Metal–Organic Frameworks (MOFs) for Adsorption and Degradation of Microplastics

Abstract

Microplastics (MPs) are currently a serious environmental problem, primarily due to their persistence in the environment, low concentration, and difficulty in detection and disposal. MPs have also been detected in humans and have been shown to be harmful. Although there are methodologies for their recovery or elimination in most water treatment plants, a significant portion still bypasses these elimination systems. It is this percentage that we must try to eliminate. In addition to finding new methodologies for the treatment of MPs, it is important to find new materials adapted to this process. In this context, metal–organic frameworks (MOFs) are high-versatility compounds that can be synthesized using different techniques to obtain materials with different properties, such as porosity, morphology, conductivity, etc. These materials can adsorb MPs in different ways, such as electrostatic interaction, bond formation, etc., or they can be obtained by containing metals that catalyze reactions for the formation of highly reactive species that can oxidize the MPs. This review examines how MOF materials have gained attention for the adsorption-based recovery and removal of MPs and discusses the problems associated with these materials and possible solutions.

1. Introduction

Plastics are synthetic materials primarily derived from petroleum. They are extensively utilized in a wide variety of products due to their low cost, versatility, and durability. However, these same characteristics make them difficult to decompose, with degradation taking centuries to complete, contributing to their accumulation in the environment [1].

In 2022, global plastic production reached 400.3 million tons [2], demonstrating the strong and continuous demand for this material in society. Consequently, the accumulation of plastics in the environment is one of the most significant challenges of the 21st century.

Every year, millions of tons of plastic residue enter the oceans, rivers, and other natural areas, causing irreparable damage to the ecosystem. This has implications for human health because plastics can fragment into smaller plastics, such as microplastics (MP, <5 mm) or nanoplastics (NP, <1 μm), eventually entering the human food chain through the ingestion of contaminated food [3]. Microplastics (MPs) can be categorized based on their origin into two groups: primary and secondary microplastics.

Primary microplastics are manufactured specifically in small sizes, such as those used in personal care products (e.g., exfoliants and toothpastes), industrial plastic pellets, and synthetic textile microfibers [4]. Secondary microplastics are formed when larger plastic items, such as bags, bottles, and fishing nets, break down into smaller particles through exposure to environmental elements like sunlight, wind, and waves [5,6,7]. In any case, the environments in which they are found pose significant potential dangers to humans and their habitats [8,9,10]. For example, MPs have been found in human samples such as stool, lung tissue, and breast milk [11].

This fragmentation makes their recovery and further decomposition more challenging due to their small size [12]. Although there has been progress in the development of biodegradable plastics, they usually require specific conditions to decompose effectively, such as high temperature and humidity, which do not frequently occur spontaneously in nature.

Wastewater treatment plants can eliminate an appreciable quantity of plastics through filtering techniques. However, a significant portion of these plastics is not captured and remains accumulated in various ecosystems for extended periods [11,13].

MPs exhibit a wide range of constitutions, chemical properties, decomposition rates, and sizes. Polyethylene (PE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), polyamide (PA), polystyrene (PS), and polypropylene (PP) are some of the most well known. All of them are used in everyday items such as bottles, plastic containers, clothes, and packaging.

Given their persistence in nature, as mentioned above, and their continuous production each year, it is realistic to forecast that the number of MPs could reach 1100 Mt by 2050 [14]. Therefore, implementing new effective strategies is crucial for dealing with the presence of the MPs in water.

2. Separation Methods

The separation of MPs from the aqueous medium is a technical challenge due to their size and wide distribution in different environments. However, various methods have been developed to recover MPs, especially in aqueous solutions. These methods change according to the particle size and complexity in the medium they are found.

The most common methods for separating MPs from aquatics environments are found below.

2.1. Chemical Methods

2.1.1. Adsorption with Adsorbent Materials

This methodology involves separating MPs from the environment using an adsorbent. The adsorption efficiency can be affected by experimental parameters such as pH, the presence of inorganic ions, functional groups, and permeability. There are many different adsorbent materials including metal–organic frameworks (MOFs), carbon-based materials, bio-based materials, or magnetic materials; activated carbon is one of the most efficient. These materials usually exhibit large surface areas and high porosity, and they can be functionalized with certain functional groups to target specific pollutants [15,16,17,18,19]. The adsorption of the MP onto the adsorbent can occur through various forces, including π–π-type bonds, hydrophobic interactions, electrostatic forces, and even magnetic forces [20]. Regardless of the type of adsorbent, it is important to mention that they must be suitable for working at low temperatures since many materials degrade and break down their structure when the temperature increases. The pH should be close to neutral, as the water requiring treatment may occasionally have very high or very low pH levels.

2.1.2. Coagulation/Flocculation or Electrocoagulation

This technique adds coagulants to the medium to facilitate clot formation, which can flocculate or settle, dragging microplastics. It is one of the most critical technologies for eliminating microplastics at drinking water treatment plants (DWTPs) and wastewater treatment plants (WWTPs).

Tang et al., 2022 [21] conducted a review of the factors that can influence the removal of MPs from water. They concluded that the coagulant, MP characteristics (such as particle size and fiber shape), water quality, and operating conditions play a significant role in the removal efficiency of MPs. Tang et al., 2023 [22] studied the removal efficiency of MPs by exploring optimal coagulation conditions and concluded that the size and concentration of MPs significantly affect the coagulation. Similar results were observed by Xue et al., 2021 [23].

Coagulation can be chemical, typically employing iron or aluminum salts [24,25], or electrochemical, applying an electrical current or potential to iron or aluminum electrodes to provide the necessary ions to form the coagulants in the medium [26]. For example, Rajala et al., 2020 [27] employed inorganic coagulants such as ferric chloride and poly- aluminum chloride, along with an organic coagulant (cationic polyamine polymer), to facilitate the flocculation/coagulation of MPs in the secondary effluent of a treatment plant. The highest efficacy reached 99.4% elimination, with iron and polyaluminum chlorides proving more efficient than polyamine.

2.2. Physical Methods

2.2.1. Flotation by Density

This technology is viable due to the potential to modify the hydrophobicity of MPs. This process involves the creation of gas bubbles that interact with the suspended particles and oil droplets. These oil droplets adhere to the suspended particles and, through the bubbles generated, lift them to the surface [24].

Zhang et al., 2021 [28] proposed a column flotation system that generates bubbles with different concentrations of terpineol (frother) to eliminate MPs. They investigated the influence of numerous conditions, such as pH and temperature, and successfully achieved 100% removal of MPs, with an aeration volume of 5.4 mL/min and a terpineol dosage of 28 mg/L. Compared with other modifications, this same group also studied [29] the implications of coating MPs with a layer of Al(OH)₃ to promote interactions and improve their elimination.

2.2.2. Filtration

Filtration is one of the most common and effective techniques for MP separation and elimination from water in wastewater treatment, as well as in the purification of potable water.

