Synthesis, characterization, and environmental implications of graphene-coated biochar

Science of the Total Environment 435–436 (2012) 567–572

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Short Communication

Synthesis, characterization, and environmental implications of graphene-coated biochar Ming Zhang a, Bin Gao a,⁎, Ying Yao a, Yingwen Xue a, b, Mandu Inyang a a b

Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL 32611, United States School of Civil Engineering, Wuhan University, Wuhan, Hubei 430072, China

H I G H L I G H T S ► A new synthesis method for graphene-wrapped biochar was developed. ► Graphene sheets were “soldered” by pyrene molecules on biochar surface. ► Graphene coating improved biochar thermal stability and sorption ability.

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Article history: Received 29 May 2012 Received in revised form 6 July 2012 Accepted 7 July 2012 Available online 18 August 2012 Keywords: Engineered biochar Graphene Adsorption Thermal stability Aromatic compound Low-cost adsorbent

a b s t r a c t Biochar has attracted much research attention recently because of its potential applications in many environmental areas. In this work, the biochar technology was combined with the emerging graphene technology to create a new engineered graphene-coated biochar from cotton wood. The biomass feedstock was first treated with graphene/pyrene‐derivative and was then annealed at 600 °C in a quartz tube furnace under N2 environment. Laboratory characterization with different microscopy and spectrometry tools showed that the graphene sheets were “soldered” by the pyrene molecules on the biochar surface during the annealing process. Thermogravimetric analysis showed that the graphene “skin” could improve the thermal stability of the biochar, making the engineered biochar a better carbon sequester for large scale land applications. Batch sorption experimental results indicated that the graphene-coated biochar has excellent adsorption ability of polycyclic aromatic hydrocarbons (PAHs) with a maximum methylene blue adsorption capacity of 174 mg g−1, which is more than 20 times higher than that of the unmodified cotton wood biochar and comparable to those of some physically or chemically activated carbons. The enhanced adsorption of methylene blue on the graphene-coated biochar is mainly controlled by the strong π–π interactions between aromatic molecules and the graphene sheets on biochar surface. It is anticipated that this novel, facile, and low-cost method can be expanded to other carbon-rich materials to create engineered biochar for various environmental applications. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Graphene, one-atom-thick sheet of carbon, has attracted vast interests from various perspectives because of its unique properties, such as high electrical and thermal conductivity, massless transportation properties, and strong mechanical properties (Allen et al., 2010; Choi et al., 2010; Hu et al., 2012; Novoselov et al., 2007). It has been applied in many areas including development of ultrafast electronic devices, molecular resolution sensors, biodevices, polymer composites, liquid-crystal devices, electromechanical systems, economic and efficient energy-related devices, and magnetoresistive/quantum Hall devices (Dutta and Pati, 2010; Guo and Dong, 2011; Shao et al.,

⁎ Corresponding author. Tel.: +1 352 392 1864x285. E-mail address: bg55@ufl.edu (B. Gao). 0048-9697/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2012.07.038

2010). To our knowledge, however, little research has been done for explore the environmental applications of graphene or its derivatives. Meanwhile, fullerenes (e.g., C60, C70, and C80) and carbon nanotubes, which are structurally composed of stacked graphene sheets of linked hexagonal rings, exhibit huge potential in many environmental applications (Silva et al., 2012a, 2012c). For example, Vecitis et al. (2011a, 2011b) reported that carbon nanotubes are effective for the removal and electro‐oxidation of aqueous chemicals and microorganisms. Fu et al. (2008) found that C60 particles can enhance the photocatalytic activity of ZnO to degrade organic dye. Several recent studies have also demonstrated that functionalized carbon nanotubes have a strong affiliation to both heavy metal and organic pollutants in water (Kumar and Wang, 2009; Peng et al., 2009; Rao et al., 2007). Large-scale environmental applications of fullerenes, however, still face many challenges, including relatively high cost and inaccessibility. Graphene exhibits similar physical and chemical performances that fullerenes

