montmorillonite composite as a highly efficient adsorbent for mercury removal from wastewater

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Research paper

Molybdenum disulfide/montmorillonite composite as a highly efficient adsorbent for mercury removal from wastewater Eustáquia De António Márioa,b, Chang Liua,b, Chizoba I. Ezugwuc, Shangjian Maoa,b, ⁎ Feifei Jiaa,b, , Shaoxian Songa,b a

Hubei Key Laboratory of Mineral Resources Processing and Environment, Luoshi Road 122, Wuhan, Hubei 430070, China School of Resources and Environmental Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan, Hubei 430070, China c School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510006, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Molybdenum disulfide Montmorillonite Mercury removal Adsorption Composite

MoS2 and montmorillonite (MMT) nanosheets were hybridized through a simple in-situ hydrothermal synthesis in order to develop an efficient adsorbent, MoS2/MMT composite, for the removal of mercury from water. Raman, XRD, and FTIR results displayed the good hybridization of MMT and MoS2. HRTEM images indicated that the active edges of MoS2 were clearly exposed in the hybrid, demonstrating the vital role of MMT as ecofriendly, cheap and stable substrate for the growth of MoS2. The overall adsorption process of Hg2+ on MoS2/ MMT proved that the composite is highly efficient for the removal of Hg2+ from water, adsorbing up to 1836 mg/g of Hg2+, which outperformed most literature reported adsorbents. The adsorption kinetic and isotherm models could be well described by pseudo-first-order and Langmuir models, respectively. Furthermore, the Langmuir dimensionless constant, RL show that Hg2+ adsorption on the composite was found favorable to occur at the temperature of 25 °C and 35 °C. The XPS analysis revealed that the S sites and slight O sites were main contributors for the remarkable adsorption performance of MoS2/MMT composite on Hg2+ removal. This work suggests that MoS2 supported on MMT substrate might be a promising adsorbent with high capacity and super efficiency for the removal of heavy metals from aqueous solution.

1. Introduction Mercury is recognized as one of the most harmful heavy metals in the environment (Wu et al., 2018). The major sources of mercury pollution are anthropogenic activities, such as agriculture, industrial wastewater discharges, gold mining activities, combustion of fuel and sludge, and non-ferrous metal smelting (Arshadi et al., 2018). Water polluted with mercury has a significant risk on the aquatic world, terrestrial environment and the entire biodiversity (Zabihi et al., 2010). The human health risks caused by contamination of mercury include: damage of kidney, central nervous system, digestive and respiratory systems, as well as mental and motor dysfunction (Liu et al., 2017a, 2017b; Wang et al., 2018). Therefore, there is vital need to reduce the concentration of mercury in wastewater. Several conventional technologies have been applied to remove mercury in contaminated waters, such as chemical precipitation, electrochemical process, adsorption, membrane separation, ion exchange, and solvent extraction (Ezugwu et al., 2013; Jia et al., 2017a, 2017b).

⁎

Adsorption method is one of the most extensively used technology because it offers feasibility in design and operation (Ezugwu et al., 2018a, 2018b; Fu et al., 2018). Also, it is sometimes reversible and the adsorbents can be regenerated (Fu and Wang, 2011; Ezugwu et al., 2018a,b). The most used adsorbents for mercury adsorption are powdered and granular activated carbon produced from different sources such as sheep bone charcoal (Dawlet et al., 2013) and organic sewage sludge (Zhang et al., 2005). Conventional adsorbents exhibit low uptake capacity and poor selectivity due to the weak affinity with mercury (Zhi et al., 2016). Mercury has high affinity with sulfur, hence enhanced adsorption could be achieved by impregnating the adsorbents with sulfur (Deng et al., 2017). However, unwelcome chemical impregnation was used in the preparation of these adsorbents and the removal efficiencies of these adsorbents were restricted because of the limited sulfurs on the surface. Molybdenum disulfide, with a chemical composition of MoS2, has abundance intrinsic sulfur atoms, hence it possesses potential active sites for the adsorption of mercury (Yan et al., 2013). MoS2 has

Corresponding author at: Hubei Key Laboratory of Mineral Resources Processing and Environment, Luoshi Road 122, Wuhan, Hubei 430070, China. E-mail address: [email protected] (F. Jia).

https://doi.org/10.1016/j.clay.2019.105370 Received 25 June 2019; Received in revised form 9 November 2019; Accepted 12 November 2019 0169-1317/ © 2019 Published by Elsevier B.V.

Please cite this article as: Eustáquia De António Mário, et al., Applied Clay Science, https://doi.org/10.1016/j.clay.2019.105370

