Surface chemistry features of spodumene with isomorphous substitution

Minerals Engineering 146 (2020) 106139

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Surface chemistry features of spodumene with isomorphous substitution a

a

b,⁎

Guangli Zhu , Yijun Cao , Yuhua Wang Xiayu Zhengb a b c

c,⁎

c

T

b

, Xuming Wang , Jan D. Miller , Dongfang Lu ,

Henan Province Industrial Technology Research Institution of Resources and Materials, Zhengzhou University, No.100 Science Avenue, Zhengzhou, Henan 450001, China School of Minerals Processing & Bioengineering, Central South University, No. 932 Lushan South Road, Changsha, Hunan 410083, China Department of Metallurgical Engineering, College of Mines and Earth Sciences, University of Utah, 135 S 1460 E 412 WBB, Salt Lake City, UT 84112, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Spodumene Isomorphous substitution Wetting characteristics MD simulation Collector adsorption

Spodumene is an important strategic mineral resource for production of lithium. Froth flotation technology is a major method for recovery of spodumene. Surface chemistry features of minerals have significant effects on flotation performance. Natural spodumene crystals have lattice defects, including isomorphous substitution of Fe, Mn, and Cr. Such substitution will affect surface features, such as hydrophobicity of minerals and collector adsorption on mineral surfaces. In this regard, the influences of lattice substitution on spodumene surface properties were studied with respect to wetting characteristics and collector adsorption using micro-flotation experiments, contact angle measurements, Fourier-transform infrared spectroscopy (FTIR) analysis, molecular dynamics (MD) simulations, and density functional theory (DFT) calculations. Micro-flotation experiments, FTIR analysis, and contact angle results showed that higher Fe contents favored for the oleate adsorption on spodumene surface. Contact angle measurement and MD simulation results revealed that spodumene surfaces with or without substitution were naturally hydrophilic, but the interfacial water structures were expected to be different with various substitutions. Therefore, the interfacial water structure was further analyzed for a better understanding of the wetting of surface. DFT calculations showed that the surfaces with substitution were more in favor of oleate adsorption than the surface without substitution. The surface with Mn substituting for Si was more favorable for the oleate adsorption. It seems that the metal ion sites at surfaces contribute to different interfacial water structures, and lattice substitution plays a significant role in the adsorption state of NaOL.

1. Introduction Lithium, best known for lithium-ion batteries, has long been of great importance in the industry such as batteries, ceramic glass, lubricant greases, aluminum, nuclear, air conditioning, and pharmaceutical industries (Conrad et al., 2007; Scrosati and Garche, 2010; Oruch et al., 2014; Swain, 2017). In spite of diverse use of lithium in metallurgical and chemical industries, large potential demand for lithium is expected due to the urgency for clean energy, which requires the replacement of internal combustion engine cars with electric vehicles (Scrosati and Garche, 2010). Available global lithium resources are primarily from brines and hard rocks (Mohr et al., 2012). Brine is generally regarded as having a much greater capacity for large-scale, long-term production as well as a lower cost than hard rock deposits (Kesler et al., 2012). However, as it is easier and more economic for hard rock deposits to obtain battery grade lithium carbonate (Evans, 2008), which is essential for the battery industry, hard rocks, especially spodumene, the most abundant lithium-bearing mineral (Gruber et al., 2011), remain a ⁎

continued area of interest. Spodumene [LiAlSi2O6] is a pyroxene mineral theoretically containing 8.03% Li2O. The spodumene crystal pertains to a monoclinic system with a single-chain structure and space group C2/c (Deer et al., 1997). In spodumene crystal, the chains of silicate tetrahedron are parallelly extended along the c-axis and laterally bound together through the ionic bonding with Li (M2) and Al (M1), both of which form distorted octahedral coordination with oxygens (Moon and Fuerstenau, 2003). The monoclinic pyroxene group has a general chemical formula of M2M1T2O6, where M2 is cations in a generally irregular octahedral coordination. Ml refers to cations in an octahedral coordination, and T is tetrahedrally coordinated cations (Morimoto et al., 1988). A variety of cations occupying different sites results in various minerals assigned to pyroxene, and therefore it is not unexpected that many lattice impurities, most of which exist as a form of substitutes, are found in pyroxene. The natural pure spodumene crystal occurs as transparent to translucent, and as colorless to yellowish, purplish, pink, and green. Various impurities, mostly Fe, Mn, and Cr which exist in the spodumene

Corresponding authors. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Wang).

https://doi.org/10.1016/j.mineng.2019.106139 Received 12 September 2019; Received in revised form 30 October 2019; Accepted 24 November 2019 0892-6875/ © 2019 Elsevier Ltd. All rights reserved.

