Facile one‐step hydrothermal syntheses of graphene oxide–MnO 2 composite and their application in removing heavy metal ions
2018; Institution of Engineering and Technology; Volume: 13; Issue: 8 Linguagem: Inglês
10.1049/mnl.2017.0761
ISSN1750-0443
AutoresChunyan Liang, Xuezhen Feng, Jin‐Gang Yu, Xinyu Jiang,
Tópico(s)Nanomaterials for catalytic reactions
ResumoMicro & Nano LettersVolume 13, Issue 8 p. 1179-1184 ArticleFree Access Facile one-step hydrothermal syntheses of graphene oxide–MnO2 composite and their application in removing heavy metal ions Chunyan Liang, Chunyan Liang School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorXuezhen Feng, Xuezhen Feng School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorJingang Yu, Jingang Yu School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorXinyu Jiang, Corresponding Author Xinyu Jiang jiangxinyu@csu.edu.cn School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this author Chunyan Liang, Chunyan Liang School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorXuezhen Feng, Xuezhen Feng School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorJingang Yu, Jingang Yu School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this authorXinyu Jiang, Corresponding Author Xinyu Jiang jiangxinyu@csu.edu.cn School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 People's Republic of ChinaSearch for more papers by this author First published: 01 August 2018 https://doi.org/10.1049/mnl.2017.0761Citations: 7AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In this work, a composite of graphene oxide (GO) supported by granulated manganese dioxide (MnO2) has been synthesised via a facile one-step hydrothermal process and then used as adsorbent to remove heavy metal ions from aqueous solution. The morphology and surface functional groups of GO–MnO2 composite were characterised by Fourier transform infrared spectroscopy, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetric analysis. The results indicated that MnO2 particles were successfully inserted into graphene sheets, and the agglomeration of GO was prevented. The effects of contact time and initial concentrations of heavy metal ions on the adsorption capacity of GO–MnO2 composite were studied. The adsorption data fitted well with the pseudo-second-order kinetic model and the Langmuir isotherm model. The maximum adsorption capacity for Pb2+ obtained from Langmuir isotherm model was about 490 mg/g. Based on the results from the removal of four heavy metal ions from smelting wastewater and the reusability experiments, it could be deduced that GO–MnO2 composite is a potential adsorbent for environmental clean-up. 1 Introduction Toxic metal pollution has been a prominent problem in the environment because of its strong toxicity, long duration and difficulty in degrading [[1]]. Heavy metal contaminations are mainly discharged by industries, such as rubber, storage batteries, leather, automobile, medicine, radioactivity shields, mining and metal processing [[2], [3]]. Heavy metals in the water are bringing a great threat to human beings, animals and plants. As the heavy metals poisoning can cause irreversible structural changes of the proteins, thus affect the function of tissue cells [[4]]. In order to remove heavy metals from wastewater, many common techniques and methods have been reported, such as precipitation [[5]], ion exchange [[6]], membrane filtration [[7]], biological treatment and adsorption [[8]]. Among these water purification techniques, adsorption has been proved to be a facile and highly-effective method [[9]]. Various adsorbents have been synthesised for the heavy metal ions removal. However, many adsorbents have suffered from low adsorption capacities and complex synthesis procedures. Therefore, there is an urgent need for developing novel adsorbents that can be synthesised more easily and possess higher adsorption capacities. Graphene, which consists of a single atomic sheet of conjugated sp2 carbon atoms [[10]], exhibits many outstanding physical and chemical properties, and many efforts have been devoted to study the application of graphene-based composites. Graphene oxide (GO), which is graphene-based material, contains a large amount of oxygen-containing functional groups, such as carboxyl, hydroxyl, epoxy groups and so on [[11]]. These oxygen-containing functional groups on the surface of the GO allow it to efficiently