Pore Space Partition Synthetic Strategy in Imine-linked Multivariate Covalent Organic Frameworks.

Partitioning the pores of covalent organic frameworks (COFs) is an attractive strategy for introducing microporosity and achieving new functionality, but it is technically challenging to achieve. Herein, we report a simple strategy for partitioning the micropores/mesopores of multivariate COFs. Our approach relies on the predesign and synthesis of multicomponent COFs through imine condensation reactions with aldehyde groups anchored in the COF pores, followed by inserting additional symmetric building blocks (with C2 or C3 symmetries) as pore partition agents. This approach allowed tetragonal or hexagonal pores to be partitioned into two or three smaller micropores, respectively. The synthesized library of pore-partitioned COFs was then applied for the capture of iodine pollutants (i.e., I2 and CH3I). This rich inventory allowed deep exploration of the relationships between the COF adsorbent composition, pore architecture, and adsorption capacity for I2 and CH3I capture under wide-ranging conditions. Notably, one of our developed pore-partitioned COFs (COF 3-2P) exhibited greatly enhanced dynamic I2 and CH3I adsorption performances compared to its parent COF (COF 3) in breakthrough tests, setting a new benchmark for COF-based adsorbents. Results present an effective design strategy toward functional COFs with tunable pore environments, functions, and properties.

Palladium(II) Modulation Enhances the Water Stability and Aqueous 99TcO4-/ReO4- Removal Performance of Metal-Organic Frameworks.

Improving the water stability of metal-organic frameworks (MOFs) is essential for their use in water pollution treatment and environmental remediation, though it remains technically challenging. Herein, we report a novel cationic MOF constructed with [Th6O4(OH)4(COO)12] units and [CoN4·Cl2] units possessing a ftw-type topology (denoted as 1-Th-Co). 1-Th-Co itself exhibited poor water stability but excellent stability following a palladium(II) modulation strategy. Experimental studies reveal that Co(II) ions in 1-Th-Co were replaced by Pd(II) ions through cation exchange in N,N-diethylformamide (yielding 1-Th-Pd). The planar PdN4 units in 1-Th-Pd were responsible for improving the water stability of the framework. As a result, 1-Th-Pd offered excellent stability, fast adsorption kinetics, and high removal ratios for 99TcO4- and ReO4- (as a 99TcO4- surrogate) in contaminated water. When used in packed columns, 1-Th-Pd can dynamically capture ReO4- from groundwater. This work provides a new avenue for improving the water stability of MOFs, offering new vistas for the decontamination of aqueous solutions containing 99TcO4- and ReO4-.

Open-Framework Vanadate as Efficient Ion Exchanger for Uranyl Removal.

The elimination of uranium from radioactive wastewater is crucial for the safe management and operation of environmental remediation. Here, we present a layered vanadate with high acid/base stability, [Me2NH2]V3O7, as an excellent ion exchanger capturing uranyl from highly complex aqueous solutions. The material possesses an indirect band gap, ferromagnetic characteristic and a flower-like morphology comprising parallel nanosheets. The layered structure of [Me2NH2]V3O7 is predominantly upheld by the H-bond interaction between anionic framework [V3O7]nn- and intercalated [Me2NH2]+. The [Me2NH2]+ within [Me2NH2]V3O7 can be readily exchanged with UO22+. [Me2NH2]V3O7 exhibits high exchange capacity (qm = 176.19 mg/g), fast kinetics (within 15 min), high removal efficiencies (>99%), and good selectivity against an excess of interfering ions. It also displays activity for UO22+ ion exchange over a wide pH range (2.00-7.12). More importantly, [Me2NH2]V3O7 has the capability to effectively remove low-concentration uranium, yielding a residual U concentration of 13 ppb, which falls below the EPA-defined acceptable limit of 30 ppb in typical drinking water. [Me2NH2]V3O7 can also efficiently separate UO22+ from Cs+ or Sr2+ achieving the highest separation factors (SFU/Cs of 589 and SFU/Sr of 227) to date. The BOMD and DFT calculations reveal that the driving force of ion exchange is dominated by the interaction between UO22+ and [V3O7]nn-, whereas the ion exchange rate is influenced by the mobility of UO22+ and [Me2NH2]+. Our experimental findings indicate that [Me2NH2]V3O7 can be considered as a promising uranium scavenger for environmental remediation. Additionally, the simulation results provide valuable mechanistic interpretations for ion exchange and serve as a reference for designing novel ion exchangers.

