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Simultaneous removal of Hg2+, Pb2+ and Cd2+ from aqueous solutions on multifunctional MoS2
Journal of Molecular Liquids xxx (xxxx) xxx
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Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2 Chang Liu a, Shilin Zeng a, Bingqiao Yang b, Feifei Jia a, *, Shaoxian Song c a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Xingfa School of Mining Engineering, Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, Hubei, 430073, China c School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2019 Received in revised form 17 October 2019 Accepted 20 October 2019 Available online xxx
In this work, multifunctional MoS2 prepared by hydrothermal method was studied as an adsorbent for the simultaneous removal of Hg2þ, Pb2þ and Cd2þ in aqueous solutions. This study was performed through the adsorption experiments in single- and multi-metal systems. It was shown that MoS2 exhibited a highest adsorption capacity for Hg2þ, and lowest for Cd2þ, which was mainly attributed to the different affinities between the S sites and the heavy metals. Based on the results of XPS and Zeta potential, the adsorption mechanisms were investigated in depth, which involved oxidation-reduction between Hg2þ and MoS2, electrostatic attraction between positively charged heavy metals and negatively charged MoS2, complexation of HgeS, PbeS, CdeS, HgeO and PbeMoO4 and ion-exchange reaction occurred on eSH and eOH. In the coexistence of Hg2þ, Pb2þ and Cd2þ, the excess MoO3 generated during the oxidation-reduction between MoS2 and Hg2þ resulted in a higher adsorption capacity for Pb2þ compared with the condition in single-metal system. However, the adsorption sites of S on MoS2 would be occupied competitively by the three heavy metals so that it caused lower adsorption capacities for Hg2þ and Cd2þ. These findings would make a great significance in the field of water treatment by MoS2. © 2019 Elsevier B.V. All rights reserved.
Keywords: Multifunctional MoS2 Heavy metals Oxidation-reduction Co-adsorption Adsorption mechanism
1. Introduction The strong release of heavy metals into the environment brings serious threat to human beings and environment because of the non-biodegradability and high toxicity of heavy metals [1]. Thereby, increased awareness and worldwide concern on the removal of heavy metal contaminants from wastewater have been aroused and many technologies such as adsorption, chemical precipitation, membrane separation and ion exchange process have been proposed [2,3]. Among these technologies, adsorption is regarded as a promising method for removing heavy metals from wastewater on the advantages of high efficiency and low cost [4]. Recent researches reported that MoS2 exhibited superior performance as adsorbent for removing heavy metals from aqueous solutions [5]. For example, the adsorption capacity of MoS2-based adsorbents are more than 50 times higher for Hg2þ [6,7], 15 times higher for Pb2þ [8,9], 12.5 times higher for Cd2þ [10,11] and 68 times higher for Cr6þ [12,13] than active carbon-based or clay-based adsorbents. The excellent adsorption behavior of MoS2 ascribed to its
* Corresponding author. E-mail address: [email protected] (F. Jia).
specific 2D structure. The SeMoeS sandwiched structure of monolayer MoS2 sheet makes rich S exposed on the surface, which provides abundant and high active adsorption sites to complex with heavy metals [14,15]. What is more, the negatively charged surface caused from the edge position could promote the adsorption with positively charged heavy metals [16]. Because of its excellent adsorption behavior, there are many studies on the modification and composition of MoS2 for the improvement of adsorption capacity and feasibility in practice. For example, in our previous work, a simple thermal treatment was used to modify MoS2 to improve the surface activity. As a result, the adsorption capacity and rate were 11-flod higher and 17.6 times faster compared with no modificatory MoS2, which was ascribed to the induced defects and oxidation [17]. Another study reported a porous Au/Fe3O4/MoS2CAs quaternary adsorbent synthesized through hydrothermal and embedding method, and it can achieve the multifunctional integration of an outstanding adsorption capacity, excellent magnetic separation and the sensitive detection for Hg simultaneously [18]. Although there are many other investigations about MoS2 as an adsorbent, these researches only analyzed in single heavy metal systems and no studies concerned on the co-adsorption of heavy metals with MoS2 to our knowledge. While it is common that heavy
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Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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metal ions coexist in the wastewater, especially in the industrial effluents [19]. They may influence each other during the adsorption due to the competitive effect. Therefore, investigation on the coadsorption of heavy metals on MoS2 is of great importance. An attempt was made to investigate the co-adsorption behavior of toxic Hg2þ, Pb2þ and Cd2þ on hydrothermal synthesized MoS2 with abundant multifunctional groups in this article. The surface morphology and property of MoS2 was investigated by highresolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy-energy dispersive spectrum (SEM-EDS), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The adsorption performance was studied through the adsorption kinetics, isotherm and pH effect experiments. The mechanism of the adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 was interpreted by XPS determination. The object of this study was to obtain a clear understanding on the behavior, as well as the mechanism, of MoS2 on the adsorption in multi-metal systems. Furthermore, it could give guidance for the application of MoS2 as an adsorbent in practice in the future. 2. Materials and methods 2.1. Materials Hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24$4H2O), thiourea (CN2H4S), lead nitrate (Pb(NO3)2), cadmium nitrate (Cd(NO3)2$4H2O), nitric acid (HNO3) and sodium hydroxide (NaOH) used in this study were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). Mercuric nitrate (Hg(NO3)2$H2O) was purchased from Shanghai Zhanyun Chemical Co., Ltd (China). All chemicals and reagents utilized in the experiment were of analytical grade. Milli-Q water with a resistivity of 18.2 MU cm was used as solvent. 2.2. Hydrothermal synthesis of MoS2 MoS2 was synthesized by hydrothermal method [12]. Firstly, 2.48 g of (NH4)6Mo7O24$4H2O and 4.56 g of CN2H4S were dissolved in 72 mL deionized water, followed by magnetic stirring the mixture to form a homogeneous solution for 30 min. After that, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and thermally treated at 220 C for 6 h. After cooling down to room temperature, the synthetic precipitate was filtered and collected by a 0.1 mm membrane, and washed with deionized water for 3 times to remove the residual chemical reagents. Subsequently, the resultant material was ultra-sonicated by a ColeParmer at 300 W for 10 min and freeze-dried for 24 h to obtain the adsorbent. 2.3. Adsorption experiments 1 g/L stock solutions of Hg2þ, Pb2þ and Cd2þ were prepared by dissolving specific amounts of Hg(NO3)2$H2O, Pb(NO3)2 and Cd(NO3)2$4H2O in Milli-Q water before the adsorption experiments and few drops of dilute nitric acid were added into the stock solutions to keep the Hg2þ, Pb2þ and Cd2þ stable. All batch adsorption experiments were performed at room temperature. Before the adsorption, heavy metals solution with desired concentration was prepared by diluting the stock solution with Milli-Q water and then the pH of the solution was adjusted by HNO3 or NaOH. After that, a given amount of adsorbent was dispersed into the heavy metal solution. The erlenmeyer flask contained the suspension was then transferred to a water bath shaker and shaken at 150 rpm for a predetermined time. Finally, 5 mL of suspension was taken and filtered with 0.22 mm filter membrane, and the filtrate was collected
for a quantitative analysis of the heavy metal concentration. As for the multi-metals system, the adsorption procedure was same except that the solution contained three heavy metal ions with same initial concentration. When studying the effect of pH on the adsorption, 10 mg of MoS2 was added into 80 mL 50 mg/L heavy metal solution with pH varying from 1.0 to 5.0 and the adsorption lasted for 16 h. No experiments were conducted at higher pH to avoid the precipitation of heavy metal ions. For the adsorption kinetic experiment, 50 mg of MoS2 was added into 1 L multi-metal solution with Hg2þ, Pb2þ and Cd2þ concentration of 60 mg/L and pH of 5.0.5 mL of samples were taken at different time intervals (0e660 min). Adsorption isotherms were performed by adding 10 mg of MoS2 to 125 mL solution containing single or three ions. The initial concentration of heavy metals varied from 25 mg/L to 200 mg/L and the adsorption was conducted at pH of 5.0 for 16 h. The adsorption capacity was calculated using the following equation:
q¼
C0 Ce V m
(1)
where q (mg/g) is the adsorption capacity of the adsorbent; C0 (mg/ L) and Ce (mg/L) represents the initial and equilibrium concentration of the heavy metal, respectively; V (L) is the volume of the solution; m (g) is the mass of the adsorbent. 2.4. Measurements Raman spectra were obtained from INVIA Raman microscope with a 514 nm Ar laser (Renishaw, UK). A Zeiss Ultra Plus field emission scanning electron microscope (FESEM) (Zeiss, Germany) was applied to obtain the morphology and energy dispersive spectra (EDS). The HRTEM images were observed by using a Titan G2 60e300 Probe Cs Corrector HRSTEM (FEI, United States). Specific surface area of MoS2 sample was measured on ASAP 2460 accelerated surface area and porosimetry analyzer (Micromeritics, United States). The concentration of Hg2þ was detected by using a contrAA700 continuum source atomic absorption spectrometer (Jena, Germany), while the concentrations of Pb2þ and Cd2þ were detected by a GBC AVANTA M atomic absorption spectrometer (GBC, Australia). AFM measurements were carried out on Bruker MultiMode 8 atomic force microscopy (Bruker, USA). The XPS was performed on a VG ESCALB MK-II Instrument (VG, UK). The XPS high-resolution spectra were calibrated by setting the C 1s photoemission at 284.8 eV. The MoS2 sample used for the measurement after adsorption was prepared by the following steps: 50 mg MoS2 was added into 500 mL 150 mg/L relative heave metals with the pH of 5.0 and shaken for 16 h. Then the MoS2 sample was filtered with 0.22 mm filter membrane and washed for three times. Finally, the sample was dried under 60 C with a vacuum freezing dryer and collected. 3. Results and discussion 3.1. Characterization of MoS2 Fig. 1a showed the HRTEM image of MoS2. The good transparency of the sheets on the fringes indicated the thin thickness of MoS2. The sheets in the middle were linked together, forming a twisted-film liked morphology with a lateral dimension of around 500 nm. The representative AFM image and the inserted figure presented in Fig. 1b showed that two individual MoS2 samples had a cross-section thickness of around 5 nm and lateral size of around 500 nm, which was in good consistent with the HRTEM result. Fig. 1c and d displays the SEM image of MoS2 and corresponding EDS spectrum. The main elements detected on MoS2 were S, Mo
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Fig. 1. (a) HRTEM image of MoS2, (b) representative AFM image and corresponding height cross-section of MoS2 marked with line, (c) SEM image of MoS2, (d) EDS spectrum and elemental content of MoS2.
