Guanidinium-Based Ionic Covalent Organic Framework for Rapid and Selective Removal of Toxic Cr(VI) Oxoanions from Water

The synthesis strategies of covalent organic frameworks and advances in their application for adsorption of heavy metal and radionuclide

Covalent organic frameworks (COFs) are a novel type of porous materials, with unique properties, such as large specific surface areas, high porosity, pronounced crystallinity, tunable pore sizes, and easy functionalization, and thus have received considerable attention in recent years. COFs play an essential role in the catalytic degradation, adsorption, and separation of heavy metals, radionuclides. In recent years, considering several outstanding characteristics of COFs, including their good thermal/chemical stability, high crystallinity, and remarkable adsorption capacity, they have been widely used in the removal of various environment pollutants. This review primarily discusses the synthesis strategies of COFs along with their diverse synthesis methods, and provides a comprehensive summary and analysis of recent research advances in the use of COFs for removing heavy metal ions and radionuclides from water bodies. Additionally, the adsorption mechanism of COFs with regard to metal ions was determined by analyzing the structural characteristics of COFs. Finally, the future research directions on COFs adsorb rare earth element was discussed.

Ionic Covalent Organic Frameworks in Adsorption and Catalysis

The ion extraction and electro/photo catalysis are promising methods to address environmental and energy issues. Covalent organic frameworks (COFs) are a class of promising template to construct absorbents and catalysts because of their stable frameworks, high surface areas, controllable pore environments, and well-defined catalytic sites. Among them, ionic COFs as unique class of crystalline porous materials, with charges in the frameworks or along the pore walls, have shown different properties and resulting performance in these applications with those from charge-neutral COFs. In this review, current research progress based on the ionic COFs for ion extraction and energy conversion, including cationic/anionic materials and electro/photo catalysis is reviewed in terms of the synthesis strategy, modification methods, mechanisms of adsorption and catalysis, as well as applications. Finally, we demonstrated the current challenges and future development of ionic COFs in design strategies and applications.

Ultrasonic-Assisted Nitrate Anion Incorporation in Triaminoguanidium Chloride Based Covalent Organic Polymer for Methylene Blue Dye Adsorption

Terephthalaldehyde-triaminoguanidium chloride covalent organic polymer, Te-TGCl COP can facilely be incorporated with NaNO3 by sonication. Te-TGCl COP incorporated with NaNO3 via ultrasonication adsorbs Methylene Blue (MB) dye. Te-TGCl COP alone shows negligible adsorption capacity for MB, however, when treated with NaNO3, its adsorption capacity emerges slightly. Moreover, ultrasonication of the NaNO3 treated COP, Te-TG-NaNO3 COP shows dramatic increase in its adsorption capacity for MB (qe for Te-TGCl COP ≈0 mg g-1; for Te-TG-NaNO3=17.65 mg g-1). Emergence of MB dye adsorption property in Te-TG-NaNO3 COP composite may be attributed primarily to the electrostatic interaction of MB dye molecules with nitrate anions and the sonochemical effect caused fibrous morphological structure of the adsorbent material. The kinetics of MB dye adsorption onto Te-TG-NaNO3 COP composite exhibits an excellent fit for the pseudo-second order model, suggesting the rate-determining step to be chemisorption. Homogeneous monolayer adsorption of MB dye onto Te-TG-NaNO3 COP composite can be suggested as the Langmuir isotherm model seemed to be fitted well.

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.

Multiscale computational simulation of pollutant behavior at water interfaces

The investigation of pollutant behavior at water interfaces is critical to understand pollution in aquatic systems. Computational methods allow us to overcome the limitations of experimental analysis, delivering valuable insights into the chemical mechanisms and structural characteristics of pollutant behavior at interfaces across a range of scales, from microscopic to mesoscopic. Quantum mechanics, all-atom molecular dynamics simulations, coarse-grained molecular dynamics simulations, and dissipative particle dynamics simulations represent diverse molecular interaction calculation methods that can effectively model pollutant behavior at environmental interfaces from atomic to mesoscopic scales. These methods provide a rich variety of information on pollutant interactions with water surfaces. This review synthesizes the advancements in applying typical computational methods to the formation, adsorption, binding, and catalytic conversion of pollutants at water interfaces. By drawing on recent advancements, we critically examine the current challenges and offer our perspective on future directions. This review seeks to advance our understanding of computational techniques for elucidating pollutant behavior at water interfaces, a critical aspect of water research.

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.

