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Response to ‘LeBoeuf’s Comment on “Evaluation of the Glassy/Rubbery Model for Soil Organic Matter’ ” SIR: We are pleased to find that LeBoeuf (1) accedes our point that a number of different structural features in soil organic matter macromolecule associations may be responsible for observed nonpartitioning-like sorption behavior. In this Rebuttal we would like to address some of the flaws in his Correspondence. Structure of Humic Macromolecules. The fact that Chien and Bleam (2) measured differences in macromolecular segment mobility for aliphatic and fused ring aromatic structures in dissolved humic acid does not contradict the view presented in our paper (3). Certainly no one has claimed that such structures may not have differing mobilities, particularly as it is well known that condensed aromatic structures are inherently rigid. Our point is that such rigid structures in dissolved humic acid cannot be considered glassy because glassiness is a characteristic of a phase and not of an individual humic macromolecule or some portion thereof. To our knowledge, no one has yet demonstrated a glass transition for dissolved humic acid. On the other hand, there have been a number of reports of nonlinear binding of nonspecifically interacting organic molecules to dissolved humic acid (e.g., refs 4-6). From this we may deduce that the ability of individual humic macromolecules to form complexes with organic compounds is not connected with a glassy phase. Glass Transitions. We are surprised to learn that LeBoeuf considers the glass transitions (Tg) he measured for Aldrich and Leonardite humic acids to be significantly different from one another. As he did not report transition temperatures for numerous samples from different batches (7), he certainly is not justified in claiming that the reported differences are significant. Cornelissen et al. (8) failed to obtain a glass transition for a different sample of Aldrich humic acid using a more sensitive instrument. They concluded that the difference between their results and those of LeBoeuf and Weber may be due to differences between batches of Aldrich humic acid. Glassy Biopolymers in Soil Organic Matter. Soil organic matter glassiness as a result of incorporated glassy biopolymers such as cellulose and starch is in direct contradiction to Weber and colleagues’ concept of glassiness due to condensed and rigid aromatic moieties in humic acid (e.g., refs 7 and 9). Further, such biopolymers have been considered “impurities” in soil organic matter which should not be confused with “true” humic substances ((10) pp 37-38). It should be noted that cellulose does not contain fused aromatic moieties nor are glucose and sucrose biopolymers. Although geologic organic matter may be a component of framework grains in certain soils and sediments and may contribute to sorption of organic compounds in those soils, geologic organic matter should not be considered the same as soil organic matter, which is generally taken as synonymous with humus ((10) p 35). Glassy and Rubbery Polymers. LeBoeuf claims that poly(n-butyl methacrylate) (PBMA) was likely to be rubbery throughout the temperature range of his experiments (5-45 °C) based on the fact that he did not detect a glass transition for hydrated PBMA above 0 °C. In fact, LeBoeuf’s DSC measurement for hydrated PBMA displayed a very large (80+ 10.1021/es992012u CCC: $18.00 Published on Web 07/02/1999
1999 American Chemical Society
mW) endothermic peak between 2 and 20 °C (LeBoeuf, personal communication), corresponding to the melting of water. This peak would hide any transition occurring in this temperature range. For reference, LeBoeuf reported a glass transition for hydrated poly isobutyl methacrylate (PIMA) of approximately 1 mW (7). We may estimate the Tg for hydrated PBMA by considering Tg values reported for dry and hydrated PIMA, which is built from an isomeric monomer of PBMA. The reported Hildebrand parameters for PBMA and PIMA are the same (11). As hydration reduced the PIMA Tg by approximately 5 °C (7), we may as a first approximation consider