New insight into the mechanism of biofouling-resistant thiazole-linked covalent organic frameworks for selective uranium capture from seawater.

The extraction of uranium from seawater is crucial for the sustainable production of nuclear fuel. Traditional amidoxime-functionalized adsorbents suffer from competitive adsorption of vanadium ion and biofouling. These challenges motivate the development of novel adsorbents for selective uranium extraction from seawater. Herein, four kinds of thiazole-linked covalent organic frameworks (COFs) were investigated to harvest uranium from seawater. The selectivity and anti-biofouling performance were systematically investigated through the molecular dynamics (MD) simulations. Driven by the pore size sieving effect and electrostatic interaction, the Ca2UO2(CO3)3 complex and vanadate anions were selectively separated by different COFs in special areas. On one hand, benefits from the small steric partition factor, the Ca2UO2(CO3)3 complex can stick on the surface of COFs. On the other hand, the dispersive negatively and positively charged areas of studied COFs work as potential binding sites for the Ca2UO2(CO3)3 complex and vanadate anions, respectively. Moreover, an analysis of pulling force and desorption time between uranium and vanadium ions further confirmed the selectivity of various thiazole-linked COFs. The anti-biofouling property was comparatively investigated by dynamic trajectory and solvent accessible surface area. Our outcomes illustrate that the hydroxyl and zwitterionic groups in the thiazole-linked COFs endow their strong surface hydrations to resist marine biofouling. In particular, the TpBdsaPa is identified as a promising candidate due to charge dispersed zwitterionic group as well as remarkable anti-biofouling ability. The present study sheds an atomic-level understanding of the thiazole-linked COFs for selective uranium uptaking from seawater, which will provide aid to design novel adsorbent with highly selective uranium extraction capacity and strong anti-biofouling property.

Alkyl modified cationic COFs for preferential trapping of charge dispersed perrhenate: Synergistic hydrophobicity and anion-recognition effects.

Charge dispersed oxoanionic pollutants (such as TcO4- and ReO4-) with low hydrophilicity are typically difficult to be preferentially extracted. Recently, cationic covalent organic frameworks (COFs) have received considerable attention for anions trapping. Two cationic COFs, denoted as Tp-S and Tp-D, were synthesized by incorporating ethyl and cyclic alkylated diquats into 2,2'-bipyridine-based COF. A synergistic effect of hydrophobic channel and anion-recognition sites were achieved by branched chains, which effectively surmounted the Hofmeister bias. Both Tp-S and Tp-D exhibited raising removal performance for surrogate ReO4- at high acidity with adsorption capacities of 435.6 and 291.4 mg g-1, respectively. Obvious variations caused by side chains were displayed in microstructures and adsorption performance. Specially, compared with Tp-D, Tp-S demonstrated desirable priority in uptake capacity and selectivity. In a real-scenario experiment, Tp-S could remove 72.8 % of ReO4- in a simulated Hanford LAW stream, which was attributed to the spatial effects and charge distribution arising from the open and flexible side chains of Tp-S. Otherwise, the rigid cyclic chains endowed pyridine-base Tp-D material an unprecedented alkaline stability. Spectra and theoretical calculations revealed a mechanism of preferential capture based on electrostatic interaction and hydrogen bonding between charge dispersed ReO4-/TcO4- and Tp-S/Tp-D. This work provides an innovative perspective to tailored materials for the treatment of oxoanionic contaminants.

Methyl position affect the fluorescence performance of HBT derivatives for the detection of hypochlorite under alkaline condition.

