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Adsorption of Suwannee River Fulvic Acid on Aluminum Oxyhydroxide Surfaces: An In Situ ATR-FTIR Study Tae Hyun Yoon,† Stephen B. Johnson,† and Gordon E. Brown, Jr.*,†,‡ Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025 Received January 9, 2004. In Final Form: April 24, 2004 The adsorption of Suwannee River fulvic acid (SRFA) on boehmite, γ-AlO(OH), has been examined by both macroscopic adsorption and in situ ATR-FTIR spectroscopic techniques. At a SRFA concentration approaching surface saturation (Γ ) 5.3 µmol m-2), adsorption is at a maximum at low pH and decreases as pH is increased. The ATR-FTIR spectral features of adsorbed SRFA are very similar to those measured approximately 1-2 pH units higher in solution, indicating that (i) the SRFA appears to be predominantly adsorbed at the boehmite/water interface in an outer-sphere complexation mode and (ii) the positively charged boehmite/water interface stabilizes SRFA molecules against protonation at low pH.
1. Introduction Interactions between oxyhydroxide mineral surfaces and organic macromolecules such as humic and fulvic acids are of importance in a variety of natural environments. In particular, mineral particles comprising major components of soils, suspended solids, and sediments commonly possess at least a partial surface coating of organic matter.1 The sorption of organic materials can alter many of the physicochemical properties of mineral particles, including the rate and extent of dissolution, the sorption capacity for other solution species, and the physical stability of colloidal-sized particles when suspended in solution.2 Similarly, sorption processes can also significantly impact the mechanism and extent of transport of organic matter in natural settings, and can even protect organic substances against biodegradation through socalled carbon sequestration processes.3 Adsorption of organic molecules on oxyhydroxide mineral surfaces can proceed through simple electrostatic and hydrogen bonding interactions with surface hydroxyl groups (outer-sphere surface complexation) and/or through ligand exchange reactions to form direct bonds between the organic ligands and surface cations (inner-sphere surface complexation). The distinction between innersphere and outer-sphere bonding is of importance in a number of different contexts. In particular, inner-sphere surface complexes are known to be less susceptible to desorption when exposed to other competing adsorbates, including simple electrolytes in solutions.4 They have also been implicated in the ligand-promoted dissolution of mineral surfaces.5,6 By contrast, outer-spherically bound ligands are not typically implicated in the enhancement of mineral dissolution kinetics and have instead been observed to hinder such dissolution processes by sterically
blocking access to dissolution-active surface sites for dissolution-enhancing species.19 The adsorption of fulvic acid, a common, highly soluble fraction of natural organic matter, on oxyhydroxide surfaces has been examined in a number of recent studies. Of particular note, Filius and co-workers7,8 have presented a detailed model of the macroscopic adsorption behavior of fulvic acid on goethite as a function of pH and ionic strength using the CD-MUSIC surface complexation model. Their results suggest that inner-sphere surface complexation of fulvic acid is of greatest importance at low pH, while outer-sphere modes of adsorption dominate at high pH. Both carboxyl and hydroxyl groups were implicated in the attachment of fulvic acid to the goethite surface, with the importance of carboxyl groups, and their propensity to form inner-sphere complexes, increasing with decreasing pH.8 In light of the recent CD-MUSIC model findings of Filius et al.,7,8 it is of interest to directly probe the surface speciation and attachment modes of fulvic acid using surface-sensitive spectroscopy. The aim of the present study was therefore to directly determine the surface speciation and dominant surface adsorption modes of a model Suwannee River fulvic acid (SRFA) on a well-defined mineral surface, boehmite (γ-AlO(OH)), using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR provides an in-situ probe of adsorbate structural changes at mineral/water interfaces and has allowed the speciation and adsorption modes of SRFA on boehmite to be examined over a wide range of solution pH conditions. To the authors’ knowledge, this study is the first to use in situ FTIR spectroscopy to directly probe the binding characteristics of a humic substance on a mineral surface. 2. Experimental Section
* Author for correspondence. E-mail: gordon@ pangea.stanford.edu. Telephone: +1 650 723-9168. Fax: +1 650 725-2199. † Stanford University. ‡ Stanford Synchrotron Radiation Laboratory. (1) Mayer, L. M.; Xing, B. S. Soil Sci. Soc. Am. J. 2001, 65, 250. (2) Hering, J. G. Adv. Chem. Ser. 1995, 244, 95. (3) Kaiser, K.; Guggenberger, G. Org. Geochem. 2000, 31, 711. (4) Persson, P.; Nordin, J.; Rosenqvist, J.; Lovgren, L.; Ohman, L. O.; Sjoberg, S. J. Colloid Interface Sci. 1998, 206, 252. (5) Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1847. (6) Stumm, W. Adv. Chem. Ser. 1995, 244, 1.
