Pyrite Oxidation Mechanism by Oxygen in Aqueous Medium - The

Article pubs.acs.org/JPCC

Pyrite Oxidation Mechanism by Oxygen in Aqueous Medium Egon Campos Dos Santos, Juliana Cecília de Mendonça Silva, and Hélio Anderson Duarte* GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte − MG, Brazil, 31.270−901 S Supporting Information *

ABSTRACT: The mechanism of the initial steps of pyrite (100) surface oxidation was investigated in detail by means of density functional theory/plane-wave calculations. Pyrite oxidation is related to many environmental and technological issues, and its mechanism has not been completely understood. A chemical picture of the pyrite oxidation process in the presence of oxygen and water was proposed in the present investigation. The reaction steps of the oxidation mechanism can be separated into two types. Type I reactions present lower activation energies and are redox processes that involve oxidation of two Fe(II) sites on the surface to form predominantly the Fe(III)−OH−. This species is formed from hydrogen transfer between the adsorbed water to the adsorbed oxygen molecule on the Fe(II) sites. Type II reactions present higher activation energies and lead to the formation of a SO bond through the hydrogen atom transference from a water molecule to the Fe(III)−OH− species, forming Fe(II)−OH2. These reactions present higher activation energies. The determinant step of this oxidation mechanism involves the formation of two adsorbed hydroxide species (OH−) on the surface. The hydroxides in the presence of water from the bulk liquid react to form two water molecules adsorbed on the surface and the first S−O chemical bond. Parallel reactions were investigated explaining the experimental detection of the O2− and OOH− species. Furthermore, the proposed mechanism explains the experimental observation that the oxygen present in the sulfate is mostly originated from water instead of an oxygen molecule. The present study strengthens the importance of the water/solid interface to understand the oxidation mechanism of pyrite in the presence of water at a molecular level.

1. INTRODUCTION Pyrite (FeS2) is the most abundant and widespread sulfide mineral on the Earth’s surface, and it plays an important role in geochemistry, biology, and environmental processes. Pyrite is the most common sulfide associated with gold, and it is frequently associated with valuable sulfide minerals such as sphalerite (ZnS), chalcopyrite (CuFeS2), and galena (PbS).1,2 Pyrite is also an undesired inorganic constituent present in coal. Flotation techniques have been developed to separate pyrite from the other valuable minerals.3 The pyrite surface has also been suggested to play a central hole in the origin of life, acting as a catalyst in the formation of prebiotic molecules.4,5 Recently, many new important technological materials have been synthesized exploring its semiconductor properties. Applications in solar cells,6 photochemical7,8 technologies, and heterogeneous catalysis9 have been reported. When pyrite is exposed to the environment, due to natural processes or anthropogenic activities, it is oxidized, forming sulfuric acid in the presence of humidity. This process consequently contributes to the acidification of natural waters, such as rivers, streams, and lakes. This phenomenon is called acid rock drainage (ARD),10 and it is considered one of the major environmental problems in regions with sulfide rich soils due to anthropogenic and natural activities. The eq 1 describes pyrite oxidation in the presence of water leading to the formation of acid (H+), sulfates (SO42−), and aqueous Fe(II) ions. © 2016 American Chemical Society

FeS2 +

7 O2 + H 2O = Fe 2 + + 2SO4 2 − + 2H+ 2

(1)

Many studies have been conducted aiming to understand the kinetics and mechanism in which pyrite is converted to the ARD products in aqueous medium.11−18 The understanding of the mechanism at a molecular level is a challenge due to the complexity of the pyrite oxidation process. Numerous elementary pathways are involved in the mechanism. The sulfur atoms have oxidation number −1, and during the reactions, they end up with oxidation number +6, as sulfates. Therefore, it is expected that several electron-transfer pathways might be accessible in the oxidative process. Basolo and Pearson19 showed that, in the pyrite oxidation process, a minimum of 7 electron-transfer steps are necessary to describe completely the oxidation on the pyrite surface. Furthermore, it is well-known that different bacteria can catalyze the pyrite oxidation at a much rapid rate, making its mechanism even more complex.20 Many works identified the formation of oxidative dissolution products on pyrite, and surface sensitive techniques have been carried out to understand the pyrite reactivity in the presence of different adsorbents. Different techniques have been used such Received: November 8, 2015 Revised: January 10, 2016 Published: January 11, 2016 2760

