Water Interfaces. 2. Outer

Outer-Sphere Adsorption of Maleate and Implications for Dissolution ... Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69,...
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Adsorption of Organic Matter at Mineral/Water Interfaces. 2. Outer-Sphere Adsorption of Maleate and Implications for Dissolution Processes Stephen B. Johnson,*,† Tae Hyun Yoon,† Benjamin D. Kocar,‡ and Gordon E. Brown, Jr.†,§ Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, Soil and Environmental Chemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025 Received December 4, 2003. In Final Form: March 25, 2004 The effects of the adsorption of a simple dicarboxylate low molecular weight organic anion, maleate, on the dissolution of a model aluminum oxide, corundum (R-Al2O3), have been examined over a range of different maleate concentrations (0.125-5.0 mM) and pH conditions (2-10). In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic measurements indicate that maleate binds predominantly as an outer-sphere, fully deprotonated complex (tAlOH2+---Mal2-) at the corundum surface over the entire range of maleate concentrations and pH conditions investigated. In accordance with the ATR-FTIR findings, macroscopic adsorption data can be modeled as a function of maleate concentration and pH using an extended constant capacitance approach and a single tAlOH2+---Mal2- species. Outersphere adsorption of maleate is found to significantly reduce the protolytic dissolution rate of corundum under acidic conditions (pH < 5). A likely mechanism involves steric protection of dissolution-active surface sites, whereby strong outer-sphere interactions with maleate hinder attack on those surface sites by dissolution-promoting species.

1. Introduction Low molecular weight (LMW) organic anions, such as oxalate, citrate, and malate, are prevalent in many natural settings due to their exudation from plant roots, production by fungi, and discharge by microorganisms.1-3 When present in environments containing oxide and/or (oxy)hydroxide minerals, such LMW organic species are often strongly associated with positively charged mineral surfaces under acidic to near-neutral pH conditions.4 Organic matter-mineral interactions are particularly important in the case of multivalent LMW organic anions, as demonstrated by a number of laboratory-based adsorption studies which have shown that a variety of multivalent LMW anions (including both aliphatic and carboxyl-substituted aromatic species) bind strongly to common iron- and aluminum-(oxy)hydroxide minerals under moderately acidic conditions.5-11 Similarly, multivalent LMW organic anions are strongly sorbed to the * To whom correspondence should be addressed. E-mail: stephen. [email protected]. Phone: +1 650 723-4152. Fax: +1 650 7252199. † Surface & Aqueous Geochemistry Group, Stanford University. ‡ Soil and Environmental Chemistry Group, Stanford University. § Stanford Synchrotron Radiation Laboratory, SLAC. (1) Gadd, G. M. Adv. Microbial Physiol. 1999, 41, 47. (2) Jones, D. L. Plant Soil 1998, 205, 25. (3) Ryan, P. R.; Delhaize, E.; Jones, D. L. Annu. Rev. Plant Physiol. Plant Mol. Biol. 2001, 52, 527. (4) Yoon, T. H.; Johnson, S. B.; Musgrave, C. B.; Brown, G. E., Jr. Geochim. Cosmochim. Acta, in press. (5) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373. (6) Filius, J. D.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 1997, 195, 368. (7) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Eur. Ceram. Soc. 1997, 17, 239. (8) Evanko, C. R.; Dzombak, D. A. Environ. Sci. Technol. 1998, 32, 2846.

