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Review

Suspension Type TiO2 Photocatalysts for Water Treatment: Magnetic TiO2/SiO2/Fe3O4 Nanoparticles and Submillimeter TiO2-Polystyrene Beads

by
Manabu Kiguchi
* and
Nobuhiro Hanada
*
Institute of Photochemistry and Photofunctional Materials, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China
*
Authors to whom correspondence should be addressed.
ChemEngineering 2026, 10(1), 3; https://doi.org/10.3390/chemengineering10010003
Submission received: 29 October 2025 / Revised: 10 December 2025 / Accepted: 16 December 2025 / Published: 4 January 2026
(This article belongs to the Special Issue Advances in Chemical Engineering and Wastewater Treatment)
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Abstract

Photocatalytic degradation of organic molecules using TiO2 has attracted attention in wastewater treatment because it can decompose organic compounds that are difficult to decompose by other methods. Meanwhile, efficient photocatalytic water treatment is difficult because it is not easy to separate nano-sized photocatalysts from water. In this review, we have described two approaches to solve the water separation challenge in the suspension type TiO2 photocatalysts, which are uniformly distributed in water: magnetic TiO2/SiO2/Fe3O4 nanoparticles and TiO2-polystyrene beads. The preparation, characterization, and photocatalytic performance of the two types of photocatalysts and their application are discussed. Finally, we compare two types of photocatalysts while focusing on the respective advantages and disadvantages of each, and the future direction of research.

Graphical Abstract

1. Introduction

Photochemical and photocatalytic reactions can convert light energy into chemical energy, and they also enable the production of useful compounds and the decomposition of harmful compounds without fossil fuels or hazardous chemicals. Titanium dioxide (TiO2) photocatalyst has been the most actively investigated material in both fundamental research and industrial applications [1,2,3,4,5,6,7,8,9,10]. TiO2 possesses a lot of advantages including biological and chemical inertness, strong oxidizing power, resistance to photo corrosion, low cost, nontoxicity, and so on. In 1972, Fujishima et al. discovered the oxygen generation on the TiO2 electrode under light irradiation, while hydrogen was generated from the counter Pt electrode [6]. Kawai et al. discovered that the water splitting reaction using TiO2 powders was improved by introduction of organic compounds [7]. In the 1990s, photocatalytic degradation of pollutants using TiO2 was widely investigated in industrial areas, leading to applications like antibacterial tiles [8]. In 1997, the photo-induced hydrophilization reaction was discovered [9]. This finding provided TiO2 coating with new functions, such as self-cleaning via rainwater and anti-fog effects. Currently, various products utilizing TiO2’s antibacterial/antiviral, anti-fogging, deodorizing, and air purification functions are commercially available [10].
There are three main crystal phases of TiO2: rutile, anatase, and brookite phases [11]. Anatase is the most used in photocatalytic applications, since it shows higher photocatalytic activity compared to other phases. The bandgaps (Eg) of the rutile and anatase phase are about 3.0 and 3.2 eV, respectively [10]. When the light (hv ≥ Eg) is irradiated on TiO2, electrons are excited from the valence band (VB) to the conduction band (CB: see Table S1), resulting in photogenerated electrons (e−) and holes (h+). When they migrate to the surface of TiO2, they undergo redox reactions with the adsorbed oxygen or water molecules to form •O2− and •OH. Other reactive oxygen species (ROS), such as hydroperoxide (•OOH) and H2O2, can also be formed by further redox processes. These ROS (e.g., •O2−, •OH, •OOH, H2O2) and hole mineralize the organic molecules into CO2 and water molecules (see Figure 1) [12,13]. Photocatalytic generation of •O2− and •OH radicals are detected by electron spin resonance (ESR) and chemical method [14,15].
Photocatalytic degradation of organic molecules using TiO2 have attracted attention in the wastewater treatment as the following reasons [16,17,18,19,20]. Chemicals such as cosmetics, dyes, medical products, pesticides, make our life enrich. Despite their usefulness, their use has raised serious concerns related to their toxicity and pollution of water, soil, and so on. Some chemicals are hardly decomposed and can remain in water for a year. Chemicals that are not decomposed enter rivers and affect downstream ecosystems, drinking water, and agriculture that uses river water. Actually, a serious herbicide contamination incident occurred in Shanghai in 2019. Effective removal methods are urgently required. Conventional water treatments utilize physical adsorption with activated carbon, membrane filtration, reverse osmosis, ion exchange, photo degradation using ultra violet (UV) light, and biological process using microorganisms. However, these conventional methods have subjects. For example, physical methods require periodic replacement as contaminants accumulate. Some compounds cannot be decomposed by light or biological treatment alone. Meanwhile, the photocatalytic reaction can decompose compounds that are difficult to decompose by other methods using ROS and hole. Furthermore, photocatalytic reaction enables the use of sunlight, making it a safe and sustainable approach.
Although photocatalytic water treatment is promising approach, it is more difficult to compare to air treatment. The photocatalytic reaction proceeds on the surface. Molecules, therefore, should be transferred to the catalyst surface. Target molecules are hardly transported to the catalyst surface in solution due to their low diffusion constant. Efficient reaction is difficult to proceed in solution. The point is effective contact between the target pollutants and photocatalysts. A nano-sized photocatalyst suspension is an ideal system to realize the effective contact because nano-sized photocatalysts are uniformly dispersed in solution, which shortens the distance between photocatalysts and target molecules. On the other hand, the separation of water from nano-sized photocatalysts becomes a problem, which prevents practical application [21]. One solution is the fixed beds, where the nano-sized photocatalysts are fixed on substrates like filters and films. However, effective contact between the target pollutants and photocatalysts is difficult compared to photocatalysts suspension. There are several other approaches to overcome the water separation challenge in suspension type photocatalysts. One is combining photocatalysts with magnetic materials [22,23,24,25,26]. Water is separated from photocatalysts with a magnet after water treatment. Another is to increase the size of the photocatalyst [27,28]. When the size of the photocatalysts is not small, the catalysts can be separated from water using a filter. By considering practical application, separable using a metal mesh is desirable. Meanwhile, smaller size is better for increasing surface area, so the appropriate size of the photocatalysts is submillimeter.
Numerous excellent reviews already exist on photocatalysts, suspension type photocatalysts, magnetic photocatalysts, and related topics. However, few reviews compare two approaches to solve water separation challenge in the suspension type of photocatalysts: specifically, the use of magnetic core and scale-up of nano-sized photocatalysts. In this review, we thus compare the two approaches while focusing on the respective advantages and disadvantages of each. In the first approach, we discuss nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts (see Figure 2). In the second approach, we discuss submillimeter scale TiO2-polystyrene beads photocatalysts. The fixed bed photocatalysts (e.g., immobilized TiO2 on glass, ceramic membranes), magnetic composites, and others are not discussed in this review. Finally, we comment on future direction of research.

