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Novel pyridine-based covalent organic framework containing N,N,N-chelating sites for selective detection and effective removal of nickel
Abstract
The development of novel materials with dual functions of simultaneous detection and removal of heavy metal ions has been an important pursuit of environmental remediation. Herein, we demonstrate that two-dimensional covalent organic frameworks (COFs) with well-defined functional chelating sites can combine the inherent advantages of COF materials to prepare novel materials with the dual functions of selective detection and effective removal of Ni²⁺ from aqueous solutions. A new pyridine-based COF material (TAPA-PCBA) containing functional N,N,N-chelating sites has been designed and synthesized by the imine condensation reactions between tris(4-aminophenyl)amine and 2,6-pyridinedicarboxaldehyde under solvothermal conditions. The newly designed TAPA-PCBA possesses high crystallinity, moderate specific surface area and robust chemical and thermal stability. These excellent properties of TAPA-PCBA, together with its extended π-conjugated framework structure and the dense distribution of functional N,N,N chelating sites, enable TAPA-PCBA to selectively detect and effectively remove Ni²⁺ from aqueous solutions. This dual function can be attributed to the coordination interactions between Ni²⁺ and N,N,N-chelating sites embedded in the skeleton structure of TAPA-PCBA. These results reveal that COFs with reasonable design have great development prospects in heavy metal ion related environmental remediation.
Critical metals play a pivotal role in driving the emerging era of renewable technologies, including wind turbines, solar panels, and water electrolysis. However, their importance has surged over the years, primarily due to diminishing primary sources and a lack of viable alternatives. This escalating disparity between demand and supply necessitates a proactive approach to explore supplementary resources. Among the potential avenues, secondary sources such as e-waste, nuclear waste, and seawater offer promising alternatives for critical metal production through the concept of urban mining. In this context, the hydrometallurgical route has emerged as a notably efficient methodology. Notably, solid-phase extraction (SPE) within hydrometallurgy presents an environmentally friendly avenue, yielding reduced hydrocarbon waste. This method showcases superior efficacy, particularly in scenarios where target metals exist in low concentrations, surpassing the conventional liquid–liquid extraction techniques. Central to the successful extraction of critical metals from waste leachate solutions, primarily in robust acidic environments, lies the development of stable and high-performing sorbents. The recent emergence of covalent organic frameworks (COFs) holds considerable promise due to their intricate architecture, porous nature, and versatile design capabilities. This comprehensive review delves into the myriad applications of COF materials as effective sorbents within the realm of solid-phase extraction, facilitating the recovery of metals from secondary sources. The discussion encompasses various manifestations of COFs, including their employment as powders, composites, and components of mixed matrix membranes. Additionally, the review elucidates the methodologies employed for desorbing the extracted metals and subsequently regenerating COFs, enabling multiple extraction cycles. By offering a holistic insight into the structural nuances of COFs and their pivotal role in recycling critical metals from waste, this review not only advances our understanding but also serves as a guidepost for the future design and development of novel COFs for this crucial endeavor.
‘Life on Earth’ depends greatly on the quality of drinkable water, influenced by its origin, flow path and exposure to several environmental conditions. The ever-increasing human population results in a rapid increase in urbanization, industrialization and chemical intensive farming that pollutes various water resources. Among these, oily wastewater discharge from several industries like, textile, tannery, petroleum, food, steel etc. is increasing day-by-day; therefore, necessitating the ethos of ‘oil/water separation’ a major issue nowadays. In addition to this, clean-up of toxic organic dyes and lethal inorganic heavy metals (HMs) is also of high importance, catering the motto of ‘Right to water and sanitation’ by UNESCO. Smart ‘Porous Materials’ can be a highly promising candidate in this regard. This chapter included brief examples and water-treatment related applications of several distinct category of porous materials, which are at the forefront of 21st century's research world owing to its modular nature, easy-to-tune functionality and tailorable porosity.
Covalent organic frameworks (COFs) have emerged as significant candidates for visible-light photocatalysis due to their ability to regulate performance which is achieved through the careful selection of building modules, framework conjugation, and post-modification. This report focused on the efficient transformation of an imine-linked I-COF into a π-conjugated quinoline-based Q-COF, which enhanced both the chemical stability and conjugation of the network. By methylating the pyridyl groups in the Q-COF, an N+-COF was obtained. Subsequently, the Ru(N^N)3-photosensitizer ([Ru(dcbpy)3]4-) was incorporated into the channels of the cationic N+-COF through electrostatic interactions, resulting in the formation of [Ru(dcbpy)3]4-⊂N+-COF. This composite exhibited exceptional photocatalytic activity, demonstrating high yields and selectivity in the oxidation of sulfides or amines to their respective products.
Covalent organic frameworks (COFs) are promising in the detection and removal of toxic heavy metal ions from water. However, achieving detection and removal functions requires systematic control of stability, porosity, pore environment, and luminescence properties, which remains a challenge for most COFs materials. Herein, we develop a bithiophene-based stable COF, TAPA-BTDC, with dual functions of simultaneous detection and removal of silver ions from aqueous solution through elaborate structural design and control of the pore environment. The obtained TAPA-BTDC has good thermal stability and chemical stability, possesses high crystallinity and moderate specific surface area (396.83 m²/g), and contains dense bithiophene-S chelating sites on the pore walls. These characteristics work together to give TAPA-BTDC the dual function of selective detection and effective removal of silver ions from aqueous solution. This dual function can be attributed to the π-complexation between bithiophene-S chelating sites and silver ions, as verified by X-ray photoelectron spectroscopy. The charge transfer between bithiophene-S and silver ions alters the weak fluorescence emission caused by the π−π layered structure, enabling TAPA-BTDC to selectively detect silver ions through fluorescence enhancement, and even trace amounts of Ag⁺ (2.4 × 10⁻⁴ mmol/L) can cause observable fluorescence emission. Meanwhile, these bithiophene-S scattered across the pore wall also provide a large number of accessible chelating sites for TAPA-BTDC, enabling it to effectively remove Ag⁺ with a maximum adsorption capacity up to 398.61 mg/g, and TAPA-BTDC can be easily regenerated five times without loss of performance. The results of this study reveal the exceptional potential of COFs with reasonably tailor-made structure in dealing with heavy metal ion contamination.
