minerals Article
Insights into the Surface Transformation and Electrochemical Dissolution Process of Bornite in Bioleaching Hongbo Zhao 1,2,3 , Xiaotao Huang 1,2 , Minghao Hu 1,2 , Chenyang Zhang 1,2 Jun Wang 1,2, *, Wenqing Qin 1,2 and Guanzhou Qiu 1,2 1
2 3
*
ID
, Yisheng Zhang 1,2 ,
School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China;
[email protected] (H.Z.);
[email protected] (X.H.);
[email protected] (M.H.);
[email protected] (C.Z.);
[email protected] (Y.Z.);
[email protected] (W.Q.);
[email protected] (G.Q.) Key Lab of Biohydrometallurgy of Ministry of Education, Changsha 410083, China State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming 650093, China Correspondence:
[email protected]; Tel.: +86-0731-8887-6557
Received: 23 March 2018; Accepted: 20 April 2018; Published: 23 April 2018
Abstract: In this work, density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) and electrochemistry analysis were combined to analyze the electrochemical dissolution process of bornite during bioleaching. DFT calculations showed that bornite was a conductor with metallic conductivity. The formula of bornite may be (Cu+ )5 Fe3+ (S2− )4 and the surface reconstruction of (111)-S surface was discussed. Electrochemistry and XPS analysis showed that bornite tended to be directly oxidized with high conductivity when the potential was higher than 0.3 V vs. Ag/AgCl. Elemental sulfur (S0 ), FeOOH and CuS were the main intermediate species on the bornite surface during the oxidation process. The production of S0 and FeOOH on bornite surface can be significantly accelerated with increased redox potential, but no insoluble sulfate (SO4 2− ) formed on bornite surface in 0.3–0.65 V vs. Ag/AgCl. The oxidative dissolution of bornite was significantly accelerated with increasing redox potential, which was one important reason why mixed culture was more effective than single strains of A. caldus and L. ferriphilum in bornite bioleaching. The insoluble SO4 2− was formed mainly through the chemical reactions in solution and then covered the bornite surface in bioleaching. Based on the obtained results, a model for interpreting the dissolution process of bornite in bioleaching was proposed. Keywords: bornite; bioleaching; surface transformation; dissolution process; moderately thermophilic microorganisms
1. Introduction Bornite (Cu5 FeS4 ) is a kind of copper resource that is relatively widespread on Earth. Bio-hydrometallurgy (bioleaching) is considered an economically promising technology for clean extraction of sulfide minerals such as chalcopyrite (CuFeS2 ), pyrite (FeS2 ) and chalcocite (Cu2 S) [1]. Hence, bioleaching of bornite is an important research topic and the dissolution mechanisms of bornite during bioleaching should be firstly studied to enhance its bioleaching efficiency. Bornite is considered as an intermediate species in chalcopyrite bioleaching, and chalcopyrite was also reported as the intermediate species during bornite bioleaching [2–5]. Therefore, the study of dissolution mechanisms of bornite during bioleaching is also essential for interpreting the dissolution mechanisms of chalcopyrite bioleaching. Some publications have reported that bornite has non-ignorable effects on chalcopyrite dissolution [6–8]. The dissolution of sulfides during bioleaching sometimes can be Minerals 2018, 8, 173; doi:10.3390/min8040173
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regarded as a reverse biomineralization process. Therefore, the research on the surface transformation and dissolution process of bornite in the presence of microorganisms can not only help improve the bioleaching kinetics of bornite and chalcopyrite, but also enhance the geberal understanding of the biomineralization of Cu-sulfides and their geo-biological-chemical circulation on Earth. Some studies on chemical leaching of bornite have been reported. Price et al. [9] analyzed the dissolution process of bornite in sulfuric acid by combining anodic constant current treatments, solution analysis, electron probe microanalysis and X-ray diffraction. They found that Cu2.5 FeS4 was an intermediate species and the reaction was controlled by the solid state diffusion at 50 ◦ C. Price and Chilton [10] also proposed that the optimum conditions for electroleaching of bornite included high temperature, low current density and the presence of chloride ions. Pesic et al. [5] studied the dissolution of bornite in sulfuric acid using oxygen as an oxidant, and they found that covellite (CuS) and Cu3 FeS4 were the main intermediate species. In addition, the copper dissolution rate was controlled by the iron dissolution rate, and the initial preferential iron dissolution caused the formation of iron-deficient bornite which was further transformed to CuS and Cu3 FeS4 due to the diffusion and depletion of labile Cu+ ions. Safarzadeh et al. [11] reported that the reactivity of sulfides in sulfuric acid solution followed the order of chalcocite, bornite, enargite, covellite, chalcopyrite and pyrite. Some efforts have also been made to study the dissolution mechanisms of bornite bioleaching. Bevilaqua et al. [12] investigated the oxidative dissolution of bornite by Acidithiobacillus ferrooxidans mainly through X-fray diffraction, and found that CuS, elemental sulfur and jarosite were detected as the secondary phases. Qin et al. [13] used electrochemical analysis to investigate the dissolution process of bornite in the presence of Acidithiobacillus ferrooxidans and Acidithiobacillus caldus, and put forward that CuS, chalcocite (Cu2 S) and nonstoichiometric copper sulfides may be the possible intermediate species. Bevilaqua et al. [14] used electrochemical noise analysis to evaluate the oxidative dissolution of bornite in the absence and presence of Acidithiobacillus ferrooxidans, and found that bacterial activity induced an accelerated corrosion process. They further used electrochemical impedance spectroscopy to monitor bornite oxidation by Acidithiobacillus ferrooxidans, and inferred that CuS and nonstoichiometric compounds (Cux S) were the main intermediate species [15]. The dissolution mechanisms of bornite during bioleaching are extremely important and some studies have been conducted. However, the proposed mechanisms are different, and the specific mechanism is still under debate and not clear. Moderately thermophilic microorganisms are considered to be promising in the future industrial application due to their advantages over mesophilic microorganisms and extremely thermophilic microorganisms [16–18]. The dissolution processes of bornite in bioleaching systems consists of complicated oxidation-reduction reactions which are dependent on the redox potentials. Therefore, density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS) analysis and electrochemistry analysis were combined in this work to analyze the surface transformation and dissolution process of bornite at different applied potentials. Based on the obtained results, the dissolution mechanisms of bornite in the presence of moderately thermophilic microorganisms were further interpreted. 2. Materials and Methods 2.1. DFT Calculation A cubic bornite unit cell with space group of F-43m was employed and its lattice parameters are shown in Figure 1 and Table 1, respectively. All DFT computations were performed using the CASTEP module in the Materials Studio 6.0 software package. The PBE density functional was chosen to account for the exchange-correlation potential [19,20]. To provide proper descriptions for the interactions between ions and their valence electrons, we adopted the ultrasoft pseudopotentials (USPP), where the valence shells were set to 3d6 4s2 , 3d10 4s1 and 3s2 3p4 for Fe, Cu and S atoms, respectively. It is well known that the d and f electrons of transition metal atoms usually exhibit such strong correlations that an improper computation method often leads to irrational electron structures
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differing largely from experimental results. Accordingly, the Hubbard + U correction was usually employed [21–23]. In this work, the energy, property geometry employed [21–23]. In this work, thecorrection correction was was employed employed ininallall energy, property andand geometry optimization calculations. The U values for Cu and Fe were both 2.5, which were automatically optimization calculations. The U values for Cu and Fe were both 2.5, which were automaticallysetset by by CASTEP. Moreover, spin polarization was taken also taken account with initial spinobtained obtainedfrom CASTEP. Moreover, spin polarization was also into into account with thethe initial spin from the parameters of the aforementioned cubiccell unit cell model the parameters of the aforementioned cubic unit model [24]. [24].
Fe
Cu S
Figure 1. The structure inDFT DFTcalculation. calculation. Figure 1. The structureofofF-43m F-43m bornite bornite in 1. Lattice constants of the bornitestructure structurein in DFT DFT calculation module of Materials TableTable 1. Lattice constants of the bornite calculation(CASTEP (CASTEP module of Materials 6.0 software package, PBE densityfunction, function,Hubbard Hubbard ++UUcorrection). StudioStudio 6.0 software package, PBE density correction).
Crystal System Space Groups Lattice Constant Crystal Angle Crystal System Space Groups Lattice Cubic F-43m a = b =Constant c = 10.710 Å CrystalαAngle = β = γ = 90° Cubic F-43m α = β = γ = 90◦ a = b = c = 10.710 Å During geometry optimizations, the convergence thresholds were set to 1.0 × 10−5 eV/atom, 0.03 eV/Å and 0.001 Å for the total energies, the max force and the max displacement, respectively. The During geometry optimizations, the convergence thresholds were set to 1.0 × 10−5 eV/atom, −6 SCF convergence threshold was 1.0 × 10 eV/atom. A 3 × 3 × 3 k-point grid with a 400 eV kinetic 0.03 eV/Å and 0.001 Å for the total energies, the max force and the max displacement, respectively. energy cutoff was used. After structural relaxations, energy and property calculations were carried −6 eV/atom. A 3 × 3 × 3 k-point grid with a 400 eV The SCF convergence was 1.0 × 10integration out using the same threshold convergence thresholds, grids and cutoff. kinetic energy cutoff used. After energy and property calculations To model thewas reconstruction of structural the (111)-S relaxations, bornite surface, we adopted the lattice plane slicedwere carried out using the same convergence thresholds, integration grids and cutoff. from the optimized unit cell (the molar ratio of Cu: Fe: S within the plane was retained). Considering themodel symmetry and computational cost, finallysurface, we constructed a cell model which plane has half To the properties reconstruction of the (111)-S bornite we adopted the lattice sliced length of the a and cell vectors unit Fe: length in the cthe axisplane (5 layers total, the bottom 3 from the optimized unitb unit cell (the molar and ratioone of Cu: S within wasinretained). Considering layers were fixed during modelling). To further eliminate the interactions between the top and the symmetry properties and computational cost, finally we constructed a cell model which has half of 10and Å was added theindirection of the c axis. The convergence lengthbottom of thesurface, a and ba vacuum unit celllayer vectors one unit along length the c axis (5 layers in total, the bottom thresholds for modelling surface reconstruction were 2.0 × 10−5 eV/atom, 0.05 eV/Å and 0.002 Å for 3 layers were fixed during modelling). To further eliminate the interactions between the top and the total energies, the max force and the max displacement, respectively. The SCF convergence bottom surface, a vacuum layer of 10 Å was added along the direction of the c axis. The convergence threshold was set to 2.0 × 10−6 eV/atom. A 3 × 3 × 1 k-point grid and a 400 eV cutoff were employed. −5 eV/atom, 0.05 eV/Å and 0.002 Å thresholds for modelling surface reconstruction were 2.0 × 10 After optimizations (i.e., reconstruction), energy and property calculations were performed using the for the total energies, the max force max displacement, respectively. The SCF convergence same convergence thresholds, gridsand andthe cutoff.
