Efficient removal of methylene blue dye by a hybrid adsorption–photocatalysis process using reduced graphene oxide/titanate nanotube composites for water reuse Accepted Manuscript Title Efficient remo[.]
Accepted Manuscript Title: Efficient removal of methylene blue dye by a hybrid adsorption–photocatalysis process using reduced graphene oxide/titanate nanotube composites for water reuse Authors: Chi Hieu Nguyen, Ruey-Shin Juang PII: DOI: Reference: S1226-086X(18)30946-8 https://doi.org/10.1016/j.jiec.2019.03.054 JIEC 4474 To appear in: Received date: Revised date: Accepted date: October 2018 24 December 2018 28 March 2019 Please cite this article as: Nguyen CH, Juang R-Shin, Efficient removal of methylene blue dye by a hybrid adsorption–photocatalysis process using reduced graphene oxide/titanate nanotube composites for water reuse, Journal of Industrial and Engineering Chemistry (2019), https://doi.org/10.1016/j.jiec.2019.03.054 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 Revisions submitted to Journal of Industrial and Engineering Chemistry (Research Article) (JIEC_2018_392_R1) Efficient removal of methylene blue dye by a hybrid adsorption–photocatalysis IP T process using reduced graphene oxide/titanate nanotube composites for water reuse Chi Hieu Nguyen1,2, Ruey-Shin Juang1,3,4* SC R Department of Chemical and Materials Engineering, Chang Gung University, Guishan, Taoyuan 33302, Taiwan Institute of Environmental Science, Engineering and Management, Industrial University of Ho Chi Minh City, N U Ho Chi Minh City, Vietnam A Division of Nephrology, Department of Internal Medicine, Chang Gung Memorial Hospital, Linkou, Taiwan M Department of Safety, Health and Environmental Engineering, Ming Chi University of Technology, Taishan, D New Taipei City 24301, Taiwan TE *Corresponding author: Professor Ruey-Shin Juang EP E-mail: rsjuang@mail.cgu.edu.tw Address: Department of Chemical and Materials Engineering, Chang Gung University, 259 Wenhua First Road, CC Guishan, Taoyuan 33302, Taiwan A Graphical Abstract This scheme proposes possible photocatalytic mechanism over rGO/TNT composites TiO2 nanotubes are dispersed well on the surface of rGO sheets Tubular structure of TiO2 improves adsorption ability for dye Under photon irradiation, electrons (e-) are excited from the valence band (VB) to conduction band (CB), leading to the formation of holes (h+) in the VB The holes with strong oxidation ability scavenge water molecules on the surface of TNTs and generate highly reactive hydroxyl radical (·OH) Also, they attack and convert dye molecules to byproducts The photoinduced electrons (e-) reduce the absorbed oxygen molecule to produce superoxide radicals (·O2-), which may directly oxidize organic pollutant into small molecules; and, part of ·O2- can react with H+ and generate H2O2, which is further excited by electrons and changed into ·OH radicals These active radicals attack dye molecules and degrade them into the intermediates and final products such as CO2 and H2O Normally, the electron-hole pairs for TNTs are ready to be recombined, resulting in poor photoactivity Herein, the photoinduced electrons can be trapped by rGO, leading to electron-hole separation Therefore, the adsorption and photocatalytic M A N U SC R IP T ability of rGO/TNT composites is enhanced TE D Highlights Reduced graphene oxide/titanate nanotube (rGO/TNT) composites are hydrothermally prepared as EP novel photocatalysts Unlike methyl orange (MO), rGO/TNT composite achieves higher decolorization/mineralization of CC methylene blue (MB) than P25-TiO2 Unlike MO, removal of MB using rGO/TNT composites is due to the synergy of adsorption and A photocatalysis GO content in rGO/TNT composite has a great effect on adsorption and photocatalytic activity for cationic dye Incorporating rGO in rGO/TNT composite will extend its photocatalytic ability to the visible light region Abstract In this study, reduced graphene oxide/titanate nanotube (rGO/TNT) composites were fabricated by a one-step facile hydrothermal process The fine structures and physicochemical properties of the prepared materials were comprehensively determined The incorporation of a small amount of rGO into the composites led to the enhancement of the absorption intensity of visible light and the separation of photogenerated charged carriers The rGO/TNT composite with the optimal amount of rGO of 3% by weight showed the highest photocatalytic activity for both methylene blue (MB) and methyl orange Moreover, wt.