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Adsorption of Organic Matter at Mineral/Water Interfaces: 5. Effects of Adsorbed Natural Organic Matter Analogues on Mineral Dissolution Stephen B. Johnson,† Tae Hyun Yoon,† 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 99, Menlo Park, California 94025 Received July 27, 2004. In Final Form: November 23, 2004 The effects of the adsorption of pyromellitate, an analogue for natural organic matter, on the dissolution behavior of corundum (R-Al2O3) have been examined over a wide range of pyromellitate concentrations (0-2.5 mM) and pH conditions (2-10). The adsorption modes of pyromellitate on corundum have first been examined using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and are shown to be dominated by a fully deprotonated, outer-sphere pyromellitate species (tAlOH2+‚ ‚ ‚Pyr4-) at pH g 5.0. At lower pH conditions, however, an additional protonated outer-sphere species (tAlOH2+‚ ‚ ‚H2Pyr2-) and an inner-sphere species are also evident. In accordance with the ATR-FTIR findings, modeling of macroscopic pyromellitate adsorption data using an extended constant capacitance treatment was possible using two outer-sphere (tAlOH2+‚ ‚ ‚Pyr4- and tAlOH2+‚ ‚ ‚H2Pyr2-) and one inner-sphere (tAlPyr3-) adsorbed pyromellitate species. The presence of adsorbed pyromellitate strongly inhibited the dissolution of corundum under acidic (pH < 5) conditions, consistent with a mechanism previously proposed by Johnson et al.1 whereby outer-spherically adsorbed Pyr4- species sterically protect dissolution-active surface sites from attack by dissolution-promoting species such as protons. A reduction in the protolytic dissolution rate of corundum results. A reference Suwannee River fulvic acid, which also adsorbs to aluminum (oxyhydr)oxide surfaces in a predominantly outer-sphere manner, was similarly shown to strongly inhibit the dissolution of corundum at pH ) 3.

1. Introduction The adsorption of organic matter on (oxyhydr)oxide mineral substrates is commonly classified in terms of two fundamental mechanisms: inner-sphere adsorption, in which surface hydroxyl groups are substituted for functional groups in the organic ligand, resulting in the formation of direct bonds between surface metal cations and the organic ligands; and outer-sphere adsorption, in which no such ligand-exchange processes occur, and the organic ligands are instead held in the vicinity of the mineral surface by electrostatic and/or hydrogen bonding mechanisms. The mode of adsorption is of significant importance to the dissolution properties of the underlying mineral substrate. For example, organic ligands that bind in a mononuclear, multidentate, inner-sphere manner tend to significantly enhance mineral dissolution rates2-6 by polarizing and destabilizing surface metal-oxygen bonds in positions trans to each ligand-surface cation bond. As a result, simple low molecular weight (LMW) anions such as oxalate, which are prevalent in many natural settings7-10 and can bind in a mononuclear, bidentate, inner-sphere fashion to minerals such as * Author for correspondence. E-mail: gordon@pangea. stanford.edu. Phone: +1 650 723-9168. Fax: +1 650 725-2199. † Stanford University. ‡ Stanford Synchrotron Radiation Laboratory. (1) Johnson, S. B.; Yoon, T. H.; Kocar, B. D.; Brown, G. E., Jr. Langmuir 2004, 20, 4996. (2) Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1847. (3) Zinder, B.; Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1861. (4) Stumm, W.; Furrer, G. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley-Interscience: New York, 1987; p 197. (5) Stumm, W. Adv. Chem. Ser. 1995, No. 244, 1. (6) Stumm, W. Colloids Surf., A 1997, 120, 143. (7) Strobel, B. W. Geoderma 2001, 99, 169. (8) Jones, D. L. Plant Soil 1998, 205, 25.

aluminum (oxyhydr)oxides,11-14 are regarded as important contributors to natural mineral weathering processes.15,16 By contrast, ligands adsorbed in a binuclear, multidentate, inner-sphere manner tend to inhibit mineral dissolution processes17-19 due to the reduced probability of the simultaneous extraction of two metal cations from a mineral (oxyhydr)oxide surface. When compared with the large number of studies that have focused on the impact of inner-sphere anion adsorption on mineral dissolution, the effects of outer spherically adsorbed ligands on mineral dissolution processes have received considerably less attention in the literature. This comparative lack of interest in the interplay between outer spherically adsorbed ligands and mineral dissolution processes has most likely been due to an expectation that, given that no direct bond(s) are formed between surface metal cations and ligand functional group(s), the effects of outer spherically adsorbed ligands on mineral surface reactivity should be small.6 In contrast to such expectations, however, in a recent study undertaken in our laboratory,1 we found that a simple outer spherically (9) Ryan, P. R.; Delhaize, E.; Jones, D. L. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2001, 52, 527. (10) Gadd, G. M. Adv. Microbiol Physiol. 1999, 41, 47. (11) Axe, K.; Persson, P. Geochim. Cosmochim. Acta 2001, 65, 4481. (12) Rosenqvist, J.; Axe, K.; Sjoberg, S.; Persson, P. Colloids Surf., A 2003, 220, 91. (13) Yoon, T. H.; Johnson, S. B.; Musgrave, C. B.; Brown, G. E., Jr. Geochim. Cosmochim. Acta 2004, 68, 4505. (14) Johnson, S. B.; Yoon, T. H.; Slowey, A. J.; Brown, G. E., Jr. Langmuir 2004, 20, 11480. (15) Johnston, C. G.; Vestal, J. R. Microb. Ecol. 1993, 25, 305. (16) Drever, J. I.; Stillings, L. L. Colloids Surf., A 1997, 120, 167. (17) Bondietti, G.; Sinniger, J.; Stumm, W. Colloids Surf., A 1993, 79, 157. (18) Biber, M. V.; Afonso, M. D.; Stumm, W. Geochim. Cosmochim. Acta 1994, 58, 1999. (19) Stumm, W. Aquat. Sci. 1993, 55, 273.

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Figure 1. The relative abundances of the fully deprotonated (Pyr4-), singly protonated (HPyr3-), doubly protonated (H2Pyr2-), triply protonated (H3Pyr-), and completely protonated (H4Pyr) forms of pyromellitate in aqueous solution as a function of pH. Data are for an ionic strength (I) of 0.01 M. Acid dissociation constants were corrected from values reported at I ) 0 M by Smith and Martell (Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1982; Vol. 5.) to those applicable at I ) 0.01 M using the Davies equation.45 The chemical structure of pyromellitic acid (H4Pyr) is also shown.

adsorbed LMW organic anion, maleate, strongly inhibited the dissolution of corundum under acidic (pH < 5) conditions. The results were rationalized in terms of a surface site blocking mechanism, whereby the strongly outer spherically adsorbed maleate anions sterically hindered access to surface sites for other solution-based species. As a result, the adsorbed maleate anions acted to protect dissolution-active surface sites against attack by dissolution-enhancing species such as protons, thereby reducing the protolytic dissolution rate. Should such a mechanism extend to analogous organic ligands in the natural settings (e.g., humic substances such as fulvic acid, which are predominantly adsorbed in outer-sphere modes at environmentally relevant pH conditions20-22), then outer spherically adsorbed organic ligands may act as significant natural inhibitors of mineral dissolution processes in environments such as soils and sediments. The aim of the present study has been to examine the applicability of the finding of our previous study1 to systems containing other outer spherically adsorbed ligands. With a focus on examining organic ligands of relevance to natural environments, this study has investigated the effects of the adsorption of pyromellitate (1,2,4,5-benzenetetracarboxylate, see Figure 1), an analogue for natural (macromolecular) organic matter,23,24 on the dissolution properties of a model aluminum oxide colloid, corundum (R-Al2O3). Pyromellitate was selected for study based on a number of previous spectroscopic studies, which have shown it to adsorb to several aluminum and iron (oxyhydr)oxide surfaces via predominantly outer-sphere adsorption modes.25-27 Corundum was considered appropriate for use due to its well-characterized (20) Filius, J. D.; Meeussen, J. C. L.; Lumsdon, D. G.; Hiemstra, T.; Van Riemsdijk, W. H. Geochim. Cosmochim. Acta 2003, 67, 1463. (21) Yoon, T. H.; Johnson, S. B.; Brown, G. E., Jr. Langmuir 2004, 20, 5655. (22) Yoon, T. H.; Johnson, S. B.; Brown, G. E., Jr. Langmuir, in revision. (23) Evanko, C. R.; Dzombak, D. A. Environ. Sci. Technol. 1998, 32, 2846. (24) Evanko, C. R.; Dzombak, D. A. J. Colloid Interface Sci. 1999, 214, 189. (25) Nordin, J.; Persson, P.; Nordin, A.; Sjoberg, S. Langmuir 1998, 14, 3655. (26) Boily, J. F.; Nilsson, N.; Persson, P.; Sjoberg, S. Langmuir 2000, 16, 5719. (27) Filius, J. D.; Meeussen, J. C. L.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 2001, 244, 31.

