Experimental and Theoretical Analysis Accounting for Differences of

Article pubs.acs.org/JPCC

Experimental and Theoretical Analysis Accounting for Differences of Pyrite and Chalcopyrite Oxidative Behaviors for Prospective Environmental and Bioleaching Applications René H. Lara,† Jorge Vazquez-Arenas,*,‡ Guadalupe Ramos-Sanchez,‡ Marcelo Galvan,‡ and Luis Lartundo-Rojas§ †

Facultad de Ciencias Químicas, Universidad Juárez del Estado de Durango, Av. Veterinaria S/N, Circuito Universitario, C.P. 34120, Durango, Dgo. México ‡ Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, C.P. 09340 México, D.F., México § Instituto Politécnico Nacional, Centro de Nanociencias y Micro y Nanotecnologías, UPALM, Zacatenco México-D.F. 07738, México S Supporting Information *

ABSTRACT: The oxidative processes of pyrite (FeS2) and chalcopyrite (CuFeS2) of interest for bioleaching and/or bioremediation applications are evaluated in growing medium conditions to account for differences in their reactions mechanisms proposed with chemical and electrochemical analysis, and their electronic structures calculated with density functional theory (DFT). Electrochemical (chronoamperometry, cyclic voltammetry), spectroscopic (Raman, XPS) and microscopic techniques (SEMEDS, AFM) are used to comprehensively characterize complex surface transformations of secondary species arising during the electrochemical oxidation of these minerals. Early oxidation steps of both sulfides involve the formation of passive polysulfide species (e.g., Fe1−xS2, Cu1−xFe1−yS2), with the additional formation of Covelite-(CuS)-like species on a more passive chalcopyrite surface. Subsequent stages indicate the formation of semiconductive compounds including elemental sulfur (S0). DFT reveals that there are significant differences between pyrite and chalcopyrite densities of states (DOS), that support the fact that pyrite oxidation is more facile than chalcopyrite, as experimentally described. The DOS shows that near to the Fermi energy level of both sulfide minerals, there are few states that explain the oxidation limitations observed in the experimental region of low overpotential. At higher energies, the oxidation of pyrite is mainly due to iron species and sulfur species to a minor extent, while the chalcopyrite passivation is attributed to sulfur species and copper. and recovery.5,7,8 Chalcopyrite also presents some environmental concerns due to the industrial disposal of Cu-bearing secondary compounds and smelting Cu-bearing subproducts (e.g., tailings, dumps), causing the spoilage of ecosystems, pollution of urban, agricultural and forest soils.9−12 Accordingly, the oxidation of these minerals have been the motivation of many studies that have modified multiple variables (e.g., pH, temperature, pressure, leaching agent) and methods, for instance to stabilize pyrite exposed to weathering conditions and enhance the copper extraction from chalcopyrite.13−17 In case studies related to environment and (bio)leaching, it has been determined that pyrite oxidation is very active under practically any condition, forming secondary compounds such as polysulfides (e.g., Fe1−xS2), low amounts of elementary sulfur (S0) and ferric precipitates (e.g., ferric oxyhydroxides [FeOOH], jarosite [Me·Fe3(SO4)2(OH)6])

1. INTRODUCTION The oxidative behaviors of pyrite (FeS2) and chalcopyrite (CuFeS2) have been extensively studied since it is well accepted that these processes limit the performance of environmental and industrial applications.1−8 Pyrite is a sulfide mineral (SM) that undergoes oxidation when discarded from mining and mineral extraction operations. This weathering process results in a significant acid rock drainage (ARD), which can release potentially hazardous elements (e.g., As, Pb, Cr, Hg) to soil and water under different environmental scenarios.1−3 On the other hand, chalcopyrite presents a prominent industrial interest since it is currently the more abundant mineral source of Cu in nature.4−8 In order to extract the valuable copper, chalcopyrite is typically oxidized in smelters, thus, removing sulfur from the mineral lattice which reacts to form sulfide dioxide (SO2). Alternatively, bioleaching operations using a biomediated oxidation of sulfide. However, CuFeS2 is refractory and rapidly passivates during oxidation, forming secondary compounds under typical (bio)leaching conditions, which hampers the efficiency of subsequent stages such as extraction, purification © XXXX American Chemical Society

Received: May 29, 2015 Revised: July 7, 2015

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DOI: 10.1021/acs.jpcc.5b05149 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C under acidic conditions.1,18−20 In contrast, chalcopyrite oxidation is rapidly limited by the formation of refractory secondary compounds (e.g., polysulfides [Cu1−xFe1−yS2‑z], covellite [CuS], S0, chalcocite [Cu2S], jarosite).1,5−7,21 Natural chalcopyrite and pyrite samples were systematically analyzed after electrochemical modifications conducted in alkaline borax solutions (pH = 9.2), in order to relate textural and chemical variations arising on their surfaces as a result of changes in electrochemical reactions of interest for copper extraction processes.17 Although some of these environmental, treatment and metallurgy processes have been successful implemented, their rate-controlling steps and structure−reactivity relationships remain unknown in most cases. Sand et al.1,4 have highlighted that the oxidation mechanisms of different SM, such as sphalerite (ZnS), pyrite, and chalcopyrite, among others, are mainly determined by their electronic structure and acid solubility. In this direction, orbitals of sulfur and metals form electronic bands with different energetic levels within their crystalline structure.1,4 For pyrite, the valence bands are orbitals from the metal atoms, while for chalcopyrite are derived from both metals and sulfur orbitals. These preliminary insights illustrate the influence of the crystallographic structure upon the semiconductive properties of these minerals, which can lead to a remarkable difference in their oxidative behavior. A more in-depth evaluation of the electronic structure of pyrite and chalcopyrite is required though, there is also a need to relate such properties with a reliable surface speciation determined during the course of oxidation. In fact, there is currently a debate of the reaction mechanisms and secondary phases arising during pyrite and chalcopyrite oxidation in a typical aqueous phase (H2SO4, HCl, NaOH, NH4OH; Na2CO3), presumably, as a result of many complex environmental and operating variables interacting each other. Thus, this work aims to correlate surface and electrochemical analysis collected during the oxidation of pyrite and chalcopyrite in growing culture medium, with computational modeling examining the electronic structure of these materials. To our current state of knowledge, a similar linkage has not been undertaken, whence it is expected to contribute to the understanding of the structure−reactivity relationships and the rate-controlling phenomena limiting such oxidative processes, in order to improve environmental and industrial applications related to them (i.e., preventing ARD, enhancing Cu extraction). A growing culture medium is selected as electrolyte since (bio)leaching operations have become attractive due to its importance to obtain valuable metals such as Cu at low operation costs, and represents a novel strategy for environmental bioremediation. To this concern, the usage of an abiotic culture growing medium synthetically expedites to mimic the multiple interfaces arising during SM oxidation in the presence of (bio)leaching bacteria, and control the formation of surface compounds. Likewise, this medium enables to envisage the alteration of pyrite and chalcopyrite under different conditions since it contains most relevant ions found for environmental and industrial applications (e.g., Cl−, SO42−, NO3−, PO43−). For these purposes, electrochemical (cyclic voltammetry, chronoamperometry), microscopic (SEM-EDS, AFM) and spectroscopic (Raman, XPS) techniques are combined to comprehensively evaluate a secondary/tertiary surface speciation. This information is complemented with first-principle calculations using density functional theory (DFT) to analyze the electronic states close to the edge of the valence band of pyrite and chalcopyrite structures. Previous DFT studies have focused on

the magnetic properties and interfacial phenomena of chalcopyrite22,23 and pyrite;24 however, a detailed analysis of the differences in electronic structure between bulk pyrite and chalcopyrite responsible of the fundamental changes in reactivity is missing. The rationale of this part of the study is to identify the contribution of each atom in the unit cell to the charge donation capability of the solids, occurring in early oxidation processes of pyrite and chalcopyrite.