% rGO/TNT exhibited higher adsorption IP T capability and photocatalytic activity for MB, a cationic dye, than TNTs and commercial TiO2 P25 The maximum amount of MB adsorbed on wt.% rGO/TNT was 26.3 mg/g at 25°C, and the adsorption rapidly reached SC R equilibrium after 40 of contact time Approximately 100% decolorization and 77.4% mineralization over 3% rGO/TNT composite were achieved under 100 W UV irradiation for h, whereas 95.0% decolorization and 78.8% mineralization were achieved under visible light irradiation for h The degradation pathways of MB over N U P25, TNTs, and wt.% rGO/TNT composite were finally proposed and compared A Keywords: Adsorption; Photocatalysis; Reduced graphene oxide; Titanate nanotubes; Cationic dye; Degradation A CC EP TE D M pathways Introduction Numerous environmental issues caused by water source pollution have recently attracted considerable interest from scientists and engineers Textile industries are well-known manufacturers that use and discharge large amounts of water resources The total annual dye production exceeds 700,000 t worldwide, and approximately 20% of this production is directly released through effluents from manufacturing processes [1,2] One of the major IP T problems of textile effluents is the abundant class of colored organic compounds in water, which can impede light transmission, consequently damaging the photosynthetic process of aquatic flora The majority of synthetic dyes SC R are toxic, no biodegradable, or likely decomposed to form harmful by-products under anoxic conditions From the viewpoint of environmental sustainability and water reuse, the efficient removal of dyes from effluents is highly desired prior to discharging into receiving waters [3,4] U Traditional physicochemical methods, such as chemical precipitation, electrocoagulation, membrane filtration, N and adsorption, are currently applied to remove the color of textile effluents [5–11] However, effluents are not A mineralized but merely transferred from one kind of pollution to another Thus, further treatments are required to remediate these concentrated pollutants In recent years, heterogeneous photocatalysis has been recognized as an M efficient process for removing organic pollutants thoroughly [12–14] Most importantly, the application of this D process to textile effluent treatment might result in the decolorization and complete mineralization of dyes [15,16] Of many heterogeneous photocatalysts used, titanium dioxide (TiO2) has been proven to be promising TE because of its high chemical reactivity and stability, nontoxicity, and cost-effectiveness [14,17,18] However, EP some drawbacks of TiO2 photocatalysis, including the high bandgap energy (3.2 eV in the anatase phase) and easy recombination of photoinduced electron–hole pairs, always limit its feasibility for practical application [14,19] CC Moreover, individual catalyst is appropriate only for degrading dyes with low concentrations but fails to treat high-strength solutions [20] Thus, modifications of the existing materials have been explored to improve the photocatalytic capability and extend the application range An efficient method is to dope transitional metals (e.g., A Cu2+ and Fe+3), noble metals (e.g., Pt, Pd, and Ag), or nonmetallic elements (e.g., S and N) on photocatalysts [21– 25] The main objectives of doping are to reduce the bandgap of TiO2 by transposing the absorption band from the UV light region to the visible light region and to improve the separation of electron–hole pairs In addition to doping of TiO2, adjusting its morphology, combining it with other metal oxides (e.g., ZnO and Al2O3) [26–28], and fixing it on mesoporous materials with high surface area, such as zeolites and activated carbon, are some alternatives to increase TiO2 photoactivity [29] Each type of modification possesses relevant mechanisms to improve the optoelectronic properties of TiO2 Recent studies have indicated that titanate nanotubes (TNTs) exhibit good photocatalytic activity because of its higher surface area than commercial TiO2 P25 [30–33] However, TNTs possess similar drawbacks to TiO2 By contrast, graphene