threshold was set to 2.0 × 10−6 eV/atom. A 3 × 3 × 1 k-point grid and a 400 eV cutoff were employed. Bioleaching Experiments After 2.2. optimizations (i.e., reconstruction), energy and property calculations were performed using the same convergence thresholds, grids and cutoff. The moderately thermophilic microorganisms Acidithiobacilluscaldus (A. caldus) (CCTC AB 206240) and Leptospirillum ferriphilum (L. ferriphilum) (CCTC AB 206239) were both initially acquired
2.2. Bioleaching Experiments from the Key Lab of Bio-hydrometallurgy of the Ministry of Education, Central South University,
Changsha, China. Bornite samples of high purity were obtained from Guangdong The moderately thermophilic microorganisms Acidithiobacillus caldus (A. Meizhou, caldus) (CCTC AB 206240) Province, China. The elements analysis indicates that the pure bornite samples contained 61.59% Cu, and Leptospirillum ferriphilum (L. ferriphilum) (CCTC AB 206239) were both initially acquired from the 10.04% Fe and 27.10% S (wt %), respectively. L. ferriphilum and A. caldus were sub-cultured into basal Key Lab of Bio-hydrometallurgy of the Ministry of Education, Central South University, Changsha, culture medium supplemented with 44.7 g/L ferrous sulfate (FeSO4·7H2O) and 10 g/L sulfur as energy China. Bornite samples Mixed of highculture purityconsisting were obtained from Meizhou, Guangdong Province, China. source, respectively. of L. ferriphilum and A. caldus was sub-cultured into The elements analysis indicates that the pure bornite samples contained 10.04% basal culture medium supplemented with 44.7 g/L ferrous sulfate (FeSO4·7H61.59% 2O) and Cu, 10 g/L sulfur Fe as and 27.10% S (wt %), respectively. L. ferriphilum and A. caldus were sub-cultured into basal culture medium supplemented with 44.7 g/L ferrous sulfate (FeSO4 ·7H2 O) and 10 g/L sulfur as energy source, respectively. Mixed culture consisting of L. ferriphilum and A. caldus was sub-cultured into
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basal culture medium supplemented with 44.7 g/L ferrous sulfate (FeSO4 ·7H2 O) and 10 g/L sulfur as energy sources. When microorganisms grew to their exponential growth phase with cell density of higher than 1.0 × 108 cells/mL, cells were harvested, centrifuged and washed. The obtained cells were inoculated into a 250-mL shake flask containing 100 mL of sterilized culture medium and 2 g of bornite samples. The shake flasks were placed into an orbital shaker at 170 rpm and 45 ◦ C, pH was regulated around 1.70 with dilute sulfuric acid, and water lost through evaporation was supplemented with deionized water periodically. 2.3. Analytic Techniques Metal ions concentrations were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (PS-6, Baird Co., Deford, MA, USA), meanwhile, pH and Oxidation-Reduction Potential (ORP) values were monitored with a pH meter (PHSJ-4A, Shanghai LEICI Co., Shanghai, China) and ORP meter (BPH-221, Dalian BELL Co., Dalian, China), respectively. Electrochemistry tests were conducted by conventional three-electrode system (Princeton Model 283 Potentiostat, EG&G of Princeton Applied Research, Princeton, NJ, USA). All the potential values mentioned were referred to the Ag/AgCl electrode (vs. Ag/AgCl) in this work. X-ray photoelectron spectroscopy (XPS) analysis was implemented on a model ESCALAB 250Xi instrument (Thermo Fisher Scientific Co., Waltham, MA, USA). The XPS spectra were recorded at a constant pass energy of 20 eV and 0.1 eV/step with Al Kα X-ray source. The Thermo Avantage 5.52 software (Thermo Fisher Scientific Co.) was used to fit the obtained XPS spectra, binding energies were referred to the C 1s level at 284.8 eV, the background of spectra was achieved by using the Shirley method, and the S 2p spectra were fitted by the Gaussian-Lorentzian line (SGL) function [25]. The S 2p spectra were fitted using a 2:1 peak area ratio and 1.2 eV splitting for S 2p3/2 and S 2p1/2 . To investigate the surface transformation of bornite at different potentials, the bornite electrode was treated by different potentials for 3 h, and the treated bornite electrode was then analyzed by XPS. Bornite samples bioleached by different microorganisms for different numbers of days were analyzed by XPS to reveal the intermediate species on bornite surface during bioleaching. 3. Results and Discussions 3.1. DFT Calculations of the Bornite Surface According to symmetry, we converted the unit cell into a primitive cell before our calculation. During our CASTEP calculation, we chose the following 3 parameters to check the convergence according to the energy cutoff and the results are shown in Table 2. The total energy of system changed slightly under 3 different settings of energy cut-off, and the value of dEtot /dElog (Ecut ) also meets the requirements. It is noted that the number of SCF iterations under 400 eV decreased significantly. Based on the above, we can reach convergence fast and accurately by setting the energy cutoff at 400 eV. Table 2. Convergence test results of energy cut-off in DFT calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, Hubbard + U correction). Cut-off Energy/eV
SCF Loops
dEtot /dlog (Ecut )
Final Energy/eV
390 395 400
115 15 15
−0.15371
−17,491.94660 −17,491.94830 −17,491.95020
Table 3 shows the lattice parameters before and after optimization and the reference results. The relative error between the optimized lattice parameters and the reference results was 3.2% [24]. In combination with the results of convergence test, the optimization results were convincing. It can
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represent the actual structure of F-43m bornite. The consideration of spin and LDA + U corrections made our computations more reliable. Table 3. Results of cell parameters optimization in DFT calculation and reference (CASTEP module of Materials Studio 6.0 software package, PBE density function, Hubbard + U correction). Parameter
Reference
Optimization Result
Relative Error/%
a=b=c α=β=γ
10.710 Å 90◦
11.0503 Å 90◦
3.2 -
To analyze various kinds of properties of the F-43m bornite, we calculated the vacuum single point energy of the optimized structure. As shown in Table 4, we can get the analytical results of Cu, Fe, S atom orbital and total Mulliken population. From this table, we can see that the Fe atom transferred 0.46 electrons to the S atom (the electron cloud on Fe atom skewed to the S atom), part of the Cu atoms (Cu1-4) transferred 0.06 electrons to the S atom, while another part of the Cu atoms (Cu4-8) transferred 0.17 electrons to the S atom. These two different kinds of Cu atoms in different chemical environments suggested that the Cu atoms in F-43m bornite may exist in two different oxidation states. Table 4. Mulliken population of bornite in DFT calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, Hubbard + U correction). Species
Ion
s
p
d
Total
Charge/e
Spin/hbar
S S S S S S S S Fe Fe Fe Fe Cu Cu Cu Cu Cu Cu Cu Cu
1 2 3 4 5 6 7 8 1 2 3 4 1 2 3 4 5 6 7 8
1.79 1.85 1.79 1.79 1.79 1.79 1.79 1.79 0.36 0.36 0.36 0.34 0.60 0.59 0.59 0.59 0.49 0.50 0.50 0.50
4.56 4.54 4.55 4.55 4.55 4.55 4.55 4.55 0.64 0.64 0.64 0.60 0.56 0.55 0.55 0.55 0.50 0.50 0.50 0.50
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.54 6.54 6.54 6.54 6.54 6.60 9.79 9.79 9.79 9.79 9.84 9.83
6.34 6.39 6.33 6.34 6.33 6.34 6.33 6.34 7.54 7.54 7.54 7.54 10.95 10.94 10.94 10.94 10.82 10.83 10.83 10.83
−0.34 −0.39 −0.33 −0.34 −0.33 −0.34 −0.33 −0.34 0.46 0.46 0.46 0.46 0.05 0.06 0.06 0.06 0.18 0.17 0.17 0.17
0.04 0.13 0.12 0.10 0.12 0.10 0.12 0.10 1.70 1.70 1.70 1.67 0.00 0.00 0.00 0.00 0.04 0.04 0.04 0.04
Further analysis of the electronic structure of the whole cell of the F-43m bornite, especially the different oxidation states of the Cu atoms, as well as the bond population analysis is listed in Table 5. The value of the bond population reflects the degree of the bond covalence. The bond covalence increased with the increase of bond population value, and it’s completely a covalent bond when the value is equal to 1. When the value is equal to 0, it represents a completely ionic bond. The bonding between S and M (stand for metal) atoms in bornite was very complicated. The Cu1-4 not only bonded with the S atoms, but also had interactions with Fe atoms. (Cu1-4) transferred less electrons to the S atoms due to the covalency, thus causing Cu1-4 atoms existed in different chemical environments and with different oxidation states. From the energy band diagram (Figure 2) of the F-43m bornite, we can find that the F-43m bornite is a conductor with metallic conductivity because its energy gap was almost 0 eV. This verified the Cu-Fe bonding effect in bond population analysis, whose behavior was similar to the electrons in metals.
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Table 5. Binding energy and FWHM value for XPS spectra of S 2p3/2 peaks of bornite after treating by different potential of 0, 0.3, 0.45, 0.55, 0.65 V, respectively (Implemented on the model of ESCALAB Table 5. Binding energy and FWHM value for XPS spectra of S 2p3/2 peaks of bornite after treating by 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo different potential of 0, 0.3, 0.45, 0.55, 0.65 V, respectively (Implemented on the model of ESCALAB Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function) (FWHM 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo means full width maximum, B.E. means binding energy value). Avantage 5.52, at C half 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function) (FWHM means full width at half maximum, B.E. means binding energy value). Peak 1 Peak 2 Peak 3 Conditions (vs. Ag/AgCl) Peak 1 Peak 2 Peak 3 Conditions (vs. Ag/AgCl) B.E. (eV) B.E. (eV) B.E. (eV) FWHM (eV) B.E. (eV) B.E. (eV) B.E. (eV) FWHM (eV) 162.3 - 0V 0V 161.3161.3 162.3 162.1 163.2 1.4 1.4 0.3 V 0.3 V 161.4161.4 162.1 163.2 162.2 163.4 1.4 1.4 0.45 V0.45 V 161.4161.4 162.2 163.4 0.55 V 161.4 162.2 163.6 1.0 1.0 0.55 V 161.4 162.2 163.6 0.65 V 161.5 163.6 1.0 1.0 0.65 V 161.5 163.6
Figure 2. The band structureofofbornite bornite(X-axis, (X-axis,High High symmetry symmetry point; Figure 2. The band structure point; Y-axis, Y-axis, Energy/eV) Energy/eV)ininDFT DFT calculation (CASTEP module of Materials 6.0 software package, PBE function, density function, calculation (CASTEP module of Materials StudioStudio 6.0 software package, PBE density Hubbard + Hubbard + U correction). U correction).
