Reimagining Textile Effluent Treatment Using Metal–Organic Framework-Based Hybrid Catalysts: A Critical Review

Abstract

Textile wastewater remains one of the most challenging industrial effluents to remediate due to its intense and persistent coloration, high organic load, elevated salinity, and fluctuating pH and the presence of recalcitrant dye structures and auxiliary chemicals. Conventional physicochemical and biological treatments frequently achieve incomplete removal, generate secondary wastes, or fail under high-salt and toxic dye matrices. Advanced oxidation processes (AOPs) provide molecular-level degradation via reactive oxygen species (ROS), yet their deployment is often constrained by narrow operating windows, catalyst instability, chemical/energy demand, and scale-up limitations. In this context, metal–organic frameworks (MOFs) have emerged as tunable porous catalytic platforms that integrate adsorption and oxidation within a single architecture through controllable metal nodes, functional linkers, and engineered pore environments. This critical review reimagines textile effluent treatment through the lens of MOF-based hybrid catalysts, synthesizing progress across Fenton/photo-Fenton catalysis, photocatalytic MOFs, persulfate activation, and MOF-derived/composite systems. Mechanistic pathways are discussed by linking pollutant enrichment, cyclic redox reactions, charge-transfer processes, and ROS-driven degradation toward mineralization, with emphasis on the distinction between rapid decolorization and true organic removal. A critical comparison highlights how hybridization improves charge transport, stability, and catalyst recovery, while persistent gaps remain in hydrolytic robustness, metal leaching control, intermediate toxicity assessment, real-wastewater validation, continuous-flow reactor integration, and techno-economic feasibility. Finally, the review outlines actionable research directions, including water-stable and defect-engineered MOFs, immobilized and structured catalysts, solar-driven operation, standardized performance metrics, and life-cycle-informed design, to accelerate translation toward scalable and sustainable textile wastewater remediation. By bridging material chemistry with reactor-level feasibility and sustainability assessment, this review provides an implementation-oriented perspective for next-generation textile wastewater treatment.

1. Introduction

The textile industry is one of the largest consumers of freshwater and a major contributor to industrial wastewater pollution worldwide [1,2]. Effluents generated during dyeing, washing, and finishing processes contain complex mixtures of synthetic dyes, auxiliary chemicals, salts, surfactants, and other recalcitrant organic compounds [3,4,5]. These wastewaters are typically characterized by intense coloration, high chemical oxygen demand (COD), variable pH, and low biodegradability, posing significant risks to aquatic ecosystems and public health [4,5,6]. In particular, azo and reactive dyes exhibit high chemical stability and resistance to microbial degradation, enabling their persistence in aquatic environments. Even at low concentrations, these dyes reduce light penetration, thereby inhibiting photosynthetic activity and disrupting aquatic life [7,8].

Conventional treatment technologies including coagulation/flocculation [9], adsorption [10], membrane filtration [11], and biological activated sludge processes [12] are widely applied in textile wastewater management [13,14]. Although effective for removing suspended solids and partially reducing organic loads, these methods generally do not achieve complete degradation or mineralization of dye molecules [15]. In addition, many physicochemical treatments generate secondary pollution, such as chemical sludge or concentrated brines that require further handling [16]. Biological processes are further limited by dye toxicity, high salinity, and the structural complexity of aromatic compounds, which hinder enzymatic degradation pathways and reduce overall treatment efficiency under real operating conditions [17,18].

To overcome these limitations, advanced oxidation processes (AOPs) have attracted considerable attention due to their ability to generate highly reactive oxygen species, particularly hydroxyl radicals (•OH) capable of the non-selective oxidation of persistent organic pollutants [19,20,21]. Technologies such as Fenton and photo-Fenton reactions, heterogeneous photocatalysis, ozonation, electrochemical oxidation, and persulfate activation have demonstrated promising performance for dye degradation and partial mineralization [22,23,24,25,26]. However, many AOP systems remain constrained by catalyst instability, narrow optimal pH ranges, high chemical or energy demands, and challenges related to catalyst recovery and large-scale implementation [27,28,29,30].

Recent advances in materials science have introduced metal–organic frameworks (MOFs) as a new class of porous crystalline materials with exceptional tunability, high surface areas, and well-defined active sites, offering unique opportunities for catalytic wastewater treatment [31,32]. The hybrid structure of MOFs, which combining inorganic metal nodes with organic linkers that enables precise control over catalytic functionality [33,34], light absorption properties [35], redox activity [36], and pollutant adsorption behavior [33,34]. These features allow MOF-based systems to enhance pollutant concentration near active sites while promoting efficient generation of reactive species, thereby addressing several limitations of conventional catalysts [37]. In addition, MOFs can be engineered into hybrid catalytic architectures through compositing, heterojunction formation, or structural modification, improving stability, recyclability, and catalytic performance under realistic treatment conditions [38].

Despite the rapid growth of MOF research, current studies remain fragmented across photocatalysis, Fenton-like oxidation, adsorption/catalysis coupling, and hybrid reactor configurations, with limited critical evaluation of their collective progress and practical limitations for textile effluent treatment [39]. Key challenges that including structural instability in aqueous environments, metal leaching, regeneration strategies, operational scalability, and techno-economic feasibility, which remain insufficiently addressed [40]. Therefore, a comprehensive assessment integrating catalyst design, reaction mechanisms, performance evaluation, and implementation constraints is essential to guide future development and facilitate industrial translation [39,41].

Recent review articles have provided valuable insights into the development of MOF-based catalysts for environmental remediation; however, most of these studies adopt a generalized perspective on water treatment without focusing on the specific challenges of textile effluents [42]. For instance, Wang et al. [43] and Li et al. [44] highlight advances in MOF synthesis, catalytic performance, and emerging hybrid systems, yet their analysis remains largely centered on material development and laboratory-scale efficiency. In contrast, the present review adopts a textile-specific perspective by directly linking dye chemistry with catalytic mechanisms and treatment performance. Furthermore, it extends beyond conventional material-focused discussions by incorporating implementation-oriented considerations, including reactor design, process integration, and techno-economic feasibility under realistic wastewater conditions. Particular emphasis is also placed on hybrid catalytic systems and the practical constraints that govern their performance, providing a more comprehensive and application-driven understanding of MOF-based technologies for textile wastewater treatment.

Accordingly, this review provides a critical and integrative assessment of MOF-based hybrid catalysts for textile wastewater treatment, extending beyond the descriptive nature of existing surveys. Unlike previous reviews that primarily address MOF applications in general water treatment, this work establishes a textile-specific framework that links dye molecular characteristics with catalyst design, hybridization strategies, and reactive oxygen species generation mechanisms. Particular emphasis is placed on critically evaluating performance limitations, including hydrolytic instability, metal leaching, and catalyst deactivation, as well as inconsistencies in reported efficiencies under varying experimental conditions. The review also explicitly distinguishes between dye decolorization and true mineralization, highlighting the need for more rigorous and standardized evaluation metrics. By integrating techno-environmental considerations with reactor-level perspectives, this work provides a comprehensive and implementation-oriented roadmap for advancing MOF-based catalytic systems from laboratory-scale studies toward scalable and sustainable textile wastewater treatment applications.

2. Characteristics and Treatment Challenges of Textile Effluents

2.1. Types of Textile Dyes

Textile effluents contain a diverse range of synthetic dyes specifically engineered to exhibit strong color intensity, chemical stability, and resistance to washing, light exposure, and chemical degradation [45,46]. These design characteristics, while essential for textile performance, significantly increase environmental persistence and treatment difficulty [47]. Textile dyes are commonly classified based on their chemical structure, application method, and fiber affinity, with azo, reactive, disperse, and vat dyes representing the dominant categories encountered in industrial wastewater [45].

2.1.1. Azo Dyes

Azo dyes constitute the largest class of synthetic dyes, accounting for approximately 60–70% of commercially produced dyes. Their chemical structure contains one or more azo bonds (–N=N–) linking aromatic rings, which act as chromophoric groups responsible for their intense coloration [48]. The presence of substituted aromatic structures and sulfonate functional groups enhances water solubility and chemical stability; however, it also renders these dyes resistant to biodegradation [49]. Under anaerobic conditions, azo bonds may undergo reductive cleavage, forming aromatic amines that are often toxic or carcinogenic, thereby increasing environmental concern [7,18].

2.1.2. Reactive Dyes

Reactive dyes are widely used in cotton and cellulose-based textile processing due to their ability to form covalent bonds with fiber surfaces. Despite their high fixation efficiency, approximately 20–40% of applied reactive dyes remain unfixed and are discharged into wastewater streams [50]. These dyes typically contain reactive groups such as monochlorotriazine or vinyl sulfone moieties, which enhance fiber bonding but also increase hydrolytic stability in aqueous environments. Their high solubility and resistance to biodegradation make removal through conventional treatment processes particularly challenging [32,46,47].

2.1.3. Disperse Dyes

Disperse dyes are nonionic, sparingly water-soluble compounds primarily used for dyeing hydrophobic synthetic fibers such as polyester and nylon [51]. During dyeing operations, these dyes are dispersed using surfactants, forming stable colloidal suspensions that resist sedimentation and conventional filtration. Their hydrophobic nature promotes adsorption onto suspended solids and sludge, often resulting in incomplete removal and secondary contamination [52].

2.1.4. Vat Dyes

Vat dyes, including indigo-based compounds, are characterized by complex polycyclic aromatic structures and exceptional chemical stability [53]. These dyes are applied through reduction–oxidation cycles during textile processing, resulting in insoluble colored forms that strongly retained within fibers [54]. Residual vat dyes entering wastewater exhibit high resistance to oxidation and biodegradation due to their condensed aromatic frameworks [6].

In addition to these primary categories, textile effluents may contain sulfur dyes, direct dyes, and auxiliary coloring agents, further increasing compositional complexity. The coexistence of multiple dye classes with varying solubility, charge, and molecular structures complicates treatment design, as no single removal mechanism is universally effective [55]. Consequently, advanced oxidation and hybrid treatment systems are increasingly required to achieve effective degradation and mineralization across diverse dye chemistries. Figure 1 presents a schematic representation of the main textile dye classes. The diagram summarizes the four dominant dye categories azo, reactive, disperse, and vat dyes highlighting their characteristic structural features, solubility behavior, and relative resistance to degradation, which collectively influence treatment strategy selection in textile wastewater management [56,57].

The coexistence of multiple dye classes with varying physicochemical properties complicates treatment design, necessitating advanced oxidation and hybrid systems capable of achieving effective degradation and mineralization [53]. The major classes of textile dyes, together with their structural characteristics, environmental behavior, and associated treatment challenges, are summarized in

Table 1. Major classes of textile dyes, structural characteristics, and treatment implications.

Dye Class	Main Chemical Features	Fiber Application	Water Solubility	Environmental Behavior	Treatment Challenges
Azo dyes	–N=N– azo bond linking aromatic rings; often sulfonated	Cotton, wool, synthetic blends	High	Persistent color; reductive cleavage forms aromatic amines	Low biodegradability; toxic intermediates
Reactive dyes	Reactive groups (vinyl sulfone, triazine) forming covalent fiber bonds	Cotton, cellulose fibers	Very high	Large fraction discharged unfixed	Difficult biological removal; stable in water
Disperse dyes	Nonionic, hydrophobic aromatic molecules	Polyester, nylon	Low	Forms colloidal suspensions	Poor settling and filtration efficiency
Vat dyes	Polycyclic aromatic structures (e.g., indigo)	Cotton, denim	Insoluble after oxidation	Highly stable compounds	Resistant to oxidation and biodegradation
Sulfur dyes	Sulfur-containing polymeric structures	Cotton	Moderate	Produces sulfide residues	Toxic reduced species formation
Direct dyes	Linear anionic molecules with sulfonate groups	Cellulosic fibers	High	Strong coloration in water	Limited adsorption removal

. This table illustrates how molecular composition directly influences degradability and process selection in wastewater treatment systems [56,57,58].

