Sulfonated Graphene Nanosheets as a Superb Adsorbent for Various

Article pubs.acs.org/est

Sulfonated Graphene Nanosheets as a Superb Adsorbent for Various Environmental Pollutants in Water Yi Shen†,‡ and Baoliang Chen*,†,‡ †

Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China

‡

S Supporting Information *

ABSTRACT: Graphene nanosheets, as a novel nanoadsorbent, can be further modified to optimize the adsorption capability for various pollutants. To overcome the structural limits of graphene (aggregation) and graphene oxide (hydrophilic surface) in water, sulfonated graphene (GS) was prepared by diazotization reaction using sulfanilic acid. It was demonstrated that GS not only recovered a relatively complete sp2-hybridized plane with high affinity for aromatic pollutants but also had sulfonic acid groups and partial original oxygen-containing groups that powerfully attracted positively charged pollutants. The saturated adsorption capacities of GS were 400 mg/g for phenanthrene, 906 mg/g for methylene blue and 58 mg/g for Cd2+, which were much higher than the corresponding values for reduced graphene oxide and graphene oxide. GS as a graphene-based adsorbent exhibits fast adsorption kinetic rate and superior adsorption capacity toward various pollutants, which mainly thanks to the multiple adsorption sites in GS including the conjugate π region sites and the functional group sites. Moreover, the sulfonic acid groups endow GS with the good dispersibility and single or few nanosheets which guarantee the adsorption processes. It is great potential to expose the adsorption sites of graphene nanosheets for pollutants in water by regulating their microstructures, surface properties and water dispersion.

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carbon skeleton eliminates the conjugated π region,9,28,29 and its hydrophilic surface rejects interaction with hydrophobic aromatic pollutants, such as polycyclic aromatic hydrocarbons (PAHs).9 Therefore, for the effective removal of various environmental pollutants in wastewater, there have been efforts to develop graphene-based material as a novel adsorbent to abate pollutants such as PAHs, dyes, and heavy metals with superior adsorption capability. One potential approach is to modify the structure of graphene nanosheets to modulate hydrophilic−hydrophobic surfaces and to regulate effective adsorption sites.9,11,16,30 Some researchers have decorated graphene to recover sp2-hybridized structures and to mitigate the heavily aggregation of hydrophobic nanosheets concurrently, thereby achieving the aim to improve the adsorption capability of graphene.31−34 For example, stable monolayer and few-layer graphene nanosheets that showed extremely high adsorption capacity for persistent aromatic pollutants in water were prepared by controlling the loading of negatively charged GO on positively charged nanosilica.31 Ionic-liquid-treated graphene sheets, prepared by exfoliating ionic-liquid-treated graphite sheets with a PF6− mass

INTRODUCTION Graphene is a two-dimensional carbon allotrope with a honeycomb structure that is sp2-hybridized with a thickness of one atom.1,2 Its discovery has attracted enormous attention because of its outstanding physical, as well as chemical, properties, increased external surface area, and modification simplification,3 which are important for various applications, including the development of electronic devices,4,5 photonics devices,6 biomedicine,7,8 and contamination purification.9−18 In view of purification, adsorption is considered as a mature and effective method, and had a lot of researches and wide foreground.19−23 Generally, graphene and its basic derivative graphene oxide have proved to be a promising adsorbent candidate for wastewater treatment or water purification.9,15,16,18,24 The conjugated π region of graphene can attract aromatic pollutants,9,11 and graphene nanosheets can be modified into graphene oxide (GO) with a rich abundance of negatively charged functionalities that are suitable for interaction with cationic pollutants.18,25,26 Meanwhile, both graphene and graphene oxide have limits as adsorbents for various types of environmental pollutant removal. The lack of ionizable groups on the surfaces of graphene nanosheets makes it easy to stack via the hydrophobic effect and π−π interaction, and then inhibits the active sites to be exposed.9,16,27 Moreover, it hinders the interaction with charged pollutants (such as heavy metals and dyes). In graphene oxide, the broken sp2-hybridized © XXXX American Chemical Society