COF-based membranes for liquid phase separation: Preparation, mechanism and perspective.

Covalent organic frameworks (COFs) are a new kind of crystalline porous materials composed of organic molecules connected by covalent bonds, processes the characteristics of low density, large specific surface area, adjustable pore size and structure, and easy to functionalize, which have been widely used in the field of membrane separation technology. Recently, there are more and more researches focusing on the preparation methods, separation application, and mechanism of COF membranes, which need to be further summarized and compared. In this review, we primarily summarized several conventional preparation methods, such as two-phase interfacial polymerization, in-situ growth on substrate, unidirectional diffusion method, layer-by-layer assembly method, mixed matrix membranes, and so on. The advantages and disadvantages of each method are briefly summarized. The application potential of COF membrane in liquid separation are introduced from four aspects: dyeing wastewater treatment, heavy metal removal, seawater desalination and oil-water separation. Then, the mechanisms including pore structure, hydrophilic/hydrophobic, electrostatic repulsion/attraction and Donnan effect are introduced. For the efficient removal of different kind of pollutions, researchers can select different ligands to construct membranes with specific pore size, hydrophily, salt or organic rejection ability and functional group. The ideas for the design and preparation of COF membranes are introduced. Finally, the future direction and challenges of the next generation of COF membranes in the field of separation are prospected.

Biomimetic Photocatalytic System Designed by Spatially Separated Cocatalysts on Z-scheme Heterojunction with Identified Charge-transfer Processes for Boosting Removal of U(VI).

Designing highly efficient photocatalysts with rapid migration of photogenerated charges and surface reaction kinetics for the photocatalytic removal of uranium (U(VI)) from uranium mine wastewater remains a significant challenge. Inspired by natural photosynthesis, a biomimetic photocatalytic system is assembled by designing a novel hollow nanosphere MnOx @TiO2 @CdS@Au (MTCA) with loading MnOx and Au nano particles (Au NPs) cocatalysts on the inner and outer surfaces of the TiO2 @CdS. The spatially separated cocatalysts efficiently drive the photogenerated charges to migrate in opposite directions, while the Z-scheme heterogeneous shell further separates the interfacial charges. Theoretical calculation identifies multiple consecutive forward charge transfers without charge recombination within MTCA. Thus, MTCA could efficiently remove 99.61% of U(VI) after 15 min of simulated sunlight irradiation within 3 mmol L-1 NaHCO3 with 0.231 min-1 of the reduction rate constant, outperforming most previously reported photocatalysts. MTCA further significantly removes 91.83% of U(VI) from the natural uranium mining wastewater under sunlight irradiation. This study provides a novel approach to designing an ideal biomimetic photocatalyst for remediating environmental pollution.

Highly Selective Extraction of U(VI) from Solutions by Metal Organic Framework-Based Nanomaterials through Sorption, Photochemistry, and Electrochemistry Strategies.

With the rapid development of nuclear technology and peaceful utilization of nuclear energy, plentiful U(VI) not only is required to be extracted from solutions for a sustainable nuclear fuel supply but also is inevitably released into the surrounding environment to result in pollution and threaten human health. Thereby, realizing selective extraction of U(VI) from aqueous solutions is crucial for U(VI) pollution control and a sustainable nuclear industry. Metal organic frameworks (MOFs) have gained multidisciplinary attention due to their excellent properties including large specific surface areas, tunable pore structures, easy functionalization, etc. This Review comprehensively summarizes the research progress of MOFs and MOF-based materials on U(VI) removal from aqueous solutions by sorption, photocatalysis, electrocatalysis, membrane separation, etc. The efficient high extraction ability is dependent on the intrinsic properties of MOFs and the techniques used. The removal properties of MOF-based materials as adsorbents, photocatalysts, and electrocatalysts for U(VI) are discussed. Information about the interaction mechanisms between U(VI) and MOF-based materials are analyzed in-depth, including experiments, theoretical calculations, and advanced spectroscopy analysis. The removal properties for U(VI) of various MOF-based materials are assessed through different techniques. Finally, a summary and perspective on the direction and challenges of MOF-based materials and various pollutant removal technologies are proposed to provide some significant information on designing and fabricating MOF-based materials for environmental pollution management.