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do, but with additional advantage of being potentially much less expensive and having wider availability. In addition, several studies have indicated that graphene can be used as a wrapping/coating agent to create novel nanocomposites with enhanced functions for various applications (Chen et al., 2011; Ju et al., 2011; Mattevi et al., 2011). In this way, graphene offers greater potential than fullerenes to be applied in large-scale environmental applications, such as environmental remediations and water treatment. Biochar is a stable solid, rich in carbon, and can sequester carbon in soils for thousands of years (Glaser et al., 2009). When applied to soils, biochar may also improve soil fertility, raise agricultural productivity, increase soil nutrients and water holding capacity, and reduce emissions of other greenhouse gasses (Beesley et al., 2011; Lehmann, 2007; Lehmann et al., 2006; Sohi et al., 2010). Moreover, recent studies also find biochar produced from different types of biomass residues can be used as a low-cost sorbent to remove various contaminants from water (Cao et al., 2009; Inyang et al., 2011; Yao et al., 2012). Because of the abundance of feedstock materials biochar is becoming a promising agent for large-scale environmental applications. Engineered methods that combine traditional biochar technology with other emerging technologies, such as biotechnology and nanotechnology, have also been promoted to create biochar-based materials (i.e., engineered biochars) with enhanced functions for environmental applications (Inyang et al., 2010, 2012; Selvakumar et al., 2010; Yao et al., 2011a, 2011b; D. Zhang et al., 2010). To take advantage of the recent developments in graphene and biochar technologies, a new engineered biochar (i.e., graphene-coated biochar) was produced in laboratory through slow pyrolysis of milled cotton wood treated with pyrene dispersed graphene sheets (Fig. 1). Previous studies have demonstrated that the hydrophilic pyrene molecules can act not only as a surfactant to disperse and stabilize individual graphite sheet in suspension, but also as electrical “glue”, soldering graphene sheets on solid surfaces during annealing processes (Su et al., 2009; M. Zhang et al., 2010). Characterization experiment was conducted to determine physicochemical properties of the graphenecoated biochar. Batch sorption experiment was also carried out to determine sorption ability of the graphene-coated biochar to aqueous methylene blue, which is commonly used to evaluate the sorption ability of carbon materials to aromatic compounds (Hameed et al., 2007). The overarching objective of this work was to develop a facile and low-cost method to produce graphene-coated biochar for environmental applications. 2. Materials and methods 2.1. Materials Synthetic graphite powder (b 20 mm particle size), methylene blue (C16H18ClN3S, molecular weight, 319.9 g/mol), and 1, 3, 6, 8‐ pyrenetetrasulfonic acid (Py–SO3) tetrasodium salt hydrate from Sigma‐Aldrich were purchased and used as received. Dry cotton wood (CW) was obtained locally from Gainesville, FL and milled into powders of ~2 mm prior to use.

