Environ. Sci. Technol. 1997, 31, 1697-1702
A Distributed Reactivity Model for Sorption by Soils and Sediments. 8. Sorbent Organic Domains: Discovery of a Humic Acid Glass Transition and an Argument for a Polymer-Based Model EUGENE J. LEBOEUF AND WALTER J. WEBER, JR.* Environmental and Water Resources Engineering Program, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
Analysis of a humic acid by differential scanning calorimetry has revealed the existence of a glass transition point. Glass transition temperatures, Tg, of water-wet and desiccatordry specimens were found to range from 43 °C for waterwet humic acid to 62 °C for dry samples. Phenanthrene sorption isotherms for these and other natural and synthetic organic matrices having known glass transition temperatures were determined to exhibit linearity and nonlinearity corresponding respectively to the rubbery (expanded) and glassy (condensed) states of the sorbent. Invoking a limiting case of the distributed reactivity model based on polymer sorption theory, we explain the observed sorption behavior as comprised in each case by a linear phasepartitioning component and a Langmuir-like nonlinear adsorption component. We conclude that polymer sorption theory provides a useful context in which to assess sorption phenomena associated with soil and sediment organic matter, providing more accurate projections of contaminant behavior in environmental systems and better informed specifications of appropriate remediation measures.
Introduction In this paper, we experimentally reinforce our view of soil and sediment organic matter (SOM) as a mixture of natural macromolecules consisting of rubbery (expanded, fluid-like) and glassy (condensed, relatively rigid) components spanning a range of glass transition temperatures, Tg; i.e., the temperature that separates the glassy state from the rubbery state. Several investigators have similarly likened soil organic matter sorption behavior to that of synthetic polymers (1-7), but no clear physical evidence linking natural macromolecular behavior in soil systems to synthetic polymers has been reported until now. To establish such evidence, we employed differential scanning calorimetry (DSC) to identify the existence of a glass transition point for a humic acid, a natural component of soils and sediments. We then performed sorption experiments with this material and other natural macromolecules having known glass transition temperatures and compared their sorption behaviors to that of rubbery and glassy synthetic polymers under similar experimental conditions. We then bring our analogy between soil organic * Corresponding author: e-mail:
[email protected]; telephone: 313-763-1464; fax: 313-763-2275.
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1997 American Chemical Society
matter and synthetic polymers full circle by showing the applicability of a polymer-based limiting case of the Distributed Reactivity Model (DRM) for sorbent organic domains; i.e., domains II and III (8).
Background Soil organic matter is a relatively complex, heterogeneous composite of partially or completely degraded biomolecules of plant and animal origin. Comprised first as carbohydrates, lipids, and proteins, these biomolecules undergo gradual decomposition through various degrees of diagenesis to form fulvic and humic acids, humin, and eventually kerogen, the primary organic component of shales and coals (9, 10). Although the final composition of soil organic matter is complex, we believe that it retains a semblance of the macromolecular character suggested by proposed structural models for fulvic and humic acids (11-13) and for humins and kerogens (9, 10, 14). Synthetic polymers, in general, have relatively homogeneous structures with clearly defined chemical and physical characteristics. They are characterized in part by Tg and by their solubility parameter, σp, a measure of their intermolecular bonding energy. Tg marks a second-order phase transition in which there is continuity of the free energy function and its first partial derivatives with respect to state variables such as temperature or pressure, but there is a discontinuity in the second partial derivatives of free energy. There is, therefore, continuity in enthalpy, entropy, or volume at the transition temperature but not in the constant-pressure heat capacity, Q°H (15). Hence, measurements of changes in Q°H with increasing temperature yield information about Tg as well as about the magnitude of change in Q°H that occurs in the transition from glassy state to rubbery state. Because the rubbery state allows greater molecular motion, it exhibits a greater ability to disperse heat and thus manifests a correspondingly higher Q°H. Such measurements can be made using DSC techniques. Because Tg is a function of macromolecular mobility, any changes to the macromolecular structure that increase or decrease this mobility will have similar