adsorb heavy metal ions in the water [[12]]. However, it is worth noting that GO are easy to aggregate due to the van der Waals interaction between graphene sheets. The surface of GO is therefore often functionalised by some organic polymers [[13]], nanosized metal oxides [[14]] or inorganic nanoparticles [[15]] in order to avoid agglomeration and facilitate the practical applications. Due to the grafted polymers or doped inorganic particles, the agglomeration of graphene sheets was prevented and the adsorption capacity of the graphene-based composite was also improved [[9]]. Manganese dioxide (MnO2), which is one of the most stable manganese oxides, has exhibited excellent physical and chemical properties [[16]]. Manganese oxides have shown strong adsorption and excellent ion-exchange capacity for a variety of metal cations [[17]]. Many researches have shown that adsorbents modified with manganese oxides showed improved adsorption abilities and higher removal efficiencies for pollutants [[18]-[21]]. Liu et al. [[21]] have reported a two-step method for the synthesis of three-dimensional (3D) graphene/δ-MnO2. First, the 3D graphene was prepared, and then MnO2 was incorporated into the 3D graphene architectures to obtain 3D graphene/δ-MnO2. In this Letter, we demonstrated a facile one-step hydrothermal process to synthesise GO–MnO2 composite, while the synthesis process was simplified, and the adsorption ability of the material towards Pb(II), Cd(II), Zn(II) and Cu(II) maintained. At the same time, the removal efficiencies of heavy metal ions from industrial wastewater by the as-prepared material were also estimated. 2 Experimental procedures Flake graphite (100 mesh, purity>95%) was purchased from Shenzhen Nanotech Port Co., Ltd. Hydrochloric acid (HCl, 37 wt%), sulphuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30 wt%) and potassium permanganate (KMnO4) were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. Metal salts including Pb(NO3)2, Cd(NO3)2 ·4H2 O, CuSO4 ·5H2 O and Zn(NO3)2 ·6H2 O were used as sources for Pb(II), Cd(II), Cu(II) and Zn(II), respectively. All the chemicals used were of analytical grade and without any further treatment. All water used was ultrapure, which was purified by a Milli-Q water purification system (Millipore, Milford, MA). 2.1 Synthesis of the GO–MnO2 composite GO was prepared from natural graphite by a modified Hummers method [[12]]. The GO–MnO2 composite was synthesised via a facile one-step hydrothermal process. First, 100 mg of KMnO4 was dissolved into ultra-pure water and then slowly added 150 μl HCl (37 wt%) before 18 ml GO dispersion was added into the solution. The obtained mixture was treated by sonication for 30 min to obtain a homogeneous dispersion. Then the homogeneous dispersion was transferred to a 25 ml Teflon-lined stainless-steel autoclave and maintained at 110°C for 2 h. Finally, the obtained products were washed with ultra-pure water and then dried by vacuum freeze-drying for 48 h. The pure GO was also hydrothermally prepared via a similar procedure without the addition of HCl and KMnO4. The process for the fabrication of the GO–MnO2 composite was presented in Fig. 1. Fig. 1Open in figure viewerPowerPoint Schematic illustration of the synthesis of GO–MnO2 composite 2.2 Characterisation methods The surface morphologies of the samples were observed by field emission scanning electron microscopy (MIRA3 TESCAN); Fourier transform infrared (FTIR) analysis was performed on a Nicolet 6700 FTIR spectroscopy; X-ray photoelectron spectroscopy (XPS) was performed on an XPS system (Pekin-Elmer ESCA 5000c); thermogravimetric analysis (TGA) was performed on a SDT Q600V8.0 Build 95 thermal analyser. The concentrations of heavy metal ions in aqueous solutions were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). 2.3 Adsorption studies In order to examine the removal efficiencies of the prepared materials for heavy metal ions, the influences of adsorption temperature, initial metal ion concentrations and equilibration time were studied. A typical experiment was operated by adding 5 mg of GO–MnO2 composite to 25 ml of solution containing heavy metal ions and then shaking the mixed solution in a thermostat shaker for a certain amount of time. The initial and the final concentrations of heavy metal ions were determined by ICP-AES, and the adsorption capacity was calculated by the following equation: (1) where C0 (mg/l) is the original concentration of metal ions; Ce (mg/l) is the equilibrium concentration of metal ions; m (g) is the mass of the adsorbent and V (l) is the volume of the solution; qe (mg/g) is the equilibrium adsorption capacity. All the experimental data were the mean of triplicate determinations. 