Photocatalytic degradation of aquatic organic pollutants with Zn- and Zr-based metal-organic frameworks: ZIF-8 and UiO-66

Water treatment has been an essential issue with the increasing population over 40 years. Researchers center on the major organic pollutants, such as dyes, pesticides, and pharmaceutical products. Photocatalytic degradation is one of the promising methods for aquatic organic pollutant treatment. Over the years, scientists have been working on developments for photocatalysts to enhance their pollutant degradation performances. From the reviewed studies, it is seen that properties like surface area, chemical, mechanical, and thermal stability, and uniform distribution of active sites are crucial, and an increase in these properties provides better degradation efficiency. In this sense, metal-organic frameworks as photocatalysts can be considered more advantageous. This study focuses on the organic aquatic pollutant degradation studies by using well-known MOFs like ZIF-8 and UiO-66 photocatalysts. Mainly the organic dye (RhB, MB, MO, etc.) degradation efficiencies of ZIF-8 and UiO-66 have been achieved to 100%. Recently, the degradation capacities of various pharmaceuticals such as diazinon, acetaminophen, levofloxacin, and sulfamethoxazole have also been investigated. According to the reviewed studies, ZIF-8 and UiO-66 can be considered remarkable photocatalysts for the degradation of organic pollutants.


Wastewater treatment methods
The types and dosages of pollutants in drinking water are of great importance. While some pollutants in small amounts are not a big problem, in some cases, even the smallest doses create severe health and environmental problems [27,44]. The maximum contaminant level goal (MCLG) is the level of pollution that will not pose any known risk to health. The maximum contaminant level (MCL) is the amount of contaminant in the water without harming the human body. Appropriate treatment techniques (TT) should be used when this amount is exceeded. These doses are specified in the United States Environmental Protection Agency's standards, and values for some pollutants are listed in Table 2.  In addition to actively installed facilities to clean contaminated water, efforts are underway to make cleaning better and more cost-effective. It is possible to examine wastewater treatments in 3 main branches: Physical, chemical, and biological methods.
Using only one of these processes does not purify wastewater completely. Because generally, wastewater does not contain only one type of pollutant. For example, real textile wastewater contains dye, heavy metals, surfactants, solids, and various compounds [47]. By arranging all these methods in a specific frame, complete cleaning can be performed.
In addition to the method specified in Figure 1, new cleaning methods are also used [48]. Water cleaning studies using membrane technologies are tried for many pollutants [49]. Granular activated carbon is another widely used water cleaning method [50]. Another cleaning method that has come to the fore recently is photocatalysis. Photocatalysis removes the contaminants on the material's surface under UV rays or sunlight.

Photodegradation of aquatic pollutants
Photocatalysis is one of the most promising methods for aquatic pollutant degradation with an unlimited energy source since it is initiated with photons with the proper energy. It is an advanced oxidation process where semiconductors are used to start a degradation reaction. Commonly known semiconducting materials are TiO 2 , ZnO, and Fe 2 O 3 . Photocatalysts can be modified by controlling morphology, synthesizing composites, and doping to enhance photocatalytic activity [51][52][53].

