- Email: [email protected]
MoS2 nanosheets decorated with magnetic Fe3O4 nanoparticles and their ultrafast adsorption for wastewater treatment
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 13896–13902 www.elsevier.com/locate/ceramint
Short communication
MoS2 nanosheets decorated with magnetic Fe3O4 nanoparticles and their ultrafast adsorption for wastewater treatment Hao Jie Songa, Shengsheng Youa, Xiao Hua Jiab,n, Jin Yanga a
Institute of Polymer Materials, School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, China b School of Environment and Safety Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China Received 24 July 2015; received in revised form 4 August 2015; accepted 5 August 2015 Available online 14 August 2015
Abstract Magnetic Fe3O4/MoS2 nanocomposites were fabricated by a simple hydrothermal route in a water–ethanol system. The Fe3O4/MoS2 nanocomposites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, magnetic hysteresis studies, and nitrogen adsorption–desorption isotherms. Successful attachment of Fe3O4 nanoparticles to MoS2 sheets has been achieved. The nanocomposites which had an excellent magnetic sensitivity, could be easily and quickly separated from the suspension by applying an external magnetic field and exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Furthermore, they presented an ultrafast adsorption for dye removal. Only about 2 min was sufficient to approach the adsorption equilibrium. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Fe3O4/MoS2 nanocomposites; Hydrothermal route; Water treatment; Ultrafast adsorption
1. Introduction The removal of toxic contaminants from water is a big challenge, especially for wastewater of industrial fields. This could destroy the environment and endanger the organisms owing to their toxicity. Recently, several technologies have been developed for dye removal from aqueous solutions [1–3]. Because of low cost, ease of operation, flexibility, small amounts of harmful substances and simplicity of design, adsorption is proved to be an effective and attractive process for the treatment of these dye-bearing wastewaters [4,5]. Nowadays, a growing number of functional nanomaterials, for example, carbon material (active carbon, carbon nanotube, graphene), polymer material (polyacrylamide, cyclodextrin), magnetic material (nickel, Fe3O4) have been chosen for new sorbents to remove organic compounds from wastewater due to its superior adsorption capability [6,7]. Layered molybdenum disulfide (MoS2), a newly emerging semiconductor, has attracted immense attention due to its n
Corresponding author. Tel.: þ86 511 88790191; fax: þ 86 511 88780191. E-mail address: [email protected] (X.H. Jia).
http://dx.doi.org/10.1016/j.ceramint.2015.08.023 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
many unusual properties, which make it has broad applications in the field such as energy devices, spintronics and optoelectronics [8–10]. In our previous work, we have found that the MoS2 nanosheets have excellent adsorption performance. While after becoming exhausted, the adsorbed MoS2 nanosheets needs to be separated from the solution phase and regenerated. Unfortunately, it is complex to separate MoS2 nanosheets from solution phase after adsorption via traditional centrifugation and filtration. This restricts the applications of MoS2 in adsorption to some extent. Recently, new separation methods based on the use of magnetic nanomaterials have been found to be simple, convenient, and powerful approaches for the separation and purification of environmental samples, and removal of toxic pollutants in water [11–13]. As these particles are superparamagnetic, they could be easily separated from the treated aqueous media by an external magnetic field [14–16]. This difficulty of the separation of MoS2 can be overcome by the magnetic nanoparticles attaching on the surface of MoS2 nanosheets. Recently, in order to separate the adsorbent from solution, several researchers have prepared magnetic nanoparticle dispersed in the adsorbent. Such as, Yao et al. have
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
developed a magnetic Fe3O4@graphene nanocomposite for removal of Methylene blue from aqueous solution [17]. Yang et al. also synthesized magnetic Fe3O4-activated carbon nanocomposite for removal of Methylene blue from solution phase [18]. However, to the best of our knowledge, the application of magnetic Fe3O4/MoS2 as adsorbent to remove organic dye was seldom reported. In this paper, superparamagnetic Fe3O4 nanoparticles with the average size of 10–15 nm distributed on MoS2 nanosheet via a simple hydrothermal route. The prepared products had an excellent magnetic sensitivity, can be easily and quickly separated from suspension by applying an external magnetic field. The nanocomposites exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Moreover, they presented the property of ultrafast adsorption for dye removal. Only about 2 min is sufficient to approach the adsorption equilibrium. This information may be useful for further research and practical applications of the novel MoS2 adsorbent in wastewater treatment. 2. Material and methods 2.1. Synthesis of MoS2 All chemicals used were analytical-grade from Shanghai Chemical Reagent Corporation and without further purification. In a typical experiment, 90 mg sodium molybdate (Na2MoO4 2H2O) and 180 mg thioacetamide (C2H5NS) were dissolved in 60 mL deionized water to form a transparent solution. Then the mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 240 1C for 24 h. After being cooled to room temperature, the products were isolated by centrifugation, repeatedly washed with de-ionized water and absolute ethanol several times to remove the impurities and dried in a vacuum oven at 60 1C for 12 h to obtain the MoS2 nanosheets. 2.2. Synthesis of Fe3O4/MoS2 nanocomposites In a typical procedure, MoS2 (50 mg) was dispersed in 60 mL of ethanol/water (1:1) and ultrasonicated for 3 h at room temperature to obtain suspension liquid. Then, FeCl3 6H2O (0.1838 g, 0.00216 mol) and FeCl2 4H2O (0.0703 g, 0.00108 mol) were dissolved in 10 mL distilled water under continuous magnetic stirring. This was followed by the addition of ferric and ferrous salts aqueous solution to the suspension and then the mixture was stirred vigorously at 80 1C for 30 min. After that, 2 mL of ammonia solution
13897
