Environ. Sci. Technol. 2000, 34, 4549-4553
Glass Transition Behavior in a Peat Humic Acid and an Aquatic Fulvic Acid KATHERINE D. YOUNG AND EUGENE J. LEBOEUF* Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, Tennessee 37235
This paper presents the results of a thermal investigation of the physical and chemical properties constituting the macromolecular structure of natural organic matter (NOM). It presents new evidence of glass transition phenomena in a peat humic acid and a stream-derived fulvic acid as identified through use of temperature-modulated differential scanning calorimetry (TMDSC) and thermal mechanical analysis (TMA). Identification of glass transition temperatures (Tgs) in both soil- and stream-derived humic materials suggests a general macromolecular structure for humic and fulvic materials in NOM. Quantified Tgs are found to be related to their elemental and chemical functional group composition, where a more aromatic peat humic acid possesses higher glass transition temperatures than Suwannee River fulvic acid. Theory of glass transition behavior is used as a backdrop to explore the potential use of thermal analysis techniques for quantifying other thermodynamic parameters of NOM, including specific heat capacity, compressibility, and thermal expansion coefficient. Use of this information is then discussed in terms of its application to developing and verifying molecular simulation modeling of NOM structures.
Introduction Risk-based approaches to contaminated site remediation involve the evaluation of potential health risks arising from chemical exposure to humans or other receptors. This approach implies that a chemical at a contaminated site must be available to a receptor in that it must be released from the soil or sediment, and transported to the receptor, before exposure can occur. Slow chemical release or decreased availability from soils and sediments (i.e., resistant desorption processes) are thus key elements in evaluating potential risk arising from chemical contamination at any one site (1). Two recent reviews (2, 3) attribute resistant desorption (or sequestration) processes to a combination of mechanisms ranging from migration of sorbed contaminants through microporous minerals (4) to their diffusion through natural organic matter (NOM) in soil and sediment organic matrices (5, 6). Considering the large time scales and consequent uncertainties associated with desorption processes, it becomes apparent that accurate modeling of desorption behavior is difficult without further knowledge of controlling mechanisms. The focus of this study is to improve our understanding of the structure of various components of NOM with which contaminant interactions can occur. This * Corresponding author phone: (615)343-7070; fax: (615)322-3365; e-mail:
[email protected]. 10.1021/es000889j CCC: $19.00 Published on Web 09/22/2000
2000 American Chemical Society
structural information can then be used to enhance our ability to distinguish mechanisms controlling sequestration behavior through subsequent sorption/desorption studies and molecular simulation modeling. NOM found in soils and sediments is comprised of a complex heterogeneous mixture of diagenetically altered biopolymers of plant and animal origin. The resultant organic material includes macromolecules such as humic and fulvic acids, kerogen, and other coal-like substances (7). These components likely retain a portion of their original macromolecular character as evidenced by structural models proposed for humic and fulvic acid (8, 9) and for humin and kerogen (10). A great deal of recent research has focused on refining the chemical and conformational models that describe humic and fulvic materials (11-14) and how the chemical and structural morphology of these materials affects sorption/desorption behavior (15-18). There is, however, limited information on the thermodynamic properties of these same materials, which can serve as important inputs or calibrations for molecular simulations of NOM structure. All macromolecules, including biopolymers such as starches and lignins, display distinct thermodynamic properties that can manifest significant differences in their physical structure and mechanical behavior. One of these characteristics is the glass transition temperature, Tg, which marks a second-order phase transition between a hard, rigid, glasslike state and a soft, flexible, rubbery state. This characteristic property may be measured using differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) (19), among