Adsorption-photocatalytic degradation of dye pollutant in water by graphite oxide grafted titanate nanotubes

Accepted Manuscript Adsorption-photocatalytic degradation of dye pollutant in water by graphite oxide grafted titanate nanotubes

Xiaoman Lei, Xiaolong Li, Zhiqiang Ruan, Tingmei Zhang, Fei Pan, Qiang Li, Dongsheng Xia, Jie Fu PII: DOI: Reference:

S0167-7322(18)32297-9 doi:10.1016/j.molliq.2018.06.053 MOLLIQ 9251

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

2 May 2018 7 June 2018 13 June 2018

Please cite this article as: Xiaoman Lei, Xiaolong Li, Zhiqiang Ruan, Tingmei Zhang, Fei Pan, Qiang Li, Dongsheng Xia, Jie Fu , Adsorption-photocatalytic degradation of dye pollutant in water by graphite oxide grafted titanate nanotubes. Molliq (2018), doi:10.1016/j.molliq.2018.06.053

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ACCEPTED MANUSCRIPT Adsorption-photocatalytic degradation of dye pollutant in water by graphite oxide grafted titanate nanotubes

Xiaoman Leia, Xiaolong Lia, Zhiqiang Ruana, Tingmei Zhanga, Fei Pana,b,*, Qiang Lia,

School of Environmental Engineering, Wuhan Textile University, Wuhan, 430073,

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a

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Dongsheng Xiaa,b, Jie Fua,c,*

P.R. China

Engineering Research Centre for Clean Production of Textile Dyeing and Printing,

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b

c

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Ministry of Education, Wuhan, 430200, P.R. China

Department of Environmental Science & Engineering, Fudan University, Shanghai

*

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200438, P.R. China

Corresponding authors

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Jie Fu, Tel. & Fax: 86-21-65647707. E-mail: [email protected]

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Fei Pan, Tel. & Fax: 86-27-59367338. E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract Aiming at synthesizing titanate-based photocatalysts with high adsorption capacity and enhanced photocatalytic activity for dyes treatment, a new type of graphite oxide grafted titanate nanotubes (TNTs@GO) was prepared through a

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one-step hydrothermal method and characterized by a series of techniques including

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HRTEM, XRD, FTIR, XPS, etc. At the optimal GO loading (1 wt.%), calcination

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temperature (300 °C) and time (0.5 h), TNTs@GO showed the highest photocatalytic activity for degrading Methylene Blue (MB, a model dye) under UV irradiation.

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Approximately 97.5% of MB was removed by TNTs@GO with 30-min

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pre-adsorption and 90-min UV irradiation. Radical scavengers quenching tests and electron paramagnetic resonance (EPR) measurements demonstrated several reactive

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species, i.e., h+, •OH and O2•-, played the critical roles in the photocatalytic process,

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and the presence of oxygen vacancy in TNTs@GO reduced the recombination rate of electron-hole pairs, thereby improving the photocatalytic activity. The effects of

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operational parameters were investigated, including TNTs@GO dosage, initial MB

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concentration, temperature, initial pH, humic acid and inorganic anions. Moreover, TNTs@GO showed a good reusability and applicability for multi-types of dyes. Therefore, TNTs@GO is a promising photocatalyst for dye wastewater treatment. Keywords: TNTs@GO; Photocatalysis; Methylene Blue; Dye wastewater treatment; Reusability

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ACCEPTED MANUSCRIPT 1. Introduction Dyes are widely used in many industries, such as textile, leather, cosmetic, printing, drug, and food processing [1]. More than 100,000 dyes of various structures including acidic, basic, azo, diazo, anthroquinone based, and metal complex dyes are

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commercially available with a production amount of approximately 700,000 tons per

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year [2], and the most amount of dyes is manufactured in the developing countries,

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e.g., China, India, Thailand, Turkey, etc. [3]. During the application, large quantities of dyes were discharged into the water environment, and it is estimated that

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approximate 10% of total spent dyes are eventually discharged into the effluent in the

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textile process [4]. Most of dyes contain aromatic ring, which makes them carcinogenic, resistant to biodegradation, highly toxic, and mutagenic to both human

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beings and aquatic life [5]. For example, some researchers have detected 50-500 ng/L

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of Disperse Red 1 dye in a river influenced by the textile industry discharges; the dye concentration level exceeds the long-term Predicted No-Effect Concentration (PNEC)

