Water Adsorption on the Reconstructed (001) Chalcopyrite Surfaces

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Water Adsorption on the Reconstructed (001) Chalcopyrite Surfaces Guilherme Ferreira de Lima, Claudio de Oliveira, Heitor Avelino de Abreu, and Helio Anderson Duarte* GPQIT, Departamento de Química, ICEx, Universidade Federal de Minas Gerais, UFMG ABSTRACT: The interaction of water molecule with the reconstructed (001) chalcopyrite surfaces has been investigated by means of density functional calculations. All of the calculations were performed using periodic boundary conditions with SIESTA code. The structural parameters were compared with those obtained through PWscf code in order to evaluate the pseudopotentials and numerical basis set developed for this work. Two different surfaces were studied, namely, sulfur-terminated, (001)-S, and metal-terminated, (001)-M. The (001)-S surface reconstructs, forming disulfide dimers with a bond length of 2.23 Å. The (001)-M surface reconstructs, reordering the metal atoms in order to form planes of metal atoms and interlaced sulfur atoms. Different adsorption sites for the water molecule were investigated. The dissociative mechanism of the water molecule has also been analyzed in detail. For the (001)-S surface, the water adsorption on the iron atom is the preferred mechanism. The dissociative mechanism leads to structures which are, at least, 13 kcal mol1 higher in energy than the water adsorbed on iron atom. For the (001)-M surface, no minima in the potential energy surface were found, and the water molecule prefers to form a hydrogen bond with the sulfur atoms. The dissociative mechanism for the water adsorption on (001)-M surface is thermodynamically unfavorable. The metal-alloy-like structure underneath of the sulfur atoms and the unfavorable water adsorption indicate that the surface presents some hydrophobic character. The influence of the water molecule in the reconstruction of the (001) chalcopyrite surface and in its reactivity is discussed.

1. INTRODUCTION Chalcopyrite (CuFeS2) is a sulfide mineral which crystallizes in the tetragonal system (space group|42d) with four formulas per unit cell (16 atoms) and lattice parameters a = 5.289 Å and c = 10.423 Å.1 In chalcopyrite, each sulfur atom is bonded to two iron atoms and two copper atoms, forming a tetrahedral, while each metallic atom is bonded to four sulfur atoms, also forming a tetrahedral.1 Chalcopyrite is an antiferromagnetic material with alternate planes of iron with spin density up and spin density down along the c direction.2 From the environmental point of view, chalcopyrite and other sulfide minerals are relevant once they are associated with the acid mine drainage (AMD), which is a critical problem for regions near mining activities. The AMD is caused by the oxidation of the sulfide minerals in the environment, producing sulfuric acid and, hence, decreasing the pH of aquifers and releasing heavy metals to the environment.3 Besides the environmental importance, chalcopyrite also has great economic relevance because 80% of the copper in Earth is available as chalcopyrite.4 Basically, there are two pathways to extract the copper from the mineral. The pyrometallurgical pathway consists in heating the mineral in the presence of an oxidant atmosphere followed by the copper reduction.4 It is very effective for high-grade minerals; however, for low-grade minerals, the results are not satisfactory. Another problem related with the pyrometallurgical treatment is the production of sulfur dioxide, a critical gas for the atmosphere. An alternative for the pyrometallurgical route, especially for low-grade ore, is the hydrometallurgical route, in which the mineral is leached in aqueous r 2011 American Chemical Society

solution followed by the electrochemical reduction of copper(II) to metallic copper. Nowadays, more than 20% of copper in the world is obtained by the hydrometallurgical process. The most popular leaching agent used in the leaching of chalcopyrite is the Fe3þ ion510 in acid solution in the presence of either sulfide9 or chloride1113 ions. Leaching in the presence of a microorganism has also been used.14,15 Much attention has been spent on the leaching process with an Fe3þ ion; this process has high copper recovery in the beginning, but it basically ceases just few hours later.7,9 The reason for the kinetics decrease is a moot question due to the variety of conditions used to perform the leaching experiments. However metal-deficient sulfides, polysulfides, elemental sulfur, and jarosites are pointed out as possible passivation agents.7,9,1621 Experiments using Raman, X-ray photoelectron spectroscopy (XPS), Auger spectroscopy, and other sophisticated techniques have been carried out to understand the surface speciation during the chalcopyrite leaching.2,19,2227 Harmer et al.25 studied the evolution of surface layers during a leaching with HClO4 at pH 1.0 using XPS, STM (scanning tunneling microscopy) and ToF-SIMS (time-of-flight secondary ionization mass spectrometry). They identified the presence of polysulfides and elemental sulfur in the chalcopyrite surface. Mikhlin et al.28 used XPS, X-ray emission, M€osbauer spectroscopy, and cyclic voltammetry to analyze the surface evolution of chalcopyrite leached by iron Received: February 2, 2011 Revised: April 20, 2011 Published: May 06, 2011 10709

