Few-Layered Graphene Oxide Nanosheets As Superior Sorbents for

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Few-Layered Graphene Oxide Nanosheets As Superior Sorbents for Heavy Metal Ion Pollution Management Guixia Zhao, Jiaxing Li, Xuemei Ren, Changlun Chen, and Xiangke Wang* Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, P.R. China

bS Supporting Information ABSTRACT: Graphene has attracted multidisciplinary study because of its unique physicochemical properties. Herein, fewlayered graphene oxide nanosheets were synthesized from graphite using the modified Hummers method, and were used as sorbents for the removal of Cd(II) and Co(II) ions from large volumes of aqueous solutions. The effects of pH, ionic strength, and humic acid on Cd(II) and Co(II) sorption were investigated. The results indicated that Cd(II) and Co(II) sorption on graphene oxide nanosheets was strongly dependent on pH and weakly dependent on ionic strength. The abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets played an important role on Cd(II) and Co(II) sorption. The presence of humic acid reduced Cd(II) and Co(II) sorption on graphene oxide nanosheets at pH < 8. The maximum sorption capacities (Csmax) of Cd(II) and Co(II) on graphene oxide nanosheets at pH 6.0 ( 0.1 and T = 303 K were about 106.3 and 68.2 mg/g, respectively, higher than any currently reported. The thermodynamic parameters calculated from temperature-dependent sorption isotherms suggested that Cd(II) and Co(II) sorptions on graphene oxide nanosheets were endothermic and spontaneous processes. The graphene oxide nanosheets may be suitable materials in heavy metal ion pollution cleanup if they are synthesized in large scale and at low price in near future.

’ INTRODUCTION Heavy metal pollution due to the indiscriminate disposal of wastewater is a worldwide environment concern. Wastewaters from many industries such as metallurgical, mining, chemical manufacturing, and battery manufacturing industries contain many kinds of toxic heavy metal ions.1 Cadmium is among the toxic metals found in some surface and subsurface waters. It is wellknown that chronic cadmium toxicity has been the cause of Japan ItaiItai disease. The harmful effects of Cd also include a number of acute and chronic disorders, such as renal damage, emphysema, hypertension, testicular atrophy, and skeletal malformation in fetus.2,3 Cobalt is a very toxic metal affecting the environment. The increased use of Co(II) in nuclear power plants and in many industries has resulted in Co(II) findings its way to the environment. In high doses it causes bone defects, diarrhea, low blood pressure, lung irritations and paralysis, and may also cause mutations (genetic changes) in living cells.4 Thereby, it is necessary to eliminate the toxic heavy metal ions from wastewater before it is released into the environment. Traditional techniques for the elimination of heavy metal ions include precipitation, membrane filtration, sorption, and ion exchange, etc.5,6 Among these methods, sorption technique has been used widely because it is simple, economical, and cost-effective. Some sorbents, such as clay minerals, oxides, and carbon materials, have been studied extensively to remove heavy metal ions from aqueous solutions.79 However, these materials suffer from either low sorption capacities or efficiencies. Nanomaterials have gradually developed important roles to resolve this problem because of their high surface area, enhanced active sites, and abundant functional groups on the surfaces. r 2011 American Chemical Society

So far, a variety of nanomaterials such as carbon nanotubes,10,11 carbon nanotube based material composites,6 and graphene1214 have been studied in the removal of different organic and inorganic pollutants from large volumes of aqueous solutions, and the results indicated that these carbon nanomaterials had high sorption capacity. Graphene, a kind of one or several atomic layered graphites, possesses special two-dimensional structure and excellent mechanical, thermal, and electrical properties.15,16 In our previous study,14 sulfonated graphene nanosheets were synthesized and used as sorbents to remove naphthalene and 1-naphthol. The sorption capacities of ∼2.32.4 mmol/g for naphthalene and 1-naphthol were the highest capabilities of today’s nanomaterials. Unlike carbon nanotubes, which require special oxidation processes to introduce hydrophilic groups to improve metal ion sorption, the preparation of graphene oxide nanosheets from graphite using Hummers method introduces many oxygen-containing functional groups such as COOH, CdO, and OH, on the surfaces of graphene oxide nanosheets. These functional groups are essential for the high sorption of heavy metal ions. Graphene oxides, which are considered as the oxidized graphene, contain oxygen-containing functional groups on the surfaces. Considering the oxygen-containing functional groups on the graphene oxide surfaces and high surface area (theoretical Received: September 29, 2011 Accepted: November 9, 2011 Revised: November 6, 2011 Published: November 09, 2011 10454