effects on Tg. For example, increased cross-linking restricts chain mobility of larger macromolecular segments, while increased attractive forces between molecules (as measured by the solubility parameter) require more thermal energy to produce molecular motion. Thus, Tg will generally increase with increased crosslinking and increased σp. In addition, an increase in the free volume of the macromolecule (i.e., that volume not occupied by the component molecules themselves) allows more room for molecular movement and thus yields an accompanying reduction in Tg. Swelling of a macromolecular sorbent by thermodynamically compatible solutes (i.e., those possessing similar σp values) will therefore tend to increase the free volume and lower Tg (16). With respect to glass transitions in natural systems, we draw an analogy to what we have earlier termed “soft” and “hard” carbons (e.g., refs 5, 8, 17, and 18). That is, we view diagenesis as a process in which relatively young, expanded, lightly cross-linked, “rubbery” organic matter is converted into more condensed, highly cross-linked, more aromatic, “glassy” structures having reduced molecular mobility and corresponding increased glass transition temperatures. Given this perspective, we expect diagenetically less mature (e.g., soft carbon) soil organic matter such as humic and fulvic acids to possess lower glass transition temperatures than diagenetically more mature (e.g., hard carbon) organic matter, such as that comprising kerogens. Coal, a form of kerogen, has been shown to undergo glass transitions at temperatures
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TABLE 1. Sorbent Characteristics other important properties and characteristics elemental analyses
(%)a
sorbent
C
H
N
S
O
ash (%)
cellulosed humic acidh PIMAi Illinois No. 6 coalj
26.0 50.7 55.0 65.7
4.0 4.5 8.3 4.2
0.0 1.2 0.0 1.2
0.0 0.2 0.0 4.8
70.0 31.4 36.7 8.6
NDe 7.5 NDe 15.5
particle diametere (µm)
N2-BET surface area (m2/g)
dry glass transition temp Tg (°C)b
wet glass transition temp Tg (°C)c
100 (av)f 38-180 300-355 38-63
2.72 3.95 0.02 5.64
225g 62 55 355k
-45g 43 50 NDe
a Determined on a mass percentage basis. b Desiccator or oven-dried. c Equilibrated with water for 7 days. d Theoretical elemental analysis values. Not determined. f Manufacturer data. g Ref 21. h Elemental analyses from ref 23. i Theoretical values for elemental analysis. j Elemental analyses from ref 24. k Approximate from ref 19. e
ranging from 307 to 359 °C (19). Additionally, in the presence of pyridine, a thermodynamically compatible solvent for coal, Tg decreases by more than 200 °C (19, 20). Biopolymers, such as cellulose, have also been shown to undergo a glass transition and have exhibited similar reductions in their values in the presence of water, a “good” swelling solvent for cellulose (21).
Experimental Section Sorbents. Three natural organic sorbents spanning a range of diagenetic alteration and one synthetic organic sorbent similar in solubility parameter and permachor values to that hypothesized for SOM (4) were selected for study. The four sorbents included cellulose, humic acid, coal, and a synthetic polymer. Highly purified cellulose was obtained from Scientific Polymer Products, Inc. (100 µm average particle size) and used as received. The humic acid was obtained in powder form from Aldrich Chemical Company, Inc. and was purified prior to use employing the technique of Kilduff and Weber (22). Illinois No. 6 coal was obtained from Argonne National Laboratories Premium Coal Sample Program. The coal was extracted extensively with pyridine before use in a manner similar to the procedure of Hall and Larsen (20); i.e., Soxhlet extraction with pyridine (99+%, Aldrich Chemical Company, Inc.) for approximately 6 days until there is no further noticeable discoloration of the pyridine. The coal was then extracted with acetone (HPLC grade, Mallinckrodt) for 24 h to assist in the removal of the pyridine and placed in an oven at 105 °C for 24 h to allow for volatilization of residual pyridine and acetone. Following baking, the coal was crushed and sieved to retain the 38-63 µm size fraction. Poly(isobutyl methacrylate) (PIMA) was obtained in bead form from Scientific Polymer Products, Inc. The polymer was cleansed of polymerization artifacts prior to use employing the manufacturer suggested technique of sequential solvent flushing. This technique involves placement of approximately 100 g of sample in a borosillicate glass column and flushing with 18 L of double-distilled, deionized, filtered water (Nanopure, Barnsted Corp.), followed by 1 L of HPLC grade methanol (Mallinckrodt Chemical), followed once more by 18 L of Nanopure water. The cleaned polymer was then freeze-dried for 24 h and placed in a desiccator prior to use. N2 (99.9995%, BOC Gases) based BET surface areas for each sample were determined using Micromeritics Accelerated Surface Area and Porosimitry (ASAP) Model 2010 with a liquid nitrogen (77.35 K) bath. Surface areas and sorbent elemental analysis are summarized in Table 1. Differential Scanning Calorimetry. Calorimetric measurements were performed on humic acid and PIMA using a Perkin-Elmer Series 7 differential scanning calorimeter in the scanning mode (from 0 to 110 °C). This instrument directly measures the difference in power applied to sample and reference cells in milliWatts (mW) as it scans a preset temperature range. Samples of desiccator-dry and waterequilibrated (7 days) samples were weighed into sealable aluminum pans having one small pin-hole punctured in the
