Benzenecarboxylate Surface Complexation at the Goethite (Î±-FeOOH

Langmuir 2000, 16, 5719-5729

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Benzenecarboxylate Surface Complexation at the Goethite (r-FeOOH)/Water Interface: I. A Mechanistic Description of Pyromellitate Surface Complexes from the Combined Evidence of Infrared Spectroscopy, Potentiometry, Adsorption Data, and Surface Complexation Modeling Jean-Franc¸ ois Boily,*,† Nils Nilsson, Per Persson, and Staffan Sjo¨berg Department of Inorganic Chemistry, Umeå University, S-901 87 Umeå, Sweden Received October 26, 1999. In Final Form: February 21, 2000

An investigation combining IR spectroscopy, potentiometric titrations, and adsorption experiments was carried out to study pyromellitate (1,2,4,5-benzenetetracarboxylate) sorption at the goethite (R-FeOOH)/ water interface. The IR spectra show evidence of outer-sphere complexation throughout the pH range from 3 to 9. Below pH 6 additional IR spectroscopic features appear, which are tentatively assigned to innersphere complexes. A normalized IR peak area plot for each peak indicative of inner- and of outer-sphere complexes as a function of pH provided a semiquantitative surface speciation scheme. This scheme was successfully reproduced using surface complexation theory with a multisite complexation model calibrated on potentiometric titration and on adsorption data. The surface speciation was described with a binuclear outer-sphere complex on the {110} plane of goethite and a mononuclear inner-sphere complex on the {001} plane. Furthermore, as the IR spectra also indicated partial protonation of pyromellitate complexes at low pH, a partially protonated outer-sphere species on the {110} plane was included in the model.

Introduction Natural organic matter (NOM) occurs in various molecular weights, sizes, and configurations and contains both aliphatic and aromatic constituents.1,2 Studies of the adsorption of low molecular weight (LMW) compounds that are compositionally similar to NOM, can reveal some determining mechanisms in which NOM and mineral surfaces interact. The surface complexation of LMW carboxylates to metal-(hydr)oxide surfaces has therefore attracted much attention in the literature,although severe limitations in using LMW organic acids as analogues to NOM may stem from the polyelectrolytic attributes of NOM. In the field of surface complexation modeling, the range of electric double layer models available has necessarily resulted in important discrepancies in modeling LMW carboxylate adsorption. One major source of discrepancy involves the coordination mode of adsorbed carboxylates to the metal-(hydr)oxide surface. Phthalate adsorption to the goethite surface, for example, has been described as inner- and/or outer-sphere complexes with a wide range of modeling parameters.3,6-9,12 In recent †Present address: Institut fu ¨ r Mineralogie und Petrographie, Eidgeno¨ssiche Technische Hochschule, ETH-Zentrum CH-8092 Zu¨rich, Switzerland.

(1) Schnitzer, M. In: Environmental Geochemisry. Short Course Handbook Mineralogical Association of Canada. 1984, 10, 237. (2) Salz-Jimenez, C. Environ. Sci. Technol. 1994, 197, 28. (3) Lo¨vgren, L. Geochim. Cosmochim. Acta 1991, 55, 3639. (4) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1992, 8, 525. (5) Gu, B.; Schmitt, J.; Chen, Z.; Liang, L.; McCarthy, J. F. Geochim. Cosmochim. Acta 1995, 59, 219. (6) Ali, M. A.; Dzombak, D. A. Environ. Sci. Technol. 1996, 30, 1061. (7) Nilsson N.; Persson, P.; Lo¨vgren, L.; Sjo¨berg, S. Geochim. Cosmochim. Acta 1997, 60, 4385. (8) Nordin, J.; Persson, P.; Laiti, E.; Sjo¨berg, S. Langmuir 1997, 13, 4085. (9) Persson, P.; Nordin, J.; Rosenqvist, J.; Lo¨vgren, L.; O ¨ hman, L.O.; Sjo¨berg, S. J. Colloid Interface Sci. 1998, 206, 252. (10) Nordin, J.; Persson, P.; Nordin, A.; Sjo¨berg, S. Langmuir 1998, 14, 3655.

studies,8-10 the coordination modes of phthalate and of pyromellitate on several metal-(hydr)oxides were deciphered using IR spectroscopy. Outer-sphere complexes were found to be predominant at high pH, while innersphere complexes are present only at low pH. In those studies, the presence of inner- and outer-sphere species was modeled using the classical 2 pK model.9 These models were successful in describing potentiometric titration and adsorption data. However they failed in accurately predicting the speciation as observed with spectroscopy. A model linking macroscopic to microscopic observation requires a common ground of interpretation, one that can be sought in the analysis of the disposition of surface functional groups on metal-(hydr)oxide surfaces. The charge-distribution multisite complexation model (CDMUSIC) of Hiemstra and van Riemsdijk13 provides such a framework. In this model, the reactivity and the distribution of the different functional groups on different crystal planes of a mineral can be taken into account to devise mechanistic surface complexation models. In the present study, pyromellitate (1,2,4,5-benzenetetracarboxylate) surface complexes at the goethite (R-FeOOH)/ water interface are investigated by means of IR spectroscopy, potentiometric titrations, and adsorption experiments. This study is the first of a series of forthcoming papers from our group where we use the CD-MUSIC model to link spectroscopic observations to potentiometric titration and adsorption data. Experimental Methods Materials. Solutions. All solutions were made from deionized and boiled water. Stock solutions of nitric acid were prepared from HNO3 (Merck p.a.) and standardized against tris(hy(11) Evanko, C. R.; Dzombak, D. A. J. Colloid Interface Sci. 1999, 214, 189. (12) Filius, J. D.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1997, 195, 368. (13) Hiemstra. T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 179, 488.

