Activated Carbon Load Equalization of Gas-Phase Toluene: Effect of

Jun 28, 2007 - Effective load equalization could decrease or eliminate many of these difficulties. In research described here, experiments were conduc...
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Environ. Sci. Technol. 2007, 41, 5478-5484

Activated Carbon Load Equalization of Gas-Phase Toluene: Effect of Cycle Length and Fraction of Time in Loading WILLIAM M. MOE,* KODI L. COLLINS, AND JOHN D. RHODES Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana 70803-6405

Fluctuating pollutant concentrations pose challenges in the design and operation of air pollution control devices such as biofilters. Effective load equalization could decrease or eliminate many of these difficulties. In research described here, experiments were conducted to evaluate effects of cycle length and fraction of time contaminants are supplied on the degree of load equalization achieved by passively operated granular activated carbon (GAC) beds. Columns packed with Calgon BPL 4 × 6 mesh GAC were subjected to a variety of cyclic loading conditions in which toluene was supplied at concentrations of 1000 or 250 ppmv during loading intervals, and uncontaminated air flowed through the columns during no-loading intervals. The fraction of time when toluene was supplied ranged from 1/2 to 1/6, and cycle lengths ranged from 6 to 48 h. Results demonstrate that passively operated GAC columns can temporarily accumulate contaminants during intervals of high influent concentration and desorb contaminants during intervals of no loading, resulting in appreciable load equalization without need for external regeneration by heating or other means. Greater load equalization was achieved as the fraction of time toluene was loaded decreased and as the cycle length decreased. A pore and surface diffusion model, able to predict the level of contaminant concentration attenuation in GAC columns with reasonable accuracy, was used to further explore the range of load equalization performance expected from columns of various packed bed depths.

Introduction Numerous industrial processes emit gas streams contaminated by volatile organic compounds (VOCs) at concentrations that fluctuate with time. In many cases, the transient changes in contaminant concentrations occur at regular intervals on a repeating basis due to the inherent nature of the processes generating the contaminated gas streams. For example, fixed-length work shifts combined with overnight process shutdowns can result in waste gas streams with regular changes over a 24 h cycle length (e.g., contaminants present 8 h/day and absent 16 h/day). Regular transient loadings can also occur at shorter time intervals (i.e., on the order of minutes or hours) due to cyclic process operations such as opening and closing press vents in the manufacture * Corresponding author phone: (225)578-9174; fax: (225)578-8652; e-mail: [email protected]. 5478

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of particleboard and other laminated wood products (1), fluctuating rates of solvent use in chemical processing or manufacturing, or paint spraying in surface coating operations (2). These unsteady-state loading conditions can pose challenges in design and operation of some air pollution control technologies. For example, when biofilters (fixed-film biological processes that are increasingly applied to remove and biodegrade VOCs present in contaminated gases) are subjected to transient periods of elevated contaminant concentrations, the dynamic mass loading rate can exceed the biological reaction capacity and result in unacceptably high contaminant emissions on a temporary basis (2-7). Intervals of low or no contaminant loading in biofilter systems are also problematic because of starvation conditions imposed on the microbial populations (7-10). Recent research has demonstrated that columns packed with granular activated carbon (GAC) can serve as passively controlled load equalization devices as a pretreatment step prior to treating contaminated gases via biofiltration (7, 11). The rationale for such a system is that during periods of high contaminant loading, the GAC adsorbent can temporarily accumulate contaminants and then subsequently desorb contaminants during intervals when concentration in the waste gas is low. When applying biological processes as the method for ultimately removing and destroying VOCs, load equalization processes can serve several purposes including (1) dampening fluctuations in organic loading to prevent shock loading of the biofilter, and (2) providing continuous feed to biological systems over periods when wastes are not being generated. If successfully implemented, such an approach could also allow smaller, and therefore less expensive, biofilters for treating discontinuously generated waste gases because the biofilter bed could be used more efficiently as a function of time (11). Similar benefits may also be achieved when utilizing GAC load equalization in conjunction with other air pollution control technologies that are designed on the basis of peak contaminant loading. Applications testing passively operated GAC load equalization systems (i.e., those operated without external GAC regeneration by heating or other means) reported to date have been conducted using a fixed cycle length and fraction of time for contaminant loading (i.e., 8 h of contaminant loading followed by 16 h of no loading on a daily basis) (7, 11, 12) despite the fact that industrial processes often emit contaminants with concentrations that fluctuate over various other time scales. The overall goal of research described in this paper was to assess the effects of cycle length and fraction of time of contaminant supply on the load equalization process. Fixed-bed adsorption/desorption experiments were conducted under a variety of cyclic loading patterns, and a pore and surface diffusion model was used to further investigate performance of GAC columns subjected to intermittent loading conditions.