Modulating Anion Nanotraps via Halogenation for High-Efficiency 99TcO4-/ReO4- Removal under Wide-Ranging pH Conditions.

Efficient and sustainable methods for 99TcO4- removal from acidic nuclear waste streams, contaminated water, and highly alkaline tank wastes are highly sought after. Herein, we demonstrate that ionic covalent organic polymers (iCOPs) possessing imidazolium-N+ nanotraps allow the selective adsorption of 99TcO4- under wide-ranging pH conditions. In particular, we show that the binding affinity of the cationic nanotraps toward 99TcO4- can be modulated by tuning the local environment around the nanotraps through a halogenation strategy, thereby enabling universal pH 99TcO4- removal. A parent iCOP-1 possessing imidazolium-N+ nanotraps showed fast kinetics (reaching adsorption equilibrium in 1 min), a high adsorption capacity (up to 1434.1 ± 24.6 mg/g), and exceptional selectivity for 99TcO4- and ReO4- (nonradioactive analogue of 99TcO4-) removal in contaminated water. By introducing F groups near the imidazolium-N+ nanotrap sites (iCOP-2), a ReO4- removal efficiency over 58% was achieved in 60 min in 3 M HNO3 solution. Further, introduction of larger Br groups near the imidazolium-N+ binding sites (iCOP-3) imparted a pronounced steric effect, resulting in exceptional adsorption performance for 99TcO4- under super alkaline conditions and from low-activity waste streams at US legacy Hanford nuclear sites. The halogenation strategy reported herein guides the task-specific design of functional adsorbents for 99TcO4- removal and other applications.

Design and Fabrication of Hypercrosslinked Covalent Organic Adsorbents for Selective Uranium Extraction.

Uranium detection and extraction are necessary for the ecological environment as the growing demand for nuclear energy. Hence, exploring stable materials with excellent performance in uranium extraction and detection is highly desired. Herein, by amidoxime-functionalizing tetrafluoroterephthalonitrile (TFTPN) crosslinked hydroquinone (bP), phloroglucinol (tP), and 4,4',4″-trihydroxytriphenylmethane (tBP), three covalent organic polymers (COPs) bPF-AO, tPF-AO, and tBPF-AO with different crosslinked architectures are fabricated. Uranium extraction and detection related to the difference in molecule construction were systemically investigated, giving some reference for the rational design and fabrication of advanced materials for the removal and monitoring of uranium in the environment. The tPF-AO with a compact steric structure achieves the highest theoretical maximum adsorption capacity of 578.9 ± 15.2 mg g-1 and the best recyclability. The scattering electron center and U(VI) selective binding sites endow tBPF-AO with excellent capability in selective detection for U(VI), with a limit of detection of 24.2 nmol L-1, which is well below the standard for U(VI) in drinking water of the World Health Organization (WHO). Moreover, the COPs possess prominent physicochemical stability and recyclability, and more importantly, the PAE-based COPs are derived from inexpensive industry materials with easy processing methods, providing an efficient and economical way for the detection and adsorption of uranium.

A review of microplastic pollution in aquaculture: Sources, effects, removal strategies and prospects.

As microplastic pollution has become an emerging environmental issue of global concern, microplastics in aquaculture have become a research hotspot. For environmental safety, economic efficiency and food safety considerations, a comprehensive understanding of microplastic pollution in aquaculture is necessary. This review outlines an overview of sources and effects of microplastics in aquaculture. External environmental inputs and aquaculture processes are sources of microplastics in aquaculture. Microplastics may release harmful additives and adsorb pollutants in aquaculture environment, cause deterioration of aquaculture environment, as well as cause toxicological effects, affect the behavior, growth and reproduction of aquaculture products, ultimately reducing the economic benefits of aquaculture. Microplastics entering the human body through aquaculture products also pose potential health risks at multiple levels. Microplastic pollution removal strategies used in aquaculture in various countries are also reviewed. Ecological interception and purification are considered to be effective methods. In addition, strengthening aquaculture management and improving fishing gear and packaging are also currently feasible solutions. As proactive measures, new portable microplastic monitoring system and remote sensing technology are considered to have broad application prospects. And it was encouraged to comprehensively strengthen the supervision of microplastic pollution in aquaculture through talent exchange and strengthening the construction of laws and regulations.

