Chemical structure of Orange II. 

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... main causes of surface water and groundwater contamination are industrial discharges [1]. The textile industry has a big pollution problem. The World Bank estimates that 17–20% of industrial water pollution comes from textile dyeing and wastewater treatment. Wastewaters generated by the textile industries are known to contain considerable amounts of non-fixed dyes, espe- cially azo-dyes, and a huge amount of inorganic salts. It has been estimated that more than 10% of the total dyestuff used in dyeing processes are released into the environment [2]. Azo dyes are the largest group of synthetic colorants used in textile industry [3] constituting 60–70% of all dyestuffs produced [4]. They have one or more azo groups (R N = N R ) having aromatic rings mostly substituted by sulfonate group ( SO 3 ), hydroxyl group ( OH), etc. [5,6]. These toxic dyes are non- biodegradable and at present are abated by some common non-destructive processes [7,8]. Among azo-dyes, Orange II as shown in Fig. 1, represents more than 15% of the world production of dyes used in the textile manufacturing industry. Orange II is an anionic monoazo textile dye of the acid class. It is resistant to light degradation, the action of O 2 and common acids or bases. In wastewater treatment plants, Orange II does not undergo biological degradation [9,10]. The stability of Orange II is useful in textile manufacturing but it causes difficulty in managing its removal. Several techniques have been used to abate the model azo-dye, Orange II, such as Fenton [11], photo-Fenton [12] and TiO 2 photocatalysis [13]. However, these methods are not able to completely remove Orange II from the wastewater system using visible light. Therefore, there is a need to find a new cost effective and efficient technique to eliminate the azo-dye from industrial wastewater. The removal of azo-dyes by advanced oxidation processes (AOPs) has been the subject of several recent studies. The mechanism of dye destruction in AOPs is based on the formation of a very reactive hydroxyl radical ( OH), that, with an oxidation potential of 2.80 V [14,15] can oxidize a broad range of organic compounds. TiO 2 is a very suitable photocatalyst but its activity is mainly confined to the UV region of solar radiation. In order to explore efficient visible light induced photocatalysts, much scientific effort has been conducted in recent years to reduce its band gap to make it suitable for harvesting visible light from solar radiation [6]. These works include: doping TiO 2 with various transition metals such as Pt, Au, and Ag [16,17,18] non metal atoms (N, S, etc.) [19,20] and anchoring an organic dye sensitizer molecule onto the surface of the photocatalyst [9]. The use of Cu and Ni for bimetallic catalyst has been reported as the effective method to improve the efficiency of various reactions. Liu and Liu [21] and Huang and Jhao [22] reported Cu–Ni/Al 2 O 3 and Ni–Cu/samaria-doped ceria catalysts for carbon dioxide hydrogenation and for steam reforming of methane, respectively. Other reported studies include decomposition of methane over Ni/SiO 2 and Ni–Cu/SiO 2 catalysts [23], Cu–Ni/TiO 2 [24] prepared via (co)impregnation with 4 wt% loading for photocatalytic reduction of nitrate; Cu–Zn/TiO 2 [25], Ni/TiO 2 [26] and Cu–Fe/TiO 2 [27] for methyl orange degradation and Cu/TiO 2 for Orange II degradation with 90% color removal in the presence of UVC light and O 2 after 150 min reaction [28]. The objective of this paper was to determine the effect of calcination temperature of the bimetallic 10 wt% Cu–Ni/TiO 2 prepared with different Cu:Ni mass compositions on the efficiency of Orange II photodegradation under visible light radiation. The goal was to remove Orange II completely using this new cost effective technique. This involved incorporation of bimetallic Cu–Ni onto TiO 2 via modified deposition–precipitation method (DP) at final pH 8.5. The introduction of Cu and Ni was with the inten- tion to reduce the band gap of the photocatalyst for enhanced visible light absorption. The prepared photocatalysts were further characterized using thermogravimetric analysis (TGA), Fourier-transformed infrared (FTIR) spectroscopy, powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), diffuse reflectance UV–Visible (DR-UV–Vis) spectroscopy, temperature programed reduction (TPR) studies and surface area analysis employing the Brunauer–Emmet–Teller method (BET). Copper nitrate trihydrate, Cu(NO 3 ) 2 · 3H 2 O and nickel nitrate hexahydrate, Ni(NO 3 ) 2 · 6H 2 O (Acros brand >98% purity) were used as dopant metal salts. Titanium dioxide, TiO 2 (Degussa P25 80% anatase, 20% rutile) was used as the support which also acts as the semiconductor in photocatalysis. Sodium hydroxide, NaOH (Merck, 95% purity) was used as precipitating agent. Glycerol used was of 95% purity (Systerm). Orange II (Acros, pure) was used as the model azo dye for photocatalytic degradation study. All chemicals were used as received. 