Oxidation Mechanism of Arsenopyrite in the Presence of Water - The

Article pubs.acs.org/JPCC

Oxidation Mechanism of Arsenopyrite in the Presence of Water Juliana C. M. Silva,†,‡ Egon C. dos Santos,† Thomas Heine,‡,§ Heitor A. De Abreu,† and Hélio A. Duarte*,† †

Grupo de Pesquisa em Química Inorgânica Teórica−GPQIT−Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG), Av. Antônio Carlos, 6627, 31270-901 Belo Horizonte, MG, Brazil ‡ Department of Physics & Earth Sciences, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany § Wilhelm-Ostwald-Institute of Physical and Theoretical Chemistry, University of Leipzig, Linnéstr. 2, 04103 Leipzig, Germany S Supporting Information *

ABSTRACT: Arsenopyrite is commonly present in mining tailings and, together with pyrite, is responsible for acid rock drainage (ARD) phenomenon. The mineral undergoes oxidation in contact with oxygen and water producing a solution containing acid and heavy metals and, hence, causing important environmental impacts. Density functional/plane wave calculations were carried out to investigate the oxidation of arsenopyrite aiming to understanding its intricate mechanism at a molecular level. Molecular oxygen is dissociatively adsorbed in the Fe−O−As adsorption sites leading to the oxidation of arsenic and iron sites in good agreement with the available experimental data. It avoids the many steps observed for the pyrite oxidation mechanism. The presence of water is extremely important for the next steps of the oxidation mechanism similarly to the oxidation mechanism of pyrite. The present work reinforces the fact that the removal of the humidity can inhibit the oxidation of the arseno(pyrites) in an aerobic condition.

1. INTRODUCTION

efficient strategies of waste control with environmental, social and economic importance. Many investigations have been performed with the purpose of identifying the products formed in arsenopyrite oxidation in different media1−8 and two studies have been made concerning the structure of arsenopyrite surface.9,10 However, since the solutions formed in this process are very complex, as well as the reactions involved, the results found for products and kinetics of arsenopyrite oxidation are not in agreement and a lack of consensus is observed.1 Buckley and Walker,3 as well as Mikhlin et al.,7 suggested a depletion in Fe and As on arsenopyrite surface after acid treatment. Costa et al.11 observed elemental sulfur formation when arsenopyrite was exposed to acid. However, Richardson and Vaughan12 found the opposite: a surface enriched in Fe and As after reaction with H2SO4. Nesbitt and Muir5 reported the absence of sulfur on arsenopyrite surface reacted in acidic mine wastewater. Corkhill et al.8 observed the species Fe(III)−OH, As(III)−O, As(IV)−O, thiosulfate, and sulfate for arsenopyrite reacted in H2SO4 solution, and As is the most rapidly oxidized element in this condition. Nesbitt et al.2 and Schaufuss et al.13 found a preferential enrichment of arsenic as As-oxides on the overlayer of oxidized arsenopyrite due to diffusion of this atom

Arsenopyrite, FeAsS, is the most important arsenic containing mineral, normally associated with noble metals such as gold and copper. The mineral processing of arsenopyrite rich ores normally leads to mining tailings with high concentration of arsenic.1,2 Arsenopyrite is stable under reducing conditions, however, like other sulfide minerals, it is oxidized by weathering effects when exposed to the atmosphere in a process called acid rock drainage (ARD). This process occurs mainly during the industrial extraction of metals from ores, in which the sulfide mineral oxidizes in contact with water and oxygen, yielding sulfuric acid. This acid can solubilize the solid mineral constituents, producing a solution containing acid and dissolved metals, which can contaminate soils and natural waters. The oxidation reaction of arsenopyrite has slow kinetics, yet it can be catalyzed by microorganisms present in the environment.1 Specifically the ARD of arsenopyrite releases arsenite and arsenate species, which requires a special treatment during the oxidation process due to its toxicity.2 Arsenic is a public health problem in many regions of the world and the main environmental concern with respect to the noble metal extraction, which is found associated with pyrites. Mitigation of the ARD problem and the development of countermeasures are facilitated by understanding its mechanism. For this reason, understanding the kinetics and mechanisms of dissolution of this mineral in different conditions is crucial for envisaging © 2017 American Chemical Society

