Environ. Sci. Technol. 2000, 34, 2779-2783
Investigating the Effect of Carbon Shape on Virus Adsorption T R A C I P O W E L L , † G A I L M . B R I O N , * ,† MARIT JAGTOYEN,‡ AND FRANK DERBYSHIRE‡ Department of Civil Engineering and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506
Batch adsorption isotherm and column breakthrough studies were conducted to compare two types of activated carbon with very different structural characteristics; granular Calgon F-400 and an activated carbon fiber composite (ACFC). Carbons were evaluated for virus adsorption capacity using a bacteriophage, MS2. Two mesh fractions of each carbon type were used in batch adsorption studies to determine empirical isotherm coefficients from linear regression analysis. Freundlich isotherm models adequately described MS2 bacteriophage adsorption to both carbon types. Adsorption capacity was found to vary with carbon size for both types of carbon but for different reasons. Although adsorption isotherm capacities would have predicted the GAC carbon to provide better removals than the ACFC, carbon breakthrough column performance for equal weights of material showed virus removal to be markedly more efficient on a gram-to-gram basis for ACFC than for GAC.
Introduction Viruses display a wide range of adsorption behavior to surfaces. Although activated carbon is not thought to be an effective process for virus removal, if the process of virus adsorption could be more fully understood, then effective water treatment processes could be designed. This research investigates the ability of a well-known bacteriophage, MS2, to sorb to very differently structured activated carbons under batch and column methodologies. There were two goals to this study: first, to investigate the applicability of comparing two very differently structured carbons upon their capacity as determined by classic batch isotherm methodology utilizing powdered materials; Second, to hypothesize that thin carbon fibers, with entirely accessible activated surfaces, would provide longer term MS2 adsorption than an equal weight of granular carbon of similar capacity in a singlepass, upflow column. Carbon adsorption of microbes could prove invaluable for the removal of disinfectant-resistant protozoa and viruses that can slip through standard treatment regimes, especially when used in point-of-use treatment devices. Expected removal efficiencies for viruses and protozoa from publicly owned treatment works is 99.99% and 99.9% removal for viruses and encysted protozoa, respectively (1), but activated carbon is not given credit for removal. Instead, disinfection is relied upon. However, viruses have been found in fully * Corresponding author phone: (606)257-4467; fax: (606)257-4404; e-mail:
[email protected]. † Department of Civil Engineering. ‡ Center for Applied Energy Research. 10.1021/es991097w CCC: $19.00 Published on Web 05/31/2000
2000 American Chemical Society
treated drinking water (2), suggesting that for sensitive members of the population more barriers are needed. It is suggested that effective application of activated carbon at the plant and at the tap could help facilities meet these goals and prevent future outbreaks of waterborne disease. New types of activated carbon have been developed whose form enhances their adsorption kinetics and overall performance while preventing the release of fines and keeping pressure drops low (3). These new carbon forms may allow for different uses of activated carbon. One of their unique characteristics is that the activated sorption surface is more accessible to larger molecules than conventional granular materials. In this work, an activated carbon fiber composite (ACFC) was compared to a conventional granular activated carbon (GAC) for the adsorption of viruses from water. The ACFCs are monolithic with an open internal structure of carbon fibers, which gives very high rates of adsorption. No abrasion can occur in the composites, and the amount of carbon fines released into the treated water is expected to be very low (4). A proprietary process makes ACFCs from carbon fibers that have been joined into a composite and activated to introduce porosity (3, 5). The composites have relatively uniform pore distributions and surface areas comparable to commercial GAC (3). The main difference in the adsorptive properties of an activated carbon fiber composite and a granular carbon lies in the rates of adsorption that can be achieved. The rate of adsorption for ACFC columns has been seen to be faster