I n d . Eng. Chem. Res. 1995,34, 575-584
575
SEPARATIONS Coupling Ion Pair Extraction with Adsorption for the Separation of Acidic Solutes from Water Gregory F. Payne* and Suresh Ramakrishnan Department of Chemical and Biochemical Engineering and Center for Agricultural Biotechnology, University of Maryland Baltimore County, Baltimore, Maryland 21228
To achieve selectivity for the adsorptive separations of acidic solutes, we examined the coupling of ion pair extraction with adsorption. Specifically, we used the quaternary ammonium extractant tricaprylmethylammonium chloride ( d i q u a t 336, Henkel), the inert solvent hexane, and a polar acrylic ester sorbent (Amberlite XAD-7, Rohm and Haas), and studied the selective extractionladsorption of benzenesulfonate relative to benzoate. At high pH, when both acids are deprotonated, benzenesulfonate was selectively removed from water in this coupled system and a separation factor of 4.8 f 0.5 was measured. This separation factor resulted because the benzenesulfonate ion pair is preferentially partitioned (2.8-fold) and preferentially adsorbed (1.7fold), compared to the benzoate ion pair. At lower p H s when benzoate, but not benzenesulfonate, is protonated, the separation factor for benzenesulfonate was considerably larger. Thus by exploiting differences in ion pair partitioning and adsorption, and differences in pKa’s,the acids could be separated using the coupled extraction and adsorption system.
Introduction For a variety of reasons, there has been a renewed interest in industrial separations. For example, increased energy costs forced industries to consider less energy-intensive alternatives to distillation (Humphrey and Seibert, 1992). Energy considerations stimulated the development of liquid phase adsorption processes for operations such as fractionating xylenes and separating linear from branched chain aliphatics (Broughton, 1984- 1985; Ruthven, 1984; Jasra and Bhat, 1988). With respect to gas phase separations, pressure swing adsorption and membrane separations provided alternatives to cryogenic separation of air (Sircar and Kratz, 1989; Mariwala and Foley, 1994; Li and Govind, 1994). In addition to energy considerations, the cost and potential liability for waste disposal are requiring the chemical process industries t o invest in separations for waste minimization (e.g., for the recovery and recycling of reusable materials) (Ciric and Huchette, 1993). Finally, an emphasis on improved product quality is mandating that chemical manufacturers (especially in the food, pharmaceutical, and agricultural chemical industries) implement separations approaches t o improve product purities. Although potentially capable of addressing many of the current separations problems, adsorption has been slow to impact industrial processing. Typically, adsorption is only considered when more established alternatives (e.g., distillation) fail to adequately meet a particular need (e.g., the fractionation of xylenes). One of the major limitations to adsorption-based separations is the low selectivity of currently available adsorption systems. Separation factors for such operations as the separations of fructose from glucose or for the SORBEX processes are low (typically less than 5) and thus multiple-equilibrium-staged processing is required to achieve separations (Ching and Ruthven, 1984; Ho et al., 1987; Cheng and Lee, 1992). Because of the difoaaa-588519512634-o575$09.00/0
ficulties of obtaining countercurrent flow with a solid sorbent, multistaged contacting is achieved through extensively engineered and expensive simulated moving-bed operations (Ruthven, 1984; Keller et al., 1987; Wankat, 1987). If higher separation factors could be achieved with adsorption systems, then the need for multistaged processing would be reduced and adsorptive separations could be economically applied t o a wider range of industrial separation problems (Streat and Cloete, 1987; Keller, 1989). A second major limitation to adsorption (especially for liquids) is the current inability to accurately predict multicomponent equilibria. This limitation prevents adsorptive separation operations from being confidently inserted into process designs as compared to more established distillationbased operations. In our studies, we are investigating how specific mechanistic interactions can be exploited to confer selectivity to the adsorption process. To improve selectivity, it is necessary both to exploit specific mechanisms for adsorption and to limit additional mechanisms which can result in the indiscriminate binding of competing compounds. To address these needs, we chose to exploit the unique and uniform surface chemistries of polymeric adsorbents. Because it is well-known that hydrogen bonding is important in conferring selectivity to binding, we began by examining how hydrogen bonding could be exploited for adsorption. Initially we observed that hydrogen bonding appeared to be responsible for the adsorption of simple solutes (e.g., phenol, benzyl alcohol, and aniline) from nonpolar solvents (e.g., hexane) onto acrylic ester sorbents (e.g., Rohm and Haas XAD-7). Competing mechanistic interactions appeared to be less important under the conditions studied (Payne et al., 1989). Because of the specificity of the hydrogen bonding adsorptive mechanism, we observed that solutes could be selectively bound from hexane onto the acrylic ester sorbent with separation factors depending on the