The basic mechanism of photocatalytic processes
Photocatalysis is a chemical reaction caused by photons that break down inorganic or organic chemicals. Reactions do not have to be carried out under visible light. Any photon with enough energy can create a transformation in the chemical bonds. Thus, X-rays, ultraviolet light, and gamma rays are some examples that can start photocatalysis reactions [54].
The photocatalysis reactions start when a photon stimulates the photocatalyst with an energy higher than or equal to the band gap energy of the photocatalyst. The smallest energy difference between the conduction and valance band is band gap energy. Three reaction steps occur in the photodegradation process: oxidation (Figure 2a  Upon irradiation, an electron transfer from the valance band to the conduction band starts (Eq. (1)). The excited electrons (e -) which left the valance band cause holes (h + ) in the valance band. The photo-generated electrons possibly couple with the holes and release heat, as seen in (Eq. (2)).
(1) (2) If enough time is supplied to the electrons and holes before coupling, they can move to the surface of the photocatalyst. These electron-hole (-) pairs are captured on the photocatalyst surface then undergo the redox reactions ( Figure 3).
The positive holes in the valance band react with organic compounds, oxidize them, and lead to CO 2 and H 2 O production. Another oxidation reaction is likely to occur when H 2 O and positive holes react to produce highly reactive hydroxyl radical (OH). This radical can oxidize almost all electron-rich organic molecules and produces H 2 O and CO 2 [55]. These reactions can be seen in (Eqs. (3)-(5)).
(3) (4) (5) The electrons that move to the conduction band also react to prevent the accumulation of excess charges. Thus, the electrons react with the oxygen, creating superoxide anions, and the further reaction produces hydrogen peroxide (H 2 O 2 ) and consequently hydroxyl radicals. Considering their oxidation potential (2.8 V), mainly the hydroxyl radicals (OH) react with the organic compounds and completely mineralize them in a nonselective way (Eqs. (6)-(11)) [56].
(6) (7) (8) (9) (10) (11) Band gap energies of the semiconductors play an essential role in photocatalysis with respect to light absorption and converting the energy taken from the light into chemical energy to be used in the oxidation and reduction half-reactions. Band gap energies of the semiconductors ensure the feasibility of the proceeding reactions. Figure 4 shows the valance and conduction band potentials of the different photocatalysts. While developing a photocatalyst according to their application, it is necessary to consider the band gap energy, and band gap levels are arranged by controlling the particle size of the semiconductor [56,57].

Catalysts for the degradation of aquatic pollutants
The most common semiconductor photocatalysts used for water pollutant degradation are nonmetals, metals, and metal oxides like TiO 2 , ZnO, Fe 2 O 3 , WO 3 , BiVO 4 , and ZrO 2, as well as metal-organic frameworks like MOF-199, MIL-53, ZIF-8, and UiO-66. Scientists have worked on photocatalysts to enhance their degradation performances and developments like metal ion doping, composite photocatalysts, morphology control, nonmetal doping, and semiconductor heterojunction. These photocatalysts can be used to degrade aquatic organic pollutants such as dyes, pesticides, phenols, antibiotics, etc. [58].TiO 2 is a photocatalyst with low cost, nontoxicity, and availability [59]. However, its wide band gap causes a decrease in its effectiveness for using solar irradiation, and researchers are trying to overcome this inefficiency. Both Hu et al. [60]  and Naik et al. [61] applied morphological change development to increase the efficiency of TiO 2 . Hu et al. [60] prepared a core-shell structured TiO 2 with a hydrogen treatment method to degrade methylene blue. Naik et al. [61] prepared a super porous TiO 2 with a basic sol-gel-assisted reflux method to degrade Amaranth dye. Consequently, in both studies, regarding the photocatalysis efficiency on the degradation of azo-dyes, morphologically changed TiO 2 resulted better than the pristine TiO 2 .
ZnO has many attractive features like having exceptional electrochemical and optical properties and low cost and availability. It is possible to obtain different nanostructures of ZnO due to its versatility. Serrano-Lázaro et al. [62] established a flower-like nanostructured ZnO film with a spray hydrolysis method to degrade the widely used pesticide temephos in water. The efficiency of the ZnO photocatalyst increases due to the defects in ZnO lattice and flower-like morphology gained with nanostructural enhancement. Results show that nanostructured ZnO films are suitable for degrading temephos pesticides in water [62]. Micheal et al. [63] developed carbon nanoplate supported ZnO nanorods with a wet chemical process to degrade methylene blue dye. Compared to pristine ZnO, carbon nanoplate supported ZnO nanorods improve photocatalyst efficiency [63].
Research on bismuth as a photocatalyst has considerably increased in the last decade [58]. Both Zhang et al. [64] and Heidari et al. [65] studied the bismuth-based photocatalysts and used the heterojunction to enhance the photocatalytic activity. The heterojunction is a semiconductor modification that contains a pair of two different semiconductor materials [66]. Zhang et al. [64] produce a 2D/3D Bi 5 O 7 Br/BiOBr heterojunction photocatalyst with a facile hydrolysis process to degrade carbamazepine drug in water. The results show that Bi 5 O 7 Br/BiOBr heterojunction shows a better photocatalytic performance than Bi 5 O 7 Br and BiOBr separately. Heidari et al. [65] also prepare BiOBr heterojunction photocatalyst, a 3D-flower-like BiOCl/BiOBr-Bi 24 O 31 Br 10 type-II nano heterojunctions with the sono-assisted solvothermal method to degrade the four different types of fluoroquinolones such as levofloxacin, ofloxacin, norfloxacin, and ciprofloxacin. Similarly, Zhang et al. [64] also show that the nanoheterojunctions provide better performance than the pure component.
Researchers also use MOFs as semiconductors to degrade organic pollutants [64,67] by photocatalysis. MOFs are generally more advantageous for their greater internal surface area and easily adjustable organic units than common semiconductors [68]. Li et al. [69] and Fakhri and Bagheri [67] worked with MOFs to degrade pollutants in water bodies under visible light irradiation. In the study by Fakhri and Bagheri [67], UiO-66@ WO 3 /graphene oxide (UiO-66@WG), which is a MOF-based nanocomposite, was produced with the solvothermal method and studied in the degradation of tetracycline and malathion. Similarly, Li et al. [69] investigated the degradation of tetracycline and carbamazepine, bisphenol, and p-nitrophenol phenolic micropollutants. They prepared the g-C3N4/PDI@MOF heterojunction with the in situ growth of NH 2 -MIL-53(Fe) onto the g-C 3 N 4 /PDI layer. Considering the mutual pollutant tetracycline, the study by Li et al. [69] resulted in a better performance with 90% efficiency in 1 h, while the efficiency obtained by Fakhri and Bagheri [67] was 84% in 70 min. In addition, both studies showed superior performances compared to their respective pristine semiconductors.

