Impact de pollutions ponctuelles sur les phytocénoses des rivières acides à neutres du Limousin (Massif Central, France)

L'impact des pollutions ponctuelles sur les phytocénoses aquatiques est étudié autour des rejets de 12 agglomérations dont 9 sont équipées d'une station d'épuration. Un échantillonnage systématique avec segmentation du cours d'eau autour de chaque rejet est réalisé. Sur chaque secteur, des relevés de végétation sont pratiqués au niveau de faciès d'écoulements homogènes dont on caractérise le milieu physique parallèlement à une analyse physicochimique de l'eau.L'ensemble des rejets provoque globalement une élévation de la conductivité, des teneurs en ammonium, nitrates et orthophosphates.Cela ce traduit par la régression de la phytocénose à Callitriche hamulata et Myriophyllum alterniflorum, par le développement de Ranunculus peltatus, Callitriche platycarpa et d'espèces cryptogames telles que Leptodyctium riparium, ou Melosira sp.Une Analyse en Composantes Principales menée sur l'ensemble des données permet d'opposer des phytocénoses propres aux secteurs amonts (Scapania undulata, Chiloscyphus polyanthus) à d'autres situées au niveau de rejets (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.,).Une Analyse Canonique de Correspondances valide le déterminisme de la qualité physicochimique de l'eau sur la végétation. La conductivité, les teneurs en ammonium, nitrates et orthophosphates deviennent prépondérants par rapport aux facteurs du milieu physique classiquement discriminants dans l'installation des phytocénoses dans les rivières limousines.The impact of located pollution on aquatic phytocénoses is studied around 12 cities discharge. Nine of them are fitted out purification plant.The sampling method is based on consecutive segments from upstream to downstream. On each sector, vegetation records are realized in homogeneous water runoff facies, which are characterized by physical factors as well as water value measures.The whole discharge leads globally to an increase of conductivity, ammonium amount, nitrates and orthophosphates. The consequence of that is a decrease of Callitriche hamulata and Myriophyllum alterniflorum phytocénoses, a development of Ranunculus peltatus, Callitriche platycarpa and cryptogams species like Leptodictyum riparium or Melosira sp.A Component Principal Analysis applied on data, distinguishes phytocénoses belonging to upstream sectors (Scapania undulata, Chiloscyphus polyanthus) from the ones of discharges (Callitriche platycarpa, Leptodictyum riparium, Melosira sp.).A Canonical Correspondence Analysis validates the impact of physico-chemical water quality on vegetation. Conductivity, ammonium amount, nitrates and orthophosphates become more preponderant in comparison with physical environments usually discriminant for phytocénoses installation in Limousin rivers

La quarantaine de canne à sucre du Cirad à Montpellier

Les échanges et les transports de la canne à sucre sous forme de boutures peuvent présenter des risques phytosanitaires graves qu'il convient de contrôler. Dans les années 70, à Montpellier, le Cirad a mis en place une unité de quarantaine. Elle est constituée de deux serres d'observation et de multiplication qui sont associées à un laboratoire de phytopathologie. Ce dernier permet la détection des maladies de la canne à sucre grâce à des outils de diagnostic performants. Les clones distribués par cette quarantaine proviennent de différentes sources (variétés internationales, clones en cours de sélection produits par le Cirad en Guadeloupe et par la West Indies Central Sugar Cane Breeding Station à Barbade). Le matériel végétal est suivi et traité (traitements antifongiques et insecticides, thermothérapie) pendant deux cycles de culture. Après deux ans de quarantaine à Montpellier, des boutures saines sont fournies principalement aux périmètres sucriers d'Afrique de l'Ouest et du Centre aux stations de création variétale de la canne à sucre des Antilles (Guadeloupe) et de l'île de La Réunion

Synthesis of Highly Porous Catalytic Layers for Polymer Electrolyte Fuel Cell Based on Carbon Aerogels

