MIL Series in MOFs for the Removal of Emerging Contaminants: Application and Mechanisms

Abstract

In global economic integration and rapid urbanization, the equilibrium between resource utilization efficiency and ecological preservation is confronted with significant challenges. Emerging contaminants have further exacerbated environmental pressures and posed threats to the ecosystem and human health. Metal–organic frameworks (MOFs) have emerged as a prominent area of research in ecological remediation, owing to their distinctive porous configuration, substantial specific surface area, and exceptional chemical stability. The Materials Institute Lavoisier (MIL) series (e.g., MIL-53, MIL-88, MIL-100, MIL-101, and MIL-125) has been shown to effectively promote the separation and migration of photogenerated carriers and significantly enhance the degradation of organic contaminants. This property renders it highly promising for the photocatalytic degradation of emerging contaminants. This paper provides a concise overview of the classification, synthesis methods, modification strategies, and application effects of MIL series MOFs in the removal of emerging contaminants. The advantages and limitations of MIL series MOFs in environmental remediation are further analyzed. Particularly, we offer insights and support for innovative strategies in the treatment of emerging contaminants, including POPs, PPCPs, VOCs, and microplastics, contributing to technological innovation and development in environmental remediation. Future development of MOFs includes the optimization of the performance of the MILs, reducing the high synthesis costs of MILs, applying MILs in real-environment scenarios, and accurate detection of degradation products of environmental pollutants.

1. Introduction

With the acceleration of global industrialization and continued population growth, environmental governance faces unprecedented challenges [1,2], particularly with the emergence of new contaminants that seriously threaten ecosystems and human health [3]. The term “emerging contaminants” is a broad category that includes Persistent Organic Pollutants (POPs), Pharmaceuticals and Personal Care Products (PPCPs), Volatile Organic Compounds (VOCs), microplastics (MPs) and nanoplastics (NPs), and other novel organic contaminants that have not yet been thoroughly investigated [4,5]. Emerging contaminants are characterized by biotoxicity, environmental persistence, and bioaccumulation [6], and their hazards far exceed those of traditional pollutants. Their complex molecular structure and extremely low ecological concentrations (usually at the level of micrograms/liter or even nanograms/liter) have exceeded the detection range of traditional monitoring technologies [7].

These contaminants have become a significant challenge for global environmental governance due to their environmental persistence, bioaccumulation, and potential ecological and health hazards. For instance, POPs, including perfluorinated compounds (PFCs) and polychlorinated biphenyls (PCBs), are resistant to environmental degradation and can bioaccumulate through the food chain, resulting in immune system disorders and reproductive toxicity [5]. PPCPs, derived from antibiotics and cosmetics, are widespread in aquatic environments, potentially contributing to the dissemination of drug-resistant genes [8]. VOCs are not only involved in atmospheric photochemical reactions but also pose potential health hazards due to bioaccumulation [9]. Microplastics and nanoplastics are ubiquitous in oceans, rivers, and soils, exhibiting strong transport capacity and bioaccumulation, which may trigger complex health risks [10,11]. The complexity and diversity of these contaminants necessitate the development of more efficacious management strategies to address the pressing environmental challenges of the present moment.

Removing emerging contaminants, which vary in nature and characteristics, has historically posed a formidable challenge in environmental science. The predominant removal methodologies currently encompass biological treatment, adsorption, and membrane technology [3,12]. Biological treatment relies on microbial metabolism to degrade contaminants; however, the high chemical stability and poor biodegradability of many emerging contaminants limit its efficiency. Adsorption technology utilizes the surface properties of adsorbents to remove contaminants; nevertheless, traditional adsorbents, such as activated carbon, do not have sufficient adsorption capacity for polar or highly water-soluble pollutants. Although it can effectively retain some emerging contaminants, it faces the challenges of membrane fouling and the high cost of membrane contamination. In this context, advanced oxidation technologies (AOTs) have emerged as a pivotal solution for addressing emerging challenges in pollutant removal. These technologies can to degrade complex organic contaminants into smaller molecules or mineralize them completely into carbon dioxide and water by generating strong oxidizing free radicals, such as hydroxyl and superoxide radicals [13]. Among these technologies, photocatalytic oxidation technology has emerged as a particularly noteworthy solution due to its notable advantages, including high efficiency, environmental sustainability, and reduced environmental impact. It has been demonstrated to efficiently degrade contaminants by utilizing photogenerated carriers produced by photocatalysts when exposed to light. These carriers catalyze the generation of free radicals, exhibiting potent oxidizing properties [14].

In recent years, with the development of advanced oxidation technology, metal–organic frameworks (MOFs) have become a research hotspot in the field of photocatalysis for environmental remediation due to their unique porous structure, high specific surface area, and excellent chemical stability [15]. Among them, the Materials Institute Lavoisier (MIL) series (e.g., MIL-53, MIL-88, MIL-100, MIL-101, and MIL-125), as an important branch of MOFs, has attracted widespread attention due to their flexible pore structure, abundant active sites, and diverse synthesis pathways [14,16]. The application of MOFs in photocatalysis is mainly achieved in three ways: First, they are directly used as photocatalysts to degrade contaminants by using their light absorption and charge separation abilities [17]; second, they are compounded with other semiconductor materials to form heterojunction structures to improve the separation efficiency of photogenerated carriers; and third, they are modified by functionalization to enhance their light absorption properties and catalytic activities [18]. However, despite the excellent performance of MIL series MOFs in laboratory studies, they still face many challenges in practical applications. For example, MIL materials have weak visible-light absorption and mainly absorb UV light, which limits the efficiency of utilizing sunlight. In addition, the lack of stability of MIL materials in complex environmental media limits their application in practical pollution control. Therefore, the development of efficient synthesis methods, the optimization of material properties, and the exploration of the application of MIL series MOFs in emerging contaminants treatment have become key research directions in environmental science.

A substantial corpus of approximately 600 journal articles has been published on the application of MILs to emerging contaminants (as demonstrated in Figure 1). These studies underscore the profound research interest in the MIL series of MOFs within emerging contaminants management and reveal their considerable potential in environmental remediation. This review aims to provide a comprehensive and mechanistically focused overview of the application of MIL series MOFs (e.g., MIL-53, MIL-88, MIL-100, MIL-101, and MIL-125) for the remediation of diverse emerging contaminants, including POPs, PPCPs, VOCs, and microplastics, thus highlighting their broad-spectrum potential. Particular attention is given to correlating the structural features of these MIL frameworks—such as the metal node type, ligand functionality, and pore architecture—with their adsorption, photocatalytic, and Fenton-like removal performance, thereby elucidating the structure–activity relationship. In addition, this work integrates recent findings on the hydrolytic stability, regeneration behavior, and performance under environmentally relevant conditions, offering a more realistic understanding of their applicability. Beyond summarizing the laboratory results, this review discusses green synthesis strategies, cost considerations, and challenges in scaling up, providing a forward-looking perspective for the rational design and practical deployment of MIL-based materials in sustainable environmental remediation.

2. Classification of MILs

To date, MILs (MIL = Materials Institute Lavoisier) have attracted considerable attention due to their flexible pore structures, excellent stability, abundant active sites, and diverse synthetic routes [19]. The numerical suffixes of the MILs denote the presence of different metal ions and specific structures in the oxidation state [20]. MILs include a variety of series (MIL-47, MIL-53, MIL-68, MIL-88, MIL-97, MIL-100, MIL-101, MIL-125, MIL-127). These five materials, MIL-53, MIL-88, MIL-100, MIL-101, and MIL-125, were chosen because they are representative in terms of the structural diversity, photocatalytic potential, research maturity, and experimental feasibility (as shown in

Table 1. Overview of the three-dimensional structure and properties of MILs.

MOF	The 3D Structure	Ligand	Central Metal Atom	Typical Shape	Ref.
MIL-53		Terephthalic acid	Fe, Al, Cr	Rod-shaped, rectangular cone-like	[21]
MIL-88		Fumarate, terephthalic acid	Fe	Fusiform, hexagonal rod-shaped	[22]
MIL-100		Tricarboxylic benzene, trimesic acid	Fe, Al, Cr	Octahedron	[23]
MIL-101		Terephthalic acid	Cr, Fe	Octahedron	[24]
MIL-125		Terephthalic acid	Ti	Disc/cake, octahedron	[25]

).

2.1. MIL-53

Férey et al. synthesized and characterized the first three-dimensional chromium (III) dicarboxylate, MIL-53(Cr³⁺) [26]. One of its structures consists of an infinite purely inorganic chain composed of octahedrally coordinated Cr³⁺ that share opposite corners so that Cr³⁺ and OH alternate and extend between neighboring chains via benzene-1,4-dicarboxylic acid cross-linking, and the other structure has the dicarboxylates bonded in the same way. Each metal center is linked to the oxygen of four different carboxylic acid groups from four different linkers [27]. The MIL-53 series is a widely known porous flexible framework for MOFs. Researchers have employed various common metal salts (Al, Fe, Cr) and linkers (terephthalic acid) in the synthesis of these frameworks [28].

2.2. MIL-88

The MIL-88 series has grown in popularity in research circles, owing to its demonstrated capacity to catalyze oxidation with remarkable efficiency [29]. Its structural composition consists of a metal ortho-octahedron, comprising diverse organic linear dicarboxylic acid ligands interconnected with an oxygen-centered trimer. The specific designations MIL-88A, B, C, and D correspond to the following ligands: fumarate, terephthalic acid, 2,6-naphthalene dicarboxylate (2,6-NDC), and 4-4′ biphenyldicarboxylic acid (4-4′ BPDC), respectively [30]. MIL-88, prepared using an iron precursor, became a potential Fenton-like catalyst. Among the various forms of MIL-88, those designated as MIL-88A (Fe) and MIL-88B (Fe) have been observed to possess both exceptional chemical stability and hydrolytic stability [29]. These characteristics have demonstrated their superior removal performance in the context of contaminant removal in water.

2.3. MIL-100

The MIL-100 series is a porous trivalent metal carboxylate, and research reports published the synthesis and structural examination of macro-porous chromium (III) carboxylic acids [31]. Subsequent studies successfully synthesized MIL-100(Cr) by combining Cr³⁺ ions with 1,3,5-benzenetricarboxylic acid (BTC). The metal unit of MIL-100 features an organically connected structure similar to other trivalent metal frameworks, enabling its extension to MIL-100(M) (M = Cr, Fe, Sc, Al, V) [32]. Horcajada et al. initially synthesized iron-based MIL-100 over the course of six days [33]. Following multiple iterations of method refinement, the researchers prepared iron-based MIL-100 under mild conditions, using FeCl₃-H₂O as the iron source and 1,3,5-benzenetricarboxylic acid as the ligand [32].

2.4. MIL-101

The porous chromium terephthalate metal–organic framework MIL-101 was first synthesized and reported in [34]. MIL-101(Cr) and MIL-101(Fe) in the MIL-101 series have been the focus of systematic and extensive investigation, particularly MIL-101(Cr), which is among the most representative and well-studied MOFs in the MIL series [35]. MIL-101 possesses a number of advantageous physicochemical properties, including elevated chemical and thermal stability and a substantial number of coordinated unsaturated metal sites, thereby rendering it superior to a series of materials [36].