Key factors affecting photocatalysis
Contaminant concentration, pH, and catalyst loading are the key factors for the organic contaminant degradation with photocatalysis. Naik et al. [70] investigated the effect of catalyst and pollutant concentrations and pH on the degradation efficiency of Amaranth azo-dye with mesoporous anatase TiO 2 . It was observed that with increasing the catalyst amount to 110 mg, the degradation efficiency increased due to the increase in the surface area for adsorption. The efficiency observed at the optimum catalyst amount was 99.1% degradation of Amaranth azo-dye in 60 min. Later, the effect of pollutant concentration was evaluated. At the lower concentrations of the pollutant, the degradation occurred faster. Increasing dye concentration blocked the light photons from reaching the catalyst surface and thus decreased the degradation efficiency significantly. Finally, they studied the effect of the pH of the pollutant solution, and the most effective degradation was observed at the pH of 2 with 97.14% [70].
Yusuff et al. [71] studied the degradation of organic textile contaminants in water with ZnO/pumice photocatalyst. Their results show that the increased pH values cause a slight decrease in the efficiency, which indicates that the dye effluent is acidic [71].
In the research by Fakhri and Bagheri [67], the effect of composite dosage and pH on the degradation of aromatic tetracycline and nonaromatic malathion pollutants by UiO-66@WG photocatalyst was investigated. Different composite dosages were studied, and the best results were obtained with the 35% WO 3 /graphene oxide dosage for tetracycline and malathion. The best photocatalytic activity was found at a pH of 7 and 9, respectively, for the degradation of tetracycline and malathion [67].
Li et al. [69] investigated the most effective way for the degradation of pharmaceutical pollutants by observing the change of different catalyst dosages and H 2 O 2 concentrations. An efficiency increment was observed when the catalyst dosages increased from 0.1 g/L to 0.2 g/L, but the efficiency remained constant after 0.2 g/L. The high amount of catalysis caused competing reactions; therefore, the photocatalytic efficiency was reduced. The optimum H 2 O 2 concentration was determined as 10 mM for the degradation of bisphenol A and p-nitrophenol [69].

