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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 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
2
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
13
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
32
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
35
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
60
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].
66 67 68 69
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
91 92 93 94
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
105
solutions of the desired concentration were obtained by further dilution. The effects of
106
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
110
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
117 118 119 120
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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
132
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
146
to 272 nm with a significant increase in intensity after the reduction, suggesting that
147
GO was reduced and the electronic conjugation was restored. Position of the
148
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
150
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
152
powder at 70 °C and H+/Ti =50. When the reduction was performed on the condition
153
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
155
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
157
the reduction of GO by Ti powder. When the ratio of H+/Ti was lower than 50, GO
158
was only partially reduced since the absorption peak only shifted from 233.0 to
159
around 256.5 nm. When H+/Ti ratio was 50, the absorption peak shifted to 272.5 nm.
160
A further increasing of H+/Ti did not cause a further shift in the absorption peak, but
161
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
166
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.
167 168 169 170
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
175
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,
179
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
181
groups have been removed. This phenomenon is consistent with those for rGO
182
reduced by chemical reductants [30].
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TGA was performed to analyze the thermal stability of the sample in a N2
184
atmosphere and the results are shown Fig. 3. Graphite does not show any mass loss
185
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
189
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
199
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
192 193 194 195
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
204
The above studies confirm that Ti powder can be used to reduce GO efficiently.
205
During the reduction of GO by the Ti powder, GO was reduced to MrGO (or rGO)
206
while Ti powder was oxidized and formed ions. In order to make use of the by-
207
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)
211
easily aggregated together during the reduction owing to the vander Waals and p–p
212
stacking interactions. As for MrGO/TiO2, the high energy of microwave increased the
213
reduction rate of GO and prevented the overlap of rGO sheets. The aggregation may
214
decrease the surface area of rGO/TiO2. TiO2 nanoparticles were found to disperse
215
uniformly on the surface of MrGO (Fig.S2). The TEM images of MrGO/TiO2 in Fig.
216
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
229
amount of TiO2 in the MrGO/TiO2 composite can be calculated based on the TGA
230
curve. It is about 38.2 wt%.
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Hence, the synthetic route of MrGO/TiO2 hybrid is presumed and represented in
232
Fig. 6. GO was reduced to MrGO while Ti powder was oxidized to ions under
233
microwave assistance. The ions were then transferred to TiO2 after hydrolysis to form
234
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
237
with that of rGO/TiO2. It also predicts a higher adsorption ability towards ions or
238
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
248
the adsorbent [33]. The impact of solution pH values on the removal of dyes was
249
determined over a pH range of 4–12, and the results are shown in Fig. 8. It was found
250
that qe and R% increased with the increase of the pH value. This phenomenon could
242 243 244 245
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
252
charge due to the protonation of the remaining oxygen-containing functional groups,
253
and the positively charged surface causes electrostatic repulsion between the
254
adsorbent and the MB cationic molecules, resulting in a decrease in the adsorption
255
capacity. What’s more, a low pH value means a relatively high concentration of H+,
256
which competes strongly with MB cationic molecules for the adsorption sites on the
257
adsorbent, qe decreased as a result. As the pH value increases, the surface charge of
258
the adsorbent became more negative due to the deprotonation. At the same time, the
259
competition between H+ and cationic molecules became less significant as well. As a
260
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).
263
It can be easily observed that the trends of the four lines are similar. The qt drastically
264
increased at the beginning, then increased slowly and finally became constant after a
265
certain time. The results can be ascribed to the fact that most vacant surface sites are
266
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
273
is 149.6 mg/L, which is larger than that of most of the traditional adsorbents [4,34].
274
On the same condition, qe of MB adsorbed onto rGO/TiO2 is 407.6 mg/g, which is
275
much smaller than that of MrGO/TiO2. The higher adsorption capacity of MrGO/TiO2
267 268 269 270
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
277
adsorption performance of MrGO/TiO2, the adsorption of MB onto MrGO/TiO2 will
278
be discussed in details.
