# Metal-organic framework electrospun nanofibers in application to dye removal from textile wastewaters: A review

Anastasiia Silina, Ahmida El Achari, Fabien Salaun

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Anastasiia Silina, Ahmida El Achari, Fabien Salaun. Metal-organic framework electrospun nanofibers in application to dye removal from textile wastewaters: A review. Journal of Environmental Chemical Engineering, 2024, J. Environ. Chem. Eng., 12, 10.1016/j.jece.2024.114819. hal-04887632

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# Metal-organic framework electrospun nanofibers in application to dye removal from textile wastewaters: A review

Anastasia Silina*, Ahmida El Achari, Fabien Salaün

Univ. Lille, ENSAIT, ULR 2461 - GEMTEX - Génie et Matériaux Textiles, Lille F-59000, France

## ARTICLE INFO

### Keywords:

- Dye removal

- Metal-organic framework

- Electrospun nanofiber

- Adsorption

- Water treatment

## ABSTRACT

Emissions of dyes with wastewater from textile industries are increasing every year. Dyes, persisting or decomposing in the environment, enter drinking water through nature's water cycle and can cause severe problems to human health. Researchers have investigated and proposed various wastewater treatment techniques to reduce dyes' emissions, including adsorption, which appears to be the most cost- and time-effective. Metal-organic frameworks (MOFs) are adsorption materials used for many applications for their high specific surface area, pore size, and volume tunability. In dye removal, MOFs are used exceptionally in batch experiments, which hinders their application on an industrial scale. In this review, a comprehensive understanding of the most recent strategies for MOF synthesis is discussed with a focus on effective adsorption of dyes. The most considered interactions between MOFs and dyes are presented with examples and an endeavor is made to understand the contribution of surface charge and size compatibility to adsorption process. Furthermore, the present survey presents the strategies for incorporation of MOF into electrospun nanofiber (ENF) carriers in detail and demonstrates examples of MOF/ENF composite's effective application towards the adsorption of dye in textile effluents. It is highlighted that dye-MOF interactions are mostly responsible for the dye adsorption process on MOF/ENF. Current limitations of MOF/ENF application in industrial textile wastewater treatment are extensively discussed and include technical and environmental concerns. These limitations need to be resolved before commercialization, and we suggest a number of studies that can be conducted to address this issue.

## 1. Introduction

### 1.1. Methods of dye removal

Textile industries are responsible for 54 % of hazardous organic pollutants, such as pigments and dyes, released in water bodies, followed by dyeing industries (21 %), paper and pulp industries (10 %), tannery and paint (8 %) and the dye manufacturing industries (7 %) [1]. Approximately 2800,000 tons of dyes are released annually into the global environment with textile industry effluents. The average dye concentration in textile wastewater is approximately 300 mg L⁻¹ when even 1 mg L⁻¹ of dye is highly undesirable and harmful to the environment [2]. Dyes interfere with natural water resources, promote severe toxicological and carcinogenic implications, and harmfully impact the environment and human health [3].

To solve the problem of textile-polluted wastewaters, methods of degradation and separation of dyes have been developed to remove pollutants [4]. Degradation methods such as oxidative and biological

degradation can be very effective [5], with several drawbacks since these techniques require strong oxidants such as ozone O₃, hypochlorite ClO⁻, or photo-Fenton reagents [6,7]. These techniques create reactive radicals that degrade the chemical structure of dyes in water down to CO₂ and H₂O. However, oxidative treatments create various byproducts that are more difficult to eliminate [8,9]. Besides, hydrogen peroxide is often used for techniques like photocatalysis and photo-Fenton, which inevitably makes the process more expensive. Biodegradation is efficient for basic dye removal; however, this technique is unsuitable for reactive and anionic dyes due to their low biodegradability [2]. It is also very slow, requiring high specific surfaces for bacteria growth [10,11]. Methods of water treatment include flocculation, coagulation, ion exchange, use of membranes, and adsorbents. The coagulation method has not proven effective for processing a wide range of synthetic dyes such as azoic, reactive, acid, and basic dyes [12]. Ion-exchange is ineffective for a wide range of dyes and is often costly [13]. Membrane filtration is considered a very effective method for dye separation, but this method is hampered by membrane fouling and cost [14].

Among all the dye removal methods, adsorption represents all the

<figure><img src="image_3.png" /></figure>

<footnote>* Correspondence to: GEMTEX, ENSAIT, 2 Allée Louise et Victor Champier, Roubaix 59100, France.</footnote>

<footnote>E-mail address: anastasiia.silina@ensait.fr (A. Silina).</footnote>
Nomenclatures

<table><tr><td>AC</td><td>Activated carbon</td></tr><tr><td>BET</td><td>Brunauer-Emmett-Teller</td></tr><tr><td>CA</td><td>Cellulose acetate</td></tr><tr><td>CR</td><td>Congo red</td></tr><tr><td>CS</td><td>Chitosan</td></tr><tr><td>CZIF</td><td>Carbonized zeolitic imidazolate framework</td></tr><tr><td>DMF</td><td>N,N'-dimethylformamide</td></tr><tr><td>DMNP</td><td>Dimethyl-4-nitrophenyl phosphate</td></tr><tr><td>ENF</td><td>Electrospun nanofiber</td></tr><tr><td>HKUST</td><td>Hong Kong University of Science and Technology</td></tr><tr><td>HT</td><td>Hollow tube</td></tr><tr><td>IRMOF</td><td>Isoreticular metal-organic framework</td></tr><tr><td>LDH</td><td>Layered double hydroxide</td></tr><tr><td>MB</td><td>Methylene blue</td></tr><tr><td>MG</td><td>Malachite green</td></tr><tr><td>MIL</td><td>Material of Institute Lavoisier</td></tr><tr><td>MO</td><td>Methyl orange</td></tr><tr><td>MOF</td><td>Metal-organic framework</td></tr><tr><td>NPC</td><td>Nanoporous carbon</td></tr><tr><td>NU</td><td>Northwestern University</td></tr><tr><td>PAN</td><td>Polyacrylonitrile</td></tr><tr><td>PCN</td><td>Pocket-channel frameworks</td></tr><tr><td>PEO</td><td>Polyethylene oxide</td></tr><tr><td>PES</td><td>Polyether sulphone</td></tr></table>

<table><tr><td>PET</td><td>Polyester</td></tr><tr><td>PFO</td><td>Pseudo-first order</td></tr><tr><td>PI</td><td>Polyimide</td></tr><tr><td>PLA</td><td>Polylactic acid</td></tr><tr><td>PNP</td><td>p-nitrophenol</td></tr><tr><td>PS</td><td>Polystyrene</td></tr><tr><td>PSO</td><td>Pseudo-second order</td></tr><tr><td>PU</td><td>Polyurethane</td></tr><tr><td>PVA</td><td>Polyvinyl alcohol</td></tr><tr><td>PVDF</td><td>Poly(vinylidene fluoride)</td></tr><tr><td>PVP</td><td>Polyvinylpyrrolidone</td></tr><tr><td>RhB</td><td>Rhodamine B</td></tr><tr><td>SA</td><td>Sodium alginate</td></tr><tr><td>SEM</td><td>Scanning electron microscopy</td></tr><tr><td>UiO</td><td>Universitetet i Oslo (University of Oslo)</td></tr><tr><td>ZIF</td><td>Zeolitic imidazolate framework</td></tr></table>

Organic linkers abbreviations

<table><tr><td>BDC</td><td>Benzenedicarboxylate</td></tr><tr><td>BTC</td><td>Benzenetricarboxylate</td></tr><tr><td>DOT</td><td>Dioxoterephthalate</td></tr><tr><td>DOBDC</td><td>Dioxidobenzenedicarboxylate</td></tr><tr><td>4,4'-BPDC</td><td>4,4'-biphenyldicarboxylate</td></tr><tr><td>4,4'-SDB</td><td>4,4'-sulfonyldibenzoate</td></tr><tr><td>2,6-NDC</td><td>2,6-napthalenedicarboxylate</td></tr><tr><td>BPZ</td><td>Bipyrazolate</td></tr></table>

important characteristics for water treatment, such as simplicity in use, cost-effectiveness, and – often – reusability of adsorbent [9,15]. Besides, the adsorption process does not contribute to the formation of toxic byproducts. The performance of the adsorbent material depends on its chemical composition and structure. Both natural and synthetic adsorbents are used for textile wastewater treatment. Natural adsorbents like fly ash, silica, wood, clay material, and cotton wastes can be used in dye effluent treatment processes [16], as well as synthetic adsorbents, such as layered double hydroxides (LDH) [17], activated carbon (AC), and metal-organic frameworks (MOFs) [18]. Despite the fact that all these classes of adsorbents are quite effective for dye removal, MOFs have shown exceptionally large adsorption capacities, high efficiency, and recyclability, which is lacking in other adsorbent materials.

## 1.2. MOF and ENF united

MOFs are crystalline materials fabricated from metallic nodes coordinated with organic linkers in the form of oxo-clusters or polyhedric chains. The fields of MOFs application include gas storage and separation, heterogeneous catalysis, chemical sensing, proton conductivity, air filtration, and water treatment [19]. The reason for the wide application of MOFs is their specific properties, such as high specific surface area,

thermal and mechanical stability, material variety, and adjustable pore size. Compared to other porous materials, some MOFs show better results for adsorption of dyes. For example, PCN-222 (Fe) was more effective at adsorbing both large (brilliant green, acid red 1, acid blue 80) and small (methylene blue, methyl orange) molecules of dyes compared to activated carbon [20,21]. In addition, activated carbon is not as easily regenerable as MOFs [15], which can be regenerated up to 5 times without a significant decrease in adsorption capacity [22,23]. MOFs are of high interest in research and application in the dye industry [18,24]. The pore size and charge of MOFs can be tailored during synthesis, making them capable of capturing various dyes simultaneously. This is particularly valuable for textile wastewater treatment, as dye-containing wastewaters are often presented by a mixture of dyes with different structures (Fig. 1). Previous reviews have extensively discussed the MOF in laboratory application for removal of dyes [18, 21]. However, when MOFs are used for water treatment on an industrial scale, dispersed MOF fine powder causes aggregation and clogging of tubing and overall difficulty of handling. Besides, the leakage of nano-sized MOFs can be further harmful to ecology and human health, which is another limitation to their industrial application [25]. These problems can be solved by loading MOFs in polymer support [26]. The synergistic effect of such composite membranes can significantly improve the

<figure><img src="image_4.png" /><figcaption>Fig. 1. Scheme of dye wastewater treatment using the MOF with different pore sizes.</figcaption></figure>
performance of MOF and polymer materials.

Electrospun nanofibers (ENFs) are nano-sized fibers with an exceptionally large surface area-to-volume ratio, which significantly increases their exposure to environment (gas or liquid) compared to microfibers. This enhanced contact with environment makes ENF-based filters more effective for water treatment compared to those made from micro-sized fibers (Fig. 2). ENFs have been widely applied for water treatment from diverse types of pollutants, including dyes. Simple production, flexibility and reusability made ENF-based membranes popular for dye removal from wastewaters. Among the most widely used for water treatment purposes, polyacrylonitrile (PAN), polyimide (PI), poly(vinylidene fluoride) (PVDF), cellulose acetate (CA), polylactic acid (PLA), polyurethane (PU), polyethylene oxide (PEO) can be named [29–35]. When MOFs are integrated on ENF surface, their charge and high surface area—the key factors in adsorption—are synergistically optimized, boosting the filter’s overall performance [36–38]. Additionally, ENFs restrain agglomeration of MOF particles in the solution of effluent and filters based on metal-organic frameworks electrospun nanofiber (MOF/ENF) composites can be easily separated from the reaction system after being used, for further regeneration and reuse. Many methods of MOF incorporation into, or onto, ENFs were developed in the past decades, including incorporation of pre-synthesized MOFs into the solution for electrospinning (MOF-first strategy), attachment of pre-synthesized MOFs onto pre-synthesized fibers, and in-situ growth of MOF particles on the surface of pre-synthesized fiber first strategy [28,39].