Effect of ZIF-8 and UiO-66 on the degradation of aquatic pollutants
In recent studies, it has been observed that MOFs are promising for breaking down aqueous organic pollutants with photocatalysts [72]. MOF is a porous material comprising inorganic metal ions and organic ligands. Some of the advantages of the photoactive MOFs are their well-defined crystalline structures, adjustable active sites, internal synthesis, and ability to design the photocatalysts properly [73]. The most common photocatalysts for the degradation of organic aquatic pollutants are graphene oxide or metal nanoparticles (i.e. Ag, Fe) supported or promoted MOFs, such as MILs, ZIF-8, and UIO-66 [73][74][75]. ZIF-8 and UiO-66 have mainly used photocatalysts for removing the aqueous organic pollutants. Deposition of metals (Ti, Ag, etc.) and/or immobilization of MOFs on the support surface have increased the photocatalytic activity [75,76].

Design of ZIF-8 and UiO-66 photocatalysts
Zeolitic imidazolate framework-8 (ZIF-8) consists of imidazolate organic ligands and Zn +2 metal ions. The higher thermal stability (up to 550 °C) and chemical stability (boiling alkaline water and organic solvents) puts it ahead of other MOFs. ZIF-8 structure can be obtained by many different synthesis methods: mechanochemical, solvothermal, sonochemical, microwave-assisted, dry gel, and microfluidic methods. Particle sizes, textural, surface, and structural properties, and production efficiency are directly dependent on the method used and the ambient conditions of the method [77]. To briefly explain some essential methods, one of them is the mechanochemical method that has been used recently due to the necessity for a more straightforward application and cheaper equipment.
Moreover, the chemical reactions during the grinding process can provide products without the need for high temperatures [78]. The mechanochemical method is a very efficient method thanks to the lower raw material loss, cost, and the resulting by-products for the production of ZIF-8 [79]. Solvothermal synthesis can be carried out with different solvents at certain temperatures. Furthermore, when the used solvent is water, which is called hydrothermal synthesis, is also a safer synthesis method for MOF production [80,81]. In sonochemical synthesis, an accelerated reaction takes place with the help of ultrasonic waves [82]. In some cases, it has become possible to harmonize the methods. For example, Nie et al. [83] used the solvothermal method in their study, benefited from ultrasonic waves' properties, and named the method high efficiency.
Besides, this eco-friendly photocatalyst can be produced easily in a short time, can be used again and again, and does not require spending lots of solvents during its production. In addition, the large surface area (BET specific surface area (SSA) (~2000 m 2 g -1 ), Langmuir surface area (~1810 m 2 g -1 )) and desired pore size (pore cavities of ~1.16 nm and pore volume of ~0.60 cm 3 g -1 ) of ZIF-8 make it attractive for photocatalytic degradation of pollutants. As proposed by Chandra et al. [84], ZIF-8 can be obtained by using hydrated Zn(NO 3 ) 2 and 2-methylimidazole as dissolved in methanol [85,86] or DMF [87,88] and then stirred. The detailed procedure is given in Figure 5a.
One of the most attractive aspects of MOF synthesis is that the organic bridging ligand can be synthetically modified to introduce a desired functionality to the framework. The thermal stability of MOFs is generally 250-400 °C, and the chemical stability is quite low [89]. However, UiO-66 has great thermal stability until 540 °C and high chemical stability for water, acetone, benzene, and dimethylformamide. Cavka et al. [90] and Venna et al. [91] reported the synthesis procedure of a Zr-based MOF. ZrCl 4 and 1,4-benzene dicarboxylate (BDC) were mixed with dimethylformamide (DMF) for a solvothermal process in an autoclave at 666 °C for 24 h and the synthesized Zr-MOF was washed and dried. The synthesis mechanism of UiO-66 is illustrated in Figure 5b. During the synthesis, the parameters are the temperature, time, pH, and the composition of the reactants (metal: ligand).

