Metal–Organic Frameworks: Next-Generation Materials for Environmental Remediation

Abstract

Contamination of water resources, particularly from industrial discharges, agricultural runoff, or hospital wastewater, poses significant environmental and public health challenges. Traditional wastewater treatment methods often fail to effectively remove the diverse and persistent pollutants present in these sources, including emerging chemical compounds or biological agents. To address these challenges, metal–organic frameworks (MOFs) have emerged as multifunctional materials offering promising advancements in wastewater remediation. These materials can be applied directly as pollutant adsorbents or used for pathogen removal due to their antimicrobial activity. Additionally, MOFs play a crucial role in Advanced Oxidation Processes (AOPs) due to their catalytic activity. When incorporated into electro-Fenton, Fenton-like, or photocatalytic processes, MOFs enhance the generation of oxidant radicals, enabling efficient wastewater decontamination. This comprehensive review explores the potential of MOFs, focusing specifically on their design, synthesis, and application as multifunctional materials for the inactivation of pathogens and the removal of organic pollutants. Moreover, it examines their characteristics, recent advances in synthesis techniques, and the mechanisms underlying their removal efficiency. The findings presented underscore the transformative potential of MOFs in achieving clean and safer water, contributing to sustainable environmental management and public health protection.

1. Introduction

The contamination of water resources emerged as a critical environmental and public health issue. The presence and concentration of persistent pollutants have been increasing year by year due to demographic growth [1] and activities such as those originating from industrial effluents, hospital wastewater, and agricultural runoff. Their persistence in water poses significant risks to the environment apart from the potential to cause adverse health effects [2]. Moreover, a particular source of concern is the wastewater from hospitals, which contains higher loads of microorganisms, many of them pathogenic, as well as pharmaceutical compounds [3]. In this context, traditional wastewater treatment methods often fall short of effectively removing these complex and diverse pollutants or pathogens [4]. To solve this drawback, innovative materials such as metal–organic frameworks (MOFs) have gained prominence due to their exceptional structural and functional properties [5]. MOFs are porous crystalline materials formed by the coordination of metal ions or clusters with organic ligands, creating highly ordered structures with tunable surface areas, pore sizes, and chemical functionalities [6], forming one-, two- or three-dimensional structures [7]. This versatility allows MOFs to be tailored for specific functions, making them powerful tools in wastewater treatment.

The ability of MOFs to adsorb, disinfect, and catalyze AOPs to generate radicals involved in the degradation of pollutants [8,9,10,11] lies in their structural diversity and chemical flexibility [12,13]. Their large surface area and porosity enable the adsorption of a wide range of contaminants, while their catalytic sites facilitate chemical reactions that degrade complex organic molecules into less harmful compounds [14].

Moreover, the design of MOFs enables precise control of their physicochemical properties, allowing the development of specialized materials for targeting microorganisms or specific pollutants [15,16]. Their multifunctionality extends beyond adsorption and catalysis, encompassing photocatalytic and antimicrobial activities, making them highly adaptable in water treatment technologies [17]. Furthermore, combining MOFs (as catalysts) with Advanced Oxidation Processes (AOPs) enhances their performance, enabling simultaneous pollutant degradation and pathogen disinfection in a single treatment step. Thus, MOFs enhance the generation of radicals from various oxidants, enabling the removal of pollutants through hydroxyl (HO^·), sulfate (SO₄^·−) [18], and superoxide (O₂^·) radicals [19].

The novelty of this review lies in its comprehensive analysis of MOFs as next-generation materials for environmental remediation, specifically addressing the dual challenge of chemical contaminants and pathogenic microorganisms in wastewater. Unlike previous reviews, this work uniquely emphasizes the multifunctional capabilities of MOFs, not only as adsorbents but also as antimicrobial agents and catalysts in AOPs. By integrating discussions about their role in adsorption or degradation processes such as electro-Fenton, Fenton-like, and photocatalytic, this manuscript offers a holistic perspective on the mechanisms that drive their efficiency in removing pollutants.

In addition, this review provides a comprehensive overview of the current state of research on MOFs as a promising alternative to traditional materials such as activated carbon and homogeneous catalysts. It focuses on their role as catalysts in AOPs, adsorbents, and antimicrobial agents for wastewater decontamination, addressing one of the most pressing environmental challenges of the current decade. By deepening the understanding of the applications of MOFs and associated technologies, this study contributes to the development of effective and sustainable solutions to mitigate the impacts of pathogens and organic pollutants on human health and the environment. This manuscript also highlights recent advances in the design and synthesis of MOFs adapted to complex wastewater matrices, filling a critical knowledge gap regarding their practical applicability. This approach positions MOFs as transformative materials for sustainable water management, offering innovative solutions to persistent environmental and public health challenges.

2. MOFs: Fundamentals, Properties and Synthesis

2.1. Fundamentals and Properties

MOFs are a class of porous crystalline materials composed of metal ions or clusters coordinated with organic ligands, forming dimensional and tunable structures [7]. These materials are known for their high porosity, large surface area, and customizable physicochemical properties, making them suitable for a wide range of applications, including environmental remediation, adsorption, or catalysis [20,21]

MOFs are composed of two main components: metal nodes or clusters and organic linkers. The metal ions can create compounds with unique coordination modes, such as tetrahedral, trigonal bipyramidal, square planar, and octahedral geometries (Figure 1). Common organic linkers include polytopic carboxylates and aromatic heterocyclic molecules, such as terephthalic acid (H₂BDC), 4′′′-(pyrazine-2,3,5,6-tetrayl), tetrabenzoic acid (H₄TCPP), or imidazolates, which facilitate coordination chemistry with the metal ions [5,22].

The structural diversity of MOFs is vast, allowing for the creation of one-, two-, and three-dimensional structures. This diversity is achieved through the selection of metal ions and organic linkers, as well as the synthesis methods employed. This has led to MOF families being developed, each named according to their place of synthesis or the specific reagents used (Table 1). Examples include UiO (University of Oslo), MIL (Materials of Institute Lavoisier), HKUST (Hong Kong University of Science and Technology), and ZIF (Zeolitic Imidazolate Framework) [23,24,25], which are commonly used for environmental remediation. These different framework structures are designed to exploit the unique properties of the metal ions and organic linkers, enabling a wide range of functionalities, such as adsorption and catalytic activity, whereas UiO and ZIF frameworks are particularly effective for adsorption applications due to their high surface area and structural stability, while MIL and HKUST are a better option for AOP applications due to its tunable metal clusters based on Fe or Cu [26,27,28].

By this, the chemical nature and functionalization of the organic linkers play a critical role [29]. Rigid, conjugated ligands—such as porphyrins or aromatic dicarboxylates—not only extend the framework’s light absorption into the visible range but also promote strong π–π interactions and hydrogen bonding with guest molecules. This can enhance the selective adsorption of pollutants or substrates. Moreover, functional groups (e.g., –NH₂, –SH) on the ligands can serve directly as catalytic sites or assist in anchoring additional catalytic species via post-synthetic modifications [30]. Moreover, the metal clusters or ions in MOFs determine the framework’s Lewis acidity and redox properties. Open metal sites, which are often exposed after activation, act as strong adsorption centers—ideal for capturing small molecules like CO₂, H₂, or heavy metal ions. In catalysis, these unsaturated metal centers can coordinate with reactant molecules, lowering activation energies and promoting reactions such as oxidation, cycloaddition, or hydrogenation [31]. For example, Ti-based MOFs like MIL-125(Ti) are prized for their robust Ti–O clusters that efficiently adsorb and activate substrates for photocatalytic or thermal redox reactions [32]. Furthermore, high porosity and a large surface area not only provide ample sites for adsorption but also facilitate the diffusion of reactants and products within the structure. The size, shape, and chemical environment of the pores can be finely tuned to match the dimensions of target molecules, enhancing selectivity and catalytic efficiency [33]. Spatial confinement within the pores can stabilize transition states or reactive intermediates, thereby increasing turnover frequencies in catalytic reactions. Finally, the spatial arrangement of the ligands and metal nodes determines the distribution of active sites. Precise chemical positioning allows for the creation of microenvironments that favor particular adsorption modes (e.g., through hydrogen bonding or Lewis acid-base interactions) and enable synergistic effects when multiple catalytic functionalities are present [34]. This rational design strategy, often aided by post-synthetic modifications, allows MOFs to be tailored for a broad spectrum of catalytic applications—from traditional thermal catalysis to photocatalysis under visible light. Additionally, the introduction of functional groups into MOFs can lead to synergistic effects when multiple catalytic functionalities are present. Fan et al. [35] discussed how defect engineering in copper paddlewheel-based MOFs could enhance catalytic properties by optimizing the arrangement of ligands and metal nodes. This study highlights the potential of combining different functionalities to improve the performance of MOFs in catalytic reactions.

In summary, by strategically varying the organic linkers, metal nodes, and overall porosity, researchers can engineer MOFs with tailored adsorption capacities and catalytic activities. These design principles have led to applications in environmental remediation (e.g., pollutant capture), energy conversion (e.g., photocatalytic CO₂ reduction [36]), and organic synthesis (e.g., selective oxidation and cycloaddition reactions). The interplay between structure and function in MOFs exemplifies how precise control at the molecular level can yield materials with advanced performance in both adsorption and catalysis.

On the other hand, depending on synthesis conditions, such as those seen in Table 1, organic linkers, such as H₂BDC, can act as bidentate or monodentate [37], with the difference between the two lying in the deprotonated state of the ligand, which depends on the pH of the mixture of solvent, ligand, and metals. This state, as Zhang et al. [38] mention in the synthesis of similar compounds known as coordination polymers, is important for MOF design, as it allows one or more metal clusters to link to it, apart from conforming the two-dimensional or three-dimensional crystalline structures.

MOFs can also be categorized based on their geometry and the type of metal ions used. Thus, in MIL-type MOFs, different trivalent or tetravalent combinations can be used, while other common types of MOFs use a metal base, which can be modified with the addition of other metals to form bi-or trimetallic MOF types. Hereby, those modifications produce changes in the geometry of the material. For example, MOFs with iron (III), copper (I), and nickel (II) ions typically exhibit square planar and octahedral geometries, while cobalt (II), zinc (II), and manganese (II) ions form octahedral and tetrahedral structures [39,40,41].

Additionally, the pore size and surface volume of MOFs can be modified by altering the organic ligands, which allows for the design of materials with specific voids, porous spaces, or structures suitable for adsorption and catalyst applications [17,42]. Moreover, the presence of metals in MOFs provides them with other important qualities, such as their catalytic capacity in various AOP processes and their antibacterial properties. MOFs containing iron as metal clusters show the highest catalytic activity in Fenton-based processes, while titanium, zinc, or zirconium offer better properties in photocatalysis. Finally, metal clusters of copper, zinc, and aluminum present superior antibacterial properties compared to others since even small amounts of these metals can cause DNA damage or protein alteration, leading to cell lysis [43].

Table 1. Synthesis conditions for the main MOFs used for environmental purposes.

