Environ. Sci. Technol. IQQP, 26, 402-404
Microorganisms; American Society for Microbiology: Washington, DC, 1991; p p 89-114. Yaks, M. V.; Yaks, S. R. CRC Crit. Rev. Environ. Control 1988, 17, 307-344. Rajagopalan, R.; Tien, C. J. Am. Inst. Chem. Eng. 1976, 22, 523-533. Yao, K. M.; Habibian, M. T.; O’Melia, C . R. Environ. Sci. Technol. 1971, 11, 1105-1112. Reynolds, M. D. Masters Dissertation, Massachusetts Institute of Technology, 1985. Harvey, R. W.; George, L. H.; Smith, R. L.; LeBlanc, D. R. Environ. Sci. Technol. 1989, 23, 51-56.
Ronald W. Harvey* US. Geological Survey Water Resources Division Boulder, Colorado 80303
adsorbent Whittlesey Black Fen Whittlesey Black Fen/ Hz0* Ashurst Garden Ashurst Field Boston silt Wyoming bentonite
BET surface area, m’/g EG surface OC, % N2 EDB HzO area, m2/g 16.43 0.29
12.7 17.5 126.0 71.0 50.5 80.0
99.0 71.0
4.55 2.41 2.66 ndb
6.3 4.6 29.6 1.9 3.3 18.9 28.6 23.2 46.6 65.0 61.0 382.0
25.8 25.8 46.0 372.0
aAdapted from Call (4).bNone detected.
Stephen Garabedian U.S. Geological Survey Water Resources Division Marlborough, Massachusetts 01752
Comment on “The Surface Area of Soil Organic Matter” SIR: In a recent article Chiou et al. (1) investigated the surface area of soil organic matter (SOM) and its relationship to the sorption of nonionic organic compounds. The surface areas of two high organic carbon content soils and oven-dried soil humic acid, determined by the Brunauer-Emmett-Teller (BET) method using N2 as the adsorbate, were found to be less than 1m2/g (I). In contrast, Bower and Gschwend (2) reported that the surface area of SOM ranged from 558 to 803 m2/g, based on the retention of ethylene glycol (EG). This discrepancy is troublesome to environmental scientists because it brings into question many paradigms regarding the role of SOM in aqueous- and vapor-phase sorption processes and cation-exchange reactions. The purpose of this comment is to further explore, from a mechanistic perspective, differences between the surface area of SOM determined by EG retention and the BET method. Although significant advances have been made in the characterization of soil materials, specific surface area remains an operational concept which is dependent upon the experimental method employed. Surface area measurements are a function of both sample pretreatments, such as drying, and the properties of the molecule utilized as a surface probe (3). Chiou et al. ( I ) provided an extensive critique of the EG method, but failed to mention that the apparent surface area of SOM calculated by Bower and Gschwend (2) was actually based on the decrease in surface atea and organic carbon content of soil treated with hydrogen peroxide. Prior to the Hz02 treatment, the organic carbon content and surface area of the four soils studied by Bower and Gschwend (2) ranged from 0.24 to 3.0170, and 56 to 246 m2/g, respectively. Thus, questions arise regarding the effect of H202treatments on the composition of SOM as well as the assumption of equivalent EG molecular coverage on the surface of SOM and montmorillonite. The same procedure, applied to Whittlesey Black Fen soil treated with H202(4), yields an SOM surface area of 92 m2/g. While the treatment of soils with H202caused a reduction in EG retention, the N,/BET surface area of Whittlesey Black 402
Table I. Comparison of the Surface Area of Soils and Clay Minerals Determined by the BET and EG Retention Methods”
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Fen soil increased from 12.7 to 71.0 m2/g following the Hz02treatment. This phenomenon has also been observed for Webster soil (4.1% OC), the Nz/BET surface area of which increased from 2.6 to 33.0 m2/g after a similar HzOz treatment (5). These data suggest that mineral surfaces have a greater capacity than SOM to adsorb nonpolar N2 molecules. Therefore, the direct comparison of SOM surface areas determined for high organic carbon soils using the Nz/BET method to those derived from soils with relatively low organic carbon content by the H202/EG method may not be valid. The absence of EG surface area measurements of Houghton muck, Florida peat, and Sanhedron soil humic acid precludes a rigorous interpretation of the data presented by Chiou et al. (1). In order to account for the differences in the Nz/BET and H20,/EG surface areas of SOM, Chiou et al. (I) proposed a partitioning model, hereafter referred to as the polymer-phase model. In this model, nonpolar Nz molecules only interact with the external surface of the polymer phase and thus provides a “rigorous”measurement of SOM surface area. In contrast, polar molecules such as EG partition into the polymer phase, yielding an ill-defined “apparent surface area”. In addition, Chiou et al. (I) attributed the sorption of nonpolar organic vapors by anhydrous SOM to partitioning. Although the latter hypothesis is quite plausible under hydrated conditions, there is a considerable body of evidence which indicates that nonpolar organic vapors adsorb onto the surface of anhydrous SOM in much the same manner as N2 molecules. For example, the BET surface area of oven-dried soils and clay