A review on the adsorption mechanism of different organic contaminants by covalent organic framework (COF) from the aquatic environment

Preparation of GO/COFs composites by interlayer-confined strategy for the adsorption of nitro aromatic pollutants

With the continuous development of industrialization, the excessive emission of nitro aromatic with strong toxicity, high carcinogenicity and non-degradability has attracted great attention. How to efficiently remove nitro aromatic pollutants is an important research topic. In this work, graphene oxide/covalent organic frameworks (GO/COFs) composites were successfully synthesized via interlayer confinement strategy selecting GO, 2,5-dimethoxybenzene-1,4-dicarboxaldehyde (DMTP) and 1,3,5-tri(4-aminophenyl)benzene (TPB) as raw materials. Due to high specific surface area, hierarchical porous structure and good thermal stability, GO/COFs were utilized to adsorb and remove nitro aromatic hydrocarbons in the water environment. The adsorption behavior of GO/COFs for o-nitrophenol, 1,3-dinitrobenzene and 2,4,6-trinitrophenol were further investigated. The GO/COFs composites showed the strongest adsorption capacity for 2,4,6-trinitrophenol, and the maximum adsorption capacity for 2,4,6-trinitrophenol, o-nitrophenol, and 1,3-dinitrobenzene were 438, 317, and 173 mg g-1, respectively. The experimental results indicated that the GO/COFs composites provided great adsorption capability for nitro aromatic pollutants and can be reused, rendering it an extremely potential adsorbent for organic pollutants.

When covalent organic frameworks meet zeolites: Enhancing rhodamine B removal through the synergy in the emerging organic-inorganic nanoadsorbents

The development of new porous materials has attracted intense attention as adsorbents for removing pollutants from wastewater. However, pure inorganic and organic porous materials confront various problems in purifying the wastewater. In this work, we integrated a covalent organic framework (TpPa-1) with an inorganic zeolite (TS-1) for the first time via a solvothermal method to fabricate new-type nanoadsorbents. The covalent organic framework/zeolite (TpPa-1/TS-1) nanoadsorbents combined the merits of the zeolite and COF components and possessed efficient adsorptive removal of organic contaminants from solution. Structural morphology and chemical composition characterization by powder X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and thermogravimetric analysis demonstrated the successful preparation of TpPa-1/TS-1 composite nanoadsorbents. The resultant composite adsorbent TpPa-1/TS-1 removed rhodamine B at 1.7 and 2.6 times the efficiency of TpPa-1 and TS-1, respectively. Additional investigation revealed that the Freundlich adsorption isotherm and the pseudo-second-order kinetic model could be employed to represent the adsorption process more appropriately. Thermodynamic calculation analysis showed that the adsorption process proceeded spontaneously and exothermically. Besides, the effects of pH, absorbent mass and ionic strength on the adsorption performance were systematically investigated. The prepared composite adsorbent showed a slight decrease in removal efficiency after eight cycles of repeated use, and real water environment experiments also showed the high stability of the adsorbent. The enhanced performance can be attributed to electrostatic interaction, acid-base interaction, hydrogen bonding and π-π interactions.

Magnetic covalent organic frameworks as sorbents in the chromatographic analysis of environmental organic pollutants

Covalent organic frameworks (COFs) are featured with large specific surface areas, good thermal stability, and abundant pores. These properties are exactly what the sorbents used for extraction or adsorption of interest substances are desired with. While, the low density and hydrophobicity of COFs often makes them difficult to be dispersed evenly and recovered from the aqueous solution. Magnetic covalent organic frameworks (MCOFs) inherit magnetic property of the magnetic particles and porous structure of COFs. They have improved dispersity in aqueous solution and phase separation can be rapidly achieved via external magnetic fields. This review summarized the synthesis strategies for MCOFs, and their application in trace environmental organic pollutants analysis by chromatography techniques. The selection of COFs types and modification with active groups for a certain adsorption purpose is discussed, along with the exploration of adsorption mechanisms, which is beneficial for the design and synthesis of MCOFs.

