Comparative study of photocatalytic activity of metals- and non-metals doped ZnO and TiO2 nanocatalysts for advanced photocatalysis

1. Introduction

Water scarcity has recently become a major problem owing to global warming, rapid industrial growth, depletion of water resources, environmental pollution, and uncontrolled development of groundwater [1,2]. The rapid industrial growth after the Industrial Revolution has remarkably increased living standard, but threatens human health [3]. Because of the development and growth of industries, industrial effluents are becoming more polluted and difficult to treat. The release of organic compounds and chemical fertilizers from various industries has polluted river water and has become increasingly a global pollution [4]. Moreover, complete removal of non-biodegradable organic contaminants is tough using biological treatment technologies [5]. Biological processes are safe, economical, and reliable, but the removal percentage of suspended solids is low, so they need better operation management [6]. The coagulation and precipitation techniques can suspend solid particles with formation of flocs after adding polymeric coagulant and/or inorganic coagulant (such as Al, Fe, etc.). Coagulants bind solids together to form larger particles (flocs), then the flocs settle and separate from the effluent. This method has a high purification performance, but utilizing chemicals and generating biological sludge cause blockage of pipes and water deterioration, thus limit the use of this process [7,8]. In the Fenton oxidation process, organic compounds are broken down using a strong oxidizer reagent. OH radicals can be produced by oxidizer reagents through a reaction between H₂O₂ and iron salts. This process is easy to apply and additional equipment is not utilized excessively in comparison to other oxidation processes or photo-oxidation process. However, this process has some drawbacks such as sludge production and high operating costs for secondary process [9,10].

An advanced water treatment process can purify water at a high rate and effectively eliminate pollutants [11]. Advanced Oxidation Technology utilizes various techniques for enhancement of oxidation power. For efficient water refinement, various contaminants must be removed economically [12]. Generally, various active species can play a key role in the photocatalytic reaction, including superoxide radicals (O₂^•−), holes (h⁺), and hydroxyl radicals (^•OH) [13]. Advanced technology using TiO₂ and ZnO photocatalysts has attracted the most attention for producing OH^• using optical energy without additional chemicals. Also, the operating cost of this technique may be significantly decreased by solar energy. The low photocatalytic performance of ZnO and TiO₂ may limit the practical application of advanced technology to treat wastewater [14]. Numerous studies have been done on modifying their surface by metal and non-metal dopants for their possible practical utilization [15].

Various analyzes characterize the structure of ZnO and TiO₂ nanocatalysts, including XRD for determining crystal phases and crystal size, AFM for determining refractive index profiles, FTIR for specifying the functional groups, and EDX for determining the constituents. All these features are critical in the photocatalytic reaction. Also, SEM and TEM analyses are used to characterize the morphology and appearance structure of a catalyst [16,17]. Figure 1 demonstrates SEM and TEM photos of ZnO and TiO₂ nanocatalysts. As shown, ZnO and TiO₂ have various morphologies, shapes and sizes.

TiO₂ and ZnO photocatalysts are extensively utilized for photodegradation of contaminants under UV light or sunlight. However, owing to their band gap range, they have limitations for the photodegradation of pollutants. For doing so, TiO₂ and ZnO photocatalysts can be combined with other compounds to synthesize an efficient photocatalyst, some of which include La/TiO₂ [20], CuO/WO₃/TiO₂ [21], Fe₃O₄/TiO₂ [22], carbon-doped TiO₂ [23], N,P-codoped carbon quantum dots/TiO₂ [24], Fe₃O₄/CuO/ZnO/graphene [25], rGO/Fe₃O₄/ZnO [26], tungsten/silver/ZnO [27], Pb/ZnO, Cd/ZnO, Ag/ZnO [28], and Bi(12)ZnO(20) [29].

The paper focuses on comparing the photocatalytic capability of TiO₂ and ZnO and their composites in the degradation of organic compounds in wastewater. Also, the advantages, disadvantages, and structural features of these photocatalysts were thoroughly investigated. Moreover, the photodegradation mechanism of contaminants using TiO₂ and ZnO as well as metals-doped and non-metals doped TiO₂ and ZnO were completely discussed. Furthermore, the photodegradation efficiency of different contaminants using these photocatalysts and their derivatives (metallic- and non-metallic- dopants in their structure) were compared. One of the main features of catalysts is their stability in the photocatalytic reaction, so the recyclability of TiO₂ and ZnO and their composites was fully studied.

2. Organic Pollutants

Municipal sewage pollutants are mainly organic matter and a variety of pathogenic microorganisms and so on with complex structures such as drugs, antioxidants, polycyclic aromatic hydrocarbons, pharmaceutical intermediates, food additives, PPCPs, steroids, and phenolic compounds [30]. Also, pollutants in industrial wastewaters include gas condensates [5], petroleum products, non-chlorinated compounds, petroleum hydrocarbons, trinitrotoluene, and polycyclic aromatic hydrocarbon [31]. The presence of hydrocarbon compounds in wastewater causes many problems for human health. Discharge of these wastewaters to sea must be performed under certain conditions. The highest permissible concentration of hydrocarbon compounds for discharge into the sea should be 40 ppm, and TDS less than 32000 ppm [32].

Another kind of contaminants in wastewater is volatile organic compounds (VOCs), which have destructive effects on human health. Volatile organic contaminants are categorized into 3 groups, including semi volatile organic compounds (SVOCs), volatile organic compounds (VOCs), and very volatile organic compounds (VVOCs). Propane and butane are some VVOCs, which are poisonous compounds in very low concentrations. VOCs are also toxic compounds found in household products and environment, including isopropyl alcohol, acetone, formaldehyde, toluene, vinyl chloride, and hexanal. Molecular weight and boiling point of SVOCs are higher than VOCs and can evaporate at room temperature. Pesticides such as chlordane and plasticizers such as phthalates are some of the SVOCs [33]. The utmost allowable limit of organic compounds for drainage into the river for utilization in agricultural irrigation should be less than 200 ppm [34].

The main source of VOCs includes incomplete burning of fossil fuels, solvents used in inks and paints (e.g., acetone, ethyl acetate, and glycol ether), utilizing biofuels (e.g., cooking oil and bioethanol), biomass combustion, especially from agricultural residuals, and VOCs released from metal working fluids [33]. The presence of these contaminants in sewages results in various problems for the environment.

Also, tetracycline, sulfamethoxazole, cefixime, amoxicillin, gentamicin, erythromycin, and ciprofloxacin are kinds of antibiotics that are present in hospital effluents. The presence of antibiotics in effluents in low and high concentrations is dangerous and causes serious damage to human health. According to reports, the concentration of antibiotics in surface and groundwater reaches 100 micrograms per liter. Also, the concentration of amoxicillin in raw effluent and urban wastewater treatment is 171 mg/L and 13 ng/L, respectively [35]. The acceptable standard of WHO for antibiotics in wastewater is 1 mg/L [36]. Therefore, their concentrations in wastewater should be attenuated.