2.5. MIL-125

Dan-Hardi’s pioneering synthesis of MIL-125 utilized Ti₈O₈(OH)₄ clusters and terephthalic acid as precursors [37]. MIL-125 has undergone extensive investigation and application as a highly regarded photocatalyst, exhibiting high stability and photocatalytic activity and a rigid framework conducive to strong metal–ligand bonding [38]. MIL-125, a titanium-based metal–organic framework (MOF), exhibits remarkable photocatalytic activity, with the Ti component transforming from Ti⁴⁺ to reduced Ti³⁺ under light irradiation, positioning it as a leading photocatalyst among its Ti-based MOF counterparts [39]. In recent studies, the photocatalytic activity of MIL-125 was further enhanced by heterojunction, loading metal ions, or making functionalization changes [40].

The five metal–organic framework materials previously mentioned possess distinct structural and performance characteristics. Structurally, MIL-53 exhibits a flexible structure and a “breathing effect.” MIL-88 features multiple interconnected pores, while MIL-100 and MIL-101 adopt an MTN topology. MIL-101 further demonstrates a larger pore size. MIL-125 presents a three-dimensional network composed of titanium centers and terephthalic acid ligands. In terms of performance, MIL-101 is distinguished by its notably high specific surface area and favorable thermal stability. Conversely, MIL-53 and MIL-88 exhibit distinctive advantages in the synergistic integration of adsorption and photocatalytic properties. In conjunction with the imperative for managing emerging contaminants, these materials exhibit considerable promise in the degradation of such compounds, a property attributable to their adaptable porosity, substantial surface area, and plentiful active sites. In practical applications, the rational selection and tailored modification of MOFs according to the contaminant type and treatment objectives are essential for achieving optimal remediation efficiency.

3. Synthesis Method of MILs

To date, a wide range of synthesis strategies for MILs has been developed and progressively refined. Among the many synthesis methods, the solvothermal/hydrothermal method has attracted much attention due to its simple operation and other advantages, and it has become the primary method of choice for the synthesis of MILs. In addition to solvothermal/hydrothermal methods, some commonly used synthesis methods, such as steam-assisted synthesis, microwave-assisted synthesis, electrochemical synthesis, and mechanical grinding synthesis (as shown in Figure 2), are also used for the synthesis of MILs, which have their advantages and disadvantages. The target MILs synthesized by different methods may have other shapes and crystal structures. Fallah prepared mercerized zeolite/MIL-101(Cr) using hydrothermal, solvent-thermal, and reflux methods. The characterizations of the mercerized zeolite/MIL-101(Cr) synthesized by different synthetic methods are different [41]. These variations subsequently affect the contaminant removal efficiency. This review will discuss and summarize the aforementioned methods for the preparation of MILs. After this section, a list of green, economical, and efficient synthesis methods will be provided to provide further reference value for reading.

Table 2. MIL preparation methods, synthesis strategies, and properties.

Synthesis Method	MIL	Metal Source	Ligand	Synthesis Conditions	Material Properties	Number of Experimental Cycles	Modification Method	Ref
Solvent synthesis/Hydrothermal synthesis	MIL-53	Cr(NO3)3·9H2O	H2bdc	220 °C, 72 h, acidification by adding 40% HF	Specific surface area of 1050 m2/g	Five times	/	[42]
	MIL-53	FeCl3·6H2O, Ce(NO3)3·6H2O	H2bdc	140 °C, 24 h, 10% citric acid instead of DMF	Specific surface area > 1000 m2/g	Eight times	Carbon-based material loading	[43]
	MIL-88B	FeCl3·6H2O	H2bdc	100 °C, 12 h, add NaOH solution to adjust the Ph of the solution	Specific surface area of 35.05 m2/g	Four times	Amino functionalization	[44]
	MIL-88A	FeCl3·6H2O	Fumarate	65 °C, 12 h	/	Five times	Z-type heterojunction	[45]
	MIL-100	FeCl3·6H2O	H3BTC	95 °C, 18 h	/	Five times	S-type heterojunction	[46]
	MIL-101	Cr(NO3)2	H2bdc	210 °C, 10 h	/	/	Postsynthetic modifier	[47]
	MIL-125	C16H36O4Ti	H2bdc	150 °C, 20 h	Specific surface area of 943.9 m2/g	Five times	Z-type heterojunction	[48]
Microwave-assisted synthesis	MIL-53	FeCl3·6H2O	H2bdc	150 °C, 30 min, 300 W	Specific surface area of 96.33 m2/g	Five times	Carbon-based material load	[49]
	MIL-88B	FeCl3·6H2O	H2bdc	150 °C, 10 min, 800 W	Specific surface area of 47 m2/g	Four times	/	[50]
	MIL-100	FeCl3·6H2O	H3BTC	150 °C, 15 min, 800 W	Specific surface area of 487 m2/g	Twenty times	Z-type heterojunction	[51]
	MIL-100	FeCl3·6H2O	H3BTC	150 °C, 30 min, 300 W	Specific surface area of 1244.62 m2/g	/	Composite modified	[52]
	MIL-101	FeCl3·6H2O	H2bdc	200 °C, 50 min, 400 W	/	Four times	Type I heterozygous junction	[53]
	MIL-101	FeCl3·6H2O	H2bdc	200 °C, 50 min, 600 W	Specific surface area of 960.3 m2/g	Four times	Composite modified	[54]
	MIL-125	C12H28O4Ti	H2bdc	200 °C, 15 min, 600 W	Specific surface area of 1030 m2/g	Four times	Amino functionalization	[55]
Mechanical grinding synthesis	MIL-53	Al2(SO4)3·18H2O	H2bdc	Ball milling frequency 30 Hz, ball milling time 1 h	Specific surface area of 1143 m2/g	/	/	[21]
	MIL-53	AlCl3·6H2O	H2bdc	Ball milling frequency 30 Hz, ball milling time 1 h	Specific surface area of 638.96 m2/g	Four times	Metal ion doping	[56]
	MIL-88A	FeCl3·6H2O	Fumarate	Ball milling at 200 °C for 5 h at a rate of 5 °C/min.	Specific surface area of 723.17 m2/g	Three times	/	[57]
	MIL-100	CrCl3·6H2O	H3BTC	Grinding at room temperature for 40 min, heating at 220 °C for 15 h	Specific surface area of 1557 m2/g	/	/	[58]
	MIL-100	Fe(NO3)3·9H2O	H3BTC	Continuous grinding at room temperature for 3–5 h	Specific surface area of 753 m2/g	/	/	[59]
Electrochemical synthesis	MIL-88A	Iron plate electrode	Fumarate	Mixed solution of 50% H2O and 50% ethanol as electrolyte 15 V voltage, 30 min	Specific surface area of 128.5 m2/g	Three times	Magnetic material load	[60]
	MIL-100	TiCl4	H3TATB	−1.20 V potential, 120 °C, 18 h	Specific surface area of 5842 m2/g	/	/	[61]
	MIL-101	TiCl4	H2BPDC	−1.20 V potential, 120 °C, 18 h	Specific surface area of 3263 m2/g	/	/	[61]
	MIL-101	FeCl3·6H2O	H2BDC-NH2	TBATFB electrolyte, 15 V, 30 min	Specific surface area of 139 m2/g	/	Amino functionalization	[62]
	MIL-101	Iron plate electrode	H2BDC	TBATFB electrolyte, 15 V, 30 min	Specific surface area of 131 m2/g	/	/	[63]
	MIL-101	FeCl2	H2BPDC	+0.75 V potential, room temperature, 8 h	Specific surface area of 1918 m2/g	/	Amino functionalization	[64]
Ultrasound-assisted synthesis	MIL-53	Fe(NO3)3·9H2O	H2BDC-NH2	80 °C, 1 h, 305 W	Specific surface area of 179.9 m2/g	/	Amino functionalization	[65]
	MIL-88	Fe(NO3)3·9H2O	H2bdc	Ultrasonic treatment 0.5 h, heating at 85 °C for 12 h	/	/	Magnetic material load	[66]
	MIL-88A	FeCl3·6H2O	Fumarate	Room temperature, 4 h, 70% power	/	Three times	/	[67]
	MIL-100	FeCl3·6H2O	H3BTC	150 °C, 10 min, 1080 W, 20.5 kHz	Specific surface area of 1033 m2/g	/	/	[68]
	MIL-101	Cr(NO3)3·9H2O	H2BDC	Room temperature, 1 h, 100 W, 30 kHz	/	Four times	/	[69]

lists the preparation methods, synthesis strategies, and properties of the MIL series of materials.

3.1. Solvothermal Synthesis/Hydrothermal Synthesis

Solvothermal and hydrothermal syntheses are two frequently employed synthetic methods for the preparation of MILs. The fundamental principles underpinning these methods are analogous, with both relying on elevated temperatures and pressures within a closed system to facilitate the desired reactions [26,70]. The solvothermal/hydrothermal synthesis method can efficiently synthesize complex materials, adjust reaction conditions, perform large-scale synthesis, and be applicable to a wide range of materials. Valekar et al. used the organic ligand H₂BDC (terephthalic acid), the metal source Al(NO₃)₃⋅9H₂O (Aluminum nitrate nonahydrate), and the solvent DIW (deionized water), which were stirred and dissolved. The mixture was transferred to a stainless steel autoclave (lined with Teflon) to provide a hermetically sealed high-pressure environment at 493 K for 3 days [71]. In the synthesis of certain MIL series, auxiliary reagents are added to regulate the reaction environment, such as to control the acidity or alkalinity of the solution or promote the formation of crystals. Examples include NaOH [72] (alkaline modifier, used in the synthesis of MIL-88) and HF [73] (hydrofluoric acid, used in the morphological control of MIL-101). However, it should be noted that the synthesis process may result in the production of byproducts or impurities, which can potentially compromise the purity of the final product [74]. It has been demonstrated that when organic solvents are employed, solvent residues and impurities can deleteriously affect the final photocatalytic performance, stability, and other relevant parameters [75].

3.2. Microwave-Assisted Synthesis

Microwave-assisted synthesis is a technique that facilitates the synthesis of MILs by directly heating solvents and reactants in the reaction system through microwave radiation. In the microwave process, the reaction system is rapidly heated by dielectric heating (the solvent or reactant molecules absorb energy in the microwave electric field, and the polar molecules (e.g., water, DMF) heat up because of the polarization oscillation) and dipole heating [76] (the polar molecules in the alternating electric field due to the change of direction and friction and collision, the release of heat energy), acting directly on the solvent and substances in the reaction system. In comparison with conventional hydrothermal and hot-solvent methodologies, microwave-assisted synthesis has been shown to markedly reduce the reaction times, enhance the energy utilization efficiency, render the synthesis process more environmentally friendly and economical, and yield MILs with high crystallinity [52]. This method has garnered significant attention in MIL research and development in recent years, and it is particularly well-suited for the rapid preparation of high-performance MIL materials [77]. Pouyanfar et al. employed a modified microwave synthesis to prepare MIL-100 (Fe) by dispersing ferric chloride and benzene tricarboxylate in deionized water. They optimized the synthesis conditions to obtain the best crystallinity and particle size by adjusting the microwave maintenance time (5.5–15 min) at 1000 W power [17]. In the synthesis of MILs using microwave, it is imperative to meticulously control the reaction parameters of temperature, time, and power level to ensure the success of the synthesis and the quality of the material.