(NH4OH) (28 wt%) was quickly injected into the mixed solution. The as-formed viscous slurry was transformed into a Teflon-lined stainless steel autoclave of 100 mL capacity and maintained at 100 1C for 2 h. After the sample was cooled to room temperature, black precipitates were collected after being rinsed with pure ethanol and water repeatedly, and it was dried in vacuum oven at 60 1C for 10 h. For comparison purposes, the pure Fe3O4 nanoparticles were also synthesized under the same experimental conditions without adding the MoS2. 2.3. Characterization The phase structure and phase compositions of the asfabricated products were identified by X-ray diffraction (XRD, Philips X'pert X-ray diffractometer with Cu-Kα radiation, λ¼ 1.5406 Å) at 40 kV, 30 mA over the 2θ range 10–801. The morphologies and microstructures of the products were examined by scanning electron microscopy (SEM, JEOL JSM6700F). High-resolution TEM (HR-TEM) was taken using the JEOL JEM 2010F microscope working at 200 kV. The Brunauer–Emmett–Teller (BET) surface areas of the products were analyzed using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The magnetic properties of the samples were investigated using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 15 kOe. 2.4. Adsorption experiments Five types of organic dyes (Congo red (CR), Methylene blue (MB), Methylene green (MG), Rhodamine B (RhB), Eosin Y (EY)) were chosen as study models in the adsorption experiment. In the adsorption test, the experiments were carried out in glass bottles at room temperature. Certain amount of Fe3O4/ MoS2 nanocomposites were added into different organic dyes solution. The suspension was ultrasonicated for several mintues at room temperature. After desired adsorption time, some specimens (5 mL) were withdrawn from the suspension and were separated by magnet to remove the adsorbent. The supernatant solutions were analyzed with UV–vis spectroscopy to obtain the concentrations of dye in the solution. 3. Results and discussion The synthesis process of the Fe3O4/MoS2 nanocomposites was schematically illustrated in Fig. 1. Iron salt was mixed with an ultrasonically dispersed in MoS2 nanosheets water– ethanol solution and then the Fe2 þ , and Fe3 þ were bound to the surface of the MoS2 nanosheets through electrostatic attraction. Then, the precursors of Fe(OH)2 and Fe(OH)3 were
Fig. 1. Schematic procedure for the synthesis of Fe3O4/MoS2 nanocomposites.
13898
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
formed after the adding of NH3 H2O. Finally, the hydrothermal reactions led to the formation of Fe3O4/MoS2 nanocomposites. Furthermore, Fe3O4 nanoparticles were deposited on the surface of MoS2 nanosheet and the heterojunction structure between MoS2 and Fe3O4 could be formed. The crystal structure and phase purity of the samples were characterized by XRD. As shown in Fig. 2a, the diffraction peaks of MoS2 nanosheets correspond well with the (002), (100), (103), (105) and (110) reflections of the pure hexagonal phase of MoS2 (JCPDS Card no. 37-1492). From The XRD patterns of pure Fe3O4 nanoparticles, the (220), (311), (222), (400), (422), (511) and (440) plane of Fe3O4 are observed at 2θ ¼ 30.121, 35.741, 37.341, 43.581, 54.161, 57.541 and 62.581 respectively, which was in good agree with the reported data for Fe3O4 and were well indexed to the standard cubic phase of Fe3O4 (space group: Fd3m (227), JCPDS no. 89-0691). The diffraction peak (002), (100), (103), (105) and (110) are the characteristic peaks of MoS2 that was also presented in XRD pattern of the Fe3O4/MoS2 nanocomposites. It was also seen that the crystal phase of Fe3O4 did not change after hybridization with MoS2, but the value of 2θ for Fe3O4 were less than those for pure Fe3O4 because of the interaction between Fe3O4 and MoS2. No peaks corresponding to impurities are detected. So it could be concluded that the Fe3O4/MoS2 nanocomposites were formed during the solvothermal reaction. Energydispersive X-ray spectrometer (EDS) results are shown in Fig. 2(b), which reveals the presence of Mo, S, Fe, and O (the presence of the Si peak in Fig. 1b because of scanning test is based on the silicon substrate) and no other element was observed. The calculated atomic ratio of Mo to S is close to 1:2 and Fe to O is close to 3:4, which further confirms that the product was a mixture of two phases: the MoS2 and the Fe3O4. SEM images of the MoS2 nanosheet, Fe3O4 nanoparticles and Fe3O4/MoS2 nanocomposites are displayed in Fig. 3. As shown in Fig. 3(a), two-dimensional ultrathin uniform sheetlike clusters of MoS2 nanosheets were successfully fabricated with large area. The SEM images of MoS2 nanosheets clearly show that the thickness of MoS2 nanosheets was about 15 nm. The Fe3O4 nanoparticles with diameters ranging from 10 to
15 nm were obtained (Fig. 3(b)), and these particles tended to be aggregated together. It can be seen in Fig. 3(c) and (d) that Fe3O4 nanoparticles distributed on the surface of MoS2 nanosheets. In other words, the nanosheets structure was not destroyed after being introduced Fe3O4 nanoparticles during the hydrothermal treatment. Moreover, the MoS2 nanosheets can prevent the aggregation of Fe3O4 nanoparticles to a certain extent, which can be of great benefit to reactions. The TEM images of the nanocomposite (Fig. 4) reveal the morphology feature, which is in agreement with the SEM result. The dark spots correspond to Fe3O4 nanoparticles, which are isolated from each other and absorbed on the surface of MoS2 nanosheets. The average size of the Fe3O4 nanoparticles is about 15 nm. In the high resolution TEM (HRTEM) image (Fig. 4d), the inter-planar distance of two kinds of nanomaterials are approximately 0.62 nm and 0.25 nm, which agrees well with the lattice spacing of the (002) and (311) plane of MoS2 and Fe3O4 nanoparticles, respectively. More importantly, even after a long time of sonication during the preparation of the TEM specimen, the Fe3O4 nanoparticles were still strongly anchored on the surface of MoS2 nanosheets with a high density. All the above data proved that Fe3O4 nanoparticles were effectively attached onto the surface of MoS2 nanosheets. Fig. 5 shows the magnetization curves of Fe3O4 and Fe3O4/ MoS2 nanocomposites, which are measured at room temperature with an applied magnetic field sweeping from 15 to 15 kOe. All curves appear nonlinear and reversible characteristic with no hysteresis (zero coercivity and no remanence), exhibiting superparamagnetic behavior. The saturation magnetization (Ms) values of Fe3O4 and Fe3O4/MoS2 are 76.85 emu/g and 60.67 emu/g respectively, which completely meet the requirement of the magnetic separation. The observed reduction of Ms values of Fe3O4/MoS2 nanocomposites was mainly due to the non-magnetic MoS2 coating on the surface of Fe3O4 particles The prepared Fe3O4/MoS2 nanocomposites can be easily and quickly separated from suspension by applying an external magnetic field. The magnetic separability of nanoparticles was tested in water by placing a magnet
Fig.2. (a) XRD patterns of Fe3O4, MoS2, and Fe3O4/MoS2 nanocomposites and (b) EDX of the Fe3O4/MoS2 nanocomposites.