The relationship between dye molecular structure and degradation behavior in advanced oxidation systems is illustrated in Figure 2. Structural features such as aromatic rings, sulfonate groups, and hydrophobicity strongly influence reactivity, intermediate formation, and treatment outcomes. Different dye classes (e.g., azo, sulfonated, hydrophobic, and vat dyes) exhibit varying resistance to degradation, with pathways initiated by reactive oxygen species (•OH, SO₄•⁻, O₂•⁻) and reductive electrons. These processes involve azo bond cleavage, radical attack, ring opening, and adsorption-assisted oxidation, leading to outcomes ranging from toxic intermediates and partial oxidation to complete mineralization into CO₂ and H₂O. This highlights the importance of hybrid oxidation processes for effective textile wastewater treatment [27,56].

2.2. Physicochemical Properties of Textile Wastewater

Textile wastewater represents one of the most chemically complex industrial effluents due to the diversity of fibers, dyes, auxiliaries, and processing stages involved in textile manufacturing. Its physicochemical composition varies depending on fabric type, dye class, and processing operations such as desizing, scouring, bleaching, dyeing, printing, and finishing; however, several common characteristics are consistently reported [57,58].

A defining feature of textile wastewater is its high chemical oxygen demand (COD) and biochemical oxygen demand (BOD), reflecting substantial concentrations of organic pollutants originating from unfixed dyes, sizing agents, lubricants, and auxiliary chemicals. COD values may range from several hundred to several thousand mg L⁻¹, indicating a heavy organic load that exceeds the treatment capacity of conventional biological systems when discharged without adequate pretreatment. The relatively low BOD/COD ratio frequently observed suggests poor biodegradability, primarily due to the presence of chemically stable dye molecules [57,58,59].

Intense coloration is another distinctive property. Even trace concentrations of dyes (≤1 mg L⁻¹) can impart visible color because dye molecules possess high molar extinction coefficients and complex chromophoric structures. Reactive, azo, anthraquinone, and sulfur dyes are commonly detected, many of which remain dissolved after dyeing due to incomplete fixation efficiencies (typically 60–90%). These dyes contain conjugated aromatic systems that confer strong optical stability and resistance to natural photodegradation [60].

Textile effluents also exhibit elevated salinity and ionic strength, mainly resulting from the use of inorganic salts such as sodium chloride (NaCl) and sodium sulfate (Na₂SO₄) during dye fixation processes. Salt concentrations may reach tens of grams per liter, increasing osmotic stress on microorganisms and inhibiting enzymatic activity in biological reactors. High conductivity additionally complicates downstream reuse applications and may promote corrosion in treatment infrastructure. Also, another important characteristic is the presence of complex auxiliary chemicals, including surfactants, dispersants, wetting agents, softeners, oxidants, reducing agents, and finishing compounds. Many of these additives are designed for chemical stability and therefore persist during treatment. Nonionic and anionic surfactants can stabilize dye molecules in solution, reducing adsorption efficiency and limiting phase separation processes [54,60,61,62].

The pH of textile wastewater varies widely from pH 4 to 11, depending on the processing stage, while suspended solids, turbidity, and residual metals from dye formulations further increase treatment complexity. Moreover, many dyes contain electron-withdrawing functional groups such as –SO₃⁻, –NO₂, –Cl, which enhance solubility but reduce susceptibility to microbial enzymatic attack. As a result, biodegradation pathways are slow and often incomplete. Hence, these physicochemical properties high organic load, persistent coloration, salinity stress, chemical complexity, and recalcitrant molecular structures limit the effectiveness of conventional physicochemical and biological treatments. Consequently, advanced oxidation processes (AOPs) and hybrid catalytic systems that generate highly reactive oxidative species (e.g., •OH and SO₄•⁻) are increasingly essential for achieving molecular-level degradation and complete pollutant mineralization [19].

Table 2. Typical Physicochemical Characteristics of Textile Wastewater.

Parameter	Typical Range	Unit	Main Source in Textile Processing	Environmental Implication	Ref.
pH	4–11	–	Dyeing, bleaching, alkaline washing, finishing operations	Extreme pH affects biological activity and requires neutralization	[59]
Chemical Oxygen Demand (COD)	500–5000	mg L−1	Unreacted dyes, sizing agents, auxiliaries, organic additives	Indicates high organic pollution load; requires advanced oxidation or combined treatment	[60]
Biochemical Oxygen Demand (BOD5)	100–1500	mg L−1	Biodegradable organics, starches, surfactants	Low BOD/COD ratio reflects poor biodegradability	[61]
BOD5/COD Ratio	0.1–0.4	–	Depends on dye class and auxiliaries	Values < 0.3 indicate recalcitrant wastewater	[62]
Total Dissolved Solids (TDS)	2000–15,000	mg L−1	Dye fixation salts (NaCl, Na2SO4)	High salinity inhibits microorganisms and limits reuse	[63]
Electrical Conductivity (EC)	3–20	mS cm−1	Dissolved salts and chemicals	Impacts membrane processes and biological treatment	[64]
Color (Absorbance at λmax)	0.5–3.5	Absorbance units	Residual dyes and chromophores	Causes aesthetic pollution and blocks light penetration	[65]
Color (Pt–Co scale)	500–3000	Pt-Co units	Reactive and azo dyes	Persistent coloration even at low concentrations	[66]
Suspended Solids (TSS)	50–1000	mg L−1	Fibers, particulates, precipitated dyes	Requires coagulation/filtration pretreatment	[67]
Total Organic Carbon (TOC)	100–1500	mg L−1	Organic dyes and auxiliaries	Indicator of mineralization requirement	[68]
Chloride (Cl−)	500–6000	mg L−1	Sodium chloride used in dye fixation	Causes salinity stress and corrosion	[69]
Sulfate (SO42−)	200–3000	mg L−1	Sodium sulfate dye baths	Contributes to ionic strength and scaling	[70]
Surfactants	10–200	mg L−1	Wetting agents, detergents, dispersants	Stabilize dyes and hinder adsorption	[71]
Temperature	25–60	°C	Hot dye baths and washing stages	Influences reaction kinetics and oxygen solubility	[6]

tabulated representative physicochemical characteristics of textile wastewater reported across dyeing and finishing industries. Values vary depending on fiber type, dye class, and processing conditions but consistently indicate high organic load, salinity, and recalcitrant coloration requiring advanced treatment strategies [6,59,60,61,62,63,64,65,66,67,68,69,70,71].

2.3. Environmental Impacts of Textile Wastewater Discharge

The uncontrolled discharge of untreated or inadequately treated textile effluents poses serious environmental, ecological, and public health risks [6]. Due to their chemical stability and strong coloration, textile dyes are among the most environmentally problematic industrial pollutants [72]. One of the primary environmental impacts is a reduction in light penetration in receiving water bodies caused by persistent coloration. Colored effluents absorb and scatter sunlight, significantly decreasing photosynthetically active radiation (PAR). This reduction inhibits photosynthesis in aquatic plants and algae, disrupting primary productivity and altering aquatic food webs. Prolonged exposure can lead to oxygen depletion and ecosystem imbalance [73].

On the other hand, many textile dyes and their transformation products exhibit toxicological effects, including acute toxicity, endocrine disruption, mutagenicity, and carcinogenicity. Azo dyes, for example, may undergo reductive cleavage under anaerobic conditions, producing aromatic amines that are recognized for their potential health hazards. These compounds can bioaccumulate and enter the food chain, posing risks to both aquatic organisms and human populations relying on contaminated water resources [74]. High salinity associated with textile effluents further exacerbates environmental stress by altering osmotic conditions in freshwater ecosystems. Elevated ionic concentrations impair physiological regulation in aquatic species, reduce biodiversity, and shift microbial community structures. In addition, surfactants and finishing agents may increase membrane permeability in aquatic organisms, enhancing pollutant uptake and toxicity [75].

Additionally, another critical concern is the persistence of partially oxidized intermediates formed during incomplete treatment processes. While color removal may be achieved, residual organic fragments can remain chemically reactive and potentially more toxic than parent compounds. Therefore, modern environmental regulations increasingly emphasize mineralization efficiency, toxicity reduction, and ecological safety rather than visual decolorization alone. Therefore, from a regulatory perspective, tightening discharge standards worldwide now require the simultaneous removal of color, organic load, toxicity, and salinity impacts [58]. These requirements have driven the development of integrated and advanced treatment technologies, including photo-Fenton oxidation, photocatalysis, electrochemical oxidation, membrane hybrid systems, and nature-based solutions. Such approaches aim to achieve sustainable textile wastewater management aligned with circular economy principles, water reuse strategies, and environmental protection goals.

3. Research Mapping Through Bibliometric Perspective

3.1. Publication Growth Trends

Because the literature on textile effluent remediation has expanded across diverse catalyst families and hybrid process configurations, bibliometric analysis is employed here as an evidence-based tool to structure the review beyond narrative selection. Specifically, publication and keyword network mapping are used to reveal research shifts from conventional treatments toward hybrid oxidation strategies and MOF-enabled platforms, thereby supporting the rationale for focusing this critical review on MOF-based hybrid catalysts and their translation-relevant constraints [76].

Assessment of the annual publication output illustrates the rapid growth of research on hybrid oxidation and nature-based treatment systems for textile dye wastewater treatment from 2000 to 2025. The bars represent the number of publications per year, while the fitted curve highlights the exponential growth trend, indicating increasing global research interest and technological transition toward integrated and sustainable treatment approaches. Figure 3 presents the temporal evolution of scientific publications related to hybrid systems for textile dye wastewater treatment between 2000 and 2025. The results reveal a slow growth phase during the early 2000s, when research primarily focused on conventional biological and physicochemical treatment methods. Beginning around 2010, publication output increased steadily, coinciding with the emergence of advanced oxidation processes (AOPs) such as photocatalysis and Fenton-based systems.

A pronounced acceleration is observed after 2016, reflecting a paradigm shift toward hybrid treatment strategies integrating catalytic oxidation, adsorption materials, and nature-based systems. The exponential increase in publications during the last decade demonstrates growing scientific and technological interest in sustainable and energy-efficient wastewater treatment solutions. This trend also highlights the transition from single-process remediation toward synergistic approaches combining oxidation technologies, advanced catalysts, and ecological treatment platforms, which form the central focus of the present review.

3.2. Keyword Co-Occurrence and Research Clusters

The keyword co-occurrence network generated using VOSviewer version 1.6.20 (Figure 4) illustrates the intellectual structure and thematic evolution of research on textile wastewater treatment. The map reveals several interconnected research clusters, highlighting the multidisciplinary nature of the field. The central position and large node size of terms such as removal, decolorization, and textile wastewater indicate their dominant role as core research drivers. The red cluster primarily represents conventional treatment and industrial wastewater management themes, including reactor systems, sludge handling, reuse strategies, and effluent treatment technologies. In contrast, the green cluster reflects emerging catalytic and advanced oxidation research, characterized by keywords such as adsorption, photocatalytic degradation, azo dyes, and nanocomposite, demonstrating a shift toward material-driven remediation approaches. The blue and yellow clusters emphasize process optimization and hybrid treatment configurations, linking membrane processes, ozonation, constructed wetlands, and system integration concepts. The strong interconnections among clusters confirm a progressive transition from standalone physicochemical treatments toward integrated hybrid systems combining adsorption, catalytic oxidation, and nature-based solutions. Overall, the network highlights a clear research trajectory toward sustainable, multifunctional treatment strategies, supporting the growing emphasis on hybrid oxidation technologies and eco-engineered wastewater treatment frameworks.

