Dissertations / Theses on the topic 'Reactive transport model'

Reactive transport models are essential tools for predicting contaminant fate and transport in the subsurface and for designing effective remediation strategies. Sound understanding of subsurface mixing in heterogeneous porous media is the key for the realistic modeling of reactive transport. This dissertation aims to investigate the extent of mixing and improve upscaling effective macroscopic models for mixing-controlled reactive transport in connected heterogeneous formations, which usually exhibit strongly anomalous transport behavior. In this research, a novel approach is developed for an accurate geostatistical characterization of connected heterogeneous formations transformed from Gaussian random fields. Numerical experiments are conducted in such heterogeneous fields with different connectivity to investigate the performance of macroscopic mean transport models for simulating mixing-controlled reactive transport. Results show that good characterization of anomalous transport of a conservative tracer does not necessarily mean that the models may characterize mixing well and that, consequently, it is questionable that the models capable of characterizing anomalous transport behavior of a conservative tracer are appropriate for simulating mixing-controlled reactive transport. In connected heterogeneous fields with large hydraulic conductivity variances, macroscopic mean models ignoring concentration variations yield good prediction, while in fields with intermediate conductivity variances, the models must consider both the mean concentration and concentration variations, which are very difficult to evaluate both theoretically and experimentally. An innovative and practical approach is developed by combining mean conservative and reactive breakthrough curves for estimating concentration variations, which can be subsequently used by variance transport models for prediction. Furthermore, a new macroscopic framework based on the dual-permeability conceptualization is developed for describing both mean and concentration variation for mixing-controlled reactive transport. The developed approach and models are validated by numerical and laboratory visualization experiments. In particular, the new dual-permeability model demonstrates significant improvement for simulating mixing-controlled reactive transport in heterogeneous media with intermediate conductivity variances. Overall, results, approaches and models from this dissertation advance the understanding of subsurface mixing in anomalous transport and significantly improve the predictive ability for modeling mixing-controlled reactive transport in connected heterogeneous media.

The thesis deals with the numerical modeling of nonreactive and nonlinearly sorbing solutes in groundwater and analysis of the effect of heterogeneity resulting from spatial variation of physical and chemical parameters on the transport of solutes. For this purpose, a hy­brid flux corrected transport (FCT) and central difference method based on operator-split approach is developed for advection-dispersion solute transport equation. The advective transport is solved using the FCT technique, while the dispersive transport is solved using a conventional, fully implicit, finite difference scheme. Three FCT methods are developed and extension to multidimensional cases are discussed. The FCT models developed are anlaysed using test problems possessing analyt­ical solutions for one and two dimensional cases, while analysing advection and dispersion dominated transport situations. Different initial and boundary conditions, which mimic the laboratory and field situations are analysed in order to study numerical dispersion, peak cliping and grid orientation. The developed models are tested to study their relative merits and weaknesses for various grid Peclet and Courant numbers. It is observed from the one dimensional results that all the FCT models perform well for continuous solute sources under varying degrees of Courant number restriction. For sharp solute pulses FCT1 and FCT3 methods fail to simulate the fronts for advection dominated situations even for mod­erate Courant numbers. All the FCT models can be extended to multidimensions using a dimensional-split approach while FCT3 can be made fully multidimensional. It is observed that a dimensional-split approach allows use of higher Courant numbers while tracking the fronts accurately for the cases studied. The capability of the FCT2 model is demonstrated in handling situations where the flow is not aligned along the grid direction. It is observed that FCT2 method is devoid of grid orientation error, which is a common problem for many numerical methods based on Cartesian co-ordinate system. The hybrid FCT2 numerical model which is observed to perform better among the three FCT models is extended to model transport of sorbing solutes. The present study analyses the case of nonlinear sorption with a view to extend the model for any reactive transport situation wherein the chemical reactions are nonlinear in nature. A two step approach is adopted in the present study for coupling the partial differential equation gov­erning the transport and the nonlinear algebraic equation governing the equilibrium sorp­tion. The suitability of explicit-implicit (EI - form) formulation for obtaining accurate solution coupling the transport equation with the nonlinear algebraic equation solved using a Newton-Raphson method is demonstrated. The performance of the numerical model is tested for a range of Peclet numbers for modelling self-sharpening and self-smearing con­centration profiles resulting from nonlinear sorption. It is observed that FCT2 model based on this formulation simulates the fronts quite accurately for both advection and dispersion dominated situations. The delay in the solute mobility and additional dispersion are anal­ysed varying the statistical parameters characterising the heterogeneity namely, correlation scale and variance during the transport of solutes and comparisons are drawn with invariant, cases. The impact of dispersion during the heterogeneous transport is discussed.

The thesis deals with the numerical modeling of nonreactive and nonlinearly sorbing solutes in groundwater and analysis of the effect of heterogeneity resulting from spatial variation of physical and chemical parameters on the transport of solutes. For this purpose, a hy­brid flux corrected transport (FCT) and central difference method based on operator-split approach is developed for advection-dispersion solute transport equation. The advective transport is solved using the FCT technique, while the dispersive transport is solved using a conventional, fully implicit, finite difference scheme. Three FCT methods are developed and extension to multidimensional cases are discussed. The FCT models developed are anlaysed using test problems possessing analyt­ical solutions for one and two dimensional cases, while analysing advection and dispersion dominated transport situations. Different initial and boundary conditions, which mimic the laboratory and field situations are analysed in order to study numerical dispersion, peak cliping and grid orientation. The developed models are tested to study their relative merits and weaknesses for various grid Peclet and Courant numbers. It is observed from the one dimensional results that all the FCT models perform well for continuous solute sources under varying degrees of Courant number restriction. For sharp solute pulses FCT1 and FCT3 methods fail to simulate the fronts for advection dominated situations even for mod­erate Courant numbers. All the FCT models can be extended to multidimensions using a dimensional-split approach while FCT3 can be made fully multidimensional. It is observed that a dimensional-split approach allows use of higher Courant numbers while tracking the fronts accurately for the cases studied. The capability of the FCT2 model is demonstrated in handling situations where the flow is not aligned along the grid direction. It is observed that FCT2 method is devoid of grid orientation error, which is a common problem for many numerical methods based on Cartesian co-ordinate system. The hybrid FCT2 numerical model which is observed to perform better among the three FCT models is extended to model transport of sorbing solutes. The present study analyses the case of nonlinear sorption with a view to extend the model for any reactive transport situation wherein the chemical reactions are nonlinear in nature. A two step approach is adopted in the present study for coupling the partial differential equation gov­erning the transport and the nonlinear algebraic equation governing the equilibrium sorp­tion. The suitability of explicit-implicit (EI - form) formulation for obtaining accurate solution coupling the transport equation with the nonlinear algebraic equation solved using a Newton-Raphson method is demonstrated. The performance of the numerical model is tested for a range of Peclet numbers for modelling self-sharpening and self-smearing con­centration profiles resulting from nonlinear sorption. It is observed that FCT2 model based on this formulation simulates the fronts quite accurately for both advection and dispersion dominated situations. The delay in the solute mobility and additional dispersion are anal­ysed varying the statistical parameters characterising the heterogeneity namely, correlation scale and variance during the transport of solutes and comparisons are drawn with invariant, cases. The impact of dispersion during the heterogeneous transport is discussed.

In this thesis we show how reactive processes can be affected by the presence of different types of spatial constraints, so much so that their nonlinear dynamics can be qualitatively altered or that new and unexpected behaviors can be produced. To understand how this interplay can occur in general terms, we theoretically investigate four very different examples of this situation. <p><p>The first system we study is a reversible trimolecular chemical reaction which is taking place in closed one-dimensional lattices. We show that the low dimensionality may or may not prevent the reaction from reaching its equilibrium state, depending on the microscopic properties of the molecular reactive mechanism. <p><p>The second reactive process we consider is a network of biological interactions between pigment cells on the skin of zebrafish. We show that the combination of short-range and long-range contact-mediated feedbacks can promote a Turing instability which gives rise to stationary patterns in space with intrinsic wavelength, without the need of any kind of motion.<p><p>Then we investigate the behavior of a typical chemical oscillator (the Brusselator) when it is constrained in a finite space. We show that molecular crowding can in such cases promote new nonlinear dynamical behaviors, affect the usual ones or even destroy them. <p><p>Finally we look at the situation where the constraint is given by the presence of a solid porous matrix that can react with a perfect gas in an exothermic way. We show on one hand that the interplay between reaction, heat flux and mass transport can give rise to the propagation of adsorption waves, and on the other hand that the coupling between the chemical reaction and the changes in the structural properties of the matrix can produce sustained chemomechanical oscillations. <p><p>These results show that spatial constraints can affect the kinetics of reactions, and are able to produce otherwise absent nonlinear dynamical behaviors. As a consequence of this, the usual understanding of the nonlinear dynamics of reactive systems can be put into question or even disproved. In order to have a better understanding of these systems we must acknowledge that mechanical and structural feedbacks can be important components of many reactive systems, and that they can be the very source of complex and fascinating phenomena.<p><br>Doctorat en Sciences<br>info:eu-repo/semantics/nonPublished

This thesis presents the development of a numerical water quality model using a general paradigm of reaction-based approaches. In a reaction-based approach, all conceptualized biogeochemical processes are transformed into a reaction network. Through the decomposition of species governing equations via Gauss-Jordan column reduction of the reaction network, (1) redundant fast reactions and irrelevant kinetic reactions are removed from the system, which alleviates the problem of unnecessary and erroneous formulation and parameterization of these reactions, and (2) fast reactions and slow reactions are decoupled, which enables robust numerical integrations. The system of species transport equations is transformed to reaction-extent transport equations, which is then approximated with two subsets: algebraic equations and kinetic-variables transport equations. As a result, the model alleviates the needs of using simple partitions for fast reactions. With the diagonalization strategy, it makes the inclusion of arbitrary number of fast and kinetic reactions relatively easy, and, more importantly, it enables the formulation and parameterization of kinetic reactions one by one. To demonstrate the general paradigm, QAUL2E was recasted in the mode of a reaction network. The model then was applied to the Loxahatchee estuary to study its response to a hypothetical biogeochemical loading from its surrounding drainage. Preliminary results indicated that the model can simulate four interacting biogeochemical processes: algae kinetics, nitrogen cycle, phosphorus cycle, and dissolved oxygen balance.<br>M.S.<br>Department of Civil and Environmental Engineering<br>Engineering and Computer Science<br>Civil Engineering

