Facile synthesis of MoS₂/reduced graphene oxide composites for efficient removal of Cr(VI) from aqueous solutions

Abstract

Cr(VI) as a common heavy metal contaminate has attracted much attention for its high toxicity and bioaccumulation. In this study, novel adsorbents, MoS₂/reduced graphene oxide composites (MoS₂/rGO), were prepared via a facile solvothermal process and effectively used for Cr(VI) adsorption. The MoS₂/rGO was characterized by SEM, TEM, XRD, BET and XPS. Results showed that the MoS₂ layers had been successfully grown on the surface of reduced graphene oxide layers. MoS₂/rGO exhibited an enhanced surface area of 81.34 m² g⁻¹, more than rGO (57.20 m² g⁻¹) and MoS₂ (1.57 m² g⁻¹). Batch adsorption experiments of Cr(VI) were also investigated. The pseudo-second order model best described the adsorption kinetics. Effective Cr(VI) removal occurred in a wide pH range from 2.0 to 10.0. The excellent adsorption capacity at 25 °C, calculated from the Langmuir isotherm model, was 268.82 mg g⁻¹ at pH 2.0 and 192.63 mg g⁻¹ at pH 4.6. The electrostatic attraction and reduction on the adsorbents surface was considered to be the possible mechanism of Cr(VI) removal. These studies revealed the potential application of MoS₂/rGO for Cr(VI) removal.

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1. Introduction

The removal of heavy metals from wastewater has attracted much attention due to their high toxicity, persistence and non-degradability in the ecosystem. Hexavalent chromium (Cr(VI)) is a widespread environmental and public health problem as it is widely used in metallurgical and chemical industries. Plenty of industrial effluents contain chromium species discharged from leather tanning, paint, steel, electroplating, wood preservation, textile industries, etc.¹ It is known that both Cr(III) and Cr(VI) exist in industrial wastewater, while Cr(VI) is about 500 times more toxic than Cr(III) due to its carcinogenic properties. It can be easily adsorbed in the human body through the digestive system, respiratory tract and skin contact.²

There are various methods to remove Cr(VI) that have been reported, such as chemical precipitation, ion exchange, electrodeposition, filtration, adsorption, membrane separation, and reduction.³ Compared to these methods, adsorption is an easy, effective and flexible method for removing Cr(VI). Adsorbents used for Cr(VI) include activated carbon, chitosan, activated alumina, biomaterials, ethyl cellulose, zeolites, and modified graphene.^3–5 Graphene, as a carbon material with a special one-atom thick structure, has received much attention. It has excellent thermal, electrical, optical, and mechanical properties. As reported, graphene is a promising adsorbent for removing heavy metals, dyes and fluoride ions from aqueous solutions.⁶ Reduced graphene oxide (rGO) is the product of incomplete reduction of graphene oxide (GO), which containing a number of the oxygen-containing functional groups. The existence of irreversible aggregation during the process of direct GO reduction in water limits the use of rGO for heavy metal adsorption. Molybdenum disulfide (MoS₂) has been investigated in several fields including electrochemical devices, hydrogen storage, catalysis, capacitors, solid lubricants, and intercalation hosts due to its unique chemical and physical properties.^7–9 It is known that MoS₂ has an analogous structure to graphite, which is like a sandwich with a Mo layer between two S layers; these three atom layers stacked together in a through weak van der Waals interactions.¹⁰ As reported, MoS₂ provides a large surface area for double-layer charge storage due to its special sheet-like structure,¹¹ which may be used for metal ion adsorption. MoS₂ embedded in graphene sheets through a simple hydrothermal method can be helpful for the reduction of overlapping and coalescing of graphene layers, enlarging the specific surface area and increasing the adsorption capacity of the material. The combination of graphene and MoS₂ has already shown to have an excellent electrochemical performance,¹² and it may also produce new properties through their interactions. In this study, we synthesized MoS₂/rGO by a simple solvothermal process, which was used for removing Cr(VI) from aqueous solutions for the first time. The present study presents a new application of MoS₂/rGO for the removal of Cr(VI) from aqueous solutions.

