Adsorption of Phenanthrene on Multilayer Graphene as Affected by

Dec 12, 2013 - Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States. ‡. College of Environmen...
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Adsorption of Phenanthrene on Multilayer Graphene as Affected by Surfactant and Exfoliation Jian Zhao,† Zhenyu Wang,*,‡,§ Qing Zhao,† and Baoshan Xing*,† †

Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States College of Environmental Science and Engineering, Ocean University of China, Qingdao, 266100 Shandong, China § Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao, 266100 Shandong, China ‡

S Supporting Information *

ABSTRACT: Surfactant mediated exfoliation of multilayer graphene and its effects on phenanthrene adsorption were investigated using a passive dosing technique. In the absence of surfactant (sodium cholate, NaC), multilayer graphene had higher adsorption capacity for phenanthrene than carbon nanotube and graphite due to the higher surface area and micropore volume. The observed desorption hysteresis is likely caused by the formation of closed interstitial spaces through folding and rearrangement of graphene sheets. In the presence of NaC (both 100 and 8000 mg/L), phenanthrene adsorption on graphene was decreased due to the direct competition of NaC molecules on the graphene surface. With the aid of sonication, multilayer graphene sheets were exfoliated by NaC, leading to better dispersion. The degree of dispersion depended on the graphene-NaC ratio in aqueous solution rather than critical micelle concentration of NaC, and the good dispersion occurred after reaching adsorption saturation of NaC molecules on graphene sheets. In addition, exfoliation weakened the competition between phenanthrene and NaC and enhanced the adsorption capacity of graphene for phenanthrene due to exposed new sites. The findings on exfoliation of graphene sheets and related adsorption properties highlight not only the potential applications of multilayer graphene as efficient adsorbent but also its possible environmental risk.

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INTRODUCTION Graphene, a new class of carbon nanomaterials, has generated intense research interest in recent years due to its excellent mechanical, optical, and electrochemical properties. A graphene sheet is extremely hydrophobic, and its theoretical specific surface area is as high as 2600 m2/g.1 Graphene is expected to have excellent adsorption capacity toward hydrophobic organic compounds (HOCs), and has the potential to be applied as a superior adsorbent in wastewater and drinking water treatments.2 However, current knowledge on adsorption characteristics (e.g., desorption, competitive adsorption) of graphene is limited. Surfactants are widely used in graphene production and application. Up to date, there are three main graphene exfoliation methods: mechanical exfoliation from graphite,3 direct chemical exfoliation of graphite in surfactant solutions (e.g., sodium dodecylbenzene sulfonate) or organic solvent (e.g., N-methyl-pyrrolidone);4,5 and exfoliation of graphite oxide followed by chemical reduction.6 Exfoliation of graphite by surfactants is particularly important because this technique (1) needs no oxidation process, (2) brings less surface defects, and (3) is achieved in aqueous solutions instead of organic solvents which are less environmentally friendly.7 By using this approach, the obtained graphene products and remaining © 2013 American Chemical Society

graphene wastes would be coated with surfactants. In addition, surfactants have been applied as important and effective stabilizers for dispersing graphene8 and other carbon nanomaterials such as carbon nanotube (CNTs).9 With increasing production and application of graphene, more and more graphene-surfactant complexes will likely enter wastewater treatment plants and natural waters. Therefore, it is essential to understand the interaction of these complexes with other pollutants such as HOCs. For HOC adsorption on nanoparticle-surfactant complexes, two processes are likely to occur: (1) competitive adsorption between HOC and surfactant molecules and (2) partitioning of HOCs into the surface of surfactant-coated nanoparticles. Due to these two opposite processes, both enhancement and suppression of HOC adsorption as affected by surfactants have been observed on nanoparticles,10,11 and this sorption alteration appears to be nanoparticle-dependent. For multilayer graphene, surfactant exfoliation may be an additional process to change HOC adsorption on the surface. However, exfoliation Received: Revised: Accepted: Published: 331