other techniques. Implications of the existence of Tg in natural materials are limited not only to yielding information about the macromolecular structure of humic substances but also can provide valuable insights into the sorptive behavior of these substances. For example, synthetic polymer literature describes sorption in the glassy state as a nonlinear, hole-filling process (e.g., dual-mode sorption) (20), with consequent nonFickian, concentration- and time-dependent diffusion coefficients (21). In the rubbery state one may expect to see linear, partition-like sorption that may follow Flory-Huggins theory for dissolution (22), with Fickian-type diffusion manifesting 2 to 4 or more orders of magnitude larger diffusion coefficients relative to the glassy state (22). Similar sorption behavior in natural organic matter has been exemplified in a number of studies, including isotherm linearity (23, 24), isotherm nonlinearity (25), fast, Fickian-type diffusion in highly expanded, fluidlike matrices (6, 26), and slower, likely nonFickian-type diffusion in more rigid, condensed segments of NOM (26). A number of studies have applied the concept of rubbery/ glassy behavior to natural systems (27-30). Recent discovery of glass transitions in sediment-derived humic acids provides direct evidence to support the hypothesis of glassy/rubbery behavior in NOM, at least for sediment-derived materials. There exists, however, limited direct evidence that soil derived NOM displays glass transition behavior. This research adds to this growing body of evidence by presenting data demonstrating glass transition behavior for a peat-derived humic acid as well as a stream-derived fulvic acid in an attempt to fill the void of information for soil or aquatic humic substances. Further, this study employs, for the first time, temperature modulated differential scanning calorimetry and thermal mechanical analysis for the characterization of soil and aquatic humic substances. VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Glass Transition Theory Glass transition behavior is arguably both a kinetic and a thermodynamic phenomenon. A thermodynamic argument derives from the observation of the change in the thermodynamics associated with the shift from glassy to rubbery behavior, as evidenced by changes in heat capacity, viscosity, dielectric relaxation, or thermal expansion coefficients. The kinetic argument derives from observation of changes in glass transition temperatures for samples exposed to different thermal histories. For example, in standard differential scanning calorimetry one often observes an upward shift in the detected glass transition temperature with increased temperature scanning rates (e.g., LeBoeuf and Weber (31) observed a 12 °C shift in Tg for a lignin when heating rates were increased from 10 °C/min to 50 °C/min). This phenomenon can be explained in terms of the relaxation rate of the macromolecule relative to the time scale of the experiment. A macromolecule possesses a characteristic time for molecular motions leading to relaxation and volume recovery (32). If the time scale of the experiment exceeds this characteristic time, then there is no observed shift in the glass transition temperature from its “true” thermodynamic base. If, however, the time scale of the experiment is shorter than the characteristic time of the macromolecule (which is always the case), then the macromolecule will lag in its ability to react to the energy input, leading to an upward shift in the glass transition away from its “true” transition point. Thus, the longer the time scale of the experiment, the closer an experimental observation will be to the true transition point. Gibbs-DiMarzio theory provides an explanation to link observed kinetic and thermodynamic phenomena. GibbsDiMarzio theory for glass transition behavior assumes that each macromolecule has a lowest energy shape at temperature T2 and that configurational deviations from this minimal energy state increases the internal energy of the macromolecule. At T2, there is not enough thermal energy to cause larger scale wriggling, and hence below this temperature, the macromolecules remain in a lowest energy configuration (32). The temperature at which a macromolecule is moved from its lowest, noncrystalline, energy configuration to the rubbery state is known as the “true” glass transition temperature. Any disturbance of this low-energy configuration due to different thermal histories, etc., however, results