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Response to Comment on “Competitive Sorption between Atrazine and Other Organic Compounds in Soils and Model Sorbents” SIR: We appreciate the comments of Borisover and Graber on our paper (1). Our dual-mode model (1-3) is based on a concept of SOM as a ‘polymer mesh’ phase that ranges continuously from ‘rubbery’ to ‘glassy’ in character. Rubbery means relatively more gel-like, expanded, flexible, and highly solvated, while glassy means more condensed, rigid, and less solvated. Small organic molecules are able to penetrate all phases and intermingle with the humic macromolecules. Solid-phase dissolution (partition) sorption occurs in regions where thermal relaxation of the polymer structure is rapid, so that ‘sites’ are ephemeral and an averaged sorption potential exists, as in a liquid. Dissolution takes place throughout SOM but increases in importance with its rubbery character. Langmuir-type sorption occurs in regions where the rigidity of the humic backbone creates sites that are relatively long-lived. This results in nonlinearity because the sites vary in energy. The concentration of such sites increases with the glassy character. We have also called this the ‘holefilling’ mechanism (3) because we envision the sites as microvoids in analogy with organic polymers; however, their exact nature is irrelevant to the present discussion. Further description and experimental support of the model are given elsewhere (2, 3). Here, we address point-by-point the comments of Borisover and Graber. (i) While we all are in agreement that specific interactions evidently do take place in SOM, Borisover and Graber argue that the Langmuir sites are on the external surface rather than inside the matrix, as we propose. They suggest (a) that the observed nonlinearity of isotherms at long contact times (2, 3) is “not relevant” to shorter times and (b) that the external surface area determined by N2 adsorption at 77 K (0.88 m2/g) is sufficient to explain the Langmuir component of dualmode sorption. Underlying this disagreement are the definitions of external and internal, which relate to the question of physical structure of SOM in the hydrated state. Is it a contiguous phase or a highly porous material? What is the dimension that distinguishes a ‘pore’ from simply the normal space between individual macromolecules? These questions are important since the surface of a pore is arguably external to the bulk matrix. Not yet having a satisfactory answer to them, we provisionally take the external surface to be that which is readily accessible by the N2 probe. Responding to their first point (a), we believe that the observed decrease in the Freundlich exponent over time (2, 3) is meaningful because it indicates that sorption becomes more energetically distributed with time; i.e., Langmuir sorption increases in importance as compared to dissolution. There are two possible reasons for slower Langmuir-type sorption: that it takes longer to reach those sites or that complexation at some or all of those sites is sterically hindered. We have argued that diffusional resistance is greater in glassy than in rubbery phases, as is true for polymers. Weber and Huang (4) used a similar rationale for the existence of embedded condensed organic matter domains that sorb hydrophobic compounds more slowly. Whether or not steric effects come into play is an open question at this point. Timedependent nonlinearity is harder to explain in terms of external surface sites. Physical adsorption on an exposed surface is generally unactivated and therefore rapid. Although less accessible external sites are not hard to imagine (such as a SOM microparticle embedded in a mineral aggregate),
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it is not immediately apparent why readily accessible external sites should be more homogeneous in energy. In regard to their second point (b), we agree that the external surface area of the peat seems to be high enough to account for Langmuir sorption in the case of atrazine. However, the same conclusion does not apply to the hydrophobic compound 1,3-dichlorobenzene (1,3-DCB), whose dual-mode parameters were obtained in peat soil (3). Taking 1,3-DCB liquid density to be 1.283 g/cm3 and molecular diameter to be 0.714 nm, we calculate the maximum possible surface coverage of 1,3-DCB to be 544 µg/g peat, which is ∼6.5-fold less than the dual-mode Langmuir capacity term, S° (3535 µg/g). Furthermore, Borisover and Graber overestimate the maximum possible external surface coverage of the organic adsorbate because they assume a smooth plane surface equally available to N2 and the adsorbate. Solid humic substances are surface factals however (6). The area covered by a monolayer on a fractal surface decreases as molecular size increases because the smaller molecule can fit into more crevices. For two spherical molecules, the ratio of the number of molecules in a monolayer is given by n2/n1 ) (a1/a2)df, where a is diameter and df is the fractal dimension (5). Since df is 2.0 for a plane surface but closer to 2.5 for humic substances (range 2.2-2.8) (6), this results in a discrepancy of 1.67-fold in the observed surface area using these two probes. Thus, the maximum possible surface coverage of atrazine assuming fractal geometry is 349 µg/g rather than 582 µg/gsstill greater than S°, but only by a factor of 2. The maximum coverage of 1,3-DCB assuming fractal geometry is 326 µg/gsabout 11 times less than S°. This is consistent with internal sites as proposed in the dual-mode model. Since the paper in question (1) appeared, we have published additional data in support of internal sites (3). By analysis of CO2 adsorption isotherms at 273 K, SOM has been shown to have a large internal nanoporosity (apertures
