Langmuir 1991,7,1241-1254
1241
Adsorption from Aqueous Solutions Based on a Combination of Hydrogen Bonding and Hydrophobic Interactions Nirmalya Maity and Gregory F. Payne' Department of Chemical and Biochemical Engineering and Center for Agricultural Biotechnology, University of Maryland Baltimore County, Baltimore, Maryland 21228 Received September 10, 1990. In Final Form: January 2, 1991 By limiting the number of adsorptive mechanisms, we believe it is possible to develop highly selective sorbents. In this work, our goal was to develop a sorbent that was capable of selectivelyadsorbing solutes from water through the formation of a hydrogen bond. When water was the solvent, a polycarboxylic ester resin was the sorbent, and a homologous series of phenylalkanols were used as solutes, it appeared that hydrophobic interactions were a predominant mechanism for adsorption. This conclusionis based on the observation that the affinity for adsorption increased with the number of methylene groups of the solute and that the free energy change for adsorption per methylene group was -0.83RT. This value is similar to free energy changes per methylene group observed for various phenomena that result from hydrophobic interactions. In contrast, when hexane was used as the solvent, we observed that the affinity for adsorption of this homologous series of phenylalkanols was independent of the number of methylene groups. Thus it appears that the mechanism of adsorption onto this polycarboxylic ester sorbent varies depending on the solvent. With hexane as the solvent, we believe adsorption results from the formation of a hydrogen bond between the hydroxyl group of the solute and the carbonyl group of the sorbent. To exploit the specificity of the hydrogen bond for the adsorption of solutes from aqueous solutions, we developed a modified sorbent in which the hydrogen bonding site of the sorbent is retained in a nonpolar environment. For this modified sorbent, we filled the pores of the sorbent with hexane. Solutes that adsorb onto this modified sorbent must first partition from the aqueous phase into the hexane pore-phase, and then the solute must adsorb from the hexane pore-phase onto the hydrogen bonding site of the sorbent. Our results demonstrate that adsorption onto this modified sorbent can be quantitatively described by these individual steps. Thus this modified sorbent is capable of adsorbing solutes from aqueous solution through a combination of hydrophobic interactions (Le. the water-hexane partitioning step) and hydrogen bonding (Le. solute adsorption from the hexane pore-phase).
Introduction In comparison to other physical methods, adsorption is an efficient separations method for the removal of organics from dilute aqueous solutions. Thus adsorption has been used for the removal of various nonpolar compounds from aqueous solutions.' Currently however, sorbents are nonspecific and have limited capabilities for selectively binding a single solute from dilute solute mixtures. In our work, we are focusing on sorbent characteristics that confer selectivity to binding. Such binding selectivity is important if adsorption is to be used for the large scale separation of an organic species from dilute aqueous mixtures. The lack of selectivity in binding has been attributed to two major reasons. First, conventionally used sorbents (e.g. activated carbon) have extremely complex surface chemistries, resulting in a wide variation in the types of binding sites available to the s ~ l u t e s . ~Thus J solutes can be indiscriminately bound to the sorbents through a wide variety of mechanistic interactions. The recent development of polymeric materials with well-defined surface chemistries has provided the opportunity to develop sorbents that can bind through only a single or a small number of mechanisms. By limiting the number of possible interaction mechanisms, it should be possible to design more selective sorbents. A second reason for the loss of selectivity in adsorption from aqueous solution is that hydrophobic interactions (1) Sircar, S.;Myers, A. L. Se . Sci. Technol. 1986,21 (6&7), 535. (2)Zettlemoyer, A.C.;Micale,%.J. In Organic Compounds in Aquatic Enuironmenta; Fauet, 5.J., Hunter, J. V., Me.; Marcel Dekker: New York, 1971;p 165. (3)Mattson, J. S.;Mark, H. B. Actiuated Carbon: Surface Chemistry and Adsorption from Solution; Marcel Dekker: New York, 1971.
are a predominant mechanism in aqueous environments. Hydrophobic interactions lead to adsorption not because of specific solute sorbent interactions but rather because of the limited solvating ability of water.' Thus when hydrophobic interactions are the major driving force for adsorption, then different solutes having similar hydrophobicities will adsorb similarly from aqueous solutions. The overall goal of our work has been to develop selective sorbents by exploiting a single binding mechanism or by specifically combining binding mechanisms. Initially we attempted to characterize adsorption mechanisms and to identify conditions in which a single mechanistic interaction is responsible for adsorption. These studies suggested that hydrogen bonding was responsible for the selective adsorption of solutes from a nonpolar solvent (hexane) onto a polycarboxylic ester sorbent.st6 The hydrogen bond is presumably established between a hydrogen donating group on the solute and the hydrogen accepting carbonyl group of the sorbent. Hydrogen bonding is an attractive mechanism for separation because it is a highly specific and a low energy binding mechanism. However it has been difficult to exploit hydrogen bonding mechanisms in aqueous environments, because of water's own ability to hydrogen bond. From aqueous solutions, hydrophobic interactions seem to be a predominant mechanism for the adsorption of nonpolar solutes onto the polycarboxylic ester sorbent.& The suggestion that the adsorption mechanism varies depending on the solvent is analogous to the suggestion by Furlong and Aston that the adsorption mechanism for nonionic surfac~~~
(4) Muller, N. Acc. Chem. Rea. 1990,23,23. (5)Payne, G.F.; Payne, N. N.; Ninomiya, Y.; Shuler, M. L. Sep. Sci. Technol. 1989,24(5&6), 457. (6) Payne, G. F.; Ninomiya, Y. Sep. Sci. Technol. ISSO,25 (11-121, 1117.