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surface chemistry28-32 and aqueous dissolution behavior1,14,33,34 and its widely studied interactions with both LMW organic anions35-39 and macromolecular organic matter.40,41 Its use also allowed direct comparison of the dissolution results obtained here with those previously obtained in the presence of outer spherically adsorbed maleate.1 In this study, the mode(s) and extent of pyromellitate adsorption on corundum, and the effects of pyromellitate adsorption on the dissolution of corundum, have been investigated as a function of solution pH and pyromellitate concentration. The adsorption mode(s) of pyromellitate have first been examined using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in order to determine its speciation at the corundum-water interface. The macroscopic adsorption properties of pyromellitate on corundum have then been quantified and modeled in accordance with the ATR-FTIR findings using the extended constant capacitance model (ECCM) of Nilsson et al.42 The dissolution properties of corundum in the presence of adsorbed pyromellitate have also been examined over the same range of experimental conditions and through a series of further kinetic experiments. Finally, a range of comparable kinetic experiments have also been undertaken in order to examine the effect of a more complex Suwannee River fulvic acid on the dissolution properties of corundum. The combined results demonstrate the potential importance of outer spherically adsorbed organic matter in inhibiting mineral dissolution processes in natural settings. 2. Experimental Section 2.1. Materials. High-purity (>99.99%) AKP-30 corundum was purchased from Sumitomo Chemical Company, Japan. It possessed a BET surface area of 7 m2/g, a mean particle size of 0.30 µm, and a density of 3.97 g/cm3 and was used as received. MicroSelect (g99%) grade pyromellitic acid hydrate and 99.9995% aluminum chloride hexahydrate were obtained from Fluka BioChemika and Alfa Aesar, respectively. A Suwannee River reference fulvic acid (1R101F-1) was obtained from the International Humic Substances Society. It contained carboxyl and aromatic hydroxyl concentrations of 11.21 and 2.89 mol/kg, respectively43 (compared with a carboxyl concentration of 15.7 mol/kg for pyromellitic acid). Analytical grade NaCl and NaOH, ACS grade HCl, and Milli-Q grade water (resistivity ) 18.2 MΩ‚ (28) Sverjensky, D. A.; Sahai, N. Geochim. Cosmochim. Acta 1996, 60, 3773. (29) Sahai, N.; Sverjensky, D. A. Geochim. Cosmochim. Acta 1997, 61, 2801. (30) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Newville, M.; Sutton, S. R.; Rivers, M. L. Science 2000, 288, 1029. (31) Trainor, T. P.; Eng, P. J.; Brown, G. E., Jr.; Robinson, I. K.; De Santis, M. Surf. Sci. 2002, 496, 238. (32) Criscenti, L. J.; Sverjensky, D. A. J. Colloid Interface Sci. 2002, 253, 329. (33) Carroll-Webb, S. A.; Walther, J. V. Geochim. Cosmochim. Acta 1988, 52, 2609. (34) Samson, S. D.; Stillings, L. L.; Eggleston, C. M. Geochim. Cosmochim. Acta 2000, 64, 3471. (35) Boily, J. F.; Fein, J. B. Geochim. Cosmochim. Acta 1996, 60, 2929. (36) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79, 1857. (37) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Eur. Ceram. Soc. 1997, 17, 239. (38) Boily, J. F.; Fein, J. B. Chem. Geol. 1998, 148, 157. (39) Scales, P. J.; Johnson, S. B.; Kapur, P. C. Min. Pro. Ext. Met. Rev. 1999, 20, 27. (40) Fein, J. B.; Boily, J. F.; Guclu, K.; Kaulbach, E. Chem. Geol. 1999, 162, 33. (41) Boily, J. F.; Fein, J. B. Chem. Geol. 2000, 168, 239. (42) Nilsson, N.; Persson, P.; Lovgren, L.; Sjoberg, S. Geochim. Cosmochim. Acta 1996, 60, 4385. (43) IHHS Standard and Reference Collection data sheet; International Humic Substances Society: St Paul, MN, 2002.

Adsorption of Organic Matter cm at 25 °C) were used in all experiments. Solution and suspension pH values were measured using a Denver Instrument model 215 pH meter equipped with a high-performance sleeve junction pH electrode. The electrode was regularly calibrated using standard pH buffer solutions. Stock suspensions containing 66.7 g/L corundum were prepared by ultrasonically dispersing corundum in 0.01 M NaCl at pH ) 5 using a Branson model 450 digital sonifier equipped with a 0.5 in. horn. The ultrasonication time was 1 min and the Sonifier power output was maintained at 50% of the limiting power (400 W) in all cases. The suspensions were then placed on a shaker table and allowed to equilibrate overnight prior to further use. Aqueous corundum-pyromellitate-NaCl and corundumNaCl samples for use in ATR-FTIR, macroscopic adsorption, and dissolution experiments were prepared by diluting fixed volumes of corundum stock suspensions with appropriate quantities and concentrations of pyromellitic acid and/or NaCl solutions in polypropylene centrifuge tubes. The final corundum concentration was 50 g/L, the pyromellitate concentration was 0-2.5 mM, and the NaCl concentration was 0.01 M in all cases. The latter background electrolyte concentration was selected in order to allow comparison with previous adsorption data (and associated surface complexation modeling) and dissolution studies of outer spherically adsorbed organic anions on corundum,1,38 which were similarly undertaken using a relatively low (0.01 M) background electrolyte concentration. A limited number of aqueous corundum-fulvic acid-NaCl suspensions for use in kinetic dissolution experiments were also prepared and contained final concentrations of major fulvic acid functional groups (both carboxyl and aromatic hydroxyl) of 2-6 µmol/m2 and a background NaCl concentration of 0.01 M. In all cases, the pH of the suspension in each tube was then adjusted to the desired value using small volumes of concentrated (0.1-1 M) HCl or NaOH, before being purged with high-purity N2 gas, wrapped in aluminum foil (to prevent photodegradation of pyromellitate and fulvic acid), and placed on a shaker table operated at 160 rpm for the desired reaction time (4-336 h). During this reaction time, the suspension pH values were regularly monitored and, where necessary, were readjusted to the target pH condition using small volumes of 0.1-1 M HCl or NaOH. Interestingly, use of the shaker table was found to generate almost identical adsorption and dissolution results to those obtained when an end-over-end tumbler device was instead used, indicating that adsorption and dissolution were surface-controlled rather than diffusion-controlled processes. After completion of the reaction period, suspension samples were centrifuged at 10 000 rpm for 20 min in a Beckman Avanti 30 benchtop centrifuge, and the supernatants were decanted and passed through sterile 20 nm syringe filters (Whatman Anotop 25 Plus). Suspension pastes and supernatants for ATRFTIR analysis were measured within several hours of completion of the centrifugation step. Filtered supernatants were stored in the dark at 4 °C for a maximum of 3 days prior to UV-vis spectroscopic analysis and for a maximum of 2 weeks prior to inductively coupled plasma spectroscopic analysis. 2.2. ATR-FTIR Spectroscopy Measurements. A Nicolet NEXUS 470 FTIR spectrometer equipped with a deuterated triglycine sulfate (DTGS) detector, a horizontal attenuated total reflectance (HATR) attachment, and a germanium crystal in a trough configuration (PIKE technologies), was used to collect all Fourier transform infrared (FTIR) spectra. Data collection and spectral calculations were performed using OMNIC (version 6.0a, Nicolet Instrument Corp.) software. A total of 500-1500 scans with a spectral resolution of 4 cm-1 were taken and averaged for each sample. Both solution samples and wet suspension pastes were applied directly to the germanium crystal, and the sample-holding region was sealed with a lid to hinder evaporation during ATR-FTIR measurements. For suspension samples, a small volume (several drops) of filtered supernatant was added on top of each paste as a further precaution against evaporation. All ATR-FTIR spectra obtained for both solution samples and wet suspension pastes were dominated by the strong infrared absorbance of water, as is typically observed for aqueous-based ATR-FTIR measurements.44 For each solution sample, this unwanted spectral response of water was removed by subtracting the ATR-FTIR spectrum of a 0.01 M NaCl solution measured at an equivalent

