Adsorption of Phospholipids on Pyrite and Their Effect on Surface

Courtney R. Usher, Curtis A. Cleveland, Jr., Daniel R. Strongin, and Martin A. Schoonen. Environmental Science & Technology 2004 38 (21), 5604-5606...
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Langmuir 2003, 19, 8787-8792

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Adsorption of Phospholipids on Pyrite and Their Effect on Surface Oxidation Xiang Zhang,† Michael J. Borda,‡ Martin A. A. Schoonen,‡ and Daniel R. Strongin*,† Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, and Geosciences Department, The State University of New York at Stony Brook, Stony Brook, New York 11794-2100 Received February 3, 2003. In Final Form: August 11, 2003 Pyrite, FeS2, oxidation in nature has severe environmental implications. The adsorption of two-tail phospholipids on polycrystalline pyrite was investigated as a means to suppress the oxidation chemistry. Attenuated total reflection Fourier transform infrared spectroscopy suggested that the binding of longchain, two-tail lipids, such as L-R-phosphatidylcholine, hydrogenated (egg, chicken) lipid and 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine occurred primarily through the phosphate group of the lipid headgroup. The adsorption of this type of lipid on pyrite resulted in a strong suppression of the oxidation of the mineral under oxic conditions down to a pH of 1.0. No such oxidation suppression was experimentally observed when 1,2-dipropionoyl-sn-glycero-3-phosphocholine, a short-chain, two-tailed lipid, was used to form an adsorbed phase. The difference in the oxidation suppression efficacy suggested that the length of the hydrophobic tail is an important attribute in this regard. Experimental observations suggested that the long-chain, two-tail lipids formed structures on the pyrite surface that were at least in part thicker than a bilayer structure.

1. Introduction Understanding the surface reactivity of pyrite, FeS2, the most ubiquitous metal sulfide on Earth, is of importance in assessing its role as absorbent and reactant in natural environments as well as in waste environments. Under anoxic conditions, pyrite is stable, but exposure to oxic conditions, through industrial applications, such as coal mining, leads to its oxidation and dissolution.1 While the microscopic detail of the reaction is still under study, the overall oxidation of pyrite with molecular oxygen can be expressed by the following chemical equation:2

FeS2 + (7/2)O2 + H2O f Fe2+ + 2SO42- + 2H+ (1) As a result of this oxidation process, acid wastewaters (i.e., sulfuric acid formation) are formed. These waters, referred to as acid mine drainage (AMD), have impacted many waterways, resulting in a severe environmental problem.3 Given the environmental concerns pyrite oxidation presents, there has been an intense scientific effort to understand the oxidation process and to develop methods to protect the pyrite surface from the deleterious effects of oxidation. Fundamental research that has addressed the oxidation of the pyrite surface with modern surface science techniques has started to elucidate the molecular controls on pyrite oxidation.4-6 Prior research suggests that the pyrite surface is decorated with defect sites, most * To whom correspondence should be addressed. E-mail: daniel. [email protected]. Tel: (215)204-7119. Fax: (215)204-1532. † Temple University. ‡ The State University of New York at Stony Brook. (1) Eberling, B.; Nicholson, R. V.; Scharer, J. M. J. Hydrol. 1994, 157, 47-60. (2) Williamson, M. A.; Rimstidt, J. D. Geochim. Cosmochim. Acta 1994, 58, 5443-5454. (3) Banks, D.; Younger, P. L.; Arnesen, R. T.; Iversen, E. R.; Banks, S. B. Environ. Geol. (Berlin) 1998, 32, 157-174. (4) Eggleston, C. M.; Ehrhardt, J.; Stumm, W. A. M. Am. Mineral. 1996, 81, 1036-1056.