2. MATERIALS AND METHODS 2.1. Mineral Samples. Relatively pure chalcopyrite and pyrite crystals were obtained from mineral sources in Chihuahua and Zacatecas (México), respectively. Representative samples of these minerals were digested in acid, and analyzed by atomic absorption spectroscopy (PerkinElmer 3100 atomic absorption spectrometer) to evaluate mineral compositions and impurities. A scanning electron microscope Philips XL 30, coupled to an energy dispersive X-ray spectrophotometer, EDAX 4Dix, (SEM-EDS), and X-ray diffraction (XRD Rigaku DMAX 2200) were used to visualize surface morphology and corroborate minerals stoichiometry, respectively. The analysis conducted for chalcopyrite indicated the following composition: ∼ 99.6% wt of chalcopyrite, ∼0.2% wt of pyrite (FeS2), and ∼0.2% wt of quartz (SiO2). For pyrite samples, the composition was as follows: ∼99.8% wt of pyrite, ∼0.1% wt of chalcopyrite, and ∼0.1% wt of sphalerite (ZnS). XRD patterns confirmed chalcopyrite (JCPDS Card # 35-0752) and pyrite (JCPDS Card # 89-3057) identity, whereas SEM− EDS analyses verified the presence of the identified impurities as inclusions in pyrite and chalcopyrite crystals. Selected crystals were used for the construction of massive chalcopyrite (MCE) and pyrite (MPE) electrodes. Hence, mineral coupons with exposed surface areas ranging from 1.2 to 1.5 cm2 were mounted in epoxy resin; silver paste was used to enhance the electrical contact of the MPE and MCE. MPE and MCE surfaces were polished to obtain a mirror-like surface. 2.2. Electrochemical Study. A typical medium with significance in bioleaching and environmental bioremediation studies was selected for electrochemical characterization,25,26 comprised of an acidified ATCC (American Type Culture Collection)-125 growth culture medium (pH = 2.0, solution free of bacteria). This (bio)leaching medium contained the following components per liter of deionized water: 3 g of KH2PO4, 0.4 g of (NH4)2SO4, 0.5 g of MgSO4·7H2O, 0.25 g of CaCl2·2H2O, and 0.01 g of FeSO4·7H2O. The electrochemical experiments were carried out in a classic Pyrex glass threeelectrode cell with inert atmosphere of nitrogen (N2), using an autolab PGSTAT 30 potentiostat. Unmodified massive chalcopyrite or pyrite were utilized as the working electrode, the counter electrode was a graphite rod (AlfaAesar, 99.9995% purity), and a saturated sulfate electrode as reference (SSE, 0.615 V vs SHE, the standard hydrogen electrode). Cyclic voltammetry (CV) and chronoamperometry were conducted using MPE and MCE. In CV experiments, different limits of anodic potential (Eλ+) were used to evaluate the regions of occurrence for potential-dependent species generated on the sulfide minerals. A scan rate of 0.02 V s−1 was used for these purposes, which is an adequate rate to evaluate solid and soluble interfacial products formed during sulfide mineral oxidation, without a significant competence with the input signal. Chronoamperometry was utilized for a more precise analysis of the aforementioned regions by applying different pulses of potential. Subsequently, charge (Q) measurements B


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The Journal of Physical Chemistry C

2.4. Computational and Systems Details. First-principles calculations were performed using the Vienna ab Initio simulation package (VASP)30,31 on the generalized gradient approximation (GGA) using the exchange correlation (XC) functional of Perdew, Burke and Enzerhof functional (PBE), and the semiempirical GGA+U method along with the projector augmented wave (PAW) treatment of the core electrons.32,33 The plane wave was expanded up to a cutoff energy of 700 eV, value needed to relax the unit cell volume reducing the Pulay stress effects arising from an incomplete basis set. For the calculation of the total energy, the tetrahedron method with Bloch corrections34 was used with a smearing width of 0.05 eV and a total of 652 and 834 inequivalent tetrahedra for sampling the Brillouin zone in FeS2 and CuFeS2, respectively. For the calculation of the Density of States (DOS), the number of inequivalent tetrahedra for sampling the Brillouin zone was increased to 2082 and 2764 for FeS2 and CuFeS2, respectively. The GGA+U method in the framework of the Dudarev approach was used,35 the Ueff for Fe in CuFeS2 was set to 1.6 eV which was obtained from previous reports for iron chalcogenides;36−38 for Cu oxides and sulfides the PBE+U has been proved to have importance.39−41 The Ueff for Cu was set in the optimal value reported in ref 40. Although the semiconductor behavior of FeS2 was obtained in standard GGA calculations, the aforementioned value of Ueff for Fe was also used in FeS 2 in order to be consistent. The antiferromagnetic properties of CuFeS2 were accounted by imposing several magnetic moments on Fe and Cu atoms, the lowest energy configuration resulted in Cu atoms with a negligible magnetic moment and Fe atoms in consecutive layers in the z direction with alternated magnetic values ±3.05 μB after relaxation. The initial unit cell for the periodic calculations was obtained from experimental databases. For FeS2, it consists of four iron and eight sulfur atoms per unit cell, where each Fe is located in the middle of an octahedron formed by S atoms, while for CuFeS2, each Cu and Fe is in the center of a tetrahedron formed by S atoms. By using the method described above, the atoms positions and cell shape and size were fully relaxed.