0 1991 American Chemical Society
1248 Langmuir, Vol. 7, No. 6, 1991
Maity and Payne
\ PORE PHASE HEXANE
\
@ 8
....
\
.?
0
2
1
3
4
5
Cw (mmollL)
Figure 1. Modified sorbent with hydrogen bonding site enclosed
in n-hexane.
tants differs depending on whether silica or methylated silica is used as the or bent.^ The goal of the present work was to develop a modified sorbent capable of exploiting the specificityof the hydrogen bond in the adsorption of organic solutes from aqueous solutions. This modified sorbent is illustrated in Figure 1, which shows the hydrogen binding site of the polycarboxylic ester sorbent enclosed within a nonpolar environment. In our studies, the model nonpolar phase was hexane, although alternative solvents of reduced water solubility and lower volatility could presumably be used. As illustrated in Figure 1, the solute (e.g., phenol) must first partition from the aqueous phase into the hexane phase, which is entrapped within the pores of the sorbent. Once in the hexane pore-phase, phenol is capable of adsorbing to the hydrogen binding site of the sorbent. To characterize adsorption to this modified sorbent, we first examined the individual steps of solute partitioning between hexane and water and solute adsorption from hexane onto the sorbent. We then examined adsorption from aqueous solution onto the modified sorbent of Figure 1.
Thermodynamic Framework for Analysis Equilibrium Relationships. For a solute to adsorb to the modified sorbent, shown in Figure 1, the solute must first partition from the aqueous to the hexane phase and then adsorb from the hexane phase onto the hydrogen bonding site of the sorbent. To describe these two phasetransfer processes (partitioning and adsorption), we chose to derive our thermodynamic expressions in a consistent manner. Thus partitioning and adsorption are viewed as pseudoreactions where solute I is either partitioned from the aqueous to the organic phase or adsorbed from the liquid onto the solid. These two processes are described by I(aq)
-
I(hex)
for partitioning, and I+R-IR for the adsorption to the binding site R on the sorbent. Thus the standard free energy change for these pseudoreactions can be described by the equations AGO = -RT In K
(1) where the equilibrium constants are given by the ratio of the activities (7) Furlong,
D.N.; Aeton, J. R.Colloids Surf. 1982, 4, 121.
for the partitioning of I from water to hexane, and for adsorption. We have chosen to consider the equilibrium constants in terms of concentrations and not mole fraction units. We believe it is useful to consider adsorption equilibria in terms of concentrations rather than mole fractions because of the uncertainty in expressing the solid phase concentrations on a mole fraction basis. Further, because of the difficulty in assessing activity coefficients in the solid phase, we have not attempted to determine the absolute free energy changes for adsorption, but rather we consider only differences in the free energy change. Also, in the dilute liquid concentration ranges, over which this series of studies were conducted, it is likely that the activity coefficients remained constant. By use of this approach, the equilibrium constant for partitioning is proportional to the concentration ratio in the hexane and aqueous phases
K
Ch/Cw D (4) where the D is defined to be the partition coefficient. For adsorption, since estimation of the concentration of the unbound sites, R, would require serious assumptions about the number of binding sites on the sorbent, we have confined our study to conditions in which the fraction of adsorption sites actually bound with I is small. Thus our studies were confined to linear regions of the isotherms. Figure 2 shows typical isotherms for the adsorption of a
Langmuir, Vol. 7, No. 6,1991 1249
Selective Sorbents
solutes from aqueous solution (Figure 2a) and from hexane (Figure 2b). Under conditions, of low surface coverage, the equilibrium constant for adsorption can be related to the slope of the linear isotherms and thus
carboxylic ester sorbent. Support for this assumption is provided in this work.
