Article pubs.acs.org/JPCC
Comparison of Multilayer Water Adsorption on the Hydrophobic Galena (PbS) and Hydrophilic Pyrite (FeS2) Surfaces: A DFT Study Jianhua Chen,†,‡ Xianhao Long,‡ and Ye Chen*,† †
College of Resources and Metallurgy and ‡School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, P.R. China ABSTRACT: To investigate the essential difference in the water adsorption on the hydrophobic galena (PbS) and hydrophilic pyrite (FeS2) surfaces, a single, mono-, and multilayer water adsorptions on the PbS and FeS2(100) surfaces have been simulated by means of density functional theory. The calculated results show that the water molecule adsorption on the hydrophobic PbS surface is via a hydrogen bond between the S and H atom and on the hydrophilic FeS2 surface is mainly via the interaction between surface Fe and the water O atom. On the galena surface, the hydrogen bond effect of multilayer water weakens the S···H bond interaction and leads to the galena surface being hydrophobic. For the pyrite surface, the hydrogen bond effect of the multilayer water enhances the activity of the O 2p orbital and favors the interaction of the O 2p and Fe 3d eg orbital, which results in the pyrite surface being hydrophilic and undergoing a greater relaxation. These variations may have a great influence on the subsequent interfacial reactions on the galena and pyrite surfaces.
1. INTRODUCTION Surface structure and properties of crystals, especially for the hydrophobic and hydrophilic surfaces, have a great influence on water adsorption and the subsequent interfacial chemical reaction. Galena (PbS) and pyrite (FeS2) are the most common sulfide minerals and are widely distributed in nonferrous metal deposits. They are totally distinct in their surface characteristics. The galena surface is almost hydrophobic, with a contact angle of 48°−52°, and the pyrite surface is strongly hydrophilic with the contact angle of 20°.1,2 The research3 shows that the outermost layer of both PbS and FeS2 surfaces is electronegative, but PbS carries more negative charge than the FeS2 surface. The surface iron atom of pyrite is more reactive than the surface lead atom of galena. Oxidation of pyrite in aqueous solution is the most important factor in the generation of acid mine drainage (AMD), which will decrease the pH of water and devastate rivers, streams, and aquatic life for years.4 Weathering of galena results in the release of lead, sulfate, and other potentially toxic trace metals in the form of lattice substitution, such as cadmium, mercury, thallium, arsenic, and other trace metals.5 These dissolved sulfate, lead, and trace metals may enter groundwater, rivers, and oceans and cause heavy metal pollution. For galena and pyrite, it has been confirmed that the oxygen in sulfate is derived from water molecules.6−10 Accordingly, the adsorption of water molecules participates in the oxidation process and determines the final oxidation product. The difference of the water adsorption structure on the hydrophobic/hydrophilic surfaces would result in the obvious variation in the mineral oxidation process. Moreover, galena is the world’s primary ore of lead; meanwhile, it is also the main carrier mineral of silver. Pyrite is an important raw material for industrial manufacture of © 2014 American Chemical Society
sulfuric acid. Both galena and pyrite are generally recovered by froth flotation, which takes advantage of the differences in wettability at particle surfaces to separate different minerals. Generally, suitable reagents are necessary to add to adjust the hydrophobicity or hydrophilicity of the mineral surface to ensure the selective adhesion to the froth. As described by Stirling,11 these reactions occur on a water-covered surface, and they either directly involve the adsorbed water molecules or are initiated by the exchange of the adsorbed water with the reactants to be bound on surface sites. Both galena and pyrite surfaces have been extensively studied using experimental methods such as X-ray photoelectron spectroscopy (XPS),12−26 ultraviolet photoelectron spectroscopy (UPS),17,27,28 Raman spectroscopy,19 scanning tunneling microscopy (STM),8,13,15,28−32 low-energy electron diffraction,23,28 photoemission of adsorbed xenon,33,34 and temperature-programmed desorption (TPD)33−35 as well as X-ray standing wave technique (XSW).36 These studies are devoted to describing the surface characteristics of galena and pyrite. Besides, some theoretical studies based on Hartree−Fock (HF)31,37,41 and DFT methods on cluster or periodic models have been reported.8,17,25,26,28,30,38−61 Furthermore, a few studies about the water adsorption on the galena or pyrite surfaces have been performed. Wright et al.51,52 studied the adsorption and dissociation of molecular water on the defective (001) surface of galena using both periodic and embedded cluster electronic structure methods and found that the sorption to steps leads to rapid dissociation of the water molecules. Zhang et al.53 studied methylamine and water Received: January 2, 2014 Revised: May 13, 2014 Published: May 14, 2014 11657
