Langmuir 1996, 12, 2989-2994
2989
Fumarate, Maleate, and Succinate Adsorption on Hydrous δ-Al2O3. 2. Electrophoresis Observations and Ionic-Strength Effects on Adsorption Hsi-Liang Yao* and Hsuan-Hsien Yeh Department of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China Received April 17, 1995. In Final Form: February 12, 1996X Although the four-carbon dicarboxylic acids (fumarate, maleate, and succinate) can be adsorbed specifically on hydrous δ-Al2O3, the isoelectric point (IEP) was not observed to shift in the three adsorption systems studied due to insignificant adsorption near the IEP and low intrinsic binding affinity of these acids for the oxide surface. Evidence of low intrinsic affinity arose from the strong ionic-strength dependence of the adsorption behavior for each of the three FCDAs. The ionic-strength effects on the adsorption behavior found for maleate were close to or slightly stronger than those of succinate, while fumarate exhibited the strongest ionic-strength dependence among the three FCDAs. These results were well related to their respective adsorption maxima (under-saturation) investigated in part 1. Therefore, it is suggested that, under the same experimental conditions, the adsorption of a particular anion will be attributed to specific adsorption if the ionic-strength effects of this anion are weaker than any of the three FCDAs studied. In addition, the lack of a shift in the isoelectric point cannot be used for judging whether the adsorption is nonspecific.
Introduction Change in the amount adsorbed of an adsorbate with a broad increase or decrease in background electrolyte concentration has been commonly used for judging the strength of chemical bonding between anionic adsorbates and hydrous metal oxides.1-3 Nonetheless, the use of the above method for judging whether the adsorption is either specific or nonspecific has recently been shown to be questionable.3 For anions having weak chemical bonding, such as sulfate and selenate,1 electrostatic attractive forces between the surface and adsorbate obviously are important. Therefore, a change in the electrostatic microenvironment in such adsorption reactions by either increasing or decreasing ionic strength should significantly influence the amount adsorbed. In part 1,4 the adsorption behavior of three, four-carbon, dicarboxylic acids (FCDAs) adsorbed on δ-Al2O3 under the same ionic strength (I ) 0.05 M) was thoroughly discussed, and the key factor for effective adsorption was found to be the electrostatic attractive force. This force significantly influences the amount of the three FCDAs adsorbed on δ-Al2O3. In addition, many adsorption characteristics (e.g., HX- was the acid species most favorably adsorbed; the formation of tAlX- would cause the negative effect of adsorption reaction; the adsorption maxima of the three FCDAs appeared at given pH values) can be explained by the existence of electrostatic attractive and repulsive forces.4 If the adsorption behavior explained above was correct, a significant change in the adsorption density with a broad increase or decrease in background electrolyte concentration could be expected. Thus, the differences in these changes among the three FCDAs and their implications were worth studying. For years, the lack of a shift in the isoelectric point (IEP) after anion adsorption on hydrous metal oxides has been commonly used as evidence for nonspecific ad* Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Hayes, K. F. Ph.D. Dissertation, Stanford University, Stanford, CA, 1987. (2) Mesuere, K.; Fish, W. Environ. Sci. Technol. 1992, 26, 2357. (3) Manceau, A.; Charlet, L. J. Colloid Interface Sci. 1994, 168, 87. (4) Yao, H. L.; Yeh, H. H. Langmuir 1996, 12, 2981.
