titania nanocomposites as an adsorbent for methylene blue adsorption

titania nanocomposites as an adsorbent for methylene blue adsorption

Accepted Manuscript Title: Microwave-assisted synthesis of reduced graphene oxide/titania nanocomposites as an adsorbent for methylene blue adsorption...

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Accepted Manuscript Title: Microwave-assisted synthesis of reduced graphene oxide/titania nanocomposites as an adsorbent for methylene blue adsorption Author: Huan Wang Haihuan Gao Mingxi Chen Xiaoyang Xu Xuefang Wang Cheng Pan Jianping Gao PII: DOI: Reference:

S0169-4332(15)02770-1 http://dx.doi.org/doi:10.1016/j.apsusc.2015.11.075 APSUSC 31792

To appear in:

APSUSC

Received date: Revised date: Accepted date:

24-8-2015 23-10-2015 8-11-2015

Please cite this article as: H. Wang, H. Gao, M. Chen, X. Xu, X. Wang, C. Pan, J. Gao, Microwave-assisted synthesis of reduced graphene oxide/titania nanocomposites as an adsorbent for methylene blue adsorption, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.11.075 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Microwave-assisted synthesis of reduced graphene oxide/titania nanocomposites

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as an adsorbent for methylene blue adsorption

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Huan Wang a, Haihuan Gaob, Mingxi Chen a, Xiaoyang, Xu a, Xuefang Wang a,

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Cheng Pan a, Jianping Gao a, c *

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a

School of Science, Tianjin University, Tianjin 300072, P. R. China.

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b

Tianjin Fourth Middle School, Tianjin 300201, P. R. China.

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c

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin

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University, Tianjin 300072, P. R. China.

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Abstract

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In this study microwave-assisted reduction (MrGO) and direct reduction of graphene

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oxide (rGO) by Ti powders were established, and the effect of the reaction conditions

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on the reduction were discussed. The results showed that GO can be effectively

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reduced by both methods, however, microwave assistance can greatly shorten the

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reduction time. The produced Ti ions from the reaction of Ti powder with GO were

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transferred to TiO2 by hydrolysis and formed MrGO/TiO2 and rGO/TiO2. They were

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used as adsorbents for the removal of methylene blue (MB). MrGO/TiO2 showed a

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higher adsorption capacity (qmax, 845.6 mg/g) than rGO/TiO2 (qmax, 467.6 mg/g). Investigation on the adsorption MB onto MrGO/TiO2 was conducted and demonstrated that adsorption kinetics followed the pseudo second-order kinetics model and the adsorption isotherm was well described by the Langmuir isotherm model. The recycling of MrGO/TiO2 was achieved by photocatalytic degradation of

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MB catalyzed by MrGO/TiO2 itself..

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1. .Introduction

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Nowadays, a variety of dyes are used in industries, such as textile, paper, printing, *

Corresponding author. Tel: +86-022-2740-3475. E-mail address: [email protected]

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food, and pharmaceuticals [1-3]. Many of the dyes and their products are harmful to

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flora and fauna and some are even mutagenic or carcinogenic [4]. It is important to

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have wastewater treated before its release to water system. Several techniques

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including chemical precipitation, ion exchange, membrane filtration, adsorption,

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photocatalysis, and electrochemical technologies have continuously been developed to

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pursue efficient dye removal from wastewater [5-8]. Among these techniques,

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adsorption was found to be superior due to its low cost and simple operation

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procedure [9-12], in addition, recycling of the adsorbent is feasible after adsorption

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[13]. A number of adsorbents have been studied for removal of dye molecules from

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water system [14-17]. However, the usage of these adsorbents is hindered by several

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inherent shortages, e.g. low capacities and difficulty for cycling. Development of new

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effective and eco-friendly adsorbents for removal of dye from water system is

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attracting more and more attention from worldwide researchers

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Graphene, with two dimensional honeycomb of carbon atoms, exhibits excellent

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mechanical and physicochemical properties [4]. It can be readily obtained from cheap

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natural graphite in large scale [18-19]. The high theoretical specific surface area

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incorporation of metal oxide nanoparticles on graphene limits their re-stacking and

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aggregation, thereby enhancing the surface area of the composite [20-21]. The

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functional groups and defect sites of graphene act as the nucleation and growth sites

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for nanoparticles. Meanwhile, the incorporation of graphene extends the life time of

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(2620m2 g-1) and surface-to-volume ratio, provides more active sites for ion adsorption and makes graphene a promising adsorbent. However, graphene nanosheets have a trend to agglomerate and hinder the adsorption process. Recently, numerous studies devoted to utilization of nanomaterials on graphene and reduced graphene oxide for removal of different water pollutants have been reported. The

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the adsorbent material by acting as support material which inhibits leaching of fine

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metal oxide particles into the treated water [22]. The hybrid of graphene with

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magnetic nanomaterials such as Fe3O4 has been exploited for removal of pollutants

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from water [22]. Hao prepared SiO2/graphene composite and investigated its

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adsorption behavior for Pb(II) ion [20]. TiO2 nanoparticle has been considered for

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widespread environmental applications because of its excellent photocatalytic

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performance, easy availability, long-term stability, and nontoxicity [4,23]. Recent

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survey shows that TiO2 is also an ideal adsorbent for water pollutants [25, 26].

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Herein, a simple, fast and environmentally friendly route for the reduction of

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graphene oxide (GO) using Ti powder as a reducing agent under household

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microwave assistance is developed. Hydrolysis was followed to synthesize reduced

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graphene oxide/titania (MrGO/TiO2) by dropping dilute Na2CO3 solution. The

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reduction of GO was traced by UV-visible (UV-vis) absorption spectroscopy, and the

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obtained MrGO/TiO2 was analyzed. The removal of methylene blue (MB) can be

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realized through strong electrostatic interaction of superficial charge of MrGO/TiO2

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with the cationic dyes. The regeneration of the adsorbent was achieved by

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grade and used as received.

