Oxydation en voie humide de la pollution organique aqueuse par le peroxyde d'hydrogène Procédé « Wet Peroxide Oxidation » (WPO®) Étude de nouveaux catalyseurs

Les effluents aqueux pollués par des matières organiques provenant d'industries chimiques présentent souvent une faible biodégradabilité. Dans certains domaines de concentration (DCO = 0,5 - 15 g/l), le procédé WPO® développé au laboratoire se substitue avantageusement à l'incinération pour traiter ce type d'effluents. La réaction, qui met en œuvre le réactif de Fenton à température élevée, conduit parfois à la formation de quantités importantes d'acides carboxyliques légers. Nous avons donc développé des systèmes catalytiques originaux remplaçant les sels de fer et conduisant à une oxydation totale des acides carboxyliques. Le système le plus efficace constitué de sels de fer, de cuivre et de manganèse permet d'obtenir, en 1 h à 100 °C, l'oxydation totale d'un mélange synthétique de ces acides (COT = 5 g/l) avec 1,5 fois la quantité de peroxyde théoriquement nécessaire à l'oxydation. Le catalyseur précipité et séparé en fin de traitement peut être recyclé et conserve la môme activité. Les unités industrielles permettant d'effectuer le traitement WPO® avec les nouveaux catalyseurs, recyclés ou non, seront similaires à celle déjà réalisée pour le traitement de « points noirs » industriels.There is an important concern about the problems occuring with wastes elimination, specially the industrial liquid wastes. Te face the problem of organic aqueous wastes coming front various branches of industry, the WPO® (wet peroxide oxidation) process was developed at the laboratory. In the WAO process (wet air oxidation), which uses gaseous oxygen, the limiting step is usually oxygen transfer. In this new process, this problem is suppressed by using a liquid oxidising agent (hydrogen peroxide). This process is adapted from the classical Fenton's reaction and iron salts are used as the catalyst in order to promote the formation of •OH radicles which are the main active species. But the reaction is carried out at about 120 °C; so, a very significant TOC (total organic carton) removal efficiency is obtained (60 to 90 %) in comparison with the low efficiency of the classical Fenton's reagent (typically 25 % at room temperature).Significant amounts of free fatty acids are formed during the reaction. They are namely oxalic, malonic, succinic and acetic acids, which are common by products obtained during audition of most industrial organic pollutants. In order to comply with the regulations requirements, it was necessary to improve the efficiency of the original process. It was also very important to obtain an efficient elimination at a temperature not greater than 100 °C in order to avoid to pressurize the treatment reactor. This could be obtained by using new catalysts which are described in this paper.Because of the related field, precious metals like Pt and potentially toxic ones like Cr were not considered. One needs a treatment process as cheap and as reliable as possible. So, only Fe, Cu, Co, Ni and Mn were used as salts in order to test their calalytic activity in the treatment by hydrogen peroxide of a synthetical mixture of oxalic, malonic, succinic and acetic acids (O, M, S, A). The experimental device is a stirred tank reactor where the organics and the catalyst are batch loaded. It is continuously fed, for 1 hour, with hydrogen peroxide. The total amount injected is 1.5 the stoechiometric amount. In table 1, it can be seen that any metal has a satisfactory activity when used alone (TOC removal efficiency cannot exceed 22 %). In table 2, it is clear that, in soma cases, the association of two or three metals with each other can lead to very important synergetic effects. When using a mixture of Fe, Cu and Mn, the removal efficiency can increase to 91 %. This Fe/Cu/Mn catalyst is studied with further details in table 4. It appears to have its best efficiency at about 100 °C because of a parasitic decomposition of the peroxide at higher temperatures. For an organic mixture coutaining 5 g TOC/l, 100 ppm of each metal is a convenient concentration. This new catalyst still needs an acidic pH, from 3 to about 5, but the dependency is not so strict than with Fe alone (original process) which needs a value from 3 to 3.5. In addition, it was observed that the treatment time could be easily reduced (down to 45 minutes) as well as the amount of peroxide injected.Very similar results have been obtained with synthetic solutions of pollutants and with real industrial ones, thus establishing the ability of the Fe/Cu/Mn mixture to catalyse the oxidation of a large variety of species and not only carboxilic acids. The difference between the efficiency of this new catalyst and the conventional one is shown in table 5. Figures 1 and 2 are related to an Industrial WPO® unit which is commonly used with the conventional catalyst (Fe). It has been possible to improve its efficiency by using the new one without any significant modification. The Fe/Cu/Mn catalyst can be easily separated alter reaction (coprecipitation effect). Thus, the treated water meets the regulation requirements and the recovered catalyst can be easily resolubilized and recycled