dx.doi.org/10.1021/jp201106e | J. Phys. Chem. C 2011, 115, 10709–10717

The Journal of Physical Chemistry C sulfate in sulfuric acid and iron chloride in chloridric acid both at 50 C, and they identified polysulfide as did Harmer et al.25 Parker et al.29 evaluated with XPS the surface after leaching with Fe3þ and Fe2þ at pH 1.31.9, and they identified sulfur, sulfate, and disulfide; however, they did not identify polysulfide like Mikhlin et al.28 and Harmer et al.25 did. To understand the chalcopyrite surface reconstruction without any external influence, Klauber et al.30,31 and Acres et al.32 have used XPS and synchrotron XPS, respectively, to analyze a chalcopyrite surface in inert atmosphere, and they identified the presence of disulfide; however, they did not identify the presence of polysulfides. de Oliveira and Duarte,33 in their theoretical work, used PW91/ plane waves to study the reconstruction of the (001) chalcopyrite surface. Disulfide dimer formation has been shown to be favored with the surface reconstruction, in good agreement with the experimental studies. Besides several works discussing the mechanism of the chalcopyrite leaching under plenty of conditions, just a little attention has been given to the role of the water molecule over the chalcopyrite surface. It is important to remember that the water molecule is the most abundant chemical species in both the AMD and hydrometallurgical process. Understanding how this water molecule interacts with the surface is a crucial point. Some experimental works indicate that the water molecule could have an influence on the surface speciation. Harmer et al.,25 for example, detected by XPS the presence of iron oxyhydroxide (Fe-OOH) and copper hydroxide on the unleached air-exposed chalcopyrite surface, while these species were not detected on the chalcopyrite fractured in an inert atmosphere.3032 Gardner and Woods34 used voltammetry to analyze the flotability of chalcopyrite, proposed to explain the anodic reaction, eq 1, where the water molecule reacts with the chalcopyrite, forming iron hydroxide species. Copper oxide or copper hydroxide species were not detected in this investigation. CuFeS2 þ 3H2 O f CuS þ S þ FeðOHÞ3 þ 3Hþ þ 3e ð1Þ Mielczarski el al.35 also used XPS to analyze the leaching of chalcopyrite in basic solution (pH 10.0) and they identified the presence of oxidized species of iron over the surface, however they did not observe any evidence of copper oxidized species. Theoretical tools have been used for studying the interaction of molecules with a plenty of surfaces.3642 Stirling et al.,42 for example, performed detailed studies using DFT/plane waves to analyze the adsorption of H2O on the (100) pyrite surface considering both the molecular and dissociative pathways. They showed that the molecular pathway is the most stable, with an adsorption energy of around 13 kcal mol1, while the dissociative pathway is not favorable. In a similar work,41 they also showed that the molecular adsorption of H2S on the (100) pyrite surface is also favorable, with an adsorption energy of 11 kcal mol1, while the dissociative path is not favorable. Although other surfaces such as (111), (101), (110), and (112) have been pointed out as relevant for chalcopyrite and similar structures,2,24,43,44 chalcopyrite is considered to have no preferential cleavage. The (001) surface was indicated by Klauber as the most adequate surface to explain his XPS data under inert conditions.31 In the present work, we explore the adsorption of one water molecule in the reconstructed (001) chalcopyrite surface using density functional calculations. The different adsorption sites have been studied, and the dissociative mechanism has also been investigated, aiming to contribute to

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the understanding the role of the water molecule in the mechanism of the chalcopyrite leaching at a molecular level.

2. METHODOLOGY We carried out calculations within the framework of density functional theory (DFT) using the generalized gradient approximations of the exchange/correlation potential proposed by Perdew et al.45 as implemented in SIESTA.46 Pseudopotentials for all atoms were constructed considering the modified scheme of Troullier and Martins.47 The valence electronic configuration were set as 4s13d10 for copper, 3p64s23d6 for iron, 3s23p3.53d0.5 for sulfur, 2s22p4 for oxygen, and 1s1 for hydrogen during the pseudopotential generation. Partial core corrections48 were used for Fe, Cu, and S, improving the results relative to electron promotion when compared with full electron calculations. For O and H atoms, core corrections were not necessary. Numerical basis sets were created using the soft confinement potential proposed by Junquera et al.49 The size for all functions was double-ζ (two radial functions for angular momentum) with polarization functions, and the confinement parameter, were adjusted to describe the bulk of chalcopyrite and the water molecule. The confinement energy was set as 0.02 Ry for all atoms except Cu, where the value of 0.01 Ry was necessary to reproduce the experimental CuS bond length in the chalcopyrite bulk.1 A mesh cutoff of 350 Ry was chosen for the auxiliary basis expansion of electron density. We performed calculations with larger cutoff values; nonetheless, the results were similar. The Brillouin zone was sampled using the MonkhorstPack method,50 and the number of k points was set according to the model. The chalcopyrite bulk was constructed with 16 atoms in the unit cell with the experimental lattice parameters. We optimized both geometry and lattice parameters using a grid with 4 4 2 k points. Other calculations using larger grids were performed; however, no significant differences were observed in both geometry and total energy. We have used the optimized bulk to check the accuracy of the basis set and to create the surfaces. The (001) surfaces were simulated using a slab model considering a supercell with two unit cells along the a and b directions and eight atomic layers in the c direction. A vacuum of 20 Å was applied along the c direction to avoid the interaction between the slabs. Both the sulfur-terminated, (001)-S, and the metal-terminated, (001)-M, were reconstructed using a grid with 2 2 1 k points. This grid leads to an accuracy of 0.05 eV in the total energy. The geometry optimizations were performed using the standard conjugated gradient method as written into the SIESTA code, with full relaxation taken when none of the ionic forces exceeded 0.01 eV/Å. 3. RESULTS AND DISCUSSION 3.1. Chalcopyrite Bulk and Water Molecule. The geometry and lattice parameters of chalcopyrite bulk have been fully optimized. The results are presented in Table 1. Both the lattice parameters a and c are in good agreement with the experimental values, with differences no larger than 0.02 Å and a larger error on the c axis. This can be explained based on the larger number of bonds along the c direction; therefore, a small error on the bond length will produce a larger difference in the lattice parameter. The CuS and FeS on the optimized chalcopyrite bulk are also 10710