A Suwannee River reference fulvic acid (1R101F-1) was obtained from the International Humic Substances Society (IHSS) and was used as received. It contained carboxyl and aromatic hydroxyl concentrations of 11.21 and 2.89 mol kg-1, respectively.9 (7) Filius, J. D.; Lumsdon, D. G.; Meeussen, J. C. L.; Hiemstra, T.; Van Riemsdijk, W. H. Geochim. Cosmochim. Acta 2000, 64, 51. (8) Filius, J. D.; Meeussen, J. C. L.; Lumsdon, D. G.; Hiemstra, T.; Van Riemsdijk, W. H. Geochim. Cosmochim. Acta 2003, 67, 1463. (9) IHSS Standard and Reference Collection data sheet; International Humic Substances Society: St. Paul, MN, 2002.
10.1021/la0499214 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004
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Figure 1. Adsorption of Suwannee River fulvic acid (SRFA) on boehmite as a function of pH. Γ ) 5.3 µmol m-2.
Figure 2. ATR-FTIR spectra of Suwannee River fulvic acid in aqueous solution as a function of pH.
Boehmite was obtained in the form of dry powder from Condea Chemie GmbH (Hamburg, Germany). It possessed a surface area of 270 m2 g-1. Milli-Q water and a background electrolyte concentration of 0.01 M NaCl were used in all experiments. Solution and suspension pH values were adjusted using small volumes of concentrated (1-10 mol dm-3) HCl and NaOH. Boehmite-fulvic acid suspensions were prepared using a standard batch equilibration technique. Briefly, quantities of a predispersed boehmite suspension and SRFA were added to polypropylene centrifuge tubes to give a final boehmite concentration of 25 g dm-3 and a total surface concentration, Γ, of major fulvic acid charge groups (both carboxyl and aromatic hydroxyl) of 5.3 µmol m-2 when fully adsorbed. Other measurements recently undertaken in our laboratory indicate that this fulvic acid concentration approximately corresponds with saturation of the boehmite surface. The pH of the samples was adjusted with HCl or NaOH, and the tubes tumbled end-over-end in the dark for 48 h. The final pH was then measured before the samples were centrifuged at 10 000 rpm for 30 min and the supernatants passed through 20 nm syringe filters. Residual fulvic acid concentrations in the supernatant were determined by UV spectroscopy at pH ) 3 and λ ) 220 nm, a wavelength that was experimentally determined to be insensitive to both slight changes in pH and the presence of Al(III) in solution. ATR-FTIR measurements were performed on aqueous SRFA and centrifuged boehmite-SRFA suspensions using a Nicolet Nexus 470 FTIR spectrometer equipped with a horizontal attenuated total reflectance (HATR) accessory and a Ge crystal (PIKE Technology). Data collection and spectral calculations were accomplished using OMNIC software (version 5.1a, Thermo Nicolet, Madison, WI). In each case, the FTIR spectrum of fulvic acid was extracted from the overall ATR-FTIR response by subtracting the spectrum of an aqueous 0.01 M NaCl solution measured at an equivalent pH condition. In this manner, the FTIR contributions of water, which would otherwise dominate the FTIR spectra, were eliminated.
