Dyes Degradation with Fe-Doped Titanium Nanotube Photocatalysts Prepared from Spend Steel Slag

TiO 2 has been studied most commonly because it has high stability, nontoxicity, high catalytic activity, and high conductivity. Many studies have shown that TiO 2 would generate electron-hole pairs illuminated with UV and surround more energy than that before being illuminated. In this study, the titanium nanotube (TNT) photocatalysts were prepared to increase the surface area and adsorption capacity. The Fe TNT was also prepared from a slag iron since many slag irons cause waste treatment problems. In this study, a different Fe loading was also assessed since TNT doped with metals can be used to improve the degradation efficiency. Furthermore, five kinds of dye concentration, including 10, 20, 100, 200, and 400 ppm, and five kinds of Fe-doped content, including 0, 0.77, 1.13, 2.24, and 4.50%, were tested. Different kinds of reaction time and dye species were also assessed. In this result, Direct Black 22 was the most difficult to be degraded, although the concentration was decreased or the dose amount was increased. The degradation efficiency of 10 ppm Direct Black 22 was below 40% with 0.04 gL TNT under 365 nm UV irradiation.


Introduction
About 15% of the total world production of dyes is lost during textile dyeing when dyes are released as textile effluents [1].Dyes are widely used in the textile industry and cause polluted water.The disposal of this colored wastewater into the environment leads to many ecological problems as well as diminished natural aesthetics [2].How to efficiently solve this urgent environmental issue has become a challenging and indispensable topic of modern research.
The traditional methods for treating dye wastewater include adsorption, coagulation, and biological treatment, but they do not have good performance [3].In contrast, photocatalytic degradation is considered to be an efficient and economical alternative for controlling dye wastewater.Currently, several new photocatalysts, most impregnated with metal oxide and composite metal oxide semiconductors have been used to overcome the drawbacks of TiO 2 , which has low surface area and adsorption capacity [4].The photocatalysts commonly used are TiO 2 , ZnO, WO 3 , CdS, ZnS, SrTiO 3 , and Fe 2 O 3 , with TiO 2 being frequently reported as the most active in organic degradation experiments [5].For example, in suitable irradiation sunlight, using titanium dioxide can decrease costs significantly [6].Photocatalysts have several obvious advantages in technique, including cleaner air, an extraction stream, and carbon adsorption.In these processes, oxygens occur OH • hit, and a rate constant is higher than the oxidation of a normal state ten billion times.Ultraviolet irradiation can be obtained from sunlight or a simulated light source.
The photocatalytical activity of the S-TiO 2 photocatalyst at 400 ∘ C for the photodegradation of L-acid is better than that of pure TiO 2 [7].A novel In 3+ -doped TiO 2 and TiO 2 /In 2 S 3 nanocomposites for photocatalytic degradation of environmental pollutants and stoichiometric degradation of warfare agents is prepared using homogeneous hydrolysis with urea and thioacetamide [8].Through high-energy ball-milling technique, the nanostructured (20 nm) TiO 2 photocatalysts with different quantities of Cr doping were synthesized for 3 hours [9].Early studies primarily focused on TiO 2 -based photocatalysts, though TiO 2 only responds International Journal of Photoenergy to ultravisible (UV) irradiation, which accounts for only about 4% of all solar energies which greatly hinders wider application [10].
In our experiment, TiO 2 was chosen for modification into TNT (titanium nanotube) because it shows relatively high chemical stability, exhibits high activity for photocatalysis, and is relatively inexpensive [11].This study used hydrothermal methods to modify the titanium dioxide into TNT.The largest difference is in the structure and surface area.The former is a spherical particle with a surface area of 50 m 2 g −1 .The latter is a mesoporous tube shape with a surface area of roughly 400 m 2 g −1 .The surface area, closely related to adsorption efficiency, is about eight times that of TiO 2 manufactured using the hydrothermal method [12].Further, TNT can simultaneously generate photodegradation and adsorption in wastewater.TNT has vast pore structures and large surface areas owing to its unique tubular nanostructure [13].This mesoporous catalyst is suitable for treating high dye concentrations in wastewater.
In this study, TNT was doped with different Fe contents to increase its absorption visible light.The Fe was extracted from slag iron in steel plant to reuse the iron.The aim of this study was to investigate the performance of TNT and Fe TNT for methyl blue (MB), Direct Blue (DB22), and Reactive Black (RBK5) removal using a photocatalytic reaction.In particular, factors such as doped Fe content, light source, initial dye concentrations, and dye species were assessed to establish the optimum operating conditions.Furthermore, the reaction kinetics analysis was also investigated in this study.

