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Pignatello and Xing’s Comment on “Evaluation of the Glassy/Rubbery Model for Soil Organic Matter” SIR: The critique by Graber and Borisover (1) is strongly critical of the rubbery/glassy concept of soil organic matter (SOM) that we and others believe has merit for explaining nonideal sorption behavior of hydrophobic chemicals, such as nonlinearity and competitive effects. Published as a “Research Article”, it is devoid of any new results and offers few new insights other than what its authors have expounded in previous publications. We believe the critique rests on highly selective use of the literature, misinterpretation, flawed reanalysis of published data, and unjustified attacks on the quality and presentation of data. In this restricted space we wish to make a few points in rebuttal. (a) Based on a single limited data set reconstructed into sorption isotherms (Figure 1 of ref 1), Graber and Borisover find that sorption to the glassy polymer state does not necessarily result in nonideal behavior and then use this finding to question the polymer model. In fact, there is an extensive literature (cited in our original works) on sorption of gases and organic liquids to polymers showing nonlinear and competitive behavior in glassy polymers and linear and noncompetitive behavior in the rubbery polymers under conditions of dilute adsorbate. Contrary to a second example they cite, isotherms in other cases (2, 3) do change from nonlinear below the glass-to-rubber transition temperature Tg to linear above it. The dual-mode model permits linearity at very low concentration (Henry’s law region of the Langmuir component) or at very high concentration (saturation of the Langmuir sites or “plasticization” of the polymer); from their discussion, however, it is not apparent they realize this. (b) Graber and Borisover disagree with statements we and others have made that rubbery SOM fills up faster than glassy SOM. Those statements were supported by the diminution of the Freundlich n value over time. To discredit them, they combine misinterpretation with a flawed mathematical argument. Upon reconstructing sets of published data, they assert that there is no experimental trend in “fractional uptake” (Xt ≡ St/Se, where Se is the final sorption) with solute concentration. This assertion is incorrect: in virtually all experiments where n(t) has been followed, the Freundlich isotherms plotted as log S as a function of log C are displaced upward and become shallower with time, thus showing log(St/Se) to increase progressing from low to high values along the log C axis. Next they contend that the declining n(t) is simply the outcome of a trivial mathematical effect resulting from inappropriate use of the Freundlich equation. For sake of argument, they assume mass transfer rate is concentrationindependent. Their analysis is flawed within the context of this hypothetical situation. The flaw lies in making a direct correspondence between fractional uptake (e.g., Xt ) 0.67) and fractional equilibrium (i.e., 0.67 of the time it takes to reach final uptake) along the isotherm. Such correspondence does not hold when uptake occurs from a finite bath where solute concentration changes, owing to the nonlinear relationship between sorbed and solution concentrations. This can be seen by considering “fractional uptake” in terms of the Freundlich relationship: Xt ) St/Se ) KtCtnt/KeCene. After substituting the mass balance relationship Ct ) (1 - Xt)kSe + Ce, where k is the soil mass 10.1021/es990036v CCC: $18.00 Published on Web 06/26/1999
1999 American Chemical Society
FIGURE 1. Sorption isotherms of 2,4-dichlorophenol in peat after 1 day (black symbols) and 180 days (white symbols) on linear scale. Dotted lines are Freundlich fits and solid lines are linear fits. This plot corresponds to Figure 2a of Xing and Pignatello (5) plotted on log scale. to liquid volume ratio, and substituting KeCene for Se, this can be rewritten as eq 1, which is identical to eq 8 of their Appendix (1).