3.3. Mechanical Grinding Synthesis

Mechanical grinding is a green synthesis technology that induces chemical reactions in solid reactants through mechanical forces (e.g., grinding, ball milling, etc.) [78], and its core principle lies in the use of friction, shear, and compression by mechanical forces to disrupt the crystal structure of the reactants and to promote the coordination reaction between the metal ions and the organic ligands, while the localized high-temperature and high-pressure environment can further accelerate the bond cleavage and reorganization [79]. The method mainly comprises two forms: solvent-free mechanical grinding and liquid-assisted mechanical grinding (LAG) [78]. The former relies entirely on mechanical force to drive the reaction without solvent intervention, while the latter involves the addition of trace amounts of polar solvents (e.g., water, ethanol) as a reaction medium to enhance the efficiency of the reaction and the crystallinity of the products. In one study, Souza et al. synthesized MIL-100(Fe) through two mechanochemical methods: manual grinding (MG) and vortex grinding (VG). The MG method entailed the manual grinding of 1,3,5-benzenetricarboxylic acid (H₃BTC), ferric nitrate (Fe(NO₃)₃), and 5-FU for a duration of 1–2 h using an onyx mortar and pestle. VG involves the processing of the mixture in a 3D-printed, customized, and fixture-fixed polypropylene container. These containers undergo rotation at elevated speeds in a vortex mixer, thereby increasing the frequency of collisions and facilitating the reaction [59]. The mechanical grinding synthesis has been demonstrated to have significant advantages in MIL synthesis. However, it has also exhibited certain shortcomings, including low crystallinity of the product, poor experimental reproducibility due to the wear and tear of the equipment, an unclarified reaction mechanism, and structural inhomogeneity and dust problems that may occur in large-scale production [80].

3.4. Electrochemical Synthesis

The electrochemical synthesis of MILs is a green synthesis technique that generates MILs directly on the surface of electrodes or in solution through electrochemical reactions [81]. The core principle of this technique is to utilize anodic oxidation to dissolve metal electrodes (e.g., Zn, Cu, Fe, etc.) [62,82,83], releasing metal ions that subsequently coordinate with organic ligands (e.g., carboxylate and imidazole ligands) in solution [84,85]. The coordination reaction occurs, and MILs crystals are eventually formed. The method principally consists of two forms: anodic dissolution and cathodic reduction. The former generates metal ions through anodic oxidation, while the latter modulates the reaction environment through cathodic reduction to promote MILs’ growth. The electrochemical synthesis boasts several notable advantages, including mild reaction conditions, controllability, the absence of the need for elevated temperatures and pressures, and its environmental friendliness. This renders it particularly well-suited for the synthesis of heat-sensitive MILs [64]. Larasati et al. synthesized NH₂-MIL-101(Fe), 2-aminoterephthalic acid (NH₂-H₂BDC), and tetrabutylammonium tetrafluoroborate (TBATFB), where the initial step involved the dissolution of these components in N, N-dimethylformamide (DMF). The mixture was then subjected to stirring for a duration of 30 min, which was deemed sufficient for complete dissolution. Subsequently, the mixture was transferred into an electrochemical cell comprising two iron plates serving as electrodes, where it underwent electrolysis for a duration of 30 min at an applied voltage of 15 V. During electrolysis, Fe²⁺ was oxidized to Fe³⁺, which coordinated with the NH₂-H₂BDC ligand to produce NH₂-MIL-101 (Fe) [62]. However, its limitations include a slower reaction rate [81], potentially lower crystallinity of the product [86], limited choice of electrode materials, and difficulty in scale-up production. In recent years, electrochemical synthesis has demonstrated promising applications in the fields of functionalized MILs synthesis [64], thin-film MILs preparation, and electrochemical sensors. It is expected that, through the optimization of electrode materials, reaction conditions, and electrolyte composition, this technology will further promote its practical application in the fields of catalysis and separation [87].

3.5. Ultrasound-Assisted Synthesis

Ultrasound-assisted synthesis is a green synthesis technique based on ultrasound energy-driven synthesis. This technique significantly accelerates the dynamic coordination process of metal ions with organic ligands [88] through the cavitation effect triggered by its propagation in liquids, microjets, and localized extreme physical conditions (e.g., transient high temperature of ~5000 K and high pressure of ~1000 atm) [89]. The result of this process is the efficient preparation of MILs. The core mechanism encompasses three aspects: the cavitation effect (release of energy from the violent collapse of bubbles in the liquid, which promotes molecular collisions and chemical bond reorganization of reactants); the microjet effect (enhancement of the mixing homogeneity of the reaction system, avoiding local concentration inhomogeneity) [90]; and mechanical vibrational effect (disruption of the crystal structure of the reactants, increasing the reactivity) [91]. The experimental procedure typically involves the dissolution of metal salts and organic ligands in a solvent, followed by ultrasonic irradiation treatment (frequency 20–40 kHz, power 50–500 W, duration minutes to hours). This is followed by centrifugal separation of the products and subsequent drying and activation steps [92]. The reaction time is brief; for instance, Ge can be synthesized within 25 min. The synthesis of MIL-53 is much faster than the 24 h required by the traditional solvothermal synthesis, with mild conditions [93] (room temperature or low-temperature operation for heat-sensitive ligands or substrates). The product’s high crystallinity is promoted by the cavitation effect, which facilitates homogeneous nucleation. The method is also environmentally friendly, as it requires reduced solvent usage, making it particularly suitable for rapidly preparing various MOFs and their composites and their composites, such as ZIF, MIL, and UiO, among others. [89]. However, its limitations include the complex optimization of ultrasound parameters (e.g., frequency, power, time) (94) and difficulty in scale-up production [94].

3.6. Other Synthetic Methods

In addition to the widely used synthesis methods mentioned above, various innovative strategies exist for synthesizing MIL series. These emerging synthetic methods offer significant efficiency, cost, and environmental protection advantages. Furthermore, they can optimize the properties and structures of MILs to meet the needs of applications in different fields. As research progresses, scientists continue to explore more efficient, controllable, and environmentally friendly synthesis pathways to overcome the limitations of traditional methods. For instance, Baluk and his team employed a thermal injection method for synthesizing NH₂-MIL-125(Ti) in octahedral shape and compared it with the conventional solvothermal synthesis. The thermal injection method substantially reduced the synthesis time, from the traditional 24 h to a mere 4 h, while preserving high crystallinity and specific surface area. This method enhanced the synthesis efficiency and was carried out at reduced temperatures and pressures, exhibiting reduced energy consumption and increased environmental sustainability. However, concerning the photocatalytic performance, both exhibited comparable efficiencies (phenol photocatalytic degradation efficiencies of 59.2% and 56.0%, respectively) [95]. Tan et al. successfully prepared MIL-100(Fe) using an aqueous-based non-thermal synthesis method. This synthesis method does not require high temperatures or pressures, nor does it necessitate strong corrosive reagents (e.g., hydrofluoric acid). It is characterized by its environmental sustainability, simplicity, and efficiency. The synthesized MIL-100(Fe) material exhibits a high specific surface area (1893 m²/g), excellent thermal stability, and good aqueous stability [16]. Unlike conventional methods, emerging synthesis methods offer distinct advantages, including reduced processing time, mild reaction conditions, controllable crystallinity and morphology, and streamlined post-processing.

4. Modification of MILs

Conventional MILs have demonstrated removal efficiency, selectivity, and stability limitations. Consequently, there is a growing need to optimize their performance through different modification strategies. Researchers have made substantial advancements in the removal capacity of MILs for organic contaminants and heavy metal ions through various methodologies, including surface modification, post-synthesis modification, doping, and heterojunction. In the domain of environmental remediation, for instance, the specific surface area of MIL-88A(Fe) before modification was 15.9 m²/g, exhibiting weak visible light absorption and low efficiency in the photocatalytic degradation of Rhodamine B (RhB). However, the modification of MIL-88A(Fe) with ultrathin graphite oxide (GO) has substantially enhanced its surface area, reaching 408.9 m²/g. This modification has resulted in enhanced visible-light absorption, accompanied by an 8.4-fold increase in photocatalytic efficiency and the retention of high efficiency over multiple cycles. This enhancement in performance is particularly noteworthy when compared to the performance of the unmodified material [96].

4.1. Elemental Doping

4.1.1. Transition Metals

Transition metals exhibit a range of oxidation states and distinctive electronic configurations. Introducing dopants into MILs can result in the formation of novel active sites. These active sites can adsorb and activate reactant molecules, thereby reducing the activation energy of the reaction and increasing the reaction rate. Liu et al. demonstrated a substantial enhancement in tetracycline (TC) degradation efficiency by applying activated PMS in conjunction with Co-doping MIL-53(Al). The degradation efficiency of TC reached 94% in the 25% Co-MIL-53(Al)/PMS system, which was significantly higher than that of the undoped MIL-53(Al)/PMS system (66%) and PMS alone (26.9%) [97]. In the domain of photocatalysis, the d-orbital electronic properties in MIL-doped transition metals have been observed to trap photogenerated electrons or holes, thereby forming shallow potential wells and hindering the formation of photogenerated carrier recombination.

Furthermore, transition metals, as electron acceptors, have been demonstrated to facilitate carrier separation, thereby enhancing the photogenerated charge migration efficiency through the metal–ligand charge transfer (MLCT) mechanism [98]. Wang et al. synthesized the composite catalyst Ru@NH₂-MIL-125/MnO, doped with Ru nanoparticles, via hydrothermal synthesis. The composite catalyst exhibited a 10-fold increase in photocatalytic efficiency compared to the unaltered pristine NH₂-MIL-125 [99]. The doped Ru became a reactive site for photocatalysis, increasing the photogenerated electron and hole transfer rates and effectively reducing the electron–hole recombination rate. The transition metal doping enhanced the light absorption, augmented the effective charge transfer, and facilitated separation.

4.1.2. Main Group Elements

Significant progress has been made in doping main-group elements (e.g., sulfur, nitrogen, phosphorus) into MILs in recent years. These doping modifications have enhanced the catalytic performance of the materials and optimized their applications in environmental treatment, gas adsorption, and energy storage, among others. Du et al. have synthesized a novel catalyst of S-MIL-53(Fe) via sulfur sublimation (Figure 3), and the S doping has been shown to increase both the pore size of the catalyst and its degradation capacity for the pollutant oxytetracycline (OTC) [100]. While the doping modifications have become a prevalent strategy to enhance the performance of catalysts, their impact is not universally beneficial. This complexity stems from the interaction between the dopant element and the catalyst matrix and the effect of doping on various aspects of the catalyst’s electronic structure, surface morphology, and pore structure. For instance, George et al. examined the impact of cation doping (Li⁺, Na⁺, K⁺) on the photocatalytic performance of MIL-53(Fe), focusing on the disparities in photocatalytic activity before and after doping [101]. The results obtained from this investigation demonstrate that the photocatalytic performance of MIL-53(Fe) diminishes considerably, despite cation doping reducing the band gap energy in the band gap energy. This phenomenon is attributed to the doping-induced structural distortion and the weakening of electron transport capacity.

4.1.3. Lanthanides

Doping lanthanide elements (e.g., lanthanum, cerium, neodymium, gadolinium) into MILs represents an effective strategy to enhance their functionality by modulating the electronic structure, pore environment, and active sites. Notably, certain lanthanides (e.g., Ce³⁺/Ce⁴⁺, Eu²⁺/Eu³⁺) possess variable valence states, which can be utilized as redox-active sites. Their substantial ionic radii (e.g., the ionic radii of La³⁺ are approximately 1.06 Å) which facilitates the formation of stable high coordination number (typically 8–12) coordination bonds with oxygen- and nitrogen-containing ligands. Jiang et al. have demonstrated that lanthanum (La)-doped NH₂-MIL-53 significantly modified the structure and performance of the initial catalyst (Figure 4), exhibiting remarkable efficacy in the removal of arsenic (V) and phosphate from water [102]. Wan et al. optimized the electronic structure of iron-based metal–organic framework material (NH₂-MIL-101) by gadolinium (Gd) doping to improve the degradation performance of arsenic (V) and phosphorus from water and to realize electrochemical regeneration [103]. While lanthanide doping has been shown to enhance the performance of materials, it does result in a cost increase. However, given the performance enhancement observed (e.g., higher catalytic activity, enhanced photoluminescence properties, and improved stability), the additional cost can be justified in specific application scenarios where high-performance materials are required.