and O. The high content of O suggested that MoS2 might be partially oxidized during the synthesis. Figure S1 showed the result of nitrogen adsorption/desorption isotherms of MoS2 under 300 C. It exhibited a IV-type isotherm and a H3-type hysteresis hoop, indicating the mesopores characteristic of the MoS2 sample [16]. Besides, according to Barrett-Emmett-Teller (BET) calculations, the specific surface area is 12.05 m2/g. Fig. 2a showed the Raman spectrum of MoS2. The characteristic E12g and A1g peaks at 377 cm1 and 405 cm1 were assigned to the in-plane and out-of-plane vibration of Mo and S atoms [20], respectively, indicating the successful synthesis of MoS2. The peaks at 285 cm1, 661 cm1, 820 cm1 and 993 cm1 represented the doublet comprised of wagging modes of the terminal oxygen atoms, the asymmetric stretching of MoeOeMo bridge along the c axis, the symmetric stretch of the terminal oxygen atoms, and the asymmetric stretch of the terminal oxygen atoms, respectively [21,22], further suggesting the partial oxidation of MoS2. Fig. 2b displayed the FT-IR spectrum of MoS2. The band at 596 cm1 corresponded to the MoeS vibration mode in MoS2 [23]. The absorption bands at 1627 cm1 and 3434 cm1 were attributed to the OeH stretching vibrations from water, while the absorption peak at 904 cm1 were assigned to the bending vibration of inner OeH groups [24]. The stretching vibrations of MoeO at 757 cm1 and 654 cm1 were observed, confirming the presence of molybdenum oxide on MoS2 [25]. In addition, the observation of sulfurated functional groups SeO at 1034 cm1 and 1386 cm1, O]S]O and 1 1 OeSeO in SO2 and 1400 cm1, and bulk4 at 1056 cm , 1141 cm like sulfate at 1191 cm1 demonstrated that MoS2 had sulfurcontaining functional groups or structures on the surface [26]. It should be mentioned that although peaks of MoeO and S-function groups with strong intensity were determined from Raman and FTIR spectra, this did not mean the main composition of MoS2 was
molybdenum oxide or abundant S-function groups existed on MoS2 because Raman and FT-IR are only able to provide a simple qualitative analysis. Fig. 2cef showed the XPS spectra of MoS2. The wide-scan spectrum, as well as the inset semi-quantitative analysis, presented in Fig. 2c further proved that the main elements of MoS2 were Mo, S and O. The O 1s spectrum in Fig. 2d could be deconvoluted into four peaks at 530.6 eV, 531.7 eV, 532.6 eV and 533.6 eV, which were assigned to MoeO in MoO3, hydroxyl groups (OH), absorbed H2O molecules and OeC bonds, respectively [27]. Fig. 2e displays the Mo 3d spectrum of MoS2, which clearly indicated that Mo presented in various compounds with its chemical state varying from þ4 to þ6. The doublet peak at 228.9 eV (Mo 3d5/2) and 232.1 eV (Mo 3d3/2) were assigned to MoS2 [27], the doublet at 229.8 eV and 233.4 eV were attributed to Mo5þ in Mo2S5, the small doublet at 231.3 eV and 234.9 eV were corresponding to Mo6þ in MoS3 [28]. Besides the MoeS compounds, characteristic peaks of MoO3 were also found at 232.9 eV and 235.9 eV [22]. Fig. 2f showed the S 2p spectrum of MoS2, which could be deconvoluted into seven peaks. The strong doublet at 161.8 eV and 163.0 eV were assigned to S 2p3/2 and S 2p1/2 orbitals in MoS2 [27]. Other peaks at 161.0 eV, 163.7 eV, 164.5 eV, 168.4 eV and 169.4 eV were attributed to C]S, CSH, MoS3, SO2 and SO2 3 4 , respectively [20,29]. Different from Raman and FT-IR, XPS could provide a semi-quantitative analysis. Therefore, it could be obtained from XPS that the synthetic sample was mainly composed of MoS2, a small quantity of Mo2S5, MoS3, MoO3 and other oxygen- and sulfur-containing functional groups. 3.2. pH effect Fig. 3 illustrated the co-adsorption capacities of Hg2þ, Pb2þ, and Cd on MoS2 as a function of pH. It can be seen that the adsorption 2þ
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Fig. 2. (a) Raman spectrum of MoS2; (b) FT-IR spectrum of MoS2; (c) Wide-scan XPS spectrum of MoS2; (d), (e) and (f): High-resolution O 1s, Mo 3d and S 2p XPS spectra of MoS2, respectively.
capacities of the three ions on MoS2 were influenced by pH obviously, which exhibited lower adsorption capacities in a lower pH, while higher adsorption capacities in a higher pH. This phenomenon could be explained by Zeta potential of MoS2. As shown in Figure S2, the surface of MoS2 showed negative charges in all pH range, and a higher absolute value of Zeta potential in a higher pH value. Thus, the enhanced electrostatic interaction between MoS2 and positively charged heavy metal ions might account for the increase of adsorption capacity when pH increased from 1.0 to 5.0 [5]. In addition, with the contribution of Hþ decreased, the protonation became weaker on the surface of MoS2, which was available for Hg2þ, Pb2þ and Cd2þ to bind with the adsorption sites [11].
What is more, the adsorption capacities for the three adsorbates was as the following of Hg2þ >Pb2þ >Cd2þ nearly in all pH values, which meant the different affinity interaction with MoS2. According to Pearson hard soft acid base theory (HSAB), as a soft base, the S sites in MoS2 preferred to react with softer acid. The hardness of acid could be quantified through the value of absolute hardness (h). In this system, Hg, Pb and Cd were regarded as the acids with the relationship of absolute hardness followed by Hg (7.7 eV)< Pb (8.46 eV)< Cd (10.29 eV) [29]. It meant that Hg was softer than Pb, and Pb was softer than Cd, which is the same order of the adsorption capacity on MoS2. Additionally, the different hydrolysis of the three ions could affect the adsorption results. As shown in
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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qt ¼
t ðt=qe Þ þ 1 k2 q2e
5
(3)
where qe and qt is the adsorbed heavy metal at equilibrium time and t time (mg/g), respectively; k1 (min1) and k2 (g$mg1$min1) are the first- and second-order rate constants, respectively. The higher correlation coefficient (R2) indicated that both the two models well-fitted the experimental data. Furthermore, based on the analysis of Chi-square, the values of which were lower of Hg2þ, higher of Pb2þ and Cd2þ in Pseudo-first-order kinetics model, indicating the better fits for Hg2þ in this model, for Pb2þ and Cd2þ in Pseudo-second-order kinetics model.
3.4. Adsorption isotherms
Fig. 3. Co-adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 as a function of pH.
Figure S3, Hg2þ is easily hydrolysed and existed mainly in the forms of metal-hydroxyl species [31e33]. As a result, it can be easily adsorbed on negatively charged MoS2 due to the less hydration effect compared with Pb2þ and Cd2þ.
Both of the adsorption isotherms of Hg2þ, Pb2þ and Cd2þ on MoS2 in single- and multi-metal systems were studied at room temperature. The experimental data was fitted by Langmiur isotherm model (Eq. (4)) and Freundlich isotherm model (Eq. (5)) for a better understanding of adsorption behavior [1].
qe ¼
qm KL Ce 1 þ KL Ce 1=n
qe ¼ KF C e 3.3. Co-adsorption kinetics The co-adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 as a function of time was illustrated in Fig. 4. It showed a fast adsorption rate at the beginning and then reached the dynamic equilibrium. It took 30, 60 and 240 min for Cd2þ, Pb2þ and Hg2þ to reach the plateau with the adsorption capacity of 14, 366 and 1200 mg/g (1, 30 and 100 mg/m2), respectively. Compared with other adsorbent, MoS2 exhibited faster adsorption rate for Cd2þ and Pb2þ, and much higher adsorption capacity for Hg2þ [1,29,30]. Pseudo-first-order kinetics model (Eq. (2)) and pseudo-secondorder kinetics model (Eq. (3)) were used to fit the experimental data and the fitting results were shown in Table S1.
qt ¼ qe ½1 expð k1 tÞ
Fig. 4. Co-adsorption kinetics of Hg2þ, Pb2þ and Cd2þ on MoS2.