2. Nano Sized Magnetic TiO2/SiO2/Fe3O4 Photocatalysts

In the case of magnetic photocatalysts, the photocatalysts are bonded to magnetic cores (e.g., Fe3O4). They can be separated from water using magnets [22,23,24]. Here, the magnetic properties of the magnetic core are crucial [29]. Bulk Fe3O4 is ferromagnetic, and thus large Fe3O4 particles tend to aggregate due to magnetic forces, making dispersion in water difficult. As the size of a particle is reduced to nano-scale (~20 nm), the particle exhibits magnetic moment as a single domain under an external magnetic field. It behaves as a super paramagnet. In the absence of the external magnetic field, it has zero residual magnetization, and its magnetization disappears. The super paramagnetic photocatalysts can be collected with a magnet and can be dispersed in the absence of an external magnetic field. Therefore, magnetic photocatalysts must be at the nanoscale to exhibit super paramagnetism. The requirement of nanoscale is another advantage because the nano-sized photocatalysts provide a large surface area, which realizes the effective contact between target molecules and photocatalysts, leading to high photocatalytic efficiency.
In this review, we discuss the TiO2/SiO2/Fe3O4 as an example of nano-sized magnetic photocatalysts [25,26,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. Here, the barrier layer SiO2 is introduced to prevent the photo dissolution of Fe3O4 core and to suppress the electron–hole recombination. There are two types of nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts, core–shell and peanut shapes (Figure 3). Core–shell shaped photocatalysts are prepared by coating TiO2 on SiO2/Fe3O4 particles using Ti precursor like tetrabutyl orthotitanate (TBOT) [31,32]. Peanut-shaped photocatalysts are prepared by mixing a SiO2/Fe3O4 particle and TiO2 particle suspensions and bonding them together via electrostatic forces under controlled pH condition.

2.1. Preparation

The core–shell-shaped TiO2/SiO2/Fe3O4 photocatalysts are prepared with the sol–gel method [33,35]. Fe3O4 nano particles are prepared with the co-precipitation method. FeCl3∙6H2O and FeCl2∙4H2O (mol ratio of Fe[III]/Fe[II] = 2:1) are dissolved in HCl aqueous solution. The iron salt solution is then added to the ammonium hydroxide solution to proceed with the reduction reaction. The obtained Fe3O4 nano particles are separated with a magnet, washed, and dried. The Fe3O4 nano particles are dispersed in ethanol, water, and an ammonia solution. Then tetraethoxysilane (TEOS) is added to the mixture. The obtained SiO2/Fe3O4 nano particles are separated with a magnet, washed, and dried. The mixture of TBOT and ethanol is added to the SiO2/Fe3O4 nano particles solution. The obtained TiO2/SiO2/Fe3O4 nano particles are separated by using a magnet and washed. Finally, the particles are dried in air and calcined. The calcination at high temperature in air could result in a change from magnetic Fe3O4 to non-magnetic α-Fe2O3. The calcination condition is important for preparation of TiO2/SiO2/Fe3O4 photocatalysts.
The optimization of core–shell-shaped TiO2/SiO2/Fe3O4 photocatalysts has been investigated by changing calcination temperature, Si/Fe3O4, Ti/Fe3O4 ratio, concentration of water, mixing order of water and TBOT, and so on [33,36]. Esfandiari et al. showed that the mixing order of water and TBOT had a significant effect on the TiO2 coating [36]. When TBOT was added to SiO2/Fe3O4 suspension and then water was added (TBOT first), uniform TiO2/SiO2/Fe3O4 particles were obtained. Meanwhile, in the reverse mixing order process (water first), coarse particles were observed. TBOT is soluble in organic solvents. In the mixing order of TBOT first, TBOT was homogeneously distributed in ethanol. By adding water to the solution, H2O was quickly mixed with ethanol. Therefore, the reaction slowly and homogenously progressed in solution. Meanwhile, in the mixing order of water first, the surface of the TBOT drop would be the dominant site for the reaction with water. Thus, TiO2 particles formed distinctly and deposited without any shell coating on the SiO2/Fe3O4 nanoparticles.
The peanut-shaped TiO2-SiO2/Fe3O4 photocatalyst is prepared as the following [34]. A sodium silicate and HCl (for pH control) are added to the Fe3O4 nanoparticles suspension. The obtained SiO2/Fe3O4 is separated from the solution using a magnet and washed. TiO2 is dispersed into (NH4)2SO4 solution in another beaker. The TiO2 suspension is mixed with the SiO2/Fe3O4 suspension under the pH control (pH = 5) using HCl. The isoelectric point of TiO2 and SiO2/Fe3O4 is 6.2 and 3, respectively [35]. Therefore, TiO2 has a positive surface charge and SiO2/Fe3O4 possesses a negative surface charge in the range 3 < pH < 6.2. The TiO2 binds to SiO2/Fe3O4 via electrostatic forces. The method does not require high heat treatment over 500 °C. High heat treatment is generally required for TiO2 coating with Ti precursor in order to crystallize, but the heat treatment can cause agglomeration, reducing surface area that means degradation of the photocatalytic performance, oxidation of the magnetite, and possible conversion to non-magnetic forms (hematite). In contrast, since the process is completed below 100 °C in the case of preparation of the peanut-shaped TiO2-SiO2/Fe3O4 photocatalyst, the photocatalytic properties of nano particle photocatalysts (e.g., P25) are preserved.

