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At least 1,099 records · Page 61Linked to original sources
PubMed2003

Effect of natural organic matter on powdered activated carbon adsorption of trace contaminants: characteristics and mechanism of competitive adsorption.

Batch adsorption experiments using powdered activated carbon (PAC) to remove trace synthetic organic chemicals (SOCs) from water containing natural organic matter (NOM) were conducted. The percentage of SOC removed at any contact time and at any PAC dose was observed to be independent of the initial SOC concentration. Equations derived from the ideal adsorbed solution theory and the pore surface diffusion model validated this observation. For the strongly adsorbing SOCs (simazine and simetryn), the percentage of SOC removed was independent only at low initial SOC concentrations. The NOM fraction competing with the weakly adsorbing SOC (asulam) constituted a larger percentage of the total NOM than that competing with the strongly adsorbing SOCs. Although the adsorptive capacities of the SOCs were greatly reduced in water containing NOM compared with those in pure water, the change in the pore diffusion coefficient was insignificant. Therefore, NOM competed with the SOCs for adsorption sites, reducing the adsorptive capacity, but the amount of NOM loading was not so severe that it blocked or filled the pores, hindering the internal diffusion of the SOCs.

Adsorption↗
PubMed2002

Pesticide adsorption by granular activated carbon adsorbers. 1. Effect of natural organic matter preloading on removal rates and model simplification.

The adsorptive removal of periodic spikes of the trace synthetic organic chemicals (SOCs) simazine and asulam from water containing natural organic matter (NOM) was studied in pilot-scale granular activated carbon (GAC) adsorbers over a period of nearly 3 years. The SOC removal percentage obtained at any preloading time and bed depth was independent of the liquid-phase SOC concentration, and equations derived from the ideal adsorbed solution theory and a pore surface diffusion model validated this observation. The pseudo-steady-state SOC removal rate, (dC/dz), at each preloading time and bed depth was therefore first order with respectto the liquid-phase SOC concentration, C. Furthermore, the removal modulus, k, in the resulting SOC removal rate expression was a reflection of the solid-phase concentration of the NOM fraction that interfered with the adsorption of SOCs. Analysis of the removal modulus values indicated that the mass transfer zone of the NOM fraction competing with asulamtraveled more rapidlythrough the GAC adsorber than that competing with simazine. Given the similar molecular sizes of the targeted SOCs, this result was primarily explained by differences in SOC adsorbabilities, where the more weakly adsorbing asulam was less capable of displacing preloaded NOM. Consequently, the NOM fraction competing with asulam constituted a larger percentage of the total NOM than that competing with simazine.

Adsorption↗
PubMed2006

Prediction of a highly activated state of CO adsorbed on an Al/Fe(100) bimetallic surface.

Using periodic slab density functional theory, we investigate CO adsorption, diffusion, and dissociation energetics on a monolayer of Al covering Fe(100) [Al/Fe(100)]. We predict a weakly chemisorbed state of CO to exist on Al/Fe(100), with CO adsorbing on the 4-fold hollow site in a very tilted fashion. This state is predicted to have an extremely low CO stretching frequency of only 883 cm(-1), indicating a dramatically weakened CO bond relative to gaseous CO, even though the molecule is predicted to bind to Al/Fe(100) quite weakly. We predict that dissociation of CO starting from this weakly adsorbed state has a barrier of only approximately 0.35 eV, which is approximately 0.70 eV lower than that on Fe(100). To understand how the underlying substrate changes the electronic properties of the supported Al monolayer, we compare CO adsorption on Al/Fe(100) to its adsorption on analogous pure Al(100) surfaces. This highly activated yet weakly bound state of CO on Al/Fe(100) suggests that Al/Fe(100) could be an effective low-temperature bimetallic catalyst in reducing environments.

Journal Article↗
PubMed2005

Wave acceleration of electrons in the Van Allen radiation belts.

The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.

Journal Article↗
PubMed2006

Characterization of solvated electrons in hydrogen cyanide clusters: (HCN)n- (n=3, 4).