notably Fe(III) sites that are implicated in the initial oxidation chemistry on pyrite.7 It has been proposed that Fe(III)-bearing sites on the pyrite surface serve as conduits for the transfer of electrons from Fe(II) to electron acceptors such as molecular oxygen.4,8 The surface picture naturally leads to the notion that if such sites on the pyrite surface can be blocked or modified, the oxidation process can be impeded. The proof-of-principle of this concept is that the adsorption of phosphate9 or citric acid10 on these sites does inhibit the oxidation of pyrite under oxic conditions at relatively high pH values (>5), where these species have been shown to adsorb onto the pyrite surface. Under the conditions of AMD where pH is generally 3 or below, “site blockers” such as phosphate show little or no interaction with the surface and the pyrite oxidation process continues unabated.9 A scientific hypothesis recently explored in our laboratory was that for a strong Fe(III)-binding ligand, such as phosphate, to be a useful oxidation suppressor, it should be chemically stabilized against detachment from the pyrite surface at low pH. In earlier work, this hypothesis was evaluated by studying the suppression efficacy of an array of lipids.11 Two-tailed lipids with a phosphate headgroup showed the highest efficacy in batch experiments down to a pH of 2. While the batch experiments were useful in evaluating the efficacy of different lipids, the experiments provided little or no insight into the (5) Schaufuss, A. G.; Nesbitt, H. W.; Kartio, I.; Kartio, I.; Laajalehto, K.; Bancroft, G. M.; Szargan, R. J. Electron Spectrosc. Relat. Phenom. 1998, 96, 69-82. (6) Schaufuss, A. G.; Nesbitt, H. W.; Kartio, I.; Laajalehto, K.; Bancroft, G. M.; Szargan, R. Surf. Sci. 1998, 411, 321-328. (7) Guevremont, J. M.; Bebie, J.; Elsetinow, A. R.; Strongin, D. R.; Schoonen, M. A. A. Environ. Sci. Technol. 1998, 32, 3743-3748. (8) Moses, C. O.; Nordstrom, D. K.; Herman, J. S.; Mills, A. L. Geochim. Cosmochim. Acta 1987, 51, 1561-1572. (9) Elsetinow, A. R.; Schoonen, M. A. A.; Strongin, D. R. Environ. Sci. Technol. 2001, 35, 2252-2257. (10) Peiffer, S.; Stubert, I. Geochim. Cosmochim. Acta 1999, 63, 31713182. (11) Elsetinow, A.; Borda, M.; Strongin, D. R.; Schoonen, M. A. Adv. Environ. Res. 2002, 7, 969-974.

10.1021/la0300479 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003

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Figure 1. Schematics of the phosphocholine lipids used in the study showing the variability of the hydrocarbon tails.

binding of the lipid to the pyrite surface. More specifically, it could not be resolved whether the lipids bind to the pyrite surface through, for example, the phosphate group and what type of surface structures were formed by the adsorbed lipid. The present contribution uses attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy to study the adsorption process of two lipids shown in the previous study11 to reduce pyrite oxidation. The main objective was to resolve how two-tail lipids with a phosphate headgroup interact with the pyrite surface. In addition, complementary aqueous batch oxidation experiments were conducted at pH values lower than 2. The objective of these experiments was to test the efficacy of a few selected two-tail lipids at the extreme pH conditions that are often encountered in AMD. 2. Experimental Section The majority of the studies reported on in this contribution used the L-R-phosphatidylcholine, hydrogenated (egg, chicken) lipid (referred to as egg PC) and 1,2-bis(10,12-tricosadiynoyl)sn-glycero-3-phosphocholine lipid (referred to as 23:2 diyne PC, where 23 refers to the length of the carbon chain and 2 refers to the number of unsaturated bonds in each chain). These two lipids showed effectiveness in the suppression of pyrite oxidation in a previous study.11 A smaller set of experiments presented in this contribution used 1,2-dipropionoyl-sn-glycero-3-phosphocholine (referred to as 3:0 PC lipid). This short-tail lipid was used in our studies to help determine the importance of tail length in pyrite oxidation suppression. Each lipid was obtained from Avanti Polar Lipids, egg PC and 23:2 diyne PC as a powder and 3:0 PC as an oil. The lipid structures vary in the length of the hydrocarbon tail, and structural schematics of each lipid are displayed in Figure 1. Solutions were prepared by adding 15 µmol of the lipid of interest to 10 mL of 0.10 M NaCl solution at room temperature. The lipid suspension was stirred and subsequently placed in an ultrasonic for 10 min, similar to procedures reported elsewhere.12 The purpose of the ultrasonic treatment was to homogenize the lipid particles, presumably helping to disrupt large multilamellar vesicles (LMVs) and to form smaller structures ranging in size down to unilamellar vesicles. All pyrite powder samples used in this study were synthesized under hydrothermal conditions.13 X-ray diffraction was used to verify the pyrite structure. The powder was cleaned prior to use, (12) Kim, J.; Kim, G.; Cremer, P. S. Langmuir 2001, 17, 7255-7260.