Figure 3 shows the total density of state (DOS) and the partial density of state (PDOS) of the Cu, showsWe thecan total state (DOS) and thebond partial density of state (PDOS) the Fe,Figure and S 3atoms. finddensity that theofbottom of the valence band from −15.0 to −12.0 eV of was Cu,mainly Fe, and S atoms. We can find that the bottom of the valence bond band from − 15.0 to − 12.0 made up of the 3s orbital of S atoms. Lying between −8.0 and 0 eV were contributions of theeV wascombination mainly made up 3d of the 3s orbital S atoms. Lying between −8.0 0 eV were of the orbitals of Cu of Atoms, 3d orbitals of Fe atom andand 3p orbitals of Scontributions atoms. The of the combination of the 3d orbitals of Cu Atoms, 3d orbitals of Fe atom and 3p orbitals S atoms. conduction band between 0 and 2eV consisted of 3d orbitals of Fe atoms. The higher energy of part was Themade conduction band between 0 and 2eV consisted of 3d orbitals of Fe atoms. The higher energy part up of the 4s orbital of Cu atom, 3p orbital of Cu atom, 4s orbital of Fe atom, 3p orbital of Fe wasatom made up3poforbital the 4soforbital of The Cu atom, 3p of orbital of Cu atom, 4s orbital of mainly Fe atom, 3ptoorbital and S atom. existence the electrical conductivity was due the 3d of Fe atom and of S atom. of the was mainly due the orbitals of 3p Fe orbital atoms crossed overThe theexistence Fermi level andelectrical interactedconductivity with the high-level orbitals ofto Cu mostcrossed active atoms in the F-43m bornite unit cell were thethe Fehigh-level atoms. 3d atoms. orbitalsThus, of Fethe atoms over the Fermi level and interacted with orbitals of Cu atoms. Thus, the most active atoms in the F-43m bornite unit cell were the Fe atoms.
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b1 14 16 b1 12 14
60 50
DOSDOS (electrons/eV) (electrons/eV)
DOS DOS (electrons/eV) (electrons/eV)
a70 a7060
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10 12
50 40
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S Ss p s p
108
40 30 30 20 20 10 100
86 64 42 20 0-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Energy/eV -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 10 Energy/eV Fe 3
b b10 8 3 8 6
30 20 20 10 100
DOS DOS (electrons/eV) (electrons/eV)
b b DOS DOS (electrons/eV) (electrons/eV)
0-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Energy/eV -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 60 Energy/eV Cu 2 60 50 s Cu 2 p 50 s 40 d p 40 d 30
6 4
s Fe p sd p d
4 2 2 0
0-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 0-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Energy/eV Energy/eV -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Energy/eV Figure 3. DOS and Energy/eV PDOS of bornite in DFT calculation (CASTEP module of Materials Studio 6.0
Figure 3. DOS and PDOS of bornite in DFT calculation (CASTEP module of Materials Studio software package, PBE density function, Hubbard + U correction): (a) Total; (b1) Cu; (b2) Fe; (b3) S. 6.0 Figure software package, PBE density function, Hubbard +(CASTEP U correction): Total; (b1Studio ) Cu; (b 3. DOS and PDOS of bornite in DFT calculation module (a) of Materials 6.02 ) Fe; package, PBE density function, Hubbard + U correction): (a) Total; (b1) Cu; (b2) Fe; (b3) S. (b3 )software S. Figure 4 shows the changes of the (111)-S surface during the surface reconstruction.
Figure 4 shows the changes of the (111)-S surface during the surface reconstruction.
Figure 4 shows the changes of the (111)-S surface during the surface reconstruction.
a1
b1
a1
b1
a2
b2
a2
b2
Figure 4. Reconstruction of bornite (111)-S surface in DFT calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, + U correction): (a1) Top and of (a2Materials ) side Figure 4. Reconstruction surfacein inHubbard DFTcalculation calculation (CASTEP module Figure 4. Reconstructionofofbornite bornite(111)-S (111)-S surface DFT (CASTEP module of Materials views ofsoftware optimized (111)-S surface; (b1) Top and (b2) side views + ofU initial (111)-S surface. Studio 6.0 package, PBE density function, Hubbard correction): (a ) Top and Studio 6.0 software package, PBE density function, Hubbard + U correction): (a1) Top side 1 and (a2) (a 2 ) side views of optimized (111)-S and (b (b22) )side sideviews views initial (111)-S surface. views of optimized (111)-Ssurface; surface;(b (b11)) Top and ofof initial (111)-S surface.
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Atthe theF-43m F-43mbornite’s bornite’s(111)-S (111)-S surface, layer consisted 4 S atoms, the second At surface, thethe firstfirst layer consisted of 4 of S atoms, whilewhile the second layer layermade was made ofatoms. 4 Cu atoms. the surface relaxation, large variation occurred the surface was up of 4up Cu After After the surface relaxation, large variation occurred on theon surface S-Cu S-Cu bond. The bond length was reduced from 2.65696 Å to 2.33817 Å . The inter-layer angle of Cubond. The bond length was reduced from 2.65696 Å to 2.33817 Å. The inter-layer angle of Cu-S-Cu ◦ ◦ S-Cu changed from 109.471° to 119.785°. The original two layers were almost rearranged be alayer. new changed from 109.471 to 119.785 . The original two layers were almost rearranged to be atonew layer. Thelength bond length the Sin atom in thelayer thirdand layer and Cu in atom the surface floor extended The bond of the Sofatom the third the Cuthe atom the in surface floor extended from from 2.29674 Å to 2.59200 after optimization. Cu-Swas bond wastoeasier break by caused by 2.29674 Å to 2.59200 Å afterÅoptimization. Such a Such Cu-S abond easier breaktocaused surface surface reactions. The bondoflength the in Fe the atom in the third theinSthe atom in thewas surface was reactions. The bond length the Feof atom third layer andlayer the Sand atom surface reduced reduced from 2.80266 Å to 2.33817 Å after optimization. Such a change of the surface morphology from 2.80266 Å to 2.33817 Å after optimization. Such a change of the surface morphology resulted in resulted in someofvariations of the of inner atoms. TheCu inner core Cu atoms were exposed, some variations the reactivity ofreactivity atoms. The core atoms were exposed, making themmaking easier them to react with The of the Fe-S enhanced the Fe-S bond to reacteasier with chemicals. The chemicals. shortening of theshortening Fe-S bond enhanced thebond Fe-S bond covalence, reducing covalence, reducing the reaction activity of the Fe atoms. the reaction activity of the Fe atoms. Figure55exhibits exhibitsthe the electron density population of first the shell first shell the (111)-S Figure electron density population of the atomsatoms of the of (111)-S surfacesurface before before and after optimization. The bonding of Cu-S revealed that the atoms were and after optimization. The bonding overlap overlap of Cu-S revealed that the inner Cuinner atomsCu were exposed exposed to the surface after relaxation, leading to the increase of electron density on the surface. This to the surface after relaxation, leading to the increase of electron density on the surface. This facilitated facilitated reaction between and (111)-S the bornite (111)-S surface. In other words,surface the (111)-S the reactionthe between reactant andreactant the bornite surface. In other words, the (111)-S lost surface lost electrons and the reactant obtained electrons, making the bornite (111)-S surface oxidized. electrons and the reactant obtained electrons, making the bornite (111)-S surface oxidized.
a
b
Figure5. 5. Electron Electron density density of of bornite bornite (111)-S (111)-S surface surface before before and and after after reconstruction reconstructionin in DFT DFT calculation calculation Figure (CASTEP module module of of Materials Materials Studio Studio 6.0 6.0 software software package, package, PBE PBE density density function, function, Hubbard Hubbard ++ U U (CASTEP correction): (a) (a) The The first firstlayer layer of of optimized optimized (111)-S (111)-S surface; surface;(b) (b)The Thefirst firstlayer layerof ofinitial initial(111)-S (111)-Ssurface. surface. correction):
Figure 66 shows shows the the electron electron density density of of the the Fe-S Fe-S bond bond from from the the core core Fe Fe atoms atoms and and the the surface surface SS Figure atoms at (111)-S surface. It can be seen from Figure 6 that the bonding overlaps of the surface atoms atoms at (111)-S surface. It can be seen from Figure 6 that the bonding overlaps of the surface SS atoms and inner Fe atoms became larger after relaxation, implying that tighter bonding occurred between and inner Fe atoms became larger after relaxation, implying that tighter bonding occurred between the the inner Fe atoms the surface S atoms. Thus, corresponding bonds were relatively hard inner Fe atoms and and the surface S atoms. Thus, the the corresponding Fe-SFe-S bonds were relatively hard to to break, causing that during the initial stages of chemical reactions on the (111)-S surface, and the break, causing that during the initial stages of chemical reactions on the (111)-S surface, and the inner innercan irons can hardly participate. irons hardly participate. Analysisof ofthe thelocal localdensity densityof ofstates states(LDOS) (LDOS)of ofthe thesurface surfaceCu Cuand andSS atoms, atoms, and and inner inner iron iron atoms atoms Analysis is shown in Figure 7. By comparing the total values of LDOS before and after relaxation, we can see is shown in Figure 7. By comparing the total values of LDOS before and after relaxation, we can thatthat thethe DOS of of (111)-S plane suggested an an see DOS (111)-S planedecreased decreasedwithin withinthe the−1 −1toto00eV eV energy energy range. range. This This suggested oxidation process occurred during relaxation. If we look more in detail into the DOS of various oxidation process occurred during relaxation. If we look more in detail into the DOS of various surface surface(Cu, atoms (Cu,S), Fewithin and S),the within the −1 to 0 eV: the ironincreased, atoms increased, atoms Fe and range ofrange −1 toof 0 eV: the DOS ofDOS iron of atoms copper copper atoms atoms decreased slightly, and sulfur atoms diminished substantially. It clearly demonstrated that decreased slightly, and sulfur atoms diminished substantially. It clearly demonstrated that the the Fe Fe atoms obtained negative charges (i.e., electrons) from the atomsand andCu Cuatoms. atoms. In In other other words, words, atoms obtained negative charges (i.e., electrons) from the S Satoms the Cu Cu and and SS atoms atoms were wereoxidized oxidized while while the theFe Featoms atomswere werereduced. reduced. This This corresponds corresponds well well to to the the the aforementionedanalysis analysisof ofelectron electrondensities. densities. aforementioned
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Figure 6. Electron density Fe-S bond of bornite (111)-S surface before and after reconstruction in Figure 6. Electron density of of Fe-S bond of bornite (111)-S surface before and after reconstruction in DFT Figure 6. Electron density of Fe-S bond of bornite (111)-S surface before and after reconstruction in DFT calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, Hubbard DFT calculation (CASTEP module of Materials Studio 6.0 software package, PBE density function, Hubbard + U correction): (a) The Fe-S optimized Fe-S bond; (b) The initial Fe-S bond. + UHubbard correction): (a) The optimized bond;Fe-S (b) The initial Fe-S bond. + U correction): (a) The optimized bond; (b) The initial Fe-S bond.