The development of new dual-functional materials for detection and removal of heavy metal ions is of great significance in environmental protection. Herein, a new amine-functionalized β-ketoenamine-linked covalent organic framework (termed as NH2–Th-Tfp COF) containing rich O,N,O′-chelating sites and free amine groups has been synthesized through the site-selective synthesis strategy under solvothermal conditions. The as-synthesized NH2–Th-Tfp COF shows strong fluorescence in water dispersion and can be used as highly sensitive and selective fluorescent probe for Cu²⁺ detection. As a sensing platform, the limit of detection for NH2–Th-Tfp COF toward Cu²⁺ ion is estimated to be 0.19 μM. Besides, the NH2–Th-Tfp COF possesses high adsorption capacity of 153 mg/g toward Cu²⁺ ion in aqueous solution. More importantly, this NH2–Th-Tfp COF exhibits higher sensitivity and larger adsorption capacity toward Cu²⁺ ion in comparison with the non-amine functionalized Th-Tfp COF, which may be attributed to the large number of free amine groups in the pore walls of NH2–Th-Tfp COF. The coordination interactions between Cu²⁺ ion with the O, N, O′-chelating sites and free amine groups in the pore walls of NH2–Th-Tfp COF can greatly enhance its recognition and absorption performances toward Cu²⁺ ion, as verified by X-ray photoelectron spectroscopy. This work may pave the way for designing new fluorescent COF materials with dual functions for simultaneous detection and removal of specific metal ions.
Following the advancements and diversification in synthetic strategies for porous covalent materials in the literature, the materials science community started to investigate the performance of covalent organic polymers (COPs) and covalent organic frameworks (COFs) in applications that require large surface areas for interaction with other molecules, chemical stability, and insolubility. Sensorics is an area where COPs and COFs have demonstrated immense potential and achieved high levels of sensitivity and selectivity on account of their tunable structures. In this review, we focus on those covalent polymeric systems that use fluorescence spectroscopy as a method of detection. After briefly reviewing the physical basis of fluorescence-based sensors, we delve into various kinds of analytes that have been explored with COPs and COFs, namely, heavy metal ions, explosives, biological molecules, amines, pH, volatile organic compounds and solvents, iodine, enantiomers, gases, and anions. Throughout this work, we discuss the mechanisms involved in each sensing application and aim to quantify the potency of the discussed sensors by providing limits of detection and quenching constants when available. This review concludes with a summary of the surveyed literature and raises a few concerns that should be addressed in the future development of COP and COF fluorescence-based sensors.
Understanding the interaction of amino acids with metal surfaces is essential for the rational design of chiral modifiers able to confer enantioselectivity to metal catalysts. We present here an investigation of the adsorption of aspartic acid (Asp) on the Ni{100} surface, using a combination of synchrotron X-Ray Photoelectron Spectroscopy (XPS) and Near-Edge X-ray Absorption Fine Structure (NEXAFS), and Density Functional Theory (DFT) simulations. Based on the combined analysis of the experimental and simulated data, we can identify the dominant mode of adsorption as a pentadentate configuration with three O atoms at the bridge sites of the surfaces, and the remaining oxygen atom and the amino nitrogen are located on atop sites. From temperature-programmed XPS measurements it was found that Asp starts decomposing above 400 K, which is significantly higher than typical decomposition temperatures of smaller organic molecules on Ni surfaces. Our results offer valuable insights for understanding the role of Asp as a chiral modifier of nickel catalyst surfaces for enantioselective hydrogenation reactions.
The cellulose-based materials are new types of advanced materials combining pristine cellulose with functional materials for specific applications. In this study, a novel cotton fiber (CF)–covalent organic framework (COF) hybrid monolith was prepared for the efficient capture of iodine in vapor and solution. The cotton fibers were first modified with (3-aminopropyl)trimethoxysilane via silylation reaction to generate amino functions on the surface of the fibers, and then the COFs grafted subsequently. The obtained CF/COF hybrid monolith has been characterized by infrared spectra, powder X-ray diffraction, elemental analysis, scanning electron microscope, thermo-gravimetric analysis, and nitrogen adsorption–desorption analysis, and then it was used as adsorbent for removing iodine. The grafting of COFs on the cotton fiber matrix was calculated to be about 12.28 wt%, which increased the BET specific surface area of the cotton fiber from 1.9 to 166 m²/g. The CF/COF monolith displayed excellent capture ability for iodine vapor with adsorption capacity up to 823.9 mg/g, and it also showed remarkable adsorption ability for iodine in cyclohexane solution. The hybrid monolith can be easily regenerated by washing with methanol, and it showed good reusability. Moreover, the CF/COF monolith presented good thermal stability with a decomposition temperature above 300 °C. This strategy for combining COFs with cotton fibers will pave the way for the development of novel efficient adsorption materials for radioiodine during nuclear waste disposal.
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By using a rapid cryogenic reaction, cotton fiber was covalently functionalized with imine-linked two-dimensional covalent organic frameworks (2D COFs), which was synthesized via an imine condensation reaction between 1,3,5-tris(4-aminophenyl)benzene and terephthaldehyde. The resulting functionalized cotton (COFs@cotton) was characterized by fourier transform infrared spectroscopy, powder X-ray diffraction, scanning electron microscope, thermo-gravimetric analysis, nitrogen adsorption–desorption analysis and elemental analysis. The presence of a lot of imine functional groups in combination with dispersed mesoporous holes results in rapid adsorption rate and relatively high iodine affinity with uptake capacities up to 533.9 mg/g, which was much higher than that of untreated cotton. In addition, COFs@cotton also shows remarkable capability as absorbent for iodine in solution. The adsorbed iodine can be completely extracted from COFs@cotton by being washed with ethanol, permitting the simple reuse of adsorbent. This new functionalization approach can be applied to different kind of imine-linked COFs, and the corresponding adsorbents show more extensive practical application prospect for iodine capture.
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Covalent organic frameworks (COFs) as drug-delivery carriers have been mostly evaluated in vitro due to the lack of COFs nanocarriers that are suitable for in vivo studies. Here we develop a series of water-dispersible polymer-COF nanocomposites through the assembly of polyethylene-glycol-modified monofunctional curcumin derivatives (PEG-CCM) and amine-functionalized COFs (APTES-COF-1) for in vitro and in vivo drug delivery. The real-time fluorescence response shows efficient tracking of the COF-based materials upon cellular uptake and anticancer drug (doxorubicin (DOX)) release. Notably, in vitro and in vivo studies demonstrate that PEG-CCM@APTES-COF-1 is a smart carrier for drug delivery with superior stability, intrinsic biodegradability, high DOX loading capacity, strong and stable fluorescence, prolonged circulation time and improved drug accumulation in tumors. More intriguingly, PEG 350-CCM@APTES-COF-1 presents an effective targeting strategy for brain research. We envisage that PEG-CCM@APTES-COF-1 nanocomposites represent a great promise toward the development of a multifunctional platform for cancer-targeted in vivo drug delivery.