Five fluorescent derivatives of hydroxyphenyl-benzothiazole (HBT) with different methyl positions at the hydroxyphenyl group were synthesized with good yield. Their reactivity and fluorescent response to hypochlorite were carefully studied. It was found that the HBT derivatives with meta-methyl (3-HBT or 5-HBT) showed the highest reactivity to hypochlorite under basic conditions, accompanied by the most efficient fluorescence quenching, whereas HBT derivatives with ortho or para methyl exhibited the least reactivity to hypochlorite. The LUMO and HOMO of 3-HBT were further verified to explain the fluorescence behavior by density functional theory (DFT) calculation. The excellent selectivity of 3-HBT toward hypochlorite against other reactive oxygen species (ROS) was also evaluated under the same conditions. The compounds emit bright green fluorescence in a solid-state, which is convenient for designing sensing devices for hypochlorite in water samples. Thus, the HBT derivatives with meta methyl (3-HBT) were successfully applied to fabricate paper sensors for the quantification of hypochlorite in tap water. Hence, the fluorescent 3-HBT exhibits great promise as a selective and sensitive hypochlorite probe in chemical and biological applications.

A mussel-pearl side chain interaction in mercury(II) and phenol removal by sulfur-functionalized covalent organic frameworks: A DFT study.

The covalent organic framework materials (COFs) with excellent chemical and physical characteristics have been rapidly developed as adsorbents in the application of environmental remediation. In the design of COFs, the selection of functional groups and side chains is of great significance. Herein, density function theory (DFT) method is used to illustrate the adsorption behavior and mechanism of three sulfur-functionalized COFs (S-COFs) for the adsorption of mercury(II) and phenol. According to the analysis of geometric configurations and electronic properties, it demonstrated that the side chains of S-COFs with high flexibility and concentrated sulfur-functional groups, acting like a closed mussel which tightly confined the contaminants, the highest adsorption was -24.32 kcal/mol. The adsorption mechanism of phenol and mercury(II) on S-COFs was elucidated. For phenol, hydrogen bonds and π-π stacking interaction played an important role in the adsorption process, while the coordination interaction was dominated for the adsorption of mercury(II). This research explains the importance of selecting appropriate functional groups and side chains for COFs in the removal of contaminants in the molecular scale, and reveals the great potential of COFs in environmental remediation applications.

Chelating effect between uranyl and pyridine N containing covalent organic frameworks: A combined experimental and DFT approach.

Covalent organic frameworks (COFs) are promising adsorbents for removing heavy metal ions, and have high crystallinity, a porous structure, and conjugated stability. N-containing functional groups are known to have great affinity for uranyl ions. In this work, to explore the peculiarity of the pyridine N structure as an efficient adsorbent, we chose 2,2'-dipyridine-5,5'-diamine (Bpy) and pyridine-2,5'-diamine (Py) as the core skeletons, and 1,3,5-triformylphloroglucinol (Tp) as the linker to synthesize two crystalline and stable N-containing COFs named TpBpy and TpPy, respectively, through a facile solvothermal method. Characterization results demonstrated that TpBpy and TpPy possessed regularly growing pore sizes, large specific surface areas and relatively strong thermal resistances. The results of batch experiments showed that both COF materials were capable of the effective removal of uranyl with uptake capacities of 115.45 mg g-1 and 291.79 mg g-1, respectively. In addition, density functional theory (DFT) simulations highlighted the beneficial chelation effect of the double N structure in pyridine monomers for removing uranyl ions. Combining systematic experimental and theoretical analyses, the adsorption process and interaction mode of porous COFs and UO22+ were revealed, to provide predictable support for the application of pyridine N-containing COFs in the field of environmental remediation.

Extremely stable amidoxime functionalized covalent organic frameworks for uranium extraction from seawater with high efficiency and selectivity.

Uranium extraction from seawater is of strategic significance for nuclear power generation. Amidoxime-based functional adsorbents play indispensable roles in the recovery of seawater uranium with high efficiency. Nevertheless, balancing the adsorption capacity and selectivity is challenging in the presence of complicated interfering ions especially vanadium. Herein, a polyarylether-based covalent organic framework functionalized with open-chain amidoxime (COF-HHTF-AO) was synthesized with remarkable chemical stability and excellent crystallinity. Impressively, the adsorption capacity of COF-HHTF-AO towards uranium in natural seawater reached up to 5.12 mg/g, which is 1.61 times higher than that for vanadium. Detailed computational calculations revealed that the higher selectivity for uranium over vanadium originated from the specific bonding nature and coordination pattern with amidoxime. Combining enhanced adsorption capacity, excellent selectivity and ultrahigh stability, COF-HHTF-AO serves as a promising adsorbent for uranium extraction from the natural seawater.