Zhang et al. by exposing the material to a HCl solution (pH 2), that was deoxygenated by bubbling nitrogen through the solution for 2 h. This procedure9 was shown in prior studies in our laboratory to create relatively contaminant free and stoichiometric surfaces. The clean pyrite particles were transferred to a nitrogen-filled glovebag and added to the lipid suspension. After 10 min, the supernatant was decanted from the reaction vial and the pyrite particles were dried in a stream of ultrapure nitrogen. To carry out the ATR-FTIR experiments presented in this contribution, dry pyrite or lipid-coated pyrite particles were placed on a Ge-45° parallelogram ATR lens. A water/ethanol solution was added sparingly to the powder while on the lens. After the system was dried in a nitrogen flow, the particles showed good adhesion to the Ge surface, which is essential for the subsequent spectroscopic analysis. A Nicolet 580 research FTIR spectrometer was used to obtain the vibrational data. All IR data were recorded at 1 cm-1 resolution with 1028 scans. Attempts to analyze the solid-water interface with ATR-FTIR failed due to the lack of adhesion of the lipid/pyrite sample to the Ge lens in the aqueous environment. Two types of experiments were carried out to determine the effect of the adsorbed lipid on pyrite oxidation. In the first, pyrite samples with and without lipid were individually deposited on the ATR lens. The sample was attached to a custom-built Teflon flow cell14 that was mounted within the spectrometer. Oxygen saturated with water vapor was flowed over the sample for varying times. At specified times, the cell was purged with nitrogen and ATR-FTIR data were obtained. The gaseous reactant mixture was obtained by bubbling oxygen through water prior to admission into the reaction flow cell. In these experiments, the concentrations of iron and sulfur oxidation products, which were formed during the exposure to the oxygen/ water gaseous mixture, were used to determine the extent of pyrite oxidation. In the second type of experiment, pyrite samples with and without lipid were exposed to solutions of varying pH. The amount of iron release from the pyrite into solution as a function of time was an appropriate progress variable to determine the extent of pyrite oxidation in these experiments (see eq 1). The amount of Fe released during the oxidation was determined by the ferrozine analytical method.15 Prior to the introduction of ferrozine, which yields a deep purple color upon complexation with Fe(II), ascorbic acid was added to the solution to reduce any Fe(III) in solution to Fe(II). The concentration of Fe in solution was determined by UV-vis spectroscopy (Perkin-Elmer) by analyzing the strong ferrozine-Fe complex absorption at 560 nm. The absorbance was calibrated using Fe standards.