showed the adsorption of fulvic acid to be at a maximum at low pH and to decrease with rising pH. The adsorption behavior shown in Figure 1 can be at least partially rationalized in terms of the strong electrostatic attraction that exists between the highly positively charged boehmite surface and the negatively charged fulvate anion at low pH. This electrostatic attraction, and therefore the extent of adsorption, will decrease as the pH increases and the positive charge on the boehmite surface is diminished and reversed at high pH. In addition to the simple electrostatic logic outlined above, Filius et al.8 have recently suggested that a difference in adsorption modes contributes significantly to the adsorption behavior of fulvic acid on mineral surfaces. More specifically, their macroscopic adsorption analyses suggest that while simple outer-sphere (electrostatic and/or hydrogen bonding) adsorption modes dominate the adsorption of fulvic acid at high pH, innersphere surface complex formation becomes increasingly important as the pH decreases. Such inner-sphere binding modes may contribute to the particularly strong adsorption observed at low pH due to the direct surface cation-fulvic acid bonds that results. 3.2. ATR-FTIR Study of Aqueous SRFA. ATR-FTIR spectra of fulvic acid in aqueous solution are shown as a function of pH in Figure 2. At the highest pH conditions displayed, three prominent peaks are observed, centered at 1745, 1565, and 1395 cm-1. The peaks at 1565 and 1395 cm-1 are present in the characteristic CsO absorption region and gradually diminish as pH decreases. They can be assigned to the asymmetric (νas: 1565 cm-1) and symmetric (νs: 1395 cm-1) stretching modes, respectively, of the deprotonated carboxyl moieties. νas shifts to gradually higher wavenumber as pH is reduced to 3.74 (νas ) 1583 cm-1) and then to 2.20 (νas ) 1610 cm-1), presumably due to the effects of intramolecular hydrogen bonding between neighboring carboxylic acid groups as the degree of fulvic acid protonation increases at low pH. By contrast, the broad asymmetric peak centered at 1745 cm-1 is present at all pH conditions investigated and spans the frequency regions characteristic of the CdO stretching vibrations of esters and ketones. It is therefore likely to contain contributions from CdO stretching in ester and ketone groups,14 both of which are present in substantial
3. Results and Discussion 3.1. Macroscopic Adsorption of SRFA by Boehmite. The macroscopic adsorption behavior of SRFA on boehmite is shown as a function of pH in Figure 1. The extent of adsorption is at a maximum (96%) at low pH and systematically decreases at pH values above 5 to reach a minimum measured extent of adsorption of 29% at pH ) 11.2. These findings are in good agreement with previous studies of fulvic acid adsorption on other oxyhydroxide mineral surfaces such as goethite,7,8,10-13 which similarly (10) Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. J. Soil Sci. 1977, 28, 289. (11) Duker, A.; Ledin, A.; Karlsson, S.; Allard, B. Appl. Geochem. 1995, 10, 197. (12) Wang, L. L.; Chin, Y. P.; Traina, S. J. Geochim. Cosmochim. Acta 1997, 61, 5313.
(13) Evanko, C. R.; Dzombak, D. A. J. Colloid Interface Sci. 1999, 214, 189. (14) Cabaniss, S. E. Anal. Chim. Acta 1991, 255, 23-30.
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Figure 3. ATR-FTIR spectra of Suwannee River fulvic acid adsorbed to boehmite as a function of pH. Spectra have been normalized to the boehmite peak observed at 1157 cm-1. Γ ) 5.3 µmol m-2.