Dyes are another kind of organic compounds, which are utilized in many industries like food, plastics, paper, cosmetics and textile. Among these, the most abundant dyes are related to the textile industry, which these dyes can enter the wastewater [37]. More than 7×10⁵ tons of dyes are produced annually, 10–15% of which enters the sewage during the dyeing process. Water pollution by dyes results in many environmental problems. The highest allowable limit of dyes in industrial discharge is in the range 0.01 to 0.05 mg/L. Different groups of dyes are considered carcinogens or mutants. Dyes can prevent sunlight from entering the water, and disrupting biological process in the water. Most dyes are poisonous to organisms and have adverse impacts on their aquatic life, and entire ecosystem. The presence of dye in water causes reproductive dysfunction, kidney damage, carcinogenesis and mutagenicity. Basic dyes are cationic dyes and most of them are crystalline compounds, which can be derived from positively charged sulfur or nitrogen atoms. Under visible and UV light irradiation, dyes have high stability and insensitivity, and do not degrade without photocatalyst [38,39,40]. Cationic, anionic and non-ionic are 3 important categories of dyes, among which, the toxicity of cationic dyes is more than the other two. There are various types of dyes in sewage including crystal violet, Congo Red (CR), methyl orange (MO), reactive orange, reactive black, reactive red, methylene blue (MB), methyl violet, brilliant cresyl blue and Safranin-T that may be generated by plastic, leather, food dye, printing, paper, and textile industries [41,42]. Figure 2 illustrates different categories of organic compounds with some examples.

3. TiO₂ Photocatalyst

TiO₂ is a non-toxic and insoluble substance in water. TiO₂ has high stability with a high catalytic performance. Another name of TiO₂ is Titania, which can be available in 3 forms such as anatase, brookite, and rutile [43,44]. The rutile (tetragonal) crystalline structure of TiO₂ is quite stable in particles greater than 35 nm [45]. TiO₂ in anatase and rutile structures have oxidation strengths of 3-3.2 eV, respectively, which are very strong oxidation strengths [14]. One of the most important applications of TiO₂ is its utilization in the photocatalytic process. To this end, UV light, halogen lamp or solar irradiation can help accelerate the photocatalytic reaction in the presence of TiO₂ nanocatalyst [46]. The photocatalyst has two important properties. For instance, the photocatalyst should not be consumed or participate directly in the reaction. Also, the photocatalyst provides other mechanism pathways. Photocatalytic reaction is based on the absorption of solar energy in the semiconductor gap. Few semiconductors can be utilized as the catalyst in the photocatalysis process, of which, TiO₂ is the most extensively utilized [44]. The various morphologies of TiO₂ consist of zero, one, two and three-dimensional structures. By optimization of the shape and size, the photocatalytic activity of TiO₂ particles in the wastewater treatment process can be maximized [47].

TiO₂ can be synthesized easily by various physical, chemical, and thermal procedures such as chemical or physical vapor deposition, sol-gel, inverse micelle, solvothermal, hydrothermal, sonochemical, microwave, and electrodeposition process [48]. Synthesizing metal-doped semiconductor oxides by conventional physical mixing or chemical deposition usually obtains insoluble materials that are inherently difficult to control the morphology, size, and dispersing metal components. These methods need long time and multi-stage processes. Sonochemistry procedure is an efficient process for preparing mesoporous materials. Ultrasound is very useful to synthesize an extensive range of nanostructured materials, which include high-specific surface area oxides, alloys, carbides, and transition metals [15]. Neppolian et al. synthesized TiO₂ nanophotocatalysts by a combination of ultrasonic and sol–gel processes. They investigated the impact of various factors on the synthesis method, including magnetic stirring, ultrasonic sources (e.g., bath and horn), ultrasonic time, power density, temperature, and reactor size [49]. Table 1 indicates the advantages and disadvantages of various processes for TiO₂ synthesis. As shown, the production of high-quality crystals as well as easy control of crystals in TiO₂ can be achieved by hydrothermal process. Also, the fabrication of TiO₂ with sol-gel process has several benefits such as high purity products, good size distribution, remarkable specific surface area, economical, uniform size of particles, fine particle size, and simple synthesis. Between these processes, utilizing the microwave method has significant benefits such as short reaction time, high reaction rate and high efficiency.

3.1. Features and Reaction Mechanism of TiO₂ Photocatalyst

Utilization of TiO₂ as a photocatalyst for elimination of contaminants from wastewater has several benefits, which include: 1) the process is done at room temperature and 1 atm, 2) complete decontamination without secondary contamination, 3) producing high surface area and high catalytic activity, and 4) using the photocatalyst in multiple cycles and decreasing the costs [52]. The decomposition process of contaminants is mainly an oxidative reaction, which depends on the features of the photocatalyst. TiO₂ is also low-cost, suitable band gap energy, long-term stability against light, and safe. Generally, the band distance and wavelength of TiO₂ are between 3-3.2 eV, and 400 nm, respectively, indicating that UV light at a wavelengths less than 400 nm leads to an inverse reaction. Also, UV irradiation at a wavelength less than 400 nm initiates a photoreaction. Unfavorable recombination of holes (h⁺) and electrons (e^-), and low yield in the visible region under irradiation are 2 important disadvantages of TiO₂ [15]. The catalytic activity of TiO₂ photocatalyst can be enhanced under UV irradiation, because TiO₂ uses only 5% of the solar energy [53].

At UV light<400 nm, the surface of TiO₂ may reach temperatures above 30000^oC, which this temperature can oxidize all substances. Hence, all organic compounds are completely decomposed into CO₂ and water. Figure 3(a) illustrate a schematic of degradation of contaminants by forming photo-induced charge carriers electron/hole (e^-/h⁺) on the TiO₂ surface. Also, Figure 3(b) shows that the catalyst surface is surrounded by the contaminant molecules. On the TiO₂ surface, oxygen molecules react with electrons to produce oxygen radicals. Also, water molecules react with h⁺ for generating hydroxide radicals and H⁺ ions. The final products will be CO₂ and H₂O [14]. In other words, electrons introduced into the conduction band from valence band can be easily transferred to the catalyst surface and are trapped by doping with noble metals, intermediate metals, and rare earth elements. Metal-doped cations increase the ability of radicals produced in the photocatalysis reaction and then decrease the recombination life of e^-/h⁺ [54].

