Inactivation of Aspergillus Species and Degradation of Aflatoxins in Water Using Photocatalysis and Titanium Dioxide

Abstract

Aflatoxins (AF) are highly toxic secondary metabolites produced by various species of Aspergillus, posing significant health risks to humans and animals. The four most prominent types are aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1), and aflatoxin G2 (AFG2). These mycotoxins are prevalent in various environments, including water sources and food products. Among these mycotoxins, AFB1 is recognized as the most toxic, mutagenic, and carcinogenic to humans. Consequently, most efforts to mitigate the impact of AF have been focused on AFB1, with photocatalysis emerging as a promising solution. Recent research has demonstrated that using semiconductor photocatalysis, particularly titanium dioxide (TiO₂), combined with UV–visible irradiation significantly enhances the efficiency of AF degradation. TiO₂ is noted for its high activity under UV irradiation, non-toxicity, and excellent long-term stability, making it a favorable choice for photocatalytic applications. Furthermore, TiO₂ combined with visible light has demonstrated the ability to reduce AF contamination in food products. This article summarizes the working conditions and degradation rates achieved, as well as the advantages, limitations, and areas of opportunity of these methodologies for the degradation of AF and preventing their production, thereby enhancing food and water safety.

1. Introduction

Aflatoxins (AF) are toxic secondary metabolites produced by Aspergillus species, such as A. flavus and A. parasiticus [1]. These compounds have been shown to be mutagenic, teratogenic, genotoxic, and carcinogenic, causing severe health issues in humans, poultry, fish, and cattle with prolonged exposure [2]. Given their high toxicity, it is not advisable to establish a safe tolerance level or non-toxic dose for AF on an international scale; therefore, minimizing ingestion remains the most effective approach for ensuring safety [3]. While other mycotoxins exist, AF are the most prevalent and widespread [4]. Among the various identified AF, AFB1, AFB2, AFG1, and AFG2 are the most significant. According to the International Agency for Research on Cancer (IARC), AFB1 is the most toxic and mutagenic, categorizing it as a Group 1 human carcinogen [2]. Additionally, it is worth noting that AFB1 can withstand high temperatures, demonstrating thermostability up to 160 °C [5]. The chemical structures of AFB1, AFB2, AFG1, and AFG2 are illustrated in Figure 1.

Over 350 AF have been identified in fungi, and their distribution in agricultural products poses a significant global challenge [7]. It is estimated that 25% of food worldwide is contaminated by mycotoxins, with 60–80% of crops affected [8]. AF in crops can contaminate both groundwater and surface water, particularly through transportation via rainfall. Additionally, AF has been detected in wastewater from the food industry, fish farms, and water tanks, which poses a risk of contaminating drinking water and crops. As a result, it has been observed that individuals often consume water that has been polluted with AF [4]. Although no regulations exist regarding the maximum concentrations of AF in water, there are established limits for their presence in food products. For instance, the European Union established a maximum limit of 2 μg/kg in cereals and derivatives and a combined concentration of 4 μg/kg for AFB1, AFB2, AFG1, and AFG2. [9]. Mycotoxin contamination drives up costs for crops, such as corn, mostly due to necessities like increased testing, lower prices received for contaminated loads, potential consumer lawsuits, and reduced production of the livestock industry. In fact, AF represents the greatest threat to the US corn industry [10].

Table 1. Economic and health effects of AF.

Aflatoxin Type	Economic and Health Effects	Reference
AF in general	In Africa, the annual cost exceeds USD 750 million, while food exporters in the European Union incur about USD 670 million each year.	[11]
AF in general	Aflatoxin concerns cost US maize farmers USD 160 million annually.	[7]
AF in general	Climate change-related AF contamination could cost the US corn industry USD 52.1 million to USD 1.68 billion annually.	[10]
AFB1	Consuming 20–120 μg/kg of AFB1 per day for one to three weeks can cause acute toxicity and may be lethal.	[12]
AFB1	Teratogenic effects were observed in rabbits when administrated AFB1 orally during gestation days 6–18 at a dose of 100 μg/kg.	[13]
AFB1	Regardless of the dose, AF increases cancer risk by intercalating into DNA and alkylating bases through an epoxide moiety, leading to p53 gene mutations.	[14]
AF in general	The liver is the primary organ affected by AF, which are metabolized into highly toxic forms that cause liver damage through various mechanisms.	[3]
AFB1, AFM1	Renal damage is caused by AFB1 (500 μg/kg), AFM1 (3500 μg/kg), and the combination of AFB1 with AFM1 through the activation of oxidative stress.	[15]
AF in general	Aflatoxicosis may occur if AF consumption reaches 1 mg/kg or higher.	[12]
AF in general	Immune alterations can occur when consumption exceeds 0.9068 pmol mg−1, leading to changes in the innate immune system.	[12]

summarizes the health and economic effects of aflatoxins.