4.2. Ligand Functionalization

The ligand functionalization modification of the MIL series is a critical strategy for optimizing their properties by introducing specific organic ligands. This modification approach enables the regulation of the pore structure and surface properties of MILs, thereby significantly enhancing their applications in fields such as gas adsorption, separation, catalysis, and environmental remediation. As illustrated by MIL-125(Ti) [95], the post-synthesis modification synthesis involving the grafting of reactive group-containing ligands onto the backbone of MILs can remarkably enhance the photoresponsivity and electron transfer efficiency of the material. The introduction of these ligands, which exhibit exceptional light absorption properties, was facilitated by a solvothermal synthesis, and the successful loading of the ligands was confirmed using characterization techniques, including Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD).

4.2.1. Amino-Functionalized Ligand

In environmental remediation, amino-functionalized ligand-modified MIL series have substantially enhanced their capacity to degrade organic contaminants by introducing amino (-NH₂) functional groups. The fundamental premise of this modification strategy is that the introduction of amino functional groups can modify the materials’ surface electron distribution and chemical properties, thereby enhancing their interactions with target molecules. Mo et al. examined phenol degradation by photocatalytic materials with amino-functionalized groups (NH2-MIL-101(Fe, Ce)) in their investigation. These materials exhibited an accelerated generation of multiple reactive radicals due to the presence of an amino-functionalized group (-NH₂) as an electron donor. The subsequent interaction of these radicals resulted in the efficient degradation of phenol [18]. In a separate study, Mo et al. investigated the defect level of NH₂-MIL-125(Ti) by systematically controlling the duration of the heat treatment. The 125(Ti) defect level was successfully managed by regulating the duration of the heat treatment process. This resulted in the gradual decarboxylation of ligands in NH2-MIL-125(Ti), occurring from the surface to the interior [104]. Consequently, this process led to the formation of ligand vacancy defects with varying concentrations. This, in turn, enhanced its photocatalytic performance, thereby facilitating the efficient photocatalytic reduction of Cr (VI).

4.2.2. Sulfonated-Functionalized Ligands

The sulfonic acid functional group, which has the chemical formula -SO₃H, contains a sulfur atom at its core. This atom is attached to three oxygen atoms, forming a planar structure. One of the oxygen atoms is connected to a hydrogen atom via a single bond, forming the acidic -SO₃H group. In a study by Nguyen et al., the epichlorohydrin conversion was 68.9% when using SO₃H-MIL-101 as the catalyst alone. This conversion was further increased to 100% after functionalization with amine groups. This observation indicates that the sulfonic acid group possesses intrinsic catalytic activity and the capacity to potentiate the catalytic efficiency when combined with other functional groups, such as amine groups [105]. The functionalization of sulfonic acid groups has been demonstrated to enhance the water stability of the material. In the study by Zhao et al., a sulfonic acid group-functionalized metal–organic framework was utilized to remove iodide ions (I-) from water. The sulfonic acid group (-SO₃H) was bonded to the MIL-101(Cr) framework through a strong covalent bond. The stability of this chemical bond ensured the material’s structural integrity in the aqueous environment and reduced its susceptibility to chemical degradation [106]. The functionalization of sulfonate and amino groups has been demonstrated to enhance the performance of photocatalysts through various chemical interaction mechanisms. The functionalization of the sulfonate group has been demonstrated to enhance the charge transfer kinetics and surface active sites, primarily through electron-withdrawal-induced effects. In contrast, amino group functionalization has been shown to improve the electron transfer efficiency and stability by providing electron donors and forming hydrogen bonds [107]. These functionalization approaches offer essential strategies for designing and developing efficient photocatalysts.

4.2.3. Other Group-Functionalized Ligands

In addition to amino (-NH₂) and sulfonic (-SO₃H) groups, the MIL series can be modified with various functionalized groups to modulate their physicochemical properties and expand their applications. Yu et al. synthesized the hydroxyl (-OH)-functionalized MIL-100 by selecting hydroxyl-rich 2,5-dihydroxyterephthalic acid (H₄DODBC) as the ligand functionalized MIL-100. The experimental results demonstrated that NiCo-ZIF@MIL exhibited excellent stability in KOH solution, with nearly unchanged potentials after 1000 cycles, and the catalytic activity could be maintained for at least 24 h [108]. Anindita et al. prepared a catalyst (p-TSA-MIL-100@SBC) functionalized with toluene sulfonic acid (p-TSA). The purpose of introducing P-TSA was twofold: firstly, to enhance the structural stability of MIL-100(Fe) in an aqueous environment, thereby preventing its collapse; secondly, to inhibit the recombination of photogenerated electron–hole pairs through its sulfonic acid moiety, which acts as an electronic hub. This effectively inhibits the recombination of photogenerated electron–hole pairs, prolongs the lifetime of photogenerated carriers, and significantly enhances the photocatalytic activity [109].

4.3. Structural Heterogeneity

A heterojunction is an interfacial structure composed of two or more different materials (typically semiconductors) that differ in energy band structure, lattice constants, or electronic properties. This results in unique physicochemical properties at the interface. The categorization of these junctions can be approached through various criteria, including the type of conductivity (homojunctions and heterojunctions) [110,111], the energy band structure (type I, II, III) [112], the structural form (planar and perpendicular) [113,114], the material composition (semiconductor–semiconductor, metal–semiconductor, 2D materials, and nanomaterials) [115], and exceptional functionality (e.g., Z-scheme and S-scheme heterojunctions) [116,117]. These classifications reflect the diverse properties of heterojunctions in terms of the charge separation, light absorption, and electron transport, which have enabled them to demonstrate a wide range of applications in the fields of optoelectronic devices, semiconductor devices, sensors, and catalyst design [118]. The fundamental properties of these junctions are attributed to the electric potential barriers [119], energy band bending, quantum effects, and spectral properties present at the interfaces. These elements directly influence their performance in photovoltaic conversion, current control, and catalytic activity. In recent years, with the advent of two-dimensional materials and nanotechnology, there has been a remarkable surge in research endeavors concerning heterojunctions, particularly in the domain of novel structure design (e.g., S- and Z-scheme heterojunctions) and high-performance applications.

4.3.1. The Z-Scheme Heterojunctions’ Structure

Z-scheme heterojunctions comprise two or more semiconductor materials that, upon photoexcitation, facilitate the migration of electrons and holes to the valence band (VB) and conduction band (CB) of another semiconductor, thereby establishing a transmission path that resembles the shape of a Z. The design of this structure, which draws inspiration from the electron transport chain in plant photosynthesis, aims to emulate the efficient light energy conversion mechanism observed in nature [120]. This structure retains the high redox capacity of semiconductors, which allows preserving the strong redox potential by spatially separating electrons and holes, thus improving the separation efficiency of photogenerated carriers. Z-scheme heterojunction photocatalysts have been shown to be highly effective in the treatment of contaminants, such as antibiotic residues in water, with both high efficiency and environmental friendliness [116]. Li et al. prepared Z-scheme heterojunction materials (MIL-100(Fe)/TpPa-1 COF, MT2 for short) by in situ solvothermal synthesis (Figure 5). Before light irradiation, charge redistribution at the interface resulted in the formation of an interfacial electric field. In the presence of light, the photogenerated electrons in MIL-100(Fe) were transferred to the valence band of TpPa-1 COF due to the action of interfacial electric field, and the recombination with holes was suppressed, thereby achieving efficient charge separation. Furthermore, MT2 demonstrated its efficacy by removing 91% of TC (20 mg/L) within 120 min, thereby exhibiting its superior photocatalytic degradation performance [121].

4.3.2. The S-Scheme Heterojunctions’ Structure

The S-scheme heterojunction is a recently discovered semiconductor heterostructure that has demonstrated the capacity to enhance the performance of photocatalysis and other photovoltaic applications by optimizing the charge separation and transport pathways. In comparison with conventional type-II and Z-scheme heterojunctions, S-scheme heterojunctions have been demonstrated to enhance the photocatalytic efficiency by recombining electron–hole pairs with low reduction/oxidation capacity and retaining carriers with high reduction/oxidation capacity to convert organic contaminants into nontoxic inorganic substances [122]. Dong et al. employed a hydrothermal synthesis to synthesize MIL-53(Fe)/FeOCl composites. MIL-53(Fe) is an n-type semiconductor with a high degree of light absorption and a large number of active sites for iron oxide clusters. FeOCl is also an n-type semiconductor that exhibits excellent photovoltaic conversion properties and a suitable energy-band structure. The SEM and TEM results demonstrate that FeOCl is distributed uniformly as nano-sheets on the surface of MIL-53(Fe), forming a close contact heterojunction structure. XPS analysis reveals that the introduction of FeOCl results in alterations in the binding energies of Fe 2p and O 1s, confirming the formation of a heterojunction and electron transfer. The composites demonstrated remarkable photocatalytic degradation performance under visible light conditions, with a degradation efficiency of oxytetracycline (OTC) reaching 90%, which was significantly higher than that of MIL-53(Fe) alone (54%) [123].

4.3.3. Double S-Scheme Heterojunctions’ Structure

The double S-scheme heterojunction is a specialized structure composed of two S-scheme heterojunctions, typically involving three or more semiconductor materials [124]. The core feature of this structure is the efficient separation and transfer of photogenerated carriers (electrons and holes) by constructing two stepped energy band structures. In the double S-scheme heterojunction, the photogenerated electrons and holes are transferred between the conduction band (CB) and valence band (VB) of different materials, forming two independent charge transfer paths. This configuration enhances the separation efficiency of the photogenerated carriers and augments the redox capacity of the photocatalyst, thereby significantly improving the photocatalytic performance [125]. Patial et al. successfully synthesized Co₃O₄/MIL-88A/Mn-STO composites. The material consists of three semiconductors: MIL-88A and Mn-STO were prepared hydrothermally to form a single S-scheme heterojunction; then, Co₃O₄ was introduced to form a double S-scheme heterojunction. The XRD and FTIR results demonstrated that the components maintained their crystalline structure, and the diffraction peaks corresponded to those of the individual materials, indicating the successful construction of heterojunctions (Figure 6). In the photocatalytic degradation of sulfamethoxazole (SMX), the composite exhibited a degradation efficiency of 95.5% within 90 min, which was significantly higher than that of the material alone and the binary composite. The catalytic degradation rate constant (k) of the composite was determined to be 0.0337 min⁻¹, which is notably higher than those of Co₃O₄ (8×), Mn-STO (5.2×), MIL-88A (4.6×), and MIL/Mn-STO (3.6×), respectively [126].