2.4 Adsorption kinetics The kinetic mechanism of the adsorption process was evaluated by pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order kinetic model is described in the following equation [[22]]: (2) The pseudo-second-order kinetic model is given as the following equation: (3) where qe (mg/g) is the adsorption capacity at equilibrium and qt (mg/g) is the adsorption capacity at a certain time t (min); k1 (1/min) and k2 (g/mg min) are the rate constants of pseudo-first-order adsorption and pseudo-second-order adsorption, respectively. 2.5 Adsorption isotherm The equilibrium adsorption isotherms are usually used to understand the adsorption mechanism. In this Letter, Langmuir and Freundlich equations were chosen to describe the equilibrium between adsorbents and adsorbates. Langmuir adsorption isotherm is applied to evaluate the adsorption taking place on homogeneous surface and explain the monolayer adsorption process, while the Freundlich adsorption isotherm is used to describe multilayer adsorption [[23]]. The liner equation of Langmuir isotherm can be expressed as follows [[24]]: (4) The liner equation of Freundlich isotherm can be expressed as follows: (5) where qe (mg/g) is the amount of heavy metal ions adsorbed at equilibrium; Ce (mg/l) is the equilibrium concentration of heavy metal ions; qm (mg/g) is the maximum adsorption capacity; b is the Langmuir constant, which is related to the adsorption energy; kf and n are Freundlich constants; kf is related to adsorption capacity and 1/n is a measurement of adsorption effectiveness. 2.6 Adsorption thermodynamics The thermodynamic parameters (, , ) are used to determine whether the adsorption is an exothermic or an endothermic procedure. The standard free energy change (), standard enthalpy change (), standard entropy change () and equilibrium distribution coefficient kd can be calculated by the following equations: (6) (7) (8) where C0 (mg/l) is the initial concentration of metal ions; Ce (mg/l) is the equilibrium concentration of metal ions; V (l) is the volume of the solution and m (g) is the dosage of the adsorbent; R (8.314 J/mol K) is the universal gas constant and T (K) is the absolute temperature. The values of and can be calculated from the intercept and slope of the linear plot of lnkd versus 1/T, respectively. 3 Results and discussion The FTIR spectra of pristine GO, GO–MnO2 composite and MnO2 are shown in Fig. 2. From the FTIR curve of pristine GO, the adsorption peaks appeared at 3402, 1734, 1620, 1400 and 1114 cm−1 could be attributed to O–H and C=O of the –COOH, aromatic C=C, C–O group and alkoxy C–O stretching vibrations, respectively. The results revealed that oxygen-containing functional groups were successfully introduced onto the surface of the exfoliated GO. Compared with the FTIR spectra of GO, the bands related to the carboxyl and hydroxyl groups almost remained in the GO–MnO2 composite, which indicated the domain of GO were retained during hydrothermal process. However, the bands related to alkoxy were disappeared, indicating GO was partially reduced during the synthesis process. The new peak at 513 cm−1 observed in GO–MnO2 composite could be ascribed to Mn–O vibration of the MnO2 [[25], [26]], which indicated that the GO–MnO2 composite was successfully synthesised. Fig. 2Open in figure viewerPowerPoint FTIR spectra of the samples a GO b GO–MnO2 c MnO2 In order to further determine the structure of GO–MnO2 composite, Raman spectra were recorded (Fig. 3). The D band was related to the vibration of sp3 carbon atom of defects and disorder in the hexagonal graphite layers, whereas the G band was ascribed to the coplanar vibration of sp2 carbon atoms [[27]]. The relative intensity ratio ID/IG was used to evaluate the degree of disorder and the quality of carbon materials [[28]]. The intensity ratio ID/IG increased from 0.86 (GO) to 1.13 (GO–MnO2 composite), and the result indicated that more defects appeared in prepared material after the introduction of MnO2 particles. Furthermore, a new peak appearing at 626 cm−1 in the Raman spectra of GO–MnO2 composite could be assigned to Mn–O stretching vibration