Development of a novel photocatalyst for the photocatalytic treatment of industrial wastewater

Abstract

Heterogeneous semiconductor photocatalysts have been shown to be efficient for the degradation of refractory organics into simple compounds. Among all photocatalysts, TiO2 is the most used one. Two issues that arise with the use of unmodified TiO2 as a photocatalyst are the unwanted fast recombination of electron/hole pairs and the lower effectiveness in the presence of visible light irradiation. Doping a transition metal or a non-metal into TiO2 and its combination with another photocatalyst have been used to enhance its photoactivity. This study is to develop a new photocatalyst by the combination of TiO2 with another semiconductor oxide (Fe2O3) and its doping with transition metal such as Ag. Combined photocatalysts of Fe2O3 /TiO2 (with different mass ratio of Fe:TiO2) is synthesized and then silver ion is doped to combine photocatalysts (with different mass ratio of Ag:TiO2) to produce a new composite photocatalyst of Ag/TiO2/Fe2O3. A new method of UV-assisted thermal synthesis is also employed to prepare the new composite photocatalyst. Methyl orange (MO) and Congo red (CR), as model pollutants, are used to test the developed photocatalyst. In addition, nitrogen-doped titanium dioxide photocatalyst (N-TiO2) with heterojunction structures is synthesized by three different approaches including new UV-assisted thermal synthesis, annealing, and microwave techniques. The novel UV-assisted thermal synthesis has produced encouraging results as a preparation method to prepare N-TiO2 at lower temperature and atmospheric pressure as well as a lower cost. Design of Experiment (DOE) along with response surface methodology (RSM) is used to optimize the photocatalytic activity of N-TiO2 as well as the affecting parameters (wavelength, light intensity, pH, and initial TOC) for decomposition of organics. The structure of all synthesized composite photocatalysts are investigated by X-ray diffraction (XRD). Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS) is employed to evaluate surface characteristics and elemental analysis of synthesized photocatalysts. Specific surface area of photocatalysts is measured based on Brunauer, Emmett and Teller (BET) technique. Results show that bare TiO2 has the lowest photocatalytic activity in degradation of organics. When silver is doped to TiO2, the degradation of MO is slightly enhanced at specific mass ratio. The presence of Fe2O3 in the new composite causes a red shift and enhances the potential to absorb higher range of visible light. Results from XRD confirmed that Fe3+ substitutes with Ti4+ in the crystal lattice of TiO2 and crystal defect occurs. The degradation of MO in presence of Ag/TiO2/Fe2O3 (Ag/TiO2=0.005 w:w and Fe:TiO2= 0.01 w:w) reached to 95.6% under sunlight in three hours with a red shift towards visible light. It is observed that there is an optimum specific surface area of photocatalysts by doping and combining photocatalysts.

Photocatalytic Treatment of An Actual Confectionery Wastewater Using Ag/TiO2/Fe2O3: Optimization of Photocatalytic Reactions Using Surface Response Methodology

“…Primary anatase and rutile TiO 2 phases were observed with 2θ peaks at 25.3 • and 27.4 • , respectively. In contrast to the work performed by Nasirian [26] where only anatase phase was observed in Ag/TiO 2 /Fe 2 O 3 , rutile phase was also observed in all prepared TiO 2 after calcination at 300 • C. Other peaks of anatase TiO 2 were located at 38.2 • , 48.0 • , and 54.3 • . Other peaks of rutile TiO 2 were located at 36.0 • , 41.2 • , 44.1 • , and 56.6 • .…”

“…Moreover, the surface doping of Ag onto TiO 2 modified the surface structure of the photocatalyst, decreasing the anatase grain size and increasing the specific surface area of the photocatalyst at relatively low concentrations of Ag/TiO 2 [35]. However, excess silver would cover up the surface of the photocatalyst leading to the reduction in the concentration of photogenerated charge carrier and hinder the contact between TiO 2 and organic pollutants [9,26,35]. As photocatalytic reactions take place on the surface of the photocatalyst, the photocatalysts tend to have higher activity when higher specific surface area presents owing to the higher number of active sites available for reactions.…”

“…Each modification method has its own strengths and shortcomings. Other physical parameters, such as specific surface area, crystalline phase, crystalline size, and pore distribution, that can be modified through these modification methods are crucial in improving the photocatalytic activity of a photocatalyst [10,13,17,26]. Among all, metal-doped and semi-conductor-combined photocatalyst, Ag/TiO 2 /Fe 2 O 3 , presents an excellent improvement in the photoactivity under ultraviolet type-A (UV-A) in the treatment of textile dyes [26].…”

“…Other physical parameters, such as specific surface area, crystalline phase, crystalline size, and pore distribution, that can be modified through these modification methods are crucial in improving the photocatalytic activity of a photocatalyst [10,13,17,26]. Among all, metal-doped and semi-conductor-combined photocatalyst, Ag/TiO 2 /Fe 2 O 3 , presents an excellent improvement in the photoactivity under ultraviolet type-A (UV-A) in the treatment of textile dyes [26]. In the present study, a TiO 2 photocatalyst was synthesized, combined with another semiconductor (Fe 2 O 3 ), and finally doped with a noble metal (Ag) using the UV-assisted thermal method established by Nasirian [26].…”

“…Among all, metal-doped and semi-conductor-combined photocatalyst, Ag/TiO 2 /Fe 2 O 3 , presents an excellent improvement in the photoactivity under ultraviolet type-A (UV-A) in the treatment of textile dyes [26]. In the present study, a TiO 2 photocatalyst was synthesized, combined with another semiconductor (Fe 2 O 3 ), and finally doped with a noble metal (Ag) using the UV-assisted thermal method established by Nasirian [26].…”