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attracted much attention on the physical and chemical fields due to its few layered structure and high functional electronic properties (Gao et al., 2016). Therefore, it can find applications in hydro-desulfurization industries, as well as in environmental remediation field (Gonzalez et al., 2016). Thermal modification of MoS2 is regarded as superb method that presents high efficiency in adsorption of Hg2+ in aqueous solution (Jia et al., 2018). The adsorption of Hg2+ on natural MoS2 in aqueous solution using Atomic Force Microscope (AFM) image was studied by (Jia et al., 2017a, 2017b) The inherent properties of MoS2 have widened its area of application (Li and Peng, 2018a, 2018b, 2018c) and Hg2+ is not the only heavy metal under contention, also MoS2 has been reported to be adsorbent of Pb2+ in aqueous solution (Liu et al., 2017a, 2017b; Zhang et al., 2018). Various methods such as electrochemical deposition method, thermal decomposition and hydrothermal synthesis have been developed in hypotheses to obtain MoS2 nanosheets (Yu et al., 2014; He and Que, 2016). MoS2 nanosheets obtained from the aforementioned methods present drawbacks of deterioration and reduction on its performance because of the high hydrophobicity that could not allow it to be well dispersed in aqueous solution. The proposed effective way to solve the problem is to prepare the MoS2 with supporting material. Mesoporous graphene (Liao et al., 2013), TiO2 nanofibers (Liu et al., 2015) and others were reported to be used as a support for the synthesis of MoS2. However, these supports are expensive and the procedures used in their preparations are complicated. Montmorillonite (MMT) is an abundant, environmentally friendly clay mineral, which has great utility owing to its economic low-cost, outstanding properties of ion-exchange, high capacity and surface area (Wilpiszewska et al., 2015; Zhu et al., 2018; Peng et al., 2019). It is layered hydrophilic aluminosilicate mineral composed of central AlO4(OH)2 and SiO4 tetrahedral sheets (Wang et al., 2016a, 2016b; Peng and Yang, 2017; Li and Peng, 2018a, 2018b, 2018c). It has been used as efficient adsorbent for the removal of methylene blue (MB) (Almeida et al., 2009), propranolol (Del Mar Orta et al., 2019), tetracycline and ciprofloxacin (Wu et al., 2019) from aqueous solution. It could be hypothesized that the low dispersibility of MoS2 in water owing to its hydrophobic nature can be improved by forming a composite with MMT, which is a hydrophilic mineral, thus augmenting the overall hydrophilicity and providing more adsorption capacity of the composite in aqueous solution (Li and Peng, 2018a, 2018b, 2018c). The utilization of hydrothermal method has achieved the composite with inherent properties due to reduced hydrophobic agglomeration. (Kang et al., 2016) successfully synthesized MoS2/MMT composite nanosheets by facile hydrothermal method for catalytic reduction reaction. However, the synthesis was achieved by growing equimolar amount of MoS2 on the surface of MMT, without exfoliation of the MMT prior to the synthesis, thus the performance of the catalyst would not be fully harnessed. Hitherto, there is still knowledge gap in employing MoS2/MMT composite for the adsorption of heavy metals contaminants and in particular for the removal of the pernicious pollutant Hg2+ from water. Herein, we report for the first time, the evaluation of MoS2/MMT composite nanosheets for the adsorptive removal of Hg2+ from water. MoS2/MMT composite was prepared by facial hydrothermal synthesis. Prior to the synthesis, the MMT was exfoliated to reduce the thickness and to achieve a thin nanosheet MMT, thereby exposing more surfaces for the composition of MoS2. The nanosheets composite displayed an ultrahigh Hg2+ uptake capacity (1836 mg/g), which is higher than most literature reported adsorbents. The kinetics and isotherm models were fully investigated, followed by elucidating the underlaying adsorption mechanism via XPS measurements. This study may shed light on the exploitation of this promising adsorbent for efficient removal of heavy metals from wastewater.

2. Experimental 2.1. Materials and reagents The Na-MMT sample was obtained from Chifeng Nanchang montmorillonite Co. (Inner Mongolia, China). Hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (CN2H4S), sodium hydroxide (NaOH), and nitric acid (HNO3) were purchased from the Sinopharm chemical reagent Co, Ltd. (China). Mercury nitrate (Hg (NO3)2.H2O) was purchased from Shanghai Zhanyun Chemical Co, Ltd. (China). All the chemical reagents were of analytical grade. The deionized water was obtained from Milli-Q Direct 8/16 water purification system. 2.2. Preparation of the adsorbent 2.2.1. Preparation of MMT suspension 100 g of MMT raw mineral was added into 2 L of deionized water and stirred at 1200 rpm for 2 h. After mixing, the solution was centrifuged at 1000 rpm for 1 min by a Sorvall ST16 centrifuge (Thermo Fisher Scientific, USA) to remove the rough particle. The supernatant was taken for a further ultrasonic exfoliation by CP505 ultrasonic dispersion instrument (Vernon Hills, USA) strength at amplitude of 60% for 5 min. After ultrasonic exfoliation, the solution was subsequently centrifuged at 4000 rpm for 2 min, and the supernatant was collected to obtain MMT suspension, the concentration of which was about 11.6 mg/mL. Furthermore, the cation exchange capacity of this Montmorillonite is about 100 cmol/Kg as it was reported in our previous study (Chen et al., 2017). 2.2.2. Synthesis of MoS2/MMT composite In the synthesis method, (NH4)6Mo7O24·4H2O (2.303 g) and CN2H4S (4.235 g) with a mass ratio of 1:1 were dissolved in H2O (10 mL) and stirred for 30 min. After that, 90 mL (1.12 g) of MMT suspension was added into the solution. After 10 min stirring, the mixture was taken into a Teflon-lined reactor tank, tightly sealed in stainless steel autoclave and heated at 220 °C for 6 h. The reactor was naturally cooled and the black precipitate was washed with ultrapure water for several times. The product was dried at −60 °C by vacuum freeze-drying to obtain the composite. 2.3. Batch adsorption experiments Batch adsorption experiments were conducted at different parameters to study the adsorption of Hg2+ by MoS2/MMT composite. The effect of pH on mercury adsorption was conducted in a pH range of 1–6. A given concentration of Hg solution was prepared in the conical flask and the pH was adjusted to a desired value using NaOH or HNO3 to prevent the precipitation of Hg2+. A given amount of MoS2/MMT composite was added into Hg2+ solution and then shaken in a mechanical shaker at a constant temperature of 25 °C or 35 °C at 150 rpm for predetermined time interval. Then, 5 mL of the suspension was filtered with 0.1 μm filter membrane, in which the first 2 mL was discarded and the remaining 3 mL was collected for the chemical analysis of Hg2+ concentration. The adsorption isotherm studies were conducted by varying the initial concentration from 25 mg/L to 200 mg/L and the adsorption time was 9 h. The kinetic adsorption experiment was performed at different time intervals (5 min to 18 h) and the initial concentration was 50 mg/L. The amount of Hg2+ uptake by MoS2/ MMT composite was calculated by the following equation:

qe =

(C0 − Ce ) × V m

(1) 2+

where qe is the amount of Hg adsorbedon MoS2/MMT composite (mg/g), C0 is the initial Hg2+ concentration (mg/L), Ce is the equilibrium concentration of Hg2+ (mg/L), V is the initial volume of Hg2+ 2

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(200)

solution (L) and m is the amount of MoS2/MMT composite used for adsorption (g).

