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ammonium salts adsorption on kaolinite
Accepted Manuscript Title: Experimental investigation and DFT calculation of different amine/ammonium salts adsorption on kaolinite Authors: Jun Chen, Fan-fei Min, Lingyun Liu, Chunfu Liu, Fangqin Lu PII: DOI: Reference:
S0169-4332(17)31222-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.213 APSUSC 35894
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Received date: Revised date: Accepted date:
12-1-2017 17-4-2017 22-4-2017
Please cite this article as: Jun Chen, Fan-fei Min, Lingyun Liu, Chunfu Liu, Fangqin Lu, Experimental investigation and DFT calculation of different amine/ammonium salts adsorption on kaolinite, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.213 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation and DFT calculation of different amine/ammonium salts adsorption on kaolinite CHEN Jun，MIN Fan‐fei*，LIU Lingyun，LIU Chunfu，LU Fangqin
Department of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China * Correspongding author: Tel: +86‐554‐6668885, Fax: +86‐554‐6668885 E‐mail address: [email protected] Graphical Abstract
Highlights
We investigate different amine/ammonium salts adsorption on kaolinite. We calculate different amine/ammonium salts adsorption on kaolinite with DFT. Different amine/ammonium salts can promote the settlement of kaolinite particles. The adsorption mechanism is hydrogen‐bond interaction and electrostatic attraction.
Abstract The adsorption of four different amine/ammonium salts of DDA (Dodecyl amine), MDA (N‐methyldodecyl amine), DMDA (N,N‐dimethyldodecyl amine) and DTAC (Dodecyl trimethyl ammonium chloride) on kaolinite particles was investigated in the study through the measurement of contact angles, zeta potentials, aggregation observation, adsorption and sedimentation. The results show that different amine/ammonium salts can adsorb on the 1
kaolinite surface to enhance the hydrophobicity and reduce the electronegativity of kaolinite particle surface, and thus induce a strong hydrophobic aggregation of kaolinite particles which promotes the settlement of kaolinite. To explore the adsorption mechanism of these four amine/ammonium salts on kaolinite surfaces, the adsorptions of DDA+, MDA+, DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface are calculated with DFT (Density functional theory). The DFT calculation results indicate that different amine/ammonium cations can strongly adsorbed on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds or C‐H…O weak hydrogen bonds, and there are strongly electrostatic attractions between different amine/ammonium cations and kaolinite surfaces. The main adsorption mechanism of amine/ammonium cations on kaolinite is hydrogen‐bond interaction and electrostatic attraction. Keywords: kaolinite；hydrophobic aggregation；amine/ammonium salts；Density functional theory；(001) surfaces；adsorption mechanism
1 Introduction With the high mechanization of coal mining and the deterioration of raw coal, lots of high muddied coal slurry water is produced with high clay mineral content in coal preparation [1], and these clay minerals are mainly composed of kaolinite, montmorillonite and illite, which usually exist at less than 2 μm in aqueous solution [2]. Moreover, difficult separation of the colloidal clay minerals from recycled water in coal washing plants can cause a high‐solid concentration in the recycle water (the washing water is need to recycle in coal preparation plant) and then a detrimental effect on the follow‐up process [3,4]. Actually the kaolinite is the main component of these clay minerals in coal slurry water, and it can easily turn into ultra‐fine particles during coal preparation process and its surface electronegativity can stably disperse in coal slurry water [5], which seriously increases the difficulty of coal slurry water sedimentation. Hydrophobic aggregation is widely used in kaolinite flotation [6‐8], and it is also an effective process for kaolinite sedimentation [9,10]. Cui, et al. [11] indicated that the solution and electrical properties had an important impact on the dispersion and aggregation of 2
kaolinite particles, and one of the important premises to achieve the effective aggregation of kaolinite is to adjust its surface properties. For the hydrophilic kaolinite particles, hydrophobic aggregation could be realized through rendering surface hydrophobicity by the adsorption of surfactants on the surfaces so as to reduce hydration repulsive force and enhance hydrophobic attraction [12,13], which can promote the effective settlement of fine kaolinite particles in aqueous solution. Cationic surfactants such as alkyl amine [14] and quaternary ammonium salts [15,16] were investigated and proved to be effective surfactants for the hydrophobic aggregation of kaolinite. However, although there are lots of researches on hydrophobic aggregation behavior of kaolinite particles, few of them are about its micro mechanism of kaolinite surfactants [14‐ 17]. Therefore, it is of great significance to study the adsorption mechanism of the surfactants on kaolinite surface. Computer simulations have been widely used for the study of the surfactant adsorption on a solid surface recently [18‐21]. The quantum chemical approach is more used to understand the atomic structure of adsorbed surfactant and its electronic structure when the molecular adsorption on clay surfaces is studied, and DFT method find its extensive application in this fields recently [24‐28]. Adsorptions of four different amine/ammonium salts, DDA, MDA, DMDA and DTAC on kaolinite particles, was investigated in this paper through the experimental measurement of contact angles, zetapotentials, aggregation observation, adsorption and sedimentation. The adsorption mechanism of different amine/ammonium cations on kaolinite (001) surfaces was investigated by DFT calculation, and the adsorption energies and geometries, band populations, electron density difference and Mulliken atomic charges, are presented and discussed in the paper.
2 Materials and methods 2.1 Materials Kaolinite samples used in this study, originally collected from the Huaibei Jinyan Kaolinite Company (China), were first purified by eliminating the organic matters in a conditioning tank with 30% H2O2/dry kaolinite powder lasting for 24 h. Next the slurry was kept in a water bath 3
at 60 to remove the remnant H2O2. Then, the impurity ions in slurry were removed by washing with a large amount of deionized water. And finally it was classified with a 200 mesh screen to obtain the size fraction at ‐75 μm. The cumulative particle size distribution of the kaolinite sample, as shown in Fig. 1, was determined with a Laser particle size analyzer (SALD‐7101, Japan), the d50 (average particle size) of the kaolinite sample was 2.004 μm. The specific surface area of the kaolinite sample was 8.04 m2/g, which was obtained by using a V‐Sorb 2800P gas adsorption analyzer (China) based on the BET method. Fig. 1. Particle size distribution of the kaolinite sample. Fig. 2. XRD pattern of the kaolinite sample. The mineral components were analyzed by an X‐ray diffraction meter (XRD‐6000, Japan) with Cu Κα radiation. Figure 2 illustrates the X‐ray diffraction pattern of the sample. And there are three types of minerals in this sample, kaolinite‐1 MD, kaolinite‐1A and Dickite‐2 M1 which belongs to kaolinite family, indicating that the kaolinite sample is a high‐purity one. DDA (Dodecylamine, CH3(CH2)11NH2), MDA (N‐methyldodecylamine, CH3(CH2)11NH‐CH3), DMDA (N,N‐dimethyldodecylamine, CH3(CH2)11NH‐(CH3)2) and DTAC (Dodecyl trimethyl ammonium chloride, CH3(CH2)11NH‐(CH3)3Cl) used in this study were from Shanghai Sinopharm Chemical Reagent Co., Ltd, China. The pH value of aqueous solutions was adjusted by adding hydrochloric or sodium hydroxide solutions. All of electrolytes are analytical reagents. The water used in this study was first distilled and then passed through resin beds and a filter (0.2 μm), and the residual conductivity of the water was less than 1 μs/cm.
2.2 Experimental methods 2.2.1 Measurement of zeta potential and contact angle Zeta potentials were measured by a Zetaprobe zetameter (USA). Aqueous kaolinite suspension with 1 wt.% of solid concentration was first conditioned at 500 r/min for 10 min, while a given dosage of different amine/ammonium salt (DDA, MDA, DMDA or DTAC) was added. The pH was adjusted to the predetermined value by 0.1 mol/L NaOH or HCl. And then 4
the kaolinite suspension was immediately stirred at 750 r/min for 10 min. After that, the measurement started while the conditioning continued. The temperature throughout the measurement was kept at 25℃. Finally the suspension was poured into the sample pool to measure its zeta potential. The arithmetic average of 3 measurements was regarded as the present result. A contact angle meter (C20, America) was used to measure the contact angle of water on the kaolinite particles at 25℃. About 0.5 g dry kaolinite particles in different conditions were pressed into a thick slice of 2 mm at 30 MPa. The contact angle of surface was the average of 6 measurements on two different locations of the surface. 2.2.2 Measurement of adsorption quantity The amount of different amine/ammonium salt adsorption on kaolinite particles surface was determined by a UV‐2000 Ultraviolet Spectrophotometry in this study. At first, the absorption spectroscopies of different amine/ammonium salts at a series of given concentrations were determined to obtain the calibration curve. Next, 2 g kaolinite sample was mixed with a given concentration of different amine/ammonium salts solution into 200 mL at a given pH on an electronic blender for 10 min at 750 r/min. Then, the suspension was kept still for 3 hours in a 250 ml measuring cylinder, and the supernatant solution was put in the centrifuge running at 3000 r/min for 30 min. And then the supernatant solution was analyzed for the final reagent concentration, based on the calibration curve. The adsorption quantity of different amine/ammonium salts on the kaolinite was decided by the following expression: q
V (C0 C ) , (1) mAsp
where q is the adsorption quantity, μmoL/m2; C0 and C are the initial and supernatant concentrations, respectively, μmoL/L; V is the solution volume, L; m is the weight of the particles, g; and Asp is the specific surface area of the kaolinite sample, m2/g. 2.2.3 Sedimentation test and aggregation observation
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In sedimentation test, 2 g kaolinite sample was mixed with different amine/ammonium salt solution with a given concentration into 200 mL at a given pH on an electronic blender for 10 min at 750 r/min. Then the resultant suspension precipitated in a 200 mL cylinder. After precipitating for 5 min, the dried mass of solids was obtained after the bottom 100 mL suspension was leached. The results of the sedimentation tests were reported in terms of sedimentation efficiency (Es), which is expressed as: Es
m0 100% , (2) m
where m0 is the dried mass of solids in the bottom 100 mL suspension; m is the total mass of the sample. A HSA10 Monocular Zoom Microscope (China) was used to observe the microstructure of aggregations. Aqueous kaolinite suspension with 1.0 wt.% of solid concentration with a certain dosage of different amine/ammonium salt was immediately mixed by electronic stirrer at 750 r/min for 10 min. And the microstructure of the aggregations was observed on a glass slide by using the HSA10 Monocular Zoom Microscope when the hydrophobic aggregation became stable.