S0743-7463(95)00305-2 CCC: $12.00
sorption.5-12 However, some investigators have been rather conservative about the appropriateness of this criterion.13 The objectives of this study were to compare the differences in ionic-strength effects on adsorption behaviors among the three FCDAs (i.e., fumarate, maleate, succinate), to examine the implications of this behavior, to observe if the IEP would shift after the three respective FCDAs were adsorbed on δ-Al2O3, and to determine the breadth of shift of the IEP in these three systems. Experimental Section Adsorption vs pH experiments were conducted by following the same procedures described in part 14 except that the NaClO4 concentration was varied instead of the initial adsorbate concentration. The initial concentration of each of the three FCDAs was fixed at 50 µM. One or two control blanks without the alumina present were also prepared in each batch adsorption experiment. The same analytical methods described in part 14 were used except that a 100-µL sample loop was employed in analyzing residual succinate at an ionic strength of 0.005 M. Electrophoretic mobilities (EM) of the alumina particles were measured as a function of pH and initial organic acid concentration at constant ionic strength (0.05 M NaClO4) using the Laser Zee meter (model 501, PenKem, Bedford Hills, NY). The EM value was converted automatically to zeta potential (in mV) by the instrument using the Smoluchowski formula.14 Since the effective diameter of the alumina particles in aqueous solution (5) Yopps, J. A.; Fuerstenau, D. W. J. Colloid Sci. 1964, 19, 61. (6) Han, K. N.; Healy, T. W.; Fuerstenau, D. W. J. Colloid Interface Sci. 1973, 44, 407. (7) Kavanagh, B. V.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1977, 61, 545. (8) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: London, 1986; pp 233-235. (9) Regazzoni, A. E.; Blesa, M. A.; Maroto, A. J. G. J. Colloid Interface Sci. 1988, 122, 315. (10) Parks, G. A. In Mineral-Water Interface Geochemistry; Hochella, M. F., Jr., White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990; Chapter 4. (11) Djafer, M.; Khandal, R. K.; Terce, M. Colloids Surf. 1991, 54, 209. (12) Hesleitner, P.; Kallay, N.; Matijevic´, E. Langmuir 1991, 7, 178. (13) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; Wiley-Interscience: New York, 1990; pp 277278. (14) Sprycha, R.; Jablonski, J.; Matijevic´, E. J. Colloid Interface Sci. 1992, 149, 561.
© 1996 American Chemical Society
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Figure 1. Ionic-strength dependence of fumarate adsorption on δ-Al2O3 as a function of pH.
Figure 3. Ionic-strength dependence of succinate adsorption on δ-Al2O3 as a function of pH.
Figure 2. Ionic-strength dependence of maleate adsorption on δ-Al2O3 as a function of pH.
Figure 4. Ionic species distribution as a function of pH at the ionic strength specified: (a) fumarate; (b) succinate; (c) fumarate; (d) maleate.
is much greater than 1 µm,15 the application of this formula seems appropriate in the adsorption systems studied. Samples were prepared and equilibrated in the same manner as the adsorption-pH studies stated in part 1,4 and the final pH was determined before the EM measurement. The IEP was determined by interpolating the data to zero EM. Before any series of measurements, a TiO2 standard colloid, as approved by PenKem, was employed to ensure that the microscope focused at the stationary layer of the cell channel. The cell was cleaned and maintained according to the user’s manual.
Table 1. The Mixed Acidity Constantsa and Their Midpoints (pHm) of the FCDAs at 25 °C and Various Ionic Strengths ionic strength (M) 0.005 0.05
Results and Discussion Ionic-Strength Effects on Adsorption. Not unexpectedly, background electrolyte concentration significantly altered the adsorption densities of the three FCDAs (Figures 1-3). For fumaric acid, when ionic strength was altered from the original value of 0.05 to 0.005 and to 0.5 M, the magnitude of change of the adsorption maximum was +32 and -33 µmol/g, respectively (Figure 1), equivalent to a 60% variation of the original adsorption density. It is worth noting that the adsorption of fumaric acid exceeded the amount of C4H3O4- plus C4H2O42- in the aqueous phase at pH e 3.5 when ionic strength was 0.005 M. For instance, at pH 3.02, the amount of C4H3O4- and C4H2O42- in the aqueous phase was 51% of the total fumaric acid in solution (Figure 4a), but the amount of the acid adsorbed was 75% of the total (Figure 1), indicating that the adsorption reaction between neutral molecule C4H4O4 and surface hydroxyl groups of δ-Al2O3 (15) Bowers, A. R.; Huang, C. P. J. Colloid Interface Sci. 1985, 105, 197.