By irradiating UV light to the TiO₂ surface, the photo-induced electrons react with dissolved oxygen for producing O₂^•. The photoinduced h⁺ in the valence band penetrate to the surface of the TiO₂ photocatalyst and react with the absorbed water molecules to form OH^•. OH• is a highly active species in the photocatalytic process. The reaction mechanism of TiO₂ and the electron–hole pair is defined as follows. As shown, TiO₂ reacts with hv to produce e^- and h⁺. Next, e^- and h⁺ can produce two important radicals such as OH^• and O₂^•, which have important role in the photocatalytic reaction [14,54].

$${\mathrm{TiO}}_2+hv \to {\mathrm{TiO}}_2( e_{CB}^{-}+h_{VB}^{+})$$

$$\mathrm{Oxidation}: e_{CB}^{-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}\mathrm{H}}^{•}+{\mathrm{H}}^{+}$$

$$\mathrm{Reduction}: {\mathrm{O}}_2+e_{CB}^{-}\to {{\mathrm{O}}_2}^{•}$$

$$e_{CB}^{-}+{{\mathrm{H}\mathrm{O}}_2}^{•}\to {{\mathrm{H}\mathrm{O}}_2}^{-}$$

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

$$Ti{O}_2\left(h^{+}\right)+H_2{O}_{ads}\to Ti{O}_2+H{O}_{ads}^{.}+H^{+}$$

$$Ti{O}_2\left(h^{+}\right)+H{O}^{-}\to Ti{O}_2+H{O}_{ads}^{.}$$

$$Ti{O}_2\left(e^{-}\right)+O_2\to Ti{O}_2+O_2^{.}$$

$$O_2^{.-}+H_{2}O\to H{O}_2^{.}+{HO}^{-}$$

$$O_2^{•}+H^{+}\to {HO}_2^{•}$$

$$2H{O}_2^{.}\to H_2{O}_2+O_2$$

$$Ti{O}_2\left(e^{-}\right)+H_2{O}_2\to Ti{O}_2+{HO}^{-}+H{O}^{.}$$

$$Ti{O}_2\left(h^{+}\right)+R_{ads}\to Ti{O}_2+R_{ads}^{.+}$$

where, R is the adsorbed pollutant [55].

Also, TiO₂ is photochemically stable and non-toxic. Under irradiation, the charge-pair on TiO₂ reacts directly with solid lattice ions. Also, TiO₂ is resistant to various acidic and alkaline pHs. TiO₂ can be utilized in the photocatalytic treatment of the environment. Hence, TiO₂ can be used as a promising photocatalyst to degrade organic contaminants. Nevertheless, TiO₂ particles are inactive in visible light. To solve this problem, numerous studies have been performed on the synthesis of various composites of TiO₂ with metals and non-metals (e.g., ceramics, zeolites, carbon materials, fibers and glasses) as a support in order to have photocatalytic activity in a wide range of visible light [14].

The recombination behavior of e^-/h⁺ decrease with decreasing the size of TiO₂ NPs, which can be because of the increase in interfacial charge transfer at TiO₂ surfaces. Also, the photocatalytic activity of TiO₂ of less than a few nanometers decreases because of the predominant recombination of electron/hole at the TiO₂ surface. According to the reports, TiO₂ nanotubes are more effective in degrading contaminants than TiO₂ particles, indicating the rapid mass transfer of contaminants on the surface of nanotubes [45].

3.2. Improved Photocatalytic Activity of TiO₂ Using Metallic and Non-Metallic Dopants

As mentioned earlier, TiO₂ photocatalyst can decompose organic contaminants under UV irradiation. Researchers have tried to improve the optical sorption range of TiO₂ catalysts from ultraviolet irradiation to visible light to boost its photocatalytic activity. Therefore, for improving the photocatalytic activity of TiO₂ in visible lights, its surface should be modified [56]. The catalytic activity of TiO₂ can be improved by placing suitable materials on the surface of TiO₂ for its application under visible irradiation [53,57,58]. Metallic particles such as Pd, Pt, Fe, Ru, Au, and Ag can be utilized on the TiO₂ surface to improve the photocatalytic activity by suppressing recombination behaviors of e^-/h⁺. The induced electrons migrate to the surface of metal particles, stabilizing the photo-induced h⁺ on TiO₂ as the life time of the charge carrier increases. To this end, more superoxide (O₂^•) and OH^• radicals can be produced. As the crystallographic aspects of TiO₂ increase, the photocatalytic activity of TiO₂ also produces more OH•, which can contribute to the photodegradation of organic pollutants. Also, the element O in the lattice of TiO₂ may be replaced by various heteroatoms such as B, S, F, N, P, and common doping B-C and N-S to perform a photocatalytic reaction in the presence of visible light. Besides, metals and non-metals introduced to the TiO₂ structure can improve its catalytic activity for the decomposition of organic pollutants [14,59]. Also, modifying the surface of TiO₂ improves the uptake capacity of contaminants, which can be useful in advanced oxidation technology. The modified TiO₂ with nanotubes, foams, and mesoporous phases has shown better photocatalytic behavior compared to unmodified TiO₂ [60].

TiO₂ photocatalyst on a nanoscale has a great surface area/volume proportion, leading to increased charge separation and ions trapping at the TiO₂ surface. TiO₂ nanoparticles show enhanced oxidative power compared to TiO₂ microparticles. However, TiO₂ nanocatalyst can not be employed directly to wastewater treatment owing to the aggregation of particles during the degradation process as well as their physical and chemical features. To this end, the catalytic activity of TiO₂ NPs can be improved by incorporating TiO₂ onto carbon materials for composite synthesis such as graphene, carbon nanotubes, and activated carbon nanofiber. Carbon materials can be excellent supports for TiO₂ NPs owing to their unique features like excellent mechanical, thermal, chemical, electrical, and optical properties, which lead to rapid charge transfer on the surface of TiO₂/carbon nanocomposites [61]. TiO₂/carbon nanocomposites are suitable catalysts for the elimination of organic contaminants from effluents. Graphene NPs act as a bridge for excellent electron transfer and electron sinks. TiO₂/graphene nanocomposites have an extensive range of band gap (2.66–3.18 eV), suggesting that long-wavelength lights in the visible region can be absorbed by the TiO₂/carbon nanocomposite. One of the most benefits of carbon nanomaterials is their high specific surface area, in which TiO₂ NPs may be distributed in their structure, leading to improved selectivity of organic contaminants. TiO₂/carbon nanocomposite has a good impact on self-purification in comparison to UV irradiation and ozonation when used for wastewater treatment. These nanocomposites can be effective industrially for wastewater purification because carbon NPs are employed as a support to stabilize the composite structure [62].

The photocatalytic activity of TiO₂ can be enhanced by enhancing the interfacial charge-transfer and reducing the recombination of e^-/h⁺. To this end, various metals can be used to improve the photocatalytic activity of TiO₂, including Ag, boron (B), Si, Ni, and so on under UV light or sun light irradiation. Generally, there are several ways for improving the photocatalytic activity of TiO₂, including enhancing the ratio of surface/volume, integrating TiO₂ with other semiconductors, optimizing its particle size, and doping metals or non-metals. The presence of metal dopants in the TiO₂ structure remarkably affects the photo-reactivity because by shifting the catalyst band gap to the visible region, it changes the interfacial electron transfer rate and the charge-carrying recombination rate [63]. A dopant ion can trap e^- or h⁺, leading to longer lifespan of the produced charge carriers and increased photocatalytic activity. Figure 4 indicates various dopants used in the TiO₂ structure. As shown, transition metals, anionic compounds, metal oxides and transition metal ceramics are the main dopants for improving the TiO₂ surface. Fe, Al, Zr, Co, Cu, and Ni are several types of transition metals. Also, Fe₂O₃, Cr₂O₃, and SiO₂ are the most important metal oxides. Moreover, WO₃, SnO₂, and MoO₃ are the most important transition metal ceramics. Eventually, C, N, F, O, and S are non-metallic dopants. The addition of transition metals to the TiO₂ surface can enhance its catalytic activity and decline the recombination of photogenerated e^- and h⁺ [15]. The photodegradation mechanism of contaminants using metals- and non-metals- doped TiO₂ is also illustrated in Figure 5 [64,65]. As shown, the presence of metals and non-metals on the TiO₂ surface can help improve its photocatalytic activity by producing more oxygen radicals. These oxygen radicals play a key role in photodegradation of organic contaminants.

