Magnetically Separable Fe3O4/SiO2/TiO2 Photocatalyst Composites Prepared through Hetero Agglomeration for the Photocatalytic Degradation of Paraquat

A photocatalyst supported on magnetic material allows the simple technique by using an external magnetic material to separate photocatalyst from the treated water. Thus, it is a magnetically separable nanoparticles photocatalyst (MSNP). The use of superparamagnetic nanoparticles that do not pose spontaneous magnetic moment thus could be dispersed in water and can be recollected easily by an external magnetic bar. We prepare Fe3O4/SiO2/TiO2 composite by hetero agglomeration of Fe3O4/SiO2 and TiO2 at a pH range of 3 to 6.2 in an aqueous slurry. The Fe3O4/SiO2 was prepared via coprecipitation of iron (II) and iron (III) ionic solution containing ammonium hydroxide and sodium silicate. The prepared composites were characterized by XRD, TEM, FTIR, and VSM, while the photocatalytic activities were tested toward paraquat in water. Based on zeta potential values, the Fe3O4/SiO2 and TiO2 were being hetero agglomerated at pH 5 to obtain Fe3O4/SiO2/TiO2 composite. The XRD characterization confirmed the presence of anatase, rutile, and magnetite crystal phases. TEM images showed that the Fe3O4 was covered by SiO2 and randomly attached to TiO2. The observed FTIR peak at 940-960 cm attributed to -Si-O-Tibonding mode, ensuring photocatalyst (TiO2) adherence to the Fe3O4/SiO2 cluster. The prepared Fe3O4/SiO2/TiO2 composite showed good photocatalytic activity for the paraquat removal and showed a good magnetic property (VSM measurement).


Introduction
Paraquat is among the most common herbicides applied for weed killer and herbage desiccant in more than 130 countries. Paraquat poisoning in humans mainly occurs via ingestion and has a mortality of 30%-70% [1,2]. Owing to its large availability, relatively low cost, and low toxic dose, the wide usage of paraquat has received major attention since it damages the marine environment and human's wellbeing. Hence, its possible contamination to groundwater near agriculture plantations must not be neglected. Maksuk et al. detected paraquat in the water near the palm plantation site in Indonesia at a level of 0.01 mg/L [3], which is higher than the allowed 1 µg/L (equal to 0.001 mg/L) maximum level of paraquat in drinking water [4]. Given the function of groundwater as a natural drinking water resource in many developing countries, proper treatment to eliminate paraquat from contaminated groundwater is needed.
Photocatalysis employing semiconductor particles is a good solution to water pollution and is effective in degrading most organic pollutants in air and water [5,6]. Among semiconductor photocatalysts, TiO2 is the most encouraging, due to its properties, namely exceptionally photoreactive, low cost, non-toxic, and its inertness, chemically and biologically [7]. Heterogeneous photocatalysis employing titania (TiO2) has been investigated extensively, including in the photocatalytic degradation of pesticides [8,9] and herbicides such as paraquat [10,11].
Nanophotocatalysts have gained great attention due to their enhanced catalytic properties [13] and high surface area facilitating broad interactions between the surfaceactive catalyst and treated compounds [12]. However, in the slurry system approach, nanoparticles cause difficulty in recovering the photocatalyst after use.
Photocatalysts supported on a magnetic material could be obtained by simply using an external magnetic material to  [14,15]. Superparamagnetic nanoparticles are beneficial because they lack spontaneous magnetic moments and thus can be dispersed in water. When provided with a slightly magnetic field, a maximum magnetic response can be achieved to facilitate recollection using a simple magnetic bar.
The use of iron oxide superparamagnets (magnetite or maghemite) is favorable because of their low price and non-toxicity. Magnetically separable photocatalysts have been prepared using sol-gel [16] and precipitates [17]; nevertheless, these processes require a calcination step (>400 °C) that might be causing the agglomeration of particles, loss of magnetic properties, and even unwanted high-temperature chemical reaction between materials [18]. Hetero agglomeration combines magnetic materials with pre-activated TiO2 photocatalyst and does not necessitate a calcination step.
Hetero agglomeration occurs when two oppositely charged particles join in a medium system through electrostatic interaction [19]. The surface of TiO2 and -Fe2O3 nanoparticles can be modified using a protective lining composed of two oppositely charged polyelectrolytes [20] or grafting with 3-aminopropyl triethoxy silane [21]. Opposite charges can be generated by adjusting the pH of an aqueous slurry solution containing Fe3O4 and TiO2 nanoparticles because of their different iso-electrostatic points [22,23].
The unwanted reaction might occur between titania and iron oxide during photocatalysis and produce a photo dissolution effect, which is detrimental to the photocatalytic activity of the composite [24]. This negative influence could be hindered using an insulating layer such as SiO2 between the magnetic and photocatalytic materials [25,26].