The ultramicropore biochar derived from waste distiller's grains for wet-process phosphoric acid purification: Removal performance and mechanisms of Cr(VI)

Solid waste and heavy metal pollution are long-term and challenging subjects in the field of environmental engineering. In this study, we propose a sustainable approach to "treating waste with waste" by utilizing the ultramicropore biochar derived from solid waste distiller's grains as a means to remove Cr(VI) from simulated wastewater and wet phosphoric acid. The biochar prepared in this research exhibit extremely high specific surface areas (up to 2973 m2/g) and a well-developed pore structure, resulting in a maximum Cr(VI) adsorption capacity of 426.0 mg/g and over 99% removal efficiency of Cr(VI). Furthermore, the adsorbent can be reused for up to eight cycles without significant reduction in its Cr(VI) adsorption performance. Mechanistic investigations suggest that the exceptional Cr(VI) adsorption capacity can be attributed to the synergistic effect of electrostatic interaction and reduction adsorption. This study offers an alternative approach for the resource utilization of solid waste distiller's grains, and the prepared biochar holds promise for the removal of Cr(VI) from wastewater and wet-process phosphoric acid.

A novel cationic covalent organic framework as adsorbent for simultaneous removal of methyl orange and hexavalent chromium

The simultaneous removal of toxic, carcinogenic organic dyes and metal ions from water by one material offers significant advantages when fast, facile, and robust water purification is required. Ionic covalent organic frameworks (ICOFs) have the combined properties of COFs and ion exchange resins and are expected to achieve simultaneous capture of heavy metal ions and organic dyes from water. Herein, a novel guanidinium-based ICOF was synthesized using a solvothermal method. Benefitting from the cationic character, porosity and nanoscale pore size of ICOFs, the adsorbent exhibited high simultaneous adsorption capacities of 290 mg g-1 and 158 mg g-1 for methyl orange (MO) and Cr(vi), respectively, and retained more than 90% adsorption capacity after six adsorption-desorption cycles. In addition, based on dual control of size-exclusion and charge-selection, precisely selective adsorption is achieved towards diverse mixed anionic and cationic pollutants. This strategy offers a practical solution for COFs to confront environmental pollution issues.

Rational Fabrication of Ionic Covalent Organic Frameworks for Chemical Analysis Applications

The rapid development of advanced material science boosts novel chemical analytical technologies for effective pretreatment and sensitive sensing applications in the fields of environmental monitoring, food security, biomedicines, and human health. Ionic covalent organic frameworks (iCOFs) emerge as a class of covalent organic frameworks (COFs) with electrically charged frames or pores as well as predesigned molecular and topological structures, large specific surface area, high crystallinity, and good stability. Benefiting from the pore size interception effect, electrostatic interaction, ion exchange, and recognizing group load, iCOFs exhibit the promising ability to extract specific analytes and enrich trace substances from samples for accurate analysis. On the other hand, the stimuli response of iCOFs and their composites to electrochemical, electric, or photo-irradiating sources endows them as potential transducers for biosensing, environmental analysis, surroundings monitoring, etc. In this review, we summarized the typical construction of iCOFs and focused on their rational structure design for analytical extraction/enrichment and sensing applications in recent years. The important role of iCOFs in the chemical analysis was fully highlighted. Finally, the opportunities and challenges of iCOF-based analytical technologies were also discussed, which may be beneficial to provide a solid foundation for further design and application of iCOFs.

Leaf-like ionic covalent organic framework for the highly efficient and selective removal of non-steroidal anti-inflammatory drugs: Adsorption performance and mechanism insights

In recent years, ionic covalent organic frameworks (iCOFs) have become popular for the removal of contaminants from water. Herein, we employed 2-hydroxybenzene-1,3,5-tricarbaldehyde (TFP) and 1,3-diaminoguanidine monohydrochloride (Dg_Cl) to develop a novel leaf-like iCOF (TFP-Dg_Cl) for the highly efficient and selective removal of non-steroidal anti-inflammatory drugs (NSAIDs). The uniformly distributed adsorption sites, suitable pore sizes, and functional groups (hydroxyl groups, guanidinium groups, and aromatic groups) of the TFP-Dg_Cl endowed it with powerful and selective adsorption capacities for NSAIDs. Remarkably, the optimal leaf-like TFP-Dg_Cl demonstrated an excellent maximum adsorption capacity (1100.08 mg/g) for diclofenac sodium (DCF), to the best of our knowledge, the largest adsorption capacity ever achieved for DCF. Further testing under varying environmental conditions such as pH, different types of anions, and multi-component systems confirmed the practical suitability of the TFP-Dg_Cl. Moreover, the prepared TFP-Dg_Cl exhibited exceptional reusability and stability through six adsorption-desorption cycles. Finally, the adsorption mechanisms of NSAIDs on leaf-like TFP-Dg_Cl were confirmed as electrostatic interactions, hydrogen bonding, and π-π interactions. This work significantly supplements to our understanding of iCOFs and provides new insights into the removal of NSAIDs from wastewater.