0 1995 American Chemical Society
676 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995
relative abilities (Payne and Ninomiya, 1990) and strengths (Maity et al., 1991)for hydrogen bonding. For instance, even though both cresol and N-methylaniline can hydrogen bond from hexane onto the acrylic ester resin, cresol was observed to be preferentially adsorbed with a separation factor of 14 because it forms a hydrogen bond which is 3 kcaVmol stronger than that of N-methylaniline. In subsequent studies, we examined how operations and mechanisms could be coupled to enhance the selectivity of adsorption from aqueous solutions. Specifically, we coupled partitioning of the solute from the aqueous phase into a nonpolar solvent (e.g., hexane) with adsorption of the solute from the solvent onto the acrylic ester sorbent. With respect t o mechanistic interactions, it appears that adsorption in this coupled extraction-adsorption system results from the hydrophobic interactions associated with partitioning and the hydrogen bonding mechanism of adsorption. Critical to the success of this coupled system is that the solvent must “protect”the hydrogen bonding sites of the sorbent from direct contact with the aqueous phase. Phenomenological data suggest that when the hydrogen bonding sites are not “protected” and the sorbent is directly contacted with water, hydrogen bonding cannot be exploited for adsorbing the organic solute (Payne et al., 1989)-presumably due to water’s propensity t o hydrogen bond (e.g., Fersht, 1985; Jencks, 1987). Supporting our hypothesis that different mechanisms are responsible for solute adsorption were observations that the adsorption trends (Maity and Payne, 1991) and the adsorption energies (Payne and Maity, 1992) differed depending on whether the solutes were adsorbed from an aqueous or a nonpolar solvent (Maity and Payne, 1991). By “protecting”the hydrogen bonding sites from water, our data further indicated that the specificity of this adsorptive mechanism could be exploited to yield improved separation factors compared to the same sorbent in which the hydrogen bonding sites were not “protected” but were in direct contact with the aqueous phase (Payne and Maity, 1992). In addition to improved separation capabilities, it was possible to quantitatively describe the overall equilibrium of this coupled system using a linear free energy relationship that equates the overall equilibrium constant for the process to the product of the individual equilibrium constants for partitioning and adsorption (Maity and Payne, 1991; Payne and Maity, 1992). In the work reported here, our goal was to combine mechanistic interactions by coupling ion pair extraction with adsorption. As illustrated in Figure 1, we studied the partitioning of an acid from an aqueous phase using a quaternary ammonium ion extractant. Once the acid is partitioned as the ion pair, it can be adsorbed onto the polar surface of the sorbent. From a mechanistic standpoint, removal of a solute in the system of Figure 1 should result from the hydrophobic and electrostatic interactions associated with ion pair extraction and the polar mechanism associated with adsorption. Ion pair extraction is commonly used to permit organic acids and bases to be partitioned from an aqueous phase into an organic solvent (e.g., Kertes and King, 1986; Schill and Persson, 1991; Tamada et al., 1990; Tamada and King, 1990a,b; Yang et al., 1991). Because much of the work with ion pair extraction is focused on enhancing partition coefficients, the solvent phase typically contains diluents such as octanol which enhance partitioning. Since our goal is to improve selectivity and not simply
Organic Phase
1.1
J’
li HX
H+
I
1
A+X-
Quaternary Ammonium ton (A+)
+
Dz
I
X-
Ka
Aqueous Phase Figure 1. Schematic illustrating various species present at equilibrium in the three (aqueous, organic, and sorbent) phases. Using equilibrium relations and overall material balances for solute and quaternary ammonium extractant, equilibrium concentrations of all species shown in this figure can be calculated.