DOI: 10.1021/acs.jpcc.5b10949 J. Phys. Chem. C 2016, 120, 2760−2768

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The Journal of Physical Chemistry C as X-ray photoelectron spectroscopy (XPS),21−34 scanning tunneling microscopy (STM),25,35,36 Fourier transform infrared spectroscopy (FTIR),37−39 and X-ray absorption near-edge structure (XANES).40 Investigations of a clean pyrite surface or in air conditions provide insights about the pyrite initial oxidation mechanism. In these experiments, initial intermediates and different reactivity sites on the surface can be identified. A molecular level view of the mechanism was studied by Sit et al.,41 in which a path for the formation of SO42− in the oxidation mechanism was proposed, although it suggested the formation of an exotic Fe(IV)O2− species, which might be readily reduced in the presence of water. This work aims to contribute for understanding the pyrite abiotic oxidation mechanism by oxygen in the presence of water using the DFT/plane-wave theoretical approach. We propose a modified mechanism, which is in good agreement with experimental observations and explains the oxidation products in a more chemically intuitive pathway. Initially, the reactivity of the pyrite surfaces with respect to water and oxygen is discussed. After, we discuss the initial steps of the reaction leading to the oxidation of the sulfur on the surface involving a water and oxygen mixture.

Pyrite crystallizes at room temperature in a face-centered cubic system (Pa3 space group), and presents four FeS2 units in the unit cell. The lattice parameter a was determined experimentally to be 5.4179(11) Å, and the Fe−S and S−S chemical bond lengths 2.262(3) and 2.177(4) Å, respectively. In the present calculation, the lattice parameter a and the Fe−S bonds were underestimated by no more than 0.037 Å (0.7%) and 0.017 Å (0.7%), respectively. S−S bonds were overestimated by about 0.002 Å (0.1%). These results show clearly that the DFT/PBE/plane-wave approach applied in this work can describe the pyrite atomic structure. From the optimized bulk, the (100) surface of pyrite was simulated. This cleavage plane is the most stable surface of pyrite.47,48 No defects were simulated in the surface structure. All atomic layers in the slabs were placed on the xy plane with periodic boundary conditions. Along the xy plane, a c(2 × 2) supercell was used concerning to bulk termination in the same direction. Vacuum spaces of 10 Å in the z-axis direction were used to avoid the interactions between two adjacent sheets. Slabs with 6 atomic layers thick were used in this work. Previous work showed that this number of layers is sufficient to reproduce the adsorption of water3 and hydrogen sulfide49 on the pyrite surface.

2. METHODOLOGY The electronic structure calculations were performed by means of the density functional theory (DFT) formalism using the generalized gradient approximation (GGA) for the exchange/ correlation (XC) functional. The XC functional proposed by Perdew, Burke, and Ernzerhof (PBE) was used.42 All calculations were performed with the plane-wave approach implemented in the Quantum-ESPRESSO package.43 Spinpolarized calculations were carried out, and the most stable spin state for all systems was reported in this work. A 0.02 eV Fermi−Dirac smearing of the occupations number around the Fermi level was employed. Vanderbilt44 pseudopotentials with Fe (4s2.03d6.04p0.0), S (3s2.03p4.03d0.0), and O (3s2.03p4.0) valence electron configurations were used. Following the Monkhost−Pack scheme, Brillouin zone integration was carried out at 1 × 1 × 1 special k-grids along the 2D Brillouin zone for all slabs, and 10 × 10 × 1 k-points were performed to obtain the density of states (DOS). For the bulk calculation, the 5 × 5 × 5 k-points mesh was used along the 3D Brillouin zone. For both, slab and bulk, the Kohn−Sham (KS) electron orbitals were expanded in a plane-wave basis set up to a kinetic cutoff of 680 eV (50 Ry). This cutoff was used to ensure convergence of the total energy within 10−5 eV. Geometry optimizations were performed using a conjugated gradient, and all atoms in the supercell were allowed to move until the maximum force component was smaller than 10−3 eV. Transition states were calculated by the CI-NEB (climbing image - nudged elastic band) method,45 also implemented in the Quantum-ESPRESSO package.43 The activation energies were calculated by the following equation Ea = E TS − E IS (2)

3. RESULTS AND DISCUSSION 3.1. Water and Oxygen Adsorption on Pyrite (100) Surface. The cleavage of pyrite occurs along the (100) plane (Figure 1a) direction through Fe−S bond breaking. As shown

Figure 1. (a) Pyrite bulk. (b) Ideal pyrite (100) surface structure. (c) 5-fold iron and 3-fold sulfur sites formed on the surface.

