Conditioning-Annealing Studies of Natural Organic Matter Solids

Dec 1, 2005 - Linking Irreversible Sorption to. Irreversible Structural Expansion ... polymer irreversibility model applies to natural organic matter ...
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Environ. Sci. Technol. 2006, 40, 170-178

Conditioning-Annealing Studies of Natural Organic Matter Solids Linking Irreversible Sorption to Irreversible Structural Expansion MICHAEL SANDER,† YUEFENG LU,‡ AND J O S E P H J . P I G N A T E L L O * ,†,‡ Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, Connecticut 06511, and Department of Soil and Water, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06511

The assumption of reversibility underpins the sorption term in current models dealing with the fate and impact of organic compounds in the environment, yet experimentally sorption of organic compounds in soils and sediments often shows “irreversible” behaviors such as hysteresis and the conditioning effect (enhanced repeat sorption). The objective of this study was to test whether a glassy polymer irreversibility model applies to natural organic matter (NOM) solids. Irreversible sorption in polymers is believed to be caused by irreversible expansion and creation of internal micropores by penetrating molecules, leading to enhanced affinity during desorption or subsequent resorption. Using chlorobenzene as a conditioning agent and polychlorinated benzenes as test compounds in a second sorption step, we observed conditioning effects for a peat soil, a soil humic acid, and a model glassy polymer, poly(vinyl chloride), but not for a model rubbery polymer, poly(ethylene). The conditioning effect for the two natural solids, probed by the enhancement in the sorption distribution coefficient of 1,2,4-trichlorobenzene, relaxed upon sample annealing between 45 and 91 °C in a manner similar to the relaxation of free volume and enthalpy of glassy polymers. Relaxation of the conditioning effect in the NOM solids depended on annealing temperature and, at a given temperature, followed a double additive exponential rate law with a nonzero constant term descriptive of the final state that depends inversely on temperature. At environmentally relevant temperatures, the conditioning effect may “never” completely relax. The results provide compelling evidence for the glassy, nonequilibrium nature of natural organic matter solids and for irreversible structural expansion as a cause of irreversible sorption.

Introduction Macromolecular soil and sediment natural organic matter (NOM), which includes humic and related materials formed and aged below charring temperatures, is a primary sorbent of nonionic, hydrophobic organic compounds in the environment (1) and has been studied intensively with respect * Corresponding author phone: 203-974-8518; fax: 203-974-8502; e-mail: [email protected]. † Yale University. ‡ The Connecticut Agricultural Experiment Station. 170

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to this role (2-4). Initially proposed linear, reversible sorption models (5-7) have evolved into conceptual models that are more consistent with observed “nonidealities” in these interactions, including isotherm nonlinearity (8-10), competitive effects in multi-solute systems (11-13), and sorption irreversibility (14-22). A model originally developed for gas sorption to glassy synthetic organic polymers (23-25)sthe dual mode model (DMM)sand the mathematically identical dual reactive domain model (DRDM) (9, 26), have provided a conceptual framework to explain nonideal uptake of compounds by NOM (9, 20, 27-30). The analogy between macromolecular forms of NOM and synthetic organic polymers has proven valuable for describing molecular interactions of pollutants with NOM (27, 29-31). The present study focuses on advancing our understanding of sorption irreversibility in NOM solids in the context of this analogy. This understanding is crucial because sorption reversibility is implicit in current models for the environmental fate and deleterious effects of organic compounds. Herein, the term “irreversible” is used in its thermodynamic connotation (i.e., indicating the existence of metastable states) and does not imply the formation of an irretrievable and/or covalently bound fraction. Macromolecular organic solids can exist, in principle, in four interchangeable states: the melted, rubbery, glassy, and crystalline states (32). The most relevant ones for NOM solids are the rubbery and glassy states. Conversion between rubbery and glassy states occurs at a glass transition temperature, Tg, specific to the solid. The rubbery state (T > Tg) is characterized by relatively high segmental flexibility of the macromolecules, allowing structural equilibrium to be readily achieved following changes in an applied external condition such as temperature. Sorption in the rubbery state is reversible and its concentration dependence is welldescribed by Flory-Huggins theory (23, 33, 34). As the solid cools through the Tg to a temperature below the Tg, macromolecular rearrangements necessary for reaching the thermodynamic state require more time than is available by the imposed cooling rate and ultimately become prohibitive (35). As a result, excess free volume in the form of poorly interconnected micropores or “holes”’ is frozen into the matrix. The glassy state is, thus, a perpetual nonequilibrium state. In the DMM and related DRDM, sorption is simplified as the combination of an adsorption process in the holes represented by a Langmuir term and a dissolution process in the matrix surrounding the holes represented by a linear term. Previous experimental support for the glassy nature of NOM (including purified humic acids), in addition to the above-mentioned nonideal sorption behaviors, includes the detection of apparent glass transitions above room temperature for humic materials, soils, and coals measured by differential-scanning calorimetry (DSC), temperature-modulated DSC (TMDSC), and thermo-mechanical analysis (30, 36-41), and the demonstration of temperature-dependent coexistence of expanded (rubbery) and condensed (glassy) domains in humic acids by nuclear magnetic resonance spectroscopy (42-45). Holes are semi-permanent entities that are affected not only by temperature but also by sorbate concentration. Increasing concentration favors sorbate-polymer over polymer-polymer interactions and increases polymer segmental flexibility, which, in turn, leads to a reduction in the Tg (46). This is referred to as plasticization. Glass transition is induced at the isothermal glass transition concentration, Sg, of sorbate. A sorption-desorption cycle carried out on a glassy solid 10.1021/es0506253 CCC: $33.50