A pH-dependent and charge selective covalent organic framework for removal of dyes from aqueous solutions

A novel imine-linked COF is synthesized by the condensation of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 2-hydroxy-5-methoxyisophthalaldehyde (HMIPA) under solvothermal conditions. This COF adsorbs preferentially the neutral dye Neutral Red (NR) over the positively charged dye Methylene Blue (MB) at pH 7, and the negatively charged Methyl Orange (MO) over the positively charged Methylene Blue (MB) at pH 3. The maximum adsorption capacities (qe) obtained within very short times (11-60 min) under optimized conditions were 108, 185 and 429 mg.g-1 for the MB, MO, and NR dyes, respectively. These adsorptions obey the Langmuir isotherm and pseudo-second-order kinetics. The prepared TAPT-HMIPA-COF is used successfully for the removal of the dyes from real water and treated wastewater samples. The adsorption data, BET, FTIR, and zeta potential measurements show that the electrostatic, π-π stacking and hydrogen bond interactions are responsible for the adsorption of organic dyes on the surface of the prepared COF. Due to recyclability, high capacity and efficiency for the adsorption of positive, negative and neutral organic dyes, this COF can be considered promising for simultaneous removal of various dyes from aqueous solutions at adjusted pHs.

Cold plasma turns mixed-dye-contaminated wastewater bio-safe

The excessive and uncontrollable discharge of diverse organic pollutants into the environment has emerged as a significant concern, presenting a substantial risk to human health. Among the advanced oxidation processes used for the purification of wastewater, cold plasma technology is superior in fast and effective decontamination but often fails facing mixed pollutants. To address these issues, here we develop the new conceptual approach, plasma process, and proprietary reactor that ensure, for the first time, that the efficiency of treatment (114.7%) of two mixed organic dyes, methylene blue (MB) and methyl orange (MO), is higher than when the two dyes are treated separately. We further reveal the underlying mechanisms for the energy-efficient complete degradation of the mixed dyes. The contribution of plasma-induced ROS and the distinct degradation characteristics and mechanism of pollutants in mixed treatment are discussed. The electron transfer pathway revealed for the first time suggest that the mixed pollutants reduce the overall redox potentials and facilitate electron transfer during the plasma treatment, promoting synergistic degradation effects. The integrated frameworks including both direct and indirect mechanisms provide new insights into the high-efficiency mixed-contaminant treatment. The degradation products for mixed degradation are revealed based on the identification of intermediate species. The plasma-treated water is proven safe for living creatures in waterways and sustainable fishery applications, using in vivo zebrafish model bio-toxicity assay. Overall, these findings offer a feasible approach and new insights into the mechanisms for the development of highly-effective, energy-efficient technologies for wastewater treatment and reuse in agriculture, industry, and potentially in urban water networks.

Ultrafast adsorption of hexavalent chromium from aqueous effluents using covalent triazine frameworks

We have synthesized low-cost high performance covalent triazine framework (CTF) through Schiff base reaction of melamine and terephthalaldehyde with different proportions of the reactants. The synthesized adsorbents showed excellent capacity for adsorption of Cr (VI) at acidic pH while almost negligible adsorption at higher pH. The adsorbent displays excellent reusability, with a little decrease in adsorption capacity with the increasing number of cycles. Moreover, Cr (VI) the adsorption is unaffected by the presence of 50-500 times higher concentration of alkali metal and halide ions in solution, while sulphate ions demonstrate shielding behavior decreasing the adsorption capacity. Mechanistic studies indicate electrostatic attractions, ion exchange and reduction being responsible for the adsorption mediated by abundant nitrogen sites that also imbibes the adsorbent with high capacity. The adsorbent was also utilized to recover chromium from an industrial electroplating effluent, which demonstrates applicability of material for practical applications.

