Metal oxide-based photocatalysts for the efficient degradation of organic pollutants for a sustainable environment: a review

Photocatalytic degradation is a highly efficient technique for eliminating organic pollutants such as antibiotics, organic dyes, toluene, nitrobenzene, cyclohexane, and refinery oil from the environment. The effects of operating conditions, concentrations of contaminants and catalysts, and their impact on the rate of deterioration are the key focuses of this review. This method utilizes light-activated semiconductor catalysts to generate reactive oxygen species that break down contaminants. Modified photocatalysts, such as metal oxides, doped metal oxides, and composite materials, enhance the effectiveness of photocatalytic degradation by improving light absorption and charge separation. Furthermore, operational conditions such as pH, temperature, and light intensity also play a crucial role in enhancing the degradation process. The results indicated that both high pollutant and catalyst concentrations improve the degradation rate up to a threshold, beyond which no significant benefits are observed. The optimal operational conditions were found to significantly enhance photocatalytic efficiency, with a marked increase in degradation rates under ideal settings. Antibiotics and organic dyes generally follow intricate degradation pathways, resulting in the breakdown of these substances into smaller, less detrimental compounds. On the other hand, hydrocarbons such as toluene and cyclohexane, along with nitrobenzene, may necessitate many stages to achieve complete mineralization. Several factors that affect the efficiency of degradation are the characteristics of the photocatalyst, pollutant concentration, light intensity, and the existence of co-catalysts. This approach offers a sustainable alternative for minimizing the amount of organic pollutants present in the environment, contributing to cleaner air and water. Photocatalytic degradation hence holds tremendous potential for remediation of the environment.


Introduction
Urbanization and industrialization are cornerstones of modern civilization, underpinning signicant advances in economic growth, technological innovation, and improved standards of living. 1 These processes have facilitated the development of cities, expanded infrastructure, and increased industrial productivity, creating myriad opportunities for societal progress. 2,34][15] The environmental impact of these pollutants is profound, as they can persist in the environment, bioaccumulate in wildlife, and enter human food chains, leading to chronic health issues and ecological damage. 16,17The complexity and resilience of these organic pollutants necessitate the development of advanced treatment technologies. 18,19raditional biological treatment methods are oen inadequate for fully degrading these pollutants due to their toxicity and chemical stability.][22] AOPs are distinguished by the production of extremely reactive species, such as hydroxyl radicals, that can indiscriminately oxidize a broad spectrum of organic pollutants.This process converts the pollutants into less dangerous chemicals or fully mineralizes them into carbon dioxide (CO 2 ) and water (H 2 O). 23mong the various AOPs, photocatalytic degradation stands out as a particularly effective method. 20Photocatalysis involves the use of semiconductor materials as catalysts to accelerate chemical reactions upon exposure to light.5][26] These reactive species possess the very capability of breaking down complex organic pollutants into less harmful, simpler molecules and fully mineralizing them. 27,28The advantages of photocatalysis are numerous and include low operational costs, the ability to accomplish full mineralization of contaminants without generating secondary pollution, and the capability to operate at ambient temperatures and pressures. 29Among the various photocatalysts, titanium dioxide (TiO 2 ) is the most extensively studied and broadly applied because of its exceptional chemical and photochemical stability, cost-effectiveness, low toxicity, and high activity under ultraviolet (UV) light.TiO 2 , with its wide band gap of approximately 3.2 eV, can mineralize a broad spectrum of organic contaminants, including herbicides, dyes, pesticides, phenolic compounds, and pharmaceuticals like tetracycline and sulfamethazine. 30,31Nevertheless, the actual utilization of TiO 2 is somewhat restricted due to its dependence on UV light, which comprises just a minor portion of the solar spectral region. 32To overcome this limitation, other semiconductor materials with broader light absorption properties are being explored.Tungsten trioxide (WO 3 ) has emerged as a promising alternative due to its capability of absorbing visible light, making it more competent for photocatalytic oxidation of volatile organic pollutants under natural sunlight. 33,34Additionally, silver nanoparticles (AgNPs) have gained signicant attention as photocatalysts due to their high photostability, environmental friendliness, and catalytic properties that are dependent on their shape and size. 35The effectiveness of photocatalytic systems in degrading organic pollutants is dependent on numerous operational parameters.These factors encompass the substrate concentration, photocatalyst quantity, pH of the solution, reaction medium temperature, light irradiation duration and intensity, photocatalyst surface area, dissolved oxygen content in the reaction medium, and the characteristics of both the photocatalyst and substrate. 29,36,37Furthermore, the doping of photocatalysts with metal and non-metal ions can enhance their photocatalytic activity by modifying their electronic properties and extending their light absorption range. 38It is important to optimize these parameters to maximize the degradation kinetics and overall efficiency of photocatalytic processes. 39For instance, the proportion of the substrate to the photocatalyst must be carefully balanced to ensure that there are enough reactive sites for pollutant molecules to adsorb and react. 37The pH of the solution can affect the charge and surface properties of the photocatalyst, inuencing its interaction with pollutants.1][42] In this review, we focused on the degradation of six specic types of organic pollutants: antibiotics, organic dyes, nitrobenzene, toluene, oil, and cyclohexane.These pollutants represent a broad spectrum of chemical structures and environmental impacts, making them ideal candidates for studying the effectiveness of various photocatalysts under different operational conditions.We will delve into the various reaction parameters that are critical to achieving maximum degradation of these pollutants using different photocatalysts.This comprehensive analysis aims to provide insights into the optimal conditions and catalyst selections for effective wastewater treatment, contributing to the mitigation of environmental pollution and the protection of aquatic ecosystems.

