Fumarate, Maleate, and Succinate Adsorption on Hydrous δ-Al2O3. 1

Langmuir 1996, 12, 2981-2988

2981

Fumarate, Maleate, and Succinate Adsorption on Hydrous δ-Al2O3. 1. Comparison of the Adsorption Maxima and Their Significance 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 Adsorption behaviors among fumarate, maleate, and succinate were compared using the same adsorbent δ-Al2O3 and initial adsorbate concentrations. The adsorption maxima were found all near the midpoint of their respective two pK values indicating that the adsorption of HX- was more favorable than adsorption of H2X and X2- and the adsorption densities were significantly influenced by the electrostatic microenvironment. The adsorption maxima of maleate were about 10% higher than those of fumarate but close to or slightly less than those of succinate before saturation was approached. However, when the adsorbent surface became crowded, the saturated adsorption maxima of maleate and succinate were ca. 1/3 and 1/2, respectively, higher than that of fumarate, indicating that the trans-form isomer occupied more binding space than the cis-form and the single bond anion (succinate) occupied the least binding space among the three four-carbon dicarboxylic acids due to its molecular flexibility.

Introduction Sorption of organic anions and their conjugate acids at mineral surfaces plays a fundamental role having great implications in many fields, such as soil chemistry,1-3 geochemistry,4-6 water and wastewater treatment processes,7-9 and crystallography.10-11 In general, the amount adsorbed (and thus the adsorption-pH relation) depends on the functional groups, molecular structure and size of the organic acids, the nature of solid surfaces (e.g., acid-base properties and adsorption site densities), and composition of the aqueous phase. Regarding anions sorbed on hydrous oxide surfaces, some investigators reported that the maximum in the adsorption-pH curve was at the pH near the pK values of the conjugate acid.12-16 Hingston17 has proposed an idea, from a thermodynamic viewpoint, to explain the maxima in the adsorption envelopes for anions with monoprotic conjugate acids and breaks of slope at pK values for anions of polyprotic conjugate acids. However, that the existence of an adsorption maximum in different systems is difficult to prove was also stressed in a later article.18 In fact, adsorption maxima do not appear near the pK values in * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Parfitt, R. L.; Farmer, V. C.; Russell, J. D. J. Soil Sci. 1977, 28, 29. (2) Parfitt, R. L.; Russell, J. D. J. Soil Sci. 1977, 28, 297. (3) Fox, T. R.; Comerford, N. B. Soil Sci. Soc. Am. J. 1990, 54, 1139. (4) Davis, J. A. Geochim. Cosmochim. Acta 1982, 46, 2381. (5) Furrer, G.; Stumm, W. Geochim. Cosmoschim. Acta 1986, 50, 1847. (6) Mesuere, K.; Fish, W. Environ. Sci. Technol. 1992, 26, 2365. (7) Davis, J. A.; Gloor, R. Environ. Sci. Technol. 1981, 15, 1223. (8) Yeh, H. H.; Chen, J. M. J. Chinese Inst. Eng. 1987, 10, 335. (9) Batchelor, B.; Dennis, R. J. Water Pollut. Control Fed. 1987, 59, 1059. (10) Cornell, R. M.; Schwertmann, U. Clays Clay Miner. 1979, 27, 402. (11) Giannimaras, E. K.; Koutsoukos, P. G. Langmuir 1988, 4, 855. (12) Hingston, F. J.; Atkinson, R. J.; Posner, A. M.; Quirk, J. P. Nature 1967, 215, 1459. (13) Davis, J. A.; Leckie, J. O. Environ. Sci. Technol. 1978, 12, 1309. (14) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373. (15) Djafer, M.; Khandal, R. K.; Terce, M. Colloids Surf. 1991, 54, 209. (16) Mesuere, K.; Fish, W. Environ. Sci. Technol. 1992, 26, 2357. (17) Hingston, F. J.; Posner, A. M.; Quirk, J. P. J. Soil Sci. 1972, 23, 177.