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2. Experimental details

crystal as lattice substitutions, are responsible for these pleasing colors (Webster, 1970; Lacerda et al., 2004; Leal et al., 2005; De Lima et al., 2008). Claffy (1953) proposed that the green color of hiddenite was ascribed to Cr element, while Souza et al. (2004) reported that the green color could be due to Fe content. However, it was believed that the spodumene with yellow color had high Fe content, and lilac kunzite was due to manganese (Claffy, 1953). Gabriel et al. (1942) indicated that Fe and other impurities such as Cr were incapable of removal by mechanical methods due to intimate association with spodumene, and the XRD results supported the inference that the impurities were present as an isomorphous replacement in spodumene crystal lattice. The effects of impurities in spodumene crystal lattice studied by researchers showed that the common Fe, Mn, and Cr substitutions variously altered the electronic and optical properties such as thermally stimulated luminescence and electron paramagnetic resonance spectra of spodumene (Holuj, 1968; Fujii and Isotani, 1983; Isotani et al., 1988; Walker et al., 1997; Souza et al., 2004; Oliveira et al., 2009; Lima et al., 2010). Although it has been a significant subject of some areas so far, there is lack of fundamental understanding about the effects of isomorphous substitution on the wetting properties and surfactant adsorption at spodumene surface. The beneficiation of spodumene from ore deposits is mainly by means of froth flotation taking advantage of the difference in the surface chemistry of spodumene and its associated aluminosilicate minerals. In the practical flotation separation of spodumene from associated aluminosilicates, the presence of multivalent cations such as Fe (III), Ca (II), and Mg (II), influences the flotation performance considerably (Wang and Yu, 2007; Yu, 2014; Yu et al., 2014; Zhang et al., 2014; Liu et al., 2015). These multivalent cations activate minerals to some extent within certain pH ranges. From another point of view, Potts (2012) suggested that the flotability of aluminosilicate minerals was related to the mineral crystal chemistry such as surface active sites, surface bond types, strength, and unsaturated bond density. It has been reported that the surface crystal chemistry of spodumene was a major factor contributing to its selective flotation separation with sodium oleate (NaOL) from other aluminosilicates. For example the Al sites at the spodumene surface is considered responsible for the chemisorption of NaOL (Moon and Fuerstenau, 2003; Rai et al., 2011; Zhu et al., 2015; Xu et al., 2017). These studies initiated that isomorphous substitution in the spodumene crystal could have significant influence in spodumene flotation, while this subject has yet to be elucidated. Molecular dynamics (MD) simulation is an efficient tool that can be employed to explore water/mineral interactions, and to illuminate the structure of water at mineral surfaces, which provides detailed information as well as fundamental understanding on mineral surface chemistry (Du and Miller, 2007; Zhang, 2014; Jin et al., 2014; Li et al., 2015; Miller et al., 2016; Shrimali et al., 2016; Wu et al., 2017). In the past decades, the researches regarding MD simulations has been reported in the study of interfacial water structures and the dynamic characteristics of water/mineral systems. Therefore, the water structure at spodumene surfaces with substitutions is appropriate for study for a better understanding of spodumene flotation chemistry. Also, the adsorption of collector molecules at spodumene surfaces with substitutions is of special interest because of its vital role in mineral processing technology. In this study, (1 1 0) surface, the prominent cleavage plane for spodumene, with Fe, Mn, and Cr substitutions, was subject to the experimental study using micro-flotation experiments, contact angle measurements, and FTIR analysis, and theoretical study of the interfacial water structure and contact angle simulations of water droplet by means of MD simulation and DFT calculations to investigate the isomorphous substitutions effects on the wetting characteristics and collector adsorption at spodumene surface, which will provide significant information with respect to spodumene flotation technology.

2.1. Samples, electron microprobe and XPS analysis Three spodumene bulk crystals with different contents of Fe and/or Mn (different colors, pink, green, and white) were obtained from Koktokay Rare Metallic Mine, Xinjiang, China. The crystals were cut into pieces, among which crystals with natural cleavage (1 1 0) plane remained intact, for different experimental purposes. The chemical composition of spodumene crystals was analyzed with a Cameca model SX-50 electron microprobe equipment. The crystals were polished with microcloth with a MasterPolish II alumina suspension obtained from Buehler, and cleaned using acetone, methanol, and DI water. The cleaned crystals were then blow dried with high-purity nitrogen. Then the dried crystals were coated with carbon film in a vacuum coater. Finally, the quantitative chemical composition analyses of the crystals was performed using a Cameca SX-50 electron microprobe equipped with four wavelength-dispersive spectrometers, at the College of Mines and Earth Sciences, University of Utah using techniques described in Nash (1992). A portion of crystal samples were crushed, and ground, and then the samples with a size fraction of −150 + 38 µm were obtained for the micro-flotation experiments. The small crystals with natural cleavage (1 1 0) plane was confirmed by X-ray diffraction system modeled D5000 from Siemens/Bruker. The elemental composition of surfaces, which were rinsed with 3% hydrochloric acid (HCl) solution and then plenty of deionized (DI) water to eliminate metal contaminant, was analyzed by X-ray photoelectron spectroscopic modeled ESCALAB 250Xi from Thermo Fisher Scientific. 2.2. Micro-flotation experiments Micro-flotation experiments were carried out using a laboratory self aspirating flotation machine modeled XFG with single cell at 1650 rpm. 2 g of samples and 20 ml of DI water were put in the flotation cell. Prior to the addition of collectors (6 × 10−4 M NaOL), HCl and NaOH solutions were used to adjust the pH of systems. Subsequently, the system was conditioned for 2 min, after which the froth was collected for 3 min. Finally, the froth and tailing products were filtered, dried, and weighed to calculate the flotation recovery. 2.3. Contact angle measurements Contact angle measurements at surfaces were carried out using the sessile drop technique with a Rame-Hart goniometer (Rame-Hart, Succasunna, NJ), in the absence and presence of sodium oleate (NaOL) (> 97.0%, TCI) which was used as a collector, at room temperature of about 23 °C. Prior to each experiment, the crystal surfaces were polished with microcloth with a MasterPolish II Alumina suspension obtained from Buehler, and cleaned using acetone, methanol, and DI water. The cleaned crystal surfaces were then blow dried with highpurity nitrogen, and treated with plasma. Then, for the contact angle measurements in the absence of collector, the cleaned crystals were subjected to the measurements directly. For the contact angle measurements in the presence of collector, the cleaned crystals were treated by conditioning in 6 × 10−4 M NaOL solutions (pH = 8.0–9.0) for 40 min prior to the measurements. The stable contact angle was measured after the attached sessile drop reached equilibrium. All measurements were repeated 10 times, and the average values were obtained. The maximum experimental variation in contact angle values was ± 1°. DI water used in all experiments had a minimum resistivity of 18.2 MΩ·cm. 2.4. Fourier-transform infrared spectroscopy (FTIR) A Thermo Scientific Nicole iS50 FT-IR Spectrometer was used to analyze the samples. Mineral crystal samples were prepared prior to the 2

