Metal Oxides In Water Research Articles

Formation of environmentally persistent free radicals from photodegradation of triclosan by metal oxides/silica suspensions and particles

Metal oxides play an essential role in the photocatalysis of contaminants and substantially increase in the environment by the engineering production. However, whether emerging contaminants will be produced during photocatalysis of contaminants remains unclear. Here, triclosan (TCS) photodegradation in metal oxides/silica suspensions and particles, simulated as the states of metal oxides in water and soil environments, were studied. The photodegradation results confirmed that metal oxides exhibited a double-effect. They promoted TCS photodegradation by generating reactive oxidizing species (ROS) in metal oxides/silica suspensions and inhibited the photodegradation by competing with TCS for irradiation in metal oxides/silica particles. In this study, the critical discovery was the formation of emerging contaminants, environmentally persistent free radicals (EPFRs), and EPFRs yields were promoted by metal oxides (Al2O3, ZnO, TiO2). They were more stable in metal oxides than silica, and the half-lives ranged from 6.7 h to 90.9 d. Although CuO did not increase EPFRs yields compared to silica, the half-lives of EPFRs were also longer. In addition, this study found that EPFRs yields were dependent on the metal oxides concentrations. Our results provided a new insight into the negative environmental impacts of metal oxides and improved our understanding of the formation and fate of EPFRs by metal oxides in soil and aquatic environments.

Zinc Oxide Nanostructures Synthesized by a Simple Hot Water Treatment Method for Photocatalytic Degradation of Organic Pollutants in Water

Abstract The use of zinc oxide (ZnO) nanostructures as a photocatalyst for the degradation of organic pollutants in water has received significant attention over the recent years. However, synthesis methods for producing ZnO nanostructures are generally costly, complicated, and hazardous to the environment. In this work, we demonstrate the synthesis of ZnO nanostructures by a simple hot water treatment (HWT) method and the photocatalytic activity of the hence produced nanostructures. HWT is a one-step, low-cost, eco-friendly, and scalable nanostructure growth method. By HWT, various metal-oxide nanostructures can be produced simply by the interaction of metals with hot water without the need for any chemical additives in the solution. Growth of metal-oxide nanostructures by HWT involves the formation of metal-oxides and their release from the surface of the metal into water, the migration of the metal-oxides in water, and their re-deposition at a different part of the metallic surface, which initiates the growth of nanostructures. In this study, we used zinc powder and plates for producing the ZnO nanostructures by HWT in DI water at 75°C. Scanning electron microscopy and X-ray diffraction were utilized to verify the formation of ZnO nanostructures. Zinc plates produced a suspension of ZnO nanostructures in water, while on the other hand, zinc powder resulted in ZnO nanostructures grown on the powder surface as well as standalone ZnO nanostructures also mixed in water. We used these nanostructures + water suspensions for our photocatalytic degradation studies. Methylene blue (MB) was used as a model organic pollutant. We mixed the ZnO nanostructure suspension with MB and exposed it to UV light. The degradation of MB was observed by measuring its absorbance values using a UV-Visible spectrophotometer over a period of 4 hours. We observed a 20% decrease in the concentration of MB in 4 hours when nanostructured Zn/ZnO powder suspension was used, and a 30% decrease was achieved when ZnO nanostructure-only suspension produced from zinc plates was used. MB alone was also exposed to UV light for the same period as a control experiment, and we did not observe any significant decrease in its concentration. These results indicate that the hot water treatment method presents a very simple, cost-effective, scalable, and eco–friendly alternative for the synthesis of ZnO nanostructures for photocatalytic water treatment applications.

Modifying Surface Chemistry of Metal Oxides for Boosting Dissolution Kinetics in Water by Liquid Cell Electron Microscopy.

