Modeling kinetic non-equilibrium using the first two moments of the residence time distribution     

R. Andricevic  E. Foufoula-Georgiou 《Stochastic Hydrology and Hydraulics》1991,5(2):155-171

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The impact of human activities in the Malia coastal area (Crete) on groundwater quality   总被引：1，自引：0，他引：1  

N. J. Lambrakis 《Environmental Geology》1998,36(1-2):87-92

The evaluation of the long-term effects of seawater intrusion into the aquifers due to negative water balance and nitrate pollution of drinking-water quality due to human activities requires detailed knowledge of both the transport of the chemical constituents and the geochemical processes within aquifers. Hydrogeological and hydrochemical studies in the unconfined aquifer of Malia have provided the necessary data to define the areas at increased risk from these phenomena. The solution of the second Fick's low under given boundary conditions gave an estimate of the propagation of groundwater pollution by NO₃ ^–. Additionally, in order to simulate the ion concentration changes during a period, for example a period of positive water balance or refreshening, groundwater transport and cation exchange reactions were modelled using the code PHREEQM. Received: 25 July 1997 · Accepted: 4 November 1997  相似文献   

Transformations of Benzene Photoinduced by Nitrate Salts and Iron Oxide     

Daniele?Borghesi  Davide?Vione  Valter?Maurino  Claudio?MineroEmail author 《Journal of Atmospheric Chemistry》2005,52(3):259-281

This paper reports the results of a study on the transformation of benzene in the presence of solid nitrate salts (NaNO₃, NH₄NO₃) under irradiation in a gas-solid photoreactor. Sodium and ammonium nitrate have been chosen as representative of the composition of atmospheric particulate, benzene as a model aromatic molecule. The purpose is to simulate the transformations that aromatic compounds undergo on the surface of dispersed particles in the atmosphere. Irradiation of sodium nitrate causes hydroxylation and nitration of benzene, yielding phenol and nitrobenzene. This is most likely due to the generation of ^•OH and ^•NO₂ radicals upon nitrate photolysis, with ^•OH + O₂ leading to the formation of phenol and ^•OH + ^•NO₂ yielding nitrobenzene. The percentage of oxygen in the reaction environment influences the transformation pathways, with phenol formation being favoured and nitrobenzene formation depressed by high O₂ concentration. In the presence of hematite (α-Fe₂O₃, another component of atmospheric particulate) very relevant formation of nitrobenzene takes place even with 21% oxygen (simulated air), indicating that the interaction between hematite and nitrate can lead to the formation of aromatic nitroderivatives on the surface of atmospheric particulate. The effect of hematite is possibly due to protonation of peroxynitrite, formed upon nitrate photoisomerisation, to peroxynitrous acid, a powerful nitrating agent. A similar effect leads to relevant formation of nitrobenzene under atmospheric conditions upon irradiation of the acid salt ammonium nitrate.  相似文献   

Kinetics of microbially mediated reactions: dissimilatory sulfate reduction in saltmarsh sediments (Sapelo Island, Georgia, USA)   总被引：1，自引：0，他引：1  

Alakendra N. Roychoudhury  Philippe Van Cappellen  Joel E. Kostka  Eric Viollier 《Estuarine, Coastal and Shelf Science》2003,56(5-6):1001-1010