K a SIC
In this work phenol, cresol, anisole, and a homologous series of phenylalkanols (1-phenyl-1-methanol, 2-phenyl-1-ethanol, 3-phenyl-1-propanol, 4-phenyl-1-butanol, and 5-phenyl-1-pentanol) were used as solutes. The nonpolar solvent used was n-hexane. Solvent phase solute concentrations were measured with a UV spectrophotometer, at the wavelengths of peak absorbances for each solution. For extraction studies aqueous solutions were contacted with pure hexane and equilibrated at 25 O C and the final liquid phase concentrations were measured. Adsorptionstudies were conducted with the microporous, polycarboxylic ester resin XAD-7 (Rohm and Haas). Prior to use, this sorbent was washed with methanol. We conducted three types of adsorption studies: two-phasestudies involving the solid sorbent and a single liquid phase of water; two-phase studies involving the solid sorbent and a single liquid phase of hexane; and three-phase studies in which water was the continuous phase and a hexane phase was retained within the pores of the solid sorbent (e.g., Figure 1). In all studies, liquid phase solute concentrations were maintained low to ensure that we operated in the linear region of the adsorption isotherms. Two-Phase Studies. Adsorption studies were conducted in which solutes were adsorbed from either hexane or water. In studies with hexane, the sorbent was rinsed with hexane and, before use, the hexane was evaporated from the pores by heating. For adsorption studies with water, sorbent was rinsed extensively with water after the methanol wash, and moist sorbent was used. The results for adsorption from hexane are thus based on a dry sorbent weight while results for adsorption from water are on a wet sorbent basis. Weighed amounts of sorbent were added to liquids containing a dilute solution of a single phenylalkanol. After equilibration at 25 OC, the liquid phase concentrations were measured by UV spectrophotometry. The adsorbed concentrations were calculated from
(5)
where q is millimoles of solute adsorbed per gram of adsorbent, C is the solvent-phase concentration, and the ratio q / C is referred to as the adsorption affinity. Hydrophobic Interactions. Hydrophobic interactions play an important role in partitioning, as well as in the adsorption of solutes from aqueous solutions. For a homologous series of solutes, it has been observed that the free energy change associated with hydrophobic interactions can be related to the number of methylene groups of the homologue by the expression8 AGO = AGO'
+ n&H,
(6)
where AGO' is the standard free energy change for the first member of the homologous series, AGO is the standard free energy change for the homologue containing n additional methylene groups, and $ C H ~is the free energy change per methylene group. By combining eqs 1 and 6, it can be seen that In K = In K'--
nkH,
RT
(7)
With respect to the partitioning of nonpolar solutes from an aqueous into an organic phase, eqs 4 and 7 can be combined to give In D = I n D'--
'@CH,
RT
When hydrophobic interactions are responsible for adsorption from aqueous solution, eqs 5 and 7 can be combined to give In
(5)= In (5)'- RT 'kH,
Modified Sorbent. For the case of the modified sorbent of Figure 1, we expect the solute to first partition into the pore-phase hexane by hydrophobic interactions, and then adsorb to the polycarboxylic ester sorbent through hydrogen bonding. If this happens, then it should be possible to describe the overall adsorption by the modified sorbent in terms of the partition coefficient for the solute between water and hexane and the hydrogen bond formation between the solute and the binding site of the sorbent. Thus the ratio of the adsorbed solute concentration to the solute concentration in the aqueous phase, (q/C,), should be related to the partition coefficient D and adsorption affinity via hydrogen bonding from hexane (Q/Ch)
A critical assumption t o our treatment is t h a t t h e mechanism of solute adsorption from water will differ depending on whether the sorbent pore is filled with water or with hexane. When the sorbent pores are filled with water, we believe adsorption results from hydrophobic interactions, while we believe hydrogen bonding is the major adsorptive mechanism when hexane fills the pores of this poly(8)Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley and Sons: New York, 1980;pp 5-28.
Materials and Methods
(11) where q is the solid phase concentration (mmol/g of sorbent), Co and C are the initial and the final (equilibrated) liquid phase concentrations (mmol/L), respectively, V is the liquid volume (L),and A is the amount of sorbent (g). Three-phase Studies. Characterization of adsorption in our three-phase system of Figure 1 required two experimental approaches. The first approach was used to provide support for our thermodynamic model and involved the use of a large hexane volume. Although greatly exceedingthe volume required to fill the sorbent pores, the use of a large hexane volume was convenient because it permitted the independent measurement of both the aqueous and hexane phase solute concentrations. Thus in the first approach we could measure solute concentrations in two of the phases and calculate the concentration in the third (sorbent) phase by the material balance
where C," and C, are the initial and equilibrated solute concentration in the aqueous phase, Vw and v h are the volumes of the aqueous and hexane phases, ch is the equilibrated solute concentration of the hexane phase, and A is the mass of sorbent. In the second experimental approach, the hexane content was reduced to a volume just sufficient to fill the pores of the sorbent. Filling the pores with hexane was accomplished by contacting dry sorbent with hexane. These modified sorbents were removed from the hexane, and prior to adding them to water, they were touched to an absorbing pad to remove excess hexane. The hexane was observed to be retained within the sorbent pores even after subsequent addition of these modified sorbents to water. Thus our depiction of the modified sorbent in Figure 1appears to be valid.