4.3.4. Stability and Recyclability

Through the aforementioned strategies of elemental doping, ligand functionalization, and structural heterogeneity, the adsorption capacity, photocatalytic efficiency, and selectivity of MIL series MOFs toward specific pollutants have been significantly enhanced. However, the ultimate goal of these modification strategies is not only to improve the single-cycle reaction efficiency but also to ensure that the materials maintain long-term structural stability and reusability under complex aqueous environments, which is crucial for practical engineering applications. Therefore, it is necessary to systematically evaluate the chemical stability and recyclability of the modified MIL series materials. In an electro-Fenton system, the D-MIL-53(Fe/Ce)/400@GF catalyst exhibited excellent cyclic stability, maintaining 90.52% of the pollutant removal efficiency after eight consecutive degradation cycles, while the leaching of Fe and Ce remained well below the internationally accepted safety thresholds, indicating outstanding chemical robustness (Figure 7a). This stability is primarily attributed to the introduction of framework defects and the optimization of electron transfer pathways, which effectively preserved the catalytic activity during repeated use. Similar improvements in stability have been observed for other modified MOF-based photocatalysts. For instance, the DNMB-5 photocatalyst exhibited only a slight decrease in degradation efficiency after four consecutive cycles, owing to defect engineering and the synergistic effect of oxygen vacancies, which not only suppressed active-site deactivation but also enhanced the separation efficiency of photogenerated charge carriers (Figure 7b). Likewise, the MTO heterojunction catalyst maintained high photocatalytic activity during 36 h of continuous degradation tests, a result ascribed to the construction of a Z-scheme heterostructure and the incorporation of conductive additives, which significantly promoted electron–hole separation and transfer (Figure 7c). Notably, the MIL-100(Fe)/TiO₂/CoOx₂₀ composite photocatalyst demonstrated excellent durability in photocatalytic oxygen evolution experiments; when the Co loading corresponded to 20 atomic layer deposition (ALD) cycles, the material achieved the highest oxygen evolution rate, and after 18 h of cycling, it still retained 91.3% of its activity with no significant morphological changes (Figure 7d). These results suggest that the synergistic combination of structural heterogeneity design and precise ALD control greatly enhances the cycling stability and structural integrity of the material.

5. The Utilization of MIL Materials in the Removal of Emerging Contaminants

The MIL series of metal–organic framework (MOF) materials remove emerging contaminants through a multi-level multi-mechanism synergistic process. Their highly porous three-dimensional framework provides an ultrahigh specific surface area and abundant active sites, allowing pharmaceuticals, microplastics, volatile organic compounds, and other organic pollutants to be strongly adsorbed via hydrogen bonding, π–π stacking, electrostatic interactions, hydrophobic interactions, and van der Waals forces within the pores and on the surface. This adsorption not only concentrates pollutants locally at the material surface but also positions them at catalytic reaction sites for subsequent degradation. Under light irradiation, the metal nodes (e.g., Fe³⁺, Cr³⁺) and organic ligands of MIL form photoactive centers, where photon energy excites electrons from the valence band to the conduction band, generating photogenerated electron–hole pairs (e⁻/h⁺). The photogenerated electrons react with dissolved or surface oxygen to produce superoxide radicals (O₂·⁻), which can further generate hydrogen peroxide (H₂O₂) and subsequently hydroxyl radicals (·OH), while photogenerated holes oxidize water or hydroxide ions to generate additional ·OH; part of the energy can also generate singlet oxygen (¹O₂). These reactive oxygen species (ROS) attack chemical bonds such as C–C, C–N, C–O, and aromatic rings in pollutant molecules, breaking them down into smaller intermediates and ultimately mineralizing them to CO₂, H₂O, and nontoxic ions. The framework structure and the synergy between metal nodes and organic linkers promote electron–hole separation, suppress recombination, and enhance ROS generation, thereby significantly improving the photocatalytic efficiency. Moreover, the adsorption and photocatalytic processes of MIL can synergize with biodegradation or chemical oxidation, for example by activating H₂O₂ or ozone, or enhancing microbial degradation, further promoting pollutant removal. Specific pollutant interactions include pharmaceuticals, which are adsorbed through hydrogen bonding and π–π stacking and oxidatively degraded at aromatic rings and functional groups; microplastics, whose surfaces are adsorbed and oxidatively cleaved by ROS; and VOCs or dyes, which undergo photocatalytic oxidation or mineralization. In summary, MIL materials achieve rapid and complete pollutant degradation through a multi-level multi-mechanism process—comprising efficient adsorption, ROS-mediated photocatalysis, pollutant decomposition, and synergistic enhancement—making them highly effective for a broad range of emerging contaminants and providing a theoretical basis for designing high-performance multifunctional MOFs (see

Table 3. Summary of MIL photocatalytic removal of emerging contaminants.

Classification of Contaminants	MIL Names	Synthesis Method	Modification Method	Target Contaminants	Initial Contaminant Concentration	Catalytic Degradation Times	Catalytic Degradation Temperature	Efficiency	Environmental Adaptation	Cycling Stability	Ref.
POPs	NH2-MIL-53(Al)/CdS	Hydrothermal synthesis	Amino functionalization, heterojunction	Trichlorophenol	10 mg/L	180 min	Room temperature	98.85%	Stable at 410 °C	Five times	[127]
	Fe3O4@Nb2CTX@NH2–MIL-88	Solvothermal synthesis	Amino functionalization, heterojunction	Polychlorinated biphenyl	0.005–50 μg/L	60 min	25 °C	87.60%	/	/	[128]
	MIL-100(Fe)@C3N4	Hydrothermal synthesis	Heterojunction	Perfluorooctanoic acid	10–150 μg/L	6 h	Room temperature	60.50%	/	/	[129]
	NH2-MIL-101(Cr)	Solvothermal synthesis	Amino functionalization	Perfluorooctanoic acid	10–50 mg/L	/	/	698.4 mg/g	/	Seven times	[130]
	CdS@NH2-MIL-125	Solvothermal synthesis	Amino functionalization, heterojunction	Carbofuran	10 g/L	90 min	Room temperature	98.40%	/	Five times	[131]
	NH2-MIL-125	Hydrothermal synthesis	Amino functionalization	Perfluorooctanoic acid	100 μg/L	24 h	20 °C	98.9%	/	Three times	[132]
PPCPs	MIL-53(Fe)	Microwave-assisted synthesis	Carbon material loads	Ciprofloxacin	50–150 mg/L	60 min	25 °C	98.53%	The effect is less in conditions where inorganic ions are present	Five times	[49]
	Co-MIL-53(Al)	Mechanical grinding synthesis	Doping of elements	Tetracycline hydrochloride	10–120 mg/L	30 min	25 °C	96.10%	Less effect in the presence of inorganic ions	Four times	[56]
	MIL-88A	Ultrasound-assisted synthesis	/	Benzoxazole	/	1 h	25 °C	84%~96%	/	/	[67]
	MIL-88A(Fe)/Ti3C2/MoO3	Solvothermal synthesis	Heterojunction	Tetracycline hydrochloride	100 mg/L	120 min	25 °C	94.90%	/	/	[45]
	Fe3O4@MIL-100(Fe)	Microwave-assisted synthesis	Magnetic material loads	Diclofenac sodium	60 mg/L	3 h	25 °C	99.40%	//	Five times	[52]
	MIL-100(Fe)/TpPa-1 COF	Solvothermal synthesis	Heterojunction	Tetracycline hydrochloride	20 mg/L	120 min	Room temperature	91%	36 h of continuous catalytic	Five times	[121]
	g-C3N4/MIL-101(Fe)	Hydrothermal synthesis	Heterojunction	Enrofloxacin	10 mg/L	60 min	25 °C	100%	Degradation efficiency above 80% at pH 3 and 9	Five times	[116]
	MIL-100(Fe)/CoFeLMO/PEG	Hydrothermal synthesis	Heterojunction	Ranitidine	1–10 mg/L	1 min	Room temperature	99.50%	Extreme pH conditions maintain 99.5% degradation efficiency	Twenty times	[133]
	MIL-100(Fe)/TpPa-1 COF	Solvothermal synthesis	Heterojunction	Tetracycline hydrochloride	20 mg/L	120 min	Room temperature	91%	36 h of continuous catalytic	Five times	[121]
	MIL-125(Ti)/g-C3N5	Solvothermal synthesis	Heterojunction	Sulfadiazine	0.04 mM	15 min	25 °C	95.60%	Degradation rate of 64.5% at pH 3	Five times	[134]
VOCs	MIL-88A(Fe)@BiVO4	Hydrothermal synthesis	Heterojunction	Formaldehyde	50 ppm	120 min	25 °C	91%	Slight decrease in degradation efficiency at higher humidity levels	Five times	[135]
	TiO2@MIL-101(Cr)	Hydrothermal synthesis	Heterojunction	Toluene	/	120 min	Room temperature	39.60%	/	/	[136]
	Pr/MIL-101(Cr)	Hydrothermal synthesis	Doping of elements	1,2-dichloroethane	1000 ppm	/	290 °C	95%	Stabilized at 290 °C for 120 h	/	[137]
	NH2-MIL-125	Solvothermal synthesis	Amino functionalization	Methylbenzene	2000 ppm	20 min	25 °C	96%	High degradation efficiency in the presence of water vapor	Four times	[138]
	N/MIL-125(Ti)	Solvothermal synthesis	Doping of elements	Benzene	0.007%	480 min	25 °C	99.10%	/	Ten times	[139]
	PMA@NH2-MIL-125	Solvothermal synthesis	Amino functionalization, compound modification	Acetone	400 ppm	/	Room temperature	78%	/	Five times	[140]
	NH2-MIL-125	Solvothermal synthesis	Carbon material loads	Dichloromethane	70 ± 1 ppmv	5 h	Room temperature	85%	Stabilizes degradation at high humidity	/	[141]
	Cu/NH2-MIL-125	Solvothermal synthesis	Doping of elements	Methyl mercaptan	100 ppm	25 min	Room temperature	100%	/	Five times	[142]
Microplastics	Fe3O4@SiO2@MIL-53(Al)	Hydrothermal synthesis	Heterojunction	Polyvinyl chloride	1 mg/mL	10 h	Room temperature	93.17%	/	Five times	[143]
	NH2-MIL-88B(Fe)/MoS2	Solvothermal synthesis	Amino functionalization, heterojunction	Polyethylene	10 g/L	48 h	Room temperature	/	High degradation efficiency over the full pH range	Seven times	[144]
	TiO2/MIL-100(Fe)	Microwave-assisted synthesis	Heterojunction	Polyethylene terephthalate	100 mg/L	5 h	Room temperature	CI = 0.99	The degradation efficiency was higher at pH 3	/	[145]
	BiOI@MIL-101	Solvothermal synthesis	Heterojunction	Polyethylene	1.0 g/L	6 h	Room temperature	/	/	Five times	[146]
	Ti3C2 MXene @MIL-125(Ti)	Hydrothermal synthesis	Heterojunction	Polyethylene	0.5 g/L	4 h	Room temperature	78%	Better removal at low pH	Five times	[147]
	MIL-125-NH2/BNQDs	Solvothermal synthesis	Amino functionalization, heterojunction	Polyethylene terephthalate	/	5 h	Room temperature	95.71%	The degradation efficiency was higher at pH 3	Four times	[148]

and Figure 8).

5.1. Persistent Organic Pollutants (POPs)

The Stockholm Convention’s objective is to eliminate or limit the production, use, and release of Persistent Organic Pollutants (POPs) on a global scale [149]. Metal–organic frameworks (MOFs), specifically MIL materials, have emerged as a novel class of materials with the potential to address the treatment of POPs. MILs can function as catalysts, facilitating the conversion of POPs into less harmful or nontoxic substances via catalytic degradation processes. This approach not only mitigates the ecological and human health hazards posed by POPs but also fosters global collaborative efforts to combat POP pollution.