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adsorption on PbS by density functional theory (DFT) and found that an exceptional strong hydrogen bonding at the solution−solid interface may lower the surface energy. Zhao et al.54 conducted a microcalorimetry experiment and the density functional theory simulations of a single water adsorption on four sulfide mineral surfaces including galena and pyrite. Bryce39 has explored the adsorption and reactivity of molecular water on the surface of PbS (galena) and its interface with aqueous solution, employing the recently proposed CECILIA62 method. Philpott et al.63 found a strong adsorption of water molecules on the pyrite surface by a molecular dynamics simulation. Stirling et al.11 studied water adsorption on the (100) surface of pyrite by using the Car−Parrinello ab initio molecular dynamics (CPMD) method. Patrick et al. 7 performed a DFT study on the adsorption and reactions of oxygen and water on the pyrite (100) surface. Leeuw et al.64 introduced a potential model to study the hydration energy of FeS2 surfaces. However, a full understanding of the difference in water adsorption mechanism on both the hydrophobic galena and hydrophilic pyrite surfaces has not been established. To understand the variation in the water adsorption mechanism on the hydrophobic and hydrophilic surfaces at an atomic level, the comparison of a single, mono-, and multilayer water adsorption on the hydrophobic galena (PbS) and hydrophilic pyrite (FeS2) (100) surfaces was performed using the density functional theory (DFT) methods. The results may give insight into the nature of the wettability of hydrophobic galena/hydrophilic pyrite surfaces and help to explain the subsequent interfacial reactions on the surfaces.
Figure 1. Slab model of a (4 × 2) galena (100) surface: (a) side view and (b) top view.
2. COMPUTATIONAL METHODS All calculations were performed in the framework of the Cambridge Serial Total Energy Package (CASTEP) developed by Payne et al.65 The DFT calculation employed plane wave (PW) basis sets and ultrasoft pseudopotentials. The exchange− correlation functional applied was the generalized gradient approach (GGA) of Perdew and Wang (PW91).66 The interactions between valence electrons and the ionic core were represented by ultrasoft pseudopotentials.67 Valence electron configurations considered in the study included Pb 5d106s26p2, S 3s23p4, Fe 3d64s2, O 2s22p4, and H 1s1 states. On the basis of the test results, the plane-wave cutoff energy of 300 eV was used throughout, and the Brillouin zone was sampled with a Monkhorst and Pack special of a 2 × 2 × 1 grid for pyrite and a 1 × 2 × 1 grid for galena surface calculations.68 For selfconsistent electronic minimization, the Pulay Density Mixing method was employed with the convergence tolerance of 2.0 × 10 −6 eV/atom. The convergence criteria for structure optimization and energy calculation were set to (a) energy tolerance of 2.0 × 10−5 eV/atom, (b) maximum force tolerance of 0.05 eV/Å, and (c) maximum displacement tolerance of 0.002 Å. The PbS(100) and FeS2(100) surfaces are chosen as they are the most stable surfaces.48,52 Surfaces were cleaved on the basis of the optimized bulk structure. The computed lattice parameters for the bulk galena and pyrite are 6.018 and 5.410 Å, respectively, which are very closed to the experimental values of 5.936 and 5.416 Å.69 After testing the slab thickness, we constructed a (4 × 2) PbS(100) surface with 8 atomic layers (64 Pb and 64 S atoms) and a (2 × 2) FeS2(100) surface with 15 atomic layers (40 Fe and 80 S atoms) separated by 15 Å of vacuum, as shown in Figure 1 and Figure 2. The supercells for the galena and pyrite slabs had the dimensions of 17.01 × 8.51
Figure 2. Slab model of a (2 × 2) pyrite (100) surface: (a) side view and (b) top view.
