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Adsorption isotherms: illusive capacity and role of oxygen
War. Res. Vol. 24, No. 10. pp. 1187-1195, 1990 Printed in Great Britain. All rights reserved
0043-1354/90 $3.00+0.00 Copyright ~, 1990 PergamonPress plc
A D S O R P T I O N ISOTHERMS: ILLUSIVE CAPACITY A N D ROLE OF O X Y G E N R. D. VIDIC ], M. T. SUIDANI'*(~, U. K. TRAEGNER 2 and G. F. NAKHLA 3 tDeparlment of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, U.S.A., 2BASF AG, ZET/EC, L544, 6700 Ludwigshafen, F.R.G. and 3Department of Civil Engineering, King Fahad University of Petroleum and Minerals, Dahran 31261, Kingdom of Saudi Arabia (First received October 1989; accepted in revised form February 1990)
Abstract--This paper examines the influence of molecular oxygen on the adsorptive capacity of GAC. A new experimental procedure for determining adsorption isotherms is introduced. This procedure, denoted as "anaerobic", differs from the currently used techniques, denoted as "aerobic", in that oxygen is repeatedly purged from the test environment. The results show that the capacity of GAC for the retention of o-cresol can increase up to 3-fold in the presence of.oxygen when compared to the anaerobic capacity. The same trend is observed for the adsorption of phenol and 3-ethyiphenol. It is shown that this increase in capacity cannot be attributed to biological degradation of these adsorbates in the presence of oxygen. It is speculated that this phenomenon is due to some chemical reactions between the adsorhates and molecular oxygen that are catalyzed by the activated carbon surface and occur at a different time scale than physical adsorption. Initial portions of breakthrough curves for o~resol are very accurately predicted using capacities depicted by the anaerobic isotherm, while the total GAC adsorptive capacity for o-cresol, as determined from breakthrough experiments, appears to agree closely with the capacity predicted from the aerobic isotherm. Key words--adsorption isotherms, adsorptive capacity, aromatics, molecular oxygen, breakthrough
prediction
INTRODUCTION Granular activated carbon (GAC) is widely used in both water and wastewater treatment as an adsorbent for the removal of organic pollutants. One of the key components needed for the scale-up of experimental data and for the prediction of adsorber performance is the capacity of G A C for the retention of organic compound. This capacity is often given by an adsorption isotherm which represents the equilibrium at constant temperature between the quantity of adsorbate retained per unit mass of adsorbent, qc, and the concentration of adsorbate in solution, Ce. Isotherm data are most commonly obtained using the, so called, bottle-point technique. A discussion of some of the pitfalls often encountered in conducting an isotherm test is given by Randtke and Snoeyink (1983). Unfortunately, a unified procedure for conducting this experiment has not yet been established. Most researchers employ varying experimental procedures for the collection of isotherm data. These procedures differ in carbon preparation, particle size (pulverized versus intact granular carbon), volume of the bottles used in the experiment, volume of adsorbate solution added, buffer application and equilibration time allowed. As a result, many different isotherms for the same compound and the same G A C
are given in literature. Peel and Benedek (1980) reported that particle size, the presence or absence of a weak buffer and the initial concentration of adsorbate, had no effect on the equilibrium relationship for phenol and o-chlorophenol. This conclusion was supported by data from several other investigators (Martin and AI-Bahrani, 1978; Randtke and Snoeyink, 1983). Contradictory findings regarding the effect of initial concentration on G A C capacity are also reported. Crittenden and Weber (1978) and van Vliet et al. (1980) found G A C capacities for phenol and p-bromophenol to be higher for lower initial adsorbate concentrations while Peel and Benedek (1980), Yonge et aL (1985) and Zogorski et al. (1976) did not observe such effects for the adsorption of phenol and o-cresol. Isotherm data can also be obtained using batch reactor and column methods. Theoretically, equilibrium data are independent of the way they are obtained and G A C capacities must be in agreement irrespective of the experimental procedure adopted. However, many researchers reported that such was not always the case (Crittenden and Weber, 1978; van Vliet et al., 1980; Yonge et aL, 1985; Liu and Weber, 1981; Reschke et al., 1986). The following explanations for this discrepancy were suggested:
*Author to whom all correspondence should be addressed. 1187
(1) continuously decreasing liquid phase adsorbate concentration during the equilibration period (Yonge et al., 1985)
1188
R.D. VIDICet al. (2) irreversible adsorption (Yonge et al., 1985) (3) decline in intraparticle diffusion rate during the later part o f breakthrough, as saturation is approached (van Vliet et al., 1980) (4) difference in diffusion processes into macropores and micropores (Peel and Bcnedek, 1981; Seidel and Gelbin, 1986; Seidel et al., 1985).
There has been no systematic study designed to elucidate the effects o f the presence of molecular oxygen on adsorption equilibrium. It has been shown that oxygen adsorbs to a significant extent on G A C ( 1 0 - 4 0 m g / g ) thus increases acidic surface oxides (Prober et al., 1975). This is usually manifested as an increased base sorption capacity, particularly in the p H ranges for the carboxylic acid groups. Coughlin and Ezra (1968) found that oxidation of the surface of an active carbon, that results in increased quantities of acidic oxygen, lowered the capacity of the carbon for adsorption of phenol and nitrobenzene. Magne and Walker (1986) showed that chemisorption of oxygen on activated carbons at 573 K prior to phenol adsorption reduces the initial capacity of the carbon for the retention of phenol. This study was undertaken in order to provide more insight relative to the effects of the presence o f molecular oxygen on the adsorptive capacity o f GAC. EXPERIMENTAL PROCEDURES
The activated carbon used in this study was Filtrasorb400 granular activated carbon supplied by the manufacturer (Calgon Carbon, Pittsburgh, Pa) in a 12 x 40 U.S. Mesh size. To produce more uniform size ranges, carbon was sieved to the 12 x 14, 16 x 20 and 20 x 30 U.S. Mesh sizes having geometric mean particle diameters of 0.154, 0.100 and 0.071 era, respectively. Prior to use, carbon was washed several times with deionized water until most of the fines were removed. It was then dried in an oven at 110°C for 2 days and stored in a desiccator until use. All solutions were prepared with deionized water buffered at pH 7.0 with a 0.1 M phosphate buffer. The sorbates used in this research were phenol, o-cresol and 3-ethylphenol obtained in their highest availa01e purity from Aldrich Chemical Company. They were selected for this study because they represent commonly encountered pollutants as well as being readily analyzed using u.v. spectroscopy. Measurements of the concentrations of adsorbates were performed on a Perkin-Elmer Lambda 3B UV/VIS spectrophotometer using I and 5 cm quartz cells for the higher and lower concentration ranges, respectively. Preliminary scans were utilized to determine the wavelength for maximum absorbance of each adsorbate. Wavelengths of 268 nm for phenol, 269 nm for o-cresol, and 270 nm for 3-cthylphenol were used to determine the solution concentration together with periodic scans in the range of 230-300 nm for control purposes. Those scans were in all cases identical with scans of standard solutions that were prepared for calibration of the instrument. lsotherm test Two different procedures, denoted henceforth as "'aerobic" and "anaerobic", were developed in this research for determining the adsorptive capacity of GAC. The aerobic procedure is similar to other procedures that are most commonly used in current research practice. Accurately weighed masses of GAC (+0.1 mg) are placed in
