MIL series-based MOFs as effective adsorbents for removing hazardous organic pollutants from water
Graphical abstract
As illustrated, MIL series-based MOFs as adsorbents show a heavier weight scale and more efficient uptake toward organic pollutants. Because it has a unique structure and extra stability and provides several adsorption mechanisms to remove organic contaminants.
Introduction
Water is essential to human existence and industrial and agricultural activity [1]. Also, water is a fundamental natural resource that is indispensable to sustaining life on our planet. According to the World Health Organization (WHO), an estimated 1.7 million people have died from water pollution [2]. Additionally, approximately-six billion people will experience a shortage of potable water by the year 2050, according to United Nations World Water Development Report (2018 edition) [3]. Thus, a slight change in water quality's physical or chemical properties is a serious matter we cannot ignore. So, the ecological equilibrium of the natural water environment has been disrupted by the hazardous organic pollutants produced by human activity, making them a severe threat to human life and the environment [4].
Recently, water pollution by organic pollutants such as dyes, phenols, pharmaceutical products, herbicides, insecticides, nitro-aromatic compounds, and other inorganic like fluorides increased due to the constant release of their effluents into aquatic media [5], [6], [7], [8]. For instance, hazardous organic contaminants degrade the quality of water sources when they are not adequately handled [9]. Organic dyes in water decrease solar light's intensity, limiting photosynthesis in microbes and aquatic plants [10]. As a result, water sources have low biological oxygen, making it difficult for organisms to break down organic molecules [11]. Besides, some of them have cancerogenic effects [12], [13]. Pharmaceutical or antibiotic-contaminated water increases the risk of bacteria developing drug-resistant genes, reducing the efficiency of present antibiotic generations [14]. On the other hand, herbicides and insecticides have many evil effects on the environment, where they stay in the soil for a long time because of biodegradability and high toxicity, destroying the quality of groundwater resources [15], [16]. Most phenol compounds are considered hazardous and toxic, so eliminating them is essential for the environment [17].
In the last two decades, most scientists have spent much effort to solve this problem to avoid future water scarcity that will cause environmental malfunctions [18], [19]. Among water treatment methods, the adsorption technique is considered the best due to its cheapness, ease of operation, eco-friendliness, and high uptake of pollutants [18], [20], [21], [22], [23]. In the realm of adsorbents, metal–organic frameworks (MOFs) offer several benefits in removing contaminants: simple design, high efficiency, and advantages such as little or no cost, portability, ease of use, insensitivity to harmful substances, the potential for reuse, etc. [24], [25]. Specifically, MIL series-based MOFs have gained recognition as promising adsorbent materials for adsorbing hazardous organic materials [26], [27], [28]. They have unique structure stability; ease of surface functionalization and high surface area gives them high adsorption capability and selectivity for organic pollutants in different aqueous environments. Besides, it has unsaturated Lewis acid sites that could adsorb pollutants through electrostatic interaction [21], [29], [30].
To date and according to our knowledge, there is no overview on using MIL series-based MOFs as adsorbents for organic pollutants from wastewater, despite their excellent uptake of organic pollutants and stability in different aqueous solutions. To satisfy this gap, the current work refers to the history of MILs, including synthesis methods and their merits. Further, it analyses the adsorption mechanism, adsorption isotherm, kinetic, thermodynamic, and regeneration studies of MIL series-based MOF adsorbents. In addition, we summarized the adsorption capacities for MIL series-based MOFs reported in articles on eliminating dyes, antibiotics, herbicides, and other organic contaminants. Finally, suggestions and recommendations for future improvements in MIL series-based MOFs' removal capacity and applicability in industrial wastewater treatment are presented.