Maity and Payne
1250 Langmuir, Vol. 7, No. 6, 1991
2a
n CH2 groups
r ADSORPTION AFFINITIES FROM AQUEOUS SOLUTIONS
D=Ch&
SOLUTE
Phenol p-cresol 1-Phenyl-1-Methanol 2-PhenyCl-Ethanol *Phenyl-1-Propanol 4-PhenyC1-Butanol
0.1 3 0.23 0.52 1.32 3.05
Phenol 1-Phenyl-1-Methanol 2-Phenyl-1-Ethanol 3-Phenyl-1-Propanol &Phenyl-1-Butanol
Figure 3. Semilogarithmic plot of partition coefficients (D)vs the number of CH2 groups for a homologous series of phenylalkanols. By minimizing the amount of hexane in the modified sorbent, the removal of solutes from water by hydrophobic interactions was reduced. However, because of the small hexane volume, it was not possible to independently determine the solute concentration in the hexane phase. Thus it was necessary to assume that the hexane phase solute concentration could be related to the aqueous phase solute concentration by the partitioncoefficient (13) Thus by measuring C,, c h could be calculated. As before, the adsorbed phase concentration was determined from a material balance. However, for this material balance it was necessary to relate the volume of the hexane phase ( v h ) in the modified sorbent to the mass of modified sorbent added (M) where f is the mass fraction of hexane in the modified sorbent and Ph is the density of hexane (650g/L). The amount of hexane in this modified sorbent was determined by the difference in weight prior to and after evaporation of the hexane. From this measurement, we observed that for the modified sorbent, f = 0.64 g of hexane/g of modified sorbent. By difference it can be seen that the amount of dry resin in the modified sorbent is A = (1 - f ) M (15) Equation 15is required because the totalmass (M)of the modified sorbent (including both the hexane and the sorbent) was actually measured in experiments with the modified sorbent while calculations were based on the mass of dry sorbent (A). Substituting eqs 13,14, and 15 into the material balance (eq 12) it can be seen that
(C," V, - C,V, (C," Q'
- DCw@) Ph
(1- f l M
(16)
Results and Discussion Hydrophobic Interactions. The partitioning of the phenylalkanols from water into hexane is believed to result from hydrophobic interactions. Figure 3 shows that the partition coefficient increases with increasing size of the
0.173 0.025 0.1 24 0.279
Figure 4. Semilo arithmic plot of adsorption affinities (q/C,) vs the number of 8H2 groups for the adsorption of phenol and a homologous series of phenylalkanols from water onto the polycarboxylic ester sorbent XAD-7. alkanol group. As expected from eq 8, Figure 3 shows that the logarithm of the distribution coefficient increases linearly with the number of methylene groups. From the slope of this line, &H, can be calculated to be equal to -0.83RT. This value is similar to those reported for hydrophobic interaction energies per CH2 group observed for the solubility of hydrocarbons! alcohols, and acids,1° the partitioning of n-carboxylic acids between heptane and water,11J2 and the formation of mi~e1les.l~ Hydrophobic interactions are also expected to be a major mechanism responsible for the adsorption of solutes from aqueous solution onto solid surfaces which do not strongly interact with the solute. To quantify the role of hydrophobic interactions in adsorption, experiments were conducted in which the phenylalkanols were adsorbed from water onto the polycarboxylic ester sorbent (note: no hexane was present in the sorbent pores in these studies). A typical experimentally obtained adsorption isotherm of 3-phenyl-1-propanolis shown in Figure 2a. Isotherm were measured for each member of the homologous series and the average values of q/Cw,the adsorption affinity, were determined. As shown in Figure 4, adsorption from aqueous solution increased with the number of methylene groups. Again as suggested by eq 9, Figure 4 shows alinear increase in the logarithm of the adsorption affinity with the number of methylene groups in the phenylalkanol series. Considering only the phenylalkanols, and not phenol, was calculated from the slope in Figure 4 to be -0.82RT.This value is similar to that observed for the partitioning of these solutes (Figure 3) and for the adsorption of surfactants fromaqueous solution^.^^-^^ The (9) McAuliffe, C. Nature 1963,200, 1092. (10) Bell, G. H. Chem. Phys. Lipids 1978,10, 1. (11) Smith, R.: Tanford, C. R o c . Natl. Acad. Sci. U.S.A. 1973.70 (2), 289. (12) Mukerjee, P. J. Phys. Chem. 1965,69 (9), 2821. (13) Mukerjee, P. Ado. Colloid Interface Sci. 1967, I , 241. (14) Somasundaran, P.;Healy,T. W.; Fuerstennu,D. W. J. Phys. Chem. 1964, 68, 3562.
