Mechanism of Advanced Oxidation Processes

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1		Jeyakumar Rajesh Banu	--	3554	2023-03-27 11:21:04	\|
2	layout & references	Sirius Huang	Meta information modification	3554	2023-03-28 03:00:31	\|

This entry is adapted from the peer-reviewed paper 10.3390/su15054235

Advanced oxidation processes (AOPs) involves the generation of powerful oxidizing radical groups, such as hydroxyl radicals, which function as oxidizing agents and mineralize organic chemical substances into CO₂ and H₂O. AOPs such as photocatalysis and photo-Fenton have been widely considered to be very effective in removing persistent organic pollutants.

advanced oxidation processes solar photo-Fenton solar photocatalysis biological process

1. Introduction

One of the essential needs of all living things, including humans, is safe and uncontaminated drinking water. However, these days, finding it has become a big issue. The water used by humans for most activities produces wastewater. Wastewater accounts for over 80% of the water provided for domestic use ^[1]. About 5–10 billion tons of industrial waste are generated each year and discharged untreated into the environment ^[2]. If released directly into the environment without treatment, the wastewater generated from various industries can cause significant environmental problems ^[3]. Therefore, removing emerging pollutants is essential to making the water safe for drinking, and the release of wastewater should not cause any adverse effect on the environment. Moreover, recycling and reusing treated wastewater is necessary to reduce freshwater consumption. There are physical, chemical, and biological treatment methods; these are the most commonly employed wastewater treatment technologies. In the case of physical treatment, the removal of organics from the wastewater is challenging. The existing biological and chemical treatment methods include ozonation, electrochemical treatment, photocatalysis, aerobic treatment, activated sludge, anaerobic treatment, coagulation–flocculation treatment, etc. ^[4]. Aerobic biological activities require a lot of energy and generate a significant amount of biomass. Anaerobic biological processes are susceptible to shock loading, which requires further treatment of the wastewater produced before the final release. Although biological processes are efficient and cost-effective, they primarily need a vast area and have a significant energy demand (for aeration) and a substantial quantity of produced sludge ^[5]. Chemical treatments can quickly oxidize and totally breakdown the organic contaminants, making them an effective wastewater treatment approach ^[6]. As a result, the development of advanced oxidation processes (AOPs), which have an excellent potential for ultimately destroying a variety of resistant pollutants, has shown to be a promising alternative ^[7].

2. Mechanism of Advanced Oxidation Processes

There are two types of AOPs: homogeneous processes and heterogeneous processes. The homogeneous processes in the AOP are defined by the chemical alterations resulting solely from the interactions between the chemical reagents and the target substances ^[8]. In heterogeneous processes, the reactants and products adsorb and desorb on the catalyst’s active sites. As the reaction occurs, the desorption of products and the absorption of new species takes place on the active sites; thereby, the efficiency is affected by the surface characteristics and the pore structure of the catalyst ^[9]. The different methods for the production of hydroxyl radicals take place via several combinations, such as the Fenton process (Fe²⁺/H₂O₂), the solar photo-Fenton process (solar/Fe²⁺/H₂O₂), solar photocatalysis (solar/TiO₂/H₂O₂), peroxone (Ozone/H₂O₂), the combination of peroxone with ultraviolet (Ozone/H₂O₂), Ozone/UV, and photolysis (H₂O₂/UV). The oxidation by-product formation mechanisms in the various AOPs are discussed.

2.1. Solar Photo-Fenton Process

Among the AOPs, the solar photo-Fenton process is particularly interesting for treating a large variety of hazardous pollutants and is one of the most environmentally benign treatment systems ^[10]. Due to the potential utilization of sunlight, this technique has drawn more attention. As a result, it may be considered to be an affordable AOP as sunlight is always available in a tropical country such as India ^[11]^[12]. Figure 1 shows the schematic diagram of the SPF.

Figure 1. Schematic diagram of a solar photo-Fenton process.