dx.doi.org/10.1021/es203439v | Environ. Sci. Technol. 2011, 45, 10454–10462

Environmental Science & Technology

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Figure 1. Characterization of few-layered graphene oxide nanosheets: (a) TEM image; (b) AFM image; (c) XRD patterns of graphite and graphene oxide nanosheets; (d) XPS C1s spectrum; (e) Raman spectrum; (f) acidbase titration curve; (g) FT-IR spectrum; and (h) TG-TGA curves.

value of 2620 m2/g), the graphene oxide nanosheets should have high sorption capacity in the preconcentration of heavy metal ions from large volumes of aqueous solutions. However, the application of graphene oxide nanosheets as sorbents in the removal of heavy metal ions from aqueous solution is still scarce,17 especially in the

presence of humic substances, which present widely in the natural environment, and have strong complexation ability with metal ions because of their abundant oxygen-containing functional groups. It is therefore important to study the sorption behaviors of metal ions on graphene oxide nanosheets in the presence of humic substances. 10455

dx.doi.org/10.1021/es203439v |Environ. Sci. Technol. 2011, 45, 10454–10462


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The objectives of this study were to (1) prepare few-layered graphene oxide nanosheets and apply them as sorbents to remove Cd(II) and Co(II) ions from aqueous solutions; (2) investigate the effects of pH and ionic strength on Cd(II) and Co(II) sorption; (3) study the effect of humic acid (HA), a kind of natural organic material, on Cd(II) and Co(II) sorption; (4) discuss the mechanism of Cd(II) and Co(II) interaction with graphene oxide nanosheets. This study demonstrated the broad applicability of this fascinating material in environmental pollution cleanup.

’ EXPERIMENTAL SECTION Materials. Few-layered graphene oxide nanosheets were prepared by using the modified Hummers method18 from the natural flake graphite (average particle diameter of 20 mm, 99.95% purity, Qingdao Tianhe Graphite Co. Ltd., China) using concentrated H2SO4 and KMnO4 to oxidize the graphite layer. With the aid of ultrasonication, the oxidized graphite layers were exfoliated from each other. Then 30% H2O2 was added in the suspension to eliminate the excess MnO4. The desired products were rinsed with deionized water. Detailed processes are described in the Supporting Information (SI). The prepared fewlayered graphene oxide nanosheets were used as sorbents to remove Cd(II) and Co(II) ions from aqueous solutions in the following experiments. All chemicals used in the experiments were analytical grade. HA was extracted from the soil of Hua-Jia county (Gansu province, China), and was characterized in detail.19 The surface site density was determined to be 6.46 103 mol/g by fitting the potentiometric acidbase titration data with the aid of FITEQL 3.1. Characterization of Graphene Oxide Nanosheets. Graphene oxide nanosheets were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), Raman spectroscopy, potentiometric acidbase titration, Fourier transformed infrared spectra (FT-IR), and thermogravimetric analysis (TGA). The TEM images were obtained with a JEM-2010. The AFM images were obtained in air using a Digital Instrumental Nanoscope III in tapping mode. The XPS measurements were conducted with an ESCALab 220IXL system. The XRD patterns were measured on a D/max2500 with a Cu Kα source (λ = 1.541 Å). Raman spectra were recorded with a Renishew in Via Raman spectrometer (Renishaw plc, UK). The laser excitation was provided by a regular model laser operated at wavelength of 514 nm. The potentiometric acidbase titration was conducted using a computer-controlled titration system (DL50 Automatic Titrator, Mettler Toledo) in 0.01 M NaClO4 background electrolyte under argon conditions. The data sets of pH versus the net consumption of H+ or OH were used to calculate intrinsic acidity constants in diffuse-layer model with the aid of FITEQL 3.1. FT-IR spectroscopy measurements were mounted by using a Perkin-Elmer 100 spectrometer in KBr pellet at room temperature. TG and DTA curves were measured by using a Shimadzu TGA-50 thermogravimetric analyzer from room temperature to 800 °C with heating rate of 10 °C/min and an air flow rate of 50 mL/min. Sorption Experiments. The batch experiments of Cd(II) and Co(II) sorption on graphene oxide nanosheets were carried out at pH 6.0 ( 0.1 and in 0.01 M NaClO4 solutions in polyethylene test tubes. For most wastewaters in the environment, the pH is

Figure 2. Sorption of Cd(II) (A) and Co(II) (B) on graphene oxide nanosheets as a function of pH in different NaClO4 concentrations. C[Cd(II)]initial = 20 mg/L, C[Co(II)]initial = 30 mg/L,T = 303K, m/V = 0.1 g/L. The vertical line on each panel indicates the pH of bulk solution precipitation for the metal ion at the total metal concentration employed.

