Sorption of Arsenic, Mercury, Selenium onto Nanostructured Adsorbent Media and Stabilization via Surface Reactions

The overall goal of this study is to evaluate the ability of novel nanostructured adsorbent media (NTAs, iron sulfides (FeS2 and FeS)) to remove arsenic, selenium and mercury from ash and scrubber pond effluents. The NTAs aim to enhance arsenic removal from solution compared to conventional adsorbents. The iron sulfides are expected to produce stable residuals for ultimate disposal after removing As, Se and Hg from solution, so that removal of these compounds from wastewaters will not result in contamination of soils and groundwaters. Methods for reliably and economically producing these materials were developed. The synthesized NTAs and iron sulfides were characterized by surface analysis techniques such as XRD, FT-IR, SEM-EDS, TEM, XPS, AFM and N2-adsorption. These analyses indicated that Ti(25)-SBA-15 has highly ordered hexagonal mesopores, MT has interparticle mesopores, pyrite (FeS2) forms crystalline, nonporous rectangular nanoparticles (<500 nm), and mackinawite (FeS) forms amorphous, nonporous nanoparticles (<100 nm). Kinetic and equilibrium tests for As(III, V) removal were conducted with NTAs over a range of pH (4, 7, 9.5). The rates of arsenic uptake were very fast and followed a bi-phasic sorption pattern, where sorption was fast for the first 10 minutes, and then slowed and was almost completed within 200 minutes. Distinct sorption maxima for As(III) removal were observed between pH 7 and pH 9.5 for MT and between pH 4 and pH 7 for Ti(25)-SBA-15. The amount of As(V) adsorbed generally decreased as pH increased. In addition, a surface complexation model (SCM) based on the diffuse layer model (DLM) was used to predict arsenic adsorption envelopes by NTAs under various environmental conditions. The SCM for As(III, V) adsorption by NTAs demonstrated the role of mono- and bidentate surface complexes in arsenic adsorption. A batch reactor system was employed in an anaerobic chamber to conduct experiments to characterize both the removal of As, Se, Hg from solution and their subsequent reactions with iron sulfides. Experiment variables for removal experiments included: contaminant valence state (As(V), As(III), Se(VI), Se(IV), Hg(II)); adsorbent/reactant type (FeS, FeS2); adsorbent/reactant concentration; pH (7, 8, 9, 10); and competing ion (SO42-) concentration (0, 1, 10 mM). Experimental variables for reaction experiments were reaction time (up to 30 days) at pH 8 and oxidation states of contaminants. In addition, the stability of iron sulfides (FeS2, FeS) combined with target compounds was investigated by measuring the ability of the target compounds to resist release to the aqueous phase after removal. These experiments showed that iron sulfides were good adsorbent/reactants for target contaminants in spite of the presence of sulfate. This was particularly true at intermediate concentrations of target compounds. The experiments also demonstrated that iron sulfides interacted with target contaminants in such a way to improve their resistance to being released back to solution as pH was changed. Therefore, this study demonstrates the ability of novel nanostructured adsorbent media to remove arsenic, selenium and mercury from ash and scrubber pond effluents and the ability of iron sulfides to produce residuals that are stable when disposed in landfills

Physical Origin and Generic Control of Magnonic Band Gaps of Dipole-Exchange Spin Waves in Width-Modulated-Nanostrip Waveguides

We report, for the first time, on a novel planar structure of magnonic-crystal waveguides, made of a single magnetic material, in which the allowed and forbidden bands of propagating dipole-exchange spin-waves can be manipulated by the periodic modulation of different widths in thin-film nanostrips. The origin of the presence of several magnonic wide band gaps and the crucial parameters for controlling those band gaps of the order of ~10 GHz are found by micromagnetic numerical and analytical calculations. This work can offer a route to the potential application to broad-band spin-wave filters in the GHz frequency range.Comment: 24 pages, 8 figure

Normal modes of coupled vortex gyration in two spatially separated magnetic nanodisks

We found from analytical derivations and micromagnetic numerical simulations that there exist two distinct normal modes in apparently complex vortex gyrotropic motions in two dipolar-coupled magnetic nanodisks. The normal modes have characteristic higher and lower single angular eigenfrequencies with their own elliptical orbits elongated along the x (bonding axis) and y axes, respectively. The superposition of the two normal modes results in coupled vortex gyrations, which depend on the relative vortex-state configuration in a pair of dipolar-coupled disks. This normal-mode representation is a simple means of understanding the observed complex vortex gyrations in two or more dipolar-interacting disks of various vortex-state configurations.Comment: 18 pages, 3 figures, 1 tabl

Total Reflection and Negative Refraction of Dipole-Exchange Spin Waves at Magnetic Interfaces: Micromagnetic Modeling Study

We demonstrated that dipole-exchange spin waves traveling in geometrically restricted magnetic thin films satisfy the same laws of reflection and refraction as light waves. Moreover, we found for the first time novel wave behaviors of dipole-exchange spin waves such as total reflection and negative refraction. The total reflection in laterally inhomogeneous thin films composed of two different magnetic materials is associated with the forbidden modes of refracted dipole-exchange spin waves. The negative refraction occurs at a 90 degree domain-wall magnetic interface that is introduced by a cubic magnetic anisotropy in the media, through the anisotropic dispersion of dipole-exchange spin waves.Comment: 13 pages, 5 figure

GaAs droplet quantum dots with nanometer-thin capping layer for plasmonic applications