partition coefficients, we have chosen to use a noninteracting solvent (e.g., hexane). In addition to improving partition coefficients, previous studies with ion pair extraction have demonstrated that “chemical theories” of equilibria can be used such that simple mass action laws can adequately describe the liquid-liquid equilibria (Tamada et al., 1990; Tamada and King, 1990a,b; Mizelli and Bart, 1994). In our work, to avoid the complex stoichiometries associated with tertiary amine extractants, we limited our studies to the quaternary ammonium extractant tricaprylmethylammonium chloride (Aliquat 336,Henkel). The overall goal of the work reported here was to examine the system in Figure 1 and determine (1)if coupled ion pair extraction-adsorption can be exploited for separations and (2) whether chemical theories and linear free energy relationships can adequately describe the equilibria in this system. To develop a mathematical framework for this coupled extraction-adsorption system, we used the assumptions listed in Table 1. In addition, as suggested in Table 1, we confined our experimental studies to relatively low concentrations of solute (less than 0.3 mM in water) and extractant (less than 0.045 w t % in hexane). By using dilute conditions, it was possible to examine the intrinsic potential of coupled ion pair extraction-adsorption without introducing complexities associated with the solute-solute interactions which would be present at higher concentrations. As our experimental model, we chose benzoic and benzenesulfonic acids since the large difference in pKa (4.0and 0.7 for benzoic and benzenesulfonic acids, respectively) could be exploited to rigorously test our mathematical description of the coupled system. Materials and Methods Materials. The solutes, benzoic acid and benzenesulfonic acid, were obtained from Aldrich Chemicals (Milwaukee, WI). The inert solvent n-hexane was obtained from Fisher Scientific (Springfield, NJ). The tricaprylmethylammonium chloride (Aliquat 336) was kindly donated by Henkel Corp. (Milwaukee, WI). The acrylic ester sorbent (Amberlite XAD-7) and the ethylvinylbenzene-divinylbenzene sorbent (AmberliteXAD16) were kindly donated by Rohm and Haas (Spring
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 577 Table 1. Assumptions and Experimental Conditions Used To Facilitate Modeling of the Coupled Extraction-Adsorption Operation of Figure 1 Assumptions 1. Activity coefficients remain constant for the dilute solutions studied (i.e., pseudoequilibrium constants expressed in terms of concentrations can be used to describe the various equilibria). 2.There is a 1:l stoichiometry for ion pair formation between the solute anion (X-) and the quaternary ammonium ion (A+). 3.The hydrophobic extractant exists only as the quaternary ammonium ion or acid-ammonium ion pair, and only in the hexane phase. 4. Adsorption of the neutral acid is negligible. 5. Adsorption of one ion pair does not affect adsorption of the other. Experimental Conditions 1.The concentrations of solutes and extractant were small such that the number of bound adsorption sites was small relative to the total number of sites (Le., adsorption was confined to the linear region of the isotherm).
Table 2. Extinction Coefficients for the Solutes Studied” aqueous phase organic phase solute benzoic acid benzoate benzenesulfonate
E225
E212
€220
€232
10.81 8.12 0.63
4.32 3.25 7.26
b 13.46 7.72
b 7.27 0.62
a Extinction coefficients expressed in AU/(mM-cm). Because of the poor partitioning, the extinction coefficients for benzoic acid in the hexane phase were not determined.
House, PA). Prior to use, the sorbents were rinsed with methanol and hexane, air-dried, and finally evaporated to dryness in a vacuum oven for a day. Analysis. Liquid phase solute concentrations were measured using a Hewlett Packard 8452A diode array spectrophotometer. In single-solute studies, concentrations were determined by measuring absorbances at 225 nm for benzoate and 212 nm for benzenesulfonate in the aqueous phase, and 220 nm for benzoate and 218 nm for benzenesulfonate in the hexane phase. As can be seen from Table 2, the extinction coefficients for benzoate and benzoic acid are different, and thus at pH values near the pKa, a Henderson-Hasselbach equation is used to relate measured absorbance (Abs) to the total aqueous phase concentration ((7,). Specifically, when a cuvette of 1 cm path length is used, the relationship is given by
In the above equation, E=,, and ex-,, are the extinction coefficients of benzoic acid and benzoate in water. In studies involving mixtures of benzoate and benzenesulfonate, absorbances were measured at two suitable wavelengths in each phase as listed in Table 2. Because of overlap in the spectra, it was necessary to relate the observed absorbance at each wavelength to the sum of the absorbance contributions from each solute. Initial studies were performed to ensure additivity of the U V spectra. Methods. In extraction studies, 10 mL aqueous solutions were contacted with 10 mL of hexane phase containing differing initial extractant concentrations. After equilibration for 12 h, both aqueous and hexane phase solute concentrations were measured by spectrophotometry. When studies were conducted at pH values near the pKa of benzoic acid, the equilibrium pH was also measured. In initial adsorption studies, each acid was first extracted as an ion pair into a hexane phase containing the extractant. After decanting away the aqueous