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for Daphnia similis (60 ng/L) and thereby will pose a potential risk to freshwater biota

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[6]. Moreover, the dyes cause undesirable esthetic problem, prevent the solar light penetration and retard the photosynthetic reaction [4]. Many of developing countries do not have strict legislation, or the discharge standard is not mandatorily executed, which makes the dye pollution worse [4]. Therefore, effectively treating such dye pollutants prior to discharge is very important and has attracted increasing attention. Recently, the development of nanotechnology offers leapfrogging opportunities for developing innovative technologies for the more efficient removal of various

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ACCEPTED MANUSCRIPT pollutants during water and wastewater treatment [7]. The extraordinary properties of nanomaterials, including large surface area, small size effect, quantum effect, photosensitivity, catalytic activity and electrochemical and magnetic properties, greatly benefit their application potentials in dye wastewater treatment [4]. Up to now,

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many nanomaterials, such as carbon nanotubes (CNTs) [8], graphene [9], titanium

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dioxide (TiO2) nanoparticles [10], and molybdenum disulfide (MoS2) nanosheets [11],

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have been used to develop novel adsorbents or photocatalysts for dye removal and showed superior performance than bulk counterparts.

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Titanate nanotubes (TNTs), which were firstly synthesized by TiO2 and sodium

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hydroxide (NaOH) in 1998 [12], are promising multifunctional nanomaterials in water/wastewater treatments and have drawn great attentions in recent years due to

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their special physicochemical properties, like uniform microstructures, small tube

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diameter, large surface area, good stability, ion-exchange properties, photo-electric function and quantum size effects [13-15]. Great efforts have been made to apply

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TNTs in the removal of various dye pollutants as adsorbents [16-18], and

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photocatalysts [19-21]. The adsorption of dyes by TNTs is a chemical sorption process and the electrostatic attraction in the initial bulk diffusion is the critical step [22], resulting in the limited adsorption capacity for basic dyes due to the negatively charged surface of TNTs. Alternatively, the photocatalysis is a more complete, widely applicable, and cost-effective technique, because it can achieve non-selective degradation and mineralization, and has high reusability and prolonged service life. However, the photocatalytic activity of TNTs is relatively limited [23], and thereby

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ACCEPTED MANUSCRIPT different attempts have been made to improve the photoactivity of TNTs, including post-thermal treatment [24], hydrogen peroxide (H2O2) modification [25], decorating with cerium oxide (CeO2) [19], zinc (Zn) surface-doping [26], nitrogen-doping [27], and hybridizing with stannic oxide (SnO2) [28].

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In recent years, graphene, a flat monolayer of carbon atoms tightly packed into a

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two-dimensional honeycomb lattice, has attracted a great deal of attentions for its

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potential applications in many fields, such as nano-electronics, fuel-cell technology, and catalysts [29, 30]. Graphite oxide (GO) is one of the most important precursors of

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graphene, and shares similar sheet structures and properties with graphene, such as

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high adsorption capacity and stability, and semiconducting characteristics [31]. GO can enhance the light absorption and photocatalytic activity via suppressing the charge

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recombination by serving as a photo-generated electron transmitter, when coupled

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with a semiconductor like TiO2 [32]. Yet, few studies have combined GO with TNTs to enhance the photocatalytic activity. Sang et al. [33] used TNTs to decorate GO and

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the resultant nanocomposites can simultaneously improve the flame retardancy and

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photodegradability of a polymer, flexible polyvinyl chloride (PVC). In the follow-up work [34], they doped platinum into the TNTs/GO nanocomposites and enhanced the visible light photodegradation performance toward PVC. As far as we know, the application of TNTs/GO nanocomposites in the photocatalytic degradation of dyes has not been explored. The information on the effectiveness, feasibility and robustness of this nanomaterial on treating dye wastewater have kept unknown. To this end, we have synthesized a new type of GO grafted TNTs (TNTs@GO)

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ACCEPTED MANUSCRIPT through hydrothermal method, and optimized the synthesis conditions based on the photocatalytic activity under UV irradiation toward a model dye, Methylene Blue (MB). Besides the material characterization and photocatalytic mechanism explanation, the effects of operational parameters on the adsorption-photocatalytic

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performance were comprehensively investigated, including TNTs@GO dosage, initial

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MB concentration, temperature, initial pH, humic acid and inorganic anions.