Study on the coordination conduct and kinetic insights within the oxo-vanadate and organic reductive nitrogen and sulfur functionalities during the reduction coupled adsorption processes: Implications in practical applications

Vanadium(V) is arising wastewater contaminant recently. Although bio-reduction of vanadium(V) is effective, the knowledge of electron transfer pathways and coordination nature by cellular organic functionalities is seriously lacking. Herein, the coordination conduct and kinetic modes for the reduction of V(V) by organic nitrogen and sulfur functionalities in working pHs are comprehensively investigated for the first time. The kinetics follow 3 steps; (1) diffusion of V(V) species, (2) reduction of V(V) to V(IV), and (3) adsorption of existing V species. The diffusion of V(V) is controlled by the protonated =NH₂⁺, -SH₂⁺, -CSH⁺ functional groups and oxo-vanadate speciation. The reduction of V(V) to V(IV) was efficient by -SH than =NH, -NH- , because of the higher oxidation potential of sulfur and which acted as the sole electron donor in the process. The coordination of V(V)/V(IV) species interacted with oxygen, nitrogen and sulfur atoms via parallel orientation and leads to multi-docking or single-ionic interactions, revealing the previously unrecognized track. Hence, the system tested in four types of wastewaters with different pHs and resulted the comprehensive practical applicability of the system. This study proposes a novel tactic to design an efficient V(V) wastewater treatment system by considering its water parameters.

Silk/Polyamidoamine Membranes for Removing Chromium VI from Water

Polyamidoamine hydrogels prepared by the radical post-polymerization of α,ω-bisacrylamide-terminated M-AGM oligomers, in turn obtained by the polyaddition of 4-aminobutylguanidine with N,N'-methylenebisacrylamide, were reinforced with raw silk fibers, which can establish covalent bonds with the polyamidoamine matrix via reaction of the amine groups in the lysine residues with the acrylamide terminals of the M-AGM oligomer. Silk/M-AGM membranes were prepared by impregnating silk mats with M-AGM aqueous solutions and subsequent crosslinking by UV irradiation. The guanidine pendants of the M-AGM units imparted the ability to form strong but reversible interactions with oxyanions, including the highly toxic chromate ions. The potential of the silk/M-AGM membranes to purify Cr(VI)-contaminated water down to the drinkability level, that is, below 50 ppb, was tested by performing sorption experiments both in static (Cr(VI) concentration 20-2.5 ppm) and flow conditions (Cr(VI) concentration 10-1 ppm). After static sorption experiments, the Cr(VI)-loaded silk/M-AGM membranes could easily be regenerated via treatment with a 1 M sodium hydroxide solution. Dynamic tests performed using two stacked membranes and a 1 ppm Cr(VI) aqueous solution reduced Cr(VI) concentration down to 4 ppb. Remarkably, the use of renewable sources, the environmentally friendly preparation process, and the goal achieved meet eco-design requirements.

Chromate Incarceration by Nanojars and Its Removal from Water by Liquid-Liquid Extraction

The unprecedented liquid-liquid extraction of the dinegative chromate ion (CrO₄^2-) from neutral aqueous solutions into aliphatic hydrocarbon solvents using nanojars as extraction agents is demonstrated. Transferring chromate from water into an organic solvent is extremely challenging due to its large hydration energy (ΔG_h° = -950 kJ/mol) and strong oxidizing ability. Owing to their highly hydrophilic anion binding pockets lined by a multitude of hydrogen bond donor OH groups, neutral nanojars of the formula [cis-Cu^II(μ-OH)(μ-4-Rpz)]_n (n = 27-33; pz = pyrazolate anion; R = H or n-octyl) strongly bind the CrO₄^2- ion and efficiently transfer it from water into n-heptane or C₁₁ - C₁₃ isoalkanes (when R = n-octyl). The extracted chromate can easily be recovered from the organic layer by stripping with an aqueous acid solution. Electrospray ionization mass spectrometric, UV-vis and paramagnetic ¹H NMR spectroscopic, X-ray crystallographic, and thermal stability studies in solution and chemical stability studies toward NH₃, methanol, and Ba²⁺ ions are employed to explore the binding of the CrO₄^2- ion by nanojars. Titration of carbonate nanojars [CO₃ ⊂ {Cu(OH)(pz)}_n]^2- with H₂CrO₄ leads to anion exchange and the formation of chromate nanojars [CrO₄ ⊂ {Cu(OH)(pz)}_n]^2-. Details of chromate binding by H-bonding based on single-crystal structures of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₂₈], four pseudopolymorphs of (Bu₄N)₂[CrO₄ ⊂ {Cu(OH)(pz)}₃₁], and also the methoxy-substituted derivative (Bu₄N)₂[CrO₄ ⊂ {Cu₃₁(OH)₃₀(OCH₃)(pz)₃₁}] are presented.

