Surface Complexation of Mellitic Acid to Goethite: An Attenuated Total

Bruce B. Johnson*, Staffan Sjöberg, and Per Persson. Department of Chemistry, Inorganic Chemistry, Umeå University, SE-901 87 Umeå, Sweden, and Col...
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Surface Complexation of Mellitic Acid to Goethite: An Attenuated Total Reflection Fourier Transform Infrared Study Bruce B. Johnson,*,†,‡ Staffan Sjo¨berg,† and Per Persson† Department of Chemistry, Inorganic Chemistry, Umeå University, SE-901 87 Umeå, Sweden, and Colloid and Environmental Chemistry Laboratory, La Trobe University, Bendigo, P.O. Box 199, Bendigo, Victoria 3552, Australia Received August 12, 2003. In Final Form: October 29, 2003 The nature of the interaction between mellitic acid (benzene hexacarboxylic acid) and the common soil mineral goethite (R-FeOOH) has been investigated as a function of pH and ionic strength by use of attenuated total reflection Fourier transform infrared spectroscopy. Molecular orbital calculations of the theoretical vibrational frequencies of the mellitate ion (L6-) and dihydrogen mellitate (H2L4-) have allowed the measured absorption frequencies to be accurately assigned. At pH values above 6, adsorption involves outer-sphere complexation of the deprotonated L6- ion. At lower pH values, there is evidence of a second outer-sphere surface complex involving a partially protonated species, although the extent of protonation of the surface species is significantly less than that found for the solution species at the same pH. While there is no evidence of inner-sphere complexation, increasing the ionic strength to 2.0 M does not displace the adsorbed species but does increase the fraction present on the surface as the fully deprotonated L6-. The small effect of ionic strength suggests that the adsorptive interaction, although outer-sphere in character, is still relatively strong, which indicates that hydrogen bonds may play a significant role. Hydrogen bonding may also help to account for the observed outer-sphere complexation at pH values above the pHiep of goethite.

Introduction The nature of the bonding between organic species and mineral surfaces has been most commonly inferred from the results of surface complexation modeling of macroscopic measurements, such as potentiometric titrations and adsorption edge data. While valuable insights can be gained this way, often more than one set of reactions can provide a reasonable fit to the data. Spectroscopic investigations offer the possibility of more direct and, therefore, more reliable information and are increasingly used to inform the choice of surface reactions when modeling adsorption systems.1-4 Adsorbed organic molecules with carboxylate substituents are particularly well-suited to attenuated total reflection Fourier transform infrared (ATR-FTIR) study because they tend to have relatively intense bands between 1200 and 1800 cm-1, where common minerals show little IR absorption. As a consequence, there have been several ATR-FTIR studies of carboxylate adsorption. Persson et al.1 found that the locations of the asymmetric and symmetric stretching frequencies of the acetate ion were virtually unchanged on adsorption onto γ-Al2O3, indicating that the force constants, bond angles, and dihedral angles of the carboxylate groups were preserved on adsorption. This result, together with the strong dependence of adsorption on the ionic strength, implied the presence of an outer-sphere complex. FTIR was also used by Roddick* Author to whom correspondence should be addressed. E-mail: [email protected]. † Umeå University. ‡ La Trobe University.

(1) Persson, P.; Karlsson, M.; O ¨ hman, L.-O. Geochim. Cosmochim. Acta 1998, 62, 3657. (2) Persson, P.; Nordin, J.; Lo¨vgren, L.; O ¨ hman, L.-O.; Sjo¨berg, S. J. Colloid Interface Sci. 1998, 206, 252. (3) Boily, J.-F.; Nilsson, N.; Persson, P.; Sjo¨berg, S. Langmuir 2000, 16, 5719. (4) Boily, J.-F.; Persson, P.; Sjo¨berg, S. Geochim. Cosmochim. Acta 2000, 64, 3453.