Organic dyes
A signicant group of synthetic organic molecules produced by a variety of industries, including the leather, plastic, food, paper, textile, and medicinal sectors, are known as dyes. 35,535][56][57] Over 700 000 tons of dyes are generated globally each year; 20% of these lost dyes reach the atmosphere and create pollution throughout processing or manufacturing, accounting for about 12% of the global total of dye generation.So the degradation of these organic dyes is necessary for maintaining the ecological balance. 58Organic dyes are very detrimental to aquatic ecosystems, even at low concentrations (less than 1 ppm).Thus, it is essential and required to remove organic dyes from effluents. 59The degradation mechanism of methylene blue dye is as follows. 60otocatalyst + hn (photon) / Photocatalyst (e cb − + h vb + ) (1) Methylene blue + cOH / Degradation products (5) Methylene blue + cO 2 − / Degradation products (6) Several metal oxides, such as ZnO, MgO, AgO, TiO 2 , Fe 2 O 3 , Mn 2 O 3 , CuO, and V 2 O 5 , are frequently employed as photocatalysts in wastewater treatment processes to degrade dyes. 61inc oxide (ZnO) is an oxidizing substance found in nature as the unusual mineral zincite.There have been attempts to use ZnO alongside other semiconductors for the photocatalytic degradation of an extensive variety of biological pollutants. 62nO-based photocatalysts work according to various parameter conditions.These parameters are mainly Ph, the initial concentration of dye or catalyst, the wavelength of the light & so on.The photocatalytic reaction rate at the outermost layer of the catalyst can be inuenced by the initial concentration of the substrate.To prevent the dispersion of light and the concentration impact of the exposed photocatalyst surface, the ideal photocatalyst concentration ought to be unique for heterogeneous photocatalysis processes. 63Velmurugan et al. stated that the rate of degradation k dropped from 0.173 to 0.012 min −1 when the dye concentration was increased from 1 × 10 −4 to 4 × 10 −4 M. 64 This is because many layers of adsorbed dye molecules have formed on the outermost layer of the catalyst, which prevents the photoreaction from occurring because there was not enough direct light interaction to produce hydroxyl radicals. 65The rst amount of dye has a signicant inuence on the degradation efficiency of MB. 66 Sobana et al. used ZnO that was manually combined with activated carbon (AC-ZnO) and solar irradiation to study the impact of initial Direct Blue 53 (DB53) concentration over the concentration range from 1 × 10 −4 to 9 × 10 −4 M. 67 Its numerous functions make it extremely difficult to determine how the pH of a solution affects the efficacy of the dye photocatalytic degradation activity. 68Velmurugan et al.
stated the impact of pH in the range of 3-11 upon the photocatalytic breakdown of Reactive Red 120 (RR 120) over ZnO during solar light irradiation. 64Photocatalytic breakdown of Reactive Orange 4 (RO4) and Black 5 (RB5) dyes occurs at various solution pH levels between 3 and 11. 69 The pH, which regulates the adsorption of organic compounds on the outermost layer of the photocatalyst, serves as one of the most crucial factors inuencing photocatalysis effectiveness. 70Electromagnetic relationships between the outermost layer of the photocatalyst and the substrate of interest can be employed to clarify how pH affects photocatalysis outcomes. 27Singh et al. stated that aer exposing ZnO nanorods to UV radiation for 120 minutes, photodegradation activity levels were 7.169% and 47.63% for pH values of 4.5 and 10.5, correspondingly. 71cientists' interest has been drawn more and more to supported TiO 2 catalyst utilization over the past few years due to its prospective uses in the photocatalytic breakdown of organic contaminants such as organic dyes in air and water.Additionally, reports have it that when adsorbents are used to support TiO 2 , an ideal condition is created for the elimination or degradation of the compounds of interest. 72,73To enhance TiO 2based photocatalysts on organic dye in wastewater, several conditions were adjusted.These crucial elements, which included light intensity, TiO 2 form and structure, target type, pH level and doping type, all had an impact on the photocatalysis method's effectiveness. 58If we want to discuss the parameters it is found that it is rather tough to comprehend how pH impacts the photodegradation process's efficacy. 29TiO 2 exhibits amphoteric properties, allowing for the development of either a positive or negative charge on its outermost layer. 74Due to this, the adsorption of dye molecules over TiO 2 surfaces may be affected by changes in pH. 75Bubacz et al. found that when pH is increased, so did the rate at which methylene blue was broken down photo-catalytically. 76 On the other hand, Neppolian et al. showed that acidic conditions do not affect the degradation rate of the Reactive Blue 4 signicantly enough. 77It has been found that organic dyes like Reactive Black 5 and Reactive Orange 4 degradation were enhanced in an acidic solution containing TiO 2 . 69Tanaka et al. discovered that at less acidic values, the positively charged TiO 2 layer absorbed more Acid Orange 7, and greater breakdown was accomplished. 78A study has been conducted on the effects of pH on the adsorption as well as decolorization of Procion Red MX-5B (MX-5B) and Cationic Blue X-GRL (CBX).It was discovered that when the pH increased, MX-5B's adsorption was reduced. 62Another key parameter for dye degradation using a TiO 2 catalyst is the dye amount or dye concentration.It has been found that the increased initial concentration of the dyes increases the degradation rate. 36,79This is because when the dye's initial concentrations rise, the dye molecules become deposited on the outermost layer of the catalyst and consume a sizable proportion of UV light instead of the TiO 2 nanoparticles.(from 1.9 × 10 −4 to 5.9 × 10 −4 M). 77 The dye degradation in a water-based solution utilizing a catalyst powder of TiO 2 within a photocatalytic reactor is inuenced by two additional parameters: the wavelength and intensity of the UV light irradiation source. 82Lower radiation wavelengths are thought to encourage the creation of electron holes, which would increase the catalyst's effectiveness. 83Ollis et al. said that at minimal light levels (0-20 mW cm −2 ), the rate would rise in an orderly manner as the intensity of light increased.The rate would rely on the square root of the light intensity at moderate light intensities (about 25 mW cm −2 ) but at intense light levels, the rate is independent of the light intensity. 29,84The degradation of Orange G was shown to be affected by light intensity in a range of 215 to 586 W cm −2 .With a rise in light magnitude, Orange G's photolysis reaction rates climbed. 85Rao et al. stated that Acid Orange 7 (AO7) photocatalytically breaks down at a pace that is roughly 1.5 times faster in direct sunlight compared to that under synthetic UV radiation. 86Another signicant operational parameter for the organic dye degradation is temperature range. 36The range of 40-50 °C was determined to be the ideal operating temperature range.Since desorption of the produced products happens more slowly at low temperatures than interface degradation as well as reactant adsorption, it restricts the reaction.Conversely, the limiting step becomes the dye's adsorption on TiO 2 at an elevated temperature. 87A table has been added showing the photocatalytic degradation of organic pollutants (Table 1) and the process is illustrated in Fig. 1.The rate constant is lowered at elevated temperatures due to the organics' and hydrated oxygen's reduced adsorptive ability.Consequently, the ideal temperature oen falls between 293 and 353 K. 108,109 Antibiotics Due to their extremely stable and non-biodegradable nature, antibiotics accumulate in the ecosystem as a result of overuse and uncontrolled environmental discharge. 110,111The release of diverse antimicrobial pollutants and their varied toxicity provide a signicant challenge for researchers trying to nd a solution. 112,113The excessive accumulation of antibiotics in natural environments has presented a signicant peril to ecological systems. 114,115Unfortunately, traditional water treatment methods such as adsorption, ltration, and biodegradation are ineffective in effectively removing antibiotics due to their signicant durability and limited biodegradability.
1][122] Furthermore, it is crucial to provide an overview of frequently utilized photocatalytic nanomaterials and their specic use in breaking down popular antibiotics.4][125] The degradation mechanism of cipro-oxacin antibiotic in the presence of different photocatalysts is provided. 126otocatalyst + hn (photon) / Photocatalyst (e cb − + h vb + ) (8) Ciprofloxacin + cOH / Degradation products (12) Yang et al. researched the degradation of ciprooxacin using g-C 3 N 4 /TiO 2 nanocomposites with the help of visible light irradiation utilizing a 300 W Xe visible lamp where the authors observed 88% of CIP degraded in 180 minutes. 127erma explored the degradation of amoxicillin (AMX) by the utilization of TiO 2 photocatalysis and sono-photocatalysis and achieved the highest degradation rate (80%) of AMX at a pH of 7.0 under UV irradiation at a power density of 672 W m −2 . 128hang examined the mechanism and kinetics of photocatalytic The study revealed that bi-titanate nanoribbons, when used at a concentration of 1 g L −1 , had the most effective photocatalytic degradation capability, achieving a rate of 88%. 130The catalytic efficiency of NiS and NiS immobilized within the magnetite polypyrrole core/shell matrix (Fe 3 O 4 @PPY) was examined for the degradation of cephalexin.The study also examined the photocatalytic breakdown of cefalexin using the NiS-PPY-Fe 3 O 4 photocatalyst, which was exposed to sunshine.The photocatalyst demonstrated a removal efficiency of over 80% over a 30 minute timeframe. 131Payan studied the creation of photocatalysts using Cu-TiO 2 @functionalized single-walled carbon nanotubes and found that sulfamethazine can be fully destroyed under solar irradiation within 300 minutes.