Covalent organic frameworks (COFs) are crystalline porous polymers formed by a bottom‐up approach from molecular building units having a predesigned geometry that are connected through covalent bonds. They offer positional control over their building blocks in two and three dimensions. This control enables the synthesis of rigid porous structures with a high regularity and the ability to fine‐tune the chemical and physical properties of the network. This Feature Article provides a comprehensive overview over the structures realized to date in the fast growing field of covalent organic framework development. Different synthesis strategies to meet diverse demands, such as high crystallinity, straightforward processability, or the formation of thin films are discussed. Furthermore, insights into the growing fields of COF applications, including gas storage and separations, sensing, electrochemical energy storage, and optoelectronics are provided.
We report the H2 uptake properties of six covalent organic frameworks (COFs) from first-principles-based grand canonical Monte-Carlo simulations. The predicted H2 adsorption isotherm is in excellent agreement with the only available experimental result (3.3 vs 3.4 wt % at 50 bar and 77 K for COF-5), also reported here, validating the predictions. We predict that COF-105 and COF-108 lead to a reversible excess H2 uptake of 10.0 wt % at 77 K, making them the best known storage materials for molecular hydrogen at 77 K. We predict that the total H2 uptake for COF-108 is 18.9 wt % at 77 K. COF-102 shows the best volumetric performance, storing 40.4 g/L of H2 at 77 K. These results indicate that the COF systems are most promising candidates for practical hydrogen storage.
The creation of defects into luminescent MOFs allows for manipulation of fluorescent properties and thus leads to the enhanced detection performance for technological applications. Herein, a modulator-induced defect formation strategy was proposed to introduce missing-linker defect into lanthanide-based metal-organic framework ([email protected]) and the enhanced detection properties was investigated. Notably, the missing-linker numbers in per Zr6 cluster were calculated to facilitate comparison and quantitively interpretation. By elaborately tailoring defect numbers, the resultant [email protected] sensor with 0.53 missing linkers per Zr-O cluster showed a remarkably enhanced response slope and realized excellent trace detection under the range of 0-10 ppm with ultralow detection limit to 5.67×10⁻⁷ M (114 ppb), acting as a powerful signal amplifier for trace detection of Cd²⁺. Moreover, the relationship between structural change and defect numbers of [email protected] has been studied to understand the positive effect of the improved enrichment for the optical behavior. As far as we know, this is the first experimental demonstration of missing-linker defects in luminescent MOFs for regulation of optical sensing, offering a unique approach to explore innovative technologies for precisely detecting of heavy metal ion pollutants.
One of the key challenges in preparing metal nanoparticle (NP)-based heterogeneous catalysts is to design host substrates with abundant high affinity anchoring sites to address the problem that metal NPs, due to their high surface energy, are easily deactivated by migration-aggregation during the catalytic process. Herein, we demonstrate that two-dimensional nitrogen-containing covalent organic frameworks can be used as host substrates with strong anchoring points to create stable and highly dispersed metal NP catalysts. Highly dispersed ultrafine Ag NPs (2.93 nm) with narrow size distribution have been successfully prepared by utilizing the abundant nitrogen-containing functional groups in the ordered channels of NCOF crystals. The prepared [email protected] exhibits superior catalytic activity toward the reduction of 4-NPh. In addition, the [email protected] is highly stable and easily reused with no significant loss of catalytic activity. The unique structure of the NCOF provides favorable nucleation sites for Ag NPs, confines the space for size-controlled growth of Ag NPs, and binds Ag NPs tightly through the interface interaction. Our work provides a promising strategy for immobilizing ultrafine metal NPs on porous supports. We hope that metal-based heterogeneous catalysts prepared by further diversification of different COFs and different metal NPs can be used in industrially important transformation reactions. This journal is
It is of great significance to establish a simple method for sensitively detecting metal ions and antibiotics for food safety. Herein, two crystalline polyimide covalent organic frameworks (COFs) were constructed via a green method, in which no organic solvents were used. Results suggested that both COF-1 and COF-2 with π-conjugated frameworks emitted strong fluorescence originating from the n-π∗ transition in N,N-dimethylformamide and the alkaline aqueous solution. More interestingly, both COF-1 and COF-2 showed excellent sensing performance for various metal ions and typical antibiotics. Taking COF-1 as the specific research object, the as-synthesized COF-1 exhibited noticeable turn-off fluorescence behaviors for 10 metal ions and 2 antibiotics. A high linear correlation coefficient, a wide linear range, and a low detection limit were achieved in real samples. The aggregation effect and π-πreaction were the possible fluorescence quenching mechanisms. In short, this work provides insights into the synthesis and application of fluorescent sensors based on COFs in food detection fields.
The use of covalent organic frameworks (COFs) to cope with environmental issues was promising but making them commercial and practical remains a challenge. In this work, we explored the crystallization of a stable nitrogen-containing covalent organic framework (TAPA-PDA COF) and used it as a porous platform for efficient and reversible volatile iodine capture. The TAPA-PDA COF, prepared by simple, rapid, low temperature reactions, possesses a high BET surface area of over 685 m²/g, good crystallinity, high thermal stability and ultrahigh capture of iodine vapor up to 5.09 g/g, which was significantly higher than most reported iodine adsorbents. Although the Raman spectroscopy confirmed the formation of polyiodide compounds through charge transfer between sorbents and iodine molecules, the TAPA-PDA COF still has the ability to be recycled five times while maintaining a high uptake capacity. This study provides promising strategy to solve the difficult problems of commercializing COFs and utilizing them for practical application.
A more robust mechanistic understanding of iminelinked two-dimensional covalent organic frameworks (2D COFs) is needed to improve their crystalline domain sizes and to control their morphology, both of which are necessary to fully realize their application potential. Here, we present evidence that 2D iminelinked COFs rapidly polymerize as crystalline sheets that subsequently reorganize to form stacked structures. Primarily, this study focuses on the first few minutes of 1,3,5-tris(4- aminophenyl)benzene and terephthaldehyde polymerization, which yields an imine-linked 2D COF. In situ X-ray diffraction and thorough characterization of solids obtained using gentler isolation and activation methods than have typically been used in the literature indicate that periodic imine-linked 2D structures form within 60 s, which then form more ordered stacked structures over the course of several hours. This stacking process imparts improved stability toward the isolation process relative to that of the early stage materials, which likely obfuscated previous mechanistic conclusions regarding 2D polymerization that were based on products isolated using harsh activation methods. This revised mechanistic picture has useful implications; the 2D COF layers isolated at very short reaction times are easily exfoliated, as observed in this work using high-resolution transmission electron microscopy and atomic force microscopy. These results suggest improved control of imine-linked 2D COF formation can be obtained through manipulation of the polymerization conditions and interlayer interactions. Qualitatively similar results were obtained for analogous materials obtained from 2,5-di(alkoxy)- terephthaldehyde derivatives, except for the COF with the longest alkoxy chains examined (OC12H25), which, although shown by in situ X-ray diffraction to be highly crystalline in the reaction mixture, is much less crystalline when isolated than the other COFs examined, likely due to the more severe steric impact of the dodecyloxy functionality on the stacking process.