(100)

(223)

Intensity (a.u.)

2.4. Measurements The X-ray diffraction (XRD) was recorded from D-max 2500/PC Xray diffractometer at 2 theta range from 10° to 80° using Cu Kα radiation (λ = 1.5406 nm) at a scan rate of 0.02 o/s to determine the phase crystal structure and the purity of MMT, MoS2 and MoS2/MMT composite. Emission scanning electron microscope (SEM) image were characterized by ZEISS MERLIN COMPACT at the acceleration rate of 10 kV to observe the general morphology of the MoS2/MMT composite, and the energy dispersive spectrometer (EDS) was adopted to identify the chemical composition and distribution of the elements. High-resolution transmission electron microscope (HRTEM) was adopted from FEI Tecnai G2 F30 to further obtain the morphological microstructure and the interface between MoS2 and MMT in the composite. X-ray photoelectron spectroscopy (XPS) measurement was performed on an ESCALB 250Xi photoelectron spectrometer using Al Kα radiation (Thermo-Fisher Scientific, USA) to obtain information about the chemical composition and oxidation state of the elements on MoS2/MMT before and after adsorption. Fourier transform infrared (FTIR) analysis was measured using a Nicolet 6700 FT-IR spectrophotometer with KBr pellets at range between 4000 and 400 cm−1 to identify the chemical vibration bands and the specific interaction of MMT, MoS2, and MoS2/ MMT after adsorption. Raman microscope was recorded with Lab RRM HR Evolution at 532 nm to analyze the molecular structure of the MoS2/MMT composite nanosheet. To detect the concentration of Hg2+, the sample was tested using a contraAA700 continuum source atomic absorption spectrometry (AAS) system (Jena, Germany).

Intensity (a.u)

(110)

20

30

40

50

60

70

80

Fig. 2. XRD pattern of exfoliated MMT, MoS2, and MoS2/MMT composite.

since no characteristic peak of impurities were observed. All the detected diffraction peaks can be systematically indexed to those of the MoS2 hexagonal phase which were in appropriate concordance with the standard values of card (JCPDS card no.37-1492) (Zhang et al., 2015). The most dominant characteristic peaks were observed at 33.5°, 35.6°, 35.8° and 57.1°, which were assigned to (210), (220), (310) and (113) phase of molybdenum disulfide, respectively. The XRD pattern of MoS2/MMT clearly exhibited the combined diffraction peaks of MoS2 and MMT. The peaks for MoS2 were more pronounced due to its higher composition; nevertheless, the peaks for MMT at (101) and (110) were also present in the MoS2/MMT spectrum. Therefore, this confirmed that the nanosheets were composed of MoS2/MMT without any impurity. Obviously, the (200) diffraction peak of MoS2/MMT becomes longer and larger than the pure MoS2, indicating that the MoS2 in the composite nanosheets contains more active edge sites. The broadened peak at (200) may be due to the preparation method of the MoS2. Similar broadened peak was reported by (Gao et al., 2016) and (Yang et al., 2014). The particle size distributions of the purified MMT and exfoliated MMT are shown in the Fig. S1. The particle size of the purified MMT is about 3.5 μm - 10.5 μm. After the exfoliation of the purified sample, the particle size reduces to about 0.065 μm - 0.75 μm. In addition, the chemical composition of the MMT is shown in Table S1. The SEM images of MMT with magnification of 1 μm shows a typical layered structure composed of multitudinous parallel sheets as shown in Fig. 3(a). In addition, it has smooth surface without contamination which can promote stimulation in the deposition of MoS2 during the hydrothermal synthesis. The SEM image of pure MoS2, Fig. 3(b), depicts the assembly of the nanosheets interconnected to each other by folded flakes with several diameters. The assembly of these nanosheets provides active sites for subsequent cross-linking with the MMT. Fig. 3(c) presents the morphology of MoS2/MMT, showing the assembly of MoS2 on the MMT layer structure. As it can easily be distinguished from morphological point of view, the vertical sheets are well dispersed and linked with some sheets laying sub-vertical which provide active edge sites. Thus, from the cross-linking of these sheets, it can be inferred that there is composition of MoS2 and MMT during the hydrothermal synthesis to form the hybrid structure. The obvious morphological difference illustrates the vital role of MMT as an excellent and stable substrate for not only the growth of MoS2 but also to eliminate their agglomeration and improve the dispersion of MoS2. The EDS spectrum inset Fig. 3(c) reveals that MoS2/MMT was mainly composed of large amount of Si (14.02 wt%) Al (4.59 wt%) S (18.40 wt%) Mo (24.97 wt%) and O (37.85 wt%). In order to have a closer observation on the morphological microstructure and the interface between MoS2 and MMT in the composite, TEM and HRTEM images of MoS2/MMT were provided, Fig. 4. The TEM

401

374.7

420

(113)

2T (degree)