2.3 DFT calculation 2.3.1 Computational method DFT (Density functional theory) calculations of different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface were implemented in the CASTEP program (Material Studio version 8.0 software, Accerly Corporation) [29,30]. The Perdew, Burke, and Ernzerhof (PBE) [31] functional with generalized gradient approximation (GGA) has been used throughout the study, and the plane‐wave cutoff here was 400 eV. The interactions between valence electrons and the ionic core were represented with ultra‐soft pseudopotentials [32]. The atomic positions were optimized by Broyden‐Fletcher‐Goldfarb‐Shanno (BFGS) algorithm [33]. Tkatchenko‐Scheffler van der Waals correction method [34,35] was used to correct density functional calculations for the missing van der Waals interactions. The convergence criteria for geometry optimization was 2.0×10−4 nm for maximum displacement, 0.05 eV/Å for 6
maximum force, 0.1 GPa for maximum stress, 2.0×10−5 eV/atom for energy and 2.0×10−6 eV/atom for self‐consistent field tolerance [36]. Owing to layered structure kaolinite microparticles exist in a form of hexagonal plates with dominant [37], almost perfectly cleavaged (001) basal surfaces [38], the cleaved (001) plane of kaolinite is formed with two sides of a hydroxylated surface and a siloxane one (as shown in Fig. 3). The periodic supercell (2×1×1) was used with a vacuum thickness of 40 Å to prevent the interaction between the adjacent level. Al‐terminated (001) surface and Siterminated ( 00 1 ) surface of the platelet were investigated in the study. The pseudopotentials were preliminarily generated for the atoms Si 3s23p2, Al 3s23p, O 2s22p4 and H1s. The Brillouin zone was sampled using 2×2×1 Monkhorst‐Pack point grid [39] for the unit cell and the cut‐off energy was 400 eV according to a series of convergence test. The optimization of the amine/ammonium cations and water molecule was calculated in a 20×20×30 Å3 cubic box with Brillouin zone sampling restricted to Gamma point, and the optimization parameters were the same for primitive unit cell optimization. Fig. 3. The layer structure of kaolinite. The optimized results of the bulk of kaolinite are shown in Table 1. The error is within 2.00% by contrasting the results with the simulation optimization and experiment test values of other researchers, indicating that the calculation results are reasonable. And Figure 4 shows the optimized geometry for configurations of different amine/ammonium cations. Chen, et al. [40] indicated that polar head base of the different amine/ammonium cation can adsorption on kaolinite (001) surface and ( 00 1 ) surface, and the energies are more negative when amine/ammonium cations adsorption on hollow sites of kaolinite (001) surface. Therefore, according to the surface symmetry, three hollow adsorption sites of the single amine/ammonium cation on kaolinite (001) surface and ( 00 1 ) surface were examined, as shown in Figure 5. Table 1 Optimized results of the bulk of kaolinite Fig. 4. The optimized geometries of different amine/ammonium cations. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) 7
Fig. 5. The initial adsorption sites of different amine/ammonium cations on kaolinite (001) surface (a) and ( 00 1 ) surface (b), where H1, H2, and H3 in the circle denoted the sites of hollow 1, hollow 2, and hollow 3, respectively. 2.3.2 Calculation of adsorption energies The stability of adsorbate on the surface can be represented by the adsorption energy. The negative values mean that the adsorbate can be adsorbed on the surface. The adsorption energies of adsorbates on kaolinite surfaces were calculated as: Eads= Etotal–(Eadsorbate+Esurface), (3) where Eads is the adsorption energy, kJ/mol; Etotal is the energy of the kaolinite (001) surface or ( 00 1 ) surface with different amine/ammonium cation adsorbed, kJ/mol; Eadsorbate is the energies of different amine/ammonium cations in 20×20×30 Å3 cubic box, kJ/mol; and Esurface is the energy of the kaolinite (001) surface or ( 00 1 ) surface, kJ/mol.
3 Results and discussion 3.1 Contact angles, Zeta potentials and aggregation observation The wettability of the prepared kaolinite samples was investigated in the study according to the different contact angles on their surfaces. Figure 6 shows the contact angles of kaolinite samples under different conditions at pH 7.0. The contact angles of kaolinite samples increase with the increase of reagent dosage of different amine/ammonium salts, and the hydrophobic modification abilities of this four amine/ammonium salts for kaolinite fell in the order of DTAC>DDA>DMDA>MDA. Fig. 6. Contact angles of kaolinite samples in different conditions at pH 7.0. 8
pH is an important factor affecting the Zeta potential of mineral particle surface [43‐45]. Figure 7 illustrates the Zeta potential of kaolinite particles as a function of reagent concentration and pH value in aqueous solution in the absence and presence of different amine/ ammonium salts. The Zeta potentials of kaolinite particles are negative and reduce with the decrease of pH. Meanwhile, the absolute value of Zeta potential decreases with the increase of reagent dosage of different amine/ammonium salts. Figure 8 shows the microscopic images of the aggregated kaolinite particles with reagent concentration of 6×10‐4 mol/L at pH 7.0. The sizes of the aggregations fell in the order of DTAC>DDA>DMDA>MDA, which indicated that the aggregating abilities of this amine/ ammonium salts for kaolinite fell in the order of DTAC>DDA>DMDA>MDA. The sizes of the aggregations were about 0.5~1 mm, which were 250~500 times of the average particle size (see Figure 1) of kaolinite particles. These results show that these four kinds of different amine/ammonium salts can enhance the hydrophobicity and reduce the electronegativity of kaolinite particle surface, and promote the fine particles to form aggregation. This aggregation was directly proportional to the negative zeta potential of the particles, which was contrary to classic DLVO theory, but in line with the extended DLVO theory. And it closely correlated with the adsorption of different amine/ammonium salts on kaolinite surface and thus the hydrophobicity of particle surface. Accordingly, it belongs to hydrophobic aggregation. Fig. 7. Zeta potential of kaolinite particles as the function of reagent concentration (a) and pH value (b) in aqueous solution in the absence and presence of different amine/ammonium salts. Fig. 8. Microscopic images of aggregated kaolinite particles with reagent concentration of 6× 10‐4 mol/L at pH 7.0. (a: DDA; b: MDA; c: DMDA; d: DTAC).
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3.2 Adsorption and sedimentation Figure 9 shows the adsorption quantity of different amine/ammonium salts on kaolinite as a function of reagent concentration and pH value. With the increase of reagent concentration the adsorption quantity presented an upward trend, and it tended to be stable when the reagent concentration reaches 8×10‐3 mol/L. However, there was a slight increase of the adsorption quantity along with the increase of the pH value. Figure 10 shows the influence of reagent concentration and pH value on hydrophobic aggregation settlement of kaolinite. The sedimentation efficiency increased with the increase of reagent concentration then gradually tended towards balance, but decreased steadily with the pH increasing from 3 to 11. The results of adsorption and sedimentation show that these four kinds of different amine/ ammonium salts can adsorb on kaolinite particles to promote the settlement of fine kaolinite particles, and that pH has a significance influence on the adsorption and sedimentation of kaolinite with different amine/ammonium salts. Namely that amine/ ammonium salts can realize the effective hydrophobic aggregation settlement of fine kaolinite particles by adjusting its surface properties, which can provide a theoretical basis to develop new technologies of sedimentation and clarification for high muddied coal slurry water. Fig. 9. Adsorption quantity of different amine/ammonium salts on kaolinite particles as a function of reagent concentration (a) and pH value (b). Conditions: pulp density=2 g/L, stirring intensity=750 r/min, stirring time=10 min. Fig. 10. The influence of reagent concentration (a) and pH value (b) on hydrophobic aggregation settlement of kaolinite. Conditions: pulp density=2 g/L, pH=7.0, stirring intensity=750 r/min, stirring time=10 min. 3.3 DFT calculation
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The configurations of the four different amine/ammonium cations adsorption on different hollow positions of kaolinite (001) and ( 00 1 ) surfaces were optimized. Position changes and adsorption energies optimized are given in Table 2, which shows that the best adsorption sites for DDA+, MDA+, DMDA+, and DTAC+ adsorbing on kaolinite (001) surface and ( 00 1 ) surface are H3 site and H1 site, respectively. Adsorption energies corresponding to H3 site on kaolinite (001) surface are ‐104.808, ‐79.808, ‐86.538 and ‐103.846 kJ/mol, respectively. Similarly, adsorption energies corresponding to H1 site on kaolinite ( 00 1 ) surface are ‐122.692, ‐119.904, ‐118.654 and ‐104.808 kJ/mol, respectively. Table 2 Summary of the different trial structure and resulting adsorption energies for different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface Fig. 11. Optimized configurations of amine/ammonium cations adsorption on kaolinite (001) surface. (a：DDA+ /H3；b：MDA+ /H3；c：DMDA+ /H3；d：DTAC+ /H3) Fig. 12. Optimized configurations of amine/ammonium cations adsorption on kaolinite ( 00 1 ) surface. (a：DDA+ /H1；b：MDA+ /H1；c：DMDA+ /H1；d：DTAC+ /H1) According to the results in Table 2, taking the optimized adsorption configurations of DDA+，MDA+，DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface as examples to analyze bonding properties. The optimized adsorption configurations of kaolinite (001) surface and ( 00 1 ) surface are given in Figure 11 and Figure 12, respectively. (The strong hydrogen bonds of N‐H…O are displayed by the blue dash line, and the weak hydrogen bonds of C‐H…O are displayed by the black dash line). We can see, the orientation of the amine/ammonium cations with respect to the kaolinite surface were perpendicular with a slight tilt. The Mulliken bond populations of the optimized adsorption configurations of DDA+, MDA+, DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface are given in Table 3 11
which shows that only DDA+ of the four adsorption configurations of H3/(001) surface can form N‐H…O strong hydrogen bond with the kaolinite surface, and the bond is 1.615 Å in length, and the Mulliken bond population is 0.16. While the other three cations only form C‐ H…O weak hydrogen bond with the kaolinite surface, their bond lengths are from 2.00 to 3.00 Å, and thier Mulliken bond populations are between 0.0 and 0.03. Only DDA+ and MDA+ of all the four adsorption configurations of H1/( 00 1 ) surface can form N‐H…O strong hydrogen bond with the kaolinite surface, their bond lengths are from 1.8 to 2.1 Å, and their Mulliken bond populations are from 0.06 to 0.09. While the other three cations only form C‐H…O weak hydrogen bonds with the kaolinite surface, their bond lengths are from 2.00 to 3.00 Å, and their Mulliken bond populations are 0.0~0.03. Contrasted with the average adsorption energies of different amine/ammonium cations adsorbed on kaolinite (001) surface and ( 00 1 ) surface (see Table 3), the adsorption stability of these four cations on kaolinite (001) and ( 00 1 ) surface are in the order of DDA+ >DTAC+ > DMDA+ >MDA+, this result is basically
consistent with the results of experimental measurement. However, since different ammonium cations have different hydrophobic groups, which have different hydrophobic modification effect of kaolinite surfaces, leading to the difference between the calculation results with the experimental values. Table 3 The Mulliken bond populations of different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface The mechanism of different amine/ammonium cations adsorption on kaolinite surface could be clarified by the electron transfer analysis among the adjacent atoms in the cation/kaolinite adsorption systems. The electron density difference can describe the electron rearrangement between two adjacent atoms after adsorption [46], and the electron density difference Δρ was defined as following: Δρ= ρcation/kaolinite–(ρcation +ρkaolinite), (4)
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Where ρcation/kaolinite , ρcation and ρkaolinite were the electron densities of the cation/kaolinite system after adsorption, free amine/ammonium cation, kaolinite slab before adsorption, respectively. Figure 13 and Figure 14 show the electron density difference of different amine/ ammonium cations adsorption on kaolinite (001) surface and kaolinite ( 00 1 ) surface, respectively. The blue area in the two figures indicates the electron accumulation, while the yellow area indicates the electron depletion. And the greater the area of electron accumulation and depletion is, the stronger the interaction between cation and kaolinite surface in the adsorption system. As seen in Figure 13 and Figure 14，a significant charge transfer has happened between atoms of the four adsorption system and the adjacent atoms of the kaolinite (001) surface and kaolinite ( 00 1 ) surface. What’s more, electrons were transferred from the hydrogen atom of cations to the oxygen atom of kaolinite surface. On kaolinite (001) surface was no significant difference in the degree of charge transfer in the adsorption systems of MDA+, DMDA+ and DTAC+, but their charge transfer degrees mentioned above were obviously less than that of DDA+ adsorption system. While on kaolinite ( 00 1 ) surface was no significant difference in the degree of charge transfer in the DMDA+ and DTAC+ adsorption systems whose charge transfer degree are obviously less than that of DDA+ and MDA+ adsorption system. In the adsorption systems, apart from the charge transfer between the hydrogen atoms and the surface oxygen atoms of hydrogen bonds, there is a certain amount of electron accumlation around the oxygen atoms approaching the hydrogen atoms of the cations. Since the charge transfer from amine/ammonium cations to the surface of kaolinite, the kaolinite (001) surface and ( 00 1 ) surface displayed the negative charge. As the reasult of electrostatic attraction, the positively charged amine/ammonium cations adsorbed on the negatively charged kaolinite (001) surface and ( 00 1 ) surface. Fig. 13. The electron density difference of amine/ammonium cations adsorbed on kaolinite (001) surface, the isosurface value is 0.006 electrons/Å3, where blue area and yellow area denoted the electron accumulation and the electron depletion, respectively. 13
(a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) Fig. 14. The electron density difference of amine/ammonium cations adsorbed on kaolinite ( 00 1 ) surface, the isosurface value is 0.006 electrons/Å3, where blue area and yellow area
denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) The Mulliken atomic charges [47] were also calculated before and after different amine/ammonium cations adsorption on kaolinite (001) surfaces and ( 00 1 ) surface, as displayed in Table 4. The hydrogen atoms lost 0.01~0.17 e owing to its interaction with the surface oxygen atoms of the four kinds of cations. Furthermore, after the adsorption of four cations onto the kaolinite surface, the Mulliken charge populations of oxygen atoms gained 0.01~0.12 e. The lost electrons of four amine/ammonium cations mainly came from its headgroup hydrogen atoms or the methyl hydrogen atoms of adjacent surface. The amount of charge transfer from four cations to the kaolinite (001) surface is 0.39e, 0.30e, 0.37e, 0.38e, respectively. And the amount of charge transfer from four cations to the kaolinite ( 00 1 ) surface is 0.70 e, 0.70 e, 0.59 e, 0.57 e, respectively. Combined with the adsorption
energy calculation results, the more electrons transferred from cations to kaolinite surface, the more negative the adsorption energy and the more stable the adsorption system. Due to a large number of electrons transferred to the kaolinite surface, the surface displays a large negative charges (‐0.30 e~‐0.70 e) and easily adsorbs the positively charged cations by the means of electrostatic attraction. Table 4 Mulliken atomic charges before and after different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface
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The DFT calculation results show that different amine/ammonium cations can strongly adsorbed on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds and C‐H…O weak hydrogen bonds, and that there are strongly electrostatic attraction between different amine/ammonium cations and kaolinite surfaces. This indicates that the main adsorption mechanism of amine/ammonium cations on kaolinite is hydrogen‐bond interaction and electrostatic attraction. Additionally, the adsorption is stronger on the siloxane surface (kaolinite ( 00 1 ) surface) than the hydroxyl surface (kaolinite (001) surface), which agrees well with those found in the study of Geatches, et al. [48].