160 ml glass bottles and purged with air for a short period of time. Bottles are then f-died with 100 ml of adsorbate solution and sealed with rubber stoppers and aluminum caps. The described procedure facilitates interference of mol~ular oxygen arising from three sources: air entrapped within the pores of GAC particles, the D.O. present in the water used in preparing the solutions and the headspace in the bottles. As discussed in later sections of the text, this procedure will lead to a significant increase in the capacity of GAC for adsorbing the compounds used in this study. On the other hand, the main feature of the anaerobic procedure is a requirement for the exclusion of the interference of molecular oxygen. After placing predetermined masses of GAC in the isotherm bottles, oxygen associated with air entrapped inside the carbon pores and reversibly adsorbed oxygen is removed by purging with nitrogen gas (high purity grade from Union Carbide Corp., Linde Div.). To ensure maximum displacement of the entrapped oxygen, purging is repeated every 12 h for a period of 3 days. There still can be some oxygen that was irreversibly adsorbed on the carbon surface during carbon preparation that will not be removed by this procedure. However, the influence of that chemisorbed oxygen is not evaluated in this study. This study was designed to determine the influence of molecular oxygen that is part of the air associated with GAC particules and the headspace in the isotherm bottles or comes into the system as dissolved in the adsorbatc solution. Oxygen dissolved in the water used in preparing the adsorbate solution is elimented by stripping with nitrogen gas prior to addition of the buffering solution and the adsorbate. These bottles are also filled with I00 ml of adsorbate solution and scaled with rubber stoppers and aluminum caps after the air from the hcadspace was replaced by nitrogen gas. Isotherm bottles are then placed on a shaker in a constant temperature room controlled at 35°C and allowed to equilibrate for 2 weeks. Each set of bottles is accompanied by two blanks without carbon. To prevent interference of carbon fines, all samples are filtered through 0.45/~m nylon filters (Micron Separations Inc.) prior to analysis. It was established earlier that no significant adsorption of phenol, o-cresol or 3ethylphenol takes place on the filters. Column studies All column studies were performed in a constant temperature room controlled at 35°C. Breakthrough curves for o-cresol were obtained using 1-in. (2.54crn) i.d. glass columns packed with 50, 100 and 200g of 12 x 14U.S. Mesh size carbon. All the experiments were performed with a concentration of o-cresol in the feed solution of approx. 200 mg/l and the columns were operated in an up-flow mode at a flow rate of 100 ml/min. The dissolved oxygen concentration in the feed averaged 4mg/I. The breakthrough experiment for 3--ethylphenol was conducted using a 1.75 em i.d glass column filled with 10.97 grams of 20 x 30 U.S. Mesh size GAC. This column was also operated in an up-flow mode at a flow rate of 144 ml/min. The feed solution contained 105 mg/l of 3-ethylphenol. Prior to charging the columns, all carbon used in the column tests was presoaked in deionized water for 2 days at 35°C to replace entrapped air from the carbon pores. This caused no significant carbon pre-loading since the dissolved organic carbon (DOC) of that water was less than 0.2 mg/l. RESULTS AND DISCUSSION
Single-solute adsorption isotherms for phenol, ocresol and 3-ethylphenol were obtained at 35°C under the experimental conditions listed in Table 1. All the isotherm data for phenol were obtained from 2 isotherm tests operated under aerobic conditions and 1 isotherm test operated under anaerobic conditions. These data are all presented in Fig. 1.
Adsorption isotherms Table 1. Experimentalisotherm conditions Initial Panicle Isotherm Adsorbate cone. size type Phenol
1000 1000 1000 1000 200 1000 200 50 1000 1000 1000 200
o-Cresol
3-Ethylphenol
(2 (2 (5 (3
12 12 16 16 16 16 16 16 12 20 20 20
tests) tests) tests) tests)
x x x x x x x x x x x x
14 14 20 20 20 20 20 20 14 30 30 30
Aerobic Aerobic Anaerobic Aerobic Aerobic Anaerobic Anaerobic Anaerobic Aerobic Aerobic Anaerobic Anaerobic
The data from 4 aerobic and 9 anaerobic isotherm runs for o-cresol are shown in Fig. 2. Anaerobic and aerobic isotherms obtained for 3-ethylphenol are presented in Fig. 3. All six groups of isotherms were found to be well described by the Freundlich isotherm (Freundlich, 1906). The two parameters, K and 1/n, for the Freundlich equation q, = K*C~/", obtained by nonlinear least-square regression analysis, are listed in Table 2. Also listed in the same table are the standard asymptotic error and 95% confidence interval for the Freundlich parameters. The various isotherms revealed no detectable influence of the different initial adsorbate concentrations or the different particle sizes on the equilibrium relationship. This observation was also noted earlier by other researchers (Peel and Benedek, 1980; Yonge et al., 1985; Zogorski et al., 1976) and was accepted as an indirect proof that an equilibration period of 2 weeks was sufficient for the achievement of true adsorption equilibria. Additional proof was obtained
1189
from isotherm tests where equilibration periods of three and four weeks were allowed. Since there was no additional measureable removal of o-cresol from the liquid phase during the third and fourth week of these experiments, the remaining absorption isotherms were run for a period of 2 weeks. Kinetic experiments described later in the text also showed that a period of 2 weeks was sufficient for equilibrium to prevail. Most adsorption isotherms reported in the literature were obtained under aerobic conditions. Oxygen that is entrapped in carbon pores is present in all experimental procedures. In some of the procedures the headspace left after filling the bottle with adsorbate solution becomes another source of oxygen. Procedures like stepwise addition of a higher concentration of adsorbate in the solution (Yonge et al., 1985) and concentration progressions measurements (Peel and .Benedek, 1980) provide periodic exposure of a sample to an additional infusion of oxygen. Figures l, 2 and 3 indicate that a much higher G A C adsorptive capacity for the compounds used in this study is realized under., aerobic conditions. The increase in the G A C adsorptive capacity for o-cresol in the presence of molecular oxygen was around 22% for an equilibrium concentration of 1000 mg/I. On the other hand, due to the fact that the aerobic isotherm had a lower value of the coefficient I/n in the Freundlich equation, this difference becomes much more pronounced in the lower concentration ranges where, for an equilibrium o-cresol concentration of 1 rag/! the aerobic isotherm yields a capacity that is almost 200% over that attainable under anaerobic conditions. The same trend is observed for the
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Fig. 2. Adsorption isotherms for o-cresol. 3-cthylphcnol isotherms where the increase in adsorption capacity under aerobic conditions ranges from 18 to 9 2 % for equilibrium 3-ethylphcnol conccntrations of I000 and l rag/l, respectively. This behavior was also observed in the case of the phenol isotherms where the aerobic isotherm depicts capacities approx. 42--85% higher than those obtained in the absence of molecular oxygen.