This technique is based on the use of filters to retain microplastics. Porous membranes, such as reverse osmosis, ultrafiltration, or microfiltration are used. They act as a physical barrier, preventing or restricting the passage of certain compounds according to their size. Generally, the size of these membranes allows the passage of the ions, water, and molecules but blocks the passage of larger compounds such as MPs [30]. There are different types of filters: mesh or sieve filters, which separate larger MPs, and filtration membranes, which can be microfiltration or ultrafiltration, employing semipermeable membranes with smaller pore sizes that can capture MP particles down to a few microns [31]. Nanofiltration using membranes with even smaller pores, typically between 1 and 10 nanometers, is effective in eliminating nanoplastics or extremely small plastic particles that can pass through other filters [32] and reverse osmosis membranes, which utilize pressure to force the passage of water through a semi-permeable membrane [33].

Conesa et al., 2022 [34] conducted a review on the elimination of MPs, focusing on filtration processes. This includes the use of sand filters [35], which serve as mechanical barriers effective for the elimination of larger MPs and activated carbon filters [36], which can retain the specific sizes of MPs.

2.2.3. Centrifugation

This technique is based on the difference in density of the materials in a sample. MPs tend to have lower densities than other particles in water, causing them to float when a centrifugal force is applied. Other particles, such as sediments or denser organic materials, tend to settle at the bottom of the centrifuge tube. In this case, centrifugation is efficient when combined with other techniques, such as density flotation or filtration. Recent research focuses on using density gradient centrifugation to identify different types of MPs based on their specific density, although it employs highly toxic media such as cesium chloride [37,38]. Majcen et al. [39] presented an affordable and less dangerous method, which could even be used for educational settings, employing a Luer-Lock system, a standardized medical system.

2.2.4. Magnetic Separation

This technique is relatively new but has a promising future. It involves using magnetic particles (MNPs) to extract MPs from water and other media [40]. This method is interesting because it facilitates efficient separation without the necessity of physical filtration, which can result in a faster and less expensive process.

The method is based on incorporating magnetic particles or magnetizable materials that bind to MPs. Once magnetized, these particles can be easily attracted and removed from the water using a magnet. There are different forms to bind MPs to magnetic particles, for example, functionalizing the surface of the MPs using magnetic nanoparticles, such as Fe₃O₄, which can adhere to their surface through chemical or physical interactions, or adsorbing the MPs onto magnetic particles, employing surfactants or chemical agents that can promote/improve this adhesion [41,42]. The magnetic nanoparticles used are iron-based, especially magnetite, maghemite, and ferrites, and the most common methods to obtain these nanoparticles are co-precipitation, microemulsion synthesis, the thermal decomposition of organometallic precursors, etc., and the functionalization of these MNPs plays a crucial role to prevent oxidation and aggregation of these and good interaction with the MPs.

Grbic et al. [41] promoted the magnetic extraction of MPs employing a hydrophobic Fe nanoparticle, reaching 93% or the recovery of different MPs, such as PE, PET, and PVC, from seawater. The essential interaction mechanism between MPs and MNPs seems to be sorption, coagulation, and subsequent sedimentation. Some iron oxide studies have revealed that chemical bonding and electrostatic interaction between oppositely charged particles play major roles in bonding. This kind of interaction has been studied by Martin et al. [43].

2.3. Biological Methods

2.3.1. Bioremediation

This method employs microorganisms and enzymes that can degrade MPs. There are more than 400 species of microorganisms identified as plastic degraders that can be used to provide eco-friendly degradation [44]. For example, Skariyachan et al. [45] employed novel microbial consortia to enhance the degradation of low-density polyethylene (LDPE) from soil samples, reaching approximately 81% and 38% degradation to LDPE strips and LDPE pellets, respectively.

Although this method does not directly separate MPs, it represents a way to eliminate plastics in a biological way [46,47].

2.3.2. Biological Sponge

Biological sponges present an emergent and promising technology for eliminating MPs from water. This method uses biodegradable or natural materials that act as filters or adsorbents to capture MPs without introducing chemical substances that could harm the environment. Some researchers are developing sponges made from biological materials, such as chitosan (derived from chitin) [48], alginate or cellulose [49], which can trap MPs. These materials also have the advantage of being biodegradable or reusable.

D. Hongbing et al. [50], very recently, developed a sponge from chitosan extracted from squid bone and cotton cellulose. This sponge can adsorb materials found in the ocean such as textile products, food packaging, and even industrial materials. Furthermore, its filtration capacity is not compromised by inorganic particles, heavy metals, pollutants, or organic derivatives. During an initial adsorption cycle, the sponge successfully removed nearly all microplastics and maintained an efficiency of over 95% after five consecutive trials. The authors point out that large-scale production is feasible, and their fabrication relies on simple and low-cost materials. In the laboratory, one square meter of sponge can be produced per week, paving the way for industrial scaling and potential future applications in household water purification.

In addition to the methodologies mentioned before, developing combined systems, such as advanced filtration, and integrated methods like mechanical filtration, followed by electrocoagulation or magnetic absorption, allows the complete elimination of the MPs across different size ranges. Figure 1 presents a scheme with the more common separation methods mentioned above.

In summary, various efficacious methodologies exist for separating MPs from the aqueous environment. When applied to contaminated waters in treatment plants, following preliminary, primary, and secondary treatments, these methods achieved over 80% efficiency. Although these techniques are usually effective, in many cases, around 20% of the MPs cannot be recovered. Therefore, it seems necessary to investigate other separation methodologies and the employment of different materials to retain the MPs more efficiently, approaching 100%.

3. Microplastic Degradation

Generally, MPs are non-biodegradable polymers with good structural and mechanical stability due to the covalent bonds in their molecules. The degradation level depends on the characteristics of the polymer, such as its structure, additives, and chemical composition, as well as the characteristics of the environment, such as temperature, humidity, the medium in which it is found (for example, water, soil, sand, etc.) and the exposure time [51].

For this reason, they can be resistant to the degradation process. As MPs accumulate in the environment, their fragmentation and decomposition have become critical areas of study due to their environmental impact and persistence in ecosystems. The complete degradation of the MPs consists of their transformation into CO₂ and H₂O.

Different methodologies have been employed to degrade the MPs; among them, it would be worth mentioning the ones listed below:

3.1. Photodegradation

Light exposure, especially sunlight, is a factor that enhances the rupture and degradation of MPs, playing a role in their elimination through degradation. In general, this process is denominated by photodegradation or photolysis, and the MPs are decomposed through sunlight exposure, especially ultraviolet radiation (UV), resulting in surface oxidation, size reduction, structural defects, and, in some cases, decomposition to produce carbon dioxide (CO₂).