This dissertation presents the design of an integrated watershed model, WASH123D version 3.0, a first principle, physics-based watershed-scale model of integrated hydrology/hydraulics and water quality transport. This numerical model is comprised of three modules: (1) a one-dimensional (1-D) simulation module that is capable of simulating separated and coupled fluid flow, sediment transport and reaction-based water quality transport in river/stream/canal networks and through control structures; (2) a two-dimensional (2-D) simulation module, capable of simulating separated and coupled fluid flow, sediment transport, and reactive biogeochemical transport and transformation in two-dimensional overland flow systems; and (3) a three-dimensional (3-D) simulation module, capable of simulating separated and coupled fluid flow and reactive geochemical transport and transformation in three-dimensional variably saturated subsurface systems. The Saint Venant equation and its simplified versions, diffusion wave and kinematic wave forms, are employed for surface fluid flow simulations and the modified Richards equation is applied for subsurface flow simulation. The reaction-based advection-dispersion equation is used as the governing equation for water quality transport. Several physically and mathematically based numerical options are provided to solve these governing equations for different application purposes. The surface-subsurface water interactions are considered in the flow module and simulated on the basis of continuity of interface. In the transport simulations, fast/equilibrium reactions are decoupled from slow/kinetic reactions by the decomposition of reaction networks; this enables robust numerical integrations of the governing equation. Kinetic variables are adopted as primary dependent variables rather than biogeochemical species to reduce the number of transport equations and simplify the reaction terms. In each time step, hydrologic/hydraulic variables are solved in the flow module; kinetic variables are then solved in the transport module. This is followed by solving the reactive chemical system node by node to yield concentrations of all species. Application examples are presented to demonstrate the design capability of the model. This model may be of interest to environmental scientists, engineers and decision makers as a comprehensive assessment tool to reliably predict the fluid flow as well as sediment and contaminant transport on watershed scales so as to evaluate the efficacy and impact of alternative watershed management and remediation techniques prior to incurring expense in the field.<br>Ph.D.<br>Department of Civil and Environmental Engineering<br>Engineering and Computer Science<br>Civil Engineering PhD

Copper-mining activities in the Pinal Creek basin have generated a plume of acidic ground water in the regional aquifer in Pinal Creek basin. From 1984 to 1994, contaminant concentrations decreased in response to contaminant source removal, remedial pumping, and uncontaminated recharge. Two multi-species reactive transport models, PHREEQC and HYDROGEOCHEM, were assessed to determine which would best aid in analysis of physical and chemical processes that control contaminant concentrations. Only PHREEQC successfully simulated transport accompanied by oxidation-reduction reactions. Reactive transport was simulated with a one-dimensional PHREEQC mixing-cell model that accounted for advection, dispersion, precipitation, dissolution, diffuse double layer adsorption, and the oxidation and reduction of iron and manganese, respectively. Simulated and measured chemistry differed in part because ground water was at times undersaturated or supersaturated with respect to gypsum. Oxidation-reduction reactions may be slow relative to advection and the water may not be in equilibrium with redox minerals.

For a numerical or analytical model to be useful it should be ensured that the model outcome matches the observations or field measurements during calibration. This process has been typically done by manual perturbation of the model input parameters. This research investigates a methodology for using non linear parameter estimation technique (the Marquardt-Levenberg technique) with the multi component reactive solute transport model SEAM3D. The reactive multi-component solutes considered in this study are chlorinated ethenes. Previous studies have shown that this class of compounds can be degraded by four different biodegradation mechanisms, and the degradation path is a function of the prevailing oxidation reduction conditions. Tests were performed in three levels; the first level utilized synthetic model-generated data. The idea was to develop a methodology and perform preliminary testing where "observations" can be generated as needed. The second level of testing involved performing the testing on a single redox zone model. The methodology was refined and tested using data from a chlorinated ethenes-contaminated site. The third level involved performing the tests on a multiple redox zone model. The methodology was tested, and statistical validation of the recommended methodology was performed. The results of the tests showed that there is a statistical advantage for choosing a subgroup of the available parameters to optimize instead of the optimizing the whole available group. Therefore, it is recommended to perform a parameter sensitivity study prior to the optimization process to identify the suitable parameters to be chosen. The methodology suggests optimizing the oxidation-reduction species parameters first then calibrating the chlorinated ethenes model. The results of the tests also proved the advantage of the sequential optimization of the model parameters, therefore the parameters of the parent compound are optimized, updated in the daughter compound model, for which the parameters are then optimized so on. The test results suggested considering the concentrations of the daughter compounds when optimizing the parameters of the parent compounds. As for the observation weights, the tests suggest starting the applied observation weights during the optimization process at values of one and changing them if needed. Overall the proposed methodology proved to be very efficient. The optimization methodology yielded sets of model parameters capable of generating concentration profiles with great resemblance to the observed concentration profiles in the two chlorinated ethenes site models considered.<br>Ph. D.

The overarching goal of this thesis is to develop a diagnostic and predictive model to quantify the estuarine CO2 dynamics across scales – from catchment to the globe – using an approach that explicitly resolves the strong physical and biogeochemical gradients typically observed in these systems.Chapter 1 provides fundamental definitions and descriptions of estuaries, as well as an assessment of their role in the global carbon cycle. It also raises the specific objectives and research questions tackled in the present study. Chapter 2 presents the rationale behind the novel modelling approach (C-GEM, Carbon-Generic Estuary Model) developed in the framework of this thesis. First, the dominant processes that control the estuarine biogeochemistry in estuaries are discussed in detail. Then, the power of reactive-transport models (RTMs) in understanding and quantifying the estuarine biogeochemical functioning is illustrated on the basis of local modelling studies. Finally, trends in estuarine biogeochemical dynamics across different geometries and environmental scenarios are briefly explored with C-GEM and results are discussed in the context of improving the modelling of estuarine carbon dynamics at regional and global scales. In Chapter 3, a detailed description of C-GEM, both in terms of structure and set-up, is provided and model’s performance is successfully evaluated through comprehensive model-data and model-model comparisons in the macro-tidal Scheldt estuary (BE/NL). In Chapter 4, C-GEM is combined with a generic set of forcing conditions and parameter values to quantify the carbon dynamics (net ecosystem metabolism, CO2 exchange at the air-water interface, carbon filtering capacity) in three idealized estuaries subject to temperate climatic conditions. Their hydro-geometrical characteristics span the wide diversity of estuarine morphological characteristics. Model results are used to upscale the estuarine CO2 dynamics under present-day conditions and to quantify the response of the estuarine filter to future atmospheric CO2, land use and climate change scenarios. In Chapter 5, C-GEM is applied to derive estimations of carbon export and CO2 outgassing from all tidal estuaries discharging in the North Sea. Overall, our results suggest that the estuarine carbon filtering capacity and the contribution of these land-ocean transition systems to the atmospheric CO2 budget might not be as high as previously thought. Finally, a conclusive chapter (Chapter 6) provides a synthesis of the key findings and arguments projected by the present research work. Moreover, recommendations are given in the light of further applications of the modelling approach developed during this thesis.<br>Doctorat en Sciences<br>info:eu-repo/semantics/nonPublished

