Flower-like hierarchical Ni-Zn MOF microspheres: Efficient adsorbents for dye removal

Advantages of Bimetallic Organic Frameworks in the Adsorption, Catalysis and Detection for Water Contaminants

The binary metal organic framework (MOF) is composed of two heterometallic ions bonded to an organic ligand. Compared with monometallic MOFs, bimetallic MOFs have greatly improved in terms of structure, porosity, active site, adsorption, selectivity, and stability, which has attracted wide attention. At present, many effective strategies have been designed for the synthesis of bimetallic MOF-based nanomaterials with specific morphology, structure, and function. The results show that bimetallic MOF-based nanocomposites could achieve multiple synergistic effects, which will greatly improve their research in the fields of adsorption, catalysis, energy storage, sensing, and so on. In this review, the main preparation methods of bimetallic MOFs-based materials are summarized, with emphasis on their applications in adsorption, catalysis, and detection of target pollutants in water environments, and perspectives on the future development of bimetallic MOFs-based nanomaterials in the field of water are presented.

Pyrolyzed magnetic NiO/carbon-derived nanocomposite from a hierarchical nickel-based metal-organic framework with ultrahigh adsorption capacity

Herein, a simple one-pot solvothermal approach is used to create magnetic porous carbon nanocomposites which obtained from a nickel-based metal-organic framework (Ni-MOF) and examined for their ability to uptake methyl orange (MO) dye. Derived carbons with exceptional porosity and magnetic properties were created during the different pyrolysis temperatures of Ni-MOF (700, 800, and 900 °C) under a nitrogen atmosphere. The black powders were given the names CDM-700, CDM-800, and CDM-900 after they were obtained. A variety of analysis methods, including FESEM, EDS, XRD, FTIR, VSM, and N₂ adsorption-desorption were used to characterize as-prepared powders. Furthermore, adsorbent dosage, contact time, pH variation, and initial dye concentration effects was investigated. The maximum adsorption capacities were 307.38, 5976.35, 4992.39, and 2636.54 mg/g for Ni-MOF, CDM-700, CDM-800, and CDM-900, respectively, which show the ultrahigh capacity of the resulted nanocomposites compared to newest materials. The results showed that not only the crystallinity turned but also the specific surface area was increased about four times after paralyzing. The results showed that the maximum adsorption capacity of MO dye for CDM-700 was obtained at adsorbent dosage of 0.083 g/L, contact time of 60 min, feed pH of 3, and temperature of 45 °C. The Langmuir model has the best match and suggests the adsorption process as a single layer. According to the results of reaction kinetic studies using well-known models, the pseudo-second-order model (R² = 0.9989) displayed high agreement with the experimental data. The synthesized nanocomposite is introduced as a promising superadsorbent for eliminating dyes from contaminated water due to strong recycling performance up to the fifth cycle.

Adsorption efficacy of functionalized Cu-BDC MOFs tethered 2-mercaptobenzimidazole analogue: A comparative study

A novel metal-organic framework [MOFs], and 2-[benzo [d]thiazol-2-ylthio)-3-hydroxy acrylaldehyde-Cu-benzene dicarboxylic acid was synthesized by solvothermal method and characterized using p-XRD, FSEM-EDX, TGA, BET, FTIR. The tethered organic linker, 2-[benzo [d]thiazol-2-ylthio)-3-hydroxyacrylaldehyde was commonly known as 2-mercaptobenimidazole analogue [2-MBIA]. Analysis of BET disclosed that addition of 2-MBIA to Cu-benzene dicarboxylic acid [Cu-BDC], reduced the crystallite size from 70.0 nm to 65.90 nm, surface area from 17.95 to 17.02 m² g^-1 and enhances the pore size from 5.84 nm with 0.027 cm³ g^-1 pore volume to 8.74 nm with 0.361 cm³ g^-1 pore volume. Batch experiments were conducted to optimize pH, adsorbent dosage, and, Congo red (CR) concentration. The adsorption percentage of CR on the novel MOFs was 54%. Adsorption kinetic studies revealed that the uptake adsorption capacity at equilibrium was 184.7 mg/g from pseudo-first-order kinetics which gave a good fit with the experimental data. Intraparticle diffusion model explained the process of the adsorption mechanism: diffusion from the bulk solution onto the porous surface of the adsorbent. Freundlich and Sips models were the best fit models of the several non-linear isotherm models. Temkin isotherm suggested the adsorption of CR on MOFs was of an exothermic nature.