were calculated for each potential (Ea) to estimate the amount of surface pyrite and chalcopyrite transformed to secondary compounds during oxidation. This information (Q vs Ea plots) is commonly used to assess the effect of secondary compounds in the mineral oxidation capacity.21 Immediately to potentiostatic modification of MCE and MPE samples, eMCE and eMPE surfaces were rinsed with deionized water, exposed to a direct flow of N2 and safely kept under inert conditions until surface analysis. Note that during this time and before mounting the sample in the chamber of the XPS equipment, multiple species (H2O, OH−, and O2−) can adsorb on the eMCE surface regardless of the exposure time. This phenomenon was suppressed to some extent with the use of a prechamber to desorb environmental gases prior to XPS analysis (refer to section 2.3). Minor impacts in the S 2p regions of the XPS spectra do not modify the determination of sulfur species,27 only the Fe 2p, which herein has not been considered. 2.3. Surface Analyses. The sulfide mineral surfaces (unmodified and electro-oxidized MCE and MPE) were characterized using microscopic and spectroscopic techniques. The AFM analysis was performed with a Nanoscope Multimode IIIa digital instrument. Narrow and wide regions were visualized to obtain topographic images by tapping mode (scan rate between 0.5 and 1 Hz). The silicon cantilever showed a free resonance frequency between 275 and 325 kHz and a constant between 31.18 and 44.536 N·m−1 during these experiments. Roughness (Ra, nm) and root-mean-square (Rq, nm) of unmodified and modified surfaces were also evaluated to generate a complete description of these surfaces. Raman spectra were recorded with a triple subtractive monochromator T64000 spectrometer Horiba Jobin Yvon equipped with a confocal microscope Olympus BH2-UMA. The samples were excited by a laser beam at λ = 514 nm emitted by an Ar+ laser emission device (Stabilite 2017, Spectra Physics). Raman performance was validated using a Si wafer disc by assuming a single sharp peak at 521 cm−1. Raman backscattering showed a signal/noise ratio greater than 100 for Si analysis, thus ensuring a good Raman performance during SM analyses. The vibrational range was 120−720 cm−1 as the S0/Sn2− species show their main active modes within this interval.28,29 At least 10 Raman spectra were collected from each surface. The formation of polysulfide (S n 2−) species is difficult to discriminate with this technique from electro-oxidized MCE (eMCE) surfaces. Hence, eMCE surfaces were successively analyzed using XPS. Prior to analysis, the pristine and modified MCE surfaces were maintained during 15 h in a prechamber directly connected to the equipment, in order to desorb environmental gases and preserve the samples from alteration. Subsequently, they were automatically transferred to the main chamber under inert conditions, and subjected to vacuum. The XPS analyses were performed using a K-Alpha Thermo Scientific spectrometer with a monochromatized Al Kα Xrays source (1487 eV), running at a power of 150 W. XPS narrow scans, using an X-ray spot size of 400 μm2, were collected at 60 eV pass energy. To detect and compensate the charge shift of the core level peaks, O 1s peak position at 531.0 eV was used as an internal standard. XPS S 2p core level spectra were analyzed with AVANTAGE v5.41 software from Thermo Scientific and fitted using a Gaussian−Lorentzian mix function and Shirley type background subtraction used to decompose spectral peaks.

3. RESULTS AND DISCUSSION 3.1. Voltammetric Study of Unmodified MPE and MCE Surfaces. Figure 1 shows cyclic voltammograms of unmodified sulfide mineral (chalcopyrite electrode, MCE and pyrite electrode MPE) in ATCC-125 medium under stagnant conditions. In these experiments (continuous black lines), the potential was scanned from the open circuit potential (the OCP reached a steady value of ∼0.44 V after 2000 s for pyrite, and ∼0.30 V after 2200 s for chalcopyrite) to 1.315 V. The corresponding cyclic voltammograms initiated in the negative direction are also included for comparison purposes (Figure 1, dashed lines). When the sweep is started in the positive direction, a sluggish oxidation process (A1) is detected for pyrite (inset of Figure 1a) and chalcopyrite (inset of Figure 1b), which suggests minimal changes in the crystal lattice of the minerals.20,21,42 However, once the activation energies have been overcome, abrupt increases of current are observed for both minerals at potentials around 0.9 V (A2). Note that the magnitudes of the current associated with these processes make imperceptible the occurrence of A1. When the potential scan is switched to more negative directions, two reduction peaks (C1 and C2) arise between ∼0.36 and ∼−0.15 V, and ∼0.38 and ∼−0.4 V for pyrite and chalcopyrite, respectively. The absence C


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of significant reduction processes in the corresponding voltammograms started in the negative direction (see insets of Figure 1, dashed lines) indicates that C1 and C2 are related to the reduction of the products formed in the forward positive scans. Additionally, the comparison of the positive and negative regions of the voltammograms obtained in both SM systems (Figure 1) suggests that most of the oxidized species are soluble (e.g., Fe2+, S2O32−, SO42−). In order to analyze the potentialdependent formation of species related to the reduction processes C1 and C2 (Figure 1), a systematic voltammetric study was conducted by switching the anodic potential limit (Eλ+) at different increasing values. This analysis (colored continuous lines in Figure 1) indicates that C1 and C2 increased progressively as Eλ+ became more positive in both systems. However, in the case of chalcopyrite, the progressive appearing of intermediate products as a function of Eλ+ indicated that A1 and A2 are complex processes, which demand an exhaustive surface analysis as a result of the formation of more surface compounds relying on Eλ+ (e.g., inset of chalcopyrite in Figure 1, Eλ+ = 0.91 V). Typically, the C2 process arises in both SM systems in the range of potential where the reduction of polysulfide (Sn2−) compounds has been reported for pyrite,18,20,43 and Sn2− along with refractory Cusulfides in chalcopyrite.21,44,45 The process C1 occurs for pyrite around 0.36 V (inset of Figure 1), and shifts by ∼0.02 V in chalcopyrite, suggesting the reduction of a similar compound in both SM. The voltammetric response involves a competition between the rate to form the surface compounds and the rate at which energetic conditions are imposed on the MPE and MCE (e.g., polarization rate). Accordingly, the regions of potential observed on pyrite and chalcopyrite for the secondary compounds (refer to Figure 1) could be shifted. Constant pulses of potential (Ea) are more suitable to establish the regions of potential where the secondary phases of MPE and MCE are formed. Likewise, this technique induces the formation of a larger amount of surface products to meet the levels of detection for microscopic and spectroscopic analyses. Thus, different anodic potentials were applied on the mineral surfaces according to the CV analysis. 3.2. Potentiostatic Oxidation of Unmodified MPE and MCE Surfaces. Figure 2 shows curves of charge as a function of applied potential (Q vs E) for pyrite and chalcopyrite. These plots were constructed from pulses of potential applied on the MPE and MCE during 3600 s, once a stationary current was attained. In general, differences observed along the potential (Ea) can be related to different electrochemical properties arising from the surface properties of the oxidized phases. These surface properties comprise three major behaviors denoted as zones I to III. Upon application of 0.516 ≤ Ea ≤ 0.761 V (zone I), the Q vs E curves indicate a rapid passivation of chalcopyrite, whereas pyrite undergoes a slow oxidation (Figure 2). The application of more positive potentials (0.761 ≤ Ea ≤ 0.961 V, zone II) leads to an important change in the pyrite and chalcopyrite oxidation as indicated by the enhancement and activation-like shape (Figure 2) of the Q vs E curves, respectively. The shape and the magnitude of these plots indicate a more electroactive process governing the corresponding oxidation processes, and the generation of different surface compounds if compared with less positive potentials. These results suggest the formation of semiactive secondary compounds in the electro-oxidized mineral phases, eMPE and eMCE surfaces (Figure 2). However, semiactive behavior of

Figure 1. Cyclic voltammograms of MPE and MCE initiated into the positive-going direction (black continuous lines) and the negativegoing direction (black dot lines) in ATCC-125 (pH 2.0) growth medium. This Figure includes cyclic voltammograms obtained from Eλ+ study in color continuous lines. Selected Eλ+ values are indicated in the figure. Scan rate of 20 mV·s−1. Quiescent solution. Ambient temperature.