contains six C atoms, is created by sp2 hybridization, and has a 2-D planar honeycomb structure with a thickness of a single atomic layer [34–36] Fabricating graphene to form inorganic composites can IP T thus produce attractive materials because of its unique electronic properties [4,36–38] Currently, the Hummers’ method is most commonly used to fabricate graphene sheets, and the final product is a single graphene oxide (GO) SC R that contains many hydrophilic groups Exfoliated GO exhibits poor electronic conductivity; therefore, the reduction of GO to form reduced graphene oxide (rGO) is required to store and shuttle electrons [39–41] Previous studies have shown that GO/TiO2 composites exhibit exothermic mixing [42,43] Moreover, TiO2 can retard the U mass loss of rGO at high temperatures because of the chemical reactions between rGO and TiO2 through Ti–O–C N bonds The beneficial effect of combining rGO with TiO2 for the photodegradation of pollutants, particularly azo A dyes, has been proven [42,44] The heterojunction between rGO and TNTs is expected to enhance photocatalysis through the synergy of both materials Such an improved photocatalytic activity is caused by the combination of M absorption and separation of photoinduced charged carriers because rGO acts as electron reservoir preventing D recombination; meanwhile, holes that remain in TNTs can initiate the oxidation [4,45] Moreover, the formation of Ti–O–C bonds and its combination with carbon-based materials extend visible light harvesting [8,14,29] TE Although a few studies reported the enhanced photocatalytic activity of rGO/TNT composites for some dyes EP [46–48], to the best of our knowledge, in-depth insight into the synergistic effects of adsorption and photocatalysis, as well as the roles of rGO/TNT composites, on the improvement of degradation in the visible light region is CC lacking For example, anionic dyes were rarely investigated, although rGO/TNT composites were observed to have the potential to treat dye-bearing effluents However, this finding is insufficiently convincing due to the key A roles of the chemical structures of pollutants on adsorption capability and photocatalytic activity This work aimed to synthesize some rGO/TNT composites through one-step hydrothermal method Methylene blue (MB, a cationic dye) was selected as the model dye to assess the hybridity of adsorption capability and photocatalytic activity of composites Methyl orange (MO, an anionic dye) was chosen as the control dye because it has comparable molar mass to MB but different chemical characteristics In particular, the currently prepared composites exhibited different adsorption abilities for MO and MB The transformation products formed during MB photocatalysis over P25, TNTs, and rGO/TNT composites were finally analyzed and the pathways were proposed and compared Materials and methods 2.1 Materials TiO2 P25, whose surface area and average particle size were 50 m2/g and 21 nm, respectively, was provided by Degussa Co (Germany) Graphite powder (99%), NaOH (99.8%), H2SO4 (98%), H3PO4 (85%), and ethanol (99%) were obtained from Sigma Aldrich Co (St Louis, MI, USA) Potassium permanganate (98%) and H2O2 IP T (30 vol%) were supplied by Showa Chemical Co (Okayama, Japan) The dyes MB and MO (purity > 98%) were obtained from Alfa Aesar Co (Heysham, England) All chemicals were used as received Deionized (DI) water SC R produced by the Millipore Milli-Q system (Darmstadt, Germany) was employed throughout this study 2.2 Preparation of rGO/TNT composites U 2.2.1 Preparation of GO sheets N GO was prepared by an improved Hummers’ method [49,50] Briefly, 400 mL of concentrated H2SO4 and A H3PO4 solutions was mixed in an ice bath under continuous agitation at less than 15°C, to which 3.0 g of graphite powder and 18.0 g of KMnO4 were added Afterward, the resulting solution was heated to and kept at 50°C and M magnetically agitated for 12 h When the reaction was completed, the suspension was poured slowly into a flask D containing 400 g of iced DI water Then, the solution was cooled to 25°C, to which 30 vol% H2O2 (6 mL) was added to remove excess KMnO4 The suspension turned brilliant yellow immediately The final mixture was TE centrifuged at 10,000 rpm and sequentially washed with 30 vol% HCl, DI water, and ethanol until the washing