Soil and groundwater pollution has become a global issue since the advent of industrialization and mechanized agriculture. Some contaminants such as PAHs may persist in the subsurface for decades and centuries. In a bid to address these issues, protection of groundwater must be based on the quantification of potential threats to pollution at the subsurface which is often inaccessible. Risk assessment of groundwater pollution may however be strongly supported by applying process-based simulation models, which turn out to be particularly helpful with regard to long-term predictions, which cannot be undertaken by experiments. Such reliable predictions, however, can only be achieved if the used modeling tool is known to be applicable. The aim of this work was threefold. First, a source strength function was developed to describe the leaching behavior of point source organic contaminants and thereby acting as a time-dependent upper boundary condition for transport models. For general application of these functions dimensionless numbers known as Damköhler numbers were used to characterize the reaction of the pollutants with the solid matrix. Two functions were derived and have been incorporated into an Excel worksheet to act as a practical aid in the quantification of leaching behavior of organic contaminant in seepage water prognoses. Second, the process based model tool SMART, which is well validated for laboratory scale data, was applied to lysimeter scale data from two research centres, FZJ (Jülich) and GSF (München) for long term predictions. Results from pure forward model runs show a fairly good correlation with the measured data. Finally, the derived source term functions in combination with the SMART model were used to assess groundwater vulnerability beneath a typical landfill at Kwabenya in Ghana. The predicted breakthrough time after leaking from the landfill was more than 200 years considering the operational time of the facility (30 years). Considering contaminant degradation, the landfill would therefore not cause groundwater pollution under the simulated scenarios and the SMART model can be used to establish waste acceptance criteria for organic contaminants in the landfill at Kwabenya<br>Seit dem Beginn der Industrialisierung und der mechanisierten Landwirtschaft wurde die Boden- und Grundwasserverschmutzung zu einem weltweiten Problem. Einige Schadstoffe wie z. B. PAK können für Jahrzehnte oder Jahrhunderte im Untergrund bestehen. Um diese Probleme behandeln zu können, muss der Schutz des Grundwassers basierend auf der Quantifizierung potentieller Gefährdungen des zumeist unzugänglichen Untergrundes erfolgen. Risikoabschätzungen von Grundwasserverschmutzungen können jedoch durch die Anwendung prozess-basierter Simulationsmodelle erheblich unterstützt werden, die sich besonders im Hinblick auf Langzeitvorhersagen als hilfreich erweisen und nicht experimentell ermittelbar sind. Derart zuverlässige Vorhersagen können jedoch nur erhalten werden, wenn das verwendete Modellierwerkzeug als anwendbar bekannt ist. Das Ziel dieser Arbeit bestand aus drei Teilen. Erstens wurde eine Quellstärke-funktion entwickelt, die das Ausbreitungsverhalten organischer Schadstoffe aus einer Punktquelle beschreibt und dadurch als zeitabhängige obere Randbedingung bei Transportmodellen dienen kann. Im Hinblick auf die allgemeine Anwendbarkeit dieser Funktion werden als Damköhler-Zahlen bekannte, dimensionslose Zahlen verwendet, um die Reaktion von Schadstoffen mit Feststoffen zu charakterisieren. Zwei Funktionen wurden abgeleitet und in ein Excel-Arbeitsblatt eingefügt, das ein praktisches Hilfsmittel bei der Quantifizierung des Freisetzungsverhaltens organischer Schadstoffe im Rahmen der Sickerwasserprognose darstellt. Der zweite Teil dieser Arbeit beinhaltet die Anwendung des prozessbasierten und mittels Laborexperimenten validierten Modellwerkzeugs SMART für Langzeitprognosen auf der Lysimeterskala anhand von Daten zweier Forschungszentren, FZJ (Jülich) und GSF (München). Ergebnisse reiner Vorwärtsmodellierungsläufe zeigten gute Übereinstimmungen mit den gemessenen Daten. Im dritten Teil wurden die erhaltenen Quellstärkefunktionen in Kombination mit dem SMART-Modell eingesetzt, um das Grundwassergefährdungspotential unter einer typischen Deponie in Kwabenya, Ghana, einzuschätzen. Die vorhergesagten Durchbruchszeiten nach einer Leckage in der Deponie betragen über 200 Jahre bei einer Betriebszeit von 30 Jahren. Unter Berücksichtigung des Schadstoffabbaus verursacht die Deponie somit keine Grundwasserverunreinigung im Rahmen der simulierten Szenarien und das SMART-Modell kann verwendet werden, um Schadstoffgrenzwerte für organische Schadstoffe in der Deponie in Kwabenya festzulegen

[Truncated abstract] As mineral deposits continue to be mined, the non economic gangue materials such as sulphides (e.g. pyrite) that are extracted as part of the ore body or overburden are deposited within the waste rock and/or milled tailings. As a result of natural weathering processes, these reactive materials represent a potential hazard to surrounding environments. A major consequence, resulting from mine-waste impoundments containing sulphidic materials, relates to the offsite movement of low pH leachates containing elevated concentrations of metal ions posing a contamination threat. The processes and mechanisms acting in the formation of acid mine drainage (AMD) are highly variable and, to a high extent, controlled by climatic conditions as the main driver of water flow and wetness of the system which in turn determines the availability of oxygen as well as water for pyrite weathering. In particular, this thesis is based on the hypothesis that in semiarid and arid climates the acid production may be water … The experiments were repeated at different water contents ranging from 0.24 to 0.33 cm3 cm-3. Breakthrough curves (BTC) of Li+, K+, Ca2+, Mg2+, Na+ and pH were measured and described with models of different complexities. This included the use of a simple linear and non-linear isotherms for Li+ alone, a binary Li+ - K+ ion exchange, and a complete multicomponent chemical equilibrium description of ion transport. The latter, by including dissolution of primary minerals which released base cations such as Mg2+, Ca2+ and K+ explained some of the elution patterns of base cations for which the Li+ - K+ exchange was the dominant process. Furthermore, under unsaturated water flow conditions, retardation of Li+ increased with decreasing water content. Thus solute mobility in mafic rock tailings appears to decrease under strongly unsaturated water flow conditions.

La gestion de sols ou sédiments contaminés par des métaux nécessite de pouvoir prédire la migration de ces métaux, dont la mobilité dépend des propriétés de transport du milieu ainsi que de la réactivité chimique de celui-ci (principalement réactions sorption/désorption). Pour étudier l'adsorption des cations métalliques (majeurs et traces) en condition dynamique, des expériences de transport en colonne ont été réalisées avec un sol pauvre en minéraux carbonatés et en matière organique. Considérant que la réactivité de ce sol était due essentiellement à la phase argileuse smectitique, un modèle d'adsorption basé sur les propriétés de rétention de la montmorillonite du Wyoming, utilisant la théorie des échangeurs d'ions multi-sites a été intégré à un code de transport 1D. Les prédictions calculées par ce modèle ont ensuite été comparées aux courbes de percées mesurées dans cette étude et reportées de la littérature. L'étude sur le transport réactif des cations majeurs a mis en évidence le rôle significatif des protons (et ce même à des pH proche de la neutralité), et a permis de valider le modèle pour le transport compétitif des cations majeurs (Na et Ca). Cependant, l'étude de la rétention de Zn(II) a mis en évidence une incohérence entre les résultats issus des expériences batch et ceux obtenus avec les expériences colonne, que l'on pourrait attribuer à la contribution d'une autre phase adsorbante (illite). Néanmoins, le modèle d'adsorption proposé a permis de reproduire avec une bonne satisfaction des données expérimentales de rétention de Zn(II) proposées dans la littérature et obtenues avec des expériences colonne<br>Management of soils or sediments contaminated by metals requires to predict the migration of metallic cations, whose mobility depends both on the transport properties of the medium and chemical reactivity of the system (principally sorption/desorption reactions). To study the sorption of metallic cations (major and trace) in dynamic condition, transport experiments using columns have been carried out with a soil poor in carbonated minerals and organic matter. Considering that the reactivity of this soil was mainly due to swelling clay minerals, a sorption model based on the sorption properties of the Wyoming montmorillonite and built according to a multi-site ion exchanger theory has been integrated into a 1D transport code. The predictions given by this model were then compared with the breakthrough curves measured in this study and those reported in the literature. The study of the reactive transport of major cations highlighted the significant role of protons (even at near neutral pH), and validated the model for major cations (Na and Ca). However, the study concerning Zn (II) showed a discrepancy between the results obtained from batch experiments and those issued from column experiments, which could be attributed to the contribution of another sorbent phase (illite). Finally, the proposed sorption model allowed reproducing with a good confidence experimental data reported from literature for sorption of Zn (II) in dynamic conditions

Au vu de leurs différentes propriétés, les matériaux cimentaires sont largement considérés dans les différents projets de gestion de déchets radioactifs. Leurs propriétés mécaniques, leurs faibles coefficients de transport ainsi que leur capacité à fixer les principaux radionucléides sont les principaux avantages qui en font un des meilleurs choix pour la conception des barrières ouvragées.Pour les études de sûreté, leur durabilité est capitale. Au cours de la vie d’un tel dépôt,via l’infiltration d’eau ou les interfaces chimiquement agressives avec les argiles, les différents matériaux vont subir des perturbations physicochimiques qui vont altérer leurs structures et potentiellement compromettre leurs fonctions de sûreté. L’étendue de ces perturbations, fondamentale pour l’étude de sûreté, est contrôlée par les propriétés de transport de ces matériaux.Pour modéliser proprement ces phénomènes, il faut pouvoir coupler les évolutions géochimiques des matériaux tout en évaluant le transport à travers ceux-ci. C’est le but de différents codes de transport réactif, qui utilisent une loi de rétroaction pour modifier les propriétés de transport lors d’une modification de microstructure. Le problème est qu’il n’existe pas de loi de rétroaction adaptée aux matériaux cimentaires, qui possèdent une structure poreuse complexe du nanomètre jusqu’à plusieurs micromètres. En général, des lois empiriques de type Archie sont utilisées. Toutefois, même l’utilisation de lois plus sophistiquées ne permet pas de reproduire sensiblement les évolutions liées à la structure porale. Cette loi de rétroaction est probablement la principale raison pour laquelle les résultats de simulation ont du mal à reproduire lesrésultats expérimentaux. Le but de cette thèse est de proposer une meilleure loi de rétroaction et de l’intégrer dans un code de transport réactif.Pour ce faire, trois approches complémentaires ont été mises en oeuvre. La première, expérimentale,consiste en la réalisation des matériaux cimentaires les plus simples possibles :des phases C-S-H pures et une pâte de ciment modèle. Ces matériaux sont ensuite caractérisés finement :leurs propriétés de transport sont évaluées et une description fine de leur microstructure est obtenue. L’approche expérimentale consiste ensuite en la dégradation (par lixiviation et carbonatation sous eau) de la pâte de ciment modèle, afin de comprendre l’impact de ces dégradations sur la microstructure et les propriétés de transport.La deuxième partie, numérique, consiste en l’obtention d’un volume élémentaire représentatif de la pâte de ciment modèle, basée sur les caractérisations expérimentales. Différentes analyses de sensibilité et de propriétés de transport permettent de comprendre les liens entre les différents paramètres et les propriétés effectives. Ensuite, l’approche numérique modélise les dégradations.Ces approches numériques démontrent pourquoi les approches empiriques fonctionnent dans certains cas, et pas dans d’autres.La dernière partie dédiée à la modélisation mathématique développe une approche d’homogénéisation statistique de la diffusion, basée sur une description du phénomène à l’échelle du pore. Cette étude met en évidence des paramètres clés qui contrôlent les propriétés effectives de diffusion.C’est ce pour quoi il est démontré que cette approche, en plus d’être très adaptée aux matériaux cimentaires, est applicable à un large spectre de microstructures. Les paramètres mis en évidences ont intrinsèquement sensibles aux propriétés de percolation et de connectivités de la structure poreuse, qui sont centrales pour la compréhension des propriétés effectives de transport ainsi que l’impact des dégradations. La finalité de la thèse consiste en le couplage de ces différentes approches et en l’incorporation de celles-ci dans un code de transport réactif. Les résultats obtenus en utilisant différentes lois de rétroaction sont comparés entre eux. L’utilisation de lois de rétroaction basée sur l’étude tri-dimensionnelle de la microstructure améliore la comparaison aux résultats expérimentaux.<br>Doctorat en Sciences de l'ingénieur et technologie<br>info:eu-repo/semantics/nonPublished