2. Materials and methods

2.1 Materials

Potassium dichromate (K₂Cr₂O₇), thioacetamide (CH₃CSNH₂) and all acid reagents (HNO₃) were of guaranteed grade. Ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) and the other chemicals and reagents used in the experiments were of analytical grade and used without further purification. All solutions were prepared using deionized water. All stopped conical flasks, beakers, centrifuge tubes, pipettes and containers were carefully cleaned and soaked in 10% HNO₃ for one day, triply rinsed with deionized water and air-dried before use.

2.2 Methods

2.2.1 Preparation of MoS₂/rGO. MoS₂/rGO was prepared via a simple hydrothermal process. Briefly, graphene oxide was prepared by a modified Hummers method.⁶ Ammonium molybdate and graphene oxide at a mass ratio of 1 : 9, ammonium molybdate and thioacetamide at a mass ratio of 1 : 2, and 1080 mg of ammonium molybdate were dissolved in 40 mL of a 3 mg mL⁻¹ graphene oxide aqueous dispersion. After ultrasonication and stirring for 1 h, aqueous NaOH was added until the pH value changed to 6.5. Furthermore, 2160 mg thioacetamide was added and ultrasonicated to form a homogeneous solution; the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed tightly, and heated at 200 °C for 24 h in an electric oven. For comparison, mass ratios of 1 : 1, 1 : 3, 1 : 5, 1 : 7, pure MoS₂ and rGO were prepared in the same manner.

2.2.2 Characterization of MoS₂/rGO. MoS₂/rGO was observed with a Merlin field emission scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and a FEI Tecnai G2 F20 transmission electron microscope (TEM) with high-resolution transmission electron microscopy (HRTEM). XRD patterns of MoS₂/rGO were collected using a D8-Advance X-ray diffractometer based on Cu Kα radiation. The Brunauer–Emmett–Teller (BET) surface area of MoS₂/rGO was determined using a Quantachrome Autosorb-IQ-MP low temperature nitrogen sorption method. The XPS spectrum was recorded on the Thermo Fisher Scientific ESCALAB 250Xi spectrometer with monochromatic Al Kα radiation.

2.2.3 Cr(VI) adsorption experiments. For batch adsorption experiments of Cr(VI), a stock solution of Cr(VI) at concentration of 1000 mg L⁻¹ was prepared. All adsorption experiments were conducted at room temperature (25 ± 2 °C) at an agitation speed of 180 rpm and the normal pH of Cr(VI) solutions (4.6 ± 0.1) unless otherwise noted. Adsorption kinetics were studied by adding 0.01 g of adsorbents into 25 mL of 50 mg L⁻¹ Cr(VI) solutions and stirring for specific time intervals. The effect of the adsorbent dose was analyzed by a dosing range from 0.005 g to 0.025 g. To observe the pH effect on the adsorption, Cr(VI) solutions with varying pH values from 2.0 (±0.1) to 10.0 (±0.1) were prepared. The effect of the initial Cr(VI) concentration was investigated from 10 mg L⁻¹ to 100 mg L⁻¹. Adsorption isotherm studies were performed at different temperatures from 25 °C to 45 °C. All experiments were repeated three times. After shaking, the samples were filtered with a 0.45 μm filter membrane. The filtrate was used for the remaining Cr(VI) determination, and the Cr(VI) removal and adsorption capacity were determined by triplicate measurements, with the average data reported.

2.2.4 Determination of Cr(VI) concentration. The concentration of Cr(VI) was analyzed by a UV-Vis spectrophotometer (UV-1750, SHIMADZU, Japan), which was measured by the purple complex of Cr(VI) with 1,5-diphenylcarbazide at 540 nm.² The details of the analytical method are presented in ESI.†

3. Results and discussions

3.1 Characterization of materials

The morphology of MoS₂/rGO was observed by SEM and is shown in Fig. 1. The SEM image of MoS₂/rGO clearly illustrates a 3D sphere-like architecture, which is quite different from the reported flake-like structure of graphene. The interaction of MoS₂ and rGO resulted in the overlapping or coalescence of graphene layers being more difficult; thus the 3D sphere-like architecture increased the specific surface area and aided the adsorption. Due to the superior strength of graphene, the 3D sphere-like architecture was also helpful for the stability of MoS₂/rGO.¹³

Fig. 1 The SEM image of MoS₂/rGO.