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Moreover, the reusability and applicability to different dyes of TNTs@GO were addressed. The overall goal of this work is to develop a promising photocatalyst with

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high adsorption capacity and excellent photocatalytic activity for dye wastewater

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treatment.

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2.1. Materials and chemicals

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2. Experimental

The chemicals of this experiment were of analytical grade or higher. Natural

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graphite, and 5,5-dimethyl-1-pyrolin-N-oxide (DMPO, >97% purity) were supplied

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by Aladdin Industrial Corporation (Shanghai, China). TiO2 (P25, 80% anatase and 20% rutile), the precursor for TNTs synthesis, was purchased from Dedussa of Germany. Methylene Blue, Disperse Blue 56 (DB56), Reactive Brilliant Red X-3B (X-3B), tert-butyl alcohol (TBA), 1,4-benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA), and all other reagents were obtained from Sinopharm Chemical Reagent Company (Shanghai, China), and used without further purification. The sample solutions throughout the experiments were prepared using deionized (DI) water.

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2.2. Synthesis of TNTs@GO nanocomposites Graphite oxide was prepared using a modified Hummers’ method [35]. Text S1 provides detailed procedures for the preparation of GO. TNTs@GO was prepared

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through a one-step hydrothermal method. Specifically, predetermined amount of GO

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and 0.6 g of TiO2 was added to 33.35 mL of a 10 M NaOH solution with stirring for

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12 h. After mixing completely, the resultant solution was transferred to a 50 mL Teflon-lined flask, and then maintained at 130 °C for 72 h. Next, the sediment was

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washed with DI water several times until the pH reached neutral, and then dried at

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80 °C in an oven for 4 h. Finally, the product was annealed at 300 °C and lasting 0.5 h. In selected experiments, the GO of predetermined mass was added to achieve

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different loadings (0.5, 1, 3 and 5 wt.%). At the optimal GO loading, the calcination at

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various temperature (200, 300, 400 and 500 C) were conducted to reveal the optimal calcination temperature. The calcination time (0.5, 1 and 2 h) was also optimized at

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the optimal GO loading and calcination temperature. For comparison, TNTs was also

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prepared by the similar procedure but without GO loading.

2.3. Material characterization and measurement X-ray powder diffraction (XRD) profiles were collected with a Rigaku Ultima IV diffractometer (Tokyo, Japan) using Cu Kα radiation and operated at 40 kV and 100 mA. Fourier transform infrared spectra (FTIR) characterizations of the samples were obtained with an Avatar 360 FT-IR spectrophotometer (Nicolet, Waltham, MA, USA).

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ACCEPTED MANUSCRIPT X-ray photoelectron spectroscopy (XPS) measurements were performed ex situ using a Perkin-Elmer PHI system (Waltham, MA, USA). The dried samples were pressed into a clean and high purity indium foil for XPS analysis. The background corrected XPS spectra were fitted by constructing the parameter-controlled Gaussian/Lorentzian

2000).

High

resolution

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(http://www.phy.cuhk.edu.hk/~surface/XPSPEAK,

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peaks for different chemical states of the element with the XPSPeak software

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Transmission electron microscope (HRTEM) images and energy dispersive X-ray spectrometry (EDS) were acquired using a JEOL JEM-2100 TEM (Tokyo, Japan).

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The specific surface area and pore volumes of the samples were determined by

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nitrogen (N2) adsorption-desorption isotherms on a Quantachrome Autosorb-1 system (Boynton Beach, FL, USA) at 77K. The photo-generated free radicals from the

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photodegradation system captured by a spin trapping agent DMPO were determined

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by the electron paramagnetic resonance (EPR, Bruker A300, Rheinstetten, Germany), working at a microwave frequency of 9.86 GHz with a power of 1 mW, center field of

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3510 G, sweep width of 100 G, and scan number of 3.

2.4. Adsorption-photocatalytic degradation experiment Adsorption-photocatalytic degradation experiments were performed in a glass reactor equipped with magnetic stirring at 25 °C. After 100 mL of dye (MB, RhB or X-3B) solution with the initial concentration of 20 mg/L was introduced into the reactor, the pH was adjusted by diluted NaOH and HCl until to the desired values. Then 20 mg of the catalyst (TNTs, GO or TNTs@GO) was dispersed in the solution

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ACCEPTED MANUSCRIPT and stirred for 30 min in darkness to reach adsorption-desorption equilibrium. Later, the solution was radiation by a 175 W mercury lamp with the main output at 365 nm and a light intensity of 1.40-1.45 mW/cm2. The samples were withdrawn at pre-determined time intervals, and centrifuged to remove the catalyst. The of

dye

was

then

immediately

determined

by

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UV-vis

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concentration

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spectrophotometer (Agilent Technologies, Santa Clara, CA, USA).