Theoretical studies of the solvated electrons (HCN)n- (n=3, 4) reveal a variety of electron trapping possibilities in the (HCN)n (n=3, 4) clusters. Two isomers for (HCN)3- and four isomers for (HCN)4- are obtained at the MP2aug-cc-pVDZ+dBF (diffusive bond functions) level of theory. In view of vertical electron detachment energies (VDEs) at the CCSD(T) level, the excess electron always "prefers" locating in the center of the system, i.e., the isomer with higher coordination number shows larger VDE value. However, the most stable isomers of the solvated electron state (HCN)3- and (HCN)4- are found to be the linear Cinfinitynu and Dinfinityh structures, respectively, but not the fullyl symmetric structures which have the largest VDE values.

Journal Article↗
PubMed1980

Interphases of chain molecules: Monolayers and lipid bilayer membranes.

Using the lattice model for a liquid, we treat the packing of short-chain molecules in interphases such as bilayer membranes. The constant density in the interphase imposes intermolecular constraints on the configurations of the flexible chains. The statistical theory here presented predicts a diffuse distribution of chain ends near the bilayer midplane; no adjustable parameters are required. Inasmuch as some of the chains terminate relatively near the polar interface, the number of chains reaching deeper planar layers is diminished. Consequently, configurational freedom increases with depth. This is the source of the well-known disorder gradient.

Journal Article↗
PubMed2002

A mathematical model for the role of cell signal transduction in the initiation and inhibition of angiogenesis.

Neovascular formation can be divided into three main stages (which may be overlapping): (1) changes within the existing vessel, (2) formation of a new channel, (3) maturation of the new vessel. In two previous papers, [Levine, H.A. and Sleeman, B.D. (1997) "A system of reaction diffusion equations arising in the theory of reinforced random walks" SIAM J. AppL Math. 683-730; Levine, H.A., Sleeman, B.D. and Nilsen-Hamilton, M. (2001b) "Mathematical modelling of the onset of capillary formation initiating angiogenesis." J. Math. Biol. 195-238] the authors introduced a new approach to angiogenesis, based on the theory o f reinforced random walks, coupled with a Michaelis-Menten type mechanism which views the endothelial vascular endothelial cell growth factor (VEGF) receptors as the catalyst for transforming into a proteolytic enzyme in order to model the first stage. It is the purpose of this paper to present a more descriptive yet not overly complicated mathematical model of the biochemical events that are initiated when VEGF interacts with endothelial cells and which result in the cell synthesis of proteolytic enzyme. We also delineate via chemical kinetics, three mechanisms by which one may inhibit angiogenesis (inhibition of growth factor, growth factor receptor and protease function).

Animals↗
PubMed2006

Effect of feed length on settleability, substrate uptake and storage in a sequencing batch reactor treating an industrial wastewater.

The paper compares the performance of two Sequencing Batch Reactors (SBRs) treating the same industrial wastewater (composed of formic acid, ethylene glycol and methanol) operated at two different lengths of the feed. The two SBRs were operated in parallel under the same conditions of organic load (0.85 gCOD l(-1) d(-1)) and sludge age (about 10 d), the only difference being the length of the feed: less than 1 min vs. 5 h. In this way the conditions of a plug flow reactor and of a completely mixed reactor were simulated. The two systems were compared on sludge settleability (related to filaments abundance and floc morphology), substrate uptake rates and polyhydroxyalkanoates (PHAs) storage rates. The main difference between the two systems was in the settling properties of the sludge: both SVI and effluent solids were higher in the system with slow feed. With regard to filamentous microorganisms, even though both reactors were inoculated with the same sludge with high concentration of filaments, they were rapidly washed out from both systems. Microscopic observations showed that the reason for the different settling properties of the two sludges was in the floc structure, which was more compact in the system operated with fast feed. These data support the theory of the role of diffusion inside the flocs in determining the settling properties of the sludge. The maximum substrate uptake rates and PHA storage rates were similar in the two systems, showing that also the microorganisms grown at a constant and low substrate concentration were able to quickly increase their activity and to store PHAs when in the presence of a sudden change in substrate concentration.

Industrial Waste↗
PubMedDate not supplied

A staged model of communication effects: evidence from an entertainment-education radio soap opera in Tanzania.

