Trichloroacetic acid-modulated synthesis of polyoxometalate@UiO-66 for selective adsorption of cationic dyes

Challenges and outlooks for the polyoxometalates, metal-organic frameworks (POMs-MOFs) hybrid materials in water treatment technologies

The importance of water for life is undeniable. However, modern industrial and urban practices have led to the pollution of water reservoirs. Efficient wastewater purification is crucial for sustainability, and several materials with specific characteristics have been investigated to improve water quality. The integration of polyoxometalates (POMs) into metal-organic frameworks (MOFs) holds significant potential for water treatment applications due to their complementary properties. POMs are renowned for their high catalytic activity, redox versatility, and resistance to harsh environments, while MOFs offer high porosity, tunable chemical environments, and enhanced stability. When immobilized within MOF structures, POMs can exhibit improved processability and recyclability, overcoming limitations such as leaching and aggregation. The resulting composites maintain the catalytic efficiency of POMs and leverage the structural and adsorptive characteristics of MOFs to target contaminants in water. These hybrid systems are up-and-coming with improved characteristics where the synergy between the POM's catalytic sites and the MOF's porous network can facilitate efficient degradation of organic pollutants, heavy metal sequestration, and enhanced adsorption of micropollutants, paving the way for sustainable water purification technologies. This review encapsulates the latest advancements in POM-MOF composites, discussing the predominant synthesis strategies and their applications, particularly in wastewater treatment. Furthermore, POM-MOF composite nanoplatforms for wastewater treatment are explored based on their high stability and large specific surface area, making them an ideal choice for waste-water treatment.

Design, synthesis, and applications of defective metal-organic frameworks in water treatment

Metal-organic frameworks (MOFs) have recently garnered significant attention for their potential in water treatment technologies, owing to their unique spatial structures and chemically tunable properties. Defects within MOFs are regarded as excellent tools for enhancing certain material properties. Intentional design and synthesis of defects in MOFs can boost the number of active sites, optimize the framework, increase material conductivity, and fine-tune both structural porosity and chemical properties, thereby elevating their performance in water treatment applications, for instance, catalysis, adsorption, and membrane separation. This review sequentially presents the classification of defective MOFs (encompassing point defects, line defects, planar defects, and mesoscale volume defects), fabrication strategies (including de novo synthesis and post-synthesis treatment), characterization techniques (spanning common spatially resolved measurement techniques and chemical analysis approaches), and their performance in water treatment applications (including catalysis, adsorption, and membrane separation). This review also examines the mechanisms behind the enhanced water treatment performance of defective MOFs and explores the prospective barriers and prospects for their application in water treatment.

Anisotropic Chitosan-nanocellulose/Zeolite imidazolate frameworks-8 aerogel for sustainable dye removal

Assembling microscopic metal-organic frameworks into macroscopic polymeric scaffolds to develop highly renewable materials has been a promising yet challenging area of research. Herein, chitosan (CS) blended with nano-cellulose (NC) was unidirectionally transformed into an aerogel with oriented macropores and then biomineralized with zeolite imidazolate frameworks-8 (ZIF-8) to form a hierarchical structured chitosan-nanocellulose/zeolite imidazolate frameworks-8 (CS-NC-ZIF-8) hybrid aerogel. Incorporating ZIF-8 significantly increases the versatility and mechanical strength with a Young's modulus of 14.18 MPa of the CS-NC aerogel. The incorporation of ZIF-8 into the aerogel not only enhances its adsorption capacity for methylene blue, rhodamine B, acid fuchsin, and methyl orange, but also facilitates the generation of electrons from water that can be transferred to degrade > 90 % of malachite green within 90 min in each catalytic cycle, and this capability was maintained for at least 10 consecutive cycles. Remarkably, the hybrid aerogel was highly renewable after the adsorption of cationic dyes and catalytic removal of malachite green. With its facile production process, high removal efficiency, affordable and green nature, and excellent regeneration feasibility, the CS-NC-ZIF-8 aerogel stands as a promising solution for addressing challenges associated with dye-contaminated water treatment.