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Figure 2. ATR-FTIR spectra of 30 mM solutions of aqueous pyromellitate in its five different states of protonation. Spectra were measured at pH ) 10.1 (Pyr4-), pH ) 4.9 (HPyr3-), pH ) 3.5 (H2Pyr2-), pH ) 2.1 (H3Pyr-), and pH ) 1.1 (H4Pyr). pH. For each corundum-pyromellitate-NaCl wet paste, the infrared absorbance of water was instead minimized by measuring and subtracting the ATR-FTIR spectrum of a corundumNaCl suspension obtained at the same pH. The latter procedure also allowed significant unwanted ATR-FTIR contributions from corundum to be removed and is described in more detail elsewhere.1,14 2.3. Bulk Adsorption Measurements. UV-vis spectroscopy was utilized to measure the residual concentrations of pyromellitate in the 20 nm filtered suspensions obtained at the completion of the reaction step described in section 2.1. Filtered supernatants were diluted to appropriate concentrations using acidified 0.01 M NaCl and the pH adjusted to approximately 3. The diluted supernatant solutions were then analyzed for pyromellitate using a Hewlett-Packard 8452A diode array spectrophotometer operated at a wavelength, λ, of 213 nm. The latter condition was experimentally found to yield results that were insensitive to small changes in pH. 2.4. Dissolution Measurements. Concentrations of dissolved aluminum in the 20-nm-filtered supernatants described in section 2.1 were measured by inductively coupled plasma (ICP) spectrometry using a TJA IRIS Advantage/1000 Radial ICAP spectrometer equipped with a solid-state charge injection device detector. Dissolution data presented are the average of results obtained at four distinct wavelengths (λ ) 308.2, 309.2, 394.4, and 396.1 nm).

3. Results and Discussion 3.1. ATR-FTIR Measurements of Aqueous and Adsorbed Pyromellitate. An examination of the acidbase solution chemistry of pyromellitate demonstrates that it can exist in five different protonation states, ranging from fully protonated (H4Pyr) to completely deprotonated (Pyr4-), depending upon the solution pH (see Figure 1). The ATR-FTIR spectra of those five protonation states of pyromellitate are shown in Figure 2. The spectra in Figure 2 were measured using relatively concentrated (30 mM) pyromellitate solutions, and as a result, the solution speciation will have varied significantly from that shown at an ionic strength, I, of 0.01 M in Figure 1. Complicating matters, the ionic strength will also have changed markedly as a function of the mean protonation state of the pyromellitate anion. To account for these ionic strength complications and to determine appropriate pH conditions under which to measure the ATR-FTIR spectra of the various protonated states of pyromellitate, solution pro(44) Hind, A. R.; Bhargava, S. K.; McKinnon, A. Adv. Colloid Interface Sci. 2001, 93, 91.

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Figure 3. ATR-FTIR spectra of pyromellitate adsorbed to corundum at pH ) 5.0 ( 0.1 as a function of total pyromellitate concentration (expressed in terms of the effective surface coverage, Γ (in µmol/m2), if fully adsorbed, and in mmol/L in parentheses). All spectra were normalized against the prominent peak at 1570 ( 2 cm-1. The spectrum of aqueous Pyr4- is also shown for the purposes of comparison and is normalized against the absorption peak at 1562 cm-1.

tonation constants were iteratively recalculated using the Davies equation,45 based on the actual ionic strength predicted for 30 mM pyromellitate solutions at each pH condition. From these calculations, optimal pH conditions for measurement were found to be pH ) 10.1 (Pyr4-), pH ) 4.9 (HPyr3-), pH ) 3.5 (H2Pyr2-), pH ) 2.1 (H3Pyr-), and pH ) 1.1 (H4Pyr). The aqueous pyromellitate spectra shown in Figure 2 are in good agreement with those previously reported by Nordin et al.25 The fully deprotonated Pyr4- species possesses D2h symmetry, and its ATR-FTIR spectrum consists predominantly of various C-O vibrational modes of the deprotonated carboxyl groups (CO2-) that have previously been identified by quantum chemical means by Nordin et al.25 These absorption modes include a strong asymmetric νC-O vibration (1562 cm-1) and three prominent symmetric νC-O vibrations (1412, 1374, and 1325 cm-1). The latter are coupled with vibrations of the aromatic ring. The peak at 1488 cm-1 is, by contrast, due primarily to an in-plane vibration of the aromatic group coupled with a small symmetric νC-O contribution.25 The minor peak at 1136 cm-1 can be attributed to an aromatic C-H bending mode.46 Markedly different ATR-FTIR spectra are obtained at higher degrees of pyromellitate protonation. In particular, the intensities of the various νC-O absorption bands decrease as the pyromellitate carboxyl groups become protonated. A number of new features also become apparent as the degree of protonation increases, including a prominent vibration at 1713 cm-1 (νCdO) and a series of strong absorption bands in the region from 1200 to 1350 cm-1 (νC-OH stretching and bending vibrations).25 Figure 3 shows ATR-FTIR spectra of pyromellitate adsorbed on corundum at pH ) 5.0 after an equilibration time of 72 h as a function of pyromellitate concentration. The background electrolyte concentration was 0.01 M NaCl (45) Perrin, D. D.; Dempsey, B.; Serjeant, E. P. pKa prediction for organic acids and bases; Chapman and Hall: London, 1981. (46) Yoon, T. H.; Brown, G. E., Jr. Abstracts of Papers of the American Chemical Society; American Chemical Society: Washington, DC, 2001, 222 (pt.1), U444.