A table has been added
showing the photocatalytic degradation of antibiotics Table 2 and Fig. 2 shows the process.The ndings indicate that the breakdown percentage of TiO 2 suspension at favorable pH conditions (pH 5) is 96.47% aer 60 minutes of irradiation. 150her industrial pollutants (toluene, nitrobenzene, cyclohexane, and refinery oil) Industrial chemical pollutants are a subgroup of chemical pollutants specically connected with industrial operations. 158hey encompass a wide spectrum of chemicals used or produced in manufacturing, rening, and other industrial processes. 159Industrial chemical pollutants, including toluene, cyclohexane, nitrobenzene, and renery oil, pose signicant environmental threats due to their widespread use and high toxicity. 160,161Toluene, an industrial solvent, pollutes air, water, and soil, causing harm to aquatic organisms and long-term environmental damage. 162Cyclohexane, used in chemical production, contributes to air and water pollution, affecting aquatic life. 163Nitrobenzene, a dye and pharmaceutical precursor, contaminates soil and water, posing toxic and carcinogenic risks. 164Renery oil, a byproduct of petroleum rening, causes extensive damage through spills and leaks, affecting marine and terrestrial ecosystems. 165Photocatalytic degradation is crucial for mitigating these pollutants, as it offers an efficient, eco-friendly method to break down these toxic substances, preventing their persistence in the environment and safeguarding both ecosystems and human health. 166