La protection et la remédiation des ressources en eau sont un enjeu sociétal majeur, ainsi il est nécessaire de comprendre l’évolution de solutés, tels les polluants, au sein de la zone saturée et non saturée. Dans ce but, de nombreux travaux ont été consacrés à la modélisation du transport réactif en milieux poreux. Son déroulement à l’échelle de Darcy dépend des hétérogénéités microscopiques du milieu. Les modèles de réseau de pores qui simplifient la géométrie en un ensemble de pores reliés par des liens de sections constantes, permettent de se placer à une échelle mésoscopique, faisant le lien entre l’échelle porale et l’échelle de Darcy. Sur de telles formes géométriques, l’écoulement admet un traitement analytique. En ce qui concerne le transport réactif des solutés, nous proposons une solution analytique dans les liens qui permet de calculer le débit de masse entre pores. Le modèle de transport se formule alors comme un système d’équations de Volterra de secondes espèces dont les noyaux de convolution sont des séries d’exponentielles décroissantes (hormis le premier terme qui est constant). Leurs temps de relaxation sont pilotés essentiellement par le temps de dispersion td. Dans la limite où td tend vers 0 à Péclet constant, les termes transitoires des noyaux se réduisent à un Dirac, débouchant sur un premier modèle simplifié à réponse instantanée c'est-à-dire un modèle de transport quasi-statique. Dans le cas où les volumes des pores sont suffisamment grands, les noyaux se réduisent à leur premier terme. Ces formulations du transport généralisent celles de la littérature. En particulier pour des Péclet petit ou grand on retrouve respectivement les modèles usuels en régime dispersif et convectif. Numériquement, la décroissance exponentielle des noyaux permet d’optimiser le calcul des convolutions avec une précision arbitrairement fixée, réduisant drastiquement le temps de résolution<br>Protection and remediation of ground water resources are a major societal challenge. It implies to understand the evolution of solutes as pollutant in the saturated and non-saturated zones. For that purpose numerous studies have been conducted for modeling the reactive transport in a porous media. At Darcy scale, the behavior of solutes depends on microscopic heterogeneity for the media. The Pore Network Models (PNM) simplifies drastically its geometry and considers pores linked by straight throats the section of which is constant. They give a description which is in between the macroscopic and the pore descriptions. With such geometry it is possible to use a Poiseuille flow modeling the flux. With respect to the reactiontransport equation, we seek the analytical solution of the CDE in throats, which in turn allows computing the mass flux in pores. The transport solution consists of a Volterra equation system. Its convolution kernels result in a summation of time function which is decreasing exponentially with time (except the first term which still constant). The time constant is driven by the diffusion time td. As td goes to zero, keeping the Peclet number fixed, each term of the summation reduces to a Dirac. The response of the system is then instantaneous. When the volume of the pore is large enough it is possible to neglect all the term of the kernel except the constant one. In the limit where the Peclet number goes to zero, usual models are recovered. Numerically, the exponential time decreasing of the kernel allow to optimize their computational time up to an arbitrary fixed precision

Cette thèse s'inscrit dans le cadre de la gestion des anciens sites miniers contaminés en métaux et de leur effet sur l'environnement. L'objectif de cette thèse réside dans la détermination et la hiérarchisation des processus régulant le transfert des métaux au sein de résidus miniers anciens, via le développement d’un modèle de transport réactif (RTM). Premièrement, un modèle biogéochimique a été construit pour définir les processus contrôlant la mobilité de Pb/Zn issus de résidus miniers. Ce modèle a été développé via des données expérimentales de lixiviation en microcosmes acquises d’une précédente étude, impliquant plusieurs traitements : (1) résidu seul, (2) ajout de boue minière, (3) ajout de boue minière et de fumier. Les résultats ont montré que les processus de dissolution des phases porteuses contrôlaient la libération de Pb/Zn. L’ajout de boue minière favorise l’immobilisation des métaux à travers l’apport d’oxydes de fer sur lesquels le Pb/Zn s’adsorbe. L’ajout de fumier, en amplifiant la respiration microbienne et la production de CO2, favorise la baisse du pH et la désorption de Pb/Zn. Deuxièmement, un RTM a été développé comprenant : (1) une description mécaniste des processus biogéochimiques et (2) l’écoulement et le transport des solutés dans un milieu 1D à saturation variable. Il se base sur des données d’expériences de lixiviation sur une colonne métrique instrumentée, avec ou sans amendement. Les résultats ont montré une similarité des processus contrôlant de la mobilité du Pb/Zn entre les microcosmes et la colonne métrique. Le devenir à long-terme des résidus miniers a pu être évalué, sans amendement la libération de Pb/Zn ne s’achèvera pas avant 100 ans. Les résidus amendés présentent néanmoins une immobilisation durable de Pb/Zn<br>This thesis is part of the management of old mining sites contaminated by metals and their effect on the environment. The objective of this thesis is to determine and prioritize the processes regulating the transfer of metals within old mine tailings, via the development of a reactive transport model (RTM). Firstly, a biogeochemical model was constructed to define the processes controlling the mobility of Pb/Zn from mine tailings. This model was developed via experimental microcosm leaching data acquired from a previous study, involving several treatments: (1) tailings alone, (2) addition of mine slurry, (3) addition of mine slurry and manure. The results showed that the dissolution processes of the carrier phases controlled the release of Pb/Zn. The addition of mining slurry promotes metal immobilization through the addition of iron oxides on which Pb/Zn adsorb. The addition of manure, by amplifying microbial respiration and CO2 production, promotes the lowering of pH and desorption of Pb/Zn. Secondly, a RTM was developed including: (1) a mechanistic description of biogeochemical processes and (2) solute flow and transport in a 1D medium with variable saturation conditions. It is based on data from leaching experiments on an instrumented metric column, with and without amendment. The results showed a similarity of the processes controlling the mobility of Pb/Zn between the microcosms and the metric column. The long-term fate of the tailings could be evaluated, without amendment the release of Pb/Zn will not be completed for 100 years. Nevertheless, the amended tailings immobilize Pb/Zn on a long-term basis

Les cinétiques de dissolution représentent un composant majeur des flux de matière sur Terre. Lors de cette thèse, plusieurs facteurs pouvant impacter la dissolution ont été étudiés afin de lier différentes échelles spatiales. Cette thèse reporte l’étude de la dissolution de minéraux modèles (enstatite et calcite) de l’échelle de l’atome à celle de la face pour le premier, et de celle de la face à celle du milieu poreux pour le deuxième. L’anisotropie de dissolution observée expérimentalement sur l’enstatite a pu être reproduite à l’aide d’un modèle probabiliste des ruptures de liaisons. L’intérêt de ce type de modèle est démontré et des propositions quant à l’extension de son utilisation ont été faites. Un modèle de substitution du taux de dissolution de l’enstatite liant l’échelle atomique à celle de la face a également été proposé. L’étude de la calcite a, quant à elle, démontré l’importance du choix de la loi de vitesse dans les études de mise à l’échelle des taux de dissolution. Une expérience en colonne et sa simulation à l’aide d’un modèle de transport-réactif ont démontré que l’hétérogénéité de l’écoulement du fluide autour d’un minéral joue un rôle de premier ordre dans la dissolution des minéraux<br>Dissolution kinetics represent a major challenge in the understanding of geological phenomena. During this thesis, several factors that could impact dissolution were studied in order to link different spatial scales. The dissolution of the model minerals of this thesis, enstatite and calcite, was studied from the atomic scale to the face scale for the first and from the face to the porous medium scale for the second. The dissolution anisotropy observed experimentally on enstatite could be reproduced using a probabilistic model. The interest of this type of model has been demonstrated and proposals for extending their use have been made. A surrogate model for the dissolution rate of enstatite linking the atomic scale to the face scale was also established. The study of calcite has demonstrated the importance of the selection of the rate law in dissolution rates upscaling studies. A column experiment and its simulation using a reactive-transport model demonstrated that the heterogeneity of the fluid flow around a mineral plays a major role in the dissolution of minerals