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2.2. Preparation of GO

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GO was prepared from purified natural graphite by a modified Hummer's method [27-

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29].

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photodegradation of the absorbed MB catalyzed by MrGO/TiO2 adsorbent itself. 2. Materials and methods 2.1. Materials

Graphite was obtained from Qingdao Graphite Factory. Ti powder, H2SO4, Na2CO3,

were purchased from Tianjin Chemical Reagent Co. All the chemicals were analytical

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2.3. Preparation of rGO, MrGO, rGO/TiO2 and MrGO/TiO2 Preparation of MrGO and MrGO/TiO2 was carried out as follows: 0.2 g Ti powder

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was added to a brown GO suspension (100 mL, 2 g/L) in a glass beaker at ambient

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temperature, then 5.5 mL concentrated H2SO4 was added dropwise into the above

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mixture with stirring for 20 min. The beaker was placed in a water bath in a household

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microwave oven (Galanze, G70F20N3P-ZS), and then irradiated by microwave at

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400W for five cycles, each cycle included 5 min ‘on’ and 1min ‘off’. The brown

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solution gradually turned black, and this illustrates that GO was reduced. After that,

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0.5 M Na2CO3 solution was added to the prepared MrGO suspension until no obvious

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gas bubbles were seen to fly out, this illustrates that pH was close to 7. After that, 10

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mL (0.1g/mL) urea suspension was added and stirred at 70 °C for 3 hours to obtain

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MrGO/TiO2. The products were then centrifuged, rinsed with distilled water and dried

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for 24 h at 60 °C to remove the water. The rGO was prepared by heating GO and Ti

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suspension at 70 °C for 3 hours without microwave assistance, and rGO/TiO2 was

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prepared in the same way as that of MrGO/TiO2 but using rGO as the starting

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material.

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voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 90°(2θ)

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with steps of 4°min-1. Raman measurements were performed with a Raman

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microscope (DXR Microscope, USA). The thermogravimetric analysis (TGA)

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diagrams of the samples were recorded with a Rigaku-TD-TDA analyzer with a

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2.4. Characterization

The UV-vis absorption spectra of the GO and MrGO suspensions and dye in the

aqueous solution were recorded with a TU-1901 UV-vis spectrophotometer. The Xray diffraction (XRD) patterns of the samples were measured using an X-ray diffractometer (BDX3300) with a reference target: Cu Ka radiation (l=1.54 Å),

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heating rate of 10°C min-1. The samples were first dried in a vacuum at 40 °C for 2

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days before the TGA was recorded. The morphologies were observed with a scanning

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electron microscopy (SEM) (Desk-II; Denton Vacuum).

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2.5 Adsorption tests

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A standardized stock solution of MB of 467.6 mg/L was prepared. Experimental

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solutions of the desired concentration were obtained by further dilution. The effects of

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solution pH, initial concentration, contact time, and temperature were investigated. A

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temperature-controlled water bath shaker (SHZ-88, Shanghai, China) was used to

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control the desired temperature. All pH measurements were carried out using a pH

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meter (Model pHS-25, Shanghai, China).The initial pH levels of the experimental

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solutions were adjusted to constant values by adding 0.1 M HCl or NaOH solutions.

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The adsorption experiments were performed by shaking 4 mg of MrGO/TiO2 with 20

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mL of the experimental solution of known concentration in a temperature-controlled

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water bath shaker. At a certain time, the supernatant was taken out and filtered, and

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measured by an UV-vis spectrometer at the wavelength of 664 nm with the pre-

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established calibration curves, respectively. To prevent photodegradation of MB by

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MrGO/TiO2, the MB solution was kept from light by being wrapped with dark paper during the adsorption tests. The amount of MB adsorbed by the adsorbent and the dye removal efficiency (R%) were calculated using the following equations: (C0 - Ce)V (C0 - Ct)V , qt = m m 100(C0 - Ct) R% = C0

(1)

qe =

(2)

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where qe and qt (mg/g) are the amount of MB adsorbed per unit weight of the

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adsorbent at equilibrium and t time; C0, Ce and Ct are the MB dye concentrations at

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initial, equilibrium and t time, respectively; V is the volume of MB solution (ca. 0.02

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L); and m is the amount of the adsorbent. 5

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2.6. Regeneration experiment For dye regeneration, the MB initial concentration was 112.2 mg/L. 20 mg

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MrGO/TiO2 was added into 100 mL MB solution. When the adsorption process was

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over, MrGO/TiO2 samples saturated with MB were collected and then washed mildly

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with distilled water to remove residual dye particles. After that, the MrGO/TiO2

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samples were dried and added into 100 mL of ethanol aqueous solution. The solution

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was stirred for 10 min and put in a 8 mL quartz tube. The tube was placed axially and

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clamped in front of a 450 W medium pressure quartz mercury vapor lamp. When the

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degradation was completed (about 3h), the MrGO/TiO2 was then collected, washed

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with water and reused for adsorption again. The degradation–adsorption processes

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were repeated for 5 times. Another way to realize the regeneration is to wash

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MrGO/TiO2 samples with ethanol aqueous solution. The suspension was stirred for

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24h to obtain dynamic equilibrium, and the equilibrium concentration was calculated.