phase, hexane phase containing the ion pair was distributed into bottles containing preweighed amounts of dry sorbent. After equilibrating for 1 day, the hexane phase ion pair concentrations were measured by spectrophotometry. The adsorbed ion pair concentration was determined using the equation
4=
CCAX,ho
- cAX,h)vh M
where q is the equilibrium adsorbed ion pair concentraand C&h are the initial tion (mmoVg dry sorbent), CAXJ~” and final concentrations of the hexane phase ion pair (“om), vh is the volume of the hexane phase (L), and M is the mass of the dry sorbent (g). To couple extraction and adsorption, aqueous solutions were contacted with a hexane phase containing the quaternary ammonium extractant and the acrylic ester sorbent. After equilibration, the total liquid phase solute concentrations in both phases were measured spectrophotometrically. The adsorbed phase ion pair concentration (a)was calculated from a material balance as
(3) where C,” and C, are initial and equilibrium aqueous phase solute concentrations, c h is the equilibrium hexane phase concentration, and V, is the volume of the aqueous phase. In these studies, equal volumes of aqueous and hexane phases were used. When studies were conducted at pH values near the pK, of benzoic acid, pH was measured after equilibration. In separation studies, an aqueous mixture of benzoic acid and benzenesulfonic acid was contacted with a hexane phase containing quaternary ammonium extractant and acrylic ester sorbent. After equilibration, the pH and absorbances at two wavelengths per liquid phase were measured. The adsorbed ion pair concentrations of each of the solutes was calculated using eq 3. Then a separation factor (a)was calculated using the definition (4) where the subscripts bs and b refer to benzenesulfonate and benzoate, respectively. To test for experimental reproducibility, we performed six separation studies at a pH of 6.5 and observed that for a 95%confidence limit the separation factor was 4.8 & 0.5. All studies reported in this work were conducted at 30 “ C .
578 Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995
Results Extraction. To describe the coupled extractionadsorption operation, it is necessary to understand the individual equilibria in Figure 1. The equilibrium dissociation of an acidic solute in an aqueous solution is described by HX(aq)
-
- i
/
Benzenesulfonate,
’fi e,
Ka = (CX-,w)(cH+,w)/cm,w( 5 )
where the subscript w indicates the aqueous phase. Using the above equilibrium relation and the definition of the PKa, the aqueous phase concentrations of the neutral (CHX,~) and anionic (CX-,,) forms of the solute can be expressed in terms of the total aqueous phase solute concentration (C,) as
(7) The equilibrium associated with the partitioning of the neutral form of the solute between the aqueous and hexane phases is shown on the left side of Figure 1 as
-
HX(hex)
D1= cm,h/cm,w (8)
where the subscript h denotes the hexane phase. The third equilibrium, partitioning of the solute as an ion pair with a quaternary ammonium extractant, is shown on the right side of Figure 1 and is described by X-(aq)
,
+ X-(aq)
H%q>
HX(aq)
*.O
+ A+(hex)
-
A’X(hex)
where CA+denotes the equilibrium concentration of the quaternary ammonium extractant that is not bound as an ion pair. Combining the above three equilibrium relations (eqs 5, 8, and 9),the overall distribution coefficient (D) is given by
Equation 10 should describe the pH dependence of the overall distribution coefficient provided the individual parameters D1, D2, and pKa are known and the initial amounts of the solute and quaternary ammonium extractant are specified. To determine D2 values, single-solute extraction studies were carried out a t high pH (pH >> PKa) such that the neutral form of the acid in either phase was negligible. Under conditions of high pH, eq 10 simplifies to
(11) where the equilibrium concentration of the unbound extractant (CA+)must be calculated from an appropriate extractant material balance. Experimentally, aqueous solutions were contacted with the organic (hexane) phase containing differing initial concentrations of the extractant. Figure 2 shows the relation between the
d
I
I-
Ind. Eng. Chem. Res., Vol. 34,No. 2,1995 579 0.6
I
I
,
I
I
a
I
0.15
0.5-
0
0.4 -
0
1
Adsorption of Benzoate
Benzoate -
0.121
-
1
L
/
-
0.1 -
0.0
I
1.2
I
'
I
I
I
I
o.03j
Styrenic
I
/
I
Benzenesulfonate
I
I
I
Adsorption of Benzenesulfonate
t
t
a/
8
U 0.6
c
a E
1
gL----A
a
0.1
0.2
0.02.0
3.0
5.0
4.0
6.0
7.0
PH Figure 3. Effect of pH on overall distribution coefficient (D)for (a)benzoate and (b) benzenesulfonate. Initial aqueous phase solute concentrations were 0.2 and 0.3 mM for benzoate and benzenesulfonate, and the initial hexane phase extractant concentration was 0.25 mM (0.015 w t % in hexane). Ten milliliters of each phase was used, and the aqueous phase pH was measured after equilibration. The solid lines were obtained from eq 10.
