bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution

Advanced Powder Technology xxx (xxxx) xxx

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Original Research Paper

Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution Wenjing Xu a,⇑, Yizhen Chen a, Wensheng Zhang a, Baojun Li b,⇑ a b

School of Science, Jiaozuo Teachers College, Jiaozuo Engineering Technology Research Center of Separation and Adsorption Materials, Jiaozuo, Henan 454000, PR China College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR China

a r t i c l e

i n f o

Article history: Received 2 December 2017 Received in revised form 25 November 2018 Accepted 30 November 2018 Available online xxxx Keywords: Adsorption Bentonite Composite materials Graphene oxide Toluidine blue

a b s t r a c t The graphene oxide/bentonite (BG) composites are prepared through graphene oxide (GO) nanosheets successfully intercalated into acid-treated bentonite interlayer and deposited onto external surface. The BG composites exhibit a higher uptake capacity of toluidine blue (TB) dye from water solutions than normal bentonite owing to the synergistic effect between bentonite and GO. The as-prepared composites are characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and N2-sorption analysis. The process parameters affecting the adsorption behaviors such as initial pH, temperature, contact time and initial concentration of dye are systematically investigated. The Langmuir isotherm model fit well with the equilibrium adsorption isotherm data and the maximum adsorption capacity is 458.7 mg
g1 at pH 8 for BG composites modified using 1% GO. The pseudo-second-order kinetic model well describes the adsorption process of TB onto BG composites. The TB adsorption on BG composites is mainly attributed to ion exchange, electrostatic interaction and intermolecular interactions. The outstanding adsorption performances of composites for the removal of TB dye from water demonstrate its significant potential for environmental applications. Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Synthetic dyes have been excessively released into the environment due to rapid industrialization that has created serious global concerns [1,2]. Most of synthetic dyes possess highly complex structures and high molecular weight, meanwhile, they are stable toward heat, light, and oxidizing agents [3,4]. More seriously, most of these dyes are toxic and even teratogenetic, carcinogenic and mutagenic, which create the serious threats to human health and marine organisms [5,6]. Hence, removal of the dye contaminants prior to their discharge into the environment is urgently necessary. In the process of dyes removal from wastewater, many treatment methods have been developed, such as chemical precipitation [7], ion exchange [8,9], membrane separation [10], physical adsorption [11–13], chemical oxidation/reduction [14], electrochemical [15], photocatalytic degradation [16]. Among these techniques, adsorption is regarded as a competitive method for dyes ⇑ Corresponding authors. E-mail addresses: [email protected] (W. Xu), [email protected] (B. Li).

removal from wastewater. A large number of synthetic and natural porous materials have been explored and commercially used, such as activated carbons, activated aluminium oxide, and synthetic silica mesoporous materials [17,18]. However, these adsorbents are often very expensive, and their laborious production involves the use and waste of toxic materials with high energy consumption [18]. To address these limitations, the development of new, cheaper, sustainable and green, and more advanced adsorbents based on natural or waste materials, such as zeolites, clays, fly ash, chitosan, is recognized as a favorable approach [17,18]. Bentonite, as representative aluminosilicate clay, is excellent choice with the potential to address these requirements. The two dimensional (2D) layered structure of bentonite consists of two silica tetrahedral sheets fused to one alumina octahedral sheet [19]. The negative charge of clays, being caused by isomorphous substitution of the layers by cations of lower valence, is balanced by exchangeable cations, such as Na+ and Ca2+ [19,20]. Bentonite has been widely used in the field of adsorption and separation due to its high cation exchange capacity, high stability, low cost, strong adsorption performance and easy to obtain [21,22]. Furthermore,

https://doi.org/10.1016/j.apt.2018.11.028 0921-8831/Ó 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028

