Environ. Sci. Technol. 2000, 34, 3623-3631
Macromolecular Characteristics of Natural Organic Matter. 1. Insights from Glass Transition and Enthalpic Relaxation Behavior E U G E N E J . L E B O E U F * ,† A N D WALTER J. WEBER, JR.‡ Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, Tennessee 37235, and Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125
This is the first of a series of papers focusing on an experimental investigation of mechanisms contributing to the sequestration of hydrophobic organic compounds (HOCs) by macromolecular natural organic matter (NOM). It presents the results of a detailed study by differential scanning calorimetry (DSC) of NOM-related glass transition and enthalpic relaxation phenomena. Parallel measurements for model synthetic organic macromolecules of well characterized structure provide a basis for interpretation of the observed behaviors of the structurally heterogeneous and relatively ill-characterized natural organic materials investigated. The effects of varied DSC heating rates reveal transition temperature and enthalpic relaxation responses consistent with the Gibbs-Dimarzio glass transition theory, thus providing definitive evidence to support our earlier reports of the occurrence of glassy/rubbery state transitions in NOM macromolecules. Quantification of maximum changes in heat capacity for enthalpic relaxation phenomena provides insights into the types of physical and chemical bonds that limit glassy-state macromolecular mobility. Marked reductions by sorbed water of the effective temperature at which glass transition phenomena occur for hydrophilic NOMs are observed, suggesting that the thermodynamic states of NOM macromolecules are influenced in natural systems by the presence of large concentrations of sorbing molecules. The thermodynamic states of the more hydrophobic components or regions of NOM macromolecules, for example, are likely influenced by large amounts of sorbed HOCs in a manner similar to that affected by sorbed water for hydrophilic macromolecules. Finally, ramifications of the findings with respect to alternative remediation end points are discussed.
Introduction A fundamental understanding of sorption and desorption mechanisms is critical for accurate prediction of the fate and transport of hydrophobic organic compounds (HOCs) in subsurface systems and for effective implementation of appropriate remediation strategies. The causes and effects of slow sorption and subsequent hysteretic desorption are * Corresponding author phone: (615)343-7070; fax: (615)322-3365; e-mail:
[email protected]. † Vanderbilt University. ‡ The University of Michigan. 10.1021/es991103o CCC: $19.00 Published on Web 07/29/2000
2000 American Chemical Society
of particular relevance and concern with respect to the availability of contaminants for remediation. Specific mechanisms controlling the sorption, desorption, and sequestration of HOCs by soils, sediments, and other natural geosorbents are still matters of open discussion and debate. It is clear, however, that the macromolecular natural organic matter (NOM) matrices commonly associated with natural geosorbents play a major role in determining and controlling many of these processes (1-9). The difficulties associated with identification of specific mechanisms by which NOM affects the sorption and sequestration behavior of geosorbents for HOCs relate in large part to the complexity and ill defined behavioral characteristics of these macromolecular organic substances. In efforts to identify the physical and chemical characteristics of NOM that are most responsible for sequestration of HOCs, several investigators (1-7, 10-12) have suggested that the organic matrices of soils and sediments can be divided into two primary categories manifesting mechanistically different sorption behavior, i.e., an amorphous, gel-like “soft carbon” matrix or domain and a condensed, glasslike “hard carbon” matrix or domain. It is believed that the latter is associated with slow desorption rates and nonlinear behavior, while the former is associated with faster rates of desorption and more nearly linear sorption. Other work (1316) has hypothesized the presence of microvoids of nanometer or smaller scale within NOM, suggesting that these nanopores, thought to be primarily associated with the more condensed fraction, may also play a significant role in nonlinear sorption and desorption and hysteretic desorption behavior. Support for this two-domain concept of NOM is provided by the discovery of glass transition phenomena in several NOMs ((7, 17) this study), including a peat humic acid and a stream fulvic acid (18), and observed linear and nonlinear sorption of HOCs in synthetic (7, 15), and natural (7) macromolecules having known rubbery and glassy states. There has been no definitive prior study, however, relating HOC sequestration by NOMs to their respective glass transition temperatures. This research investigates mechanisms contributing to HOC sequestration behavior in natural organic matrices. A number of structural features of NOM may contribute to HOC sequestration. The heterogeneity of sorbents employed in prior studies generally precludes clear identification of controlling structural features, so we here concentrate on well-characterized macromolecular synthetic polymers, biopolymers, and a range of diagenetically altered NOM. The particular purpose of the study described in this paper, the first in a series on this research topic, was to provide a more thorough characterization of glass transition phenomena through various thermal analyses employing differential scanning calorimetry. A specific effort is made to relate glass transition-related heat capacity changes and enthalpic relaxation behavior to the physical and chemical properties of the macromolecules investigated. We also present evidence of glass transition behavior for an organosolv lignin. Subsequent papers in the series will discuss the influence of macromolecular characteristics of NOMs on the sorption and desorption behavior of HOCs under equilibrium and nonequilibrium saturated aqueous phase conditions.