for removing pesticide than that for GAC columns (3). There is significant diffusion resistance for large molecules adsorbing on GAC because the adsorbate has to diffuse to the interior of the particle before being adsorbed in micropores. Others have found virus adsorption to activated carbon to be diffusion limited (6). It was hypothesized that by using a custom developed ACFC in columns where contact time is limited that greater virus removals could be achieved relative to conventional GAC. Although virus adsorption is quite individual, surrogate bacteriophage are often used to predict and model enteric virus behavior since they are easier and cheaper to grow and assay (7), and they more adequately reflect virus behavior than indicator bacteria (8, 9). This research used MS2; a singlestranded, RNA-containing, F-specific coliphage belonging to the family Leviviridae. These viruses infect and replicate in Escherichia coli bacterial strains with sex pili and are detected by plaque formation in bacterial-laden semisolid agar media. (10). The MS2 viruses are relatively small (250 Å, 3.6 × 106 Da), unenveloped, and icosahedral shaped making them similar in overt physical characteristics to pathogenic enteroviruses such as poliovirus. However, virus adsorptive behavior is type and strain dependent. While MS2 is often used as a model virus in water treatment and disinfection studies (7, 9), others have shown that MS2 phage generally exhibits poorer adsorptive behavior to soils than Poliovirus, Echovirus group 7, Coxsackievirus B3, and tailed bacteriophage T2 and T4 (11). Consequently, using MS2 as a surrogate in this study should provide a conservative estimate of adsorptive behavior for other enteric viruses and bacteriophage to activated carbon.
Methods and Materials Activated Carbons. Two types of carbon were compared, granular Calgon F-400 and an ACFC. The ACFC was developed at the University of Kentucky Center for Applied Energy Research and manufactured by published methods (3, 5). In its raw form, the fiber composite is composed of a rigid mass VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of interlocked fibers of an average length of 0.1-0.4 mm and an average width of between 5 and 100 µm. For use in batch adsorption experiments whole, raw carbons were broken into smaller pieces, ground, washed, and sieved into mesh size fractions of 10 × 40 mesh and passing (-) 100 mesh before overnight drying at 105 °C. Carbons were cooled and stored with desiccant until use. Liquid Matrix. Phosphate-buffered saline (PBS) solution was used as the suspending liquid matrix (pH 7.4) for all experiments. The PBS solution contained 8.0 g of sodium chloride (NaCl), 0.2 g of potassium chloride (KCl), 1.44 g of sodium hydrogen phosphate (Na2HPO4), and 0.24 g of potassium dihydrogen phosphate (KH2PO4) per liter. Ionic strength ) 0.1 M. Virus and Host. Stocks of MS2 bacteriophage were prepared as described elsewhere (7). E. coli strain C-3000 was used as the host for bacteriophage MS2 with standard, double-layer agar assay technique (10). Nutrient broth and agars (Difco) were used for log-phase propagation of the host and plaque assay. Clearly defined plaques that formed in the bacterial lawn of the top layer of 0.7% agar resting on a support layer of 1.5% agar were counted and recorded as plaque forming units (PFU) after 18-24 h with the aid of a magnified backlit counter. All samples were assayed in triplicate for each dilution examined with the average number of plaques present in the most countable dilution used to calculate PFU per milliliter in the suspending matrix. Batch Adsorption Studies. MS2 virus stock was added to PBS to obtain initial concentrations ranging from 2.0 × 105 to 2.0 × 107 PFU/mL immediately before experiments. Adsorption reactors were sterilized, and polypropylene containers were filled with 8 mL of virus-spiked PBS into which known weights of clean, sized carbons were added followed by mixing on a test tube rotator at low setting. After 3 h of adsorption time, aliquots were withdrawn from the reactors, and carbon fines were removed by filtering through 6% beef extract-soaked 13 mm × 0.8 µm cellulose acetate membranes held in Swinnex polypropylene holders (Millipore). The aliquots were then serially diluted and assayed. Control reactors that measured the initial amount and the final amount of virus in the PBS fluid matrix were set up and processed identically to the adsorption reactors. Column Studies. Comparative studies were performed with two