279
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
281
experimental data, the pseudo-first-order equation and the pseudo-second-order
282
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
288
and qe (mg/g) have the same meaning as those in Eq. (1). The values of k1 and qe were
289
obtained from the slopes and the intercepts of the plots of ln(qe− qt) versus t in Fig.
290
10a, and the data are presented in Tab. 1. the correlation coefficient values (R2) at the
291
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]:
292 293 294 295
298
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,
301
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-
304
second-order model was more suitable for explaining the kinetics for the adsorption of
305
MB onto MrGO/TiO2. Similar results have been reported for the adsorption of MB
306
onto Na2Ta2O6 [37].
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To test the diffusion mechanism between MB and MrGO/TiO2, an intra-particle
308
diffusion model proposed by Weber and Morris has been used and rewritten in the
309
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
312
constant, is the slope of the straight line of qt versus t1/2, as shown in Fig. 11. There
313
are three slopes for each curve, indicating that there were at least three diffusion steps
314
during the adsorption process. At the first step, the external surface adsorption or
315
diffusion in macro-pores occurred until the exterior surface reached saturation. Then,
316
the second step which is controlled by intraparticle diffusion, was the gradual
317
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
324
third step was the slowest.
325
3.3.1.2. Adsorption isotherms
318 319 320 321
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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
328
design of adsorption systems. Adsorption equilibrium is a dynamic concept achieved
329
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
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406
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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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[27] P.G. Ren, D.X. Yan, X. Ji, T. Chen, Z.M. Li, Temperature dependence of
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Fabrication of gold nanoparticle/graphene oxide nanocomposites and their excellent
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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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492
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493 494 495 496
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an
us
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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]
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remove cationic dyes from aqueous solutions, J. Chem. Eng. 257 (2014) 299–308.
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cationicmethyl violet and methylene blue dyes onto sepiolite, Dyes Pigm. 75 (2007)
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[36] S.S. Gupta, K.G. Bhattacharyya, Removal of Cd(II) from aqueous solution 21
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derivatives, J. Hazard. Mater. 128 (2006) 247–257.
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distribution: High cationic selectivity and well-regulated recycling, J. Hazard. Mater
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[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.
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[39] E.N. El Qada, S.J. Allen, G.M. Walker, Adsorption of Methylene Blue onto
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equilibrium adsorption isotherm, Chem. Eng. J. 124 (2006)103–110.
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[40]D.X. Wang, L.L Liu, X.Y. Jiang, J.Q. Yu, X.H Chen, X.Q. Chen, Adsorbent for
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p-phenylenediamine adsorption and removal based on graphene oxide functionalized
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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
Page 30 of 30
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
2
as an adsorbent for methylene blue adsorption
3
Huan Wang a, Haihuan Gaob, Mingxi Chen a, Xiaoyang, Xu a, Xuefang Wang a,
4
Cheng Pan a, Jianping Gao a, c *
5
a
School of Science, Tianjin University, Tianjin 300072, P. R. China.
6
b
Tianjin Fourth Middle School, Tianjin 300201, P. R. China.
7
c
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
8
University, Tianjin 300072, P. R. China.
9
Abstract
an
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1
In this study microwave-assisted reduction (MrGO) and direct reduction of graphene
11
oxide (rGO) by Ti powders were established, and the effect of the reaction conditions
12
on the reduction were discussed. The results showed that GO can be effectively
13
reduced by both methods, however, microwave assistance can greatly shorten the
14
reduction time. The produced Ti ions from the reaction of Ti powder with GO were
15
transferred to TiO2 by hydrolysis and formed MrGO/TiO2 and rGO/TiO2. They were
16
used as adsorbents for the removal of methylene blue (MB). MrGO/TiO2 showed a
18 19 20 21
d
te
Ac ce p
17
M
10
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
22
MB catalyzed by MrGO/TiO2 itself..
