Fabricating compact covalent organic framework membranes with superior performance in dye separation

Applications of covalent organic frameworks for the elimination of dyes from wastewater: A state-of-the-arts review

Covalent organic frameworks (COFs) are class of porous coordination polymers made up of organic building blocks joined together by covalent bonding through thermodynamic and controlled reversible polymerization reactions. This review discussed versatile applications of COFs for remediation of wastewater containing dyes, emphasizing the advantages of both pristine and modified materials in adsorption, membrane separation, and advanced oxidations processes. The excellent performance of COFs towards adsorption and membrane filtration has been centered to their higher crystallinity and porosity, exhibiting exceptionally high surface area, pore size and pore volumes. Thus, they provide more active sites for trapping the dye molecules. On one hand, the photocatalytic performance of the COFs was attributed to their semiconducting properties, and when coupled with other functional semiconducting materials, they achieve good mechanical and thermal stabilities, positive light response, and narrow band gap, a typical characteristic of excellent photocatalysts. As such, COFs and their composites have demonstrated excellent potentialities for the elimination of the dyes.

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.

Urchin-like fluorinated covalent organic frameworks decorated fabric for effective self-cleaning and versatile oil/water separation

Oil contamination and industrial organic pollutants emission have been a serious problem affecting the ecological and residential environment. Membrane-based separation shows great application prospect due to its low-cost, environmental-friendly and easy operation. Therefore, the development of efficient oil-water separation membranes is highly desirable. Herein, a fabric filter with superwettability was prepared by coating urchin-like fluorinated covalent organic frameworks (COFs) on fabric, which was well utilized in filtering immiscible oil-water mixture and surfactant-stabilized water-in-oil emulsion driven only by gravity for the first time. The as-prepared COF fabric filter (defined as fabric@u-FCOF) possessed many outstanding properties, including superhydrophobicity with the water contact angle of approximately 151.6°, satisfactory resistance for alkaline, acidic and saline environments, as well as superior mechanical durability under harsh conditions. Because of the super-micropore of fabric@u-FCOF and the nanopore in the COF coating, the obtained fabric@u-FCOF exhibited excellent performances in terms of separation efficiency and permeability, in which the oil flux was up to 16964 L·m-1·h-2 and separation efficiency for the mixed o-dichlorobenzene/water was higher than 99.4%. In addition, the fabric@u-FCOF also showed excellent self-cleaning performance due to the micro/nano hierarchical structure of its surface. These excellent properties make it an ideal candidate for applications of oil/water separation and water purification.

The covalent organic framework based nylon membrane extraction coupled with UHPLC-MS/MS for highly efficiency determination of hexabromocyclododecanes in environmental water

Hexabromocyclododecanes (HBCDs) have given their adverse effects on environment and human health, and highly sensitive analysis of HBCDs in water is urgent. In this study, a new method for the determination of trace HBCDs in water was established by covalent organic framework (COF) based nylon membrane extraction (ME) coupled with ultrahigh-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). The COF had been self-assembled onto the nylon membrane in a gentle strategy to fabricate COF nylon membrane. Several important ME parameters including the dosage of COF, pH, eluent condition and salinity were systematically investigated. The limits of detection and quantification were 0.011-0.014 and 0.038-0.047 ng/L for three HBCDs, respectively. The linear ranges were from 0.04 to 20 ng/L, and the relative standard deviations were 5.7-17.8 % (intra-day) and 5.2-14.1 % (inter-day). In addition, density functional theory (DFT) calculations on adsorption energy proved that the introduction of halogen bond (XB) made a key contribution to high extraction efficiency and excellent selectivity of COF nylon membrane for HBCDs. The 500 mL of samples, including tap water and reservoir water, could be extracted only in 23 min. The established method presented highly sensitive for ultra-trace analysis of HBCDs in environmental water.