Figure 2. Charge (Q) vs potential (E) curves obtained from chronoamperometric studies of the MPE and MCE anodic oxidation in ATCC-125 (pH 2.0) growth medium. 3600 s. This figure includes different oxidative behavior of MPE and MCE characterized as zones (from I to III). Quiescent solution. Ambient temperature. D


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Figure 3. Raman spectra collected from unmodified MPE and MCE (a and b, respectively), and eMPE and eMCE obtained after application of Ea in zone I (a′ and b′, respectively), zone II (a″ and b″, respectively) and zone III (a‴ and b‴, respectively). Incises i and ii indicate different kind of species in the same modified surface. This figure includes Raman spectrum from mineral covellite and chalcocite (c′), S0 compounds (JT Baker, c″) and Si wafer disk (c) for comparison purposes. Laser wavelength = 514 nm. Collection time of 60 s.

pyrite is remarkably higher, in comparison with that observed for chalcopyrite, probably due to more important semiconductive features of the secondary compounds formed on the eMCE surface. The latter processes are enhanced upon the application of more positive potentials, Ea ≥ 0.961 V (zone III). In the case of pyrite, the rapid surface activation over zones I to

III suggests a further transformation of surface secondary compounds resulting in a severe alteration of crystal lattice of the mineral. The charge associated with zone III (Figure 2) mostly indicates a continuous dissolution of pyrite to form soluble species (e.g., Fe2+, Fe3+, S2O32−, SO42−). While for chalcopyrite, the slow rise of the charge suggests the continuous E


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Table 1. Summary of Raman Transitions Identification in Terms of Main Surface Sulfur Species on MPE, MCE, eMPE, and eMCE Surfaces (vi Indicates the Transition Order) surface species −

pyrite (monosulfide, S ) chalcopyrite (monosulfide, S22−) polysulfides (Sn2−) elemental sulfur (S0) covellite (S−) chalcocite (S2−)

Raman peak position (cm‑1)

references

344 (v2), 380 (v1), 433 (v3) 292 (v1), 320 (v2) 292 (v1) from 443 to 475, main at 470 (v1) 155 (v3), 220 (v2), 470 (v1) 471 (v1) 267 (v2), 471 (v1) 476 (v1)

18, 50, 58, and this study 28, 47, and this study 18, 47, 50, 58−60, and this study 18, 47, 50, 58−60, and this study 47 and this study 28 and this study

potential (Ea) in zone I (e.g., 0.71 V), zone II (e.g., 0.961 V), and zone III (1.21 V, Figure 3). A summary of Raman transitions to identify pyrite, chalcopyrite, CuS, Cu2S, Sn2−, and S0 compounds is provided in Table 1. Broad Raman peaks at 472−480 cm−1 and 472−475 cm−1 can be associated with eMPE and eMCE (Figure 3) in zone I, respectively, indicating the formation of Sn2− in both mineral surfaces (e.g., Cu1−xFe1−yS2 and/or Fe1−xS2, respectively). An additional Raman peak at 471 cm−1 indicated the formation of CuS-like species for oxidized chalcopyrite (Figure 3ii. Thus, the low and negligible Q observed in zone I for eMPE and eMCE (Figure 2) is associated with the formation of Sn2− and Sn2−/CuS-like compounds, respectively. These phases result on their turn in a slow pyrite oxidation, and the chalcopyrite passivation. Chalcopyrite passivation has been related to the initial formation of passive Sn2− compounds in metal-deficient layers under (bio)hydrometallurgical conditions.5,28,42,47 While secondary sulfur compounds have been associated with enhanced oxidation capacity in iron sulfides samples.48 On the other hand, the Raman spectrum recorded for the eMPE generated after the application of Ea (e.g., 0.915 V) in zone II shows the formation of S0, along with Sn2−, as indicated by peaks at 153, 220, 248, 273 and the asymmetric shape of the peak at 453− 472 cm−1 (Figure 3, Table 1). Similarly, peaks at 152, 217, 470, and 471 cm−1 indicated the formation of S0 accompanied by CuS-like on the eMCE in zone II (Figure 3ii, Table 1). Hence, the increasing Q in the transition zone II is attributed to the oxidation of a mixture of Sn2− and S0 for eMPE, and a mixture of S0 and CuS-like for eMCE. Raman spectra obtained for zone III show also the formation of S0, as indicated by peaks at 155, 223, 250 and 470−475 cm−1 in the eMPE (Figure 3, Table 1) and at 151, 217, and 470 cm−1 in the eMCE (Figure 3, Table 1). The oxidation of S0 is mainly associated with the highest Q obtained in zone III for both oxidized minerals. Shifts in the Raman peaks for S0 compounds can be mainly related to specific sulfur species (e.g., S8, S0).28,47,49 The Raman analysis confirmed that the eMPE and eMCE surfaces contain multiple chemical species, whose formation depends on Ea. The identification of Sn2− species during pyrite oxidation has been documented by Raman spectroscopy.18,50 In contrast, a more sensitive and powerful technique is often required for chalcopyrite due to different surface compounds that can be formed (e.g., Sn2−, CuS, Cu2S, CuS1.2),2,42,51 since the Raman vibrations are most of the times not well distinguished. Consequently, XPS has been used to identify the oxidation states of sulfur, copper and iron in chalcopyrite, before and after potentiostatic modification of the MCE. In addition, this technique has become invaluable to elucidate sulfur surface species, which are involved in the oxidation mechanisms proposed for chalcopyrite. Figure 4 shows the global comparison of narrow resolution spectra (S 2p) for the MCE (Figure 4a), and eMCE (parts a′ to a‴) after potentiostatic

Figure 4. General comparison of undeconvoluted XPS core level spectra of S 2p obtained from unmodified MCE (a), and eMCE obtained after application of Ea in the zone I (a′), zone II (a″1 and a″2) and zone III (a‴). Main chemical states of sulfur species are indicated in the figure.