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will result in an increase in the excess free volume, whether or not Sg had been exceeded (47-51). Two processes are believed responsible for this increase: creation of new holes as polymer strands part to create a cavity to volumetrically accommodate the incoming sorbate, and dilation of existing holes as thermal motions of sorbate exert pressure on the surrounding walls. Since segmental flexibility decreases as the sorbate becomes diluted, these processes are only partly reversible. Consequently, the solid exhibits sorption irreversibility, which may be manifested macroscopically as hysteresis or the “conditioning effect”. Hysteresis is the nonsingularity of the sorption and desorption branches of the isotherm (i.e., enhanced affinity during desorption). The conditioning effect refers to enhanced affinity in a second sorption experiment following a prior sorption-desorption cycle involving the same or another compound. Any increase in the excess free volume of a glassy polymer, induced either thermally or via penetrant addition and removal, is followed by structural relaxation tending toward the hypothetical equilibrium state. However, at temperatures much below the Tg, the equilibrium state can never be reached. The process of (incomplete) relaxation is referred to as “physical aging” (52). Physical aging has been extensively studied in polymer and material sciences, predominantly via isothermal annealing experiments in which a polymer is cooled through its Tg to an annealing temperature, Ta, at which the temporal changes in selected properties of the polymer are followed. These properties include total volume measured by dilatometry, or heat capacity measured by DSC (52). Recently, more advanced technologies, such as positron annihilation lifetime spectroscopy, have been used to study microstructural changes during aging (53, 54). Relaxation in glassy materials is usually nonexponential. Existing models differ in complexity and are often partly empirical (55). The effect of physical aging on sorption by NOM was first reported by Lu and Pignatello (20), who observed the conditioning effect for trichloromethane sorption in dichloromethane-conditioned Pahokee peat. They showed that the memory of the conditioning effect persisted after sample storage for 3 months at room temperature but was nearly eliminated when the sample was annealed at 100 °C. Such results clearly warranted further study, since an observation of structural relaxation on sorption would provide both supporting evidence for the glassiness of NOM and a mechanistic foundation for sorption irreversibility. The objective of the present study was to test the hypothesis that irreversible matrix expansion is the cause of irreversible sorption of organic compounds to NOM solids through conditioning-annealing experiments. We compared the conditioning effect on two NOM solids (i.e., a soil humic acid and a high-organic reference peat soil) with glassy poly(vinyl chloride) and rubbery poly(ethylene) using chlorobenzene (CB) as conditioning agent and polychlorinated benzenes as test compounds. The synthetic polymers were used as homogeneous reference sorbents in an attempt to identify processes causative of irreversible sorption of organics to chemically highly complex and heterogeneous NOM solids. We further carried out thermal annealing experiments to study structural relaxation in the conditioned NOM solids probed by affinity of the test chemical for the annealed solid. Our results not only clearly demonstrate the glassy character of NOM but also link irreversible sorption of HOC in NOM to irreversible matrix expansion of the solid.

for the presence of black carbon in the HA sample (28, 29). The polymers, purchased from Aldrich, included mediumdensity poly(vinyl chloride) (PVC, nominal diameter 200 µm), which was ground to 1-5 µm in a cryogenic vibratory ball mill at 77 K (10), and poly(ethylene) (PE, nominal diameter 100 h) is either fixed fitted, qfitted , or fixed, qfixed was taken to be the ∞ ∞ . The q∞ average of 6 measurements from the final 3 annealing time points (duplicates at each point), over which no systematic change in qadjusted(t) was observed. An exception was the curve for Ta ) 65 °C on PP which showed continued relaxation up to t ) 100 h. In that case, qfixed was taken as the average of ∞ duplicate measurements at the final annealing time-point. Fixing q∞ reduces the number of fitting parameters, and the difference between qfitted and qfixed inversely reflects model ∞ ∞ robustness. 176

