Article pubs.acs.org/JPCC
Green One-Step Approach to Prepare Sulfur/Reduced Graphene Oxide Nanohybrid for Effective Mercury Ions Removal Suman Thakur,† Gautam Das,‡ Prasanta Kumar Raul,§ and Niranjan Karak†,* †
Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Tezpur-784028, India Department of Material Science and Engineering, Yonsei University, Seoul, South Korea § Defence Research Laboratory, Post Bag No. 2, Tezpur-784001, India ‡
S Supporting Information *
ABSTRACT: A one-step, eco-friendly strategy is developed to prepare sulfur/reduced graphene oxide nanohybrid (SRGO) (sulfur nanoparticles of average size ∼20 nm) using the combined effect of the polyphenolic compounds and acids present in Citrus limon juice. The prepared nanohybrid is characterized with Fourier transform infrared, Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. The detailed plausible mechanism of the simultaneous reduction of graphene oxide and formation of sulfur nanoparticles is also established. The nanohybrid demonstrated a fast and efficient Hg2+ removal at around pH 6−8. The adsorption kinetics followed pseudosecond-order and the isotherm could be well described by the Langmuir model. The thermodynamic parameters (ΔG0, ΔS0, and ΔH0) are calculated from the temperature-dependent isotherms and indicate that the adsorption process is endothermic and spontaneous. The nanohybrid also has excellent reusability and high selectivity of Hg2+. Therefore, this nanohybrid has use as a promising toxic Hg2+ adsorbent.
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INTRODUCTION Mercury, one of the most poisonous metals, has drawn much attention due to its toxicity and adverse impact on the environment. It has been reported that Hg2+ causes permanent harmful effects to living organisms even at minimal doses.1 Conventional techniques used for removal of aqueous mercury are precipitation, coagulation, reduction, amalgamation, membrane separation, ion exchange, adsorption, and so forth.2 Among these, adsorption is one of the most effective methods owing to its cost-effectiveness and ease of operational setup.3 A large number of adsorbents such as activated carbons, silicates, iron oxides, polymers, and biomass have been studied for Hg2+ removal.4−7 However, advances in nanoscience and engineering are providing new opportunities to develop more cost-effective and environmentally acceptable absorbents and adsorption processes. Nanomaterials with high aspect ratios are particularly attractive for this application. In this context, graphene, a twodimensional carbon nanomaterial is highly efficient to detect and remove toxic elements/gases from contaminated systems.8 It is reported in the literature that graphene or its nanohybrids are used to remove heavy metal ions like arsenic, chromium, and so forth.9,10 Moreover, it is pertinent to mention that materials which carry sulfur, nitrogen, and oxygen-containing functional groups as major binding sites are effective for mercury removal.11 Among the above-mentioned materials, sulfur-containing groups bind more effectively due to soft−soft © 2013 American Chemical Society
interaction with mercury ion. Therefore, it can be presumed that a sulfur nanoparticle decorated graphene nanohybrid may be an apt option for efficient removal of mercury ion. A variety of methods are employed for preparation of sulfurgraphene nanohybrids. For example, (i) sulfur can be precipitated on an aqueous suspension of graphene through a disproportionation reaction of thiosulfate solution in the presence of acid, (ii) sulfur nanoparticles can be dispersed in graphene oxide (GO) sheet followed by reduction of the GO using reducing agent such as hydrazine, and (iii) melted sulfur can also be deposited on graphene to prepare this nanohybrid.12−14 However, these conventional methods are multistep as well as environmentally hazardous, which limits their use. On the contrary, a one-step synthetic technique is in accordance to the tenets of green chemistry as it minimizes the energy, resources, and time. Therefore, a greener and one step preparative technique will be more appreciable for the simultaneous reduction of GO and formation of sulfur nanoparticles. It is reported that weak organic acids are more efficient to form finer sulfur nanoparticles by disproportionation reaction of thiosulfate compared to inorganic strong acid.15 In this context, lemon (Citrus limon) juice contains a high amount of Received: January 8, 2013 Revised: February 14, 2013 Published: March 25, 2013 7636