Toluene
As one of the pollutants that pose a risk to human health and the ecosystem, toluene has been classied as a priority pollutant; for this reason, emission management is required. 167,168Owing to the serious issues that toluene causes, various methods for toluene abatement have been developed. 169he rapid growth in industrialization and urbanization has played a notable role in the emergence of severe environmental issues. 170,171][174][175][176] Therefore, it is necessary to enhance the efficacy of eliminating indoor toluene vapors.Methods to counteract atmospheric pollution can be classied as either chemical or physical approaches. 177,178Physical approaches include adsorption, the process of condensation, and separating membranes.Chemical approaches encompass combustion, low-temperature plasma, biological, and photocatalytic treatments. 179,180Photocatalysis is regarded as a very promising option for environmental cleaning among these techniques.Photocatalytic technologies provide the benets of being non-toxic and cost-effective, requiring gentle reaction conditions, and producing no secondary pollutants. 136,181Almost all the hydrocarbon degrades via the following mechanism. 182,183otocatalyst + hn (photon) / Photocatalyst (e cb − + h vb + ) (15) in a nanoscale form.This nanomaterial was then employed to fabricate a composite photocatalyst consisting of In 2 S 3 and g-C 3 N 4 .The process of toluene photocatalytic decomposition was investigated, and a feasible mechanism was proposed.The In 2 S 3 /g-C 3 N 4 heterojunctions exhibited the highest photocatalytic degradation when a 40% loading of In 2 S 3 was used.184 B. N. R. Winayu et al. enhanced the TiO 2 catalyst by introducing sulfur and nitrogen (S, N) components and reduced graphene oxide (rGO) through doping.The most efficient photocatalytic degradation of toluene was achieved using a combination of 1 wt% reduced graphene oxide (rGO) and 0.05 wt% nitrogen-doped titanium dioxide (N 0.1 TiO 2 ).185 V. T. T. Ho et al. stated that the nanostructured Ir-doped TiO 2 is a highly effective photocatalyst that produces a superb material for reducing the risk of gaseous toluene.The material had a large surface area and had a consistently spherical shape of 10-15 nm diameter.186 The composite of PIL (polyionic liquid)@TiO 2 was formed using two different concentrations of polymerized ionic liquid (low and high).The composite was then assessed for its ability to degrade toluene.The ndings indicated that the PIL(low)@TiO 2 composite exhibited higher activity compared to the PIL(high) @TiO 2 composites.187    Photocatalysts that were articially created were utilized for the process of breaking down gaseous toluene dynamically using photocatalysis while being exposed to UV radiation.191 Rostami synthesized a TiO 2 and bentonite photocatalyst by a method called co-precipitation and evaluated its catalytic efficiency in degrading para-nitrotoluene (PNT).192 Oxygen vacancies (OVs) can regulate photocatalytic activity by altering their electrical and/or band structures.A wide bandgap p-block metal combination containing OVs, indium oxyhydroxide (InOOH), produced using a one-pot hydrothermal approach, was used to investigate the effect of OVs on photocatalytic decomposition and toluene ring breakage.Validated modied InOOH improves photocatalytic potency by decreasing the energy limitation of critical intermediates for reaction during toluene degradation.nanorods for the degradation of toluene using vacuum ultraviolet (VUV) catalytic oxidation.CeO 2 nanorods were utilized in a system that involved VUV-photolysis, UV-PCO, OZCO, and UVOZCO processes.Utilizing VUV light instead of ozone catalytic oxidation can signicantly enhance the efficiencies, increasing them from 12.9% to 83.2% when combined with the suggested catalyst.194 An efficient electrochemical method consisting of two steps was devised to produce a nanotube array of atomically dispersed Au-loaded WO 3 /TiO 2 for the oxidation of volatile organic compounds (VOCs).The presence of vacancies (OVs) on the surface of WO 3 greatly improved the separation and movement of photogenerated carriers, as well as the adsorption of toluene.This resulted in an 85.5% mineralization and 95.4% degradation rate for the removal of toluene. 195J. Lyu et al. fabricated a hollow heterophase junction by applying a layer of amorphous TiO 2 onto anatase TiO 2 hollow spheres.The ndings demonstrated that the application of the amorphous TiO 2 coating resulted in an augmentation of ne pores and intermediate pores in the photocatalyst, leading to an improved capacity for toluene adsorption.196 By adding nanodiamonds to ZnO, the photocorrosion problem can be solved for photocatalytic degradation of gaseous toluene.A table has been added showing the photocatalytic degradation of toluene Table 3 and Fig. 3 shows the process.Nanodiamond decoration resulted in lowered photoluminescence intensity and electrochemical impedance, enhancing ZnO light absorption, charge transfer, and photocatalytic toluene oxidation efficiency.197