formation of passive secondary compounds which hinder the dissolution of this mineral in zone III (Figure 2). These results confirm the generation of different surface properties on eMPE and eMCE as a function of Ea. Studies concerning pyrite oxidation under acidic conditions1,2,18,20,46 have proposed as initial reaction the transformation of pyrite to sulfur-rich metal deficient layer including polysulfides (e.g., Sn2− associated with Fe1−xS2), preceding the formation of S0 (10−20% wt), and mostly soluble sulfur species (e.g., S2O32−, SO42−), along with iron precipitates (e.g., oxyhydroxide ferric compounds, FeOOH). On the other hand, for the chalcopyrite oxidation under acidic conditions, mainly phases conferring passivation have been suggested: polysulfides (e.g., Cu1−xFe1−yS2), S0, covellite (CuS), soluble sulfur species (e.g., S2O32−, SO42−), and ferric precipitates such as K-jarosite.2,5−7,21,42 Therefore, the potentiostatic formation of these surface compounds could arise during the oxidation of pyrite and chalcopyrite in zones I, II, and III, and the probable generation of soluble sulfur species mainly in zone III (Figure 2). However, only electrochemical measurements have been conducted up to this point, whence the chemical characterization of the electrooxidised surfaces is needed to identify the composition of the secondary compounds involved on these minerals. 3.3. Physicochemical Characterization of Pyrite and Chalcopyrite. Figure 3 shows Raman spectra for unmodified and oxidized MPE and MCE (Figure 3). The oxidized samples were generated potentiostatically by imposing a pulse of F


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Figure 5. Fitted XPS core level spectra of S 2p acquired from unmodified MCE (a), and eMCE obtained after application of Ea in the zone I (a′), zone II (a″1 and a″2), and zone III (a‴).

The presence of disulfide (S22−) species was related with the second 2p3/2 component at 162.4 eV (Figures 4a and 5a, Tables 2 and 4) while, Sn2−/S0 compounds (Figures 4a and 5a, Tables 2 and 4) were associated with the third minor doublet with 2p3/2 at 163.7 eV. The S 2p spectrum for the pristine MCE was similar to those found in refs.,51,52 and therefore, it was considered a typical comparison basis. In addition, the eMCE surface obtained after application of the Ea in zone I shows a minor increase in the Sn2−/S0 contribution (163.9 eV) (Figures 4a′ and 5a′, Tables 2 and 4), thus, confirming the formation of Sn2− on the eMCE in zone I, in agreement with the Raman and

modification in zones I, II, and III. Figure 5 shows the corresponding peak fitting procedures of these XPS spectra. Although Cu 3d, Fe 3d, and O 1s spectra were also acquired (data not shown), essential information on sulfur species is directly determined from the analysis of the S 2p spectra in chalcopyrite systems.51,52 A summary of S 2p peak parameters and the corresponding binding energies (eV) for identification of sulfur compounds are listed in Table 2. The pristine MCE surface shows a major doublet, with 2p3/2 component located at 161.2 ± 0.2 eV that is commonly assigned to monosulfide (S2−) from chalcopyrite lattice (Figures 4a and 5a, Tables 2 and 4). G


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Table 2. Binding Energy (BE), FHWM (Full Width at Half-Maximum), Peak Areas, and Chemical State for S 2p3/2 Spectra of MCE and eMCE Surfaces this work surface MCE (pristine) S 2p3/2a

eMCE zone I (0.715 V vs SHE) S 2p3/2a

zone II (0.865 V vs SHE) S 2p3/2a

zone II (0.965 V vs SHE) S 2p3/2a

zone III (1.115 V vs SHE) S 2p3/2a

literature

BE (eV)

FHWM (eV)

area (At %)

chemical state

161.1 162.4 163.7 161.1

1.1 0.9 1.7 1.0

65.9 20.7 13.4 54.2

S2− S22− Sn2−/S0 S2−

162.4 163.9 160.7

1.2 1.5 1.4

29.8 16.0 3.7

S22− Sn2−/S0 S2−

162.3 163.6 167.2 161.1

1.2 1.1 2.1 1.4

53.7 24.1 18.5 13.2

S22− Sn2−/S0 SOx2− S2−

162.4 163.5 166.8 160.6

1.0 1.1 2.3 1.4

50.5 28.2 8.1 18.6

S22− Sn2−/S0 SOx2− S2−

162.8−163.8 168.5−169.3

162.1 163.6

1.2 1.3

39.9 28.0

S22− Sn2−/S0

164.5−166.6

167.0

2.0

13.5

SOx2−

BE (eV)

chemical state

reference

160.8−161.7

chalcopyrite (S2−)

42, 44, 52, 61, and 62

161.9−162.3

chalcopyrite (S22−)

42, 52, 61, and62

160.9−161.6

covellite (S2−)

42, 53, and 60

161.7−162.2

covellite (S22−)

53

163.7−164.6

elemental sulfur (S0)

18,42,53,58,61,63

polysulfides (Sn2−)

18, 42, 44, 52, and 62

sulfate (SO42−)

18, 42, 44, 52, 62, and 63

undetermined sulfoxyspecies (SOx2−) to SO32−

64−66

Spin−orbit doublets were used to fit the data of narrow scan S 2p. Only the S 2p3/2 peak position is indicated. S 2p3/2 and S 2p1/2 contributions were summed to obtain the total proportion of each sulfur oxidation state.

a

cyclic voltammetry studies (see above). These values and the surface characterization closely agree with published data concerning the formation of sulfur-rich lattice layers in Cu2S samples after minor alteration.2 The S 2p spectra for eMCE after application of Ea in zones II and III show a strong increase in the Sn2−/S0 and S22− species contribution, and a concomitant decrease of S2− species (Figures 4 and 5, a″1-a″2 and a‴, respectively, Tables 2 and 4), indicating enrichment of surface compounds (e.g., Cu1−xFe1−yS2‑t-like/S0, CuS), previously found in the Raman analysis. According to Yin et al.,53 CuS presents two forms of sulfur (S22− and S2−), and the presence of the S22− is the main indicative for its formation. Additional emissions with 2p3/2 components located at 167.2 eV reflected the presence of undetermined sulfoxy species (e.g., SOx2−) in the eMCE generated in zones II and III (Figures 4 and 5, a″1a″2 and a‴, Tables 2 and 4). Accordingly, the XPS technique provided a deeper analysis of the formation of Sn2− (e.g., Cu1−xFe1−yS2‑t, structures containing Sn2−/S0species) and CuS on the eMCE obtained in zone I, and mainly in zones II and III, thus, confirming the Raman and electrochemical studies shown above. The previous analysis indicates the differences in oxidation capacity between pyrite and chalcopyrite. There is convincing experimental data to consider that structural imperfections such as edges, vacancies, holes and defects enhance minerals reactivity, as it has been generally reported for heterogeneous catalysis. However, this reactivity is local and does not account for major oxidation (i.e., massive dissolution) of sulfide minerals.54,55 Conversely, these differences in oxidation capacity are mainly driven by semiconductive properties associated with the initial limitations of the mineral electronic structure and the formation of secondary compounds at