Total Total
50 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 105 50 0 -5 -5-18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 Energy/eV Energy/eV 30 30 Cu Before optimization 2 Cu Before 225 Before optimization optimization Before optimization 25
DOS (electrons/eV) DOS (electrons/eV)
b b
20 20 15 15 10 10 5 5
10
b10 b118 8 DOS (electrons/eV) DOS (electrons/eV)
Before optimization Before optimization After optimization After optimization
Before optimization Before optimization After optimization After optimization
S S
6 6 4 4 2 2 0 0
-2 -2 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 Energy/eV Energy/eV 3.5 3.5 Before optimization 3 Before Before optimization optimization 33.0 Before optimization 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 -18 -16 -14 -12 -10 -8 Energy -6 -4 (eV) -2 0 Energy (eV)
b b DOS (electrons/eV) DOS (electrons/eV)
DOS (electrons/eV) DOS (electrons/eV)
a 55 a 5550
2 4 6 8 10 2 4 6 8 10
Fe Fe
0 0 2 4 6 8 10 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 2 4 6 8 10 -18 -16 -14 -12 -10 -8 Energy -6 -4 (eV) -2 0 2 4 6 8 10 Energy (eV) Figure 7. Local DOS of bornite before and after optimization (CASTEP module of Materials Studio 6.0 Figure LocalDOS DOSof of bornite bornite before and after (CASTEP module of Materials StudioStudio 6.0 Figure 7. 7.Local andHubbard afteroptimization optimization (CASTEP module software package, PBE densitybefore function, + U correction): (a) Total; (b1) S;of (b2Materials ) Cu; (b3) Fe in package, PBE density function, Hubbard + U+ correction): (a) Total; (b1)(b S; (b 2) Cu; (b3) Fe in 6.0software software package, PBE density function, Hubbard U correction): (a) Total; ) S; (b ) Cu; (b 1 2 3 ) Fe DFT calculation. in DFT DFT calculation. calculation.
3.2. Surface Transformation of Bornite at Different Potentials Electrochemistry analysis was carried out to interpret the electrochemical dissolution processes of bornite. The cyclic voltammograms of bornite in positive scan route and negative scan route are
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3.2. Surface Transformation of Bornite at Different Potentials Electrochemistry analysis was carried out to interpret the electrochemical dissolution processes of bornite. are Minerals 2018,The 8, 173cyclic voltammograms of bornite in positive scan route and negative scan route 10 of 22 presented in Figure 8. Peaks a, b and c were considered as a continuous process composed of the formation and further oxidation of Cu2S [26–29]. Peak c can also represent the oxidation of hydrogen presented in Figuresulfur 8. Peaks a, b and c were considered a continuous process composed of the sulfide to elemental as shown in Equation (4) [27,30].asPeak d was the decomposition of bornite formation and further oxidation of Cu S [26–29]. Peak c can also represent the oxidation of hydrogen 2 as shown in Equation (5) [26]. Peak e was considered to be the reduction of Fe3+ and bornite shown sulfide to elemental sulfur as shown in Equation (4) the [27,30]. Peak dofwas of bornite in Equations (6) and (7) [27,31,32]. Peak f should be reduction Cu2the S todecomposition metal copper [31,33]: as shown in Equation (5) [26]. Peak e was considered to be the reduction of Fe3+ and bornite shown in 2Cu + HS − → + H + + 2eof− Cu S to metal copper [31,33]: (1) Equations (6) and (7) [27,31,32]. Peak f should be Cu the2 Sreduction 2
a30
(2) (1)
− Cu2−x S 2→ CuSCu +2(1 − x)Cu2+2+++ 2(1 Cu S→ 2xe−−x)e −x S + xCu
(3) (2)
+ 2+ + − 2 (1 − x )e− Cu2−x S → H CuS + (01 − x)Cu 2 S → S + 2H + 2e
(4) (3)
H2 S → S0 + 2H+ + 2e− 2Cu5 FeS4 + 6H + → CuS + (4 − x)Cu2+ + 3H2 S + Fe3+ + (5 − x)e− 2Cu5 FeS4 + 6H+ → CuS + (4 − x)Cu2+ + 3H2 S + Fe3+ + (5 − x)e− Fe3+ + e− → Fe2+ Fe3+ + e− → Fe2+
(4) (5)
2+ 2Cu5FeS FeS4++6H 6H++ + + 2e 2e−− → → 5Cu 2Cu 5Cu22SS++3H 3H22SS++2Fe 2Fe2+ 5 4
(5) (6) (6) (7) (7)
+ − Cu 2Cu++HS HS−− Cu2 S +HH + + + 2e 2e− → → 2Cu 2 S+
(8) (8)
b1200
Negative route Positive route
1000
20 15 10 5
800
d a
b
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2
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25
− − 2−x S + xCu2+ + Cu2+ S→ 2xe → Cu2 S + H+ + 2e− 2Cu HSCu
c
600 400
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-5
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-15 -1000-800 -600 -400 -200 0 200 400 600 800 1000 E/mV vs. Ag/AgCl
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0.6
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Figure analysis of bornite in 9K culture medium (Conventional threeFigure 8.8. Electrochemistry Electrochemistry analysis of bornite inbacterial 9K bacterial culture medium (Conventional electrode systemsystem on a Princeton Model 283 Potentiostat, EG&G of Princeton Applied Research): (a) three-electrode on a Princeton Model 283 Potentiostat, EG&G of Princeton Applied Research): Cyclic voltammograms of bornite electrode (Scan (a) Cyclic voltammograms of bornite electrode (Scanrate rate2020mV/s); mV/s);(b) (b)Relationship Relationshipbetween betweenthe the total total charges charges (evaluated (evaluated from from the current-time curves with duration time of 240 s) and applied potentials.
The relationship between the total charge (Figure 8) and applied potential was calculated based The relationship between the total charge (Figure 8) and applied potential was calculated based on potentiostatic polarization testes. It was found that bornite has a high conductivity, especially at on potentiostatic polarization testes. It was found that bornite has a high conductivity, especially relatively high potentials. The oxidation rate was significantly higher than the reduction rate, at relatively high potentials. The oxidation rate was significantly higher than the reduction rate, indicating that bornite tended to be directly oxidized. This was different with chalcopyrite which was indicating that bornite tended to be directly oxidized. This was different with chalcopyrite which was difficult to be oxidized and Ag++ is required to enhance the oxidation rate [34,35]. difficult to be oxidized and Ag is required to enhance the oxidation rate [34,35]. Bornite electrode was treated by different potentials for 3 h to further investigate the Bornite electrode was treated by different potentials for 3 h to further investigate the electrochemical dissolution process of bornite. Figure 9 shows the current-time curves of bornite electrochemical dissolution process of bornite. Figure 9 shows the current-time curves of bornite electrode at different applied potentials for 3 h. The curves sharply declined and then kept steady, electrode at different applied potentials for 3 h. The curves sharply declined and then kept steady, indicating that the oxidation reactions of bornite reached chemical equilibrium. Figure 10 presents indicating that the oxidation reactions of bornite reached chemical equilibrium. Figure 10 presents the open circuit potentials (OCP) of bornite after treating by different potentials. The values of OCP the open circuit potentials (OCP) of bornite after treating by different potentials. The values of OCP significantly increased with the increase of applied potentials, indicating that higher potential was significantly increased with the increase of applied potentials, indicating that higher potential was required to further oxidize the formed oxidation species on bornite surface. This may be mainly attributed to the accumulation of oxidation species on bornite species with the increase of potential.
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required to further oxidize the formed oxidation species on bornite surface. This may be mainly Minerals 2018, 173 11 of 22 required to 8,further oxidize the formed oxidation species on bornite surface. This may be mainly attributed to the accumulation of oxidation species on bornite species with the increase of potential. attributed to the accumulation of oxidation species on bornite species with the increase of potential.
2 Current Currentdensity density(mA/cm (mA/cm)2)
9 9 8 8 7 7 6 6 5 5 4 4 0.55 V 0.65 V 0.65 V 3 0.55 V 3 2 2 0.45 V 1 0.45 V 1 0.3 V 0.3 V 0 0 -1 -1 0 2000 4000 6000 8000 10000 12000 0 2000 4000 6000 Time/s 8000 10000 12000 Time/s Figure 9. Current-time curves of bornite electrode at different applied potentials potentials for for 33 h h in 9K bacterial Figure 9.Current-time Current-timecurves curvesof ofbornite borniteelectrode electrode at atdifferent different applied applied in9K 9Kbacterial bacterial Figure 9. potentials for 3 h in culture medium (Conventional three-electrode system on a Princeton Model 283 Potentiostat, EG&G culture medium medium (Conventional EG&G of culture (Conventional three-electrode three-electrodesystem systemon onaaPrinceton PrincetonModel Model283 283Potentiostat, Potentiostat, EG&G of Princeton Applied Research). Princeton Applied Research). of Princeton Applied Research).
0.65 V 0.65 V
E/mV E/mVvs. vs.Ag/AgCl Ag/AgCl
500 500 480 480 460 460 440 440 420 420 400 400 380 380 360 360 340 340 320 320 300 300 0 0
0.55 V 0.55 V 0.45 V 0.45 V 0.3 V 0.3 V 100 100
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400 400
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600 600
Figure 10. The open circuit potential of bornite electrode after treating by different potentials for 3 h Figure of of bornite electrode after treating by different potentials for 3for h Figure 10. 10. The Theopen opencircuit circuitpotential potential bornite electrode after treating by different potentials in 9K bacterial culture medium (Conventional three-electrode system on a Princeton Model 283 in 9K bacterial culture medium (Conventional three-electrode system on a Princeton Model 283 3 h in 9K bacterial culture medium (Conventional three-electrode system on a Princeton Model Potentiostat, EG&G of Princeton Applied Research). Potentiostat, EG&G of Princeton Applied Research). 283 Potentiostat, EG&G of Princeton Applied Research).