To address the issue of AF pollution, various methods have been proposed for its degradation. These methods include gamma and UV irradiation, the use of oxidoreductases, and ozone treatment. Additionally, the effective use of materials such as minerals, polyvinyl alcohol, durian peel, synthetic polymers, and activated carbon could be used as membranes in pipes to adsorb AF in water flows [4].

Table 2. Common methodologies used for the degradation of AF.

Method|Aflatoxin Type|Sample Type	Working Conditions	Percentage of Degradation| Elimination|Detoxification|Toxicity Assays	Reference
UV-A LED irradiation (physical)|AFB1 and AFM1|Aqueous solution	365 nm, 4 °C, 1200 mJ/cm2, 156 s, initial concentrations of 1 µg/mL and 2 µg/mL for AFB1 and AFM1	70%|Assays conducted with HepG2 cells|No significant cytotoxicity	[16]
Co60 gamma ray (physical) |AFB1|Ethanol–water solution	Initial concentration at 20 mg/L, with a dose rate of 0.31 Gy s−1, measured at room temperature	Toxicity analyses of radiolytic products were based on the quantitative structure–activity relationship. A double bond equivalent (DBE) of 70% for the radiolytic products was lower than that of AFB1.	[17]
Ozonolysis (physical)|AFB1	Ozone solutions at a concentration of 20 mg/L, maintained at temperature range of 19.5–20.5 °C	Toxicity analysis conducted by DBE revealed low values, specifically between 6.5 and 12.5.	[18]
Electron beam (physical)|AFB1 |Aqueous solution	5 MeV, 2 kGy/s, 4 °C, and an initial concentration of 5 ppm	100%|In assays conducted with HepG-2 cells, there was an observed decrease in cell viability of 13%.	[19]
Ultrasound (physical)|AFB1|Aqueous solution	20 kHz, 6.6 W/cm3, 25 °C, 80 min, and an initial concentration of 10 mg/L	85.1%|Toxicity analysis through an index of hydrogen deficiency (IHD) indicated that 75% of the AFB1 reaction products had values lower than 11.5.	[6]
Water-assisted microwave irradiation (WMI) (physical)|AFB1|Aqueous solution	500 W, 140 °C, 35 min, and an initial concentration of 0.1 mg/L	74%|Toxicity was determined by DBE, with values ranging from 6.5 to 11.5 considered low.	[20]
Degradation with lactic acid bacteria (LAB) Levilactobacillus spp. 2QB383 (biological)|AFB1|Potassium phosphate buffer and Aspergillus parasiticus NRRL 2999 cultures	Heat-killed LAB cultures were prepared by heating at 100 °C for 1 h. A 25 µL inoculum of each micro-organism was added to 200 µL yeast extract sucrose broth, resulting in a final volume of 250 µL. This setup achieved a final concentration of 5.0 log10 CFU/mL for the LAB and/or 5.0 log10 spores/mL for the fungus. The cultures were maintained under static conditions in the dark for a duration of 7 days.	100% inhibition of AFB1 production	[21]
Ozone as an oxidizing agent (chemical)|AFB1 and AFG1|Aqueous solution	AFB1 and AFG1 had initial concentrations of 2 ppb and 10 ppb, respectively. The temperature was 308.15 K, and an ozone concentration of 40 ppm caused the greatest degradation in both aflatoxins.	100 %|No toxicity assays were conducted.	[22]
Lactic acid–heat (chemical–physical)|AFB1|Aqueous solution	80 °C, 120 min, pH 3–4, and an initial concentration of 1 µg/mL	85%|Toxicity assays were conducted using HeLa cells, revealing a decrease in toxicity of the degradation products ranging from 1.2 to 15.4%	[23]
Plant extract from Ocimum basilicum|AFB1 and AFB2|Aqueous solution	30 °C, 72 h, pH 8. Initial concentrations of AFB1 and AFB2 were 100 and 50 µg/L, respectively.	90.4 and 88.6% for AFB1 and AFB2, respectively|Growth inhibition of aflatoxigenic isolates was recorded at 82–87% for AFB1 and AFB2, respectively|Toxicity bioassays using brine shrimps (Artemia salina) indicated a mortality rate of only 9.2–22.5%

provides a summary of the most common methods for degrading AF and the levels of degradation achieved.

Furthermore, photocatalysis employing semiconductors like ZnO, Fe₂O₃, C₃N₄, bismuth-based semiconductors, and TiO₂ has demonstrated significant potential in degrading a wide array of pollutants in water. This process ultimately converts these pollutants into less toxic compounds that can be further mineralized into harmless CO₂ and H₂O [24]. Examples of the types of pollutants that TiO₂-based photocatalytic methods have successfully degraded include dyes, petroleum hydrocarbons, phenolic compounds, antibiotics, anti-inflammatories, lipid regulators, and pesticides. In addition, these methods have led to the inactivation of microorganisms and the removal of heavy metals [24].