∼6.0 because of the dissociation of CO2. The stock suspensions of graphene oxide nanosheets, Cd(II) or Co(II) solution, and NaClO4 solution were added in the polyethylene test tubes to achieve the desired concentrations of different components. The desired pH of the suspensions in each tube was adjusted by adding 0.01 mol/L HClO4 or NaOH solution. It was necessary to notice that the graphene oxide nanosheets were equilibrated with HA before the addition of Cd(II) or Co(II) solution in the presence of HA. After the suspensions were shaken for 24 h to achieve sorption equilibrium, the solid phase was separated from the solution using 0.22-μm membrane filters. The results of kinetic sorption suggested that Cd(II) and Co(II) sorption on graphene oxide nanosheets achieved equilibrium in several hours. The concentrations of Cd(II) or Co(II) in the filtrate were determined by atomic absorption spectroscopy. All the experimental data were the average of duplicate determinations, and the relative errors were about 5%. The amounts of Cd(II) or Co(II) ions adsorbed on graphene oxide nanosheets were calculated from the difference between the initial concentration 10456


Environmental Science & Technology (C0) and the equilibrium one (Ce) (Sorption % = (C0 Ce)/ C0 100%, and Cs = (C0 Ce)/m V, where Cs is the concentration of metal ion adsorbed on graphene oxide nanosheets, V is the volume of the suspension, and m is the mass of graphene oxide nanosheets).

’ RESULTS AND DISCUSSION Characterization of Graphene Oxide Nanosheets. The results of TEM, AFM, XRD, XPS, Raman, FT-IR, TGA, and acidbase titration characterization of the prepared graphene oxide nanosheets are shown in Figure 1. The synthesized graphene oxide nanosheets have lateral dimensions of several micrometers. The TEM image (Figure 1a) shows that fewlayered graphene oxides are formed, although the TEM image does not estimate the layer numbers of the graphene oxide nanosheets exactly. From the AFM image (Figure 1b), the thickness of graphene oxide nanosheets is about 2.98 nm, suggesting that few-layered graphene oxide nanosheets are formed.20,21 The thickness of one layer graphene oxide nanosheet is ∼0.81.0 nm.21 In the XRD patterns (Figure 2c) of graphene oxide nanosheets and graphite, the diffraction peak at 2θ = 26.40° (d = 0.34 nm), which corresponds to the normal graphite spacing (002) of graphite plane, disappears in the graphene oxide nanosheets. The broad and relatively weak diffraction peak at 2θ = 10.03° (d = 0.87 nm), which corresponds to the typical diffraction peak of graphene oxide nanosheets, is attributed to the (002) plane. The c-axis spacing increases from 0.34 to 0.87 nm after the graphite is modified to graphene oxide nanosheets, which is due to the creation of the abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets.22,23 The C1s XPS spectrum (Figure 1d) indicates a considerable degree of oxidation with different functional groups, i.e., the nonoxygenated ring C (284.5 eV, 71.4%), the C atom in CO bond (286.2 eV, 18.6%), the carbonyl C (287.8 eV, 9.8%), and the carboxylate carbon (OCdO) (289.0 eV, 0.2%).24 From the XPS analysis, it is clear that the graphene oxide nanosheets are highly oxidized by the oxidant. The specific peak area noted in Figure 1d shows that the main oxygen-containing groups are CO and CdO, which are expected to form strong surface complexes with metal ions on the solid surfaces. The synthesized graphene oxide nanosheets also have very high dispersion properties in aqueous solutions. The suspension did not form any aggregation even after several months of aging time with a brown yellow color. The high dispersion property of graphene oxide nanosheets in aqueous solution is favorable for the surface oxygenfunctional groups to freely form strong complexes with metal ions. In the Raman spectrum (Figure 1e), the G band at ∼1580 cm1 is associated with the vibration of sp2 carbon atoms in a graphitic 2D hexagonal lattice, and the D band at ∼1350 cm1 is related to the vibrations of sp3 carbon atoms of defects and disorder. The weak and broad 2D peak at ∼2700 cm1 is another indication of disorder as the result of an out-of-plane vibration mode. These strong G, D, and 2D bands are very similar to previous results of graphene oxide characterization.25,26 The pHpzc (point of zero charge) value is calculated to be 3.9 and the surface site density is 2.36 103 mol/g from the acid base titration (Figure 1f). The surface charge is positive at pH < pHpzc, and is negative at pH > pHpzc. The surface site density of graphene oxide nanosheets is about five times higher than that