10.1021/la991407o CCC: $19.00 © 2000 American Chemical Society Published on Web 05/27/2000

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droxymethyl)-aminomethane (Trizma-base). NaOH solutions were prepared from a saturated NaOH solution mixture (50% NaOH, 50% H2O) which was filtered immediately before use, thereby excluding carbonate impurities. NaOH solutions were standardized against solutions of HNO3 of known concentrations. Solutions of 1,2,4,5-benzenetetracarboxylic acid (pyromellitic acid, H4L) were prepared from C10H6O8 (Merck zur Synthese) and standardized in potentiometric titrations with NaOH solutions or with coulometrically generated hydroxide ions. The background electrolyte used in all experiments was 0.1 M in Na(NO3), prepared with sodium nitrate (Merck p.a., dried at 353 K) and distilled and boiled water. However, due to spectral interference with nitrate those for IR spectroscopy were prepared in 0.1 M NaCl (Merck p.a., dried at 453 K). Goethite. Goethite (R-FeOOH) was prepared according to the method of Atkinson et al.14 by mixing 400 mL of 2.5 M KOH (EKA p.a.) to 1650 mL of 0.15 M Fe(NO3)3‚9H2O (Merck p.a.) in one addition and thoroughly mixing the freshly precipitated solids. The precipitates were aged for 48 h at 358 K and washed repeatedly in the following 6 months with deionized and boiled water until the pH of the supernatant reached 7-8. The stock suspensions were prepared and stored only in polyethylene bottles. The precipitates were identified as goethite by X-ray powder diffraction. The surface area of the goethite particles was determined by the B.E.T. N2(g) adsorption method to be 43 m2/g. Potentiometric Titrations and Adsorption Experiments. The present investigation was carried out as a series of potentiometric titrations in a constant ionic medium of 0.1 M Na(NO3). The potentiometric titrations were performed with an automatic system for precise emf titrations developed from the concept of Ginstrup.15 The cell arrangement, which was immersed in an oil thermostat at 298.15 ( 0.05 K, was similar to that described by Forsling et al.16 A glass electrode (Ingold type U262S7) and a Ag,AgCl-reference electrode, prepared according to Brown17 were used for the measurement of H+ concentrations. This implies that given pH values correspond to -log[H+], and not -log aH+. All experiments were performed in rooms thermostated at 298.2 ( 0.5 K. During the titrations, an inert atmosphere was maintained by a continuous flow of moisturized and purified argon gas above the suspension in an airtight titration vessel, thereby excluding CO2. A propeller stirrer maintained effective stirring. The H-L4- System. Solutions of pyromellitic acid were titrated with NaOH solutions or with coulometrically generated hydroxide ions. The glass electrode was calibrated either at the beginning of each titration in a H+ solution of known composition before the addition and titration of H4L or immediately after each titration in separate H+ solutions of known composition. The agreement between these calibration procedures was better than 0.5 mV. H-tFeOH and H+-L4--tFeOH System. Titrations in the two component system were carried out with an internal calibration of the glass electrode in the pH range 2.7-3.0. The titrations of the goethite suspension were carried out with dilute standardized solutions of HNO3 and NaOH in concentrations of less than 15% of the total 0.1 M ionic strength. The titrations in the three component system were started by the addition of coulometrically generated OH- until the pH reached about 10. A small volume of the pyromellitate stock solution was then added and the increasing adsorption of pyromellitate with decreasing pH was studied by titration with acid. An aliquot of the suspension was sampled and centrifuged at each titration point in this part of the titration. The total concentration of pyromellitate in the supernatant was determined by UV-absorption measurements of acidified solutions at 291 nm. As a criterion for stable readings that must be fulfilled before successive additions of H+ or OH-, a drift in the electrode potential of less than 0.3 mV per hour was used (corresponding to 0.005 pH units per hour). Typical values for the time between two additions were 10-20 h. (14) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Phys. Chem. 1967, 71, 550. (15) Ginstrup, P. Chem. Instrum. 1973, 4, 141. (16) Forsling, W.; Hietanen, S.; Sille´n, L. G. Acta Chem. Scand. 1952, 6, 901. (17) Brown, A. S. J. Am. Chem. Soc. 1934, 56, 646.

Boily et al. To obtain information about the kinetics of the adsorption reactions and to test the reversibility, a titration was performed in the following manner: an excess of H+ was added to the goethite suspension and the acidified suspension was neutralized with coulometrically generated OH-. The ligand was added (to give pH ) 3.6) and aliquots of the suspension were taken, centrifuged, and analyzed. Twenty-four h after the addition of pyromellitate, hydroxide ions were added (to reach pH 9.6) and the sampling was continued. The kinetic experiment showed that both the adsorption at pH 3.6 and the desorption at pH 9.6 were fast reactions. Three minutes after the addition of ligand approximately 90% of the total adsorption had occurred and after 1 h no more ligand was adsorbed. The desorption was even faster and the reactions were completely reversible. The complexation of Fe3+ with pyromellitate in solution is not well characterized. To check the importance of such complexes the total concentration of Fe3+ was measured in samples from the titrations using a spectrophotometric method with ferrozine as an indicator. The total concentration of aqueous Fe3+ was found to be less than 1 µM indicating dissolution was negligible and consequently complexation between Fe3+ and pyromellitate could be neglected. IR Spectroscopy. Preparation of Samples. The wet pastes of goethite samples in equilibrium with aqueous pyromellitate solutions were studied by IR spectroscopy as a function of pH and ionic strength in NaCl. To remove dissolved bicarbonate, the goethite stock suspensions were first made weakly acidic by an initial addition of acid followed by purging with argon. Aliquots of the degassed suspensions were transferred to 50 mL polyethene centrifuge tubes to which given amounts of pyromellitate and NaCl had been added, and pH was adjusted to the desired values by addition of acid or base. After equilibration for at least 24 h (end-over-end rotation), the pH of the suspension was measured while purging with argon. If the measured value was significantly different from the desired one, the pH was readjusted and the suspension was allowed to equilibrate for another 24 h period. After the final pH measurement, the samples were centrifuged and the solid material was used for IR spectroscopic measurements. Collection of IR Spectra. The IR spectra were collected with a Perkin-Elmer Spectrum 2000 FTIR spectrometer, which was equipped with a deuterated triglycine sulfate (DTGS) detector. All adsorption samples were analyzed as wet pastes with the attenuated total reflection (ATR) technique. The spectra were recorded with a horizontal ATR accessory and an AMTIR crystal as the reflection element (Perkin-Elmer). The angle of incidence for this setup is approximately 45° and the total number of reflections is 12. The samples for IR analysis were prepared in agreement with the adsorption experiments described above. After centrifugation, the supernatant was decanted and the wet pastes were uniformly applied directly to the AMTIR crystal, and a small volume of the supernatant was applied on top of this layer. A lid with two gas valves was placed over the crystal and the samples were protected from the atmosphere during data collection by a slow flow of moisturized argon. Typically, 500 scans were collected per sample. For each sample a spectrum of the supernatant was recorded using the same setup and procedure as for the paste. The supernatant spectra were used to check for possible contributions to the paste spectra from ligands remaining in solution. The samples prepared at 2 M ionic strength and high pH (i.e., samples with low total adsorption) showed band intensities of the supernatant between 10 and 20% of the intensities of the corresponding paste spectra. For all samples with ionic strengths of 0.1 M or below, the supernatant intensities were less than 5% of those for the pastes. Also, to isolate the spectra of the ligands at the water-solid interface, the supernatant spectra were subtracted from the paste supernatant spectra to remove the strong contributions from the water bands and those from the unadsorbed ligands remaining in solution. All calculations and plotting were accomplished with Spectrum 2000 for Windows, Perkin-Elmer.