Materials and Methods Activated Carbon and Experimental Apparatus. Activated carbon employed in this research was BPL 4 × 6 mesh GAC (Calgon Carbon Corp., Pittsburgh, PA). This is a bituminous coal-based activated carbon designed for use in vapor phase applications, and it has been extensively characterized in previous studies (13, 14). The GAC was rinsed with deionized water to remove fines and dried at 105 °C prior to use. Configuration of the apparatus used to test the GAC in fixedbed toluene adsorption/desorption experiments is depicted in Figure 1. A mass flow controller (model GFMC37, Aalborg 10.1021/es070336+ CCC: $37.00

 2007 American Chemical Society Published on Web 06/28/2007

FIGURE 1. Schematic diagram of apparatus used for fixed-bed adsorption/ desorption experiments.

TABLE 1. Experimentally Tested Cyclic Loading Conditions. Each Loading Condition was Tested at Influent Toluene Concentrations of Both 1000 ppmv and 250 ppmv. fraction of time toluene loaded

tloading (hours)

tno loading (hours)

cycle length (hours)

0.500

3 6 12 24

3 6 12 24

6 12 24 48

0.333

2 4 8 16

4 8 16 32

6 12 24 48

0.167

1 2 4 8

5 10 20 40

6 12 24 48

Instruments, Orangeburg, NY) regulated flow of contaminantfree air entering the system. Liquid toluene (ACS reagent grade) was delivered by syringe pumps (KD Scientific, Boston, MA) equipped with glass gastight syringes and evaporated into the air stream. Packed-columns, operated in down-flow mode, were constructed of PVC (ID 7.62 cm). A mesh screen installed at the bottom of each column supported 6 cm depth of glass beads (5 mm diameter) to evenly distribute air flow, a thin layer of glass wool to retain GAC particles, 33.3 cm depth of GAC (mass 720 g, packed bed volume 1.52 L), another thin layer of glass wool, and another 6 cm depth of glass beads. A microprocessor-based controller (Chron-Trol, San Diego, CA) turned syringe pumps on and off as necessary. Inlet and outlet toluene concentrations were measured using a model 1312 photoacoustic multigas analyzer (California Analytical, Orange, CA) as described previously (7). Gas sampling lines were constructed using Teflon tubing. Initial tests conducted prior to placement of activated carbon demonstrated that column components other than GAC had little or no adsorption capacity for toluene. All experiments were conducted at ambient laboratory temperature of 23 ( 2 °C. Experimental Tests of Intermittent Toluene Loading. The frequency and duration of intervals when toluene was present (tloading) and absent (tno loading) in the synthetic waste gas in the various cyclic-loading experiments are summarized in Table 1. The fraction of time when toluene was present in the synthetic waste gas [tloading/(tloading + tno loading)] ranged from 0.5 (i.e., one-half) to 0.167 (i.e., one-sixth), at cycle lengths (tloading + tno loading) ranging from 6 to 48 h. The air flow rate was 22.8 L/min during both loading and non-loading

intervals; the only difference between the two periods was that syringe pumps were turned on to supply toluene as a contaminant during loading intervals. The gas flow rate corresponds to a superficial velocity of 300 m/hr through the GAC column with an empty bed contact time (EBCT) of 4.0 s. For each loading condition summarized in Table 1, separate experiments were conducted with target influent toluene concentrations (Co) of 1000 and 250 ppmv. Effluent toluene concentrations were measured at 5 min intervals during at least five cycles after quasi-steady state was reached at each loading condition tested. Pore and Surface Diffusion Model. The pore and surface diffusion model (PSDM) described by Crittenden et al. (15) and Hand et al. (16), calibrated with parameter values reported by Moe and Li (11), was used to simulate the degree of load dampening achieved by GAC columns under discontinuous loading conditions identical to those experimentally tested. The PSDM is a dynamic fixed-bed model that incorporates assumptions and governing equations described elsewhere (16, 17). Simulations using the PSDM were performed using the AdDesignS software package (Michigan Technological University). Dynamic adsorption calculations using this model require equilibrium parameters, kinetic parameters, physical properties of adsorbing compound and adsorbent, fluid properties, influent concentrations, flow characteristics, and column dimensions. A summary of parameter values and sources of data input to the model is presented in Table S1 (Supporting Information).