( ) KeXt Kt
1/nt
) Ce1-(ne/nt) + kKeCene-(ne/nt)(1 - Xt)
(1)
While not possible to express explicitly, clearly Xt ) f(Ce) except when nt ) ne ) 1. Thus, at any t, fractional uptake from a finite bath varies with position along the concentration axis. Reinterpretation of literature data in terms of fractional uptake as a measure of equilibrium (their Figures 2 and 4) must therefore be dismissed. A diminution in n(t) is fully consistent with the presence of both rubbery and glassy domains in SOM. That small moleclules diffuse faster in the rubbery state has ample basis in the polymer literature (4). Two additional causes of timedependent changes in the value of n include the nonlinearity of the solute chemical potential within the glassy SOM domains and the plasticizing (softening) effect of the solute on glassy SOM at high loadings, which may increase the solute’s diffusivity. (c) The critique makes unjustified claims on the quality of published data. Citing Figures 2a and 3a of ref 5, among other examples, they charge “...many instances in the literature of poor Freundlich model fits...” which are “...obscured by the condensed scale and nonproportionately sized data points”. Frankly, we are astonished by this claim in regard to our data and invite readers to see for themselves. The four log-log Freundlich plots illustrated in ref 5 are plotted full-scale. The r 2 values are 0.999, 0.999, 0.996, and 0.999 (18 data each). The symbol size reflects the deviation between duplicate determinations. They further state “the use of log-log plots is particularly misleading because very large deviations between model fit and data can be masked”. We offer Figure 1 here, which reproduces two of the curves in question on a linear scale. VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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One can readily see that the Freundlich expression fits the 1 day and 180 day data quite well, certainly better than a linear expression. Despite its shortcomings, the Freundlich equation has a long and useful history in soil and environmental science because it describes heterogeneity with a minimum number of parameters. There is no a priori reason it cannot be used in time-dependent phenomena. We also wish to address the alternative concepts offered by Graber and Borisover. 1. Complex Formation with a SOM Macromolecule. Graber and Borisover suggest that the nonlinearity of PAH sorption to “dissolved organic matter” (DOM) observed in some cases (e.g., ref 6) is evidence that nonideal behavior does not require a “solid state”, as they presume the rubbery/ glassy concept must. This argument presupposes that DOM is not solid. Yet DOM may encompass everything from truly dissolved individual macromolecules to macromolecular aggregates to organic colloids to coatings on inorganic colloids. The study they cite (6) used water extracts of various soils that were unpurified beyond a centrifugation step. Such extracts likely contained all of the above materials. Furthermore, whether organic colloids are solids or not, and whether they can exist in rubbery and glassy states, is open to question. Using 2D NMR Chien and Bleam (7) concluded that “dissolved” humic acid is comprised of rigid and flexible domains. Graber and Borisover offer an alternative mechanism labeled in their Abstract as “complexation of sorbate with a humic macromolecule”. We note that this mechanism is similar to the one we had in mind for sorption to Langmuir sites when we made the analogy of a host-guest inclusion complex (2). 2. Interactions at the [External] Surface of SOM. Graber and Borisover draw a mechanistic distinction between sorption of “large” molecules such as phenanthrene and atrazine and “small” molecules such as substituted benzenes. They calculate that SOM has sufficient external surface area (by N2 BET) to fully account for the adsorption component of the “large” molecules; hence, no need to invoke the rubbery/glassy concept. We find the distinction based on molecular size to be shaky. Chien et al. (8) recently studied atrazine sorption to dissolved humic acids by NMR using paramagnetic probes that either partitioned or stayed in solution; they found atrazine resides in the hydrophobic interior. Nevertheless, we feel that the N2 BET measurement probably overestimates the hard surface area available to organic molecules for adsorption under typical environmental conditions. Under conditions of the BET isotherm (dry material, 77 K) the SOM surface is so hard (inflexible) that it actually presents an energy barrier to passage of N2 molecules to the interior, whereas at 20 °C in the presence of water the organic surface is thermally flexible and expanded by hydration. Last, we note that if large molecules such as
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phenanthrene were excluded from the interior, sorption and desorption would take place rapidly, the opposite of what is commonly found. 3. Cross-Linking of SOM Macromolecules Resulting in a Finite Sorption Domain. In a recent paper (9), Graber and Borisover attribute Langmuir sorption of small, nonpolar molecules to “...a limited sorption domain imposed on nonpolar solutes by virtue of cross-linking at polar functional groups [of humic molecules]...” (9) “...without rupturing polar contacts [between humic molecules]” (1). Without directly addressing the validity of this hypothesis, we note that it seems to have little in conflict with the rubbery/glassy concept and dual-mode model, which envisions thermally dynamic and thermally rigid sites. The “polar contacts” they invoke would, in a sense, increase the degree of cross-linking of macromolecules in SOM, which in turn would lead to more rigid domains.
Acknowledgments The authors acknowledge prior contacts by Ellen Graber and Mikhail Borisover to clarify some of the issues in their critique. We regret that discussions with them were not more fruitful.
Literature Cited (1) Graber, E. R.; Borisover, M. D. Environ. Sci. Technol. 1998, 32, 3286-3292. (2) Xing, B.; Pignatello, J. J. Environ. Sci. Technol. 1997, 31, 792799. (3) Berens, A. R. J. Membrane Sci. 1978, 3, 247-264. (4) Berens, A. Makromol. Chem., Macromol. Symp. 1989, 29, 95108. (5) Xing, B.; Pignatello, J. J. Environ. Toxicol. Chem. 1996, 15, 12821288. (6) Maxin, C. R.; Ko¨gel-Knabner, I. Eur. J. Soil Sci. 1995, 46, 193204. (7) Chien, Y.-Y.; Bleam, W. F. Environ. Sci. Technol. 1998, 32, 36533658. (8) Chien, Y.-Y.; Kim, E.-G.; Bleam, W. F. Environ. Sci. Technol. 1997, 31, 3204-3208. (9) Graber, E. R.; Borisover, M. D. Environ. Sci. Technol. 1998, 32, 258-263.
Joseph J. Pignatello* Department of Soil and Water The Connecticut Agricultural Experiment Station P.O. Box 1106, New Haven, Connecticut 06504
Baoshan Xing Department of Plant and Soil Sciences University of Massachusetts Amherst, Massachusetts 01003 ES990036V * Corresponding author phone: (203)974-8518; fax: (203)974-8502; e-mail:
[email protected].