U(VI) adsorption on hematite nanocrystals: Insights into the reactivity of {001} and {012} facets.

Predicting the environmental behavior of U(VI) relies on identification of its local coordination structure on mineral surfaces, which is also an indication of the intrinsic reactivity of the facet. We investigated the adsorption of U(VI) on two facets ({001} and {012}) of hematite (α-Fe2O3) by coupling experimental, spectroscopic and theoretical studies. Batch experiments results indicate higher removal capacity of the hematite {012} facet for U(VI) with respect to the {001} facet, due to the existence of extra singly and triply coordinated oxygen atoms with higher reactivity on the {012} facet while only doubly coordinated oxygen atoms exist on the {001} facet. The formation of surface complexes containing U(VI) is responsible for the appearance of a new sextuplet by Mössbauer spectra. The local structures of an inner-sphere edge-sharing bidentate complex on the hematite {001} and a corner-sharing complex on the {012} facet was deciphered by extended X-ray absorption fine structure spectroscopy. The chemical plausibility of the proposed structures was further verified by density functional theory calculation. This finding reveals the important influence of surficial hydroxyl groups reactivity on ions adsorption, which is helpful to better understand the interfacial interactions and to improve the prediction accuracy of U(VI) fate in aquatic environments.

Comparative adsorption capabilities of rubbish tissue paper-derived carbon-doped MgO and CaCO3 for EBT and U(VI), studied by batch, spectroscopy and DFT calculations.

Water pollution due to organic dyes and radionuclides is a challenging issue to the modern world. Cheap and efficient adsorbents are needed for their removal from wastewaters. Carbon-doped magnesium oxide (C-MgO) and calcium carbonate (C-CaCO3) were synthesized by the in situ hydrothermal treatment of Mg(OH)2 and Ca(OH)2 with carbon, and applied for the removal of eriochrome black T (EBT) at pH = 2.0 and uranium (U(VI)) at pH = 6.0. The Langmuir monolayer adsorption capacities of C-MgO (3.62 × 10-4 mol/g for EBT and 8.10 × 10-4 mol/g for U(VI)) were higher than those of C-CaCO3 (2.53 × 10-4 mol/g for EBT and 5.92 × 10-4 mol/g for U(VI)). The high adsorption capacity of C-MgO was also evidenced with DFT calculations which showed that the sorption energies (ΔE) of C-MgO for EBT (20.62 kcal/mol) and U(VI) (63.41 kcal/mol) were higher than those of C-CaCO3 for EBT (10.21 kcal/mol) and U(VI) (34.29 kcal/mol). In all cases, the electrostatic interactions were involved in the adsorption process. The sorption kinetic data followed pseudo-second-order kinetics. The results demonstrate that both C-MgO and C-CaCO3 are reusable and can be effectively applied for the elimination of EBT and U(VI) from wastewater.

Adsorption Properties of Hydrated Cr3+ Ions on Schiff-base Covalent Organic Frameworks: A DFT Study.

Considering the superior physiochemical property, increasing efforts have been devoted to exploiting the covalent organic frameworks (COFs) materials on the environmental remediation of heavy metal ions. Water pollution caused by Cr3+ metal ions is of special concern for scientists and engineers. Notwithstanding all the former efforts made, it is surprising that very little is known about the interaction mechanisms between the hydrated Cr3+ metal ions and COF materials. In present context, density functional theory (DFT) method is used to elucidate geometric and electronic properties with the purpose of putting into theoretical perspective the application values and interaction mechanisms for COF materials on Cr3+ capture. The results showed that all the five selected Schiff-base COFs materials displayed good adsorption performance on Cr3+ removal while the phenazine-linked and imine-COFs possessed the most favorable adsorption capacity due to the optimal chemical units and frameworks. The hydration effect was found to play a two-side role in the adsorption process and interaction mechanisms, involving coordination, hydrogen bonds, as well as weak non-covalent interactions, have been illuminated to explain the observed different adsorption behaviors. This study provides a general guidance for the design and selection of efficient COF materials as high-capacity Cr3+ adsorbents.