× 36.05 Å3 and 10.77 × 10.77 × 27.03 Å3, respectively. For the FeS2 surface, the six outermost atomic layers of the substrate were allowed to relax, while the nine bottom-most atomic layers of the substrate were fixed to the bulk coordinates. For the PbS surface the three outermost atomic layers of the substrate were allowed to relax, while the five bottom-most atomic layers of the substrate were fixed to the bulk coordinates in the adsorption calculations. The optimizations of the H2O molecule were performed in a 10 × 10 × 10 Å cubic cell. The calculated dO−H and ∠H−O−H of optimized H2O are 0.976 Å and 103.8°, respectively, which are close to the experimental values of 0.958 Å and 104.5°.70 Adsorption energy can be expressed by the following equation ΔEads =
1 [Esurf + nH2O − Esurf − nE H2O] n
(1)
where ΔEads is the adsorption energy; Esurf+nH2O is the total energy of the surface with the water molecules; Esurf is the total energy of the mineral surface; and EH2O is the energy of one water molecule which is calculated in the cell with the same cell size and k-point grid as used in the surface. 11658
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Figure 3. Optimized geometry for the adsorption of a single water molecule on the PbS surface.
3. RESULTS AND DISCUSSION 3.1. Adsorption of an Isolated Water Molecule. First, an isolated water molecule was placed on the PbS and FeS2(100) surfaces to simulate a single water adsorption, and the most stable adsorption geometries are shown in Figure 3 and Figure 4, respectively. It is shown in Figure 3 that the adsorption of a single water molecule is mainly via its H atoms and surface S atoms with the S−H distance of 2.386 and 2.237 Å. The calculated adsorption energy is −29.1 kJ/mol, which indicates that the interaction between the water molecule and galena surface is weak. In an earlier DFT study, Wright et al.51 found adsorption energies on the PbS (001) surface with −31 and −44 kJ/mol for an embedded 4 × 4 × 2 atom slab and a 2 × 2 × 6 atom periodic slab. Moreover, the dissociative adsorption of water on the galena (100) surface is not observed in our study, which is similar to the earlier findings of Wright et al.51 They suggested that on the perfect surface no obvious sites would interact with water or facilitate dissociative reactions. However, water would readily dissociate if surface defects were present.52 The adsorption of water at full coverage was also conducted, and the calculated adsorption energy per molecule is −42.9 kJ/mol. In addition, hydrogen bonds between water molecules are observed. On the FeS2(100) surface, both the molecular and the dissociative adsorptions have been considered, and the optimized geometries are shown in Figure 4(a) and (b). For the molecular adsorption, it is found that water prefers to adsorb on the surface Fe atom via its O atom with the Fe−O length of 2.108 Å. The calculated adsorption energy is −56.2 kJ/mol, which agrees well with the result of −54.4 kJ/mol calculated by Stirling et al.11 and the result of −54.8 kJ/mol calculated by Zipoli et al.71 Other theoretical studies from Leeuw64 and Rodriguesz58 predicted energies of −47 and −45 kJ/mol. These results are consistent with the experimental value of −42 kJ/mol determined by TPD experiments.35 Other theoretical studies predicted an energy of −65.7 kJ/mol from spin-polarized DFT calculations7 or −62 kJ/mol using ab initio molecular dynamics simulations.72 The dissociative adsorption is energetically unfavorable with the energy of 82.4 kJ/mol.
Figure 5. Optimized structures for the adsorption of monolayer water molecules on (a) PbS and (b) FeS2 surfaces.