of the sorbent's surface chemistry on the adsorption equilibrium (i.e., the adsorption isotherm). For this we partitioned the solute as an acid-ammonium ion pair into hexane, decanted the organic phase, and then added dry sorbent to this organic. After equilibration, the isotherms were measured, and as indicated in Figure 4a, the benzoate ion pair was observed to adsorb onto the polar acrylic ester sorbent (Amberlite XAD-7), but not onto the ethylvinylbenzene-divinylbenzene sorbent (Amberlite XAD-16). Similarly, Figure 4b shows that the benzenesulfonate ion pair adsorbs onto the acrylic ester but not onto the aromatic sorbent. Thus the results in Figure 4 demonstrate the importance of the sorbent's surface chemistry, with ion pairs being observed to bind only to the more polar acrylic ester sorbent. As seen in Figure 4, the adsorption studies were confined to the linear regions of the adsorption isotherm. In such cases, the equilibrium for adsorption of the ion pair to the binding sites (SIcan be described by
where q is the adsorbed ion pair concentration in mmoVg calculated by eq 2, C&h is the hexane phase ion pair concentration in mmoVL, and the ratio Kads is a pseudoequilibrium constant for adsorption in Ug.
-
Styrenic
1
I
I
0.0
0.1
0.2
0.0
C,,,
t I
0.3
("OW
Figure 4. Isotherms for adsorption of (a) benzoate and (b) benzenesulfonate ion pairs from organic (hexane plus extractant) phase onto acrylic ester sorbent (AmberliteXAD-7) and aromatic, styrene-type sorbent (Amberlite XAD-16).
For direct comparison, Figure 5 shows the isotherms for the adsorption of the benzoate and the benzenesulfonate ion pairs onto the acrylic ester sorbent. As can be seen from Figure 5, the benzenesulfonate adsorbs with a higher affinity than the benzoate ion pair. Kads values were measured to be 2.12 and 1.25 Ug for benzenesulfonate and benzoate ion pairs, respectively. Coupled Extraction and Adsorption. To accurately describe the coupled extraction and adsorption, it is necessary to combine the four equilibria shown in Figure 1. Specifically,the adsorbed concentration of the ion pair, q, can be related to the total aqueous phase concentration of the acidic solute, Cw,by
4 = Karjs[C~,dCwlCw
(14)
where the bracketed term in eq 14 describes the partitioning step. At pH values much greater than the pKa, when eq 10 can be simplified to eq 11, eq 14 becomes
where D2 and Kads values were obtained from the twophase studies of Figures 2 and 5, and CA+must be calculated from an appropriate quaternary ammonium extractant material balance. To test ifeq 15 can predict three-phase (aqueous, hexane and sorbent) behavior using two-phase data, experiments were conducted in which aqueous phase anionic solutions were equili-
580 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 I
0.40
1
1
,
I
0.12'
Adsorption onto Acrylic Ester Sorbe%t
'
'
"
'
'
/
,
'
Benzoate
'
"
'
'
o
O /
Y o
d
0.20
O.l01 0.08
Benzenesulfonate
dg 0.04 0.02
I
I
1
I
I
0.00
0.04
0.08
0.12
0.16
0.00
I
0.20
CAX,h( " O W
Figure 5. Comparison of adsorption isotherms for adsorption of benzoate and benzenesulfonate ion pairs from organic phase onto acrylic ester sorbent (AmberliteXAD-7).Adsorption affinities ( K d ) were observed to be 1.25 Ug for benzoate and 2.12 Ug for benzenesulfonate ion pairs.
brated with hexane solutions containing extractant and acrylic ester sorbent (Amberlite XAD-7). To ensure that the sorbent surface was exposed to hexane, and not to water, dry resin was contacted with hexane to allow the pores to be filled with this solvent. Once the pores are filled, the hexane is not displaced by water (Maity and Payne, 1991). Figure 6 shows q as a function of the aqueous phase concentration (C), for benzoate and benzenesulfonate. The solid lines in Figure 6 were generated from eq 15 using the solute's previously measured parameters D2 and Kads. As can be seen from Figure 6, there is good agreement between experimental and predicted values demonstrating the model's ability to predict three-phase behavior using equilibrium data derived from two-phase studies. It should be noted that since different initial quaternary ammonium extractant concentrations were used in the benzoate and benzenesulfonate studies, we have displayed the results in separate plots. To more rigorously test the model in Figure 1, we conducted coupled extraction-adsorption studies over a pH range. For any pH, eqs 7, 9, and 14 can be combined to yield
where the bracketed term is the ratio of the hexane phase concentration of the ion pair to the total aqueous phase concentration. Again Kadsand D2 were previously determined parameters and CA+must be calculated from an appropriate quaternary ammonium extractant material balance. It should be noted that adsorption of the neutral species is neglected in eq 16 because it is small relative to adsorption of the ion pair. The presence of the CA+term in eq 16 precludes us from expressing (q/ C ), as an explicit function of pH alone. Thus, to predict equilibrium concentrations for the individual species in Figure 1, it is necessary to solve all four equilibrium relations (eqs 5,8,9, and 13) and two material balances (for the solute and extractant) for each pH. Using the Maple Waterloo software package (Version 5.0.2, University of Waterloo, Canada) and eq 16, we calculated q/C, values expected for a pH range of 2.0-6.0 with increments of 0.25 pH unit. These predicted values are shown as the smooth curves in Figure 7.