4 Conclusion (1) Fine kaolinite particles strongly aggregates with amine/ammonium salts, which can enhance the hydrophobicity and reduce the electronegativity of particle surfaces. This situation is in line with the extended DLVO theory, and closely correlated with the adsorption of amine/ammonium salts on particles and the hydrophobicity of particle surfaces accordingly. Therefore, it belongs to hydrophobic aggregation. The aggregating abilities of the four amine/ammonium salts for kaolinite fell in the order DTAC > DDA > DMDA > MDA. (2) Different amine/ammonium salts can adsorb on kaolinite particles, and realize the effective hydrophobic aggregation settlement of fine kaolinite particles. With the increase of reagent concentration, the adsorption quantity of amine/ammonium salts on kaolinite particles tends to increase, reaches equilibrium when the reagent concentration is about 8× 10‐3 mol/L. (3) The different amine/ammonium cations can strongly adsorbe on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds or C‐H…O weak hydrogen bonds, and there is a strongly electrostatic attraction between different amine/ammonium cations and kaolinite surfaces. Additionally, the adsorption is stronger on the siloxane surface than the hydroxyl surface. (4) The main adsorption mechanism of amine/ammonium cations on kaolinite is a hydrogen‐bond interaction and electrostatic attraction.
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Acknowledgements The financial supports for this work from the National Natural Science Foundation of China under the grant No. 51474011 and the National Natural Science Foundation of China under the grant No.51504011 are gratefully acknowledged.
REFERENCES [1] J. Chen, F. Min, L. Liu, C. Peng, F. Lu, Hydrophobic aggregation of fine particles in high muddied coal slurry water, Water Sci. Technol. 73 (2016) 501‐510. [2] F. Uddin, Clays, Nanoclays, and Montmorillonite Minerals, Mater. Trans. A. 39 (2008) 2804‐2814. [3] E. Sabah, I. Cengiz, An evaluation procedure for flocculation of coal preparation plant tailings, Water Res. 38 (2004) 1542‐1549. [4] B. Wang, Y. Peng, The behavior of mineral matter in fine coal flotation using saline water, Fuel, 109 (2013) 309‐315. [5] J. Chen, F. Min, L. Liu, C. Peng, Y. Sun, J. Du, Study on hydrophobic aggregation settlement of high muddied coal slurry water, J. China Coal Soc. 39 (2014) 2507‐2512. [6] A. Koca, H. Koca, Carrierflotation of alunite from kaolin clay, Developments in Mineral Processing.13 (2000) c11‐1‐c11‐8. [7] H. Li, J. Long, Z. Xu, J.H. Masliyah, Flocculation of kaolinite clay suspensions using a temperature‐sensitive polymer, AICHE. J. 53 (2007) 479‐488. [8] K.E. Lee, N. Morad, B.T. Poh, T.T. Teng, Comparative study on the effectiveness of hydrophobically modified cationic polyacrylamide groups in the flocculation of kaolin, Desalination. 270 (2011) 206‐213. [9] F. Min, C. Peng, S. Song, Hydration layers on clay mineral surfaces in aqueous solutions: a review, Arch. Min. Sci. 59 (2014) 373‐384. [10] M. Żbik, R.G. Horn, Hydrophobic attraction may contribute to aqueous flocculation of clays, Colloid. Surface. A. 222 (2003) 323‐328. 16
[11] J. Cui, Q. Fang, G. Huang, Crystal structures and surface properties of diaspore and kaolinite, Nonferr. Metal. 51 (1999) 25‐30. [12] S. Song, A. Lopez‐Valdivieso, J. L. Reyes‐Bahena, H.I. Bermejo‐Perez, Hydrophobic Flocculation of Galena Fines in Aqueous Suspensions, J. Colloid Interf. Sci. 227 (2000) 272‐281. [13] F. Merzel, J. C. Smith, Is the first hydration shell of lysozyme of higher density than bulk water? P. Natl. Acad. Sci. USA. 99 (2002) 5378‐5383. [14] Y. Hu, L. Liu, F. Min, M. Zhang, S. Song, Hydrophobic aggregation of colloidal kaolinite in aqueous suspensions with dodecylamine. Colloid. Surface, A. 434 (2013) 281‐286. [15] X. Zhang, Y. Hu, R. Liu, Hydrophobic aggregation of ultrafine kaolinite, J. Cent. South Univ. T. 15 (2008) 368‐372. [16] H. Jiang, Z. Sun, H. Long, Y. Hu, K. Huang, S. Zhu, A comparison study of the flotation and adsorption behaviors of diaspore and kaolinite with quaternary ammonium collectors, Miner. Eng. 65 (2014) 124‐129. [17] Y. Hu, P. Chen, W. Sun, Study on quantitative structure‐activity relationship of quaternary ammonium salt collectors for bauxite reverse flotation, Miner. Eng. 26 (2012) 24‐ 33. [18] Y. Xu, Y. Liu, D. He, G. Liu, Adsorption of cationic collectors and water on muscovite (001) surface: a molecular dynamics simulation study, Miner. Eng. 53 (2013) 101‐107. [19] L. Wang, Y. Hu, W. Sun, Y. Sun, Molecular dynamics simulation study of the interaction of mixed cationic/anionic surfactants with muscovite, Appl. Surf. Sci. 327 (2015) 364‐370. [20] S.S. Rath, H. Sahoo, B. Das, B.K. Mishra, Density functional calculations of amines on the (101) face of quartz, Miner. Eng. 69 (2014) 57‐64. [21] H.C. Greenwell, S. Stackhouse, P.V. Coveney, W. Jones, A Density Functional Theory Study of Catalytictrans‐Esterification bytert‐Butoxide MgAl Anionic Clays, J. Phys. Chem. B. 107 (2003) 3476‐3485.
17
[22] J.A. Greathouse, R.T. Cygan, Molecular dynamics simulation of uranyl(VI) adsorption equilibria onto an external montmorillonite surface, Phys. Chem. Chem. Phys. 7 (2005) 3580‐ 3586. [23] J.L. Suter, R.L. Anderson, H.C. Greenwell, P.V. Coveney, Recent advances in large‐ scale atomistic and coarse‐grained molecular dynamics simulation of clay minerals, J. Mater. Chem. 19 (2009) 2482‐2493. [24] X. Wang, Y. Huang, Z. Pan, Y. Wang, C. Liu, Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations, J. Hazard. Mater. 295 (2015) 43‐54. [25] X. Wang, P. Qian, K. Song, C. Zhang, J. Dong, The DFT study of adsorption of 2,4‐ dinitrotoluene on kaolinite surfaces, Comput. Theor. Chem. 1025 (2013) 16‐23. [26] C.I. Sainz‐Díaz, V. Timón, V. Botella, E. Artacho, A. Hernández‐Laguna, Quantum mechanical calculations of dioctahedral 2:1 phyllosilicates: Effect of octahedral cation distributions in pyrophyllite, illite, and smectite, Am. Mineral. 87 (2002) 958‐965. [27] E. Escamilla‐Roa, V. Timón, A. Hernández‐Laguna, DFT study of the adsorption of Ni on Anatase (001) surface, Comput. Theor. Chem. 981 (2012) 59‐67. [28] D.L. Geatches, S.J. Clark, H.C. Greenwell, DFT+Uinvestigation of the catalytic properties of ferruginous clay, Am. Mineral. 98 (2013) 132‐140, 2013. [29] S.J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. I. J. Probert, K. Refson, M. C. Payne, First principles methods using CASTEP, Z. Kristallogr. 220 (2005) 567‐570. [30] M.D. Segall, P. J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, First‐principles simulation: ideas, illustrations and the CASTEP code, J. Phys.: Condens. Matter. 14 (2002) 2717‐2744. [31] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865‐3868. [32] D. Vanderbilt, Soft self‐consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B. 41 (1990) 7892‐7895.