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of oxidized multiwalled carbon nanotubes (MWCNTs) (4.2 104 mol/g).27 The oxygen-containing groups on the surfaces of graphene oxide nanosheets are characterized by FT-IR analysis (Figure 1g). Different functional groups are found in the FTIR spectrum, i.e., CO group at 1220 cm1 and 1100 cm1, CdO group at 1730 cm1 and CdC at 1620 cm1, which indicates that large amounts of oxygen-containing functional groups exist on graphene oxide nanosheets. The TG curve (Figure 1h) shows significant weight loss before 115 °C, which is due to desorption of absorbed water molecules on graphene oxide nanosheets. The graphene oxide nanosheets exhibit two steps of mass loss at 200 and 550 °C, which are attributed to the loss of CO and CO2 from the decomposition of oxygen functional groups and carbon oxidation, respectively.28,29 The graphene oxide nanosheets show the mass loss, starting at the temperature of 200 °C, illustrating a much lower thermal stability compared to the natural flake graphite.28 Effect of pH and Ionic Strength. Figure 2 shows Cd(II) and Co(II) sorption on graphene oxide nanosheets as a function of pH in different NaClO4 solutions. The sorption of Co(II) increases slowly at pH < 6, quickly at pH 69, and then maintains high level at pH > 9. The sorption of Cd(II) increases with increasing pH at pH < 9, and about ∼98% Cd(II) is adsorbed on graphene oxide nanosheets at pH > 9. From the results of acidbase titration (Figure 1f), the pHpzc value of graphene oxide nanosheets is ∼3.9. At pH < pHpzc, the surface charge of graphene oxide nanosheets is positive because of the protonation reaction (SOH + H+ f SOH2+, where S represents the surface of graphene oxide nanosheets, and OH represents the oxygen-containing functional groups). The positive metal ions are difficult to adsorb on the positively charged surface of graphene oxide nanosheets because of the electrostatic repulsion. At pH > pHpzc, the surface charge of graphene oxide nanosheets is negative because of the deprotonation reaction ( SOH f SO + H+). The reaction scheme for hydroxide formation of metal ion (M2+) could be set out as below: M2þ T MðOHÞþ T MðOHÞ02 T MðOHÞ 3 T :::

ð1Þ M(II) could be combined with deprotonated surface sites, and the sorption of Co(II) and Cd(II) on graphene oxide nanosheets could be expressed by the following reaction: 2þ 2þ 2 SO ðsÞ þ MeðaqÞ T ½ð SO Þ2 Me ðsÞ

ð2Þ

The solution pH affected the degree of deprotonation and the speciation of the surface functional groups. With increasing pH, the surface charge is more negative and the electrostatic interactions between the metal ions and graphene oxide nanosheets become stronger, and thereby result in the increase of metal ion sorption. The relative proportion of Cd(II) species is calculated from the stability constants (log β1 = 3.9, log β2 = 7.7, and log β3 = 8.7) and the results demonstrate that Cd(II) presents in the form of Cd2+, Cd(OH)+, Cd(OH)20, and Cd(OH)3 at various pH values (Figure S1). At pH < 8.0, the predominant Cd(II) species is Cd2+ and the removal of Cd(II) is mainly accomplished by sorption reaction. The precipitation curve of Cd(II) calculated from the precipitation constant of Cd(OH)2(s) (Ksp = 2.50 1014) and the initial Cd(II) concentration (i.e., 1.78 104 mol/L) is also shown in Figure 2A. Cd(II) begins to form precipitation at 10457