Lanzilotta and McQuillan5 to study glutamate and aspartate adsorption onto TiO2. Their results provided evidence of bridging bidentate coordination of both molecules to two titanium ions via the carboxylate groups. However, for glutamate, the spectra of adsorbed species at lower pH values suggested that chelation to a single titanium ion also occurs. Hidber et al.6 investigated the adsorption of citric acid by R-Al2O3 by ATR-FTIR and proposed from the relative shifts in the asymmetric and symmetric COO- bands that adsorption occurred via bridging bidentate coordination of the carboxylate group. Recently, the adsorption of dicarboxylic acids, hydroxybenzoic acids, and resorcinol onto kaolinite using ATRFTIR7 has also been investigated. Band shifts for the dicarboxylic acids were consistent with inner-sphere complexation, but the spectral changes on adsorption of hydroxybenzoic acids and resorcinol were too small to provide definitive information on the type of surface complexes formed. A series of studies have investigated the adsorption of benzene carboxylates onto goethite. In these, ATR-FTIR has been used to provide information on the nature of adsorbed species. Benzene carboxylates may be regarded as analogues of fulvic and humic acids as aromatic structures substituted by COOH and OH groups comprise a substantial fraction of the mass of humic substances.8 These surface species identified using IR spectroscopy have then provided the basis for the modeling of potentiometric titration and adsorption data. Persson et al.2 investigated (5) Roddick-Lanzilotta, A. D.; McQuillan, A. J. J. Colloid Interface Sci. 2000, 227, 48. (6) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79, 1857. (7) Specht, C. H.; Frimmel, F. H. Phys. Chem. Chem. Phys. 2001, 3, 5444. (8) Schnitzer, M. In Interactions of Soil Minerals with Natural Organics and Microbes; Huang, P. M., Schnitzer, M., Eds; SSSA Special Publication 17; Soil Science Society of America: Madison, WI, 1986; p 77.

10.1021/la035471o CCC: $27.50 © 2004 American Chemical Society Published on Web 12/31/2003

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the phthalate/goethite system using ATR-FTIR and identified two adsorbed species, an outer-sphere complex involving deprotonated phthalate ions and an inner-sphere complex, also assumed to be deprotonated. The outersphere complex completely dominated adsorption at pH values above 7, while the fraction of phthalate adsorbed as the inner-sphere complex increased as the pH decreased. The adsorption of the benzene tri- and tetracarboxylates trimellitate and pyromellitate on goethite both showed evidence of three surface complexes.3,4 The first was a fully deprotonated outer-sphere complex, which was the dominant adsorbed species at higher pH values. The second was a partially protonated outer-sphere complex, apparent only at pH values below 5, and comprising a relatively small fraction of the adsorbed species even at low pH. The third adsorbed species was an inner-sphere complex that became of increasing importance as the pH decreased. In each case, the species determined from analysis of the ATR-FTIR spectra were used successfully to model potentiometric titration and batch titration data. Herein, we extend the series of investigated benzene carboxylates by studying the hexacarboxylic acid, mellitic acid. Its adsorption properties may provide insight into the behavior of highly charged natural organic matter. This paper investigates the ATR-FTIR spectra of mellitic acid adsorbed onto goethite with the aim of providing information on the nature of the adsorbed species as a function of the pH and ionic strength. Experimental Section Materials. The preparation and characterization of the goethite sample has been described previously.9 The sample was stored as a dry powder and was from the same batch as that used in previous studies of the adsorption of mellitic acid,10,11 with a surface area of 49.6 m2 g-1. Previous XRD analysis showed only goethite lines. Milli-Q reagent-grade water was used in the preparation of all suspensions and solutions. Unless otherwise stated, all chemicals used were of analytical reagent grade. Preparation of Suspensions. A total of 0.500 g of goethite was suspended in a solution of 5 mM KNO3 and stirred under Ar at 25 ( 1 °C for at least 5 h. Sufficient mellitic acid solution was then added to provide an initial concentration of 2.0 mM, and the volume of the suspension was adjusted to 50.0 mL. The total mellitate concentration was chosen so that there was a 1:1 mellitate/surface site ratio if all mellitate was adsorbed. Previous adsorption results10,11 indicate that more than 80% of the mellitate is adsorbed when the concentration ratio is 0.25:1, so it is reasonable to assume that the majority of the mellitate is adsorbed under the conditions used in this study. The very weak mellitate supernatant spectra at pH values below 8 confirm this. The pH was adjusted to the required value by use of a KOH or HNO3 solution. After a further equilibration time of at least 3 h, a 2.5mL sample of the suspension was taken and centrifuged at 3750 rpm for 20 min. FTIR spectra of the supernatant and paste were taken separately, as described below. The pH of the suspension was then adjusted to a new value, the system equilibrated for at least 3 h, a 2.5 mL sample was taken, and the process was repeated. ATR-FTIR spectra of 10 mM solutions of mellitic acid in 5 mM KNO3 were measured at several pH values between 3 and 10. Adsorption at Different Ionic Strengths. The effect of the ionic strength on the adsorption of mellitic acid was also studied. Because the nitrate ion absorbs strongly in the wavelength range of interest, NaCl was used as the background electrolyte. A total of 0.500 g of goethite was suspended in 45.0 mL of 5 mM NaCl (9) Angove, M. J.; Wells, J. D.; Johnson, B. B. J. Colloid Interface Sci. 1999, 211, 281. (10) Angove, M. J.: Wells, J. D.; Johnson, B. B. Colloids Surf., A 1999, 146, 243. (11) Angove, M. J.; Fernandes, M. B.; Ikhsan, J. J. Colloid Interface Sci. 2002, 247, 282.