continuous, up-flow, column systems containing equal weights (12.8 g) of either F-400 or ACFC to assess how each carbon form would perform as a single-pass, pointof-use device. A flow rate of 4.33 mL/min of virus-spiked PBS was established with a peristaltic pump and passed through columns of 1.9 cm diameter and length-to-width ratios of greater than 3:1. The F-400 column was constructed of 1.9 cm diameter PVC pipe with glass microfiber filters at both ends to hold 26.7 cm3 of granular 30 × 40 mesh F-400 carbon in place. The ACFC column was constructed of five, preformed, molded disks of 1.9 cm diameter stacked on top of each other, held together by heat shrinkable PVC wrap for a final length of 3.5 in. and volume of 26.7 cm3. Similarly, glass fiber filters were used on each end. Sterile tubing was used to connect the columns, pump, and fluid reservoir together. Samples were taken at the outlet of each column over a time period of 20-36 h. Statistical Analysis. To closely scrutinize the underlying model, outliers were evaluated and eliminated by standard techniques where possible from the regression data sets. Regression diagnostics to flag outliers were performed by studentized residuals analysis (95% CI, R ) 0.05). Points that were flagged as outliers were checked for leverage and Cook’s distance. All points with calculated Cook’s distance g 1 and/ or calculated leverage values g 2 times the expected (1/n) were retained in the datasets for regression. Once outliers were eliminated, regression analysis was redone on the culled 2780
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FIGURE 1. Sorption kinetics for MS2 and powdered activated carbon. data sets. Comparisons of the slopes and intercepts between linear regression fits to the data for the different carbons and size fractions were accomplished on a pairwise basis by hypothesis testing using the Student t distribution (null hypothesis of equal slope, or equal intercept, two-sided, at R ) 0.05).
Results and Discussion Batch Studies. Establishing Equilibrium. The fluid matrix selected for suspending virus in was PBS (ionic strength ) 0.1 M). This medium was selected to enhance virus survival and stability while maximizing adsorption by reducing the thickness of electric double layers (12). In earlier studies of MS2 with coal, adsorption was seen to increase linearly with ionic strength (13). By using a relatively high ionic strength solution, adsorption kinetics of MS2 to activated carbon should have been maximized. All batch studies were allowed 3 h of contact time between virus and carbon materials in PBS so that equilibrium conditions were established. The 3-h time was determined from an evaluation of the literature and initial kinetic studies that showed sorption to coal be essentially complete within the first 2 h of contact (13, 14). Figure 1 shows that at 150 min 90% of the available 3.5 × 104 virus in solution had been adsorbed to powdered F-400 carbon ((-)100 mesh) with 99% adsorption by 180 min. This was in agreement with the findings of others that found virus adsorption to coal or cell monolayers essentially complete within 2 h (13-15). Cookson (16) found that only 0.37% additional virus adsorbed to activated carbon at 16 h after the initial contact time of 12 h. For our experiments, >90% adsorption of available virus was obtained for even the smallest weight of carbon used in the 3-h contact time. So the results presented in this paper are classified as approaching equilibrium. Adsorption Isotherms. Although the carbons used in this study have comparable surface areas per unit mass (1050 m2/g for granular F-400 and 840 m2/g for the ACFC), it was unclear what the relative proportions of those surface areas are actually available for virus adsorption. The large size of MS2 (250 Å) relative to the pore sizes of ACFC (20 Å) used for this research limited virus adsorption to the surface of the fiber. While the GAC F-400 has some mesoporosity (pores of diameter of 20-500 Å), adsorption was presumed to be limited to the granules’ surface as other researchers have found (13, 16). Cookson (17) determined that the maximum surface area of activated carbon used for adsorption of T4 phage was 18%, while Oza and Chadhuri (13) found only 0.1% of coal surface area used for MS2 adsorption. The difference in shape between the two carbon types chosen for study, cylindrical versus spherical, was expected to have an impact upon empirical parameters derived from linearlized isotherms as the surface-to-volume ratio changed.