Received: March 2, 2015 Revised: May 2, 2015 Accepted: May 26, 2015

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DOI: 10.1021/acs.est.5b01057 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Adsorption Experiments. PHE, MB, and Cd2+ were selected as representative pollutants to investigate the adsorption process of graphene materials. PHE is a typical hydrophobic organic contaminant. MB is a representative organic cationic dye with wide applications in dyeing cottons and silks and coloring paper.41 Cd2+ is a common heavy metal in the environment. Batch adsorption experiments of PHE and MB to GS, rGO and GO were performed in PTFE screw cap vials sealed with aluminum foil at 25 ± 1 °C, and the background solution (pH 7.0) contained 0.01 mol/L CaCl2 as a mimicking ionic strength of natural water with 200 mg/L NaN3 as a bioinhibitor in deionized water. In the adsorption of Cd2+, the solution pH was adjusted to approximately 5 with 0.01 mol/L NaOH or HNO3, in order to control solution pH value and to prevent metal ions of Cd2+ to precipitate. The sorption kinetic studies were conducted using initial PHE, MB, and Cd2+ concentrations of 1, 28, and 10 mg/L, respectively. For PHE, the solid-to-water ratios for GS, rGO, and GO were 1 mg per 400 mL, 1 mg per 240 mL and 1 mg per 8 mL, respectively. For MB, the solid-to-water ratios for GS, rGO, and GO were 1 mg per 37.5 mL. For Cd2+, the solid-to-water ratios for GS, rGO, and GO were 2 mg per 8 mL. The remaining concentrations in a series of independent samples were measured from 10 to 2880 min for PHE adsorption by GS, rGO, and GO; 1 to 240 min for MB adsorption by GS and GO, and 1 to 120 h by rGO; and 1 to 720 min for Cd2+ adsorption by GS, rGO, and GO. Isotherm experiments were performed by a batch equilibration method. To reach equilibrium, the vials were placed on a shaker and agitated in the dark at 120 rpm for 24, 5, and 24 h for PHE, MB, and Cd2+, respectively. The samples were filtered using 0.22 μm membrane filters. After filtration, the PHE concentrations in the supernatants were analyzed using an Agilent 1200 HPLC equipped with a fluorescence detector, and the apparent equilibrium concentration of the MB solutions was measured using an UV/vis spectrometer and calculated by the absorbance at 664 nm,42 while Cd2+ was analyzed using a PerkinElmer Analyst 700 (PE700, USA) atomic absorption spectrometer (For more detail, see the SI). Data Analysis. Pseudo-first-order and pseudo-second order kinetic models were used to analyze the adsorption kinetics of the graphene materials. The Langmuir and Freundlich models were used to fit the adsorption isotherms. More details on the adsorption kinetics and the Langmuir and Freundlich models are presented in the SI. The Kd (mL/g) value is the distribution adsorption coefficient calculated by dividing the adsorbed amounts (qe) by the equilibrium concentration (Ce) of adsorbate.

fraction of 30%, were used to remove Pb(II) and Cd(II) ions from wastewater with high adsorption capacities.32 However, the target pollutants of the reported graphene materials were still limited. Si et al. first obtained sulfonated graphene,35 which not only recovers relatively sp2 carbon domains but also introduces charged −SO3− units to the graphene surfaces. Thus, the sulfonated graphene was water-soluble and was prevented from heavy aggregation in water. Presumably, the sulfonated graphene may have multiple adsorption sites for different types of pollutants compared with graphene and graphene oxide. For example, sulfonated graphene was applied as an adsorbent to remove naphthalene and 1-naphthol from aqueous solution with the maximum adsorption capacities of 298.66 mg/g for naphthalene and 347.45 mg/g for 1-naphthol, which were the highest adsorption capacities observed at that time.33,34 However, even though there have been a handle of researches about the synthesis and characterization of sulfonated graphene,36−38 the adsorption properties and mechanisms of sulfonated graphene with varying environmental pollutants require further investigation to pursue its potential environmental applications as an effective adsorbent. In the current study, we prepared and characterized sulfonated graphene (GS), reduced graphene oxide (rGO) and graphene oxide (GO) to examine GS’s adsorption capacity and mechanism. Phenanthrene (PHE), methylene blue (MB) and cadmium (Cd2+) were selected as adsorbates to study the different adsorption mechanism and site contributions of synthesized graphene materials. Atomic force microscopy (AFM) was used to observe the localized sheet morphology and to monitor the layer numbers and sizes of graphene sheets. Raman and X-ray photoelectron spectroscopy (XPS) were informative techniques to probe the hybridization of carbon atoms and the content of chemical bonds in various materials. Fourier transform infrared spectroscopy (FTIR) analysis and zeta potential analysis were also performed. In comparison with rGO and GO, the superior structure and multiple adsorption sites of GS were revealed to exploit multifunctional graphenebased adsorbents.