Johnson et al. and stirred under Ar for 18 h at the natural pH (ca. 8.2) and 25 °C. A total of 5 mL of a 20 mM mellitic acid solution was then added, giving an initial concentration of 2 mM, the pH was adjusted to 3.5 with HCl, and the suspension was left stirring under Ar overnight. A total of 3 mL of the suspension was removed and centrifuged for ATR-FTIR analysis as before. The ionic strength of the suspension was increased to 0.050 M by the addition of solid NaCl, the pH was adjusted to 3.5 with NaOH, and the suspension was left to equilibrate under Ar for 3 h. After sampling, the ionic strength was adjusted again with solid NaCl, providing samples with background electrolyte concentrations of 0.50 and 2.0 M. ATR-FTIR spectra of 10.0 mM solutions of mellitic acid were also measured after the addition of NaCl to provide background electrolyte concentrations of 0.005, 0.050, 0.50, and 2.0 M. (For the 0.005 M system, the ionic strength will be significantly affected by the extent of deprotonation of the mellitic acid.) FTIR Spectroscopy. A Perkin-Elmer Spectrum 2000 FTIR spectrometer equipped with a deuterated triglycine sulfate detector was used to collect the IR spectra. All samples were analyzed with a 9 reflection DiComp/ZnSe Diamond DuraSamplIR ATR system. For each sample, 500 scans were collected from the cell, supernatant, and paste, with the supernatant and an approximately 1-mm thickness of paste applied uniformly to the ATR crystal. A Perspex lens on top of an O-ring gasket was pressed over the sample to prevent drying or CO2 contamination during measurement of the spectra. Supernatant spectra were collected to check for possible contributions to the paste spectra from mellitic acid remaining in solution. Only in spectra taken at pH values above 8.0 did the supernatant band compromise more than 5% of the intensities of the corresponding paste spectra. This was not surprising because previous studies had shown strong adsorption of mellitic acid at pH values up to 8.0.10,11 The supernatant spectra were subtracted from the paste spectra to remove any contribution from ligands remaining in solution. Spectra were also taken of goethite suspensions at several pH values and of a 5 mM KNO3 solution. These were subtracted from the spectrum of the mellitate/goethite paste to obtain the spectra of the adsorbed mellitic acid. All calculations were accomplished with the Perkin-Elmer software, Spectrum 2000 for Windows. For the spectra of the 10 mM mellitic acid solutions, 100 scans were collected together with spectra from the cell and Milli-Q water. The spectra from the cell, water, and, where necessary, water vapor, were subtracted from those of the mellitic acid solutions to yield spectra of mellitic acid solution species as a function of the pH. Molecular Orbital Calculations. Theoretical vibrational frequencies of the geometry-optimized mellitate (L6-) and dihydrogen mellitate (H2L4-) species were calculated with the Hartree-Fock theory using the 3-21G* basis set. The calculations were performed with the program Gaussian 98 by Gaussian, Inc., Pittsburgh, PA, U.S.A.,12 and visualization of the calculated vibrational modes was accomplished with GaussView. The potential energy minimum structures of the complexes were obtained without applying symmetry restrictions; that is, all bond lengths, angles, and dihedral angles were allowed to vary. Calculations were preformed on solvated molecules using six explicit water molecules to mimic solvation. All frequencies were scaled by the factor 0.93 to account for systematic errors. This scale factor has been found to be appropriate for similar systems (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, N.; Rega, P.; Salvador, J. J.; Dannenberg, D. K.; Malick, A. D.; Rabuck, K.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.11.2; Gaussian, Inc.: Pittsburgh, PA, 2001.

