Search

Les hydrocarbures aromatiques polycycliques (HAP) sont des contaminants produits notamment dans les processus de combustion. Leur caractère ubiquiste et leur génotoxicité sont à l'origine d'une activité de recherche importante. Après avoir présenté les structures chimiques et les propriétés physico-chimiques et biologiques principales de ces composés, on résume les connaissances actuelles concernant leur présence dans l'environnement. Les critères géochimiques de leurs différentes origines pyrolytique, diagénétique ou pétrolière, sont exposés. On examine la contribution des différentes sources d'émission, le transport et la diffusion dans l'environnement de ces composés, ainsi que les modifications qu'ils subissent et leur sort ultime. La distribution qualitative et quantitative des HAP de combustion dans les sols d'environnements variés est présentée. Polycyclic aromatic hydrocarbons (PAH) are environmental contaminants produced in particular in combustion processes. As a consequence of their genotoxicity and ubiquity, they are the subject of an important research activity. After a presentation of the chemical structures and of the main physico-chemical and biological properties of these compounds, the current knowledge regarding their presence in the environment is summarized. The geochernical criteria of the different,origins, pyrolytic, diagenetic and petroleum of PAH are presented. The respective contributions of their various emission sources are discussed , as well as the transfer and diffusion in the environment, the modifications undergone and the ultimate fate of these compounds. The qualitative and quantitative distribution of combustion PAH in soils in different environmental situations is presented.

Après définition des grandes lignes d'un procédé de base d'hydrolyse-fermentation de la paille de blé, l'analyse de sensibilité montre que le rendement de l'hydrolyse a une grande importance sur les bilans énergétique et économique. Des rendements de l'ordre de 85 % permettent d'obtenir des valeurs d'investissement par tonne de pétrole économisée tout à fait comparables à celles obtenues par d'autres techniques de valorisation de la biomasse en alcool, telle la synthèse du méthanol obtenu après gazéification du bois à l'oxygène. The basic features of a process for production from cereal straw of an acetone-butanol mixture for use as a gasoline substitute are described. They include pretreatment and enzymatic hydrolysis of the substrate followed by fermentation of the sugars produced. A cost evaluation based on the performances of a reference process is presented. Then, an analysis of the sensitivity of the cost price of the process to the variation of the important parameters such as production capacity, enzyme productivity, hydrolysis yield is carried out. The energy balance of the process is presented.

La microbiologie de la dégradation des hydrocarbures aromatiques polycycliques (HAP) est un domaine de recherche en plein développement. C'est à la fois le devenir dans l'environnement de ces composés ubiquistes et génotoxiques et l'utilisation de procédés microbiologiques de dépollution des sols industriels contaminés par ces produits qui motivent cet intérêt. On présente ici une synthèse des connaissances actuelles dans ce domaine. Les organismes dégradeurs sont essentiellement les bactéries et les champignons. Le processus de dégradation, aérobie, est initié par des oxygénases. Les bactéries utilisent les HAP de deux à quatre cycles comme substrats de croissance, ce qui conduit à leur minéralisation. Les champignons, lignolytiques et non lignolytiques, attaquent les HAP par cométabolisme, ce que font également les bactéries. L'ensemble des micro-organismes dégradant les HAP, et les voies métaboliques impliquées, sont présentés. Le mode d'accession des micro-organismes à leurs substrats très peu solubles est un point important. Les études menées avec les bactéries ont montré l'existence de deux mécanismes, le transfert par solubilisation dans la phase aqueuse et l'accession interfaciale directe. Un autre aspect présenté est le devenir des HAP, en termes de bilans carbone, lors de la dégradation bactérienne de HAP individuels et de mélanges de HAP. Des taux de minéralisation élevés peuvent être obtenus. Dans le cas des mélanges, ces taux élevés impliquent la mise en oeuvre de microflores complexes où le cométabolisme joue un rôle important. Les progrès accomplis ces dernières années conduisent à ne plus considérer les HAP comme des composés intrinsèquement récalcitrants à la biodégradation. Dans l'environnement, un facteur important limitant la dégradation des HAP est leur accessibilité aux micro-organismes. The microbiology of the degradation of polycyclic aromatic hydrocarbons (PAH) is a rapidly progressing area. Research aims both at developing the knowledge of the fate of these ubiquitous and genotoxic pollutants in the environment and at promoting the utilization of microbiological processes for the remediation of industrial soils contaminated with these compounds. A review of present knowledge in this field is presented here. Degradative organisms are essentially bacteria and fungi. The biodegradation processes involved are aerobic and are initiated by oxygenases. Bacteria utilize two to four cycle-PAH as growth substrates, leading to their mineralization. In addition, lignolytic fungi, as well as bacteria, carry out a cometabolic attack of PAH. A survey of the various microorganisms degrading PAH and of the metabolic pathways involved is presented. The mechanisms by which these scarcely soluble compounds are made accessible to the degradative microorganisms is discussed. Studies completed with bacteria had shown that two distinct mechanisms are involved, namely transfer by dissolution in the aqueous phase and direct interfacial uptake. Another important aspect is the fate of PAH in terms of carbon balances during the bacterial degradation of individual PAH and of PAH mixtures. High levels of mineralization can be obtained. In the case of PAH mixtures such high mineralization levels are obtained using complex microflora involving a large participation of cometabolism. Progress achieved in recent years leads to give up considering PAH as intrinsically biodegradation-recalcitrant compounds. In the environment, an important parameter limiting PAH degradation is their accessibility to microorganisms.