The fabrication of phosphotungstate@UIO-Au/reduced graphene oxidation for electrochemical ultrasensitive detection of alpha-fetoprotein

As an early multi-purpose tumor marker of hepatocellular carcinoma, alpha-fetoprotein (AFP) plays a vital role in early diagnosis and treatment. To achieve the early and accurate determination of AFP, the POMOF was fabricated by embedding H3PW12O40 (PW12) into UIO-66-NH2, further immobilized on reduced GO (rGO) and fabricated an innovative POMOF nanocomposite (PW@UIO-Au/rGO) as an electrochemical immunosensor (ECI-sensor). The PW@UIO-Au/rGO achieved 17-fold signal enhancement owing to their synergistic effect, enabling PW@UIO-Au/rGO exhibit high oxidase-like catalytic activity, facilitating their sensing performance. Under optimal experimental conditions, the proposed PW@UIO-Au/rGO ECI-sensor presented excellent sensing performance over a wide range from 0.01 ng mL-1 to 500 ng mL-1 with ultra-low detection of 4.0 pg mL-1. Notably, sensing results in real serum samples were verified by the clinical enzyme-linked immunosorbent assay (ELISA) and electrochemiluminescence immunoassay (ECL) methods with excellent accuracy and consistency, indicating the excellent environmental tolerance of the proposed ECI-sensor. This work provided a promising strategy for designing feasible ultra-sensitive probe for sensing AFP in clinical test.

Innovations in metal-organic frameworks (MOFs): Pioneering adsorption approaches for persistent organic pollutant (POP) removal

Adsorption is a promising way to remove persistent organic pollutants (POPs), a major environmental issue. With their high porosity and vast surface areas, MOFs are suited for POP removal due to their excellent adsorption capabilities. This review addresses the intricate principles of MOF-mediated adsorption and helps to future attempts to mitigate organic water pollution. This review examines the complicated concepts of MOF-mediated adsorption, including MOF synthesis methodologies, adsorption mechanisms, and material tunability and adaptability. MOFs' ability to adsorb POPs via electrostatic forces, acid-base interactions, hydrogen bonds, and pi-pi interactions is elaborated. This review demonstrates its versatility in eliminating many types of contaminants. Functionalizing, adding metal nanoparticles, or changing MOFs after they are created can improve their performance and remove contaminants. This paper also discusses MOF-based pollutant removal issues and future prospects, including adsorption capacity, selectivity, scale-up for practical application, stability, and recovery. These obstacles can be overcome by rationally designing MOFs, developing composite materials, and improving material production and characterization. Overall, MOF technology research and innovation hold considerable promise for environmental pollution solutions and sustainable remediation. Desorption and regeneration in MOFs are also included in the review, along with methods for improving pollutant removal efficiency and sustainability. Case studies of effective MOF regeneration and scaling up for practical deployment are discussed, along with future ideas for addressing these hurdles.

Multifunctional In-MOF and Its S-Scheme Heterojunction toward Pollutant Decontamination via Fluorescence Detection, Physical Adsorption, and Photocatalytic REDOX

A novel doubly interpenetrated indium-organic framework of 1 has been assembled by In3+ ions and highly conjugated biquinoline carboxylate-based bitopic connectors (H2L). The isolated 1 exhibits an anionic framework possessing channel-type apertures repleted with exposed quinoline N atoms and carboxyl O atoms. Owing to the unique architecture, 1 displays a durable photoluminescence effect and fluorescence quenching sensing toward CrO42-, Cr2O72-, and Cu2+ ions with reliable selectivity and anti-interference properties, fairly high detection sensitivity, and rather low detection limits. Ligand-to-ligand charge transition (LLCT) was identified as the essential cause of luminescence by modeling the ground state and excited states of 1 using DFT and TD-DFT. In addition, the negatively charged framework has the ability to rapidly capture single cationic MB, BR14, or BY24 and their mixture, including the talent to trap MB from the (MB + MO) system with high selectivity. Moreover, intrinsic light absorption capacity and band structure feature endow 1 with effective photocatalytic decomposition ability toward reactive dyes RR2 and RB13 under ultraviolet light. Notably, after further polishing the band structure state of 1 by constructing the S-scheme heterojunction of In2S3/1, highly efficient photocatalytic detoxification of Cr(VI) and degradation of reactive dyes have been fully achieved under visible light. This finding may open a new avenue for designing novel multifunctional MOF-based platforms to address some intractable environmental issues, i.e., detection of heavy metal ions, physical capture of pony-sized dyes, and photochemical decontamination of ultrastubborn reactive dyes and highly toxic Cr(VI) ions from water.