IR Spectroscopy The objectives of the IR spectroscopic part of this work are threefold: (i) To identify the number of dominating surface complexes. (ii) To propose bonding mechanisms

Spectroscopy and Modeling of Pyromellitate Surface Complexes

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Figure 1. Attenuated total reflectance FTIR spectra of pyromellitate sorbed on goethite at pH (a) 7.3, (b) 5.9, (c) 5.2, (d) 4.3, and (e) 3.3. The samples were prepared at a total pyromellitate surface concentration of 2.2 µmoles/m2 in 0.1 M NaCl. The ordinate scale is identical for all spectra. The features around 1650 cm-1 are due to imperfections in the subtraction of the strong water contributions.

for the identified surface complexes. (iii) To estimate the relative concentrations of the surface complexes from IR data. We try to accomplish this by studying a rather limited part of the spectra, namely the frequency region where the characteristic carboxylate stretching vibrations (νC-O) appear. Assignment of peaks in this region for the protonated and nonprotonated molecules have been published in a previous paper.10 Identification of Surface Complexes. To identify the number of structurally different pyromellitate surface complexes we have studied samples prepared under a wide range of experimental conditions. These data show that surface speciation varies both as a function of pH and ionic strength. The effect of pH is shown in Figure 1. At pH values around 7 and above the spectrum of adsorbed pyromellitate is almost identical to that of the completely deprotonated molecule in aqueous solution. Of particular interest for the following discussion is the strong νC-O (sym.) peak at 1377 cm-1. A decrease in pH is accompanied by distinct changes in the spectra of the adsorbed species. Most notable are the appearances of new shoulders at the high and low frequency sides of the 1377 cm-1 peak, and two peaks at 1254 and 1271 cm-1; these features are clearly seen in the spectrum of the pH 3.3 sample (Figure 1). The two latter peaks indicate the presence of protonated pyromellitate at the interface (see previous assignment in ref 10). The observed shoulders are resolved into two peaks at 1364 and 1384 cm-1 in the second derivative spectra (Figure 2). The former peak is detected at higher pH values. However, from these data it is not clear whether the two peaks originate from the same surface complex or from two different species. This question is resolved by the experiments conducted at fixed pH and variable ionic strength. A narrow spectral region of samples prepared at pH 3.5 and ionic strengths from 10 mM to 2 M (NaCl) are shown in Figure 3. At low ionic strength the peak at 1384 cm-1 together with peaks at 1254 and 1271 cm-1 are prominent. As the ionic strength is increased the intensity of these features decrease; this behavior is not observed

for the 1364 cm-1 peak. Thus, the ionic strength experiments show that the peaks at 1254, 1271, and 1384 cm-1 most probably originate from one and the same surface complex and the peak at 1364 cm-1 from a structurally different one. To summarize, by varying pH and ionic strength we have identified three different pyromellitate surface complexes. In agreement with findings in related phthalate systems, the surface complexes are most readily distinguished by peaks in the frequency region where motions involving symmetric νC-O vibrations appear.9 Bonding Mechanisms and Structures. The striking similarity between the spectra of the surface complex formed at the onset of adsorption at high pH and L4- (aq) implies that the adsorption process induces very little distortion in the molecule (Figure 4). Inner sphere coordination between Fe(III) and carboxylates is known to have a significant effect on the IR spectra,9 thus our data indicate that pyromellitate adsorbs to a large extent as a nonprotonated outer-sphere ligand. The only noticeable consequence of adsorption at high pH for the IR spectrum is broadening and a slight shift of the asymmetric νC-O around 1570 cm-1. The full width at half-height for this peak is 66 cm-1 for the surface complex and 53 cm-1 for the solution species. These changes can be attributed to changes in the hydrogen bond configuration around the oxygen atoms of the carboxylate groups. In aqueous solution pyromellitate is solvated by a shell of water molecules. The solvation shell is most probably stabilized by hydrogen bonds where the hydrogen atoms in water interact with the carboxylate oxygens. A possible adsorption mechanism is then a partial desolvation of pyromellitate and replacement of solvating water molecules with surface bound water molecules or hydroxyl groups. This will induce an asymmetry in the hydrogen bonds to pyromellitate and distort the D2h symmetry of the free ion,10 which in turn might explain the change suggested by the IR spectra. According to this adsorption mechanism pyromellitate is only separated from the surface by one hydration layer (i.e., the hydration layer is -OH2 in t

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Figure 2. Second derivative spectra multiplied by -1 of pyromellitate sorbed on goethite at pH (a) 7.3, (b) 5.2, and (c) 3.3.

Figure 3. Attenuated total reflectance FTIR spectra of pyromellitate sorbed on goethite at pH 3.5 in (a) 10 mM, (b) 0.1 M, and (c) 2 M NaCl.

