Adsorption of Organic Matter at Mineral/Water Interfaces: 7. ATR-FTIR and Quantum Chemical Study of Lactate Interactions with Hematite Nanoparticles Juyoung Ha,* ,† Tae Hyun Yoon, †,§ Yingge Wang, † Charles B. Musgrave, ‡,⊥ and Gordon E. Brown, Jr. †,| Surface & Aqueous Geochemistry Group, Department of Geological & EnVironmental Sciences, and Department of Chemical Engineering Stanford UniVersity, Stanford, California 94305-2115, Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305-5025, Department of Chemistry, Hanyang UniVersity, Seoul, 133-791, Korea, and Photon Science Department and SSRL, 2575 Sand Hill Road, SLAC, MS 69, Menlo Park, California 94025 ReceiVed January 14, 2008. ReVised Manuscript ReceiVed March 6, 2008 The interaction of the L-lactate ion (L-CH 3 CH(OH)COO - , Lact -1 ) with hematite (R-Fe 2 O 3 ) nanoparticles (average diameter 11 nm) in the presence of bulk water at pH 5 and 25 °C was examined using a combination of (1) macroscopic uptake measurements, (2) in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and (3) density functional theory modeling at the B3LYP/6-31+G* level. Uptake measurements indicate that increasing [Lact -1 ] (aq) results in an increase in Lact -1 uptake and a concomitant increase in Fe(III) release as a result of the dissolution of the hematite nanoparticles. The ATR-FTIR spectra of aqueous Lact -1 and Lact -1 adsorbed onto hematite nanoparticles at coverages ranging from 0.52 to 5.21 µmol/m 2 showed significant differences in peak positions and shapes of carboxyl group stretches. On the basis of Gaussian fits of the spectra, we conclude that Lact -1 is present as both outer-sphere and inner-sphere complexes on the hematite nanoparticles. No significant dependence of the extent of Lact -1 adsorption on background electrolyte concentration was found, suggesting that the dominant adsorption mode for Lact -1 is inner sphere under these conditions. On the basis of quantum chemical modeling, we suggest that inner-sphere complexes of Lact -1 adsorbed on hematite nanoparticles occur dominantly as monodentate, mononuclear complexes with the hydroxyl functional group pointing away from the Fe(III) center. 1. Introduction The nature of bonding between organic species and metal oxide surfaces can substantially alter the properties of metal oxide substrates and thereby affect the geochemical cycling of metals, the dissolution of redox-sensitive metal oxides, and the aggregation of colloids and nanoparticles. Low-molecular-weight (LMW) organic acids can adsorb onto metal oxide surfaces either by specific chemical interactions (chemisorption) to form inner- sphere complexes or by nonspecific interactions (physisorption) via hydrogen bonding and/or electrostatic interactions to form outer-sphere complexes. 1–11 Such organic materials can also interact to form a surface precipitate, 12,13 as well as ternary surface complexes with cations. 14 Among the LMW organic acids, lactate plays a significant environmental role because of its prevalence in soil environments resulting from its exudation by plant roots, production by fungi, and discharge by microorganisms. 15–19 However, only a few studies have investigated the sorption modes of lactate onto metal oxide surfaces. Cornell and Schindler 7 performed infrared spectroscopic experiments on lactate adsorp- tion on goethite (R-FeOOH) and amorphous Fe(III) hydroxide. On both iron (hydr)oxide surfaces studied by Cornell and Schindler, 7 lactate was suggested to adsorb as monodentate inner- sphere surface complexes. They concluded that the carboxyl group of lactate is involved in binding to goethite surfaces and that the deprotonated alcoholic hydroxyl groups also participate in binding to amorphous Fe(III) hydroxide surfaces. Filius et al. 2 studied the adsorption of lactate along with other LMW organic acids on goethite and fit the uptake data using the CD-MUSIC model. Based on their fits, they suggested that lactate adsorbs onto goethite predominantly as outer-sphere complexes. However, Filius et al. 2 also considered and included a number of inner- sphere species as minor surface complexes at the goethite/water interface in order to improve the goodness of their model fit to the experimental uptake data as a function of pH. It should be * To whom correspondence should be addressed. E-mail: jyha@ stanford.edu. Phone: 650-723-7513. Fax: 650-725-2199. † Department of Geological & Environmental Sciences, Stanford Uni- versity. ‡ Department of Chemical Engineering, Stanford University. § Hanyang University. | SLAC. ⊥ Present address: Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215. (1) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (2) Filius, J. D.; Hiemstra, T.; van Riemsdijk, W. H. J. Colloid Interface Sci. 1997, 195, 368. (3) Awatani, T.; Dobson, K. D.; McQuillan, A. J.; Ohtani, B.; Uosaki, K. Chem. Lett. 1998, 849. (4) Axe, K.; Persson, P. Geochim. Cosmochim. Acta 2001, 65, 4481. (5) Johnson, S. B.; Yoon, T. H., Jr. Langmuir 2005, 21, 2811. (6) Yoon, T. H.; Johnson, S. B., Jr. Langmuir 2004, 20, 5655. (7) Cornell, R. M.; Schindler, P. W. Colloid Polym. Sci. 1980, 258, 1171. (8) Davis, J. A. Geochim. Cosmochim. Acta 1982, 46, 2391. (9) Johnson, S. B., Jr.; Healy, T. W.; Scales, P. J. Langmuir 2005, 21, 6356. 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Med. 2004, 42, 1341. 6683 Langmuir 2008, 24, 6683-6692 10.1021/la800122v CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008