Envlron. Sci. Technol. 1985, 79, 690-894
Single-Solute Irreversible Adsorption on Granular Activated Carbon David R. Yonge,* Thomas M. Kelnath, Kazlmera Poznanska, and Zhan Peng Jiang
Department of Environmental Systems Engineering, Clemson University, Clemson, South Carolina 29631 Irreversible adsorption has been shown to influence activated carbon adsorption equilibria. To gain a better understanding of irreversible adsorption and its influence on adsorption equilibria, batch and continuous-flow single-solute adsorption experiments were conducted on five low molecular weight substituted phenols. Sorbate functional group type and position were shown to influence the degree of irreversible adsorption. Furthermore, the occurrence of irreversible adsorption was shown to influence the shape of the isotherm trace depending on the experimental procedure used for isotherm development. Introduction Numerous citations in the literature have demonstrated that multisolute adsorption equilibria models frequently fail to accurately predict solid-phase loadings on activated carbon for certain system conditions (1-5). This is because they do not account for irreversible adsorption and unequal competition for adsorption sites. Equal competition for adsorption sites is expected to occur only for a homogeneous adsorbent. Activated carbon, however, is heterogeneous with respect to adsorption sites (6-8). Moreover, because the majority of adsorption on activated carbon occurs in the microporous structure, access to certain adsorption sites is limited to the smaller sorbates. The extent of irreversibility is a function of the strength of the sorbent/sorbate bond and the method of sorbate introduction to an adsorber. When several sorbates are introduced simultaneously to an adsorber at a constant concentration, irreversible adsorption probably does not play a significant role in multisoluble adsorption equilibria. This situation, however, rarely occurs in an operating granular activated carbon (GAC) adsorber where dynamic changes in both sorbate type and sorbate concentration are commonplace. Such dynamic changes accentuate the effects of irreversible adsorption and concomitant adsorption hysteresis. On the basis of the findings of Mattson et al. (9),which indicated the formation of charge-transfer complexes between sorbates and activated carbon surface functional groups, chemisorption appears to be the most logical explanation for irreversible adsorption. Depending on the type of surface functional group as well as the type of sorbate, a sufficiently strong bond that resists desorption can be formed. The degree of irreversibility then would be directly related to the number of high-energy (chemisorptive) bonds. That fraction of adsorption that was reversible would then likely be that fraction of adsorption that occurred as a result of van der Waals forces and/or weaker charge-transfer complexes that occur at other adsorption sites on the activated carbon surface. Most research pertaining to desorption has been performed on single-solutesystems by using batch desorption procedures. Snoeyink et al. (10) have shown that only 50% of adsorbed phenol was desorbed from a granular activated carbon. Conversely, Schultz (11 ) and Robertaccio (12) obtained virtually 100% adsorption reversibility for phenol
* Address correspondence to this author at the Department of Civil and Environmental Engineering, Washington State University, Pullman, WA 99164-2912. 690
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adsorbed on a powdered activated carbon produced by a different manufacturer. Pirbazari and Weber (13,14) have shown that benzene exhibits complete reversibility while polychlorinated biphenyls were found to be completely irreversibly adsorbed in both batch and continuous-flow column studies on granular activated carbon. These results indicated that the ocurrence of irreversible adsorption is a function of both sorbent and sorbate type. To develop a predictive model which accounts for the effects of irreversible adsorption, it is first necessary to gain a more complete understanding of single-solute irreversibility. This paper presents data that were generated for this purpose. Experimental Methods The sorbent used in this research was Calgon Filtrasorb 400 granular activated carbon supplied by the manufacturer in a 12 X 40 mesh size. To facilitate shorter equilibration times during experimentation, the GAC was ground and sieved to yield a 100 X 140 mesh size fraction. This GAC was washed with distilled water to remove carbon fines, dried at 105 "C, and stored in a sealed glass bottle. All of the adsorption experiments were performed with this batch of GAC. All solution water was prepared with deionized tap water buffered at pH 7.0 with a 0.002 M phosphate buffer. Prior to the addition of the buffer, the deionized tap water was allowed to equilibrate at room temperature in a 300-L Nalgene holding tank and was passed through 2 kg of 1 2 X 40 mesh Calgon Filtrasorb 400 activated carbon contaihed in a 5.08 cm i.d. glass column operated in a downflow, packed-bed mode to remove potential interfering organic materials. The sorbates used in this research, purchased in their highest available purity from Aldrich Chemical Co., included 2-ethylphenol (2-EP), o-cresol (o-C), o-methoxyphenol (o-M),4-isopropylphenol(4-IPP),and phenol (PH). Sorbate stock solutions were prepared at 1 g/L concentrations with buffered solution water. Sorbate solutions required for each