The photocatalysis process involves activating a photocatalyst by exposing it to light, which triggers a chemical reaction. This activation results in the formation of electron-hole e⁻ and h⁺ pairs, leading to the generation of free radicals essential for the reaction to take place. When light energy surpasses the band gap, an electron in the valence band is promoted to the conduction band in the semiconductor [25]. A visual representation of this process can be observed in Figure 2, which offers a concise overview of the general photocatalytic mechanism.

1.1. Photocatalysis Combined with TiO₂ Is Used for the Degradation of AF

Photocatalysis has emerged as an advanced oxidation technique that is eco-friendly, cost-effective, and non-detrimental for wastewater treatment and environmental remediation [26]. Recent research indicates that combining UV–visible irradiation and semiconducting photocatalysts can significantly improve the efficiency of degrading AF in a liquid matrix. It has been demonstrated that the photocatalysis process can effectively degrade AFB1 [27]. During this process, the photo-generated valence band holes (h⁺), hydroxyl radicals (^•OH), and superoxide radicals (O₂^•−) directly oxidize the AF. Unfortunately, TiO₂ reported in the literature is UV-active due to its larger band gap, but solar light spectra consist of only approx. 5% UV and 45% visible light radiation [26]. Therefore, coupling TiO₂ with narrow bandgap materials that respond to visible light is essential for creating an effective nanocomposite for photocatalysis. The combined use of these materials can create a synergic effect, enhancing visible light response and improving charge separation, thus increasing photocatalytic activity [28]. By utilizing these materials, the significant limitation of TiO₂, namely, its low quantum efficiency under solar radiation, can be successfully addressed [29].

1.2. Advantages of the Use of TiO₂ as a Photocatalyst for AF Degradation

The photocatalytic oxidation process can be initiated at normal pressure and temperature by highly reactive oxidative species, particularly hydroxyl radicals. Among the various semiconductors utilized for environmental remediation, TiO₂ stands out as a significant photocatalyst due to its excellent activity and resistance to decomposition [30]. Titanium dioxide (TiO₂) is commonly employed as a photocatalyst due to several key features. It demonstrates high activity when exposed to UV light, is non-toxic and highly efficient, and exhibits long-term photostability [31]. Research conducted by Sun et al. indicated that the degradation of AFB1 can be effectively described by a pseudo-first-order kinetic model. They also discovered that the catalyst can be easily separated from the solution while retaining good activity. In their findings, hole (h⁺) and hydroxyl radicals (^•OH) were identified as critical contributors to the degradation of AFB1. The authors concluded that AC/TiO₂ exhibits a synergy of high absorption capacity and photoactivity, providing a simple, efficient, and environmentally friendly method for AFB1 degradation [27]. In addition, TiO₂ possesses several significant advantages, including a high refractive index, a bandgap of 3.0–3.2 eV, high chemical and photostability, and, notably, low cost [32]. Furthermore, it has been shown that using TiO₂ in conjunction with visible light can effectively reduce contamination levels of Aspergillus flavus and inhibit AF production in food products like peanuts [33]. Yang et al. also proposed an inhibition mechanism in which reactive species like O₂^•−, ^•OH, h⁺, and e⁻ generated from photoreactions damage cell structures, leading to decreased viability of Aspergillus flavus spores [33]. Additionally, TiO₂-based photocatalysts can be reused [27], enhancing the sustainability of this technology. Another advantage of TiO₂ is that it can be synthesized through eco-friendly processes. For example, Alluqmani et al. produced TiO₂/carbon nanoparticles derived from oil fly ash (TiO₂/COFA), which not only improved light conversion and photocatalytic efficiency in the visible range but also has potential applications as solar energy materials [34].

1.3. Recent Findings on the Use of Photocatalysis and TiO₂ to Degrade AF in Different Matrices

The potential of TiO₂ and photocatalysis for degrading AF has been demonstrated in several studies. For instance, a glass tube coated with TiO₂ and exposed to UV light was used to detoxify AFB1 in contaminated peanut oil, resulting in a 60.4% removal of AFB1 within 120 min. This result significantly surpasses the degradation achieved through UV photolysis alone, which only resulted in 35.1% removal [35]. Moreover, a magnetic graphene oxide/TiO₂ (MGO/TiO₂) nanocomposite was utilized to effectively reduce AFB1 in corn oil, attaining a remarkable 96.4% reduction after 120 min of UV–vis light exposure [36]. In another study, Magzoub et al. successfully removed AFB1 and AFB2 from Sudanese peanut oil, achieving reductions of ≥99.4 and ≥99.2%, respectively, within just 4 min of UV–vis treatment. Notably, this process did not significantly alter the oil’s physicochemical characteristics, such as its fatty acid composition, free fatty acid content, peroxide value, saponification value, acid value, iodine value, moisture, volatile matters, and refractive index [37]. Furthermore, Xu et al. proposed using an iodine-doped supported TiO₂ thin film combined with UV light for detoxifying AFB1 in peanut oil, achieving a degradation rate of 81.96% [38].