of MnO6 groups [[29]]. The results also could prove that the MnO2 particles were successfully introduced onto the graphene sheets. Fig. 3Open in figure viewerPowerPoint Raman spectra of a GO b GO–MnO2 composite Detailed elemental composition of GO–MnO2 composite was investigated by XPS, and the corresponding XPS results are shown in Fig. 4. From the wide scan XPS profiles (Fig. 4a), there were three major peaks with binding energies of about 288, 532 and 640 eV which could be attributed to C1s, O1s and Mn2p, respectively. The O1s narrow scan is shown in Fig. 4b, the spectra of the O1s are separated from each other by three major peaks at 530.6, 531.5 and 532.6 eV, which could be assigned to metal oxide (Mn–O–Mn), hydroxyl bonded to metal (Mn–OH) and C–O/C=O in GO–MnO2 composite, respectively [[21], [30]]. The presence of Mn–OH indicated that Mn atoms might form bond with O atoms of the functional groups via a covalent coordination bond or an intermolecular hydrogen [[30]]. The spectra of Mn2p were presented in Fig. 4c, where the doublet peaks at 642.4 and 654.3 eV were observed, and they had a spin energy separation of 11.9 eV in line with previous reports [[31]], which indicated the existence of MnO2 phase in GO–MnO2 composite. Fig. 4Open in figure viewerPowerPoint XPS spectra of GO–MnO2 composite a Survey spectrum b O1 s c Mn2p The TGA was used to evaluate the thermal stability of the materials. The TGA curves of MnO2 and GO–MnO2 are shown in Fig. 5. As it can be seen, two weight-loss steps were observed for MnO2: the first weight loss below 250°C was probably due to the loss of water adsorbed in its external surface and internal pores; the weight loss at 400–600°C was attributed to the conversion of MnO2 to Mn2O3 [[30], [32]]. For GO–MnO2 composite, the weight loss below 250°C was due to the dehydration of the physically absorbed water and the thermal decomposition of residual oxygenate groups on the surface of GO. A sharp weight loss was observed in the range of 300–400°C, which could be attributed to the removal of the carbon skeleton of GO. Due to the transition of MnO2 to Mn2O3, the slight weight loss also could be observed at the temperature above 400°C. At 800°C, the Mn2O3 with weight per cent of 20% was remained. Therefore, the amount of MnO2 bonded to the GO was estimated to be about 40%. Fig. 5Open in figure viewerPowerPoint TGA curves of MnO2 and GO–MnO2 composite The surface morphology and structural details of GO–MnO2 composite were studied by scanning electron microscopy (SEM). The SEM image of GO–MnO2 composite is shown in Fig. 6. It can be seen that the MnO2 particles with the size of about 1 µm are uniformly distributed on GO sheets and are not aggregated. It was speculated that the GO could act as spacers to prevent MnO2 particles from aggregating, while the incorporation of MnO2 particles also avoided the restacking of graphene sheets. The above properties might benefit the improved adsorption capacity of GO–MnO2 composite. Fig. 6Open in figure viewerPowerPoint SEM images of GO–MnO2 composite at different magnification (a magnified 20.0k×; b magnified 2.00k×) 3.1 Adsorption kinetics The effects of contact time on the adsorption of GO–MnO2 for Pb2+, Cd2+, Zn2+ and Cu2+ are shown in Fig. 7. As can be seen from Fig. 7a, the adsorption process could be divided into two steps. The adsorption increased rapidly during the first 5 min and then slowly approached to the equilibrium within 120 min. The maximum adsorption capacities of GO–MnO2 for Pb2+, Cd2+, Zn2+ and Cu2+ were 429.01 mg/g (2.07 mmol/g), 183.75 mg/g (1.63 mmol/g), 140.50 mg/g (2.15 mmol/g) and 141.35 mg/g (2.22 mmol/g), respectively. In order to ensure that the adsorption process reached equilibrium, the adsorption time was chosen at 120 min in the subsequent experiments. The calculated parameters of the pseudo-first-order and pseudo-second-order kinetic models were summarised (Table 1) and the fitting lines of pseudo-second-order kinetic models are shown in Fig. 7b. From Table 1, we could see that the coefficient of determination (R 2) of the pseudo-second-order kinetic model (R 2 = 0.9948 − 0.9994) were better than pseudo-first-order kinetic model (R 2 = 0.4364 − 0.9266). Furthermore, the adsorption capacities (432.90, 179.86, 137.74 and 143.68 mg/g) calculated from the pseudo-second-order kinetic equation were closer to the experimental data (429.01, 183.75, 140.50 and 141.35 mg/g) than