This phenomenon occurs when light energy is absorbed by plastic polymers, causing the rupture of their chemical bonds and gradually degrading the material into smaller molecules or fragments. The efficiency of this process depends on the polymer type and its ability to absorb these wavelengths.

This UV energy can generate free radicals that attack carbon–carbon bonds (C–C) in its polymer chain. This results in a phenomenon known as chain scission, in which the chain breaks apart, breaking the plastic into smaller fragments. In the presence of oxygen, previously formed free radicals react with plastic molecules, oxidizing them, and generating compounds such as CO₂, water, and other organic or inorganic intermediates. This means that photodegradation typically does not convert MPs directly into harmless molecules, and it can generate toxic substances, including nanoparticles [52].

In general, these aging processes through sunlight exposure (chemical bond rupture, fragmentation, oxidation, nanoparticle formation) have low short-term efficiency. Therefore, photocatalysts are usually employed to accelerate them. In this context, when using a light catalyst, active species that accelerate the degradation process are generated [53]. To obtain complete photodegradation, factors such as the light source, plastic properties, and reaction conditions play a significant role [54]. For example, He et al. [55] studied the photocatalytic degradation of polyethylene and polyester using TiO₂-coated ZnO tetrapods. They observed that the morphology of microplastics affects the degradation process, and an electron scavenger was necessary to maintain the reactivity of the catalysts over a prolonged period. Their study achieved a complete mass loss of MPs within 816 h.

3.2. Advanced Oxidation Processes (AOPs)

This technique is based on generating reactive oxygen species (ROS), such as hydroxyl radicals (•OH), which have high oxidizing power and can decompose dangerous organic and inorganic compounds, including MPs and other persistent contaminants.

AOPs are mostly applied to eliminate water, air, and soil contaminants. These techniques are considered efficient for removing contaminants that are very difficult to treat through conventional methods such as pesticides, pharmaceutical products, volatile organic compounds (VOCs), and, recently, MPs. Regarding them, this technique is used to break the MPs into smaller, less harmful components or even achieve complete mineralization (CO₂ + H₂O).

Depending on the method of ROS generation, AOPs for the degradation of MPs can be classified into Fenton oxidation, photo-Fenton oxidation, (homogeneous or heterogeneous) photocatalytic oxidation, electrocatalytic oxidation, and ozonation [51,56,57].

Examples of homogeneous Fenton oxidation can be found in the studies conducted by Luo et al. [58] and Kang et al. [59], which employed a heterostructure based on carbon nanotubes (Mn@NCNTs-800) for the degradation of MPs. In optimal conditions and the presence of peroxymonosulfate (PMS), they achieved 54 wt% elimination of MPs, through carbocatalysis and hydrothermal hydrolysis. In most cases, smaller-sized intermediate products were produced during the degradation process, which can occur due to the fragmentation of the original polymer that did not achieve complete mineralization or from the decomposition of H₂O₂ and S₂O₈Na₂, which produce ROS.

Lin et al. [60] degraded 91.3% of polyethylene terephthalate MPs in water within 12 h, using SO₄^•− and •OH, radicals through an electro-Fenton/persulfate technique in the presence of ${\mathrm{FeS}}_2^{-}$ modified carbon felt as the cathode. Radicals can be produced through an electrochemical process called electrochemical oxidation. Under these circumstances, a certain quantity of energy is applied to a solution that contains H₂O₂ and persulfate to produce the respective radicals that will oxidize MPs.

Another work that employed a similar methodology was Ning et al. [61], who employed an anode of PbO₂ modified with CeO₂, to degrade polyvinyl chloride MPs. They observed that, when the concentration of CeO₂ was 0.005 M, better electrocatalytic activity occurred through the electrodes, eliminating 38.67%, compared with 16.67% obtained from the PbO₂ electrode. In the same way, Mais et al. [62] studied the elimination of PE and PET through electrolysis with mixed metallic oxides, eliminating 70% of the MPs.

The radical generation process can be carried out by electrochemical decomposition, e.g., by the reduction of oxygen in solutions to produce •OH. These radicals can, as in other methodologies, oxidize MPs in solutions. Normally, this process is not very efficient, so it can be enhanced by using catalysts that activate the production of radicals by a photoelectrodegradation process, where the production of ROS is combined electrochemically and photochemically, simultaneously [63,64].

3.3. Biodegradation

Biodegradation is a process that decomposes organic material using living organisms, such as bacteria. Some studies have been carried out to identify microbes that can digest the MPs in the environment, whether in soils, landfills, sediments, or water [62,63].

For example, Torena et al. [65] investigated the employment of activated sludge as a biocatalyst source for eliminating polyethylene terephthalate in water. The authors found that, in 168 days of incubation, 17% of the MPs were eliminated. On the other hand, Cunha et al. [66] studied exopolymer substances excreted by microalgae to form hetero-aggregates with MPs (poly(methyl methacrylate) (PMMA) and polystyrene). The authors employed different microalgae species (such as freshwater and marine microalgae), achieving more than 42% MP elimination. Yolanda et al. [67] focused on marine ecosystems, analyzing the use of the microbial community of Jakarta Bay to degrade polystyrene MPs. It was observed that, after 60 days of incubation, between 4 and 6.4% polystyrene elimination was achieved.

Different MPs can be degraded by microorganisms such as bacteria, fungi, or algae. The major inconvenience of this technique is the time required to achieve partial or total degradation, which may be months or even years.

Figure 2 presents some of the methodologies most used to degrade MPs.

The materials employed to provide MP adsorption, as well as the catalysts for their degradation, are many and diverse; their use depends on the type of MPs that will be eliminated or degraded, as well as the medium conditions. This review highlights the use of modern metal–organic frameworks (MOFs) in the adsorption and degradation of microplastics.

4. Metal–Organic Frameworks (MOFs)

MOFs are porous crystalline structures formed of metallic ions or metal-containing clusters interconnected by an organic linker, coordinate bonds, giving rise to a framework characterized by a huge surface area and adjustable properties, such as tunable pore size and customizable chemical functionality, making them valuable for various applications [68,69,70,71,72].

These compounds could form porous networks of one-dimensional, two-dimensional, or three-dimensional structures. Consequently, they can be generated with a wide range of different porosity, pore sizes, compositions, and chemical stability changing the metallic ions or organic linker. They also can exhibit various metals in their structure and present them in different forms, such as powder, films, and pellets [68,69,70].

At the end of 1990, research on MOFs experienced rapid growth; the pioneer was Omar Yaghi from UC Berkeley [73]. Since then, more than 90,000 MOF structures have been reported, with this number increasing every year (Figure 3) [74].