Received: September 29, 2017 Revised: November 12, 2017 Published: November 13, 2017 26887

DOI: 10.1021/acs.jpcc.7b09706 J. Phys. Chem. C 2017, 121, 26887−26894

Article

The Journal of Physical Chemistry C

Figure 1. Adsorption of O2 on (001) arsenopyrite surface: (a, b) end-on; (c, d) side-on. Yellow atoms are sulfur, violet are arsenic, brown are iron and red are oxygen.

from the bulk. In aqueous solution this layer can be leached. Nesbitt et al.2 also observed the same type of chemical species in air-oxidation and water-oxidation of arsenopyrite, however greater proportions of oxide species was found in the presence of water, especially S-oxides, which indicates that the oxidation process is more intense in this medium. Walker et al.14 did not find a dependence of the arsenopyrite oxidation rate on the concentration of O2, in disagreement with the results of Yu et al.15 This difference might be due to the different pH used in the previous studies. In fact, Yu et al.15 found a decrease in the release rate of As in pH above 6.5, reaching a minimum between pH 7 and 8. Nesbitt et al.2 and Schaufuss et al.13 agreed that As is more readily oxidized than Fe, and S, while McKibben et al.16 results show that Fe is dissolved quicker than As. Walker et al.14 proposed that the rate-determining step of the oxidation reaction is the attachment of oxygen from water to As and S species, while Corkhill et al.1 suggested that it could be the transference of electrons to the oxidant agent. These differences might come from the reaction conditions used in the investigation. Therefore, more information about arsenopyrite surface reactivity is necessary. The comparison with other sulfides might also help in the study of arsenopyrite. Mckibben et al.16 found that arsenopyrite dissolution is 3 to 4 times faster than pyrite. Pyrite is a similar sulfide mineral and has been extensively investigated, including DFT/plane waves calculations of water,17 H2S,18 and As(OH) 3 19 adsorbed on its surfaces, and its oxidation mechanism.20,21 Adsorption of water in different sulfides has also been investigated by DFT.22−24 The global oxidation reaction is described according to the following equation:14,15

4FeAsS(s) + 11O2 (aq) + 6H 2O(S) → 4Fe2 +(aq) + 4H3AsO3 (aq) + 4SO4 2 ‐(aq)

(1)

.The role of arsenic in the oxidation mechanism of arsenopyrite remains to be understood. In the present work, DFT/plane wave methods have been applied to investigate the oxidation mechanism of arsenopyrite. The preferential arsenopyrite (001) surface cleavage10 was used to study the coadsorption of water/O2 and the first steps of the oxidation mechanism.

2. METHODOLOGY The density functional theory (DFT)/plane waves calculations have been performed with periodic boundary conditions as implemented in the Quantum Espresso package.25 The (DFT)/plane waves method has been extensively tested in previously publications, and it is capable to describe electronic and structural properties of sulfide minerals.10,17,20,26,27 The ultrasoft pseudopotentials proposed by Vanderbilt28 with the following valence configurations: Fe (3s2 3p6 3d6.5 4s1 4p0), As (4s2 4p3), S (3s2 3p4) together with the PW9129 exchange/ correlation (XC) functional were used. After being tested, a wave function cutoff energy of 30 Ry, and 240 Ry for density were chosen, as well as a 2 × 2 × 1 K-point mesh sampling based on the Monkhorst−Pack scheme,30 besides a smearing of 0.02 Ry according to the model of Marzari-Vanderbilt.31 The energy conversion threshold was set to10−6 Ry. Spin polarization was applied initially on the iron surface atoms in all calculations. The Damped dynamics method32 was used in geometry optimization with Parrinello−Rahman extended Lagrangian,33 keeping a force tolerance criterion of 10−3 Ry Bohr−1. No symmetry constraint was applied to any atom in the structure. 26888


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Figure 2. Dissociation of O2 molecule on (001) arsenopyrite surface. Yellow atoms are sulfur, violet are arsenic, brown are iron and red are oxygen. Energy values are in kcal mol−1.

