Incidence d'une technique de décontamination par tensioactifs sur la conductivité hydraulique d'un aquifère contrôlé pollué par du gazole

Les nappes d'eau souterraine constituent des ressources vulnérables que l'activité industrielle croissante contribue à polluer trop fréquemment. Les produits les plus manipulés se voient donc directement concernés dans les cas de pollution d'aquifères. A ce titre, les produits pétroliers, notamment ceux que l'on considère comme étant domestiques (gazole, fuel, essence, etc.) se situent au premier rang.Dans le but de mieux aborder ce type de contamination, des études sont menées sur un site expérimental de grandes dimensions et parfaitement contrôlé, dans lequel 476 l de gazole routier (GOR86) ont été déversés. Une première tentative de récupération du polluant, par infiltration de tensioactifs depuis la surface, démontre la capacité du mélange utilisé à mobiliser le polluant mais conduit à la réduction de la perméabilité du milieu poreux faisant ainsi apparaître une limitation des performances pour ce type de procédés de décontamination.Des expériences au laboratoire, conduites sur des colonnes de milieu poreux, ont été entreprises pour rechercher les causes de cette perte de conductivité hydraulique. Avec le sable considéré ici, les argiles ne jouent pas un rôle prépondérant dans le colmatage. Il est démontré également que le gradient de charge, quand il est augmenté, permet d'injecter une quantité plus importante de tensioactifs sans pour autant pallier ce problème de colmatage. Enfin, il apparaît que les tensioactifs, en présence des ions calcium, peuvent s'agglomérer pour former des associations de micelles lamellaires, cylindriques et/ou mixtes, assez volumineux pour être filtrés à la surface du sol à traiter. Au bout d'un certain temps, le milieu poreux est totalement colmaté et par conséquent le débit d'infiltration de la solution devient nul. Sur le plan de la récupération du polluant, ce paramètre s'avère être particulièrement important. Sa variation influence donc fortement les quantités de polluant mobilisé.Cette étude démontre la faisabilité du procédé. Toutefois elle souligne aussi que, sous peine de perdre de leur efficacité, les solutions de tensioactifs doivent conserver leurs caractéristiques initiales (stabilité de la taille des particules colloïdales notamment) pendant toute la durée du traitement. Dans la mesure où l'agglomération des micelles est en cause, il faudra rechercher les moyens d'obtenir une dispersion plus stable, par exemple par addition d'agents solvants.In case of groundwater contamination by hydrocarbon spills, one of the main problems is how to recover residual hydrocarbons trapped in the porous medium, forming a long term pollution source. In order to develop a better approach to such problems, experiments were conducted in a large experimental controlled site, called SCERES (Site Contrôlé Expérimental de Recherche pour la Réhabilitation des Eaux et des Sols) made of an impervious concrete basin (25 m x 12 m x 3 m) packed with two layers of quartz sand and fitted with specific instrumentation.This research site was experimentally polluted with diesel oil (476 litres), then the shape of the impregnation body was identified and oil saturation values were quantified thanks to a specific coring programme before and after each step of the experiments (water table fluctuations, hydraulic pumping, surfactant infiltration, etc.). The principle of the remediation technique is based on a surfactant flushing from the soil surface. This process was carried out when all of the removable diesel oil had been recovered by hydraulic pumping in the central recovery well. The surfactant infiltration allowed a displacement of the main part of the residual pollutant in the vertical zone of the impregnation body, in spite of a progressive reduction of hydraulic conductivity leading to a plugging of the porous matrix. This phenomenon has been observed by many scientists (Allred et Brown, 1994 ; Celik et al., 1982) and explained as being caused by a precipitation of the anionic surfactant in presence of calcium ions.This decrease of hydraulic conductivity, due to surfactant infiltration and its effects, was studied in the laboratory by the implementation of experiments using columns filled with the same sand as in the SCERES basin. The results showed that the reduction of the hydraulic conductivity of the porous medium cannot be due to the precipitation of the anionic surfactants in presence of calcium ions of the sand. If this were situation, all the porous medium in the column would have exhibited this loss of hydraulic conductivity. In our case, only the upstream part of the physical model was influenced by this phenomenon. Furthermore, the sand devoid of its clay minerals has nearly the same behaviour as a natural sand, with respect to the infiltration flowrate of the surfactants. This proves that the clay minerals, present in small proportion in this matrix, are not mainly responsible for the plugging problem. The influence of the hydraulic gradient, another studied parameter, was observed and permitted to apprehend the behaviour of the surfactant solution during its infiltration through the porous medium. . The increase of this parameter, even if it allows the injection of more surfactant, could not resolve the problem of the reduction of hydraulic conductivity. Finally, all the experiments indicated that the plugging effect (cancellation of the flowrate) happened after nearly 20 hours of surfactant infiltration through the porous matrix. The time parameter seems to be important since its variation can influence the stability of the surfactant solution. The results indicated that we can infiltrate a greater volume of the solution if it has been recently prepared. In other words, this is certainly due to the time necessary for the formation of liquid crystals obtained in presence of the calcium ions in the water used for the preparation of the solutions (tap water). After nearly 20 hours these crystals should have attained a size sufficient to be retained by a filtration effect, near the soil surface. We suggest that this filtration is the major factor responsible for the reduction of hydraulic conductivity and the plugging.The present study shows the feasibility of this process and it highlights the need to conserve the initial characteristics of the surfactants (especially the size stability of the colloidal particles) during all the treatment. Because of the agglomeration of the micelles, we have to search for means to ensure a good dispersion in the aqueous medium, for example by adding a solvent agent