solid phases of natural soil samples over a broad pH range,12 with the sorbed concentrations often substantially exceeding those found in free soil solution.13 The adsorption of LMW organic acids on mineral particles is generally thought to occur through either of two fundamental interactions: inner-sphere adsorption, in which the organic anion forms a direct bond with a surface cation via a ligand exchange process, and outersphere adsorption, in which no direct anion-surface bond is formed and the anion is instead held at the surface through a combination of hydrogen bonding and electrostatic interactions. The importance of inner-sphere adsorption for processes such as dissolution at mineralwater interfaces has been widely investigated and has been comprehensively reviewed by Stumm.14,15 By contrast, the dissolution-enhancing and/or -inhibiting effects of outer-spherically adsorbed organic anions have received comparatively little attention to date but are of significant potential importance given that LMW analogues for natural organic matter, such as pyromellitate,9,16,17 bind to (oxy)hydroxide mineral surfaces predominantly through the formation of outer-sphere adsorption complexes under environmentally relevant (near-neutral) pH conditions. (9) Boily, J. F.; Persson, P.; Sjoberg, S. Geochim. Cosmochim. Acta 2000, 64, 3453. (10) Filius, J. D.; Meeussen, J. C. L.; Hiemstra, T.; Van Riemsdijk, W. H. J. Colloid Interface Sci. 2001, 244, 31. (11) Rosenqvist, J.; Axe, K.; Sjoberg, S.; Persson, P. Colloids Surf., A 2003, 220, 91. (12) Jones, D. L.; Brassington, D. S. Eur. J. Soil Sci. 1998, 49, 447. (13) Strobel, B. W. Geoderma 2001, 99, 169. (14) Stumm, W. Adv. Chem. Ser. 1995, 244, 1. (15) Stumm, W. Colloids Surf., A 1997, 120, 143. (16) Boily, J. F.; Nilsson, N.; Persson, P.; Sjoberg, S. Langmuir 2000, 16, 5719. (17) Yoon, T. H.; Brown, G. E., Jr. Abstr. Pap. Am. Chem. Soc. 2001, 222 (pt. 1), U444.

10.1021/la036288y CCC: $27.50 © 2004 American Chemical Society Published on Web 05/12/2004

Organic Matter at Mineral/Water Interfaces

It is the outer-sphere adsorption of simple LMW organic acids and the resulting effects on mineral dissolution that are the focus of the present study. For completeness, however, we briefly review work undertaken to date on the effects of LMW anions, including those bound in innersphere adsorption modes, on the dissolution properties of mineral surfaces. Inner-sphere adsorption of LMW anions is widely considered to be of importance in mineral dissolution processes. Of particular note, a number of studies by Stumm and co-workers18-20 have focused on the effects of a range of LMW organic anions on the dissolution kinetics of various colloidal minerals including common aluminum and iron oxides and (oxy)hydroxides. These studies have shown that simple multivalent LMW organic anions such as oxalate, malonate, citrate, and salicylate can dramatically increase mineral dissolution rates. Furrer and Stumm18,19 proposed that dissolution kinetics are enhanced due to the LMW anions chelating surface metal cations in a bidentate, mononuclear, inner-sphere fashion. Transfer of considerable electron density into the coordination sphere of the surface metal cations will result. Consequently, bridging substrate metal-oxygen bonds in positions trans to each metal-anion bond are polarized and weakened, leaving the complexed metal cations more susceptible to release from the surface via a ratedetermining detachment step. A number of spectroscopic studies have since demonstrated that oxalate,4,11,21-25 citrate,26 malonate,11,23,25 and salicylate27-29 are indeed capable of forming inner-sphere complexes with a range of mineral oxides and (oxy)hydroxides, thus providing support for the dissolution mechanism of Furrer and Stumm.18,19 Since these seminal studies, the influence of innersphere binding of LMW acids on the dissolution of mineral oxides and (oxy)hydroxides has received considerable attention in the literature and has been the subject of several reviews.14,15,30,31 Further studies of the role of innersphere adsorption on dissolution have shown that ligands which adsorb in an inner-sphere, bidentate, binuclear manner can inhibit mineral dissolution,32-34 in contrast with the dissolution-enhancing effects of inner-sphere, bidentate, mononuclear complexes discussed above. The stabilizing effects against dissolution imparted by such binuclear complexes can be largely attributed to the low probability of (and unfavorably high activation energies (18) Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1847. (19) Stumm, W.; Furrer, G. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley-Interscience: New York, 1987; p 197. (20) Zinder, B.; Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1861. (21) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (22) Degenhardt, J.; McQuillan, A. J. Chem. Phys. Lett. 1999, 311, 179. (23) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta, Part A 1999, 55, 1395. (24) Axe, K.; Persson, P. Geochim. Cosmochim. Acta 2001, 65, 4481. (25) Duckworth, O. W.; Martin, S. T. Geochim. Cosmochim. Acta 2001, 65, 4289. (26) Hidber, P. C.; Graule, T. J.; Gauckler, L. J. J. Am. Ceram. Soc. 1996, 79, 1857. (27) Yost, E. C.; Tejedor-Tejedor, I.; Anderson, M. A. Environ. Sci. Technol. 1990, 24, 822. (28) Biber, M. V.; Stumm, W. Environ. Sci. Technol. 1994, 28, 763. (29) Dobson, K. D.; McQuillan, A. J. Spectrochim. Acta, Part A 2000, 56, 557. (30) Hering, J. G. Adv. Chem. Ser. 1995, 244, 95. (31) Drever, J. I.; Stillings, L. L. Colloids Surf., A 1997, 120, 167. (32) Bondietti, G.; Sinniger, J.; Stumm, W. Colloids Surf., A 1993, 79, 157. (33) Stumm, W. Aquat. Sci. 1993, 55, 273. (34) Biber, M. V.; Afonso, M. D.; Stumm, W. Geochim. Cosmochim. Acta 1994, 58, 1999.