FeOH20.5+), but since there is no direct interaction between the ligand and Fe(III) we use the term outer-sphere surface complex. The peaks at 1254 and 1271 cm-1 originate from motions involving C-(OH) stretching and C-O-H bending motions,10 and therefore constitute direct evidence for protonation of pyromellitate at the interface. As discussed above these peaks are accompanied by the appearance of the peak at 1384 cm-1. A similar observation can be made in the spectra of L4-(aq) and HL3-(aq), Figure 5, where protonation causes a shift of the main νC-O peak to higher wavenumbers. Thus the 1384 cm-1 peak is also an

indicator for protonation. This similarity together with the observation that significant amounts of nonprotonated surface complexes are formed suggest that the protonated surface complexes also exist as outer-sphere species. However the extent of the protonation reaction is not a direct reflection of the protonation state of the molecule in aqueous bulk solution. For example, at pH 4.3 the solution contains a mixture between HL3- and H2L2-, while the surface speciation is dominated by a nonprotonated complex (Figure 1). Thus the interface seems to have a stabilizing effect on the more highly charged surface complex in this particular pH range. This observation was


Langmuir, Vol. 16, No. 13, 2000 5723

Figure 4. Attenuated total reflectance FTIR spectra of (b) pyromellitate sorbed on goethite at pH 7.3 and (a) deprotonated pyromellitate in aqueous solution.

Figure 5. Attenuated total reflectance FTIR spectra of (a) protonated pyromellitate (HL3-) and (b) deprotonated pyromellitate (L4-) in aqueous solution.

also made in the related pyromellitate-boehmite (γAlOOH) system.10 Protonation of the outer-sphere complex also probably means a weakening of the attachment to the surface since it lowers the total charge of pyromellitate and the number of carboxylate groups available as acceptors in hydrogen bonds. The third surface complex identified above is characterized by the peak at 1364 cm-1, which increases in significance with decreasing pH and increasing ionic strength. In the related o-phthalate-goethite system, the inner sphere surface complex was found to follow the same trends.9 This together with the fact that the peak at 1364

cm-1 cannot be explained by protonation effects suggests that pyromellitate also forms an inner-sphere complex on goethite at low pH values. We have not been able to isolate a spectrum of the pure inner-sphere complex, and therefore a structural assignment is not possible at present. To make this assignment, further data of relevant model compounds and results from theoretical frequency calculations (Hartree-Fock and/or density functional) are needed. This work is in progress. Surface Speciation Evaluated from IR Data. In the preceding analysis of the IR data three types of surface complexes were identified: a nonprotonated outer-sphere

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Figure 6. Curve fitting results of the spectrum of pyromellitate sorbed on goethite at pH 4.3.

complex, a protonated outer-sphere complex, and an innersphere complex. Additional information on how the relative concentrations of these species vary as a function of pH is contained in the IR data. As will be shown below, such information is valuable for modeling purposes. In this work we have used a rather simple curve-fitting procedure to estimate these relative changes in surface speciation. The following peaks, identified in the second derivative spectra discussed above, were used as indicators for the complexes: (i) 1377 cm-1 (nonprotonated outersphere), (ii) 1254 and 1388 cm-1 (protonated outer-sphere), and (iii) 1362 cm-1 (inner sphere). As noted these frequencies vary slightly from those reported above. This is due to the fact that in this case the peak positions were determined by curve fitting spectra of samples with a high concentration of the complex of interest. The obtained peak positions were then fixed in the fits of the rest of the spectra. To reduce the number of variables a pure Gaussian model was used and the widths were constrained to be equal for all peaks, thus only peak amplitudes were varied. All fits were performed with PeakFit 4.0. The obtained peak areas were normalized against the area of the goethite peaks at 791 and 890 cm-1 (not shown). An example of a fitted spectrum is shown in Figure 6 and the fit results are visualized in the diagram in Figure 7. Additional bands are needed to fit the analyzed frequency range. These are included based on the second derivative spectra, and both peak positions and amplitudes are varied in the fits. They are not used in the analysis of surface speciation, but introduce uncertainties caused by overlapping peaks. Note also that the diagram shows peak areas and not concentrations; i.e., peak absorptivities have not been taken into account. From our experience with the pyromellitate studied herein and with other carboxylate systems, the absorptivities of the analyzed peaks do not show dramatic differences. For example, the peak at 1254 cm-1 originating from C-O-H has approximately 3/4 of the absorptivity of the νC-O (sym.) at 1377 cm-1 as evaluated the solution spectra of L4- and H4L. Thus, if absorptivities were available they would perhaps slightly

Figure 7. Normalized peak area diagram of pyromellitate sorbed on goethite at a total pyromellitate surface concentration of 2.2 µmoles/m2 in 0.1 M NaCl. The symbols refer to the wavenumber of the peaks.

change the relative positions and the slopes of the curves, but the general trends observed in the peak area diagrams still provide good descriptions of the molecular level surface speciation. This means that a surface complexation model should be able to describe these trends, namely the predominance of an outer-sphere complex above pH 5, protonation below pH 5, and increasing amounts of an inner-sphere complex with decreasing pH. Surface Complexation Modeling The objective of the modeling part of this work is to present a surface complexation model that can predict the potentiometric titration and adsorption data invoking surface complexes that are consistent with the interpretations of the IR spectra. Modeling Approach. Goethite Surface Sites. The goethite surface contains oxygens which are singly, doubly, and triply coordinated to underlying Fe(III).18,19 These sites, denoted as tFeO1.5-, tFe2O1.0-, and tFe3O0.5-, can accept protons and take part in complexation reactions


