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Combined DFT and XPS investigation of enhanced adsorption of sulfide species onto cerussite by surface modification with chloride
Applied Surface Science 425 (2017) 8–15
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Combined DFT and XPS investigation of enhanced adsorption of sulfide species onto cerussite by surface modification with chloride Qicheng Feng a , Shuming Wen a,∗ , Jiushuai Deng a , Wenjuan Zhao b a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China b Kunming Metallurgical Research Institute, Kunming 650031, China
a r t i c l e
i n f o
Article history: Received 5 May 2017 Received in revised form 8 June 2017 Accepted 4 July 2017 Available online 6 July 2017 Keywords: Cerussite Chloride Sulfide species Sulfidization products DFT XPS
a b s t r a c t This study systematically investigates the enhanced adsorption of sulfide species onto cerussite by surface modification with chloride through density functional theory calculations and X-ray photoelectron spectroscopy (XPS) measurements. Calculation results demonstrate obvious differences in the surface structures and electronic properties of cerussite after HS− adsorption in the absence and presence of chloride species. Surface modification with chloride promotes the stable adsorption of HS− onto the cerussite surface, and the hybridization of Pb 6p orbital at the surface layer and S 3p orbital from HS− is enhanced. Moreover, the reactivity of cerussite surfaces is enhanced by the occurrence of new DOS peaks of Pb 6p and S 3p near the Fermi level and by the transfer of additional electrons between the bonding atoms in the presence of chloride. Meanwhile, Mulliken population and XPS analysis results indicate that a slight oxidation is involved in the interaction between sulfide species and cerussite surfaces because of the presence of disulfide and polysulfide in the sulfidization products. In addition, a higher proportion of disulfide and polysulfide relative to the overall S is exhibited in the presence of chloride, facilitating the sulfidization flotation of cerussite. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Lead oxide minerals are a potential lead resource that can fill the gap caused by the gradual reduction of lead sulfide minerals [1–3]. Among multitudinous beneficiation methods, surface sulfidization flotation is the most promising to concentrate lead oxide minerals in consideration of technical and economic factors [4–6]. The surface hydration and solubility of lead sulfide minerals are particularly weaker than those of lead oxide minerals. As such, the sulfidizing agent sodium sulfide (Na2 S) is usually added into pulp suspensions containing lead oxide minerals to generate lead sulfide species on their surfaces [7–10]. Hydrolysis easily occurs in Na2 S solutions, and the dissociated product varies as a function of solution pH. For instance, H2 S is the dominating S species at pH lower than 7.0, HS− is predominant within the pH range of 7.0–13.9, and S2− is the primary S species at pH higher than 13.9 [11–13]. The optimum flotation recovery of cerussite occurs at mild alkaline pH [8,10,13]. Thus, HS− is a substantial reactant with cerussite sur-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Wen). http://dx.doi.org/10.1016/j.apsusc.2017.07.017 0169-4332/© 2017 Elsevier B.V. All rights reserved.
faces. The sulfidization reaction occurring at the cerussite surface can be depicted as follows: PbCO3(surface) + HS − ↔ PbS(surface) + HCO− 3
(1)
Despite many experimental studies on surface sulfidization flotation, this process remains to have limitations, such as unsatisfactory sulfidization efficiency, inferior sulfidization products, massive consumption of sulfidizing agents and collectors, as well as inadequate flotation recovery of lead. To address these issues, chloride ions were introduced into the pulp solution to modify the surface composition and property of cerussite, which is the most representative lead oxide mineral [10,14,15]. The positive contribution of chloride ions to the sulfidization flotation of cerussite was confirmed through surface analysis, liquid detection and solution chemistry calculation. In addition, the flotation recovery was increased by 10%–16% after cerussite was pretreated with chloride ions before addition of Na2 S [10,14]. Such an improvement was attributed to the increase in the number of active sites and enhancement in the activity of active sites on cerussite surfaces as determined by dissolution experiments, zeta potential measurements, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations [15]. These experimental
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results provide a macroscopic foundation for the positive effect of chloride ions on the sulfidization of cerussite. Recently, computational chemistry has developed into an important research field that complements and consummates experimental studies. It focuses on the atomic arrangement, adsorption, and interaction between flotation reagents and mineral surface. Wen et al. [16] found through DFT calculations that the surface relaxation and reconstruction occurring on the natural chalcopyrite (001) surface could significantly affect its flotation performance. Chen et al. [17] studied the interactions among pyrite, galena surfaces, and xanthate species with and without oxygen by using first-principles calculations and indicated that dixanthogen could be generated on the pyrite (100) surface but not on the galena (100) surface. Sarvaramini et al. [18] simulated the adsorption of collectors on the activated sphalerite surface through DFT. Long et al. [19,20] conducted a DFT study on the attachment of collectors to sphalerite (110) and galena (100) surfaces in the presence of H2 O. The adsorption of flotation reagents on quartz and kaolinite surfaces was also simulated based on DFT [21–23]. Recently, a DFT simulation of the adsorption of hydrogen sulfide ions onto the cerussite (110) surface has been reported, and the sulfidization mechanism has been elucidated at the atomic level [24]. These studies played a significant role in the computational chemistry applied to the flotation field for demonstrating the interaction mechanism of flotation reagents with the mineral surface. In the present work, we systematically investigated the effect of chloride on the adsorption of sulfide species onto the cerussite surface by using DFT and XPS. DFT calculations were performed to explore the adsorption configuration, electronic structures, and charge transfer of interactions between HS− and cerussite (110) surface in the presence of chloride. Meanwhile, XPS measurements were conducted to provide insights into the chemical states of S in sulfide-treated cerussite samples in the presence of chloride. This study provides a comprehensive understanding of the enhanced sulfidization of cerussite induced by chloride species.
2. Computational and experimental methods 2.1. Computational details The calculations in this work were performed using DFT on the basis of the generalized gradient approximation with the Perdew–Burke–Ernzerhof for solids (GGA–PBESOL) functional implemented in the CASTEP 8.0 program [25]. A norm-conserving pseudo potential was employed to describe electron–ion interactions with a kinetic energy cutoff of 340 eV [26]. In energy calculations, the K-point grid was set to 4 × 3 × 1. With respect to self-consistent electronic minimization, the Pulay density mixing method was adopted with an energy tolerance of 1.0 × 10−5 eV/atom, a force tolerance of 3.0 × 10−2 eV/Å, and a displacement tolerance of 1.0 × 10−3 Å. The configuration of valence electrons in this work involved C 2s2 2p2 , O 2s2 2p4 , Pb 5d10 6s2 6p2 , H 1s1 , and S 3s2 3p4 states. The cerussite crystal with cell parameters of a = 5.1807 Å, b = 8.5698 Å, c = 6.1067 Å, and ␣ = ˇ = = 90◦ was utilized to model the primitive unit cell in this work [27]. The adsorption surface was acquired through a cleavage along the (110) plane of an optimized bulk cerussite structure [15,24]. The slab model of the cerussite (110) surface with 20 Å atomic layers was used to simulate the adsorption of flotation reagents on the mineral surface in the absence and presence of chloride. An optimized vacuum space of 20 Å along the z-axis perpendicular to the surface was determined to avoid the spurious interaction between nearby slabs. Adsorption energy is an indicator in estimating the adsorption stability of sulfide species on the cerussite surface. A negative
9
Fig. 1. Adsorption configurations of HS− on the cerussite (110) surface in the presence of chloride.
value of calculated adsorption energy signifies the occurrence of adsorption, and a more negative value implies easier and stronger adsorption of sulfide species on the cerussite surface. HS− was taken as the dominating sulfide species, and PbCl+ was calculated as the primary lead chloride complex in the cerussite sulfidization flotation enhanced by chloride ions [10,14,15]. Thus, the adsorption energies of sulfide species on the mineral surface in the absence and presence of chloride were calculated as follows: Eads−1 = Esurface+HS− − Esurface − EHS−
(2)
Eads−2 = Esurface+PbCl+ +HS− − Esurface − EPbCl+ − EHS−
(3)
where Eads-1 and Eads-2 denote the adsorption energy in the absence and presence of PbCl+ , respectively; Esurface+HS− and Esurface+PbCl+ +HS− denote the energy of the cerussite (110) surface with HS− adsorbed in the absence and presence of PbCl+ , respectively; Esurface denotes the energy of the cerussite (110) surface without HS− adsorbed; and EPbCl+ and EHS− denote the energy of PbCl+ and HS− before interaction with the mineral surface.