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
13899
Fig. 3. SEM images of (a) Fe3O4; (b) MoS2 nanosheet and (c, d) Fe3O4/MoS2 nanocomposites.
Fig. 4. TEM (a–c) and HRTEM (d) images of Fe3O4/MoS2 nanocomposites.
beside the glass bottle. The black sample was attracted toward the magnet within 20 s (in inset of Fig. 5), demonstrating directly that the prepared products had an excellent magnetic
sensitivity. The magnetic sensitive property would favor the separation of Fe3O4/MoS2 nanocomposites from dyecontaining effluents in the future.
13900
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
Fig. 5. Room-temperature magnetic hysteresis loops Fe3O4, and Fe3O4/MoS2 nanocomposites.
Fig.6. N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of Fe3O4/MoS2 nanocomposites.
Fig. 6 shows N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of Fe3O4/MoS2 nanocomposites. Using the BJH method and the adsorption branch of the nitrogen isotherm, the calculated pore size distribution indicates that the material contains an average pore size of 16.0819 nm. The specific surface area of the Fe3O4/MoS2 nanocomposites is calculated to be 72.0727 m2/g by the BET equation. As-synthesized Fe3O4/MoS2 nanocomposites have a high special surface area, which may provides more surface active sites and pore-channels for the adsorptions and diffusion of reactants. The adsorption activities of Fe3O4/MoS2 nanocomposites were evaluated by the degradation of different organic dyes in aqueous solution. The samples of 20 mg were dispersed respectively in 100 mL different organic dyes aqueous solution with an initial concentration of 20 mg/L. Five types of organic dyes (Congo red (CR), Methylene blue (MB), Methylene green (MG), Rhodamine B (RhB), Eosin Y (EY)) were chosen as study models in the adsorption experiments. Fig. 7(a) shows the adsorption capacities of different dyes by the Fe3O4/MoS2
nanocomposites, which indicates that the nanocomposites have higher adsorption capacities for CR than for RhB, MG, MB and EY. In other words, the Fe3O4/MoS2 nanocomposites adsorbent has good selectivity to CR. In order to investigate the adsorption capacities of Fe3O4/MoS2 nanocomposites for CR, the adsorption experiment were carried out respectively in the different initial concentration of Congo red (5 mg/L, 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L) and the adsorption time is 20 min. The adsorption isotherm of Congo red by the as-synthesized nanocomposites can be seen from Fig. 7(b), it was demonstrated that the adsorption capacity of Fe3O4/MoS2 nanocomposites for Congo red is 71 mg/g. The rate of adsorption is very important for adsorbent. In order to directly observe the quick removal of pollutants by the Fe3O4/MoS2 nanocomposites, 50 mg of sample was added to 30 mL of CR solution with an initial concentration of 40 mg/L. The suspension was ultrasonicated for 2 min at room temperature, then the Fe3O4/MoS2 nanocomposites were separated by magnet and the supernatant solutions were analyzed with UV– vis spectroscopy to obtain the concentrations of dye in the solution (Fig.7(c)). Fig. 7(c) shows that more than 99% of CR was quickly removed from the solution within 2 min. The red color of CR solution almost disappeared, and finally becomes nearly transparent which can be seen from the inset of Fig.7(c). Furthermore, to illustrate the quick adsorption nature of the nanocomposites, the percentage removal for Congo red at different time intervals was recorded after 50 mg of Fe3O4/ MoS2 nanocomposites was added into 50 mL of CR solution with an initial concentration of 100 mg/L (Fig. 7(d)). About 65.2% of CR was removed after the sample was added into the solution for 2 min, whereas only 6.2% additional CR was removed after the next 120 min adsorption. The adsorption equilibrium of Fe3O4/MoS2 nanocomposites can be almost achieved in very short time, which is significantly faster than that of the previously reported some traditional nanostructures [19–21].
4. Conclusions In summary, the superparamagnetic nanocomposites with magnetic Fe3O4 nanoparticles decorated on MoS2 nanosheets were prepared by a simple hydrothermal route. This is a simple, effective, low-cost, and environment friendly approach. The experimental results clearly demonstrated the successful attachment of Fe3O4 nanoparticles to MoS2 nanosheets. The prepared products had an excellent magnetic sensitivity and can be easily and quickly separated from suspension by applying an external magnetic field. The nanocomposites exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Furthermore, they presented the property of ultrafast adsorption for dye removal. Only about 2 min was sufficient to approach the adsorption equilibrium. This information may be useful for further research and practical applications of the novel MoS2 adsorbent in anilines wastewater treatment.