2.2. Characterization

The nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts have been characterized with TEM, SEM, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and so on [25,26,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. The crystal structure can be characterized by XRD. Figure 4a shows the XRD patterns of peanut-shaped TiO2-SiO2/Fe3O4 photocatalysts together with P25 (TiO2) and SiO2/Fe3O4 [26]. In the case of P25, peaks appeared at 2θ at 25.38°, 27.52°, which corresponded to the anatase and rutile phases, respectively. The XRD pattern of SiO2/Fe3O was similar to that of magnetite. The XRD pattern of the TiO2-SiO2/Fe3O4 photocatalysts was a superposition of that of SiO2/Fe3O4 nanoparticles and P25, which meant the crystal structure of TiO2 for TiO2-SiO2/Fe3O4 was the same as that of TiO2 introduced in the preparation.
The particle size can be determined by the XRD, TEM, and BET technique. Three combined measurements were performed for the same TiO2-SiO2/Fe3O4 photocatalysts [26]. By analyzing XRD peak with Scherrer’s formula, the crystal size of TiO2 was determined to be 19 nm for both anatase and rutile phase. The average particle size was 23 nm by analyzing TEM image. The surface area was 79 (±1) m2/g by BET analysis. Assuming a spherical particle and its density (4 g/cm3), the particle size was determined to be 19 nm. All XRD, TEM, and BET analysis showed the particle size was about 20 nm [26].