However, despite everything, MOFs present certain limitations for some potential applications, such as thermal and chemical stability, the cost of synthesis, and scalability. Therefore, in recent years, studies have focused on combining MOFs with other compounds, creating MOF-based materials. They are derived from MOFs that either incorporate them directly or are produced by modifying or transforming them. These materials leverage the unique properties of MOFs, such as high surface area, tunable porosity, and modular chemistry, but are tailored for specific applications or to overcome the inherent limitations of MOFs. There are different strategies, such as the production of metallic oxides from the MOF by calcination [75,76]; the integration of MOFs with polymers for improving mechanical stability, flexibility, or processability [77,78,79]; embedding nanoparticles (e.g., metal, metal oxide) within MOFs to enhance conductivity, catalytic activity, or adsorption capacity [80,81,82,83], combining with carbon materials (e.g., graphene, C₃N₅, carbon nanotubes) to create synergistic composites [84,85,86]; dispersing in liquids or formulated into gels for specific applications like drug delivery or flexible electronics [87]; combining with enzymes because MOFs are ideal enzyme support due to their porous structural superiority [88,89,90] or combining with proteins [91]; and growing or encapsulating on other substrates (e.g., silica, alumina, fibers) [92,93,94], among others.

MOF-based materials are a growing field, with ongoing research to optimize their design and performance for diverse technological and industrial applications.

Due to the large number of MOFs synthesized and new ones to be prepared in the future, their nomenclature is based primarily on the laboratory in which they were first synthesized. Accordingly, the most widely known and widely used MOFs for different applications, among which we can find the recovery and/or elimination of PMs, are as follows:

(a)

Zeolitic Imidazolate Frameworks (ZIFs): This is an important subclass of MOFs, employing tetrahedrally coordinated divalent cations and imidazolate-based organic ligands [95]. They are characterized by their zeolite-like structure and thermal and chemical stability. ZIFs inherit the structural diversity of MOFs and the thermal and chemical robustness typical of zeolites, making them highly versatile materials that can be applied to energy storage, gas separation, and catalysis [96,97,98]. Their characteristics include acid sensibility, large surface area, and low toxicity [99,100].

(b)

Material Institute Lavoisier (MIL) MOFs: These materials, as the name indicates, were synthesized at the Lavoisier Institute [101,102]. They are constituted by trivalent metal cations and carboxylic acid ligands and contain large pores and permanent porosity [103,104]. They are widely used in biomedical applications [105,106,107], hydrogen storage, and remediation processes [108,109,110,111,112].

(c)

Porous Coordination Polymers (PCPs): These materials are MOFs with high specific surface areas and modulable pore sizes. Typically synthesized through carboxylic acids, pyridine, and its derivatives as primary building units (PBUs) and transition metal ions as secondary building units (SBUs) [113,114], these materials are employed in gas storage [115,116] and photocatalysis [117,118].

(d)

University of Oslo (UiO) MOFs: In this case, carboxylic acids are used as PBUs and Zr₆(μ3-O)₄(μ3-OH)₄ as SBUs; these materials were synthesized for the first time by Cavka et al. [119]. UiO-MOFs can be found as UiO-66, UiO-67, UiO-68, and UiO-69. They have a large and relatively uniform pore size and specific surface area, with high strength and active ZrO groups, which have been widely studied and employed in post-modification and other applications. The post-modification of UiO-MOFs with functionalized specific groups in their linker is carried out to improve the active site for the adsorption of heavy metal ions [120]. UiO-MOFs and UiO-based materials are being studied as photocatalysts in the degradation of emergent contaminants or for CO₂ reduction [121,122,123,124].

These are some examples of the possible nomenclatures utilized depending on where they were created. For instance, MOFs synthesized by other research groups are the HKUST-n, which is so named because it was synthesized at the Hong Kong University of Science and Technology [125], and the group CAU-n was synthesized at Christin Albrecht University [126].

4.1. Metal–Organic Framework Synthesis

Similarly to the numerous existing MOF structures, a large variety of methodologies are employed in their production, considering the cost, biocompatibility, safety, construction unities, energy quantity, use of an innocuous medium, easy activation, and possibility to be produced continuously [127,128,129,130,131,132,133].

4.1.1. Solvothermal Method

This simple technique is one of the most used methods for synthesizing MOFs [130,134]. The process combines a metal salt with an organic ligand in an organic aprotic or protic solvent medium. Among the aprotic solvents are dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and N,N−dimethylacetamide (DMA), while the protic solvents include methanol, ethanol, or a mix of both [135]. If, instead of organic solvents, water is employed, the methodology is called hydrothermal synthesis. The mixture of all the compounds is typically introduced into a stainless-steel autoclave lined with Teflon, and the system is heated to a temperature above the boiling point of the solvent, leading to an increase in internal pressure. The changes in the solvent and the temperature at which the process is carried out are parameters that can be modified to obtain different structures with distinct surface areas.

4.1.2. Electrochemical Method

MOFs can be produced on a substrate or in solution through an electrochemical method. This method is carried out in the absence of any metal salt at the beginning of the process; as oxidation occurs, the metal ions from the reaction are incorporated into the solution [136,137]. The solution must contain a linker and an electrolyte to enhance conductivity. Anodic dissolution, introduced by Muller et al. [138], is very well known. In this method, the voltage or current is applied between the metal electrode and another electrode (graphite, platinum, etc.); when the process begins, the metal dissolves, and the produced ions react with the linker to produce the MOF on the metal surface or in the solution [139]. The factors that affect the properties of the MOFs are diverse, including the solvent nature, applied voltage, electrolyte concentration, time, and distance between the electrodes [140]. With this methodology, a continuous quantity of MOFs can be obtained by increasing the solid content.

4.1.3. Other Methodologies

Other methods employed for the synthesis of these materials include the use of microwaves. The methodology is like solvothermal, but the Teflon container is placed in a microwave. In this method, an oscillating electromagnetic field is employed to induce molecular rotations, followed by a rapid increase in the temperature of the liquid phase. It is considered a faster synthesis procedure, with significantly shorter crystallization times and easy control of the morphology, as well as a uniform particle size distribution [130,141,142,143]. The biggest drawback of this technique is its difficulty in industrial applications.

Another possible method is the employment of the sonication or sonochemical method. In this method, mechanical vibration (from 20 KHz to 10 MHz) is used to produce the cavitation phenomenon, which consists of the collapse of bubbles in the solution; these bubbles can reach temperatures of up to thousands of Kelvins, at very high pressures, allowing the modulation of homogeneous nucleation of the material [144,145,146]. Other physical methods, such as mechanochemical synthesis, involve using mechanical forces (e.g., grinding) to induce intramolecular bond breakage. The main advantage of this method is that it operates under solvent-free conditions and does not require heating, as it works at room temperature [147,148]. Other synthesis methods, although they are less often employed, include ionothermal [149], and microfluidic synthesis [150], diffusion [151], and spray-drying [152].

Figure 4 shows a summary of the different methods employed in the synthesis of MOFs.