Removal of cationic dyes from aqueous solution using polyacrylic acid modified hemp stem

Water pollution caused by dyes is a pressing environmental challenge due to their persistence and difficulty in degradation. Herein, an anionic adsorbent (HS-PAANa) was synthesized by grafting polyacrylic acid (PAA) onto the agricultural waste-hemp stem (HS). The obtained HS-PAANa adsorbent exhibited rapid adsorption kinetics, high adsorption capacity, and a favorable preference for cationic dyes, such as methylene blue (MB) and crystal violet (CV). The experimental data fit well with the pseudo-second-order kinetic model and Langmuir isotherm, demonstrating the efficiency of HS-PAANa in dye removal. Notably, the optimal adsorption capacities of HS-PAANa for MB and CV were found to be 1296.65 mg/g and 1451.43 mg/g, respectively. In the cationic/anionic dyes (MB/MO) binary systems, HS-PAANa exhibited enhanced selective adsorption of cationic dyes (MB), indicating its potential for targeted removal of specific dyes from mixed solutions. Moreover, HS-PAANa adsorption shows an excellent recyclability, after five cycles, HS-PAANa still maintained MB and CV removal rates of 93.85% and 95.08%, respectively. Therefore, the bioadsorbent HS-PAANa exhibits high potential as a highly efficient adsorbent for the effective treatment of cationic pollutants in wastewater.

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.

Cage-Based Covalent Organic Framework for the Effective and Efficient Removal of Malachite Green from Wastewater

A cage-covalent organic framework (COF)-TP {T = bis(tetraoxacalix[2]arene[2]triazine); P = piperazine}, a novel two-dimensional covalent organic skeleton substituted with a nucleophilic cyanuric chloride analogue, was synthesized by a simple polymerization process. Cage-COF-TP is advantageous owing to its good structural order, permanent porosity, and low preparation cost. This skeleton was employed as a cost-effective adsorbent for the intermittent adsorption of an organic dye from aqueous solutions. Adsorption experiments were carried out at different initial dye concentrations, contact times, and solution pH. The adsorption kinetics followed the pseudo-second order model, and the results of thermodynamic studies were consistent with the Langmuir isotherm model. The high degree of matching between the size and shape of malachite green (MG) and the shrunken channels present in Cage-COF-TP were responsible for the enhanced adsorption ability of this material. Furthermore, theoretical calculations indicated that the high adsorption of the studied adsorbent can be attributed to the presence of nitrogen-rich triazine units in the Cage-COF-TP, which are expected to strengthen its affinity to guest molecules. The obtained results showed that the developed adsorbent is an efficient adsorbent that is theoretically capable of stimulating the removal of ∼2000 mg/g MG from wastewater at ambient temperature. This study will therefore be expected to promote the development of new functional materials based on COFs.

Synthesis and Characterization of Hydroxyethyl Cellulose Grafted with Copolymer of Polyaniline and Polypyrrole Biocomposite for Adsorption of Dyes

The emerging concepts of sustainable textiles and controlled production strategy demands ideally zero emissions of contaminants into the aquatic environment. However, the currently in-practiced conventional processes in textiles dispose of a number of contaminants especially super toxic synthetic dyes as effluents. In recent years, nanomaterials have become attractive choice for eco-friendly removal of organic dyestuff. Accordingly, this article reports synthesis and characterization of biocomposite wherein copolymer of polyaniline (PANI) and Polypyrrole (PPY) was grafted onto hydroxyethyl cellulose (HEC). Further, adsorption properties of as-prepared composite were evaluated using textile dyes Rhodamine B (RhB) and methyl Orange (MO)- as model adsorbate. The characterization of novel biocomposite (HEC/PANI-PPy) was carried out using Fourier Transform Infrared (FT-IR), Brunauer-Emmett-Teller analyzer (BET), Scanning Electron Microscope (SEM), and powder X-ray diffraction (XRD). The operational parameters such as dye initial concentration, adsorbent amount, pH and contact time were also studied to evaluate the efficiency level of the prepared biocomposite. Interestingly, the composite-mediated adsorption of RhB and MO followed pseudo-second order and the Langmuir isotherm. It is found that the adsorption capacity HEC/PANI-PPy is 30.06 and 29.3 for RhB and MO respectively. Thus, HEC/PANI-PPy is an inexpensive and highly efficient adsorbent that could be employed for could be employed for the separation and removal of toxic organic dyes from polluted textile effluents.

