Challenges and Prospects of TiO₂-Based Photocatalysis for Wastewater Treatment: Keyword Analysis

Abstract

Environmental pollution driven by socioeconomic development has intensified the need for advanced and sustainable wastewater treatment technologies. Herein, TiO₂-based photocatalysis emerged as a promising solution due to its oxidative potential, chemical stability, and eco-friendliness but does have unavoidable immobilized recoverability challenges. Therefore, this study explored the challenges and prospects of TiO₂-based photocatalysis for the degradation of emerging contaminants in wastewater. A comprehensive keyword analysis was conducted by using a decade of publications retrieved from Google Scholar, Scopus, and Web of Science (WOS) databases via Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework. From a pool of 518 refined publications, 318 significant keyword occurrences related to TiO₂-based photocatalysis advanced oxidation processes (AOPs) were revealed. The review delved into various types of AOP mechanisms and catalysts and highlighted the synergistic effect of process parameters and magnetization as recoverability potential for TiO₂-based photocatalysts. Furthermore, emerging strategies including surface modifications, doping, and hybrid AOP integrations were discussed to improve photocatalysis performance and industrial scalability. The study underscores the economic opportunity and environmental sustainability of degrading persistent organic pollutants by integrating a TiO₂-based photocatalytic system with a regenerative magnetic field into the water sector.

1. Introduction

The rapid expansion of urbanization and industrialization has produced substantial amounts of organic wastewater, which poses significant and remarkable threats to the environment and human health [1,2]. Notwithstanding the social, economic, and environmental effects of poor sanitation, good water quality is essential for the well-being of all [3]. It is projected that 10 to 20 million individuals die each year due to waterborne and nonfatal contamination. Consistently, around 5000 to 6000 kids die owing to water-related problems of diarrhea [4]. About 0.78 billion individuals globally lack access to potable water resources, causing significant health issues that require drastic measures to meet water needs [5].

Advanced oxidation processes (AOPs) are widely regarded as the most effective technology to alleviate the issues mentioned above and treat high-organic wastewater [6,7]. AOPs have the benefits of a high mineralization efficiency, a rapid oxidation reaction rate, and no secondary contamination [8]. AOPs include hydrogen peroxide (H₂O₂), ozone (O₃), Fenton, TiO₂, and an ultraviolet radiation agent, which generate highly active radicals that can completely degrade organic pollutants [9,10]. These free radicals have high reactivity and redox capability to react non-selectively with the organic compounds in water [11]. Photocatalysis involving oxidation and reduction reactions occurs under a light source and semiconductor-based nano-photocatalysts, including metallic, non-metallic, and nanomaterials. Titanium dioxide (TiO₂), zinc oxide (ZnO), tin oxide (SnO₂), tungsten oxide (WO₃), cadmium sulfide (CdS), and carbon nitride (C₃N₄) are among the common semiconductor-based nano-photocatalysts that have been studied and applied [12,13,14,15,16]. TiO₂ nanoparticles are among the most promising and fastest growing photocatalysts for purifying water. The mechanism of non-toxicity semiconductor-based photocatalysts, like low-cost TiO₂, includes the production of highly responsive oxidants, like hydroxyl radicals (OH^●), for removing microorganisms like viruses, algae, fungi, bacteria, among others [17]. TiO₂ has been reported to decrease several waterborne microorganisms such as Pseudomonas aeruginosa, E. coli, fungi, protozoa, and other things after 8 h of simulated exposure to sunlight [14,18,19]. A total inactivation of fecal coliforms under solar irradiation is reported in research expressing the photocatalytic disinfection efficiency of TiO₂ [20].

Titanium dioxide (TiO₂) has been a photocatalyst since the 1970s because of its stable semiconductor properties for natural applications. It has been considered one of the most commonly reported nanomaterials found in consumer products (for example, clothing and food additives). A photocatalysis system happens when a light source meets a semiconductor like TiO₂, prompting or leading to the separation of electrons. Therefore, these electrons scatter on the photocatalyst’s surface, which intends to react with external substances, leading to oxidations and reductions [17,21]. The nano-sized particle of TiO₂, with its high surface area, makes it conceivable to achieve high response rates. Recent research shows the action of TiO₂ in degrading typical organic pollutants like methyl orange emanating from textile industries. The outcomes suggest that a synergetic impact between the intensity of the ultraviolet (UV) ratio and electrocatalyst advances the degradation of methyl orange by nearly 90% [22,23,24]. TiO₂ has additionally demonstrated the ability to adsorb phosphate ions from aqueous solutions within an hour [25].

Iron (III)-doped TiO₂ NPs can also degrade phenol under UV light radiation within 4 h [23]. Another advantage of using this material is that disinfection is commonly unnecessary since UV light helps eliminate microorganisms. However, the significant limitations of this technology are its lower photocatalytic impact within complex organic mixtures and the considerable power consumption needed for longer illumination times in large-scale applications [26,27]. Over the last ten (2013–2023) years, the use of TiO₂ in the fields of environmental management (wastewater treatment, water, and air) and energy production (water splitting) has gained considerable attention from researchers worldwide.

This study analyzed a comprehensive review based on the keywords “Advanced oxidation process” and “TiO₂ photocatalysis” to determine the emerging trends of scientific publications in water and wastewater treatment. This was carried out to reveal current research on AOPs, their mechanisms, and potential opportunities in the water field. Also, it will highlight the challenges and prospects of TiO₂ photocatalysis as a considerable technique for treating emerging contaminants from wastewater due to their prevalence, hazards, and potential risks to human health and the environment.

2. Literature Search Process and Eligibility Criteria

This study employed a well-known published protocol for systematic reviews, the Preferred Reporting Items for Systematic Reviews and meta-analyses (PRISMA). This was driven by investigating recent progress on TiO₂-based photocatalysis and advanced oxidation processes for wastewater treatment. For precise and accurate information gathering, the Google Scholar, Scopus, and Web of Science (WOS) databases were consulted for publications from 2013 to 30 November 2023. Figure 1 shows the publication trend retrieved from the three databases, whereby, upon screening, cumulative published documents of 13,773, 696, and 468, respectively, were obtained from Google Scholar, Scopus, and WOS. It was deduced that progressive interest in research in photocatalysis had gained attention within the decade.

Most importantly, between 2020 and 2022, there was a massive increase in publications focusing on addressing emerging contaminants in wastewater settings. The refined database result was obtained by considering the exclusion criteria such as (i) book series and book chapter, (ii) conference proceedings, (iii) non-English language, (iv) publications before 2013, (v) articles in the press, and (vi) articles not open access. The retrieved publications were merged using a common library format (EndNote). In the screening process, duplicated articles were excluded from the database to make them eligible. A database with 518 articles containing titles, abstracts, keywords, and full texts was created using EndNote reference management software Version 21. Some of the potential research areas identified included chemistry (159), environmental science (140), chemical engineering (66), water resources (57), energy (51), and other multidisciplinary (45) fields. The authorship results showed that India is the most affiliated country, with 83 publications linked to 30 countries. India was followed by China (79 publications with 28 linked countries), Spain (79 publications with 20 linked countries), Brazil (58 publications with 18 linked countries), Italy (51 publications with 20 linked countries), USA (51 publications with 15 linked countries), Canada (44 publications with 10 linked countries), Mexico (36 publications with 10 linked countries), South Korea (30 publications with 8 linked countries, and South Africa (7 publications with 5 linked countries). The strong alliance between China and India can be attributed to the number of postgraduate scholarships offered by their academic institutions worldwide. This has significantly impacted their ability to collaborate globally, as evidenced by their diverse research partners and many foreign postgraduates and researchers. By investing in education and research, both countries have paved the way for advancing knowledge and promoting cross-cultural exchange, making them a force to be reckoned with in the international community. Consequently, 75 articles with research focused on TiO₂-based photocatalytic and AOP activities in the water sector were finally selected and analyzed.