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in all cases. The spectrum of aqueous Pyr4- is also given for the purposes of comparison. The spectra shown at the lowest pyromellitate concentrations (0.125 and 0.250 mM) are of the lowest quality due to the low surface coverage (Γ ) 0.36-0.71 µmol/m2) of adsorbed pyromellitate. In addition, the spectrum obtained for 0.250 mM pyromellitate contains a substantial noise-to-signal ratio due to the highly flocculated nature of the aqueous corundum suspensions obtained under that experimental condition. Such flocculated suspensions are difficult to centrifuge to high solids concentrations due to the large compressive yield stress that exists when compared with the same suspensions under dispersed conditions.47 As a result, the centrifuged paste obtained at a pyromellitate concentration of 0.250 mM contained a relatively low particle concentration, and therefore a reduced ATR-FTIR signal from the corundum surface, compared with the other pyromellitate concentrations investigated. In all cases, Figure 3 shows that the ATR-FTIR spectra obtained for pyromellitate adsorbed on corundum possess close similarities with that of Pyr4- in solution. A number of subtle differences are, however, apparent between the spectra of the adsorbed and solution-based species. For example, the strong asymmetric νC-O peak present at 1562 cm-1 in solution is shifted to higher wavenumber (by 6-10 cm-1), significantly broadened (with the full width at halfmaximum increasing by 8-29 cm-1), and gradually altered in shape from Lorenzian to Gaussian in the adsorbed versus the solution pyromellitate spectra. Similar findings can also be observed for other major ATR-FTIR peaks through a comparison of the adsorbed pyromellitate versus the solution Pyr4- spectra (see Figure 3). Comparable results have previously been reported in a number of ATRFTIR studies of simple carboxylate-containing LMW anions,1,12,48-50 including pyromellitate,25,26 adsorbed on mineral (oxyhydr)oxide surfaces, and have been interpreted as evidence for outer-sphere mode/s of anion adsorption. Applying the findings of those previous studies, the wavenumber increase of peaks in the ATR-FTIR spectra of the adsorbed versus the solution-based pyromellitate can be attributed to a solvation asymmetry that accompanies replacement of a “bulk” water molecule with a surface-bound hydroxyl group or water molecule in the anionic solvation shell.11 Similarly, the significant peak broadening observed in the ATR-FTIR spectra of the adsorbed Pyr4- is a reflection of the wide range of hydrogen bond strengths that can exist between the outer spherically bound pyromellitate carboxyl groups and different surfacebound hydroxyl groups and water molecules.12 Such results are therefore consistent with pyromellitate adsorbing to corundum predominantly as a fully deprotonated, outersphere complex at pH ) 5.0. Similar findings have previously been reported for pyromellitate adsorbed at boehmite-water25 and goethite-water26 interfaces. Interestingly, the data of Figure 3 show that the extent of peak broadening is greatest at the highest pyromellitate concentrations examined. For example, the full width at half-maximum of the asymmetric νC-O peak (present at 1570 ( 2 cm-1 in the adsorbed pyromellitate spectra) gradually increases from 59 cm-1 at 0.125 mM pyromellitate to 62 cm-1 at 0.250 mM pyromellitate, 69 cm-1 at 0.500 mM pyromellitate, and finally to 80 cm-1 at both (47) Green, M. D.; Boger, D. V. Ind. Eng. Chem. Res. 1997, 36, 4984. (48) Nordin, J.; Persson, P.; Laiti, E.; Sjoberg, S. Langmuir 1997, 13, 4085. Johnson, B. B.; Sjoberg, S.; Persson, P. Langmuir 2004, 20, 823. (49) Persson, P.; Nordin, J.; Rosenqvist, J.; Lovgren, L.; Ohman, L. O.; Sjoberg, S. J. Colloid Interface Sci. 1998, 206, 252. (50) Boily, J. F.; Persson, P.; Sjoberg, S. Geochim. Cosmochim. Acta 2000, 64, 3453.

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Figure 4. ATR-FTIR spectra of solution-based HPyr3- in its uncomplexed and Al(III) complexed (AlHPyr) forms. The spectrum of HPyr3- is from Figure 2, while the spectrum of AlHPyr was measured at pH ) 3.4 using a solution containing 10 mM pyromellitate and 20 mM AlCl3. For ease of comparison, both spectra have been normalized against their maximum absorbances.

1.25 and 2.50 mM pyromellitate. Such findings are consistent with previous results obtained for the outersphere adsorption of maleate on corundum1 and indicate that an increase in the range of hydrogen bonding strengths at the surface (presumably driven by an increase in the heterogeneity of the total occupied surface binding sites) occurs as a function of rising pyromellitate concentration. Figure 3 also shows the gradual emergence of minor spectral features not observed in the solution-based Pyr4- spectrum, most notably a peak positioned at 1703 ( 3 cm-1. This feature may be indicative of other (very minor) adsorbed pyromellitate complexes or, alternatively, may be due to small contributions from nonadsorbed pyromellitate species such as HPyr3- (which forms the dominant pyromellitate species in solution at pH ) 5.0, see Figure 1). As the spectra shown in Figure 3 were obtained after subtraction of the filtered supernatant for each corundum-pyromellitate suspension, the latter explanation requires that solution-based pyromellitate species accumulated to a greater degree in the diffuse electrical double layer surrounding the corundum particles than in bulk solution. Such behavior appears possible given that the corundum particles possess an underlying positive electrical charge at pH ) 5.0,51,52 and outer spherically adsorbed LMW anions possess a very limited ability to reverse the interfacial charge under acidic pH conditions.53 The spectral changes induced by outer-sphere adsorption of pyromellitate can be contrasted with those generated by formation of inner-sphere pyromellitate complexes by measuring ATR-FTIR spectra for Al(III)-pyromellitate species in solution. Unfortunately, the solubility of aluminum in solution proved to be too low to allow measurement of the unprotonated AlPyr- or Al(OH)Pyr2species that dominate the speciation of Al(III)-pyromellitate solutions at pH values in excess of approximately 4.25 It was, however, possible to obtain a reasonable spectrum for the protonated AlHPyr species, which is prevalent at lower pH where the solubility of Al(III) is considerably higher. That spectrum, along with the ATRFTIR spectrum of the uncomplexed HPyr3- anion (shown for the purposes of comparison), is presented in Figure 4. (51) Johnson, S. B.; Russell, A. S.; Scales, P. J. Colloids Surf., A 1998, 141, 119. (52) Johnson, S. B.; Scales, P. J.; Healy, T. W. Langmuir 1999, 15, 2836. (53) Johnson, S. B.; Brown, G. E., Jr.; Healy, T. W.; Scales, P. J. Submitted for publication in Langmuir.

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It should be noted that the ATR-FTIR spectrum of AlHPyr was obtained at a substantially higher NaCl concentration (0.25 M) than previously utilized in order to provide a relatively constant ionic strength, and thereby allow the complex solution chemistry of the Al(III)-pyromellitate system to be accurately controlled. To determine solution conditions under which AlHPyr was prevalent, the pyromellitate and Al(III) speciation constants determined by Nordin et al.25 for mixed Al(III)-pyromellitate systems were corrected to an ionic strength of 0.25 M using specific interaction theory.54 On the basis of the resulting solution speciation calculations, ATR-FTIR measurements were performed on an aqueous solution containing 10 mM pyromellitate and 20 mM AlCl3 at pH ) 3.4, a condition under which AlHPyr is predicted to be the major pyromellitate species in solution. As is shown in Figure 4, inner-sphere complexation of HPyr3- by Al(III) generates a number of significant changes in the measured ATR-FTIR spectrum compared with that of uncomplexed HPyr3-. For example, the prominent peak positioned at 1568 cm-1 in the ATR-FTIR spectrum of HPyr3- is shifted to higher wavenumber by 19 cm-1 and broadened significantly in the spectrum of AlHPyr. A second derivative analysis of the spectrum of AlHPyr reveals that these spectral changes are due to the presence of a new peak at 1594 cm-1. Similarly, the peaks positioned at 1490, 1411, and 1378 cm-1 for HPyr3- are shifted to higher wavenumber by up to 14 cm-1 in the spectrum of AlHPyr, while the intensities of several peaks (most notably that positioned at 1418 ( 7 cm-1) are significantly enhanced in the spectrum of AlHPyr compared with that of HPyr3-. The latter behavior is at least in part due to the presence of a new peak (evidenced by a shoulder which a second derivative analysis reveals is centered at 1454 cm-1) in the spectrum of AlHPyr, which overlaps with other prominent absorption peaks in its vicinity. Finally, an examination of Figure 4 in the frequency region from 1100 to 1300 cm-1 shows a reduction in the number of absorption peaks present in the spectrum of AlHPyr versus that of HPyr3-. In total, the spectral changes induced by such innersphere complexation of HPyr3- by Al(III) are far more marked than are typically observed for outer-sphere complexation of carboxyl-containing LMW organic anions.1,12,25,26,48-50 They are, however, consistent with the changes in the number of spectral peaks and/or relatively large alterations in peak positions that are commonly observed when other carboxyl-containing LMW organic anions bind in an inner-sphere fashion to (oxyhydr)oxide mineral surfaces11-13,55-58 or metal cations11,59 in solution. Such changes can arise due to significant alterations of molecular symmetry and/or bond strengths and angles that typically accompany inner-sphere adsorption processes. Compared with the relatively dramatic spectral changes observed for the inner-sphere complexation of HPyr3- by Al(III) in Figure 4, the minor changes in the spectrum of adsorbed pyromellitate compared with that of Pyr4- in solution (Figure 3) are taken as additional evidence that pyromellitate complexes corundum via a predominantly outer-sphere adsorption mode at pH ) 5.0. (54) Pettit, L. D. Ionic strength corrections using specific interaction theory, 1.2 ed.; IUPAC: London, 2002. (55) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (56) Degenhardt, J.; McQuillan, A. J. Chem. Phys. Lett. 1999, 311, 179. (57) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta, Part A 1999, 55, 1395. (58) Duckworth, O. W.; Martin, S. T. Geochim. Cosmochim. Acta 2001, 65, 4289. (59) Persson, P.; Karlsson, M.; Ohman, L. O. Geochim. Cosmochim. Acta 1998, 62, 3657.