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MATERIALS AND METHODS Preparation of GS, rGO, and GO. Graphene oxide (GO) was synthesized from natural graphite flake (325 mesh, 99.8%, Alfa Aesar) using a modification of the Hummers method.39 The adhesive GO layers were exfoliated through ultrasonication. After dialysis to remove the acid and other impurities, GO nanosheets were obtained. Reduced graphene oxide (rGO) was prepared by a chemical reduction method using GO as the precursor.40 Sulfonated graphene (GS) by diazotization reaction was prepared from GO in three steps:35 (1) prereduction of GO with sodium borohydride at 80 °C for 1 h to remove a portion of the oxygen-containing groups; (2) sulfonation with the aryl diazonium salt of sulfanilic acid in an ice bath for 2 h; and (3) postreduction with hydrazine at 100 °C for 24 h to remove any remaining oxygen-containing groups. The detailed processes and the scheme of the preparation are presented in the Supporting Information (SI). Structural Characterization of GS, rGO, and GO. The structures of GS, rGO, and GO were characterized by the BETN2 specific surface areas, AFM and Raman spectra. The surface functional groups were observed by XPS and FTIR. Surface charges were monitored by the zeta potential method. The detailed methods are presented in the SI.

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RESULTS AND DISCUSSION Characterization of Graphene Materials. The BET specific surface areas (SA) of GS, rGO, and GO were 616, 399, and 261 m2/g, respectively. The high SA may directly reflect the exfoliation degree of graphene materials compared with graphite (4.5 m2/g). However, the obtained SA is much lower than the theoretical value (∼2630 m2/g), which is likely related to incomplete exfoliation and aggregation during the sample preparation process. The SA of GS is larger than the SA of rGO because the charged HSO3− units prevent the graphitic sheets from aggregating,35 resulting in less π−π stacking and more single/few nanosheets of GS than those of rGO. The FTIR spectra of GS, rGO and GO are shown in SI Figure S-1. The peaks of GS at 1175 and 1126 cm−1 were assigned to the two νS−O, and the weak peak of 1040 cm−1 was B


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Environmental Science & Technology assigned to νS‑phenyl.35 These peaks are absent in the spectra of rGO and GO, confirming the presence of sulfonic acid groups in GS. The zeta potentials of the aqueous dispersions of GS, rGO and GO are presented in SI Figure S-2. Similar to GO, GS was also negatively charged. The zeta potential of GO was higher than the zeta potential of GS and rGO, and the negative charges decreased with increasing solution pH values. When the pH was higher than 5, the zeta potentials of GO and GS were both higher than 40 mV (negative), ideal values for stabilizing conventional colloidal particles. The aqueous dispersions of GO and GS showed good stability. There was no sign of coagulation of graphene sheets of GO or GS after more than two months. In contrast, the graphene nanosheets of rGO self-stacked quickly within a few minutes. In the Raman spectra (Figure 1), two prominent peaks at 1350 and 1580 cm−1 correspond to the D band and the G

GO is hardly distinguishable, indicating that GO may form more than 5 layers.49 XPS studies revealed the nature of the carbon bonds present on the surfaces of GS, rGO, and GO (Figure 2). GS contains sp2- (31.5%) and sp3- (56.8%) hybridized carbon atoms (SI Table S-1). The rest of the carbonaceous material is bonded to O (SI Figure S-3) and consists of carbonyl groups (CO) and carboxylic groups (COO).53,54 The 1.8:1 ratio of sp3-/sp2hybridized carbon atoms of GS may explain the high intensity of the D band observed in the Raman spectra, as GS is not