According to our previous works,10,27 the (001) plan is arsenopyrite’s most stable cleavage surface, therefore it was used in this work as a model of the surface. A slab model of a (2 × 2) unit cell, built from the atomic positions of an optimized bulk structure, was used for the surface model, in order to avoid lateral interactions between the adsorbed molecules in the neighbor cells. For the same reason, a vacuum of 15 Å was applied between slab layers to create the surface in the threedimensional cell. This model creates two surfaces, top and bottom, which were built symmetrically. The appropriate thickness of this slab surface was also investigated to ensure there are no edge effects, and a number of 12 atomic layers was chosen, for the surface energy convergence within 0.01J m−2. All adsorption energies were calculated using the eq 2. Ead = Esurf + mol − Esurf − Emol

modes. Figure 1 shows two of them: (a, b) end-on and (c, d) side-on adsorption. The calculated adsorption energy (Ead) on the first position was −16.6 kcal mol−1, similar to the values calculated for pyrite: −14.1 kcal mol−1 by Sit et al.,20 −14.9 kcal mol−1 by Dos Santos et al.,21 and −13.1 kcal mol−1 by Sacchi et al.35 The Fe−O distance of 1.78 Å was slightly shorter than the one calculated by Dos Santos et al.21 for pyrite, 1.87 Å, and the O−O distance was identical and equal to 1.30 Å. This is the same O−O distance as in superoxide ion, O2−, according to Table S1 in the Supporting Information, calculated at the same level of theory. This is an evidence that the bonded oxygen received an electron from the iron atom of the surface. The molecule axis is tilted in a 50° angle from the surface normal. It is 5° more than the same molecule on the surface of pyrite.35 For the side-on structure, the calculated adsorption energy was −6.2 kcal mol−1, about 10 kcal mol−1 higher than the one calculated for pyrite independently by Sit et al.20 and Dos Santos et al.,21 and 14.3 kcal mol−1 higher than the value calculated by Rozgonyi and Stirling.36 However, the Fe−O distances of 1.90 and 1.94 Å were shorter on arsenopyrite than the ones on pyrite21 (1.96 and 2.02 Å) and the O−O distance was longer, about 1.40 Å, compared to 1.37 Å for pyrite. This O−O distance is in the range between the O−O distances of peroxide ion, O22−, and hydrogen peroxide, H2O2 (see Table S1), which means that both oxygen atoms must receive an electron from each iron atom, oxidizing them. There are two different iron sites on arsenopyrite (001) surface. Figure S1 in the Supporting Information, shows the possible adsorption sites on the surface. Fe1 and Fe3 are more steric exposed, and Fe2 and Fe4 less exposed, as can be observed in Figure S1. The end-on adsorption on site Fe2 is less stable, −3.6 kcal mol−1, shown in Figure S2. Although the distance between Fe3 and Fe4 is the shortest one, and possibly the best for side-on oxygen adsorption, it is hindered by the S2 surface atom, which is in between and in a higher position than

(2)

,where Ead is the adsorption energy, Esurf+mol is the total energy of the surface with a molecule adsorbed, Esurf is the total energy of the relaxed isolated surface, and Emol is the total energy of the isolated molecule calculated in a box identical to the one used for the surface calculation. For the case of the O2 molecule, spin polarized calculation consistent to the triplet ground state were performed. The activation energies,E‡, were calculated using the NEB (nudged elastic band) method with CI (climbing image) option. These calculations were performed in Γ point using 14 images converged until the norms of the forces orthogonal to the path were below 0.05 eV·Å−1.

3. RESULTS AND DISCUSSION 3.1. Adsorption of Oxygen. It has been shown in our previous calculations that the Fe atom is the preferential adsorption site on arsenopyrite surface34 for different leaching agents. The adsorption of molecular oxygen on arsenopyrite (001) surface was investigated for different adsorption sites and 26889