The TEM image of MoS₂/rGO is shown in Fig. 2. From Fig. 2, it is clear that the MoS₂ (black stripes) layers are homogeneously embedded in the graphene layers. The thickness of pure MoS₂ is reported to be tens of layers,¹⁴ while the thickness of MoS₂ embedded in graphene layers, shown in the HRTEM image, is smaller than pure MoS₂ and about 4–5 layers. The difference in thickness reveals that during the self-assembled process of MoS₂/rGO, MoS₂ is grown in situ on the surface of the graphene layers, and the combination of MoS₂ and graphene greatly inhibits the restacking of MoS₂ and graphene layers.¹⁴

Fig. 2 The TEM and HRTEM images of MoS₂/rGO.

Fig. 3 shows the XRD patterns of rGO, MoS₂ and MoS₂/rGO. The XRD pattern of rGO exhibits a clear diffraction peak at 2θ = 24.8°, corresponding to the characteristic of graphene. At about 2θ = 12.3°, there is a diffraction peak that may be characteristic of graphene oxide because of the incomplete elimination of oxygen-containing functional groups during the synthesis of MoS₂/rGO. The XRD patterns of MoS₂ and MoS₂/rGO show a similar trend because of the big mass ratio of MoS₂ in the composites. The diffraction peak of MoS₂/rGO shows very weak, at (100), (110) diffraction line is slightly stronger than the diffraction peak of MoS₂, indicating the poor crystallinity of MoS₂ and MoS₂/rGO.

Fig. 3 The XRD patterns of MoS₂, rGO, and MoS₂/rGO.

The rGO, MoS₂ and MoS₂/rGO were investigated by the BET method to calculate the specific surface area. The nitrogen adsorption/desorption isotherms are shown in Fig. 4. The specific surface area of rGO, MoS₂ and MoS₂/rGO is 57.20 m² g⁻¹, 1.57 m² g⁻¹ and 81.34 m² g⁻¹, respectively. The specific surface area of MoS₂/rGO is higher than those of both rGO and MoS₂, meaning the combination of MoS₂ and rGO can greatly reduce the restacking of MoS₂ layers and graphene layers.

Fig. 4 The nitrogen adsorption/desorption isotherms of rGO, MoS₂ and MoS₂/rGO.

To further understand the chemical composition and valence states of MoS₂/rGO, the XPS spectrum was recorded and shown in Fig. 5. The C 1s peak associated with the binding energy of 284.8 eV represents the sp² carbon of graphene (oxide). Two peaks at 229.2 eV and 232.4 eV can be assigned to Mo 3d_5/2 and Mo 3d_3/2, respectively. The S 2p and S 2s can be fitted with the peaks at 162.3 eV, 163.2 eV and 227.5 eV.

Fig. 5 (a) XPS survey spectrum of MoS₂/rGO and high resolution scans for (b) the C 1s, (c) Mo 3d, and (d) S 2p.

3.2 Effect of contact time

Fig. 6 shows the effect of contact time on the adsorption of Cr(VI) onto MoS₂/rGO. The adsorption of different initial concentrations of Cr(VI) solutions showed a similar trend. The adsorption capacity at an initial concentration of 50 mg L⁻¹ was better than 30 mg L⁻¹ and 70 mg L⁻¹. In the first 15 min, the adsorption rapidly increased, which was due to the abundant, available vacant sites of MoS₂/rGO. With further increases in time, the rate of adsorption slowed and reached equilibrium within 180 min. Moreover, the rapid adsorption process is an important feature of physical adsorption, which revealed that the adsorption of Cr(VI) onto MoS₂/rGO is a combination of physical adsorption and chemical adsorption. The optimum contact time was 180 min according to the above results.

Fig. 6 Effect of contact time on the adsorption of Cr(VI) by MoS₂/rGO.