A novel thiol-functionalized covalent organic framework (COF-S-SH) was prepared as an adsorbent for the simultaneous removal of BTEX (benzene, toluene, ethyl benzene and xylenes) and Hg(II) ion from water. The COF-S-SH was obtained via a Schiff-base reaction between a new triamino-monomer (TABPB) and 2,5-divinylterephthalaldehyde, followed by the thiol-ene “click” reaction between 1,2-ethanedithiol and the vinyl groups. The COF-S-SH was characterized by FT-IR spectrum, energy-dispersive X-ray spectrum, ¹³C MAS NMR spectrum, nitrogen adsorption-desorption isotherms, thermo-gravimetric analysis, scanning electron microscope, X-ray diffraction, and zeta potential. The new adsorbent showed high binding ability with the maximum adsorption capacities (Qmax) of 150.2-255.8 mg/g for BTEX and 588.2 mg/g for Hg(II). The adsorbent also presented fast adsorption kinetics with rate constants of 0.01043-0.05002 g/mg/min for BTEX and 7.254×10⁻⁴ g/mg/min for Hg(II). Moreover, the adsorbent had good binding selectivity for Hg(II) against common metal ions in water. The adsorbent also showed good simultaneous uptake ability and good reusability for BTEX and Hg(II), and it has been applied to simultaneously removing BTEX and Hg(II) in the simulated sewage with the removal efficiencies of 63.6, 82.1, 94.6, 95.3 and 94.1% for benzene, toluene, ethyl benzene, m-xylene and Hg(II), respectively. Especially, it can reduce the low-concentration Hg(II) in the simulated sewage to a level lower than the drinking water limit. Therefore, COF-S-SH can be used as an excellent adsorbent material for simultaneously removing BTEX and Hg(II) in from the produced water and wastewater in petroleum chemistry industries.
Removal of heavy metal ions from waste water has drawn intense attention due to their toxicity, bio-accumulation tendency and persistency in nature. Adsorption is regarded as one of the most promising method because of its simplicity and efficiency. In the present work, we report the preparation of a novel EDTA-Functionalized covalent organic framework (COF) for removal of heavy metal ions. Firstly, a COF named TpPa-NO2 was reduced to TpPa-NH2 by using Na2S2O4 as a reductant, and then EDTA dianhydride was grafted onto TpPa-NH2 to obtain [email protected] through post-modification. Both the COF morphology and structure kept unchanged after post-modification. The [email protected] showed excellent performance in adsorbing different kinds of heavy metal ions, such as soft Lewis acid (Ag+, Pd2+), hard Lewis acid (Fe3+, Cr3+) and borderline Lewis acid (Cu2+, Ni2+), and the removal efficiencies are all above 85% within 5 minutes due to the strong chelation effect of EDTA. The [email protected] also showed high adsorption ability in pH ≥ 3 environment and have an adsorption capacity of over 50 mg/g for the six representative heavy metal ions. This work provides a new idea for the application of COF materials in the removal of heavy metal ions from waste water.
Interlayer interactions play an important role in the formation of two-dimensional covalent organic frameworks (2D COFs), yet the effect of monomer structure on COF formation, crystallinity, and susceptibility to exfoliation are not well understood. Here we probe these effects by studying the stacking behavior of imine-linked macrocycles that represent discrete models of 2D COFs. Specifically, mac-rocycles based on terephthaldehyde (PDA) or 2,5-dimethoxyterephthaldehyde (DMPDA) stack upon cooling molecularly dissolved solutions. Both macrocycles assemble cooperatively with similar ΔHe values of -97 kJ/mol and -101 kJ/mol, respectively, although the DMPDA macrocycle assembly process showed a more straightforward temperature dependence. Density functional theory calculations of the stacking of PDA macrocycles suggested two stable configurations that were close in energy. Circular dichroism spectroscopy per-formed on macrocycles bearing chiral side chains revealed a helix reversion process for the PDA macrocycles that was not observed for the DMPDA macrocycles. Given the structural similarity of these monomers, these findings demonstrate that the stacking processes associat-ed with nanotubes derived from these macrocycles, as well as for the corresponding COFs, are complex and susceptible to kinetic traps, casting doubt on the relevance of thermodynamic arguments for improving materials quality. Rather, a deeper understanding of the mechanism of supramolecular polymerization and its interplay with polymerization and error correction during COF synthesis is needed for improved control of the crystallinity and morphology of these emerging materials.
Covalent organic frameworks (COFs) materials with designed skeleton structure and controlled pore size are a new class of adsorbent materials for environment remediation. Here, we synthesize two amide-based COFs materials via a polymerization reaction of acyl chloride and amino groups by mechanical ball milling at room temperature for Pb ²⁺ removal as adsorbents. Two types of diamine monomers are selected to construct COFs with different framework structure and functional groups contents, i.e. COF-TP from aromatic diamine and COF-TE from linear diamine. The as-prepared COFs materials display typical lamellar structure, and the amide groups on COFs skeleton are confirmed to behave as active adsorption sites for Pb ²⁺ capture via multi-coordination. COF-TE exhibits superior Pb ²⁺ adsorption capacity to COF-TP, because that less aromatic skeleton and weak π-π stacking of COF-TE facilitate higher internal diffusion of Pb ²⁺ adsorption. Meanwhile, the higher amide group content on COF-TE leads to its higher saturated adsorption capacity of 185.7 mg/g than COF-TP of 140.0 mg/g. Finally, great recycling stability of Pb ²⁺ adsorption allows the amide-based COFs materials to be promising adsorbents for heavy metal Pb ²⁺ removal or recovery.
There are several researches on preparation and application of hydrazone-linked COFs, and all of them generally necessitate rigid aromatic amines. Herein, we report a strategy for design and synthesis of COF with flexible alkyl-amine as building block and intramolecular hydrogen bonding as knot in the network. The proof-of-concept design was demonstrated by exploring 1,3,5-triformylphloroglucinol (Tp) and oxalyldihydrazide (ODH) as precursors to synthesize a novel COF material (TpODH), in which the different organic building units are combined through hydrazone bonds to form 2D porous frameworks. It should be pointed that irreversible enol-to-keto tautomerism and intramolecular N-H···O=C hydrogen bonding of TpODH would enhance the crystallinity and chemical stability, leading to large specific surface area of 835 m2/g. However, another COF synthesized with 1,3,5-triformylbenzene (TFB) and ODH exhibited less crystallinity and low special surface area (94 m2/g). Representatively, the resulting TpODH afforded Cu (II) and Hg (II) capacities of 324 and 1692 mg g-1, respectively, which exceeded most COFs previously reported. Moreover, the FT-IR and XPS spectra analyses were taken to demonstrate the adsorption mechanism. These results suggested that the materials could be applied to the removal of metallic ions in the future.