Langmuir, Vol. 7, No. 6,1991 1251
Selective Sorbents similarity in @cH2for adsorption with those values observed for other phenomenon known to result from hydrophobic interactions supports the contention that hydrophobic interactions play a key role in the adsorption of nonpolar solutes from aqueous solutions onto the XAD-7 sorbent.6 It should be noted that although all the phenylalkanols followed the expected trends, phenol, an aromatic alcohol, adsorbed better than expected from the trends of Figure 4. To further examine the discrepancy between the adsorption of the alkanols and phenol, a second aromatic alcohol, p-cresol, was studied. It was also observed that p-cresol adsorbed better than phenol from aqueous solutions (data shown in the table accompanying Figure 4). Since p-cresol differs from phenol by a single methylene group, a $JCH~for p-cresol relative to phenol could be calculated. This calculated @ C H ~(=1.05RT) was observed to be the same order of magnitude as that observed for the phenylalkanols. This observation suggests that a semilogarithmic plot of the adsorption affinities versus the number of methylene groups for the aromatic alcohols would have a similar slope although a different intercept from the phenylalkanols. Thus we believe that the apparent anomaly between the adsorption of phenol and the phenylalkanols is due to differences between aromatic and aliphatic alcohols. Hydrogen Bonding. Previous studies have shown that phenol was adsorbed from a hexane solvent onto the polycarboxylic ester resin.b Since neither benzene nor anisole were adsorbed under the same conditions, it was presumed that adsorption was the result of hydrogen bond formation. This hydrogen bond was assumed to be formed between the hydrogen-donating hydroxyl group of the solute and the hydrogen-acceptingcarbonyl group of the sorbent. In support of this assumption it was observed that primary and secondary but not tertiary amines were also adsorbed onto the sorbent.6 To further characterize this hydrogen bonding mechanism, we studied the adsorption of the homologous series of phenylalkanols from hexane onto the polycarboxylic ester sorbent (note: no water waspresent in these studies). A typical adsorption isotherm for 3-phenyl-1-propanol from n-hexane is shown in Figure 2b. Isotherms were measured for each phenylalkanol and the semilogarithmic plot of the adsorption affinities (Q/Ch) is shown in Figure 5 as a function of the number of CH2 groups. Figure 5 shows that the adsorption of the series of phenylalkanols was independent of the number of methylene groups. The aromatic alcohols, phenol, and p-cresol (data shown in the table accompanying Figure 5) were however observed to bind with a greater affinity than any of the phenylalkanols. The fact that the adsorption affinities for the phenylalkanols from hexane are not affected by the number of CH2 groups suggests two important conclusions. First, the independence of adsorption on the number of methylene groups, and therefore solute polarity, contradicts suggestions that the adsorption from a nonpolar solvent will be dependent on the polarity of the molecule just as adsorption from aqueous solution is dependent on the nonpolarity of the molecule^.'^ Sincewe only studied a narrow range of solutes and a single sorbent, we cannot make firm conclusions concerning the generality of this observation. (15) Wakamatau, T.; Fuerstenau, D. W. In Adsorption from Aqueous Solutions; Gould, R. F., Ed.; American Chemical Society: Washington, DC,1968; p 161. (16) Healy, T.W. In Organic Compounds in Aquatic Environments; Faust, S . J., Hunter, J. V., Eds.; Marcel Dekker: New York, 1971; p 187. (17) Adamson, A. W. Physical Chemistry ofSurfaces;John Wileyand Sons: New York, 1982; pp 373-376.
-
-1
-
0" . -2-
-Ec
. P
a
-3-
Y
I
Y
4-
-5
1
.