Highly Selective Recovery of Lanthanides by Using a Layered Vanadate with Acid and Radiation Resistance.

It is of vital importance to capture lanthanides (nuclear fission products) from waste solutions for radionuclide remediation owing to their hazards. The effective separation of lanthanides are achieved by an acid/base-stable and radiation-resistant vanadate, namely, [Me2 NH2 ]V3 O7 (1). It exhibits high adsorption capacities for lanthanides (qm Eu =161.4 mg g-1 ; qm Sm =139.2 mg g-1 ). And high adsorption capacities are maintained over a pH range of 2.0-6.9 (qm Eu =75.1 mg g-1 at low pH of 2.5). It displays high selectivity for Eu3+ (simulant of An3+ ) against a large excess of interfering ions. It can efficiently separate Eu3+ and Cs+ (or Sr2+ ) with the highest separation factor SFEu/Cs of 156 (SFEu/Sr of 134) to date. The adsorption mechanism is revealed by calculations and XPS, EXAFS, Raman, and elemental analyses. These merits combined with facile synthesis and convenient elution makes the title vanadate a promising lanthanide scavenger for environmental remediation.

Synthesis of orthogonally assembled 3D cross-stacked metal oxide semiconducting nanowires.

Assemblies of metal oxide nanowires in 3D stacks can enable the realization of nanodevices with tailored conductivity, porous structure and a high surface area. Current fabrication methods require complicated multistep procedures that involve the initial preparation of nanowires followed by manual assembly or transfer printing, and thus lack synthesis flexibility and controllability. Here we report a general synthetic orthogonal assembly approach to controllably construct 3D multilayer-crossed metal oxide nanowire arrays. Taking tungsten oxide semiconducting nanowires as an example, we show the spontaneous orthogonal packing of composite nanorods of poly(ethylene oxide)-block-polystyrene and silicotungstic acid; the following calcination gives rise to 3D cross-stacked nanowire arrays of Si-doped metastable ε-phase WO3. This nanowire stack framework was also tested as a gas detector for the selective sensing of acetone. By using other polyoxometallates, this fabrication method for woodpile-like 3D nanostructures can also be generalized to different doped metal oxide nanowires, which provides a way to manipulate their physical properties for various applications.

Coagulation mechanisms of humic acid in metal ions solution under different pH conditions: A molecular dynamics simulation.

Humic acid (HA) exerts a variety of significant environmental and geochemical influences on the soils, sediments and aqueous environments. The interaction with metal ions induces strong HA-metal complexation, thus effecting the transport of the toxic metals as well as the colloidal aggregation of HA. In the present work, we systematically report and analyze the aggregation mechanisms of HAs in solutions filled with different heavy cations (Ag+, Cd2+ and Cr3+) or common metal ions (Na+, Ca2+ and Al3+) under neutral and low pH conditions by using molecular dynamic simulations. We aim to explore the effects of pH, metal ions type and other possible weak interactions on the aggregation capabilities of HA. Scrutiny of the simulation results showed that the aggregation of HAs under neutral condition was driven by the HA-metal complexation which combined the effects of electrostatic attraction and inter-molecular bridging between cations and COO- groups. Larger extent of aggregation was found in heavy metal ions compared with the common ones. On the other hand, under low pH condition, due to the protonation states of carboxyl and phenolic group, the aggregation of HAs was stabilized mainly by weak forces, such as hydrogen bonds between different functional groups. In addition, other weak interactions such as the hydrophilic and hydrophobic effects, the cation-π interactions have also been proposed to be progressive effects on the coagulation behavior. Our computational studies give supplement to the experimental observation and provide insights into the intrinsic mechanisms of the aggregation behavior of HAs and their complexation with metal ions. Such computational modelling supplied a highly effective tool for qualitatively evaluating their roles in environmental remediation.

