Ind.Eng. Chem. Res. 1993,32, 2269-2276
2269
Sorption Equilibrium Isotherms for Volatile Organics in Aqueous Solution: Comparison of Head-Space Gas Chromatography and On-Line UV Stirred Cell Results Edward J. Simpson, Ramzi K. Abukhadra, William J. Koros,’ and Robert S. Schechter Department of Chemical Engineering, The University of Texas, Austin, Texas 78712
Sorption equilibrium isotherms from aqueous solution of benzene, toluene, chlorobenzene, p-xylene, carbon tetrachloride, trichloroethylene, and chloroform for various sorbents have been measured by two independent techniques: head-space gas chromatography (HSGC) and a stirred cell with on-line UV detection. Isotherms obtained by each technique showed close agreement at all concentrations. Isotherms are presented for XAD4, XAD2, AC-F400, and solid poly(styrene/ divinylbenzene) copolymer microspheres. For solution concentrations 25 % saturated in organic) industrial waste streams, XAD4 would appear to be an attractive alternative to activated carbon. Sorption data for this study were best represented for solutes onto XAD4 by normalized volumetric isotherms and AC-F400 by a Polanyi potential theory correlation plot.
Introduction Several researchershave measured equilibrium sorption isotherms from aqueous solution of volatile organic compounds (VOCs) on activated carbons and commercial macroporouspolymeric resins (Itaya et al., 1984,De Sonier, 1990; No11 et al., 1992). Most comparisons have shown activated carbons to be more effective than the polymeric sorbents (Weber and van Vliet, 1981a,b). Some studies showed sorption isotherms of the polymeric resins had higher slopes than activated carbon, suggesting at higher solute concentrations the polymeric resins might surpass activated carbon in sorption capacity (Browne and Cohen, 1990). Most comparative studies have focused on the “environmental” concentration range ( 30 (Sontheimer et al., 1988).
the solutes were common to both studies (chloroform, toluene, p-xylene, and trichloroethylene),others were also included in the regression analysis (various chlorinated and brominated alkanes and alkenes). Nevertheless, both sets of data predict an equivalent "adsorption volume" of approximately 0.62 cm3/g for each of the solutes as the adsorption potential tends to zero (i.e., as CL C L , ~ ~ ) . Interestingly, this value is close to reported values of the total pore volume for AC-F400. Weber and van Vliet (1988b) used a two-coefficient Dubinin isotherm (a "modified" Polanyi potential plot) to analyze data for various solutes onto XAD4 and AC-F400. While their isotherms for carbon tetrachloride showed good agreement with the current study (see Figure 5) and were measured to reasonably high solute concentrations (CL 0.5C~,,t), their data analysis technique dramatically underestimates the maximum capacity of both AC-F400 and XAD4 for carbon tetrachloride (the only solute common to both studies). Their analysis suggests maximumcapacities of 0.419 cm3/g (vs 0.62 cm3/g in this study) for AC-F400 and 0.41 cm3/g (vs -1.7!) for XAD4. As discussed below, it appears that a pure adsorption model like that used for adsorbents such as activated carbon is inappropriate for the XAD resins. Figure 8 shows a plot used to determine the Freundlich constants for chlorobenzene adsorption onto AC-F4OO. Over the complete concentration range, the data fit the Freundlich isotherm rather poorly (R2= 0.686-0.964). However, one can see that a better fit can be obtained by splitting the concentration range into two regions. A summary of the Freundlich parameters, along with some literature parameters, is presented in Table V. It is widely recognized (Sontheimer et al., 1988) that isotherms extending wide concentration ranges may require adjustment of the KF and n Freundlich parameters. Owing to the differing concentration ranges between this study and the literature data shown in the table, the lack of agreement is not surprising. The data in Table V suggest a trend that as concentration decreases, KF decreases and n increases. Figure 9 shows a plot used to determine the Langmuir constants for carbon tetrachloride adsorption onto ACF400. As shown in Table IV, the data for each solute fit this isotherm quite well over the entire concentration range (R2 = 0.984-0.998). A calculation was performed to estimate the expected monomolecular coverage, qm, for benzene. Using a maximum radius of 3 &molecule (Reid et al., 19771, and assuming a circular coverage area (28.3 A2/molecule), the estimated monomolecular coverage
-
-
9
10
11
12
13
Figure 8. Example plot for determination of Freundlich isotherm parameters for chlorobenzene adsorption on AC-F400 at 25 O C . The full line is obtainedfrom regression over the full concentration range of this study. The dashed lines illustrate the improved fit (as noted in Table V) obtained by splitting the concentration range. CLJis in pg/L in this plot for ease of comparison with previously published Freundlich parameters.