Covalent Triazine Framework C6N6 as an Electrochemical Sensor for Hydrogen-Containing Industrial Pollutants. A DFT Study

Industrial pollutants pose a serious threat to ecosystems. Hence, there is a need to search for new efficient sensor materials for the detection of pollutants. In the current study, we explored the electrochemical sensing potential of a C₆N₆ sheet for H-containing industrial pollutants (HCN, H₂S, NH₃ and PH₃) through DFT simulations. The adsorption of industrial pollutants over C₆N₆ occurs through physisorption, with adsorption energies ranging from -9.36 kcal/mol to -16.46 kcal/mol. The non-covalent interactions of analyte@C₆N₆ complexes are quantified by symmetry adapted perturbation theory (SAPT0), quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses. SAPT0 analyses show that electrostatic and dispersion forces play a dominant role in the stabilization of analytes over C₆N₆ sheets. Similarly, NCI and QTAIM analyses also verified the results of SAPT0 and interaction energy analyses. The electronic properties of analyte@C₆N₆ complexes are investigated by electron density difference (EDD), natural bond orbital analyses (NBO) and frontier molecular orbital analyses (FMO). Charge is transferred from the C₆N₆ sheet to HCN, H₂S, NH₃ and PH₃. The highest exchange of charge is noted for H₂S (-0.026 e^-). The results of FMO analyses show that the interaction of all analytes results in changes in the E_H-L gap of the C₆N₆ sheet. However, the highest decrease in the E_H-L gap (2.58 eV) is observed for the NH₃@C₆N₆ complex among all studied analyte@C₆N₆ complexes. The orbital density pattern shows that the HOMO density is completely concentrated on NH₃, while the LUMO density is centred on the C₆N₆ surface. Such a type of electronic transition results in a significant change in the E_H-L gap. Thus, it is concluded that C₆N₆ is highly selective towards NH₃ compared to the other studied analytes.

Guanidine-Functionalized Fluorescent sp2 Carbon-Conjugated Covalent Organic Framework for Sensing and Capture of Pd(II) and Cr(VI) Ions

Palladium is a key element in fuel cells, electronic industries, and organic catalysis. At the same time, chromium is essential in leather, electroplating, and metallurgical industries. However, their unpremeditated leakage into aquatic systems has caused human health and environmental apprehensions. Herein, we reported the development of an sp² carbon-conjugated fluorescent covalent organic framework with a guanidine moiety (sp² c-gCOF) that showed excellent thermal and chemical stability. The sp² c-gCOF showed effective sensing, capture, and recovery/removal of Pd(II) and Cr(VI) ions, which could be due to the highly accessible pore walls decorated with guanidine moieties. The fluorescent sp² c-gCOF showed higher selectivity for Pd(II) and Cr(VI) ions, with an ultra-low detection limit of 2.7 and 3.2 nM, respectively. The analysis of the adsorption properties with a pseudo-second-order kinetic model showed that sp² c-gCOF could successfully and selectively remove both Pd(II) and Cr(VI) ions from aqueous solutions. The polymer also showed excellent capture efficacy even after seven consecutive adsorption-desorption cycles. Hence, this study reveals the potential of fluorescent sp² c-gCOF for detecting, removing, and recovering valuable metals and hazardous ions from wastewater, which would be useful for economic benefit, environmental safety, human health, and sustainability. The post-synthetic modification of sp² c-COF with suitable functionalities could also be useful for sensing and extracting other water pollutants and valuable materials from an aqueous system.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

One-Step Synthesis of Cationic Covalent Organic Frameworks

Ionic covalent organic frameworks (iCOFs) have attractive properties that make them suitable for use as ion transport materials, as energy storage media, and for metal sorption. However, the synthetic pathways to prepare iCOFs are limited. Herein, we prepare an iCOF via a single-step reaction. The synthesized materials were isolated as polycrystalline nanowires. The theoretical and experimental data reveal that the synthesized iCOFs are predominately assembled into staggered configurations. The materials exhibit an uptake capacity of 3.5 g·g^-1 for iodine. The ab initio calculations point to the role of bromide counterions, forming I₂Br^- as stable ions within the framework.

