I n d . Eng. Chem. Res. 1990,29, 1338-1345
1338
factor in determining energy costs; a high concentration of solute in the extract will allow for a more efficient process. Coupling of diluent and temperature swing may be advantageous. The additional energy cost of the distillation over simple temperature swing may be offset by the greater concentration of the product and the fact that diluent swing offers less pinched, more favorable equilibrium curves.
Acknowledgment We are grateful to the late A. Steven Kertes for helpful discussions. We thank Eric Chan, Roy Kamimura, and Maninderpal Grewal, who made significant contributions toward the experimental portion of this work. This work was supported by a National Science Foundation Graduate Fellowship and by the Assistant Secretary for Conservation and Renewable Energy, Office of Energy Systems Research, Energy conversion and Utilization Technologies (ECUT) Division, U S . Department of Energy, under Contract DE-AC03-76SF00098. Registry No. MIBK, 108-10-1;succinic acid, 110-15-6;lactic acid, 50-21-5; dichloromethane, 75-09-2;nitrobenzene, 98-95-3; chloroform, 67-66-3; 1-octanol, 111-87-5; water, 7732-18-5.
Literature Cited Apelblat, A. Enthalpy of Solution of Oxalic, Succinic, Adipic, Maleic, Malic, Tartaric, and Citric Acids, Oxalic Acid Dihydrate, and
Citric Acid Monohydrate in Water at 298.15 K. J.Chem. Thermodyn. 1986, 18, 351-357. Baniel, A. M.; Blumberg, R.; Hajdu, K. Recovery of Acids from Aqueous Solutions. U.S.Patent 4,275,234, 1981; Chem. Abstr. 1982, 97, 109557.
Holten, C. H. Lactic Acid: Properties and Chemistry of Lactic Acid and Derivatives; Verlag Chemie: Copenhagen, 1971; pp 20-58. Kertes, A. S.; King, C. J. Extraction Chemistry of Fermentation Product Carboxylic Acids. BiotechnoL Bioeng. 1986,28, 269-282. King, C. J. Binary Multistage Separations: General Graphical Approach. In Separation Processes, 2nd ed.; McGraw-Hill Book Co.: New York, 1980; pp 258-296. Sato, T.; Watanabe, H.; Nakamura, H. Extraction of Lactic, Tartaric, Succinic, and Citric Acids by Means of Trioctylamine. Bunseki Kagaku 1985,34, 559-563. Saville, G.; Gundry, H. A. The Heats of Combustion, Solution and Ionization of Lactic Acid. Trans. Faraday SOC.1959, 55, 2036-2038. Smith, J. M.; Van Ness, H. C. Phase Equilibria. In Introduction to Chemical Engineering Thermodynamics, 3rd ed.; McGraw-Hill Book Co.: New York, 1975; Chapter 8, pp 290-375. Tamada, J. A. PhD. Dissertation, University of California, Berkeley,
1989.
Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids by Amine Extractants. Report LBL-25571; Lawrence Berkeley Laboratory: Berkeley, CA, Jan 1989. Wennersten, R. The Extraction of Citric Acid from Fermentation Broth Using a Solution of a Tertiary Amine. J . Chem. Technol. Biotechnol. 1983, 33B, 85-94. Received for review August 30, 1989 Revised manuscript received February 12, 1990 Accepted February 21, 1990
Aqueous-Phase Adsorption of Trichloroethene and Chloroform onto Polymeric Resins and Activated Carbon Thomas E. Browne and Yoram Cohen* Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024-1592
The adsorption of the EPA priority pollutants trichloroethene (TCE) and chloroform (CHC13)from aqueous solutions onto activated carbon and macroporous polymeric resins was investigated over a wide concentration range. Over much of the concentration range studied, activated carbon adsorbed more TCE and CHC13 on a weight basis and on a surface area basis than did the polymer resins. A t concentrations greater than 1000 hg/L, the adsorption capacity, on a surface area basis, for the different resins and for activated carbon was similar. The adsorption isotherms were fitted by the empirical Freundlich isotherm and the semiempirical isotherm of Jossens et al. The affinity of the solutes for the different resins was qualitatively described by a simple Hildebrand solubility parameter correlation.
Introduction Over the past 2 decades, it has often been suggested that macroporous polymeric resins could be useful for the adsorptive removal of a variety of organic solutes from aqueous systems (Gustafson et al., 1968; Paleos, 1969; Chriswell et al., 1977; Chudyk et al., 1979; Fox, 1979 Neely, 1980; Van Vliet and Weber, 1981; Chanda et al., 1983; Cornel and Sontheimer, 1986). In general, it has been found that polymeric resins have a lower adsorption capacity than does activated carbon for most organic solutes (McGuire and Suffet, 1978; Van Vliet and Weber, 1981; No11 and Gounaris, 1988). Several studies, however, demonstrated that for the solutes p-chlorophenol and ptoluenesulfonate (Van Vliet and Weber, 1981), p-chloro-
* Author to whom correspondence should be addressed.