2.2. Preparation of graphene suspension Stable graphene suspension was prepared from the synthetic graphite powder using a method similar to that of M. Zhang et al. (2010). A stock solution of Py–SO3 with a concentration of 0.2 mg/mL was first prepared in deionized (DI) water by vigorous stirring the solution for 1 h. Graphite powder was then added into the Py–SO3 solutions with a weight ratio of 2:1 between the Py–SO3 and the graphite powder. Direct exfoliation of graphite to graphene sheets was made through bath sonication of the mixture for 1 h with Misonix S3000 ultrasonicator (QSonica, Newtown, CT). The obtained gray colored graphene suspension was used directly to prepare the graphene-coated biochar by annealing. 2.3. Preparation of biochar Graphene sheets were first coated on the biomass feedstock (i.e., milled cotton wood) using a dip coating procedure similar to that of Schoen et al. (2010). Herein, 10 g of the cotton wood was dipped into the graphene suspension and was then oven dried at 80 °C. The graphene treated feedstock was placed in a quartz tube inside a tube furnace (MTI, Richmond, CA) to produce the graphene-coated biochar through slow pyrolysis (annealing) in a N2 environment at temperatures of 600 °C for one hour. Untreated cotton wood was also used as feedstock to produce biochar without graphene coating in the furnace with the same pyrolysis conditions. The resulted biochar sample was washed with DI water for several times to remove impurities, oven dried, and sealed in a container for further testing. The untreated and graphene-coated biochar samples were referred as BC and GCBC, respectively. 2.4. Characterizations A Cary-Eclipse fluorescence spectrophotometer (Varian, Inc, Palo Alto, CA) was used to monitor the exfoliation process of graphite dispersed in the Py–SO3 solution by comparing the emission spectra (excited at 340 nm) at the beginning and end of the exfoliation period. The exfoliated graphene sheet suspension was pipetted onto a mica sheet and dried. These sheets were then characterized with a tapping mode Nanoscope IIIa atomic force microscopy (AFM) (Veeco instrument, Santa Barbara, CA). The AFM was operated in a tapping mode with silicon nitride cantilever tips and the area of scan was on the middle of mica sheet where the graphene suspension was applied. Pore structures and surface elemental compositions of the GCBC were determined with a field emission gun scanning electron microscopy (FEG-SEM, JEOL 6335F) equipped with an energy-dispersive X-ray analyzer. X-ray diffraction (XRD) analysis was also carried out to identify any crystallographic structure in the samples using a computer-controlled X-ray diffractometer (Philips Electronic Instruments) equipped with a stepping motor and graphite crystal monochromator. Thermogravimetric analysis (TGA) of the biochar samples was carried out in a stream of air at a heating rate of 10 °C/min with a Mettler TGA/DSC1 analyzer (Columbus, OH).

Fig. 1. Illustration of procedures to produce graphene-coated biochar.

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2.5. Sorption of methylene blue A stock solution of methylene blue (1000 ppm) was prepared by dissolving it in DI water. Batch sorption experiment was carried out at room temperature (22 ± 0.5 °C) in 68 mL digestion vessels (Environmental Express) containing about 0.1 g of sorbent (i.e., BC or GCBC) and 50 mL methylene blue solution with concentration ranging from 5 to 1000 ppm. After being shaken at 60 rpm in a mechanical shaker for 24 h, the vials were withdrawn and the mixtures were filtered through 0.22 μm pore size nylon membrane filters (GE cellulose nylon membrane). The methylene blue concentrations of the liquid phase samples were then determined by measuring light absorbance at wavelength of 665 nm with aid of a UV–vis spectrophotometer (Thermo Scientific EVO 60). The amount of methylene blue sorbed on the biochar samples was calculated based on the difference between initial and final aqueous concentrations. All the experimental treatments were performed in duplicate and the average values were reported. Additional analyses were conducted whenever two measurements showed a difference larger than 5%. 3. Results and discussion 3.1. Graphene AFM analysis showed the presence of the pyrene-treated graphene sheets on the mica surface and the size of the graphene patches was in the micrometer range (Fig. 2a). Although a graphene sheet is thin, the AFM could easily characterize the morphological feature of the graphene patches. Cross-sectional images of the AFM also revealed that the thickness of a single-layer graphene/pyrene hybrid on the