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kcal mol−1 for arsenopyrite. Our predicted Fe−O distance in FeAsS was shorter by about 0.06 Å and the As−O distance longer by about 0.02 Å than the previous results. Sacchi et al.35 observed a more stable structure on the dissociation of adsorbed O2 with the O adatom adsorbed on S (−30.9 kcal mol−1) than on Fe (−17.8 kcal mol−1) at the (001) pyrite surface. Table 1 shows the effect of oxygen adsorption on the surface bonds in arsenopyrite. In general the bonds are elongated due

the two iron atoms. The adsorption between Fe4 and Fe1 is also hindered by As2 surface atom. The S2 and As2 are also more reactive than the other anion sites, since they have one dangling bond due to the bond breaking due to the surface cleavage. Since the atoms Fe2 and Fe3 are connected by the subsurface S1 atom, and the distance between them is shorter, it should be the favored site for side-on oxygen adsorption. In fact, a structure with the side-on oxygen adsorbed on sites Fe1 and Fe2 is not a minimum in the potential energy surface. It breaks the O−O bond and forms FeO bonds (Figure S5, parts a and b). Other positions of O2 adsorption on FeAsS surface were also investigated and are presented in the Supporting Information, Figures S2−S9. Although the side-on adsorption is less favorable than the end-on type, the dissociative energy of the side-on structure is −52.9 kcal mol−1, which makes the total process more favorable (Ead = −59.1 kcal mol−1) when the end-on adsorption is followed by the molecule dissociation (Figure 2). The breakage of O−O bond is energetically favorable, yielding 24.9 kcal mol−1 of energy, with low activation energy of 4.5 kcal mol−1 (see Figure S10). Sit et al.20 found a slightly higher barrier of 6.1 kcal mol−1 for the dissociation of O2 on the pyrite surface, and a more favorable energy change (ΔE) of −26.2 kcal mol−1. Dos Santos et al.21 found a barrier of 5.4 kcal mol−1 for the dissociation of O2 on the pyrite surface, comparable to our result, and an energy change of −23.9 kcal mol−1. Rozgonyi and Stirling36 found a similar barrier of 6.9 kcal mol−1 for pyrite, and observed that the migration of the O atoms to sulfur sites leads to more stable structures with an energy barrier of 13.4 kcal mol−1. Therefore, the O migration was considered as the determining step in this dissociation. The intermediate structure in Figure 2b presents a Fe4+=O species, which was proposed in the pyrite oxidation mechanism.20 However, recently, Dos Santos et al.21 showed that this intermediate is not necessary and in aqueous medium it would not exist or have a very short lifetime. On pyrite,21 this type of adsorption has an energy of −40.2 kcal mol−1, while for arsenopyrite, it is −31.1 kcal mol−1, about 9 kcal mol−1 higher in energy. However, in the case of arsenopyrite, the O atoms can bind to neighbor As atoms, yielding a Fe−O−As bridge site, shown in Figure 2c. The adsorption energy is predicted to be −59.1 kcal mol−1. This bond formation increases its stability and has not been predicted for pyrite. Both Fe and As are oxidized to Fe3+ and As0 and the O atoms are reduced to O2−. The calculated ∠ Fe−O−As angle was 80.7° on Fe1 site and 74.2° on Fe2 site. This structure is more stable than the similar one, in which O binds to Fe1 and Fe2 by 0.6 kcal mol−1 (Figure S6a). The structures that form Fe−O−S bonds were also investigated and are at least 19.2 kcal mol−1 less stable, as presented in Figure S7. The displacement of the oxygen atom from the bridge site to the As site (Figure 2d) is not favorable by about 18.9 kcal mol−1, indicating that the bridge Fe−As sites are the most stable for the oxygen atom adsorption. Therefore, the most stable structure is the one in which the O atoms bind to Fe and As surface sites. The energy barriers of the reactions have the same magnitude of about 4−5 kcal mol−1. The case in which one O atom adsorbs to a Fe atom and the other one to an As atom was observed by Li et al.37 for pyrite doped with As using DFT/PW91, similar to what we have calculated for FeAsS in Figure S4a. The adsorption energy was −58.2 kcal mol−1 for the pure surface and −59.3 kcal mol−1 for the surface doped with As, which must be compared to −43.6

Table 1. Atomic Distances on the Arsenopyrite (001) Surface as an Effect of the Oxygen Adsorption (All Distances in Å) Fe−Asa Fe−Sa Fe−As axial Fe−S axial As−Sa Fe−O a

pristine surface

molecular

dissociative

2.323 2.202 2.308 2.123 2.374 −

2.360 2.220 2.464 2.119 2.375 1.783

2.372 2.227 2.450 2.143 2.372 2.065, 1.882

Average of the bonds around the adsorbed atom.