MOF	Ligand Linker	Metal Clusters	Synthesis Method	Temperature (°C)	Time (h)	Solvent	Reference
MIL53	H2BDC	Zn, Fe	Solvothermal	90–150	15	DMF *1/EtOH *2	[44]
	H2BDC	Fe	Sonochemical	70	2	DMF *1	[45]
MIL101	NH2-BDC	Fe	Hydrothermal	110–150	24	H2O	[46]
MIL125	H2BDC	Ti	Solvothermal	150–220	12	DMF *1/EtOH *2	[47]
ZIF-8	2-methylimidazole	Zn	Solvothermal	100–150	6	DMF *1/MeOH *3	[48,49]
	2-methylimidazole	Zn	Microwave	100–150	4	DMF *1/MeOH *3	[50]
HKUST	BTC	Cu	Solvothermal	150	24	DMF *1/EtOH *2	[51]
	BTC	Cu, Ru	Mechanochemical	25	0.33	-	[52]
UiO-66	H2BDC	Zr	Solvothermal	120–220	12	DMF *1	[53]
UiO-67	BPDC	Zr	Solvothermal	120–220	48	DMF *1	[54]
MOF-5	H2BDC	Zn	Electrochemical	25	2	DMF *1/EtOH *2	[55]

*¹ DMF: N,N-Dimethylformamide. *²: EtOH: Ethanol. *³ MeOH: Methanol.

Table 1. Synthesis conditions for the main MOFs used for environmental purposes.

MOF	Ligand Linker	Metal Clusters	Synthesis Method	Temperature (°C)	Time (h)	Solvent	Reference
MIL53	H2BDC	Zn, Fe	Solvothermal	90–150	15	DMF *1/EtOH *2	[44]
	H2BDC	Fe	Sonochemical	70	2	DMF *1	[45]
MIL101	NH2-BDC	Fe	Hydrothermal	110–150	24	H2O	[46]
MIL125	H2BDC	Ti	Solvothermal	150–220	12	DMF *1/EtOH *2	[47]
ZIF-8	2-methylimidazole	Zn	Solvothermal	100–150	6	DMF *1/MeOH *3	[48,49]
	2-methylimidazole	Zn	Microwave	100–150	4	DMF *1/MeOH *3	[50]
HKUST	BTC	Cu	Solvothermal	150	24	DMF *1/EtOH *2	[51]
	BTC	Cu, Ru	Mechanochemical	25	0.33	-	[52]
UiO-66	H2BDC	Zr	Solvothermal	120–220	12	DMF *1	[53]
UiO-67	BPDC	Zr	Solvothermal	120–220	48	DMF *1	[54]
MOF-5	H2BDC	Zn	Electrochemical	25	2	DMF *1/EtOH *2	[55]

*¹ DMF: N,N-Dimethylformamide. *²: EtOH: Ethanol. *³ MeOH: Methanol.

Furthermore, properties such as the Brunauer–Emett–Teller (BET) surface area are crucial for characterizing and understanding their mechanism of action. For example, MOFs like MIL present significant diversity in metals and tunable options, exhibiting a BET surface area with an average value of around 600–900 m²/g [56]. In comparison, other MOFs such as ZIF or UiO present BET surface areas of around 800–1000 m²/g, while HKUST presents an average BET surface area of 1000–1900 m²/g, respectively [57,58]. Since BET is closely related to adsorption capacities, it indicates that ZIF, UiO, or HKUST are generally better options for enhanced adsorption capacities [59].

In recent times, bimetallic and trimetallic MOFs have emerged as an advanced class of MOFs designed to optimize their properties in wastewater treatment. By incorporating two or three metal centers into the MOF framework, these porous materials achieve synergistic effects that enhance their adsorption and catalytic and structural stability [60]. The combination of metals allows for the tuning of the electronic, chemical, and physical properties, enabling dual or even multiple functionalities within a single MOF. For instance, bimetallic MOFs, combining iron and cobalt, have shown enhanced performance in Fenton-like reactions due to improved reactive oxygen species (ROS) generation and the regeneration of the active sites of the catalyst [61]. Moreover, trimetallic MOFs, such as those incorporating iron, titanium, and zirconium, also exhibit superior robustness and versatility compared to monometallic MOFs in the catalytic reduction in organic contaminants.

Furthermore, advanced characterization techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Energy-Dispersive X-ray spectroscopy (EDX) [62] help in understanding the molecular structure and elemental composition of MOFs, which are critical for tailoring their properties for specific applications in AOPs [63]. Moreover, elucidating the structure and verifying the successful incorporation of metal clusters into MOFs to ensure their correct synthesis is essential. For example, the crystal structure of MOFs can be confirmed through XRD analysis. As shown in Figure 2 the XRD analysis of the synthesized Zn-MIL53(Fe) and MIL53(Fe) revealed a pattern of continuous diffraction peaks in the low-angle region, typical of carboxylic materials. The diffraction pattern closely resembled MIL-53(Fe), with peaks at 2Theta (θ) values of 9.13°, 10.4°, 13.9°, and 19.7°, consistent with the MIL-53(Fe) crystal structure reported by Nguyen et al. [64]. Additional peaks at 14.16° and 17.5° indicated successful Zn (II) incorporation into the pyrrolic ring of MIL-53(Fe). Notable intensity changes at 13.9°, 19.7°, and 25.6°, along with new peaks at 17.51°, 19.54°, and 28.63°, suggested a distinct crystal structure.

The XRD data highlighted the integrity and composition of the Zn-MIL53(Fe) material, with higher intensity peaks in the lower 2Theta region indicating the primary MOF crystal structure. Overall, XRD analyses are particularly useful in providing detailed insights into the structural integrity and composition of the synthesized MOFs, whether monometallic or bimetallic, with high-intensity peaks in the lower 2Theta range (below 10) highlighting the crystal conformation of the materials. These findings were supported by SEM analysis, which showed morphological changes due to Zn introduction (Figure 2A,B), confirming the structural variation and the crystal structure. Moreover, TEM results demonstrate the particle size of the material, which varies depending on the synthesis method used. Additionally, FTIR analysis, as depicted in Figure 2D, is used to obtain a representation pattern and to analyze the distribution and status of functional groups of the synthesized MOFs, being crucial to the presence of 1600 and 1300 cm⁻¹ range bands that indicate the ligand has deprotonated, forming a carboxylate that has coordinated with metal clusters to form the MOF and the low part below 500 cm⁻¹ range bands that represent the metal cluster presence [44].

In addition, these bimetallic crystalline structures not only broaden the functional scope of MOFs but also address limitations associated with monometallic counterparts, such as stability in harsh environments or limited catalytic activity in some AOP techniques [44].

2.2. Synthesis

In the synthesis of MOFs, various techniques—including hydrothermal, solvothermal, mechanochemical, electrochemical, microwave-assisted, and ultrasound methods—are employed to design materials with specific physicochemical properties that directly impact their adsorption capacity and catalytic performance. For example, hydrothermal synthesis carried out in aqueous media is particularly suitable for producing MOFs with high crystallinity and water stability, attributes essential for adsorption and catalysis applications [66]. In contrast, solvothermal synthesis, which utilizes organic solvents, allows for precise control over crystal size and morphology, thereby optimizing the exposure of active sites for catalytic reactions [67]. Similarly, mechanochemical methods—operating under solvent-free conditions—often yield MOFs with controlled defects and variable crystallinity, which, in turn, modify the pore architecture and surface reactivity [68]. Electrochemical synthesis provides detailed control over the formation process, resulting in conductive structures that are ideal for electrocatalytic or sensing applications [69]. Additionally, microwave-assisted synthesis significantly accelerates nucleation and growth, producing MOFs with well-defined pores and enhanced mass transfer efficiency, while the use of ultrasound facilitates the formation of nanometer-scale MOFs with enlarged surface areas, thereby boosting catalytic reactivity [70]. Collectively, the choice of synthesis method plays a decisive role in determining key properties such as crystallinity, defect density, and pore size distribution, which ultimately govern the efficiency of adsorption and catalytic processes.

As seen in

Table 2. Advantages and disadvantages of the different synthesis methods.

Synthesis Method	Advantages	Disadvantages
Solvothermal	High crystallinity and purity Tunable morphology Scalable	Long reaction times High temperatures and organic solvents
Sonochemical	Fast synthesis Energy efficient Nanosized	Possible structural defects Limited scalability
Hydrothermal	Environmentally friendly Good crystallinity Scalable	High temperatures Long reaction times
Microwave-Assisted	Rapid synthesis Uniform heating for better crystallinity	Limited morphology Potential side reactions Scarce solvents for synthesis Limited scalability
Mechanochemical	Environmentally friendly Fast and cost-effective Scalable	Limited control over morphology
Electrochemical	Precise control over crystal growth Low temperatures	Require conductive substrates Limited scalability

, MOF synthesis encompasses a variety of methods, each presenting distinct advantages and disadvantages in terms of reaction speed, environmental impact, crystal quality, and scalability [71,72]. For instance, solvothermal synthesis is valued for its ability to yield highly crystalline MOFs with tunable morphology; however, it typically involves extended reaction times and high-temperature conditions using organic solvents, which may raise cost and environmental concerns [73]. In contrast, hydrothermal methods use water as the solvent, providing a greener alternative while still necessitating high temperatures and prolonged durations [74]. Sonochemical synthesis accelerates the process through ultrasound-induced cavitation, producing nanoscale MOFs rapidly and with energy efficiency, yet it can introduce structural defects and faces limitations in scaling [75]. Microwave-assisted synthesis offers rapid and uniform heating that can enhance crystallinity in a matter of minutes, though this approach is often restricted by limited scalability [71]. In contrast, electrochemical synthesis allows for precise control over crystal growth without the need for high temperatures but is hindered by its reliance on conductive substrates and scalability challenges [76]. Meanwhile, mechanochemical synthesis eliminates the need for solvents entirely, marking a green and cost-effective route, albeit sometimes at the expense of crystallinity and morphological control. The simplicity of the equipment required and shorter reaction times further enhance the cost-effectiveness and sustainability of this approach, making it a viable choice for industrial-scale synthesis. Collectively, these methods underscore the importance of aligning the synthesis strategy with the targeted MOF properties and application requirements, as each approach balances distinct advantages against inherent limitations.

On the other hand, the development of synthesis techniques for MOFs has significantly advanced their applications in environmental remediation. These techniques aim to enhance the structural properties, stability, and sustainability of MOFs, making them more effective and eco-friendly for various applications.

Among the most common methods are hydrothermal synthesis, solvothermal synthesis, and mechanochemical synthesis [77]. In addition, there are more specialized synthesis methods, such as layer-by-layer (LbL) synthesis and liquid phase synthesis [78], which allow for the formation of MOFs with more complex and specific structures.

Moreover, eco-synthesis based on these synthesis methods and capable of competing with them is being developed. For example, Yuan et al. [79] demonstrated a one-pot synthesis of Al-based eco-MOFs, mainly MIL53. It used high alumina fly ash (HAFA) as a sustainable aluminum source, produced by a sintered process with an alkali reaction to activate the HAFA for 90 min at 890 °C. Additionally, the obtained product was mixed with H₂BDC, water, and HCl at a molar ratio of 1:1:160 and heated for 3 h at 220 °C. Finally, the solid was calcined at 300 °C for 3 h for activation to obtain eco-MIL-53(Al). This research of eco-synthesis demonstrated the vast capacity of MOFs to be obtained by transforming waste materials into them and being able to achieve similar results as the common synthetic synthesis.

Electrochemical and mechanochemical processes are faster than other synthesis methods [52,55], such as solvothermal, sonochemical, or hydrothermal, and they also consume less energy. In the case of the mechanochemical method, no solvent is required, although this technique requires the use of a steel ball mill to control and conduct the reactions in sealed vessels, allowing the desired MOF to be efficiently obtained while maintaining the material stability. Furthermore, the homogeneity between different batches is a crucial aspect in the synthesis of MOFs, with solvothermal and hydrothermal methods achieving the best results in this aspect compared to the other techniques [46,47]. As seen in Table 1, the solvothermal method is widely used for synthesizing MOFs, but other techniques such as ultrasonic, microwave-assisted, electrochemical, and mechanochemical methods are also employed to achieve the desired structures [80], resulting in their different final uses and applications.