Covalent Organic Frameworks with Ionic Liquid-Moieties (ILCOFs): Structures, Synthesis, and CO2 Conversion

CO₂, an acidic gas, is usually emitted from the combustion of fossil fuels and leads to the formation of acid rain and greenhouse effects. CO₂ can be used to produce kinds of value-added chemicals from a viewpoint based on carbon capture, utilization, and storage (CCUS). With the combination of unique structures and properties of ionic liquids (ILs) and covalent organic frameworks (COFs), covalent organic frameworks with ionic liquid-moieties (ILCOFs) have been developed as a kind of novel and efficient sorbent, catalyst, and electrolyte since 2016. In this critical review, we first focus on the structures and synthesis of different kinds of ILCOFs materials, including ILCOFs with IL moieties located on the main linkers, on the nodes, and on the side chains. We then discuss the ILCOFs for CO₂ capture and conversion, including the reduction and cycloaddition of CO₂. Finally, future directions and prospects for ILCOFs are outlined. This review is beneficial for academic researchers in obtaining an overall understanding of ILCOFs and their application of CO₂ conversion. This work will open a door to develop novel ILCOFs materials for the capture, separation, and utilization of other typical acid, basic, or neutral gases such as SO₂, H₂S, NOx, NH₃, and so on.

Fluorescence turn on amine detection in a cationic covalent organic framework

Ionic covalent organic frameworks (iCOFs) are new examples of porous materials and have shown great potential for various applications. When functionalized with suitable emission sites, guest uptake via the ionic moieties of iCOFs can cause a significant change in luminescence, making them excellent candidates for chemosensors. In here, we present a luminescence sensor in the form of an ionic covalent organic framework (TGH⁺•PD) composed of guanidinium and phenanthroline moieties for the detection of ammonia and primary aliphatic amines. TGH⁺•PD exhibits strong emission enhancement in the presence of selective primary amines due to the suppression of intramolecular charge transfer (ICT) with an ultra-low detection limit of 1.2 × 10^‒7 M for ammonia. The presence of ionic moieties makes TGH⁺•PD highly dispersible in water, while deprotonation of the guanidinium moiety by amines restricts its ICT process and signals their presence by enhanced fluorescence emission. The presence of ordered pore walls introduces size selectivity among analyte molecules, and the iCOF has been successfully used to monitor meat products that release biogenic amine vapors upon decomposition due to improper storage.

Ionic covalent organic frameworks (iCOFs) are new examples of porous materials and show great potential for various applications. Here, the authors demonstrate functionalization of an iCOFs with suitable emission sites and application as chemosensor for amine detection with high sensitivity which can be used to monitor meat spoilage.

Facile synthesis of a novel magnetic covalent organic frameworks for extraction and determination of five fungicides in Chinese herbal medicines

A novel magnetic covalent organic framework was synthesized via one step coating approach with solvothermal reaction employing 2,4,6-tris(4-aminophen-yl)-1,3,5-triazine and 2,4,6-triformylphloroglucinol as two building blocks by covalent bonding. The prepared magnetic covalent organic frameworks were properly characterized by different techniques and employed as adsorbent of magnetic solid phase extraction. An analytical method was developed for simultaneous determination of five fungicides in two Chinese herbal medicine samples via magnetic solid phase extraction coupled to UHPLC-MS/MS analysis. Under optimized magnetic solid phase extraction conditions, the method exhibited satisfactory recoveries (74.0-109.6%) with the relative standard deviations of 0.4-4.6%, low limits of detection (0.003-0.015 μg kg^-1 ), and good linearity (R² > 0.9960). Compared with the traditional extraction method, the proposed method required a lower amount of adsorbent (3 mg) and extraction time (5 min). The adsorbent also had favourable reusability (not less than 8 times). Therefore, the magnetic covalent organic frameworks could be a promising adsorbent for the extraction and quantitation of pesticide residues in Chinese herbal medicines. This article is protected by copyright. All rights reserved.

Porous Organic Polymers via Diels-Alder Reaction for the Removal of Cr(VI) from Aqueous Solutions

The Diels-Alder reaction is striking as a prominent synthetic tool for the rapid construction of complex molecular frameworks, but the synthesis of porous organic polymers (POPs) via the Diels-Alder reaction is rare. Herein, we report the solvothermal synthesis of a new type of POPs (DA-POPs) via the furan/alkynyl Diels-Alder reaction. These polymers show favorable porous properties and high specific surface areas (up to 1041 m²·g^-1). Meanwhile, the high porosity in conjuction of ether bridges in the DA-POPs enable a fine adsorption performance for the removal of Cr(VI) from aqueous solutions.

