Etude de la dégradation de quelques composés organochlorés volatils par photolyse du peroxyde d'hydrogène en milieux aqueux

Le travail a eu pour but d'étudier l'efficacité de la photolyse du peroxyde d'hydrogène sur la dégradation de quelques composés organochlorés aliphatiques saturés (chiorométhanes et chloroéthanes) en milieu aqueux (pH 7,5). Les expériences ont été réalisées en réacteur statique, avec une Lampe basse pression à vapeur de mercure et avec des concentrations initiales en produit chloré de l'ordre de 10-6 mol l-1 et en H202 comprises entre 10-5 et 10-3 mol L-.Les résultats montrent que le système H202/UV peut oxyder les composés organochlorés étudiés à l'exception des composés ne possédant pas d'atome d'hydrogène (CCL4 et C2 CL6). Les rendements d'oxydation obtenus avec Le réacteur utilisé dépendent du temps de réaction, de la concentration initiale en H202, du flux photonique et peuvent être nettement diminués par la présence de pièges à radicaux (ions bicarbonates) dans le milieu réactionnel.Par ailleurs, une étude cinétique de la photolyse du peroxyde d'hydrogène en absence de matière organique est également présentée.The aim of this work was to study oxidation of certain volatile polychlorinated hydrocarbons, using hydrogen peroxide photoactivated by UV. This research was carried out with different mixtures of diluted aqueous solutions of chloromethanes (CHCl3, CCl4) and chloroethanes (C2H3Cl3, C2H2Cl4, C2HCl5, CCl6), which are typical halogenated compounds most frequently found in contaminated groundwater. The effect of the hydrogen peroxide concentration, the light intensity and the bicarbonate concentration on the rate of 1,1,2-trichloroethane (TCE) oxidation was determined. A kinetic study on hydrogen peroxide photolysis in a solution free of organic compounds was also carried out.EXPERIMENTATIONExperiments were conducted in a batch reactor (V = 4 l), equipped with an immersed mercury low-pressure lamp. The intensity emitted at 253.7 nm was roughly 2 1019 photons s-1. The temperature of the reaction mixture was maintained with a regulation system at 16 ± 0.5 °C (figure 1).The solutions were prepared in a phosphate buffer µ = 2 10-2 M, pH = 7.5). The outer surface of the lamp was masked with strips of aluminium, so as to obtain various percentages of initial energy (20 to 100 %).The concentration of the hydrogen peroxide of the samples was determined by spectrophotometry and the chlorinated compounds were analysed by electron capture gas chromatography.RESULTKinetics of hydrogen peroxide photolysis : H202 was decomposed by UV tb produce two hydroxyl radicals. In diluted solutions ([H202] < 10-3 M), the concentration decreases in accordante with a first order law. The rate constant depends on the initial light intensity (Io), on the characteristics of the reactor (volume and distance between the lamp and the watt. of the reactor) and on the motar extinction coefficient of the irradiated solution (equation C). The decomposition rate appears to be dependent on pH, the rate of constant rire has been found to be proportional to the dissociation of hydrogen peroxide into its basic form (EH2O2 = 20 mol-1 cm-1,EH2O2_ = 240 mol-1 cm-1) (figure 2 and 3).Oxidation of the chlorinated compounds : H202/UV is very efficient for the removal of organic compounds. Preliminary experiments showed that both UV and H2O2 treatments do not decompose halogenated compounds. Hydroxyl radicals are extremly reactive and attack organic compounds preferentialty by abs-tracting a hydrogen atom from an organic molecule. This is confirmed by the results which show that chloromethanes and chloroethanes with an H atom are eliminated, but net compounds such as tetrachloride and hexachloroethane (figure 4 to 7).The effectiveness of an H202/UV system depends on various parameters. Studies on the TCE elimination show that the oxidation yields an increase when the reaction time, the UV irradiation dose (figure 9b) and the hydrogen peroxide concentration (figure 8) increase. However, the efficiency decreases in the presence of radical traps such as bicarbonate and carbonate ions (figure 10)