experiment were prepared from these stock solutions by dilution. The standard static bottle procedure was used for all adsorption isotherm studies. This involved placing accurately weighed portions of GAC into a series of 118-mL square specimen bottles. To provide an estimate of experimental error, replicate weights of GAC were placed in three bottles for each of two different equilibria positions on the isotherm. After the GAC had been weighed and added to each bottle, 100-mL aliquots of a 2.5 mM sorbate solution were transferred to each bottle by using a volumetric pipet, Also, a dilution of the 2.5 pM sorbate solution was made to provide concentration of approximately 40 pM. A 100-mL aliquot of this solution was transferred to an isotherm bottle containing no GAC. This bottle served as a blank to check for sorbate volatilization and/or adsorption onto the walls of the container during the equilibration period. Additional samples of both the concentrated and the diluted feed solution were placed in 20-mL glass culture tubes, sealed with Teflon-lined screw caps, and stored at 5 "C. These refrigerated samples served as a reference to determine the initial sorbate concentration (C,) added to the isotherm bottles and the
0013-936X/85/0919-0690$01.50/0
0 1985 American Chemical Society
initial sorbate concentration in the blank. The isotherm bottles, including the blank, were sealed with Teflon-lined screw caps, placed on a rotary shaker, and agitated at 200 rpm unit1 equilibrium was attained. Preliminary adsorption tests showed the liquid-phase sorbate concentration to exhibit no detectable change after an 8-day equilibration period. A 2-day safety factor was added, resulting in the 10-day equilibration period. Following equilibration, samples for sorbate analysis were prepared by filtration to separate the liquid and solid phases. The separation was performed by using an all-glass Millipore filtration unit designed to hold 47-mm diameter filter disks. Because preliminary tests showed that the sorbates were adsorbed by polycarbonate membrane filters, Whatman GF/F glass-fiber filters were evaluated for sorbate adsorption. Although these showed no adsorption during filtration, it was necessary to use three filters, one on top of another, in the Millipore filtration apparatus to prevent the passage of GAC fines into the filtrate. The influence of experimental procedure on final adsorption equilibrium conditions was evaluated by conducting several adsorption tests using different initial sorbate concentrations. Procedurally, these experiments were identical with those discussed previously except that a more dilute initial sorbate concentration was used (ca. 400 pM). Experiments were also performed in which an isotherm was developed by using the same mass of GAC. This is in contrast to the standard static bottle tests in which each isotherm data point is developed by using different quantities of GAC. This isotherm data were collected by separating the liquid and solid phases by filtration after initial equilibration, returning the GAC to the bottle, and adding a new sorbate solution. This sequential sorbate addition procedure was repeated until the maximum solid-phase capacity for the sorbate was attained. In addition, single-solute adsorption isotherms were developed by using a continuous-flow,tubular reactor (1.3 cm i.d. glass tubing) containing 0.4 g of GAC operated in a down-flow, packed-bed configuration. Sorbate solution wm supplied from and collected in 19-L flint-glass bottles. The flow to the column was maintained at 17 mL/min by a variable speed peristaltic pump and was monitored by a rotameter. Flow to the column was continued until the influent and effluent sorbate concentrations were not significantly different statistically. Wherever possible, glass tubing was used for all feed and collection lines to facilitate acid washing to prevent biological growth. Each isotherm was developed by initially equilibrating the GAC at a low sorbate concentration (approximately 30 pM). Following equilibration, the mass adsorbed was determined by taking the difference between the mass of sorbate in the collection bottle and the mass of sorbate pumped through the activated carbon contactor. A new feed solution was then prepared at a slightly higher concentration (approximately 50 pM) and the process repeated. This was continued until the GAC had ultimately been equilibrated at a 500 pM feed solution concentration. Single-solute desorption isotherms were developed to determine the extent of single-solute irreversible adsorption for each sorbate used in this research. The desorption isotherm data were generated by using 1-L rather than the 118-mL square specimen bottles. In this manner a larger mass of GAC could be used so as to minimize the effects of unintentional loss of GAC which could occur during the desorption procedure. The experimental protocol consisted of equilibrating a GAC sample at a liquid-phase sorbate concentration of approximately 500 pM. Following
Table I. Langmuir and Freundlich Isotherm Equation Constants
sorbate 4-IPP PH 2-EP 0-C
O-M
Freundlich . equation K n 1.204 0.313 1.954 2.035 2.200
RSS
0.124
0.017
0.296 0.069 0.066 0.052
0.039 0.034 0.021 0.059
Langmuir equation Q, b, mmol/g L/mol 2.452 2.026 2.867 2.834 3.018
0.110 0.021 0.415 1.037 0.230
RSS
0.133 0.200 0.143 0.402 0.124