Production of polyacrylonitrile/ionic covalent organic framework hybrid nanofibers for effective removal of chromium(VI) from water

Hexavalent Cr(VI) found in industrial wastewater is a proven carcinogen which causes serious health issues in humans around the world. This study presents a novel method to enhance the Cr(VI) oxyanion removal from wastewater by polyacrylonitrile (PAN) nanofibers through incorporation of a guanidinium-based ionic covalent organic framework (BT-DG) in the nanofibers structure. Simple electrospinning technique was employed to produce PAN nanofibers and BT-DG was synthesized through condensation between benzene-1,3,5-tricarbaldehyde and N,N'-diaminoguanidine monohydrochloride. In-situ polymerization of BT-DG onto PAN nanofibers resulted in generation of hybrid PAN-BT-DG nanofibers. This modified PAN-BT-DG was characterized by obtaining its point of zero charge (PZC), differential scanning calorimeter (DSC), scanning electron microscopy (SEM) morphology and surface elements and oxidation states by X-ray photoelectron spectroscopy (XPS). PAN-BT-DG exhibited positive surface charge below pH 4, making it an outstanding adsorbent, for Cr(VI) removal. Cr(VI) adsorption onto PAN-BT-DG followed pseudo second order kinetics and adsorption data fitted well to Freundlich isotherm model. Highest Cr(VI) removal was obtained at 55 ℃ with a maximum Langmuir adsorption capacity of 173 mg/g at pH 3. Kinetic studies revealed that Cr(VI) adsorption onto PAN-BT-DG is endothermic and thermodynamically feasible. Desorption studies were conducted on PAN-BT-DG using 1 M NaOH as the stripping solvent and PAN-BT-DG exhibited excellent regeneration after five consecutive cycles.

Porous polyelectrolyte frameworks: synthesis, post-ionization and advanced applications

Porous organic polymers (POPs), which feature high surface areas, robust skeletons, tunable pores, adjustable functionality and versatile applicability, have constituted a designable platform to develop advanced organic materials. Endowing polyelectrolytes with the distinct characteristics of POPs will attract mounting interest as the structural diversity of polyelectrolytes will bring the new hope of intriguing applications and potential benefits. In this review, the striking progress in ionized POPs (i-POPs) has been systematically summarized with regard to their synthetic strategies and applications. In the synthesis of i-POPs, we illustrate the representative ionic building blocks and charged functional groups capable of constructing the polyelectrolyte frameworks. The synthetic methods, including direct synthesis and post-modification, are detailed for the i-POPs with amorphous or crystalline structures, respectively. Subsequently, we outline the distinctive performances of i-POPs in adsorption, separation, catalysis, sensing, ion conduction and biomedical applications. The survey concerns the interplay between the surface chemistry, ionic interaction and pore confinement that cooperatively promote the performance of i-POPs. Finally, we conclude with the remaining challenges and promising opportunities for the on-going development of i-POPs.

Ionic Covalent Organic Frameworks for Energy Devices

Covalent organic frameworks (COFs) are a class of porous crystalline materials whose facile preparation, functionality, and modularity have led to their becoming powerful platforms for the development of molecular devices in many fields of (bio)engineering, such as energy storage, environmental remediation, drug delivery, and catalysis. In particular, ionic COFs (iCOFs) are highly useful for constructing energy devices, as their ionic functional groups can transport ions efficiently, and the nonlabile and highly ordered all-covalent pore structures of their backbones provide ideal pathways for long-term ionic transport under harsh electrochemical conditions. Here, current research progress on the use of iCOFs for energy devices, specifically lithium-based batteries and fuel cells, is reviewed in terms of iCOF backbone-design strategies, synthetic approaches, properties, engineering techniques, and applications. iCOFs are categorized as anionic COFs or cationic COFs, and how each of these types of iCOFs transport lithium ions, protons, or hydroxides is illustrated. Finally, the current challenges to and future opportunities for the utilization of iCOFs in energy devices are described. This review will therefore serve as a useful reference on state-of-the-art iCOF design and application strategies focusing on energy devices.