would be 492 mg of benzene/g of AC-F400, versus the reported q m of 528 mg/g. These values are in reasonable agreement, particularly given the uncertainty in both the molecular coverage area of benzene and the specificsurface area of AC-F400 (literature value, not measured on the actual carbon used in the study). In any event, the good fit for this isotherm over the complete concentration range of this study may prove useful in modeling of adsorption systems. Previous researchers have suggested that sorption in polymeric resins is due to combined ad- and absorption contributions (Cornel and Sontheimer, 1986; Garcia and King, 1989). Figure 10 shows the normalized volumetric sorption curves for PSDVB microspheres prepared with 80% DVB. The microspheres were included in the study as potential sorbent candidates and to measure the amount of absorption (or swelling) of the organic solutes into PSDVB copolymers. The sorption levels for the microspheres were found to be a function of the amount of DVB cross-linking agent as expected. The microspheres are not competitive with XAD4 as sorbent candidates. The XAD4 resin is a high surface area agglomeration of 80% DVB microspheres (Maity et al., 1992),albeit the smallest domains in XAD4 are significantly smaller microspheres (-0.03 pm). The PSDVB microspheres have a calculated surface area 1/85th that of the XAD4 (10 vs 870 m2/g). Thus, the sorption levels for the 80% DVB microspheres are due largely to absorption, and provide an estimate of the amount of absorption occurring in the XAD-4 resin. The 80% DVB microspheres suggest the uptake due to absorption in the XAD4 resin is 0.6-0.7 cm3/g. The reported pore volume for XAD-4 is 1.14 cm3/g, while the total uptake from the volumetric sorption plot is within the range of 1.6-1.8 cm3/g. Thus, combining the estimated amount of absorption and the amount required to completely fiil the pores comes reasonably close to the quantity sorbed. Further studies will be aimed at more accurate quantification of the ad- and absorption contributions in XAD4 and XAD2 (including volume dilation measurements). At this point, the above data can be viewed as qualitatively showing significant contribution to sorption by absorption (swelling) in the XAD4 resin. The normalized volumetric sorption and Polanyi potential theory isotherms presented do not necessarily represent "universal" plots for sorption onto XAD4 and
Ind. Eng. Chem. Res., Vol. 32,No. 10, 1993 2275 Table V. Langmuir and Freundlich Isotherm Parameters for Noted Concentration Ranges. ~~
Langmuir isotherm params solute benzene
KL(L/mg)
(mg/g)
528.4
8.48 X 10-8
R2 0.976
carbon tetrachloride
753.4
2.96 X 1k2
0.998
chlorobenzene
589.8
4.53 x
10-2
0.984
chloroform
920.6
1.36 X 10-8
0.984
trichloroethylene
789.2
2.13 X 1k2
0.994
toluene
552.9
3.01 X 1 t 2
0.992
qm
_ _ _ ~
Freundlich isotherm parame
KF(mg/g)/(rcg/L)"
n
R2
CL, range (&L)
8.05 0.265 5.64 9.24 X 1(F2 134.5 6.29 6.9 X lo-' 112.8 3.81 0.248 20.96 3 x 10-2 (1) 0.114 (2) 16.87 2.2 (1) 35.31 7.7 7.28
0.295 0.617 0.38 0.808 0.125 0.368 1.37 0.124 0.348 0.57 0.234 0.748 0.704 0.286 0.47 0.307 0.366 0.36
0.904
1X 1W1.7 X 108 1X 1 6 1 . 7 X 1@ 1X 104-8.5 X l @ 1X 1W8.5 X 104 8 X 168.5 X l@ 5 X 108-5.1 X l @ 5 X 109-5 X 104 5 X 1W5.1 X 106 3 X 1 W 8 X 108 1X 1W1.7 X 106 3 X l W X 108 13-2 X 104 10-757 1X 1 6 1 . 2 X 108 1.3-1.2 X 108 5 X 109-5.8 X l @ 5 X 108-5.8 X 104 1X 109-2.1 X 104
0.929 0.812 0.966 0.884 0.686 0.935 0.939 0.951 0.998 0.963 0.99 0.924 0.991
0.964 0.986 0.933
p-xylene 576.5 5.67 X 1k2 0.99 0 For theFreundlichisotherm, CLjisinpg/Lfor easeof comparisonwithliterature values. Freundlichparameterswithnumbersinparentheses are published values from (1) Browne and Cohen (1990) and (2) Alben et al. (1988). 1
of the solute. Further studies will investigate influence of the above factors.