3.3. Emerging Research Frontiers

The keyword co-occurrence network presented in Figure 5 reveals conceptual organization and research evolution within the field of metal–organic framework (MOF)-assisted dye wastewater treatment. The map demonstrates a highly interconnected knowledge structure centered around dominant keywords such as metal–organic framework, adsorption, performance, and aqueous solution, indicating that adsorption-driven remediation remains the foundational research theme. The large node sizes associated with these terms reflect their high occurrence frequency and central importance in catalyst development and wastewater purification studies. Distinct thematic clusters can be identified, highlighting the multidisciplinary expansion of the field. The green cluster primarily represents adsorption and material-performance studies, focusing on adsorption capacity, optimization, and water purification efficiency. In contrast, the blue cluster emphasizes material engineering and fabrication strategies, including graphene oxide integration, nanosheet design, and photoanode development, reflecting increasing attention toward engineered hybrid materials. The red cluster relates to photocatalysis and pollutant degradation mechanisms, demonstrating the transition from passive adsorption toward catalytic oxidation pathways capable of degrading refractory dye molecules.

Notably, another cluster involves advanced oxidation processes and dye/salt separation technologies, indicating growing interest in coupling MOF-based materials with hybrid treatment approaches. The presence of keywords such as photocatalysis, advanced oxidation process, and azo dye highlights the shift toward multifunctional catalytic systems capable of simultaneous adsorption and oxidative degradation. Strong interconnections among clusters suggest that recent research increasingly integrates material synthesis, catalytic performance, and process optimization within unified treatment frameworks. Visualization of author keyword co-occurrence illustrating thematic clusters and research interactions in MOF-assisted dye wastewater treatment. Node size represents keyword frequency, while link strength reflects co-occurrence relationships. The clusters highlight dominant research themes including adsorption performance, photocatalytic degradation, advanced oxidation processes, and material fabrication strategies, demonstrating the evolution toward hybrid catalytic treatment systems.

4. Emerging Catalysts and Functional Materials

Advanced catalytic materials play a pivotal role in the development of next-generation oxidation systems for treating textile effluents characterized by intense coloration, high salinity, and recalcitrant organic content [58]. In contrast to conventional catalysts, which typically perform a single function (e.g., adsorption or radical generation), emerging catalytic platforms are rationally engineered to integrate multiple functionalities, including pollutant enrichment, efficient oxidant activation, and enhanced interfacial electron transfer, while preserving stability under realistic wastewater conditions. This multifunctional design is particularly critical for textile wastewater treatment, where complex dye structures, the presence of surfactants and salts, and matrix-induced radical scavenging collectively hinder the performance of standalone treatment processes [77].

Within this landscape, metal–organic frameworks (MOFs) have gained prominence as tunable catalytic scaffolds capable of integrating adsorption and oxidation within a single porous architecture [78]. The hybrid organic–inorganic nature of MOFs enables rational control over pore environment, redox-active metal nodes, and linker functionality, allowing optimization of catalytic pathways such as Fenton-like oxidation, photo-Fenton processes, photocatalysis, and persulfate activation [39]. Importantly, MOFs can be engineered into hybrid catalytic architectures via compositing with conductive carbon, magnetic phases, or secondary semiconductors, which enhances charge mobility, suppresses recombination, enhances ROS generation, and facilitates catalyst recovery key requirements for scalable implementation [79].

Other functional materials, including biochar-based supports and magnetite nanostructures, remain valuable components within MOF-centered hybrid catalysts, primarily playing stabilizing and enabling roles. Biochar and other carbonaceous matrices enhance adsorption capacity and facilitate electron shuttling, while magnetite provides redox-active Fe²⁺/Fe³⁺ cycling and enables magnetic separation. When integrated with MOFs or MOF-derived structures, these materials can mitigate intrinsic limitations such as hydrolytic instability, metal leaching, and recovery challenges, thereby improving catalyst durability and operational practicality [80].

Accordingly, their role in this review is considered mainly within the context of MOF hybridization strategies rather than as independent treatment approaches. The following sections therefore critically examine MOF-based catalytic platforms and their derived or composite architectures for textile dye oxidation, with emphasis on structure–property relationships, dominant oxidation mechanisms, stability and reusability constraints, and performance under realistic effluent conditions. A comparative summary of representative catalyst classes and their functional roles in hybrid oxidation systems is provided in

Table 3. Comparative Evaluation of MOF-Centered Hybrid Catalysts for Textile Dye Wastewater Treatment.

Catalyst Category	Role in Hybrid System	Key Structural Features	Primary Function	Advantages	Limitations	Ref.
MOF-based catalysts	Core catalytic platform	Metal nodes coordinated with organic ligands forming tunable porous frameworks	ROS generation, adsorption–oxidation coupling	High surface area; tunable active sites; multifunctionality	Hydrolytic instability (some MOFs); synthesis cost	[81]
MOF-derived carbon/metal composites	Stability-enhanced catalysts	Pyrolyzed MOFs forming metal–carbon hybrids	Enhanced electron transfer and heterogeneous Fenton activity	Improved conductivity; strong stability	Requires thermal processing; structural control needed	[82]
Biochar-supported MOFs	Support and electron mediator	Porous carbon matrix with oxygen functional groups	Adsorption enhancement and electron shuttling	Low cost; waste-derived; improved catalyst dispersion	Feedstock variability; moderate intrinsic activity	[83]
Magnetite–MOF composites	Magnetic recovery and redox enhancement	Fe3O4 nanoparticles integrated with MOF structures	Fenton-like redox cycling and catalyst separation	Magnetic recyclability; improved redox activity	Potential Fe leaching under acidic conditions	[84]
Metal-loaded carbon supports	Auxiliary catalytic component	Metal nanoparticles anchored on conductive carbon	Synergistic adsorption–oxidation	High recyclability; improved ROS generation	Metal aggregation over repeated cycles	[85]

, contextualizing the performance and limitations of MOF-centered materials relative to conventional benchmarks [81,82,83,84,85].

5. Fundamentals of Oxidation Processes

Oxidation processes constitute a cornerstone of advanced wastewater treatment technologies due to their ability to transform complex and recalcitrant organic pollutants into simpler, less harmful compounds. In textile effluent treatment, oxidation reactions are particularly important because many dye molecules, especially azo and reactive dyes, possess stable aromatic structures and electron-withdrawing functional groups that resist conventional biological degradation [86]. Oxidative pathways enable the cleavage of chromophoric bonds, destruction of aromatic rings, and eventual mineralization into carbon dioxide, water, and inorganic ions. At the molecular level, these processes involve electron transfer reactions in which reactive oxidizing species attack pollutant molecules. Among these species, hydroxyl radicals (•OH) are considered the most powerful non-selective oxidants, with a high oxidation potential (E° ≈ 2.8 V) [7]. These radicals react rapidly with organic contaminants through hydrogen abstraction, electrophilic addition, and electron transfer mechanisms, initiating chain reactions that progressively degrade complex dye structures. Other reactive oxygen species (ROS), including superoxide radicals (•O₂⁻), hydroperoxyl radicals (HO₂•), and singlet oxygen (¹O₂), also contribute to pollutant transformation depending on the oxidation system employed [7,87].

Advanced oxidation processes (AOPs) are characterized by the in situ generation of highly reactive species under controlled conditions. Common AOPs include Fenton and photo-Fenton reactions, photocatalysis, ozonation, persulfate activation, and electrochemical oxidation. In Fenton-based systems, ferrous ions react with hydrogen peroxide to produce hydroxyl radicals via cyclic Fe²⁺/Fe³⁺ redox reactions, whereas photocatalytic processes rely on semiconductor excitation under light irradiation to generate electron–hole pairs that initiate oxidative pathways [86].

The effectiveness of oxidation processes is governed by several operational parameters, including pH, oxidant concentration, catalyst properties, light intensity, and temperature. Acidic conditions traditionally favor homogeneous Fenton reactions; however, recent advances in heterogeneous catalysts and hybrid systems enable efficient oxidation under near-neutral conditions compatible with biological treatment units. Catalyst surface characteristics, electron mobility, and pollutant adsorption behavior strongly influence radical generation efficiency and overall reaction kinetics [77]. Despite their high degradation capability, standalone oxidation processes may suffer from limitations such as excessive oxidant consumption, incomplete mineralization, and high energy demand. Consequently, current research increasingly focuses on integrating oxidation processes with adsorption-based materials and nature-based systems to develop hybrid treatment platforms. Such integration enables oxidation to initiate pollutant breakdown, while biological and ecological processes complete mineralization and nutrient removal, thereby enhancing overall sustainability and operational stability [58].

5.1. Reactive Oxygen Species and Oxidation Mechanisms

Reactive oxygen species (ROS) are the primary driving force behind advanced oxidation processes (AOPs), enabling the degradation of persistent organic pollutants present in textile wastewater. These highly reactive intermediates are generated in situ through catalytic, photochemical, or electrochemical activation pathways and possess strong oxidation potentials capable of attacking stable dye molecules. The effectiveness of oxidation systems largely depends on the type, concentration, and lifetime of ROS produced during treatment [22,23].

Among the various oxidizing species, hydroxyl radicals (•OH) are considered the most powerful and non-selective oxidants, with an oxidation potential of approximately 2.8 V. Hydroxyl radicals rapidly react with organic compounds through hydrogen abstraction, electron transfer, and electrophilic addition mechanisms, leading to cleavage of chromophoric bonds such as azo (–N=N–) linkages and subsequent fragmentation of aromatic structures [26]. These reactions initiate a cascade of oxidative transformations that convert complex dye molecules into smaller intermediates and ultimately mineralize them into CO₂, H₂O, and inorganic ions. In addition to •OH radicals, other ROS including superoxide radicals (•O₂⁻), hydroperoxyl radicals (HO₂•), singlet oxygen (¹O₂), and sulfate radicals (SO₄•⁻) contribute to pollutant degradation depending on the oxidation system employed. Superoxide radicals are commonly generated during photocatalytic reactions via a reduction in dissolved oxygen by photogenerated electrons, while singlet oxygen may form through energy-transfer pathways. Sulfate radicals, produced via persulfate activation, exhibit higher selectivity and longer lifetimes compared with hydroxyl radicals, enabling effective degradation under broader pH conditions [23,33,34,35,36,37].

The generation pathways of ROS vary among oxidation technologies. In Fenton systems, Fe²⁺ reacts with hydrogen peroxide to produce hydroxyl radicals through cyclic redox reactions:

Fe²⁺ + H₂O₂ → Fe³⁺ + •OH + OH⁻

(1)

Fe³⁺ + e⁻ → Fe²⁺

(2)

H₂O₂ + hν → 2•OH

(3)

O₂ + e⁻ → •O₂⁻

(4)

•O₂⁻ + H⁺ → HO₂•

(5)

HO₂• + HO₂• → H₂O₂ + O₂

(6)

H₂O + h⁺ → •OH + H⁺

(7)

Organic dye + •OH → Oxidized intermediates → CO₂ + H₂O

(8)

The subsequent regeneration of Fe²⁺ sustains continuous radical production. In photocatalysis, semiconductor excitation under light irradiation generates electron–hole pairs (e⁻/h⁺), where photogenerated holes oxidize water or hydroxide ions to produce •OH radicals, while electrons reduce oxygen to form •O₂⁻ species. These coupled redox reactions enhance oxidative efficiency and extend degradation pathways [16]. The interaction between ROS and pollutants is strongly influenced by catalyst surface properties and adsorption behavior. Effective catalysts promote pollutant pre-concentration near reactive sites, facilitating rapid radical attack and improving reaction kinetics. Consequently, modern catalyst design focuses on enhancing electron transfer, suppressing charge recombination, and stabilizing active sites to maximize ROS generation efficiency under environmentally compatible conditions. Hence, understanding ROS formation and reaction pathways provides the mechanistic foundation for designing hybrid oxidation systems, where catalytic reactions initiate pollutant breakdown and biological or ecological processes complete mineralization [25,44]. This mechanistic synergy underpins the development of advanced catalytic platforms discussed in the subsequent sections.