2.2. Experimental details Natural cerussite samples were purchased from Yunnan Province, China, and all reagents employed in the present work were of analytical grade and used as received. The samples used in the XPS measurement were obtained by dispersing 5 g of pure cerussite particles with sizes ranging from −74 mm to +45 m into 500 mL of aqueous solution. First, a desired concentration of NaCl stock solution was added into the mineral suspension and then stirred for 10 min as necessary. Then, a fresh Na2 S·9H2 O stock solution was poured and stirred for 30 min. Subsequently, the mixture was filtered, and the produced solid particle was collected, dried, and stored to assure the following XPS measurements. The interaction product was examined using PHI5000 Versa Probe II (PHI5000, ULVAC-PHI, Japan) with an Al K␣ X-ray source. A survey scan of the analyzed sample was first conducted to detect elemental compositions, and then a precise scan was performed to obtain the XPS spectrum of a specific element. Subsequently, the MultiPak Spectrum software was used to calculate and analyze each spectrum and the atomic proportions of the measured samples. The C1s spectrum at 284.8 eV was obtained and used as an internal standard to calibrate the measured spectra for charge compensation.
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Table 1 Changes in bond lengths after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Chloride
Absence Presence Changes Chloride
Absence Presence Changes
Table 2 Changes in bond angles after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Chloride
Bond lengths (Å) C1–O1
C2–O4
C3–O5
C4–O2
Pb1–O4
Pb2–O2
1.31 1.29 −0.02
1.32 1.32 0.00
1.30 1.30 0.00
1.30 1.29 −0.01
2.31 2.38 0.07
2.95 4.77 1.82
O1–Pb1–O4 O3–Pb1–O4 O2–Pb2–O5 O2–Pb2–O6 O5–Pb2–O6 Absence 81.31 Presence 58.67 Changes −22.64 Chloride
Bond lengths (Å) Pb2–O5
Pb3–O1
Pb–O1
2.70 2.91 0.21
2.73 2.64 −0.09
– 2.69 –
3. Results and discussion 3.1. Adsorption configuration of sulfide species on the cerussite (110) surface in the presence of chloride HS− is the dominating sulfide species in the sulfidization flotation of cerussite, and PbCl+ is the primary lead chloride complex when chloride is employed to improve cerussite sulfidization [10,14,15]. Thus, HS− and PbCl+ are presented in this work to represent the corresponding sulfide and chloride species. After various available adsorption configurations in the presence of chloride are compared, the adsorption configuration presented in Fig. 1 is considered to be the most stable. Meanwhile, the adsorption configurations of HS− onto the cerussite (110) surfaces in the absence and presence of chloride are compared to provide a basis for distinguishing the subsequent surface structures and electronic properties. The adsorption configuration in the absence of chloride is shown in Fig. 4 in our previous published literature [24]. The serial numbers of different atoms labeled in Figs. 1 and 4 in our previous published literature [24] are uniform. In other words, the Pb1, Pb2, Pb3, and Pb4 atoms in Figs. 1 and 4 in our previously published literature [24] refer to the same atoms from the cerussite crystal, as is the case with the C and O atoms. The only difference is that an additional Pb atom appears at the surface layer of cerussite in the presence of chloride due to the involvement of PbCl+ on the mineral surface (i.e., Pb represents the Pb atom from PbCl+ ), and it may interact with the S atom from the sulfide species. Normally, the interacting distance between two atoms is close to or shorter than the sum of their atomic radii when a covalent bonding is formed between them. Thus, the strength of interaction between two atoms can be estimated by comparing their interacting distance and the sum of their atomic radii [20]. In other words, no chemical adsorption occurs between the S atom from HS− and the surface Pb atom when the interaction distance between these two atoms is longer than the sum of their atomic radii. As shown in Fig. 1, the distances between the Pb atoms at the surface layer and the S atom from the sulfide species are 2.82 Å for Pb–S, 3.72 Å for Pb1–S, 2.76 Å for Pb2–S, and 5.36 Å for Pb3–S in the presence of chloride, whereas the distances between the Pb atoms and the S atom are 3.65 Å for Pb1–S, 2.61 Å for Pb2–S, and 4.46 Å for Pb3–S in the absence of chloride as shown in Fig. 4 in our previous published literature [24]. The sum of Pb and S atomic radii is 2.79 Å, which is close to the Pb–S and Pb2–S distances in the presence of chloride, and is slightly longer than the Pb2–S distance in the absence of chloride. This result suggests that the S atom can chemically adsorb onto the Pb and Pb2 atoms on the cerussite surface in the presence of chloride and onto the Pb2 atom on the cerussite surface in the absence of chloride. In other words, the sulfide species can chemically adsorb onto the cerussite surface more strongly through the interaction of more electrons in the presence of chloride than in the
Bond angles (◦ )
Absence Presence Changes
43.35 54.63 11.28
84.74 47.79 −36.95
72.60 44.92 −27.68
42.91 32.59 −10.32
Bond angles (◦ ) O1–Pb3–O5
O1–Pb–O4
O3–Pb–O4
102.66 103.29 0.63
– 54.54 –
– 30.16 –
absence of chloride. This result agrees with previous experimental results [10,14]. Meanwhile, an additional Pb atom, which is derived from lead chloride complexes, demonstrates that more available active sites are provided for the adsorption of sulfide species in the presence of chloride. Our previous study confirmed that the attachment of PbCl+ to cerussite surfaces can increase the number of active sites and enhance the activity of active sites on the mineral surface [15]. The adsorption energies are used in the present work to reveal the adsorption stability of HS− on the mineral surface, and the calculated values in the absence and presence of chloride are −500.83 and −1389.74 kJ/mol, respectively. This result further indicates that the adsorption of HS− on the cerussite (110) surface in the absence and presence of chloride can occur spontaneously (i.e., HS− can chemically adsorb onto the cerussite surface), and the adsorption of HS− is more stable in the presence than in the absence of chloride due to a more negative value of the adsorption energy [28,29]. 3.2. Effect of chloride on the surface structures and electronic properties of cerussite The surface structures and electronic properties of cerussite significantly influence the interaction between HS− and cerussite surfaces. Thus, changes in the bond lengths and bond angles as well as in electronic properties after HS− adsorption onto the cerussite (110) surface in the absence and presence of chloride were investigated. Table 1 shows almost no difference in bond lengths between the C and O atoms after HS− adsorption regardless of the presence of chloride. This result indicates that the interaction between HS− and the mineral surface in the absence and presence of chloride does not significantly influence the carbonate group of cerussite. In other words, the C and O atoms are not involved in the sulfidization reaction, and HS− prefers Pb sites on the cerussite surface for adsorption. As shown in Table 1, the bond lengths between the Pb and O atoms are 2.31 Å for Pb1–O4, 2.95 Å for Pb2–O2, 2.70 Å for Pb2–O5, and 2.73 Å for Pb3–O1 in the absence of chloride, and are 2.38 Å for Pb1–O4, 4.77 Å for Pb2–O2, 2.91 Å for Pb2–O5, and 2.64 Å for Pb3–O1 in the presence of chloride. The presence of chloride changes the bond lengths between the Pb and O atoms on the mineral surface, especially for Pb2–O2 and Pb2–O5. Furthermore, the Pb atom from PbCl+ bonds to the O atom on the cerussite surface, indicating that the additional Pb atom can tightly attach to the mineral surface and serve as the active site. The presence of chloride considerably changes the bond angles of different atoms at the surface layers (Table 2). The bond angles of O1–Pb1–O4, O2–Pb2–O5, O2–Pb2–O6, and O5–Pb2–O6 decrease by 22.64◦ , 36.95◦ , 27.68◦ , and 10.32◦ , respectively, whereas those of O3–Pb1–O4 and O1–Pb3–O5 increase by 11.28◦ and 0.63◦ , respectively. These phenomena disclose that the presence of chloride
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Fig. 2. DOS of atoms after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride.