n C H ~ groups
I
I
ADSORPTION AFFINITIES FROM n-HEXANE
~~~~
SOLUTE
Phenol p-CmSOl 1-Phenyl-1-Methanol P-PhenYl-l-EthanOl %Phenyl-l-Propanol 4-Phenyl-1-Butanol 5-Phenyl-1-Pentanol
qlch 0.251 0.251 0.065 0.052 0.0055 0.055 0.063
Secondly, if the adsorption affinity and equilibrium constant are unchanged by the additional methylene groups (as observed in Figure 5 ) , then the standard free energy change for adsorption (AGO) is also unchanged by the additional methylene groups. Also, Armstead et al. reported that the enthalpy change (AHo)for the adsorption of fatty acids from n-hexane onto silica was not a function of the chain length.18 Thus if both the standard free energy and enthalpy changes (AGO and AHo) for adsorption are independent of the number of methylene groups, then the entropy change for adsorption from hexane would also be independent of the number of methylene groups. Again this would be in contrast to hydrophobic interactions, which are currently believed to be driven by entropy changes, and these entropy changes increase with increasing number of methylene group^.^^^ At this point we believe we have shown that the adsorption mechanism is different depending on whether water or hexane surrounds the binding site of the sorbent (compare Figures 4 and 5). When hexane surrounds the binding site, we believe adsorption results from hydrogen bond formation. Since hydrogen bonding is a very specific interaction, it should be possible to develop selective sorbents if this hydrogen bonding ability can be exploited. However, when the binding site is surrounded by water, hydrogen bonding does not appear to be an important adsorption mechanism, presumably due to the hydrogen bonding ability of water.s Thus we proposed to develop a modified sorbent (Figure 1) to isolate the hydrogen binding site from the solvent water. Adsorption from Water onto the Modified Sorbent. In the modified sorbent of Figure 1 we expect that any solute which is adsorbed must first partition into the hexane pore-phase and then adsorb onto the hydrogen bonding site of the sorbent. In our initial studies we wished to test (18) Armstead, C.G.;Tyler, A. J.; Hockey, J. A. Faraday SOC.Tione. 1971,67,600.
1252 Langmuir, Vol. 7,No. 6, 1991
Maity and Payne
Table I. Equilibrium Distribution of Solute (3-Phenyl-1-propanol)between Aqueous, Hexane, and Solid Sorbent Phases. A, g C,, mmol/L c h , mmol/L c h / c w q, mmol/g q / C h 0.457 1.2272 1.7099 1.393 0.1043 0.061 1.2938 1.426 0.766 0.9070 0.0760 0.058 1.2463 1.431 0.0650 0.914 0.8709 0.052 1.250 0.8150 0.9580 1.175 0.0507 0.052 0.9059 1.232 0.0457 1.435 0.7353 0.051
av a
1.33
0.055
Sorbent was retained in 10 mL of hexane.
0.151
0.00 0.00
0.25
0.50
0.75
1.00
1.25
1.50
Cw (mmollL)
Figure 6. Adsorption isotherm (q vs Cw) for 3-phenyl-1-propanol adsorption from water onto sorbent (XAD-7) contained in 10 mL of n-hexane. Experimentalmeasurements are compared with predicted isotherm (solid line)from eq 10and the previously determined partition coefficient (Figure3) and adsorption affinity from hexane (Figure 5 ) .
whether eq 10 could characterize the equilibrium for adsorption onto this modified sorbent. To test eq 10,our initial studies were conducted such that solute concentration in both the aqueous and hexane phases could be measured. In the first study aqueous solutions of 3-phenyl1-propanol were contacted with 10 mL of n-hexane containing varying weights of the sorbent. It should be noted that the sorbent was retained in the hexane phase and thus we believe the adsorptive binding site was surrounded by hexane. After equilibration, concentrations in both the aqueous and the n-hexane phases were measured and a material balance was used to calculate the amount of solute that had been adsorbed. The concentrations obtained in the three phases are shown in Table I. Table I also shows that the average values of the partition coefficient (D)and the adsorption affinityfrom hexane ( q / C h )were computed to be 1.33and 0.055, respectively. These values are in good agreement with the corresponding values of 3-phenyl-1-propanol observed in Figures 3 and 5. The above results in Table I are also shown in Figure 6 as an adsorption isotherm of the adsorbed concentration q vs the equilibrium aqueous phase concentration, C,. The theoretical line in Figure 6 is eq 10 where the partition coefficient and adsorption affinity were obtained from Figures 3 and 5. The agreement between our experimental results and the results expected from eq 10 supports our physical interpretation of the adsorption process onto this modified sorbent (i.e. solute partitioningfollowed by hydrogen bond formation). To demonstrate that hydrogen bonding can be exploited to confer selectivity to adsorption from aqueous solution, a second experiment was conducted. In this experiment,
Table 11. Equilibrium Distribution of Solute (Anisole) between Aqueous, Hexane, and Solid Sorbent Phases. A, g Cw, mmol/L c h , mmol/L c h / c w q, mmol/g q/Ch 0.226 0.1146 1.9402 16.92 0.0022 0.0011 0.339 0.1077 1.9257 17.89 0.0023 0.0012 0.411 0.1072 1.9308 18.01 0.0018 O.OOO9 0.522 0.1160 1.9272 16.98 0.0017 O.OOO8 1.9277 0.625 0.1089 17.70 0.0015 O.OOO8
17.5 a Sorbent was retained in 10 mL of hexane.