to oxygen adsorption, either molecularly or dissociatively, except for the As−S bond, which keeps almost the same length since it is the strongest bond on the surface. 3.2. Co-Adsorption of Oxygen and Water. Nesbitt et al.2 observed that the oxidation of FeAsS proceeds in a greater extent in the presence of water than in air. Therefore, the coadsorption of water and oxygen on arsenopyrite surface was investigated and it is also very favorable. The adsorption of one water molecule between two adsorbed oxygen atoms forming hydrogen bonds, Figure 3a, has energy of −7.1 kcal mol−1. When the water molecule adsorbs to a neighbor iron, it is −23.3 kcal mol−1 (see Figure 3b), compared to the value of −32.3 kcal mol−1 calculated by Sit et al.20 in a similar structure for pyrite. The adsorption energy of two water molecules on the same surface, in which one molecule adsorbs to a Fe atom and the other one forms hydrogen bonding to the adsorbed water, Figure 3c, is −28.3 kcal mol−1. It is reasonable to think that these species must act together in a surface oxidation process mimicking the bulk water at the solid/water interface. If the water molecules from the bulk water bind through hydrogen bonds to the adsorbed oxygen, the adsorption is weaker, about −10.7 kcal mol−1 (Figure 3d). Nesbitt et al.2 based on XPS experiments found the species O2−, OH−, H2O, and FeOOH on an oxidized arsenopyrite surface, besides the arsenic and sulfur compounds. An attempt to investigate the formation of the oxihydroxide species is presented in Figure 4, where a water molecule is adsorbed dissociatively to the oxygen end-on structure of Figure 1a in a stable compound. The adsorption energy is predicted to be −30.8 kcal mol−1, about −14.2 kcal mol−1 more stable than the end-on adsorbed oxygen. A value very close to pyrite 21 adsorption energy for the same species (−33.5 kcal mol−1). Then this type of adsorption structure is favorable. This species is involved in the oxidation path proposed by Dos Santos et al. 21 for pyrite. Table 2 lists the adsorption energies and metal−oxygen distances of the most stable structures for O2 adsorption on arsenopyrite surface. The most stable structure is formed in the dissociative adsorption, where the O atom binds to Fe and As atoms forming a bridge (Figures 2c, S6c, and S6d). The 26890


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Figure 3. Co-adsorption of water and oxygen to (001) arsenopyrite surface. Yellow atoms are sulfur, violet are arsenic, brown are iron, red are oxygen, and white are hydrogen.

Figure 4. Adsorption of oxihydroxide species to (001) arsenopyrite surface (see text for details). Yellow atoms are sulfur, violet are arsenic, brown are iron, red are oxygen, and white are hydrogen.

adsorbed O atoms on the surface, as considered by Dos Santos et al.21 and by Sit et al.20 for pyrite (see Figure 5). The process starts with the dissociative adsorption of an O2 molecule on the arsenopyrite surface. By this, the Fe and As atoms are oxidized to Fe3+ and As0. Subsequently, a water molecule approaches the O adsorbed, with ΔE = −1.2 kcal mol−1. This water molecule donates a hydrogen to the O adsorbed and the OH− binds to the neighbor As, oxidizing it to +1 and forming the As−OH bond. The energy change is ΔE = −15.2 kcal mol−1 and the energy barrier 8.5 kcal mol−1 (Figure S12a). This step is similar to the one calculated by Dos Santos et al.21 for pyrite, in which the OH− binds instead to a Fe atom, with energy barrier of 5.0 kcal mol−1 and ΔE = −5.3 kcal mol−1. Sit et al.20 found values of ΔE = 0.6 kcal mol−1 and a barrier of 2.9 kcal mol−1 in a similar hydrogen atom transfer. In the oxidation of arsenopyrite, the previous step is repeated with the second O adsorbed

coadsorption of water and O2 shows that the most stable structure is the one that a water molecule not only binds to an O atom in a hydrogen bond, but also adsorbs to a Fe surface atom. Starting from a surface fully covered by water, it is also possible to adsorb oxygen on the iron sites present in the arsenopyrite (001) surface, as shown in the Figure S11. The replacement of an adsorbed water molecule by an end-on oxygen is favored by −6.0 kcal mol−1 (see Figure S11, parts a and b). In the case of the water replacement by the dissociated oxygen, the process is favored by −43.8 kcal mol−1. 3.3. Oxidation Reaction. The reaction path was investigated based on the oxidation mechanism proposed for pyrite, taking into account how the water and oxygen are preferentially coadsorbed on arsenopyrite. One possibility is the donation of a hydrogen atom from water molecules to the 26891