with metal ions and ligands. The MUSIC model considers the contributions on each site of each crystal plane in surface complexation reactions. The dominant crystal planes of goethite needles have been previously identified to be the {110} and the {021} planes.20,21 Our TEM pictures show that the {110} plane represents about 90% of the total surface area of the particles. The terminations of the particles however tend to be rounded, potentially displaying a large range of crystal planes more or less perpendicular to the {001} axis. Therefore as a first-order approximation we model the terminations of our particles with the {001} plane. The {110} plane is made up of double rows of edgesharing Fe(III) octahedra (Figure 8) aligned along the {001} axis. Two types of double rows may be distinguished on the basis of the types of surface functional groups present. Rows with doubly coordinated oxygens, tFe2OH0.0, display an equal quantity of triply coordinated sites, denoted as tFe3OI0.5-. Those with singly coordinated oxygens, tFeOH0.5-, also display an equal quantity of tFe3OI0.5-, but also of a different kind of triply coordinated site, denoted as tFe3OII0.5-. The bond valence of Fe-O bonds in tFe3OI0.5- is significantly lower than those in tFe3OII0.5-.22 On the {001} plane, all sites form edges of Fe(III) octahedra composed of either two singly coordinated oxygens or two doubly coordinated oxygens. Triply coordinated oxygens are absent on this plane. Reactivity of Goethite Surface Sites. In our modeling approach we assign different surface complexes to different crystallographic planes of the goethite particles. This is motivated by the fact that previously, simpler models were unable to explain the quantitative changes of the surface speciation, as observed with infrared spectroscopy, when pH was varied.8 As will be shown below, with the current approach such data can be satisfactorily modeled. It should however be stressed that the actual assignment of the adsorption sites is not based on any spectroscopic information. Instead, it should be regarded as one of perhaps several mechanistic approaches that can be used to model adsorption and potentiometric titration data, resulting in a surface complexation model in agreement with the molecular level observations. The surface sites tFeOH0.5- and tFe3OI0.5- have been proposed as those responsible for the acid/base properties of the goethite surface.22 The site tFe3OI0.5- would have a particularly high affinity for protons, reflected in its predicted acidity constant, log β1, of about -12.22 The speciation of this site would thus be entirely dominated by the tFe3OIH0.5+ species (6.0 sites/nm2 on the {110} plane) in circumneutral to low pH. Conversely, the site tFe3OII0.5- would be of low proton affinity and would display a permanent negative charge (3.0 sites/nm2 on the {110} plane) throughout the same pH range. Provided triply coordinated sites do not participate in surface complexation reactions, besides proton adsorption, the MUSIC model can thus be simplified by canceling 3.0 sites/ nm2 of tFe3OIH0.5+ with 3.0 sites/nm2 of tFe3OII0.5-. The doubly coordinated site, tFe2OH0, with a neutral formal charge, would also not react with protons in the normal working pH range.22 Once more, provided the doubly coordinated site does not participate in surface complex(18) Hiemstra, T.; van Riemsdijk, W. H.; Bolt, G. H. J. Colloid Interface Sci. 1989, 133, 91. (19) Hiemstra, T.; De Wit, J. C. M.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1989, 133, 105. (20) Weidler, P. G.; Schwinn, T.; Guab, H. E. Clays Clay Min. 1996, 44, 437. (21) Schwertmann, U. Thermochim. Acta 1984, 78, 39. (22) Venema, P.; Hiemstra, T.; Weidler, P. G.; van Riemsdijk, W. H. J. Coll. Int. Sci. 1998, 198, 282.

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Figure 8. Schematic representation of the goethite surface at the (a) {110} plane and (b) {001} plane. The different surface sites are identified with the symbols shown in the legend. An example of a binuclear outer-sphere complex on the {110} plane is illustrated in (a), whereby the singly coordinated sites are considered to be the most reactive. Given the size of the molecule, secondary interactions with doubly coordinated sites may be possible, although these are not considered in the surface complexation model (Table 3). An example of a mononuclear inner-sphere complex on the {001} plane is illustrated in (b). The inner-sphere complex is not expected to lie flat on the surface. The uncoordinated carboxyl groups of this complex are therefore assigned to the 1-plane (Figure 9). The dimensions of the pyromellitate molecule were determined with the computer program PC SpartanPro v. 1.0.1. The shaded volume is defined by the 0.002 isodensity surface calculated on the molecule optimized by the Hartree-Fock/3-21G* method. Typically g98% of the electron density is within this surface, therefore it is a good estimate of the size of the molecule. The goethite surface was constructed with ATOMS. The diagrams are close but not entirely to scale. Note also that ortho-positioned carboxyl groups can rotate to vary the distance of separation of two oxygens from 4.1 Å, as shown in the diagram, to smaller distances to allow bonding (e.g., 3.0 Å, the distance between oxygens of two adjacent [tFeOH0.5-]{110} sites, as indicated in (a)).

ation reactions, we are left with a simplified MUSIC model, whereby we consider only 3.0 sites/nm2 of tFeOH0.5- and 3.0 sites/nm2 of tFe3OI0.5- on the {110} plane and 9.1 sites/nm2 of tFeOH0.5- on the {001} plane (Table 1). One last simplification is adopted by resorting to the 1 pK

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Table 1. Modeling Parameters for the Basic Charging Properties of Goethitea [tFeOH 0.5-]{110}b [tFeOH 0.5-]{001}b [tFe3OI 0.5-]{110}b log β1c log βMc log βAc CSternd {110} planee {001} planee

3.0 9.1 3.0 9.3 -0.6 -0.6 1.05 90% 10%

pH+ + q [tFeOH0.5-]{110}/{001} + r L4- a [Hp(tFeOH)qLr(p-0.5q-4r)]{110}/{001} βp,q,r (3)

a Taken from refs 23, 24. b Site density, site/nm2. c Formation constants of eqs 1 and 2. d Capacitance of the Stern layer (F/m2). e Percentage of the surface area represented by the crystal plane.

approximation whereby the acidity constants of the tFeOH0.5- and the tFe3O0.5- are set to the pH of point of zero charge, i.e. 9.3.22-24 The basic charging properties of the goethite surface are therefore described using the singly and the triply coordinated site which can undergo protonation reactions and form ion pairs with the background electrolyte ions:23,24

[tFeOH0.5-]{110}/{001} + H+ a [tFeOH20.5+]{110}/{001} β1 (1a) [tFeOH0.5-]{110}/{001} + Na+ a [tFeOH0.5--Na+]{110}/{001} βM (1b) [tFeOH20.5+]{110}/{001} + NO3- a [tFeOH20.5+-NO3-]{110}/{001} βA (1c) and with

[tFe3OI0.5-]{110} + H+ a [tFe3OIH0.5+]{110}

ation equation is therefore written as:

β1 (2a)