In this work, Fe3O4/SiO2/ TiO2 catalyst was produced by merely adjusting the pH of the aqueous media, in which the as-prepared Fe3O4/SiO2 has an opposite charge to that of the pre-activated TiO2. Fe3O4/SiO2 was prepared by hydrolyzing sodium silicate in the presence of freshly prepared Fe3O4 nanoparticles.
This new type of magneto-photocatalyst was investigated for paraquat photodegradation in water solution and compared with two kinds of commercial nanoparticle TiO2 photocatalysts.

Materials and Methods
Synthesis of Fe3O4. Fe3O4 nanoparticles were prepared with the co-precipitation method of Choi et al. [22]. In brief, 5. After the reaction, the obtained Fe3O4 precipitate was removed from the solution using a magnetic field and re-dispersed into de-ionized water; this process was repeatedly performed until the pH of washing water became neutral. The solid produced was dried at 60 °C overnight and at 100 °C for another 2 h in an oven.
Synthesis of Fe3O4/SiO2. Fe3O4/SiO2 nanoparticles were prepared using the procedure of Zhao et al. [27] with minor modifications. The prepared iron salt solution was dropwise added to the 250 mL of 1.5 M ammonium hydroxide solution with intense stirring. Fe3O4 precipitate was dropwise added with 3.2 mL sodium silicate solution (27 wt % SiO2, Aldrich), and the pH was fixed to 10 through 2 M HCl addition. The mixture was stirred intensely for 4 h and left overnight. Lastly, the obtained Fe3O4/SiO2 nano-composite was removed from the solution using a magnetic field and rinsed with deionized water frequently until the pH of the washing water became neutral. The solid produced was dried at 60 °C overnight and at 100 °C for another 2 h in an oven.
Synthesis of Fe3O4/SiO2/TiO2. Fe3O4/SiO2/TiO2 nanoparticles were prepared through hetero agglomeration. The formed Fe3O4/SiO2 nanocomposite was removed from the solution by a magnetic field and rinsed with deionized water frequently until the pH of the washing water became neutral. In another flask, 4.64 g of TiO2 was dispersed into 100 mL of 0.02 M (NH4)2SO4 0.02 M and stirred for 30 min at room temperature. Two types of TiO2, namely, TiO2 (mixture of anatase and rutile) nanoparticle Aldrich and TiO2-P25 Evonik were used. The dispersed TiO2 was mixed into Fe3O4/SiO2 suspension, and the pH was fixed to 5 through 1M HCl addition. The suspension was sonicated for 30 min at room temperature and centrifuged at 4000 rpm. The solid produced was dried in an oven at 60 °C overnight and at 100 °C for another 3 h.
Characterization. Composite phases were confirmed using X-ray diffractometer (XRD, XD-610, Cu-Kα source, λ= 0.154 nm, Phillip). Transmission electron microscope (TEM) images were recorded with a transmission electron microscope (JEM-1400, JEOL). Infrared (IR) spectra were measured using a Fourier transformation infrared spectroscope (FTIR) type Tensor 27 Bruker. Powdered samples of approximately 10 mg were placed on the sample holder in the FTIR device and irradiated with IR light within 500-4000 cm −1 wavelength. Magnetic properties were measured using a vibrating sample magnetometer Oxford type 1.2 T at room temperature. Surface area was determined using QuandraSorb SI-4-Kr/MP, Quantochrome). The surface December 2021  Vol. 25  No. 4 charges of Fe3O4, TiO2, and Fe3O4/SiO2 were determined using zeta potential (Zetasizer Nano-Zs, Malvern).