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size of 103.6 Å × 125.6 Å, were expanded from the 2 × 2 × 1 optimized isomorphous substituted (1 1 0) surfaces as shown in Fig. 1. Thus, a 25% of isomorphous substitution was created. Also, the simulation systems of 12.5%, 25%, 50%, 75%, and 100% Fe substituting Al were built to study if the amount of isomorphous substitution has effects in the surface hydrophobicity. The vertical extent for all periodic boxes was set at about 140 Å to eliminate the influence of periodic conditions on the water droplets. Geometry optimization, or energy minimization using Universal force field, was done for each system after initial configuration, prior to MD simulations. For each simulation, an NVT ensemble was employed (Martyna et al., 1996), and the Universal force field was used. The Andersen barostat was adopted to obtain a constant pressure fixed at 0 Gpa, and a constant temperature at 298 K was maintained using the Andersen thermostat. The time step for all MD simulations was 1 fs. A total simulation time of 1 ns, including a 500 ps equilibration period, was performed. The last 500 ps of simulation period was used for analysis.

measurements using the same procedure as in the aforementioned contact angle measurements. Spectra (4000–450 cm−1) were collected at a resolution of 4 cm−1. 3. Computational details 3.1. Spodumene surfaces The monoclinic C2/c spodumene crystal structure obtained from the American Mineralogist Crystal Structure Database (Cameron et al., 1973) was used as the initial structure for further geometry optimizations. The Cambridge Serial Total Energy Package (CASTEP), which is based on the density functional theory, was employed for geometry optimizations (Payne and York, 1992). The exchange-correlation functional Local Density Approximation (LDA) and the CA-PZ (Ceperley-Alder and Perdew-Zunger), exchange-correlation potentials for the LDA, were used (Vanderbilt, 1990; Perdew and Wang, 1992; Chen et al., 2011; Jiang et al., 2013; He et al., 2014). The BFGS, a modified Broyden-Fletcher-Goldfarb-Shanno method for optimization, was adopted using line search method (Rai, 2014). The Pulay Density Mixing method used as a self-consistent electronic minimizer was set with a convergence tolerance of 1.5 × 10−6 eV/atom. The lattice parameters of the optimized spodumene crystal structure are a = 9.486922, b = 8.237576, c = 5.181515, β = 110.7417 , consistent with the previous report (Dana and Dana, 1883). According to the previous reports, the (1 1 0) surface of spodumene is a prominent cleavage plane, as well as the one with lowest surface energy (Moon and Fuerstenau, 2003; Rai et al., 2011). The (1 1 0) surface, therefore, was selected for the study. The (1 1 0) surface was built by cleaving the optimized spodumene crystal structure at a (1 1 0) Miller plane and undergone further geometry optimization. Consequently, a 2 × 2 × 1 perfect (1 1 0) surface of spodumene with a 20 Å vacuum slab was modeled. It is well accepted that transition elements primarily substitute for Al ions in M1 sites, and thus Fe or Cr atoms may predominantly substitute for Al atoms because of the same valence state that they are liable to (Manoogian et al., 1965; Isotani et al., 1988; Ito and Isotani, 1991). With regard to Mn substitution, the previous reports proposed that in one case, Mn may substitute for Si, and in another case, Mn4+ is found substituting Al3+, with one Mn2+ substituting for another Al3+ neighboring to the Mn4+ for charge compensation (Souza et al., 2007). However, there is also the report indicating that Mn2+ is shown to be mainly in Li-sites rather than Al-sites (Walker et al., 1997). In nature, however, the states of impurities in spodumene lattice are far more complex, and not yet clear. To simplify the study, the focus is on the isomorphous substitution of the top layer of surfaces. The (1 1 0) surfaces with Fe or Cr substitutions were built by substituting one Fe or Cr atom for one Al atom at the surface. In the case of Mn substitution, one type of the (1 1 0) surface with Mn substitution was created by substituting one Mn atom for one Si atom, and the other isomorphous substituted (1 1 0) surface was built by substituting two Mn atoms for two Al atoms. The crystal structure of the isomorphous substituted (1 1 0) surfaces were further optimized with the top four layers of the structure relaxed, and the remaining layers fixed. The models of the isomorphous substituted (1 1 0) surfaces of spodumene are depicted in Fig. 1.