2.3. Photocatalytic Performance

The photocatalytic performance has been investigated with dye molecules (e.g., methyl orange (MO), methylene blue (MB) [37], acid fuchsine [32], acid Blue 161 [38]), acetic acid [39], nitrophenol [40], herbicides, pesticide [29,35], bacteria [41], and so on. CO2 reduction and formation of ethanol have also been reported [42]. MB is one of the most frequently studied compounds. Figure 5a shows the UV-Vis spectra of the MB solution containing 0.1% nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts at different UV irradiation times [16]. Before measurement, the solution was stirred for 60 min to reach the adsorption equilibrium. A clear peak appeared at 665 nm in the UV-Vis spectra, and its intensity decreased with irradiation time of UV light. Figure 5b shows the irradiation time dependence of MB concentration. The MB concentration exponentially decreased with time. The degradation was not observed in solution without light and/or without the photocatalysts, which meant that the observed degradation of MB was photocatalytic reaction. For comparison, the result of the nano-sized TiO2 photocatalyst (P25) is shown in Figure 5b. The reaction rate for the TiO2-SiO2/Fe3O4 photocatalysts was close to that for P25, showing high efficiency of the TiO2-SiO2/Fe3O4 photocatalysts. The good dispersion and small size of the photocatalysts could explain high photocatalytic performance.
The photocatalytic performance is affected by the co-dissolved organic and inorganic compounds. Vahidian et al. investigated the effect of sodium sulfite, sodium sulfate, and sodium nitrite on degradation of P-Nitrophenol [40]. Sodium sulfite, sodium sulfate, and sodium nitrite were chosen because they were contained within the redwater effluents. Sulfite and sulfate decreased the degradation rate, which could be explained by the radical scavenging characteristic. Furthermore, sulfite decreased the degradation rate more than sulfate. In contrast, nitrite increased the degradation rate. The increase was explained by that nitrite produced hydroxyl radicals by the irradiation of UV light.
The nano-sized magnetic TiO2-SiO2/Fe3O4 photocatalysts have been applied to herbicide degradation. Figure 6 shows the time course of herbicide concentration (simetryn, prometryn, metolachlor, and sulfometuron methyl) [26]. Triazine herbicides simetryn and prometryn are applied as both pre- and post-emergence treatments. Chloroacetanilide herbicide metolachlor is typically utilized as a pre-emergence herbicide. Sulfometuron methyl is a pre-emergence herbicide. The herbicide concentration decreased with the irradiation time of UV light. After irradiation of UV light for 120 min, all four herbicide concentrations were smaller than 1 ppm. Notably, the change in the UV-vis spectra was not so significant compared to the change in concentration of herbicide, especially for simetryn and prometryn. The peak around 200–400 nm in UV-Vis spectra indicated the presence of aromatic rings. The triazine backbone of the herbicides was not completely destroyed and preserved after irradiation of UV light. As for the simetryn and prometryn, two photodecomposition pathways, hydroxylation of triazines and dealkylation have been reported [43,44]. The gas chromatography mass spectroscopy (GC-MS) study showed the dealkylation process in which the two CH3 and CH2-CH3 units attached to the triazine ring detached one by one. Hydroxylation and dealkylation for metolachlor, and amide hydrolysis has been reported for the sulfometuron methyl [44,45,46].
In case of herbicide degradation, not only the reduction in herbicide concentrations but also the reduction in the toxicity is important. In some herbicides, decomposition products are more toxic than herbicides themselves. Nomura et al. evaluated the toxicity of treated water with plant bio assay validation [30]. Figure 7 shows the survival rate of pak choi using photocatalytic treated water at different UV irradiation times. In the case of simetryn solutions, the survival rate increased with the UV irradiation time, which meant the reduction in toxicity of the water. Simetryn solutions, which were treated for 4 h (h) and 1 day showed growth close to that of pure water. In contrast, prometryn solution that was treated for 1 h and 2 h was more toxic than non-treated prometryn solution. The plant bio assay validation clearly showed that intermediates were more toxic than the herbicide. For practical feasibility for irrigation-water treatment, a large-scale photocatalytic reactor has been investigated [30]. The 5 L unit, fitted with 96 light emitting diode (LED)s (total power draw = 192 W) and 2.5 g of nano-sized magnetic TiO2-SiO2/Fe3O4 photocatalysts, achieved complete herbicide degradation of a 10 ppm herbicide solution within 4 h, which corresponded to a throughput of 1.25 L/h.
The nano-sized magnetic TiO2-SiO2/Fe3O4 photocatalysts have also been applied to photokilling of bacteria. Kiguchi et al. studied the photokilling of Gram-positive bacteria, Lactobacillus casei (L. casei) with the peanut-shaped TiO2-SiO2/Fe3O4 [26]. Figure 8a shows the McFarland unit of L. casei de Man, Rogosa, and Sharpe (MRS) solution as a function of the incubation time for samples at different UV irradiation times. As for non-treated water, the McFarland unit started to increase at 10 h. The increase in the McFarland unit was delayed for photocatalytic treated water. The increase in the McFarland unit was not observed for irradiation times of more than 50 min. The number of bacterial cells exponentially increased with incubation time in the incubation process, through the lag phase. The delay in the increase meant that the bacteria were killed by photocatalytic treatment, and the absence of the increase indicated the complete photokilling of the bacteria. The concentration of viable bacterial cells before incubation is shown in Figure 8b. The number of viable bacterial cells decreased with the UV irradiation time, and that the effect of UV irradiation was more pronounced with the catalysts.
The bacterial sterilization progressed even in the absence of photocatalysts. It can be explained by the following reasons. First, ultra violet A (UVA) light induces oxidative stress on cells, leading to damage to DNA and cell membranes. L. casei is a Gram-positive bacterium that lacks an outer membrane. Therefore, L. casei is sensitive to UV damage. Second, •OH and H2O2 can be generated by irradiation of UV light on water. These species can damage DNA and bacterial cell membranes. In the system containing only the photocatalysts without light irradiation, bacterial sterilization did not progress, indicating that only presence of photocatalysts did not affect bacterial sterilization. While bacterial sterilization progressed with UV light alone, bacterial sterilization progressed significantly with the photocatalyst. The difference between the systems with and without the photocatalyst represented the contribution of ROS to bacterial sterilization.
Much of the photocatalytic performance discussion is based on dye molecules. However, their photocatalytic decomposition mechanism is complex, and light screening and different adsorption behavior on photocatalysts can sometimes prevent accurate evaluation of photocatalytic performance under certain experimental conditions, such as high concentration ranges. L. casei can play important roles as model bacteria in photocatalytic sterilization research. Currently, Escherichia coli (E. coli) is used as gold standard for sterilization evaluation [10]. E. coli is an aerobic bacterium that enables highly accurate quantification with the colony-counting method. However, this approach is labor-intensive and time-consuming due to the need for long time incubation, plating, and counting. On the other hand, L. casei, classified as a facultative anaerobe, can be cultured under static conditions and evaluated by turbidity measurements using a spectrophotometer, allowing efficient kinetic analysis. Thus, the choice between E. coli and L. casei should be made according to the specific experimental objectives and desired data accuracy.
A comparison is made between the core–shell-shaped and the peanut-shaped magnetic nano photocatalysts (see Table 1). Regarding size, the core–shell type tends to be larger, since multiple layers are stacked for the core–shell. Smaller catalyst size results in greater specific surface area and allows for a shorter distance between the target pollutants and the photocatalysts, improving catalytic performance. Regarding limitations during photocatalyst fabrication, the peanut type allows TiO2 to be prepared separately from the magnetic core. This enables various treatments on the TiO2, such as high-temperature treatment and ion implantation, and facilitates the use of high-performance catalysts like P25. In contrast, the core–shell type has limitations due to the presence of Fe3O4. For example, when heating after coating with TBOT, the temperature cannot be raised too high. Calcination at high temperature in air could result in a change from magnetic Fe3O4 to non-magnetic alpha-Fe2O3. Finally, we compare the stability of two types of photocatalysts. In case of the peanut type, electrostatic force binds TiO2 and the magnetic core together within an appropriate pH range. Therefore, if the solvent pH exceeds the appropriate range, the attraction force between two particles disappears, potentially causing the TiO2 and magnetic core to separate. In contrast, in the core–shell type, the TiO2 is connected to the magnetic core via a strong chemical bond. This makes the TiO2 difficult to peel off from the magnetic core, making it considered more chemically stable than the peanut-shaped photocatalysts.

3. Submillimeter Scale TiO2-Polystyrene Beads Photocatalysts

Polystyrene (PS) is an inexpensive material that is chemically and mechanically stable. Furthermore, its specific gravity is slightly greater than water, making it easy to disperse in water. PS is, therefore, one of the most suitable support materials of TiO2 [27,28,47,48,49,50,51,52,53,54,55]. Considering practical separation from water and photocatalytic performance, the appropriate size for the PS beads catalysts is submillimeter. There are two types of submillimeter-sized PS beads photocatalysts: floatable photocatalysts and those dispersed within the water. Natural water is often not completely transparent but rather turbid, and oxygen supply is readily available at the air/water interface. Therefore, floatable photocatalysts is a smart approach considering practical application where the photocatalysts are simply spread in water without mechanical stirring. Expanded polystyrene (EPS), which has a low specific gravity is used for support of floatable photocatalysts. Regarding the suspension type PS beads photocatalysts, the specific gravity of the beads is critically important. Large mechanical force is required to disperse the photocatalysts in water if its specific gravity is high. Applying large mechanical force to the catalyst risks detaching it from the PS support. Meanwhile, the photocatalysts cannot be dispersed in water, if its specific gravity is smaller than that of water. The specific gravity should be slightly larger than 1, so that they can be easily dispersed by weak perturbations like water or air flow. In this regard, PS is an optimal material with a specific gravity of 1.06.