23
1． ．Introduction
24
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]
1
Page 1 of 30
food, and pharmaceuticals [1-3]. Many of the dyes and their products are harmful to
26
flora and fauna and some are even mutagenic or carcinogenic [4]. It is important to
27
have wastewater treated before its release to water system. Several techniques
28
including chemical precipitation, ion exchange, membrane filtration, adsorption,
29
photocatalysis, and electrochemical technologies have continuously been developed to
30
pursue efficient dye removal from wastewater [5-8]. Among these techniques,
31
adsorption was found to be superior due to its low cost and simple operation
32
procedure [9-12], in addition, recycling of the adsorbent is feasible after adsorption
33
[13]. A number of adsorbents have been studied for removal of dye molecules from
34
water system [14-17]. However, the usage of these adsorbents is hindered by several
35
inherent shortages, e.g. low capacities and difficulty for cycling. Development of new
36
effective and eco-friendly adsorbents for removal of dye from water system is
37
attracting more and more attention from worldwide researchers
d
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25
Graphene, with two dimensional honeycomb of carbon atoms, exhibits excellent
39
mechanical and physicochemical properties [4]. It can be readily obtained from cheap
40
natural graphite in large scale [18-19]. The high theoretical specific surface area
45
Ac ce p
te
38
46
incorporation of metal oxide nanoparticles on graphene limits their re-stacking and
47
aggregation, thereby enhancing the surface area of the composite [20-21]. The
48
functional groups and defect sites of graphene act as the nucleation and growth sites
49
for nanoparticles. Meanwhile, the incorporation of graphene extends the life time of
41 42 43 44
(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
2
Page 2 of 30
the adsorbent material by acting as support material which inhibits leaching of fine
51
metal oxide particles into the treated water [22]. The hybrid of graphene with
52
magnetic nanomaterials such as Fe3O4 has been exploited for removal of pollutants
53
from water [22]. Hao prepared SiO2/graphene composite and investigated its
54
adsorption behavior for Pb(II) ion [20]. TiO2 nanoparticle has been considered for
55
widespread environmental applications because of its excellent photocatalytic
56
performance, easy availability, long-term stability, and nontoxicity [4,23]. Recent
57
survey shows that TiO2 is also an ideal adsorbent for water pollutants [25, 26].
us
cr
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50
Herein, a simple, fast and environmentally friendly route for the reduction of
59
graphene oxide (GO) using Ti powder as a reducing agent under household
60
microwave assistance is developed. Hydrolysis was followed to synthesize reduced
61
graphene oxide/titania (MrGO/TiO2) by dropping dilute Na2CO3 solution. The
62
reduction of GO was traced by UV-visible (UV-vis) absorption spectroscopy, and the
63
obtained MrGO/TiO2 was analyzed. The removal of methylene blue (MB) can be
64
realized through strong electrostatic interaction of superficial charge of MrGO/TiO2
65
with the cationic dyes. The regeneration of the adsorbent was achieved by
70
Ac ce p
te
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an
58
71
grade and used as received.
72
2.2. Preparation of GO
73
GO was prepared from purified natural graphite by a modified Hummer's method [27-
74
29].
66 67 68 69
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
3
Page 3 of 30
75
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
77
was added to a brown GO suspension (100 mL, 2 g/L) in a glass beaker at ambient
78
temperature, then 5.5 mL concentrated H2SO4 was added dropwise into the above
79
mixture with stirring for 20 min. The beaker was placed in a water bath in a household
80
microwave oven (Galanze, G70F20N3P-ZS), and then irradiated by microwave at
81
400W for five cycles, each cycle included 5 min ‘on’ and 1min ‘off’. The brown
82
solution gradually turned black, and this illustrates that GO was reduced. After that,
83
0.5 M Na2CO3 solution was added to the prepared MrGO suspension until no obvious
84
gas bubbles were seen to fly out, this illustrates that pH was close to 7. After that, 10
85
mL (0.1g/mL) urea suspension was added and stirred at 70 °C for 3 hours to obtain
86
MrGO/TiO2. The products were then centrifuged, rinsed with distilled water and dried
87
for 24 h at 60 °C to remove the water. The rGO was prepared by heating GO and Ti
88
suspension at 70 °C for 3 hours without microwave assistance, and rGO/TiO2 was
89
prepared in the same way as that of MrGO/TiO2 but using rGO as the starting
90
material.