Solvothermal and hydrothermal techniques involve dissolving metal salts and organic ligands in a solvent and heating the mixture under pressure to form crystalline MOFs, with the principal difference between them being that solvothermal uses organic solvents and hydrothermal requires water. Recent advancements in these methods have focused on optimizing reaction conditions to improve the crystallinity and stability of MOFs. Adjusting the temperature, solvent type, and reaction time can significantly affect the morphology and size of the resulting MOFs. For example, in the MIL53(Fe) synthesis, temperatures about 150 °C and organic solvents such as DMF (which allows for obtaining a fully homogeneous solution of the mixture) are the most used conditions because they achieve better crystallinity and homogeneity, typically resulting in rhomboid or diamond-shapes MOFs, as seen in Figure 3. In contrast, the use of lower temperatures, such as 110 or 90 °C, results in MOFs with rod-like or square morphologies, as depicted in Figure 2A. Moreover, hydrothermal synthesis produced small squares or dots in the superficial layers of MOFs due to the water present during the synthesis. These methods have been particularly effective for synthesizing MIL, UiO, and HKUST-1 MOFs, providing high-quality crystals with well-defined porosity and stability [17,81]

Microwave synthesis is a technique that offers several advantages over the previously mentioned methods, as it requires shorter reaction times and lower energy consumption. This method involves using microwave radiation to heat the reaction mixture, which accelerates the nucleation and growth of MOFs. The power applied typically varies between 100 and 1000 W, and higher power accelerates the reaction but can cause decomposition. The microwave application time ranges from 5 to 60 min, while the temperature is typically set between 100 and 250 °C.

Moreover, studies have shown that microwave-assisted synthesis can produce MOFs with higher surface areas as it provides uniform and rapid heating, which leads to faster nucleation, resulting in smaller, well-defined crystals with fewer defects, enhancing the BET [71]. The precisely controlled reaction environment ensures higher porosity compared to other methods, as Nguyen et al. [83] reported a 5% improvement in BET and better catalytic properties compared to those synthesized by traditional methods [84,85]. ZIF-8 and ZIF-67 are examples of MOFs that have benefited from microwave-assisted synthesis, demonstrating improved performance in pollutant adsorption and catalytic degradation [86,87].

Additionally, ultrasound synthesis uses ultrasonic waves to enhance the chemical reactions involved in MOF formation. The cavitation effect generated by ultrasonic waves can increase the reaction rate and promote the formation of MOFs with unique morphologies. Typical conditions include a frequency between 20 and 40 kHz, continuous wave mode, or pulsed mode (1–10 s on and 1–5 s off), depending on promoting a fast reaction time or enhancing the particle uniformity, respectively [88]. This technique has been successfully used to synthesize ZIF-type MOFs with enhanced gas and organic pollutant adsorption capacities and [89] catalytic activities for pollutant degradation [90]. The use of ultrasound has been particularly effective in forming composites, such as MIL and HKUST-1 MOFs combined with biochar, resulting in improved stability and performance in water treatment [91].

Furthermore, electrochemical synthesis is a versatile method that uses electrical energy to drive the formation of MOFs. This technique allows for precise control over the oxidation states of metal ions, enabling the synthesis of MOFs with tailored properties by typically using a current density lower than 0.6 mA/cm² in most cases and an average deposition time of 180 min [92]. A typical mechanism of the anodic deposition of MOF is disclosed in Figure 4. The process consists of nucleation (Figure 4a), island growth (Figure 4b), intergrowth (Figure 4c), and detachment (Figure 4d) [93]. Initially, metal ions are released into the solution, leading to the formation of nuclei. These nuclei grow into micrometer-sized crystals on the surface. As more crystals nucleate and grow, an intergrown layer forms. Finally, parts of the MOF layer detach from the substrate and are released into the solution due to internal stress and crystal undercutting. Moreover, organic electrolytes typically exhibit low conductivity and reduced energetic efficiency. These limitations can be mitigated by raising the temperature or incorporating conductive salts. Among the commonly used conductive salts for the electrochemical deposition of MOFs are tributyl methylammonium methyl sulfate (MTBS), tetrabutylammonium tetrafluoroborate (TBATFB), LiClO₄, and NaNO₃ [92]. Electrochemical synthesis has been employed to produce MOFs with high electrical conductivity and catalytic activity. This fact makes them suitable for applications in energy storage, hydrogen production, catalyst, and environmental analysis remediation [76,94]. The electrochemical approach has also been effective in the synthesis of HKUST-1 MOFs, where the copper center’s oxidation state can be finely tuned to enhance its catalytic performance in AOPs [95].

On the other hand, mechanochemical synthesis involves grinding metal salts and organic ligands together in the absence of solvents, using mechanical force to induce chemical reactions. This solvent-free approach is environmentally friendly and can produce MOFs with high purity and crystallinity, as is depicted in Figure 5. XRD patterns of the synthesized Ni/UiO-66 by Gómez-López et al. [96] showed two main peaks located at 7.5° and 8.6°, corresponding to the (111) and (002) crystallographic planes, respectively, which are typically observed in UiO-66 MOF. The magnified XRD diffractograms (Figure 5B–E) revealed the characteristic UiO-66 pattern and an evident fall in crystallinity in the increase in Ni content samples with 3 and 5% Ni. These results confirmed both the successful synthesis of UiO-66 MOFs and the appropriateness of the mechanochemical process for modifying MOFs with metal entities.

Recent advancements in mechanochemical synthesis have focused on developing scalable processes for large-scale MOF production [52,97]. This method has been employed to synthesize ZIF and UiO MOFs, providing an efficient and green alternative for large-scale production while maintaining good structural integrity and catalytic properties [98,99].

Additionally, greener solvents, such as recycled H₂BDC from plastic bottles [100] or STEPOSOL MET-10U [101], a bio-derived solvent produced via olefin metathesis using renewable feedstocks like plant oils, have been introduced as new options in the synthesis process.

3. MOFs for Wastewater Treatment

MOFs have shown great potential and interest in research from 2020 to 2024, particularly in wastewater treatment, as can be seen in Figure 6. They are increasingly used to remove organic pollutants, such as pharmaceuticals and pathogens, from wastewater through catalytic processes. Figure 6 presents the results of the Scopus literature search from 2020 to 2024, using the keywords ‘MOF’ and ‘wastewater’, along with abbreviations of specific MOFs (e.g., MIL, ZIF, etc.).

The data indicate a significant rise in the number of publications, highlighting the growing progress and feasibility of MOFs in wastewater treatment, particularly with MIL and ZIF MOFs. Considering the trend observed between 2020 and 2024, it is likely that the upward trajectory will persist in 2025.

The utilization of MOFs in large-scale wastewater treatment presents a myriad of technical challenges alongside promising solutions and future research directions. MOFs are characterized by their high surface area, tunable pore sizes, and chemical versatility, making them attractive candidates for the adsorption and removal of various pollutants from wastewater [102,103,104]. However, their practical application presents drawbacks due to several factors, including stability issues in aqueous environments, difficulties in recovery after use, and the complexity of large-scale synthesis.

One of the primary challenges associated with MOFs in wastewater treatment is their stability in water. Many MOFs, such as MOF-5 and MIL-101-V, are susceptible to hydrolysis, which can lead to ligand displacement and degradation of the framework [105]. This instability not only limits the operational lifespan of MOFs in aqueous environments but also affects their adsorption capacity and efficiency. Research has indicated that integrating MOFs with other materials, such as polymers, can enhance their water stability and overall performance [106,107]. For instance, the development of MOF–polymer composites has shown improved flexibility and resilience, allowing for more effective wastewater treatment applications [107].

Another significant challenge is the recovery of MOFs after they have been utilized for pollutant adsorption. Many studies have conducted experiments using MOFs in fine powder form, which complicates the recovery process and can lead to losses of the material [108]. To address this issue, researchers are exploring the fabrication of MOFs into structured forms, such as membranes or composites, which can facilitate easier recovery and reuse [109,110]. The creation of MOF-coated substrates, for example, has been shown to enhance the ease of separation while maintaining high adsorption efficiency [111].

The synthesis of MOFs at a large scale also poses challenges, particularly regarding the reproducibility and consistency of the material properties. Traditional synthesis methods often involve complex procedures that can be costly and time-consuming [112]. However, recent advancements in modular functionalization techniques have demonstrated the potential for scalable production of high-quality MOFs tailored for specific wastewater treatment applications [113,114]. Future research should focus on optimizing these synthesis methods to ensure that they are economically viable and can produce MOFs with consistent properties suitable for large-scale deployment. Moreover, the versatility of MOFs allows for the incorporation of various functional groups and metal centers, which can enhance their adsorption capabilities for specific pollutants [102,115]. This tunability is a double-edged sword; while it offers the potential for targeted pollutant removal, it also complicates the design and optimization of MOFs for specific applications. Future research directions should emphasize the systematic exploration of different metal–organic combinations and their effects on pollutant adsorption, as well as the development of predictive models to streamline the design process [116].

In addition to these challenges, the environmental impact of MOF disposal after its use in wastewater treatment is an area that requires further investigation. As the use of MOFs expands, understanding the long-term effects of their degradation products in the environment will be crucial [117]. Research into the biodegradability of MOFs and the potential for recycling or repurposing spent materials could provide sustainable solutions to mitigate environmental risks associated with their use. Furthermore, the integration of MOFs with AOPs and photocatalytic systems presents an exciting avenue for future research. These hybrid systems can enhance the degradation of organic pollutants beyond simple adsorption, potentially leading to more efficient wastewater treatment solutions [113,118]. The exploration of MOF-based photocatalysts, for instance, has shown promise in the degradation of dyes and other organic contaminants, suggesting that combining adsorption with catalytic processes could yield superior treatment outcomes [119].

In conclusion, while the application of MOFs in large-scale wastewater treatment faces several technical challenges, ongoing research is paving the way for innovative solutions. Enhancing the stability of MOFs, improving recovery methods, optimizing synthesis processes, and exploring hybrid systems are critical areas that warrant further investigation. By addressing these challenges, the potential of MOFs as effective materials for wastewater treatment can be fully realized, contributing to more sustainable water management practices.

3.1. MOFs as Adsorbents for Pollutants

From 2020 to 2024, as highlighted in Figure 7, the number of scientific articles focusing on MOFs and their applications in adsorption has demonstrated a notable upward trend. Among the various MOF families, ZIF and UiO frameworks consistently dominate the research landscape, holding the top position due to their remarkable stability and adsorption efficiency. HKUST frameworks follow in second place, owing to their versatile structural properties and ease of synthesis. Meanwhile, MIL-based MOFs, though widely recognized for their unique adsorption capabilities, are the least represented in the literature.

The interaction mechanisms between pollutants and MOFs or MOF-based adsorbents are diverse, including electrostatic forces, hydrogen bonding, π–π stacking, physical adsorption, chemisorption, Lewis acid-base interactions, hydrophobic effects, ion exchange, and surface complexation [120]. Lately, researchers have increasingly explored the potential of MOFs to address pressing environmental challenges such as the new EU wastewater normative [121]. This hierarchy reflects the evolving priorities and challenges in the field as researchers seek to optimize MOF performance for specific contaminants in sustainable wastewater treatment technologies. Notably, MOFs such as MOF-525 or MOF-545 can achieve uptake of 585 and 690 mg/g, respectively, to adsorb emerging contaminants such as sulfamethoxazole [122].