Numerous studies have been performed on the expansion of the solar absorption band by combining TiO₂ with new metals, intermediates and anions, which maximizes the performance of the photocatalyst [45]. Table 2 presents the influence of various metal dopants on the photocatalytic activity of TiO₂. Accordingly, Wang et al. (2019) synthesized 5% Pd-doped TiO₂ photocatalyst to degrade 2,2',4,4'-tetrabromodiphenyl ether from water under UV lamp and showed 100% photodegradation efficiency, which is a remarkable amount [70]. Also, Sescu et al. (2020) synthesized Au/TiO₂ photocatalyst using two different procedures such as incipient wet impregnation (IWI) and ultrasound impregnation (UI). Then, Au/TiO₂ (IWI) and Au/TiO₂ (UI) were used for degradation of 2,4 dinitrophenol under UV irradiation. According to their outcomes, the photodegradation efficiency of 2,4 dinitrophenol using Au/TiO₂ (IWI) and Au/TiO₂ (UI) were 50 and 37%, respectively, which were less than the photodegradation efficiency of pure TiO₂ (60%). Comparing these two methods displays that the IWI procedure is more efficient than the UI procedure. Therefore, they used the IWI procedure to synthesize Pd/TiO₂ for degradation of 2,4 dinitrophenol and Rhodamine 6G contaminants under UV irradiation. The photodegradation efficiency of 2,4 dinitrophenol and Rhodamine 6G using Pd/TiO₂ photocatalyst was 67% and 96%, respectively, indicating that the Pd/TiO₂ photocatalyst is able to remove Rhodamine 6G dye with a significant efficiency [72]. In another work, Yadav et al. (2020) utilized B-doped TiO₂ nanophotocatalyst for 4-nitrophenol removal from water. The utmost photodegradation efficiency using TiO₂ and B-doped TiO₂ was obtained 79 and 90%, respectively, indicating that the photodegradation efficiency of TiO₂ enhanced with its doping by boron [71]. In general, previous studies show that most photocatalytic processes are performed under UV light. Also, Ag and Pd dopants showed greater potential for photodegradation of organic contaminants than other dopants.

4. ZnO nano-photocatalyst

ZnO nanocatalyst is one of the most efficient catalysts in the degradation of organic contaminants due to its unique features like excellent oxidation ability, direct and extensive band gap in the spectral region close to UV, high photocatalytic activity, and high binding energy. UV light can be absorbed on ZnO at a wavelength lower than 385 nm [73]. Rock salt, wurtzite and cubic structure are important crystalline structures of ZnO. Among the 3 structures, the wurtzite structure of ZnO is the most common structure and has the utmost stability. Also, ZnO in the form of rock salt is wholly rare. At room temperature and pressure, the crystalline structure of ZnO is a hexagonal wurtzite. Moreover, ZnO can crystallize in the structure of wurtzite. The ZnO wurtzite structure is the most common form of ZnO due to its stability in environmental conditions [74]. Different structures of ZnO are exhibited in Figure 6. In the ZnO wurtzite structure, each Zn atom is surrounded by four O atoms. ZnO has many benefits compared to TiO₂, including low-cost, chemical stability, no toxicity, and abundance [75]. Other properties of ZnO include insoluble in water, odorless, and bitter taste. ZnO can be used in catalysis processes, fertilizers, rubber industry, paint industry, and cosmetics. The development of nano-ZnO with precisely controllable properties has recently gained considerable scientific attention [76].

Different zinc salts can be utilized to fabricate ZnO, including Zn(C₂H₃O₂)₂.2H₂O, Zn(SO₄)₂.7H₂O, Zn(NO₃)₂·6H₂O, and ZnCl₂ [77]. ZnO nanoparticles and doped ZnO can be synthesized by various techniques, including pulsed-laser deposition, chemical coprecipitation, hydrothermal, thermal decomposition, sol-gel, liquid-solid solution, vapor condensation, microwave, and spray pyrolysis process [73,78]. Hydrothermal procedure is a well-known and efficient process for synthesizing ZnO nanoparticles, which is performed at high pressure and temperature [78]. Chemical coprecipitation is one of the most successful processes to synthesize ZnO nanoparticles with a fine particle size distribution. Chemical coprecipitation can prevent complex steps like alkoxide reflux and therefore takes less time than other methods [73]. Also, synthesizing ZnO nanoparticles by sol-gel and hydrothermal methods have several important advantages such as uniform particle distribution of particles and synthesis at low temperatures. However, microwave-based synthesis has attracted a lot of attention due to its advantages such as simpler, faster, and more energy efficient. The precursor solution is irradiated by the microwave source. Energy transfer via relaxation and/or resonance can lead to a relatively fast heating process. Also, heating process by the microwave source leads to uniform heating in a short time, which results in uniform distribution of particles [79].

4.1. Characterization of ZnO Nanocatalyst

ZnO nanocatalyst is a semiconductor with broad band-gap as well as high binding energy in environmental conditions. ZnO NPs have a better optical, electrical, and magnetic feature than ZnO microparticles. ZnO NPs also have unique physical, thermal, and chemical properties like biocompatibility, low cost, and non-toxic. ZnO is an environmentally friendly substance, which is well suited for a wide range of daily applications that pose no risk to human health. Also, ZnO NPs have longer shelf life compared to other metal oxides like Fe₂O₃, SiO₂, WO₃, and TiO₂. Because of its exceptional features, ZnO nanocatalyst can be utilized as a valuable catalyst to treat wastewater [77]. Owing to the band gap energy (3.2 eV) similar to ZnO with TiO₂, their photocatalytic capability is similar. Also, ZnO nanocatalyst is relatively cheaper than TiO₂ nanocatalyst, the use of ZnO is more economical in the large-scale water purification process. ZnO can adsorb a broad range of solar spectra and more optical quantum than many semiconductor metal oxides. However, the main disadvantage of ZnO is its wide band gap energy, therefore, the absorption of light by ZnO is limited to the visible light range. This leads to rapid recombination of light-generated charges, resulting in low photocatalytic performance [80]. The addition of doping affects the optical features of ZnO and possibly shifts the amplitude of optical absorption to the visible region. Therefore, the photocatalytic activity of ZnO can be enhanced by doping it with other materials, which is described in detail in Section 4.3 [72].