3.3. Spodumene-sodium oleate complex The NaOL molecule was built and optimized using DMol3 package. The optimized atomic structure of NaOL molecule is shown in Fig. 2. The optimized molecule was docked on the (1 1 0) surfaces of spodumene with and without substitutions. Taking possible interactions of functional groups of NaOL with surface atoms into account, several initial configurations (~20) of the spodumene-NaOL complex were sampled and assessed to locate the most stable configuration which should have the minimum energy. Many different configurations of the spodumene-water complex were sampled to find the lowest energy configuration of a water molecule on surfaces. The most favorable configuration was chosen for further computations. The interaction energy of NaOL with the mineral surface was computed using Eq. (1):

ΔE = ETotal − (Esurface + Ecollector )

(1)

where ETotal is the total energy of the spodumene-NaOL complex, Esurface and Ecollector is the total energy of surface in the absence of collector molecule, and of the collector, respectively. Although the interaction energy calculated depends on the force field and parameters used in the computation, the interaction energy obtained can be a comparative measure to contrast the strength of spodumene surface-collector interactions under the condition that the same force field and same parameters set are used. 4. Results and discussion 4.1. Electron microprobe analysis and XPS results The chemical composition of spodumene samples analyzed by electron microprobe is presented in Table 1 and the surface elemental composition of spodumene samples is listed in Table 2. As can be seen from Tables 1 and 2, spodumene samples with different colors mainly have Na, Fe, and Mn metal ion impurities, and metal ion impurity contents are distinct. All samples have similar low Na content and relatively low Mn contents. It is obvious that the green spodumene has a higher Fe content of 0.45%, compared to the pink and white spodumene which has Fe content of 0.03% and 0.30%, respectively. The different metal element content is primarily due to the varied isomorphous substitutions. In the case of surface elemental composition of samples with different colors, the Fe contents follows the sequence of pink < white < green as well, as shown in Table 2.

3.2. Simulated contact angles on surfaces To simulate the contact angles of water droplets at the isomorphous substituted (1 1 0) surfaces of spodumene, the systems containing a water droplet positioned at the surface within a periodic box were modeled for MD simulations. A water droplet containing 400 water molecules was packed into a cubic cell and was simulated using the extended simple point charge (SPC/E) model (Berendsen et al., 1987; Du and Miller, 2007). The supercells, or the periodic simulation boxes, of isomorphous substituted (1 1 0) surfaces, with a horizontal crystal

4.2. Micro-flotation experiments The flotation results of three different spodumene samples are shown in Fig. 3. It is noteworthy that the recoveries of all the samples 3

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Fig. 1. The slab models of the (1 1 0) surfaces of spodumene with isomorphous substitutions. The color code of the atoms is as follows: yellow-Si, red-O, violet-Li, pink-Al, white-H. The atoms to which the arrow directs are Fe, Cr and Mn, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

had the similar variation trend as a function of pH, and reached the peak at a pH value of around 8.5. The green spodumene sample responses flotation well with a highest recovery of 82%, while the pink sample had poor response with a highest recovery of 49%. These results implied that the spodumene samples with different colors had different response to the flotation, which might be due to the different contents of Fe (and/or Mn) elements. Furthermore, the spodumene with higher Fe content has better flotability.

Table 1 Electron microprobe data for spodumene crystals. Spodumene samples

Pink (Low in iron) White (Middle in iron) Green (High in iron)

Relative contents (W%) Si

Al

Li

Na

Fe

Mn

O

Total

30.24 30.18

14.94 14.75

3.73 3.73

0.09 0.08

0.03 0.30

0.09 0.07

52.10 51.98

101.22 101.09

30.34

14.59

3.73

0.07

0.45

0.04

52.02

101.24

4.3. Contact angle measurements Table 2 Surface elemental composition for spodumene samples.

Contact angles at the (1 1 0) surfaces of the three spodumene crystals are shown in Table 3. As can be seen from Table 3, in DI water, the three surfaces have nearly the same contact angles with a value of 16°, 15.0°, and 15.5°, respectively, exhibiting hydrophilic properties. In the presence of collector, the contact angle of the green spodumene (1 1 0) surface was 76.0°, larger than that at the white spodumene (1 1 0) surface, at which the contact angle was 65.0°. The pink spodumene has a smallest contact angle of 54°. The results revealed that all the spodumene (1 1 0) surfaces with different Fe contents are naturally hydrophilic, while distinction on the hydrophobicity occurs in the presence of collectors. NaOL shows a slight preferential adsorption on the spodumene (1 1 0) surface with high Fe contents.

Spodumene samples

Pink (Low in iron) White (Middle in iron) Green (High in iron)

Relative contents (%) Si(2p)

Al(2p)

Li(1s)

Fe(2p)

O(1s)

20.14 20.44 19.95

10.80 10.10 10.70

11.15 11.78 10.35

0.01 0.28 0.41

57.90 57.40 58.59

4.5. Simulated contact angles To understand the natural hydrophobicity of the isomorphous substituted (1 1 0) surfaces with various substitutions, the wetting characteristics of spodumene with substitutions were evaluated by MD simulations in the absence of collector. The interaction of a water droplet with the isomorphous substituted spodumene (1 1 0) surfaces at the molecular level was studied using MD simulation. Snapshots of water droplets at the surfaces in the y–z plane are seen in Fig. 5. To determine the contact angles, the contact angles of the two-dimensional droplets were measured in the x-z plane and the y-z plane, respectively, and then averaged, as listed in Table 4. As can be seen from Table 4, generally, the water droplets spread at the surfaces. The simulated contact angles at the (1 1 0) surface with Fe and Cr substitutions are 17° and 12°, respectively, as compared with the (1 1 0) surface without isomorphous substitution, which has a contact angle of 13°. In the case of the (1 1 0) surface with Mn substituting for Al and Si, however, both of their simulated contact angles are zero, indicating the water droplet has completely wet the (1 1 0) surfaces with Mn substitutions, as shown in