Preparation of TNT.
Commercial TiO 2 (P25) nanoparticles (5 g) were mixed in an aqueous 10 M NaOH solution and charged into a Teflon-lined autoclave.The autoclave was then oven-heated at 135 ∘ C for 3 days.The precipitate was filtered, and the pH value of the slurry was adjusted with a diluted 0.1 M HNO 3 by washing.The final products were obtained by filtration with subsequent drying at 100 ∘ C overnight.Finally, the TNT samples were obtained through calcination at the temperatures of 400 ∘ C.

Preparation of Fe TNT by
Photodeposition.An appropriate amount of TNT suspension solution was weighed in a Pyrex glass container and then purged with nitrogen for 2 h to remove any dissolved oxygen (DO) from the solution.Then an appropriate amount of Fe solution was weighed and mixed with 10 mL methanol and dissolved in TNT solution.Photodeposition was performed for 12 h under a continuous nitrogen purge using a 254-nm ultraviolet light.Once the Fe-TNT suspension solution was formed, the subsequent clearing was applied using methods similar to those used with the TNT photocatalyst.

Characterization of TNT and Fe TNT.
A UV spectrometer (Hitachi, U-3900) was applied to analyze the reflectance spectra of TNT samples, ranging from 200 nm to 800 nm in wavelength.High-resolution transmission electron microscopy (HR-TEM, JEOL, EM2100 High Resolution TEM) was used for morphological observations of the TNT.Specifically, surface areas and pore volumes of the derived nanotubes were determined by N 2 adsorption/desorption isotherm (Micromeritics, BET ASAP 2020N).
2.4.The Performance Assessment of TNT and Fe TNT.The photocatalytic degradation of MB, DB22, and RBK5 for initial concentrations (10,20,100,200, and 400 ppm), Fedoped content (0, 0.77, 1.13, 2.24, and 4.50%) and light source (254 nm UV, 365 nm UV, 380 nm, and 490 nm) was determined by means of UV-Vis spectrophotometer analysis through indication of color disappearance.A calibration curve of methyl blue solution was obtained at 600 nm wavelength in order to correlate the concentration of MB at different reaction times by converting the absorbance of the sample to MB concentration.During the reaction time the solutions were centrifuged at 300 rpm to mix the dye well.

Kinetics Analysis.
Pseudo-first-order reaction kinetics is the most commonly used kinetic expression to explain the kinetics of the heterogeneous catalytic processes.The Pseudo-first-order expression that explains the kinetics of heterogeneous catalytic systems is given by (1) [14] where   is the reaction rate constant (mol dm −1 min −1 ),  is the adsorption coefficient of the reactant on catalyst (mol dm −3 ) −1 , and  is the concentration of the solute (ppm or mg L −1 ).Pseudo-first-order expression reduces to first-order equation when  is small: where  1 =   .

Results and Discussion
3.1.UV-Visible Spectrum.In order to determine the photo absorbance properties, the commercial Fe TNT and TNT were analyzed by UV-Vis for wavelengths of 200-800 nm.The band gap (, eV) of the six samples was calculated by (4) [16] as follows: where  is the wavelength of UV absorption thresholds.After calculation, P25, TNT (0% Fe TNT), and 1.13% Fe TNT had band gaps of 3.20, 3.10, and 2.7 eV, respectively, International Journal of Photoenergy as shown in Figure 1.The results showed that less energy was needed for reacting with pollutants with the 1.13% Fe TNT.It was found that the Fe-doped TiO 2 can get higher photocatalytic activity with visible light than that of P25.The differences between photocatalytic activity of different Fedoped TiO 2 and pure TiO 2 were illustrated in Figure 7 and described later.
The TNT surface can be blocked with increasing the Fe content.It causes the surface of Fe TNT and band gap to decrease because the amount of Fe increases with increasing red shift.
The peaks of Fe 2p 3/2 can be decomposed into two contributions corresponding to the different oxidation of iron, respectively.The main contribution is attributed to Fe 3+ ions (binding energy at 709.1 eV), and the other is to Fe 2+ ion (binding energy at 710.9 eV) [17].However, the Fe cannot be detected with no element diffraction peak since the Fe content is too small as shown in Figure 3.
The typical tube structure of the TNT was shown in the TEM image.The tube size, judged from the TEM image, ranged from 5-10 nm and was smaller than the results of Park et al. [18], but similar to the results of Song et al. [19].The end of the tube was open, which is extremely critical for TNT absorption and photocatalysis ability [20].This shows that the hydrothermal method successfully changed the p25 nanopowder into nanotubes.As observed in Figure 4, the nanotubes were scrolls which can be explained by the hydrothermal mechanism.