phenol and phenol (No11 and Gounaris, 1988), p-nitrophenol (Kim et al., 1976), benzaldehyde (Furusawa and Smith, 1974),and phenol and carbon tetrachloride (Weber and Van Vliet, 1981), the slopes of the adsorption isotherms for the polymeric resins were steeper than the slopes of the corresponding isotherms for activated carbon. The results of these latter studies suggested that at sufficiently high solute concentrations the polymeric resins may have a higher capacity than does activated carbon. It is well-known that the pore size distribution and surface area affect the resin’s capacity for solute adsorption (Paleos, 1969; Kunin, 1977; Feeney, 1979; Chudyk et al., 1979; Slejko, 1985; Cornel and Sontheimer, 1986). Yet most of the studies on water decontamination rarely report adsorption capacity based on the measured surface area (i.e., moles of solute/squared meter), and even fewer studies have considered the possible influence of the in-
0888-5885190J 2629-1338$02.50/0 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1339 accessible pore volume. Thus, it is not always possible to compare, based on reported adsorption data, the true adsorption affinity of the solute for a particular resin surface. The focus of this work is on comparing the diffrence in the adsorption affinity of the two priority pollutants trichloroethene (TCE) and chloroform (CHCl,) for different polymeric resins in relation to the physical and chemical properties of the resins. TCE and chloroform are volatile organic contaminants (VOC's) and are classified as priority pollutants by the United States Environmental Protection Agency. TCE, which is a common industrial solvent and ingredient of household products (Love and Eilers, 1982), has found its way into aquifers in many parts of the United States. Chloroform in drinking water, as well as other trihalomethanes, may originate from the reaction of humic substances with dissolved chlorine used to disinfect the water (Bellar et al., 1974; Weber, 1977; Suffet, 1980; Boening et al., 1980; Nicholson et al., 1984). The absorptive removal of chlorinated VOC's such as trichloroethene and chloroform by activated carbon and the carbonaceous resin XE-340 (Kong and DiGiano, 1986) received much attention in the literature (Chriswell et al., 1977; McGuire and Suffet, 1978; El-Dib et al., 1978; Chudyk et al, 1979; Isacoff and Bitner, 1979; Boening et al., 1980; Youssefi and Faust, 1980; Neely, 1980; Weber and Pirbazari, 1982; Love and Eilers, 1982; O'Brien et al., 1983; Endicott and Weber, 1985; Kong and DiGiano, 1986; Speth, 1986; Smith et al., 1987; Sakoda et al., 1987; Thakker and Manes, 1987; Lykins et al., 1988; Weber and Smith, 1986; Smith and Weber, 1988; Summers et al., 1989),and excellent reviews have been provided by Weber (1977) and Suffet (1980). In contrast, only limited data exist for the adsorption of trichloroethene and chloroform onto polymeric resins. TCE adsorption onto polystyrene resins was previously reported by Cornel and Sontheimer (1986) for laboratory-synthesized resins made by Bayer A. G. Leverkusen, and chloroform adsorption was reported by Neely (1980) for the commercial resin XAD-4.
Experimental Section Materials. Trichloroethene was obtained from Mallinckrodt Inc. (St. Louis, MO), and CHC1, was obtained from Burdick & Jackson Laboratories, Inc. (Muskegon, MI). Both chemicals were reagent grade and were used to make calibration standards and saturated aqueous solutions without further purification. The methanol used to make the calibration standards was OPTIMA grade, available from Fisher Scientific (Tustin, CA). ACS reagent grade methanol and diethyl ether, used in the preparation of the polymeric resins, were also obtained from Fisher Scientific. Acetonitrile (pesticide analysis grade) and pentane (suitable for trihalomethane analysis) were obtained from Burdick & Jackson. Reagent grade potassium dihydrogen phosphate (KH2P04)and disodium hydrogen phosphate (Na2HP04),used to buffer the aqueous solutions, were obtained from the J. T. Baker Chemical Co. (Phillipsburg, NJ). Activated carbon (F-400, Calgon Corp., Pittsburgh, PA) and the four macroporous polymeric resins (XAD-2, XAD-4, XAD-8, Rohm and Haas, Inc., Philadelphia, PA); Reillex-425 (Reilly Tar and Chemical Co., Indiannapolis, IN) as listed in Table I were used as the adsorbents. The resins XAD-2 and XAD-4 are matrices of polystyrene cross-linked with divinylbenzene. XAD-8 is a cross-linked polyacrylate matrix. Reillex-425 is a matrix of poly(vinylpyridine) cross-linked with divinylbenzene. Adsorbent Preparation and Adsorption Studies. The polystyrene (XAD-2 and XAD-4) and poly(viny1pyridine) (Reillex-425) resins were prepared according to
Table I. Results of the Nitrogen BET Analysis total area, m2/g this total pore adsorbent study manufacturer vol, cm3/g F-400 1075 1050-12000 0.617 XAD-4 873 760b 1.02 XAD-2 3506 354 0.764 125 140b XAD-8 0.642 Reillex-425 53.8 9OC 0.657
av pore radius, nm 14.7 23.2 48.3 98.0 156
"McGuire and Suffet (1978). *Van Vliet and Weber (1981). 'Reilly Tar and Chemical Co. (1987).
the procedure recommended by the respective manufacturers (Rohm and Haas, Inc., 1978; Reilly Tar and Chemical Co., 1987). The resins were first soaked in methanol (500 mL for a 30-g batch) for a period of about 1 day. Subsequently, the resins were exhaustively washed with deionized, distilled water (6 L per batch of approximately 30 g of resin). Activated carbon of mesh size 40150 U S . sieve was first washed with deionized, distilled water. The activated carbon and the above polymeric resins were then allowed to air dry for 2 days at ambient conditions in order to drive off the bulk of the water. Subsequently, the above resins were dried under vacuum at 105 "C for 1 day. The adsorbents were subsequently allowed to cool to room temperature in a desiccator. The cross-linked polyacrylic resin (XAD-8)was cleaned following the procedure of Ram and Morris (1982) and Thurman et al. (1978). The XAD-8 resin was washed sequentially in a Soxhlet extraction column with three solvents: methanol, acetonitrile, and diethyl ether. The XAD-8 resin was then allowed to dry at ambient temperature in the fume hood for 1 day followed by vacuum drying for 1day at 80 "C. All adsorbents were stored in air-tight jars at room temperature. Adsorption Experiments. Carefully weighed quantities of a given polymeric resin or activated carbon were added to 14- or 18-mL glass vials with Teflon-lined screw cap tops. The vials were then filled with previously prepared saturated aqueous solutions of TCE or chloroform such that the headspace was negligible, and the volume of added solution was recorded. Agitation of the solution-resin vials was accomplished by using a 3-ft-long rack (with a capacity for holding 60 adsorption vials) that rotated the tubes end-over-end at 12 rpm. Adsorption experiments in which the allowed equilibration time was varied for up to a period of 2 weeks indicated that a 7-day equilibration period was sufficient to assure adsorption equilibrium. Thus, in all of the adsorption experiments reported in this work, the TCE and chloroform adsorption vials were allowed at least 7 days to equilibrate at room temperature (22 f 1 "C). At least one blank vial containing water and the adsorbent (but no solute) was run simultaneously with each solution-resin set (i.e., the adsorption vials). Also, at least one control vial containing the water and solute (and no adsorbent) was run for each of the different initial concentrations. For each set of adsorption experiments, blanks and controls were rotated on the same racks with their respective set of adsorption vials. Subsequent analyses of the blanks revealed that the resincleaning procedure indeed rendered the resins free of any contaminants. Finally, it is worth noting that according to the present procedure for resin preparation, the resins may have been partially nonwetted. According to the studies of Rixey and King (1987, 19891, the rate of adsorption onto nonwet resins is slower than for wet resins except when the solutes are of high volatility, as indeed is the case in this work. The study of Rixey and King (1989) also suggests that selective solute adsorption may be feasible when adsorbing from multisolute systems