3.1. Preparation

The PS beads photocatalysts have been prepared by several methods including thermal treatment, the solvent-cast method, the sol–gel method, and liquid phase deposition [27,28,47,48,49,50,51,52,53,54,55]. Altın et al. prepared the TiO2/PS photocatalysts with thermal treatment [38]. The TiO2 and PS mixture was heated in an oven. As the temperature gradually increased to glass transition point of EPS beads (~150 °C), their surface softened so TiO2 adhered to the soft surface of the PS. Singh et al. prepared TiO2/EPS photocatalysts with the solvent-cast method [52,53]. TiO2 (Degussa P25) powder was mixed in PS-dissolved xylene. Then, the suspension was casted into a Petri dish and left for solvent to evaporate. Miądlicki et al. [27] and Kiguchi et al. [28] prepared the TiO2/SiO2/PS beads photocatalysts with the sol–gel method. First, PS beads were added to a mixture of ammonia and ethanol solution. Subsequently, TEOS mixed with ethanol was added to form SiO2/PS beads. For impregnation of TiO2 on the SiO2/PS beads, the SiO2/PS beads were added to the TiO2 water suspension.
The dispersion state of the PS beads photocatalyst has been examined for the TiO2/SiO2/PS beads photocatalysts [28]. Figure 9 shows the PS beads photocatalysts dispersed in water with water flow (water stirring) and air flow (air stirring). In water stirring, the inlet tube was placed at the bottom and the outlet tube was placed at the top. A metal mesh (SUS304 stainless) was attached to the end of the tubes to prevent the beads from leaving the reactor. In air stirring, air was injected at the bottom of the reactor. In both cases, the PS beads photocatalysts were uniformly distributed in the solution at a flow rate of 0.9 mL/s. The experiments were also performed with silica whose specific gravity was 2.2 g/cm3. Figure 9c shows the photo image for silica beads with a flow rate of 0.9 mL/s. Half of the silica beads stayed at the bottom, indicating that the distribution of the beads was affected by specific gravity of the support material.

3.2. Photocatalytic Performance

The photocatalytic performance has been investigated with dye molecule (e.g., MB [50,51,52,53], MO [53], drimaren red [48]), ethylene [27], phenol [55], L. casei, E. coli and Aspergillus niger [49], tetracycline, and so on (see Table 2). Here again, most of studies have been performed with dye molecules. Figure 10 shows the UV-Vis spectra of the MB solution at different UV irradiation time. The MB concentration decreased exponentially with the UV irradiation time. The degradation of MB was not observed for solution without the photocatalysts and/or without light, supporting that the observed degradation of MB was photocatalytic degradation. Regarding catalyst stability, direct connecting PS and TiO2 causes damage to PS due to holes and ROS generated in TiO2. As is the case with magnetic nano photocatalysts, introducing a SiO2 barrier layer between PS and TiO2 is effective for enhancing catalyst stability. Actually, the TiO2/SiO2/PS beads photocatalysts have been confirmed to work without degradation even after 100 h of UV light irradiation [28].
Magalhaes et al. showed high performance of the floating photocatalysts using dye molecules [48]. They compared the TiO2/EPS and pure TiO2 with Hg lamp under the NON-STIIRRING and no-oxygenation condition. Pure TiO2 in an equivalent amount showed much lower activity compared to TiO2 (18 wt%)/EPS. They explained the result by decantation of dense TiO2 to the bottom and non-illuminated parts of the reactor under the no-stirring condition. No-stirring condition is better in practical use. So, their study showed the usefulness of the floating photocatalysts with EPS.
Altın et al. studied the effect of catalyst concentration, pH, and dye concentration on the photocatalytic reaction with the TiO2/PS photocatalyst with MB solution [49]. The removable rate of MB did not decrease at high catalyst mass regime. In a conventional TiO2 suspension system, the turbidity of the suspensions increases with the catalyst’s concentration. The photocatalytic removal efficiency, therefore, decreases with a catalyst concentration above certain threshold concentration due to the enhanced light scattering effect. Meanwhile, a turbidity problem was not observed in the TiO2/PS system because TiO2 nanoparticles were immobilized on PS beads [49]. Regarding the effect of pH, the removal efficiency was no good at low pH regime and did not significantly change between 6 and 10. The isoelectric point of the TiO2 photocatalyst is at pH 6.25 and TiO2 surface is positively charged at lower pH (pH < 6.25), whereas it is negatively charged under alkaline conditions (pH > 6.25). MB is cationic dye molecule. Therefore, the efficiency of the MB removal increased at high pH because of the electrostatic interactions between the negative charged TiO2 surface and the MB cations [49]. Complete removal between pH = 6 and 10 is an advantage because removal or degradation processes do not require additional pH arrangement. Photocatalytic degradation of the MB decreased with increasing dye concentration. Dye molecules can absorb more UV light than catalyst at higher concentrations and, therefore, the amount of light irradiated onto the photocatalysts decreased with the initial dye concentration.
The submillimeter scale TiO2 PS beads photocatalysts has been applied to photokilling of bacteria [28]. Figure 11a shows the time course of McFarland unit for L. casei MRS solution at different UV irradiation time. During the incubation, the number of bacteria exponentially increased with time through the lag phase. The McFarland unit increased at 8–10 h for UV irradiation times shorter than 25 min. The rise in McFarland unit was delayed with the irradiation time of UV light. The rise was not observed for UV irradiation times longer than 30 min. The delay of rise showed that the bacteria were killed by UV light. Figure 11b shows the concentration of viable bacterial cells before incubation as a function of UV irradiation time. The number of bacterial cells decreased with the UV irradiation time.
Varnagiris et al. conducted inactivation experiments targeting E. coli under UV irradiation [47]. The results showed over 90% of the bacteria were inactivated within 45 min, and high photocatalytic performance was also confirmed in a MB decomposition experiment. This study demonstrates that the suspended photocatalyst is effective for both bacterial sterilization and organic matter decomposition, further supporting the practical significance of the PS bead photocatalyst. In addition to photocatalytic applications, PS beads have been widely utilized in various biomedical and biotechnology fields. For instance, PS beads functionalized with specific ligands or antibodies are commonly employed as solid supports for immunoassays, enzyme immobilization, and cell separation techniques [56,57,58]. Their uniform size, high surface area, and ease of surface modification make them ideal platforms for biosensing applications [56,57]. Furthermore, PS microbeads have been investigated as drug delivery carriers [59] and scaffolds for tissue engineering [60], demonstrating their versatility in biomedical research. These diverse applications highlight the multifunctional nature of PS bead-based materials [61], extending their utility beyond photocatalysis to encompass a broad spectrum of biological and medical applications.