Modification of ZIF-8 and UiO-66 4.2.1. Metal nanoparticles in ZIF-8 and UiO-66: doping
MOFs become up-and-coming alternatives thanks to their uniform distribution of active sites and topological structure [92]. To increase the photocatalyst effect, they are loaded with metal nanoparticles. Thus, effective heterogeneous catalysts are created with the high thermal and mechanical stability of the MOFs. Metal nanoparticles can be loaded into ZIF-8 with several methods. The reaction can occur in several steps [93] or in one step [85]. While metal nanoparticles were loaded into ZIF-8, some researchers produced MOF and nanoparticles separately and then added them together [76,94,95]; the others produced doped ZIF-8 in a single process [96]. Thanh et al. [97] achieved a higher photodegradation efficiency using doped ZIF-8.
For the doping of metal on UiO-66, metal nanoparticles are dissolved in deionized water, added to the UiO-66, and the solution is stirred. The mixture is heated, and the solids formed in this process are centrifuged. Precipitates are washed with distilled water or ethanol, and resulting solids are dried in air or under a vacuum. Doped ZIF-8 and UiO-66 photocatalysts and photocatalysis conditions for various aquatic organic pollutants are comprehensively reviewed in Table 3.

Immobilization of ZIF-8 and UiO-66
The combinations of supporting materials to the MOFs can have higher catalytic activity [98]. It is an essential method to immobilize some functional areas and dynamic groups to increase the porous structure where MOF interacts with target pollutants [99]. Many different substances, such as zeolite, silica, graphene, cotton fabrics, can immobilize MOFs. In addition, high photocatalytic activity and stabilization can also be achieved by immobilization [100]. The deposition of semiconductors on the ZIF-8 is highly influential on photocatalytic activity [101]. Coating and encapsulating ZIF-8 by TiO 2 or CuO shell increase the organic guest molecules in the micropores of ZIF-8 [64,98].
Cotton fabrics (CF) are frequently used for immobilization due to their flexibility, air permeability, and adsorption ability. Lan et al. [101] studied CF supported ZIF-8 composite (CF@ZIF-8), and the photocatalytic degradation was reached 89%. This composite could be used in large-scale applications with low costs [101]. The CF@ZIF-8 composite was prepared by pretreatment and synthesis steps. Pretreatment for CF follows the steps: sonication of cotton fabric in acetone solution, washing with distilled water, and drying. 2-methylimidazole and methanol are mixed with an ultrasonic mixture, immersing the pretreated CF in the solution, washing with methanol, and drying the solution. The immobilization technique is applied to produce CuO nanoparticles/ZIF-8 composite [98]. For green and cleaner products, Xie et al. [102] produced the organic-inorganic hybrid recyclable catalyst (AILs/HPW/ UiO-66-2COOH). These catalysts have both Bronsted and Lewis acid sites, effective for one-pot biodiesel production from low-cost oils.
The acid catalysts were characterized by several techniques, and the results demonstrated that 12-tungstophosphoric heteropolyacid (HPW), a polyoxometalate (POM) acid, was encapsulated. The polyoxometalate-based sulfonated acidic ionic liquids (AILs) were virtually immobilized on the UiO-66-2COOH. Hassabo et al. [103] synthesized a UiO-66-COOH for purified L-Methionine-ɣ-lyase (METase) enzyme from Wickerhamomyces subpelliculosus. Subsequently, a new composite (METase@UiO-66) was prepared. The results revealed that greater stability was achieved with the composite as compared to the free enzyme. Moreover, the storage stability of METase was significantly improved after immobilization [103].