[tFe3OI0.5-]{110} + Na+ a [tFe3OI0.5--Na+]{110} βM (2b) [tFe3OIH0.5+]{110} + NO3- a [tFe3OIH0.5+-NO3-]{110} βA (2c) The subscripts denote the crystal plane of the surface species considered. Note also that the singly coordinated sites of the {110} and {001} planes are treated separately. The values of the formation constants are shown in Table 1. The IR spectra show evidence of the presence of innersphere and outer-sphere complexes. The formation of the inner-sphere complex can be described as a ligand exchange reaction whereby surface hydroxyls or water molecules are replaced by carboxylate oxygen atoms. We argue that this reaction is most likely to take place at sites with singly coordinated surface groups. For simplicity, we model outer-sphere complexes to take place with singly coordinated oxygens as well. The general complex(23) Boily, J.-F. The Surface Complexation of Ions at the Goethite/ Water Interface: A Multisite Complexation Approach. Ph.D. Thesis, Umeå University, Sweden, 1999. (24) Boily, J.-F.; Lu¨tzenkirchen J.; Balmes, O.; Beattie, J.; Sjo¨berg, S. submitted to Coll. Surf.

where p, q, and r are the stoichiometric coefficients, L4is pyromellitate and βp,q,r is the formation constant. Calculations. The equilibrium constants βp,q,r of eq 3 were evaluated using the potentiometric titration and the adsorption data, simultaneously, with a modified version of the computer program FITEQL 2.0.25 This version (J. Lu¨tzenkirchen, personal communication) correctly accounts for dilution factors on solid concentration. The input of experimental adsorption data into FITEQL 2.0 was made using dummy components for the pyromellitate and proton balances. The parameter optimization procedure in FITEQL 2.0 is based on minimizing a weighted sum of squares of the errors in the mass balances of pyromellitate and of protons at each experimental point. Each experimental value is weighed according to the calculated error as a result of the estimated experimental errors. Relative errors of 1% were used for pyromellitate and the absolute error in the total concentration of H+ was set to 10-5. The quality of the obtained fit is estimated by comparing the averaged variance of the models under trial to the experimental data, V(Y), a value calculated by FITEQL 2.0. The formation constants βp,q,r were corrected to a standard state defined as a hypothetical complex with zero surface potential using the three plane model (TPM).13 FITEQL 2.0 was also modified (J. Lu¨tzenkirchen, personal communication) to allow calculations to be carried out using this model. The TPM allows the coexistence of innersphere and outer-sphere complexes, electrolyte ion pairs, and a diffuse layer. Inner-sphere adsorption takes place at the 0-plane, outer-sphere adsorption at the 1-plane, and electrolyte adsorption at the 2-plane of the goethite/ water interface (Figure 9). The net charge of these three planes is counterbalanced by point charges in the diffuse layer, according to Gouy-Chapman theory. We calculate the intrinsic formation constant, βp,q,r (int) with:

βp,q,r(int) ) βp,q,r e∆z0FΨo/RT e∆z1FΨ1/RT e∆z2FΨ2/RT

(4)

where F is Faraday’s constant, R is the molar gas constant, T is the absolute temperature and ∆z and Ψ are the change in charge and the electric potential, respectively, at the plane of adsorption indicated in subscript. For simplicity, the distribution of the charges of adsorbed pyromellitate was constrained as follows: (i) All the charges of outersphere pyromellitate complexes were set in the 1-plane. (ii) Charges involved in forming the inner-sphere complex were set in the 0-plane, while those remaining were set in the 1-plane (Figure 9). Justifications to this specific charge distribution model may also be found in the caption of Figure 8. The values of the capacitance of the inner charge-free layer, C1, and of the outer charge-free layer, C2 were optimized according to:

1 1 1 ) + CStern C1 C2

(5)

The species combination with the C1 and C2 pairs giving the lowest value of V(Y) is considered to be the best model, (25) Westall, J. C. FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data. Version 2.0 Report 82-02, Department of Chemistry, Oregon State University, Corvallis, OR, 1982.


Langmuir, Vol. 16, No. 13, 2000 5727

Figure 9. Schematic representation of the electric double layer on the {110} plane of the goethite/water interface. The different surface sites are illustrated with the symbols defined in the legend. The charges of the inner-sphere complex (upper part of the diagram) are distributed between the 0- and the 1-plane. In the outer-sphere complex (lower part of the diagram) all charges are assigned to the 1-plane. Electrolyte ion pairs are set to the 2-plane. The diagram is not entirely to scale. Note that we only illustrate the inner-sphere complex on the {110} plane for practical purposes; in the models this complex forms on the {001} plane.

as long as it is in agreement with the interpretations of the IR data. The value of CStern is given in Table 1. The formation constants for a reaction consuming two surface sites tFeOH0.5- (q ) 2) results in a goethite concentration-dependent value, i.e. βp,q,r,gt. This constant is corrected to a goethite mass-independent formation constant with:23,26

βp,q,r ) βp,q,r,gt ‚ [tFeOH0.5-]Tot,{110}/{001}

(6)

These corrections are carried out on each experimental data point in FITEQL 2.0 with a dummy component which corresponds to the correction coefficient, [tFeOH0.5-]Tot,{110}/{001}, of eq 6. The equilibrium constants βp,0,1 of the two component system H+-L4- were evaluated using the computer program LAKE.27 The optimization of the equilibrium constants was based on minimizing the sum of squares of the error in the mass balance for H+ in each point, i.e. the difference between the calculated and the experimental values of the total concentration of H+. The quality of the obtained fit was estimated from σ(H), the standard deviation in the total concentration of H+. Subsystem Models: H-L4- and H-tFeOH. Modeling pyromellitate adsorption at the goethite/water interface requires knowledge of the acidity constant of pyromellitate in 0.1 M NaNO3. The protonation of pyromellitate (L4-) was thus studied in 10 titrations in the range 2.2 < pH < 9.9. Data treatment was based on 273 experimental points using LAKE27 and the resulting equilibrium constants are shown in Table 2. This study is also the first from our group where we employ a different acid-base model than the one devised in ref 28. Justifications for using a different model can be found in refs 23, 24, and 29. Briefly, the acid-base (26) Venema, P.; Hiemstra, T.; and van Riemsdijk, W. H. J. Colloid Interface Sci. 1996, 183, 515. (27) Ingi, N.; Andersson, I.; Pettersson, L.; Yagasaki, A.; Andersson, L.; Holmstro¨m, K. Acta Chem. Scand. 1996, 50, 717. (28) Lo¨vgren, L.; Sjo¨berg, S.; Schindler, P. W. Geochim. Cosmochim. Acta 1990, 54, 1301. (29) Lu¨tzenkirchen, J.; Boily, J.-F.; Lo¨vgren, L.; Sjo¨berg, S. Manuscript.