equilibration, the liquid and solid phases were separated as previously discussed. The GAC retained on the glassfiber filters was rinsed back into the bottle with 750 mL of sorbate free buffer solution, Preliminary experiments were performed to determine if a significant quantity of GAC remained on the filters following this rinse procedure. This was done by drying the filters, following the rinse procedure, at 105 "C to a constant weight and comparing this weight to the tare weight of the filters. No detectable mass of carbon was found to adhere to the filters. The bottles were then agitated for another 10 days, after which the filtration and rinse procedure was repeated. This process was continued until the resultant liquid-phase sorbate concentration was less than the detectable limit, approximately 0.3 mg/L. The solid-phase loading on the GAC was calculated after each desorption step by using M A - E (MD)j m where MA = mass of sorbate adsorbed in the initial adsorption step (moles), (MD)i = mass of sorbate desorbed after each desorption step, i (moles), m = mass of GAC in desorption bottle (grams), and (q& = equilibrium solid-phase loading at each desorption step, i (moles per gram). A Hewlett-Packard 5830A gas chromatograph (GC) equipped with a flame ionization detector (FID) was used for sorbate quantification. The GC column used was 1.52 m long, had a 6.35 mm outside diameter and a 2 mm inside diameter, and was packed with 15% Carbowax 20M on 80/100 Chromasorb WAW-DMCS. This packing allowed for direct aqueous injection. The minimum detectable limit for the sorbates used in this research was approximately 0.3 mg/L with a sample injection volume of 2 pL. The GC operating conditions were the following: (1) oven temperature 195 "C; (2) injection temperature 150 "C; (3) N2carrier gas flow rate 25 mL/min; (4) FID temperature 275 "C. The internal standard technique was used for sorbate quantification. Each sorbate sample was prepared in triplicate, and replicate injections were performed on each sample. (qD)i =
Results and Discussion Single-solute adsorption isotherm data, developed for 4-IPP, 2-EP, 0-C, PH, and o-M, were modeled by using the Freundlich and Langmuir equations using the nonlinear models procedure (NLIN) of the Statistical Analysis Subroutine (SAS) programming procedure (15). The Freundlich equation was found to provide the best description of the data as indicated by a lower residual sum of squares (RSS). The Freundlich and Langmuir constants and the RSS for each model are given in Table I. It is not possible to perform a rigorous analysis of variance to determine whether the Freundlich equation provides a statistically significant improved description of the data over the Langmuir equation, because the solid-phase Envlron. Scl. Technol., Vol. 19, No. 8, 1985
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Table 11. Single-Solute Desorption Irreversibility sorbate
irreversible adsorption, %
sorbate
4-IPP 2-EP 0-M
87 91 97
PH
0
100
200
300
400
500
Ce (/AM)
Figure 1. Single-solute isotherm data for 4-IPP, 2-EP, 04,PH, and o-M with their Freundllch equation lines of best fit.
irreversible adsorption, %
0-c
94 85
loading (q,) is determined by using the equilibrium liquid-phase concentration (C,). The variables q, and C,, therefore, are not mutually independent. Nonetheless, as shown in Table I, the difference between the RSS values is sufficiently great to conclude that the Freundlich equation provides a better description of the data. The isotherm data are shown in Figure 1,along with the Freundlich equation lines of best fit. One can observe that 2-EP, o-M, and 0-C have virtually identical single-solute adsorption isotherms. Furthermore, the shape of the isotherms indicates that the sorbates have a strong affinity for GAC a t low liquid-phase equilibrium values, C,. Single-Solute Desorption Studies. The data listed in Table I1 summarizes the relative degree of irreversibility for each sorbate. Percent irreversibility represents the percentage of the mass of sorbate initially adsorbed at a 500 MMequilibrium, solution-phase concentration that remained adsorbed when C, was less than the minimum detectable limit. These data indicate that there is a high degree of irreversibility associated with the adsorption of each of the sorbates used in this research, implying that the dominant adsorption mechanism is high-energy bonding to specific functional groups on the GAC surfaces. PH and 4-IPP yielded the lowest degree of irreversible adsorption while 2-EP, 0-C, and o-M were the most irreversible. This suggests that the addition of the methyl, ethyl, and methoxy groups in the ortho position of the phenol molecule intensifies the binding energy to the GAC surface functional groups. The close proximity of these groups to the hydroxyl of the phenol molecule can result in an inductive effect that is stronger than that exhibited by the same group in the para position. This more intense inductive effect could result in a stronger sorbate/sorbent bond, and therefore, a higher degree of irreversibility as was observed for the orho-substituted phenols. This hypothesis, however, cannot be conclusively substantiated by the present data. The difference between the adsorption and desorption isotherms, shown in Figure 2, indicates the occurrence of hysteresis due to irreversible adsorption. The fact that the adsorption and desorption data do not coincide implies that the adsorption and/or desorption data do not depict a true equilibrium condition. That is, if true equilibrium 692
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Figure 2. Adsorption and desorptions isotherms for 4-IPP.