2.1. Research Development and Authors’ Keyword Occurrence Network

The co-occurrence analysis of the keyword’s appearance concerning the strength of an author’s keywords was explored. There were 5491 keywords obtained from the 518 published articles that emerged. Setting the minimum number for a keyword to be repeated with an author’s keyword at 10 resulted in 318 keywords linked to others. Figure 2 shows the overlay visualization of the keywords, year of publication, and the link strength. It was found that the average publications with the first 10 high-linked keywords was between 2017 and 2022. “photocatalysis” was the most representative keyword used, with 495 occurrences and 9087 linked strengths to other keywords. The second most linked keyword was “Titanium dioxide”, with 485 occurrences and 9475 links to others. Catalysts were the third, with 308 occurrences and 5973 links to others. The rest included “advanced oxidation process” (275 occurrences, 6514 links) > “ultraviolet radiation” (236 occurrences, 5927 links) > “wastewater treatment” (209 occurrences, 4195 links) > “photodegradation “(155 occurrences, 3727 links) > “photolysis” (135 occurrences, 3222 links) > “organic pollutants” (112 occurrences, 3539 links) > “oxidation-reduction” (84 occurrences, 2692 links) and others. Keywords associated with research focused on ozonation, degradation of emerging contaminants (antibiotics), catalyst synthesis (sol–gel process), characterization (surface area, X-ray diffraction, scanning electron microscopy, FTIR, etc.), energy gap, and light sources were observed.

2.2. Research Interest Themes and Keyword Occurrence

TiO₂-based photocatalysis and advanced oxidation processes for wastewater treatment were used as a search string to obtain information. Using the Google Scholar, Scopus, and Web of Science (WOS) databases resulted in cumulative published documents of 13,773, 696, and 468, respectively. Screening and merging of the published articles resulted in 518 articles containing titles, abstracts, keywords, and full texts. Almost 40% of the total publications on this subject under study globally were contributed by China and India, followed by other countries. There were keywords 5491 obtained from the 518 published articles that emerged, whereby 318 keywords were linked to photocatalysis. The keywords obtained were further processed to ascertain themes in connection with wastewater treatment via photocatalysis. As shown in Figure 3, there are five distinct clusters linked to the keywords network.

Cluster 1 (Red) shows the prevalence of TiO₂-related keywords, which is extensively studied as a photocatalyst in photocatalytic applications. This cluster, with 121 keywords linked to TiO₂, highlights research studies on the drawbacks, synthesis, and characterization approach to producing either photocatalysts or nanocomposites with other narrow band gap semiconductors. The sol–gel approach was the most regularly used method for synthesizing the photocatalysts. Also, the nanoparticle/nanomaterial-related keywords are used in photocatalysis to provide a substantial surface area to enhance adsorption and photocatalytic reactions. It was found that most of the photocatalytic degradation activities were carried out experimentally with the use of UV light and dyes (azo dyes).

Cluster 2 (Green), with 69 keywords linked to wastewater treatment, demonstrated photocatalysis as an effective technique for degrading organic pollutants and emerging contaminants via redox reactions. In wastewater treatment plants, municipal wastewater is usually polluted with pharmaceuticals (ibuprofen, carbamazepine, diclofenac), pesticides, amides, and micropollutants. Aside from photocatalysis, ozonation, and ultraviolet radiation, other keywords for alternative methods for the detoxification of emerging contaminants were observed.

Cluster 3 (Blue), with 56 keywords linked to photocatalysis, highlights the metabolism, process optimization, kinetics, detoxification, and analysis of drugs under ultraviolet radiation. Cluster 4 (Yellow) had 53 keywords linked with photocatalysis using oxides for the degradation of organic matter. Additional techniques such as membranes, microfiltration, chemical treatment, disinfection, and biological processes for wastewater treatment (surface water) for reusability or drinking water are presented.

Cluster 5 (Violet), with 14 keywords linked to photocatalysis, showed that researchers must pay attention to emerging contaminants and wastewater purification. Thus, the presence of antibiotics in wastewater settings contributes to antibacterial resistance. Therefore, emphasis was placed on magnetic photocatalysis to ensure complete mineralization of wastewater organic pollutants.

2.3. Fate of Emerging Contaminants in Wastewater

Nanotechnology-based material industries that do not have centralized systems to mitigate their waste result in the unsustainable management of emerging contaminants in wastewater settings [28]. Nonetheless, the most significant groups of organic pollutants potentially found in wastewater are within the scale of nano- and micropollutants [29]. For example, nanomaterials (photocatalysts) with a minimal size of less than a hundred nanometres (nm), with excellent surface morphology and chemical reactivity, have shown great potential for the removal of contaminants [30,31,32].

Table 1. Types and properties of nanomaterials, adapted from [28].

Examples of Nanomaterials	Novel Properties of Nanomaterials
Carbon nanotubes (CNTs), magnetite, zeolites, TiO2, and nano-Ag	Photocatalytic activity, high selectivity and permeability, hydrophilicity, low toxicity to humans, intense antimicrobial activity, high chemical and mechanical stability, among others.
Fullerene derivatives and nano-TiO2	Low cost, high selectivity and stability, low human toxicity, photocatalytic activity in solar spectrum, among others.
CNTs, titanium dioxide (Ag/TiO2), and nano-silver	Ease of use, high chemical stability, low cost and toxicity, potent antimicrobial activity, among others.
Nanofibers, metal oxide, and nanoscale or CNTs	Easy reuse, tunable surface chemistry, short intraparticle diffusion distance, more adsorption and selective sites, accessible adsorption sites, and high specific surface area.

presents some nanomaterials and their properties. However, most of the nanomaterials used in petrochemical industries, agrochemicals, textiles, food processing, cosmetics, among others end up in wastewater [30]. Nanoparticles’ toxicity is affected by a broad spectrum of factors, including surface properties like coating, reactivity, and area [33]. Also, their chemical composition, shape, and size could likely affect their toxicity. Nanoparticle types with the same chemical composition could have several toxicity levels. This is becoming very alarming due to the industry’s successive inability to conduct risk assessment on nanoparticles [11].

There is also vast uncertainty about nanoparticle legality and economic and social issues, including the capacity to control nano-risks, the government’s right to reject nano-applications, intellectual property, liability, among others [34]. The bioaccumulation of nanoparticles in wastewater streams can result in long-term pathogenic resistance. For instance, published research on carbon nanotubes revealed that they are a causative agent of asbestos-like pathogenicity at the beginning of mesothelioma in test mice. Also, some nanoparticles appear to have the potential for bioaccumulation and biomagnification in the environment [35,36]. Due to this, the United Kingdom’s Royal Society, the world’s oldest scientific organization, has suggested that the rising proof of serious nanotoxicity risks, new safety evaluations, and treatability solutions should be considered. Industries should treat nanoparticles with the assumption that they are hazardous and releasing them into the environment ought to be avoided as much as possible.

In essence, emerging contaminants such as nanomaterials, antibiotics, dyes, micropollutants, pharmaceuticals, pesticides, and others directly discharged into wastewater and aquatic environments are detrimental to human health. Removing these recalcitrant pollutants or biotransformation in conventional wastewater treatment plants is very challenging [11]. This necessitates a study on advanced oxidation processes (AOPs), with emphasis on photocatalysis, to provide information that will enhance their complete elimination.