10
The possibility that biological degradation under aerobic conditions was responsible for the higher observed capacities was negated by showing that dissolved organic carbon (DOC) measurements conducted on liquid samples agreed very well with the theoretical DOC computed for the concentration of o-cresol measured by u.v. spectroscopy. Companion plate count studies conducted by Calgon Carbon
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Adsorption isotherms Table 2. Freundlich isotherm equation parameters Compound
Isotherm type
Phenol
Aerobic Anaerobic
o-Cresol
Aerobic Anaerobic
3-Ethylphenol
Aerobic Anaerobic
K*
1/n *
74.02 _+ 1.99 (69.96-78.09) 40.03 + 1.24 (37.34-42.72) 237.4 +_ 5.5 (226.3-248.5) 81.64 _+ 1.84 (77.96-85.31) 210.90 _+ 8.96 (191.2-230.6) 106.8 _+ 2.6 (101.2-112.3)
0.201 + 0.005 (0.191-0.210) 0.239 _+0.006 (0.226-0.251) 0.074 _+0.005 (0.064--0.084) 0.200 _+0.005 (0.190-0.210) 0.113 + 0.008 (0.095-0.131) 0.189 + 0.005 (0.179--0.200)
*95% Confidence interval is given in parentheses.
(Pittsburgh, Pa) on samples of GAC equilibrated by the authors under aerobic and anaerobic conditions with o-cresol, revealed the absence of o-cresol biodegrading organisms. This provided an independent confirmation that the mechanism under investigation was chemical and not biological. Excellent agreement was obtained between spectroscopic scans conducted in the range of 230-300 nm at the beginning and the end of equilibrium period. This provided additional proof that negates the role of biological activity on the observed phenomenon. Furthermore, the concentration of dissolved inorganic carbon did not increase during the isotherm runs indicating the lack of production of carbon dioxide as a result of biological activity. This does not exclude an increase in the GAC adsorptive capacity as a result of a chemical reaction that can take place on the carbon surface in the presence of molecular oxygen. The possibility of the occurrence of such a chemical reaction that can lead to the increased adsorptive capacity of GAC for o-cresol (above the capacity predicted by the anaerobic isotherm) became apparent from the results of kinetic experiments. Two batch tests were performed using the same mass of GAC, the same particle size, the same initial concentration of o-cresol, and the same mixing conditions. One of the experiments was conducted under aerobic conditions and the other one under anaerobic conditions. The rates at which o-cresol was removed from solution were identical in both experiments during the first 10h. After that, liquid phase concentration of o-cresol in the anaerobic batch test leveled off at the value predicted by the anaerobic isotherm. On the other hand, the concentration of o-cresol measured in the aerobic test continued to drop at a reduced rate until it leveled off, after an additional period of l0 days, at the value predicted by the aerobic isotherm. Since it was already established that biological degradation of o-cresol under aerobic conditions can not be responsible for the observed additional removal of o-cresol, it was assumed that a chemical transformation was responsible for the observed behavior. Several breakthrough profiles from GAC adsorbers were obtained under the conditions listed in Table 3. Breakthrough curves for o-cresol and 3ethylphenol are given in Figs 4 and 5, respectively.
I 191
The data in Fig. 6 represent a plot of the breakthrough curves for o-cresol normalized to the masses of GAC used in the respective columns. The breakthrough curves obtained for o-cresol were observed to have a characteristic tail. The adsorptive capacity of GAC that is tied up in the tail of the breakthrough curve is not negligible and represents a significant increase in the overall adsorptive capacity of a GAC adsorber. The fact that the effluent concentration does not reach the influent concentration during the time of observation was previously reported by Liu and Weber (1981), Peel and Benedek (1981) and Seidel and Gelbin (1986). These authors, however, did not make any attempts to further examine this phenomenon. Most modeling of GAC adsorbers was aimed at predicting the time of breakthrough and, consequently, little attention was paid to the latter section of the breakthrough profile. Few attempts were made to understand the observed tailing. This behavior was attributed to a decrease m the intraparticle diffusion rate during the latter part of breakthrough (van Vliet et al., 1980) or by associating part of.the GAC capacity with small and more difficult to reach pores (Peel and Benedek, 1981; Scidel and Gelbin, 1985). In light of the discoveries made in this study, it is proposed that the observed tailing can be attributed to a chemical reaction between the adsorbate and molecular oxygen that is catalyzed by the carbon surface. This chemical reaction appears to occur at a different (slower) time scale from physical adsorption. Since there were no changes in the concentration of adsorbate in the blanks that accompanied every isotherm test, it can be concluded that the presence of carbon is necessary to catalyze these reactions. As a result, the adsorption process is modified under aerobic conditions and an increase in the adsorptive capacity of GAC is observed. Early portions of the breakthrough curves for both compounds are very accurately predicted by the anaerobic isotherms. This becomes apparent when the GAC adsorptive capacities, as calculated from a square wave passing through the 50% breakthrough point, are compared with those capacities predicted from the anaerobic isotherm (see Table 4). Also included in Table 4 are the total GAC adsorptive capacities calculated for each column experiment by integrating over the entire breakthrough curves. The adsorptive capacities of GAC under aerobic conditions (calculated from the aerobic isotherms) corresponding to each column influent concentrations are also given in Table 4.
Table 3. Experimental conditions for column runs Run number 1 2 3 4
Compound o-Cresol o-Cresol o-Cresol 3-Ethylphenol
Average feed cone. (rag/l)
Carbon mass (g)
Particle size
218.5 214.8 214.9 105.0
200 100 50 10.97
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values given in Table 4, it is evident that a considerable amount of the capacity is retained in the tail portion of breakthrough curves. Insufficient time of observation or very short column runs are usually responsible for tailing not to be observed. This is demonstrated very well by the data from the breakthrough experiment with 3-ethylphenol (run number 4). In this instance, the total GAC adsorptive capacity obtained by integrating over the entire breakthrough curve agrees closely with the capacity given by the anaerobic isotherm. Since the total time of the experiment is only around 10 h the presence of molecular oxygen did not exert any significant influence on the adsorption process. Both total capacity and the capacity base on the square wave passing through the 50% breakthrough point are accurately predicted by the anaerobic isotherm (see Table 4).