The authors draw on (1) the hierarchy-of-effects (HOE) model, (2) the stages-of-change (SOC) model, (3) social learning theory (SLT), and (4) the diffusion of innovations (DOI) to synthesize a staged model through which communication messages have effects on individual behavior change by stimulating (1) involvement with media characters and role modeling of their actions, and (2) interpersonal communication. Data from a field experiment in Tanzania on the effects of an entertainment-education radio soap opera, Twende na Wakati (Let's Go With the Times), on the adoption of family planning, are analyzed in light of a six-staged model of communication effects. It is found that (1) the model provides a useful framework for understanding the effects of an entertainment-education program, and (2) the radio soap opera promoted progress through the stages for family planning adoption in the treatment area in three of the four years of broadcast, and in the comparison area after broadcasts of the radio program began there.

Adult↗
PubMed2003

Dissemination of heart health promotion in the Ontario Public Health System: 1989-1999.

This paper reports the results of an analysis of the dissemination of community-based heart health promotion strategies. The research draws on diffusion and socio-ecological theories to study the first 10 years of heart health promotion in the public health system in Ontario, Canada. Using case description and interpretive analysis, the study describes developments in five stages of dissemination, and examines the interplay of factors operating in the internal organizational setting and the external environment in order to explain these developments. Findings demonstrate that dissemination of health promotion is a long-term, iterative process involving multiple stages. Dissemination is influenced by a complex interplay of factors operating within the public health system (especially traditional public health practice and champions) and factors in the environment in which the public health system operates (especially research, practice information and health policies). Implications are that policy makers should take a long-term view of dissemination, identify intermediate and long-term goals consistent with dissemination stages, and capitalize on internal and external forces supporting dissemination goals. Similar case-study research in other public health systems and time periods, and in more advanced stages of dissemination, will add further insight into the dissemination process.

Health Promotion↗
PubMed2004

Radon progeny size distributions and enhanced deposition effects from high radon concentrations in an enclosed chamber.

Prior work studying radon progeny in a small enclosed chamber found that at high (222)Rn concentrations an enhanced surface deposition was observed. Subsequent measurements for unfiltered air showed minimal charged particle mobility influence. Progeny particle size measurements reported here, performed at the US Department of Energy Environmental Measurement Laboratory (now with Home Security Department), using the EML graded screen array (GSA) system show in unfiltered air that the high (222)Rn levels causes a reduction in the attached (218)Po progeny airborne particulates and formation of additional normal sized unattached ( approximately 0.80 nm) and also even smaller (218)Po below 0.50 nm. At a (222)Rn level of 51 kBq m(-3), 73% of all (218)Po are of a mean particle diameter of about 0.40 +/- 0.02 nm. At this (222)Rn level, the ratio of (218)Po to (222)Rn airborne concentrations is reduced significantly from the concentration ratio at low (222)Rn levels. Similar reductions and size reformations were observed for the (214)Pb and (214)Bi/Po progeny. The particle size changes are further confirmed using the plateout rates and corresponding deposition velocities. The Crump and Seinfeld deposition theory provides the corresponding particle diffusion coefficients. With the diffusion coefficient to ultrafine clustered particle diameter correlation of Ramamurthi and Hopke, good agreement is obtained between EML GSA and deposition velocity data down to 0.40 nm. Strong evidence is presented that the progeny size reduction is due to, as a result of air ionization, the increased neutralization rate (primarily from electron scavenging of OH molecules) of the initially charged progeny. This is shown to increase with the (1/2) power of (222)Rn concentration and relative humidity as well as increased air change rate in the chamber. These results imply that at (222)Rn levels above 50 kBq m(-3), at relative humidity of 52%, a considerable reduction in lung dose could occur from preferential deposition of the progeny in the nasal and oral passages.

Adsorption↗
PubMed2000

Near-field diffraction tomography with diffuse photon density waves.

An angular spectrum algorithm is presented for fast, near-field diffraction tomographic imaging with diffuse photon density waves in highly scattering media. A general relation in K space is derived that connects the spatial variations of the optical properties of heterogeneities to the spatial spectra of the measured scattered diffuse photon density waves. The theory is verified experimentally for situations when boundary effects can be neglected. We further describe how to reconstruct absorption and scattering properties simultaneously, and how to incorporate boundary conditions into this angular spectrum algorithm for a turbid medium of finite size (e.g., the slab medium). Limitations and potential improvements of the near-field diffraction tomography are also discussed.