Rubbery and Glassy States of Macromolecules NOM in the environment varies widely in composition, ranging from newly deposited bits and pieces of biopolymers through moderately aged humins, humic acids, and fulvic acids to well aged coatings of kerogens on mineral surfaces VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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constituting particles of shale or bits of coal formed over millions of years. Whether natural or of anthropogenic origin, soot and ash from combustion processes also contribute to environmental NOMs. Although complex in nature, most of these NOM components maintain some semblance of macromolecular structure regardless of age or origin. For example, molecular weights for biopolymers range from a few thousand to several hundred thousand daltons for polysaccharides, lignins, and lipids and to several million daltons for complex proteins such as deoxyribonucleic acid (19). Humic molecular weights range from 500 to greater than 250 000 daltons (20), while more diagenetically altered kerogens and coals may exhibit molecular weights in the millions (21). It is also reasonable to expect that soil and sediment NOM will exhibit such macromolecular properties as rubbery and glassy states. This is supported by numerous reports of glass transitions in biopolymers (e.g., refs 22 and 23), highly diagenetically altered NOM such as coals (e.g., ref 24), and the more recent demonstrations of glass transition behavior of Aldrich and Leonardite humic acids (7, 25), including a peat humic acid and a stream fulvic acid (18). NOM also possesses a remarkable range of chemical and structural features that may influence glass transition behavior, including chemical composition and degrees of cross-linking. These variations arise largely from differences in origin and degree of humification or other physicochemical processes (e.g., diagenesis). Both the rubbery and glassy states of natural organic macromolecules are amorphous in nature, the distinction being one of degree. Thus, while all structural features influence the mobility of such macromolecules, it is only those of the amorphous (noncrystalline) regions that influence the magnitude of the glass transition temperature. While glassy states constitute structures with void spaces created by the presence of fixed microvoids, they are amorphous rather than crystalline. Crystallinity is characterized by regular, ordered, three-dimensional structures (26). Crystalline regions are formed from the alignment of amorphous structures and thus only configure when there is sufficient mobility of the amorphous portion of the macromolecule (i.e., over very extended time periods for glassy NOMs or very rapidly for NOMs above their glass transition temperature). Macromolecules of relatively homogeneous composition are more likely to form crystalline regions than macromolecules with irregular chain structures and protruding side functional groups (27) or so-called atactic macromolecules. Given the relative heterogeneity of humified soil or sediment organic macromolecules, one would thus expect the occurrence of total crystallinity in these natural (likely atactic) systems to be relatively infrequent, with only very small or so-called “microcrystalline regions” forming under glassy state conditions. However, for more homogeneous biopolymers or largely diagenetically altered natural organic matter (e.g., coals), it may be possible to find significantly larger regions of crystallinity. While it is possible that regions of microcrystallinity may influence observed equilibrium sorption behavior (11), it is likely that such crystalline areas do not contribute significantly to sorption capacity and do not play significant roles in nonequilibrium sorption behavior due to the inability of most solutes to penetrate these regions (28). It is, more likely, the void spaces between the crystallites, in the amorphous rubbery and glassy regions of the macromolecule, that control nonequilibrium sorption behavior. The large disparity in macromolecular mobility between rubbery and glassy states can result in widely varying sorption behavior, where nonlinear sorption (29), slow, non-Fickian diffusion (30), and competitive multisolute sorption (2, 3, 31) is attributed to glassy or “hard carbon” regions; and linear, partition-like sorption (7, 32-34), relatively fast, Fickiantype diffusion (35), and no multisolute competitive sorption 3624
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(2, 3, 31) is attributed to rubbery or “soft carbon” regions. The temperature (or, more accurately, range of temperatures) at which increased molecular motions lead to rubbery behavior is referred to as the glass transition temperature or T g.
