1,690 research outputs found
Die konsep van biobeskikbaarheid en die neerlĂŞ van eenvormige standaarde vir toelaatbare chemiese kontaminante in die grond
The original publication is available at http://www.satnt.ac.za/Soils are very heterogeneous substrates providing an environmental matrix with varying spatial and temporal
gradients of pH, organic carbon, particle size distribution, moisture content as well as biological factors
associated with soil organisms. These chemical, physical as well as biological factors determine the
bioavailability of chemicals to soil-dwelling invertebrates. This review of recent literature on the use of the
bioavailability concept in soil ecotoxicology indicates that the concept is often used unqualified and
indiscriminately to mean different things to different authors. A clear understanding of the concept is crucial
for toxicity testing, environmental monitoring, risk assessment and the setting of soil quality criteria since
knowledge of the actual exposure of organisms, and not merely the total amount of the chemical, is required.
The aim of this paper is to contribute towards a clarification of the concept. Apart from defining or
describing bioavailability, the problems related to the comparability of toxicity data between soils and species, inter-soil and inter-species comparisons of toxicity data are discussed. The potential role that
biomarkers can play in assessing bioavailability, is touched upon. In an effort to prescribe uniform criteria or
standards for environmental quality, both biotic and abiotic characteristics, which determine the
bioavailablity of contaminants should be considered. This requires a dynamic approach which takes both
uptake processes as well as a variety of other biological factors into consideration. It is concluded that
bioavialiblity should be interpreted qualitatively and that the rate of uptake of a contaminant could possibly
serve as a measure of bioavailability. The development of standardised protocols for exposure of selected
species and the measurement of biological responses with the aid of biomarkers could serve to refine and
take risk assessment a step further.Publishers' Versio
The concept of bioavailability and establishing uniform standards for permissible chemical contamination of soil
The original publication is available at http://www.satnt.ac.zaSoils are very heterogeneous substrates providing an environmental matrix with varying spatial and temporal gradients of pH, organic carbon, particle size distribution, moisture content as well as biological factors associated with soil organisms. These chemical, physical as well as biological factors determine the bioavailability of chemicals to soil-dwelling invertebrates. This review of recent literature on the use of the bioavailability concept in soil ecotoxicology indicates that the concept is often used unqualified and indiscriminately to mean different things to different authors. A clear understanding of the concept is crucial for toxicity testing, environmental monitoring, risk assessment and the setting of soil quality criteria since knowledge of the actual exposure of organisms, and not merely the total amount of the chemical, is required. The aim of this paper is to contribute towards a clarification of the concept. Apart from defining or describing bioavailability, the problems related to the comparability of toxicity data between soils and species, inter-soil and inter-species comparisons of toxicity data are discussed. The potential role that biomarkers can play in assessing bioavailability, is touched upon. In an effort to prescribe uniform criteria or standards for environmental quality, both biotic and abiotic characteristics, which determine the bioavailablity of contaminants, should be considered. This requires a dynamic approach which takes both uptake processes as well as a variety of other biological factors into consideration. It is concluded that bioavialiblity should be interpreted qualitatively and that the rate of uptake of a contaminant could possibly serve as a measure of bioavailability. The development of standardised protocols for exposure of selected species and the measurement of biological responses with the aid of biomarkers could serve to refine and take risk assessment a step further.Grond is ’n besonder heterogene substraat en ’n matriks vir variërende ruimtelike en temporale gradiënte van pH, organiese koolstof, partikelgroottes, voginhoude asook biologiese faktore geassosieer met grondlewende organismes. Hierdie chemiese, fisiese sowel as biologiese faktore bepaal die biobeskikbaarheid van chemiese stowwe vir organismes wat in die grond lewe. Hierdie oorsig van onlangse literatuur oor die gebruik van die konsep van biobeskikbaarheid in die ekotoksikologie, lei tot die slotsom dat hierdie konsep dikwels ongekwalifiseerd en teenstrydig gebruik word omdat dit verskillende dinge vir verskillende outeurs beteken. ’n Heldere begrip van die konsep is van wesenlike belang vir die interpretasie van toksisiteitstoetse, omgewingsmonitering en risiko-assessering van omgewingskontaminante. Dit is ook nodig vir die stel van grondkwaliteitskriteria omdat kennis van die werklike blootstelling van organismes nodig is, en nie bloot van die totale hoeveelheid van die chemiese stof in die omgewing nie. Die doel van hierdie bydrae is om die konsep duidelik toe te lig ter wille van konsekwente en verantwoordbare aanwending daarvan. Afgesien van die definiëring of omskrywing van biobeskikbaarheid, word die probleme rondom die vergelykbaarheid van toksisiteitsgegewens tussen gronde en tussen spesies bespreek. Verder word die potensiële rol wat biomerkers in grondorganismes kan speel om biobeskikbaarheid te bepaal, aangeroer. In ’n poging om eenvormige kriteria of standaarde vir omgewingskwaliteit neer te lê, sal rekening gehou moet word met beide biotiese sowel as abiotiese eienskappe wat die biobeskikbaarheid van chemiese stowwe vir grondorganismes bepaal. Dit vereis dus ’n dinamiese benadering wat beide fisies-chemies gedrewe desorpsieprosesse en fisiologies gedrewe opnameprosesse, sowel as ’n verskeidenheid ander biologiese faktore, in ag moet neem. Die bevinding is dat biobeskikbaarheid kwalitatief geïnterpreteer moet word en dat die opnamekoers van ’n chemiese stof moontlik as maatstaf van biobeskikbaarheid kan dien. Die ontwikkeling van gestandaardiseerde protokolle vir blootstelling van geselekteerde spesies en meting van biologiese response met behulp van biomerkers kan risiko-assessering verfyn en ’n stap verder neem.Publishers' Versio
Surface detonation in type Ia supernova explosions?