TABLE 1. Experimental Ranges of Equilibrium Concentrations and Carbon Weights carbon type
mesh size
n
limits
carbon (g)
log Ceq (PFU/mL)
F-400
(-) 100
24
F-400
30 × 40
10
ACFC
(-) 100
12
ACFC
30 × 40
12
max min max min max min max min
0.0032 0.0300 0.0111 0.1153 0.0031 0.0304 0.0032 0.0656
4.09 2.44 5.35 2.95 5.12 3.04 6.01 4.38
The maximum and minimum equilibrium concentrations of virus and carbon weights used for all batch adsorption experiments are shown in Table 1. The log-transformed equilibrium solution concentrations of the batch experiments for various mesh fractions spanned approximately 1.252.00 log units, providing sufficient data separation for analysis. This range of equilibrium virus concentrations agree with that utilized by other researchers (18). Although there are several adsorption isotherm models, the two most commonly used are the Langmuir and the Freundlich models. Both have been used to describe virus adsorption to solid surfaces, but the Freundlich equation allows for heterogeneity in adsorption sites. (12, 19). The data obtained from these batch adsorption studies on both carbon types and for all size fractions conformed to the Langmuir model, but the Freundlich isotherm described the MS2 phage adsorption better as determined by comparison of both linearlized fits to raw, unculled data (data not shown). The Freundlich equation can be expressed in the following linearlized form:
FIGURE 2. Freundlich isotherms for MS2 adsorption to Calgon F-400.
FIGURE 3. Freundlich isotherms for MS2 adsorption to ACFC.
log Qeq ) log K + (1/n) log Ceq
(1)
where Qeq is the amount of adsorbate per unit of adsorbent at equilibrium and Ceq is the solute solution concentration at equilibrium. K and 1/n are experimental coefficients that have been determined by a linearlized least-squares method. The magnitude of the slope (1/n) is related to the strength of the adsorption forces between the adsorbate and the adsorbent. The y-intercept yields the log of the empirical constant, K, describing carbon adsorptive capacity (20). The difference in the model fits may be related to their respective mathematical forms. The Freundlich isotherm is an analytical relationship for a general parabola, while the Langmuir isotherm is a rectangular hyperbola. Consequently, the Langmuir isotherm will reach a limit where increasing equilibrium solution concentration does not correspond to increasing surface concentration, whereas the Freundlich isotherm theoretically increases without limit as concentration increases (21). The Langmuir maximum surface monolayer coverage, and adsorption rate constants may not remain constant over a broad concentration ranges due to potential interactions between adsorbates at adjacent adsorption sites or possible heterogeneity in adsorption sites (20, 21). Additionally, the assumption of monolayer coverage may not be valid, as viruses are known to clump to each other spontaneously if concentrations become great enough. Figures 2-4 show the data and linear fits for both activated carbon types and size fractions graphed according to the Freundlich equation. The Freundlich adsorption coefficients extracted from each linear fit and the overall goodness of fit are summarized in Table 2. Statistical testing of observed differences in the adsorption coefficients will be discussed in detail later, but by visual examination of the isotherms presented in Figures 2 and 4 and comparison of K values in
FIGURE 4. Freundlich isotherms for MS2 adsorption to (-) 100 mesh Calgon F-400 and ACFC. Table 2, it is clear that the powdered (-) 100 mesh F-400 carbon adsorbed viruses to a greater extent than all other fractions tested. The capacity (K) predicted from extrapolation to the y axis was the highest (1.45 × 107 PFU/g) for (-) 100 mesh F-400. The predicted capacity of the (-) 100 mesh ACFC (8.91 × 106 PFU/g) was between that of the (-) 100 and the 30 × 40 mesh F-400. However, the capacity of granular 30 × 40 mesh ACFC was least of all. The decrease in capacity for the F-400 as the particle size increases was thought to be due to decreasing area per unit weight of material. Oza and Chaudhuri (13) reported a linear relationship between particle size and adsorption capacity, suggesting that MS2 adsorption was restricted to exterior VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Summary of Freundlich Adsorption Coefficients carbon type
mesh size
log Ka
K
1/na
R2
F-400 F-400 ACFC ACFC
(-) 100 30 × 40 (-) 100 30 × 40
7.2 6.4 7.0 4.7
1.45 × 107 2.55 × 106 8.91 × 106 5.47 × 104
0.4 0.4 0.3 0.7
0.897 0.995 0.948 0.959
a
Values have been rounded.