5.2. Radical Generation Pathways

5.2.1. Fenton and Photo-Fenton Oxidation Mechanisms

Fenton and photo-Fenton reactions represent some of the most effective advanced oxidation processes for degrading refractory organic pollutants in textile wastewater. These systems rely on the catalytic decomposition of hydrogen peroxide (H₂O₂) in the presence of iron species to generate highly reactive hydroxyl radicals (•OH), which initiate rapid oxidation of dye molecules. Due to their strong oxidative capacity, operational simplicity, and relatively low chemical cost, Fenton-based processes have been extensively investigated for the treatment of azo dyes, reactive dyes, and other persistent textile contaminants.

5.2.2. Classical Fenton Reaction

The classical Fenton reaction involves the reaction between ferrous ions (Fe²⁺) and hydrogen peroxide under acidic conditions, producing hydroxyl radicals. The generated •OH radicals non-selectively attack organic molecules through oxidation, hydrogen abstraction, and aromatic ring opening. Simultaneously, ferric ions (Fe³⁺) can be reduced back to Fe²⁺ through secondary reactions, maintaining the catalytic redox cycle. This cyclic Fe²⁺/Fe³⁺ transition sustains continuous radical generation, enabling efficient degradation of complex dye structures and rapid decolorization [1,2,3].

Despite its effectiveness, the homogeneous Fenton process exhibits several intrinsic limitations, including a narrow optimal pH range (≈2.8–3.5), continuous iron consumption, and the generation of iron-containing sludge. These drawbacks restrict large-scale implementation and motivate the development of heterogeneous and hybrid Fenton systems [22].

5.2.3. Photo-Fenton Enhancement

The photo-Fenton process enhances classical Fenton chemistry through light irradiation (UV or solar), which accelerates iron redox cycling and increases radical production. Under irradiation, ferric complexes undergo photoreduction reaction, such reaction regenerates Fe²⁺ more rapidly, sustaining continuous hydroxyl radical formation while improving oxidant utilization efficiency. Additionally, photolysis of hydrogen peroxide contributes to further radical generation [42].

The synergistic interaction between photochemical activation and catalytic oxidation significantly enhances degradation kinetics, mineralization efficiency, and treatment performance compared with dark Fenton reactions. Solar-driven photo-Fenton systems have gained increasing attention due to their ability to utilize natural sunlight as a renewable energy source, thereby reducing operational costs and energy demand. Such systems are particularly attractive for decentralized wastewater treatment and integration with nature-based treatment platforms.

5.2.4. Heterogeneous Fenton and Catalyst Development

To overcome the limitations of homogeneous systems, heterogeneous Fenton catalysts have been developed by immobilizing iron species onto solid supports such as biochar, magnetite (Fe₃O₄), metal oxides, and metal–organic frameworks (MOFs). These materials enable catalyst recovery, reduce iron leaching, and extend operational pH ranges toward near-neutral conditions compatible with biological treatment processes. Magnetite-based catalysts facilitate electron transfer between Fe²⁺ and Fe³⁺ sites, while conductive carbon supports enhance charge mobility and pollutant adsorption near catalytic sites. Similarly, MOF-based catalysts provide tunable coordination environments and high surface areas, promoting efficient oxidant activation and improved light harvesting. These advancements allow Fenton-like reactions to operate within hybrid systems combining adsorption, photocatalysis, and biological polishing mechanisms [25].

5.2.5. Role in Hybrid Treatment Systems

In modern wastewater treatment strategies, Fenton and photo-Fenton reactions increasingly function as the primary oxidation stage within hybrid treatment architectures. Oxidative processes initiate fragmentation of recalcitrant dye molecules into biodegradable intermediates, which are subsequently mineralized through microbial activity or nature-based systems such as constructed wetlands. This sequential oxidation–biodegradation synergy improves overall treatment efficiency while minimizing chemical consumption and sludge generation [23]. Consequently, Fenton-based oxidation has evolved from a standalone chemical process into a core component of integrated catalytic–ecological treatment platforms, aligning with sustainable wastewater management and circular-economy principles [26].

Figure 6 summarizes the primary radical generation and propagation mechanisms occurring in Fenton, photo-Fenton, and photocatalytic systems [24]. The process is initiated by the activation of oxidants such as hydrogen peroxide or dissolved oxygen, leading to the formation of highly reactive species including hydroxyl radicals (•OH), superoxide radicals (•O₂⁻), and hydroperoxyl radicals (HO₂•). In Fenton-based systems, Fe²⁺ reacts with H₂O₂ to generate •OH, while Fe³⁺ is subsequently reduced back to Fe²⁺, sustaining the catalytic redox cycle. Under irradiation, photo-Fenton processes accelerate Fe³⁺ photoreduction and may directly photolyze H₂O₂, enhancing radical yield. In photocatalytic systems, semiconductor excitation produces electron/hole pairs; photogenerated holes oxidize water or hydroxide ions to form •OH, while electrons reduce oxygen to generate •O₂⁻ [25]. These radicals attack dye molecules through hydrogen abstraction, electrophilic addition, and aromatic ring opening, leading to decolorization and progressive mineralization into CO₂, H₂O, and inorganic ions. The interplay between radical formation, propagation, and termination reactions determines the overall degradation efficiency and selectivity of oxidation systems [5,19,22].

5.3. Mechanistic Probing and Reactive Oxygen Species Identification Methods

The elucidation of degradation mechanisms represents a critical requirement for understanding the catalytic behavior of metal–organic framework (MOF)-based oxidation systems in textile wastewater treatment. Because advanced oxidation processes typically involve multiple reactive oxygen species (ROS) operating simultaneously, reliable identification of dominant oxidative pathways is essential to distinguish adsorption-driven discoloration from true catalytic oxidation. Mechanistic probing therefore plays a central role in validating catalyst functionality, optimizing material design, and enabling meaningful comparison among reported studies [22].

5.3.1. Radical Scavenger-Based Mechanistic Probing

Radical scavenger quenching experiments are widely employed as indirect yet practical tools for identifying active species involved in catalytic oxidation reactions (

Table 4. Mechanistic Probing and ROS Identification Methods.

Scavenger	Target Reactive Species	Mechanistic Role	Ref.
Tert-butanol (TBA)	Hydroxyl radicals (•OH)	Selective quencher for hydroxyl radicals; commonly used to verify Fenton-like pathways.	[25]
Methanol (MeOH)	•OH and SO4•−	Broad radical scavenger; helps distinguish sulfate-radical systems when compared with TBA.	[26]
p-Benzoquinone (BQ)	Superoxide radicals (•O2−)	Captures electron-derived oxygen radicals in photocatalytic mechanisms.	[26]
Furfuryl alcohol (FFA)	Singlet oxygen (1O2)	Selective probe for non-radical oxidation pathways.	[76]
L-Histidine	Singlet oxygen (1O2)	Alternative quencher confirming energy-transfer oxidation mechanisms.	[77]

). In these tests, selective quenching agents are introduced into the reaction system to suppress specific ROS, and the resulting change in degradation performance is used to infer mechanistic contributions. Although qualitative in nature, comparative inhibition analysis provides valuable insight into dominant oxidation pathways when carefully interpreted [25].

Common scavengers used in MOF-assisted dye oxidation studies include tert-butanol (TBA), methanol (MeOH), p-benzoquinone (BQ), furfuryl alcohol (FFA), and L-histidine. TBA is widely recognized as a selective hydroxyl radical (•OH) scavenger due to its rapid reaction kinetics with surface-bound radicals, making it particularly useful for identifying Fenton-like mechanisms. Methanol acts as a broader radical quencher capable of reacting with both hydroxyl and sulfate radicals (SO₄•⁻), allowing differentiation between radical systems when compared with TBA inhibition behavior. In photocatalytic systems, p-benzoquinone selectively captures superoxide radicals (•O₂⁻), providing evidence of electron-mediated oxygen activation pathways. Furfuryl alcohol and L-histidine are commonly applied probes for singlet oxygen (¹O₂), enabling identification of non-radical oxidation routes that increasingly emerge in engineered catalytic systems [26].

Significant reduction in degradation efficiency following the addition of a specific scavenger indicates the involvement of the corresponding reactive species. For example, strong inhibition by TBA typically confirms hydroxyl radical dominance characteristic of Fenton or photo-Fenton processes, whereas pronounced suppression by BQ suggests photocatalytic electron transfer pathways generating superoxide radicals. Detection of singlet oxygen participation through FFA or L-histidine quenching indicates alternative oxidation mechanisms governed by energy-transfer processes rather than classical radical reactions.

5.3.2. Interpretation Considerations and Methodological Limitations

Despite their widespread use, scavenger experiments require cautious interpretation. Many quenching agents exhibit partial selectivity and may influence adsorption behavior or catalyst surface chemistry, potentially introducing experimental bias. Consequently, mechanistic conclusions should rely on comparative inhibition trends rather than single-scorer observations. Reliable interpretation generally involves evaluation of multiple scavengers under identical conditions, correlation with catalyst composition and oxidant type, and consistency with known reaction chemistry [25]. For instance, comparable inhibition by methanol and TBA may indicate sulfate radical participation, while dominant BQ inhibition under irradiation conditions supports photocatalytic pathways involving oxygen reduction. Such comparative analysis allows identification of synergistic ROS systems frequently observed in MOF-based hybrids, where radical and non-radical pathways coexist.

5.3.3. Complementary Techniques for ROS Verification

To strengthen mechanistic reliability, scavenger tests are increasingly combined with spectroscopic and electrochemical characterization techniques. Electron paramagnetic resonance (EPR/ESR) spectroscopy with spin-trapping agents (e.g., DMPO or TEMP) enables the direct detection of transient radical species and provides stronger experimental evidence for ROS formation. Fluorescence probe methods, such as terephthalic acid assays for hydroxyl radicals, offer additional indirect confirmation. Furthermore, photoluminescence spectroscopy, electrochemical impedance spectroscopy, and transient photocurrent measurements are commonly used to evaluate charge separation and electron-transfer behavior in photocatalytic MOF systems. Integration of quenching experiments with these complementary analyses significantly enhances mechanistic confidence and reduces ambiguity in assigning degradation pathways.

5.3.4. Relevance to MOF-Based Catalyst Design

Mechanistic probing not only clarifies reaction pathways but also informs rational catalyst engineering strategies. The identification of dominant ROS allows the optimization of metal-node selection, linker functionalization, and hybridization approaches to favor desired oxidation routes. Iron-based MOFs typically promote hydroxyl radical formation through cyclic redox reactions, whereas semiconductor-coupled MOFs enhance superoxide-mediated photocatalytic pathways. Recognition of singlet oxygen participation has further expanded catalyst design toward selective, non-radical oxidation mechanisms less susceptible to radical scavenging in saline textile wastewater matrices. Accordingly, systematic ROS identification should be regarded as a fundamental requirement in evaluating MOF-based catalytic systems. Adoption of standardized mechanistic probing approaches will improve reproducibility, enable cross-study comparison, and support the development of robust hybrid catalysts capable of achieving efficient and reliable textile wastewater remediation [25,54].

6. MOFs as Catalytic Platforms

MOFs have emerged as a versatile class of crystalline porous materials that bridge coordination chemistry and heterogeneous catalysis. Constructed from metal ions or metal-oxo clusters interconnected by organic linkers, MOFs possess highly ordered architectures with tunable physicochemical properties that distinguish them from conventional catalytic materials. Their structural diversity, exceptionally high surface areas, and adjustable pore environments enable precise control over adsorption behavior, active site accessibility, and catalytic reactivity. These characteristics have positioned MOFs as promising platforms for advanced oxidation and hybrid catalytic systems targeting complex industrial effluents, including textile wastewater [24,27].

6.1. Structural Features and Tunable Properties of MOFs

The defining feature of MOFs lies in their modular construction, where inorganic secondary building units (SBUs) are coordinated with organic linkers to form extended porous networks. This modularity enables nearly unlimited structural variability through selection of metal centers, ligand functionality, topology, and synthesis conditions. As a result, MOFs can be engineered with specific pore sizes, surface chemistries, and functional groups tailored for targeted pollutant interactions [32].