Photocatalytic degradation and mechanism
ZIF-8 and its derivatives are commonly used to degrade organic dyes and pharmaceuticals. The degradation of organic aquatic pollutant studies is generally about organic dyes such as methylene blue (MB) and rhodamine B (RhB). Moreover, there are successful studies for methyl orange, rhodamine 6G, Congo red, Reactive Black KN-B, levofloxacin, diazinon, acetaminophen, etc. ZnO-doped on ZIF-8 photocatalysts has been studied by Ökte et al. [76], Liu et al. [104], and Yu et al. [95] for degradation of MB. Yu et al. [95] reported the maximum MB degradation efficiency of 94.1% after 240 min. TiO 2doped ZIF-8 photocatalysts for degradation of MB dye were investigated by different researchers [76,84,94]. Chandra and Nath [94] obtained successful results with 93.4% MB degradation in 120 min. The Ag nanoparticles (AgNPs)doped ZIF-8 photocatalysts for MB degradation were also studied by numerous researchers [104,105]. Chandra and Nath [105] achieved 97.25% MB degradation under UV-visible light at pH 7.89 after 120 min. They produced the most efficient and stable catalyst by using a 300 μL suspension of Ag nanoparticles. When the suspension amount increased, lower degradation efficiency and stability were observed. The charge was transferred from HOMO to LUMO in ZIF-8, so electrons effortlessly passed from the valence band (VB) to the conduction band (CB). Moreover, AgNPs hold the electrons; thus, photo-generated electron transfer became easier in ZIF-8. e-and h+ could react with oxygenated water under the influence of light. Therefore, some of the necessary reactive oxidative species (H+, OH•) were produced for MB degradation.
The photodegradation mechanism of RhB is given in Figure 6. TiO 2 -doped ZIF-8 was synthesized for degradation of RhB [84,94]. Moreover, Jing et al. [106] and Zuo et al. [107] studied the performances of Ag/AgCl@ZIF-8 for RhB degradation. Among these studies, Jing et al. [106] succeeded with a remarkable 99.12% RhB degradation in 60 min with high stability. The reusability study of Ag/AgCl@ZIF-8 indicated that the photocatalyst was stable and reusable. Visible light could not affect the AgCl molecule directly due to its large band gaps. However, AgNPs could absorb visible light owing to their surface plasmon resonance properties. AgNPs were transferred to the conduction band of AgCls. Then, these particles came together with adsorbed O 2 and •O 2 active molecules. The holes observed on the surface were integrated with OH-to obtain •OH. Therefore, •O 2 and •OH were acquired to degrade RhB dye [106]. The reactions of RhB degradation are given in (Eqs. (12)-(17)).
(12) (13) (14) (15) (16) (17) The degradation of diazinon [100], acetaminophen (ACT) [86], and levofloxacin [101] as pharmaceuticals were investigated by using ZIF-8 catalysts. Diazinon was degraded 64.1% after 100 min by Fe 3 O 4 -COOH@ZIF-8/Ag/Ag3PO4 Levofloxacin was degraded 87% after 60 min by Ag/AgCl@ZIF-8/g-C3N4 composite. Furthermore, acetaminophen (ACT) degradation was 99% after 90 min by Ag/AgCl@ZIF-8 catalyst. The photocatalytic mechanism has been examined using different scavengers for trapping holes; superoxide radicals and hydroxyl radicals, ammonium oxalate, benzoquinone, and isopropanol were utilized, respectively. The electrons of ZIF-8 cannot be inspired by visible light irradiation; however, it is possible for Ag/AgCl. Thus, when the energy of an incident photon is larger than the Ag/AgCl band gap value, the electrons on the conduction band pass to the valance band by forming h + and e -. Ag nanoparticles located on the AgCl surface could behave as a bridge to supply eto the surface of ZIF-8 by preventing the reunion of electronic hole pairs. ZIF-8 structure leads to a large specific surface area and provides strong adsorption characteristics. The enrichment of acetaminophen to the catalyst's surface was considerably accelerated by the efficient adsorption process. The adsorbed acetaminophen was being decomposed.
The primary active substance was O 2 -• for the photocatalytic degradation of acetaminophen by Ag/AgCl at ZIF-8. Heterostructure had several roles, such as enhancing the separation efficiency, improving the migration rate of electron hole pairs, decreasing the recombination rate, and enhancing the stability of the catalyst [86].
The band structure of UiO-66 is similar to a semiconductor [108]. UiO-66 and its metal-doped or immobilized variations have been used as photocatalysts for the degradation of aquatic organic pollutants, such as pharmaceuticals and dyes in water [75,108].
RhB degradation has been investigated by using different UiO-66-based catalysts [75,109]. The SnO 2 @UiO-66/rGO catalyst achieved 95.5% degradation in 150 min under visible light with 3.3 eV band gap energy [75]. Using the CdS/UiO-66-NH 2 catalyst with a band gap energy of 2.28 eV, 95% degradation of RhB was obtained at 60 min under a xenon lamp [109]. Feng et al. [110] also analyzed RhB degradation under xenon lamp irradiation with pure and doped UiO-66 catalyst.
The photodegradation mechanism for methyl orange (MO) via In 2 Sn 3 @UiO-66 was assessed by Gan et al. [111] and 98% degradation was obtained in 60 min. The band gap energy of the catalyst was determined as 2.2 eV [111]. The potential photocatalysis reactions for MO degradation by GO@In 2 S 3 @UiO-66 catalyst under visible light are illustrated in Figure 7 [111].

