Interactions of Oxygen and Water Molecules with Pyrite Surface: A

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Interactions of Oxygen and Water Molecules with Pyrite Surface: A New Insight Yuqiong Li, Jianhua Chen, Ye Chen, Cuihua Zhao, Yibing Zhang, and Baolin Ke Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04112 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Interactions of Oxygen and Water Molecules with Pyrite Surface: A New Insight Yuqiong Li†, Jianhua Chen*,‡,§, Ye Chen†, Cuihua Zhao*,†, Yibing Zhang†, Baolin Ke† †

School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China

‡Innovation

Center for Metal Resources Utilization and Environment Protection, Guangxi University, Nanning, China.

§Guangxi

Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, China. ABSTRACT: Pyrite is the most common sulfide in nature, and it is well known for its roles in the acid mine drainage (AMD), flotation separation of useful metal (Cu, Pb, Zn, Mo) sulfide minerals, optoelectronic and photovoltaic application, pneumoconiosis, even role in origin of life. However, the detailed oxidation behaviors of pyrite are still unclear and not well understood. New oxidation pathways by O2 on pyrite (100) surface are found in this work for the first time using density functional (DFT) theory simulation, i.e. besides Fe sites, S sites are also possible oxidation sites in the initial oxidation state of pyrite, where easier and stronger oxidation may occur. This is the first time to confirm the other researchers' conjecture on the direct oxidation of S sites, which explains the isotopic composition experiments that minor O2 is permanently incorporated into SO42– during pyrite oxidation (O in SO42– is mainly derived from water). We constructed various H2O–O2 co–adsorption models on pyrite surface considering the adsorption sequence of H2O and O2. It is found that H2O molecule undergoes step–wise dissociation in the presence of O2 molecule. Hydroxyl radical ∙ OH is the reactive oxygen species during H2O dissociating. Cyclic voltammetric measurements confirm the presence of ∙ OH. In addition, H2O2 may also be formed on the surface in terms of H2O–then–O2 sequence adsorption. Keywords: pyrite; oxidation; water molecule; oxygen molecule; hydroxyl radical ■ INTRODUCTION Pyrite (FeS2) is the most common and widely distributed sulfide mineral on the earth. It has been pointed out that acid mine drainage (AMD) from coal mines and metal–sulfide mines can be largely resulted from pyrite oxidation which is very hazardous and causes serious environmental problems. In addition, it has been suggested that pneumoconiosis is related to the presence of FeS2 in the coals because the oxidative decomposition processes of pyrite could damage lung tissue.1–2 The oxidation of pyrite is also very important and desired for its flotation separation from useful metal sulfide minerals (Cu, Pb, Zn, and so on), and for the leaching process of precious metals such as gold because pyrite is the main gold– bearing mineral. Recently over three decades, pyrite has gotten lots of attention because of its suitable band gap and high light absorption coefficient for applications as an optoelectronic and photovoltaic materials.3–7 However, the oxidation of pyrite is a problem that limits its application. More important, pyrite role in origin of life is now gotten more and more attention by scientists because its reaction with water may have formed hydroxyl radicals (∙ OH), which could have limited the stability of prebiotic biomolecules critical to the emergence and evolution of life.8 Oxidation of pyrite is needed to be understood in terms of reaction details with oxygen and water. Many oxidation studies have been carried out experimentally using X–ray photoelectron spectroscopy (XPS),9–17 UV photoelectron spectroscopy (UPS),16,18–19 and scanning tunneling microscopy and spectroscopy (STM and STS),16,19–20 photoemission of adsorbed xenon (PAX),14,21–22 temperature– ACS Paragon Plus Environment

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programmed desorption (TPD).14,21–22 Generally, low reactivity of the pristine pyrite surface toward water is established;9–10,13–15,18–19 however, it is also pointed that nonstoichiometric or sulfur deficient surface sites could lead to the oxidation of pyrite.13,23 Exposing pyrite to oxygen there was oxidation of the iron component, while co–adsorption of oxygen and water can enhance the pyrite surface oxidation compared to water or oxygen alone.13,19,24–25 O isotopic composition of sulfate has suggested that SO42– produced from aqueous pyrite oxidation mainly contributed from water–derived O and minor from atmospherically–derived O.26–33 At the beginning, Heidel et al. mentioned that it remains uncertain if (and how) O2 is permanently incorporated into SO42– during pyrite oxidation,34 while Heidel and Tichomirowa later confirmed the permanent existence of O2–derived O in SO42–.26,33 Tichomirowa and Junghans35 calculated the bulk contribution of atmospheric O2 in the dissolved SO42– and found a value reaching up to 50% during initial oxidation stages (first 5 days, pH 2, fine–grained pyrite fraction) and decreased to less than 20% after about 100 days. After about 10 days, they found that the O of all newly–formed sulfates is originated only from water. They proposed that the greater proportion of molecular oxygen in the early formed sulphate was caused by initially adsorbed on S sites, in addition to chemisorption on Fe sites. While for the later stages they suggested that it was only occurred at Fe sites because the attack of water–derived hydroxyl groups at S site was more favorable, and molecular O2 was acted as electron acceptor only at the Fe site for oxidation of Fe(II) to Fe(III). However, in their experiments they did not observe the time trends for dissolved oxygen concentration. Hubbard et al.36 also suggested the direct incorporation of O2 via pyrite–S sites, but they mentioned that such a mechanism is not favored within the literature. Kendelewicz et al.37 using synchrotron–based PES showed the exposure to O2 alone leaded to S oxidation; however, they explained this was associated in part with the oxidation of the mono sulfide component because surface S–dimer was much greater stability than the mono sulfide component in the presence of the O2 reactant. Rosso and Becker et al.19,25 proposed a proximity effect theory suggesting that O2 pulled electron density from underlying and surrounding Fe and S surface atoms adjacent to the O2–adsorbed Fe sites, causing increase in the affinity of these surrounding Fe atoms for the lone pair electrons on H2O molecules. Then H2O was strongly adsorbed and dissociated at these surrounding Fe sites forming –OH group due to the proximity effect, and finally –OH would attack the S sites forming the S–O groups with O from H2O. Sulfate formed by this process contains only O derived from water. By theoretical modelling Sit et al.38 also gave a model that O2 dissociated at Fe sites and H2O dissociated at S sites forming S–O species, and the O in sulfate was only derived from water. It is worth mentioning that step–wise oxidation of S has been proposed by Luther39-40 and Rimstidt and Daughan41; however, they suggested that S was interacted with water and hence the O resource of sulfite was only from water, similar to the results by Rosso and Sit et al.19,25,38 Based on the above statements, it is suggested that H2O would dissociate into OH– and H+, and then OH– attacked the S sites forming S–O species. In fact, pyrite has been detected to spontaneously form reactive oxygen species (ROS), i.e., hydrogen peroxide (H2O2) and hydroxyl radicals (∙ OH), 2,8,24,42-48 which are strong oxidative regents. Especially, ∙ OH is extraordinary reactive. The first experimental verification of the presence of ∙ OH in the pyrite–water system was done by Borda et al.23 They proposed that accompanied by the conversion of Fe(III) to Fe(II) H2O dissociated at defect pyrite surface producing ∙ OH and then the combination of two ∙ OH produced H2O2. However, they suggested that the dissociation of H2O to ∙ OH may be energetically unfavorable. In contrast, Cohn et al. proposed that O2 can be catalyzed by surface bonded Fe or dissolved Fe into H2O2 and then into ∙ OH.44,48 All these researches have pointed out the important role of Fe2+/Fe3+ ions in producing H2O2 and ∙ OH. However, the generation details of these ROS are still unclear and not well established. 2 ACS Paragon Plus Environment

