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Berlin Brandenburg

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  • 1
    Language: English
    In: The journal of physical chemistry. B, 07 June 2012, Vol.116(22), pp.6233-49
    Description: We simulate spin relaxation processes, which may be measured by either continuous wave or pulsed magnetic resonance techniques, using trajectory-based simulation methodologies. The spin-lattice relaxation rates are extracted numerically from the relaxation simulations. The rates obtained from the numerical fitting of the relaxation curves are compared to those obtained by direct simulation from the relaxation Bloch-Wangsness-Abragam-Redfield theory (BWART). We have restricted our study to anisotropic rigid-body rotational processes, and to the chemical shift anisotropy (CSA) and a single spin-spin dipolar (END) coupling mechanisms. Examples using electron paramagnetic resonance (EPR) nitroxide and nuclear magnetic resonance (NMR) deuterium quadrupolar systems are provided. The objective is to compare those rates obtained by numerical simulations with the rates obtained by BWART. There is excellent agreement between the simulated and BWART rates for a Hamiltonian describing a single spin (an electron) interacting with the bath through the chemical shift anisotropy (CSA) mechanism undergoing anisotropic rotational diffusion. In contrast, when the Hamiltonian contains both the chemical shift anisotropy (CSA) and the spin-spin dipolar (END) mechanisms, the decay rate of a single exponential fit of the simulated spin-lattice relaxation rate is up to a factor of 0.2 smaller than that predicted by BWART. When the relaxation curves are fit to a double exponential, the slow and fast rates extracted from the decay curves bound the BWART prediction. An extended BWART theory, in the literature, includes the need for multiple relaxation rates and indicates that the multiexponential decay is due to the combined effects of direct and cross-relaxation mechanisms.
    Keywords: Molecular Dynamics Simulation
    ISSN: 15206106
    E-ISSN: 1520-5207
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  • 2
    Language: English
    In: Environmental science & technology, 2014, Vol.48(3), pp.1370-1
    Keywords: Environmental Policy ; Industry ; Policy Making ; Research ; Commerce -- Legislation & Jurisprudence
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 3
    Language: English
    In: Environmental Science & Technology, 09/2007, Vol.41(18), pp.6343-6349
    Description: The response of microorganisms to metal contamination of soils varies significantly from one investigation to another. One explanation is that metals are heterogeneously distributed at spatial scales relevant to microbes and that microoorganisms are able to avoid zones of intense contamination. This article aims to assess the microscale distribution of Cu in a vineyard soil. The spatial distribution of Cu was measured at two resolutions (0.3 mm and 20 mm) in thin sections of the surface 4 cm of undisturbed soil by electron microprobe and synchrotron X-ray microfluorescence spectroscopy. Bulk physicochemical analyses of Cu, pH, organic matter, texture, and mineralogy were performed. The results indicate that the Cu distribution is strongly heterogeneous at both scales of observation. Entire regions of the thin sections are virtually devoid of Cu, whereas highly localized "hotspots" have Cu signal intensities thousands of times higher than background. The distribution of Rb, or Al and Si, indicators of clay minerals, or Fe (iron (hydr)oxides), show that Cu is not preferentially associated with these mineral phases. Instead, Cu hotspots are associated with particulate organic matter. These observations suggest modification of current sampling protocols, and design of ecotoxicological experiments involving microorganisms, for contaminated soils.
    Keywords: Vineyards ; Fluorescence ; Spatial Distribution ; Contamination ; Heavy Metals ; Hot Spots ; Organic Matter ; Electron Microprobe ; Retinoblastoma Protein ; Copper ; Spectroscopy ; Clays ; Soil Microorganisms ; Soil ; Soil Pollution ; Particulate Organic Matter ; Ionizing Radiation ; Microorganisms ; Mapping ; Sampling ; Ph Effects ; Iron ; Minerals ; Hot Spots ; Metals ; Clay ; Fluorescence ; Organic Matter ; Soil Contamination ; Particulates ; Mineralogy ; Spatial Distribution ; Vineyards ; Microorganisms ; Minerals ; Iron ; Land Pollution ; Antibiotics & Antimicrobials;
    ISSN: 0013-936X
    E-ISSN: 1520-5851
    Source: American Chemical Society (via CrossRef)
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  • 4
    Language: English
    In: Environmental science & technology, 01 March 2009, Vol.43(5), pp.1354-9
    Description: The extensive use of titanium dioxide nanoparticles (nano-TiO2) in many consumer products has raised concerns about possible risks to the environment The magnitude of the threat may depend on whether nano-TiO2 remains dispersed in the environment, or forms much larger-sized aggregates or clusters. Currently, limited information is available on the issue. In this context, the purpose of the present article is to report initial measurements of the morphology and rate of formation of nano-TiO2 aggregates in aqueous suspensions as a function of ionic strength and of the nature of the electrolyte in a moderately acid to circumneutral pH range typical of soil and surface water conditions. Dynamic light scattering results show that 4-5 nm titanium dioxide particles readily form stable aggregates with an average diameter of 50-60 nm at pH approximately 4.5 in a NaCl suspension adjusted to an ionic strength of 0.0045 M. Holding the pH constant but increasing the ionic strength to 0.0165 M, leads to the formation of micron-sized aggregates within 15 min. At all other pH values tested (5.8-8.2), micron-sized aggregates form in less than 5 min (minimum detection time), even at low ionic strength (0.0084-0.0099 M with NaCl). In contrast, micron-sized aggregates form within 5 min in an aqueous suspension of CaCl2 at an ionic strength of 0.0128 M and pH of 4.8, which is significantly faster than observed for NaCI suspensions with similar ionic strength and pH. This result indicates that divalent cations may enhance aggregation of nano-TiO2 in soils and surface waters. Optical micrographs show branching aggregates of sizes ranging from the 1 microm optical limit of the microscope to tens of micrometers in diameter.
    Keywords: Cations, Divalent -- Chemistry ; Cations, Monovalent -- Chemistry ; Nanoparticles -- Chemistry ; Titanium -- Chemistry
    ISSN: 0013-936X
    E-ISSN: 15205851
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  • 5
    Language: English
    In: Environmental Science & Technology, 01 December 2007, Vol.41(23), pp.8056-8061
    Description: The fate of organic herbicides applied to agricultural fields may be affected by other soil amendments, such as copper applied as a fungicide. The effect of copper on the leaching of diuron and glyphosate through a granitic and a calcareous...
    Keywords: Adsorption ; Copper ; Diuron ; Drug Interactions ; Environmental Monitoring ; Glycine ; Herbicides ; Protons ; Soil ; Soil Pollutants ; Life Sciences ; Agricultural Sciences ; Soil Study ; Sciences of the Universe ; Continental Interfaces, Environment ; Environmental Sciences ; Global Changes ; Sciences of the Universe ; Earth Sciences ; Geochemistry
    ISSN: 0013-936X
    E-ISSN: 1520-5851
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