Figure 1. Raman spectra of sulfonated graphene (GS), reduced graphene oxide (rGO) and graphene oxide (GO).

band, respectively. The D band originates from the stretching vibration of sp3-hybridized carbon atoms, which induces defects and disorders, whereas the G band originates from the stretching vibration of sp2-carbon atoms. The intensity ratio of the D band to the G band (ID/IG, SI Table S-1) of rGO (1.31) was larger than the corresponding values for GS (1.16) and GO (0.892), suggesting that the newly formed sp2hybridized domains were smaller in size but more prevalent40 with increasing reduction degree, which is caused by the increased number of smaller graphitic domains formed during the reduction process.43,44 Meanwhile, the intensity of the D band becomes stronger with decreasing crystallite size of the graphene, which proved that the sizes of rGO and GS were smaller than that of GO. Raman spectroscopy was also utilized to investigate the single-, bi-, and multilayer characteristics of graphene materials. It was shown that the GS band of the single-layer graphene located at 1582 cm−1 was red-shifted by 2 cm−1 to the lower wavenumber 1580 cm−1 for rGO after stacking 2−3 graphene layers.45−49 Moreover, the shape and position of the 2D band are known as key parameters for judging both the formation and the layer numbers of the graphene sheets.45,47−50 The obtained sharp 2D peak of GS is at 2679 cm−1, which corresponds to single-layer graphene sheets,51,52 whereas the peak of rGO is blue-shifted to 2698 cm−1 and provides evidence that the layers stack.49,51 However, the 2D shape of

Figure 2. XPS spectra of GS (a), rGO (b), and GO (c) for C 1s. C


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Figure 3. AFM images of GS (a) and GO (b).

of 1−2 layers of graphene oxide sheets.58,59 The decrease in the lateral dimensions of GS is attributed to the severe reaction conditions and excessive sonication during each step of the reduction process. Based on the structural characterization above, the atomic structure models of GS, rGO, and GO are shown in SI Figure S-4. rGO has a structure of packed sp2-bonded carbon atoms with a few functional groups and usually stacks in multilayers. In comparison, GO usually exists as single sheet, and its structure of sp2-bonded carbon atoms is destroyed, but it has the most oxygen-containing groups. It is generally believed that carboxyl groups exist at the edges of carbon layers, whereas epoxy and hydroxyl groups are incorporated into the layers,56,57 resulting in an acidic or negatively charged surface. Interestingly, GS disperses readily in water as single or few layers to form a stable colloidal suspension, which not only recovers the skeleton of the packed sp2-bonded carbon atoms but also exhibits −SO3H and −COOH groups on the edge of graphene33 and −OH on the plane. Meanwhile, the sp2hybridized structure of GS is less complete than that of rGO but more relevant than the fragmentary sp2-hybridized carbon skeleton of GO. The multiple structures make double adsorption sites available in GS, namely, strong conjugate π region and functional groups. Therefore, GS is expected to interact well with the delocalized π−bonds of the organic aromatic compounds and the charges of cationic dyes as well as with heavy metals, as will be elucidated next. Adsorption Kinetics of GS, rGO, and GO Nanosheets. The effects of time on the adsorption of PHE, MB, and Cd2+ on graphene materials are presented in Figure 4. The adsorption process of PHE, MB and Cd2+ onto GS is usually fast. For example, the adsorption of PHE to GS reached equilibrium in approximately 120 min. For MB, the adsorption capacity of GS increased rapidly in the first 10 min (see SI Figure S-5) and reached equilibrium in approximately 30 min, whereas the adsorption equilibrium of rGO was difficult to reach even after 72 h. Moreover, the adsorption of Cd2+ to GS increased more rapidly in the initial 60 min than the adsorption to rGO and GO. The regression kinetic parameters for the adsorption of GS, rGO, and GO are presented in SI Table S-2. For PHE, the

