Hydrophobic-Polymer-Grafted Graphene Oxide Nanosheets as an

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Hydrophobic-Polymer-Grafted Graphene Oxide Nanosheets as an Easily Separable Adsorbent for the Removal of Tetrabromobisphenol A Xubo Zhao and Peng Liu* State Key Laboratory of Applied Organic Chemistry and Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China S Supporting Information *

ABSTRACT: Hydrophobic polymer brushes have been grafted from graphene oxide nanosheets (GO) via the facile surface-initiated redox radical polymerization of tert-butyl acrylate (tBA) from the GO with cerium ammonium nitrate (CAN) as an oxidant. After the hydrophobic surface modification, the poly(tert-butyl acrylate) (PtBA)-grafted graphene oxide nanosheets (GO-PtBA) could still be dispersed in water because of the remaining oxygencontaining groups but deposited within 40 min. The feature makes it an easily separable adsorbent for environmental pollutants. For example, tetrabromobisphenol A (TBBPA) could be removed from aqueous solution via hydrogen bonds (between hydroxyl groups of TBBPA and hydroxyl and carboxyl groups of GO) and π−π interactions (between the benzene ring of TBBPA and GO), with an adsorption capacity of 22.2 mg g−1 at pH 7.0. The TBBPA-adsorbed GO-PtBA could be deposited completely within 30 min, and the adsorbed TBBPA could be easily desorbed with ethanol, demonstrating its good recyclability.

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INTRODUCTION Recently, graphene oxide (GO) has been widely used as the adsorbent for various environmental pollutants such as heavy metal ions and organics.1 These environmental pollutants could be adsorbed onto the GO nanosheets via complexation,2 hydrogen bonds,3 or π−π interactions4 owing to the functional groups and benzene ring structure of GO. To improve their adsorption performance (adsorption rate, capacity, or selectivity), the GO nanosheets usually have been modified with certain substances.5,6 However, one drawback of the nanoscaled GO-based adsorbents is their serious agglomeration and restacking during application via π−π interaction of the sheets, which reduces the effective surface area and consequently the adsorption capacity.7 To effectively reduce or prevent the restacking and agglomeration of the GO nanosheets, water-soluble polymer brushes such as poly(ethylenimine) (PEI)8 and poly(acrylamide) (PAM)9 have been grafted onto the GO nanosheets. At the same time, their dispersibility and adsorption performance have also been enhanced. It is well known that the stably functionalized GO derivatives are much less toxic than the unfunctionalized one.10 The low toxicity of the polymer-grafted GO nanosheets is another advantage for water treatment. Even more serious is that the separation of the GO-based nanoscaled adsorbents is very difficult after adsorption because of their excellent dispersibility and stability in water.8 Therefore, high-speed centrifugation is usually required. Obviously, this is not practical for water treatment because of the large quantity of wastewater. Most recently, researchers have focused © 2014 American Chemical Society

on magnetic nanomaterials. Once the GO-based nanoscaled adsorbents are modified with the magnetic nanoparticles, the obtained magnetic GO-based nanoscaled adsorbents could be easily separated and recovered with external magnetic fields.11,12 Tetrabromobisphenol A (TBBPA), a brominated flame retardant (BFR), is extensively used in the fire safety treatment of paper, textiles, plastics, circuit boards, and many other consumer goods. Because of its extensive application, TBBPA ubiquitously occurred in the environment.13 Additionally, in vitro experiments showed that it might induce the disruption of endocrine function, cytotoxicity, immunotoxicity, and neurotoxicity.14,15 Therefore, it is necessary to establish approaches to remove TBBPA from the environment by adsorption16,17 or photocatalytic degradation.18 To simultaneously prevent the restacking and agglomeration of the GO nanosheets and realize their easy separation after adsorption, the hydrophobic polymer brushes of poly(tert-butyl acrylate) (PtBA) have been grafted from the graphene oxide nanosheets (GO) via the facile surface-initiated redox radical polymerization of tert-butyl acrylate (tBA) in the present work. After the hydrophobic surface modification, the grafted graphene oxide nanosheets (GO-PtBA) could still be dispersed into water as well as the unmodified GO, and the dispersion was stable with slight concussion for days. On standing, they could deposit completely within 40 min. Interestingly, the GObased nanoscaled adsorbent could deposit completely within Received: October 15, 2014 Published: October 22, 2014 13699