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top to allow volatilization of water. The non-wet humic acid sample was dried for 30 min in the DSC cell under N2 (99.998%, BOC Gases) at 110 °C; all other samples were analyzed without further preparation. Each sample was cooled to 0 °C, and calorimetric analyses were performed to a temperature of 110 °C. Confirmation of no further volatilization of the sample or of physically-sorbed water after drying under N2 at 110 °C for 30 min was determined through independent thermal gravimetric studies (TA Instruments Hi-Res 2950 thermal gravimetric analyzer) scanning a temperature range of 25110 °C. Additional DSC experiments confirmed that the Tg of the water-wet humic acid samples returned to their original dry Tg ((4 °C) after volatilization of all physically-sorbed water. Isotherms. (A) Chemicals. Spectrophotometric grade phenanthrene (98%, Aldrich Chemical Co., Inc.) was used as the target or probe solute for the sorption isotherm measurements. Stock solutions of phenanthrene were prepared as noted in ref 8 and stored at -5 °C in glass bottles with aluminum crimp caps containing Teflon-lined silicone septa. Solute solutions for the isotherms consisted of appropriate amounts of phenanthrene stock solution added to a buffered solution of double-distilled, filtered water (Nanopure, Barnsted Corp.) containing 0.005 M CaCl2 and 100 mg/L NaN3 (for biological control), buffered at pH 7 with NaHCO3. Methanol concentrations within these solutions were maintained at less than 0.2% by volume in all experiments to reduce possible co-solvency effects (25). (B) Sorption Experiments. Established procedures (5, 8, 26) employing a bottle-point, fixed-sorbent dosage technique were utilized for conducting all sorption experiments in completely-mixed batch reactors (CMBRs). The CMBRs consisted of 40-mL glass centrifuge tubes (for cellulose, PIMA, and humic acid) and 125-mL glass bottles (for coal), each sealed with screw caps, Teflon-lined silicone septa, and silver foil to minimize system losses to the Teflon liner. Each reactor contained enough sorbent to ensure 35-70% sorbate uptake. The sorbents were pre-equilibrated with 5 mL of buffered aqueous solution (described above) in the reactors for 72 h prior to addition of the phenanthrene sorbate to ensure thorough wetting of sorbent surfaces. All reactors were placed in rotating tumblers and, based on preliminary sorption rate and equilibrium studies, equilibrated at 5 or 45 °C ((0.5 °C) for 4 weeks, except those for the humic acid, which were equilibrated for 2 weeks. Solids were separated by centrifugation at 2000 rpm for 10 min for the 40-mL tubes and by sedimentation for 1 h for the 125-mL bottles. Control reactors containing no sorbent were prepared and operated in the same manner as described for the sorbent-containing reactors. After centrifugation, an approximately 2.0-mL sample of supernatant was immediately withdrawn from each of the reactors and placed in 4-mL glass vials containing 2.0 mL of methanol. Phenanthrene concentrations in the supernatant/ methanol mixtures were analyzed using a Hewlett-Packard Model 1050 HPLC in the manner described in ref 8. Average system losses to control reactors were consistently less than
A
A
B B
FIGURE 1. Calorimetric analysis of the glass transitions of poly(isobutyl methacrylate). (A) Desiccator-dried specimen with normal endothermic over-relaxation peak and observed glass transition of 55 °C. (B) Water-wet specimen (equilibrated for 7 days) with reduced over-relaxation response and lowered glass transition of 50 °C. Note the large endothermic response near 100 °C due to the volatilization of water. 3% of initial concentration, thus no corrections in isotherm calculations were required. Isotherm model parameters were estimated using SYSTAT software (Version 5.2.1, SYSTAT, Inc.). It is important to note that subsequent extended time or “aging” sorption studies have revealed a very small but continual increase in phenanthrene uptake in the glassy coal and PIMA in time periods extending beyond three months. True thermodynamic equilibrium thus may not be reached in the case of these sorbents, especially at 5 °C. Equilibrium measurements in this study are therefore more accurately referred to as “practical” isotherms.