3.3 Effect of the adsorbent dose

The relation between the adsorbent dose of MoS₂/rGO and removal efficiency is shown in Fig. 7. With an increase in the MoS₂/rGO dosage, the efficiency of the removal of Cr(VI) increased. In addition, the adsorption capacity decreased. It is clear that the efficiency of the removal of Cr(VI) sharply increased from 59.55% to 98.14%, when the adsorbent dose increased from 0.20 g L⁻¹ to 0.40 g L⁻¹. Correspondingly, the adsorption capacity decreased from 148.88 mg g⁻¹ to 122.67 mg g⁻¹. This might be due an increased dose that led to an enhancement in the surface and the available active sites for adsorption.¹⁵ Based on these results, 0.40 g L⁻¹ was selected as an optimum adsorbent dose.

Fig. 7 Effect of adsorption dose on adsorption capacity and removal of Cr(VI) by MoS₂/rGO.

3.4 Effect of pH

Fig. 8 shows the effect of pH on the adsorption of Cr(VI) by MoS₂/rGO. The pH of the solution was an important factor in the adsorption. It indicates that the change between the initial pH of the solution and adsorption capacity of Cr(VI) tends to be similar. At a pH range of 2.0–10.0, MoS₂/rGO (mass ratio of 1 : 9) had a much higher adsorption capacity (>87.59 mg g⁻¹) than the others in this study. MoS₂/rGO (mass ratio of 1 : 9) showed a great adsorption capacity not only at pH 2.0 (124.67 mg g⁻¹) but also at pH 4.6 (118.31 mg g⁻¹). The adsorption capacity decreased with the value of pH increasing until the pH value was 10. Cr(VI) exists in various different ionic forms in solutions. Chromate (CrO₄²⁻), dichromate (Cr₂O₇²⁻) and hydrogen chromate (HCrO₄⁻) are the main ionic forms of Cr(VI) in solutions. At a pH lower than 6.8, HCrO₄⁻ is the major species, while above 6.8, only CrO₄²⁻ is stable. Under acidic conditions, the adsorption is facilitated by electrostatic attraction between the sorbents and sorbates, which might due to the protonation on the sorbents surface. In fact, much chromium-containing water is acidic, such as electroplating wastewater at pH 4–6 and acid pickling wastewater at a pH lower than 1.5. Most of the reported adsorbents of Cr(VI) show a high adsorption capacity only under strong acidic conditions, such as pH 2.0. In this case, pH 4.6 was selected for the subsequent experiments unless otherwise noted. This was done to avoid adjusting the pH value of Cr(VI) solutions to reduce experimental error and to show the great potential of MoS₂/rGO for removing Cr(VI) compared to the other reported adsorbents at pH 4.6.

Fig. 8 Effect of pH on the adsorption of Cr(VI) by MoS₂/rGO.

3.5 Effect of initial concentration

The effect of the initial concentration on Cr(VI) adsorption by rGO, MoS₂ and MoS₂/rGO is shown in Fig. 9. It shows that the adsorption capacity of MoS₂/rGO is quite higher than the adsorption capacity of rGO and MoS₂, signifying that the combination of rGO and MoS₂ not only exhibited the properties of constituents, but also produced new properties through the interaction.

Fig. 9 Effect of initial concentration on the adsorption of Cr(VI) by rGO, MoS₂ and MoS₂/rGO.

3.6 Adsorption kinetics study

The pseudo-first order¹⁶ model and the pseudo-second order¹⁷ model were employed to investigate the controlling mechanism. The linearized forms of the adsorption kinetic models are given in eqn (1) and (2), respectively:where q_e is the amount adsorbed at equilibrium (mg g⁻¹), q_t is the amount adsorbed at time t (mg g⁻¹), and k₁ and k₂ are the pseudo-first order and pseudo-second order, respectively.

Fig. 10a and b show the linearized plots of eqn (1) and (2), respectively. The fitting results of kinetic models are shown in Table 1. The results suggest that the calculated q_e values by a pseudo-second order model are close to the experimental values and the correlation coefficient (r² ≈ 0.9990–0.9996). Meanwhile, the calculated q_e values by a pseudo-first order model greatly differ from the experimental values, and the correlation coefficient (r² ≈ 0.6363–0.9455), which indicates that the Cr(VI) adsorption onto MoS₂/rGO fits well with the pseudo-second order model than the pseudo-first order model.