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mica surface ranged from 0.5 to 1.3 nm with an average of 0.9 ± 0.4 nm (Fig. 2b). The theoretical thickness of a graphene sheet should be around 0.34 nm; however, its actual AFM measurement could be higher because of not only the differences in tip attraction/repulsion between the insulating substrate and semimetallic graphene but also preferential adsorption of a thin layer of water on graphene under ambient conditions (Allen et al., 2010). In addition, the large graphene thicknesses of this work could be attributed to the possible inhomogeneous coverage of Py–SO3 molecules on the graphene surface or the AFM system noises (M. Zhang et al., 2010). AFM image also showed some holes (diameter 2–500 nm) randomly arranged on the graphene sheets (Fig. 2a). These were probably caused by high power sonication or by incomplete healing. In summary, the AFM analysis demonstrated that the Py–SO3 dispersion method can be used to produce graphene sheets from graphite powders. Comparison of the fluorescence spectra of the pyrene dispersed graphite solution at the beginning and end of the exfoliation period confirmed the formation of the graphene pyrene hybrid (Fig. 3). Pyrene is a fluorophore that has four rings with a special spectral property to form an excited dimer (i.e., excimer), when two monomers are in close proximity (within about 1–2 Å). Prior to sonication, the fluorescence spectrum of the suspension showed a large peak at 500 nm, which could be attributed to the excimer emission of pyrene derivatives (Fig. 3) (Sposito, 1989; Steiner et al., 2009). At the end of the exfoliation period, this peak shifted to a wavelength of 374 nm and its intensity decreased significantly (Fig. 3). This fluorescence behavior is virtually the same as that of pure pyrene solution when its concentration is below the critical micelle concentration, suggesting that the fluorescence spectrum of the graphene/pyrene solution arose from the non-bound (free) pyrene monomers. These results further confirmed the exfoliation and stabilization effect of pyrene on the graphene sheets in the suspension. 3.2. Graphene-coated biochar XRD analysis of the GCBC displayed a (002) diffraction peak at around 23°, corresponding to a layer-to-layer distance (d-spacing) of about 0.39 nm (Fig. 4). This d-spacing of the film on the GCBC surface was only slightly bigger than, but quite close to, the theoretical values (i.e., 0.34 nm) of the thickness of a pristine graphene sheet, indicating that the GCBC was coated with a single layer of pristine graphene. The slightly increased d-spacing of this graphene layer could be ascribed to the presence of a small amount of residual oxygen-containing functional groups or other structural defects that

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Fig. 3. Fluorescence spectra of the pyrene/graphite suspension at the beginning (0 h) and the end (1 h) of the exfoliation process.

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were formed during the annealing by the slow pyrolysis (M. Zhang et al., 2010). Unfortunately, we could not quantify the amount of graphene on the biochar surfaces because both of them are carbon materials. The SEM images of GCBC showed that graphene sheets were grown up on the biochar surface uniformly and smoothly so that the char kept the original morphological structure (Fig. 5a and b). This indicates that pyrene is a good annealing agent in “gluing” the graphene sheets to form a seamless membrane on the biochar surface. Similar phenomena were observed for carbon nanotubes and

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coal secondary products (Oliveira et al., 2012; Quispe et al., 2012; Ribeiro et al., 2010; Silva et al., 2012b). Fig. 5b shows the crosssection of a broken structure where the graphene sheets can be seen on the circumference (pointed by the arrows in Fig. 5b). The EDS spectrum of the GCBC surface indicated that almost all of the elements on the surface were carbon, confirming that the graphene sheets were ‘soldered’ by the fusion of graphene sheets and Py–SO3 on the biochar surface. In addition, it also suggested the Py–SO3 molecules were completely consumed by the slow pyrolysis during the annealing process. The EDS results also showed a small peak of oxygen, which could come from few residual oxygen-containing functional groups on graphene surface or from the biochar inside the graphene sheets because EDS usually has a penetration depth of about 100 nm at the surface of the samples. A remarkable property of graphene is its high thermal stability, which was demonstrated in the TGA of the biochar samples (Fig. 6). The TGA indicated that the GCBC had a much better thermal stability than the BC. In general, thermal degradation of a char samples could be divided into three stages. The first stage is in the range of 50–100 °C, reflecting loss of surface water. The second degradation stage was in the range of 100–350 °C, where the surface functional groups degrade. Once the temperature is higher than 350 °C, the final stage starts and the carbon skeletons begin to disappear. Both biochar samples were stable in the first two stages, indicating they contain little water or surface functional groups. Compared with the BC, the GCBC showed better thermal stability in the third stage. The gradation temperature of the GCBC was about 64 °C higher than that of the BC sample when 50 wt.% losses was used as a point of comparison, indicating graphene coating can protect biochar from thermal degradation. Because land application of biochar is often

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Fig. 5. SEM-EDS analysis of the graphene-coated biochar: (a) morphological structure, (b) cross section; and (c) EDS spectrum. The arrows denote the graphene sheets around the biochar and the EDS spectrum was obtained at the same location of (b).