XPS was used to analyze the surface species of bornite after treating by different potentials, XPS was used to analyze the surface species of bornite after treating by different potentials, XPS used to analyze iron-containing, the surface species of bornite afterand treating by different potentials, including was copper-containing, sulfur-containing oxygen-containing species. including copper-containing, iron-containing, sulfur-containing and oxygen-containing species. including copper-containing, sulfur-containing oxygen-containing species. Figure 11 shows the XPS spectrairon-containing, of Cu peaks of bornite surface after and treatment at different potentials, Figure 11 shows the XPS spectra of Cu peaks of bornite surface after treatment at different potentials, Figure 11 shows the XPS spectra of Cu peaks of bornite surface after treatment at different potentials, including Cu 2p peaks and Cu LMM peaks. It was reported that Cu 2p3/2 peak with a shake-up peak including Cu 2p peaks and Cu LMM peaks. It was reported that Cu 2p3/2 peak with a shake-up peak 2+), with including Cu 2p peaks andthe Cumajor LMM XPS peaks. It was reported that Cu 2p3/2(Cu peak a shake-up peak (at about 939–944 eV) was characteristic of cupric species and the Cu 2p3/2 peak (at about 939–944 eV) was the major XPS characteristic of cupric species (Cu2+2+), and the Cu 2p3/2 peak (at aboutshake-up 939–944 eV) was thethe major XPS characteristic of cupric species 2p3/2 peak without peak was characteristics of cuprous species (Cu++(Cu ) [36].),Itand canthe be Cu found that all without shake-up peak was the characteristics of cuprous species (Cu ) [36]. It can be found that all + ) [36]. without shake-up peak was the characteristics of cuprous species (Cu It can be found that all the the Cu 2p3/2 peaks of bornite were centered at 932.2 eV without shake-up peak, which was in the Cu 2p3/2 peaks of bornite were centered at 932.2 eV without shake-up peak, which was in Cu 2p3/2 peaks bornite werevalue centered at 932.2 eV without shake-up peak, which in accordance accordance withofthe reported of bornite [36,37]. Therefore, the elemental Cuwas of bornite should accordance with the reported value of bornite [36,37]. Therefore, the elemental Cu of bornite should with theexist reported of bornite the elemental of bornite should mainly exist mainly in thevalue valence state of[36,37]. +1. The Therefore, Cu 2p3/2 peaks of borniteCu surface after treating by different mainly exist in the valence state of +1. The Cu 2p3/2 peaks of bornite surface after treating by different in the valence state of +1. The Cu 2p peaks of bornite surface after treating by different potentials 3/2 potentials were all centered at 932.2–932.4 eV, which were in accordance with the binding energy of potentials were all centered at 932.2–932.4 eV, which were in accordance with the binding energy of were all centered at 932.2–932.4 which were in increased accordance with binding CuS [38]. CuS [38]. In addition, the values eV, of binding energy with thethe increase of energy appliedofpotentials, CuS [38]. In addition, the values of binding energy increased with the increase of applied potentials, In addition, the values of binding energy increased with the increase of applied potentials, indicating indicating that Cu element of bornite tended to be oxidized due to the increase of applied potentials. indicating that Cu element of bornite tended to be oxidized due to the increase of applied potentials. that Cu element of bornite to be oxidized due potentials to the increase potentials. Cu LMM Cu LMM peaks of bornite tended after treating by different wereofallapplied centered at 568.5–569.0 eV, Cu LMM peaks of bornite after treating by different potentials were all centered at 568.5–569.0 eV, peaks bornitewell after with treating different value potentials were[39]. all centered 568.5–569.0 agreed whichofagreed thebyreported of CuS Hence,atCuS should eV, be which the possible which agreed well with the reported value of CuS [39]. Hence, CuS should be the possible well with thecopper reported value ofspecies CuS [39]. Hence, surface CuS should bethe theoxidation possible intermediate copper intermediate containing on bornite during process. intermediate copper containing species on bornite surface during the oxidation process. containing species on bornite surface during the oxidation process.
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0.3 V 0.3 V 0.65 V Untreated 0.65 V Untreated 940 945 950 955 960 965 970 940 945 energy/eV 950 955 960 965 970 Binding Binding energy/eV
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565 570 575 energy/eV575 565 Binding 570 Binding energy/eV
580 580
Figure 11. XPS XPS spectra of of Cu peaks peaks of of bornite bornite surface surface after after treating treating by by different different potential potential of 0, 0, 0.3, 0.45, 0.45, Figure Figure 11. 11. XPS spectra spectra of Cu Cu peaks of bornite surface after treating by different potential of of 0, 0.3, 0.3, 0.45, 0.55, 0.65 V, respectively: (a) Cu 2p peak; (b) Cu LMM peak (Implemented on the model of ESCALAB 0.55, 0.55, 0.65 0.65 V, V, respectively: respectively: (a) (a) Cu Cu 2p 2p peak; peak; (b) (b) Cu Cu LMM LMM peak peak (Implemented (Implemented on on the the model model of of ESCALAB ESCALAB 250Xi of of Al Al Kα KαX-ray X-raysource sourcewith with20 20eV eVconstant constantpass passenergy energyand and0.1 0.1eV/step; eV/step; Fitted Fitted by by Thermo Thermo 250Xi 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). Avantage Avantage 5.52, 5.52, CC 1s 1s 284.8 284.8eV eVas asreference, reference,Shirley Shirleymethod, method,Gaussian-Lorentzian Gaussian-Lorentzianfunction). function).
The Fe 3p spectra of bornite surface after treatment at different potentials are presented in Figure The Fe 3p bornite surface after treatment at different potentials are presented in Figure The 3pspectra spectraofof bornite surface after treatment at different potentials are presented in 12. Two main peaks can be detected at about 53.0–53.6 eV and 55.5–56.5 eV, respectively. The first 12. Two12. main peaks be detected at about at 53.0–53.6 eV and 55.5–56.5 eV, respectively. The first Figure Two maincan peaks can be detected about 53.0–53.6 eV and 55.5–56.5 eV, respectively. 2+ bonded to sulfur in the species of FeS2 or FeS, peak centered at 53.0–53.6 eV can be assigned to Fe2+ peakfirst centered at 53.0–53.6 eV can be to Fe bonded to sulfurto3+ insulfur the species of FeS2 of orFeS FeS, The peak centered at 53.0–53.6 eVassigned can be assigned to Fe2+ bonded in the species 2 while the second peak centered at 55.5–56.5 eV can be associated with Fe3+ in FeOOH [40]. Hence, Fe-S 3+ while the second peak centered at 55.5–56.5 eV can be associated with Fe in FeOOH [40]. Hence, Fe-S or FeS, while the second peak centered at 55.5–56.5 eV can be associated with Fe in FeOOH [40]. species and FeOOH should be the possible intermediate iron containing species during the oxidation species Fe-S and FeOOH should be theshould possible iron containing species during the oxidation Hence, species and FeOOH be intermediate the possible intermediate iron containing species during process of bornite. Additionally, the percentage of FeOOH in iron containing species increased with process of bornite. Additionally, percentagethe of FeOOH in iron containing species increased with the oxidation process of bornite. the Additionally, percentage of FeOOH in iron containing species the increase of applied potentials from analyzing the peak area. the increase of the applied potentials frompotentials analyzingfrom the peak area. the peak area. increased with increase of applied analyzing
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Figure 12. Cont.
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Figure 12. XPS spectra peaks of bornite surface after treating different potential 0, of 0.3, Figure 12.12. XPS spectra ofFe Fe peaks bornite surface after treating by different potential 0,0.45, 0.3, Figure XPS spectraofof Fe peaks ofof bornite surface after treating byby different potential of 0,of0.3, 0.45, 0.55, 0.65 V, respectively: (a) Fe 2p peak; (b 1–b5): Fe 3p peak (Implemented on the model of ESCALAB 0.55, 0.65 V, respectively: (a) Fe 2p peak; (b 1 –b 5 ): Fe 3p peak (Implemented on the model of ESCALAB 0.45, 0.55, 0.65 V, respectively: (a) Fe 2p peak; (b1 –b5 ): Fe 3p peak (Implemented on the model of 250Xi of of Al250Xi KαKαX-ray with 20 constant pass energy and eV/step; Fitted byFitted Thermo 250Xi Al X-ray source with 20 eV eV constant energy and 0.10.1 eV/step; Fitted by Thermo ESCALAB of Al source Kα X-ray source with 20 eVpass constant pass energy and 0.1 eV/step; by Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function).
The S 2p peakswere werefitted fitted(Figure (Figure 13) 13) and binding energy andand fullfull width at at The S 2p peaks and the theSS2p 2p3/23/2values valuesofof binding energy width Themaximum S 2p peaks were fitted (Figure 13)inand the5.S The 2p3/2types values of binding energyspecies and full width at half (FWHM) are presented Table of sulfur containing can be half maximum (FWHM) are presented Table 5. The types of sulfur containing species can be halfobtained maximum (FWHM) arewith presented in Table 5. The types of sulfur containing species can(Sbe 2−)obtained 2−) and comparing the references references [41–45]. monosulfide obtained bybycomparing with the [41–45]. ItItcan canbebefound foundthat that (Sand 2monosulfide − ) and disulfide by disulfide comparing with the references [41–45]. It can be found that monosulfide (S (S2 2− ) 2− 2− (S 2 ) were the main sulfur containing species on untreated bornite surface. S , S2− 22− and 2− disulfide (S2 ) were the main sulfur containing species on untreated 2bornite surface. S , S 22− and − , S 2− and elemental sulfur 0)containing were the main sulfur species oncontaining untreated borniteon surface. Ssurface elemental sulfur werethe themain main sulfur 2 after treatment with 0) were elemental sulfur (S(S sulfur containingspecies species onbornite bornite surface after treatment with 0 2− 2− 0 (S )different were thepotentials, main sulfur containing surface after treatment with was different potentials, indicating that species S2− and Son 2 bornite can be oxidized to S when potential higher than different potentials, indicating that S and S 22− can be oxidized to S0 when potential was higher than 0.3 V vs.that Ag/AgCl. indicating S2− and S2 2− can be oxidized to S0 when potential was higher than 0.3 V vs. Ag/AgCl.
0.3 V vs. Ag/AgCl.
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60000 60000 40000 40000 20000 20000
S0 S0
S2S20 158 160 162 164 166 168 170 172 174 0 energy/eV 158 160 162 Binding 164 166 168 170 172 174 Binding energy/eV Figure 13. XPS spectra of S 2p peaks of bornite surface after treating by different potential of 0, 0.3, Figure 13.0.65 XPSV, spectra peaks bornite surface treating by different potential 0.3, 0.45, 0.55, respectively: (a) Untreated; (b) 0.3 V;after (c)after 0.45 V; (d) V; (e)potential 0.65V (Implemented Figure 13. XPS spectra ofofSS2p2ppeaks ofof bornite surface treating by0.55 different of 0, of 0.3,0,0.45, 0.45, 0.55, respectively: (a) Untreated; (b) 0.3 V; (c) 0.45 V; V; (e)(Implemented 0.65Venergy (Implemented on the0.65 model ofV,ESCALAB of Al(b) Kα0.3 X-ray source 20 (d) eV constant pass and 0.55, V,0.65 respectively: (a) 250Xi Untreated; V; (c) 0.45 V;with (d) 0.55 V;0.55 (e) 0.65V on 0.1 the on the model of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussianmodel of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, GaussianLorentzian by Thermofunction). Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). Lorentzian function).