those (78.92, 22.98, 20.59 and 49.75 mg/g) calculated from pseudo-first-order kinetic equation. Therefore, the adsorption process fitted with pseudo-second-order kinetic model better. In addition, the adsorption capacity followed the sequence of Cu2+ (2.22 mmol/g) > Zn2+ (2.15 mmol/g) > Pb2+ (2.07 mmol/g) > Cd2+ (1.63 mmol/g). The adsorption capacities for metal ions are closely related to the ionic radius, hydration energy and valence of metal ions. In view of the same valence, the adsorption capacity mainly depends on ionic radius and hydration energy. In general, the ions with smaller radius would be more inclined to the ion exchange process and the metal ions with lower hydration energy could be separated from the combined water molecules more easily and then exchanged with –OH and –COOH of GO–MnO2 [[21]]. That was to say, metal ions with smaller ionic radius and lower hydration energy would be more likely to be adsorbed on GO–MnO2. In the present case, the ionic radius of four heavy metals were ranked as Cu2+ (0.71 Å) < Zn2+ (0.74 Å) < Cd2+ (0.97 Å) < Pb2+ (1.20 Å) [[21], [33]], and hydration energy followed the sequence of Pb2+ (1481 kJ/mol) < Cd2+ (1807 kJ/mol) < Zn2+ (2044 kJ/mol) < Cu2+ (2100 kJ/mol) [[21], [34]]. Therefore, the adsorption capacities for Cu2+ and Zn2+ were close due to the similar ionic radius and hydration energy. Bigger ionic radius and higher hydration energy of Cd2+ resulted in the lowest adsorption capacity. In conclusion, the adsorption capacity was affected by the ionic radius and hydration energy of metal ions in this adsorption process. Fig. 7Open in figure viewerPowerPoint Adsorption of metal ions onto GO-MnO2 a Effect of reaction time on Pb2+, Cd2+, Zn2+ and Cu2+ adsorption onto GO–MnO2 composite b Pseudo-second-order kinetic plots of Pb2+, Cd2+, Zn2+ and Cu2+ adsorption (initial concentration: 100 mg/l, temperature: 35°C) Table 1. Kinetic parameters for the adsorption of Pb2+, Cd2+, Zn2+ and Cu2+ onto GO–MnO2 composite at 35°C Equations Parameters Pb(II) Cd(II) Zn(II) Cu(II) first-order-kinetic qe, mg/g 78.92 22.98 20.59 49.75 k1, min−1 0.0340 0.0074 0.0113 0.0193 R 2 0.7755 0.8137 0.4364 0.9266 second-order-kinetic qe, mg/g 432.90 179.86 137.74 143.68 k2, min−1 0.0013 0.0025 0.0038 0.0012 R 2 0.9994 0.9959 0.9962 0.9948 3.2 Adsorption isotherm The Pb2+ was chosen to study the adsorption isotherm and the adsorption isotherm was shown in Fig. 8 and the related parameters of Langmuir and Freundlich isotherm were summarised in Table 2. It was found that the coefficient of determination obtained from Langmuir isotherm was higher than that obtained from Freundlich isotherm. In addition, theoretical adsorption capacities at different temperature, which calculated from Langmuir model, were closer to the experimental values. Therefore, the adsorption process fitted Langmuir model well, indicating that the adsorption occurred at specific homogeneous sites within the GO–MnO2 composite. That is to say, the adsorption process was mainly considered as monolayer adsorption. Fig. 8Open in figure viewerPowerPoint Pb2+ adsorption onto GO-MnO2 a Pb2+ adsorption isotherms on GO–MnO2 composite at three different temperatures b Fitting of isotherms data with linear Langmuir model (initial concentration: 10–180 mg/l, adsorption time: 2 h) Table 2. Langmuir and Freundlich isotherms parameters for the adsorption of Pb2+ onto GO–MnO2 composite Model Parameter Parameter value 25°C 35°C 45°C Langmuir isotherm qm, mg/g 448.43 476.19 490.19 b, l/mg 1.4575 1.3125 1.0355 R 2 0.9993 0.9991 0.9989 Freundlich isotherm n 14.71 12.60 10.34 kf (mg/g•(mg/l)n) 342.12 347.32 334.43 R 2 0.9356 0.9106 0.8872 3.3 Adsorption thermodynamic parameters The thermodynamic parameters (, and ) could be calculated from the temperature-dependent adsorption isotherms. The thermodynamic parameters and were obtained, respectively, from the slope and intercept of lnkd versus 1/T plots in Fig. 9, and the obtained thermodynamic parameters (, and ) were shown in Table 3. The positive value of indicated that the Pb2+ adsorption onto GO–MnO2 composite was an endothermic process. In addition, the positive value suggested that the degree of freedom increased at the solid/solution interface during the adsorption process. The negative value of indicated that the adsorption was a favourable and spontaneous process. Furthermore, the lower value of was obtained at higher temperature, indicating greater