in Figure 1b,c, 5-fold iron sites and 3-fold sulfur sites are formed on the surface. All S−S bonds remain intact in the ideal cleavage. Figure 1 shows the optimized structure of the pyrite surface. The a lattice parameter for the 2D slab was estimated in 5.358 Å, 0.023 Å less than the bulk parameter. As the a parameter does not vary significantly, the Fe−Seq bond distance, oriented along the xy plane, was found to be 2.226 Å, 0.019 Å less than the bulk Fe−S bond. This small difference in the parameters is in good agreement with STM measurements carried out by Rosso et al.,50 which showed that no modification occurs on the xy plane of (100) pyrite surface. The structural modifications of the pyrite surface occur perpendicular to the plane of the surface, and it is directly related to the breaking of Fe−S bonds in this direction. In the cleavage, the Fe−Sax bond becomes stronger and shorter. The Fe−Sax bond was estimated to be 2.194 Å, 0.101 Å less than in bulk. Hung et al.,48 using PBE/plane-wave calculations, noticed the same behavior in the relaxation of the pyrite structure.

where ETS is the transition state energy and EIS is the energy of the initial state. Fourteen images were used in all calculations, and in the optimization process, each image converged to the minimum energy pathway using a convergence accuracy of 0.05 eV·Å−1. The velocity Verlet algorithm was used in all CI-NEB optimizations. The unit cell used in the bulk calculations was obtained by Brostiger and Kjekshus46 from crystallographic refinement data. 2761


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The Journal of Physical Chemistry C The electronic structure of the FeS2(100) surface is in good agreement with experimental data27,30,51,52 and other theoretical3,48 reported works. Figure 2a,b shows the ligand field theory

Figure 2. Pyrite (100) surface electronic structure. (a) Ligand field theory scheme showing the pyrite 3d orbital split. (b) DOS for the pyrite (100) surface.

scheme of the pyrite 3d orbital split and the density of states (DOS) of this surface, respectively. The top of the valence band consists predominantly of Fe 3d t2g states and the bottom of the conduction band of Fe 3d eg states, though S 3p states are present in both the conduction and the valence bands. The band gap obtained by our DFT calculations is 0.06 eV, in good agreement with STM measurements, which indicated a small band gap of approximately 0.04 eV.50 The FeS2(100) surface does not present spin polarization, as it was observed by other theoretical works.3,41,48 Experimental works identified spin polarization on the pyrite surface, but most of the authors attribute it to the nonstoichiometric portion of the surface. Several studies of water adsorption on the pyrite surface have already been reported.27,29,51,53−58 In these studies, there is a consensus that water molecules adsorb molecularly over the Fe(II) sites on the surface. Some authors27,33,51,53−56,59,60 have observed the presence of dissociated water; however, the results were interpreted considering that the water dissociates on nonstoichiometric areas of the surface. Theoretical works are in agreement with this issue,3,32,61 and the simulations show that water molecules adsorb molecularly on the Fe(II) sites on the surface. In our calculations, no minimum was found on the potential energy surface (PES) for molecular adsorption of water on the S22− surface sites. Guevremont et al.53 showed that the water molecules on the surface do not react with sulfur dimers and adsorb preferentially on the Fe(II) sites, as shown in Figure 3a. The Fe(II)−OH2 bond interaction occurs through the iron dz2 eg-like orbital that receives an electron pair from water and weakens the Fe−Sax bonds. The estimated Fe(II)− Sax bond size of 2.205 Å is 0.061 Å smaller than the same bond in the pristine surface. The magnitude of Fe(II)−Seq and S−S bonds does not change significantly after water adsorption. The Fe(II)−OH2 bond length is estimated to be 2.152 Å. This value must be compared to other published values in the range of 1.98 and 2.23 Å.3,32,41,61 The adsorption energy calculated for one water molecule adsorbed on the FeS2(100) surface is −14.5 kcal·mol−1. This value is in good agreement with the experimental value of −10 kcal·mol−1 obtained using temperature-programmed desorption analysis.55 Unlike what is observed for the adsorption of water molecules, in the presence of oxygen gas, the pyrite surface reacts and forms sulfates. Experimental studies27,32,62 have shown that, after some time of exposure to oxygen gas, the iron

Figure 3. Water and oxygen molecules adsorption on the pyrite surface. (a) Adsorption of water; (b) end-on adsorption of oxygen gas; (c) side-on adsorption of oxygen; (d) dissociative adsorption of oxygen on Fe(II) sites; (e) dissociative adsorption of oxygen on Fe(II) and S(−I) sites; (f) dissociative adsorption of oxygen on S(−I) sites. Values are in kcal·mol−1.