International audienceRecent studies of the PEMFC catalytic layer electrocatalystsupport showed that high surface area carbon blacks cannot beused efficiently (1) because they have necessarily small primarypores that yield high diffusion limitation for the oxygenreduction reaction (ORR). In the present paper, we firstsynthesized and characterized carbon aerogels which exhibit highsurface area, high porous volume and adjustable pore-sizedistribution. In that way, they enable to decouple betweensurface area and porosity, while they are also promisingelectrocatalyst supports (2). Second, we elaborated newcatalytic layers made from 2 different carbon aerogels with 2different Nafion loadings. Finally, we characterized thestructure of such catalytic layers, and evaluated their activitytowards the oxygen reduction reaction (ORR). Finally, anappropriate catalyst support should display large pore-size andhigh surface area in order to reduce the PEMFC cathode platinumloadings without loosing activity, following the reduction ofoxygen diffusion limitation

Influence of carbon aerogel texture on PEMFC performances

International audienceTo improve Proton Exchange Membrane Fuel Cells (PEMFC), it is necessary to understand the phenomena occurring in operating conditions. The objective of this study is to determine how carbon support architecture can impact PEMFC performances, particularly diffusive limitations. In this context, as they have a controllable texture, carbon aerogels were used as catalyst supports in PEM fuel cell cathodes. Three carbon aerogels with different morphologies were synthesized. Fuel cell measurements show that the carbon support architecture has a significant impact on diffusive limitations. Moreover, they confirm that Nafion® loading must be optimized in order to preserve the catalytic layers architecture. This work finally highlights the impact of the catalytic layers architecture on the PEMFC performances

Elaboration and characterizations of platinum nanoparticles supported on cellulose-based carbon aerogel

International audienceThis work investigates the deposition of Pt nanoparticles onto carbon aerogels (CA), derived from microcrystalline cellulose. Nanoparticles are synthesised via impregnating the CA with H2PtCl6 followed by reduction either under H2 at 300 C or in a basic NaBH4 solution. H2 reduction yields uniform Pt nanoparticles (average diameter < 2nm) dispersed over the CA surface as revealed by Transmission electron microscopy (TEM). Larger agglomerates can be seen in TEM images of NaBH4 reduced samples, which is confirmed by powder X-ray diffraction (XRD). A rotating disk electrode was employed to analyse the electrochemical properties of the Pt nanoparticles. The active area of the platinum nanoparticles was evaluated using hydrogen adsorption/desorption cyclic voltammetry and CO stripping measurements. The oxygen reduction reaction was also studied to (i) obtain the kinetic parameters of oxygen reduction for the Pt/CA materials and (ii) compare them with commercial Pt/Carbon Black

Pt/carbon xerogel catalysts for PEM fuel cells

International audienceCarbon xerogels have been used to replace carbon black as catalyst support at the cathode of proton exchange membrane (PEM) fuel cells in order to decrease the mass transport limitations in this electrode. Carbon xerogels are very clean nanostructured carbons with well-defined pore texture, which allows forbetter reactant/product diffusion. Pt/carbon xerogel catalysts with high metal dispersion (nanoparticles ca. 2 nm in size) and high metal content (~ 25 wt.%) can be engineered via rational synthesis methods such as the 'Strong Electrostatic Adsorption' technique. The results show that choosing correctly the average pore texture of the carbon xerogel allows for minimizing the diffusion overpotential of the H2/air cell. However, the catalyst characterization indicates that the presence of chlorine, coming from H2PtCl6, induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer. To remove chlorine, a reduction of the catalyst at 450°C, at least, is necessary

Oxygen reduction at the silver/hydroxide-exchange membrane interface

A solid-state cell is used to study the electrocatalysis of oxygen reduction at the silver/hydroxide-exchange membrane interface. The catalyst/membrane interface exhibits improved performance in comparison to a catalyst/aqueous sodium hydroxide interface. Surprisingly, the half-wave potential for oxygen reduction is shown to shift 185 mV higher at the silver/hydroxide-exchange membrane interface than for the silver/aqueous hydroxide solution interface, and the exchange current density is significantly higher at 1.02 × 10−6 A m−2. On a cost per performance basis, silver electrocatalysts in a hydroxide-exchange membrane fuel cell may provide better performance than platinum in a proton-exchange membrane fuel cell. Keywords: Oxygen reduction reaction, Electrocatalyst, Alkaline membrane, Solid-state cell, Silve