dx.doi.org/10.1021/jp201106e |J. Phys. Chem. C 2011, 115, 10709–10717

The Journal of Physical Chemistry C

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Table 1. Comparison of Geometric, Lattice, and Vibrational Parameters of Chalcopyrite Bulk, Water Molecule, and (001) Chalcopyrite Surfaces Calculated Using Siesta and/or PWscf with Experimental Values Chalcopyrite Siesta PWscf

experimental1

Lattice Parametersa a

5.277

5.263

5.289

c

10.447

10.418

10.423

Distance on Bulka dFeS dCuS

2.250 2.300

2.248 2.300

Water Molecule Siesta PWscf

2.257 2.302

Figure 1. (a) Top view and (b) side view of the reconstructed (001)-S chalcopyrite surface. The dark red balls represent the iron atoms, the blue are the copper atoms, and the yellow are the sulfur atoms. Dashed lines indicate the supercell. All of the distances are in Angstroms.

experimental54

Geometric Parametersa,b dOH

0.969

0.958

ÅHOH

105.0

104.5

Vibrational Frequenciesc ν1

1592

1594

ν2

3683

3657

ν3

3849

3755

(001)-Slab Siesta

PWscf33,55

experimental

Bond Length (001)-S Surfacea SS

2.23

2.158

SFe

2.24

2.319

SCu

2.30

2.326

FeFe FeCu

2.64 2.65

2.61 2.63

CuCu

2.63

2.61

a

Bond Length (001)-M Surface

a

Units: Angstroms. b Units: degrees. c Units: cm1.

in good agreement with the experimental value, as shown on Table 1. To compare our results, we have performed the geometry and lattice parameters optimization using a plane wave methodology as implemented in the PWscf Quantum Espresso package.51 The optimizations were done using the PW91 functional,52 an ultrasoft pseudopotential,53 and a cutoff energy of 30 Ry. As shown in Table 1, our results using localized basis functions as implemented in SIESTA are similar to those obtained through the delocalized plane wave calculations. The water molecule was inserted in a large cubic box, and the calculation was performed at the Γ point. The OH bond length, as shown in Table 1, differs only 0.01 Å from the experimental value,54 and the HOH angle is also in good agreement with a difference of around 2. The vibrational frequencies were calculated using the harmonic approach, with good agreement with the experimental value for the ν1 and ν2. The ν3, the OH stretching, differs from the experimental value by more than 90 cm1 following the same tendency as other DFT calculations using Gaussian basis sets.

Figure 2. (a) Top view and (b) side view of the reconstructed (001)-M chalcopyrite surface. (c) Zoom of the highlighted part on (a). The dark red balls represent the iron atoms, the blue are the copper atoms, and the yellow are the sulfur atoms. Dashed lines indicate the supercell. All of the distances are in Angstroms.

These results indicate that the developed pseudopotential and numerical basis sets are able to describe the chalcopyrite bulk and the water molecule in good agreement with other calculation methods and experimental results. 3.2. (001) Surface Reconstruction. To investigate the (001) surface reconstruction, a model with two unit cells along the a and b axes and a vacuum of 20.0 Å along the c axis has been used. Both the sulfur-terminated and the metal-terminated surfaces were relaxed, and the results are presented in Figures 1 and 2, respectively. The (001)-S surface reconstructs, forming disulfide dimers with a bond length of 2.23 Å, as shown in Figure 1. The bond lengths between the copper and iron on the second atomic layer with the disulfides on the first atomic layer are 2.30 and 2.24 Å, respectively, as indicated in Table 1. de Oliveira and Duarte33 analyzed the reconstruction of the (001)-S surface using DFT/ plane waves with ultrasoft pseudopotentials. In Table 1, we compared our results with those indicated by de Oliveira and Duarte,33 obtaining good agreement and, hence, indicating that the pseudopotential and numerical basis sets developed in the 10711