(2)
(4)
(5)
where qe is the adsorption capacity at equilibrium concentration (mg/g); qm is the maximum adsorption capacity (mg/g) from Langmiur model; Ce is the equilibrium concentration of Hg2þ, Pb2þ and Cd2þ (mg/L); KL and KF represents the Langmuir constant (L/ mg) and Freundlich constant (mg/g), respectively; n is a constant, which represents the sorption intensity. From the data shown in Table S2, the lower Chi-square values of Hg2þ and Cd2þ in Langmiur isotherm model indicated that the adsorption of the two metals fitted better in this model. Similarly, Freundlich isotherm model could better describe the adsorption of Pb2þ compared with Langmiur isotherm model. The adsorption isotherms of MoS2 in single-metal system were illustrated in Figure S4. Based on the fitting data of Langmuir model in Table S2, qm for Hg2þ, Pb2þ and Cd2þ are 2409, 293 and 30 mg/g (200, 24 and 2 mg/m2), respectively. Fig. 5 showed the adsorption isotherms result in multi-metal system. Compared with the single-metal system, the adsorption capacity for Hg2þ decreased dramatically,
Fig. 5. Co-adsorption isotherm of Hg, Pb and Cd on M.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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which might be because of the competitive effect. Unexpectedly, the adsorption for Pb2þ enhanced in multi-metal system, which indicated that there might be other mechanisms except the competitive effect during this co-adsorption procedure. While the adsorption capacity for Cd2þ was as low as the phenomenon in single-metal system. The semiquantitative analysis of XPS was used for the further study of the adsorption capacities for the three ions in the two systems. Fig. 6 showed the wide-scan XPS spectra of MoS2 after the adsorption of Hg, Pb and Cd in single- or multi-metal solutions (named as M-Hg, M-Pb, M-Cd and M-HgPbCd, respectively). Strong peaks of Hg and Pb were detected on MoS2, indicating the good adsorption. Compared with the single-metal solutions, the contents of Hg and Cd were decreased, while that of Pb was increased (Table 1), which was in good agreement with the results of adsorption isotherm experiments. Fig. 7 was the results of SEM-EDS of MoS2 after the adsorption, which indicated that plenty of Hg and Pb, as well as a small amount of Cd, were equably distributed on MoS2. 3.5. Adsorption mechanism The high-resolution XPS spectra of MoS2 loading with heavy metals were given in Fig. 8 to explore the chemical reaction between MoS2 and heavy metals. Fig. 8a showed the spectrum of Mo 3d. The peaks at 228.9, 229.8, 231.3, 232.1, 232.9, 233.4, 234.9, 235.9 eV could be assigned to MoS2, Mo2S5, MoS3, MoS2, MoO3, Mo2S5, MoS3, MoO3, respectively [22,27,28]. The Mo 3d spectrum indicated a great increase of MoO3 on the sample of MoS2eHg, MoS2ePb, and MoS2eHgPbCd, which might be related to the reaction of MoS2 with Hg and Pb. The redox potential of MoS2/MoO3 is 0.429 V, which is lower than that of Hg2þ/Hg0, while higher than that of Pb2þ/Pb and Cd2þ/Cd. It meant that MoS2 was able to reduce Hg2þ into Hg0 by itself while was inactive for Pb2þ and Cd2þ [34]. Thus, a redox reaction might occur between Hg2þ and MoS2, resulting in the increase of MoO3 in sample MoS2eHg. As for MoS2ePb, the peaks of Mo (VI) were located in 236.2 eV, which was 0.3 eV higher than those in other samples. The shift of the peaks might originate from the complexation between Pb2þ and MoO3 according to Eq. (6) [35], and the peaks of Mo (VI) in 236.2 eV might be assigned to PbMoO4. Therefore, the increase of Mo (VI) peak might be highly because of the chemical reaction between Pb2þ and MoO3. The Mo (VI) peaks in MoS2eHgPbCd were stronger than those in both MoS2eHg and MoS2ePb, which might be attributed that additional MoO3 would generate during the loading of Hg2þ on
Table 1 The elemental composition of M adsorb Hg, Pb and Cd (wt %) by XPS semiquantitative analysis. Adsorbent
Mo
S
O
Hg
Pb
Cd
M-Hg M-Pb M-Cd M-HgPbCd
26.07 42.64 49.88 21.3
21.95 31.71 39.65 16.12
5.34 9.18 10.12 8.98
46.64 / / 32.63
/ 16.48 / 20.98
/ / 0.35 /
MoS2, and these MoO3 can serve as adsorption sites for Pb2þ and promote the loading of Pb2þ through the formation of PbMoO4.
MoO3 þ H2 O þ Pb2þ ¼ PbMoO4 þ 2Hþ
(6)
Fig. 8b presented the Hg 4f spectrum of MoS2eHg and MoS2eHgPbCd. The doublet with binding energy of 101.0 eV and 105.0 eV were assigned to HgeS, while the other doublet at 102.2 eV and 106.0 eV were HgeO [20]. It suggested that both S and O served as the adsorption sites for the loading of Hg2þ on the surface of MoS2, while S sites were the main contributors. In addition, a small Hg0 peak at 99.5 eV was detected, which confirmed the redox reaction between Hg2þ and MoS2 [36]. Fig. 8c was the Pb 4f spectrum of MoS2ePb and MoS2eHgPbCd. Both of the Pb 4f5/2 and Pb 4f7/2 could be deconvoluted into two peaks. The doublet with binding energy of 144.5 eV and 139.6 eV could be attributed to PbeS [37], while the other with binding energy of 144.0 eV and 139.2 eV can be assigned to PbeO in PbMoO4 [38]. The obvious PbeS and PbeO peaks further illustrated the complexation of Pb2þ with S sites and MoO3. Compared with MoS2ePb, the intensity of PbeS peak in MoS2eHgPbCd decreased, while that of PbeO peak increased, making a slight shift of Pb 4f. The decrease of PbeS peak was because some S sites were occupied by Hg2þ and Cd2þ in the multi-metal system, while the increase of PbeO peak might be ascribed to more MoO3 existing in the multimetal system due to the redox reaction of Hg2þ and MoS2 and then removed more Pb2þ with the formation of PbMoO4. The Cd 3d spectrum of MoS2eCd and MoS2eHgPbCd was presented in Fig. 8d. The Mo 3p1/2 peak at 412.2 eV has a slight shift in the multi-metal system, which might be attributed to the adsorption of Hg2þ and Pb2þ on MoS2. A small CdeS peak was observed at 405.5 eV in sample MoS2eCd [11], indicating that there was a weak interaction between S and Cd2þ in single-metal system. However, the peak disappeared in MoS2eHgPbCd, which is because of the occupation of the S sites by Hg2þ and Pb2þ. Fig. 9 was the release amount of proton during the adsorption of Hg2þ, Pb2þ, and Cd2þ, which was calculated through comparing the pH before and after adsorption. A blank group with no heavy metals was set to eliminate the released proton which generated by the oxidation of MoS2 in water [25]. It could be seen that about 0.23 103, 0.54 103 and 0.03 103 moL/L Hþ released in the adsorption of Hg2þ, Pb2þ and Cd2þ, respectively. According to other researches, the heavy metal ions would bond with S on MoS2 surface by replacing the protonated Hþ, it was therefore Hþ was released during the adsorption for heavy metals on MoS2 [5,7]. What is more, other ion exchange reactions between the functions such as eSH and eOH on MoS2 and heavy metals might also occur as shown in equations (6)e(8) [29,39]. It was worthy to mention that the reaction in equation (6) which involved double release amount of Hþ was the reason of the most release amount of proton in Pb2þ solution.
SH þ Hg2þ / S Hgþ þ Hþ
(7)
Fig. 6. XPS spectra of M after the adsorption of Hg, Pb and Cd.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Fig. 7. SEM images of Hg, Pb and Cd adsorbed on M: (a) SEM morphological image; (bed) EDS elemental mapping of Hg, Pb and Cd, respectively.
Fig. 8. High-resolution of M after the adsorption of Hg, Pb and Cd both with the blank group: (a) Mo 3d spectrum; (b) Hg 4f spectrum; (c) Pb 4f spectrum; (d) Cd 3d spectrum.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2019.111987. References
Fig. 9. The release amount of proton in Hg2þ, Pb2þ and Cd2þ solution during the adsorption.
OH þ Pb2þ / O Pb2þ þ Hþ
(8)
4. Conclusion In this work, multifunctional MoS2 adsorbent with nanometer thickness was prepared by hydrothermal synthesis for the removal of Hg2þ, Pb2þ and Cd2þ from water. The adsorbent exhibited different adsorption capacity for the three heavy metals, which mainly ascribed to their affinities for S sites on MoS2 according to HSAB. The adsorptions for Hg2þ, Pb2þ and Cd2þ both showed significant pH dependence from 1 to 5 which were influenced by the electrostatic attraction and protonation. XPS analysis of multifunctional MoS2 after the adsorption revealed that the adsorption mechanisms might involve oxidation-reduction between MoS2 and Hg2þ both with the complexation to form HgeS, PbeS, CdeS, HgeO and PbeMoO4 on MoS2. Under the multi-metal systems of the three heavy metals, the adsorption capacities for Hg2þ and Cd2þ were decreased compared with the condition in single-metal systems because of the competitive adsorption on their common adsorption sites. On the contrary, the adsorption for Pb2þ was increased which was because the excess MoO3 generated by the oxidation-reduction between MoS2 and Hg2þ removed more Pb2þ with the formation of PbMoO4.
Conflicts of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgement The financial supports for this work from the National Natural Science Foundation of China (Nos. 51704220, 51704212 and 51674183), the Natural Science Foundation of Hubei Province (No. 2016CFA013), and the Research Fund Program of Key Laboratory of Rare Mineral, Ministry of Land and Resources (KLRM-KF201802) were gratefully acknowledged.