Since 50% breakthrough in most column experiments occurs within a day or two from the start of an experiment, the influence of molecular oxygen is not very pronounced by that time. Therefore, there is excellent agreement between the capacities obtained from square waves passing through the 50% breakthrough points and values given by the anaerobic isotherms. On the other hand, total GAC adsorptive capacities for o-cresol calculated from the entire breakthough curves are somewhat lower than those predicted by the aerobic isotherms. This is due to the fact that complete breakthrough in all three column experiments was not achieved during the time of observation. If better agreement is desired, breakthrough experiments should be monitored for the same period of time that was allowed for equilibration to be reached in the isotherm tests. From the 1.0
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Adsorption isotherms Table 4. G A C adsorptive capacity
Run number I 2 3 4
be parallel to the ones determined experimentally. The anaerobic isotherms given in Figs 2 and 3 that accurately predict the initial portions of breakthrough curves arc not parallel to the isotherms obtained using standard procedure, i.c.aerobic conditions. Consequently, the use of a constant correction factor applied to the aerobic isotherm will not always provide an accurate measure for column capacity. Peel and Bcnedek (1981) showed that models that describe adsorption kinetics using one film diffusion coc~cient and one diffusivity within the GAC particle, predicted column breakthroughs that are delayed when compared to measured ones. They suggested that the adsorption process can be described as being controlled by two diffusive processes that occur simultaneously: rapid diffusion of adsorbates in the macroporc region that accounts for approx. 70% of total aerobic isotherm capacity and is dominant during the early period of breakthrough, and a slower diffusion rate that takes place in the region of micropores and becomes important during the latter part of breakthrough. Peel and Bcnedek (1981) concluded that the distribution of macropores and micropores as determined from rate studies depends on the initial concentration of adsorbatc. This observed dependency on initial concentration has no physical interpretation, but was necessary to obtain hettcr agreement of the model pre.dictions with experimental values. Keeping in mind the results of this research, a more justifiable approach might be to use the anaerobic isotherm and to include additional terms that would account for the chemical reaction that is stimulated in the presence of oxygen. This approach is, unfortunately, still not feasible and more work is needed for understanding the nature of the factors that influence the reaction and to determine parameters that adequately characterize this behavior. Different compounds and different carbons may
Capacity based Isotherm capacity on the 50% Total breakthrough capacity Anaerobic Aerobic (mg/g) (rag/g) (rag/g) (rag/g) 229.4 236.2 219.2 255.0
296.9 341.5 310.8 270.1
236.7 235.9 235.9 257.9
353.6 353.1 353.1 356.8
The GAC capacity for o-cresol in the three column runs varied between 84 and 97% of the capacities determined from the aerobic isotherms. It is interesting to note that when the three breakthrough curves were normalized to the mass of GAC in the appropriate column (see Fig. 6), all three curves showed identical 50% breakthrough points and the tail portions of the curves collapsed together. The data in Fig. 6 indicate that the early portions of the breakthrough profiles become sharper with increasing carbon mass. This, in reality, may be an artifact of the normalization procedure since such differences were not as pronounced in the real time profiles shown in Fig. 4. Although, this difference in the early breakthrough profile may be due to the longer contact between GAC, o-cresol, and oxygen provided in the columns containing the larger masses of carbon, it is still premature to attribute this phenomenon to the effects of the presence of oxygen. The additional GAC adsorptive capacity observed in the presence of oxygen presents a major problem in modeling adsorption columns. Existing adsorption models used for the prediction of breakthrough patterns from a GAC reactor include certain assumptions required to achieve better agreement between measured and predicted values. Crittenden and Weber (1978) introduced the use of reduced isotherms which account for only a fraction of the aerobic adsorptive capacity. These "pseudo"-isotherms were assumed to 1.0
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be affected differently and this phenomenon should be investigated using a large number of organic compounds. SUMMARY
A new experimental procedure for obtaining adsorption equilibrium data is introduced in this work. The new procedure emerged as a result of observations that indicated that the presence of molecular oxygen had a strong influence on the adsorptive capacity of GAC for some organic compounds. This procedure, denoted as "anaerobic", differs from currently used methods, denoted as "aerobic", in that it recommends the elimination or minimization of interferences of molecular oxygen with the adsorption process. The GAC adsorption capacity for the compounds used in this study increases appreciably in the presence of molecular oxygen. The GAC capacity for the retention of o-cresol, for example, increased as much as 3-fold for an equilibrium adsorbate concentration of 1 mg/l. Due to the difference in slopes of adsorption isotherms obtained under aerobic and anaerobic conditions, this increase in capacity is not as pronounced for an equilibrium o-cresol concentration of 1000 mg/l where the aerobic adsorptive capacity is only 22% greater than the capacity under anaerobic conditions. The same phenomenon is observed for phenol and 3-ethylphenoi. The aerobic GAC adsorptive capacity for phenol and 3-ethyiphenoi exceeds the anaerobic one in the range of 42-85 and 18-92% for the equilibrium adsorbate concentrations of 1000 and 1 mg/l, respectively. The possibility that biological degradation under aerobic conditions was responsible for the higher observed capacities was discounted because of the following facts: (I) The DOC measurements conducted on liquid samples from both procedures agreed very closely with the theoretical DOC computed for the concentration of o-cresol measured by u.v. analysis. (2) Biological analysis of the samples from both aerobic and anaerobic isotherm tests revealed the absence of any microorganisms capable of degrading o-cresol aerobically or anaerobically. (3) Spectroscopic scans, in the range of 230300 nm, of liquid samples obtained from both procedures conducted at the beginning and end of an adsorption isotherm run compared very well with the scans determined for pure o-cresol solutions. (4) No increase in the concentration of inorganic carbon was observed during isotherm tests. The phenomenon advanced in this study appears to be due to a chemical reactions between the adsorbate and molecular oxygen that is catalyzed by the carbon surface.
The importance of the new, anaerobic, isotherm discovery is fully emphasized when the breakthrough curves for o-cresol and 3-ethylphenol are examined. The early portions of the breakthrough curves for o-cresol are very accurately predicted by the anaerobic isotherm while the total capacity obtained by integrating over the entire breakthrough curve compares well with the capacity given by the aerobic isotherm. On the other hand, when the time permitted for the occurrence of initial breakthrough was short, the chemical reactions that involve molecular oxygen do not appear to exert a significant impact on breakthrough. As a result, both the early portion of a breakthrough curve and the total GAC adsorptive capacity are well predicted by the anaerobic isotherm. This was evident with the breakthrough experiment for 3-ethylphenol. REFERENCES
Coughlin R. W. and Ezra F. S. (1968) Role of surface acidity in the adsorption of organic pollutants on the surface of carbon. Envir. Sci. Technol. 2, 291-298. Crittenden J. C. and Weber W. J. Jr (1978) Predictive model for design of fixed bed adsorbers: parameter estimation and model development. J. envir. Engng Div., Am. Soc. cir. Engrs EE2, 185-197. Freundlich H. (1906) Ueber die Adsorption in Leosungen. Z. Phys. Chem. 57, 385-470. Liu K. T. and Weber W. J. Jr (1981) Characterization of
mass transfer parameters for adsorber modeling and design. J. Wat. Pollut. Control Fed. 53, 1541-1550. Magne P. and Walker P. L. Jr (1986) Phenol adsorption on activated carbon: application to the regeneration of activated carbons polluted with phenol. Carbon 24, I01-I07. Martin R. J. and AI-Bahrani K. S. (1978) Adsorption studies using gas-liquid chromatograpy--II. Experimental factors influencing adsorption. Wat. ICes.12, 879--885. Peel R. G. and Benedek A. (1980) Attainment of equilibrima in activated carbon isotherm studies. Envir. Sci. TechnoL 14, 66-71. Peel R. G. and Benedek A. (198 I) Dual rate kinetic model of fix bed adsorber. J. envir. Engng Div., Am. Soc. cir. Engrs EE4, 797-813. Prober R., Pyeha J. J. and Kidon W. E. (1975) Interaction of activated carbon with dissolved oxygen. A.LCh.E. Jl 21, 1200-1204. Randtke S. J. and Snoeyink V. L. (1983) Evaluating GAC adsorptive capacity. J. Am. Hat. Wks Ass. 75, 406-413. Reschke G., Radeke K. H. and Gelbin D. (1986) Breakthrough curves for single solutes in beds of activated carbon with a broad pore-size distribution--II. Measuring and calculating breakthrough curves of phenol and indole from aqueous solutions on activated carbon. Chem. Engng Sci. 41, 549-554. Seidel A. and Gelbin D. (1986) Breakthrough curves for single solutes in beds of activated carbon with a broad pore-size distribution--l. Mathematical models of breakthrough curves in beds of activated carbon. Chem. Engng Sci. 41, 541-548. Seidel A., Tzscheutschler E., Radeke K. H. and Gelbin D. (1985) Adsorption equilibria of aqueous phenol and indole solutions on activated carbon. Chem. Engng Sci. 40, 215-222. van VIiet B. M., Weber W. J. Jr and Hozumi H. (1980) Modeling and prediction of specific compound adsorption by activated carbon and synthetic adsorbents. Wat. Res. 14, 1719-1728.