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Selected properties of graphene and other carbon materials are presented in Table S2. Morphology of graphene was investigated using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For SEM observation, the graphene in the suspension was dried in a vacuum freeze-dryer, mounted on stubs, sputter coated with gold, and then viewed using SEM (S4800, Hitachi, Japan).17 Sample for TEM (H-7650, Hitachi, Japan) imaging was prepared by evaporating a drop of graphene suspension on a nickel grid. Passive Dosing Vials Preparation and Phenanthrene Loading. Passive dosing vials were prepared following the method of Birch et al.18 In brief, the prepolymer and catalyst were mixed in a weight ratio of 10:1, and then 1000 ± 5 mg of the mixture were carefully added onto the bottom of 20-mL vials. After the curing procedure (4 °C for 48 h, 25 °C for 72 h, and 110 °C for another 72 h), the silicone in vials became hard and translucent. Then, all vials were rinsed with methanol (HPLC grade, Fisher) for three times and distilled water for another three times. The passive dosing vials were wiped with lint-free tissues after the cleaning procedure. The prepared passive dosing vials were loaded by equilibrium partitioning phenanthrene from a loading solution,19 and the detailed procedures are described in the Supporting Information. After loading, all vials were rinsed with distilled water for four times to remove the remaining methanol in silicone (Figure S1). The washed vials were added with 19.0 mL of distilled water and shaken at 25 °C for 48 h to reach equilibrium. The free phenanthrene in water (Cfree) was measured by mixing 1.6 mL of test solution with 4.0 mL of Ultima Gold XR cocktail (Perkin-Elmer) for liquid scintillation counting (Beckman, LS6500). Passive dosing vials with different Cfrees were obtained by adding different concentrations of phenanthrene loading solutions (22−1762 mg/L). Phenanthrene Adsorption by Graphene in Passive Dosing Vials. For phenanthrene adsorption in distilled water, 1.00 mg of graphene was added into passive dosing vials which already contained 19.0 mL of distilled water, and then the vials were sealed and shaken at 25 °C for 5 days. After equilibration, the solution containing graphene in each vial was completely transferred to another vial. The original passive dosing vials were quickly rinsed and amended with 19 mL of distilled water. Phenanthrene concentrations in these amended solutions (Cfree) were measured after equilibration for 48 h. Graphene samples in the transferred solutions were dispersed by sonication, and then phenanthrene concentrations (Ctotal) in the suspensions were measured by liquid scintillation counter. To ensure the uniform distribution of graphene, all solutions were shaken for 30 s prior to measurements. The presence of graphene did not interfere with the radioactive measurement in phenanthrene solutions (Figure S2). The equilibrium concentration of adsorbed phenanthrene on graphene (qe) was determined by mass difference: qe = (Ctotal−Cfree)/Cgraphene. The adsorption experiment was also conducted using a traditional batch/centrifugation technique described by Yang et al.20 In comparison with graphene, phenanthrene adsorption on MWCNTs and graphite was investigated. The same procedures were followed except for the adsorbent amounts (1.00 and 5.00 mg for MWCNTs and graphite, respectively). Desorption of phenanthrene from a graphene sheet was also investigated using a batch/centrifugation equilibration technique as described in our previous study.16

of the multilayer has not been addressed yet. As introduced above, graphite flakes could be exfoliated into individual graphene sheets by surfactants. The layers in multilayer graphene are stacked similar to graphite, but the interlayer distance differs from graphite;12 the strength of interlayer interaction in multilayer graphene is also different from graphite and likely depends on the layer number and monolayer arrangement.13,14 We hypothesized that surfactants could exfoliate multilayer graphene with the assistance of sonication. Adsorption capacity of graphene is controlled by the surface area and layer number.2,3 Thus, this hypothesis could be quantitatively verified by the enhancement of adsorption capacity of multilayer graphene from a series of HOC adsorption experiments. In addition, the nanoparticle-surfactant complexes in the previous investigations were prepared by freeze-drying,10,11 but drying could change the orientation of surfactant molecules on the surface. In this work, we will employ a passive dosing technique which is based on equilibrium partitioning of HOCs from a silicone to an adsorbent-contained solution.15 By using this technique, we are able to directly quantify the adsorption of graphene-surfactant complexes in aqueous solution without the need of pretreatments (e.g., drying, centrifugation). The main objectives of this work were therefore to investigate (1) adsorption−desorption of HOCs on multilayer graphene; (2) HOC adsorption on graphene-surfactant complexes and competition between surfactant and HOC molecules; and (3) surfactant exfoliation of multilayer graphene and its influence on HOC adsorption. Phenanthrene, composed of three fused benzene rings, was employed as a model HOC. Sodium cholate (NaC), an anionic surfactant, was chosen in this work because of its effective performance in exfoliation of graphite.7 The information from this study is expected to help better understand the adsorption characteristics of graphene in complicated aquatic environments and potential environmental risk of multilayer graphene.