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bentonite is easily intercalated with organic molecules or cations, such as methylene blue, cetyltrimethylammonium and dodecylammonium cations to promote its adsorption [23–25], unfortunately its smaller specific surface area affects its adsorption properties. Graphene oxide (GO) is considered to be a revolutionary material for its excellent optical, electrical and mechanical properties. In recent years, GO and its composites have been studied in many fields, such as photocurrent [26,27], photo-degradation [28], solar cells [29], photovoltaic and NIR-detector [30], energy storage [31] and adsorption [32]. Due to abundant oxygen atoms in the forms of epoxy, hydroxyl, and carboxyl groups, very high surface area and sheet-like structure, GO often has high adsorption capacity for environmental contaminants and has gained considerable attention as a significant adsorbent [33,34]. GO and GO composite materials have become a hot spot in adsorption research [35–37], for example, some graphene/GO composites combined with natural mineral (beta zeolite, diatom silica) have attracted tremendous attention for the design of advanced adsorbents [18,38]. In this work, in order to improve the specific surface area and adsorption property of bentonite, we combined bentonite with GO, which has large specific surface area. GO nanosheets were intercalated into the interlayer of acid-treated bentonite and external surface. The preparation of GO/bentonite (BG) and possible mechanisms of toluidine blue (TB) adsorption onto BG composites are graphically presented in Fig. 1. In the present study, the TB cation exchange with the exchangeable hydrated cations of metals (such as Na+, Ca2+, and K+) between the layers. At the same time, the mechanisms between TB and BG composite materials also include the electrostatic interaction and intermolecular interactions. The structure and properties of the as-prepared BG composites with different GO contents were confirmed by a series of characterizations. The effect of the adsorption conditions on the adsorption capacity of TB onto the BG composites was extensively investigated.

powder and toluidine blue were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China), The chemicals including potassium permanganate (KMnO4), sodium nitrate (NaNO3), hydrogen peroxide (H2O2, 30%), concentrated sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl), sodium hydroxide (NaOH) and all other chemicals were analytical reagent grade and purchased from Aladdin Chemicals (Shanghai, China). All solutions were prepared using distilled water. 2.2. Synthesis of graphene oxide Graphene oxide was prepared from natural graphite powder using modified Hummer’s methods [39]. In detail, graphite powder (1.0 g), NaNO3 (0.5 g) and KMnO4 (3.0 g) were slowly added to a concentrated H2SO4 solution (23 mL) within an ice bath. After removing the ice bath, the above mixture was intensely stirred at 35 ± 3 °C for 30 min. When the color of the mixture changed from dark green to brown and its viscosity increased significantly, the reaction was completed. Then, deionized water (46 mL) was added to above mixture while keeping the temperature at 98 °C for 15 min, followed by reducing the temperature to 60 °C with the addition of warm deionized water (140 mL) and H2O2 (30%, 10 mL) while stirring continuously for a further 2 h. The obtained mixture was filtrated to collect the solid product and washed with 4 wt% HCl solution 5 times and then with deionized water until the pH of the supernatant was neutral. Finally the material was dried in a vacuum to obtain a loose brown powder. 2.3. Acid treatment of bentonite powder Bentonite powder (1.0 g) was suspended in deionized water (40 mL). Then, HCl (36 wt%, 0.1 mL) was dropwisely added into and stirred the suspension for 30 min. The above bentonite powder was filtrated and rinsed with distilled water until the pH of the powder was neutral. Finally, the derived powder was dried in a vacuum drying oven at 60 °C for 24 h.

2. Materials and methods 2.4. Preparation of graphene oxide/bentonite (BG) 2.1. Materials Bentonite, chemical pure, light yellow powder, was obtained from Shanghai Shisihewei Chemical Co. Ltd (China). Graphite

Different wt% of graphene oxide within deionized water (20 mL) was sonicated in a flask for 2 h. 1.0 g of acid-treated bentonite was added to the flask. The above suspension was sonicated

Fig. 1. Illustration of the synthesis of BG composites and TB adsorption using BG composites.

Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028

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for 10 min, stirred for 30 min at 90 °C constant water bath. Then the mixture was filtrated and placed in an oven at 60 °C for 24 h. Finally, the residual powders were cooled down for further use. The resulting products are still powdered, while their color becomes light gray compared with the light yellow bentonite before preparation. The product obtained by variation of graphene oxide content were marked as BG-1 (1 wt% of GO) and BG-2 (2 wt% of GO), respectively. When the mass fraction of GO was over than 2%, the graphene oxide/bentonite (BG) composites cannot be prepared successfully due to part of graphene oxide are not uniformly dispersed in bentonite. 2.5. Characterization Fourier transform infrared (FT-IR) spectroscopies of the samples were recorded on a spectrum 100 FT-IR spectrophotometer (Perkin-Elmer, USA) using a KBr disk method and scanned in the range of 4000–450 cm1. The scanning electron microscopy (SEM) images were examined with JSM-6700F scanning electron microscopy (JEOL Ltd., Japan). The specific surface area and pore size of the adsorbents were carried out at 77 K (boiling point of nitrogen) with the aid of volumetric adsorption analyzer (ASAP 2420, Micrometrics, USA). The established Brunauer-EmmettTeller (BET) and Barrett-Joyner-Halenda (BJH) techniques were employed for area determination and pore calculation. The Raman spectra of samples were executed by applying Renishaw RM-2000 with an Ar-ion laser (k = 514 nm, power = 50 mW). X-ray photoelectron spectra (XPS) of samples were researched by using a PHI quantera SXM spectrometer, and the binding energies were adjusted according to C 1s peak (284.8 eV). Powder XRD data were collected on a PANalytical X’Pert PRO diffractometer with Cu Ka = 1.5418 Å over the angular range from 5° to 80°. TB concentration of the test samples was analyzed using a UV–vis spectrophotometer (Lambda 35, Perkin-Elmer, USA). 2.6. Batch adsorption The adsorption behaviors of TB on BG composites were performed in 250 mL conical flask by batch equilibrium method. A desired amount of adsorbents were added into each flask containing TB solution with appropriate concentrations. Subsequently, the mixture was agitated at 150 rpm in a SHA-B waterbath constant temperature shaker for a desired time to reach the equilibrium. The effect of pH on the adsorption was studied in the range of 3.0–13.0 that were adjusted with 0.10 mol
L1 HCl or NaOH solution at 25 °C. Adsorption isotherms were studied at different initial TB concentration (200–800 mg
L1, 50 mL) with adsorbent (50 mg) under temperature of 25 °C and pH = 8 for 24 h, respectively. Adsorption kinetics were carried out at 25 °C and pH = 8 with adsorbent (50 mg) and TB solution (100 mL) (concentration: 100 mg
L1 and 200 mg
L1). The samples were collected at predetermined time intervals immediately. The concentration of TB was measured by UV–vis spectrophotometer at a wavelength of 632 nm. The adsorption amount (q, mg
g1) was calculated by the following formula:

q¼

ðc0  cÞ  V m

ð1Þ

where c0 (mg
L1) and c (mg
L1) are the initial and final concentrations of TB, respectively, V (L) is the volume of the solution, and m (g) is the mass of adsorbent, respectively. The TB removal percentage was calculated by the following formula:

% Remov al ¼

c0  c  100 c0

ð2Þ

3

2.7. Desorption and regeneration studies 50 mg of BG-1 adsorbent was added into 50 mL of TB solution (100 mg
L1) under temperature of 25 °C and pH = 8 for 24 h. The TB-loaded BG-1 composite was separated by centrifugation and the concentration of TB was measured by UV–vis spectrophotometer at a wavelength of 632 nm. The desorption of the TB-loaded composite was done using a mixture solution of ethanol and 0.1 mol
L1 HCl (v:v = 1:1) as the eluent. 15 mL eluent and 50 mg of TB-loaded BG-1 composite was constantly stirred for 15 min, then the solution was separated by centrifugation. After three times of washing with eluent, the solid was washed with distilled water for three times and then employed for another adsorption. The consecutive adsorption-desorption process was performed for five times, and the removal rates of TB by the adsorbent regenerated for different times was obtained.