ATR-FTIR Study of Mellitic Acid Adsorption

Figure 1. ATR-FTIR spectra of 10 mM solutions of mellitic acid in 0.005 M KNO3 at the pH values indicated. Included are also the theoretical spectra of L6- (middle) and H2L4- (bottom). using the same computational method and basis set.13 For all molecules investigated, only real frequencies were obtained, which show that the optimized structures represent minima at the potential energy surface.

Results and Discussion Mellitate Solution Spectra. The five ATR-FTIR spectra presented in Figure 1 show the effect of progressive protonation of the carboxylate groups of mellitate as the pH is decreased. Assuming the pKa values for mellitic acid given by Evanko and Dzombak,14 the dominant species at each pH represented in the figure are pH 8.75, L6- (98%); pH 6.6, L6- (26%) and HL5- (62%); pH 5.5, HL5- (26%) and H2L4- (62%); pH 4.6, H2L4- (37%) and H3L3- (50%); pH 2.5, H4L2- (90%). As expected, the bands found in the IR spectra show a strong resemblance to those found previously for the o-phthalate, trimellitate, and pyromellitate systems.4,15,16 Furthermore, the theoretical spectra of L6- and H2L4reproduce the main features of the experimental spectra (Figure 1). In the theoretical spectra, the vibration mode for each frequency is known; thus, by direct comparison this information can be used to assign the experimental frequencies. These assignments are summarized in Table 1. (13) Clause´n, M.; O ¨ hman, L.-O.; Kubicki, J. D.; Persson, P. J. Chem. Soc., Dalton Trans. 2002, 2559. (14) Evanko, C. R.; Dzombak, D. A. J. Colloid Interface Sci. 1999, 214, 189. (15) Nordin, J.; Persson, P.; Laiti, E.; Sjo¨berg, S. Langmuir 1997, 13, 4085. (16) Nordin, J.; Persson, P.; Nordin, A.; Sjo¨berg, S. Langmuir 1998, 14, 3655.

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At high pH, where speciation is dominated by L6-, the spectrum is comparatively simple and characterized by three strong bands. These are all vibration modes of the carboxylate group that are more or less mixed with modes of the aromatic ring. The broad and strong band at 1577 cm-1 is an asymmetric stretching vibration of the carboxylate group (νCsOas) and is the most pure group mode of the three bands. Theory predicts this band to consist of two closely spaced frequencies (Figure 1), one (the most intense signal) is a pure νCsOas mode while the other is a νCsOas slightly mixed with a CsC stretch involving carbon atoms of the aromatic ring (νring). The remaining strong bands originate from more complex vibration modes. The 1426 cm-1 band is assigned to a mixed vibration between a CsC stretch vibration (νCsCO2), a νring mode, and a bending motion of the carboxylate groups (δCO2). Note that this vibration has no major contribution from the symmetric stretching vibration of the carboxylate group (νCsOs). Instead, the 1334 cm-1 band is a νCsOs mode that is coupled with νring. In addition to the three strong bands, a weak and rather broad band is observed around 1050 cm-1. This is predicted to be a mixed mode between an out-of-plane bend of the carboxylate groups and a wagging motion of the water molecules hydrogen bonded to these groups. The theoretical over-prediction of the intensities of these latter bands might be a consequence of inadequacies in modeling the hydration of L6-. As the pH is decreased, the L6- ion becomes gradually protonated, and this is accompanied by the appearance of new bands in the IR spectra. The most significant changes are the band at 1716 cm-1, which is absent at higher pH values but increases in intensity on protonation of the carboxylate groups, and the concurrent intensity increase of the band at 1254 and the shoulder at 1160 cm-1. Furthermore, we observe an upward shift of the νCsOas band and broadening/splitting of the 1426 and 1334 cm-1 bands of L6-. The agreement between the experimental and the theoretical IR spectra is worse for the protonated species. This, we believe, is primarily due to problems in modeling hydration. Nonetheless, the theoretical results can be used to make a tentative assignment of the experimental frequencies (Table 1). The 1716 cm-1 band is characteristic for protonated carboxylate groups and is commonly assigned to a carbonyl-like stretching vibration (νCdO). The results from the theoretical calculations on H2L4- support this assignment but also show mixing between νCdO and δCOH and that this band most probably is composed of two closely spaced and similar modes (Table 1 and Figure 1). The upward shift from 1577 to about 1590 cm-1 of νCsOas of the protonated species is predicted by the frequency calculations and can be ascribed to the influence of the adjacent protonated carboxylate groups and the fact that the vibration modes are slightly different from those in L6with stronger mixing with νring. The changes and the more complex pattern of bands from 1450 cm-1 and below are primarily caused by the existence of sCOH groups and the mixing of δCOH with other modes of the protonated molecules. Both the broadening/splitting of the 1426 and 1334 cm-1 bands and the appearance of new bands at 1254 and 1160 cm-1 can be ascribed to this effect. Thus, while the 1716 cm-1 band is evidence for sCdO, these latter features are indicative of the presence of sCOH, and together they indicate the existence of protonated carboxylate groups. The effect on the spectrum of mellitic acid at pH 3.5 of variation in the ionic strength from about 0.01 to 2.0 M using NaCl is shown in Figure 2. From these spectra, we can see that there is a tendency for the solution species