Evaluation of photocatalytic activity. Photocatalytic reactions were conducted on a slurry reactor system, and photocatalytic activity was studied for the degradation of paraquat solution in water. Approximately 250 mg of tested materials were dispersed into 250 mL of 20 mg/L paraquat. The suspension was stirred under a UV lamp 2 x 18 W with main radiation at 254 nm at room temperature. Paraquat concentration in water was measured by a UV-vis spectrophotometer at the wavelength of 258 nm and a certain time interval. Photocatalytic activity under UV irradiation was determined under varying experimental conditions, namely, pH range, catalyst type, and catalyst loading in the water

Results and Discussion
Crystalline phase structural characterization. Figure  1a displays the diffraction patterns of commercial TiO2 used to prepare the magneto-photocatalyst nanocomposite. The XRD spectra of both TiO2 samples revealed the occurrence of anatase (JCPDF file no. 21-172) and rutile JCPDF file no. 21-1276) phases. According to peak intensity observations, the rutile phase content in TiO2 Aldrich was higher than that in TiO2-P25 Evonik. TiO2-P25 Evonik has a particle size of  20 nm and consists of above 70% anatase with minor rutile and a small amorphous phase [28]. Meanwhile, TiO2 nanoparticle mixture anatase and rutile Aldrich has a particle size of < 150 nm. Figure 1b shows the peaks related to Fe3O4 (magnetite) in the XRD spectrum, which resembles the standard diffraction pattern of magnetite (JCPDF file no. 03-0863). The crystallite size of Fe3O4 is estimated at 12.7 nm according to Scherrer equation and employed to the (311) plane. The XRD pattern of Fe3O4/SiO2 was similar to that of Fe3O4, indicating the stability of the crystalline phase of Fe3O4 after being coated by silica. The lack of any XRD peak related to the silica crystal phase confirmed the amorphous phase of silica [29]. Magnetite, anatase, and rutile phase peaks were observed in the two magnetically separable photocatalyst (MSNP) composites (Fe3O4/SiO2/TiO2-Aldrich and Fe3O4/SiO2/TiO2-Evonik). This finding indicates the preservation of these phases that are essential for the photocatalysis and magnetic separation of composites in treated water.   Figure 2 reveals the TEM images of the prepared samples. The mean diameter size of Fe3O4 from the TEM image is less than 20 nm, that is close to the value obtained using the Scherrer formula in the XRD patterns. The magnetite particles incline to aggregate as clusters because of the magnetic dipole interaction between neighboring particles [16]. These clusters of superparamagnetic nanoparticles are beneficial for magnetic separation and retain the superparamagnetic property of the individual nanoparticles; their relatively large volume allows their effective high-gradient magnetic separation [30]. In Fe3O4/SiO2, a SiO2 coating layer (bright) covered the surface of magnetic iron oxide clusters (dark).
FTIR characterization. Figure 3 shows the FTIR spectra of Fe3O4/SiO2, Fe3O4/SiO2/TiO2-P25, and Fe3O4/SiO2/TiO2-Aldrich. All curves possess a peak at ~1085 cm −1 , which can be assigned to the asymmetric stretching vibration of Si-O-Si. The strong peak found at This surface hydroxylation is beneficial for the photocatalytic activity of the sample because it gives a high capacity for oxygen adsorption [34]. Figure 4 shows the magnetic properties of Fe3O4 and composites determined at room temperature. Magnetization versus applied field is illustrated in the hysteresis curves. All the prepared samples possessed superparamagnetic behavior. Table 1 lists the saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) values. The saturation magnetization value of prepared Fe3O4 was 57.7 emu/gr. Compounding processes reduced the saturation magnetization due to the low magnetite phase (Fe3O4) in the composite with a non-magnetic coating layer and due to SiO2 and TiO2 in Fe3O4/SiO2 and Fe3O4/SiO2/TiO2 composites, respectively. Superparamagnetic behavior was evidenced by the low values of remanent and coercivity magnetization, which can be attributed to particle aggregation during separation by an external magnetic bar. Hence, December 2021  Vol. 25  No. 4  the photocatalysts can be simply dispersed in a solution and recollected for recycling. Reusing the composite once and four times in repetition only slightly decreased its saturation magnetization (SI-1). Therefore, this phenomenon will not affect composite recollection in water, even after repeated usages.

Magnetic and surface charge properties.
Zeta potential is the surface charge derived from the magnitude of electrostatic repulsive interactions between particles in water. The iso-electric point can be found at the pH level where zeta potential is zero. Figure 5 shows the surface charge (zeta potential) of TiO2, Fe3O4, and Fe3O4/SiO2 determined in the pH range of 2-12. The iso-  [35], and the iso-electric point of TiO2 is at pH 6.2. At pH >3 and <6.2, Fe3O4/SiO2 has a negative surface charge, whereas TiO2 possesses a positive surface charge. Thus, the electrostatic interaction force between these two particles attracts them together. Thus, pH 5 was selected to disperse these two particles in an aqueous solution and consequently form the MSNP Fe3O4/SiO2/TiO2 nanocomposite. Figure 6 shows the Fe3O4/SiO2/TiO2 suspension in water. The Fe3O4/SiO2/TiO2 formed a stable suspension in water for a long time (left). When the magnetic bar was immersed in the suspension, the Fe3O4/SiO2/TiO2 particles moved and attached to the surface of the magnetic bar (center). The attachment of Fe3O4/SiO2/TiO2 on the magnetic bar could be simply separated from water by pulling up the magnetic bar to the top of the container (right).