Dissolution of metal oxides is fundamentally important for understanding mineral evolution and micromachining oxide functional materials. In general, dissolution of metal oxides is a slow and inefficient chemical reaction. Here, by introducing oxygen deficiencies to modify the surface chemistry of oxides, we can boost the dissolution kinetics of metal oxides in water, as in situ demonstrated in a liquid environmental transmission electron microscope (LETEM). The dissolution rate constant significantly increases by 16-19 orders of magnitude, equivalent to a reduction of 0.97-1.11 eV in activation energy, as compared with the normal dissolution in acid. It is evidenced from the high-resolution TEM imaging, electron energy loss spectra, and first-principle calculations where the dissolution route of metal oxides is dynamically changed by local interoperability between altered water chemistry and surface oxygen deficiencies via electron radiolysis. This discovery inspires the development of a highly efficient electron lithography method for metal oxide films in ecofriendly water, which offers an advanced technique for nanodevice fabrication.

Green, Multi-Gram One-Step Synthesis of Core-Shell Nanocomposites in Water and Their Catalytic Application to Chemoselective Hydrogenations.

We devise a new and green route for the multi-gram synthesis of core-shell nanoparticles (NPs) in one step under organic-free and pH-neutral conditions. Simply mixing core and shell metal precursors in the presence of solid metal oxides in water allowed for the facile fabrication of small CeO2 -covered Au and Ag nanoparticles dispersed on metal oxides in one step. The CeO2 -covered Au nanoparticles acted as a highly efficient and reusable catalyst for a series of chemoselective hydrogenations, while retaining C=C bonds in diverse substrates. Consequently, higher environmental compatibility and more efficient energy savings were achieved across the entire process, including catalyst preparation, reaction, separation, and reuse.

Dark ambient degradation of Bisphenol A and Acid Orange 8 as organic pollutants by perovskite SrFeO3−δ metal oxide

Current advanced oxidation processes (AOPs) are chemically and energetically intensive processes, which are undesirable for cost-effective and large-scale system water treatment and wastewater recycling. This study explored the Strontium Ferrite (SFO) metal oxide on the degradation of highly concentrated organic pollutants under dark ambient condition without any external stimulants. The SFO particles with single perovskite structure were successfully synthesized with a combined high temperature and high-energy ball milling process. An endocrine disruptor, Bisphenol A (BPA) and an azo dye, Acid Orange 8 (AO8) were used as probe organic pollutants. BPA was completely degraded with 83% of mineralization in 24h while rapid decoloration of AO8 was achieved in 60min and complete breakdown into primary intermediates and aliphatic acids occurred in 24h under the treatment of dispersed SFO metal oxide in water. Such efficient degradation could be attributed to the enhanced adsorption of these anionic pollutants on positively charged ball-milled SFO metal oxide surface, resulted in higher degradation activity. Preliminary degradation mechanisms of BPA and AO8 under the action of SFO metal oxide were proposed. These results showed that the SFO metal oxide could be an efficient alternative material as novel advanced oxidation technology for low cost water treatment.

A three-dimensional microstructural model of reactions and transport in aqueous mineral systems

A stochastic three-dimensional microstructure model is introduced for simulating spatial and temporal variations in aqueous mineral systems. Dissolution, nucleation, precipitation and solute transport are governed by local probabilistic rules applied on a regular computational lattice. The model is shown to accurately simulate ion diffusion in a dilute electrolyte. The reaction algorithms faithfully reproduce kinetics expected from standard rate equations, and reversible reactions are shown to converge to the correct equilibrium state determined by detailed balance of forward and reverse reaction rates, or the law of mass action. Accounting for the exponential temperature dependence of the reaction rate constants is shown to provide accurate predictions of the influences of temperature on both the kinetics and equilibrium of reactions. A simulation of the hydration of a generic metal oxide in water demonstrates the important relationships between microstructure development and the mechanisms of nucleation, growth and solute diffusion.