Adsorption isotherms Weber W. J. Jr and Morris J. C. (1964) Equilibria and capacities for adsorption on carbon. J. sanit. Engng Div., Am. Soc. cit'. Engrs EE2, 185-197. Yonge D. R., Keinath T. M., Poznanska K. and Jiang Z. P. (1985) Single-solute irreversible adsorption on
1195
granular activated carbon. Envir. Sci. Technol. 19, 690-694. Zogorski J. S., Faust S. D. and Haas J. H. Jr (1976) The kinetics of adsorption of phenols by granular activated carbon. J. Colloid Interface Sci. 55, 329-341.
0043-1354/90 $3.00+0.00 Copyright ~, 1990 PergamonPress plc
A D S O R P T I O N ISOTHERMS: ILLUSIVE CAPACITY A N D ROLE OF O X Y G E N R. D. VIDIC ], M. T. SUIDANI'*(~, U. K. TRAEGNER 2 and G. F. NAKHLA 3 tDeparlment of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, U.S.A., 2BASF AG, ZET/EC, L544, 6700 Ludwigshafen, F.R.G. and 3Department of Civil Engineering, King Fahad University of Petroleum and Minerals, Dahran 31261, Kingdom of Saudi Arabia (First received October 1989; accepted in revised form February 1990)
Abstract--This paper examines the influence of molecular oxygen on the adsorptive capacity of GAC. A new experimental procedure for determining adsorption isotherms is introduced. This procedure, denoted as "anaerobic", differs from the currently used techniques, denoted as "aerobic", in that oxygen is repeatedly purged from the test environment. The results show that the capacity of GAC for the retention of o-cresol can increase up to 3-fold in the presence of.oxygen when compared to the anaerobic capacity. The same trend is observed for the adsorption of phenol and 3-ethyiphenol. It is shown that this increase in capacity cannot be attributed to biological degradation of these adsorbates in the presence of oxygen. It is speculated that this phenomenon is due to some chemical reactions between the adsorhates and molecular oxygen that are catalyzed by the activated carbon surface and occur at a different time scale than physical adsorption. Initial portions of breakthrough curves for o~resol are very accurately predicted using capacities depicted by the anaerobic isotherm, while the total GAC adsorptive capacity for o-cresol, as determined from breakthrough experiments, appears to agree closely with the capacity predicted from the aerobic isotherm. Key words--adsorption isotherms, adsorptive capacity, aromatics, molecular oxygen, breakthrough
prediction
INTRODUCTION Granular activated carbon (GAC) is widely used in both water and wastewater treatment as an adsorbent for the removal of organic pollutants. One of the key components needed for the scale-up of experimental data and for the prediction of adsorber performance is the capacity of G A C for the retention of organic compound. This capacity is often given by an adsorption isotherm which represents the equilibrium at constant temperature between the quantity of adsorbate retained per unit mass of adsorbent, qc, and the concentration of adsorbate in solution, Ce. Isotherm data are most commonly obtained using the, so called, bottle-point technique. A discussion of some of the pitfalls often encountered in conducting an isotherm test is given by Randtke and Snoeyink (1983). Unfortunately, a unified procedure for conducting this experiment has not yet been established. Most researchers employ varying experimental procedures for the collection of isotherm data. These procedures differ in carbon preparation, particle size (pulverized versus intact granular carbon), volume of the bottles used in the experiment, volume of adsorbate solution added, buffer application and equilibration time allowed. As a result, many different isotherms for the same compound and the same G A C
are given in literature. Peel and Benedek (1980) reported that particle size, the presence or absence of a weak buffer and the initial concentration of adsorbate, had no effect on the equilibrium relationship for phenol and o-chlorophenol. This conclusion was supported by data from several other investigators (Martin and AI-Bahrani, 1978; Randtke and Snoeyink, 1983). Contradictory findings regarding the effect of initial concentration on G A C capacity are also reported. Crittenden and Weber (1978) and van Vliet et al. (1980) found G A C capacities for phenol and p-bromophenol to be higher for lower initial adsorbate concentrations while Peel and Benedek (1980), Yonge et aL (1985) and Zogorski et al. (1976) did not observe such effects for the adsorption of phenol and o-cresol. Isotherm data can also be obtained using batch reactor and column methods. Theoretically, equilibrium data are independent of the way they are obtained and G A C capacities must be in agreement irrespective of the experimental procedure adopted. However, many researchers reported that such was not always the case (Crittenden and Weber, 1978; van Vliet et al., 1980; Yonge et aL, 1985; Liu and Weber, 1981; Reschke et al., 1986). The following explanations for this discrepancy were suggested:
*Author to whom all correspondence should be addressed. 1187
(1) continuously decreasing liquid phase adsorbate concentration during the equilibration period (Yonge et al., 1985)
1188
R.D. VIDICet al. (2) irreversible adsorption (Yonge et al., 1985) (3) decline in intraparticle diffusion rate during the later part o f breakthrough, as saturation is approached (van Vliet et al., 1980) (4) difference in diffusion processes into macropores and micropores (Peel and Bcnedek, 1981; Seidel and Gelbin, 1986; Seidel et al., 1985).