that the decrease in Tg for hydrated PBMA would be of similar extent. In other words, if the glass transition for dry PBMA is 20 °C (as reported by the manufacturer (12)), the Tg for hydrated PBMA should be approximately 15 °C. A transition at 15 °C would be masked by the observed large response for water melting. In fact, LeBoeuf measured a Tg of 30 °C for his sample of dry PBMA (LeBoeuf, personal communication); we are not in a position to speculate as to the possible causes of discrepancies between his measurement and that supplied by the manufacturer. LeBoeuf’s contention that sorption of phenanthrene can lower the PBMA Tg cannot explain the linear isotherm he obtained at 5 °C for sorbed concentrations from 20 to 12 000 µg/g. In a system displaying a sorbate-induced transition, the isotherm is strongly nonlinear and sigmoidal (e.g., ref 13). Conclusions. We would like to conclude this Rebuttal by reiterating for the benefit of the three sets of correspondents some important points in our paper. First, we showed that (a) a glassy phase does not necessarily result in nonlinear sorption behavior, (b) high sorption capacity is not necessarily a function of a glassy phase, and (c) nonlinear sorption may be observed for systems without a glassy phase. Second, we presented some alternative explanations for nonlinear sorption behavior of non- and low polar compounds. Third, we demonstrated definitively that trends in apparent Freundlich n should not be used to conclude anything about the effect of concentration on rate of attainment of apparent equilibrium (see also refs 14 and 15). Fourth, we reiterated the importance of statistical testing for isotherm models, showing, for example, that inadequate Freundlich model descriptions can result in a lack of correspondence between modelcalculated and experimentally-determined fractional uptake curves (see also ref 15). Finally, the protagonists of the glassy/ rubbery theory for soil organic matter provided their own extensive discussions examining the evidence they feel supports their theory. What was missing was an unbiased evaluation of the available data, the rigor of the interpretations, and a presentation of alternatives to the theory based on the same data. None of the three correspondents has, in fact, successfully identified any flaws in our analysis of this material.
Literature Cited (1) LeBoeuf, E. J. Environ. Sci. Technol. 1999, 33, xxxx-xxxx. (2) Chien, Y.-Y.; Bleam, W. F. Environ. Sci. Technol. 1998, 32, 36533658. (3) Graber, E. R.; Borisover, M. D. Environ. Sci. Technol. 1998, 32, 3286-3292. (4) Maxin, C. R.; Kogel-Knabner, I. Eur. J. Soil Sci. 1995, 46, 193204. (5) Schlebaum, W.; Badora, A.; Schraa, G.; van Riemsdijk, W. H. Environ. Sci. Technol. 1998, 32, 2273-2277. (6) Tramonti, V.; Zienius, R. H.; Gamble, D. S. Intern. J. Environ. Anal. Chem. 1986, 24, 203-212. (7) LeBoeuf, E. J.; Weber, W. J., Jr. Environ. Sci. Technol. 1997, 31, 1697-1702. VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(8) Cornelissen, G.; Hassell, K. A.; van Noort, P. C. M.; Kraaij, R.; van Ekeren, P. J.; Dijkema, C.; de Jager, P. A.; Govers, H. A. J. Environ. Pollut. 1999, in press. (9) Huang, W.; Weber, W. J., Jr. Environ. Sci. Technol. 1997, 31, 2562-2569. (10) Stevenson, F. J. Humus Chemistry; John Wiley & Sons: New York, 1982. (11) Barton, A. F. M. CRC Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press, Inc.: FL, 1990. (12) LeBoeuf, E. J.; Weber, W. J., Jr. American Chemical Society Division of Environmental Chemistry Preprints of Papers, 214th National Meeting of the American Chemical Society; Las Vegas, NV, 1997; American Chemical Society: Washington, DC, 1997; Vol. 37, 211-214.
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(13) Kamiya, Y.; Bourbon, D.; Mitoguchi, K.; Naito, Y. Polymer J. 1992, 24, 443-449. (14) Graber, E. R.; Borisover, M. Environ. Sci. Technol. 1999, 33, xxxx-xxxx. (15) Borisover, M.; Graber, E. R. Environ. Sci. Technol. 1999, 33, xxxx-xxxx.
Mikhail Borisover and Ellen R. Graber* Institute of Soil, Water, and Environmental Sciences The Volcani Center, A.R.O. P.O.B. 6, Bet Dagan 50250, Israel ES992012U
