High Efficient Photocatalytic Degradation of Methyl Orange Dye in an Aqueous Solution by CoFe2O4-SiO2-TiO2 Magnetic Catalyst

This study successfully synthesized a core-shell-shell in the form of CoFe2O4-SiO2-TiO2 catalyst magnetic and recyclable. The catalyst was employed for the photocatalytic degradation of methyl orange (MO) dye. Subsequently, the catalyst was subjected to XRD, FTIR, SEM-EDS, VSM, as well as UV-DRS characterizations. The photocatalytic degradation was studied as a function of the solution pH, MO concentration, and irradiation time, while the kinetics of photocatalytic degradation and the catalyst reusability were also evaluated. On the basis of the XRD, FTIR, and SEM-EDS characterizations, the CoFe2O4 coating was successfully carried out using SiO2 and TiO2. CoFe2O4-SiO2-TiO2 was discovered to possess magnetic properties with a saturation magnetization of 17.59 emu/g and a bandgap value of 2.4 eV. The photocatalytic degradation of MO followed the Langmuir-Hishelwood model. The optimum degradation was obtained at the MO concentration of 25 mg/L, solution pH of 4, catalyst dose of 0.05 g/L, irradiation time of 160 minutes, MO removal efficiency achieved 93.46%. The regeneration study showed CoFe2O4-SiO2-TiO2 after 5 cycles were able to catalyze the photocatalytic degradation with an MO removal efficiency of 89.96%.


INTRODUCTION
The continuous discharge of industrial liquid waste containing toxic compounds into water bodies tends to cause environmental pollution and presents several health risks [Ojemaye et al., 2015]. The previous studies by Chan et al. [2008] and Trabelsi et al. [2016] described dyes as toxic compounds produced by several industries, including the textile, pharmaceutical, chemical, paper, foodstuff, soap, cosmetic, and leather industries, where over 50% of the dyes used are azo-based. According to Koohestani et al. [2016], azo dyes are the compounds with an azo bond in the form of -N=N-. Over 15% of the dyes are discharged as liquid waste during the dyeing and coloring process [Nair et al., 2014;Ahmad et al., 2014]. Azo dyes and their intermediates, for instance aromatic amines, are highly stable, toxic, carcinogenic, mutagenic, and not easily degraded [Konstantinou and Albanis, 2004;Alghamdi et al., 2019]. A report by Huang et al. [2008] showed that dyes block the penetration of light into the water, consequently lowering the photosynthetic efficiency and impeding the growth of aquatic organisms. These dyes also cause aesthetic changes which are harmful to the environment. Methyl orange (MO) is an azo dye with the molecular formula C 14 H 14 N- 3 SO 3 Na and is classified as an anionic dye with High Efficient Photocatalytic Degradation of Methyl Orange Dye in an Aqueous Solution by CoFe 2 O 4 -SiO 2 -TiO 2 Magnetic Catalyst a sulfonic group. In addition to being used as an industrial coloring agent, MO is also used as a pH indicator in the laboratory, with a pH indicator range of 3.1 to 4.4 [Alghamdi et al., 2019].
Recently, advanced oxidative processes (AOPs) were discovered to be the most effective method for degrading organic matter from water and wastewater [Suzuki et al., 2015;Mrotek et al., 2020]. These processes are based on the formation of highly reactive radicals, including hydroxyl groups, which oxidize and convert organic compounds into harmless products, for instance, CO 2 Jurek et al. [2017], the formation of a layer on the magnetic core helps to prevent degradation, photo-dissolution, and adverse effects of the magnetic core on TiO 2 . Silica protects the magnetic core, prevents the transmission of electron holes from the photocatalyst layer to the magnetic part, and is, therefore, often used as an intermediary [Awazu et al., 2008;Cheng et al., 2012].
In this study, a core-shell-shell in the form of CoFe 2 O 4 /SiO 2 /TiO 2 was prepared as a photocatalyst for the degradation of MO. CoFe 2 O 4 was selected as the core due to its high thermal and chemical stability, low toxicity, high coercivity, as well as moderate magnetization [Rajput and Kaur, 2014;El-Shobaky et al., 2010]. Subsequently, the photocatalytic efficiency of the degradation, including the effect of solution pH, irradiation time, as well as catalyst dose, were investigated, and the reusability of catalysts was evaluated.

CoFe 2 O 4 preparation
CoFe 2 O 4 was synthesized using the coprecipitation method. For this process, 9.517 g of CoCl 2 .6H 2 O and 21.623 g of FeCl 3 .6H 2 O were dissolved in 100 mL of distilled water. Subsequently, 2 M NaOH was added to the solution in drops, while nitrogen gas was passed across until a pH of 10 was obtained. The precipitate obtained was then collected by magnetic separation, washed several times with distilled water until the pH was neutral, and oven-dried at 110 °C for 1 hour. This was followed by subjecting the CoFe 2 O 4 obtained to further calcination at 800 °C for 2 hours.