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associated with) simultaneous detachment of two metal cations from mineral surfaces. As a result, in complex systems consisting of multiple LMW anionic species, the overall dissolution behavior is given by the balance of the dissolution-enhancing and -inhibiting properties of anions adsorbed in different modes35,36 (e.g., mononuclear versus binuclear and monodentate versus multidentate adsorption complexes). These adsorption modes are in turn critically dependent upon factors such as the individual, relative, and overall anion concentrations and the solution pH. Despite the considerable attention that has been given to the role of inner-sphere binding of multivalent LMW anions in promoting or hindering mineral dissolution, comparatively few studies have addressed the potential role(s) of outer-spherically adsorbed LMW organic anions in mineral dissolution processes. Three potential outcomes exist for outer-sphere adsorption of multivalent LMW organic anions to the surfaces of mineral oxides or (oxy)hydroxides. (1) Organic anions adsorbed as outer-sphere complexes do not transfer significant electron density into the coordination sphere of the surface cations and so have little influence on the bridging metal-oxygen bonds in the mineral surface. They therefore have little direct effect on the dissolution kinetics. This hypothesis is supported by measurements of aqueous metal complex reactivities (which can be closely correlated with ligand-induced mineral dissolution rates37) cited by Stumm,15 which show that ligands bound as outer-sphere complexes have little impact on the kinetics of water exchange around solutionbased cations. (2) Outer-spherically bound LMW organic anions reduce the effective electrical charge generated by positively charged mineral surfaces and ions present in the Stern layer. This charge reduction promotes higher proton concentrations in the diffuse electrical double layer adjacent to the surface-water interface, thus inducing an increased rate of protolytic attack on bridging metaloxygen bonds in the mineral surface. An increased overall dissolution rate results, as suggested by Persson et al.38 (3) Although adsorbed in an outer-sphere mode, LMW multivalent organic anions tightly bind to specific surface sites via electrostatic and/or hydrogen bonding mechanisms. As a result, they effectively hinder or block attack at these sites by dissolution-promoting species such as protons, reducing the overall rate of dissolution. In reality, each of the three possible mechanisms outlined above is likely to exert some influence upon the overall dissolution kinetics. The aim of this study is to establish the net effect of outer-spherically bound LMW organic anions on the dissolution of mineral oxides and (oxy)hydroxides. A simple multivalent LMW organic anion, maleate, was selected for study. In its acid form, maleate is an important industrial chemical, being a raw material for the manufacture of plasticizers, surface coatings, agricultural chemicals, lubricant additives, polyester resins, and copolymers.39 It is also of environmental significance, being commonly found in a variety of vegetated soil environments.13 Structurally, maleate consists of two carboxylate groups (pKa1 ) 1.81, pKa2 ) 6.05 at an ionic strength of 0.01 M) linked by a three(35) Eick, M. J.; Peak, J. D.; Brady, W. D. Soil Sci. Soc. Am. J. 1999, 63, 1133. (36) Campbell, J. L.; Eick, M. J. Clays Clay Miner. 2002, 50, 336. (37) Ludwig, C.; Casey, W. H.; Rock, P. A. Nature 1995, 375, 44. (38) Persson, P.; Nordin, J.; Rosenqvist, J.; Lovgren, L.; Ohman, L. O.; Sjoberg, S. J. Colloid Interface Sci. 1998, 206, 252. (39) Felthouse, T. R.; Burnett, J. C.; Horrell, B.; Mummey, M. J.; Kuo, Y. J. Maleic Anhydride, Maleic Acid, and Fumaric Acid. In KirkOthmer Encyclopedia of Chemical Technology Online; Wiley-Interscience: New York, 2001.