Three-dimensional graphene/titanium dioxide composite for enhanced U(VI) capture: Insights from batch experiments, XPS spectroscopy and DFT calculation.

Efficient containment and capture of uranium (U(VI)) from aqueous solution is an essential component to ensure socially and environmentally sustainable development. Herein, the three-dimensional graphene/titanium dioxide composite (3D GA/TiO2) was synthesized and applied as an effective adsorbent to remove U(VI) from wastewater as a function of contact time, temperature, pH and ion strength. The 3D GA/TiO2 material was characterized by X-ray diffraction, Raman spectroscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The batch experiments results indicated that the adsorption of U(VI) on materials were fitted with the pseudo-second order kinetics and Langmuir models. More specifically, 3D GA/TiO2 (441.3 mg/g) was observed to outperform the GO (280.0 mg/g), rGO (140.9 mg/g) and TiO2 (98.5 mg/g) at pH 5.0, which was attributable to the excellent cooperative effects. Furthermore, XPS analyses and DFT calculations confirmed the formation of surface complexes between oxygen-containing group and U(VI) with the U-O bonds length of 2.348 Š(U-O1) and 2.638 Š(U-O2). Meanwhile, the adsorption energy was calculated to be 1.60 eV, which showed a very strong chemisorption during the interaction process. It is believed that the 3D GA/TiO2 revealed good removal performance for uranyl ions, which showed a great potential application to control the nuclear industrial pollution.

Systematic studies on the binding of metal ions in aggregates of humic acid: Aggregation kinetics, spectroscopic analyses and MD simulations.

The binding of metal ions with humic acid (HA) plays an important role in the aggregation of HA and the migration of metal ions in the environments. The effects of common cations (Na+, Mg2+, Ca2+ and Al3+) and heavy metal ions (Ag+, Cd2+, Cu2+, Cr3+ and Eu3+) on the aggregation of HA were investigated systematically by aggregation kinetics, spectroscopic techniques and molecular dynamic (MD) simulations. The critical coagulation concentration (CCC) of mono-, di- and trivalent cations could be predicted by the Schulze-Hardy rule. The aggregation of HA in the presence of Na+ and Ag+ was mainly due to the reduction of repulsive force and the hydrogen bonds between HA molecules. While the complexation of di- and trivalent cations with carboxylic/phenolic groups, or the cation-π interactions enhanced the intra- or inter-molecular bridges in HA and then contributed greatly to the aggregation of HA. Heavy metal ions could easily pass through the electric double-layer of HA compared with common cations. MD simulations further signified the strong aggregation ability of HA molecules in solutions containing high valence metal ions. These findings are important for understanding not only how the influence of metal ions on the aggregation of HA, but also the conditions which ions more efficient for aggregation.

Anaerobic Alcohol Conversion to Carbonyl Compounds over Nanoscaled Rh-Doped SrTiO3 under Visible Light.

Photocatalytic oxidation of organic compounds on semiconductors provides a mild approach for organic synthesis and solar energy utilization. Herein, we identify the key points for the photocatalytic oxidation over Pt-loaded Rh-doped strontium titanate allowing the conversion of alcohols efficiently and selectively to aldehydes and ketones under anaerobic conditions and visible light with an apparent quantum efficiency of pure benzyl alcohol oxidation at 420 nm of ≤49.5%. Mechanistic investigations suggest that thermodynamically the controlled valence band edge position via Rh doping provides a suitable oxidation ability of photogenerated holes, avoiding the powerful hydroxyl radical intermediates prone to overoxidation resulting in high selectivity. Kinetically, oxygen vacancies induced by Rh3+ substitution in the SrTiO3 lattice not only favor the dissociative adsorption of alcohols yielding alkoxy species but also induce the weakening of the α-C-H bond facilitating its cleavage by the photogenerated holes. Pt nanoparticles deposited as a cocatalyst contribute to the final hydrogen evolution.