1340 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
containing solutes of significantly different vapor pressures. Analysis. The amount of adsorbed solute was calculated from the following mass balance:
-
This study, T
A
G Crittenden,
0) 0)
_I
E
-
22 "C
et rl., (1985), T
Kong and DiGiano, (1986)
-
10-12 'C
1
v
(C, - Cq) 9 =
(1)
in which Co and C , are the initial and final (equilibrium) solute concentrations (milligrams/liter), respectively, V is the volume of the solution (liters), m is the mass of the adsorbent (grams), and q is the amount adsorbed (milligrams/gram). A summary of the analytical procedure is given below. Aqueous TCE and chloroform concentrations were determined by the liquid/liquid extraction method followed by gas chromatographic (GC) analysis similar to the method used by Mieure (1977) and Richard and Junk (1977). TCE or CHCISwas extracted from the aqueous phase with an equal volume of pentane by hand-shaking for 2 min in a glass vial. The solute concentration in the pentane phase was determined from GC analysis. TCE and chloroform concentrations in the aqueous phase were then calculated from calibration curves (peak area versus concentration in the aqueous phase) that covered the concentration range 2-5000 pg/L for each solute. A Hewlett-Packard gas chromatograph (Model 5890) equipped with an electron capture detector, using 63Nias the isotope source, was used. Samples were injected directly in the splitless mode. Carrier and make-up gas were high-purity nitrogen (9.9995%), which was passed sequentially through an activated carbon adsorber, a 13-?, zeolite adsorber to remove water, and an oxygen trap. The column was a 10-m-longmacrobore silica column (0.53-mm i.d.) coated with a 5-pm thick poly(dimethylsi1oxane) stationary phase (Alltech Assoc., Inc., Deerfield, IL). The column temperature was 30 "C, and the injector and detector temperatures were set at 150 and 200 "C, respectively. Surface Area and Pore Size Analyses. Surface area and pore size analyses were done via nitrogen adsorption in an Autosorb-1 sorption system (Quantachrome, Corp., Syosset, NY). The samples used for analysis were taken from the same adsorbent batch used for the TCE and CHC1, adsorption experiments without further modification. The total nitrogen BET (Brunauer-Emmett-Teller) surface area was calculated from .measurements of the volume of adsorbed nitrogen as a function of relative pressure, p/psat(Gregg and Sing, 1982; Lowell and Shields, 1984). The total pore volume and average pore radius were determined from the same nitrogen isotherm (Lowell and Shields, 1984). For the activated carbon, which is a microporous resin with the majority of the surface area in pores 20 A in radius and smaller, the tottal BET surface area was determined from the nitrogen adsorption data obtained from the relative pressure range 0.0 < p / p m t < 0.15 (Lowell and Shields, 1984). For the polymeric resins, the BET surface area was determined from the relative pressure range 0.0 PIPsat < 0.30, which is the recommended range for mesoporous materials (i.e., 20 A < rp < 500 A) (Lowell and Shields, 1984). The total surface area, pore volume, and average pore radius for the five adsorbents investigated are given in Table I. The results for activated carbon agree well with those previously published by McGuire and Suffet (1978). The BET surface areas for XAD-2 and XAD-8 also agree well with those reported by the manufacturer (see Paleos, 1969). The total surface area of XAD-4 was found to be about 15% larger than that reported by the manufacturer (see Paleos, 1969);however, batch-to-batch variation in the
IF0 1
5E-i
I iEO
1Ei
iE2 AOUEOUS CONCENTRATION TCE
iE3
2E3
(pgll)
Figure 1. Adsorption of TCE onto activated carbon F-400: comparison with other investigations.
product could explain this difference. The surface area determined for Reillex-425 was lower than the manufacturer-reported value by approximately a factor of 1.6. Reillex-425 used in this study was of particle size 53-150 pm (100/270-mesh U S . sieve) ground from a size fraction of 8/30-mesh U.S. sieve. It is plausible that the deformation of the polymeric resin upon grinding may have contributed to the lowering of the specific surface area.
Results and Discussion Adsorption isotherms for TCE and CHC1, on activated carbon and four polymeric resins were determined on a surface area basis and on a weight basis over a concentration range of approximately 1pg/L C, IO00 mg/L. The results for the adsorption of TCE (on a weight basis) onto activated carbon F-400, shown in Figure 1, fall between those of Crittenden et al. (1985) and Kong and DiGiano (1986). The slight difference between the results reported herein and those of Crittenden et al. (1985) is most likely due to the difference in temperature: 22 "C in the current study compared to 10-12 "C in the study done by Crittenden et al. (1985). Aqueous-phase adsorption onto activated crbon is an exothermic process (Weber and Morris, 1964; Radke and Prausnitz, 1972), and thus, the amount adsorbed (per unit mass of resin) should increase with decreasing temperature. A quantitative comparison of the present isotherms with that of Kong and DiGiano (1986) is not possible since these authors did not report the adsorption temperature. Nonetheless, we note that above a concentration of about 40 ppb, the slopes of the isotherm from this work and from the Kong and DiGiano study differed by only 1.9%. The aqueous-phase isotherm for the adsorption of CHC1, onto activated carbon F-400 compared well with the results of Crittenden et al. (1985) and Weber (1977), as illustrated in Figure 2. The adsorption capacity of activated carbon for TCE, on a weight basis, was found to be higher than that of the polymeric resins studied (Figure 3), consistent with previous studies using different organic solutes (Chriswell et al., 1977; Chudyk et al., 1979; Boening et al., 1980; Van Vliet and Weber, 1981). A t 10 pg/L, the adsorption capacity of activated carbon for TCE, on a weight basis, was approximately 300 times greater than for the XAD-8 and Reillex-425 resins. It is worth noting that the isotherms for TCE and CHC13 on Reillex-425 (Figures 3 and 5, respectively) were "concave upward" while the isotherms for TCE and CHC13 adsorption on XAD-2, XAD-4, and XAD-8 were "concave downward". A concave-upward shape is known as "unfavorable adsorption" (Weber and
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1341
-
E4 0
This study, T = 22 OC
a.,
- - - Crittenden,
-
&t (1989, T Weber, (1977), T = 20 O C
-
C
20-22
%B E3
-
OC
a.