The The percentages percentages of of sulfur sulfur containing containing species species on on bornite bornite surface surface after after treatment treatment with with different different The percentages of sulfur containing species on bornite surface after treatment with different potentials were calculated from the peak areas, which are shown in Figure 14. It can be found potentials were calculated from the peak areas, which are shown in Figure 14. It can be found thatthat the potentials were calculated from the peak areas, which are shown in Figure 14. It can be found that 2− 2− 2 − 2 − the total percentage and decreased significantly significantly from from 100% 100% to total percentage of Sof Sand S2S2 decreased to about about 10%, 10%, and and the theproportion proportion the0total percentage of S2− and S022−todecreased significantly from 100% to about 10%, and the 0proportion of of SS0increased increasedobviously obviouslyfrom from 0 toabout about90% 90%when whenthe theapplied appliedpotentials potentialsincreased increasedfrom from 0 to to0.65 0.65VV 0 of SAg/AgCl. increased obviously from 0main to about 90% whensulfur the applied potentials increased from 0 to 0.65 V 0 0was vs. Hence, S the intermediate containing species on the bornite surface vs. Ag/AgCl. Hence, S was the main intermediate sulfur containing species on the bornite surface 0 was the main intermediate sulfur vs. Ag/AgCl. Hence, S containing species on the bornite surface 0 during during the the oxidation oxidation process process and and the the production production of of SS0 can can be be enhanced enhanced with with the the increase increase of of applied applied during the In oxidation process and the production ofinsoluble S0 can besulfate enhanced with the increase of applied 2− 2 − potentials. addition, no significant amount of (SO 4 ) formed on the potentials. In addition, no significant amount of insoluble sulfate (SO4 ) formed on the bornite bornite potentials.0.3–0.65 In addition, no significant amount of insoluble sulfate (SO42−) formed on the bornite surface surface in in 0.3–0.65 V V vs. vs. Ag/AgCl. Ag/AgCl. surface in 0.3–0.65 V vs. Ag/AgCl.
Percentage/% Percentage/%
110 110 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 0 0Untreated 0.3 V 0.45 V 0.55 V 0.65 V Untreated 0.3 V 0.45 V 0.55 V 0.65 V
S00 SS22-2S S2-22S
Figure 14. 14. Distribution Distributionofofsulfur sulfur containing containingspecies species on on bornite bornite surface surface after after treating treating by by different different Figure Figure 14. of sulfur species on bornite surface after treating by different potential 0.65 respectively (Implemented ononthe model ofof ESCALAB 250Xi of of Al potential of Distribution 0, 0.3, 0.45, 0.55, 0.55, 0.65V, V,containing respectively (Implemented the model ESCALAB 250Xi potential of 0, 0.3, 0.45, 0.55, 0.65 V, respectively on Fitted the model of ESCALAB 250Xi of Kα X-ray source with 2020 eVeVconstant pass and by Thermo Thermo Avantage 5.52, Al Kα X-ray source with constant passenergy energy(Implemented and0.1 0.1eV/step; eV/step; by Avantage 5.52, Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function).
The The XPS XPS spectra spectra of of the the O O1s 1speaks peaksof ofthe thebornite bornitesurface surfaceafter aftertreatment treatmentwith withdifferent different potentials potentials The XPS spectra the binding O 1s peaks of the surface treatment with are in Figure binding energy ofbornite O1s 1sat at529.6 529.6±± after 0.1 eV, eV, 531.5± ± 0.1 eVdifferent and 532.8potentials ± 0.1 are shown shown 15.ofThe energy of O 531.5 ± 0.1eV eV 2 − − are shown in Figure 15. The binding energy of O 1s at 529.6 ± 0.1 eV, 531.5 ± 0.1 eV and 532.8 ± 0.1 eV 2− − can can be be mainly attributed to O in in oxide oxide phases, phases, OH OH ininhydroxide hydroxidespecies speciesand and oxygen oxygen in in sulfate sulfate 2− in oxide phases, OH22− −−in hydroxide − can be mainly attributed to O species and oxygen in sulfate − or/and O and or/andwater, water,respectively respectively [36]. [36]. It It can be found that O and OH OH were werethe themain mainoxygen oxygen containing containing − were the main oxygen containing or/and on water, respectively [36].treating It can beat O2− and OH species surface 0.3 vs. and sulfate water formed at at a species on bornite bornite surfaceafter after treating atfound 0.3 VV that vs. Ag/AgCl, Ag/AgCl, and sulfateor/and or/and water formed species on bornite surface after treating at 0.3 V vs. Ag/AgCl, and sulfate or/and water formed at higher potential. The percentage ofofoxygen a higher potential. The percentage oxygencontaining containingspecies specieson onbornite bornitesurface surfaceafter aftertreating treatingby bya higher potential. The percentage of oxygen containing species on bornite surface after treating 2− different potentials calculated from the peak area is shown in Figure 16. The percentages of Oby different potentials calculated from the peak area is shown in Figure 16. The percentages of O2−
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different potentials calculated from the peak area is shown in Figure 16. The percentages15of O2 − Minerals 2018, 8, x FOR PEER REVIEW of 22 − − increased decreased significantly but the theproportion proportionofofOH OH increased decreased significantlywith withthe theincrease increaseof ofapplied applied potential, potential, but − increased decreased significantly with the increase of applied potential, but the proportion of OH with increase appliedpotential. potential.This This was was consistent consistent with that thethe percentage of of with thethe increase ofof applied withthe theconclusion conclusion that percentage within the of applied potential. Thiswith was consistent with conclusion that the percentage ofthe FeOOH inincrease iron containing species increased with the increase applied potentials from analyzing FeOOH iron containing species increased the increase ofofthe applied potentials from analyzing FeOOH in iron containing species increased with the increase of applied potentials from analyzing the Fe 3p peaks. Fe 3p peaks. the Fe 3p peaks.
a a
0.3 V 0.3 V
40000 40000 35000 35000 30000 30000 25000 25000 20000 20000 15000 15000 10000 528 10000 528
OHOH-
25000 25000 20000 20000
OHOH-
15000 15000 530 532 534 536 530 Binding 532 energy/eV 534 536 Binding energy/eV
c c
Counts/s Counts/s
30000 30000
538 538
0.55 V 0.55 V OHOH-
0.45 V 0.45 V
b b
Counts/s Counts/s
Counts/s Counts/s
O2O2-
35000 35000
10000 528 10000 528
35000 35000 30000 30000
530 532 534 530 Binding 532 energy/eV 534 Binding energy/eV
O O2-
15000 15000
538 538
OHOH-
20000 20000
2-
536 536
0.65 V 0.65 V
d d
25000 25000
H2O/SO42H2O/SO42-
H2O/SO42H2O/SO42-
O2O2-
Counts/s Counts/s
24000 24000 22000 22000 20000 20000 18000 18000 16000 16000 14000 14000 12000 528 12000 528
O2O2-
H2O/SO42H2O/SO42-
10000 530 532 534 536 538 528 530 532 534 536 538 10000 Binding energy/eV 530 532 534 536 538 528 530 Binding 532 energy/eV 534 536 538 Binding energy/eV Binding energy/eV Figure XPS spectraofofOO1s1speaks peaksof of bornite bornite surface potential of 0, Figure 15.15. XPS spectra surfaceafter aftertreating treatingbybydifferent different potential of0.3, 0, 0.3, Figure 15. XPS spectra of O 1s peaks of bornite surface after treating by different potential of 0, 0.3, 0.45, 0.55, 0.65 V, respectively: (a) 0.3 V; (b) 0.45 V; (c) 0.55 V; (d) 0.65 V (Implemented on the model of 0.45,0.45, 0.55,0.55, 0.650.65 V, respectively: (a) 0.3 V; (b) 0.45 V; (c) 0.55 V; (d) 0.65 V (Implemented on the model V, respectively: (a) 0.3 V; (b) 0.45 V; (c) 0.55 V; (d) 0.65 V (Implemented on the model of of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by ESCALAB 250Xi of Al KαKα X-ray source and0.1 0.1eV/step; eV/step; Fitted ESCALAB 250Xi of Al X-ray sourcewith with2020eV eVconstant constant pass pass energy energy and Fitted by by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). Thermo Avantage 5.52, C 1s 284.8 eVeV asas reference, Gaussian-Lorentzianfunction). function). Thermo Avantage 5.52, C 1s 284.8 reference,Shirley Shirleymethod, method, Gaussian-Lorentzian
Percentage/% Percentage/%
110 110 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 0 0
OH- OH
H2O/SO42-2H2O/SO42-2H2O/SO4 H2O/SO 24 H2O/SO4 2OH- H O/SO OH 2 4
OH- OH O2-2O
0.3 V 0.3 V
O2-2O
0.45 V 0.45 V
OH- OH O2-2O 0.55 V 0.55 V
O2-2O 0.65 V 0.65 V
Figure Distributionofofoxygen oxygen containing species on bornite surface treating byby different Figure 16.16. Distribution surfaceafter after treating different Figure 16. Distribution of oxygencontaining containingspecies species on on bornite bornite surface after treating by different potential of 0, 0.3, 0.45, 0.55, 0.65 V, respectively (Implemented on the model of ESCALAB 250Xi of of potential of 0,of0.3, 0.45,0.45, 0.55,0.55, 0.650.65 V, respectively (Implemented onon the potential 0, 0.3, V, respectively (Implemented themodel modelofofESCALAB ESCALAB250Xi 250Xi of Al Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, Kα X-ray with 20 eV20constant passpass energy and 0.10.1eV/step; Fittedby byThermo Thermo Avantage 5.52, Al Kαsource X-ray source with eV constant energy and eV/step; Fitted Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function). 1s 284.8 eVreference, as reference, Shirley method,Gaussian-Lorentzian Gaussian-Lorentzian function). function). C 1s C284.8 eV as Shirley method,
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3.3. 3.3. Intermediate Intermediate Species Species of of Bornite Bornite Surface Surface during during Bioleaching Bioleaching 3.3. Intermediate Species of Bornite Surface during Bioleaching Bioleaching Bioleaching of of bornite bornite by by A. A. caldus, caldus, L. L. ferriphilum ferriphilum and and mixed mixed culture culture was was conducted. conducted. The The Bioleaching of bornite by A. caldus, L. ferriphilum and mixed culture was conducted. The potential variation variation of copper extraction and redox potential is presented in Figure 17. The variation of copper extraction and redox potential is presented in Figure 17. The redox redox potential of copper extraction and redox potential is presented Figurewas 17. higher The redox during bornite during bornite bioleaching in presence of culture than that presence of during bornite bioleaching in the the presence of mixed mixed in culture was higher thanpotential that in in the the presence of bioleaching in the presence of mixed culture was higher than that in the presence of single strains of single strains of A. caldus and L. ferriphilum. Accordingly, the copper extraction of bornite bioleaching single strains of A. caldus and L. ferriphilum. Accordingly, the copper extraction of bornite bioleaching A. caldus and L. of ferriphilum. Accordingly, copper extraction of bornite bioleaching in the presence of in was higher than that presence of of in the the presence presence of mixed mixed culture culture was also alsothe higher than that in in the the presence of single single strains strains of A. A. caldus caldus mixed culture was also higher than that in the presence of single strains of A. caldus and L. ferriphilum. and L. ferriphilum. Redox potential during bornite bioleaching was in the range of 0.25–0.65 V vs. and L. ferriphilum. Redox potential during bornite bioleaching was in the range of 0.25–0.65 V vs. Redox potential bioleaching was in the range 0.25–0.65 V potential vs. Ag/AgCl bornite Ag/AgCl where bornite was directly oxidized, and aa high redox was beneficial for Ag/AgCl whereduring bornitebornite was mainly mainly directly oxidized, and of high redox potential waswhere beneficial for was mainly directly oxidized, and a high redox potential was beneficial for accelerating the oxidative accelerating the oxidative dissolution of bornite. This was different with chalcopyrite bioleaching accelerating the oxidative dissolution of bornite. This was different with chalcopyrite bioleaching dissolution ofaccelerated bornite. This was different with bioleaching be accelerated at which at low potential passivated at redox potential which can can be be accelerated at relatively relatively low redox redoxchalcopyrite potential and and passivatedwhich at high highcan redox potential [46– [46– relatively low redox potential and passivated at high redox potential [46–48]. 48]. 48].