dx.doi.org/10.1021/jp400221k | J. Phys. Chem. C 2013, 117, 7636−7642
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Scheme 1. Plausible Mechanism of Simultaneous Reduction of GO and Formation of Sulfur Nanoparticles
Preparation of Graphene Oxide (GO). GO was prepared by oxidizing the graphite flakes using a mixture of concentrated sulfuric acid and KMnO4 based on modified Hummers method.17 Briefly, a 2-g portion of graphite flakes was stirred in 35 mL 98% H2SO4 for 2 h. Then 6 g of KMnO4 was gradually added to the above solution below 20 °C. The mixture was then stirred at 35 °C for 2 h in an oil-bath. The resulting solution was diluted by adding 90 mL of water under vigorous stirring, a dark brown suspension was obtained, which was stirred continuously for another 1 h. The suspension was treated again by adding 30% H2O2 solution dropwise until the color of the solution became bright yellow. The resulting GO suspension was washed by repeated centrifugation, first with 5% HCl aqueous solution to remove excess of manganese salt and then with Millipore water until the pH of the solution became neutral. The purified GO was finally dispersed in water (0.5 mg/mL) and ultrasonically exfoliated in an ultrasonic bath. The dispersion was found to be stable for a long time. Preparation of Sulfur Nanoparticle Decorated Reduced Graphene Oxide (SRGO). In this experiment, 64 mg of Na2S2O3 was dissolved in 50 mL Millipore water to prepare a thiosulfate solution. Then 35 mg GO was added to the solution, which was sonicated for 1 h to yield a GO dispersed thiosulfate solution. Then 10 mL of Citrus limon juice was added into this
weak organic acids such as citric and ascorbic acids which assist to the disproportionation reaction of thiosulfate.16 In addition, the reduction of GO may be envisaged in parallel lines due to the combined effects of polyphenolic compounds as well as ascorbic acid, which abound in lemon juice, and thiosulfate in the reaction mixture. So, lemon juice is a good choice for achieving a double reward in the same pot. Here, we report an eco-friendly, one-step approach to prepare sulfur nanoparticles decorated reduced graphene oxide (SRGO) using lemon juice. The prepared SRGO was also used as an effective adsorbent for Hg2+ removal from water, and the Langmuir and Freundlich isotherm models were used to examine its adsorption isotherm.
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EXPERIMENTAL SECTION
Material. Graphite flakes (60 meshes, 99% pure) were obtained from Loba Chemie, Kolkata, India. Sodium thiosulfate, mercuric acetate, concentrated sulfuric acid (98%), hydrogen peroxide (H2O2, 30%), and concentrated hydrochloric acid were purchased from Merck, India and used as received. Potassium permanganate (KMnO4) was obtained from Analytical Rasayan, India, and used as an oxidizing agent to prepared graphene oxide. Citrus limon was collected from the local area. 7637
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Figure 1. (a) FTIR spectra of (i) GO and (ii) SRGO, (b) Raman spectra of (i) GO and (ii) SRGO, (c) XRD patterns of (i) graphite, (ii) GO (iii) sulfur nanoparticles and (iv) SRGO, (d) TGA (black lines) and DTG (red lines) thermograms of (i) GO and (ii) SRGO.
spectrophotometer using KBr pellets. UV-spectra were analyzed in Hitachi (U-2001, Tokyo, Japan) UV spectrophotometer. The X-ray diffraction (XRD) study was carried out at room temperature (ca. 25 °C) by a Rigaku X-ray diffractometer (Miniflex, UK) over a range of 2θ = 10−70°. Thermogravimetric analysis (TGA) was done by a thermal analyzer, TGA50 (Simadzu, Japan) with a nitrogen flow rate of 30 mL/min at heating rate of 10 °C/min. The surface morphology was studied by a JEOL scanning electron microscope (SEM) of model JSM-6390LV after the platinum was coated on the surface. Transmission electron microscope (TEM) analysis was performed with JEOL 2100X electron microscope at operating voltage of 200 kV. The mercury concentrations prior to and after adsorption were determined by an inductively coupled plasma-emission spectrometer (ICP-ES).