0.5
pK1 pK2 pHm pK1 pK2 pHm pK1 pK2 pHm
fumaric acid
maleic acid
succinic acid
3.020 4.395 3.708 2.962 4.220 3.591 2.93 4.07 3.50
1.877 6.233 4.055 1.819 6.058 3.939 1.79 5.76 3.775
4.174 5.537 4.856 4.116 5.362 4.739
a Values listed here were extrapolated from zero ionic strength to the appropriate ionic strength by means of the Gu¨ntelberg or Davies approximation.16,17
occurred. Thus, the adsorption of fumaric acid should be attributed to specific adsorption. In addition, even if the distance between the two pK values of fumaric acid is less than 1.4 pH units and shortened with an increase in ionic strength (Table 1),16,17 the adsorption maxima always appear within this small pH range (Figure 1), indicating that C4H3O4- was the most favorable acid species adsorbed on the surface sites of hydrous δ-Al2O3 and that this characteristic was not (16) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1977; Vol. 3. (17) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley-Interscience: New York, 1981.
Adsorption of Organic Anions
affected by the broad increase or decrease of ionic strength. However, it should be noted that, at ionic strength of 0.05 M, the adsorption maxima appeared at a pH 0.2 pH unit to the right of the pHm. This is likely due to the fact that the adsorptive affinity of C4H2O42- for the surface of δ-Al2O3 was stronger than C4H4O4, as described in part 1.4 As the ionic strength was decreased to 0.005 M, the adsorption maximum occurred 0.3 pH unit to the right of the pHm (Figure 1), indicating that the preponderance of the adsorptive affinity of C4H2O42- over C4H4O4 remained. Conversely, when the ionic strength was increased to 0.5 M, the adsorption maximum appeared 0.2 pH unit to the left of the pHm (Figure 1), revealing that the adsorption affinity of C4H4O4 for the surface may be stronger than C4H2O42- at a very high ionic strength. This might be caused by a decrease in positive surface potential18 and a mass distribution of ClO4- near the surface of δ-Al2O3 (i.e., the outer Helmholtz layer and the diffuse layer) as increasing ionic strength decreasing the double layer thickness. This results in a weaker adsorption affinity for C4H2O42-. When the positive surface potential was high (i.e., as in a low ionic strength of 0.005 M), difference in adsorptive affinity between the divalent anion C4H2O42and the neutral molecule C4H4O4 was significant because electrostatic contribution was important. In contrast, when the positive surface potential was low (i.e., as in a high ionic strength of 0.5 M), the adsorptive affinity of C4H2O42- became close to that of the neutral molecule C4H4O4. However, for C4H2O42- it has to penetrate the barriers of ClO4- before adsorption can occur; this gets more difficult at high ClO4- concentration. Under such circumstances, adsorption of the neutral molecule C4H4O4 becomes easier than adsorption of C4H2O42-. In fact, it can be verified by the results shown in Figure 1 that C4H2O42- is difficult to adsorb at high ionic strength. For instance, at pH 5.9 (i.e., the fraction of C4H2O42- is greater than 97% due to pH g pK2 + 1.5 (parts a and c of Figure 4)), the adsorption density could be 50 µmol/g at ionic strength of 0.005 M, but was only 3 µmol/g at ionic strength of 0.5 M, indicating that C4H2O42- is difficult to adsorb at high ionic strength. For maleic acid, when the ionic strength was altered from the original value of 0.05 to 0.005 and to 0.5 M, the magnitude of change of the adsorption maximum was +23 and -20 µmol/g, respectively (Figure 2). It was found that the concentration of maleic acid in the blank declined in the adsorption experiment at the ionic strength of 0.5 M, indicating that maleic acid was probably oxidized by NaClO4. Thus, the magnitude of the adsorption density decrease from ionic strength of 0.05 to 0.5 M might be larger than that indicated (Figure 2). As described in part 1,4 the adsorption maximum of maleic acid appeared at pH 