Microscopic Insights into pH-dependent Adsorption of Cd(II) on Molybdenum Disulfide Nanosheets

DONG Lijia, GUO Xiaojie, LI Xue, CHEN Chaogui, JIN Yang, AHMED Alsaedi, TASAWAR Hayat,4, ZHAO Qingzhou, SHENG Guodong

Microscopic Insights into pH-dependent Adsorption of Cd(II) on Molybdenum Disulfide Nanosheets

DONG Lijia1, GUO Xiaojie2, LI Xue1, CHEN Chaogui1, JIN Yang1, AHMED Alsaedi3, TASAWAR Hayat3,4, ZHAO Qingzhou5, SHENG Guodong6

(1. School of Life Science, Shaoxing University, Shaoxing 312000, China; 2. College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China; 3. NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia; 4. Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan; 5. College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China; 6. College of Chemistry and Chemical Engineering, Shaoxing Univeristry, Shaoxing 312000, China)

Herein, the retention mechanisms and microstructure of Cd(II) on MoS2nanosheets were evaluated by batch experiments and EXAFS technology. The sorption of Cd(II) on MoS2was strongly affected by solution pH, contact time, and temperature, but not by the ionic strength. The solution pH could only promote the sorption capacity, but does not improve the sorption rates and change the sorption isotherms and thermodynamics in the pH range of 3.3–9.6. The pseudo-second-order model could fit the equilibrium data better and the intra-particle diffusion model showed three typical stages in the sorption process. The isotherms and thermodynamics analysis indicated that the heterogeneity sorption of Cd(II) onto MoS2was a spontaneous, endothermic, and irreversible process. The EXAFS spectra revealed the coexistence of two sorption types. The inner-sphere complexation was formed in the form of Cd–S bond at lower pH (3.56, 6.48), while the Cd(OH)2precipitation occurred in the form of Cd–O and Cd–Cd bonds at higher pH (9.57). These results provide new insights into the interaction mechanisms between metal ions and MoS2nanosheets.

MoS2nanosheets; pH; microstructure; EXAFS; cadmium(II)

Cadmium (Cd(II)) is one of the toxic heavy metals from industrial activity such as nickel-cadmium batteries, mining, paint pigmentations, alloy, and electroplating[1-2]. With the input flux increasing, the Cd pollution is posing a great pressure on environmental safety[3]. Especially for the human public health, Cd(II) is very nasty due to its long biological half-life, non-biodegradable nature, and carcinogenicity[4-5]. Then appropriate removal technologies become necessary. Adsorption is considered as one of the promising techniques owing to its low cost, reliability, simplicity and high efficiency[6-9]. Since the adsorption reaction, mobility and species of metal ions in the environment determined their fate[10-16], it is of great importance to explore the interaction mechanism and microstructure of heavy metals at the solid/water interface in various environmental conditions.

Molybdenum disulfide (MoS2), a “rising star” material, has attracted tremendous interest in remediation of heavy metals from water over past decades[17-20]. MoS2nano-sheets have layered structure with strong in-plane bonding in layers and weak out-of-plane van der Waals interactions between the individual sandwiched S-Mo-S layers[21-23]. Based on the anisotropic structure of MoS2, two-dimensional (2D) structure with large surface area and permeable channels for ion adsorption and tran-sportation is prone to form[23]. It is noted that both surfaces of each layer of MoS2are fully occupied by S atoms, which may serve as binding sites[19-21]. In addition, MoS2offers some excellent properties like superior mechanical flexibility, excellent chemical and thermal stability. All above properties endow MoS2nanosheets with good adsorption performance for the removal of heavy metals. In several cases, the maximum sorption capacities of MoS2nanosheets for Hg(II), Ag(I), U(VI), Cd(II), Th(IV) are up to 2506, 1348, 493, 477, 454 mg/g, respectively[21,24-26].

Many mechanisms have been proposed for the interaction of metals with MoS2, including ion exchange, inner layer metal-S complexation, outer layer electrostatic attraction, surface precipitation[25-28]. Extended X-ray adsorption fine structure (EXAFS) spectroscopy provides an effective way to distinguish the interaction mechanism and microstructure[29]. In the past, the EXAFS technology was widely applied to reveal the interaction mechanisms and microstructures of heavy metals and adsorbents, such as Eu(III) and-MnOOH, Eu(III) and-MnO2, Cd(II) and kaolinite, and Cd(II) and HAs[29-33]. However, few studies[34-36]about the interaction mechanism of heavy metals and MoS2nanosheets have been conducted at the molecular level.