In-situ synthesis of aluminum/nano-quasicrystalline Al-Fe-Cr composite by using selective laser melting

In this research, Al-Fe-Cr quasicrystal (QC) reinforced Al-based metal matrix composites were in-situ manufactured by using selective laser melting (SLM) from the powder mixture. The parametrical optimization based on our previous work was performed with focus on laser scanning speed. From the optimized parameters, an almost dense (99.7%) free-crack sample was fabricated with an ultra-fine microstructure. A phase transition from decagonal QC Al65Cu25Fe10Cr5 to icosahedral QC Al91Fe4Cr5 could be observed as laser scanning speed decreases. Differential scanning calorimetry curves show that the QC phase is quiet stable until 500 °C. And then, the effects of annealing temperature on the microstructural and mechanical properties were determined. The results indicate that the recrystallization and growth behavior of α-Al grains could be prevented by QC particle during annealing. Furthermore, the growth of QC particle, which tends to form a porous structure, leads an improvement of Young modulus and decline of ductility

Effect of Zn- and Ca-oxides on the structure and chemical durability of simulant alkali borosilicate glasses for immobilisation of UK high level wastes

Compositional modification of United Kingdom high level nuclear waste (HLW) glasses was investigated with the aim of understanding the impact of adopting a ZnO/CaO modified base glass on the vitrified product phase assemblage, glass structure, processing characteristics and dissolution kinetics. Crystalline spinel phases were identified in the vitrified products derived from the Na2O/Li2O and the ZnO/CaO modified base glass compositions; the volume fraction of the spinel crystallites increased with increasing waste loading from 15 to 20 wt%. The spinel composition was influenced by the base glass components; in the vitrified product obtained with the ZnO/CaO modified base glass, the spinel phase contained a greater proportion of Zn, with a nominal composition of (Zn0.60Ni0.20Mg0.20)(Cr1.37Fe0.63)O4. The addition of ZnO and CaO to the base glass was also found to significantly alter the glass structure, with changes identified in both borate and silicate glass networks using Raman spectroscopy. In particular, these glasses were characterised by a significantly higher Q3 species, which we attribute to Si–O–Zn linkages; addition of ZnO and CaO to the glass composition therefore enhanced glass network polymerisation. The increase in network polymerisation, and the presence of spinel crystallites, were found to increase the glass viscosity of the ZnO/CaO modified base glass; however, the viscosities were within the accepted range for nuclear waste glass processing. The ZnO/CaO modified glass compositions were observed to be significantly more durable than the Na2O/Li2O base glass up to 28 days, due to a combination of the enhanced network polymerisation and the formation of Ca/Si containing alteration layers

Stabilité photochimique des liquides ioniques en solution aqueuse diluées

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Relations entre les propriétés physico-chimiques du support et la dégradation des pesticides à l'état adsorbé: cas de la silice

Our investigations have shown that even in a given type of support: silica, the obtained degradation of the phenmedipham, a bisphenylcarbamat, is variable. Thus, we studed the role of different physico-chemical parameters: type of preparation (sol-gel, aqueous precipitation, pyrogenic silica),composition and presence of impurities, specific area, Ph, porosity, quantity and nature of reactive sites and amount of water. Among those parameters, some do not condition the degradation, such as the type of preparation or the porosity. Pesticide molecule's modelisation gives a limit size largely smaller than the different silicas' porosity. At the contrary, some parameters are determinant. We are talking about the OH sites and in an other way the presence of impurities. Parallel investigations (IRFT and kinetic) have shown that for such products as carbamat, it isn't the number of OH sites but their density ( or number of OH sites/ m2 ) wich influences the silica's reactivity. They act on the speed of the degradation but also on the obtained products. The specific area have not a direct relation with the support's reactivity but its activity is linked to its relation on the density and the nature of the reactive sites (OH groups). As regard to the impurities, the composition of the support influences the degradation only in the case that they are laying on the grain's surface. / Cette étude aborde le rôle des caractéristiques physico-chimiques du support sur la dégradation chimique du phenmédiphame. Certains de ces paramètres ne sont pas déterminants, comme: le mode de préparation ( précipitation, flamme), la porosité, la quantité d'eau intrinsèque, le pH. La surface spécifique n'a pas un rôle direct dans la réactivité de la silice, mais par son influence sur la densité de sites actifs, elle a une action indirecte. Contrairement aux observations de réactivité classique du solide, la silice n'est réactive que pour des valeurs relativement faibles de la surface spécifique, et pour lesquelles le nombre de sites OH est suffisant. Le seul paramètre permettant de prévoir la réactivité de la silice lors du dépôt de phenmédiphame est la densité de sites OH libres (ou nombres de sites OH au m2). Celle-ci a été mesurée à partir de la bande à 3750 cm-1 des spectres IRTF des silices, rapportée à la surface spécifique. La densité de sites OH intervient à la fois sur la vitesse de dégradation du produit mais aussi sur la nature des sous-produits obtenus. Pour les silices à forte densité de OH on a une grande proportion de produit de dimérisation