2.5. Reusability evaluation

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To evaluate the recyclability of TNTs@GO, five successive repeatability

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experiments were performed through the following method. After each photocatalytic process, the mixture solution was still for 1 h, allowing the spent TNTs@GO to settle

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by gravity. Then, about 90% of the supernatant was discarded and the remaining

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liquid-solid mixture was transferred to another glass photoreactor. The reactor was irradiated with UV light for 1 h and washed with methanol, then dried at 80 °C. The

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product was subjected to the photocatalysis again.

3. Results and discussion 3.1. Adsorption-photocatalytic performance of TNTs@GO prepared under different conditions The adsorption-photocatalytic performance of TNTs@GO prepared with different GO loadings (0.5-5 wt.%, with a fixed calcination temperature of 300 C and calcination time of 0.5 h), calcination temperatures (200-500 C, with a fixed GO

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ACCEPTED MANUSCRIPT loading of 1 wt.% and calcination time of 0.5 h), and calcination times (0.5-2 h, with a fixed GO loading of 1 wt.% and calcination temperature of 300 C) were evaluated using MB as a model dye pollutant. GO exhibited a remarkable adsorption capacity for MB, and the removal

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efficiency reached 93.2% after 30-min adsorption; after 90-min UV irradiation,

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another more 4.4% of MB in the solution were removed by photocatalytic degradation

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(Fig. 1(a)). TNTs showed considerable adsorption capacity for MB, and 77.6% of MB can be removed by adsorption; TNTs also had limited photocatalytic activity and 9.3%

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of MB was removed by TNTs facilitated photodegradation (Fig. 1(b)). Increasing the

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GO loading in TNTs@GO from 0.5 to 1 wt.% gradually decreased the adsorption percentage of MB to 63.6%, however, the photocatalytic removal of MB was

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significantly increased to 33.9%. Further increasing GO loading to 5 wt.% decreased

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the photocatalytic removal of MB to 5.5%, though the adsorption removal of MB was enhanced. Consequently, the optimum GO loading for TNTs@GO was set at 1 wt.%,

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when both photocatalytic removal and total removal of MB were the largest. The

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results suggest that the GO loading is of crucial importance for the photocatalytic activity of TNTs@GO. The GO can promote the charge separation in the TNTs@GO, and enhance the photocatalytic activity [36]. However, Excessive GO can cover the active surface of TNTs and hinder its light absorbance, thereby reducing the photocatalytic activity. Increasing the calcination temperature from 200 to 300 C decreased the adsorption capacity of TNTs@GO but enormously promoted the photocatalytic

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ACCEPTED MANUSCRIPT activity (Fig. 1(c)). The maximum total removal of MB (97.5%) was achieved with a calcination temperature of 300 C. However, continuously increasing the calcination temperature to 400 and 500 C would impact the total removal of MB. Thus, the optimal calcination temperature was determined to be 300 C. Extending the

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calcination time from 0.5 to 1 and 2 h showed a modest effect to promote the

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adsorption capacity TNTs@GO but an evident effect to decline the photocatalytic

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activity (Fig. 1(d)).

Taken together, the TNTs@GO with GO loading of 1%, calcination temperature

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of 300 C and calcination time of 0.5 h showed the optimum performance on the

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removal of MB, and was further characterized and tested in the following sections.

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3.2. Characterization of as-synthesized photocatalysts

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Fig. 2 displays the XRD patterns of GO, TNTs and TNTs@GO. For GO, the peak at 10.1° (2θ) corresponds to the (002) interlayer distance of 0.950 nm. Because of the

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presence of oxygen functional groups attached on both sides of the graphite flakes, it

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creates atomic defects (sp3 bonding) in the graphite structure and tends to exfoliate to a few layers or individual layers of GO in an aqueous medium [37]. The as-synthesized TNTs exhibits five characteristic peaks at 2θ = 9.8°, 24.3°, 28.4°, 48.4° and 62.5°, which can be attributed to sodium tri-titanate and assigned as the (200), (110), (310), (020) and (202) planes of crystalline H2Ti2O5·H2O (JCPDS 47-0124) [38, 39]. The peaks of TNTs@GO are the same as TNTs, and the sample shows no diffraction peaks of GO layered structure. This is due to the layer-stacking or the little