As shown in Equation (1), catalysis by ferrous iron breaks down hydrogen peroxide in an acidic medium to produce highly reactive species such as OH^● radicals without requiring high pressure or temperature. When solar irradiation is used, Fe³⁺ hydroxy complexes (Fe(OH)²⁺) and Fe³⁺ organic complexes (FeOOCR²⁺) can perform the photo-reduction in Equation (2) and the photodecarboxylation in Equation (3), enabling the regeneration of iron and generating extra OH^● radicals ^[13]. The hydroxyl radicals break down the complex organic structures into simpler compounds, and the partial oxidation of the non-biodegradable organics contributes to the biodegradability of the wastewater.

Fe²⁺ + H₂O₂ → Fe³⁺ + OH^● + HO (1)

[Fe(OH)]²⁺ + hν → Fe²⁺ + OH^● + H⁺(2)

Fe(OOCR)²⁺ + hν → Fe²⁺ + CO₂ + R^•(3)

In the solar photo-Fenton process, the significant characteristic of iron is that it undergoes cyclic oxidation and reduction. Table 1 shows the treatment of wastewater by various solar photo-Fenton processes.

Xu et al. (2007) ^[14] used the solar photo-Fenton technique to treat a pulp and paper mill’s bleaching wastewater. According to the reports, the TOC removal was usually faster for 15 min using a cost-effective source of solar light irradiation. After that, there was a slow increase for 3 h. Sheik et al. (2008) ^[15] explored the photocatalytic oxidation of organic contaminants such as p-nitroaniline, p-aminophenols, and acetanilide. It was found that the organic compounds were entirely oxidized and degraded into CO₂ and H₂O. Karimi et al. (2011) ^[16] focused on the discoloration of wastewater from a paper mill by the photo-Fenton process. The research revealed that after 15 min of photo-Fenton treatment, the soda effluent had dropped nearly 65% of its original color.

Lucas et al. (2012) ^[17] reported experimentally that about 90% of DOC degradation was attained with 5 mg/L of Fe²⁺ and 50 mM of H₂O₂ for the treatment of pulp and paper mill wastewater. The same DOC degradation can be achieved by solar photo-Fenton using less H₂O₂ and a shorter time. Bernabeu et al. (2012) ^[18] employed the photo-Fenton process to degrade emerging pollutants. According to the findings, emerging contaminants at high concentrations are removed at an acidic pH rather than a neutral pH. Trovo et al. (2012) ^[19] evaluated the photodegradation of paracetamol using ferrous sulphate and potassium ferrioxalate under artificial solar light. In contrast to FeO_x, FeSO₄ decreased Fe³⁺ to Fe²⁺, enhancing the decomposition by generating higher concentrations of hydroxylated intermediates. Garcia and Buitron (2013) ^[13] proposed the photo-Fenton process and obtained 97% decolorization of each azo dye and the development of chemometric tools, a strategy that reduced the peroxide from 33% to 65%. Turbay et al. (2013) ^[20] examined the photo-Fenton reaction-based tetracycline antibiotic degradation under varying hydrogen peroxide doses. The outcome demonstrated the improved effectiveness of the treatment and a more effective use of the hydroxyl radicals generated in the reaction medium.

