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Equilibrium Assumption (equilibrium + assumption)
Selected AbstractsPartitioning of metals (Cd, Co, Cu, Ni, Pb, Zn) in soils: concepts, methodologies, prediction and applications , a reviewEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2009F. Degryse Summary Prediction of the fate of metals in soil requires knowledge of their solid,liquid partitioning. This paper reviews analytical methods and models for measuring or predicting the solid,liquid partitioning of metals in aerobic soils, and collates experimental data. The partitioning is often expressed with an empirical distribution coefficient or Kd, which gives the ratio of the concentration in the solid phase to that in the solution phase. The Kd value of a metal reflects the net effect of various reactions in the solid and liquid phases and varies by orders of magnitude among soils. The Kd value can be derived from the solid,liquid distribution of added metal or that of the soil-borne metal. Only part of the solid-phase metal is rapidly exchangeable with the solution phase. Various methods have been developed to quantify this ,labile' phase, and Kd values based on this phase often correlate better with soil properties than Kd values based on total concentration, and are more appropriate to express metal ion buffering in solute transport models. The in situ soil solution is the preferred solution phase for Kd determinations. Alternatively, water or dilute-salt extracts can be used, but these may underestimate in situ concentrations of dissolved metals because of dilution of metal-complexing ligands such as dissolved organic matter. Multi-surface models and empirical models have been proposed to predict metal partitioning from soil properties. Though soil pH is the most important soil property determining the retention of the free metal ion, Kd values based on total dissolved metal in solution may show little pH dependence for metal ions that have strong affinity for dissolved organic matter. The Kd coefficient is used as an equilibrium constant in risk assessment models. However, slow dissociation of metal complexes in solution and slow exchange of metals between labile and non-labile pools in the solid phase may invalidate this equilibrium assumption. [source] Forced convective heat transfer for fluid flowing through a porous medium with internal heat generationHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 3 2001Hua Du Abstract Forced convection in channels filled with packed beads with internal heat source was numerically analyzed by using the extended Darcy model for the flow and energy conservation equations with nonthermal equilibrium (NTE). The temperature difference between the matrix and fluid phases was discussed with three dimensionless parameters (Rep, H/dp, and ks/kf). It is concluded that the thermal equilibrium assumption may be adopted for the situation when the solid conduction dominated over the convection or when the heat transfer coefficient between the two phases is large at large Rep. The Nusselt number at the wall with the variation of Rep for different ks/kf was also investigated. © 2001 Scripta Technica, Heat Trans Asian Res, 30(3): 213,221, 2001 [source] A three-scale computational model of reactive pollutant transport in smectitic claysINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 8 2006Márcio A. Murad Abstract A three-scale model of dual-porosity type is proposed to describe contaminant transport in swelling clays. The swelling medium is characterized by three separate length scales (nano, micro and macro) and two levels of porosity (micro- and macro-pores). At the nanoscale the medium is composed of charged clay particles saturated by a binary monovalent aqueous electrolyte solution occupying the micro-pores. At the intermediate (micro) scale the two-phase system is represented in a homogenized fashion with averaged microscopic equations governing the behaviour of the clay clusters (or aggregates) regarded a two-phase mixture composed of clay particles and electrolyte solution. At the macroscale, the microscale mixture of clay clusters is homogenized with the bulk solution containing non-electrolyte species lying in the macro-pore system. The resultant macroscopic picture appears governed by a dual-porosity model wherein the clay clusters act as sources/sinks of mass to the macro-pore system. Under a local equilibrium assumption between the clay clusters and macro-pores, a quasi-steady version of the dual-porosity model is derived. This framework combined with a three-scale picture of the colloidal system allows to provide nanoscopic representations for the retardation coefficient, governing the instantaneous adsorption/desorption of the ionic species in the micro-pores, in terms of the local behaviour of the electrical double layer potential which satisfies a Poisson,Boltzmann-type problem at the nanoscale. Copyright © 2006 John Wiley & Sons, Ltd. [source] A reliability-based network design problemJOURNAL OF ADVANCED TRANSPORTATION, Issue 3 2005Piya Chootinan Abstract This paper presents a reliability-based network design problem. A network reliability concept is embedded into the continuous network design problem in which travelers' route choice behavior follows the stochastic user equilibrium assumption. A new capacity-reliability index is introduced to measure the probability that all of the network links are operated below their capacities when serving different traffic patterns deviating from the average condition. The reliability-based network design problem is formulated as a bi-level program in which the lower level sub-program is the probit-based stochastic user equilibrium problem and the upper level sub-program is the maximization of the new capacity reliability index. The lower level sub-program is solved by a variant of the method of successive averages using the exponential