The Journal of Physical Chemistry C present work describe the chalcopyrite surface as well as the DFT/plane waves methodology. Experimentally, Klauber31 and Harmer et al.24 used XPS and synchrotron XPS to analyze chalcopyrite fractured under inert atmosphere, and both attributed a binding energy of 161.9 eV in the S2p spectrum to the disulfide dimer, S22. In our slab, the unrelaxed (001)-M surface presents eight metallic atoms (four iron atoms and four copper atoms) at the first atomic layer, eight sulfur atoms at the second atomic layer, and eight more metallic atoms at the third layer. After the reconstruction, the atoms on the first layer go down, while the atoms on the third atomic layer go up, forming a plane with these metallic atoms. In this plane, the metallic atoms form squares with FeFe, FeCu, and CuCu bonds, as shown in Figure 2. The sulfur atoms on the second atomic layer on the relaxed (001)-M surface occupy alternate positions over and under these squares. Each square is composed of one FeFe bond, one CuCu bond, and two FeCu bonds, but all of the bonds have similar length of 2.632.65 Å, as shown in Table 1, forming a regular square. Each square has a sulfur atom over or under the metal plane with a SFe distance of 2.23 Å and a SCu distance of 2.35 Å. There are small deviations, especially on the distant CuS; however, these differences are not larger than 0.1 Å. de Oliveira et al. obtained similar reconstruction using the PW91 functional within the plane waves formalism.55 This reconstruction can be explained based on the dangling bond on the metals after the cleavage. On the bulk, each metallic atom is coordinated to four sulfur atoms in a tetrahedral geometry,1 and in this case, the dxy, dyz, and dxz orbitals of Fe and Cu overlap with the sp3 orbitals of the sulfur atoms. On the surface, the dxy, dyz, and dxz orbitals do not have the orbital of sulfur atoms to overlap forming dangling bonds. To avoid the dangling bonds, the metal atoms on the surface go down, while the metallic atoms on the third atomic layer go up to the same plane. The atoms on the same plane allow overlap of these dxy, dyz, and dxz orbitals, forming δ bonds between the metallic atoms. This is in accordance with the short bond lengths between the metallic atoms on the surface shown in Figure 2. It also explains the sulfur atoms over and under the squares once the dz2 and dx2y2 orbitals on the Fe and Cu atoms have components upward and downward, which allows the overlap with the sulfur orbitals. To the best of our knowledge, this kind of reconstruction has not been detected experimentally yet. 3.3. Water Adsorption on the (001) Reconstructed Chalcopyrite Surface. Adsorption on the (001)-S Surface. In our model, the reconstructed (001)-S surface is represented by a supercell with four iron and four copper atoms on the second atomic layer and eight sulfur atoms forming four disulfide dimers on the first atomic layer, as shown in Figure 1. First, the interaction of one water molecule with different adsorption sites in the (001) surfaces was investigated. Three different adsorption sites were considered, one iron center and one copper center in the second atomic layer and one sulfur center in the first atomic layer. Once the calculations were performed with reduced symmetry, atoms which were equivalent in the unrelaxed surface, such as, for example, the four iron atoms in the second atomic layer, were not equivalent in the relaxed surface. However, the chemical environment in these atoms is very similar, and consequently, one might not observe important differences in the water adsorption process. We performed a detailed conformational search for the water adsorbed on the iron atom. Several structures were optimized by

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Figure 3. Top view of a water molecule adsorbed on different sites on the (001)-S chalcopyrite surface: (a) adsorption on the iron atom, (b) adsorption on the copper atom, (c) adsorption on the sulfur atom, and (d) side view of the water molecule adsorbed on the sulfur atom. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens. All of the distances are in Angstroms.

considering, as starting point, the water molecule coordinated by the oxygen atom in different conformations. Different minima were obtained with the adsorption energy (discussed in detail below) ranging from 22.8 to 18.4 kcal mol1. Figure 3a shows the most stable structure obtained for this adsorption site. The bond length between the iron and oxygen is 2.38 Å, and one of the hydrogens of the water molecule points toward one sulfur atom, making a hydrogen bond with a distance of 2.18 Å and an angle of — SHO equal to 172. The water molecule has an influence in the disulfide bonds, as shown in Figure 3a. The bond length of the disulfide, which has the sulfur atom making a hydrogen bond with the water molecule, increases from 2.23 to 2.37 Å. Another disulfide bond (see Figure 3a) decreases its bond length from 2.23 to 2.13 Å. The other two disulfide bonds basically are not affected by the water adsorption. Figure 3b shows the optimized structure with the water adsorbed on the copper atom. The optimized CuO bond length is 2.82 Å. As we did for the water molecule adsorbed on the iron atom, we explored the potential energy surface (PES) surrounding this adsorption site. The starting point structures for geometry optimization were carefully set up by changing the orientation of the water molecules and the angles and distances. Despite all starting geometries used, only one minimum was found in the PES, in which the water molecule remained bound to the copper center. Analyzing the structure shown in Figure 3b, just one of the disulfide bonds has an important change, decreasing the SS bond length from 2.23 to 2.15 Å. The other disulfide bonds do not change significantly. Strong interaction between the hydrogen of the water molecule with the sulfur dimer was not observed for the copper adsorption site. The distance between the hydrogen and sulfur atoms is calculated to be 2.35 Å, and the — SHO angle is 127. The other hydrogen has a distance from the sulfur atom of 2.72 Å and — SHO angle of 142, as shown in Figure 3b. 10712