A sediment disk reactor was tested in once flow-through mode to retrieve kinetic parameters for the Monod rate law that describes sulfate reduction. The experimental method was compared with a previously described procedure by the authors where a sediment plug-flow reactor was operated in a recirculation mode. In recirculation mode, accumulation of metabolic byproducts in certain cases may result in negative feedback, thus preventing accurate determination of kinetic information. The method described in this article provides an alternative to the recirculation sediment plug-flow-through reactor technique for retrieving kinetic parameters of microbially mediated reactions in aquatic sediments.For sulfate reduction in a saltmarsh site, a maximum estimate of the half-saturation concentration, K_s, of 204±26 μM and a maximum reaction rate, R_m, of 2846±129 nmol cm_{(wetsediment)}³ d⁻¹ was determined. The K_s value obtained was consistent with the one estimated previously (K_s=240±20 μM) from a different site within the same saltmarsh mud flat using a recirculating reactor. From the R_m value and reduction rates determined using ³⁵SO₄²⁻ incubation experiments, we infer that sulfate reduction is limited in the field. Substrate availability is not the main contributor for the limitation, however. Competition from other microbes, such as iron reducers affects the activity of sulfate reducers in the suboxic to anoxic zones, whereas aerobes compete in the oxic zone. High sulfide concentration in the pore water may also have acted as a toxin to the sulfate reducers in the field.  相似文献   

Some aspects of the hydrothermal reactions of tin during skarn formation     

P.J. Eadington  K. Kinealy 《Australian Journal of Earth Sciences》2013,60(3-4):461-471

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http://www.sciencedirect.com/science/article/pii/S1674987114000449     

Oleg G.Safonov  Leonid Y.Aranovich 《地学前缘(英文版)》2014,5(5):711-727

We review petrologic observations of reaction textures from high-grade rocks that suggest the passage of fluids with variable alkali activities. Development of these reaction textures is accompanied by regular compositional variations in plagioclase, pyroxenes, biotite, amphibole and garnet. The textures are interpreted in terms of exchange and net-transfer reactions controlled by the K and Na activities in the fluids. On the regional scale, these reactions operate in granitized, charnockitized, syenitized etc. shear zones within high-grade complexes. Thermodynamic calculations in simple chemical systems show that changes in mineral assemblages, including the transition from the hydrous to the anhydrous ones, may occur at constant pressure and temperature due only to variations in the H2O and the alkali activities. A simple procedure for estimating the activity of the two major alkali oxides, K2O and Na2O, is imple- mented in the TWQ software. Examples of calculations are presented for well-documented dehydration zones from South Africa, southern India, and Sri Lanka. The calculations have revealed two end-member regimes of alkalis during specific metamorphic processes： rock buffered, which is characteristic for the precursor rocks containing two feldspars, and fluid-buffered for the precursor rocks without K-feldspar. The observed reaction textures and the results of thermodynamic modeling are compared with the results of available experimental studies on the interaction of the alkali chloride and carbonate-bearing fluids with metamorphic rocks at mid-crustal conditions. The experiments show the complex effect of alkali activities in the fluid phase on the mineral assemblages. Both thermodynamic calculations and experiments closely reproduce paragenetic relations theoretically predicted by D.S. Korzhinskii in the 1940s.  相似文献   

Isobar Attenuation Using Anion–Gas Reactions for Accelerator Mass Spectrometry and Application to 36Cl     

John Eliades  Xiaolei Zhao  William E. Kieser  Albert E. Litherland 《Geostandards and Geoanalytical Research》2010,34(2):107-115

Preliminary tests of a prototype radio-frequency quadrupole (RFQ) collision cell system, known as an isobar separator for anions (ISA), for the removal of isobaric interferences for accelerator mass spectrometry (AMS) and for studies of anion–gas interactions are reported. The ISA decelerated a mass-analysed beam of anions from an energy (∼ 20 keV) typically generated by an AMS ion source to < 10 eV. RFQs and electrostatic lenses then guided the ions through the collision cell where ion-gas collisions reduced both the energy and energy spread of the ion beam (cooled the ions) and ion-gas reactions attenuated most of the unwanted isobars. The anions were then re-accelerated to their original energy for injection into the rest of the AMS system. With the ISA installed on a full 3 MV AMS system, attenuations of ³²S^-, ¹²C₃^- and ³⁹K^- by six, seven and greater than ten orders of magnitude, respectively were achieved using 0.7–1 Pa NO₂ gas in the collision cell, while maintaining approximately 10–30% of the chlorine anion transmission. A further measurement of a ³⁶Cl/Cl = 4.1 × 10^-11 RM is also described. The results suggest that the ³⁶Cl/Cl lower detection limit of the current system was 10^-14–10^-15 for samples that could be prepared with S/Cl ratios below 10 μg g^-1.  相似文献   