CoFe 2 O 4 -SiO 2 preparation
The synthesis of CoFe 2 O 4 -SiO 2 was carried out using the sol-gel method. For this process, 0.8 g of CoFe 2 O 4 and 0.8 g of Trisodium citrate dihydrate were dissolved in 20 mL of ethanol and 8 mL of distilled water. The mixture was then homogenized by sonification for 10 minutes, and 4 mL of ammonium hydroxide, as well as 3.2 mL of TEOS, were added to the solution, and sonification was continued for 3 hours at 40 °C to form a silica layer around CoFe 2 O 4 . Subsequently, the precipitate obtained was separated by centrifugation, washed severally with ethanol, and dried using rotary evaporation.

CoFe 2 O 4 -SiO 2 -TiO 2 preparation
The CoFe 2 O 4 -SiO 2 -TiO 2 composites were prepared using the method reported by Habila et al. [2015], with some modifications. For this process, 2 g of CoFe 2 O 4 -SiO 2 was suspended in a mixture of 24 mL ethanol, 30 mL distilled water, and 800 L ammonium hydroxide (28%), using sonification, for 30 minutes. Subsequently, 20 mL of ethylene glycol and 2 mL of TBT solution were slowly added and the mixture was homogenized using a magnetic stirrer at 45 °C, for 24 hours. The CoFe 2 O 4 -SiO 2 -TiO 2 obtained was washed with distilled water, as well as ethanol, then ovendried at 110 °C for 1 hour and calcined at 450 °C for 3 hours.

Characterization
The crystal structure and phase of catalyst were analyzed using X-Ray Diffraction (XRD PANalytical), while the functional groups were identified using Fourier Transform Infra-Red (FTIR Prestige 21 Shimadzu). In addition, the morphology and elemental composition were analyzed using a Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS JOEL JSM 6510 LA). The magnetic moment was determined using a Vibrating Sample Magnetometer (VSM Oxford Type 1.2 T). At the same time, the wavelength and band gap were analyzed using Diffuse Reflectance Ultra Violet-Visible Spectroscopy (UV-Vis DRS Pharmaspec UV-1700). The radiation source for photocatalytic degradation was UV light (12 W Phillips) and the MO absorbance was measured using a UV-Vis Spectrophotometer (Type Orion Aquamate 8000). Mineralization degree was measurement by Total Organic Carbon (TOC Teledyne Tekmar). Determination of pHpzc was carried out following a modification of the technique reported by Bezahdi et al. [2020] using NaNO 3 solution as an electrolyte.

Photocatalytic degradation
In this experiment, 50 mL of MO was mixed with CoFe 2 O 4 -SiO 2 -TiO 2 at a dose of 0.05 g/L in separate quartz pipes, with the MO concentrations of 25, 50, 75, and 100 mg/L. Using UV light as the irradiation source, the mixture was placed in a photoreactor with a vessel distance of 30 cm from the light source. Furthermore, the effects of the pH and irradiation time were studied by varying the pH in the range of 2 to 7, as well as the irradiation time between 0 to 200 minutes. Subsequently, the MO removal (%) was calculated using the following formula (Eq. 1).
where: C o and C t are the initial and final concen- The reusability of the catalyst was also investigated using the same method, under the optimum conditions for photocatalytic degradation. For this evaluation, the CoFe 2 O 4 -SiO 2 -TiO 2 was separated using a permanent magnet after the photocatalytic degradation, then washed with ethanol and distilled water, dried in an oven for 60 minutes at 80 °C, and reused for photocatalytic degradation [Ajabshir and Niasari, 2019]. This experiment was repeated 5 times, and the catalyst efficiency was measured after each cycle.  The SEM and EDS analyses are used to investigate the morphology and composition of the catalyst elements. Figure 3 shows the morphology of CoFe 2 O 4 , CoFe 2 O 4 -SiO 2 , and CoFe 2 O 4 -SiO 2 -TiO 2 with the same magnification. The morphology of CoFe 2 O 4 appears spherical because due to the small size which tends to agglomerate and the morphology of CoFe 2 O 4 -SiO 2 is similar to CoFe 2 O 4 but more homogeneous. This is consistent with the XRD results which show the SiO 2 coating is not observed in the spectra due to the amorphous nature of the compound. Meanwhile, a heterogeneous and rough surface is visible in the morphology of CoFe 2 O 4 -SiO 2 -TiO 2 where the TiO 2 aggregates appear to be round and coat the CoFe 2 O 4 -SiO 2 . Table 2 shows the elemental composition of CoFe 2 O 4 , CoFe 2 O 4 -SiO 2 , and CoFe 2 O 4 -SiO 2 -TiO 2 from the EDS analysis, where the presence of Si and Ti elements in CoFe 2 O 4 -SiO 2 -TiO 2 indicates a successful synthesis. Furthermore, no other elements were detected as impurities. Figure 4 shows the magnetization curves of CoFe 2 O 4 , CoFe 2 O 4 -SiO 2, and CoFe 2 O 4 -SiO 2 -TiO 2 obtained using VSM. According to the results, the saturation magnetization of CoFe 2 O 4 is 57.05 emu/g, and this value is close to the saturation magnetization of CoFe 2 O 4 synthesized using the combustion, coprecipitation, and precipitation methods, which are 56.7, 55.8, and