adsorption capacity at higher temperatures. Fig. 9Open in figure viewerPowerPoint Plots of lnkd versus 1/T for the adsorption of Pb2+ Table 3. Thermodynamic parameters for the adsorption of Pb2+ onto GO–MnO2 composite ΔH θ, kJ/mol ΔS θ, J/K·mol T, K ΔG θ, kJ/mol R 2 7.2589 38.15 298 −4.1098 0.9822 308 −4.4913 318 −4.8728 3.4 Reusability of GO–MnO2 composite For practical application, an advanced adsorbent should not only possess high adsorption capacity but also have excellent reusability. In order to evaluate the practical reusability of GO–MnO2 composite, the consecutive adsorption–desorption experiments were operated. After adsorption, the Pb2+ loaded GO–MnO2 composite was filtered and then immersed into 0.1 mol/l HCl solution for 3 h. Then, the adsorbent was filtered, washed several times with ultra-pure water and vacuum-freeze dried for 48 h. Finally, the obtained adsorbent was reused to remove the target contaminants. The adsorption capacity as a function of recycle number was shown in Fig. 10. It could be found that an obvious loss in adsorption capacity took place after one cycle and almost no loss happened in subsequent cycles of adsorption–desorption. As seen in Fig. 11, after one cycle, the SEM image indicated that some MnO2 particles were dislodged and left holes on the surface of graphene sheet. Even though there was a big drop in the first cycle, the adsorption capacity GO–MnO2 composite was still above 200 mg/g after five consecutive cycles of adsorption–desorption. The adsorption capacity of acid washed GO–MnO2 was much higher than that of pure GO with adsorption capacity about 120 mg/g for Pb2+. Accordingly, GO–MnO2 composite could be repeatedly used as an efficient adsorbent to remove Pb2+ although the reuse process was complicated. Fig. 10Open in figure viewerPowerPoint Reusability of GO–MnO2 composite for Pb2+ adsorption Fig. 11Open in figure viewerPowerPoint SEM image of the first acid washed GO–MnO2 3.5 Application for real samples In order to investigate whether the GO–MnO2 composite was efficient for treating real wastewater, water sample was collected from a local smelter. The sample was filtered through 0.45 µm membrane filter before adsorption experiment. Then 0.02 g GO–MnO2 composite was added into 20 ml water sample and the mixed solution was shaken in a thermostat shaker for 120 min at 35°C. The initial concentrations of Pb2+, Cd2+, Zn2+ and Cu2+ in the sample were 18.92, 23.48, 9.69 and 34.44 mg/l, respectively. The results of adsorption were shown in Fig. 12. The removal efficiencies of Pb2+, Cd2+, Zn2+ and Cu2+ by GO–MnO2 composite were 100, 80, 80 and 100%, respectively. For comparison, the removal efficiencies of four metal ions by GO also were studied. The results showed that the removal efficiencies of the four heavy metal ions by GO–MnO2 composite were much higher than those by GO. It indicated that MnO2 particles effectively arrested the agglomeration of GO nanosheets and improved the adsorption capacity of materials. In addition, although matrices of practical sample were complex, the GO–MnO2 composite could effectively remove heavy metal ions in the real wastewater. Fig. 12Open in figure viewerPowerPoint Removal efficiencies of four metal ions by GO–MnO2 and GO in metal smelting wastewater 4 Conclusion GO–MnO2 composite was successfully synthesised by facile one-step hydrothermal method and used as adsorbent for removing Pb2+, Cu2+, Cd2+ and Zn2+ in aqueous solution. The kinetic and isotherm study indicated that the adsorption process followed the pseudo-second-order kinetic model and Langmuir isotherm model, respectively. The thermodynamic parameters calculated from the temperature-dependent adsorption isotherms showed that the adsorption process was spontaneous and endothermic. The adsorption capacity of GO–MnO2 composite for Pb2+ remained above 200 mg/g after five cycles of adsorption–desorption. In addition, the adsorption experiment of real wastewater demonstrated the efficient application of the composite for removing heavy metal ions, which showed much higher adsorption capacity than that of GO. In conclusion, GO–MnO2 composite could be used as a promising adsorbent for removing heavy metal ions. 5 Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 21571191 and 51674292) and Provincial Natural Science Foundation of Hunan (2016JJ1023). 6 References [1]Yadanaparthi S. 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