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Johnson et al.

Table 1. Assignments of the Major IR Frequencies of Deprotonated Mellitate (L6-) and Diprotonated Mellitate (H2L4-) Based on the Results from the Theoretical Frequency Calculations L6-, cm-1 1577a 1426 1334 a

assignment

H2L4-, cm-1

assignment

νCsOas, νCsOas + νring νCsCO2 + δCO2 + νring νCsOs + νring

1710 1585b 1428a 1338a 1258

νCdO + δCOH 2νCsOas, νCsOas+νring νCsCO2 + δCO2 + δCOH + νring, δCOH + νring νCsOs + δCOH + νring + δring, δCOH + νring νCsOs + νCsCO2 + δCOH + νring

Predicted to be composed of two vibration modes. b Predicted to be composed of three vibration modes.

Figure 2. ATR-FTIR spectra of 10 mM solutions of mellitic acid at pH 3.5 and a range of ionic strengths. The ionic strength was adjusted with NaCl.

to deprotonate as the ionic strength increases so that, in the presence of 0.5 M NaCl, the spectrum resembles that at pH 4.6 in Figure 1, while when I ) 2.0 M the spectrum suggests a solution speciation similar to that found at pH 5 at low ionic strength. This corresponds to a shift from H4L2- as the dominant solution species at low ionic strength to H2L4- when I ) 2.0 M. This increased trend in acidity with increasing I of the different mellitate species reflects effects of specific ion interactions involving the Na+ ion. These interactions become important for highly charged ions in solutions with I g 0.1 M. Spectra of Mellitate Adsorbed on Goethite. Spectra of pure goethite pastes were recorded at several pH values between 3 and 8 with no discernible change evident over this pH range. While the major goethite bands, located at 792, 895, and 3092 cm-1, do not fall in the spectral region of interest for adsorbed mellitate species, bands with relatively low intensities were found at 1115, 1328, 1600, 1663, and 1790 cm-1. The goethite spectrum was, therefore, subtracted from mellitate/goethite paste spectra to obtain spectra of the adsorbed species as a function of the pH. Figure 3 shows the IR spectra for mellitate species adsorbed on goethite in the pH range from 3 to 9.5. As seen, pronounced bands are detected in all spectra, which indicates significant adsorption over the whole pH range. At pH 6 and above, the spectra are almost identical. Each shows three major bands at about 1327, 1428, and 1580 cm-1, which closely parallel the frequencies at 1334, 1426, and 1577 cm-1 of L6- in the solution spectra. This close similarity shows that there is very little structural change when mellitate is transferred from solution to an adsorbed state at the water-goethite interface. As has been discussed in previous studies, this strongly suggests that adsorption at pH > 6 occurs via a deprotonated outersphere complex.3,4 Though the band positions and intensities are similar to those in solution, the bandwidths at 1339 and 1580 cm-1 increase from 13 and 48 cm-1 in

Figure 3. ATR-FTIR spectra of mellitic acid adsorbed to goethite at pH values at and below 7. All samples contained 2 mM mellitic acid and 10 g/L goethite in 0.005 M KNO3. Bands have been scaled to the area of the goethite band at 895 cm-1.