0.001 ' I ' I ' I ' I ' I 0.00 0.02 0.04 0.06 0.08 0.10 Cw(mmoUL) 0.35'
a
'
'
"
I
'
I
'
I
Benzenesulfonate
0.301
1
'
0.12
'
I
/
0.25
0
0.02
0.04
0.06
0.08
0.1
0.12
Cw(mmol/L)
Figure 6. Plot of q versus C, for adsorption of (a) benzoate and (b) benzenesulfonate in a three-phase system. For studies with benzoate, the initial aqueous phase benzoate and hexane phase extractant concentrations were 0.2 and 0.4 mM (0.024 wt % in hexane), respectively. For studies with benzenesulfonate, initial aqueous phase benzenesulfonate and hexane phase extractant concentrations were 0.3 and 0.37 mM (0.022 wt % in hexane), respectively. Equal volumes of the aqueous and hexane phases were used in all studies. The solid lines were obtained from eq 15.
Model predictions for benzoic acid are shown in Figure 7a. At low pH, the qIC, for benzoic acid is predicted to be small because of the poor partitioning of the neutral acid and the assumption that adsorption of the neutral acid from hexane onto the acrylic ester sorbent is negligible. As the aqueous phase pH is increased near the PKa of benzoic acid (PKa = 4.0),Figure 7a shows that (q/C,) is expected to increase. At these intermediate pH values, significant dissociation of benzoic acid occurs with a corresponding increase in the partitioning and adsorption of the benzoate ion pair. At pH values above the pKa of benzoic acid, Figure 7a shows that (q/ C ), is predicted to approach (Kad&CA+). Thus the predicted dependence of (q/C,) on pH is dominated by the partitioning step, or the bracketed term in eq 16. For benzenesulfonic acid, the pKa is significantly below the pH values considered, and thus eq 16 predicts q/C, to be independent of pH. To experimentally test the model predictions of Figure 7, single-solute aqueous solutions of differing pH were contacted with a hexane phase containing quaternary ammonium extractant and the sorbent. After equilibration, the aqueous phase concentration (C), and pH were measured, and q was calculated by eq 3. These experimental measurements which are indicated by the
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 581 1
0.5
1
1
1
1
,
1
1
1
1
0
0.4-
-
0.3 -
-
0.2
-
-
0.1
-
-
3
24
1
A\
A \ A
A
0.0
I
O
'
I
'
I
'
I
0 )
'
2.5
A
I
I
I
1
3.5
4.5
5.5
6.5
PH Figure 8. pH dependence of separation factor for selective removal of benzenesulfonate from an aqueous mixture in a threephase system. The aqueous phase initially contained 0.2 mM of benzoate and 0.2 mM of benzenesulfonate while the initial hexane phase quaternary ammonium extractant concentration was 0.74 mM (0.045 wt % in hexane). The aqueous phase pH was measured after equilibration. The solid line was obtained from eq 17.
Benzenesulfonate
f 1.2
VE
0.8
0.0
t
-1 I
Ft
1 2
I
I
I
I
I I
3
4
5
6
I
PH Figure 7. pH dependence of q/Cwfor (a) benzoate adsorption and (b) benzenesulfonate adsorption in a three-phase system. For studies with benzoate, initial aqueous phase benzoate and hexane phase extractant concentrations were 0.2 and 0.25 mM (0.015 w t % in hexane), respectively. For studies with benzenesulfonate, initial aqueous phase benzenesulfonate and hexane phase extractant concentrations were 0.2 and 0.3 mM (0.018 w t % in hexane), respectively. Equal volumes of the aqueous and hexane phases were used in all studies, and the aqueous phase pH was measured after equilibration. The solid lines were obtained from eq 16 as described in the text.