Results Following the start of contaminant loading to each of the activated carbon columns, there was an initial period during which toluene accumulated in the column and no toluene was detected in the effluent (see the example in Figure S1, Supporting Information). Eventually, however, breakthrough occurred and a consistent pattern of attenuated effluent concentration was exhibited on a cyclic basis with a frequency corresponding to the total cycle length. Hereafter, this condition is referred to as the quasi-steady state behavior. Experimental measurements and model simulations of quasi-steady state breakthrough curves for GAC columns receiving gas flow with toluene intermittently loaded at influent concentrations of 1000 and 250 ppmv are shown in Figures 2 and 3, respectively. Each graph depicts data from a 48 h interval with contaminant concentrations measured at 5 min intervals. The y axis of each graph is the dimensionless effluent concentration (effluent concentration divided by corresponding target influent concentration entering during the loading interval). Measured inlet toluene concentrations were found to closely match target concentrations (within 5% of target values, data not shown). As shown in Figures 2 and 3, the experimentally determined maximum dimensionless toluene concentration exiting the GAC columns at any time during the loading cycle (Cmax/Co) was appreciably lower than the influent concentration during the loading (C/Co ) 1) for most loading conditions tested. Likewise, the minimum dimensionless toluene concentration exiting the GAC columns at any time during the loading cycles (Cmin/Co) was appreciably higher than the influent concentration during the period of no loading (C/Co ) 0). Minimum and maximum effluent toluene concentrations each cycle were consistently reproducible, and mass balance calculations verified that contaminant mass entering and exiting GAC columns on a daily basis were essentially the same after quasi-steady state was reached at each loading condition (average mass balance closure of 97%). Two trends are readily apparent from data depicted in Figures 2 and 3. First, the degree of load equalization (characterized by lower Cmax/Co and higher Cmin/Co) increased as the cycle length decreased. For example, when toluene VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Experimental measurements (filled symbols) and model simulations (open symbols) of quasi-steady state breakthrough curves for various fractions of time in loading and cycle lengths at influent toluene concentration of 1000 ppmv during loading intervals. was supplied at an influent concentration of 1000 ppmv during one-third of the cycle length [tloading/(tloading + tno loading) ) 0.333], Cmax/Co was experimentally observed to be 0.823 ( 0.006 (mean ( standard deviation) at a cycle length of 48 h, while it was 0.506 ( 0.004, 0.344 ( 0.007, and 0.347 ( 0.004 at cycle lengths of 24, 12, and 6 h, respectively (see middle column in Figure 2). Second, the degree of load equalization increased as the fraction of time during which toluene was supplied decreased. For example, when toluene was supplied at an influent concentration of 1000 ppmv at a cycle length of 24 h, Cmax/Co was experimentally observed to be 0.826 ( 0.005 when toluene was supplied during one-half of the cycle length, while Cmax/Co was 0.506 ( 0.004 and 0.209 ( 0.003 when toluene was supplied during one-third and one-sixth of the cycle length, respectively (see second row in Figure 2). Consistent with results reported previously (11), the degree of load dampening also increased as the influent toluene concentration decreased. For example, when toluene was supplied during one-third of a 48 h cycle length, Cmax/Co was experimentally determined to be 0.823 ( 0.006 when the influent toluene concentration was 1000 ppmv, while at an influent toluene concentration of 250 ppmv, Cmax/Co was 0.386 ( 0.009 (see middle graph, top row, in Figures 2 and 3, respectively). To further assess the relationship between cycle length and degree of load equalization, plots of Cmax/Co versus cycle length were constructed as shown in Figure 4. Each measured data point in the figure represents the average of at least five replicate loading cycles measured after the columns reached quasi-steady state. Also depicted in Figure 4 is the “ideal buffering” level for each fraction of time in loading tested (see dashed lines). “Ideal buffering” refers to the situation in which dynamically varying contaminant concentrations 5480