present on the surface is oxidized. Theoretical calculations32,54,63,64 show three stable forms of oxygen adsorption over the iron surface sites: (1) end-on approximation (Figure 3b), (2) side-on approximation (Figure 3c), and (3) dissociative adsorption (Figure 3d). In the end-on approximation, the calculated Fe−O distance was 1.872 Å, which is in good agreement with other theoretical studies that found distances in a range of 1.83−1.88 Å.41,64 The calculated O−O distance was 1.298 Å, also in agreement with the reported value of 1.29 Å.41 The estimated O−O distance for the superoxide, O2−, calculated in the same level of theory in vacuum, is 1.293 Å. It suggests that the oxygen in the end-on approximation receives one electron from the surface, and an Fe(III)−O2− group is formed. Superoxide was suggested to form on the surface by theoretical models and experimental observations.25,41 Side-on adsorption of the oxygen molecule was also investigated and compared with the previous published results. It forms two Fe−O bonds [Fe(III)−O−O−Fe(III)] with two adjacent iron(II) sites on the surface (Figure 3c). The observed Fe−O distances for side-on oxygen interaction are 2.017 and 1.960 Å, higher than those bonds distances observed for end-on adsorption, which might be compared to the reported values of 1.97 and 1.99 Å.41 The estimated O−O distance is about 1.365 Å, in good agreement with the value (1.38 Å) reported in the literature using the DFT/PBE level of theory.41 Oxygen dissociative adsorption calculated in this work is in good agreement with the previous theoretical study. The estimated Fe−O distance is 1.664 Å, compared to 1.67 Å, estimated by Sit et al.41 The adsorption energy for end-on, side-on, and dissociative oxygen adsorption is −14.9, −16.3, and −40.2 kcal·mol−1, respectively. As shown in Figure 2, the pyrite conduction band is predominantly composed by the eg orbital from iron surface atoms with a smaller participation of the sulfur orbitals. 2762


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The Journal of Physical Chemistry C Therefore, it is expected that both sites are adequate to adsorb the oxygen molecule. There is no information in the literature concerning the adsorption of an oxygen molecule on the sulfur sites in spite of its importance. Different initial adsorption structures for the oxygen molecule were tried; however, no minimum were found on the PES (see Figure S6 in the Supporting Information). On the sulfur sites, only a dissociative mechanism of the oxygen adsorption was found. S−O bonds are formed on the surface, as shown in Figure 3e,f. The structure of Figure 3f presents an adsorption energy of −53.8 kcal·mol−1, approximately −13.6 kcal·mol−1 lower than the dissociative adsorption involving two iron sites on the surface of pyrite. It is interesting to observe that, in the absence of water, oxygen will prefer to react with the sulfur atoms to form sulfur oxides, as it was observed experimentally by Kendelewicz et al.27 using synchrotron-based photoemission spectroscopy. The activation energies for the adsorption of oxygen species on the surface were evaluated using the NEB approach, as shown in Figure 3. The SO bond formed on the surface presents more stability than the Fe−O bonds. The activation energy for the conversion of the end-on to side-on adsorbed oxygen was estimated to be about 0.3 kcal·mol−1 and the dissociation of the oxygen molecule to be about 5.4 kcal·mol−1. The migration of the dissociated oxygen on the iron sites to the sulfur sites on the surface was simulated considering the formation of an intermediate (Figure 3e) containing an oxo group coordinated to a S(−I) site and another oxo group coordinated to an Fe(II) site. The activation energy for the formation of this intermediate is 11.4 kcal·mol−1. On the next step (Figure 3e,f), the activation energy to form a structure with the oxygen atoms bonded on top to two S(−I) sites was estimated at 23.8 kcal·mol−1. 3.2. The Initial Oxidation Mechanism of Pyrite Surface in the Presence of H2O/O2 Mixture. Several studies reported the success of using surface sensitive spectroscopic techniques for identifying species (OH−, OOH−, O2−, OH2) present on the initial steps of the oxidation of pyrite in the presence of water and oxygen.25,27,29,32,33,35,51,53−56 Nesbitt et al.29 showed that the formation of sulfates occurs after a long period of surface exposure to a humid air atmosphere. Therefore, more attention was given to the initial stages of the pyrite oxidation, and various mechanisms were tested to identify the activation energy required for the formation of the first S−O bond. Experiments with isotopically labeled water (H216O/H218O),65−70 have shown that most of the oxygen atoms incorporated into sulfates are derived from water molecules. Consequently, it was assured that all mechanisms proposed were coherent with the experimental observations. According to our observations, the oxidation reactions on the pyrite surface in the presence of water molecules and oxygen might be classified into two types: I and II (as shown in Figure 4). Type I reaction is a redox process that involves two iron sites on the surface. In this reaction step, the iron sites are oxidized to form Fe(III)−OH− (or Fe(IV)O2−). For that to happen, one Fe(II)−OH2 surface group transfers a radical hydrogen to OH− or O2− adsorbed species, reducing the iron sites. Schaufuss et al.33 proposed a mechanistically oxidation process involving a redox reaction on the surface iron sites. In this work, a redox process was proposed involving the electron transference between iron and adsorbed species on the pyrite surface. The key point to understand the pyrite oxidation mechanism is how the oxygen can act over the surface oxidizing the iron centers. Eggleston et al.25 proposed the initial reaction