C
z
C
1E2
x:
E
/
9 /c
8
V
El
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/'
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W
2
E O
B
b
v)
w E-I
1E-I IEO
1El
1E2
1E3
1E4
A ~ ' 0
-I
E-2 11
5E4
1El
AQUEOUS CONCENTRATION CHCiS (pgli)
Figure 2. Adsorption of CHCl, onto activated carbon F-400: comparison with other investigations. 1E3 T
-
5
1E2
5E1
22 o c
E2
A 0
0
O
1
I
3
-
v
vv
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41 AB
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c
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T = 22 OC carbon F-400
-0- activated A XAD-4 0 XAD-2 0 XAD-8
v Reillex -
425 XAD-4, Neely (1980)
vgV
0
A
1E-2 v
v vvv
v
p
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El
IEB
p-I
o
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1E-2 IEO
1E5 AQUEOUS CONCENTRATION TCE ((rgli)
I
XAD-4 > XAD-2 > XAD-8 > Reillex-425. The isotherms for CHCl, adsorption onto XAD-2, XAD-4, XAD-8, Reillex-425, and F-400 are shown in Figure
"Crittenden et al. (1985).
5. The adsorption data for CHC13onto XAD-4 compare very favorably with the data of Neely (1980) in further support of the accuracy of our analytical technique. As the results of Figure 5 clearly indicate, on a weight basis, activated carbon has a higher capacity for CHC1, although to a lesser extent compared to TCE adsorption. I t is interesting to note that the slopes of the isotherms for CHC13 adsorption onto the F-400, XAD-4, and XAD-2 resins differed by no more than 10-15% over most of the concentration range investigated. The Freundlich parameters for the different resins, along with those activated carbon, are given in Table 111. On a surface area basis, activated carbon F-400 had a slightly greater affinity for CHC13than did the polystyrene resins (Figure 6). In addition, the slope of the CHCl, isotherm for XAD-8 was close to the slopes of the XAD-2
1342 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 "-- I
-
22 'C activated carbon F-400 XAD-4 XAD-2 XAD-8 Relllex 425 T
Table V. Estimated Fractional Pore Volume Filling by CHCIJat Highest Surface Loading" adsorbent C,, mg/L q , mg/g 1.5 10.0 1.03 X F-400 2.8 10.0 0.56 X XAD-4 XAD-2 11.5 10.0 0.85 X 10" 27.4 10.0 1.07 X XAD-8 Reillex-425 425 10.0 1.07 X
**
,P 8
J
Highest surface loading where all the adsorption isotherms overlap. *Volume of CHCla/total pore volume.
0
zj I
j
3-
w
0
1EO
1El
E2
E9
E4
AQUEOUS CONCENTRATION CHCl
E5 (pgII)
1E6
Figure 6. Adsorption of CHCl:, based on surface area: activated carbon and polymeric resins. Table IV. Estimated Fractional Pore Volume Filling by TCE at Highest Surface Loadinrr" adsorbent C,, mg/L q, mg/g *b F-400 0.77 50 0.112 XAD-4 4.0 50 0.618 17.3 50 0.0934 XAD-2 50.5 50 0.116 XAD-8 Reillex-425 234 50 0.116 ~
~
a Highest surface loading where all the adsorption isotherms overlap. Volume of TCE/total pore volume.
and XAD-4 isotherms, which was also the case for TCE adsorption onto these resins. It is worth noting that the study of Cornel and Sontheimer (1986) revealed that in the case of polystyrene-divinylbenzene gel resins (of a lower degree of cross-linking compared to the XAD-2 and XAD-4 resins) the adsorption capacity of the gel resins for TCE did not correlate with nitrogen BET surface areas. Those authors argued that the lack of correlation with surface area for the gellike resins was in part due to swelling of the resin by trichloroethene. The present results indicate that on a mass basis the adsorption capacity of XAD-4 and XAD-2 scale with the surface area as depicted in Figures 5 and 6 in contrast with the behavior observed with the resins used by Cornel and Sontheimer (1986). It is argued that if swelling effects were significant, one would expect to find that the total volume of solute retained by the resin should approach or exceed the available pore volume (Cornel and Sontheimer, 1986). For example, even at a relatively high adsorption loading (for the range of solute concentrations employed in this study), the fraction of the polymeric resin's pore volume occupied by the solute (Tables IV and V) was low, suggesting that the majority of the resin's pore space is free. Thus, it is unlikely that the highly cross-linked resins used in the current study swelled over the concentration range investigated. Adsorption capacity and the apparent affinity are also affected by the pore size distribution of the adsorbent. If an adsorbent has a large fraction of its available surface area in pores too small for the solute to penetrate into and diffuse along the pore, then size exclusion effects will occur. Based on bond lengths alone, the largest characteristic dimension of the TCE molecule is 8 A, while the largest characteristic dimension of the CHCI, molecule is 7 A (Weast, 1978). From the results of the BET analysis, both XAD-2 and XAD-4 had approximate1 12% of their total surface area in pores smaller than 20 in diameter, while XAD-8 and Reillex-425had only 7% and 5%, respectively,
w
Table VI. Hildebrand Solubility Parameters for Polymers and Solutes polymer 6, ( c a l / ~ m ~ ) ' / ~ polystyrene (cross-linked) 9.10 poly(methylmethacry1ate) 9.45b poly(viny1pyridine) 9.7' trichloroethene 9.Zd chloroform 9.3d phenol 10.6' p-nitrophenol 9.8' p-cresol 9.9' m-cresol 9.9e p-chlorophenol 10.1' aBoyer and Spencer (1948). bMangaraj et al. (1963). 'Estimated from the method given in Burrell and Immergut (1966). Normal boiling point calculated from Weast (1978). dBurrell and Immergut (1966), pp 341-367. 'Reid et al. (1977).