18
[33] B. Pfrommer, M. Côté, S. Louie, M. Cohen, Relaxation of crystals with the quasi‐ Newton method, J. Comput. Phys. 131 (1997) 233‐240. [34] A. Tkatchenko, M. Scheffler, Accurate molecular van der Waals interactions from ground‐state electron density and free‐atom reference data, Phys. Rev. Lett. 102 (2009) 073005‐1‐4. [35] T. Bučko, S. Lebègue, J. Hafner, J. G. Ángyán, Tkatchenko‐Scheffler van der Waals correction method with and without self‐consistent screening applied to solids, Physical Review B, 87 (2013) 064110‐1‐15. [36] Y. Han, W. Liu, J. Chen, DFT simulation of the adsorption of sodium silicate species on kaolinite surfaces, Appl. Surf. Sci. 370 (2016) 403‐409. [37] C.M.P. Vaz, P.S.P. Herrmann, S.Crestana, Thickness and size distribution of clay‐sized soil particles measured through atomic force microscopy, Powder Technol. 126 (2002) 51‐58. [38] R. Šolc, M.H. Gerzabek, H. Lischka, D. Tunega, Wettability of kaolinite (001) surfaces‐ Molecular dynamic study, Geoderma. 169 (2011) 47‐54. [39] H.J. Monkhorst, J.D Pack, Special points for Brillonin‐zone integrations, Phys. Rev. B. 13 (1976) 5188‐5192. [40] J.Chen, F. Min, L. Liu, C. Peng, DFT calculations of different amine/ammonium cations adsorption on kaolinite (001) surface, J. China Coal Soc. 41 (2016) 3115‐3121. [41] X.L. Hu, A. Michaelides, Water on the hydroxylated (001) surface of kaolinite: From monomer adsorption to a flat 2D wetting layer, Surf. Sci. 602 (2008) 960‐974. [42] D.L. Bish, Rietveld refinement of the kaolinite structure at 1.5 K, Clay. Clay. Miner. 41 (1993) 738‐744. [43] A. Syed, D. Şahinde, G. Özbayoğlu, Zeta potential measurements on three clays from Turkey and effects of clays on coalflotation, J. Colloid Interf. Sci. 184 (1996) 535‐541. [44] Y. Yukselen, A. Aksoy, A study of factors affecting on the zeta potential of kaolinite and quartz powder, Environ. Earth. Sci. 62 (2011) 697‐705. 19
[45] K.G. Satyanarayana, Clay surfaces: fundamentals and applications (Vol. 1). F. Wypych (Ed.), Academic Press, 2004. [46] C. Peng, F. Min, L. Liu, J. Chen, A periodic DFT study of adsorption of water on sodium‐montmorillonite (001) basal and (010) edge surface, Appl. Surf. Sci. 387 (2016) 308‐ 316. [47] R.S. Mulliken, Electronic population analysis on LCAO‐MO molecular wave functions, I. J. Chem. Phys. 23 (1955) 1833‐1840. [48] D.L. Geatches, A. Jacquet, S.J. Clark, H. C. Greenwell, Monomer Adsorption on Kaolinite: Modeling the Essential Ingredients, J. Phys. Chem. C. 116 (2012) 22365‐22374. Figure Caption
20
Cumulative undersize /%
100
80
d25 =0.498 m
60
d50 =2.004 m
40
d75 =12.030m
20
0
0
10
20
30
40
50
60
70
80
Particle size /m
Fig. 1. Particle size distribution of the kaolinite sample. 300 1
Intensity / (a.u.)
250
1- Al2Si2O5(OH)4, Kaolinite-1Md 2- Al2Si2O5(OH)4, Kaolinite-1A 3- Al2Si2O5(OH)4, Dickite-2M1
200
1
150
1 2
100
2
12
11
50 0
20
30
1
1 1
3
2 2
40
50
60
3
10
1
1
2
70
80
2/()
Fig. 2. XRD pattern of the kaolinite sample.
21
(a) Al‐OH surface Si‐O surface Hydrogen bond Octahedral AlO2(OH)4 Tetrahedral SiO4
(b) Silicate ring (lower surface) Aluminate ring (upper surface) Al
Si
O
H
Fig. 3. The layer structure of kaolinite.
(a)
(b)
(c)
(d)
N
C
H
Fig. 4. The optimized geometries of different amine/ammonium cations. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
22
(a)
(b)
H1 H2 H3
H2
H3
H1
Al
Si
O
H
Fig. 5. The initial adsorption sites of different amine/ammonium cations on kaolinite (001) surface (a) and ( 00 1 ) surface (b), where H1, H2, and H3 in the circle denoted the sites of hollow 1, hollow 2, and hollow 3, respectively.
Contact angle /°
52
48
44
n= 750 r/min t = 10 min pH = 7.0
40
36
0
2
4
6
DDA MDA DMDA DTAC
8
10
-4
Reagent concentration /(10 mol· L)
Fig. 6. Contact angles of kaolinite samples in different conditions at pH 7.0.
23
10
-4
C= 210 mol/L
5
0
Zeta potential /mV
Zeta potential /mV
5
-5 -10 -15
DDA MDA DMDA
pH = 7.0
DTAC
5
10
15
20
25
30
35
-5 -10 -15
DDA MDA DMDA DTAC Original kaolinite
-20 -25 -30
-20 0
0
-35
40
-4
Reagent concentration /(10 mol· L)
(a)
3
4
5
6
7
8
9
10
11
pH value
(b)
Fig. 7. Zeta potential of kaolinite particles as the function of reagent concentration (a) and pH value(b) in aqueous solution in the absence and presence of different amine/ammonium salts.
（a）
1 mm
（b）
1 mm
（c）
1 mm
（d）
1 mm
Fig. 8. Microscopic images of aggregated kaolinite particles with reagent concentration of 6× 10‐4 mol/L at pH 7.0. (a: DDA; b: MDA; c: DMDA; d: DTAC). 24
-2
Adsorption quantity /µmol· m
-2
Adsorption quantity /µmol· m
15
10
12 9
n= 750 r/min t = 10 min pH = 7.0
6 3 0
11
0
20
40
DDA MDA DMDA DTAC
60
80
100
9 8 7 6
DDA MDA DMDA DTAC
n= 750 r/min t = 10 min -3 C= 210 mol/L
5 4
3
4
5
-4
6
7
8
Reagent concentration /(10 mol· L)
pH value
(a)
(b)
9
10
11
Fig. 9. Adsorption quantity of different amine/ammonium salts on kaolinite particles as a function of reagent concentration (a) and pH value (b). Conditions: pulp density=2 g/L, stirring intensity=750 r/min, stirring time=10 min.
95
Sedimentation efficiency /%
Sedimentation efficiency /%
90 85
n= 750 r/min t = 10 min pH = 7
80 75
DDA MDA DMDA DTAC
70 65
0
2
4
6
8
-4
10
Reagent concentration /(10 mol· L)
(a)
96 DDA MDA DMDA DTAC
92 88 84
n= 750 r/min t = 10 min -4 C= 210 mol/L
80 76
2
4
6
8
10
12
pH value
(b)
Fig. 10. The influence of reagent concentration (a) and pH value (b) on hydrophobic aggregation settlement of kaolinite. Conditions: pulp density=2 g/L, pH=7.0, stirring intensity=750 r/min, stirring time=10 min.
25
(a)
(b) H2 H1 H3
H2 H3 O2
H1 O1
O2 O1 O3
O3
(c)
(d) H2 H3
H1
H2
H1
H3
O2
O2
O1
O1 O3
O3
Fig. 11. Optimized configurations of amine/ammonium cations adsorption on kaolinite (001) surface. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
(a)
(b)
H1
H3 H1 H2 O1
H3 H2
O1
O2
O2 O3
O3
(c)
(d) H1
H3 H1 H2 O1
H3 H2
O1
O2
O2 O3
O3
26
Fig. 12. Optimized configurations of amine/ammonium cations adsorption on kaolinite ( 00 1 ) surface. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
Fig. 13. The electron density difference of amine/ammonium cations adsorbed on kaolinite (001) surface, the isosurface value is 0.006 electrons / Å3, where blue area and yellow area denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
27
Fig. 14. The electron density difference of amine/ammonium cations adsorbed on kaolinite ( 00 1 ) surface, the isosurface value is 0.006 electrons / Å3, where blue area and yellow area
denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
28
Table 1 Optimized results of the bulk of kaolinite Lattice parameters /Å
Computational
Cell Angles /°
Current cell
parameter
a
b
c
α
β
γ
volume /Å3
GGA/400 eV
5.196
9.007
7.372
93.029
105.983
89.866
331.221
(GGA/700 eV)[41]
5.196
9.021
7.485
91.70
104.72
89.78
339.17
experimental test value[42]
5.153
8.942
7.391
91.926
105.046
89.797
329.910
Table 2 Summary of the different trial structure and resulting adsorption energies for different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface Adsorption configuration
DDA+
MDA+
DMDA+
DTAC+
On (001) surface
On ( 00 1 ) surface
Initial position
Final position
Eads /(kJ/mol)
Initial position
Final position
Eads /(kJ/mol)
H1
H1
‐98.077
H1
H1
‐122.692
H2
H2
‐95.192
H2
H2
‐94.038
H3
H3
‐104.808
H3
H3
‐88.461
H1
H1
‐67.308
H1
H1
‐119.904
H2
H2
‐61.538
H2
H2
‐99.712
H3
H3
‐79.808
H3
H3
‐52.885
H1
H1
‐72.501
H1
H1
‐118.654
H2
H2
‐68.632
H2
H2
‐101.346
H3
H3
‐86.538
H3
H3
‐67.308
H1
H1
‐96.273
H1
H1
‐104.808
H2
H2
‐91.346
H2
H2
‐101.923
H3
H3
‐103.846
H3
H3
‐89.808
29
Table 3 The Mulliken bond populations of different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface (001) surface / H3
( 00 1 ) surface / H1
Adsorption configuration
DDA+
MDA+
DMDA+
DTAC+
Bond
Length /Å
Bond population
Bond
Length /Å
Bond population
N‐H1∙∙∙O1
1.615
0.16
N‐H1∙∙∙O1
1.727
0.09
C‐H2∙∙∙O2
2.136
0.02
N‐H2∙∙∙O2
2.086
0.03
C‐H3∙∙∙O3
2.907
0.00
C‐H3∙∙∙O3
1.983
0.01
C‐H1∙∙∙O1
2.037
0.03
N‐H1∙∙∙O1
1.858
0.06
C‐H2∙∙∙O2
2.581
0.00
C‐H2∙∙∙O2
2.444
0.00
C‐H3∙∙∙O3
2.718
0.00
C‐H3∙∙∙O3
2.482
0.00
C‐H1∙∙∙O1
2.092
0.03
C‐H1∙∙∙O1
2.195
0.00
C‐H2∙∙∙O2
2.785
0.00
C‐H2∙∙∙O2
2.599
0.00
C‐H3∙∙∙O3
2.878
0.00
C‐H3∙∙∙O3
2.432
0.00
C‐H1∙∙∙O1
2.529
0.00
C‐H1∙∙∙O1
2.190
0.01
C‐H2∙∙∙O2
2.452
0.01
C‐H2∙∙∙O2
2.640
0.00
C‐H3∙∙∙O3
2.206
0.02
C‐H3∙∙∙O3
2.389
0.00
The atomic numbers in Table 4 are corresponding to the atomic numbers in Fig. 10 and Fig. 11; the same below.
Table 4 Mulliken atomic charges before and after different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface Mulliken charge /e Atomic number H3 / (001) surface
H1 / ( 00 1 ) surface
Adsorption states DDA+
MDA+
DMDA+
DTAC+
DDA+
MDA+
DMDA+
DTAC+
before
0.30
0.27
0.27
0.25
0.30
0.27
0.27
0.25
after
0.39
0.27
0.28
0.27
0.43
0.45
0.29
0.29
before
0.30
0.28
0.25
0.27
0.30
0.28
0.25
0.27
after
0.31
0.28
0.26
0.28
0.44
0.30
0.28
0.29
before
0.26
0.28
0.27
0.25
0.26
0.28
0.27
0.25
after
0.29
0.28
0.27
0.27
0.30
0.31
0.30
0.29
before
‐0.93
‐0.68
‐0.42
‐0.16
‐0.93
‐0.68
‐0.42
‐0.16
after
‐0.85
‐0.66
‐0.40
‐0.14
‐0.80
‐0.60
‐0.39
‐0.16
before
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
after
‐1.07
‐1.06
‐1.06
‐1.08
‐1.13
‐1.15
‐1.15
‐1.15
H1
H2
H3
N
O1
30
before
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
after
‐1.06
‐1.06
‐1.07
‐1.06
‐1.16
‐1.18
‐1.18
‐1.17
before
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
after
‐1.05
‐1.05
‐1.05
‐1.06
‐1.16
‐1.16
‐1.17
‐1.16
before
0
0
0
0
0
0
0
0
after
0.39
0.30
0.37
0.38
0.70
0.70
0.59
0.57
before
0
0
0
0
0
0
0
0
after
‐0.39
‐0.30
‐0.37
‐0.38
‐0.70
‐0.70
‐0.59
‐0.57
O2
O3
Cations
(001) surface
31
S0169-4332(17)31222-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.213 APSUSC 35894
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-1-2017 17-4-2017 22-4-2017
Please cite this article as: Jun Chen, Fan-fei Min, Lingyun Liu, Chunfu Liu, Fangqin Lu, Experimental investigation and DFT calculation of different amine/ammonium salts adsorption on kaolinite, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.213 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Experimental investigation and DFT calculation of different amine/ammonium salts adsorption on kaolinite CHEN Jun，MIN Fan‐fei*，LIU Lingyun，LIU Chunfu，LU Fangqin
Department of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China * Correspongding author: Tel: +86‐554‐6668885, Fax: +86‐554‐6668885 E‐mail address: [email protected] Graphical Abstract
Highlights
We investigate different amine/ammonium salts adsorption on kaolinite. We calculate different amine/ammonium salts adsorption on kaolinite with DFT. Different amine/ammonium salts can promote the settlement of kaolinite particles. The adsorption mechanism is hydrogen‐bond interaction and electrostatic attraction.