3. Advanced Oxidation Processes (AOPs)

Different types of advanced oxidation processes (AOPs) produce hydroxyl radicals (OH^●) to remove emerging contaminants from wastewater that have a high chemical balance and limited biodegradability. AOPs have high efficiency in promoting the complete mineralization of pollutants into inorganic compounds, water, and CO₂ [37]. The diversity of AOPs includes chemical oxidation processes (Fe²⁺/H₂O₂, H₂O₂/O₃, O₃), photocatalysis (photo-Fenton reactions, UV/TiO₂), and photochemical processes (H₂O₂/UV, O₃/UV), which all produce OH^● radicals [22,23,24]. Numerous cutting-edge advanced oxidation systems that are presently being investigated for potential application in wastewater treatment are described in the following sections. These sophisticated, advanced oxidation processes fall into one of two categories: heterogeneous or homogeneous. It is possible to further categorize homogeneous processes into energy-using and non-energy-using processes (Figure 4).

3.1. Hydrogen Peroxide Coupled with UV Radiation

This advanced oxidation process entails the formation of hydroxyl radicals generated by the photolysis of H₂O₂ and the corresponding propagation reactions [23]. The photolysis of hydrogen peroxide occurs when UV radiation (hv) is applied, as shown in Equation (1):

$${\mathrm{H}}_2{\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }2{\mathrm{O}\mathrm{H}}^{●}$$

(1)

This photolysis rate is not dependent on the pH and increases under more alkaline conditions. This is probably because the wavelength 253.7 nm to the peroxide anion HO⁻² has an absorption coefficient with a higher value (240 as compared to 18.6 M⁻¹ cm⁻¹) [38]. The propagation reactions are expressed in Equations (2)–(4):

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}\mathrm{H}}^{●}\to {\mathrm{O}\mathrm{H}}_2^{●}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}\mathrm{H}}_2^{●}\to {\mathrm{O}\mathrm{H}}^{●}+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$2{\mathrm{O}\mathrm{H}}_2^{●}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$$

(4)

Hydrogen peroxide decomposes in one way or another, depending on the reaction expressed in Equation (5), and thereafter radicals recombine, as shown in Equation (6):

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}\mathrm{H}}_2^{-}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{O}\mathrm{H}}^{●}+{\mathrm{O}}_2$$

(5)

$$2{\mathrm{O}\mathrm{H}}^{●}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(6)

3.2. Photo-Fenton (Fe₂+/H₂O₂/UV)

Hydrogen peroxide (H₂O₂), iron salts, and UV-visible light are the three main components of the photo-Fenton process (Fe²⁺/H₂O₂/UV) of the AOP system that produces hydroxyl radicals (OH^●). The photo-Fenton AOP requires a wavelength of less than 580 nm, since light in this instance accelerates the processes [23]. The process utilizes solar-powered or UV light for the conversion of Fe (III) oxalate back to Fe (II) oxalate. However, to maintain the iron in the solution, the pH level in the photo-Fenton AOP needs to be kept acidic. The chemicals employed in conventional Fenton reactions are supplemented with UV-visible radiation power in homogenous photo-Fenton reaction processes [39]. In the process of the degradation of pollutants in wastewater, hydrogen peroxide at a wavelength of more than 300 nm can produce hydroxyl radicals to enhance the reaction. The reaction mechanism is expressed in Equations (7)–(9).

$${\mathrm{F}\mathrm{e}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {{\mathrm{F}\mathrm{e}}^{3+}+\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}\mathrm{H}}^{●}$$

(7)

$${\mathrm{F}\mathrm{e}}^{3+}+{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{h}\mathrm{v}}{\to }{{\mathrm{F}\mathrm{e}}^{2+}+\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{●}$$

(8)

$${\mathrm{H}}_2{\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }2{\mathrm{O}\mathrm{H}}^{●}$$

(9)

3.3. Electrochemical Oxidation

Fundamentally, electrochemical AOPs are techniques for the remediation of organic compound-contaminated wastewater (emerging contaminants) by producing reactive oxygen species (ROS) in situ, such as hydroxyl radicals (OH^●) [40]. Electrochemical oxidation occurs via direct and indirect anodic reactions expressed in Equations (10)–(13), producing electron transfers to enhance water purification. In anodic oxidation, a current density is applied to the cell’s anode to produce hydroxyl radicals on the anode surface [17,18]. Hydrodynamic parameters, such as the type of cell and electrodes usually incorporated in an electrochemical AOP system, raise concerns about the oxidation capacity and energy required. This makes the cost of operating electrochemical AOP higher compared to the conventional system. However, it is more efficient if the operating conditions are optimized [23]. A combination of methods has been proposed as a solution to all these drawbacks. To make physicochemical treatments (including electrochemical AOPs) effective and robust, bioremediation, nanotechnology, and other processes are commonly integrated [41]. Consequently, the biodegradable pollutants are degraded via the biological processes, granting the removal of the recalcitrant pollutants via the electrochemical AOPs [39].

[Anode]

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+{\left({\mathrm{O}\mathrm{H}}^{●}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{e}}^{-}$$

(10)

$${\left({\mathrm{O}\mathrm{H}}^{●}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to {\left(\mathrm{O}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}} \mathrm{o}\mathrm{r} {\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(11)

$${\left(\mathrm{O}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{O}}_2\to {\mathrm{O}}_3$$

(12)

$$\left[\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d}\mathrm{e}\right]$$

$${\mathrm{O}}_2+{2\mathrm{H}}^{+}+2\mathrm{e}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(13)

4. Photocatalysis Process

Photocatalysis has become a viable alternative post-treatment process to conventional water and wastewater treatment technologies. The illumination of photocatalytic semiconductor particles with solar light or UV radiation results in the formation of electron–hole pairs (e⁻/h⁺) [11]. TiO₂ is classified as a photocatalyst due to its lone electron properties in the particle’s outer orbital [42]. The crystal structures of TiO₂ are categorized as rutile (stable at higher temperatures), brookite (usually found in minerals with an orthorhombic crystal structure), and anatase (stable at low temperatures) [43]. A photocatalyst, as described by Umar and Aziz [44], is a material that can be activated by the absorption of a photon and is then capable of speeding up a reaction without itself being consumed in the reaction. TiO₂ can be activated by UV light with a wavelength less than 400 nm, as it has a band gap of 3.2 eV [45].

Table 2. Summary of some common photocatalyst band gaps.

Semiconductor	Crystal Structure	Band Gap Structure = 7			Ref.
		Conduction Band (CB)	Valence Band (VB)	Eg/eV
TiO2	Anatase	−0.50	2.70	3.20	[46]
ZnO		−0.31	2.89	3.20	[47]
CuO		−1.16	0.85	2.00	[48]
CdS		−0.90	1.50	2.40	[49]
ZnS		−1.04	2.56	3.60	[50]
g-C3N4		−1.30	1.40	2.70	[51,52]
g-C3N4		−1.53	1.16	2.70	[53]
Ta3N5		−0.75	1.35	2.10	[54]
TaON		−0.75	1.75	2.50	[55]
Fe2O3		0.28	2.48	2.20	[56]
Bi2O3		0.33	3.13	2.80	[57]
BiVO4		−0.30	2.10	2.40	[58]
WO3		−0.10	2.70	2.80	[59]
Ag3PO4	Cubic	0.04	2.49	2.45	[60]

summarizes the bandwidths of various commonly used photocatalysts, as semiconductor bandwidth is a significant factor restricting the wide application of photocatalysts.