changes the surface structures of cerussite after HS− adsorption on the cerussite (110) surface. The electronic properties of the cerussite (110) surface after HS− adsorption in the absence and presence of chloride are compared to determine the positive contribution of chloride to the reactivity of surface atoms. Fig. 2 demonstrates the density of state (DOS) of the C, O, and Pb atoms at the surface layers for the cerussite (110) surface after HS− adsorption in the absence and presence of chloride. The EF value, the position of Fermi level, is set at 0 eV. As shown in the DOS curves, the 2p and 2 s orbitals of the C and O atoms are distant from the Fermi level, indicating that they contribute insignificantly to the sulfidization reaction of cerussite, and the presence of chloride slightly affects their reactivity. The DOS of the Pb 6p orbital near the Fermi level is very intense, suggesting the high reactivity of the Pb atoms on the mineral surface. Accordingly, the main contribution of the reactivity of the atoms at the surface layers for the cerussite (110) surface is attributed to Pb atoms when HS− is involved. The projected DOS (PDOS) of the surface Pb atom remarkable changes near the Fermi level in the absence and presence of chloride. In the Pb 6p orbital, a new DOS peak occurs near the Fermi level in the presence of chloride, indicating a stronger interaction between the surface Pb atom and the S atom from sulfide species. The involvement of chloride species benefits the reactivity of cerussite surfaces, and Pb atoms are the main adsorption sites for HS− . To determine the contribution of Pb atoms on the mineral surface to the reactivity, the electronic properties of different Pb atoms numbered in Fig. 1 are investigated. Fig. 3 provides the PDOS curves of the Pb atoms on the cerussite (110) surface after HS− adsorption in the presence of chloride. The DOSs of the various Pb atoms at the surface layers show evident differences, including the difference occurring near the Fermi level. The DOS of atoms occurring near the Fermi level significantly facilitates the reactivity of the interacting elements. The results shown in Fig. 3 indicate that the Pb3 and Pb4 atoms contribute insignificantly to the DOS of the Fermi level, whereas the Pb, Pb1, and Pb2 atoms contribute positively to the reactivity of cerussite surfaces. The Pb atom from chloride species exhibits the most prominent contribution.
Fig. 3. PDOS of Pb atoms on the cerussite (110) surface after HS− adsorption in the presence of chloride.
3.3. DOS of interactions between HS− and the cerussite (110) surface in the absence and presence of chloride The presence of chloride changes the reactivity of atoms at the (110) surface layers after HS− adsorption, which facilitates sulfidization. To probe the microscopic sulfidization mechanism of cerussite, the DOS of the interaction between Pb, Pb1, Pb2, and S atoms in the presence of chloride is investigated (Fig. 4). The Pb–S DOS curve shows an obvious overlap between the Pb 6p and S 3p
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Fig. 6. DOS of Pb atoms at the surface layers after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride. Fig. 4. DOS of interactions between S atom from HS− and Pb atoms on the cerussite (110) surface in the presence of chloride.
Fig. 7. PDOS of S atom after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride. Fig. 5. DOS of interactions between S atom from HS− and Pb atoms at the surface layers in the absence (a) and presence (b) of chloride.
states near the Fermi level. The relevant hybridization peaks of Pb 6p and S 3p appear at −2.8, −1.5, −0.8, and 0 eV, suggesting a strong interaction between Pb and S atoms. The DOSs of Pb1–S and Pb2–S are similar to that of Pb–S, but the overlapped DOSs of their Pb 6p and S 3p orbitals are smaller than that of Pb–S. This result discloses that a stronger interaction occurs between the Pb atom from chloride species and S atoms than between other Pb atoms and S atoms. In other words, the presence of chloride species strengthens the interaction between HS− and the cerussite surface. Hence, the DOS curves of the interactions between the S atom from HS− and the Pb atoms at the surface layers in the absence and presence of chloride are given in Fig. 5 to further understand the contribution of chloride to the sulfidization reaction of cerussite. Fig. 5 shows an obvious difference in the DOS of interactions between the Pb and S atoms in the absence and presence of chlo-
ride. The DOSs of the Pb 6p and S 3p orbitals remarkably change in the presence of chloride, implying a difference in electron transfer between the Pb and S atoms. Meanwhile, the overlapping range of the Pb 6p and S 3p states near the Fermi level increases, indicating that the presence of chloride strengthens the bonding interaction between the surface Pb atom and the S atom of sulfide species. In addition, the number of hybridization peaks of Pb 6p and S 3p near the Fermi level increases, suggesting that the presence of chloride enhances the reactivity of HS− with the cerussite surface. These results confirm that HS− easily interacts with the Pb sites on the mineral surface in the presence of chloride and that the formed lead sulfide species are more active. The DOSs of the Pb and S atoms after HS− adsorption on the cerussite (110) surface are compared in the absence and presence of chloride (Figs. 6 and 7). After HS− adsorption, an obvious difference in the DOS of surface Pb atom is observed in Fig. 6 when the cerussite surface is in the absence and presence of chloride species.
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Table 3 Mulliken charge populations of atoms after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Atoms
Chloride
s
p
d
Total
Charge (e)
Pb1
Absence Presence Absence Presence Absence Presence Absence Presence
1.98 1.89 1.92 1.98 1.90 1.88 1.88 1.89
1.70 1.47 1.82 1.58 1.21 1.04 4.63 4.51
10.01 10.01 10.01 10.00 10.00 10.00 0.00 0.00
13.69 13.37 13.75 13.56 13.12 12.92 6.51 6.40
0.31 0.63 0.25 0.44 0.88 1.08 −0.51 −0.40
Pb2 Pb3 S
The DOS of the Pb 6p orbital increases generally, demonstrating that chloride improves the ability of the surface Pb atom to capture electrons, thereby strengthening its interaction with HS− . In addition, the presence of chloride widens the distribution of the Pb 6p and Pb 6 s orbitals and increases the number of their DOS peaks near the Fermi level. In other words, several new DOS peaks of Pb 6p and Pb 6 s occur near the Fermi level. These results prove that the presence of chloride on the cerussite surface increases the binding capacity to sulfide species. As is shown in Fig. 7, the DOSs of the S 3p and S 3s orbitals on the cerussite surface after HS− adsorption in the absence of chloride are different from those in the presence of chloride. After interacting with HS− , the DOSs of the S 3p and S 3s orbitals slightly decrease and shift to the low energy direction, implying the loss of more electrons for S atoms during electron transfer to Pb atoms in the presence of chloride. Compared with the absence of chloride, the presence of chloride widens the distribution range of the S 3p and S 3 s orbitals and increases the number of their DOS peaks near the Fermi level. In particular, several new DOS peaks of S 3p and S 3s appear near the Fermi level, which coincide well with the newly generated DOS peaks of Pb 6p and Pb 6s, hybridizing each other and forming an additional stable lead sulfide species on the mineral surface (Fig. 5). The flotation performance of minerals significantly depends on their surface properties, and their surface properties are significantly influenced by their electronic properties. Accordingly, the detailed discussion on the electronic properties of interactions between HS− and the cerussite (110) surface further verifies the positive effect of chloride species on the sulfidization of cerussite at the atomic level, which agrees with the experimental results [10,14,15].