Despite the effectiveness of MOF/ENF composites, studies on their usage in water treatment are limited. This is due to the great difficulties these materials face when applied for water treatment, like aqueous medium with a certain pH, large volumes of water, and strong water pressure.

## 1.3. Purpose of this review

Among the wide range of possible MOF/ENF combinations, only a few are suitable for dye wastewater treatment due to various challenges, such as poor water resistance, inadequate mechanical stability, and limited adsorption capacity due to occupied MOF's pores by melted ENF polymer. While the idea of combining the synergistic properties of MOFs with the enhanced water exposure of ENFs is not new, there are relatively few studies focused on integrating MOFs into ENFs for dye removal. The goal of this review is to outline strategies for synthesizing effective MOF/ENF-based filters for dye adsorption from textile wastewater, with a focus on understanding adsorption mechanisms, ensuring water stability, and enhancing adsorbent efficiency.

In this review we focus on MOF/ENFs application for dye wastewater treatment. Previous reviews have already discussed on the application of MOF/ENF composites for a wide variety of pollutants [39]. Others have discussed the use of MOF/ENF composites in dye removal, without detailing the challenges of the MOF-in-ENF incorporation [25,40]. In this paper, we not only highlight the difficulties associated with the incorporation of MOFs into ENFs and the eventual application of MOF/ENF composites for dye removal, but also outline solutions to these concerns.

One of the goals of this work is to identify the pairs of MOF/ENF composites that have already been successfully applied in dye wastewater treatment. Here, we outline strategies for improving their stability in water and their performance under the harsh conditions of dye wastewater treatment processes and highlight the further needed research on the MOF/ENF composites.

This review paper aims to present a comprehensive overview of the application of the MOF/ENF in textile wastewater treatment as an adsorbent for dye removal. It systematizes the known data on strategies for synthesizing MOFs, such as functionalizing organic linkers, defect engineering, and carbonization. The most important interactions between MOFs and dyes are presented, and their effects are elaborated and discussed. Examples include electrostatic, acid-base interactions, π-π stacking/conjugation, hydrogen bonding, hydrophobicity, or hydrophilicity of molecules, and the importance of fitting in molecule sizes. In this work, we highlight for the first time whether size compatibility between adsorbate's molecule and adsorbent's pores or their surface charge has a greater impact on adsorption process. The introduction of MOF into ENF is discussed. Electrospinning as a high-tech method for the manufacturing of nanofibers is presented briefly. The examples of strategies of MOF incorporation into ENF are discussed, i.e., (1) MOF addition into a solution for electrospinning, (2) MOF attachment on the polymer fiber surface, and (3) MOF in-situ growth on the fibers. Differences of MOF-in-ENF incorporation are discussed, as well as the common ways to overcome them. Different challenges that MOF/ENF composites meet when applied for textile water treatment are presented in examples. Finally, several MOF/ENF pairs suitable for dye removal from wastewaters are outlined. Character and mechanism of dye adsorption on MOF/ENF are discussed. Current limitations, technical and environmental, are presented, and the further needed research on MOF/ENF for dye removal are suggested.

# 2. Strategies for effective MOFs synthesis

## 2.1. Methods of synthesis

Synthesis approaches for MOFs comprise several techniques, from conventional, like solvo/hydrothermal and microwave-assisted synthesis, to more eco-friendly, greener, and energy-efficient techniques, like electrochemical, ultrasound-assisted, mechanochemical, and sol-gel synthesis [41,42] (Fig. 3).

The solvothermal process is one of the most used methods of MOF synthesis due to its simplicity and processability. In this method, metal salt is mixed with the solution of organic ligands in a proper solvent. The synthesis is called hydrothermal when water is used as a solvent. After stirring the precursor solution, it is placed and closed in a Teflon-lined steel vessel at temperatures from 120 to 220 °C. This method commonly synthesizes MOF series such as UiOs [43], MILs [44], and ZIFs [45]. Hydrothermal synthesis under normal pressure is considered on an industrial scale because it is a simple and undemanding method that does not require mechanical or thermal energy input, leading to a smaller final cost of the material [46,47].

Microwave-assisted synthesis follows the same steps as solvothermal

<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Micro- and (b) nano-sized fibers [27,28]; (c) synthetic nanofibers on cellulose microfibers.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Methods of MOFs synthesis on example of Zn-based MOF-5 (IRMOF-1).</figcaption></figure>

synthesis, except the mixture is heated by microwave instead of a con-
ventional oven. Some typical MOFs can be synthesized using the this
method, such as, UiO-66, ZIF-67 and HKUST-1 [48]. Microwave-assisted
synthesis permits the uniform and rapid heating of the mixture, which
means homogeneous nucleation and well-defined size distribution [44,
49]. The main advantage of this method over the solvothermal synthesis
is the reaction time up to 1 hour compared to 24 hours in the sol-
vothermal technique.

Electrochemical synthesis uses the principle of electrolysis and in-
volves two electrodes, galvanic cells, and a reactor. When oxidizing
voltage or current is applied, the electrode (anode) starts to dissolve in a
solution containing a supporting electrolyte and an organic linker. As
the electrode dissolves, metal ions are quickly released near its surface.
These metal ions then react with the organic ligands in the solution,
leading to the formation of a MOF layer on the surface of the electrode.
Electrochemical synthesis is used for applications requiring thin layers
of MOFs, like sensors and electronic devices. HKUST-1, ZIF-8, UiO-66
and NU-1000 can be prepared by electrochemical synthesis [50].
Although electrochemical synthesis is not as popular as solvothermal
and microwave methods, it has some important advantages: the syn-
thesis is fast under mild conditions (ambient temperature and pressure),
the anions are removed from metal salts, and the solution consists of
metallic ions and organic linkers, which increases the reaction yield. The
electric potential is precisely controlled, allowing its scaling up [41–43].

The sonochemical or ultrasound-assisted method is based on the
effect of acoustic cavitation, which involves sequential generation,
growth, and collapse of bubbles in a so-called hot spot. This process
leads to high local heating and high pressures, accumulating an amount
of energy sufficient to accomplish the reaction of MOF synthesis. Pop-
ular MOFs, like MIL-53, MOF-5 and HKUST-1 can be synthesized
sonochemically [51]. This method permits the obtaining of smaller
particle sizes at milder conditions, facilitates the narrow size contribu-
tion of particles, increases the purity of particles, and shortens the re-
action time [41,45,52].

Mechanochemical synthesis is achieved by simple ball milling or
manual grinding of the reagents. During the grinding, the initial
chemical bonds are broken, exposing reactive layers of atoms for
chemical reaction and new bond formation. The main advantage of this
method is the possibility of using safer, environmentally friendly

reagents, like metal oxides, instead of metal salts [53,54]. The
well-known MOFs, including MOF-5, MIL-101, ZIF-8, HKUST-1, and
UiO-66, can be synthesized using the mechanochemical approach [55].

Sol-gel synthesis is a technique of generating a colloidal suspension of precursors within a solvent, followed by the formation of a gel upon the addition of a catalyst. The resulting material consists of a solid backbone that encloses the liquid phase. Further, the solvent phase is removed by evaporation, affording highly porous structures that are inaccessible via previously described methods. The sol-gel method allows the formation of different pore size regimes within the same material [56,57].

## 2.2. *Structure and stability*

The structure and properties of MOFs are controlled by the nature of
metallic cations, the nature and geometry of organic units (linkers/li-
gands), and conditions of synthesis, such as concentration and ratio of
precursors, temperature, pressure, time of synthesis, and solvents. An
unlimited number of materials with the requested size and topology are
designed by changing metal ions and organic linkers. The inorganic part
of MOF is represented by metallic ion of transition, rare earth metal, or
3p-element of various charges (2+, 3+, 4+, etc.). Organic linkers
include carboxylates [58–63], azolates (imidazolates, pyrazolates, tri-
azolates, etc.) [64–66], phosphonates [67–69], etc.

Functional MOFs, metal nodes, and organic linkers are designed from
organic ligands with functional groups, as illustrated in Fig. 4(a).
Organic functional groups are classified in two categories depending on
their role and position in the framework, i.e., coordinating sites and
guest-interactive sites (Fig. 4(b)) [70]. Coordinating functional groups
coordinate with metal ions during the construction of MOF and often
include functions such as carboxylate, sulfonate, phosphonate, and
heterocyclic compounds. On the other hand, guest-interactive sites
remain intact during the self-assembly of MOF and are free to interact
with guest molecules. Like coordinating groups, the second class in-
cludes carboxylate, sulfonate, phosphonate, urea, amide, oxalamide,
imide, and triazine, which are only applied as guest-interactive sites.

The stability of MOF is its property in maintaining its structure and
coordination bonds under thermal and chemical conditions [71,72].
Most MOFs are thermally stable until 350–400 °C and selected ones are
<figure><img src="image_7.png" /><figcaption>Fig. 4. (a) Illustration of the construction elements of functional MOFs [19] Copyright © 2019 Materials Today; (b) Classification of organic functional groups based on their role and position in framework [70] Copyright © 2019 Coordination Chemistry Reviews.</figcaption></figure>

stable above 500 °C (MOF-5, UiO-66, MIL-53) [19,73]. The increase of rigidity of organic linker leads to an increase in MOF's rigidity, and results in a higher thermal and chemical stability of MOFs [74–76].

Many MOFs degrade in water due to hydrolysis, making them difficult to use for adsorption in aqueous solutions [19,71,77]. The force of the coordination bond is one of the most important parameters assuring the stability of MOF, and it depends on the couple of Lewis acids and bases composing the structure of the framework [19]. The coordination bonds between a strong acid and a strong base, or weak acid and a weak base, are the strongest. Therefore, metal ions of greater valency, such as $Cr^{3+}$, $Zr^{4+}$, or $Ti^{4+}$, are preferred to create bonds with strong carboxylic acid, and the metal ions of weaker valency, such as $Cu^{2+}$ or $Zn^{2+}$, create bonds with weak base ligands, such as imidazolates [21].

## 2.3. Functionalization of organic linkers

Isoreticular (having the same topology) chemistry within MOFs is one of the promising solutions for the selective adsorption of analytes. Functionalization of organic linkers can be used for different purposes; one is to increase the pore size of MOF. Functional groups presented in the main chain can be widely varied. Structures isoreticular to MOF-5 and MOF-74 are obtained by replacing BDC with other ditopic carboxylate linkers, as shown in Fig. 5. Linker size change extends MOF's pore

size from 0.38 to 2.88 nm for MOF-5 and from 1.4 to 9.8 nm for MOF-74 [78–80].

Another reason for the functionalization of organic linkers is the appropriation of physico-chemical attraction properties to adsorbent. For that, functional groups in the side chain of the organic linker can be changed depending on the properties of the adsorbate. The most common example is the use of amino-containing side chain, apporting basic properties to MOF (Fig. 6) [81]. $NH_2$-BDC-based MOF have a more negative zeta potential, which causes a higher removal rate of cationic dyes. Dye-MOF interactions are discussed in detail further.

## 2.4. Defect engineering

Defects in MOFs are defined as sites of local disruption of the regular arrangement of atoms or ions in the framework caused by missing organic linkers or metallic ions. Defect engineering is considered an effective method for increased porosity and surface area of MOF and is based on (1) the use of modulators or (2) post-synthetic treatment [82]. The interest of post-synthetic treatment is the possibility of introducing a greater range of functionalities to MOFs because it is not restricted by the solvothermal synthesis conditions of parent MOF. However, post-synthetic treatment is commonly used in gas separation ($CO_2$, $N_2$, $H_2$) [83], and less in dye removal.