1.4. Using Photocatalysis and TiO₂ to Eliminate Aspergillus sp. And AF in Water

The efficiency of the photocatalytic degradation process of AFB1 using TiO₂ is notably affected by several factors, including irradiation time, initial levels of AFB1, the concentration of catalysts, and pH [27,39]. TiO₂ can be utilized both as a standalone catalyst and in conjunction with other materials. Table 3 provides a summary of TiO₂-based photocatalysts, detailing the percentage of degradation achieved, the duration of the process, and the specific working conditions employed in these studies.

1.4.1. TiO₂ Nanoparticles

In a study conducted by Babaei et al., TiO₂ nanoparticles combined with UV light were employed to eliminate Staphylococcus aureus, Escherichia coli, and A. flavus. The TiO₂ nanoparticles used in the research exhibited a spherical shape with an average diameter of approximately 30 nm. The researchers prepared a suspension of TiO₂ nanoparticles by weighing 5 g of the nanoparticle powder and transferring it into a 250 mL Erlenmeyer flask, diluting it to a total volume of 100 mL, and then autoclaving it at 121 °C for 15 min. They subsequently created three different concentrations (0.1, 0.5, and 1 g L⁻¹) from the aqueous stock solution. Following a 60 min treatment with a 1 g L⁻¹ concentration on suspensions of 1.5 × 10⁸ UFC, the concentrations of all the strains were recorded as <50 UFC. While the precise mechanism of action is not entirely understood, the authors hypothesized that the nanoparticles interacted directly with microbial cells or indirectly caused DNA damage in these microorganisms. The study assessed various factors, including the presence or absence of TiO₂, the presence or absence of irradiation, the TiO₂ concentration, and the irradiation time. Notably, it was determined that the TiO₂ concentration and irradiation time were the most significant factors: as both the TiO₂ concentration and irradiation duration time increased, there was a marked decrease in the population of A. flavus. The optimal results were achieved with the longest irradiation time of 60 min. This could be attributed to the composition of A. flavus’ cell wall, which consists of long carbohydrate layers, polysaccharides, glycoproteins, and lipids that confer a degree of resistance. However, this resistance can be mitigated by the generation of oxidative agents during the photocatalytic process [40]. The authors attributed the decrease in viability and degradation to the formation of hydroxyl radicals [40].

1.4.2. Activated Carbon and TiO₂

Photocatalysis utilizing TiO₂ has shown significant effectiveness in degrading AF and Aspergillus cells. For instance, research conducted by Sun et al. demonstrated the degradation of AFB1 using an activated-carbon-supported TiO₂ catalyst (AC/TiO₂) under UV–vis light. The TiO₂ particles exhibited sizes ranging from 0.5 to 4 μm, attributed to partial agglomeration, and were nearly uniformly distributed on the activated carbon surface. The material showed planes of anatase type, and no peaks attributed to rutile type. The synthesis of AC/TiO₂ involved a straightforward hydrothermal method, including the purification of activated carbon through treatment with 1 mol L⁻¹ of HNO₃ for 2 h, which facilitated a milling process to attain a grain size between 0.2 and 0.3 mm. Following this, the activated carbon was dried for 3 h at 95 °C for further use. The preparation protocol for AC/TiO₂ was comprised of mixing 5 mL of butyl titanite with 50 mL of distilled water, which was stirred for 1 h. Subsequently, 2.5 g of activated carbon were added, and the mixture was stirred for an additional 30 min, ensuring the pH was adjusted to 6. This homogeneous mixture was then sealed in a 100 mL container. The authors systematically evaluated the effects of various factors, including the initial catalyst concentration, light source (UV or UV–vis), and irradiation duration on the degradation process. Modeling the degradation kinetics of AFB1 through the pseudo-first-order kinetic model revealed that the optimal degradation rate (95%) occurred at pH 7 under prolonged irradiation (120 min) using UV–vis light in the presence of AC/TiO₂, particularly at the highest catalyst concentration (6 mg mL⁻¹). This was notably more effective than using UV–vis irradiation alone, which achieved only 50% degradation [27]. In elucidating the mechanism of action, the study highlighted the significant role of the holes (h⁺) and hydroxyl radical (OH) in the degradation process, while the superoxide radical (O₂^•−) exhibited minimal impact. The enhanced degradation efficiency of AFB1 (98%) with the AC/TiO2 composite, compared to the lower efficiency observed with TiO₂ alone (76%), was attributed to the higher surface area and increased visible light intensity [27].