Nitrobenzene
Since aromatic nitro compounds are frequently employed in industrial processes (such as the production of explosives, dyes and insecticides), they are present as contaminants in a variety of liquid sources, particularly surface water, and wastewater from industries. 203Since nitrobenzene (NB) is identied as a signicant contaminant, it is selected as a model pollutant.It is an extremely hazardous material and the highest permitted level of NB is 1 mg L −1 in wastewater. 204,205Numerous factors, including the presence of anions, pH, light wavelength, and others, have an impact on nitrobenzene photocatalytic degradation utilizing UV radiation. 206The degradation working mechanism of nitrobenzene in the presence of several photocatalysts is described. 207,208otocatalyst + hn (photon) / Photocatalyst (e cb − + h vb + ) (22) Nitrobenzene / Catalyst surface (24) The study of the impacts of several factors, such as pH, anions, starting concentration, etc., has been done because the rate of breakdown of nitrobenzene utilizing controlled UV radiation is quite signicant when compared to that utilizing solar radiation, and a small amount of TiO 2 (0.05%, w/v) was used. 209,210Degussa P-25 TiO 2 was utilized as the photocatalyst in the majority of the nitrobenzene photocatalytic tests.Aldrich-TiO 2 (pure anatase with a BET surface area of roughly 250 m 2 g −1 ) was used in a few tests. 206Matthews et al. used immobilized TiO 2 in a spiral-shaped reactor for the photocatalytic degradation of NB and other chemicals and accomplished around 95-100% degradation at the initial concentration between 1.75 and 4.25 mg L −1 . 211Degussa P-25 was applied as the catalyst in photocatalytic degradation tests, and UV lamps with lights radiating at l max of 253 and 365 nm, respectively, were used.The two bulbs produced nearly identical deterioration. 212When it comes to 4-chlorophenol degradation, it has been discovered that utilizing pulsed photocatalysis makes little distinction in terms of TOC elimination at shorter and longer wavelengths.It should be mentioned that 387 nm is the l min for anatase TiO 2 . 213The pH has an impact on the ionizable organic molecules' photocatalytic breakdown.The signicance of pH on the photocatalytic destruction of NB was assessed within a pH value range of 4-10, in a solution containing 2.52 × 10 −4 M of pollutants.The ideal photocatalyst concentration was determined to be 0.5 wt% Fe-TiO 2 = 250 mg L −1 , with an irradiation period of 60-240 minutes. 214A table has been added showing the photocatalytic degradation of nitrobenzene Table 4 and Fig. 4 shows the process.It has been discovered that, given the specied conditions, pH 7 is ideal for NB photocatalytic breakdown. 205clohexane A common volatile organic compound (VOC) that presents signicant dangers to both humans and the environment is cyclohexane. 225An extremely signicant industrial procedure is the breakdown of cyclohexane to produce cyclohexanol and cyclohexanone which are utilized globally as chemical precursors for the synthesis of caprolactam and adipic acid. 226,227hotocatalytic techniques for the degradation of cyclohexane in both solid heterogeneous and homogeneous stages have received a lot of research attention in recent years. 228In heterogeneous environments, semiconductors along with oxides are being used as photocatalysts to oxidize cyclohexane.A number of semiconductors have been used, including CeO 2 , WO 3 , Sn/Sb, ZrO 2 , ZnO, V 2 O 5 , SnO 2 , Sb 2 O 4 and mixed oxides. 229n the presence of various types of photocatalysts, cyclohexane degrades via the following mechanism. 230,231ble 5 Data for the catalytic degradation of cyclohexane using various catalysts Cyclohexane conc.Xiao et al. discussed the photocatalytic characteristics of silver nanoparticles loaded on the nanocrystals of tungsten oxide when cyclohexane was being photo-catalytically degraded. 232In standard manufacturing processes, cyclohexane is degraded at 150 °C using a homogeneous cobaltbased catalyst. 228Variations in the emitted photon ux and the irradiation wavelength during continuous irradiation result in notable variations in substance outputs and selectivity values during the photocatalytic degradation of cyclohexane by the help of TiO 2 in a pure liquid organic phase. 233The photodegradation of cyclohexane proceeded with hydrogen peroxide at ambient temperature, assisted by a copper(II)-exchanged Y zeolite (CuY).A table has been added showing the photocatalytic degradation of cyclohexane Table 5 and Fig. 5 shows the process.Following 6 hours of processing, cyclohexanol and cyclohexyl hydroperoxide with 37% and 54% selectivities, respectively, were obtained as the major products. 247