<figure><img src="image_8.png" /><figcaption>Fig. 5. (a) Some representative ditopic carboxylate linkers and illustration of formation of extended IRMOF network by replacing acetate with rigid dicarboxylate [79] Copyright © 2014 Royal Society of Chemistry; (b) Perspective views of a single one-dimensional channel of several IRMOF-67. Color scheme: C (grey); O (red); O (red); Mg (blue); Zn (green) [80] Copyright © 2012 American Association for the Advancement of Science.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Zr-MOFs with (a) 1,4-benzene-dicarboxylate (BDC) linker, UiO-66; (b) 2-amino-1,4-benzene-dicarboxylate (H₂N-BDC) linker [73] Copyright © 2010 American Chemical Society.</figcaption></figure>

Modulators such as formic, acetic, and trifluoroacetic acids change porosity through cluster defects [84,85]. Modulators form competitive relationships with ligands to play the role of coordination modulation, breaking the coordination balance in the crystal growth process. Modulators compete with organic linkers for the metal coordination site, creating voids (defects) in the original structure of MOF. The defect modulation of MOFs is influenced by the synthesis conditions, synthesis temperature, the acidity and concentration of the modulator, and the ratio of the modulator to the ligand.

Tan et al. studied the impact of the ratio of hydrofluoric acid as a modulator in iron-based MIL-100 on the adsorption of methylene blue [86]. The modulator increase led to a remarkable increase in the Langmuir surface and pore volume. As a result, the adsorption of MB was more effective on the MIL-100, which had the highest amount of modulator among those studied. The smaller particles of MIL-100 (Fe)-4-0F in the first hour of an experiment performed a two times faster sorption rate than larger particles of MIL-100(Fe)-4-3F (Fig. 7).

The type of modulator used in a modified synthesis influences the sorption ability of the obtained adsorbent. In the study of Vo et al., toluene was adsorbed on a series of UiO-67(Zr) with different modulators [85]. Formic (Fo), acetic (Ac), and propionic (Pro) acids were used as defect modulators. The authors showed that the toluene adsorption occurred more readily on the MOF modulated with formic acid. The adsorption capacity of Fo-modulated MOF was 11 % and 16 % higher than that of Ac-modulated and Pro-modified MOF, respectively. Besides, higher formate concentrations in the solution favored the competition between formic acid and organic linker, which resulted in a more porous MOF.

## 2.5. Carbonization

Carbonization is another strategy that allows the synthesis of a porous framework by changing the physio-chemical properties of the

initial one, especially by the evaporation of metals or complete MOF. Liu et al. used MOFs as templates for porous carbon synthesis [87]. MOF-5, used as a template, was heated at 150°C in the atmosphere of furfuryl alcohol used as carbon precursor and polymerized in the pores of MOF-5 during the reaction. During the carbonization process, the MOF-5 framework decomposes, behaving as a self-sacrificed template and resulting in the formation of the nanoporous carbon (NPC). The increase of carbonization temperature from 800 to 1000 °C increases the pore volume more than three times, and Brunauer-Emmett-Teller (BET) specific surface area by almost seven times.

Carbonization can be used to change the charge of MOF. In the study of Abbasi et al., the Zeolitic Imidazolate Framework-8 (ZIF-8) was carbonized to result in negatively charged NPC [88]. The carbonization of ZIF-8 increased its adsorption capacity towards cationic MB dye by ten times, compared to the non-carbonized ZIF-8.

## 3. Dye-MOF interactions

Adsorption of organic pollutants on MOFs occurs *via* different mechanisms, i.e., acid-base, hydrophobic interactions, and H-bonding, precisely described previously [21]. In the context of dye adsorption on MOFs, notably electrostatic interactions, π-π stacking and H-bonding are of higher importance [20]. It is well-documented that more than one interaction is possible during adsorption. Characteristics of pore size and volume of MOF play a significant role in dye adsorption since compatibility in dye and pore sizes assures high rates of adsorption. Different adsorbents applied to dyes of different nature and molecule sizes are presented in Table 1, as well as dye-MOF interactions driving the adsorption process.

### 3.1. Electrostatic interactions

Jia et al. estimated the adsorption capacity of iron-based MIL-100 on

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Adsorption capacity of methylene blue on MIL-100(Fe) synthesized with different amounts of HF and SEM images of MIL-100(Fe) samples [86] Copyright 2015 © Chemical Engineering Journal.</figcaption></figure>
<caption>**Table 1**</caption>

Structure of dyes, MOFs used for their adsorption and adsorption driving forces; (a) Maximum adsorption capacity, q<sub>m</sub>; (b) Equilibrium amount of adsorbed dye, q<sub>e</sub>. *Only in this table PS is for Pore Size.

<table><thead><tr><th>Dye structure and molecule dimensions</th><th>Adsorbent</th><th>Pore width (PW),<br>pore size (PS*),<br>pore volume (PV),<br>total pore volume (TPV)</th><th>Zeta potential of MOF at<br>experimental pH</th><th>Adsorption rate, qe etc.</th><th>pH of experiment</th><th>Adsorption driving force</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="10">Methylene blue<br><img src="methyleneblue_molecule.png" alt="Methylene blue molecule structure"></td><td>MIL-68 (Al)</td><td>PW 2.90 nm<br>PV 0.7 cm<sup>3</sup> g<sup>-1</sup></td><td>-</td><td>1666 mg g<sup>-1</sup> (a)</td><td>7.96</td><td>Electrostatic<br>interaction<br>Acid-base<br>interaction<br>π-π interaction</td><td>[89]</td></tr><tr><td>BUT-29</td><td>PW 1.24 nm x<br>0.71 nm</td><td>-</td><td>1119 mg g<sup>-1</sup> (a)</td><td>7</td><td>Size fitting<br>Electrostatic<br>interaction</td><td>[90]</td></tr><tr><td>MIL-100 (Fe)</td><td>PS 1.33-2.7 nm<br>PV 0.72 cm<sup>3</sup> g<sup>-1</sup></td><td>-<br>-34 mV at pH 10</td><td>1105 mg g<sup>-1</sup> (a)<br>492 mg g<sup>-1</sup> (b)</td><td>7<br>10</td><td>Size fitting<br>Electrostatic<br>interaction</td><td>[86]</td></tr><tr><td>Co/NPC-800</td><td>PS 0.5-4.0 nm<br>TPV 0.25 cm<sup>3</sup><br>g<sup>-1</sup></td><td>-</td><td>500 mg g<sup>-1</sup> (b)</td><td>7</td><td>Electrostatic<br>interaction<br>(complexation)<br>π-π interaction</td><td>[91]</td></tr><tr><td>Zn-MOF</td><td>PS 3.9 nm<br>PV 0.465 cm<sup>3</sup><br>g<sup>-1</sup></td><td>-23.25 mV at pH<br>6.7</td><td>326 mg g<sup>-1</sup> (b)<br>(98 %)</td><td>7</td><td>Electrostatic<br>interaction</td><td>[92]</td></tr><tr><td>MOF-235 (Fe)</td><td>-</td><td>+12 mV at pH<br>6.5<br>-22 mV at pH 11</td><td>187 mg g<sup>-1</sup> (a)<br>220 mg g<sup>-1</sup> (a)</td><td>7<br>10.5</td><td>Electrostatic<br>interaction with<br>negative charge of<br>charge balancing<br>anion [FeCl<sub>4</sub>]<sup>-</sup></td><td>[92]</td></tr><tr><td>CZIF 1000</td><td>PW 0.34 nm<br>PS 1.14 nm<br>TPV 0.98 cm<sup>3</sup><br>g<sup>-1</sup><br>Micro pore<br>volume 0.4 cm<sup>3</sup><br>g<sup>-1</sup></td><td>-60 mV at pH 7</td><td>186.3 mg g<sup>-1</sup> (b)</td><td>7</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[88]</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub>@-<br>SiO<sub>2</sub>@UiO-66</td><td>PV 0.47 cm<sup>3</sup> g<sup>-1</sup></td><td>-</td><td>128 mg g<sup>-1</sup><br>(98 %)</td><td>11</td><td>Electrostatic<br>interaction<br>π-π interaction<br>Hydrogen bonding</td><td>[93]</td></tr><tr><td>UiO-68-NH<sub>2</sub></td><td>PS 2-50 nm</td><td>-4.91 mv at pH 7</td><td>96.45 mg g<sup>-1</sup><br>(98 %)</td><td>7</td><td>Electrostatic<br>interaction</td><td>[82]</td></tr><tr><td rowspan="2">Safranin<br><img src="safranin_molecule.png" alt="Safranin molecule structure"></td><td>UiO-66</td><td>PS 1.15-1.5 nm<br>(max<br>distribution of<br>1.2 nm)<br>TPV 0.88 cm<sup>3</sup><br>g<sup>-1</sup></td><td>-</td><td>366 mg g<sup>-1</sup></td><td>&gt;5.5</td><td>Size fitting</td><td>[94]</td></tr><tr><td>ZIF-8@CS/<br>PVA-2</td><td>-</td><td>-</td><td>1000 mg g<sup>-1</sup> (a)</td><td>6</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[95]</td></tr><tr><td rowspan="2">Malachite Green<br><img src="malachiteGreen_molecule.png" alt="Malachite Green molecule structure"></td><td>Ce-doped UiO-67</td><td>PW 2.047 nm<br>PV 0.878 cm<sup>3</sup><br>g<sup>-1</sup></td><td>0 at pH 5.05</td><td>589 mg g<sup>-1</sup></td><td>5</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[96]</td></tr><tr><td>MIL-100 (Fe)</td><td>PW 0.55 and<br>0.86 nm<br>PS 2.5, 2.9 and<br>3.16 nm<br>PV 0.79 cm<sup>3</sup> g<sup>-1</sup></td><td>-30 at pH 5</td><td>205 mg g<sup>-1</sup> at<br>25 °C<br>458 mg g<sup>-1</sup> at<br>50 °C</td><td>5</td><td>Electrostatic<br>interaction<br>π-π interaction<br>Electrostatic<br>interaction<br>Surface<br>complexation</td><td>[97]</td></tr><tr><td rowspan="3">Crystal Violet<br><img src="crystalViolet_molecule.png" alt="Crystal Violet molecule structure"></td><td>Mn-MOF</td><td>PW along a, b, c<br>axis are 0.6<br>×1.4, 0.4 ×0.84<br>and 0.7 ×0.5 nm</td><td>-0.158</td><td>938 mg g<sup>-1</sup></td><td>6</td><td>Size fitting<br>Electrostatic<br>interactions</td><td>[98]</td></tr><tr><td>BUT-29</td><td>PS 1.24<br>×0.71 nm</td><td>-</td><td>832 mg g<sup>-1</sup></td><td>-</td><td>Electrostatic<br>interaction</td><td>[90]</td></tr><tr><td>Zn-MOF</td><td>PS 3.9 nm<br>PV 0.465 cm<sup>3</sup><br>g<sup>-1</sup></td><td>-23.35 mV at 6.7</td><td>91 %</td><td>6.7</td><td>Electrostatic<br>interaction</td><td>[99]</td></tr></tbody></table>