S0743-7463(95)00304-0 CCC: $12.00

certain systems such as protocatechuate sorbed on hydrous ferric oxide (HFO),13 phosphate on γ-Al2O3,4 lactate on R-FeOOH,19 arsenate and arsenite on HFO,20 arsenate on amorphous aluminum hydroxide,21 and borate on activated alumina,22 but such adsorption characteristics for each of these respective systems were not discussed in-depth. By performing fundamental studies on the adsorption characteristics of organic compounds differing in number, type, or position of the functional groups, one may acquire important information concerning the mechanism of their adsorption.23,24 For instance, high adsorption of catechol on boehmite as contrast to the lack of adsorption of phenol implies that both functional groups of catechol may be involved in the surface bond.23 Although this idea may also be applied to aliphatic compounds by suitably altering the approach in order to obtain different information, as illustrated in this series of papers, few reports on this subject have appeared in the literature.19,25 In contrast to many studies utilizing a two-carbon dicarboxylic acid (i.e., the oxalic acid) as a model adsorbate, however, there has been little discussion about the differences in adsorption characteristics of the four-carbon dicarboxylic acids (FCDAs), such as fumaric, maleic, and succinic acids. Since fumaric and maleic acids are geometric isomers, while succinic acid possesses no double bond between the middle carbon atoms, we might expect to see a difference in adsorption behavior arising from the disparity of the carboxylic group orientation and differences in their pK values. Furthermore, as compared to oxalic acid, the three FCDAs do not tend to dissolve hydrous δ-Al2O3 as readily,5 and thus, the complexity in interpreting the adsorption results is reduced. (18) Hingston, F. J. In Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; Chapter 2. (19) Cornell, R. M.; Schindler, P. W. Colloid Polym. Sci. 1980, 258, 1171. (20) Pierce, M. L.; Moore, C. B. Water Res. 1982, 16, 1247. (21) Anderson, M. A.; Ferguson, J. F.; Gavis, J. J. Colloid Interface Sci. 1976, 54, 391. (22) Choi, W. W.; Chen, K. Y. Environ. Sci. Technol. 1979, 13, 189. (23) McBride, M. B.; Wesselink, L. G. Environ. Sci. Technol. 1988, 22, 703. (24) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6, 979. (25) McBride, M. B. J. Colloid Interface Sci. 1980, 76, 393.

© 1996 American Chemical Society

2982 Langmuir, Vol. 12, No. 12, 1996

The objectives of this study were to compare the differences in adsorption behavior among the three FCDAs (i.e., fumarate, maleate, succinate) using the same adsorbent and initial adsorbate concentrations in the aqueous phase, and to draw the significance from the above differences. The model oxide δ-Al2O3 (sometimes called γ-Al2O326-28) used represented the aluminol and Lewis acid edge sites of kaolinite and has been commonly used in the related studies.5,29 Furthermore, like oxalic acid, the three model compounds selected were also widely used industrial chemicals (and thus might be industrial pollutants)30,31 as well as naturally occurring organics.3,32,33 Thus, the selected systems are likely to have implications in soil and geochemistry.

Yao and Yeh Table 1. Some Information about the Dicarboxylic Acids Used in This Study

Materials and Methods Materials. The adsorbent δ-Al2O3 (Aluminum Oxide C) used was obtained from Degussa Corp. (Frankfurt, Germany) and without further purification. According to the manufacturer, the average particle size is 0.02 µm and the specific N2-BET surface area is 100 ( 15 m2/g. Other properties of this adsorbent were determined by Bowers and Huang.28 Stock suspensions of Al2O3 (10 g/L) were stored in tightly-capped, 1000-mL HDPE bottles (Nikko, Osaka, Japan) for at least 2 weeks before use to assure complete hydration of the solid. The adsorbent concentration was fixed at 0.5 g/L. Purities, structural formulas, acidity constants and the companies supplying these adsorbates are listed in Table 1.34,35 A batch of fresh stock organic solution was prepared for each experiment and kept refrigerated until use. Sodium perchlorate (G.R., Merck, Germany) was used as background electrolyte to maintain a constant ionic strength of 0.05 M. Analytical reagent grade HClO4 and NaOH were chosen to prepare solutions for the pH adjustments as necessary. Merck standard buffers (pH 2, 4, 7, 9, and 10) were used to calibrate and doubly check the pH electrode. Deionized, 0.2-µm-filtered water (Milli-Q SP, MilliQ, or similar system, resistivity >17.8 MΩ cm) was employed to prepare solutions and suspensions. Methods. Adsorption Measurements as a Function of Time. Kinetic experiments were run in order to determine the sorption equilibrium. Predetermined amounts of deionized water, 1.0 M NaClO4 stock solution, 1 mM organic acid, and 10 g/L stock suspension were added to a glass beaker (with cover) and stirred intermittently. After mixing, a 10-mL aliquot (in the case of succinate adsorption, 20 mL) was removed from the beaker and filtered immediately using 25-mm Nuclepore syringe filter (Swin-Lok Filter Holder) containing a Gelman 0.45-µm membrane (Supor-450). (Supor-200 membrane was inserted when succinate adsorption was studied in order to meet the requirement of 0.2-µm prefiltration before ion chromatographic analysis.) After a 30 min mixing period, another aliquot was removed and filtered. The remainder of the suspension was then transferred to a series of 125-mL HDPE bottles (Nalgene, New York), each ca. 80% filled and placed on a reciprocating shaker for further reaction. Temperature was maintained at 25 ( 0.2 °C and samples were continuously shaken throughout the entire experiment. Under different reaction periods, aliquots of sus(26) Ettlinger, M. Technical Bulletin Pigments, No. 56; The Inorganic Chemical Products Division, Degussa AG: Frankfurt, Germany, 1990. (27) Stone, A. T.; Torrents, A.; Smolen, J.; Vasudevan, D.; Hadley, J. Environ. Sci. Technol. 1993, 27, 895. (28) Bowers, A. R.; Huang, C. P. J. Colloid Interface Sci. 1985, 105, 197. (29) Haderlein, S. B.; Schwarzenbach, R. P. Environ. Sci. Technol 1993, 27, 316. (30) Robinson, W. D.; Mount, R. A. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley & Sons: New York, 1981; Vol. 14, pp 770-793. (31) Winstrom, L. O. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley & Sons: New York, 1983; Vol. 21, pp 848-864. (32) Pohlman, A. A.; McColl, J. G. Soil Sci. Soc. Am. J. 1988, 52, 265. (33) Thurman, E. M. Organic Geochemistry of Natural Waters; Martinus Nijhoff: Dordrecht, The Netherlands, 1985; pp 138-140. (34) Martell, A. E.; Smith, R. M. Critical Stability Constants; Plenum Press: New York, 1977; Vol. 3. (35) Stumm, W.; Morgan, J. J. Aquatic Chemistry; Wiley-Interscience: New York, 1981.