Abstract The adsorption of four different amine/ammonium salts of DDA (Dodecyl amine), MDA (N‐methyldodecyl amine), DMDA (N,N‐dimethyldodecyl amine) and DTAC (Dodecyl trimethyl ammonium chloride) on kaolinite particles was investigated in the study through the measurement of contact angles, zeta potentials, aggregation observation, adsorption and sedimentation. The results show that different amine/ammonium salts can adsorb on the 1
kaolinite surface to enhance the hydrophobicity and reduce the electronegativity of kaolinite particle surface, and thus induce a strong hydrophobic aggregation of kaolinite particles which promotes the settlement of kaolinite. To explore the adsorption mechanism of these four amine/ammonium salts on kaolinite surfaces, the adsorptions of DDA+, MDA+, DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface are calculated with DFT (Density functional theory). The DFT calculation results indicate that different amine/ammonium cations can strongly adsorbed on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds or C‐H…O weak hydrogen bonds, and there are strongly electrostatic attractions between different amine/ammonium cations and kaolinite surfaces. The main adsorption mechanism of amine/ammonium cations on kaolinite is hydrogen‐bond interaction and electrostatic attraction. Keywords: kaolinite；hydrophobic aggregation；amine/ammonium salts；Density functional theory；(001) surfaces；adsorption mechanism
1 Introduction With the high mechanization of coal mining and the deterioration of raw coal, lots of high muddied coal slurry water is produced with high clay mineral content in coal preparation [1], and these clay minerals are mainly composed of kaolinite, montmorillonite and illite, which usually exist at less than 2 μm in aqueous solution [2]. Moreover, difficult separation of the colloidal clay minerals from recycled water in coal washing plants can cause a high‐solid concentration in the recycle water (the washing water is need to recycle in coal preparation plant) and then a detrimental effect on the follow‐up process [3,4]. Actually the kaolinite is the main component of these clay minerals in coal slurry water, and it can easily turn into ultra‐fine particles during coal preparation process and its surface electronegativity can stably disperse in coal slurry water [5], which seriously increases the difficulty of coal slurry water sedimentation. Hydrophobic aggregation is widely used in kaolinite flotation [6‐8], and it is also an effective process for kaolinite sedimentation [9,10]. Cui, et al. [11] indicated that the solution and electrical properties had an important impact on the dispersion and aggregation of 2
kaolinite particles, and one of the important premises to achieve the effective aggregation of kaolinite is to adjust its surface properties. For the hydrophilic kaolinite particles, hydrophobic aggregation could be realized through rendering surface hydrophobicity by the adsorption of surfactants on the surfaces so as to reduce hydration repulsive force and enhance hydrophobic attraction [12,13], which can promote the effective settlement of fine kaolinite particles in aqueous solution. Cationic surfactants such as alkyl amine [14] and quaternary ammonium salts [15,16] were investigated and proved to be effective surfactants for the hydrophobic aggregation of kaolinite. However, although there are lots of researches on hydrophobic aggregation behavior of kaolinite particles, few of them are about its micro mechanism of kaolinite surfactants [14‐ 17]. Therefore, it is of great significance to study the adsorption mechanism of the surfactants on kaolinite surface. Computer simulations have been widely used for the study of the surfactant adsorption on a solid surface recently [18‐21]. The quantum chemical approach is more used to understand the atomic structure of adsorbed surfactant and its electronic structure when the molecular adsorption on clay surfaces is studied, and DFT method find its extensive application in this fields recently [24‐28]. Adsorptions of four different amine/ammonium salts, DDA, MDA, DMDA and DTAC on kaolinite particles, was investigated in this paper through the experimental measurement of contact angles, zetapotentials, aggregation observation, adsorption and sedimentation. The adsorption mechanism of different amine/ammonium cations on kaolinite (001) surfaces was investigated by DFT calculation, and the adsorption energies and geometries, band populations, electron density difference and Mulliken atomic charges, are presented and discussed in the paper.
2 Materials and methods 2.1 Materials Kaolinite samples used in this study, originally collected from the Huaibei Jinyan Kaolinite Company (China), were first purified by eliminating the organic matters in a conditioning tank with 30% H2O2/dry kaolinite powder lasting for 24 h. Next the slurry was kept in a water bath 3
at 60 to remove the remnant H2O2. Then, the impurity ions in slurry were removed by washing with a large amount of deionized water. And finally it was classified with a 200 mesh screen to obtain the size fraction at ‐75 μm. The cumulative particle size distribution of the kaolinite sample, as shown in Fig. 1, was determined with a Laser particle size analyzer (SALD‐7101, Japan), the d50 (average particle size) of the kaolinite sample was 2.004 μm. The specific surface area of the kaolinite sample was 8.04 m2/g, which was obtained by using a V‐Sorb 2800P gas adsorption analyzer (China) based on the BET method. Fig. 1. Particle size distribution of the kaolinite sample. Fig. 2. XRD pattern of the kaolinite sample. The mineral components were analyzed by an X‐ray diffraction meter (XRD‐6000, Japan) with Cu Κα radiation. Figure 2 illustrates the X‐ray diffraction pattern of the sample. And there are three types of minerals in this sample, kaolinite‐1 MD, kaolinite‐1A and Dickite‐2 M1 which belongs to kaolinite family, indicating that the kaolinite sample is a high‐purity one. DDA (Dodecylamine, CH3(CH2)11NH2), MDA (N‐methyldodecylamine, CH3(CH2)11NH‐CH3), DMDA (N,N‐dimethyldodecylamine, CH3(CH2)11NH‐(CH3)2) and DTAC (Dodecyl trimethyl ammonium chloride, CH3(CH2)11NH‐(CH3)3Cl) used in this study were from Shanghai Sinopharm Chemical Reagent Co., Ltd, China. The pH value of aqueous solutions was adjusted by adding hydrochloric or sodium hydroxide solutions. All of electrolytes are analytical reagents. The water used in this study was first distilled and then passed through resin beds and a filter (0.2 μm), and the residual conductivity of the water was less than 1 μs/cm.
2.2 Experimental methods 2.2.1 Measurement of zeta potential and contact angle Zeta potentials were measured by a Zetaprobe zetameter (USA). Aqueous kaolinite suspension with 1 wt.% of solid concentration was first conditioned at 500 r/min for 10 min, while a given dosage of different amine/ammonium salt (DDA, MDA, DMDA or DTAC) was added. The pH was adjusted to the predetermined value by 0.1 mol/L NaOH or HCl. And then 4
the kaolinite suspension was immediately stirred at 750 r/min for 10 min. After that, the measurement started while the conditioning continued. The temperature throughout the measurement was kept at 25℃. Finally the suspension was poured into the sample pool to measure its zeta potential. The arithmetic average of 3 measurements was regarded as the present result. A contact angle meter (C20, America) was used to measure the contact angle of water on the kaolinite particles at 25℃. About 0.5 g dry kaolinite particles in different conditions were pressed into a thick slice of 2 mm at 30 MPa. The contact angle of surface was the average of 6 measurements on two different locations of the surface. 2.2.2 Measurement of adsorption quantity The amount of different amine/ammonium salt adsorption on kaolinite particles surface was determined by a UV‐2000 Ultraviolet Spectrophotometry in this study. At first, the absorption spectroscopies of different amine/ammonium salts at a series of given concentrations were determined to obtain the calibration curve. Next, 2 g kaolinite sample was mixed with a given concentration of different amine/ammonium salts solution into 200 mL at a given pH on an electronic blender for 10 min at 750 r/min. Then, the suspension was kept still for 3 hours in a 250 ml measuring cylinder, and the supernatant solution was put in the centrifuge running at 3000 r/min for 30 min. And then the supernatant solution was analyzed for the final reagent concentration, based on the calibration curve. The adsorption quantity of different amine/ammonium salts on the kaolinite was decided by the following expression: q
V (C0 C ) , (1) mAsp
where q is the adsorption quantity, μmoL/m2; C0 and C are the initial and supernatant concentrations, respectively, μmoL/L; V is the solution volume, L; m is the weight of the particles, g; and Asp is the specific surface area of the kaolinite sample, m2/g. 2.2.3 Sedimentation test and aggregation observation
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In sedimentation test, 2 g kaolinite sample was mixed with different amine/ammonium salt solution with a given concentration into 200 mL at a given pH on an electronic blender for 10 min at 750 r/min. Then the resultant suspension precipitated in a 200 mL cylinder. After precipitating for 5 min, the dried mass of solids was obtained after the bottom 100 mL suspension was leached. The results of the sedimentation tests were reported in terms of sedimentation efficiency (Es), which is expressed as: Es
m0 100% , (2) m
where m0 is the dried mass of solids in the bottom 100 mL suspension; m is the total mass of the sample. A HSA10 Monocular Zoom Microscope (China) was used to observe the microstructure of aggregations. Aqueous kaolinite suspension with 1.0 wt.% of solid concentration with a certain dosage of different amine/ammonium salt was immediately mixed by electronic stirrer at 750 r/min for 10 min. And the microstructure of the aggregations was observed on a glass slide by using the HSA10 Monocular Zoom Microscope when the hydrophobic aggregation became stable.