Renery oil
5][256] Conventional methods like adsorption or membrane separation produce an inferior contaminant by moving the contamination from one phase to another, and the reusability of adsorbents is uncertain. 257,2580][261] Photocatalytic degradation techniques have attracted signicant attention due to their ability to break down a wide range of organic compounds utilizing suitable photocatalysts. 52,262,263he degradation of pollutant chemicals is caused by the hydroxyl radical (OH), which can react with organic compounds and break them down and degrade them. 264,265The mechanism for renery oil degradation in the presence of various photocatalysts is given.) nanocomposites with varying amounts of BiOI deposited via sequential ionic layer adsorption and reaction (SILAR) and found that they perform well in water under visible (>400 nm) irradiation for crude oil degradation.The BiOI/TiO 2 heterojunction separates photogenerated charges, improving degradation efficiency. 268Actual wastewater from a renery, containing a variety of aromatic and aliphatic organic compounds, was treated using nanoparticles (specically TiO 2 and ZnO).The degradation ability of the organic contaminants was reduced from 98.57% to 89.482% when the photocatalysts changed from TiO 2 to ZnO. 267 Data for the photocatalytic degradation of refinery oil using various catalysts Nanoscale Advances Review light (1000 W m −2 ), to decrease the total organic carbon (TOC) content in the actual petroleum wastewater obtained from Sohar Renery Company (SRC).The treatment efficiency for total organic carbon (TOC) at pH 5.5 increased signicantly compared to that of the TiO 2 procedure. 269Z. Ghasemi et al. examined the photocatalytic oxidation of organic contaminants in petroleum renery wastewater (PRWW) utilizing synthesized nano-TiO 2 incorporated into Fe-ZSM-5 zeolite and UV light.Results indicate optimal photodegradation efficiency at 3 g L −1 photocatalyst concentration, pH 4, 45 °C temperature, and 120 min UV irradiation. 270Shahrezaei investigated TiO 2 photocatalysis for the primary degradation of phenol and phenolic compounds in renery wastewater.Under optimal conditions, 90% phenol removal was achieved in 2 hours. 271he user created a composite membrane by combining polyvinylidene and titanium dioxide (PVDF/TiO 2 ) and then treated it using the hot-pressing method.A table has been added showing the photocatalytic degradation of cyclohexane Table 6 and Fig. 6 shows the process.This treatment was done to increase the bonding between the TiO 2 and the membrane surfaces, to employ the membrane to degrade oil in wastewater.

Effects of crystal size and surface area on photocatalytic degradation
Organic chemicals and the photocatalyst's surface coverage are directly correlated, and therefore surface morphology, such as crystal size and the surface area, must be taken into account during the photocatalytic degradation procedure. 287,288Since every chemical process occurs at the surface, the surface morphology of any photocatalyst is essential to its efficacy as a catalyst. 289The anatase phase with a range of 2.59 to 12.00 nm in TiO 2 crystallite dimensions is visible in metal-doped TiO 2 products.TiO 2 has a specic surface area of between 100 and 500 m 2 g −1 . 290,291Sivalingam et al. used a solution combustion process where 8-10 nm pure anatase phase TiO 2 with 156 m 2 g −1 BET surface area was created.This TiO 2 is commonly utilized for photocatalytic degradation of many dyes, including Orange G, Methylene Blue, Alizarin S, Methyl Red, and Congo Red.In this analysis, the crystal size of the photocatalyst was found to be 8 ± 2 nm. 292The photoactivity of the photocatalysts increased due to the higher surface area.It has been found that the photoactivity of the TiO 2 while degrading the dye-like MB increased when the surface area of the catalyst increased from 63 m 2 g −1 to 156 m 2 g −1 . 293For the maximum degradation of antibiotics like cefoxitin sodium, a novel BN/CdAl 2 O 4 composite with a surface area of 14.34 m 2 g −1 is used. 133Mushtaq et al. found a decrease in the degradation rate of the ooxacin antibiotic due to the increase in the particle size and decrease in the surface area of the photocatalysts. 294The same scenario was also found during the advanced degradation of tetracycline antibiotics by graphene-ordered mesoporous silica. 295Zhou et al. used highly photoactive mesoporous anatase nanospheres that have a high specic surface area of 609 m 2 g −1 for the degradation of toluene. 296The highest specic surface area (130.3 m 2 g −1 ) of