minerals determined from the adsorption of nonpolar organic vapors, such as p-xylene, toluene, and ethylene dibromide (EDB), are almost identical to those based on Nz adsorption isotherms (Tables I and 11). In addition, the surface area estimated by the retention of polar molecules, such as EG or ethylene glycol monoethyl ether (EGME), is similar to those derived from water adsorption isotherms using the BET equation. Jurinak and Volman (7) also reported that the surface area of oven-dried Staten peaty muck (35% SOM), based on EG retention and the adsorption of EDB using the BET equation, was 264.0 and 11.7 m2/g, respectively. Thus, it is not necessarily the size of the adsorbate molecule, but rather the magnitude of adsorbate-adsorbent interactions, which are strongly correlated to adsorbate polarity, that dictates the degree to which an adsorbate explores the internal surface area of a dry adsorbent. It is widely recognized that weakly adsorbed molecules, such as Nz and nonpolar organic vapors, do not penetrate the interlayer surfaces of oven-dried smectite, whereas polar molecules such as EG solvate exchangeable cations associated with internal surfaces, thereby providing a measure of total surface area. To examine this point with
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Table 11. Comparison of the Surface Area Soils and Clay Minerals Determined by the BET and EGME Retention Methods'
adsorbent
oc, 70
NZ
Webster soil Oldsmar soil Bentonite Kaolinite Lula aquifer
3.02 1.09 0.48
4.2 0.2 14.4 13.6 7.7
0.07 0.01
BET surface area, m2/g p-xylene toluene 5
17 10 4
16 9
H2O 69 8 15 10
EGME surface area, m2/g 101.0 3.8 690.0 3'0.9 10.5
'Adapted from Rhue et al. (6).
1.2
1
0 2.5% OC 0 4.0% OC
that complete monolayer coverage may not have been attained prior to the onset of multilayer formation, the existence of a distinct "kneen in the adsorption isotherm is indicative of surface adsorption phenomena. In addition, N2 adsorption on the freeze-dried soil humic acid yielded a surface area of 18 m2/g and a value of C of approximately 30 (1). The adsorption of benzene and toluene vapors on the freeze-dried soil humic acid conformed to type I11 isotherms, which are indicative of weak gas-solid interactions. Type I11 adsorption isotherms have also been obtained for adsorption of hexane vapors on dry polytetrafluoroethylene (PTFE) (10) and for the adsorption of benzene vapors on silica, whose surface was treated with trimethylchlorosilane to replace hydroxyl groups with nonpolar Si(CHJ3 groups (11). For systems exhibiting type I11 behavior, the magnitude of the adsorbateadsorbent interactions is so small that limited adsorption occurs at low relative vapor pressures, but once a molecule is adsorbed on the surface, adsorbate-adsorbent interactions result in surface condensation which increases exponentially at high relative vapor pressures (12). In contrast, Chiou et al. (1,9) contended that the type I11 isotherms obtained for the sorption of nonpolar organic vapors on freeze-dried soil humic acid are the result of solute partitioning, followed by condensation on the external surface of the soil humic acid. On the basis of this assumption, a "limiting sorption capacity" was extrapolated from the linear portion of the type I11 isotherm (9). The discrepancy between the limiting sorption capacity and the monolayer sorption capacity, based on the N,/BET surface area of the adsorbent, was taken as evidence of vapor-phase partitioning into soil humic acid (1,9). However, the assumption that the linear portion of either a type I1 or type I11 adsorption isotherm can be ascribed to partitioning phenomena has no theoretical basis. Thus, the comparison of a sorption capacity term derived from a type I11 isotherm, for which multilayer formation occurs prior to the completion of monolayer coverage, to one calculated on the basis of the assumption of monolayer coverage may not be valid. The model of SOM envisaged by Chiou et al. (I) is controversial and will undoubtedly force environmental scientists to reconsider their own views regarding the surface area of SOM as it pertains to sorption mechanisms. However, the polymer-phase model fails to account for the similar adsorption of N2 and nonpolar organic vapors on anhydrous SOM and does not incorporate surface adsorption phenomena which could occur on external as well as internal surfaces of SOM. In addition, the polymerphase model does not explicitly account for the high cation-exchange capacity of SOM, which presumably could only exist on the external surface of the polymer phase. Therefore, we propose that SOM behaves as a three-dimensional system, in which a multipolymer phase exists in a semistructured framework, similar to that presented by Hayes et al. (13) and Wershaw (14). The polymers, which possess hydrophobic moieties and polar functional
o ' 2
0.0 0.0
0.2
0.6
0.4
0.8
1.o
PIP0 Figure 1. Benzene vapor adsorption on dry TMA-smectRe normalized to the corresponding BET surface area of each TMA-smectite. The organic carbon (OC)content is related to the degree of TMA saturation. Adapted from Lee et al. (8).