MOFs are made by reticular fusion in which metals or clusters of metals are joined to organic linkers with strong coordination bonds. They are crystalline materials with a porous structure, where the metal is created stiffly, and the pore geometry results from organic linkers [31]. The porosity of MOFs reaches>90 % of its volume and a tall specific surface area of over 7000 m2/g [32]. MOFs with permanent porosity were first introduced in 1995 by O. M Yaghi [33]. Recently, many attempts have been made to name MOF materials according to standard systems. Unfortunately, we found the naming of MOFs based on where they were found instead of how similar their structures are. As a result, we mentioned the most well-known MOFs used in all research studies [34]. Namely, the Zeolitic imidazolate framework (ZIF) set [35], the isoreticular metal–organic framework (IRMOF) set [36], the Materials of Institute Lavoisier (MIL) set [37], the Hong Kong University of Science and Technology (HKUST) set [38], and the University of Oslo (UiO) set. [39]. Lastly, many studies reported the synthesis, characterization, and activation methods of MOFs for different applications [32], [40]. In addition, Ashlee J. Howarth wrote the best practice in MOFs from synthesis to activation [41]. On the other hand, Olaniran and colleagues provided further insight into the fabrication, characterization, and activation of MOFs to eliminate newly identified organic pollutants from aquatic environments via adsorption [24].
Consequently, their unique structural distinctiveness and large surface area make them have widespread uses in numerous research areas (Fig. 1a) [42], [43], [44], [45], [46], [47]. For water treatment fields, MOFs are used in several technologies (Fig. 1b), including adsorption [48], [49], photocatalysis [37], biological [50], and membrane filtration [26], [27]. The adsorption process is extensively utilized as a means of separation, particularly in environmental remediation, owing to its significant efficacy and cost-effectiveness [19]. Adsorbents with higher surface areas, pore volumes, and sufficient functionalities are required for effective adsorption processes [49]. Porous absorbent materials, such as porous organic polymers (POPs), MOFs, carbon-organic frameworks (COFs) [51], covalent triazine frameworks (CTFs) [52], activated carbons (AC) [53], and MOFs [12] can meet all of these requirements. MOFs are novel adsorbents that have aroused scientists' interest worldwide [45], [54], [55], [56]. Fig. 1c displays the network analysis (Web of Science database) for the reports that used MOFs and MILs as adsorbents.
MIL series-based MOFs as adsorbents have a lot of distinctive advantages: (1) they possess enduring porosity and an extensive specific surface area, thereby affording a profusion of active sites and superior uptake proficiency [57], [58]. (2) they exhibit exceptional stability against air, water, and heat, rendering them highly suitable adsorbents for water purification applications [59]. (3) they have a multi-dimensional network structure with high order that facilitates its surface modification and functionalization to absorb specific pollutants [60]. However, the practical application of these innovative adsorbents has been limited due to some drawbacks, such as high production expenses [61] and challenges in the recycling/regeneration process. However, changing the pH and considering other factors can solve this problem [62]. Also, the production cost can be solved by making the preparation depend on waste and choosing some green and cheap agents.
MOFs based on the MIL series are 3D structures built by fusing M+ ions (such as Cr, Fe, Al, Ti, In, V, and Ga) with carboxylate ligands (Table 1). MILs materials exceptional stability and 3D structure have piqued researchers' interest. From another point of view and based on how their metal centres and organic ligand precursors are arranged, MIL series-based MOFs can be classified into two groups. The first MIL series-based MOF uses lanthanide, transition metal oxides as metal sources, and galacturonic and succinic acids as organic ligands. Cr, Fe, Al, and V serve as metal nodes and terephathalic acid (BDC) as a linker in the second category of MILs series-based MOF [37].
Due to their capacity to reduce contaminants through adsorption and catalytic processes, MILs series-based MOFs encourage adsorption and catalytic processes [21], [42]. Also, MIL-based composites have been developed for water purification [70]. The MILs series are the most popular among the many types of MOFs. They have extraordinarily high chemical, thermal, and water stability, large pores, unsaturated Lewis's acid locations in some of their structures, and enormously specific surface areas, which make them the perfect adsorbents for organic pollutants from wastewater, as shown in Fig. 1d. Using an autoclave that heated to 220 °C for 8 h, Gérard Férey and his colleagues developed the first MIL (MIL-101) by the hydrothermal treatment of BDC, hydrofluoric acid, and Cr (NO3)3·9H2O [71]. It is ranked among the best-known MIL series-based MOFs kinds, particularly in terms of potential applications. MIL-101 is also written when the organic ligand is BDC, and the metal ion/clusters are Fe3+ or Al3+ [72]. At the same time, the zeolite architecture of the MILs series-based MOF is similar in its surface morphologies, densities, and pore sizes. For instance, MIL-101(Fe) and MIL-101(Cr) have always been the subject of extensive investigation because their structural and topological are identical. As a catalyst, MIL-101(Fe) performs admirably. Some iron content in Fe-based MIL-101 can be reduced from Fe3+ to Fe2+ under the right conditions, making it a promising catalyst activator.