Fig. 8. S2p XPS spectra of cerussite sulfidized with: (a) 5 × 10−5 mol/L Na2 S·9H2 O, (b) 5 × 10−3 mol/L NaCl and 5 × 10−5 mol/L Na2 S·9H2 O, (c) 5 × 10−4 mol/L Na2 S·9H2 O, and (d) 5 × 10−3 mol/L NaCl and 5 × 10−4 mol/L Na2 S·9H2 O.
After HS− interacts with the mineral surface, the charge values of the surface S atom show a difference in the absence and presence of chloride. The 3p orbital of the S atom has 4.63 electrons in the absence of chloride and 4.51 electrons in the presence of chloride, i.e., the presence of chloride diminishes the electrons of the S 3p orbital. The decrease in negative charge of the surface S atom in the presence of chloride may be related to the oxidation of HS− when it interacts with cerussite surfaces. The oxidation of sulfide species is involved in the sulfidization of cerussite, and the higher proportion of disulfide and polysulfide relative to the overall S positively contributes to the activity of sulfidization products [13,24]. Thus, the formed sulfidization products in the presence of chloride may include higher contents of lead disulfide and lead polysulfide. 3.5. XPS studies
3.4. Charge transfer of interactions between HS− and the cerussite (110) surface in the absence and presence of chloride The Mulliken population analysis is employed to analyze the related transfers of charge and evaluate the bonding nature of the Pb atoms at the surface layer and the S atom from HS− . The charge values of the Pb1, Pb2, Pb3, and S atoms on the cerussite (110) surface after HS− adsorption in the absence and presence of chloride are listed in Table 3. The table shows that the absence and presence of chloride cause an obvious difference in charge transfer between the bonding atoms. Compared with the absence of chloride, the presence of chloride allows the Pb atoms to hold a more positive charge after HS− adsorption on the cerussite surface. In specific, the charges of the Pb1, Pb2, and Pb3 atoms increase from 0.31 e to 0.63 e, 0.25 e to 0.44 e, and 0.88 e to 1.08 e, respectively. The lost electrons for the Pb atoms are primarily derived from the Pb 6p orbital, and the presence of chloride renders more electrons lost in this orbital. These results demonstrate that the interaction between HS− and the cerussite (110) surface is stronger in the presence than in the absence of chloride, and the Pb sites positioned in the formed lead sulfide species easily interact with the subsequent collector, thereby improving the flotation behavior of cerussite.
The S2p XPS spectra were fitted to determine the effect of chloride on the component of sulfidization products. Figs. 8a and b presents the S2p XPS spectra of cerussite sulfidized with Na2 S·9H2 O at a concentration of 5 × 10−5 mol/L in the absence and presence of chloride. The result shows that the entire asymmetric S2p peak can be fitted into three symmetric peaks regardless of the presence of chloride. The first peak at a binding energy of 160.73 eV in Fig. 8a is assigned to the S2− state, i.e., the S in lead monosulfide films formed on the cerussite surface [13,30]. The second peak is 1.40 eV higher in the binding energy than the first one, which corresponds to the S2 2− state, i.e., the S in disulfide. The third peak at a binding energy of 163.43 eV is attributed to the Sn 2− state, i.e., the S in polysulfide, and its peak is 2.70 eV higher in the binding energy than the first one. Accordingly, the sulfidization product of cerussite surfaces is composed of PbS, PbS2 , and PbSn . The higher binding energy of S2 2− and Sn 2− may be ascribed to the slight oxidation of HS− when interacting with the Pb sites on the mineral surface [13,24]. As shown in Fig. 8b, a slight shift of binding energy occurs in the three fitted peaks compared with the data in the absence of chloride, but their peak areas exhibit a remarkable difference. The peak area can be converted into the species distribution of different chem-
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Table 4 S2p quantification of cerussite sulfidized with: (a) 5 × 10−5 mol/L Na2 S·9H2 O, (b) 5 × 10−3 mol/L NaCl and 5 × 10−5 mol/L Na2 S·9H2 O, (c) 5 × 10−4 mol/L Na2 S·9H2 O, and (d) 5 × 10−3 mol/L NaCl and 5 × 10−4 mol/L Na2 S·9H2 O. Samples
Species
S2p3/2 binding energy (eV)
Species distribution (%)
a
S2− S2 2− Sn 2− S2− S2 2− Sn 2− S2− S2 2− Sn 2− S2− S2 2− Sn 2−
160.73 162.13 163.43 160.70 162.15 163.45 160.74 162.07 163.49 160.36 161.78 163.09
63.06 24.74 12.20 13.91 59.27 26.82 39.03 41.30 19.67 18.66 38.53 42.81
b
c
d
3p orbitals is stronger in the presence than in the absence of chloride. (2) An obvious difference in surface structures and electronic properties of cerussite is observed after HS− adsorption in the absence and presence of chloride. Several new DOS peaks of Pb 6p and S 3p appear near the Fermi level, and the number of electrons transferred between the bonding atoms increases, indicating that chloride enhances the reactivity of cerussite surfaces. (3) A slight oxidation occurs in the interaction between sulfide species and the cerussite surface, and disulfide and polysulfide are the primary oxidation products. The presence of chloride increases the proportion of disulfide and polysulfide relative to the overall S, which is advantageous to the sulfidization flotation of cerussite. Acknowledgement
ical states. Table 4 lists the compositions of S species on cerussite surfaces at different Na2 S concentrations in the absence and presence of chloride. The fractions of S2− , S2 2− , and Sn 2− relative to the overall S are 63.06%, 24.74%, and 12.20% in the absence of chloride, respectively, and are 13.91%, 59.27%, and 26.82% in the presence of chloride, respectively. This result indicates that the presence of chloride increases the percentages of S2 2− and Sn 2− . In general, the S S bond in S2 2− and Sn 2− can be considered a short-chain oligomer, and the S atoms are equivalent in S2 2− but non-equivalent in Sn 2− [13,31]. Given the chain structure of Sn 2− , the S atoms located at both ends and in the middle exhibit a different charge distribution along the chain, namely, the former and the latter possess formal charges of −1 and 0, respectively [13,30,31]. In addition, the presence of S2 2− and Sn 2− (especially Sn 2− ) on the mineral surface is beneficial to the hydrophobicity of minerals [30,32,33]. Accordingly, greater proportions of S2 2− and Sn 2− in the sulfidization products mean better floatability of cerussite particles. Combining the data in Fig. 8 and Table 4 as well as the previous discussion, it is estimated that the presence of chloride results in the formation of more S2 2− and Sn 2− species on the mineral surface after HS− adsorption, thereby improving the reactivity of the sulfidization products. For further understanding of the identity of surface species, the S2p XPS spectra of cerussite sulfidized with a higher Na2 S concentration in the absence and presence of chloride are demonstrated in Figs. 8c and d. The result in the presence of chloride at a higher concentration of Na2 S verifies the reliability of the above statement, and all fitted S2p peaks shift to a lower binding energy due to the change in chemical environment. Meanwhile, the percentage of Sn 2− in the sulfidization product in the presence of chloride increases by 23.14% compared with that in the absence of chloride. These results conformably reveal that the presence of chloride is advantageous to the sulfidization of cerussite, leading to the enhanced hydrophobicity on the mineral surface. 4. Conclusions The atomic interaction mechanism of sulfide species with the cerussite surface in the presence of chloride was studied by DFT simulations, and the chemical environment of the sulfidization products was characterized by XPS measurement. On the basis of the aforementioned results, the primary conclusions are summarized as follows: (1) Sulfide species may adsorb on the cerussite surface, and the adsorption of HS− in the presence of chloride is more stable than that in the absence of chloride. The hybridization of the Pb 6p and S 3p orbitals is the main interaction between HS− and the cerussite surface, and the hybridization of the Pb 6p and S
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Combined DFT and XPS investigation of enhanced adsorption of sulfide species onto cerussite by surface modification with chloride Qicheng Feng a , Shuming Wen a,∗ , Jiushuai Deng a , Wenjuan Zhao b a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China b Kunming Metallurgical Research Institute, Kunming 650031, China
a r t i c l e
i n f o
Article history: Received 5 May 2017 Received in revised form 8 June 2017 Accepted 4 July 2017 Available online 6 July 2017 Keywords: Cerussite Chloride Sulfide species Sulfidization products DFT XPS