Results and Discussion In Figures 1 and 2, we present results from our calorimetric investigation of glass transitions for dry and water-wet samples of our reference synthetic polymer, poly(isobutyl methacrylate), and the purified humic acid. As shown in Figure 1A, the measured Tg of the dry PIMA agrees closely with the manufacturer-specified value of 55 °C. Equilibration of the PIMA with water, however, results in a reduction of approximately 5 °C in the Tg to a value of 50 °C (Figure 1B). Glass transitions for humic acid are presented in Figure 2. As evident by a comparison of Figures 1A and 2A, the dry humic acid shows a less sharp phase transition than that of the synthetic polymer. We believe the spreading of the glass transition over a greater temperature range is the result of the heterogeneous nature of the humic acid as compared to the relatively homogeneous synthetic polymer, with respect to both chemical structure and molecular weight. As illustrated by comparing panels A and B of Figure 2, equilibration of the humic acid with water brought about a reduction in Tg from approximately 62 to 43 °C, as well as an almost
FIGURE 2. Calorimetric analysis of the glass transitions of humic acid. (A) Desiccator-dried humic acid with significantly reduced over-relaxation response and broad glass transition peak at 62 °C. (B) Water-wet specimen (equilibrated at pH 7.0 for 7 days) with little over-relaxation response and lowered glass transition of 43 °C. complete elimination of an endothermic peak immediately following the phase transition. The discrepancy between the 5 °C drop in Tg for the waterwet synthetic polymer and the observed 19 °C drop for the water-wet humic acid relative to their respective dry states can be attributed to different degrees of interaction of water molecules with the respective sorbing matrices. Water, with a solubility parameter (σp) of 23.4 (cal/cm3)0.5 (27), interacts more favorably with humic acid, which has a σp value of approximately 11.5 (cal/cm3)0.5 (28), than it does with PIMA, which has a σp value of 8.63 (cal/cm3)0.5 (29). This increased interaction results in greater water uptake (we observed approximately 75% greater uptake of water by the humic acid as compared to PIMA on a mass of water/mass of sorbent basis), causing greater swelling within the humic acid matrix and a greater resultant lowering of Tg. Additional work with a water-wet poly(methyl methacrylate), σp ) 10.5 (cal/cm3)0.5 (29) has shown a Tg lowering of approximately 15 °C. The environmental remediation implications of phase transition phenomena in natural organic matter become most apparent when placed in the context of synthetic polymer sorption theory. Rubbery polymers, with their relative ease of molecular motion, behave similarly to fluids, within which simple Brownian motion and Fickian diffusion of solute molecules occurs. This allows successful modeling of the sorption/desorption behavior of such materials in terms of phase partitioning theory (30, 31). Glassy polymers, with decreased molecular mobility, are described as containing fixed free-volume microvoids within which sorbing molecules are adsorbed and immobilized, resulting in nonlinear adsorption behavior. If the microvoid characteristics are more or less homogeneous throughout the glassy state matrix, the nonlinear adsorption will exhibit a Langmuir-type character. Diffusion within these matrices is often non-Fickian in nature, with diffusion coefficients depending upon both the con-
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centration of the solute and the relaxation of the polymer matrix. In such cases, solute diffusion coefficients are generally 2-3 orders of magnitude smaller than their corresponding values for polymers in their rubbery state (32). In the first paper in this series (17), we proposed a model designed to account for multiple sorption domains of different reactivity at the soil-sediment particle scale. This model, termed the Distributed Reactivity Model (DRM), has the form k
qe,T )
∑(q i)1
k
e,L)i
+
∑(q
e,NL)i
) KD,LCe + KFCne
(1)
i)1