a The mixed acidity constants listed here were extrapolated from zero ionic strength to 0.05 M by means of the Gu¨ntelberg approximation.34,35 b pHm ) pK1 + 1/2(pK2 - pK1).

pension were taken from the bottles and filtered. The filtrates were then analyzed for the residual organic acid. During the experiment, blanks were also prepared without adding the alumina to see if the organic acids were adsorbed on the walls of containers or filtration apparatus or reacted with NaClO4. Adsorption vs pH Experiments. Batch adsorption experiments were conducted in order to determine the amounts adsorbed at different pH and initial adsorbate concentrations. Predetermined quantities of deionized water, a NaClO4 stock solution, the organic acid, and a stock suspension were added to a glass beaker (with cover) and stirred. After 5-10 min, the contents were transferred to a series of 125-mL Nalgene bottles, each 70-80% filled, kept closed, and shaken occassionally. The pH was then adjusted to between 3.0 and 10.5 using 0.5 and 0.1 N HClO4 or 0.5 and 0.1 N NaOH. During pH adjustment, less than 1% of the original sample volume was added. An Orion Ionalyzer (Model EA940) and a Ross combination electrode (Model 8104) were used for pH measurements. Samples after pH adjustment were immediately placed on a reciprocating shaker at 25 °C for at least 27 h (at least 44 h in succinate sorption experiments) of further reaction. At the end of the experiment, the final pH was measured, and an aliquot was taken and filtered as stated earlier. The filtrates were analyzed for the residual organic acids, and the quantities adsorbed were then calculated. About half of the adsorption experiments were conducted in duplicate or triplicate to ensure the validity of the results. One or two control batches with no alumina present were also prepared in each adsorption experiment and adjusted to the pH near the adsorption maximum predicted (or one near the adsorption maximum, another in the alkaline region) to check for other possible mechanisms of organic acid removal that had not been found in the kinetic experiments. Analytical Techniques. Maleic and fumaric acids were determined using a spectrophotometer (Beckman Model DU-50, CA) at λ 210 and 204 nm, respectively. The pH of calibration standards and samples should be adjusted within 3.5-4.8 for maleate (5.5-8.5 for fumarate) before analysis. The calibration curves gave straight lines up to 75 µM for maleic acid and up to 70 µM for fumaric acid with minimum R2 0.9997 and 0.9996, respectively. If the organic acid concentrations were needed to fall below the above upper limits, an Eppendorf Model 4710 Varipette (Hamburg, Germany) was employed to pipet the filtrates and deionized water for precise dilution. When the initial

Adsorption of Organic Anions

Langmuir, Vol. 12, No. 12, 1996 2983

Table 2. Ion Chromatographic Conditions Used for Quantifying Succinate precolumn separator column eluent for succinatea eluent for perchlorateb eluent flow rate suppressor suppressor regenerant regenerant flow rate dector sample injection volume integrator

Dionex IonPac AG4A Dionex IonPac AS4A 2.4 mM Na2CO3/3.0 mM NaHCO3c 6.0 mM Na2CO3c 2.3 mL/min Dionex AMMS-2, anion micromembrane 22 mN H2SO4 4 mL/min Dionex CDM-2, conductivity detector 50-µL (sample loop) Spectra Physics SP4600 (Data Jet)

a Retention time of succinate was typically 4.3 min. b Retention time of perchlorate was typically 12 min. c Step gradient elution was performed by switching to the second eluant at 5.7 min after injection.

Figure 2. Fumarate adsorption as a function of final pH for various levels of total fumaric acid concentrations.