highly crystalline and does not show AB stacking of the graphene layers. Compared with GS, the sp2-hybridized carbon proportion in rGO increased. Interestingly, the sp2-hybridized carbon proportion obtained from XPS data followed the order of rGO > GS > GO, which is contrary to the order of sp2hybridized domains (rGO < GS < GO) in the Raman spectra. The quantity of sp2-hybridized carbon increased with the intensification of the reduction reaction, while the sp2hybridized domains became more intensive and prevalent, namely, complete.40 In Figure 2, the deconvolution of the C 1s peak of GS resolved to main peaks at 283.9 and 284.6 eV, which were attributed to the presence of CC and C−C.55 The remaining peaks correlated to the carbon in C−O, C−S, and C−N, the carbonyl carbon in CO, and the carboxyl carbon in O−C O. The contents and intensities of the functional groups in GS, rGO and GO were different. The surface functional groups of GS, rGO, and GO were also analyzed (SI Table S-1). The π−π* domains increase with intensification of the reduction reaction, indicating that the sp2-hybridized domain increased. Meanwhile, GO included more −OH and −COOH groups and was more hydrophilic than GS and rGO (SI Figure S-3). According to the calculations (SI Table S-1), GS had 2.57 mmol/g −SO3H, 1.72 mmol/g −COOH, and 11.8 mmol/g −OH. Because the reduction by N2H4·H2O was directed mainly at the −OH, compared with GO, the decrease of −OH in rGO was more significant than that of −COOH. In GS, the circumstance was different, perhaps due to the competitive location of −SO 3 H and −COOH at the edge of graphene.33,56,57 The AFM images of GS and GO are shown in Figure 3. It is clear that the evaporated dispersions of GS and GO are composed of isolated graphitic sheets, while rGO lacks a good figure due to serious aggregation. The lateral dimensions of GS ranged from several hundred nanometers to several micrometers. The thickness of the GS was only ∼1 nm, that is, a single graphene nanosheet. These observations indicate that GS is similar to single or few graphene sheets peeled from pyrolytic graphite (0.9 nm thick).2 GO has lateral dimensions of several micrometers and a thickness of 1−2 nm, which is characteristic D


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Figure 4. Adsorption kinetics of phenanthrene, methylene blue and Cd2+ onto GS, rGO and GO (The initial concentrations of PHE, MB, and Cd2+ were 1, 28, and 10 mg/L, respectively.).

Figure 5. Adsorption isotherms of phenanthrene, methylene blue and Cd2+ onto GS, rGO, and GO. Solid lines represent isotherms fitted by the Freundlich model, and dotted lines represent isotherms fitted by the Langmuir model.

adsorption kinetic curves of GS, rGO, and GO fitted better with the pseudo second-order model than with the pseudo firstorder model. For MB, the pseudo first-order model fitted the adsorption kinetics of GS and GO well, and both the pseudo first-order model and the pseudo second-order model fitted the kinetic results of rGO well, which implies that the adsorption mechanisms of MB onto GS and GO are different from the adsorption mechanism onto rGO. The adsorption kinetics of Cd2+ onto GS, rGO, and GO were described well by the pseudo first-order and the pseudo second-order models. Based on the adsorption kinetics of graphene materials, GS was the most efficient adsorbent for various pollutants. Adsorption Isotherms of PHE, MB, and Cd2+ onto GS, GO, and rGO. The adsorption isotherms of PHE, MB and Cd2+ onto GS, rGO and GO are shown in Figure 5. The regression parameters of the isotherms by the Langmuir and Freundlich models are listed in Table 1. On the whole, the

isotherms of three graphene materials were fitted better by the Langmuir than by the Freundlich model. It was distinct for MB, indicating that the adsorption of MB to graphene materials tends toward monolayers rather than multilayers. Meanwhile, the superiority of Langmuir to Freundlich model was not so obvious in the isotherms of PHE and Cd2+ adsorption, which were also fitted well by Freundlich models. Based in Figure 5, GS was the best graphene material to abate PHE, MB, and Cd2+. The saturated adsorption capacity (Qo) of PHE onto GS was 400 mg/g, which was much higher than that onto rGO (152 mg/g) and GO (6 mg/g) in this study. As reported, the saturated adsorption capacity of PHE was 255 mg/g on monolayer and few-layer graphene nanosheets through loading on nanosilica substrates,31 136 mg/g on reduced graphene,9 and 29.8 mg/g on multilayer graphene.16 The Qo of MB onto GS was 906 mg/g, which was higher than that onto rGO (136 E


