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Hydroxamate Complexes in Solution and at the Goethite-Water Interface: A Cylindrical Internal Reflection Fourier Transform Infrared Spectroscopy Study Britt A. Holme´n,*,† M. Isabel Tejedor-Tejedor,‡ and William H. Casey† Department of Land, Air and Water Resources, University of California, Davis, California 95616, and Water Chemistry Program, University of Wisconsin, Madison, Wisconsin 53706 Received September 30, 1996. In Final Form: January 27, 1997X The infrared spectra of aqueous solutions of acetohydroxamic acid (aHA), an analogue for important iron(III) chelating ligands in soils and groundwaters, and of suspensions of goethite with adsorbed aHA were measured using a cylindrical internal reflectance cell. Using molecular orbital theory and data from the literature on the infrared spectra of solid hydroxamates and metal-hydroxamate complexes, we identify the spectral changes occurring upon complexation with both aqueous Fe(III) and Fe atoms on the surface of goethite. The effect of pH upon the solution and surface spectra and the possibility of aHA hydrolysis at the goethite surface at pH ) 3 were also examined. There was no significant difference in adsorbed aHA spectra between pH ) 3 and pH ) 6, indicating identical surface complexes form over the pH range where the solution phase complex changes from the bis- to the tris(hydroxamato)iron(III) complex. The infrared technique was not capable of detecting the low concentrations of hydrolysis products that form at the mineral surface over a week-long incubation.
Introduction Adsorption of organic solutes to mineral surfaces is critical to understanding solute transport in aquatic environments because many key chemical reactions are enhanced by adsorption. Some important surface-mediated processes are ligand-promoted mineral dissolution,1-7 photocatalysis,8 pesticide hydrolysis,9-14 and redoxpromoted mineral dissolution and degradation of organic contaminants.15-17 The surface complex stoichiometry and †
University of California. University of Wisconsin. X Abstract published in Advance ACS Abstracts, March 1, 1997. ‡
(1) Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 18471860. (2) Stumm, W.; Wollast, R. Rev. Geophys. 1990, 28, 53-69. (3) Ludwig, C.; Casey, W. H.; Rock, P. A. Nature 1995, 375, 44-47. (4) Ludwig, C.; Devidal, J.-L.; Casey, W. H. Geochim. Cosmochim. Acta 1996, 60, 213-224. (5) Ludwig, C.; Casey, W. H. J. Colloid Interface Sci. 1996, 178, 176185. (6) Zinder, B.; Furrer, G.; Stumm, W. Geochim. Cosmochim. Acta 1986, 50, 1861-1869. (7) Nowak, B. Behavior of EDTA in Groundwatersa study of the surface reactions of metal-EDTA complexes. Ph.D., Swiss Federal Institute of Technology, 1996. (8) Sulzberger, B.; Laubscher, H.; Karametaxas, G. Photoredox reactions at the surface of iron(III) (hydr)oxides. In Aquatic and Surface Photochemistry; Helz, G., Zepp, R., Crosby, D. G., Eds.; Lewis Publ.: Boca Raton, FL, 1994; pp 53-73. (9) Stone, A. T. J. Colloid Interface Sci. 1988, 132, 81-87. (10) Stone, A. T. J. Colloid Interface Sci. 1989, 127, 429-441. (11) Stone, A. T.; Torrents, A. The role of dissolved metals and metalcontaining surfaces in catalyzing the hydrolysis of organic pollutants. In Environmental Impact of Soil Component Interactions: Natural and Anthropogenic Organics; Huang, P. M., Berthelin, J., Bollag, J.-M., McGill, W. B., Page, A. L., Eds.; CRC Press: Boca Raton, FL, 1995; Vol. I, pp 275-298. (12) Torrents, A.; Stone, A. T. Environ. Sci. Technol. 1991, 25, 143149. (13) Torrents, A.; Stone, A. T. Soil Sci. Soc. Am. J. 1994, 58, 738745. (14) Baldwin, D. S.; Beattie, J. K.; Coleman, L. M.; Jones, D. R. Environ. Sci. Technol. 1995, 29, 1706-1709. (15) Xyla, A. G.; Sulzberger, B.; Luther, G. W., III; Hering, J. G.; van Cappellen, P.; Stumm, W. Langmuir 1992, 8, 95-103. (16) LaKind, J. S.; Stone, A. T. Geochim. Cosmochim. Acta 1989, 53, 961-971. (17) Klausen, J.; Trober, S. P.; Haderlein, S. B.; Swarzenbach, R. P. Environ. Sci. Technol. 1995, 29, 2396-2404.