Photocatalytic properties
In photocatalytic degradation, the solution pH influences the charge of the catalyst surface. The solution pH is an important parameter.
A report by Behzadi et al. [2020] showed the optimum pH depends on the type of pollutant and the pHpzc, which shows the pH on the material surface in total is zero or the catalyst surface is neutrally charged [Amulya et al., 2020]. The pHpzc value must be investigated to determine the appropriate pH for an effective photocatalytic degradation process. The pHpzc of CoFe 2 O 4 -SiO 2 -TiO 2 is 5.2. Figure 7 shows the effect of solution pH on the removal of MO, which has a pH range of 3.1-4.4 with a pKa of 3.7. At a solution pH < pHpzc, CoFe 2 O 4 -SiO 2 -TiO 2 is positively charged, while MO is an anionic dye, and consequently, the attraction is more effective. MO removal increases at pH 2 to 4 and subsequently decreases at pH 5. Meanwhile, at a solution pH > pHpzc, there is a repulsion of electrostatic charges between the anionic dye and the negatively charged CoFe 2 O 4 -SiO 2 -TiO 2 . The highest MO removal was obtained at pH 4 with variations in the initial MO concentrations of 25, 50, 75, and 100 mg/L. Figure 8 shows the effect of irradiation time on the photocatalytic degradation of MO at concentrations of 25, 50, 75, and 100 mg/L, the catalyst dose of 0.05 g/L, as well as pH of 4, under UV light. The results showed MO removal increases along with irradiation time; however, at 160 minutes of irradiation, there was no increase in the amount of degraded MO. Furthermore, the highest MO removal was obtained at a concentration of 25 mg/L (93.46%). This is because higher concentrations tend to block light from reaching the catalyst, consequently, reducing the rate of removal.    Table 3 shows the kinetics parameters of MO photocatalytic degradation where the t 1/2 value is calculated using 0.693/k.

Reusability of the photocatalyst
The regenerability and reusability of catalysts are highly significant in industrial contexts, because these properties are related to cost, pilotscale remediation systems, and environmental safety [ Figure 10,  [Pourzad et al., 2020]. In this study, the efficiency of TOC removal for photocatalytic degradation of MO under optimal conditions with a concentration of 25 mg/L, catalyst dose of 0.05 g/L, solution pH of 4, and irradiation time of 160 minutes is 82.68%. This result indicates a successful dye decomposition process.

CONCLUSIONS
A core-shell-shell composite in the form of CoFe 2 O 4 -SiO 2 -TiO 2 was successfully synthesized and effectively used for photocatalytic degradation of methyl orange dye under UV light irradiation. On the basis of the XRD analysis, CoFe 2 O 4 was discovered to possess a cubic spinel structure and TiO 2 was in the anatase phase. In addition, the FTIR and SEM-EDS analyses confirmed the presence of SiO 2 and TiO 2 shells. The CoFe 2 O 4 -SiO 2 -TiO 2 composite possessed magnetic properties with a saturation magnetization of 47 emu/g, as well as a bandgap of 2.4 eV. Furthermore, the removal efficiency of MO using CoFe 2 O 4 -SiO 2 -TiO 2 was discovered to be 93.46% with a MO concentration of 25 mg/L, solution pH of 4, catalyst dose of 0.05 g/L, and irradiation time of 160 minutes under UV light irradiation. In addition, the photocatalytic degradation followed the Langmuir-Hinshelwood model expressed in pseudo-first-order. These results show that CoFe 2 O 4 -SiO 2 -TiO 2 has the potential for use in wastewater treatment, especially for organic pollutant removal. The catalyst effectiveness decreased by only 3.74% after 5 cycles of photocatalytic degradation.