MOFs for water purification

<正>Metal-organic frameworks (MOFs),as emerging porous crystalline materials,were constructed from metal ions or clusters as nodes and organic ligands as linkers.Due to the superior features like diverse structure,abundant functional groups,tunable pore channels,large specific surface area and so on,MOFs have received increasing attentions in various fields like gas separation&storage,catalysis/photocatalysis,adsorption,sensor,electrochemistry as well as biochemistry.     

When is water not water? Exploring water confined in large reverse micelles using a highly charged inorganic molecular probe

The interior water pool of aerosol OT (AOT) reverse micelles tends toward bulk water properties as the micelle size increases. Thus, deviations from bulk water behavior in large reverse micelles are less expected than in small reverse micelles. Probing the interior water pool of AOT reverse micelles with a highly charged decavanadate (V(10)) oligomer using (51)V NMR spectroscopy shows distinct changes in solute environment. For example, when an acidic stock solution of protonated V(10) is placed in a reverse micelle, the (51)V chemical shifts show that the V(10) is deprotonated consistent with a decreased proton concentration in the intramicellar water pool. Results indicate that a proton gradient exists inside the reverse micelles, leaving the interior neutral while the interfacial region is acidic.     

Monitoring three typical phenol endocrine disrupting compounds in drinking water of Suzhou urban area – from raw water to tap water

In this work, three typical phenol endocrine-disrupting compounds (EDCs), namely Bisphenol A (BPA), 4-nonylphenol (4-NP) and 4-tert-octyphenol (4-t-OP), were detected in water samples from the whole drinking water supply chain of seven water plants at three hydrographic periods in Suzhou, China. Almost all of the drinking water in urban area of Suzhou city is provided by the seven water plants. Every link of drinking water supply, including raw water, finished water and tap water were monitored. The results indicated that BPA and 4-NP were detected in all samples with the concentration of 0.067–0.118 μg L^?1 and 0.111–0.350 μg L^?1, respectively, while 4-t-OP could be detected in only a few raw water samples with the concentration lower than its limit of quantitation (LOQ). The levels of BPA and 4-NP in raw water were significantly different among three hydrographic periods and the highest concentration can be observed in dry period. The difference in removal efficiency of BPA and 4-NP in seven water plants was also observed among three hydrographic periods, e.g. relative higher in dry period for BPA and in flow period for 4-NP. Potential risk assessment hinted that BPA and 4-NP in raw water showed a low to medium and a medium to high risk respectively to aquatic environment, while in tap water, the health risk from BPA and 4-NP could be ignored on human.     

Mass accommodation mechanism of water through monolayer films at water∕vapor interface

The mass transfer dynamics at water∕vapor interface through monolayer films was theoretically investigated by a combination of molecular dynamics and Langevin dynamics simulations. The rare events of mass accommodation are sampled by the Langevin simulation with sufficient statistical accuracy, on the basis of the free energy and friction profiles obtained by the molecular dynamics simulation. The free energy profiles exhibit a barrier in the long-chain monolayers, and the mechanism of the barrier is elucidated in relation to the "water finger" formation. The present Langevin simulation well described the remarkable dependence of the mass accommodation coefficient on the chain length and surface density. The transition state theory for the barrier passage remarkably overestimates the mass accommodation coefficient, and the Kramers or Grote-Hynes theory may not be appropriate, due to large variation of the friction in the entrance channel and∕or broad barrier.     

Stimulus-responsive water soluble synthetic nanographene

Nanographenes(NGs)are exceptionally hydrophobic.They are insoluble in water,preventing the exploration and utilization of their photophysical properties under aqueous conditions.This work discloses an atomically precise water-soluble synthetic NG1,featuring a 2 nm sp² carbon skeleton appended with 12 branched triethylene glycol chains.It synergistically combines low critical solution temperature(LCST)behavior and a photothermal effect to create the first thermo-and photo-responsive atomically precise NG functioning in an aqueous solution.The LCST behavior can be attributed to a delicate balance of hydrophobic-hydrophilic interactions,providing a sensitive thermal response to changes over a temperature range of physiological interest(close to 37℃).Moreover,1 has considerable photothermal conversion capability,with irradiation of 1 in water by red or near infrared light increasing the solutions temperature to above the clouding point within seconds,leading to a reversible clear-to-turbid transition over many cycles without evident fatigue.     

Vitrification and crystallization of low-temperature amorphous condensates of water and methane—water mixture

Low-temperature condensates of water and water-methane mixture are studied in the temperature range of 65–200 K. Amorphous samples are obtained by molecular beam deposition under vacuum conditions on the substrate cooled with liquid nitrogen. The vitrification and crystallization temperatures are determined from the changes in the dielectric properties of the condensates upon heating. The kinetics of crystallization of amorphous water layers is studied by differential thermal analysis. The temperature conditions for the growth of thick methane crystalline hydrate layers during the low-temperature condensation of molecular water-gas mixture beams are found.     