symbols in Figure 7 show good agreement with predicted results. This agreement between theory and experiment demonstrates that mass action relationships determined from two-phase studies can be extrapolated to predict three-phase behavior in this system. Separation Studies. Extending the model developed in eq 16 to a system containing both benzenesulfonate and benzoate, the separation factor defined in eq 4 can be expressed as
above expression. At high pH, eq 17 simplifies to DZ,bs
a=
Kads,bs
7.4 2.12 = 4.82
(%)(G) = (26)(1.25)
which predicts that because the benzenesulfonate ion pair is both preferentially partitioned (2.8-fold) and preferentially adsorbed (1.7-fold1, it can be selectively bound compared t o the benzoate ion pair. As the pH is reduced and the protonation of benzoate becomes important, eq 17 predicts that the separation factor will increase. This prediction is shown as the solid line in Figure 8. To experimentally verify the above model, aqueous mixtures of benzoate and benzenesulfonate were contacted with a hexane phase containing the extractant and the adsorbent. Solute concentrations in the two liquid phases were measured, and eq 3 was used to calculate adsorbed ion pair concentrations. Using the definition given by eq 4,a separation factor at high pH (pH = 6.5) was experimentally observed to be 4.8 k 0.5, which is in good agreement with eq 18. At intermediate pHs, the separation factors were calculated from experimental measurements and are shown as triangles in Figure 8. The agreement between theory and experiment in Figure 8 demonstrates that the coupled extraction-adsorption system in Figure 1can be exploited for separations, and that simple mass action laws can adequately be extrapolated to predict separation factors.
Discussion where the subscripts bs and b refer to benzenesulfonate and benzoate, respectively. In eq 17, the equilibrium values of D Zand &Is, which were determined from twophase studies (either extraction or adsorption) for the individual solutes, are being extrapolated t o predict separation factors in a three-phase, dual-solute system. The pKa in eq 17 refers to that for benzoic acid. Because of benzenesulfonic acid's low PKa, its partitioning is independent of pH in the range of pH investigated and thus benzenesulfonate's pKa does not appear in the
As illustrated in Figure 1, ion pair extraction was coupled with adsorption with the goals of achieving selectivity and predictability for this adsorption-based separation. Specifically, we believed these goals could be realized if mechanistic interactions could be coupled in a controlled manner and if nonspecific interactions could be limited. Partitioning of the ion pair is believed to result from a combination of electrostatic interactions involved in ion pair formation and hydrophobic interactions which tend to drive nonpolar species from water (Tanford, 1980). Exploiting electrostatic interactions to
582 Ind. Eng. Chem. Res., Vol. 34, No. 2 , 1995
yield an ion pair is analogous to the formation of salt bridges which are known t o be important in molecular recognition of natural systems (Fersht, 1972, 1985; Glockshuber et al., 1991; Economou et al., 1992). Although the importance of the hydrophobic solvent phase was not specifically studied, it seems reasonable that an inert and nonpolar solvent such as hexane may serve three functions for improving the overall selectivity: to limit ion pair partitioning t o more hydrophobic acids by reducing nonspecific solvent-solute (i.e., solvation) interactions; to limit solvent-adsorbent interactions which would compete with the ion pair for adsorption sites; to “protect” the polar binding site of the sorbent from water. Adsorption of the ion pair from the solvent onto the sorbent surface appears to result from a polar interaction-although the exact mechanism is unknown. The involvement of polar interactions in adsorption of the ion pair is indicated by the observation that both the benzoate and benzensulfonate ion pairs were capable of adsorbing onto the polar acrylic ester sorbent but not onto the nonpolar aromatic sorbent (Figure 4). To assess the ability of the coupled extractionadsorption system t o achieve selectivity, we examined solutions of benzoate and benzenesulfonate. As illustrated in Figures 2 and 5, partitioning and adsorption of the benzensulfonate ion pair was 2.8 and 1.7 times more favorable than for the benzoate ion pair. Thus, at high pHs, the coupling of ion pair extraction and adsorption resulted in a separation factor of 4.8 f 0.5 (Figure 8). At lower pH’s (with respect to the pKa of benzoic acid), the separation factor was enhanced considerably because of the protonation of benzoate in the aqueous phase and the poor partitioning of this neutral acid. Thus by coupling mechanistic interactions and exploiting differences in pKLs of the solutes t o be separated, the coupled system in Figure 1 offers a flexible means to separate acidic solutes. To describe the equilibria in this system, we employed “chemical theories” and linear free energy relationships. Chemical theories are commonly used when interactions between compounds result from polar, “chemical”forces, as opposed t o lower energy physical (i.e., van der Waals) forces (Prausnitz et al., 1986). To employ chemical theories, reactions and products are postulated and equilibrium constants are used to describe the thermodynamic behavior