Table 3. Summary of research utilizing semiconductor heterojunction photocatalysis at photocatalyst load (0.25–6 g/L) for photocatalytic degradation of several organic compounds.

Photocatalyst	Photocatalyst Load	Irradiation Source	Contaminant	Exposure Time (Minutes)	Photodegradation Efficiency (%)	Ref.
Fe3O4@Al2O3-PMo	0.5–3 g/L	UV (360 nm)	Cibacron brilliant yellow 3G-P	300	>90	[61]
Pt-TiO2	0.5–6.0 g/L	UV	Methyl orange	Not given	90.5	[62]
Sn/TiO2/AC	5.0–15.0 g/L	UV	Orange G	60	99.1	[63]
Cu-doped ZnO	0.1–1 g/L	125 W medium-pressure UVC lamp	Diazinon	120	96.97	[64]
Fe3O4@ZnO/PMOs	0.25–1.5 g/L	40 W white LED lamps	Methyl orange	180	98.2	[61]
Fe3O4@SiO2@Ag2WO4@Ag2S	0.5–3 g/L	Xenon + LED lamps (95 W)	Methyl blue	60	99.9	[65]
Bi2O3/SnO2	(20–60) mg/50 mL	350 W Xenon lamp	Bisphenol A	60	93.42	[66]
P-ZnO1.8%	0.5–3 g/L	300 W halogen lamp	Rhodamine B	180	99	[67]
Fe0@PEDOT\PW12	0.5–3 g/L	Visible light	Oxidative desulfurization	60	98.4	[68]

provides a summary of additional research that explains how the photocatalyst dose affects the breakdown of organic contaminants by photocatalysis.

4.1. TiO₂ Photocatalytic Mechanism

Photo-induced reactions are activated by the absorption of a photon with energy equal to or higher than the band gap energy of the catalyst [69]. This absorption results in charge separation as the electron (e⁻) moves from the valence band to the conduction band of the semiconductor catalyst, resulting in a hole (h⁺) in the valence band [70]. For the photocatalytic reaction to be favoured, the recombination of the electron and hole must be prevented. This allows for a reaction of electrons with an oxidant, resulting in a reduced product, as well as a hole reacting with a reductant to produce an oxidized product [71]. Hydroxyl radicals produced (OH^●) are very reactive species and are therefore able to oxidize a wide variety of organic pollutants in a quick, efficient, and non-selective manner [72]. Akpan and Hameed [69] and Chong et al. [70] explain the series of chain oxidative–reductive reaction equations that take place at the photon-activated surface. The photocatalytic reaction mechanism (Figure 5) indicates that the degradation method is a clean technology and can be applied to the treatment of wastewater, where unwanted organics may exist [73]. These reactions are given by Equations (14)–(20).

$$\mathrm{P}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{T}\mathrm{i}{\mathrm{O}}_2+\mathrm{h}\mathrm{v}\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

(14)

$$\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}-\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{r} \mathrm{t}\mathrm{r}\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{o}\mathrm{f} {\mathrm{e}}^{-}:\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}\to {\mathrm{e}}_{\mathrm{T}\mathrm{R}}^{-}$$

(15)

$$\mathrm{C}\mathrm{h}\mathrm{a}\mathrm{r}\mathrm{g}\mathrm{e}-\mathrm{c}\mathrm{a}\mathrm{r}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{r} \mathrm{t}\mathrm{r}\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{o}\mathrm{f} {\mathrm{h}}^{+}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\to {\mathrm{h}}_{\mathrm{T}\mathrm{R}}^{+}$$

(16)

$$\mathrm{E}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}-\mathrm{h}\mathrm{o}\mathrm{l}\mathrm{e} \mathrm{r}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}:\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{e}}_{\mathrm{T}\mathrm{R}+{\mathrm{h}}_{\mathrm{V}\mathrm{B}}^{+}\left({\mathrm{h}}_{\mathrm{T}\mathrm{R}}^{+}\right)\to {\mathrm{e}}_{\mathrm{C}\mathrm{B}}^{-}+\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{t}}^{-}$$

(17)

$$\mathrm{P}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{e}\mathrm{d} {\mathrm{e}}^{-}-\mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g}:\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\left({\mathrm{O}}_2\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{e}}^{-}\to {\mathrm{O}}_2^{●-}$$

(18)

$$\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{y}\mathrm{l} {\mathrm{h}}^{+} \mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g}:\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{h}}^{+}\to {\mathrm{O}\mathrm{H}}^{●}$$

(19)

$$\mathrm{P}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{b}\mathrm{y} {\mathrm{O}\mathrm{H}}^{●}:\hspace{1em}\hspace{1em}\hspace{1em}\mathrm{R}-\mathrm{H}+{\mathrm{O}\mathrm{H}}^{●}\to {\mathrm{R}}^{\prime ●}+{\mathrm{H}}_2\mathrm{O}$$

(20)

4.2. Factors Affecting Photocatalytic Process

There are a multitude of factors that influence the performance of photocatalytic degradation with TiO₂, which include, but are not limited to, catalyst type and loading, pH of the solution, light intensity or wavelength, the concentration of the photocatalyst, the presence of dissolved oxygen, and reaction time [44,71].

4.2.1. Catalyst Loading

The quantity of catalyst utilized is a major determinant of photodegradation efficiency and total cost. Increasing the TiO₂ load increases the number of e⁻/h⁺ pairs, which accelerates the reaction rate, in addition to the number of active sites for pollutant adsorption [74]. The rise in TiO₂ loading is proportional to the photocatalytic reaction, and the concentration of catalyst particles affects the reaction rate remarkably, especially in the heterogeneous regime [44]. A higher catalyst concentration means a more significant number of active sites available for adsorption. As catalyst concentration increases above a specific limit, the degradation efficiency decreases [75]. Several researchers have indicated that when catalyst loading exceeds the optimum quantity, a significant reduction in efficiency is observed as excess particles tend to scatter light [76,77,78]. This could be due to a light scattering effect where light penetration of the effluent was retarded as there was a large number of solid particles present in the solution [79]. The increase in the catalyst amount increases the number of active sites on the photocatalyst surface, thus causing an increase in the formation of several OH^● radicals. However, particle–particle interaction or agglomeration generally takes place at high solid concentrations, which results in a lower surface area for light absorption, thus reducing the degradation efficiency [80].

