Al Layered Double

Research Article pubs.acs.org/journal/ascecg

Preparation of Molybdenum Disulfide Coated Mg/Al Layered Double Hydroxide Composites for Efficient Removal of Chromium(VI) Jian Wang,† Pengyi Wang,† Huihui Wang,† Junfei Dong,† Wanying Chen,† Xiangxue Wang,† Suhua Wang,*,†,‡ Tasawar Hayat,‡ Ahmed Alsaedi,‡ and Xiangke Wang*,†,‡,§ †

College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, People’s Republic of China ‡ NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia § Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions and School for Radiological and Interdisciplinary Sciences, Soochow University, Suzhou, 215123, People’s Republic of China

ABSTRACT: With the rapid development of industry, heavy metal pollution has become a potential hazard to public health and the ecological system. Herein, molybdenum disulfide coated Mg/Al layered double hydroxide composites (LDHs@MoS2) were prepared via a simple hydrothermal method and applied for the adsorption of Cr(VI) from a water solution. The removal capacity of Cr(VI) on LDHs@MoS2 reached 76.3 mg/g at pH = 5.0, and the removal process relied on ionic strength and pH. The results confirmed that the uptake of Cr(VI) on LDHs@MoS2 followed a spontaneous endothermic process. In contrast to the LDHs, LDHs@MoS2 showed excellent chemical stability, which was beneficial for practical applications. Specifically, the coexisting ions had little influence on the uptake of Cr(VI). The interaction of Cr(VI) with the LDHs@MoS2 composites was mainly controlled by electrostatic attraction and outer-sphere surface complexation. The findings can provide new insights into the uptake of heavy metal ions in a natural aquatic environment pollution cleanup. KEYWORDS: Cr(VI), Layered double hydroxides, Molybdenum disulfide, LDHs@MoS2 composites

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ions.12,13 All kinds of materials including layered double hydroxides, activated carbon, activated alumina, chitosan, and zeolite have been modified by different methods for the uptake of Cr(VI) from aquatic systems.14,15 For instance, Liu et al.16 synthesized the Fe-modified activated carbon from a low-cost aquatic plant residue for Cr(VI) removal from sewage and reached the maximum adsorption capacity of 11.8 mg/g. Xiao et al.17 found that the calcined layered double hydroxides could be used to the uptake of Cr(VI). However, most of the existing materials are not appropriate for the trapping of Cr(VI) because of low efficiency and may result in secondary pollution in extreme conditions, which will hinder the sewage treatment in practical applications. Thereby, it is essential to explore new material with excellent stability for the effective elimination of Cr(VI). In the past few years, two-dimensional (2D) layered nanomaterials have attracted wide public concern in catalysis, optoelectronics, photochemistry and hydrogen storage owing to their excellent chemical stability, designability, and high surface area.18,19 Among which, molybdenum disulfide (MoS2)

INTRODUCTION With the expansion of industrialization, more and more heavy metal ions such as cadmium, lead, copper, chromium, and mercury have been excessively released into the environment and caused serious environmental pollution.1−4 Among the most toxic heavy metals, hexavalent chromium (Cr(VI)) is a common pollutant arriving from numerous industrial applications such as metal plating, pigment manufacturing, leather tanning, textile manufacturing, steel fabrication, corrosion control, wood treatments, and so on.5−7 It has been demonstrated that long-term exposure to Cr(VI) can result in arthritis, bronchitis, nerve tissue damage, brain damage, and even cancer.8 Unfortunately, the movement and accumulation of Cr(VI) in a natural aquatic environment is quite easy, and it may enter the human body by the food chain.9 Thereby, it is urgent to eliminate Cr(VI) from wastewater to decrease its toxicity. Many technologies have so far been carried out to trap Cr(VI) from sewage including adsorption, ion exchange, chemical sedimentation, biodegradation, solvent extraction, membrane separation, chemical oxidation, electrochemical precipitation, phytoremediation, flocculation, and photocatalytic degradation.10−12 Among these methods, adsorption has been extensively applied for the removal of heavy metal © 2017 American Chemical Society