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A 100 mL 20 mg L−1 dosage of GO-PtBA5 was added to the TBBPA solution (10 mg mL−1 for the pH effect and adsorption kinetics, 5−40 mg mL−1 for the adsorption thermodynamics). The mixture was shaken in a water bath mechanical shaker for certain time intervals at 140 rpm and 30 °C until equilibrium. The adsorbents were separated by standing for 30 min, and residual TBBPA was detected by UV/vis spectrophotometry at 277 nm to calculate the adsorption capacity of TBBPA (Q (mg g−1)). All experiments were carried out in triplicate. The separated TPPBA-adsorbed adsorbents were washed thoroughly with water and shaken in 100 mL of ethanol at 140 rpm for 6 h for desorption. The recycled adsorbent was separated by standing for 30 min. Characterization. The morphologies of the GO-PtBA samples were analyzed with a transmission electron microscope (JEM-1200 EX/S, JEOL, Tokyo, Japan). The samples were dispersed in water and then deposited on a copper grid covered with a perforated carbon film, followed by drying in vacuum at room temperature. The Fourier transform infrared (FTIR) spectroscopy analysis of the GO-PtBA samples was characterized with an infrared spectrometer (IFS 66 v/s, Bruker, Karlsruhe, Germany) in the range of 400−4000 cm−1 with a resolution of 4 cm−1 by the KBr pellet technique. Raman spectra were recorded out with a Horiba Jobin-Yvon LabRAM HR 800 UV Raman spectrometer using an excitation laser with a wavelength of 532 nm. Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (Diamond TG, Pyris Diamond TG/DTA, PE Instruments, USA) at a heating rate of 10 °C min−1 from 30 to 600 °C in an N2 atmosphere. The zeta potentials of GO-PtBA5 were analyzed with a Zetasizer Nano ZS (Malvern Instruments Ltd., U.K.) in media at the desired pH values. The behavior of adsorption and desorption of the GO-PtBA5 adsorbent was assessed with a UV−vis spectrometer (PerkinElmer Lambda 35, PE Instruments, USA) at room temperature.

20 min, after the adsorption of a lipophilic molecule such as TBBPA. This feature makes it an easily separable adsorbent for environmental pollutants. The adsorption performance of TBBPA from aqueous solution was also investigated in detail.