4.2. Photocatalysis Mechanism of ZnO

In a desirable photocatalytic reaction in the presence of zinc oxide particles and active oxidizing species like oxygen, organic contaminants can be converted to H₂O, CO₂, and inorganic acids. Photocatalytic reaction begins when ZnO NPs absorb photons from light with energies greater than their band gap energy. Thus, the photo-induced electron rises from the valence band to the conduction band, creating h⁺‏ and e^- on the ZnO surface [81]. The presence of O atom as an electron absorber lengthens the pair of recombinant electron cavities and the formation of superoxide radicals. The reaction between h⁺‏ and OH^- results in the production of hydroxyl radicals. Figure 7(a) indicates the photocatalytic mechanism of anionic- and cationic doped ZnO catalyst and photodegradation mechanism of the contaminant on the ZnO surface under sun light is illustrated in Figure 7(b) [82]. The reaction mechanism of ZnO in the photocatalytic process has several steps, which are presented below [83]:

$$ZnO+hv\to e^{-}+h^{+}$$

$$\mathrm{Absorbed} \mathrm{oxygen}: O_2+e^{-}\to O_2^{.-}$$

$$\mathrm{Water} \mathrm{ionization}: H_{2}O\to {OH}^{-}+H^{+}$$

$$\mathrm{Superoxide} \mathrm{protonation}: O_2^{.-}+H^{+}\to {HOO}^{.}$$

$${HOO}^{.}+e^{-}\to {HOO}^{-}$$

$${HOO}^{-}+H^{+}\to H_2{O}_2$$

$$H_2{O}_2+e^{-}\to {OH}^{-}+{OH}^{.}$$

$$H_{2}O+h^{+}\to H^{+}+OH$$

4.3. Improving the Photodegradation Efficiency of ZnO

The recombination of photo-produced h⁺ and e^- is one of the main disadvantages of semiconductor photocatalysis. This recombination stage reduces quantum efficiency and wastes energy. Thus, the recombination process of e^-/h⁺ must be inhibited to have an impressive photocatalysis process. Metal doping can counteract the recombination problem by increasing the isolation of charge between h⁺ and e^-. Also, dopants can trap e^- and reduce the chance of electron/hole recombination, which inactivates the photocatalytic process. Moreover, the production of active oxygen species and hydroxyl radicals significantly increases the charge separation efficiency. In general, the dopant concentration, operating conditions, and the synthesis procedure have significant impacts on the metal-doped semiconductor photocatalysis process [84].

Anion-doped zinc oxide photocatalyst has shown higher photocatalytic degradation efficiency than pure zinc oxide. The presence of isolated N2p modes above the utmost ZnO valence band in the Ne ZnO sample increases its ability to absorb visible light. Under visible light irradiation, narrow-band gaps in N- ZnO require lower energy to induce the charge carriers (e^- and h⁺). Increased photocatalytic activity of C-doped zinc oxide can be owing to many reasons, including increased adsorption of contaminants on the surface of catalyst, higher UV uptake than pure ZnO, creating new energy levels under the ZnO conduction band, where photoexcited e^- by these new energy levels are removed to avoid the electron hole from recombining. All these factors increase the number of charge carriers and improve the photodegradation performance. Also, the number of oxygen vacancies in ZnO have a key role in influencing the photo-activity of ZnO doped with S. In the photocatalytic reaction, oxygen vacancies become centers for trapping e^-. Therefore, the more the oxygen vacancies there are, the greater the catalytic activity [85,86]. Figure 8 shows the mechanism of the pollutant degradation by metals and non-metals doped ZnO [87]. As shown in Figure 8(a), the metal doped on the ZnO surface contributes to generate e^- and subsequently O₂^- radicals, resulting in better photodegradation of organic contaminants. The same role is played by non-metals doped with ZnO to generate O₂^- radicals followed by the photodegradation of pollutants (Figure 8(b)).

Despite the high photodegradation efficiency of ZnO, studies show that the doped ZnO has a significant photocatalytic efficiency compared to the undoped ZnO. ZnO surface modification using cationic dopants has attracted much attention. The chemical, electrical, magnetic, and structural features of ZnO can be adjusted by adding cationic dopants such as Ni, Mn, Co, Al, and Sb. The doped elements are usually iso-morphic to Zn ion, like Cu(II)‏, Co(II),‏ Ni(II)‏, and Mn(II)‏. The doped ZnO photocatalyst shows a faster response to degradation of organic contaminants than pure ZnO [88]. Table 3 reports the impact of different metallic dopants on the photocatalytic efficiency of ZnO. As shown, Adeel and coworkers (2021) studied the degradation process of MO using Co-doped ZnO. The photodegradation efficiency of ZnO and Co-doped ZnO were 46 and 93%, showing that the photodegradation efficiency of ZnO doubled after its doping with Co [89]. In another work, Vallejo et al. (2020a) compared the photodegradation efficiency of MB using ZnO and Ag-doped ZnO. Their results displayed photodegradation efficiencies of 2.7 and 45.1% for ZnO and Ag-doped ZnO, respectively, indicating a remarkable increment in photodegradation efficiency after ZnO doping with Ag. These photodegradation efficiencies were attained after 120 min visible irradiation [90]. Also, Vallejo et al. (2020b) synthesized Co-doped ZnO and Cu-doped ZnO photocatalysts and compared their photodegradation efficiencies with ZnO in MB removal under visible light after 140 min. Their findings demonstrated that the photodegradation efficiencies of ZnO, Co-doped ZnO and Cu-doped ZnO were 2.7, 62.6, and 42.5%, respectively [91]. Moreover, Cr-doped ZnO was able to decompose MO dye with excellent photodegradation efficiency of 99.8% under UV-vis light illumination after 100 min [92]. Among these works, Cr-doped ZnO showed the highest photodegradation efficiency (99.8%) for removing methyl orange after 100 min under UV light.

Table 4 also reports the impact of various non-metallic dopants on the ZnO photocatalytic activity for some organic contaminants. As shown, Wu and coworkers (2014) studied the photodegradation performance of MB using N-doped ZnO under visible light irradiation and compared its results with pure ZnO. Their outcomes indicated that the photodegradation efficiency of MN using pure ZnO and N-doped ZnO photocatalysts are 76.2 and 81.6%, respectively [97]. Also, Li and coworkers (2012) used pure ZnO and C-doped ZnO for photodegradation of MB under visible light. After 60 min, C-doped ZnO was able to decompose MB with the photodegradation efficiency of 98.1%, which is a significant amount compared to pure ZnO (54.3%) [99]. Moreover, the photodegradation efficiency of Bisphenol A using C-doped ZnO under UV irradiation was 100%, indicating high degradation efficiency [101]. Furthermore, Fu et al. (2012) studied the photocatalytic reaction of MB dye using C-doped ZnO under visible light and compared the photodegradation efficiency of ZnO and C-doped ZnO. The highest photodegradation efficiency of ZnO and C-doped ZnO was achieved 26 and 80% under optimal conditions such as 2.5% catalyst and 200 ^oC, which showed a significant increase in the photodegradation efficiency after ZnO doping with C [100]. In general, most studies have performed by N- and C-doped ZnO photocatalysts and both dopants showed significant photodegradation activity. Therefore, these studies demonstrate that utilizing C- and N-doped ZnO photocatalysts is more attractive to researchers than other dopants.