Recent advances in application of metal-organic frameworks (MOFs) as adsorbent and catalyst in removal of persistent organic pollutants (POPs)

The presence of persistent organic pollutants (POPs) in the aquatic environment is causing widespread concern due to their bioaccumulation, toxicity, and possible environmental risk. These contaminants are produced daily in large quantities and released into water bodies. Traditional wastewater treatment plants are ineffective at degrading these pollutants. As a result, the development of long-term and effective POP removal techniques is critical. In water, adsorption removal and photocatalytic degradation of POPs have been identified as energy and cost-efficient solutions. Both technologies have received a lot of attention for their efforts to treat the world's wastewater. Photocatalytic removal of POPs is a promising, effective, and long-lasting method, while adsorption removal of persistent POPs represents a simple, practical method, particularly in decentralized systems and isolated areas. It is critical to develop new adsorbents/photocatalysts with the desired structure, tunable chemistry, and maximum adsorption sites for highly efficient removal of POPs. As a class of recently created multifunctional porous materials, Metal-organic frameworks (MOFs) offer tremendous prospects in adsorptive removal and photocatalytic degradation of POPs for water remediation. This review defines POPs and discusses current research on adsorptive and photocatalytic POP removal using emerging MOFs for each type of POPs.

Bimetallic MOFs loaded cellulose as an environment friendly bioadsorbent for highly efficient tetracycline removal

Due to the increasingly serious antibiotic-related pollution, it is crucial to develop novel green bioadsorbents to effectively remove antibiotics from aqueous solutions. In this study, Fe doped zeolitic imidazolate frameworks-8 loaded cellulose (Fe/ZIF-8@cellulose) aerogels were prepared. The synthesized Fe/ZIF-8@cellulose aerogels were characterized experimentally including morphology observation and chemical compositions determination. The effects of bioadsorbent dosage, solution pH, adsorption time, initial TC concentration and adsorption temperature on the TC adsorption behaviors were systematically studied. Due to the introduction of Fe in the ZIF-8, the maximum adsorption capacity of Fe/ZIF-8@cellulose for TC could reach as high as 1359.2 mg/g, which is higher than the reported ZIF-8 loaded polysaccharide based adsorbents. The adsorption kinetics and isotherm of TC adsorption were also determined. With the cellulose as the matrix to load Fe/ZIF-8, the obtained Fe/ZIF-8@cellulose aerogels exhibited good reusability. Most importantly, the TC adsorption mechanism was proposed. The results of our finding suggest that the Fe doping into MOFs is an effective strategy to improve the antibiotics adsorption performance and the application of cellulose as the matrix is a valuable method to increase the cyclic utilization. This study highlights the potentials of applying the Fe/ZIF-8@cellulose aerogels in the antibiotics removal for practical wastewater.

Two-dimensional magnetic bimetallic organic framework nanosheets for highly efficient enrichment of phosphopeptides

Highly selective enrichment and sensitive detection of phosphopeptides is pivotal for comprehensive phosphoproteomics analysis; however, it also poses a long-standing challenge. Here, a novel two-dimensional (2D) magnetic bimetallic organic framework (MOF) nanosheet with Zr-O clusters and Ti-O clusters (denoted as the Fe₃O₄@Zr-Ti BPDC nanosheet) is prepared via a solvothermal method and in situ deposition of Fe₃O₄ nanoparticles for the first time. Taking advantage of the abundant dual affinities of Zr-O and Ti-O clusters for phosphopeptides, large surface area and high chemical stability, the Fe₃O₄@Zr-Ti BPDC nanosheets exhibit excellent enrichment performance for phosphopeptides. Within the framework of density functional theory, the interaction between Zr-O clusters, Ti-O clusters and phosphorylated molecules was studied to find the possible reason behind the superior adsorption performance of the bimetallic MOF nanosheets. We found that electrons would migrate from Ti to Zr spontaneously after doping Ti element and enhance the electrostatic traction between Zr species and phosphorylated molecules, demonstrating that the synergistic effect of Zr-Ti was helpful to improve the enrichment efficiency for phosphopeptides. Furthermore, the Fe₃O₄@Zr-Ti BPDC nanosheets showed good enrichment performance in complex bio-samples, including nonfat milk, human saliva, and a breast cancer cell lysate, indicating their tremendous potential in the analysis of trace phosphorylated biomolecules in complex bio-samples.