Pristine or modified MOFs have been tested for the removal by adsorption of several pollutants and metals of concern (Table 3). Xiao et al. [123] demonstrated that water-stable iron-based MIL101(Fe) exhibited physical adsorption of cationic dyes such as Rhodamine B, Malachite Green, and Methylene Blue, reaching removals of 24%, 12%, and 35%, respectively, after 24 h. As demonstrated by Rojas et al. [124], nickel bispyrazolate MOF (Ni-BDP) has shown high adsorption capacity for atenolol (97%, 72 mg/g). Additionally, MOF’s stability operation in continuous flow can be improved by introducing extra basic sites (OH) into the Ni-BDP structure, resulting in modified material (KOH@N_i8BDP₆). Terron et al. [44] tested MIL53(Fe) for the adsorption of Rhodamine B, reaching similar uptakes (75 mg/g). Moreover, they established that including other metals in the synthesis process, such as Zn, enhances the adsorption rate by 75%. Similarly, Li et al. [125] demonstrated enhanced adsorption capacity of UiO-66 modified by Ni for tetracycline removal, achieving 90% (120 mg/g). They found that the main mechanisms for the adsorption are electrostatic attraction, hydrogen–bond interaction, and π–π interaction.

However, most MOF materials are not used directly as adsorbents due to their low stability in aqueous environments, and improvements in the synthesis process are being developed. Accordingly, Pan et al. [126] synthesized porous carbon derived from HKUST-1, attained by carbonization at 350 °C of pristine MOF, showing excellent tetracycline affinity with an adsorption capacity of 136.88 mg/g and improving water stability. Moreover, MOFs can also be modified or combined with materials like graphene oxide, biochar, or covalent organic frameworks (COF) to form composites [127,128], which enhance their adsorption efficiency, stability, and applicability. Such composites improve pollutant removal performance and reduce issues like metal leaching. Accordingly, Yuan et al. [129] synthesized a composite Fe-based MOF in a COF, achieving uptakes of 1250 mg/g for dyes as congo red and an adsorption of 70% of pharmaceuticals as tetracycline with a maximum adsorption quantity of 118 mg/g. In the same way, Ouyan et al. [130] developed a UiO-66 composite by combining MOFs with corncob-based biochar and demonstrated the improvement in microporosity and stability, reaching uptake of 60 mg/g (90% removal) for sulfamethoxazole adsorption.

The inclusion of MOF into membranes also improves their performance in the remediation of aqueous environments. In this regard, Hassan et al. [131] synthesized ZIF-8-PVDF membranes for ibuprofen removal, achieving performance enhancements by encapsulating laccase or peroxidase enzymes within the coated membrane. These membranes retained nearly 60% ibuprofen removal efficiency for 1 h at room temperature when stored in 0.1 M phosphate buffer. However, the reusability and stability of the Enzyme-ZIF-8-PVDF membranes remain the most challenging for their practical application in wastewater treatment. Recently, Pen et al. [132] developed hybrid membranes of MIL-based MOFs grown on electrospun PAN nanofibers (NH₂-MIL-53Al/PAN). They achieved a maximum Co (II) adsorption capacity of around 59 mg/g. All these achievements can further extend the MOF membrane’s functionality for removal systems such as nanofiltration and reverse osmosis.

Table 3. Main MOFs and their derivatives used for adsorption of pollutants.

MOF MOF-Derived	Dosage (g/L)	Efficiency (%)	Pollutant	Time (min)	Reference
HKUST-1-derived porous carbon	0.33	80	Tetracycline	360	[126]
Ni8BDP6	0.73	97	Atenolol	60	[124]
MOF-525	0.20	97	Sulfamethoxazole	360	[122]
MOF-545	0.20	95	Sulfamethoxazole	360	[122]
MIL101(Fe)	0.40	32 11 22	Methylene Blue Malachite Green Rhodamine B	25 25 25	[123]
MIL53(Fe)	0.43	35	Rhodamine B	90	[44]
Zn-MIL53(Fe)	0.43	48	Rhodamine B	90	[44]
ZIF-8	1.10	80	Ibuprofen	360	[50]
ZIF-67	0.50	96	Ibuprofen	240	[133]
ZIF-67/PVDF	1.50	99	Congo Red	60	[134]
ZIF-8/PVDF	0.40	60	Ibuprofen	60	[131]
ZIF-8/PVDF	1.20	94	Paper	540	[135]
Ni-UiO-66	0.20	90	Tetracycline	500	[125]
UiO-66-biochar composite	0.40	35	Sulfamethoxazole	600	[130]
UiO-67/Biochar	1.00	89	B3+	160	[136]
UiO-66/GO	0.50	94	Tetracycline	600	[137]
NH2-MIL53(Al)/PAN	0.20	95	Co2+	360	[132]
TiO2-MIL101(Fe)	0.50	88	Fluoxetine	120	[138]
MIL101(Fe)/GO	0.05	82	As5+	300	[127]

Table 3. Main MOFs and their derivatives used for adsorption of pollutants.

MOF MOF-Derived	Dosage (g/L)	Efficiency (%)	Pollutant	Time (min)	Reference
HKUST-1-derived porous carbon	0.33	80	Tetracycline	360	[126]
Ni8BDP6	0.73	97	Atenolol	60	[124]
MOF-525	0.20	97	Sulfamethoxazole	360	[122]
MOF-545	0.20	95	Sulfamethoxazole	360	[122]
MIL101(Fe)	0.40	32 11 22	Methylene Blue Malachite Green Rhodamine B	25 25 25	[123]
MIL53(Fe)	0.43	35	Rhodamine B	90	[44]
Zn-MIL53(Fe)	0.43	48	Rhodamine B	90	[44]
ZIF-8	1.10	80	Ibuprofen	360	[50]
ZIF-67	0.50	96	Ibuprofen	240	[133]
ZIF-67/PVDF	1.50	99	Congo Red	60	[134]
ZIF-8/PVDF	0.40	60	Ibuprofen	60	[131]
ZIF-8/PVDF	1.20	94	Paper	540	[135]
Ni-UiO-66	0.20	90	Tetracycline	500	[125]
UiO-66-biochar composite	0.40	35	Sulfamethoxazole	600	[130]
UiO-67/Biochar	1.00	89	B3+	160	[136]
UiO-66/GO	0.50	94	Tetracycline	600	[137]
NH2-MIL53(Al)/PAN	0.20	95	Co2+	360	[132]
TiO2-MIL101(Fe)	0.50	88	Fluoxetine	120	[138]
MIL101(Fe)/GO	0.05	82	As5+	300	[127]

MOFs, such as MIL-101 and UiO-66, are well known for their high porosity and stability [139,140]. When functional groups like –NH₂ are introduced (e.g., MIL-101–NH₂), the frameworks often exhibit enhanced adsorption capacities for organic pollutants, such as dyes and pharmaceuticals. This enhancement is primarily attributed to the formation of hydrogen bonds and Lewis acid–base interactions between the –NH₂ groups and the target molecules. Quantitatively, the modified MIL-101 framework reported by Zhang et al. [141] demonstrated adsorption capacities exceeding 250 mg/g for congo red in 90 min, improving 20% over the base MOF, underscoring the impact of functionalization on adsorption performance.

On the other hand, MOFs such as ZIF-8, with their predominantly hydrophobic channels and aromatic linkers, preferentially interact with organic contaminants via π–π stacking and hydrophobic interactions. Yoo et al. [142] demonstrated that ZIF-8 effectively adsorbs organic dyes and pharmaceuticals, even in competitive conditions, due to these interactions. Their study underscores the versatility of MOFs in targeting various pollutants based on their structural characteristics.

For heavy metal ions, frameworks like Cu–BTC (HKUST-1) or those modified with –SH or –SO₃H groups demonstrate strong affinities through coordination bonds. The exposed, unsaturated metal sites in these MOFs act as Lewis acids, interacting robustly with metal cations (e.g., Pb²⁺, Hg²⁺) through Lewis acid–base interactions and ion-exchange mechanisms [143], leading to high selectivity and capacity.

Overall, the adsorption mechanisms in MOFs are multifaceted. They typically involve a synergy of physical adsorption (van der Waals forces, hydrophobic interactions, π–π stacking) and chemical adsorption (hydrogen bonding, electrostatic attractions, coordination/chelation). The ability to modify the pore environment by introducing specific functional groups allows for the design of MOFs that can be optimized for the selective removal of various pollutants, thereby broadening their applicability in environmental remediation.

Compared to activated carbon, MOFs generally exhibit higher adsorption capacities due to their tunable pore structures and specific functional groups that enhance selectivity for target pollutants. Studies have demonstrated that MOFs such as MIL-101 and UiO-66 [144] can achieve higher uptake efficiencies for pharmaceuticals and organic pollutants compared to conventional activated carbon. However, repeated regeneration cycles, by solving washing, can lead to partial degradation of MOFs, affecting their long-term stability and adsorption efficiency. Structural integrity analyses, including XRD and BET studies, have shown that some MOFs maintain stability over multiple cycles, losing a median of 14% in metal content peruse in the monometallic MOFs and reduced to 5, 7, and 9% in the bimetallic counterpart [44,145,146]. Nevertheless, others suffer from framework collapse and are not reusable. Additionally, the majority of the research does not include information about metal leaching in MOFs.

In contrast, activated carbon retains its structural framework but experiences a loss in adsorption capacity due to pore blockage and irreversible adsorption of contaminants [147]. MOFs, however, can be designed with functionalized linkers that enhance selectivity toward specific contaminants, enabling targeted removal in complex water matrices. This selectivity is particularly beneficial in multi-contaminant scenarios where competing species may interfere with adsorption. Activated carbon, on the other hand, primarily relies on hydrophobic interactions and van der Waals forces, making it less selective.

Although MOFs show promising adsorption capabilities, at the present time, their large-scale application could be limited by synthesis costs and stability issues in real-world environments. Thus, activated carbon, being widely available and cost-effective, is currently the preferred choice for industrial applications [144]. However, advances in MOF synthesis, such as mechanochemical and the immobilization of MOF powders, may improve their economic viability, making large-scale implementation a key challenge for the future [148,149,150].

The MOF regeneration is a critical step in their application for wastewater remediation. Various techniques have been developed to facilitate this process, including the use of specific solvents, thermal conditions, and electrochemical methods [148]. Each of these techniques has unique implications for the efficiency of regeneration and the structural integrity of the MOFs involved, apart from being more energy-efficient than activated carbon [151]. In relation to solvent use for desorption, the choice of solvent plays a significant role in this process. Acidic solvents, such as formic acid, have been shown to enhance desorption efficiency by maintaining the stability of the analytes and improving extraction performance. Feng et al. [152] demonstrated that the concentration of formic acid used to acidify the desorbing solvent significantly influenced the desorption performance of flavonoids from a magnetic/zeolitic imidazolate framework-67 (ZIF-67) nanocomposite. In addition to acidic solvents, other polar solvents, such as ethanol and methanol, are commonly employed for desorption processes. These solvents can effectively disrupt the interactions between the pollutants and the MOF, allowing for the release of adsorbed species. Beamish-Cook et al. [153] noted that polar aprotic solvents like dimethylformamide (DMF) were less effective compared to protic solvents like methanol in the context of MOF-74 synthesis. This observation suggests that the solvent’s polarity and ability to interact with the MOF framework are crucial for optimizing desorption.