Toxicity of PAHs and jelly protection of eggs in the Common frog Rana temporaria

International audiencePolycyclic Aromatic Hydrocarbons (PAHs) are damaging for aquatic organisms such as amphibians. In this study, toxicity of a mixture of three PAHs (naphthalene (2 rings), phenanthrene (3 rings) and pyrene (4 rings)) was tested on Common frog (Rana temporaria) embryos. The protective role of the jelly coat surrounding the eggs was studied by exposing embryos with and without jelly coat to PAHs dissolved in an aqueous solution without organic solvent. Results showed that the mixture of these three PAHs significantly increase embryonic mortality rate after a few hours of exposure. Embryos with jelly coat tend to suffer a lower mortality rate than embryos without jelly. The jelly surrounding eggs is filled by water of the breeding site, which can contain pollutants. Because jelly characteristics vary among species, sensitivity to environmental pollutants and levels of embryonic protection could be different among amphibian species

Photochemical degradation of methylalkyl imidazolium chloride salts in aqueous solution by UV radiation

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Direct and indirect photolysis of Alkyl Methyl Imidazolium salts in diluted aqueous solutions

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Solvent toxicity to amphibian embryos and larvae

International audienceOrganic micropollutants are often damaging for aquatic organisms. Being usually hydrophobic compounds, they are often dissolved in an organic co-solvent which increases their solubility in water. The aim of this study was to study the toxicitv of various solvents on embryos (protected or not by jelly coat) and on tadpoles of the common frog (Rana temporaria). Tested solvents were methanol (MeOH), methylene chloride (CH2Cl2), dimethyl sulfoxyde (DMSO), acetone (Ac) and ethanol (EtOH). Embryos exhibited higher mortality rates than tadpoles. Embryos with jelly were more sensitive to high concentration of solvents than embryos without jelly (except for acetone). According to these results, Ac, DMSO and CH2Cl2 can be used as co-solvents in water to help the dissolution of micropollutants at concentration equal to or lower than 0.001 ml/l for frog embryos, and EtOH, Ac and CH2Cl2 at concentration equal to or lower than 0.01 ml/l for Rana temporaria tadpoles. (c) 2005 Elsevier Ltd. All rights reserved

Photodegradation de l’acifluorfene et de l’oxyfluorfene en phase aqueuse, influence des substances humiques

L'acifluorfene et l'acifluofene sont deux herbicides qui appartiennent à la famille des diphényléthers. Nous avons étudié leur comportament photochimique dans differents solvants afin de mettre en évidence leurs mécanismes de dedégradation. L'étude en phase aqueuse a èté faite en absence et en presence d'acides humiques. Ces deux herbicides adsorbent dans le proche ultraviolet avec des coefficients d'absorption molaire compris enter 4000 et 5000 mol-1 cm-1. Il sont donc susceptibles de se transformer par photoreactions directe et indirecte. Leur stabilité à la lumiere est relativament faible puisque leur efficacité quantique de photodegardation est comprise entre 10-4 et 10-2 molecule degradée par photon absorbé en solution aqueuse ou alcoliqu