Materials and Methods Sorbents. A total of 14 sorbents (9 sources of purified natural organic matter (2 biopolymers, 3 humic acids, 2 shalekerogens, and 2 coals) and 5 synthetic organic macromolecules) were selected for study. General sorbent characteristics are summarized in Tables 1 through 3. Natural Organic Matter Sorbents. Nine natural organic matter sorbents were selected to represent different degrees of organic diagenetic alteration, ranging from biopolymers (cellulose and lignin (organosolv)), humic acids (peat, Aldrich, and Leonardite humic acids), Type I kerogen (Green River Oil Shale), Type II kerogen (Ohio Shale II from an Upper Devonian formation in Northwest Ohio (36)), and 2 coals (Illinois No. 6 and Wyoming) comprising Type III kerogens. Highly purified cellulose was obtained from Scientific Polymer Products, Inc. in 100-µm average particle size form. Lignin (organosolv) was obtained from Aldrich Chemical Co. Each biopolymer was stored without further preparation in airtight, amber-colored glass bottles and placed in desiccators until used in subsequent characterizations and sorption studies. Peat (reference grade) and Leonardite (standard grade) humic acids were used as received from the International Humic Substance Society (IHSS). Aldrich humic acid (Aldrich Chemical Co., Lot No. 01816HH) was obtained from Dr. Tanju Karanfil of Clemson University following a purification procedure described elsewhere (37). Upon receipt from Dr. Karanfil, the purified Aldrich humic acid was crushed and sieved to retain the 38-180 µm particle size fraction. Each humic acid was stored in airtight glass bottles and placed in desiccators for future use. Two kerogens (Green River and Ohio Shale II) were obtained through a standard demineralization process (38) of the two parent shales (Green River Shale and Ohio Shale II, respectively). After crushing and sieving to pass a 250-µm sieve, approximately 30 to 40 g of sample was placed in cellulose thimbles and sequentially Soxhlet extracted to remove extractable organic matter (e.g., small, nonbound hydrocarbons, asphaltenes, and waxes) using toluene, hexane, and methanol until each solvent cycled clear in the Soxhlet system (approximately three to 6 days). Following methanol evaporation, the contents of each cellulose thimble was poured into 250 mL Teflon beakers containing approximately 150 mL of Nanopure water. Following a solidswetting stage of 1 day, a concentrated solution (37 wt %) of HCl was added to each beaker to dissolve acid-soluble mineral components. Following no further evidence of reaction, the solution was decanted. This process was repeated as needed until no further reaction was observed upon addition of fresh HCl for approximately one week. After decanting a final time, the solids were exposed to a concentrated mixture of one part (on a volume basis) of HCl (37 wt %) to two parts of HF (48 wt %) (for dissolution of silicates) and placed in a water bath at approximately 25 °C. Again, the supernatant mixture was decanted periodically (approximately every one to 3 days), followed by addition of the concentrated HCl and HF mixture until all silicate minerals were removed (approximately four to six weeks). Following a final decant of the HCL/HF mixture, each sorbent mass was placed in a large centrifuge bottle (Nalgene) and sequentially rinsed and decanted with Nanopure water approximately five times (over a period of approximately 3 days) until the pH of the supernatant was brought within the pH 6.0 to 7.0 range. Following a final decant of the Nanopure supernatant, the
TABLE 1. NOM Sorbent Characteristics
sorbent cellulose Peat Humic Acid Aldrich Humic Acid Leonardite Humic Acid Lignin (Organosolv) Green River Shale Kerogen (Type I) Ohio Shale Kerogen (Type II) Wyoming Coal Illinois No. 6 Coal
elemental analysis [mass %] C H N S O 26.0a 56.84e 50.7f 63.25e 65.8h 66.37 39.88 75.01i 77.67i
4.0a 3.60e 4.5f 3.64e 5.37h 8.21 3.65 5.35i 5.00i
0.0a 0.0a 3.74e 0.7e 1.2f 0.2f 1.17e 0.84e 0.08h NDb 2.25 3.84 1.26 9.98 1.12i 0.47i 1.37i 2.38i
ash [%]
70.0a NDb 36.62e 1.72e 31.4f 7.5f 31.0e 2.47e 28.7h 0.0h 12.35 5.91 9.15 29.24 18.02i 8.8 13.51i 15.5
atomic mass ratio O/C H/C 2.69 0.64 0.62 0.49 0.44 0.19 0.23 0.24 0.17
particle N2 BET specific diameter [µm] surface area [m2/g] 100 (av)c NDb 38-180 NDb NDb 38-180 38-180 38-63 38-63
0.15 0.63 0.09 0.06 0.08 0.12 0.09 0.07 0.06
3.1 0.7 1.1 NDb 1.8 26.3 37.4 6.9 4.9
a Theoretical approximation based on chemical structure. b Not determined. c Manufacturer data. d Reference 46. e International Humic Substance Society, 1996. f Reference 41. g Reference 39. h Reference 49. i Reference 50 (dry, ash-free basis). j Approximate, from ref 24.
kerogen was freeze-dried and stored in desiccators for future use in airtight, amber-colored glass bottles. Illinois No. 6 (bituminous) and Wyoming (subbituminous) coals were obtained from the Argonne National Laboratories Premium Coal Sample Program. Before use, each coal was extracted extensively following the procedure described by LeBoeuf and Weber (7). Following extraction, the coal was crushed and sieved to retain the 38-63 µm size fraction and stored for future use in desiccators in amber-colored, airtight glass bottles. Synthetic Organic Sorbents. Synthetic sorbents representing surrogates to SOM were selected on the basis of the following criteria: (i) Hildebrand solubility parameters (a measure of the enthalpy change upon mixing solvents with macromolecules) within the approximate ranges hypothesized for NOMs (between 9.0 and 13.5 (cal/cm3)0.5) (4); (ii) permachors (forces holding the macromolecule chains