3. Results and Discussion 3.1. ATR-FTIR of the Two-Tail Lipid. Figure 2 exhibits a typical ATR-FTIR spectrum for egg PC deposited on a Ge ATR crystal. The sample was prepared by completely solvating the lipid in chloroform, transferring a drop of sample solution to the ATR crystal, where it was allowed to dry, depositing a thick layer of lipid. Table 1 summarizes the assignments for the vibrational modes. In the following discussion, we will generally derive many of our conclusions concerning the bonding of the lipid to the pyrite surface from the P-O stretching region [υa (1250 cm-1) and υs (1093 cm-1)], C-O stretching (1720 cm-1), and the C-H stretching region in the range of 28003000 cm-1. 3.2. FTIR of Two-Tail Lipids on Pyrite. Figure 3 exhibits ATR-FTIR vibrational data of egg PC for two experimental conditions. In the first experiment, pyrite was exposed to a lipid suspension. After 30 min, a time in which all the pyrite settled to the bottom, the supernatant was decanted to remove the vast majority of lipid (13) Bebie, J.; Schoonen, M. A. A.; Fuhrmann, M.; Strongin, D. R. Geochim. Cosmochim. Acta 1998, 62, 633-642. (14) Borda, M. J.; Strongin, D. R.; Schoonen, M. A. Spectrochim. Acta, Part A 2003, 59A, 1103-1106. (15) Stookey, L. L. Anal. Chem. 1970, 42, 779-781.

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Figure 2. IR spectrum of the egg PC lipid deposited on a Ge ATR crystal. The clean Ge ATR crystal was used as the reference spectrum. Table 1. Assignments of the IR Absorption of Lipid (Egg PC) label a b c d e f g h i j k l m n o p

frequency (cm-1) 3028 2955 2936 2919 2896 2870 2850 1739 1470 1253 1175 1093 1064 967 816 722

group vibrations choline methylene methyl methylene methylene methylene methylene carbonyl methylene phosphate ester phosphate phosphate-ester choline phosphate methylene

assignment N+-(CH

ν(C-H) in 3) νa(CH3) CH3 Fermi resonance νa(CH2) CH2 Fermi resonance νs(CH3) νs(CH2) ν(CdO) δ(CH2) scissoring νa(PO2-) νa(C-O) νs(PO2-) ν(C-O-PO2-) νa(N+-(CH3)3) νa(P-(OC)2) γr(CH2)

ref 28 29 30 20 30 29 20 20 20 20, 31 24 23, 31 17 20 20 20

that remained free in solution. In essence, these FTIR data are associated with the fraction of lipid that adsorbed to the pyrite in the aqueous environment (referred to as thin in the figure). Data also are presented in the figure that were obtained by allowing the pyrite/lipid solution to dry under a stream of nitrogen. This treatment resulted in a thick lipid layer on the pyrite surface (referred to as thick in the figure) that was useful as a reference. Figure 3a displays FTIR data for the C-H and P-O stretching regions for a thin and thick layer of egg PC on pyrite. The C-H stretching modes for the thin and thick data are similar to within the resolution of our spectrometer. The P-O stretching region, however, shows differences between the two coverage regimes. The differences are emphasized in Figure 3b, which highlights the P-O vibration region and shows that there are two contributions to the P-O asymmetric (υa) and symmetric (υs) stretching features for the thin adsorbed lipid layer. To better illustrate these contributions, the symmetric (υs) stretching region is further highlighted in Figure 4. The thin layer lipid data are fitted with two Gaussians at 1110 cm-1 (feature A) and 1095 cm-1 (feature B). We believe that the smaller contribution, feature A, shifted 15 cm-1 from the major contribution, results from the interaction of the phosphate group of the lipid with the pyrite surface. This contribution is less noticeable in the thick layer spectrum, presumably due to the relatively small contribution of lipid that is directly bound to the pyrite surface, relative to the lipid making up the bulk of the surface layer. This interpretation of these data led to the conclu-

Figure 3. IR data exhibiting the C-H and P-O vibrational regions (a) for a thin and a thick egg PC layer on pyrite. (b) Data exhibiting an expanded view of the P-O stretching region for a thin and thick lipid layer of egg PC on pyrite. For reference, the absorbance scale is included in the figure as an inset.