EP solution became neutral Finally, the recovered precipitate was dried in a vacuum oven at 60°C for 24 h CC 2.2.2 Preparation of rGO/TNT composites The rGO/TNT composite was hydrothermally synthesized as stated earlier [48] but with some modifications A Briefly, 3.0 g of P25 and a certain amount of GO (i.e., 1%, 2%, 3%, and 5% by weight) were added to 90 mL of NaOH (10 M) under vigorous agitation for 1.5 h and sonicated for h to make it homogeneous The resulting solution was shifted into a stainless-steel autoclave (Teflon-lined), which was kept in an oven at 135°C for 24 h Afterward the autoclave was cooled to 25°C The obtained solid was washed with 0.1 M HNO3 until the pH of the washing solution became 1.5 Then, the mixture was agitated continuously for 24 h to ensure ion exchange The suspended solids were recovered by centrifugation at 10,000 rpm and rinsed with DI water several times until the washing solution became neutral The final products were dried in a vacuum oven at 80°C for 24 h and calcined at 300°C for h in a tube furnace under Ar atmosphere to refine the crystal form The solid catalysts were denoted as 1%, 2%, 3%, and 5% rGO/TNT Pure TNTs were prepared by the same procedures used previously but without the addition of GO 2.3 Characterization of the synthesized catalysts The morphology of the prepared catalysts was observed under a field emission scanning electron microscope IP T (FE-SEM; SU8220, Hitachi, Japan) and a transmission electron microscope (TEM; JEOL JEM1230, Tokyo, Japan) The crystalline structure was identified by an X-ray powder diffractometer (XRD; Bruker D2 PHASER, SC R Germany) with CuKα radiation source Functional groups in the prepared samples were identified by a Fourier transform infrared spectroscope (FTIR; Bruker Tensor 27 IR, Germany) In addition, the surface composition of the catalysts was determined by an X-ray photoelectron spectroscope (XPS), and the XPS spectra were recorded U by a Fison VG ESCA210 spectrometer with MgKα radiation N Optical properties of as-prepared catalysts were analyzed according to the UV–visible diffuse reflectance A spectra recorded by a Jasco V650 spectrophotometer (Tokyo, Japan) equipped with an ISV-722 integrating sphere The photoluminescence (PL) spectra were recorded by a F-4500 fluorescence spectrophotometer (Hitachi, Japan) M at 25°C Photoelectrochemical tests of the synthesized catalysts were conducted on a CHI 660D workstation with D a three-electrode mode, following the procedures described earlier [51] The BET surface area of the catalysts was determined from N2 adsorption–desorption isotherms at −196°C on an accelerated surface area and porosimetry EP TE analyzer (Micromeritics ASAP2020, USA) The sample was degassed at 300°C for h before measurements 2.4 Adsorption of dyes on the synthesized catalysts CC Batch adsorption of dyes was conducted in the dark to prevent photocatalysis and photolysis Briefly, 0.1 g of catalysts was mixed with 100 mL of dye solution (10–60 mg/L) under magnetic stirring at 25±1°C The pH was A kept constant at 6.8±0.2 Samples (3 mL) were taken at preset time intervals and filtered through a 0.22 µm syringe filter (Chromophil® Xtra, Germany) to remove catalyst particles The concentrations of dyes in the sample were determined by a UV–visible spectrophotometer at the wavelength of 665 nm for MB and 464 nm for MO The amount of dye adsorbed on the catalysts at equilibrium, qe (mg/g), is calculated as follows: qe C0 Ce V (1) W where C0 and Ce are the initial and equilibrium concentrations of dye in the solution (mg/L), respectively, V is the volume of the working solution (L), and W is the mass of the catalyst (g) [52,53] 2.5 Photodegradation experiments The photocatalytic activity of TNTs and rGO/TNT composites was assessed via the oxidation of dyes under UV and visible light irradiation A mercury lamp (Sen Lights Co., Japan) without and with a UV cutoff filter, which had a light intensity of 6.5 and 0.009 mW/cm2, was used as the source of UV and visible light, respectively For comparison, P25 was adopted as a reference catalyst Photodegradation experiments were conducted in a IP T batch reactor as described previously [54] Briefly, the lamp was placed in a quartz tube, which was immersed in the reactor The initial solution pH was adjusted using 0.1 M NaOH or HNO3 