together) similar in range to that proposed for NOMs by Carroll et al. (4) (45 to 60 (cal/cm3)); (iii) span a range of glass transition temperatures (e.g., 20 °C to 110 °C); (iv) little to no cross-linking (to limit variables used for correlating sorption results); (v) densities greater than 1.0 cm3/g at 25 °C (for ease in aqueous-phase sorption experiments); (vi) obtainable in bead (spherical) form; and (vii) readily available, well-characterized, and inexpensive. The five macromolecules selected include poly(butyl methacrylate) (PBMA), poly(isobutyl methacrylate) (PIMA), poly(butyl methacrylate/ isobutyl methacrylate) (PBIMA), a 50/50 copolymer blend of PBMA and PIMA, poly(methyl methacrylate) PMMA, and poly(phenyl methacrylate) (PPMA)). Each polymeric sorbent was obtained in bead form from Scientific Polymer Products, Inc. Prior to use, each macromolecule was cleansed of polymerization artifacts (e.g., monomers) through the manufacturer suggested technique of sequential solvent flushing described by LeBoeuf and Weber (7). Selected properties of these five polymeric sorbents are summarized in Table 3. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) was used to identify Tg values for all sorbents, including the synthetic macromolecules (thus allowing confirmation of manufacturer information). DSC is a thermal analysis technique that measures the difference in heat capacities between a reference cell and a sample cell while scanning a programmed temperature range (further information on DSC is provided in ref 7). Samples were tested under both desiccator-dry and water-equilibrated conditions. The latter measurement providing a more accurate portrayal of the thermodynamic state of the sorbent while in aqueous solution. Each DSC experiment was performed using a PerkinElmer Series 7 Analyzer; temperature axis and enthalpy measurements were calibrated with indium. Samples of approximately 20 [mg] of desiccator-dry and water-equilibrated (7 days) sorbent were weighed into sealable aluminum
TABLE 2. NOM Solid-State 13C NMR Spectral Data
sorbent Peat Humic Acidb Aldrich Humic Acidc Leonardite Humic Acidc Lignin (Organosolv)d Green River Shale Kerogene Ohio Shale II Kerogene Illinois No. 6 Coalf Wyoming Coalf
integrated areas [%] percent 0-50 50-108 108-165 165-230 aromaticitya ppm ppm ppm ppm [%] 35 45 21
12 14 9
32 28 45
19 13 25
32 28 45
ND 76.2
ND ND
ND 23.8
ND ND
ND 23.8
51.5
ND
48.5
ND
48.5
31.0 36.0
ND ND
69.0 64.0
ND ND
69.0 64.0
a Percent aromaticity ) (peak area of 13C NMR spectrum 108-165 ppm)/(total peak area of 13C NMR spectrum 0-230 ppm). b Integrated area estimated from International Humic Substance Society reference spectra, 1996. c Reference 51. d Not determined. e Reference 39. f Reference 52.
pans with one small pinhole punctured in the top to allow volatilization of water. For the nonwet natural organic matter specimens, the sample was dried for 30 min in the DSC cell under N2 (99.998%, BOC Gases) at 110 °C prior to analysis. All synthetic macromolecule samples and all water-equilibrated samples were analyzed without further preparation. Each sample was cooled to 0 °C, and DSC was performed to a temperature of 110 °C using a standard heating rate of 10 °C per min (heating rates of 2.5, 10, 25, and 50 °C per min were used to investigate the impacts of different heating rates). Specifics of the experimental procedure for DSC analyses are available elsewhere (39). Thermal Gravimetric Analysis. Natural organic matter sorbents displaying identifiable DSC glass transition behavior were also scanned using thermal gravimetric analysis. A TA Instruments Hi-Res 2950 Thermal Gravimetric Analyzer scanning a similar temperature program to that of the DSC experiments, (i.e., temperature range of 25 °C to 110 °C at a heating rate of 10 °C/min) was used to confirm that no further volatilization of the sorbent or physi-sorbed water occurred after drying under N2 at 110 °C for 30 min.
Results and Discussion Synthetic Macromolecule Glass Transitions. The Tg of PIMA depicted in Figure 1 (A) illustrates a typical response of homogeneous macromolecules to a DSC heating rate that is faster than the rate at which the macromolecules can relax, or accommodate the increased energy. The DSC thermoscan thus returns a fairly sharp transition signal (often indicative of the sample’s relative chemical and molecular weight homogeneity) and is highlighted by a large enthalpic VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Synthetic Polymer Sorbent Characteristics molecular weight [g/mol]a
melting point Tm [K]a
solubility elemental parameter, σp permachor P distribution 3 0.5 b 3 c [mass fract.]d [(cal/cm ) ] [cal/cm ]
polymer
monomer
polymer
density, G [g/cm3]a
poly(butyl methacrylate) PBMA
142.2
100 000
1.055
noncryst
8.75
71
C 0.68 O 0.23 H 0.09
poly(isobutyl methacrylate) PIMA
142.2
260 000
1.045
noncryst
8.65
50
C 0.64 O 0.28 H 0.08
poly(methyl methacrylate) PMMA
100.2
75 000
1.20
>573
11.08
48.5
C 0.68 O 0.23 H 0.09
poly(phenyl methacrylate) PPMA
162.19
100 000
1.21
noncryst
10
40.3
C 0.74 O 0.20 H 0.06
carbon distribution [mass fract.]d aromatic 0.00 aliphatic 0.63 ester 0.13 other 0.24 aromatic 0.00 aliphatic 0.66 ester 0.17 other 0.17 aromatic 0.00 aliphatic 0.37 ester 0.13 other 0.50 aromatic 0.60 aliphatic 0.20 ester 0.10 other 0.10
polarity (O/C ratio) C/H [mass fract.]d ratiod 0.33
0.57
0.44
0.67
0.33
0.57
0.26
1.00
a Manufacturer data (Scientific Polymer Products, Inc.), g/mol. b Reference 53. c Calculated values based on fucntional group contribution method developed in ref 54. d Theoretical approximation based on chemical structure.