95
Ac ce p
te
d
M
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76
96
voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 90°(2θ)
97
with steps of 4°min-1. Raman measurements were performed with a Raman
98
microscope (DXR Microscope, USA). The thermogravimetric analysis (TGA)
99
diagrams of the samples were recorded with a Rigaku-TD-TDA analyzer with a
91 92 93 94
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),
4
Page 4 of 30
heating rate of 10°C min-1. The samples were first dried in a vacuum at 40 °C for 2
101
days before the TGA was recorded. The morphologies were observed with a scanning
102
electron microscopy (SEM) (Desk-II; Denton Vacuum).
103
2.5 Adsorption tests
ip t
100
A standardized stock solution of MB of 467.6 mg/L was prepared. Experimental
105
solutions of the desired concentration were obtained by further dilution. The effects of
106
solution pH, initial concentration, contact time, and temperature were investigated. A
107
temperature-controlled water bath shaker (SHZ-88, Shanghai, China) was used to
108
control the desired temperature. All pH measurements were carried out using a pH
109
meter (Model pHS-25, Shanghai, China).The initial pH levels of the experimental
110
solutions were adjusted to constant values by adding 0.1 M HCl or NaOH solutions.
111
The adsorption experiments were performed by shaking 4 mg of MrGO/TiO2 with 20
112
mL of the experimental solution of known concentration in a temperature-controlled
113
water bath shaker. At a certain time, the supernatant was taken out and filtered, and
114
measured by an UV-vis spectrometer at the wavelength of 664 nm with the pre-
115
established calibration curves, respectively. To prevent photodegradation of MB by
117 118 119 120
us
an
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d
te
Ac ce p
116
cr
104
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）
121 122
where qe and qt (mg/g) are the amount of MB adsorbed per unit weight of the
123
adsorbent at equilibrium and t time; C0, Ce and Ct are the MB dye concentrations at
124
initial, equilibrium and t time, respectively; V is the volume of MB solution (ca. 0.02
125
L); and m is the amount of the adsorbent. 5
Page 5 of 30
2.6. Regeneration experiment For dye regeneration, the MB initial concentration was 112.2 mg/L. 20 mg
128
MrGO/TiO2 was added into 100 mL MB solution. When the adsorption process was
129
over, MrGO/TiO2 samples saturated with MB were collected and then washed mildly
130
with distilled water to remove residual dye particles. After that, the MrGO/TiO2
131
samples were dried and added into 100 mL of ethanol aqueous solution. The solution
132
was stirred for 10 min and put in a 8 mL quartz tube. The tube was placed axially and
133
clamped in front of a 450 W medium pressure quartz mercury vapor lamp. When the
134
degradation was completed (about 3h), the MrGO/TiO2 was then collected, washed
135
with water and reused for adsorption again. The degradation–adsorption processes
136
were repeated for 5 times. Another way to realize the regeneration is to wash
137
MrGO/TiO2 samples with ethanol aqueous solution. The suspension was stirred for
138
24h to obtain dynamic equilibrium, and the equilibrium concentration was calculated.
139
3. Results and discussion
140
3.1. Reduction of GO by Ti powder
141 142 143 144 145
te
d
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127
Ac ce p
126
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
146
to 272 nm with a significant increase in intensity after the reduction, suggesting that
147
GO was reduced and the electronic conjugation was restored. Position of the
148
absorption peak reflects the degree of the reduction in the rGO [30].
149
The effects of several factors such as H2SO4 amount and the mass ratio of Ti/GO on
150
the reduction of GO by Ti powder were tested and the results are shown in Fig. 1(a)
6
Page 6 of 30
and (b). Figure 1a shows the effects of Ti/GO ratio on the reduction of GO by Ti
152
powder at 70 °C and H+/Ti =50. When the reduction was performed on the condition
153
of Ti/GO=0.5:1, the peak position did not change much. When the Ti/GO was 1:1, the
154
peak red shifted to 272.5 in 3h, a further increasing in the Ti/GO ratio did not result in
155
a further shift, but the reaction time was shortened. For example, the reduction time is
156
only 110 min when the Ti/GO was 5:1. Figure 1b shows the effect of H+/Ti ratio on
157
the reduction of GO by Ti powder. When the ratio of H+/Ti was lower than 50, GO
158
was only partially reduced since the absorption peak only shifted from 233.0 to
159
around 256.5 nm. When H+/Ti ratio was 50, the absorption peak shifted to 272.5 nm.