Figure 10. Experimental data from 5 titrations in the H-pyromellitate-goethite system at 4 different surface coverages indicated in the legend (µmoles/m2 total pyromellitate; initial goethite concentration of 10.8 g/L). (a) Proton balance data shown in terms of ZB (eq 7). The thin dashed line shows the proton balance data in the absence of pyromellitate. (b) Adsorption data shown in terms of sorption density in µmoles of the ligand adsorbed per m2. Table 2. Proposed Model for the Acid/Base Properties of 1,2,4,5-benzenetetracarboxylate (298.2 K, I ) 0.1 M NaNO3) H+ + L4- a HL32H+ + L4- a H2L23H+ + L4- a H3L14H+ + L4- a H4L1-

log β1,0,1 ( 3σ ) 5.193 ( 0.009 log β2,0,1 ( 3σ ) 9.294 ( 0.008 log β3,0,1 ( 3σ ) 11.911 ( 0.012 log β4,0,1 ( 3σ ) 13.698 ( 0.015

properties of the goethite surface were reevaluated in ref 24 on new data at various ionic strengths using the basic Stern model. The modeling parameters are shown in Table 1, using the reactions of eq 1 a-c and 2 a-c. Pyromellitate Surface Complexation. Experimental Data. The surface complexation of pyromellitate at the goethite/water interface was studied in 5 titrations in the range 3.0 < pH < 9.0 at four different [L4-]Tot/ [tFeOH0.5-]Tot ratios. Data treatment was based on 39 experimental points. The titration data is plotted in terms of ZB curves (Figure 10a):

ZB ) (H - [H+] + [H+]-1Kw) ([[tFeOH0.5-]{001},Tot] +

/

[[tFeOH

0.5-

]{110},Tot] + [[tFe3O0.5-]{110},Tot]) (7)

where H is the total concentration of acid (mol/L), and Kw is taken from ref 30. The denominator represents the sum of the total concentration (mol/L) of tFeOH0.5- on the {110} and the {001} planes and of tFe3OI0.5- on the {110} plane, as considered in the simplified MUSIC approach (30) Sjo¨berg, S.; Ha¨gglund, Y.; Nordin, A.; Ingri, N. Mar. Chem. 1983, 13, 35.

5728


Boily et al.

Table 3. Proposed Model for Pyromellitate Complexes at the Goethite/Water Interface (298.2K, I ) 0.1 M NaNO3)a 2H+ + 2[tFeOH0.5-]{110} + L4- a [(tFeOH20.5+)2-L4-]{110} 3H+ + [tFeOH0.5-]{110} + L4- a [tFeOH20.5+ -H2L2-]{110} 2H+ + 2[tFeOH0.5-]{001} + L4- a [(tFe2)L1-,2-]{001}a + 2H2O C1 ) 1.60 F m-2 C2 ) 3.05 F m-2

logβ2,2,1 (int) ) 20.88 ( 0.06 logβ3,1,1(int) ) 19.91 ( 0.09 logβ2,2,1(int) ) 13.86 ( 0.20

a Best-fitting species combinations. Inner-sphere pyromellitate complexes with a charge distribution between the 0- and the 1-plane, denoted as 1-,2-, respectively. Note that the inner-sphere complex on the {001} plane is a mononuclear bidendate chelate, hence (tFe2). Illustrations are shown in Figure 8.

Figure 11. Distribution of pyromellitate surface species for a surface coverage of 2.24 µmoles/m2 (10.8 g/l goethite) generated with the model shown in Table 3. The complexes are identified with respect to their p,q,r composition, modes of coordination (i.s. ) inner-sphere; o.s. ) outer-sphere), and the crystal plane.

(Table 1). The dashed line in Figure 10a shows the proton balance data of the goethite suspension in the absence of pyromellitate. The adsorption data are displayed as surface coverage ([L4-]adsorbed/nm2) in Figure 10b. Surface Species. The analysis of IR spectra has constrained the speciation search to three surface complexes. These are also partially constrained with respect to stoichiometric composition. In the following paragraphs this information will be used together with crystallographic data for goethite to propose a set of surface equilibria. As noted previously, pyromellitate surface complexes are assumed to take place with or at sites of singly coordinated oxygens. Inner-sphere complexation is likely to proceed through ligand exchange with singly coordinated hydroxyls and/or water molecules. Based on the similarities with the formation of phthalate inner-sphere complexes we infer that only two carboxylate groups of pyromellitate are coordinated to Fe(III).23 Recent spectroscopic results in combination with molecular modeling and frequency calculations have shown that phthalate forms mononuclear inner-sphere surface complexes. These data show no evidence for bridging phthalate complexes.31 In analogy, a mononuclear chelate is suspected to be a likely complexation mechanism for the pyromellitate inner-sphere complex. The edge sites formed by the singly coordinated sites of the {001} plane are the only sites on the goethite surface planes considered herein where a mononuclear chelate can form. We thus write:

2H+ + 2[tFeOH0.5-]{001} + L4- a [(tFe2)L1-,2-]{001} + 2H2O (8) The notation 1-,2- in superscript refers to the charges of the complex in the 0-plane (z0 ) 1-) and those of the unbound carboxyl groups in the 1-plane (z1 ) 2-). Note that the reaction describes a mononuclear bidendate (31) Persson, P. manuscript.