1.4.3. Biochar and TiO₂

In a study conducted by Zhang et al. [41], soybean dreg-based Biochar@TiO₂ composites were prepared using hydrothermal synthesis. They evaluated several factors affecting degradation, including different photocatalysts, the photocatalytic dosage, light sources, initial AFB1 concentration, and pH levels. For the synthesis, 2 mL of butyl titanate solution were added to 22 mL of isopropyl alcohol while stirring magnetically for 30 min. Then, 10 mL of distilled water were added to form a white suspension, which was magnetically stirred for an additional 10 min. Following this, the prepared SDB-6-K-9 was added in varying mass ratios under ultrasonic exposure. The mixture was then transferred to a hydrothermal reactor and maintained at 180 °C for 18 h in an oven. After the reaction, the resulting precipitate was rinsed with ethanol and deionized water until the filtrate was nearly neutral. It was then dried at 105 °C for 24 h and ground using a 120-mesh sieve for later use. The composite exhibited an irregular spherical granular shape and was uniformly supported on the surface of SDB-6-K-9. The TiO₂ particles showed good dispersibility and maintained their shape. An energy-dispersive spectroscopy (EDS) analysis revealed that the dominant elements in TiO₂ were Ti and O, confirming the successful loading of TiO₂ onto the SDB-6-K-9 surface. X-ray diffraction (XRD) patterns of the TiO₂ and SDB-6-K-9@TiO₂ composites displayed crystal planes characteristic of anatase-type TiO₂. The resulting photocatalyst demonstrated enhanced efficiency in reducing AFB₁, with the holes playing a crucial role in the degradation process. It exhibited excellent performance, high stability, and reusability, even after five cycles of photocatalytic experiments (J. Zhang et al., 2024 [41]). The working conditions are detailed in Table 3.

1.4.4. Composite and TiO₂

In a study by Zhang et al., a TiO₂/UiO-67 photocatalyst was used in conjunction with visible light. The catalyst was analyzed by X-ray diffraction and showed crystal planes of anatase TiO₂. In addition, according to the high-resolution transmission electron microscopy, TiO₂ had a thin nanosheet morphology, and a small octahedral nanocrystal was found for UiO-67, a commercially available Zr-metal–organic frameworks (MOFs) used for its large surface area, good photocatalysis activity, and lower band gap (3.14 eV), and had better light response than TiO₂. The preparation of UiO-67 involved dispersing 270 mg biphenyl-4,4-dicarboxylic acid (the structure is shown in Figure 3) and 201 mg of zirconium (IV) chloride in 45 mL of N, N-Dimethylformamide (DMF), followed by the addition of 4.2 mL of ethanoic acid.

The resulting mixture was subjected to continuous stirring for 10 min, then autoclaved at 120 °C for 24 h. Afterward, the samples were washed with DMF and treated at 120 °C for 12 h to remove any remaining DMF in the pores of UiO-67. The TiO₂/UiO-67 heterojunctions were prepared by mixing 0.2 mL tetrabutyl titanate, 30 mL ethanol, and 3 mL acetic acid to form a transparent solution. Different mass ratios of UiO-67 were then added to the solution and subjected to stirring for 30 min. The resulting mixture was autoclaved at 220 °C for 4 h, and the precipitates were collected by centrifugation, washed with ethanol, and dried at 60 °C for 12 h. The effects of factors like the absence or presence of visible light and irradiation time were evaluated in the study. The authors reported a remarkable 98.9% degradation rate of AFB1 in 80 min using the TiO₂/UiO-67 photocatalyst, which outperformed commercial P25, commercial TiO₂, and most reported photocatalysts under visible light irradiation. Furthermore, the TiO₂/UiO-67 photocatalyst exhibited excellent recyclability, highlighting its potential for preventing the production of AF. Additionally, the authors found that superoxide radicals (CO₂), holes (h⁺), and hydroxyl radicals (−OH) were the main active species and that oxidized AFB1 to small molecules [42]. The mechanism of action was described by the authors as follows [42]:

TiO₂/UiO-67 + hv --> TiO₂ (h⁺) + UiO-67(e⁻)

(1)

H₂O/(OH⁻) + TiO₂ (h⁺) --> •OH

(2)

O₂ + UiO-67(e⁻) --> •O₂⁻

(3)

•O₂⁻ + e⁻ + 2H₂O(H⁺) --> H₂O₂

(4)

H₂O₂ + e⁻ --> •OH + OH⁻

(5)

•O₂⁻ + h⁺ + •OH + AFB1 --> small molecules

(6)