Regulation pore size distribution for facilitating malachite green removal on carbon foam

Malachite green (MG) is widely used as a textile dye and an aquacultural biocide, and become a serious pollution of drink water, but effectually isolating and removing it from wastewater are still a challenge. Here we report a new strategy to prepare a carbon foam with tunable pore size distribution by a one-pot lava foam process. We find that uniform micropore size is beneficial to the formation of C-OH coordination on the pore surface, increasing MG adsorption rates via H⁺ ionization. As a result, carbon foam with uniform pore size distribution demonstrates an optimum MG removal efficiency of 1812 mg g^-1 and a higher partition coefficient of 3.02 mg g^-1 μM^-1, which is twice that of carbon foams with irregular pore size distribution. The adsorption of MG onto these adsorbents was found to be an endothermic monolayer chemical adsorption process, and the Gibbs free energy of adsorption process was decreased obviously by regulating micropore size distribution. The experiment results are in good agreement with pseudo-second-order kinetic and Langmuir isotherm models. Revealed the pore size distribution was the critical factor of MG removal by carbon foam. It should be and inspiration for the design and development of highly efficiency adsorbents for dyes removal.

Freestanding 2D NiFe Metal-Organic Framework Nanosheets: Facilitating Proton Transfer via Organic Ligands for Efficient Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is crucial to electrochemical hydrogen production. However, designing and fabricating efficient electrocatalysts still remains challenging. By confinedly coordinating organic ligands with metal species in layered double hydroxides (LDHs), an innovative LDHs-assisted approach is developed to facilely synthesize freestanding bimetallic 2D metal-organic framework nanosheets (2D MOF NSs), preserving the metallic components and activities in OER. Furthermore, the research has demonstrated that the incorporation of carboxyl organic ligands coordinated with metal atoms as proton transfer mediators endow 2D MOF NSs with efficient proton transfer during the electrochemical OH_ads  → O_ads transition. These freestanding NiFe-2D MOF NSs require a small overpotential of 260 mV for a current density of 10 mA cm^-2 . When this strategy is applied to LDH nanosheets grown on nickel foam, the overpotential can be reduced to 221 mV. This outstanding OER activity supports the capability of multimetallic organic frameworks for the rational design of water oxidation electrocatalysts. This strategy provides a universal path to the synthesis of 2D MOF NSs that can be used as electrocatalysts directly.

Integration of Phosphotungstic Acid into Zeolitic Imidazole Framework-67 for Efficient Methylene Blue Adsorption

To enhance the dye adsorption capacity of zeolitic imidazolate framework-67 (ZIF-67), phosphotungstic acid (HPW) was integrated into ZIF-67 to prepare composite adsorbents. Characterization results demonstrated that the electronegative HPW was uniformly and tightly deposited on the electropositive ZIF-67. Methylene blue (MB) was selected as a model contaminant to evaluate the adsorption performance of hybrid adsorbents. Results showed that HPW@ZIF-67 had excellent adsorption capacity toward cationic MB. The optimal ZIF-67-0.2 HPW sample with a HPW dosage of 9.9 wt % presented an adsorption capacity of 446.4 mg g^-1. ZIF-67-0.2 HPW displayed good reusability, and the adsorption data can be well described by pseudo-second order and Langmuir isotherm models. The adsorption mechanism was ascribed to the preferred electrostatic attraction and π-π stacking between MB and composite adsorbents. This work provides a route to enhance organic dye removal efficiency of ZIF materials through regulation of surface charge property and sheds light on the development of ZIF-based adsorbents.

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.