Figure 4. Scheme showing the different reaction types (I and II) investigated on the pyrite surface.

steps involving the oxygen adsorption and the transference of one electron of the surface to the oxygen molecule. Therefore, this initial step must be studied in detail. Type II reactions occur with the transference of a radical hydrogen, from a water molecule involved in a hydrogen bond, to the Fe(III)−OH− or Fe(IV)O2− groups formed by a previous Type I reaction. After the transference, S−OH and Fe(II)−OH2 (or Fe(III)− OH−) groups are formed on the surface. We will show that, if two OH− (or O2−) groups are available, one SO bond is formed and two iron sites are reduced on the surface. The S−O first bond mechanism formation starting from the end-on oxygen adsorption was investigated. The water and oxygen molecules adsorb in adjacent iron sites on the pyrite surface, forming a hydrogen bond (Figure 5a). The adsorption

Figure 5. First S−O bond forming from end-on oxygen adsorption. Values are in kcal·mol−1.

of the species was estimated in −35.3 kcal·mol−1. Comparing this value with the adsorption energies of a water molecule (−14.5 kcal·mol−1) and end-on oxygen molecule (−14.9 kcal· mol−1), the hydrogen bond energy was estimated to be −5.9 kcal·mol−1. This value is close to the experimental value of the hydrogen bonding in water dimers formed at gas phase (−5.4 kcal·mol−1).71 The second step (Figure 5b) is a Type I reaction with a radical hydrogen transference from the water molecule to the oxygen molecule, and the groups Fe(III)−OOH− and Fe(III)−OH− are formed. The activation energy (Ea) and the energy change (ΔE) for this reaction are 1.8 and 3.0 kcal·mol−1, respectively. The S−OH bond formation on the surface occurs by a Type II reaction. First, the water molecule adsorbs on the surface and interacts with OOH− and OH− groups by hydrogen 2763


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The Journal of Physical Chemistry C bonding (Figure 5c). The distances between water oxygen and the oxygen from OOH− and OH− groups are 3.819 and 3.063 Å, respectively. The distance between water oxygen and the surface sulfur atom (S−OH2) is 3.092 Å. The ΔE for the water in this atomic configuration is −0.3 kcal·mol−1. The Ea and ΔE for the formation of the S−OH are 18.2 and 8.4 kcal·mol−1, respectively, see Figure 5d. The reactions with bulk water to form hydroxyl radical (·OH) from Fe(III)−O2− (Figure 6a,b) and from Fe(IV)O2− (Figure 6c,d) have also been calculated.

Figure 7. Hydroxide (OH−) formation from the dissociative and sideon oxygen adsorption. Values are in kcal·mol−1.

−35.1 kcal·mol−1, respectively. In dissociative oxygen adsorption, a hydrogen bond formation stabilizes the system in −6.1 kcal·mol−1 per water molecule. The activation energy for the oxygen dissociation on the surface in the presence of water molecules is about 1.4 kcal·mol−1 smaller than in the absence of water molecules (Figure 3). This might be due to the stabilization of the transition state by hydrogen bonding formed between water molecules and O2− species. To form OH− species, a Type I reaction with transference of two radical hydrogens to two O2− species occurs. Thus, four OH− groups are formed on the surface. As shown in Figure 7b,c, this reaction is energetically favorable with an energy change of −5.2 kcal·mol−1 and an activation energy of 5.0 kcal·mol−1, slightly higher value than the breaking of the O−O bond reaction of the side-on oxygen molecule. Starting from the structure of the adsorbed side-on oxygen, the reaction of direct formation of OH− groups was evaluated, without the oxygen molecule dissociation; see Figure 7a,c. The activation energy for this reaction was estimated at 4.1 kcal·mol−1, the same value than in the reaction involving the dissociated oxygen as an intermediate. Another theoretical work41 simulated a phase diagram of the adsorbed species on the surface under standard conditions of temperature and pressure, indicating that the OH− groups over the surface presents larger stability. The authors observed the formation of the iron surface with all sites bonded to OH− species on pyrite. To the best of our knowledge, no experiment using surface science techniques indicated the presence of Fe(IV)O2− groups on the (100) pyrite surface. The difference between the activation energies of the reactions may not be used to unambiguously justify the presence of the Fe(IV)O2− group on the surface during the oxidation process. Probably, the O−O bond dissociation reaction occurs on the surface; however, experiments do not identify this species. A possible explanation for this lies in the reactivity of the O2− group. Even if it is formed, it will react immediately to form the OH− species in the presence of water. Looking again at Figure 7, it can be seen that the O−O bond cleavage activation energy has the same magnitude than the forming step of the OH− species. This shows that, if the O−O bond cleavage occurs, the system will have enough energy to form OH− groups by Type I reaction over the pyrite surface. We also evaluated the formation of Fe(III)−OH− species on the surface by the reaction between Fe(IV)O2− groups and the bulk water with the formation of hydroxyl radical in the medium; see

