Migration Of Metal Cations Research Articles

Comprehensive Evaluation of Poly-Phosphate Release from Sewage Sludge with Thermal Treatment

Phosphorus is a necessary nutrient element to all lives in the world. Distribution and phosphate rock reserves in the world disclose shortage crisis of phosphorus. Thus phosphorus recovery has been a research hot spot in the world. Mr.Ohtake’ group [1] recommended a new method to recover phosphorus from sewage sludge during heating process. It seems that nearly 90% of the poly-phosphate (poly-P) and 87% of total phosphate (T-P) can be released from cultivated sludge by heating at 70 °C for 1 hour. Meanwhile, released poly-P can precipitate with Ca in neutral condition without any pH adjustment. Sewage sludge was used in this research to identify its performance during thermal treatment. With the increase of temperature and heating time, release of poly-P and T-P increased obviously. However, poly-P would be hydrolyzed when temperature was higher than 80 °C. With addition of EDTA, release of poly-P increased nearly 100%. Migration of metal cations during thermal treatment can provide indirect evidence which kinds of metal cations would make impediment on poly-P release. A novel phenomenon was observed poly-P mixed with ortho-phosphate can precipitate with NH4+ and Mg2+ as struvite at pH 8.5.

Resistive Switching in Conductive-Bridging Random-Access Memory Structure with Nanocrystalline Silicon Films

In this work, intrinsic nanocrystalline silicon thin films were deposited on indium tin oxide/glass substrates by plasma-enhanced chemical vapor deposition and used as a conduction material for conductive-bridging random-access memory devices. The resistive switching characteristics of nanocrystalline silicon thin films were investigated. Experimental results show a stable bipolar resistive switching of the nanocrystalline silicon films with a retention time of over 1×104 s. In addition, the current conduction mechanism of the nanocrystalline silicon films was examined by X-ray photoelectron spectroscopy depth profiling and transmission electron microscopy analysis. Results clearly indicate that the conduction mechanism for resistive switching is the formation of metallic bridges due to the metal cation migration in the nanocrystalline silicon films.

Resistive Switching Characteristics in Nanocrystalline Silicon Films for Conductive-Bridging Resistive Random-Access Memory Applications

In this paper, intrinsic nanocrystalline silicon thin films were deposited onto the ITO/glass substrates by PECVD and were used as the conduction material for the conductive-bridging random-access memory devices. The resistive switching characteristics of the nanocrystalline silicon thin films were investigated. Experimental results have shown that the stable bipolar resistive switching of the nc-Si films and retention time over 104s. In addition, the current conduction mechanism of the nanocrystalline silicon films was examined with XPS depth file analysis. It clearly indicates that the conduction mechanism for the resistive switching is formation metallic bridges come form metal cation migration in the nanocrystalline silicon films.

NOx storage/reduction over alkali-metal-nitrate impregnated titanate nanobelt catalysts and investigation of alkali metal cation migration using XPS

We reported earlier that KNO3- and Pt-impregnated potassium titanate nanobelt (KTN) catalysts exhibit high storage capacity and excellent catalytic cycles for NOx storage-reduction (NSR) reaction. In the present study, we compared the NSR behavior of various alkali-metal-nitrate impregnated KTN catalysts, which revealed that the KNO3-impregnated catalyst exhibited superior performance. The XPS and XRD analyses of the NaNO3-impregnated KTN catalyst showed that the migration of K+ from KTN bulk to the surface nitrate salt took place more predominantly than the migration of Na+. Probably for this reason, KNO3-impregnated KTN materials exhibit the best performance for NSR catalytic cycles.

Electrode dependence of filament formation in HfO2 resistive-switching memory

This study investigates bipolar and nonpolar resistive-switching of HfO2 with various metal electrodes. Supported by convincing physical and electrical evidence, it is our contention that the composition of conducting filaments in HfO2 strongly depends upon the metal electrodes. Nonpolar resistive-switching with the Ni electrode is attributed to the migration of metal cations and the corresponding electrochemical metallization. Conversely, oxygen-deficient filaments induced by anion migration are responsible for bipolar resistive-switching. It was also found that the characteristic nature of the conducting filaments influences many aspects of switching characteristics, including the switching power, cycling variations, and retention at elevated temperatures.