Raman spectrum of MoS2/MMT composite is shown in Fig. 1. The morphology exhibits two strong characteristic peaks positioned at 374.79 cm−1 and 401 cm−1, which correspond to the vibration modes in-plane E12g and out-of-plane A1g vibration of MoeS bonds, respectively (Peng et al., 2017). The two strong peaks indicate that MoS2 was successfully prepared after the hydrothermal synthesis. The crystallography structure of the samples was analyzed using XRD pattern as illustrated in Fig. 2. The spectrum of MMT displays clearly the visible peaks at 7.07°, 19.76°, 35.12° and 61.8°, affirming the purity of the as-prepared sample. For the MoS2, it can be seen that pure molybdenum disulfide structure with high crystallinity was synthesized

400

(310) b) MoS2

(101)

10

3.1. Characterization of MoS2/MMT composite

380

(220) (210)

a) MMT

3. Results and discussion

360

c) MoS2/MMT

440

Wavenumber (cm-1) Fig. 1. Raman spectrum of MoS2/MMT composite. 3

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Fig. 3. SEM of (a) MMT, (b) MoS2 and (c) MoS2/MMT.

image, Fig. 4(a) revealed the uniform growth and improved dispersibility of MoS2 nanosheets on the MMT support. The active edges of the MoS2 nanosheets were clearly exposed as illustrated in the HRTEM Fig. 4(b), hence, more active sites would be available for adsorption. Fig. 4(c) shows the composition of MoS2 on MMT which displayed a well-stacked layered structure of MoS2 with a d spacing of 0.61 nm, assigned to the (002) lattice plane (Kang et al., 2016). FTIR analysis was carried out to identify the chemical vibrational bands and the specific interaction of MMT, MoS2, and MoS2/MMT as shown in Fig. 5. The spectrum of MMT presents vibration bands at 464 cm−1 and 1033 cm−1 assigned to the bending vibration bond and stretching-vibration band of SieO and O-Si-O respectively, indicating that the layered structure of MMT consists of silicon‑oxygen tetrahedron (Peng and Yang, 2017). The bands at 3453 cm−1 and 1638 cm−1 were due to H-O-H stretching vibration and the –OH bending vibration of the hydrogen bonded water respectively. The band at 3621 cm−1 could be ascribed to be the –OH stretching vibration corresponding to the Al-O-H group, and the band at 1430 cm−1 can be attributed to the stretching vibration of carbonate in the mineral (Liu et al., 2013). The vibration bands in the FTIR spectrum of MoS2 were assigned to the stretching vibration of MoeS. From the FTIR spectrum of MoS2/MMT, it can be observed that the main vibration band of MMT and MoS2 were present. The vibration at 464 cm−1 and 1033 cm−1 in MMT shifted to 460 cm−1 and 1025 cm−1 in the MoS2/MMT composite respectively, due to the existence of interaction between the two components in the composite. More so, the plateau in-between 1033 cm−1 and 464 cm−1 in MMT drastically decreased in the MoS2/ MMT spectrum. The peaks at 1638 cm−1 appeared in low intensity in MoS2/MMT at 1627 cm−1. Similarly, other vibration stretching bands observed in MMT and MoS2 were obviously weakened in MoS2/MMT composite, which might be due to the interaction of MMT with MoS2 under hydrothermal condition (Li and Peng, 2018a, 2018b, 2018c).

MoS2

460

1025

1399

Transmittance (% T)

1627

MoS2/MMT

3500

3000

2500

2000

1500

Wavenumber (cm

-1

1000

464

1033

3621

3453

1638

1430

MMT

500

)

Fig. 5. FTIR spectrum of MoS2/MMT composite.

1400

qe(mg/g)

1200 1000 800 600 400 qe pseudo-first-order-kinetics model pseudo-second- order- kinetics model

200 0 0

3.2. Adsorption kinetics and performance of MoS2/MMT composite nanosheets on Hg2+

200

400

600 800 Time (min)

1000

Fig. 6. Adsorption kinetic of Hg2+ on MoS2/MMT.

The adsorption of Hg2+ on MoS2/MMT as a function of time were

Fig. 4. (a) TEM of MoS2/MMT, (b) and (c) HR-TEM of MoS2/MMT. 4

1200

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Table 1 Pseudo-first- and pseudo-second-order kinetic model parameters for Hg2+ adsorption on MoS2/MMT. Experimental data

Pseudo-first-order kinetic model

Pseudo-second-order kinetic model −1

qe,exp(mg/g)

qe,cal (mg/g)

k1(mim

1053.95

1119.94

0.00526

)

R

2

0.98164

investigated for a better understanding of the mechanism of the adsorption and the adsorption results are shown in Fig. 6. It can be observed that the adsorption capacity increased gradually up to 240 min and then maintained when the equilibrium was attained. After 360 min, the adsorption capacity was not showing significant change due to the attainment of maximum equilibrium conditions. The MoS2/MMT composite exhibited high adsorption capacity for Hg2+ and a long time to reach the adsorption saturation due to the extremely high adsorption capacity. The experimental data were simulated and fitted using pseudo-firstorder (Eq. (2)) and pseudo-second-order (Eq. (3)) kinetic model equations. The fitting results are listed in Table 1.

ln(qe − qt ) = ln qe − k1 t

qe,cal (mg/g)

k2(g/mg mim−1)

R2

1390.81

3.84037–10−6

0.96089

1200

q e ( m g /g )

1000

k1

800

k3

k2

600 400 200 0

(2)

0

5

10

15

20

25

30

35

t1/2 (min1/2)

t 1 t = + qt qe k2 ×qe2

(3)

Fig. 7. Intraparticle diffusion kinetic model for Hg2+ adsorption on MoS2/ MMT.