4.2.2. pH

The effect of pH on the photodegradation process can be attributed to three possible reaction mechanisms. This involves the (i) charge neutralization attack by the photolysis hydroxyl radicals, (ii) the direct oxidation by the positive hole, and (iii) direct reduction by the electron in the conducting band [13,14,17]. The pH effect associated with the surface-charge properties of the photocatalysts could be described at the point of zero charges (PZC). The point of zero charges (PZC) for TiO₂ particles is (pH = 6.8). Therefore, below this value, the photocatalyst surface is positively charged, and above this value, the photocatalyst surface is negatively charged. The surface of the photocatalyst becomes positively charged in an acidic solution or at pH values lower than PZC (pH < 6.8), and vice versa, according to the equations below, where Khan et al. [81] explain that the adsorption of anions is favoured when the pH is less than the point of zero charges (PZC). The pH is known to affect the surface of titania by either protonation or deprotonation, as expressed in Equations (21) and (22).

$$\mathrm{P}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{T}\mathrm{i}\mathrm{O}\mathrm{H}}_{\left(\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\right)}+{\mathrm{H}}^{+}\to {\mathrm{T}\mathrm{i}\mathrm{O}{\mathrm{H}}^{2+}}_{\left(\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\right)}$$

(21)

The adsorbent surface is positively charged when pH is lower than the PZC value, and the surface becomes cation-repelling/anion-attracting.

$$\mathrm{D}\mathrm{e}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{\mathrm{T}\mathrm{i}\mathrm{O}\mathrm{H}}_{\left(\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\right)}+\mathrm{O}{\mathrm{H}}^{-}\to {\mathrm{T}\mathrm{i}{\mathrm{O}}^{-}}_{\left(\mathrm{S}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}\right)}+{\mathrm{H}}_2$$

(22)

On the contrary, the surface is negatively charged above PZC, and the surface becomes anion-repelling/cation-attracting [82], further iterating that pH is a significant factor influencing the degradation rate. The importance of pH was further emphasized by Saien and Shahrezaei [83], who explain that pH influences the catalyst charge, the pollutant molecule, and the degradation mechanism and generation rate of the hydroxyl radical.

4.2.3. Light Wavelength and Intensity

The photochemical impacts of light sources with various wavelength emission ranges will have a significant effect on the photocatalytic process [84]. The UV light used in the process needs to have sufficient energy to promote electron–hole formation [85]. The corresponding electromagnetic spectrum for UV irradiation is categorized as UV-A, UV-B, and UV-C based on the wavelength of light that it emits [86]. The UV-A range has its light wavelength range from 315 to 400 nm, which corresponds to a (3.10–3.94 eV) band gap [87]. UV-B has a wavelength range of 280–315 nm, which corresponds to a (3.94–4.43 eV) band gap, and the UV-C ranges from 100 to 280 nm, which corresponds to a (4.43–12.4 eV) band gap [88]. In most previous studies, the UV ranges from 254 nm to 315 nm and provides light photons sufficient for the photonic activation of titanium dioxide (TiO₂) [12,13,14].

TiO₂ has a wide band gap (about 3.2 eV), with optical absorption in the 310–400 nm-wavelength region [66]. A series of modifications to TiO₂ is usually carried out, thereby improving the utilization of visible light by TiO₂ and reducing the photo-generated electron–hole recombination rate on the surface of TiO₂ [89], which is discussed in the next section. However, recombining electron–hole pairs is usually a problem experienced in photocatalysis [66]. Electron–hole pair separation struggles with recombination at a lower light intensity, which decreases the creation of free radicals, and, hence, the degradation of the organic molecules is reduced [86]. The efficiency of isopropanol degradation by TiO₂ nanotubes increased to 56% with increasing UV light intensity from 0 to 3.0 mW/cm² [90].

4.2.4. Initial Concentration Nature of Pollutants

The photocatalysis process depends on the adsorption of organic pollutants on the surface of the photocatalyst. This can also be influenced by the rate of photodegradation, the initial concentration of pollutants, nature, and other existing compounds in the wastewater matrix [17]. A high concentration of pollutants in water saturates the TiO₂ surface and hence reduces the photonic efficiency (less radical OH^● ions are generated) resulting in the deactivation of the photocatalyst and a smaller degradation percentage. Consequently, the photocatalytic activity of the produced photocatalyst is decreased. Zhu et al. [91] reported that the path length of the photons entering the solution decreases by increasing the initial amount of methyl orange (MO) azo dye, and the opposite effect occurs for the lower amount. Kertèsz et al. [92] studied the effect of different initial amounts of reactive yellow 125 dye on the process of photodegradation, and their results revealed a decrease in degradation efficiency by increasing the amount of dye. This dependence is most likely associated with generating numerous monolayers of the adsorbed dye on the surface of the photocatalyst, which is preferred at high dye concentrations [86]. On the other hand, dye molecules are adsorbed on the photocatalyst surface by increasing the initial concentrations of the dye, and a considerable amount of UV is absorbed by the dye molecules instead of the photocatalyst particles. Consequently, the light penetration decreases on the catalyst surface [21].

4.2.5. Morphology

Surface morphology (surface area, structure, and size of the photocatalyst) is an important physicochemical factor to be considered in the photocatalytic degradation process. The number of photons striking the photocatalyst controls the rate of reaction, which signifies that the reaction takes place only in the absorbed phase of the photocatalyst [93,94]. However, the type of photocatalyst, size, and shape enhance the adsorption capacity and the photocatalytic activity. Additionally, the crystallite size of a catalyst plays a significant role in the photocatalytic process [95,96]. The surface area of the photocatalyst materials also has a significant impact on photocatalytic activity. Thus, a large surface area contains more active sites for the oxidative reactions [97].

The efficiency of the photodegradation of organic molecules is gradually enhanced by raising the temperature. An increase in reaction temperature generally results in increased photocatalytic activity; however, reaction temperature above >80 °C favours the recombination of charge carriers and demotes the adsorption of organic compounds on the titania surface [86]. Also, in the photocatalytic process, increasing the contact time leads to the increasing production of active radicals due to more exposure to the light, thus improving photocatalytic degradation efficiency [98].

Table 4. Different photocatalysts are used in photocatalytic degradation.

Type of Wastewater	Type of Catalyst	Target Pollutant	Operating Conditions	Light Source	Efficiency (%)	Reference
Textile industry wastewater	TiO2/anthill	Colour	IC (3.01); CL (2.5 g/L); pH (2); T (67 min)	Sunlight	70.92	[99]
Petroleum refinery	TiO2	COD and SO42−	COD: (1226 mg/L); pH (8); CL (1.5 g/L); T (150 min)	UV light	92	[100]
Industrial wastewater	ZnO	Phenol and benzoic acid	IC (50 mg/L); CL (1.0 g/L); T (120 min)	UV light	69.75	[101]
Industrial wastewater	Ce-doped ZnO	Rhodamine B (RhB)	IC (10 mg/L); CL (0.7 g/L); pH (9.0); T (120 min)	Sunlight	97.66	[102]
Textile industry wastewater	ZnO/bentonite	Solophenyl Red 3BL (SR 3BL)	IC (0.75 mg/L); CL (0.75 g/L); pH (6), T (160 min)	Sunlight	92	[103]
Industrial wastewater	Kaolinite/TiO2/ZnO	Remazol Red (RR), an anionic azo dye	IC (100 mg/L); CL (100 mg/L); pH (2.5); T (120 min)	Sunlight	98	[104]
Synthetic wastewater	ZnO/RGO	Congo red (CR) and Eosin yellow (EY) contaminate	IC (10 mg/L); CL (0.05 g/L); pH (3.0); T (120 min)	Visible light	51	[105]
Textile industry wastewater	ZnO/pumice composite	Dye-containing wastewater	IC (3.01 mg/L); CL (3.0 g/L); pH (4.01); T (45.04 min)	Sunlight	90.17	[106]
Synthetic wastewater	CuS flowers	Rhodamine B (Rh B)	CL (1 g/L); T (12 min)	Visible light	∼99	[107]
Synthetic wastewater	Gelatin–cerium–copper sulfide Ge-Ce-CuS nanoparticles	Malachite green oxalate dye (MGO dye)	IC (3 × 10−4 M); CL (1.5 g/L); pH (9); T (120 min);	Sunlight	90.7	[108]
Pharmaceutical industry treatment	Persulfate Sodium (PPS)	Penicillin G (PG)	IC (5 mg/L); CL (0.8 g/L); t pH (6), (22 °C); MS (500 rpm)	UV light	72.72	[109]
Textile industry wastewater	Lanthanum Vanadate (LaVO4)	Methylene blue (MB)	IC (10 mg/L); CL (0.3 g/L); pH (7)	UV light	91	[110]
Synthetic wastewater	TiO2/GO graphene oxide composite	Methylene blue (MB)	IC (5 mg/L); CL (0.2 g/L); pH (10); T (4 h).	UV–vis	99	[111]
Pharmaceutical industry treatment	Nanobiocomposite CoFe2O4@methycellulose (MC)	Metronidazole (MNZ) antibiotic	IC (5 mg/L); CL (0.2 g/L); pH (11); T (120 min)	UV light	85.3	[112]

IC = initial concentration of pollutant; CL = catalyst load; T = expo-*sure time; t = temperature; MS = mixing speed; UV = ultraviolet.

summarizes some investigations for the photocatalytic degradation of organic pollutants.