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3. Results and discussion

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3.1. Reduction of GO by Ti powder

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When the GO suspension was mixed with Ti powder and heated at 70 °C, the color

of the GO solution gradually turned from brown to black, implying that GO was reduced by Ti powder. The transition from GO to reduced GO (rGO) can be monitored by UV-vis spectroscopy. The UV-vis absorption peak of the GO dispersion is at 233 nm, which corresponds to the π-π* transition. This peak gradually red-shifted

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to 272 nm with a significant increase in intensity after the reduction, suggesting that

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GO was reduced and the electronic conjugation was restored. Position of the

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absorption peak reflects the degree of the reduction in the rGO [30].

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The effects of several factors such as H2SO4 amount and the mass ratio of Ti/GO on

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the reduction of GO by Ti powder were tested and the results are shown in Fig. 1(a)

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and (b). Figure 1a shows the effects of Ti/GO ratio on the reduction of GO by Ti

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powder at 70 °C and H+/Ti =50. When the reduction was performed on the condition

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of Ti/GO=0.5:1, the peak position did not change much. When the Ti/GO was 1:1, the

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peak red shifted to 272.5 in 3h, a further increasing in the Ti/GO ratio did not result in

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a further shift, but the reaction time was shortened. For example, the reduction time is

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only 110 min when the Ti/GO was 5:1. Figure 1b shows the effect of H+/Ti ratio on

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the reduction of GO by Ti powder. When the ratio of H+/Ti was lower than 50, GO

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was only partially reduced since the absorption peak only shifted from 233.0 to

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around 256.5 nm. When H+/Ti ratio was 50, the absorption peak shifted to 272.5 nm.

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A further increasing of H+/Ti did not cause a further shift in the absorption peak, but

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the reduction required less time.

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To investigate the effect of microwave on the reduction of GO by Ti powder, the

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GO reduction was conducted under microwave assistance and the results are shown in

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Fig. 1(c) and (d). Figure 1c shows the reduction of GO at different Ti/GO. When the

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mass ratio of Ti/GO increased to 1:1, the absorption peak of the MrGO suspension

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could get 267.0 nm in 30 min. As is shown in Fig. 1e, when H+/Ti increased to 50, the

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conditions for GO reduction were: Ti/GO =1.0, H+/Ti =50:1, 400W, and the samples

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in the following study are MrGO.

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absorption peak of the MrGO suspension shifted from 233 to 267.0 nm. A further increasing of H+/Ti did not cause a further shift in the absorption peak. We can conclude that the reduction of GO under microwave assistance was much faster in comparison with that by thermal reduction. So GO can be efficiently reduced by Ti powder under acidic conditions. In order to reduce GO in a short time, the selected

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To characterize the structure of GO reduced by Ti powder under microwave

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assistance, XRD patterns of graphite, GO and MrGO were measured and shown in Fig.

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2. Graphite has a narrow and strong diffraction peak at around 2θ=26.5° (d-spacing is

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0.34 nm), but GO has a broad peak at about 11.4° (the interlayer spacing is 0.78 nm).

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The larger interlayer distance can be attributed to the formation of hydroxyl, epoxy,

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and carboxyl groups, which increases the distance between the layers. However, this

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peak disappeared in MrGO. It indicates that some of the oxygen-containing functional

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groups have been removed. This phenomenon is consistent with those for rGO

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reduced by chemical reductants [30].

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TGA was performed to analyze the thermal stability of the sample in a N2

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atmosphere and the results are shown Fig. 3. Graphite does not show any mass loss

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from room temperature to 600°C. GO shows two main weight losses. The first rapid

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weight loss (15%) was at temperatures up to 100 °C and can be attributed to the

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removal of water molecules absorbed on the GO surface. The second weight loss

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(25%) between 200 and 250 °C is due to decomposition of the oxygen-containing

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functional groups [31]. The MrGO has a mass loss of 10% between 200 and 250 °C,

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which is much smaller than that of GO. This indicates that the thermal stability of

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MrGO was improved which is due to a decrease in the amount of oxygen containing

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atoms (usually observed at 1596 cm-1) and the D mode due to the breathing mode of

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k-point phonons with A1g symmetry (at 1360 cm-1). Changes in the relative intensities

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of the D and G bands (D/G) indicate changes in the electronic conjugation state of GO.

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The Raman spectra of GO and MrGO shown in Fig. 4 demonstrate that the D/G ratio

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functional group. So the data again indicate that the GO was reduced by Ti powder. Raman spectroscopy is also widely used to analyze carbon materials and can

provide information about defects density, disorder, defect structures, and doping levels. Generally, the Raman spectrum of graphene is characterized by two main features, the G mode arising from the first order scattering of the E2g phonons of sp2 C

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of MrGO increased to 1.28 compared with that of GO (0.99). This increase suggests

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that sp2 domains were formed owing to the reduction by Ti powder [32]. 3.2. Fabrication of MrGO/TiO2 and rGO/TiO2 hybrids

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The above studies confirm that Ti powder can be used to reduce GO efficiently.

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During the reduction of GO by the Ti powder, GO was reduced to MrGO (or rGO)

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while Ti powder was oxidized and formed ions. In order to make use of the by-

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products (ions), Na2CO3 solution was introduced to prepare rGO/TiO2 and

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MrGO/TiO2.

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The SEM photos of the rGO/TiO2 and MrGO/TiO2 were shown in Fig.5. rGO/TiO2

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is larger and thicker when compared with MrGO/TiO2, because GO sheets (Fig.S1)

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easily aggregated together during the reduction owing to the vander Waals and p–p

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stacking interactions. As for MrGO/TiO2, the high energy of microwave increased the

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reduction rate of GO and prevented the overlap of rGO sheets. The aggregation may

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decrease the surface area of rGO/TiO2. TiO2 nanoparticles were found to disperse

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uniformly on the surface of MrGO (Fig.S2). The TEM images of MrGO/TiO2 in Fig.