Atmospheric tropical modes are important drivers of Sahelian springtime heatwaves

Heatwaves pose a serious threat to human health worldwide but remain poorly documented over Africa. This study uses mainly the ERA5 dataset to investigate their large-scale drivers over the Sahel region during boreal spring, with a focus on the role of tropical modes of variability including the Madden–Julian Oscillation (MJO) and the equatorial Rossby and Kelvin waves. Heatwaves were defined from daily minimum and maximum temperatures using a methodology that retains only intraseasonal scale events of large spatial extent. The results show that tropical modes have a large influence on the occurrence of Sahelian heatwaves, and, to a lesser extent, on their intensity. Depending on their convective phase, they can either increase or inhibit heatwave occurrence, with the MJO being the most important of the investigated drivers. A certain sensitivity to the geographic location and the diurnal cycle is observed, with nighttime heatwaves more impacted by the modes over the eastern Sahel and daytime heatwaves more affected over the western Sahel. The examination of the physical mechanisms shows that the modulation is made possible through the perturbation of regional circulation. Tropical modes thus exert a control on moisture and the subsequent longwave radiation, as well as on the advection of hot air. A detailed case study of a major event, which took place in April 2003, further supports these findings. Given the potential predictability offered by tropical modes at the intraseasonal scale, this study has key implications for heatwave risk management in the Sahel

The Saharan heat low and moisture transport pathways in the central Sahara-multiaircraft observations and Africa-LAM evaluation

We present a characterization of the Saharan heat low (SHL) based on dropsonde observations made on 22 June 2011 by two simultaneously flying aircraft during the Fennec project. The observations are used to identify moisture transport pathways and to validate the UK Met Office limited area model for northern Africa (Africa-LAM). The observations capture the SHL, harmattan, and monsoon surge. The SHL has a northeast-southwest orientated elongated shape centered over northern Mauritania. The SHL core is associated with a 950 hPa temperature minimum (36.4°C) in the morning caused by the monsoon surge and a maximum (42.6°C) in the afternoon. The monsoon surge east of the SHL core splits into two transport pathways: (a) curving around the SHL core in the north, especially pronounced in a morning near-surface layer, and (b) northeastward transport within the ~2km deep monsoon surge (afternoon observations only). In the morning the model forecasts the harmattan, monsoon surge, and the SHL geographic location and northeast-southwest orientation well but the model represents the SHL flatter and more spatially extended and overestimates the convective boundary layer (CBL) by up to ~0.3 km. The simulated afternoon SHL location appears shifted westward by up to ~1°. The model overestimates the shallow afternoon monsoon surge CBL depth of ~1.8km by >2kmresulting in southwestward transport of vertically mixed moisture above ~2.5km contrasting observed northeastward-only transport at lower levels. This moisture distribution model error is likely to have consequences for simulations of Saharan thermodynamics and dust emissions caused by convection-driven cold pools

Intercomparison of methods of coupling between convection and large-scale circulation. 2: comparison over non-uniform surface conditions