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Figure 1. The chemical structure of maleate and the relative abundances of its fully deprotonated (Mal2-), singly protonated (HMal-), and doubly protonated (H2Mal) forms 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 (ref 56) to those applicable at I ) 0.01 M using the Davies equation (ref 81).

carbon backbone (see Figure 1), such that monodentate or bidentate, inner-sphere binding with surface metal cations would require formation of energetically unfavorable seven- or eight-membered chelate rings, respectively. As a result, maleate is anticipated to adsorb in a predominantly outer-sphere fashion, an expectation that has been borne out by recent attenuated total reflectance Fourier transform infrared (ATR-FTIR) measurements of maleate on gibbsite undertaken by Rosenqvist et al.11 A common aluminum oxide substrate, corundum, was selected as a model mineral colloid on which to study the adsorption properties and resulting dissolution effects of maleate. Corundum, R-Al2O3, is of industrial importance due to its use as an abrasive and as a raw material for the production of advanced ceramic components. Interestingly, in the latter application, corundum has recently been investigated for use in conjunction with multivalent LMW organic acid dispersants in aqueous-based (colloidal) ceramics production.26,40,41 The surface chemistry of corundum in aqueous media42-46 and its interactions with adsorbates including LMW organic acids7,26,47-49 have been widely investigated, as has its aqueous dissolution behavior.50-52 As a result, corundum was found to be a suitable colloidal mineral for investigation in the present work. This study has investigated the mode and extent of adsorption of maleate on corundum as a function of pH (40) Sacks, M. D.; Tseng, T. Y. J. Am. Ceram. Soc. 1983, 66, 242. (41) Luther, E. P.; Yanez, J. A.; Franks, G. V.; Lange, F. F.; Pearson, D. S. J. Am. Ceram. Soc. 1995, 78, 1495. (42) Sverjensky, D. A.; Sahai, N. Geochim. Cosmochim. Acta 1996, 60, 3773. (43) Sahai, N.; Sverjensky, D. A. Geochim. Cosmochim. Acta 1997, 61, 2801. (44) 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. (45) Trainor, T. P.; Eng, P. J.; Brown, G. E., Jr.; Robinson, I. K.; De Santis, M. Surf. Sci. 2002, 496, 238. (46) Criscenti, L. J.; Sverjensky, D. A. J. Colloid Interface Sci. 2002, 253, 329. (47) Boily, J. F.; Fein, J. B. Geochim. Cosmochim. Acta 1996, 60, 2929. (48) Boily, J. F.; Fein, J. B. Chem. Geol. 1998, 148, 157. (49) 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. (50) Carroll-Webb, S. A.; Walther, J. V. Geochim. Cosmochim. Acta 1988, 52, 2609. (51) Hirata, Y.; Otsubo, Y.; Arimura, Y. J. Ceram. Soc. Jpn. 1995, 103, 769. (52) Samson, S. D.; Stillings, L. L.; Eggleston, C. M. Geochim. Cosmochim. Acta 2000, 64, 3471.

Johnson et al.