Received: April 30, 2017 Revised: June 11, 2017 Published: June 15, 2017 7165

DOI: 10.1021/acssuschemeng.7b01347 ACS Sustainable Chem. Eng. 2017, 5, 7165−7174

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of LDHs (a) and LDHs@MoS2 (b). TEM images of LDHs (c) and LDHs@MoS2 (d). Elemental mapping images of different elements in LDHs@MoS2 (e).

characterized by X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The influence of different factors including solid content, coexisting ions, temperature, and adsorption time on the uptake of Cr(VI) by the LDHs@MoS2 composites were systematically studied by batch experiments, and the adsorption mechanism was discussed. This study can provide new insights into the trapping of Cr(VI) from water solution and broaden the practical applications of the LDHs@MoS2 composites for environmental pollution control.

is an important binary transition-metal dichalcogenide material, which is similar to the layer-structured graphene and Mo layer which is sandwiched between two sulfur layers through covalent force.20 Because of the good application prospects, a number of methods have been used to prepare MoS2 such as electrochemical deposition, precursor thermal decomposition, and hydrothermal methods, which can reflect the efforts toward the design of new functional materials and obtain different morphologies such as spherical, tubular, and 3D flowerlike structures.21 However, to the best of our knowledge, little attention has been given to its use in environmental pollution cleanup.22 Herein, we reported the preparation of the molybdenum disulfide coated Mg/Al layered double hydroxide composites (LDHs@MoS2) through a simple hydrothermal method for Cr(VI) removal. The physicochemical properties were

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MATERIALS AND METHODS

Chemicals. All the reagents used in this study were acquired from Sinopharm Chemical Regent Co., Ltd. (Beijing, China). All chemicals 7166


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Figure 2. XRD patterns (a), FT-IR spectra (b), and zeta potentials (c) of LDHs and LDHs@MoS2. N2 adsorption−desorption isotherms for LDHs@MoS2 (d). were analytical purity and used as received. K2Cr2O7 was used to prepare Cr(VI) standard stock solution (180 mg/L), and deionized water was used in all of the removal experiments. Synthesis of Composites. Typically, 30 mL deionized water containing MgCl2·6H2O (26.7 mmol) and AlCl3·6H2O (8.9 mmol) was added dropwise to 30 mL of NaOH (73.3 mmol) and Na2CO3 (3.3 mmol) solution under continuous stirring for 60 min. The mixed solution was moved to a Teflon-lined stainless-steel autoclave and then placed at 100 °C for 10 h. After centrifugation, the samples were washed with ethanol and deionized water three times and vacuumdried at 60 °C for 24 h. Thus, the achieved product was named LDH. The above LDHs (0.1 g) were added to 50 mL deionized water containing a certain amount of Na2MoO4 (2.7 mmol) as well as CH3CSNH2 (5 mmol) and stirred for 60 min to get a uniform suspension. Afterward, the mixed suspension was transferred into autoclave and reacted at 200 °C for 24 h, then followed by washing with ethanol and deionized water several times through a centrifugation−redispersion process. The obtained composites were vacuum-dried at 60 °C for 24 h and used for the following experiments. For comparison, the LDHs was also used for Cr(VI) removal. Characterization. The morphology of the LDHs@MoS2 was acquired from Hitachi S-4800 scanning electron microscope operated at 15.0 kV. The transmission electron microscopy (TEM) image was obtained by JEM-2011 at an acceleration voltage of 200 kV. Zetasizer (Nano-ZS) was applied to confirm the particle size and pHPZC of the LDHs@MoS2. The phases of the composites were characterized by Xray diffraction (XRD) using a Scintag XDS-2000 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu Kα radiation (λ =