5.1.1. Pesticides

In recent years, pesticides and insecticides have played an important role in ensuring food security and controlling pests and diseases [150]. However, there is growing concern about their negative impact on the environment. The persistence, bioaccumulation, and biomagnification of pesticides in the environment enable their transmission through the food chain, thereby posing risks to human health [151]. In order to address this challenge, researchers are developing efficient catalysts for the removal of pesticide residues from the environment. These materials demonstrate not only the efficient adsorption of POPs, including organochlorine pesticides, but also the capacity to convert them into less harmful or nontoxic substances through the process of photocatalytic degradation [152]. For instance, Reda et al. successfully prepared composites Fe₃O₄@NH₂-MIL-125 and CuO/Cu₂O@NH₂-MIL-125, which completely degraded 1.23 × 10⁻⁴ mol/L solutions of metanaphos and methylpyrimidinphos, respectively, within 150 min [153]. Vigneshwaran et al. synthesized the TiO₂/CS/MIL-88(Fe) (TCS@MOF) composites via solvothermal synthesis. The TCS@MOF composites demonstrated remarkable photocatalytic performance, achieving 98.79% degradation efficiency of Monocrotophos (MCP) within a span of 30 min. Furthermore, the material exhibited notable durability (Figure 9), with a mere 4.1% decrease in photocatalytic efficiency after five cycles of experimentation [154]. However, the MIL is confronted with challenges, including high removal costs, limited regeneration capacity, and insufficient stability in complex environmental media. In practical applications, photocatalytic technology can be used synergistically with other water treatment technologies (e.g., adsorption, filtration, biological treatment, etc.) to improve the treatment efficiency and lower costs.

5.1.2. Industrial Chemicals

The harm caused by industrial chemicals to current ecosystems is multidimensional and far-reaching. The presence of industrial chemicals in the environment, resulting from various anthropogenic activities such as industrial effluents, agricultural runoff, and atmospheric deposition, has the potential to disrupt the ecological balance and contribute to biodiversity loss [155]. In light of the deleterious effects of industrial chemicals on ecosystems, the development of effective removal technologies, such as MIL, is crucial for mitigating these impacts. In industry, perfluorooctanoic acid (PFOA), an organic compound with unique chemical properties, is utilized in the synthesis of fluoropolymers, including polytetrafluoroethylene (PTFE), and functions as an emulsifier [156]. However, its environmental persistence has led to its classification as a new type of POP, and researchers are exploring more efficient techniques for PFOA removal. Su et al. achieved a degradation efficiency of 60.5% for PFOA, which was significantly higher than that of pure C₃N₄, through the use of MIL-100(Fe)@C₃N₄ via the synergistic effect of adsorption and photocatalysis [129]. This provides an effective technological means to deal with difficult-to-degrade organic contaminants [157]. Kong et al. prepared the material F- through the in situ growth of TiO₂ and surface fluorine functionalization. The photocatalytic degradation of perfluorooctanoic acid (PFOA) was studied, and it was found that the reaction rate constant for TiO₂@MIL-125 was 1.221 × 10⁻⁴/s [158]. This result indicates that the in situ growth of TiO₂ and surface fluorine functionalization in MIL-125 is an effective method for achieving efficient photocatalysis. As demonstrated by Wen et al., NH₂-MIL-125 exhibited remarkable efficacy in the photocatalytic degradation of PFOA, achieving a PFOA degradation rate of up to 98.9% within a 24 h period. This study also reported an overall defluorination rate of 66.7% [132].

In the primary process of the photocatalytic degradation of PFOA (Figure 10), the photocatalyst generates photogenerated holes (h⁺) and electrons (e⁻) upon excitation by UV or visible light (EP.1). The photogenerated holes initially oxidize (C₇F₁₅COO⁻) to an unstable PFOA radical (C₇F₁₅COO·) (Figure 10 (1)). This radical subsequently undergoes decarboxylation, resulting in the formation of a perfluoroalkyl radical (C₇F₁₅·) (Figure 10 (2)). C₇F₁₅· can undergo oxidation through two distinct pathways: pathway I, representing the hydroxyl radical oxidation route (Figure 10 (3)), produces a perfluorinated primary alcohol (C₇F₁₅OH); pathway II, denoting the oxygen-involved oxidation route (Figure 10 (4–7)), generates a similar intermediate. The subsequent decomposition of C₇F₁₅OH yields reactive acyl fluoride (C₆F₁₃COF) (Figure 10 (8)) and hydrolyses to short chain perfluorocarboxylic acids (e.g., PFHpA) (Figure 10 (9)). By repeating the aforementioned steps, PFOA is progressively degraded to shorter chain perfluorocarboxylic acids and eventually mineralized to CO₂ and F⁻ [159,160].

M + hv → e⁻ + h⁺

(1)

While MIL materials demonstrate considerable promise in the degradation of POPs, certain photocatalytic materials exhibit deficiencies in terms of their stability in practical applications and are prone to deactivation by light, acidity, alkalinity, and other factors. Furthermore, the photocatalytic degradation process may yield toxic byproducts, which, if inadequately treated, can lead to secondary environmental contamination. In order to address these challenges, researchers have recently implemented improved measures. For instance, Mahmoud Babalar et al. designed a PES@PDA&β-CD@PMACZ-MOF composite membrane that has achieved efficient removal of polychlorinated biphenyls (PCBs) by combining multiple materials with a 100% retention rate and very low release rate (0.3804% to 0.8544%), effectively preventing the secondary release of contaminants [161].

5.2. Pharmaceuticals and Personal Care Products (PPCPs)

Pharmaceuticals and Personal Care Products (PPCPs) have emerged as an emerging class of environmental contaminants of great concern. Their widespread use and relatively stable chemical properties make them difficult to degrade rapidly in the environment, resulting in potential long-term impacts on ecosystems and human health [162]. PPCPs consist mainly of various types of pharmaceuticals, such as antibiotics, anti-inflammatory drugs, and painkillers. They enter water bodies, soils, and sediments through various pathways, such as domestic sewage, hospital wastewater, and agricultural runoff, and continue to accumulate in the environment [5]. Due to their high bioaccumulation and possible ecotoxicity, PPCPs not only persist in the environment but may also be transmitted through the food chain, posing a potential threat to ecosystems and human health [8].

In the context of emerging technologies for environmental governance, the relationship between MILs and PPCPs is evident not only in the degradation mechanism but also in the synergistic effect on environmental governance. The high photocatalytic capacity and catalytic activity inherent in MIL materials enable the precise degradation of specific functional groups in PPCPs, thereby significantly reducing the environmental mobility and bioavailability of these contaminants [163]. This property demonstrates the significant potential of MILs in mitigating the risks posed by PPCPs to ecosystems and human health. An example is sulfadiazine (SDZ), which is a sulfonamide antibiotic that is widely present in the aquatic environment. Nguyen et al. investigated Z-scheme-based Ti-MIL@g-C₃N₅ composite catalysts for the activation of peroxymonosulfate (PMS) for the removal of SDZ from wastewater under visible light. In the context of optimized conditions (100 mg/L catalyst and 0.4 mM PMS), the catalyst demonstrated a 95.6% degradation efficiency for 0.04 mM SDZ within a span of 15 min. Notably, the removal efficiency persisted beyond five cycles, retaining over 76.25% efficacy [134]. It has been determined that sulfonamides and tetracyclines are among the most prevalent antibiotic contaminants in the environment, exhibiting high levels of residual activity. In their study, Li et al. examined a novel Z-scheme heterojunction photocatalyst, designated as MIL-100(Fe)/TpPa-1 COF (abbreviated as MT2), for the photocatalysis of tetracycline (TC) in water. MT2 was utilized to photocatalyze 20 milligrams per liter of tetracycline (TC) in water over the course of 120 min. MT2 demonstrated a 91% degradation efficiency of TC molecules within 120 min, which is 1.26 times that of MIL-100(Fe) alone. The TC molecule generates intermediates through the pathways of hydroxylation, demethylation, ring-opening, and oxidation and is ultimately mineralized to CO₂ and H₂O. The genotoxicity of the degradation intermediates decreases with the progress of the reaction, as analyzed by the Toxicity Evaluation Software, version 5.1.2. (T.E.S.T.) [121].

In the primary process of the photocatalytic degradation of tetracycline (TC) (Figure 11), the TC molecule undergoes a series of chemical reactions, including demethylation, ring cleavage (decarbonylation), and deamination, resulting in the conversion of the molecule into two intermediates, TC 1 and TC 2 (Figure 11 (1) (2)). Subsequently, TC 2 is further decomposed by reactions of dehydroxylation and decarbonylation to form TC 3 and TC 4 (Figure 11 (3) (4)). The reaction continues, resulting in the formation of TC 5, TC 6, and TC 7 through cyclic esterification, ring-opening reactions, and demethylation steps (Figure 11 (5) (6) (7)). These intermediates undergo further degradation, ultimately resulting in the formation of the ring-opening structure of TC 8 (Figure 11 (8)). TC 8, a pivotal intermediate in the degradation chain, undergoes oxidation and decomposition, yielding H₂O and carbon dioxide (CO₂) (Figure 10 (9)), thereby completing the mineralization of tetracycline [164].

The aforementioned studies provide novel concepts for the development of efficient photocatalytic degradation of antibiotics and offer material design paradigms and process optimization strategies for scaled-up water treatment technologies. These strategies will promote the transition of photocatalytic technology from laboratory to engineering applications.

5.3. Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs), including hydrocarbons, oxygenated organics, halogenated hydrocarbons, and nitrogen- and sulfur-containing organics, contribute to atmospheric photochemical reactions that generate ozone and PM2.5 and pose health risks through bioaccumulation and toxicity. Developing efficient and environmentally friendly VOC treatment technologies is therefore a key challenge. The MIL series of metal–organic frameworks (MOFs) has emerged as a promising solution due to their porous structure, high specific surface area, and tunable chemical composition, which facilitate both the efficient adsorption and catalytic degradation of VOC molecules. Numerous studies have demonstrated the superior adsorption performance of MIL series MOFs compared with conventional adsorbents. For example, MIL-101 exhibits significantly higher adsorption capacities for VOCs such as acetone, benzene, and toluene than zeolites or activated carbon, with enhanced selectivity toward molecular size and polarity [165], while MIL-101(Cr) achieves markedly greater toluene uptake than silica gel under low-concentration conditions, showing improved breakthrough and saturation behavior [166]. These results collectively highlight the substantial advantages of MIL series MOFs in both adsorption capacity and selectivity, underscoring their potential for efficient removal of gaseous and aqueous VOC pollutants.