As part of an international intercomparison project, the weak temperature gradient (WTG) and damped gravity wave (DGW) methods are used to parameterize large-scale dynamics in a set of cloud-resolving models (CRMs) and single column models (SCMs). The WTG or DGW method is implemented using a configuration that couples a model to a reference state defined with profiles obtained from the same model in radiative-convective equilibrium. We investigated the sensitivity of each model to changes in SST, given a fixed reference state. We performed a systematic comparison of the WTG and DGW methods in different models, and a systematic comparison of the behavior of those models using the WTG method and the DGW method. The sensitivity to the SST depends on both the large-scale parameterization method and the choice of the cloud model. In general, SCMs display a wider range of behaviors than CRMs. All CRMs using either the WTG or DGW method show an increase of precipitation with SST, while SCMs show sensitivities which are not always monotonic. CRMs using either the WTG or DGW method show a similar relationship between mean precipitation rate and column-relative humidity, while SCMs exhibit a much wider range of behaviors. DGW simulations produce large-scale velocity profiles which are smoother and less top-heavy compared to those produced by the WTG simulations. These large-scale parameterization methods provide a useful tool to identify the impact of parameterization differences on model behavior in the presence of two-way feedback between convection and the large-scale circulation

Intercomparison of methods of coupling between convection and large-scale circulation: 1. Comparison over uniform surface conditions

As part of an international intercomparison project, a set of single column models (SCMs) and cloud-resolving models (CRMs) are run under the weak temperature gradient (WTG) method and the damped gravity wave (DGW) method. For each model, the implementation of the WTG or DGW method involves a simulated column which is coupled to a reference state defined with profiles obtained from the same model in radiative-convective equilibrium. The simulated column has the same surface conditions as the reference state and is initialized with profiles from the reference state. We performed systematic comparison of the behavior of different models under a consistent implementation of the WTG method and the DGW method and systematic comparison of the WTG and DGW methods in models with different physics and numerics. CRMs and SCMs produce a variety of behaviors under both WTG and DGW methods. Some of the models reproduce the reference state while others sustain a large-scale circulation which results in either substantially lower or higher precipitation compared to the value of the reference state. CRMs show a fairly linear relationship between precipitation and circulation strength. SCMs display a wider range of behaviors than CRMs. Some SCMs under the WTG method produce zero precipitation. Within an individual SCM, a DGW simulation and a corresponding WTG simulation can produce different signed circulation. When initialized with a dry troposphere, DGW simulations always result in a precipitating equilibrium state. The greatest sensitivities to the initial moisture conditions occur for multiple stable equilibria in some WTG simulations, corresponding to either a dry equilibrium state when initialized as dry or a precipitating equilibrium state when initialized as moist. Multiple equilibria are seen in more WTG simulations for higher SST. In some models, the existence of multiple equilibria is sensitive to some parameters in the WTG calculations

Directed hypergraph connectivity augmentation by hyperarc reorientations

The orientation theorem of Nash-Williams states that an undirected graph admits a k-arc-connected orientation if and only if it is 2k-edge-connected. Recently, Ito et al. showed that any orientation of an undirected 2k-edge-connected graph can be transformed into a k-arc-connected orientation by reorienting one arc at a time without decreasing the arc-connectivity at any step, thus providing an algorithmic proof of Nash-Williams' theorem. We generalize their result to hypergraphs and therefore provide an algorithmic proof of the characterization of hypergraphs with a k-hyperarc-connected orientation originally given by Frank et al. We prove that any orientation of an undirected (k,k)-partition-connected hypergraph can be transformed into a k-hyperarc-connected orientation by reorienting one hyperarc at a time without decreasing the hyperarc-connectivity in any step. Furthermore, we provide a simple combinatorial algorithm for computing such a transformation in polynomial time

Large‐Scale Drivers of Tropical Extreme Precipitation Events: The Example of French Overseas Territories