1.54 Å) in the range of 5−70°. The Fourier transform infrared (FTIR) spectrum (Tensor 27) was applied to detect the surface functional groups. The X-ray photoelectron spectroscopy (XPS) spectra were derived from VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers. Adsorption Experiment. The trapping of Cr(VI) on LDHs@ MoS2 was systematically studied by batch experiments. Typically, 1.0 mL NaCl solution (0.06 M), 3.0 mL Milli-Q water, 1.0 mL LDHs@ MoS2 suspension (3.0 g/L), and 1.0 mL Cr(VI) stock solution (180 mg/L) were moved into 10 mL polyethylene centrifuge tube. The negligible volume of 0.01−1.0 M HCl or NaOH was moved to the tube to adjust the pH values. Afterward, the suspension was shaken at 150 rpm for 24 h to ensure adsorption equilibrium. After centrifugation, the residual Cr(VI) concentration in the supernatant was detected at 540 nm by UV−vis spectrophotometer (UV-2600, Shimadzu).23 The samples after Cr(VI) adsorption for characterization were collected by centrifugation and dried at 60 °C for 24 h. Such dried solid samples of LDHs@MoS2-Cr(VI) were characterized by different techniques like FT-IR, XRD, and XPS to examine the removal mechanisms. All adsorption experiments were conducted in duplicate and the removal efficiency was assessed via the average value. The removal percentage was calculated by the following formula:

removal percentage =

C0 − Ce × 100% C0

(1)

where C0 and Ce (mg/L) represent the initial and equilibrium concentrations of Cr(VI) ions. 7167


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Figure 3. Effect of pH and ionic strength on Cr(VI) removal onto LDHs (a) and LDHs@MoS2 (b) (Cr(VI) initial concentration = 30 mg/L, m/V = 0.5 g/L, adsorption time = 24 h, T = 298 K). Cr(VI) removal on LDHs and LDHs@MoS2 as a function of solid content (c) and contact time (d) (Cr(VI) initial concentration = 30 mg/L, pH = 5.0 ± 0.1, I = 0.01 M NaCl, T = 298 K). Effect of coexisting cations (e) and anions (f) on Cr(VI) removal onto LDHs and LDHs@MoS2 (Cr(VI) initial concentration = 30 mg/L, m/V = 0.5 g/L, adsorption time = 24 h, pH = 5.0 ± 0.1, I = 0.01 M, T = 298 K).

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RESULTS AND DISCUSSION

more lucid (Figure 1c). Compared to LDHs, the morphology and shape of LDHs@MoS2 changed obviously (Figure 1d). It is worth noting that LDHs were successfully coated by MoS2. Furthermore, the elemental mapping was performed to further study the composition of LDHs@MoS2, and the elements of Mo, S, Mg, Al, C, and O were well dispersed on the composites surface (Figure 1e). The crystal structures of the materials were characterized by XRD. From Figure 2a, one can see that the diffraction peaks of

Characterization. The morphology of the composites was acquired by scanning electron microscopy (SEM) techniques. From Figure 1a, it can be observed that the LDHs revealed a relatively uniform hexagonal morphology, similar to that found in other reports.24,25 From Figure 1b, one can see that LDHs@ MoS2 lost the hexagonal structure and presented a smooth surface with the average particle size of 0.19 μm. Furthermore, the TEM image demonstrated the flaky morphology of LDHs 7168


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Figure 4. Adsorption isotherms for Cr(VI) removal on LDHs (a) and LDHs@MoS2 (b) (the solid lines represent the Langmuir model, the dashed lines represent the Freundlich model, pH = 5.0 ± 0.1, I = 0.01 M NaCl, m/V = 0.5 g/L, adsorption time = 24 h); thermodynamics plots for Cr(VI) removal on LDHs (c) and LDHs@MoS2 (d).

