Correspondence Comment on “Adsorption of CO2 and N2 on Soil Organic Matter: Nature of Porosity, Surface Area, and Diffusion Mechanism” SIR: In a recent paper of De Jonge and MittelmeijerHazeleger (1), the structure of soil organic matter (SOM) was studied using CO2 gas adsorption at 273 K. The authors concluded that the surface area of SOM (94-174 m2 g-1) is mainly (>95%) formed by micropores with maximum restrictions of approximately 0.5 nm and that the microporous structure is not strongly affected by hydration of the samples. Both statements are important with respect to sorption and diffusion of organic contaminants in soils and sediments, in particular to the ongoing debate on the mechanism of the sorption process, that is, absorption vs adsorption. Our conclusion from these statements would be that large organic molecules have no access to the interior of the SOM. This, however, is in contrast to many observations from sorption investigations. Many correlations between Koc and Kow of solutes have been published (e.g., refs 2-6). They are linear in log Koclog Kow coordinates for a great variety of organic matters in soils and sediments as well as for a wide range of nonpolar organic compounds. A decrease of the slope of such correlations making them curvilinear at high Kow would be expected when the sorbate size approaches the pore dimensions. The correlations involve solutes with very different molecular size, from benzene up to five-ring PAHs, DDT, and hexachlorobiphenyls. These compounds have van der Waals diameters (7), which are by far beyond the pore restrictions proposed in ref 1 for SOM. However, no decline was observed up to the largest sorbate molecules. Therefore, the steric restrictions for sorbates in wet SOM must be assumed to be above 1.0 nm. Similar conclusions can be drawn from recent studies (8-10), wherein sorption parameters are determined for large sorbate molecules. Some researchers describe steric effects for the sorption of organic compounds on humic organic matter (11-14) that occur regardless of the physical state of the sorbents, particulate or dissolved. The latter can hardly develop a fixed pore structure. It is worth noting that steric effects were observed only for very large sorbates such as five-ring PAHs. Compounds up to four-ring PAHs and PCBs (15), whose van der Waals diameters (7) are clearly above the micropore dimensions proposed in ref 1, are not affected. Schulten et al. presented computerized models of humic acid molecules on the basis of comprehensive knowledge from many analytical disciplines (16-18). They proposed voids of various dimensions in the polymer network. These voids, however, have sufficient volume to trap bulky biomolecules such as carbohydrates, lipids, proteinaceous materials, etc. The estimated void dimensions are in the scale of 1.15 × 0.80 nm. According to recent ideas from literature (cf. refs 1923), SOM is best considered to be a mixture of polymers,
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which are partly in a condensed or glassy state and partly in the rubbery state. The sorption process may range from simple phase partitioning to purely surface adsorption phenomena. Although steric restrictions are evident in the condensed SOM phase, sorption sites are accessible for large sorbates such as phenanthrene (23) and PCBs up to tetrachlorobiphenyl (24). Brusseau et al. (25, 26) give exerimental evidence that intraorganic matter diffusion should not be interpreted in terms of fixed-pore models: instead, the polymer should be considered as a flexible, dynamic ‘mesh’, a term utilized in polymer science. For a given polymer and solvent, the diffusion coefficient decreases (exponentially) with increasing size of the diffusant when this is similar to the size of the mesh, whereas in a fixed-pore system oversized molecules are completely excluded. Obviously, the image of a flexible polymer matrix meets the experimental results. At the end of their paper, De Jonge and MittelmeijerHazeleger (1) themselves expressed some doubts on the interpretation of CO2-derived surface areas and pore dimensions, but did not take the above arguments into consideration. Most of the sorption studies on humic organic matters were conducted with aqueous solutions or suspensions. This is because the hydrated sorbent state is important for various natural conditions. Surface measurements, however, inevitably refer to the dry sorbent. Differences between the two states can possibly explain some of the discrepancies discussed above. Chiou et al. (27, 28) measured sorption capacities of dry SOM by loading with organic compounds from the gas phase. They determined a maximum volume fraction of small nonpolar compounds sorbed in a peat organic matter of 0.052 ( 0.002 (28). This is close to the micropore volumes of 0.043-0.063 mL/mL estimated by De Jonge and Mittelmeijer-Hazeleger from CO2 adsorption isotherms. The results can be understood assuming that nonpolar compounds such as hexane, benzene, etc. are taken up by filling the free pore volume of the dry humic polymer. In contrast to the above, polar compounds, from ethers up to the most polar molecule H2O, enter into stronger interactions with the polymer network. This causes swelling and significant alterations of the polymer properties. The high uptakes of polar compounds (e.g., φ ) 0.33 for water and φ ) 0.51 for methanol (28)) are better visualized by a partition model than an adsorption model. It is likely that hydrated SOM has a much more flexible network. Any definite pore structure, which may have developed in the dry state, merges. The humic polymer can be considered more or less a continuum. This now applies even for nonpolar sorbates. Therefore, sorption of organic compounds from water is frequently properly described as a partition process with less steric restrictions than would follow from the statements of De Jonge and Mittelmeijer-Hazeleger. The following observations from our laboratory, described only briefly here (29), illustrate the decisive role of water in the diffusion kinetics: After loading an air-dried
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1996 American Chemical Society
humic acid (6 wt % water) with toluene and stripping the losely bound fraction with a flow of air, the residual sorbates (about 1000 ppm) desorbed very slowly with a half-life of about 1000 h. The desorption rate could not be increased by vacuum (