In present work, both batch experiment and EXAFS technique were employed to analyze the sorption behavior and mechanism of Cd(II) on MoS2. Firstly, MoS2were characterized by SEM, TEM, XRD, FT-IR,. Secondly, effects of solution pH, ionic strength, and temperature, contact time on the sorption of Cd(II) and the adsorption kinetics, isotherms, thermodynamics in solutions with various pH were evaluated. Finally, EXAFS technique was used to analyze the sorption mechanisms of Cd(II) on MoS2as a function of pH.

1 Materials and methods

1.1 Materials

Overall chemical reagents including cadmium nitrate (Cd(NO3)2·2H2O), sodium nitrate (NaNO3), sodium hydroxide (NaOH), nitrite acid (HNO3), and molyb-denum disulfide (MoS2) were analytical grade purchased from Nanjing XFNANO Materials Tech. Co. Ltd. (China) without further purification. Cd(II) stock solution was obtained by dissolving Cd(NO3)2·2H2O in distilled water.

1.2 Characterization

The intrinsic properties of MoS2greatly determinate its sorption capacity, thereby it is helpful to investigate its characterization for revealing the sorption mechanism. Herein, TEM, SEM, EDX analysis and elemental distribution mappings of the MoS2sample were carried out by using a transmission electron microscope (JEM-1011, Japan) instrument and a field emission scanning electron microscope (JSM-6360LV, Japan). XRD pattern of the sample was tested by the D8 Discover X-ray diffractometer (Bruker, Germany) with Cu Kαradiation (=0.1541 nm) and distinguished according to the JCPD standards. FT-IR spectrometer (NEXUS, America) was employed to evaluate surface functional groups of MoS2in the wavelength range of 400–4000 cm–1. Zeta potential analyzer (Zetasizer Nano ZS, Malvern Co., UK) was used to locate the pHpzcof the adsorbent,, the suspension of MoS2and NaNO3adjusted to an appropriate pH was test.

1.3 Batch experiment

A set of sorption experiments of Cd(II) onto MoS2were conducted under N2in polyethylene tubes. The stock suspensions of Cd(II), MoS2, NaNO3, and distilled water were mixed in polyethylene tubes in order to gain the desired concentrations. The pH was adjusted by add-ing 0.1 or 0.01 mol/L HNO3or NaOH solution with negligible volumes. The tubes containing above mentioned mix-tures were shaken for more than 12 h to achieve sorption equilibrium, and the the solid was separated by centrifu-gation method. Finally, the Cd concentration in the su-pernatant was tested. The Cd(II) sorption percentage, distribution coefficient (d), and sorption amount (e) on MoS2were calculated based on the follow-ing equations:

Sorption%=(0–e)/0×100% (1)

d=(0–e)/e×/(2)

e=(0–e)×/(3)

where0anderepresent the initial and equilibrium concentrations of Cd(II) (mg/L), respectively.is the suspension volume (L) andis the MoS2mass (g).

1.4 EXAFS data collection and analysis

EXAFS data were collected at room temperature on BL14W at Shanghai Synchrotron Radiation Facility (SSRF, China). The obtained EXAFS data was analyzed by using Athnea software. The raw, averaged data were processed to isolate the EXAFS oscillations by removal of the pre-edge background. The3-weighted EXAFS spectra of Cd(II) were Fourier transformed (FT). The code FEEF7 and the as-known crystal structure of Cd(NO3)2, Cd(OH)2, CdS were used to calculate the theore-tical scatter-ing phases and amplitudes. The bond distance () and coordination number (CN), and the Debye-Waller factor of sample were optimized for each single peak by performing curve fitting with nonlinear least-squares.

2 Results and discussion

2.1 Characterization of MoS2 nanosheets

The morphologies and microstructures of MoS2were characterized by SEM, TEM,, and the results are shown in Fig. S1 in detail. SEM images, EDX spectra, and elemental distribution mapping of MoS2are shown in Fig. 1. It is evident that two non-target elements are present in MoS2nanosheets,, Cu and Zn. However, the elemental distribution percentages of Mo and S atoms are much higher than those of Cu and Zn, indicating that MoS2contains negligible impurities. In other words, the effects of Cu and Zn in MoS2materials on sorption can be ignored.