of the system. With respect to the partitioning of acids by tertiary amine extractants, Tamada et al. (Tamada et al., 1990; Tamada and King, 1990a,b) provided spectroscopic evidence for the existence of hydrogen-bonded chemical “products” and used chemical theories to describe partition equilibrium. In our work we postulated the formation of an ion pair in the organic phase and described this ion pairing, phase transfer step by the equilibrium partition coefficient D2. To quantify equilibria in the coupled extractionadsorption system of Figure 1,we employed a linear free energy relationship such that the overall equilibrium is related to the equilibrium of the individual steps. Linear free energy relationships are commonly used to describe physicochemical behavior which results from multiple mechanistic interactions. For instance, linear free energy relationships have been used t o describe solute transfer to micelles (Bunton and Sepulveda, 1979; Cabrera and Sepulveda, 19901, solute adsorption to inorganics (Fuersteneau and Jang, 1991), and solute adsorption to organic polymers (Slama-Schwok and Rabani, 1988; Alexandratos and Kaiser, 1990; Rabani
et al., 1991; Garcia, 1991). The advantage of using chemical theories and linear free energy relationships is that these approaches employ models which can reasonably be expected to reflect the physical reality of the system. When physically-reasonable models can be postulated, the subsequent mathematical descriptions generally yield better predictive capabilities. Using the model in Figure 1, it was possible to extrapolate equilibrium constants (D1, D2,and &ds) measured for the individual solutes in two-phase studies to predict separation factors when two solutes and three phases were present (Figure 8). For perspective, it is useful to contrast the approach in this work with alternative approaches t o improve selectivity in adsorption systems. There has been considerable success in conferring selectivity by developing zeolite and carbon sorbents which impose steric limitations at the binding sites. However, not all separation problems can be solved by imposing steric or kinetic limitations, and it would be desirable if binding systems could be developed which confer selectivity through differences in binding energies. To obtain differences in binding energies, specialized binding sites have been built in which specific mechanisms are exploited and coupled such that binding of the desired compound is thermodynamically more favorable than binding of competing species (Pirkle et al., 1986; Cram, 1988; Pirkle and Pochapsky, 1989; Rebek, 1990; Schneider, 1991). Of particular interest is the recent development of molecular imprinting to build specific binding sites into highly specialized sorbents (Mosbach, 1994). Although such specialized sorbents are likely to have significantly improved separation capabilities, the highly ordered three-dimensional arrangement of interactions at the binding site will likely limit their use to individual, high-value applications (i.e., medical applications). In contrast, the approach used in this study is targeted at coupling mechanistic interactions in a generic way without defining a three-dimensional binding site. Obviously, by not defining a threedimensional binding site, the selectivity of such generic adsorption systems will be limited compared to the potential of molecularly imprinted polymers (Ramakrishnan and Payne, 1994). Nevertheless, this generic approach permits concepts of molecular recognition (Le., exploiting and coupling specific mechanistic interactions) to be exploited with readily-available, inexpensive extractants and adsorbents and thus can be applied to a wider range of lower-valued products. Thus, we believe this approach may provide an inexpensive and flexible means to address a range of separation problems. To understand the capabilities and limitations of such a generic approach, however, it is critical to understand the contributions of individual mechanistic interactions and the capabilities for mixing-and-matching interactions t o confer selectivity.
Acknowledgment This work was partially supported by the Rohm and Haas Co. The authors would like to acknowledge the helpful suggestions of Dr. R. L. Albright.
Nomenclature C , = aqueous phase solute concentration, m m o n c h = hexane phase solute concentration, m m o n Cm = neutral species concentration, mmoVL Cx- = ionic species concentration, m m o n
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 583
CAX= ion pair concentration, mmoYL CA+= free (unbound) quaternary ammonium extractant concentration, mmol/L V, = volume of aqueous phase, L Vh = volume of hexane phase, L D = overall distribution coefficient q = adsorbed ion pair concentration, mmoVg M = mass of dry sorbent, g E = extinction coefficient, AU/(mM*cm) a = separation factor Equilibrium Constant
D1 = partition coefficient for neutral acid Dz = partition coefficient for ion pair, IJmmol K, = dissociation constant for acid, IJmmol K,ds = pseudoequilibrium constant for adsorption of the ion pair, L/g Subscripts w = water h = hexane b = benzoate bs = benzenesulfonate Superscript ’ = initial
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Received for review May 10, 1994 Revised manuscript received September 16, 1994 Accepted October 18, 1994@
IE940301F
* Abstract published in Advance ACS Abstracts, January 1, 1995.
