solvents that can dissolve significantly Environ. Sci. Technol., Vol. 23, No. 8 . 1989
981
Table 11. Physicochemical Constants Needed for Predicting Binding Constants by the Modified Flory-Huggins Model"
Table 111. Comparison of Predicted and Observed Solute Binding Constants for Commercial Aldrich and Fluka Humic Acids"
compound
6
Vi
log (Vw/ Vi)
log (Yi")
compound
TCE toluene naphthalene phenanthrene anthracene fluorene biphenyl 2,5,2'-PCB 2,4,4'-PCB 2,2',5,5'-PCB a-chlordane p,p'-DDT 1,4-DCB 1,2,4-TCB
9.2 8.9 9.9 9.8 9.9 9.7 8.3 9.9 9.9 10.4 10.4 8.8 9.7 9.3
90.2 106.8 111.5 158 150 138 177 168 168 179 223 222 112.8 159
-0.70 -0.77 -0.79 -0.94 -0.92 -0.885 -0.99 -0.97 -0.97 -0.99 -1.09 -1.09 -0.80 -0.95
3.81 3.97 4.82 6.25 6.25 6.00 5.72 7.36 7.42 7.98 7.78 8.04 4.79 5.49
TCE toluene naphthalene phenanthrene anthracene fluorene biphenyl 2,2',5-PCB 2,2',4-PCB 2,2',5,5'-PCB a-chlordane p,p'-DDT 1,4-DCB 1,2,4-TCB
Pesticides and 2,2',5,5'-PCB solubility parameters were calculated from Small's molar attraction constants. All other values were estimated from heat of vaporization data or taken from Barton (23). TCE,trichloroethylene. Molar volumes were determined from reported formula weights and compound densities or reported in Barton (23). Activity Coefficients from Chin et al. (31, 32).
log
(Kb)pred
log
1.98 2.04 2.59 3.59 3.62 3.57 3.37 4.55 4.62 4.76 4.08 5.29 2.61 3.02
(Kb,w)pred
2.28 2.34 2.89 3.89 3.92 3.87 3.67 4.85 4.91 5.06 4.38 5.59 2.91 3.32
1%
(Kb,oe)obad
2.20 2.27 3.02 4.00 4.21 f 0.11 3.95 3.27 4.57 4.84 4.97 f 0.35 4.77 5.61 f 0.11 2.92 3.11
"Methyl salicylate parameters: X = 7.8 (cal/mL)0,6;7 = 7.16 (~al/mL)O.~. The value 7 incorporates contributions from both polar and hydrogen-bonding type interactions [taken from Barton (23)1.
Regression analysis between observed and predicted binding constants yields
1%
(Kb,oc)obsd
= 1.01 1%
(Kb,oc)pred
+ 0.036 r = 0.98; n = 14 (17)
1 1
2
3
4
5
6
Log Kb,m@red)
Figure 3. Correlation between observed and Flory-Huggins model predicted solute-commercial humic acid binding constants.
more water than octanol. Miller and co-investigators (28) further support this position by noting that octanol and organic geopolymers associated with natural solids are different in both size and structure, and that the behavior of hydrophobic solutes within these structurally different types of phases can be expected to differ. It is rather apparent that the relationship between K b and K,, although qualitatively attributable to their mutual dependence upon the aqueous-phase activity coefficients of the target solutes, is largely empirical in nature. Equation 16, however, does provide a means for making initial estimates of binding constants for systems similar to those tested. Estimation of Binding Constants with the Modified Flory-Huggins Model. The modified Flory-Huggins model given by eq 9 was applied to predict the equilibrium binding constants for 14 target compounds in Aldrich or Fluka humic acid/ water system. Methyl salicylate was used as the humic surrogate polymer surrogate, and its solubility parameters were employed for calibrating the model. The physicochemical properties of the target compounds and methyl salicylate are reported in Table 11. The model output and experimental binding constants are tabulated in Table I11 and reproduced graphically in Figure 3. Inspection of this information reveals that the modified Flory-Huggins model predictions come within less than */z order of magnitude of observed values for all cases studied. Binding constants, for which more than one reported value was found, averages, and standard deviations are provided to illustrate ranges of reported values. 982