a b s t r a c t This study systematically investigates the enhanced adsorption of sulfide species onto cerussite by surface modification with chloride through density functional theory calculations and X-ray photoelectron spectroscopy (XPS) measurements. Calculation results demonstrate obvious differences in the surface structures and electronic properties of cerussite after HS− adsorption in the absence and presence of chloride species. Surface modification with chloride promotes the stable adsorption of HS− onto the cerussite surface, and the hybridization of Pb 6p orbital at the surface layer and S 3p orbital from HS− is enhanced. Moreover, the reactivity of cerussite surfaces is enhanced by the occurrence of new DOS peaks of Pb 6p and S 3p near the Fermi level and by the transfer of additional electrons between the bonding atoms in the presence of chloride. Meanwhile, Mulliken population and XPS analysis results indicate that a slight oxidation is involved in the interaction between sulfide species and cerussite surfaces because of the presence of disulfide and polysulfide in the sulfidization products. In addition, a higher proportion of disulfide and polysulfide relative to the overall S is exhibited in the presence of chloride, facilitating the sulfidization flotation of cerussite. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Lead oxide minerals are a potential lead resource that can fill the gap caused by the gradual reduction of lead sulfide minerals [1–3]. Among multitudinous beneficiation methods, surface sulfidization flotation is the most promising to concentrate lead oxide minerals in consideration of technical and economic factors [4–6]. The surface hydration and solubility of lead sulfide minerals are particularly weaker than those of lead oxide minerals. As such, the sulfidizing agent sodium sulfide (Na2 S) is usually added into pulp suspensions containing lead oxide minerals to generate lead sulfide species on their surfaces [7–10]. Hydrolysis easily occurs in Na2 S solutions, and the dissociated product varies as a function of solution pH. For instance, H2 S is the dominating S species at pH lower than 7.0, HS− is predominant within the pH range of 7.0–13.9, and S2− is the primary S species at pH higher than 13.9 [11–13]. The optimum flotation recovery of cerussite occurs at mild alkaline pH [8,10,13]. Thus, HS− is a substantial reactant with cerussite sur-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Wen). http://dx.doi.org/10.1016/j.apsusc.2017.07.017 0169-4332/© 2017 Elsevier B.V. All rights reserved.
faces. The sulfidization reaction occurring at the cerussite surface can be depicted as follows: PbCO3(surface) + HS − ↔ PbS(surface) + HCO− 3
(1)
Despite many experimental studies on surface sulfidization flotation, this process remains to have limitations, such as unsatisfactory sulfidization efficiency, inferior sulfidization products, massive consumption of sulfidizing agents and collectors, as well as inadequate flotation recovery of lead. To address these issues, chloride ions were introduced into the pulp solution to modify the surface composition and property of cerussite, which is the most representative lead oxide mineral [10,14,15]. The positive contribution of chloride ions to the sulfidization flotation of cerussite was confirmed through surface analysis, liquid detection and solution chemistry calculation. In addition, the flotation recovery was increased by 10%–16% after cerussite was pretreated with chloride ions before addition of Na2 S [10,14]. Such an improvement was attributed to the increase in the number of active sites and enhancement in the activity of active sites on cerussite surfaces as determined by dissolution experiments, zeta potential measurements, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations [15]. These experimental
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results provide a macroscopic foundation for the positive effect of chloride ions on the sulfidization of cerussite. Recently, computational chemistry has developed into an important research field that complements and consummates experimental studies. It focuses on the atomic arrangement, adsorption, and interaction between flotation reagents and mineral surface. Wen et al. [16] found through DFT calculations that the surface relaxation and reconstruction occurring on the natural chalcopyrite (001) surface could significantly affect its flotation performance. Chen et al. [17] studied the interactions among pyrite, galena surfaces, and xanthate species with and without oxygen by using first-principles calculations and indicated that dixanthogen could be generated on the pyrite (100) surface but not on the galena (100) surface. Sarvaramini et al. [18] simulated the adsorption of collectors on the activated sphalerite surface through DFT. Long et al. [19,20] conducted a DFT study on the attachment of collectors to sphalerite (110) and galena (100) surfaces in the presence of H2 O. The adsorption of flotation reagents on quartz and kaolinite surfaces was also simulated based on DFT [21–23]. Recently, a DFT simulation of the adsorption of hydrogen sulfide ions onto the cerussite (110) surface has been reported, and the sulfidization mechanism has been elucidated at the atomic level [24]. These studies played a significant role in the computational chemistry applied to the flotation field for demonstrating the interaction mechanism of flotation reagents with the mineral surface. In the present work, we systematically investigated the effect of chloride on the adsorption of sulfide species onto the cerussite surface by using DFT and XPS. DFT calculations were performed to explore the adsorption configuration, electronic structures, and charge transfer of interactions between HS− and cerussite (110) surface in the presence of chloride. Meanwhile, XPS measurements were conducted to provide insights into the chemical states of S in sulfide-treated cerussite samples in the presence of chloride. This study provides a comprehensive understanding of the enhanced sulfidization of cerussite induced by chloride species.
2. Computational and experimental methods 2.1. Computational details The calculations in this work were performed using DFT on the basis of the generalized gradient approximation with the Perdew–Burke–Ernzerhof for solids (GGA–PBESOL) functional implemented in the CASTEP 8.0 program [25]. A norm-conserving pseudo potential was employed to describe electron–ion interactions with a kinetic energy cutoff of 340 eV [26]. In energy calculations, the K-point grid was set to 4 × 3 × 1. With respect to self-consistent electronic minimization, the Pulay density mixing method was adopted with an energy tolerance of 1.0 × 10−5 eV/atom, a force tolerance of 3.0 × 10−2 eV/Å, and a displacement tolerance of 1.0 × 10−3 Å. The configuration of valence electrons in this work involved C 2s2 2p2 , O 2s2 2p4 , Pb 5d10 6s2 6p2 , H 1s1 , and S 3s2 3p4 states. The cerussite crystal with cell parameters of a = 5.1807 Å, b = 8.5698 Å, c = 6.1067 Å, and ␣ = ˇ = = 90◦ was utilized to model the primitive unit cell in this work [27]. The adsorption surface was acquired through a cleavage along the (110) plane of an optimized bulk cerussite structure [15,24]. The slab model of the cerussite (110) surface with 20 Å atomic layers was used to simulate the adsorption of flotation reagents on the mineral surface in the absence and presence of chloride. An optimized vacuum space of 20 Å along the z-axis perpendicular to the surface was determined to avoid the spurious interaction between nearby slabs. Adsorption energy is an indicator in estimating the adsorption stability of sulfide species on the cerussite surface. A negative
9
Fig. 1. Adsorption configurations of HS− on the cerussite (110) surface in the presence of chloride.
value of calculated adsorption energy signifies the occurrence of adsorption, and a more negative value implies easier and stronger adsorption of sulfide species on the cerussite surface. HS− was taken as the dominating sulfide species, and PbCl+ was calculated as the primary lead chloride complex in the cerussite sulfidization flotation enhanced by chloride ions [10,14,15]. Thus, the adsorption energies of sulfide species on the mineral surface in the absence and presence of chloride were calculated as follows: Eads−1 = Esurface+HS− − Esurface − EHS−
(2)
Eads−2 = Esurface+PbCl+ +HS− − Esurface − EPbCl+ − EHS−
(3)
where Eads-1 and Eads-2 denote the adsorption energy in the absence and presence of PbCl+ , respectively; Esurface+HS− and Esurface+PbCl+ +HS− denote the energy of the cerussite (110) surface with HS− adsorbed in the absence and presence of PbCl+ , respectively; Esurface denotes the energy of the cerussite (110) surface without HS− adsorbed; and EPbCl+ and EHS− denote the energy of PbCl+ and HS− before interaction with the mineral surface.