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entering a GAC column are attenuated to such an extent that contaminants exit the GAC system at a constant concentration equal to the time weighted average influent concentration. For a gas stream with intermittent contaminant loading at a fixed periodicity and amplitude with constant influent concentration during the loading interval (as was the case for experiments described here), the dimensionless concentration exiting a GAC column at ideal buffering is equal to the fraction of time during which contaminants are loaded to the system. Differences between measured and modeled Cmax/Co values averaged 0.049 and 0.021 for influent toluene concentrations of 1000 and 250 ppmv, respectively (corresponding to differences of 49 and 5.3 ppmv, respectively). Discrepancies between measured and modeled Cmax/Co were largest for the longest cycle length tested (i.e., 48 h), with a maximum difference of 0.159 (corresponding to a difference of 159 ppmv, for the case of 1000 ppmv toluene loading during one-third of a 48 h cycle). At cycle lengths of 24 h or shorter, differences between measured and modeled Cmax/Co were consistently small (average difference 0.032 and maximum difference 0.126 (for the case of 1000 ppmv toluene loading during one-half of a 24 h cycle)). To further explore the range of load equalization behavior expected from GAC columns subjected to intermittent loading conditions, additional model simulations were performed for the loading conditions summarized in Table 1 but for a variety of GAC bed depths longer and shorter (ranging from 2 to 150 cm) than those experimentally tested. Input parameters other than column dimensions and GAC mass were identical to those used in simulations corresponding to experimental tests. To represent the data in a form amenable to direct use as a tool for design or analysis, Cmax/Co and Cmin/Co were

FIGURE 3. Experimental measurements (filled symbols) and model simulations (open symbols) of quasi-steady state breakthrough curves for various on:off ratios and cycle lengths at influent toluene concentration of 250 ppmv during loading intervals. “ideal” buffering. Simulation results demonstrate that the bed depth necessary to approach the ideal buffering level decreases as the cycle length decreases, as the fraction of time for contaminant loading decreases, or as the influent contaminant concentration decreases.

Discussion Data presented in Figures 2 and 3 support the previously reported finding (7, 11, 12) that toluene mass can be temporarily accumulated in a GAC column during intervals when influent concentrations are high and then desorb within a sufficiently short time interval (i.e., during each loading cycle when influent contaminant concentrations are low or zero) to be of practical benefit as a passively operated load dampening equalization mechanism. The only driving force necessary for contaminant desorption was the decrease in influent contaminant concentration imposed by the gas stream. Regeneration of the GAC columns through other means (e.g., heating) was not necessary. FIGURE 4. Experimentally determined (filled symbols) and model simulation values (open symbols) of Cmax/Co as a function of cycle length at quasi-steady state for influent toluene concentrations of 1000 ppmv (top) and 250 ppmv (bottom). 2/4 Loading one-half of the cycle length, (/) Loading one-third of the cycle length, 9/0 Loading one-sixth of the cycle length. plotted as a function of bed depth as shown in Figures 5 and 6 for influent toluene concentrations of 1000 and 250 ppmv, respectively. For all loading conditions, toluene was more attenuated (i.e., characterized by lower Cmax/Co and higher Cmin/Co) as bed depth increased, asymptotically approaching

Visual comparison of dynamic model predictions shown in Figures 2 and 3 with experimental measurements reveals a good fit in terms of overall breakthrough curve shape at quasi-steady state. In fact, in many cases the symbols denoting model simulations are obscured by the experimentally measured data points because of the close fit, particularly for the shorter cycle lengths tested (i.e., 6 or 12 h). At the longest experimentally tested cycle length (i.e., 48 h), the model consistently over-predicted the maximum effluent toluene concentrations exiting the GAC columns, but by a relatively small amount. Considering the fact that all model simulations reported herein were conducted VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Maximum (Cmax/Co, denoted by filled symbols) and minimum (Cmin/Co, denoted by open symbols) dimensionless effluent toluene concentrations as a function of bed depth at quasi-steady state for influent toluene concentration of 1000 ppmv. Dashed lines denote ideal buffering levels. exclusively using parameter values reported in the literature without any additional calibration, the predictive capability of the model is impressive. This provides further validation that the PSDM model is able to reasonably well-describe the process of load equalization in GAC columns receiving intermittent loading of toluene vapors as reported previously (11). As shown in Figure 4, experimental data, supported by model predictions, demonstrate that load attenuation very close to ideal buffering can be achieved in GAC columns with relatively low EBCT (i.e., 4.0 s) subjected to loading for a relatively large fraction of the cycle length (e.g., one-half) when the cycle length is sufficiently short (e.g., 6 h). For an equal size GAC column loaded for a smaller fraction of the cycle length (e.g., one-sixth rather than one-half), load attenuation close to ideal buffering can be achieved even when the cycle length is relatively long (e.g., 48 h). Regardless of the fraction of time in loading, experimental data, supported by model predictions, demonstrate that Cmax/Co asymptotically approaches ideal buffering as the cycle length decreases. Model predictions also indicate that Cmax/Co asymptotically approaches a value of 1.0 (effluent equal to influent concentration) at long cycle lengths, with the cycle length necessary to reach a value of 1.0 increasing as the fraction of time contaminant is loaded decreases or as the influent contaminant concentration decreases (data not shown). If an integrated system consisting of a GAC buffering column followed by a biofilter or other air pollution control device is to be designed, there is an obvious need to relate GAC column size to the degree of load equalization that will 5482