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5c-d clearly showed that TiO2 nanoparticles (Fig. 5c-d) with sizes of 10-30 nm were

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(004), (200), (105), (211), (204) and (215) of anatase TiO2 (JCPDS, card no. 21-1272),

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respectively. This indicates that Ti was transformed to TiO2 nanocrystals and

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MrGO/TiO2 was formed.

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found uniformly deposited on the surface of MrGO sheets. Interplanar lattice spacing of TiO2 is 0.352 nm, corresponding to the (101) plane of anatase phase of TiO2. This suggests that the TiO2 in the composite was in the anatase phase. The XRD diagram of MrGO/TiO2 is shown in Fig. 2. The diffraction peaks at

25.24°, 37.04°, 48.06°, 53.95°, 55.10°, 62.75° and 75.07° corresponds to the (101),

TGA was performed to analyze the thermal stability of TiO2, MrGO and

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MrGO/TiO2 under flowing air (Fig. S3). There was a small loss (about 10.7%) for the

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TiO2. MrGO had a rapid weight loss around 550°C and retained a residual mass of

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9.1% at 800°C. The residual mass of MrGO/TiO2 was about 39.7% at 550 °C. The

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amount of TiO2 in the MrGO/TiO2 composite can be calculated based on the TGA

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curve. It is about 38.2 wt%.

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Hence, the synthetic route of MrGO/TiO2 hybrid is presumed and represented in

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Fig. 6. GO was reduced to MrGO while Ti powder was oxidized to ions under

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microwave assistance. The ions were then transferred to TiO2 after hydrolysis to form

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MrGO/TiO2 hybrid

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3.3. Adsorption of MB by MrGO/TiO2 and rGO/TiO2

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The above SEM has demonstrated the superior structure of MrGO/TiO2 compared

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with that of rGO/TiO2. It also predicts a higher adsorption ability towards ions or

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organic compounds. Figure 7 shows the adsorption of MB onto GO, rGO, rGO/TiO2,

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MrGO, MrGO/TiO2. The adsorption capacity of rGO/TiO2 was lower than those of

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GO and rGO, and the adsorption capacity of MrGO/TiO2 was also lower than that of

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MrGO. This is because the direct reduction of GO can cause aggregation and

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influence the surface charge of the adsorbent as well as the surface binding-sites of

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the adsorbent [33]. The impact of solution pH values on the removal of dyes was

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determined over a pH range of 4–12, and the results are shown in Fig. 8. It was found

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that qe and R% increased with the increase of the pH value. This phenomenon could

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detriment the adsorption process, and the removal of TiO2 can create more space for MB and decrease the weight of adsorbent. However, the introduction of TiO2 can realize photocatalytic degradation of the adsorbed MB. Therefore, MB was used as a model organic compound to study the adsorption ability of MrGO/TiO2. The adsorption is affected by several factors, such as solution pH, because it can

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be explained as follows, at a low pH value, MrGO/TiO2 acquires a surface of positive

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charge due to the protonation of the remaining oxygen-containing functional groups,

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and the positively charged surface causes electrostatic repulsion between the

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adsorbent and the MB cationic molecules, resulting in a decrease in the adsorption

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capacity. What’s more, a low pH value means a relatively high concentration of H+,

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which competes strongly with MB cationic molecules for the adsorption sites on the

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adsorbent, qe decreased as a result. As the pH value increases, the surface charge of

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the adsorbent became more negative due to the deprotonation. At the same time, the

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competition between H+ and cationic molecules became less significant as well. As a

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result, qe dramatically increased.

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Figure. 9 shows the effect of contact time on the adsorption capacity of MB onto

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MrGO/TiO2 at different MB initial concentrations (37.4, 74.8, 112.2 and 149.6 mg/L).

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It can be easily observed that the trends of the four lines are similar. The qt drastically

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increased at the beginning, then increased slowly and finally became constant after a

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certain time. The results can be ascribed to the fact that most vacant surface sites are

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available for adsorption at the initial adsorption stage. At the end stage, the remaining

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overcome the mass transfer resistance of the dye. The qe reaches 675.9 mg/g when C0

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is 149.6 mg/L, which is larger than that of most of the traditional adsorbents [4,34].

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On the same condition, qe of MB adsorbed onto rGO/TiO2 is 407.6 mg/g, which is

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much smaller than that of MrGO/TiO2. The higher adsorption capacity of MrGO/TiO2

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vacant surface sites are hard to be utilized due to repulsive forces between the MB molecules adsorbed onto MrGO/TiO2 and those in the solution. The results also demonstrates that the adsorption is highly dependent on initial MB concentration. The adsorption capacities of MB present an increasing trend as MB concentration increases, since high initial MB concentration can provide a strong driving force to

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can be attributed to its special morphology discussed above. Due to the excellent

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adsorption performance of MrGO/TiO2, the adsorption of MB onto MrGO/TiO2 will

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be discussed in details.

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3.3.1. Kinetics and thermodynamics of MrGO/TiO2 adsorbent

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To understand the adsorption mechanism, two kinetic models were used to test the

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experimental data, the pseudo-first-order equation and the pseudo-second-order

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equation.

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3.3.1.1. The pseudo-first-order and pseudo-second-order kinetic model

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The pseudo-first-order kinetic model is more suitable for low concentration of solute. It can be written in the following form [35]: ln (qe − qt ) = ln qe − k1t

(3)

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Where k1 (min-1) is the rate constant of the pseudo first-order adsorption (min−1), qt

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and qe (mg/g) have the same meaning as those in Eq. (1). The values of k1 and qe were

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obtained from the slopes and the intercepts of the plots of ln(qe− qt) versus t in Fig.