This is in accordance with both the experimental73,35 and theoretical studies.11,8 The adsorption of water at full coverage was considered, and the pyrite surface exposed four iron atoms; therefore, we placed four water molecules on the surface. The computed adsorption energy per molecule is −56.8 kJ/mol, which is consistent with the result of −59.8 kJ/mol calculated by Zipoli et al.71 employing a Car−Parrinello FPMD simulation. In previous studies, Stirling et al.11 found an energy of −48.5 kJ/mol using the Car−Parrinello ab initio molecular dynamics (MD), and Leeuw et al.64 calculated an energy of −52.2 kJ/mol using a potential model for FeS2. 3.2. Excess Water Molecule Adsorption. In aqueous environment, the adsorption of excess water is interesting because both the H bonding and water−surface interactions are involved in the adsorbed water molecules. Studying these excess water molecules at surfaces may help us understand the competition between the two interactions.71,74,75 Therefore, we build monolayer and multilayer excess water adsorption models to simulate the influence of two-dimensional hydrogen bonding and three-dimensional hydrogen bonding. In the monolayer excess water adsorption model, the amount of water molecules is more than adsorption sites on the surface and is determined by the surface area of minerals and the hydrogen bonding distance between water molecules. After testing, it is found that nine water molecules are sufficient to cover the (2 × 2) FeS2(100) surface and 18 water molecules are sufficient to cover the (4 × 2) PbS(100) surface.
Figure 4. Optimized geometries for the adsorption of a single water molecule on the FeS2(100) surface: (a) molecular adsorption and (b) dissociative adsorption. 11659
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Figure 8. PDOS of Fe−O bonding before and after water adsorption (Gaussian smearing parameter of 0.05 eV).
Figure 6. Optimized structures for the adsorption of multilayer water molecules on (a) PbS and (b) FeS2 surfaces.
Figure 9. DOS of the O atom of free, two-dimensional (2D), and three-dimensional (3D) water molecules.
structure of water on the pyrite surface. The bond population of Fe−O increases from 0.12 without a hydrogen bond to 0.16 with two-dimensional hydrogen bonds, suggesting that the twodimensional hydrogen bonding strengthens the covalent characteristic of the Fe−O bond. The three-dimensional multilayer water adsorption could provide the information about the influence of the hydrogen bond at the z direction on the adsorption of H2O at the mineral surface. The adsorption of multilayer water was simulated by placing 54 water molecules on the PbS(100) surface and 27 water molecules on the FeS2(100) surface, respectively, and the optimized configurations are shown in Figure 6(a) and (b). For the PbS surface, the additional water at the z direction changes the configuration of water adsorption. The average values of the S···H bond length increase from 2.468 Å for the monolayer to 2.524 Å for the multilayer water, indicating that the formation of a hydrogen bond at the z direction weakens the adsorption of water. This is ascribed to the hydrogen bonding interaction between water molecules at the z direction. It is found from Figure 6(a) that the average values of the O··· H bond at the z direction (1.755 Å) are shorter than that of the S···H bond (2.208 Å), suggesting the stronger interaction
Figure 7. PDOS of S and H atoms with different S···H bond lengths on the PbS surface (Gaussian smearing parameter of 0.05 eV).
For the PbS(100) surface, the optimized structure of monolayer excess water adsorption is shown in Figure 5(a). It is found that the adsorption structure of monolayer water is very different from that of a single water molecule due to the formation of hydrogen bonds between water molecules (O−H distances: 1.650−1.812 Å). Except the interaction of S···H bonding, the interaction between O and the surface Pb atoms is also observed with the Pb−O distances of 2.933, 2.950, and 3.085 Å. Particularly, strong hydrogen bonds are formed with the S−H distances of ≈2.3 Å. In the case of the FeS2(100) surface, the optimized configuration of monolayer excess water adsorption is shown in Figure 5(b). It is found that the formation of twodimensional hydrogen bonds influences the adsorption 11660
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Table 1. Comparison of the Relaxation for the PbS and FeS2(100) Surfaces
PbS(100) surface FeS2(100) surface
bare surface
monolayer water adsorption
multilayer water adsorption
Pb−Saxb
2.813
2.864
2.836
Fe−Saxa
2.120
2.188
2.191
a
Fe−Sax: average value of the three interacting axial Fe−S bond lengths (Å). bPb−Sax: average value of the three interacting axial Pb−S bond lengths (Å).