prolonged reaction time, rather than structural imperfections.56,57 Experimental evidence of such behavior is the passivation of chalcopyrite. Figure 6 shows AFM images collected on the unmodified MPE (Figure 6a), the unmodified MCE (Figure 6b), and the eMPE and the eMCE surfaces after the application of Ea in zones I to III (Figures 6a′ to 6a‴ and 6b′ to 6b‴, respectively). After the potentiostatic oxidation of the MPE and the MCE, a contrasting surface transformation can be observed by AFM; the formation of widespread nanoscale size structures on the eMPE (Figure 6a′) and the eMCE (Figure 6b′) surfaces in zone I, than those found on the MPE (Figure 6a) and the MCE (Figure 6b). The surface of the eMCE shows larger grains than those observed on the eMPE, suggesting the accumulation of a higher amount of secondary compounds on eMCE. These nanoscale size structures were well distributed on the eMPE and the eMCE surfaces suggesting that pyrite and chalcopyrite oxidation was well-ordered in the zone I (Figure 2a′ and 2b′, respectively). From the results determined from the former section, it was concluded that such nanoscale size structures are low active Sn2− and passive Sn2−/CuS-like compounds, on the eMPE and the eMCE, respectively. Nava et al. (2008) identified by XPS inactive secondary Sn2− species (e.g., Cu1‑xFe1‑yS2‑z), during the first stage of chalcopyrite potentiostatic oxidation under hydrometallurgical conditions, such secondary species were progressively active enhanced, as a function of Ea. Afterward, AFM images collected on the eMPE and the eMCE after application of Ea in zone II show significant development of the secondary compounds, from the nanoscale to the microscale size (Figure 6, parts a″ and b″, respectively). These microscale size structures are responsible for semiactive Sn2−/S0 and low active CuS/S0 secondary compounds oxidation, H


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Figure 6. AFM (3D) images from unmodified MPE and MCE (a and b, respectively), and eMPE and eMCE after the application of Ea in zone I (a′ and b′, respectively), zone II (a″ and b″, respectively), and zone III (a‴ and b‴, respectively). Tapping mode in air. Scan rate of 0.5−1 Hz.

Tables 3 and 4 show the Ra, Rq on the MPE and the MCE before and after application of Ea in zones I to III (e.g., 0.71, 0.91, and 1.21 V, respectively). It is well-known that Ra, Rq are parameters that can be indicative of a coarse or uneven surface, and in this way reflect changes in the surface topography as a result of external perturbations. Ra and Rq parameters are

respectively. AFM images collected on the eMPE (Figure 6a‴) and eMCE (Figure 6b‴) after application of Ea in zone III show microscale size structures divergently distributed on the eMPE and the eMCE, suggesting that S0 oxidation (main secondary compound) is greatly heterogeneous in the last stage of the processes. I


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Table 3. Root Mean Square (Rq) and Roughness (Ra) Values Collected from Different MPE, MCE, eMPE, and eMCE Surface Areas pyrite and/or chalcopyrite surface MPE eMPE, zone I eMPE, zone II eMPE, zone III MCE eMCE, zone I eMCE, zone II eMCE, zone III

area (μm × μm)

number of areas

× × × × × × × × × × × × × × × ×

10 20 15 30 15 30 15 30 15 30 15 30 15 30 15 30

5 2 5 2 5 2 5 2 5 2 5 2 5 2 5 2

5 2 5 2 5 2 5 2 5 2 5 2 5 2 5 2

RMS (Rq) (nm) 2.78 3.05 24.35 24.65 75.25 71.56 65.89 63.25 10.23 10.02 56.88 54.65 138.98 135.63 186.23 175.58

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.18 0.22 2.18 1.49 3.36 2.69 3.87 2.69 1.26 1.10 2.36 1.78 4.36 5.36 7.89 6.12

roughness (Ra) (nm) 2.12 2.05 19.41 19.63 83.26 73.15 78.12 74.56 9.98 10.01 55.01 53.96 138.56 137.78 190.78 189.59

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.32 0.45 1.78 1.03 4.23 3.25 4.56 4.23 1.78 1.88 2.19 1.89 4.23 4.11 6.78 6.58

was expected by interaction of the soluble iron released during pyrite and/or chalcopyrite oxidation with the phosphate ions present in the ATCC-125 medium. The chemical and electrochemical characterizations have exposed a connection existing between the different oxidative behaviors of pyrite and chalcopyrite, and the secondary compounds formed on their surfaces (Tables 1 to 4). To this concern, ab initio methods are used below to disclose their structure−reactivity relationships established for their early oxidation stages. 3.4. Theoretical Analysis. Periodic density functional theory (DFT) was used to determine the electronic structure of pyrite and chalcopyrite in order to account for their early oxidation stages, as well as the influence of their crystal structure upon their oxidation capacity. This analysis constitutes the first insight into understand the oxidation mechanism of these SM in the aforementioned acidified (pH 2.0) ATCC-125 medium. Note that experimentally, the main electrochemical oxidation of these materials occurs within the bulk. Certainly, the presence of chemical species (H2SO4, HCl, NaOH, NH4OH; Na2CO3) in the interface induces the formation of compounds generated from the SM oxidation and the chemical reaction with these species. There should be an impact of these interface modifications in the general behavior of the oxidation process. However, the present theoretical analysis focuses on bulk properties which control early oxidation stages of the minerals, rather than interfacial phenomena which are more dominant during the course of the processes. Thus, it is assumed that at these stages, where the amount of bulk material prevails with respect to that of the interfaces, the oxidative capacities of the minerals rely more on the electronic structure of the bulk rather than on the properties of the surface secondary compounds. Consequently, electronic structure information is important to identify the regions of the structure responsible for the activation processes. The theoretical analysis of the solvent effects and subsequent oxidation steps of pyrite and chalcopyrite demand the simulation of a polycrystalline material involving multiplephases to capture their physicochemical properties in the presence of these ions (e.g., electrochemical response). Likewise, the influence of solvent on the surface states entails additional experimental evaluation, and a simulation protocol