High surface areas, often exceeding 1000–5000 m² g⁻¹, facilitate the enhanced adsorption of dye molecules, effectively concentrating pollutants near catalytic active sites. This adsorption–catalysis coupling is particularly advantageous for textile effluents containing large aromatic compounds that suffer from mass-transfer limitations in conventional systems. Additionally, functionalization of organic linkers with electron-donating or electron-withdrawing groups including –NH₂, –COOH, –SO₃H that enables tuning of hydrophilicity, light absorption properties, and charge-transfer behavior. Structural flexibility in certain MOFs, such as breathing or gate-opening behavior, further enhances pollutant accessibility and diffusion within the framework. Moreover, post-synthetic modification strategies allow incorporation of additional catalytic species, formation of heterojunctions, or anchoring of nanoparticles, thereby transforming pristine MOFs into hybrid catalytic architectures with enhanced activity and stability [32].

6.2. Catalytically Active Sites and Electronic Properties

The catalytic performance of MOFs originates primarily from their metal nodes, ligand environments, and metal–ligand electronic interactions. Unsaturated metal coordination sites can act as Lewis acid centers capable of activating oxidants such as hydrogen peroxide, persulfate, or dissolved oxygen, promoting the formation of reactive oxygen species (ROS) including hydroxyl radicals (•OH), sulfate radicals (SO₄•⁻), and superoxide radicals (•O₂⁻) [64]. Additionally, in photocatalytic systems, MOFs exhibit semiconductor-like behavior in which light absorption induces electron excitation from ligand orbitals to metal centers through ligand-to-metal charge transfer (LMCT) or metal-to-ligand charge transfer (MLCT) processes. These electronic transitions facilitate efficient charge separation and suppress recombination losses when appropriately engineered. Amino-functionalized MOFs, for example, exhibit narrowed band gaps that enable visible-light activation, improving solar utilization for environmental applications [23].

Hybridization with conductive materials or secondary semiconductors further enhances electron mobility and interfacial charge transfer. Such hybrid MOF systems promote synergistic catalytic pathways by combining adsorption capacity with enhanced redox cycling, thereby accelerating degradation kinetics and improving mineralization efficiency for persistent textile dyes.

6.3. Stability and Water Compatibility of MOFs

Despite their promising catalytic characteristics, the practical application of MOFs in aqueous environments remains strongly influenced by structural stability. Hydrolytic degradation can occur through cleavage of metal–ligand coordination bonds, particularly in MOFs constructed from low-valence metal centers. Exposure to varying pH conditions, dissolved salts, and oxidizing agents commonly present in textile wastewater may accelerate framework decomposition or induce metal leaching [25].

To address these challenges, significant research has focused on developing water-stable MOFs based on high-valence metal clusters such as Zr⁴⁺, Ti⁴⁺, and Al³⁺, which form stronger coordination bonds and enhanced structural robustness. Surface functionalization, composite formation, and MOF-derived materials have also been explored to improve resistance to hydrolysis and maintain catalytic performance under realistic operating conditions. Water compatibility further depends on factors such as pore hydrophilicity, particle morphology, and catalyst immobilization strategies. Immobilized or supported MOFs can reduce structural collapse while facilitating catalyst recovery and reuse. Nevertheless, long-term stability, regeneration efficiency, and prevention of metal ion release remain critical considerations that must be addressed before large-scale implementation of MOF-based catalytic systems in textile effluent treatment [64].

6.4. MOF-Based Catalytic Mechanism in Textile Wastewater Oxidation

MOF-based catalysts enable textile wastewater treatment through a synergistic mechanism integrating pollutant adsorption, redox catalysis, and photo-induced reactive oxygen species (ROS) generation within a highly ordered porous framework as shown in Figure 7. Their hybrid organic–inorganic architecture provides spatially distributed active sites, where metal nodes act as verified redox-active centers and organic linkers contribute to light absorption and charge transfer, collectively enhancing catalytic efficiency and charge separation [54,66,69].

The process is initiated by the adsorption of dye molecules onto the MOF surface and within its pore network, where strong π–π interactions, electrostatic attraction, and coordination with unsaturated metal sites promote pollutant enrichment near active centers. This adsorption step facilitates efficient electron transfer and accelerates subsequent oxidation reactions [32,66]. Catalytic activation occurs primarily at the metal nodes (e.g., Fe, Cu, Co, Ti), which activate oxidants such as H₂O₂, persulfate, or dissolved oxygen. In Fenton-like systems, cyclic redox transitions (e.g., Fe²⁺/Fe³⁺) sustain continuous generation of hydroxyl radicals (•OH), forming the basis of oxidative degradation pathways [23,55].

Under light irradiation, photocatalytic and photoelectrocatalytic systems exhibit enhanced performance through ligand-to-metal or metal-to-ligand charge transfer, generating electron–hole pairs. Photogenerated electrons reduce oxygen to produce superoxide radicals (•O₂⁻), while holes oxidize water or hydroxide ions to generate •OH radicals. In photoelectrocatalytic systems, the application of an external bias further improves charge separation efficiency and suppresses recombination, leading to increased ROS generation and extended catalyst activity [28,54].

Importantly, the involvement of ROS including •OH, •O₂⁻, SO₄•⁻, and ¹O₂ has been experimentally validated using electron paramagnetic resonance (EPR/ESR) spectroscopy and radical scavenger quenching tests. For instance, tert-butanol and methanol are commonly used to confirm •OH and SO₄•⁻ pathways, while p-benzoquinone and furfuryl alcohol are employed to identify •O₂⁻ and singlet oxygen contributions, respectively. These techniques provide direct and indirect evidence of ROS generation, enabling reliable identification of dominant oxidation mechanisms [25,26,54].

The generated ROS attack dye molecules through non-selective oxidative pathways, initiating the cleavage of chromophoric groups (e.g., azo bonds), followed by the progressive oxidation of aromatic rings and side-chain functional groups. Intermediate products are subsequently transformed into smaller oxygenated compounds and ultimately mineralized into CO₂, H₂O, and inorganic ions. The hierarchical pore structure of MOFs promotes repeated interaction between intermediates and active sites, enhancing mineralization efficiency [32,66]. It is critical to distinguish between proposed and experimentally validated mechanisms. While many studies suggest reaction pathways based on theoretical or indirect observations, only those supported by EPR analysis, scavenger experiments, and complementary spectroscopic techniques can be considered fully validated. This distinction is essential for accurate mechanistic interpretation and rational catalyst design.

Hybridization strategies further enhance catalytic performance by improving electron transport, stabilizing active sites, and increasing structural robustness under aqueous conditions. However, challenges such as metal leaching, pore blockage, and catalyst deactivation remain, highlighting the need for controlled redox cycling and stability-oriented MOF design for practical applications [66,69].

6.5. Comparative Analysis of MOF-Based Catalysts with Emerging Catalytic Systems

MOF-based catalysts have attracted significant attention due to their tunable structure, high surface area, and ability to integrate adsorption with catalytic oxidation. However, a critical assessment relative to other emerging catalysts such as covalent organic frameworks (COFs), carbon-based materials, and semiconductor photocatalysts is essential to evaluate their practical applicability in textile wastewater treatment.

From a reactive selectivity perspective, MOF-based systems exhibit strong versatility due to the presence of well-defined metal nodes capable of activating multiple oxidants [88]. For example, Fe- and Cu-based MOFs predominantly generate hydroxyl radicals (•OH) through Fenton-like mechanisms, while persulfate activation enables production of sulfate radicals (SO₄•⁻), allowing operation across a broader pH range. In contrast, semiconductor photocatalysts such as TiO₂ primarily rely on photogenerated electron–hole pairs to produce •OH and •O₂⁻, often limited by rapid charge recombination and lower selectivity under visible light. COFs, being metal-free frameworks, typically exhibit photocatalytic activity through π-conjugated structures and favor selective pathways such as singlet oxygen (¹O₂) generation [89], offering higher selectivity but generally lower oxidation strength compared to radical-driven systems. Carbon-based catalysts (e.g., biochar, graphene, carbon nanotubes) often operate through non-radical pathways or electron transfer mechanisms, providing improved selectivity and resistance to radical scavenging but limited mineralization capability [88,89].

In terms of quantitative performance, MOF-based hybrid systems frequently achieve degradation efficiencies exceeding 90% and significant total organic carbon (TOC) removal (typically 50–80%) under optimized conditions, outperforming many standalone carbon-based and COF systems. Hybrid MOF composites further enhance reaction kinetics, often demonstrating 2–4-fold increases in degradation rates compared to pristine materials due to improved charge separation and electron transport [58]. Regarding stability under practical conditions, semiconductor catalysts such as TiO₂ exhibit superior structural robustness and long-term durability across a wide pH range. Similarly, carbon-based materials offer excellent chemical stability, low cost, and resistance to fouling in complex wastewater matrices. In contrast, MOF stability remains highly dependent on metal–ligand coordination strength. While Zr- and Ti-based MOFs demonstrate high hydrolytic stability, many transition-metal MOFs (e.g., Fe-, Cu-based) are susceptible to structural degradation and metal leaching under acidic or oxidative conditions. COFs generally show improved chemical stability compared to MOFs but may suffer from lower catalytic activity and limited redox functionality [90].

Under real wastewater conditions, additional challenges such as high salinity, competing ions, and organic matter significantly influence catalyst performance. Radical scavenging by chloride and carbonate species can reduce the efficiency of ROS-driven systems, particularly for •OH-based pathways [30]. In this context, MOF-based hybrid catalysts demonstrate improved adaptability due to their combined adsorption–oxidation functionality, which enhances pollutant concentration near active sites and mitigates matrix effects. However, issues related to catalyst recovery, long-term stability, and economic feasibility remain critical barriers to large-scale implementation. Hence, MOF-based catalysts offer a unique balance between high catalytic activity, tunable selectivity, and multifunctionality, particularly when integrated into hybrid systems. Nevertheless, no single catalyst platform simultaneously optimizes activity, stability, cost, and scalability. Future research should therefore focus on hybrid design strategies, stability enhancement, and standardized performance evaluation under realistic conditions to enable meaningful comparison and facilitate practical deployment [88,90].

Whereas Figure 8 provides a detailed illustration of the adsorption behavior, reactive oxygen species generation, and cyclic redox pathways governing MOF-based catalytic degradation, a broader perspective is necessary to place these mechanisms within the overall context of advanced oxidation technologies and practical implementation. In this regard, Figure 8 presents an integrated overview that connects fundamental reaction pathways with system-level considerations, including photocatalytic processes, comparative performance among emerging catalyst systems, and reactor configurations for scalable wastewater treatment [91]. This comprehensive representation highlights the progression from molecular-level mechanisms to engineering applications, emphasizing the role of MOF-based catalysts as a bridge between high catalytic efficiency and real-world deployment in sustainable wastewater treatment systems.

7. Progress and Constraints in MOF-Based Dye Oxidation Catalysis

The growing diversity of MOF-based catalysts developed for textile wastewater treatment necessitates systematic performance evaluation beyond simple reporting of degradation efficiencies. While numerous studies demonstrate rapid dye removal using MOF systems, meaningful comparison requires consideration of mineralization capability, catalytic mechanisms, operational stability, and adaptability to realistic wastewater conditions [90]. Comparative assessment therefore provides critical insight into the practical potential and limitations of different MOF architectures. Representative examples summarized in Table 4 reveal clear relationships between metal center chemistry, oxidation pathway, and overall catalytic performance [75,76,77,78,79,80,81,82,83].

A dominant trend observed across the literature is the superior oxidation activity of iron-based MOFs operating via Fenton and photo-Fenton mechanisms. Frameworks such as MIL-53(Fe) and MIL-100(Fe) exhibit efficient Fe²⁺/Fe³⁺ cyclic redox behavior, enabling continuous activation of hydrogen peroxide and sustained hydroxyl radical generation [30]. This mechanism promotes rapid chromophore cleavage and high discoloration efficiencies, frequently exceeding 90%. In addition, the porous structure of these MOFs enhances adsorption-assisted oxidation, facilitating repeated interactions between dye molecules and reactive species. However, despite their strong catalytic activity, iron-based MOFs often suffer from partial framework degradation and iron leaching during prolonged operation, particularly under circumneutral pH conditions typical of industrial effluents. These observations highlight a recurring trade-off between catalytic reactivity and structural stability [58].