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structure change to reflect different solution conditions (i.e., pH or ligand concentration) and affect the mechanism, rate, and extent of the subsequent surface reactions. There are, unfortunately, very few techniques for determining adsorbate structures directly. Cylindrical internal reflection Fourier transform infrared (CIR-FTIR) is a relatively new technique that is capable of determining the structure of mineral-water interface complexes in situ under favorable conditions.18-20 The present study was conducted to establish the structure of the goethite surface complex with acetohydroxamic acid, a simple analog compound for the highmolecular-weight hydroxamate siderophores produced by soil microbes to sequester iron from the environment. Hydroxamate Siderophores and Hydroxamate Solution Chemistry. Hydroxamate siderophores, having paired carbonyl and hydroxylamino (-NOH) functional groups (Figure 1), are used in the transport of iron to microbial cells in soils.21-32 These microbial metabolites (18) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6, 979-987. (19) Yost, E. C.; Tejedor-Tejedor, M. I.; Anderson, M. A. Environ. Sci. Technol. 1990, 24, 822-828. (20) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587-3597. (21) Bossier, P.; Hofte, M.; Verstraete, W. Adv. Microbial Ecol. 1988, 10, 385-414. (22) Boyer, G. L.; Aronson, D. B. Iron uptake and siderophore formation in the actinorhizal symbiont Frankia. In Biochemistry of Metal Micronutrients in the Rhizosphere; Manthey, J. A., Crowley, D. E., Luster, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; pp 41-54. (23) Crowley, D. E.; Reid, C. P. P.; Szaniszlo, P. J. Microbial siderophores as iron sources for plants. In Iron Transport in Microbes, Plants and Animals; Winkelmann, G., van der Helm, D., Neilands, J. B., Eds.; VCH Pub.: New York, 1987; pp 375-385. (24) Gran, L. Appl. Environ. Microbiol. 1994, 60, 2132-2136. (25) Hersman, L.; Lloyd, T.; Sposito, G. Geochim. Cosmochim. Acta 1995, 59, 3327-3330. (26) Kurzak, B.; Kozlowski, H.; Farkas, E. Coord. Chem. Rev. 1992, 114, 169-200. (27) Matzanke, B. F. Structures, coordination chemistry and function of microbial iron. In Handbook of Microbial Iron Chelators; Winkelmann, G., Ed.; CRC Press: Boca Raton, FL, 1991; pp 15-65. (28) Raymond, K. N.; Mueller, G.; Matzanke, B. F. Top. Curr. Chem. 1984, 123, 49-102. (29) Waid, J. S. Hydroxamic acids in soil systems; Marcel Dekker: New York, 1975; Vol. 4. (30) Walter, A.; Romheld, V.; Marschner, H.; Crowley, D. E. Soil Biol. Biochem. 1994, 26, 1023-1031.
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Figure 2. Speciation diagram for Fe(III) and aHA at a ligand to metal ratio of 10. Note the change in species from bis- to tris(hydroxamate) complex at pH ) 4. Fe(III) hydrolysis species (i.e., FeOH+2, Fe(OH)2+) concentrations do not exceed 1% of the total Fe(III) over the pH range 3 to 10.