There has been no systematic study designed to elucidate the effects o f the presence of molecular oxygen on adsorption equilibrium. It has been shown that oxygen adsorbs to a significant extent on G A C ( 1 0 - 4 0 m g / g ) thus increases acidic surface oxides (Prober et al., 1975). This is usually manifested as an increased base sorption capacity, particularly in the p H ranges for the carboxylic acid groups. Coughlin and Ezra (1968) found that oxidation of the surface of an active carbon, that results in increased quantities of acidic oxygen, lowered the capacity of the carbon for adsorption of phenol and nitrobenzene. Magne and Walker (1986) showed that chemisorption of oxygen on activated carbons at 573 K prior to phenol adsorption reduces the initial capacity of the carbon for the retention of phenol. This study was undertaken in order to provide more insight relative to the effects of the presence o f molecular oxygen on the adsorptive capacity o f GAC. EXPERIMENTAL PROCEDURES
The activated carbon used in this study was Filtrasorb400 granular activated carbon supplied by the manufacturer (Calgon Carbon, Pittsburgh, Pa) in a 12 x 40 U.S. Mesh size. To produce more uniform size ranges, carbon was sieved to the 12 x 14, 16 x 20 and 20 x 30 U.S. Mesh sizes having geometric mean particle diameters of 0.154, 0.100 and 0.071 era, respectively. Prior to use, carbon was washed several times with deionized water until most of the fines were removed. It was then dried in an oven at 110°C for 2 days and stored in a desiccator until use. All solutions were prepared with deionized water buffered at pH 7.0 with a 0.1 M phosphate buffer. The sorbates used in this research were phenol, o-cresol and 3-ethylphenol obtained in their highest availa01e purity from Aldrich Chemical Company. They were selected for this study because they represent commonly encountered pollutants as well as being readily analyzed using u.v. spectroscopy. Measurements of the concentrations of adsorbates were performed on a Perkin-Elmer Lambda 3B UV/VIS spectrophotometer using I and 5 cm quartz cells for the higher and lower concentration ranges, respectively. Preliminary scans were utilized to determine the wavelength for maximum absorbance of each adsorbate. Wavelengths of 268 nm for phenol, 269 nm for o-cresol, and 270 nm for 3-cthylphenol were used to determine the solution concentration together with periodic scans in the range of 230-300 nm for control purposes. Those scans were in all cases identical with scans of standard solutions that were prepared for calibration of the instrument. lsotherm test Two different procedures, denoted henceforth as "'aerobic" and "anaerobic", were developed in this research for determining the adsorptive capacity of GAC. The aerobic procedure is similar to other procedures that are most commonly used in current research practice. Accurately weighed masses of GAC (+0.1 mg) are placed in
160 ml glass bottles and purged with air for a short period of time. Bottles are then f-died with 100 ml of adsorbate solution and sealed with rubber stoppers and aluminum caps. The described procedure facilitates interference of mol~ular oxygen arising from three sources: air entrapped within the pores of GAC particles, the D.O. present in the water used in preparing the solutions and the headspace in the bottles. As discussed in later sections of the text, this procedure will lead to a significant increase in the capacity of GAC for adsorbing the compounds used in this study. On the other hand, the main feature of the anaerobic procedure is a requirement for the exclusion of the interference of molecular oxygen. After placing predetermined masses of GAC in the isotherm bottles, oxygen associated with air entrapped inside the carbon pores and reversibly adsorbed oxygen is removed by purging with nitrogen gas (high purity grade from Union Carbide Corp., Linde Div.). To ensure maximum displacement of the entrapped oxygen, purging is repeated every 12 h for a period of 3 days. There still can be some oxygen that was irreversibly adsorbed on the carbon surface during carbon preparation that will not be removed by this procedure. However, the influence of that chemisorbed oxygen is not evaluated in this study. This study was designed to determine the influence of molecular oxygen that is part of the air associated with GAC particules and the headspace in the isotherm bottles or comes into the system as dissolved in the adsorbatc solution. Oxygen dissolved in the water used in preparing the adsorbate solution is elimented by stripping with nitrogen gas prior to addition of the buffering solution and the adsorbate. These bottles are also filled with I00 ml of adsorbate solution and scaled with rubber stoppers and aluminum caps after the air from the hcadspace was replaced by nitrogen gas. Isotherm bottles are then placed on a shaker in a constant temperature room controlled at 35°C and allowed to equilibrate for 2 weeks. Each set of bottles is accompanied by two blanks without carbon. To prevent interference of carbon fines, all samples are filtered through 0.45/~m nylon filters (Micron Separations Inc.) prior to analysis. It was established earlier that no significant adsorption of phenol, o-cresol or 3ethylphenol takes place on the filters. Column studies All column studies were performed in a constant temperature room controlled at 35°C. Breakthrough curves for o-cresol were obtained using 1-in. (2.54crn) i.d. glass columns packed with 50, 100 and 200g of 12 x 14U.S. Mesh size carbon. All the experiments were performed with a concentration of o-cresol in the feed solution of approx. 200 mg/l and the columns were operated in an up-flow mode at a flow rate of 100 ml/min. The dissolved oxygen concentration in the feed averaged 4mg/I. The breakthrough experiment for 3--ethylphenol was conducted using a 1.75 em i.d glass column filled with 10.97 grams of 20 x 30 U.S. Mesh size GAC. This column was also operated in an up-flow mode at a flow rate of 144 ml/min. The feed solution contained 105 mg/l of 3-ethylphenol. Prior to charging the columns, all carbon used in the column tests was presoaked in deionized water for 2 days at 35°C to replace entrapped air from the carbon pores. This caused no significant carbon pre-loading since the dissolved organic carbon (DOC) of that water was less than 0.2 mg/l. RESULTS AND DISCUSSION
Single-solute adsorption isotherms for phenol, ocresol and 3-ethylphenol were obtained at 35°C under the experimental conditions listed in Table 1. All the isotherm data for phenol were obtained from 2 isotherm tests operated under aerobic conditions and 1 isotherm test operated under anaerobic conditions. These data are all presented in Fig. 1.