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
13901
Fig. 7. (a) The adsorption capacities of different dyes by the Fe3O4/MoS2 nanocomposites; (b) the adsorption capacities for Congo red at varied concentrations by the Fe3O4/MoS2 nanocomposites, the initial concentration of Congo red is 5–50 mg/L and the adsorption time is 20 min; (c) the adsorption spectra of Congo red aqueous solution of Fe3O4/MoS2 nanocomposites. (d) Percentage removal for Congo red by the Fe3O4/MoS2 nanocomposites. The adsorption rate test was conducted with an initial CR concentration of 100 mg/L. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51202092 and 51372103). References [1] H. Mittal, S.B. Mishra, Gum ghatti and Fe3O4 magnetic nanoparticles based nanocomposites for the effective adsorption of rhodamine B, Carbohydr. Polym. 101 (2014) 1255–1264. [2] T.Y. Li, C. Yang, X.H. Rao, F. Xiao, J.D. Wang, X.T. Su, Synthesis of magnetically recyclable Fe3O4@NiO nanostructures for styrene epoxidation and adsorption application, Ceram. Int. 41 (2015) 2214–2220. [3] D. Chen, Y. Li, J. Zhang, J.Z. Zhou, Y. Guo, H. Liu, Magnetic Fe3O4/ ZnCr-layered double hydroxide composite with enhanced adsorption and photocatalytic activity, Chem. Eng. J. 185–186 (2012) 120–126. [4] X. Zhang, P.Y. Zhang, Z. Wu, L. Zhang, G.M. Zeng, C.J. Zhou, A dsorption of methylene blue onto humic acid-coated Fe3O4 nanoparticles, Colloids Surf. A: Physicochem. Eng. Asp. 435 (2013) 85–90. [5] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interface Sci. 209 (2014) 172–184. [6] H. Mittal, N. Ballav, S.B. Mishra, Gum ghatti and Fe3O4 magnetic nanoparticles based nanocomposites for the effective adsorption of methylene blue from aqueous solution, J. Ind. Eng. Chem. 20 (2014) 2184–2192.
[7] R.H. Wu, J.H. Liu, L.Q. Zhao, X.L. Zhang, J.R. Xie, B.W. Yu, X.L. Ma, S.T. Yang, H.F. Wang, Y.F. Liu, Hydrothermal preparation of magnetic Fe3O4@C nanoparticles for dye adsorption, J. Environ. Chem. Eng. 2 (2014) 907–913. [8] X.Z. Wang, S.X. Yang, Q. Yue, F.M. Wu, J.B. Li, Response of MoS2 nanosheet field effect transistor under different gas environments and its long wavelength photoresponse characteristics, J. Alloy. Compd. 615 (2014) 989–993. [9] A.L. Friedman, F.K. Perkins, E. Cobas, G.G. Jernigan, P.M. Campbell, A.T. Hanbicki, B.T. Jonker, Chemical vapor sensing of two-dimensional MoS2 field effect transistor devices, Solid-State Electron. 101 (2014) 2–7. [10] C.N.R. Rao, U. Maitra, U.V. Waghmare, Extraordinary attributes of 2dimensional MoS2 nanosheets, Chem. Phys. Lett. 609 (2014) 172–183. [11] Z.G. Liu, F.S. Zhan, R. Sasai, Arsenate removal from water using Fe3O4loaded activated carbon prepared from waste biomass, Chem. Phys. Lett. 160 (2010) 57–62. [12] W.L. Liao, Y.Q. Ma, A.Y. Chen, Y.L. Yang, Preparation of fatty acids coated Fe3O4 nanoparticles for adsorption and determination of benzo(a) pyrene in environmental water samples, Chem. Eng. J. 271 (2015) 232–239. [13] L. Liang, Q.C. Zhu, T.B. Wang, F.X. Wang, J. Ma, L.Q. Jing, J.M. Sun, The synthesis of core–shell Fe3O4@mesoporous carbon in acidic medium and its efficient removal of dye, Microporous Mesoporous Mater. 197 (2014) 221–228. [14] S. Rostamnia, E. Doustkhah, Synthesis of water-dispersed magnetic nanoparticles (H2O-DMNPs) of β-cyclodextrin modified Fe3O4 and its
13902
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
catalytic application in Kabachnik–Fields multicomponent reaction, J. Magn. Magn. Mater. 386 (2015) 111–116. [15] T.J. Yao, Q. Zuo, H. Wang, J. Wu, B.F. Xin, F. Cui, T.Y. Cui, A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis, J. Colloid Interface Sci. 450 (2015) 366–373. [16] Y. Wu, Z.M. Li, J. Chen, C.G. Yu, X. Huang, C.Z. Zhao, L.F. Duan, Y. Yang, W. Lü, Graphene nanosheets decorated with tunable magnetic nanoparticles and their efficiency of wastewater treatment, Mater. Res. Bull. 68 (2015) 234–239. [17] Y.J. Yao, S.D. Miao, S.Z. Liu, L.P. Ma, H.Q. Sun, S.B. Wang, Synthesis, characterization, and adsorption properties of magnetic Fe3O4@graphene nanocomposite, Chem. Eng. J. 184 (2012) 326–332.