Algorithms↗
PubMed2002

Conjugate gradient filtering of instantaneous normal modes, saddles on the energy landscape, and diffusion in liquids.

Instantaneous normal modes (INM's) are calculated during a conjugate-gradient (CG) descent of the potential energy landscape, starting from an equilibrium configuration of a liquid or crystal. A small number (approximately equal to 4) of CG steps removes all the Im-omega modes in the crystal and leaves the liquid with diffusive Im-omega which accurately represent the self-diffusion constant D. Conjugate gradient filtering appears to be a promising method, applicable to any system, of obtaining diffusive modes and facilitating INM theory of D. The relation of the CG-step dependent INM quantities to the landscape and its saddles is discussed.

Journal Article↗
PubMed2002

Relaxation time hierarchy in a two-component quasiparticle gas.

A quasiparticle description of various condensed media is a very popular tool in the study of their transport and thermodynamic properties. I present here a microscopic theory for the description of diffusion processes in a two-component gas of quasiparticles with arbitrary dispersion law and statistics. In particular, I analyze the role of interaction within each subsystem (i.e., between identical quasiparticles) in relaxation of the whole system. The approach for solving such kinetic problems allows one to study the most important limiting cases and to clarify their physical sense. Classical results for diffusion coefficients of light particles in a massive gas (Lorentz model) and of massive particles in a light gas (Rayleigh model) are obtained directly from the general solution without using artificial approaches, as was done earlier. This provides a possibility to generalize these popular models on quasiparticle systems.

Journal Article↗
PubMed2002

Mixing in fully chaotic flows.

Passive scalar mixing in fully chaotic flows is usually explained in terms of Lyapunov exponents, i.e., rates of particle pair separations. We present a unified review of this approach (which encapsulates also other nonchaotic flows) and investigate its limitations. During the final stage of mixing, when the scalar variance decays exponentially, Lyapunov exponents can fail to describe the mixing process. The failure occurs when another mixing mechanism, first introduced by Fereday et al. [Phys. Rev. E 65, 035301 (2002)], leads to a slower decay than the mechanism based on Lyapunov exponents. Here we show that this mechanism is governed by the large-scale nonuniformities of the flow which are different from the small scale stretching properties of the flow that are captured by the Lyapunov exponents. However, during the initial stage of mixing, i.e., the stage when most of the scalar variance decays, Lyapunov exponents describe well the mixing process. We develop our theory for the incompressible and diffusive baker map, a simple example of a chaotic flow. Nevertheless, our results should be applicable to all chaotic flows.

Journal Article↗
PubMed2002

Generalized correlated states in a ring of coupled nonlinear oscillators with a local injection.

In this paper, we study the spatiotemporal dynamics of a ring of diffusely coupled nonlinear oscillators. Floquet theory is used to investigate the various dynamical states of the ring, as well as the Hopf bifurcations between them. A local injection scheme is applied to synchronize the ring with an external master oscillator. The shift-invariance symmetry is thereby broken, leading to the emergence of generalized correlated states. The transition boundaries from these correlated states to spatiotemporal chaos and complete synchronization are also derived. Numerical simulations are performed to support the analytic approach.

Journal Article↗
PubMed2005

Diffusive transport of light in a Kelvin foam.

Although diffusing-wave spectroscopy has already been successfully applied to study dynamic properties of foams, we still lack a clear understanding of the diffusive transport of photons in foams. In this paper, we present a thorough study of photon diffusion in the Kelvin structure as an example for a three-dimensional model foam. We consider the photons' random walk as they are reflected or transmitted by the liquid films according to the rules of ray optics. For constant reflectance and special one- and two-dimensional photon paths, we are able to calculate diffusion constants analytically. Extensive numerical simulations reveal a remarkable similarity with our previous two-dimensional investigations. To implement a more realistic model, we use thin-film reflectances. The simulated diffusion constants exhibit oscillations for varying film thickness d which vanish when disorder is introduced in d. Absolute values and the behavior at small d agree with measurements in very dry foams providing a strong argument for the importance of liquid films in the diffusive photon transport. An analytical theory with a minimum of input parameters reproduces the numerical results.

Journal Article↗