Environ. Sci. Technoi., Voi. 23, No. 8. 1989
Linearity exists between the pooled sets of data over a span of 3 orders of magnitude. The correlation clearly given in eq 17 illustrates relatively good agreement between model output and observed values, in that the slope of the line is approximately unity and the y-intercept value approaches zero. The absolute deviation between predicted and experimentalK b ranges from 0.40 to 0.01 log unit, with an absolute average deviation of 0.16. In terms of predictive capabilities, the model can estimate the log equilibrium binding constant for the hydrophobic organic compounds tested to a commercial humic substrate to within 4.3 f 3.4% of experimentally determined values. The modeling approach is conceptually satisfying in that such estimations of Kb are more strongly rooted in thermodynamic principles than are those resulting from correlations or other "curve-fitting" models (such as the octanol/water approach). In addition, the model parameters are derived from knowledge of the molecular properties of both solute and polymer and are not empirically elucidated through the use of regression-derived constants. Model Sensitivity Analysis: Relative Importance of the Entropic and Enthalpic Flory Parameters. Many of the model coefficients used to characterize the target compounds can be accurately determined from their physicochemical properties (e.g., heat of vaporization, molar volume) or through the use of analytical measurements (e.g., aqueous-phase activity coefficient). The properties of the polymers are much more difficult to quantify, although rough estimates can be attained through the use of surrogate substances or back-extrapolation techniques (16,22). As part of the current investigation, an analysis was performed in which model outputs based upon various input values of xs,A, and 7, were compared to experimental values. Model outputs for different entropic contribution parameters are illustrated in Figure 4. An "ideal line" or relationship in which estimated Kb,oc values are exactly equal to observed Kb,oc values is included as a reference to illustrate the relative sensitivity of the model's predictive capability. The parameter xs becomes relatively more important than Xh, if the heats of interaction between the
“ I
Table IV. Comparison of Observed Binding Capacities of the Dispersed Organic Polymer Phases Present in Several Natural Waters and Fulvic Acid to Model Predictions Based upon PMA As a Humic Surrogate Compound’
/I
PbI 0 ChiW =0.1 0 Chi($ = 0.25 0 Chi(s)=0.5 A Ideal Line A Chi(s) = 0.34
2
3
5
4
6
L o g Kb,oc (pred.)
Figure 4. Sensitivity of the model to changes in the entropic Flory parameter The ideal line is defined as K,,(obsd) = Kb,,(pred).
(x,).
6 5
a
s4
organic polymer source
DDT
log Kb,, 2,4,4’PCB
PMA predictions Sopchoppy River Waterb Suwanee River Waterb Suwanee River F.A.b Pakim Pond Waterc Lake Michigand Huron Rivere
4.56 4.39 4.39 4.40 4.84 4.26 f 0.44 4.23
4.21 3.57 3.53 3.57 no data no data no data
p,p’-
2,2’,5,5’PCB 4.35 no data no data no data no data 3.88 i 0.53 3.87
“Polymaleic acid parameters: 6, has a reported value of 13.6 (~al/mL)O.~ based on the solubility parameter of maleic anhydride [taken from Barton (23)]. The value h is 8.09 (~al/mL)O,~ based upon refractive index studies, and T is computed from h and 6,. bChiou and co-workers (12). CCarterand Suffet (7). dEadie and co-workers (30). e Landrum and co-workers (5).
Y
$ 3
n Y
0 PMA
-g 2
0
A
1
LIGNIN ideal Line
0
0
1
2
3
4
5
6
7
L o g Kb,W (pred.) Figure 5. Sensitivity of the model to humic polymer surrogates with different physicalhhemical properties.