average to represent the learning process of network users on a daily basis that results in the daily variation of traffic-flow pattern, and Monte Carlo stochastic loading. The upper level sub-program is tackled by means of genetic algorithms. A numerical example is used to demonstrate the concept of the proposed framework. [source] HETEROZYGOTE EXCESS IN SMALL POPULATIONS AND THE HETEROZYGOTE-EXCESS EFFECTIVE POPULATION SIZEEVOLUTION, Issue 9 2004Franclois Balloux Abstract It has been proposed that effective size could be estimated in small dioecious population by considering the heterozygote excess observed at neutral markers. When the number of breeders is small, allelic frequencies in males and females will slightly differ due to binomial sampling error. However, this excess of heterozygotes is not generated by dioecy but by the absence of individuals produced through selfing. Consequently, the approach can also be applied to self-incompatible monoecious species. Some inaccuracies in earlier equations expressing effective size as function of the heterozygote excess are also corrected in this paper. The approach is then extended to subdivided populations, where time of sampling becomes crucial. When adults are sampled, the effective size of the entire population can be estimated, whereas when juveniles are sampled, the average effective number of breeders per subpopulations can be estimated. The main limitation of the heterozygote excess method is that it will only perform satisfactorily for populations with a small number of reproducing individuals. While this situation is unlikely to happen frequently at the scale of the entire population, structured populations with small subpopulations are likely to be common. The estimation of the average number of breeders per subpopulations is thus expected to be applicable to many natural populations. The approach is straightforward to compute and independent of equilibrium assumptions. Applications to simulated data suggest the estimation of the number of breeders to be robust to mutation and migration rates, and to specificities of the mating system. [source] TIME TO THE MOST RECENT COMMON ANCESTOR AND DIVERGENCE TIMES OF POPULATIONS OF COMMON CHAFFINCHES (FRINGILLA COELEBS) IN EUROPE AND NORTH AFRICA: INSIGHTS INTO PLEISTOCENE REFUGIA AND CURRENT LEVELS OF MIGRATIONEVOLUTION, Issue 1 2002Cortland K. Griswold Abstract We analyzed sequences from a 275-bp hypervariable region in the 5, end of the mitochondrial DNA control region in 190 common chaffinches (Fringilla coelebs) from 19 populations in Europe and North Africa, including new samples from Greece and Morocco. Coalescent techniques were applied to estimate the time to the most recent common ancestor (TMRCA) and divergence times of these populations. The first objective of this study was to infer the locations of refugia where chaffinches survived the last glacial episode, and this was achieved by estimating the TMRCA of populations in regions surrounding the Mediterranean that were unglaciated in the late Pleistocene. Although extant populations in Iberia, Corsica, Greece, and North Africa harbor haplotypes that are basal in a phylogenetic tree, this information alone cannot be used to infer that these localities served as refugia, because it is impossible to infer the ages of populations and their divergence times without also considering the population genetic processes of mutation, migration, and drift. Provided we assume the TMRCAs of populations are a reasonable estimate of a population's age, coalescent-based methods place resident populations in Iberia, Corsica, Greece, and North Africa during the time of the last glacial maximum, suggesting these regions served as refugia for the common chaffinch. The second objective was to determine when populations began diverging from each other and to use this as a baseline to estimate current levels of gene flow. Divergence time estimates suggest that European populations began diverging about 60,000 years before present. The relatively recent divergence of populations in North Africa, Italy, and Iberia may explain why classic migration estimates based on equilibrium assumptions are high for these populations. We compare these estimates with nonequilibrium-based estimates and show that the nonequilibrium estimates are consistently lower than the equilibrium estimates. [source] A two-step procedure for constructing confidence intervals of trait loci with application to a rheumatoid arthritis datasetGENETIC EPIDEMIOLOGY, Issue 1 2006Charalampos Papachristou Abstract Preliminary genome screens are usually succeeded by fine mapping analyses focusing on the regions that signal linkage. It is advantageous to reduce the size of the regions where follow-up studies are performed, since this will help better tackle, among other things, the multiplicity adjustment issue associated with them. We describe a two-step approach that uses a confidence set inference procedure as a tool for intermediate mapping (between preliminary genome screening and fine mapping) to further localize disease loci. Apart from the usual Hardy-Weiberg and linkage equilibrium assumptions, the only other assumption of the proposed approach is that each region of interest houses at most one of the disease-contributing loci. Through a simulation study with several two-locus disease models, we demonstrate that our method can isolate the position of trait loci with high accuracy. Application of this two-step procedure to the data from the Arthritis Research Campaign National Repository also led to highly encouraging results. The method not only successfully localized a well-characterized trait contributing locus on chromosome 6, but also placed its position to narrower regions when compared to their LOD support interval counterparts based on the same data. Genet. Epidemiol. 30:18,29, 2006. © 2005 Wiley-Liss, Inc. [source] | |||||||||||||