2.2. Experimental details Natural cerussite samples were purchased from Yunnan Province, China, and all reagents employed in the present work were of analytical grade and used as received. The samples used in the XPS measurement were obtained by dispersing 5 g of pure cerussite particles with sizes ranging from −74 mm to +45 m into 500 mL of aqueous solution. First, a desired concentration of NaCl stock solution was added into the mineral suspension and then stirred for 10 min as necessary. Then, a fresh Na2 S·9H2 O stock solution was poured and stirred for 30 min. Subsequently, the mixture was filtered, and the produced solid particle was collected, dried, and stored to assure the following XPS measurements. The interaction product was examined using PHI5000 Versa Probe II (PHI5000, ULVAC-PHI, Japan) with an Al K␣ X-ray source. A survey scan of the analyzed sample was first conducted to detect elemental compositions, and then a precise scan was performed to obtain the XPS spectrum of a specific element. Subsequently, the MultiPak Spectrum software was used to calculate and analyze each spectrum and the atomic proportions of the measured samples. The C1s spectrum at 284.8 eV was obtained and used as an internal standard to calibrate the measured spectra for charge compensation.
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Table 1 Changes in bond lengths after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Chloride
Absence Presence Changes Chloride
Absence Presence Changes
Table 2 Changes in bond angles after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Chloride
Bond lengths (Å) C1–O1
C2–O4
C3–O5
C4–O2
Pb1–O4
Pb2–O2
1.31 1.29 −0.02
1.32 1.32 0.00
1.30 1.30 0.00
1.30 1.29 −0.01
2.31 2.38 0.07
2.95 4.77 1.82
O1–Pb1–O4 O3–Pb1–O4 O2–Pb2–O5 O2–Pb2–O6 O5–Pb2–O6 Absence 81.31 Presence 58.67 Changes −22.64 Chloride
Bond lengths (Å) Pb2–O5
Pb3–O1
Pb–O1
2.70 2.91 0.21
2.73 2.64 −0.09
– 2.69 –
3. Results and discussion 3.1. Adsorption configuration of sulfide species on the cerussite (110) surface in the presence of chloride HS− is the dominating sulfide species in the sulfidization flotation of cerussite, and PbCl+ is the primary lead chloride complex when chloride is employed to improve cerussite sulfidization [10,14,15]. Thus, HS− and PbCl+ are presented in this work to represent the corresponding sulfide and chloride species. After various available adsorption configurations in the presence of chloride are compared, the adsorption configuration presented in Fig. 1 is considered to be the most stable. Meanwhile, the adsorption configurations of HS− onto the cerussite (110) surfaces in the absence and presence of chloride are compared to provide a basis for distinguishing the subsequent surface structures and electronic properties. The adsorption configuration in the absence of chloride is shown in Fig. 4 in our previous published literature [24]. The serial numbers of different atoms labeled in Figs. 1 and 4 in our previous published literature [24] are uniform. In other words, the Pb1, Pb2, Pb3, and Pb4 atoms in Figs. 1 and 4 in our previously published literature [24] refer to the same atoms from the cerussite crystal, as is the case with the C and O atoms. The only difference is that an additional Pb atom appears at the surface layer of cerussite in the presence of chloride due to the involvement of PbCl+ on the mineral surface (i.e., Pb represents the Pb atom from PbCl+ ), and it may interact with the S atom from the sulfide species. Normally, the interacting distance between two atoms is close to or shorter than the sum of their atomic radii when a covalent bonding is formed between them. Thus, the strength of interaction between two atoms can be estimated by comparing their interacting distance and the sum of their atomic radii [20]. In other words, no chemical adsorption occurs between the S atom from HS− and the surface Pb atom when the interaction distance between these two atoms is longer than the sum of their atomic radii. As shown in Fig. 1, the distances between the Pb atoms at the surface layer and the S atom from the sulfide species are 2.82 Å for Pb–S, 3.72 Å for Pb1–S, 2.76 Å for Pb2–S, and 5.36 Å for Pb3–S in the presence of chloride, whereas the distances between the Pb atoms and the S atom are 3.65 Å for Pb1–S, 2.61 Å for Pb2–S, and 4.46 Å for Pb3–S in the absence of chloride as shown in Fig. 4 in our previous published literature [24]. The sum of Pb and S atomic radii is 2.79 Å, which is close to the Pb–S and Pb2–S distances in the presence of chloride, and is slightly longer than the Pb2–S distance in the absence of chloride. This result suggests that the S atom can chemically adsorb onto the Pb and Pb2 atoms on the cerussite surface in the presence of chloride and onto the Pb2 atom on the cerussite surface in the absence of chloride. In other words, the sulfide species can chemically adsorb onto the cerussite surface more strongly through the interaction of more electrons in the presence of chloride than in the
Bond angles (◦ )
Absence Presence Changes
43.35 54.63 11.28
84.74 47.79 −36.95
72.60 44.92 −27.68
42.91 32.59 −10.32
Bond angles (◦ ) O1–Pb3–O5
O1–Pb–O4
O3–Pb–O4
102.66 103.29 0.63
– 54.54 –
– 30.16 –
absence of chloride. This result agrees with previous experimental results [10,14]. Meanwhile, an additional Pb atom, which is derived from lead chloride complexes, demonstrates that more available active sites are provided for the adsorption of sulfide species in the presence of chloride. Our previous study confirmed that the attachment of PbCl+ to cerussite surfaces can increase the number of active sites and enhance the activity of active sites on the mineral surface [15]. The adsorption energies are used in the present work to reveal the adsorption stability of HS− on the mineral surface, and the calculated values in the absence and presence of chloride are −500.83 and −1389.74 kJ/mol, respectively. This result further indicates that the adsorption of HS− on the cerussite (110) surface in the absence and presence of chloride can occur spontaneously (i.e., HS− can chemically adsorb onto the cerussite surface), and the adsorption of HS− is more stable in the presence than in the absence of chloride due to a more negative value of the adsorption energy [28,29]. 3.2. Effect of chloride on the surface structures and electronic properties of cerussite The surface structures and electronic properties of cerussite significantly influence the interaction between HS− and cerussite surfaces. Thus, changes in the bond lengths and bond angles as well as in electronic properties after HS− adsorption onto the cerussite (110) surface in the absence and presence of chloride were investigated. Table 1 shows almost no difference in bond lengths between the C and O atoms after HS− adsorption regardless of the presence of chloride. This result indicates that the interaction between HS− and the mineral surface in the absence and presence of chloride does not significantly influence the carbonate group of cerussite. In other words, the C and O atoms are not involved in the sulfidization reaction, and HS− prefers Pb sites on the cerussite surface for adsorption. As shown in Table 1, the bond lengths between the Pb and O atoms are 2.31 Å for Pb1–O4, 2.95 Å for Pb2–O2, 2.70 Å for Pb2–O5, and 2.73 Å for Pb3–O1 in the absence of chloride, and are 2.38 Å for Pb1–O4, 4.77 Å for Pb2–O2, 2.91 Å for Pb2–O5, and 2.64 Å for Pb3–O1 in the presence of chloride. The presence of chloride changes the bond lengths between the Pb and O atoms on the mineral surface, especially for Pb2–O2 and Pb2–O5. Furthermore, the Pb atom from PbCl+ bonds to the O atom on the cerussite surface, indicating that the additional Pb atom can tightly attach to the mineral surface and serve as the active site. The presence of chloride considerably changes the bond angles of different atoms at the surface layers (Table 2). The bond angles of O1–Pb1–O4, O2–Pb2–O5, O2–Pb2–O6, and O5–Pb2–O6 decrease by 22.64◦ , 36.95◦ , 27.68◦ , and 10.32◦ , respectively, whereas those of O3–Pb1–O4 and O1–Pb3–O5 increase by 11.28◦ and 0.63◦ , respectively. These phenomena disclose that the presence of chloride
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Fig. 2. DOS of atoms after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride.