adsorption sites of coal, and the results of our study indicate this may be true for activated carbon as well. Surface-tovolume ratio varies inversely with the particle radius for granules. MS2 virus is a small, icosahedral virus (250 Å) and would be excluded from reaching the majority of adsorption sites inside the F-400 granules where about 55% of the pore volumes are under 250 Å. Therefore, virus adsorption to carbon granules can be modeled as surface restricted, and the increase in capacity must be correlated with the increase in surface area per gram of carbon as the particle diameter shrinks. Both F-400 size fractions have essentially the same density of 0.48 g/cm3. Assuming spherical particles, a rough calculation using mean particle diameters of 0.09 and 0.51 mm for the (-) 100 mesh and 30 × 40 mesh F-400 carbons would result in surface areas of 0.067 and 0.012 g/cm3, respectively. This difference in surface areas closely matches the 5.7 times increase in capacity indicated by the K values. The slopes for both F-400 carbon isotherms are identical, at 0.4 regardless of the size of the particle. This indicated that the magnitude of attractive forces did not change appreciably when the carbon was ground finer, supporting the assumption that this material was homogeneous throughout the granule. Hypothesis testing was done to evaluate if either slopes or intercepts were statistically different. Calculated t values against values from pairwise comparisons of regression coefficients were compared to values from the Student t distribution at R ) 0.05 for two-sided testing with (n1 + n2 - 4) degrees of freedom. The results support the observations that the F-400 carbon attractive forces do not change with size while the capacity does (Table 3). However, the comparisons for the ground ACFC material do not show the same trends. The ACFC material changes in both capacity and attraction as the size fraction changes. Inspection of Figure 3 and Tables 2 and 3 shows that the ACFC composite fractions have different slopes and capacities for the mesh fractions tested, but this was not thought to be due to increased activated carbon surface area as it was for the F-400 carbon. This was most probably due to nonhomogeneity of the material and the difference in grinding effects. The ACFC composite is a mesh of fibers of 0.010-0.040 mm in width of varying lengths. Microscopic examination of the different size fractions showed that crushing the composite material shortened the fibers but did not change the diameter of the individual fibers. This kept the activated carbon surface to total carbon fiber volume ratio constant. However, unlike the F-400 carbon, which was a fairly homogeneous mix of activated material throughout the granule, the composite fibers are not of homogeneous
composition. Fibers are shafts of pure carbon that have been activated only on their surfaces not to the interior core. Therefore, when they are broken, the ends of the fiber are not the same material as the outside of the shaft. So as the fiber gets shorter, the relative amounts of activated surface to nonactivated carbon area changes, with the nonactivated areas becoming more predominant as the fiber was shortened. In essence, the process of grinding creates a different type of adsorbent material. This causes the material to absorb viruses differently for different size fractions. Judging from the increase in capacity as the relative amount of virgin, unactivated carbon surface increases, it would seem that the virgin carbon material was quite attractive to