FIGURE 1. DSC scans at heating rates of 10 °C/min for (A) PIMA; (B) PBMA; (C) PMMA; and (D) PPMA. relaxation over a narrow temperature range. Because the heating rate of 10 °C per min is faster than the macromolecular relaxation rate, excessive energy (greater than that required for the occurrence of a glass transition) is temporarily stored in the macromolecule. However, once the macromolecules “catch-up” (relax), they over-relax or expand excessively, thus allowing for greater molecular motions, with a resulting endothermic shift of the DSC scan, and the consequent enthalpic relaxation. Over time, however, the macromolecule relaxes once more into a more stable, relatively open, fluidlike configuration particular to the rubbery state. 3626
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The Tg of PBMA, shown in Figure 1(B), shows a much less sharp transition response (i.e., the Tg is spread over tens of degrees) and a reduced enthalpic relaxation. The spreading of the PBMA Tg is likely attributable to a larger molecular weight distribution relative to PIMA. Additionally, given similar heating rates, the smaller enthalpic relaxation can be also attributable to relative heterogeneity of the molecular weight distribution (i.e., each molecular weight will transcend the Tg at a different temperature; hence not enough molecules are transitioning at the same time to result in a large enthalpic relaxation). Faster macromolecular relaxation rates relative to the PIMA may also contribute to the observed behavior.
TABLE 4. Sorbent Glass Transition Temperatures total organic carbon [mass %]a
solubility parameter, σp [cal/cm3]0.5
cellulose
26.0
13.8b
Lignin (Organosolv)
65.8
10.8b
Aldrich Humic Acid
50.7
11.5f
Leonardite Humic Acid
63.3
11.5f
Illinois No. 6 Coal
77.7
10.6 g
PBMA
68.0
8.75b
PBMA/PIMA
68.0
8.70b
PIMA
68.0
8.65b
PMMA
60.0
11.08b
PPMA
74.0
10.0j
sorbent
desiccator-dry Tg [°C] (∆Q°H) 225c (NDd)e 69-70 (0.45)e 57-62 (0.04)e 70-73 (0.03)e 355h (ND)e 20/30i (0.09)e (20)55/(ND)57i (0.19)e 55/55-59i (0.24)e 105/107i (0.28)e 110/105i (0.17)
water-wet Tg [°C] (∆Q°H) -45c (ND)e ND 43 (0.03)e ND ND ND ND 50 (0.12)e 90 (0.20)e 96 (ND)
desiccator-dry relaxation peak [mW] (temp range [°C])
water-wet relaxation peak [mW] (temp range [°C])
ND (ND) 0.3 (22) 0.2 (30) 0.0 (15) ND (ND) 0.3 (28) 6.9 (20) 3.7 (25) 0.4 (25) 0.0 (12)
ND (ND) ND (ND) ND (ND) ND (ND) ND (ND) ND (ND) ND (ND) 1.5 (25) 0.3 (15) 0.0 (8)
a From Tables 2 and 3. b Reference 53. c Reference 46. d ND ) not determined. e Values in parentheses represent change in heat capacity (∆Q° ) H from the glassy state to the rubber state [J/g‚°C]. f Estimated based on average literature values. g Reference 28. h Reference 24. i Manufacturer j data/experimental data. Estimated from chemical functional group contribution method (ref 54).
TABLE 5. Heating Rate Effects on Sorbent Glass Transition Temperatures
sorbent
10 °C/min heating rate Tg [°C]
25 °C/min heating rate Tg [°C]
50 °C/min heating rate Tg [°C]
PIMA PIMA IIb Lignin (Organosolv) Aldrich Humic Acid Leonardite Humic Acid
58.8 59.79 70 57.1 72.8
63.9 59.84 ND 57.4 73.6
69.1 68.3 82.8 58.8 75.8
Ia
a Heating rate for the PIMA I was 10.0, 2.5, and 50.0 °C/min (cycled in that order). b Heating rate for PIMA II was 2.5, 10.0, and 50.0 °C/min (cycled in that order).