5.0 (i.e., pHm + 1.1) when the initial adsorbate concentration was 50 µM (I ) 0.05 M). This was because the adsorptive affinity of C4H2O42- for the surface of δ-Al2O3 was stronger than that of C4H3O4in the pH range of 4.0-5.0. However, the preponderance of the adsorptive affinity of C4H2O42- will gradually disappear if the adsorption density is increased by increasing the initial adsorbate concentration.4 It is worth noting what the effects will be by decreasing ionic strength. In Figure 2, one finds that the amount of adsorbed maleic acid remained unchanged over the pH range of 4.3-5.0 (I ) 0.005 M), showing that the amount of adsorption decreased owing to the decreased fraction of C4H3O4- was equivalent to that increased owing to the increased fraction of C4H2O42- in this pH range (Figure 4d). This indicated (18) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; Wiley-Interscience: New York, 1990; Chapter 9.
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Figure 5. Comparison of the adsorption behavior of the three FCDAs at low ionic strength (I ) 0.005 M).
that the preponderance of the adsorptive affinity of C4H2O42- has reduced to about the same as C4H3O4- when the adsorption density was increased by decreasing ionic strength. This finding was in accord with the viewpoint stated in part 1 that low surface coverage (low initial dicarboxylic acid concentrations) and low divalent anion fractions were the main factors influencing X2- over HXadsorption.4 For succinic acid, when ionic strength was decreased from 0.05 to 0.005 M, the adsorption maximum increased by 20 µmol/g (Figure 3). The adsorption experiment performed under a background electrolyte concentration of 0.5 M was not conducted due to restrictions imposed by the analytical method. It is worth noting that even a 10-fold alteration in ionic strength did not change the position of the adsorption maximum of succinic acid (i.e., it remained at the pH equal to the pHm (Figure 3 and Table 1)). These results indicated that the ionic strength decrease did not affect not only the preponderance of the adsorptive affinity of C4H5O4- but also the equivalent adsorptive affinities of C4H4O42- and C4H6O4.4 By comparing the three FCDAs, one notes that the ionicstrength effects on adsorption found for maleic acid were close to or slightly stronger than those of succinic acid, while fumaric acid exhibited the strongest ionic-strength dependence among the three (Figures 1-3). These results reflect the strength of chemical bonding between the δ-Al2O3 surface and the acids and suggest that fumaric acid was the weakest among the three, while maleic acid was close to or slightly weaker than succinic acid. This finding was in agreement with the results given in part 14 (i.e., under unsaturation conditions, the adsorption maxima of fumaric acid were all lower than those of the other two FCDAs, while those of maleic acid were close to or slightly lower than succinic acid). Although the adsorption density of a FCDA was sensitively influenced by the fractions of HX-, X2-, and H2X in the aqueous phase,4 the main factors influencing an adsorption maximum of a FCDA to be higher or lower than that of the other at the same initial adsorbate concentration were the intrinsic affinity of this adsorbate for the surface and ionic strength of the aqueous phase. As can be found in Figure 5, the adsorption maxima of the three FCDAs were close to one another at a low ionic strength of 0.005 M (high surface potential), although the degrees of acid ionization of the three FCDAs at the pH of adsorption maxima were quite different (parts a, b, and d of Figure 4). When the ionic strength was increased to 0.05 M (the surface potential decreased), fumaric acid exhibited the greatest reduction in the adsorption maxi-
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Figure 6. Comparison of the adsorption behavior of the three FCDAs at median ionic strength (I ) 0.05 M).