[1] J. Deng, Y. Liu, S. Liu, G. Zeng, X. Tan, B. Huang, X. Tang, S. Wang, Q. Hua, Z. Yan, Competitive adsorption of Pb(II), Cd(II) and Cu(II) onto chitosan-pyromellitic dianhydride modified biochar, J. Colloid Interface Sci. 506 (2017) 355e364. [2] M. Li, Q. Liu, L. Guo, Y. Zhang, Z. Lou, Y. Wang, G. Qian, Cu(II) removal from aqueous solution by Spartina alterniflora derived biochar, Bioresour. Technol. 141 (2013) 83e88. [3] L. Dong, L. Hou, Z. Wang, P. Gu, G. Chen, R. Jiang, A new function of spent activated carbon in BAC process: removing heavy metals by ion exchange mechanism, J. Hazard Mater. 359 (2018) 76e84. [4] F.M. Mohammed, E.P.L. Roberts, A. Hill, A.K. Campen, N.W. Brown, Continuous water treatment by adsorption and electrochemical regeneration, Water Res. 45 (2011) 3065e3074. [5] Z. Wang, B. Mi, Environmental applications of 2D molybdenum disulfide (MoS2) nanosheets, Environ. Sci. Technol. 51 (2017) 8229e8244. [6] T. Phothitontimongkol, N. Siebers, N. Sukpirom, F. Unob, Preparation and characterization of novel organo-clay minerals for Hg(II) ions adsorption from aqueous solution, Appl. Clay Sci. 43 (2009) 343e349. [7] K. Ai, C. Ruan, M. Shen, L. Lu, MoS2 nanosheets with widened interlayer spacing for high-efficiency removal of mercury in aquatic systems, Adv. Funct. Mater. 26 (2016) 5542e5549. [8] Y.-H. Li, J. Ding, Z. Luan, Z. Di, Y. Zhu, C. Xu, D. Wu, B. Wei, Competitive adsorption of Pb2þ, Cu2þ and Cd2þ ions from aqueous solutions by multiwalled carbon nanotubes, Carbon N. Y. 41 (2003) 2787e2792. [9] C. Liu, F. Jia, Q. Wang, B. Yang, S. Song, Two-dimensional molybdenum disulfide as adsorbent for high-efficient Pb(II) removal from water, Appl. Mater. Today. 9 (2017) 220e228. [10] B. Xiao, K.M., Thomas Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon, Langmuir 20 (2004) 4566e4578. [11] Q. Wang, L. Yang, F. Jia, Y. Li, S. Song, Removal of Cd(II) from water by using nano-scale molybdenum disulphide sheets as adsorbents, J. Mol. Liq. 263 (2018) 526e533. [12] Y. Gao, C. Chen, X. Tan, H. Xu, K. Zhu, Polyaniline-modified 3D-flower-like molybdenum disulfide composite for efficient adsorption/photocatalytic reduction of Cr(VI), J. Colloid Interface Sci. 476 (2016) 62e70. [13] K. Selvi, Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon, Bioresour. Technol. 80 (2001) 87e89. [14] X. Li, K. Peng, Hydrothermal synthesis of MoS2 nanosheet/palygorskite nanofiber hybrid nanostructures for enhanced catalytic activity, Appl. Clay Sci. 162 (2018) 175e181. [15] X. Li, K. Peng, MoSe2/Montmorillonite composite nanosheets: hydrothermal synthesis, structural characteristics, and enhanced photocatalytic activity, Minerals 8 (2018), https://doi.org/10.3390/min8070268. [16] F. Jia, K. Sun, B. Yang, X. Zhang, Q. Wang, S. Song, Defect-rich Molybdenum Disulfide as Electrode for Enhanced Capacitive Deionization from Water Desalination, vol. 446, 2018, pp. 21e30. [17] F. Jia, C. Liu, B. Yang, X. Zhang, H. Yi, J. Ni, S. Song, Thermal modification of the molybdenum disulfide surface for tremendous improvement of Hg2þ adsorption from aqueous solution, ACS Sustain. Chem. Eng. 6 (2018) 9065e9073. [18] L. Zhi, W. Zuo, F. Chen, B. Wang, 3D MoS2 composition aerogels as chemosensors and adsorbents for colorimetric detection and high-capacity adsorption of Hg2þ, ACS Sustain. Chem. Eng. 4 (2016) 3398e3408. [19] B.K. Adebola, S.J. Kayode, O.A., Akeem Integrated assessment of the heavy metal pollution status and potential ecological risk in the Lagos Lagoon, South West, Nigeria, Hum. Ecol. Risk. Assess. 24 (2018) 377e397. [20] F. Jia, Q. Wang, J. Wu, Y. Li, S. Song, Two-Dimensional molybdenum disulfide as a superb adsorbent for removing Hg2þ from water, ACS Sustain. Chem. Eng. 5 (2017) 7410e7419. [21] B.C. Windom, W.G. Sawyer, D.W. Hahn, A Raman spectroscopic study of MoS2 and MoO3: applications to tribological systems, Tribol. Lett. 42 (2011) 301e310. [22] F. Jia, C. Liu, B. Yang, S. Song, Microscale control of edge defect and oxidation on molybdenum disulfide through thermal treatment in air and nitrogen atmospheres Appl, Surf. Sci. 462 (2018) 471e479. [23] S.S. Karade, D.P. Dubal, B.R., Sankapal MoS2 ultrathin nanoflakes for high performance supercapacitors: room temperature chemical bath deposition (CBD), RSC Adv. 6 (2016) 39159e39165. [24] M. Ghanei-motlagh, M.A. Taher, A novel electrochemical sensor based on silver/halloysite nanotube/molybdenum disulfide nanocomposite for efficient nitrite sensing, Biosens. Bioelectron. 109 (2018) 279e285. [25] F. Jia, X. Zhang, S. Song, AFM study on the adsorption of Hg2þ on natural molybdenum disulfide in aqueous solutions, Phys. Chem. Chem. Phys. 19 (2017) 3837e3844. [26] J. Wang, C. Li, SO2 adsorption and thermal stability and reducibility of sulfates formed on the magnesiumealuminate spinel sulfur-transfer catalyst, Appl. Surf. Sci. 161 (2000) 406e416. [27] X. Zhang, F. Jia, B. Yang, S. Song, Oxidation of molybdenum disulfide sheet in
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
C. Liu et al. / Journal of Molecular Liquids xxx (xxxx) xxx
[28]
[29]
[30]
[31]
[32]
[33]
water under in situ atomic force microscopy observation, J. Phys. Chem. C 121 (2017) 9938e9943. J. Myeong, C. Sung, H. Kim, S. Jae, L. Seong, S. Kim, Effects of pressure and temperature in hydrothermal preparation of MoS2 catalyst for methanation reaction, Catal. Lett. 148 (2018) 1803e1814. D. Saha, S. Barakat, S.E. Van Bramer, K.A. Nelson, D.K. Hensley, J. Chen noncompetitive and competitive adsorption of heavy metals in sulfurfunctionalized ordered mesoporous carbon, ACS Appl. Mater. Interfaces 8 (2016) 34132e34142. W. Wang, Y. Zhao, H. Yi, T. Chen, S. Kang, T. Zhang, F. Rao, S. Song, Pb(ІІ) removal from water using porous hydrogel of chitosan-2D montmorillonite, Int. J. Biol. Macromol. 128 (2019) 85e93. €berg, K.J. Powell, P.L. Brown, R.H. Byrne, T. Gajda, G. Hefter, A. Leuz, S. Sjo H. Wanner, Chemical speciation of environmentally significant metals with 2e 3e inorganic ligands. Part 3: the Pb2þþ OHe, Cle, CO2e 3 , SO4 , and PO4 systems (IUPAC Technical Report), Pure Appl. Chem. 81 (2009) 2425e2476. F. Di Natale, A. Erto, A. Lancia, D. Musmarra, Mercury adsorption on granular activated carbon in aqueous solutions containing nitrates and chlorides, J. Hazard Mater. 192 (2015) 1842e1850. D. Borah, K. Senapati, Adsorption of Cd(II) from Aqueous Solution onto Pyrite
9
Fuel., vol. 85, 2006, pp. 1929e1934. [34] Z. Wang, A. Sim, J.J. Urban, B. Mi, Removal and recovery of heavy metal ions by two-dimensional MoS2 nanosheets: performance and mechanisms environ, Sci. Technol. 52 (2018) 9741e9748. [35] Y. Wu, X. Cheng, X. Zhang, Y. Xu, S. Gao, H. Zhao, L. Huo, High efficient and selective removal of Pb2þ through formation of lead molybdate on a-MoO3 porous nanosheets array, J. Colloid Interface Sci. 491 (2017) 80e88. [36] J.J. Ehrhardt, P. Behra, P. Bonnissel-Gissinger, M. Alnot, PS study of the sorption of Hg(II) onto pyrite FeS2, Surf. Interface Anal. 30 (2000) 269e272. [37] M. Zhang, F. Jia, M. Dai, S. Song, Combined electrosorption and chemisorption of low concentration Pb(II) from aqueous solutions with molybdenum disulfide as electrode Appl, Surf. Sci. 455 (2018) 258e266. [38] G. Gyawali, R. Adhikari, B. Joshi, T.H. Kim, V. Rodríguez-Gonz alez, S.W. Lee, Sonochemical synthesis of solar-light-driven Ag-PbMoO4 photocatalyst, J. Hazard Mater. 263 (2013) 45e51. [39] H. Pei, J. Wang, Q. Yang, W. Yang, N. Hu, Y. Suo, D. Zhang, Z. Li, J. Wang, Interfacial growth of nitrogen-doped carbon with multi-functional groups on the MoS2 skeleton for efficient Pb(II) removal, Sci. Total Environ. 631e632 (2018) 912e920.