Rocha et al. (2014) ^[21] treated extracted petroleum wastewater utilizing sunlight as the irradiation source. After 7 h of exposure to sunlight, the experimental results showed a reduction of about 92.7% and 96.2% of the polycyclic aromatic hydrocarbons and the aromaticity, respectively. Rodriguez et al. (2014) ^[22] explored the potential of the photo-Fenton process to decolorize and mineralize simulated wastewaters from wool dyeing tanks. The results indicated a high level of efficiency in decreasing TOC, COD, and BOD₅. Velegraki and Mantzavinos (2015) ^[23] studied the solar photo-Fenton technique for the treatment of winery effluent and achieved mineralization at a low catalyst dose (Fe²⁺ = 5 mg/L) and relied on an oxidant consumption of 500 mg/L. Chueca et al. (2015) ^[24] assessed the efficacy of a mild solar photo-Fenton system for disinfecting the actual effluents containing fecal bacteria. Despite the complexity and variety of the pollutants in the effluents, the tests conducted on the real effluents yielded highly encouraging results. Soares et al. (2015) ^[25] treated the textile dye wastewater using ferric organic ligands with the solar photo-Fenton process. It was reported that 87% of the mineralization was achieved at neutral pH values with biodegradability enhancement. Guzman et al. (2016) ^[26] assessed solar photo-Fenton treatment and various ozone-based processes for the pretreatment of synthetic samples of citrus wastewater. According to the findings, the photo-Fenton method removes COD and DOC at rates of 76.9% and 53.3%, respectively, which are higher than those of ozone-based systems.

Benitez and Penuela (2018) ^[27] examined the complete removal of benzophenone and the biodegradability increase using the solar photo-Fenton process under simulated solar irradiation. Costa et al. (2018) ^[28] treated biodiesel effluent, employing the solar photo-Fenton approach to decrease COD by adding oxalate at 50 kJ m⁻² of accumulated UVA radiation. About 72% of COD and 76% of BOD removal were achieved using 1 mmol L⁻¹ of ferrioxalate. Garcia et al. (2021) ^[29] investigated an advanced method for post-treating laundry effluent using the solar photo-Fenton process. It was found that a high removal efficiency was achieved, reaching 89% and 96%, respectively, in just 8 min. Arka et al. (2021) ^[30] investigated the solar photo-Fenton method for the treatment of institutional wastewater. According to the study, the solar UV/Fe²⁺/H₂O₂ process was very efficient in treating the wastewater from institutions, achieving a greater pollutant removal rate. The removal of six representative pharmaceuticals (PHCs) in raw hospital wastewater was investigated by Lumbaque et al. (2021) ^[31]. It was observed how the molar ratios (1:1 and 1:2) affected the breakdown of PHCs and the production of their transformation products during the treatments. The primary findings showed that PHC degradation is greatly favored in Fe³⁺:EDDS (1:2) due to the overall H₂O₂ consumption and the enhanced iron stability, with degradation efficiencies of above 77% in most PHCs.

Lin and Lin (2021) ^[32] used a solar photo-Fenton technique to remove cytostatic medicines. In comparison to three other Fe(III)-ligand complexes, the addition of EDDS caused the degradation of cyclophosphamide and 5-fluorouracil to occur more quickly. Gualda-Alonso et al. (2022) ^[33] carried out a continuous flow solar photo-Fenton process, employing a 100 m² raceway pond reactor (RPR), for the elimination of emerging contaminants. The RPR was run in continuous flow mode with 0.1 mM FeSO₄ and 1.47 mM H₂O₂ at an acidic pH and 1 h of hydraulic residence time. The liquid depth was fixed at 10 cm for winter and 18 cm for summer, removing >85% of the CEC in both cases. Pandey et al. (2023) ^[34] treated contaminated water by solar photo-Fenton using Fe-rich catalysts and obtained 82% COD removal.

The discussions mentioned above provide a general overview of the refractory compounds degraded by the photo-Fenton process and the effectiveness of the mineralization of these chemical pollutants under observed optimal conditions.