solute and polymer are zero or very small (Le., can be quantified accurately with the unmodified regular solution equation). Values of xe were varied from 0.1 to 0.5, which defines the critical Flory parameter. It was found that the model was relatively insensitive to deviations from the ideal entropy of mixing. This is not unexpected given that x exceeded the critical value of 0.5. This is in large part caused by the nonideal enthalpic molecular interactions than can occur between the target compound and its host polymer. These findings are consistent with those observed by Chiou and co-workers (16),who found that the term Xh comprised a significant portion of the total Flory parameter. For reasons stated previously, polymaleic acid was selected as a surrogate compound to represent polar constituents such as fulvic acids commonly found in natural waters. The solubility parameters of PMA are based upon its subunit, maleic anhydride, which has a bt (total solubility parameter) value of 13.6 (~al/mL)O.~ (23). The nonpolar solubility parameter was estimated to be 8.09 (~al/mL)O.~ based upon index of refraction data and correlations developed by Karger and co-workers (29). The polar solubility parameter was subsequently determined from the relationship 6: = r 2
+ X2
p ( A H v - RT)/MW
(18)
and was found to have a value of 10.9. Lignin, an aromatic substance found in all woody plants, and a likely precursor for many humic materials (26), was selected to represent a somewhat less polar surrogate compound. Reported nonpolar and polar solubility parameters for lignin are 9.1 and 5.23, respectively (23). Figure 5 illustrates model output for the other two surrogate compounds. Binding constants are consistently overestimated by the model when it is based upon the solubility parameters for lignin. Regression analysis be-
tween the two pooled sets of data for this humic substance surrogate yields 1% (Kb,oc)obsd = 0.856 1% (Kb,oc)pred + 0.0217 r = 0.99; n = 14 (19) Related predictions of K b suggest that solutes are more compatible in the lignin phase than in commercial-grade humic acid phase. This is due in part to lignin’s structure, which is primarily phenolic in nature and deficient in more polar functional groups, such as carboxylic acids, which can interact more specifically with water as well as with each other. Inspection of the relative contributions of the polar and nonpolar solubility parameter terms also indicates that the cohesive energy density (i.e., the solubility parameter squared) is largely comprised of dispersion forces (A2 = 82.8 cal/mL) compared to ? = 27.35 cal/mL). Hence, while lignin is considered to be a precursor to many humic and fulvic acids, it differs from these latter substances in structure and property and would be unsuitable for use as a model surrogate compound. Predictions of the binding constant based upon PMA as a surrogate humic substance compound are consistently lower than observed values. Regression analysis of the data yields the following equation for the correlation curve: 1% (Kl,,m)obd = 1.15 log (Kb,W)pred + 0.179 r = 0.98; n = 14 (20) Inspection of the nonpolar and polar solubility parameter contributions of PMA to the cohesive energy density indicates a reversal in their relative importance (A2 = 65.45 cal/mL, while r2 = 119.42 cal/mL). This makes PMA an overall more polar and less efficient substrate with respect to nonpolar solute partitioning potential. The structural properties of PMA revealed by spectroscopic and degradative methods suggest the existence of a large number of carboxylic functional groups attached to olefins and possibly aromatic structures. PMA thus may be used as a suitable surrogate compound for fulvic acids and other polar constituents of the dispersed organic polymer matrix present in natural waters. Attempts to predict natural water binding constants for three hydrophobic compounds (p,p’-DDT, 2,4,4’-PCB, 2,2’,5,5’-PCB) and polymaleic acid as a polymer surrogate are reported in Table IV along with observed values for several fulvic acids and natural waters. Agreements between model outputs and measured values are generally good, with the exception of 2,4,4’-PCB for which there is a deviation of 0.68 log unit between estimated and obEnviron. Sci. Technol., Vol. 23, No. 8 , 1989
983
served Kb values. All the natural water binding data display a lower binding efficiency relative to commercial humic substances, and this is predicted by the model using PMA as a polar humic substances surrogate. This suggests that dispersed organic polymers typical of natural waters are more polar than commercially available humic acids and are, as a result, less efficient at partitioning hydrophobic organic compounds. These findings are in agreement with conclusions reached by Chiou and co-workers (12). Leo and co-workers (27) reported an analogous finding when they compared organic solvent/water partition coefficients tQ corresponding K , values. They noted that for all target solutes tested, polar organic solvent/ water systems yielded Kp values that were lower than their respective octanol/water partition coefficients. Summary and Conclusions (1)The modified Flory-Huggins equation was employed with some success as a means for estimating binding constants of hydrophobic organic compounds to dispersed organic polymers of humic nature. The modifications incorporated terms to account for polar and inductive molecular interactions. (2) Methyl salicylate, utilized as a characterizable compound similar in structure to commercial humic acids, was found to be a reasonable surrogate for purposes of model calibration. Model predictions based upon this surrogate agreed quite well with experimentally determined binding constants for a variety of different target solutes. (3) Analysis of the sensitivity of the model to selected input parameters and coefficients revealed that model output was relatively insensitive to the entropy contribution term, but varied more widely with changes in the enthalpy contribution term. (4) Predictions of binding using polymaleic acid as a surrogate for polar dispersed organic polymers were generally lower than those using methyl salicylate. However, in all cases studied, the model overestimated binding constants for the limited pool of observed data provided. Acknowledgments We express appreciation to Ingrid Padilla, Kathy Smee, and Stephen Westenbroek for their contributions to the experimental and data acquisition stages of this project, as well as to Phil Gschwend, Cary Chiou, and Bruce Brownawell for their valuable comments and discussions. Registry No. PMA, 26099-09-2;TCE, 108-88-3;2,2’,5-PCB, 37680-65-2; 2,2’,4-PCB, 37680-66-3; 2,2’,5,5’-PCB, 35693-99-3; p,p’-DDT, 50-29-3; 1,4-DCB, 106-46-7; 1,2,4-TCB, 120-82-1; toluene, 91-20-3; naphthalene, 79-01-6; phenanthrene, 85-01-8; anthracene, 120-12-7; fluorene, 86-73-7; biphenyl, 92-52-4; achlordane, 5103-71-9;methyl salicylate, 119-36-8;lignin,9005-53-2.
