Fe-TiO 2 /zeolite H-A photocatalyst for degradation of waste dye (methylene blue) under UV irradiation

: Industrial wastewater contains non-biodegradable dyes that are highly toxic to humans and aquatic life. As solution from photocatalytic degradation, TiO 2 is one of the effective photocatalysts for wastewater degradation, but it has low adsorption power. To overcome this deficiency, this study synthesized a new photocatalyst by Fe-TiO 2 /zeolite H-A. The photocatalyst was successfully synthesized by the impregnation method and was systematically characterized by XRD, XRF, SEM, FT-IR and UV-Vis DRS. XRD diffractogram at 2θ = 25.3° showed anatase phase of the photocatalyst. SEM results showed a rough and soft surface with a size of 491.49 nm. FT-IR analysis obtained the zeolite-A characteristic band, vibration of Ti-O-Ti groups and the vibration of the Fe-O group. The bandwidth of the band gap was 3.16 eV. The photocatalytic efficiency of methylene blue degradation reached 89.58% yield with optimum conditions: irradiation time of 50 min, pH 9 and concentration of methylene blue about 20 mg/L. Fe-TiO 2 /zeolite H-A as a new photocatalyst can be an alternative photocatalyst to purify methylene blue.


Introduction
Textile dye come from the dyeing process, in which 10-15% of the used dyes cannot be recycled that still contain non-degradable dyes [1,2]. This is because the structure of the textile dye has one or more azo groups (R-N=N-R′) and an aromatic ring which are mostly replaced by a degradation was carried out with several parameters of contact time, pH and initial concentration of dye. In addition, until now, the development of photocatalysts with the addition of metal and a porous matrix (zeolite) as well as with TiO 2 is still rarely done.

Preparation of zeolite A
Preparation of zeolite A was carried out using the ion exchange method. The process was assisted with an acid solution to obtain zeolite H-A. First, 5 g of zeolite Na-A was added, and then 50 mL of 1M NH 4 Cl (1:10) was added with stirring using a shaker for 1 h at 150 rpm. After that, it was allowed to stand for 24 h. The mixture was filtered and neutralized with demineralized water to get a neutral pH. After that, the powder (residue) was dried in an oven at 115 ℃ for 2 h. The dried powder was calcined at 450 ℃ for 4 h. The calcined powder was zeolite H-A.

Synthesis of Fe-TiO 2 /zeolite H-A
Synthesis of Fe-TiO 2 /zeolite H-A photocatalyst was begun with the synthesis of Fe-TiO 2 . The synthesis of Fe-TiO 2 was carried out with a modified method [25]. The synthesis method was the impregnation method. Fe(NO 3 ) 3 •9H 2 O powder was added to TiO 2 powder with 3% of TiO 2 weight in 100 mL of demineralized water. The solution was stirred with a magnetic stirrer for 1 hour and then heated at a temperature of 80-90 ℃. The residue was centrifuged and washed with demineralized water to pH 7. The obtained solid was then calcined at 400 ℃ for 2 h. Furthermore, the synthesis of Fe-TiO 2 /zeolite H-A was carried out by a modified method [26]. 1.4 g of Fe-TiO 2 and 8 g of zeolite H-A were added into 10 mL of ethanol with stirring for 5 h. The mixture was filtered and neutralized. The residue was dried in an oven at 120 ℃ for 5 h. The solid was then calcined at 450 ℃ for 3 h.

Characterization for Fe-TiO 2 /zeolite H-A
Crystal structure and phase were identified by Powder X-ray Diffraction (PXRD) (PANanalytical E'xpert Pro). The crystalline element content was analyzed by X-ray Fluorescence (XRF) (brand: PANalytical; type: Minipal 4). Surface morphology analysis with various magnifications was performed by Scanning Electron Microscopy (SEM) (FEI Inspect-S50). Functional group investigations were carried out by Fourier Transform Infra-Red (FT-IR) (Shimadzu IRPrestige 21). UV-Vis DRS (Analytik Jena Specord 200 plus) with a wavelength of 1100-200 nm was used to obtain band gap values, absorbance data. The obtained data was then processed using the Tauc plot equation with Origin software.

