Single-step uncalcined N-TiO2 synthesis, characterizations and its applications on alachlor photocatalytic degradations
Graphical abstract
Introduction
The harmful organic compounds in wastewater such as agricultural pollutants, dyes and cosmetic sources are topics of worldwide concerns. Organic compounds in water effected to the character of water, inhibit the permeability of light into the water and reduce the photosynthetic activity [1], [2], [3]. Alachlor (2-chloro-2ʹ,6ʹ-diethyl-N-(methoxymethyl) acetanilide), one of the most important agricultural pollutants, is widely used for chloroacetanilide herbicides. It is toxic at very low concentration. Remarkably, it has been detected in drinking water, groundwater and rivers [4], [5], [6]. The maximum concentration level of alachlor for drinking water established by the United Stated Environmental Protection Agency (EPA) is set at 0.002 mg/L, while in the European Union (EU) allowed maximum contaminant is 0.0001 mg/L [7], [8].
Most of the herbicide-organic pollutant compounds including alachlor are very hard to degrade by a microorganism in the biological treatment process. Additionally, the conventional treatment methods such as flocculation, adsorption and membrane separation are unable to remove alachlor completely and could not mineralize the alachlor pollutant to harmless products [8]. Recently, many researchers obviously reported that photocatalysis using titanium dioxide (TiO2) catalyst, one of the advance oxidation processes, can be utilized for alachlor decomposition [9], [10].
Titanium dioxide (TiO2) is a very important semiconductor photocatalyst. It is extensively used by many researchers to degrade the environmental organic pollutions because of its unique properties including non-toxicity, inexpensive, strong oxidizing power, high photocatalytic activity and long-term stability against photo and chemical corrosion [11], [12], [13]. Besides, TiO2 can be synthesized in various nanostructure such as nanoparticles, nanotubes, nanorods and nanowires by different preparation methods including sol-gel, electrochemical anodic oxidation, vapor deposition, solvothermal and hydrothermal [14]. The hydrothermal method is regarded as a convenient, simplicity and environmentally harmless method. Also, it is one of the most appropriate methods for preparation of high quality TiO2 such as well crystalline, high specific surface area and narrow particle size distribution. Moreover, this method can be produced TiO2 without calcination process at high temperature [13], [15].
On the other hand, some researchers have concentrated in synthesis TiO2 to develop their photocatalytic properties [16], [17], [18], [19] such as narrowing the energy band-gap by doping various metal and non-metal impurities [20], [21], [22], [23], [24], [25]. The goal is to produce an active TiO2 photocatalyst, which can be used under visible light region. Doping of TiO2 with a non-metal elements such as nitrogen, carbon, fluorine, chlorine, bromine or sulfur has become the most important approach for the enhancement of TiO2 photoactivity. Among these most of the non-metal elements, nitrogen doped titanium dioxide (N-TiO2) which shows a significant photocatalytic activity in various reactions performed under visible light irradiation [26].
The aims of this paper were (i) to synthesize N-TiO2 with single-step doped nitrogen from ammonia source (NH3·H2O) by using hydrothermal preparation method and (ii) to study the effect of aging temperature and time variables in preparation process on the specific surface area and the performance of alachlor degradations. The central composite design (CCD), one of the designs of experiments (DOE) methods based on response surface methodology (RSM), was used to design preparation conditions of the hydrothermal process. Since, CCD can be reducing costs and times comparing to full experimental runs. Therefore, CCD was interested and chosen by many researchers in previous studies on photocatalytic processes [27], [28]. In this study, we use CCD to determine the optimal condition, main effects and interaction of their effects by using specific surface area, percent alachlor removal and observed first-order rate constant (k) as responses. The physical and chemical properties, for example, crystallite phases, particle sizes, elemental compositions and specific surface areas were done by X-ray diffraction, Transmission Electron Microscopy, Energy Dispersive X-ray spectrometer and adsorption isotherms using the Brunauer–Emmett–Teller (BET) method, respectively.
Section snippets
Experimental design
In this study, central composite design (CCD), created by using Minitab 16 statistical software (Minitab, Inc., Pennsylvania, USA), was used to design catalyst preparations in the hydrothermal condition in various preparation parameters (i.e. temperature and time of hydrothermal process) presented in Table 1.
Reagents and catalyst preparations
Titanium tetrachloride (TiCl4, 99%) from Merck Schuchardt OHG was used as a precursor to prepare the amorphous TiO2. Hydrogen peroxide (H2O2, 30%) and ammonia solution (NH3, 28%) were
Phase structures and morphologies of uncalcined N-TiO2
Fig. 1 shows the XRD patterns of uncalcined TiO2 catalysts prepared by hydrothermal method. All the samples contain only anatase crystallite phase comparing with JCPDS no. 21-1272. The phase transformation from anatase to rutile or brookite does not occur in our N-TiO2 photocatalysts in different preparation conditions. Considerably, the results of the main anatase peak (1 0 1) show that the crystallinity becomes stronger and higher with increasing of both hydrothermal temperatures and times. The
Conclusions
The N-TiO2 can be synthesized in nanoparticles and nanocrystals by one-step hydrothermal technique. The particle sizes and pure anatase crystallite sizes were about 14–26 nm and 16–24 nm, respectively. The energy band-gaps of photocatalysts have been shown about 2.95 eV. Since, nitrogen content about 10% could replace oxygen in the lattice of TiO2. The optimal condition for alachlor decomposition on N-TiO2 was about 145 °C and 12 h of both alachlor removal and reaction rate constant. The activity
Acknowledgements
Authors would like to thank the Research Center for Environmental and Hazardous Substance Management (EHSM), Faculty of Engineering, Khon Kaen University for their financial support.
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