5.3.1. Hydrocarbons

Hydrocarbon VOCs, which include alkanes, olefins, alkynes, and aromatic hydrocarbons—are present in industrial emissions, fuel combustion, and daily life activities. These compounds are important precursors of air pollution, photochemical smog, and fine particulate matter (PM2.5), due to their high volatility and chemical stability. Aromatic hydrocarbons, including benzene and toluene, are not only involved in photochemical reactions but also pose a threat to human health due to their toxicity [167]. In the study of the catalytic degradation of benzene, Zhao et al. prepared disc-shaped mesoporous N-TiO₂ photocatalysts (MM-500) by high-temperature calcination of a mixture of MIL-125(Ti) and melamine. The photocatalytic degradation efficiency of MM-500 reached 99.1% after 480 min of visible light irradiation. Following the completion of ten cycles of experimentation, the degradation efficiency of benzene was found to be 84.8%, thereby demonstrating its stability and reusability. The findings indicated an impressive mineralization efficiency of 72.0%, suggesting that benzene could be efficiently decomposed into carbon dioxide and water [139]. This observation also reflected the effective separation of photogenerated carriers, further emphasizing the system’s functionality. As a benzene congener, toluene can also be catalytically degraded efficiently with the MIL series catalysts. Sun et al. investigated the degradation performance of TiO₂@MIL-101(Cr)-composite photocatalysts for toluene by experimental analysis, which showed a degradation efficiency 23.28% higher than that of MIL-101(Cr) alone and 26.88% higher than that of TiO₂ alone. During the degradation process, a significant increase in degradation efficiency was observed during the initial 40 min. Subsequently, a gradual decline in the growth rate was noted, suggesting that TiO₂@MIL-101(Cr) predominantly underwent adsorption as the primary mechanism for toluene degradation in the early stages of the reaction. This was followed by a transition to photocatalysis as the dominant degradation pathway [136]. The MILs exhibited notable efficacy in the treatment of aromatic hydrocarbon contaminants, effectively adsorbing and degrading aromatic hydrocarbon VOCs, including benzene, toluene, xylene, and others [168]. However, relatively few studies have been conducted on non-aromatic VOCs, such as alkanes, olefins, and alkynes. This paucity of research is likely attributable to the strong chemical inertness of these compounds and the absence of π-π interactions, which enhance adsorption and degradation efficiency, as observed with aromatic hydrocarbons [169].

5.3.2. Oxygenated Organic Compounds (OVOCs)

Oxygenated Volatile Organic Compounds (OVOCs) are important components of VOCs, mainly including aldehydes (e.g., formaldehyde, acetaldehyde), ketones (e.g., acetone, methyl ethyl ketone), alcohols (e.g., ethanol, isopropanol), ethers (e.g., diethyl ether, ethylene glycol monomethyl ether), and acids (e.g., acetic acid). These compounds have low boiling points and high volatility and are widely present in the atmosphere, posing direct or indirect hazards to human health [170]. Formaldehyde, for example, is a known carcinogen, while substances such as acetone and acetaldehyde are highly irritating and toxic. Jiang et al. constructed Z-type heterojunction photocatalysts MIL-88A(Fe)-CA/BiVO₄-OVs (MIBVOS-8) enriched with oxygen vacancies for formaldehyde degradation. In the context of simulated sunlight irradiation, formaldehyde degradation efficiency attained 91.0% within 120 min, and MIBVOS-8 demonstrated 86.3% degradation efficiency after five cycle experiments, indicating notable stability and reusability. DRIFTS analysis revealed that formaldehyde was adsorbed on the catalysts’ surface and converted to dioxymethylene (DOM) and formate intermediates, which were subsequently oxidized to CO₂ and H₂O [135]. For acetone, which is recognized as a class I carcinogen by the International Agency for Research on Cancer (IARC), Yang et al. designed a phosphomolybdic acid (PMA)@NH₂-MIL-125 composite bridged by Ti-O-Mo bonds for acetone degradation. With respect to the degradation performance, the degradation efficiency of PMA@NH₂-MIL-125 was 62% for 400 ppm acetone, which was higher than the 45% efficiency of pure NH₂-MIL-125. Furthermore, the degradation efficiency for 25 ppm acetone increased from 64% to 78%. PMA@NH₂-MIL-125 demonstrated a pseudo-primary reaction rate constant of 0.0426 min⁻¹, which was higher than that of NH₂-MIL-125 (0.0278 min⁻¹). These results suggest that PMA loading significantly enhances the photocatalytic performance of the material [140].

Degradation mechanism studies have demonstrated that acetone (CH₃COCH₃) is initially oxidized to acetaldehyde (CH₃CHO) (Figure 12 (1)), which can subsequently be further oxidized to yield acetic acid (CH₃COOH) and formic acid (HCOOH) (Figure 12 (2) (3)). Acetic acid and formaldehyde (HCHO) are also oxidized to formic acid (Figure 12 (4) (5)), and eventually formic acid is mineralized to carbon dioxide (CO₂) and water (H₂O) (Figure 12 (5) (6)). Throughout the process, the photocatalyst generates hydroxyl radicals (·OH) and peroxyl radicals (·O²⁻) in the presence of light (Figure 12). These reactive species drive the successive oxidation reactions of organic matter [171].

While MILs demonstrate considerable potential in their capacity to degrade OVOCs, it is important to note that they are also subject to certain limitations. In environments characterized by elevated levels of oxygenated OVOCs, particularly in the context of treating substantial concentrations of industrial exhaust gases, the adsorption sites of MILs may be rapidly saturated. This phenomenon has been observed to result in a substantial decline in the degradation efficiency of these materials. To address this challenge, the specific surface area and porosity of MIL materials can be enhanced through the optimization of synthesis or the incorporation of additional materials, such as activated carbon or carbon nanotubes [172,173]. These modifications provide additional reaction sites, thereby promoting the degradation of OVOCs.

5.3.3. Chlorinated Volatile Organic Compounds (CVOCs)

Chlorinated Volatile Organic Compounds (CVOCs) consist primarily of chlorinated alkanes, chlorinated olefins, and aromatic hydrocarbons. Due to their high chemical stability, toxicity, and resistance to natural degradation, these compounds have wide application in industrial contexts, including pharmaceuticals, chemicals, pesticides, and solvent production. However, it is important to note that many chlorinated alkanes (e.g., 1,2-dichloroethane) have been identified as carcinogenic, and prolonged exposure to these substances may result in an elevated risk of developing cancer. The substance has been demonstrated to be acutely toxic to both the central nervous system and the respiratory system. Potential symptoms associated with its use include but are not limited to headaches, fatigue, and respiratory irritation [174].

Furthermore, during catalytic degradation, the incomplete degradation of CVOCs has been observed to result in the generation of more toxic byproducts, such as dioxins, further exacerbating the environmental risks associated with CVOCs [175]. In recent years, the MIL series of MOFs materials has demonstrated noteworthy efficacy in the adsorption and degradation of chlorinated volatile organic compounds (CVOCs). Zhu et al. prepared praseodymium (Pr)-doped Cr₂O₃ catalysts (M-PrCr) by in situ pyrolysis of MIL-101(Cr) and applied them to the efficient catalytic oxidative degradation of 1,2-dichloroethane (1,2-DCE). The 1,2-DCE was subjected to complete oxidation by M-PrCr, forming environmentally benign CO₂ and HCl. The M-PrCr catalyst demonstrated satisfactory overall durability, notwithstanding a decline in activity attributable to water vapor. The PrCr catalyst demonstrated satisfactory overall durability following 100 h of operation at 290 °C, with a conversion of 1,2-DCE that remained above 70%, despite a decline in activity attributable to the impact of water vapor [137]. In Liu et al., carbon nanolayer-encapsulated TiO₂ nanocomposites were prepared by the precise regulation of the pyrolysis process of NH₂-MIL-125(Ti) composites (CNWT-x). These composites were subsequently utilized in a photocatalytic dichloromethane (DCM) degradation process under UV–Vis irradiation. Among the samples examined, those designated as CNWT-2, following a two-hour exposure to air, exhibited remarkable photocatalytic performance. CNWT-2 exhibited 85% DCM conversion and 90% CO₂ selectivity under 5 h of UV–visible irradiation. Furthermore, DRIFTS analysis revealed that no polychlorinated byproducts (e.g., CHCl₂ and CCl₄) were detected during the degradation process, suggesting that CNWT-2 effectively prevented the generation of hazardous byproducts and ensured that the degradation process was environmentally friendly [141].

MILs are confronted with the challenge of chlorine toxicity in the degradation of CVOCs. This challenge is characterized by the strong adsorption of chlorine species, the formation of chloride compounds, the inhibition of reactive oxygen species, the corrosion of the catalyst, and the accumulation of chlorine species. The combined effect of these factors is the deactivation of the catalyst’s active sites. In order to enhance the resistance of catalysts to chlorine poisoning, researchers have proposed various strategies, including the construction of a protective layer [176], the promotion of chlorine species desorption [174], the enhancement of catalyst stability [177], the introduction of sacrificial sites, and acidification treatment [178]. These strategies have been demonstrated to offer effective protection for the catalyst’s active sites, thereby enhancing its stability and catalytic efficiency.

5.3.4. Sulfur-Containing Volatile Organic Compounds (S-VOCs)

Sulfur-Containing Volatile Organic Compounds (S-VOCs) from petroleum have been identified as a primary contributor to air pollution. These compounds are constantly escaping into the environment during the extraction, processing, storage, and transportation of petroleum [179]. Due to their remarkable volatility and chemical activity, they are capable of rapidly spreading in the atmosphere and becoming involved in a series of complex chemical reactions. These reactions have been demonstrated to directly degrade air quality and potentially generate secondary contaminants, including sulfate aerosols and ozone, under the influence of sunlight. Additionally, these reactions have been observed to exhibit synergistic effects with other air contaminants [9]. These secondary contaminants have been identified as contributing factors to environmental issues such as haze and acid rain, which have been demonstrated to exert deleterious effects on human health, ecological systems, and infrastructure [180]. During the atmospheric distillation and decompression distillation of crude oil, sulfur-containing compounds in crude oil are volatilized with the steam [179]. Methyl mercaptan, a low-boiling point mercaptan, is readily released into the gas phase during distillation. He et al. investigated copper-doped Cu/NH₂-MIL-125 photocatalytic materials for the efficient degradation of methyl mercaptan under sunlight. It has been demonstrated that under the condition of visible light (λ > 420 nm), the degradation rate constant of 0.2Cu/80% NH₂-MIL-125 (300 °C) was found to be 0.264 min⁻¹, which is 4.65 times higher than that of pure NH₂-MIL-125 (Figure 13a–c). NH₂-MIL-125 exhibited consistent and efficient methyl mercaptan removal over the course of five cycles (Figure 13d), demonstrating its stability and effective degradation [142]. This result indicates an efficient degradation process. This property underscores its considerable potential for practical applications. Thiophene, a prevalent constituent in petroleum refining, is a notable pollutant that must be addressed and eliminated during petroleum processing. In the study by McNamara et al., TBHP was utilized as an oxidizing agent, and MIL-125 was employed as a catalyst for thiophene degradation. Furthermore, MIL-125 demonstrated notable catalytic activity at 80 °C, exhibiting a rate constant of 0.7 × 10⁻³ min⁻¹ for the thiophene degradation reaction. Subsequent to the reaction, thiophene underwent a transformation to the more polar thiophene sulfoxide and thiophene sulfone, thereby enabling its separation from the reaction system and facilitating the effective removal of sulfur-containing compounds [181].

The utilization of MILs in the fields of adsorption and catalysis is promising due to their high specific surface area and porous structure. However, concerns regarding their stability in practical applications must be addressed to fully realize their potential. In complex environments, such as those characterized by high humidity and high concentrations of S-VOC, the structures of these enzymes are prone to collapse. Active sites become easily inactivated, and the catalytic efficiency is reduced. Additionally, organic ligands exhibit sensitivity to strong acids, strong bases, or high concentrations of organics, which further exacerbate the stability problems. The structural stability of MILs can be enhanced by post-synthesis treatment or the introduction of more stable ligands to improve their adaptability and service life in complex environments.