in a movement in the observed Tg to values away from the true thermodynamic transition, because less energy will need to be absorbed to move the macromolecule into a rubbery state (e.g., the swelling of a humic acid matrix due to the sorption of water lowers the glass transition temperature since less energy is required to move the macromolecule from its more open (swelled) configuration). Similarly, a macromolecule heated at a slower rate will have a lower observed Tg than its more quickly heated counterpart due to the relaxation time of the macromolecule heated at the faster rate is much longer than the time scale of the shorter experiment (32). The reader is referred to McKenna (32) for a more complete review of Gibbs-DiMarzio and other theories for glassy/ rubbery behavior. Although observed glass-transition behavior is clearly influenced by the time scale of the experiment, we concentrate here on the fundamental thermodynamic argument for glassy behavior, for it serves as the basis of detection of glass transition temperatures for the instruments employed in this work. Because of the underlying effects of relaxation rates of macromolecules, the glass transitions quantified in this work will deviate, albeit very slightly, from the true transition temperature. Comparisons of glass transition temperatures from different works should thus only be made using similar samples with similar thermal histories and experimental conditions. 4550
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Methods used to measure Tg take advantage of the thermodynamic relationships of second-order transitions. Considering a single component system, free energy of the system may be expressed in terms of the Gibbs free energy equation
G ) H - TS
(1)
where H is enthalpy, T is absolute temperature, and S is entropy. Noting that H ) U + PV, where U is internal energy, P is pressure, and V is molar volume, then
G ) -TS + PV + U
(2)
For first order thermodynamic transitions, such as the melting or boiling points, the entropy, volume, and therefore enthalpy change markedly. This may be mathematically expressed in terms of the first derivative of the Gibbs free energy with respect to temperature (at constant pressure) or pressure (at constant temperature):
) -S (∂G ∂T ) )V (∂G ∂P )
(3)
P
(4)
T
Entropy is a property which may not be measured directly, so, a more convenient form of the first derivative with respect to inverse temperature may be used:
( ) ∂(G/T) ∂(1/T)
)H
(5)
P
Second derivatives of the free energy equation yield expressions for second order thermodynamic transitions, including specific heat capacity (Cp), compressibility (k), and thermal expansion coefficient (R); each of which may be determined experimentally:
[ ( )] ( ) [ ( )] ( ) [ ( )] ( ) ∂ ∂G ∂T ∂T
)
P P
∂ ∂G ∂T ∂P
)
T T
∂2 G ∂T2
)
P
∂2 G ∂P2
∂ ∂(G/T) ∂T ∂(1/T)
(6)
∂ (V) ) -kV ∂T T
(7)
∂H ∂T
(8)
)
T
)
P P
CP ∂ (-S)P ) ∂T T
∂V )( ) [∂T∂ (∂G ∂T ) ] ∂T T P
P
) CP
) RV P
(9)
The free energy function and its first partial derivatives with respect to temperature and pressure are continuous. However, the second derivatives of the Gibbs free energy, R and Cp, exhibit discontinuities (33). Thus, if we assume that glass transitions are fundamentally thermodynamic in nature, then Tg may be determined by evaluating the temperature at which a discontinuity occurs in Cp (via evaluation of heat capacity changes as a function of temperature through differential scanning calorimetry) or R (via measurement of dimension changes as a function of temperature through thermal mechanical analysis). Because the rubbery state allows greater macromolecular mobility relative to the glassy state, it exhibits a greater ability to disperse heat as reflected by a corresponding shift to a higher Cp as well as an increased thermal expansion coefficient due to larger molecular motions of the macromolecule at increased temperatures (34). This increased free volume eventually leads to a decrease
TABLE 1. Elemental and Molecular Weight Data for PHA and SRFA elemental analysisa (%) sample
C
H
O
N
S
ash
MWb
peat HA Suwannee River FA
56.84 53.04
3.60 4.36
36.62 43.91
3.74 0.75
0.70 0.46
1.72 0.98
104-105 103
a Elemental analyses were performed by Huffman Laboratories, Wheat Ridge, CO, U.S.A. for the International Humic Substances Society on humic substances prepared as described in ref 43. b Molecular weight information is derived from ref 7.
FIGURE 1. Heat flow versus temperature curve. in macromolecule viscosity as provided in the Doolittle expression (33).