4.4. FTIR analysis FTIR spectra of spodumene surfaces treated with NaOL solutions at pH = 8.0–9.0 are given in Fig. 4. There are two peaks at about 2925 cm−1 and 2850 cm−1 attributed to the eCH2e asymmetric and symmetric stretching frequencies, respectively. Qualitatively speaking, in the case of the green spodumene (1 1 0) surface, the intensities of eCH2e stretching frequencies are the highest among the three crystals, showing that NaOL was well adsorbed at the green crystal surface. As expected, the intensities from the pink crystal surface are the lowest, which reveals that less NaOL was adsorbed. These results are consistent with the micro-flotation and contact angle measurement results, revealing that NaOL showed a slight preferential adsorption on the spodumene (1 1 0) surface with high Fe contents.

Fig. 2. The optimized structure of sodium oleate with the charge of each atom. The color code of the atoms is as follows: red-O, blue-H, gray-C, violet-Na. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4

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Fig. 3. Flotation recovery of spodumene with different iron contents as a function of pH using 6 × 10−4 M sodium oleate as a collector. Table 3 Contact angles at spodumene (1 1 0) surfaces in DI water (pH = 5.0–7.0), and in collector systems (pH = 8.0–9.0). (1 1 0) surfaces

Pink (Low in iron) White (Middle in iron) Green (High in iron)

Fig. 5. MD simulation snapshot of equilibrated water droplets at the (1 1 0) surfaces of spodumene: (a) Fe substituting Al, (b) Cr substituting Al, (c) Mn substituting Al, (d) Mn substituting Si, and (e) surface without substitution.

Contact angles (°) DI water

Sodium oleate(6 × 10−4 M)

16.0 15.0 15.5

54.0 65.0 76.0

Table 4 Simulated contact angles of water droplets at the (1 1 0) surfaces of spodumene with and without isomorphous substitution. (1 1 0) Surfaces

Contact angle (°)

No substitution

Fe(Al)

Cr(Al)

Mn(Al)

Mn(Si)

13.0

17.0

12.0

0

0

Table 5 Simulated contact angles of water droplets on the (1 1 0) surface of spodumene with Fe substitutions: (a) 12.5% Fe substituting Al, (b) 25% Fe substituting Al, (c) 50% Fe substituting Al, (d) 75% Fe substituting Al, (e) 100% Fe substituting Al. (1 1 0) Surfaces

Contact angle (°)

(a)

(b)

(c)

(d)

(e)

22.0

16.0

15.0

14.0

17.0

the amount of substitution has effects on the natural hydrophobicity of the isomorphous substituted (1 1 0) surfaces, MD simulation of contact angles at the surfaces with different concentrations of Fe substitutions, as a typical example, were carried out, and the results are presented in Table 5. As shown in Table 5, at the surface with about 12.5% of Fe substitution, the contact angle is about 22°, larger than those at the other surfaces, which is 16°, 15°, 14°, and 17°, for the surfaces with 25%, 50%, 75%, and 100% Fe substituting Al, respectively. The results, however, suggest that the distinct amount of Fe substitutions do not significantly influence the hydrophobicity of the (1 1 0) surface. In order to further understand the wetting characteristics of isomorphous substituted surfaces, the interfacial water structure at the isomorphous substituted (1 1 0) surfaces will be discussed in the following section.

Fig. 4. FTIR spectra for spodumene crystal surfaces with different iron contents treated in 6 × 10−4 M sodium oleate solution (pH = 8.0–9.0).

Fig. 5. It is obvious that the Fe substitution makes the (1 1 0) surface more hydrophobic, while Mn substitutions give rise to complete hydrophilic surfaces, to some extent. The hydrophobicity of the (1 1 0) surface with Cr substitutions, having a simulated contact angle of 12°, which is similar to that for the (1 1 0) surface without isomorphous substitution. These surfaces, however, are naturally hydrophilic due to small values of contact angles. The simulated contact angle results reveal that the substitutions do not make significant differences in the hydrophobicity of the (1 1 0) surface, consistent with the experimental contact angle measurement results, to some extent. In order to study if

4.6. Water structures on surfaces 4.6.1. Water density profile The simulated contact angle results show that Fe substitutions 5

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Fig. 7. Intermolecular H2O-H2O radial distribution functions at the (1 1 0) surfaces with and without isomorphous substitutions.