Functionally modified metal–organic frameworks for the removal of toxic dyes from wastewater

This review highlights recent advancement in functional modified (FM) MOFs as superior adsorbents for the removal of dyes, classifying them by various modification strategies. The adsorption interactions affected by the FM approach are summarized.

Ultra-efficient catalytic degradation of malachite green dye wastewater by KMnO4-modified biochar (Mn/SRBC)

In this work, KMnO₄-modified biochar was prepared from spirulina residue as the research object. Herein, we report the synthesis, characterization, and catalytic degradation performance of KMnO₄-modified biochar, given that heterogeneous catalytic oxidation is an effective way to treat dye wastewater rapidly. The Mn/SRBC catalyst prepared by KMnO₄ modification was characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, nitrogen adsorption–desorption and laser Raman spectroscopy. In addition, we compared the results with that of the unmodified SRBC. The results showed that the Mn/SRBC catalyst prepared by KMnO₄ modification had a rich pore structure, which provided sufficient contact area for the catalytic reaction. In the presence of H₂O₂, the catalyst could be used to catalyze the oxidative degradation of malachite green in aqueous solution with ultra-high efficiency. In the experiment, the initial pH values of the reaction system had a significant influence on the reaction rate. The removal effect of biochar on the malachite green was poor in an alkaline environment. Within a specific range, the removal rate of malachite green was proportional to the concentration of H₂O₂ in the reaction system. The degradation rate of malachite green dye at 8000 mg L⁻¹ was about 99% in the presence of the catalyst over 5 mmol L⁻¹ hydrogen peroxide for 30 min. These results show the potential application of algae residue biochar and carbon-based composite catalysts for degrading and removing dye wastewater.

In this work, KMnO₄-modified biochar was prepared from spirulina residue as the research object.

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.

Facile and scalable preparation of ZIF-67 decorated cotton fibers as recoverable and efficient adsorbents for removal of malachite green

Recently, metal–organic frameworks (MOFs) have received considerable attention as highly efficient adsorbents for dye wastewater remediation. However, the immobilization of MOFs on the substrate surfaces to fabricate easy recyclable adsorbents via a facile route is still a challenge. In this work, ZIF-67/cotton fibers as adsorbents for dye removal were prepared in a large-scale using a simple coordination replication method. The successful fabrication of the ZIF-67/cotton fibers was confirmed by FTIR, XRD, XPS, SEM and BET analysis, respectively. As expected, the as-prepared ZIF-67/cotton fibers exhibited high adsorption capacity of 3787 mg/g towards malachite green (MG). Meanwhile, the adsorption kinetics and isotherm obeyed the pseudo-second-order kinetics and Langmuir model, respectively. Moreover, its removal efficiency towards MG was not significantly influenced by the pH and ionic strength of aqueous solution. Most importantly, the ZIF-67/cotton fibers can remove MG from synthetic effluents, and it can be easily regenerated without filtration or centrifugation processes, with the regeneration efficiency remaining over 90% even after 10 cycles. Additionally, the ZIF-67/cotton fibers presented excellent antimicrobial performance against E. coli and S. aureus. Hence, the distinctive features of the as-prepared ZIF-67/cotton fibers make it promisingly applicable for the colored wastewater treatment.

Synergistic Photocatalytic-Adsorption Removal of Basic Magenta Effect of AgZnO/Polyoxometalates Nanocomposites

The bifunctional photocatalytic-adsorbent AgZnO/polyoxometalates (AgZnO/POMs) nanocomposites were synthesized by combining AgZnO hybrid nanoparticles and polyoxometalates [Cu(L)₂(H₂O)]H₂[Cu(L)₂(P₂Mo₅O₂₃)]⋅4H₂O (HL = C₆H₆N₂O) into nanostructures via a sonochemical method. Transmission electron microscopy (TEM) indicated that AgZnO/POMs nanocomposites were uniform with narrow particle size distribution and without agglomeration. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the nanostructure and composition of AgZnO/POMs nanocomposites. The ultraviolet-visible spectra (UV-Vis) and photoluminescence spectra (PL) confirmed excellent optical properties of the AgZnO/POMs nanocomposites. 94.13% ± 0.61 of basic magenta (BM) in aqueous solution could be removed using the AgZnO/POMs nanocomposites through adsorption and photocatalysis. The kinetic analysis showed that both the adsorption and photocatalysis process conform to pseudo-second-order kinetics. In addition, the removal rate of AgZnO/POMs nanocomposites was found to be almost unchanged after 5 cycles of use. The bifunctional photocatalytic-adsorbent AgZnO/POMs nanocomposites with high stability and cycling performance have broad application prospects in the treatment of refractory organic dye wastewater containing triphenylmethane.