5. Factors Affecting Photodegradation Efficiency

5.1. Photocatalyst Dosage

Catalyst dosage or catalyst loading is a critical factor on the photocatalytic efficiency of organic contaminants. Increasing the catalyst dose leads to an enhancement in the specific surface area of the catalyst, the creation of more active sites, and ultimately the formation of more hydroxyl and superoxide radicals. Thus, degradation of organic contaminants will be facilitated and a higher photodegradation efficiency will be attained. The photodegradation efficiency initially enhances by enhancing the photocatalyst dosage until it reached an optimal value. At the photocatalyst concentration beyond the optimal concentration, the photodegradation efficiency decreases because of light scattering. Increasing the photocatalyst dosage beyond the optimal value leads to the agglomeration of catalyst particles and then decrease the specific surface area of the catalyst to absorb light, which ultimately leads to a reduction in the degradation efficiency. On the other hand, it prevents light from penetrating into the sewage. As a critical result, an optimal value of catalyst must be determined to prevent overuse of the catalyst and to achieve maximum degradation efficiency [40].

In one study, Zhang and co-workers (2015) studied the photodegradation efficiency of MB using TiO₂ in the range of photocatalyst dosage from 1-10%. According to their outcomes, the maximum photodegradation efficiency of MB (93.78%) was attained at an optimal value of 7% [103]. Also, Notodarmojo et al. (2017) studied degradation efficiency of RB-5 dye from water and found that the degradation efficiency increases with increasing the TiO₂ photocatalyst concentration from 0.5 to 2.5 g/L. Therefore, 2.5 g/L was considered as the optimum photocatalyst dosage [104]. Moreover, Saiful Amran and co-workers (2019) investigated the maximum degradation efficiency of MB using carbon-doped TiO₂ in the photocatalyst dosage range of 1-3%. According to their outcomes, the highest photodegradation efficiency (82.67%) was obtained at 2 wt.% photocatalyst dosage [105]. Furthermore, Erdemoğlu et al. studied the influence of nano-TiO₂ dosage (0.1-1 wt.%) on photodegradation of CR under visible light. The nanocatalyst could remove 94% of CR at the catalyst dosage of 0.25 wt.%. By enhancing catalyst dosage from 0.25 to 1 wt.%, the photodegradation efficiency decreased [106].

5.2. Photocatalyst Structure

Photocatalytic efficiency is effectively influenced by the photocatalytic structure. The synthesis of nanostructured photocatalysts has recently received much attention because of their unique structures and features. There are various morphologies for nanostructured photocatalysts, including nanoflowers, nanosheets, nanowires, nanorods, nanodumbbells, nanobelts, and nanospiral disks. Some structures of these photocatalysts are illustrated in Figure 9. Nanostructured photocatalysts have a high specific active area with great catalytic strength. A high surface/volume proportion presents a better physical and chemical feature. Each of these structures has shown various photocatalytic features [107,108].

5.3. Contaminant Concentration on Photodegradation Efficiency

Contaminant concentration has an important influence on the photocatalytic process. Previous studies show that the concentration of organic pollutant in aqueous solution has an undesirable impact on photodegradation efficiency. The higher the concentration of contaminants in the effluent, the lower the photodegradation efficiency, which is because the concentration of the target contaminant becomes more and more. Organic contaminants can be adsorbed on the photocatalyst surface. The number of contaminant molecules increases, while the number of active sites remains constant [113]. Hence, generating hydroxyl radicals are not enough and there will be only a few active sites on the photocatalyst surface to absorb hydroxyl ions. Consumption of hydroxyl radicals (OH^.) by the produced intermediates decreases the photodegradation efficiency in solutions with high contaminant concentration. Therefore, the lower the pollutant concentration, the less competition there is for consumption [114,115]. Parida and Parija (2006) investigated the impact of phenol concentration on the degradation efficiency using ZnO in various irradiation strength. Under solar irradiation, the photodegradation yield decreased from 100% to 60% with enhancing the phenol dosage, while under ultraviolet light, the photodegradation efficiency reduced from 94% to 52% with enhancing the phenol concentration [116]. Also, Benhabiles et al. (2016) studied the influence of MB dye concentration (10-30 mg/L) on the photocatalytic process by commercial TiO₂. After 5 h, 70% of MB dye was degraded by TiO₂ at MB concentration of 10 mg/L, while at MB concentration of 30 mg/L, the catalyst was able to remove only 30% of MB [117]. Moreover, Shelar et al. (2020) surveyed the impact of MB dye concentration (10-40 mg/L) on photodegradation using Ag-doped ZnO. The photodegradation efficiency reduced from 95% to 65% with enhancing MB dosage from 10 to 40 mg/L, indicating that the highest photodegradation efficiency occurs at the lowest dosage of MB dye [118].

Generally, previous studies demonstrate that the highest photodegradation efficiency of contaminants occurs at the lowest contaminant concentration.

5.4. pH

Solution pH plays a vital role in the photocatalytic reaction of water purification. pH can change the surface charge of the photocatalyst [119]. Determination of optimal pH in the photodegradation process depends on the zero-point charge (pH_zpc) of the photocatalyst. pH and the photodegradation rate do not have a specific relation. The TiO₂ photocatalyst will be negatively charged, if pH> pH_ZPC. In this case, TiO⁻ anion will be formed. Also, if pH<pH_ZPC, the TiO₂ photocatalyst will be positively charged and form TiOH⁺₂ cation. Depending on the catalyst used, the surface charge of the photocatalyst at 4.5<pH_ZPC<7 is neutral. When operating at pHzpc, the surface charge of TiO₂ photocatalyst is positively charged and absorbs negatively charged molecules electrostatically over time. Industrial effluents may be discharged at different pHs, which complicates the photocatalytic reaction. Also, hydroxyl radicals are rapidly eliminated at high pHs, inhibiting their reaction with the contaminant [40,115]. There are many effective factors on pH in the photocatalytic reaction, including electrostatic charge of catalyst particles, band structure, and crystals size. Each of these factors can alter the photocatalyst surface charge and generally alter the photodegradation efficiency [115]. Generally, the optimal pH in photodegradation depends on the contaminant and the photocatalyst. For example, Hameed et al. (2009) studied the impact of pH on the photodegradation efficiency of carbofuran using TiO₂/Ultraviolet and pH 7 was obtained as the optimal pH [120]. Also, Lopez-Alvarez et al. (2011) surveyed the impact of pH on the photodegradation efficiency of carbofuran using TiO₂/solar light and their outcomes showed that pH 7.6 is the best pH [121]. Moreover, in the work done by Saien and Khezrianjoo (2008) for degradation of carbendazim using TiO₂/Ultraviolet, the optimal pH was 4 [122]. Furthermore, Elmolla and Chaudhuri (2010) found the optimal pH of 11 for photodegradation of amoxicillin, ampicillin and claxocillin using ZnO/Ultraviolet [123]. In another work, the highest photodegradation efficiency of TiO₂ for MB removal was attained at pH of 11 [117]. Also, Shelar et al. studied the impact of pH on photodegrading MB dye in the range of 2-12 and found that the utmost photodegradaion efficiency is obtained at pH 8 [118].