160
A further increasing of H+/Ti did not cause a further shift in the absorption peak, but
161
the reduction required less time.
M
an
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cr
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151
To investigate the effect of microwave on the reduction of GO by Ti powder, the
163
GO reduction was conducted under microwave assistance and the results are shown in
164
Fig. 1(c) and (d). Figure 1c shows the reduction of GO at different Ti/GO. When the
165
mass ratio of Ti/GO increased to 1:1, the absorption peak of the MrGO suspension
166
could get 267.0 nm in 30 min. As is shown in Fig. 1e, when H+/Ti increased to 50, the
171
Ac ce p
te
d
162
172
conditions for GO reduction were: Ti/GO =1.0, H+/Ti =50:1, 400W, and the samples
173
in the following study are MrGO.
167 168 169 170
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
174
To characterize the structure of GO reduced by Ti powder under microwave
175
assistance, XRD patterns of graphite, GO and MrGO were measured and shown in Fig.
7
Page 7 of 30
2. Graphite has a narrow and strong diffraction peak at around 2θ=26.5° (d-spacing is
177
0.34 nm), but GO has a broad peak at about 11.4° (the interlayer spacing is 0.78 nm).
178
The larger interlayer distance can be attributed to the formation of hydroxyl, epoxy,
179
and carboxyl groups, which increases the distance between the layers. However, this
180
peak disappeared in MrGO. It indicates that some of the oxygen-containing functional
181
groups have been removed. This phenomenon is consistent with those for rGO
182
reduced by chemical reductants [30].
cr
ip t
176
TGA was performed to analyze the thermal stability of the sample in a N2
184
atmosphere and the results are shown Fig. 3. Graphite does not show any mass loss
185
from room temperature to 600°C. GO shows two main weight losses. The first rapid
186
weight loss (15%) was at temperatures up to 100 °C and can be attributed to the
187
removal of water molecules absorbed on the GO surface. The second weight loss
188
(25%) between 200 and 250 °C is due to decomposition of the oxygen-containing
189
functional groups [31]. The MrGO has a mass loss of 10% between 200 and 250 °C,
190
which is much smaller than that of GO. This indicates that the thermal stability of
191
MrGO was improved which is due to a decrease in the amount of oxygen containing
196
Ac ce p
te
d
M
an
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183
197
atoms (usually observed at 1596 cm-1) and the D mode due to the breathing mode of
198
k-point phonons with A1g symmetry (at 1360 cm-1). Changes in the relative intensities
199
of the D and G bands (D/G) indicate changes in the electronic conjugation state of GO.
200
The Raman spectra of GO and MrGO shown in Fig. 4 demonstrate that the D/G ratio
192 193 194 195
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
8
Page 8 of 30
201
of MrGO increased to 1.28 compared with that of GO (0.99). This increase suggests
202
that sp2 domains were formed owing to the reduction by Ti powder [32]. 3.2. Fabrication of MrGO/TiO2 and rGO/TiO2 hybrids
204
The above studies confirm that Ti powder can be used to reduce GO efficiently.
205
During the reduction of GO by the Ti powder, GO was reduced to MrGO (or rGO)
206
while Ti powder was oxidized and formed ions. In order to make use of the by-
207
products (ions), Na2CO3 solution was introduced to prepare rGO/TiO2 and
208
MrGO/TiO2.
us
cr
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203
The SEM photos of the rGO/TiO2 and MrGO/TiO2 were shown in Fig.5. rGO/TiO2
210
is larger and thicker when compared with MrGO/TiO2, because GO sheets (Fig.S1)
211
easily aggregated together during the reduction owing to the vander Waals and p–p
212
stacking interactions. As for MrGO/TiO2, the high energy of microwave increased the
213
reduction rate of GO and prevented the overlap of rGO sheets. The aggregation may
214
decrease the surface area of rGO/TiO2. TiO2 nanoparticles were found to disperse
215
uniformly on the surface of MrGO (Fig.S2). The TEM images of MrGO/TiO2 in Fig.