Where qe and qt (mg/g) are the capacity of mercury adsorbed on MoS2/MMT at equilibrium and at t (min) times, respectively; k1 (min−1) and K2 (g·mg−1·min−1) are the pseudo-first-order and pseudosecond-order rate constants respectively. Compared to pseudo-second-order kinetic model with R2 of 0.96, Fig. 6 and Table 1 showed that the pseudo-first-order kinetic model fitted the experimental data better with a higher R2 value of 0.98. Moreover, the calculated equilibrium adsorption capacity qe, cal (1119.94 mg/g) from pseudo-first-order model was agreeing well with the experimental value (1053.95 mg/g) than that from pseudo-secondorder kinetic model. These results confirmed that the overall adsorption process followed the pseudo-first-order kinetic model (Manap et al., 2018). Fig. S2 shows the adsorption kinetics of MMT and MoS2, and the kinetic model parameters are presented in Table S2. The results show that MoS2 has a much higher Hg adsorption capacity than MMT and further unveil the synergic effect of MoS2 and MMT on improving adsorption rate of the composite. Also, MoS2 that contain the predominant active sites maintained pseudo-first-order kinetic model as the composite (Table 2). To have a good knowledge of the diffusion mechanism as well as the rate limiting step of the Hg2+ adsorption on the MoS2/MMT composite nanosheets, intraparticle diffusion kinetic model was applied and the corresponding parameter was represented by the following equation:

qe = ki t + ci

second step known as intraparticle diffusion stage occurred when adsorption on the external surface reached saturation and Hg2+ diffused into the pores of the MoS2/MMT (Zhu et al., 2015) resulting in nearly all of the active sites in the pores of the MoS2/MMT being occupied by Hg2+ (Lei et al., 2018). The third step was the equilibrium stage with very slow intraparticle diffusion due to the decreased Hg2+ concentration. For the MoS2/MMT adsorbent, the order of their diffusion rate constant was K2 > k1 > k3. Owing to the lower boundary layer diffusion rates, k1 < k2 the rate-limiting adsorption step might be the external transfer (Beheshtian et al., 2013). Furthermore, adsorption experiments were carried out to compare the capacities of MMT, MoS2 and MoS2/MMT on adsorption of Hg2+. It is observed that the adsorption capacity of MoS2 is relatively higher than some normal literature reported MoS2. This may be attributed to the exfoliation of the sample and the longer dispersion time during the adsorption process. Expectedly, the results in Fig. 8 proves that the MoS2/MMT composite possesses a high Hg2+ adsorption capacity, adsorbing up to 1053.9 (mg/g), which is more than that of pure synthesized MoS2 and the adsorption capacity of MMT is the lowest. This result prove that the use of MMT as supporting substrate for the

(4) −1

(mg/g)

-1/2

Adsorption capacities for Hg

2+

Where ki (mg·min ·g ) is the diffusion rate constant and ci is the intercept of the plot of qe versus t1/2. The intraparticle diffusion kinetic model for Hg2+ adsorption on MoS2/MMT is shown in Fig. 7. It illustrated that three diffusion steps (the three linear segments) influenced the adsorption process. The instantaneous adsorption stage, also called boundary layer diffusion, occurred when Hg2+ migrated to the external surface of the MoS2/MMT composite, which was driven by high initial Hg2+ concentration. The Table 2 Intraparticle diffusion. Sample

Kd1 (mg/ gmin-1/2)

R1 2

Kd2 (mg/ gmin-1/2)

R2 2

Kd3 (mg/ gmin-1/2)

MoS2/MMT

18.10844

0.9435

68.60913

0.92506

1.10125

1000 800 600 400 200 0

MoS2/MMT

MoS2

MMT

Fig. 8. Comparison of adsorption capacity of MoS2/MMT, MoS2 and MMT on Hg2+. 5

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2000

(a)

0.16

(b)

1750 0.12 Ce/qe (mg/L)

qe (mg/g)

1500 1250 1000 750 500

25 °C 35 °C

250

0.08

0.04

25 ° C 35 ° C

0.00

0 0

50

100 150 Ce (mg/L)

200

0

250

50

100 150 Ce (mg/L)

200

250

2000 8

(c)

(d) 1600

qe (mg/g)

ln qe

6

1200

4

800

2

°C

25 35 °C 0 0

1

2

3 ln Ce

4

5

25 ° C 35 °C

400 6

2.5

3.0

3.5

4.0 ln Ce

4.5

5.0

5.5

Fig. 9. (a) Adsorption isotherm of Hg2+ on MoS2/MMT, (b) exponential data Langmuir isotherms, (c)Freundlich isotherm and(d) Temkin isotherm.

and 35 °C respectively, while 243.77 mg/g Hg2+ adsorption in fish bone charcoal was reported by (Wu et al., 2018). In addition, the maximum adsorption capacity recorded of a nano composite materials was 179.74 mg/g (Awual, 2017). Therefore, MoS2/MMT composite is more efficient adsorbent for removal of Hg2+ from wastewater than most other literature reported adsorbents. Fig. S3 presents the isotherm behavior MoS2 and MMT. The low dispersibility of MoS2 in aqueous solution affected their mercury adsorption capacity. For a thorough interpretation of the interaction between the mercury ions and the MoS2/ MMT composite at equilibrium adsorption, three equilibrium isotherm models namely Langmuir (eq. (5)), Freundlich (eq. (6)) and Temkin models (eq. (7)) were employed.

preparation of MoS2 to form MoS2/MMT composite was ideal method and notable, the hydrophobicity was totally reduced and the hybrid was able to disperse in aqueous solution and interacted more with Hg2+ ions. Since MoS2 play important role in enhancing the adsorption capacity of the composite, the effect of MoS2 loading amounts on the structure and adsorption capacity of MoS2/MMT composite are evaluated as shown in Fig. S5 and Fig. S6. It is observed that as the percentage of MoS2 in the composite increase, the Hg2+ adsorption capacity increases.