Figure 4. An expanded view of the symmetric P-O stretching region for thin and thick egg PC layers on pyrite. Two contributions contribute to the region, and they are associated with phosphate binding to the pyrite surface (peak A) and phosphate contained in lipid that is not intimately bound to the pyrite surface (peak B).

sion that a P-O υs mode shift to higher energy (and the υa mode to lower energy based on Figure 3b) is associated with lipid directly bound to the surface. Note that comparing intensities associated with features A and B between the thin and thick data is not possible on a quantitative level. Differences in the geometry of how particles rest on the ATR lens greatly affect the observed intensities.16 In our experiments, while the relative intensities between features A and B remained relatively constant for similar lipid surface concentrations, the absolute intensities between experimental runs varied, presumably due to subtle differences in how the pyrite adhered to the ATR lens. (16) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587-3597.

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Figure 5. IR data exhibiting the C-H and P-O vibrational regions (a) for a thin and thick 23:2 diyne PC layer on pyrite. (b) Data exhibiting an expanded view of the P-O stretching region for a thin and thick layer of 23:2 diyne PC on pyrite. For reference, the absorbance scale is indicated in the figure.

Different hydration states of the lipid could lead to shifts in the P-O modes, but in this scenario both P-O modes should shift to lower energy,17,18 which is contrary to our experimental observation. Furthermore, prior studies have shown that changes in the lipid hydration may also lead to changes in the C-H stretching region, something that is not experimentally observed in our experiment.19,20 Hence, we infer from our data that the experimentally observed shift in P-O modes is due to the adsorption process and not due to differences in lipid hydration.17,18 We also carried out similar ATR-FTIR studies of 23:2 diyne PC to determine whether similar shifts in the P-O stretching region occurred. Figure 5 exhibits these data, and similar to the egg PC circumstance, there is no noticeable shift in the C-H stretching region (Figure 5a). There are differences in the P-O stretching region between the thin and thick lipid layer data, but the differences appear to be more subtle than in the egg PC case. Figure 5b shows that as in the case of the egg PC data, there is spectral weight in the υs and υa mode contributions that are associated with lipid directly bound to the pyrite surface. The υa and υs modes are shifted by no more than 10 cm-1 to lower and higher energy, respectively, relative to the position associated with the majority of the lipid in the thick surface layer. On the basis of these data, we suspect that, as was the case for egg PC, the phosphate group in the headgroup of the 23:2 (17) Hubner, W.; Blume, A. Chem. Phys. Lipids 1998, 96, 99-123. (18) Pohle, W.; Bohl, M.; Bohlig, H. J. Mol. Struct. 1991, 242, 333342. (19) Gauger, D. R.; Selle, C.; Fritzsche, H.; Pohle, W. J. Mol. Struct. 2001, 565-566, 25-29. (20) Binder, H.; Anikin, A.; Kohlstrunk, B.; Klose, G. J. Phys. Chem. B 1997, 101, 6618-6628.

Zhang et al.

