29,349 research outputs found
Adsorption of two pesticides on a clay surface: a theoretical study
The contamination of water resources with many organic xenobiotic compounds poses a challenge to environmental sciences and technologies [1]. Although in many cases these contaminants are present only in small concentrations, the large variety of such compounds (some of which are classified as priority pollutants) is a matter of concern. Adsorption, alone or as part of a more complex water or wastewater treatment process, has been seen as playing a very important role in the removal of many of these pollutants [2]. In this regard, the choice of adsorbent materials is crucial, which requires an understanding of the details involved in the adsorption of more or less complex organic molecules by a variety of surfaces of different types. In addition to laboratory studies, computational studies may be valuable in this study [3].
MCPA (2-methyl-4-chlorophenoxyacetic acid, a herbicide) and Clofibric acid (2-(4-chlorophenoxy)-2-methylpropanoic, the metabolite of a pharmaceutical, clofibrate, and also a herbicide) are two phenoxy acids that differ only slightly in their structures. However, a quite distinct behavior in adsorption phenomena on clay materials has been observed in past studies [4]. By relating those differences with the molecules' structural features through atomistic computational studies, some insight may be gained into the respective adsorption processes of this type of compounds.
In the present work quantum chemical calculations at density functional theory level have been performed to study the adsorption of MCPA and Clofibric acid by a clay surface model. Since hydration plays an important role for the adsorption process of these species, solvent effects were considered by inclusion of water molecules explicitly into the quantum chemical calculations.
The deprotonated negatively charged species were found to strongly interact with the surface and the distinct behavior of both species upon adsorption was compared with experimental evidences
Dynamic nonlinear analyses for the 4-storey infilled R/C frame: study of a retrofitting solution
A research project on assessment and retrofitting of R/C frame structures is currently being developed under the research programme of the ICONS TMR-research network. This paper presents and discusses the preliminary experimental results from a 4-storey bare frame representative of the common practice of 40~50 years ago in most south European countries and devotes special attention to the study of a retrofitting solution based on bracing and rubber dissipaters, which intends to increase stiffness and damping reducing consequently the earthquake deformation demands.O estudo aqui apresentado concentra-se numa solução de reforço de um pórtico utilizando contraventamentos (k-bracing) com perfis de aço em conjunto com elementos elastoméricos de dissipação. Os resultados das análises não lineares da estrutura com e sem alvenaria e com reforço são apresentados e discutidos. Na segunda parte da comunicação apresentam-se os resultados experimentais já disponíveis e discute-se o problema da modelação recorrendo aos resultados experimentais e comparando os resultados obtidos com diferentes tipos de modelos
Seismic analyses of a R/C building: study of a retrofitting solution
The preliminary experimental results from the tests on a 4-storey R/C frame structure are presented and discussed. The full-scale model is representative of the common practice of 40~50 years ago in most south European countries. Special attention is devoted to the study of a retrofitting solution based on bracing and rubber dissipaters, which intends to increase stiffness and damping reducing consequently the earthquake deformation demands
Berry phases and zero-modes in toroidal topological insulator
An effective Hamiltonian describing the surface states of a toroidal
topological insulator is obtained, and it is shown to support both bound-states
and charged zero-modes. Actually, the spin connection induced by the toroidal
curvature can be viewed as an position-dependent effective vector potential,
which ultimately yields the zero-modes whose wave-functions harmonically
oscillate around the toroidal surface. In addition, two distinct Berry phases
are predicted to take place by the virtue of the toroidal topology.Comment: New version, accepted for publication in EPJB, 6 pages, 1 figur
Ground-state configurations in ferromagnetic nanotori
Magnetization ground states are studied in toroidal nanomagnets. The
energetics associated to the ferromagnetic, vortex and onion-like
configurations are explicitly computed. The analysis reveals that the vortex
appears to be the most prominent of such states, minimizing total energy in
every torus with internal radius (for Permalloy). For
the vortex remains the most favorable pattern whenever
( is the torus external radius and is
the exchange length), being substituted by the ferromagnetic state whenever
.Comment: 16 pages, 9 figures, 3 apendices, Revtex forma
Differential temporal beta‐diversity patterns of native and non‐native arthropod species in a fragmented native forest landscape
An important factor that hinders the management of non‐native species is a general lack of information regarding the biogeography of non‐natives, and, in particular, their rates of turnover. Here, we address this research gap by analysing differences in temporal beta‐diversity (using both pairwise and multiple‐time dissimilarity metrics) between native and non‐native species, using a novel time‐series dataset of arthropods sampled in native forest fragments in the Azores. We use a null model approach to determine whether temporal beta‐diversity was due to deterministic processes or stochastic colonisation and extinction events, and linear modelling selection to assess the factors driving variation in temporal beta‐diversity between plots. In accordance with our predictions, we found that the temporal beta‐diversity was much greater for non‐native species than for native species, and the null model analyses indicated that the turnover of non‐native species was due to stochastic events. No predictor variables were found to explain the turnover of native or non‐native species. We attribute the greater turnover of non‐native species to source‐sink processes and the close proximity of anthropogenic habitats to the fragmented native forest plots sampled in our study. Thus, our findings point to ways in which the study of turnover can be adapted for future applications in habitat island systems. The implications of this for biodiversity conservation and management are significant. The high rate of stochastic turnover of non‐native species indicates that attempts to simply reduce the populations of non‐native species in situ within native habitats may not be successful. A more efficient management strategy would be to interrupt source‐sink dynamics by improving the harsh boundaries between native and adjacent anthropogenic habitats.Portuguese FCT‐NETBIOME – ISLANDBIODIV grant 0003/2011.info:eu-repo/semantics/publishedVersio
White dwarfs with a surface electrical charge distribution: Equilibrium and stability
The equilibrium configuration and the radial stability of white dwarfs
composed of charged perfect fluid are investigated. These cases are analyzed
through the results obtained from the solution of the hydrostatic equilibrium
equation. We regard that the fluid pressure and the fluid energy density follow
the relation of a fully degenerate electron gas. For the electric charge
distribution in the object, we consider that it is centralized only close to
the white dwarfs' surfaces. We obtain larger and more massive white dwarfs when
the total electric charge is increased. To appreciate the effects of the
electric charge in the structure of the star, we found that it must be in the
order of with which the electric field is about
. For white dwarfs with electric fields close to the
Schwinger limit, we obtain masses around . We also found that in
a system constituted by charged static equilibrium configurations, the maximum
mass point found on it marks the onset of the instability. This indicates that
the necessary and sufficient conditions to recognize regions constituted by
stable and unstable equilibrium configurations against small radial
perturbations are respectively and .Comment: This is a preprint. The original paper will be published in EPJ
Gluon saturation and the Froissart bound: a simple approach
At very high energies we expect that the hadronic cross sections satisfy the
Froissart bound, which is a well-established property of the strong
interactions. In this energy regime we also expect the formation of the Color
Glass Condensate, characterized by gluon saturation and a typical momentum
scale: the saturation scale . In this paper we show that if a saturation
window exists between the nonperturbative and perturbative regimes of Quantum
Chromodynamics (QCD), the total cross sections satisfy the Froissart bound.
Furthermore, we show that our approach allows us to describe the high energy
experimental data on total cross sections.Comment: 6 pages, 5 figures. Includes additional figures, discussion and
reference
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