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Figure 5. ATR-FTIR spectra of pyromellitate adsorbed to corundum as a function of pH. The pyromellitate concentration was 1.25 mM (Γ ) 3.6 µmol/m2) in all cases. For the purposes of comparison, all spectra were normalized against the prominent peak at 1570 ( 3 cm-1.

Figure 6. Macroscopic adsorption properties of pyromellitate on corundum as a function of pyromellitate concentration and pH. The corundum concentration was 50 g/L, the background electrolyte was 0.01 M NaCl, and the equilibration time was 72 h in all cases. The total pyromellitate concentrations were 0.125 mM (b), 0.250 mM (O), 0.500 mM (2), 1.25 mM (4), and 2.50 mM ([). Plotted lines represent optimal extended constant capacitance model42 fits to experimental adsorption data using the model parameters listed in Table 1.

ATR-FTIR spectra of pyromellitate adsorbed on corundum are shown as a function of pH in Figure 5. The pyromellitate concentration was 1.25 mM, the background electrolyte concentration was 0.01 M NaCl, and the equilibration time was 72 h in all cases. Spectra obtained at pH ) 5.0 and pH ) 6.6 bear strong similarities with the spectrum of aqueous Pyr4- (see Figure 2) and again indicate that pyromellitate adsorbs to corundum predominantly as a fully deprotonated, outer-sphere species under those conditions. At lower pH conditions, however, significantly different ATR-FTIR spectra are obtained. For example, at pH ) 3.5, a significant additional feature is observed at 1710 cm-1, several minor spectral features begin to emerge in the spectral region from 1210 to 1300 cm-1, and a new peak is also evident at 1126 cm-1. Comparison with the aqueous spectra shown in Figure 2 indicates that these features are similar to those possessed by both HPyr3- and H2Pyr2- and are therefore consistent with the formation of an additional protonated outersphere pyromellitate species. A further comparison of the relative peak intensities in the aqueous HPyr3- and H2Pyr2- spectra with those of the adsorbed pyromellitate spectrum in Figure 5 suggests that pyromellitate either is outer spherically adsorbed predominantly as HPyr3- or is present mostly as outer-sphere Pyr4- with a small contribution from outer-sphere H2Pyr2-. Analysis of the peak positions in the adsorbed pyromellitate spectrum obtained at pH ) 3.5 sheds further light on the likely surface speciation, as the position of the prominent peak at 1572 cm-1 is identical to that observed in the adsorbed pyromellitate spectrum obtained at pH ) 5.0 (where outersphere Pyr4- is the dominant surface species). By contrast, the position of this asymmetric νC-O vibration is shifted to higher wavenumber by 7 cm-1 in the spectrum of aqueous HPyr3- compared with that of Pyr4- and, by analogy, would be expected to lead to a similar change in peak position if HPyr3- were the dominant surface species at pH ) 3.5. As a result, we conclude that outer-sphere Pyr4- remains the dominant surface species at pH ) 3.5, with a minor contribution from outer-sphere H2Pyr2-. At the lowest pH condition examined (pH ) 2.0), Figure 5 shows that the new spectral features noted at pH ) 3.5 become far more significant. These features are consistent with an increased contribution from outer-sphere H2Pyr2with one notable exception: the greatly increased intensity of the peak in the vicinity of 1420 cm-1. Interestingly, an

increase in the intensity of this peak compared with that expected of the predicted outer-sphere surface species was also observed at pH ) 3.5, and even (though to a much lesser extent) at pH ) 5.0. Its presence could conceivably be explained by the presence of an additional protonated outer-sphere species, such as H3Pyr-, for which (in solution) a substantial increase in the magnitude of the major peak in this spectral region is noted (see Figure 2). The position of that peak in the aqueous H3Pyr- spectrum (1403 cm-1) is, however, substantially lower than that in the spectrum of adsorbed pyromellitate at pH ) 2.0. In addition, positively charged mineral (oxyhydr)oxide surfaces tend to stabilize LMW organic anions, including pyromellitate,26 against protonation, leading to the outer spherically adsorbed anions possessing significantly lower degrees of protonation than those in bulk solution at the same pH.1,21 In combination, these factors suggest that an additional protonated outer-sphere species such as H3Pyr- is unlikely to be responsible for the increased peak intensity observed in the vicinity of 1420 cm-1 in the adsorbed pyromellitate spectra obtained at pH ) 2.0 and 3.5. Instead, we assign this spectral feature to the formation of an inner-sphere pyromellitate species at low pH. Similar inner-sphere pyromellitate species have been proposed under low pH conditions in previous ATR-FTIR studies of pyromellitate adsorbed on boehmite25 and goethite26 surfaces. In summary, the ATR-FTIR spectra presented in this section indicate that pyromellitate adsorbs to corundum predominantly as an outer-sphere, fully deprotonated species at pH ) 5.0 over the range of surface coverages examined. Its surface complexation is similarly dominated by this outer-sphere, fully deprotonated species at pH ) 6.5. At pH ) 3.5, however, minor contributions from outersphere H2Pyr2- and an inner-sphere species are also apparent, with these complexes becoming more significant at the lowest pH condition examined (pH ) 2.0). The nature of these complexes is further examined in the following section. 3.2. Macroscopic Adsorption of Pyromellitate on Corundum. The macroscopic adsorption properties of pyromellitate on corundum are shown as a function of pH and pyromellitate concentration in Figure 6. The background electrolyte concentration was 0.01 M NaCl, and the equilibration time was 72 h in all cases. At the lowest pyromellitate concentrations investigated (0.125 and 0.250


Langmuir, Vol. 21, No. 7, 2005 2817

Table 1. Parameters Used in Surface Complexation Modeling of Macroscopic Pyromellitate Adsorption Data (See Figure 6) Using the Extended Constant Capacitance Model (ECCM) of Nilsson et al.42 parameter site density (Ns) CT H+ + OH- T H2O Pyr4- + H+ T HPyr3Pyr4- + 2H+ T H2Pyr2Pyr4- + 3H+ T H3PyrPyr4- + 4H+ T H4Pyr tAlOH0 + H+ T tAlOH2+ tAlOH0 T tAlO- + H+

valuea

source

5.0 sites/nm2 0.92 F m-2

from Johnson et al.1 from Boily and Fein38,41

Solution Reactions log K ) 13.9 log K ) 5.9 log K ) 10.5 log K ) 13.5 log K ) 15.1

from Lideb and Smith and Martell,c corrected to I ) 0.01 M using the Davies equation45

Surface Protonation Reactions log K ) 6.55 from Johnson et al.,1 based on Sverjensky log K ) -12.25 and Sahai28,29

Surface Complexation Reactions tAlOH + H+ + Pyr4- T tAlOH2+‚‚‚Pyr4log K ) 13.8 model fits + + 42tAlOH + 3H + Pyr T tAlOH2 ‚‚‚H2Pyr log K ) 18.8 tAlOH + H+ + Pyr4- T tAlPyr3log K ) 2.3 V(Y) ()WSOS/DF)