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The most economical, single-exponential model (n ) 1; eq 5) proved inadequate to describe relaxation curves of both tc-PP and tc-HA (not shown). For all Ta, the model greatly overestimated initial relaxation rates. Relaxation in PP and HA thus exhibits nonexponential behavior, consistent with relaxation in synthetic organic polymers. The KWW model 2 2 gave acceptable fits for qfixed ∞ , R > 0.877 for PP and R > 0.863 for HA (not shown). However, β-values (β ) 0.12-0.29 for PP and β ) 0.15-0.26 for HA) are low compared to β-values determined for annealed polymers, typically with 0.6 < β < 0.9 (62, 63). Our results indicate that if structural relaxation in NOM and HA is described by assuming a unimodal distribution of relaxation rates, this distribution is substantially broader for NOM solids than for synthetic polymers, reflecting the greater heterogeneity of NOM. The double additive exponential model (n ) 2; eq 5), which assumes a fast-relaxing fraction of sites ffast with rate constant kfast and a slow-relaxing fraction of sites fslow with rate constant kslow, describes the relaxation curves well. This model proved to be very robust with respect to reduction from four to three fitting parameters. With one exception, qfitted values deviate ∞ by less than 3% from qfixed with little or no effect on R2 (data ∞ not shown). The exception (Ta ) 45 °C on PP) gave an unrealistically low value of qfitted (2.27 × 10-21 mol L-1) even ∞ though it resulted in a slightly better R2. The three-parameter model fits are shown as solid lines in Figure 5 and the parameters are listed in Table 2. The R2 are > 0.93 for PP and R2 > 0.88 for HA. For PP, ffast is approximately 0.7 for all Ta, permitting a comparison of the rate constants at different temperatures. This comparison shows that (i) kfast and kslow differ by more than an order of magnitude; and (ii) both kfast and kslow increase with Ta. Both findings are consistent with theory of structural relaxation in synthetic organic glasses: the activation energy is lowered by an increase in the flexibility of the polymeric backbone with Ta. Relaxation rates in HA at different Ta cannot be directly compared due to a nonconstant ffast. Environmental Implications. In addition to advancing our understanding of the nonequilibrium state of NOM, the relaxation kinetics provide a basis to assess the effects of structural relaxation on contaminant sorption under environmentally relevant conditions. Relaxation in NOM has a slow component. For example, given the fitted kslow of 1.16 10-2 h-1 for PP at Ta ) 45 °C, it takes approximately 11 days for 95% of the slow sites to relax. Relaxation rates in PP at environmentally relevant temperatures (e40 °C) are even slower. More significant, however, is the fact that relaxation is generally incomplete as long as the solid remains glassy. Figure 6 shows the fraction F of sites relaxed during annealing of the tc-sample, qtc - qfixed (Ta), relative to the total number ∞ of sites created by thermal conditioning, qtc - qo, in PP and HA with respect to the probe TCB. The solid lines represent

Research and Education Foundation (EREF) for support (Francois Fiessinger Scholarship 2003).

Supporting Information Available Method of adjusting ordinate values in Figure 5a and b to account for nonlinearity effects. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 6. Extrapolation of the fraction of sites annealed relative to the total number of sites created during thermal conditioning of PP and HA to environmental temperatures (∼0-40 °C) with respect to sorption of the probe 1,2,4-trichlorobenzene at 20 °C. Annealing is taken to include heating to the annealing temperature Ta, isothermal annealing at Ta, and cooling the sample back to 20 °C.

linear fits of F vs T in the experimental range. Extrapolations toward environmentally relevant temperatures (0-40 °C) suggest that, for both PP and HA, less than 50% of the sites produced in the employed thermal conditioning experiment would have been lost during annealing. Thus, we conclude that deformation of the microstructure by sorption is not spontaneously reversible under environmental conditions. It is reasonable to assume that the irreversible effects demonstrated here on selected systems apply qualitatively to a broad spectrum of probe chemicals and NOM solids from different sources. Additional work is clearly warranted to study the effects of other environmental factors such as moisture content, pH, redox potential, ionic strength, and composition on the structure of NOM. These factors were deliberately kept constant in the present study. Our study has several important implications with regard to the environmental fate of organic compounds: (i) The conditioning-annealing characteristics of the selected systems provide compelling evidence of the glassy character of these particular NOM samples. (ii) Our results are consistent with a broad distribution of Tg in the NOM solids of this study. (iii) The internal microstructure of NOM sorbents responds to changes in environmental stress such as sorbate concentration and/or temperature. The assumption of either invariant sorbent structure and/or complete reversibility of structural changes in diffusion/transport models is therefore questionable when applied to natural, deformable particles. (iv) Glassy NOM has a “memory” of its exposure history to these environmental stresses. (v) Conditioning and annealing clearly demonstrate that irreversible sorbate-induced structural expansion is a cause of sorption irreversibility in NOM. Structural relaxation times, as demonstrated here, are on the order of hours to weeks and are comparable to sorbate diffusion times. Molecular diffusion in deformable solids is thus coupled with structural deformation. We have shown this in another study using isotope exchange under conditions of static bulk chemical concentration (64). Consequently, slow matrix deformation may well contribute to slow sorption and desorption kinetics commonly observed for HOC uptake to NOM-containing sorbents (2).

Acknowledgments This study was funded principally by the National Science Foundation, Bioengineering Program (BES-0122761) with assistance from federal Hatch funds administered by the U.S. Department of Agriculture. M.S. thanks the Environmental

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Received for review April 1, 2005. Revised manuscript received September 8, 2005. Accepted October 19, 2005. ES0506253