where qe,T, qe,L, and qe,NL refer to the total, linear contribution, and nonlinear contribution solid-phase concentrations, respectively (in units of µg/g; KD,L is the sum of the distribution coefficients for all linearly sorbing soil-sediment components (L/g); Ce is the residual solution-phase concentration of solute at equilibrium (µg/L); KF is the Freundlich capacity factor [(mg/g)(L/mg)n]; and n is the Freundlich exponent (unitless), a joint index of the cumulative magnitude and diversity of energies associated with the adsorption of solute by the components of a heterogeneous condensed phase (33) and a readily evident measure of the degree of linearity of a sorption isotherm. As we noted in the original DRM paper (17) and have since discussed in more detail (33), the Freundlich term in eq 1 represents a summation of several distinct Langmuir-type (i.e., capacity-limited and relatively constant energy) nonlinear adsorptions at different sites in a heterogeneous matrix. Vieth and Sladek (34) earlier proposed a more restrictive two-domain model for describing the combined rubbery and glassy state sorption behavior of chemically homogeneous polymers. That model, which included a linear phase partitioning component and a single, limited-site Langmuirtype isotherm component, represents a limiting case of the DRM; i.e.
qe,T ) qe,L + qe,NL ) KD,CCe +
Q°abCe 1 + bCe
(2)
where qe,T, qe,L, qe,NL, and Ce are as defined previously; KD,C is the distribution coefficient of the linear component of the limiting case DRM; Q°a represents the adsorbed phase solute concentration that corresponds to saturation of a relatively homogeneous site-limited sorption domain (µg/g); and b is a coefficient related to the enthalpy of sorption in that domain (L/µg). This model has been successfully applied in a number of different applications involving relatively homogeneous polymer structures (35-37), including descriptions of solute sorption and transport in molecular sieves, dye transport in textile fibers (38), and most recently sorption of hydrophobic organic chemicals to soils and sediments (7). The simplifications of the DRM that lead to the polymerbased expression given in eq 2 are 2-fold. The first simplification assumes that a single partitioning reaction into a homogeneous rubbery phase accounts for all of the linear component, whereas the more robust DRM addresses linear sorption reactions onto mineral surfaces as well as partitioning into highly amorphous organic matter. Most significantly, the second simplification treats sorption by a glassy polymer phase as a singular site-limited and relatively constant energy process, i.e., a Langmuir-type adsorption. The DRM treats the nonlinear component of adsorption as a set of multiple reactions involving different sites of different energy, thus manifesting Freundlich-type behavior, i.e., a summation of several Langmuir-type adsorptions (17, 33). Based on our finding of a glass transition point for a soil derived humic acid, we here draw an analogy between SOM sorption behavior and that of synthetic polymers and advance
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FIGURE 3. Apparent equilibrium isotherms for aqueous-phase phenanthrene sorption on Illinois no. 6 coal [Tg ) 355 °C (19), σp ) 10.6 (29)], humic acid [Tg ) 62 °C dry, 43 °C water-wet, σp ) 11.5 (28)], PIMA [Tg ) 55 °C dry, 50 °C water-wet, σp ) 8.63 (29)], and cellulose [Tg ) 225 °C dry, -45°C water-wet (21), σp ) 13.8 (29)]. (A) Freundlich model fit with 95% confidence interval for sorption at 5 °C. (B) Freundlich model fit with 95% confidence interval for sorption at 45 °C. the limiting case form of the DRM given in eq 2, the Dual Reactive Domain Model (DRDM), for characterizing soil and sediment domains II and III (8). In Figure 3, the Freundlich term on the right-hand side of eq 1 is used to emphasize the overall linearity/nonlinearity of aqueous-phase sorption of phenanthrene [σp ) 9.8 (cal/ cm3)0.5, ref 27] by three samples of natural organic matter and PIMA. An overall linear sorption would be reflected by an n value of unity, while values of n lower than unity signify increasing heterogeneity of sites and increased sorption energies (33). Based on polymer sorption theory and each specimen’s water-wet Tg, we would expect that at 5 °C all sorbents except cellulose will exhibit nonlinear sorption behavior. This expectation is borne out in Figure 3A, in which it is clearly evident that the Freundlich n value is significantly less than 1 for all samples except cellulose. As noted in Figure 3B, however, only the coal, which has a relatively high Tg, continues to demonstrate significantly nonlinear sorption at 45 °C. All other samples, including the PIMA and humic acid, which have water-wet Tg values close to the experimental temperature, show linear or nearlinear sorption behavior. Table 2 summarizes the Freundlich model parameters. Because the humic acid sample studied and the PIMA are both expected to have relatively homogeneous organic matrices, we can apply the DRDM introduced above to evaluate their respective sorption data. The results, presented in Figure 4, illustrate large nonlinear contributions to sorption for both sorbents at 5 °C, while at 45 °C (i.e., at or near their water-wet Tg values) these nonlinear contributions disappear
TABLE 2. Freundlich Model Parameters 5 °C
45 °C
sorbent