MS2. This change can been seen by inspection of the slope impact and capacity coefficients calculated for 30 × 40 mesh ACFC as compared to (-) mesh ACFC. Hypothesis testing supports the observed differences between the slopes and the capacities for the two size fractions of the ACFC composite. The null hypothesis of equality cannot be accepted with calculated t values greater than 2.2. When comparing the (-) 100 mesh fractions of F-400 and ACFC, the slopes and capacities are not statistically different from each other, yet at the 30 × 40 mesh size they are different with the F-400 carbon predicted to have more capacity than the ACFC. Although this finding would lead one to think that the F-400 carbon would be a superior material for virus removal under column studies, our results do not show this to be true. Column Studies. Column breakthrough studies were conducted to compare the two types of activated carbon, GAC (F-400, 30 × 40 mesh) and an ACFC for continuous MS2 removal under a single pass through scenario, upflow configuration similar to that of a point-of-use water treatment device. This part of the study was designed to evaluate the effect of replacing GAC with monolithic material of similar density and pressure drop in commercial applications. The comparison was on a per weight basis at a very slow flow rate of 4.33 mL/min. The GAC column was packed with 30 × 40 mesh of the F-400 carbon, while the ACFC column was composed of molded disks of ACFC material stacked upon each other. The initial concentrations in the virus reservoir were somewhat variable over the entire time of the column runs; therefore, they were tabulated as a moving average of control samples. The inlet concentration of virus to the F-400 column was 2.8 × 106 PFU/mL suspended in PBS. It was anticipated that the ACFC column might take longer to reach breakthrough from preliminary research on other organic contaminants (3). Consequently, the inlet concentration to the ACFC column was increased by a factor of 10-2.7 × 107 PFU/mL. Column breakthrough studies showed that the ACFC had superior adsorption capacity relative to the F-400 on a oncepass-through basis. The breakthrough time for ACFC was 2.5 times longer than that for the equivalent bed of F-400. The ACFC column clearly outperformed the F-400 column despite the 10 times greater inlet concentration for the ACFC column (Figure 5). Using a conservative value of 10% of the average initial concentration, the F-400 column reached breakthrough after about 12 h, while the ACFC reached breakthrough after about 26.5 h. The amount of virus
TABLE 3. Hypothesis Testing on Freundlich Regression Coefficients carbon isotherm F-400 (-) 100 ACFC (-) 100 F-400 (-) 100 F-400 30 × 40 2782
9
vs vs vs vs
carbon isotherm
tcalc slope
tcalc intercept
slope equal?
capacity equal?
F-400 30 × 40 ACFC 30 × 40 ACFC (-) 100 ACFC 30 × 40
0.24 5.40 1.80 5.51
4.79 6.36 0.95 6.38
Y N Y N
N N Y N
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materials need to be evaluated by multiple methods so that all factors that play into adsorption can be evaluated with respects to eventual application and process design. Although granular activated carbon has not been an economic or recognized method to remove waterborne viruses, as new materials with the ability to overcome diffusion limitations become available their applicability should be rethought. ACFC materials, with the majority of surface area easily available for adsorption, may well be at the heart of pointof-use devices for individual water treatment within a few years.