Tuning Local Charge Distribution in Multicomponent Covalent Organic Frameworks for Dramatically Enhanced Photocatalytic Uranium Extraction.

Optimizing the electronic structure of covalent organic framework (COF) photocatalysts is essential for maximizing photocatalytic activity. Herein, we report an isoreticular family of multivariate COFs containing chromenoquinoline rings in the COF structure and electron-donating or withdrawing groups in the pores. Intramolecular donor-acceptor (D-A) interactions in the COFs allowed tuning of local charge distributions and charge carrier separation under visible light irradiation, resulting in enhanced photocatalytic performance. By optimizing the optoelectronic properties of the COFs, a photocatalytic uranium extraction efficiency of 8.02 mg/g/day was achieved using a nitro-functionalized multicomponent COF in natural seawater, exceeding the performance of all COFs reported to date. Results demonstrate an effective design strategy towards high-activity COF photocatalysts with intramolecular D-A structures not easily accessible using traditional synthetic approaches.

Selective and Efficient Photoextraction of Aqueous Cr(VI) as a Solid-State Polyhydroxy Cr(V) Complex for Environmental Remediation and Resource Recovery.

Aqueous hexavalent chromium (Cr(VI)) treatment and chromium resource recovery toward Cr-containing wastes are of significant importance and necessity to both wastewater remediation and resource recovery. Herein, via mild photoreaction conditions with isopropanol (IPA) as an electron donor, a catalyst-free strategy for aqueous Cr(VI) extraction to form an insoluble polyhydroxy Cr(V) complex is developed for the first time. Aqueous Cr(VI) with concentration from 5 to 150 ppm can be efficiently extracted with high selectivity even in the presence of coexisting ions, and the total Cr concentration in residue solution can be as low as 0.5 ppm. The Cr resource could be efficiently recovered as pure Cr2O3 by calcinating the resulting Cr(V) precipitate. Outstanding extraction efficiency could be realized with various IPA concentrations (1.3-12.0 mol/L) by coordinately tuning the pH value to promote the formation of Cr(VI)-IPA ester. The formed ester undergoes intramolecular electron transition under visible light irradiation, resulting in a polyhydroxy solid-state Cr(V) intermediate complex. The controlled pH value blocks further reduction of Cr(V) to soluble Cr(III); thus the insoluble Cr(V) intermediate complex is stabilized thermodynamically under ambient conditions. Because of its electric neutrality property and the strong intermolecule interaction via hydrogen bonds, a dioxo-bridged di-nuclear Cr(V) complex {Cr2(µ-O)2(OH)4[OCH(CH3)2]2} is finally precipitated as the main product. Satisfactory extraction and recovery of Cr from chromium-plating wastewater and discarded stainless steel verify that this approach is ideal for both one-step purification of Cr(VI)-containing wastewater and selective resource recovery from Cr-containing solid wastes in practical application.

Green-emitting carbon quantum dots as a dual-mode fluorescent and colorimetric sensor for hypochlorite.

In this work, green-emitting carbon quantum dots were successfully prepared through a facile one-step solid-state reaction method. The obtained green-emitting carbon dots (G-CDs) showed good fluorescence stability in NaCl aqueous solution and different pH values. Moreover, the G-CDs showed high sensitivity and selectivity for detecting hypochlorite by both fluorometry and colorimetry. Under the optimized condition, a highly sensitive detection of hypochlorite was established in the range of 0.2-100 µM and 10-150 µM for fluorescent and colorimetric methods, respectively. The corresponding limits of detection (LOD) were 0.0781 µM and 1.82 µM, respectively. Therefore, the G-CDs were successfully applied to determinate hypochlorite in actual water samples. In addition, a paper-based sensor loading with the G-CDs was also developed for rapid visual detection of hypochlorite. The results suggested that the G-CDs could be a promising candidate to detect hypochlorite.

Low-temperature plasma induced phosphate groups onto coffee residue-derived porous carbon for efficient U(VI) extraction.