Literature Cited Voice, T. C.; Weber, W. J., Jr. Enuiron. Sei. Technol. 1985, 19, 789. Gschwend, P. M.; Wu, S. Enuiron. Sei. Technol. 1985,19, 90. Curtis, G. P.; Reinhard, M.; Roberts, P. V. In Geochemical Processes a t Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; p 191. Morel, F. M. M.; Gschwend, P. M. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley-Interscience: New York, 1987; p 405.
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( 5 ) Landrum, P. F.; Nihart, S. R.; Eadie, B. J.; Gardner, W. S. Environ. Sei. Technol. 1984, 18, 187. (6) Landrum, P. F.; Reinhold, M. D.; Nihart, S. R.; Eadie, B. J. Enuiron. Toxicol. Chem. 1985, 4, 459. (7) Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982,16, 735. (8) Carter, C. W.; Suffet, I. H. In Fate of Chemicals in the Environment Compartmental Modeling for Predictions; Swann, R. L., Eschenroder, A., Eds.; American Chemical Society: Washington, DC, 1983; p 215. (9) Gauthier, T. D.; Shane, E. C.; Guerin, W. F.; Seitz, W. R.; Grant, C. L. Environ. Sei. Technol. 1986, 20, 1162. (10) Garbarini, D. R.; Lion, L. W. Enuiron. Sci. Technol. 1985, 19, 1122. (11) Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sei. Technol. 1986, 20, 502. (12) Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. A.; MacCarthy, P. Enuiron. Sei. Technol. 1987, 21, 1231. (13) Hassett, J. P.; Millicic, E. Enuiron. Sei. Technol. 1985,19, 638. (14) McCarthy, J. F.; Jimenez, B. D. Enuiron. Sci. Technol. 1985, 19, 1072. (15) Khan, S. U.; Schnitzer, M. Geochim. Cosmochim. Acta 1972, 36, 745. (16) Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Enuiron. Sei. Technol. 1983, 17, 227. (17) Flory, P. J. J. Chem. Phys. 1942, 10, 51. (18) Huggins, M. L. Ann. N.Y. Acad. Sci. 1942, 43, 1. (19) Flory, P. J. Principles of Polymer Chemistry; Cornel1 University Press: Ithaca, NY, 1953. (20) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions; Van Natrand R e i o l d New York, 1970. (21) Blanks, R. F.; Prausnitz, J. M. Ind. Eng. Chem. Fund. 1964, 3, 1. (22) Karickhoff, S. W. J . Hydraul. Eng. 1984, 10, 707. (23) Barton, A. F. M. Handbook of Solubility Parameters, and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. (24) Welch, D. I., Ph.D. Dissertation, The Macauley Institute for Soil Research, 1981. (25) Spiteller, M.; Schnitzer, M. J . Soil. Sei. 1983, 34, 525. (26) Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P. Humic Substances in Soil, Sediment, and Water; Wiley-Interscience: New York, 1985. (27) Leo, A.; Hansch, C.; Elkins, D. Chem. Reu. 1971, 71, 6. (28) Miller, M. M.; Wasik, S. P.; Huang, G.; Shiu, W.; MacKay, D. Environ. Sci. Technol 1985, 19, 522. (29) Karger, B. L.; Snyder, L. R.; Horvath, C. J. Chromatogr. 1976, 125, 71. (30) Eadie, B. J.; Morehead, N. R.; Landrum, P. F., submitted to Environ. Sei. Technol. (31) Chin, Y. P.; Weber, W. J., Jr.; Voice, T. C. Water Res. 1986, 20, 1443. (32) Chin, Y. P., Ph.D. Dissertation, University of Michigan, 1988. Received for review December 12, 1988. Accepted April 7,1989. This publication is a result of work sponsored by the Michigan Sea Grant College Program, Project No. RITS-29under Grant No. NA86AA-O-SG043 from the Office of Sea Grant, National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce, and funds from the State of Michigan. This work is also supported in part by Research Grant No. ECE-8503903from the National Science Foundation, Dr. Edward H. Bryan, Environmental Engineering Program Director. The US.government is authorized to produce and distribute reprints for governmental purposes notwithstanding any copyright notation appearing hereon.