Conclusion
The aim of this review article was to focus on the increasing water pollution and treatment of wastewater by using the photocatalytic degradation method. Especially studies on the removal of organic pollutants from wastewater have gained importance recently. Since scientists discovered the role of photocatalysts on degradation performance, they have been working on modification methods of photocatalysts. These methods include metal ion doping, immobilization (on the support surface), morphology control, and semiconductor heterojunction. In the studies, it has been seen that the surface area of the semiconductor is significant, and the increase in the surface area causes an increase in the decomposition efficiency. In this sense, metal-organic frameworks like photocatalysts can be more advantageous in larger internal surface areas and easily tunable organic units. In this study, ZIF-8 and UiO-66 photocatalysts were investigated in detail. The synthesis, modification, and photodegradation mechanisms of ZIF-8 and UiO-66 for organic aquatic pollutants were summarized. Using these photocatalysts, many organic dyes, various pharmaceuticals, and pesticides have been degraded almost completely. According to the researchers, these two photocatalysts are promising for the degradation of aquatic organic pollutants and can be widely applicable in the future, especially with some essential modifications.

Future prospects
In this study, it is observed that ZIF-8 and UiO-66 are efficient photocatalysts for the degradation of aquatic organic pollutants. By using modification methods such as doping and immobilization, more stable (e.g., in water) and more active photocatalysts with larger surface areas were obtained. At the same time, the band gap can be decreased, which helps to work under visible light [17]. Some issues to be developed are listed below: Metals with high-value electrons (Fe 3+ , Cr 3+ , Al 3+ , etc.) and transition metals (V 4+ , Ti 4+ , Zr 4+ , etc.) have been used to regulate and increase stability. MOFs containing redox-active metals and organic binders should be produced. Pathways should be sought to produce reusable and cost-effective MOFs [72]. MOFs produced in different amounts, and contents should be examined and optimized under different conditions [45].
There are primarily dye-related studies and fewer studies on other organic pollutants. Thus, it is necessary to increase the studies on other organic pollutants. Studies should be initiated to enlarge the experiments and assist in treating wastewater (containing heavy metals, biological residues, toxic substances) in actual environmental conditions [112].
In order to provide a cleaner and healthier environment and energy saving, studies on this subject should be supported and continued rapidly.