Comparison of platinum deposit methods on carbon aerogels used in Proton Exchange. Membrane Fuel Cells (PEMFC)

International audienceWith the rarefaction and price increase of fossil fuels along with the consequences of greenhouse effect, many challenges have to be taken up. Consequently, a strong research effort is devoted to cleaner energy converters like fuel cells. In the car industry, Proton Exchange Membrane Fuel Cells (PEMFC) are chosen by a majority. But, remaining problems must be solved before a development at a large scale, among which: reducing the costs and increasing the power density and durability. Costs reduction mostly implies both diminishing the platinum quantity required for the oxygen reduction reaction at the cathode and increasing its activity. For these reasons, researches are devoted to the impact study of various methods of platinum deposit on the performances of new electrocatalysts and to the understanding of phenomena occurring in fuel cells

Variability of aerosol vertical distribution in the Sahel

In this work, we have studied the seasonal and inter-annual variability of the aerosol vertical distribution over Sahelian Africa for the years 2006, 2007 and 2008, characterizing the different kind of aerosols present in the atmosphere in terms of their optical properties observed by ground-based and satellite instruments, and their sources searched for by using trajectory analysis. This study combines data acquired by three ground-based micro lidar systems located in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analysis (AMMA), by the AEROsol RObotic NETwork (AERONET) sun-photometers and by the space-based Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard the CALIPSO satellite (Cloud-Aerosol Lidar and Infrared Pathfinder Observations). &lt;br&gt;&lt;br&gt; During winter, the lower levels air masses arriving in the Sahelian region come mainly from North, North-West and from the Atlantic area, while in the upper troposphere air flow generally originates from West Africa, crossing a region characterized by the presence of large biomass burning sources. The sites of Cinzana, Banizoumbou and M'Bour, along a transect of aerosol transport from East to West, are in fact under the influence of tropical biomass burning aerosol emission during the dry season, as revealed by the seasonal pattern of the aerosol optical properties, and by back-trajectory studies. &lt;br&gt;&lt;br&gt; Aerosol produced by biomass burning are observed mainly during the dry season and are confined in the upper layers of the atmosphere. This is particularly evident for 2006, which was characterized by a large presence of biomass burning aerosols in all the three sites. &lt;br&gt;&lt;br&gt; Biomass burning aerosol is also observed during spring when air masses originating from North and East Africa pass over sparse biomass burning sources, and during summer when biomass burning aerosol is transported from the southern part of the continent by the monsoon flow. &lt;br&gt;&lt;br&gt; During summer months, the entire Sahelian region is under the influence of Saharan dust aerosols: the air masses in low levels arrive from West Africa crossing the Sahara desert or from the Southern Hemisphere crossing the Guinea Gulf while in the upper layers air masses still originate from North, North-East. The maximum of the desert dust activity is observed in this period which is characterized by large AOD (above 0.2) and backscattering values. It also corresponds to a maximum in the extension of the aerosol vertical distribution (up to 6 km of altitude). In correspondence, a progressive cleaning up of the lowermost layers of the atmosphere is occurring, especially evident in the Banizoumbou and Cinzana sites. &lt;br&gt;&lt;br&gt; Summer is in fact characterized by extensive and fast convective phenomena. &lt;br&gt;&lt;br&gt; Lidar profiles show at times large dust events loading the atmosphere with aerosol from the ground up to 6 km of altitude. These events are characterized by large total attenuated backscattering values, and alternate with very clear profiles, sometimes separated by only a few hours, indicative of fast removal processes occurring, likely due to intense convective and rain activity. &lt;br&gt;&lt;br&gt; The inter-annual variability in the three year monitoring period is not very significant. An analysis of the aerosol transport pathways, aiming at detecting the main source regions, revealed that air originated from the Saharan desert is present all year long and it is observed in the lower levels of the atmosphere at the beginning and at the end of the year. In the central part of the year it extends upward and the lower levels are less affected by air masses from Saharan desert when the monsoon flow carries air from the Guinea Gulf and the Southern Hemisphere inland