Photocatalytic evaluation. Photocatalytic oxidation for paraquat was conducted with a 20 ppm aqueous solution of paraquat and 1 g/L of Fe3O4/SiO2/TiO2 catalyst suspension under UV illumination for up to 240 min. For a specific time, the solution was subjected to UV-vis spectrometry measurement (Figure 7). Paraquat has a strong UV signal at 257 nm corresponding to -* transition in the pyridinium ring. Figure 8 shows that the absorption band at 257 nm decreased during irradiation time, whereas the absorption at ca. 220 nm slightly increased and then decreased because of the forming of intermediate (SI-2) and reaction products [10].
The pH of the medium plays a significant part in the adsorption and photocatalytic oxidation of pollutants [8]. Figure 9 shows the result of the photocatalytic degradation of paraquat solutions at different initial pH values Experiments with different initial pH levels were performed to investigate the influence of pH on paraquat degradation rate. Paraquat degradation was least effective at acidic pH [36]. A high pH medium effectively degraded paraquat by increasing the adsorption capacity for this chemical [10] and approaching the point of zero charge (pzc) of TiO2. The best pH value for degradation is above the pzc point [37]. At the same pH, the prepared composite containing TiO2-P25 showed higher activity than the composite containing TiO2-Aldrich due to differences in their surface areas (Table 1). A high surface area greatly facilitates the reactions/interactions between the surface-active catalyst and the interacting media [12]. Moreover, a high anatase phase in TiO2-P25 may lead to photocatalysis under UV irradiation [13], which may be responsible for this observation.   Figure 10 shows the photocatalytic degradation of paraquat in an aqueous solution with various types of photocatalyst composites. No degradation took place during the solution was irradiated without photocatalyst and with only magnetite (Fe3O4). Fe3O4/ TiO2 exhibited a photocatalytic effect for paraquat degradation due to TiO2; however, its photocatalytic rate was quite low because of the photodissolution effect. This influence occurs during electronic interaction and causes charge carrier transfer across the junction between the two semiconductors in contact [24].
Photodissolution can be overcome by introducing SiO2 as a layer between Fe3O4 and TiO2 [25,26]. In addition, SiO2 can act as an adsorbent in the composite [38]. The presence of SiO2 in Fe3O4/SiO2/TiO2 composite plays a role in paraquat adsorption and may be responsible for the reduction in paraquat concentration. Amorphous silica contains silanol (groups) that has ion-exchange capabilities via the weakly acidic silanol groups [39]. The photocatalytic activity of composite dramatically intensified because of the presence of SiO2 barrier between Fe3O4 and TiO2. The synergistic influence of adsorption and photocatalytic properties was evident from the increase in paraquat removal. However, the photocatalytic activity was still lower than that of single TiO2-P25. A single TiO2-P25 contains only pure active material, and Fe3O4/SiO2/TiO2 contains non-photoactive material.
For any practical applications, the optimum loading of catalyst in wastewater treatment is necessary to prevent the catalyst's excess and assures the total absorption of efficient photons. The influence of catalyst loading in solution at 0.1-2 g/L on paraquat degradation was investigated. Figure 11 shows the relationship between catalyst loading and the first-order kinetics of paraquat. The first-order kinetics increased with catalyst loading up to 1 g/L for Fe3O4/SiO2/TiO2-P25 and up to 1.5 g/L for Fe3O4/SiO2/TiO2-Aldrich. Further increase in catalyst loading only led to a minor increase in the first-order kinetics of paraquat. However, the excessive particle loading may result in solution turbidity, thus causing the shading effect. Hence, the light transmission through the solution will be non-uniform and even reduced [40].
Photocatalyst reusability is a key point for practical applications, e.g., economically attractive. The reusability of the prepared catalysts for paraquat degradation is displayed in Figure 12. Fe3O4/SiO2/TiO2-P25 and Fe3O4/SiO2/TiO2-Aldrich particles in paraquat solution were repeatedly used for five cycles under UV irradiation for 90 min. The photocatalyst maintained a stable performance up to five cycles. For approximately 3 hrs, the photocatalyst eliminated 16 mg of paraquat per 1 L of treated water (equal to 5.3 mg/h/L) (Figure 10), and its activity almost remained the same after being used for at least five cycles ( Figure 12).

Conclusion
A MSNP Fe3O4/SiO2/TiO2 system was successfully prepared through hetero agglomeration and showed a good photocatalytic activity while maintaining its magnetic properties. This feature facilitates easy recollection by an external magnetic bar from the treated water after use. Thus, the new Fe3O4/SiO2/TiO2 composite system is a potential candidate for a photocatalytic slurry reactor with a simple recollection system. The MSNP composite prepared using P-25 Evonik has more photocatalytic activity than that prepared with TiO2 Aldrich. The Fe3O4/SiO2/TiO2 composite can degrade paraquat in water (5.3 mg/hr/L) under acidic conditions (pH ≤ 6) and is retrieved with ease from treated water with almost the same activity using an external magnetic bar.