International audienceDue to their severity and lack of predictability, understanding and forecasting extreme precipitation events (EPEs) is critical for disaster risk reduction. The present work documents the large‐scale environment of tropical EPEs based on a 42‐year data set combining dense rain‐gauge networks that cover several tropical small islands and coastal regions. Approximately 10%–30% of EPEs are associated with a tropical storm or cyclone (TC), except for Reunion, for which its high topography makes it reach 55%. TCs multiply the EPE probability by a factor of 4–15, especially during TCs of category 1 or higher. A composite analysis demonstrates that the remaining large part of EPEs occurs within large‐scale and strong moist, convective, and cyclonic wind anomalies resulting from the superimposition of intraseasonal, seasonal‐to‐annual, and interannual timescales. These intense anomalies come essentially from intraseasonal variability, and lower frequencies improve the effect of intraseasonal events in creating a favorable environment for EPEs.En raison de leur gravité et de leur manque de prévisibilité, la compréhension et la prévision des événements de précipitations extrêmes (EPE) sont essentielles pour réduire les risques de dommages humains et matériels. Ce travail documente sur la période 1979-2021 l'environnement à grande échelle des EPE tropicaux sur la base d'un réseau de pluviomètres couvrant plusieurs petites îles tropicales et régions côtières des territoires outre-mer français dans les tropiques. Environ 10 à 30 % des EPE sont associés à une tempête tropicale ou à un cyclone, sauf à la Réunion, où la topographie élevée permet d'atteindre 55 %. Ces systèmes multiplient la probabilité d'EPE par un facteur de 4 à 15, en particulier lors des cyclones de catégorie 1 ou plus. Une analyse composite montre que la partie restante des EPE se produit au sein de fortes anomalies humides et convectives -- liée à du cyclonisme et un flux équatorial -- à grande échelle, résultant de la superposition d'échelles de temps intrasaisonnières, saisonnières à annuelles et interannuelles. Ces anomalies intenses proviennent essentiellement de la variabilité intrasaisonnière, les fréquences plus basses apportant plutôt un environnement de fond où les anomalies intrasaisonnières peuvent se développer

Recherche de complexes dinucleaires du ruthenium et du rhodium en phase homogene ou sur supports polymeres pour l'hydroformylation selective des alcenes

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 83318 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Seasonal forecasts of the Saharan Heat Low characteristics: a multi-model assessment

This work is supported by the French National Research Agency in the framework of the “Investissement d’avenir”program (ANR-15-IDEX-02) with the project PREDISAHLIM(2019–2021) and under grant with the projectSTEWARd (2020–2024).International audienceThe Saharan heat low (SHL) is a key component of the West African Monsoon system at the synoptic scale and a driver of summertime precipitation over the Sahel region. Therefore, accurate seasonal precipitation forecasts rely in part on a proper representation of the SHL characteristics in seasonal forecast models. This is investigated using the latest versions of two seasonal forecast systems namely the SEAS5 and MF7 systems from the European Center of Medium-Range Weather Forecasts (ECMWF) and Météo-France respectively. The SHL characteristics in the seasonal forecast models are assessed based on a comparison with the fifth ECMWF Reanalysis (ERA5) for the period 1993–2016. The analysis of the modes of variability shows that the seasonal forecast models have issues with the timing and the intensity of the SHL pulsations when compared to ERA5. SEAS5 and MF7 show a cool bias centered on the Sahara and a warm bias located in the eastern part of the Sahara respectively. Both models tend to underestimate the interannual variability in the SHL. Large discrepancies are found in the representation of extreme SHL events in the seasonal forecast models. These results are not linked to our choice of ERA5 as a reference, for we show robust coherence and high correlation between ERA5 and the Modern-Era Retrospective analysis for Research and Applications (MERRA). The use of statistical bias correction methods significantly reduces the bias in the seasonal forecast models and improves the yearly distribution of the SHL and the forecast scores. The results highlight the capacity of the models to represent the intraseasonal pulsations (the so-called east–west phases) of the SHL. We notice an overestimation of the occurrence of the SHL east phases in the models (SEAS5, MF7), while the SHL west phases are much better represented in MF7. In spite of an improvement in prediction score, the SHL-related forecast skills of the seasonal forecast models remain weak for specific variations for lead times beyond 1 month, requiring some adaptations. Moreover, the models show predictive skills at an intraseasonal timescale for shorter lead times