Adsorption isotherms Table 1. Experimentalisotherm conditions Initial Panicle Isotherm Adsorbate cone. size type Phenol
1000 1000 1000 1000 200 1000 200 50 1000 1000 1000 200
o-Cresol
3-Ethylphenol
(2 (2 (5 (3
12 12 16 16 16 16 16 16 12 20 20 20
tests) tests) tests) tests)
x x x x x x x x x x x x
14 14 20 20 20 20 20 20 14 30 30 30
Aerobic Aerobic Anaerobic Aerobic Aerobic Anaerobic Anaerobic Anaerobic Aerobic Aerobic Anaerobic Anaerobic
The data from 4 aerobic and 9 anaerobic isotherm runs for o-cresol are shown in Fig. 2. Anaerobic and aerobic isotherms obtained for 3-ethylphenol are presented in Fig. 3. All six groups of isotherms were found to be well described by the Freundlich isotherm (Freundlich, 1906). The two parameters, K and 1/n, for the Freundlich equation q, = K*C~/", obtained by nonlinear least-square regression analysis, are listed in Table 2. Also listed in the same table are the standard asymptotic error and 95% confidence interval for the Freundlich parameters. The various isotherms revealed no detectable influence of the different initial adsorbate concentrations or the different particle sizes on the equilibrium relationship. This observation was also noted earlier by other researchers (Peel and Benedek, 1980; Yonge et al., 1985; Zogorski et al., 1976) and was accepted as an indirect proof that an equilibration period of 2 weeks was sufficient for the achievement of true adsorption equilibria. Additional proof was obtained
1189
from isotherm tests where equilibration periods of three and four weeks were allowed. Since there was no additional measureable removal of o-cresol from the liquid phase during the third and fourth week of these experiments, the remaining absorption isotherms were run for a period of 2 weeks. Kinetic experiments described later in the text also showed that a period of 2 weeks was sufficient for equilibrium to prevail. Most adsorption isotherms reported in the literature were obtained under aerobic conditions. Oxygen that is entrapped in carbon pores is present in all experimental procedures. In some of the procedures the headspace left after filling the bottle with adsorbate solution becomes another source of oxygen. Procedures like stepwise addition of a higher concentration of adsorbate in the solution (Yonge et al., 1985) and concentration progressions measurements (Peel and .Benedek, 1980) provide periodic exposure of a sample to an additional infusion of oxygen. Figures l, 2 and 3 indicate that a much higher G A C adsorptive capacity for the compounds used in this study is realized under., aerobic conditions. The increase in the G A C adsorptive capacity for o-cresol in the presence of molecular oxygen was around 22% for an equilibrium concentration of 1000 mg/I. On the other hand, due to the fact that the aerobic isotherm had a lower value of the coefficient I/n in the Freundlich equation, this difference becomes much more pronounced in the lower concentration ranges where, for an equilibrium o-cresol concentration of 1 rag/! the aerobic isotherm yields a capacity that is almost 200% over that attainable under anaerobic conditions. The same trend is observed for the
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10
The possibility that biological degradation under aerobic conditions was responsible for the higher observed capacities was negated by showing that dissolved organic carbon (DOC) measurements conducted on liquid samples agreed very well with the theoretical DOC computed for the concentration of o-cresol measured by u.v. spectroscopy. Companion plate count studies conducted by Calgon Carbon
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Adsorption isotherms Table 2. Freundlich isotherm equation parameters Compound
Isotherm type
Phenol
Aerobic Anaerobic
o-Cresol
Aerobic Anaerobic
3-Ethylphenol
Aerobic Anaerobic
K*
1/n *
74.02 _+ 1.99 (69.96-78.09) 40.03 + 1.24 (37.34-42.72) 237.4 +_ 5.5 (226.3-248.5) 81.64 _+ 1.84 (77.96-85.31) 210.90 _+ 8.96 (191.2-230.6) 106.8 _+ 2.6 (101.2-112.3)
0.201 + 0.005 (0.191-0.210) 0.239 _+0.006 (0.226-0.251) 0.074 _+0.005 (0.064--0.084) 0.200 _+0.005 (0.190-0.210) 0.113 + 0.008 (0.095-0.131) 0.189 + 0.005 (0.179--0.200)
*95% Confidence interval is given in parentheses.
(Pittsburgh, Pa) on samples of GAC equilibrated by the authors under aerobic and anaerobic conditions with o-cresol, revealed the absence of o-cresol biodegrading organisms. This provided an independent confirmation that the mechanism under investigation was chemical and not biological. Excellent agreement was obtained between spectroscopic scans conducted in the range of 230-300 nm at the beginning and the end of equilibrium period. This provided additional proof that negates the role of biological activity on the observed phenomenon. Furthermore, the concentration of dissolved inorganic carbon did not increase during the isotherm runs indicating the lack of production of carbon dioxide as a result of biological activity. This does not exclude an increase in the GAC adsorptive capacity as a result of a chemical reaction that can take place on the carbon surface in the presence of molecular oxygen. The possibility of the occurrence of such a chemical reaction that can lead to the increased adsorptive capacity of GAC for o-cresol (above the capacity predicted by the anaerobic isotherm) became apparent from the results of kinetic experiments. Two batch tests were performed using the same mass of GAC, the same particle size, the same initial concentration of o-cresol, and the same mixing conditions. One of the experiments was conducted under aerobic conditions and the other one under anaerobic conditions. The rates at which o-cresol was removed from solution were identical in both experiments during the first 10h. After that, liquid phase concentration of o-cresol in the anaerobic batch test leveled off at the value predicted by the anaerobic isotherm. On the other hand, the concentration of o-cresol measured in the aerobic test continued to drop at a reduced rate until it leveled off, after an additional period of l0 days, at the value predicted by the aerobic isotherm. Since it was already established that biological degradation of o-cresol under aerobic conditions can not be responsible for the observed additional removal of o-cresol, it was assumed that a chemical transformation was responsible for the observed behavior. Several breakthrough profiles from GAC adsorbers were obtained under the conditions listed in Table 3. Breakthrough curves for o-cresol and 3ethylphenol are given in Figs 4 and 5, respectively.
I 191
The data in Fig. 6 represent a plot of the breakthrough curves for o-cresol normalized to the masses of GAC used in the respective columns. The breakthrough curves obtained for o-cresol were observed to have a characteristic tail. The adsorptive capacity of GAC that is tied up in the tail of the breakthrough curve is not negligible and represents a significant increase in the overall adsorptive capacity of a GAC adsorber. The fact that the effluent concentration does not reach the influent concentration during the time of observation was previously reported by Liu and Weber (1981), Peel and Benedek (1981) and Seidel and Gelbin (1986). These authors, however, did not make any attempts to further examine this phenomenon. Most modeling of GAC adsorbers was aimed at predicting the time of breakthrough and, consequently, little attention was paid to the latter section of the breakthrough profile. Few attempts were made to understand the observed tailing. This behavior was attributed to a decrease m the intraparticle diffusion rate during the latter part of breakthrough (van Vliet et al., 1980) or by associating part of.the GAC capacity with small and more difficult to reach pores (Peel and Benedek, 1981; Scidel and Gelbin, 1985). In light of the discoveries made in this study, it is proposed that the observed tailing can be attributed to a chemical reaction between the adsorbate and molecular oxygen that is catalyzed by the carbon surface. This chemical reaction appears to occur at a different (slower) time scale from physical adsorption. Since there were no changes in the concentration of adsorbate in the blanks that accompanied every isotherm test, it can be concluded that the presence of carbon is necessary to catalyze these reactions. As a result, the adsorption process is modified under aerobic conditions and an increase in the adsorptive capacity of GAC is observed. Early portions of the breakthrough curves for both compounds are very accurately predicted by the anaerobic isotherms. This becomes apparent when the GAC adsorptive capacities, as calculated from a square wave passing through the 50% breakthrough point, are compared with those capacities predicted from the anaerobic isotherm (see Table 4). Also included in Table 4 are the total GAC adsorptive capacities calculated for each column experiment by integrating over the entire breakthrough curves. The adsorptive capacities of GAC under aerobic conditions (calculated from the aerobic isotherms) corresponding to each column influent concentrations are also given in Table 4.