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ACCEPTED MANUSCRIPT amount of GO-doping or the reduction of GO to graphene [40, 41]. The FTIR spectrum of GO shows several strong peaks (Fig. 3). We can observe the peaks of oxygen functional groups at 1067 cm-1, 1366 cm-1 and 1724 cm-1, which are attributed to C-O, C-O-C and C=O, respectively. The peak at 1620 cm-1 can be

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assigned to in-plane vibrations of C=C group in graphene [29]. Furthermore, the wide

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absorption ranging from 3038 cm-1 to 3571 cm-1 derives from the O-H stretching

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vibration. Compared to GO, the C=O peak at 1724 cm-1 and other sharp peaks at 600-1100 cm-1 are disappeared in the TNTs@GO, which confirms the reduction of

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GO.

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Based on N2 adsorption-desorption isotherms, the BET surface area was calculated to be 94.4 m2/g and 139.9 m2/g for TNTs and TNTs@GO, and the pore

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volume was 0.18 cm3/g and 0.244 cm3/g, respectively. The increase of BET surface

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area and pore volume may be owed to that the introduction of GO has resulted in a progressive change on the structure of support. The high BET surface and porous

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structure of photocatalysts are beneficial for adsorption and photodegradation of

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targeted pollutants. The detailed descriptions for N2 adsorption-desorption isotherms are shown in Text S2 and Fig. S1. The HRTEM was used to investigate the morphology of the synthesized GO and TNTs@GO. The as-prepared GO exhibited a typical layered, flaky degree, and crumpled structure owing to its thin and larger sheetlike morphology (Fig. 4(a-b)). The lateral dimensions of GO nanosheets are several micrometers [42]. For TNTs@GO, we can see that TNTs occupy most of the available surface area of GO,

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ACCEPTED MANUSCRIPT and an interconnected network of TNTs floated on the surface of the GO (Fig. 4(c-d)), which shows that GO serves as a matrix for the densely packed TNTs. To confirm that the TNTs was successfully distributed on the GO, the EDS was carried out to quantitative analysis for the presence of elements in the materials. The EDS spectrum

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shows peaks corresponding to C, O and Ti (Fig. 4(e)), which ensures that the TNTs

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are grown on the GO surface.

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XPS was investigated to examine the elemental composition and chemical state of the elements that exist within TNTs@GO. It can be seen from the Ti 2p spectrum

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that the significant peaks were located in 458.52 eV and 464.17 eV (Fig. 5(a)), which

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are the characteristics of Ti 2p3/2 and Ti 2p1/2, respectively [15], and both are derived from Ti4+-O bonds [43]. The O 1s XPS spectrum of TNTs@GO (Fig. 5(b)) contains

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three dominated peaks at 529.97 eV, 531.22 eV and 535.22 eV, which corresponds to

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O-Ti4+, O-Ti3+, and C-O bonds, respectively. Fig. 5(c) shows the XPS spectrum of C 1s in the TNTs@GO, and the strong peak at 284.72 eV is ascribed to C-C bond.

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Additionally, the oxygen functionalities attached to the carbons show deconvoluted

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peaks for C-O (285.57 eV) and C=O (288.72 eV) at higher binding energies [44]. The quantitative results and parameters of the XPS fit are listed in Table 1. To further identify the presence of oxygen vacancy (Vo) and Ti3+ in TNTs or TNTs@GO under UV irradiation, EPR was employed. We can see the TNTs has no characteristic EPR resonances (Fig. 6). However, TNTs@GO yields a perfect data fit to the EPR resonance at g = 2.0079, indicating the existence of Ti3+, and the low field signal corresponds to the electrons trapped at the Vo [45]. The reduction of Ti4+ to Ti3+ 13

ACCEPTED MANUSCRIPT owes to the efficient photo-induced interfacial charge transfer. Hence, the recombination rate of electron-hole pairs in TNTs@GO could be reduced, thereby improving the photocatalytic activity for the decomposition of dye pollutants.

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3.3. Effects of operational parameters on removal of Methylene Blue

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We studied the effect of TNTs@GO dosage on the removal of MB and the

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results are shown in Fig. 7. When the dosage of TNTs@GO was raised from 50 to 400 mg/L, the removal efficiency of MB was enhanced significantly; nearly complete

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removal of MB was achieved when the TNTs@GO dosage was higher than 200 mg/L.