5. Use the MOFs to Adsorb Microplastics

MOFs are exceptionally suitable for adsorbents, due to their porous structure, polar surface, and electronic activity. All these properties can play a significant role in the adsorption and elimination of MPs. They can be synthesized in a particular way depending on the composition of the MP that is to be adsorbed. The ligands employed in MOF synthesis are responsible for increasing the adsorption of the MPs on them through different mechanisms such as electrostatic interaction of the functional groups and hydrophobic interactions. This interaction arises from non-polar hydrophobic molecules to repel water molecules in a polar medium. The molecules are not bound by strong covalent, hydrogen, or ionic bonds but rather by weak forces, such as van der Waals forces and π–π interactions. This kind of interaction refers to a type of multiple bond formed between two atoms, typically involving p-orbitals, and it is especially associated with the molecules containing heteroatoms, like nitrogen or oxygen, and hydrogen bonds. Figure 5 shows some of these kinds of interactions.

Chen et al. [154] published one of the first works developing a composite in which a melamine foam was employed as a substrate for synthesizing Zr-MOFs due to its porosity, flexibility, sturdiness, and stability properties. Different functionalized Zr-MOFs (UiO-66-X, X=N, NH₂, OH, Br, and NO₂) were prepared. The UiO-66OH@MF-3 had the highest recovery efficiency, above 95.5% compared to the other composites obtained, maintaining this efficiency after 10 reuse cycles. Figure 6 shows the recovery efficiency of different functionalized Zr-MOFs and the efficiency of UiO-66-OH@MF-3 after repeated reuse cycles.

These compounds have high uniformity, sturdiness, flexibility, durability, and scalability. The ζ potential plays an important role in adsorption, which is why the authors studied its variation with the temperature from 80 to 110 °C, identifying that the surface charge of the MP is essential for determining the possible interaction between the MOF and the MPs.

An interesting aspect of this work was the investigation of MP recovery in seawater on a small scale, simulated through a microfiltration system employing sunlight. As a proof of concept, a lab-scale simulated automatic filtration system was designed and successfully powered by sunlight to pump water through three filtration units. Figure 7 shows a photograph of the system and a scheme of possible interactions between the MOF and MPs.

The authors indicated the four most important advances shown by Zr-MOF-based foam materials for removing MPs: (1) melamine foam (MF) contains a very porous structure with many functional sites, which allows the anchoring of the Zr-MOF and adequate contact of the MPs with the skeleton; (2) the prepared Zr-MOFs have high stability and uniformity throughout the MF, which results in high contact between the MPs and the MOFs; (3) the prepared MOFs possess numerous defects and are positively charged, which results in a high affinity for the negatively charged MPs; and (4) the functional groups in the Zr-MOFs cause hydrogen bonding type interactions or Van der Waals interactions with the MPs.

Within the MOFs employed to adsorb MPs, the MOF-545 with oxime covering has demonstrated excellent physical-chemical properties, high adsorption capacity, and high selectivity [155], with an adsorption capacity of 294.6 mg/g at pH 6. A comparison between the MOF-545 and MOF-545 with oxime covering shows higher adsorption due to the presence of oximes. These functional groups, which contain lone electron pairs, have low toxicity, making this coating environmentally friendly. Figure 8 shows the MP concentration effect on the adsorption capacity of MOF covering and non-covering, as well as the velocity of microplastic removal. In both cases, the adsorption capacity rose with the growth of MP concentration, and a saturated adsorption capacity was observed at 300 mg/L. The removal rates gradually diminished when C₀ rose, and the values obtained were 24.5% and 40.6% for 25 mg/L. At these concentrations, the coordination with the MPs was maximum, and the adsorption process reached saturation.

Similarly, the ZIF-67 [156], which has Co²⁺ as the central metal ion and 2-Methylimidazole as an organic ligand, presented strong adsorption to MPs, especially polystyrene. An increase in the recovery efficiency of 65.4% for quantities of 0.1 g/L to 90.2% of 0.6 g/L was observed. It was concluded that higher amounts of MPs do not imply an increase in recovery efficiency; the interaction between the positive charge of the ZIF-67 and the negative charge of the PS produces an electrostatic attraction that is responsible for the adsorption of the MPs, as well as π–π interactions and hydrogen bonding. These high levels of efficiency are maintained at pHs between 3 and 10 but decrease at pH 11–12 due to the repulsion between the same charges generated in both materials. The authors concluded that the ZIF-67 is a viable material for contaminated water treatment, especially when it contains MPs.

Modak et al. [157] investigated the adsorption capacity and recovery efficiency of polystyrene nanoplastics (PSNPs) using the Cr-MOF. Adsorption was found to be primarily driven by the electrostatic attraction between the negatively charged PSNPs and the positively charged Cr-MOF. Figure 9 shows schematically possible interactions. Mainly, the adsorption process was dominated by three mechanisms: (i) electrostatic interactions between the positively charged Cr-MOF and the negatively charged PSNPs, (ii) π–π interactions involving the benzene ring in the terephthalic acid of Cr-MOF and polystyrene, and (iii) acid–base interactions between the chromium node and the sulfate ester group.

Recovery efficiency reaches 96% when employing 5 to 70 ppm concentrations. It was observed that the adsorption capacity increased with increasing initial PSNP concentration, reaching a maximum of 800 mg/g. In addition, it was possible to promote the regeneration of the MOF by employing sodium hydroxide and ethanol, although the adsorption capacity decreased with the number of regenerations. Figure 10 illustrates efficiency and its decline with successive regenerations.

Another composite example was published by Pasanen et al. [158], in which they synthesized a magnetic nanocomposite formed of a ZIF of Zn and Fe nanoparticles, named Fe@ZIF-8, for the simultaneous elimination of polystyrene microspheres (1.1 μm in diameter) and phenol endocrine disruptors (bisphenol A and 4-tert-Butylphenol). A measure of 20 mg of this nanocomposite could eliminate 98% of highly concentrated (25 mg/L) polystyrene microspheres in 5 min, as well as 94% of bisphenol A (1 mg/L) and 4-tert-Butylphenol (1 mg/L) in the same period. Figure 11 shows the removal efficiency of the contaminants studied.

Harris et al. [159] developed a composite with magnetic properties, consisting of C@FeO nanopillars on a 2D-MOF, to simultaneously remove both solid and dissolved contaminants from water matrices. This tricomponent was able to remove 100% of MPs in just 60 min, with the same results when a dye, such as methylene blue MB, was in the medium, eliminating both. This material exhibits great stability, as it was employed consecutively in six adsorption cycles, maintaining a removal efficiency of 90%.

In this same line, Liu et al. [160] synthesized a tricomponent magnetic material based on an MOF of Al with magnetite and silica Fe₃O₄@SiO₂@MIL-53(Al). This novel material was evaluated to eliminate four types of MPs (PVC, PS, PP, and polyethersulfone), optimizing the time effect, material mass/MP ratio, temperature, and pH. Depending on the MP type, MP removal efficiencies were between 54.10 and 97.14%. Furthermore, they proved that this material can be reutilized up to five times, making it suitable for use in food systems.