Efficient elimination of Cr(VI) from aqueous solutions using sodium dodecyl sulfate intercalated molybdenum disulfide.

In recent years, the heavy metal ions have been immoderately released into the ecological system and result in potential hazardous to public health. Herein, the sodium dodecyl sulfate intercalated molybdenum disulfide (SDS-MoS2) was synthesized for the adsorption of Cr(VI). The SDS molecule was flat and vertically intercalated into the interlayer of MoS2, which was further evidenced by density functional theory calculations. The capture of Cr(VI) on the sphere-like SDS-MoS2 relied on solution pH. The retention of Cr(VI) on SDS-MoS2 attained 63.92 mg/g, and the removal process was endothermic, spontaneous and increased with temperature increasing. The main removal mechanism of Cr(VI) onto SDS-MoS2 was Cr(VI) fixing on the surface of the composites by chemisorption involving possible Cr-S coordination bonding. More importantly, Cr(VI) passed into the increased interlamination and reacted at the interlamination of SDS-MoS2, which was further proved at molecular level. The results can provide critical information for the application of SDS-MoS2 in Cr(VI) elimination or other kinds of pollutants removal in natural aquatic environment.

N, P, and S Codoped Graphene-Like Carbon Nanosheets for Ultrafast Uranium (VI) Capture with High Capacity.

The development of functional materials for the highly efficient capture of radionuclides, such as uranium from nuclear waste solutions, is an important and challenging topic. Here, few-layered N, P, and S codoped graphene-like carbon nanosheets (NPS-GLCs) that are fabricated in the 2D confined spacing of silicate RUB-15 and applied as sorbents to remove U(VI)ions from aqueous solutions are presented. The NPS-GLCs exhibit a large capacity, wide pH suitability, an ultrafast removal rate, stability at high ionic strengths, and excellent selectivity for U(VI) as compared to multiple competing metal ions. The 2D ultrathin structure of NPS-GLCs with large spacing of 1 nm not only assures the rapid mass diffusion, but also exposes a sufficient active site for the adsorption. Strong covalent bonds such as P-O-U and S-O-U are generated between the heteroatom (N, P, S) with UO2 2+ according to X-ray photoelectron spectroscopy analysis and density functional theory theoretical calculations. This work highlights the interaction mechanism of low oxidation state heteroatoms with UO2 2+, thereby shedding light on the material design of uranium immobilization in the pollution cleanup of radionuclides.

Computational Study on the Excited-State Decay of 5-Methylcytosine and 5-Hydroxymethylcytosine: The Common Form of DNA Methylation and Its Oxidation Product.

5-Methylcytosine (5mC) is the predominant epigenetic modification of DNA. 5mC and its sequential oxidation product, 5-hydroxymethylcytosine (5hmC), are crucial epigenetic markers which have a profound impact on gene stability, expression, and regulation. In the present work, ab initio electronic structure computations were performed to investigate the excited-state decay pathways for 5mC and 5hmC in both the neutral and protonated forms. Based on the theoretical quantities, four nonradiative decay pathways via conical intersections (CIs) were identified: ring distortion, ring opening, N-H dissociation, and intersystem crossing (ISC) pathways. Additional calculated potential energy surfaces revealed that ring distortion and ISC pathways were the most effective routes for 5mC and 5hmC, respectively. The influence of environmental factors, such as the solution and an acidic environment, was also explored in this study. Our study demonstrated that excited-state decay pathways via CIs are indispensable for the photostability of DNA epigenetic modifications and may be involved in ingenome stability and mammalian development.