Table 1. Langmuir and Freundlich Parameters for Cr(VI) Adsorption Langmuir model adsorbent LDHs

LDHs@MoS2

Freundlich model

T (K)

qmax (mg/g)

b (L/mg)

R2

Kf ((mg1−n Ln)/g)

n

R2

298 313 328 298 313 328

36.8 42.1 46.6 76.3 88.7 95.6

0.02 0.03 0.04 0.07 0.03 0.02

0.969 0.973 0.972 0.922 0.964 0.936

1.8 3.1 4.4 2.9 5.7 11.8

1.53 1.74 1.92 1.41 1.68 2.32

0.939 0.943 0.953 0.901 0.941 0.923

characteristic peaks disappeared and some new peaks appeared after recombination. It is apparent that the symmetric reflections at (002), (100), (103), and (110) can be assigned to MoS2.20 The surface functional groups of the as-prepared materials were tested by FT-IR. Figure 2b showed that the band at approximately 3565 cm−1 was ascribed to hydroxyl stretching vibration and the peak at 1630 cm−1 was attributed to hydroxyl bending vibration.27 The absorption peaks at 1502 and 1398 cm−1 were attributed to CO32−.28 Likewise, the peaks of LDHs@MoS2 at 1627 and 3432 cm−1 were caused by the bending and stretching vibrations of hydroxyl.27 The bands at 1388 and 1126 cm−1 were caused by the stretching vibration of CO32−,29 and the peaks in low-frequency region were ascribed to Mo−S vibrations.30 According to the above analysis, it can be considered that LDHs was successfully coated by MoS2.

Table 2. Adsorption Capacity of Cr(VI) on LDHs@MoS2 Compared with Other Materials materials

pH

qmax (mg/g)

ref

natural Akadama clay Fe-modified activated carbon tannin-immobilized activated clay newspaper Tannin resin amino-functionalized mesoporous alumina LDHs LDHs@MoS2

2.0 6.0 2.5 1.0 5.0 2.0 5.0 5.0

4.3 11.8 24.1 55.1 55.6 59.5 36.8 76.3

37 16 38 39 40 41 this study this study

LDHs at (003), (006), (009), (015), (018), (110), and (113) indicated the typical hydrotalcite structure.26 However, those 7169


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ACS Sustainable Chemistry & Engineering Table 3. Thermodynamics Parameters for Cr(VI) Adsorption ΔG (kJ/mol) adsorbent

ΔH (kJ/mol)

ΔS (J/(mol K))

298 K

313 K

328 K

LDHs LDHs@MoS2

10.27 11.84

21.39 38.53

−6.37 −11.48

−6.69 −12.06

−7.01 −12.64

Figure 5. XRD patterns (a) and FT-IR spectra (b) of LDHs@MoS2 before and after Cr(VI) removal. EDS spectrum of LDHs@MoS2 after Cr(VI) removal (c) and the peak for Cr(VI) (d).

Adsorption Property. Effect of Ionic Strength and pH. The influence of pH on the trapping of Cr(VI) onto the composites was illustrated in Figure 3. Figure 3a indicated that the removal percentage of Cr(VI) on LDHs increased at pH of 2.0−5.0 and, then, declined gradually with the pH increasing further. In contrast to LDHs, the removal percentage of Cr(VI) on LDHs@MoS2 decreased obviously with the increase of pH from 2.0 to 11.0, indicating that the adsorption of Cr(VI) on LDHs@MoS2 was dependent on pH (Figure 3b). This phenomenon can be ascribed to the surface properties of LDHs@MoS2. In acidic environment, the surface of LDHs@ MoS2 was protonated and presented positive charge, consistent with the zeta-potential result (Figure 2c), which was beneficial to adsorb the negatively charged Cr(VI) ions by electrostatic attraction. While in neutral and alkaline environment, the surface of LDHs@MoS2 showed a neutral or negative charge due to the deprotonation, resulting in the decrease of removal efficiency by electrostatic repulsion. Crucially, it is clear that the leaching out of metal ions from the composites in the elimination process was not detected in the whole pH scope

due to excellent chemical stability, which could broaden its reallife applications. The Cr(VI) removal trends influenced by pH further confirmed that electrostatic attraction played a decisive role in the removal process. The influence of ionic strength on the trapping of Cr(VI) was performed in different concentrations of NaCl solutions (Figure 3a and b). The removal of Cr(VI) was influenced by ionic strength at all pH values, and a relatively low concentration of NaCl was conducive to Cr(VI) removal, implying that the adsorption process was controlled by outersphere surface complexation rather than innersphere surface complexation.31 This phenomenon is consistent with Cr(VI) removal onto kaolin.32 Effect of Solid Content. As illustrated in Figure 3c, the influence of solid content (m/V) on the trapping of Cr(VI) was systematically studied. It is clear that the Cr(VI) trapping on LDHs and LDHs@MoS2 increased significantly with solid content increasing. As solid content increased, more functional groups are available and then more active sites are supplied for Cr(VI) removal, leading to the increase of removal efficiency. It 7170