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Table 2. Water Adsorption Energy (in kcal mol1) for the Reconstructed (001)-S and (001)-M Surfaces adsorption site

Reconstructed (001-S) surface ΔEads

figure

Molecular (one water molecule) Fe

22.8

3a

Cu

17.0

3b

S

13.4

3c and d

Molecular (four water molecule) Fe (water molecules parallel) Fe (water molecules near)

21.0 24.8

5a 5b

Dissociative Adsorption ∼8

Fe*

1.68

Cu

adsorption site for Hþ

Reconstructed (001)-M ΔEads

7a 6

figure

Dissociative Adsorption S (first neighbor)

33.9

10a

S (second neighbor)

31.0

10b

The adsorption energy is in the range from 8.97 to 1.08 kcal mol1 according to the adsorption site of Hþ. For more details see Table 3. *

The adsorption of the water molecule on the disulfide dimer forming a covalent bond between the sulfur and oxygen atoms is not expected due to the large energy difference in the frontier orbitals. Nevertheless, it is possible that the water molecule could be adsorbed on the surface, making hydrogen bonds with the disulfide. All tries with the oxygen coordinated to the sulfur atom converged, as expected, to structures in which the water molecule was not bonded to the sulfur atom. We performed calculations testing different interaction modes of the water with the disulfide bonds, and the most stable is presented in Figure 3c and d, in which the water molecule interacts by hydrogen bonding with two sulfur atoms on two different disulfide dimers. The distances between the hydrogen and the sulfur atoms are 2.34 and 2.37 Å, with — SHO angles of 161 and 167, respectively. The adsorption energy was calculated considering eq 2 1 ΔEads ¼ ½Esurf þnH2 O Esurf nEH2 O n

ð2Þ

where ΔEads is the adsorption energy, n is the number of water molecules on the surface, EsurfþnH2O is the total energy of the surface with the water molecules, Esurf is the total energy of the surface, and EH2O is the total energy of one water molecule calculated in a box with the same dimension used to calculate the surface total energy. The adsorption energies of all calculated adsorption sites are summarized in Table 2. The most stable adsorption site is the iron atom with an adsorption energy of 22.8 kcal mol1. The adsorption energy for the water molecule adsorbed on the copper site is calculated to be 17.0 kcal mol1, about 6 kcal mol1 larger than the iron adsorption site. The water adsorbed on the disulfide dimer forming hydrogen bonding has an adsorption energy calculated to be 13.4 kcal mol1, about 9.3 kcal mol1 higher in energy.

Figure 4. (a) DOS projected over the superficial atoms of the (001)-S chalcopyrite surface. (b) Integration of the electronic states from EF to EF þ 1 eV. The dark red balls represent the iron atoms, the blue are the copper atoms, and the yellow are the sulfur atoms.

Stirling et al.42 performed detailed analysis using DFT/plane waves about the adsorption of water on the (100) pyrite (FeS2) surface. They showed that the water molecule adsorbs on the iron site of the pyrite through the overlap of the dz2 orbital of iron with the sp3 orbitals of oxygen forming an FeO bond a 2.124 Å bond length and an adsorption energy of around 13 kcal mol1. This is almost 10 kcal mol1 larger than the adsorption energy of water on the iron atom of the chalcopyrite surface. We believe that one of the reasons for this difference is the hydrogen bond with the disulfide dimers; they obtained distances of 2.3632.698 Å for the S 3 3 3 H interaction, while we obtained the distance of 2.18 Å for the chalcopyrite. On the chalcopyrite (001)-S surface, the iron atom is coordinated in a tetrahedral symmetry with four sulfur atoms (two from the bulk and two others bonded to two disulfide dimers (Figure 1)). The orbitals dxy, dxz, and dyz of iron are involved in the bond with the sulfur atoms forming the tetrahedral. After the reconstruction, there is slight deviation from the ideal tetrahedral. From the molecular orbital analysis, it is possible to conclude that the interaction between the iron atom in chalcopyrite and the oxygen of the water molecule is due to the overlap of the dz2 orbital available on the iron center and the hybrid sp3 orbital in the oxygen of the water molecule, similar to pyrite.42 To explain the preference for the adsorption on the iron atom we analyzed the density of states (DOS) projected on the atoms of the surface, Figure 4a. At the conduction band, most of the unoccupied states close to the Fermi level have larger components on the iron center, in agreement with the results obtained by de Oliveira and Duarte in their PW91/plane waves study.33 The basic character of the water lone electron pairs will prefer to bond to the iron atom, which can readily receive the electron density 10713



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Figure 6. (001)-S chalcopyrite surface with one hydroxyl bonded to a bimetallic center. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens. All of the distances are in Angstroms.