The treatment of low-level radioactive wastewater is a critical and considerable challenge. Bacterial cellulose membrane (BCM) modified with ethylenediaminetetraacetic acid (EDTA) using (3-aminopropyl) triethoxysilane (APTES) as a crosslinker were used to remove Sr²⁺ in this work. SEM, XPS, and FTIR were used to characterize the morphology, structure, chemical shift, and functional groups of the as-prepared adsorbent. The synthesized [email protected] presented a three-layer structure of membrane-net-membrane with nano-sized fibers (<100 nm). The adsorption of Sr²⁺ onto [email protected] was investigated as a function of contact time and initial concentration of Sr²⁺. Results showed that the adsorption of Sr²⁺ followed the pseudo second-order kinetic model (R² = 0.999), and fitted well with the Langmuir isotherm model (R² = 0.996). The maximum adsorption capacity was calculated to be 44.86 mg g⁻¹, which was comparable to other adsorbents. Additionally, the mechanism of Sr²⁺ adsorbed by the as-prepared adsorbent was studied through FTIR and XPS analysis, which indicated that the tertiary amines and carboxylate from grafted EDTA participated in the adsorption of Sr²⁺.
Two-dimensional (2D) covalent organic framework (COF) materials have the most suitable microstructure for membrane applications in order to achieve both high flux and high selectivity. Here, we report the synthesis of a crystalline TFP-DHF 2D COF membrane constructed from two precursors of 1,3,5-triformylphloroglucinol (TFP) and 9,9-dihexylfluorene-2,7-diamine (DHF) through the Langmuir–Blodgett (LB) method, for the first time. A single COF layer is precisely four-unit-cell thick and can be transferred to different support surfaces layer-by-layer. The TFP-DHF 2D COF membrane supported on anodic aluminum oxide (AAO) porous supports displayed remarkable permeabilities for both polar and nonpolar organic solvents, which were approximately 100 times higher than that of the amorphous membranes prepared by the same procedure and similar to the best of the reported polymer membranes. The transport mechanism through the TFP-DHF 2D COF membrane was found to be a viscous flow coupled with a strong slip boundary enhancement, which was also different from those of the amorphous polymer membranes. The membrane exhibited a steep molecular sieving with a molecular weight retention onset of approximately 600 Da and a molecular weight cut-off of approximately 900 Da. The substantial performance enhancement was attributed to the structural change from an amorphous structure to a well-defined ordered porous structure, which clearly demonstrated the high potential for the application of 2D COFs as the next generation of membrane materials.
Schiff-condensation reactions carried out between 1,6-diaminopyrene (DAP) and the tritopical 1,3,5 benzenetricarbaldehyde (BTCA) or 1,3,5-triformilfloroglucionol (TP) ligands give rise to the formation of two-dimensional imine-based covalent-organic frameworks (COFs), named IMDEA-COF-1 and -2, respectively. These materials show dramatic layer-packing-driven fluorescence in solid-state arising from the three-dimensional arrangement of the pyrene units among layers. Layer-stacking within these 2D-COF materials to give either eclipsed or staggered conformations can be controlled, at an atomic, level through chemical design of the building blocks used in their synthesis. Theoretical calculations have been used to rationalize the different preferential packing between both COFs. IMDEA-COF-1 shows green emission with absolute PL quantum yield of 3.5 % in solid state. This material represents the first example of imine-linked 2D-COF showing high-emission in solid state.
Carbon-based nanomaterials, especially carbon nanotubes and graphene, have drawn wide attention in recent years as novel materials for environmental applications. Notably, the functionalized derivatives of carbon nanotubes and graphene with high surface area and adsorption sites are proposed to remove heavy metals via adsorption, addressing the pressing pollution of heavy metal. This critical revies assesses the recent development of various functionalized carbon nanotubes and graphene that are used to remove heavy metals from contaminated water, including the preparation and characterization methods of functionalized carbon nanotubes and graphene, their applications for heavy metal adsorption, effects of water chemistry on the adsorption capacity, and decontamination mechanism. Future research directions have also been proposed with the goal of further improving their adsorption performance, the feasibility of industrial applications, and better simulating adsorption mechanisms.
In this study, a novel mesoporous conjugate adsorbent (MCA) was fabricated by directly anchoring the organic ligand of 2-hydroxyacetophenone-4N-pyrrolidine thiosemicarbazones (HAPT) onto highly ordered mesoporous ZSM-5 for efficient and selective mercury (Hg(II)) ions capturing from aqueous solution. The effect of solution pH, interference of foreign metal ions, contact time, initial Hg(II) ions concentration, and elution-regeneration of the MCA were evaluated in the case of detection and removal operations. The MCA was exhibited an obvious color change from colorless to yellow in the presence of Hg(II) ions at optimum pH conditions. In addition, the Hg(II) ions was detected and removed from water samples with naked-eye observations. The sensitive detection was determined under specific optimum conditions and the lower detection limit was determined by the MCA was 3.69 µg/L of Hg(II) ions. The foreign ions effect was evaluated and the MCA was exhibited high selectivity towards the Hg(II) ions with optimum color formation and the Hg(II) ions can be detected easily on-site Hg(II) ions monitoring. The adsorption efficiency was determined in terms of initial Hg(II) ions concentration. The data well fitted with the Langmuir isotherm model, and the maximum adsorption capacity of Hg(II) ions was 172.61 mg/g by the MCA. Moreover, the adsorption results of the MCA were compared with the other forms of different materials. The effective eluent of 0.10 M thiourea-0.10 M HCl was used to elute the Hg(II) ions from the MCA, and the MCA was simultaneously regenerated into the initial form for next process without significant loss in its initial performance. The data clarified that the MCA is an efficient and eco-friendly for simultaneous detection and removal of Hg(II) ions from wastewater samples.