av
0.001
the solute used was anisole, which does not contain a hydrogen atom bonded to an electronegative atom and hence has no hydrogen donating ability. Thus, anisole adsorption from hexane onto the polycarboxylic ester is extremelylow.6 In this experiment, an anisole-containing aqueous phase was contacted with 10mL of hexane which contained the sorbent. After equilibration, the aqueous and hexane phase anisole concentrations were measured and the sorbent phase concentration was calculated from the material balance. The results, which are shown in Table 11, show that the average partition coefficient (D) and adsorption affinity from hexane ( q / C h ) were 17.6 and 0.0009, respectively. These values are in good agreement with individually determined values of the partition coefficient (17.9) and the adsorption affinity from hexane (0.001). From the values shown in Table 11, the average value of q/C, was observed to be 0.0175,while the predicted value (calculated by using the individually determined values of partition coefficient and adsorption affinity in eq 10) is 0.0179. Again there is good agreement between observed and predicted results. More importantly, this experiment demonstrates that solute which are unable to hydrogen bond cannot be adsorbed to this modified sorbent. These initial studies demonstrate that when the binding site of the sorbent is surrounded by hexane, hydrogen bonding appears to be the major mechanism for adsorption. However, to be practical, the amount of hexane in the modified sorbent must be minimized, otherwise the hydrophobic interactions that are responsible for the partitioning process would predominate with respect to the removal of solutes from the aqueous phase. In other words, in the previously described system, where the sorbent is contained in a large solvent volume, the hydrogen bonding ability of the sorbent acts to enhance a partitioning process rather than to confer selectivity to the removal of solutes from the aqueous solution. Ideally, it is desirable to minimize the hexane volume such that the hexane is just sufficient to surround the hydrogen bonding site of the sorbent. To minimize the hexane volume, we used only enough hexane to fill the pores of the sorbent as illustrated in Figure 1. To examine adsorption onto this modified sorbent, varying amounts of this sorbent were added to aqueous solutions containing a phenylalkanol. After equilibration, the aqueous phase solute concentration was measured, the hexane phase solute concentration was calculated by using the partition coefficients determined in Figure 3, and the adsorbed solute concentration was calculated from the material balance of eq 16. Adsorption isotherms (q vs C,) for these phenylalkanols are shown in Figure 7. The solid lines are theoretical lines generated from eq 10 using the D values from Figure 3 and the q / c h values from Figure 5 for the individual solutes. As shown in Figure 7, there is good agreement between the experimentally observed adsorption in the three-phase modified sorbent and predictions based on our model which uses parameters
Langmuir, Vol. 7, No. 6, 1991 1253
Selective Sorbents
Cw (mmollL)
Figure 7. Adsorption isotherms (q vs Cw)plots for the adsorption of three phenylalkanols onto the modified sorbent. Experimental results are compared with predicted isotherms(solid lines)from eq 10and the previously determinedpartition coefficient (Figure 3) and adsorption affinity from hexane (Figure 5 ) . measured from two phase studies. This agreement again supports our interpretation that solutes first partition from the aqueous into the hexane phase and then are adsorbed through the formation of a hydrogen bond. The agreement between the observed and predicted adsorption onto this modified sorbent suggests that the mathematical treatment can be generalized. If eq 8 is solved for D and then substituted into eq 10, it can be seen that
where @.JCH*is -0.83RT and the q / C h term can be assumed to be constant with the average value from Figure 5 of 0.58. D' was determined from the intercept of the best fit line in Figure 3 to be 0.1. Equation 17 can then used to generate the theoretical line of the semilogarithm of the adsorption affinity for this modified sorbent ( q / C w )as a function of the number of methylene groups. This theoretical line is shown in Figure 8to be in good agreement with experimentally obtained values. As illustrated in eq 17, adsorption onto the modified sorbent results from two effects, the partitioning process which we believe to be driven by hydrophobic interactions, and the formation of the hydrogen bond between the solute and the sorbent. Because of the dependence of this process on hydrophobic interactions, Figure 8 and eq 17 show that solute adsorption is enhanced by additional methylene groups. In contrast, Figure 5 shows that hydrogen bonding (Le. q / c h ) is unaffected by the number of methylene groups. Thus the above results demonstrate that the adsorption onto this modified sorbent results from a combination of mechanistic interactions.
Conclusions There are three major conclusions from this work. First, the mechanism by which solutes are adsorbed from liquids appears to depend on the solvent. When water was used as the solvent, hydrophobic interactions appear to play an important role in the adsorption onto the polycarboxylic ester sorbent used in this study. Support for this conclusion is provided by the observation that the affinity for adsorption increases systematically with the number of methylene groups in the homologous series of phenylalkanols (Figure 4). As discussed, the change in free energy for adsorption per methylene group is quantitatively
ADSORPTION AFFINITIES FROM AQUEOUS SOLUTION ONTO THE MODIFIED SORBENT.