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Table 2. Adsorption Energies, Ead, and M−O Distances for the O2 and O2 + H2O Adsorption on the Arsenopyrite (001) Surfacea species O2

O2 + H2O

O2 + 2H2O a

type

sites

Ead/kcal mol−1

end-on side-on dissociative dissociative dissociative dissociative dissociative, H bond dissociative, adsorbed end-on, dissociative dissociative, adsorbed dissociative, H bond

on top Fe1 bridge Fe2, Fe3 on top Fe2, Fe3 on top As, As on top S, S bridge Fe2−As, Fe3−As Fe2, Fe3 Fe2, Fe3 Fe1, As Fe2, Fe3 Fe2, Fe3

−16.6 −6.2 −31.1 −40.2 −26.3 −59.1 −66.2 −82.4 −30.8 −87.4 −69.8

M−O distance/Å 1.78 1.90, 1.68, 1.67, 1.52, 2.07, 2.09, 2.13, 1.80, 2.13, 2.10,

1.94 1.64 1.67 1.52 1,75; 1.88, 1.78 1.88 1.89 1.83 1.92 1.92

M = Fe or As.

Figure 5. Proposed oxidation reaction for the arsenopyrite (001) surface. All energies are in kcal mol−1. Yellow atoms are sulfur, violet are arsenic, brown are iron, red are oxygen, and white are hydrogen.

with a water adsorption energy of −5.5 kcal mol−1. The hydrogen transfer yields −9.1 kcal mol−1 of energy and has an energy barrier of 21.9 kcal mol−1. Compared to pyrite,21 the energy changes are more favorable in the last steps, but the barriers are higher. Next, another water molecule adsorbs to a neighbor Fe atom, with −22.6 kcal mol−1 of energy and donates a hydrogen atom to the adsorbed OH group, forming water and reducing the initial oxidized Fe again to Fe2+. This process yields −1.3 kcal mol−1 of energy, with a very low barrier of 0.2 kcal mol−1 (Figure S12c). Again another water molecule approaches, yielding −9.6 kcal mol−1 of energy and donates a H to the other OH group with an energy barrier of 1.5 kcal mol−1 (Figure

S12d) and ΔE = −11.6 kcal mol−1. In this process a new surface As atom is oxidized to As0. Compared to pyrite,21 this second H donation is less favorable, but with lower energy barriers. All the steps investigated for FeAsS oxidation are energetically favorable, and the results for the energy barrier indicate that this reaction mechanism is relatively low compared to the pyrite oxidation mechanism. It is also in agreement with isotopic labeling experiments on pyrite38 that suggested the oxygen atoms of the sulfate come from water molecules. A variation of the previous mechanism includes the bonding of the OH groups to the S atom instead of the As atom (see Figure S13a). As expected, this situation is less stable than the 26892


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Figure 6. Migration of the OH group from an Fe atom to an As atom on the arsenopyrite surface. Yellow atoms are sulfur, violet are arsenic, brown are iron, red are oxygen, and white are hydrogen.