Figure 1. Extent of adsorption of the three FCDAs on δ-Al2O3 as a function of time at initial concentration of 100 µM. The pH values of the fumarate, maleate, and succinate systems after an 8 h reaction period were 4.3, 4.6, and 4.6, respectively. organic acid concentrations for the batch sorption experiments were high (250 µM or higher), another spectrophotometer (Hitachi Model U-1100, Tokyo, Japan) with higher precision was used, while the Beckman spectrophotometer was used for scanning the diluted filtrates simultaneously. Blanks were also treated and analyzed in the same way. Succinate was quantified by ion chromatography36 using a Dionex Model 4500i chromatograph (Sunnyvale, CA). In order to obtain precise and reproducible results, perchlorate should be completely eluted after each injection. The operation conditions of eluant composition and flow rate used by Williams37 for eluting perchlorate were found to be effective in our system, and a step gradient elution program was thus established. The chromatographic conditions used are outlined in Table 2. The concentrations of succinic acid of the filtrates in each run were determined by comparing the peak areas with an external standard curve. Calibration was repeated as a check after the analysis had been completed. The minimum R2 of calibration curves (four or five levels) was 0.9997.

Results Kinetic Experiments. The amount of the three FCDAs (i.e., fumaric acid, maleic acid, succinic acid) adsorbed on hydrous δ-Al2O3 for different reaction times is shown in Figure 1. After a 24-h reaction time, the adsorption of the three FCDAs (at initial concentration of 100 µM) on δ-Al2O3 was close to equilibrium, and the three FCDAs were found not be oxidized by NaClO4 during the entire period of blank and kinetic experiments. In addition, the adsorption of the three FCDAs on the walls (36) Weiss, J. Handbook of Ion Chromatography; Johnson, E. L., Ed.; Dionex Corporation: Sunnyvale, CA, 1986. (37) Williams, R. J. Anal. Chem. 1983, 55, 851.

of containers of filtration apparatus were not observed in any experiments. Adsorption vs pH Experiments. Fumaric Acid. When the initial concentrations of fumaric acid ranged from 50 to 500 µM, the adsorption maxima always appeared in the pH range of 3.6-4.0 (Figure 2). From an equilibrium chemistry viewpoint, over this small pH range fumaric acid was mainly in the form of C4H3O4-. Thus, the position of the adsorption maxima was likely to imply that C4H3O4- was the favorable ionic species of fumaric acid adsorbed on the surface sites of hydrous δ-Al2O3. Moreover, in Figure 2, the amount of fumaric acid adsorbed always declined to less than 10 µmol/g when the pH was greater than 8. This was probably because, at this high pH, the percentage of positively charged surface hydroxyl groups (tAlOH2+) of δ-Al2O3 was less than 10% while the neutral groups tAlOH represented greater than 90% of the surface species,26 and fumaric acid was mainly in the form of C4H2O42-. This implied that, with a shortage of electrostatic attractive forces, the negative charge of fumarate (C4H2O42-) should be very difficult to adsorb on the uncharged surface sites. It is possible that hydrogen bonds may be formed between C4H2O42- and tAlOH;28 however, this seems unlikely to have occurred in this adsorption system because only little was adsorbed at pH greater than 8 (Figure 2). In order to determine if these results were caused by some reasons due to the fact that C4H2O42- could not displace H2O molecules on neighboring tAlOH sites,38 an experiment was further conducted by reacting 500 µM fumaric acid with hydrous δ-Al2O3 at pH 3.4 for an 8-h period and then the pH was raised to a level greater than 8 for another 27 h of shaking. The final amount adsorbed was still found to be less than 10 µmol/ g, indicating that the adsorption mode between fumaric acid and δ-Al2O3 was not likely due to hydrogen bonding. In addition, the amount of fumaric acid adsorbed increased with an increase in initial concentration (Figure 2), and the increment was markedly decreased when the initial adsorbate concentration was raised from 250 to 500 µM, indicating that the saturation of adsorption had been approached. Maleic Acid. When the initial concentration of maleic acid was 1000 µM, under alkaline condition, the concentration of maleic acid in the blank was found somewhat (38) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203.

2984 Langmuir, Vol. 12, No. 12, 1996

Yao and Yeh

Figure 3. Maleate adsorption as a function of final pH for various levels of total maleic acid concentrations.

declined at the end of the experiment, revealing that the oxidation of maleate by perchlorate becomes observable under these conditions. Therefore, the amount of maleic acid adsorbed at pH values greater than 7.8 as shown in Figure 3 might be slightly higher than the actual amount adsorbed. However, for initial concentrations lower than 1000 µM, the amount of maleic acid adsorbed would all decline to less than 15 µmol/g when the pH was equal to or greater than 9. This implied that, with a shortage of an electrostatic attractive force, the negative charge of maleate (C4H2O42-) makes it difficult to be adsorbed on δ-Al2O3. Similar to the experiment with fumaric acid, 500 µM of maleic acid was reacted with δ-Al2O3 at pH 3.5 for a 13-h period and then the pH was raised to a level greater than 9 for another 25 h of shaking. The final amount adsorbed was still found to be less than 15 µmol/ g, indicating that the adsorption between maleic acid and δ-Al2O3 was not due to hydrogen bonding. Furthermore, since the adsorption maxima appeared within a pH range of 3.5-5.0 (Figure 3), where maleic acid was mainly in the form of C4H3O4-, which seemed to be the most favorable acid species adsorbed on δ-Al2O3. However, the adsorption maximum tended to become sharper with an increase in the initial concentration until the saturation of adsorption was approached. In addition, the adsorption maximum for maleic acid gradually shifted in the direction of lower pH with an increase in the initial concentration of this acid. Therefore, there must be some factor influencing this phenomenon (see discussion section). Nonetheless, this was not due to the oxidation of maleic acid by NaClO4 because the concentrations of maleic acid in the blanks remained unchanged at (or near) the pH of the adsorption maxima. Succinic Acid. Although the pK1 value of succinic acid is higher than that of fumaric and maleic acids and close to the pK2 value of fumaric acid (note that the quantity of tAlOH2+ of δ-Al2O3 decreases with an increase in pH and probably affects the position of the adsorption maximum to lie between the pK values of a FCDA if this factor is strong enough), the adsorption maxima of succinic acid again appeared between its two pK values (pH 4.7-4.8, Figure 4), where succinic acid was mainly in the form of C4H5O4-. Thus, the position of the adsorption maxima implied that C4H5O4- was the most favorable ionic species adsorbed on δ-Al2O3.