Figure 1. Structures of two natural siderophores (a, b) and acetohydroxamic acid (c, d). (a) Ferrioxamine B, a linear trihydroxamate siderophore, forms a complex that encapsulates Fe(III). (b) Ferrichrome is a cyclic trihydroxamate siderophore shown here in its “desferri” form. Acetohydroxamic acid (aHA) structures include two keto (c) and two iminol (d) conformers.
usually exist with three hydroxamate moieties in molecules such as ferrioxamine B and desferriferrichrome (Figure 1a,b). Acetohydroxamic acid (aHA), our model compound, can exist in the keto (Figure 1c) and iminol (Figure 1d) forms, but the keto form is the dominant tautomer in aqueous solutions.33-37 Furthermore, rotation about the C-N bond gives the cis and trans conformers of each tautomer. In aqueous solution, the cis rotation allows intramolecular hydrogen bonding and it is the stable conformation.38 There has been considerable debate in the literature concerning the site of deprotonation of hydroxamates.33-39 Some hydroxamates deprotonate at nitrogen (e.g., benzohydroxamic acid in methanol34), and others lose the hydroxyl proton (e.g., acetohydroxamic acid34). Infrared spectra will differ considerably depending on which site deprotonates. (31) Watteau, F.; Berthelin, J. Symbiosis 1990, 9, 59-67. (32) Watteau, F.; Berthelin, J. Eur. J. Soil Biol. 1994, 30, 1-9. (33) Exner, O.; Kakac, B. Collect. Czech. Chem. Commun. 1963, 28, 1656-1663. (34) Bagno, A.; Comuzzi, C.; Scorrano, G. J. Am. Chem. Soc. 1994, 116, 916-924. (35) Bauer, L.; Exner, O. Angew. Chem., Int. Ed. Engl. 1974, 13, 376-384. (36) Yamin, L. J.; Ponce, C. A.; Estrada, M. R.; Vert, F. T. THEOCHEM 1996, 360, 109-117. (37) Ventura, O. N.; Rama, J. B.; Turi, L.; Dannenberg, J. J. J. Am. Chem. Soc. 1993, 115, 5754-5761. (38) Hadzi, D.; Prevorsek, D. Spectrochim. Acta 1957, 10, 38-51. (39) Shenderovich, V. A.; Ryaboi, V. I.; Kriveleva, E. D.; Ionin, B. I.; Vainshenker, I. A.; Dogadina, A. V. Zh. Obsh. Khim. 1979, 49, 174651.
Hydroxamate complexation with Fe(III) in solution is a strong function of pH (Figure 2). At pH 4 the tris(hydroxamate) complex is formed. Of significance to our earlier work40 is the observation that there is a change in the number of hydroxamate ligands bound to Fe(III) at pH ) 4. At this pH we found a significant drop in the rate of goethite dissolution in the presence of 0.01 M aHA.40 It is conceivable that the type of surface complex on goethite changes at this pH either via (a) changes in the protonation state of the ligand,20 (b) changes in the number of bonds to the surface metal atom, or (c) a change in the adsorbed species due to ligand hydrolysis or oxidation prior to adsorption. Infrared examination of the surface complex is used here to evaluate these hypotheses. Methods Goethite Preparation. Goethite was prepared via the method of Atkinson et al. (1967).41 Briefly, 100 g of ferric nitrate (Fisher Scientific) was added to 1650 mL of 18 MΩ, ultrapure water (Barnstead Nanopure), which had been filtered through a 0.2 µm, in-line filter. Slowly, 400 mL of 2.5 M potassium hydroxide was added to the vessel, causing precipitation of an amorphous Fe(III) solid. The suspension was incubated at 60 °C for 27 h with intermittent mixing in order to crystallize the solid. The supernatant was removed and the solid was repeatedly washed with 18 MΩ water (via centrifugation/decantation) until the pH of the supernatant solution was near neutral. Solids were stored as stirred aqueous suspensions under argon. Transmission-electron microscopy indicated that the particles are lath-shaped, approximately 25 nm wide and up to 0.5 mm long (see Figure 3, ref 40). The surface area was determined to be 43.3 m2 g-1 using the static BET method42 and a Micromeritics Gemini 2360 analyzer. X-ray diffractometry confirmed that the (40) Holme´n, B. A.; Casey, W. H. Geochim. Cosmochim. Acta 1996, 60, 4403-4416. (41) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Phys. Chem. 1967, 71, 550-558. (42) Lowell, S. Introduction to Powder Surface Area; WileyInterscience: New York, 1979.