Sunflower-like missing-linker covalent organic framework for efficient extraction of non-steroidal anti-inflammatory drugs

As excellent crystalline materials, covalent organic frameworks (COFs) are widely used in drug adsorption. In this work, a defective engineering strategy was proposed for designing and preparing the functionalized end-capping monomer and missing-linker COFs. The missing-linker COF 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde compound with glycidyltrimethyl ammonium chloride modified benzene-1,4-diamine (TpPa-GTA) was synthesized through Schiff base reaction with wide pore size distribution for adsorption of four nonsteroidal anti-inflammatory drugs (NSAIDs). The adsorption process follows pseudo-second-order kinetics, and the four drugs reached adsorption equilibrium within 10 min. The sunflower-like structure helps to promote intraparticle diffusion during the adsorption process, thereby realizing the rapid adsorption of TpPa-GTA. The equilibrium isotherms fit well with both the Freundlich and Langmuir models, with a maximum adsorption capacity of 83.3-315 mg g-1 calculated from the Langmuir model. Based on the detection results of Zeta potential and XPS, the adsorption mechanism was inferred, and the rapid capture of NSAIDs in the wide pH range of 4.0 to 7.5 was realized under electrostatic interaction, hydrogen bonding, and π-π interaction. The detection of lake and river samples using the missing adapter TpPa-GTA has a recovery rate of 84.2-117%, which provides a new approach to the adsorption of pollutants with COFs.

Magnetic molecular imprinted covalent organic framework composite for the magnetic solid-phase extraction of bisphenol AF

A magnetic molecular imprinted covalent organic framework composite (MCOF-MIP) that possessed the 'dual-selectivity' of a covalent organic framework and molecular imprinted polymer (MIP) with rapid response performance was successfully prepared for the removal of bisphenol AF (BPAF) from real water and blood samples. First, the MCOF was separately synthesized using magnetic Fe3O4 as the magnetic core, 1,3,5-triaminobenzene and 2,5-dibromobenzene-1,4-diformaldehyde as precursors and a deep eutectic solvent (DES) as the solvent using a solvothermal synthesis method. The MCOF showed high crystallinity and good adsorption capacities for BPAF (107.4 mg g-1), bisphenol A (113.6 mg g-1), bisphenol S (120.0 mg g-1) and bisphenol F (82.1 mg g-1). To further improve the selectivity for BPAF, an MIP, which uses BPAF as a template, was introduced to form the MCOF-MIP. Due to the dual selectivity of MCOF and MIP, the MCOF-MIP exhibited relatively high selective adsorption capacity to BPAF (243.1 mg g-1) compared to that for the MCOF (107.4 mg g-1), while the adsorption capacities (149.7-109.4 mg g-1) for the other three compounds were not significantly improved. Furthermore, a magnetic solid-phase extraction (MSPE) method was established, and MSPE parameters such as adsorbent dosage, adsorption time, desorption solvent and desorption time were optimized. Combined with high-performance liquid chromatography with diode-array detection (HPLC-DAD) analysis, a rapid and sensitive method was developed to detect BPAF, which showed good linearity (r > 0.9969) ranging from 0.1 to 400 μg mL-1. Low limits of detection (0.04 μg mL-1, S/N = 3) and quantitation (0.1 μg mL-1, S/N = 10) and good precision with low relative SDs (<1.2 % for intra-day and <1.1 % for inter-day) were also obtained. Finally, MSPE coupled with HPLC-DAD was employed for the analysis of BPAF in water and blood samples, and the recoveries of BPAF were satisfactory (91.1-112.6 %).