Figure 5. (a) Top and (b) side views of the full covered (001)-S surface with four water molecules disposed in a symmetric way. (c) Top and (b) Side views of the full covered (001)-S surface with water molecules pointing toward each other. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens. All of the distances are in Angstroms.

from the water molecule. Figure 4b shows the integrated electronic states from the Fermi energy (EF) to EF þ 1 eV. The virtual electronic states available to receive electrons and make a covalent bond with a ligand such as water or a hydroxyl ion are mostly localized on the iron atoms. We have also analyzed the full covered surface with four water molecules adsorbed on iron atoms. We have studied two distinct situations. First, we optimized one structure with four water molecules with translational symmetry. The water molecules were added keeping the most stable conformation obtained in the study of adsorption of one water molecule (Figure 3a). Second, we optimized a structure with four water molecules coordinated on iron by oxygen, but in a nonsymmetric way. In Figure 5a and b, we present the optimized structure with the four water molecules with the same orientation (first situation). The four water molecules remained coordinated to iron with a bond length ranging from 2.43 to 2.47 Å, almost 0.08 Å larger than when we considered just one water molecule. In the full covered surface, all of the disulfide bonds decreased to 2.17 Å. The adsorption energy was calculated using eq 2 to be 21.0 kcal mol1, an increase of almost 2.0 kcal mol1 when compared with the adsorption of one water molecule. We believe that the difference in the adsorption energy is due to electron lone pairs in the oxygen atom repelling the sulfur atoms and making the disulfide bond shorter. In Figure 5c and d, we show the most stable structure considering four water molecules in a nonsymmetric way. The structure presented in Figure 5c had been proposed with the four water molecules coordinated by oxygen to the iron atoms; however, after the optimization process, just two water molecules remained coordinated to iron with a bond length of 2.30 Å. The other two water molecules left the iron atom to form hydrogen bonds with the waters that remained bonded. In this case, two disulfides near the iron with water adsorbed have a bond length

Figure 7. (a) Scheme with the possible adsorption sites for the proton. (b) The most stable structure with the proton adsorbed on site 6. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens. All of the distances are in Angstroms.

of 2.14 Å, while the other two disulfide bonds are 2.32 Å. The adsorption energy was calculated to be 24.8 kcal mol1, around 4 kcal mol1 lower in energy than the situation presented in Figure 5a. These results suggest a strong hydrogen bond between the adsorbed water molecule and the water molecules in a second solvation shell. The possibility for the water molecule to adsorb in a dissociative pathway has also been investigated. The dissociative pathway for water adsorption has been found for oxides surfaces such as TiO2,5658 SiO2,56 and Fe3O4.59 However, for sulfides, this kind of adsorption is not expected due to the stronger OH bond that has to be broken to form a weaker SH bond. Stirling et al.42 investigated this type of adsorption pathway on the (100) surface of pyrite with the OH adsorbed on the iron and the hydrogen atom adsorbed on the disulfide dimer. As a result, they found it not favorable for 19 kcal mol1 at a DFT/plane waves level of theory. In our study, we also investigated the dissociative pathway for the water adsorption on the (001)-S chalcopyrite surface. We considered the OH adsorbed on either Cu or Fe and the Hþ adsorbed on the disulfide. Considering the OH adsorbed on Cu, we obtained an adsorption energy of 1.68 kcal mol1 for the water adsorption in a dissociative pathway, very small if compared with a nondissociative pathway. In Figure 6, we present the most stable structure obtained with the OH coordinated to the copper. The hydroxyl migrates toward the iron atom, forming a bimetallic hydroxide due to the largest electrophilic character of iron atom. The bond lengths were evaluated as 2.35 and 1.92 Å for the CuO and FeO bonds, respectively. 10714



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Table 3. Adsorption Energy (in kcal mol1) and Geometrical Parameters (in Å) for the Water in a Dissociative Pathway Considering the OH Adsorbed on Iron Atoms and Different Adsorption Sites of Hþ on the Reconstructed (001)-S Surface sitea

ΔEads

dOFe

d(H)SS

1

6.57

1.94

3.45

2

7.54

1.93

3.42

4

5.77

1.91

3.42

5

8.97

1.94

3.45

6

8.97

1.94

3.39

7 8

1.88 1.08

2.00 2.18

2.18 2.53

3

a

Adsorption site for the Hþ. See Figure 7a.

Figure 8. DOS projected over the superficial atoms of the (001)-M chalcopyrite surface.