Figure 3. Effect of humic acid on the sorption of Cd(II) (A) and Co(II) (B) on graphene oxide nanosheets. C[HA]initial = 10 mg/L, C[Co(II)]initial = 30 mg/L, C[Cd(II)]initial = 20 mg/L, I = 0.01 M NaClO4, T = 303K, m/V = 0.1 g/L.

pH ∼9.1 in the absence of graphene oxide nanosheets. However, more than 90% Cd(II) is adsorbed on graphene oxide nanosheets at pH 9.0. Thereby, it is impossible to form precipitation because of the very low concentration of Cd(II) remained in solution. The relative proportion of Co(II) species is calculated from the stability constants (log β1 = 4.3, log β2 = 8.4, and log β3 = 8.4) and the results demonstrate that Co(II) presents in the form of Co2+, Co(OH)+, Co(OH)20, and Co(OH)3 at various pH values (Figure S2). At pH < 8.0, the predominant Co(II) species is Co2+ and the removal of Co(II) is mainly accomplished by sorption reaction. The precipitation curve of Co(II) calculated from the precipitation constant of Co(OH)2(s) (Ksp = 2.50 1016) and the initial Co(II) concentration (i.e., 5.08 104 mol/L) is also shown in Figure 2B. Co(II) begins to form

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Figure 4. Sorption isotherms of Cd(II) (A) and Co(II) (B) on graphene oxide nanosheets at different temperatures. m/V = 0.1 g/L, pH = 6.0 ( 0.1, I = 0.01 M NaClO4. The solid lines are Langmuir model simulation, and the dashed lines are Freundlich model simulation.

precipitation at pH ∼8.2 in the absence of graphene oxide nanosheets. Therefore, at pH > 8.2, Co(II) sorption on graphene oxide nanosheets takes place partly though precipitation reaction. The sorptions of Cd(II) and Co(II) are weakly dependent on NaClO4 concentrations. The sorption curves shift to left at lower NaClO4 concentrations as compared to those at higher NaClO4 concentrations. This phenomenon can be attributed to the following: (1) The formed electrical double layer complexes between Cd(II)/Co(II) ions and graphene oxide nanosheets favor metal ion sorption when the concentration of NaClO4 is decreased. The sorption interactions between the functional groups and metal ions are mainly ionic interaction, which is in accordance with ion exchange mechanism. (2) The activity coefficient of metal ions is affected by NaClO4 concentrations, which then limits the 10458



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Table 1. Parameters for Langmuir and Freundlich Models of Cd(II) and Co(II) Sorption on Graphene Oxide Nanosheets Langmuir

Freundlich

Qmax (mg/g)

KL (L/mg)

R2

KF (mg1n 3 Ln/g)