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Figure 6. XPS spectra: the wide scan of LDHs@MoS2 before and after Cr(VI) adsorption (a); the peak for Cr 2p (b); the high Mo 3d deconvolution of LDHs@MoS2 before (c) and after (d) Cr(VI) removal; the high S 2p deconvolution of LDHs@MoS2 before (e) and after (f) Cr(VI) removal.

is important to note that the removal percentage increased from ∼7.4% to ∼29.7% for LDHs and from ∼15.3% to ∼50.6% for LDHs@MoS2 with the solid content increase from 0.1 to 0.8 g/L, revealing that LDHs@MoS2 possessed high removal capacity for Cr(VI) from wastewater. Moreover, Figure 3c also showed that the removal capacity (qe) declined with increasing solid content, which was consistent with the removal of fluoride ions on cellulose@hydroxyapatite nanocomposites.29 At low solid content, almost all of the active sites were binding to Cr(VI), resulting in a higher removal capacity. But with the increase of solid content, the amount of Cr(VI) in solution greatly decreased and quickly reached adsorption equilibrium,

Table 4. High Deconvolution of Mo 3d and S 2p Spectra of LDHs@MoS2 before and after Cr(VI) Removal before adsorption

after adsorption

peak

component

peak position (eV)

area %

peak position (eV)

area %

Mo 3d

Mo 3d3/2 Mo 3d5/2 S 2s S 2p1/2 S 2p3/2

233.0 229.8 226.9 163.8 162.6

33.9 46.9 19.2 25.3 74.7

233.1 229.9 227.1 163.9 162.7

34.5 44.7 20.8 27.5 72.5

S 2p

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The adsorption data and fitting plot of Freundlich and Langmuir models for Cr(VI) removal on the two materials are given in Figure 4. The removal of Cr(VI) on LDHs (Figure 4a) and LDHs@MoS2 (Figure 4b) increased obviously with temperature increasing, suggesting an endothermic process.35 From the calculated isotherm parameters (Table 1), one can see that the Langmuir model simulated the removal of Cr(VI) better than Freundlich model, revealing that the removal process was mainly controlled by monolayer adsorption. According to the Langmuir adsorption simulation, the maximum removal capacities of Cr(VI) on LDHs and LDHs@MoS2 reached to 36.8 and 76.3 mg/g at 298 K, respectively, revealing that LDHs@MoS2 had better removal capacity for Cr(VI) from large volumes of sewage as compared with LDHs. It can be seen from Figure 2d, the specific surface area of LDHs@MoS2 reached 85.5 m2/g, indicating that the composites could provide more functional groups and active sites for the uptake of Cr(VI) from wastewater. As shown in Table 2, although a direct comparison of LDHs@MoS2 with other materials is difficult due to the various experimental conditions, it is important to highlight that the removal capacity of Cr(VI) on LDHs@MoS2 is larger than other adsorbents, suggesting greater advantage in practical applications. To investigate the thermodynamic performance and confirm the relationship between adsorption process and temperature, the thermodynamic study was conducted (Figure 4). The corresponding thermodynamic parameters can be obtained by the following formulas:36

thus some of the active sites were left and still available for further binding, which resulted in the decrease of the removal efficiency.7 Effect of Contact Time. The influence of removal time was illustrated in Figure 3d. It is apparent that most Cr(VI) was adsorbed in the first short contact time, and no further reduction of Cr(VI) concentration was observed with the increase of adsorption time after 120 min, implying the chemical adsorption process. To identify removal rate and potential rate-controlling step of the removal process, the pseudo-second-order and pseudo-first-order kinetic models were used to simulate the data. The pseudo-first-order kinetic model is described as33 ln(qe − qt ) = ln qe − k1t