and the effects of these variables on the dissolution behavior of corundum. The adsorption mode(s) of maleate at the corundum/water interface have been examined as a function of maleate concentration and pH using in situ ATR-FTIR spectroscopy. The macroscopic adsorption behavior of maleate was examined over the same range of experimental conditions and modeled based on the ATRFTIR spectroscopic findings and using the Extended Constant Capacitance Model (ECCM) of Nilsson et al.53 Finally, the dissolution behavior of corundum has been investigated over a broad range of pH conditions as a function of maleate concentration. The study concludes by briefly outlining the potential impact of outer-spherically bound LMW organic anions for mineral dissolution processes in the environment. 2. Experimental Section 2.1. Material. High purity (>99.99%) AKP-30 corundum was obtained from Sumitomo Chemical Co., Japan. It had a Brunauer-Emmett-Teller (BET) surface area of 7 m2/g, a mean particle diameter of 0.30 µm, and a density of 3.97 g/cm3. Reagent grade maleic acid was obtained from Fisher Scientific. Maleic acid monopotassium (>99%) and dipotassium (>98%) salts were purchased from Sigma-Aldrich. Analytical grade potassium nitrate or potassium chloride and Milli-Q grade water (resistivity ) 18.2 MΩ cm at 25 °C) were used in all experiments. Solution and suspension pHs were measured using a Denver Instrument model 215 pH meter equipped with a high-performance sleeve junction pH electrode, which was regularly calibrated using standard pH buffer solutions. Stock corundum suspensions were prepared by ultrasonically dispersing corundum (66.7 g/L) in aqueous solution at pH ) 5 using a Branson model 450 digital sonifier equipped with a 0.5 in. horn. The suspensions were then slowly tumbled end-overend for 12-24 h prior to use. A relatively low background electrolyte concentration (of 0.01 M KNO3 or 0.01 M KCl) was used in all cases in order to allow direct comparison of our results with both previous adsorption experiments (and associated surface complexation modeling) of outer-spherically bound LMW organic anions on corundum48 and past electrokinetic measurements of corundum-maleate systems,49 all of which were similarly conducted at a low (0.01 M) background electrolyte concentration. Aqueous corundum-KNO3 (or KCl) and corundum-maleateKNO3 (or KCl) samples for ATR-FTIR, adsorption, and dissolution analysis were prepared by diluting stock suspensions with appropriate volumes and concentrations of 0.01 M KNO3 (or KCl) solutions and/or maleate solutions. In all cases, the final corundum concentration was 50 g/L, the background KNO3 or KCl concentration was 0.01 M, and the maleate concentration ranged from 0 to 5 mM. The suspension samples were rapidly transferred to polypropylene centrifuge tubes, and the pH of individual tubes was adjusted to the desired value using small volumes of 1 M HNO3 (or HCl) and 1 M KOH. The tubes were then purged with high purity nitrogen gas, wrapped in aluminum foil (to prevent photodegradation of maleate), and slowly tumbled end-over-end for 72 h (except in the case of kinetic dissolution runs, for which the run time ranged from 3 to 168 h). During the “equilibration” period, the pH of the suspension in each tube was periodically checked and, if necessary, readjusted to the target pH condition. In general, however, only minor adjustments were required once the target pH condition was initially established. Finally, to separate and concentrate the corundum pastes, each sample was centrifuged at 10 000 rpm for 20 min. The supernatant was then decanted and passed through a 20 nm syringe filter in order to remove any residual corundum particles. 2.2. ATR-FTIR Measurements. ATR-FTIR measurements were made using a Nicolet NEXUS 470 FTIR spectrometer equipped with a DTGS detector and a horizontal attenuated total reflectance attachment (germanium crystal). Data collection and spectral calculations were performed using OMNIC (version 6.0a, (53) Nilsson, N.; Persson, P.; Lovgren, L.; Sjoberg, S. Geochim. Cosmochim. Acta 1996, 60, 4385.