n

R2

T = 303 K

106.3

0.180

0.999

35.83

0.27

0.944

T = 313 K

153.6

0.089

0.997

30.70

0.377

0.997

T = 333 K

167.5

0.104

0.998

36.22

0.367

0.995

T = 303 K

68.2

0.088

0.993

12.11

0.417

0.983

T = 313 K

69.4

0.133

0.999

20.90

0.283

0.968

T = 333 K

79.8

0.133

0.999

22.41

0.298

0.928

experimental conditions Cd(II), pH 6.0

Co(II), pH 6.0

metal ion transfer from solution to solid surfaces. (3) At high ionic strength, the electrostatic repulsion is cut down and the particle aggregation increases, which then reduces the available sites to bind metal ions on graphene oxide surfaces.21,32 The effect of ionic strength on Cd(II) and Co(II) sorption is more obvious at low pH as compared to that of ionic strength at high pH values. On the basis of the above theory, one can deduce that Cd(II) and Co(II) sorption on graphene oxide nanosheets is mainly attributed to outer-sphere surface complexation or ion exchange at low pH, and is attributed to inner-sphere surface complexation at high pH values.6,33 Effect of Humic Acid. Effect of HA on Cd(II) and Co(II) sorption on graphene oxide nanosheets is shown in Figure 3. The presence of HA reduces Cd(II) sorption on graphene oxide nanosheets. For Co(II) sorption, the presence of HA reduces Co(II) sorption at pH < 8, and at pH > 8 no obvious difference is found in the presence and absence of HA. For most materials, such as carbon nanotubes, clay minerals, and oxides, the presence of HA enhances metal ion sorption on solid phase at low pH values, and reduces metal ion sorption at high pH values.6,19,34,35 The increase of metal ion sorption at low pH is generally attributed to the strong complexation of metal ion with surface adsorbed HA on solid particles, whereas the decrease of metal ion sorption is interpreted by the formation of soluble MHA complexes in aqueous solution. Herein, the presence of HA decreases Cd(II) and Co(II) sorption on graphene oxide nanosheets at pH < 8, which may be attributed to the strong surface complexation and high surface site density of graphene oxide nanosheets. The surface site density of graphene oxide nanosheets is calculated to be 2.36 103 mol/g from the acidbase titration, whereas the surface site density of HA is 6.46 103 mol/g.19 The high surface site density of graphene oxide nanosheets assures the high sorption of Cd(II) and Co(II) ions on graphene oxide nanosheets. HA can be bound to graphene oxide nanosheets through strong ππ interactions. HA can interact with graphene oxide nanosheets in aquatic systems, thereby greatly changing their properties in such systems.6,36 Although the surface site density of graphene oxide nanosheets is lower than that of HA, the strong interaction of HA with graphene oxide nanosheets occupies parts of surface sites on graphene oxide nanosheets and also reduces the available binding sites of HA, and thereby results in the decrease of Cd(II) and Co(II) sorption on graphene oxide nanosheets. It is necessary to note that the effect of HA on Co(II) sorption at pH > 8 is not obvious, this is maybe attributed to the formation of Co(OH)2 precipitation at pH > 8.2.

Sorption Isotherms and Thermodynamic Data. Figure 4 shows Cd(II) and Co(II) sorption isotherms on graphene oxide nanosheets at three different temperatures. The experimental data are simulated with the Langmuir (Qs = Qs max 3 KL 3 Ce/1 + KL 3 Ce) and Freundlich (Qs = KF 3 Ce1/n) models, respectively (where Ce is the equilibrium concentration of metal ions in aqueous solution (mg/L), Qs is the amount of metal ions adsorbed on graphene oxide (mg/g), Qsmax is the maximum amount of metal ions adsorbed per unit weight of graphene oxide to form a complete monolayer coverage on the surface, KL represents enthalpy of sorption and should vary with temperature, and KF and n are the Freundlich constants related to the sorption capacity and sorption intensity, respectively. The relative parameters calculated from the two models are listed in Table 1. The sorption isotherms are fitted better by the Langmuir model than by the Freundlich model, suggesting that Cd(II) and Co(II) sorption on graphene oxide nanosheets are monolayer coverage. The Qsmax values of Cd(II) and Co(II) sorption on graphene oxide nanosheets are 68.2 and 106.3 mg/g, respectively. Comparing to Qsmax values of Cd(II) and Co(II) sorption on other sorbents, such as granular activated carbon (10.1 mg/g Cd(II) at pH 5 and T = 298 K),2 activated carbon fiber (13.6 mg/g Cd(II) at pH 5 and T = 298 K),2 activated carbon cloth (23.5 mg/g Cd(II) at pH 5 and T = 298 K),2 granular activated carbon oxide (30.8 mg/g Cd(II) at pH 5 and T = 298K),2 activated carbon fiber oxide (50.0 mg/g Cd(II) at pH 5 and T = 298 K),2 activated carbon cloth oxide (50.0 mg/g Cd(II) at pH 5 and T = 298 K),2 filtrasorb 400 (9.5 mg/g Cd(II) at pH 6 and T = 298 K),37 carbon aerogel (15.5 mg/g Cd(II) at pH 6 and T = 333 K),3 Indonesian peat (14.0 mg/g Cd(II) at pH 6 and T = 296 K),38 zeolite (14.4 mg/g Co(II) at pH 6 and T = 298 K),37 sepiolite (4.7 mg/g Co(II) at pH 7.8 and T = 293 K),38 Al-pillared bentonite (38.6 mg/g Co(II) at pH 6 and T = 303 K),39 lemon peel (25.6 mg/g Co(II) at pH 6 and T = 298 K),40 activated carbon (1.2 mg/g Co(II) at pH 6 and T = 303 K),41 one can see that the graphene oxide nanosheets have the highest sorption capacity of today’s materials. What’s more important, the abundant oxygen-containing functional groups on the surfaces of graphene oxide nanosheets make the adjacent oxygen atoms available to bind metal ions. From the XRD analysis, the c-axis spacing of graphene oxide nanosheets is ∼0.87 nm, which is large enough for the metal ions to enter into the interlayer space of graphene oxide nanosheets. Also, the graphene oxide nanosheets are the Lewis base and the metal ions are the Lewis acid.42 The delocalized π electron systems of graphene layer can act as Lewis base to form electron donor acceptor complexes with metal ions. Strong surface complexation 10459