Covalent Organic Framework Membranes and Water Treatment

Covalent organic frameworks (COFs) are an emerging class of highly porous crystalline organic polymers comprised entirely of organic linkers connected by strong covalent bonds. Due to their excellent physicochemical properties (e.g., ordered structure, porosity, and stability), COFs are considered ideal materials for developing state-of-the-art separation membranes. In fact, significant advances have been made in the last six years regarding the fabrication and functionalization of COF membranes. In particular, COFs have been utilized to obtain thin-film, composite, and mixed matrix membranes that could achieve effective rejection (mostly above 80%) of organic dyes and model organic foulants (e.g., humic acid). COF-based membranes, especially those prepared by embedding into polyamide thin-films, obtained adequate rejection of salts in desalination applications. However, the claims of ordered structure and separation mechanisms remain unclear and debatable. In this perspective, we analyze critically the design and exploitation of COFs for membrane fabrication and their performance in water treatment applications. In addition, technological challenges associated with COF properties, fabrication methods, and treatment efficacy are highlighted to redirect future research efforts in realizing highly selective separation membranes for scale-up and industrial applications.

Recent progress in chromium removal from wastewater using covalent organic frameworks - A review

Covalent organic frameworks (COFs) offer a pivotal solution to urgently address heavy metal removal from wastewater due to their exceptional attributes such as high adsorption capacity, tunable porosity, controllable energy band structures, superior photocatalytic performance, and high stability-reusability. Despite these advantages, COFs encounter certain challenges, including inefficient utilization of visible light, rapid recombination of photogenerated carriers, and limited access to active sites due to close stacking. To enhance the photocatalytic and adsorptive performance of COF-based catalysts, various modification strategies have been reported, with a particular focus on molecular design, structural regulation, and heterostructure engineering. This review comprehensively explores recent advancements in COF-based photocatalytic and adsorptive materials for chromium removal from wastewater, addressing kinetics, mechanisms, and key influencing factors. Additionally, it sheds light on the influence of chemical composition and functional groups of COFs on the efficiency of hexavalent chromium [Cr (VI)] removal.

Efficient decontamination of organic pollutants from wastewater by covalent organic framework-based materials

Covalent organic frameworks (COFs), assembling through covalent bonds, are a rising class of porous materials. Nowadays, various COFs are widely applied in organic pollutants decontamination due to the outstanding capabilities of large surface area, multiple functional groups, porous structure, excellent absorptivity, flexible design and so on. This review concentrates on the applications of COFs in different decontamination technologies such as solid-phase extraction, membrane filtration and sieving, adsorption, and catalysis reaction. The factors influencing water chemistry, such as pH, temperature, salt concentration and natural organic matter, are summarized in terms of their impact on decontamination performance and the extraction mechanisms for the diverse analytes. The interaction mechanisms between COFs and organic pollutants were hydrogen bonding, π-π stacking, hydrophilic, hydrophobic, and electrostatic interactions. Furthermore, a perspective on current obstacles and upcoming developments of COFs for organic pollutant removal has been provided. Due to their adaptable and versatile design as well as elaborate and diverse functionalization, COFs possess significant possibility in ameliorating environmental pollution.

Covalent organic frameworks as highly versatile materials for the removal and electrochemical sensing of organic pollutants

The presence of persistent organic compounds in water has become a worldwide issue due to its resistance to natural degradation, inducing its environmental resilience. Therefore, the accumulation in water bodies, soils, and humans produces toxic effects. Also, low levels of organic pollutants can lead to serious human health issues, such as cancer, chronic diseases, thyroid complications, immune system suppression, etc. Therefore, developing efficient and economically viable remediation strategies motivates researchers to delve into novel domains within material science. Moreover, finding approaches to detect pollutants in drinking water systems is vital for safeguarding water safety and security. Covalent organic frameworks (COFs) are valuable materials constructed through strong covalent interactions between blocked monomers. These materials have tremendous potential in removing and detecting persistent organic pollutants due to their high adsorption capacity, large surface area, tunable porosity, porous structure, and recyclability. This review discusses various synthesis routes for constructing non-functionalized and functionalized COFs and their application in the remediation and electrochemical sensing of persistent organic compounds from contaminated water sources. The development of COF-based materials has some major challenges that need to be addressed for their suitability in the industrial configuration. This review also aims to highlight the importance of COFs in the environmental remediation application with detailed scrutiny of their challenges and outcomes in the current research scenario.