mum owing to its weakest chemical bonding (intrinsic binding affinity) to the surface, maleic acid exhibited the next reduction, and succinic acid exhibited the smallest, resulting in larger differences between the adsorption maxima (Figure 6). However, differences in adsorption densities between the FCDAs would still be reflected if their degrees of acid ionization at the same pH were very different. For instance, at pH 3.04, the fraction of C4H5O4for succinic acid was less than 8%, but the fraction of C4H3O4- for maleic acid was greater than 93% (parts b and d of Figure 4). At this pH (3.04), the percent adsorbed of succinic acid was 21% lower than that of maleic acid (Figure 5). When the ionic strength was increased to 0.05 M, the adsorptive affinity of maleate (C4H3O4-) for the surface became closer to that of the neutral molecule of succinic acid (C4H6O4) due to a decrease in positive surface potential. In this case, the difference in percent adsorbed between maleic and succinic acids reduced to 13% (Figure 6). From the data illustrated above, one realizes to what extent the difference in acid ionization could reflect in the difference in adsorption densities between the FCDAs. Therefore, the reason why the adsorption maxima of the three FCDAs appeared close to each other at low ionic strength can be made clear by introducing the concept of relative proportion between the intrinsic (or chemical) part of the Gibbs free energy change of adsorption (∆Gint) and the Coulombic (electrostatic) part of the Gibbs free energy change (∆Gcoul) to that of the total (∆Gtotal ) ∆Gint + ∆Gcoul). When the positive surface potential was high (I ) 0.005 M), the differences in ∆Gint/∆Gtotal between the three FCDAs were small because high values of |∆Gcoul| gave high values of |∆Gtotal|,18,19 resulting in small differences in the adsorption maxima between the three FCDAs (Figure 5). However, when the positive surface potential decreased (I ) 0.05 M), the proportion of ∆Gcoul in ∆Gtotal also decreased, resulting in larger differences in ∆Gint/∆Gtotal and in the adsorption maxima between the three FCDAs (Figure 6). Furthermore, in contrast to the effect of the differences in ∆Gint, which were independent of pH and adsorbate concentration,19 reflected in the differences in the adsorption maxima between the three FCDAs at I ) 0.05 M (or higher), |∆Gcoul| reflected in the shapes of the adsorption-pH curves for each of the FCDA as a function of pH and adsorbate concentration, having their respective maxima at the respective adsorption maxima.4 In addition, the existence of a chemical component (∆Gint) in the total free energy change further supports the specific adsorption of these FCDAs. (19) Hansmann, D. D.; Anderson, M. A. Environ. Sci. Technol. 1985, 19, 544.
Yao and Yeh
As the pH is increased near the pK2 of a FCDA, the divalent anion of this acid in the aqueous phase will greatly increase. Therefore, the adsorption density of a FCDA having lower pK2 value will become less than that of the other having a higher pK2 value due to the decreasing amount of the most favorably adsorbed acid species HXand the increasing amount of the surface complexes tAlXas rising the pH.4 In Figures 5 and 6, fumaric acid, the FCDA with the weakest intrinsic binding affinity and the lowest pK2 value, showed a remarkably lower adsorption than the other two acids at pH values greater than its pK2. Also, succinic acid, the FCDA with the strongest binding affinity and the second highest pK2 value, showed its adsorption higher than maleic acid, but gradually decreased to slightly lower than maleic acid at pH greater than 6. These results further support the viewpoint stated in part 1 (i.e., a decrease in the adsorption density deviated from an adsorption maximum was owing to a decrease in the concentration of the most favorably adsorbed ionic species of HX- and the appearance of an unfavorable electrostatic repulsion between the surface complexes tAlX- and the anions).4 Since the model compounds selected have two carboxylic groups, bidentate binuclear surface complexes may be formed.20 In general, the binding force of bidentate surface complexes is stronger than those of monodentate. Due to the fact that the amount of succinic acid adsorbed was found much less than that of oxalic and phthalic acids20 and that the ionic-strength effects on adsorption for the three FCDAs were very significant, the adsorption behavior of the three FCDAs may not be attributed to bidentate binding.21,22 