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K0, can be calculated by plotting lnKd versus Ce (Figures S3 and S4) and extrapolating Ce to zero. The standard enthalpy change (ΔH0) and the standard entropy change (ΔS0) are calculated from the following equation: ln K 0 ¼

Figure 5. Linear plot of lnK0 vs 1/T for the sorption of Cd(II) (A) and Co(II) (B) on graphene oxide nanosheets at 303, 313, and 333 K. m/V = 0.1 g/L, pH = 6.0, I = 0.01 M NaClO4.

between the graphene oxide nanosheets and metal ions occurs through the Lewis acidbase interaction, which also contributes to metal ion sorption on graphene oxide nanosheets. The sorption isotherm is the highest at T = 333 K and is the lowest at T = 303 K, indicating that Cd(II) and Co(II) sorption on graphene oxide nanosheets are promoted at higher temperature. The thermodynamic parameters (ΔH0, ΔS0, and ΔG0) for Cd(II) and Co(II) sorption on graphene oxide nanosheets can be calculated from the temperature dependent sorption isotherms. The standard free energy change (ΔG0) can be calculated from the following equation: ΔG0 ¼ RTln K 0

ð3Þ

where R is the universal gas constant (8.314 J 3 mol1 3 K1), T is the temperature in Kelvin. The sorption equilibrium constant,

ΔS0 ΔH 0 R RT

ð4Þ

Linear plots of lnK0 vs 1/T for Cd(II) and Co(II) sorption on graphene oxide nanosheets are shown in Figure 5. The thermodynamic parameters are calculated from the plot of lnK0 vs 1/T using eqs 3 and 4. The positive value of ΔH0 (10.49 kJ 3 mol1) for Co(II) sorption indicates that Co(II) sorption on graphene oxide nanosheets is an endothermic process. The interpretation to the endothermicity of ΔH0 is that Co(II) ions are well solvated in aqueous solution. For Co(II) ions to adsorb on graphene oxide nanosheets, they have to have their hydration sheath denuded to some extent, and this dehydration process needs energy. The energy of dehydration exceeds the exothermicity of Co(II) ions to attach to graphene oxide nanosheets. The removal of water molecules from Co(II) ions is essentially an endothermic process, and the endothermicity of the desolvation process exceeds that of the enthalpy of Co(II) sorption.1 The positive ΔS0 value (103.1 J 3 mol1 3 K1) of Co(II) also indicates that the sorption process is spontaneous with high affinity. The negative ΔG0 values (20.73 kJ 3 mol1 at 303 K, 21.83 kJ 3 mol1 at 313 K, and 23.83 kJ 3 mol1 at 333 K) of Co(II) also indicate the spontaneous process of Co(II) sorption under the conditions applied. The decrease of ΔG0 with increasing temperature indicates more efficient sorption at higher temperatures. At higher temperature, Co(II) ions are readily dehydrated, and therefore the sorption becomes more favorable. The thermodynamic parameters reflect the affinity of graphene oxide nanosheets toward Co(II) ions in aqueous solutions and may suggest some structural changes in the sorbents.1,43 The thermodynamic parameters of Cd(II) sorption on graphene oxide nanosheets are ΔH0 = 7.39 kJ 3 mol1, ΔS0 = 100.1 J 3 mol1 3 K1, and ΔG0 = 2.97 kJ 3 mol1 at 303 K, 23.92 kJ 3 mol1 at 313 K and 25.97 kJ 3 mol1 at 333 K, respectively. The thermodynamic data of Cd(II) sorption on graphene oxide nanosheets also suggest that Cd(II) sorption is a spontaneous and endothermic process. The standard free energy change (ΔG0) values of Cd(II) sorption are more negative as compared to those of Co(II) sorption, suggesting that Cd(II) sorption on graphene oxide nanosheets is more spontaneous and easy than Co(II) sorption. The results are in good agreement with the pH-dependent sorption of Cd(II) and Co(II) as shown in Figure 2. At pH 6.0, about 50% Cd(II) is adsorbed on graphene oxide nanosheets, whereas only about 25% Co(II) is adsorbed from aqueous solution to graphene oxide nanosheets. From the literature query,2,3,3739 the prepared graphene oxide nanosheets have much higher sorption capacity for Cd(II) and Co(II) removal from aqueous solutions than any of today’s materials. This paper highlights the application of few-layered graphene oxide nanosheets as sorbents in environmental pollution management. The graphene oxide nanosheets may be suitable materials for ex situ environmental remediation of heavy metal ions. The solution pH and humic acid can affect the removal of Cd(II) and Co(II) ions from aqueous solution to graphene oxide nanosheets. Although the graphene oxide nanosheets are relatively more expensive than other natural materials and other carbon materials such as active carbon, carbon nanotubes, the graphene 10460