Figure 4. Succinate adsorption as a function of final pH for various levels of total succinic acid concentrations.

Similarly, the amount of succinic acid adsorbed always declined to less than 10 µmol/g when the pH was equal to or greater than 9 (Figure 4). This implied that the adsorption mechanism of succinic acid on δ-Al2O3 was similar to that of maleic and fumaric acid. As to the previous two FCDAs, an experiment was conducted by reacting 500 µM of succinic acid with δ-Al2O3 at pH 3.9 for an 8-h period and then the pH was raised to a level greater than 9 for another 35 h of shaking. Still, the amount adsorbed was found to be less than 10 µmol/g, indicating that the adsorption mode between succinic acid and δ-Al2O3 should not be due to hydrogen bonding. This finding was also supported by the fact that methanol was very difficult to adsorb on aluminum oxide.28,39 In addition, the adsorption had become saturated when the concentration of succinic acid was raised to 1000-1500 µM (Figure 4). Discussion The speciation of both the dicarboxylic acids used and the surface hydroxyl groups of δ-Al2O3 (tAlOH2+, tAlOH, tAlO-) are functions of the solution pH. In order to understand the effects of adsorbate and adsorbent speciation on adsorption, fractions of the surface hydroxyl groups distribution and acid ionization as well as the adsorbed amount of FCDAs as a function of pH are all listed in Figures 5-7. The speciation of surface hydroxyl groups as a function of pH shown in part a of Figures 5-7 was quoted from the experimental results of Bowers and Huang,28 although their background electrolyte and adsorbent concentrations were somewhat different from our experimental conditions. However, these differences would have a minor influence on the explanation of our results given below.40 The values of the adsorption densities in part b of Figures 5-7 were the same as those presented in Figures 2-4 for a 50 µM initial concentration of FCDA. Succinic Acid. When the pH was greater than 4.8, the amount of succinic acid adsorbed and the fraction of (39) Kosmulski, M. J. Colloid Interface Sci. 1993, 156, 305. (40) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; Wiley-Intersceince: New York, 1990; pp 249284.

Adsorption of Organic Anions

Langmuir, Vol. 12, No. 12, 1996 2985

Figure 5. Comparison of the pH dependence of the surface hydroxyl group distribution (experimental results of Bowers and Huang28) (a), succinate adsorption (b), and the ionic species distribution of succinic acid (c).

Figure 6. Comparison of the pH dependence of the surface hydroxyl group distribution (experimental results of Bowers and Huang28) (a), fumarate adsorption (b), and the ionic species distribution of fumaric acid (c).

tAlOH2+ both decreased with an increase in pH (Figure 5a,b). When the pH was less than 4.8, although the density of tAlOH2+ increased as the pH decreased, the amount of succinic acid adsorbed on δ-Al2O3 also declined. If the decrease of the adsorbed amount with increasing pH in the former (i.e., at pH > 4.8) was attributed to the decreasing fraction of tAlOH2+, the decrease of the adsorbed amount in the latter (i.e., at pH < 4.8) must be due to other factors. By comparing Figure 5b and Figure 5c, one notes that a similar shape was found between the amount adsorbed and fraction of C4H5O4- which varies with pH, implying that C4H5O4- plays an important role in the adsorption reaction. Moreover, the amount adsorbed, at pH equal to or less than 4.1, was found higher than the amount of C4H5O4- plus C4H4O42- in the aqueous phase, indicating that the neutral molecule C4H6O4 also participated in the adsorption reaction. In other words, in addition to electrostatic forces, chemical bonding was also involved in the adsorption reaction. Thus, adsorption of succinic acid should be attributed to specific adsorption, and ligand exchange may be its appropriate adsorption mechanism, which can be shown below.16,14,41