2.3 DFT calculation 2.3.1 Computational method DFT (Density functional theory) calculations of different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface were implemented in the CASTEP program (Material Studio version 8.0 software, Accerly Corporation) [29,30]. The Perdew, Burke, and Ernzerhof (PBE) [31] functional with generalized gradient approximation (GGA) has been used throughout the study, and the plane‐wave cutoff here was 400 eV. The interactions between valence electrons and the ionic core were represented with ultra‐soft pseudopotentials [32]. The atomic positions were optimized by Broyden‐Fletcher‐Goldfarb‐Shanno (BFGS) algorithm [33]. Tkatchenko‐Scheffler van der Waals correction method [34,35] was used to correct density functional calculations for the missing van der Waals interactions. The convergence criteria for geometry optimization was 2.0×10−4 nm for maximum displacement, 0.05 eV/Å for 6
maximum force, 0.1 GPa for maximum stress, 2.0×10−5 eV/atom for energy and 2.0×10−6 eV/atom for self‐consistent field tolerance [36]. Owing to layered structure kaolinite microparticles exist in a form of hexagonal plates with dominant [37], almost perfectly cleavaged (001) basal surfaces [38], the cleaved (001) plane of kaolinite is formed with two sides of a hydroxylated surface and a siloxane one (as shown in Fig. 3). The periodic supercell (2×1×1) was used with a vacuum thickness of 40 Å to prevent the interaction between the adjacent level. Al‐terminated (001) surface and Siterminated ( 00 1 ) surface of the platelet were investigated in the study. The pseudopotentials were preliminarily generated for the atoms Si 3s23p2, Al 3s23p, O 2s22p4 and H1s. The Brillouin zone was sampled using 2×2×1 Monkhorst‐Pack point grid [39] for the unit cell and the cut‐off energy was 400 eV according to a series of convergence test. The optimization of the amine/ammonium cations and water molecule was calculated in a 20×20×30 Å3 cubic box with Brillouin zone sampling restricted to Gamma point, and the optimization parameters were the same for primitive unit cell optimization. Fig. 3. The layer structure of kaolinite. The optimized results of the bulk of kaolinite are shown in Table 1. The error is within 2.00% by contrasting the results with the simulation optimization and experiment test values of other researchers, indicating that the calculation results are reasonable. And Figure 4 shows the optimized geometry for configurations of different amine/ammonium cations. Chen, et al. [40] indicated that polar head base of the different amine/ammonium cation can adsorption on kaolinite (001) surface and ( 00 1 ) surface, and the energies are more negative when amine/ammonium cations adsorption on hollow sites of kaolinite (001) surface. Therefore, according to the surface symmetry, three hollow adsorption sites of the single amine/ammonium cation on kaolinite (001) surface and ( 00 1 ) surface were examined, as shown in Figure 5. Table 1 Optimized results of the bulk of kaolinite Fig. 4. The optimized geometries of different amine/ammonium cations. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) 7
Fig. 5. The initial adsorption sites of different amine/ammonium cations on kaolinite (001) surface (a) and ( 00 1 ) surface (b), where H1, H2, and H3 in the circle denoted the sites of hollow 1, hollow 2, and hollow 3, respectively. 2.3.2 Calculation of adsorption energies The stability of adsorbate on the surface can be represented by the adsorption energy. The negative values mean that the adsorbate can be adsorbed on the surface. The adsorption energies of adsorbates on kaolinite surfaces were calculated as: Eads= Etotal–(Eadsorbate+Esurface), (3) where Eads is the adsorption energy, kJ/mol; Etotal is the energy of the kaolinite (001) surface or ( 00 1 ) surface with different amine/ammonium cation adsorbed, kJ/mol; Eadsorbate is the energies of different amine/ammonium cations in 20×20×30 Å3 cubic box, kJ/mol; and Esurface is the energy of the kaolinite (001) surface or ( 00 1 ) surface, kJ/mol.
3 Results and discussion 3.1 Contact angles, Zeta potentials and aggregation observation The wettability of the prepared kaolinite samples was investigated in the study according to the different contact angles on their surfaces. Figure 6 shows the contact angles of kaolinite samples under different conditions at pH 7.0. The contact angles of kaolinite samples increase with the increase of reagent dosage of different amine/ammonium salts, and the hydrophobic modification abilities of this four amine/ammonium salts for kaolinite fell in the order of DTAC>DDA>DMDA>MDA. Fig. 6. Contact angles of kaolinite samples in different conditions at pH 7.0. 8
pH is an important factor affecting the Zeta potential of mineral particle surface [43‐45]. Figure 7 illustrates the Zeta potential of kaolinite particles as a function of reagent concentration and pH value in aqueous solution in the absence and presence of different amine/ ammonium salts. The Zeta potentials of kaolinite particles are negative and reduce with the decrease of pH. Meanwhile, the absolute value of Zeta potential decreases with the increase of reagent dosage of different amine/ammonium salts. Figure 8 shows the microscopic images of the aggregated kaolinite particles with reagent concentration of 6×10‐4 mol/L at pH 7.0. The sizes of the aggregations fell in the order of DTAC>DDA>DMDA>MDA, which indicated that the aggregating abilities of this amine/ ammonium salts for kaolinite fell in the order of DTAC>DDA>DMDA>MDA. The sizes of the aggregations were about 0.5~1 mm, which were 250~500 times of the average particle size (see Figure 1) of kaolinite particles. These results show that these four kinds of different amine/ammonium salts can enhance the hydrophobicity and reduce the electronegativity of kaolinite particle surface, and promote the fine particles to form aggregation. This aggregation was directly proportional to the negative zeta potential of the particles, which was contrary to classic DLVO theory, but in line with the extended DLVO theory. And it closely correlated with the adsorption of different amine/ammonium salts on kaolinite surface and thus the hydrophobicity of particle surface. Accordingly, it belongs to hydrophobic aggregation. Fig. 7. Zeta potential of kaolinite particles as the function of reagent concentration (a) and pH value (b) in aqueous solution in the absence and presence of different amine/ammonium salts. Fig. 8. Microscopic images of aggregated kaolinite particles with reagent concentration of 6× 10‐4 mol/L at pH 7.0. (a: DDA; b: MDA; c: DMDA; d: DTAC).
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3.2 Adsorption and sedimentation Figure 9 shows the adsorption quantity of different amine/ammonium salts on kaolinite as a function of reagent concentration and pH value. With the increase of reagent concentration the adsorption quantity presented an upward trend, and it tended to be stable when the reagent concentration reaches 8×10‐3 mol/L. However, there was a slight increase of the adsorption quantity along with the increase of the pH value. Figure 10 shows the influence of reagent concentration and pH value on hydrophobic aggregation settlement of kaolinite. The sedimentation efficiency increased with the increase of reagent concentration then gradually tended towards balance, but decreased steadily with the pH increasing from 3 to 11. The results of adsorption and sedimentation show that these four kinds of different amine/ ammonium salts can adsorb on kaolinite particles to promote the settlement of fine kaolinite particles, and that pH has a significance influence on the adsorption and sedimentation of kaolinite with different amine/ammonium salts. Namely that amine/ ammonium salts can realize the effective hydrophobic aggregation settlement of fine kaolinite particles by adjusting its surface properties, which can provide a theoretical basis to develop new technologies of sedimentation and clarification for high muddied coal slurry water. Fig. 9. Adsorption quantity of different amine/ammonium salts on kaolinite particles as a function of reagent concentration (a) and pH value (b). Conditions: pulp density=2 g/L, stirring intensity=750 r/min, stirring time=10 min. Fig. 10. The influence of reagent concentration (a) and pH value (b) on hydrophobic aggregation settlement of kaolinite. Conditions: pulp density=2 g/L, pH=7.0, stirring intensity=750 r/min, stirring time=10 min. 3.3 DFT calculation
10
The configurations of the four different amine/ammonium cations adsorption on different hollow positions of kaolinite (001) and ( 00 1 ) surfaces were optimized. Position changes and adsorption energies optimized are given in Table 2, which shows that the best adsorption sites for DDA+, MDA+, DMDA+, and DTAC+ adsorbing on kaolinite (001) surface and ( 00 1 ) surface are H3 site and H1 site, respectively. Adsorption energies corresponding to H3 site on kaolinite (001) surface are ‐104.808, ‐79.808, ‐86.538 and ‐103.846 kJ/mol, respectively. Similarly, adsorption energies corresponding to H1 site on kaolinite ( 00 1 ) surface are ‐122.692, ‐119.904, ‐118.654 and ‐104.808 kJ/mol, respectively. Table 2 Summary of the different trial structure and resulting adsorption energies for different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface Fig. 11. Optimized configurations of amine/ammonium cations adsorption on kaolinite (001) surface. (a：DDA+ /H3；b：MDA+ /H3；c：DMDA+ /H3；d：DTAC+ /H3) Fig. 12. Optimized configurations of amine/ammonium cations adsorption on kaolinite ( 00 1 ) surface. (a：DDA+ /H1；b：MDA+ /H1；c：DMDA+ /H1；d：DTAC+ /H1) According to the results in Table 2, taking the optimized adsorption configurations of DDA+，MDA+，DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface as examples to analyze bonding properties. The optimized adsorption configurations of kaolinite (001) surface and ( 00 1 ) surface are given in Figure 11 and Figure 12, respectively. (The strong hydrogen bonds of N‐H…O are displayed by the blue dash line, and the weak hydrogen bonds of C‐H…O are displayed by the black dash line). We can see, the orientation of the amine/ammonium cations with respect to the kaolinite surface were perpendicular with a slight tilt. The Mulliken bond populations of the optimized adsorption configurations of DDA+, MDA+, DMDA+ and DTAC+ on kaolinite (001) surface and ( 00 1 ) surface are given in Table 3 11
which shows that only DDA+ of the four adsorption configurations of H3/(001) surface can form N‐H…O strong hydrogen bond with the kaolinite surface, and the bond is 1.615 Å in length, and the Mulliken bond population is 0.16. While the other three cations only form C‐ H…O weak hydrogen bond with the kaolinite surface, their bond lengths are from 2.00 to 3.00 Å, and thier Mulliken bond populations are between 0.0 and 0.03. Only DDA+ and MDA+ of all the four adsorption configurations of H1/( 00 1 ) surface can form N‐H…O strong hydrogen bond with the kaolinite surface, their bond lengths are from 1.8 to 2.1 Å, and their Mulliken bond populations are from 0.06 to 0.09. While the other three cations only form C‐H…O weak hydrogen bonds with the kaolinite surface, their bond lengths are from 2.00 to 3.00 Å, and their Mulliken bond populations are 0.0~0.03. Contrasted with the average adsorption energies of different amine/ammonium cations adsorbed on kaolinite (001) surface and ( 00 1 ) surface (see Table 3), the adsorption stability of these four cations on kaolinite (001) and ( 00 1 ) surface are in the order of DDA+ >DTAC+ > DMDA+ >MDA+, this result is basically
consistent with the results of experimental measurement. However, since different ammonium cations have different hydrophobic groups, which have different hydrophobic modification effect of kaolinite surfaces, leading to the difference between the calculation results with the experimental values. Table 3 The Mulliken bond populations of different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface The mechanism of different amine/ammonium cations adsorption on kaolinite surface could be clarified by the electron transfer analysis among the adjacent atoms in the cation/kaolinite adsorption systems. The electron density difference can describe the electron rearrangement between two adjacent atoms after adsorption [46], and the electron density difference Δρ was defined as following: Δρ= ρcation/kaolinite–(ρcation +ρkaolinite), (4)
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Where ρcation/kaolinite , ρcation and ρkaolinite were the electron densities of the cation/kaolinite system after adsorption, free amine/ammonium cation, kaolinite slab before adsorption, respectively. Figure 13 and Figure 14 show the electron density difference of different amine/ ammonium cations adsorption on kaolinite (001) surface and kaolinite ( 00 1 ) surface, respectively. The blue area in the two figures indicates the electron accumulation, while the yellow area indicates the electron depletion. And the greater the area of electron accumulation and depletion is, the stronger the interaction between cation and kaolinite surface in the adsorption system. As seen in Figure 13 and Figure 14，a significant charge transfer has happened between atoms of the four adsorption system and the adjacent atoms of the kaolinite (001) surface and kaolinite ( 00 1 ) surface. What’s more, electrons were transferred from the hydrogen atom of cations to the oxygen atom of kaolinite surface. On kaolinite (001) surface was no significant difference in the degree of charge transfer in the adsorption systems of MDA+, DMDA+ and DTAC+, but their charge transfer degrees mentioned above were obviously less than that of DDA+ adsorption system. While on kaolinite ( 00 1 ) surface was no significant difference in the degree of charge transfer in the DMDA+ and DTAC+ adsorption systems whose charge transfer degree are obviously less than that of DDA+ and MDA+ adsorption system. In the adsorption systems, apart from the charge transfer between the hydrogen atoms and the surface oxygen atoms of hydrogen bonds, there is a certain amount of electron accumlation around the oxygen atoms approaching the hydrogen atoms of the cations. Since the charge transfer from amine/ammonium cations to the surface of kaolinite, the kaolinite (001) surface and ( 00 1 ) surface displayed the negative charge. As the reasult of electrostatic attraction, the positively charged amine/ammonium cations adsorbed on the negatively charged kaolinite (001) surface and ( 00 1 ) surface. Fig. 13. The electron density difference of amine/ammonium cations adsorbed on kaolinite (001) surface, the isosurface value is 0.006 electrons/Å3, where blue area and yellow area denoted the electron accumulation and the electron depletion, respectively. 13
(a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) Fig. 14. The electron density difference of amine/ammonium cations adsorbed on kaolinite ( 00 1 ) surface, the isosurface value is 0.006 electrons/Å3, where blue area and yellow area
denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+) The Mulliken atomic charges [47] were also calculated before and after different amine/ammonium cations adsorption on kaolinite (001) surfaces and ( 00 1 ) surface, as displayed in Table 4. The hydrogen atoms lost 0.01~0.17 e owing to its interaction with the surface oxygen atoms of the four kinds of cations. Furthermore, after the adsorption of four cations onto the kaolinite surface, the Mulliken charge populations of oxygen atoms gained 0.01~0.12 e. The lost electrons of four amine/ammonium cations mainly came from its headgroup hydrogen atoms or the methyl hydrogen atoms of adjacent surface. The amount of charge transfer from four cations to the kaolinite (001) surface is 0.39e, 0.30e, 0.37e, 0.38e, respectively. And the amount of charge transfer from four cations to the kaolinite ( 00 1 ) surface is 0.70 e, 0.70 e, 0.59 e, 0.57 e, respectively. Combined with the adsorption
energy calculation results, the more electrons transferred from cations to kaolinite surface, the more negative the adsorption energy and the more stable the adsorption system. Due to a large number of electrons transferred to the kaolinite surface, the surface displays a large negative charges (‐0.30 e~‐0.70 e) and easily adsorbs the positively charged cations by the means of electrostatic attraction. Table 4 Mulliken atomic charges before and after different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface
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The DFT calculation results show that different amine/ammonium cations can strongly adsorbed on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds and C‐H…O weak hydrogen bonds, and that there are strongly electrostatic attraction between different amine/ammonium cations and kaolinite surfaces. This indicates that the main adsorption mechanism of amine/ammonium cations on kaolinite is hydrogen‐bond interaction and electrostatic attraction. Additionally, the adsorption is stronger on the siloxane surface (kaolinite ( 00 1 ) surface) than the hydroxyl surface (kaolinite (001) surface), which agrees well with those found in the study of Geatches, et al. [48].