0.8
Conclusions HSGC and the UV stirred cell are two accurate techniques for aqueous-phase sorption isotherm measurement. These studies demonstrated the following: 1. AC-F400 had higher sorption capacity than XAD4 and XAD2 at concentrationsless than 25 76 of the aqueousphase solubility limit for the solutes in this study. XAD4 had higher sorption capacity at higher concentrations, and therefore may be an attractive alternative in some cases. The absorption levels for PSDVB microspheres were not competitive at any concentration. 2. Presenting the results for XAD4 on normalized volumetric sorption isotherms showed solutes of widely varying physical properties had essentially the same volumetric sorption level at equal normalized concentrations. Thus, one should be able to estimate sorption levels of some volatile organic compounds by XAD resins with acceptable accuracy. 3. The results for AC-F400 were usefully represented by a Polanyi potential theory correlation plot. For the concentration ranges of this study, the data for AC-F400 fit the Langmuir isotherm better than the Freundlich isotherm. 4. Data for PSDVB microspheres of the same polymeric composition as XAD4 qualitatively show a significant absorption contribution in XAD4 isotherms. These data indicate the total sorption level for XAD4 is close to the combined amount required to fill the pores of the XAD4 plus the sorption level in the PSDVB microspheres. Acknowledgment Specials thanks go to research assistants Ying Sun and Sanjay Barman Roy, whose efforts were key to the HSGC and UV stirred cell isotherm measurements. Fred Gadelle also worked on the validation of the HSGC technique. Thanks go to Rohm and Haas Inc. and Calgon Corp. for providing sorbent samples. This research was funded in part by the State of Texas Energy Research in Applications Program Project No. 151 and the Separations Research Program, The University of Texas, Austin. Nomenclature CL,~= liquid-phase concentration of component i, mg/L = aqueous-phase solubility of component i, mg/L
0.6 0.4 0.2 0 0
100
200
400
300
500
700
600
CL,CC14 (mgCC14/L) Figure 9. Example plot for determination of Langmuir isotherm parameters for carbon tetrachloride adsorption on AC-F400 at 25 OC. 0.71
0.3
I
n
,
7
I
i
I
I
,
i
r
,
I
I
Toluene-80%DVB TCE-80%DVB
0.5
0.4
I
I 1
0
*.
0.2
0.4
0.6
I
I
I
.
0.8
1
CL/CL,sa t Figure 10. Normalized volumetric sorption of solutes for 80%DVB microspheres at 25 OC.