N 2 Adsorption and Desorption
Analyzer.The catalyst surface area was determined by an accelerated surface area and porosimetry analyzer (ASAP 2000) from Micromeritics company using nitrogen at a constant temperature (−196 ∘ C).The data obtained from the N 2 adsorption and desorption analyzer showed that the surface area of the TNT was 331 m 2 g −1 , which was larger than that of 4.50% Fe TNT.

The Effect of Initial Concentration.
The temperature and catalysis loading were controlled at 25 ∘ C and 0.04 gL −1 , respectively, to test the effect of initial concentration.Five kinds of concentrations, including 10, 20, 100, 200, and 400 ppm (mg L −1 ), were assessed to understand the effects of initial concentrations on the degradation of MB.The effect of initial concentration on degradation efficiency is shown in Figure 6.It can be seen that the photodegradation efficiency of methylene blue was inversely proportional to its concentration, which means that the lower the dye concentration, the higher the efficiency of the dye photodegradation [21].In addition, the degradation efficiency of the MB decreased with increasing solution concentration [20].

The Effect of Dye Species.
The three types of dyes were analyzed in this study.The degradation results under UV light are shown in Figure 5.After 180 min, the degradation efficiency of MB reached 99%, but the degradation efficiency of RBK5 was only 68% under 254 nm with and 0.04 gL −1 of 1.13% Fe TNT.These phenomena can be explained by the chemical structure of RBK5.
The test results under illuminated UV light with the three types of dyes are shown in Figure 7.The photodegradation efficiency of the three dyes ranked in order was MB, RBK5, and DB22.The MB degradation efficiency reached almost 100% after 180 min, whereas, the DB22 decolorizing efficiency was only 38% after 180 min because the chemical bonds of DB22 are longer.To increase degradation efficiency, length of exposure to UV light needs to be increased.

The Effect of Fe Content.
The photocatalytic degradation of methylene blue (MB, 10 ppm) by 0.77-4.5 wt% Fedoped TNT was conducted under a 14-W UV and visible light.The power of a 14-W UV and visible light is a very low source compared to conventional visible sources or solar light at 300-500 watts [22].The 1.13% Fe TNT displayed the best properties in this experiment, as shown in Figure 8.
The photodegradation efficiency abruptly decreased to 62% due to an agglomeration and sedimentation of the catalyst particles when the Fe content exceeded 4.50%.This caused an increase in the particle size and a decrease in a specific surface area which lead to a decrease in the number of surface active sites [23].El-Sharkawy et al. [24].indicate that MB was not degraded completely when irradiated with UV in the absence of catalyst after 25 hr.This study has the same .

The Effect of Light Source.
There are four kinds of light sources, including 254 nm UV, 365 nm, 380 nm, and 490 nm light, for assessing the effect of light sources.The decolorizing efficiency reached almost 100% after 180 min of illumination time under 254 nm UV, as shown in Figure 9. Decolorizing efficiency reached 90% after 180 min under near visible light ( = 380 nm).In addition, the decolorizing efficiency was 95% under 365 nm UV.Several minutes were spent before each reaction to maintain the temperature stability.The temperature was kept at 25 ∘ C by constant temperature water bath circulation system.

Kinetic Analysis.
The photocatalytic degradation of MB, RBK5, and DB22 dyes as a function of the irradiation time was observed to follow an exponential decay as depicted in Figure 10.These results suggest that the photodegradation reaction follows the pseudo-first-order reaction kinetics.
The pseudo-first-order reaction rate constant and half-life parameters for MB dyes are listed in Table 1.The values of  2 for different dyes show that a good compliance with the pseudo-first-order equation and the regression coefficients for the linear plots all exceeded 0.94.

Conclusion
In this study, the characterization of TNT and Fe TNT prepared using a hydrothermal and photodeposition method was performed.The time of decolorization increased with an increase in the initial MB concentration.The decolorizing efficiency decreased with increasing initial MB concentration, and a higher efficiency was obtained under UVlight illumination.However, excessive loading decreased the efficiency, and 1.13% weight Fe loading was found to be the optimal addition.Fe TNT using different Fe loadings was well characterized by the pseudo-first-order kinetic model.

Figure 6 :
Figure 6: The effect of initial concentration on degradation efficiency under 254 nm UV and 1.13 % Fe-TNT of 0.04 gL −1 .

Figure 7 :Figure 8 :
Figure 7: The effect of dye species on degradation efficiency under 254 nm UV and 1.13% Fe TNT of 0.04 gL −1 .

Figure 9 :Figure 10 :
Figure 9: The effect of light source on degradation efficiency for 10 ppm MB removal and catalysts loading of 0.04 gL −1 with 1.13% Fe TNT.

Table 1 :
Pseudo-first-order kinetic parameters of different dyes.