solution to 22 and 61 cm-1 for the surface species. The increase in bandwidths found can be attributed to a range of slightly different states, including all adsorbed outersphere mellitate ions. Thus, this encompasses both mellitate ions with an intact hydration shell and ions that are partly desolvated and hydrogen-bonded to surface hydroxyl groups and surface-coordinated water molecules. At pH 5.0 and below, the spectra show evidence of additional bands at 1710 and 1279 cm-1, which increase in intensity as the pH is decreased (Figure 3). The frequencies of these bands agree closely with the bands originating from the protonated carboxylate groups of the solution species, where the former frequency indicates sCdO and the latter sCOH groups. This shows that protonated surface species are formed at lower pH values. Apart from bands originating from the nonprotonated outer-sphere complex and from protonated carboxylate groups, no other major features can be detected under the chemical conditions investigated. Thus, the most likely explanation of the pH-dependent changes is that the nonprotonated mellitate ions adsorbed at high pH are gradually protonated as the pH is lowered while retaining their outer-sphere coordination. Compared with previously investigated benzene carboxylate systems, the lack of strong evidence for innersphere complexes at low pH is surprising. There is only one weak band at 1360 cm-1 that is not explained by comparison with the solution species. Hence, this could indicate a small amount of inner sphere complexes but it may also correspond to a ring mode vibration that is absent

ATR-FTIR Study of Mellitic Acid Adsorption

Figure 4. Effect of pH on the adsorption of mellitic acid: (b) adsorption data;7 (4) values obtained from the ratio of the band area of the mellitate band at 1428 cm-1 to the area of the goethite band at 895 cm-1, arbitrarily scaled for comparison with the adsorption data.

in the solution spectra because of the high symmetry of the mellitic acid molecule. This suggests that the proximity of the surface on adsorption, even as an outer-sphere complex, may cause sufficient restriction in the motion of the molecular ion in the direction of the surface to upset the molecular symmetry. Irrespective of the interpretation of this band, it is clear that inner-sphere complexation is much more important for the previously investigated benzene carboxylates than for mellitate. The reason for this is not known, but one explanation might be that the additional carboxylate groups stabilize the outer-sphere complex (by creating more sites for hydrogen bonding) with the result that for the hexacarboxylate the outersphere complex is simply stable enough to out-compete the inner-sphere species. We can obtain an estimate of the amount of mellitic acid adsorbed to goethite from the paste spectra by use of the ratio of the area of a band due to adsorbed mellitate to the area of a goethite band. For this purpose, the band at 1428 cm-1 was chosen as representative of adsorbed mellitate species, and the band at 895 cm-1 was chosen for goethite. This measure will tend to underestimate mellitate adsorption at pH values below 6 because the spectra show that the intensity of the mellitate band at 1428 cm-1 is reduced as the extent of protonation of adsorbed species increases. The adsorption curve predicted from the spectra is compared with the measured pH variation of mellitate adsorption on goethite10,11 in Figure 4. The concordance between the measured and the predicted pH dependence is encouraging and suggests that a reasonable estimate of relative surface concentrations can be found from the relative intensities of spectral bands. A surprising feature of this study is the spectral evidence for bonding of the L6- ion as an outer-sphere complex to goethite at pH 9.5. Although relatively less intense than that at lower pH values, the only bands evident at pH 9.5 are those at 1339, 1428, and 1580 cm-1. The pHiep of goethite is reported to be between 9.1 and 9.4,17-19 so the (17) Johnson, B. B. Environ. Sci. Technol. 1990, 24, 112. (18) Venema, P.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1997, 192, 94. (19) Boily, J.-F.; Lutzenkirchen, J.; Balme´s, O.; Beattie, J.; Sjo¨berg, S. Colloids Surf. 2001, 179, 11.