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10a, and the data are presented in Tab. 1. the correlation coefficient values (R2) at the

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initial concentration of 37.4, 74.8, 112.2 and 149.6 mg/L were 0.9719, 0.9023, 0.9382

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can be represented in the following form [36]:

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and 0 .9385, respectively, which were far from 1. The calculated values of qe were 5.0, 72.7, 201.1, and 257.5 mg/g, respectively, which were smaller than the experimental ones, indicated that the experimental data did not agree well with this model. The pseudo-second-order equation is dependent on the amount of the solute

adsorbed on the surface of adsorbent and the amount adsorbed at equilibrium [35]. It

t 1 = qt k2 qe

2

+

t qe

(4)

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Where k2 (g mg-1 min-1) is the rate constant of pseudo-second-order equation, qt, qe,

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and t have the same meaning as that in Eq. (1). From the slope and intercept of the 12

Page 12 of 30

plot of t/qt versus t as shown in Fig. 10b, the values of k2 and qe can be obtained. all

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the correlation coefficients (R2) were higher than 0.97, which indicated that pseudo-

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second-order model was more suitable for explaining the kinetics for the adsorption of

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MB onto MrGO/TiO2. Similar results have been reported for the adsorption of MB

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onto Na2Ta2O6 [37].

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To test the diffusion mechanism between MB and MrGO/TiO2, an intra-particle

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diffusion model proposed by Weber and Morris has been used and rewritten in the

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following form:

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(5)

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where C is the value of intercept, ki (mg g-1 min1/2), intra-particle diffusion rate

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constant, is the slope of the straight line of qt versus t1/2, as shown in Fig. 11. There

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are three slopes for each curve, indicating that there were at least three diffusion steps

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during the adsorption process. At the first step, the external surface adsorption or

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diffusion in macro-pores occurred until the exterior surface reached saturation. Then,

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the second step which is controlled by intraparticle diffusion, was the gradual

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adsorption step. The third step was the final equilibrium step, for which MB moved

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dimensions decreased [38, 39]. The ki,3 is significantly lower than the others, so the

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third step was the slowest.

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3.3.1.2. Adsorption isotherms

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slowly from larger pores to micro-pores and caused a slow adsorption rate. As shown in Tab. 1, ki increased with an increase in MB concentration, as a result of the fact that multitude MB molecules interacted with active sites on adsorbent (high adsorption intensity) at a high initial concentration. For all initial concentrations, ki,1>ki,2, indicated that the free path available for diffusion became smaller and the pore

The equilibrium adsorption isotherm is studied in detail, since it can provide

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information about the surface properties of adsorbent, the adsorption behavior and the

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design of adsorption systems. Adsorption equilibrium is a dynamic concept achieved

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as the rate of dye adsorption is equal to the desorption rate. The adsorption isotherms

330

of MB onto MrGO/TiO2 adsorbent (Fig.S4) were investigated by fitting the

331

experimental data with Freundlich and Langmuir isotherm models, respectively. The

332

Langmuir model is based on the assumption that adsorption is localized on a

333

monolayer and all adsorption sites at the adsorbent are homogeneous. Whereas the

334

Freundlich isotherm presumes that the multilayer of the adsorption process occurs on

335

a heterogeneous surface.

336

The Freundlich can be represented as follows:

337

ln qe = 1/n ln Ce + ln kF

(6)

M

an

us

cr

ip t

327

where Ce (mg/L) is the equilibrium concentration of the dyes in the solution, qe (mg/g)

339

is the amount of MB adsorbed at the equilibrium, kF is the Freundlich constant, and n

340

gives an indication of how favorable the adsorption process is. The plots of ln qe

341

versus ln Ce are illustrated in Fig. 12a

342

The linearized form of the Langmuir isotherm can be given as follows:

344 345 346

te

Ac ce p

343

d

338

Ce Ce 1 = + qe qmax qmaxkL

(7)

Where qmax (mg/g) is the maximum capacity of the adsorbent, and kL (L/mg) is the Langmuir adsorption constant; Ce, qe have the same meaning as that in Eq. (6). The plots of Ce/qe versus Ce are illustrated in Fig. 12b. According to the correlation

347

coefficients and parameter values in Tab. 2. The R2 (0.9986) of Langmuir model is

348

very close to 1 and larger than Freundlich model (0.9890), it indicated that the

349

adsorption of MB onto MrGO/TiO2 followed the Langmuir model. Besides, the

350

monolayer adsorption capacity calculated from the Langmuir isotherm is 684.9 mg/g,

351

which approaches the experimental data (675.9 mg/g). It also suggests the adsorption 14

Page 14 of 30

352

of MB onto MrGO/TiO2 follows the Langmuir isotherm. It means that once a MB

353

molecule occupies homogeneous sites within the adsorbent surface, the adsorption is

354

completed and monolayer of MB is formed. The separation factor (RL) related to Langmuir isotherm is used to evaluate the

356

feasibility of adsorption on adsorbent. It can be calculated from the following

357

equation:

cr

358

ip t

355

(8)

RL=1/ (1+ bC0)

where C0 (mg/L) is initial dye concentration and b (L/mg) is Langmuir constant. The

360

value of RL indicates the type of the isotherm: irreversible (RL = 0), favorable (0 < RL

361

< 1), linear (RL = 1), unfavorable (RL > 1). The RL of MB adsorption onto MrGO/TiO2

362

is in the range of 0.004–0.08. It can demonstrate the MB adsorption onto MrGO/TiO2

363

is favorable

364

3.3.1.3. Adsorption thermodynamics

d

M

an

us

359

It is confirmed that in the range of 298–328 K the maximum adsorption capacity

366

increases by increasing the temperature (see Tab.3.), which specifies an endothermic

367

nature of the existing process [38]. The values of thermodynamic parameters such as

369 370 371 372 373 374

Ac ce p

368

te

365

change in enthalpy (△H0), change in entropy (△S0) and change in free energy (△G0) were determined from the slope and intercept of the van’t Hoff plots of ln (kL) versus 1/T using the following Van’t Hoff equations:

△G0 = −RT ln kL

lnkL = −

(9)

△ H 0 △S 0 + RT R

(10)

where R (8.314 J mol-1 K-1) is the gas constant, T (K) is the absolute temperature, and

15

Page 15 of 30

kL (L mol-1) is the Langmuir constant. The calculated △S0 is 180.14 (J mol-1 K-1) and

376

its positive value corresponds to a increase in adsorbed species degree of freedom at

377

the solid/solution interface of the whole adsorption process (Fig. 13). Also, the

378

calculated △G0 at 298, 308, 318 and 328 K are -22.74, -25.43, -27.04 and -28.68 (kJ

379

mol-1), respectively, the negative △G0 suggests spontaneity and feasibility of the

380

Ac ce p

381

endothermic adsorption in accordance with the increasing adsorption capacity with an

382

increasing adsorption temperature [40].

383

3.3.2. Regeneration of MrGO/TiO2

te

d

M

an

us

cr

ip t

375

adsorption process. Finally, the calculated △H0 (34.25 kJ mol-1) confirmed an

16

Page 16 of 30

Regeneration capacity of an adsorbent decides the cost of over-all process and plays

385

a key role in its commercial application of an adsorbent. The regeneration of

386

MrGO/TiO2 were carried out by two methods: photocatalytic degradation of the dye,

387

desorption by washing with ethanol solution for five times, and the results are shown

388

in Fig. 14. The removal % of the MrGO/TiO2 recovered by washing with ethanol

389

decreased to 42.4% after five cycles, while photo-treated MrGO/TiO2 kept a higher

390

adsorption capacity of 86.6%. So the regeneration of MrGO/TiO2 can be realized by

391

photocatalytic degradation of MB on the adsorbent. The regeneration does not use and

392

consume any chemicals, so it doesn’t cause the secondary pollution to the

393

environment and provides a new way for regeneration of MrGO/TiO2 adsorbents.

394

4. Conclusions

cr

us

an

M

In the present work, a new adsorbent, MrGO/TiO2 composite was synthesized

396

under microwave-assisted reduction of GO and used for the removal of

397

Compared with rGO/TiO2 synthesized by thermal reduction, MrGO/TiO2 was

398

prepared in shorter time and showed better adsorption performance. Regeneration of

399

Ac ce p

395

ip t

384

400 401 402 403

te

d

MB.

the adsorbent was achieved by photocatalytic degradation, free from secondary environmental contamination. The kinetic and equilibrium of the adsorptions were well-modeled using pseudo second-order kinetics and Langmuir isotherm model, respectively. The adsorption was found to be a spontaneous and endothermic process, and an increased randomness occurred at the solid or solution interface.

404

References

405

[1] D. Pokhrel, T. Viraraghavan, Treatment of pulp and paper mill wastewater – a

406

review, Sci. Total Environ. 333 (2004) 37–58.

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[3] A. Cassano, R. Molinari, M. Romano, E. Drioli, Treatment of aqueous effluents of

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the leather industry by membrane processes: a review, J. Membr. Sci. 181 (2001)

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methylene blue, Bioresour. Technol. 114 (2012) 703–706.

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421

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[17] F.A. Pavan, S.P.L. Dias, E.C. Lima, E.V. Benvenutti, Removal of Congo red from aqueous solution by anilinepropylsilica xerogel, Dyes Pigments. 76 (2008) 64-

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in Graphene Based Polymer Composites, Prog. Polym. Sci. 35 (2010) 1350–1375.

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Assisted Photocatalytic Reduction of Graphene Oxide, ACS Nano. 2 (2008) 1487–

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[22] P. Zong, S. Wang, Y. Zhao, H. Wang, H. Pan and C. He, Synthesis and

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from aqueous solutions, J. Chem. Eng. 220 (2013) 45–52.

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TiO2 Nanofibers: Synthesis, Characterization and Photocatalytic Properties J.

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468

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Nanosci. Nanotechnol.14 (2014) 3034–3040. [25] D. Pan, C.Y. Chen, F. Yang, Y.M. Long, Q.Y. Cai, S.Z. Yao, Titanium wirebased SPE coupled with HPLC for the analysis of PAHs in water samples, Analyst. 136 (2011) 4774–4779.

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removal by novel TiO2/adsorbent nanocomposites, Water Science & Technology. 61

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[27] P.G. Ren, D.X. Yan, X. Ji, T. Chen, Z.M. Li, Temperature dependence of

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graphene oxide reduced by hydrazine hydrate, Nanotechnology. 22 (2011) 055705.

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[29] N.N. Zhang, H.X. Qiu, Y. Liu, W. Wang, Y. Li, X.D. Wang, J.P. Gao,

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Fabrication of gold nanoparticle/graphene oxide nanocomposites and their excellent

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catalytic performance, J. Mater. Chem. 21 (2011) 11080-11083.

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[30] M.X. Chen, C.C. Zhang, L.Z. Li, Y. Liu, X.C. Li, X.Y. Xu, F.L. Xia, W. Wang,

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J.P. Gao, Sn Powder as Reducing Agents and SnO2 Precursors for the Synthesis of

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SnO2-Reduced Graphene Oxide Hybrid Nanoparticles, ACS Appl. Mater. Interfaces.

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Synthesis and Characterization of Graphene Nanosheets, J. Phys. Chem. C 112 (2008)

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8192-8195.

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[32] S.F. Pei, H.M. Cheng, The reduction of graphene oxide, Carbon. 50 (2012)

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3210−3228.

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[33] K.S.W. Singh, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J.

492

Ac ce p

493 494 495 496

te

d

M

an

us

cr

ip t

477

Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603–619. [34]

J.S.