An aliquot of the catalysts was SC R added to 20 mg/L of dye solution (1.0 L) Prior to photocatalysis, the suspension was agitated in the dark for h to ensure adsorption equilibrium Samples (3 mL) were taken at preset time intervals, and the concentrations of dyes were measured spectrophotometrically to evaluate the decolorization efficiency and degradation kinetics In U addition, the total organic carbon (TOC) was measured by a TOC Torch analyzer (Teledyne Tekmar, Mason, OH, N USA) to determine the mineralization efficiency A Reaction intermediates formed during MB photocatalysis over P25, TNTs, and rGO-TNT composite were identified by a high-resolution UPLC®-MS/MS system (Waters Xevo TQ-XS, Milford, MA, USA) equipped with M an ACQUITY UPLCđ BEH C18 column (2.1ì50 mm, particle size of 1.7 µm) Mass spectrometry was conducted D with electrospray ionization in positive ion mode (ESI+) The assigned parameters, as well as the operating procedures and conditions, were the same as those described previously [51] TE The reusability of 3% rGO/TNT composite was typically studied for five repeated cycles The catalysts used EP in each cycle were washed several times with DI water Then, 30 mL of 30 vol% H2O2 was added to the catalyst solution (3 g/L), which was irradiated with UV for h and rinsed several times with DI water The catalysts were CC regenerated by drying and calcining in an oven at 300°C for h before the beginning of the next cycle A Results and discussion 3.1 Characterization of as-prepared catalysts 3.1.1 Structural and textural characterization Fig depicts the XRD patterns of as-prepared catalysts For GO, the diffraction peaks that appeared at 2θ values of 11.3° and 42.5° (inset) were attributed to the (002) and (001) planes of GO, respectively, which indicate the existence of O-containing groups, such as hydroxyl, carbonyl, and epoxy Pristine rGO was similarly fabricated by hydrothermal treatment of GO in the absence of P25 In the rGO spectrum, the main diffraction peak at 2θ of 11.3° disappeared and the broad peak of rGO was observed at 25.1° [39], indicating that many Ocontaining groups were removed This finding shows that GO was successfully reduced to rGO by the hydrothermal method The XRD patterns of rGO/TNT composites present diffraction peaks at 2θ = 25.4°, 37.6°, 48.1°, 53.5°, 55.1°, 62.6°, 68.7°, 70.2°, and 75° assigned to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of the anatase phase of TiO2, respectively (JCPDS Nos 21-1272 and 21-1276) The original rutile peaks in P25 disappeared completely for hydrothermally prepared catalysts Typical diffraction IP T peaks of GO (Fig 1, inset) or carbon species cannot be detected, even for 5% GO Moreover, rGO with diffraction peak at 25.1°, which was formed after the hydrothermal process, could be hidden by a strong anatase SC R peak at 25.4° [29,46,55] This phenomenon is caused by the relatively small amount and weak intensity of rGO Hence, the hydrothermal process successfully converted TiO2 nanoparticles into TNTs and simultaneously reduced GO to rGO The heights of the peaks of rGO/TNT composites were lower than those of TNTs, indicating U that the crystalline structure of TNTs was less developed than that of rGO/TNT composites The average crystal N size of rGO/TNT composites and TNTs was estimated by the Debye–Scherrer formula [51] As presented in Table A 1, the crystal size of rGO/TNT composites decreased because of the addition of rGO compared with that of TNTs Fig 2a shows the N2 adsorption–desorption isotherms of the prepared catalysts, which were classified as type M IV on the basis of the IUPAC classification Such an isotherm is commonly observed for mesoporous materials, D and its characteristic feature is the hysteresis loop, which is related to the occurrence of pore condensation [56] The limiting adsorption regime occurs over a range of high P/P0 values, but the hysteresis loop in the isotherm of TE as-prepared catalysts is detected at P/P0 = 1, indicating the presence of macropores The shape of the adsorption EP branches is rather similar to type II, which is typical for nonporous or macroporous adsorbents, where monolayer– multilayer adsorption occurs This reveals that mesopores and macropores were created due to the agglomeration CC of TNTs and rGO sheets, whereas the smaller pores (