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Figure 5. ATR-FTIR spectra of mellitic acid adsorbed to goethite at pH 3.5 and a range of ionic strengths. All samples contained 2 mM mellitic acid and 10 g/L goethite. The ionic strength was adjusted with NaCl. Bands have been scaled to the area of the goethite band at 895 cm-1.

particle surfaces have a net negative charge when pH > 9.4. Outer-sphere complexation of L6- to goethite against this electrostatic barrier may be the result of the structure of the goethite surface. The surface of goethite is known to contain singly, doubly, and triply coordinated surface hydroxyl groups.20 Venema et al.21 propose, on the basis of predicted proton affinity constants, that the singly coordinated tFeOH-0.5 sites are largely responsible for the acid/base properties of the goethite surface in the pH range from 1 to 11. They propose that the doubly coordinated tFe2OH0 sites are not proton active over the normal pH range. These doubly coordinated sites are present in rows across the dominant {110} crystal plane of goethite, thereby providing possible sites for outersphere coordination of mellitate ions at high pH values involving hydrogen bonding. The large negative charge of the mellitate ion may also cause a redistribution of positively charged sites to enhance local electrostatic forces. In a final set of experiments, we studied the influence of ionic strength on the adsorption of mellitate ions at the water-goethite interface. The spectra of the adsorbed species at pH 3.5 were measured at ionic strengths between 0.01 and 2.0 M, generating the results shown in Figure 5. Increasing the ionic strength to 0.05 M caused a significant change in the spectrum with the band at 1278 cm-1 decreasing in intensity while there was a corresponding increase in the intensity of the band at 1328 cm-1. This suggests that increasing ionic strength causes a decrease in the amount of adsorbed protonated species relative to the deprotonated mellitate ion. Further increase in ionic strength enhanced these effects. By the time the ionic strength was increased to 2.0 M, the spectrum suggested that little protonated mellitate remained on the surface. Thus the trend in the extent of protonation of the surface complexes matched that found for the mellitate species in solution where deprotonation at higher ionic strengths was observed also. The relative intensity of the mellitate band at 1428 cm-1 to the goethite band at 895 cm-1 indicates that there was little change in the total amount of mellitate adsorbed as the ionic strength was increased to 2.0 M. Hence, high (20) Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature 1974, 248, 220. (21) Venema, P.; Hiemstra, T.; Weidler, P. G.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1998, 198, 282.

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ionic strength appears to cause a net increase in the concentration of the deprotonated surface species with little or no change of the total amount of adsorbed mellitate. Accordingly, for mellitate adsorption on goethite, increasing ionic strength does not displace outer-sphere complexes from the surface. These results indicate that the use of changes in the ionic strength to differentiate between inner- and outer-sphere complexation22 is inappropriate where electrostatic or hydrogen-bonding forces are strong, as is expected for highly charged species. An interesting trend is revealed from the experiments conducted at variable ionic strength in solution and suspension. Comparison of the relative ratios of bands indicative of protonated and nonprotonated carboxylate groups in Figures 2 and 5 shows that, at identical ionic strengths at pH 3.5, mellitate is significantly less protonated at the interface than in solution. Thus, mellitate ions adsorbed as nonprotonated outer-sphere complexes exhibit resistance toward protonation and, in other words, are more acidic than their solution counterparts. This is not expected for classical outer-sphere complexes, which retain the 1st hydration sphere, thus supporting the suggestion that at least part of the mellitate ions are adsorbed to goethite through comparatively strong hydrogen bonds. Summary and Conclusions

Johnson et al.

L6- ion. At lower pH values, there is evidence of outersphere complexation of partially protonated species, though the extent of protonation of the adsorbed species is always significantly less than that found in solution at the same pH. Increasing the ionic strength to 2.0 M does not displace the outer-sphere L6- species from the surface. However, it does decrease the fraction of the surface covered with protonated mellitate ions. These results indicate that the effect of ionic strength should not be used to differentiate between inner- and outer-sphere complexes formed by highly charged species. Mellitate ions (L6-) adsorb as outer-sphere complexes to goethite at relatively high pH values where the surface has a net negative charge. While this seems unreasonable in electrostatic terms, it shows the significance of hydrogenbonding interactions for polycarboxylic acids such as mellitate when adsorbed to goethite surfaces. Finally, the spectroscopic evidence for the predominance of outersphere complexes (nonprotonated as well as protonated), irrespective of the ionic strength, provides important constraints for the surface complexation modeling of mellitate adsorption to goethite. This will be the subject of a forth-coming publication.23 Acknowledgment. B.B.J. thanks Kristina Axe for her patient help with the use of the FTIR spectrometer.

Adsorption of mellitic acid to goethite at pH values above 6 involves outer-sphere complexation of the deprotonated

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(22) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; John Wiley and Sons: New York, 1996; p 542.

(23) Angove, M. J.; Wells, J. D.; Johnson, B. B. Langmuir, submitted for publication.