of the total surface area in the same pore size range. It is likely that, in small pores (less than about 20 A), once a single solute molecule penetrates a pore, the passage of additional molecules may be hindered. Thus, it is plausible that, especially for the XAD-2 and XAD-4 resins, size exclusion of the solute from some of the pores may have slightly affected the adsorption capacity of the polystyrene resins for TCE and CHC13. In contrast, activated carbon F-400 has about 26% of its available surface area in pores smaller than 20 A in diameter, suggesting that size exclusion effects for CHC13and TCE may be more significant in activated carbon. However, it is likely that the larger specific surface area of activated carbon compensates for any adverse size exclusion effects. The affinity of the solute for the polymeric resins can be qualitatively related to the enthalpy of mixing, A H ~ x , (Billmeyer, 1984) Mmix
=
' ~ s ~ p ( 6-a 6p)2
(2)
in which 6, and 6, are the Hildebrand solubility parameters for the polymer and solvent, respectively, and cps and pP are the solute and polymer volume fractions, respectively. The solubility parameter, 6, is defined by 6 = E vaP/u,
(3) where E vap is the change in internal energy upon vaporization of the substance and u, is its molar volume. In general, an organic solute will have an increasing affinity for the polymer as the difference in the Hildebrand solubility parameters for the polymer and solute decreases (Burrell and Immergut, 1966; Cornel and Sontheimer, 1986). The Hildebrand solubility parameters for TCE, chloroform, and the different polymer resins used in this study are given in Table VI. The relative affmities of TCE and chloroform for the different polymeric resins can be ascertained once the adsorption capacities are expressed on a surface area basis (Figures 4 and 6). For example, one can compare the adsorption capacities a t a given concentration as a function of the difference in the solute and adsorbent solubility parameters. At low concentrations (where C 0), the adsorption isotherms are expected
-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1343 Table VII. Parameters for the Jossens et al. (1978) Isotherm (Equation 5) for the Polymeric Resins K,(mol/m2)/ k, svstem (mZ/mol)p (mol/L) TCEIXAD-4 2.45 X 307 TCE/XAD-2 2.77 X 23.3 6.17 X 2.92 TCE/XAD-8 TCE/Reillex-425 2.52 X 63.3 CHCl,/XAD-4 1.06 X 149 9.90 x 10-3 CHClS/XAD-2 71.50 CHCIJXAD-8 8.46 x 10-3 45.9 CHC13/Reillex-425 1.86 X 17.1 phenol/XAD-4 5.07 X lo4 337 2E-1
Table VIII. Linear Adsorption Coefficient for Other Systems Used in the Correlation of Figure 7 P 0.430 0.216 0.207 0.247 0.311 0.560 0.499 0.165 0.357
phenol/EX-86ln p-chlorophenol/EX-861’ m-cresol/ES-86Ia p-cresol/ES-861a phenol/XAD-4* p-~hlorophenol/XAD-4~ p-cresol/XAD-4b ~henol/XAD-7~ p-chlorophenol/XAD-7* p-cresol/XAD-7*
I
5.71 X 5.29 X 1.69 X 1.99 x 1.09 x 3.61 x 8.40 x 8.91 X 7.07 x 3.48 x
IO-‘ lo-‘
10-3 10-3 10-3 10-3 lo4
10-3 10-3
a Costa and Rodriguez (1982). The specific surface area of ES861 used was 500 mz/g. *Itaya et al. (1984). The specific surface areas of XAD-4 and XAD-7 used were 870 and 450 m2/g, respectively.
4
c E-1 t
I
I)
y’
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8
s
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B
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ES 881, Corti m d Rodriguez, (1982)
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Figure 7. Correlation of the linear adsorption coefficient with the difference in solubility parameters.
to be linear and one can characterize the adsorption isotherm with the equivalent of the Henry’s law constant, q = KC (4) in which K is the linear adsorption coefficient and q is the adsorption capacity based on the surface area. A semiempirical isotherm that reduces to eq 4 at low concentrations was proposed by Jossens et al. (1978), C / q = (1/K) e x p ( k p )
(5) The parameters k,p, and K as determined from the fit of eq 5 (where q, in this work, is expressed on a surface area basis) to the present experimental adsorption data are given in Table VII. The linear adsorption coefficient, K , was correlated with the square of the difference between the solubility parameter of the solute, 6, and the solubility parameter of the resin, 6 as shown in Figure 7. Also shown in Figure 7 are the ?j values extracted from the data of Costa and Rodriguez (1982) for aqueous-phase adsorption of phenol, p-nitrophenol, and p-cresol on a crosslinked polystyrene-divinylbenzene resin (ES-861) and the data reported by Cohen (1989) for phenol adsorption. Also included in Figure 7 are the K values extracted from the study of Itaya et al. (1984) for aqueous-phase adsorption of phenol, p-chlorophenol, and p-cresol on XAD-4 and XAD-7 (the latter resin being chemically similar to XAD-8 but having a specific surface area of 450 m2/g). The K values used from other investigators are listed in Table VIII. The solubility parameters of the solutes and the resins used in the correlation (Figure 7) are givin in Table VI. As Figure 7 illustrates, the resin-solute compatibility is qualitatively reflected by the increase in the value of K with a decrease in the value of (6, It is important to note that accurate adsorption data at sufficiently low concentrations are essential in order to determine the K values. The adsorption data in the current work were
obtained down to a concentration level of about 1-10 pg/L relative to the lowest concentration limit of 660 pg/L for the Itaya et al. (1984) data. Therefore, it is possible that the K values obtained from the data of Itaya et al. (1984) are not accurate representations of the linear region of the adsorption isotherms, which may explain the deviation of the Itaya et al. K values from the data of others for similar compounds (see Figure 7). In another attempt to correlate the adsorption characteristics for polymeric resins, Abe et al. (1983) proposed the use of the molecular parachor (Quayle, 1953; Meisner, 1949) as a correlating parameter. Their study included homologous series of alcohols, ketones, esters, and carboxylic acids and used the molecular parachlor, P, as a correlating parameter. The molecular parachlor is defined as (Meisner, 1949)
P = u”4Mw/(~liq- ~ v a p ) (6) where u is the surface tension of the liqui/air interface (Dynes/centimeter) of the solute a t a given temperature, Mwis its molecular weight, and pliqand pva are the liquid and vapor densities of the solute, respective& (grams/cubic centimeter). The molecular parachlor is a secondary thermodynamic quantity that was first proposed by MacLeod in 1923 as a means of estimating surface tension from a generalized correlation (Reid et al., 1977). On the basis of their adsorption studies, Abe et al. (1983) proposed the following correlations between P and the parameters of the Freundlich isotherm for solute adsorption onto XAD-4: l / n = -0.002624P + 1.290 (7) (8) log K f = 0.01582P - 3.758 For systems for which the surface tension (see eq 6) is unknown, the functional group additivity approach of Quayle (1953) can be used to estimate the molecular parachor. For CHCl,, for example, the value of the parachor calculated from eq 6 differs from the value calculated by the group additive method by less than 1%. The method yields values of the molecular parachor for TCE and CHC1, of 212.8 and 184.8, respectively. By using the above values for P, the estimated Freundlich parameter, l / n (slope on a log-log plot), for TCE and CHC13 adsorption onto XAD-4 differed from the experimental results (Table 11) by only 0.5% for CHC1, and 4% for TCE. In contrast, the predicted values for the Freundlich Kf parameter (based on eq 8) were underpredicted by a factor of 29 for CHC1, and a factor of 33 for TCE. The parachor correlation did, however, predict the preference for TCE over CHC1, in agreement with the experimental results. Although the overall performance of the parachor correlation for XAD-4 appears to be questionable, it may be worth
1344 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 exploring for correlating adsorption affinity.