[18] N. Yang, S.M. Zhu, D. Zhang, S. Xu, Synthesis and properties of magnetic Fe3O4-activated carbon nanocomposite particles for dye removal, Mater. Lett. 62 (2008) 645–647. [19] Y.H. Li, Q.J. Du, T.H. Liu, X.J. Peng, J.J. Wang, J.K. Sun, Y.H. Wang, S.L. Wu, Z.H. Wang, Y.Z. Xia, L.H. Xia, Comparative study of methylene blue dye adsorption onto activated carbon, graphene oxide, and carbon nanotubes, Chem. Eng. Res. Des. 91 (2013) 361–368. [20] L. Xiao, S.S. Zhang, J.H. Huang, Effective removal of organic dyes by tungstate oxide nanourchins, Powder Technol. 258 (2014) 297–303. [21] C.Z. Li, Y.H. Dong, J.J. Yan, Y.Y. Li, C.C. Huang, Modified nanographite/Fe3O4 composite as efficient adsorbent for the removal of methyl violet from aqueous solution, J. Mol. Liq. 196 (2014) 348–356.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 13896–13902 www.elsevier.com/locate/ceramint
Short communication
MoS2 nanosheets decorated with magnetic Fe3O4 nanoparticles and their ultrafast adsorption for wastewater treatment Hao Jie Songa, Shengsheng Youa, Xiao Hua Jiab,n, Jin Yanga a
Institute of Polymer Materials, School of Materials Science & Engineering, Jiangsu University, Zhenjiang 212013, China b School of Environment and Safety Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China Received 24 July 2015; received in revised form 4 August 2015; accepted 5 August 2015 Available online 14 August 2015
Abstract Magnetic Fe3O4/MoS2 nanocomposites were fabricated by a simple hydrothermal route in a water–ethanol system. The Fe3O4/MoS2 nanocomposites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, magnetic hysteresis studies, and nitrogen adsorption–desorption isotherms. Successful attachment of Fe3O4 nanoparticles to MoS2 sheets has been achieved. The nanocomposites which had an excellent magnetic sensitivity, could be easily and quickly separated from the suspension by applying an external magnetic field and exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Furthermore, they presented an ultrafast adsorption for dye removal. Only about 2 min was sufficient to approach the adsorption equilibrium. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Fe3O4/MoS2 nanocomposites; Hydrothermal route; Water treatment; Ultrafast adsorption
1. Introduction The removal of toxic contaminants from water is a big challenge, especially for wastewater of industrial fields. This could destroy the environment and endanger the organisms owing to their toxicity. Recently, several technologies have been developed for dye removal from aqueous solutions [1–3]. Because of low cost, ease of operation, flexibility, small amounts of harmful substances and simplicity of design, adsorption is proved to be an effective and attractive process for the treatment of these dye-bearing wastewaters [4,5]. Nowadays, a growing number of functional nanomaterials, for example, carbon material (active carbon, carbon nanotube, graphene), polymer material (polyacrylamide, cyclodextrin), magnetic material (nickel, Fe3O4) have been chosen for new sorbents to remove organic compounds from wastewater due to its superior adsorption capability [6,7]. Layered molybdenum disulfide (MoS2), a newly emerging semiconductor, has attracted immense attention due to its n
Corresponding author. Tel.: þ86 511 88790191; fax: þ 86 511 88780191. E-mail address: [email protected] (X.H. Jia).
http://dx.doi.org/10.1016/j.ceramint.2015.08.023 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
many unusual properties, which make it has broad applications in the field such as energy devices, spintronics and optoelectronics [8–10]. In our previous work, we have found that the MoS2 nanosheets have excellent adsorption performance. While after becoming exhausted, the adsorbed MoS2 nanosheets needs to be separated from the solution phase and regenerated. Unfortunately, it is complex to separate MoS2 nanosheets from solution phase after adsorption via traditional centrifugation and filtration. This restricts the applications of MoS2 in adsorption to some extent. Recently, new separation methods based on the use of magnetic nanomaterials have been found to be simple, convenient, and powerful approaches for the separation and purification of environmental samples, and removal of toxic pollutants in water [11–13]. As these particles are superparamagnetic, they could be easily separated from the treated aqueous media by an external magnetic field [14–16]. This difficulty of the separation of MoS2 can be overcome by the magnetic nanoparticles attaching on the surface of MoS2 nanosheets. Recently, in order to separate the adsorbent from solution, several researchers have prepared magnetic nanoparticle dispersed in the adsorbent. Such as, Yao et al. have
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
developed a magnetic Fe3O4@graphene nanocomposite for removal of Methylene blue from aqueous solution [17]. Yang et al. also synthesized magnetic Fe3O4-activated carbon nanocomposite for removal of Methylene blue from solution phase [18]. However, to the best of our knowledge, the application of magnetic Fe3O4/MoS2 as adsorbent to remove organic dye was seldom reported. In this paper, superparamagnetic Fe3O4 nanoparticles with the average size of 10–15 nm distributed on MoS2 nanosheet via a simple hydrothermal route. The prepared products had an excellent magnetic sensitivity, can be easily and quickly separated from suspension by applying an external magnetic field. The nanocomposites exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Moreover, they presented the property of ultrafast adsorption for dye removal. Only about 2 min is sufficient to approach the adsorption equilibrium. This information may be useful for further research and practical applications of the novel MoS2 adsorbent in wastewater treatment. 2. Material and methods 2.1. Synthesis of MoS2 All chemicals used were analytical-grade from Shanghai Chemical Reagent Corporation and without further purification. In a typical experiment, 90 mg sodium molybdate (Na2MoO4 2H2O) and 180 mg thioacetamide (C2H5NS) were dissolved in 60 mL deionized water to form a transparent solution. Then the mixed solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 240 1C for 24 h. After being cooled to room temperature, the products were isolated by centrifugation, repeatedly washed with de-ionized water and absolute ethanol several times to remove the impurities and dried in a vacuum oven at 60 1C for 12 h to obtain the MoS2 nanosheets. 2.2. Synthesis of Fe3O4/MoS2 nanocomposites In a typical procedure, MoS2 (50 mg) was dispersed in 60 mL of ethanol/water (1:1) and ultrasonicated for 3 h at room temperature to obtain suspension liquid. Then, FeCl3 6H2O (0.1838 g, 0.00216 mol) and FeCl2 4H2O (0.0703 g, 0.00108 mol) were dissolved in 10 mL distilled water under continuous magnetic stirring. This was followed by the addition of ferric and ferrous salts aqueous solution to the suspension and then the mixture was stirred vigorously at 80 1C for 30 min. After that, 2 mL of ammonia solution
13897