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Higher catalyst dosage probably leaded to more active sites for adsorption and oxidation, and produced more reactive radical species to attack dye pollutants.

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However, the enchantment effect will be limited when the system is saturated due to

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the shielding of partial UV illumination and self-consumption of radicals by excess catalysts [39, 46]. Since highest MB removal efficiency was got at 200 mg/L, we

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chose this material dosage in the following tests for economy and cost consideration.

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The MB removal experiments were carried out with different initial MB concentrations (10-40 mg/L), and the results are shown in Fig. 8. The adsorption process was dominant and the photodegradation process can be ignorable with an initial MB concentration of 10 mg/L. As the initial MB concentration was increased from 20 to 40 mg/L, the total MB removal efficiency was gradually decreased from 97.5% to 69.7% after 90-min UV irradiation. The possible reason was that adsorption sites and reactive species approached saturation with an increase in initial MB 14

ACCEPTED MANUSCRIPT concentration [5]. Additionally, higher initial MB concentration introduced more chloride anions (Cl-), which competed with organic substances for reaction with reactive species [47]. In order to ensure a sufficient reaction rate, we selected 20 mg/L as the initial MB concentration in the adsorption-photocatalytic degradation

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experiments.

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The effect of reaction temperature on photocatalytic degradation of MB by

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TNTs@GO was investigated, and the results are displayed in Fig. 9. The calculated pseudo-first-order rate constant (k1) of MB photodegradation was 0.02064 min-1 (R2 =

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0.9749) at 15 °C，0.02845 min-1 (R2 = 0.9844) at 25 °C, and 0.02465 min-1 (R2

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=0.9861) at 35 °C, respectively. Thus, increasing temperature had a limited promoting effect on the photodegrdation rate of MB, which can be explained by the classical

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collision theory where the higher temperature accelerates the molecular collision rate,

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and the change in interatomic interactions weakens the chemical bonds, resulting in faster reaction rates [29]. Due to the limited effect and economy consideration, the

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photocatalytic experiments can be conducted in room temperature (25 °C).

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Chloride (Cl-), sulfate (SO42-), bicarbonate (HCO3-) and phosphate (HPO42-) anions are commonly existed in wastewater. As illustrated in Fig. 10, in the presence of these anions of 200 mg/L, a significant inhibitory effect on MB adsorption and photodegradation was observed, resulting in approximately 31-37% decrease in MB removal after 90-min UV irradiation. When these inorganic anions were introduced into the reaction solution, they were adsorbed on the TNTs@GO surface and competed with MB for limited adsorption sites. What’s more, inorganic anions are 15

ACCEPTED MANUSCRIPT generally regarded as scavengers of photo-generated holes (h+) and/or hydroxyl radicals (•OH), and can pose inhibitory effects through following reactions [39, 48]: Cl- + h+ → •Cl (1)

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Cl- + •OH → •ClOH-

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(2)

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SO42- + •OH → •SO42- + OH-

HCO3- + •OH → •CO3- + H2O

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(4)

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(3)

HPO42- + •OH → •PO43- + H2O

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(5)

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The effects of initial solution pH and humic acid (HA) on the removal of MB were also studied. The removal efficiency of MB in the acidic and neutral conditions

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(pH 3-7) was slightly higher than that in the alkaline conditions (pH 8-10), and the

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neutral condition (pH = 7) provided the highest removal efficiency of MB (Text S3; Fig. S2). The addition of HA (2.5-10 mg/L) promoted the adsorption of MB on TNTs@GO, but inhibited the photodegradation efficiency, which may be due to competition of HA for the light energy and reactive species, and shielding MB molecules from irradiation and reactive species (Text S3; Fig. S3) [29].

3.4. Reusability of TNTs@GO 16

ACCEPTED MANUSCRIPT To evaluate the recyclability of TNTs@GO, five successive repeatability experiments were performed. As shown in Fig. 11, in the first run, 97.5% of MB removal efficiency was achieved within 2 h, and in the second run, the removal of MB was 91.3% within a longer time of 5 h. After five cycles, the photocatalytic efficiency

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performance of the material in water treatment application.