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MATERIALS AND METHODS Materials. Graphene (AO-3), multiwalled CNTs (MWCNT) and graphite were purchased from Graphene Supermarket (Reading, MA, USA), Chengdu Organic Chemistry Co. (China), and Fisher Scientific Co., respectively. The surfactant NaC and 14C labeled (55 μCi/μmol) and unlabeled phenanthrene were obtained from Sigma-Aldrich Chemical Co. Selected physicochemical properties of NaC and phenanthrene are listed in Table S1. Elastomer kit (Silastic MDX4−4210) which contains a poly(dimethylsiloxane) (PDMS) silicone prepolymer and a catalyst was purchased from Dow Corning Co. (Midland, USA). Characterization of Graphene, MWCNTs, and Graphite. Surface area and pore distribution of all the test materials were analyzed using an Autosorb-1 (Quantachrome, USA).16 Average surface area, volume, and surface area of mesopores were calculated from the adsorption−desorption isotherm of N2 by multipoint Brunauere-Emmette-Teller (BET) and Barrette-Joynere-Halenda (BJH) methods, respectively. Volume and surface area of micropores were calculated from CO2 adsorption−desorption isotherm by the Dubinine-Rudaushkevich (DR) method. Elemental composition and surface functional property of graphene sheets were analyzed using elemental analyzer (MicroCube, Elementar, Germany) and Fourier transform infrared (FTIR) spectroscopy, respectively. 332

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qe = Q 0Ce/(KL + Ce)

Critical micelle concentration (CMC) of NaC was determined based on the distinct solubilization difference between surfactant monomers and micelles using the passive dosing vials which had the same Cfrees (0.064 mg/L) of phenanthrene. The detailed procedures are in the Supporting Information. For phenanthrene adsorption in the presence of surfactant, NaC concentrations both below and above CMC (100 and 8000 mg/L, respectively) were tested. The procedures were the same as those of adsorption experiments in the absence of NaC; Ctotal and Cfree were therefore obtained. Solubilization of phenanthrene in NaC solutions (100 and 8000 mg/L) was determined as described in the Supporting Information to obtain solubilized phenanthrene concentration (Csolubilized). The equilibrium phenanthrene concentration on graphene (qe) in the presence of NaC was then determined as follows: qe = (Ctotal−Csolubilized−Cfree)/Cgraphene. For all phenanthrene adsorption experiments, there were also two sonication pretreatments conducted before shaking to reach equilibrium: (1) sonication for 5 min using a sonic probe (Fisher, Model 100) at an output wattage of 23 w and (2) sonication for 2 h using a sonic bath (Fisher, FS30) at an output wattage of 100 w. Due to the long-time sonication in sonic bath, bath water was continuously refilled in order to maintain the sonication temperature and efficiency of exfoliation.7 During the experiments, sonication as mentioned above did not significantly change the partitioning of phenanthrene from silicone to aqueous solution (Figure S3). Surfactant Adsorption Studies and Dispersion Stability Tests. The surfactant NaC was dissolved in distilled water to obtain a series of solutions (150−1500 mg/L). A volume of 40 mL of NaC solution was added into each 40-mL vial which already had 100 mg of graphene. Then, all vials were allowed to equilibrate for 5 days. The two sonication treatments (sonic probe and bath) as described above were also conducted before shaking. Then, all vials were centrifuged (3000 rpm, 20 min) twice, and the supernatants were filtered (Teflon membrane, 0.22 μm) before analysis. NaC concentrations were determined using a UV−vis spectrophotometer (Agilent 8453, USA).21 Briefly, 0.2 mL of supernatant was mixed with 6.0 mL of concentrated sulfuric acid (98%), and the mixture was shaken for 1 h and then determined at a wavelength of 389 nm. The adsorbed amounts on graphene were calculated directly through the mass difference between the initial and equilibrium concentrations. The dispersion of graphene by NaC was examined following the same procedures as in the surfactant adsorption experiments. Briefly, after shaking for 5 days and two cycles of centrifugation, all vials were left to stand for 24 h, and then the turbidity of suspensions were measured at 800 nm using a UV− vis spectrophotometer.22 Data Modeling. All experiments were run at least in duplicate. Data were compared by one-way analysis of variance (ANOVA) (p < 0.05) using SPSS 13.0. Nonlinear DubininAshtakhov (DA, eq 1) and Freundlich models (eq 2) were employed to fit the isotherms on phenanthrene adsorption by graphene, while the Langmuir model was used for NaC adsorption by graphene (eq 3)

qe =

where qe (mg/kg) is the equilibrium adsorbed concentration of solute; Cfree and Ce (mg/L) are the equilibrium aqueous concentrations of phenanthrene and NaC, respectively; Q0 (mg/kg) is the maximum adsorption capacity; ε (kJ/mol) = −RT ln(Cfree/Cs) is the effective adsorption potential, where Cs (mg/L) is the water solubility of solute, R [8.314 × 10−3 kJ/ (mol K)] and T (K) are universal gas constant and absolute temperature, respectively; E (kJ/mol) is the “correlating divisor” and b is a fitting parameter; Kf [(mg/kg)/(mg/L)n] is the Freundlich affinity coefficient, and n is the Freundlich exponential coefficient; KL is the Langmuir affinity coefficient.