Fig. 1 SEM image (a), EDX spectra, and corresponding ele-mental maps (b) for MoS2 nanosheets

2.2 Batch sorption

Fig. 2 shows the pH dependence of Cd(II) sorption on MoS2in 0.1, 0.01 and 0.001 mol/L NaNO3solutions, respectively. It is observed that the sorption is strongly dependent on pH. The percentage of Cd(II) sorption sharply increased at pH 3.3–6.5 and then creeped until a plateau level at pH>6.5. At pH 3.3–6.5, owing to protonation reaction, the number of protonated sites decreased with the increase of pH, leading to stronger affinity between the ions and MoS2. However, the saturated surface of MoS2immobilized more cadmium ions at pH>6.5. It is noted that the species of Cd(II) are highly dependent on the solution pH. As provided by Wang[25], the main species were Cd2+, CdOH+at pH<8.5, whereas the main species were Cd(OH)2and Cd(OH)–at pH>8.5. Therefore, both electrostatic interaction and precipitation might be the main mechanisms of the sorption. Ionic strength is another important factor influencing sorption. Fig. 2 shows that the effect of ionic strength on Cd(II) sorption is not significant, implying inner-sphere complexation dominates the sorption behavior at different pH. The complexation type of Cd(II) with MoS2is different from that with other materials, such as montmorillonite[37], kaolinite[31], manganese oxide[38], fibre fruit lufa[39], magnetic polyvinyl alcohol/ laponite[40]. The interactions between Cd(II) and these materials were found to contain both outer-sphere com-plexation at lower pH and inner-sphere complexation at higher pH. The difference may be attributed to various mi-crostructure and intrinsic properties among these ad-sorbents.

Fig. 2 Effects of pH and ionic strength on Cd(II) adsoprtion onto MoS2 nanosheets

Cd(II) initial concentration=10 mg/L,/=0.15 g/L,=293 K

To understand the sorption kinetics and to determine their phenomenological coefficients, the sorption capacity as a function of contact time was shown in Fig. 3(a). The sorption rates at different pH were similar and the contact time needed for complete adsorption of Cd(II) on MoS2was 2 h. The final sorption capacities at pH 4.55, 5.34, and 6.12 reached 28.4, 37.2 and 49.3 mg/g, respectively. These results illustrate that pH only enhances the sorption capacity but does not promote the sorption rate. Besides, 3 kinetic models were employed to analyze the kinetic data, including pseudo-first- kinetic model[41], pseudo-second-kinetic model[42], and intra-particle diffusion model[43], which was described by follows:

Fig. 3 Cd(II) adsorption on MoS2 nanosheets as a function of contact time (a) and the fitting of pseudo-first-order kinetic model (b), pseudo-second-order kinetic model (c) and intra-particle diffusion model (d) at different pH

Cd(II) initial concentration=10 mg/L,/=0.15 g/L,=0.01 mol/L NaNO3,=293 K

Table 1 Parameters of kinetic models for the adsorption of Cd(II) on MoS2 as a function of pH

Cd(II) initial concentration=10 mg/L,/=0.15 g/L,=0.01 mol/L NaNO3,=293 K

whereq(mg/g)represents the sorption capacity of Cd(II) at time(h);1(h–1),2(g/(mg·h)),i(g/(mg·h1/2)) are the rate constants of 3 models, respectively;(mg/L) indicates the thickness of boundary layer. The linear plots of 3 models went with Fig. 3(b-d), respectively. The corresponding parameters can be found in Table 1. We can see that the sorption kinetics could be fittedbetter by the pseudo-second-order kinetic model than pseudo-first-order kinetic model due to the higher correlation coefficients (2). The result implied the chemical sorption process[44]. More remarkable, all the sorption processes on MoS2at different pH contained 3 stages (Fig. 3(d)). The initial stage might be ascribed to Cd(II) ions diffusion from solution to the surface of MoS2. Due to enough binding sites, high concentration Cd(II) ions and strong affinity between metal ions and S atoms[24-25], great numbers of Cd(II) were rapidly adsorbed onto the MoS2surface. The second stage is the intra-particle diffusion, which donated as Cd(II) intrapar-ticle diffusion through 2D structure of MoS2with permeable channels[23]. The third stage on MoS2was the final equilibrium stage, which might be attributed to lack of binding sites or low concentration Cd(II) ions. Interestin-gly, the turning points in time at three stages were the same at solution pH 4.55, 5.34, 6.12. The intra-particle diffusion process mainly occurred after 1 h and the sorption reached equilibrium after 2 h. The sorption isotherms studies were carried out at different solution pH (, 4.55, 5.34, 6.12) and 3 temperatures (, 293, 313, 333 K), and the detailed results are shown in Fig. S2, Fig. S3 and Table S1, Table 2. The sorption isotherms were consistent with previous studies[45-48]. The thermodynamics implied that the sorption was a spontaneous, endothermic process[49-51].