Table 1. Treatment of wastewaters by various solar photo-Fenton processes.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Pulp and paper mill	Solar photo-Fenton	Fe(II) concentration from 31 to 310 mg L⁻¹ (initial pH 3.0, 30 °C), initial H₂O₂ concentration from 0.5 to 3 Dth	TOC—82% Time—120 min	Xu et al., 2007 ^[14]
2	p-nitroaniline	Solar photo-Fenton			Sheik et al. (2008) ^[15]
3	Soda pulping effluent	Fenton and photo-Fenton	FeSO₄—1 mM H₂O₂—570 μL	Color—65%	Karimi et al. (2011) ^[16]
4	Pulp and paper mill	Solar photo-Fenton	5 mg/L of Fe²⁺ and H₂O₂ of 50 mM	90% of DOC mineralization	Lucas et al. (2012) ^[17]
5	Azo dye, Acid Blue 161	Solar photo-Fenton	H₂O₂/Fe²⁺ = 12, pH = 2.5 to 4.0	Degradation 40 %	Trovo et al., 2016 ^[19]
6	Sulfonated azo dyes	Solar photo-Fenton	Fe²⁺ = 5 to 38 mg/L, H₂O₂ = 98 to 828 mg/L	Color—97%	Garcia and Buitron (2013) ^[13]
7	Petroleum extraction	Photo-Fenton	485.3 mmol/L of H₂O₂	Hydrocarbons and aromaticity of about 92.7% and 96.2%	Rocha et al. (2014) ^[21]
8	Winery effluent	Photo-Fenton	(Fe²⁺ = 5 mg/L)	-	Velegraki and Mantzavinos (2015) ^[23]
9	Citrus wastewater	Photo-Fenton	H₂O₂ = 1017 mg/L, pH = 7	COD—77%	Guzman et al., 2016 ^[26]
10	Biodiesel effluent	Solar photo-Fenton	1 mmol L⁻¹ of ferrioxalate	72% of COD and 76% of BOD	Costa et al. (2018) ^[28]
11	Laundry effluent	Solar photo-Fenton	H₂O₂ (50–400 mg/L), Fe²⁺ (2.75–10 mg/L)	SDS removal—96%	Garcia et al. (2021) ^[29]
12	Industrial effluent	Solar photo-Fenton	(H₂O₂)—0.25 to 1.25 g/L, Fe²⁺—0.005 to 0.12 g/L, pH (2 to 10), time (30 to 180 min)	color (91%), turbidity (90%), and COD—86%)	Arka et al. (2021) ^[30]
13	Hospital wastewater	Solar photo-Fenton	Fe³⁺:EDDS = 1:2, H₂O₂ (230 mg L⁻¹)	Degradation = 77%	Lumbaque et al., 2021 ^[31]
14	Cytostatic drugs	Solar photo-Fenton	pH 3.0–8.5, 0.1 mM Fe(III)-EDDS, 1 mM H₂O₂		Lin and Lin, 2021 ^[32]
15	Emerging contaminants	Solar photo-Fenton	0.1 mM FeSO₄ and 1.47 mM H₂O₂ at an acidic pH and 1 h of hydraulic residence time	>85%	Gualda-Alonso et al., 2022 ^[33]
16	Paraquat (PQ)-contaminated water	Photo-Fenton	Fe-containing industrial waste	COD—82%	Pandey et al., 2023 ^[34]

2.2. Solar Photocatalytic Process

Solar photocatalysis has been widely used to degrade organic compounds. According to numerous studies, many organic contaminants have been totally oxidized in irradiated semiconductor solutions ^[35]. Photocatalysts remove pollutants from wastewater primarily through hydroxyl radical (OH^●) attacks by transforming them into harmless compounds such as water and CO₂ ^[36]. The photocatalyst that has been researched the most is TiO₂. However, its usage is restricted to ultraviolet light because of the wide energy band gap (3.2 eV) ^[37]. When exposed to ultraviolet light, a semiconductor catalyst such as TiO₂ or another transition metal oxide is initiated by the photon absorption with enough energy to be equal to or greater than the catalyst’s bandgap energy, stimulating an electron to move from the valence band to the conduction band, creating a hole in the valence band while using just 4% of the solar radiation ^[38]^[39]^[40].