On the other hand, thermal desorption is another widely used method for removing pollutants from MOFs. The thermal stability of a MOF is a key factor in determining the maximum temperature that can be applied without causing structural degradation. For example, Zr-based MOFs, such as UiO-66, exhibit exceptional thermal stability due to their strong Zr–O bonds, allowing them to withstand elevated temperatures during desorption processes. However, excessive heating can lead to framework collapse, particularly when water molecules are released from the MOF channels, as indicated by studies on the capillary forces affecting Zr-based MOFs [154]. In practice, thermal desorption is typically conducted at temperatures ranging from 100 °C to 300 °C, depending on the specific MOF and the nature of the adsorbed pollutants. For example, the desorption of CO₂ from MOF-808 has been reported to occur at temperatures around 143 °C, highlighting the importance of carefully selecting thermal conditions to optimize desorption while maintaining structural integrity [155].

Moreover, electrochemical methods represent an innovative approach to pollutant desorption from MOFs, particularly for heavy metals and organic pollutants. These techniques utilize redox reactions to facilitate the release of adsorbed species. Wang et al. investigated bioinspired neuron-like adsorptive networks that utilize electrochemical processes to enhance the heavy metals desorption from MOFs [156]. This method allows for the selective recovery of valuable metals while minimizing the risk of structural damage to the MOF framework. Electrochemical desorption can be particularly advantageous in scenarios where traditional methods may lead to the degradation of the MOF. By applying an electrical potential, it is possible to induce desorption without the need for harsh solvents or extreme thermal conditions. This approach not only enhances the efficiency of pollutant recovery but also promotes the reusability of the MOF material across multiple cycles.

In summary, MOF regeneration involves a careful selection of solvents, thermal conditions, and electrochemical techniques. Acidic solvents like formic acid can enhance desorption efficiency, while thermal desorption must be conducted within the thermal stability limits of the MOF to prevent structural collapse. Electrochemical methods offer a promising alternative, allowing for selective recovery of pollutants while maintaining the integrity of the MOF framework. Ongoing research into optimizing these desorption techniques will be essential for advancing the practical applications of MOFs in environmental remediation.

3.2. Enhancements Strategies for MOF Adsorption

Enhancement strategies for the adsorption applications of MOFs in wastewater remediation have been a key research focus due to their unique properties, including high surface area, tunable porosity, and chemical versatility.

A key strategy for enhancing the adsorption performance of MOFs is functionalization, which involves modifying the surface chemistry to introduce specific functional groups that interact favorably with target pollutants. For instance, Valverde et al. [157] investigated the in situ functionalization of MOF nanoparticles with cysteine, significantly improving their adsorption capacity for mercury ions (Hg²⁺) in aqueous solutions. Their results indicated an increase in adsorption capacity from 8 mg/g to 30 mg/g, demonstrating the effectiveness of surface modification in improving MOF performance in real water matrices.

Another effective strategy is the development of MOF composites, which combine the advantageous properties of MOFs with other materials to enhance adsorption performance. Chowdhury et al. [158] reported an aluminum-based MOF–graphene oxide (GO) nanocomposite that exhibited superior adsorption capabilities for lead ions (Pb²⁺) in aqueous solutions. The incorporation of GO, known for its high surface area and abundant active sites, significantly improved the overall adsorption efficiency compared to the MOF alone.

The optimization of synthesis conditions is also crucial for enhancing the adsorption properties of MOFs. Mohammadi et al. [159] compared the removal efficiency of hazardous cationic dyes using MOF-5 and modified graphene oxide, noting that the adsorption kinetics and capacities were significantly influenced by the synthesis parameters, such as temperature and precursor concentrations. By optimizing these conditions, researchers can tailor the properties of MOFs to maximize their adsorption capabilities for specific contaminants. Moreover, the incorporation of magnetic properties into MOFs has emerged as a promising strategy for enhancing their adsorption performance and facilitating easy recovery after use. Yu et al. [160] demonstrated that a magnetic metal–organic framework (MMOF) achieved a maximum removal rate of 72.15% for ciprofloxacin, an antibiotic, due to its large pore size and abundant adsorption sites. The magnetic properties of the MMOF allowed for easy separation from the aqueous phase, making it a practical option for wastewater treatment applications.

The use of advanced characterization techniques to understand the adsorption mechanisms in MOFs is another critical strategy. Chen et al. [161] systematically studied the interaction mechanisms between MOFs and various pollutants, providing insights into how structural characteristics influence adsorption performance. By elucidating these mechanisms, researchers can design MOFs with enhanced selectivity and capacity for specific contaminants.

Additionally, the exploration of new MOF structures and compositions is vital for improving adsorption capabilities. For example, Xu et al. [162] developed a robust Fe-based MOF platform for removing endocrine-disrupting chemicals (EDCs) from wastewater. Their findings indicated that the incorporation of different functional groups significantly affected the adsorption kinetics and capacities for various EDCs, showcasing the importance of structural diversity in enhancing MOF performance.

Furthermore, the integration of MOFs into membrane systems has shown promise in enhancing adsorption applications. Capsoni et al. [163] highlighted the potential of zinc-based MOFs as effective adsorbents for pharmaceuticals in polluted waters when incorporated into membrane systems. The combination of MOFs with membrane technology allows for the simultaneous removal of multiple contaminants, improving the overall efficiency of wastewater treatment processes.

3.3. MOF as Catalyst for AOP of Organic Pollutants

AOPs are powerful treatment technologies used to degrade organic pollutants in water through the generation of highly reactive species such as hydroxyl radicals (^•OH), sulfate radicals (SO₄^•−), and superoxide radicals (O₂^•−). MOFs have shown significant potential as catalysts in AOPs due to their tunable structures, high surface areas, and the ability to incorporate catalytic active sites [164]. Their versatility, ease of functionalization, and ability to form composites further enhance their performance in these processes, as highlighted in previous reviews. Specifically, Fdez-Sanromán et al. [164] and Ramalingam et al. [165] provided a general overview discussing MOFs and their derivatives for catalytic applications in AOPs aimed at pollutant degradation. Moreover, Du and Zhou [63] reported the efficiency of MOFs in a Fenton-like process, activating persulfate or peroxymonosulfate to generate SO₄^·− radicals, while Xiao et al. [166] and Li et al. [167] demonstrated the capacity of MOFs as efficient catalysts for Fenton and electro-Fenton reactions, respectively.

Between 2020 and 2024, research on AOPs using MOFs has grown significantly, as depicted in Figure 8, by the increasing number of publications. Studies highlight the prominent role of MOFs in enhancing contaminant degradation. MIL-based MOFs are the most studied in this area and are valued for their outstanding catalytic performance, structural stability, and efficiency in producing ROS. ZIF MOFs follow as the second most researched group, appreciated for their chemical durability and adaptable porosity. In comparison, UiO and HKUST frameworks are tied for the least representation, reflecting a more limited focus on their application in AOPs. This distribution emphasizes the critical role of MIL MOFs in advancing AOP technologies and their growing relevance in environmental remediation efforts. The upcoming sections present information on the use of MOFs as catalysts in different AOPs.

3.3.1. Photocatalytic Processes

MOFs are effective in photocatalytic oxidation processes, where they facilitate the generation of reactive species under light irradiation, as shown in the graphical abstract depicted in Figure 9. This process involves photon energy to produce radicals like O₂^·− or ^·OH, due to an energy band gap given by the metal cluster, that will degrade the organic pollutant into oxidized intermediates and mineralize them into final products such as CO₂ and H₂O. MOFs such as Ti-based MOFs have been used to degrade a variety of organic pollutants, including pharmaceuticals (Table 4). These photocatalytic processes benefit from the high surface area and the ability to tune the bandgap of MOFs, enhancing their light absorption and catalytic activity [168]. ZIF-based MOFs, such as ZIF-8 and their derivatives, have demonstrated significant photocatalytic activity, effectively degrading contaminants due to their stability and adjustable electronic properties by including enhanced substances. Thus, Mohamed et al. [169] synthesized an Ag₂WO₄-supported ZIF-8 hybrid photocatalyst, which denoted high photocatalytic methylene blue degradation under visible light and excellent photostability. This performance was attributed to the synergic mechanism, which proposed the efficient charge separation efficiency and the presence of plasmon Ag in the Ag₂WO₄/ZIF-8 Z-scheme heterojunction [169]. Moreover, Sepehrmansourie et al. [170] developed a novel MOF-on-MOF composite (UiO-66(Zr) on NH₂-MIL-125(Ti)) that grew in g-C₃N₄ nanosheets, obtaining a new type of double Z-scheme MOF/MOF/g-CN heterojunction to degrade over 99% of ofloxacin in 50 min under visible light irradiation by the photodegradation reactions (Equations (1)–(4)). This fact shows the capability of MOFs to be designed and mixed with other composites, such as g-C₃N₄, that present catalytic properties to improve a photocatalytic AOP process.

MOF + visible light → e⁻ + h⁺

(1)

h⁺ + H₂O → ^•OH + H⁺

(2)

e⁻ + O₂ → O₂^•−

(3)

O₂^{• −}/^•OH/h⁺ + pollutant → intermediates → CO₂ + H₂O

(4)

In the context of MOF-induced photocatalysis, the structural stability and high specific surface area of MOFs facilitate the diffusion of reactants to active sites, thereby improving reaction kinetics. For instance, the incorporation of metal nodes within the MOF structure can create active sites that enhance photocatalytic activity. Additionally, the ligands in MOFs can also play a crucial role by chelating metals that act as co-catalysts, further enhancing the surface reactions involved in photocatalysis [172,173]. This synergy between metal nodes and organic linkers allows for the design of MOFs with tailored properties that can be optimized for specific photocatalytic applications.

Moreover, the ability of MOFs to capture photons and induce charge separation is a significant advantage in photocatalytic processes. MOF linkers can absorb light and facilitate the generation of charge carriers, which drive redox reactions on the surface of the photocatalyst [173]. This capability is particularly important in visible-light-driven photocatalysis, where the efficient utilization of solar energy is paramount. Recent studies have demonstrated that MOF structure engineering can lead to improved charge separation and transport, both of which are critical for enhancing photocatalytic performance [174,175].

In addition to their structural advantages, MOFs can be combined with other materials to form heterojunctions that further enhance photocatalytic activity. For example, integrating noble metals or semiconductor materials with MOFs can create synergistic effects that improve charge separation and increase the overall photocatalytic efficiency [176,177]. The formation of these heterojunctions allows for the effective utilization of a broader spectrum of light, thereby enhancing the photocatalytic activity under various light conditions.

The photocatalytic mechanisms in MOFs can be categorized into three primary processes: photo-absorption, charge carrier generation and separation, and surface redox reactions. Photo-absorption involves the absorption of light by the MOF, leading to the excitation of electrons. These excited electrons can then migrate to the surface of the MOF, where they participate in redox reactions with adsorbed substrates [175]. The efficiency of these processes is influenced by factors such as the electronic structure of the MOF, the nature of the metal nodes, and the configuration of the organic linkers.

Recent advancements in the field have also highlighted the potential of MOFs for specific photocatalytic applications, such as the degradation of organic pollutants and the reduction in carbon dioxide. For instance, certain MOFs have been shown to effectively photodegrade nitrophenolic compounds, demonstrating their utility in environmental remediation [178]. Additionally, the use of MOFs in photocatalytic hydrogen production has garnered significant attention, as they can facilitate the conversion of solar energy into chemical energy [179].

The versatility of MOFs extends to their ability to be modified post-synthetically, allowing for the tuning of their optical properties and enhancing their photocatalytic performance. Techniques such as doping with heteroatoms or incorporating metal nanoparticles can significantly alter the electronic properties of MOFs, leading to improved light absorption and charge separation [180,181]. This adaptability is crucial for developing next-generation photocatalysts that can operate efficiently under varying environmental conditions.