Fig. 6. Density profiles of water molecules at the (1 1 0) surfaces with and without isomorphous substitutions along the z-axis.

valid enough to understand the characteristics of water molecules at the surfaces. Thus, the H2O-H2O radial distribution functions for water molecules on mineral surfaces is discussed in the following section to provide complementary information.

slightly increase the contact angle at the (1 1 0) surface, while Mn substitutions cause the contact angle at the (1 1 0) surface to be zero. The contact angle at the (1 1 0) surface with Cr substitutions is similar to that of the (1 1 0) surface without substitutions. Therefore, the water structure was analyzed for detailed information on the wetting characteristics of isomorphous substituted surfaces to investigate differences in these isomorphous substituted surfaces. The density profiles of oxygen and hydrogen atoms of water molecules are shown in Fig. 6. As expected, the distances between water molecules and surfaces are negative, and this result shows that some water molecules penetrated these surfaces, an indication of a strong interaction of water molecules with atoms at these hydrophilic surfaces. The peak of the relative density distribution of water molecules suggests that water molecules in this region is more closely packed than in the bulk. As shown in Fig. 6, the distances from the primary density peaks of hydrogen to the surface are smaller for the surface with Cr or Mn substituting Al than those for the other three surfaces, from which the water density peaks are located at similar distances, indicating that hydrogen atoms in this region are densely packed at these two surfaces; Furthermore, there is a second hydrogen density peak for the surface with Cr or Mn substituting Al, followed by the primary oxygen density peak along the surface normal, which means that in this region, water molecules are ordered with one hydrogen atom orientated toward the surface and the other one away from the surface. These results indicate that there is stronger attraction of surface to hydrogen than oxygen atoms in this region at the surface with Cr or Mn substituting Al. The curves for the relative number density of oxygen and hydrogen atoms at the surface without substitution and the surface with Fe substitution appear to be similar. At these two surfaces, the oxygen relative number density begins to increase from zero, which is ahead of that for hydrogen, implying that some oxygen atoms are oriented at a smaller distance than hydrogen atoms from the surface. In the case of surface with Cr and Mn substitutions, the oxygen and hydrogen relative number density appears to be overlapped near these surfaces, indicative of similar distance of oxygen and hydrogen of the primary water molecule layer at the surface. These results indicate that different substitutions as electron donor/acceptor sites contribute to distinct orientations of water molecules at surfaces. It seems that Fe substitution has no significant effect in the interfacial water structure, while Cr and Mn substitutions have changed the interfacial water structure at the (1 1 0) surface, which will be further discussed in the following section. Furthermore, it appears that the interaction of water molecules at the hydrophilic surfaces is very strong, and the minor difference in water density profiles is not

4.6.2. Radial distribution functions The H2O-H2O radial distribution functions for water molecules on mineral surfaces give information about the molecular order and water structure. From Fig. 7, all the curves have two remarkable peaks at radii of about 1.00 Å and 1.50 Å, respectively. The distance between these two water layers is about 0.50 Å, showing that the two closest atoms are from different water molecules. It is obvious that the first peaks for the water molecules on the (1 1 0) surface with Mn substitutions substituting Al and Si, are higher than those for the other systems, indicating that the number of water molecules at this distance away from one referenced water molecule is the largest and water molecules are more densely arranged on the (1 1 0) surface with Mn substitutions. The first peak for the (1 1 0) surface without substitutions is the lowest, indicating that water molecules are loosely packed and the H2O-H2O interaction is the weakest. Based on the water density profiles and radial distribution functions results, the interaction of H2O-H2O molecules, and H2O-surface at different surfaces is distinct and complicated. The radial distribution functions provide a piece of information about the packing density of water molecules at surfaces, but it cannot effectively describe more precise structures of water molecules at different isomorphous substituted surfaces. The orientation of each water molecule is a helpful addition to the analysis of water structure, which will be analyzed in the following section. 4.6.3. The orientation of water molecules The water structure at spodumene surfaces can be studied with respect to the orientation of water molecules from water dipole moments and hydrogen position distributions. Angle α (0° to 180°) is defined as the angle between the water dipole moment and the surface normal, and angle β (0° to 90°) is the angle of the two hydrogen atom positions in relation to the surface normal (Zhang, 2014). The contours of water dipole moment and hydrogen position distribution for the (1 1 0) surfaces with and without different substitutions are shown in Fig. 8. From Fig. 8, the distribution of dipole moment (angle α) at the (1 1 0) surface without substitutions covers a wide range from 30° to 160° with two distinct peaks assigned to the primary and second water layers respectively, one ranging from 30° to 50° with a center around 40° and the other ranging from 100° to 150° centering at about 130°. This result suggests that most of water molecules at the primary water 6

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Fig. 8. The distribution of water dipole moment (angle α) (1) and hydrogen position (angle β) (2) of water molecules at the (1 1 0) surfaces with and without isomorphous substitutions: (a) Fe substituting Al, (b) Cr substituting Al, (c) Mn substituting Al, (d) Mn substituting Si, and (e) surface without substitution.

oxygen and hydrogen atoms are oriented towards the surface with Mn substituting Al. In the case of the surface with Mn substituting Al, the orientation distribution of dipole moment of water molecules ranges from 20° to 160°, and the highest density happens at the range of 50° to 130°, indicating that partial water molecules are orientated with oxygen atoms towards the surface and partial water molecules are orientated with hydrogen atoms towards the surface. The angle β of the primary water layer ranges from 35° to 90°, implying a random orientation of water molecules in this region. These results indicate that the orientation of interfacial water molecules at the (1 1 0) surface is influenced with the transition metal substitutions. The attraction of atoms at the surface without substitution and Fe substitution to oxygen of water molecules at the primary water layer is stronger than that of atoms to hydrogen of water molecules. The orientation of interfacial water at the surface with Cr and Mn substituting for Al is similar, while water molecules at the surface with Mn substituting for Si is more randomly orientated as compared with those at other surfaces.