changes the surface structures of cerussite after HS− adsorption on the cerussite (110) surface. The electronic properties of the cerussite (110) surface after HS− adsorption in the absence and presence of chloride are compared to determine the positive contribution of chloride to the reactivity of surface atoms. Fig. 2 demonstrates the density of state (DOS) of the C, O, and Pb atoms at the surface layers for the cerussite (110) surface after HS− adsorption in the absence and presence of chloride. The EF value, the position of Fermi level, is set at 0 eV. As shown in the DOS curves, the 2p and 2 s orbitals of the C and O atoms are distant from the Fermi level, indicating that they contribute insignificantly to the sulfidization reaction of cerussite, and the presence of chloride slightly affects their reactivity. The DOS of the Pb 6p orbital near the Fermi level is very intense, suggesting the high reactivity of the Pb atoms on the mineral surface. Accordingly, the main contribution of the reactivity of the atoms at the surface layers for the cerussite (110) surface is attributed to Pb atoms when HS− is involved. The projected DOS (PDOS) of the surface Pb atom remarkable changes near the Fermi level in the absence and presence of chloride. In the Pb 6p orbital, a new DOS peak occurs near the Fermi level in the presence of chloride, indicating a stronger interaction between the surface Pb atom and the S atom from sulfide species. The involvement of chloride species benefits the reactivity of cerussite surfaces, and Pb atoms are the main adsorption sites for HS− . To determine the contribution of Pb atoms on the mineral surface to the reactivity, the electronic properties of different Pb atoms numbered in Fig. 1 are investigated. Fig. 3 provides the PDOS curves of the Pb atoms on the cerussite (110) surface after HS− adsorption in the presence of chloride. The DOSs of the various Pb atoms at the surface layers show evident differences, including the difference occurring near the Fermi level. The DOS of atoms occurring near the Fermi level significantly facilitates the reactivity of the interacting elements. The results shown in Fig. 3 indicate that the Pb3 and Pb4 atoms contribute insignificantly to the DOS of the Fermi level, whereas the Pb, Pb1, and Pb2 atoms contribute positively to the reactivity of cerussite surfaces. The Pb atom from chloride species exhibits the most prominent contribution.
Fig. 3. PDOS of Pb atoms on the cerussite (110) surface after HS− adsorption in the presence of chloride.
3.3. DOS of interactions between HS− and the cerussite (110) surface in the absence and presence of chloride The presence of chloride changes the reactivity of atoms at the (110) surface layers after HS− adsorption, which facilitates sulfidization. To probe the microscopic sulfidization mechanism of cerussite, the DOS of the interaction between Pb, Pb1, Pb2, and S atoms in the presence of chloride is investigated (Fig. 4). The Pb–S DOS curve shows an obvious overlap between the Pb 6p and S 3p
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Fig. 6. DOS of Pb atoms at the surface layers after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride. Fig. 4. DOS of interactions between S atom from HS− and Pb atoms on the cerussite (110) surface in the presence of chloride.
Fig. 7. PDOS of S atom after HS− adsorption on the cerussite (110) surface in the absence (a) and presence (b) of chloride. Fig. 5. DOS of interactions between S atom from HS− and Pb atoms at the surface layers in the absence (a) and presence (b) of chloride.
states near the Fermi level. The relevant hybridization peaks of Pb 6p and S 3p appear at −2.8, −1.5, −0.8, and 0 eV, suggesting a strong interaction between Pb and S atoms. The DOSs of Pb1–S and Pb2–S are similar to that of Pb–S, but the overlapped DOSs of their Pb 6p and S 3p orbitals are smaller than that of Pb–S. This result discloses that a stronger interaction occurs between the Pb atom from chloride species and S atoms than between other Pb atoms and S atoms. In other words, the presence of chloride species strengthens the interaction between HS− and the cerussite surface. Hence, the DOS curves of the interactions between the S atom from HS− and the Pb atoms at the surface layers in the absence and presence of chloride are given in Fig. 5 to further understand the contribution of chloride to the sulfidization reaction of cerussite. Fig. 5 shows an obvious difference in the DOS of interactions between the Pb and S atoms in the absence and presence of chlo-
ride. The DOSs of the Pb 6p and S 3p orbitals remarkably change in the presence of chloride, implying a difference in electron transfer between the Pb and S atoms. Meanwhile, the overlapping range of the Pb 6p and S 3p states near the Fermi level increases, indicating that the presence of chloride strengthens the bonding interaction between the surface Pb atom and the S atom of sulfide species. In addition, the number of hybridization peaks of Pb 6p and S 3p near the Fermi level increases, suggesting that the presence of chloride enhances the reactivity of HS− with the cerussite surface. These results confirm that HS− easily interacts with the Pb sites on the mineral surface in the presence of chloride and that the formed lead sulfide species are more active. The DOSs of the Pb and S atoms after HS− adsorption on the cerussite (110) surface are compared in the absence and presence of chloride (Figs. 6 and 7). After HS− adsorption, an obvious difference in the DOS of surface Pb atom is observed in Fig. 6 when the cerussite surface is in the absence and presence of chloride species.
Q. Feng et al. / Applied Surface Science 425 (2017) 8–15
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Table 3 Mulliken charge populations of atoms after HS− adsorption on the cerussite (110) surface in the absence and presence of chloride. Atoms
Chloride
s
p
d
Total
Charge (e)
Pb1
Absence Presence Absence Presence Absence Presence Absence Presence
1.98 1.89 1.92 1.98 1.90 1.88 1.88 1.89
1.70 1.47 1.82 1.58 1.21 1.04 4.63 4.51
10.01 10.01 10.01 10.00 10.00 10.00 0.00 0.00
13.69 13.37 13.75 13.56 13.12 12.92 6.51 6.40
0.31 0.63 0.25 0.44 0.88 1.08 −0.51 −0.40
Pb2 Pb3 S
The DOS of the Pb 6p orbital increases generally, demonstrating that chloride improves the ability of the surface Pb atom to capture electrons, thereby strengthening its interaction with HS− . In addition, the presence of chloride widens the distribution of the Pb 6p and Pb 6 s orbitals and increases the number of their DOS peaks near the Fermi level. In other words, several new DOS peaks of Pb 6p and Pb 6 s occur near the Fermi level. These results prove that the presence of chloride on the cerussite surface increases the binding capacity to sulfide species. As is shown in Fig. 7, the DOSs of the S 3p and S 3s orbitals on the cerussite surface after HS− adsorption in the absence of chloride are different from those in the presence of chloride. After interacting with HS− , the DOSs of the S 3p and S 3s orbitals slightly decrease and shift to the low energy direction, implying the loss of more electrons for S atoms during electron transfer to Pb atoms in the presence of chloride. Compared with the absence of chloride, the presence of chloride widens the distribution range of the S 3p and S 3 s orbitals and increases the number of their DOS peaks near the Fermi level. In particular, several new DOS peaks of S 3p and S 3s appear near the Fermi level, which coincide well with the newly generated DOS peaks of Pb 6p and Pb 6s, hybridizing each other and forming an additional stable lead sulfide species on the mineral surface (Fig. 5). The flotation performance of minerals significantly depends on their surface properties, and their surface properties are significantly influenced by their electronic properties. Accordingly, the detailed discussion on the electronic properties of interactions between HS− and the cerussite (110) surface further verifies the positive effect of chloride species on the sulfidization of cerussite at the atomic level, which agrees with the experimental results [10,14,15].