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RESULTS AND DISCUSSION Adsorption of Phenanthrene on Graphene, Carbon Nanotube, and Graphite. Similar adsorption results were obtained by the passive dosing and conventional batch/ centrifugation techniques (Figure S4, Table S3). The passive dosing technique was therefore employed to study phenanthrene adsorption on graphene, MWCNTs, and graphite. In the passive dosing vials, the Cfrees were within the range of 0.001− 0.4 mg/L (Figure S1). Adsorption isotherms for phenanthrene on the test carbon materials are presented in Figure 1A. Better fitting was observed using Freundlich and DA models (Tables S4 and S5) compared to the Langmuir model (Table S6). Therefore, the following discussion is based on the fitting results of Freundlich and DA models. As indicated by Freundlich-fitted n values, adsorption isotherm on graphene was less nonlinear than MWCNTs. This finding was consistent with that reported by Yang et al.,20 in which the n values increased with decreasing curvatures of MWCNTs. Obviously, the maximum adsorption capacity (Q0) followed an order: graphene > MWCNTs > graphite (Table S5). Surface areas and micropores volumes of the three carbon materials are presented in Table S2. Due to narrow space (∼0.34 nm),16 the interlayer or interwall spacing of graphene and MWCNT are not accessible for both N2 and phenanthrene molecules. Also, inner cavities of MWCNTs are inaccessible because of the interior impurities such as catalysts and amorphous carbons,16 which is one possible reason for the lower surface area of MWCNTs than graphene (Table S2). Micropores of graphene are believed to originate from the structural defects of graphene.23 Folding/aggregation of graphene sheets may also form micropores (the folding effect is discussed in the “Desorption” section). For the three carbon materials, surface areas and micropore volumes exhibited the same order with their adsorption capacities (Tables S2 and S5). However, MWCNTs had higher adsorption capacity after normalizing with surface area or micropore volume (Q0/Asurf, Q0/Vmicro) than graphene (Table S5). The employed graphene sheets had more oxygen-containing functional groups than MWCNTs as indicated by the elemental compositions of these two materials (6.3% vs 0.1%) (Table S2), likely due to their different synthesis methods. Therefore, the hydrophobic sites (per square meter) for phenanthrene sorption on the graphene sheets are not as abundant as MWCNTs. Adsorption capacities of carbon adsorbents including graphene, CNTs, biochar, and activated carbon for phenanthrene were summarized and compared (Table S7). Activated carbon had the highest Q0 (mg/g) value, while Q0/Asurf (mg/m2) value of graphene was comparable with activated carbon and other carbon materials such as CNTs. This suggests that graphene could be potentially

Q0 b

10(ε / E)

(1)

qe = K f Cfree n

(2)

(3)

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sheets. Then, a closed interstitial space would be formed as a polyhedron at the edge of this sheet (Figure 1C, State II). This closed interstitial space could also be created around the whole edge of a small sheet when it was overlapped by other big sheets. Thus, phenanthrene molecules could be entrapped in these interstitial spaces (State I and II) due to rearrangement of graphene sheets, and this fraction of phenanthrene would not release into aqueous solution during the desorption process. Effect of Sonication on Phenanthrene Adsorption by Graphene. Sonication is a commonly used process during graphene production and application. Figure 2A compared the

Figure 1. Adsorption of phenanthrene on graphene sheets: (A) Phenanthrene sorption on graphene, carbon nanotube, and graphite; (B) Desorption of phenanthrene from graphene sheets; (C) TEM images of graphene sheets. In panel C, the left image showed the curling of graphene sheets (State I), while the right image showed the overlapping between two sheets (State II).