Mercury contamination is a global concern because of its high toxicity, persistence, bioaccumulative nature, long distance transport and wide distribution in the environment. In this study, the efficiency and multiple-pathway remediation mechanisms of Hg(2+) by a selenite reducing Escherichia coli was assessed. E. coli can reduce Hg(2+) to Hg(+) and Hg(0) and selenite to selenide at the same time. This makes a multiple-pathway mechanisms for removal of Hg(2+) from water in addition to biosorption. It was found that when the original Hg(2+) concentration was 40μgL(-1), 93.2±2.8% of Hg(2+) was removed from solution by E. coli. Of the total Hg removed, it was found that 3.3±0.1% was adsorbed to the bacterium, 2.0±0.5% was bioaccumulated, and 7.3±0.6% was volatilized into the ambient environment, and most (80.6±5.7%) Hg was removed as HgSe and HgCl precipitates and Hg(0). On one hand, selenite is reduced to selenide and the latter further reacts with Hg(2+) to form HgSe precipitates. On the other hand Hg(2+) is successively reduced to Hg(+), which forms solid HgCl, and Hg(0). This is the report on bacterially transformation of Hg(2+) to HgSe, HgCl and Hg(0) via multiple pathways. It is suggested that E. coli or other selenite reducing microorganisms are promising candidates for mercury bioremediation of contaminated wastewaters, as well as simultaneous removal of Hg(2+) and selenite.
In this paper, new quaternized cellulose derivative based on Ethylenediaminetetraacetic acid (EDTA) and hydroxyethyl cellulose (HEC) is successfully prepared in homogeneous medium. The resulted product is characterized using spectroscopy techniques (FTIR, 1H NMR and 13C NMR). At the supramolecular level, the x-ray patterns show that a high hydrogen bond density occurs by grafting EDTA on the HEC fibers. The new adsorbent (HEC-EDTA) shows a high adsorption capacity of heavy metals (Pb (II) and Cu (II)) from aqueous metals solutions. The adsorption of the both metal ions follows the pseudo-second-order kinetic model, while the adsorption isotherms are well described by the Langmuir model. The qm values are determined for Pb (II) and Cu (II), respectively. For each metal, the equilibrium adsorption time is found to be 30 min. Moreover, the HEC-EDTA adsorption capacity is strongly dependent on the pH value; and the adsorption is favorable for pH values between 4 and 6. Moreover, the results show a high affinity toward Cu (II) than Pb (II).
Two new polyimide based porous covalent organic frameworks(PI-COF 201 and PI-COF 202) were synthesized in high yields by directly heating the mixtures of melamine and pyromellitic dianhydride and naphthalenetetracarboxylic dianhydride in N2 atmosphere, respectively. Their structures and properties were characterized by elemental analysis, Fourier transform infrared spectra, X-ray photoelectron spectroscopy, X- ray powder diffraction, N2 adsorption-desorption and scanning electron microscopy and the fluorescence properties were studied in detail. These two COFs emitted strong fluorescence in proper solvents originated from π*→n transition caused by high electro-delocalization and inherent rigid structure of the COF materials. Strong quenching effects of Fe³⁺ on the fluorescene of the COFs allowed selective chemosensing for Fe³⁺.
Conjugated covalent networks
Although graphene and related materials are two-dimensional (2D) fully conjugated networks, similar covalent organic frameworks (COFs) could offer tailored electronic and magnetic properties. Jin et al. synthesized a fully π-conjugated COF through condensation reactions of tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile. The reactions were reversible, which provides the self-healing needed to form a crystalline material of stacked, π-bonded 2D sheets. Chemical oxidation of this semiconductor with iodine greatly enhanced its conductivity, and the radicals formed on the pyrene centers imparted a high spin density and paramagnetism.
Science , this issue p. 673
Imine-linked two-dimensional covalent organic frameworks (2D COFs) are crystalline polymer networks with enhanced stability and broader monomer scope compared to boronate ester-linked systems. They are traditionally prepared by condensing polyfunctional aldehydes and amines at elevated temperature in a mixture of organic solvents and aqueous CH3CO2H, which catalyzes imine formation and exchange. Under these conditions, an amorphous imine-linked polymer network precipitates quickly and then crystallizes after extended reaction times (hours to days). Here we employ metal triflates, which are water-tolerant Lewis acids, to vastly accelerate 2D imine-linked COF synthesis and improve their materials quality. Low catalyst loadings provide crystalline polymer networks in nearly quantitative yields. The generality of these conditions is demonstrated for several monomer combinations, including heteroatom-containing aromatic systems of interest for optoelectronic applications.
β-Zeolite was synthesized and modified with ethylenediamine (EDA). The synthesized materials were characterized and used for removal of Ni(II) from aqueous solutions. The influences of pH, contact time and temperatur on Ni(II) adsorption onto synthesized β-zeolite and modified β-zeolite by ethylenediamine (β-zeolite-EDA) were studied by batch technique, and XPS was employed to analysed the experimental data. The dynamic process showed that the adsorption of Ni(II) onto β-zeolite and β-zeolite-EDA matched the pseudo-second-order kinetics model, and the adsorption of NI(II) were significantly dependent on pH values. Through simulating the adsorption isotherms by Langmuir, Freundlich and Dubini-Radushkevich (D-R) models, it could be seen respectively that the adsorption patterns of Ni(II) onto β-zeolite and β-zeolite-EDA were mainly controlled by surface complexation, and the adsorption processes were endothermic and spontaneous. The modification of β-zeolite by ethylenediamine improved the adsorption capacity of Ni(II) significantly, it shows a novel material for the removing of Ni(II) from water environment for industrialized application.
A key challenge in environmental remediation is the design of adsorbents bearing an abundance of accessible chelating sites with high affinity, to achieve both rapid uptake and high capacity for the contaminants. Herein, we demonstrate how two-dimensional covalent organic frameworks (COFs) with well-defined mesopore structures display the right combination of properties to serve as a scaffold for decorating coordination sites to create ideal adsorbents. The proof-of-concept design is illustrated by modifying sulfur derivatives on a newly designed vinyl-functionalized mesoporous COF (COF-V) via thiol–ene “click” reaction. Representatively, the material (COF-S-SH) synthesized by treating COF-V with 1,2-ethanedithiol exhibits high efficiency in removing mercury from aqueous solutions and the air, affording Hg²⁺ and Hg⁰ capacities of 1350 and 863 mg g–1, respectively, surpassing all those of thiol and thioether functionalized materials reported thus far. More significantly, COF-S-SH demonstrates an ultrahigh distribution coefficient value (Kd) of 2.3 × 10⁹ mL g–1, which allows it to rapidly reduce the Hg²⁺ concentration from 5 ppm to less than 0.1 ppb, well below the acceptable limit in drinking water (2 ppb). We attribute the impressive performance to the synergistic effects arising from densely populated chelating groups with a strong binding ability within ordered mesopores that allow rapid diffusion of mercury species throughout the material. X-ray absorption fine structure (XAFS) spectroscopic studies revealed that each Hg is bound exclusively by two S via intramolecular cooperativity in COF-S-SH, further interpreting its excellent affinity. The results presented here thus reveal the exceptional potential of COFs for high-performance environmental remediation.
