Research Article pubs.acs.org/journal/ascecg
Two-Dimensional Molybdenum Disulfide as a Superb Adsorbent for Removing Hg2+ from Water Feifei Jia,†,‡ Qingmiao Wang,† Jishan Wu,† Yanmei Li,§ and Shaoxian Song*,†,∥ †
School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China ‡ Hubei Key Laboratory of Mineral Resources Processing and Environment, Luoshi Road 122, Wuhan, Hubei 430070, China § Engineering Division, Universidad Autonoma de Guanajuato, Ex Hacienda. de San Matias S/N, Guanajuato, Gto 36000, Mexico ∥ Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources, Luoshi Road 122, Wuhan, Hubei 430070, China S Supporting Information *
ABSTRACT: One feature of two-dimensional (2D) molybdenum disulfide nanosheets is the huge sulfur-rich surface area, which might lead to the strong adsorption of Hg2+ in water, because the sulfur on the surfaces could strongly bind to Hg2+. In this work, the adsorption of Hg2+ on 2D molybdenum disulfide sheets in water has been studied in order to develop a novel and efficient adsorbent for removing Hg2+ from water. The study was performed through the measurements of adsorption isotherm and kinetics, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy−energy-dispersive spectroscopy (SEM-EDS). The partially oxidized 2D molybdenum disulfide sheets with less than five S−Mo−S layers were prepared through the exfoliation of natural molybdenite. AFM observations illustrated a fast and multilayer Hg2+ adsorption on the surface of 2D molybdenum disulfide. The results of adsorption tests and SEM-EDS have indicated that 2D molybdenum disulfide was a superb adsorbent. The adsorption followed the Freundlich isotherm model and fitted well with pseudo-second-order kinetics model. The excellent Hg2+ capture property was mainly attributed to the complexation of Hg2+ with intrinsic S and oxidationinduced O atom exposed on 2D molybdenum disulfide surfaces, as well as the electrostatic interaction between negatively charged 2D molybdenum disulfide and cation Hg2+. KEYWORDS: Two-dimensional molybdenum disulfide, Mercury, Removal, Hg−S complexation, Multilayer adsorption
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2-mercaptobenzimidazole-clay5 and sulfide minerals8−10 were better scavengers for mercury in aqueous solutions than general absorbents. Despite the improved adsorption capacity after the involving of sulfur ligands, these adsorbents were subjected to unwelcome chemical impregnation in the preparation and restricted removal efficiency, because of the limited sulfurs on the surface. Thereby, it is imperative to explore an adsorbent that is free from these disadvantages but still has high mercury capture capacity. Molybdenum disulfide, with a chemical composition of MoS2, is a typical layered transition-metal dichalcogenite (TMD). Each layer consists of one molybdenum sheet sandwiched by two sulfur sheets, forming a S−Mo−S structure (see Figure S1 in the Supporting Information). Molybdenum disulfide has strong chemical bonding in layers and weak van der Waals forces between layers. As shown in Figure S1, both
INTRODUCTION Mercury contamination in water has been a worldwide environmental problem, because of its high toxicity.1 Longterm exposure to mercury would terribly threaten public health.2 Therefore, it is of critical importance to remove mercury from aqueous solutions. Conventional technologies include chemical precipitation, electrochemical process, adsorption, membrane separation, ion exchange, and solvent extraction. Adsorption is regarded as one promising technology, compared with the other technologies, in terms of its ease of operation and low cost. However, because of the weak affinity of the conventional adsorbents with mercury, adsorption suffers from low mercury uptake capacity and poor selectivity.3−5 According to the Pearson Hard Soft Acid Base (HSAB) principle, mercury, which is a soft acid, could complex readily with soft bases such as reduced-S ligands.6 Therefore, sulfur was introduced into the adsorbents to improve their adsorption capacity. Previous research indeed revealed that sulfurcontaining materials, such as sulfurized activated carbons,4,7 © 2017 American Chemical Society