1.4.5. Metals and TiO₂

Yang et al. utilized Ag-loaded TiO₂ and visible light to degrade AFB1, AFB2, and AFG2. The Ag/TiO₂ was synthesized using a photo deposition method, in which 625 mg TiO₂ powder were dispersed in 200 mL of a methanol aqueous solution (10%) following 30 min of sonication. Subsequently, 868 µL of a AgNO₃ solution (0.1 mol L⁻¹) were added drop by drop. After vigorous stirring for 1 h, the suspension liquid was exposed to an ultraviolet halogen lamp (365–405 nm) for 1 h. Then, the precipitate was centrifuged, washed thrice with ethanol and water, and dried at 80 °C for 18 h. To assess the impact of silver concentrations on the photocatalytic inhibition performance, they prepared a range of Ag/TiO₂ samples with varying silver concentrations, labeled 0.5%, 1%, 1.5%, 2%, and 2.5% Ag/TiO₂. The catalyst was characterized and showed characteristic diffraction peaks of anatase, and rutile were observed, and the catalyst had a crystal plane of Ag. Factors like the absence or presence of visible light, the absence or presence of catalyst, and the concentration of Ag were evaluated. The authors achieved a decrease of 96.02 ± 0.19%, 92.50 ± 0.45%, and 89.81 ± 0.52%, respectively, and an inhibition higher than 90% against A. flavus under visible light for 15 min, with a silver concentration of 1.5%. In higher concentrations of Ag, the inhibition decreased, maybe because the high concentrations of Ag nanoparticles had a masking effect on the surface of the TiO₂. The authors proposed a possible mechanism of action for inhibiting spores. When the Ag/TiO₂ was irradiated by visible light, the deposition of Ag on TiO₂ enhanced light absorption through the surface plasmon resonance (SPR) effect [20]. This resulted in the generation of hot electrons, which were then transferred to the conduction band of TiO₂, creating additional Fermi levels and reducing the band gap energy near the conduction band [33]. Additionally, the intense interfacial contact between anatase, rutile, and Ag led to a more positive conduction band potential of anatase, compared with rutile. The hot electrons were primarily directed toward the conduction band of rutile before flowing into the conduction band of anatase. This prolonged the lifetime of carriers improved the separation and transfer abilities of electron-hole pairs and enhanced the photocatalytic activity. Moreover, O₂ was reduced to •O²⁻ by e⁻, while h⁺ oxidized H₂O/OH⁻ to •OH. These active species attacked the spore cell membranes, causing damage to cell structures and reducing spore viability. In this process, •O²⁻ was the main active species for inhibiting A. flavus [33].

On the other hand, the research conducted by Huang et al. involved the use of Pd-C-TiO₂ as a catalyst in combination with visible light. The composites were prepared using an isobutyl alcohol solution containing 0.1 N hydrochloric acid and 0.1 N sodium hydroxide. Various amounts of TiO₂ composite (0.05–1.0 wt%) were added to the solution and stirred until evenly dispersed. Subsequently, poly (methyl methacrylate) film (PMMA) served as the substrate for the deposition of the photocatalyst. In the study, factors like the initial A. niger concentration, catalyst concentration, and visible light intensity were evaluated. The antibacterial testing materials were prepared by coating different photocatalysts onto the PMMA film through a sol–gel process in an aqueous solution using an immersion deep coater device. After aging the sol for 24 h, it was dried at 378 K for 24 h to evaporate the solvent, resulting in a residual aerogel crushed into a fine powder. The prepared immersion was loaded into a dip coater device, which was utilized to create a template of PMMA composites, using liquid-phase deposition to deposit a tightly packed layer of TiO₂ composites onto the template. The catalyst was characterized, and its structure showed the crystallite contents of TiO₂ composites after doping, including anatase, rutile, and brookite, with a particle size of 17 nm and a pore volume of 0.15 cm³/g. As a result, the authors achieved 100% A. niger mortality in 96 h at pH 3.97. The authors attributed the inhibition effect to the reactive oxygen species and the energy band matching and P-N junction, which can decrease the recombination rate of electron-hole pairs and increase the quantum yield. Additionally, it is possible that the relatively higher specific surface area of Pd-C-TiO₂ could enhance light-absorption capacity compared to other materials [39].

Table 3. Methods for the degradation of AF using photocatalysis combined with TiO₂ and relevant working conditions.