between water molecules at the z direction than that of water and the galena surface. For the FeS2 surface, the distances between the adsorbed H2O molecules and surface Fe atoms become shorter, with the Fe···O distances of 2.074, 2.090, and 2.253 Å, indicating that the interaction between H2O and the Fe atom is strengthened. In addition, the S···H distances become shorter, implying that the S···H bonds are also strengthened after adsorbing multilayer water. Compared to the monolayer water adsorption, the average value of the Fe−O bond length shortens from 2.21 to 2.14 Å, and the bond population increases from 0.16 to 0.19. It is indicated that the additional water molecules enhance the covalent characteristic of the Fe−O bond. On the PbS surface, as the adsorption of water molecules is mainly via the hydrogen bonds between the H and surface S atoms, the partial density of states (PDOS) of S and H atoms with different S···H bond length is investigated (Figure 7) to give insight into the nature of the bonding mechanism. From the PDOS of the S···H bond with the length of 2.097 Å, it is clearly seen that the H 1s−S 3s bonding DOS peak is located at
Figure 10. Geometry configurations of (a) bare PbS surface, (b) monolayer, and (c) multilayer water adsorption on the PbS surface.
Figure 11. Geometry configurations of (a) bare FeS2 surface, (b) monolayer, and (c) multilayer water adsorption on the FeS2 surface. 11661
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Figure 12. DOS of (100) surfaces before and after water adsorption: (a) PbS and (b) FeS2 (Gaussian smearing parameter of 0.05 eV).
−11.3 eV and the antibonding DOS peak at −8.7 eV; meanwhile, the H 1s−S 3p bonding DOS peak appears between −4.5 and 0 eV, and the antibonding DOS peak resonances are at 1.3−4.8 eV. The considerable decreases of bonding and antibonding DOS peaks are observed when the S···H bond is 3.097 Å long, implying that the longer the length of the S···H bond, the weaker the hydrogen bond between the S and H atoms. In the case of the FeS2 surface, the adsorption of water molecules is mainly via the interaction of the O atom and surface Fe atom, hence we show the PDOS of Fe−O bonding before and after multilayer water adsorption in Figure 8. It is clearly shown that the DOS of Fe and O atoms is changed dramatically after water adsorption. Three DOS peaks of the O 2p orbital decrease greatly and shift greatly to the lower energy direction, indicating that the O 2p orbital is losing electrons. The DOS of Fe 3d t2g near the Fermi level becomes localized,
and two peaks of Fe 3d eg* appearing around 0.3−2.8 eV merge into one great peak. The bands from −1.7 to −7.6 eV are the bonding DOS of Fe 3d eg and O 2p orbitals, and hybridization DOS peaks are observed around −6.5, −5.8, and −3.9 eV, implying a relatively strong interaction between Fe 3d eg and O 2p orbitals. The bands from 0.4 to 2.8 eV are the antibonding DOS of Fe 3d eg and O 2p orbitals. The results above suggest that the additional water enhances the Fe−O interaction. To further investigate the influence of hydrogen bonding effect, we discuss the DOS of the O atom of free, two-dimensional (2D), and three-dimensional (3D) water molecules (Figure 9). It is interesting to note that considerable changes of the DOS of the O atom are observed for 2D and 3D water structures, which may be ascribed to the effect of hydrogen bonding. The bands of O 2p states in the presence of a 2D hydrogen bond become broadened and shift to the lower energy direction. 11662
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When a hydrogen bond appears at the z direction, O 2p states move much lower from 0.7 ∼ −6.4 to −1.2 ∼ −9.3 eV, which is nearer to the Fe 3d eg orbital located from −1.7 to −7.6 eV. It suggests that the hydrogen bonding between additional water molecules can enhance the interaction between the O 2p orbital and Fe 3d eg orbital. Perhaps this phenomenon may be applied for other systems, in which surfaces contain 3dtransition metals. 