3. Results and discussion 3.1. Structural and characterization of materials Fig. 2a shows the FTIR spectra of GO, bentonite, BG-1 and BG-2. The spectrum of GO shows the following characteristic functional groups: CAOAC (1054 cm1), OAH (1388 cm1), C@C (1619 cm1), and C@O (1721 cm1) bonds. The OAH stretching vibrations in the region of 3600–3300 cm1 correspond to the hydroxyl and carboxyl groups of GO. This spectrum is consistent with the data reported in the literature for GO [30]. For bentonite and BG composite materials, the peak at 3629 cm1 and the broad peak centered at 3419 cm1 are assigned to the OAH stretching vibration of the SiAOH groups from the clay and OAH vibration of the water molecules adsorbed on the silicate surface. The peak at 1635 cm1 is due to the bending OAH bond of water molecules, which is retained in the silicate matrix. The broad peak centered at 1038 cm1 reflects the SiAOASi groups of the tetrahedral sheet. The peaks originating from the external bentonite components (for example, quartz) are located at 797 and 694 cm1. Also, 521 and 467 cm1 correspond to AlAOASi and SiAOASi bending vibrations, respectively [20,23]. The peak at 1430 cm1 could be ascribed to the stretching vibration of CO2 anion associated in 3 dolomite, which was an impurity in the bentonite clay [40]. The FTIR spectra of BG-1 and BG-2 are similar to the original bentonite except for the peak at 1430 cm1 disappearing, indicating no formation of new functional groups in the process of GO modification bentonite. Fig. 2b shows the Raman spectra of GO, bentonite and BG composite materials. The two characteristic peaks at 1357 cm1 and 1597 cm1 are observed for the GO, BG-1 and BG-2. The peak at 1597 cm1 (G-band) corresponds to the in-plane vibration of sp2 to sp3 hybridized carbon originating from the destruction of the sp2 structures of the graphite or due to the covalent attachment of functional groups. The peak at 1357 cm1 (D-band) is associated with the disorder in the graphitic structure [18]. Compared to the ID/IG intensities of the BG-1 (0.94), BG-2 (0.98), the two ratios are larger than that of the GO sheets (0.91), indicating that the amount of the disordered carbon has an obvious increasing in preparation of BG composites [18,30]. The N2 adsorption-desorption isotherms and corresponding BJH pore size distributions of GO, bentonite and BG composites are shown in Fig. 3. All isotherm plots of the samples exhibit similar type IV isotherms with an obvious H3-type hysteresis loop at relative pressure P/P0 above 0.45 caused by capillary condensation of N2, indicating the existence of slit-shaped pore and mesopores characteristic in four samples [41]. As summarized in Table 1,

Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028

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Fig. 2. (a) FTIR and (b) Raman spectra of GO, bentonite, BG-1 and BG-2 composites.

The BET surface area of the BG composites are obviously increased due to an exfoliated structure resulting from the GO addition while a slight rise with the augment of GO content from 1% to 2%, indicating that the compatibility of bentonite with GO is limited, which are in agreement with SEM images and XRD analysis of these samples. In order to compare the surface morphologies and microstructures changes before and after modification, SEM analyzes were conducted. The GO sheets (Fig. 4a) with many wrinkles through all the surfaces have the typical morphology characteristics of standard GO. Fig. 4b shows that the bentonite displays rock-like macroparticles with rough and compact surfaces. After GO modification, some cracks (long narrow opening) and lamellae (thin plate) are observed on the surface of the BG composites (Fig. 4c–f), indicating the partial exfoliation of the bentonite layer during GO modification process. It should be believed that this change is closely related to the intercalation of the GO into the bentonite layers [19]. The further proof about the intercalation of the GO into the bentonite layers was provided by the XRD patterns of the BG composites. After being used for adsorption TB, it can be observed that the gray adsorbents turned into a dark blue powder. From SEM images (Fig. 4g and h), more and deeper cracks and lamellae were observed on the surface of TB loaded BG-1 adsorbent, indicating the more exfoliation of BG-1 composite layer during TB adsorption process.

Table 1 The textural properties of samples. Samples

BET surface area (m2
g1)

Total pore volume (cm3
g1)

Average pore diameter (nm)

GO Bentonite BG-1 BG-2

424.8 38.0 56.8 63.4

1.680 0.070 0.098 0.094

14.76 9.98 8.95 7.76

The XRD patterns of the samples are shown in Fig. 5a. After graphite oxidation, a diffraction peak at 26.5° vanished, and a new diffraction peak appeared at 11.6° corresponding to the typical diffraction peak of GO nanosheets, which is due to the insertion of oxygen functional groups and water molecules into graphene layer [42]. This indicates graphite was almost oxidized into GO [27]. The XRD patterns can also be used to evaluate the basal spacing between layers of the clay, as summarized in Table 2. The four curves of bentonite, BG-1, BG-2 and BG-1-TB all show a large peak at 2h = 26.6°, corresponding to the pattern of quartz phase, suggesting that the bentonite consists of quartz phase [19,43]. During modification processes, the (0 0 1) reflection of the bentonite shifts gradually from 7.00° to 5.88°, implying the formation of an exfoliated structure resulting from the GO addition. After adsorption TB using BG-1 composite, the (0 0 1) reflection of the bentonite also shifts from 6.04° to 5.96°. From Bragg’s Law, it could be determined

Fig. 3. (a) N2 adsorption-desorption isotherms and (b) adsorption pore size distributions curves of GO, bentonite, BG-1 and BG-2 composites.