Mechanism of photocatalytic degradation
Photocatalytic degradation is a process where light energy, typically from UV or visible light, activates a photocatalyst, such as titanium dioxide (TiO 2 ).When the photocatalyst absorbs light, it generates electron-hole pairs.These electron-hole pairs can initiate redox reactions that produce reactive oxygen species (ROS) like hydroxyl radicals and superoxide anions.These ROS are highly reactive and can break down organic pollutants, converting them into less harmful substances like water, carbon dioxide, and inorganic ions.The overall mechanism involves light absorption, generation of electron-hole pairs, formation of ROS, and degradation of pollutants (Fig. 7).

Conclusion
Various photocatalysts are used depending on the variation in organic pollutants.Titanium dioxide (TiO 2 ) is the most broadly applied photocatalyst, known for its maximum ability, stability, and non-toxicity.It is primarily activated by UV light.Zinc oxide (ZnO) is another effective photocatalyst with properties similar to those of TiO 2 but with some advantages under certain conditions.Recent research includes materials like cadmium sulde (CdS), tungsten oxide (WO 3 ), and various metal-organic frameworks (MOFs) as effective photocatalysts.Scientists are working on photocatalysts that are triggered by visible light in order to improve the process's applicability and reduce energy consumption in the real world.This review scrutinizes the variance in the degradation rate of organic pollutants under different conditions such as different pH levels, different concentration levels, various composites of the photocatalysts, different surface areas and sizes of the photocatalysts, and so on.This review will help to identify the optimum parameters for the maximum amount of organic pollutant degradation.The goal of this eld's ongoing research and development is to broaden the use of catalytic technologies and overcome current obstacles to ensure cleaner soil and water thus leading to a more sustainable environment.Greater prospects for the use of photocatalysis in the destruction of dangerous organic pollutants may arise from a greater understanding of the process and its operating parameters.

Fig. 1
Fig. 1 Working procedure of the photocatalyst for dye degradation.
132 R. Kumar et al. synthesized BN/CdAl 2 O 4 composites and evaluated their photocatalytic ability to degrade cefoxitin sodium (CFT) antibiotic in an aqueous solution.The ndings demonstrated that a nearly complete degradation of CFT, reaching approximately 100%, occurred within 240 minutes at a concentration of 15 mg L −1 and a pH of 7. 133 A bismuth oxybromide (BiOBr) photocatalyst capped with PVP was produced by a solvothermal technique.The PVP-capped BiOBr exhibits a removal efficiency of 94% and 99.8% for the antibiotics ooxacin (OFL) and noroxacin (NOR) respectively, when exposed to visible light. 134Y. Gong prepared Z-scheme CdTe/ TiO 2 heterostructure photocatalysts decomposing 78% tetracycline hydrochloride (TC-H) within 30 min of irradiation under visible light. 135W. Wang examined the photocatalytic efficiency of BiVO 4 /TiO 2 /RGO composites for four tetracycline antibiotics.The BiVO 4 /TiO 2 /RGO photocatalyst demonstrated signicant photocatalytic activity and compatibility, providing efficient separation of photo-generated carriers with oxidation capabilities and high reduction. 136N. Askari synthesized a novel heterojunction Z-scheme MnWO 4 /Bi 2 S 3 using a hydrothermal technique to study the photocatalytic behavior of catalysts in the decomposition of metronidazole (MTZ) and cephalexin (CFX) under LED light exposure where a maximum degradation efficiency of 78.8% was achieved for CFX and 83.3% for MTZ. 137A. Mohammad et al. studied manufactured nanostructured photocatalysts composed of tin oxide (SnO 2 )and cerium oxide (CeO 2 ).These photocatalysts were employed to degrade the antibiotic tetracycline hydrochloride (TC) under visible light.The most optimal outcome seen among the examined photocatalysts had a TC removal effectiveness of approximately 97% within a 120 minute timeframe under visible-light exposure.138An investigation was conducted on the photocatalytic degradation of pharmaceutical micropollutants of Penicillin G (PG) in a photoreactor.The prociency of the photocatalytic process was increased by the inclusion of persulfate sodium (PPS).The inclusion of PPS greatly enhanced the efficiency of the photolysis process, resulting in a considerable improvement of 72.72% compared to the traditional photocatalysis system, which achieved 56.71% efficiency.139Bouyarmane synthesized TiO 2 -hydroxyapatite nanocomposites precipitating a re-dissolved natural phosphate mineral in ammonia using the concurrent gelation of titanium alkoxide.These nanocomposites were then subjected to degradation for drug testing in a solution under ultraviolet light.When utilizing 40TiHAp as a photocatalyst, ciprooxacin and ooxacin were destroyed through photodegradation in 15 minutes and 120 minutes, respectively. 140A simple solvothermal technique was employed to synthesize a novel Cu 3 P-ZSO-CN p-n-n heterojunction photocatalyst for the degradation of the antibiotic tetracycline (TC) under exposure to visible light.The degradation efficiency for TC was found to be 98.45%. 141M. Abdullah et al. synthesized ACT-X nanocomposites using activated carbon and TiO 2 to enhance the inherent characteristics of TiO 2 , resulting in improved light absorption in the visible area.The ACT-4 photocatalyst has demonstrated the maximum level of photocatalytic degradation (99.6%) for the ceriaxone (CEF) antibiotic.142The very rst 3D hierarchical ZnO/Bi 2 MoO 6 heterojunctions were synthesized using an in situ solvothermal technique.These heterojunctions exhibited a remarkable efficiency of 100% in the photodegradation of the ooxacin (OFL) antibiotic.This exceptional performance can be ascribed to their reduced electron-hole recombination rate and large surface area. 143A novel heterojunction photocatalyst (MoO 3 /g-C 3 N 4 ) was synthesized using a straightforward hydrothermal calcination technique.The catalytic efficiency of