Marine sediments are a key component of the global carbon cycle and climate system. They host one of the largest carbon reservoirs on Earth, provide the only long-term sink for atmospheric CO2, recycle nutrients and represent the most important climate archive. Early diagenetic pro- cesses in marine sediments are thus central to our understanding of past, present and future biogeochemical cycling and climate. Because all early diagenetic processes can be directly or indirectly linked back to the degradation of organic matter (OM), advancing this understand- ing requires disentangling the different factors that control the fate of OM (sedimentation, degradation and burial) on different spatial and temporal scales. In general, the heterotrophic degradation of OM in marine sediments is controlled by the quantity and, in particular, by the ap- parent reactivity of OM that settles onto marine sediments. While the potential ((micro)biological, chemical and physical) controls on OM reactivity are increasingly well understood, their relative significance remains difficult to quantify. Traditionally, integrated data-model approaches are used to quantify apparent OM reactivity (i.e. OM degradation rate constants) at well-studied drill-sites. These approaches rely on Reaction-Transport Models (RTMs) that typically account for transport (advection, molecular diffusion, bioturbation, and bioirrigation) and reaction (pro- duction, consumption, equilibrium) processes, but vary in complexity. Apparent OM reactivity (i.e. the OM degradation rate constant) is generally considered as a free parameter that is used to fit observed depth-profiles, reaction rates or benthic-pelagic exchange fluxes. Currently, no quantitative framework exists to predict apparent OM reactivity in areas where comprehensive benthic data sets are not available.To evaluate the impact of this knowledge gap, the sensitivity of benthic biogeochemical reaction rates, as well as benthic-pelagic exchange fluxes to variations in apparent OM reactivity (i.e. reactive continuum model parameters a and ν) is explored by means of a complex, numerical diagenetic model for shelf, slope and deep sea depositional environments. Model results show that apparent OM reactivity exerts a dominant control on the magnitude of biogeochemical reaction rates and benthic-pelagic exchange fluxes across different environments. The lack of a general framework to quantify OM reactivity thus complicates the parametrization of regional and global scale diagenetic models and, thus, compromises our ability to quantify global benthic-pelagic coupling in general and OM degradation dynamics in particular.To make a first step towards an improved systematic and quantitative knowledge of OM reac- tivity, apparent OM reactivity (i.e. reactive continuum model parameters a and ν) is quantified by inverse modelling of organic carbon, sulfate (and methane) sediment profiles, as well as the location of the sulfate-methane transition zone using a complex, numerical diagenetic model for 14 individual sites across different depositional environments. Model results highlight again the dominant control of OM reactivity on biogeochemical reaction rates and benthic exchange fluxes. In addition, results show that, inversely determined ν-values fall within a narrow range (0.1 < ν < 0.2). In contrast, determined a-values span ten orders of magnitude (1 · 10−3 < a < 1·107) and are, thus, the main driver of the global variability in OM reactivity. Exploring these trends in their environmental context reveals that apparent OM reactivity is determined by a dynamic set of environmental controls rather than traditionally proposed single environmental controls (e.g. water depth, sedimentation rate, OM fluxes). However, the high computational demand associated with such a multi-species inverse model approach, as well as the limited availability of comprehensive pore water data, limits the number of apparent OM reactivity estimates. Therefore, while providing important primers for a quantification of OM reactivity on the global scale, inverse model results fall short of providing a predictive framework.To overcome the computational limitations and expand the inverse modelling of apparent OM reactivity to the global scale, the analytical model OMEN-SED is extended by integrating a nG- approximation of the reactive continuum model that is fully consistent with the general structure of OMEN-SED. The new version OMEN-SED-RCM thus provides the computational efficiency required for the inverse determination of apparent OM reactivity (i.e. reactive continuum model parameters a and ν) on the global scale. The abilities of the new model OMEN-SED-RCM in capturing observed local, as well as global patterns of diagenetic dynamics are rigorously tested by model-data, as well as model-model comparison.OMEN-SED-RCM is then used to inversely determine apparent OM reactivity by inverse modelling of 394 individual dissolved oxygen utilisation (DOU) rate measurements. DOU is commonly used as a proxy for OM reactivity, it is more widely available than comprehensive porewater data sets and global/regional benthic maps of dissolved oxygen utilisation rates (DOU) have been derived based on the growing DOU data set. Sensitivity test show that, while inverse modelling of DOU rates fails to provide a robust estimate of RCM parameter ν, it is a good indicator for RCM parameter a. Based on previous findings, parameter ν was thus assumed to be globally constant. Inversely determined a-values vary over order of magnitudes from a = 0.6 years in the South Polar region to a = 5.6 · 106 in the oligotrophic, central South Pacific. Despite a high intra- as well as interregional heterogeneity in apparent benthic OM reactivity, a number of clear regional patterns that broadly agree with previous observations emerge. High apparent OM reactivities are generally observed in regions dominated by marine OM sources and characterized by efficient sinking of OM and a limited degradation during sinking. In contrast, the lowest apparent OM reactivities are observed for regions characterized by low marine primary production rates, in combination with a great distance to the continental shelf and slope, as well as deep water columns. Yet, results also highlight the importance of lateral transport processes for apparent OM reactivity. In particular, deep sea sediments in the vicinity of dynamic continental margin environments or under the influence of strong ocean currents can receive comparably reactive OM inputs from more productive environments and, thus, reveal OM reactivities that are higher than traditionally expected. Finally, based on the observed strong link between apparent OM reactivity (i.e. RCM parameters a) and DOU rate, a transfer function that predicts the order of magnitude of RCM parameter a as a function of DOU is used to derive, to our knowledge, the first global map of apparent OM reactivity.Finally, we use the new global map of apparent OM reactivity to quantify biogeochemical dynamics and benthic-pelagic coupling across 22 benthic provinces that cover the entire global ocean. To this end, the numerical diagenetic model BRNS model is set-up for each province and forced with regionally averaged boundary conditions derived from global data sets, as well as apparent OM reactivities informed by the global OM reactivity map. The 22 regional model set-ups were then used to quantify biogeochemical process rates, as well as benthic carbon and nutrient fluxes in each province and on the global scale. Model results of regional and global fluxes and rates fall well within the range of observed values and also agree with general globally observed patterns. Results also highlight the role of the deeper ocean for benthic-pelagic cycling and indicate towards a large regional variability in benthic cycling at great depth. This is a first step towards a more refined global estimate of benthic biogeochemical cycling that accounts for the global heterogeneity of the seafloor environment. This aspect is critical to improve our understanding of benthic feedbacks on benthic-pelagic coupling and on the carbon-climate system, which can then be incorporated in benthic processes in Earth System Models.<br>Les sédiments marins sont un élément clé du cycle mondial du carbone et du système climatique. Ils abritent l’un des plus grands réservoirs de carbone sur Terre, fournissent le seul puits à long terme pour le CO2 atmosphérique, recyclent les nutriments et constituent les archives climatiques les plus importantes. Les processus de la diagénèse précoce dans les sédiments marins sont donc au cœur de notre compréhension des cycles et du climat biogéochimiques passés, présents et futurs. Étant donné que tous les processus diagénétiques précoces peuvent être directement ou indirectement liés à la dégradation de la matière organique (MO), faire progresser cette compréhension nécessite de démêler les différents facteurs qui contrôlent le devenir de la MO (sédimentation, dégradation et enfouissement) à différentes échelles spatiales et temporelles. En général, la dégradation hétérotrophique de la MO dans les sédiments marins est contrôlée par la quantité et, en particulier, la réactivité apparente de la MO qui se dépose sur les sédiments marins. Bien que les contrôles potentiels ((micro) biologiques, chimiques et physiques) de la réactivité de la MO soient de mieux en mieux compris, leur importance relative reste difficile à quantifier. Traditionnellement, des approches de modèle de données intégrées sont utilisées pour quantifier la réactivité apparente de la MO (c’est-à-dire les constantes de vitesse de dégradation de la MO) sur des sites de forage bien étudiés. Ces approches reposent sur des modèles de réaction-transport (RTM) qui tiennent généralement compte des processus de transport (advection, diffusion moléculaire, bioturbation et bio-irrigation) et de réaction (production, consommation, équilibre), mais leur complexité varie. La réactivité apparente de la MO est généralement considérée comme un paramètre libre qui est utilisé pour ajuster les profils de profondeur, les taux de réaction ou les flux d’échange benthique-pélagique observés. À l’heure actuelle, aucun cadre quantitatif n’existe pour prédire la réactivité apparente de la MO dans les zones où aucun ensemble complet de données benthiques n’est disponible.Pour évaluer l’impact de ce manque de connaissance, nous avons exploré la sensibilité des taux de réaction biogéochimiques benthiques, ainsi que des flux d’échange benthique-pélagique aux variations de la réactivité apparente de la MO (c.-à-d. les paramètres du modèle de con- tinuum réactif a et ν) au moyen d’un modèle diagénétique numérique complexe appliqué aux zones de dépôts sur les plateaux, les talus et en haute mer. Les résultats du modèle montrent que la réactivité apparente de la MO exerce un contrôle dominant sur l’ampleur des taux de réaction biogéochimiques et des flux d’échange benthique-pélagique dans différents environ- nements. L’absence d’un cadre général pour quantifier la réactivité de la MO complique donc la paramétrisation des modèles diagénétiques à l’échelle régionale et mondiale et, ainsi, compromet notre capacité à quantifier le couplage benthique-pélagique global en général et la dynamique de dégradation de la MO en particulier.Pour tendre à meilleure connaissance systématique et quantitative de la réactivité de la MO, la réactivité apparente OM (c.-à-d. les paramètres du modèle de continuum réactif a et ν) est quantifiée par modélisation inverse des profils de sédiments organiques de carbone, de sulfate (et de méthane), ainsi que localisation de la zone de transition sulfate-méthane à l’aide d’un modèle diagénétique numérique complexe pour 14 sites individuels à travers différents environnements de dépôt. Les résultats du modèle mettent à nouveau en évidence le contrôle dominant de la réactivité de l’OM sur les taux de réaction biogéochimiques et les flux d’échanges benthiques. De plus, les résultats montrent que les valeurs déterminées inversement déterminées se situent dans une plage étroite (0,1 <ν<0,2). En revanche, les valeurs déterminées s’étendent sur dix ordres de grandeur (1 ·10−3 <ν< 1·107) et sont donc le principal moteur de la variabilité globale de la réactivité OM. L’exploration de ces tendances dans leur contexte environnemental révèle que la réactivité apparente de l’OM est déterminée par un ensemble dynamique de contrôles environnementaux plutôt que par des contrôles environnementaux uniques traditionnellement proposés (par exemple, la profondeur de l’eau, le taux de sédimentation, les flux OM). Cependant, la forte demande de calcul associée à une telle approche de modèle inverse multi-espèces, ainsi que la disponibilité limitée de données complètes sur l’eau interstitielle, limitent le nombre d’estimations apparentes de la réactivité OM. Par conséquent, tout en fournissant des amorces importantes pour une quantification de la réactivité de l’OM à l’échelle mondiale, les résultats du modèle inverse sont loin de fournir un cadre prédictif.Pour surmonter les limites de calcul et étendre la modélisation inverse de la réactivité apparente de l’OM à l’échelle mondiale, le modèle analytique OMEN-SED est étendu en intégrant une approximation nG du modèle de continuum réactif qui est pleinement cohérente avec la structure générale d’OMEN-SED. La nouvelle version OMEN-SED-RCM fournit ainsi l’efficacité de calcul requise pour la détermination inverse de la réactivité apparente de l’OM (c’est-à-dire les paramètres du modèle de continuum réactif a et ν) à l’échelle mondiale. Les capacités du nouveau modèle OMEN-SED-RCM à capturer les modèles locaux et globaux de dynamique diagénétique observés sont rigoureusement testés par les données du modèle, ainsi que la comparaison modèle- modèle.OMEN-SED-RCM est ensuite utilisé pour déterminer inversement la réactivité apparente de l’OM par modélisation inverse de 394 mesures individuelles du taux d’utilisation de l’oxygène dissous (DOU). Le DOU est couramment utilisé comme indicateur de la réactivité de l’OM, il est plus largement disponible que les ensembles de données exhaustifs sur l’eau interstitielle et les cartes benthiques mondiales/régionales des taux d’utilisation de l’oxygène dissous (DOU) ont été dérivées sur la base de l’ensemble de données DOU croissant. Le test de sensibilité montre que, bien que la modélisation inverse des taux de DOU ne fournisse pas une estimation robuste du paramètre RCM ν, c’est un bon indicateur pour le paramètre RCM a. Sur la base des résultats précédents, le paramètre ν a donc été supposé être globalement constant. Les valeurs a déterminées à l’inverse varient selon l’ordre de grandeur, de a = 0,6 an dans la région polaire sud à a = 5, 6 · 106 dans le Pacifique sud oligotrophique central. Malgré une forte hétérogénéité intra et interrégionale dans la réactivité apparente de la MO benthique, un certain nombre de schémas régionaux clairs qui correspondent largement aux observations précédentes émergent. Des réactivités apparentes élevées de l’OM sont généralement observées dans les régions dominées par des sources marines de MO et caractérisées par un naufrage efficace de l’OM et une dégradation limitée pendant le naufrage. En revanche, les réactivités MO apparentes les plus faibles sont observées pour les régions caractérisées par de faibles taux de production primaire marine, en combinaison avec une grande distance du plateau continental et de la pente, ainsi que des colonnes d’eau profonde. Pourtant, les résultats mettent également en évidence l’importance des processus de transport latéral pour la réactivité apparente de l’OM.En particulier, les sédiments des mers profondes au voisinage d’environnements de marge continentale dynamiques ou sous l’influence de forts courants océaniques peuvent recevoir des apports OM de réactivité comparable provenant d’environnements plus productifs et, ainsi, révéler des réactivités OM plus élevées que ce qui était traditionnellement prévu. Enfin, sur la base du lien fort observé entre la réactivité apparente de l’OM (c’est-à-dire le paramètre RCM a) et le taux DOU, une fonction de transfert qui prédit l’ordre de grandeur du paramètre RCM a en fonction de DOU est utilisée pour dériver, pour nos connaissances, la première carte mondiale de la réactivité apparente de l’OM. Les résultats du modèle des flux et des taux régionaux et mondiaux se situent bien dans la gamme des valeurs observées et également d’accord avec les tendances générales observées au niveau mondial. Les résultats mettent également en évidence le rôle de l’océan profond pour le cycle benthique-pélagique et indiquent une grande variabilité régionale du cycle benthique à grande profondeur. Il s’agit d’une première étape vers une estimation mondiale plus précise du cycle biogéochimique benthique qui tient compte de l’hétérogénéité mondiale du milieu marin. Cet aspect est essentiel pour améliorer notre compréhension des rétroactions benthiques sur le couplage benthique-pélagique et sur le système carbone-climat, qui peuvent ensuite être incorporées aux processus benthiques dans les modèles du système terrestre.<br>Doctorat en Sciences<br>info:eu-repo/semantics/nonPublished