5.4. Microplastics

As an emerging global pollutant, microplastics have been found in various environments, including oceans, rivers, soils, and the atmosphere [182]. Their sources are highly diverse, including, but not limited to, the fragmentation of large plastic wastes in the natural environment, the leakage of plastic particles during industrial production, the addition of microbeads to personal care products (e.g., cosmetics, toothpaste, etc.), and the abrasion of synthetic fibers during use [183]. The classification of microplastics is determined by their size, with particles ranging from the micrometer scale (1 µm to 5 mm) to the nanometer scale (less than 1 µm) [184]. In recent years, the MIL series of metal–organic framework materials (MOFs) has demonstrated considerable potential in microplastic pollution control. This is due to their distinctive porous structure and photocatalytic properties. MIL materials exhibit not only the capacity to adsorb microplastic particles with efficiency but also the capability to expedite the oxidative degradation process of microplastics through the effective separation of electron–hole pairs and the generation of free radicals under light conditions. Consequently, MILs facilitate the efficient degradation and removal of microplastics.

5.4.1. Micrometer Scale

The term “microplastics” (MPs) is commonly used to refer primarily to micron-sized particles, with particle sizes ranging from 1 micron to 5 mm [184]. This variety of microplastics is the most prevalent and extensively studied type in the contemporary environment. Due to its comparatively substantial particle size, the migration and diffusion of MPs in the environment is relatively straightforward to observe and investigate. Nevertheless, they represent a potential hazard to the ecosystem and human health [185]. The utilization of MIL adsorption and the degradation of MPs possesses a wide range of possible applications, garnering the interest and investment of numerous researchers. Yang et al. synthesized a titanium-based metal–organic framework material (MX@MIL-125(Ti)) on the surface of Ti₃C₂ MXene utilizing a hydrothermal synthesis. The study’s findings demonstrated that the composite material exhibited the capacity to attain a removal efficiency that exceeded 75% for polyethylene (PE) within 4 h under conditions of ultrasonic radiation [147]. However, achieving this level of efficiency necessitates elevated ultrasonic power (650 W), an extended action time (4 h), and a heightened catalyst concentration (2.0 g/L). These exacting conditions result in a substantial escalation in energy consumption and treatment costs, thereby constraining its feasibility and economic viability in large-scale water treatment applications. Feng et al. prepared heterostructured photocatalysts via the hydrothermal synthesis, namely NH₂-MIL-88B(Fe)/MoS₂, which exhibited highly efficient photocatalytic degradation of HDPE under simulated sunlight irradiation. This process also resulted in a significant change in the surface morphology and chemical structure of HDPE (Figure 14) [144].

Gu et al. prepared BiOI/MIL-101 composites for the photocatalytic degradation of PE using hydrothermal and co-precipitation synthesis. The carbonyl index (CI) of the composite exhibited an increase of 0.127 after six hours of exposure to light conditions, which is 5.3 and 3.7 times higher than that of BiOI and MIL-101 alone, respectively. The efficient degradation was primarily attributed to the photo-Fenton reaction, whereby Fe (II) catalyzes the decomposition of H₂O₂ to generate strongly oxidizing hydroxyl radicals (·OH) within the composite [146]. In the photocatalytic degradation of PE (Figure 15), the hydroxyl radical initially oxidizes the C-C bond of PE, thereby initiating the degradation process (EP.2). In the presence of oxygen, the generated C-C bond is rapidly converted to peroxyl radicals, which further react with other polymer chains to form hydrogen peroxide (EP.3,4). The decomposition of hydrogen peroxide under light exposure results in the formation of alkoxy radicals and hydroxyl radicals, a process facilitated by the weak O-O bonds present in the compound (EP.5). Subsequently, the alkoxy radicals promote the formation of polymers containing ketone and aldehyde groups by reacting with oxygen and undergoing β-cleavage (EP.6). Under UV irradiation, these ketone and aldehyde groups undergo α-cleavage, primarily through Norrish-type reactions, which generate carbon monoxide and result in mass loss (EP.7,8). This process is identified as the rate-determining step in the degradation process. However, the high efficiency of the Fenton reaction is contingent upon acidic conditions and elevated H₂O₂ concentrations, which can increase the operational complexity and cost in practical applications. Furthermore, the iron ions produced by the Fenton reaction have the potential to induce secondary pollution in water bodies, necessitating additional treatment.

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5.4.2. Nanoscale

The unique physicochemical properties and environmental behavior of nanoplastics (NPs) result from their minute size, which is less than 1 micron. These particles demonstrate augmented mobility within the environment and possess the capacity to penetrate cell membranes, gain entry into organisms, and accumulate within these hosts, which can manifest more intricate health hazards [186]. The elevated specific surface area of NPs enables their capacity to adsorb a greater quantity of contaminants and heavy metals, thereby amplifying their toxicity [187]. To effectively manage NP pollution, MILs have also demonstrated significant potential in the photocatalytic degradation of NPs. This process involves the activation of peroxymonosulfate (PMS) under light conditions, which results in the generation of highly reactive free radicals (·SO₄²⁻) capable of efficiently degrading NPs. Sharmin et al. have developed a novel catalytic membrane that combines a mixed metal oxide (CoFeLMO) with MIL-100 (Fe) and polyethylene glycol (PEG) to promote the PMS-activated advanced oxidation process, to degrade NPs efficiently. The removal efficiency of NPs was 98.5% when the PMS concentration was increased to 0.2 mM [133]. This result indicated that the LMO-MOF-PEG catalytic membrane exhibited high efficiency in removing NPs, even at lower PMS concentrations, demonstrating its remarkable efficacy in addressing NPs contamination. However, the regeneration of these MILs after adsorption saturation is challenging; their reuse is limited, and their reuse may lead to clogging, due to membrane contamination, increasing the operating cost. To enhance their viability for practical water treatment applications, further optimization of their performance and cost reduction are necessary.

6. Summary

This paper provides a comprehensive overview of the current state of MIL series MOFs materials’ application in emerging contaminants’ management. A thorough examination of MIL series MOFs materials offers a specific perspective on future research directions. A comprehensive compendium of MIL series classification, synthesis methods, and modification strategies, along with their application effects in novel pollutant management was presented. This compendium also revealed the advantages and limitations of the MIL series in environmental remediation. The findings of this study offer a multifaceted contribution to the field. On the one hand, they provide a robust theoretical foundation and practical guidance for managing emerging contaminants. On the other hand, they serve as a valuable reference point and a source of inspiration for future research. In the following section, we outline four key areas that we believe will be the focus of future research:

Firstly, the optimization of the performance of the MIL series is pivotal to achieving its wide application. In the contemporary context, the catalytic properties of MILs can be substantially enhanced by implementing elemental doping, ligand functionalization, and heterostructure design. For instance, transition metal doping can introduce new active sites, thereby improving the catalytic performance of the materials. In addition, ligand functionalization can modulate the surface properties of the materials by introducing specific functional groups (e.g., amino and sulfonic acid groups) to improve their adsorption capacity for particular contaminants. Future research should explore novel modification methods, such as multi-element synergistic doping and multi-ligand functionalization, for more efficient pollutant degradation. In addition, the construction of heterojunction structures has been demonstrated to enhance the separation efficiency of photogenerated carriers, thereby improving the photocatalytic performance.

While various synthesis methods have been employed to enhance the MIL series, it is noteworthy that the materials synthesized by different methods exhibit substantial disparities in performance. For instance, the solvothermal synthesis is characterized by its simplicity and low energy consumption; however, it is susceptible to impurity introduction. In contrast, microwave-assisted synthesis significantly reduces the reaction time, yet its equipment costs are substantial. Future research endeavors should prioritize the development of greener, more efficient, and scalable synthesis techniques to reduce production costs and enhance the material reproducibility and consistency. For instance, the crystallinity and purity of MILs can be further enhanced by optimizing the reaction parameters of the microwave-assisted synthesis. Furthermore, integrating synthesis methods, such as mechanical grinding and solvothermal techniques, is anticipated to improve material properties while reducing costs.

Secondly, a cost–benefit comparison of the MIL series with other established technologies (e.g., activated carbon adsorption, bioremediation, etc.) in treating emerging contaminants is warranted. Despite the notable advantages of MILs in adsorption efficiency and selectivity, their economic viability in practical applications is hindered by their relatively high synthesis costs. For instance, a conventional adsorbent, activated carbon, possesses a comparatively modest adsorption capacity; however, its cost-effectiveness and ease of regeneration make it the preferred choice in numerous application scenarios. Conversely, the synthesis of the MIL series necessitates the utilization of costly organic ligands and metal salts, and the energy consumption and solvent usage during the synthesis process contribute to the overall cost. Nevertheless, MIL materials generally exhibit superior overall performance compared to other adsorption materials, including higher stability, adsorption capacity, and recyclability, which can justify their use in targeted applications. A critical factor in enhancing the economic viability of MIL series synthesis is the reduction in the price of raw materials. A significant body of research is currently exploring inexpensive industrial waste as a source of ligands. For instance, a recent study successfully synthesized MIL-101 using waste plastics as a ligand source, substantially reducing the synthesis costs. Additionally, using more economical metal salts, such as iron salts, can contribute to the cost reduction.

Thirdly, the majority of research in the MIL series has centered on laboratory simulation experiments, and its application in real environments continues to face significant challenges. For instance, a limited array of contaminants in actual environments has been demonstrated to impede the efficacy of detection and removal processes. To address these challenges, future research should prioritize the translation of MILs from laboratory settings to real-world applications. This transition necessitates the development of efficient detection techniques and large-scale treatment processes. The development of composite materials and multi-technology coupling processes holds great potential for enhancing the effectiveness of MILs in real-world applications by facilitating a synergistic treatment of multiple contaminants. Achieving enhanced environmental adaptability in MILs is imperative for their practical applications. For instance, in conditions of high humidity, high salinity, and strong acid or alkali, the structure and performance of MILs may be significantly impacted. Future research endeavors should prioritize enhancing material stability in complex environments through optimizing synthesis and modification strategies. For instance, the water and salt resistance of MILs can be substantially improved by incorporating hydrophobic ligands and constructing heterojunction structures. Furthermore, developing efficient regeneration techniques, such as heat treatment, chemical cleaning, and ultrasonic treatment, is crucial for restoring the activity of the materials and enhancing their reuse.

Fourthly, the accurate detection of degradation products is paramount for evaluating the environmental remediation efficacy of MILs. Presently, the detection of degradation products is chiefly accomplished through techniques such as gas chromatography–mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC). However, these techniques have limitations in detecting low concentrations and complex degradation products. To this end, future research should prioritize the development of more efficient detection means, such as high-resolution mass spectrometry (HRMS) and nanosensors, to achieve accurate detection of the degradation products. Furthermore, enhancement of microscopic characterization tools is imperative for comprehending the degradation mechanism of MIL materials. Currently, techniques such as X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy are widely used to analyze the surface and structure of materials.

Nevertheless, these techniques are still inadequate for resolving complex reaction mechanisms. Future research endeavors should prioritize the in-depth elucidation of the degradation mechanism of MILs through the integration of in situ characterization techniques and computational simulations. For instance, in situ XPS and FTIR techniques enable real-time observation of surface alterations and the breaking and formation of chemical bonds within the materials during the reaction process.

Furthermore, developing MILs with multifunctional capabilities and integrating adsorption, catalysis, and photocatalysis into a unified structure holds promise for the synergistic management of multiple contaminants. This, in turn, would enhance the overall performance of MILs in practical applications. By pursuing research in the four directions above, it is anticipated that significant advancements will be made in applying MILs to treat emerging contaminants. These advancements will facilitate the widespread implementation of these materials from the laboratory to the field of environmental remediation. Consequently, this will provide substantial support for the sustainable development of the global environment.