Materials and Methods NOM. Suwannee River fulvic acid (SRFA) (reference grade, lot no. 1R101F) and peat humic acid (PHA) (reference grade, lot no. 1R103H) samples were obtained from the International Humic Substances Society and were used as received. Characterization information for these materials is presented in Table 1. TGA. Thermal gravimetric analysis was used to determine the maximum temperature prior to sample thermal degradation. This technique evaluates the thermal stability of a sample by measuring the amount and rate of weight change of a material as a function of temperature. A TA Instruments TGA 2950 was used to scan each sample from 25 °C to 450 °C at a heating rate of 10 °C per minute. Standard DSC. DSC measures the difference in energy input required to maintain a sample cell and a reference cell at the same temperature while scanning at a controlled rate across a range of temperatures. The Tg is defined as the discontinuity in the heat capacity between the rubbery and glassy states as described above. A TA Instruments differential scanning calorimeter Model DSC 2920 was used to perform standard differential scanning calorimetry. One scan was performed for each sample from -30 °C to 120 °C at a standard heating rate of 10 °C per minute after first heating the sample at 10 °C per minute to 110 °C, holding for 30 min to evolve physi-sorbed water, and cooling at 10 °C per minute to -30 °C. Data were analyzed using TA Instruments TA5200 Advantage Software Suite. Briefly, this method involves the identification of the maximum of the derivative of the heat flow versus temperature curve as shown in Figure 1. Points of slope deviation before (T1) and after (T2) the transition are selected and tangent lines to the data are determined for each of these points. A midpoint temperature (Tm) is determined based on a tangent with the Tg, which is determined as described above. The transition onset (To) and endset (Te) are defined at the intersection of the tangent lines. The change in specific heat capacity is then defined as the vertical difference between the onset and endset temperatures (35). TMDSC. Temperature-modulated DSC (TMDSC) involves the superposition of a nonlinear sinusoidal modulation over
the traditional DSC heating ramp in which the temperature increases with time. The resultant experimental heat flow and heat rate signals are then deconvoluted by discrete Fourier transformation to yield thermodynamic quantities such as heat capacity, glass transition temperature, and heats of vaporization and melting (see ref 35 for a more detailed description of TMDSC). The TMDSC technique offers a number of advantages over standard DSC including increased sensitivities for weak glass transitions, opportunity to measure heat capacity and heat flow during the same experiment, and reduction of enthalpic relaxation events which can obscure the detection of Tg (35). Heat flow measurements were taken for SRFA and PHA samples using a TA Instruments Model DSC 2920 in the temperature modulation mode (-60 to 110 °C). The underlying heating rate used was 2 °C per minute with an 80 s period of modulation, and a (0.5 °C temperature amplitude. Figure 1S (Supporting Information) shows the TMDSC heating profile used in this work. Although sample temperature changes were sinusoidal, the analyzed signals were plotted against the linear average temperature. Three scans were performed for each sample to ensure reproducibility of results. TMA. TMA involves applying a stress to a sample while its deformation is measured as a function of temperature. Sample thermal expansion characteristics and/or resistance to deformation are measured against temperature, thus yielding information about Tg. Because this technique is a more sensitive method of detecting Tg, it is often used to confirm a suspected Tg detected by conventional DSC (36). A number of various natural polymer systems have been successfully evaluated for Tg using TMA including proteins (37), starches (38), and coal tar pitch (39). Dimensional changes of PHA and SRFA were measured using a TA Instruments TMA 2940 thermomechanical analyzer to confirm results obtained via TMDSC. Three heating and cooling cycles were performed on each sample, each at a 5 °C per minute heating (cooling) rate, scanning from 25 to 120 °C. Humic acid and fulvic acid powders were prepared for TMA by placing 50-75 mg of powder into a 13 mm die which was pressed under 5000 lbf for 1 min. Dimension change versus temperature data were analyzed in a method similar to that described for DSC using the second derivative of heat flow to determine Tg.
Results and Discussion A summary of Tg results using all methods is shown in Table 2. A TGA scan for SRFA is shown in Figure 2 indicating that thermal degradation begins around 120 °C. Similar results (not shown) were found for PHA. This information suggests that thermal degradation does not occur below 120 °C, therefore allowing one to conclude that observed glass transition behavior detected below this temperature is likely a true transition and not an anomaly resulting from thermal degradation or evolution of physi-sorbed water. TMDSC results are presented in Figures 3 and 4. Standard DSC (data not shown) does not reveal the existence of a Tg for PHA and only a weak indication of a Tg for SRFA near 97 °C. TMDSC data show marginally improved resolution for VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Glass Transition Temperatures of PHA and SRFA as Determined by Standard DSC, TMDSC, and TMA Tg (°C) sample
standard DSC
TMDSC
TMA
peat HA Suwannee River FA
NDa
43-62 36-49
62 36
a
97
ND ) not detected.