Figure 6. Hydroxyl radical (·OH) formation from Fe(III)−OO− and Fe(IV)O2− pyrite surface groups. Values are in kcal·mol−1.

The reaction energy estimated in the same theoretical level to form Fe(III)−OOH− is 53.1 kcal·mol−1, a rather higher value found for the reaction involving two iron sites on the surface. Besides the reaction shown in Figure 5, other reactions involving Fe(III)−OOH− and Fe(III)−OH− groups were tested. Other mechanisms for the S−O formation from sideon oxygen were evaluated, and the results are available in the Supporting Information. The barriers and reaction energies are at least 4.4 kcal·mol−1 larger in energy than the reaction path shown in Figure 5. Through a Type I reaction, a water molecule could adsorb on an Fe(II) site near the OOH− group; then, the hydrogen radical is transferred and a hydrogen peroxide (HOOH) molecule is formed. This possibility was evaluated, but HOOH species do not present stability in such a medium. Experimental studies have shown that peroxides could be formed on the surface of pyrite,59 but probably it would be through a different mechanism. The other possibility is an adsorbed water molecule at the Fe(II) site neighboring the Fe(III)−OH− group. Reactions with hydrogen radical transference to Fe(II)−OH− and transference of hydroxyl radical to sulfur atoms were tested. However, it presented a rather unfavorable activation and reaction energy of 40.2 and 30.0 kcal·mol−1, respectively. This reaction has an activation energy 2 times higher than the one proposed in Figure 5, and it shows that the reaction to form the S−OH bond on the surface involves water from the bulk. The S−O bond formation mechanism, involved in the sideon and dissociative adsorption, was evaluated. To investigate the reactivity of Fe(III)−O−O−Fe(III) and Fe(IV)O2− groups on the surface, we initially considered the end-on oxygen adsorption with two water molecules adsorbed on adjacent Fe(II) surface sites, as shown in Figure 7a. The O···O distances between water molecules and the side-on oxygen atoms are 3.572 Å, and the hydrogen bonding stabilization does not happen. From this structure, the formation of four Fe(III)− OH− groups might happen for the dissociation of the oxygen O−O bond, followed by a Type I reaction. The O−O bond dissociation leads to the oxo group adsorption on the surface; see Figure 7b. The Ea and ΔE for this reaction are 4.1 and 2764


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The Journal of Physical Chemistry C Figure 6c,d. The ΔE for this reaction is 85.2 kcal·mol−1, and the formation of a ·OH radical is not favorable. The formation mechanism of the first S−O bond was established considering the dissociated oxygen on the surface, as shown in Figure 8a. This reaction was also previously