650 650 600 600
A. A. caldus caldus L. L. ferriphilum ferriphilum Mixed Mixed culture culture 55
10 10
15 15 20 20 Time/day Time/day
25 25
30 30
Redoxpotential/mV potential/mV Redox
a
Copperextraction/% extraction/% Copper
110 110 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00 00
b
550 550 500 500
A. A. caldus caldus L. L. ferriphilum ferriphilum Mixed Mixed culture culture
450 450 400 400 350 350 300 300 250 2500 0
35 35
55
10 10
15 15 20 20 Time/day Time/day
25 25
30 30
35 35
Figure 17. of bornite by A. caldus, L. ferriphilum and mixed culture consisting of A. caldus 17. Bioleaching caldus Figure 17. Bioleaching of bornite by A. caldus, L. ferriphilum and mixed culture consisting of A. caldus and L. ferriphilum: (a) Copper potential. ferriphilum: (a) Copper extraction; (b) Redox Redox potential. potential. and L. ferriphilum: extraction; (b) (b) Redox
XPS XPS spectra spectra of of Cu Cu 2p 2p peaks peaks and and Cu Cu LMM LMM peaks peaks of of bornite bornite leached leached by by different different microorganisms microorganisms XPS spectra of Cu 2p peaks and Cu LMM peaks of bornite leached by different microorganisms for different numbers of days are shown in Figure 18. for different numbers of days are shown in Figure 18. for different numbers of days are shown in Figure 18.
100000 100000 Intensity/Counts Intensity/Counts
80000 80000
aa
A. A. caldus-7 caldus-7 days days A. A. caldus-21 caldus-21 days days L. L. ferriphilum-7 ferriphilum-7 days days L. L. ferriphilum-21 ferriphilum-21 days days Mixed Mixed culture-7 culture-7 days days Mixed Mixed culture-21 culture-21 days days
60000 60000 40000 40000 20000 20000 925 940 925 930 930 935 935Binding 940 945 945 950 950 955 955 960 960 965 965 970 970 Binding energy/eV energy/eV
40000 40000 35000 35000
bb
30000 30000
Intensity/Counts Intensity/Counts
120000 120000
25000 25000
A. A. caldus-7 caldus-7 days days A. caldus-21 A. caldus-21 days days L. L. ferriphilum-7 ferriphilum-7 days days L. L. ferriphilum-21 ferriphilum-21 days days Mixed Mixed culture-7 culture-7 days days Mixed Mixed culture-21 culture-21 days days
20000 20000 15000 15000 10000 10000 5000 5000560 560
565 575 565 Binding570 570 575 Binding energy/eV energy/eV
580 580
Figure 18. spectra Cu peaks of bornite surface leached by A. caldus, L. ferriphilum and mixed Figure 18. 18. XPS XPS spectra spectra of of Cu Cu peaks peaks of of bornite bornite surface surface leached leached by by A. A. caldus, caldus, L. L. ferriphilum ferriphilum and and mixed mixed Figure XPS of culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the model of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; model of of ESCALAB ESCALAB 250Xi 250Xi of of Al Al Kα Kα X-ray X-ray source source with with 20 20 eV eV constant constant pass pass energy energy and and 0.1 0.1eV/step; eV/step; model Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function): function): (a) (a) Cu Cu 2p 2p peak; peak; (b) (b) Cu Cu LMM LMM peak. peak. function): (a) Cu 2p peak; (b) Cu LMM peak.
Cu peaks of leached bornite samples were all centered at about 932.0–932.5 eV, and the Cu Cu 2p 2p3/2 3/2 peaks of leached bornite samples were all centered at about 932.0–932.5 eV, and the Cu LMM peaks were LMM peaks were all all centered centered at at about about 568.0–568.7 568.0–568.7 eV, eV, indicating indicating that that CuS CuS should should be be the the main main
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Minerals 2018, PEER Cu 2p8, x FOR peaks of REVIEW leached
17 the of 22 bornite samples were all centered at about 932.0–932.5 eV, and Cu LMM peaks were all centered at about 568.0–568.7 eV, indicating that CuS should be the main intermediate bornite bioleaching bioleaching[38,39]. [38,39].The TheSS2p 2ppeaks peakswere werefitted fitted intermediatecopper copper containing containing species species during during bornite (Figure 19) and the S 2p 3/2 values of binding energy and full width at half maximum (FWHM) are (Figure 19) and the S 2p3/2 values of binding energy and full width at half maximum (FWHM) are presented presentedin in Table Table 6. 6. 3/2
15000 13500
12000
a1
10000
12000
S22-
10500
S22-
8000 Counts/s
Counts/s
a2
9000 7500
S
22
S0
6000
S2-
6000
S0
4000
4500 3000 158
160
13000 12000 1 11000 10000 9000 S228000 7000 S26000 5000 4000 3000 158 160
162 164 166 168 Binding energy/eV
170
13000 12000 2 11000 10000 S229000 8000 7000 S26000 5000 4000 3000 158 160
S0
162 164 166 168 Binding energy/eV
Counts/s
Counts/s
8000
SO42-
170
172
7000
c1
6000
S22-
5000
S2-
162 164 166 168 Binding energy/eV
170
172
SO42-
S0
162 164 166 168 Binding energy/eV
170
172
c2
5000 S0
SO42-
4000
Counts/s
Counts/s
7000 6000
160
b
b
9000
2000 158
172
4000 3000
SO42-
3000
2000
2000 158 160 162 164 166 168 170 172 174 Binding energy/eV
1000 158 160 162 164 166 168 170 172 174 Binding energy/eV
Figure surface leached leached by by A. A.caldus, caldus,L.L.ferriphilum ferriphilumand andmixed mixed Figure19. 19.XPS XPSspectra spectra of of SS 2p 2p peaks peaks of of bornite bornite surface culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the model ESCALAB250Xi 250XiofofAlAl X-ray source 20constant eV constant pass energy 0.1 eV/step; model of of ESCALAB KαKα X-ray source withwith 20 eV pass energy and 0.1and eV/step; Fitted Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function): function): (a1)-A. and days; (a2)-A. and (b1)-L. and ferriphilum 7 days; (b2)-L. (a1 )-A. caldus andcaldus 7 days; (a27)-A. caldus and caldus 21 days; (b121 )-L.days; ferriphilum 7 days; and (b2 )-L. ferriphilum ferriphilum and days; culture (c1)-mixed 7 days;culture (c2)-mixed and 21 days. and 21 days; (c121 )-mixed andculture 7 days;and (c2 )-mixed and culture 21 days. Table 6. Binding energy and FWHM value for XPS spectra of S 2p3/2 peaks of bornite leached by A. caldus, L. ferriphilum and mixed culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the model of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function) (FWHM means full width at half maximum, B.E. means binding energy).