You et al. [161] synthesized a composite based on an MOF of Zn and aerogel, ZIF-8@Aerogel. They grew the MOF on the aerogel fibers for MP removal in simulated water and seawater. This material reached a removal rate of 91.4% for micro/nanoplastics, including poly(1,1-difluoroethylene) (60–110 nm), and 85.5% for polystyrene (90–140 nm).

In another way, Peng et al. [162] employed a functional superhydrophobic and superoleophilic sponge based on ZIF-67 MOF and layered CuCo-LDH spheres, labeled as OMCTS/ZIF-67/CuCo-LHDs@Sponge. The innovative aspect was the employment of MOF ZIF-67 for the growth of Co²⁺ in the CuCo-LHD. They obtained a high adsorption capacity for three different MPs: 85 mg/g for PP-2000, 94 mg/g for PS-100, and 93 mg/g for PE-2000. This material demonstrated high mechanical durability, maintaining a mechanical separation capacity for MPs above 98% after 40 cycles of consecutive adsorption.

The MOF/graphene oxide composites represent another strategy evaluated for the adsorption of cationic dyes such as methylene blue (MB) and Rhodamine B (RhB), as well as the recovery of polyurethane (PU), with IRMOF-1/rGO being the composite formed [163]. The adsorption capacity was evaluated through a molecular dynamic simulation, and it was found that the Van Der Waals interactions (vdW) have a dominant impact on the stability of the simulation system, with the effect of the electrostatic interactions considered inferior. The high vdW interactions of the PU with the IRMOF-1 can be attributed to the high molecular aggregation of MPs around the composite, while the interaction with the rGO is minimal or negligible.

In recent years, separation through membranes has aroused great interest in residual water treatments due to its excellent benefits, including low power consumption, high separation efficiency, and convenient operating procedures [137]. Using membranes is one of the most employed techniques for contaminant removal in wastewater treatment plants. Additionally, due to the beneficial properties of the MOFs (high porosity and surface area, tunnel-shaped pore structure, good affinity for polymers, the great abundance of functional groups, and high adsorption capacity), they are excellent candidates for the manufacture of composite membranes compared to polymeric membranes [164].

The surface morphology and pore size can be changed and controlled by employing different metal ions, ligands, solvents, and temperatures [165]. Although the use of MOFs to produce membranes has been studied for dyes and heavy metals in contaminated waters [166], the use of membranes based on MOFs for the separation of MPs in contaminated water has also been published, to a lesser extent [167]. For example, a superhydrophobic MOF-based coated sponge developed by Chen et al. [168] was utilized to facilitate the elimination of MPs through their retention. The material called ODSOSS/TiO₂/Ni-MOF/PDA@sponge, where the ODSOSS is an octakis(dimethylsilyloxy)octasilsequioxane and PDA is polydopamine, is a complex composite that has the qualities that each of its components gives it. The PDA increases the adherence between the sponge and the other components, although it increases MOF growth through complexation. The Ni-MOF shows a large area on the microscale, which promotes an increase in adsorption, and TiO₂ in nanoparticle form can promote the photodegradation process. The results indicated excellent recovery efficiency, reaching 100% for different MPs including PS, ABS, PVC, PE, and PP.

A novel composite combined with a hydrophilic MOF, MIL-100 (Fe) incorporated in a polysulfone (PSF) matrix demonstrated excellent efficiency in recovering MPs in the presence of other textile contaminants. A scheme of the formation of these composites in the membrane is represented in Figure 12. The membranes were prepared using the phase inversion technique. A casting solution was formulated with 17.5 wt% PSF and 2 wt% PVP in NMP. Initially, MIL-100 (Fe) was dispersed in NMP at 0, 0.25, 0.5, and 1 wt% concentrations and subjected to ultrasonication for 30 min. Subsequently, PSF and PVP were gradually added to the solution. The resulting mixtures were placed in a thermally insulated shaker at 55 °C for 12 h.

The well-mixed solutions were then cast onto a clean glass plate using a film applicator set to a thickness of 200 µm and immediately immersed in a bath of deionized (DI) water maintained at 4 °C. Before use, the membranes were thoroughly washed several times with DI water.

Different quantities of the MIL-100 (Fe) in the PSF matrix were analyzed, and it was found that 0.5 wt% was the ideal quantity to obtain the best results. Using this composite as a membrane, they achieved an efficiency 10.3 times greater than that of the pristine PSF membrane. Additionally, they obtained 99% rejection of methylene blue (MB), moderate salt rejection, and reusability for at least six cycles [169].

Seeking greater robustness and increased surface area for MP adsorption, hybrid compounds of covalent organic frameworks (COFs) together with MOFs have been obtained. FS-50/COF(MATPA)-MOF(Zr/PDA@PVDF) was synthesized through solvothermal synthesis combined with surface modification of the FS-50 molecule. Solvent regulation was used to generate smaller needle-like whiskers during the “in situ” growth of MOFs on COFs, and high pressures were employed to induce directional crystallization. In addition, the PDA increases adhesion between the deposited and PVDF membrane, and the use of the functional group FS-50 provides the membrane with hydrophobicity combined with superoleophobicity [170]. This composite could adsorb different colorants and pesticides and recover simultaneously distinct MPs, such as PP 2000 and 100 and PS-100. They obtained 100% recovery with 0.2 g/300 mL quantity.

In another way, the Ni-MOF membrane has been employed to recover multicompounds through gravity. The operation of this membrane requires low energy input, low transmembrane pressure, and a facility of operation. In a previous study, Ni-MOF particles were incorporated into a nylon membrane. This type of membrane showed excellent hydrophilic and underwater superoleophobic properties. Their employment demonstrated a significant improvement in the rejection rate of different colorants and showed an MP rejection rate of 99% in contaminated water. In addition, it exhibited high stability over extended periods of operation [171].

6. Degradation of Microplastics Employing MOFs

In previous cases, MPs have been removed from aqueous matrices using adsorption techniques, either directly or through membranes. However, they were not eliminated; instead, they were retained in solid matrices. These solid matrices require thermal treatment at high temperatures to eliminate the MPs, which can produce more harmful environmental compounds, such as greenhouse gases. Therefore, recent advances in MP removal are focused on the total removal in one step. Thus, investigations that employ degradation processes for MP total removal have been gaining prominence. For greater effectiveness of the degradation process, it is necessary to separate the MPs from the rest of the contaminants in the environment, using techniques already described such as adsorption, flotation, and filtration. In this way, the degradation process can be studied and easily understood. Therefore, there is a need for increased research on new methodologies for degradation. The use of MOFs for the degradation of MPs is an area that has recently begun to develop, although this material has already proven to be an efficient and environmentally friendly alternative. The mechanisms through which MOFs degrade microplastics are varied, with catalytic processes and photodegradation playing a particularly important role.