Cr and Zr/Cr nitride CAE-PVD coated aluminum bipolar plates for polymer electrolyte membrane fuel cells

In this work, two nitride coatings deposited on aluminum-based bipolar plates via cathodic arc evaporation physical vapor deposition (CAE-PVD) have been evaluated using two different techniques. The coating materials, a multi-layer chromium-zirconium nitride (ZrN–CrN) and a monolayer chromium nitride (CrN) have been exposed to electrochemical polarization tests for corrosion resistance simulating the typical environment in the anode and cathode sides in polymer electrolyte membrane fuel cells (PEMFC). Besides, two 3-cell PEMFC stacks, one per each coating material, have been formed. The migration of metal cations toward both the gas diffusion layers (GDL) and catalyst layers have been analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), after 100 h of continuous operation of the stacks. Results have shown that the two coatings applied over the Al-plates satisfy the corrosion resistance requirements in the short-term tests performed at the two stacks. Moreover, results obtained from electrochemical polarization tests have revealed that the CrN-coating confers a good corrosion resistance to the Al-based metal plate, achieving values of corrosion potential and corrosion current two orders of magnitude lower than the ones obtained for the Al alloy as-received.

The Field Dependence of Aging Processes: Ion Migration in Anodic Oxide Films after Potentiostatic Formation

Anodic oxidation of thin aluminum and tantalum films was performed in an electrochemical droplet cell. Thicknesses from 2 up to were employed on thick aluminum and tantalum films. Under ultrahigh vacuum conditions the conductivity of the remaining metal film is monitored as function of time. Equally prepared oxide films were accomplished in metal-insulator-metal capacitors, which were used to monitor the capacitance as a function of time. The combination of the two experimental setups shows clearly, that small losses in the capacitance of aluminum oxide capacitors are due to slight thickening of the oxide even under vacuum conditions. Up to one monolayer of the base aluminum electrode can be oxidized during . Modeling in terms of migration of either oxygen anion interstitials or metal cation migration points preferably to the latter one. Tantalum oxide films of the same thickness show a much larger stability indicating either a lower content of mobile ions or a higher activation barrier for ionization and migration.

Reaction Force Analysis of the Effect of Mg(II) on the 1,3 Intramolecular Hydrogen Transfer in Thymine

The 1,3 intramolecular hydrogen transfer reaction in free thymine and in Mg(II)-thymine have been studied at the density functional theory level. The mechanism of intramolecular proton transfer in these systems emerges from the analysis of the reaction force profile along the reaction path; it is rationalized in terms of structural and electronic reorganizations that take place during the chemical transformation. Results show that the presence of Mg(II) monocoordinated to thymine activates the hydrogenic motion by inducing structural and electronic changes in the molecular backbone. In the metallic complex, it is found that the hydrogen transfer is followed by a relaxation process that facilitates the metal cation migration to form a bicoordinated complex.

The influence of impregnating pH on the postnatal and steam reforming characteristics of a Co-Ni/Al 2O 3 catalyst

Bimetallic alumina supported Co-Ni catalysts were prepared by impregnation under low (2) and high (8) pH values. Support dissolution due to acid attack appeared to be responsible for the low BET surface area for catalyst obtained at pH 2. However, this low-pH catalyst possesses higher dispersion and superior metal surface area. This is ascribed to the charged-induced migration of metal cations towards the grain centre where adsorption sites are located as a result of the formation of positively charged alumina surface at low pH. Ammonia NH 3-TPD analysis showed that the surface of both catalysts was populated with weak Lewis acid sites though a higher site concentration was found on the high-pH catalyst. TEM images further revealed an eggyolk profile for the catalyst impregnated at pH 2 with metal species located in the particle interior; while in the catalyst synthesised at pH 8, the impregnant metal is concentrated around the external surface of the particle. XRD analysis of the catalysts before and after reduction indicates that the basic catalyst was more difficult to reduce probably because of higher metal aluminate content. This was further confirmed by the lower degree of reduction shown for this catalyst during the thermogravimetric TPR–TPO runs. Solid-state kinetic data of the catalyst calcination, reduction and oxidation conformed with the Avrami–Erofeev model. In particular, the ratios of the associated kinetic rate constants for calcination and oxidation parallel those obtained for the deactivation and steam reforming constants, respectively, in both catalysts. Thus, it may be possible to have an a priori knowledge of the comparative reaction and deactivation behaviour of different catalysts from the temperature-programmed kinetics of their nascent solid states.