5. Challenges and Prospects of TiO₂-Based Photocatalysis

The photocatalytic process has shown significant promise as a sustainable treatment method in the water and wastewater treatment settings. However, among the various photocatalysts available, TiO₂ is the most extensively studied and utilized due to its cost-saving benefits, non-hazardous properties, and oxidation performance [113,114]. Although many studies have applied TiO₂ photocatalysis in wastewater treatment, TiO₂ possesses several shortcomings that limit its commercial or industrial applications such as wide bandgaps, inability to absorb visible light or visible light utilization from solar sources, stability or poor separation potential (fast electron–hole recombination), recovery, recyclability, and significant attention demand [115]. Figure 6 presents a schematic SWOT (strengths, weaknesses, opportunities, and threats) analysis of TiO₂-based AOPs in the context of wastewater treatment, highlighting critical insights that inform both the current state and future directions of this emerging technology. The SWOT also underscores the need for an integrated research approach that addresses material limitations, process design, environmental implications, and regulatory frameworks. It serves as a roadmap for enhancing the performance, safety, and application of TiO₂-based AOPs in future wastewater treatment solutions. Despite the strong fundamental knowledge of TiO₂ photocatalysis, persistent barriers are hindering its large-scale industrial adoption and successful transformation from laboratory to real-world applications.

Table 5. Summary of knowledge gaps and prospects of TiO₂-based photocatalysis.

Thematic Area	Key Findings	Considerable Remarks	Future Research	Reference
TiO2 photocatalysis	Synergistic effect of metal-doped TiO2/AC for efficient visible light-driven cationic dye degradation, Ag-doped TiO2/AC with enhanced surface functionality, reactivity, and cyclic stability showed 87.25% Rh B degradation.	Advances in environmental remediation using metal-doped TiO2/AC heterojunctions (e.g., Ag-TiO2 or TiO2-ZnO, TiO2-graphene) for charge separation.	Doping with metals/non-metals to extend visible light activity.	[116]
	Multi-element synergy in photocatalytic material-integrated mechanism–design–preparation strategies	Quantitative analysis of the complex interactions between elements and the integrated design of low-cost materials, elemental doping, and surface plasma.	Immobilization of supports to improve recovery potential. Scalability of eco-friendly synthesis routes (bio-mediated TiO2 nanostructures).	[117]
	Degradation of antibiotic oxytetracycline using surface-reconstituted TiO2 photocatalyst	The synergistic effect of co-doping compared to single-component doping.	Understanding of charge transfer mechanism and improving band gap energy reduction under responsive light.	[118]
Advanced oxidation process (AOP)	Commercialization aspects for TiO2-based indoor air purification	Designing strategies to improve their photon utilization and deactivation resistance, and regeneration of TiO2-based photocatalysts.	TiO2-based air purification is proposed to demonstrate the innovative commercialization direction.	[119]
	Application of Uv/O3/Tio2/hydrogen peroxide-based advanced oxidation processes for wastewater treatment	Hybrid system (UV/TiO2/O3) for pollutant removal with pathogen inactivation. Also, ozone-based AOPs are efficient at detoxifying a variety of resistant effluents.	To establish a solid theoretical foundation for the implementation of UV/H2O2- and O3/H2O2-based AOPs in wastewater management.	[120]
	Amalgamation of advanced oxidation process with biological techniques for the treatment of tannery wastewater	Pre-treatment of wastewater by AOP converts the recalcitrant organic pollutants to simpler and biodegradable compounds, allowing the wastewater to be treated by subsequent biological treatment.	A cost-effective methodology for the treatment of tannery wastewater to comply with applicable current regulations worldwide must explore the use of AOP coupling with biological treatment technologies.	[121]
	Preparation of the Ti/TiO2-RNTs/SnO2–Sb–Ni-La electrode and its electrochemical degradation of oily wastewater	The electrochemical oxidation method is expected to reduce energy loss.	Electrochemical AOPs using renewable energy sources and exploring suitable cost-effective electrode materials.	[122]
Magnetic nanomaterials	Evaluation of photocatalytic degradation of Bisphenol A by reusable Fe3O4/SiO2/TiO2 magnetic nanocomposite: optimization by response surface methodology	The potential of Fe3O4/SiO2/TiO2 using both UVA and solar light. Agglomeration and loss of surface activity over time.	Magnetic TiO2 for easy recovery and reuse with magnetic separation technology. Environmental toxicity and long-term stability have not been fully assessed for scalability applications.	[123]
Emerging materials	Emerging frontiers of nickel–aluminum-layered double hydroxide heterojunctions for photocatalysis	Ni–Al LDH-based heterojunctions in photocatalytic applications, such as H2 evolution, CO2 reduction, and pollutant removal.	Exploit new Ni–Al LDH-based heterojunctions for high-performance photocatalytic applications, large surface area, tunable band gap and morphology, abundant reaction sites, and high activity, selectivity, and photostability.	[124]
	ZIF-8 metal–organic frameworks and their hybrid materials: Emerging photocatalysts for energy and environmental applications	Applications of ZIF-8-based photocatalysts in light-driven H2 evolution, H2O2 evolution, CO2 reduction, and dye and drug degradation.	Developing cost-effective, scalable, and environmentally friendly ZIF-8 composites for industrial applications.	[125]
	Ionic liquids: The emerging “cardiotonic” for photocatalytic materials	Ionic liquids (ILs) and photocatalytic materials (PMs) enhance the properties (hydrogen bonds effect, electrostatic effect, polarity effect, and coordination effect).	Research in high-powered PMs using the strategy of IL modification.	[126]
Environmental relevance	Toward sustainable photocatalysis: Addressing deactivation and environmental impact of anodized and sol–gel photocatalysts	Photocatalytic coating preparation, like chemical sol–gel and electrochemical anodic oxidation, generates the oxide directly from a titanium substrate. Life cycle assessment (LCA) was used to quantify and compare the potential environmental impacts associated with the two different TiO2 production processes.	Developing more eco-friendly application strategies, like magnetic photocatalysts and synthesis routes (sol–gel photocatalysts). Future research on iron oxide-based composites for environmental remediation.	[127]
AI-integrated process	Artificial intelligence (AI) techniques	Fe3O4/TiO2 nanocomposites were synthesized and optimized for enhanced photocatalytic degradation of organic pollutants. An AI-assisted model was developed to predict removal efficiency based on experimental data.	The potential of integrating AI with advanced photocatalysts for wastewater treatment and suggests a scalable strategy for industrial applications.	[128]

shows a summary of extensive studies on TiO₂-based photocatalysis and future research directions. With continued innovation and research, TiO₂ photocatalysis stands poised to play a critical role in achieving sustainable and efficient water treatment solutions in the 21st century. As shown in Figure 6, visible light activation, recovery techniques (magnetic separation), hybrid system integration, and AI-enhanced optimization were highlighted as future opportunities for TiO₂-based photocatalysis.