Furthermore, the exploration of MOF-derived nanocomposites has opened new avenues for enhancing photocatalytic activity. By combining MOFs with nanostructured materials, researchers have been able to create composites that exhibit superior photocatalytic properties compared to their individual components [177]. These nanocomposites benefit from the unique characteristics of both MOFs and nanomaterials, resulting in enhanced surface area, improved charge carrier dynamics, and increased stability under operational conditions.

The ongoing research into the mechanisms of MOF-induced photocatalysis continues to reveal new insights into how these materials can be optimized for specific applications. For instance, the role of band bending in MOFs has been identified as a critical factor influencing charge separation and photocatalytic efficiency [176,182]. Understanding these mechanisms is essential for the rational design of MOFs that can effectively harness solar energy for various photocatalytic processes.

Moreover, Photocatalytic degradation can also be coupled with adsorption, as demonstrated by Rad et al. [138], who synthesized TiO₂-MIL-101(Fe) composite for the removal of fluoxetine. Their findings revealed that the composite showed great adsorption capacity and synergistic photocatalytic role of MIL-101(Fe) and TiO₂ under UV and visible light irradiation using a combined adsorption/photocatalysis method. Furthermore, MOFs can be synthesized and engineered to catalyze various AOPs. For example, Wang et al. [183] developed single-component piezo-photocatalytic nanoparticles based on UiO-66-NH₂(Hf). These nanoparticles achieved ultrafast degradation of Rhodamine B. Under UV irradiation and ultrasonic vibrations, net positive and negative charges were generated on the opposite faces of the piezoelectric material, resulting in an internal electric field. Through this mechanism, a degradation efficiency of 91% was achieved for an initial Rhodamine B concentration of 5 mg/L in only 60 s.

3.3.2. Fenton and Fenton-Based Processes

The Fenton process is the origin of many AOPs that are widely used for the degradation of organic pollutants in water. It involves the generation of highly reactive hydroxyl radicals (^•OH) through the reaction of hydrogen peroxide (H₂O₂) with ferrous ions (Fe²⁺) under acidic conditions (Equations (5)–(7)).

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2 {\displaystyle \frac{acid}{ }}\to {\mathrm{Fe}}^{3+}+{\mathrm{OH}}^{-}+•\mathrm{OH}$$

(5)

Fe³⁺ + H₂O₂ → Fe²⁺ + O₂• + 2H⁺

(6)

R-H + OH• → R• + H₂O

(7)

Fe-based MOFs are particularly effective in Fenton-base reactions, where MOFs catalyze the decomposition of hydrogen peroxide (H₂O₂) to produce hydroxyl radicals or sulfate radicals. The enhanced performance is attributed to the large surface area and the high density of accessible iron sites within the Fe-MOF [167].

The viability of the system H₂O₂ with MIL-53(Fe) has been demonstrated by Dinh Du and Ngoc Hoai [28], who reported excellent performance. They achieved nearly complete removal of Methylene Blue, Phenol, and Rhodamine B after 20 min at pH 2. The authors noted that MIL-53(Fe) exhibited catalytic activity across a wide pH range (2–12), confirming its role as a heterogeneous Fenton catalyst. This means that the iron in the material remains stable and does not dissolve into the solution to form a homogeneous Fenton system.

Sulfate radical-based AOPs (Fenton-Like) utilize persulfate (PS) or peroxymonosulfate (PMS) as oxidants to generate sulfate radicals. The process relies on a series of interconnected reactions involving redox cycles and the generation of reactive radicals to degrade organic pollutants radicals [63]. Initially, persulfate ions are activated by ferrous ions, producing sulfate radicals, ferric ions, and other reactive species. This activation step is critical as it generates the highly reactive sulfate radicals that drive the oxidation process. Ferric ions, formed during the reaction, are reduced back to ferrous ions, ensuring the continuation of the catalytic cycle (Equations (8) and (9)). Oxygen also plays a key role, interacting with ferrous ions to produce superoxide radicals, which further participate in subsequent transformations (Equations (10) and (11)). These radicals can form hydroperoxyl radicals through reactions with protons, adding to the system’s oxidative potential. Hydrogen peroxide, if present, can be decomposed by ferrous ions in a Fenton-like reaction, generating hydroxyl radicals, which are among the most powerful oxidants (Equation (12)). Additionally, superoxide radicals can react to form hydrogen peroxide and oxygen, regenerating oxidizing agents and sustaining radical chain reactions (Equation (13)). Sulfate and hydroxyl radicals can interact with persulfate to produce additional reactive species, maintaining the system’s oxidative efficiency (Equations (14)–(16)). These radicals can also react with water, producing hydroxyl radicals and protons, further contributing to the breakdown of contaminants (Equation (17)). Through this intricate network of reactions, the process achieves high oxidative power, making it effective for the treatment of various organic pollutants in water. The process’s ability to regenerate reactive species and sustain radical propagation ensures efficiency in advanced water treatment applications [26].

Fe-MIL-based MOFs, like MIL-101(Fe), have also been utilized in Fenton-like processes for the degradation of dyes, such as methylene blue, demonstrating the catalytic potential for sulfate radical production and, at the same time, hydroxyl radical [123]. In addition, its removal efficiency remains almost unchanged without any regeneration treatments, indicating that this MOF can be stable and reusable.

The use of bimetallic MIL-MOF has demonstrated a synergistic effect when incorporating metal pairs such as Co and Fe, Mn and Fe, or Zn and Fe (Table 4). These combinations have been shown to enhance the catalytic performance of MOFs for H₂O₂ or PMS/PS activation for drug removal [61,123,166].

In addition, MOFs can be grown in situ on different materials to enhance their applicability for the treatment of water streams. Accordingly, Peng et al. [184] synthesized a zeolitic imidazolate framework (ZIF)-67, grown in situ on commercial nickel foam (NF) for the efficient activation of peroxymonosulfate and degradation of Rhodamine B (Figure 10). The catalytic activity of NF/ZIF-67 composite was evaluated in the catalytic oxidation of Rhodamine B by PMS activation in an aqueous solution. Since the unique and porous structure and abundant active sites, the as-prepared three-dimensional heterogeneous catalyst exhibited high PMS activation efficiency and good stability. Moreover, cobalt is responsible for activating the PMS to degrade Rhodamine B into less harmful intermediates, following the equations mentioned below and, at the end of the process, obtaining a complete mineralization of the pollutant into H₂O and CO₂. Similarly, magnetic MnFe₂O₄/ZIF-67 nanocomposites were successfully prepared by Lu et al. [185], who combined the magnetic separation characteristics of MnFe₂O₄ with the high catalytic activity of ZIF-67 for PMS activation.

≡Fe²⁺ + S₂O₈²⁻ → ≡Fe³⁺ + SO₄⁻² + SO₄^−•

(8)

≡Fe³⁺ + S₂O₈²⁻ → ≡Fe²⁺ + S₂O₈⁻·

(9)

≡Fe²⁺ + O₂ ↔ ≡Fe³⁺ + O₂^−•

(10)

O₂^−• +H⁺↔ HO• + 2O₂^−• +2H⁺↔ HO^• + H₂O₂ + O₂

(11)

≡Fe²⁺ + H₂O₂ → ≡Fe³⁺ +OH^• + OH⁻

(12)

2O₂^−• + 2H⁺↔ H₂O₂ + O₂

(13)

O₂^−• + S₂O₂⁻⁸→SO₄²⁻ + S₂O₈⁻·

(14)

SO₄^−• + S₂O₈⁻² → SO₄⁻² + S₂O₈^−•

(15)

OH• + S₂O₈⁻² → OH⁻ + S₂O₈⁻²

(16)

SO₄^−• + H₂O → SO₄⁻²+H⁺ + OH•

(17)

Electro-Fenton processes combine electrochemical and Fenton reactions to generate hydroxyl radicals (Equations (18)–(20)) and regenerate the Fe³⁺/Fe²⁺. Among the different approaches the use of MOF-based heterogeneous catalysts in this process is increasingly utilized [167]. Figure 10 illustrates the scheme and main reactions of the electro-Fenton treatment employing MOF as catalysts. Hydrogen peroxide is produced by the oxygen reduction reaction (ORR) at the cathode, and the MOF, either in the bulk media of the electrochemical reactor (Figure 11) or on the cathode surface (Figure 11), catalyzes the reaction to produce hydroxyl radicals. Using these configurations, electro-Fenton has been used effectively to eliminate a wide variety of emerging pollutants. Terrón et al. [43] demonstrated that under optimized conditions (25 mA and 4.32 g/L of Zn-MIL53(Fe)), removal of over 90% was achieved for fluoxetine and sulfamethoxazole. Moreover, the use of HKUST-1-derived Cu@C decorated in a 3D graphene network has also been explored in electro-Fenton systems, where the copper center plays a role in the effective regeneration of reactive species, enhancing the degradation efficiency of Rhodamine B dye [186]. Fe-MOF-derived cathodes and Fe-MOF-CB@PAN fibers have been used by Fdez-Sanroman et al. [187] to degrade fluoxetine and other pharmaceuticals, achieving a degradation of over 90% and demonstrating good reusability [187]. The immobilization of porous fibers, such as PAN, offers a highly hydrophilic material that enhances the accessibility of Fe-MOF to facilitate reactions.

Cathode: O₂ + 2H⁺ +2e⁻→H₂O₂

(18)

Anode: OH^• + RH→ R• + H₂O

(19)

≡Fe²⁺+H₂O₂+H⁺→≡Fe³⁺ +OH• + H₂O

(20)

In photo-Fenton processes, light irradiation is used to enhance the production of hydroxyl radicals. MOFs can have a dual role in this process: they can act as Fenton catalysts and photocatalysts, absorbing light and transferring energy to the catalytic sites. Therefore, the synergetic effect of both processes increases the efficiency of pollutant degradation. For instance, the presence of transition metals such as Fe in MOFs in photo-Fenton systems led to a more effective degradation of contaminants such as sulfamethoxazole under visible light irradiation [188,189]. MIL-101(Fe) and MIL-53 have been particularly effective in photo-Fenton reactions (Equations (21)–(23)), where the UV photon interacts with the metal clusters of the MOF and reduces it (Equation (21)), catalyzing the generation of ROS as hydroxyl radical is produced from hydrogen peroxide (Equation (22)). Moreover, hydroxyl radicals can interact with water and hydrogen peroxide, producing superoxides (Equation (23)). Finally, simultaneously, the UV photon can reduce the metal of the MOF while transforming the hydrogen peroxide into hydroxyl radicals, where the metal becomes oxidized by unpaired electrons and reactivated to produce both superoxides and hydroxyl radicals (Equations (24) and (25)). These radicals are the main ones responsible for degrading complex organic pollutants like tetracycline, sulfamethoxazole, or fluoxetine in this process [81,190]. In addition, other metals can be used for activation. For example, calcinated HO-UIO-66 (Zr) has also been employed in photo-Fenton reactions, where its metal centers facilitate the production of ROS under light irradiation, resulting in improved degradation rates of various pollutants [23].

MOF + hv→ h⁺ +e⁻

(21)

H₂O₂ + e⁻→OH• + OH

(22)

H₂O₂+OH•→H₂O+HO₂^•

(23)

≡M³⁺ +H₂O₂ + hv→≡M²⁺+HO• + 2H⁺

(24)

≡M³⁺ + H₂O₂ + hv→≡M²⁺+HO₂•+H⁺

(25)

≡M refers to a transition metal.