diyne lipid plays a significant role in the adsorption process. Prior calculations on substituted phosphate suggest that the direction of the υa and υs shifts experimentally observed may be due to a decrease of the O-P-O angle in the adsorbed layer.18 Such a perturbation of the phosphate group is not unexpected, since prior research in our laboratory indicates that phosphate adsorbs strongly to Fe(III)-containing sites on the pyrite surface.9 A surface picture having the phosphate group of the lipid binding with surface Fe(III) might also be expected. Another important fundamental issue concerning the adsorption of the two-tail lipid is the structure of the adsorbed phase. Prior studies have shown that two-tail lipids not only form a bilayer in solution but often form such structures on solid surfaces.12,21,22 In the case of egg PC, we have used a protocol for the preparation of the lipid that is similar to that used in a prior study that showed that this lipid formed a bilayer structure on a silicon oxide surface.12 The sonication of an egg PC suspension is thought to reduce a majority of any multilamellar lipid vesicle structures to unilamellar ones. In our experimental studies, sonication of the egg PC above its gel temperature produced marked visual changes in the lipid suspension (a reduction in the translucence of the solution), consistent with the prior study and the production of a preponderance of unilamellar vesicles. Nevertheless, our experiments cannot unambiguously show the nature of the adsorbed egg PC thin layer structure. ATR-FTIR data lend some insight into the structure of the adsorbed lipid on pyrite. It must be kept in mind, however, that ATR-FTIR data were obtained on the samples after removal from the aqueous phase, so we cannot be certain that the FTIR data of the lipid are representative of its structure at the mineral-liquid interface. A prior study does show that phospholipid bilayer formation occurring at the solid-liquid interface can survive even after the liquid phase is removed.23 FTIR data obtained in the present study do resolve at least two lipid environments: one presumably is lipid directly bound to pyrite, and the other is coordinated to neighboring lipid rather than the pyrite surface. An analysis of Figure 4, which deconvolutes the two contributions to the υs mode associated with egg PC, suggests that the latter contribution (feature B) is approximately 3 times greater in spectral area. An interpretation of the ATR-FTIR data in this manner would suggest that the egg PC lipid forms at least some structures on the pyrite surface that are thicker than a single bilayer. In the case of a bilayer structure, one might expect instead approximately equal contributions from each lipid environment. A visual inspection of the P-O stretching modes for the 23:2 diyne PC shown in Figure 5 leads to a similar conclusion. The experimental observation that both egg PC and 23:2 diyne PC form similar lipid structures on pyrite is interesting. The 23:2 diyne PC suspension, which pyrite was exposed to, showed a much greater degree of cloudiness than the egg PC, consistent with its lower solubility in water and the presence of multilamellar vesicles. While the mechanism by which lipid growth proceeds at the solid-liquid interface is not certain, a prior study has suggested that the growth may proceed through the interaction of a multilamellar vesicle, through the rolling (21) Sackmann, E. Science 1996, 271, 43-48. (22) Raedler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 45394548. (23) Stephens, S. M.; Dluhy, R. A. Thin Solid Films 1996, 284-285, 381-386.