L'Institut de Radioprotection et de Sûreté Nucléaire (IRSN) s'intéresse à l'étude des réactions de gonflement interne, dont les Réactions Sulfatiques, et à leur impact sur l'évolution des propriétés du matériau cimentaire. Les Réactions Sulfatiques sont caractérisées par la précipitation de l'ettringite, dans les pores du matériau durci entraînant des gonflements locaux et une fissuration par déformations différentiées. Les fissures créées constituent alors le lieu privilégié de la précipitation d'ettringite et accélèrent le transport des espèces chimiques au sein du milieu poreux. La modification locale des phénomènes de transport induit une accélération de la dégradation du matériau.Ce travail de thèse modélise à l'échelle mésoscopique d'une collection de granulats, le gonflement du béton par les Réactions Sulfatiques et la cinétique de dégradation. Un modèle chimio-mécanique basé sur une description du transport réactif (diffusion d'espèces et réactions chimiques) et mécanique (Modèle de Zones Cohésives) dans un milieu poreux fissuré est proposé et résolu à l'aide d'un couplage étagé générique.Les paramètres chimiques et mécaniques initiaux sont estimés par un calcul d'hydratation et d'homogénéisation analytique.La modélisation chimio-mécanique tridimensionnelle est validée de façon modulaire et appliquée aux Réactions Sulfatiques Externe et Interne. Les effets de la composition du béton et des conditions environnementales chimiques sur la cinétique d'expansion et le faciès de rupture sont étudiés. Les applications mettent en évidence l'influence des granulats et des fissures dans la répartition spatiale inhomogène des zones de précipitation de l'ettringite et les contraintes de gonflement associées<br>The French "Institut de Radioprotection et de Sûreté Nucléaire" (IRSN) conducts researches on the impact of internal swellings reactions on concrete, such as Sulfate Reactions. Such reactions are characterized by the precipitation of ettringite which induces swellings and cracks by differential strain. These cracks are preferential location for ions diffusion and further ettringite precipitations.The aim of the study is to model the degradation of a mature material by ettringite pressure at the aggregate scale.A chemo-mechanical model based on a coupling between reactive transport (species diffusion and chemical reactions) and mechanics in cracked porous medium is developed and is solved with a generic staggered approach.The initial microstructure and poro-mechanical and diffusion parameters are estimated by hydration computing and analytical homogenization.The coupled chemo-mechanical model is validated and then applied to Sulfate External and Internal Attack.The impact of the concrete composition and the chemical environments on the swelling kinetics and crack path is taken into account. Furthermore, our simulations highlight the influences of inclusions and cracks on the inhomogeneous spatial distribution of precipitation areas of ettringite and associated swelling stress

Thesis (MSc)--Stellenbosch University, 2013.<br>ENGLISH ABSTRACT: Flow and reactive transport of chemical species is a very common phenomenon that occurs in natural and artificial systems. However in this study, the topic is related to acid mine drainage in the South African mining environment. Due to the hazards associated with acid mine drainage, prevention or treatment of mine effluent water before discharging to receiving waters and other environments is a necessity. A new time-dependent mathematical model is developed for a passive treatment method, based on multi-scale modelling of the coupled physico-chemical processes such as diffusion, convection, reactions and filtration, that are involved in the treatment process. The time-dependent model is simulated on a two-dimensional domain using finite volume discretization to obtain chemical species distributions.<br>AFRIKAANSE OPSOMMING: Vloei en reagerende transport van chemiese spesies is ’n baie algemene verskynsel wat in natuurlike en kunsmatige stelsels plaasvind. In hierdie studie is die onderwerp egter verwant aan suurmyndreinering in die Suid-Afrikaanse mynbou-omgewing. As gevolg van die gevare wat verband hou met suurmyndreinering, is die voorkoming of die behandeling van die afval-mynwater voor dit in opvangswaters en ander omgewings beland ’n noodsaaklikheid. ’n Nuwe tydafhanklike wiskundige model vir ’n passiewe behandelingsmetode is ontwikkel. Dit is gebaseer op die multi-skaal modulering van gekoppelde fisies-chemiese prosesse soos diffusie, konveksie, reaksies en filtrasie, wat by die behandelingsproses betrokke is. Die tydafhanklike model word gesimuleer op ’n twee-dimensionele domein met behulp van eindige volume diskretisasie om die verspreiding van chemiese spesies te bepaal.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2007.<br>Includes bibliographical references.<br>Solute transport models are essential tools for understanding and forecasting chemical concentrations in groundwater. Advection-dispersion based models can adequately predict spatial averages of conservative solute concentrations without using explicit maps of pore structures or variations in hydraulic conductivity. However, coupling advection-dispersion based transport models to chemical reaction models is inaccurate because it implicitly assumes complete mixing. Mixing in natural porous media is a slow process that can control the overall rate of chemical reactions, and the lack of mixing causes concentrations to be spatially variable. This thesis develops and experimentally validates a new solute transport modeling framework that approximates the correct amount of chemical reaction and provides concentration probability density functions, which are needed to address laws and regulations based on maximum contaminant levels. To study solute mixing and reaction in porous media, we conducted highly detailed lab-scale experiments by digitally imaging the movement of colored dye tracers and colorimetric chemical reactions through illuminated clear homogeneous and heterogeneous porous media.<br>(cont.) The resulting sequence of solute concentration maps demonstrates the problem of conventional solute transport models and shows that concentrations can be well approximated with Beta distributions. Conservative Beta distributions can be modeled with partial-differential equations for concentration mean and variance. These conservative distributions can then be transformed into joint reactant distributions, which produces product and remaining reactant distributions. This upscaling approach is verified by modeling the product and reactant means, variances, and distributions in heterogeneous media and product means in homogeneous media from our lab-scale experiments. We found that (co)variance production-destruction balances can approximate aqueous species covariance matrixes, which are necessary to form multivariate reactant distributions of complex reactive transport scenarios. Alternatively, these second moments can be used in upscaled reaction expressions derived from a second order Taylor series expansion. Incomplete mixing, parameterized by variance and covariance, causes an upscaled reaction rate to be almost an order of magnitude smaller compared to the conventional reaction rate that implicitly assumes complete mixing.<br>(cont.) Finally, manipulating the flow field to be perpendicular to its original direction would increase the rate of reactive mixing by an order of magnitude. Thus generating a transient flow field would be a practical way to accelerate natural attenuation and bioremediation.<br>by Peter M. Oates.<br>Ph.D.