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EXPERIMENTAL SECTION

Materials. Graphite powder was obtained from Huatai Chemical Reagent Co. Ltd. (Shandong, China). tert-Butyl acrylate (tBA) was purchased from J & K Chemical Ltd. TBBPA was purchased from Sichuan Chemical Co. Ltd. (China). Cerium ammonium nitrate (CAN) and other reagents used were analytical reagent grade. Deionized water was used throughout the experiments. Preparation of Graphene Oxide (GO). Graphene oxide (GO) was prepared through a modified Hummers method from the graphite powder.19 Briefly, graphite flakes (1.25 g) were added to a mixture containing 8.02 mL of concentrated H2SO4, 1.00 g of K2S2O8, and 1.04 g of P2O5 in a round-bottomed flask and then vigorously stirred at 82 °C for 8 h to pretreat the graphite flakes. The product was then dried, after being washed with deionized water until neutral, and filtered. This pretreated graphite was then subjected to further oxidation by the Hummers method by adding 1.01 g of the pretreated graphite powder to 50.1 mL of concentrated H2SO4 containing 1.00 g NaNO3 in a two-round-bottomed flask. After 4.01 g of KMnO4 was added gradually with stirring over about 1 h at around 0 °C in an ice− water bath, the mixture was vigorously stirred for 2 days at room temperature. Afterward, 100 mL of a 5 wt % H2SO4 aqueous solution was added within 1 h under stirring and stirred for another 2 h at 98 °C. After the mixture was cooled to 60 °C, 3.2 mL of 30% H2O2 was added. As a result, the color of the mixture changed to bright yellow.20 The mixture was stirred for 2 h at room temperature, centrifuged, and washed with 10% HCl solution and then with deionized water to remove any residual metal ions until the solution became neutral, after which individual GO nanosheets were stably dispersed in deionized water. The resulting GO nanosheets were dried at 65 °C in vacuum. Surface-Initiated Redox Radical Polymerization. The PtBA brushes were grafted from the surface of the GO nanosheets via the surface-initiated redox radical polymerization technique with cerium ammonium nitrate (CAN) as an oxidant. The procedure was referenced to the valuable report by Tu’s group.21 In a typical polymerization, a 200 mL dispersion of GO nanosheets (0.10 mg mL−1) was prepared in deionized water with the aid of ultrasound for 30 min; a certain amount of tBA monomer was then added to the dispersion. CAN (3.0 mL, 0.10 mol L−1) in nitric acid solution was added to the dispersion dropwise for 3 min, and then the mixture was stirred at 40 °C under an N2 atmosphere. The polymerization process was carried out with stirring at 40 °C for 4 h. A series of tBA feeding ratios were utilized to modify the GO nanosheets in order to obtain the desired product along with the same amount of CAN (Table 1). After

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RESULTS AND DISCUSSION Synthesis of GO-PtBA. Figure 1a shows the TEM image of the as-prepared GO that well indicates a sheet structure with a size of about 200 nm. The transparent GO sheets have exhibited a monolayer or few-layer planar sheet.22 Then the hydroxyl groups on GO surfaces were used as the reducing agents to initiate the polymerization, coupled with Ce(IV) salts in nitric acid solution at moderate temperatures. Another advantage of this surface-initiated redox radical polymerization is that the free ungrafted polymers can be neglected because the initiation sites were formed on the GO nanosheets. Compared to previous work,21 the products in this work could deposit completely within 40 min because of the hydrophobic polymer brushes. Thus, high-speed centrifugation is not needed for separation. After the surface-initiated redox radical polymerization of tBA with different feeding ratios (Table 1), the GO nanosheets became larger and wrinkled, especially for the products with higher tBA feeding ratios (GO-PtBA4 and GO-PtBA5). A thin coating could be clearly observed on the graphene oxide, revealing the grafted PtBA polymer brushes.23 To investigate the grafting further, we compared the FTIR spectra of GO and GO-PtBA in Figure 2. GO shows the characteristic absorbance of O−H stretching at 3427 cm−1, CO stretching at 1722 cm−1, C−O stretching at 1066 cm−1, and the characteristic band at 1622 cm−1 for graphite.24 After the surface-initiated redox polymerization, a strong CO stretching absorbance at 1728 cm−1 of the carbonyl characteristic band introduced by PtBA and the well-defined characteristic absorbances at 1453 and 1369 cm−1 owing to the symmetrical deformation vibration of the tertiary butyl groups in

Table 1. Polymerizing Conditions samples

tBA (mL)

CAN (g)

GO-PtBA1 GO-PtBA2 GO-PtBA3 GO-PtBA4 GO-PtBA5

0.125 0.25 0.50 1.00 2.00

0.1589 0.1589 0.1589 0.1589 0.1589

polymerization, the suspension was separated by centrifugation (6000 rpm, 4 min), and the sediments were washed extensively with THF to remove the unreacted monomer and possible free polymers. The resulting products (GO-PtBA) were dried at 35 °C under vacuum to constant weight. Adsorption and Desorption Experiments. The adsorption experiments were carried out by a batch method. The dispersion of the GO-PtBA5 adsorbent was adjusted to the desired pH value with 0.1 mol L−1 HCl or 0.1 mol L−1 NaOH at room temperature. 13700

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Figure 1. TEM images of GO (a), GO-PtBA1 (b), GO-PtBA2 (c), GO-PtBA3 (d), GO-PtBA4 (e), and GO-PtBA5 (f).