The pseudo-second-order kinetic model is described as qt =

(2) 33

k 2qe 2t 1 + k 2qet

(3)

where k1 and k2 represent the pseudo-first- and pseudo-secondorder rate constants, qe and qt (mg/g) denote the adsorption of Cr(VI) at equilibrium and time t (min). In Figure 3d, the solid lines represented the pseudo-secondorder model simulation (R2 = 0.995), whereas the dashed lines represented the pseudo-first-order model simulation (R2 = 0.958). It can be seen that the pseudo-second-order model is more suitable for describing the adsorption kinetics than the pseudo-first-order model. Effect of Coexisting Ions. The removal experiments were conducted in the presence of different kinds of cations with the initial concentration of 0.01 M. From Figure 3e, one can see that the coexisting cations (i.e., K+, Na+, Ca+, Mg2+, Zn2+, and Al3+) showed little impact on the elimination of Cr(VI) onto the two materials. Similarly, the effect of coexisting anions on Cr(VI) adsorption was also examined in 0.01 M Na2C2O4, Na3PO4, NaClO4, NaCl, Na2SO4, and Na2CO3 solutions, respectively (Figure 3f). The results revealed that the trapping of Cr(VI) on the samples was independent of the coexisting anions, indicating a huge advantage in practical applications. Adsorption Isotherms and Thermodynamic Study. To explore the possible removal mechanism, the adsorption isotherms of Cr(VI) were performed at different temperature. The Freundlich and Langmuir models were used to depict the data. The Langmuir model is suitable for monolayer adsorption properties, which can be expressed as34

qe =

Kc =

ΔG° = −RT ln Kc

ln Kc =

(4)

The Freundlich model is appropriate for multilayer adsorption, which can be described as34 qe = K f Ce1/ n

ΔS ° ΔH ° − R RT

(6) (7)

(8)

where C e and C Ae (mg/L) represent the equilibrium concentration in solution and on the material; Kc denotes the thermodynamic equilibrium constant; ΔG° (kJ/mol) denotes free energy change; R (8.314 J/(mol K)) represents the ideal gas constant; T (K) denotes absolute temperature; ΔS° (J/ (mol K)) and ΔH° (kJ/mol) are the entropy change and enthalpy change, respectively. As illustrated in Table 3, the ΔG° implied that the removal of Cr(VI) on the materials was a feasible and spontaneous process. The ΔH° confirmed that the adsorption process was endothermic and high temperature was beneficial to remove Cr(VI). Furthermore, the ΔS° suggested the increased randomness in Cr(VI) removal process. Interaction Mechanism. The XRD patterns of LDHs@ MoS2 before and after Cr(VI) removal were investigated to evaluate the possible interaction mechanism. From Figure 5a, one can see that the typical characteristic peaks of LDHs@ MoS2 at (002), (100), (103), and (110) had no change in adsorption process. However, a sharp peak at 42.8° appeared after Cr(VI) removal. This is because that the S groups on the surface of LDHs@MoS2 can form the strong Cr−S bond, which is conducive to the effective capture of Cr(VI).42 Similarly, Ding et al.43 found that the intensity of the sharp peak increased obviously with the increase of Cr(VI) content in the composites. As shown in Figure 5b, the FT-IR spectra implied almost no change for the position of the peaks attributed to O− H and CO32−. Interestingly, the intensity of the peaks in low-

qmax Ceb 1 + Ceb

CAe Ce

(5)

where qe (mg/g) represents the adsorption capacity at equilibrium; qmax (mg/g) denotes the theoretical saturated adsorption capacity; Ce (mg/L) represents the equilibrium concentration of Cr(VI) in solution; b (L/mg) denotes the Langmuir adsorption constant; and n and Kf ((mg1−n Ln)/g) are Freundlich adsorption coefficients in relation to heterogeneity factor and removal capacity, respectively. 7172