Organic Matter at Mineral/Water Interfaces Nicolet Instrument Corp.) software. To prevent spectral complications introduced from absorption by the background electrolyte, 0.01 M KCl was used in place of 0.01 M KNO3 for all solution and suspension samples. Solution samples were applied directly to the germanium crystal. For alumina-KCl and alumina-maleate-KCl suspensions, a thin layer of concentrated paste was evenly applied to the germanium ATR crystal, a small volume of supernatant was added on top of the paste samples, and the sample-holding region was sealed with a lid to prevent evaporation during ATR-FTIR measurements. Suspension pastes were not washed between the completion of the equilibration step (see section 2.1) and ATR-FTIR analysis. Five hundred scans (with a spectral resolution of 2 cm-1) were taken and averaged for each solution and suspension sample. As is usual for ATR-FTIR measurements in aqueous media,54 all solution and wet paste spectra were dominated by the strong infrared absorbance of water. For each aqueous maleate sample, the spectral contribution of water was removed by measuring the spectrum of a 0.01 M KCl solution at the same pH and subtracting this from the aqueous maleate spectrum. For corundum-maleate-KCl pastes, we measured and subtracted the spectrum of the supernatant solution in order to eliminate the bulk water signal. In addition to the spectral contribution from bulk water, however, the ATR-FTIR spectra of corundum pastes were found to contain another strong signal due to physisorbed water and some additional absorbances attributable to the hydrated corundum surface. To remove these unwanted signals for each corundum-maleate-KCl sample, the spectrum of a corundum-KCl suspension was measured at an identical pH in the absence of maleate. This spectrum was then subtracted from the overall corundum-maleate-KCl signal to yield the spectrum of maleate species at or near the corundum-water interface. The latter procedure was found to impart a slight negative slope to the extracted spectra at wavenumbers less than 1500 cm-1, with the gradient of the slope rising as a function of increasing maleate concentration. The occurrence of this baseline gradient is presumably due to a maleate-induced disruption of the layer of physisorbed water associated with corundum, leading to a slight overcorrection for the absorbance from chemisorbed hydroxyls (at wavenumbers of 9) conditions. Such increases in solubility as the pH moves away from near-neutral conditions are consistent with an increased attack on the corundum surface by dissolution-enhancing proton and hydroxyl species, respectively, and are typical of aluminum-(oxy)hydroxide minerals.82-84 Interestingly, however, at pH values less than 4, Figure 7 also shows that in the absence of maleate, the solution concentration of dissolved Al(III) reaches a maximum value in the vicinity of pH ) 3.5 and decreases slightly at lower pH values. These findings are consistent with the relative pH-insensitivity of the corundum dissolution rate at low pH that has been previously observed by Carroll-Webb and Walther.50 It can be attributed to the slow regeneration rate of dissolution-active sites at low pH, which is in turn driven by the low rate of water exchange at surface Al(III) sites under acidic conditions.52 Figure 7 also shows that as the concentration of maleate is increased, the concentration of dissolved Al(III) in solution decreases for pH < 5 but is not significantly affected at pH > 9. These findings indicate that maleate is inhibiting dissolution, but only when it is strongly adsorbed to the corundum surface (see Figure 6a,b). To further investigate the effects of adsorbed maleate on corundum dissolution, the dissolution kinetics of corundum have been investigated at a single acidic pH condition (pH ) 3.0) over the same range of maleate concentrations (82) Bethke, C. M. The Geochemist’s Workbench, release 4.0; University of Illinois: Urbana-Champaign, 2002. (83) Baes, C. F.; Mesmer, R. E. The hydrolysis of cations; WileyInterscience: New York, 1976. (84) Wesolowski, D. J.; Palmer, D. A. Geochim. Cosmochim. Acta 1994, 58, 2947.

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Figure 8. Concentration of Al(III) dissolved from corundum at pH ) 3.0 as a function of maleate concentration and time. The corundum concentration was 50 g/L, and the background electrolyte was 0.01 M KNO3 in all cases. For the purposes of comparison with Figure 7, all Al(III) concentrations have been normalized against the concentration of dissolved Al(III) measured after 72 h in the absence of maleate. Slight differences between the data presented here and those of Figure 7 are due to two different batches of AKP-30 corundum being utilized. The total maleate concentration was 0 mM, b; 0.125 mM, O; 0.250 mM, 2; 0.500 mM, 4; 1.25 mM, [; 2.50 mM, ]; and 5.00 mM, 1.