The structural tunability of MOFs allows for the design of photocatalysts with enhanced light absorption, improved charge separation, and reduced recombination rates. The following revised literature provides insights into how these structural modifications influence the photocatalytic performance of MOFs, highlighting the role of organic linkers, metal clusters, and co-catalysts.

On the one hand, Qiu et al. [191] unraveled the photocatalytic electron transfer mechanism in a Ti-MOF/g-C₃N₄ heterojunction, demonstrating that spatial separation of charge carriers improves photocatalytic quantum efficiency. Their study emphasizes the importance of structural design in enhancing photocatalytic performance.

Moreover, Kinik et al. [192] investigated the photocatalytic hydrogen evolution rates of pyrene-based metal–organic frameworks (MOFs) under visible light irradiation. Their findings highlight the role of organic linkers in extending light absorption into the visible range, which is crucial for effective photocatalysis.

Additionally, Ma et al. [193] reviewed recent advances in MOF-based photocatalysts, focusing on design strategies that modulate the electronic structure to suppress charge recombination. The authors emphasized the versatility of MOFs in creating optimal photocatalysts for heavy metal control.

On the other hand, Karmakar et al. [194] explored the development of post-modified Ce-MOF as a photocatalyst for CO₂ reduction. Their study highlights how structural modifications can enhance photocatalytic activity by improving light absorption and charge separation.

Moreover, Li et al. [195] discussed the regulation of the electronic band structure in Ti-based MOFs to boost light-driven hydrogen evolution. Their findings indicate that the structural properties of MOFs significantly influence photocatalytic performance. Furthermore, Wang et al. emphasized the rational design of titanium-based MOFs to enhance photocatalytic performance. Their findings indicate that structural tunability is key to optimizing photocatalytic activity.

On the other hand, Fang et al. [196] discussed the advancements in electrically conductive MOFs for photocatalytic energy conversion. They noted that the structural properties of these MOFs facilitate efficient charge transfer, enhancing photocatalytic performance.

Additionally, Chen et al. [197] focused on fine-tuning the metal oxo cluster composition in bimetallic MOFs for efficient CO₂ reduction. Their findings illustrate how structural modifications can enhance photocatalytic activity.

In summary, the structural tunability of MOFs allows for the design of photocatalysts with enhanced light absorption, improved charge separation, and reduced recombination rates. The references provided illustrate the diverse strategies employed to optimize MOFs for photocatalytic applications, emphasizing their potential for environmental remediation and energy conversion.

Table 4. Main MOFs and their derivatives used as catalysts of AOPs to degrade pollutants.

MOF MOF-Derived	Process	Dosage (g/L)	Efficiency (%)	Pollutant	Time (min)	Reference
Fe-BTC	Electro-Fenton	0.75	100	Tetracycline	40	[190]
Zn-MIL53(Fe)	Electro-Fenton	0.43	100/100	Sulfamethoxazole/Fluoxetine	90	[43]
HKUST-1@C/Graphene	Electro-Fenton	0.25	100	Rhodamine B	150	[186]
MIL53(Fe)	Fenton	0.50 0.50 0.50	100 100 97	Methylene Blue Phenol Rhodamine B	20 20 30	[28]
Co-MIL53(Fe)	Fenton	0.13	90	Oxytetracycline	90	[61]
FeCu-MOF	Fenton-like (PMS)	1.19	81	Sulfamethoxazole	90	[164]
Ni-MOF	Fenton-Like (PS)	0.2	15	Tetracycline	45	[198]
MnFeO-ZIF-8	Fenton-like (PMS)	0.5	100	Bisphenol A	15	[199]
HKUST-1	Fenton-like (PMS)	0.66	100	Antipyrine	300	[200]
Zn-MIL53(Fe)	Fenton-like (PMS)	0.43	85/100	Sulfamethoxazole/Fluoxetine	240	[44]
Mn-MIL53(Fe)	Fenton-like (PMS)	0.2	90	Tetracycline	60	[201]
Ni-MIL101(Fe)	Fenton-like (PS)	0.2	96	Tetracycline	30	[198]
MIL101(Fe)	Fenton-like (PS)	0.2	31	Tetracycline	45	[198]
MIL101(Fe)	Fenton-like (PMS)	0.4	72	Methylene Blue	25	[123]
Cu-MIL101(Fe)	Fenton-like (PS) photocatalysis	0.3/0.05	91	Tetracycline	120	[202]
MIL101(Fe)	Fenton-like (PS) photocatalysis	0.3/0.05	52	Tetracycline	120	[202]
UiO-66(Zr)/MIL125(Ti)@GN	Photocatalysis	0.1	99	Ofloxacin	50	[170]
AgWO-ZIF-8	Photocatalysis	0.1	98	Methylene Blue	120	[169]
MIL101(Fe)/TiO2	Adsorption-Photocatalysis	0.5	92	Fluoxetine	20	[138]
HF-UiO-67	Photocatalysis-Ultrasound	0.6	91	Rhodamine B	1	[183]
OH-UiO-66	Photo-Fenton	8	95	Sulfamethoxazole	120	[23]
Ag-ZIF-8	Photo-Fenton	0.2	68	Methylene Blue	90	[203]
UiO-66	Photo-Fenton	0.1	95	Tetracycline	120	[204]

Table 4. Main MOFs and their derivatives used as catalysts of AOPs to degrade pollutants.

MOF MOF-Derived	Process	Dosage (g/L)	Efficiency (%)	Pollutant	Time (min)	Reference
Fe-BTC	Electro-Fenton	0.75	100	Tetracycline	40	[190]
Zn-MIL53(Fe)	Electro-Fenton	0.43	100/100	Sulfamethoxazole/Fluoxetine	90	[43]
HKUST-1@C/Graphene	Electro-Fenton	0.25	100	Rhodamine B	150	[186]
MIL53(Fe)	Fenton	0.50 0.50 0.50	100 100 97	Methylene Blue Phenol Rhodamine B	20 20 30	[28]
Co-MIL53(Fe)	Fenton	0.13	90	Oxytetracycline	90	[61]
FeCu-MOF	Fenton-like (PMS)	1.19	81	Sulfamethoxazole	90	[164]
Ni-MOF	Fenton-Like (PS)	0.2	15	Tetracycline	45	[198]
MnFeO-ZIF-8	Fenton-like (PMS)	0.5	100	Bisphenol A	15	[199]
HKUST-1	Fenton-like (PMS)	0.66	100	Antipyrine	300	[200]
Zn-MIL53(Fe)	Fenton-like (PMS)	0.43	85/100	Sulfamethoxazole/Fluoxetine	240	[44]
Mn-MIL53(Fe)	Fenton-like (PMS)	0.2	90	Tetracycline	60	[201]
Ni-MIL101(Fe)	Fenton-like (PS)	0.2	96	Tetracycline	30	[198]
MIL101(Fe)	Fenton-like (PS)	0.2	31	Tetracycline	45	[198]
MIL101(Fe)	Fenton-like (PMS)	0.4	72	Methylene Blue	25	[123]
Cu-MIL101(Fe)	Fenton-like (PS) photocatalysis	0.3/0.05	91	Tetracycline	120	[202]
MIL101(Fe)	Fenton-like (PS) photocatalysis	0.3/0.05	52	Tetracycline	120	[202]
UiO-66(Zr)/MIL125(Ti)@GN	Photocatalysis	0.1	99	Ofloxacin	50	[170]
AgWO-ZIF-8	Photocatalysis	0.1	98	Methylene Blue	120	[169]
MIL101(Fe)/TiO2	Adsorption-Photocatalysis	0.5	92	Fluoxetine	20	[138]
HF-UiO-67	Photocatalysis-Ultrasound	0.6	91	Rhodamine B	1	[183]
OH-UiO-66	Photo-Fenton	8	95	Sulfamethoxazole	120	[23]
Ag-ZIF-8	Photo-Fenton	0.2	68	Methylene Blue	90	[203]
UiO-66	Photo-Fenton	0.1	95	Tetracycline	120	[204]

3.4. Enhancing Catalyst Performance of MOFs

Improving the catalytic performance of MOFs relies on a multi-pronged strategy that integrates both structural and chemical modifications. One primary approach is to engineer MOF architectures by designing hierarchical pore structures that increase the number of active sites and enhance mass transfer [205,206]. Simultaneously, functionalizing the MOF surface with catalytic groups or incorporating metal nanoparticles into the framework can tailor active sites for specific reactions, thereby boosting catalytic efficiency. Additionally, modifying the ligands—either by choosing electron-rich or electron-deficient linkers—allows fine-tuning of the electronic environment within the framework, which is critical for optimizing reactivity [207]. Post-synthetic modifications, such as ion exchange or the covalent attachment of catalytic species, offer further versatility by enhancing catalytic performance without compromising the framework’s structural integrity. Finally, forming composite materials that combine MOFs with other supports or conductive substrates can improve electron transfer and overall stability, leading to additional gains in catalytic efficiency [205]. Collectively, these integrated strategies—whether employed individually or in combination—enable systematic enhancement of MOF catalytic properties for a broad range of targeted applications, apart from improvements in solutions to MOF drawbacks and future research directions.

3.5. Antibacterial and Antimicrobial Properties of MOFs

In recent times, MOFs have also shown significant promise as antibacterial and antimicrobial agents. Their ability to release metal ions, generate ROS, and disrupt bacterial membranes makes them effective against a wide range of bacterial pathogens. Moreover, those metal ions diffuse over the agar, creating an inhibition area [208,209,210,211]. Their high surface area, tunable porosity, and ability to incorporate metal ions with known antimicrobial properties make them ideal candidates for combating bacterial infections, including antibiotic-resistant strains. MOFs and MOF-derived materials of MIL, ZIF, HKUST-1, and UiO (Table 5) have all demonstrated effective antibacterial properties, which are further enhanced when combined with other materials to form composites [208,209].

MOFs can exhibit antibacterial properties through several mechanisms. One primary method is the release of metal ions, such as Cu²⁺, Ag⁺, and Zn²⁺, which can disrupt bacterial cell membranes and interfere with vital cellular processes [212,213]. However, this mechanism raises a significant concern regarding the potential for secondary pollution due to uncontrolled ion release [214]. Recent studies [215,216] suggest that while a controlled, gradual release of metal ions can enhance antibacterial efficacy without markedly affecting the environment, excessive or rapid ion liberation might lead to the accumulation of toxic metals in ecosystems, thereby causing secondary contamination. To address these challenges, modern strategies focus on fine-tuning MOF synthesis to achieve a stable framework with optimized porosity and defect engineering, which allows for the controlled leaching of metal ions. Moreover, the development of biodegradable MOF matrices and composites that prevent ion leaching is being explored to further mitigate the risk of environmental accumulation, ensuring that their antibacterial benefits are harnessed without compromising environmental safety. Ongoing research in this area is crucial to fully understand the long-term environmental impacts and to refine the design of MOF-based antibacterial systems.

Accordingly, Elmehrath et al. [217] established the antibacterial mechanisms of Cu-BTC and Cu-GA MOF (Figure 12), consisting of copper ions bridged by deprotonated gallate ligands (H₂gal²⁻), against Escherichia coli (E. coli) and Lactobacillus bacteria involve the release of Cu²⁺ ions from the frameworks, promoting their attachment to and penetration into the surfaces of both Gram-negative and Gram-positive bacteria (Figure 12). Moreover, Cu-GA also releases H₂gal²⁻ ions from its structure, amplifying its antibacterial effect specifically against E. coli. Similarly, Ag-loaded MOFs have demonstrated broad-spectrum antibacterial activity due to the release of silver ions, which interact with bacterial cell components, leading to cell death [211]. UiO-66 and ZIF-8 have also demonstrated effective antibacterial activity by the release of metal ions that disrupt cellular processes, making them effective against multiple bacterial strains, including resistant pathogens [210,218,219].