3.3. Structure and Electronic Properties of Galena and Pyrite Surfaces. Compared with the bare surfaces, the adsorptions of water molecules have an apparent effect on the surface structure of both PbS and FeS2 surfaces and thus may influence the interaction between the mineral surface and other coadsorbed molecules. To investigate the effects of water adsorption on the surface characteristics of galena and pyrite, the optimized geometry configurations of bare and water adsorption are shown in Figure 10 and Figure 11, respectively. In the case of monolayer water adsorption, it is found that on the galena surface the average value of the Pb−S bond length increases from 2.813 to 2.864 Å (see Table 1). In addition, the formation of H bonds between surface S and H atoms results in the axial S1−Pb, S3−Pb, and S4−Pb bonds become longer (from 2.835 to 2.975, 2.879, and 2.994 Å). Compared with the monolayer water adsorption, it is interesting to notice that Pb−S bond length decreases from 2.864 to 2.836 Å (see Table 1). As we discussed before, the hydrogen bonding at the z direction is stronger than S−H bonding and weakens the interaction of water and the surface, which results in the smaller relaxation of the galena surface covered by multilayer water. In the case of the FeS2 surface, the adsorption of monolayer water causes the surface axial Fe−S bond to lengthen greatly; especially the Fe1, Fe2, and Fe3 atoms interacting with O atoms are significantly moved upward, and the average value of these Fe−S bond lengths increases from 2.120 to 2.188 Å (Table 1). For the multilayer water adsorption, the Fe−S bond length becomes longer than that of monolayer adsorption (from 2.188 to 2.191 Å), which is ascribed to hydrogen bonding at the z direction mentioned above. The effect of water adsorption on the electronic properties of PbS and FeS2 surfaces is also investigated by analyzing the DOS of the first layer of PbS and the adsorbed Fe atoms at FeS2(100) surfaces (Figure 12). The adsorptions of both monolayer and multilayer water lead to the change of Pb 6p, Pb 6s, and S 3p states. The three DOS peaks of S 3p states around 0 ∼ −5 eV are merged into two peaks. A DOS peak of Pb 6p located around 4.3 eV increases noticeably. Meanwhile, Pb 6s states become localized, and the two weak DOS peaks around 3.4 and 4.3 eV are merged into one strong DOS peak appearing at 4.1 eV. In addition, the peak value of monolayer water adsorption is higher than that of multilayer water, indicating that the effect of multilayer water adsorption on the electronic properties of PbS seems weaker. For the FeS2 surface, the adsorption of water leads to the decrease of Fe 3d eg states around −4 ∼ −1 eV, and the Fe 3d eg states become broader; two separated DOS peaks of Fe 3d eg* appearing around 0−3 eV merge into one strong DOS peak at 1.7 eV. An intensified DOS peak is observed around −7 eV due to the adsorption of multilayer water.
by using periodic models in the density functional theory framework. On the hydrophobic galena surface water adsorption is mainly via the strong hydrogen bonds between water H atoms and the surface S atoms, while on the hydrophilic pyrite surface, the adsorption of water is mainly via the interaction between the surface Fe atom and O atom. For the hydrophobic galena surface, the two-dimensional hydrogen bonding of monolayer water strengthens the S···H bonding; however, three-dimensional hydrogen bonding is stronger than S···H bonding and consequently weakens the interaction of water and the galena surface. However, for the hydrophilic FeS2 surface, the hydrogen bonding among additional water molecules including twodimensional and three-dimensional structure promotes the activity of the O 2p orbital and consequently enhances the interaction of the O 2p and Fe 3d eg orbital. The obvious difference in relaxation and electronic properties between hydrophobic galena and hydrophilic pyrite surfaces after multilayer water adsorption may have great effects on the subsequent interfacial reactions.
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AUTHOR INFORMATION
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[email protected]. Tel.: +86-771-3232200. Fax: +86771-3233566. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of People’s Republic of China (51164001) and (51364002) and Guangxi Natural Science Foundation (2011GXNSFB018010). The authors are thankful for this support.
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REFERENCES
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4. CONCLUSION The adsorption of water molecules on the hydrophobic galena (PbS) and hydrophilic pyrite (FeS2) surfaces has been studied 11663
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