AGB星氦燃烧壳层源出现非稳定热核反应的判据     

蒋苏云 《天文学报》2005,46(4):380-397

建立了热脉动AGB星氦燃烧壳层源出现非稳定热核反应的判据,新判据包含 了丰富的物理信息,它不仅与热脉动AGB星氦燃烧壳层源的几何性质有关,而且与氦燃 烧壳层源的力学、热学和化学的性质都有关． 提出了热脉动AGB星氦燃烧壳层源非稳定热核反应的发生和消失的机理,它可表述 为:热脉动AGB星氦燃烧壳层源的局部区域出现对流不稳定区会触发非稳定热核反应的 发生,非稳定热核反应会促使氦燃烧壳层源急速膨胀,氦燃烧壳层源的急速几何形变会消 除非稳定热核反应． 用改进后的Kippenhahn恒星演化程序对5M(?)恒星进行了从主序星到热脉动AGB 星的演化模型计算,结果表明新判据能很好地反映5M(?)AGB星氦燃烧壳层源的热核反应 情况．并得出5M(?)热脉动AGB星在第6次热脉动周期阶段,被挖掘到热脉动AGB星 表面的元素主要是在温度lgT2／K<8．155和密度4．0相似文献   

On variations in the fine-structure constant and stellar pollution of quasar absorption systems     

Y. Fenner  M. T. Murphy  B. K. Gibson 《Monthly notices of the Royal Astronomical Society》2005,358(2):468-480

At redshifts   z _abs≲ 2  , quasar absorption-line constraints on space–time variations in the fine-structure constant, α, rely on the comparison of Mg  ii and Fe  ii transition wavelengths. One potentially important uncertainty is the relative abundance of Mg isotopes in the absorbers, which, if different from solar, can cause spurious shifts in the measured wavelengths and, therefore, α. Here we explore chemical evolution models with enhanced populations of intermediate-mass (IM) stars, which, in their asymptotic giant branch phase, are thought to be the dominant factories for heavy Mg isotopes at the low metallicities typical of quasar absorption systems. By design, these models partially explain recent Keck/HIRES evidence for a smaller α in   z _abs < 2  absorption clouds than on Earth. However, such models also overproduce N, violating observed abundance trends in high- z _abs damped Lyman-α (DLA) systems. Our results do not support the recent claim of Ashenfelter et al. that similar models of IM-enhanced initial mass functions (IMFs) may simultaneously explain the HIRES varying-α data and DLA N abundances. We explore the effect of the IM-enhanced model on Si, Al and P abundances, finding it to be much less pronounced than for N. We also show that the ¹³C/¹²C ratio, as measured in absorption systems, could constitute a future diagnostic of non-standard models of the high-redshift IMF.  相似文献   

Rosseland and Planck mean opacities for primordial matter     

Michael Mayer  Wolfgang J. Duschl 《Monthly notices of the Royal Astronomical Society》2005,358(2):614-631

We present newly calculated low-temperature opacities for gas with a primordial chemical composition. In contrast to earlier calculations, which took a pure metal-free hydrogen/helium mixture, we take into account the small fractions of deuterium and lithium as resulting from standard big bang nucleosynthesis. Our opacity tables cover the density range  −16 < log ρ[g cm⁻³] < −2  and the temperature range of  1.8 < log  T [K] < 4.6  , while previous tables have usually been restricted to   T > 10³ K  . We find that, while the presence of deuterium does not significantly alter the opacity values, the presence of lithium gives rise to major modifications of the opacities, at some points increasing it by approximately two orders of magnitude relative to pure hydrogen/helium opacities.  相似文献