3.3. Adsorption isotherm

ce 1 1 = ce + qe qm qm kl

The adsorption isotherms of Hg2+ on MoS2/MMT composite were studied at 25 and 35 °C and the results are presented in Fig. 9. The adsorption capacities increased with the increase in Hg2+ equilibrium concentration until a steady adsorption was attained. At 35 °C, the adsorption capacity was significantly higher than that at 25 °C. For instance, the composite adsorbed 984 mg/g of mercury at 25 °C at equilibrium concentration of 132 mg/L, while up to 1655 mg/g was adsorbed at 35 °C at the same equilibrium concentration of 131 mg/L. Hence, the increase in temperature had a positive influence on the adsorption process. This could be due to the increase in the diffusion rate. When the temperature is higher, more Hg2+ ions move faster to surface of the adsorbent (Syafiqah and Yussof, 2018). Notably, MoS2/ MMT composite had the potentials to adsorb up to 1836 mg/g of Hg2+ at 35 °C. The maximum adsorption capacities of Hg2+ with other adsorbents reported in most literatures were < 500 mg/g. For example, (Jia et al., 2017a, 2017b) reported that two-dimensional molybdenum disulfide had a Hg2+ removal capacity of 254 and 305 mg/g at 20 °C

ln q e =

1 ln ce + ln kF n

qe = B × ln kT + B × ln ce

(5)

(6) (7)

where qe is the adsorption capacity at equilibrium concentration (mg/ g), qm is the maximum adsorption capacity of the adsorbent (mg/g) from Langmuir model, ce is the equilibrium concentration of Hg2+ (mg/ L), while kf, kl and kt are Langmuir constant (L/mg), Freundlich constant (mg/g) and Temkin constant (L/g), respectively, n is the adsorption intensity, B is a constant related to sorption heat (J/mol). Moreover, an essential dimensionless constant, RL, referred to as separation factor or equilibrium parameter, could be obtained from the Langmuir isotherm, which is used to predict the favorability of the adsorption process and is represented by (eq. (8)). When the value of RL 6

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of Hg2+ on MoS2 and MMT, Fig. S4, as the adsorption capacity increased until pH 3. Notably, the adsorption capacity is higher on MoS2/ MMT than MoS2 and MMT. The diminishing of the adsorption capacity from pH 3 to 6 can be due to the changes of the existence form of mercury (Wu et al., 2018). Previous studies reported that predominant mercury species in the solution is Hg2+ at pH˂ 3, Hg(OH)2 at pH ˃ 5.0, and both of the two species exist between pH of 3 and pH of 5. Little amount of HgOH+ also could be detected between pH of 2 and pH of 6 (Hahne and Kroontje, 1973; Zhang et al., 2005). The adsorption capacity of Hg2+ on MoS2/ MMT reached maximum at pH 3, which might be due to electrostatic attraction. The Zeta potential illustrated in Fig. 10(b) could confirm this opinion. The surface of MoS2/MMT composite was negatively charged and the negativity continuously increased as the pH increased during the entire pH range. Since Hg2+ species dominated at pH ≤ 3, the increasing negative charge of the composite in pH of 1.0–3.0 led to a stronger electrostatic interaction between Hg2+ and MoS2/MMT, hence there was more pulling of Hg2+ by the active sites on the composite. It is worthy to note that, although MoS2/MMT surfaces were more negative at higher pH, the presence of Hg(OH)2 and HgOH+ at pH > 3 decreased and competed with Hg2+ species, thereby resulting in less electrostatic interaction and that decreased adsorption capacity of MoS2/MMT at higher pH values (Zhang et al., 2005).

is between 0 and 1, the adsorption process is favorable, when RL > 1, the adsorption is unfavorable. RL = 1 means irreversible adsorption and RL = 0 suggests that the adsorption process is linear (Wang et al., 2016a, 2016b).

RL =

1 1 + Cm KL

(8)

Where Cm is the maximum initial concentration in all experimental solutions (mg/L), and KL is the Langmuir isotherm constant (L/mg). The related parameters and correlation coefficients (R2) of the three models are presented in Table 4. The higher value of the correlation coefficient for the Langmuir model (R2 = 0.995 at 25 °C and 0.944 at 35 °C) than that of Freundlich (R2 = 0.784 at 25 °C and 0.838 at 35 °C), together with the better fitting of the adsorption data, indicated that the adsorption of Hg2+ on MoS2/MMT was controlled by the Langmuir isotherm model, suggesting monolayer adsorption of Hg2+ on MoS2/ MMT composite. The values of the separation factor confirmed that the adsorption of Hg2+ on MoS2/MMT was favorable and the interaction between MoS2/MMT and Hg2+ was relatively strong at both temperatures, since RL = 0.004 and 0.003 at 25 °C and 35 °C respectively (Wang et al., 2016a, 2016b). In addition, the adsorption process was more favorable at 35 °C since the RL value was lower than that at 25 °C. Temkin isotherm model assumes that the heat of adsorption of all the molecules in the layer decreases linearly due to adsorbent-adsorbate interaction and the adsorption is characterized by uniform distribution of banding energies, up to some maximum binding energy (Etim et al., 2016). The parameters of Temkin model as represented in Fig. 7(d) and Table 3, showed a high correlation coefficient, R2 > 0.98 and R2 > 0.97, at 25 °C and 35 °C respectively, revealing a good linearity within the adsorption capacities. In addition, the adsorption of Hg2+ on MoS2/MMT was exothermic as revealed from the positive value of the constant, B, (Table 3), which was related to heat of adsorption.