Hydroxamate Complexes in Solution solid is goethite, and we have no evidence for other minerals or amorphous solids in the suspension. Infrared Technique. A Nicolet 750 Series II spectrometer was used to collect spectra interferometrically for both aqueous solutions and goethite suspensions. This spectrometer has a Hg-Cd-Te (MCT/A) detector and cylindrical internal reflection (CIR) cell employing a 35° ZnSe crystal rod in a stainless-steel sample boat. Single-beam spectra were averages of 1000 and 3000 scans for solutions and suspensions, respectively, collected over the 700-4000 cm-1 range with a resolution of 4 cm-1 and were compared to an empty cell background spectrum to give sample absorbance. All sample spectra were corrected by subtracting a reference spectrum. For aqueous solutions, the reference was either pH-adjusted 18 MΩ (Milli-Q) water or D2O. For suspensions, the reference was the supernatant obtained after final centrifugation of the suspension. Spectra were also corrected for water vapor within the spectrometer using a measured water vapor spectrum. All sample spectra were collected immediately after the reference spectra without removing the CIR cell from the instrument. Infrared spectra intensities depend strongly on the degree of dispersivity of the suspension, which is determined by goethite surface charge. Since solution variables such as pH, ionic strength, and the presence of adsorbing ligands affect goethite surface charge, these variables affect the characteristics of the double layer at the mineral surface which in turn affect the IR spectra of interfacial solvent and solute species.18,43 Thus, it is very difficult to obtain representative goethite-only reference spectra (for subtraction from the suspension spectra) without introducing spectral artifacts in the regions of the ligand’s IR bands. For this reason, supernatant solutions over the goethiteaHa suspensions were used as reference spectra and allow qualitative comparisons of suspension spectra as a function of pH. Any changes in complex structure with pH will be indicated by shifts in the frequency of the ligand’s IR bands under different experimental conditions. In other words, changes in band locations from solution-to-suspension and in suspensions as a function of pH, rather than changes in peak intensities, are useful, albeit qualitative, indicators of changes in bonding and in the formation of new types of surface complexes. Preparation of Solutions. Aqueous solutions of acetohydroxamic acid (Aldrich, 98%), hydroxylamine hydrochloride (Aldrich, 99%), sodium acetate, and sodium perchlorate were made up at 0.1 M in Milli-Q water. The pH of these solutions was adjusted using either 0.1 M HClO4, 0.1 M HCl , or 0.1 M KOH. Solutions of aHA and hydroxylamine hydrochloride in D2O were made up by first exchanging protons for deuterium by drying the required amount of solid with a small amount of D2O in a vacuum desiccator. The solutions in D2O were analyzed by FTIR prior to diluting the solutions for use in the adsorption experiments with goethite. Ferric chloride/aHA solutions (0.12 M/0.1 M) were prepared in both water and D2O and pH(D) ) 2. Under these conditions, the monoacetohydroxamate-Fe(III) complex, [Fe(H2O)4CH3C(O)N(O)H]2+, is dominant. The reference solution for the water Fe-aHA sample was 0.12 M ferric chloride at pH 2. The reference for the heavy water Fe-aHA complex sample was pure D2O. Goethite Suspensions. Goethite suspensions from the aHA adsorption experiments were analyzed by CIR-FTIR at suspension concentrations of 60 g/L. This solid-to-water ratio was achieved by centrifuging two 20 mL 6 g/L goethite-0.02 M aHA suspensions that had been equilibrated for 1 h at the pH of interest at 3700g and combining the pellets in 4 mL of supernatant. The ionic strength in all suspensions was 0.1 M, achieved using NaClO4 in H2O and NaCl in the D2O samples. All suspensions were sonicated prior to FTIR analysis to ensure dispersal of the solids. In some cases, the suspension was reanalyzed after diluting to 30 g/L with the supernatant in order to improve the intensity of the goethite bands. Goethite suspensions in D2O were prepared by twice washing, centrifuging, and decanting D2O from an aqueous goethite suspension. These goethite/D2O suspensions were then used in the D2O-aHA-goethite adsorption experiments. The aHA ligand hydrolyzes slowly to acetate and hydroxylamine at pH