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Table 1. Regression Parameters of Adsorption Isotherms of Phenanthrene (PHE), Methylene Blue (MB) and Cadmium (Cd2+) onto Sulfonated Graphene (GS), Reduced Graphene Oxide (rGO) and Graphene Oxide (GO) Fitted by Freundlich and Langmuir models Freundlich Model 1/n

Langmuir Model Q (mg/g)

KL (L/g)

R2

0.977 0.960 0.952

400 ± 9 152 ± 4 6.03 ± 0.14

8.14 ± 0.51 9.91 ± 0.85 21.0 ± 2.1

0.996 0.992 0.985

0.155 ± 0.019 0.399 ± 0.037 0.0630 ± 0.0087

0.759 0.877 0.704

906 ± 14 136 ± 4 623 ± 4

6.43 ± 0.55 1.11 ± 0.11 17.5 ± 1.3

0.921 0.969 0.914

0.257 ± 0.019 0.409 ± 0.026 0.246 ± 0.019

0.959 0.970 0.954

57.6 ± 2.0 10.5 ± 0.2 43.7 ± 1.6

0.28 ± 0.05 1.01 ± 0.06 0.375 ± 0.078

0.969 0.996 0.957

pollutant

adsorbent

Kf ((mg/g)/(mg/L) )

N

R

PHE

GS GR GO

500 ± 22 200 ± 12 7.02 ± 0.28

0.505 ± 0.028 0.502 ± 0.037 0.341 ± 0.026

MB

GS GR GO

754 ± 11 65.1 ± 2.8 579 ± 4

Cd2+

GS GR GO

17.5 ± 1.4 4.63 ± 0.18 14.4 ± 1.2

2

o

adsorption sites of conjugate π region plane in GS are about 1 mmol/g or 0.002 mmol/m2. Furthermore, the planar molecular area of PHE was calculated using the van de Waals radius (SI Table S-3). If the PHE molecules form a face-to-face adsorption on the surface of GS, rGO, and GO, the theoretical adsorbed amounts of PHE on GS, rGO, and GO are 182, 118, and 77.3 mg/g, respectively. The experimental value of the adsorbed amount of PHE on GO (6.03 mg/g) was much lower than the predicted adsorbed amount (77.3 mg/g), suggesting that the availability of adsorption sites on GO is significantly inhibited by the hydrophilic surface. Interestingly, the experimental adsorbed amount of PHE on GS (400 mg/g) was much higher than the theoretical calculation (182 mg/g), indicating that the available surface area of GS in the aqueous solution surpasses the measured data by N2 adsorption−desorption in a solid state. In aqueous solution, −SO3H allows GS to form a stable suspension and introduces adsorption sites on the edge and plane. As GS edges consisting of sp3-hybridized carbons with unpaired electrons contain dangling bonds, weak charge transfer interactions with donor molecules and/or strong surface electric fields occurred upon interaction with the molecular dipoles of H2O. Moreover, H2O molecules should be selectively adsorbed only on the sp3-hybridized edge surfaces rather on than the sp2-hybridized planes, surrounding the graphene layer by a long-range of looping lines of H2O molecules.64 The powerful interaction with H2O molecules around the abundant hydrogen bonds at the edge of the graphene layer may benefit the planar unfolding and gradual exposure of sites. In the AFM images (Figure 3), the relatively smooth surface of GS may be attributed to the unfolding of graphene nanosheets by −SO3H at the edge; meanwhile, GO had higher wrinkles. In analogy, the sp2-hybridized plane of GS looks like a fishing net hunting for the pollutant, while the −SO3H acts as netting twine that favors the stretching of the net (Figure 6). Therefore, although the π−π interaction and hydrophobic effect both contribute to adsorption onto GS and rGO, the π−π interaction sites of GS are more effective than those of rGO. Furthermore, the calculated peak Kd/KHW ratios of PHE on GS, rGO and GO are 148, 30.5, and 8.51, respectively, suggesting that the effective π−π interaction with PHE of GS is very powerful. Second, the functional groups at the sp3-hybridized edge of graphene are active for electrostatic interaction, and the exposed extent of the adsorption sites determines the adsorption extent for ionic pollutants. In addition to unfolding