Received: June 11, 2017 Published: July 2, 2017 7410
DOI: 10.1021/acssuschemeng.7b01880 ACS Sustainable Chem. Eng. 2017, 5, 7410−7419
Research Article
ACS Sustainable Chemistry & Engineering
times with deionized water, followed by freeze-drying for 12 h. The resulting molybdenum disulfide sheets were called 2D-M. Some of the 2D-M sheets were taken and dispersed in deionized water (shown in Figure S3c) for the subsequent characterization and Hg2+ adsorption. Some of the 2D-M sheets were heated in a muffle furnace at 500 °C with a heating rate of 10 °C/min. After 2 h of thermal treatment, the sample was cooled to room temperature, followed by dispersing the sample in deionized water (pH 10) and stirring for 12 h to remove the surface oxides that were produced during the thermal treatment. Successively, the sample was filtered using a 0.1 μm micropore filter and washed four times with deionized water. The sample then was freeze-dried for 12 h, and the obtained sample was called 2D-M-500. AFM Study on Hg2+ Adsorption. Sample E-M with a fresh and smooth surface cleaved with Scotch tape was first prepared prior to experimental analysis for a better observation under AFM. After that, several drops of 100 mg/L Hg2+ solution prepared with Hg(NO3)2· H2O was placed on the surface of E-M for a predetermined time, followed by washing the sample with deionized water several times (to remove the unadsorbed Hg2+) and drying in air. The surface morphology was then observed using a Bruker MultiMode 8 AFM system. The measurement was performed in PeakForce Tapping mode with a ScanAsyst-Air silicon nitride probe (nominal tip radius of 2 nm) on a V-shaped cantilever, during which the images were captured with 512 pixels and automatically optimized scan parameters (set point, feedback response, and scan rate). The obtained images then were analyzed with NanoScope Analysis 1.5 software, in which the images were flattened in second order without further processing. Routine Hg2+ Adsorption Experimental. Bath tests were conducted to study the Hg2+ adsorption on 2D-M and 2D-M-500. First, a given amount of adsorbent and Hg(NO3)2·H2O solution with a desired Hg2+ concentration and pH were added into conical flasks. The pH was adjusted by 0.1 and 1.0 mol/L NaOH or HNO3. The flask then was placed into a water bath shaker at a constant temperature of 20 or 35 °C and shaken at 150 rpm for predetermined time intervals. Finally, 5 mL of the suspension was filtered with 0.1 μm filter membrane, during which the first 2 mL of filtrate was discarded and another 3 mL was collected for the chemical analysis of Hg2+. For the adsorption kinetic experiment, 5 mL of suspension containing 16 mg of 2D-M was added into 500 mL 100 mg/L Hg2+ solution and adsorbed for different time intervals (0−180 min). For the adsorption isotherm experiments, 1 mL of suspension containing 1.9 mg of 2D-M or 2D-M-500 was added into 20 mL of Hg2+ solution with a concentration of 30−250 mg/L. The pH of solutions were maintained at 6.0 ± 0.1. AS an experiment to determine the pH influence, 1 mL of suspension containing 1.9 mg of 2D-M and 20 mL of 10 mg/L Hg2+ solution was mixed and shaken at 25 °C for 1 h at the desired pH values. Measurements. XRD patterns were recorded on a D8 Avance system (Bruker AXS, Germany), using Cu Kα radiation (0.15406 nm). Raman spectra were obtained from an INVIA Raman microscope with a 514 nm Ar laser (Renishaw, U.K.). A Zeiss Ultra Plus field-emission scanning electron microscopy (FESEM) system (Zeiss, Germany) was applied to obtain the morphology and the energy-dispersive spectra (EDS). XPS analysis was performed on an ESCALB MK-II instrument (VG, U.K.). The concentration of Hg2+ was detected using a contrAA700 continuum source atomic absorption spectrometry (AAS) system (Jena, Germany). AFM measurements were carried out on a MultiMode 8 AFM system (Bruker, USA). When obtaining the thickness distribution, a dilute dispersion of exfoliated molybdenum disulfide was deposited dropwise on a Si substrate, followed by drying the sample in air for 4 h. Two hundred fifty (250) exfoliated molybdenum disulfide sheets were randomly selected and measured to give the thickness distribution.