Table 3. Experimental conditions for column runs Run number 1 2 3 4
Compound o-Cresol o-Cresol o-Cresol 3-Ethylphenol
Average feed cone. (rag/l)
Carbon mass (g)
Particle size
218.5 214.8 214.9 105.0
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values given in Table 4, it is evident that a considerable amount of the capacity is retained in the tail portion of breakthrough curves. Insufficient time of observation or very short column runs are usually responsible for tailing not to be observed. This is demonstrated very well by the data from the breakthrough experiment with 3-ethylphenol (run number 4). In this instance, the total GAC adsorptive capacity obtained by integrating over the entire breakthrough curve agrees closely with the capacity given by the anaerobic isotherm. Since the total time of the experiment is only around 10 h the presence of molecular oxygen did not exert any significant influence on the adsorption process. Both total capacity and the capacity base on the square wave passing through the 50% breakthrough point are accurately predicted by the anaerobic isotherm (see Table 4).
Since 50% breakthrough in most column experiments occurs within a day or two from the start of an experiment, the influence of molecular oxygen is not very pronounced by that time. Therefore, there is excellent agreement between the capacities obtained from square waves passing through the 50% breakthrough points and values given by the anaerobic isotherms. On the other hand, total GAC adsorptive capacities for o-cresol calculated from the entire breakthough curves are somewhat lower than those predicted by the aerobic isotherms. This is due to the fact that complete breakthrough in all three column experiments was not achieved during the time of observation. If better agreement is desired, breakthrough experiments should be monitored for the same period of time that was allowed for equilibration to be reached in the isotherm tests. From the 1.0
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be parallel to the ones determined experimentally. The anaerobic isotherms given in Figs 2 and 3 that accurately predict the initial portions of breakthrough curves arc not parallel to the isotherms obtained using standard procedure, i.c.aerobic conditions. Consequently, the use of a constant correction factor applied to the aerobic isotherm will not always provide an accurate measure for column capacity. Peel and Bcnedek (1981) showed that models that describe adsorption kinetics using one film diffusion coc~cient and one diffusivity within the GAC particle, predicted column breakthroughs that are delayed when compared to measured ones. They suggested that the adsorption process can be described as being controlled by two diffusive processes that occur simultaneously: rapid diffusion of adsorbates in the macroporc region that accounts for approx. 70% of total aerobic isotherm capacity and is dominant during the early period of breakthrough, and a slower diffusion rate that takes place in the region of micropores and becomes important during the latter part of breakthrough. Peel and Bcnedek (1981) concluded that the distribution of macropores and micropores as determined from rate studies depends on the initial concentration of adsorbatc. This observed dependency on initial concentration has no physical interpretation, but was necessary to obtain hettcr agreement of the model pre.dictions with experimental values. Keeping in mind the results of this research, a more justifiable approach might be to use the anaerobic isotherm and to include additional terms that would account for the chemical reaction that is stimulated in the presence of oxygen. This approach is, unfortunately, still not feasible and more work is needed for understanding the nature of the factors that influence the reaction and to determine parameters that adequately characterize this behavior. Different compounds and different carbons may
Capacity based Isotherm capacity on the 50% Total breakthrough capacity Anaerobic Aerobic (mg/g) (rag/g) (rag/g) (rag/g) 229.4 236.2 219.2 255.0
296.9 341.5 310.8 270.1
236.7 235.9 235.9 257.9
353.6 353.1 353.1 356.8
The GAC capacity for o-cresol in the three column runs varied between 84 and 97% of the capacities determined from the aerobic isotherms. It is interesting to note that when the three breakthrough curves were normalized to the mass of GAC in the appropriate column (see Fig. 6), all three curves showed identical 50% breakthrough points and the tail portions of the curves collapsed together. The data in Fig. 6 indicate that the early portions of the breakthrough profiles become sharper with increasing carbon mass. This, in reality, may be an artifact of the normalization procedure since such differences were not as pronounced in the real time profiles shown in Fig. 4. Although, this difference in the early breakthrough profile may be due to the longer contact between GAC, o-cresol, and oxygen provided in the columns containing the larger masses of carbon, it is still premature to attribute this phenomenon to the effects of the presence of oxygen. The additional GAC adsorptive capacity observed in the presence of oxygen presents a major problem in modeling adsorption columns. Existing adsorption models used for the prediction of breakthrough patterns from a GAC reactor include certain assumptions required to achieve better agreement between measured and predicted values. Crittenden and Weber (1978) introduced the use of reduced isotherms which account for only a fraction of the aerobic adsorptive capacity. These "pseudo"-isotherms were assumed to 1.0
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R.D. VIDICet al.
be affected differently and this phenomenon should be investigated using a large number of organic compounds. SUMMARY
A new experimental procedure for obtaining adsorption equilibrium data is introduced in this work. The new procedure emerged as a result of observations that indicated that the presence of molecular oxygen had a strong influence on the adsorptive capacity of GAC for some organic compounds. This procedure, denoted as "anaerobic", differs from currently used methods, denoted as "aerobic", in that it recommends the elimination or minimization of interferences of molecular oxygen with the adsorption process. The GAC adsorption capacity for the compounds used in this study increases appreciably in the presence of molecular oxygen. The GAC capacity for the retention of o-cresol, for example, increased as much as 3-fold for an equilibrium adsorbate concentration of 1 mg/l. Due to the difference in slopes of adsorption isotherms obtained under aerobic and anaerobic conditions, this increase in capacity is not as pronounced for an equilibrium o-cresol concentration of 1000 mg/l where the aerobic adsorptive capacity is only 22% greater than the capacity under anaerobic conditions. The same phenomenon is observed for phenol and 3-ethylphenoi. The aerobic GAC adsorptive capacity for phenol and 3-ethyiphenoi exceeds the anaerobic one in the range of 42-85 and 18-92% for the equilibrium adsorbate concentrations of 1000 and 1 mg/l, respectively. The possibility that biological degradation under aerobic conditions was responsible for the higher observed capacities was discounted because of the following facts: (I) The DOC measurements conducted on liquid samples from both procedures agreed very closely with the theoretical DOC computed for the concentration of o-cresol measured by u.v. analysis. (2) Biological analysis of the samples from both aerobic and anaerobic isotherm tests revealed the absence of any microorganisms capable of degrading o-cresol aerobically or anaerobically. (3) Spectroscopic scans, in the range of 230300 nm, of liquid samples obtained from both procedures conducted at the beginning and end of an adsorption isotherm run compared very well with the scans determined for pure o-cresol solutions. (4) No increase in the concentration of inorganic carbon was observed during isotherm tests. The phenomenon advanced in this study appears to be due to a chemical reactions between the adsorbate and molecular oxygen that is catalyzed by the carbon surface.
The importance of the new, anaerobic, isotherm discovery is fully emphasized when the breakthrough curves for o-cresol and 3-ethylphenol are examined. The early portions of the breakthrough curves for o-cresol are very accurately predicted by the anaerobic isotherm while the total capacity obtained by integrating over the entire breakthrough curve compares well with the capacity given by the aerobic isotherm. On the other hand, when the time permitted for the occurrence of initial breakthrough was short, the chemical reactions that involve molecular oxygen do not appear to exert a significant impact on breakthrough. As a result, both the early portion of a breakthrough curve and the total GAC adsorptive capacity are well predicted by the anaerobic isotherm. This was evident with the breakthrough experiment for 3-ethylphenol. REFERENCES
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