Figure 2. FTIR spectra of GO and GO-PtBA5 measured in KBr pellets.

Figure 3. TGA curves of GO, GO-PtBA1, GO-PtBA2, GO-PtBA3, GOPtBA4, and GO-PtBA5 at a heating rate of 10 °C min−1 in N2.

the t-BA units appeared in the spectrum of the product,25 indicating that the hydrophobic brushes of PtBA had been successfully grafted onto the GO nanosheets. The TGA results also proved the success of the grafting process (Figure 3). The mass loss near 100 °C resulted from the moisture.26 A relatively higher mass loss of about 35% occurred around 200 °C as a result of the outgassing of the oxygencontaining groups. The weight losses occurred at 300−500 °C after polymerization. Furthermore, the weight losses increased with the increasing feed ratio of tBA in the polymerization.27

A maximum percentage of grafting (PG%, mass ratio of the grafted polymer to the GO nanosheets) of 37% was achieved for the GO-PtBA5 sample, synthesized with 2.00 mL of tBA added during polymerization. The GO nanosheets could be dispersed well in water because of their oxygen-containing groups. After the grafting of the hydrophobic polymer brushes, the GO-PtBA samples with different numbers of PtBA brushes (GO-PtBA1, GO-PtBA2, GOPtBA3, GO-PtBA4, and GO-PtBA5) can also dispersed in water (Figure 4A). Therefore, hydrophilic groups such as hydroxyl 13701

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Figure 4. Digital photographs of the dispersion status of GO: (a) GO-PtBA1, (b) GO-PtBA2, (c) GO-PtBA3, (d) GO-PtBA4, (e) and GO-PtBA5 (f) in aqueous solution and (g) in ethanol solution, respectively, for different dispersion times of (A) 0, (B) 20, and (C) 40 min. The concentration of all samples was around 0.5 mg mL−1 at pH 7.00.

and carboxyl groups in the GO nanosheets have remained during the polymerization. However, the dispersion stability of the GO-PtBA samples is much lower than that of the GO nanosheets, except for the GO-PtBA1 sample with the lowest PG%. The GO-PtBA samples with higher PG% (GO-PtBA2, GO-PtBA3, GO-PtBA4, and GO-PtBA5) deposited completely within 40 min because of the grafting of the hydrophobic polymer brushes, whereas the GO nanosheets remained an excellent dispersion (Figure 4C). The good dispersibility of the products is favorable to their adsorption property, and their deposition within a certain time renders them easily separated after adsorption. This is the most valuable improvement for the graphene-based adsorbents. An interesting phenomenon has also been noticed in which GO-PtBA samples with a relatively low PG% (especially for the GO-PtBA2 and GO-PtBA3) are deposited as matter visible to the naked eye (Figure 4B), which might be due to the aggregates of the GO nanosheets. Therefore, it could be concluded that the lower PG% of the hydrophobic grafted polymer could not efficiently prevent the GO nanosheets from restacking and agglomerating. On the basis of the above discussion, the GO-PtBA5 sample with the highest PG% was selected as the adsorbent for TBBPA. Adsorption Performance with Respect to TBBPA. As is well known, the π−π interaction is usually employed to explain the sorption mechanism of organic pollutants containing CC groups or benzene rings on flat graphene via the interaction

Figure 5. Raman spectra of TBBPA-adsorbed GO-PtBA5 and GOPtBA5 at a laser excitation wavelength of 532 nm.

between their π electrons and the π electrons of graphene.28 The Raman spectra of the GO-PtBA5 adsorbent and adsorbed TBBPA are compared in Figure 5. The G band (1583 cm−1) corresponding to the sp2-hybridized carbon shifted to 1609 cm−1 after the adsorption, indicating that the adsorption should occur via π−π stacking interactions between TBBPA and the aromatic structure on GO-PtBA5,21 although the powerful groove regions formed by wrinkles on the GO surfaces might 13702