Zirconium-based MOFs, exemplified by NH₂-UiO-66, exhibit an alternative performance profile characterized by exceptional hydrolytic stability arising from strong Zr–O coordination bonds. Such materials maintain structural integrity across a wide pH range and under oxidative conditions, making them attractive candidates for aqueous applications. Nevertheless, their catalytic efficiency is primarily governed by photocatalytic charge-transfer processes rather than strong redox cycling, resulting in comparatively slower oxidation kinetics. Although functionalization strategies can enhance visible-light absorption, reactive oxygen species (ROS) generation remains lower than that observed in Fenton-active MOFs. This contrast highlights a fundamental design challenge in MOF catalysis: balancing long-term structural stability with high oxidative performance [92].

Zeolitic imidazolate frameworks (ZIFs), particularly cobalt-based ZIF-67, have attracted attention due to their ability to activate persulfate and generate sulfate radicals, which retain high oxidation potential over broader pH ranges than hydroxyl radicals. These systems often exhibit rapid reaction kinetics and strong adaptability to variable wastewater conditions [93]. However, concerns regarding cobalt ion release and environmental toxicity persist, raising questions about long-term sustainability. Zinc-based ZIF-8, while exhibiting excellent adsorption capacity and rapid dye removal, primarily functions through adsorption-assisted pathways and therefore may not achieve complete mineralization without additional catalytic modification.

Hybrid MOF architectures represent a significant advancement in performance optimization [94]. Composites formed by coupling MOFs with semiconductors, including g-C₃N₄, conductive carbon materials, or magnetic phases, enhance electron mobility and improve charge separation efficiency. These hybrid systems frequently demonstrate accelerated degradation rates, improved catalyst recyclability, and reduced metal leaching compared with pristine MOFs. The enhanced performance arises from synergistic interactions that facilitate electron transfer, stabilize intermediate oxidation states, and extend the lifetime of reactive species. Such findings suggest that hybridization is not merely a modification strategy but a necessary step toward the practical deployment of MOF-based catalysts [90].

When benchmarked against conventional catalysts, MOF-based systems exhibit both advantages and remaining limitations. Compared with TiO₂ photocatalysts, MOFs offer tunable electronic structures and enhanced adsorption capacity, enabling improved interaction with large dye molecules. Relative to homogeneous Fenton systems, MOFs reduce sludge formation and facilitate catalyst recovery. However, traditional catalysts still outperform many MOFs in terms of cost-effectiveness, mechanical robustness, and industrial scalability [90]. The complexity of MOF synthesis and concerns regarding long-term stability continue to limit widespread adoption. Hence, comparative analyses indicate that MOF-based catalysts provide a highly adaptable platform capable of integrating adsorption, redox catalysis, and photocatalytic oxidation within a single material framework. Nevertheless, no single MOF system simultaneously maximizes activity, durability, and economic feasibility [90]. Future progress will therefore depend on rational catalyst engineering that integrates hybrid design strategies with enhanced stability and scalable synthesis approaches. Such developments are essential for translating promising laboratory-scale dye oxidation results into viable textile wastewater treatment technologies (

Table 5. MOF-Based Catalysts for Dye Oxidation in Textile Wastewater Treatment.

MOF Catalyst	Metal Center	Target Dye	Oxidation System	Dominant Reactive Species	Typical Performance	Advantage	Limitation	Ref.
MIL-53(Fe)	Fe	Reactive Black 5, Methylene Blue	Fenton/Photo-Fenton	•OH	>90% degradation	Strong redox cycling	Stability near neutral pH	[95]
MIL-100(Fe)	Fe	Rhodamine B, Congo Red	Photo-Fenton	•OH, •O2−	High mineralization (TOC reduction)	Large pore volume	Iron leaching risk	[96]
NH2-UiO-66(Zr)	Zr	Rhodamine B	Photocatalysis	h+, •O2−	Enhanced visible-light activity	Excellent water stability	Moderate ROS generation	[97]
ZIF-67	Co	Azo dyes	Persulfate activation	SO4•−	Rapid oxidation kinetics	Wide pH applicability	Metal ion leaching	[93]
ZIF-8	Zn	Cationic dyes	Adsorption–oxidation hybrid	•OH (indirect)	Fast color removal	High surface area	Limited hydrolytic stability	[93]
MIL-125(Ti)	Ti	Reactive dyes	Photocatalysis	h+, •OH	Good photostability	Solar activation potential	Slow charge transfer	[98]
HKUST-1	Cu	Dye intermediates	Catalytic oxidation	•OH	High catalytic activity	Accessible active sites	Water instability	[99]
MOF/g-C3N4 Composite	Fe/Zr hybrid	Mixed textile dyes	Photocatalytic hybrid	•OH, •O2−	2–4× rate enhancement	Improved charge separation	Complex synthesis	[100]
MOF-derived Fe3O4/C	Fe	Textile dye mixtures	Heterogeneous Fenton	•OH	High recyclability	Magnetic recovery	Structural transformation	[101]

) [93,95,96,97,98,99,100,101].

7.1. Comparative Performance Benchmarking of Catalytic Systems

Although numerous catalytic systems have been explored for textile dye wastewater treatment, direct comparison among different technologies remains challenging due to variations in experimental conditions, pollutant composition, and evaluation methodologies reported across studies. To provide an integrated and critical perspective, a qualitative benchmarking analysis was conducted to compare representative catalyst classes, including MOF-based hybrid catalysts, TiO₂ photocatalysts, homogeneous Fenton systems, and biochar-based catalysts, across key performance and sustainability criteria relevant to practical wastewater treatment [21].

The comparative framework evaluates catalyst performance according to six parameters: catalytic activity, structural stability, economic cost, recyclability, scalability potential, and environmental safety. These criteria were selected because they collectively determine technological feasibility beyond laboratory-scale degradation efficiency, incorporating engineering, environmental, and economic considerations. Performance scores were assigned using a normalized qualitative scale ranging from 1 (very poor) to 5 (excellent), based on consistent trends and consensus observations reported across the literature rather than individual experimental datasets.

As illustrated in Figure 9, MOF-based hybrid catalysts exhibit superior catalytic activity, attributed to their tunable active sites, high surface area, and efficient generation of reactive oxygen species. However, comparatively lower scores in cost and scalability reflect current challenges associated with synthesis complexity and large-scale production. In contrast, TiO₂ photocatalysts demonstrate excellent structural stability and environmental compatibility but moderate catalytic performance due to charge recombination limitations [21]. Homogeneous Fenton systems provide high oxidation efficiency yet suffer from poor recyclability and environmental drawbacks related to iron sludge formation. Biochar-based catalysts achieve strong performance in cost-effectiveness, scalability, and sustainability owing to their waste-derived origin, although their intrinsic catalytic activity is generally lower than that of MOF-centered systems. Hence, the benchmarking highlights a central design dilemma: catalytic activity and mineralization potential often increase with catalyst complexity, while cost, scalability, and environmental reliability become more challenging [102,103]. The purpose of this semi-quantitative visualization is to summarize literature-consistent trade-offs and relative technology readiness rather than to provide absolute performance metrics. These findings reinforce the strategic value of MOF-centered hybrid architectures, particularly when stability enhancement, immobilization, and life-cycle-informed design are integrated to bridge laboratory performance with practical textile wastewater deployment [102].

7.2. Standardized Evaluation Metrics for MOF-Based Oxidation Systems

Despite the rapid expansion of metal–organic framework (MOF)-based catalysts for textile dye wastewater treatment, meaningful comparison among reported studies remains challenging due to inconsistent evaluation criteria and non-uniform experimental reporting. Many investigations emphasize rapid color removal as the primary performance indicator; however, decolorization alone does not necessarily reflect complete pollutant degradation or environmental safety [74]. In textile effluents, chromophoric bond cleavage may occur rapidly, while dissolved organic carbon and potentially toxic intermediates persist in solution. Consequently, standardized evaluation metrics are required to distinguish between visual discoloration, partial oxidation, and true mineralization, while simultaneously assessing catalyst stability, environmental compatibility, and process feasibility. Establishing unified reporting protocols is therefore essential to advance MOF-based oxidation systems from laboratory demonstrations toward practical wastewater treatment applications [104].

7.2.1. Treatment Performance Indicators: Decolorization Versus Mineralization

Color removal determined by UV–Vis spectroscopy should be considered a preliminary indicator rather than a definitive measure of treatment efficiency. Accurate assessment requires reporting spectral evolution across a broad wavelength range to confirm degradation of aromatic structures rather than attenuation at a single absorption maximum. More importantly, bulk organic removal must be quantified using chemical oxygen demand (COD) and total organic carbon (TOC), which provide complementary information on oxidizable load and carbon mineralization, respectively. Reporting both parameters enables differentiation between molecular fragmentation and genuine mineralization processes. Whenever feasible, identification of degradation intermediates using chromatographic techniques (e.g., LC–MS or GC–MS) is recommended to verify reaction pathways and evaluate the persistence of potentially hazardous byproducts such as aromatic amines formed during azo dye cleavage [105].

7.2.2. Catalyst Stability, Metal Leaching, and Mass Balance

The long-term applicability of MOF catalysts strongly depends on their structural stability under aqueous and oxidative conditions typical of textile wastewater. Quantification of metal leaching should therefore be systematically reported using analytical techniques such as ICP-OES, ICP-MS, or atomic absorption spectroscopy, expressed relative to the total metal content of the catalyst [91]. In addition, catalyst recovery efficiency and mass balance should be evaluated after each reuse cycle to distinguish true heterogeneous catalytic activity from homogeneous contributions arising from dissolved metal species. Structural integrity should be verified through pre- and post-reaction characterization (e.g., XRD, FTIR, XPS, and surface area analysis) to assess framework preservation, pore blockage, or phase transformation. Reusability studies should include multiple cycles with clearly defined regeneration procedures, with performance evaluated based on mineralization metrics rather than discoloration alone [106,107].

7.2.3. Ecotoxicity and Environmental Safety Assessment

Because incomplete oxidation may generate transformation products with equal or greater toxicity than parent dyes, treatment performance should incorporate toxicity evaluation alongside chemical removal metrics. Standardized bioassays such as Microtox (Vibrio fischeri inhibition), algal growth inhibition, or Daphnia magna immobilization tests provide valuable insight into ecological safety. Reporting toxicity reduction before and after treatment allows verification that catalytic oxidation leads to detoxification rather than accumulation of reactive intermediates. Particular attention should be given to textile wastewater containing high chloride concentrations, where secondary oxidized or chlorinated byproducts may form under oxidative conditions [108].

7.2.4. Energy Efficiency and Process Intensity

The assessment of energy demand is essential for evaluating the scalability of MOF-based oxidation technologies, especially in photo-assisted systems. Energy consumption should be expressed using normalized metrics such as electrical energy per order (EEO), energy consumption per treated volume (kWh m⁻³), or energy per unit COD or TOC removed. For solar-driven systems, reporting irradiation intensity, cumulative solar dose, reactor geometry, and optical path length is necessary to enable reproducibility and comparison across studies. Such metrics facilitate objective assessment of process sustainability beyond laboratory-scale efficiency [109].

7.2.5. Oxidant Utilization Efficiency

Oxidant consumption represents a major operational and environmental cost in advanced oxidation processes. Accordingly, studies should quantify oxidant utilization efficiency by relating hydrogen peroxide or persulfate consumption to pollutant removal or mineralization performance. Reporting oxidant dosing strategy (single addition versus continuous or stepwise dosing) and residual oxidant concentrations provides insight into reaction efficiency and radical utilization [109]. Consideration of matrix effects, including radical scavenging by salts and background organic matter commonly present in textile effluents, is essential for realistic performance evaluation [56].