4. Comparison Between Two Type Photocatalysts and Future Perspective

Magnetic nano photocatalysts have significantly smaller particle sizes than PS beads photocatalysts, resulting in larger specific surface areas and shorter distances between target pollutants and the photocatalyst, making their catalytic properties superior. Regarding dispersibility, PS beads photocatalysts can be dispersed in water with minor perturbations thanks to their specific gravity (1.06). However, magnetic nano photocatalysts are easier to disperse in water compared to PS beads photocatalysts due to their smaller particle size. From the perspective of separation from water, while magnetic nano photocatalysts can be recovered using magnets, complete recovery is difficult without significant optimization of magnet placement. There is a risk of residual magnetic nano photocatalysts remaining in the water. In contrast, submillimeter scale PS beads photocatalysts can be completely separated from water using filters. Therefore, PS beads photocatalysts are preferable for food and medical applications where catalyst contamination is unacceptable, while magnetic nano photocatalysts are more suitable for wastewater treatment, where efficiency is more important. In terms of photocatalyst manufacturing costs, PS beads photocatalysts are cheaper than magnetic nano photocatalysts, which need various chemical compounds for synthesis. However, if PS beads photocatalysts incorporate SiO2 block layers or complex structures, PS bead’s cost advantage is lost. Regarding scaling up, magnetic photocatalysts require magnets for water separation, making large-scale water treatment systems difficult to implement compared to PS beads photocatalysts. Magnetic nano photocatalysts are better suited for relatively small-scale applications. Furthermore, unless the magnet placement is carefully designed, their use in flow systems is also difficult, making large-scale water treatment challenging. On the other hand, PS beads photocatalysts can be easily separated using filters and their use in flow system is possible, making the scaling up of water treatment systems easier. Magnetic nano photocatalysts are suitable if photocatalytic efficiency is prioritized. PS beads photocatalysts are suitable if treatment capacity is prioritized.
Although these two types of TiO2 photocatalyst system are useful, further improvement is needed to overcome the inherent subject of TiO2, relatively large bandgap and high charge recombination rate. These two properties limit the application of pure TiO2 in visible light-driven processes, such as those using natural sunlight. Utilizing visible light is currently a hot topic and it is also a research direction to pursue going forward for magnetic nano photocatalysts and PS beads photocatalysts. Regarding modification of TiO2 towards visible light utilization, there are various studies including the utilization of semiconductors (e.g., WO3, bismuth based oxide, carbon nitride, ZnS) with smaller band gap [62,63,64,65,66,67], doping with non-metal (e.g., N, C [68,69]), metal (e.g., Cr, Mn, Fe, Ni) and novel metal (Ag, Cu) [13,70,71], deposition of noble metal nano particles on the photocatalysts [72], introduction and modification of surface defects [73] and oxygen through optimized heat treatment [74], forming heterojunction [75], and so on. One research direction aims to enhance photocatalytic properties by combining these modified TiO2 with magnetic nano photocatalysts and PS beads photocatalysts.
As for the PS beads photocatalysts, B and Ce doping and Bi2O3/TiO2 heterojunctions have been reported [61,76]. Zhang et al. prepared a water-floating PS-sphere-supported TiO2/Bi2O3 S-scheme heterojunction by oxidizing Bi(NO3)3·5H2O [76]. Photocatalytic degradation of tetracycline (TC) using a xenon lamp was investigated. The TC removal rate of the optimal-Bi2O3/TiO2 reached 88.4% under 1 h illumination, which was higher than that of pristine Bi2O3 (60.8%) and PS@TiO2 (40.1%). Manjunatha et al. prepared a PS photocatalysts using a B and Ce co-doped TiO2 photocatalysts [61]. Photocatalytic activity using sunlight improved with increasing B and Ce doping levels. They confirmed that ciprofloxacin was decomposed by 89.17% in 240 min on B0.8Ce0.2TiO2.
Regarding magnetic nano photocatalysts, nitrogen doping, composite formation with carbon materials, and the loading of metal nanoparticles have been reported [72,77,78,79]. Kumar et al. prepared an N-TiO2@SiO2@Fe3O4 catalyst using the sol–gel method with urea solution. The decomposition reaction of ibuprofen under visible light irradiation improved with increasing nitrogen doping [77]. Zhan et al. prepared Fe3O4/TiO2 catalysts supported with Ag nanoparticles via electroless plating [72]. They observed that the photocatalytic reaction rate of MB under visible light increased from 0.24/h to 1.30/h due to the Ag coating. Enhanced catalytic properties through Ag support were also reported by Osanloo et al. [78]. Here, Ag was supported on the photocatalyst via photoelectrolysis. Under 150 min of irradiation using a high-pressure mercury lamp, naproxen was treated at 92.56% and Rhodamine B at 92%.