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Fig. 4. SEM images of (a) GO, (b) pristine bentonite, (c and d) BG-1, (e and f) BG-2, and (g and h) BG-1 composites after adsorption TB.

that the GO addition had an impact on the spacing of the bentonite layers. Bragg’s Law is expressed as in the following formula:

2d sin h ¼ nk

ð3Þ

where d is the spacing between the planes, h is the angle between the incident and reflected rays, n is the order of the reflection, and k is the X-ray wavelength. It was determined that the d001 spacing in natural bentonite was 1.26 nm. After the additions of 1%GO and 2%GO, the d001 values increased to 1.46 and 1.50 nm, respectively. These results suggest an exfoliated structure resulting from the GO addition, and confirm that the BG-1 and BG-2 composites were successfully synthesized. After adsorption TB using BG-1 composite, the d001 values increased from 1.46 to 1.48 nm, implying TB molecules were intercalated into the interlayer of BG-1 composite by cation exchange. Fig. 5(b–d) shows the XPS survey and C 1s spectra of GO and GB-1 composite. For GB-1, the percentage of carbon atoms in the composite is 17.11%. Considering the earlier reports on the XPS studies of grapheme [43], the C 1s photoelectron peak is decomposed in five symmetrical peaks located at 284.6, 285.2, 286.2, 287.2 and 288.4 eV, respectively. Two peaks located at 284.6 and 285.2 eV were assigned to nonfunctionalized carbon sp2 and sp3 atoms, respectively. Three other peaks located at 286.2, 287.2

and 288.4 eV correspond to carbon atoms bound with oxygen atoms, representing CAOA, C@O and COOA groups, respectively. Furthermore, the peak intensity percentages of intact carbon (CAC/C@C) and oxygenated carbon atoms (CAOA, C@O and COOA) reflect the oxidation degree of GO [30]. Compared to the peaks of GO at 284.6, 285.2, 286.2 and 287.2 eV, these peaks of BG-1 located at 285.0, 285.8, 286.6 and 288.0 eV, respectively, presenting a slight higher binding energy. The shift values of the binding energy are calculated about 0.4, 0.6, 0.4 and 0.8 eV, indicating that the active groups of GO has been hybridized with the surface groups of bentonite [30]. The peak intensity percentages of oxygenated carbon atoms (CAO and C@O) in BG-1 composites are increased, indicating that some possible chemical reactions have occurred between bentonite and GO in preparation.

3.2. Effect of pH and temperature The effect of pH values (ranging from 3 to 13) on the adsorption capacity of TB on bentonite, BG-1 and BG-2 are shown in Fig. 6a with an initial concentration of 200 mg
L1 TB. The adsorption capacity of TB increased obviously with increasing of pH values from 3 to 8 and changed little between pH 9 and 13. The three adsorbents exhibited maximum adsorption capacity at pH 8. This

Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028

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Fig. 5. (a) XRD patterns of graphite, GO, bentonite, BG-1, BG-2 and BG-1 after adsorption TB (BG-1-TB), (b) XPS survey and (c), (d) C 1s spectra of GO and BG-1.

Table 2 The diffraction angle (2h) and interlayer spacing (d001) of samples. Samples

2h (°)

d001 (nm)

Bentonite BG-1 BG-2 BG-1-TB

7.00 6.04 5.88 5.96

1.26 1.46 1.50 1.48

phenomenon can be explained by the electrostatic action. At low pH values, the electrostatic repulsion between protonated adsorption sites on the three adsorbents and cation dye TB restricted the

interaction between TB molecules and these adsorbents, so the adsorption capacity is less at low pH values. With increasing pH values, the level of protonation decreased and the electrostatic repulsion weakened, more TB molecules could be adsorbed onto the three adsorbents. From Fig. 6a, the adsorption capacity of bentonite varies from 164.0 mg
g1 to 177.4 mg
g1, increased by 8.2% in a range from pH 3 to 8. Similar effect could also be seen in BG-1 and BG-2. The results indicate that electrostatic interaction played a vital role in the adsorption process. The maximum adsorption capacity of TB on bentonite, BG-1 and BG-2 at pH 8 were 177.4 mg
g1, 197.0 mg
g1 and 199.8 mg
g1, respectively. The

Fig. 6. Effect of (a) solution pH at 25 °C and (b) temperature on the adsorption of TB (amount of adsorbent = 1.0 g
L1).