(NH4OH) (28 wt%) was quickly injected into the mixed solution. The as-formed viscous slurry was transformed into a Teflon-lined stainless steel autoclave of 100 mL capacity and maintained at 100 1C for 2 h. After the sample was cooled to room temperature, black precipitates were collected after being rinsed with pure ethanol and water repeatedly, and it was dried in vacuum oven at 60 1C for 10 h. For comparison purposes, the pure Fe3O4 nanoparticles were also synthesized under the same experimental conditions without adding the MoS2. 2.3. Characterization The phase structure and phase compositions of the asfabricated products were identified by X-ray diffraction (XRD, Philips X'pert X-ray diffractometer with Cu-Kα radiation, λ¼ 1.5406 Å) at 40 kV, 30 mA over the 2θ range 10–801. The morphologies and microstructures of the products were examined by scanning electron microscopy (SEM, JEOL JSM6700F). High-resolution TEM (HR-TEM) was taken using the JEOL JEM 2010F microscope working at 200 kV. The Brunauer–Emmett–Teller (BET) surface areas of the products were analyzed using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The magnetic properties of the samples were investigated using a vibrating sample magnetometer (VSM) with a maximum magnetic field of 15 kOe. 2.4. Adsorption experiments Five types of organic dyes (Congo red (CR), Methylene blue (MB), Methylene green (MG), Rhodamine B (RhB), Eosin Y (EY)) were chosen as study models in the adsorption experiment. In the adsorption test, the experiments were carried out in glass bottles at room temperature. Certain amount of Fe3O4/ MoS2 nanocomposites were added into different organic dyes solution. The suspension was ultrasonicated for several mintues at room temperature. After desired adsorption time, some specimens (5 mL) were withdrawn from the suspension and were separated by magnet to remove the adsorbent. The supernatant solutions were analyzed with UV–vis spectroscopy to obtain the concentrations of dye in the solution. 3. Results and discussion The synthesis process of the Fe3O4/MoS2 nanocomposites was schematically illustrated in Fig. 1. Iron salt was mixed with an ultrasonically dispersed in MoS2 nanosheets water– ethanol solution and then the Fe2 þ , and Fe3 þ were bound to the surface of the MoS2 nanosheets through electrostatic attraction. Then, the precursors of Fe(OH)2 and Fe(OH)3 were
Fig. 1. Schematic procedure for the synthesis of Fe3O4/MoS2 nanocomposites.
13898
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
formed after the adding of NH3 H2O. Finally, the hydrothermal reactions led to the formation of Fe3O4/MoS2 nanocomposites. Furthermore, Fe3O4 nanoparticles were deposited on the surface of MoS2 nanosheet and the heterojunction structure between MoS2 and Fe3O4 could be formed. The crystal structure and phase purity of the samples were characterized by XRD. As shown in Fig. 2a, the diffraction peaks of MoS2 nanosheets correspond well with the (002), (100), (103), (105) and (110) reflections of the pure hexagonal phase of MoS2 (JCPDS Card no. 37-1492). From The XRD patterns of pure Fe3O4 nanoparticles, the (220), (311), (222), (400), (422), (511) and (440) plane of Fe3O4 are observed at 2θ ¼ 30.121, 35.741, 37.341, 43.581, 54.161, 57.541 and 62.581 respectively, which was in good agree with the reported data for Fe3O4 and were well indexed to the standard cubic phase of Fe3O4 (space group: Fd3m (227), JCPDS no. 89-0691). The diffraction peak (002), (100), (103), (105) and (110) are the characteristic peaks of MoS2 that was also presented in XRD pattern of the Fe3O4/MoS2 nanocomposites. It was also seen that the crystal phase of Fe3O4 did not change after hybridization with MoS2, but the value of 2θ for Fe3O4 were less than those for pure Fe3O4 because of the interaction between Fe3O4 and MoS2. No peaks corresponding to impurities are detected. So it could be concluded that the Fe3O4/MoS2 nanocomposites were formed during the solvothermal reaction. Energydispersive X-ray spectrometer (EDS) results are shown in Fig. 2(b), which reveals the presence of Mo, S, Fe, and O (the presence of the Si peak in Fig. 1b because of scanning test is based on the silicon substrate) and no other element was observed. The calculated atomic ratio of Mo to S is close to 1:2 and Fe to O is close to 3:4, which further confirms that the product was a mixture of two phases: the MoS2 and the Fe3O4. SEM images of the MoS2 nanosheet, Fe3O4 nanoparticles and Fe3O4/MoS2 nanocomposites are displayed in Fig. 3. As shown in Fig. 3(a), two-dimensional ultrathin uniform sheetlike clusters of MoS2 nanosheets were successfully fabricated with large area. The SEM images of MoS2 nanosheets clearly show that the thickness of MoS2 nanosheets was about 15 nm. The Fe3O4 nanoparticles with diameters ranging from 10 to
15 nm were obtained (Fig. 3(b)), and these particles tended to be aggregated together. It can be seen in Fig. 3(c) and (d) that Fe3O4 nanoparticles distributed on the surface of MoS2 nanosheets. In other words, the nanosheets structure was not destroyed after being introduced Fe3O4 nanoparticles during the hydrothermal treatment. Moreover, the MoS2 nanosheets can prevent the aggregation of Fe3O4 nanoparticles to a certain extent, which can be of great benefit to reactions. The TEM images of the nanocomposite (Fig. 4) reveal the morphology feature, which is in agreement with the SEM result. The dark spots correspond to Fe3O4 nanoparticles, which are isolated from each other and absorbed on the surface of MoS2 nanosheets. The average size of the Fe3O4 nanoparticles is about 15 nm. In the high resolution TEM (HRTEM) image (Fig. 4d), the inter-planar distance of two kinds of nanomaterials are approximately 0.62 nm and 0.25 nm, which agrees well with the lattice spacing of the (002) and (311) plane of MoS2 and Fe3O4 nanoparticles, respectively. More importantly, even after a long time of sonication during the preparation of the TEM specimen, the Fe3O4 nanoparticles were still strongly anchored on the surface of MoS2 nanosheets with a high density. All the above data proved that Fe3O4 nanoparticles were effectively attached onto the surface of MoS2 nanosheets. Fig. 5 shows the magnetization curves of Fe3O4 and Fe3O4/ MoS2 nanocomposites, which are measured at room temperature with an applied magnetic field sweeping from 15 to 15 kOe. All curves appear nonlinear and reversible characteristic with no hysteresis (zero coercivity and no remanence), exhibiting superparamagnetic behavior. The saturation magnetization (Ms) values of Fe3O4 and Fe3O4/MoS2 are 76.85 emu/g and 60.67 emu/g respectively, which completely meet the requirement of the magnetic separation. The observed reduction of Ms values of Fe3O4/MoS2 nanocomposites was mainly due to the non-magnetic MoS2 coating on the surface of Fe3O4 particles The prepared Fe3O4/MoS2 nanocomposites can be easily and quickly separated from suspension by applying an external magnetic field. The magnetic separability of nanoparticles was tested in water by placing a magnet
Fig.2. (a) XRD patterns of Fe3O4, MoS2, and Fe3O4/MoS2 nanocomposites and (b) EDX of the Fe3O4/MoS2 nanocomposites.