TiO₂ photocatalysis uses UV-A radiation (λ < 387 nm) in the presence of oxygen and water to produce hydroxyl radicals for the excitation of the photocatalyst ^[41]. The overall rates of pollutant degradation are significantly influenced by the surface area of the TiO₂ catalysts that contains active sites. Thus, increasing the TiO₂-specific surface area effectively increases the pollutant adsorption capacity on the catalyst. As a result, a larger surface area with a more significant number of active sites will lead to a faster and more extensive reaction ^[42].

The activation Equation (4) can be written as

TiO₂ + hν → h⁺ + e⁻(4)

It is possible to express the oxidation and reduction reactions as given in Equations (5) and (6)

h⁺ + OH⁻ → -OH^●(5)

e⁻ + O_2ads⁻ → O_2ads⁻(6)

Another critical factor in the process of photocatalytic oxidation is the pH level. In alkaline media, the reaction of OH^● with holes on the TiO₂ surface causes the formation of hydroxyl radicals. Thus, hydroxyl radicals accelerate photocatalytic oxidation at higher pH solutions and vice versa ^[43]. Figure 2 shows the schematic diagram of the SPC.

Figure 2. Schematic diagram of the solar photocatalytic process.

Adish and Kanmani (2010) ^[44] used a single baffle reactor to photo-catalyze H₂O₂ to treat phenolic wastewater. According to reports, the solar/Fe²⁺/H₂O₂ process degrades phenol two to three times more quickly than the solar/TiO₂ process. Kumar et al. (2011) ^[45] studied the degradation of pulp and paper mill wastewater by a photocatalytic process. The findings showed that the increase in the BOD/COD ratio after photocatalytic oxidation was 0.09 for the primary wastewater with the removal of the COD (57.9%), BOD (42.9%), and color (89.2%). Dimitrakopoulou et al. (2012) ^[46] exploited eight different TiO₂ catalysts for the degradation of amoxicillin, and it was reported that Degussa P25 was the most active of the catalysts, with 93% mineralization after 25 and 95 min of reaction time at the amoxicillin concentrations of 10 mg/L and 250 mg/L of titania. Singh et al. (2013) ^[43] examined the photocatalytic oxidation of the organic content in synthetic wastewater at the pH values 2, 4, 6, 8, and 10 and at normal pH. The outcomes demonstrated clearly that an 86% reduction in COD was achieved at a normal pH level (pH = 6.8), and it was reported that an acidic pH was unfavorable for the reduction of organic content. Nagpure et al. (2013) ^[47] studied the photocatalytic decomposition of paper mill wastewater with ultraviolet and catalysts such as TiO₂. The degradation efficiency was observed as 91.34% using TiO₂ as a photocatalyst, with an optimum catalyst dose of 80 mg/50 mL for an irradiation time of 8 h under continuous stirring with maximum intensity. Ruiz et al. (2013) ^[48] studied the photodegradation of trimethoprim by various nanoparticles deposited on TiO₂. It was reported that the degradation of trimethoprim was only 50% of the organic matter mineralization with pure TiO₂-P25.

Shao et al. (2013) ^[49] produced mesoporous TiO₂ nanotubes with a large diameter for the photodecomposition of effluent from paper manufacturing. The result indicated that after 12 h of photodegradation, the chemical oxygen demand and chroma percent degradations of the paper manufacturing wastewater were approximately 73% and 99.5%, respectively. Metribuzin, a common herbicide, was transformed and mineralized in extensive detail by TiO₂-driven photocatalysis under simulated sunlight, as described by Antonopoulou et al. (2014) ^[50]. The results pointed out that the complete transformation of metribuzin was attained within 40 min at a metribuzin concentration of 10 mg/L, a TiO₂ concentration of 100 mg/L, and I = 750 W/m², whereas 80% mineralization was achieved within 300 min of irradiation.