KF a
n
R2
Nb
KF a
n
R2
Nb
cellulose humic acid PIMA Illinois No. 6 coal
1.20 ( 0.77 51.00 ( 0.62 3.06 ( 0.80 839 ( 244
0.995 ( 0.050 0.758 ( 0.025 0.766 ( 0.052 0.635 ( 0.066
0.999 1.000 0.999 1.000
16 18 20 16
0.592 ( 1.33 9.91 ( 1.15 11.22 ( 2.57 582.7 ( 1.2
0.977 ( 0.050 0.951 ( 0.031 1.038 ( 0.084 0.640 ( 0.029
0.998 1.000 1.000 1.000
24 34 22 24
a
Units: (µg/g)/(µg/L)n.
b
Number of observations.
process becomes increasingly slower (8). As noted previously, diffusion into glassy regions of synthetic polymers is several orders of magnitude slower than diffusion into rubbery matrices. A related phenomenon commonly observed for polymers used as ion exchange resins is the increased difficulty in regeneration that occurs as the polymer becomes more fully exhausted over extended periods of operation, i.e., as the diffusing exchange ion migrates into increasingly more highly cross-linked regions of the polymer (39). This “aging effect”, reported in the polymer literature almost 40 years ago (40), is attributed to the slow relaxation of glassy macromolecules, eventually providing for a reduction in resistance to solute migration and thus to further movement of the contaminant into the organic matrix. The process can occur over periods of days, weeks, months, or even years. The environmental remediation practice of actively desorbing contaminants over short time periods thus likely impacts only those contaminants contained in the more expanded, rubbery fractions of soil organic matter. Mass transfer out of the more condensed fractions is hampered not only by slow diffusion out of those matrices but also by continued diffusion into them, i.e., true sorption equilibrium has not likely been achieved. The longer the contaminant is able to diffuse into a condensed organic matter matrix, the more difficult it is to reverse its flow and, thus, to completely desorb from that matrix. Failure to recognize and account for nonequilibrium and/or nonlinear sorption and desorption behavior of more condensed or glassy organic carbon associated with soils and sediments can thus result in large errors in contaminant fate and transport modeling (41).
Acknowledgments FIGURE 4. Practical equilibrium isotherms for aqueous-phase phenanthrene sorption on PIMA and humic acid. (A) DRDM for sorption at 5 °C. (B) DRDM for sorption at 45 °C. Note the disappearance of the Langmuir contribution to the 45 °C PIMA isotherm and the subsequent coincidence of the linear and DRDM sorption models. for the PIMA and are several orders of magnitude less than the linear contributions for the humic acid, giving rise to almost complete partitioning behavior, consistent with existence of a predominantly rubbery state. Although we readily admit that comparison of the simple linearity or nonlinearity of isotherms may not in itself constitute a basis for drawing definitive conclusions with respect to the existence of separate rubbery or glassy states within soil organic matter, we believe that these data, when placed in the context of our differential scanning calorimetry results, provide strong evidence to support the hypothesis. Considering this new evidence, we find general agreement between the model that treats soil organic matter as two distinctly different domains (8) and the rubbery and glassy state behavior of synthetic polymers. This lends support to our hypothesis that the phenomenon of aging can be attributed principally to slow sorption into, and correspondingly slow desorption out of, the condensed or glassy fractions of organic matter. As the most highly condensed soil organic fractions are accessed over extended time frames, the diffusion
We thank Brett Bolan and Dr. Albert Yee of the Materials Science Engineering Department at The University of Michigan for informative discussions on DSC and for the use of their DSC system and David Peevers, Colleen O’Brien, and Tina Katopodes, undergraduate research assistants in our laboratories, for their assistance in the experimental aspects of this work. We also thank the anonymous reviewers of our manuscriipt for their detailed comments and Dr. Alok Bhandari and Dr. Tanju Karanfil, both post-doctoral research associates in our program, for their helpful suggestions and for providing the humic acid sample. This research was funded in part by the U.S. Environmental Protection Agency, Office of Research and Development, Great Lakes and MidAtlantic Center (GLMAC) for Hazardous Substance Research R2D2 Program and in part by The University of Michigan through a University of Michigan Regents Fellowship to E.J.L. Partial funding of the research activities of GLMAC and thus of this research was also provided by the State of Michigan Department of Environmental Quality.