For the continuous utilization of nuclear energy and efficient control of radioactive pollution, low-cost materials with high efficient U(VI) removal are of great importance. In this study, low temperature plasma method was applied for the successful modification of O-phosphorylethanolamine (O-PEA) on the porous carbon materials. The produced materials (Cafe/O-PEA) could adsorb U(VI) efficiently with the maximum sorption capacity of 648.54 mg/g at 1 hr, T=298 K, and pH=6.0, much higher than those of most carbon-based composites. U(VI) sorption was mainly controlled by strong surface complexation. From FTIR, SEM-EDS and XPS analyses, the sorption of U(VI) was related to the complexation with -NH2, phosphate and -OH groups on Cafe/O-PEA. The low temperature plasma method was an efficient, environmentally friendly and low-cost method for surface modification of materials for the effective enrichment of U(VI) from aqueous solutions.

Application of aluminosilicate clay mineral-based composites in photocatalysis.

Aluminosilicate clay mineral (ACM) is a kind of typical raw materials that used widely in manufacturing industry owing to the abundant reserve and low-cost exploring. In past two decades, in-depth understanding on unique layered structure and abundant surface properties endows ACM in the emerging research and application fields. In field of solar-chemical energy conversion, ACM has been widely used to support various semiconductor photocatalysts, forming the composites and achieving efficient conversion of reactants under sunlight irradiation. To date, classic ACM such as kaolinite and montmorillonite, loaded with semiconductor photocatalysts has been widely applied in photocatalysis. This review summaries the recent works on ACM-based composites in photocatalysis. Focusing on the properties of surface and layered structure, we elucidate the different features in the composition with various functional photocatalysts on two typical kinds of ACM, i.e., type 1:1 and type 2:1. Not only large surface area and active surface hydroxyl group assist the substrate adsorption, but also the layered structure provides more space to enlarge the application of ACM-based photocatalysts. Besides, we overview the modifications on ACM from both external surface and the inter-layer space that make the formation of composites more efficiently and boost the photo-chemical process. This review could inspire more upcoming design and synthesis for ACM-based photocatalysts, leading this kind of economic and eco-friendly materials for more practical application in the future.

High Sorption and Selective Extraction of Actinides from Aqueous Solutions.

The selective elimination of long-lived radioactive actinides from complicated solutions is crucial for pollution management of the environment. Knowledge about the species, structures and interaction mechanism of actinides at solid-water interfaces is helpful to understand and to evaluate physicochemical behavior in the natural environment. In this review, we summarize recent works about the sorption and interaction mechanism of actinides (using U, Np, Pu, Cm and Am as representative actinides) on natural clay minerals and man-made nanomaterials. The species and microstructures of actinides on solid particles were investigated by advanced spectroscopy techniques and computational theoretical calculations. The reduction and solidification of actinides on solid particles is the most effective way to immobilize actinides in the natural environment. The contents of this review may be helpful in evaluating the migration of actinides in near-field nuclear waste repositories and the mobilization properties of radionuclides in the environment.

Correction: Plasma-induced grafting of polyacrylamide on graphene oxide nanosheets for simultaneous removal of radionuclides.

Correction for 'Plasma-induced grafting of polyacrylamide on graphene oxide nanosheets for simultaneous removal of radionuclides' by Wencheng Song et al., Phys. Chem. Chem. Phys., 2015, 17, 398-406.

Metal-Organic Framework Enhances Aggregation-Induced Fluorescence of Chlortetracycline and the Application for Detection.

The development of an analytical method for selective and sensitive detection of chlortetracycline (CTC), an often overused broad spectrum antibiotic, is important and challenging in environmental and health monitoring. This paper reports a zinc based metal-organic framework of pyromellitic acid (Zn-BTEC), which has been found to greatly enhance the aggregation-induced emission (AIE) of chlortetracycline. The unique emission response of CTC on Zn-BTEC has been extensively examined and applied for the sensitive detection of CTC on the basis of fluorescence intensity of AIE, and a limit of detection (LOD) was estimated to be 28 nM. A rational mechanism has been proposed based on the porous structure of Zn-BTEC, and the CTC molecules would defuse into the rigid MOF structure and assemble or aggregate, leading to fluorescence enhancement of CTC. Interestingly, the Zn-BTEC materials could discriminate CTC from other TC antibiotics with high selectivity. We have further demonstrated that the Zn-BTEC materials are successfully applied for the sensitive and selective determination of CTC in real samples of fish and urine.