6, = Hildebrand solubility parameter of the solvent, (cal/
Conclusions Aqueous-phase adsorption of TCE and chloroform was investigated for four macroreticular resins and activated carbon. For both solutes, activated carbon was found to have a higher adsorption capacity when the adsorption capacity was expressed on a mass basis. However, when loading was expressed on a surface area basis,the following conclusions regarding solute/adsorbent affuity were noted (1)For TCE adsorption, the polymeric resin isotherms all had similar slopes over 4 decades of solute concentration. The values of the slopes for the polymeric resins' isotherms were about a factor of 2 greater than the slope of the activated carbon adsorption isotherms. Thus, it is plausible that at sufficiently high concentrations the adsorption capacity of the polymeric resins m a y exceed that of activated carbon. (2) The nonpolar polystyrene resins (XAD-2 and XAD4) had almost the same affinity for CHC13 as did F-400, based on the results for adsorption on a surface area basis. The slopes of the resin isotherms for CHC13on XAD-8 and Reillex-425 differed significantly from that of the polystyrene isotherms, but the isotherms overlapped over nearly the entire concentration r a n g e studied, on both a mass basis and a surface area basis. Finally, the preference of TCE and chloroform for the polymeric resins was qualitatively related to the square of the difference in the Hildebrand solubility parameters for the solute-resin pairs. The molecular parachor correlation may be useful for correlating the adsorption of different single solutes onto a given resin, but further testing of the a p p r o a c h is required.
plis = density of the liquid, g/cm3 pvap = density of the vapor, g/cm3
cm3)1/2
Acknowledgment
This work was funded in part by the National Science Foundation Grant CBT-8416719, the National Science Foundation Engineering Research Center on Hazardous Substance Control Grant CDR-86-22184, the United States Geological Survey Department of the Interior Award 1408-0001-G1315,the Water Resources Center (University of California) Project W-600, the University of California University-Wide Energy Research Group, and the University Academic Senate. Nomenclature C, = equilibrium concentration of solute, m o l / L Co = initial concentration of solute, m o l / L
E vsP = change in internal energy upon vaporization, cal/mol AHmix= enthalpy change upon mixing, cal/mol k = parameter in t h e adsorption isotherm of Jossens et al. (1978) (eq 51, (m2/mol)p K = linear adsorption coefficient, L/m2 Kf = parameter in the Freundlich adsorption isotherm, (mg/g) (L/mg)'/*
m = mass of adsorbent, g M, = molecular weight of solute, g/mol l / n = parameter in t h e Freundlich adsorption isotherm, dimensionless p = parameter in t h e adsorption isotherm of Jossens e t al. (1978) (eq 5 ) , dimensionless P = molecular parachor, (g1I4 ~ m ~ ) / ( s mol) '/~ q = amount adsorbed, mg/g, mol/g, pg/m2, or mol/m2 r p = pore radius, 8, V = volume of solution, L u, = molar volume, cm3/mol
Greek Letters 6 , = Hildebrand solubility parameter of t h e polymer, (cal/ cm3)lI2
u = surface tension 'p,
at t h e vapor/liquid interface, dyn/cm
= volume fraction of t h e solvent (eq 2)
= volume fraction of t h e polymer (eq 2) Registry No. TCE, 79-01-6; CHCl,, 67-66-3; C, 7440-44-0; Amberlite XAD-2, 9060-05-3; Amberlite XAD-4, 37380-42-0; Amberlite XAD-8, 11104-40-8; Reillex-425, 37273-66-8. (op
Literature Cited Abe, I.; Hayashi, K.; Hirashima, T. Adsorption of Organic Compounds from Aqueous Solution onto Hydrophobic High-Surface Area Copolymer. J. Colloid Znt. Sci. 1983, 94, 577. Bellar, T. A.; Lichtenberg, J. J.; Kroner, R. C. The Occurrence of Organohalides in Chlorinated Drinking Waters. J. Am. Water Works Assoc. 1974,66, 703. Billmeyer, F. W. Textbook of Polymer Science, 3rd ed.; Wiley-Interscience: New York, 1984. Boening, P. H.; Beckman, D. D.; Snoeyink, V. L. Activated Carbon Versus Resin for the Adsorption of Humic Substances. J. Am. Water Works Assoc. 1980, 72, 54. Boyer, R. F.; Spencer, K. S. Some Thermodynamic Properties of Slightly Cross-Linked Styrene-Divinylbenzene Gels. J. Polym. Sci. 1948, 3, 97. Burrell, H.; Immergut, B. Solubility Parameter Values. In Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; John Wiley and Sons: New York, 1966. Chanda, M.; Rempel, G. L.; O'Driscoll, K. F. Sorption of Phenolics onto Cross-Linked Poly(4-Vinylpyridine).React. Polym. 1983, I , 281. Chriswell, C. D.; Ericson, R. L.; Junk, G . A.; Lee, K. W.; Fritz, J. S.; Svec, H. J. Comparison of Macroreticular Resin and Activated Carbon as Sorbents. J. Am. Water Works Assoc. 1977,69,669. Chudyk, W. A,; Snoeyink, V. L.; Brinkman, D.; Temperly, T. J. Activated Carbon Versus Resin for the Adsorption of 2-Methylisoborneal and Chloroform. J.Am. Water Works Assoc. 1979, 71, 529. Cohen, Y. Selective Removal of Organics. Annual Report; the National Science Foundation Engineering Research Center for Hazardous Substances Control, University of California: Los Angeles, Oct 15, 1989. Cornel, P.; Sontheimer, H. Sorption of Dissolved Organics from Aqueous Solution by Polystyrene-I. Resin Characterization and Sorption Equilibrium. Chem. Eng. Sci. 1986, 41, 1791. Costa, C. A,; Rodriguez, A. E. "Adsorption of Phenol on Adsorbent Resins and Activated Carbon: Equilibrium and Kinetic Studies in a Batch and Open Systems". In Adsorption at the Gas-Solid and Liquid-Solid Interface; Roquerol, J., Sing, K. S. W., Eds.; Elsevier Scientific Publ. Co.: Amsterdam, Netherlands, 1982. Costa, C. A.; Rodriguez, A. E. Dynamics of Phenol Adsorption on Polymeric Supports. In Fundamentals of Adsorption; Belfort, G., Myers, A. L., Eds.; Engineering Foundation: New York, 1986. Crittenden, J. C.; Luft, P.; Hand, D. W.; Oravitz, J. L.; Loper, S. W.; Ari, M. Prediction of Multicomponent Adsorption Equilibria Using Ideal Adsorbed Solution Theory. Enuiron. Sci. Technol. 