presented in Figure 7. The resulting data are shown in Figure 8. In all cases, the concentrations of dissolved Al(III) are several orders of magnitude below the saturation concentration of Al(III) in solution at pH ) 3.82,83 They are similarly well below the concentrations of Al(III) and maleate that we were able to dissolve in solution at pH ) 3 ([Al(III)] and [maleate] > 0.1 M), indicating that the dissolved Al(III) and maleate concentrations shown in Figure 8 are far less than those required to generate bulk formation of an Al(III)-maleate precipitate. As a result, saturation phenomena should not have significantly influenced the dissolution data presented in Figure 8. Figure 8 shows that the rate of Al(III) dissolution gradually decreases as a function of time and approaches approximately steady-state dissolution kinetics after ∼100 h in all cases. Beyond this time, the relative rates of surface site elimination and regeneration (which is rate-determining for the dissolution of corundum at low pH52) are presumably equivalent. Figure 8 also shows that the concentration of Al(III) in solution systematically decreases as a function of increasing maleate concentration, with both the initial and steady-state rates of dissolution also being significantly reduced as the maleate concentration is increased. The change in the steady-state dissolution kinetics is particularly noteworthy, with the rate of corundum dissolution at maleate concentrations of 1.25 mM and greater being only 39 ( 1% of that observed in the absence of maleate. The likely mechanism underlying the maleate-driven inhibition of corundum dissolution observed in Figures 7 and 8 is one based on steric protection of surface sites, with the outer-sphere association of maleate with tAlOH2+ sites blocking access to those surface groups for dissolutionenhancing H+ ions. As a result, the protolytic dissolution rate of corundum is substantially reduced. Importantly, under this mechanism, the absence of a direct surfacemaleate bond means that maleate does not have a significant polarizing effect on bridging Al-O bonds in the corundum surface. As a result, outer-spherically bound maleate does not destabilize Al-O bonds in the mineral surface in the manner of LMW organic anions bound in an inner-sphere, mononuclear bidentate fashion.18,19

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Figure 9. Concentration of Al(III) dissolved from corundum at pH ) 3.0 as a function of maleate concentration and time. Data are derived from Figure 8. The equilibration time was 6 h, b; 20 h, O; 44 h, 2; 96 h, 4; and 168 h, [.

Figure 7 further shows that the concentration of dissolved Al(III) in solution reaches a minimum at a total maleate concentration of 1.25 mM and remains at approximately that level for the higher concentrations of maleate (2.50-5.00 mM) examined. The latter finding indicates that maleate has saturated the surface sites most susceptible to dissolution (including particularly reactive defect sites19) by a total maleate concentration of 1.25 mM. While maleate adsorption continues to rise at total concentrations greater than 1.25 mM (see Figure 6a,b), the surface sites affected at these higher maleate concentrations evidently do not contribute significantly to the overall dissolution behavior of corundum. As a result, the dissolution properties remain relatively unchanged at maleate concentrations in excess of 1.25 mM. The dissolved Al(III) data shown Figure 8 can be replotted as a function of maleate concentration in order to more accurately determine the point above which the dissolution data become insensitive to maleate concentration at pH ) 3.0. As is shown in Figure 9, the dissolved Al(III) versus maleate concentration data follow an approximately linear form at low maleate concentrations. Above a critical maleate concentration, however, the concentration of dissolved Al(III) is almost constant within each of the experimental time frames shown. As demonstrated in Figure 9, the intersection of these two data domains yields a critical maleate concentration of 0.60 ( 0.04 mM, above which maleate has no significant effect on the corundum dissolution kinetics. ECCM simulations (see section 3.2) using the surface complexation parameters listed in Table 2 indicate that at a total maleate concentration of 0.60 mM, 86% of the total maleate should be adsorbed to corundum in the form of tAlOH2+---Mal2-. If it is assumed that this adsorbed maleate concentration ()0.51 mM) corresponds to the concentration of highly dissolution-active sites on the corundum surface, a dissolution-active site density of 1.5 ( 0.1 µmol/m2 (or 0.9 sites/nm2) is obtained. Based on a total corundum surface site density of 5.0 sites/nm2 (see section 3.2), this estimate implies that approximately 17-19% of the total corundum surface sites are controlling its aqueous dissolution behavior, at least at pH ) 3.0. The inhibition of corundum dissolution by maleate is interesting in light of other, competing processes that are also driven by the adsorption of maleate and which could be anticipated to promote mineral dissolution. For example, the electrokinetic study of Johnson49 has shown that the adsorption of maleate substantially reduces the ζ potential (generated predominantly by the net electrical charge of the surface and any tightly associated ions) of