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frequency region increased significantly after Cr(VI) adsorption (the dashed area), indicating the strong interaction between S groups and Cr(VI).21 Furthermore, the composition of the composites after Cr(VI) elimination was studied by EDS (Figure 5c). As can be seen from Figure 5d, a new peak at approximately 5.4 keV appeared after Cr(VI) removal, implying the successful capture of Cr(VI) onto LDHs@MoS2. It is worth noting that the elements of S (45.7%), Mo (21.2%), Mg (6.3%), Al (2.2%), C (11.9%), O (10.3%), and Cr (2.4%) were confirmed by the EDS analysis. To further reveal the interaction mechanism, the as-prepared composites before and after Cr(VI) removal were characterized by XPS spectroscopy. It is clear that a variety of peaks such as Mo 3p, Mo 3d, S 2p, Mg 2p, Al 2p, O 1s, and C 1s can be found for the composites before and after Cr(VI) adsorption (Figure 6a). Compared to the pristine sample, a distinct peak at 578.3 eV was observed after adsorption, which could be ascribed to Cr(VI) (Figure 6b).44 As illustrated in Figure 6c, the high resolution of the Mo 3d and S 2s spectra of LDHs@ MoS2 can be divided into three components located at 233.0, 229.8, and 226.9 eV, corresponding to the Mo 3d3/2, Mo 3d5/2, and S 2s, respectively.20 After adsorption, the three peaks changed obviously (Figure 6d), and the relevant binding energy values of the multifarious elements in the composites were tabulated in Table 4. It can be seen that the ratio of the peak area attributed to Mo 3d3/2 slightly increased from 33.9% to 34.5% and that of Mo 3d5/2 decreased from 46.9% to 44.7%, which were ascribed to the removal of Cr(VI) onto LDHs@ MoS2. Meanwhile, the peak position of S 2s moved from 226.9 to 227.1 eV, indicating that S groups played a significant role in the removal process. As can be seen from Figure 6e, the high resolution of the S 2p can be decomposed into two characteristic components located at 163.8 (S 2p1/2) and 162.6 (S 2p3/2), respectively.20 As given in Table 4, the peak position of S 2p1/2 changed from 163.8 to 163.9 eV after adsorption (Figure 6f). The peak position of S 2p3/2 shifted from 162.6 to 162.7 eV, suggesting the strong interaction between S groups and Cr(VI). According to the above analysis and combined with the batch experiments, it can be considered that the interaction mechanism of Cr(VI) with the composites was governed by electrostatic attraction and outersphere surface complexation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-10-61772890. Fax: 86-10-61772890 (X.W.). *E-mail: [email protected]. Phone: 86-10-61772890. Fax: 86-10-61772890 (S.W.). ORCID

Suhua Wang: 0000-0001-8257-4937 Xiangke Wang: 0000-0002-3352-1617 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (91326202, 21577032), the Fundamental Research Funds for the Central Universities (JB2015001, JB2016166), the National Special Water Programs (2015ZX07203-011, 2015ZX07204-007), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development of Jiangsu Higher Education Institutions. X.W. acknowledged the CAS Interdisciplinary Innovation Team of Chinese Academy of Sciences.

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CONCLUSIONS

With the development of industry, heavy metal pollution is threatening the human health. In this study, the molybdenum disulfide coated Mg/Al layered double hydroxide composites were prepared via a simple hydrothermal method for the efficient elimination of Cr(VI). The results implied that the removal of Cr(VI) on LDHs@MoS2 was rely on ionic strength and solution pH. The interaction mechanism of Cr(VI) with the composites was mainly ascribed to electrostatic attraction and outersphere surface complexation. Compared to LDHs, the LDHs@MoS2 indicated excellent chemical stability in the whole pH range, which is beneficial for practical applications. The removal capacity of Cr(VI) on LDHs@MoS2 achieved 76.3 mg/g at pH = 5.0, and the removal process was a spontaneous endothermic process. Specifically, coexisting ions had little impact on the removal process. The findings in this study can improve the application of LDHs@MoS2 composites in the current understanding of Cr(VI) removal or other kinds of metal ions in environmental pollution cleanup. 7173


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