216
5c-d clearly showed that TiO2 nanoparticles (Fig. 5c-d) with sizes of 10-30 nm were
221
Ac ce p
te
d
M
an
209
222
(004), (200), (105), (211), (204) and (215) of anatase TiO2 (JCPDS, card no. 21-1272),
223
respectively. This indicates that Ti was transformed to TiO2 nanocrystals and
224
MrGO/TiO2 was formed.
217 218 219 220
225
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
9
Page 9 of 30
MrGO/TiO2 under flowing air (Fig. S3). There was a small loss (about 10.7%) for the
227
TiO2. MrGO had a rapid weight loss around 550°C and retained a residual mass of
228
9.1% at 800°C. The residual mass of MrGO/TiO2 was about 39.7% at 550 °C. The
229
amount of TiO2 in the MrGO/TiO2 composite can be calculated based on the TGA
230
curve. It is about 38.2 wt%.
ip t
226
Hence, the synthetic route of MrGO/TiO2 hybrid is presumed and represented in
232
Fig. 6. GO was reduced to MrGO while Ti powder was oxidized to ions under
233
microwave assistance. The ions were then transferred to TiO2 after hydrolysis to form
234
MrGO/TiO2 hybrid
235
3.3. Adsorption of MB by MrGO/TiO2 and rGO/TiO2
an
us
cr
231
The above SEM has demonstrated the superior structure of MrGO/TiO2 compared
237
with that of rGO/TiO2. It also predicts a higher adsorption ability towards ions or
238
organic compounds. Figure 7 shows the adsorption of MB onto GO, rGO, rGO/TiO2,
239
MrGO, MrGO/TiO2. The adsorption capacity of rGO/TiO2 was lower than those of
240
GO and rGO, and the adsorption capacity of MrGO/TiO2 was also lower than that of
241
MrGO. This is because the direct reduction of GO can cause aggregation and
246
Ac ce p
te
d
M
236
247
influence the surface charge of the adsorbent as well as the surface binding-sites of
248
the adsorbent [33]. The impact of solution pH values on the removal of dyes was
249
determined over a pH range of 4–12, and the results are shown in Fig. 8. It was found
250
that qe and R% increased with the increase of the pH value. This phenomenon could
242 243 244 245
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
10
Page 10 of 30
be explained as follows, at a low pH value, MrGO/TiO2 acquires a surface of positive
252
charge due to the protonation of the remaining oxygen-containing functional groups,
253
and the positively charged surface causes electrostatic repulsion between the
254
adsorbent and the MB cationic molecules, resulting in a decrease in the adsorption
255
capacity. What’s more, a low pH value means a relatively high concentration of H+,
256
which competes strongly with MB cationic molecules for the adsorption sites on the
257
adsorbent, qe decreased as a result. As the pH value increases, the surface charge of
258
the adsorbent became more negative due to the deprotonation. At the same time, the
259
competition between H+ and cationic molecules became less significant as well. As a
260
result, qe dramatically increased.
an
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251
Figure. 9 shows the effect of contact time on the adsorption capacity of MB onto
262
MrGO/TiO2 at different MB initial concentrations (37.4, 74.8, 112.2 and 149.6 mg/L).
263
It can be easily observed that the trends of the four lines are similar. The qt drastically
264
increased at the beginning, then increased slowly and finally became constant after a
265
certain time. The results can be ascribed to the fact that most vacant surface sites are
266
available for adsorption at the initial adsorption stage. At the end stage, the remaining
271
Ac ce p
te
d
M
261
272
overcome the mass transfer resistance of the dye. The qe reaches 675.9 mg/g when C0
273
is 149.6 mg/L, which is larger than that of most of the traditional adsorbents [4,34].