Fig. 8. S2p XPS spectra of cerussite sulfidized with: (a) 5 × 10−5 mol/L Na2 S·9H2 O, (b) 5 × 10−3 mol/L NaCl and 5 × 10−5 mol/L Na2 S·9H2 O, (c) 5 × 10−4 mol/L Na2 S·9H2 O, and (d) 5 × 10−3 mol/L NaCl and 5 × 10−4 mol/L Na2 S·9H2 O.
After HS− interacts with the mineral surface, the charge values of the surface S atom show a difference in the absence and presence of chloride. The 3p orbital of the S atom has 4.63 electrons in the absence of chloride and 4.51 electrons in the presence of chloride, i.e., the presence of chloride diminishes the electrons of the S 3p orbital. The decrease in negative charge of the surface S atom in the presence of chloride may be related to the oxidation of HS− when it interacts with cerussite surfaces. The oxidation of sulfide species is involved in the sulfidization of cerussite, and the higher proportion of disulfide and polysulfide relative to the overall S positively contributes to the activity of sulfidization products [13,24]. Thus, the formed sulfidization products in the presence of chloride may include higher contents of lead disulfide and lead polysulfide. 3.5. XPS studies
3.4. Charge transfer of interactions between HS− and the cerussite (110) surface in the absence and presence of chloride The Mulliken population analysis is employed to analyze the related transfers of charge and evaluate the bonding nature of the Pb atoms at the surface layer and the S atom from HS− . The charge values of the Pb1, Pb2, Pb3, and S atoms on the cerussite (110) surface after HS− adsorption in the absence and presence of chloride are listed in Table 3. The table shows that the absence and presence of chloride cause an obvious difference in charge transfer between the bonding atoms. Compared with the absence of chloride, the presence of chloride allows the Pb atoms to hold a more positive charge after HS− adsorption on the cerussite surface. In specific, the charges of the Pb1, Pb2, and Pb3 atoms increase from 0.31 e to 0.63 e, 0.25 e to 0.44 e, and 0.88 e to 1.08 e, respectively. The lost electrons for the Pb atoms are primarily derived from the Pb 6p orbital, and the presence of chloride renders more electrons lost in this orbital. These results demonstrate that the interaction between HS− and the cerussite (110) surface is stronger in the presence than in the absence of chloride, and the Pb sites positioned in the formed lead sulfide species easily interact with the subsequent collector, thereby improving the flotation behavior of cerussite.
The S2p XPS spectra were fitted to determine the effect of chloride on the component of sulfidization products. Figs. 8a and b presents the S2p XPS spectra of cerussite sulfidized with Na2 S·9H2 O at a concentration of 5 × 10−5 mol/L in the absence and presence of chloride. The result shows that the entire asymmetric S2p peak can be fitted into three symmetric peaks regardless of the presence of chloride. The first peak at a binding energy of 160.73 eV in Fig. 8a is assigned to the S2− state, i.e., the S in lead monosulfide films formed on the cerussite surface [13,30]. The second peak is 1.40 eV higher in the binding energy than the first one, which corresponds to the S2 2− state, i.e., the S in disulfide. The third peak at a binding energy of 163.43 eV is attributed to the Sn 2− state, i.e., the S in polysulfide, and its peak is 2.70 eV higher in the binding energy than the first one. Accordingly, the sulfidization product of cerussite surfaces is composed of PbS, PbS2 , and PbSn . The higher binding energy of S2 2− and Sn 2− may be ascribed to the slight oxidation of HS− when interacting with the Pb sites on the mineral surface [13,24]. As shown in Fig. 8b, a slight shift of binding energy occurs in the three fitted peaks compared with the data in the absence of chloride, but their peak areas exhibit a remarkable difference. The peak area can be converted into the species distribution of different chem-
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Q. Feng et al. / Applied Surface Science 425 (2017) 8–15
Table 4 S2p quantification of cerussite sulfidized with: (a) 5 × 10−5 mol/L Na2 S·9H2 O, (b) 5 × 10−3 mol/L NaCl and 5 × 10−5 mol/L Na2 S·9H2 O, (c) 5 × 10−4 mol/L Na2 S·9H2 O, and (d) 5 × 10−3 mol/L NaCl and 5 × 10−4 mol/L Na2 S·9H2 O. Samples
Species
S2p3/2 binding energy (eV)
Species distribution (%)
a
S2− S2 2− Sn 2− S2− S2 2− Sn 2− S2− S2 2− Sn 2− S2− S2 2− Sn 2−
160.73 162.13 163.43 160.70 162.15 163.45 160.74 162.07 163.49 160.36 161.78 163.09
63.06 24.74 12.20 13.91 59.27 26.82 39.03 41.30 19.67 18.66 38.53 42.81
b
c
d
3p orbitals is stronger in the presence than in the absence of chloride. (2) An obvious difference in surface structures and electronic properties of cerussite is observed after HS− adsorption in the absence and presence of chloride. Several new DOS peaks of Pb 6p and S 3p appear near the Fermi level, and the number of electrons transferred between the bonding atoms increases, indicating that chloride enhances the reactivity of cerussite surfaces. (3) A slight oxidation occurs in the interaction between sulfide species and the cerussite surface, and disulfide and polysulfide are the primary oxidation products. The presence of chloride increases the proportion of disulfide and polysulfide relative to the overall S, which is advantageous to the sulfidization flotation of cerussite. Acknowledgement
ical states. Table 4 lists the compositions of S species on cerussite surfaces at different Na2 S concentrations in the absence and presence of chloride. The fractions of S2− , S2 2− , and Sn 2− relative to the overall S are 63.06%, 24.74%, and 12.20% in the absence of chloride, respectively, and are 13.91%, 59.27%, and 26.82% in the presence of chloride, respectively. This result indicates that the presence of chloride increases the percentages of S2 2− and Sn 2− . In general, the S S bond in S2 2− and Sn 2− can be considered a short-chain oligomer, and the S atoms are equivalent in S2 2− but non-equivalent in Sn 2− [13,31]. Given the chain structure of Sn 2− , the S atoms located at both ends and in the middle exhibit a different charge distribution along the chain, namely, the former and the latter possess formal charges of −1 and 0, respectively [13,30,31]. In addition, the presence of S2 2− and Sn 2− (especially Sn 2− ) on the mineral surface is beneficial to the hydrophobicity of minerals [30,32,33]. Accordingly, greater proportions of S2 2− and Sn 2− in the sulfidization products mean better floatability of cerussite particles. Combining the data in Fig. 8 and Table 4 as well as the previous discussion, it is estimated that the presence of chloride results in the formation of more S2 2− and Sn 2− species on the mineral surface after HS− adsorption, thereby improving the reactivity of the sulfidization products. For further understanding of the identity of surface species, the S2p XPS spectra of cerussite sulfidized with a higher Na2 S concentration in the absence and presence of chloride are demonstrated in Figs. 8c and d. The result in the presence of chloride at a higher concentration of Na2 S verifies the reliability of the above statement, and all fitted S2p peaks shift to a lower binding energy due to the change in chemical environment. Meanwhile, the percentage of Sn 2− in the sulfidization product in the presence of chloride increases by 23.14% compared with that in the absence of chloride. These results conformably reveal that the presence of chloride is advantageous to the sulfidization of cerussite, leading to the enhanced hydrophobicity on the mineral surface. 4. Conclusions The atomic interaction mechanism of sulfide species with the cerussite surface in the presence of chloride was studied by DFT simulations, and the chemical environment of the sulfidization products was characterized by XPS measurement. On the basis of the aforementioned results, the primary conclusions are summarized as follows: (1) Sulfide species may adsorb on the cerussite surface, and the adsorption of HS− in the presence of chloride is more stable than that in the absence of chloride. The hybridization of the Pb 6p and S 3p orbitals is the main interaction between HS− and the cerussite surface, and the hybridization of the Pb 6p and S
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