Liu,

G.N.

Liu,

W.X.

Liu,

Preparation

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cyclodextrin/poly(acrylic acid)/grapheme oxide nanocomposites as new adsorbents to

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remove cationic dyes from aqueous solutions, J. Chem. Eng. 257 (2014) 299–308.

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[35] M. Do˘gan, Y. Özdemir, M. Alkan, Adsorption kinetics and mechanism of

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cationicmethyl violet and methylene blue dyes onto sepiolite, Dyes Pigm. 75 (2007)

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701–713.

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[36] S.S. Gupta, K.G. Bhattacharyya, Removal of Cd(II) from aqueous solution 21

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bykaolinite, montmorillonite and their poly(oxo zirconium) and tetrabutylamm-onium

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derivatives, J. Hazard. Mater. 128 (2006) 247–257.

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[37] X.Q. Liu, S.S. Huang, Y.G. Su, Z.L. Chai, H. Zhai, X.J. Wang, A novel

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adsorbent of Na2Ta2O6 porous microspheres with F−gradient concentration

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distribution: High cationic selectivity and well-regulated recycling, J. Hazard. Mater

507

265 (2014) 226– 232.

508

[38] M.C. Somasekhara Reddy, L. Sivaramakrishna, A. Varada Reddy, The use of an

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agricultural waste material, Jujuba seeds for the removal of anionic dye (Congored)

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from aqueous medium, J. Hazard. Mater. 203–204 (2012) 118–127.

511

[39] E.N. El Qada, S.J. Allen, G.M. Walker, Adsorption of Methylene Blue onto

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activated carbon produced from steam activated bituminous coal: A study of

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equilibrium adsorption isotherm, Chem. Eng. J. 124 (2006)103–110.

514

[40]D.X. Wang, L.L Liu, X.Y. Jiang, J.Q. Yu, X.H Chen, X.Q. Chen, Adsorbent for

515

p-phenylenediamine adsorption and removal based on graphene oxide functionalized

516

with magnetic cyclodextrin, Applied Surface Science. 329 (2015) 197–205.

518 519 520

cr

us

an

M

d

te

Ac ce p

517

ip t

502

22

Page 22 of 30

520

figures 275

275

a

b

270

270 265

260 255 250

Ti/GO=0.5:1 Ti/GO=1:1 Ti/GO=2:1 Ti/GO=3:1 Ti/GO=4:1 Ti/GO=5:1

240 235 230 0

50

100

150

200

250

245

n(H+/Ti)=0:1 n(H+/Ti)=25:1 n(H+/Ti)=50:1 n(H+/Ti)=75:1

240 235 230 0

300

50

100

270

c

d

265 260

Wavelength (nm)

260

250 245

Ti/GO=0.5:1 Ti/GO=1:1 Ti/GO=2:1 Ti/GO=3:1 Ti/GO=4:1 Ti/GO=5:1

240 235

255

an

255

250

300

250

n(H+/Ti)=0:1 n(H+/Ti)=25:1 n(H+/Ti)=50:1 n(H+/Ti)=75:1

245 240 235

M

Wavelength (nm)

200

us

265

150

Time (min)

270

230

230 0

10

20

30

40

Time (min)

50

0

10

20

30

40

50

Time( min)

Fig. 1. Effects of Ti/GO (a) and H+/Ti (b) on the reduction of GO by Ti powder

d

523

250

Time (min)

521

522

255

cr

245

260

ip t

Wavelength (nm)

Wavelength (nm)

265

directly; and effects of Ti/GO (c) H+/Ti ratios (d) on the reduction of GO by Ti

525

powder under microwave assistance (400W).

(101)

(215)

(116) (220)

(204)

(105) (211)

(200)

(004)

MrGO/TiO2

Intensity

Ac ce p

526 527 528

te

524

MrGO GO Graphite

20

529 530

40

60

80

2 Theta (degree)

Fig. 2. XRD diagrams of graphite, GO, MrGO and MrGO/TiO2

23

Page 23 of 30

531

Graphite

100

90

Weight(%)

ip t

MrGO

80

70

60 GO

cr

50

40 100

200

300

400

600

us

532 533 534

Fig. 3. TGA profiles of graphite, GO and MrGO.

an

535 D

te

d

Intensity (a.u.)

M

G

1000

1500

MrGO

GO

2000

2500

3000

3500

-1

Raman shift (cm )

Ac ce p

500

536 537 538 539

500

Temperature (°C)

Fig. 4. Raman spectra of GO and MrGO.

a

b

540

c

c

d

24

Page 24 of 30

ip t

541

Fig. 5. SEM images of MrGO/TiO2 (a), rGO/TiO2 (b), TEM images of MrGO/TiO2 (c,

543

d).

us

cr

542

M

an

544

Fig. 6. Schematic procedure for MrGO/TiO2 preparation.

te

546

d

545

Ac ce p

547

1000

qt (mg/g)

800

600

400

GO rGO rGO/TiO2

200

MrGO MrGo/TiO2

0

0

548

50

100

150

200

250

300

t (min)

549

Fig. 7. The adsorption of MB onto GO, rGO, rGO/TiO2, MrGO and MrGO/TiO2.

550

Conditions: C0: 200 mg/L ; dose of adsorbent: 0.2 mg/mL; PH=10.8; at 25°C.