3.6. Characterization of MoS2/MMT after Hg2+ adsorption The adsorption of Hg2+ on MoS2/MMT was measured by EDS as shown in Fig. 11. The EDS mapping shows that S, Mo, Si, Al, O and Hg elements is present on the composite after adsorption. More so, it can be obviously observed that large amount of elemental mercury is uniformly distributed on the surface of MoS2/MMT, demonstrating an excellent uptake of Hg2+ on the active sites of the adsorbent. To further prove the adsorption of Hg2+ on the composite, FT-IR spectra of MoS2/MMT before and after Hg2+ adsorption was analyzed as illustrated in Fig. 12. For MoS2/MMT before adsorption, the peaks at 460 cm−1, 1025 cm−1, 1399 cm−1, and 1627 cm−1 were characteristic peaks of MoS2 or MMT as it was discussed in Fig. 5. For Hg2+ loaded MoS2/MMT, new bands located at 616 cm−1 and 1109 cm−1 emerged. The new peaks were assigned to HgeS and HgS2OH, indicating the adsorption and interaction between Hg2+ and MoS2/MMT.

3.4. Reusability of MoS2/MMT composite The reusability analysis of an adsorbent is of great significant in order to guarantee a cost effective adsorbent that has practical industrial application. The MoS2/MMT composite can easily release the adsorbed Hg2+ through simply washing the MoS2/MMT with excess EtOH (ethanol) under ultrasonication. After each cycle, the MoS2/MMT was centrifuged, ultrasonicated and washed with excess EtOH for the subsequent cycle. Fig. S8 revealed that MoS2/MMT composite conserved a good Hg2+ adsorption capacity even after four cycles of reuse and the XRD result, Fig. S9, after reuse confirms the stability of the adsorbent.

3.7. Adsorption mechanism of Hg2+ on MoS2/MMT In order to elucidate the adsorption mechanism of Hg2+ on MoS2/ MMT composite, XPS surface chemistry analysis was conducted and the results are presented in Fig. 13. The survey spectrum in Fig. 13(a) shows that the composite was comprised of Mo, Al, Si, O, C and high concentration of Hg2+ as shown by the strong peaks of Hg4d and Hg4f after adsorption. Fig. 13(b) represents the high resolution XPS spectrum of Hg4f, which can be deconvoluted into two sets of doublet peaks. The characteristic doublet peak at 103.6 and 107.5 eV were attributed to HgeS and the remaining doublets at 104.9 and 108.4 eV were ascribed to HgeO (Jia et al., 2017a, 2017b). The existence of HgeS and HgeO indicates that S and O were the active binding sites for Hg2+ adsorption. The peak intensities of HgeS were higher than that of HgeO, suggesting that the S in the composite were the main sites for the

3.5. Effect of pH on Hg2+ adsorption on MoS2/MMT The influence of pH is an important parameter for adsorption studies because it is one of the factors that can determine the adsorption properties of the adsorbent, ionization, speciation and surface charge of the adsorbent (Li et al., 2011; Xu et al., 2012). Fig. 10(a) illustrates the adsorption capacity of Hg2+ on MoS2/MMT as a function of pH ranging from 1 to 6. It can be noticed that the adsorption capacity increased dramatically before pH 2, and continued increasing gradually until it achieved a plateau at pH 3. Then, the capacity decreased after pH of 3. The same phenomenon was also observed on effect of pH on adsorption

Table 3 Fitting results of Langmuir, Freundlich and Temkin linear models for adsorption isotherm of Hg2+ on MoS2/MMT. Temp

25 °C 35 °C

Langmuir

Freundlich

Temkin

qm(mg/g)

KL(L/mg)

R2

n

KF(mg/g)

R2

B(J/mol)

KT(L/g)

R2

1031.89 2055.92

0.1571 0.0296

0.995 0.944

0.770 0.759

2.797 3.204

0.784 0.838

179.083 484.387

1.847 0.196

0.983 0.980

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Transmittance (%)

2+

loaded MoS2/MMT

616

Hg

3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

460

1025

3441

1627

461

MoS2/MMT 1399

adsorption of Hg2+. The peak for Hg0 at around 99.9 eV was not detected, confirming that there was no reduction of Hg2+ during the adsorption process (Jia et al., 2017a, 2017b). The best fitted XPS results of S2p Fig. 13(c) can be deconvoluted into four peaks. The characteristic peaks at 165.3 and 166.3 eV were associated with the binding energies assigned to S2p3/2 and S2p1/2 of the MoS2. The peak related to S2p3/2 of HgeS, centered at the binding energy of 164.2 eV, further confirmed the uptake ability of Hg2+ by the active S sites. The peak at 170.9 eV was ascribed to S4+ species in SO32− group, which might be due to the oxidation of sulfur at the edge of the composite. The best fitted XPS results of O1s as presented in Fig. 13(d) showed peaks at 534, 532.5 and 533.3 eV, which were attributed to HgeO, MoO3 and OH– of H2O, respectively, further suggesting the interaction between Hg and O. Notably, Fig. S7(a) and (b) present the XPS spectra of Mo 3d before and after the adsorption of mercury, showing two peaks assigned to the Mo4+ state and two other corresponding peaks of oxidized Molybdenum, Mo+6, which may still occur at the exposed edge of the composite. Concisely, on the adsorption mechanism, the surface of the composite was completely covered with uniformly distributed S sites which interacted with the Hg2+ during adsorption to form SeHg complexes. Saturated surface adsorption resulted to the penetration of the adsorbates into the internal surface of the composite, followed by being adsorbed on the internal sulfur layers. In addition, the complexation between Hg2+ and O sites on the composite, as well as the electrostatic interaction between the positive mercury and the negative surface of

1109

Fig. 10. (a) Adsorption of Hg2+ on MoS2/MMT as a function of pH, (b) Zeta potential of MoS2/MMT.