Facile preparation of robust dual MgO-loaded carbon foam as an efficient adsorbent for malachite green removal

This study developed a facile approach for the fabrication of dual MgO-loaded carbon foam (DMCF) via carbonization of a cured MgO/cyanate ester resin mixture, which underwent self-foaming of the resin followed by the carbothermal reduction of MgO. The features of the prepared DMCF prepared were characterized by FESEM, TEM, XRD, FTIR, XPS and so on, and the effects of adsorption conditions, adsorption isotherms, kinetics, and thermodynamics on malachite green (MG) removal using the DMCF as adsorbents were investigated through batch adsorption experiments. Results demonstrate that the DMCF possesses a unique dual loading of MgO particles which are not only loaded onto its foam walls but also filled within the walls with a graphene-wrapped core-shell structure. The experimental maximum adsorption capacity of MG reaches up to 1874.18 mg/g with a partition coefficient of 10.87 mg/g/μM. The adsorption process can be better described with Langmuir, pseudo-second-order, and intraparticle diffusion models. Moreover, the DMCF exhibits a removal percentage of 84.85% after five reuses, indicating that it is an efficient and promising adsorbent for MG adsorption.

New catalyst comprising Silicotungstic acid and MCM-22 for degradation of some organic dyes

A heterogeneous catalyst comprising Keggin type polyoxometalate, silicotungstic acid (SiW₁₂), and MCM-22 was synthesized using wet impregnation method and characterized by acidity measurement, BET, FT-IR, XRD, and SEM. Their catalytic activity was evaluated for the degradation of cationic organic dyes like methylene blue (MB), crystal violet (CV), and an azo dye Chryosidine Y (CY) in an aqueous solution. The experimental parameters such as catalyst amount, initial dye concentration, and contact time were studied for the degradation of dyes, and it was found that the cationic dyes like methylene blue and crystal violet show better activity as compared to azo dye Chryosidine Y. This may be attributed to better electrostatic interaction of these cationic dyes with the residual negative surface charge of the catalyst, due to presence of SiW₁₂ ion as it is rich in surface oxygens and surface hydroxyl groups. The control experimental results showed that the presence of SiW₁₂ at the surface of MCM-22 promoted the degradation reactions, and presence of multiple W-O bonds in polyoxometalate also played a key role in this reaction. The catalyst exhibits recycling ability without any significant loss in activity up to four cycles.

Engineered Fe3 triangle for the rapid and selective removal of aromatic cationic pollutants: complexity is not a necessity

In this study, a low-cost oxo-bridged {Fe3} triangular cluster was constructed based on a benzoate ligand via slow evaporation. The cluster was thoroughly characterized by FTIR and UV-visible spectroscopy, TGA, and PXRD, and the exact structure was elucidated by single-crystal XRD. The formation of C-H⋯π and π-π interactions is responsible for the extra stability of {Fe3} clusters, which further enhances the dye adsorption property. The dye adsorption experiments performed on cationic [methylene blue (MB) and rhodamine-B (Rh-B)] as well as anionic [methyl orange (MO) and congo red (CR)] dyes revealed the ultimate selectivity of the present cluster towards the cationic ones. The {Fe3} cluster exclusively adsorbs the cationic dyes, i.e., MB and Rh-B even in the presence of anionic dyes, i.e., CR and MO. The extra stability, reusability and high efficiency of the {Fe3} molecular ensemble make it an attractive and fascinating material of importance. The kinetics analysis was evaluated employing different kinetics models. Furthermore, the plausible adsorption mechanism was also proposed, which suggests the interplay of cation-π and π-π interactions consolidating the efficient adsorption. Thus, the present work opens new doors for coordination chemists to further tune the structural features to modulate the adsorption/separation capacities of simple low-cost clusters for environmental protection for future efforts.