I
1
SOLUTE 1- Phenyl+ Methanol 3- Phenyl-1- Propanol 4- Phenyl-1- Butanol
I
7
q/cw
I
0.0131 0.0794
0.1730
Figure 8. Semilogarithmic plot of (PIC,) vs the number of CH2 groups for the adsorption of phenylalkanols onto the modified sorbent. The experimental measurements are compared with predictions (solid line) based on eq 17. consistent with values measured for various phenomena known to result from hydrophobic interactions. Prior results5suggestedthat hydrogen bonding mechanisms were unimportant for solute adsorption from dilute aqueous solution, presumably due to the extensive hydrogen bonding ability of the solvent water. It should be noted that in aqueous environments the polycarboxylicester sorbent does not appear to interact strongly with solutes and thus specific solute-sorbent interactions appear to be small. When a nonpolar solvent (hexane) is used, it appears that adsorption of the phenylalkanols onto the polycarboxylic ester sorbent results from hydrogen bonding. This was suggested from prior studies in which solutes that had hydrogen-donating groups were adsorbed, while similar solutes which lacked hydrogen donating groups could not be a d s ~ r b e d In . ~the ~ ~present study we observed that the affinity for solute adsorption from hexane was independent of the number of methylene groups (Figure 5). This observation is in marked contrast to results in which water was used as the solvent (Figure 4). The second conclusion from this work is that sorbents can be designed to limit adsorption to a single or small number of mechanistic interactions. By exploiting specific mechanisms, it should be possible to design more selective separating agents. In this study we examined hydrogen bonding because we believe that the low energy and specificity of this mechanism could be exploited for separations. Although an attractive mechanism, it is difficult to exploit hydrogen bonding in aqueous systems because of the hydrogen-bonding ability of water. To overcome this problem we surrounded the hydrogenbonding site of the sorbent in a nonpolar environment. In initial studies (Tables I and II), we demonstrated that adsorption from water onto such a sorbent only occurred if the solute was able to hdyrogen bond to the sorbent. Thus, it should be possible to exploit the specificity of the hydrogen bond for separations. For instance, it should be possible to use this modified sorbent to separate solutes of similar polarity but which differ in their hydrogen bonding abilities.
1254 Langmuir, Vol. 7, No. 6, 1991 In addition to exploiting a single mechanistic interaction, we demonstrated that it is possible to design sorbents in which adsorption results from more than one mechanism. For solutes to be adsorbed by the modified sorbent of Figure 1,they must first partition into the hexane phase by hydrophobic interactions and then adsorb onto the hydrogen bonding site of the sorbent. Thus it should be possible to use this sorbent to separate solutes that have similar hydrogen bonding abilities but differ in their polarities (i.e. differ in their abilities to partition into hexane). The final conclusion from this work is that because we are using a sorbent of well-defined surface chemistry and because adsorption appears to result from a single, wellcharacterized mechanism, then it is possible to develop simple mathematical models to describe the adsorption equilibria. In our case we used simple thermodynamic relationships, invoked various assumptions, and used data obtained from two-phase experiments to predict the results of experiments involving three phases. The success of this approach can be seen by comparing the "theoretical" lines with the experimental data in Figures 6, 7, and 8. Currently, there are very limited capabilities for the extrapolation of results from simple studies to predict adsorptive behavior in more complex systems. Such predictive capabilities are important for the design and scaleup of adsorption processes. Acknowledgment. This work was supported by the National Science Foundation Grant CTS-8912141. Also the helpful suggestions from Dr. Robert L. Albright of Rohm and Haas are greatly appreciated. Nomenclature A = amount of sorbent (g) ( q ) h = activity of solute in hexane phase
Maity and Payne ( a ~ ) ,=
activity of solute in aqueous phase
a1 = activity of the unadsorbed solute in the liquid phase QIR = activity of the adsorbed solute in the solid (sorbent)
phase
UR = activity of the unbound adsorption site in the solid (sorbent) phase C = solute concentrationin the liquid phase at equilibrium (mmol/L) C" = initial solute concentration in the liquid phase (mmol/ L) c h = solute concentration in the hexane phase at equilibrium (mmol/L) C, = solute concentration in the aqueous phase at equilibrium (mmol/L) C," = initial solute concentration in the aqueous phase (mmol/L) D = partition coefficient for a solute between hexane and aqueous phases LY = partition coefficient for the first member of the homologous series of solutes f = mass fraction of hexane in the modified sorbent AGO = standard free energy change AGO' = standard free energy change for the first member of a homologous series of solutes K = equilibrium constant K' = equilibriumconstantthe first member of a homologous series of solutes M = total mass of the modified sorbent (g) n = number of methylene (CH2) groups in a solute q = adsorbed solute concentration (mmol/g) R = universal gas constant T = absolute temperature (K) V = volume of liquid (L) v h = volume of hexane phase (L) V , = volume of aqueous phase (L) &H, = free energy change per additional methylene (CH2) group Ph = density of hexane (g/L)