Model Fit Statistics 28.9

output from FITEQL for optimal ECCM fits to pyromellitate adsorption data

a K is the formation constant for each surface or solution species. b Lide, D. R. CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994. c Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1982; Vol. 5.

mM), Figure 6 shows that pyromellitate is completely adsorbed to the corundum surface under acidic pH conditions but is adsorbed to a progressively lesser extent as the pH becomes increasingly basic. Similarly, pyromellitate is fully adsorbed below pH ) 4 at an intermediate concentration of 0.500 mM but again adsorbs to a lower extent as the pH is progressively increased to higher values. At the highest pyromellitate concentrations examined (1.25 mM and 2.50 mM), the extent of pyromellitate adsorption is at a maximum at low pH, and then systematically decreases as a function of rising pH. Interestingly, for all pyromellitate concentrations examined, the extent of pyromellitate adsorption on corundum approaches zero at pH conditions above the isoelectric point (iep) of corundum (iep of AKP-30 corundum ) 9.49.8 in the presence of 0.01 M indifferent background electrolytes51,52,60). The latter finding demonstrates the importance of the electrostatic component to adsorption and is consistent with the predominant formation of outersphere pyromellitate surface complexes under nearneutral to basic pH conditions (see section 3.1). Similar macroscopic adsorption results have previously been obtained for the interaction of pyromellitate with aluminum25,38 and iron (oxyhydr)oxide23,24,26,27 materials. To link the macroscopic adsorption data of Figure 6 with the ATR-FTIR results previously presented in section 3.1, surface complexation modeling has been undertaken based on the extended constant capacitance model (ECCM) of Nilsson et al.42 The latter model was selected for two reasons: first, to maintain consistency with (and allow comparison with) surface complexation modeling treatments previously undertaken for other outer spherically adsorbed LMW organic anions on corundum;1,41 and second, to minimize the complexity of the modeling process. The ECCM is equivalent to the triple layer model (TLM) in the absence of a diffuse electrical double layer and, as such, allows for the formation of both inner-sphere complexes (located in the surface, or 0, plane) and outersphere complexes (positioned at the inner Helmholtz, or β, plane) at mineral-water interfaces. Compared with the TLM, the lack of a diffuse electrical double layer means (60) Johnson, S. B. The Relationship Between the Surface Chemistry and the Shear Yield Stress of Mineral Suspensions. Ph.D. Thesis, The University of Melbourne, Melbourne, Australia, 1998.

that the modeling results are insensitive to the concentration of indifferent electrolyte and are therefore specific to the experimental ionic strength utilized. The ECCM treats the charging behavior of the corundum surface in terms of two fundamental surface protonation reactions, namely

tAlOH0 + H+ T ≡AlOH2+, log K+ ) 6.55

(1)

tAlOH0 T tAlO- + H+, log K- ) -12.25 (2) where the protonation constants K+ and K- are from Johnson et al.1 and are based on the detailed Born solvation, electrostatic, and Pauling bond strength considerations of Sverjensky and Sahai.28,29 A discussion concerning the use of the simplified tAlOH0-based surface species shown in eqs 1 and 2 is given elsewhere.1 ECCM simulations were undertaken using a surface site density, Ns, of 5 sites/nm2. This value falls in the midrange of experimentally determined values38,61 and at the lower end of crystallographic estimates62 for corundum and has recently been found to be appropriate for modeling the outer-sphere adsorption of maleate on corundum.1 A total capacitance, CT, of 0.92 F m-2 was also utilized, as has been determined by Boily and Fein38,41 to be appropriate for corundum particles dispersed in aqueous 0.01 M NaNO3. These and other parameters used in ECCM simulations are listed in Table 1. Surface complexation modeling was performed with the optimization programs GRFIT63 and FITEQL 4.0.64 Relative and absolute errors in the adsorbed pyromellitate concentrations were defined as 2% and 1 × 10-6 M, respectively. Errors in pH were left at the default FITEQL (61) Hayes, K. F.; Redden, G.; Ela, W.; Leckie, J. O. J. Colloid Interface Sci. 1991, 142, 448. (62) Koretsky, C. M.; Sverjensky, D. A.; Sahai, N. Am. J. Sci. 1998, 298, 349. (63) Ludwig, C. GRFIT - A computer program for solving speciation problems: Evaluation of equilibrium constants, concentrations and other physical parameters; University of Berne: Berne, Switzerland, 1996. (64) Herbelin, A.; Westall, J. FITEQL: A computer program for determination of chemical equilibrium constants from experimental data; Version 4.0; Report 99-01; Department of Chemistry, Oregon State University: Corvallis, OR, 1999.

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Figure 7. Predicted surface speciation of pyromellitate adsorbed on corundum as a function of pH. The pyromellitate concentration was 2.5 mM.

values.64 Optimal fits to the experimental adsorption data were evaluated using the relationship

V(Y) )

WSOS ) DF

(

∑

)

Ycalc - Yexp sexp npnII - nu

(3)

where WSOS is the weighted sum of squares, DF is the degrees of freedom, Ycalc and Yexp are the calculated and experimental data, sexp is the default FITEQL error associated with the experimental data, np is the number of data points, nII is the number of components for which both total and free concentrations are known, and nu is the number of adjustable parameters. Optimal model fits to the pyromellitate adsorption data are shown in Figure 6. The use of three different pyromellitate surface species was found to give the best model agreement with the experimental adsorption results. Consistent with the ATR-FTIR results shown in section 3.1, these complexes consisted of two outer-sphere species,tAlOH2+‚‚‚Pyr4- (logK)13.8)andtAlOH2+‚‚‚H2Pyr2(log K ) 18.8), and one inner-sphere species, tAlPyr3(log K ) 2.3). An example of the resulting surface speciation of adsorbed pyromellitate is given in Figure 7. Significantly, the three surface species found to best fit the experimental adsorption data in this study are largely analogous to those recently invoked by Boily et al.26,50 to best describe the adsorption of pyromellitate on goethite. The ECCM fits shown in Figure 6 yielded an average value of V(Y) ) 28.9, with this value being in reasonable accordance with the range of 0.1-20 suggested by Herbelin and Westall64 as indicating a good fit to experimental data. A considerably lower average value of V(Y) ) 6.4 was obtained when the relatively poor model fits to experimental data at pH ) 10.0 ( 0.1 were disregarded. Interestingly, the formation constant for the outer-sphere tAlOH2+‚‚‚Pyr4- species (log K ) 13.8) is significantly greater than those previously determined for outer-sphere maleate (tAlOH2+‚‚‚Mal2-, log K ) 11.16) and phthalate (tAlOH2+‚‚‚Pht2-, log K ) 11.2) complexes from ECCM fits of macroscopic adsorption data on corundum. The latter finding is consistent with the substantially greater electrostatic attraction that will exist between the Pyr4anion and surface tAlOH2+ groups compared with that present between divalent anions (such as Mal2- and Pht2-) and the same surface tAlOH2+ groups. Boily et al.50 have similarly reported substantially higher surface binding constants for pyromellitate than for lower-charged aromatic carboxylates on goethite. The optimal ECCM fit to experimental data shown in Figure 6 was calculated based on a total ionic strength of

Figure 8. Concentration of Al(III) dissolved from corundum after a reaction time of 72 h as a function of pH and pyromellitate concentration. The corundum concentration was 50 g/L and the background electrolyte was 0.01 M NaCl in all cases. The total pyromellitate concentrations were 0 mM (b), 0.125 mM (O), 0.250 mM (2), 0.500 mM (4), 1.25 mM ([), and 2.50 mM (]).