Pre-oxidation coupled with charged covalent organic framework membranes for highly efficient removal of organic chloramines precursors in algae-containing water treatment

Organic chloramines in water would pose both chemical and microbiological risks. It is essential to remove the precursors of organic chloramine (amino acids and decomposed peptides/proteins) to limit its formation in disinfection. In our work, nanofiltration was chosen to remove organic chloramines precursors. To solve the "trade-off" effect and low rejection of small molecules in algae organic matter, we synthesized a thin film composite (TFC) nanofiltration (NF) membrane with a crumpled polyamide (PA) layer via interfacial polymerization on polyacrylonitrile (PAN) composite support loaded with covalent organic framework (COF) nanoparticles (TpPa-SO₃H). The obtained NF membrane (PA-TpPa-SO₃H/PAN) increased the permeance from 10.2 to 28.2 L m^-2 h^-1 bar^-1 and the amino acid rejection from 24% to 69% compared to the control NF membrane. The addition of TpPa-SO₃H nanoparticles decreased the thickness of PA layers, increased the hydrophilicity of the membrane, and increased the transition energy barrier for amino acids transferring through the membrane, which was identified by scanning electron microscope, contact angle test, and density functional theory computations, respectively. Finally, pre-oxidation coupled with PA-TpPa-SO₃H/PAN membrane nanofiltration on the limitation of organic chloramines formation was evaluated. We found that the combined application of KMnO₄ pre-oxidation and PA-TpPa-SO₃H/PAN membranes nanofiltration in algae-containing water treatment could minimize the formation of organic chloramines in subsequent chlorination and maintain a high flux during filtration. Our work provides an effective way for algae-containing water treatment and organic chloramines control.

Covalent organic frameworks@ZIF-67 derived novel nanocomposite catalyst effectively activated peroxymonosulfate to degrade organic pollutants

Metal organic frameworks-Covalent organic frameworks (MOFs-COFs) nanocomposites could improve the catalytic performance. Herein, a novel nanocomposite catalyst (CC@Co₃O₄) derived from MOFs-COFs (COF@ZIF-67) was prepared on peroxymonosulfate (PMS) activation for bisphenol A (BPA) and rhodamine B (RhB) degradation. Owing to the Co species, oxygen vacancy (O_V), surface hydroxyl (-OH), graphite N and ketone groups (C=O), the CC@Co₃O₄ exhibited higher catalytic degradation performance and total organic carbon (TOC) for BPA (93.8% and 22.3%) and RhB (98.2% and 82.5%) with a small quantity of catalyst (0.10 g/L) and low concentration of PMS (0.20 g/L) even without pH adjustment. Sulfate radicals (•SO₄^-), hydroxyl radicals (•OH), single oxygen (¹O₂), superoxide radicals (•O₂^-) and electron transfer process were all involved in the degradation of BPA and RhB. Among them, the degradation of BPA and RhB mainly depended on •O₂^- and ¹O₂, respectively. Meanwhile, the degradation pathways of BPA and RhB were proposed, and the biotoxicity of the degradation products was evaluated by freshwater chlorella. The results illustrated that the degradation products were environmentally friendly to organisms. In addition, the role of COF in the nanocomposites was also studied. The addition of COF remarkably improved the catalytic performance of CC@Co₃O₄ due to the faster electron transfer, more graphite N and C=O. Overall, this work may open the door to the development of COF-based catalysts in the field of water pollutant remediation.

A Review of Advancing Two-Dimensional Material Membranes for Ultrafast and Highly Selective Liquid Separation

Membrane-based nanotechnology possesses high separation efficiency, low economic and energy consumption, continuous operation modes and environmental benefits, and has been utilized in various separation fields. Two-dimensional nanomaterials (2DNMs) with unique atomic thickness have rapidly emerged as ideal building blocks to develop high-performance separation membranes. By rationally tailoring and precisely controlling the nanochannels and/or nanoporous apertures of 2DNMs, 2DNM-based membranes are capable of exhibiting unprecedentedly high permeation and selectivity properties. In this review, the latest breakthroughs in using 2DNM-based membranes as nanosheets and laminar membranes are summarized, including their fabrication, structure design, transport behavior, separation mechanisms, and applications in liquid separations. Examples of advanced 2D material (graphene family, 2D TMDs, MXenes, metal-organic frameworks, and covalent organic framework nanosheets) membrane designs with remarkably perm-selective properties are highlighted. Additionally, the development of strategies used to functionalize membranes with 2DNMs are discussed. Finally, current technical challenges and emerging research directions of advancing 2DNM membranes for liquid separation are shared.