In this context, MOFs can be employed in the degradation process by generating reactive oxygen species (ROS). The presence of metallic ions in their structure can catalyze the species oxidation as H₂O₂ or persulfate to produce hydroxyl or sulfate radicals, promoting the degradation of contaminants in the environment. The quantity of ROS can be regulated by changing the structure or medium to enhance selectivity control to different MPs.

The principal mechanism studied for generating radical species is the Fenton-like reaction. Different metal ions can be employed in this process, with Fe²⁺ ions in the presence of H₂O₂ being the most widely studied [172,173]. This process generates hydroxyl and hydroperoxyl radicals, which are highly oxidizing species that facilitate degradation by altering functional groups such as hydroxyl and carbonyl, thus modifying the C–H bonds of the MPs. However, this technique has significant drawbacks that minimize its practical application, such as strict pH conditions, high oxidant (H₂O₂) consumption, and the presence of the iron catalyst.

Tagg et al. [173] exposed four different types of MPs (PP, PE, PVC, and nylon) to three different doses of H₂O₂ and iron sulfate as the iron source. They observed that this treatment did not have a significant influence on the MP surfaces of PE, PP, and PVC. However, there was a variation in the surface of MPs from nylon, likely due to the inherent variation in the nylon particles among the fragments employed in their study, and not because of an effect caused by the Fenton reagents on particle sizes.

The Fenton process can be enhanced by high temperatures, ultraviolet light, and the presence of iron ions. Employing peroxymonosulfate as a catalyst and Fe₃O₄-PVP@ZIF-67 as an MOF-based material in a Fenton-like process, Cui et al. [174] studied the degradation of bisphenol F (BPF) and concluded that this system could produce sulfate radicals through iron catalysis contained in the MOF, achieving 100% degradation of BPF within a wide pH range in 1 h. The synthesized material was versatile for a broad range of contaminants and organic ions as well as recyclable.

Heterostructures of NH₂-MIL-88B (Fe) to form the composite MIL-88B(Fe)/(MoS₂) were employed to promote the efficient photodegradation of high-density polyethylene fragments in water [63]. The formation of this composite improved the light absorption, resistance to recombination, carrier lifetime, and specific surface area of the hybrid system. The proposed mechanism indicated that highly active species (h⁺, ^•O₂⁻ y ^•OH) are produced through the photoinduction of the prepared composite.

Following a similar methodology, but in their case, synthesizing an MIL-101 modified with 40% bismuth-based BiOI, Gu et al. [175] studied the degradation of polyethylene. Thanks to its combination, this composite demonstrates a high surface area, exceptional photoactivity, and the efficient separation of electrons and holes. In addition, the presence of Fe(II) allows the photo-Fenton reaction with the presence of H₂O₂ in the medium. The degradation mechanism under simulated light can be observed in Figure 13.

In this mechanism, electrons generated in the conduction band (CB) of BiOI under irradiation can migrate to the CB of MIL-101. Simultaneously, photogenerated h⁺ are transferred from the valence band (VB) of MIL-101 to the VB of BiOI, resulting in an efficient separation of e⁻ and h⁺. Furthermore, the VB potential of BiOI (2.71 eV) was higher than that of H₂O/HO• (2.38 eV), allowing h⁺ on BiOI to react with H₂O or OH⁻ to form HO• (Equation (1)). Through this mode of transport, the photogenerated e⁻ could not reduce O₂ to O₂•⁻ because the CB potential of MIL-101 (0.73 eV) was more positive than the standard value of O₂/O₂• (−0.33 eV). Additionally, the e⁻ generated in the CB could directly react with H₂O₂ to form HO• (Equation (2)) and reduce Fe(III) to the more active Fe(II) (Equation (3)). Moreover, ¹O₂ was generated through the conversion of HO• (Equation (4)). These generated reactive radicals attacked MPs, leading to polymer chain breakage (Equation (5)). During the photocatalytic process, the heterojunction constructed between the composite catalysts enhanced the rate-limiting step of the conventional Fenton reaction, resulting in further improvement of the overall catalytic activity of the system.

h⁺ + OH⁻ → HO•

(1)

H₂O₂ + e⁻ → HO• + OH⁻

(2)

Fe(III) + e⁻ → Fe(II)

(3)

4 HO• → ¹O₂ + 2 H₂O

(4)

OH•/h⁺/¹O₂/O₂•⁻+ microplastics → byproducts

(5)

A recently developed proof-of-concept involved the use of enzymes. Although enzymes are capable of degrading polymers, they face challenges related to stability, reduced cyclability, and economic problems. Rincon et al. [176] studied the adsorption of Candida rugosa-CrL on different MOFs for Bis-(hydroxyethyl)terephthalate (BHET) degradation. They studied the enzyme quantity in the composite, the degradation of the MOF in the medium, the reusability of each one, and the degradation efficiency of the BHET. Considering all the results, the authors concluded that the best composite was the CrL_UiO-66-NH₂, reaching 37.4% degradation, which means the degradation of 2.0 mg of degraded BHET per g of immobilized CrL. Although this proof-of-concept resulted in an attractive system for the biodegradation of contaminated waters, it was necessary to prove the economic viability.

The degradation process can be carried out through electrochemistry and, more specifically, by employing catalytic and photoelectrochemical processes. This union results in photocatalytic degradation, which typically refers to the separation of a hole (h⁺)/electron (e⁻) pair in a semiconductor excited through photonic energy (E).

When the e⁻ is transferred from the VB of the semiconductor catalyst to the CB, this process produces a h⁺ in the VB. Both e⁻ and h⁺ react with OH⁻, O₂ or H₂O resulting in large amounts of reactive oxygen species (ROS), including superoxide radicals as •O₂⁻, O₂/•O₂⁻ = −0.33 V vs. NHE [177], singlet oxygen (¹O₂, ¹O₂/O₂ = 1.88 V vs. NHE) [178], hydroxyl radical (HO^•, H₂O/HO^• = 2.22 V vs. NHE) [179]. In this line, Zanaty et al. [180] synthesized a nanocomposite combining ZIF of Co (ZIF-67) with hydrogen titanate nanotubes (HTNTs) at room temperature. This material was evaluated for the adsorption and catalytic oxidation of contaminants in water, such as organic dyes (fluorescein acid, Rhodamine B, direct blue I, and basic fuchsin) and MPs from cosmetics and personal care. It achieved removal efficiencies of 99% and 97% for adsorption and catalytic oxidation by H₂O₂.