FIGURE 5. Thermal mechanical analysis (TMA) results for PHA.
FIGURE 2. Thermal gravimetric analysis (TGA) scan of Suwannee River fulvic acid (SRFA).
FIGURE 6. Thermal mechanical analysis (TMA) results for SRFA.
FIGURE 3. Temperature modulated differential scanning calorimeter (TMDSC) scan of a peat humic acid (PHA).
FIGURE 4. Temperature modulated differential scanning calorimeter (TMDSC) scan of SRFA. PHA, with Tg appearing near 43-62 °C, and that SRFA possesses a Tg between 36 and 49 °C. TMA data for PHA and SRFA samples are shown in Figures 5 and 6, respectively. These results confirm that PHA and 4552
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SRFA exhibit glass transition behavior, further establishing the Tg near 62 °C and 36 °C, respectively. Because TMA measures volume changes with temperature, the Tg is represented by a change in the viscosity or resistance to penetration of the dilatometer. Further support for the Tg occurring at these respective temperatures derives from the remarkable repeatability of Tg for each of the samples for each heating and cooling cycle. Although one often expects a thermal expansion of a macromolecular matrix during heating, it is possible, for some samples, and under certain sample preparation protocols, for one to observe a reduction in the volume upon heating. In describing the molecular level dilatometric behavior of a bituminous coal, Larsen (40) explained that as coal particles transition to the rubbery state, pores collapse to achieve more efficient packing. The likely presence of microporosity in both fulvic and humic acids (41), combined with the protocol for compression of the NOM pellets, may thus explain the volume reduction behavior observed for NOM samples used in this study. Nonetheless, the important observation is the repeatable, abrupt change in the dilatometric response, which coincides with behavior normally associated with Tgs in TMA (39), and further coincides with the aforementioned independent TMDSC measurements. The Tg observed for SRFA was approximately 26 °C lower than that found for PHA. Comparing the average molecular structural unit of fulvic acid to that of humic acid, the less complex fulvic acid possesses a lower molecular weight, less aromaticity, and less likelihood to coil than its humic acid counterpart (7, 8, 11). A lower molecular weight could cause a reduction of Tg as shorter polymer chains give rise to more free-end groups, which contribute to increased molecular wriggling (42). The decreased aromaticity observed in FA
compared to HA could also contribute to the lower Tg displayed for the SRFA compared to the peat humic substance. Polymer chains that have aromatic compounds in their backbones have extremely stiff bonds, resulting in reduced molecular mobility and increased glass transition temperatures (42). In addition to the structural differences noted above, humic acids are also more likely to coil, thus providing for increased opportunities for the formation of cross-links, resulting in a decrease in molecular mobility with a concurrent increase in Tg (7, 33). This paper presents evidence of glass transition behavior in soil- and stream-derived humic materials, contributing to the growing body of evidence of glass transitions in soil and sediment-derived NOMs. These findings have potentially important implications for refined interpretations of the physical and chemical morphology of humic materials and for an improved understanding of the effects of varying mechanical behavior of humic materials on the sorption and transport of environmental contaminants. Similarly, it is important to recognize that identification of a Tg in both soil- and stream-derived humic materials suggests a general macromolecular structure for humic and fulvic materials in NOM. Further, Tg along with other thermodynamic characteristics such as thermal expansion coefficient and heat capacity can be quantified for use in developing and verifying molecular simulation modeling of NOM structures that can be further employed to predict molecular scale interactions between contaminants and NOM.
Acknowledgments The authors gratefully acknowledge the assistance of Jeff Groh from TA Instruments for sample analyses. The research in this report was funded in part by a grant from the University Research Council at Vanderbilt University and the Vanderbilt School of Engineering. We also wish to thank two anonymous reviewers for their helpful comments.
Supporting Information Available A detailed illustration of the heating profile utilized in temperature modulated differential scanning calorimetry experiments is provided in Figure 1S. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 10, 2000. Revised manuscript received August 9, 2000. Accepted August 11, 2000. ES000889J
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