Aflatoxin Type|Sample Type	TiO2-Based Photocatalyst	Percent of Degradation|Time	Light Source	Reference
AFB1|Aqueous solution	20 mL of AFB1 solution at a concentration of 1 μg/mL and 6 mg of AC/TiO2 irradiated with UV–vis light	95%|120 min at pH 7	High-pressure mercury lamp (130 w, 350–450 nm wavelengths)	[27]
AFB1|Aqueous solution	TiO2/UiO-67 under visible light, with 10 mg photocatalyst and 100 mL of an AFB1 aqueous solution at a concentration of 0.5 mg/mL	98.9%|80 min	300 W high-pressure xenon lamp with a UV-cutoff filter	[42]
AFB1|Aqueous solution	6 mg of 4% SDB-6-K-9@TiO2 in a 100 µg/mL AFB1 solution, pH 7, under simulated sunlight	95%|180 min	Simulated sunlight (300–1000 nm)	[41]
Aqueous suspension of A. flavus spores	25 mg of Ag/TiO2 powder suspended in 50 mL of spore suspension containing 106 CFU/mL	>90%|15 min	300 W Xenon lamp (PLS SXE300, Beijing Perfectlight Inc., Beijing, China) with a visible light filter (λ > 420 nm)	[33]
Aqueous suspension of A. niger spores	Pd-C-TiO2 was tested under visible light using an A. niger suspension containing 105 spores/mL and a catalyst concentration of 1%.	100% fungal mortality|96 h, pH 3.97 in a batch-type reactor	7.25 mW cm−2; 9 tubes of T5 fluorescent lamps (FL8D-EX; OA lighting Kanjin Supply Co., Ltd., Taoyuan City, Taiwan) with a wavelength-cutting filter underneath the lamps at 0.5 cm were used. Radiation wavelength below 410 nm entirely cutting	[39]
Aqueous cultures of S. aureus and E. coli|Aqueous suspension of A. flavus spores	A concentration of 1 g/L of TiO2 nanoparticles was applied to strain suspensions containing 1.5 × 108 UFC/mL under UV light	Decrease in culture population from 1.5 × 108 to less than 50 UFC/mL within 60 min	UV lamp (details not specified)	[40]

Table 3. Methods for the degradation of AF using photocatalysis combined with TiO₂ and relevant working conditions.

Aflatoxin Type|Sample Type	TiO2-Based Photocatalyst	Percent of Degradation|Time	Light Source	Reference
AFB1|Aqueous solution	20 mL of AFB1 solution at a concentration of 1 μg/mL and 6 mg of AC/TiO2 irradiated with UV–vis light	95%|120 min at pH 7	High-pressure mercury lamp (130 w, 350–450 nm wavelengths)	[27]
AFB1|Aqueous solution	TiO2/UiO-67 under visible light, with 10 mg photocatalyst and 100 mL of an AFB1 aqueous solution at a concentration of 0.5 mg/mL	98.9%|80 min	300 W high-pressure xenon lamp with a UV-cutoff filter	[42]
AFB1|Aqueous solution	6 mg of 4% SDB-6-K-9@TiO2 in a 100 µg/mL AFB1 solution, pH 7, under simulated sunlight	95%|180 min	Simulated sunlight (300–1000 nm)	[41]
Aqueous suspension of A. flavus spores	25 mg of Ag/TiO2 powder suspended in 50 mL of spore suspension containing 106 CFU/mL	>90%|15 min	300 W Xenon lamp (PLS SXE300, Beijing Perfectlight Inc., Beijing, China) with a visible light filter (λ > 420 nm)	[33]
Aqueous suspension of A. niger spores	Pd-C-TiO2 was tested under visible light using an A. niger suspension containing 105 spores/mL and a catalyst concentration of 1%.	100% fungal mortality|96 h, pH 3.97 in a batch-type reactor	7.25 mW cm−2; 9 tubes of T5 fluorescent lamps (FL8D-EX; OA lighting Kanjin Supply Co., Ltd., Taoyuan City, Taiwan) with a wavelength-cutting filter underneath the lamps at 0.5 cm were used. Radiation wavelength below 410 nm entirely cutting	[39]
Aqueous cultures of S. aureus and E. coli|Aqueous suspension of A. flavus spores	A concentration of 1 g/L of TiO2 nanoparticles was applied to strain suspensions containing 1.5 × 108 UFC/mL under UV light	Decrease in culture population from 1.5 × 108 to less than 50 UFC/mL within 60 min	UV lamp (details not specified)	[40]

1.5. Comparison of the Performance of TiO₂ Photocatalyst and Other Methods for the Degradation of Aflatoxins in Water

Table 3 shows that, with the exception of the work by Huang et al. [39], most photocatalytic processes were conducted over short durations (ranging from 15 to 120 min), achieving degradation percentages between 90 and 100%. In contrast, other technologies, particularly biological degradation [21], while also reaching 100% degradation, typically require several days. Furthermore, Table 2 indicates that the TiO₂ photocatalysts demonstrated greater efficiency compared to Co⁶⁰ gamma rays [17], UV-A LED irradiation [16], ultrasound [6], water-assisted microwave irradiation [20], and lactic acid–heat methods [23]. None of these alternative methodologies exceeded a degradation rate above 85.1%; their degradation rates ranged from 70% to 85.1%, although their degradation times were comparable to those of the TiO₂ photocatalysts.

Only electron beam [19] and ozone-based technologies achieved a 100% degradation rate. However, these technologies cannot be directly compared to TiO₂-based photocatalysts, as their reaction times were not specified. The superior efficiency of the TiO₂-based photocatalysts may stem from the synergic effects of UV/UV–visible light and the formation of free radicals [25]. Another advantage of TiO₂-based photocatalysts is their ability to degrade AF concentrations as high as 10 g L⁻¹ (see Table 3), while the other techniques are limited to degrading concentrations up to 0.04 g L⁻¹ (see Table 2).