obtained from the AFM images presented in Figure 6, but using the respective 2D images with several areas (Tables 3 and 4). In the unmodified MPE and MCE surfaces, the Ra, average was ∼2.2 and ∼10 nm, respectively. The Ra, Rq of the eMPE (∼20 and 24 nm, respectively) and the eMCE (∼57 nm and ∼59 nm, respectively) are enhanced after application of Ea in zone I. These parameters are significantly enhanced on the eMPE (∼75.23 nm and ∼83.3 nm, respectively) and the eMCE (∼138.6 nm and ∼139 nm, respectively) after application of Ea in zone II. These findings confirm the progressive accumulation of the secondary compounds from zone I to III, especially on the eMCE. The Ra and Rq values slightly diminish on the eMPE after the application of Ea in zone III (∼66 nm and ∼78.12 nm, respectively), whereas the highest values for these parameters are measured on the eMCE in zone III (∼186.26 nm and ∼191 nm, respectively). Similar Ra and Rq values on the eMPE (from zone II to III) indicate the equilibrium point between the formation via oxidation and dissolution, and suggest that S0 compounds are progressively active from zones I to III. On the other hand, the enhancement of Ra and Rq values on the eMCE is connected to the largest surface modifications and accumulation of secondary compounds at the microscale level. Figure 7 shows SEM images collected on the unmodified MPE (Figure 7a), unmodified MCE (Figure 7b), and the eMPE and eMCE generated after application of Ea in zone I (e.g., 0.71 V, Figure 7, parts a′ and b′, respectively), zone II (e.g., 0.91 V, Figure 7, parts a″ and b″, respectively) and zone III (e.g., 1.21 V, Figure 7, parts a‴ and b‴, respectively). EDS analyses (n = 10) were performed on the aforementioned surfaces, indicating the formation of secondary compounds labeled on the Figure. In addition to XPS and Raman studies (refer to previous sections), SEM-EDS analyses suggest the formation of patches of ferric phosphate (e.g., FePO4)-like compounds, randomly dispersed on eMPE and eMCE formed in zone III (Figure 7, parts a‴ and b‴, respectively). The formation of a ferric precipitate was previously suggested in the voltammety analysis, such ferric compound is progressively enhanced during pyrite and chalcopyrite oxidation (Figure 1). However, most of the eMPE and eMCE are found to be covered by sulfur-rich (Sn2−, S0, Figure 7a′−a‴) and sulfur-rich/CuS-like compounds (Sn2−, S0 and CuS; Figure 7b′−b‴), in agreement with the previous evaluations. The formation of ferric phosphate-like compound J


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Table 4. Electrochemical Reactions Induced by Imposed Anodic Potentials Ea, on the MPE and MCE Surfaces, in Acidified (pH 2.0) ATCC-125 Medium potential range or zone (V/SHE) eMPE zone I (0.515 < Ea < 0.761 V)

electrochemical reactions

eq no.

FeS2(s) ⇔ Fe1 − xS2(s) + x Fe2 +(l) + (2x)e−

1

Fe(II) ⇔ Fe(III)

2

Fe1‑xS2(s) Low Active Compound zone II (0.761 < Ea < 0.961 V)

FeS2(s) + 1.5y H2 O(l) ⇔ Fe1 − t S2 − y(s) + t Fe2 +(l) + 0.5yS0(s) + 0.5yS2 O32 −

3

(l)

+ 3y H+ + 2(t + y)e−

Fe2 +(l) + PO4 3 −(l) ⇔ FePO4(s) + e−

4

S2O32 −(l) + 5H 2O(l) ⇔ 2SO4 2 −(l) + 10H+ + 8e−

5

Fe1‑tS2‑y(s) Active Compound, FePO4 Passive Compound zone III (0.961 < Ea < 1.215 V)

FeS2(s) + PO4 3 −(l) + 4H 2O(l) ⇔ FePO4(s) + S0(s) + SO4 2 −(l) + 8H+ + 9e−

6

S0(s) + 4H 2O(l) ⇔ SO4 2 −(l) + 8H+ + 6e−

7

S0 Active Compound eMCE zone I (0.515 < Ea < 0.761 V)

CuFeS2(s) ⇔ Cu1 − xFeS2(s) + xCu 2 +(l) + (2x)e−

8

Cu1 − xFeS2(s) + zCu 2 +(l) + y PO4 3 −

9

(l)

⇔ Cu1 − xFe1 − yS2 − z(s) + zCuS(s) + y FePO4(s) + (3y + 2z)e− Cu1‑xFe1‑xS2‑z(s) and FePO4(s) Passive Compounds, x = z zone II (0.761 < Ea < 0.961 V)

CuFeS2(s) ⇔ Cu1 − sFeS2(s) + sCu 2 +(l) + (2s)e−

10

Cu1 − sFeS2(s) + t Cu 2 +(l) + y PO4 3 −

11

(l)

⇔ Cu1 − sFe1 − yS2 − t(s) + 0.5t CuS(s) + 0.5t Cu 2 +(l) + 0.5t S0(s) + y FePO4(s) + (3y + t )e− Cu1‑sFe1‑yS2‑t(s), S0(s) and CuS Semiactive Compounds, t = s zone III (0.961 < Ea < 1.215 V)

CuFeS2(s) + y PO4 3 −

(l)

12

+ 0.75H 2O(l)

⇔ Cu1 − sFe1 − yS2 − w(s) + sCu 2 +(l) + 0.5wS0(s) + 0.25wS2O32 −(l) + y FePO4(s) + 1.5H+ + ([2s + 1.5 − 0.5w] + 3y)e− CuS(s) + 0.75H 2O(l) + Cu 2 + ⇔ + 0.5S0(s) + 0.25S2O32 −(l) + 1.5H+ + e−

13

S0(s) + 4H 2O(l) ⇔ SO4 2 −(l) + 8H+ + 6e−

14

S2O32 −(l) + 5H 2O(l) ⇔ 2SO4 2 −(l) + 10H+ + 8e−

15

(l)

Cu1‑sFe1‑yS2‑w(s), S0(s) Semiactive Compounds

more difficult to be involved in oxidation processes (unless a high oxidation potential is applied). In connection to this idea, it is important to recall that when these SM experience an oxidation process, the fluctuation of the electric field across them will accelerate electrons to higher energies than they do not pose at open circuit conditions. Therefore, in order to compare the initial response capabilities of pyrite and chalcopyrite to oxidation, an analysis of the electronic structure close to Ef was perfomed. With the aim of focusing the analysis on the states near to Ef, Figure 8 shows the total and projected DOS, calculated with the PBE+U method, for pyrite and chalcopyrite at two different energy ranges (partial DOS) referred to the Fermi level. As observed in Figure 8, parts a and b, there exist more states situated in pyrite than in chalcopyrite in all the energy range; in addition, from the analysis of Figure 8, parts a′ and b′, it is evident the presence of higher values of

that takes account of the interfacial polarization will be the motivation of a forthcoming analysis. Optimized structures were found to have similar lattice parameters to those found in our X-ray diffraction analysis (refer to section 2.1), the relaxed structures employing the PBE functional differ in the lattice parameter, as compared to the experimental structure, in −0.22% and −0.31% for pyrite and chalcopyrite respectively; using the PBE+U method the same differences are +0.31% and +0.18%. Therefore, both methods represent well the size and shape of the unitary cell. The obtained band gap of pyrite was 0.44 and 0.92 eV as calculated with PBE and PBE+U respectively; for chalcopyrite a metallic behavior is obtained using the PBE functional and a band gap of 0.52 eV using the PBE+U method. The initial stages of a solid bulk material oxidation should be related to the electrons in occupied states near Ef; while those far away from Ef are K


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Figure 7. SEM images from MPE and MCE (a and b, respectively), and eMPE and eMCE after the application of Ea in the zone I (a′ and b′, respectively), zone II (a″ and b″, respectively) and zone III (a‴ and b‴, respectively). This figure includes main secondary phases according to EDS analyses (n = 10).

