Environ. Sci. Technol. 1992,26,401-402
Literature Cited Harvey, R. W.; Garabedian, S. P. Environ. Sci. Technol. 1991,25, 178-185.
Rittmann, B. E. Biotechnol. Bioeng. 1982, 24, 501-506. Taylor, S. W.; Jaffe, P. R. Water Resour. Res. 1990, 26, 2181-2194.
Yao, K. M.; Habibian, M. T.; O’Melia, C. R. Environ. Sci. Technol. 1971,5, 1105-1112.
Rajagopalan,R.; Tien, C. J. Am. Inst. Chem. Eng. 1976, 22, 523-533.
Scherer, P. J . Appl. Bacteriol. 1983, 55, 481-486. Kubitschek, H. E.; Baldwin, W. W.; Graetzer, R. J. Bacteriol. 1983, 155, 1027-1032. Kuhn, A. H. U.; Jutta, H.; Kellenberger, E. J. Virol. 1983, 47,540-552.
Kubitschek, H. E.; Baldwin, W. W.; Schroeter, S. J.; Graetzer, R. J. Bacteriol. 1984, 158, 296-299. Baldwin, W. W.; Kubitschek, H. E. J. Bacteriol. 1984,159, 393-394.
Dicker, D. T.;. Higgins, M. L. J . Bacteriol. 1987, 169, -1200-1204.
O’Melia, C. R. Colloids Surf. 1989, 39,255-271. Edward J. Bouwer” Department of Geography and Environmental Engineering The Johns Hopkins University Baltimore, Maryland 2 12 18 Bruce E. Rittmann Department of Civil Engineering University of Illinois at Urbana-Champaign Urbana, Illinois 6 1801
SIR: The letter from Bouwer and Rittmann focuses on three important topics regarding the modeling of bacterial transport through sandy aquifers. We agree that the mathematical descriptions concerning bacterial sorption/detachment, the calculation of the so-called “collector efficiency”,and the specific gravity of indigenous bacteria used in our recent article ( I ) are topics worthy of further discussion. As noted earlier ( I ) , all three topics are subject to various degrees of uncertainty. A more detailed discussion of each is given below. Regarding the mathematical descriptions of bacterial removal and detachment in our first equation ( I ) , we do not agree that the bacteria removed from solution are being double counted. Our approach assumes that removal of bacteria traveling through sandy aquifer sediments occurs at two types of sites, i.e., one type of site at which bacteria permanently adhere (irreversible adsorption) and another type of site at which bacteria are more weakly held (reversibly sorbed). Hence, it is assumed that there is permanent removal of bacteria at some sites and a rapid exchange at others. It should be noted that the irreversible sorption process in our model involves a first-order kinetic term, identical to the Rd term proposed by Bouwer and Rittmann. The difference between our model and that proposed by Bouwer and Rittmann involves the desorption process. It should also be pointed out that the second modeling approach applied by Harvey and Garabedian ( I ) employs kinetic terms to model the sorption process. We agree with Bouwer and Rittman that the kinetic approach may be more conceptually consistent. However, the two-site model is not inconsistent with existing theory. Both reversible and irreversible sorption of bacteria are known to occur in the natural environment, although much is unknown about the processes controlling bacterial removal in sandy aquifers (2). Examples of other two-site models involving porous media appear in the virus transport literature (3).
The model proposed by Harvey and Garabedian was offered as an example of how filtration theory may be incorporated into a mathematical description of subsurface bacterial transport in a field experiment. However, as noted earlier ( I ) ,these simple models may not be adequate to describe transport on a larger scale. More accurate data-based models will undoubtedly be developed as more information about the controlling processes of bacterial migration in aquifers becomes available and the assumptions behind the various models are more adequately tested. Bouwer and Rittmann are correct in pointing out that a better way to calculate collector efficiency (7) involves use of the equation proposed by Rajagopalan and Tien (4), which accounts for close-approach effects. They are also correct in stating that the manner in which r] is determined affects the calculated value of the collision efficiency factor (a).We chose to use the older equation published by Yao et al. (5), so that we could more directly compare the a values we determined for bacterial transport at our field site with that calculated earlier by another investigator (6) (refer to Table 11, Harvey and Garabedian ( I ) ) . The specific gravity (buoyant density) for the indigenous population of bacteria used in our field experiment is uncertain ( I ) . Undoubtedly, our morphologically diverse population, which we obtained directly from the aquifer WITHOUT culturing, represents a range of buoyant densities that would be difficult to determine experimentally to a very high degree of accuracy. However, it is evident that the mean effective specific gravity for our indigenous population is lower than the published values for the buoyant densities of laboratory-grownbacteria cited by Bouwer and Rittmann. Unlike bacteria-sized microspheres (sp gr 1.05) used in an earlier experiment (7) and bacteria cultured successively in nutrient broth, the uncultured indigenous bacteria from the aquifer at our field site form very stable suspensions. Settling during months of storage in the laboratory and during the month-long field transport experiment was insignificant. Our choice of 1.002 was somewhat arbitrary, but correctly suggests that our indigenous bacteria behave as though they are almost neutrally buoyant. Although a specific bacterial specific gravity of 1.10 (suggested by Bouwer and Rittmann) may not have been appropriate in our transport experiments with indigenous groundwater bacteria, high buoyant densities may have important ramifications for proposed field experiments involving microorganisms cultured in the laboratory (e.g., genetically engineered bacteria) for aquifer biorestoration. Clearly, the role of settling upon the transport behavior of larger, less neutrally buoyant bacteria would have a much more significant effect upon subsurface mobility. We thank Drs. Bouwer and Rittmann for their interest in our article and the useful information provided in their correspondence. It is clear that more research is needed in the many areas of uncertainty involving microbial transport in contaminated aquifers. Studies involving reversibility of bacterial and viral sorption to subsurface material are presently being conducted in several laboratories and more field experiments are being carried out to evaluate a t least some of the controls of subsurface microbial mobility. Progress in modeling subsurface transport of microorganisms will undoubtedly result from new information provided by this research.
Literature Cited (1) Harvey, R. W.; Garabedian, S. P. Enuiron. Sci. Technol. 1991,25, 178-185. (2)
Harvey, R. W. In Modeling the Environmental Fate
Not subject to U.S.Copyright. Published 1992 by the American Chemical Society
of
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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
OC, %
BET surface area, m’/g EG surface N2 EDB HzO area, m2/g
Whittlesey Black Fen Whittlesey Black Fen/ Hz0* Ashurst Garden Ashurst Field Boston silt Wyoming bentonite
16.43 0.29
12.7 71.0
4.55 2.41 2.66 ndb
6.3 4.6 1.9 3.3 28.6 23.2 65.0 61.0
17.5 126.0 50.5 80.0 29.6 18.9 46.6 382.0
99.0 71.0 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”
Environ. Sci. Technol., Vol. 26, No. 2, 1992
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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