quantities in SRFA.15 It should be noted that the apparent change in the intensity of this peak as a function of pH does not have a chemical basis but is instead an experimental artifact resulting from the dilution of the fulvic acid solutions during the pH adjustment process. Figure 2 also shows that at the lowest pH condition examined two additional prominent features are present, centered at 1250 and 1715 cm-1. These peaks can be assigned to the CsO and CdO stretches, respectively, of the protonated carboxyl groups. In addition, a minor peak is present at 1265 cm-1 in the spectra of aqueous fulvic acid at all but the highest pH values ()12.44) examined. On the basis of previous FTIR studies of phenolic-type groups in aqueous media,16-18 it is assigned to the CsO stretch of protonated phenol groups that are prevalent in Suwannee River fulvic acid molecules.15 On the basis of the above peak assignments, our further FTIR investigations have been limited to a study of the role of the most prevalent SRFA functional groups, the carboxyls, in the adsorption of SRFA on boehmite. This is primarily due to the experimental difficulty of extracting accurate, interpretable spectra for the hydroxyl and phenolic moieties of SRFA from measured spectra that contain very strong (swamping) contributions from the aqueous environment and both the surface and bulk boehmite hydroxyl groups. In addition, other functional groups such as amino acids, sulfate esters, and phosphate esters are present in only minor concentrations in SRFA15 and are therefore not expected to contribute significantly to the overall adsorption characteristics. 3.3. ATR-FTIR Study of SRFA Adsorbed at the Boehmite/Water Interface. The ATR-FTIR spectra of SRFA adsorbed at the boehmite/water interface are shown in Figure 3. For consistency of data presentation, all spectra have been normalized to the strong boehmite peak observed at 1157 cm-1. The FTIR spectra displayed for adsorbed SRFA in Figure 3 bear strong similarities to those previously shown for aqueous fulvic acid in Figure 2. For example, at pH values of 3.66 and above, the spectra (15) Leenheer, J. A.; McKnight, D. M.; Thurman, E. M.; MacCarthy, P. In Humic substances in the Suwannee River, Georgia: Interactions, properties and proposed structures; Averett, R. C., Leenheer, J. A., McKnight, D. M., Thorn, K. A., Eds.; U.S. Geological Survey: Denver, CO, 1994. (16) Tejedor-Tejedor, M.; Yost, E.; Anderson, M. Langmuir 1990, 6, 979-987. (17) Yost, E. C.; Tejedor-Tejedor, I.; Anderson, M. A. Environ. Sci. Technol. 1990, 24, 822-828. (18) Connor, P.; Dobson, K.; Mcquillan, A. Langmuir 1995, 11, 41934195.
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shown in Figure 3 all possess a strong peak around 1568 cm-1 (νas), another prominent peak at 1398 cm-1 (νs), and a broad feature in the spectral region of approximately 1650-1800 cm-1. All of these features are similarly observed for aqueous fulvic acid at pH values of 6.29 and above (Figure 2). At pH 2.44, an additional strong peak is shown to emerge at 1712 cm-1, in close proximity to the 1715 cm-1 peak observed at low pH for the aqueous fulvic acid molecule. The position of νas is also observed to move to higher energy at pH 2.44, with its position (1590 cm-1) lying between those measured for pH ) 2.20 and pH ) 3.74 in aqueous solution (Figure 2). Unfortunately, a comparison between the C-O stretching vibration for the protonated carboxyl group of aqueous fulvic acid (1250 cm-1) and the corresponding vibration for adsorbed fulvic acid is not possible, as the relevant spectral region is obscured by the strong boehmite peak in the case of the adsorbed complex. The spectra in Figure 3 are most notable for the absence of additional peaks observed in the aqueous fulvic acid spectra, which would have been consistent with the occurrence of an inner-sphere adsorption mode for the fulvic acid carboxyl groups. Instead, the very similar positions of the absorption peaks observed for the aqueous and adsorbed fulvic acid molecules suggest that, at least over the range of pH conditions and the surface coverage investigated in this study, the adsorption of fulvic acid through its carboxyl groups is predominantly outer-sphere in nature. Interestingly, the very slight shifts in the peak positions observed in the spectra of adsorbed versus aqueous SRFA are significantly smaller than those that we have recently observed for more simple organic ligands (e.g., maleate19) but very similar to those observed for larger organic ligands (e.g., pyromellitate) adsorbed at boehmite/water interfaces via predominantly outer-sphere adsorption modes.20 We interpret these findings as indicating that only a small fraction of the carboxyl groups present in each fulvic acid molecule participate in outersphere adsorption through direct interactions with surface hydroxyl groups. The spectra obtained are then the average of those generated by the outer-sphere bound carboxyls and the remaining (presumably solution-facing) carboxyl groups, which do not engage in hydrogen bonding with surface hydroxyls. Interestingly, comparison of the low pH data in Figures 2 and 3 shows a similarity between the adsorbed fulvic acid spectra and those obtained at 1-2 pH units higher in aqueous solution. For example, the relative magnitudes of the major absorption peaks obtained for adsorbed fulvic acid at pH ) 2.44 closely resemble those obtained for fulvic acid in solution at pH ) 3.74. The position of νas (1590 cm-1) is also significantly closer to that obtained in solution at pH ) 3.74 (νas ) 1583 cm-1) than it is to that obtained at pH ) 2.20 (νas ) 1610 cm-1). Similarly, the spectrum obtained for adsorbed fulvic acid at pH ) 3.66 more closely resembles the aqueous fulvic acid spectrum obtained at pH ) 6.29 than that measured at pH ) 3.74. These findings indicate that under these acidic pH conditions the boehmite/water interface is promoting deprotonation of the fulvic acid molecule. Two possible mechanisms may underlie the lower degree of carboxyl protonation exhibited by the adsorbed versus the solution-based fulvic acid molecules. First, as the boehmite surface possesses a substantial positive charge at low pH, protons will be repelled from the surface in the (19) Johnson, S. B.; Yoon, T. H.; Kocar, B. D.; Brown, G. E., Jr. Langmuir 2004, 20, 4996. (20) Nordin, J.; Persson, P.; Nordin, A.; Sjoberg, S. Langmuir 1998, 14, 3655.
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ing with the boehmite surface is expected to be significantly greater). Those results will be presented in a future publication.
surrounding electrical double layer, leading to a reduced proton activity in the solution region near the surface (i.e., a higher effective interfacial pH). This reduced proton activity may promote deprotonation of the carboxyl groups in the SRFA molecule. Second, proton transfer may take place between the adsorbed fulvic acid molecules and the boehmite surface, leading to an enhancement of the positive surface charge and a decreased degree of fulvic acid protonation. The latter mechanism has previously been proposed by Vermeer et al.21 for the interaction of humic acid with hematite. As a final comment, it should be noted that while the results presented in this study indicate a dominant outersphere mode of adsorption for the carboxyl groups of SRFA on boehmite, the broad nature of the SRFA spectral peaks may have obscured minor spectral contributions arising from possible inner-sphere interactions, as have been previously predicted by Filius et al.7,8 for the interaction of fulvic acid carboxyl groups with goethite under acidic conditions. To assess this possibility, additional work is currently underway to examine the interactions of SRFA with boehmite at much lower surface coverages (where the proportion of SRFA carboxyl groups directly interact-
The adsorption of Suwannee River fulvic acid on boehmite has been examined as a function of pH at a single surface coverage of 5.3 µmol m-2. Macroscopic adsorption results show that adsorption is at a maximum at low pH and decreases at pH values in excess of 5. Comparison of ATR-FTIR spectra obtained for adsorbed and solution-based fulvic acid indicates that adsorption via the carboxyl groups appears to occur predominantly by an outer-sphere mode over the range of pH conditions examined. ATR-FTIR spectra also indicate that the boehmite/water interface promotes deprotonation of the fulvic acid molecule.
(21) Vermeer, A. W. P.; van Riemsdijk, W. H.; Koopal, L. K. Langmuir 1998, 14, 2810.
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4. Conclusions
Acknowledgment. We wish to acknowledge the support of NSF Grant CHE-0089215 (Stanford University NSF-CRAEMS on Chemical and Microbial Interactions at Environmental Interfaces). We also wish to thank Prof. Scott Fendorf (Stanford University) for the use of the Nicolet Nexus 470 FTIR spectrometer.