the DOS in pyrite than in chalcopyrite, particularly at values below 0.05 eV from Ef. This suggests that there are more electronic states just below Ef in pyrite than in chalcopyrite that indeed can be used to transfer these electrons in order to

oxidize the material. Thus, the electron transfer occurs more readily in pyrite than in chalcopyrite, whence its oxidation will be more facile, such as observed in Figure 2 (zone I). L


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Figure 8. Partial density of states with respect to Ef of (a) pyrite and (b) chalcopyrite at two different energy ranges. Partial contributions to the total DOS are labeled in the figure. Spin up and down contributions are represented as positive and negative values of DOS respectively.

there are few states near to Ef (i.e., range from −0.03 to Ef). This low value of projected DOS could account for the oxidation limitations observed in Figure 2 in zone I. When states with higher energies are explored, around (E − Ef)= −0.04 eV, it is apparent that FeS2 even presents more states than CuFeS2, but there is a minor contribution to these states arising from sulfur atoms in comparison to iron ones. This analysis suggests the oxidation of each atom within the SM structures at a determined energy (e.g., bias voltage imposed). This argument may be used to connect those energies with the potential value in the experimental data (sections 3.1 and 3.2). The oxidation of pyrite is mainly due to the iron species (Figure 8a), and the sulfur species to a minor extent, limiting the electronic conduction of the structure as detailed in the reaction mechanism proposed in Table 4. The incorporation of Cu into the lattice modifies considerably the DOS and the atoms connected to it. Specifically, the oxidation of chalcopyrite is attributable to sulfur species and Cu, but since there is a lower DOS compared to FeS2, the extent of Fe oxidation of this mineral is very limited. Further analysis comes from the calculation of partial electronic density evaluated at different energy ranges from −0.15 to Ef and −1.0 to Ef (eV). In Figure 9, partial electron density from −0.15 to Ef is depicted at two isosurface levels. At the higher isosurface value, Fe and S atoms in pyrite present electronic density volume (Figure 9a) while in chalcopyrite a small electronic density isosurface can be observed only in copper atoms (Figure 9b), suggesting the higher reactivity of pyrite in comparison to chalcopyrite. At the lower isosurface value both S and Fe contributions are observed in pyrite extended all over the unitary cell, while for chalcopyrite a lower volume of electron density is observed with main contributions from Cu and S atoms; however, it is observed a small

Figure 9. Partial electron density contributions of pyrite (top) and chalcopyrite (bottom) to the total electron density from −0.05 to Ef, with the isosurface values on the side. Yellow, brown, and blue spheres represent sulfur, iron, and copper atoms, respectively.

Another relevant information displayed in Figure 8 comes from the projected DOS. Parts a′ and b′ of Figure 8 reveal that M


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The Journal of Physical Chemistry C contribution from Fe atoms. Expanding the analysis to a higher energy range from −1.0 to Ef (eV) (Supporting Information, Figure S1), it is more evident the contributions from Fe and S atoms in pyrite; for chalcopyrite iron states become evident (note the higher electronic density isosurface value). In general the analysis from the DOS is confirmed, Fe electrons are more available than S in pyrite at values close to the Fermi level, while, for chalcopyrite, a lower amount of electrons are available for being transferred, mainly located in Cu and S. However, when the electronic density is analyzed to a lower value the participation of S and Fe in pyrite and chalcopyrite become available which is also observed when the electronic density and DOS is analyzed at lower energy states; i.e., when a more oxidizing potential is applied those states would become available. These calculations allow a rationalization of the passive features observed for the secondary phases formed on chalcopyrite (Figure 2), limiting its oxidation from 0.51 to 1.21 V, and the activation of pyrite surface as the potential energy is increased and the states become dominated by Iron species.

potential energy was increased and the states became dominated by iron species. The complex effect of the electrolyte (e.g., incorporation of SO42−, PO43−) was not explicitly considered in the theoretical analysis, whereby the influence of the counterion in the oxidative behavior will be the motivation of a forthcoming analysis, as well as the evaluation of secondary phases formed in subsequent oxidation stages.

CONCLUSIONS A detailed analysis of the oxidative processes of pyrite and chalcopyrite of interest for bioleaching and/or bioremediation applications was carried out. The effort was focused on accounting for the differences existing in their oxidation mechanisms, on the basis of their electronic structure. Thus, the study attempted to relate surface analysis (XPS, Raman, SEM-EDS, AFM) and electrochemical characterization (cyclic voltammetry, Q vs E curves) conducted with pristine and oxidized samples, along with computational modeling of the pristine structures using density functional theory. Pyrite oxidation was found to be highly active, and the generation of secondary compounds such as deficient-metal sulfur rich compounds (Sn2−, e.g., Fe1‑xS2‑y), elemental sulfur (S0), and ferric phosphate (FePO4-like) compounds was respectively spread over three mayor oxidation stages (e.g., zones I to III). On the other hand, chalcopyrite oxidation presented a very passive behavior and therefore, experienced a slow dissolution associated with secondary compounds including S0, deficientmetal sulfur rich compounds (e.g., Cu1‑xFe1‑yS2‑y), FePO4-like, among others, in the previous oxidation stages. According to the theoretical results, the main differences existing in the bulk electronic properties of these materials rely on a significant higher density of states found in pyrite close to the Fermi level, indicating that there are more electronic states just below the Fermi energy level in pyrite than in chalcopyrite. This indeed can be used to transfer electrons in order to oxidize the material. Thus, the electron transfer occurs more readily in pyrite than in chalcopyrite, whence its oxidation will be more facile, as formerly determined by experimental data. A projected density of states of these structure computed few states near the Fermi energy level of both sulfide minerals, which account for the oxidation limitations experimentally observed in the region of low overpotential (zone I). At higher energies, the oxidation of pyrite was mainly due to iron species, and sulfur species to a minor extent, limiting the electronic conduction of these structures, as described in the proposed reaction mechanism. The incorporation of Cu into the lattice considerably modified the density of states and the atoms connected to it, attributing its sluggish oxidation to sulfur species and copper. Accordingly, these calculations allow a rationalization of the passive features observed for the secondary phases formed on chalcopyrite from 0.51 to 1.21 V, and the activation of pyrite surface as the

Notes

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ASSOCIATED CONTENT

* Supporting Information S

Partial electronic density isosurfaces evaluated for pyrite and chalcopyrite from 1.0 to Ef (eV). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05149.

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AUTHOR INFORMATION

Corresponding Author

*(J.V.-A.) E-mail: [email protected]; jorge_gva@hotmail. com. Telephone: +52 (55) 58044600 Ext 2686.

■

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support is greatly appreciated from CONACYT (Grants No. 2010-155698, 2012-183230, 2013-205416 and 2014-237343). The authors acknowledge the assistance of Erasmo Mata (UASLP) during mineral preparation, Donato Valdez (UASLP) to conduct the AFM measurements, and ́ Á ngel G. Rodriguez (UASLP) to facilitate access to the Raman analysis.

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REFERENCES

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