The Application of Modified Natural Polymers in Toxicant Dye Compounds Wastewater: A Review

The utilization of various types of natural and modified polymers for removing toxicant dyes in wastewater generated by the dye industry is reviewed in this article. Dye wastewater contains large amounts of metals, surfactants, and organic matter, which have adverse effects on human health, potentially causing skin diseases and respiratory problems. The removal of dyes from wastewaters through chemical and physical processes has been addressed by many researchers. Currently, the use of natural and modified polymers for the removal of dyes from wastewater is becoming more common. Although modified polymers are preferred for the removal of dyes, due to their biodegradability and non-toxic nature, large amounts of polymers are required, resulting in higher costs. Surface-modified polymers are more effective for the removal of dyes from the wastewater. A survey of 80 recently published papers demonstrates that modified polymers have outstanding dye removal capabilities, and thus have a high applicability in industrial wastewater treatment.

Microwave assist sorption of crystal violet and Congo red dyes onto amphoteric sorbent based on upcycled Sepia shells

A new sorbent based on Sepia shells (cuttlefish bones) has been synthesized (SSBC) and tested for the sorption of cationic dye (crystal violet, CV) and an anionic dye (congo red, CR). SSBC was produced by reaction of sepia shells powder with urea in the presence of formaldehyde. In the first part of the work, the sorbent was characterized using scanning electron microscopy, energy dispersive X-ray analysis, Fourier-transform infra-red spectrometry and titration (for determining pH_PZC). In a second step, sorption properties were tested on the two dyes through the study of pH effect, sorbent dosage, temperature and ionic strength; the sorption isotherms and uptake kinetics were analyzed at the optimum pH: Langmuir equation fits isotherm profiles while the kinetic profile can be described by the pseudo-second order rate equation. Maximum sorption capacities reach up to 0.536 mmol g^-1 for CV and 0.359 mmol g^-1 for CR, at pH 10.6 and 2.4, respectively. The comparison of sorption properties at different temperatures shows that the sorption is endothermic. Processing to the sorption under microwave irradiation (microwaved enforced sorption, MES) increases mass transfer and a contact time as low as 1 min is sufficient under optimized conditions (exposure time and power) reaching the equilibrium, while 2-3 h were necessary for "simple" sorption. Dye desorption was successfully tested using 0.5 M solutions of NaOH and HCl for the removal of CR and CV, respectively. The sorbent can be re-used for a minimum of four cycles of sorption/desorption. Finally, the sorbent was successfully tested on spiked tap water and real industrial wastewater.

Nano-engineered Adsorbent for the Removal of Dyes from Water: A Review

Background:

The huge quantity of wastewater, containing poisonous and hazardous dyes, is released by various industries which pollute water in direct and indirect ways. Most of the dyes are a dangerous class of water contaminants which have affected the environment drastically. Some dyes such as congo red, rhodamine B, methylene blue, methyl violet, and crystal violet are a serious threat to human beings.

Remediation Method:

Numerous methods are available for the removal of dyes from water. Adsorption, being a superior and eco-friendly technique, has advantage of eliminating organic dyes because of the availability of materials as adsorbents. The inexpensive nanomaterials are a more attractive choice for remediation of various dyes due to their unique properties and offer an adequate pathway to adsorb any organic dye from water to overcome its hazardous effects on human health.

Results:

In this review, we have discussed the latest literature related to various types of synthesis, characterization and uses as adsorbent for highly adsorptive removal capacity of nanoparticles for organic dyes.

Conclusion:

Adsorption technology provides an attractive pathway for further research and improvement in more efficient nanoparticles, with higher adsorption capacity, for numerous dyes to eliminate the dyes discharged from various industries and thus reduce the contamination of water. Therefore, nanocomposites may contribute to future prospective water treatment process.