AC-Fm. While the properties of the solutes studied here varywidely,more diverse solutes could show more marked variation in sorption levels. Also, parameters such as temperature and solution ionic strength warrant further investigation since they can strongly influence several aspects of the system,namely the aqueous-phasesolubility
2276 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
Cv,i = vapor-phase concentration of component i, mg/L = Polanyi adsorption potential, cal/mol KF = Freundlich isotherm parameter, (mg/g)/(L/pg)n kH,i = Henry's law constant of component i, atm KL = Langmuir isotherm parameter, L/mg mi, = mass of organic solute initially added, mg mL,i= mass of organic solute in liquid phase at equilibrium, mg mv,i = mass of organic solute in vapor phase at equilibrium, mg m, = mass of sorbent, g n = Freundlich isotherm exponent parameter, dimensionless P = pressure, atm Pmt,i = vapor pressure of pure component i, atm q = equilibrium sorption level, mg of solute/g of sorbent q, = monomolecular coverage level from Langmuir isotherm, mg/g qmol = equilibrium molar sorption level, mol of solute/g of sorbent qvol = equilibrium volumetric sorption level, cm3of solute/g of sorbent T = temperature, K V , = molar volume of solute (molecular weight/density),cm3/ mol xi = liquid-phase mole fraction of component i , mole fraction xmt,i = aqueous-phase mole fraction of solute i at saturation, mole fraction yi = vapor-phase mole fraction of component i, mole fraction e
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Keeley, D. F.; et al. Solubility of Aromatic Hydrocarbons in Water and Sodium Chloride Solutions of Different Ionic Strengths: Benzene and Toluene. J. Chem. Eng. Data 1988,33,87. Kunin, R. The Use of Macroreticular Polymeric Adsorbents for the Treatment of Waste Effluents. Pure Appl. Chem. 1976,46,205. Mackay, D.; Shiu, W. Y. A Critical Review of Henry's Law Constants for Chemicals of Environmental Interest. J. Phys. Chem. Ref. Data 1981, 10, 1175. Maity, N.; et al. Caffeine Adsorption from Aqueous Solutions onto Polymeric Sorbents: The Effect of Surface Chemistry on the Adsorptive Affinity and Adsorption Enthalpy. React. Polym. 1992, 17, 273. Manes, M.; Hofer, L. J. E. Applicationof the Polanyi PotentialTheory to Adsorption from Solution on Activated Carbon. J. Phys. Chem. 1969, 73, 584. Montgomery, J. H.; Welkom, L. M. Groundwater Chemicals Desk Reference; Lewis Publishers, Inc.: Chelsea, MI, 1990. Noll, K.E.;etal.Adsorption TechnologyforAirand WaterPollution Control; Lewis Publishers, Inc.: Chelsea, MI, 1992. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill, New York, NY, 1977. Rohm and Haas, Inc. 'Fluid Process Chemicals: Amberlite XAD2 and XAD4"; Technical Bulletin; Rohm and Haas: Philadelphia, PA, 1990. Sakoda, A.; et al. Trihalomethane Adsorption of Activated Carbon Fibers. Water Res. 1991,25, 219. Schoene, K.; et al. Determination of Adsorption Isotherms by Automated Headspace Analysis. J. Colloid Interface Sci. 1983, 91, 595. Sontheimer, H.; Crittenden, J. C.; Summers, R. S. Activated Carbon for Water Treatment;DVGW-Forschungsstelle: Karlsruhe, FRG, 1988. Treiner, C.; Khodja, A. A.; Fromon, M. Micellar Solubilization of 1-Pentanol in Binary Surfactant Solutions: A Regular Solution Approach. Langmuir 1987, 3,729. Tucker, E. E.; Christian, S. D. APrototype Hydrophobic Interaction. The Dimerization of Benzene in Water. J. Phys. Chem. 1979,83, 426. Weber, W. J.; van Vliet, B. M. Synthetic Adsorbents and Activated Carbons for Water Treatment: Overview and Experimental Comparisons. J. Am. Water Works Assoc. 1981a, 73, 420. Weber, W. J.; van Vliet, B. M. Synthetic Adsorbents and Activated Carbons for Water Treatment: Statistical Analyses and Interpretations. J. Am. Water Works Assoc. 1981b, 73, 426. Wylie, P. L. 'Analysis of Volatile Compounds in Water Using the HP 19395A Headspace Sampler"; Hewlett Packard Gas Chromatography Application Note 228-40, 1985.
Received for review January 25, 1993 Revised manuscript received June 22, 1993 Accepted June 25, 1993'
Abstract published in Advance ACS Abstracts, September 15, 1993.