used as an efficient adsorbent after exfoliation to expose more surface sites. Desorption is another important interfacial process in aqueous environments. Desorption of phenanthrene from graphene sheets was therefore investigated (Figure 1B). Obvious hysteresis was observed during the two cycles of desorption. For carbon materials, there are two possible explanations for the hysteresis phenomenon: (1) deformation and rearrangement between aggregates of carbon materials (e.g., C60)16 and (2) formation of chemical bonds on the surface (e.g., CNTs).24,25 In the present study, no covalent bonds could form on the graphene sheets due to the absence of functional groups on phenanthrene molecules. Hence, deformation and rearrangement of graphene sheets are likely responsible for the observed hysteresis phenomenon. Due to the strong van der Waals attraction, the parallel graphene sheets tend to aggregate together in aqueous solution.26 This aggregation (folding) could occur on an individual sheet because of its large two-dimension surface. As shown in Figure 1C, a graphene sheet was curled or folded by itself, thus the closed interstitial spaces could be formed inside (State I). When several individual multilayer graphene sheets aggregated together, the edge of a sheet could be enclosed by other

Figure 2. Phenanthrene sorption by original graphene and graphene treated with sonic bath or probe (A) and SEM images of original graphene (B) and graphene after sonication with probe (C).

adsorption of phenanthrene on graphene sheets before and after sonication (probe and bath). The three adsorption isotherms almost overlapped together. The maximum adsorption capacity increased slightly but was not significantly altered by sonication (p < 0.05), with a Q0 value from 28.2 ± 1.1 mg/g (without sonication) to 29.5 ± 1.2 mg/g (by probe) and 29.8 ± 1.2 mg/g (in sonic bath), respectively (Table S5). Before sonication, the big aggregates were around 500 μm (Figure 2B). After sonication by probe and bath, a large number of small particles around 20 μm were observed (Figures 2C and S5). These findings indicated that the available adsorption sites for phenanthrene were not highly increased by the sonication intensity used in this study, although big aggregates could be 334

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broken down to relatively small pieces. The deconstruction of big aggregates may alter the adsorption site distribution of graphene; we therefore further analyzed the surface area and pore distribution of graphene before and after sonication (Table S2). Interestingly, the sonicated graphene samples had higher micropore volume and lower mesopore volume. Thus the sonication treatment decreased mecopore volume due to the deconstruction of large aggregates, while creating more micropores. The overall effect of sonication was the slight increase in the surface area of graphene from original 137 m2/g to sonicated 148−150 m2/g. This increase could explain the slight increase of Q0 after sonication. Oxidization of carbon materials (e.g., CNTs) was reported during sonication.27 The increase of oxygen-functional groups could induce better dispersion of CNTs and more sites for HOC adsorption. On the other hand, the functionalized sites are unavailable for phenanthrene adsorption, thus the sites for phenanthrene adsorption may be decreased. Therefore, the elemental compositions of graphene sheets before and after sonication were determined (Table S2). Clearly, the oxygen contents were not increased after sonication. We further analyzed the change of functional groups using FTIR, whereas no clear change was observed (Figure S6). Therefore, oxidization of graphene did not occur significantly (if any) in the present work. Our finding also revealed that the direct sonication (both sonic probe and bath) was unable to effectively exfoliate multilayer graphene as indicated by the similar surface areas and adsorption capacities before and after sonication. This ineffective exfoliation can be explained by (1) resonant oscillations of graphene sheets28 and (2) high energy barriers between single layers due to strong interlayer conjugation and van der Waals attractive forces.29 Dispersion of Graphene by the Surfactant NaC and Its Adsorption on Graphene. The well-exfoliated graphene sheets from graphite tend to disperse in aqueous solutions with the aid of surfactant.7 Therefore, we investigated the dispersion of sonicated graphene in the presence of NaC by measuring the turbidity of graphene suspensions. A positive linear relationship exists between dispersed graphene concentrations and turbidity of suspensions (Figure S7). As shown in Figure 3A, a larger amount of graphene was dispersed by NaC under sonication compared to the nonsonication treatment. Sonic probe (23 w, 5 min) was more effective than sonic bath (100 w, 2 h), even though the sonic probe used lower nominal output wattage and shorter time. It is probably because the sonic probe was directly inserted into graphene suspension without hindrance by the glass wall of vials. The centrifugation (3000 rpm, 20 min) could remove the large and thick graphene sheets. It was reported that 20% of dispersed graphene sheets (number fraction) were monolayer and 80% were less than five layers after being separated at a centrifugation rate of 1500 rpm.7 So it is reasonable that the dispersed graphene sheets after sonication by probe has been well-exfoliated (five layers or less). Besides exfoliation, graphene sheets may be shortened to small sheets induced by sonication, and probe is more effective than sonic bath.7,30 In our work, this sonication-induced fragmentation could be another reason for the observed good dispersion of graphene (Figures 3A and S8). CMC of NaC was measured using the passive dosing technique based on the different solubilization performance of NaC monomers and micelles. NaC monomer has no solubilization effect, while a high amount of phenanthrene could release from silicone and partition into NaC micelles. As