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Table 6. Binding energy and FWHM value for XPS spectra of S 2p3/2 peaks of bornite leached by A. caldus, L. ferriphilum and mixed culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented on the model of ESCALAB 250Xi of Al Kα X-ray source with 20 eV constant pass energy and 0.1 eV/step; Fitted by Thermo Avantage 5.52, C 1s 284.8 eV as reference, Shirley method, Gaussian-Lorentzian function) (FWHM means full width at half maximum, B.E. means binding energy). Minerals 2018, 8, x FOR PEER REVIEW
Time Conditions
Time
/days
Conditions
/days
7 7 21
A. caldus
A. caldus
L. ferriphilum
L. ferriphilum
Mixed culture Mixed culture
21 7 21 7 21
7 21 7 21
18 of 22
Peak 1
Peak 2
B.E. Peak 1FWHM (eV) (eV) FWHM
B.E. PeakFWHM 2 (eV) (eV) B.E. FWHM (eV) (eV) 162.3 0.9 162.3 0.9 162.1 0.8 162.1 0.8 162.3 0.9 162.3 0.9 162.0 0.9 162.0 0.9 162.0 0.9 162.0 0.9 ---
B.E. (eV)
161.4
161.4 161.2 161.2 161.4 161.4 161.0 161.0 161.1 161.1 - -
(eV) 0.7 0.7 0.9 0.9 0.7 0.7 0.7 0.7 0.8 0.8 --
Peak 3
Peak 4
B.E. PeakFWHM 3 (eV) (eV) B.E. FWHM
B.E. Peak FWHM 4 (eV) (eV) B.E. FWHM
(eV) 163.8 163.8 163.5
(eV) -168.4 168.4 168.5 168.5 168.6 168.6
163.5
163.7 163.7 163.4 163.4
163.1 --
(eV) 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.0 1.0 --
(eV) -1.5 1.5 1.1 1.1 1.3 1.3
2− 2− and S0 were the SS2−, ,SS2 2 2− and S0 were themain mainintermediate intermediatesulfur sulfurcontaining containingspecies specieson on bornite bornite surface surface during during 2− 2− 0 2− 2− and 04, SO bioleaching by S2, Sand S , SO was2−detected as the main intermediate species bioleaching by A. A.caldus. caldus.Except Exceptfor forS S,2− S was detected as the main intermediate 2 4 on bornite surfacesurface duringduring bioleaching by L.byferriphilum andand byby mixed of species on bornite bioleaching L. ferriphilum mixedculture. culture.The Thedistribution distribution of sulfur-containing species on the bornite surface leached by different microorganisms for different sulfur-containing species on the bornite surface leached by different microorganisms for different days 2−, S20 2− and S0 on bornite surface 2− Sand dayscalculated was calculated as shown in 20. Figure 20. The proportions was as shown in Figure The proportions of S2− , S2of S on bornite surface changed changedduring slightly during bioleaching byand A. the caldus, and theofpercentage of S028%. keptThe around 28%. The slightly bioleaching by A. caldus, percentage S0 kept around percentages of percentages of sulfur containing species on the bornite surface during bioleaching by L. ferriphilum sulfur containing species on the bornite surface during bioleaching by L. ferriphilum changed slightly 2− formed on the bornite surface. Particularly, the distributions of changed about 5% 2− formed and aboutslightly 5% SO4and on SO the4 bornite surface. Particularly, the distributions of sulfur containing sulfur containing species on varied the bornite surfaceduring variedbioleaching significantly bioleaching by 2mixed − was species on the bornite surface significantly byduring mixed culture and SO 4 2− culture and SO4 sulfur was thecontaining predominant sulfur at the Hence, end of the predominant species oncontaining the bornitespecies surfaceon atthe thebornite end of surface bioleaching. bioleaching. Hence, the oxidative dissolution of bornite can be significantly accelerated at high redox the oxidative dissolution of bornite can be significantly accelerated at high redox potential, which was potential, which was one important reason why mixed culture was more effective than single strains one important reason why mixed culture was more effective than single strains of A. caldus and of ferriphilum. A. caldus and L. ferriphilum. L.
SO42S0 S22-
Percentage/%
110 100 90 80 70 60 50 40 30 20 10 0
S2-
A. caldus 7 days
A. caldus L. ferriphilum L. ferriphilum Mixed culture Mixed culture 21 days 21 days 7 days 7 days 21 days
Figure 20. 20.Distribution Distributionof of sulfur containing species on bornite by A. caldus, L. sulfur containing species on bornite surfacesurface leached leached by A. caldus, L. ferriphilum ferriphilum and mixed culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively and mixed culture consisting of A. caldus and L. ferriphilum for 7 and 21 days, respectively (Implemented (Implemented the model of ESCALAB 250Xisource of Al Kα X-ray source withpass 20 eV constant energy on the model ofon ESCALAB 250Xi of Al Kα X-ray with 20 eV constant energy and pass 0.1 eV/step; and 0.1 eV/step;Avantage Fitted by Thermo Avantage 5.52, CShirley 1s 284.8 eV asGaussian-Lorentzian reference, Shirley function). method, Fitted by Thermo 5.52, C 1s 284.8 eV as reference, method, Gaussian-Lorentzian function).
3.4. The Proposed Model for Interpreting the Dissolution Process of Bornite during Bioleaching The proposed model for interpreting the dissolution process of bornite during bioleaching is provided in Figure 21. Bornite is a conductor with high metallic conductivity. The Eredox of redox 3+
2+
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3.4. The Proposed Model for Interpreting the Dissolution Process of Bornite during Bioleaching The proposed model for interpreting the dissolution process of bornite during bioleaching is provided in Figure 21. Bornite is a conductor with high metallic conductivity. The Eredox of redox couple of Fe3+ /Fe2+ was located below the Fermi energy, so bornite can transfer electrons to the Fe3+ /Fe2+ couple and be oxidized. The Eredox of redox couple of O2 /H2 O was always situated below the Fermi energy and in the range of valence band, but the content of dissolved O2 was extremely low at normal pressure, that was why redox potential in bioleaching system was mainly determined by 3+ /Fe2018, 2+ couple Fe [46,49]. Bornite tended to be directly oxidized to CuS, FeOOH and S0 on the surface, Minerals 8, x FOR PEER REVIEW 19 of 22 H2 S and Fe2+ in the solution at potential of higher than 0.3 V vs. Ag/AgCl. The production of S0 and 2+ in the solution at potential of higher than 0.3 V vs. Ag/AgCl. The production of S 0 and H2S andonFebornite FeOOH surface can be significantly accelerated with the increase of redox potential but the FeOOHelemental on bornite surface significantly the H increase of further redox potential formed sulfur didcan not be further oxidize accelerated to insolublewith sulfate. oxidizedbut to 2 S can be 2 − 2+ 3+ the formed elemental sulfur did not further oxidize to insoluble sulfate. H 2 S can be further oxidized SO4 through series of intermediate species, and Fe can be oxidized to Fe by microorganisms. 3+ by microorganisms. 2+ acted as the to SO 42− through series intermediate species, Fe2+ in can be oxidized to Febornite, The redox couple of Fe3+of /Fe main and oxidant directly oxidizing and insoluble 3+ 2+ 3+ 2−, H O The redox couplecan of Fe acted through as the main oxidant inreaction directlyofoxidizing and insoluble sulfate of jarosite be /Fe produced the chemical Fe , SObornite, and M+ in 4 2 +, K + +and sulfate of be produced through thea chemical reaction ofsuch Fe3+, as SOH 42−,O H+2,O and M in solution solution asjarosite shown can in Equation (9), where M is monovalent cation, Na NH4 + : 3 + + + + as shown in Equation (9), where M is a monovalent cation, such as H3O , Na , K and NH4 : + M+ ++ 3Fe3+3++ 2SO4 2− + 6H O → MFe (SO )2 (OH)6 + 6H (9) + (9) M + 3Fe + 2SO4 2− + 6H22 O → MFe33(SO4 )42 (OH) 6 + 6H
Conduction band
e-
H2S
CuS
S0
S0 0 2- H2SO2
2-
S5O6
S2O3
Bornite Cu5FeS4
Gap Fe3+/Fe2+
FeOOH Fe2+
O2/H2O
Bacteria
HS2O4H2SO30
Valence band S2O62H2O SO42-
Fe3+
M+
Figure21. 21.The Theproposed proposedmodel modelfor forinterpreting interpretingthe thedissolution dissolutionprocess processof ofbornite borniteininbioleaching bioleachingby by Figure moderatelythermophilic thermophilicmicroorganisms. microorganisms. moderately
4. Conclusions 4. Conclusions Bornite with formula of (Cu+)5Fe3+(S2−) is a conductor with metallic conductivity whose behavior Bornite with formula of (Cu+ )5 Fe3+ (S2− ) is a conductor with metallic conductivity whose behavior was similar to that of metals. The existence of the electrical conductivity was mainly due to the 3d was similar to that of metals. The existence of the electrical conductivity was mainly due to the 3d orbitals of Fe atoms crossed over the Fermi level and interacting with the high-level orbitals of Cu orbitals of Fe atoms crossed over the Fermi level and interacting with the high-level orbitals of Cu atoms, thus making Fe atoms the most active species in the bornite unit cell. After reconstruction of atoms, thus making Fe atoms the most active species in the bornite unit cell. After reconstruction (001)-S surface, the inner core Cu atoms were exposed, making them easily react with chemicals. The of (001)-S surface, the inner core Cu atoms were exposed, making them easily react with chemicals. shortening of the Fe-S bond enhanced the Fe-S bond covalence, reducing the reaction activity of the The shortening of the Fe-S bond enhanced the Fe-S bond covalence, reducing the reaction activity of the Fe atoms. The inner Cu atoms were exposed to the surface after relaxation, which increased the Fe atoms. The inner Cu atoms were exposed to the surface after relaxation, which increased the electron electron density on the surface and facilitated the reaction between reactant and the bornite (111)-S density on the surface and facilitated the reaction between reactant and the bornite (111)-S surface. surface. Tighter bonding occurred between the inner Fe atoms and the surface S atoms. Thus, the Tighter bonding occurred between the inner Fe atoms and the surface S atoms. Thus, the corresponding corresponding Fe-S bonds were relatively hard to break. Therefore, during the initial stages of Fe-S bonds were relatively hard to break. Therefore, during the initial stages of chemical reactions on chemical reactions on the (111)-S surface, the inner irons can hardly participate. The Cu and S atoms were oxidized while the Fe atoms were reduced in surface reconstruction. Bornite tended to be directly oxidized to CuS, FeOOH and S0 on the surface, H2S and Fe2+ in the solution at potential of higher than 0.3 V vs. Ag/AgCl. The redox couple of Fe3+/Fe2+ acted as the main oxidant in directly oxidizing bornite. H2S can be further oxidized to SO42− through series of intermediate species. The formed elemental sulfur on bornite surface did not further oxidize to insoluble sulfate which can be produced through the chemical reaction of Fe3+, SO42−, H2O and M+ in
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the (111)-S surface, the inner irons can hardly participate. The Cu and S atoms were oxidized while the Fe atoms were reduced in surface reconstruction. Bornite tended to be directly oxidized to CuS, FeOOH and S0 on the surface, H2 S and Fe2+ in the solution at potential of higher than 0.3 V vs. Ag/AgCl. The redox couple of Fe3+ /Fe2+ acted as the main oxidant in directly oxidizing bornite. H2 S can be further oxidized to SO4 2− through series of intermediate species. The formed elemental sulfur on bornite surface did not further oxidize to insoluble sulfate which can be produced through the chemical reaction of Fe3+ , SO4 2− , H2 O and M+ in solution. High redox potential was beneficial for the oxidative dissolution of bornite, which was one important reason why mixed culture was more effective than single strains of A. caldus and L. ferriphilum. This work can help to improve the bioleaching kinetics of bornite and chalcopyrite as well as help to elucidate the biomineralization of Cu-sulfides and the geo-biological-chemical circulation on the Earth. Acknowledgments: This work was supported by the National Natural Science Foundation of China (project No. 51704331, 51774332 and 51374248), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001) and Innovation-Driven Project of Central South University (2018CX019). Author Contributions: Hongbo Zhao conceived and designed the experiments; Xiaotao Huang, Minghao Hu, Yisheng Zhang and Hongbo Zhao performed the experiments; Hongbo Zhao, Minghao Hu and Chenyang Zhang analyzed the data; Jun Wang, Wenqing Qin and Guanzhou Qiu contributed reagents/materials/analysis tools; Hongbo Zhao wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The Funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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