Quantitative Analysis of Surface Sites on Carbon Dots and Their Interaction with Metal Ions by a Potentiometric Titration Method.

Carbon dots (CDs) possess abundant functional groups on their surface which are related to their application in various fields such as sensing, imaging, and catalysis. Understanding the amount and properties of these functional groups and their interaction with metal ions is essential but has posed longstanding challenges because of the diverse and complex structures of CDs. In this work, potentiometric titration is demonstrated as an effective method to figure out the categories and amounts of functional groups. Surface complexation modeling with the FITEQL program was applied to the quantification of the surface sites on CDs with the titration data. Then with the obtained molar concentrations of the surface sites, the pKas of these surface sites were calculated with the Hyperquad program. Finally, titration experiments of CDs with and without Fe(III) were carried out and the stability constants of Fe(III) and ArgCDs were simulated on the Hyperquad program. By utilizing the stability constants, the distribution of Fe(III) species at different pHs and the concentrations of Fe(III) and CDs were also investigated. This potential method might be used for characterizing the surface sites on other CDs or even other soluble nanoparticles as well as for investigating the interactions of the surface sites with different metal ions.

Enhanced Photocatalytic Simultaneous Removals of Cr(VI) and Bisphenol A over Co(II)-Modified TiO2.

To enhance the electron-hole separation and boost the practical performance of commercial titania (Degussa P25) under natural solar light, in this work, P25 was modified with Co(II) species (CoP25) through post-treatment with decomposition of Co-ethylenediaminetetraacetic acid precursors in a wet chemical anchoring process. With appropriate Co(II) loading amount as molecular cocatalyst, the resulted CoP25-4 showed significantly improved photocatalytic performance for Cr(VI) reduction and bisphenol A (BPA) oxidation under UV-light irradiation. The coexistence of Cr(VI) and BPA promoted mutually the degradation of both pollutants. Under simulated solar light (AM 1.5G) illumination, the Cr(VI) reduction rate over CoP25-4 was 8.5 times enhanced compared with that over P25, whereas the simultaneous degradation rate of BPA over CoP25-4 was 8 times higher than that over P25. Further investigations indicated that the covalent atomic Co(II) anchoring on P25 significantly promoted the photogenerated electron-hole separation and facilitated Cr(VI) reduction via the formation of a Co(I) intermediate and simultaneously boosted BPA oxidation. Our results demonstrated a facile strategy to modify P25 with remarkably improved performance for the practical application in environmental pollution management under natural light excitation.

Enhanced Photoreduction of U(VI) on C3N4 by Cr(VI) and Bisphenol A: ESR, XPS, and EXAFS Investigation.

The effect of Cr(VI) and bisphenol A (BPA) on U(VI) photoreduction by C3N4 photocatalyst was demonstrated by the batch experiments, electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) techniques. The batch experiments manifested that Cr(VI) and BPA enhanced the photocatalytic activity of C3N4 for U(VI) photoreduction, whereas U(VI) photoreduction was significantly diminished with increased pH from 4.0 to 8.0. According to radical scavengers and ESR analysis, U(VI) was photoreduced to U(IV) by photogenerated electrons of conduction band edge, whereas Cr(VI) was reduced to Cr(III) by H2O2. BPA and its products such as organic acid and alcohols can capture photoinduced holes, which resulted in the enhancement of U(VI) photoreduction to U(IV). XPS and XANES analyses demonstrated that U(VI) was gradually photoreduced to U(IV) by C3N4 within irradiation 60 min, whereas U(IV) was reoxidized to U(VI) with increasing irradiation time. EXAFS analysis determined that the dominant interaction mechanisms of U(VI) on C3N4 after irradiation for 240 min were reductive precipitation and inner-sphere surface complexation. This work highlights the synergistic removal of radionuclides, heavy metals, and persistent organic pollutants by C3N4, which is crucial for the design and application of a high-performance photocatalyst in actual environmental cleanup.