A hierarchical LDH/MOF nanocomposite: single, simultaneous and consecutive adsorption of a reactive dye and Cr(vi)

The design and development of an environmentally benign porous adsorbent for effective simultaneous adsorption of organic dyes and heavy metals from water are important but remain a big challenge. Herein, we have designed a layered double hydroxide/metal-organic framework-based hierarchical nanocomposite (LDH/MOF HNC) by a facile, room-temperature in situ approach. This paper for the first time reports a hierarchical trimodal micro-meso-macroporous LDH/MOF composite with a high surface area (surface area 1282 m2 g-1 and pore volume 0.93 cm3 g-1), synthesised by uniformly growing MOF nanocrystals on the surface of LDH nanosheet ultrathin films. An attempt is made to quantitatively demonstrate the adsorption data via suitable nonlinear kinetic and isotherm equations for single, simultaneous, and consecutive adsorption of the orange II reactive dye and Cr(vi). Experiments were performed at various values of pH (6.0-11.0), adsorbent dosages (1.0-8.0 mg), adsorbate concentrations (5-500 mg L-1), and temperatures (293-323 K). The Langmuir model revealed a satisfactory fit to the equilibrium data of the LDH/MOF HNC (correlation coefficients R2 > 0.98) with a calculated maximum adsorption capacity of 1173 and 733 mg g-1 for orange II and Cr(vi), respectively, in a simultaneous adsorption system. The results of the study demonstrated that LDH/MOF HNCs could potentially be applied as a promising nanoadsorbent for the simultaneous removal and extraction of toxic dyes and metals from water.

Octamolybdate-based hybrid constructed by flexible bis-triazole ligands: synthesis, photocatalytic and electrochemical properties

A novel polyoxometalate-based organic–inorganic hybrid compound was synthesized via a facile in situ hydrothermal reaction. 1 exhibits selective photocatalytic degradation of dyes and the great property to electrocatalytically reduce H₂O₂ and NaNO₂.

One-step synthesis of Cu(II) metal-organic gel as recyclable material for rapid, efficient and size selective cationic dyes adsorption

Efficient removal of non-biodegradable and hazardous dyes from wastewater remains a hot research topic. Herein, a rationally designed a Cu(II)-based metal-organic gel (Cu-MOG) with a nanoporous 3D network structure prepared via a simple one-step mixing method was successfully employed for the removal of cationic dyes. The Cu-MOG exhibited high efficiency, with an adsorption capacity of up to 650.32 mg/g, and rapid adsorption efficiency, with the ability to adsorb 80% of Neutral Red within 1 min. The high adsorption efficiency was attributed to its large specific surface area, which enabled it to massively bind cationic dyes through electrostatic interaction, and a nanoporous structure that promoted intra-pore diffusion. Remarkably, the Cu-MOG displayed size-selective adsorption, based on adsorption studies concerning dyes of different sizes as calculated by density functional theory. Additionally, the adsorption performance of the Cu-MOG still maintained removal efficiency of 100% after three regeneration cycles. These results suggested that the Cu-MOG could be expected to be a promising and competitive candidate to conveniently process wastewater.

Selective Dye Adsorption by Zeolitic Imidazolate Framework-8 Loaded UiO-66-NH2

In this study, Zeolitic Imidazolate Framework-8 (ZIF-8)-loaded UiO-66-NH₂ was synthesized, characterized, and analyzed for its potential to efficiently remove dyes. The selective adsorption on ZIF-8-loaded UiO-66-NH₂ or its parent MOFs (UiO-66-NH₂ and ZIF-8) in the mixed dyes solution was explored, including anionic dye (methyl orange (MO)) and cationic dyes (methylene blue (MB) and rhodamine B (RhB)). ZIF-8-loaded UiO-66-NH₂ displayed much better selectivity to MB than its parent MOFs. Adsorption capacity of ZIF-8-loaded UiO-66-NH₂ (173 mg/g) toward MB was found to be 215% higher than UiO-66-NH₂ (55 mg/g). A kinetics study based on adsorption data demonstrated that the adsorption process most closely matched with the model of pseudo-second-order kinetic and Langmuir isotherm. The adsorption was an exothermic and spontaneous physical process as revealed by the values of thermodynamic parameters. Furthermore, reusability of ZIF-8-loaded UiO-66-NH₂ was investigated and revealed the significant regeneration efficiency in adsorption capacity for MB even after four adsorption cycles. Experimental results proved that the interaction between ZIF-8-loaded UiO-66-NH₂ and MB was mainly affected by the mechanism, for instance, electrostatic interaction as well as π–π stacking interactions.