respectively. Note that in the above adsorption reactions, reaction 2b is the only one that can be promoted by an electrostatic attractive force. Nevertheless, as mentioned earlier, the neutral molecule C4H6O4 also participated in the adsorption probably through the reactions given in eqs 1a and 1b. When the pH is less than 4.74 (i.e., the pHm of succinic acid), the fraction of C4H5O4- decreases with a decrease in pH, while the effect of decreasing pH on the fraction of C4H6O4 is adverse (Figure 5c). If the negative contribution to adsorption in the former could be balanced by the positive contribution to adsorption in the later (especially in 3.0 e pH e 4.1, where the fraction of C4H4O42- is insignificant), then the overall adsorption density would not decrease when the pH was lower than 4.74. The results in Figure 5b indicated that the adsorptive affinity of C4H6O4 for δ-Al2O3 was weaker than that of C4H5O4-. This can be explained by the fact that adsorption of C4H5O4- was assisted by an electrostatic attractive force (eq 2b). In addition, even if the quantity of tAlOH2+ decreased with an increase in pH between 3.0 and 4.74 (Figure 5a), the amount of succinic acid adsorbed still increased with an increase in pH (Figure 5b), indicating that the quantity of tAlOH2+ should not be a critical factor in the adsorption reactions. This can be explained by the fact that the quantity of tAlOH2+ occupied by succinic acid was far below its total amount since the total site density of δ-Al2O3 was about 340 µmol/g28 (Figure 5a,b). When the pH was greater than 5.45, the amount of succinic acid adsorbed was also higher than the amount of C4H5O4-. For instance, at pH 5.9, the amount adsorbed was 50% of the initial concentration of succinic acid, but the fraction of C4H5O4- was only 22% (Figure 5b,c). This indicated that C4H4O42- also participated in the adsorption

tAlOH + H2X a tAlXH + H2O

(1a)

tAlOH2+ + H2X a tAlXH + H3O+

(1b)

tAlOH + HX- a tAlX- + H2O

(2a)

tAlOH2+ + HX- a tAlXH + H2O

(2b)

HX-

In eqs 1 and 2, H2X and represent the neutral molecules and monovalent anions of dicarboxylic acids, (41) Yao, H. L.; Yeh, H. H. Langmuir 1996, 12, 2989.

2986 Langmuir, Vol. 12, No. 12, 1996

Yao and Yeh

Figure 7. Comparison of the pH dependence of the surface hydroxyl group distribution (experimental results of Bowers and Huang28) (a), maleate adsorption (b), and the ionic species distribution of maleic acid (c).

reaction. The probable adsorption reaction is shown below:

tAlOH2+ + X2- a tAlX- + H2O

(3)

In eq 3, X2- represents the divalent anions of dicarboxylic acids. When the pH is greater than 7.0, succinic acid will be mainly in the form of C4H4O42-, and the quantity of tAlOH2+ would decrease from ca. 30% to zero (Figure 5a). Meanwhile, the percent adsorbed would decline from 17% to zero (Figure 5b). Obviously, the decrease in the amount adsorbed can be simply attributed to the decrease in the site density of tAlOH2+. It seems that the most favorably adsorbed species of succinic acid was C4H5O4- because the adsorption maxima at pH 4.7-4.8 were found right between pK1 (4.116) and pK2 (5.362). However, even if the pH is in the midpoint of pK1 and pK2 (i.e., pHm ) pK1 + 1/2(pK2 - pK1) ) 4.74), the percentage of C4H5O4- in solution cannot reach 100% since the contributions of C4H6O4 and C4H4O42- were each ca. 16%. As shown in Figure 5c, when the pH shifts to the base side of the pHm, the fraction of C4H6O4 will decrease but C4H4O42- will increase. If the amount of adsorption decreased owing to the decreased fraction of C4H6O4 was equivalent to that increased owing to the increased fraction of C4H4O42-, the overall adsorption density dereased would be simply attributed to the decreased fraction of C4H5O4because the influence due to the decreased fraction of tAlOH2+ was insignificant as mentioned earlier. Similarly, this idea can be applied to the situation existing on the acid side of the pHm. Thus, the implication that the adsorption maxima appeared at a pH equal to the pHm was made clear. The reason why C4H5O4- was preferentially adsorbed on δ-Al2O3 over C4H4O42- was probably due to an unfa-