In the MILs series, MIL-101(Cr) is among the most typical examples. The Cr3O ionic cluster is coordinated with BDC, yielding the formula [Cr3 (O) X (BDC)3 (H2O)2] where X is either OH− or F−. The structure of MIL-101(Cr) (Fig. 2a) is a combination of dodecaedric (512) and hexacaidecaedric (512 64) mesoporous cages. Its diameter ranges from 29 to 34 Å, forming a zeotypic MTN network. Five-membered (12 Å) and six-membered (16 Å) cages with a precise BET of 4100 m2/g provide access to the cages' pore windows [71], [73]. Unsaturated MIL-101(Cr) sites (Lewis structure) were achieved under vacuum drying [73]. Due to its outstanding thermal stability up to 270 °C, there is a lot of porosity, excellent physical and chemical properties, and chemical resistance. Hence, using different techniques, it has been used widely in wastewater treatment applications [74], [75], [76], [77].
Section snippets
Construction materials for MIL series-based MOFs
The construction of MIL series-based MOFs based on structural aspects is classified into primary building units (PBUs) that consist of metals and organic linkers that result in porous three-dimensional structures in MOFs. Besides SBUs, metal–oxygen-carbon clusters have inherent symmetrical properties that facilitate MOF's topology [78].
Synthesis routes of MILs series-based MOFs
Routinely, MOF synthesis embraces solvents and temperatures ranging from room temperature to 250 °C [89]. Most procedures for fabricating MIL series-based MOF occur in the liquid phase, with the metal and ligand solutions either being synthesized independently or as a mixture. MOF synthesis has progressed in leaps and bounds during the past two decades, with numerous new synthesis pathways devised [90]. MIL series-based MOFs are just one example of the many that may be synthesized using active
Adsorption mechanisms of hazardous organic pollutants over MIL series-based MOFs
MIL series-based MOF adsorbents generally have adsorption mechanisms comparable to those of other adsorbents, except for ligand functional groups and metal ions or metal clusters in MOF structure. Many organic contaminants, including dyes, pesticides, and antibiotics, interact with MILs MOF in multiple ways, resulting in enhanced adsorption efficacy in some situations. Electrostatic interactions, π-π staking, hydrogen bonding, acid-base interactions, and hydrophilic contacts are the most
Dyes and textiles
has a superior feature to MOF by its high stability in acidic and organic environments. Therefore, a hybrid of MOF and COF can improve the sorption performance toward dye removal from aqueous solutions. This combination outperforms conventional materials with hydrophobic nature, resonant electrostatic forces, π-π stacking, and hydrogen bonding. M. Dinari and F. Jamshidian created a unique composite of MIL-101-NH2 and COF-based triazine (MIL-101-NH2@COF) as a superior adsorbent for acid blue 9
Regeneration, reusability, and stability of MIL series-based MOFs
No doubt, the adsorbents that have suitable stability without high ion leaching and a facile and green regeneration process will attract a lot of researchers who are interested in water treatment [162], [163], [164]. They are always thinking about finding the best adsorbents to meet the high uptake of organic pollutants and, at the same time, be practical in wastewater treatment. Recently, MOFs had high applications in the removal of organic pollutants, especially dyes [165], [166].
Conclusion and future perspectives
MIL series-based MOFs as adsorbents for organic pollutants attract researchers interested in water treatment because of their advantages, such as high stability, numerous surface areas and porosities, and a variety of surface functionalization. For the first time, this study presents a comprehensive analysis of the utilization of MIL series-based MOFs as adsorbents for organic contaminants. Additionally, we shed light on the adsorption mechanisms for MIL series-based MOFs for organic
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.
Acknowledgement
Financial support from the National Natural Science Foundation of China (51873189 and 51811530097) is gratefully acknowledged.
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