mg/g) and GO (623 mg/g). In previous reports, the adsorptions of MB onto graphene nanosheet/magnetite (Fe3O4) composite and graphene oxide were 43.82 mg/g60 and 714 mg/g,61 respectively. The Qo of Cd2+ onto GS was 58 mg/g, which was higher than that onto rGO (11 mg/g) and GO (44 mg/g). Considering other graphene materials, subnanometer-thick poly dopamine (PD) layer coated GO (PD/GO) composites can adsorb Cd2+ 33.3 mg/g,62 and graphene sheets modified with 1-pyrenebutyric acid (PBA) can adsorb Cd2+ 85.5 mg/g.63 Clearly, rGO is a good adsorbent only for PHE, and GO is an effective adsorbent for MB and Cd2+, while GS displays the highest experimental adsorption capacity for all the three tested pollutants, which suggests that GS is a novel graphene-based adsorbent for various environmental pollutants. Relationship between Structure and Adsorption Property of GS, rGO, and GO. The available adsorption sites on GS, rGO, and GO are regulated by the surface properties, the recovered conjugate π region and the aqueous dispersion of graphene material colloids, which were probed by representative environmental pollutants, such as PHE, MB, and Cd2+. The high adsorption mechanism of GS will be elucidated in comparison with rGO and GO based on their microstructures. First, π−π interaction and the hydrophobic effect are the dominant mechanisms of PHE adsorption onto graphene nanosheets.9 Generally, the conjugate π region depends on the completion content of sp2-hybridized structure recovery. The adsorption capacity of GO was extremely low because the serious damage to the sp2-hybridized structure prevented π−π interaction, and the polar nanosheet on the surface limited the hydrophobic effect. In contrast, the high affinity of PHE for rGO was dominated by the π−π interaction with the flat sp2hybridized carbon surface. In view of the excellent adsorption capacity of PHE to GS, we analyzed the adsorption sites from the comparison with naphthalene (NAPH) as another representative PAH among GS,33 rGO9 and GO,9 as presented in SI Table S-3. Although the sp2-hybridized carbon structure of rGO is more complete than that of GS, the amounts of adsorbed NAPH and PHE converted into unit weight mmol/g all follow the order of GS > rGO, which may be due to the increased SA in GS. According to the measured SA, the conjugate π region adsorption sites standardized by unit area increased after sulfonation, that is, 0.002−0.003 mmol/m2 for rGO and 0.003−0.005 mmol/m2 for GS. The calculated new F