Fig. 2 3
Fig. 2 Working procedure of the photocatalysts for antibiotic degradation.

Fig. 3
Fig.3Working procedure of the photocatalysts for toluene degradation.
to modify activated carbon bers (ACFs).Subsequently, titanium dioxide (TiO 2 ) was loaded onto the modied ACFs.The study found that the photocatalytic performance and adsorption of TiO 2 /ACF-Ac modied by Zn(CH 3 COO) 2 were highest for the removal of toluene.189The presence of a three-dimensional (3D) and directed structure enables efficient absorption of photons and rapid diffusion of volatile organic compounds (VOCs), surpassing the capabilities of catalysts in powder form.The researchers successfully created uniform and free-standing nanowire (NW) arrays of p-type Cu 2 O by subjecting Cu(OH) 2 NWs to heat treatment.The Cu 2 O NWs, as they are created, exhibit exceptional performance in degrading 30 ppm toluene, with a degradation rate of 99.9% achieved within 120 minutes. 190P. Mohammadi et al. used a hydrothermal technique to deposit synthesized SrTiO 3 onto graphene oxide (GO).
183 X. Zhao et al. enhanced the performance of the C-USTiO 2 photocatalyst by applying it to carbon cloth and conducted a study on its ability to continuously degrade toluene under LED light exposure.The results demonstrated that the removal of the degraded toluene can exceed 80% when a large concentration of CO 2 is produced, and it exhibits exceptional cycle stability lasting for over 180 minutes. 193M. Wu et al. researched the use of CeO 2

Fig. 4
Fig. 4 Working procedure of the photocatalysts for nitrobenzene degradation.

Fig. 6
Fig. 6 Working procedure of the photocatalysts for refinery oil degradation.

Fig. 7
Fig. 7 Illustration of (a) formation of free radicals, (b) degradation of the organic pollutants by radicals, and (c) overall photocatalytic degradation mechanism.

Table 1
80,81 Neppolian et al. investigated how the original dye concentration affected the percentage of degradation.With the best possible catalyst loading, they changed the starting concentrations of Reactive Yellow 17 (from 8.9 × 10 −4 to 1.29 × 10 −3 M), Reactive Red 2 (from 4.169 × 10 −4 to 1.259 × 10 −3 M), and Reactive Blue 4Data for the photocatalytic degradation of organic dyes using various catalysts a Z. Sun et al. synthesized a novel hierarchical heterostructured photocatalyst consisting of TiO 2 /Bi/ Bi 2 MoO 6 using a solvothermal technique.On the outermost layer of ower-like Bi 2 MoO 6 nanospheres, the TiO 2 nanoparticles were evenly dispersed.The results suggest that the combination of TiO 2 can greatly improve the effectiveness of the photocatalytic oxidation of toluene using the hierarchical heterostructure TiO 2 /Bi/Bi 2 MoO 6 . 188Y. Bi et al. used zinc chloride (ZnCl 2 ), zinc nitrate (Zn(NO 3 ) 2 ), and zinc acetate (Zn(CH 3 COO) 2 )

Table 4
Data for the catalytic degradation of nitrobenzene using various catalystsNitrobenzene conc.(mg L −1 266,267 D. A. Aljuboury et al. investigated the application of ZnO/TiO 2 /H 2 O 2 using visible