5.5. Light Intensity and Wavelength

Photodegradation efficiency depends on the absorption of light by a catalyst [124]. Light intensity specifies how much light can be adsorbed by a photocatalyst at a given wavelength. The higher the light intensity, the more radiation is deposited on the photocatalyst surface, leading to the production of more hydroxyl radicals and enhances the reaction rate. At higher light intensities, the photodegradation reaction depends on the mass transfer between the reactants, because the photocatalyst surface is completely covered by saturated solids, limiting the mass transfer for sorption and desorption. Therefore, the photocatalytic reaction speed remains constant in spite of an increase in the intensity of light. Previous studies have shown that there is a relationship between the photodegradation efficiency of the contaminant and light intensity for different organic compounds. The generation of hydroxyl radicals increases with increasing light intensity, which leads to an improvement in the degradation rate [115,125]. Elaziouti et al. investigated the impact of light intensity in the range of 50-90 j/cm² on the photodegradation efficiency of CR and benzopurpurine 4B. The photodegradation efficiency of CR and benzopurpurine 4B increased with enhancing the light intensity from 50 to 80 j/cm² and 50 to 90 j/cm², respectively. Because more photons at higher light intensities will be available for excitation on the catalyst surface and more electron-hole pairs can be produced [126].

Based on the wavelength of UV irradiation, there are three electromagnetic spectra, including UV-A, UV-B and UV-C. These are categorized based on the wavelength range. For example, the light wavelength range for UV-C, UV-B, and UV-A is between 100-280, 280-315, and 314-400 nm, respectively [127]. Because of the shorter penetration of higher energy photons, the photodegradation rate is greater at 254 nm, which increases the number of pairs of electron holes produced to decompose the target contaminant [128].

5.6. Temperature

It is better to perform the photodegradation reaction at 25 ^oC and 1 bar because of photonic activation, which is useful to purify water, by which the heating stage can be eliminated to save energy. However, the optimum temperature for photocatalytic reaction can occur at 20-80 ℃. The results show that the photodegradation efficiency of organic pollutants increases with enhancing the reaction temperature, however, it may decline the adsorption capacity of reactive species and dissolved oxygen, leading to lower photodegradation efficiency [129]. Chen and coworkers studied the impact of temperature (0-50 ^oC) on the photocatalytic activity of TiO₂ and Pd/TiO₂ photocatalysts for MB elimination under UV light, and their outcomes demonstrated that the photodegradation efficiency increases with raising temperature [130]. Also, Hu and coworkers investigated the impact of temperature on photodegradation of MO dye using TiO₂. According to their results, under UV-vis irradiation, the photocatalytic activity of TiO₂ increased by enhancing temperature from 38 to 100 ^oC. Hence, the reaction was endothermic. Also, the reaction rate constant increased from 0.00031 to 0.00217 min^-1 with enhancing temperature from 34 to 100 ^oC, respectively [131]. Moreover, Barakat and coworkers studied the influence of temperature (5-55 ^oC) for degradation of rhodamine B using Ag-doped TiO₂. The highest photodegradation efficiency under irradiation occurred at 55 ^oC [132]. Generally, the photodegradation efficiency of organic contaminants increases with enhancing temperature and the highest removal performance occurs at high temperatures.

5.7. Reaction Time

Irradiation time or reaction time is a critical factor in the photodegradation process and is when the contaminant is exposed to light or photon energy. Many researches have investigated the impact of reaction time on the photodegradation efficiency of organic contaminants using TiO₂ and ZnO photocatalysts. The longer the reaction time, the higher the photodegradation efficiency of the contaminant [40]. According to the work done by Shi et al. (2019), eliminating MO using TiO₂/chitosan increased with enhancing the reaction time and finally reaches equilibrium. At the beginning of the photocatalytic reaction, there is a large number of active sites at the photocatalyst surface to attach with MO molecules, resulting in a high degradation efficiency. After the equilibrium reaction time, no change in the photodegradation efficiency was observed [133]. Elaziouti et al. studied the impact of reaction time (0-80 min) on the photodegradation efficiency of CR and benzopurpurine 4B. 95% of CR dye and 97.2% of benzopurpurine 4B were degrade using 1 g/L ZnO after 60 and 80 min, respectively [126]. Also, Erdemoğlu et al. studied the influence of irradiation time on photodegradation of CR dye using TiO₂. Under visible irradiation, CR was completely decomposed after 30 min [106].

6. Recyclability of ZnO and TiO₂ Photocatalysts

The most important factor for the practical application of catalyst is its recyclability and stability. Catalyst recyclability shows how many times a catalyst can be utilized in the catalytic process. After repeated reuse, the degradation efficiency of the catalyst is reduced, which may be due to saturating active sites, inactivating active sites, and chemical decomposition of the catalyst structure. If the degradation efficiency changes slightly after repeated use of the catalyst, it indicates that the catalyst is highly reusable. Thus, determining the reusability of a catalyst is critical [18,134]. Table 5 reports the reusability of various composites of ZnO and TiO2 in different cycles.

Nikoofar et al. (2015) synthesized ZnO nanorodes for imidazole removal. They studied the reusability of ZnO nanorodes in 4 runs and their results showed that the elimination efficiencies of imidazole after the first, second, third and fourth cycles were 83%, 83%, 81% and 80%, respectively, which indicates that the elimination efficiency has declined slightly (less than 3% after 4 reuse cycles), showing remarkable reusability of ZnO nanorodes [135]. In another study, the reusability of Ag/ZnO nanocomposite was studied in eliminating MB, MO and CR in 5 successive steps. A slight decrease in dye removal efficiency was observed after 5 cycles, indicating significant recyclability of Ag/ZnO for eliminating organic dyes [144]. Also, Shelar et al. (2020) investigated the stability and recyclability of Ag-doped ZnO nanocatalyst for eliminating MB dye. The photodegradation efficiency of MB was 95%. Then, the nanocatalyst was eluted 3 times with deionized water and reused in the photocatalytic process. After 4 cycles, the photodegradation efficiency reduced to 89.5%, indicating high stability of Ag-doped ZnO nanocatalyst [118]. Moreover, Fe₃O₄@S-doped ZnO was utilized for degrading ofloxacin. The recyclability study illustrated that the photodegradation efficiency of ofloxacin was above 90% after 6 cycles, indicating remarkable stability and reusability of Fe₃O₄@S-doped ZnO [136]. Furthermore, Khan and coworkers (2021) studied the reusability of ZnO NPs for photodegradation of MB and Rhodamine B dyes after 5 consecutive cycles. The photodegradation efficiencies of MB and Rhodamine B dyes were decreased from 93.25 to 86.63% and 91.06 to 83.61% after 5 cycles, respectively, indicating that the efficiency reduction percentage is less than 10%. ZnO indicates excellent photocatalytic activity and high stability after 5 cycles [18]. Besides, Ikram et al. (2021) studied the reusability of Mg-doped ZnO nanorods for eliminating a mixture of MB and ciprofloxacin. After 4 cycles, the photodegradation efficiency slightly reduced from 82 to 75% after 40 min [137].