(continued on next page)
<table><caption>Table 1 (continued)</caption><thead><tr><th>Dye structure and molecule dimensions</th><th>Adsorbent</th><th>Pore width (PW),<br>pore size (PS*),<br>pore volume (PV),<br>total pore<br>volume (TPV)</th><th>Zeta potential<br>of MOF at<br>experimental<br>pH</th><th>Adsorption<br>rate, qe etc.</th><th>pH of<br>experiment</th><th>Adsorption<br>driving force</th><th>Ref.</th></tr></thead><tbody><tr><td>1.23—1.39 nm [79]<br>Rhodamine B</td><td>MIL-68(Al)</td><td>PV 0.7 cm<sup>3</sup> g<sup>-1</sup></td><td>–</td><td>1111.1 mg g<sup>-1</sup><br>(a) (97 %)</td><td>7.96</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[89]</td></tr><tr><td rowspan="2">1.44 ×1.09 ×0.64 nm [70]</td><td>Ce-doped UiO-67</td><td>PW 2.047 nm<br>PV 0.878 cm<sup>3</sup><br>g<sup>-1</sup></td><td>0 at pH 5.05</td><td>754.4 mg g<sup>-1</sup></td><td>5</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[96]</td></tr><tr><td>UiO-66-NH<sub>2</sub></td><td>PS 2–50 nm</td><td>–</td><td>82.69 %</td><td>–</td><td>Electrostatic<br>interaction</td><td>[81]</td></tr><tr><td rowspan="2">Methyl Red</td><td>MIL-68(In)-NH<sub>2</sub>/<br>GO</td><td>PW 0.5 and<br>1.6 nm<br>Interlayer<br>distance<br>0.82 nm<br>PV 0.389 cm<sup>3</sup><br>g<sup>-1</sup></td><td>–12.5 mV at pH<br>6</td><td>268 mg g<sup>-1</sup></td><td>6</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[100]</td></tr><tr><td>Fe<sub>3</sub>O<sub>4</sub>@MIL-100<br>(Fe)</td><td>–</td><td>–</td><td>586.34 mg g<sup>-1</sup><br>(b)</td><td>4</td><td>Electrostatic<br>interaction<br>π-π interaction</td><td>[101]</td></tr><tr><td rowspan="2">Methyl Orange</td><td>MIL-53 (Fe)</td><td>PW 9.8 nm<br>PV 0.0215 cm<sup>3</sup><br>g<sup>-1</sup></td><td>–</td><td>80 mg g<sup>-1</sup> (b)</td><td>3</td><td>Electrostatic<br>interaction<br>π-π interaction<br>Breathing effect of<br>MIL-53 (Fe)</td><td>[102]</td></tr><tr><td>MOF-235 (Fe)</td><td>–</td><td>+12 mV at pH<br>6.5<br>+22 mV at pH<br>3.5</td><td>477 mg g<sup>-1</sup> (a)<br>490 mg g<sup>-1</sup> (a)</td><td>7<br>3.5</td><td>Electrostatic<br>interaction<br>with positively charged<br>framework</td><td>[92]</td></tr><tr><td rowspan="2">1.31 nm x 0.55 nm x 0.18 nm [70]</td><td>Fe<sub>3</sub>O<sub>4</sub>@-<br>SiO<sub>2</sub>@UiO-66</td><td>PV 0.47 cm<sup>3</sup> g<sup>-1</sup></td><td>–</td><td>219 mg g<sup>-1</sup><br>(98 %)</td><td>3</td><td>Electrostatic<br>interaction<br>π-π interaction<br>Hydrogen bonding</td><td>[93]</td></tr><tr><td>CZIF 1000</td><td>PW 0.34 nm<br>PS 1.14 nm<br>TPV 0.98 cm<sup>3</sup><br>g<sup>-1</sup><br>Micropore V<br>0.4 cm<sup>3</sup> g<sup>-1</sup></td><td>–60 mV at pH 7</td><td>153.5 mg g<sup>-1</sup></td><td>7</td><td>Size fitting</td><td>[88]</td></tr><tr><td></td><td>Fe<sub>3</sub>O<sub>4</sub>/MIL-101<br>(Cr)</td><td>PW 2.9–3.4 nm<br>PS 1.2–1.6 nm<br>PV 1.78 cm<sup>3</sup> g<sup>-1</sup></td><td>+39.8 mV at 7</td><td>80 mg g<sup>-1</sup><br>(90 %)</td><td>7</td><td>Electrostatic<br>interaction</td><td>[86]</td></tr></tbody></table>

three different dyes: cationic methylene blue, neutral isatin, and anionic methyl blue [103]. MIL-100(Fe) showed slight adsorption ability for isatin compared to methylene blue and methyl blue, which is explained by its electro-neutrality. Adsorption of cationic methylene blue on MIL-100 was more effective than that of anionic methyl blue (97.9 % and 44.9 %, respectively) due to a stronger electrostatic interaction of methylene blue with negatively charged network. The plausible reason for the adsorption of anionic dye on negatively charged MOF is the π-π interactions between the benzene rings of methyl blue and MIL-100. The poor adsorption of isatin is also explained by hindered π-π interaction due to a smaller number of benzene rings than that of methyl blue. The impact of π-π interaction is discussed below.

Haque et al. synthesized functionalized chromium MOFs. MIL-101,

ethylenediamine-grafted MIL-101 (ED-MIL-101), and protonated ED-MIL-101 (PED-MIL-101) were studied for the adsorption of anionic MO dye (Fig. 8) [22]. The results showed that the increase of positive charge on the MOF leads to an increase of adsorption capacity towards anionic dye.

The framework is not the only possible source of charge. Haque synthesized an iron terephthalate MOF-235 composed from both cationic framework and anionic charge-balancing ion [FeCl<sub>4</sub>]<sup>-</sup> [92]. The MOF-235 was found to effectively adsorb both negatively charged MO and positively charged MB dyes, all due to its dual properties.

The pH level is closely related to electrostatic adsorption. Typically, the adsorption of cationic dyes on anionic MOFs occurs more readily at higher pH and vice versa. In the studies of Haque et al. the adsorption of
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Schematic interactions between anionic dye methyl orange and MIL-101, ethylenediamine-grafted MIL-101, and protonated ED-MIL-101 [22]. Copyright © 2010 Chemical Engineering Journal.</figcaption></figure>

anionic MO decreased with the increase of pH due to the decrease of positively charged adsorbent sites [22,92]. In electrostatic interactions, the zeta potential is an important adsorbent characteristic, dependent on the pH level. Karmakar et al. showed the importance of zeta potential on the adsorption of four reactive dyes, i.e., reactive black 5, reactive yellow 15, reactive red 24, and reactive blue 2 [104]. The zeta potential of these four dyes and MIL-101(Cr) varies with the pH level and lies in the range from 49 mV (pH 3) to ~33 mV (pH 10). At lower pH, the adsorption is purely due to the strong physisorption. At neutral pH, physisorption is facilitated by electrostatic interactions, whereas at high pH, the adsorption is hindered due to a strong electrostatic repulsion. Furthermore, the surface area of the adsorbent can play an integral role in the adsorption process [43,105].

### 3.2. π-π stacking

π-π interactions between dyes' benzene rings and MOF's aromatic backbone can be responsible for adsorption. In the study of Torad et al., the Co-doped nanoparticles of carbon were tested to adsorb MB dye [91]. Compared to AC, the Co/NPC provided a much larger adsorption capacity for MB dye, almost ten times higher than the original ZIF-67. This result is due to high nanoporosity, open pore network, and highly graphitized composition of Co/NPC, facilitating fast molecular diffusion and more effective interaction of MB molecule with the graphitic carbon (sp²-carbon).

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Dimensions of several dye molecules: front and side views [70,108].</figcaption></figure>

Multiple interactions are possible in the adsorption process [14,97, 106]. For instance, in the work of Huang et al., MO and MB were adsorbed on Zr-MOF via electrostatic and π-π interactions [93]. MO with pKa of 3.4 exists in a mixture of neutral and anionic species, while the surface of Zr-MOF is positively charged at pH 3. Therefore, the negative form of MO is adsorbed via electrostatic interactions with positive sites of MOF, and the neutral form is adsorbed via π-π stacking of benzene rings. When MB is presented in neutral form (solution pH > pKa<sub>MB</sub>), the π-π interaction and hydrogen bonding are the main factors driving adsorption.

### 3.3. Hydrogen bonding

Hydrogen bonding is another important type of interaction between MOF and dye. In the study of Gao et al., the adsorption of Congo red (CR) on a bifunctional porous Zn-MOF with a pore size of 2.94 nm was investigated [107]. The molecule of CR (2.59 nm x 0.93 nm x 0.14 nm [108]) could not easily penetrate the pores of Zn-MOF (Fig. 9). Instead, it was adsorbed via hydrogen bonding between the amino group of CR and the hydroxyl group in Zn-MOF.

Comparatively inert adsorbates, which do not easily interact with other species, can be removed or separated through hydrogen bonding. Phenol and p-nitrophenol are industrial waste effluents for dye & textile and tannery industries. Liu et al. studied the adsorption of phenol and p-nitrophenol (PNP) on MIL-101 (Fe, Cr), NH₂-MIL-100(Al), and AC [109]. The adsorption of phenol was low for all MOFs, even for NH₂-MIL-100(Al), which was expected to show high affinity between its weak basic sites of amino group and weakly acidic phenol. However, the adsorption of PNP on NH₂-MIL-100(Al) was superior to that on AC. The solubility of phenol in water is seven times higher than that of PNP. It was, therefore, expected that the adsorption capacity of PNP would be higher than that of phenol for all MOFs. However, MIL-100(Fe, Cr) showed little adsorption capacity towards PNP, meaning that the most important interaction between PNP and NH₂-MIL-100(Al) was the hydrogen bonding between oxygens of the nitro group of PNP and hydrogens of the amino group of the MOF.

Song et al. showed that free carboxyl ligands positively influence the adsorption of heterocyclic amines indole and pyrrole [110]. Four Zr-based UiO-66 with different ratios of terephthalic/isophthalic acid ligands were synthesized and used for the adsorption of indole and pyrrole. The authors showed that the greater the quantity of isophthalic ligands in the framework, the greater the adsorption. Therefore, the hydrogen bonding was the main reason for the increased adsorption of indole and pyrrole with the increased isophthalic content in the linker used to synthesize MOF. The plausible adsorption mechanism of indigo via H-bonding is illustrated in Fig. 10.
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Plausible way of H-bonding between indole and UiO-66 [110] Copyright © 2016 American Chemical Society.</figcaption></figure>

### 3.4. Size compatibility

The size and accessibility of pores are particularly important characteristics of an adsorbent that influence the isotherm and kinetics of the adsorption process. To investigate the size-dependent behavior of adsorption on MOF, Zhao et al. applied an anionic MOF, to adsorb MB and Rhodamine B (RhB) – two dyes of positive charge but of different sizes [111]. The removal efficiency for MB was almost seven times greater than that for RhB, which was attributed to the smaller size dimensions of MB molecule allowing their penetration into the network of this MOF.

The size and accessibility of pores can be changed in the presence and absence of water. The phenomenon of the fully reversible hydration-dehydration process is called the “breathing effect.” This effect is presented in zeolites and some MOFs. In particular, the MIL-53-n series has a flexible framework that can open pores for guest introduction [112]. MIL-53 consists of a three-dimensional structure of chromium(III) octahedra chains linked through terephthalate dianions with an array of one-dimensional large pore channels (Fig. 11). After synthesis, the terephthalate molecules are trapped in pore channels (MIL-53as) and removed by evaporation. The transition between the hydrated form (MIL-53lt) and the anhydrous solid (MIL-53ht) is fully reversible and followed by a large breathing effect (more than 0.5 nm). The pores are clipped in the presence of water molecules and reopened when the channels are empty. Therefore, MIL-53as and MIL-53lt (filled with free dicarboxylic acid and water within the pores) do not exhibit any accessible porosity, while MIL-53ht possesses pores of free aperture.

Trung et al. studied chromium-based MOFs of different pore sizes, MIL-100 (Cr(VI)-based) and MIL-101 (Cr(III)-based), for adsorption of different linear alkanes from n-pentane to n-nonane (C5–C9) [113]. The sizes of mesoporous cages are 2.4 and 2.9 nm for MIL-100 and 2.7 and 3.4 nm for MIL-101, and the microporous windows are 0.50 and 0.86 nm, and 1.17 and 1.60 nm, respectively (Fig. 12(a–c)). The adsorption isotherms for both MOFs were of type I and revealed a slight sub-step in adsorption of alkanes C6–C9 at very low pressure (Fig. 12(d, e)), which was previously attributed to the breathing effect, caused by the structural transition between two phases. However, because of the rigidity of MIL-100 and MIL-101 frameworks, this effect could not explain the sub-step. Instead, it was attributed to different cages filling at different relative pressures. The first part of the isotherm, before the sub-step, describes the adsorption in both smaller and bigger cages. When the smaller cages are filled, the molecules continue to fill the larger cages until saturated, which is described by the rest of the

isotherm.