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Theoretically simulating studies on the oxidation process of pyrite are also performed based on molecular dynamics (MD) method and density functional theory (DFT).19,25,38,49-52 It is shown that dissociative adsorption of water molecule alone is energetically unfavorable, while co–adsorption of oxygen and water molecules causes the dissociation of water. Stirling et al.49 reported ab initio simulations of the reaction of water with pyrite (1 0 0); however, they did not considered the role of O2. Rosso and Becker et al.19,25 using small cluster sizes predicated theoretically a strong dissociative reaction of molecular oxygen at Fe sites and H2O molecularly adsorbing at Fe sites. While for the mixtures they suggested that chemisorption of O2 on Fe sites promoted the adsorption and dissociation of water molecules to nearby pyrite Fe sites. So far, the most detailed study is carried by Sit et al. by DFT calculations.38 They gave a model of successive surface oxidation reactions with molecular oxygen and water, where finally the complete oxidation of surface sulfur to SO42− occurs. They proposed that oxygen dissociated at Fe site and water dissociated at S sites, which is different from Rosso and Becker et al. where water molecule dissociated at nearby Fe site due to proximity effect.19,25 In addition, they suggested that O in sulfate was only derived from water. This can not well explain the O isotopic experimental results that there exists a small part of O in sulfate derived from molecular O2. Just like these authors stated “it should be noted that the mechanism we have investigated in this work is not the only possible pathway in the reaction of pyrite with oxygen and water”, it may have another pathway for pyrite oxidation. It is noted that the surface structure of pyrite for the complex reaction of water and oxygen is not fully considered. Not only their research, have all the other works not paid attention to the surface structure of pyrite and its influence on the interaction of water and oxygen on it. For the present work, we found different ring structures consisting of Fe and S atoms, different Fe sites for O2 adsorption, and even S sits for O2 adsorption, which will be discussed in detail in the text. It is noted that the surface structure of pyrite for the complex reaction of water and oxygen is not fully considered. Not only their research, have all the other works not paid attention to the surface structure of pyrite and its influence on the interaction of water and oxygen on it. For the present work, we found different ring structures consisting of Fe and S atoms, different Fe sites for O2 adsorption, and even S sits for O2 adsorption, which will be discussed in detail in the text. The subject of this work focuses on the initial stage of oxidation of pyrite surface with H2O and O2 by density functional theory calculations, and aims to explore all the possible pathways for the reaction of pyrite with water and oxygen. In addition, the generating process and mechanism of hydroxyl radical was studied. ■COMPUTATIONAL METHODS AND EXPERIMENTAL DETAILS Computational methods. The calculations were performed based on density functional theory (DFT) using CASTEP, GGA–PW91.53–55 Only the valence electrons (Fe 3d6 4s2 and S 3s2 3p4) were considered using ultrasoft pseudopotentials.56 A plane wave cut–off energy of 350 eV was determined by tests. The self–consistent field (SCF) convergence tolerance was set to 2.0×10–6 eV·atom–1. The slab model was cleaved from the optimized bulk crystal. The slab thickness was tested to determine the slab size that produced a convergence of the surface energy to within 0.005 J/m2, and slab sizes with 15 atomic layers and 20 Å thickness of vacuum was placed between two surfaces to avoid the minor effect. Four types of sites, hollow Fe–Fe sites, hollow Fe–S sites, hollow S–S sites and Fe–S bond sites, were tested for the adsorptions of O2 molecule on pyrite (100) surface which is terminated with Fe–S bonds and perfect S–S bonds. Fe atom is five–coordinated with S atoms, and one top S atom (S1, S2, S3, or S4) is three–coordinated with one bottom S atom (S5, S6, S7, or S8) and two Fe atoms. The surface is found existing four hollow Fe–Fe sites (Fe1–Fe2, Fe1–Fe3, Fe1–Fe4, and Fe1–Fe5), two hollow Fe–S sites (Fe1–S1 and Fe1–S2), two hollow S–S sites (S3–S2 and S3–S4) and one Fe–S bond site (Fe1–S3), as shown in Figure 1. It is shown that, the distances between hollow Fe–Fe atoms are differ3 ACS Paragon Plus Environment

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ent (including two kinds of distances, lower or higher than 3.80 Å), while the distances between hollow S–S or Fe–S atoms are the same. In addition, two different ring structures are found, Fe1–S6–Fe5–S1– S5 and Fe1–S6–S2–Fe2–S3. The former is consisted of one top S (S1), two bottom S (S5 and S6), and two Fe (Fe1 and Fe5), and the latter is consisted of two top S (S2 and S3), 1 bottom S (S6), and two Fe (Fe1 and Fe2). All these surface sites are likely to be the adsorption sites for O2. However, it is clear that they are located in different environments, which may affect the adsorption of oxygen and/or water molecule.

Figure 1. Top view of pyrite (100) surface, O2 and H2O molecules. The numbers in the figure are the atomic distances in Å

The adsorbates, H2O and O2 were placed inside a cubic cell with lengths of 10×10×10 Å to optimize before adsorption on the mineral surface. The adsorption energy of adsorbates on the mineral surface was calculated as: E /

(1)

where E is the adsorption energy, is the energy of the H2O or O2, is the energy of the pyrite slab, and / is the energy of the H2O and/or O2–adsorbed pyrite slab. A larger negative value of Eads indicates stronger adsorption of molecule on the surface. Experimental details. The electrochemical behavior of pyrite electrode was studied by cyclic voltammetry using a Chi660e electrochemical workstation in the solution of tert–butanol with pH 7.0 at room temperature of 25℃. The current with respect to potential in the potential ranged from –1 V to 1 V with a scan rate of 0.05 V/sec. Saturated calomel electrode and platinum electrode were used as reference electrode and counter electrode, respectively.