Figure 1(C,D) illustrates Tgs for PMMA and PPMA, respectively. PMMA shows a relatively sharp transition and small enthalpic relaxation, while PPMA also shows a sharp transition, but no enthalpic relaxation, indicating that the macromolecular relaxation rate is similar to or less than the heating rate of the experiment. Quantitative values of the change in heat capacity (Tg) (depicted on the figures by the difference between lower (glassy state) and upper (rubbery state) parallel heat flow lines) for the synthetic macromolecules ranged from 0.09 J/g‚°C for PBMA to 0.276 J/g‚°C for PMMA. Each of the experimentally determined synthetic macromolecule glass transitions agreed fairly closely with the manufacturer values (Table 2), except for the PBMA/PIMA copolymer where no values (other than the theoretical values) are known. Results are summarized in Table 5. Natural Organic Matter Glass Transitions. Figure 2(A) illustrates Tg of a purified biopolymer, lignin (organosolv) at approximately 70 °C. Note that this transition is relatively sharp (even compared to PBMA), and it possesses a relatively small enthalpic relaxation. The change in heat capacity with this transition is 0.45 J/g‚°C. Figure 2(B,C) depicts previously reported glass transitions for Aldrich (7) and Leonardite humic acids (25), respectively. Aldrich humic acid manifests a Tg at approximately 63 °C with a ∆Q°H of 0.03 J/g‚°C. Leonardite humic acid manifests a Tg at 72 °C and a corresponding similar ∆Q°H of 0.03 J/g‚°C. Although the ∆Q°H
is relatively small in comparison to each of the synthetic macromolecules except PBMA, it is generally accepted that glass transitions will manifest a ∆Q°H of 11 J/mol‚°C or greater for aliphatic-like structures (40). Using reported molecular weights and carbon content of 4000 g/mol and 50.7% and 5800 g/mol and 63.1% for Aldrich and Leonardite humic acids (41), respectively, results in a ∆Q°H of at least 60 J/mol‚°C, well above the aforementioned criteria for glass transitions. Because each NOM was subjected to the same thermal history by heating at 10 °C per min to 110 °C, holding for 30 min, and cooling at 10 °C per min to 0 °C, we may make a comparison between magnitudes of enthalpic relaxations by integrating the area under the relaxation response relative to the rubbery state heat flow (upper parallel line). With respect to the structure of lignin, Aldrich, and Leonardite humic acids, one can clearly see a difference in the larger enthalpic relaxation of lignin (4.53 J/g) compared to the relatively smaller enthalpic response of Aldrich (1.23 J/g) and Leonardite (0.28 J/g) humic acids. This important result may indicate that the transition for lignin may result from either stronger or a larger number of intermolecular bond breaks compared to the two humic acids. Given the relative homogeneity of the lignin compared to the humic acid, it is also possible that other Tg values may exist for the humic acids, and the observed change in heat capacity is only associated with the breaking of specific bonds at that activation energy. For example, in addition to Tg values normally reported in the range of 300 °C, second-order transitions have also been reported for some coals at temperatures as low as 110 °C (42). In an effort to further confirm glass transition behavior of NOMs, Aldrich humic acid was selected for additional analyses involving repeated heating and cooling cycles at the same scanning rate of 10 °C. If the observed transition is indeed a true thermodynamic phase change, one should observe replicatable transitions from the glassy state to the rubbery state, and vice versa, as one cycles above and below the Tg. Figure 3 in the Supporting Information supports the idea of a thermodynamic transition, indicating a completely VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. DSC scans at heating rates of 10 °C/min for (A) Lignin (organosolv); (B) Aldrich Humic Acid; and (C) Leonardite Humic Acid. reversible phase change through two separate heating and cooling cycles. Furthermore, comparison of the cooling cycle Tg depicted in Figure 3(B), to the heating cycle Tg in Figure 3(A) reveals an apparent shift in the Tg, and a disappearance of the enthalpic relaxation during the cooling cycle. This shift in Tg between heating and cooling cycles is associated with the relaxation rate of the macromolecule (43). Finally, we here report an inability to detect glass transitions for the two shale kerogens (Green River and Ohio II) while scanning a temperature range of 0 °C to 110 °C using standard DSC. One explanation is that phase transitions are still occurring (all macromolecules exhibit glass transitions), but the NOM is so heterogeneous in chemical composition and molecular weight distribution that glass transitions are too weak or occur over such a large temperature range so as to allude detection using standard DSC. A second explanation is that the Tg occurs above 110 °C. Additional thermal analysis work is ongoing to evaluate these and other NOMs using more sensitive techniques. Influence of Heating Rate and Thermal History. GibbsDiMarzio theory for glass transition behavior predicts a true 3628
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second-order transition to occur at one temperature (at constant pressure), T2, where the configurational entropy contribution to free energy approaches zero (43). GibbsDiMarzio assumes that each macromolecule chain has a lowest energy shape, and that configurational deviations from this minimal energy state increases the internal energy of the molecule. This internal energy is comprised of the energy associated with the flexing of macromolecule chains out of their lowest energy state, as well as “hole” energy attributed to the number of intermolecular bonds broken by introduction of vacancies, or “holes” into a macromolecule lattice due to increased flexing or wriggling action. At temperature T2, there is not enough thermal energy to cause the larger scale wriggling, and hence below this temperature, the molecules remain in their low energy configuration (43). With respect to differential heating rate experiments, Gibbs-DiMarzio theory allows for an explanation of why one observes glass to rubber transitions at different temperatures for the same macromolecule if the macromolecule is aged in different