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Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2 Chang Liu a, Shilin Zeng a, Bingqiao Yang b, Feifei Jia a, *, Shaoxian Song c a
Hubei Key Laboratory of Mineral Resources Processing and Environment, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China Xingfa School of Mining Engineering, Wuhan Institute of Technology, Xiongchu Avenue 693, Wuhan, Hubei, 430073, China c School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei, 430070, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 September 2019 Received in revised form 17 October 2019 Accepted 20 October 2019 Available online xxx
In this work, multifunctional MoS2 prepared by hydrothermal method was studied as an adsorbent for the simultaneous removal of Hg2þ, Pb2þ and Cd2þ in aqueous solutions. This study was performed through the adsorption experiments in single- and multi-metal systems. It was shown that MoS2 exhibited a highest adsorption capacity for Hg2þ, and lowest for Cd2þ, which was mainly attributed to the different affinities between the S sites and the heavy metals. Based on the results of XPS and Zeta potential, the adsorption mechanisms were investigated in depth, which involved oxidation-reduction between Hg2þ and MoS2, electrostatic attraction between positively charged heavy metals and negatively charged MoS2, complexation of HgeS, PbeS, CdeS, HgeO and PbeMoO4 and ion-exchange reaction occurred on eSH and eOH. In the coexistence of Hg2þ, Pb2þ and Cd2þ, the excess MoO3 generated during the oxidation-reduction between MoS2 and Hg2þ resulted in a higher adsorption capacity for Pb2þ compared with the condition in single-metal system. However, the adsorption sites of S on MoS2 would be occupied competitively by the three heavy metals so that it caused lower adsorption capacities for Hg2þ and Cd2þ. These findings would make a great significance in the field of water treatment by MoS2. © 2019 Elsevier B.V. All rights reserved.
Keywords: Multifunctional MoS2 Heavy metals Oxidation-reduction Co-adsorption Adsorption mechanism
1. Introduction The strong release of heavy metals into the environment brings serious threat to human beings and environment because of the non-biodegradability and high toxicity of heavy metals [1]. Thereby, increased awareness and worldwide concern on the removal of heavy metal contaminants from wastewater have been aroused and many technologies such as adsorption, chemical precipitation, membrane separation and ion exchange process have been proposed [2,3]. Among these technologies, adsorption is regarded as a promising method for removing heavy metals from wastewater on the advantages of high efficiency and low cost [4]. Recent researches reported that MoS2 exhibited superior performance as adsorbent for removing heavy metals from aqueous solutions [5]. For example, the adsorption capacity of MoS2-based adsorbents are more than 50 times higher for Hg2þ [6,7], 15 times higher for Pb2þ [8,9], 12.5 times higher for Cd2þ [10,11] and 68 times higher for Cr6þ [12,13] than active carbon-based or clay-based adsorbents. The excellent adsorption behavior of MoS2 ascribed to its
* Corresponding author. E-mail address: [email protected] (F. Jia).
specific 2D structure. The SeMoeS sandwiched structure of monolayer MoS2 sheet makes rich S exposed on the surface, which provides abundant and high active adsorption sites to complex with heavy metals [14,15]. What is more, the negatively charged surface caused from the edge position could promote the adsorption with positively charged heavy metals [16]. Because of its excellent adsorption behavior, there are many studies on the modification and composition of MoS2 for the improvement of adsorption capacity and feasibility in practice. For example, in our previous work, a simple thermal treatment was used to modify MoS2 to improve the surface activity. As a result, the adsorption capacity and rate were 11-flod higher and 17.6 times faster compared with no modificatory MoS2, which was ascribed to the induced defects and oxidation [17]. Another study reported a porous Au/Fe3O4/MoS2CAs quaternary adsorbent synthesized through hydrothermal and embedding method, and it can achieve the multifunctional integration of an outstanding adsorption capacity, excellent magnetic separation and the sensitive detection for Hg simultaneously [18]. Although there are many other investigations about MoS2 as an adsorbent, these researches only analyzed in single heavy metal systems and no studies concerned on the co-adsorption of heavy metals with MoS2 to our knowledge. While it is common that heavy
https://doi.org/10.1016/j.molliq.2019.111987 0167-7322/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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metal ions coexist in the wastewater, especially in the industrial effluents [19]. They may influence each other during the adsorption due to the competitive effect. Therefore, investigation on the coadsorption of heavy metals on MoS2 is of great importance. An attempt was made to investigate the co-adsorption behavior of toxic Hg2þ, Pb2þ and Cd2þ on hydrothermal synthesized MoS2 with abundant multifunctional groups in this article. The surface morphology and property of MoS2 was investigated by highresolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy-energy dispersive spectrum (SEM-EDS), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The adsorption performance was studied through the adsorption kinetics, isotherm and pH effect experiments. The mechanism of the adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 was interpreted by XPS determination. The object of this study was to obtain a clear understanding on the behavior, as well as the mechanism, of MoS2 on the adsorption in multi-metal systems. Furthermore, it could give guidance for the application of MoS2 as an adsorbent in practice in the future. 2. Materials and methods 2.1. Materials Hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24$4H2O), thiourea (CN2H4S), lead nitrate (Pb(NO3)2), cadmium nitrate (Cd(NO3)2$4H2O), nitric acid (HNO3) and sodium hydroxide (NaOH) used in this study were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). Mercuric nitrate (Hg(NO3)2$H2O) was purchased from Shanghai Zhanyun Chemical Co., Ltd (China). All chemicals and reagents utilized in the experiment were of analytical grade. Milli-Q water with a resistivity of 18.2 MU cm was used as solvent. 2.2. Hydrothermal synthesis of MoS2 MoS2 was synthesized by hydrothermal method [12]. Firstly, 2.48 g of (NH4)6Mo7O24$4H2O and 4.56 g of CN2H4S were dissolved in 72 mL deionized water, followed by magnetic stirring the mixture to form a homogeneous solution for 30 min. After that, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, and thermally treated at 220 C for 6 h. After cooling down to room temperature, the synthetic precipitate was filtered and collected by a 0.1 mm membrane, and washed with deionized water for 3 times to remove the residual chemical reagents. Subsequently, the resultant material was ultra-sonicated by a ColeParmer at 300 W for 10 min and freeze-dried for 24 h to obtain the adsorbent. 2.3. Adsorption experiments 1 g/L stock solutions of Hg2þ, Pb2þ and Cd2þ were prepared by dissolving specific amounts of Hg(NO3)2$H2O, Pb(NO3)2 and Cd(NO3)2$4H2O in Milli-Q water before the adsorption experiments and few drops of dilute nitric acid were added into the stock solutions to keep the Hg2þ, Pb2þ and Cd2þ stable. All batch adsorption experiments were performed at room temperature. Before the adsorption, heavy metals solution with desired concentration was prepared by diluting the stock solution with Milli-Q water and then the pH of the solution was adjusted by HNO3 or NaOH. After that, a given amount of adsorbent was dispersed into the heavy metal solution. The erlenmeyer flask contained the suspension was then transferred to a water bath shaker and shaken at 150 rpm for a predetermined time. Finally, 5 mL of suspension was taken and filtered with 0.22 mm filter membrane, and the filtrate was collected
for a quantitative analysis of the heavy metal concentration. As for the multi-metals system, the adsorption procedure was same except that the solution contained three heavy metal ions with same initial concentration. When studying the effect of pH on the adsorption, 10 mg of MoS2 was added into 80 mL 50 mg/L heavy metal solution with pH varying from 1.0 to 5.0 and the adsorption lasted for 16 h. No experiments were conducted at higher pH to avoid the precipitation of heavy metal ions. For the adsorption kinetic experiment, 50 mg of MoS2 was added into 1 L multi-metal solution with Hg2þ, Pb2þ and Cd2þ concentration of 60 mg/L and pH of 5.0.5 mL of samples were taken at different time intervals (0e660 min). Adsorption isotherms were performed by adding 10 mg of MoS2 to 125 mL solution containing single or three ions. The initial concentration of heavy metals varied from 25 mg/L to 200 mg/L and the adsorption was conducted at pH of 5.0 for 16 h. The adsorption capacity was calculated using the following equation:
q¼
C0 Ce V m
(1)
where q (mg/g) is the adsorption capacity of the adsorbent; C0 (mg/ L) and Ce (mg/L) represents the initial and equilibrium concentration of the heavy metal, respectively; V (L) is the volume of the solution; m (g) is the mass of the adsorbent. 2.4. Measurements Raman spectra were obtained from INVIA Raman microscope with a 514 nm Ar laser (Renishaw, UK). A Zeiss Ultra Plus field emission scanning electron microscope (FESEM) (Zeiss, Germany) was applied to obtain the morphology and energy dispersive spectra (EDS). The HRTEM images were observed by using a Titan G2 60e300 Probe Cs Corrector HRSTEM (FEI, United States). Specific surface area of MoS2 sample was measured on ASAP 2460 accelerated surface area and porosimetry analyzer (Micromeritics, United States). The concentration of Hg2þ was detected by using a contrAA700 continuum source atomic absorption spectrometer (Jena, Germany), while the concentrations of Pb2þ and Cd2þ were detected by a GBC AVANTA M atomic absorption spectrometer (GBC, Australia). AFM measurements were carried out on Bruker MultiMode 8 atomic force microscopy (Bruker, USA). The XPS was performed on a VG ESCALB MK-II Instrument (VG, UK). The XPS high-resolution spectra were calibrated by setting the C 1s photoemission at 284.8 eV. The MoS2 sample used for the measurement after adsorption was prepared by the following steps: 50 mg MoS2 was added into 500 mL 150 mg/L relative heave metals with the pH of 5.0 and shaken for 16 h. Then the MoS2 sample was filtered with 0.22 mm filter membrane and washed for three times. Finally, the sample was dried under 60 C with a vacuum freezing dryer and collected. 3. Results and discussion 3.1. Characterization of MoS2 Fig. 1a showed the HRTEM image of MoS2. The good transparency of the sheets on the fringes indicated the thin thickness of MoS2. The sheets in the middle were linked together, forming a twisted-film liked morphology with a lateral dimension of around 500 nm. The representative AFM image and the inserted figure presented in Fig. 1b showed that two individual MoS2 samples had a cross-section thickness of around 5 nm and lateral size of around 500 nm, which was in good consistent with the HRTEM result. Fig. 1c and d displays the SEM image of MoS2 and corresponding EDS spectrum. The main elements detected on MoS2 were S, Mo
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Fig. 1. (a) HRTEM image of MoS2, (b) representative AFM image and corresponding height cross-section of MoS2 marked with line, (c) SEM image of MoS2, (d) EDS spectrum and elemental content of MoS2.