values observed for the end-on (18.2 kcal·mol−1) and dissociative (19.0 kcal·mol−1) oxygen adsorption. 3.3. Discussion. From the mechanism model proposed in this paper, we predicted the formation of the initial oxidation species observed experimentally on the pyrite (100) ideal surface. It was possible to clearly understand the role of oxygen and water molecules in the oxidation mechanism of pyrite. An important aspect, not considered in the simulations, is that there is no perfect crystalline pyrite surface in nature, and moreover, the presence of other minerals and microorganisms activity can significantly influence the oxidation process. The aim of this work is to provide insights about the first steps of the oxidation mechanism of the (100) surface in the presence of oxygen and water. The oxygen molecule has a pivotal role in the initial stages of the pyrite oxidation mechanism. The oxygen acts by oxidizing the iron sites of the pyrite surface to form O2− and O2− species, which readily react with adsorbed water molecules. The reaction of these species with the bulk water is less favorable than reactions involving water species molecularly adsorbed on the surface. On the basis of this, we proposed a reaction mechanism involving two iron sites (Type I), and another reaction mechanism involving one iron and one sulfur surface site (Type II). Reactions with the formation of hydroxide radical are not favorable on the pyrite ideal surface. The O2− group has been suggested in the literature25,29,41 as a group capable of reacting with water molecules on the surface and forming OOH− species. Our calculations show that the formation of this species can be formed on the surface, but it is not energetically favorable, which explains that only a small portion of the surface is covered by these species in the oxidation process.29 Another fact is that the OOH− group has no reactivity on the surface of pyrite. Then, when this species is formed, it can shift the chemical equilibrium to the formation of the Fe(III)−OH− group, which reacts by Type II reaction to form S−O bonds on the surface. In unveiling the oxidation mechanism of pyrite, the most difficult step is to understand the interaction between the oxygen molecule and the pyrite surface. We showed that oxygen molecules adsorb on the iron surface sites in three different ways: end-on, side-on, and dissociative. These three adsorption sites were described in the literature, and our results are in good agreement with these observations.3,41,63,64 The oxygen molecule adsorbs on the pyrite surface, forming S−O bonds with sulfur atoms on the surface. This is in agreement with Kendelewicz et al.,27 who observed the formation of S−O on the pyrite surface after exposure of it to atmospheric oxygen. Their findings are in contrast with the observations of Guevremont et al.,55 but they argued that the formation of S−O bonds was not observed because of the different exposure times in the earlier experiments. Our results show that the activation energy for the formation of the S−O bond in the presence oxygen molecules is about 18 kcal·mol−1, and it is energetically favorable, suggesting that the formation of the S O bond from oxygen molecules might occur on the surface. Our results do not discard the formation of an intermediate containing a ferryl−oxo (Fe(IV)O2−) group, where Fe(IV) site is present. However, our calculations, along with the experimental results interpretation, have clearly showed that this group can readily react to form the Fe(III)−OH− group, which will be predominant on the surface during the initial stages of the oxidation process. We showed that the formation of the Fe(III)−OH− groups may occur directly by horizontal oxygen adsorption. This reaction has a slightly lower activation

Figure 8. First S−O bond forming from dissociative and side-on oxygen adsorption. Values are in kcal·mol−1.

investigated, and our results are in good agreement with the reported values.41 In this adsorption, two Fe(IV)O2− groups are formed on the surface. The water molecule is stabilized by hydrogen bonds with the Fe(IV)O2− groups formed on the surface from the oxygen dissociative adsorption (Figure 8a). The distance between the water and O2− oxygen atoms was found to be 2.940 Å. The energy estimated for the formed hydrogen bond is about −7.7 kcal·mol−1. The Type II reaction was simulated to evaluate the formation of the first S−O bond from oxygen dissociated on the surface. In this reaction, two O2− groups are available, and two hydrogen radicals are transferred over the surface, forming SO and Fe(III)−OH− groups (Figure 8b). The activation and reaction energies are 19.0 and −19.3 kcal·mol−1, respectively. The activation energy is slightly greater than the value for the formation of the first S−OH bond from the end-on oxygen adsorption. We also evaluated the stability of the intermediate species formed after the hydrogen radical transference, forming the S−OH bond. However, this intermediate is not a minimum in the potential energy surface (see Figure S9 in the Supporting Information). In the geometry optimization process, the hydrogen atom from the S−OH group is rather transferred to the oxygen group of Fe(IV)O2−, forming the structure shown in Figure 6b. The third possibility for the S−O first bond formation mechanism involves the formation of OH− species on the surface. From the structure containing four OH− groups, the step of S−O bond formation was made following a Type II reaction step. This reaction occurs from an adsorbed water molecule near two OH− groups on the surface (Figure 8c). The adsorption energy of the water molecule is −0.4 kcal·mol−1. The O−O and S−OH2 were estimated to be about 2.937 and 3.092 Å, respectively. The next step is the transference of two hydrogen radicals from the water molecule to two OH− groups. The SO and Fe(II)−OH2 are formed on the surface (Figure 8d). This step has an energy change of −23.3 kcal·mol−1. The activation energy estimated by the calculation is 11.6 kcal· mol−1, a value about twice lower than the activation energy 2765