Furthermore, each of the three FCDAs has its two carboxylic groups different in orientation to the others but can appear at almost the same adsorption density at the adsorption maximum (Figure 5) indicating that the adsorption mode of the three FCDAs on the surface of δ-Al2O3 should be the same (i.e., 1:1 surface complexes). In addition, according to the result calculated by using the highest adsorption density of succinic acid (166 µmol/g)4 and the surface site density of δ-Al2O3 (340 µmol/g),15 almost all of the surface hydroxyl groups of δ-Al2O3 were occupied by succinic acid if bidentate complexation had occurred. This seems unlikely if lateral hindrances are considered. A similar situation has also been found for oxalic acid in that its adsorption behavior on the surface of goethite can be changed from the original bidentate binding to monodentate at higher adsorption densities.21 If succinic acid, the FCDA having the strongest binding affinity, adsorbed as a monodentate surface complex at low and at high initial adsorbate concentrations as deduced above, the other two FCDAs should be as well. Electrophoresis Observations. At an ionic strength of 0.05 M, the pHpzc of δ-Al2O3 found was 9.65 (Figure 7), which was consistent with the results reported by Bowers and Huang.15 As can be found in Figures 7-9, the IEP (or point of zero charge) was not observed to shift (i.e., pHiep ) 9.65 ( 0.10) after the adsorption of each FCDA. Since adsorption of a sufficient amount of specifically adsorbed ions, or alternatively, adsorption at a sufficiently high initial adsorbate concentration for an adsorbent is one of the two necessary conditions for the shift of the IEP15 (another condition is an adsorbate having a strong enough intrinsic binding affinity for the surface13), the experimental conditions including the initial adsorbate (20) Furrer, G.; Stumm, W. Geochim. Cosmoschim. Acta 1986, 50, 1847. (21) Parfitt, R. L.; Farmer, V. C.; Russell, J. D. J. Soil Sci. 1977, 28, 29. (22) McBride, M. B. Clays Clay Miner. 1982, 30, 438.
Adsorption of Organic Anions
Figure 7. Zeta potential of Al2O3 particles as a function of pH (with and without maleic acid present).
concentrations corresponding to those approaching the saturation of adsorption4 ensure that the shift in the IEP will not occur by further increase in the initial adsorbate concentrations. Therefore, the lack of a shift in the IEP for the FCDA-Al2O3 systems should be attributed to their insignificant adsorption near the IEP or to the weak intrinsic affinities of these acids for the oxide surface.13 Furthermore, the lack of a shift in the IEP can also be considered as a result of (nearly) complete desorption along the Γ-pH curves given in part 14 as rising the pH from the adsorption maxima to the point of zero charge. Once a desorption process was achieved, the zeta potential of the adsorbent should return to its blank status (without the addition of an adsorbate) and therefore no shift in the IEP. In contrast, strongly adsorbed anions such as arsenate23 and oxalate24 show incomplete desorption at the PZC of the adsorbent resulting in a shift in the IEP. In Figure 7, the ζ-pH curves for various initial concentrations of maleic acid moved closer to one another at pH greater than 8 and finally overlapped that of the blank reflected the desorption processes as stated above. Note that fumaric acid having a pK2 value 2 pH units lower than that of maleic acid (Table 1) resulting in its ζ-pH curves nearly overlapped that of the blank at pH greater than 6 (Figure 8). In addition, it is interesting to notice that the zeta potential of fumaric and succinic acids declined to a minimum at a pH value slightly greater than their respective pK2 value (i.e., pH Z 4.6 and pH Z 5.5 for the adsorption systems of fumaric and succinic acids, respectively), and then raised and finally overlapped that of the blank as rising the pH (Figures 8 and 9). Based on the desorption concept stated above and the Γ-pH curves of fumaric and succinic acids reported in part 1,4 one finds that these minima appeared in the positions where the corresponding Γ-pH curves began to have a steep drop and thus these succeeding abrupt desorption processes caused the corresponding zeta potential increase. Meanwhile, the surface potential of δ-Al2O3 would be influenced by the following reactions: (23) Anderson, M. A.; Ferguson, J. F.; Gavis, J. J. Colloid Interface Sci. 1976, 54, 391. (24) Zhang, Y.; Kallay, N.; Matijevic´, E. Langmuir 1985, 1, 201.