chelate (on one Fe(III)) on the {001} plane, which we denote as (tFe2) (Figure 8b). The IR spectra indicated the presence of both nonprotonated and partially protonated outer-sphere pyromellitate complexes. The spectrum of the nonprotonated species showed features which suggested partial desolvation of the molecule and direct bonding to protons in the first hydration layer. This in turn suggests some site specificity for the outer-sphere complex. At present there is no information on which and how many sites are involved in this sorption reaction. We assume that the nonprotonated pyromellitate ion binds to two protonated singly coordinated sites and the protonated complex to only one of these, as preliminary modeling attempts were most successful with this scheme. Accordingly, we can write:

2H+ + 2[tFeOH0.5-]{110} + L4- a [(tFeOH20.5+)2 - L4-]{110} (9) pH+ + [tFeOH0.5-]{110} + L4- a [tFeOH20.5+ - Hp-1L-4+p-1]{110} (10) where the charges of the adsorbed pyromellitate are assigned to the 1-plane. For modeling purposes we furthermore assume outer-sphere complexes form only at the {110} plane. Modeling Attempts. Surface speciation modeling was carried out by optimizing formation constants for pyromellitate surface complexes. The constants are optimized using adsorption and proton data, simultaneously. The search for a speciation model is conducted by (i) testing various stoichiometries of inner- and outer-sphere surface complexes, and (ii) varying the values of C1 and C2 according to eq 5. Preliminary modeling attempts revealed problems related to the protonated surface complex (eq 10). Attempts to optimize the constants for the protonated outer-sphere complex of p,q,r composition 2,1,1 failed. The complex of p,q,r composition 3,1,1 provided the best description of the data to explain protonation of pyromellitate surface complexes, although it is likely that the 2,1,1 species is present. At this stage a quantitative treatment of the normalized infrared peak area would be required to constrain the stabilities of the 3,1,1 and of the 2,1,1 species of eq 10, simultaneously. However, as the absorptivities of the complexes are unknown we adopt the simplest speciation scheme. The model with C1 ) 1.6 Fm-2 (Table 3) provides a good fit to experimental data and the speciation diagram (Figure 11) is in very good agreement with the surface speciation as observed with IR spectroscopy. Discussion and Conclusions The diversity of surface complexation models available in the literature that describe the same systems illustrates how models can be poorly constrained by traditional wet


Langmuir, Vol. 16, No. 13, 2000 5729

chemistry data, regardless of their quality and the range of adsorbent/adsorbate ratios studied. This observation is not surprising given the complexity of the surface of metal-(hydr)oxides and the various assumptions required in the different models of the charged water/metal-(hydr)oxide interface. In this study, we applied an elaborate surface complexation model to describe pyromellitate adsorption in order to provide a mechanistic-like description of pyromellitate surface complexation. This was necessary to achieve one of our main objectives: to develop a model in agreement with molecular level observations. The proposed model is however still an oversimplification. The stronger intrinsic affinity of pyromellitate for the goethite surface at high pH, compared to phthalate,23,32 for example, may suggest that pyromellitate adsorbs through additional processes that have not been resolved in this study. Given the size of the molecule, it could be possible for an outer-sphere pyromellitate complex to be stabilized by hydrogen bonding with neighboring tFe2OH0 sites (Figure 8b), although at present the number of participating sites in surface complexes is uncertain. Another uncertainty is how the charges of pyromellitate surface complexes are distributed between planes of adsorption. Charge distribution can be a determining factor in controlling the extent of proton coadsorption33,34 and, at present, it is only estimated with knowledge of the coordination modes of the adsorbates and assumptions on the number of participating sites. Filius et al.12 attempted to do so for a series of organic acids adsorbed onto goethite. Their study involved the use of elaborate charge distribution factors in the CD-MUSIC and TPM frameworks that were justified on the basis of literaturebased spectroscopic information. In this study charge distribution factors for the different pyromellitate complexes were kept at their simplest in order to avoid complications in an already complicated modeling approach. Elaborate charge distribution factors and electric

double layers will be the object of a forthcoming publication from our group. This study adds to the evidence that carboxylates, in the absence of other functional groups, sorb predominantly as hydrogen bonded outer-sphere complexes on metal hydroxides at pH values above approximately 6. This behavior has been observed for mono-, di-, tri, tetra-, and hexacarboxylates,23,35,36 and thus seems to be rather general, at least for molecules that can only form chelates with a ring size greater than or equal to 7. Hence previous surface complexation models neglecting outer-sphere sorption for such molecules most probably provide a poor description of the actual molecular level reactions. Another interesting trend is the stability of nonprotonated polycarboxylate outer-sphere surface complexes. Despite predictions based on the pH of bulk the solution, pyromellitate, and to an even greater extent mellitate (1,2,3,4,5,6-benzenehexacarboxylate), exhibit a resistance protonation at the interface.36 This, perhaps, is not expected for classical outer-sphere complexes, i.e., where the sorbing molecules retain the 1st hydration sphere, again indicating that outer-sphere pyromellitate sorbs to goethite through comparatively strong hydrogen bonds. A consequence of the hydrogen bonding of multifunctional carboxylic acids is the significant surface complexation at high pH. Thus, the common argument that phenol groups in NOM are responsible for adsorption in the pH above 7-8 is not necessarily entirely correct. Based on the results presented in this study we propose that hydrogen bonding with carboxyl groups may be a significant process in NOM adsorption at elevated pH.

(32) Boily, J.-F.; Persson, P.; Sjo¨berg, S. Accepted Geochim. Cosmochim. Acta, in press. (33) Rietra, R. P. J. J.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1999, 208, 511. (34) Rietra, R. P. J. J.; Hiemstra, T.; van Riemsdijk, W. H. Geochim. Cosmochim. Acta 1999, 63, 3009.

LA991407O

Acknowledgment. We would like to thank Mrs. Agneta Nordin for help with the titrations. This work was financially supported by the Swedish Natural Science Research Council (NFR), the National Swedish Environmental Protection Board (SNV), the Waste Research Council (AFR), and a FCAR scholarship to J.-F.B.

(35) Persson, P.; Karlsson, M.; O ¨ hman, L.-O. Geochim. Cosmochim. Acta 1998, 62, 3657. (36) Johnson, B. B.; Persson, P., manuscript.

Benzenecarboxylate Surface Complexation at the Goethite (Î±-FeOOH

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