vorable electrostatic repulsion. Since the surface complex tAlX- formed by the adsorption of X2- (eq 3) would cause a negative effect on the succeeding adsorption of X2- anions onto the neighboring tAlOH2+ due to the electrostatic repulsive force occurred between the tAlX- and X2-,41 while the neutral surface complex (tAlXH) formed by the adsorption of HX- (eq 2b) did not have this negative effect. Due to the favorable electrostatic attraction (eq 2b) and the experimental results which showed that all the adsorption maxima appeared at a pH equal to the pHm of succinic acid (Figures 4 and 5b,c), it is likely that the adsorption of succinic acid contributed by reaction 2a will be insignificant as compared to that of reaction 2b. Fumaric Acid. In Figure 6, one finds that, similar to succinic acid, C4H3O4- was the acid species most favorably adsorbed, and C4H2O42- also participated in the adsorption reaction. In addition, by changing the experimental conditions, it was shown that C4H4O4 participated in the adsorption reaction (see part 2).41 Therefore, in a mechanism similar to succinic acid, the adsorption of fumaric acid should be attributed to specific adsorption and eqs 1-3 were the probable adsorption reactions. However, if the position of the adsorption maxima (at pH 3.8) in Figure 2 was closely examined, it was found shifted to the base side about 0.2 pH unit of the pHm, unlike the case of succinic acid which the adsorption maxima fell right between its pK1 and pK2. In addition, the amounts of fumaric acid adsorbed in two sides of the adsorption maximum were not symmetrical, especially when the initial concentrations were 250 and 500 µM. These revealed that the adsorbability of C4H2O42- was likely higher than that of C4H4O4. Moreover, although the density of tAlOH2+ decreased with an increase in pH between 3.0 and 3.8 (Figure 6a), the amount of fumaric acid adsorbed still increased with an increase in pH (Figure 6b); again, indicating that the quantity of tAlOH2+ should not be a critical factor in the adsorption reaction. Maleic Acid. Similar to the previous two FCDAs but using Figure 7 and Figure 3, we can find that C4H3O4was the most favorably adsorbed species when the pH was lower than 4.0 or higher than 5.0, and that C4H2O42also participated in the adsorption reaction. However, the significant difference between the previous two FCDAs and maleic acid was that the adsorption maxima of the latter shifted in the direction of lower pH when the initial concentration of maleic acid was increased from 50 to 1000 µM. This may be due to the existence of a wide pH range between the pK1 and pK2 of maleic acid (i.e., more than 4 pH units), resulting in a competitive adsorption between two acid species only. In fact, when the initial adsorbate concentration was 50 µM, the adsorption maximum of maleic acid appeared at pH 5.0 (i.e., pHm + 1.1) (Figure 3). Under low initial adsorbate concentration, many surface sites of δ-Al2O3 would be available for the adsorption of negatively charged ions (C4H3O4- and C4H2O42-) because the adsorption density was relatively small. In other words, the physical distance between anions adsorbed on the surface sites of δ-Al2O3 is larger in this case, and the electrostatic repulsive force generated between tAlX- (formed between C4H2O42- and tAlOH2+) and C4H2O42- is relatively weak. Besides, at the same surface potential, the electrostatic attractive force exerted on C4H2O42- was twice as much as C4H3O4-. Therefore, the former species could be more favorably adsorbed than the latter (i.e., the adsorptive affinity of C4H2O42- for the surface of δ-Al2O3 was stronger than that of C4H3O4-) over the pH range of 3.94 (i.e., the pHm of maleic acid) to 5.0 (Figure 7b,c). With an increase in pH, the amount of C4H2O42- adsorbed on the surface sites of δ-Al2O3 should be restricted by the electrostatic repulsive force owing to

Adsorption of Organic Anions

Figure 8. Comparison of the three FCDAs adsorption on δ-Al2O3 as a function of final pH at initial concentration of 250 µM.

the formation of tAlX- although the fraction of C4H2O42was abruptly increased, while the amount of C4H3O4adsorbed would decrease since the fraction of C4H3O4was steeply decreased. Consequently, the overall adsorption density of maleic acid at pH > 5 declined. Moreover, the preponderance of the adsorptive affinity of C4H2O42will gradually disappear when the adsorption density increases to a certain level with an increase in initial adsorbate concentration (Figure 3 and Figure 7c). In short, low initial dicarboxylic acid concentrations (low surface coverage) and low divalent anion fractions were the main factors influencing C4H2O42- over C4H3O4- adsorption.41 Note that shifting of the point, where the adsorption density begins to decrease, to a lower pH value as the initial adsorbate concentration was increased had been observed for arsenate, which also has a wide pH range between its pK1 and pK2 (i.e., pK2 - pK1 > 4.5), adsorbed on amorphous iron and aluminum hydroxides.20,21 It is likely that this shifting is caused by the similar effects as in the case of the maleate-Al2O3 system.20 Since the left side of a sharp adsorption maximum in the narrow pH range stated above (i.e., pHm e pH e 5.0) was probably reflecting the adsorption of C4H2O42- in addition to that of C4H3O4- (Figure 3 and Figure 7c), it accounted for the adsorption maximum of maleic acid tending to become sharper at higher initial concentrations before the saturation of adsorption was approached because the number of moles of C4H2O42- was also increased in the same order along with the initial concentration of maleic acid over this narrow pH range. Significance of Molecular Structure. Comparison of the amounts of the three FCDAs adsorbed on δ-Al2O3 under the same initial adsorbate concentration of 250 µM is shown in Figure 8. As can be found from the other two initial adsorbate concentrations that are below saturation adsorption conditions (i.e., 50 and 100 µM, Figures 2-4), the adsorption maximum of maleic acid was ca. 10% higher than that of fumaric acid. Moreover, except for an initial concentration of 250 µM, the adsorption maxima of succinic acid all tended to be close to or even slightly higher than that of maleic acid before saturation adsorption had been approached (Figures 8-9 and 2-4). This indicated that the adsorption maxima of the three FCDAs were not significantly influenced by their different molecular

Langmuir, Vol. 12, No. 12, 1996 2987

Figure 9. Comparison of the three FCDAs adsorption on δ-Al2O3 as a function of final pH at initial concentration of 500 µM.