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Figure 8. Zeta potential of Al2O3 particles as a function of pH (with and without fumaric acid present).
Figure 9. Zeta potential of Al2O3 particles as a function of pH (with and without succinic acid present).
tAlOH2+ + OH- f tAlOH + H2O (for increasing pH) (1) tAlXH + OH- f tAlX- + H2O (significant when pH > pK2) (2a) tAlX- + H2O f tAlOH2+ + X2- (desorption) (2b) The desorption reaction in eq 2b was due to the lateral electrostatic repulsion, where X2- represents the divalent anions of dicarboxylic acids. Equation 1 can cause a decrease in surface potential of δ-Al2O3, while eq 2b can cause its increase. When the amount of X2- desorbed via eq 2b exceeds the amount of tAlOH2+ consumed by eq 1, the surface potential will thus be increased by an increase in pH. In Figures 8 and 9, the quantities of the desorbed anions were large enough to force the surface potential to rise, giving important evidence that the surface complexes
2994 Langmuir, Vol. 12, No. 12, 1996
tAlX- formed do cause a negative effect on the anion adsorption and are responsible to the steep decrease in adsorption densities of fumaric and succinic acids as rising the pH.4 However, owing to the fact that the amount of tAlOH2+ is decreasing with increasing pH,4,15 the surface potential rise becomes unfavorable at high pH. Therefore, for maleic acid, which has the highest pK2 value, its potential to exhibit a rising surface potential should be the worst among these FCDAs. In Figure 7, the ζ-pH curves for the initial concentrations of 500 and 1000 µM of maleic acid showed that the zeta potential tended to become constant at pH greater than 6.5, reflecting the effect of eq 2b to be equivalent to that of eq 1. This behavior was probably due to a higher pK2 value of maleic acid. Comparison with Oxalate and Chromate. In previous studies it was found that under the conditions of ionic strength of 0.05 M, initial oxalate concentrations of 50 and 100 µM, and an adsorbent concentration of 1 g/L, the maximum adsorption of oxalate on the surface of δ-Al2O3 approached 100% of the acid added.25 Also, under the conditions of ionic strength of 0.05 M, an initial oxalate concentration of 50 µM, and an adsorbent concentration of 0.9 g/L, the maximum adsorption of oxalate on the surface of R-FeOOH (surface area ) 66 m2/g, site density ) 1.5 sites/nm2) reached 95% of the acid added.2 This implied that the intrinsic binding affinity of oxalate for metal oxides was much stronger than that of the three FCDAs studied. In addition, the maximum adsorption of chromate on goethite was much higher than that of oxalate under the same conditions, reflecting that the intrinsic binding affinity of chromate for goethite was much stronger than that of oxalate.2 Since the adsorption behaviors of these FCDAs on the surface of δ-Al2O3 have been shown attributed to specific adsorption, therefore, the adsorption of oxalate and chromate on the surface of either δ-Al2O3 or R-FeOOH should also be attributed to specific adsorption, although the calculated results using the triple-layer model with an assumption of nonspecific adsorption were better than those with an assumption of specific adsorption.2 Moreover, according to the facts that the ionic-strength effects on adsorption for oxalate were more pronounced than those for chromate and the ionicstrength effects for the three FCDAs studied were more marked than those for oxalate,2 the intrinsic binding affinities of the above five anions for the δ-Al2O3 and R-FeOOH surface sites can thus be compared as follows: chromate > oxalate > succinate ∼ maleate > fumarate. These results are in agreement with those obtained from the comparison between their maximum adsorption described above and also in accord with the facts mentioned earlier that the ionic-strength effects can be used for judging the strength of chemical bonding between anionic adsorbates and hydrous metal oxides. Therefore, it is suggested that, under the same experimental conditions, the adsorption of a particular anion will be attributed to specific adsorption if the ionic-strength effects of this anion are weaker than any of the three FCDAs studied. From the facts that the IEP was found to shift after oxalate was adsorbed on goethite,11 hematite,24 and δ-Al2O3,25 the IEP should also shift after chromate adsorption due to its stronger binding affinity for metal oxides, although the simulated results using the diffuse (25) Yeh, C. C. Master Thesis, Department of Environmental Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C., 1988 (in Chinese).