Nickel and iron-based metal-organic frameworks for removal of organic and inorganic model contaminants

Metal-organic frameworks (MOFs) are a promising class of porous nanomaterials in the field of environmental remediation. Ni-MOF and Fe-MOF were chosen for their advantages such as structural robustness and ease of synthesis route. The structure of prepared MOFs was characterized using FE-SEM, XRD, FTIR, and N₂ adsorption-desorption. The efficiency of MOFs to remove organic model contaminants (anionic Alizarin Red S (ARS) and cationic malachite green (MG) and inorganic fluoride was studied. Fe-MOF and Ni-MOF adsorbed 67, 88, 6% and 32, 5, and 9% of fluoride, ARS, and MG, respectively. Further study on ARS adsorption by Fe-MOF showed that the removal efficiency was high in a wide range of pH from 3 to 9. Moreover, dye removal was directly increased by adsorbent mass (0.1-0.75 g/L) and decreased by ARS concentration (25-100 mg/L). The pseudo-first-order kinetic model and Langmuir isotherm model with a q_max of 176.68 mg/g described the experimental data well. The separation factor, K_L, was in the range of 0-1, which means the adsorption process was favorable. In conclusion, Fe-MOF showed remarkable adsorption of organic and inorganic model contaminants.

Flower-Shaped Ni/Co MOF with the Highest Adsorption Capacity for Reactive Dyes

Reactive dyes are widely used in textile industry, but their excessive use has caused several water pollution problems. In order to reasonably treat printing and dyeing wastewater, the highly efficient adsorbent for reactive dyes employed in this study is a new type metal-organic framework (MOF) material. Ni/Co MOF (NCM) was synthesized using the solvothermal method; then, the materials were analyzed by a series of characterization methods. This study mainly investigated the adsorption properties of NCM toward reactive dyes, and the adsorption capacities of NCM toward reactive red 218 were up to 200 mg·g^-1. The results were found to conform to the Langmuir isotherm model, and the pseudo-second-order kinetic model by performing kinetic and isotherm studies on the adsorption process of reactive red 218 on NCM. The results of the intraparticle diffusion model suggest that the binding of reactive red 218 to NCM was mainly divided into three steps: adsorption, diffusion, and saturation. Moreover, it was concluded by thermodynamic fitting of the adsorption process that the adsorption of reactive red 218 by NCM proceeded spontaneously and was accompanied by an endothermic reaction, in which the adsorption of both occurred mainly by electrostatic attraction. The NCM has good reusability and still has good adsorption performance after being reused 5 times. Therefore, NCM is a very promising and excellent adsorbent for the treatment of dye wastewater because of its high efficiency and reusability.

Microporous hierarchically Zn-MOF as an efficient catalyst for the Hantzsch synthesis of polyhydroquinolines

A three-dimensional walnut-like Zn-based MOF microsphere system was designed and synthesized via hydrothermal reaction of zinc salt with 4,6-diamino-2-pyrimidinethiol as a tridentate ligand. Besides, Zn ions were coordinated to the functional groups of the ligand to give a novel Zn-MOF microsphere material. Afterward, the resultant material was thoroughly characterized using various analysis and physico-chemical methods; including, FT-IR, XRD, TGA, EDX, X-ray mapping, SEM, TEM, and BET analysis. The Zn-MOF microspheres were utilized in the Hantzsch reaction for a selective synthesis of asymmetric polyhydroquinolines, using various aromatic aldehydes. Our strategy aims at providing a controlled synthesis of hierarchically nanoporous Zn-MOF microspheres with a well-defined morphology, structure, and excellent catalytic properties. Besides, it would result in having a promising heterogeneous catalyst for a selective synthesis with good yields, short reaction time, a low limit of steric hindrance and electronic effects. Moreover, the heterogeneity of the catalyst is further tested with hot filtration and also the reusability results point.