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
13899
Fig. 3. SEM images of (a) Fe3O4; (b) MoS2 nanosheet and (c, d) Fe3O4/MoS2 nanocomposites.
Fig. 4. TEM (a–c) and HRTEM (d) images of Fe3O4/MoS2 nanocomposites.
beside the glass bottle. The black sample was attracted toward the magnet within 20 s (in inset of Fig. 5), demonstrating directly that the prepared products had an excellent magnetic
sensitivity. The magnetic sensitive property would favor the separation of Fe3O4/MoS2 nanocomposites from dyecontaining effluents in the future.
13900
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
Fig. 5. Room-temperature magnetic hysteresis loops Fe3O4, and Fe3O4/MoS2 nanocomposites.
Fig.6. N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of Fe3O4/MoS2 nanocomposites.
Fig. 6 shows N2 adsorption–desorption isotherm and BJH pore size distribution plots (inset) of Fe3O4/MoS2 nanocomposites. Using the BJH method and the adsorption branch of the nitrogen isotherm, the calculated pore size distribution indicates that the material contains an average pore size of 16.0819 nm. The specific surface area of the Fe3O4/MoS2 nanocomposites is calculated to be 72.0727 m2/g by the BET equation. As-synthesized Fe3O4/MoS2 nanocomposites have a high special surface area, which may provides more surface active sites and pore-channels for the adsorptions and diffusion of reactants. The adsorption activities of Fe3O4/MoS2 nanocomposites were evaluated by the degradation of different organic dyes in aqueous solution. The samples of 20 mg were dispersed respectively in 100 mL different organic dyes aqueous solution with an initial concentration of 20 mg/L. Five types of organic dyes (Congo red (CR), Methylene blue (MB), Methylene green (MG), Rhodamine B (RhB), Eosin Y (EY)) were chosen as study models in the adsorption experiments. Fig. 7(a) shows the adsorption capacities of different dyes by the Fe3O4/MoS2
nanocomposites, which indicates that the nanocomposites have higher adsorption capacities for CR than for RhB, MG, MB and EY. In other words, the Fe3O4/MoS2 nanocomposites adsorbent has good selectivity to CR. In order to investigate the adsorption capacities of Fe3O4/MoS2 nanocomposites for CR, the adsorption experiment were carried out respectively in the different initial concentration of Congo red (5 mg/L, 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L) and the adsorption time is 20 min. The adsorption isotherm of Congo red by the as-synthesized nanocomposites can be seen from Fig. 7(b), it was demonstrated that the adsorption capacity of Fe3O4/MoS2 nanocomposites for Congo red is 71 mg/g. The rate of adsorption is very important for adsorbent. In order to directly observe the quick removal of pollutants by the Fe3O4/MoS2 nanocomposites, 50 mg of sample was added to 30 mL of CR solution with an initial concentration of 40 mg/L. The suspension was ultrasonicated for 2 min at room temperature, then the Fe3O4/MoS2 nanocomposites were separated by magnet and the supernatant solutions were analyzed with UV– vis spectroscopy to obtain the concentrations of dye in the solution (Fig.7(c)). Fig. 7(c) shows that more than 99% of CR was quickly removed from the solution within 2 min. The red color of CR solution almost disappeared, and finally becomes nearly transparent which can be seen from the inset of Fig.7(c). Furthermore, to illustrate the quick adsorption nature of the nanocomposites, the percentage removal for Congo red at different time intervals was recorded after 50 mg of Fe3O4/ MoS2 nanocomposites was added into 50 mL of CR solution with an initial concentration of 100 mg/L (Fig. 7(d)). About 65.2% of CR was removed after the sample was added into the solution for 2 min, whereas only 6.2% additional CR was removed after the next 120 min adsorption. The adsorption equilibrium of Fe3O4/MoS2 nanocomposites can be almost achieved in very short time, which is significantly faster than that of the previously reported some traditional nanostructures [19–21].
4. Conclusions In summary, the superparamagnetic nanocomposites with magnetic Fe3O4 nanoparticles decorated on MoS2 nanosheets were prepared by a simple hydrothermal route. This is a simple, effective, low-cost, and environment friendly approach. The experimental results clearly demonstrated the successful attachment of Fe3O4 nanoparticles to MoS2 nanosheets. The prepared products had an excellent magnetic sensitivity and can be easily and quickly separated from suspension by applying an external magnetic field. The nanocomposites exhibited excellent performance for water treatment. The maximum adsorption capacity for Congo red was 71 mg/g. Furthermore, they presented the property of ultrafast adsorption for dye removal. Only about 2 min was sufficient to approach the adsorption equilibrium. This information may be useful for further research and practical applications of the novel MoS2 adsorbent in anilines wastewater treatment.