regard to SOM, let us consider a smectite exchanged with tetramethylammonium (TMA)ions. Lee et al. (8)reported that the adsorption of benzene vapors on oven-dried TMA-smectite increased with the degree of TMA saturation (25,50, and loo%), and thus organic carbon content (1.7,2.5 and 4.0%). Initially, these data suggest that the increase in benzene adsorption was due to uptake by organic carbon. However, normalization of the benzene adsorption isotherms with respect to the corresponding N,/BET surface area of each TMA-smectite essentially yields a single adsorption isotherm (Figure 1). These data indicate that the observed increase in benzene adsorption was primarily due to expansion of the interlayer space by TMA ions (pillaring), which allowed nonpolar organic vapors to explore the internal surfaces of TMA-smectite. Thus, under anhydrous conditions the adsorption of benzene vapors occurred primarily on exposed mineral surfaces while the hydrophobic moieties of the exchanged TMA ions contributed little, if any, to the adsorption of either N2 or benzene vapors. In fact, the nonlinear relationship between N2/BET surface area and the extent of TMA exchange suggests that at high TMA loadings a portion of the mineral surface was inaccessible to N2 molecules. Unfortunately, Chiou et al. ( I ) did not present relevant sorption data for Houghton muck, Florida peat, and oven-dried soil humic acid. However, in a related paper Chiou et al. (9) reported data for the adsorption of several nonpolar and polar vapors on freeze-dried Sanhedron soil humic acid. The adsorption of l,l,l-trichloroethane and water on the freeze-dried soil humic acid conformed to type I1 adsorption isotherms and yielded BET surface areas of 10 and 298 m2/g, respectively. Although the relatively low values of C obtained for these adsorbates (C = 9) suggests
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Environ. Sci. Technol. 1992, 26, 404-406
groups, are linked together by weak interactions, such as hydrogen bonding, hydrophobic interactions, and T bonding (14). Thus, the multipolymer phase possesses an internal porosity, which may expand and contract depending on solvent properties and can account for the large cation-exchangecapacities characteristic of SOM via polar (ionic) functional groups. When the multipolymer phase is air-dried, polar functional groups orient toward the interior of the structure, while the external surface becomes primarily hydrophobic in nature (13). Thus, under anhydrous conditions nonpolar organic vapors condense on the external surface of SOM, whereas polar molecules explore the internal surfaces and solvate exchangeable cations in a manner analogous to that observed for expandable clay minerals. Under saturated conditions, the multipolymer phase functions as a conventional partition medium for nonionic organic compounds. The interpretation of surface area measurements is a formidable task and should be considered in the context of both the probe molecule, or adsorbate, and the adsorbent. The data presented herein indicate that the adsorption of nonpolar molecules occurs primarily on external surfaces of SOM, whereas polar adsorbates explore both the external and internal surfaces of SOM. On the basis of the N,/BET surface areas measured by Chiou et al. ( I ) , it is reasonable to conclude that the external surface area of SOM is quite small. However, the N,/BET surface area of freeze-dried soil humic acid (18 m2/g) is considerably greater than that of oven-dried soil humic acid (-0.7 m2/g) (1). In general, air-drying results in the collapse and shrinkage of soil humic acids, whereas freeze-dried soil humic acid maintains an intricate structural network (15, 16). Thus, under natural conditions the external surface area of SOM is likely to be greater than 1m2/g. We also contend that (a) SOM does in fact possess an internal surface area, albeit ill-defined, which plays an important role in many soil processes and (b) a simple polymer phase cannot be used to describe vapor-phase sorption on anhydrous SOM. However, surface area measurements of clay minerals, based on the retention of EG or EGME and the adsorption of water, have been shown to be dependent upon the exchangeable cation species (17,18). Given the potential for specific interactions between polar molecules and SOM, it is likely that experimental error will be associated with measurements of the total surface area of SOM. Nevertheless, the retention of polar adsorbates by SOM yields a measure of surface area that may be quite useful to environmental scientists, provided the operational nature of surface area measurements is recognized.