layer are oriented with oxygen towards the surface, while most water molecules of the second water layers are aligned with oxygen atoms oriented away the surface. The angle β ranges from 15° to 80° with the highest density located from 30° to 60° centering at around 50°, indicating that the water molecules at this region are oriented in disorder. Based on the analysis, it can be concluded that the majority of oxygen of water molecules at the primary layer are oriented towards the surface. The spodumene (1 1 0) surface constitutes of oxygen atoms in the topmost layer, followed by silicon and aluminum in a sublayer, and then lithium atoms. Therefore, a stronger attraction of silicon, aluminum, and lithium to oxygen of the primary water layer compared to the weaker repulsive force of oxygen at the surface to the oxygen of water molecules contributes to this orientation. Similar to the (1 1 0) surface without substitutions, the orientation distribution of dipole moment of water molecules at the (1 1 0) surface with Fe substitutions ranges from 10° to 150° with two distinct peaks ranging from 40° to 60° with a center around 50°, and from 100° to 145° centering at about 120°, respectively. The distribution range of H position orientation is from 10° to 90°, with peaks from 30° to 65° centering at 50°. These results imply that most oxygen of water molecules at the primary water layer are oriented towards the (1 1 0) surface with Fe substitutions, with most water molecules nearly paralleling to the surface. In the case of the (1 1 0) surface with Cr substitutions, the dipole moment of water molecules also covers a wide range from 0° to 168°, and two distinct peaks are located from 20° to 50° centering at 30° and from 130° to 170° with a center at 145°, respectively. The location of these two peaks are overlapped along the surface normal, which means that both two peaks are assigned to the primary water layer. It demonstrates that a partial water molecules of the primary water layer is oriented with oxygen towards the surface while the oxygen atoms are oriented away from the surface for the other water molecules. The angle β ranges from 20° to 90° with the highest density from 55° to 75° centering at 65°. These results suggest that a portion of water molecules of the primary water layer are oriented with hydrogen atoms towards the surface while the other portion of water molecules are oriented with oxygen towards the surface. Similar to the surface with Cr substitution, the orientation distribution of dipole moment of water molecules at the surface with Mn substituting Al ranges from 10° to 140°, and the highest density happens at the range of 20° to 50° and 130° to 170°, indicating that a partial water molecules of the primary water layer is oriented with oxygen towards the surface while the oxygen atoms are oriented away from the surface for the other water molecules. The angle β peak of the primary water layer is between 55° to 70°. Therefore, both

4.6.4. Hydrogen bonds Hydrogen bonding network in water molecules is another important parameter for describing the molecular characteristics of water molecules, apart from the water density profiles, the H2O-H2O radial distribution functions and the orientation of water molecules at the interface. For the SPC/E water model, one hydrogen bond is formed if the distance between two oxygen atoms corresponding to two water molecules respectively is less than 3.5 Å and meanwhile the O···OeH angle is less than 30° (Luzar and Chandler, 1996). The average number of hydrogen bonds per water molecule for water droplets on the isomorphous substituted (1 1 0) surfaces and the number of hydrogen bonds for hydrogen atoms of water molecules and oxygen atoms on these surfaces were calculated, and the results are shown in Fig. 9 and Table 6, respectively. As shown in Fig. 9, the average number of hydrogen bonds per water molecule increases to reach peaks and then remains unchanged with a value about 3.50. In addition, it is noteworthy that hydrogen bonds starts to form at a closer location from the surface for Mn substituting Si than the other substitutions, indicating that in this region there is a strong hydrogen bond network for water molecules at the surface with Mn substituting Si. The hydrogen bonds of surface oxygen atoms reveal the extent of their interaction with water molecules. From Table 6, there is a strong interaction of water molecules with oxygen atoms on the (1 1 0) surfaces without substitutions, with 8 surface oxygen atoms possessing 2 hydrogen bonds and 200 7

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Fig. 9. The distribution of hydrogen bonds per water molecule along the surface normal for the spodumene (1 1 0) surface with isomorphous substitutions.

previously reported (Moon and Fuerstenau, 2003; Rai et al., 2011; Zhu et al., 2015), the Al sites on spodumene surfaces account for the chemisorption of NaOL. The interaction of eCOOe group with Al is responsible for adsorption of NaOL on spodumene surfaces. Therefore, the multivalent ions, transition metal substitutions, might make a critical difference in the chemisorption of NaOL on spodumene surfaces. In order to evaluate the affinity of the isomorphous substituted (1 1 0) surfaces with NaOL, molecular modeling computations were carried out, and the interaction energies for surface-collector were computed. Since flotation occurs in the aqueous environment, the effect of water molecules on mineral-collector interactions was considered, and interaction energies of water with surfaces were calculated. The results are shown in Fig. 10. It is obvious that the interaction energies for the collector are much smaller compared to those for water, suggesting that the NaOL is capable of replacing water and interacting with atoms on these surfaces (Pradip et al., 2002). As shown in Fig. 10, it is obvious that the magnitude (-Kcal/mol) of the interaction energy of NaOL with the (1 1 0) surfaces with different substitutions is higher than that of the (1 1 0) surface without substitutions, indicating that the collector adsorption on the isomorphous substituted surface is more favored. The results are consistent with the experimental micro-flotation tests, contact angle measurements and FTIR results which indicated that NaOL shows a preferential adsorption on the spodumene (1 1 0) surface with a higher content of Fe. The (1 1 0) surface with Mn substituting Si have the highest negative interaction energy (-Kcal/mol) with NaOL among the five surfaces, followed by the (1 1 0) surface with Fe substitutions, and then the isomorphous substituted (1 1 0) surface with Mn substituting Al. Furthermore, the interaction energy results reveal that the (1 1 0) surface with Mn substituting Si is the most favorable for the adsorption of NaOL, which might be due to the ratio of a total number of multivalent cations including Mn and Al to number of oxygen atoms is higher, consistent with the viewpoint of the previous report. Also, the interfacial water molecules are more randomly orientated at the surface with Mn substituting Si, and this disordered orientation should help for the collector to break through water molecules and adsorb on the