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of TNTs@GO was still maintained about 86%, indicating a promising reusing

3.5. Mechanistic investigation of photocatalytic degradation of Methylene Blue

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In order to clarify the mechanism of photocatalytic degradation of MB by

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TNTs@GO under UV irradiation, TBA, BQ and EDTA were added into the reaction system as radical scavengers to capture •OH (reaction rate constant k2 = 6.0×108

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M-1·s-1 [49]), superoxide radical (O2•-) (k2 = 0.0-1.0×1010 M-1·s-1 [50]) and h+,

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respectively. With the addition of 20 mg/L EDTA, the final removal efficiency of MB was reduced by 20% (Fig. 12(a)), indicating that h+ plays an important role for the

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photocatalytic activity of TNTs@GO. Combining the discussion of Fig. 6, the

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presence of Vo-Ti3+ can be further confirmed. The photocatalytic activity was also greatly inhibited by the addition of the other two radical scavengers, suggesting that both •OH and O2•- play a significant role in the photocatalytic degradation of MB. Typically, •OH and O2•- are important ROS in the photocatalytic degradation process. •OH is usually generated by the reaction of photo-generated h+ with adsorbed water, whereas O2•- is formed through photo-generated electrons (e-) activating dissolved oxygen [7]. DMPO is commonly used to trap •OH and O2•- in EPR 17

ACCEPTED MANUSCRIPT measurements. As illustrated in Fig. 12(b), a typical four signals of DMPO-•OH adduct with a peak intensity ratio of 1:2:2:1 was observed for TNTs and TNTs@GO. The signals of •OH for TNTs@GO was stronger than that for TNTs, suggesting that TNTs@GO can improve the photocatalytic activity for the degradation of MB. Fig.

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12(c) shows the EPR spectra of DMPO-O2•- adduct, and we can see the characteristic

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signal peaks of O2•-, indicating that O2•- also plays a very important role in the

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photocatalytic degradation of MB.

In summary, the enhanced photocatalytic degradation of MB by TNTs@GO is a

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complicated multifunctional process and many kinds of reactive species are involved

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such as h+, •OH and O2•-. Fig. 13 presents the mechanism of enhanced photocatalytic ability for TNTs@GO. GO can enhance the photocatalytic activity of TNTs by e-

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transportation [36]. The following equations present the proposed photocatalytic

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process:

TNTs + hv → TNTs(e- + h+)

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(6)

(7)

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TNTs(e-) + GO → TNTs + GO(e-)

h+ + H2O → •OH + H+ (8) e- + O2 → O2•(9)

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ACCEPTED MANUSCRIPT h+ + Pollutants → Degradation products (10) •OH + Pollutants → Degradation products (11)

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O2•- + Pollutants → Degradation products

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(12)

3.6. Adsorption-photocatalytic removal of different types of dye pollutants

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Besides MB (cationic dye), we also tested the removal efficiency of DB56

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(neutral dye) and X-3B (anionic dye) by TNTs@GO. The adsorption capacity of TNTs@GO for various dyes is in the order of MB > X-3B > DB56 (Fig. 14). The high

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adsorption capacity of MB onto TNTs@GO is due to the electric attraction between

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MB cations and negatively charged surface of TNTs@GO. As a consequence, the total dye removal after 90-min UV irradiation is in the same order, and the removal

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efficiency reached to 97.5%, 87% and 72%, respectively. Therefore, TNTs@GO has

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an excellent adsorption-photocatalytic performance on various types of dyes, and is a promising photocatalyst for treatment of dye wastewater.

4. Conclusions A series of TNTs@GO nanocomposites with different GO loadings, calcination temperatures

and

calcination

times

were

prepared

and

evaluated

their

adsorption-photocatalytic performance on the removal of the model dye MB. The 19

ACCEPTED MANUSCRIPT optimization results indicated the TNTs@GO with GO loading of 1%, calcination temperature of 300 C and calcination time of 0.5 h showed the optimum performance. Thus, the as-prepared TNTs@GO at this condition was further characterized their morphology, crystal phase, functional groups, elemental composition and chemical

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state by HRTEM, EDS, XRD, FTIR, XPS, EPR as well as N2 adsorption-desorption

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isotherms. For the purpose of practical use, effects of various operational parameters

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including TNTs@GO dosage, initial MB concentration, temperature, initial pH, HA, inorganic anions were comprehensively studied. By using the radical scavengers and

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EPR measurements, several reactive species, i.e., h+, •OH and O2•-, were demonstrated

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to play the critical roles in the photocatalytic process. Overall, TNTs@GO showed an excellent performance on the removal of different types of dyes, a robustness for wide

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range of pH and temperature, and a good reusability, and therefore has the application

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potential in dye wastewater treatment. However, before the broader engineered

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application, the economy and safety of TNTs@GO should be carefully addressed.