Yao and Yeh
layer model indicated that the IEP remained unchanged.13 Nevertheless, it is conclusive that specific adsorption does not necessarily cause a shift in the IEP of an adsorbent even if the adsorption is up to a saturated level. Summary and Conclusions The ionic-strength effects on adsorption of fumaric, maleic, and succinic acids were compared using the same adsorbent and initial adsorbate concentrations. The results showed that the ionic-strength effects of the three FCDAs were all very significant, indicating weak intrinsic binding affinities of these acids for the surface sites of δ-Al2O3. Among the three FCDAs, fumaric acid exhibited the strongest ionic-strength dependence of adsorption, while maleic acid was close to or slightly stronger than succinic acid. These results were well related to the strength of chemical bonding between the δ-Al2O3 surface sites and the acids, demonstrating that fumaric acid was the weakest among the three, while maleic acid was close to or slightly weaker than succinic acid. These findings were also well related to their respective adsorption maxima (under-saturation) investigated in part 14 and clearly revealed that the ionic-strength effects can be used for judging the strength of chemical bonding between anionic adsorbate and hydrous metal oxides. Since the adsorption behaviors of these FCDAs on the surface of δ-Al2O3 were attributed to specific adsorption, therefore, under the same experimental conditions, if the ionicstrength effects of a particular anion were weaker than any of the FCDAs studied, the adsorption behavior of this anion should also be attributed to specific adsorption. Owing to the weak binding strength of these FCDAs to the surface sites of δ-Al2O3, the electrostatic part of the Gibbs free energy change (∆Gcoul) thus played an important role in the adsorption reactions and reflected in the variation of adsorption density as a function of pH and adsorbate concentration. Therefore, an adsorption maximum represented the position of a corresponding maximum value of |∆Gcoul| which varied with pH. On the other hand, the differences in the intrinsic binding affinities (∆Gint) reflected in the differences in the adsorption maxima between the three FCDAs at higher ionic strength (i.e., I ) 0.05-0.5 M). Although a shift in the IEP can be treated as evidence for specific adsorption, the contraries are unreliable. Since there are a range of ∆Gint corresponding to anions which have their intrinsic binding affinities for the surface sites of hydrous metal oxides between those of oxalate and perchlorate (or nitrate) and exhibit their ionic-strength dependence of adsorption more pronounced than oxalate, some of them do not cause a shift in the IEP after adsorption. Therefore, the lack of a shift in the IEP cannot be used for judging whether the adsorption is nonspecific. Acknowledgment. This work was partly supported by the National Science Council of the Republic of China (Project No. NSC 77-0414-E006-06Z). We gratefully acknowledge Professor Ju-Hsien Huang (Department of Environmental Engineering, N.C.K.U.) for giving much help in preparing this manuscript and Meng-Hsiu Shih for typing the manuscript. The authors also wish to thank two anonymous reviewers for their enhancement of this paper. LA9503055