Bifunctional Ionic Covalent Organic Networks for Enhanced Simultaneous Removal of Chromium(VI) and Arsenic(V) Oxoanions via Synergetic Ion Exchange and Redox Process

Chromium (VI) and arsenic (V) oxoanions are major toxic heavy metal pollutants in water threatening both human health and environmental safety. Herein, the development is reported of a bifunctional ionic covalent organic network (iCON) with integrated guanidinium and phenol units to simultaneously sequester chromate and arsenate in water via a synergistic ion-exchange-redox process. The guanidinium groups facilitate the ion-exchange-based adsorption of chromate and arsenate at neutral pH with fast kinetics and high uptake capacity, whereas the integrated phenol motifs mediate the Cr(VI)/Cr(III) redox process that immobilizes chromate and promotes the adsorption of arsenate via the formation of Cr(III)-As(V) cluster/complex. The synergistic ion-exchange-redox approach not only pushes high adsorption efficiency for both chromate and arsenate but also upholds a balanced Cr/As uptake ratio regardless of the change in concentration and the presence of interfering oxoanions.

A review on water treatment technologies for the management of oxoanions: prospects and challenges

Oxoanions are a class of contaminants that are easily released into the aquatic systems either through natural or anthropogenic activities. Depending on their oxidation states, they are highly mobile, resulting in the contamination of underground water. Above the permissible level in groundwater, they pose as threats to mammals when the contaminated water is consumed. Some of the health challenges caused are cancer, neurological, cardiac, gastrointestinal, and skin disorders. Several treatment technologies have been adopted over the years for the management of these oxoanions present in the aquatic systems. However interesting these treatment technologies might be, they also have their limitations such as cost-effectiveness, the complexity of the process, and generation of secondary pollutants. This work focused on some of the water treatment technologies applied for the removal of oxoanions. Some of the advantages and disadvantages of these treatment technologies are also highlighted. Amongst all the treatment technologies, adsorption is the most applied method for the removal of oxoanions. However, photocatalysis has a higher prospect since it is non-selective and secondary pollutants are not generated after the treatment process. Also, photocatalysis can simultaneously reduce and oxidise oxoanions as well as organic pollutants respectively.

Field-scale studies on the change of soil microbial community structure and functions after stabilization at a chromium-contaminated site

Various remediation strategies have been developed to eliminate soil chromium (Cr) contamination which challenges the ecosystem and human health, and chemical stabilization is the most popular one. Limited work focuses on the change of soil microbial community and functions after chemical stabilization. The present study examined the diversity and structure of bacterial, fungal and archaeal communities in 20 soils from a Cr-contaminated site in China after chemical stabilization and ageing. Cr contamination significantly reduced microbial diversity and shaped microbial community structure. After chemical stabilization, bacterial and fungal communities had higher richness and evenness, whereas archaea behaved oppositely. Microbial community structure after stabilization were more similar to uncontaminated soils. Among all environmental variables, pH and Al explained 25.2% and 9.4% of the total variance of bacterial diversity, whereas the major variable affecting fungal community was pH (29.3%). Cr, organic matters, extractable-Al and moisture explained 25.8%, 22.4%, 9.9% and 9.9% of the total variance in archaeal community, respectively. This work for the first time unraveled the change of the whole soil microbial community structures and functions at Cr-contaminated sites after chemical stabilization on field scale and proved chemical stabilization as an effective approach to detoxicate Cr(VI) and recover microbial communities in soils.

Highly efficient Lewis acid catalytic activity of the tritylium ion at the node of a tensile organic framework

Tritylium salts have been used as Lewis acid catalysts in organic synthesis for a long time. In this work, we found that the Lewis acid catalytic activity of tritylium ions at the node of a tensile framework is significantly improved compared to that of the free tritylium salts. The tritylium-based framework, PAF-201 (PAF, porous aromatic framework), was prepared by acidification of a semi-rigid triphenylcarbinol-based parent framework, PAF-200. When PAF-200 was alternately exposed to HCl and NH3 gas, a fast allochroic cycle was observed due to repeated formation of tritylium species. Interestingly, the pseudo-first-order reaction rate of a Povarov model reaction catalyzed by PAF-201 as a Lewis acid was ∼3.7 times and ∼4.7 times as those of tritylium tetrafluoroborate and tri(4-biphenyl)carbonium tetrafluoroborate, respectively. Theoretical calculations revealed that the tritylium ion at the node of PAF-201 has a quasi-planar structure. The transformation of triphenylcarbinol in PAF-200 to tritylium in PAF-201 can make the framework taut, and the rebounding force toward the tetrahedral structure is stored. This is favorable for tritylium to activate the imine substrate along with a deformation of the quasi-plane to tetrahedron. PAF-201 could be easily recycled at least three times without evident loss of catalytic activity. This work presents the catalytic activity of the tritylium ion under stress.