Once iron atom on the (001)-S surface is the most electrophilic site, as shown in Figure 4a, it is probably the most stable site for the OH adsorption, considering a dissociative pathway for the water adsorption. Considering the OH adsorption on this site, there are eight different sites for the proton adsorption, as shown in Figure 7a. All possibilities were calculated, and the adsorption energies for the water molecule, in a dissociative pathway, considering the OH adsorbed on Fe and the distinct sites for Hþ are presented in Table 3. Our results indicate that in a dissociative pathway, the OH will coordinate on iron, and the proton will be adsorbed on the disulfide that contains sites 5 and 6. Considering these sites, we obtained an adsorption energy of 8.97 kcal mol1. In Figure 7b, the optimized structure considering the Hþ adsorbed on site 6 is shown. The proton adsorption on sites 1, 2, and 4 results in an adsorption energy of 6.57, 7.54, and 5.77 kcal mol1, respectively. When the proton is adsorbed on sites 7 and 8, the adsorption energy is close to 0, as indicated in Table 3. We are not able to evaluate the water adsorption in a dissociative pathway keeping the proton on site 3. This site is very close to the hydroxide, and during the optimization procedure, the proton migrates toward OH, forming a water molecule. The OFe bond length, presented in Table 3, is around 1.90 Å for the structures with lower adsorption energies, 2.00 and 2.18, when the proton is adsorbed on sites 7 and 8, respectively. The effects of protonation on the disulfide bond were also analyzed, and the SS distance is presented in Table 3. This is an interesting point,because the chalcopyrite leaching process by

Figure 9. (a) Top view and (b) side view of the water molecule adsorbed on the (001)-M chalcopyrite surface. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens. All of the distances are in Angstroms.

Figure 10. Adsorption of a water molecule in a dissociative pathway on the (001)-M chalcopyrite surface. (a) The proton is located in a first neighboring sulfur atom, and (b) the proton is located in a second neighboring sulfur atom. The dark red balls represent the iron atoms, the blue are the copper atoms, the yellow are the sulfur atoms, the red are the oxygen atoms, and the white are the hydrogens.

Fe2(SO4)3 is performed in very acidic solution to avoid jarosites precipitation.9 However, electrophoresis studies indicate that in pH below 1.8, chalcopyrite has a positive charge surface,60 which means that on the (001) surface, the disulfides are protonated. The results in Table 3 indicate an elongation of the SS bond to values larger than 2.53 Å, resulting, in some cases, in a break of the SS bond, such as, for example, when the proton is adsorbed on sites 1 and 2. A clear exception is when the proton is adsorbed on site 7, however, in this case, the hydroxyl strength the disulfide bond. Adsorption on the (001)-M Surface. As discussed in section 3.2, the (001)-M surface reconstructs, forming squares with metals, as shown in Figure 2. Some squares have a sulfur atom in an upper position, while others have sulfur atoms below the square plane. The DOS projected over the superficial atoms shown in Figure 8 indicates electronic states to receive electrons available on iron atoms. Therefore, a water molecule was placed to coordinate the iron in the squares that do not have sulfur occupying an upper position as the starting point for geometry optimization. However, to our surprise, the water molecule did not remain coordinated to iron, and it migrated to form hydrogen bonds with sulfur, as shown in Figure 9. The adsorption energy was evaluated as 14.0 kcal mol1. Although the electronic states available to receive electrons are localized on iron, the water molecule does not adsorb on it. The main reason for this is the steric hindrance due to the sulfur atoms in the first layer. The dissociative pathway for water adsorption on the reconstructed (001)-M surface was also analyzed. The results are indicated in Table 2. Considering the OH adsorbed on iron, the most electrophilic site, there are two possibilities for the proton 10715


The Journal of Physical Chemistry C adsorption. The first one is on a sulfur atom localized near the OH group, as a first neighbor, and the second one is on a sulfur atom localized far from the OH group, as a second neighbor. Both possibilities were evaluated, and the most stable arrangements are presented in Figure 10a and b. The adsorption energy for the water molecule in this dissociative pathway was evaluated as 33.9 kcal mol1 when the proton was adsorbed in the sulfur atom near the OH group (Figure 10a) and 31.0 kcal mol1 when the proton was far from the OH group (Figure 10b). In both cases, the adsorption is not favorable, with a large positive adsorption energy. This is due to the reconstruction of the (001)-M surface. As discussed in section 3.2, this surface reconstructs, forming almost perfect squares with metal atoms making δ bonds. The sulfur atoms are over and under the squares, forming pyramids. The OH and Hþ adsorption breaks the regularity of these squares, hence leading to some deviation in the plane formed by the metal centers. The overlap between the metal d orbitals will not be as efficient as that in the reconstructed surface. This could explain why the adsorption of OH and Hþ on the (001)-M surface is not favorable.