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of juxtaposed bilayers.22 Hence, the formation of a similar layer structure from the 23:2 diyne lipid suspension may be reasonable. To obtain a better understanding of the extent of lipid adsorption, we carried out experiments to estimate the coverage of the 23:2 diyne lipid on the pyrite surface. The goal here was to determine whether the lipid adsorbed uniformly over pyrite or whether it was restricted to a fraction of the pyrite surface area. Pyrite, which was coated with lipid and yielded an ATR-FTIR spectrum similar to the thin layer spectrum displayed in Figure 5, was exposed to chloroform. The high solubility of the lipid in chloroform resulted in the removal of the vast majority of the adsorbed lipid from the pyrite surface. Analysis of the pyrite after this procedure showed that an insignificant amount of lipid remained on the surface. The pyrite was then separated from the chloroform/lipid solution, and the concentration of lipid was determined with UV-vis spectrophotometry (via comparison of the 255 nm wavelength absorption with a standard curve). This analysis showed that approximately 2.04 × 10-9 mol of lipid was adsorbed to every milligram of pyrite. The experimentally determined surface area of the pyrite sample from Brunauer-Emmett-Teller (BET) measurements was 0.75 m2/g. If the cross-sectional area of the lipid is assumed to be 70 Å2,24 then 1.78 × 10-9 mol of lipid per milligram of pyrite would be required to form a single lipid high layer on the entire pyrite surface. If we assume that a single lipid layer is not present, the calculation suggests that there is less lipid adsorbed than would be needed to cover the entire pyrite surface with bilayer or thicker structures. This inference from the calculation suggests that the lipid covers only a fraction of the total pyrite surface. While the lipid forms an incomplete surface layer, it leads to a significant change in the oxidation properties of pyrite, as discussed below. 3.3. Effect of the Adsorbed Lipid Layer on Pyrite Oxidation. 3.3.1. Exposure of Pyrite to Gaseous O2/H2O. Panels a and b of Figure 6 exhibit ATR-FTIR data for pyrite and pyrite coated with 23:2 diyne PC lipid, respectively, that was exposed to gaseous H2O/O2 for various time intervals. Visual inspection of Figure 6a shows that there is a progressive increase in spectral weight near 1100 cm-1 and in the 750-1000 cm-1 range, features which are interpreted as being due to the evolution of sulfate25 and iron oxyhydroxide,26 respectively. The intensity of the sulfate and iron oxyhydroxide modes during the oxidation of pyrite is significantly different when lipid is present as an adsorbed layer. We base this assertion on a comparison of data in Figure 6a with that data presented in Figure 6b. Such a comparison leads to the conclusion that sulfate oxidation is largely inhibited and iron oxyhydroxide production occurs at a much slower rate when lipid is present as an adsorbed phase on the pyrite surface. Note that the intensity of the iron oxyhydroxide modes shown in Figure 6b is approximately 10 times smaller than in part a of the figure. 3.3.2. Pyrite Oxidation as a Function of Solution pH. Figure 7 exhibits the iron concentration in solution as a function of time during pyrite (with and without 23:2 diyne lipid) oxidation at pH values of 2 and 1. At all the pH values, the adsorbed lipid suppressed the amount of pyrite oxidation, relative to clean pyrite. An analysis of the pH 2 data shows that after 115 h, the extent of (24) Hunt, R. D.; Mitchell, M. L.; Dluhy, R. A. J. Mol. Struct. 1989, 214, 93-109. (25) Hug, S. J. J. Colloid Interface Sci. 1997, 188, 415-422. (26) Ishikawa, T.; Cai, W. Y.; Kandori, K. J. Chem. Soc., Faraday Trans. 1992, 88, 1173-1177.

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Figure 6. FTIR spectra of (a) clean pyrite and (b) lipid/pyrite after exposure to oxygen saturated with water vapor for various times. Pyrite shows a much higher concentration of oxidation products in the absence of lipid (sulfate near 1100 cm-1 and iron oxyhydroxide near 860 cm-1). Notice the difference in absorbance scales for panels a and b. The spectra in panel b are presented using lipid/pyrite at “0” time as the reference spectra (i.e., any lipid modes are canceled out from the spectra). For reference, the absorbance scale is indicated in the figure.

Figure 7. The amount of Fe released into solution as a function of time for pyrite, with and without adsorbed lipid, at two different pH values. At both pH values, egg PC and 23:2 diyne PC lipid show a strong suppression of pyrite oxidation. FTIR data showed that the concentration of lipid on the pyrite surface remained constant over the entire time period under the different pH environments.

oxidation has been reduced by 70% when lipid is present, noting that the Fe concentration for clean and lipid-covered pyrite is 1 and 0.3 µmol L-1, respectively, after this period of time. After 160 h of reaction at pH 1, the aqueous Fe concentration is 0.3 and 0.1 µmol L-1 for clean and lipidcovered pyrite, respectively, a suppression of 67%, roughly the same lipid-induced suppression exhibited at pH 2. The experimental observation that the oxidation rate is slower at lower pH is consistent with previous kinetics

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Figure 8. The amount of Fe released into solution as a function of time at pH 2 for pyrite without adsorbed lipid (control) and pyrite with adsorbed egg PC, 23:2 diyne PC, and 3:0 PC. The headgroups of the three lipids are similar, but the much stronger suppression for the egg PC and 23:2 PC suggests that the long hydrocarbon tail is a necessity for effective oxidation suppression under the experimental conditions used in this study.

studies showing that the total reaction rate is proportional to the inverse of proton concentration (with an order