Figure 3. Dispersion of graphene by NaC (A) and sorption of NaC (B) on graphene under sonication and nonsonication treatments. In panel A, the turbidity of suspensions was measured at 800 nm (OD800) for all supernatants as a function of initial NaC concentration. The graphene supernatants were obtained by two circles of centrifugation (3000 rpm, 20 min).

shown in Figure S9, the CMC value of NaC calculated through the solubilization curve is 5700−5800 mg/L. Consistent values in the range of 13−14 mM (5600−6000 mg/L) were reported using different methods such as capillary chromatography, spectrophotometry and conductometry.31,32 Therefore, the passive dosing technique could be applied as an effective approach for CMC determination. With the radioactive phenanthrene as guest molecule/probe, the determination could be conducted under complicated solution conditions (e.g., high ionic strength, in the presence of another organic compound). In Figure 3A, dispersion sharply increased with increasing NaC concentration and then reached a relative steady state at an initial NaC concentration of 300 mg/L. This turning point was much lower than CMC of NaC (5700−5800 mg/L), indicating that the dispersion of graphene is related to the graphene/surfactant ratio, independent of the CMC value. Adsorption isotherms of NaC on graphene are presented in Figure 3B and fitted well with the Langmuir model (Table S8). The fitted adsorption capacities showed that NaC adsorption by graphene sonicated with probe was higher than the other treatments, which was in agreement with the dispersion results. It should be noted that even a low percentage of suspended graphene could lead to a high turbidity value. Therefore, the difference of adsorption capacities between probe and other treatments was not as obvious as turbidity (Figure 3A). On the other hand, an insignificant difference (even slightly lower) of adsorption capacity was observed for graphene sonicated with bath compared to the original graphene, probably due to the similar low-dispersion for both treatments; however, presently we do not have any plausible explanation for the slightly 335

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reduced adsorption capacity. Calculated from the fitting results, the equilibrium aqueous concentrations of NaC for the three treatments were 293−295 mg/L when the initial concentration was 300 mg/L. Clearly, aqueous NaC concentration at the turning point (293−295 mg/L) of the dispersion curves (Figure 3A) was higher than the concentrations (around 90 mg/L) at which adsorption of NaC reached a plateau (Figure 3B). This means that the effective dispersion of graphene by NaC occurred after the achievement of adsorption saturation. This finding is reasonable because the graphene surface would be highly hydrophilic after adsorption saturation due to the uniform coating of NaC molecules. Effects of NaC on Phenanthrene Adsorption: Competition and Exfoliation. Solubilization of phenanthrene by the surfactant NaC is shown in Figure S10. Clearly, sonication did not influence the solubilization effect of NaC. All solubilization curves were fitted well with a linear model, consistent with the previous studies on bile salts and phospholipid.19,33 Taking into account the possible partitioning of polar organic molecules from aqueous solution into silicone,34 we examined the partitioning of NaC molecules into silicone in the passive dosing vials. The result showed that the partitioning of NaC molecules was insignificant (Figure S11). Unlike phenanthrene, NaC has higher solubility and bigger molecular size (Table S1), which are responsible for this negligible partitioning. Figure 4 shows the adsorption isotherms