and O. The high content of O suggested that MoS2 might be partially oxidized during the synthesis. Figure S1 showed the result of nitrogen adsorption/desorption isotherms of MoS2 under 300 C. It exhibited a IV-type isotherm and a H3-type hysteresis hoop, indicating the mesopores characteristic of the MoS2 sample [16]. Besides, according to Barrett-Emmett-Teller (BET) calculations, the specific surface area is 12.05 m2/g. Fig. 2a showed the Raman spectrum of MoS2. The characteristic E12g and A1g peaks at 377 cm1 and 405 cm1 were assigned to the in-plane and out-of-plane vibration of Mo and S atoms [20], respectively, indicating the successful synthesis of MoS2. The peaks at 285 cm1, 661 cm1, 820 cm1 and 993 cm1 represented the doublet comprised of wagging modes of the terminal oxygen atoms, the asymmetric stretching of MoeOeMo bridge along the c axis, the symmetric stretch of the terminal oxygen atoms, and the asymmetric stretch of the terminal oxygen atoms, respectively [21,22], further suggesting the partial oxidation of MoS2. Fig. 2b displayed the FT-IR spectrum of MoS2. The band at 596 cm1 corresponded to the MoeS vibration mode in MoS2 [23]. The absorption bands at 1627 cm1 and 3434 cm1 were attributed to the OeH stretching vibrations from water, while the absorption peak at 904 cm1 were assigned to the bending vibration of inner OeH groups [24]. The stretching vibrations of MoeO at 757 cm1 and 654 cm1 were observed, confirming the presence of molybdenum oxide on MoS2 [25]. In addition, the observation of sulfurated functional groups SeO at 1034 cm1 and 1386 cm1, O]S]O and 1 1 OeSeO in SO2 and 1400 cm1, and bulk4 at 1056 cm , 1141 cm like sulfate at 1191 cm1 demonstrated that MoS2 had sulfurcontaining functional groups or structures on the surface [26]. It should be mentioned that although peaks of MoeO and S-function groups with strong intensity were determined from Raman and FTIR spectra, this did not mean the main composition of MoS2 was
molybdenum oxide or abundant S-function groups existed on MoS2 because Raman and FT-IR are only able to provide a simple qualitative analysis. Fig. 2cef showed the XPS spectra of MoS2. The wide-scan spectrum, as well as the inset semi-quantitative analysis, presented in Fig. 2c further proved that the main elements of MoS2 were Mo, S and O. The O 1s spectrum in Fig. 2d could be deconvoluted into four peaks at 530.6 eV, 531.7 eV, 532.6 eV and 533.6 eV, which were assigned to MoeO in MoO3, hydroxyl groups (OH), absorbed H2O molecules and OeC bonds, respectively [27]. Fig. 2e displays the Mo 3d spectrum of MoS2, which clearly indicated that Mo presented in various compounds with its chemical state varying from þ4 to þ6. The doublet peak at 228.9 eV (Mo 3d5/2) and 232.1 eV (Mo 3d3/2) were assigned to MoS2 [27], the doublet at 229.8 eV and 233.4 eV were attributed to Mo5þ in Mo2S5, the small doublet at 231.3 eV and 234.9 eV were corresponding to Mo6þ in MoS3 [28]. Besides the MoeS compounds, characteristic peaks of MoO3 were also found at 232.9 eV and 235.9 eV [22]. Fig. 2f showed the S 2p spectrum of MoS2, which could be deconvoluted into seven peaks. The strong doublet at 161.8 eV and 163.0 eV were assigned to S 2p3/2 and S 2p1/2 orbitals in MoS2 [27]. Other peaks at 161.0 eV, 163.7 eV, 164.5 eV, 168.4 eV and 169.4 eV were attributed to C]S, CSH, MoS3, SO2 and SO2 3 4 , respectively [20,29]. Different from Raman and FT-IR, XPS could provide a semi-quantitative analysis. Therefore, it could be obtained from XPS that the synthetic sample was mainly composed of MoS2, a small quantity of Mo2S5, MoS3, MoO3 and other oxygen- and sulfur-containing functional groups. 3.2. pH effect Fig. 3 illustrated the co-adsorption capacities of Hg2þ, Pb2þ, and Cd on MoS2 as a function of pH. It can be seen that the adsorption 2þ
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Fig. 2. (a) Raman spectrum of MoS2; (b) FT-IR spectrum of MoS2; (c) Wide-scan XPS spectrum of MoS2; (d), (e) and (f): High-resolution O 1s, Mo 3d and S 2p XPS spectra of MoS2, respectively.
capacities of the three ions on MoS2 were influenced by pH obviously, which exhibited lower adsorption capacities in a lower pH, while higher adsorption capacities in a higher pH. This phenomenon could be explained by Zeta potential of MoS2. As shown in Figure S2, the surface of MoS2 showed negative charges in all pH range, and a higher absolute value of Zeta potential in a higher pH value. Thus, the enhanced electrostatic interaction between MoS2 and positively charged heavy metal ions might account for the increase of adsorption capacity when pH increased from 1.0 to 5.0 [5]. In addition, with the contribution of Hþ decreased, the protonation became weaker on the surface of MoS2, which was available for Hg2þ, Pb2þ and Cd2þ to bind with the adsorption sites [11].
What is more, the adsorption capacities for the three adsorbates was as the following of Hg2þ >Pb2þ >Cd2þ nearly in all pH values, which meant the different affinity interaction with MoS2. According to Pearson hard soft acid base theory (HSAB), as a soft base, the S sites in MoS2 preferred to react with softer acid. The hardness of acid could be quantified through the value of absolute hardness (h). In this system, Hg, Pb and Cd were regarded as the acids with the relationship of absolute hardness followed by Hg (7.7 eV)< Pb (8.46 eV)< Cd (10.29 eV) [29]. It meant that Hg was softer than Pb, and Pb was softer than Cd, which is the same order of the adsorption capacity on MoS2. Additionally, the different hydrolysis of the three ions could affect the adsorption results. As shown in
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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qt ¼
t ðt=qe Þ þ 1 k2 q2e
5
(3)
where qe and qt is the adsorbed heavy metal at equilibrium time and t time (mg/g), respectively; k1 (min1) and k2 (g$mg1$min1) are the first- and second-order rate constants, respectively. The higher correlation coefficient (R2) indicated that both the two models well-fitted the experimental data. Furthermore, based on the analysis of Chi-square, the values of which were lower of Hg2þ, higher of Pb2þ and Cd2þ in Pseudo-first-order kinetics model, indicating the better fits for Hg2þ in this model, for Pb2þ and Cd2þ in Pseudo-second-order kinetics model.
3.4. Adsorption isotherms
Fig. 3. Co-adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 as a function of pH.
Figure S3, Hg2þ is easily hydrolysed and existed mainly in the forms of metal-hydroxyl species [31e33]. As a result, it can be easily adsorbed on negatively charged MoS2 due to the less hydration effect compared with Pb2þ and Cd2þ.
Both of the adsorption isotherms of Hg2þ, Pb2þ and Cd2þ on MoS2 in single- and multi-metal systems were studied at room temperature. The experimental data was fitted by Langmiur isotherm model (Eq. (4)) and Freundlich isotherm model (Eq. (5)) for a better understanding of adsorption behavior [1].
qe ¼
qm KL Ce 1 þ KL Ce 1=n
qe ¼ KF C e 3.3. Co-adsorption kinetics The co-adsorption of Hg2þ, Pb2þ and Cd2þ on MoS2 as a function of time was illustrated in Fig. 4. It showed a fast adsorption rate at the beginning and then reached the dynamic equilibrium. It took 30, 60 and 240 min for Cd2þ, Pb2þ and Hg2þ to reach the plateau with the adsorption capacity of 14, 366 and 1200 mg/g (1, 30 and 100 mg/m2), respectively. Compared with other adsorbent, MoS2 exhibited faster adsorption rate for Cd2þ and Pb2þ, and much higher adsorption capacity for Hg2þ [1,29,30]. Pseudo-first-order kinetics model (Eq. (2)) and pseudo-secondorder kinetics model (Eq. (3)) were used to fit the experimental data and the fitting results were shown in Table S1.
qt ¼ qe ½1 expð k1 tÞ
Fig. 4. Co-adsorption kinetics of Hg2þ, Pb2þ and Cd2þ on MoS2.