1985, 19, 1037. El-Dib, M. A.; Moursy, A. S.; Badang, M. I. Role of Adsorbents in the Removal of Soluble Aromatic Hydrocarbons from Drinking Water. Water Res. 1978, 12, 1131. Endicott, D. D.; Weber, W. J., Jr. Lumped-Parameter Modelling of MulticomponentAdsorption in the Treatment of Coal Conversion Wastewater by GAC. Environ. Prog. 1985, 4, 105. Feeney, E. C. Removal of Organic Materials from Wastewaters with Polymer Adsorbenta. In Ion Exchange for Pollution Control; Calmon, C., Gold, H., Eds.; CRC Press: Boca Raton, FL, 1979; Vol. 11, Chapter 4. Fox, C. R. Removing Toxic Organics from Wastewater. Chem. Eng. Prog. 1979, 75, 70. Furusawa, T.; Smith, J. M. Intraparticle Mass Transport in Slurries by Dynamic Adsorption Studies. AIChE J. 1974,20,88. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, 2nd ed.; Academic Press: London, 1982. Gustafson, R. L.; Albright, R. L.; Heisler, J.; Lirio, J. A.; Reid, 0. T., Jr. Adsorption of Organic Species by High Surface Area Styrene-Divinylbenzene Copolymers. Ind. Eng. Chem. Prod. Des. Deu. 1968, 7, 107.
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1345 Isacoff, G.; Bitner, J. A. Resin Adsorbents Take on Chlororganics from Well Water. Water Sewage Works 1979, 126, 41. Itaya, A.; Kato, N.; Yamamoto, J.; Okamoto, K. I. Liquid Phase Adsorption Equilibrium of Phenol and its Derivatives on Macromolecular Adsorbents. J. Chem. Eng. Jpn. 1984,17(4), 389-395. Jossens, L.; Prausnitz, J. M.; Fritz, W.; Schlunder, E. U.; Myers, A. L. Thermodynamics of Multi-Solute Adsorption from Dilute Aqueous Solutions. Chem. Eng. Sci. 1978,33,1097. Kim, B. R.; Snoeyink, V. L.; Saunders, F. M. Adsorption of Organic Compounds by Synthetic Resins. J. Water Pollut. Control Fed. 1976, 48, 120. Kong, E. J.; DiGiano, F. A. Competitive Adsorption Among VOCs on Activated Carbon and Carbonaceous Resin. J. Am. Water Works Assoc. 1986, 78, 181. Kunin, R. Polymeric Adsorbents for Treatment of Waste Effluents. Polym. Eng. Sci. 1977, 17, 58. Love, 0. T., Jr.; Eilers, R. G. Treatment of Drinking Water Containing Trichloroethylene and Related Industrial Solvents. J. Am. Water Works Assoc. 1982, 78, 413. Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 2nd ed.; Chapman and Hall: New York, 1984. Lykins, B. W.; Clark, R. M.; Adams, J. Q. Granular Activated Carbon for Controlling THMs. J. Am. Water Works Assoc. 1988,80,85. Mangaraj, D.; Bhatnagar, S. K.; Rath, S. B. Cohesive-Energy Densities of High Polymers Part 111. Estimation of C.E.D. by Viscosity Measurements. Makromol. Chem. 1963,67, 75. McGuire, M. J.; Suffet, I. H. Adsorption of Organics from Domestic Water Supplies. J. Am. Water Works Assoc. 1978, 70, 621. Meisner, H. P. Critical Constants from Parachor Refraction. Chem. Eng. Prog. 1949,45, 149. Mieure, J. P. A Rapid and Sensitive Method for Determining Volatile Organohalides in Water. J. Am. Watr. Works. Assoc. 1977, 69, 60. Neely, J. W. A Model for the Removal of Trihalomethanes from Water by Ambersorb XE-340. In Actiuated Carbon Adsorption of Organics from the Aqueous Phase; McGuire, M. J., Suffet, I. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. 11. Nicholson, B. C.; McGuire, B. P.; Bursill, D. B. Henry’s Law Constants for the Trihalomethanes: Effects of Water Composition and Temperature. Enuiron. Sci. Technol. 1984, 18, 518. Noll, K. E.; Gounaris, V. Gravimetric Measurement of Adsorption Parameters in Water Systems with a Quartz Spring Balance. Water Res. 1988, 22, 815. O’Brien, R. P.; Clemens, M. M.; Schuliger, W. G. Treatment of Contaminated Groundwaters with Activated Carbon. Adsorption and Ion-Exchange; AIChE Symposium Series 230; AIChE: New York, 1983; Vol. 83, p 44. Paleos, J. Adsorption from Aqueous and Non-aqueous Solutions on Hydrophobic and Hydrophilic High Surface-Area Copolymers. J. Colloid Int. Sci. 1969, 31, 7. Quayle, 0. R. The Parachors of Organic Compounds. Chem. Reu. 1953, 53, 439. Ram, N. M.; Morris, J. C. Selective Passage of Hydrophilic Nitrogenous Organic Materials through Macroreticular Resins. Enuiron. Sci. Technol. 1982, 16, 170-174. Radke, C. J.; Prausnitz, J. M. Adsorption of Organic Solutes from Dilute Aqueous Solutions onto Activated Carbon. Ind. Eng. Chem. Fundam. 1972, 11,445. Reid, R.; Prausnitz, J. M.; Shemood, T. K. The Properties of Cases and Liquids, 3rd ed.; McGraw-Hill Book Co.: New York, 1977. Reilly Tar and Chemical Co. Reillex Report 2, Indianapolis, IN, 1987. Richard, J. J.; Junk, G. A. Liquid Extraction for the Rapid Determination of Halomethanes in Water. J. Am. Water. Works Assoc. 1977, 69, 62. Rixey, G. W.; King, C. J. Non-Wetting Adsorbents for the Recovery of Solutes from Dilute Solutions. In Fundamentals of Adsorption; Liapis, A. I., Ed.; Engineering Foundation: New York, 1987. Rixey, G. W.; King, C. J. Fixed-Bed Multisolute Adsorption Characteristics of Nonwet Adsorbents. AIChE J. 1989, 35, 69.