7.2.6. Minimum Reporting Parameters for Reproducibility

To improve comparability and reproducibility, future studies should provide a consistent set of experimental details, including wastewater composition, initial pH and ionic strength, catalyst dosage and characteristics, oxidant concentration, reactor configuration, irradiation conditions, and reaction temperature [110]. Time-resolved measurements of color removal, COD/TOC reduction, catalyst stability, and reusability should be reported together with uncertainty estimates whenever possible. The adoption of such standardized reporting practices will significantly enhance cross-study benchmarking and accelerate technological development [111].

7.2.7. Implications for MOF-Centered Hybrid Catalyst Development

The implementation of standardized evaluation protocols will enable clearer differentiation between adsorption-assisted discoloration and true oxidative remediation, thereby strengthening mechanistic understanding and performance assessment of MOF-based hybrid catalysts. By integrating mineralization metrics, stability analysis, toxicity evaluation, and energy efficiency indicators, researchers can more effectively identify design strategies that balance catalytic activity with durability, safety, and scalability. Ultimately, harmonized reporting standards will facilitate objective comparison between MOF systems and conventional catalysts, supporting the rational advancement of hybrid oxidation technologies toward sustainable textile wastewater treatment [112].

8. Current Challenges and Research Gaps

8.1. Technical and Implementation Challenges of MOF-Based Catalytic Systems

Despite significant progress in the development of MOF-based hybrid catalysts for textile wastewater treatment, several scientific and technological challenges continue to limit their transition from laboratory investigations to large-scale implementation. Addressing these limitations is essential to realize the full potential of MOF-driven oxidation systems for sustainable effluent remediation [91]. One of the primary challenges lies in structural stability under realistic aqueous environments. Textile effluents contain fluctuating pH conditions, high ionic strength, surfactants, and oxidizing agents that can weaken metal–ligand coordination bonds, leading to framework degradation or partial collapse. Although water-stable MOFs based on high-valence metals such as Zr⁴⁺ and Ti⁴⁺ have demonstrated improved robustness, their long-term stability under continuous operation remains insufficiently explored. Many studies evaluate catalyst performance over only a few cycles, which does not adequately represent industrial operating conditions [113].

Another critical concern involves metal leaching and environmental safety. Transition-metal-based MOFs may release active metal ions during catalytic reactions, particularly under acidic or oxidative conditions. Such leaching not only reduces catalytic efficiency but may also introduce secondary contamination risks. Standardized protocols for evaluating catalyst stability, metal release, and toxicity impacts are still lacking, making cross-study comparison difficult [112]. A further research gap relates to incomplete mineralization and intermediate formation. While high dye decolorization efficiencies are frequently reported, fewer studies quantify total organic carbon (TOC) removal or identify transformation products. Partial oxidation may generate intermediate compounds with unknown or increased toxicity, highlighting the need for comprehensive degradation pathway analysis and ecotoxicological assessment [90].

In addition, mass-transfer limitations and real wastewater complexity remain underexplored. Many catalytic studies rely on model dye solutions rather than authentic textile effluents containing salts, surfactants, and mixed contaminants that compete for reactive oxygen species. Matrix effects can significantly reduce catalytic efficiency, indicating the necessity of testing under realistic treatment scenarios. From an engineering perspective, scalability and reactor integration represent major barriers. MOF synthesis often involves complex solvothermal methods, expensive precursors, or low production yields, raising concerns regarding economic feasibility. Furthermore, most studies employ batch reactors, whereas industrial systems require continuous-flow operation with stable catalyst immobilization and minimal pressure loss [90]. Finally, limited attention has been given to techno-economic analysis and life-cycle sustainability. Energy consumption, chemical usage, catalyst regeneration costs, and environmental footprints are rarely quantified, preventing objective comparison with established treatment technologies. Bridging these gaps requires interdisciplinary research integrating materials science, reaction engineering, environmental assessment, and process systems engineering design [114].

8.2. Environmental Safety, Leaching, and Byproduct Risk

While MOF-based hybrid catalysts demonstrate significant potential for efficient textile wastewater oxidation, environmental safety considerations remain a critical factor governing their practical deployment. Beyond catalytic performance, the long-term ecological implications associated with catalyst stability, metal release, and oxidation byproducts must be systematically evaluated to ensure sustainable application [115].

8.2.1. Metal Leaching and Secondary Contamination

One of the primary environmental concerns associated with MOF catalysts is the potential leaching of metal ions during oxidative treatment. Many catalytically active MOFs incorporate transition metals such as Fe, Cu, Co, or Ti, which participate in redox cycling during advanced oxidation processes. Under acidic conditions, high ionic strength, or prolonged oxidant exposure conditions commonly encountered in textile effluents partial framework degradation may occur, releasing dissolved metal species into treated water. Metal leaching not only reduces catalytic lifetime but may also introduce secondary contamination risks, particularly for cobalt- and copper-based frameworks where toxicity thresholds are relatively low [58,116]. Consequently, quantification of dissolved metal concentrations using techniques such as ICP-OES or ICP-MS should be considered a mandatory evaluation parameter alongside catalytic efficiency. Establishing acceptable leaching thresholds and reporting metal mass balance across reaction cycles are essential for realistic environmental assessment [116].

8.2.2. Toxicity of Organic Linkers and Degradation Fragments

In addition to metal release, the environmental fate of organic linkers and framework degradation products requires careful consideration. MOF structures rely on organic ligands that may undergo oxidative fragmentation during advanced oxidation reactions. Partial decomposition could generate low-molecular-weight aromatic compounds or oxygenated intermediates with unknown toxicity profiles. Although many studies emphasize dye removal efficiency, limited attention has been given to identifying linker-derived transformation products or assessing their ecological impacts. Comprehensive characterization using LC–MS or GC–MS, coupled with toxicity screening, is therefore necessary to verify that catalytic degradation does not introduce new environmental hazards [58].

8.2.3. Formation of Chlorinated Oxidation Byproducts

Textile wastewater frequently contains elevated chloride concentrations due to the extensive use of NaCl and Na₂SO₄ during dye fixation processes. Under strong oxidative conditions, particularly in radical-driven systems, chloride ions may participate in secondary reactions leading to the formation of reactive chlorine species (Cl•, Cl₂•⁻, HOCl). These species can promote the formation of chlorinated organic intermediates, some of which may exhibit increased persistence or toxicity compared with parent dye molecules. The potential generation of halogenated byproducts highlights the importance of evaluating oxidation selectivity under realistic wastewater matrices rather than simplified model solutions. Monitoring total organic halogens (TOX) or chlorinated intermediates should therefore be incorporated into advanced catalyst assessment protocols [117].

8.2.4. Need for Integrated Ecotoxicity and Safety Evaluation

To ensure environmentally responsible implementation, catalytic performance evaluation must extend beyond decolorization or COD removal toward integrated safety assessment. Standardized ecotoxicity testing using representative bioassays such as Daphnia magna, algal growth inhibition, bacterial luminescence assays (Microtox), or aquatic microbial activity tests that should accompany degradation studies. These analyses provide direct evidence of toxicity reduction and confirm whether oxidation pathways achieve genuine detoxification rather than partial transformation [58].

8.3. Scalability and Techno-Economic Considerations

Despite the promising performance of MOF-based catalysts at the laboratory scale, several critical barriers limit their translation to large-scale wastewater treatment applications. A primary challenge lies in material synthesis and cost. Many MOFs are synthesized via solvothermal routes that require high-purity precursors, organic solvents, and tightly controlled conditions, leading to elevated production costs and limited scalability. Although recent studies have explored greener synthesis approaches and low-cost feedstocks, economically viable large-scale production remains a significant constraint [90].

Structural stability under realistic conditions represents another major limitation. Textile wastewater is characterized by high salinity, fluctuating pH, surfactants, and competing ions, all of which can destabilize MOFs through hydrolysis or ligand dissociation. In addition, metal leaching, particularly from Fe-, Cu-, and Co-based MOFs, not only reduces catalytic performance but also introduces risks of secondary contamination, raising environmental and regulatory concerns. Additionally, catalyst recovery and reuse further complicate practical implementation. Most MOFs are used in powdered form, making separation from treated water difficult and leading to material loss and increased operational costs. To address this issue, various immobilization strategies including coating on substrates, incorporation into membranes, and integration with magnetic components have been developed to improve recyclability and enable continuous operation [118].

From an engineering perspective, reactor design and process integration remain underdeveloped. The majority of studies are conducted in batch systems using model dye solutions, whereas industrial applications require continuous-flow reactors capable of handling complex wastewater matrices. Challenges such as mass transfer limitations, pressure drop, catalyst fouling, and long-term operational stability must be resolved to ensure reliable performance under practical conditions [58].

Regarding techno-economic analysis (TEA), only a limited number of studies have quantitatively assessed the cost and energy requirements of MOF-based systems. Available analyses suggest that, despite their high catalytic efficiency, economic feasibility is strongly influenced by synthesis cost, catalyst lifetime, and oxidant consumption. Integration with solar-driven processes and hybrid treatment systems has been proposed as a strategy to reduce energy demand and enhance sustainability [119].

Similarly, pilot-scale demonstrations remain scarce, with most investigations confined to laboratory-scale experiments. Although a few studies have explored structured catalysts and continuous-flow configurations, comprehensive validation under real wastewater conditions is still lacking. This gap underscores the need for interdisciplinary approaches that integrate materials design, reaction engineering, and environmental assessment [120].

A comparative techno-economic assessment (

Table 6. Techno-Economic and Scalability Assessment of MOF-Based Catalysts Compared with Emerging Systems.

Parameter	MOF-Based Catalysts	COFs	Semiconductor Catalysts	Carbon-Based Catalysts
Synthesis Cost	High [12,32]	Moderate–High [94]	Low–Moderate [102]	Low [14,122]
Raw Material Availability	Moderate [31]	Moderate [94]	High [29]	Very high [14]
Catalytic Activity	Very high [37,42]	Moderate [94]	Moderate–High [29,102]	Moderate [122]
Reactive Selectivity	Tunable [38,44]	Selective [94]	Limited [29]	Selective [14]
Mineralization Efficiency	High (50–80%) [56,79]	Moderate [94]	Moderate [29]	Low–Moderate [14]
Stability in Water	Variable [89,92]	High [94]	Excellent [102]	Excellent [14]
Metal Leaching Risk	Moderate–High [26,93]	Low [94]	Very low [102]	None [14]
Recyclability	Moderate [116]	Moderate [94]	High [102]	High [14]
Catalyst Recovery	Challenging [116]	Moderate [94]	Easy [102]	Easy [14]
Scalability Potential	Limited [91,119]	Emerging [94]	High [29]	Very high [122]
Sensitivity to Matrix	Moderate [25,30]	Moderate [94]	High [29]	Low [14]
Energy Demand	Moderate–High [30,117]	Moderate [94]	High [29]	Low [122]
Techno-Economic Maturity	Low [91,119]	Low [94]	High [29]	High [122]
Pilot-Scale Studies	Very limited [91]	Rare [94]	Established [29]	Emerging [122]
Environmental Safety	Concern [72,103]	Safer [94]	Safe [29]	Very safe [14]
Practical Readiness	Medium–Low [91]	Low [94]	High [29]	High [122]

) indicates that MOF-based catalysts offer superior catalytic activity and tunable reactive selectivity compared with other emerging systems, primarily due to their multifunctional active sites and ability to generate diverse reactive oxygen species [90]. However, their large-scale implementation is constrained by high synthesis costs, structural instability, and metal leaching risks. In contrast, semiconductor catalysts such as TiO₂ exhibit excellent structural stability and technological maturity, although their performance is often limited by charge recombination and energy requirements under UV irradiation [121]. Carbon-based materials provide high scalability, low cost, and strong environmental compatibility, but generally exhibit lower mineralization efficiency due to limited intrinsic catalytic activity [122]. Similarly, COFs demonstrate improved chemical stability and selective oxidation pathways but lack the redox versatility of MOF systems [58]. Hence, bridging the gap between laboratory research and industrial application requires addressing key challenges related to cost reduction, structural stability, catalyst immobilization, and reactor scalability, alongside the development of standardized techno-economic and life-cycle assessment frameworks. Such efforts are essential to enable the practical deployment of MOF-based catalytic systems for sustainable textile wastewater treatment.