274
On the same condition, qe of MB adsorbed onto rGO/TiO2 is 407.6 mg/g, which is
275
much smaller than that of MrGO/TiO2. The higher adsorption capacity of MrGO/TiO2
267 268 269 270
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
11
Page 11 of 30
can be attributed to its special morphology discussed above. Due to the excellent
277
adsorption performance of MrGO/TiO2, the adsorption of MB onto MrGO/TiO2 will
278
be discussed in details.
279
3.3.1. Kinetics and thermodynamics of MrGO/TiO2 adsorbent
ip t
276
To understand the adsorption mechanism, two kinetic models were used to test the
281
experimental data, the pseudo-first-order equation and the pseudo-second-order
282
equation.
283
3.3.1.1. The pseudo-first-order and pseudo-second-order kinetic model
286
us
an
285
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)
M
284
cr
280
Where k1 (min-1) is the rate constant of the pseudo first-order adsorption (min−1), qt
288
and qe (mg/g) have the same meaning as those in Eq. (1). The values of k1 and qe were
289
obtained from the slopes and the intercepts of the plots of ln(qe− qt) versus t in Fig.
290
10a, and the data are presented in Tab. 1. the correlation coefficient values (R2) at the
291
initial concentration of 37.4, 74.8, 112.2 and 149.6 mg/L were 0.9719, 0.9023, 0.9382
296
Ac ce p
te
d
287
297
can be represented in the following form [36]:
292 293 294 295
298
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)
299 300
Where k2 (g mg-1 min-1) is the rate constant of pseudo-second-order equation, qt, qe,
301
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
303
the correlation coefficients (R2) were higher than 0.97, which indicated that pseudo-
304
second-order model was more suitable for explaining the kinetics for the adsorption of
305
MB onto MrGO/TiO2. Similar results have been reported for the adsorption of MB
306
onto Na2Ta2O6 [37].
ip t
302
To test the diffusion mechanism between MB and MrGO/TiO2, an intra-particle
308
diffusion model proposed by Weber and Morris has been used and rewritten in the
309
following form:
us
qt =kit1/2 + C
(5)
an
310
cr
307
where C is the value of intercept, ki (mg g-1 min1/2), intra-particle diffusion rate
312
constant, is the slope of the straight line of qt versus t1/2, as shown in Fig. 11. There
313
are three slopes for each curve, indicating that there were at least three diffusion steps
314
during the adsorption process. At the first step, the external surface adsorption or
315
diffusion in macro-pores occurred until the exterior surface reached saturation. Then,
316
the second step which is controlled by intraparticle diffusion, was the gradual
317
adsorption step. The third step was the final equilibrium step, for which MB moved
322
Ac ce p
te
d
M
311
323
dimensions decreased [38, 39]. The ki,3 is significantly lower than the others, so the
324
third step was the slowest.
325
3.3.1.2. Adsorption isotherms
318 319 320 321
326
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
13
Page 13 of 30
information about the surface properties of adsorbent, the adsorption behavior and the
328
design of adsorption systems. Adsorption equilibrium is a dynamic concept achieved
329
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
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406
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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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Fabrication of gold nanoparticle/graphene oxide nanocomposites and their excellent
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SnO2-Reduced Graphene Oxide Hybrid Nanoparticles, ACS Appl. Mater. Interfaces.
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493 494 495 496
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an
us
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ip t
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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]
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remove cationic dyes from aqueous solutions, J. Chem. Eng. 257 (2014) 299–308.
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cationicmethyl violet and methylene blue dyes onto sepiolite, Dyes Pigm. 75 (2007)
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derivatives, J. Hazard. Mater. 128 (2006) 247–257.
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distribution: High cationic selectivity and well-regulated recycling, J. Hazard. Mater
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[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.
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[39] E.N. El Qada, S.J. Allen, G.M. Walker, Adsorption of Methylene Blue onto
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[40]D.X. Wang, L.L Liu, X.Y. Jiang, J.Q. Yu, X.H Chen, X.Q. Chen, Adsorbent for
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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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