A 1D numerical reaction-transport model (RTM) that is a coupled system of partial differential equations is created to simulate prominent physical and biogeochemical processes and interactions in limnological environments. The prognostic variables considered are temperature, horizontal velocity, salinity, and turbulent kinetic energy of the water column, and the concentrations of phytoplankton, zooplankton, detritus, phosphate (H3PO4), nitrate (NO3-), ammonium (NH4+), ferrous iron (Fe2+), iron(III) hydroxide (Fe(OH)3(s)), and oxygen (O2) suspended within the water column. Turbulence is modelled using the k-e closure scheme as implemented by Gaspar et al. (1990) for oceanic environments. The RTM is used to demonstrate how it is possible to investigate limnological trophic states by considering the problem of eutrophication as an example. A phenomenological investigation of processes leading to and sustaining eutrophication is carried out. A new indexing system that identifies different trophic states, the so-called Self-Consistent Trophic State Index (SCTSI), is proposed. This index does not rely on empirical measurements that are then compared to existing tables for classifying limnological environments into particular trophic states, for example, the concentrations of certain species at certain depths to indicate the trophic state, as is commonly done in the literature. Rather, the index is calculated using dynamic properties of only the limnological environment being considered and examines how those properties affect the sustainability of the ecosystem. Specifically, the index is calculated from a ratio of light attenuation by the ecosystem’s primary biomass to that of total light attenuation by all particulate species and molecular scattering throughout the entire water column. The index is used to probe various simulated scenarios that are believed to be relevant to eutrophication: nutrient loading, nutrient limitation, overabundance of phytoplankton, solar-induced turbulence, and wind-induced turbulence.

In view of the worsening air quality in the world, more concerns are focused on the environment. This thesis uses the technique of CFD and develops the computer model to investigate the wind and pollutant transport, as well as the chemistry of reactive pollutants in idealized two-dimensional (2D) street canyons. Three scientific questions are raised in this thesis. The first task is to find out the po- sition with the most favorable pollutant removal along the ground level over 2D idealized street canyon of different building-height-to-street-width (aspect) ratios (ARs). The di- mensionless parameter, C, represents the pollutant removal performance. In the isolated roughness regime, the two local maximum C locate at the reattachment point and the windward corner. In the wake interference regime, C is peaked on the windward side. The number of vertically aligned recirculations depends on the street depth in the skimming flow regime. The sizes of the secondary recirculation upstream and downstream deter- mine how the maximum C shifts from the street centre. After identifying the position of peaked pollutant removal rate at the ground level, the emission source should be placed with the highest constant C in order to remove the pollutants upward more quickly to safeguard the street-level air quality. After understanding the best pollutant removal in the street canyon of different ARs, the second task is to find out what AR is the most favorable for the ventilation and pollutant removal across the roof level. The three parameters, namely friction factor, air exchange rate (ACH) and pollutant exchange rate (PCH), are introduced to quantify the pressure difference to sustain the mean flow, the ventilation and pollutant removal, respectively. The turbulence contributes more than 70% to the total ACH and PCH in all the three flow regimes. By increasing the atmospheric turbulence in building geometry as well as the surface roughness, the ventilation and pollutant removal performance can be improved. The linear relation between the friction factor and ACH demonstrates the larger resistance that in turn promotes the air exchange over the roof level. The physical dispersion is studied; however atmospheric pollutants are seldom in- ert but chemically reactive instead. The last task is to include the three common air pollutants, NO, NO2 and O3, in the simple NOx ?O3 mechanism in terms of the photo- stationary state and reaction rates. The Damkohler numbers of NO and O3, DaNO and DaO3, are parameterized by the concentrations of the sources NO and O3. The normalized mean and fluctuation NO, NO2 and O3 are separately considered. The integrated pho- tostationary state (PSS) in the first canyon increases with DaO3 under the same DaNO. The integrated PSS of the second to the twelveth street canyons are compared with each case, the monotonic increase in the PSS from the second to twelveth canyon is perceived in DaNO/DaO3 1, 0.03, 0.02, 0.001 and 0.000333. Further decreases the DaNO/DaO3 to 0.000143, 0.000125, 0.000118, 0.000111 and 0.0001, the PSS is found to be non-linear and the trough appears in the fourth and fifth canyons.<br>published_or_final_version<br>Mechanical Engineering<br>Master<br>Master of Philosophy

En esta tesis doctoral se analiza el papel del estrés oxidativo en el transporte iónico en distintos modelos, mediante técnicas de imagen y de electrofisiología. El trabajo se subdivide en cuatro capítulos. El primero, a modo de marco teórico, introduce el transporte iónico en condiciones de estrés oxidativo. La interacción entre especies reactivas de oxígeno (ROS) y el transporte iónico ha sido poco estudiada hasta el momento, por lo que los principales objetivos de la tesis se han centrado en el estudio del transporte iónico en condiciones de estrés oxidativo en diferentes modelos, con la finalidad de contribuir a un mejor conocimiento de los mecanismos subyacentes a la regulación del transporte iónico por parte de estas ROS. En el segundo capítulo se analizan diferencias entre fuentes primarias y secundarias (salinidad) de estrés oxidativo sobre la homeostasis de potasio en dos especies halófitas, Chenopodium quinoa and Atriplex lentiformis. En el tercero el modelo empleado es Arabidopsis thaliana y diferentes mutantes con pérdida/ganancia de función en relación a su capacidad de transportar cobre. El capítulo se centra en la generación de radicales hidroxilo, una ROS altamente tóxica, a partir del cobre, y en su implicación en la regulación de canales iónicos de calcio y potasio. Por último, en el capítulo cuarto, se introduce la activación de los canales de calcio de tipo anexina (Anexina 1 de Arabidopsis y anexina 5 humana) en condiciones de estrés oxidativo.<br>In this PhD Thesis, a range of electrophysiological and imaging techniques are applied in different models in order to get a better understanding of oxidative stress-induced ion transport. The work is subdivided in 4 chapters. In the first one, ion transport under oxidative stress conditions are reviewed. Taking into account this potential, but still not well characterized interaction between reactive oxygen species (ROS) signalling and ion channel activity, the objectives of this Ph. D. Thesis were to study the influence of oxidative stress on ion transport in different models, in order to contribute to a better knowledge of the mechanisms underlying the possible role of ROS in ion transport regulation. The second chapter analyses differences between salinity-induced oxidative stress and primary oxidative sources in the disturbance of potassium homeostasis in two halophytes, Chenopodium quinoa and Atriplex lentiformis. For the third chapter, seedlings of the plant model Arabidopsis thaliana differing in their ability of copper transport were used. The chapter focuses on copper-induced hydroxyl radical, a highly toxic reactive oxygen species, and its implications on calcium and potassium homeostasis. The last chapter refers to the activation of annexin-type calcium channels (Arabidopsis thaliana annexin 1 and human annexin V) under oxidative stress conditions.

Biogeochemical reactions and vadose zone transport, in particular gas phase transport, are inherently coupled processes. To explore feedback mechanisms between these processes in a quantitative manner, multicomponent gas diffusion and advection are implemented into an existing reactive transport model that includes a full suite of geochemical reactions. Multicomponent gas diffusion is described based on the Dusty Gas Model, which provides the most generally applicable description for gas diffusion. Gas advection is described by Darcy's Law, which in the current formulation, is directly substituted into the transport equations. The model is used to investigate the interactions between geochemical reactions and transport processes with an emphasis to quantify reaction-induced gas migration in the vadose zone. Simulations of pyrite oxidation in mine tailings, gas attenuation in partially saturated landfill soil covers, and methane production and oxidation in aquifers contaminated by organic compounds demonstrate how biogeochemical reactions drive diffusive and advective transport of reactive and non-reactive gases. Pyrite oxidation in mine tailings causes a pressure reduction in the reaction zone and drives advective gas flow into the sediment column, enhancing the oxidation process. Release of carbon dioxide by carbonate mineral dissolution partly offsets pressure reduction, and illustrates the role of water-rock interaction on gas transport. Microbially mediated methane oxidation in landfill covers reduces the existing upward pressure gradient, thereby decreasing the contribution of advective methane emissions to the atmosphere and enhancing the net flux of atmospheric oxygen into the soil column. At an oil spill site, both generation of CH4 in the methanogenic zone and oxidation of CH4 in the methanotrophic zone contribute to drive advective and diffusive fluxes. The model confirmed that non-reactive gases tend to accumulate in zones of gas consumption and become depleted in zones of gas production. In most cases, the model was able to quantify existing conceptual models, but also proved useful to identify data gaps, sensitivity, and inconsistencies in conceptual models. The formulation of the model is general and can be applied to other vadose zone systems in which reaction-induced gas transport is of importance.