Bulk properties and hydration numbers of components of water–salt,water–urea,and water–urea–salt systems

With an increase in the concentration of additives, the hydration numbers of compounds decrease. Thus, in a saturated 54.6% solution, urea loses approximately 3/4 of the initial amount of water, forming an aquacomplex of the composition (NH₂)₂CO?H₂O. In a supersaturated 44% solution, the sodium chloride aquacomplex is dehydrated by 2/3, and in a supersaturated 67% solution, sodium sulfate is dehydrated by 5/6. The density of these solutions is 1.354÷1.360 g/cm³ (44% NaCl) and 1.800÷1.849 g/cm³ (67% Na₂SO₄). In a saturated urea solution, NaNO₃, NaCl, and Na₂SO₄ complexes lose 53÷55% of hydration water. It is shown that the interactions in the binary water–urea system somewhat increase the hydration number of the salts (structural hydration). The hydration water density, a structurally important characteristic, increases in the series of solutions of urea, NaNO₃, NaCl, and Na₂SO₄. In the same series of additives, the excess volume of binary water–urea and water–salt systems becomes more negative.     

An enhanced β turn in water

Aiming to design short linear peptides featuring strong intramolecular hydrogen bonds in water, a series of tetrapeptides based on the sequence Ac-Ala-Pro-Ala-Ala-NH(2) containing all possible combinations of L- and D-amino acids was synthesized. A regiospecific combination of heterochiral residues (DDLL or its mirror image LLDD) can be used to increase turn formation and stability within short peptides in water.     

Can water float on oil?

The floatability of water on oil surface was studied. A numerical model was developed from the Young-Laplace equation on three interfaces (water/oil, water/air, and oil/air) to predict the theoretical equilibration conditions. The model was verified successfully with an oil/water system. The stability of the floating droplet depends on the combination of three interface tensions, oil density, and water droplet volume. For practical purposes, however, the equilibrium contact angle has to be greater than 5° so the water droplet can effectively float. This result has significant applications for biodegrading oil wastes.     

Solid water at room temperature?

Previously, we have shown that water adjacent to many hydrophilic substances excludes colloidal and molecular solutes. It was labelled “exclusion zone” (EZ) or “fourth phase” water. A salient feature is its characteristic light absorbance at or near 270 nm. In this study, EZ water formed against three chemically distinct surfaces, Nafion, ghee, and Whatman-5 filter paper was extracted, characterized by UV–Visible absorbance spectroscopy, and solidified either by lyophilizing or evaporation in an oven. The resulting highly stable solid was dissolved in water and confirmed as EZ water by its characteristic absorbance at 270–280 nm. We used mass spectroscopy to verify the absence of ionizable contaminants that could reproduce the characteristic “signature EZ” spectra in the three liquid preparations, or in the solids formed from desiccated EZ water that had been reconstituted in deionized water. Hence, a solid form of EZ water may, indeed, exist at room temperature.     

Iron analysis in atmospheric water samples by atomic absorption spectroscopy (AAS) in water–methanol

To distinguish between Fe(II) and Fe(III) species in atmospheric water samples, we have adapted an analytical procedure based on the formation of a specific complex between Fe(II) and ferrozine (FZ) on a chromatographic column. After elution of Fe(III), the Fe(II) complex is recovered with water–methanol (4:1). The possibility of trace iron measurements in this complex medium by graphite-furnace atomic-absorption spectrometry has been investigated. A simplex optimization routine was required to complete the development of the analytical method.     

Surface oxidation and water desorption of CdS

The surface oxidation and HP desorption of powder CdS were studied by means of X-ray photoetectron spectroscopy (XPS), quadrupole mass spectrometry (QMS) and in-situ FTIR. The results show that with the changes of surface composition and the elongation of store time of CdS there are four types of H2O thermally desorbed at different temperatures. It has also been found that through high-temperature air treatment for a short time the oxidized surface layer of CdS can prevent O2 and H2O in air from further attacking the inner CdS molecules.     

Does water condense in carbon pores?

Using grand canonical Monte Carlo (GCMC) simulations of molecular models, we investigate the nature of water adsorption and desorption in slit pores with graphitelike surfaces. Special emphasis is placed on the question of whether water exhibits capillary condensation (i.e., condensation when the external pressure is below the bulk vapor pressure). Three models of water have been considered. These are the SPC and SPC/E models and a model where the hydrogen bonding is described by tetrahedrally coordinated square-well association sites. The water-carbon interaction was described by the Steele 10-4-3 potential. In addition to determining adsorption/desorption isotherms, we also locate the states where vapor-liquid equilibrium occurs for both the bulk and confined states of the models. We find that for wider pores (widths >1 nm), condensation does not occur in the GCMC simulations until the pressure is higher than the bulk vapor pressure, P0. This is consistent with a physical picture where a lack of hydrogen bonding with the graphite surface destabilizes dense water phases relative to the bulk. For narrow pores where the slit width is comparable to the molecular diameter, strong dispersion interactions with both carbon surfaces can stabilize dense water phases relative to the bulk so that pore condensation can occur for P < P0 in some cases. For the narrowest pores studied--a pore width of 0.6 nm--pore condensation is again shifted to P > P0. The phase-equilibrium calculations indicate vapor-liquid coexistence in the slit pores for P < P0 for all but the narrowest pores. We discuss the implications of our results for interpreting water adsorption/desorption isotherms in porous carbons.     