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■ RESULTS AND DISCUSSION Isolated H2O molecule adsorption. Experimental and theoretical studies have shown that dissociative adsorption of H2O is not thermodynamically favored and molecular adsorption is the dominant binding mode,13,18–19,35,49 and our previous study also showed an unenergetically dissociated adsorption with an adsorption energy of +82.4 kJ/mol.50 Because the calculation parameters setting for the present study are different from Ref. 50, here we re–examined the non–dissociative adsorption of H2O. Optimized configuration is shown in Fig. 2. The total Mulliken charges of H2O molecule is calculated as +0.08 e, compared to +0.03 e obtained by Rosso et al.19. The less net charge of H2O molecule suggests weak interaction of H2O molecule with the mineral surface.. Adsorption energy is calculated as –61.8 kJ/mol, very close to –65.6 kJ/mol calculated by Sit et al.38 Combined with the adsorption energy and net charge H2O molecule, it is suggested that water molecule is physically adsorbed on pyrite surface. Water molecularly adsorbs with Fe–O distance of 2.16 Å and two S…H bonds of 2.57 Å and 2.58 Å, close to 2.12 Å of Fe–O distance but a slightly different for S…H distance (2.36 and 2.70 Å) calculated by Stirling et al..49 Additionally, the O–Fe–S angle (this S is located in the inner surface, see it in Figure 2 where it is connected with Fe by blue dotted line) is 172.2°, very close 180° in bulk pyrite. This suggests that the binding of O restores the distorted octahedron of Fe atom. For further investigation of interaction of H2O molecule with surface, partial density of states (DOS) of atoms is plotted, as shown in Figure 2b–c. XPS study by Knipe et al.9 has shown the water species detected on pyrite interact with the Fe 3d eg molecular orbital. Our calculation is consistent with the experiment. It is clear from the DOS pattern that hybridization between O 2p state and Fe 3d eg bonding state occurs at the energy range of –7.5 to –1.5 eV.

Figure 2. Adsorption configuration of isolated H2O molecule (a), and the densities of states of interacted Fe-O before (b) and after H2O adsorption (c) . H2O molecule adsorption on the surface. eg, t2g and eg* represent the bonding state, non–bonding state and anti–bonding state of Fe 3d, respectively.

Isolated O2 molecule adsorption. Although many studies have done on the O2 adsorption on pyrite, including experimental and theoretical, we found it is still incomplete and needed to be learnt more. On the pyrite surface, we have found two types of hollow Fe–Fe sites (see Figure 1), which have different distance, i.e. the type Fe1–Fe2 and Fe1–Fe3 with distance 3.72 and 3.77 Å and the other type Fe1– Fe4, Fe1–Fe5 with distance 3.89 and 3.86 Å. In addition, one type of hollow Fe–S (Fe1–S1 and Fe1–S2 5 ACS Paragon Plus Environment

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with distance of 3.56 and 3.55 Å, respectively) and one type of hollow S–S (S3–S2 and S3–S4 with distance of 3.01 Å) site were also found. These Fe–Fe sites, S–S sites and Fe–S sites are likely to become oxygen adsorption sites. However, previous publications have not noted these situations and only one type of Fe–Fe site were considered. In order to test the impact of these sites on the adsorption of oxygen molecule, oxygen were placed on these sites and the corresponding adsorption energy was calculated shown in Table 1 and the corresponding adsorption configurations are shown in Figure 3. Table 1. Adsorption energies of O2 molecule on different surface sites Adsorption types

Configuration

Adsorption sites

Adsorption energy /kJ/mol

a

Fe1–Fe2

–202.6

b

Fe1–Fe3

–173.7

c

Fe1–Fe4

–115.8

d

Fe1–Fe5

–111.9

e

Fe1–S1

–194.9

f

Fe1–S2

–213.2

Hollow

g

S3–S2

–284.6

S–S sites

h

S3–S4

–283.7

Fe–S bond site

i

Fe1–S3

–61.8

Hollow Fe–Fe sites

Hollow Fe–S sites


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Figure 3. Configurations of O2 adsorbing on pyrite surface. (a)–(f) is numbered the same as numbered in Table 1. (a)– (d) corresponding to the adsorption patterns on hollow Fe–Fe sites, (e) –(f) corresponding to the patterns on hollow Fe– S sites, (g)–(h) corresponding to the patterns on hollow S–S sites, and (i) corresponding to the patterns on Fe–S bond. The non–bold number next to the dotted line is the bond length and the bold number on the arcuate line is the angle formed by three atoms.

It is found that the adsorption energies of O2 on Fe1–Fe2 and Fe1–Fe3 sites are close, and are also close to that on Fe1–S1 and Fe1–S2 sites, with adsorption energy about –200 kJ/mol, while the adsorption energies on Fe1–Fe4 and Fe1–Fe5 are much lower and is only about –110 kJ/mol. It is well worth noting that the adsorption energies of O2 on S–S (S3–S2 and S3–S4) sites have the maximum negative value down to –280 kJ/mol. The Fe–S bond site was also considered and the adsorption energy is only – 61.8 kJ/mol. These results suggest that O2 molecule can strongly and chemically adsorb on one type of hollow Fe–Fe (Fe1–Fe2 and Fe1–Fe3) sites, hollow Fe–S sites and hollow S–S sites on pyrite, whereas weakly adsorb on the other hollow type of Fe–Fe (Fe1–Fe4 and Fe1–Fe5) sites and Fe–S bond sites. These are new findings which have not been published previously. Firstly, it is generally considered that O2 molecule would adsorb on Fe–Fe sites while no other sites were considered. Secondly, it is not reported previously that there exits two types of Fe–Fe sites, i.e. one (Fe1–Fe2 and Fe1–Fe3) is the active site for O2 molecule adsorption but the other (Fe1–Fe4 and Fe1–Fe5) is not. Combining with the adsorption energies results, it is found that when O2 is strongly adsorbed it is dissociated on the surface (configurations a, b, e, f, g and h in Figure 3), while when weakly adsorbed it is not dissociated (configurations c, d and i). In the former, the Fe–O and S–O bonds lengths are about 1.69 and 1.50Å, respectively, much shorter than that in the latter with Fe–O and S–O bonds lengths about 2.00 and 1.68 Å, respectively. In addition, in the latter, except configuration i, the O–O bond length is about 1.38 Å, a typical superoxide bond. Sit et al.38 also gave a modelling result for the O2 adsorption; however their work is not intact because they have not done many tests on the adsorption sites 7 ACS Paragon Plus Environment