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(3) Ball, W. P.; Roberts, P. V. Organic Substances and Sediments in Water; Lewis Publishers: Chelsea, MI, 1991; p 273. (4) Carroll, K. M.; Harkness, M. R.; Bracco, A. A.; Balcarcel, R. R. Environ. Sci. Technol. 1994, 28, 253. (5) Young, T. M.; Weber, W. J., Jr. Environ Sci. Technol. 1995, 28, 92. (6) Pignatello, J. J.; Xing, B. Environ. Sci. Technol. 1996, 30, 1. (7) Xing, B.; Pignatello, J. J.; Gigliotti, B. Environ. Sci. Technol. 1996, 30, 2432. (8) Weber, W. J., Jr.; Huang, W. Environ. Sci. Technol. 1996, 30, 881. (9) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence; Springer Verlag: New York, 1984. (10) Durand, B., Ed. Kerogen: Insoluble Organic Matter for Sedimentary Rocks; Editions Technip: Paris, 1980. (11) Schnitzer, M.; Khan, S. U. Humic Substances in the Environment; Marcel Dekker, Inc.: New York, 1972; Chapter 5. (12) Schnitzer, M. Soil Organic Matter; Schnitzer, M., Khan, S. U., Eds.; Elsevier: Amsterdam, 1978. (13) Hayes, M. B. H.; MacCarthy, P.; Malcom, R.; Swift, R. S. Humic Substances II: in Search of Structure; Hayes, M. B. H., et al., Eds.; John Wiley and Sons: New York, 1989; p 689. (14) Green, T.; Kovas, J.; Brenner, D.; Larsen, J. W. Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982; Chapter 6. (15) McKenna, G. B. Comprehensive Polymer Science: Vol. 2, Polymer Properties; Booth, C., Price, C., Eds.; Pergamon: Oxford, 1989; Chapter 10. (16) Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983; p 409. (17) Weber, W. J., Jr.; McGinley, P. M.; Katz, L. E. Environ. Sci. Technol. 1992, 26, 1955. (18) McGinley, P. M.; Katz, L. E.; Weber, W. J., Jr. Environ. Sci. Technol. 1993, 27, 1524. (19) Lucht, L. M.; Larson, J. M.; Peppas, N. A. Energy Fuels 1987, 1, 56. (20) Hall, P. J.; Larsen, J. W. Energy Fuels 1991, 5, 228. (21) Akim, E. L. Chemtech 1978, 8, 676. (22) Kilduff J.; Weber, W. J., Jr. Environ. Sci. Technol. 1992, 26, 589. (23) Karanfil, T.; Kilduff, J. E.; Schlautman, M. A.; Weber, W. J., Jr. Environ. Sci. Technol. 1996, 30, 2187. (24) Vorres, K. S. Users Handbook for the Argonne Premium Coal Sample Program; Argonne National Laboratory: Argonne, IL, 1993; p 17. (25) Wauchope, R. D.; Savage, K. E.; Koskinen, W. C. Weed Sci. 1983, 31, 744.
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Received for review July 17, 1996. Revised manuscript received December 10, 1996. Accepted December 16, 1996.X ES960626I X
Abstract published in Advance ACS Abstracts, April 1, 1997.