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Acknowledgements

The authors thank the Collaborative Innovation Plan of Hubei Province for Key Technologies in the Eco-Ramie Industry. This work was financially supported by the Natural Science Foundation of Hubei Province, China (No. 2018CFB515). Dr. Jie Fu thanks the financial support from the Shanghai Pujiang Program (17PJ1400900).

Supplementary materials 20

ACCEPTED MANUSCRIPT Supplementary material associated with this article can be found, in the online version, at doi:.

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Fig. 1. Adsorption-photocatalytic performance of TNTs@GO prepared under different conditions for removal of MB: (a-b) Effect of GO loading, (c) effect of calcination temperature, and (d) effect of calcination time. Experimental conditions: [MB]0 = 20

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mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, and T = 25 °C. Fig. 2. XRD patterns of GO, TNTs and TNTs@GO.

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Fig. 3. FTIR spectra of GO and TNTs@GO.

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Fig. 4. (a-b) TEM pictures of GO sheets, (c-d) HRTEM pictures of TNTs@GO, and (e)

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EDS analysis of TNTs@GO.

Fig. 5. XPS spectra of the synthesized TNTs@GO: (a) Ti 2p, (b) O 1s, and (c) C 1s.

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Fig. 6. EPR spectra of TNTs and TNTs@GO.

Fig. 7. Effect of TNTs@GO dosage on removal of MB. Experimental conditions:

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[MB]0 = 20 mg/L, catalysts dosage = 50-400 mg/L, pH0 = 7.0±0.1, and T = 25 °C. Fig. 8. Effect of initial MB concentration on removal of MB. Experimental conditions:

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[MB]0 = 10-40 mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, and T = 25 °C. Fig. 9. Effect of temperature on photocatalytic degradation of MB. Experimental

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conditions: [MB]0 = 20 mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, and T = 15-35 °C. The inset is the pseudo-first-order kinetic model fitting. Fig. 10. Effect of inorganic anions on removal of MB. Experimental conditions: [MB]0 = 20 mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, T = 25 °C, and [anion]0 = 200 mg/L.

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ACCEPTED MANUSCRIPT Fig. 11. Recycling experiments of TNTs@GO for photocatalytic degradation of MB. Experimental conditions: [MB]0 = 20 mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, and T = 25 °C. Fig. 12. (a) Effects of TBA, BQ and EDTA on photocatalytic degradation of MB. (b)

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EPR spectra of DMPO-•OH adduct with water dispersion. (c) EPR spectra of

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DMPO-O2•- adduct with methanol dispersion. Experimental conditions: [MB]0 = 20

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mg/L, catalysts dosage = 200 mg/L, pH0 = 7.0±0.1, T = 25 °C, [scavenger]0 = 20 mg/L, and DMPO = 5 mM.

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Fig. 13. Proposed mechanisms for the enhanced photocatalytic activity of TNTs@GO

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under UV irradiation.

Fig. 14. Adsorption-photocatalytic performance of TNTs@GO for removal of

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different dyes. Experimental conditions: [dye]0 = 20 mg/L, catalysts dosage = 200

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mg/L, pH0 = 7.0±0.1, and T = 25 °C.

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Table 1. XPS curve fitting results and parameters for Ti 2p, C 1s and O 1s spectra of TNTs@GO Ti 2p

C 1s

O 1s

Ti 2p1/2 C-C/C-H

C-O

C=O

O-Ti4+

O-Ti3+ C-O bonds

Position

458.52

464.17

284.72

285.57

288.72

529.97

531.22

FWHM

1.216

2.151

1.132

2.467

1.811

1.296

2.818

2.29

Area

164502

81102

28820

16392

4444

193092

63291

17038

535.22

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CE

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SC

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Ti 2p3/2

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ACCEPTED MANUSCRIPT Highlights 

Graphite oxide grafted titanate nanotubes was prepared through hydrothermal method. Synthesis condition was optimized to achieve the excellent photocatalytic activity.



Oxygen vacancy in TNTs@GO reduced the recombination rate of electron-hole

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

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pairs.

The effects of operational parameters were comprehensively investigated.



TNTs@GO showed a good reusability and applicability for multi-types of dyes.

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

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Graphics Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14