existed, the method of obtaining the isotherm data should not influence the equilibrium values obtained. Nonetheless, before it can be stated that hysteresis does indeed exist, it must be determined whether the experimental procedure employed influenced the results obtained. Rate Effects. The apparent difference between the adsorption and desorption data at low C, values could be due to a low rate of approach to adsorptive or desorptive equilibrium. In either case equilibrium may not have been attained during the period of time allowed for equilibration. To investigate this possibility in the case of desorption, the contents of two desorption bottles (2-EP and 4-IPP) were analyzed after 10- and 35-day equilibration periods. No desorption of either sorbate was observed after the initial 10-day equilibration period. This indicates that a 10-day equilibration period was sufficient for the attainment of equilibrium in the desorption isotherm studies. Although preliminary adsorption rate studies showed that a 10-day equilibration period was sufficient, it is possible that adsorption in the low C, region is so slow that changes in concentration are not detected within the time scale of measurement. If an extended period of equilibration were provided, more adsorption could occur, thereby increasing the solid-phase concentration for low C, values. Unfortunately, the detection of small changes in liquid-phase sorbate concentration at low concentration levels is analytically difficult. It is virtually impossible, therefore, to draw any conclusions regarding the effect of adsorption rate effects on adsorption hysteresis. It may, however, be possible to substantiate the occurrence of hysteresis by altering the experimental procedure used in the development of an adsorption isotherm. Influence of Initial Sorbate Concentration. The generation of isotherm data using different initial sorbate concentrations (C,) is one method of determining the influence of experimental procedure on isotherm data. This has been done by various researchers with conflicting results. Crittenden and Weber (16) used two initial sorbate concentrations to obtain isotherm data. The solid-phase loadings obtained for the case in which a comparatively low initial sorbate concentration was used were found to lie above those obtained for the case in which a higher initial sorbate concentration was used. Conversely, Peel and Benedek (17) found no influence of C, on adsorption equilibria. Because of these conflicting observations, three sorbates (CIPP, 0-C, and o-M) were used to evaluate the influence of Co on adsorption equilibrium. It was found that different initial sorbate concentrations had no detectable influence on the resulting isotherm for the sorbates used in this research. This suggests that the difference between the adsorption and desorption isotherms shown in Figure 2 is due to irreversible adsorption and is not an
A STEPWISE ADDITION 0 BOTTLE TEST
0
100
200
300
400
500
Ce ( p M ) Flgure 3. Isotherms for 4-IPP generated by the bottle and stepwlse sorbate addition procedures.