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adsorption capacity of BG composites increased by more than 11% modified using GO. This was due to the increase of the layer spacing, oxygen-containing groups and adsorption active sites of bentonite after modification, which improved the surface affinity of BG composites for TB molecules and is beneficial to adsorption. Fig. 6b shows the effect of temperature on adsorption of TB onto BG-1 and BG-2 with initial concentration of 200 mg
L1. Within the total adsorption temperature (3060 °C), the removal efficiencies of TB on BG-1 and BG-2 are slightly decreased from 99.0% to 97.5%, and from 99.7% to 98.0%, respectively, inferring that the adsorption is exothermic and mainly dominated by the physical adsorption [30]. 3.3. Adsorption isotherm The effects of initial TB concentration on adsorption capacity were investigated in a range from 200 to 800 mg
L1 at pH 8.0 for 24 h. The dye adsorption capacities onto BG-1 and BG-2 increased with the increase of TB concentration (Fig. 7a). The maximum adsorption capacities for TB onto BG-1 and BG-2 reached to 455 mg
g1 and 470 mg
g1 at the initial concentration of 800 mg
L1, respectively. Equilibrium adsorption isotherms, namely Langmuir and Freundlich, are applied to analyze the adsorption data of TB on BG-1 and BG-2, and their linear equations are presented below [44,45].

Langmuir : qce ¼ q 1 b þ qce

ð4Þ

1 Freundlich : ln qe ¼ ln K F þ lnce n

ð5Þ

e

m

m

1

where qe and qm (mg
g ) are the adsorption capacity at equilibrium and the maximum adsorption capacity according to Langmuir monolayer adsorption, ce (mg
L1) is the equilibrium concentration of dye, b (L
mg1) is the Langmuir adsorption constant related with the adsorption energy; KF (L
mg1) is the Freundlich constant indicating the adsorption capacity and n is the heterogeneity factor representing the adsorption intensity. The relative parameter values calculated from the Langmuir and the Freundlich models are listed in Table 3. By comparing the results obtained from the two isotherm models, due to higher R2 values, the Langmuir model was the better fit, indicating that TB molecules were mainly adsorbed in monolayer coverage manner. The proposed adsorption mechanism of monolayer coverage is consistent to the minor structure change demonstrated by the

XRD patterns (Fig. 5a and Table 2). Thus, the values of qm deduced from the Langmuir model could reflect the adsorption capacity of BG composite materials. The values of qm was calculated as 458.7 mg
g1 for BG-1 and 471.7 mg
g1 for BG-2. The values of b were found to be within the range from 0 to 1, indicating that the two as-prepared adsorbents were suitable for adsorption of TB. For Freundlich model, the values of KF showed that adsorption of TB on BG-1 and BG-2 was easy and n > 1 confirmed favorable adsorption conditions. 3.4. Adsorption kinetics The effect of contact time on the adsorption of TB onto BG composites was investigated to determine the kinetic process. The adsorption kinetics were conducted using 100 mL of TB solution with initial concentration of 100 and 200 mg
L1 at pH 8.0 and 50 mg of BG-1. As Fig. 7b shows, a fast adsorption process was observed in the initial 30 min for 100 mg
L1 solution and 60 min for 200 mg
L1 solution, and then basically reached adsorption equilibrium in the first 2 h. Two kinetic models namely pseudofirst-order and pseudo-second-order models were used to explore the adsorption kinetics behavior as shown in Eqs. (6) and (7), respectively [44,45].

lgðqe  qt Þ ¼ lg qe  t 1 t ¼ þ qt k2 q2e qe

k1 t 2:303

ð6Þ

ð7Þ

where qt and qe (mg
g1) are the adsorbed amounts of TB at time t (min) and equilibrium time, respectively. The k1 (min1) and k2 (g
mg1
min1) are the pseudo-first-order and pseudo-secondorder rate constants, respectively. The kinetic parameters in the above linear models were determined by plotting log(qe-qt) versus t and t/qt versus t, respectively. The fitting results of adsorption kinetics data by pseudo-firstorder and pseudo-second-order models are shown in Table 4. Compared with the correlation coefficient R2 and the experimental adsorption capacity (qe,exp) values, the adsorption process could be described well with the pseudo-second-order model. This indicates that the adsorption kinetics was mainly controlled by chemical action, more than material transport step. The pseudo-secondorder constant (k2) was higher for TB concentration of 100 mg
L1, indicating a greater adsorption rate for this condition.