Evaluation of Photocatalytic, Antioxidant, and Antibacterial Efficacy of Almond Oil Capped Zinc Oxide Nanoparticles

In this study, ZnO nanoparticles (NPs) were synthesized in the presence of almond oil at various molar ratios of zinc acetate and sodium hydroxide, including 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1, to obtain pH values of 11, 10, 9, 8, and 7, respectively. The XRD results revealed that ZnO NPs exhibit a hexagonal structure, with high crystallinity. SEM results showed that dense and large sized ZnO NPs were formed at pH 11, and relatively small (~30-40 nm) NPs were obtained at pH 9. The size distribution can be explained in terms of the presence of OH- ions at different pH levels. However, the larger size of the NPs at pH 7 compared to those at pH 8-11 were due to the coalescence of NPs suitable for antioxidant/antibacterial activities. ZnO NPs demonstrated a high degradation efficiency (~93%) in 90 min, with a high rate constant for Methyl Orange (MO), which is better than the previously reported rate. The larger sized almond oil capped ZnO NPs also showed excellent radical scavenging activity (94%) and are proven to be good carriers to resist Escherichia coli (E. coli) bacteria.

Latest advances in layered covalent organic frameworks for water and wastewater treatment

This review provides an overview of recent progress in the development of layered covalent organic frameworks (LCOFs) for the adsorption and degradation of pollutants in water and wastewater treatment. LCOFs have unique properties such as high surface area, porosity, and tunability, which make them attractive adsorbents and catalysts for water and wastewater treatment. The review covers the different synthesis methods for LCOFs, including self-assembly, co-crystallization, template-directed synthesis, covalent organic polymerization (COP), and solvothermal synthesis. It also covers the structural and chemical characteristics of LCOFs, their adsorption and degradation capacity for different pollutants, and their comparison with other adsorbents and catalysts. Additionally, it discussed the mechanism of adsorption and degradation by LCOFs, the potential applications of LCOFs in water and wastewater treatment, case studies and pilot-scale experiments, challenges, and limitations of using LCOFs, and future research directions. The current state of research on LCOFs for water and wastewater treatment is promising, however, more research is needed to improve their performance and practicality. The review highlights that LCOFs have the potential to significantly improve the efficiency and effectiveness of current water and wastewater treatment methods and can also have implications for policy and practice.

Recent advances in covalent organic frameworks (COFs) for wound healing and antimicrobial applications

Covalent organic frameworks (COFs) are crystal-like organic structures such as cartography buildings prepared from appropriately pre-designed construction block precursors. Moreover, after the expansion of the first COF in 2005, numerous researchers have been developing different materials for versatile applications such as sensing/imaging, cancer theranostics, drug delivery, tissue engineering, wound healing, and antimicrobials. COFs have harmonious pore size, enduring porosity, thermal stability, and low density. In addition, a wide variety of functional groups could be implanted during their construction to provide desired constituents, including antibodies and enzymes. The reticular organic frameworks comprising porous hybrid materials connected via a covalent bond have been studied for improving wound healing and dressing applications due to their long-standing antibacterial properties. Several COF-based systems have been planned for controlled drug delivery with wound healing purposes, targeting drugs to efficiently inhibit the growth of pathogenic microorganisms at the wound spot. In addition, COFs can be deployed for combinational therapy using photodynamic and photothermal antibacterial therapy along with drug delivery for healing chronic wounds and bacterial infections. Herein, the most recent advancements pertaining to the applications of COF-based systems against bacterial infections and for wound healing are considered, concentrating on challenges and future guidelines.

Electronic structure evolution induced by the charge redistribution during the construction of two-dimensional polymer networks from monomers to crystal frameworks

The electronic structure of COFs is dominated by the relative energy level between the frontier orbitals of building units, and the charge carrier mobility within the 2D structure is dominated by the charge transfer between core and linker units.