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result from its use. For a defined influent contaminant loading condition, graphs such as those shown in Figures 5 and 6 can be used to directly estimate the minimum bed depth required for a GAC column to achieve a desired level of load equalization (11). For example, if a waste gas contained 1000 ppmv toluene for 8 h followed by no toluene for 16 h (cycle length of 24 h) on a continuously repeating basis and a designer wished to decrease the maximum toluene concentration entering a downstream pollution control device to 500 ppmv, the middle graph in the second row of Figure 5 indicates that a GAC bed depth of roughly 50 cm would be necessary to achieve this objective (see point on the curve corresponding to Cmax/Co of 0.5). As shown in the third column of Figure 3, toluene was well buffered near the ideal level at all four experimentally tested cycle lengths (48, 24, 12, and 6 h) when the influent concentration was 250 ppmv and loading was for one-sixth of the cycle length. Inspection of curves shown in the third column of Figure 5 indicates that GAC bed depths substantially smaller than the experimentally tested 33.3 cm would have been able to achieve roughly the same degree of load equalization. Model simulations indicate that GAC bed depths of 21.6, 7.6, 3.8, and 3.6 cm, would be sufficient to decrease Cmax/Co to a level of 0.2 (corresponding to 50 ppmv) for cycle lengths of 48, 24, 12, and 6 h, respectively. The fact that passively operated GAC columns with short EBCTs (on the order of a few seconds or less) can decrease Cmax/Co to such a large extent could provide an obvious practical benefit in design and operation of biofilters or other air pollution control devices. If the air pollution control

FIGURE 6. Maximum (Cmax/Co, denoted by filled symbols) and minimum (Cmin/Co, denoted by open symbols) dimensionless effluent toluene concentrations as a function of bed depth at quasi-steady state for influent toluene concentration of 250 ppmv. Dashed lines denote ideal buffering levels. device’s design is to be based on peak loading, as advocated by some authors for design of biofilters (5, 11, 18), the downstream control device could be markedly reduced in size compared to a system without buffering. Alternately, the GAC load equalization system could be used to increase the safety factor used in design (11). In cases where low pollutant concentrations are also a concern, for example, because they impose starvation during periods of low or no contaminant loading to biofilters (7-10), GAC load equalization systems could also be utilized to minimize or entirely avoid such problems. From a design perspective, there are a number of constraints in applying curves such as those presented in Figures 5 and 6 for sizing GAC columns. Such curves will generally be applicable only to the specific adsorbent, contaminant, contaminant concentration, superficial gas velocity, temperature, relative humidity, and load periodicity for which it was generated. Pilot-scale testing may be necessary to verify model predictions and test load equalization under real-world operating conditions. Additionally, because of competitive adsorption, different contaminants can exhibit different levels of attenuation in systems treating multicomponent contaminant mixtures (as opposed to the single-contaminant waste gas examined here) (7, 15, 16). More extensive modeling and/or experimental testing may be necessary for such systems (7). Nevertheless, the data and approach presented here may prove useful for preliminary sizing/analysis of passively operated GAC load equalization systems for use in conjunction with biofilter treatment or other air pollution control technologies.

Acknowledgments We gratefully acknowledge the Louisiana Board of Regents and BioReaction Industries for financial support and Calgon Carbon Corp. for providing activated carbon.