Difunctional covalent organic framework hybrid material for synergistic adsorption and selective removal of fluoroquinolone antibiotics

Due to the low efficiency of traditional sewage treatment methods, the effective removal of zwitterionic fluoroquinolone (FQs) antibiotics is of vital significant for environment protection. In this work, a SO₃H-anchored covalent organic framework (TpPa-SO₃H) was deliberately designed by linking phenolic trialdehyde with triamine through Schiff reaction, then low-content Tb³⁺ ions were loaded onto covalent organic framework according to wet-chemistry immersion dispersion method which benefitting for efficient FQs antibiotics uptaking. Tb@TpPa-SO₃H functionalized with regularly distributed sulfonic acid groups and terbium ions which could provide difunctional binding sites. Tb³⁺ sites could capture carboxylic acid group of FQs molecules according to the complexes coordination effect and sulfonic acid sites play a significant role in the adsorption of FQs molecules through electrostatic interaction with amine group. Tb@TpPa-SO₃H with dual complementary function sites exhibited ultra-fast adsorption kinetics (< 2 min, average over 99% removing rate) and high adsorption capacities of 989, 956, and 998 mg g^-1 for Norfloxacin (NOR), ciprofloxacin (CIP), enrofloxacin (ENR), respectively. Furthermore, Tb@TpPa-SO₃H showed excellent selectivity for the adsorption of FQs in tanglesome system. This work not only explored synergistic adsorption in ion-functionalized 2D covalent organic framework with dual binding sites, but also delineated a promising strategy for the elimination of organic pollutants in environmental remediation.

Electrochemical recovery and high value-added reutilization of heavy metal ions from wastewater: Recent advances and future trends

Wastewater treatment for heavy metals is currently transitioning from pollution remediation towards resource recovery. As a controllable and environment-friendly method, electrochemical technologies have recently gained significant attention. However, there is a lack of systematic and goal oriented summarize of electrochemical metal recovery techniques, which has inhibited the optimized application of these methods. This review aims at recent advances in electrochemical metal recovery techniques, by comparing different electrochemical recovery methods, attempts to target recycling heavy metal resources with minimize energy consumption, boost recovery efficiency and realize the commercial application. In this review, different electrochemical recovery methods (including E-adsorption recovery, E-oxidation recovery, E-reduction recovery, and E-precipitation recovery) for recovering heavy metals are introduced, followed an analysis of their corresponding mechanisms, influencing factors, and recovery efficiencies. In addition, the mass transfer efficiency can be promoted further through optimizing electrodes and reactors, and multiple technologies (photo-electrochemical and sono-electrochemical) could to be used synergistically improve recovery efficiencies. Finally, the most promising directions for electrochemical recovery of heavy metals are discussed along with the challenges and future opportunities of electrochemical technology in recycling heavy metals from wastewater.

Recent Advances in Covalent Organic Frameworks for Heavy Metal Removal Applications

Covalent organic frameworks comprise a unique class of functional materials that has recently emerged as a versatile tool for energy-related, photocatalytic, environmental, and electrochromic device applications. A plethora of structures can be designed and implemented through a careful selection of ligands and functional units. On the other hand, porous materials for heavy metal absorption are constantly on the forefront of materials science due to the significant health issues that arise from the release of the latter to aquatic environments. In this critical review, we provide insights on the correlation between the structure of functional covalent organic frameworks and their heavy metal absorption. The elements we selected were Pb, Hg, Cr, Cd, and As metal ions, as well as radioactive elements, and we focused on their removal with functional networks. Finally, we outline their advantages and disadvantages compared to other competitive systems such as zeolites and metal organic frameworks (MOFs), we analyze the potential drawbacks for industrial scale applications, and we provide our outlook on the future of this emerging field.

Covalent organic frameworks as robust materials for mitigation of environmental pollutants

Today, one of the main leading global problems is the presence of different pollutants in the environment. These pollutants not only affect human health but also overshadow the life of other creatures. Thus, pollutant treatment has become a challenging issue among the researchers and the scientific community. Different adsorbents and catalysts have been applied to the removal of pollutants. However, the associated limitations like poor chemical and physical stability, low surface area and low binding capacity revived researchers' attention to exploring alternative materials. Covalent organic frameworks (COFs) are versatile materials created based on the strong covalent interactions between blocked monomers. Unique features, including high specific surface area, high chemical-physical stability and crystallinity render COFs an intriguing sorbent and catalyst in treating pollutants. This review spotlights the applications of COFs as distinguished adsorbents to remove hazardous pollutants from the environment. At first, COFs and their properties as alternative materials were introduced. Then, different synthesis approaches of COFs and their advantages and disadvantages were discussed. Furthermore, the applications of COFs outlined to remove a wide variety of pollutants based on adsorption and degradation. Finally, the prospects of COFs for the treatment of pollutants were evaluated.