5.1. Challenges of TiO₂’s Large Energy Band Gap

Titanium dioxide (TiO₂) is a widely used semiconductor with significant applications in photocatalysis, solar cells, and environmental purification. However, its large energy band gap (~3.2 eV) presents several challenges that limit its practical applications, particularly under visible light. The key challenges of TiO₂’s large energy band gap are as follows. (i) Limited Light Absorption: The large band gap of TiO₂ restricts its light absorption to the ultraviolet (UV) region, which constitutes only a small fraction of the solar spectrum. This results in poor utilization of visible light, significantly limiting its efficiency in solar energy applications and photocatalysis [129]. (ii) Low Quantum Efficiency: Due to the wide band gap, TiO₂ exhibits low quantum efficiency. This means that the number of charge carriers generated per photon absorbed is relatively low, reducing the overall efficiency of photocatalytic processes. (iii) High Charge Recombination Rate: TiO₂ suffers from a high rate of recombination of photo-generated electron–hole pairs. This recombination reduces the number of charge carriers available for photocatalytic reactions, further limiting their efficiency [13].

5.1.1. Enhancing TiO₂ Visible/Sunlight and Near-Infrared (NIR) Light Absorption

To enhance the visible/sunlight and near-infrared (NIR) light absorption capacity of titanium dioxide (TiO₂), several strategies have been explored and proven effective. The following are the key approaches:

Doping with Metal and Non-Metal Elements

(i) Metal Doping: Incorporating metal elements such as copper (Cu), iron (Fe), and manganese (Mn) into TiO₂ can significantly improve its visible light absorption. For instance, Cu substitution in TiO₂ has been shown to enhance visible light absorption, especially when combined with an applied electric field [130]. Fe doping broadens the light absorption spectrum and improves charge separation, enhancing photocatalytic performance [131]. Mn-doped TiO₂ nanotubes also demonstrate improved visible light absorption and efficient separation of photo-generated electron–hole pairs [132]. (ii) Non-Metal Doping: Non-metal elements like nitrogen (N), sulfur (S), and carbon (C) can also be used to extend the absorption edge of TiO₂ into the visible region. N, S-co-doped TiO₂ nanoparticles exhibit strong visible light absorption and enhanced photocatalytic activity [133]. Anion doping with elements such as nitrogen and carbon in TiO₂ nanotube membranes has been shown to improve light absorption and efficiency in solar-to-fuel conversion [134].

Creating Oxygen Vacancies, Defects, and Introducing Dopants

(i) Oxygen Vacancies: Introducing oxygen vacancies in TiO₂ narrows its band gap, enhancing light absorption in the NIR region. Techniques such as laser ablation in liquid (LAL) can create abundant oxygen vacancies efficiently [135]. (ii) Ti³⁺ Defects: Introducing Ti³⁺ defects in TiO₂ can make it responsive to visible light. A novel approach involves generating surface Ti³⁺ sites without creating oxygen vacancies, which significantly enhances visible light response [136]. (ii) Photon Localization Effects: Utilizing photonic crystals to localize photons can enhance light harvesting efficiency. For example, coupling SiO₂ opal photonic crystals with Fe-doped TiO₂ films improves N₂ photo-fixation performance [133].

Combining with Conjugated Polymers

Polymer Composites: Using conjugated polymers with TiO₂ nanoparticles can extend light absorption to the visible range and enhance photocatalytic reactions. For instance, PVA/TiO₂ composites with a porous morphology show efficient degradation of phenol under visible light [137].

Structural and Morphological Modifications

(i) Nanostructuring: Creating nanostructures such as nanotubes, nanorods, and aerogels can increase the surface area and improve light absorption. For example, nitrogen-doped TiO₂ aerogels loaded with palladium nanoparticles show significant enhancement in visible light-driven H₂ production [138]. (ii) Core–Shell Nanostructures: Creating core–shell nanostructures, such as NaYF₄:b, Tm@NaYF₄:b, Nd@TiO₂, can enhance NIR absorption and energy transfer, improving the overall photocatalytic performance [139]. (iii) Plasmonic Nanostructures: Integrating metallic plasmonic nanostructures like aluminum and gold with TiO₂ can activate the structures plasmonically, increasing optical absorption in the visible and NIR spectral range [140]. Embedding plasmonic nanostructures such as silver nanoplates (Ag NPLs) into TiO₂ can significantly enhance NIR absorption. The surface plasmon resonance (SPR) effect of Ag NPLs generates additional photo-generated carriers and improves the separation of electron–hole pairs, thus enhancing the overall light absorption [141].

Co-Doping and Synergistic Effects

Co-doping: Combining multiple dopants can synergistically enhance light absorption and photocatalytic activity. For instance, co-doping with phosphorus (V) and titanium (III) in a leaf-architectured TiO₂ structure significantly enhances visible light harvesting and solar photocatalysis [142].

Hydrogenation and Phase Transformation

(i) Hydrogenation: Hydrogenating TiO₂ to form hydrogenated TiO₂ (H-TiO₂) improves its absorption in the Vis-NIR region. This process can be achieved through a one-step hydrogen reduction, which also enhances the material’s magnetic and photoconversion properties [143]. (ii) Phase Transformation: Using pulsed laser irradiation to induce phase transformations in TiO₂ can create defect-engineered black TiO₂, which shows improved light absorption in the visible and NIR regions [144].

Up-Conversion Materials

Rare-Earth Doping: Incorporating rare-earth-doped luminescent materials that perform NIR-to-UV-VIS spectral conversion can enhance the photocatalytic activity of TiO₂. This approach leverages the energy from NIR radiation to drive photocatalytic reactions [145,146].

5.2. Complex Oxidizable Organic Substrates

The critical challenge identified as a constraint for photocatalysis application is the treatment scope for contaminants. Jiang et al. [147] reported that obtaining selective oxidation of the target pollutant or pollutants in the presence of other oxidizable organic substrates is relatively complex, which has become a significant setback for photocatalysis systems. It is challenging to oxidize benzene or other BTEX chemicals selectively in the presence of a high concentration of DOM (dissolved organic matter) [27,148]. Also, the intermediates of some emerging contaminants (antibiotics, phenols, pharmaceuticals, etc.) can result in antibiotic resistance and environmental risks [149]. Consequently, future technical research should focus on the intermediate formed as well as increasing the material efficiency [150,151,152].

5.3. Energy-Intensive/Recovery, Recyclability, and Reusability

The conventional method for the use of TiO₂ involves slurry suspension, where the TiO₂ nanoparticle powder is dispersed in contaminated water in the form of a slurry or suspension. However, the TiO₂ particles in the form of a slurry can utilize the maximum light absorption and mass transfer of pollutants. The method has some major drawbacks, such as the power consumption needed for continuous mixing to keep the particles suspended during photolysis [153,154,155]. After the photocatalytic process, the TiO₂ nanoparticles are difficult to separate, which affects the sustainability of the environment. The post-photocatalysis separation of the TiO₂ particles usually requires complex and high-energy-consuming processes such as ultrafiltration, ultracentrifugation, and adequate precipitation [19]. The main flaw associated with the slurry suspension method is that photocatalytic water treatment processes are not receiving widespread acceptance in practical applications [153,156,157,158]. An approach such as the immobilization of titanium dioxide onto supporting materials in a solid matrix has become an intriguing alternative to avoid the challenge of coagulation and the drawbacks of the slurry suspension method [159,160,161].