Photocatalytic degradation of Methylene Blue
The photocatalytic degradation process with UV irradiation was carried out in a cube-shaped box. All sides of the box covered in the dark (black light) box, and a 20-watt UV lamp was used. A stirrer was provided in the cube as a stirring device.
The degradation process was carried out as follows: 50 mg of Fe-TiO 2 /zeolite H-A photocatalyst was put into a 50 mL Beaker glass. 10 mg/L of methylene blue was pipetted and diluted to 25 mL. Then, methylene blue was added to the photocatalyst. The mixture was stirred and irradiated with UV for 10-50 min with an interval of 10 min. After the irradiation process, the suspension was filtered to separate the supernatant and the precipitate. The supernatant obtained from each beaker was measured for absorbance with a UV-Vis spectrophotometer at a wavelength of 664 nm. The absorbance value obtained was then analyzed to get the linear regression equation, so the optimum time was obtained. The obtained data was then used in the formula for the percentage of degradation (Efficiency percentage) through the following equation.
C o and C t are the initial concentration of methylene blue and the concentration after the degradation process, respectively. The same procedure was repeated for pH (3,5,7,9) and concentration of methylene blue (10,20,30,40, 50 mg/L).

Activation of zeolite A
In Zeolite H-A form, the acidic site in the zeolite framework transformed into a soft acid site (zeolite H-A), so it is more easily substituted by Ti 4+ ions [27]. On the other hand, zeolite pretreatment or zeolite activation is to reduce impurities that cover the external surface and pores of the zeolite [28]. The modification of zeolite A to zeolite H-A was carried out using a solution of ammonium chloride (NH 4 Cl) through the ion exchange method. The ion exchange process occurs between metal cations alkali metals (Na + , K + ) and alkaline earths (Ca + , Ba 2+ ) with ammonium cations (NH 4+ ) from HH 4 Cl, because metal cations have weak ion-dipole interactions with zeolites. The interactions support ion-exchange as shown in XRF results (Table 1) [29].
The XRD results are shown in Figure 1. Based on Figure 1, diffraction peaks of raw material appear at 2θ = 10.24°, 12.53°, 16.19°, 20.51°, 21.76°, 24.09°, 26.22°, 27.22°, 30.95°, 32.66°, 33.49° and 34.29°, which have the same crystal characteristics as zeolite A or zeolite Linde Type A (LTA) as the ICSD database No. 150097. This confirmed that the used raw material of zeolite was zeolite A. The XRD pattern of the zeolite H-A ( Figure 1b) showed no peak shift and no significant diffractogram changes. This showed that the treatment of zeolite A to zeolite H-A did not change the crystal structure of the zeolite and the crystal phase compared to the initial zeolite. In addition, Figure 1 showed a fairly sharp diffraction peak, and this indicated the formation of zeolite H-A.
Based on the FT-IR results (Figure 1), the spectrum of zeolite LTA material appeared. The presence of the OH group of H 2 O was indicated by the absorption band at 3365 cm −1 (bending vibration). The stretching vibration of the OH group of the silanol group (Si-OH) was shown in the absorption band of 1648 cm −1 . The absorption bands of the O-T-O group (T = Si, Al) were shown at 677 cm −1 (symmetrical) and 1000 cm −1 (asymmetrical). The absorption at 552 cm −1 indicated the external vibration of the double ring (D4R or D6R), which is a characteristic of the zeolite LTA structure [30]. The absorption band at 475 cm −1 indicated a bending vibration of the T-O group (T = Si, Al), and the absorption band at 400-370 cm −1 indicated the open pores of the zeolite framework material [31]. Furthermore, the FT-IR spectrum of the zeolite H-A showed a slight shift in the absorption wavenumber and intensity, so it did not have much different results from the initial zeolite A. The absorption band at 1400-400 cm −1 appeared in the zeolite H-A spectrum; the absorption band is a typical absorption band of LTA zeolite (zeolite A) [32]. This shows that the preparation process of zeolite A to zeolite H-A was successful without damaging the structure of the zeolite.
Supporting data confirmed by looking at changes in the level of elemental composition in the zeolite using XRF characterization. The XRF results are shown in Table 1. The data was used to calculate the Si/Al zeolite ratio. It was known that the ratio of Si/Al zeolite from zeolite A to H-A has changed from 2.03 to 1.98, but this zeolite has a ratio value of more than 1, which is different from the molar ratio of zeolite A in general. For example, zeolite A with a Si/Al molar ratio more than 1 was zeolite ZK-4 [30]. Thus, the used zeolite material is still a group of zeolite A. This study still uses the term zeolite A because the complexity of dimensions, structure and impurities are the same as zeolite A [32].  The SEM results are shown in Figure 2. Figure 2a presented a typical cube-shaped micrograph with zeolite LTA morphology [30]. Figure 2b presented a micrograph of the pretreated zeolite that is not different from the initial zeolite. The zeolite has a uniform size even though there is a small crystal size around it. This shows the zeolite pretreatment from Na-A to H-A does not damage the initial zeolite structure. The average particle sizes of zeolite Na-A and H-A were 516.50 nm and 511.74 nm.