The predesignable porous structures found in covalent organic frameworks (COFs) render them attractive as a molecular platform for addressing environmental issues such as removal of toxic heavy metal ions from water. However, a rational structural design of COFs in this aspect has not been explored. Here we report the rational design of stable COFs for Hg(II) removal through elaborate structural design and control over skeletons, pore size, and pore walls. The resulting framework is stable under strong acid and base conditions, possesses high surface area, has large mesopores, and contains dense sulfide functional termini on the pore walls. These structural features work together in removing Hg(II) from water and achieve a benchmark system that combines capacity, efficiency, effectivity, applicability, selectivity, and reusability. These results suggest that COFs offer a powerful platform for tailor-made structural design to cope with various types of pollutions.
An environmental friendly and economic natural biopolymer-sodium humate (HA-Na) was used to capture Hg(II) from aqueous solutions, and the trapped Hg(II) (HA-Na-Hg) was then removed by aluminum coagulation. The best Hg(II) capturing performance (90.60%) was observed under the following conditions: initial pH of 7.0, coagulation pH of 6.0, HA-Na dosage of 5.0 g L−1, Al2(SO4)3.18H2O dosage of 4.0 g L−1, initial Hg(II) concentration of 50 mg L−1 and capturing time of 30 min. The HA-Na compositions with the molecular weight beyond 70 kDa showed the most intense affinity toward Hg(II). The results showed that the reaction equilibrium was achieved within 10 min (pH 7.0), and could be well fitted by the pseudo-second-order kinetics model. The capturing process could be well described by the Langmuir isotherm model and the maximum capturing capacity of Hg(II) was high up to 9.80 mg g−1 at 298 K (pH 7.0). The FTIR and XPS analysis showed that the redox reaction between Hg(II) and HA-Na and the coordination reaction of carboxyl and hydroxy groups of HA-Na with Hg(II) were responsible for Hg(II) removal. The successive regeneration experiment showed that the capturing efficiency of humates for Hg(II) was maintained at about 51% after five capture-regeneration recycles.
There is an increasing interest to use SP-ICP-MS to help quantify exposure to engineered nanoparticles, and their transformation products, released into the environment. Hindering the use of this analytical technique for environmental samples is the presence of high levels of dissolved analyte which impedes resolution of the particle signal from the dissolved. While sample dilution is often necessary to achieve the low analyte concentrations necessary for SP-ICP-MS analysis, and to reduce the occurrence of matrix effects on the analyte signal, it is used here to also reduce the dissolved signal relative to the particulate, while maintaining a matrix chemistry that promotes particle stability. We propose a simple, systematic dilution series approach where by the first dilution is used to quantify the dissolved analyte, the second is used to optimize the particle signal, and the third is used as an analytical quality control. Using simple suspensions of well characterized Au and Ag nanoparticles spiked with the dissolved analyte form, as well as suspensions of complex environmental media (i.e., extracts from soils previously contaminated with engineered silver nanoparticles), we show how this dilution series technique improves resolution of the particle signal which in turn improves the accuracy of particle counts, quantification of particulate mass and determination of particle size. The technique proposed here is meant to offer a systematic and reproducible approach to the SP-ICP-MS analysis of environmental samples and improve the quality and consistency of data generated from this relatively new analytical tool.
Heavy metal ions are highly toxic and widely spread as environmental pollutants. New strategies are being developed to simultaneously detect and remove these toxic ions. Herein, we take the intrinsic advantage of covalent organic frameworks (COFs) and develop the fluorescent COFs for sensing applications. As a proof-of-concept, a thioether-functionalized COF material, COF-LZU8, was "bottom-up" integrated with multi-functionality for the selective detection and facile removal of mercury(II): the π-conjugated framework as the signal transducer, the evenly-and-densely distributed thioether groups as the Hg2+ receptor, the regular pores to facilitate the real-time detection and mass transfer, together with the robust COF structure for the recycle use. The excellent sensing performance of COF-LZU8 was achieved in terms of high sensitivity, excellent selectivity, easy visibility, and real-time response. Meanwhile, the efficient removal of Hg2+ from water and the recycle use of COF-LZU8 offered the possibility for practical applications. In addition, XPS and solid-state NMR investigations verified the strong and selective interaction between Hg2+ and the thioether groups of COF-LZU8. This research not only demonstrates the utilization of fluorescent COFs for both sensing and removal of metal ions, but also highlights the facile construction of functionalized COFs for environmental applications.
We explore the crystallization of a high surface area imine-linked two-dimensional covalent organic framework (2D COF). The growth process reveals rapid initial formation of an amorphous network that subsequently crystallizes into the layered 2D network. The metastable amorphous polymer may be isolated and resubjected to growth conditions to form the COF. These experiments provide the first mechanistic insight into the mechanism of imine-linked 2D COF formation, which is distinct from that of boronate-ester linked COFs.
Metal ion separation is crucial to environmental decontamination, chromatography and metal recovery and recycling. Theoretical studies have suggested that the ion distributions in the electric double-layer (EDL) region depend on the nature of the ions and the characteristics of the charged electrode surface. We believe that rational design of the electrode material and device structure will enable EDL-based devices to be utilized in separation of aqueous metal ions. Based on this concept, we fabricate an EDL separation (EDLS) device based on sandwich-structured N-functionalized graphene sheets (CN-GS) for selective separation of aqueous toxic heavy metal ions. We demonstrate that the EDLS enables randomly distributed soluble ions to form a coordination-driven layer and electrostatic-driven layer in the interfacial region of the CN-GS/solution. Through tuning the surface potential of the CN-GS, the effective separation of heavy metal ions (coordination-driven layer) from alkali or alkaline earth metal ions (electrostatic-driven layer) can be achieved.
Magnetic Fe3O4 nanoparticles were synthesized, surface modi fied with an amino-terminated silane coupling agent, 3-aminopropyltrimethoxysilane (APTMS), and characterized by Fourier transform infrared spectroscopy (FT-IR),field scanning electron microscopy (FESEM), and X-ray di ffraction (XRD). A copolymer of methyl methacrylate (MMA) and maleic anhydride(MA), poly(MMA-co -MA), was synthesized by radical polymerization and transformed into magnetic nanocomposite (MNC) by chemical immobilization of APTMS-Fe3O4 with the anhydride groups of poly(MMA-co -MA) chains. The MNC was characterized by FT-IR, XRD, FESEM, TEM, and atomic force microscopy (AFM) and used for the removal of metal ions from water. Various factors infl uencing adsorption capacity such as contact time, absorbent dosage, pH, and initial concentration of ions were investigated. The adsorption kinetics showed a pseudo-second-order rate law, indicating chemical sorption as the rate-limiting step mechanism. Sorption of metal ions to MNC agreed well with the Langmuir adsorption model with the maximum adsorption capacity of 90.09, 90.91, 109.89, and 111.11 mg g−1 for Co2+,Cr3+,Zn2+, and Cd2+, respectively.