Article

The Journal of Physical Chemistry C

also important to note that the reaction on the pyrite surface is multichannel and many different mechanisms can contribute to the global reaction. However, the adsorption of oxygen on the Fe(II) surfaces and the role of water in the mechanism are clear. It is also well characterized that the formation of Fe(IV)O2− species is not necessary or its lifetime must be very small. Actually, the pathway through radical hydrogen transfer from the adsorbed water molecules to the adsorbed oxygen, Fe(III)−OH− or Fe(IV)O2−, is predicted to have the same activation barrier and the reaction energy about 5 kcal·mol−1 more exothermic for the Fe(III)−OH−. Furthermore, one cannot forget that the reaction happens in the interface of water/solid, and the water is rather available on the surface. Understanding the mechanism of the oxidation of minerals at the environment abiotic conditions is not an easy task for experimentalists nor theoreticians. To model the surface of pyrite, it is necessary to make sharp assumptions such as purity, ideal structure, and well-defined phase. Actually, the surface of pyrite is normally nonstoichiometric, with the presence of different kinds of defects and impurities, which might have an important role in the mechanism, lowering the activation barriers. Furthermore, it is well-known that the incrustations of different sulfide minerals such as arsenopyrite72 are very common and a galvanic pair is formed. Modeling such kind of complex system is actually an open field, and much development at the theoretical methodology and experimental techniques is still necessary. However, this work shows clearly two classes of reactions that happen on the surface of pyrite in the presence of oxygen and water.

energy than the one in which Fe(IV) group-containing intermediates are formed. Therefore, the formation of Fe(IV)O2− groups cannot be disregarded based on the present accuracy of the DFT/plane-wave calculations. Furthermore, the presence of water on the pyrite surface was modeled in the present work with water molecules adsorbed on the specific surface, without taking into account the water bulk and the molecular dynamics. We believe that, in the presence of liquid water, the direct formation of the Fe(III)−OH− species will be favored, decreasing the activation energy. However, in the case of Fe(IV)O2− formation, our calculations clearly indicate that its lifetime might be very small, preventing one to detect it through spectroscopy. A valuable strategy for understanding the pyrite oxidation at a molecular level is the experiments with isotopically labeled water (H218O) in acid conditions. Bailey and Peters65 were the first to perform such analysis, and Taylor et al.68 confirmed their results in a broad range of temperature using mass spectrometry. Later, Usher et al.69,70 performed an in situ horizontal attenuated total reflectance infrared (HATR-IR) isotope study leading to the same conclusions. All the authors found that water is the primary source of oxygen atoms present in the sulfates observed in the medium. However, observing eq 1, only one water molecule (containing only one oxygen atom) reacts to form two sulfate molecules (containing a total of eight oxygen atoms). Our mechanism can explain this experimental observation (see Figure 4). The Type I reactions take place consuming the water adsorbed on the surface of pyrite, leading to the formation of the Fe(III)−OH− groups. As previously noted, these reactions have low activation energies. On the other hand, Type II reactions occur with the formation of S−O bonds on the surface from the reaction of Fe(III)−OH− to form Fe(II)−OH2, regenerating the water molecules on the surface. Another important aspect of the latter reaction is that it reduces the Fe(III) sites to Fe(II). This is in agreement with the experimental observations described by eq 1, where the species that is released in the medium is Fe(II). Another important aspect is the fact that the water molecules are formed after the reactions of Type II, which have the oxygen atoms originating from the oxygen molecules. As the sulfate formation occurs on the surface, the amount of these water molecules will increase considerably on the surface. As the water surface molecules are constantly being exchanged with the bulk molecules, the water containing oxygen atoms from oxygen molecules, can react with the Fe(III)−OH− groups to form S−O bonds. This mechanism can be used to justify the presence of some sulfates formed in between. Another justification is the mechanism shown in Figure 3, where it is shown that the formation of S−O bonds can occur directly from the oxygen molecules. This result clearly shows that parallel reactions might occur on the surface of pyrite, although in less extension. The mechanism involving the S−O bonds directly from oxygen molecules has a higher activation energy than the mechanism involving the formation of S−O bonds from bulk water molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10949. Structural parameters and surface energies for different slab models of (100) pyrite surface, residual dipole test, different reaction paths investigated, details about the methodology used for finding the position of water molecule in the intermediates of Type II reactions, and the minimum energy pathways versus reaction coordinate curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +55-31-3409-5748. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Prof. Heitor Avelino de Abreu for the fruitful discussions in the beginning of this project. The support of the Brazilian agencies Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional para ́ o Desenvolvimento Cientifico e Tecnológico (CNPq), and Coordenaçaõ de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES) is gratefully acknowledged. The National Institute of Science and Technology for Mineral Resources Water and Biodiversity has also supported this work − ACQUA-INCT (http://www.acqua-inct.org).

4. CONCLUSIONS The results shown in this work were able to establish a chemical picture of the pyrite oxidation process in the presence of water and oxygen in its initial stage. It is clear that the reactions of Type I and Type II are the main mechanism in which the oxygen, in the presence of water molecules, reacts on the surface, leading to the sulfate formation and Fe(II) species. It is 2766


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Pyrite Oxidation Mechanism by Oxygen in Aqueous Medium - The

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