artifact of the particular value of Cochoosen for adsorption isotherm development. In both the high and low C,, experiments, each equilibrium point was developed by using a different mass of GAC. Consequently, an additional adsorption isotherm experiment was conducted in which isotherm data were derived from a system in which the same mass of GAC was used and increasing concentrations of sorbate were added sequentially following each equilibration period. The results of this experiment are shown in Figure 3 along with the isotherm for 4-IPP as derived by the static bottle procedure. Two points are noteworthy. First, the only datum point that coincides with the isotherm trace obtained by the static bottle procedure in the low C, region is the initial one. This is expected since the procedure for the initial point is identical for both experimental protocols. Each subsequent datum point lies above the static bottle isotherm trace. Second, the data generated by sequential sorbate addition can be seen to merge with the data obtained by using the static bottle procedure for values of C, equal to or greater than 300 pM. These observations may be due to the combined effects of (1) irreversible adsorption and (2) a decreasing liquid-phase sorbate concentration during the equilibration period in the case of the static bottle tests. The influences of these effects are discussed below. Immediately after the GAC has been contacted with a sorbate solution in a static bottle, a period of rapid adsorption ensues due to the large concentration gradient that exists between the liquid and solid phases. Initially, the majority of adsorption occurs on the outer pure surfaces of the GAC. If adsorption were fully reversible, a portion of the sorbate initially adsorbed at the outer pore surfaces would desorb, migrate inward, and readsorb. Concurrent with this inward diffusion would be the continual adsorption of sorbate from the liquid phase, albeit at a lower rate. This inward diffusion within the GAC and removal of sorbate via adsorption from the liquid phase would continue until equilibrium is attained for which a uniform concentration profile should exist throughout the GAC particle if particle activity were radially uniform. Conversely, if significant irreversibility exists, as shown by the single-solute desorption experiments, migration of sorbate from the outer to the inner pore surfaces would not occur. This would lead to a higher solid-phase concentration in the outer pores as compared with the inner pores; i.e., a radial concentration profile approximately parabolic in shape would result. Upon reaching equilibrium, the solid-phase concentration in the outer pores would “appear” to have resulted from a higher liquid-phase
concentration than actually exists in the isotherm bottle at equilibrium. This results from the combined effects of (1)a continually decreasing liquid-phase sorbate concentration during the equilibration period and (2) irreversible adsorption. If a radial concentration profile that is approximately parabolic in shape does indeed exist, the data in Figure 3 can be explained as follows. Due to irreversible adsorption, the pores at or near the surface of the particle are nearly saturated on account of exposure to a high initial sorbate concentration. Upon addition of the second solution of the same sorbate in sequence, rapid adsorption occurs just as during the first equilibration step with the exception that adsorption occurs on the pore surfaces that are located more toward the interior of the GAC. The adsorption that occurs on these surfaces also exhibits a high degree of irreversibility preventing desorption and subsequent inward diffusion. The net result is a more uniform radial concentration profile due to the increase in solid-phase concentration on the surfaces of the inner pores. This process continues upon subsequent sorbate additions until the solid-phase concentration becomes uniform throughout the GAC particle at a level equal to the maximum solid-phase capacity. This is shown to occur for the last two sequential additions of solute that yield C, values of 117 and 198 pM with corresponding solidphase concentrations of 2.54 and 2.56 mmol/g. These values indicate that the isotherm derived by using the sequential sorbate addition procedure approaches the same maximum solid-phase capacity achieved by using the static bottle procedure. It is apparent, therefore, that a significant fraction of adsorption on GAC is irreversible for the compounds investigated in this study. Because this influences the shape of the isotherm depending on the procedure used for isotherm development, no “true isotherm” exists for the case of irreversible adsorption. For low equilibrium concentrations, the static bottle and sequential sorbate addition procedures result in the outer surfaces of the GAC having a higher solid-phase concentration than would be expected on the basis of the value of C, that exists at the end of the equilibration period. This phenomenon causes the shape of the isotherm trace to differ, in the low C, region, as a function of the experimental procedure employed. For high equilibrium solution-phase concentrations, however, the experimental procedure has no influence on the isotherm trace, because the radial concentration profile at or near the maximum solid-phase capacity is constant throughout. These experimental artifacts can be prevented, however, by using an experimental protocol that leads to the development of a uniform solid-phase concentration profile for low solution-phase concentrations. Fortunately, this can be done by continually contacting GAC with a fixed sorbate concentration until equilibrium is attained. This can be accomplished by using a continuous-flow contactor. Preliminary tests were performed by using the continuous-flow procedure by continually contacting GAC at a specified influent concentration until the influent and effluent concentrations were not significantly different relative to experimental error. The influent concentration was then increased to a new predetermined level and equilibrium again established. The results of these preliminary tests showed this isotherm trace to lie beneath that of the static bottle isotherm trace for low C, values. This concurs with what would be expected if a constant solid-phase radial concentration profile were attained. It is not possible, however, to confirm the results of the Environ. Sci. Technol., Vol. 19, No. 8, 1985
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continuous-flow method due to experimental error problems inherent in the procedure. In a continuous-flow carbon adsorber, adsorption is initially rapid, resulting in a significant difference between the feed and effluent concentrations. As the solid-phase sorbate concentration increases, the rate of adsorption decreases due to the decreasing concentration gradient between the solid and liquid phases. Ultimately, the difference between the influent and effluent sorbate concentrations becomes undetectable even though adsorption may still be proceeding. During this period it is not possible to account for the adsorption that does occur. This would cause the isotherm trace to be positioned erroneously low, making it difficult to ascertain the significance of a constant radial concentration profile on adsorption equilibria. In addition, the continuous-flow procedure suffers from error accumulation. Since each equilibrium datum point of the isotherm trace is developed by using data from a previous equilibrium point, any errors incurred are accumulated throughout the entire isotherm. Consequently, although it is theoretically possible to develop an adsorption isotherm that is not influenced by irreversibility, error propagation problems render this technique unacceptable as well.