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of O2 molecule on the surface. Only two models were considered in their work, i.e. the end–on configuration and the side–on configuration (hollow Fe–Fe site), with adsorption energies of –58.9 and –69.4 kJ/mol, respectively. They thought the latter was more stable. In the side–on configuration, they gave a Fe–O distance and the O–O distance about 1.99 and 1.38 Å, respectively. This configuration is the same as the weak adsorption configuration of O2 on Fe1–Fe4 and Fe1–Fe5 in our study. They thought this O– O binding configuration was a key reaction intermediate in pyrite surface oxidation and could readily break exothermically by –109.5 kJ/mol. This suggests that the dissociative adsorption of O2 on pyrite surface went through two steps and the total adsorption energy was –178.9 kJ/mol (sum of –69.4 kJ/mol and –109.5 kJ/mol), close to our calculation on configurations a and b. However, our calculation shows that O2 can directly and dissociatively adsorb on pyrite surface at Fe–Fe sites, Fe–S sites, even S–S sites. By observing the dissociative adsorption configurations of O2, it is found that the angles formed by O–Fe–S and O–S–S (the bold number labeled on the arcuate line shown in Figure 3) closely approximate to that in bulk pyrite of 180° and 102°, respectively, whereas in the undissociated configurations a part of these angles are greatly deviated from the bulk values. These suggest that the dissociative adsorption of O2 can well restore the distorted octahedron of Fe and the tetrahedron of S as found in bulk; however, the undissociated adsorption can not well restore the shapes of all the distorted surface atoms. Moreover, electron density difference can give clear information in the electrons transfer on atoms. By plotting the electron density difference of surface Fe and S atoms cleaved from a same plane without O2 adsorption (Figure 4a–b) and with O2 adsorption (Figure 4c–d), it is clearly shown that charge is uniformly distributed around full–coordinated Fe atom in bulk pyrite (Figure 4a), while the charge distributed around pure surface Fe atom is diffused into the vacuum due to the lack of one sulfur ligand (Figure 4b). On the undissociated O2 adsorption surface (Figure 4c), part of the charges on Fe atom are still diffused into vacuum because adsorbed O atom deviates from the plane and can not well restore the distorted octahedron of Fe atom. On the dissociated O2 adsorption surface (Figure 4d), the bonding of O atom enables complete coordination of Fe atom where the distorted octahedron is well restored, and hence the positive charge is gathered around Fe atom, similar to that in the bulk phase (Figure 4a).


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Figure 4. Electron density difference on the atoms cleaved from a same plane. In bulk pyrite (a), without O2 adsorption surface (b), on undissociated O2 adsorption surface (c) and on dissociated O2 adsorption surface (d). Blue represents electrons loss, red represents electrons enrichment, and white represents zero electron density.

Based on the calculation results, this is the first confirmation that O2 molecule can be directly chemisorbed on S site by DFT simulation. Generally, it is thought that O2 is dissociated at Fe sites, but not at S sites because of the electrostatic repulsion between negatively charged O and S atoms. As suggested in the introduction, both water–derived and atmospheric oxygen could be incorporated into sulfate;26–32 however, such a mechanism that the direct incorporation of O2 via pyrite–S sites such is not favored within the literature.34 Our study firstly shows that at the initial oxidation stage, O2 can be easily dissociatively adsorbed at Fe–S and S–S sites, in addition to chemisorption on Fe–Fe sites. Chemisorption on Fe–S and S–S sites is one of the schemes of the initial oxidation of pyrite surface by O2, causing part of O in sulfate derived from O2. Co-adsorption of H2O-O2 on pyrite surface. H2O molecule was put on the surface preferentially adsorbed O2 to investigate the interactions of H2O–O2 on the surface (O2–then–H2O sequence adsorption). All the nine kinds of oxygen adsorption models were considered and the final adsorption configurations are shown in Figure 5. It is shown that H2O molecule is dissociated completely on the surface after O2 molecule adsorbing on all the four Fe–Fe sites (Figures 5a–d). It is noted that the undissociated O2 molecule on pure surface (see Figure 3c–d) is dissociated due to the adsorption of H2O molecule (Figures 5c–d). The two dissociated H atoms are bonded to the two dissociated O atoms in O2, forming two Fe–OH species with Fe–O distance about 1.83 Å, and the dissociated O in H2O is bonded to top S atom, forming S–O specie with distance about 1.52 Å. When O2 is adsorbed at Fe–S sites (Figures 5e– f), H2O molecule is dissociated into one –H and one –OH. –H is bonded to the O of O2 molecule preferentially adsorbed at Fe site, forming Fe–OH specie with Fe–O distance of 1.82 Å, and –OH is adsorbed at Fe site (Figure 5e) or S site (Figure 5f), forming Fe–OH specie with Fe–O distance of 1.85 Å (Figure 4e) and S–OH specie with S–O distance of 1.65 Å (Figure 5f). When O2 is adsorbed at S–S sites (Fig9 ACS Paragon Plus Environment

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ures 5g–h)，H2O molecule can not dissociate on the surface; however, the Fe–O distance (2.10 Å and 2.06 Å) decreases compared to that on pure H2O adsorption surface (2.16 Å), suggesting the adsorption of H2O is enhanced due to the presence of O2. This is caused by the formation of hydrogen bonding between H and O of O2 molecule. When O2 is adsorbed on Fe–S bond site (Figure 5i), O2 is dissociated (undissociated when no H2O is present, see Figure 3i) and H2O is dissociated into one –H and one –OH, forming Fe–OH specie with Fe–O distance of 1.93 Å; however, no S–OH specie is formed.

Figure 5. Configurations of H2O adsorbing on pyrite surface preferentially adsorbed O2. (a)–(d) corresponding to the adsorption patterns of O2 at hollow Fe–Fe sites, (e)–(f) corresponding to the patterns of O2 at hollow Fe–S sites, (g)–(h) corresponding to the patterns of O2 at hollow S–S sites, and (i) corresponding to the patterns of O2 at Fe–S bond. The number next to the dotted line is the bond length in Å.