5. Conclusions

In this review, we have described the suspension type photocatalysts, mainly focused on nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts and submillimeter scale TiO2-PS beads photocatalysts, which can solve the water separation problem. The nano-sized magnetic photocatalysts can be separated from the water using a magnet. Thanks to the small size of the photocatalysts, its photocatalytic performance is close to the nano size TiO2 photocatalysts (e.g., P25) suspension. Submillimeter scale TiO2-PS beads photocatalysts can be separated from water using a filter (e.g., metal mesh). The beads photocatalysts can be effectively dispersed in solution due to their small specific gravity. Nano-sized magnetic TiO2/SiO2/Fe3O4 photocatalysts are better than submillimeter scale TiO2-PS beads photocatalysts in terms of specific surface area and dispersibility. Meanwhile, submillimeter scale TiO2-PS beads photocatalysts are better in terms of water separation. We should choose appropriate photocatalysts by considering the utilization area. One of the subjects in photocatalysts is the utilization of visible light. There are various studies on the modification of TiO2 towards visible light utilization. One research direction aims to enhance photocatalytic properties by combining these modified TiO2 with magnetic nano photocatalysts and PS beads photocatalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemengineering10010003/s1, Table S1: List of abbreviation used in the paper.

Author Contributions

Writing—original draft, review, and editing, M.K. and N.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by Shanghai Agricultural Science and Technology Innovation Program, “Development and application of technology and equipment for the safe treatment of river irrigation water used in facility vegetable fields” (Grant No. A2024004).

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic image of the photocatalytic reaction on TiO2 photocatalysts. Photo-generated hole and ROS (e.g., •O2−, •OH) react with organic molecules or bacteria, leading to photocatalytic degradation and photo killing.
Figure 1. Schematic image of the photocatalytic reaction on TiO2 photocatalysts. Photo-generated hole and ROS (e.g., •O2−, •OH) react with organic molecules or bacteria, leading to photocatalytic degradation and photo killing.
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Figure 2. (a) Schematic image, (b) Transmission electron microscope (TEM) image of nano-sized magnetic photocatalysts, (c) Photo and schematic image of photocatalyst separated from water by using a magnetic disc. (d) Schematic image and (e) Scanning electron microscope (SEM) image of TiO2-polystyrene beads photocatalysts. (f) Photo and schematic image of photocatalysts separated from water by using a metal mesh filter. Reprinted with permission from Refs. [26,28]. 2025, Manabu Kiguchi.
Figure 2. (a) Schematic image, (b) Transmission electron microscope (TEM) image of nano-sized magnetic photocatalysts, (c) Photo and schematic image of photocatalyst separated from water by using a magnetic disc. (d) Schematic image and (e) Scanning electron microscope (SEM) image of TiO2-polystyrene beads photocatalysts. (f) Photo and schematic image of photocatalysts separated from water by using a metal mesh filter. Reprinted with permission from Refs. [26,28]. 2025, Manabu Kiguchi.
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Figure 3. (a) Core–shell-shaped and (b) peanut-shaped magnetic photocatalysts. In the case of peanut-shaped magnetic photocatalysts, photocatalyst is connected to magnetic particles via electric force.
Figure 3. (a) Core–shell-shaped and (b) peanut-shaped magnetic photocatalysts. In the case of peanut-shaped magnetic photocatalysts, photocatalyst is connected to magnetic particles via electric force.
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Figure 4. (a) XRD pattern of nano TiO2-SiO2/Fe3O4 photocatalysts, SiO2/Fe3O4, and TiO2 (P25). (b) TEM image of the nano TiO2-SiO2/Fe3O4 photocatalysts. (c) Adsorption (black) and desorption (red) isotherm. Inset figure: BET analysis of nano TiO2-SiO2/Fe3O4 photocatalysts. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
Figure 4. (a) XRD pattern of nano TiO2-SiO2/Fe3O4 photocatalysts, SiO2/Fe3O4, and TiO2 (P25). (b) TEM image of the nano TiO2-SiO2/Fe3O4 photocatalysts. (c) Adsorption (black) and desorption (red) isotherm. Inset figure: BET analysis of nano TiO2-SiO2/Fe3O4 photocatalysts. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
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Figure 5. (a) UV-Vis spectra for the MB solution at different irradiation times using TiO2-SiO2/Fe3O4 photocatalysts. (b) Time course of MB concentration. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
Figure 5. (a) UV-Vis spectra for the MB solution at different irradiation times using TiO2-SiO2/Fe3O4 photocatalysts. (b) Time course of MB concentration. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
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Figure 6. Time course of concentrations of (a) simetryn, (b) prometryn, (c) metolachlor, (d) sulfometuron methyl. Inset figure: HPLC data. UV-Vis spectra of solution containing (e) simetryn, (f) prometryn, (g) metolachlor, (h) sulfometuron methyl at different irradiation times. Reprinted with permission from Refs. [26,28]. 2025, Manabu Kiguchi.
Figure 6. Time course of concentrations of (a) simetryn, (b) prometryn, (c) metolachlor, (d) sulfometuron methyl. Inset figure: HPLC data. UV-Vis spectra of solution containing (e) simetryn, (f) prometryn, (g) metolachlor, (h) sulfometuron methyl at different irradiation times. Reprinted with permission from Refs. [26,28]. 2025, Manabu Kiguchi.
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Figure 7. Survival rate of pak choi with photocatalytic treated water (a) simethryn and (b) prometryn herbicide. Reprinted with permission from Ref. [30]. 2025, Manabu Kiguchi.
Figure 7. Survival rate of pak choi with photocatalytic treated water (a) simethryn and (b) prometryn herbicide. Reprinted with permission from Ref. [30]. 2025, Manabu Kiguchi.