Another observation of this work is that the interactions governing the sorption process are stronger in the case of MIL-100 because of the smaller pore size enhancing confinement effect due to the stronger interactions between the surface of the adsorbent and the sorbate. It is worth noting that the kinetics of the sorption process were not influenced by the alkane chain length, at least up to n-nonane. The adsorption of n-nonane is a slower process, which is explained by a higher number of possible conformations than the other alkanes, contributing to an extended relaxation time for the system and to a slower diffusion of the molecule compared to other lighter alkanes.

### 3.5. Predominant factor: molecular size or surface charge

Byun et al. studied the separation of water-soluble organic molecules and showed that the adsorption depended on the charge and size of adsorbates and the pore size of adsorbents [96]. The authors showed that the covalent organic polymer (COP-99) had almost no uptake of bisphenol A, an uncharged molecule with dimensions like MB. This confirms that the lack of charge hinders the adsorption despite size compatibility.

On the other hand, there was no significant difference in the adsorption of 4-nitrophenol (4-NP) in its neutral (99 % removal) and anionic (97 %) forms at pH 4 and 9, respectively. This indicates that the contribution of size compatibility was bigger than that of the charge interaction if the size of the target molecules is small enough to penetrate the pore network. Abbasi et al. presented similar observations [71]. Negatively charged CZIF 1000 had almost two times higher adsorption capacity to anionic MO than to cationic RhB dye. The molecular dimensions of MO are 1.31 nm x 0.55 nm x 0.18 nm, which is smaller than those of RhB (1.44 nm x 1.09 nm x 0.64 nm) (Fig. 9). The MO molecules could fit into adsorbent pore windows ranged from 0.34 to 1.14 nm more easily. Size compatibility compensated for the negative charge of MO dye and allowed its adsorption on anionic MOF. However, the adsorption capacity of CZIF 1000 for cationic MB is bigger than that for anionic MO, regardless of the slightly bigger size of MB molecule (1.43 nm x 0.61 nm x 0.40 nm). When the adsorbate molecule is small enough to penetrate the framework, the size compatibility is predominant factor of dye adsorption on MOF. In this case, the adsorption can be promoted by employing the electrostatic nature of substances. However, for molecules of larger dimensions electrostatic, π-π, and hydrogen interactions will play a greater role in dye-MOF adsorption.

### 3.6. Hydrophobicity and hydrophilicity

Hydrophobicity/hydrophilicity of adsorbent, as well as solvent polarity, are characteristics that may be important yet not well-researched. In the case of adsorption of naphthalene (non-polar compound) over AC, the adsorption capacity decreased in the presence of organic non-polar solvents (cyclohexane and heptane) [97]. It was suggested that the competitive adsorption between adsorbate and solvent occurs, and solvent molecules occupy the AC adsorption sites. Besides, the affinity between solvent and adsorbate can be an important factor influencing adsorption [98,99].

Thiophene, pyrrole, and nitrobenzene are blocks used for dye synthesis, and dyes decompose under environmental or degradative

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** View of the pore systems of (a) MIL-53as, (b) MIL-53ht, and (c) MIL-53lt [112] Copyright © 2002 American Chemical Society.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Structures of MIL-100(Cr) and MIL-101: (a) Small cages, (b) large cages, (c) pentagonal and hexagonal windows. Chromium octahedra, carbon atoms are in green and black, respectively. (d) Adsorption of (♦) n-hexane (C6), (o) n-heptane (C7), (Δ) n-octane (C8), (η) n-nonane (C9) on MIL-100(Cr), and (e) on MIL-101(Cr) at 313 K [113]. Copyright © 2010 Microporous and Mesoporous Materials.</figcaption></figure>

<figure><img src="image_16.png" /></figure>

treatment. In the study of Bhadra et al. the adsorption of these com-
pounds was studied on AC and several MOFs [100]. The hydrophobicity
of AC and ZIF-8 was beneficial for adsorption from water or polar sol-
vents due to hydrophobic interactions between adsorbates and adsor-
bents. On the other hand, MOFs such as MIL-101 are effective for
adsorbing aromatics from non-polar solvents due to the polar in-
teractions between adsorbate and hydrophilic MIL-101.

More than one interaction is possible during the adsorption process.
Compounds of both ionic and neutral nature adsorb on the surface of
MOFs, then penetrate their pores and stay captured. When different
types of interactions are involved, dye adsorption on MOFs is enhanced.
Therefore, for the adsorption of dyes with specific characteristics, such
as charge, molecule structure, and size, the pragmatic synthesis of MOFs
must be involved, and valuable properties of the framework must be
considered for the most effective outcome of the adsorption process.

<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Number of articles investigating dye removal with MOFs. From Web of ScienceDirect. Trends in the number of new scientific articles on the Web of ScienceDirect. Search query: (metal-organic framework* OR MOF*) AND (dye removal*).</figcaption></figure>

4. Integration of MOFs on supports

During the past decade, the number of papers where MOFs were used for dye removal on a laboratory scale increased dramatically (Fig. 13). The MOFs' fine powder form, used for experiments on a laboratory scale, has important inconveniences for its use on an industrial scale, such as aggregation, difficulty in handling, and clogging of tubing [24]. Attempts have been made to facilitate the separation of MOFs after the adsorption cycle. Torad et al. synthesized magnetic Co/NPC (nano-porous carbon) particles by thermal decomposition of ZIF-67 [78]. The obtained adsorbent was successfully used for methylene blue removal, and the adsorbent could be easily removed from the solution with a magnet, as shown in Fig. 14. Zhang et al. reported the fabrication of magnetic Fe₃O₄@carbon@ZIF-8 core-shell nanocomposite, where carbon-based iron (II, III) oxide plays the role of a magnetic core, and ZIF-8 acts as a water-stable outer shell [101]. The adsorbent was used for effective adsorption of CR dye (adsorption capacity of 806.45 mg g⁻¹) and could be recovered after adsorption by magnetic separation. The powder form of magnetic MOF particles can be appropriately used on a laboratory scale, but it is inconvenient for industry applications. MOFs should be integrated into support for scaling up to simple industrial usage and not promoting secondary pollution.

MOFs can be introduced in polymer matrix membranes [114-117],
fabrics [27], or nanofibers [28,118,119]. Yu et al. showed that MOF

<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Photographs of the initial MB solution, the solution after mixing with Co/NPC-800, and the solution after removal of MB dyes [91] Copyright © 2014 John Wiley and Sons.</figcaption></figure>
containing membranes presented better dye removal capacity than the single substrate [120]. Nonetheless, both these forms of support often have smaller surface areas compared to those that nanofibers can offer. For example, a common MOF-5 was incorporated on cotton fabrics [27], cellulose acetate (CA) membrane [121], and chitosan (CS)/PEO nanofibers [122], which gave the BET surface areas of 1.43 m² g⁻¹, 19.87 m² g⁻¹, and 78.42 m² g⁻¹, respectively. Nowadays, nanofibers are used in clothing, personal protective equipment, air purification and filtration, energy storage, biomedical equipment, drug delivery, detection and sensing, and many other applications. Nanofibrous filtration media have the advantages of low weight, small pore size, and high permeability, making them attractive for various filtration applications.

The routes to make nanofibers include drawing, template synthesis, phase separation, and self-assembly, and in the last 30 years, electrospinning has drawn a lot of attention. This technique, unlike others, permits the formation of infinitely long and uniform nanofibers with tunable diameters (down to the range of a few nanometers) and requested morphologies and enables a wide range of MOF loadings in or on polymer fibers [123].

## 4.1. Electrospinning

Electrospinning offers fiber formation not based on mechanical but rather on electrical forces. Self-assembly of fiber in the electrospinning process is controlled by Coulomb interactions between charged elements of the fluid body (solution of polymer).

The experimental approach is simple. A solution of a polymer dissolved in an appropriate solvent is placed in a syringe with a needle with an inner diameter of several hundred micrometers. Droplets of polymer solution fall off the tip of the syringe onto the aluminum foil collector if no high voltage is applied. However, when a sufficient voltage is applied between the syringe tip acting as an electrode and the aluminum foil acting as a counter electrode, the formation of thin steadily and smoothly downstream sets in. The thin fluid jet becomes unstable as it falls towards the counter electrode and spirals violently in large loops. The solid fiber is, therefore, deposited on the collector. Due to the instabilities and their superpositions, the morphology of obtained fibers can vary, from thin, smooth fibers and porous fibers to fibers with fractal surface structures and bandlike or odd-shaped fibers [123–125].

Nanofibers of a target diameter (uniformed or not), surface topology, porosity, branches, and cross sections can be formed using electrospinning. Different key parameters governing electrospinning control these and other fiber characteristics, such as polymer-solution/polymer-melt properties like concentration, viscosity, and conductivity and technical features like voltage, flow rate, and tip-to-collector distance. These parameters of electrospinning system impact the fiber diameter, their length, and beads formation.

Polymer concentration, viscosity and molecular weight are interrelated parameters of the electrospinning solution. At the optimum polymer concentration, bead-free nanofibers are produced. With an increase of polymer concentration, the increase of viscosity occurs, resulting in the appearance of spherical beads. With the following increase of viscosity, the form of beads changes to spindle-like until it results in the formation of smooth bead-free fibers with increased diameter. Solutions with high polymer concentration are not suitable for electrospinning due to their high viscosity, which can cause the clogging of the needle. The increase of molecular weight results in generation of smooth bead-free fibers, while decrease of molecular weight at a maintained polymer concentration results in formation of beads. The increase of the solution conductivity generally causes thinning of fibers and can be reached by adding salts. However, the increase of salt concentration leads to an increase of viscosity, which causes bead formation and increased fiber diameter [126].

Other than solution characteristics, the parameters of the electrospinning process also cause morphological changes in nanofibers. Electrospinning occurs when applied voltage is higher than the critical

voltage value. The critical value of applied voltage is a parameter identified experimentally, and it varies from one polymer to another [127–129]. Evaluation of fiber diameter under applied voltage can be divided into two stages. In the first stage, the increase of applied voltage causes thinning fiber diameter, attributed to the stretching of the polymer solution under electrical force. When the increase of applied voltage continues, it results in formation of multiple jets, which merge and form thicker fibers with increased diameter. For formation of small diameter narrow fibers, an appropriate value of applied voltage must be tailored [126]. The tip-to-collector distance is the path that the fiber travels between the needle tip of syringe and the surface of collector. If this distance is too short, jet stretching is limited, resulting in bead formation and large diameter. If the distance is too far, again, beaded structures are formed because of the weak strength of electric field. When the optimum distance is set, the fiber is bead-free and has a narrow diameter. The flow rate is another parameter influencing morphology and diameter of ENF. Generally, reducing the flow rate results in thinning of fiber diameter, since it allows sufficient time for solvent evaporation. Increasing of the flow rate results in instability of the Taylore cone and, therefore, bead formation and diameter increase [130,131]. These solution and process parameters must be adjusted individually when a MOF is being integrated in ENFs [132].

## 4.2. Incorporation of MOFs in ENFs

ENFs have infinite lengths and small diameters; membranes are porous and very light (about 0.1 g cm⁻³). All these characteristics lead to an inevitable advantage – ENFs have extremely high specific surface area. The ratio between the surface and volume is extraordinarily high, which benefits air and water treatment applications. In air filtration, ENFs with functional groups on the surface can deactivate harmful gases and warfare agents into relatively harmless by-products [133]. The electrospun polyurethane (PU) rapidly adsorbed volatile organic compounds, such as toluene and acetone [134]. The authors revealed that adsorption occurred not only on the surface but also in the polymer matrix of the PU fibers themselves, which resulted in adsorption capacity comparable to that of AC. In water filtration, ENFs are used because their morphology is tunable, and their surface can be controlled, modified, or functionalized [28,135]. Dyes can be degraded using photocatalytic reaction [136,137], or adsorbed on non- or modified ENFs [138–140]. Like MOF-dye electrostatic interaction, oppositely charged nano-fibrous membranes can successfully capture dyes carrying ionic charge [141]. MOF/ENF composites have been shown to be few times more effective for dye removal than MOF and ENF separately. For example, Zr-MOF polyvinyl alcohol (PVA) ENFs showed almost four times higher adsorption capacity for MO dye adsorption compared to that of pure Zr-MOF powder [142].