For the co–adsorption of O2 and H2O, besides the model of H2O adsorbing on the surface preferentially adsorbed O2 (O2–then–H2O sequence adsorption) was considered, the models of O2 adsorbing on the surface preferentially adsorbed (H2O–then–O2 sequence adsorption) and simultaneous adsorption of H2O and O2 were also investigated (see Supporting Information). It is found that O2 adsorbing on the surface preferentially adsorbed H2O may lead to the formation of H2O2 (see Figures S1 and S2), while the simultaneous adsorption configurations of H2O and O2 on pyrite surface (see Figure S3) are the same as the configurations for the O2–then–H2O sequence adsorption. Here the cases of O2–then–H2O sequence adsorption (O2–H2O simultaneous adsorption has the same result) will be discussed, and the case of H2O–then–O2 sequence adsorption is shown in Supporting Information. Our calculations has suggested that co–adsorption of O2 and H2O can enhance the oxidization of pyrite. This result is consistent with the other theoretical calculations and experimental studies.15,19 Table 2 lists the surface species and numbers derived from Figure 5. Configurations a, b, c, d, e have the same surface species and specie numbers, 1 of S–O and 2 of Fe–OH, and the co–adsorption energy of H2O–O2 for these configurations are close, ranging from –328.1 to –346.4 kJ/mol. Configurations f and i have the same surface species and numbers, 1 of S–O, 1 of Fe–OH and 1 of S–OH, and also 10 ACS Paragon Plus Environment

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have close adsorption energy for O2–H2O (about –270 kJ/mol). Configurations g and h have the same surface species and numbers, 2 of S–O, and have close adsorption energy for O2–H2O low to –390 kJ/mol. These results reflect that if the surface productions are the same after adsorbates adsorption, the adsorption energy, i.e. the adsorption strength of adsorbates are nearly the same. In addition, based on the adsorption energy of adsorbates on the surface it is found that the production of S–OH species on the surface is not energetically favorable, while the production of Fe–OH and S–O species are favorable. Consequently, configurations f and i where O2 locates on hollow Fe–S site and Fe–S bond site are not favorable, although O2 molecule can dissociate on the surface in the presence of H2O molecule for these two configurations. It is noted that for configurations g and h where O2 dissociatively adsorbs at hollow S–S sites and H2O adsorbs at Fe site, although H2O molecule is not dissociated and no Fe–OH specie is produced, the co–adsorption energies of O2 and H2O are the lowest to –390 kJ/mol, compared to the adsorption energy of isolated H2O and isolated O2 of about –60 kJ/mol and –280 kJ/mol, respectively. This suggests that the adsorption of H2O molecule in the presence of O2 is enhanced, and this can be ascribed to the formation of hydrogen bonds between H derived from H2O and O derived from dissociated O2 molecule. However, this co–adsorption model can not cause any H2O molecules to be dissociated.

Table 2. Surface species, numbers and adsorption energies at different adsorption sites Adsorption types

Configuration

Adsorption sites

Surface species and numbers S–O

Fe–OH

S–OH

Adsorption energy of O2+H2O /kJ/mol

a

Fe1–Fe2

1

2

0

–346.4

b

Fe1–Fe3

1

2

0

–334.8

c

Fe1–Fe4

1

2

0

–328.1

d

Fe1–Fe5

1

2

0

–341.6

e

Fe1–S1

1

2

0

–344.5

f

Fe1–S2

1

1

1

–269.2

g

S3–S2

2

0

0

–395.6

h

S3–S4

2

0

0

–391.7

i

Fe1–S3

1

1

1

–274.0

Hollow Fe–Fe sites

Hollow Fe–S sites

Hollow S–S sites Fe–S bond site

Based on the above results, for these two co–adsorption cases of O2 and H2O (simultaneous adsorption and O2–then–H2O sequence adsorption) it can be concluded that O2 can be dissociated at Fe–Fe sites, Fe–S sites, and even S–S sites, and correspondingly, H2O can thoroughly, partially, and completely not be dissociated on pyrite surface. Consequently, S–O and Fe–OH species are produced and energetically favored. When O2 adsorbs at Fe–Fe sites, 2 H is thoroughly dissociated from H2O molecule and bonded to 2 O of dissociated O2 molecule forming 2 of Fe–OH species, leaving O bonded to surface S atom. Hence, for this adsorption scheme, the O of sulfate is derived from H2O molecule. When O2 adsorbs at Fe–S sites, only 1 H is dissociated from H2O molecule and then bonded to 1 O dissociated from O2 molecule forming 1 of Fe–OH specie, leaving –OH bonded to surface Fe atom forming 1 of Fe–OH, and another dissociated O from O2 molecule is bonded to surface S atom. Consequently, for this adsorp11 ACS Paragon Plus Environment

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tion scheme, the O of sulfate is derived from O2 molecule. When O2 adsorbs at S–S sites, H2O molecule is not dissociated, and the O of sulfate is derived from O2 molecule. The atomic Mulliken charge was calculated and labeled in Figure S4. It is found that on pure O2 molecule adsorption surface (processes a, b and c in Figure S4), O atoms capture electrons from the surface and are negatively charged. Moreover, O atoms adsorbing at S–S site are the most negatively charged, followed by at Fe–S site, and then at Fe–Fe site. When H2O molecule is adsorbed (processes d, e and f), O2 molecule continues to capture electrons and is more negatively charged. However, the captured electrons numbers at the Fe–Fe site is the most, followed by at Fe–S site and then at S–S site. It is shown that from process a to d, the charge of O2 is changed from –0.92 e to –1.49 e (capturing the most electrons), and from process b to e, the charge of O2 is changed from –1.16 e to –1.48 e and from process c to f, the charge of O2 is changed from –1.52 e to –1.61 e (nearly not changed). These results suggest that the O2 adsorbed at Fe site has greater activity than at S site for the dissociation of H2O molecule because it is very conducive to form Fe–OH specie which is a more energetically favored surface specie than S–OH specie. Moreover, observing from process a to d (Figure S4) it is shown that the charge on Fe atom is not changed obviously after H2O adsorption, while the total charge of H2O is close to zero and the S bonded to O in H2O loses large numbers of electrons and is positively charged. Hence, it is clear that O2 captures electrons from this S atom. This is consistent with the result by Rimstidt and Vaughan41 that the electrons were transferred from sulfur atoms at an anodic site through the crystal to cathodic Fe sites and were acquired by the oxidant species (O2). We compared the bond strength between O (both from O2 molecule and H2O molecule) and surface Fe and S by calculating the Mulliken bond populations including bond length and covalent strength (See Table S1). The larger the bond population value, the stronger covalent property of the bond is. It is shown that the Fe–OO bond length increases and the bond population decreases after H2O molecule adsorption, while the bond lengths and populations of S–OO are not changed after H2O molecule adsorption. This result suggests that the covalent strength of Fe–OO weakens due to the adsorption of H2O molecule, while the covalent strength of S–OO is not changed. Moreover, it is found that the bond lengths and populations of S–OO and S–OW are the same, suggesting that the bond properties of S–O are the same regardless of the resource of O (from H2O molecule or O2 molecule). Similarly, the H–O properties are also the same regardless of the resource of O. Additionally, we compared the results of isolated H2O adsorption on Fe site with the results of H2O adsorption on Fe site after O2 adsorption on S–S sites (i.e. O2–then–H2O sequence adsorption) and found that the adsorption energy of the latter (–110.9 kJ/mol) is lower than the former (–61.8 kJ/mol), suggesting the latter behaved chemisorption property. We further compared the Fe–OW bonds and found that the Fe–OW bond length forming on O2–then–H2O sequence adsorption is shorter than that forming on isolated H2O adsorption. The former also has larger bond population value. These results suggest that the adsorption of O2 on S–S site can greatly enhance the adsorption of H2O molecule, even causing chemisorbed H2O. Becker et al. proposed a proximity effect theory to explain the enhancing oxidation rate of pyrite by H2O and O2 compared with O2 alone.25 They predicted that O2 pulls electron density from underlying and surrounding Fe and S surface atoms adjacent to the O2–adsorbed Fe sites, causing increase in the affinity of these surrounding Fe atoms for the lone pair electrons on H2O molecules; hence H2O interacts more strongly with available Fe sites adjacent to Fe–O groups and finally dissociates forming Fe– OH species. For the dissociation reason, however, we found that it is not ascribed to the electrons loss of the proximity atoms. We found that the adsorption of O2 on Fe sites has small influence on the charge 12 ACS Paragon Plus Environment