Fig. 10 (a) The pseudo-first-order, (b) the pseudo-second-order kinetics model for the adsorption of Cr(VI) by MoS₂/rGO.

Table 1 Kinetic parameters for the adsorption of Cr(VI) by MoS₂/rGO

Kinetic models		Parameters
		qe,exp (mg g^−1)	qe,cal (mg g^−1)	k1 (min^−1)	r^2
Pseudo-first-order model	30 mg L^−1	74.59	31.83	2.46 × 10^−2	0.6363
	50 mg L^−1	113.49	164.38	5.09 × 10^−2	0.9461
	70 mg L^−1	98.74	63.92	3.59 × 10^−2	0.9455

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Kinetic models		Parameters
		qe,exp (mg g^−1)	qe,cal (mg g^−1)	k2 [g mg^−1 min^−1]	r^2
Pseudo-second-order model	30 mg L^−1	74.59	75.87	0.22 × 10^−2	0.9990
	50 mg L^−1	113.49	116.82	0.09 × 10^−2	0.9985
	70 mg L^−1	98.74	100.70	0.20 × 10^−2	0.9996

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3.7 Adsorption isotherms study

The Langmuir¹⁸ and Freundlich¹⁹ isotherm models were employed to understand the adsorption mechanism. The Langmuir isotherm assumes a homogeneous adsorption surface with all the adsorption sites having an equal adsorbate affinity. The equation of the Langmuir isotherm is given as follows:where q_m is the maximum adsorption capacity (mg g⁻¹), and k_L is the Langmuir constant related to the free energy of adsorption. In addition, a dimensionless separation constant R_L can be expressed as follows:where the value of R_L indicates the type of isotherm that is unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1), or irreversible (R_L = 0).

The Freundlich isotherm assumes a heterogeneous adsorption surface during the adsorption process. The Freundlich isotherm model is represented by the following equation:

where 1/n and k_F are the constants related to the intensity of adsorption and capacity of adsorption, respectively.

Fig. 11a and b show the linearized plots of the Langmuir and Freundlich isotherm models, and the values of the parameters are listed in Table 2. Based on the high value of the correlation coefficient by the Langmuir isotherm model (r² ≈ 0.9998–0.9601) compared to the Freundlich isotherm model (r² ≈ 0.1247–0.8377), the Langmuir isotherm model gave a better description of Cr(VI) adsorption by MoS₂/rGO. The maximum capacity at pH 2.0 and pH 4.6 at 25 °C were calculated to be 268.82 mg g⁻¹ and 192.63 mg g⁻¹, respectively. The value of R_L at three different temperatures was in the range of 0.0011–0.0234 and revealed that the Cr(VI) adsorption process was favorable. The comparison of the maximum adsorption capacity of MoS₂/rGO with various reported adsorbents in the Langmuir isotherm is shown in Table 3. The dose of MoS₂/rGO was only 0.4 g L⁻¹, which is much less than some materials, such as reduced graphene oxide–montmorillonite³ (6.0 g L⁻¹) and polypyrrole/F₃O₄ magnetic nanocomposite⁵ (2.0 g L⁻¹), among others.^6,20–22 Based on the results, the MoS₂/rGO had a much higher maximum adsorption capacity than others, which can be considered as a promising material for the removal of Cr(VI) in wastewater.

Fig. 11 (a) The Langmuir, and (b) the Freundlich isotherm models for the adsorption of Cr(VI) by MoS₂/rGO.

Table 2 Isotherm parameters for the adsorption of Cr(VI) by MoS₂/rGO

Temp (°C)	pH	Langmuir					Freundlich
		qm (mg g^−1)	kL (L mg^−1)	r^2	RL	1/n	kF (mg g^−1)	r^2
25	2.0	268.82	6.2000	0.9998	0.0011	0.1209	162.4061	0.1247
	4.6	192.63	0.7414	0.9799	0.0089	0.1842	67.2805	0.7730
35	4.6	134.95	0.4074	0.9720	0.0161	0.1874	64.3599	0.8030
45	4.6	148.15	0.2787	0.9601	0.0234	0.1917	65.3149	0.8377

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Table 3 Comparison of adsorption capacity for Cr(VI) on different adsorbents at 25 °C