Characterization of Fe-TiO 2 /zeolite H-A photocatalyst
XRD patterns of TiO 2 , Fe-TiO 2 , zeolite H-A, TiO 2 /zeolite H-A and Fe-TiO 2 /zeolite H-A are shown in Figure 3. The used TiO 2 showed sharp peaks at 2θ = 25.34°, 37.83°, 48.07°, 53.92°, 55.09°, 62.72°, and these peaks indicated an anatase structure, which is in accordance with JCPDS card No. 21-1272 [33,34]. The XRD pattern of Fe-TiO 2 did not show a significant diffractogram difference with pure TiO 2 , as well as the characteristic peak of anatase at 2θ = 25.3274°. After the impregnation, no diffractogram pattern related to the characteristics of Fe element was observed. This is possible because the amount of the added Fe(NO 3 ) 3 •9H 2 O powder was too low, so it is difficult to detect in XRD. On the other hand, the size of the Fe 3+ ion radius (0.69 Å) is almost the same as the Ti 4+ ion (0.74 Å), so Fe 3+ ions can easily replace Ti 4+ ions and combine into the structure of TiO 2 [35]. The XRD results show that there is no significant difference between TiO 2 /zeolite H-A and Fe-TiO 2 /zeolite H-A. XRD peaks at 2θ = 25.34° (TiO 2 /zeolite H-A) and 2θ = 25.31° (Fe-TiO 2 /zeolite H-A) indicate the presence of an anatase phase of TiO 2 and a typical zeolite diffractogram pattern, respectively. This shows that the addition of Fe 3+ impregnation does not change the phase of TiO 2 . Further characterization was carried out by looking at the elemental composition using XRF. The results of XRF characterization can be seen in Table 2.   Table 2, the composition of the synthesized Fe-TiO 2 /zeolite H-A photocatalyst consisted of Ti (TiO 2 ), Fe, Si and Al (zeolite component) with elemental contents of 41.1%, 0.723%, 23.2% and 33.1%, respectively. This confirmed that the synthesis of Fe-TiO 2 /zeolite H-A photocatalyst was successfully carried out. However, from XRF and XRD results, it is not certain whether Fe-TiO 2 and TiO 2 have been evenly dispersed into the external surface of the zeolite.
The FT-IR measurement is carried out with a wavenumber range of 4000-400 cm −1 . The FT-IR spectra of TiO 2 , Fe-TiO 2 , zeolite H-A, TiO 2 /zeolite H-A and Fe-TiO 2 /zeolite are presented in Figure 4. The absorption band at 1400-400 cm −1 was a typical absorption band for zeolite A. In addition, absorption bands appeared at 705.91 cm −1 (symmetric) and 1002. 16 [36]. The impregnation of Fe 3+ ions into the TiO 2 structure gave rise to new absorption at a wavenumber of 1063 cm −1 , which indicated the presence of vibrations from the Fe-O group. Previous studies reported that absorption at 1054-1065 cm −1 indicated Fe-O vibrations [37]. Overall, the spectrum can be observed to have a typical absorption of each component and does not overcome a significant spectrum shift in the final structure of the synthesized Fe-TiO 2 /zeolite H-A photocatalyst. This indicated that the photocatalyst was reacted without any particular reaction [9]. The SEM results are presented in Figure 5. The surface morphologies of TiO 2 and Fe-TiO 2 have no significant differences in shape and changes. The surface morphologies were spherical, and agglomerate with average particle sizes of 40.96 nm (TiO 2 ) and 40.56 nm (Fe-TiO 2 ), respectively. Figure 5c shows the surface morphology of the zeolite H-A with an average particle size of 511.74 nm. Figure 5d shows a change in the surface morphology of the zeolite after TiO 2 addition. It was observed that the TiO 2 agglomerates spread over the surface of the zeolite framework [38,39]. The photocatalyst micrograph of Fe-TiO 2 /zeolite H-A in Figure 5e shows that more agglomerates are spread over the surface of the zeolite and cover the surface of the zeolite H-A framework, so the surface becomes rough and uneven after the addition of Fe in TiO 2 . This indicates that Fe-TiO 2 has been well distributed on the surface of the zeolite and there is no typical change in morphology of zeolite after the addition of Fe-TiO 2 [5]. The average particle size of the Fe-TiO 2 /zeolite H-A photocatalyst was 491.49 nm, and TiO 2 /zeolite H-A was 487.29 nm. DRS analysis was used to investigate the optical properties of the synthesized photocatalyst. UV-Vis DRS analysis ( Figure 6) was carried out at a wavelength of 200-1100 nm, and then the Tauc plot shown in Figure 7 was created. The energy band gaps of Fe-TiO 2 , TiO 2 /zeolite H-A and Fe-TiO 2 /zeolite H-A obtained from the calculation of the Tauc plot (Figure 7a-c) were 3.18 eV, 3.24 eV and 3.16 eV, respectively. These results indicated that the energy band gap of the Fe-TiO 2 /zeolite H-A photocatalyst has the smallest value. In addition, there was a decrease in the band gap after TiO 2 was added with Fe 3+ ion and the zeolite. This indicates that the reducing of the energy band gap has been achieved.