Rohm and Haas, Inc. Fluid Process Chemicals: Amberlite XAD-2 and Amberlite XAD-4. Technical Bulletin, Philadelphia, PA, 1978. Sakoda, A.; Kawazoe, K.; Suzuki, M. Correlation of Adsorption Equilibrium Data of Volatile Chlorinated Hydrocarbons from Aqueous Solutions to Activated Carbon Fibers by the DubininAstakhov Equation. J. Chem. Eng. Jpn. 1987, 20, 199. Slejko, F. L. Adsorption Technology; Marcel Dekker: New York, 1985. Smith, E. H.; Weber, W. J., Jr. Modeling Activated Carbon Adsorption of Target Organics from Leachate-Contaminated Groundwaters. Enuiron. Sci. Technol. 1988,22, 313. Smith, E. H.; Tseng, S. K.; Weber, W. J., Jr. Modelling the Adsorption of Target Compounds by GAC in the Presence of Background Dissolved Organic Matter. Enuiron. Prog. 1987, 6, 18. Speth, T. F. Predicting Equilibrium for Single and Multicomponent Aqueous Phase Adsorption onto Activated Carbon. M.S. Thesis, Department of Civil Engineering, Michigan Technological University, Houghton, 1986. Suffet, I. H. An Evaluation of Activated Carbon for Drinking Water Treatment: A National Academy of Sciences Report. J. Am. Water Works Assoc. 1980, 72, 41. Summers, R. S.; Hoist, B.; Koehler, J.; Ritz, J.; Zimmer, G.; Sontheimer, H. The Influence of Background Organic Matter on GAC Adsorption. J. Am. Water Works Assoc. 1989,81, 66. Thakker, S.; Manes, M. Adsorptive Displacement of Many-Component Priority Pollutants on Activated Carbon. Enuiron. Sci. Technol. 1987,21, 546. Thurman, E. M.; Malcolm, R. L.; Aiken, G. R. Prediction of Capacity Factors for Aqueous Organic Solutes Adsorbed on a Porous Acrylic Resin. Anal. Chem. 1978, 50, 775-778. Van Vliet, B. M.; Weber, W. J., Jr. Comparative Performance of Synthetic Adsorbents and Activated Carbon for Specific Compound Removal from Wastewaters. J. Water Pollut. Control Fed. 1981,53, 1585. Weast, R. C., Ed. Handbook of Chemistry and Physics, 59th ed.; CRC Press, Inc.: Boca Raton, FL, 1978; pp F215-F218, D228D229. Weber, W. J., Jr. Effectiveness for Removal of Volatile Halogenated Hydrocarbons from Drinking Water. In Viruses and Trace Contaminants in Water and Wastewater; Borchardt, J. A., et al., Eds.; Ann Arbor Science: Ann Arbor, MI, 1977. Weber, W. J., Jr. Adsorption Theory, Concepts, and Models. In Activated Carbon Adsorption of Organics from the Aqueous Phase; McGuire, M. J., Suffet, I. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. I. Weber, T. W.; Chakravortz, R. K. Pore and Solid Diffusion Models for Fixed Bed Adsorbers. AIChE J. 1974,20(2), 228-238. Weber, W. J., Jr.; Morris, J. C. Equilibrium and Capacities for Adsorption on Carbon. J. Am. SOC. Ciuil Eng. 1964, 90 (SA 31, 79. Weber, W. J., Jr.; Pirbazari, M. Adsorption of Toxic and Carcinogenic Compounds from Water. J. Am. Water Works Assoc. 1982, 74, 203. Weber, W. J., Jr.; Smith, E. H. Removing Dissolved Organic Contaminants from Water. Enuiron. Sci. Technol. 1986, 20, 970. Weber, W. J., Jr.; Smith, E. H. Simulation and Design Models for Adsorption Processes. Enuiron. Sei. Technol. 1987, 21, 1040. Weber, W. J.; Van Vliet, B. Synthetic Adsorbents and Activated Carbons for Water Treatment Statistical Analysis and Interpretations. J. Am. Water Works Assoc. 1981, 73, 426. Youssefi, M.; Faust, S. D. Adsorption and Formation of Light Halogenated Hydrocarbons and Humic Acid in Water by GAC. In Activated Carbon Adsorption of Organics from the Aqueous Phase; McGuire, M. J., Suffet, I. H., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. I. Receiued for review August 24, 1989 Reuised manuscript received February 5, 1990 Accepted March 8, 1990