Ultrasonic vibration potential and point of zero charge of some rare earth oxides in water

The surface charge of rare earth metal oxides in water was examined using ultrasonic vibration potential (UVP) technique for the dilute colloid. Sonic irradiation induced an alternative current in suspensions of oxide powders in water. The observed potential largely varied, depending on the surface charge density of oxide powders, for suspensions at different pH as well as the kind of rare earth metals used. The paper described the measurement results and comparison for several rare earth oxides. The effect of pH and its swing and the kinds of rare earth element in oxides were reported. The point of zero charge (p.z.c., pH of zero charge for powders in water) strongly depended on the kinds of metal elements forming oxides and valence of metal ions. For a series of rare earth elements, dependence of atomic number can be observed; however, the distinguishing feature was the difference between trivalent rare earth metal oxides and tetravalent Ce oxide. The p.z.c. of CeO 2 was 6.7, while those of oxides of trivalent rare earth were 8.3–8.8.

Surface Hydroxyl Site Densities on Metal Oxides as a Measure for the Ion-Exchange Capacity

Hydroxyl groups on metal oxide in water are the sites for ion exchange, and the surface hydroxyl site density on oxides is a measure of the ion-exchange capacity. Here, the Grignard reagent method was applied to determine the surface hydroxyl site density of oxide samples. The results were similar to those reported for different oxides with other methods (dehydration by heating, tritium exchange, crystallographic calculations, etc.), and they are comparable with those calculated from the closest packing of hydroxide ions. A mechanism of hydroxylation is proposed: lattice oxide ions (extremely strong bases) are exposed to aqueous solutions and are neutralized by water to become hydroxide ions. Also, the saturated deprotonation method was applied to hematite, and it was found that all the acid hydroxyl groups on hematite were deprotonated in very high concentrations of alkali solutions (∼5 mol dm−3NaOH), and from the saturated amount of OH−consumed by deprotonation, the same result as that by the Grignard method was obtained. It is shown that all hydroxyl groups take part in ion exchange and that the unusually small values reported elsewhere with the saturated (de)protonation method may contain errors. Hetero- or homogeneity of hydroxyl groups in contact with water as ion-exchange sites is also discussed. It is suggested that intensely hydrated layers at the oxide/water interface may result in homogeneous discrete sites. The development of microstructures in the oxides was suggested from the measured values of specific surface areas, and the effect of the microstructure environments on the reactivity of internal surface hydroxyl sites is discussed.

Studies of Organic Catalytic Reactions. V. The Function of the Chelates formed in the Hydrolysis of 2-Cyanopyridine with Metal Oxide Catalysts

Abstract When 2-cyanopyridine is refluxed with water in the presence of nickel or copper oxide, it is easily hydrolyzed and affords a chelate compound of the acid with nickel or copper. On the other hand, 3- or 4-cyanopyridine affords only the corresponding free amide by the same reaction. The interfering effect of thiophene is not observed in the reaction of 2-cyanopyridine, but it is observed in the case of the 3- or 4-cyanopyridine. When 2-cyanopyridine is refluxed with an aqueous solution of nickel or cupric chloride, it is hydrated to form a chelate of the amide with nickel or copper. These chelates are hydrolyzed with the metal oxides in water to afford the chelates of the acid. Pyridine-2-carboxylic acid amide is also hydrolyzed to the chelate of the acid, but the hydrolysis of the 3- or 4-amide isomer does not occur under the same conditions. The four chelates of the acid and amide are excellent catalysts for the hydration of 2-cyanopyridine, giving the free amide in a high yield. From these facts, it may be presumed that: 1) The adsorption of 2-cyanopyridine or its amide on the catalysts is stereospecific and results in the formation of a kind of chelate. Upon forming the acid, the metal atom of the catalyst is drawn out of the catalyst to form the chelate. 2) The metal oxide is the most suitable catalyst for the chelate-type adsorption and for the chelate formation. 3) The chelate of the acid formed during the hydrolysis of 2-cyanopyridine acts as another catalyst in the formation of the free amide, then, the amide is further converted into the chelate of the acid by means of the metal oxide catalyst.