Adsorbent	Dose (g L^−1)	pH	qmax (mg g^−1)	Reference
MoS2/rGO	0.4	2.0	268.82	This study
MoS2/rGO	0.4	4.6	192.63	This study
Reduced graphene oxide–montmorillonite	6.0	2.0	12.86	3
Polypyrrole/F3O4 magnetic nanocomposite	2.0	2.0	169.4–243.9	5
Graphene modified with cetyltrimethylammonium bromide	4.0	2.0	21.57	6
Graphene oxide/Fe3O4/SO3H nanohybrid	0.5	3.0	222.22	20
Magnetic ionic liquid/chitosan/graphene oxide composite	1.0	3.0	145.35	21
β-Cyclodextrin–chitosan modified biochars	2.0	2.0	206	22

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3.8 Thermodynamic study

To investigate the thermodynamic study of MoS₂/rGO, the Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰), and entropy (ΔS⁰)²³ were determined at temperatures from 298 K to 318 K, as shown in Table 4. At all three temperatures, the negative value of ΔG⁰ indicated that the Cr(VI) adsorption process by MoS₂/rGO was spontaneous. The negative ΔH⁰ value suggested that the removal of Cr(VI) was an exothermic reaction.²⁴

Table 4 Thermodynamic parameters for the adsorption of Cr(VI) by MoS₂/rGO

T (K)	ΔG^0 (kJ mol^−1)	ΔH^0 (kJ mol^−1)	ΔS^0 (J mol^−1 K^−1)
298	−7.0085	−30.7309	−79.6053
308	−6.2124
318	−5.4164

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3.9 Adsorption mechanism

The EDS pattern (Fig. 12a) of MoS₂/rGO after Cr(VI) adsorption at pH 4.6 revealed the existence of C, O, Mo, S and Cr. Furthermore, it was noted that the peaks of Mo and S were overlapped. To further understand the adsorption mechanism of MoS₂/rGO, Cr ions loaded on the surface of MoS₂/rGO after Cr(VI) adsorption at pH 4.6 were employed to XPS. The XPS spectra are shown in Fig. 12b. There are two obvious energy bands at about 577.5 eV and 587.5 eV, which correspond to the orbital binding energies of Cr(2p_3/2) and Cr(2p_1/2), respectively. This indicated that the Cr ions exist as Cr(VI) and Cr(III) on the MoS₂/rGO surface.⁵ The Cr(VI) on the adsorbents surface, due to the sorption of Cr(VI), may be due to the specific surface area of MoS₂/rGO and the electrostatic attraction between adsorbents and Cr(VI) ions. The Cr(III) on the adsorbents surface suggests that some fraction of adsorbed Cr(VI) on the MoS₂/rGO surface is reduced to Cr(III), and the Cr(III) species can be precipitated on the surface of MoS₂/rGO due to the co-electron cloud, which formed by the electrons of the S atom layer and its adjacent carbon layer.¹⁸ The reduction was accompanied by the consumption of protons (eqn (6)).

HCrO₄⁻ + 7H⁺ + 3e⁻ → Cr³⁺ + 4H₂O (6)

Fig. 12 (a) EDS pattern and (b) XPS spectra of the MoS₂/rGO after Cr(VI) adsorption at pH 4.6.

4. Conclusions

In this study, MoS₂/rGO was synthesized via a facile, one-step solvothermal process and successfully showed new properties for Cr(VI) removal. It had an excellent adsorption capacity from pH 2.0 to 10.0 compared to some of the reported adsorbents. The adsorption kinetics fitted better to a pseudo-second-order model than a pseudo-first-order model. The Langmuir isotherm was found to describe the experimental data better than the Freundlich isotherm. The maximum adsorption capacity of MoS₂/rGO at pH 2.0 and at pH 4.6 was 268.82 mg g⁻¹ and 192.63 mg g⁻¹, respectively. A thermodynamic study suggested that Cr(VI) adsorption was spontaneous and exothermic. The abovementioned results revealed that MoS₂/rGO can be an effective adsorbent for the removal of Cr(VI).

Acknowledgements

Financial support from the Key Natural Science Foundation of Guangdong Province (No. 2015A030311002) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra03531d

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