4 Conclusion (1) Fine kaolinite particles strongly aggregates with amine/ammonium salts, which can enhance the hydrophobicity and reduce the electronegativity of particle surfaces. This situation is in line with the extended DLVO theory, and closely correlated with the adsorption of amine/ammonium salts on particles and the hydrophobicity of particle surfaces accordingly. Therefore, it belongs to hydrophobic aggregation. The aggregating abilities of the four amine/ammonium salts for kaolinite fell in the order DTAC > DDA > DMDA > MDA. (2) Different amine/ammonium salts can adsorb on kaolinite particles, and realize the effective hydrophobic aggregation settlement of fine kaolinite particles. With the increase of reagent concentration, the adsorption quantity of amine/ammonium salts on kaolinite particles tends to increase, reaches equilibrium when the reagent concentration is about 8× 10‐3 mol/L. (3) The different amine/ammonium cations can strongly adsorbe on kaolinite (001) surface and ( 00 1 ) surface by forming N‐H…O strong hydrogen bonds or C‐H…O weak hydrogen bonds, and there is a strongly electrostatic attraction between different amine/ammonium cations and kaolinite surfaces. Additionally, the adsorption is stronger on the siloxane surface than the hydroxyl surface. (4) The main adsorption mechanism of amine/ammonium cations on kaolinite is a hydrogen‐bond interaction and electrostatic attraction.
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Acknowledgements The financial supports for this work from the National Natural Science Foundation of China under the grant No. 51474011 and the National Natural Science Foundation of China under the grant No.51504011 are gratefully acknowledged.
REFERENCES [1] J. Chen, F. Min, L. Liu, C. Peng, F. Lu, Hydrophobic aggregation of fine particles in high muddied coal slurry water, Water Sci. Technol. 73 (2016) 501‐510. [2] F. Uddin, Clays, Nanoclays, and Montmorillonite Minerals, Mater. Trans. A. 39 (2008) 2804‐2814. [3] E. Sabah, I. Cengiz, An evaluation procedure for flocculation of coal preparation plant tailings, Water Res. 38 (2004) 1542‐1549. [4] B. Wang, Y. Peng, The behavior of mineral matter in fine coal flotation using saline water, Fuel, 109 (2013) 309‐315. [5] J. Chen, F. Min, L. Liu, C. Peng, Y. Sun, J. Du, Study on hydrophobic aggregation settlement of high muddied coal slurry water, J. China Coal Soc. 39 (2014) 2507‐2512. [6] A. Koca, H. Koca, Carrierflotation of alunite from kaolin clay, Developments in Mineral Processing.13 (2000) c11‐1‐c11‐8. [7] H. Li, J. Long, Z. Xu, J.H. Masliyah, Flocculation of kaolinite clay suspensions using a temperature‐sensitive polymer, AICHE. J. 53 (2007) 479‐488. [8] K.E. Lee, N. Morad, B.T. Poh, T.T. Teng, Comparative study on the effectiveness of hydrophobically modified cationic polyacrylamide groups in the flocculation of kaolin, Desalination. 270 (2011) 206‐213. [9] F. Min, C. Peng, S. Song, Hydration layers on clay mineral surfaces in aqueous solutions: a review, Arch. Min. Sci. 59 (2014) 373‐384. [10] M. Żbik, R.G. Horn, Hydrophobic attraction may contribute to aqueous flocculation of clays, Colloid. Surface. A. 222 (2003) 323‐328. 16
[11] J. Cui, Q. Fang, G. Huang, Crystal structures and surface properties of diaspore and kaolinite, Nonferr. Metal. 51 (1999) 25‐30. [12] S. Song, A. Lopez‐Valdivieso, J. L. Reyes‐Bahena, H.I. Bermejo‐Perez, Hydrophobic Flocculation of Galena Fines in Aqueous Suspensions, J. Colloid Interf. Sci. 227 (2000) 272‐281. [13] F. Merzel, J. C. Smith, Is the first hydration shell of lysozyme of higher density than bulk water? P. Natl. Acad. Sci. USA. 99 (2002) 5378‐5383. [14] Y. Hu, L. Liu, F. Min, M. Zhang, S. Song, Hydrophobic aggregation of colloidal kaolinite in aqueous suspensions with dodecylamine. Colloid. Surface, A. 434 (2013) 281‐286. [15] X. Zhang, Y. Hu, R. Liu, Hydrophobic aggregation of ultrafine kaolinite, J. Cent. South Univ. T. 15 (2008) 368‐372. [16] H. Jiang, Z. Sun, H. Long, Y. Hu, K. Huang, S. Zhu, A comparison study of the flotation and adsorption behaviors of diaspore and kaolinite with quaternary ammonium collectors, Miner. Eng. 65 (2014) 124‐129. [17] Y. Hu, P. Chen, W. Sun, Study on quantitative structure‐activity relationship of quaternary ammonium salt collectors for bauxite reverse flotation, Miner. Eng. 26 (2012) 24‐ 33. [18] Y. Xu, Y. Liu, D. He, G. Liu, Adsorption of cationic collectors and water on muscovite (001) surface: a molecular dynamics simulation study, Miner. Eng. 53 (2013) 101‐107. [19] L. Wang, Y. Hu, W. Sun, Y. Sun, Molecular dynamics simulation study of the interaction of mixed cationic/anionic surfactants with muscovite, Appl. Surf. Sci. 327 (2015) 364‐370. [20] S.S. Rath, H. Sahoo, B. Das, B.K. Mishra, Density functional calculations of amines on the (101) face of quartz, Miner. Eng. 69 (2014) 57‐64. [21] H.C. Greenwell, S. Stackhouse, P.V. Coveney, W. Jones, A Density Functional Theory Study of Catalytictrans‐Esterification bytert‐Butoxide MgAl Anionic Clays, J. Phys. Chem. B. 107 (2003) 3476‐3485.
17
[22] J.A. Greathouse, R.T. Cygan, Molecular dynamics simulation of uranyl(VI) adsorption equilibria onto an external montmorillonite surface, Phys. Chem. Chem. Phys. 7 (2005) 3580‐ 3586. [23] J.L. Suter, R.L. Anderson, H.C. Greenwell, P.V. Coveney, Recent advances in large‐ scale atomistic and coarse‐grained molecular dynamics simulation of clay minerals, J. Mater. Chem. 19 (2009) 2482‐2493. [24] X. Wang, Y. Huang, Z. Pan, Y. Wang, C. Liu, Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations, J. Hazard. Mater. 295 (2015) 43‐54. [25] X. Wang, P. Qian, K. Song, C. Zhang, J. Dong, The DFT study of adsorption of 2,4‐ dinitrotoluene on kaolinite surfaces, Comput. Theor. Chem. 1025 (2013) 16‐23. [26] C.I. Sainz‐Díaz, V. Timón, V. Botella, E. Artacho, A. Hernández‐Laguna, Quantum mechanical calculations of dioctahedral 2:1 phyllosilicates: Effect of octahedral cation distributions in pyrophyllite, illite, and smectite, Am. Mineral. 87 (2002) 958‐965. [27] E. Escamilla‐Roa, V. Timón, A. Hernández‐Laguna, DFT study of the adsorption of Ni on Anatase (001) surface, Comput. Theor. Chem. 981 (2012) 59‐67. [28] D.L. Geatches, S.J. Clark, H.C. Greenwell, DFT+Uinvestigation of the catalytic properties of ferruginous clay, Am. Mineral. 98 (2013) 132‐140, 2013. [29] S.J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. I. J. Probert, K. Refson, M. C. Payne, First principles methods using CASTEP, Z. Kristallogr. 220 (2005) 567‐570. [30] M.D. Segall, P. J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M.C. Payne, First‐principles simulation: ideas, illustrations and the CASTEP code, J. Phys.: Condens. Matter. 14 (2002) 2717‐2744. [31] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865‐3868. [32] D. Vanderbilt, Soft self‐consistent pseudopotentials in a generalized eigenvalue formalism, Phys. Rev. B. 41 (1990) 7892‐7895.
18
[33] B. Pfrommer, M. Côté, S. Louie, M. Cohen, Relaxation of crystals with the quasi‐ Newton method, J. Comput. Phys. 131 (1997) 233‐240. [34] A. Tkatchenko, M. Scheffler, Accurate molecular van der Waals interactions from ground‐state electron density and free‐atom reference data, Phys. Rev. Lett. 102 (2009) 073005‐1‐4. [35] T. Bučko, S. Lebègue, J. Hafner, J. G. Ángyán, Tkatchenko‐Scheffler van der Waals correction method with and without self‐consistent screening applied to solids, Physical Review B, 87 (2013) 064110‐1‐15. [36] Y. Han, W. Liu, J. Chen, DFT simulation of the adsorption of sodium silicate species on kaolinite surfaces, Appl. Surf. Sci. 370 (2016) 403‐409. [37] C.M.P. Vaz, P.S.P. Herrmann, S.Crestana, Thickness and size distribution of clay‐sized soil particles measured through atomic force microscopy, Powder Technol. 126 (2002) 51‐58. [38] R. Šolc, M.H. Gerzabek, H. Lischka, D. Tunega, Wettability of kaolinite (001) surfaces‐ Molecular dynamic study, Geoderma. 169 (2011) 47‐54. [39] H.J. Monkhorst, J.D Pack, Special points for Brillonin‐zone integrations, Phys. Rev. B. 13 (1976) 5188‐5192. [40] J.Chen, F. Min, L. Liu, C. Peng, DFT calculations of different amine/ammonium cations adsorption on kaolinite (001) surface, J. China Coal Soc. 41 (2016) 3115‐3121. [41] X.L. Hu, A. Michaelides, Water on the hydroxylated (001) surface of kaolinite: From monomer adsorption to a flat 2D wetting layer, Surf. Sci. 602 (2008) 960‐974. [42] D.L. Bish, Rietveld refinement of the kaolinite structure at 1.5 K, Clay. Clay. Miner. 41 (1993) 738‐744. [43] A. Syed, D. Şahinde, G. Özbayoğlu, Zeta potential measurements on three clays from Turkey and effects of clays on coalflotation, J. Colloid Interf. Sci. 184 (1996) 535‐541. [44] Y. Yukselen, A. Aksoy, A study of factors affecting on the zeta potential of kaolinite and quartz powder, Environ. Earth. Sci. 62 (2011) 697‐705. 19
[45] K.G. Satyanarayana, Clay surfaces: fundamentals and applications (Vol. 1). F. Wypych (Ed.), Academic Press, 2004. [46] C. Peng, F. Min, L. Liu, J. Chen, A periodic DFT study of adsorption of water on sodium‐montmorillonite (001) basal and (010) edge surface, Appl. Surf. Sci. 387 (2016) 308‐ 316. [47] R.S. Mulliken, Electronic population analysis on LCAO‐MO molecular wave functions, I. J. Chem. Phys. 23 (1955) 1833‐1840. [48] D.L. Geatches, A. Jacquet, S.J. Clark, H. C. Greenwell, Monomer Adsorption on Kaolinite: Modeling the Essential Ingredients, J. Phys. Chem. C. 116 (2012) 22365‐22374. Figure Caption
20
Cumulative undersize /%
100
80
d25 =0.498 m
60
d50 =2.004 m
40
d75 =12.030m
20
0
0
10
20
30
40
50
60
70
80
Particle size /m
Fig. 1. Particle size distribution of the kaolinite sample. 300 1
Intensity / (a.u.)