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suggested to be an effective way for carbon sequestration, the enhanced thermal stability of the GCBC make it a better carbon sink for large scale applications for mitigating global warming. 3.3. Sorption of methylene blue Compared to the BC, the GCBC also demonstrated enhanced sorption ability to aqueous methylene blue. The isotherm of GCBC was much higher than that of the BC (Fig. 7), indicating the presence of graphene on the biochar surface improved its adsorption ability to methylene blue greatly. The Langmuir model was used to describe 1 1 1 the two isotherms: ¼ þ , where qe is the amount of qe qm Kqm C e sorbate on the sorbent at equilibrium (mg/g), qm the Langmuir maximum sorption capacity (mg/g), K is a constant related to the interaction energies (L/mg), and Ce the equilibrium solution concentration. The model simulations matched the experimental data very well with R 2 equal to 0.95 and 0.99 for GCBC and BC, respectively. The qm of methylene blue adsorption on the GCBC was around 174 mg/g, which is more than 20 times higher than that of BC (8 mg/g) and comparable to that of activated carbon adsorbents, such as bamboodust activated carbon (143 mg/g) and groundnut-shell activated carbon (165 mg/g) that were activated with either chemicals (KOH or ZnCl2) or gasses (H2O or CO2) of 700–1000 °C (Bhadusha and Ananthabaskaran, 2011; Qiu et al., 2009; Valdes et al., 2002). The enhanced adsorption of methylene blue by the GCBC biochar could be mainly attributed to the strong interactions between the methylene blue molecules and the graphene sheets on the biochar surface through the π–π interactions. Previous studies have demonstrated aqueous methylene blue and other aromatic organic compounds can form strong π–π electron interaction with the graphite/graphene surfaces (Rochefort and Wuest, 2009). Thus, the GCBC can be also applied as an effective and low-cost adsorbent to remove other aromatic compounds from water. In addition, the novel, facile, and low-cost technology can be expanded to produce graphene-coated biochar/ carbon from other carbon-rich biomass, such as agricultural and forest residues. 4. Conclusions In this work, a new method to synthesize GCBC through “soldering” (pyrolysis) gaps between graphene sheets on a biomass template (cotton wood) by “glue” molecules (pyrene, an aromatic molecular surfactant) in a nitrogen environment at 600 °C. Laboratory characterizations confirmed the successful formation of graphene sheets on

Fig. 7. Adsorption isotherms of methylene blue on the original biochar (a) and on the graphene-coated biochar (b).

biochar surface. The resulted GCBC was thermally more stable than the original BC derived directly from the cotton wood. Compared to the original BC, the GCBC also showed enhanced adsorption ability to aqueous methylene blue, a commonly used evaluator for adsorbents. Findings of this study can provide a basis for designing graphene wrapping/coating technology to open new possibilities in producing innovative and effective engineered biochars for various environmental applications, including water treatment and carbon sequestration. Acknowledgments This research was partially supported by the NSF through grant CBET-1054405 and the USDA through grant 58-3148-1-179. The authors also thank the anonymous reviewers for their invaluable insight and helpful suggestions. References Allen MJ, Tung VC, Kaner RB. Honeycomb carbon: a review of graphene. Chem Rev 2010;110:132–45. Beesley L, Moreno-Jimenez E, Gomez-Eyles JL, Harris E, Robinson B, Sizmur T. A review of biochars' potential role in the remediation, revegetation and restoration of contaminated soils. Environ Pollut 2011;159:3269–82. Bhadusha N, Ananthabaskaran T. Adsorptive removal of methylene blue onto ZnCl(2) activated carbon from wood apple outer shell: kinetics and equilibrium studies. E-J Chem 2011;8:1696–707. Cao XD, Ma L, Gao B, Harris W. Dairy-manure derived biochar effectively sorbs lead and atrazine. Environ Sci Technol 2009;43:3285–91.

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