Conjugated polymers are attractive materials for the detection of chemicals because of their remarkable π-conjugation and photoluminescence properties. In this article, we report a new strategy for the construction of molecular detection systems with conjugated microporous polymers (CMPs). The condensation of a carbazole derivative, TCB, leads to the synthesis of a conjugated microporous polymer (TCB-CMP) that exhibits blue luminescence and possesses a large surface area. Compared with a linear polymer analogue, TCB-CMP showed enhanced detection sensitivity and allowed for the rapid detection of arenes upon exposure to their vapors. TCB-CMP displayed prominent fluorescence enhancement in the presence of electron-rich arene vapors and drastic fluorescence quenching in the presence of electron-deficient arene vapors, and it could be reused without a loss of sensitivity and responsiveness. These characteristics are attributed to the microporous conjugated network of the material. Specifically, the micropores absorb arene molecules into the confined space of the polymer, the skeleton possesses a large surface area and provides a broad interface for arenes, and the network architecture facilitates exciton migration over the framework. These structural features function cooperatively, enhancing the signaling activity of TCB-CMP in fluorescence-on and fluorescence-off detection.
With a low optical background, high loading capacity, and good biocompatibility, hydrogels are ideal materials for immobilization of biopolymers to develop optical biosensors. We recently immobilized mercury and lead binding DNAs within a monolithic gel and demonstrated ultrasensitive visual detection of these heavy metals. The high sensitivity was attributed to the enrichment of the analytes into the gels. The signaling kinetics was slow, however, taking about 1 h to obtain a stable optical signal because of a long diffusion distance. In this work, we aim to understand the analyte enrichment process and improve the signaling kinetics by preparing hydrogel microparticles. DNA-functionalized gel beads were synthesized using an emulsion polymerization technique and most of the beads were between 10 and 50 μm. Acrydite-modified DNA was incorporated by copolymerization. Visual detection of 10 nM Hg(2+) was still achieved and a stable signal was obtained in just 2 min. The gel beads could be spotted to form a microarray and dried for storage. A new visual sensor for adenosine was designed and immobilized within the gel beads. The adenosine aptamer binds its target about 1000-fold less tightly compared to the mercury binding DNA, allowing a comparison to be made on analyte enrichment by aptamer-functionalized hydrogels.
Condensation of 2,5-diethoxyterephthalohydrazide with 1,3,5-triformylbenzene or 1,3,5-tris(4-formylphenyl)benzene yields two new covalent organic frameworks, COF-42 and COF-43, in which the organic building units are linked through hydrazone bonds to form extended two-dimensional porous frameworks. Both materials are highly crystalline, display excellent chemical and thermal stability, and are permanently porous. These new COFs expand the scope of possibilities for this emerging class of porous materials.
Functionalized magnetic nanoparticles, composed of both inorganic and organic components, have recently been examined as promising platforms for detection and separation applications. This unique class of nanomaterials can retain not only beneficial features of both the inorganic and organic components, but can also provide the ability to systematically tune the properties of the hybrid materials through the combination of appropriate functional components. This tutorial review focuses on the recent development of functionalized magnetic nanoparticles for use in biological and environmental applications, in which these chromogenic and fluorogenic chemosensors can selectively detect and separate specific toxic metal ions.
Mercury is a highly toxic environmental pollutant with bioaccumulative properties. Therefore, new materials are required to not only detect but also effectively remove mercury from environmental sources such as water. We herein describe a polyacrylamide hydrogel-based sensor functionalized with a thymine-rich DNA that can simultaneously detect and remove mercury from water. Detection is achieved by selective binding of Hg(2+) between two thymine bases, inducing a hairpin structure where, upon addition of SYBR Green I dye, green fluorescence is observed. In the absence of Hg(2+), however, addition of the dye results in yellow fluorescence. Using the naked eye, the detection limit in a 50 mL water sample is 10 nM Hg(2+). This sensor can be regenerated using a simple acid treatment and can remove Hg(2+) from water at a rate of approximately 1 h(-1). This sensor was also used to detect and remove Hg(2+) from samples of Lake Ontario water spiked with mercury. In addition, these hydrogel-based sensors are resistant to nuclease and can be rehydrated from dried gels for storage and DNA protection. Similar methods can be used to functionalize hydrogels with other nucleic acids, proteins, and small molecules for environmental and biomedical applications.
Laboratory studies were conducted to investigate the feasibility of using ion-exchange resins in permeable reactive barriers (PRBs) for the remediation of groundwater contaminated by heavy and transition metals. Ion-exchange resins represent an essentially neglected class of materials which may, in addition to iron, activated carbon, and zeolites, prove effective for use in PRBs. Four resins were considered: two commercially available resins, Duolite GT-73 (Rohm and Haas) and Amberlite IRC-748 (Rohm and Haas), and two solvent-impregnated resins (SIRs). The SIRs were prepared from Amberlite IRA-96 (Rohm and Haas) and two different thiophosphoric extractants. All four resins are able to reduce cadmium, lead, and copper concentrations from 1000 microg/L (typical for contaminated groundwaters) to below 5 microg/L. Significantly, all of the resins are effective for the capture of cadmium, copper, and lead, even in the presence of CaCl2 and clay. Because of their high hydraulic conductivity, the use of these resins in clusters of wells, as an alternative to continuous walls, is considered in the design of effective PRBs. Numerical solution of the groundwater flow equations shows that, depending on the well configuration, most (or all) of the contaminated groundwater can pass through the resins. These results demonstrate the possibility of using selective ion-exchange resins as an effective, active material in PRBs for in situ groundwater remediation.
Applications of second-order kinetic models to adsorption systems were reviewed. An overview of second-order kinetic expressions is described in this paper based on the solid adsorption capacity. An early empirical second-order equation was applied in the adsorption of gases onto a solid. A similar second-order equation was applied to describe ion exchange reactions. In recent years, a pseudo-second-order rate expression has been widely applied to the adsorption of pollutants from aqueous solutions onto adsorbents. In addition, the earliest rate equation based on the solid adsorption capacity is also presented in detail.
- Faghihian