Thomas et al. (2014) ^[51] compared the efficiency of TiO₂ and few-layer graphene nanocomposites for degrading rhodamine B under solar radiation. The titania incorporation on the few-layer graphene improved titania’s visible light photocatalytic activity, decreased electron–hole recombination, and increased electron–hole mobility. Fernandez et al. (2015) ^[41] investigated the photocatalytic treatment of polluted wastewater. The effects of many factors, including the type of microorganism, the water temperature, the dissolved oxygen concentration, and the water matrix composition were studied, and the results showed that this process has great potential for the chemical reduction and disinfection of pathogens. Murgolo et al. (2015) ^[52] investigated a novel photocatalyst based on nanosized TiO₂ upon radiation by both UV and solar-simulated light, resulting in degradation efficiencies ranging from 9 to 87% and 9 to 96%, respectively.

Solano et al. (2018) ^[53] evaluated the removal efficiency of hexavalent chromium (Cr⁶⁺) and divalent zinc (Zn²⁺) using heterogeneous photocatalysis with TiO₂. The result revealed that this process was efficient for the total removal of a Cr⁶⁺ concentration of 5 mg/L and was less effective for higher concentrations (15, 25 ppm); however, this technology was ineffective for Zn²⁺ removal. Yang and Yang (2018) ^[54] addressed the decomposition of rhodamine B using photocatalyst TiO₂. The result indicated that the catalyst reuse was found to be efficient up to 8 times with the same degradation efficiency. Aljouboury et al., 2021 ^[55] studied the ideal conditions for natural petroleum wastewater, utilizing TiO₂/ZnO/Fenton/solar and TiO₂/ZnO/air/solar. According to the reports, the TiO₂/ZnO/air/solar process achieved the highest treatment effectiveness with the COD removal of 74% and the TOC removal of 99% under ideal circumstances.

Kader et al. (2022) ^[56] investigated the synthesis of TiO₂-based photocatalysts, the production of TiO₂-based immobilized borosilicate glass reactors, and the use of the reactors to the treatment of methyl orange dye under UV light. The results revealed that the photocatalyst effectiveness dropped with the increase in pH and the initial dye concentrations. Total organic halogen was utilized as an analytical tool by Abusallout and Hua (2022) ^[57] to assess the effectiveness of solar TiO₂ photocatalysis in dehalogenating disinfection by-products in the water. Compared to pH 5, the TOX dehalogenation was improved at pH 9, and the addition of hydrogen peroxide slightly improved the TOX elimination. The findings demonstrated that compared to the anatase and rutile TiO₂ particles, the mixed-phase TiO₂ (Aeroxide P25) was significantly more efficient at removing TOX.

To immobilize TiO₂ for the removal of paraben from wastewater, Martins et al., 2022 ^[58] tested the suitability of utilizing polymeric supports. According to the results, polydimethylsiloxane is a suitable material to support TiO₂ for wastewater treatment by solar photocatalytic oxidation. Using two nanocomposites (TiO₂ and TiO₂ (Ag) doped), Behera et al. (2022) ^[59] compared the photocatalytic activity on the breakdown of phenol from water. The nanocomposites were created using the UV photo-reduction method with silver loadings of 0.25, 0.5, 0.75, and 1% (w/w). At pH 7, about 98% of the phenol was degraded in 180 min with 0.5 g L⁻¹ photo-catalyst TiO₂ (Ag-1.0) EY. Rueda-Marquez et al., 2020 ^[60] examined the solar photocatalytic process using immobilized TiO₂ for treating urban wastewater and obtained a maximum removal of greater than 40% of the pharmaceutically active compounds. Gai et al., 2021 ^[61] synthesized potassium and iodide TiO₂ and observed the enhancement of photocatalytic activity under simulated sunlight. Giang et al., 2023 ^[62] synthesized sulfur-doped TiO₂ for the degradation of crystal violet. The findings revealed the potential uses of the material in environmental treatments. Rapti et al., 2022 ^[63] treated hospital wastewater using TiO₂ with the catalyst loading between 150 mg/L and 200 mg/L and attained the degradation efficiency of 73%.