10 mM NaCl. In addition to the background electrolyte, however, nonadsorbed pyromellitate can also contribute significantly to the ionic strength of the suspensions. Considering the most extreme case of the highest pyromellitate concentration (2.5 mM) examined at high pH (where the extent of pyromellitate adsorption is very low (Figure 6) and solution pyromellitate is predominantly in the form of Pyr4- (Figure 1)), a total solution ionic strength of 35 mM could conceivably result (including the 10 mM NaCl background and NaOH used for pH adjustments). On the basis of this upper extreme, ECCM simulations were repeated on the adsorption data of Figure 6 using solution protonation constants adjusted to an ionic strength of 35 mM. Other surface-based protonation constants, site densities, and capacitances were maintained at the values listed in Table 1. The resulting ECCM simulations yielded a similar goodness of fit (V(Y) ) 27.1) to that obtained when using an ionic strength of 10 mM (V(Y) ) 28.9). Furthermore, compared with the surface complexation results obtained using an ionic strength of 10 mM, only a slightly altered distribution of pyromellitate surface species was observed when ECCM simulations were instead undertaken at an ionic strength of 35 mM (i.e., a slightly decreased concentration of tAlPyr3- (log K ) 1.66) relative to the concentrations of tAlOH2+‚‚‚Pyr4(log K ) 13.7) and tAlOH2+‚‚‚H2Pyr2- (log K ) 18.5) at low pH). Such results indicate that ionic strength contributions from nonadsorbed pyromellitate have only a minor effect on the surface complexation modeling results obtained in this study. 3.3. Effects of Adsorbed Pyromellitate and Fulvic Acid on the Dissolution of Corundum. The dissolution behavior of corundum after a reaction time of 72 h is shown as a function of both pyromellitate concentration and pH in Figure 8. The background electrolyte concentration was 0.01 M NaCl in all cases. In the absence of pyromellitate, Figure 8 shows that very low concentrations of Al(III) are dissolved from corundum in the pH range from 5 to 9, consistent with the low solubility of corundum65 and/or the low rate of corundum dissolution33 that is typically observed in this pH domain. The concentrations of dissolved Al(III) become systematically greater at progressively more basic (pH > 9) and acidic (pH < 5) conditions, consistent with an increased attack on the corundum surface by dissolution-enhancing OH- and H+ (65) Bethke, C. M. The Geochemist’s Workbench, Release 4.0; University of Illinois: Urbana-Champaign, IL, 2002.


species. Such dissolution behavior is typical of aluminum (oxyhydr)oxide minerals.65-67 The dissolution behavior at pH < 4 is, by contrast, unique to corundum, with Figure 8 showing that in the absence of pyromellitate, the concentration of dissolved Al(III) in solution reaches a maximum at pH ) 3.5 and then slightly decreases at lower pH conditions. The latter behavior is believed to be driven by the very slow regeneration rate of dissolution-active sites on the corundum surface at low pH and is discussed in greater detail elsewhere.1,14,34 Figure 8 also shows that as the concentration of pyromellitate is increased, the concentration of dissolved Al(III) progressively decreases at pH < 5. By contrast, at pH > 9 (where the extent of pyromellitate adsorption is very low, see section 3.2), the presence of pyromellitate does not have a significant effect on the extent of corundum dissolution. These findings indicate that the presence of pyromellitate is inhibiting the dissolution of corundum, but only when it is strongly associated with the corundum surface. Importantly, the dissolution-inhibiting effects of pyromellitate cannot be attributed to solution saturation and/or precipitation phenomena. For example, at pH < 4.5, the concentrations of dissolved Al(III) shown in Figure 8 are significantly below the saturation concentrations of Al(III) in solution,65 which are predicted to increase as a function of rising pyromellitate concentration.25 Furthermore, in aqueous solutions containing the highest pyromellitate concentration examined (2.50 mM), we were able to dissolve g100 mM AlCl3 at pH ) 3.0, g10 mM AlCl3 at pH ) 3.3, and g2 mM AlCl3 at pH ) 4.0. These dissolved Al(III) concentrations are several orders of magnitudes higher than those shown at equivalent pH conditions in Figure 8 and indicate that the dissolved pyromellitate and Al(III) concentrations in Figure 8 are far below those required to form a bulk Al(III) pyromellitate precipitate. As a result, saturation phenomena are not expected to have significantly affected the results presented in Figure 8. The results of Figure 8 are similar to those previously obtained for the interaction of maleate with corundum by Johnson et al.1 In that case, maleate was also found to inhibit the dissolution of corundum under acidic pH conditions, an effect that was attributed to the outer spherically adsorbed maleate anion (Mal2-) blocking access to corundum surface sites by dissolution-promoting species such as protons. As a result, the protolytic dissolution rate of corundum was significantly reduced. The results shown in Figure 8 are consistent with the findings of Johnson et al.1 and similarly suggest that outer spherically adsorbed pyromellitate anions are hindering protolytic attack on the corundum surface at pH < 5. The dissolution-inhibiting properties of outer spherically adsorbed pyromellitate are more directly compared with those of outer spherically bound maleate in Figure 9. The pyromellitate and maleate concentrations were 2.50 mM, and the reaction time was 72 h. Data were collected using different background electrolytes (0.01 M NaCl for pyromellitate and 0.01 M KNO3 for maleate), which were found to lead to slightly different extents of dissolution under quite acidic (pH < 4) conditions. As a result, both “blank” dissolution runs (i.e., in the absence of pyromellitate and maleate) are shown in Figure 9. The data of Figure 9 clearly show that except at the most acidic conditions examined, pyromellitate inhibits the dissolution of corundum to a substantially greater extent than does (66) Baes, C. F.; Mesmer, R. E. The hydrolysis of cations; WileyInterscience: New York, 1976. (67) Wesolowski, D. J.; Palmer, D. A. Geochim. Cosmochim. Acta 1994, 58, 2947.

Langmuir, Vol. 21, No. 7, 2005 2819

Figure 9. Concentration of Al(III) dissolved from corundum after a reaction time of 72 h in the presence of pyromellitate or maleate and as a function of pH. The corundum concentration was 50 g/L in all cases. The LMW organic anion and background electrolyte concentrations were 0 mM pyromellitate, 0.01 M NaCl (b), 0 mM maleate, 0.01 M KNO3 (O), 2.50 mM pyromellitate, 0.01 M NaCl ([), and 2.50 mM maleate, 0.01 M KNO3 (]).

maleate. For example, the dissolved Al(III) concentration in solution in the absence of pyromellitate was reduced by 46% in the presence of 2.50 mM pyromellitate at pH ) 2.5 and by 86 ( 1% at pH ) 3.5 and 4.5. The dissolved Al(III) concentrations in the presence of 2.5 mM maleate were also significantly reduced when compared with those obtained in the absence of maleate, but to a lesser extent than for systems containing pyromellitate (i.e., by 39% at pH ) 2.5 and by 53 ( 1% at pH ) 3.5 and 4.5). These results indicate that pyromellitate is a more effective inhibitor of corundum dissolution than is maleate over most of the acidic pH conditions examined. The results of Figure 9 can be rationalized with reference to the ECCM results previously presented in section 3.2 and by Johnson et al.,1 which demonstrate that the surfacecomplexed Pyr4- species (which is dominant at all but the lowest pH conditions examined, see sections 3.1 and 3.2) is more strongly outer spherically adsorbed at the corundum surface (log K ) 13.8) than is the outer-sphere Mal2- species (log K ) 11.16). As a result, the Pyr4- is less readily displaced by dissolution-enhancing species such as protons, and so the protolytic dissolution rate of corundum is lower in the presence of outer spherically adsorbed Pyr4- than when outer spherically complexed by Mal2-. Interestingly, as the pH is progressively lowered, the abundance of the outer-sphere tAlOH2+‚‚‚Pyr4species is gradually reduced, with surface species such as outer-spheretAlOH2+‚‚‚H2Pyr2- andinner-spheretAlPyr3becoming increasingly prevalent (see section 3.2). The outer spherically adsorbed H2Pyr2- species has a lower surface binding strength than Pyr4- (due to its lower charge, resulting in a lower electrostatic attraction for surface tAlOH2+ groups), while the inner-sphere tAlPyr3species may actually promote surface dissolution in a manner similar to other organic anions adsorbed to (oxyhydr)oxide mineral surfaces in an inner-sphere, mononuclear fashion.2-6 As a result, the dissolutioninhibiting impact of adsorbed pyromellitate is expected to become less significant at low pH, an effect that is clearly demonstrated in Figures 8 and 9. The dissolution kinetics of corundum are shown as a function of pyromellitate concentration at pH ) 3.0 in Figure 10. At each reaction time, Figure 10 demonstrates that the concentration of dissolved Al(III) in solution is progressively reduced as a function of pyromellitate concentration up to 1.25 mM, beyond which constant Al(III) concentrations are observed. The effects of adsorbed

2820


Figure 10. Concentration of Al(III) dissolved from corundum at pH ) 3.0 as a function of pyromellitate concentration and time. The corundum concentration was 50 g/L and the background electrolyte was 0.01 M NaCl in all cases. Slight differences between the data presented here and those of Figures 8 and 9 are due to two different batches of AKP-30 corundum being utilized. The total pyromellitate concentrations were 0 mM (b), 0.125 mM (O), 0.250 mM (2), 0.500 mM (4), 1.25 mM ([), and 2.50 mM (]).