Conclusions A significant fraction of adsorption on GAC was found to be irreversible for the sorbates investigated in this study, indicating that adsorption onto specific sites on the GAC surface was an important mechanism of adsorption. The type and position of functional groups on the phenol molecule were indicated as influencing the degree of single-solute irreversibility. The occurrence of irreversibility, furthermore, was shown to influence the shape of the isotherm depending on the procedure used for isotherm development. I t was determined that a continuous-flow isotherm procedure would be used to develop an isotherm that may not be affected by irreversibility. This technique, however, was found to suffer from error propagation problems rendering it unacceptable. Registry No. 2-EP, 90-00-6; o-C,95-48-7;o-M, 90-05-1;4-IPP, 99-89-8; PH, 108-95-2; carbon, 7440-44-0.
Literature Cited (1) Martin, R. J.; Al-Bahrani, K. S. Water Res. 1979, 13, 1301-1304.
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(2) Keinath, T. M.; Karesh, H.; Lowry, S.; Abdo, M. “Mathematical Modeling of Heterogeneous Sorption in Continuous Contactors for Wastewater Decontamination: Influences of Reversibility and Chromatographic Effects on System Design and Operations”. U.S. Army, Medical Research and Development Command Washington, DC, 1976; Contract DADA-17-73-C-3154,Clemson University, Clemson, SC. (3) Carnahan, R. P. Ph.D. Dissertation, Clemson University, Clemson, SC, 1973. (4) DiGiano, F. A.; Frye, W. H.; Baxter, C. W. “A Rational Approach to Utilization of Carbon Beds in Reducing Microorganic Contamination in Drinking Water”. Water Resources Research Center, University of Massachusetts, Amherst, MA, 1979, Publication No. A-091. (5) Crittenden,J. C. Ph.D. Dissertation, University of Michigan, 1976. (6) Snoeyink,V. L.; Weber, W. J. Environ. Sci. Technol. 1967, I, 228-234. (7) Coughlin, R. W., Ezra, F. S. Environ. Sci. Technol. 1968, 2, 291-297. (8) Mattson, J. S.; Mark, H. R. J . Colloid Interface Sci. 1969, 31, 131-144. (9) Mattson, J. S.; Mark, H. B.; Malbin, M. D.; Weber, W. J.; Crittenden, J. C. J . Colloid Interface 1969, 31, 116-130. (10) Snoeyink, V. L.; Weber, W. J.; Mark, H. B. Environ. Sci. Technol. 1969,3,918-926. (11) Schultz, J. R. Ph.D. Dissertation, Clemson University, Clemson, SC, 1982. (12) Robertacio, F. L. Ph.D. Dissertation, University of Delaware, 1976. (13) Pirbazari, M.; Weber, W. J. “Adsorption of Polycholrinated Biphenyls from Water by Activated Carbon; Cooper, W. J., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1981; Chem. Water Reuse 2, 309. (14) Pirbazari, M.; Weber, W. J., “Adsorption of Benzene from Water by Activated Carbon”;Cooper, W. J., Ed.; Ann Arbor Science Publishers; Ann Arbor, MI, 1981; Chem. Water Reuse 2, 285-307. (15) Barr, A. J.; Goodnight, J. H.; Sall, J. P.; Helwig, J. T. “A User’s Guide to SAS-76”;SAS Institute Inc.: Raleigh, NC, 1976. (16) Crittenden, J. C., Weber, W. J., ”Predictive Model for Design of Fixed-Bed Adsorbers: Parameter Estimation and Model Development”. Environ. Eng. Diu. (Am. SOC.Civ. Civ. Eng.) ASCE, EE2, 185-197. (17) Peel, R. G.; Benedek, A. Environ. Sci. Technol. 1980,14, 66-71.
Received for review April 25,1984. Revised manuscript received December 8, 1984. Accepted January 15, 1985.