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the corundum-water interface under acidic conditions. As a result, in the presence of adsorbed maleate, a higher proton concentration will exist in the diffuse electrical double layer adjacent to the corundum-water interface than will exist in the absence of maleate. Under such conditions, an increase in the protolytic dissolution rate of corundum may be expected to occur. In addition, the ATR-FTIR results previously presented in section 3.1 indicate the possible presence of some minor inner-sphere maleate surface complexes on corundum under low pH and high surface coverage conditions. Such inner-spherically bound LMW organic species have been widely implicated in the enhancement of (oxy)hydroxide mineral dissolution rates due to their ability to polarize and weaken surface metal-oxygen bonds.14,15,18-20,30,31 The presence of maleate adsorbed in an inner-sphere manner may therefore be expected to significantly enhance the dissolution kinetics of corundum. In fact, the two potential dissolution-enhancing mechanisms outlined above probably do have some impact upon the dissolution rate of corundum. However, their combined effects are clearly minor compared with the surface siteblocking role also played by outer-spherically bound maleate anions. As a result, the overall effect of maleate adsorption is to inhibit corundum dissolution. A similar site-blocking mechanism has been previously proposed by Compton and Brown85 for the interaction of Mal2- with calcite. In addition to maleate, our preliminary investigations have found that other organic anions that bind in a predominantly outer-sphere fashion similarly inhibit dissolution of aluminum-(oxy)hydroxides. For example, pyromellitate, a LMW organic anion considered to be a reasonable analogue for fulvic acid,8 binds predominantly in an outer-sphere fashion to mineral surfaces9,16,17 including aluminum-(oxy)hydroxides86 and can significantly inhibit the dissolution of corundum under acidic conditions.87 Similarly, our recent studies on fulvic acid adsorption on boehmite (γ-AlOOH) have indicated that its predominant outer-sphere mode of adsorption88 also leads to inhibition of dissolution under acidic conditions.57 Given the prevalence of such high molecular weight (85) Compton, R. G.; Brown, C. A. J. Colloid Interface Sci. 1995, 170, 586. (86) Yoon, T. H. Unpublished data. (87) Johnson, S. B.; Yoon, T. H.; Brown, G. E., Jr. In preparation. (88) Yoon, T. H.; Johnson, S. B.; Brown, G. E., Jr. Langmuir, submitted. (89) Lide, D. R. CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994.

Johnson et al.

organic species in the environment, it is likely that they, along with LMW organic anions such as maleate, will play a role in reducing mineral dissolution rates. Further work is, however, required to better understand the true environmental significance of outer-spherically bound anions in inhibiting mineral dissolution. 4. Conclusions The adsorption of maleate on corundum and its impacts on the dissolution of corundum have been examined using ATR-FTIR spectroscopic and macroscopic (batch) techniques. Over the broad range of maleate concentrations and pH conditions examined, ATR-FTIR spectra of adsorbed maleate exhibit strong similarities with that of aqueous Mal2-, but with slight peak frequency shifts (to higher energy) and significant peak broadening. These findings are consistent with maleate binding predominantly as a fully deprotonated, outer-sphere complex to corundum surface sites (tAlOH2+---Mal2-). Macroscopic adsorption data can similarly be accurately described over a broad range of maleate concentrations (0.125-5.0 mM) and pH conditions (2-10) using a single tAlOH2+---Mal2surface complex and the Extended Constant Capacitance Model of Nilsson et al.53 (Kf ) 11.16). Under acidic conditions, adsorbed maleate is found to significantly inhibit the dissolution of corundum, but does not have a discernible effect at high pH, where the adsorption of maleate approaches zero. Such results are consistent with the strongly adsorbed maleate anions sterically protecting corundum surface sites against attack by dissolutionpromoting species such as protons at low pH. The findings of this study are likely to have a broader environmental significance given that macroscopic organic species such as fulvic acids have also been found to adsorb in a predominantly outer-sphere manner on positively charged mineral surfaces and may therefore similarly affect mineral dissolution processes. Acknowledgment. The authors thank Dr. Scott Fendorf for the use of FTIR, ion chromatography, and ICP spectrometry equipment in his laboratory and Guangchao Li for his help with performing ICP measurements. We also thank Dr. Mike Angove (Latrobe University, Australia) for his assistance with surface complexation modeling. This manuscript benefited from comments by two anonymous reviewers. This work was supported by EPA-STAR Grant EPA-R827634-01-1 (G.E.B.) and NSF Grant CHE-0089215 (G.E.B.). LA036288Y