Retention of clay-solidified grouting curtain to Cd2+, Pb2+ and Hg2+ in landfill of municipal solid waste

The effects of components and their ratio of groups on anti-seepage capability of clay-solidified grouting curtain and its permeability of heavy metal cations were investigated by permeating experiments, using reactive solute transport model to study the permeation of heavy metals (Cd2+, Pb2+ and Hg2+). The study of permeating for different mixture ratios of cement and clay indicates that hydraulic conductivity of clay-solidified groupting curtain with different ratios of solid to liquid or with the same ratio of solid to liquid but with different ratios of cement to clay is changed. The laboratory simulation test results also show that precipitates produced in heavy metal cation migration process in curtain block up water flowing passage which makes the hydraulic conductivity of the solution-permeated curtain decrease with the leakage time. The permeation velocities for different heavy metal cations vary with ionic concentration, exchange capacity and ion radius etc. The test results indicate that the permeation rapidity order of heavy metals cations in clay-solidified grouting curtain is Hg2+ > Pb2+ in the same experimental circumstance. In addition, permeability for different mixture ratios and antisepsis capabilities of clay-solidified grouting curtain were studied in tests.

Metaphosphate in the Active Site of Fructose-1,6-bisphosphatase

The hydrolysis of a phosphate ester can proceed through an intermediate of metaphosphate (dissociative mechanism) or through a trigonal bipryamidal transition state (associative mechanism). Model systems in solution support the dissociative pathway, whereas most enzymologists favor an associative mechanism for enzyme-catalyzed reactions. Crystals of fructose-1,6-bisphosphatase grow from an equilibrium mixture of substrates and products at near atomic resolution (1.3 A). At neutral pH, products of the reaction (orthophosphate and fructose 6-phosphate) bind to the active site in a manner consistent with an associative reaction pathway; however, in the presence of inhibitory concentrations of K+ (200 mm), or at pH 9.6, metaphosphate and water (or OH-) are in equilibrium with orthophosphate. Furthermore, one of the magnesium cations in the pH 9.6 complex resides in an alternative position, and suggests the possibility of metal cation migration as the 1-phosphoryl group of the substrate undergoes hydrolysis. To the best of our knowledge, the crystal structures reported here represent the first direct observation of metaphosphate in a condensed phase and may provide the structural basis for fundamental changes in the catalytic mechanism of fructose-1,6-bisphosphatase in response to pH and different metal cation activators.

Stability constant determination and characterizationn of the complexation of trivalent lanthanides with polyoxometalates

Polyoxometalates (POMs), water-soluble metal oxides, can be considered small fragments of oxide lattices similar to the principle components of clay surfaces. Consequently, POMs may be used to model the thermodynamic aspects of clay–metal interactions to develop an understanding of the importance of clay colloids in the migration of metal cations in an environmental setting. The stability constants for the complexation of lanthanide(III) cations with P 2W 18O 62 6− and the lacunary α 2-P 2W 17O 61 10− polyoxometalates have been measured using laser-induced fluorescence techniques at 25 °C and an ionic strength of 0.10 M (NaClO 4). The values of log β 1 correlate well with the ligand p K a values, reflecting the strong ionic nature of the metal–ligand interaction. The stability constants were used to estimate the localized effective charges sensed by the metal ions. The luminescent lifetimes of the 1:1 and 1:2 complexes were measured to evaluate the residual hydration of the metal ions in these complexes.