calculated as follows: Csolubilized = (0.4812Cfree−0.0003) × CNaC, where CNaC (g/L) was the free concentration of NaC. CNaC was assumed to be the equilibrium aqueous concentration of NaC and obtained from the isotherms in Figure 3B and their fitting results in Table S8 because phenanthrene had an insignificant influence on the adsorption of NaC by graphene (Figure S12). It should be noted that for the isotherms in Figure 4B, the initial NaC concentration (100 mg/L) was much lower than its CMC value, so the solubilization effect of NaC (Csolubilized) was negligible (less than 0.2%) and not taken into account. It was reported that the adsorbed surfactant (cetylpyridinium chloride, CPC) could form hemimicelles for naphthalene partitioning.10 However, unlike conventional “head-and-tail” surfactants such as CPC, NaC molecules are rigid facial amphiphiles with a polar face and a nonpolar face.9,35 The nonpolar face of a NaC molecule should adsorb on a graphene sheet while leaving the polar face exposed in aqueous solution due to hydrophobic interaction with graphene and hydrogen bonding with water. This conformation will not result in the formation of hemimicelles due to lack of loose hydrophobic spaces among NaC monomers. Moreover, the exposing surface of adsorbed NaC is highly hydrophilic and would not adsorb phenanthrene significantly. Therefore, we interpret the calculated qe in isotherms (Figure 4) as the phenanthrene directly adsorbed on graphene sheets. The adsorption isotherms for phenanthrene in the presence of NaC were fitted well with Freundlich and DA models (Table 1) rather than the Langmuir model (Table S9). For the treatments without sonication, NaC increased the linearity of adsorption isotherm for phenanthrene on graphene as indicated by Freundlich-fitted n values (Table 1, S4). This indicated that the competition between phenanthrene and NaC occurred on graphene sheets, which is supported by the reduction in adsorption (as discussed below) and the same phenomenon observed between different polycyclic aromatic hydrocarbon molecules.36 Similar n value increase and competitive adsorption were reported when natural organic matter coexisted with phenanthrene on the graphene surface.37 In the presence of NaC (both 100 and 8000 mg/L), the adsorption isotherms were more nonlinear under sonication (both sonic probe and bath), indicating that sonication increased the distribution of heterogeneous adsorption sites. The DA-fitted Q0 values (Table 1) were significantly increased under sonication (p < 0.05), and the order followed probe > sonic bath > nonsonication for NaC above and below its CMC. Moreover, for each sonication treatment, all fitted Q0 values in the presence of NaC (Table 1) were lower than those in the absence of NaC (Table S5), further confirming the occurrence of competitive adsorption between phenanthrene and NaC molecules, regardless of whether the graphene sheets were sonicated or not. The observed competition resulted from the occupation of hydrophobic sites by NaC molecules, thus causing less available adsorption sites for phenanthrene. It is noted that the competition effect was stronger at lower NaC concentration (100 mg/L) for both sonication and nonsonication treatments, while adsorption of NaC molecules on graphene sheets was higher at higher NaC concentrations (Figure 3B). Thus, more sites for phenanthrene adsorption on graphene were created at the higher NaC concentration (8000 mg/L). This finding is expected and supported by the dispersion data (Figure 3A), in which obviously higher dispersion was observed at a NaC concentration of 8000 mg/ L compared to that of 100 mg/L.

Figure 4. Sorption of phenanthrene on graphene in the presence of surfactant NaC under sonication treatments. The tested concentrations of NaC were 8000 (A) and 100 mg/L (B), respectively.

for phenanthrene by graphene in the presence of NaC. In these isotherms, the adsorbed concentration of phenanthrene on graphene (qe, mg/g) at equilibrium was calculated using the following equation: qe = (Ctotal−Csolubilized−Cfree)/Cgraphene, where Cgraphene (g/L) was the concentration of graphene in the aqueous solution. In this equation, Csolubilized (mg/L) was obtained from the isotherms for phenanthrene in NaC 336

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Table 1. Fitting Results of Phenanthrene Sorption Isotherms on Graphene in the Presence of NaC by Freundlich and DubininAshtakhov (DA) Models Freundlich model surfactant

treatment

NaC 8000 mg/L

probe bath nonsonication probe bath nonsonication

NaC 100 mg/L

Kfa ( × 103) 30.0 27.1 28.3 28.8 25.5 22.3

± ± ± ± ± ±

0.854 0.353 1.17 0.926 0.828 1.13

n 0.422 0.430 0.525 0.346 0.340 0.460

± ± ± ± ± ±

0.012 0.006 0.019 0.012 0.012 0.022

DA model radj2b

KFc (L/g)

0.990 0.998 0.985 0.984 0.983 0.973

98.0 87.1 74.9 110 98.5 67.4

Q0 ( × 103 mg/kg) 28.1 26.0 25.7 23.6 21.4 21.1

± ± ± ± ± ±

1.2 1.2 1.2 1.1 1.1 1.3

b 1.22 1.15 1.26 1.39 1.38 1.15

± ± ± ± ± ±

0.17 0.15 0.17 0.10 0.15 0.20

E (KJ/mol)

radj2b

KDAc (L/g)

13.8 13.9 11.4 17.2 17.2 12.9

0.985 0.993 0.987 0.994 0.990 0.983

100 87.8 76.7 111 100 66.3

a

Kf unit is [(mg/kg)/(mg/L)n]. bradj2 is the adjusted coefficient of determination, and it is influenced by both the number of data points (m) and the number of fitting parameters (p). radj2 =1 − (m−1)(1−r2)/(m−p−1). cKF and KDA are the single point adsorption coefficients calculated on the basis of the Freundlich and DA models, respectively when Cfree is 0.1Cs. Cs is water solubility of phenanthrene at 25 °C.