We explore the evolution of thermonuclear supernova explosions when the
progenitor white dwarf star ignites asymmetrically off-center. Several
numerical simulations are carried out in two and three dimensions to test the
consequences of different initial flame configurations such as spherical
bubbles displaced from the center, more complex deformed configurations, and
teardrop-shaped ignitions. The burning bubbles float towards the surface while
releasing energy due to the nuclear reactions. If the energy release is too
small to gravitationally unbind the star, the ash sweeps around it, once the
burning bubble approaches the surface. Collisions in the fuel on the opposite
side increase its temperature and density and may -- in some cases -- initiate
a detonation wave which will then propagate inward burning the core of the star
and leading to a strong explosion. However, for initial setups in two
dimensions that seem realistic from pre-ignition evolution, as well as for all
three-dimensional simulations the collimation of the surface material is found
to be too weak to trigger a detonation.Comment: 5 pages, 3 figures, in: Proceedings of the SciDAC 2006 Meeting,
Denver June 25-26 2006, also available at
http://herald.iop.org/jpcs46/m51/gbr//link/40
Multi-spot ignition in type Ia supernova models
We present a systematic survey of the capabilities of type Ia supernova
explosion models starting from a number of flame seeds distributed around the
center of the white dwarf star. To this end we greatly improved the resolution
of the numerical simulations in the initial stages. This novel numerical
approach facilitates a detailed study of multi-spot ignition scenarios with up
to hundreds of ignition sparks. Two-dimensional simulations are shown to be
inappropriate to study the effects of initial flame configurations. Based on a
set of three-dimensional models, we conclude that multi-spot ignition scenarios
may improve type Ia supernova models towards better agreement with
observations. The achievable effect reaches a maximum at a limited number of
flame ignition kernels as shown by the numerical models and corroborated by a
simple dimensional analysis.Comment: 14 pages, 12 figures with reduced resolution to meet astro-ph file
size restriction, full-resolution version available from
http://www.mpa-garching.mpg.de/~fritz/publications/astro-ph/multispot.pdf
submitted to A&
Splotch: Visualizing Cosmological Simulations
We present a light and fast, public available, ray-tracer {\tt Splotch}
software tool which supports the effective visualization of cosmological
simulations data. We describe the algorithm it relies on, which is designed in
order to deal with point-like data, optimizing the ray-tracing calculation by
ordering the particles as a function of their ``depth'' defined as a function
of one of the coordinates or other associated parameter. Realistic
three-dimensional impressions are reached through a composition of the final
color in each pixel properly calculating emission and absorption of individual
volume elements. We describe several scientific as well as public applications
realized with {\tt Splotch}. We emphasize how different datasets and
configurations lead to remarkable different results in terms of the images and
animations. A few of these results are available online.Comment: 19 Pages, 8 Figures, to appear in New Journal of Physics, Focus Issue
on "Visualisation in Physics", edited by B. Sanders, T. Senden and V.
Springe
Low-cost Sensor System for Non-invasive Monitoring of Cell Growth in Disposable Bioreactors
AbstractTo ensure productivity and product quality, the parameters of biotechnological processes need to be monitored. Along temperature or pH, one important parameter is the cell density in the culture medium. In this work, we present a low-cost sensor system for online cell growth monitoring in bioreactors via permittivity measurements based on coplanar transmission lines. To evaluate the sensor, E. coli cultivations are performed. We found a good correlation between optical density of the culture medium and the effective permittivity at a frequency of 1kHz when the sensor is submerged into the culture medium. Measurements at higher frequencies additionally allow monitoring the osmolarity. Furthermore, an improved sensor was successfully used for first non-invasive measurements through the polymer wall of a disposable bioreactor
- …