surfaces of molybdenum disulfide were fully occupied by intrinsic sulfur atoms, which could serve as the binding sites for mercury, indicating that it might be a potential mercury adsorbent. Our previous work indicated that Hg2+ could be highly immobilized on the surface of natural molybdenum disulfide and present in the form of multilayers by the mechanism of Hg−S complexation and electrostatic interaction.11 However, the neighboring S−Mo−S layers of bulk molybdenum disulfide leave a narrow gallery of ∼0.30 nm,12 which is unfavorable for the access of hydrated Hg2+ ions into the interior spaces. Therefore, the vast majority of S atoms located in the interlayer of molybdenum disulfide cannot bind mercury. However, the great force difference between layers and interlayers of molybdenum disulfide might give rise to the easy cleavage of molybdenum disulfide along the (001) plane, which would bring more surfaces with sulfur exposed outside. Therefore, two-dimensional (2D) molybdenum disulfide exfoliated from bulk molybdenum disulfide might be an efficient adsorbent for the removal of Hg2+ from aqueous solutions. In this work, an attempt was made to explore the adsorption behavior and mechanism of 2D molybdenum disulfide as an adsorbent for the removal of Hg2+ from water. The adsorption efficiency was performed through routine experimental analysis (adsorption isotherm, adsorption kinetics, temperature, and pH effect) and atomic force microscopy (AFM) observation. The adsorption mechanism was studied via X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and analysis of the zeta potential. The objective is to obtain a clear understanding in the adsorption behavior and mechanism of Hg2+ on 2D molybdenum disulfide, as well as to explore the possibility of using 2D molybdenum disulfide as a superb adsorbent for the removal of Hg2+ from aqueous solutions.
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MATERIALS AND METHODS
Materials. Natural molybdenite collected from the Wuzhou Mine, Guangxi Province, China, was used in this work as molybdenum disulfide. The sample is referenced in this paper as M. The X-ray diffraction (XRD) pattern (shown in Figure S2 in the Supporting Information) indicated that the main composition of the sample was molybdenum disulfide. X-ray fluorescence (XRF) revealed an assayed composition of 53.53% Mo, 43.33% S, 1.17% Bi, 0.88% Si, and 0.54% Cl, indicating a molybdenum disulfide content of ∼89.22% in sample M, based on the fact that pure molybdenite contains 60% Mo. Mercuric nitrate (Hg(NO3)2·H2O) used in this study was purchased from Shanghai Zhanyun Chemical Co., Ltd. (China), while nitric acid and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All of the reagents were of analytical grade. Milli-Q water (Millipore, Bedford, MA) with a resistivity of 18.2 MΩ cm was used in the experiments. Methods. Preparation of 2D Molybdenum Disulfide. First, a bulk molybdenite with dimensions of ∼2 cm × 2 cm × 0.1 cm, a Pt foil, and a 0.5 mol/L Na2SO4 solution were used as an anode electrode, a cathode electrode, and an electrolyte, respectively, as shown in Figure S3a in the Supporting Information. The electrochemical expansion was then conducted initially at a constant voltage of 1.0 V for 15 min to wet the bulk molybdenite, followed by increasing the voltage to 10.0 V for 24 h to expand the sample.13 The bulk molybdenite after electrochemical expansion was called E-M. Molybdenite before and after expansion was shown in Figure S3b, revealing that the roughness of sample E-M was higher than sample M. It might be attributed to the oxidation in the expansion, which will be discussed in detail later. E-M then was ultrasonicated in deionized water, using a Sonic Vibracell with 150 W for 20 min. The suspension was successively centrifuged at 1500 rpm for 30 min to obtain the supernatant. Successively, the supernatant was filtered with 0.1 μm micropore filter and washed four
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RESULTS AND DISCUSSION Characterization. Figure 1 shows high-resolution transmission electron microscopy (HRTEM) images of 2D-M at magnifications of 30 000× (Figure 1a), 120 000× (Figures 1b and 1c), and 300 000× (Figure 1d). The lateral size of most exfoliated molybdenum disulfide nanosheets ranged from 7411
DOI: 10.1021/acssuschemeng.7b01880 ACS Sustainable Chem. Eng. 2017, 5, 7410−7419
Research Article