Although numerous research studies on the photocatalytic degradation of wastewater pollutants using TiO₂ as a semiconductor have been reported, its industrial application has some constraints, including the recombination of the photogenerated electron–hole pair, the lack of and the inability to produce a catalyst with high photon efficiency and low cost that can absorb a broader range of the solar spectrum, and the surface structure ^[38]. Moreover, the aggregation of the TiO₂ catalyst reduces the surface area and hence the effectiveness of the catalyst ^[64]. Because of the large amounts of chemicals required for proper performance, photocatalysis is only sometimes considered a viable treatment to be used alone; however, their combination with biological processes may improve the overall process efficiency, increasing their viability. Table 2 shows the treatment of wastewater by various solar photocatalytic processes.

Table 2. Treatment of wastewater by various solar photocatalytic processes.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Pulp and paper mill	Solar photocatalytic (TiO₂)	-	COD—57.9%, BOD—42.9%, color—89.2%	Kumar et al. (2011) ^[45]
2	Amoxicillin	Solar photocatalytic (TiO₂)	TiO₂ catalysts 100 to 750 mg/L	93% mineralization	Dimitrakopoulou et al. (2012) ^[46]
3	Synthetic wastewater	Photocatalytic oxidation	pH values 2, 4, 6, 8, 10	COD—86%	Singh et al. (2013) ^[43]
4	Trimethoprim	Photocatalytic oxidation	-	50% mineralization	Ruiz et al. (2013) ^[48]
5	Pulp and paper mill	Photocatalytic oxidation	-	COD—73%, chroma percent—95%	Shao et al. (2013) ^[49]
6	Metribuzin	Photocatalytic oxidation	TiO₂—100 mg/L and I = 750 W/m²	80% mineralization Reaction time—300 min	Antonopoulou et al. (2014) ^[50]
7	Organic pollutants	Nanosized TiO₂ supported on single-wall carbon nanotubes	-	UV—9 to 87%, simulated solar light—9 to 96%	Murgolo et al. (2015) ^[52]
8	Hexavalent chromium	Solar photocatalytic (TiO₂)	pH = 2, 6 and 10, TiO₂—2.0, 2.5, and 3.0 g/L	Complete removal for 5 mg/L Cr⁶⁺	Solano et al. (2018) ^[53]
9	Rhodamine B	Solar photocatalytic (TiO₂)	Film thickness = 361 nm Specific surface area of 47.72 m² g⁻¹	98.33% in 30 min	Yang and Yang (2018) ^[54]
10	Urban wastewater	Immobilized TiO₂		High removal (>40%)	Rueda-Marquez et al., 2020 ^[60]
11	Petroleum wastewater	TiO₂/ZnO/Fenton/Solar and TiO₂/ZnO/Air/Solar	ZnO dosage of 54 g/L and TiO₂ dosage 50 g/L	COD—74%, TOC—99%	Aljouboury et al., 2021 ^[55]
12	Organic pollutant	Solar TiO₂ photocatalysis		Perfect acid and alkali resistance	Gai et al., 2021 ^[61]
13	Methyl orange dye	TiO₂-based photocatalysts	pH—7.0, reaction time—5.5 h of UV irradiation, photocatalyst (0.12 g)	Degradation—75.8%	Kader et al. (2022) ^[56]
14	Dehalogenation disinfection by-products	Solar TiO₂ photocatalysis	pH 9, TiO₂—100 mg/L	Effective removal of TOX	Abusallout and Hua (2022) ^[57]
15	Pharmaceutical residues	Solar TiO₂ photocatalysis	TiO₂—200 and 300 mg L⁻¹	Higher than 56%	Rapti et al. (2022) ^[63]
16	Crystal violet	Titanium dioxide/graphene aerogel-doped sulfur		Enhanced photo-degradation	Giang et al., 2023 ^[62]

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