2. Final Considerations

It is essential to highlight that in degradation processes, ensuring that the resulting products or by-products are not more toxic than the original pollutant is crucial. Despite the high degradation rates reported in the studies of AF degradation, none of them mentioned conducting a toxicity study [27,42]. Assessing toxicity can be achieved through bioassays. A commonly used and cost-effective bioassay for analyzing toxicity in effluents involves using Daphnia magna [43]. Bioassays with Lepidium sativum, Eisenia fetida, and Vıbrio fischeri can also be conducted for this purpose. Additionally, toxicity can be estimated indirectly by analyzing the chemical structure, utilizing techniques such as chromatography–mass spectrometry data, and examining information about degradation products, including their retention times, proposed formulae, accurate experimental masses, double bond equivalents, and mass errors [20]. It is also essential to conduct assays with actual wastewater samples, as the proposed techniques were primarily tested on synthetic samples, while real environmental samples often exhibit very complex matrices [44]. Moreover, more tests should be carried out to evaluate the impact of different pH levels. Not all studies have assessed the effect of pH; however, the findings of Sun et al. suggest a stronger adsorption of AFB1 on the photocatalyst surface in weak alkaline environments [27]. Furthermore, there is a lack of understanding regarding the impact of temperature in the studies mentioned. Authors like Chen et al. found that, within a reaction temperature range of 0–50 °C, the activity of TiO₂ and Pd/TiO₂ increased with rising temperatures. However, when the temperature reached 70 °C, the reaction rate of TiO₂ slightly declined, and Pd/TiO₂ became less effective. In contrast, photocatalysts like Cu/TiO₂ exhibited greater activity at room temperature compared to other temperatures. This indicates that photocatalytic activity is influenced by both reaction temperature and the type of co-catalyst [45]. Combining photocatalysis with other technologies can enhance degradation performance. Treatment trains for wastewater are often employed due to the complex combinations of pollutants, some of which are challenging to degrade. Therefore, it is necessary to apply various technologies [46,47]. The integration of photocatalysts with adsorbent materials has shown promise in improving the degradation of different pollutants. For instance, various biochar-based photocatalysts containing Zn, Bi, Fe, and C₃N₄ have been utilized for the removal of Cr (III) and Cr (VI) [48]. Additionally, Zr-based materials, like UiO (UiO-66(Zr) and NH₂-UiO-66(Zr)), have been effective, achieving rapid adsorption of polycyclic aromatic hydrocarbons (PHAs) within a short period (30 min) [49]. On the other hand, activated carbon (AC) has been shown to adsorb pollutants on the catalyst’s surface, enhancing degradation. For example, in the study by Sun et al., an AC/TiO₂ photocatalyst was able to degrade 95% of AFB1 in a solution [27]. In this context, TiO₂-based photocatalysts could represent an effective option for degrading AF in water, provided that more efforts are directed toward large-scale production. All the reported degradation studies were conducted on a laboratory scale. Once these processes are scaled up, they could be applicable in treating industrial wastewater and irrigation water, thereby recycling water and preventing AF contamination of crops. Finally, modified TiO₂ has various applications, not only in the degradation of pollutants for wastewater treatment but also in hydrogen production, CO₂ reduction, antibacterial activity [26], and even in solar energy production [34].

3. Conclusions

The use of photocatalysis with TiO₂-based processes has shown great potential for degrading AF in various matrices, such as water and oils, with AFB1 being the main target of these studies. The process involves the generation of free radicals through the activation of the photocatalyst by light exposure, leading to the degradation of AF into less toxic molecules. The initial amount of catalyst, the light source, and the irradiation time had more impact on the degradation efficiency. TiO₂ exhibits high efficiency, stability, and cost-effectiveness, making it a promising option for AF degradation and Aspergillus sp. elimination. One of the main challenges associated with TiO₂ as a photocatalyst is its low quantum efficiency under solar radiation. However, this limitation can be addressed through modifications with other materials. Recent studies have demonstrated a significant reduction in AF levels when TiO₂ was combined with other materials and components, highlighting its potential for practical food and water safety applications, as well as other fields, such as hydrogen production, CO₂ reduction, antibacterial activity, and solar energy generation. Photocatalysis with TiO₂-based materials present advantages over alternative technologies, including shorter times and higher degradation percentages. Further research and development in this area could lead to the industrial implementation of TiO₂ photocatalysis as an effective method for AF elimination in real-world applications. Additionally, the use of TiO₂-based photocatalysts in conjunction with other technologies in treatment trains could enhance their effectiveness through increased adsorption capacity, resulting in synergistic effects. Finally, it is essential to conduct studies assessing the toxicity of the degradation products generated by TiO₂-based photocatalysts, as well as experiments utilizing real wastewater samples.