solution in an acidic environment, wherein sodium thiosulphate underwent a disproportionation reaction to form sulfur and sulfur dioxide. After the reactants were mixed, 30 min of equilibrium time was allowed for the completion of the reaction. After equilibration, the sample was sonicated in a bath for 5 min. The resulting suspension was washed by repeated centrifugation with Millipore water and acetone. Then it was dried in a vacuum oven at 40 °C. Batch Adsorption Experiments. Mercury acetate was used as the mercury source. A stock solution was prepared by dissolving 88 mg salt into 100 mL Millipore water. Then the adsorbent, SRGO, was dispersed into the Hg2+ solution. After being stirred for 3 h, the solution was filtered for toxic metal ion analysis. Mercury ion removal efficiency (E) was calculated by measuring the mercury concentration before and after adsorption, respectively. Four mercury adsorption experiments were carried out with different amounts of adsorbent (SRGO), different metal ion concentrations, different pH, and variation of time. The adsorbing capacities of Hg2+ were calculated using the equation,
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RESULTS AND DISCUSSION In this one-step preparation of SRGO, sodium thiosulfate solution and GO aqueous dispersion were mixed well before addition of lemon juice. The basic sodium thiosulfate can undergo a redox reaction with the graphene oxide to form a slightly more graphitic carbon than GO and initiate the formation of sulfur nanoparticles.18 These nucleating sites grow further to form sulfur nanoparticles by disproportionation reaction of the remaining thiosulfate. As lemon juice mainly contains citric and ascorbic acids, which are responsible for the formation of sulfur nanoparticles through a disproportionation reaction, as shown in first part of Scheme 1 (inset). The plausible mechanisms of the simultaneous reduction of the GO and formation of sulfur nanoparticles are shown in Scheme 1. During the disproportionation reaction of thiosulfate, sulfur
Q e = (C0 − Ce) × V /m
where C0 and Ce are the initial and equilibrium concentrations of Hg2+ (mg L−1), V is the volume of the solution (L), and m is the mass of SRGO (mg). A detailed study method of adsorption kinetics, isotherms, and thermodynamics parameters are given in the Supporting Information, SI. Characterization. Fourier transform infrared spectroscopy (FT-IR) was performed over the wavenumber range of 4000− 400 cm−1 by a Nicolet (Madison, WI) FT-IR impact 410 7638
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and do not restack upon the decoration of sulfur nanoparticles. Therefore, in the diffraction pattern of SRGO, peaks for sulfur nanoparticles were only found. Further, the amount of sulfur is sufficiently high, which may decorate the surface of the reduced GO almost completely. TGA thermograms of GO and SRGO are shown in Figure 1d. GO exhibits a two-step degradation where the first step began at 202 °C due to the loss of hydroxyl, carbonyl functional groups and remaining water molecules. The second step degradation (550−650 °C) involves the pyrolysis of the remaining oxygen-containing groups as well as the breakdown of ring carbon.20 In the thermogram of SRGO, an almost similar two-step degradation was observed. But in the first step, the maximum degradation occurred at around 200−240 °C due to the loss of sulfur and some remaining oxygenated groups. The morphology of SRGO was inspected by transmission electron microscopy. From the TEM micrograph, it is clearly revealed that sulfur nanoparticles (average particle size, 23 nm) are uniformly distributed on the surface of graphene sheets (Figure 2a). The SAED pattern indicated that the sulfur
dioxide (SO2) was formed along with sulfur nanoparticles. This SO2 transforms to HSO3− and H+ in the aqueous medium. The generated H+ helps to further the disproportionation reaction of thiosulfate and HSO3− helps to reduce the GO sheet by following the deoxygenation mechanism as shown in path 1 of Scheme 1. During this reduction, HSO3− is oxidized to SO3 (as shown in Scheme 1), which formed sulfuric acid by reaction with water.19 The H+ generated from H2SO4 further helps to advance the above disproportionation reaction. The reduction of GO is also accompanied by the polyphenolic compounds of lemon juice and the conjugated base of ascorbic acid, as shown in path 2 of Scheme 1.20 As all of the above-mentioned reactions were occurring simultaneously, the overall time for the formation of SRGO was minimal (30 min). FTIR spectra of graphite, GO, and SRGO are shown in Figure 1a. The presence of strong bands at 1720 cm−1 (for C O stretching), 1204 cm−1 (for C−O−C stretching), 1049 cm−1 (for C−O stretching) and a broad band at around 3400 cm−1 for hydroxyl group clearly suggested the existence of a variety of oxygenated groups such as carbonyl, carboxyl, and hydroxyl groups in GO.21 In SRGO, the absence of a carbonyl band at 1720 cm−1 clearly indicated deoxygenation of such groups. Also in the spectrum of SRGO, C−H band stretching was found at 2919 and 2842 cm−1, which suggested that some polyphenolic compounds are present in the surface of nanohybrids. In addition to that weak evidence of the