Dye contaminated wastewater treatment through metal–organic framework (MOF) based materials

A complete discussion of MOFs and MOF composites such as MOF-based membranes, magnetic MOFs, and metal–organic gels (MOGs) used for dye removal along with their adsorption efficiency has been done.

Adsorptive removal of dyes from wastewater using a metal-organic framework: A review

Water pollution from synthetic dyes is a growing environmental concern because many dyes have carcinogenic effects on humans and aquatic life. Adsorption is a widely used technology for the separation and removal of dyes from wastewater. However, the dye removal process using conventional adsorbents is not sufficiently efficient for industrial wastewater. Metal-organic frameworks (MOFs) addresses these drawbacks. MOF showed excellent dye removal and degradation capacity owing to its multifunctionality, water-stability, large surface area, tunable pore size and recyclability. Magnetic MOFs retained excellent performance up to several consecutive cycles. Modified MOFs performed as Fenton-like catalysis process which generated abundant reactive radicals that degraded complex organic dyes into simple and less toxic forms which were further adsorbed onto the MOF. This review systematically compiles in-depth studies on the adsorptive removal of dyes from wastewater, MOF adsorption mechanisms, major influencing factors, to adsorption efficiency of MOFs. While all MOFs adsorb dyes through electrostatic attraction, the type of MOF, presence of functional groups, ligands, and pH significantly control the adsorption mechanism. Before developing an MOF, optimization and upgradation of factors and interaction between available adsorption site and adsorbate is needed. Finally, the prospects and new frontiers of MOFs in sustainable water treatment is discussed.

Bimetallic Ag-Zn-BTC/GO composite as highly efficient photocatalyst in the photocatalytic degradation of reactive yellow 145 dye in water

Ag_x-Zn_100-x-BTC/GO composites (BTC: benzene-1,3,5-tricarboxylic, GO: graphene oxide) with different Ag/Zn molar ratios were synthesized using microwave-assisted hydrothermal treatment. The Ag_x-Zn_100-x-BTC/GO exhibited excellent photocatalytic performance in the reactive yellow 145 dye (RY-145) degradation under irradiation of visible light with nearly 100% of RY-145 removal after 35 min, as compared to Zn-BTC/GO and Ag-BTC/GO. Reactive oxygen species scavenging assays have shown that the holes (h⁺) and superoxide radical anion (O₂^-•) play a primary role in RY-145 degradation. Based on the band structure of materials, the Z-scheme photocatalytic mechanism was suggested. The effect of catalyst dosage, pH and dye concentration on the efficiency of photocatalytic activity of bimetallic Ag₅₀-Zn₅₀-BTC/GO was also investigated. The improvement in photocatalytic activity of bimetallic Ag₅₀-Zn₅₀-BTC/GO could be given by the synergism of (i) absorption of visible light confirmed by UV-Vis diffuse reflectance spectra; (ii) the increased lifetime as evidenced by photoluminescence spectra and transient photocurrent response; (iii) the increased oxygen vacancy defects as confirmed by X-ray photoelectron spectroscopy results. The degradation pathway of RY-145 dye was also predicted based on liquid chromatography-mass spectrometer analysis. The removed chemical oxygen demand, biological oxygen demand, total organic carbon outcomes indicated the high mineralization ability for RY-145 degradation over Ag₅₀-Zn₅₀-BTC/GO.