In this paper, we present a theoretical study and a numerical simulation of a classical long wavelength top illuminated PIN photodetector for microwave applications under very high optical power. The modeling includes a monodimensional drift-diffusion model for the device and takes into account the external circuit. At first, this modeling is validated using experimental results from the literature. Second, we consider a classical InP/GaInAs/InP photodiode grown on [N.sup.+] InP substrate. The presented results show that the distortion and the saturation of the microwave signal at 20 GHz are due to the space F charge effect in the photodetector and also to the depolarization of the device because of the external circuit. The main parameter influencing these phenomena are the optical power, the bias voltage, the optical spot c width and the modulation depth. In case of small optical spot, the effect of the external circuit is neglectable, while it contributes to the decrease of the microwave responsivity in case of large spot. The maximum output power is calculated in different cases and we can expect up to 12 dBm microwave output power for a 5 V reverse bias voltage.

Gordonia terrae strain IFP 2001 was selected from activated sludge for its capacity to grow on ethyl t-butyl ether (ETBE) as sole carbon and energy source. ETBE was stoichiometrically degraded to t-butyl alcohol (TBA) and the activity was inducible. A constitutive strain, G. terrae IFP 2007, derived from strain IFP 2001, was also selected. Methyl t-butyl ether (MTBE) and t-amyl methyl ether (TAME) were not used as carbon and energy sources by the two strains, but cometabolic degradation of MTBE and TAME was demonstrated, to TBA and t-amyl alcohol (TAA) respectively, in the presence of a carbon source such as ethanol. No two-carbon compound was detected during growth on ETBE, but formate was produced during cometabolic degradation of MTBE or TAME. A monooxygenase was involved in the degradation of ethers, because no degradation of ETBE was observed under anaerobic conditions and the presence of a cytochrome P-450 was demonstrated in G. terrae IFP 2001 after induction by cultivation on ETBE. [ABSTRACT FROM AUTHOR]

The parameters that control fermentation performance of butyrate production have been studied with a selected strain of Clostridium tyrobutyricum. Fed-batch supply of glucose increased productivity for butyrate. The ratio of butyrate to total acids was strongly influenced by the growth rate of the bacteria, acetate being produced along with butyrate at higher growth rates. In glucose-limited, fed-batch cultures, initially produced acetate was re-utilized, resulting in exclusive production of butyrate. In cultures with non-limiting glucose feeding, the butyrate concentration reached 42.5 g·1 with a selectivity of 0.90, a productivity of 0.82 g· per hour and a yield of 0.36 g·g The effects of the mode of supply of glucose on the production of butyrate and acetate are discussed in relation with the energy requirements for cell growth. [ABSTRACT FROM AUTHOR]