The main challenge in applying ENFs in water treatment is that many are hydrophobic. There exist ways to overcome this problem. For example, Yoon et al. reported the modification of polyester sulphone (PES) nanofiber with a strong oxidant 3 % w v⁻¹ ammonium persulfate, which increased the membranes' hydrophilicity [143]. However, different surface treatment types must be considered for specific applications [144]. The incorporation of MOFs into ENFs can enhance the framework's stability in water, too. When MOF-5, exhibiting poor water stability, is modified with hydrophobic functional groups, like methyl, the framework's water stability is increased, and in combination with PAN ENFs the composite increases π-π interactions, hydrogen bonding, and hydrophobic interactions between the sorbate and sorbent [145].

A large number of methods allowing the functionalization of polymer nanofibers were presented by researchers, including (1) physical coating or blending, when functional molecules are blended into the bulk polymer or coated on the polymer surface, (2) graft copolymerization and plasma treatment where radical groups are produced via irradiation, and (3) chemical treatment, where polymers with functional groups like hydroxyl, carboxyl, amino, ester, etc. can be modified
directly. However, strong reaction conditions such as plasma or UV
irradiation and acidic or basic environment may destroy the polymer
nanofibers. They must be protected from being disrupted. Depending on
the incorporating compound, some modification methods remain that
can be performed under moderate conditions, like molecular self-
assembly and "layer-by-layer" techniques. These methods can be used
especially to increase the hydrophilicity of polymer nanofibers, most of
which are hydrophobic [146].

MOFs can be introduced into the polymer solution either before
electrospinning (so-called MOF-first strategy), they can be attached to
the functionalized surface of electrospun fibers, or else they can be
synthesized directly on the surface of the preliminary electrospun fibers
(so-called fiber-first strategy) (Fig. 15) [39,147]. Integrating MOFs with
electrospun polymer nanofibers enables the formation of fibrous com-
posite material with both high specific surface area and high potential

for adsorption.

4.2.1. MOF-first strategy

In the MOF-first technique, pre-synthesized MOFs are mixed with a
polymer solution, and the resulting suspension is then electrospun. For
water filtration, the concentration of MOF in a polymer solution and the
size of MOF particles are important as they can influence the MOF
loading and the morphology of obtained fibers. The load of MOFs can be
affected by the MOFs' size, the electrospun fiber's diameter, and the
polymer's polarity. Precise control of the MOF concentration is essential
because excess MOF in polymer solution can lead to particle clustering
due to MOF-MOF interactions and poor fiber formation, resulting in
increased viscosity, needle clogging, and decreased stability of MOF
particles. If MOF particles are smaller or about the size of the diameter of
the polymer fiber, they can form a fine attached morphology on the

<figure><img src="image_19.png" /><figcaption>Fig. 15. Strategies for the integration of MOFs into and onto electrospun nanofibers.</figcaption></figure>
surface of polymer fibers. However, when MOF crystals are significantly larger than the diameter of polymer fiber, they form assemblies of agglomerated sites on the fiber surface, considerably reducing the effectiveness of water treatment.

The MOF-first technique can be challenging. Particle clustering, agglomerated morphology, and the possibility that the MOF crystals can stay embedded within the fibers can significantly decrease the adsorption properties and reduce the effectiveness of treatment. However, this technique can create fibers with suitable, effective morphologies for water treatment. This type of attached morphology is achieved using thinner polymer fibers concerning MOF crystal size, direct mixing of MOF and polymer, moderate-to-high MOF loadings, and reduced MOF-polymer compatibility, the lack of which ‘pushes’ the MOF crystals outside of the fiber during its formation. This effect was shown in the work of Rose et al., where HKUST-1 and MIL-100(Fe) were loaded in polyacrylonitrile (PAN), polystyrene (PS), and polyvinylpyrrolidone (PVP) [148]. The MOF loading in the polymeric fiber was up to 80 % due to a small diameter compared to the particle size. However, even at such high loadings, it is possible that MOF particles can stay embedded within ENFs, surrounded by polymer acting as a diffusion barrier [149]. For example, in the study of Ostermann et al., the access to nitrogen of ZIF-8 fraction decreased almost two times after electrospinning of MOF with PVP [150].

### 4.2.2. Fiber functionalization

Another technique is MOF integration by particle deposition. In this technique, pre-synthesized MOFs are attached to prefabricated nanofibers using attachment techniques, such as employing binding polymers or reactive groups capable of covalent and non-covalent bonding [132, 147]. The choice of the correct binding polymer is essential to avoid the polymer blocking access to the pores inside MOFs. Wang et al. used polyvinylidene fluoride as a binder for ZIFs with limited reduction in surface area [151]. Further research is needed in this area since if the proper binder between fiber and MOF is found, this technique can potentially become both cost and time effective. Otherwise, the organic ligands of MOFs can be intentionally changed to promote covalent or non-covalent interactions with the polymer interface [152].

### 4.2.3. Fiber-first strategy

In fiber-first technique, MOFs grow directly on pre-synthesized nanofibers. The fiber-first strategy includes three techniques of MOF in-situ growth on the surface of fiber (Fig. 16) [153]. The first approach is in-situ direct growth when the fiber is immersed in a metal and ligand precursor solution and heated in a closed vessel. Ma et al. coated Zr-based MOF on polyester fabrics in this manner without polymer surface modification [154]. Importantly, the MOF particles were intact after washing due to robust intercrystallite interactions and strong binding between crystals and fiber. With this approach, uniformly distributed MOF crystals are obtained on the surface of fibers (Fig. 17 (a–f)).

The second approach is stepwise growth, when the metal precursors are first added to form the initial coordination between metal ions and functional groups of fibers. Then, organic linkers are added for MOF nucleation and growth. Lian et al., for the first time, coated ZIF-8 crystals on polyurethane nanofibers [156]. A polymer electrospun mat was immersed into the metal solution, after which the ligand solution was added. The highest loading capacity was 63 %, and the surface area reached 566 cm² g⁻¹. Functionalization of polymer fiber is often needed to enhance the MOF loading onto the surface of electrospun fibers. Meilikhov et al. functionalized polyester surface using polyvinylamine and additional bromoacetic acid (BAA), which gave both anchoring and seeding effects for the [Cu₃(BTC)₂] deposition (Fig. 17(g)) [155].

Lastly, the layer-by-layer strategy is used when a pre-synthesized ENF is repeatedly immersed into a MOF precursor solution. This method is like stepwise growth, with the difference that the nucleation and growth stages repeat several times. Laurila et al. have evaluated the

<figure><img src="image_20.png" /><figcaption>Fig. 16. Three types of the fiber-first strategy for MOF growth on ENF.</figcaption></figure>

HKUST-1 growth on the surface of pretreated anionic electrospun cellulose using the layer-by-layer technique [149]. It was shown that the distribution of HKUST-1 on the nanofiber surface highly depended on the type of anionic pretreatment and that the BET surface area could be increased by a factor of ten. The number of layering cycles is important since, with their increase, the size of resulting particles increases gradually, which can cause an overall decrease in adsorption. Mahmoodi et al. presented a synthesis of ZIF-8@chitosan/ /polyvinyl alcohol (CS/PVA) nanofibers with one to three layering cycles [95]. The nanofibers with three layers of ZIF-8 had lower adsorption capacity to malachite green (MG), than those with two layers of coated MOF, which was attributed to the blockage of pores between the strings with increased ZIF-8 diameter.

Since the heterogeneous nucleation of MOF crystals on supports can be poor, some variations of the mentioned strategies are possible. For example, Wu et al. presented a two-step HKUST-1 and ZIF-8 integration in polymer nanofibers [157]. In the first step, a MOF was mixed with a polystyrene solution for the following electrospinning, during which MOF particles were embedded within the polymer. In the second step, the electrospun polymer membrane was immersed in the solution of MOF precursors. Nitrogen adsorption-desorption isotherm showed a significant increase in BET surface area and micropore volume after the seeded secondary growth of MOFs. Raw electrospun polystyrene nanofibers without embedded MOF seeds were tested for MOF growth for a control experiment. The growth of MOF crystals in the polymer network was impossible, indicating that embedded MOF nano seeds played a crucial role in the heterogeneous nucleation of MOF crystals.

### 4.2.4. Influence of MOF/ENF morphology on properties and applications

To summarize, the integration of MOFs and polymers can occur either via particle deposition of MOFs to the fiber before fiber formation (MOF-first) or by in situ growth of MOF on polymer fiber (fiber-first). For applications requiring adsorption, sensing, and catalysis, the strategies for obtaining MOF crystals exposed on the fiber surface are favored over those that entrap MOFs within fibers. In contrast, techniques of embedding generally reduce the BET surface area of MOFs
<figure><img src="image_21.png" /><figcaption>**Fig. 17.** SEM images of uniformly distributed MOF-808 particles on the PET fiber surface. (a) Pristine PET fiber, after reaction of (b) 10 min, (c) 20 min, (d) 30 min, (e) 60 min, and (f) 120 min; all scale bars are 5 µm [154] Copyright © 2019 American Chemical Society; (g) schematic functionalization of polyester surface with polyvinylamine and BAA [155] Copyright © 2019 Royal Society of Chemistry.</figcaption></figure>

[158], since the powder appears to be enclosed inside of the fiber and adsorption sites become inaccessible, decreasing overall adsorption. Nevertheless, embedding MOF crystals are favored for applications requiring enhanced mechanical strength of fibers [159], as, for instance, battery separators [160]. For all listed approaches, the growth of MOF crystals directly on the fiber surface can be complicated because most untreated fibers have few reactive sites. Thus, the surface of fibers often needs to be modified to make the anchoring of MOFs easier [149].

### 4.3. MOF/ENF stability and compatibility

Some fibers, like PVP and PVA, have low mechanical strength and dissolve when in contact with even small amounts of water [148]. MOF-based filters usually have weak robustness, which is a drawback for their application in industry [161]. Therefore, special attention must be paid to the everyday challenges that MOF-fiber composite materials face when used in filtration: their water stability and mechanical strength. Adjusting MOF and polymer compounds can increase interfacial compatibility and the composite fiber's chemical and mechanical stability. Table 2 summarizes examples of MOFs incorporated in nanofibers, pretreatment, application, and stability to thermal heat and/or solvents.

Reinforced interfacial MOF-ENF interactions can be reached by introducing anionic groups in the fiber surface. This method permits the synthesis of more chemically and thermally stable nanofibers. Lian et al. synthesized ZIF-8 on PU nanofibers after oxidation treatment activation of the PU surface [156]. They used concentrated sulfuric acid and chromium anhydrate to create anionic sites on PU nanofibers and to provide more binding sites for the adsorption of Zn²⁺, the ZIF-8 coordination center. ZIF-8-PU exhibited thermal stability up to 80 °C for prolonged periods and 150 °C for short periods and was stable in methanol, acetone, toluene, and water.

Another method of improving water stability of fibers is crosslinking. Commonly used crosslinking agents are ethylene glycol diglycidyl ether, diphenyl carbonate, and hydrogen sulfide, which, however, are not environmentally friendly. Zhang et al. crosslinked β-cyclodextrin with a more environmentally friendly citric acid to improve its water stability [165]. Chen et al. performed crosslinking of MOF/SA (sodium alginate) hollow tubes (HT) using Ca²⁺ ions [161].