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limit the adsorption via π−π stacking interactions29 after the grafting of the hydrophobic polymer brushes. Additionally, the hydrogen bonds between the oxygen-containing groups of GOPtBA5 and the phenolic hydroxyl groups of TBBPA17 and the hydrophobic interaction between the adsorbate and adsorbent might also occur during the adsorption. The effect of pH on the removal efficiency of TPPBA was investigated. As shown in Figure 6, the highest adsorption

Figure 7. Adsorption capacity of GO-PtBA5 toward TPPBA at different adsorption times: 1.10 mg of GO-PtBA5 in 55 mL of 10 mg L−1 TBBPA at pH 7.0 at different temperatures.

(Figure 8C). After 30 min, all of the dispersion deposited completely, as well as the dispersion in ethanol (sample g). The complete deposition of the GO-PtBA5 adsorbent after adsorption and desorption within 30 min indicates that GO-PtBA5 is an easily separable adsorbent. Figure 9 shows the adsorption capacity of the GO-PtBA adsorbents with different PG%. With the increasing in PG% from the GO-PtBA1 to GO-PtBA5 adsorbent, the adsorption capacity decreased gradually from 33.6 to 22.0 mg g−1. With increasing PG%, the GO nanosheets wrinkled heavily versus the adsorption via the π−π interactions between the aromatic rings of the polycyclic aromatic hydrocarbons (PAHs) and the aromatic structure of the graphene layers.32 However, the higher PG% is favorable to its separation after both adsorption and desorption (Figure 8). Thus, the GO-PtBA5 adsorbent was optimized for further experiments. For kinetic analysis, two kinetic models including the pseudo-first-order and pseudo-second-order models were employed to illustrate the adsorption mechanism (Table 2).33 The adsorption of TBBPA on the GO-PtBA5 adsorbent perfectly fits the pseudo-second-order model better than the other model. The R2 values for the pseudo-second-order kinetic models of TBBPA were higher than those from the pseudofirst-order kinetic models (Figure S2a,b). Moreover, the calculated equilibrium adsorption capacity, Qe,cal, was much closer to the experimental value, Qe,exp. Thus, it could be concluded that the rate-limiting step during the adsorption was chemisorption, involving the valence forces via the sharing or exchanging of electrons between TBBPA and GO-PtBA5.33 The adsorption activation energy was established with the linearized Arrhenius equation on the basis of the rate constant in the pseudosecond-order model (K2).34 A plot of ln K2 versus 1/T gives a straight line (Figure S3), and the adsorption activation energy for TBBPA onto GO-PtBA5 was calculated to be 9.15 kJ mol−1 from the slope of the linear plots. The linear plots of Ce/Qe against Ce in the Langmuir isotherm and ln(Ce) against ln(Qe) in the Freundlich isotherm are shown in Figure S4.33 The maximum monolayer adsorption capacity QL and the Freundlich constants nF of the adsorption of TBBPA on the GO-PtBA5 adsorbent were calculated from the slopes of the linear plots. RL2 of the fitted lines with the linear Langmuir isotherm equation and RF2 of the fitted lines with the linear Freundlich isotherm equation were both higher than 0.95 (Table 3), indicating that the adsorption of TBBPA on the GO-PtBA5 adsorbent could be evaluated by both Langmuir and Freundlich models. The Langmuir isotherm

Figure 6. Effect of initial pH on the adsorption of TBBPA on GO-PtBA5: 1.10 mg of GO-PtBA5 in 55 mL of 10 mg L−1 TBBPA.