Table 6 Hydrogen bonding of oxygen atoms at surfaces with hydrogen atoms of water molecules. Number of hydrogen bonds per oxygen atom

2 1

Number of oxygen atoms Fe (Al)

Cr (Al)

Mn (Al)

Mn (Si)

No substitution

3 87

14 110

15 91

0 49

8 200

surface oxygen atoms having 1 hydrogen bond. In the case of the (1 1 0) surface with substitutions, water molecules have a stronger interaction with oxygen atoms on the surface with Cr substitutions, with 14 surface oxygen atoms possessing 2 hydrogen bonds and 110 surface oxygen atoms having 1 hydrogen bond, compared with that on the surface with Fe substitutions, which have 3 surface oxygen atoms with 2 hydrogen bonds and 87 surface oxygen atoms with 1 hydrogen bond, indicating that the hydrogen atoms of water molecules are more attracted by the oxygen atoms at the surface with Cr substitutions than that with Fe substitutions, which is consistent with the orientation results. There are 15 surface oxygen atoms having 2 hydrogen bonds and 91 surface oxygen atoms having 1 hydrogen bond for the (1 1 0) surface with Mn substituting Al, and zero surface oxygen atoms having 2 hydrogen bonds and 49 surface oxygen atoms having 1 hydrogen bond for the (1 1 0) surface with Mn substituting Si, indicating that the (1 1 0) surfaces with Mn substitutions have weaker interaction with hydrogen atoms of water molecules. On the other hand, there is a stronger attraction of atoms at the surfaces with Mn substitution to oxygen atoms of water molecules at the primary water layer than that of atoms to hydrogen of water molecules, as revealed by the orientation analysis.

4.7. Interaction of spodumene-sodium oleate MD simulation results reveal that the (1 1 0) surfaces with and without substitutions are all naturally hydrophilic, while water structures on these surfaces are very different in molecular level. As 8

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Fig. 10. Interaction energies of sodium oleate and water molecules at the (1 1 0) surfaces with and without isomorphous substitutions.

Declaration of Competing Interest

surface. In conclusion, the NaOL adsorption on the isomorphous substituted surfaces is more favored than on the surface without substitutions, and the (1 1 0) surface with Mn substituting Si is the most favorable for the adsorption of NaOL. It seems that the metal ion sites at the surfaces contribute to different interfacial water structures and the substitutions play a significant role in the adsorption state of NaOL.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors are thankful for the support of the National Natural Science Foundation of China (Grant No. 51674290; No. 51804341), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province of China (Grant No. 19IRTSTHN028), and the Natural Science Foundation of Hunan Province of China (Grant No. 2016JJ3150 and No. 2019JJ5083).

5. Conclusions The micro-flotation experiments, contact angle measurement and FTIR analyses results reveal that NaOL shows a slight preferential adsorption on the spodumene (1 1 0) surface with high Fe contents. Contact angle measurement results and MD simulation of water droplets at the perfect and defective (1 1 0) surfaces reveal that these spodumene surfaces are naturally hydrophilic. The Fe impurity increases the contact angle at the (1 1 0) surface, while Mn substitutions give rise to complete hydrophilic surfaces, to some extent. The hydrophobicity of the defective surface with Cr substitutions is similar to that of the perfect surface. MD simulation of water droplets at the defective (1 1 0) surfaces with different contents of Fe substitutions reveals that the distinct contents of Fe substitutions do not significantly influence the hydrophobicity of the (1 1 0) surface. The water density distributions reveal that there is a strong interaction of water molecules with atoms at surfaces, because of hydrophilic surfaces. Water molecules are more densely arranged on the (1 1 0) surface with Mn substitutions, while the H2O-H2O interaction is the weakest at the surface without substitution. The orientation of interfacial water molecules at the (1 1 0) surface is influenced with the transition metal substitutions. The attraction of atoms at the surface without substitution and Fe substitution to oxygen of water molecules at the primary water layer is stronger than that to hydrogen of water molecules. The orientation of interfacial water at the surface with Cr and Mn substituting for Al is similar, while water molecules at the surface with Mn substituting for Si is more randomly orientated as compared with those at other surfaces. The interaction energy results reveal that the oleate adsorption on the surfaces with substitution is more favored than on the surface without substitutions, and the (1 1 0) surface with Mn substituting Si is the most favorable for the oleate adsorption. It seems that the metal ion sites at the surfaces contribute to different interfacial water structures and the substitutions play a significant role in the adsorption state of NaOL.

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