Higher MOF loadings correlate with higher adsorption capacity of the final material. However, a significant difference in polarities between MOF and polymer can hinder the increase in MOF loading. Rose et al. synthesized HKUST-1/PS ENF with a maximum HKUST-1 loading of 40 wt% [148]. Attempts to increase MOF loading were unsuccessful due to the nature of non-polar PS and HKUST-1 with a more polar surface. This polarity difference made the suspension of HKUST-1/PS in tetrahydrofuran unstable at loadings higher than 40 wt%.

To increase the mechanical stability of composite material, Yoon et al. used a mixture of high boiling solvent N-Methyl-2-Pyrrolidone

with DMF (50 %:50 %) to modify poly (ether sulfone) [143]. Young's modulus and tensile strength improved to 570 % and 360 %, respectively, compared to the DMF solution. Wang et al. reported a polyethylene membrane with different loadings of NH₂-UiO-66, where the mechanical performance decreased with increased loading of MOF [166]. The fiber remained flexible up to 86 % of MOF loading. A higher MOF loading leads to a brittle breakdown of composite nanofiber material. The fiber thickness is another parameter that limits the material's flexibility; below a specific diameter, the nanofibrous mat changes mechanical behavior from ductile to brittle [167]. High chemical and mechanical stability are the factors that researchers attempt to improve to increase the efficiency of nanofibrous materials. However, it is essential to notice that the high stability of composite can lead to a decrease in its adsorption capacities because higher stability is also conjugated with a higher inertness [147].

### 4.4. MOF/ENF composites for dyes removal

ENFs modified with MOFs can be used for water filtration from different contaminants, like warfare agents, pathogens, heavy metal ions, etc. MOF/ENF membranes were shown to efficiently purify water from dyes and could be regenerated and reused multiple times with a minor or negligible decrease in adsorption effectiveness. However, weak robustness and weak water stability are electrospun membranes' main drawbacks for their industry application. Some examples of effective dye effluent treatment with MOF/ENF membranes exist, and researchers continue to develop this field. In this chapter, we feature the MOF/ENF pairs that have been successfully applied in dye removal.

Sodium alginate is a water-soluble polymer rich in functional hydroxylic and carboxylic sodium groups. Chen et al. cross-linked MIL-101-NH₂@SA-HT membrane with Ca²⁺ cations, using CaCl₂·2 H₂O solution, to stabilize the adsorbent in water. The membrane was then used for water treatment from MO dye (Fig. 18(a)) [161]. Moreover, ZIF-8@SA-HT was used for adsorption AsO₄³⁻, and the membrane could be regenerated and reused up to three times with no evident decrease in adsorption effectiveness [168].

In 2020, Mahmoodi et al. coated ZIF-8 on CS/PVA ENF using a layer-by-layer technique for MG dye adsorption on an industrial scale [95]. Three adsorbent membranes were synthesized with one, two, and three layers of ZIF-8, denoted as ZIF-8@CS/PVA-ENF-1, ZIF-8@CS/PVA-ENF-2, and ZIF-8@CS/PVA-ENF-3, respectively. After five adsorption-desorption cycles, ZIF-8@CS/PVA-ENF-2 performed sustainable stability and recyclability. With each subsequent cycle, the adsorption decreased, but at a slower rate than pristine ZIF-8. After the fifth cycle, the adsorption rate of ZIF-8@CS/PVA-ENF-2 was almost 90 %. Close attention must be paid to the number of MOF layers covering the ENF. In this study, the adsorption capacity of ZIF-8@CS/PVA-ENF-3 was smaller than that of ZIF-8@CS/PVA-ENF-2.

Higher loadings of MOF correlate with increased porosity and
<table><caption>**Table 2**
Examples on MOF-polymer fiber compatibility, stability, and morphology of composite.</caption><thead><tr><th>MOF</th><th>Polymer for electrospinning</th><th>Incorporation strategy</th><th>Pretreatment</th><th>Application</th><th>Structure, appearance, morphology</th><th>Remark on stability</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="5">HKUST-1</td><td>Cellulose</td><td>In-situ layer-by-layer growth</td><td>Introduction of anionic carboxyl groups</td><td>Gas adsorption</td><td>Uniformly distributed MOF on the fiber interface</td><td>Not reported</td><td>[149]</td></tr><tr><td>Polyacrylonitrile</td><td>MOF-first strategy</td><td>–</td><td></td><td>Network-like fiber structure</td><td>Stable in water</td><td>[148]</td></tr><tr><td>Polystyrene</td><td>MOF-first strategy</td><td>–</td><td>Protective clothing</td><td>Pearl-necklaces fiber structure</td><td>Not reported</td><td>[148]</td></tr><tr><td>Polystyrene</td><td>MOF-first, then in-situ growth</td><td>–</td><td>Gas separation, catalysis, sensing</td><td>Neckless-like structure with encapsulated MOF particles</td><td>Stable, reinforced, and self-supporting membrane</td><td>[157]</td></tr><tr><td>Polyester</td><td>In-situ stepwise growth</td><td>Functionalization with polyvinylamine and bromoacetic acid</td><td>Gas separation, protection layers for working clothes</td><td>Uniform distribution of MOF on the fiber interface</td><td>Mechanically stable</td><td>[155]</td></tr><tr><td>ZIF-8</td><td>Polyvinylpyrrolidone</td><td>MOF-first strategy</td><td>–</td><td>Gas adsorption and separation</td><td>Homogeneous distribution of nanoparticles inside of the fibers with a smooth polymeric surface</td><td>Stable up to 150°C and in non-polar solvents; Sensitive to polar solvents</td><td>[150]</td></tr><tr><td rowspan="5">ZIF-L</td><td>Polystyrene</td><td>MOF-first strategy</td><td>–</td><td>Adsorption in water and alcohols</td><td>Not reported</td><td>Stable in water and alcohols</td><td>[150]</td></tr><tr><td>Polyethylene oxide</td><td>In-situ direct growth</td><td>–</td><td>Catalyst for the Knoevenagel reaction</td><td>Even distribution, continuous morphology, and good crystallinity</td><td>Thermally stable up to 300 °C</td><td>[162]</td></tr><tr><td>Polyimide</td><td>In-situ direct growth</td><td>–</td><td>Catalyst for the Knoevenagel reaction</td><td>Even distribution, continuous morphology, and good crystallinity</td><td>Thermally stable up to 300 °C</td><td>[162]</td></tr><tr><td>Poly(pyromellitic dianhydride-co-4,4'-oxydianiline)</td><td>In-situ direct growth</td><td>Poly(pyromellitic dianhydride-co-4,4'-oxydianiline)</td><td>Gas adsorption</td><td>Even distribution, continuous morphology, and good crystallinity</td><td>Stable at 80°C for long periods of time, at 150°C for short periods of time. Does not degrade in methanol, acetone, toluene, and water</td><td>[156]</td></tr><tr><td>Polyurethane</td><td>In-situ stepwise growth</td><td>Introduction of anionic groups on PU using H<sub>2</sub>SO<sub>4</sub>+CrO<sub>3</sub> solution</td><td></td><td>Even distribution, continuous morphology, and good crystallinity</td><td>Stable, reinforced, and self-supporting membrane</td><td>[157]</td></tr><tr><td rowspan="2">MOF-808</td><td>Polystyrene</td><td>MOF-first, then in-situ growth</td><td>–</td><td>Gas separation, catalysis, sensing</td><td>Neckless-like fiber structure</td><td>Stable in dye removal under harsh environment</td><td>[163]</td></tr><tr><td>Polyacrylonitrile: Polyvinylpyrrolidone (10:1)</td><td>In-situ direct growth</td><td>–</td><td>Adsorption of malachite green</td><td>ZIF-L nanosheets densely and uniformly assembled on nanofibers</td><td>Particles intact after vigorous washing</td><td>[154]</td></tr><tr><td>MIL-100 (Fe)</td><td>Polyester</td><td>In-situ direct growth, fiber-first strategy</td><td>–</td><td>Destruction of a nerve agent simulant Dimetyl-4-nitrophenyl phosphate (DMNP)</td><td>Continuous morphology</td><td>Low mechanical stability, desolvation in water</td><td>[148]</td></tr><tr><td>MIL-47</td><td>Polyacrylonitrile</td><td>In-situ direct growth under microwave irradiation</td><td>Introduction of anionic carboxyl groups</td><td>Filtration, chemical protective coatings, and catalysis</td><td>Closely related and disordered phase</td><td>Not reported</td><td>[164]</td></tr><tr><td>UiO-66-NH<sub>2</sub></td><td>Polyester</td><td>In-situ direct growth, fiber-first strategy</td><td>–</td><td>Hydrolysis of DMNP</td><td>Continuous morphology</td><td>Stable after treatment with HCl diluted in acetone</td><td>[154]</td></tr></tbody></table>

surface area of polymer films, which is beneficial for the adsorption of dyes [150,156]. Tati et al. synthesized electrospun PVA/CS nanofibers (EPCNF) with different amounts of Ce-based UiO-66, of 0 (EPCNF), 3 (EPCNF-3), 7 (EPCNF-7), and 10% (EPCNF-10) [169]. The adsorption experiments (Fig. 18(b)) with both anionic (congo red, methyl orange, methyl red) and cationic (methylene blue) dyes demonstrated that with the increase of Ce-UiO-66 percentage embedded in ENF, the adsorbed quantities of all dyes enlarged, which was apparently due to the increased BET surface area. Hydrogen bonding, electrostatic interactions and π-π stacking are hypothesized mechanisms binding

EPCNF with dyes. Interestingly enough, the EPCNF (Ce-UiO-67 free) was more consistent with the Langmuir isotherm model, while the EPCNF-10 (with 10% Ce-UiO-67) was more consistent with the Freundlich model. This means that the loading of MOF particles on the ENF changed the character of adsorption process from monolayer and homogeneous to multilayer and heterogeneous.

In 2023, Ahmadijokani et al. clearly showed the synergic effect of MOF incorporation in ENF. They synthesized a series of PVA/CS electrospun composite nanofibers (ECNF) for MO dye removal [142]. Composites were named ECNF-m, where m = 0, 3, 5, 7, 10,
<figure><img src="image_22.png" /><figcaption>Fig. 18. (a) Comparison of the filtration efficiency of four different samples [161]; (b) Adsorption capacity of the fabricated ENFs toward Congo red dye as a function of contact time [170]; (c) Kinetic of MO adsorption [142]; (d) ZIF-L MOF coated on PAN/PVP ENF [163]; (e) Schematic diagram of macroscopic and microscopic morphology of metal ions adsorbed by nanofibers before and after cross-linking [176].</figcaption></figure>

corresponding to the percentage of post-modified UiO-66-NH₂ (Zr-MOF). The hydrogen bonding was responsible for a decent interaction between Zr-MOF and polymer matrix, which prevented agglomeration and embedding of MOF particles and enabled effective and homogeneous distribution of MOF particles in ECNF. When Zr-MOF and PVA/CS ENF were used separately, the MO dye removal was lower than in the case of ECNF with any loading percentage (Fig. 18(c)). The adsorption effectiveness gradually increased from ECNF-3 to ENCF-7. ECNF-10, with the highest loading of Zr-MOF, exhibited a smaller adsorption capacity than ECNF-7 because some of the interfibrous voids were enclosed, which decreased the adsorbent's overall surface area. Besides, with the increase of MOF loading, the fibers tend to become less ductile and more brittle, reducing the mechanical resistance.