Exfoliation and fragmentation of multilayer graphene sheets are two possible reasons for the increase of adsorption sites induced by sonication. Therefore, we introduced the other two materials, MWCNT and graphite, for further examination. MWCNT can be broken down under sonication38 but be hardly exfoliated due to its rolled-up structure and much less exposed edges. Graphite is known to be easily exfoliated and broken down with the aid of surfactant.30 Figure 5 shows their adsorption isotherms for phenanthrene in the presence of NaC. All isotherms were well-fitted using the DA model (Table S10). Interestingly, phenanthrene adsorption on MWCNT in the presence of NaC (100 mg/L) had no difference between sonication and nonsonication treatments from both adsorption isotherms and their fitted Q 0 values, suggesting that fragmentation of MWCNT and deconstruction of big aggregates (if any) had no effect on phenanthrene adsorption. For graphite, Q0 values increased from 2.48 mg/g (without sonication) to 3.15 and 3.71 mg/g (sonicated with bath and probe, respectively). Combined with the finding on graphene (Figure 4, Table 1), it is confirmed that multilayer graphene sheets were truly exfoliated under sonication with the aid of NaC, and the exfoliation was responsible for the mitigation of competitive adsorption between phenanthrene and NaC under sonication. Moreover, we examined the exfoliation of multilayer graphene using TEM (Figure S13); a large amount of singlelayer (or only a few) graphene sheets were observed for graphene samples with sonication in the presence of NaC. Taking the data together, the effect of surfactant NaC on phenanthrene adsorption is clearly related to competition and exfoliation, and a schematic illustration is presented in Figure 5C. Competition of NaC suppressed phenanthrene adsorption on multilayer graphene (Process I), and the competition was weakened with the aid of sonication due to graphene exfoliation to create more adsorption sites (Process II). It should be noted that the exfoliation of multilayer graphene in this work was incomplete because DA-fitted Q0 for phenanthrene in the presence of NaC increased only 10% with the assistance of sonication (probe) (Table 1). This incomplete exfoliation could be supported by the same extent of the increased adsorption capacities for NaC (Table S8). Higher exfoliation may be obtained by longer and/or more powerful sonication with a more effective-dispersing surfactant.

Figure 5. Sorption of phenanthrene on MWCNT (A) and graphite (B) in the presence of NaC (100 mg/L) under sonication treatments and schematic illustration for the effect of surfactant NaC on phenanthrene adsorption on multilayer graphene (C). In panel C, the hydrophilic face of the NaC molecule (blue area) would be exposed in water after adsorption, thus phenanthrene would not be adsorbed on the surface of NaC molecules. Therefore, sorption of NaC will lower the amount of phenanthrene adsorbed on graphene without sonication (i.e., I: competition process in Panel C). However, sonication in the presence of NaC will exfoliate graphene sheets to create additional surface and sites for elevated adsorption (i.e., II: exfoliation in Panel C).

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ENVIRONMENTAL IMPLICATION For the first time, we investigated the effects of surfactants on HOC adsorption by graphene sheets. In the absence of NaC, graphene had much higher adsorption capacity for phenan337

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threne than MWCNT and graphite. This result demonstrated the potential of graphene to be used as a superior adsorbent. The observed desorption hysteresis implied that graphene could trap HOC molecules as a sink after adsorption, thus decreasing the availability of HOCs. In the presence of NaC, dispersion of graphene depended on the ratio of graphene to NaC rather than the CMC of NaC. The competition of NaC molecules (both below and above CMC) resulted in reduced adsorption of phenanthrene (likely other HOCs as well) on multilayer graphene sheets. However, when graphene-surfactant complexes enter aquatic environments, the adsorption capacity of graphene for HOCs could likely increase because of desorption of surfactants from graphene sheets and more available sites for phenanthrene adsorption. More importantly, we found that multilayer graphene could be readily exfoliated by NaC with the aid of sonication to mitigate the competition between phenanthrene and NaC molecules and increase adsorption of HOCs. Therefore, the results of this research suggest a new approach to enhance the adsorption potential of graphene materials as adsorbents. Finally, by overcoming the disadvantages of the batch/centrifugation technique such as incomplete separation of sorbents and dissolved materials (e.g., graphene and NaC) from aqueous phase, the passive dosing technique is demonstrated to be an effective way for complicated adsorption studies.

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ASSOCIATED CONTENT

S Supporting Information *

Thirteen figures and ten tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 413 545 5212. E-mail: [email protected] (B.X.). *E-mail: [email protected] (Z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was in part supported by NSFC (41120134004, 41325013, 41328003) and USDA-AFRI Hatch program (MAS 00978).

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