manners, or subjected to different thermal histories. For example, the raising of the observed glass transition temperature as the rate of initial cooling decreases can be attributed to the ability of the less quickly cooled macromolecule to have more time to move more closely to equilibrium configurations (and lower total volumes). This results in a movement in the Tg to values closer to the true thermodynamic transition, T2. In similar fashion, an “aged” macromolecule will have a higher Tg than a younger macromolecule (given similar thermal histories). In addition, the rate, or duration that the glass transition experiment takes place has an impact on the observed values of Tg. In this case, a macromolecule heated at a slower rate will have a lower Tg than its more quickly heated counterpart (given similar “ages” and previous thermal history) due to the fact that the relaxation time of the macromolecule heated at the faster rate is much longer than the time scale of the shorter experiment (43). Figure 4 illustrates the impacts of differential heating rates on Tg and enthalpic relaxations for PIMA, Aldrich humic acid, and Leonardite humic acid. Notice that at lower heating rates each of the macromolecule samples manifest lower Tg than their more quickly heated counterparts. This is once more attributed to the difference in the relaxation time of the macromolecule relative to the time scale of the experiment. In other words, at a slower heating rate, the macromolecule has a chance to “keep up” with the experiment and thus display a transition that is more consistent with its true transition, compared to the faster heating rates which are certainly functions of the relaxation rate of the macromolecule. Furthermore, it is well established in DSC literature that higher heating rates will sharpen heat flow events (increase their intensity) and make them more obvious (44). Table 5 summarizes the changes in Tg with heating rate. Notice that the 10 °C transition for the PIMA in Figure 4(A) shows a sharper enthalpic relaxation compared to the 10 °C transition in Figure 4(B), because, at least for this sample, the 10 °C/min heating cycle is the initial cycle in a three cycle (2.5, 10, and 50 °C/min) experiment using the same sample, while in Figure 4(B), the 10 °C/min heating cycle follows an initial heating cycle of 2.5 °C/min. This reflects the importance of considering the initial temporal and thermal history of the sample. In other words, it is likely that over periods of weeks or months, the PIMA has relaxed over time (albeit very slowly, as explained by the Gibbs-DiMarzio glass transition theory, considering it was stored at temperatures approximately 30 °C below its “dry” Tg) to a more condense configuration, leading to increased intermacromolecule contacts and resultant increase in the number of bonds (possibly either van der Waals or hydrogen). Upon first heating, the PIMA manifests a sharp relaxation. However,
FIGURE 4. DSC scans of (A) heating rates of 10 °C/min, 25 °C/min, and 50 °C/min for PIMA I and heating rates of 2.5 °C/min, 10 °C/min, and 50 °C/min for (B) PIMA II; (C) Aldrich Humic Acid; and (D) Leonardite Humic Acid. subsequent cooling over short time periods (at a rate of 10 °C/min) causes the macromolecules to have less time to move more closely to more condensed, minimal energy configurations, leaving some macromolecules “frozen” into more open configurations. Hence, upon reheating, the macromolecules are in a different initial state compared to the first heating cycle, and thus manifest different enthalpic relaxations. It should also be recognized that any activated process will give a DSC thermogram that shifts when exposed to differential heating rates (44). For example, covalent bond breakage or macromolecular realignment will each show shifts upon differential heating rates. Thus, our data does not prove that the curve shifts are solely due to glass transition behavior; however, the shapes of the thermograms coupled with the repeatability of the change in heat capacity upon repeated heating and cooling cycles makes reconciliation with other reaction processes rather difficult. Influence of Water on the Glass Transition. Exposure of sorbents to solvents or good swelling compounds can also impact the glass transition temperature. Free volume theory for glass transition behavior suggests that the glass transition temperature is observed for macromolecules when their viscosity approaches that of their liquid state. Following a derivation based on the Doolittle expression for macromolecule viscosity as a function of free volume (45), this theory provides that substantially less viscous behavior occurs at a free volume fraction, f, for a wide range of materials
f)
Vf = 0.25 Vp + Vf
(1)
where Vp is the volume of the macromolecule itself, and Vf is the free volume. The usefulness of this theory lies in one’s ability, through use of this rather simple relationship, to easily visualize the impacts of a number of variables affecting free volume on determination of the glass transition temperature
(e.g., swelling of the macromolecule due to sorption of a solute or thermal expansion due to heating). For example, coal, a form of kerogen, has been shown to undergo glass transitions at temperatures ranging from 307 °C to 359 °C (24), but in the presence of pyridine, a thermodynamically compatible solvent for coal, Tg decreases by more than 200 °C (24). Cellulose has also exhibited a similar reduction (225 °C to -45 °C) in its Tg values in the presence of water, a “good” swelling solvent for cellulose (46). Figure 5 (Supporting Information) illustrates the influence of water on lowering glass transition temperatures for four synthetic macromolecules and Aldrich humic acid. Compared to its “dry” Tg, the water causes a reduction in the Tg by 5 °C, 15 °C, 10 °C, and 19 °C for the PIMA, PMMA, PPMA, and the previously reported (7) Aldrich humic acid, respectively. Note the large endothermic response near 100 °C associated with the volatilization of water in Figure 5(A-C). The discrepancy between the magnitude in drop of the Tg for the water-wet sorbents relative to their respective “dry” states can be attributed to the interaction of water molecules with the sorbing matrices. Water, with a solubility parameter (σp) of 23.4 (cal/cm3)0.5 (47), interacts