4. FINAL REMARKS The effect of the water molecule adsorption on the (001) surface reconstruction of the chalcopyrite (CuFeS2) has been investigated by means of density functional calculations. The (001) chalcopyrite surfaces undergo important reconstructions, as has been pointed out in previous published results.33 All calculations were performed using periodic boundary conditions with the SIESTA program package and the numerical basis sets and pseudopotentials developed in the present work. The (001)-S and (001)-M surfaces were investigated using (2 2) unit cells. The reconstruction of the (001)-S surface leads to the formation of sulfur dimers on it, in agreement with de Oliveira and Duarte.33 This is very important because it can play an important role in the mechanism responsible for the low kinetic oxidation process of chalcopyrite. The (001)-M surface reconstructs, forming a series of two alternate different planes, one containing only metal atoms and the other one composed only of sulfur atoms. Concerning the water adsorption on the (001)-S surface, we have studied three distinct sites of adsorption on iron, copper, and sulfur atoms. The most stable site for adsorption was the iron atom, presenting an adsorption energy equal to 22.7 kcal mol1, followed by copper and sulfur atoms that present adsorption energies equal to 17.0 and 13.4 kcal mol1, respectively. The presence of other water molecules on the surface affects slightly the adsorption energy. The iron atom remains the most stable adsorption site, with an energy equal to 24.8 kcal mol1. The dissociative mechanism for the water adsorption is less favorable than the nondissociative mechanism. The adsorption energy for the (001)-S surface is about 8 kcal mol1. The iron atom is the most probable adsorption site for the hydroxyl group, while the sulfur atom is the preferred site for the proton on the (001)-S terminated surface. For the (001)-M terminated surface, all possible adsorption sites lead to a nonthermodynamically favorable process (ΔEads > 0). The metal-alloy-like structure underneath of the sulfur atoms and the unfavorable water adsorption indicate that the surface presents some hydrophobic character. This structure deserves more attention from experimentalists because it is probably related to the low reactivity of the surface chalcopyrite. However, it is important to note that the

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hydration of a surface is a complex process that can involve many water layers and modify its thermodynamic stability.61 In highly acidic medium, protons may adsorb on the sulfur dimer, leading to important structure effects such as enlarging the SS bond. As the sulfur dimer is the precursor of polysulfide formation on the surface, the proton adsorption prevents it from being formed and drives the reaction toward the H2S formation. Certainly, other anionic species might also have important effects on the reactivity of the chalcopyrite surfaces and may deserve our attention in the future.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work is supported by the Brazilian Initiative National Institute of Science and Technology for Mineral Resources, Water and Biodiversity, INCT-ACQUA (http://www.acqua-inct.org). The financial support from the Brazilian Agencies Conselho Nacional para o Desenvolvimento Científico e Tecnologico — CNPq and Fundac-~ao de Amparo a Pesquisa do Estado de Minas Gerais — FAPEMIG is gratefully acknowledged. C.d.O. would like to thank the CNPq for the postdoctoral fellowship (PNPD program). ’ REFERENCES (1) Hall, S. R.; Stewart, J. M. Acta Crystallogr., Sect. B 1973, B 29, 579. (2) Von Oertzen, G. U.; Harmer, S. L.; Skinner, W. M. Mol. Simul. 2006, 32, 1207. (3) Valente, T. M.; Gomes, C. L. Sci. Total Environ. 2009, 407, 1135. (4) Davenport, W. G.; King, M.; Schlesinger, M.; Biswas, A. K. Extractive Metallurgy of Copper; Pergamon: Oxford, U.K., 2002. (5) Acero, P.; Cama, J.; Ayora, C. Eur. J. Mineral. 2007, 19, 173. (6) Acero, P.; Cama, J.; Ayora, C.; Asta, M. P. Geol. Acta 2009, 7, 389. (7) Cordoba, E. M.; Munoz, J. A.; Blazquez, M. L.; Gonzalez, F.; Ballester, A. Hydrometallurgy 2008, 93, 81. (8) Kimball, B. E.; Rimstidt, J. D.; Brantley, S. L. Appl. Geochem. 2010, 25, 972. (9) Klauber, C. Int. J. Miner. Process. 2008, 86, 1. (10) Li, J.; Kawashima, N.; Kaplun, K.; Absolon, V. J.; Gerson, A. R. Geochim. Cosmochim. Acta 2010, 74, 2881. (11) Nicol, M.; Miki, H.; Velasquez-Yevenes, L. Hydrometallurgy 2010, 103, 86. (12) Velasquez-Yevenes, L.; Nicol, M.; Miki, H. Hydrometallurgy 2010, 103, 108. (13) Yevenes, L. V.; Miki, H.; Nicol, M. Hydrometallurgy 2010, 103, 80. (14) Pradhan, N.; Nathsarma, K. C.; Rao, K. S.; Sukla, L. B.; Mishra, B. K. Miner. Eng. 2008, 21, 355. (15) Watling, H. R. Hydrometallurgy 2006, 84, 81. (16) Dutrizac, J. E. Hydrometallurgy 1990, 23, 153. (17) Gomez, C.; Figueroa, M.; Munoz, J.; Blazquez, M. L.; Ballester, A. Hydrometallurgy 1996, 43, 331. (18) Hackl, R. P.; Dreisinger, D. B.; Peters, E.; King, J. A. Hydrometallurgy 1995, 39, 25. (19) Klauber, C.; Parker, A.; van Bronswijk, W.; Watling, H. Int. J. Miner. Process. 2001, 62, 65. (20) Linge, H. G. Hydrometallurgy 1976, 2, 51. (21) Munoz, P. B.; Miller, J. D.; Wadsworth, M. E. Metall. Trans. B 1979, 10, 149. (22) Cordoba, E. M.; Munoz, J. A.; Blazquez, M. L.; Gonzalez, F.; Ballester, A. Miner. Eng. 2009, 22, 229. 10716


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Water Adsorption on the Reconstructed (001) Chalcopyrite Surfaces

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