551 25

Page 25 of 30

600 100 550

qe (mg/g)

85

qe Removal %

450

80

400 75 350

70 5

6

7

8

9

10

11

Initial solution pH

552

12

cr

4

ip t

90

500

Removal of MB (%)

95

Fig. 8. Effect of pH values on the adsorption of MB onto MrGO/TiO2. Conditions: C0:

554

112.2 mg/L; dose of MrGO/TiO2: 0.2 mg/mL; at 25°C.

us

553

an

555

M

700 600

qt(mg/g)

500 400

d

300

37.4 mg/L 74.8 mg/L 112.2 mg/L 149.6 mg/L

te

200 100

0

556 557 558 559 560

50

100

150

200

250

t (min)

Ac ce p

0

Fig. 9. Effect of contact time on the adsorption of MB onto MrGO/TiO2 at different C0. Conditions: dose of MrGO/TiO2: 0.2 mg/mL; pH=10.8; at 25°C.

7

a

0.7

6

37.4 mg/L 74.8 mg/L 112.2 mg/L 149.6 mg/L

5

b

0.5 0.4

3

t/qt

ln (qe-qt)

4

0.6

2

0.3

37.4 mg/L 74.8 mg/L 112.2 mg/L 149.6 mg/L

0.2

1 0.1

0 0.0

-1 -0.1

0

561

50

100

150

200

250

300

350

0

50

100

150

200

250

300

350

t (min)

t (min)

26

Page 26 of 30

562

Fig. 10. Plots of pseudo-first-order model (a) and pseudo-second-order model (b) for

563

the adsorption MB onto MrGO/TiO2 .

564

ip t

565

700 C=574.58

cr

600 C=373.41 C=470.95 C=364.40

400 C=239.77

us

qt (mg/g)

500

C=345.22

C=192.23 C=275.37

300 C=143.09

200

37.4 mg/L 74.8 mg/L 112.2 mg/L 149.6 mg/L

C=172.87

100

C=87.85

-2

0

2

4

6

8

t

566

1/2

an

C=171.15

10

12

14

16

18

1/2

(min

)

Fig. 11. Intra-particle diffusion kinetic model fit for the adsorption of MB onto

568

MrGO/TiO2 . Conditions: dose of MrGO/TiO2: 0.2 mg/mL; pH=10.8; at 25 °C.

M

567

te

d

569

6.8

a 6.7

0.12

0.08

6.5 6.4

570 571 572

2

3

25°C 35°C 45°C 55°C

0.02 0.00

6.2

1

0.06 0.04

25°C 35°C 45°C 55°C

6.3

0

b

0.10

ce/qe

ln qe

Ac ce p

6.6

0.14

-0.02 4

0

5

20

40

60

80

100

Ce

ln Ce

Fig. 12. Freundlich adsorption isotherm of MB onto MrGO/TiO2 (a) Langmuir adsorption isotherm of MB onto MrGO/TiO2 (b)

573

27

Page 27 of 30

10.6 10.4 10.2

ln KL

10.0 9.8

ip t

9.6 9.4 9.2 9.0 0.0032

0.0034

1/T

574

Fig. 13. Plot of ln KL versus 1/T for the adsorption of MB onto MrGO/TiO2

us

575

0.0033

cr

0.0031

photocatalytic degradation washing with ethanol

an

100

60

M

Removal of MB (%)

80

40

20

0

578 579

4th

5th

te

577

3rd

Cycle

Fig. 14. Reusability of MrGO/TiO2 for MB

Ac ce p

576

2nd

d

1st

28

Page 28 of 30

Graphical Abstract

579

ip t

580

an

us

cr

581

M

582 700

d

600

qt(mg/g)

500

te

400 300

37.4 mg/L 74.8 mg/L 112.2 mg/L 149.6 mg/L

586

Ac ce p

200

587

The absorbents’ recycling was achieved by photocatalytic degradation.

100 0

0

583 584 585

50

100

150

200

250

t (min)

Highlights

A fast way to synthesize MrGO/TiO2 under microwave assistance was developed. The MrGO/TiO2 was an efficient adsorbent for the removal of MB.

588 589

29

Page 29 of 30

589

Tables

590 591

Table 1

592

Kinetic parameters for the adsorption of MB onto MrGO/TiO2 at 25 °C.

593

C0 (mg/L) qe.exp(mg/g)

Second-order kinetics

-1

qe.cal(mg/g) k1(min )

R

2

-1

qe.cal(mg/g) k2(min )

Intra-particle diffusion model R

2

ki,1

ip t

594

First-order kinetics

ki,2

595

37.4

177.3

5.0

0.05213

0.9719

163.4

596

74.8

372.5

72.7

0.03360

0.9023

373.1

0.0011

0.9999

597

112.2

542.5

201.1

0.01232

0.9382

561.8

0.0004

0.9979

598

149.6

675.9

257.5

0.01085

0 .9385

704.2

0.0002

0.9787

599

.

600

Table 2

601

Adsorption isotherm parameters for the adsorption of MB onto MrGO/TiO2

602

Temp (°C)

603

qmax(mg/g)

0.9999

38.23

0.96

0.61

11.68

1.72

66.89

15.33

3.52

73.55

18.12

4.56

us

cr

63.73

an

Langmuir constants

Freundlich constants

M

qe, exp (mg/g)

0.0138

kL(L/mg)

R2

kF

n

R2

25

675.9

684.9

0.2593

0.9992

428.49

8.93

0.9872

605

35

728.0

724.6

0.5524

0.9987

446.96

8.52

0.9890

606

45

795.8

806.4

0.7381

0.9987

445.71

6.93

0.9649

607

55

845.6

847.5

0.9986

469.85

6.84

0.9807

610 611 612 613 614 615

te

Ac ce p

609

d

604

608

ki,3

0.9894

Table 3

Thermodynamic parameters for the adsorption of MB onto MrGO/TiO2. Temp (K)

△G0(kJ.mol-1)

298

-22.74

308

-25.43

318

-27.04

328

-28.68

△S0(J.mol-1K-1)

180.14

△H0(kJ.mol-1)

34.25

616 617 618

30

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