Formation of a 1-D metal–water chain and a 2-D water–sulfate layer containing cyclic water octamers in a 3-D supramolecular network

A new tetranuclear complex, {[Cu(phen)(OH)]₄(H₂O)₂}?·?(SO₄)₂?·?8H₂O (1) (phen = 1,10-phenanthroline), has been synthesized and characterized by elemental analysis, UV-Vis, IR, TG, and single-crystal X-ray diffraction. Complex 1 exhibits a 3-D supramolecular network with a 1-D metal–water chain consisting of tetranuclear hydroxo-bridged copper(II) clusters and water octamers and a 2-D water–sulfate layer containing cyclic water octamers with five types of O–H?···?O hydrogen-bonding motifs. The free ligand and its complex were screened for antibacterial activity against Gram-positive and Gram-negative bacteria.     

Nanoparticle-catalyzed green synthetic chemistry … in water

Pre-catalyst nanoparticles (NPs) can be derived from FeCl₃ and methyl Grignard that contain ppm levels of Pd and an associated ligand. By varying the amount of precious metal and the nature of the phosphine ligand, different C–C bond-forming reactions can be catalyzed in an aqueous medium containing very small amounts (2 wt %) of the designer surfactant TPGS-750-M. Reactions mediated by these NPs include Suzuki–Miyaura, Sonogashira, Mizoroki–Heck, and Negishi couplings, as well as other couplings that have appeared in the recent literature. Also included in the discussion are nitro group reductions that require even less Pd and no ligand for activity, as well as ppm Cu-containing NPs that catalyze click reactions in water at rt.     

Effect of water droplet in solvent sublation

Aqueous phase layer around bubble and water droplet are two additional processes in solvent sublation. In the dynamic process of mass transfer, they are always neglected, but they are very important in the investigation of thermodynamic equilibrium. In this paper, the effect of water droplet in solvent sublation was discussed in detail, and the previous mathematical model of solvent subaltion was improved. Matlab 6.5 was used to simulate the process of water droplets, and the comparison between the previous hypothesis and the improvement in this paper showed the superiority, especially in the investigation of thermodynamic equilibrium. Moreover, the separation and concentration of the complex compound dithizone-Co（Ⅱ） from aqueous phase to n-octanol by solvent sublation also proved the improved mathematical model was reasonable.     

“Binding” of water by ions

Summary We confirmed the inadequacy of the correction for, binding of water by ions for explaining the discrepancy between the electrostatic theory in [2] and experiment.     

“Surface water” and “strong-bonded water” in cyclodextrins: a Karl Fischer titration approach

Cyclodextrins are some of the most used carriers for bioactive compounds (as host–guest complex) and many factors influence the association–dissociation of this complex, some of them being related to hydrophobicity. In the solid state, cyclodextrins contain two types of water molecules: “surface” water molecules (especially close to the crystal surface) and “strong-bonded” water molecules (especially from the cyclodextrin cavity), but the classification is hard to do, and the concentration of these water molecules are relatively difficult to estimate by simple methods. In the present study we used the volumetric Karl Fischer titration to estimate these types of water molecules in cyclodextrins by means of the rate of water reaction (related to diffusion from cyclodextrin crystals). “Surface” water molecules are titrated with rates between 1.8–2.8 mM/s for α-cyclodextrin, while for β-cyclodextrin these rates are little bit higher (2.9–3.4 mM/s). The rates corresponding to “strong-bonded” water molecules are approximately tens fold lower (0.05–0.3 mM/s for α-cyclodextrin and 0.15–0.33 mM/s for β-cyclodextrin). The approximate ratio between “surface” and “strong-bonded” water molecules could also be estimated by this simple and rapid method.     

Argon–water system at low temperatures

The molecular dynamics method is used to simulate argon solutions in water and a thin water film–argon system at low temperatures. The correlation in motions of two closely spaced argon atoms is of another nature than the correlation of two neon atoms in a neon solid solution in ice II. The structure of hydrate shells of argon atoms contains five-membered rings composed of water molecules. The solubility of argon in a water film at low temperatures is noticeably higher than at room temperature. If a water film is first cooled to the glassy state and then argon atoms are added to it, then approximately as many argon atoms are absorbed on the film surface as they are present in a cooled film in equilibrium with the argon atmosphere. Argon atoms migrate from one pit to another on the rough surface of a solid water film.