Thermodynamic constant determination for the complexation of trivalent lanthanides with polyoxometalates

Polyoxometalates (POM), which are large, water-soluble metal oxides, can be considered as small fragments of metal lattices similar to the principle components on clay or inorganic colloid surfaces. Consequently, POMs can be used to model the thermodynamic aspects of metal–clay interactions to develop an understanding of clay colloids in the migration of metal cations within an environmental setting. The binding constants for the lacunary P 2W 17 and the plenary P 2W 18 POMs with trivalent lanthanide cations have been previously reported. In this study, the thermodynamic constants for the complexation of Ln(III) cations with these polyoxometalates have been measured using both enthalpy calorimetric titrations and the entropy titration method at 25 °C and an ionic strength of 1.0 M (NaClO 4). The results indicate weak binding interaction for the plenary P 2W 18 species, but stronger binding to the lacunary P 2W 17 complexes with significant entropy effects for both POMs.

Conductivity of fullerene-doped optical glasses

Appropriate amount doping of fullerenes caused insulating phosphate and fluorophosphate optical glasses to be conductive at room temperature. Microphotoluminescence measurements indicated the existence of ordered and disordered microstructures. A large number of fullerene-related medium-range microdomains aggregated as self-organized islands along the glass networks, and brought about connective tissues in the vitreous matrices for percolation migration of metal cations. The variation of microphotoluminescence induced by electric field presented us an indirect way to characterize the random activation, percolation migration, and retrap of metal cations near fullerene-related amorphous islands.

Interparticle migration of metal cations in stream sediments as a factor in toxics transport

Sorption of metal cations by stream sediments is an important process affecting the movement of released contaminants in the environment. The ability of cations to desorb from one sediment particle and subsequently sorb to another can greatly affect metal transport rates but rates for this process have not been reported. The objective of this study was to determine the rate at which sorbed metals can migrate from contaminated sediment particles to uncontaminated sediment particles as a function of the concentration of the contaminating solution and the duration of the contact with the contaminating solution. Samples of small sediment particles were exposed to solutions containing cobalt, after which they were rinsed and combined with larger uncontaminated sediment particles in the presence of stream water. Initial concentrations of the contaminating solution ranged from 1 ng/l to 1000 mg/l and exposures to the contaminating solution ranged from 6 h to 14 days. The rate of the migration increased with increasing concentrations in the contaminating solution and with decreasing times of exposure to the contaminating solution. Under the conditions of these experiments, the time required for the migration to reach equilibrium was on the order of months or longer. In separate experiments, the kinetics of adsorption and desorption of cobalt were measured as a function of concentration of the contaminating solution. The time required to reach adsorption equilibrium increased with increasing concentration in the contaminating solution. Times to sorption equilibrium were on the order of months. Desorption was much slower than adsorption and, together with intraparticle diffusion, probably controls the rate of migration from contaminated to uncontaminated sediment. The results of this study show that interparticle migration of metal cations can proceed at significant rates that are strongly influenced by the length of time that the metal has been in contact with the sediment.

Prediction and optimization of the separation of metal cations by capillary electrophoresis with indirect UV detection

The migration of metal cations in capillary electrophoresis with indirect UV detection was investigated. A mathematical model was used to relate the electrophoretic mobility of a metal cation with two background electrolyte factors: the pH and the concentration of a complexing agent. Imidazole, which has an electrophoretic mobility close to those of metal cations, was used as the visualization agent, and 2-hydroxyisobutyric acid (HIBA) was used as the complexing agent. Through computer modeling, the electrophoretic mobilities of fourteen metal cations were predicted on the basis of five initial experiments within the factor space of pH and HIBA concentration. Good agreement was observed between the predicted electrophoretic mobilities and the observed values of the metal cations. Subsequently, the separation of a complex mixture of metal cations was successfully optimized. A complete separation of 14 metal cations including alkali, alkaline earth and transition metal(II) cations was achieved within 4 min. In addition, the effectiveness of other visualization agents and complexing agents for the separation of metal cations was examined.