We report on the growth and optical characterisation of droplet GaAs quantum dots with extremely-thin (11 nm) capping layers. To achieve such result, an internal thermal heating step is introduced during the growth and its role in the morphological properties of the quantum dots obtained is investigated via scanning electron and atomic force microscopy. Photoluminescence measurements at cryogenic temperatures show optically stable, sharp and bright emission from single quantum dots, at near-infrared wavelengths. Given the quality of their optical properties and the proximity to the surface, such emitters are ideal candidates for the investigation of near field effects, like the coupling to plasmonic modes, in order to strongly control the directionality of the emission and/or the spontaneous emission rate, crucial parameters for quantum photonic applications.Comment: 1 pages, 3 figure

Novel Adsorbent-Reactants for Treatment of Ash and Scrubber Pond Effluents

The overall goal of this project was to evaluate the ability of novel adsorbent/reactants to remove specific toxic target chemicals from ash and scrubber pond effluents while producing stable residuals for ultimate disposal. The target chemicals studied were arsenic (As(III) and As(V)), mercury (Hg(II)) and selenium (Se(IV) and Se(VI)). The adsorbent/reactants that were evaluated are iron sulfide (FeS) and pyrite (FeS{sub 2}). Procedures for measuring concentrations of target compounds and characterizing the surfaces of adsorbent-reactants were developed. Effects of contact time, pH (7, 8, 9, 10) and sulfate concentration (0, 1, 10 mM) on removal of all target compounds on both adsorbent-reactants were determined. Stability tests were conducted to evaluate the extent to which target compounds were released from the adsorbent-reactants when pH changed. Surface characterization was conducted with x-ray photoelectron spectroscopy (XPS) to identify reactions occurring on the surface between the target compounds and surface iron and sulfur. Results indicated that target compounds could be removed by FeS{sub 2} and FeS and that removal was affected by time, pH and surface reactions. Stability of residuals was generally good and appeared to be affected by the extent of surface reactions. Synthesized pyrite and mackinawite appear to have the required characteristics for removing the target compounds from wastewaters from ash ponds and scrubber ponds and producing stable residuals

A gigahertz-range spin-wave filter composed of width-modulated nanostrip magnonic-crystal waveguides

We found a robust magnonic-crystal waveguide structure for use as an efficient gigahertz-range spin-wave filter that passes only spin waves of chosen narrow band frequencies and filters out the other frequencies. The structure consists of the serial combinations of various width modulations with different periodicities and motifs in planar-patterned thin-film nanostrips composed of a single soft magnetic material. The observed magnonic band gaps result from both the translation symmetry of the one-dimensional width modulation and the higher-quantized width-mode spin waves excited from scattering at the periodic edge-steps of the width modulation. This work brings us one step closer to practical implementations of spin waves in information transmission and processing devices.open452

Versatile Bifunctional and Supported IrNi Oxide Catalyst for Photoelectrochemical Water Splitting

Designing a high-performance electrocatalyst that operates with photon-level energy is of the utmost importance in order to address the world’s urgent energy concerns. Herein, we report IrNi nanoparticles uniformly distributed on cost-effective activated carbon support with a low mass loading of 3% by weight to drive the overall water splitting reaction under light illumination over a wide pH range. The prepared IrNi nanomaterials were extensively characterized by SEM/EDX, TEM, XRD, Raman, and UV-visible absorption spectroscopy. The experimental results demonstrate that when the Ir:Ni ratio is 4:1, the water splitting rate is high at 32 and 25 mA cm−2 for hydrogen (at −1.16 V) and oxygen evolution reactions (at 1.8 V) in alkaline electrolyte, respectively, upon the light irradiation (100 mW cm−2). The physical and electrochemical characterization of metal and alloy combinations show that the cumulative effect of relatively high crystallinity (among the materials used in this study), reduced charge recombination rate, and improved oxygen vacancies observed with the 4Ir1Ni@AC electrode is the reason for the superior activity obtained. A high level of durability for hydrogen and oxygen evolution under light illumination is seen in the chronoamperometric study over 15 h of operation. Overall water splitting examined in 0.1 M of NaOH medium at a 50 mV s−1 scan rate showed a cell voltage of 1.94 V at a 10 mA cm−2 current density.Qatar National Research Fund (QNRF) - Grant No. NPRP13S-0202-200228

In Situ-Generated Reactive Oxygen Species in Precharged Titania and Tungsten Trioxide Composite Catalyst Membrane Filters: Application to As(III) Oxidation in the Absence of Irradiation

This study demonstrates that in situ-generated reactive oxygen species (ROSs) in prephotocharged TiO₂ and WO₃ (TW) composite particle-embedded inorganic membrane filters oxidize arsenite (As(III)) into arsenate (As(V)) without any auxiliary chemical oxidants under ambient conditions in the dark. TW membrane filters have been charged with UV or simulated sunlight and subsequently transferred to a once-through flow-type system. The charged TW filters can transfer the stored electrons to dissolved O₂, producing ROSs that mediate As(III) oxidation in the dark. Dramatic inhibition of As(V) production with O₂ removal or addition of ROS quenchers indicates an ROS-mediated As(III) oxidation mechanism. Electron paramagnetic spectroscopic analysis has confirmed the formation of the HO₂•/O₂•– pair in the dark. The WO₃ fraction in the TW filter significantly influences the performance of the As(III) oxidation, while As(V) production is enhanced with increasing charging time and solution pH. The As(III) oxidation is terminated when the singly charged TW filter is fully discharged; however, recharging of TW recovers the catalytic activity for As(III) oxidation. The proposed oxidation process using charged TW membrane filters is practical and environmentally benign for the continuous treatment of As(III)-contaminated water during periods of unavailability of sunlight