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number of surrounding Fe and S atoms nearby the O2–adsorbed Fe sites (see processes a, b and c in Figure S4); however, the density of states (DOS) of the surrounding S atom is changed significantly. We investigated the changes of DOS of surrounding S atom and O atoms from process a to d in Figure S4, as shown in Figure 6. It is found that on pure O2–adsorbed surface (Figure 6b), the surrounding S 3p state increases near energy –1.5 eV compared to that on pure surface (Figure 6a), suggesting that the electronic state of S atom is changed and the electronic activity of S 3p increases. And then observing the densities of states (DOSs) of the S and O atoms (derived from H2O) in S–O specie formed on the surface after co–adsorption of H2O and O2 (Figure 6c and d), it is found that there is a strong hybridization peak near energy –1.5 eV. These results suggest that it is the change of electronic density state but not the electronic density loss of surrounding S atom that leads to the affinity of the surrounding S atom for the lone pair electrons on H2O molecule.

Figure 6. Density of states of S and O atoms changes from process a to d in Figure S5.

The change of DOS of S atom can be ascribed to the structural change of the atom. Figure 7 shows the structure of surface atoms before and after O2 adsorption. It is found that Fe2–S1, Fe1–S2 and Fe2– S2 bonds distances are increased significantly from 2.23 Å, 2.21 Å and 2.16 Å to 2.30 Å, 2.30 Å and 2.24 Å after the adsorption of O2 molecule, respectively, while S1–S3 and S2–S4 bonds distances are decreased from 2.21 Å and 2.20 Å to 2.15 Å and 2.14 Å, respectively. This means that Fe–S bonds are weakened while S–S bonds are strengthened due to the adsorption of O2 on Fe sites. Hence, it is predicted that further oxidation will firstly lead to the rupture of Fe–S bonds. In fact, Fe–S bond is believed to be weaker than S–S bond.


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Figure 7. Influences of O2 adsorption on the surrounding atomic distances. Distance in Å marked next to the bond.

In addition, for the dissociation sites of H2O on pyrite surface in the presence of O2, Becker et al.25 proposed that due to the proximity effect H2O molecule was firstly strongly adsorbed at Fe site adjacent to the Fe–O group (formed after O2 adsorption), and then H2O dissociated at this Fe site producing –OH at Fe site forming Fe–OH, and this –OH then would attack the S site common to Fe–OH and Fe–O groups (i.e., HO–Fe–S–Fe–O linkages) finally forming S–O groups with O derived from H2O. However, we found that H2O is dissociated directly at the S sites adjacent to the Fe–O group (formed after O2 adsorption) but not at the nearby Fe sites. This is consistent with the result of Sit et al..38 In addition, we also found the formation of S–OH specie is not energetically favored. For the co–adsorption behaviors of H2O and O2, our result is agreement with Sit et al.38 when O2 is adsorbed at Fe sites, i.e. O2 dissociated at Fe sites forming Fe–O specie and H2O dissociated at S site forming S–O specie leaving the dissociated H bonding to Fe–O forming Fe–OH. However, they only considered one adsorption site for O2 adsorption, i.e. adsorption at Fe site. We found S site can also be an active site for O2 adsorption, which may explain the O isotopic composition of sulfate indicating that both water–derived and atmospheric oxygen could be incorporated into sulfate, especially, explain the high percentage of O from O2 at the initial oxidation of pyrite. In summary, it can be concluded that for the interaction of O2–H2O on pyrite surface, including simultaneous adsorption, O2–then–H2O sequence adsorption and H2O–then–O2 sequence adsorption, the O of sulfate can be only derived from H2O or both from H2O and O2 at the initial oxidation stage of pyrite. This result is consistent with O isotope experiments on pyrite oxidation that both the O derived from molecular oxygen and water could be incorporated into sulphate, with most derived from water. Exactly, Usher et al.30 showed that exposing pyrite to gaseous O2 prior to pure H2O vapor, both SO42– and iron oxyhydroxide became significant products, and isotopic labeling experiments showed that the O in the SO42– product was derived from both H2O and O2. However, if pyrite was exposed to pure H2O vapor prior to gaseous O2, it led to the formation of sulfur oxyanions that included SO42–, and isotopic labeling experiments showed that the O in the sulfate product was primarily derived from the H2O reactant. Formation of hydroxyl radical. It is now concerned about the dissociation process of H2O and O2 on pyrite and the surface oxidation mechanism. Here we discuss the cases of complete dissociation of H2O and O2 (in cases of O2–then–H2O sequence adsorption and simultaneous adsorption of H2O and O2, corresponding to process a–d in Figure S4) and generation of hydrogen peroxide (in case of H2O–then– 14 ACS Paragon Plus Environment