500

Fig. 12. FT-IR spectra of MoS2/MMT before and after Hg2+ adsorption.

the composites contributed towards the adsorption. The negatively charged MoS2/MMT surface is confirmed by the Zeta potential result and the maximum mercury adsorption capacity of the composite is achieved at a pH 3 due to dominant of Hg2+ species, resulting in higher interaction. Due to the low concentration of MMT in the composite and that MoS2 constitutes the main active S adsorption sites, Fig. 8 and S3,

Fig. 11. SEM image of (a) Hg2+ loaded on MoS2/MMT, (b), EDS elemental mapping of combined S, Mo, Si, Al, O, and Hg; panel (c) Mo, panel (d) S, panel (e) Si, panel (f) Al, panel (g) O, and panel (h) Hg2+. 8

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Fig. 13. XPS survey spectrum (a), Hg4f spectrum (b), S2p spectrum (c), O1sspectrum (d) of MoS2/MMT after adsorption of Hg2.

manner to show the individual arrangement of the MoS2 and MMT in the composite. As demonstrated, assemble of MoS2 on the surface of MMT can well be depicted. The MoS2 was well dispersed and the active sulfur sites (yellow element) are uniformly distributed and clearly exposed on the surface, hence making them more available for Hg2+ capturing. The mobilization of Hg2+ on the MoS2/MMT can be observed as the Hg2+ (red dots) covered all the external surface of the composite, which is in accordance with the ultra-high adsorption

the contribution of MMT towards Hg adsorption is minimal. This contribution result from the electrostatic interaction owing to the cation exchange capacity of MMT. In addition, MMT known as a soft acid according to Lewis theory can interact with Hg, a soft acid (Brigatti et al., 2005). The structure of MoS2/MMT and the mobilization of Hg2+ on the surface of the adsorbent are shown schematically in Fig. 14 and the selfassemble mechanism (network) is clearly illustrated in a magnified

Fig. 14. The structure of MoS2/MMT and the mobilization of Hg2+ on the surface of the adsorbent. 9

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capacity recorded during the adsorption studies. Assigned to the unlimited external surface of the composite, the adsorption of Hg2+ was not limited to the external surface only but also to the available internal sulfur active sites. After the Hg2+ covered the external surface-active sulfur sites, the mercury ions penetrated inside the internal surface of the composite, hence resulting to more adsorption in the internal sulfur layers until saturation was attained.

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4. Conclusion MoS2 and MMT could be successfully hybridized through hydrothermal synthesis. The as-prepared composite exhibited a superb adsorption capacity (1836 mg/g) for the removal of the toxic heavy metal Hg2+ from water, hence demonstrating to be one of the most promising adsorbents for the treatment of wastewater and drinking water polluted with mercury. Little rise in temperature favored largely the affinity of MoS2/MMT composite for Hg2+ uptake and also the performance was better at pH < 3. The results of the kinetics and equilibrium isotherm data showed that the overall adsorption process was fitted well with pseudo-first-order kinetic and Langmuir isotherm models. The high adsorption behavior of the composite was attributed to the uniform distribution of high-density sulfur active sites on the surface of the adsorbent, the exposed edges of MoS2 and the synergistic effect of the double active sites of S and O. This work illustrates that MoS2/MMT composite has potential application for the purification of water contaminated with Hg2+ and other related heavy metals. Declaration of Competing Interest There are no conflicts of interest to declare. Acknowledgements The financial support for this work from the National Natural Science Foundation of China (Nos. 51704220 and 51674183), the Natural Science Foundation of Hubei Province (2016CFA013), and the Research Fund Program of Key Laboratory of Rare Mineral, Ministry of Land and Resources (KLRM-KF201802) were gratefully acknowledged. Author contributions section Eustáquia De António Mário. Performed most of the experiment. Feifei Jia. Corresponding author. Chang Liu, Chizoba I. Ezugwu, Shangjian Mao and Shaoxian Shaoxian Song: The authors contributed equally. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105370. References Almeida, C.A.P., Debacher, N.A., Downs, A.J., Cottet, L., Mello, C.A.D., 2009. Removal of methylene blue from colored effluents by adsorption on montmorillonite clay. J. Colloid Interface Sci. 332, 46–53. Arshadi, M., Eskandarloo, H., Karimi Abdolmaleki, M., Abbaspourrad, A., 2018. A biocompatible nanodendrimer for efficient adsorption and reduction of Hg(II). ACS Sustain. Chem. Eng. 6, 13332–13348. Awual, M.R., 2017. Novel nanocomposite materials for efficient and selective mercury ions capturing from wastewater. Chem. Eng. J. 307, 456–465. Beheshtian, J., Peyghan, A.A., Bagheri, Z., 2013. Formaldehyde adsorption on the interior and exterior surfaces of CN nanotubes. Struct. Chem. 24, 1331–1337. Brigatti, M.F., Colonna, S., Malferrari, D., Medici, L., Poppi, L., 2005. Mercury adsorption by montmorillonite and vermiculite: a combined XRD, TG-MS, and EXAFS study. Appl. Clay Sci. 28, 1–8. Chen, T., Zhao, Y., Song, S., 2017. Correlation of electrophoretic mobility with exfoliation

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