Microstructure and mechanical behavior in dissimilar 13Cr/2205 stainless steel welded pipes

Thiswork aims to investigate the microstructure and themechanical behavior of dissimilar 13Cr Supermartensitic/2205 Duplex stainless steelwelded pipes. A wide variety ofmicrostructures resulting fromboth solidification and solid state transformation is induced by the fusion welding process across the weld joint. The tensile tests show that the deformation process of the dissimilarweld joint ismainly controlled by the two basematerials: the duplex steel at the beginning of the deformation and the supermartensitic one at its end. This is confirmed by the microtensile tests showing the overmatching effect of the weld metal. The fatigue tests conducted on dissimilar welded specimens led us to conclude that the weld metal is considered as a weak link of the weld joint in the high cycle fatigue regime. This is supported by its lower fatigue limit compared to the two basematerials that exhibit a similar fatigue behavio

An Enhanced Hybrid Key Management Protocol for Secure Multicast in Ad Hoc Networks

Colloque avec actes et comité de lecture. internationale.International audienceAn ad hoc network is a collection of wireless mobile hosts forming a temporary network without the aid of any established infrastructure or centralized administration. This exibility in space and time induces new challenges towards the security infrastructure needed to support secure unicast and multicast communications. Especially, traditional group key management architectures meant for wired networks are not appropriate in such environment due to high dynamics and mobility of nodes. In this paper, we propose an enhanced hybrid key management protocol for secure multicast dedicated to operate in ad hoc networks. Built on a protocol called BAAL dedicated to key distribution in wired networks, our approach integrates threshold cryptography and the services of the AKMP protocol to deliver fast, efficient and mobility aware key distribution in a multicast service

The Hydrogen Spillover Effect—A Misunderstanding Study II: Single Oxide and Zeolite Supports

This investigation confirms that the existence of the hydrogen spillover effect (HSPE) in the case of metal catalysts supported on non-reducible monoxides or zeolites is based on a strong corpus of experimental studies, enlarging and deepening previous statements. The structure of hydrogen spillover consists of H/OH pairs conjugated with Mm+/Op− pairs (p = 1 or 2). It is formed by dehydroxylation followed by OH/OH exchange or by the hydrogenation of conjugated pairs. Such a structure imposes the following chemical processes: (i) hydrogenations take place over OH Brönsted acid sites (BAS); (ii) they are excluded over Mm+/Op− Lewis acid sites (LASs), which are deactivating or dehydrogenating; (iii) surface diffusion of hydrogen spillover proceeds through the migration of H/H pairs from LASs to LASs; (iv) the diffusion rates are determined by the oxide supports’ basicity; and (v) H/D exchange is proof of the existence of hydrogen spillover. The nature of hydrogen spillover (radical/ionic) depends on the polarity of the H/OH pairs, which in turn, is determined by the basicity of the support. Our concept of conjugated active sites is a good descriptor of the reaction paths at the molecular level. The view of LASs bringing about additional activity to BAS is not pertinent

Étude de la réactivité de l’acide isobutyrique par spectroscopie infrarouge et résonance paramagnétique électronique du sel de sodium et de l’acide de l’ion 1-vanado-11-molybdophosphorique

Le sel alcalin de l’ion 1-vanado-11-molybdophosphorique est étudié par spectroscopies IR et RPE sous cycles réactionnels d’absorption - désorption - réoxydation par l’oxygène d’acide isobutyrique, en comparaison au comportement de l’acide. La stabilité de la phase active en catalyse semble liée à la formation d’une phase cubique dont l’existence dépend fortement de la présence d’eau dans le mélange réactionnel. Cette phase peut être générée à partir d’un sel alcalin comme précurseur soumis à un traitement thermique modéré conduisant à la dispersion d’ion dioxovanadium (+) dans le réseau d’un 12-molybdophosphate alcalin

Impact of palladium substitution for copper in Bi

An extensive new solid solution was synthesized by substitution of Pd+2 for Cu+2 in Bi2CuO4 compound [J.-C. Boivin et al., C.R. Hebd. Seances Acad. Sci., Ser. C 276, 1105 (1973)]. Polycrystalline ceramic powder with formula Bi2Cu$_{1-x}$PdxO4 were prepared by sol-gel method using (Bi, Cu, Pd) nitrates and pure citric acid. X-ray patterns show a single phase with tetragonal symmetry and space group P4/ncc (No. 130). In cases, the tetragonal cell c parameter and therefore the metal-metal distance (dM-M = c/2) increases linearly with increasing palladium concentration. Electrical conductivity measurements show that the resistivity decreases regularly with pd2+ substitution. It is possible to obtain materials with an ambient temperature conductivity ranging from 10-6 to 10-3 S cm-1