existence of C−O−C bonds at about 1200 cm−1 in the SRGO was also noticed, which may be due to the presence of some polyphenolic compounds or partially existence with C−O bonds at the edges of SRGO even after the reduction. Raman spectroscopy is well-known as a valuable tool to study the ordered/disordered crystal structures of carbonaceous materials. The well-known characteristics of Raman spectra of carbon materials are D and G bands (typically located at∼1350 and 1580 cm−1), which are usually assigned to the local defects/ disorders (especially located at the edges of graphitic and graphene platelets) and the sp2 graphitic structure, respectively. Raman spectra of GO and SRGO were taken to evaluate the nature of the carbon and sulfur in them (Figure 1b). In the spectrum of GO, the characteristic bands were found at 1322 and 1584 cm−1 for the D and G bands, respectively.20 After the reduction of GO, the intensity ratio of D-band to G-band (ID/ IG) increased from 0.98 to 1.21 in SRGO spectrum. The presence of characteristic band due to the A1 symmetry mode of the sulfur−sulfur bond at 520 cm−1 and the increase of the ID/IG ratio in SRGO clearly indicated the decoration of sulfur nanoparticles on the graphene sheets and the extent of reduction, respectively.22 The XRD patterns of graphite, graphene oxide, sulfur, and SRGO are shown in Figure 1c. After the oxidation of natural graphite by modified Hummers’ method, the (002) reflection peak shifts to the lower angle at 2θ = 10.1° (d-spacing = 8.75 Å) from the 2θ = 26.6° (d-spacing = 3.35 Å) which indicated the formation of oxygen-containing functional groups between the layers of the graphite and presence of intercalated water molecules.23,24 As a result, the space between the layers as well as d-spacing increased. In the diffraction pattern of the sulfur, the presence of peaks at 2θ = 23°, 25.8°, 27.7°, and 31.39° for the (111), (004), (103), and (113) planes, respectively, clearly reflected that the sulfur nanoparticles were in anorthic form (JCPDS#89−6764). It is noteworthy that no peak for graphite or graphene oxide appeared in the SRGO pattern, which proves that the graphene sheets are in a substantially exfoliated state
Figure 2. TEM images (a) at low magnification (scale bar = 200 nm) showing sulfur nanoparticle distribution and inset shows semi crystalline SAED patterns of SRGO and (b) at high magnification (scale bar = 10 nm) showing the presence of the lattice planes and distance of lattice plane is 28 nm (shown by red line in the inset).
nanoparticles are semi-crystalline in nature (inset of Figure 2a). The crystal lattice fringes with d-spacing of 0.28 nm can be assigned to the (113) plane of the sulfur nanoparticles, which is consistent with the XRD results (Figure 2b). In order to evaluate Hg2+ removal efficiency of SRGO, the kinetics of the adsorption process was investigated. The effect of exposure time on the adsorption of Hg2+ was carried out at neutral pH under ambient conditions. Figure 3a shows that the initial adsorption rate of Hg2+ was high. About 90% of Hg2+ removal was observed during the initial period of 15 min. Then there was a gradual diminution in the rate of removal leading to an equilibrium condition, which was achieved within 30 min with almost complete Hg2+ removal. This rapid adsorption might be attributed to two factors: (1) a strong interaction between Hg2+ ions and the sulfur nanoparticle present in the surface of graphene sheets and (2) an electrostatic attraction between the free exposed surface of the adsorbent and the metal ions.3 To illustrate the adsorption kinetics, four kinetic models, viz., pseudofirst-order, pseudosecond-order, Elovich, and intraparticle diffusion model were employed. The results obtained by fitting experimental data of the adsorption in these models are summarized in Table 1. Calculated and observed Qe values for different adsorbate concentrations are compared and found that pseudo second order kinetics is more suitable (Table S2 of the SI). Also, this model provides an excellent correlation for 7639
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Figure 3. Hg2+ removal efficiency of SRGO (a) with variation of time and (b) at different pH.
Table 1. Kinetic Parameters and Corelation Factors of Hg2+ Adsorption on SRGO Obtained from Different Models models
parameters
R2
pseudofirst-order pseudosecond-order Elovich intraparticle diffusion
K = 0.1354, Qe = 9.6 K = 0.0583, Qe= 14.083 α = 0.9797, β = 0.9756 Kdif = 1.6173, C = 0.11854
0.9823 0.9994 0.9801 0.9569
the adsorption of Hg2+ on SRGO with the highest correlation coefficient of R2 = 0.999. The slopes and the intercepts of each linear plot are used to calculate the kinetic parameters for adsorption of Hg2+ (Table 1). Solution pH, one of the most important variables, plays a vital role in the adsorption of metal ions by changing the surface charge density on both the adsorbent and adsorbate. The adsorption properties of Hg2+ on SRGO are dependent on pH (Figure 3b). The maximum adsorption by SRGO was observed at pH >6 (Figure 3b). Mercury ions exist as Hg2+ in a solution of pH 6, and both of these species along with Hg(OH)+ at pH 3−6.23 The small decrease of adsorption observed at acidic pH (