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
13901
Fig. 7. (a) The adsorption capacities of different dyes by the Fe3O4/MoS2 nanocomposites; (b) the adsorption capacities for Congo red at varied concentrations by the Fe3O4/MoS2 nanocomposites, the initial concentration of Congo red is 5–50 mg/L and the adsorption time is 20 min; (c) the adsorption spectra of Congo red aqueous solution of Fe3O4/MoS2 nanocomposites. (d) Percentage removal for Congo red by the Fe3O4/MoS2 nanocomposites. The adsorption rate test was conducted with an initial CR concentration of 100 mg/L. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51202092 and 51372103). References [1] H. Mittal, S.B. Mishra, Gum ghatti and Fe3O4 magnetic nanoparticles based nanocomposites for the effective adsorption of rhodamine B, Carbohydr. Polym. 101 (2014) 1255–1264. [2] T.Y. Li, C. Yang, X.H. Rao, F. Xiao, J.D. Wang, X.T. Su, Synthesis of magnetically recyclable Fe3O4@NiO nanostructures for styrene epoxidation and adsorption application, Ceram. Int. 41 (2015) 2214–2220. [3] D. Chen, Y. Li, J. Zhang, J.Z. Zhou, Y. Guo, H. Liu, Magnetic Fe3O4/ ZnCr-layered double hydroxide composite with enhanced adsorption and photocatalytic activity, Chem. Eng. J. 185–186 (2012) 120–126. [4] X. Zhang, P.Y. Zhang, Z. Wu, L. Zhang, G.M. Zeng, C.J. Zhou, A dsorption of methylene blue onto humic acid-coated Fe3O4 nanoparticles, Colloids Surf. A: Physicochem. Eng. Asp. 435 (2013) 85–90. [5] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: a review, Adv. Colloid Interface Sci. 209 (2014) 172–184. [6] H. Mittal, N. Ballav, S.B. Mishra, Gum ghatti and Fe3O4 magnetic nanoparticles based nanocomposites for the effective adsorption of methylene blue from aqueous solution, J. Ind. Eng. Chem. 20 (2014) 2184–2192.
[7] R.H. Wu, J.H. Liu, L.Q. Zhao, X.L. Zhang, J.R. Xie, B.W. Yu, X.L. Ma, S.T. Yang, H.F. Wang, Y.F. Liu, Hydrothermal preparation of magnetic Fe3O4@C nanoparticles for dye adsorption, J. Environ. Chem. Eng. 2 (2014) 907–913. [8] X.Z. Wang, S.X. Yang, Q. Yue, F.M. Wu, J.B. Li, Response of MoS2 nanosheet field effect transistor under different gas environments and its long wavelength photoresponse characteristics, J. Alloy. Compd. 615 (2014) 989–993. [9] A.L. Friedman, F.K. Perkins, E. Cobas, G.G. Jernigan, P.M. Campbell, A.T. Hanbicki, B.T. Jonker, Chemical vapor sensing of two-dimensional MoS2 field effect transistor devices, Solid-State Electron. 101 (2014) 2–7. [10] C.N.R. Rao, U. Maitra, U.V. Waghmare, Extraordinary attributes of 2dimensional MoS2 nanosheets, Chem. Phys. Lett. 609 (2014) 172–183. [11] Z.G. Liu, F.S. Zhan, R. Sasai, Arsenate removal from water using Fe3O4loaded activated carbon prepared from waste biomass, Chem. Phys. Lett. 160 (2010) 57–62. [12] W.L. Liao, Y.Q. Ma, A.Y. Chen, Y.L. Yang, Preparation of fatty acids coated Fe3O4 nanoparticles for adsorption and determination of benzo(a) pyrene in environmental water samples, Chem. Eng. J. 271 (2015) 232–239. [13] L. Liang, Q.C. Zhu, T.B. Wang, F.X. Wang, J. Ma, L.Q. Jing, J.M. Sun, The synthesis of core–shell Fe3O4@mesoporous carbon in acidic medium and its efficient removal of dye, Microporous Mesoporous Mater. 197 (2014) 221–228. [14] S. Rostamnia, E. Doustkhah, Synthesis of water-dispersed magnetic nanoparticles (H2O-DMNPs) of β-cyclodextrin modified Fe3O4 and its
13902
H.J. Song et al. / Ceramics International 41 (2015) 13896–13902
catalytic application in Kabachnik–Fields multicomponent reaction, J. Magn. Magn. Mater. 386 (2015) 111–116. [15] T.J. Yao, Q. Zuo, H. Wang, J. Wu, B.F. Xin, F. Cui, T.Y. Cui, A simple way to prepare Pd/Fe3O4/polypyrrole hollow capsules and their applications in catalysis, J. Colloid Interface Sci. 450 (2015) 366–373. [16] Y. Wu, Z.M. Li, J. Chen, C.G. Yu, X. Huang, C.Z. Zhao, L.F. Duan, Y. Yang, W. Lü, Graphene nanosheets decorated with tunable magnetic nanoparticles and their efficiency of wastewater treatment, Mater. Res. Bull. 68 (2015) 234–239. [17] Y.J. Yao, S.D. Miao, S.Z. Liu, L.P. Ma, H.Q. Sun, S.B. Wang, Synthesis, characterization, and adsorption properties of magnetic Fe3O4@graphene nanocomposite, Chem. Eng. J. 184 (2012) 326–332.
[18] N. Yang, S.M. Zhu, D. Zhang, S. Xu, Synthesis and properties of magnetic Fe3O4-activated carbon nanocomposite particles for dye removal, Mater. Lett. 62 (2008) 645–647. [19] Y.H. Li, Q.J. Du, T.H. Liu, X.J. Peng, J.J. Wang, J.K. Sun, Y.H. Wang, S.L. Wu, Z.H. Wang, Y.Z. Xia, L.H. Xia, Comparative study of methylene blue dye adsorption onto activated carbon, graphene oxide, and carbon nanotubes, Chem. Eng. Res. Des. 91 (2013) 361–368. [20] L. Xiao, S.S. Zhang, J.H. Huang, Effective removal of organic dyes by tungstate oxide nanourchins, Powder Technol. 258 (2014) 297–303. [21] C.Z. Li, Y.H. Dong, J.J. Yan, Y.Y. Li, C.C. Huang, Modified nanographite/Fe3O4 composite as efficient adsorbent for the removal of methyl violet from aqueous solution, J. Mol. Liq. 196 (2014) 348–356.