Literature Cited
FIGURE 5. Comparative column studies for MS-2 adsorption to F-400 and ACFC. adsorbed per gram of carbon was 1.4 × 108 and 3.4 × 107 PFU for the ACFC and F-400, respectively. One of the reason virus removal was markedly more efficient on a weight basis for ACFC than for the F-400 was simply due to the amount of available surface area available for contact with the virus. Although gas testing shows each carbon to have similar adsorptive surface areas, 1050 and 840 m2/g for F400 and ACFC, respectively, more of this total area was available on the surface of the fiber carbon than it was on the surface of the granular. Packing 1 cm3 with 0.051 cm diameter spheres results in 0.012 m2 of surface area, while packing the same volume with fibers 0.003 cm diameter and 0.51 cm in length results in 0.133 m2. This 10-fold difference in area should have been partially compensated for by the 10-fold virus increase in the dosing solution used for the ACFC column, but even with the higher doses, the ACFC column lasted twice as long before 10% breakthrough. Another reason for the performance difference may have been due to the difference in rates of adsorption. The open architecture of the ACFC composite, combined with the narrow fiber diameter, essentially negates diffusion limitations. This makes the surface of the active carbon fibers more accessible to the MS2 viruses. Gas adsorption studies have shown that the rate of adsorption in a GAC bed approaches that of a ACFC only when the particle size of the granules are smaller than 14 µm, i.e., a very fine powder (3); but a 3-in. column of fine powder would have large pressure drops. The flow pattern through composite materials is thought to be more uniform than through the GAC bed giving a more efficient contact between adsorbing virus and the carbon surface. The shape of activated carbon can either inhibit or enhance the adsorption of larger particles such as viruses, as has been shown in this study. New types of activated carbon
(1) National Primary Drinking Water Regulations: Disinfection; Turbidity, Giardia lamblia, Viruses, Legionella, and Heterotrophic Bacteria, Final Rule; U.S. Environmental Protection Agency, Office of Water, U.S. Government Printing Office: Washington, DC, 1989; 54 CFR 27486, June 29, 1989. (2) Rose, J. B.; Gerba, C. P.; Singh, S. N.; Toranzos, G. A.; Keswick, B. J. Am. Water Works Assoc. 1986, 78, 56. (3) Jagtoyen, M.; Derbyshire, F.; Brubaker, N.; Fei,Y. Q.; Kimber, G.; Matheny, M.; Burchell, T. Proceedings Materials Resource Society Symposium; Materials Resource Society: Pittsburgh, PA, 1996; Vol. 344, p 77. (4) Suzuki, M. Water Sci. Technol. 1991, 23, 1649. (5) Thwaites, M. W.; et al. Fuel Process. Technol. 1993, 34, 137. (6) Cookson, J. T., Jr. Environ. Sci. Technol. 1967, 1 (2), 157. (7) Brion, G. M.; Silverstein, J. Water Res. 1999, 33 (1), 169. (8) Berg, G.; Dahling, D.; Brown, G. A.; Berman, D. Appl. Environ. Microbiol. 1978, 36 (6), 880. (9) IAWPRC Study Group on Health Related Water Microbiology. Water Res. 1991, 25 (5), 529. (10) Adams. M. H. Bacteriophage; Interscience: New York, 1959. (11) Gerba, C. P.; Goyal, S. M. Environ. Sci. Technol. 1981, 15 (8), 940. (12) Gerba, C. P. Adv. Appl. Microbiol. 1984, 30, 133. (13) Oza, P. P.; Chaudhuri, M. J. Environ. Eng. Div. ASCE 1976, 6, 1255. (14) Murray, J. P.; Parks, G. A. Particulates in water: fate, effects, and removal; American Chemical Society: Washington, DC, 1980; pp 97-133. (15) Gerba, C. P.; Sobsey, M. D.; Wallis, C.; Melnick, J. L. Environ. Sci. Technol. 1975, 9 (8), 727. (16) Cookson, J. T., Jr. J. Am. Water Works Assoc. 1967, 61, 52. (17) Cookson, J. T., Jr.; North, W. J. Environ. Sci. Technol. 1967, 1 (1), 46. (18) Zerda, K. S. Ph.D. Dissertation, Baylor College of Medicine, 1982. (19) Bitton, G. Phage Ecology; John Wiley & Sons: New York, 1987; Chapter 7. (20) Suffet, I. H.; McGuire, M. J. Activated Carbon Adsorption of Organics from Aqueous Phase; Ann Arbor Publishers: Ann Arbor, NY, 1981; Vol. 1. (21) Hartman, Robert J. In Colloid Chemistry; Houghton Mifflin Company: Boston, 1930.
Received for review September 23, 1999. Revised manuscript received April 3, 2000. Accepted April 17, 2000. ES991097W
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