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the plane. The inappropriate low adsorption capacity of GO was due to the abundant in-plane hydrophilic −OH of GO, and the whole layers were confined in the water film,67,68 failing to expose the adsorption sites, as indicated by the smaller Kd/Khw than GS (see SI Figure S-6). For GS, the decrease in −OH diminishes the cover of water film above the graphene plane and helps to expose the adsorption sites for MB. Furthermore, the decrease in the lateral dimensions of GS indicated the smaller particles than GO, which makes it more available to the pollutants. Third, the adsorption sites in GS can be concluded to consist of the conjugate π region sites and the functional group sites for different pollutants. In view of the adsorption of Cd2+, except for negatively charged sites, GS exhibits cation−π interaction with heavy metals, which is lacking for GO. Thus, the superior adsorption capacity of Cd2+ onto GS is also obvious. Besides typical heavy metals, there have been some studies on the sorption of radionuclides (Eu(III), Sr(II), and Cs(I)) on graphene-based materials,26,69 which were all with high adsorption capacity. This indicates that the modification of graphene has broad scope and bright perspective future in the environmental pollution management. As a superb adsorbent, GS possesses high adsorption capacities and fast adsorption rates for various environmental pollutants, which can be briefly attributed to three reasons. The adsorption sites of PHE, MB, and Cd2+ onto GS are schematized in Figure 6. First, the stable dispersion and decreasing aggregation of nanosheets derived from −SO3H provide the foundation for adsorption by GS, which facilitates all three types of pollutants tested. Second, the recovery of the available structure, including the sp2-hybridized plane and functional groups, is highly important. The recovered sp2hybridized plane directly favors the adsorption of PHE and also indirectly strengthens the adsorption of MB and Cd2+. The available functional groups promote the adsorption onto GS of both MB and Cd2+. Third, the functional groups of −SO3H unfold the stacking plane to expose the effective sites, consisting of planar conjugate π region sites for PHE and edge functional group sites for MB and Cd2+. This paper shows that GS is a powerful graphene-based adsorbent for various pollutants. This study provides a basis for obtaining effective allpurpose graphene materials by regulating the microstructure and interfacial properties of graphene nanosheets.

Figure 6. Schematic adsorption sites of PHE, MB, and Cd2+ onto GS.

the planar π−π surface, the −SO3H functional groups can serve as potential adsorption sites. The adsorption of MB (cationic dye) onto GS, rGO, and GO is mainly due to the electrostatic attractions between the positive charge of MB and the negative charge of graphene materials, in distinct contrast to PHE. The hyperslow adsorption of MB onto rGO indicates that π−π interaction is not the main force. Clearly, the −COOH and −SO3H functional groups of graphene materials are critical in the adsorption of MB. Meanwhile, these two types of functional groups located at the edge of graphene layers are more accessible and reactive than the groups incorporated into the layers, such as epoxy and hydroxyl groups.56 The adsorption capacities of graphene materials correlated with the quantity of functional groups. For rGO, the −OH groups were greatly reduced by the chemical reduction of N2H4·H2O, and the quantity of −COOH in rGO was 4.57 mmol/g, which did not contribute to the considerable adsorption capacity of MB. However, the stacking layered structure of rGO cannot expose −COOH effectively as available adsorption sites for MB. GO has the lowest Freundlich N value among the three graphene materials, suggesting that GO displayed the most inhomogeneous surface with a wide adsorption site distribution for MB.65 The adsorption capacity of MB onto GO was lower than that onto GS, and GO had 5.14 mmol/g −COOH, which was far greater than the adsorbed 1.96 mmol/g MB, indicating that most −COOH groups were not available for MB adsorption. GS had 1.72 mmol/g −COOH and 2.57 mmol/g −SO3H, and their sum was slightly larger than the adsorbed 2.83 mmol/g MB. Compared with rGO and GO, the adsorbed amount of MB to GS was excellent and approached the total amount of −COOH and −SO3H. Because both −SO3H and −COOH can dissociate, lose electrons, and carry negative charges, the main difference in MB adsorption between GS and GO can be concluded to arise from the differences in the sp3-hybridized domains and planar functional groups of −OH. Primarily, the sp3-hybridized domains of GS were larger than those of GO, so the longer edge of GS had a higher probability of interacting with surrounding molecules. Additionally, the sp3-hybridized edge surfaces should have an electron-acceptor nature66 as a result of the presence of extra unpaired spins.64 Therefore, GS attracted more cationic molecules through weak charge-transfer interactions, favoring the contact of −SO3H and −COOH with MB molecules and the electrostatic adsorption process. Another factor is the water film, influenced by the −OH in

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis methods, structural characterization, and adsorption properties of GS, rGO and GO are presented. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b01057.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0086-571-88982587; fax: 0086-571-88982587; e-mail: [email protected] . Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Science Foundation for Distinguished Young Scholars of China (Grant 21425730), the National Basic Research Program of China (Grant G


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2014CB441106), the National Natural Science Foundation of China (Grants 21277120 and 41071210).

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