Further complexation of MOFs can upgrade the class of MOF/ENF as adsorbents. MOF nanosheets are a two-dimensional (2D) form of frameworks. The structure of MOF nanosheets presents metallic nodes coordinated with organic ligands only along the vertical direction [170]. Zeolite-imidazole frameworks are popular for water treatment due to their stability and simplicity of synthesis, which does not require heating. In 2022, Sun et al. incorporated ZIF-L nanosheets on PAN/PVP electrospun nanofibers using an in-situ growth technique [163]. ZIF-L assembled homogeneously and densely on the surface of electrospun nanofibers, and the final product was denoted ZIF-L/NF. The maximum adsorption capacity of ZIF-L/NF for MG dye reached a tremendous 5103 mg g⁻¹ under the MG concentration of 1600 mg L⁻¹. The adsorbent was stable at low pH and slightly decreased adsorption capacity with the increase of the regeneration cycle. ZIF-L/NF has showed an extraordinary adsorption capacity, which, however, is not a rule. Lu et al. synthesized a 2D Co-MOF for basic fuchsin adsorption, and the adsorption capacity was only 11.483 mg g⁻¹ after 24 hours [171].

In 2023, Heidari et al., for the first time, published the method of synthesis of magnetic Zr-MOF functionalized with third-generation hyperbranched poly(amidoamine) dendrimer (PAMAM) [38]. This magnetic nanopowder was named Fe₃O₄@MOF-PAMAM and was added to the cellulose acetate (CA) solution to be electrospun (MOF-first

strategy). PAMAM, having amide and amine functionalities, enhanced the adsorption of MB cationic dye at pH higher than pH<sub>ZPC</sub> (8). The MB removal reached 87 % in real wastewater and over 95 % in test samples at optimized conditions. The adsorbent was regenerated using ethanol, and the removal rate remained high for the fourth reuse time (over 80 %).

It appears that water-soluble polymers (CS, PVP, PVA) can be used in water treatment applications, when they are completely coated with MOF particles or when they are crosslinked (Fig. 18(d,e)). As an example, CS/PVA is a popular combination of polymers for ENF synthesis. Both CS and PVA are water soluble polymers. In acidic conditions (pH < 6), the amino groups of chitosan are partially protonated, which causes repulsion between positively charged macrochains, which eventually leads to the solvation of CS molecules. When PVA is used as cross-linker, it increases the water resistance of CS by making hydrogen bonding with the amino groups of CS molecules. As a result, the cross-linked ENF is stable in water, and its morphology is conserved after the adsorption process (Fig. 18(e)).

It is interesting that the incorporation of MOF in water-stable polymers is not so present in the literature. At the same time, water-stable polymers, like PVDF, PEO, PU, PS, are quite often used for MOF-containing membrane synthesis [172–175]. More research on the electrospinning of water-stable polymers for ENF synthesis could resolve the main challenge of MOF/ENF composites – their water-instability.

MOF/ENF composites regeneration is mainly performed in an ultrasonic bath with use of polar organic solvents, such as ethanol and methanol, permitting the extraction of adsorbed dyes. To the best of our knowledge, there is little study on the process of desorption of dyes from MOF/ENF composites. Researchers focus on performance of regenerated MOF/ENFs rather than on investigation of the mechanism of desorption. More research needs to be done to establish the mechanism of desorption process.
## 4.5. Character and mechanisms of dye-MOF/ENF adsorption process

Determining the nature of the adsorption process involves identifying both its kinetic order and the appropriate isotherm model. The kinetic order is crucial for discerning whether the rate-controlling step is governed by physical or chemical interactions. In cases where adsorption is primarily physical, the process is characterized by the diffusion of the adsorbate onto the surface of the adsorbent, following pseudo-first order (PFO) kinetics. Conversely, when the rate-controlling step involves chemical interactions between the adsorbate and adsorbent, the process is described by pseudo-second order (PSO) kinetics. To determine whether the adsorption involves monolayer or multilayer coverage, the Langmuir and Freundlich isotherm models are commonly used. The Langmuir isotherm assumes monolayer adsorption on a surface with uniform energy sites, while the Freundlich isotherm describes multilayer adsorption on a surface with heterogeneous energy sites.

To compare the dye adsorption isotherms of MOFs before and after MOF-in-ENF incorporation, uniform data were collected from four studies involving dye adsorption on MOF particles and MOF/ENF. Table 3 presents a comparison of the isotherm models for various dyes adsorbed by MOFs and MOF/ENF composites, based on separate studies.

For instance, ZIF-8 exhibits multilayer adsorption consistent with the Freundlich model when used for cationic MG dye removal. However, when ZIF-8 is incorporated into a PVA/CS ENF, the adsorption behavior shifts to the monolayer Langmuir model. An opposite shift is observed for CR and MB dyes. Ce-based UiO-66 follows the Langmuir model, but when combined with PVA/CS ENF, the adsorption behavior aligns with the Freundlich model.

Regardless of the isotherm model, studies have found that adsorption generally follows pseudo-second order kinetics, indicating that chemisorption is the rate-controlling step and that dye-MOF interactions are central to the adsorption process.

Regarding the adsorption mechanism of dye MOF/ENF composites, most studies, once again, agree that dyes are primarily adsorbed through dye-MOF interactions, including electrostatic forces, π-π interactions between the conjugated systems of MOFs and dyes, and hydrogen bonding. However, by selecting a suitable polymer for electrospinning, the interactions between the dye and the ENF can be strengthened, complementing the inherent interactions between the MOF and the dye. Further research is needed to explore the role of ENF polymers in the dye adsorption process.

## 5. Current limitations and future of MOF/ENF research for dye removal

Industrial wastewaters present harsh conditions for adsorbents.

<caption>**Table 3**</caption>

Isotherm models and kinetic orders of adsorption reactions of several MOFs and MOF/ENF pairs.

<table><thead><tr><th>Dye</th><th>MOF</th><th>ENF</th><th>Isotherm model</th><th>Kinetic order</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="2">Malachite green</td><td>ZIF-8</td><td>-</td><td>Freundlich</td><td>PSO</td><td>[176]</td></tr><tr><td>ZIF-8</td><td>PVA/CS</td><td>Langmuir</td><td>PSO</td><td>[95]</td></tr><tr><td rowspan="3">Methylene blue</td><td>Ce-UiO-66</td><td>-</td><td>Langmuir</td><td>PSO</td><td>[177]</td></tr><tr><td>-</td><td>PVA/CS</td><td>Langmuir</td><td>PSO</td><td>[169]</td></tr><tr><td>Ce-UiO-66</td><td>PVA/CS</td><td>Freundlich</td><td>PSO</td><td>[169]</td></tr><tr><td rowspan="3">Congo red</td><td>Ce-UiO-66</td><td>-</td><td>Langmuir</td><td>PSO</td><td>[177]</td></tr><tr><td>-</td><td>PVA/CS</td><td>Langmuir</td><td>PSO</td><td>[169]</td></tr><tr><td>Ce-UiO-66</td><td>PVA/CS</td><td>Freundlich</td><td>PSO</td><td>[169]</td></tr></tbody></table>

Particularly, large volumes of effluents place significant mechanical stress on filtration systems due to the continuous flow and pressure. Therefore, the proper choice of polymer material for electrospinning is a complex task. Both polymer and the incorporated MOF particles must also be water, pH, and mechanically stable. Besides, MOF must retain its structure and open porosity for the optimal dye uptake. In addition, strong adhesion between MOF and ENF must be attained, which is ensured by selecting compatible materials and adjustment of the MOF loading. These constraints highly impact the sustainability of MOF/ENF composites and restrain their large application in wastewater treatment. On the other hand, considering the nano dimensions of MOFs, their release to environment must not remain overlooked. Synthesis of MOFs with enhanced stability should be paid more attention in the future, and their release into wastewater after adsorption cycle must be studied and minimized. As for the ENFs, because of certain requirements for the electrospinning process (electrospinning parameters, like voltage, flow rate, etc.), it is difficult to ensure the uniformity and consistency of the synthesized nanomaterial, which is why it is difficult to industrialize and commercialize nanofibers.

Nevertheless, as demonstrated in the studies listed in previous chapters, it is possible to synthesize adsorbents for effective dye removal with the proper choice of a polymer for electrospinning and incorporated MOFs into nanofibrous mats. These materials overcome the drawbacks of clogging and agglomeration of MOF powders. They are also recyclable alternatives to AC, demonstrating a slight or no evident decrease in adsorption capacities after several adsorption-desorption cycles. Many researchers agree that MOF/ENF composites have great potential for treating dye-contaminated wastewater in the future. However, to ensure their effective and safe use, several key aspects need further investigation before these materials can be widely applied.

First, it is crucial to focus on producing more stable MOFs and ENFs with continuous fiber structures. More research is also needed to better understand the MOF/ENF dye adsorption process. Most studies indicate that the adsorption mechanism primarily involves interactions between the dye and MOFs, with little attention given to dye-ENF interactions. Further investigation is required to clarify the role of ENFs in the adsorption process. Since MOF particles are considered responsible for dye adsorption in MOF/ENF composites, more research is needed on incorporating high-performance MOFs into water-stable polymers, such as PAN, PET, PVDF, PU, PS, crosslinked PEO and CS.

## 6. Conclusion

Adsorption of dyes on MOFs is one of the most effective techniques for dye effluent purification. The most common methods of MOF synthesis are solvo/hydro-thermal and microwave-assisted methods, differing from other methods with their simplicity. Strategies for synthesis of effective MOFs are based on the porosity development and on the enhancing of framework's surface charge. The adsorption of dyes on MOFs is mainly due to their intermolecular interactions, such as electronic interactions, π-π stacking, and hydrogen bonding. The size compatibility between dye molecules and pore sizes of MOF can determine the adsorption process. The dye molecules of the same charge as the framework can be absorbed if their size is small enough to penetrate the pores of framework. In other cases, the intermolecular interactions determine the adsorption process. This tendency is conserved when MOFs are integrated in ENFs: the adsorption of dyes on MOF/ENF composites is promoted by the dye-MOF interactions.

Electrospinning enables the incorporation of a wide range of MOF in polymer fibers. As a result, MOF/ENF membranes can adsorb dyes of different natures. The MOF/ENF composites can be synthesized by different methods. The choice of the synthesis strategy determines the properties of the final composite. When mechanically resistant ENF is developed, the strategy of embedding the MOFs in polymer are favored (MOF-first). For the applications, demanding a high surface area of composite, like adsorption, sensing, or catalysis, MOFs must be exposed
on the surface of the ENFs. For that, strategies of MOF attachment and in situ MOF growth on the fiber surface are prioritized. In these strategies, the surface of fibers often needs to be modified to increase the MOF anchoring. The most important ENF surface modifications include the oxidation treatment, the integration of different functional groups, like hydroxyl, carboxyl, amino, etc. on fibers, and intentional modification of organic ligands of MOFs. These modifications promote covalent and non-covalent interactions, keeping the MOF/ENF composites inseparable.

Not every polymer for electrospinning can be an effective support for MOF incorporation, due the several crucial factors, like polymer's instability in water, and poor mechanical resistance. The most popular MOF/ENF pairs, discussed in the dye removal application, are Zn-based ZIF-8, and UiO-66 (Zr, Ce) with PAN/PVP and PVA/CS ENFs. In the majority of studies, the adsorption on MOF/ENF was attributed to the dye-MOF interactions, whilst the dye-ENF interactions are hardly mentioned. At the same time, ENFs can adsorb dyes without MOF particles. Besides, we showed that the isotherm model of the dye adsorption process changes after the MOFs are integrated in ENFs. Further research is needed to clarify the contribution of ENFs in the MOF/ENF dye adsorption process.

When MOFs are coupled with electrospun polymer fibers, the resulting MOF/ENF membranes become a great alternative to MOF nanopowder, with the advantages of non-clogged filter tubes, facile filter replacement, and easy regeneration. The perspectives of MOF/ENF membranes for the textile industry are based on cost-effectiveness, reusability, and ease of manipulation. The search and adjustment of MOF/ENF adhesion, water stability, permeability, and mechanical resistance of both materials will eventually play the crucial role in deblocking their usage for effective textile water treatment.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The French state funded this project as part of France 2030, which is operated by ADEME.

## Data availability

No data was used for the research described in the article.

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