Figure 10. Comparison of maleate and succinate adsorption on δ-Al2O3 as a function of final pH at initial concentration of 1000 µM.

structures. Therefore, if the adsorption of succinic and fumaric acids can be attributed to specific adsorption as mentioned earlier, the adsorption of maleic acid should be as well. However, when the initial adsorbate concentration was increased to 500 µM, the adsorption maximum of maleic acid became ca. 27% higher than that of fumaric acid but still close to that of succinic acid (Figure 9), since the adsorption maximum of fumaric acid at initial concentration of 250 µM had been close to the saturated adsorption maximum, but this was not the case for maleic and succinic acids (Figures 2-4). Consequently, when the initial adsorbate concentration was increased to 500 µM, the increment of the adsorption density of fumaric acid would be smaller than those of the other two FCDAs. The relative lowliness of the saturated adsorption maximum of fumaric acid was probably due to the fact that its two carboxylic groups are oriented on different sides of the double bond,

2988 Langmuir, Vol. 12, No. 12, 1996

resulting in a significant steric hindrance between the adsorbed and approaching species. In contrast, the two carboxylic groups of maleic acid are oriented on the same side of the double bond, resulting in less steric hindrance. This finding revealed that the trans-form isomer (fumaric acid) occupied more binding space than the cis-form (maleic acid). In addition, for succinic acid, the middle two carbons at two ends of the single bond can freely rotate, resulting in the least steric hindrance among the three FCDAs. The merit of this argument was that the average distance of succinic acid adsorbed on δ-Al2O3 can be reduced, leading to succinic acid having the least binding space compared to the other two FCDAs. In fact, when the initial adsorbate concentration was increased to 1000 µM, the adsorption maximum of succinic acid became ca. 10% higher than that of maleic acid (Figure 10). Furthermore, since the adsorption maximum of succinic acid at this initial concentration had been close to the saturated adsorption maximum (Figure 4), the adsorption maximum of maleic acid should be as well (compared Figure 3 to Figure 4). Finally, from Figures 2-4, the relation of the saturated adsorption maxima of the three FCDAs can be disclosed:

fumaric acid:maleic acid:succinic acid ) 43 110:148:166 ) 1:1.35:1.5 Z 1: : 32 Summary Experiments were carefully designed and conducted to compare the differences in adsorption behaviors among fumaric, maleic, and succinic acids using the same adsorbent δ-Al2O3 and initial adsorbate concentrations. When the initial concentration was gradually increased from 50 µM to that of saturated adsorption, adsorption maxima were found for the three FCDAs in each of the initial concentrations. However, the adsorption maxima did not correspond to the position of pK1 or pK2. Instead, the maxima all fell near the midpoint of their respective pK1 and pK2 values. This phenomenon seemed to be a result of the competitive adsorption of the three acid species (H2X, HX-, X2-) for the adsorption sites of δ-Al2O3. In most cases, HX- was the most preferentially adsorbed acid species and the amount adsorbed was significantly influenced by the concentration of HX-. In contrast,

Yao and Yeh

influence on the adsorption denisties by the relative proportion of the three surface hydroxyl groups of δ-Al2O3 (tAlOH2+, tAlOH, tAlO-) was insignificant, except for pH values equal to or greater than pK2 + 2, where the FCDAs are mainly in the form of X2-, which were found adsorbed on the positively charged surface sites (tAlOH2+) only. In fact, the key factor for favorable adsorption of an acid species over the others was the existence of a favorable electrostatic microenvironment, and much of the adsorption behavior can be understood using the concept of electrostatic attractive and repulsive forces. This also revealed that the ∆Gcoul made up a significant proportion of the ∆Gtotal (∆Gtotal ) ∆Gcoul + ∆Gint).16,41 Although the orientation of the two carboxylic groups of maleic and fumaric acids was different, the adsorption maxima were not significantly influenced by this difference in molecular structure (i.e., the adsorption maxima of the former were only ca. 10% higher than those of the latter). Also, the adsorption maxima of succinic acid were close to or slightly higher than those of maleic acid. However, the effect of functional group orientation was finally reflected in the situation when the adsorbent surface became crowded. The saturated adsorption maxima of maleic and succinic acids were ca. 1/3 and 1/2, respectively, higher than that of fumaric acid, indicating that the transform isomer (fumaric acid) occupied more binding space than the cis-form (maleic acid), and the single bond FCDA (succinic acid) occupied the least binding space among the three FCDAs due to its molecular flexibility. Lastly, the principal adsorption mechanism for the three FCDAs was shown not due to hydrogen bonding but was attributed to ligand exchange instead. 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. LA950304C