Ion-Exchange Resins for Efficient Removal of Colorants in Bis(hydroxyethyl) Terephthalate

Bis(hydroxyethyl) terephthalate (BHET) obtained from waste poly(ethylene terephthalate) (PET) glycolysis often have undesirable colors, leading to an increased cost in the decoloration of the product and limiting the industrialization of chemical recycling. In this work, eight types of ion-exchange resins were used for BHET decoloration, and resin D201 showed an outstanding performance not only in the decoloration efficiency but also in the retention rate of the product. Under the optimal conditions, the removal rate of the colorant and the retention efficiency of BHET were over 99% and 95%, respectively. D201 showed outstanding reusability with five successive cycles, and the decolored BHET and its r-PET showed good chromaticity. Furthermore, the investigations of adsorption isotherms, kinetics, and thermodynamics have been conducted, which indicated that the decoloration process was a natural endothermic reaction. Adsorption interactions between the colorant and resin were extensively examined by various characterizations, revealing that electrostatic force, π-π interactions, and hydrogen bonding were the dominant adsorption mechanisms.

Electropolymerization of poly(methylene blue) on flower-like nickel-based MOFs used for ratiometric electrochemical sensing of total polyphenolic content in chrysanthemum tea

A ratiometric electrochemical sensor for caffeic acid (CAE) detection was constructed using a glassy carbon electrode modified with poly(methylene blue) and flower-like nickel-based metal organic frameworks (PMB@Ni-TPA/GCE). The electrochemical behavior of CAE was investigated at the PMB@Ni-TPA/GCE, and was found to follow a two-electron, two-proton electrooxidation process. PMB was used as the internal reference probe, and Ni-TPA can enhance the electrochemical signals of both CAE and PMB. As the CAE concentration increases, the oxidation peak current of CAE is enhanced but that of PMB keeps almost unchanged. The oxidation peak current ratio between CAE and PMB recorded by differential pulse voltammetry changes linearly with CAE concentration over the range of 0.25-15.0 μM, with a detection limit of 0.2 μM. The proposed sensor was successfully employed to evaluate the total polyphenolic content as CAE equivalent in chrysanthemum tea, and the results were comparable with those given by the reference Folin-Ciocalteu spectrophotometry.

A Critical Review on Metal-Organic Frameworks and Their Composites as Advanced Materials for Adsorption and Photocatalytic Degradation of Emerging Organic Pollutants from Wastewater

Water-borne emerging pollutants are among the greatest concern of our modern society. Many of these pollutants are categorized as endocrine disruptors due to their environmental toxicities. They are harmful to humans, aquatic animals, and plants, to the larger extent, destroying the ecosystem. Thus, effective environmental remediations of these pollutants became necessary. Among the various remediation techniques, adsorption and photocatalytic degradation have been single out as the most promising. This review is devoted to the compilations and analysis of the role of metal-organic frameworks (MOFs) and their composites as potential materials for such applications. Emerging organic pollutants, like dyes, herbicides, pesticides, pharmaceutical products, phenols, polycyclic aromatic hydrocarbons, and perfluorinated alkyl substances, have been extensively studied. Important parameters that affect these processes, such as surface area, bandgap, percentage removal, equilibrium time, adsorption capacity, and recyclability, are documented. Finally, we paint the current scenario and challenges that need to be addressed for MOFs and their composites to be exploited for commercial applications.

The research trends of metal-organic frameworks in environmental science: a review based on bibliometric analysis

Metal-organic frameworks, an emerging class of porous material, have developed rapidly in recent years. In order to clarify the application of metal-organic frameworks in the field of environmental science, 1386 articles over the last 20 years were obtained from Scopus and analysed by the bibliometric method. And the collaboration between countries, institutions and authors and the co-occurrence of keywords were also conducted using VOSviewer. The results indicated that this area of research has entered a fast-developing stage. The number of articles published has grown from 7 articles in 1999 to 378 articles in 2018. The most productive country was China with 626 articles published. The most productive institution was the Chinese Academy of Sciences, and the most productive author was Jhung SH from Kyungpook National University of South Korea. Although metal-organic frameworks have been widely used in adsorption and catalytic degradation of pollutants from the environment, the removal mechanism of pollutants by MOFs, the stability improvement and the cost reduction of metal-organic frameworks are still the main challenges for their practical applications.