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O2 sequence adsorption, corresponding to process a–b in Figure S6) on the surface due to interaction of H2O–O2. Figures 8 and 9 show these two cases, respectively. In Figure 8, the atomic distances are plotted as a function of optimization step (reflecting the calculation process), from which the changes in distances between atoms can be clearly observed during the interaction process between molecules with pyrite surface and interaction process between molecules themselves. The complete dissociation process of H2O–O2 can be divided into three regions, Ⅰ, Ⅱ, and Ⅲ zones. Correspondingly, three representative adsorption configurations are plotted to illustrate the reactions taking place in these three regions, as indicated by the arrows. In region Ⅰ, S–O1 (in H2O) distance increases dramatically, indicating that H2O molecule is repelled from the surface. The distance of O2–O3 (in O2) is also increased. In region Ⅱ, H1 dissociates from H2O molecule accompanying with O2 dissociating when H2O molecule is far away from the surface (large S–O1 distance), forming the first Fe–OH (O3H1) specie and leaving –OH (O1H2) far away from the surface S. In region Ⅲ, –O1H2 is down towards surface S with short S–O1 distance and H2 dissociates from –O1H2 forming the second Fe–OH (O2H2) specie.

Figure 8. Complete dissociation process of H2O and O2 on pyrite surface. Corresponding to the process a–d in Figure S4. The numbers in the Figure are the atomic distances in Å

The atomic distances changes corresponding to process a–b in Figure S6 are shown in Figure 9. It is found that the generation process of hydrogen peroxide (H2O2) is the same as the above process shown in Figure 8, i.e. H2O is firstly repelled from the surface and one H dissociates forming one –OH, 15 ACS Paragon Plus Environment

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and then –OH is down towards the surface S and finally also dissociates. The two dissociated H interact with O2 forming H2O2.

Figure 9. Formation of H2O2 on pyrite surface. Corresponding to the process a–b in Figure S6. The numbers in

the Figure are the atomic distances in Å

It is noted that the dissociation of H from H2O is a step–wise process. One H is dissociated when H2O molecule is far away from the surface (with S–O1 distance even larger than 2.0 Å), while the other H is dissociated when the leaving –OH is down towards the surface S. The question is that why the leaving –OH which is far away from the surface can be down towards the surface S and finally forms strong S–O bond (with a final distance below 1.6 Å). We speculate the hydroxide radical (∙ OH) mechanism could give an explanation on this result. The dissociation of H2O molecule may go through the following reactions on the surface: H O → OH H OH h → ∙ OH

(2) (3)

py S ∙ OHpy SOH 4 16 ACS Paragon Plus Environment

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py SOH → py SO H 5 where py represents pyrite. Reaction (2) indicates that H2O molecule dissociates into OH and H due to interaction with dissociated oxygen, although the molecule is away from the surface. For reaction (3), OH captures surface hole (h ), causing the production of hydroxyl radical ∙ OH. We know that ∙ OH has great activity leading to it down towards the surface and then interacting with the surface S. Finally, under the effect of the dissociated O atom, the H in ∙ OH is dissociated. Consequently, it is the hydroxyl radical, ∙ OH, that makes the reaction continued on the surface. The reaction of O2 molecule can be expressed as follows: py 2FeⅡ O 2H → py 2FeⅢ 2OH 6 O → ∙ O ∙ O 7 In fact, the first experimental verification of the presence of ∙ OH in the pyrite–water system was done by Borda et al.23. To verify the presence of ∙ OH, tert–butanol was used as the reactant with ∙ OH in our study. Electrochemical behavior of pyrite electrode was studied by cyclic voltammetry at different pH values by addition of different concentrations of tert–butanol. Figure 10 shows the scanned cyclic voltammetric curves. Here only the oxidation of S is considered. The oxidation for S is believed to occur around potential of 0.2 V. It is found that the oxidation peak of S is very obvious under all pH conditions when there is no addition of tert–butanol (0% tert–butanol), while addition of tert–butanol (5% or 10%) leads to significant decrease (even disappearance at pH 7) of oxidation peak. The corrosion current (Icorr) of pyrite scanned around 0.2 V under different pH conditions and tert–butanol concentrations listed in Table 3 shows that the Icorr is lowered due to the addition of tert–butanol. These results suggest that the oxidation of S is weakened due to the decrease of ∙ OH which is reacted with tert–butanol. This result confirms the presence of ∙ OH when pyrite is oxidation under H2O–O2 circumstance. Table 3. Corrosion current (Icorr, in 10–6 A.m–2) of pyrite scanned around 0.2 V under different pH conditions and tert–butanol concentrations. pH value

0% tert–butanol

5% tert–butanol

10% tert–butanol

4

3.963

3.368

3.174

7

1.349

1.127

0.722

10

19.810

4.774

––

Noted: –– represents no detection.


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Figure 10. Cyclic voltammetric curves in the potential ranging from –1 V to 1 V with a scan rate of 0.05 V/sec under different pH conditions.

■ CONCLUSIONS This is the first time that directive dissociative chemisorption of O2 on the S site on pyrite surface has been confirmed, besides dissociative chemisorption on the Fe sites. This gives explanation for the small amount of O in sulfate derived from O2 besides from water. However, H2O can not be dissociated when O2 is adsorbed on S–S sites, while it can be partially dissociated when O2 is adsorbed on Fe–S sites and completely dissociated when O2 is adsorbed on Fe–Fe sites accompanying by the dissociation of O2. Consequently, O in sulfate can be derived from O2 when O2 is adsorbed on S–S sites and Fe–S sites, while derived from H2O when O2 is adsorbed on Fe–Fe sites. We believed that all these cases can occur during the pyrite surface oxidation. Differences in adsorption sequence of H2O and O2 cause different surface productions. It is found that simultaneous adsorption of H2O and O2 and O2–then–H2O sequence adsorption make the same results on the adsorption behaviors of H2O and O2 on the surface, while H2O–then–O2 sequence adsorption causes different results from the former two. S–O, Fe–OH and S–OH species are formed when simultaneous adsorption of H2O and O2 and O2–then–H2O sequence adsorption are occurred, while H2O2 may be formed on the surface when H2O–then–O2 sequence adsorption is occurred. However, S–OH specie is found energetically unfavorable on the surface. Finally, H2O is found going through a step– wise dissociation process when interacting with O2 on pyrite surface, and hydroxyl radical ∙ OH is the 18 ACS Paragon Plus Environment