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Figure 8. (a) Time course of McFarland unit of the L. casei solution at different irradiation times of UV light. (b) Viable bacterial cells concentration before incubation as a function of irradiation time of UV light, with (black curve) and without (red curve) photocatalysts. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
Figure 8. (a) Time course of McFarland unit of the L. casei solution at different irradiation times of UV light. (b) Viable bacterial cells concentration before incubation as a function of irradiation time of UV light, with (black curve) and without (red curve) photocatalysts. Reprinted with permission from Ref. [26]. 2025, Manabu Kiguchi.
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Figure 9. Schematic and photo image of the reactor of (a) water stirring and (b) air stirring at different flow rates. Photocatalysts are dispersed by water flow or air flow. (c) Photo image of the reactor with silica beads at flow rate of 0.9 mL/s. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
Figure 9. Schematic and photo image of the reactor of (a) water stirring and (b) air stirring at different flow rates. Photocatalysts are dispersed by water flow or air flow. (c) Photo image of the reactor with silica beads at flow rate of 0.9 mL/s. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
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Figure 10. (a) UV-Vis spectra of the MB solution at different UV irradiation time. (b) MB concentration as a function of UV irradiation time. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
Figure 10. (a) UV-Vis spectra of the MB solution at different UV irradiation time. (b) MB concentration as a function of UV irradiation time. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
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Figure 11. (a) Time course of McFarland unit of the Lactobacillus casei solution at different irradiation time of UV light. (b) Viable bacterial cells concentration before incubation as a function of irradiation time of UV light without (black) and with (red) photocatalysts. Inset: Plot of a logarithmic scale on the vertical axis. The blue line corresponds to the results without the UV light. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
Figure 11. (a) Time course of McFarland unit of the Lactobacillus casei solution at different irradiation time of UV light. (b) Viable bacterial cells concentration before incubation as a function of irradiation time of UV light without (black) and with (red) photocatalysts. Inset: Plot of a logarithmic scale on the vertical axis. The blue line corresponds to the results without the UV light. Reprinted with permission from Ref. [28]. 2025, Manabu Kiguchi.
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Table 1. Photocatalytic performance comparison table for nano sized magnetic TiO2/SiO2/Fe3O4 photocatalysts.
Table 1. Photocatalytic performance comparison table for nano sized magnetic TiO2/SiO2/Fe3O4 photocatalysts.
ShapeSynthesis MethodParticle SizeSurface Area (BET)Saturation MagnetizationTarget PollutantsLight SourceApparent Rate Constants or Degradation Percentage/TimeRecyclabilityReference
Core-shellSol-gel200 nm446.87 m2/g MB, MOUV lamp 20 W
Energy intensity 200 mW/cm2		0.129/min5 times 95%[37]
Core-shellSolvothermal240 nm 44 emu/gAcid BlueUV lamp91% 180 min [38]
Core-shellSol-gelPomegranate like
structure		 MBHigh-pressure mercury lamp 100 W Wavelength 290–450 nm78% 5 min [39]
Core-shellSol-gel22 nm 12 emu/gNitrophenolUV-C light 150 W
Wavelength 254 nm		0.032/min4 cycle 92%[40]
Core–shellMicrowave assisted sol-gelPorous microstructure 17 emu/gCiprofloxacinUVA lamp
Wavelength 365 nm		0.0158/min3 cycle 97%[29]
Core-shellSol-gel JRS4
S. Saprophyticus
S. pyogenes M9022434
S. pyogenes M9141204
S. aureus		UVB lamp
Main wavelength 306 nm
Energy intensity
0.412 mW/cm2		20 min Survival ratio
JRS4 6%
S. saprophyticus 0.5%
S. pyogenes M9022434 4%
S. pyogenes M9141204 26%
S. aureus 7%		 [41]
PeanutHetero Agglomeration
P25		20 nm79 m2/g MBLED lamp: 365 nm (40 mW/cm2)0.055/min5 cycles[26]
PeanutHetero Agglomeration
P25		25 nm86.1 m2/g17.43 emu/gParaquatUV lamp 2 × 18 W
Main wavelength 254 nm		 [35]
Table 2. Photocatalytic performance comparison table for submillimeter scale TiO2-polystyrene beads photocatalysts.
Table 2. Photocatalytic performance comparison table for submillimeter scale TiO2-polystyrene beads photocatalysts.
Floating vs. SuspendedBead Size and DensityTiO2 LoadingTarget PollutantsLight SourceStirring or no StirringDegradation PerformanceRecyclabilityReference
Floating
EPS		5–12 nm MBUV light
wave length 254 nm		stirring9.05 × 10−3/min [50]
Suspended290 nm MBUV lamp (365 nm) with (150 mW cm−2)stirring3.67%/min [51]
Suspended615 μm
Density 0.62 gm/cm3		10:1 volume ratio of TiO2 to polystyreneMBLow-pressure mercury lamp 30 W
Main wavelength 254 nm		stirring0.3 μmol/min10[52]
Floating2–4 mm18%MBHg lamp
Main wavelength 254 nm 15 W		no stirring10 × 10−5 g/L min5[48]
Suspended500–800 μm3%Escherichia coliUV-A lamp
Wave length 365 nm		stirring89% of E. coli after 60 min exposure5[49]
Suspended200 nm MBLED light 216 W
Wave length 365 nm		stirring0.11/min3[28]
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Kiguchi, M.; Hanada, N. Suspension Type TiO2 Photocatalysts for Water Treatment: Magnetic TiO2/SiO2/Fe3O4 Nanoparticles and Submillimeter TiO2-Polystyrene Beads. ChemEngineering 2026, 10, 3. https://doi.org/10.3390/chemengineering10010003

AMA Style

Kiguchi M, Hanada N. Suspension Type TiO2 Photocatalysts for Water Treatment: Magnetic TiO2/SiO2/Fe3O4 Nanoparticles and Submillimeter TiO2-Polystyrene Beads. ChemEngineering. 2026; 10(1):3. https://doi.org/10.3390/chemengineering10010003

Chicago/Turabian Style

Kiguchi, Manabu, and Nobuhiro Hanada. 2026. "Suspension Type TiO2 Photocatalysts for Water Treatment: Magnetic TiO2/SiO2/Fe3O4 Nanoparticles and Submillimeter TiO2-Polystyrene Beads" ChemEngineering 10, no. 1: 3. https://doi.org/10.3390/chemengineering10010003

APA Style

Kiguchi, M., & Hanada, N. (2026). Suspension Type TiO2 Photocatalysts for Water Treatment: Magnetic TiO2/SiO2/Fe3O4 Nanoparticles and Submillimeter TiO2-Polystyrene Beads. ChemEngineering, 10(1), 3. https://doi.org/10.3390/chemengineering10010003

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