2.3 EXAFS analysis

Fig. 4 Normalized, background-subtracted and k3-weighted EXAFS spectra (a) and corresponding RSF magnitudes and imaginary parts (b) of Cd reference samples

3 Conclusions

In this work, the interaction mechanisms and microscopic structure of Cd(II) and MoS2nanosheets were investi-gated by batch experiments and EXAFS technology. In batch experiments, the pH-dependent and ionic strength- independent sorption of Cd(II) onto MoS2implied an inner-sphere complexation in the range of pH 3.3–9.6. The better fitted pseudo-second-order kinetics confirmed the chemical nature of the sorption and the intra-particle diffusion model reflected the sorption process from the surface to intra-particle diffusion to final equili-brium. Theisotherms could be simulated better by Freun-dlich isotherms than by Langmuir isotherms model, indicat-ing the hetero-geneity of active sites on MoS2. The ther-mody-namics implied the sorption at each pH was a spontaneous, endothermic, and irreversible process. The EXAFS spectra revealed the coexistence of two sorption types. The inner-sphere complexation was formed in the form of Cd–S bond similar to CdS complex at pH 3.56 and 6.48, while the precipitation occurred in the form of Cd–O and Cd–Cd bonds similar to Cd(OH)2at pH 9.57. In summary, the interaction mechanisms and local structure of Cd(II) and MoS2are strongly affected by the solution pH.

Supporting materials

Supporting materials related to this article can be found at https://doi.org/10.15541/jim20190381.

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不同pH條件下硫化鉬納米片吸附Cd(II)的微觀機(jī)制研究

董麗佳1, 郭筱潔2, 李雪1, 陳朝貴1, 金陽1, AHMED Alsaedi3, TASAWAr Hayat3,4, 趙輕舟5, 盛國棟6

(1. 紹興文理學(xué)院 生命科學(xué)院, 紹興 312000; 2. 杭州電子大學(xué) 材料與環(huán)境工程學(xué)院, 杭州 310018; 3. NAAM Research Group, Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Sudi Arabia; 4. Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan; 5. 中國科學(xué)院大學(xué) 資源與環(huán)境學(xué)院, 北京 100049; 6. 紹興文理學(xué)院 化學(xué)與化工學(xué)院, 紹興 312000)

本研究結(jié)合靜態(tài)實驗和X射線吸收精細(xì)結(jié)構(gòu)譜學(xué)(EXAFS)評估了硫化鉬納米片對重金屬Cd(II)的吸附行為和微觀機(jī)制。結(jié)果表明: Cd(II)在硫化鉬納米片上的吸附受溶液pH、反應(yīng)時間和溫度的顯著影響, 但不受離子強(qiáng)度的影響。在pH 3.3~9.6范圍內(nèi), pH升高顯著促進(jìn)了硫化鉬對Cd(II)的吸附量, 但不改變吸收速率、吸附等溫線和熱力學(xué)。二級動力學(xué)模型能更好地擬合該吸附平衡, 且內(nèi)表面顆粒擴(kuò)散模型顯示了吸附過程中的三個典型階段。等溫線和熱力學(xué)分析說明Cd(II)在硫化鉬上的吸附是異質(zhì)性的、自發(fā)的、吸熱的和不可逆的過程。EXAFS光譜學(xué)分析揭示了該吸附存在兩種類型: 在較低的pH(3.56, 6.48)條件下, 內(nèi)表面絡(luò)合以Cd–S配位鍵為主; 在較高的pH(9.57)條件下, 出現(xiàn)Cd(OH)2沉淀, 且配位鍵以Cd–O和Cd–Cd的形式存在。這些研究結(jié)果對于評估重金屬離子和硫化鉬納米片在分子水平上的作用機(jī)理提供了新的視野。

硫化鉬納米片; pH; 微觀結(jié)構(gòu); EXAFS; Cd(II)

TQ174

A

1000-324X(2020)03-0293-08

10.15541/jim20190381

date:2019-07-24;

date: 2019-09-11

National Natural Science Foundation of China (31700476)

DONG Lijia(1984–), female, PhD. E-mail: Donglijia@126.com

董麗佳(1984–), 女, 博士. E-mail: Donglijia@126.com

Corresponding author:JIN Yang, professor. E-mail: jyk@usx.edu.cn; SHENG Guodong, PhD. E-mail: gdsheng@usx.edu.cn.

金陽, 教授級高級工程師. E-mail: jyk@usx.edu.cn; 盛國棟, 博士. E-mail: gdsheng@usx.edu.cn.