(2)
(4)
(5)
where qe is the adsorption capacity at equilibrium concentration (mg/g); qm is the maximum adsorption capacity (mg/g) from Langmiur model; Ce is the equilibrium concentration of Hg2þ, Pb2þ and Cd2þ (mg/L); KL and KF represents the Langmuir constant (L/ mg) and Freundlich constant (mg/g), respectively; n is a constant, which represents the sorption intensity. From the data shown in Table S2, the lower Chi-square values of Hg2þ and Cd2þ in Langmiur isotherm model indicated that the adsorption of the two metals fitted better in this model. Similarly, Freundlich isotherm model could better describe the adsorption of Pb2þ compared with Langmiur isotherm model. The adsorption isotherms of MoS2 in single-metal system were illustrated in Figure S4. Based on the fitting data of Langmuir model in Table S2, qm for Hg2þ, Pb2þ and Cd2þ are 2409, 293 and 30 mg/g (200, 24 and 2 mg/m2), respectively. Fig. 5 showed the adsorption isotherms result in multi-metal system. Compared with the single-metal system, the adsorption capacity for Hg2þ decreased dramatically,
Fig. 5. Co-adsorption isotherm of Hg, Pb and Cd on M.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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which might be because of the competitive effect. Unexpectedly, the adsorption for Pb2þ enhanced in multi-metal system, which indicated that there might be other mechanisms except the competitive effect during this co-adsorption procedure. While the adsorption capacity for Cd2þ was as low as the phenomenon in single-metal system. The semiquantitative analysis of XPS was used for the further study of the adsorption capacities for the three ions in the two systems. Fig. 6 showed the wide-scan XPS spectra of MoS2 after the adsorption of Hg, Pb and Cd in single- or multi-metal solutions (named as M-Hg, M-Pb, M-Cd and M-HgPbCd, respectively). Strong peaks of Hg and Pb were detected on MoS2, indicating the good adsorption. Compared with the single-metal solutions, the contents of Hg and Cd were decreased, while that of Pb was increased (Table 1), which was in good agreement with the results of adsorption isotherm experiments. Fig. 7 was the results of SEM-EDS of MoS2 after the adsorption, which indicated that plenty of Hg and Pb, as well as a small amount of Cd, were equably distributed on MoS2. 3.5. Adsorption mechanism The high-resolution XPS spectra of MoS2 loading with heavy metals were given in Fig. 8 to explore the chemical reaction between MoS2 and heavy metals. Fig. 8a showed the spectrum of Mo 3d. The peaks at 228.9, 229.8, 231.3, 232.1, 232.9, 233.4, 234.9, 235.9 eV could be assigned to MoS2, Mo2S5, MoS3, MoS2, MoO3, Mo2S5, MoS3, MoO3, respectively [22,27,28]. The Mo 3d spectrum indicated a great increase of MoO3 on the sample of MoS2eHg, MoS2ePb, and MoS2eHgPbCd, which might be related to the reaction of MoS2 with Hg and Pb. The redox potential of MoS2/MoO3 is 0.429 V, which is lower than that of Hg2þ/Hg0, while higher than that of Pb2þ/Pb and Cd2þ/Cd. It meant that MoS2 was able to reduce Hg2þ into Hg0 by itself while was inactive for Pb2þ and Cd2þ [34]. Thus, a redox reaction might occur between Hg2þ and MoS2, resulting in the increase of MoO3 in sample MoS2eHg. As for MoS2ePb, the peaks of Mo (VI) were located in 236.2 eV, which was 0.3 eV higher than those in other samples. The shift of the peaks might originate from the complexation between Pb2þ and MoO3 according to Eq. (6) [35], and the peaks of Mo (VI) in 236.2 eV might be assigned to PbMoO4. Therefore, the increase of Mo (VI) peak might be highly because of the chemical reaction between Pb2þ and MoO3. The Mo (VI) peaks in MoS2eHgPbCd were stronger than those in both MoS2eHg and MoS2ePb, which might be attributed that additional MoO3 would generate during the loading of Hg2þ on
Table 1 The elemental composition of M adsorb Hg, Pb and Cd (wt %) by XPS semiquantitative analysis. Adsorbent
Mo
S
O
Hg
Pb
Cd
M-Hg M-Pb M-Cd M-HgPbCd
26.07 42.64 49.88 21.3
21.95 31.71 39.65 16.12
5.34 9.18 10.12 8.98
46.64 / / 32.63
/ 16.48 / 20.98
/ / 0.35 /
MoS2, and these MoO3 can serve as adsorption sites for Pb2þ and promote the loading of Pb2þ through the formation of PbMoO4.
MoO3 þ H2 O þ Pb2þ ¼ PbMoO4 þ 2Hþ
(6)
Fig. 8b presented the Hg 4f spectrum of MoS2eHg and MoS2eHgPbCd. The doublet with binding energy of 101.0 eV and 105.0 eV were assigned to HgeS, while the other doublet at 102.2 eV and 106.0 eV were HgeO [20]. It suggested that both S and O served as the adsorption sites for the loading of Hg2þ on the surface of MoS2, while S sites were the main contributors. In addition, a small Hg0 peak at 99.5 eV was detected, which confirmed the redox reaction between Hg2þ and MoS2 [36]. Fig. 8c was the Pb 4f spectrum of MoS2ePb and MoS2eHgPbCd. Both of the Pb 4f5/2 and Pb 4f7/2 could be deconvoluted into two peaks. The doublet with binding energy of 144.5 eV and 139.6 eV could be attributed to PbeS [37], while the other with binding energy of 144.0 eV and 139.2 eV can be assigned to PbeO in PbMoO4 [38]. The obvious PbeS and PbeO peaks further illustrated the complexation of Pb2þ with S sites and MoO3. Compared with MoS2ePb, the intensity of PbeS peak in MoS2eHgPbCd decreased, while that of PbeO peak increased, making a slight shift of Pb 4f. The decrease of PbeS peak was because some S sites were occupied by Hg2þ and Cd2þ in the multi-metal system, while the increase of PbeO peak might be ascribed to more MoO3 existing in the multimetal system due to the redox reaction of Hg2þ and MoS2 and then removed more Pb2þ with the formation of PbMoO4. The Cd 3d spectrum of MoS2eCd and MoS2eHgPbCd was presented in Fig. 8d. The Mo 3p1/2 peak at 412.2 eV has a slight shift in the multi-metal system, which might be attributed to the adsorption of Hg2þ and Pb2þ on MoS2. A small CdeS peak was observed at 405.5 eV in sample MoS2eCd [11], indicating that there was a weak interaction between S and Cd2þ in single-metal system. However, the peak disappeared in MoS2eHgPbCd, which is because of the occupation of the S sites by Hg2þ and Pb2þ. Fig. 9 was the release amount of proton during the adsorption of Hg2þ, Pb2þ, and Cd2þ, which was calculated through comparing the pH before and after adsorption. A blank group with no heavy metals was set to eliminate the released proton which generated by the oxidation of MoS2 in water [25]. It could be seen that about 0.23 103, 0.54 103 and 0.03 103 moL/L Hþ released in the adsorption of Hg2þ, Pb2þ and Cd2þ, respectively. According to other researches, the heavy metal ions would bond with S on MoS2 surface by replacing the protonated Hþ, it was therefore Hþ was released during the adsorption for heavy metals on MoS2 [5,7]. What is more, other ion exchange reactions between the functions such as eSH and eOH on MoS2 and heavy metals might also occur as shown in equations (6)e(8) [29,39]. It was worthy to mention that the reaction in equation (6) which involved double release amount of Hþ was the reason of the most release amount of proton in Pb2þ solution.
SH þ Hg2þ / S Hgþ þ Hþ
(7)
Fig. 6. XPS spectra of M after the adsorption of Hg, Pb and Cd.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
C. Liu et al. / Journal of Molecular Liquids xxx (xxxx) xxx
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Fig. 7. SEM images of Hg, Pb and Cd adsorbed on M: (a) SEM morphological image; (bed) EDS elemental mapping of Hg, Pb and Cd, respectively.
Fig. 8. High-resolution of M after the adsorption of Hg, Pb and Cd both with the blank group: (a) Mo 3d spectrum; (b) Hg 4f spectrum; (c) Pb 4f spectrum; (d) Cd 3d spectrum.
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molliq.2019.111987. References
Fig. 9. The release amount of proton in Hg2þ, Pb2þ and Cd2þ solution during the adsorption.
OH þ Pb2þ / O Pb2þ þ Hþ
(8)
4. Conclusion In this work, multifunctional MoS2 adsorbent with nanometer thickness was prepared by hydrothermal synthesis for the removal of Hg2þ, Pb2þ and Cd2þ from water. The adsorbent exhibited different adsorption capacity for the three heavy metals, which mainly ascribed to their affinities for S sites on MoS2 according to HSAB. The adsorptions for Hg2þ, Pb2þ and Cd2þ both showed significant pH dependence from 1 to 5 which were influenced by the electrostatic attraction and protonation. XPS analysis of multifunctional MoS2 after the adsorption revealed that the adsorption mechanisms might involve oxidation-reduction between MoS2 and Hg2þ both with the complexation to form HgeS, PbeS, CdeS, HgeO and PbeMoO4 on MoS2. Under the multi-metal systems of the three heavy metals, the adsorption capacities for Hg2þ and Cd2þ were decreased compared with the condition in single-metal systems because of the competitive adsorption on their common adsorption sites. On the contrary, the adsorption for Pb2þ was increased which was because the excess MoO3 generated by the oxidation-reduction between MoS2 and Hg2þ removed more Pb2þ with the formation of PbMoO4.
Conflicts of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Acknowledgement The financial supports for this work from the National Natural Science Foundation of China (Nos. 51704220, 51704212 and 51674183), the Natural Science Foundation of Hubei Province (No. 2016CFA013), and the Research Fund Program of Key Laboratory of Rare Mineral, Ministry of Land and Resources (KLRM-KF201802) were gratefully acknowledged.