capacity toward TBBPA was obtained at pH 7.0. There are two proton-binding sites in TBBPA, carboxyl and piperazinyl groups with pKa values of 7.5 and 8.5, respectively. The zeta potentials of the GO-PtBA5 adsorbent increased from −8.9 ± 0.62 to −25.23 ± 1.46 (Figure S1), with increasing pH values from 2.0 to 9.0 resulting from the deprotonation of carboxyl groups in GO, with increasing media pH values. With increasing pH values from 2.0 to 7.0, the carboxyl groups of GO were deprotonated to produce the surface negative charges, and the electrostatic repulsions between the GO nanosheets made them disperse much better in water. So the adsorption capacity for TBBPA increased. At pH 7.0, the adsorption capacity for TBBPA declined with increasing pH values because the anion of TBBPA was much less hydrophobic than the TPPBA molecule.30 When the media pH was increased, the TBBPA molecular fraction decreased and its anion increased, so the π−π interaction with GO was weakened. Furthermore, the electrostatic repulsions between the negatively charged GO and TBBPA species hinder the adsorption. The sorption behavior of TBBPA on the GO-PtBA5 adsorbent versus the contact time was performed at pH 7.0 at three different temperatures (293, 303, and 313 K). The results demonstrated that the adsorption equilibrium time was less than 120 min, with a maximum adsorption capacity of 22.2 mg g−1 (Figure 7). The value in the present work for the GO-PtBA5 adsorbent is not as high as the 115 mg g−1 reported previously for GO,17 mainly because of the severely wrinkled GO morphology (Figure 1f) and the PtBA brushes grafted. It was higher than for loamy clay (0.07 mg g−1) and silt loam (0.27 mg g−1),31 near the MWCNTs/Fe3O4 (22.04 mg g−1).16 After the adsorption, the TBBPA-adsorbed adsorbents could deposit faster, as shown in Figure 8. Within 4 min, the flocculation of the TBBPA-adsorbed adsorbents (especially for GO-PtBA4 and GO-PtBA5 with the higher PG%) occurred obviously (Figure 8B). The larger pieces of matter in the TBBPA-adsorbed GO-PtBA5 deposited within about 12 min 13703

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Figure 8. Digital photographs of the dispersion status of TBBPA-adsorbed GO: (a) TBBPA-adsorbed GO-PtBA1, (b) TBBPA-adsorbed GO-PtBA2, (c) TBBPA-adsorbed GO-PtBA3, (d) TBBPA-adsorbed GO-PtBA4, (e) and TBBPA-adsorbed GO-PtBA5 (f) in aqueous solution and (g) in ethanol solution, respectively, for different dispersion times at (A) 0, (B) 4, (C) 12, and (D) 30 min. The concentrations of all samples were around 0.5 mg mL−1 at pH 6.94−7.01.

and standard Gibbs free energy change (ΔG0, kJ mol−1) could be calculated from the linear plot between ln K versus 1/T × 103 (where K is the intercept of the straight line of (Qe/Ce) versus Qe (Figure S5 a)) (Figure S5b), as reported previously.17 As shown in Table 4, the standard enthalpy change was negative, indicating that the adsorption of TBBPA on the GO-PtBA5 adsorbent was an exothermic process. The standard Gibbs free energy changes were negative at all temperatures, indicating that the adsorption of TBBPA on the GO-PtBA5 adsorbent was thermodynamically feasible and a spontaneous process. Furthermore, all of the standard Gibbs free energy changes were close with rising temperature from 293 to 313 K, meaning that the temperature had only a slight effect on the adsorption. After the adsorption, the TBBPA-adsorbed GO-PtBA5 was eluated with ethanol with shaking at 140 rpm because TBBPA

results indicated that the maximum monolayer adsorption capacities of TBBPA on the surface of the GO-PtBA5 adsorbent were 54.17 mg g−1 at 293 K, 60.57 mg g−1 at 303 K, and 71.63 mg g−1 at 313 K, respectively, indicating that the higher temperature was favorable to the adsorption. In addition, increasing the initial concentration of TBBPA results in an increase in osmotic pressure, and subsequently an increased equilibrium absorption capacity was also observed upon increasing the initial TBBPA concentrations at a constant temperature (Figure S4). The Freundlich isotherm showed that the nF values of the GOPtBA5 adsorbent toward TBBPA increased with increasing temperature, indicating that favorable adsorption occurred at higher temperature. The thermodynamics parameters, standard enthalpy change (ΔH0, kJ mol−1), standard entropy change (ΔS0, J mol−1 K−1), 13704