In this thesis, a new aerosol module is developed for the STEM model (the Sulfur Transport and dEposition Model) to better understand the chemical aging of dust during long range transport and assess the impact of heterogeneous reactions on tropospheric chemistry. The new aerosol module is verified and first applied in a box model, and then coupled into the 3-Dimentional STEM model. In the new aerosol model, a non-equilibrium (dynamic or kinetic) approach to treat gas-to-particular conversion is employed to replace the equilibrium method in STEM model. Meanwhile, a new numerical method solving the aerosol dynamics equation is introduced into the dynamic aerosol model for its improved computational efficiency and high accuracy. Compared with the equilibrium method, the new dynamic approach is found to provide better results on predicating the different hygroscopicity and chemical aging patterns as a function of size. The current modeling study also takes advantage of new findings from laboratory experiments about heterogeneous reactions on mineral oxides and dust particles, in order to consider the complexity of surface chemistry (such as surface saturation, coating and relative humidity). Modeling results show that the impacts of mineralogy and relative humidity on heterogeneous reactions are significant and should be considered in atmospheric chemistry modeling with first priority. Finally, the upgraded 3-D STEM model is utilized to explore the observations from the Intercontinental Chemical Transport Experiment - Phase B (INTEX-B). The new dynamic approach for gas-to-particular conversion and RH-dependent heterogeneous uptake of HNO3 improve the model performance in term of aerosol predictions under different conditions. It is shown that these improvements change the modeled nitrate and sulfate concentrations, but also modify their size distributions significantly.

Packed bed reactors are widely used in the chemicals industry and have been studied carefully in the last century. Several reaction engineering models have been developed in order to predict the behavior of such reactors under specified conditions, in order to assist in the sizing during an industrial process conception. These reactors can be categorized using different parameters, and the bed-to- particle diameter ratio - N - is one of them. It has been shown that this parameter influences greatly the transfer phenomena that occur in the bed, and that for ratios under 10, particular attention is needed when considering the wall effects. An impor- tant point that has to be evaluated is the accuracy of the actual chemical reaction engineering models when simulating such beds as it is valid to question the hypoth- esis of a pseudo-continuum model when considering a low bed-to-particle diameter ratio bed. Through high precision Computational Fluid Dynamics calculations, several beds of particles are modeled and studied in term of mass dispersion and reaction rate distribution. Two reaction engineering models - a simple pseudo-continuum model with effectiveness factor, and a model we refer to as "Single pellet" model - and several correlations regarding Peclet numbers are then evaluated under the same conditions in order to determine their accuracy and reliability for that particular kind of bed. Two beds of N = 5.96 and N = 7.99 are studied for dispersion phenomena, and the bed of N = 5.96 is studied for reaction rate distribution. It is shown that the pseudo- continuum model of dispersion stands valid for the higher N, but that none of the correlations we used were able to correctly predict the behavior of the N = 5.96 bed at any of the Reynolds number we considered, only giving close behaviors. We were confronted with some difficulties regarding the reaction simulation under Fluent, but some comparisons were successfully made regarding species and reaction rate distribution in the bed.

Dans cette thèse, nous développons le modèle phénoménologique optimisé de lessivage chimique in situ (ISL) de l'uranium par l'injection d'acide sulfurique, en prenant en compte la précipitation des espèces non-solubles telles que le gypse, qui réduisent la récupération de l'uranium. Le modèle proposé décrit le transport de masse avec des réactions chimiques hétérogènes entre le liquide et les roches solides, qui mènent à la dissolution des oxydes d'uranium et à la récupération de l'uranium sous forme liquide. Ce modèle comprend à la fois des réactions utiles, qui décrivent la dissolution de divers types d'oxydes d'uranium, et les réactions néfastes qui conduisent à la précipitation des sédiments solides (gypse), dont les flocons couvrent la surface de canaux poreux et réduisent l'efficacité des réactions utiles. Parmi les résultats qualitatifs, nous avons révélé l'existence d'un taux critique de sédimentation de gypse, en dessous duquel la récupération ultime de l'uranium est complète. En revanche, elle tend à une valeur limite inférieure à 100% lorsque le taux de sédimentation est supérieur à la valeur critique. Ce taux de récupération limite dépend de divers paramètres du processus. La théorie et la méthodologie développées dans ce travail peuvent être facilement étendues et appliquées aux autres types de minerais qui sont récupérés par la méthode de lessivage in situ, et autres types de solvant<br>In the present thesis we develop the optimized phenomenological model of in-situ chemical leaching (ISL) of uranium by the injection of sulfuric acid, with special account for the precipitation of non-soluble species as gypsum, which reduces the uranium recovery. The suggested model describes the mass transport with heterogeneous chemical reactions between liquid and solid rocks, leading to dissolve uranium oxides and recover uranium in liquid form. It includes both useful reactions, describing the dissolution of various kinds of uranium oxides, and detrimental reactions, leading to the precipitation of solid sediments (gypsum), whose flakes cover the surface of porous channels and reduce the efficiency of useful reactions. Among the qualitative results we revealed the existence of a critical rate of gypsum sedimentation, below which the ultimate uranium recovery is complete. In contrast, it tends to a limit value lower than 100% when the sedimentation rate is higher than the critical value. This limit recovery depends on various parameters of the process. The theory and the methodology developed in this work can be easily extended and applied on other type of ores that are recovered by in-situ leaching method and other types of solvents

O estudo dos mecanismos que desencadeiam as reações alérgicas é um tema de grande interesse científico na atualidade. A anafilaxia, reação alérgica sistêmica severa, tem ocupado um lugar de destaque nas pesquisas. Diferentes experimentos em laboratório, tanto in vivo quanto in vitro, assim como diferentes modelos matemáticos baseados nos resultados experimentais, têm procurado investigar a participação ou não dos mastócitos nesse mecanismo. No entanto, os resultados obtidos não são conclusivos, dividindo a comunidade científica em dois grupos: os que consideram determinante o papel dos mastócitos responsáveis pela liberação de histamina e os que consideram que a histamina não é o neurotransmissor determinante na reação anafilática. Trabalhos anteriores propuseram modelos diferenciais para simular processos relacionados com a reação anafilática na escala celular para o mecanismo de geração de potencial na membrana das células. Mais recentemente foi proposto, a nível de tecido, um modelo probabilístico para simular a resposta in vitro a antígenos. A nível de organismo têm sido propostos modelos de multi compartimentos para a cinética da histamina no fluido sanguíneo. Contudo, nenhum trabalho até o momento abordou a construção de um modelo capaz de descrever os processos que participam no mecanismo de reação anafilática nas diversas escalas. Neste trabalho propomos um modelo que integra as escalas celular e do tecido, que permite modelar experimentos in vitro, e que pode ser estendido para escala do organismo incluindo o fluxo sanguíneo para modelar experimentos in vivo. O modelo proposto integra o mecanismo de resposta elétrica a nível celular com o processo de reação-difusão da histamina e dos antígenos no tecido, considerando o mecanismo de reação mediado por mastócitos. Para integrar as duas escalas propomos uma relação constitutiva baseada em resultados experimentais da resposta mecânica (contração do tecido) a estímulos elétricos. Este modelo permite o desenho de novos experimentos especificamente direcionados ao estudo da reação anafilática, indicando os parâmetros a serem estimados. Utilizando-se o modelo proposto, foram realizadas simulações numéricas para uma ampla faixa de variação dos parâmetros visando identificar domínios com diferentes comportamentos do modelo. Uma análise dos resultados obtidos baseada em parâmetros adimensionais é apresentada.<br>The study of the mechanisms that set off allergic reactions is being a subject of great scientific interest. Anaphylaxis, severe systemic allergic reaction, occupies a prominence place in researches. Different laboratory experiments, in vivo as well as in vitro, and also different mathematical models based on experimental results, tries to investigate if mast cells takes part in those mechanisms or not. However, the obtained results are inconclusive, dividing the scientific community in two groups: one considering that mast cells have a prime role in releasing histamine, and another one which considers that histamine is not the determinative neurotransmitter in the anaphylactic reaction. Previous works proposed differential models to simulate processes related to anaphylactic reactions in the cellular scale for the cell membrane potential generation mechanism. More recently, it has been proposed a probabilistic model, in the tissue scale, to simulate an in vitro antigen response. In the organism level scale, multi-compartimental models have been proposed for the kinetics of histamine in the blood. Nevertheless, no work, until now, has proposed the construction of a model that is able to describe the processes that participate in the mechanism of anaphylactic reaction in different scales. In this work, a model is proposed that integrates the cellular and the tissue scales, allowing to model in vitro experiments, being capable to be extended to the organism scale by the inclusion of the blood flow to model in vivo experiments. The proposed model couples the electric response in the cellular level with the reaction-diffusion of histamine and antigens in the tissue, considering the reaction mechanism mediated by the mast cells. To integrate these two scales, it is proposed here a constitutive relation based on experimental results for the mechanical response (tissue contraction) to electric stimulus. This model allows to design experiments specifically related to the anaphylaxis reaction, indicating the parameters that should be estimated. With this model, numerical simulations have been performed for a wide variation range of the parameters to identify the different domains of the model. A dimensionless parameter based analysis is presented for the obtained results.

Les techniques ultramicroélectrochimiques engageant des électrodes de très faibles dimensions ont été appliquées à l'étude des phénomènes de transport de matière en absence d'électrolyte support lors des réactions électrochimiques. L'analyse des effets de migration a porté d'abord sur des transformations électrochimiques engageant une seule étape. Le comportement des systèmes i-/i#3#, br#/br#3#, br#/br#2 et le système du diphenylpicrylhydrazyle dpph-/dpph dans les solvants organiques a été précisé. La variation des courants limites en présence ou absence d'électrolyte a pu être quantifiée simplement. L'analyse a été étendue à des systèmes engageant plusieurs etapes, les uns faisant intervenir des espèces chargées (i#/i#3#/i#2, br#/br#3#/br#2, br#/br#2/br#+ et dpph#/dpph/dpph#+), les autres faisant intervenir des espèces moléculaires (la réduction du tetracyanoquinodimethane (tcnq), l'oxydation de la tetramethyl-p-phenylenediamine (tmppd) et la réduction de l'iode). Les effets d'exaltation sont également prévisibles à partir du modèle proposé