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main reactive oxygen species (ROS) which can oxidize the surface S to sulfate. Experimental results confirm the presence of radicals. ■ ASSOCIATED CONTENT Supporting Information ■ AUTHOR INFORMATION Corresponding Author *[email protected], [email protected] Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors are grateful for the financial support provided by the National Natural Science Foundation of China (NSFC. 51764002, 51574092, and 51364002), the Guangxi Natural Science Foundation (2017GXNSFAA198216), and the Special Program for Applied Research on Super Computation of the NSFC–Guangdong Joint Fund (the second phase). ■ REFERENCES (1) Huang, X.; Finkelman, R. B. Understanding the chemical properties of macerals and minerals in coal and its potential application for occupational lung disease prevention. J. Toxicol. Env. Heal B 2008, 11, 45–67. (2) Cohn, C. A.; Laffers, R.; Simon, S. R.; O'Riordan, T.; Schoonen, M. A. A. Role of pyrite in formation of hydroxyl radicals in coal: possible implications for human health. Part. Fibre. Toxicol. 2006, 3, 16–. (3) Ennaoui, A.; H. Tributsch. Iron sulphide solar cells. Solar. Cells. 1984, 13, 197–200. (4) Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonso-Vante, N.; Buker, K.; Bronold, M.; Hopfner, C.; Tributsch, H. Iron disulfide for solar energy conversion. Sol. Energ. Mat. Sol. C. 1993, 29, 289–370. (5) Nakamura, S.; Yamamoto, A. Operation control of photovoltaic/diesel hybrid generating system considering fluctuation of solar radiation. Sol. Energ. Mat. Sol. C. 2001, 65, 79–85. (6) Altermatt, P. P.; Kiesewetter, T.; Ellmer, K.; Tributsch, H. Specifying targets of future research in photovoltaic devices containing pyrite (FeS2 ) by numerical modelling. Sol. Energ. Mate. Sol. C. 2002, 71, 181–181. (7) Khalid, S.; Malik, M. A.; Lewis, D. J.; Kevin, P.; Ahmed, E.; Khan, Y.; O’Brien, P. Transition metal doped pyrite (FeS2) thin films: structural properties and evaluation of optical band gap energies. J. Mater. Chem. C. 2015, 3, 12068–12076. (8) Cohn, C. A.; Borda, M. J.; Schoonen, M. A. RNA decomposition by pyrite-induced radicals and possible role of lipids during the emergence of life. Earth. Planet. Sc. Lett. 2004, 225, 271–278. (9) Knipe, S. W.; Mycroft, J. R.; Pratt, A. R.; Nesbitt, H. W; Bancroft, G. M. X-ray photoelectron spectroscopic study of water adsorption on iron sulphide minerals. Geochim. Cosmochim. Ac. 1995, 59, 1079–1090.


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(10) Nesbitt, H. W.; Muir, I. J. X-ray photoelectron spectroscopic study of a pristine pyrite surface reacted with water vapour and air. Geochim. Cosmochim. Ac. 1994, 58, 4667–4679. (11) Nesbitt, H. W.; Muir, I. J. Oxidation states and speciation of secondary products on pyrite and arsenopyrite reacted with mine waste waters and air. Miner. Petrol. 1998, 62, 123–144. (12) Mattila, S.; Leiro, J. A.; Heinonen, M. XPS study of the oxidized pyrite surface. Sur. Sci. 2004, 566-568, 1097– 1101. (13) Guevremont, J. M.; Bebie, J.; Elsetinow, A. R.; Strongin, D. R.; Schoonen. M. A. A. Reactivity of the (100) plane of pyrite in oxidizing gaseous and aqueous environments: effects of surface imperfections. Environ. Sci .Technol. 1998, 32, 1453–. (14) Guevremont, J. M.; Strongin, D. R.; Schoonen, M. A. A. Structure sensitivity of pyrite oxidation; comparison of the (100) and (111) planes. Am. Mineral. 1998, 83, 1246–1255. (15) Guevremont, J. M.; Elsetinow, A. R.; Strongin, D. R.; Bebie, J.; Schoonen, M. A. A. Structure sensitivity of pyrite oxidation; comparison of the (100) and (111) planes. Am. Mineral. 1998, 83, 1353–1356. (16) Eggleston, C. M.; Ehrhardt, J. J.; Stumm, W. Eggleston, C. M.; Ehrhardt, J. J.; Stumm, W. Surface structural controls on pyrite oxidation kinetics; an XPS-UPS, STM, and modeling study. Am. Mineral. 1996, 81, 1036–1056. (17) Schaufusz, A. G.; Nesbitt, H. W.; Kartio, I.; Laajalehto, K.; Bancroft, G. M.; Szargana, R. Incipient oxidation of fractured pyrite surfaces in air. J. Electron. Spectrosco. 1998, 96, 69–82. (18) Pettenkofer, C.; Jaegermann, W.; Bronold, M. ChemInform abstract: site specific surface interaction of electron donors and acceptors on FeS2(100) cleavage planes. Cheminform. 1991, 22, 560–565. (19) Rosso, K. M.; Becker, U.; Hochella, M. F. The interaction of pyrite (100) surfaces with O2 and H2O: Fundamental oxidation mechanisms, Am. Mineral. 1999, 84, 1549–1561. (20) Rosso, K. M.; Becker, U.; Hochella, M. F. Atomically resolved electronic structure of pyrite {100} surfaces: An experimental and theoretical investigation with implications for reactivity. Am. Mineral. 1999, 84, 1535–1548. (21) Guevremont, J. M.; Strongin, D. R.; Schoonen, M. A. A. Effects of surface imperfections on the binding of CH3OH and H2O on FeS2(100): using adsorbed Xe as a probe of mineral surface structure. Sur. Sci. 1997, 391, 109–124. (22) Guevremont, J. M.; Strongin, D. R.; Schoonen, M. A. A. Photoemission of adsorbed xenon, X-ray photoelectron spectroscopy, and temperature-programmed desorption studies of H2O on FeS2(100). Langmuir 1998, 14, 1361– 1366. (23) Borda, M. J.; Elsetinow, A. R.; Strongin, D. R.; Schoonen, M. A. A mechanism for the production of hydroxyl radical at surface defect sites on pyrite. Geochim. Cosmochim. Ac. 2003, 67, 935–939. (24) Raikar, G. N.; Thurgate S. M. An Auger and EELS study of oxygen adsorption on FeS2. J. Phys-Condens. Mat. 1991, 3, 1931–1939. (25) Becker, U.; Rosso, K. M.; Hochella, M. F. The proximity effect on semiconducting mineral surfaces: a new aspect of mineral surface reactivity and surface complexation theory? Geochim. Cosmochim. Ac. 2001, 65, 2641–2649.


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