250
1- Al2Si2O5(OH)4, Kaolinite-1Md 2- Al2Si2O5(OH)4, Kaolinite-1A 3- Al2Si2O5(OH)4, Dickite-2M1
200
1
150
1 2
100
2
12
11
50 0
20
30
1
1 1
3
2 2
40
50
60
3
10
1
1
2
70
80
2/()
Fig. 2. XRD pattern of the kaolinite sample.
21
(a) Al‐OH surface Si‐O surface Hydrogen bond Octahedral AlO2(OH)4 Tetrahedral SiO4
(b) Silicate ring (lower surface) Aluminate ring (upper surface) Al
Si
O
H
Fig. 3. The layer structure of kaolinite.
(a)
(b)
(c)
(d)
N
C
H
Fig. 4. The optimized geometries of different amine/ammonium cations. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
22
(a)
(b)
H1 H2 H3
H2
H3
H1
Al
Si
O
H
Fig. 5. The initial adsorption sites of different amine/ammonium cations on kaolinite (001) surface (a) and ( 00 1 ) surface (b), where H1, H2, and H3 in the circle denoted the sites of hollow 1, hollow 2, and hollow 3, respectively.
Contact angle /°
52
48
44
n= 750 r/min t = 10 min pH = 7.0
40
36
0
2
4
6
DDA MDA DMDA DTAC
8
10
-4
Reagent concentration /(10 mol· L)
Fig. 6. Contact angles of kaolinite samples in different conditions at pH 7.0.
23
10
-4
C= 210 mol/L
5
0
Zeta potential /mV
Zeta potential /mV
5
-5 -10 -15
DDA MDA DMDA
pH = 7.0
DTAC
5
10
15
20
25
30
35
-5 -10 -15
DDA MDA DMDA DTAC Original kaolinite
-20 -25 -30
-20 0
0
-35
40
-4
Reagent concentration /(10 mol· L)
(a)
3
4
5
6
7
8
9
10
11
pH value
(b)
Fig. 7. Zeta potential of kaolinite particles as the function of reagent concentration (a) and pH value(b) in aqueous solution in the absence and presence of different amine/ammonium salts.
（a）
1 mm
（b）
1 mm
（c）
1 mm
（d）
1 mm
Fig. 8. Microscopic images of aggregated kaolinite particles with reagent concentration of 6× 10‐4 mol/L at pH 7.0. (a: DDA; b: MDA; c: DMDA; d: DTAC). 24
-2
Adsorption quantity /µmol· m
-2
Adsorption quantity /µmol· m
15
10
12 9
n= 750 r/min t = 10 min pH = 7.0
6 3 0
11
0
20
40
DDA MDA DMDA DTAC
60
80
100
9 8 7 6
DDA MDA DMDA DTAC
n= 750 r/min t = 10 min -3 C= 210 mol/L
5 4
3
4
5
-4
6
7
8
Reagent concentration /(10 mol· L)
pH value
(a)
(b)
9
10
11
Fig. 9. Adsorption quantity of different amine/ammonium salts on kaolinite particles as a function of reagent concentration (a) and pH value (b). Conditions: pulp density=2 g/L, stirring intensity=750 r/min, stirring time=10 min.
95
Sedimentation efficiency /%
Sedimentation efficiency /%
90 85
n= 750 r/min t = 10 min pH = 7
80 75
DDA MDA DMDA DTAC
70 65
0
2
4
6
8
-4
10
Reagent concentration /(10 mol· L)
(a)
96 DDA MDA DMDA DTAC
92 88 84
n= 750 r/min t = 10 min -4 C= 210 mol/L
80 76
2
4
6
8
10
12
pH value
(b)
Fig. 10. The influence of reagent concentration (a) and pH value (b) on hydrophobic aggregation settlement of kaolinite. Conditions: pulp density=2 g/L, pH=7.0, stirring intensity=750 r/min, stirring time=10 min.
25
(a)
(b) H2 H1 H3
H2 H3 O2
H1 O1
O2 O1 O3
O3
(c)
(d) H2 H3
H1
H2
H1
H3
O2
O2
O1
O1 O3
O3
Fig. 11. Optimized configurations of amine/ammonium cations adsorption on kaolinite (001) surface. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
(a)
(b)
H1
H3 H1 H2 O1
H3 H2
O1
O2
O2 O3
O3
(c)
(d) H1
H3 H1 H2 O1
H3 H2
O1
O2
O2 O3
O3
26
Fig. 12. Optimized configurations of amine/ammonium cations adsorption on kaolinite ( 00 1 ) surface. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
Fig. 13. The electron density difference of amine/ammonium cations adsorbed on kaolinite (001) surface, the isosurface value is 0.006 electrons / Å3, where blue area and yellow area denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
27
Fig. 14. The electron density difference of amine/ammonium cations adsorbed on kaolinite ( 00 1 ) surface, the isosurface value is 0.006 electrons / Å3, where blue area and yellow area
denoted the electron accumulation and the electron depletion, respectively. (a：DDA+；b：MDA+；c：DMDA+；d：DTAC+)
28
Table 1 Optimized results of the bulk of kaolinite Lattice parameters /Å
Computational
Cell Angles /°
Current cell
parameter
a
b
c
α
β
γ
volume /Å3
GGA/400 eV
5.196
9.007
7.372
93.029
105.983
89.866
331.221
(GGA/700 eV)[41]
5.196
9.021
7.485
91.70
104.72
89.78
339.17
experimental test value[42]
5.153
8.942
7.391
91.926
105.046
89.797
329.910
Table 2 Summary of the different trial structure and resulting adsorption energies for different amine/ammonium cations on kaolinite (001) surface and ( 00 1 ) surface Adsorption configuration
DDA+
MDA+
DMDA+
DTAC+
On (001) surface
On ( 00 1 ) surface
Initial position
Final position
Eads /(kJ/mol)
Initial position
Final position
Eads /(kJ/mol)
H1
H1
‐98.077
H1
H1
‐122.692
H2
H2
‐95.192
H2
H2
‐94.038
H3
H3
‐104.808
H3
H3
‐88.461
H1
H1
‐67.308
H1
H1
‐119.904
H2
H2
‐61.538
H2
H2
‐99.712
H3
H3
‐79.808
H3
H3
‐52.885
H1
H1
‐72.501
H1
H1
‐118.654
H2
H2
‐68.632
H2
H2
‐101.346
H3
H3
‐86.538
H3
H3
‐67.308
H1
H1
‐96.273
H1
H1
‐104.808
H2
H2
‐91.346
H2
H2
‐101.923
H3
H3
‐103.846
H3
H3
‐89.808
29
Table 3 The Mulliken bond populations of different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface (001) surface / H3
( 00 1 ) surface / H1
Adsorption configuration
DDA+
MDA+
DMDA+
DTAC+
Bond
Length /Å
Bond population
Bond
Length /Å
Bond population
N‐H1∙∙∙O1
1.615
0.16
N‐H1∙∙∙O1
1.727
0.09
C‐H2∙∙∙O2
2.136
0.02
N‐H2∙∙∙O2
2.086
0.03
C‐H3∙∙∙O3
2.907
0.00
C‐H3∙∙∙O3
1.983
0.01
C‐H1∙∙∙O1
2.037
0.03
N‐H1∙∙∙O1
1.858
0.06
C‐H2∙∙∙O2
2.581
0.00
C‐H2∙∙∙O2
2.444
0.00
C‐H3∙∙∙O3
2.718
0.00
C‐H3∙∙∙O3
2.482
0.00
C‐H1∙∙∙O1
2.092
0.03
C‐H1∙∙∙O1
2.195
0.00
C‐H2∙∙∙O2
2.785
0.00
C‐H2∙∙∙O2
2.599
0.00
C‐H3∙∙∙O3
2.878
0.00
C‐H3∙∙∙O3
2.432
0.00
C‐H1∙∙∙O1
2.529
0.00
C‐H1∙∙∙O1
2.190
0.01
C‐H2∙∙∙O2
2.452
0.01
C‐H2∙∙∙O2
2.640
0.00
C‐H3∙∙∙O3
2.206
0.02
C‐H3∙∙∙O3
2.389
0.00
The atomic numbers in Table 4 are corresponding to the atomic numbers in Fig. 10 and Fig. 11; the same below.
Table 4 Mulliken atomic charges before and after different amine/ammonium cations adsorption on kaolinite (001) surface and ( 00 1 ) surface Mulliken charge /e Atomic number H3 / (001) surface
H1 / ( 00 1 ) surface
Adsorption states DDA+
MDA+
DMDA+
DTAC+
DDA+
MDA+
DMDA+
DTAC+
before
0.30
0.27
0.27
0.25
0.30
0.27
0.27
0.25
after
0.39
0.27
0.28
0.27
0.43
0.45
0.29
0.29
before
0.30
0.28
0.25
0.27
0.30
0.28
0.25
0.27
after
0.31
0.28
0.26
0.28
0.44
0.30
0.28
0.29
before
0.26
0.28
0.27
0.25
0.26
0.28
0.27
0.25
after
0.29
0.28
0.27
0.27
0.30
0.31
0.30
0.29
before
‐0.93
‐0.68
‐0.42
‐0.16
‐0.93
‐0.68
‐0.42
‐0.16
after
‐0.85
‐0.66
‐0.40
‐0.14
‐0.80
‐0.60
‐0.39
‐0.16
before
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
after
‐1.07
‐1.06
‐1.06
‐1.08
‐1.13
‐1.15
‐1.15
‐1.15
H1
H2
H3
N
O1
30
before
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
‐1.06
after
‐1.06
‐1.06
‐1.07
‐1.06
‐1.16
‐1.18
‐1.18
‐1.17
before
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
‐1.05
after
‐1.05
‐1.05
‐1.05
‐1.06
‐1.16
‐1.16
‐1.17
‐1.16
before
0
0
0
0
0
0
0
0
after
0.39
0.30
0.37
0.38
0.70
0.70
0.59
0.57
before
0
0
0
0
0
0
0
0
after
‐0.39
‐0.30
‐0.37
‐0.38
‐0.70
‐0.70
‐0.59
‐0.57
O2
O3
Cations
(001) surface
31