The MOFs employed in this technology act as catalysts in the solution, assisting the electrochemical process. The degradation of MPs takes place, as previously indicated, through the photogeneration of radical species, occurring on the electrocatalyst and in the presence of peroxide or persulfate, which can be degraded employing a Fenton-like process through the MOF [181]. The great variety of reactive oxygen species (ROS) led to the breaking of the bonds in the MP polymers, resulting in their destruction or degradation [182]. A scheme of photoelectrocatalytic (PEC) oxidation can be observed in Figure 14 [183].

In this figure, it can be observed that the PEC process is highly complex. Initially, various light sources, such as ultraviolet, visible, or solar light, can excite electrons from the VB to the CB of the material used as the photoanode. However, a recombination process may occur at the photoanode, potentially reducing the efficiency of the process. Two primary reactions can take place: first, the holes generated in the VB can oxidize water to produce radical species, and, second, the electrons in the CB can reduce oxygen to generate ROSs, which facilitates the degradation of organic compounds such as MPs.

Furthermore, with an appropriate electrocatalyst, hydrogen peroxide or hydrogen can be produced in the cathode cell. In this complex system, several factors must be considered to optimize the process for industrial applications. These factors include the type of reactor and electrodes employed, the operational parameters such as applied voltage and current, and overall energy consumption.

There is no doubt that this methodology is viable for the degradation of MPs, but there are some problems that must be resolved before use on a large scale, such as the high potential employed, the lack of a standardized process, and the need to improve the knowledge of the processes involved. For this reason, few studies have used this methodology in the literature. For example, Kiendrebeogo et al. [184] studied the degradation of PS MPs in water, employing dual approximation using a boron-doped diamond (BDD) and an electrooxidation process for degradation. The degradation mechanism involved radical formation and gave an MP degradation efficiency of 90%, producing gases such as CO₂ and smaller molecules in the solution.

Although degradation processes usually involve the conversion of MPs in CO₂, the transformation of them into high-value-added chemicals must be explored. This process is known as “upcycling” or “resource-based recycling” [185], and electro-photodegradation, in addition to the use of MOFs, can be a feasible way to achieve these valuable products and contribute to the circular economy. In this way, Qin et al. [186] synthesized photocatalysts of Ag₂O/Fe-MOF through a new synthesis method of in situ conversion of unstable metal sites on bimetallic MOFs. The result was particles with nanometric size and semiconductor properties confined into the framework of MOFs to prevent agglomeration. These photocatalysts could be employed in photocatalytic MP conversion into high-value-added chemicals and hydrogen production. This study offered insights into the design of advanced heterojunction photocatalysts, the conversion of microplastics into value-added chemicals, and the generation of green energy.

However, this technique requires catalyst development in MOF synthesis to promote total synergy between both. On the other hand, employing MOFs directly as electrocatalysts is possible. Undoubtedly, a new frontier of research is emerging in this area, demanding the attention of the scientific community.

7. Conclusions

The development of new MOFs and new strategies for their use suggest that these materials can be effectively applied for MP adsorption and catalytic degradation. Furthermore, it also opens a new and promising route for developing new and existing methodologies employing MOFs. They, as adsorbents, present indisputable advantages, not only as powders but also in the investigation of new routes for the design of more complex structures. They present several drawbacks that can limit their practical applications, such as thermal and chemical stabilities, because many MOFs can be degraded at high temperatures, limiting their use in high-temperature applications, and are sensitive to moisture, acids, or bases, which can lead to structural collapse or loss of functionality. The production costs of synthesis compared to other porous materials are another disadvantage due to often requiring expensive precursors, such as high-purity metal salts and complex organic linkers or large volumes of solvents used, being not cost-effective or energy-efficient for industrial-scale production. Related to this, scaling up MOF production and maintaining high purity and structural integrity is an important challenge, as well as using non-toxic or non-biodegradable organic linkers and solvents, through a green method to minimize environmental risks. They can also present other potential risks, particularly related to their stability and the degradation of their components in water. The structural integrity of MOFs can be compromised under aqueous conditions, leading to the release of metal ions or organic linkers into the environment. These byproducts may pose toxicological risks to aquatic ecosystems and human health. Additionally, the long-term behavior of MOFs in water is not yet fully understood, raising concerns about their accumulation and the formation of harmful degradation products. Ensuring the safe application of MOFs requires comprehensive studies on their environmental stability and the potential impacts of their breakdown products. One of the disadvantages is their reusability; in a few cases, they have been able to reach more than five or six cycles of use, which is why their use can be limited. To solve this problem, the protection of the structure through the formation of composites can increase the number of cycles of use, in addition to improving the properties by combining two materials simultaneously. The manipulation of MOF structures through coupling with magnetic nanoparticles has allowed for the recovery of MPs in a very efficient way. In addition, this strategy allows for the recovery of the MOF by employing a magnet, increasing its possible reuse and eliminating the problem of recovery from the adsorbent medium. Another use in which MOFs have gained special attention is in the fabrication of membranes, which can allow the recovery of MPs and other micropollutants from the medium and have been shown, in some cases, to be better than polymeric membranes. Although there are many examples in the literature, future research is needed to build membranes that can act effectively in media contaminated with a wide range of pollutants, since some of them may deteriorate or not be retained if the membranes used are not properly prepared. It is important to ensure they have a solid structure, appropriate loading, efficient water permeability, and antifouling properties. Their catalytic properties and, therefore, their use for MP degradation is a largely unexplored field. Fenton-like and photodegradation processes that lead to the formation of strongly oxidizing radicals require the presence of iron ions, either in the MOF structure or in solution. This means that not all MOFs can be used. Future studies in the field of MP recovery and removal should be aimed at solving the aforementioned problems in using MOFs by synthesizing more robust materials via green methods with multi-functional properties adaptable to the required needs.

In summary, future research in MOFs should prioritize strategies to mitigate potential environmental and health risks while enhancing their functional performance. One key area is the design and synthesis of MOFs with improved water stability, ensuring they maintain structural integrity under aqueous conditions. Researchers should focus on substituting potentially toxic metals and organic linkers with environmentally friendly and non-toxic alternatives to reduce the risks associated with degradation. Additionally, a deeper understanding of MOF degradation mechanisms is critical to predict and control the release of harmful byproducts, such as metal ions or linker fragments, into water systems.

Long-term studies on the environment and behavior of MOFs are essential to assess their impact on ecosystems and human health. This includes examining their bioaccumulation potential, interactions with aquatic organisms, and the formation of secondary products under various environmental conditions. Furthermore, integrating green chemistry principles, such as renewable feedstocks and solvent-free synthesis, can make MOF production more sustainable.

In parallel, advancements in computational modeling and simulation can provide insights into the design of MOFs with tailored properties, enhancing their durability and safety. Collaborative efforts between material scientists, environmental researchers, and regulatory bodies will play a pivotal role in developing guidelines and standards for the responsible use of MOFs. These interdisciplinary approaches will ensure that MOFs can be safely and sustainably applied in areas such as water purification, including MP adsorption and degradation.