Also, Padikkaparambil and coworkers (2013) studied the reusability of 2%Au-doped TiO₂ nanocatalyst for MO photodegradation under UV irradiation in 11 reuse cycles. Under operating conditions (3 g/L catalyst, 10 mg/L MO and 1 h irradiation time), the photocatalytic activity of Au/TiO₂ did not change (about 100% photodegradation efficiency) even after 10 consecutive tests, which indicates the high reusability of 2%Au-TiO₂ [138]. Also, Wu and Zhang (2019) studied the reusability and stability of Samarium/Nitrogen co-doped TiO₂/diatomite for eliminating tetracycline under visible light. After 5 cycles, the photodegradation efficiency reduced from 87.1% to 83.2% (less than 5% reduction), indicating high stability and significant reusability of the Samarium/Nitrogen co-doped TiO₂/diatomite catalyst [139]. Moreover, Song et al. studied the reusability of C-doped TiO₂/carbon nanofibrous in removing rhodamine B. The nanocatalyst showed the photodegradation efficiency of 94.2% under UV light. After six reusing cycles, the photodegradation efficiency reached to 92%, showing significant reusability and stability of C-doped TiO₂/carbon nanofibrous [140]. In another work, Bahrudin and coworkers (2019) studied the reusability of TiO₂ and TiO₂/polyaniline for MO dye photodegradation. After ten reuse cycles, the photodegradation efficiency of MO using TiO₂ and TiO₂/polyaniline decreased from 58.3 to 36.1% and 86 to 46.2%, respectively [141]. Moreover, Moosavi et al. (2020) studied the recyclability and stability of Fe₃O₄/AC/TiO₂ for 7 cycles of MB photodegradation. After 7 reuse cycles, the photodegradation efficiency reduced from 98 to 93% (about 5% reduction). They attributed the decrease in the photodegradation efficiency after 7 cycles to the following reasons: i) Particles losses may take place in washing and drying steps, which result in lower doses in the next cycle, leading to the decrease in the surface catalytic activity as well as the efficiency. ii) Characteristics of the catalyst like aggregation may change during different cycles, because aggregation of particles reduces the specific surface area as well as number of active sites. iii) The catalytic activity of the catalyst dwindles after each cycle due to blockage of active sites and pores [142]. Furthermore, the reusability of 2-(methacryloyloxy) ethyltrimethylammonium chloride/TiO₂ photocatalyst was studied for MB removal under UV irradiation. The photodegradation efficiency of the aforementioned photocatalyst after 270 min was 99.66% and after 20 reuse cycles, the photodegradation efficiency reached 98.7%, i.e., a reduction of less than 1% in the photodegradation efficiency, indicating remarkable stability of the photocatalyst [143]. Besides, TiO₂/2NiO photocatalyst was used to decompose MB in 5 reuse cycles and the outcomes showed that the photodegradation efficiency of MB using TiO₂/2NiO reduced from 100% to 72.6% after 5 cycles, indicating its high stability [134].

In general, examining the reusability of ZnO and TiO₂ nanocatalysts demonstrates that they have high reusability and their utilization in industrial applications is cost-effective. Also, adding metallic and non-metallic dopants to the structure of TiO₂ and ZnO catalyst increased their stability.

7. Literature Review

So far, many works have been done on the removal of organic compounds by catalysts. Most of these catalysts are based on TiO₂ or ZnO, because these catalysts have shown high performance compared to other catalysts. These catalysts are able to degrade organic contaminants in the presence of light or UV. For example, TiO₂ was able to eliminate phenol, nitrobenzene and MO with degradation efficiencies of 100%. But the degradation efficiency of toluene by TiO₂ photocatalyst was 71% [145]. Also, Razip and coworkers (2019) synthesized Fe₃O₄/TiO₂ nanocatalyst and used it in the removal of MO from wastewater. After 1h UV irradiation, the photodegradation efficiency of MO dye was obtained 90.3% [22]. In another study, Nagaraju et al. (2020) investigated the photocatalytic degradation of chlorobenzene from water using several catalysts such as ZnO, Pb/ZnO, Cd/ZnO and Ag/ZnO in the presence of LED light and tungsten light. Among these photocatalysts, Pb/ZnO was able to decompose chlorobenzene with 100% degradation efficiency in the presence of both lights. Also, pure ZnO showed the minimum degradation efficiency (71%) in the presence of LED light [28]. Also, carbon-doped TiO₂ nanocatalyst was able to eliminate methyl ethyl ketone under UV light with a photodegradation efficiency of 94% [23]. Bi₁₂TiO₂₀ nanophotocatalyst could remove cefixime with a degradation efficiency of 94.93% after 3 h, while Bi₁₂ZnO₂₀ removed 80% of cefuroxime from water after 4 h [146]. Moreover, Akhlaghian and Najafi (2018) synthesized CuO/WO₃/TiO₂ nanocatalyst for eliminating 4-Chlorophen. According to their outcomes, the aforementioned nanocatalyst was able to remove the pollutant with a degradation efficiency of 94.8%, which was obtained in the catalyst dosage of 0.75 g/L and H₂O₂ amount of 563.16 mmol/L, after 3 h [21]. Furthermore, Elmolla and Chaudhuri (2010) utilized ZnO nanophotocatalyst for removing Amoxicillin, Ampicillin, and Claxocillin and their outcomes demonstrated that ZnO could eliminate all contaminants with significant photodegradation percentage of 100% under ultraviolet after 180 min [123]. Table 6 reports the performance of TiO₂ and ZnO nanocatalysts in photodegradation of organic contaminants.

8. Conclusion and Future Perspectives

TiO₂ and ZnO photocatalysts under either UV light or solar irradiation were studied for degradation of organic compounds from wastewater. These catalysts have outstanding features like low cost, excellent degradation efficiency, high photocatalytic activity, etc. and are known as important catalysts in the water and wastewater industry. Both catalysts have been shown to be usable in the presence of sunlight and UV light, but the results generally indicate that using UV light is more efficient. Previous studies have shown that a mix of these catalysts with other materials can enhance the degradation efficiency of organic compounds. In this review study, the impact of various factors like temperature, time, photocatalyst loading, photocatalyst shape, pH, light wavelength and light intensity was investigated on degrading organic compounds and the outcomes demonstrated that pH, photocatalyst dosage, temperature and light intensity have significant impact on the photodegradation efficiency. Also, impacts of these photocatalysts on the degradation rate of various organic compounds from wastewater were fully surveyed. Moreover, various dopants can be utilized in the structure of TiO₂ and ZnO nanocatalysts, so the impact of adding metal and non-metal dopants on their structures were fully studied. Studies show that the addition of some metal dopants such as Ag, Pd, and Co and non-metal dopants such as C- and N- in the structure of ZnO and TiO₂ significantly increases the photodegradation performance of organic contaminants. Furthermore, the reusability of ZnO and TiO₂ catalysts cleared that they have high stability and significant reusability. Therefore, they can be used in several cycles without significant decrease in their photocatalytic activity. 2%Au-doped TiO₂ nanocatalyst with a photodegradation efficiency of 100% after 11 reuse cycles showed outstanding stability for elimination of methyl orange dye. Also, the photodegradation efficiency of 2-(methacryloyloxy) ethyltrimethylammonium chloride/TiO₂ photocatalyst reduced from 99.66% to 98.7% after 20 reuse cycles for elimination of MB dye, indicating remarkable stability.

Generally, previous studies have shown that they can remove more than 90% of most organic contaminants. Therefore, both metals and non-metals doped TiO₂ and ZnO are strongly recommended for elimination of organic compounds from municipal and industrial wastewaters.