more favorably with chemically similar matrices (i.e., sorbents with solubility parameters closest to its own value). In order of polarity (higher solubility parameter normally constitutes greater polarity), Aldrich humic acid’s (σp ) 11.5 (cal/cm3)0.5) ∆Tg ) 19 °C (∆Tg ) Tg(dry) - Tg(wet)) > PMMA (σp ) 11.08 (cal/ cm3)0.5) ∆Tg ) 17 °C > PPMA (σp ) 10.0 (cal/cm3)0.5) ∆Tg ) 9 °C > PIMA (σp ) 8.65 (cal/cm3)0.5) ∆Tg ) 5 °C. This increased interaction results in greater water uptake (e.g., there was approximately 75% greater uptake of water in the humic acid compared to PIMA on a mass of water/mass of sorbent basis), causing additional swelling within the humic acid matrix, and hence a greater lowering of Tg. Results are summarized in Table 4. VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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It is also important to note that the change in the magnitude of heat capacity and enthalpy relaxation upon transitioning from the glassy state to the rubbery state is apparently reduced in the presence of sorbed water. Discounting the water content (assuming that it is approximately equal in either the glassy state or rubbery state for a particular sorbent (a reasonable assumption over narrow temperature ranges near the Tg)), the ∆Q°H is reduced since the relative heat capacity of the partially swollen, water-wet glassy macromolecule will be larger compared to its “dry” counterpart (due to larger molecular motions in the swollen matrix); while the water-wet rubbery macromolecule’s ∆Q°H will likely remain the same as its “dry” rubbery counterpart since the macromolecule is already in a fluidlike state, regardless of water content. The overall change in the heat capacity during the transition is thus reduced in the presence of a sorbing solute. In other words, the sorbed water likely disrupts many intermolecular bonds; thus less thermal energy is required for the macromolecule to transcend to the rubbery state. This last piece of information may provide especially important insights to the physical and chemical structure of macromolecules, particularly the type and strength of bonds likely responsible for holding glassy macromolecules in their more rigid configuration. For the synthetic macromolecules used in this study, all intermacromolecule interactions are expected to be simple van der Waals forces; in the case of Aldrich humic acid, the water may not only play a role in swelling of the matrix and disruption of van der Waals forces but it may also play a role in disrupting some hydrogen bonds; thus the larger reduction in Tg compared to the synthetic counterparts. Further support of this behavior may be exemplified by pyridine swelling of coal. Pyridine is largely believed to disrupt hydrogen bonds in coal networks, thus leading to large reductions in the glass transition temperature of coal (42). In summary, differential scanning calorimetry is a useful technique for partially characterizing natural organic matter in terms of its macromolecular properties. Observations of glass transitions in model synthetic organic macromolecule systems provide a foundation from which to base observations of glass transition behavior in more heterogeneous natural systems. The exploration of experimental heating rates on glass transition temperatures furnish additional evidence to support the existence of glass transitions in natural organic matter. Increased heating rates result in increased glass transition temperatures and increased enthalpic relaxation response; each characteristic behavior being consistent with Gibbs-Dimarzio glass transition theory. Quantification of the maximum change in heat capacity for enthalpic relaxation phenomenon provides insights to the type of physical or chemical bonds limiting macromolecular mobility while in the glassy state. The impact of water on more hydrophilic sorbents was shown to cause an increased reduction in the Tg relative to more hydrophobic matrices. This highlights the influence that large concentrations of sorbing molecules (e.g., pyridine in coal) can have on the thermodynamic state of a sorbent. It is quite possible, therefore, that sorbents with large sorption capacities for a particular solute may actually transcend from a glassy state to a rubbery state with increasing sorbate uptake. This can have profound impacts on the rate and extent of desorption; i.e., a highly swollen, “rubbery” sorbent (resulting from large uptakes of a contaminant, especially in environments of nonaqueous phase liquids) may transcend from a rubbery state to a glassy state as the sorbate is released from the matrix. Because diffusion in glassy matrices is often 2 to 3 orders of magnitude slower than in rubbery matrices (48), this could lead to sharp reductions in rates of contaminant desorption as the desorption process continues over extended 3630
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time frames. This has obvious important ramifications with respect to the determination and justification of alternative remediation end points.
Acknowledgments We thank Brett Bolan and Albert Yee of the Material Sciences Engineering Department at The University of Michigan for informative discussions on DSC and for the use of their DSC system. This research was funded in part by the Great Lakes and Mid-Atlantic Center for Hazardous Substance Research R2D2 Program under a grant from the Office of Research and Development, U.S. Environmental Protection Agency. Partial funding of the research activities of the Center is also provided by the State of Michigan Department of Environmental Quality. The content of this publication does not necessarily represent the views of either agency. Partial support was also provided by a University of Michigan Regents Fellowship award to E.J.L. We also thank three anonymous reviewers for their helpful comments.
Supporting Information Available Figure 3 provides evidence of glass transition behavior of Aldrich humic acid through heating and cooling cycles, and Figure 5 provides glass transition data for water-wet PIMA, PMMA, PPMA, and Aldrich humic acid. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review September 27, 1999. Revised manuscript received May 15, 2000. Accepted May 16, 2000. ES991103O
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