[1] J. Deng, Y. Liu, S. Liu, G. Zeng, X. Tan, B. Huang, X. Tang, S. Wang, Q. Hua, Z. Yan, Competitive adsorption of Pb(II), Cd(II) and Cu(II) onto chitosan-pyromellitic dianhydride modified biochar, J. Colloid Interface Sci. 506 (2017) 355e364. [2] M. Li, Q. Liu, L. Guo, Y. Zhang, Z. Lou, Y. Wang, G. Qian, Cu(II) removal from aqueous solution by Spartina alterniflora derived biochar, Bioresour. Technol. 141 (2013) 83e88. [3] L. Dong, L. Hou, Z. Wang, P. Gu, G. Chen, R. Jiang, A new function of spent activated carbon in BAC process: removing heavy metals by ion exchange mechanism, J. Hazard Mater. 359 (2018) 76e84. [4] F.M. Mohammed, E.P.L. Roberts, A. Hill, A.K. Campen, N.W. Brown, Continuous water treatment by adsorption and electrochemical regeneration, Water Res. 45 (2011) 3065e3074. [5] Z. Wang, B. Mi, Environmental applications of 2D molybdenum disulfide (MoS2) nanosheets, Environ. Sci. Technol. 51 (2017) 8229e8244. [6] T. Phothitontimongkol, N. Siebers, N. Sukpirom, F. Unob, Preparation and characterization of novel organo-clay minerals for Hg(II) ions adsorption from aqueous solution, Appl. Clay Sci. 43 (2009) 343e349. [7] K. Ai, C. Ruan, M. Shen, L. Lu, MoS2 nanosheets with widened interlayer spacing for high-efficiency removal of mercury in aquatic systems, Adv. Funct. Mater. 26 (2016) 5542e5549. [8] Y.-H. Li, J. Ding, Z. Luan, Z. Di, Y. Zhu, C. Xu, D. Wu, B. Wei, Competitive adsorption of Pb2þ, Cu2þ and Cd2þ ions from aqueous solutions by multiwalled carbon nanotubes, Carbon N. Y. 41 (2003) 2787e2792. [9] C. Liu, F. Jia, Q. Wang, B. Yang, S. Song, Two-dimensional molybdenum disulfide as adsorbent for high-efficient Pb(II) removal from water, Appl. Mater. Today. 9 (2017) 220e228. [10] B. Xiao, K.M., Thomas Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon, Langmuir 20 (2004) 4566e4578. [11] Q. Wang, L. Yang, F. Jia, Y. Li, S. Song, Removal of Cd(II) from water by using nano-scale molybdenum disulphide sheets as adsorbents, J. Mol. Liq. 263 (2018) 526e533. [12] Y. Gao, C. Chen, X. Tan, H. Xu, K. Zhu, Polyaniline-modified 3D-flower-like molybdenum disulfide composite for efficient adsorption/photocatalytic reduction of Cr(VI), J. Colloid Interface Sci. 476 (2016) 62e70. [13] K. Selvi, Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon, Bioresour. Technol. 80 (2001) 87e89. [14] X. Li, K. Peng, Hydrothermal synthesis of MoS2 nanosheet/palygorskite nanofiber hybrid nanostructures for enhanced catalytic activity, Appl. Clay Sci. 162 (2018) 175e181. [15] X. Li, K. Peng, MoSe2/Montmorillonite composite nanosheets: hydrothermal synthesis, structural characteristics, and enhanced photocatalytic activity, Minerals 8 (2018), https://doi.org/10.3390/min8070268. [16] F. Jia, K. Sun, B. Yang, X. Zhang, Q. Wang, S. Song, Defect-rich Molybdenum Disulfide as Electrode for Enhanced Capacitive Deionization from Water Desalination, vol. 446, 2018, pp. 21e30. [17] F. Jia, C. Liu, B. Yang, X. Zhang, H. Yi, J. Ni, S. Song, Thermal modification of the molybdenum disulfide surface for tremendous improvement of Hg2þ adsorption from aqueous solution, ACS Sustain. Chem. Eng. 6 (2018) 9065e9073. [18] L. Zhi, W. Zuo, F. Chen, B. Wang, 3D MoS2 composition aerogels as chemosensors and adsorbents for colorimetric detection and high-capacity adsorption of Hg2þ, ACS Sustain. Chem. Eng. 4 (2016) 3398e3408. [19] B.K. Adebola, S.J. Kayode, O.A., Akeem Integrated assessment of the heavy metal pollution status and potential ecological risk in the Lagos Lagoon, South West, Nigeria, Hum. Ecol. Risk. Assess. 24 (2018) 377e397. [20] F. Jia, Q. Wang, J. Wu, Y. Li, S. Song, Two-Dimensional molybdenum disulfide as a superb adsorbent for removing Hg2þ from water, ACS Sustain. Chem. Eng. 5 (2017) 7410e7419. [21] B.C. Windom, W.G. Sawyer, D.W. Hahn, A Raman spectroscopic study of MoS2 and MoO3: applications to tribological systems, Tribol. Lett. 42 (2011) 301e310. [22] F. Jia, C. Liu, B. Yang, S. Song, Microscale control of edge defect and oxidation on molybdenum disulfide through thermal treatment in air and nitrogen atmospheres Appl, Surf. Sci. 462 (2018) 471e479. [23] S.S. Karade, D.P. Dubal, B.R., Sankapal MoS2 ultrathin nanoflakes for high performance supercapacitors: room temperature chemical bath deposition (CBD), RSC Adv. 6 (2016) 39159e39165. [24] M. Ghanei-motlagh, M.A. Taher, A novel electrochemical sensor based on silver/halloysite nanotube/molybdenum disulfide nanocomposite for efficient nitrite sensing, Biosens. Bioelectron. 109 (2018) 279e285. [25] F. Jia, X. Zhang, S. Song, AFM study on the adsorption of Hg2þ on natural molybdenum disulfide in aqueous solutions, Phys. Chem. Chem. Phys. 19 (2017) 3837e3844. [26] J. Wang, C. Li, SO2 adsorption and thermal stability and reducibility of sulfates formed on the magnesiumealuminate spinel sulfur-transfer catalyst, Appl. Surf. Sci. 161 (2000) 406e416. [27] X. Zhang, F. Jia, B. Yang, S. Song, Oxidation of molybdenum disulfide sheet in
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987
C. Liu et al. / Journal of Molecular Liquids xxx (xxxx) xxx
[28]
[29]
[30]
[31]
[32]
[33]
water under in situ atomic force microscopy observation, J. Phys. Chem. C 121 (2017) 9938e9943. J. Myeong, C. Sung, H. Kim, S. Jae, L. Seong, S. Kim, Effects of pressure and temperature in hydrothermal preparation of MoS2 catalyst for methanation reaction, Catal. Lett. 148 (2018) 1803e1814. D. Saha, S. Barakat, S.E. Van Bramer, K.A. Nelson, D.K. Hensley, J. Chen noncompetitive and competitive adsorption of heavy metals in sulfurfunctionalized ordered mesoporous carbon, ACS Appl. Mater. Interfaces 8 (2016) 34132e34142. W. Wang, Y. Zhao, H. Yi, T. Chen, S. Kang, T. Zhang, F. Rao, S. Song, Pb(ІІ) removal from water using porous hydrogel of chitosan-2D montmorillonite, Int. J. Biol. Macromol. 128 (2019) 85e93. €berg, K.J. Powell, P.L. Brown, R.H. Byrne, T. Gajda, G. Hefter, A. Leuz, S. Sjo H. Wanner, Chemical speciation of environmentally significant metals with 2e 3e inorganic ligands. Part 3: the Pb2þþ OHe, Cle, CO2e 3 , SO4 , and PO4 systems (IUPAC Technical Report), Pure Appl. Chem. 81 (2009) 2425e2476. F. Di Natale, A. Erto, A. Lancia, D. Musmarra, Mercury adsorption on granular activated carbon in aqueous solutions containing nitrates and chlorides, J. Hazard Mater. 192 (2015) 1842e1850. D. Borah, K. Senapati, Adsorption of Cd(II) from Aqueous Solution onto Pyrite
9
Fuel., vol. 85, 2006, pp. 1929e1934. [34] Z. Wang, A. Sim, J.J. Urban, B. Mi, Removal and recovery of heavy metal ions by two-dimensional MoS2 nanosheets: performance and mechanisms environ, Sci. Technol. 52 (2018) 9741e9748. [35] Y. Wu, X. Cheng, X. Zhang, Y. Xu, S. Gao, H. Zhao, L. Huo, High efficient and selective removal of Pb2þ through formation of lead molybdate on a-MoO3 porous nanosheets array, J. Colloid Interface Sci. 491 (2017) 80e88. [36] J.J. Ehrhardt, P. Behra, P. Bonnissel-Gissinger, M. Alnot, PS study of the sorption of Hg(II) onto pyrite FeS2, Surf. Interface Anal. 30 (2000) 269e272. [37] M. Zhang, F. Jia, M. Dai, S. Song, Combined electrosorption and chemisorption of low concentration Pb(II) from aqueous solutions with molybdenum disulfide as electrode Appl, Surf. Sci. 455 (2018) 258e266. [38] G. Gyawali, R. Adhikari, B. Joshi, T.H. Kim, V. Rodríguez-Gonz alez, S.W. Lee, Sonochemical synthesis of solar-light-driven Ag-PbMoO4 photocatalyst, J. Hazard Mater. 263 (2013) 45e51. [39] H. Pei, J. Wang, Q. Yang, W. Yang, N. Hu, Y. Suo, D. Zhang, Z. Li, J. Wang, Interfacial growth of nitrogen-doped carbon with multi-functional groups on the MoS2 skeleton for efficient Pb(II) removal, Sci. Total Environ. 631e632 (2018) 912e920.
̊
Please cite this article as: C. Liu et al., Simultaneous removal of Hg2þ, Pb2þ and Cd2þ from aqueous solutions on multifunctional MoS2, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.111987