Supporting Information Available A table shows the parameter values using in the pore and surface diffusion model simulations. A figure shows the target influent and measured effluent toluene concentrations. This material is available free of charge via the Internet at http:// pubs.acs.org.

Literature Cited (1) Dirk-Faitakis, C.; Allen, D. G. Biofiltration of cyclic air emissions of alpha-pinene at low and high frequencies. J. Air Waste Manage. Assoc. 2003, 53, 1373-1383. (2) Wright, W. F.; Schroeder, E. D.; Chang, D. P. Y. Regular transient loading response in a vapor-phase flow-direction-switching biofilter. J. Environ. Eng. 2005, 131, 1649-1658. (3) Martin, F. J.; Loehr, R. C. Effects of periods of non-use on biofilter performance. J. Air Waste Manage. Assoc. 1996, 46, 539-546. (4) Moe, W. M.; Irvine, R. L. Effect of nitrogen limitation on performance of toluene degrading biofilters. Water Res. 2001, 35, 1407-1414. (5) Martin, R. W.; Li, H. B.; Mihelcic, J. R.; Crittenden, J. C.; Lueking, D. R.; Hatch, C. R.; Ball, P. Optimization of biofiltration for odor control: Model calibration, validation, and applications. Water Environ. Res. 2002, 74, 17-27. (6) Atoche, J. C.; Moe, W. M. Treatment of MEK and toluene mixtures in biofilters: Effect of operating strategy on performance during transient loading. Biotechnol. Bioeng. 2004, 86, 468-481. (7) Li, C.; Moe, W. M. Activated carbon load equalization of discontinuously generated acetone and toluene mixtures treated by biofiltration. Environ. Sci. Technol. 2005, 39, 2349-2356. VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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(8) Park, J.; Kinney, K. A. Evaluation of slip feed system for vaporphase bioreactors. J. Environ. Eng. 2001, 127, 979985. (9) Cox, H. H. J.; Deshusses, M. A. Effect of starvation on the performance and re-acclimation of biotrickling filters for air pollution control. Environ. Sci. Technol. 2002, 36, 3069. (10) Moe, W. M.; Qi, B. Performance of a fungal biofilter treating gas-phase solvent mixtures during intermittent loading. Water Res. 2004, 38, 2259-2268. (11) Moe, W. M.; Li, C. A design methodology for activated carbon load equalization systems applied to biofilters treating intermittently generated toluene concentrations. Chem. Eng. J. 2005, 113, 175-185. (12) Weber, F. J.; Hartmans, S. Use of activated carbon as a buffer in biofiltration of waste gases with fluctuating concentrations of toluene. Appl. Microbiol. Biotechnol. 1995, 43, 365-369. (13) Russell, B. P.; LeVan, M. D. Pore size distribution of BPL activated carbon determined by different methods. Carbon 1994, 32, 845855. (14) Gusev, V. Y.; O’Brien, J. A. A self-consistent method for characterization of activated carbons using supercritical adsorption and grand canonical Monte Carlo simulations. Langmuir 1997, 13, 2815-2821.

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(15) Crittenden, J. C.; Hutzler, N. J.; Geyer, D. G.; Oravitz, J. L.; Friedman, G. Transport of organic compounds with saturated groundwater flow: Model development and parameter sensitivity. Water Resour. Res. 1986, 22, 271-284. (16) Hand, D. W.; Crittenden, J. C.; Hokanson, D. R.; Bulloch, J. L. Predicting the performance of fixed-bed granular activated carbon adsorbers. Wat. Sci Techol. 1997, 35, 235-241. (17) Mertz, K. A.; Gobin, F.; Hand, D. W.; Hokanson, D. R.; Crittenden, J. C. Manual: Adsorption Design Software for Windows (AdDesignS); Michigan Technological University: Houghton, MI, 1999. (18) Martin, R. W.; Mihelcic, J. R.; Crittenden, J. C. Design and performance characterization strategy using modeling for biofiltration control of odorous hydrogen sulfide. J. Air Waste Manage. Assoc. 2004, 54, 834-844. (19) Crittenden, J. C.; Cortright, R. D.; Rick, B.; Tang, S. R.; Perram, D. Using GAC to remove VOCs from air stripper off-gas. J Am. Water Works Assoc. 1988, 80, 73-84.

Received for review February 9, 2007. Revised manuscript received May 17, 2007. Accepted May 25, 2007. ES070336+