Literature Cited (1) Chiou, C. T.; Lee, J.-F.; Boyd, S. A. Enuiron. Sci. Technol. 1990, 24, 1164. (2) Bower, C. A.; Gschwend, F. B. Soil Sci. SOC.R o c . 1952,16, 342. (3) Sposito, G. The Chemistry of Soils; Oxford University Press: New York, 1984. (4) Call, F. J. Sci. Food Agric. 1957, 8 , 630. (5) Pennell, K. D. Ph.D. Dissertation, University of Florida, 1990. (6) Rhue, R. D.; Rao, P. S. C.; Smith, R. E. Chemosphere 1988, 17, 727. (7) Jurinak, J. J.; Volman, D. H. Soil Sci. 1957, 33, 487. (8) Lee, J.-F.; Mortland, M. M.; Boyd, S. A.; Chiou, C. T. Chem. SOC.,Faraday Trans. 1 1989,85, 2953. (9) Chiou, C. T.; Kile, D. E.; Malcolm, R. L. Environ. Sci. Technol. 1988, 22, 298. (10) Kiselev, A. V. Q. Rev. Chem. SOC.1961, 15, 99. (11) Whalen, J. W. J. Colloid Interface Sci. 1968, 28, 443. 404
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(12) Greg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd Ed.; Academic Press: New York, 1982. (13) Hayes, M. B. H.; MacCarthy, P.; Malcolm, R. L.; Swift, R. S. In Humic Substances II: I n Search of Structure; Hayes, M. B. H., MacCarthy, P., Malcolm, R. L., Swift, R. S., Eds.; John Wiley & Sons: New York, 1989; Chapter 24, pp 689-733. (14) Wershaw, R. L. J. Contam. Hydrol. 1986, 1, 29. (15) Chen, Y.; Schnitzer, M. In Humic Substances II: I n Search of Structure; Hayes, M. B. H., MacCarthy, P., Malcolm, R. L., Swift, R. S., Eds.; John Wiley & Sons: New York, 1989; Chapter 22, pp 622-638. (16) Tan, K. H. Soil Sci. SOC.Am. J. 1985, 49, 1185. (17) McNeal, B. L. Soil Sci. 1964, 97, 96. (18) Quirk, J. P. Soil Sci. 1955, 80, 423.
Kurt D. Pennell,”*+P. Suresh C. Raoz Department of Civil and Environmental Engineering The University of Michigan Ann Arbor, Michigan 48109-2 125 Soil Science Department University of Florida Gainesville, Florida 326 1 1-0 15 1 Corresponding author: Department of Civil and Environmental Engineering, 1351 Bed St., 181 Engineering Building l A , The University of Michigan, Ann Arbor, MI 48109-2125. t University of Florida.
SIR: Pennell and Rao (1)considered the surface area of a bulk solid to be an “operationally defined” quantity. By this assumption, they postulated the existence of a “semistructured multipolymer phase” in soil organic matter (SOM) to account for the large discrepancy of the measured surface areas of SOM by the standard BET/N2 method and by the ethylene glycol (EG) retention method. In their opinion SOM possesses an internal porosity which can either expand or contract depending on the solvent (adsorbate) properties and thus results in different measured surface areas of SOM. To support their contention, Pennell and Rao (I) drew an analogy to the behavior of smectites in uptake of N2 and polar liquids (such as EG and water) and cited results from other sorption studies of organic vapors with soils and organoclays. It is rather unfortunate that Pennell and Rao take a view of surface area that is at odds with accepted terminology in allied fields; this leads to an unnecessarily pessimistic view of the complexity of the sorption of organic compounds by SOM. The argument of Pennell and Rao is essentially a redefinition of the surface area to fit the experimental data produced by different methods. The concept of the “surface areas” for divided solids is quite straightforward. They are the values customarily measured by determining the adsorption of gases or vapors that can reach the solid’s surfaces (i.e., they are not excluded by molecular sieving) but do not penetrate into the bulk solid. These surfaces represent the vacuum-solid interfaces that exist before the measurement is taken and are external to the bulk material. Some porous solids may have “internal surfaces” in the sense that they are internal to the (outer) granule boundary (as in activated carbon) but are nevertheless external to the bulk solid. These “internal surfaces” make sense only if they are accessible to gases of low molecular weights.
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