Study of the photocatalytic degradation
Methylene blue degradation with Fe-TiO 2 /zeolite H-A through a photocatalytic process can be seen in Figure 8. Based on Figure 8, the optimum irradiation time of the Fe-TiO 2 /zeolite H-A photocatalyst to degrade methylene blue was 50 min with a degradation of 87.87%. After it, the decrease of irradiation time occurred due to the Fe-TiO 2 /zeolite H-A photocatalyst being saturated. Moreover, pH plays a very important role in the formation of hydroxyl radicals (OH•) [40]. The optimum pH was pH 9, because methylene blue is a cationic dye (positive charge), so at alkaline pH it can increase the photocatalytic degradation. At alkaline pH, the surface of TiO 2 is negatively charged, which causes an increase in the electrostatic attraction between methylene blue and TiO 2 in the photocatalyst. This can be explained by the theory of pH pzc (pH Point of Zero Charge ). TiO 2 has a pH pzc value of 6.8 when the oxide surface is uncharged. The surface of TiO 2 is positively charged when the pH value is less than 6.8 (acidic pH) and negatively charged when pH more than 6.8 (alkaline pH). Finally, efficiency percentage of methylene blue degradation increased with increasing initial methylene blue concentration until it reached a certain limit and then decreased with increasing concentration. Optimum concentration of Methylene Blue was 20 mg/L. If a small amount of UV light reaches the photocatalyst, then the number of electrons will reduce [41]. The presence of zeolite as a support for TiO 2 will result in the preservation of nearby dye molecules, thereby increasing the rate of degradation [5]. Another reason is that, with the presence of Fe element and zeolite H-A as a photocatalyst, Fe-TiO 2 /zeolite H-A has a smaller band gap energy than pure TiO 2 . The narrower energy band gap means that the distance between the valence band and the conduction band is narrower, so electron excitation is easier, and consequently photocatalytic activity is increasing [42]. So, the degradation performance of the modified TiO 2 was higher than the pure TiO 2 [43].

Conclusions
Fe-TiO 2 /zeolite H-A photocatalyst was synthesized using the impregnation method. The synthesized photocatalyst was used for the cationic degradation of methylene blue as a model pollutant in wastewater. The successful synthesis of Fe-TiO 2 /zeolite H-A was proven by XRD, SEM, FT-IR and UV-Vis/DRS. XRD results at 2θ = 25.3° showed an anatase phase of the photocatalyst, SEM results showed particle size about 491.49 nm, FT-IR analysis obtained typical bands of zeolite at wavenumbers 490 and 1720 cm −1 , and the absorption appeared at 491.85-1049.28 cm −1 , which was the vibrational absorption of the Ti-O-Ti group. The addition of Fe 3+ ions into the TiO 2 structure gave rise to new absorption at 1063 cm −1 , which indicated the presence of vibrations from the Fe-O group. The band gap width of the synthesized nanocomposite was 3.16 eV, and this value was very close to the band gap of pure TiO 2 . Furthermore, the photocatalytic efficiency of Fe-TiO 2 /zeolite H-A against methylene blue was 89.58% with optimum conditions: irradiation time of 50 minutes, pH 9 and concentration of methylene blue about 20 mg/L. The Fe-TiO 2 /zeolite H-A photocatalyst has been successfully synthesized and can be developed as a promising photocatalyst for the degradation of wastewater dyes.