ACS Sustainable Chemistry & Engineering
typical thickness of monolayer molybdenum disulfide exfoliated in a solution process,14 proving the existence of monolayer molybdenum disulfide. The above results demonstrated that molybdenite had been successfully exfoliated into molybdenum disulfide with a 2D structure, and the exfoliated molybdenum disulfide consisted of a monolayer or multiple layers of S−Mo− S. Figures 2a and 2b show a representative AFM image of the mica surface with deposition of 2D-M and the cross-sectional analysis, relative to the line in the AFM image in Figure 2a, respectively. Figure 2a shows that most 2D-M nanosheets had a thin thickness. The step height from the substrate to the nanosheet was measured to be ∼1.7 nm (shown in Figure 2b). The thickness of a monolayer S−Mo−S structure was ∼0.7 nm;14−16 therefore, these nanosheets might belong to bilayer molybdenum disulfide sheets. To obtain a general understanding regarding the thickness distribution of 2D-M, 250 nanosheets were selected randomly to measure the thickness. The distribution is shown in Figure 2c, revealing that monolayer molybdenum disulfide nanosheets accounted for ∼3%. Approximately 34%, 22%, and 15% of the nanosheets had thicknesses in the range of 1.0−2.0 nm, 2.0−3.0 nm, and 3.0− 4.0 nm, respectively, suggesting that the majority of 2D-M belonged to bilayer, trilayer, or four-layer molybdenum disulfide nanosheets. 2D-M with a thickness of >5.0 nm occupied 20%, indicating that 80% of the 2D-M nanosheets had less than five S−Mo−S unit layers. Raman spectra of molybdenum disulfide before and after exfoliation are shown in Figure 3. Two characteristic peaks at 382 and 407 cm−1 were observed on sample M, which corresponded to the E12g and A1g of molybdenum disulfide,
Figure 1. HRTEM images of 2D-M at different magnifications: (a) 30 000×, (b and c) 120 000×, and (d) 300 000×.
hundreds of nanometers to micrometers (Figure 1a). The transparency of molybdenum disulfide nanosheets to visible light indicated their thin thickness (Figure 1b). Molybdenum disulfide nanosheets with a thickness of several nanometers were observed in Figure 1c, which might be derived from the halfway exfoliation or the pileup of the thin sheets. Figure 1d shows a folded edge of one molybdenum disulfide nanosheet. The thickness was ∼0.7 nm, which was consistent with the
Figure 2. AFM characterization of 2D-M: (a) typical AFM image of 2D-M, (b) corresponding height cross-section profile of the 2D-M nanosheet marked in panel (a), and (c) histogram of the thickness distribution of 2D-M. 7412
DOI: 10.1021/acssuschemeng.7b01880 ACS Sustainable Chem. Eng. 2017, 5, 7410−7419
Research Article
ACS Sustainable Chemistry & Engineering
AFM Observation of Hg2+ Adsorption on E-M. AFM was used to investigate the immobilization of Hg2+ on the surface of E-M. Two-dimensional (2D) and three-dimensional (3D) AFM images of E-M at different Hg2+ adsorption times are illustrated in Figures 5a−e. An atomically smooth surface with some dwarf slices was captured before adsorption (Figure 5a). These small bumps might originate from the oxidation of natural molybdenum disulfide. After being exposed to Hg2+ solution for 5 min, needlelike formations in the range of 30−70 nm were observed on the surface of E-M (Figure 5b). These formations were confirmed Hg2+ by XPS result inserted in Figure 5b. Figure 5b indicated that the adsorption of Hg2+ on E-M was a quick process and most of the surface was occupied by Hg2+ in 5 min. As the adsorption time increased to 15 min, a second mercury layer appeared above the first adsorbed layer (Figure 5c). More mercury was observed and the height of the newly formed layer increased with the further exposure to Hg2+ solution (Figures 5d and 5e). The AFM observation demonstrated a fast and multilayer adsorption of Hg2+ on EM, which could indicate that 2D-M might be highly capable of capturing Hg2+, because it had much greater specific surface area and more exposed S atoms. Adsorption Kinetics of Hg2+ on 2D-M. The adsorption of Hg2+ on 2D-M, as a function of time, is illustrated in Figure 6. It showed fast adsorption kinetics when adding 16.0 mg of 2DM into 500 mL of 100 mg/L Hg2+ solution. The adsorption proceeded at a high rate in the first 60 min and reached a capacity of ∼183.579 mg/g after ∼60 min, while the equilibrium adsorption capacity of sulfur-supported activated carbon was