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Figure 10. Desorption ratio (TBBPA) of ethanol from TBBPA-loaded GO-PtBA5. Figure 9. Effect of PG% on the adsorption of TBBPA: 1.1 mg of GO-PtBA in 55 mL of 10 mg L−1 TBBPA.

Table 2. Parameters of the Adsorption Kinetics Models parameters

pseudo first order

Qe,exp (mg g−1) 293 K

303 K

313 K

pseudo second order 22.13

Qe,cal (mg g−1) K2 (g mg−1 min−1) R2 Qe,cal (mg g−1) K2 (g mg−1 min−1) R2 Qe,cal (mg g−1) K2 (g mg−1 min−1) R2

29.65

25.06 0.0441 0.9956 26.74 0.0585 0.9976 28.57 0.0654 0.9953

0.9720 35.09 0.9489 46.04 0.9448

Figure 11. Reusability of GO-PtBA5 for TBBPA.

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CONCLUSIONS In this work, the separation matter of the nanoscaled GO-based adsorbents was resolved by grafting hydrophobic polymer brushes via surface-initiated redox radical polymerization. In the strategy, the dispersibility of the GO nanosheets remained because of the remaining hydrophilic groups (hydroxyl and carboxyl groups) in GO. The hydrophobic polymer brushes rendered them an easily separable property after both adsorption and desorption. The GO-PtBA5 adsorbent with the highest PG% exhibited the highest adsorption capacity of 22.2 mg g−1 toward TBBPA at pH 7.0 via both the hydrogen bonds and π−π interactions. The TBBPA-adsorbed GOPtBA5 could deposit completely within 30 min. Furthermore, the adsorbed TBBPA could be desorbed with ethanol, and GO-PtBA5 could also deposit completely in ethanol within 30 min. These features make the hydrophobic-polymergrafted GO nanosheets easily separable and an efficient adsorbent for PAHs.

Table 3. Parameters of the Adsorption Isotherms temperature parameters Langmuir isotherm QL (mg g−1) RL2 Freundlich isotherm nF RF 2

293 K

303 K

313 K

54.17 0.9689

60.57 0.9682

71.63 0.9934

1.38 0.9916

1.39 0.9893

1.50 0.9917

Table 4. Parameters of Adsorption Thermodynamics T (K) thermodynamic constants

293

303

313

ΔG0 (kJ mol−1) ΔH0 (kJ mol−1) ΔS0 (J mol−1 K−1)

−19.70 −10.13 32.66

−20.03

−20.35

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ASSOCIATED CONTENT

S Supporting Information *

could be dissolved in ethanol and the GO-PtBA5 adsorbent could also be dispersed well in ethanol. It was found that 97% of the adsorbed TBBPA could be eluted from the adsorbent within 110 min (Figure 10), demonstrating the possible reusability of the easily separable GO-PtBA5 adsorbent. The adsorption−desorption cycles of the GO-PtBA5 adsorbent for the removal of TBBPA were repeated six times, and its reusability is presented in Figure 11. The GO-PtBA5 adsorbent still possessed 95% adsorption capacity for TBBPA after six cycles of reuse, indicating its good reusability.

Zeta potentials of the GO-PtBA5 adsorbent and dynamics and thermodynamics adsorption models. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: 86 0931 8912582. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 13705

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ACKNOWLEDGMENTS This project was granted financial support from the Fundamental Research Funds for the Central Universities (no. lzujbky-2014-245).

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dx.doi.org/10.1021/la504077x | Langmuir 2014, 30, 13699−13706