Figure 11. Steady-state corundum dissolution rates as a function of pyromellitate (b) and Suwannee River fulvic acid (SRFA) (O) concentrations at pH ) 3.0. Dissolution rates are based on data obtained for reaction times g130 h in Figures 10 and 12.

pyromellitate on the corundum dissolution kinetics are further examined in Figure 11, where the steady-state corundum dissolution rate (obtained at reaction times g130 h) at pH ) 3.0 is presented as a function of pyromellitate concentration. In this case, the dissolution rate behavior observed is slightly different to that previously reported in Figures 8 and 9 due to a different batch of AKP-30 corundum being utilized. As a result, the dissolved Al(III) concentrations shown after 72 h in Figure 11 are only approximately 75% of those previously shown in Figures 8 and 9. Figure 11 shows that the corundum dissolution rate decreases in a linear manner with pyromellitate concentration at [pyromellitate] e 0.5 mM, while at higher pyromellitate concentrations (1.25 and 2.50 mM), an approximately constant dissolution rate is evident. The intersection of these two dissolution domains is found to occur at a pyromellitate concentration of 0.64 mM and indicates that above this critical concentration, those surface sites primarily responsible for the dissolution behavior of corundum are fully complexed by outer spherically adsorbed Pyr4-. Similar findings have previously been reported for the interaction of maleate with corundum (where a critical maleate concentration of 0.60 ( 0.04 mM was reported).1 Comparable findings are also observed at pH < 5 in Figure 8 but with some minor

Johnson et al.

Figure 12. Concentration of Al(III) dissolved from corundum at pH ) 3.0 as a function of a reference Suwannee River fulvic acid (SRFA) concentration and time. The corundum concentration was 50 g/L and the background electrolyte was 0.01 M NaCl in all cases. The SRFA concentration is expressed in terms of the surface coverage, Γ (in µmol of functional groups/m2), of its dominant carboxyl and aromatic phenolic functional groups, and in terms of its molecular concentration (in millimoles/liter) in brackets. The total SRFA concentration was 0 µmol/m2 (0 mM) (b), 2 µmol/m2 (0.06 mM) (O), 4 µmol/m2 (0.12 mM) (2), and 6 µmol/m2 (0.19 mM) (4).

discrepancies, e.g., a gradual increase in dissolved Al(III) concentrations at [pyromellitate] g 0.5 mM at pH ) 2.0. The latter results are presumably attributable to an increase in the concentration of the dissolution-enhancing inner-sphere tAlPyr3- species, which ECCM simulations (not shown) show to become increasingly important at the highest pyromellitate concentrations examined. While pyromellitate is often treated as a reasonable analogue for natural organic matter in the environment,23,24 it is also of interest to directly examine the effects of a natural macromolecular organic substance on (oxyhydr)oxide mineral dissolution behavior. To this end, the effects of a reference Suwannee River fulvic acid (SRFA) on the dissolution of corundum have been examined at a single pH condition (pH ) 3.0) and a range of different SRFA concentrations. Those results are presented in Figure 12, where the concentration of SRFA is presented in terms of the concentrations of its dominant (carboxyl and aromatic phenolic) functional groups.43 SRFA has recently been shown to adsorb to aluminum (oxyhydr)oxides in a predominantly outer-sphere manner,21 forming only minor inner-sphere species under quite acidic pH conditions.22 As a result, SRFA is also expected to inhibit the dissolution of corundum should it act to sterically protect dissolution-active surface sites against protolytic attack. This effect in shown in Figure 12, where at each reaction time (4-336 h), the concentration of dissolved Al(III) in solution decreases as a function of the concentration of SRFA (which is >95% adsorbed to the corundum surface over the range of SRFA concentrations and reaction times investigated). The effects of adsorbed SRFA on the dissolution of corundum are further demonstrated in Figure 11, where the steady-state corundum dissolution rate (obtained at reaction times g130 h) is shown as a function of SRFA concentration (this time expressed in terms of its molecular concentration, based on an average SRFA molecular weight of approximately 80068,69). As was the case for pyromellitate, the corundum dissolution rate decreases in an approximately linear fashion over the range of low (68) Aiken, G. R.; Brown, P. A.; Noyes, T. I.; Pinckney, D. J. 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, 1994.


(0.06-0.19 mM) SRFA concentrations examined. Interestingly, however, the dissolution-inhibiting effect of SRFA is substantially greater than that observed for pyromellitate, indicating that (1) SRFA is more strongly outer spherically bound to specific dissolution-active surface sites, (2) free (uncomplexed) SRFA charge groups are binding to (and therefore effectively trapping) Al(III) cations as they are extracted from the corundum surface, and/or (3) each SRFA anion is interacting with multiple dissolution-active surface sites on the corundum surface. The last explanation in particular is certainly understandable given the substantially larger predicted sizes of SRFA molecules compared with that of the pyromellitate anion.69 4. Conclusions The effects of pyromellitate, a LMW analogue for natural organic matter, on the dissolution behavior of a model aluminum oxide, corundum, have been investigated using a combination of ATR-FTIR spectroscopy, macroscopic adsorption, and direct dissolution measurements. ATRFTIR experiments conducted at pH g 5.0 indicate that adsorbed pyromellitate is predominantly in the form of a single outer-sphere species, tAlOH2+‚‚‚Pyr.4- At lower pH conditions, additional features in the ATR-FTIR spectra of adsorbed pyromellitate indicate the presence of two additional species: a partially protonated outer(69) 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, 1994.

Langmuir, Vol. 21, No. 7, 2005 2821

sphere complex, tAlOH2+‚‚‚H2Pyr2-, and an inner-sphere species. Consistent with these findings, optimal extended constant capacitance model42 fits to batch pyromellitate adsorption data were obtained using three surface species: outer-sphere tAlOH2+-Pyr4- (log K ) 13.8) and tAlOH2+‚‚‚H2Pyr2- (log K ) 18.8), and inner-sphere tAlPyr3- (log K ) 2.3). Adsorbed pyromellitate was observed to dramatically inhibit the dissolution of corundum under acidic (pH < 5) conditions. The latter finding is consistent with a reduction in the protolytic dissolution rate caused by steric protection of dissolution-active surface sites by the outer spherically adsorbed Pyr4species. A similar mechanism has previously been proposed by Johnson et al.1 to justify the dissolution-inhibiting effects of outer spherically adsorbed maleate. Kinetic dissolution measurements performed at a single acidic condition (pH ) 3.0) further demonstrate that Suwannee River fulvic acid (SRFA) inhibits the rate of corundum dissolution to an even greater extent than does pyromellitate at the same pH condition. The latter finding is most likely due to the ability of each SRFA molecule to simultaneously strongly interact with multiple surface sites on the corundum surface. Acknowledgment. The authors wish to thank Professor Scott Fendorf for the use of FTIR and inductively coupled plasma spectrometry (ICP) equipment in his laboratory, and Guangchao Li for his help in performing ICP measurements. This work was supported by EPASTAR Grant EPA-R827634-01-1 (GEB) and NSF Grant CHE-0089215 (GEB). LA0481041

Water Interfaces: 5. Effects of

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