Furthermore, TiO₂ photocatalysts can be enhanced using dye sensitization supports, magnetization, and surface modification by doping with non-metals, metals, and transition metals, as well as coupling with other semiconductors [162,163]. The TiO₂ ternary/quaternary-based composites and hybrid materials are also a major trend, primarily to attain higher photonic efficiency by synergistically modifying physical, chemical, and electronic properties [164]. Researchers have used various substrates or other uncommon materials, such as microporous cellulosic membranes, alumina clays, ceramic membranes, monoliths, zeolites, and even stainless steel, as a support for TiO₂ [165,166,167]. The recyclability of catalysts is one of the critical steps toward the sustainable application of photocatalysts and the development of heterogeneous photocatalysis technology for water and wastewater treatment [168]. Recently, several studies have demonstrated satisfactory recyclability of nanomagnetic nanoparticles via magnetic separation processes using a magnetic field [169]. Krishna et al. [170] reported the reusability of the CoFe₂O₄/TiO₂ nanocatalysts for acid blue 113 (AB113) dye degradation through magnetic separation, where their photocatalytic activity was found to be retained up to six consecutive cycles without considerable loss of photocatalytic activity and stability.

5.4. Industrial/Large-Scale Application of Semiconductor Photocatalysts

The relatively slow transfer of photocatalysis treatment techniques from bench to industrial scale is likely due to the system’s design currently used in water treatment [114,171]. A disadvantage of large-scale applications could be an unsuitable choice of catalyst for pollutant degradation, such as utilizing a photocatalyst that is ineffective or incompatible with the contaminants [114]. This necessitates the modification of the photocatalyst’s structure to adjust its band gap and effectively harness visible light irradiation. The primary obstacle to large-scale photocatalysis is the retrieval of the photocatalyst after each cycle. For instance, powdered photocatalysts tend to settle at the bottom of the tank, making their full recovery unfeasible [172]. However, further research should be carried out, as the upscaling of photocatalytic processes may be challenging without adequate studies on more economical and available materials. Also, the design of more reliable photocatalysts through coating or doping on fixed supports would allow for the simultaneous degradation and separation of contaminants from the effluent stream [114].

5.5. The Challenges and Prospects of Magnetized TiO₂-Based Photocatalysts

Magnetic photocatalytic systems have emerged as a promising route for environmental remediation due to its dual advantages of photocatalytic degradation and magnetic recoverability. However, the stability of core–shell magnetic photocatalysts (Fe₃O₄-TiO₂) under real wastewater conditions has some setbacks. For instance, under extreme pH, high ionizing strength, and oxidizing conditions, leaching of semiconductors and corrosive ions (Cl and SO₄) can occur. The exposure to the magnetic core diminishes the structural integrity and the catalytic performance, thereby rendering the magnetic serration ineffective. The environmental and health concerns due to the potential ecotoxicological impacts of leached magnetic nanomaterials require research attention.

Consequently, ongoing research work is being conducted on the application of TiO₂-based photocatalysis. However, its industrial applications and efficient use of sunlight to mitigate its energy challenge are still limited.

Table 6. Action plan to improve TiO₂-based photocatalyst challenges.

Conditions	Current Challenges	Prospective Way Forward
Cost-effective synthesis route	* Nanomaterials required to improve TiO2 production * Cheap design method * Synthesis route with eco-friendly and non-toxic precursors	* Developing and optimizing cost-effective TiO2 synthesis methods to maximize production yield * Regenerative and reusable photocatalyst approach
Morphology control	* Synthesis of nanoparticles with good crystallinity, surface area, and small particle size	* Develop a synthesis method to maximize morphology * Develop a kinetic model for the predictive morphology of photocatalysts
Visible light absorption	* Modification with cheap co-catalysts * Concentration of doped materials	* Develop and optimize a cost-effective modification process
Product selectivity and optimization	* Sensitivity and selectivity of nanomaterials to improve product yield* Optimizing the selective product	* Tuning the band structure and accumulation of illumination * TiO2 modification and regeneration * Optimizing the synthesis process, multi-factors, or operating conditions
Charge separation efficiency	* Difficult to separate photocatalysts after being used	* Develop semiconductors with suitable band positions to couple with TiO2 * Develop magnetized TiO2 to enhance magnetic separation and regeneration usability
Cost-effective photocatalytic reactor design	* Design of a suitable photocatalytic reactor	* Establishment of hybridized solar/UV–photocatalytic reactor * Commercialization of reactors

, therefore, presents an action plan to develop reusable and regenerative photocatalysts with an affordable, environmentally friendly, and non-toxic precursor. Moreover, doping of tunable materials for suitable morphological structures via a synthetic route has a bottleneck. Hence, adapting optimization and life cycle assessment (LCA) concepts may aid in developing environmentally friendly and sustainable green synthesis techniques. Although laboratory-based research has demonstrated efficient magnetic recovery, the costs associated with rare-earth magnetics, the energy required to generate a magnetic field, and the complexity of sludge handling pose substantial barriers for commercial implementation.

Furthermore, the operational and economic viability of magnetic separation systems requires a higher loading of the magnetic core, which is photocatalytically inert. Hence, proposing reactor design and development of ion-resistant coatings to enhance material stability, establishing testing protocols for metal leaching and toxicity under realistic wastewater matrices, and integrating magnetic photocatalysis with complementary technologies such as membrane, filtration, adsorption, or electrochemical systems come in handy. These interdisciplinary efforts are needed to bridge the gap between material synthesis and reactor engineering to ensure that promising laboratory materials can be translated into scalable and robust treatment solutions.

6. Conclusions

This study provides insight into the research trends, challenges, and prospects of advanced oxidation processes and photocatalysis in wastewater treatment. A comprehensive review was conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) approach. The keyword occurrence and research interest assessment revealed the research gap and prospects of TiO₂-based photocatalysis on emerging contaminants. Moreover, to improve TiO₂ photocatalytic efficiency, energy band gap, morphology, recyclability, and reusability, schemes such as metal, non-metal, magnetic, and plasma doping can be employed via optimized sol–gel techniques. Also, the integration of photocatalysis with biological processes was found to be very favourable than other conventional methods of mitigating recalcitrant pollutants. Also, studies on the life cycle assessment of photocatalysts at the preparatory and application stages should be intensified. Multidisciplinary and application-driven research can unlock the full potential of TiO₂-based photocatalysis in wastewater treatment. Future research should explore more of the following:

• Magnetic nanocomposite photocatalysis, which involves coupling TiO₂ with magnetic materials (Fe₃O₄), allows for easy recovery and regeneration of the catalyst with a magnetic field and enhances its practical applicability.
• TiO₂ doped with emerging materials such as nitrogen, ionic liquids, sulfur, metal ions, carbon, and hybrid systems with carbon-based materials can increase the photocatalytic activity under visible light response systems.
• The adoption of environmentally friendly synthesis methods of TiO₂ nanoparticles with plant or biodegradable-based materials can contribute to the broader goals of sustainable nanotechnology.
• The integration of TiO₂ photocatalysis with biological treatment, electrocatalysis, or membrane systems offers multifunctional effects to enhance overall treatment performance.
• The development of standard testing protocols, regulatory frameworks, and environmental risk assessments is critical to support the safe and responsive implementation of TiO₂-based photocatalysis.