10 wt% Bimetallic Cu–Ni/TiO 2 photocatalysts with varying Cu:Ni mass composition were prepared via precipitation method using NaOH as precipitating agent. The monometallic 10 wt% Cu/TiO 2 and 10 wt% Ni/TiO 2 were also prepared as references. Cu and Ni salts were weighed in appropriate amount and dissolved in 100 ml of distilled water followed by the addition of glycerol in 2:1 mol ratio of glycerol:total metals with continuous stirring [29]. TiO 2 was added to the solution and stirred for 1 h. The pH of the solution was adjusted to pH 8.5 with 0.25 M NaOH. The mixture was stirred at 10 ◦ C for 1 h before filtering and the precipitate was dried in an oven at 75 ◦ C overnight. The dried photocatalyst was ground with a mortar and pestle, kept in air-tight glass bottle as raw photocatalyst and stored in a desiccator at room temperature prior to calcination. In order to estimate suitable calcinations temperatures for the raw photocatalysts, thermal gravimetric analysis (TGA) was carried out using Perkin Elmer (Pyris 1 TGA) instrument. The dried photocatalyst was loaded in a sample cup and weighed using a built-in microbalance attached to the instrument which automatically pro- vide the weight of the sample (in the range of 5–10 mg). The sample was heated in flowing N 2 from 30 ◦ C to 800 ◦ C at a ramp rate of 20 ◦ C/min. Results from TGA were reported as thermograms which are plots of the relative weight of the photocatalyst vs. temperature. Based on the TGA results, calcination was conducted at selected temperatures for 1 h duration. The calcined photocatalysts were given denotation: x Cu– y Ni– T , where ‘ x ’ and ‘ y ’ represent the mass composition of Cu and Ni, respectively, with ‘ x ’ + ‘ y ’ = 10; while ‘ T ’ represents calcination temperature in ◦ C. It is important to characterize the calcined photocatalysts in order to determine their chemical and physical properties and then to relate these properties to their photocatalytic performance. In this study, photocatalysts were characterized using FTIR (Shimadzu FTIR-8400S), XRD (Bruker D8 Advance Diffractometer), FESEM (Supra55VP), HRTEM (Model: Zeiss Libra 200), DR-UV–Vis (Shimadzu), TPR (Thermo Finnigan TPDRO 1100) and BET surface area analyzer (Micromeritics ASAP 2000). The phases present in the photocatalysts were investigated using XRD with Cu K ␣ radiation (40 kV, 40 mA) at 2 Â angles from 10 ◦ to 80 ◦ , with a scan speed of 4 ◦ min − 1 . The morphology of the photocatalysts such as crystallite particle shape, size, and particle size distribution were analyzed using FESEM. DR-UV–Vis measurement was performed using a UV–Vis spectrophotometer equipped with an integrating sphere. Reflectance spectrums were recorded at 190–800 nm wavelength. The band gap energies of the photocatalysts were determined from the reflectance using Kubelka–Munk function, F ( R ), and the extrapolation of Tauc plot, which is a plot of ( F ( R ) · h ) 1/2 against h . Barium sulfate (Ba 2 SO 4 ) powder was used as a standard, an internal reference. Reflectance spectrum was collected as R-sample/R-Reference and then plotted applying Kubelka–Munk Theory in order to determine bandgap energy [32]. Photocatalytic degradation of 50 ppm Orange II was conducted using halogen lamp (500 W) as the visible light source at 25 ◦ C with an initial solution pH 6.8. Photocatalysts with loading of 1 mg/ml was added to 10 ml of distilled water and sonicated for 10 min in an ultrasonic bath at 25 ◦ C followed by the addition of Orange II solution to give rise to a final concentration of 50 ppm and volume of 30 ml. The suspension was stirred using a magnetic stirrer for 2 h in the dark and later this suspension was illuminated for 1 h using 500 W halogen lamp as the visible light source at a dis- tance of 25 cm (intensity 30856.66 lux) and the temperature of the reactor was controlled at 25 ± 2 ◦ C by continuous cooling air. During the irradiation experiments, aliquots were withdrawn from the suspension at different intervals and immediately centrifuged to monitor the Orange II concentration and further analysis. The Orange II degradation during photoreaction was monitored by measuring the solution absorbance from 400 to 800 nm using a Shimadzu UV-3101 UV/Visible spectrophotometer. A calibration curve was obtained beforehand using 1, 10, 20, 30, 50 and 60 ppm standard solutions. Prior to absorbance measurement, the reaction samples were centrifuged twice at 3500 rpm for 10 min to remove the suspended solid photocatalyst. The absorbance peak at 485.0 nm was used as the representative peak for Orange II concentration [30,31]. The photodecolorization efficiency in terms of % Orange II removal was calculated as ...