MOFs can also enhance antibacterial activity through Fenton and Fenton-like reactions, where hydrogen peroxide (H₂O₂) or PMS/PS is decomposed to produce highly reactive radicals such as hydroxyl (·OH) or sulfate (SO₄^·−). These radicals can cause oxidative damage to bacterial cell membranes, leading to increased permeability and cell death. For example, trimetallic MOF and NiCoFe-MOFs with enhanced Fenton catalytic activity have been shown to inhibit bacterial growth effectively by producing radicals that damage bacterial membranes and increase their susceptibility to environmental conditions, achieving full disinfection in 120 min for E. coli and S. aureus [27,220,221]. MIL-100(Fe) has also been used in the Fenton process, enhancing their antibacterial effects through the generation of ROS, which disrupts bacterial cells [222], which resulted in an inactivation of over 99% of S. aureus and Pseudomonas aeruginosa. Fdez-Sanromán et al. [223] demonstrated the effect on the synthesis of bimetallic CuFe-MOFs achieved by varying the ratios of solvents, salts, and reaction temperatures. A higher antibacterial activity was attained with the system CuFe(BDC-NH₂)R/PMS due to the high copper content. On the other hand, Terrón et al. [43] showed the ability of Zn-MIL53(Fe) in electro-Fenton treatment to inactivate two Gram-negative and a Gram-positive pathogen such as E. coli, P. aeruginosa and Lactobacillus crispatus, respectively, both in 60 min.

The performance of MOFs can be further enhanced by their encapsulation within polymeric matrices, such as polyacrylonitrile (PAN). For instance, Giráldez et al. [224] demonstrated the efficiency of HKUST-1 encapsulated in PAN, which successfully inactivated E. coli within 30 min through a Fenton-like process using PMS. This innovative approach holds significant potential for scaling up to enable continuous treatment applications.

Recent advancements in the development of composites, such as biochar and nanoparticles, have further enhanced the stability and antibacterial efficiency of MOFs, paving the way for their broader implementation in clinical and environmental applications. Continued research and development in this field will be essential to fully realize the potential of MOFs in clinical and environmental applications [15,225].

3.6. Influence of Real Water Matrices on the Performance of MOFs in AOPs and Antimicrobial Applications

The performance of MOFs in adsorption, AOPs, and antimicrobial applications is significantly influenced by the composition of the water matrices. The interaction of MOFs with various contaminants in these matrices can enhance or inhibit their effectiveness, depending on the specific environmental conditions and the nature of the pollutants present. While numerous studies demonstrate the efficacy of MOFs in AOPs and antimicrobial applications, most investigations have been performed using spiked water samples—synthetic matrices enriched with specific contaminants or model microorganisms. Although these controlled experiments allow for precise evaluation of catalytic and antimicrobial properties, they do not fully capture the complexity of real wastewater. In actual operational environments, wastewater typically contains a mixture of dissolved organic matter (DOM), various salts, and competing ions, which can alter reactive species generation and hinder direct interactions between MOFs and target contaminants or pathogens [226].

In the context of a real water matrix, the major presence of DOM and inorganic ions (e.g., chloride and bicarbonate) can act as radical scavengers, thereby reducing the availability of hydroxyl radicals (·OH) essential for pollutant degradation. For instance, studies have shown that these wastewater matrix components may interfere with the redox reactions catalyzed by MOFs, resulting in altered degradation kinetics of pharmaceuticals and other persistent organic pollutants [200].

Similarly, while antimicrobial evaluations are often conducted using single-species cultures of pathogens such as E. coli and P. aeruginosa under controlled conditions, real wastewater hosts diverse microbial communities that may shield bacteria from direct contact with MOF surfaces.

In this context, recent studies have demonstrated that MOFs can effectively capture antibiotics from aquatic environments, particularly in wastewater streams. Moreover, the integration of MOFs into hybrid systems has shown promise in enhancing their antimicrobial properties. For example, Hu et al. [227] developed molecularly imprinted MOF/PAN hybrid membranes that not only selectively adsorb bisphenol A but also exhibit antibacterial properties, thereby addressing both organic pollutants and microbial contamination in sewage treatment plant water. This dual functionality is particularly beneficial in real water matrices where both chemical and biological contaminants are present.

For instance, a multivariate MOF developed by Negro et al. [228] shows remarkable efficiency in removing antibiotics, surpassing traditional materials like activated carbon. This performance is attributed to its unique structural properties, which allow for enhanced interaction with antibiotic molecules from Albufera Nature Park and sewage treatment plant water. The recyclability of such MOFs further underscores their potential as sustainable solutions for environmental remediation.

In addition to antibiotic removal, MOFs have been employed in the treatment of heavy metals from real wastewater, such as electroplating effluents. Ali et al. [229] reported the development of MOF-mixed matrix membranes that effectively eradicate toxic metals, showcasing the versatility of MOFs in addressing multiple contaminants in real-world scenarios. This adaptability is crucial, as real water matrices often contain a mixture of pollutants that can affect the performance of treatment technologies.

The selective capture of specific pollutants, such as mercury ions, has also been achieved using MOFs in real wastewater. Yao et al. [230] fabricated 2D MOF nanosheets that demonstrated high efficiency in capturing Hg²⁺ from acidic groundwater and mercury-oxide battery industry wastewater, highlighting the potential of MOFs to target specific contaminants in complex matrices. This selectivity is essential for optimizing the removal processes in diverse environmental conditions.

Furthermore, the catalytic capabilities of MOFs have been explored for the degradation of organic pollutants in wastewater. Gao et al. [231] reported a heterostructure that promotes the degradation of phenol, a common contaminant in industrial coking wastewater, demonstrating the potential of MOFs in AOPs. The ability of MOFs to facilitate catalytic reactions in real water matrices enhances their applicability in environmental remediation.

In summary, the performance of MOFs in AOPs and antimicrobial applications is significantly influenced by the characteristics of real water matrices. Their ability to capture a wide range of contaminants, including antibiotics and heavy metals, as well as their catalytic and antimicrobial properties, positions MOFs as promising materials for addressing complex environmental challenges. Finally, future work will expand upon these promising pilot-scale findings by investigating the long-term stability, regeneration, and scalability of MOF-based systems in actual wastewater treatment methods. By bridging the gap between controlled laboratory studies and real operational conditions, aiming to validate MOFs as robust, cost-effective technologies for disinfection and wastewater remediation.

Table 5. Main MOFs used for pathogen removal.

MOF MOF-Derived	Process	Dosage (g/L)	Efficiency (%)	Pathogen	Time (min)	Reference
CuO-MOF2/MMT	Agar-diffusion	-	Inhibition	E. coli, B. subtilis	24 h	[232]
ZnO-MIL53(Fe)	Agar-diffusion	-	Inhibition	E. coli	24 h	[208]
CuO-MIL53(Fe)	Agar-diffusion	-	Inhibition	E. coli	24 h	[208]
CuBTC	Agar-diffusion	-	Inhibition	E. coli, Lactobacillus	90	[217]
ZIF-8	Agar-diffusion	-	Inhibition	E. coli	24 h	[210]
SH-MOF(UiO-66)@Cotton	Membrane	1	>90	S. aureus	60	[218]
SrTiO/CuFeO-MIL101(Co)	Agar-diffusion	-	Inhibition	S. aureus, Candida albicans, P. aeruginosa	24 h	[220]
MIL53(Fe)	Photo-electro-Fenton	0.68	>85	Pseudomonae, Enterobacteriae	60	[221]
MIL100(Fe)	Fenton	0.1	>99	S. aureusP. aeruginosa	24 h	[222]
NiCoFe-MOF	Fenton	0.1	100/100	E. coli, S. aureus	120	[27]
HKUST-1	Fenton-Like (PMS)	0.66	100	E. coli	5	[200]
PAN-HKUST-1	Fenton-Like (PMS)	0.61	100	E. coli	30	[224]
CuFe-MOF	Fenton-Like (PMS)	0.25	100	E. coli	60	[223]
Zn-MIL53(Fe)	Electro-Fenton	0.43	100	E. coli, P. aeruginosa, L. crispatus	5	[43]

Table 5. Main MOFs used for pathogen removal.

MOF MOF-Derived	Process	Dosage (g/L)	Efficiency (%)	Pathogen	Time (min)	Reference
CuO-MOF2/MMT	Agar-diffusion	-	Inhibition	E. coli, B. subtilis	24 h	[232]
ZnO-MIL53(Fe)	Agar-diffusion	-	Inhibition	E. coli	24 h	[208]
CuO-MIL53(Fe)	Agar-diffusion	-	Inhibition	E. coli	24 h	[208]
CuBTC	Agar-diffusion	-	Inhibition	E. coli, Lactobacillus	90	[217]
ZIF-8	Agar-diffusion	-	Inhibition	E. coli	24 h	[210]
SH-MOF(UiO-66)@Cotton	Membrane	1	>90	S. aureus	60	[218]
SrTiO/CuFeO-MIL101(Co)	Agar-diffusion	-	Inhibition	S. aureus, Candida albicans, P. aeruginosa	24 h	[220]
MIL53(Fe)	Photo-electro-Fenton	0.68	>85	Pseudomonae, Enterobacteriae	60	[221]
MIL100(Fe)	Fenton	0.1	>99	S. aureusP. aeruginosa	24 h	[222]
NiCoFe-MOF	Fenton	0.1	100/100	E. coli, S. aureus	120	[27]
HKUST-1	Fenton-Like (PMS)	0.66	100	E. coli	5	[200]
PAN-HKUST-1	Fenton-Like (PMS)	0.61	100	E. coli	30	[224]
CuFe-MOF	Fenton-Like (PMS)	0.25	100	E. coli	60	[223]
Zn-MIL53(Fe)	Electro-Fenton	0.43	100	E. coli, P. aeruginosa, L. crispatus	5	[43]

4. Conclusions

MOFs have rapidly emerged as transformative materials in environmental remediation, particularly wastewater treatment. Their intrinsic properties—such as high porosity, tunable pore dimensions, and versatile surface functionalities—allow them to function as both adsorbents and catalysts. This dual functionality allows MOFs not only to capture a wide array of organic pollutants and pathogens but also to degrade them through AOPs, including Fenton, photo-Fenton, and electro-Fenton reactions.

Recent advances in synthesis methods, spanning hydrothermal and solvothermal to microwave-assisted and mechanochemical techniques, have significantly improved MOF structural precision and operational stability. These innovative approaches have led to materials with enhanced catalytic activity, improved regeneration capabilities, and better adaptability to complex aqueous environments. Moreover, the integration of MOFs with complementary materials—such as graphene oxide, biochar, and functional membranes—has further broadened their applicability, paving the way for hybrid systems that can achieve simultaneous adsorption and catalytic degradation of contaminants.

Despite these promising developments, several challenges remain. Key issues include long-term water stability, cost-effective large-scale synthesis, and effective regeneration after repeated use. Future research should prioritize eco-friendly synthesis routes and the design of composite systems that mitigate these limitations. Furthermore, comprehensive environmental and economic assessments are essential to validate the sustainability of MOF-based technologies for real-world water treatment applications.

In summary, while significant progress has been made, continued optimization of MOF structures and synthesis strategies is essential. By addressing current limitations and exploring novel composite materials, MOFs have the potential to meet the escalating demands of sustainable water management and public health protection. The development of these advanced materials holds great promise for transforming wastewater remediation into a more efficient, selective, and environmentally benign process.