Fig. 7. Effects of (a) initial concentration (25 °C, pH = 8, amount of adsorbent = 1.0 g
L1) and (b) contact time on the adsorption capacity of TB onto BG-1 composite (25 °C, pH = 8, solution volume = 100 mL).

Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028

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Table 3 Fitting results of adsorption isotherms by Langmuir and Freundlich models. Materials

Langmuir RL

BG-1 BG-2

Freundlich 1

2

qm (mg
g

0.9988 0.9998

)

458.7 471.7

1

b (L
mg

)

0.175 0.533

RF2

n

KF (L
mg1)

0.9490 0.9369

9.09 8.79

241.5 261.2

Table 4 Kinetic parameters for TB adsorption onto BG-1. qe,exp (mg
g1)

Solution

Pseudo-first-order model

Pseudo-second-order model

R12

qe (mg
g1)

k1 (min1)

R22

qe (mg
g1)

k2 (g
mg1
min1)

100 mg
L1 200 mg
L1

0.7547 0.9224

47.84 163.8

0.028 0.016

0.9995 0.9969

200.4 369.0

2.51  103 5.88  104

198.0 390.6

Fig. 8. (a) Reusability of BG-1 for TB adsorption (25 °C, pH = 8, amount of adsorbent = 1.0 g
L1) and (b) XRD patterns of BG-1 and its regenerated samples.

3.5. Desorption and reusability Desorption experiments can be used to evaluate the possibility of desorbing the target contaminant and reusing the adsorbents. In this study, TB adsorbed BG-1 composite was treated with a mixture solution of ethanol and 0.1 mol
L1 HCl (v:v = 1:1) as the eluent to investigate and evaluate the regeneration ability of BG adsorbents, as shown in Fig. 8. From Fig. 8a, the removal rates of TB in the 100 mg
L1 TB solution are over 97% in the first three cycles and more than 93% in the fourth and fifth cycles, indicating the adsorption-desorption performance of BG-1 composite is excellent for TB dye. From Fig. 8b, the XRD patterns of the third and fourth regenerated BG-1 are highly similar to that of the original BG-1, while the XRD pattern of the fifth regenerated BG-1 change obviously. This indicates that the structure of BG-1 remains unchanged after the fourth regeneration and deforms after the fifth regeneration, which results in a low removal rate for the sixth adsorption (Fig. 8a). This fast deformation indicates that there is a critical point for the surface property of GO and bentonite. As discussed above, the as-prepared adsorbent has the potential to be developed into an efficient adsorbent for TB dye removal. 4. Conclusion In conclusion, the BG composite materials were prepared through GO sheets successfully intercalated into acid-treated bentonite interlayer and composited onto external surface. The

as-prepared adsorbents exhibit a faster adsorption equilibrium and higher adsorption capacity for TB dye from the water solutions. The maximum adsorption capacities according to Langmuir model were 458.7 mg
g1 for BG-1 and 471.7 mg
g1 for BG-2, respectively. Based on the results of the adsorption studies, the TB adsorption on the composites is mainly attributed to ion exchange, electrostatic interaction and intermolecular interactions between the TB and the adsorbents. BG composites intercalated 1% GO have promising potential for the removal of TB dye from effluents and may be promising applicable in dye industry and other energy, environment fields for their high efficiency, facile and green process, low toxicity and cost. Acknowledgements This work was supported by Foundation of Henan Scientific and Technological Committee (No. 182102210420), National Natural Science Foundation of China (No. 21401168) and Foundation of Jiaozuo Scientific and Technological Bureau (No. 20150103). References [1] M.A. Adebayo, L.D.T. Prola, E.C. Lima, M.J. Puchana-Rosero, R. Cataluna, C. Saucier, C.S. Umpierres, J.C.P. Vaghetti, L.G. Silva, R. Ruggiero, Adsorption of procion blue MX-R dye from aqueous solutions by lignin chemically modified with aluminium and manganese, J. Hazard. Mater. 268 (2014) 43–50. [2] W.J. Xu, W.S. Zhang, Y. Li, W. Li, Synthesis of acrylic-lignosulfonate resin for crystal violet removal from aqueous solution, Korean J. Chem. Eng. 33 (2016) 2659–2667.

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Please cite this article as: W. Xu, Y. Chen, W. Zhang et al., Fabrication of graphene oxide/bentonite composites with excellent adsorption performances for toluidine blue removal from aqueous solution, Advanced Powder Technology, https://doi.org/10.1016/j.apt.2018.11.028