9. Future Perspectives and Implementation Roadmap

Future progress in MOF-based textile wastewater treatment requires the integration of material innovation, reactor engineering, and sustainability considerations into a unified implementation framework. Beyond catalyst development, practical deployment depends on rational selection strategies, scalable process configurations, and techno-economic feasibility. The following subsections outline a forward-looking roadmap linking catalyst design with real-world treatment requirements [123].

9.1. Roadmap for MOF-Based Hybrid Catalyst Selection in Textile Wastewater Treatment

The increasing diversity of metal–organic framework (MOF) catalysts and hybrid oxidation strategies has created significant opportunities for tailored textile wastewater treatment; however, the absence of systematic selection criteria often limits practical implementation. To bridge the gap between material innovation and engineering deployment, a conceptual decision framework is proposed for guiding the rational selection of MOF-based catalytic systems according to wastewater characteristics, operational constraints, and treatment objectives. Rather than promoting a single catalyst class, this roadmap emphasizes adaptive design principles that align catalyst functionality with real treatment requirements [90].

The proposed framework integrates five key decision parameters that collectively determine optimal catalyst and process configuration. First, wastewater chemistry, particularly pH and salinity, governs material stability and radical chemistry. Highly saline or chloride-rich effluents favor water-stable MOFs constructed from high-valence metal clusters (e.g., Zr- or Ti-based frameworks), whereas iron-based MOFs are advantageous under mildly acidic conditions where Fenton-like reactions can operate efficiently. Near-neutral pH systems benefit from hybrid architectures that combine adsorption and heterogeneous oxidation to compensate for reduced radical generation rates [27].

Second, pollutant composition and organic load, represented by dye class and COD level, influence the dominant treatment mechanism. Effluents containing high concentrations of azo or reactive dyes typically require strong oxidative pathways capable of chromophore cleavage and aromatic ring opening, favoring Fe-based MOFs operating through Fenton or photo-Fenton mechanisms. In contrast, mixed dye matrices with moderate COD may benefit from photocatalytic MOFs or adsorption/oxidation hybrids that enhance pollutant enrichment prior to oxidation [49].

Third, energy availability, particularly access to solar irradiation, determines the activation strategy used. Regions with strong solar resources support visible-light-responsive MOFs and photo-Fenton hybrids that reduce operational energy demand. In low-light or indoor treatment scenarios, persulfate activation or catalytic oxidation systems may provide more reliable performance independent of irradiation conditions [37].

Fourth, catalyst recovery and operational configuration influence material selection and reactor design. Suspended nanoparticulate catalysts offer high activity but require separation steps, whereas magnetically recoverable MOF composites or immobilized MOF coatings enable continuous-flow operation with improved process stability. Structured catalysts integrated into membranes, monoliths, or packed-bed reactors represent promising pathways toward industrial scalability [124].

Finally, treatment objectives, particularly the desired degree of mineralization, dictate process intensity. Applications targeting rapid color removal for reuse pretreatment may employ adsorption-assisted systems with moderate oxidation strength, whereas discharge compliance and toxicity reduction require high mineralization efficiency supported by hybrid oxidation–biological polishing strategies.

Figure 10 schematically summarizes this decision framework, illustrating how wastewater characteristics, operational conditions, and sustainability targets converge to guide the selection of the MOF composition, oxidant type, and hybridization strategy. The roadmap highlights that optimal performance is achieved not through maximizing catalytic activity alone, but through balancing stability, energy efficiency, recovery capability, and environmental safety.

By translating material-level innovation into a practical decision-oriented framework, this graphical roadmap provides a strategic tool for researchers and engineers seeking to design scalable MOF-based treatment systems tailored to diverse textile wastewater scenarios. Such integrative approaches are expected to accelerate the transition from laboratory-scale catalyst development toward real-world implementation aligned with sustainable water management and circular economy principles.

9.2. Reactor Integration and Operational Strategy Considerations

While catalyst selection provides the foundation for treatment optimization, successful implementation further depends on reactor configuration and operational strategy. Continuous-flow operation, catalyst immobilization, hydraulic stability, and energy integration strongly influence system performance at scale. Structured MOF catalysts incorporated into membranes, packed beds, monoliths, or catalytic wetland systems offer promising routes toward industrial deployment by improving catalyst recovery, reducing pressure losses, and enabling long-term operation under realistic wastewater conditions. Integration with solar irradiation and hybrid oxidation–biological polishing systems further enhances sustainability and operational resilience [125].

9.3. Techno-Economic and Life-Cycle Considerations

The long-term viability of metal–organic framework (MOF)-based hybrid catalysts for textile wastewater treatment depends not only on catalytic efficiency but also on techno-economic feasibility and environmental sustainability across the full process life cycle. While laboratory studies frequently emphasize degradation performance, practical implementation requires comprehensive evaluation of material cost, operational energy demand, chemical consumption, catalyst lifetime, and overall environmental footprint. Integrating techno-economic analysis (TEA) and life-cycle assessment (LCA) perspectives is therefore essential to guide the realistic translation of MOF technologies toward industrial deployment [90].

9.3.1. Cost Drivers in MOF Production and Operation

A major economic challenge arises from the synthesis complexity of many MOF materials. Key cost contributors include organic linker precursors, solvent consumption, thermal energy requirements during solvothermal synthesis, and post-synthetic processing steps such as activation, washing, and drying. Aromatic multicarboxylate linkers and high-purity organic ligands can significantly increase material costs compared with conventional catalysts such as metal oxides or activated carbon. In addition, the use of large solvent volumes that commonly N,N-dimethylformamide (DMF) or similar organic media introduces both financial and environmental burdens associated with solvent recovery and waste handling [126].

Energy consumption during synthesis represents another critical factor. Elevated temperatures and extended reaction times typical of solvothermal routes increase production costs and carbon footprint, particularly when scaled beyond laboratory batches. Consequently, simplified synthesis approaches, room-temperature methods, and continuous-flow fabrication strategies are increasingly recognized as key pathways toward economically viable MOF production.

9.3.2. Catalyst Lifetime and Regeneration Assumptions

Economic feasibility is strongly linked to catalyst durability and regeneration efficiency. Many reported studies evaluate catalytic performance over limited reuse cycles, which may not accurately reflect operational lifetimes required in industrial wastewater treatment. Catalyst deactivation caused by metal leaching, pore blockage, or accumulation of inactive oxidation states directly increases replacement frequency and operational cost [126]. Regeneration strategies, including washing, thermal treatment, and chemical reactivation, must be evaluated in terms of energy demand, material loss, and performance recovery. Accurate techno-economic assessment should incorporate realistic assumptions regarding catalyst lifetime, regeneration efficiency, and performance decay, rather than relying on single-cycle degradation efficiency [127].

9.3.3. Chemical Consumption and Process Footprint

Advanced oxidation systems employing MOF catalysts often rely on oxidants such as hydrogen peroxide or persulfate, whose production and consumption significantly influence overall environmental impact. Excess oxidant dosing increases operational cost and contributes to indirect emissions associated with chemical manufacturing. Optimization of oxidant utilization efficiency, radical selectivity, and catalytic turnover frequency is therefore essential for reducing chemical footprint [128].

Metrics such as oxidant utilization efficiency, electrical energy per order (EEO), or energy consumption per treated volume (kWh m⁻³) should be incorporated into performance evaluation to enable comparison with established treatment technologies. Solar-driven photo-Fenton or photocatalytic systems offer particular advantages by partially replacing electrical energy demand with renewable irradiation.

9.3.4. Life-Cycle Assessment Levers for Sustainable MOF Design

Life-cycle thinking highlights several opportunities for improving the sustainability profile of MOF-based catalysts. Green synthesis strategies, including the use of water or ethanol as solvents, mechanochemical synthesis, and microwave-assisted methods, can significantly reduce environmental burden compared with conventional solvothermal processes. The development of waste-derived or bio-based linkers represents another promising pathway for lowering material cost while aligning catalyst production with circular economy principles.

The integration of MOF catalysts into solar-driven treatment systems further reduces operational emissions by minimizing external energy requirements. Additionally, immobilized or structured catalysts that extend operational lifetime and simplify recovery can substantially improve life-cycle performance by reducing material losses and downstream separation steps [129].

10. Research Outlook and Emerging Directions

The future development of MOF-based hybrid catalytic systems for textile wastewater treatment will depend on advancing both material design and process integration toward scalable and sustainable solutions. Several promising research directions can be identified [90].

First, the rational design of water-stable and defect-engineered MOFs is expected to play a central role in improving catalytic durability. Strategies such as linker functionalization, mixed-metal nodes, and hierarchical pore engineering may enhance hydrolytic resistance while maintaining high catalytic activity. The development of MOF-derived catalysts through controlled thermal transformation also offers a promising pathway toward structurally robust materials with preserved active-site distribution [90].

Second, hybridization strategies will likely dominate future catalyst innovation. Coupling MOFs with conductive carbon materials, semiconductor photocatalysts, or magnetic phases can improve charge transfer, suppress electron–hole recombination, and enable efficient catalyst recovery. Such multifunctional architectures may allow operation under visible light or solar irradiation, reducing energy requirements and supporting decentralized treatment applications [37].

Third, increasing emphasis should be placed on solar-driven and low-energy oxidation systems. Integration of MOFs into photo-Fenton or photocatalytic platforms powered by natural sunlight strongly aligns with circular economy principles and sustainable water reuse strategies. Combining catalytic oxidation with biological polishing units or nature-based systems may further enhance mineralization while minimizing chemical consumption [130].

Another important direction involves reactor and process engineering. Immobilized MOF catalysts, structured monoliths, membrane-supported systems, and catalytic wetlands represent promising configurations for continuous-flow treatment. Such designs can address catalyst recovery challenges while improving operational stability and scalability [58,90].

Moreover, future studies should incorporate standardized performance metrics, including mineralization efficiency, toxicity reduction, catalyst lifetime, and energy efficiency indicators. The integration of artificial intelligence and data-driven optimization approaches may further accelerate catalyst discovery by correlating structural descriptors with catalytic performance.

11. Conclusions

MOF-based hybrid catalysts have emerged as a transformative platform for reimagining textile effluent treatment by integrating adsorption, redox catalysis, and advanced oxidation processes within highly tunable porous architectures. Their unique structural versatility enables efficient oxidant activation, enhanced pollutant–catalyst interactions, and improved generation of reactive oxygen species capable of degrading recalcitrant dye molecules. This review has critically examined the characteristics of textile wastewater, underlying oxidation mechanisms, and the evolving role of MOF-centered hybrid catalysts in addressing the limitations of conventional treatment technologies. Comparative analysis demonstrates that MOF systems offer significant advantages in catalytic tunability and multifunctionality; however, challenges related to hydrolytic stability, metal leaching, scalability, and economic feasibility remain key barriers to practical implementation.

Progress in the field increasingly highlights hybrid catalyst architectures, in which MOFs serve as central platforms integrated with conductive supports, magnetic components, or secondary semiconductors to enhance stability and operational performance. Future success will depend on the development of water-stable materials, improved reactor integration, and the establishment of standardized evaluation protocols that emphasize mineralization efficiency and environmental safety. Ultimately, the future of textile wastewater remediation will rely not on a single catalytic material but on intelligently engineered MOF-centered hybrid systems that balance catalytic efficiency, structural stability, environmental safety, and techno-economic viability within integrated treatment frameworks.