previous one, by 17.8 kcal mol−1. The situation where two OH groups are bonded to S atoms is 47.8 kcal mol−1 less stable than in the case of As bond (Figure S13b). An attempt to form OH groups that are only connected to the Fe and S atoms was unsuccessful (Figure S13c). One of the OH groups transfers a hydrogen to form water on top of Fe adsorption site. The energy involved is estimated to be 30.8 kcal mol−1 higher than for the structure involving As, i.e., it is less stable. Therefore, the first steps in the arsenopyrite oxidation must involve the reaction of the As rather than the S atom. If the OH group bound to the Fe atom in the last structure of Figure 5 is bound to an As atom, thereby oxidizing it and reducing the Fe, the structure is 5.0 kcal mol−1 more stable than the previous one (see Figure 6). This shows that in the beginning of the oxidation process of arsenopyrite, the surface Fe atoms are oxidized and then reduced to Fe2+, in accordance with eq 1 and with the experimental results of Corkhill et al.,8 Nesbitt et al.,2 and Schaufuss et al.,13 who observed that As is more readily oxidized than Fe and S. The highest energy barrier observed in the previous reaction steps was 21.9 kcal mol−1 for the hydrogen transfer from water to the adsorbed O to form Fe(III)−OH and As−OH species. This is in agreement with Walker et al.,14 who proposed that the rate-determining step of FeAsS oxidation reaction is the bonding of oxygen from water to the As and S surface sites. This value is in the range of the reaction energies calculated by Sit et al.20 for the oxidation of pyrite. As discussed by these authors, this value of energy barrier makes the reaction process slow, but it is still possible at room temperature. It should be noted that the method used in this study considers the system as a perfect crystal at 0 K. Finite temperature and defects on the surface can decrease even more this value. Experimentally, Yu et al.15 found a value of 13.6 kcal mol−1 for the activation energy of arsenopyrite in the reaction with O2 at pH 5.9. It is important to note that defects on the surface are more reactive and, therefore, the oxidation process usually starts at defective sites. However, an ideal surface such as the used in the present work to model the oxidation mechanism provide important insights and relevant information about the arsenopyrite surface oxidation.

the oxidation of Fe and sulfur centers. The presence of more reactive arsenic sites on the surface is the most important difference compared to the pyrite system. The coadsorption of water is necessary to the next oxidation steps leading to the formation of arsenite and releasing Fe(II) species. Following Dos Santos et al.,21 the type 1 reactions in which a hydrogen atom is transferred from the adsorbed water molecules to the Fe(III)−OH, reducing the Fe sites to the +2 oxidation states are also present with similar energy barriers and reaction energies. The type 2 reactions, in which a hydrogen is transferred from the water and the OH· oxidizes the sulfur, are also present in the arsenopyrite but, in that case, it prefers to oxidize the arsenic atoms. The oxidation mechanism proposed in the present study is in agreement with the experimental findings of Corkhill et al.,8 Nesbitt et al.2 and Schaufuss et al.13 and highlights the role of the arsenic in the oxidation mechanism. Furthermore, it suggests theoretical evidence of why arsenopyrite is preferentially oxidized with respect to pyrite16 leading to the formation of arsenite. The oxygen molecules readily dissociates on the surface leading to the oxidation of the Fe(II) and As sites avoiding the many steps observed for the pyrite oxidation mechanism. Furthermore, it reinforces why the removal of the humidity is necessary to avoid the oxidation of arseno(pyrites).

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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.7b09706. Structural details of the adsorbed oxygen and water in the different adsorption sites on arsenopyrite (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(H.A.D.) E-mail: [email protected]. ORCID

Thomas Heine: 0000-0003-2379-6251 Heitor A. De Abreu: 0000-0001-5324-6010 Hélio A. Duarte: 0000-0001-8164-4454 Notes

4. FINAL REMARKS On the arsenopyrite surface, the oxygen molecule is preferentially adsorbed dissociatively on bridge Fe−As sites. The Fe and As sites are readily oxidized to +3 and 0 oxidation states, respectively. This mechanism is very different compared to the pyrite analogue, where the oxygen is molecularly adsorbed and water has an important role in the first steps of

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of the Brazilian agencies Conselho Nacional para o ́ Desenvolvimento Cientifico e Tecnológico (CNPq), Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), and Coordenaçaõ de Aperfeiçoamento de Pessoal de Ensino 26893


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Superior (CAPES) are gratefully acknowledged. In addition, the National Institute of Science and Technology for Mineral Resources, Water and BiodiversityINCT-ACQUA (http:// www.acqua-inct.org), and the European Commission FP7PEOPLE-2011-IRSES TEMM1P, GA 295172, who have supported this work, are thanked.

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REFERENCES

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Oxidation Mechanism of Arsenopyrite in the Presence of Water - The

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