Environmental Science & Technology oxide nanosheets will be synthesized in large scale and at low price in the near future with the development of technology. The graphene oxide nanosheets will be very suitable materials for the preconcentration and solidification of heavy metal ions from large volumes of aqueous solutions in environmental pollution cleanup in the near future.

’ ASSOCIATED CONTENT

bS

Supporting Information. Preparation of graphene oxide nanosheets. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +86-551-5592788; fax: +86-551-5591310; e-mail: xkwang@ ipp.ac.cn.

’ ACKNOWLEDGMENT Financial support from 973 projects of MOST (2011CB933700), NSFC (20971126, 21071147, 21071107, 91126020), and open foundation of State Key Lab of Pollution Control and Resource Reuse is acknowledged. Thanks are also extended to Prof. D.A. Dzombak and the anonymous reviewers for their helpful comments to improve the quality of our manuscript. ’ REFERENCES (1) Chen, C. L.; Wang, X. K. Adsorption of Ni(II) from aqueous solution using oxidized multi-walled carbon nanotubes. Ind. Eng. Chem. Res. 2006, 45, 9144–9149. lvarez-Merino, M. A.; Lopez-Ramon, (2) Moreno-Castilla, C.; A M. V.; Rivera-Utrilla, J. Cadmium ion adsorption on different carbon adsorbents from aqueous solutions. Effect of surface chemistry, pore texture, ionic strength and dissolved natural organic matter. Langmuir 2004, 20, 8142–8148. (3) Goel, J.; Kadirvelu, K.; Rajagopal, C.; Garg, V. K. Cadmium(II) uptake from aqueous solution by adsorption onto carbon aerogel using a response surface methodological approach. Ind. Eng. Chem. Res. 2006, 45, 6531–6537. (4) Rengaraj, S.; Moon, S. H. Kinetics of adsorption of Co(II) removal from water and wastewater by ion exchange resins. Water Res. 2002, 36, 1783–1793. (5) Matlock, M. M.; Howerton, B. S.; Atwood, D. A. Chemical precipitation of lead from lead battery recycling plant wastewater. Ind. Eng. Chem. Res. 2002, 41, 1579–1582. (6) Yang, S. B.; Hu, J.; Chen, C. L.; Shao, D. D.; Wang, X. K. Mutual effect of Pb(II) and humic acid adsorption onto multiwalled carbon nanotubes/poly(acrylamide) composites from aqueous solution. Environ. Sci. Technol. 2011, 45, 3621–3627. (7) Fonseca, B.; Figueiredo, H.; Rodrigues, J.; Queiroz, A.; Tavares, T. Mobility of Cr, Pb, Cd, Cu and Zn in a loamy sand soil: A comparative study. Geoderma 2011, 164, 232–237. (8) Tan, X. L.; Fang, M.; Chen, C. L.; Yu, S. M.; Wang, X. K. Counterion effects of Ni2+ and sodium dodecylbenzene sulfonate adsorption to multiwalled carbon nanotubes in aqueous solution. Carbon 2008, 46, 1741–1750. (9) Tan, X. L.; Fan, Q. H.; Wang, X. K.; Grambow, B. Eu(III) sorption to TiO2 (anatase and rutile): Batch, XPS, and EXAFS study. Environ. Sci. Technol. 2009, 43, 3115–3121. (10) Long, R. Q.; Yang, R. T. Carbon nanotubes as superior sorbent for dioxin removal. J. Am. Chem. Soc. 2001, 123, 2058–2059.

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