Contrasted role of disorder for magnetic properties in an original mixed valency iron Phosphate

We have measured the magnetic properties of a mixed valency iron phosphate. It presents an original structure with crossed chains containing Fe II and orthogonal to the longest direction of the crystallites. Microstructural investigations using electron microscopy show the presence of random nano-twinning. The ac susceptibility measurements demonstrate similarities with the kinetics of a disordered magnetic, spin-glass like, state but are shown to be essentially due to this peculiar disorder. Scaling properties are characteristics of 3D second order transition implying that this disorder at a small scale does not influence significantly long range magnetic ordering. At low temperature, a decrease of the spontaneous magnetization and an irreversible metamagnetic transition is observed, and is attributed to a canting of the spins in the iron chain.Comment: accepted for publication in PR

Effect of a Grazing Intensity Gradient on Primary Production and Soil Nitrogen Mineralisation in a Humid Grassland of Western France

Large herbivores have a major influence on the structure and the functions of humid grasslands and especially on primary production. Earlier work on the study site showed that grazing intensity was spatially varied and created a diversity of vegetation patches in the grassland (Loucougaray, 2003). The first objective of this study was to determine whether the variation in grazing intensity led to variation of primary production within the three plant communities located at three topographic levels in the grassland. The second objective was to determine whether a relationship linked primary production variation and net soil nitrogen (N) mineralisation

EFFECT OF CALCINATION TEMPERATURE OF TUNISIAN CLAYS ON THE PROPERTIES OF GEOPOLYMERS

Geopolymers are amorphous three dimensional aluminosilicate materials that may be synthesized at room or slightly higher temperature by alkaline activation of aluminosilicates obtained from industrial wastes, calcined clays and natural minerals. Among the different family of geopolymers, two Tunisian clays (a kaolinite clay from Tabarka and illito/kaolinitic clay from Medenine) are tested for their feasibility of geopolymers at low temperature. The unfired and calcined clays were dissolved in strongly alkaline solution in order to produce consolidated materials whose pastes were characterized by their compressive strength. Hardened geopolymer samples were also submitted to X-Ray diffraction, FTIR spectroscopy and scanning electron microscopy analyses. The geopolymer strength is related to the structure and reactivity of the clay generated by thermal treatment and to the role of associated minerals in clays. The amorphous character of obtained geopolymers and the displacement of the IR wavenumber are signature of geopolymerisation reaction

Sub-Terahertz Monochromatic Transduction with Semiconductor Acoustic Nanodevices

We demonstrate semiconductor superlattices or nanocavities as narrow band acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics experiments in the transmission geometry with pump and probe incident on opposite sides of the thick substrate, phonon generation and detection processes are fully decoupled. Generating with the semiconductor device and probing on the metal, we show that both superlattices and nanocavities generate spectrally narrow wavepackets of coherent phonons with frequencies in the vicinity of the zone center and time durations in the nanosecond range, qualitatively different from picosecond broadband pulses usually involved in picosecond acoustics with metal generators. Generating in the metal and probing on the nanoacoustic device, we furthermore evidence that both nanostructured semiconductor devices may be used as very sensitive and spectrally selective detectors

Influence de l'adsorption d'alginates sur les propriétés de membranes organiques d'ultra et de microfiltration

Les applications potentielles des cultures de microalgues et cyanobactéries en dépollution d'effluents dans des photobioréacteurs à membrane souffrent de performances limitées par un colmatage de l'élément filtrant dû en grande partie aux exopolysaccharides sécrétés par ces micro-organismes. Cette étude du laboratoire quantifie les effets de l'adsorption de ces polysaccharides sur des membranes organiques d'ultra et microfiltration tangentielle de matériaux et charges de surface différents. L'alginate de sodium est utilisé comme adsorbat modèle. Les membranes propres sont d'abord testées par une mesure de flux à l'eau pure, puis mises en contact avec une solution d'alginate durant un temps choisi. Le flux à l'eau pure des membranes après adsorption est ensuite à nouveau mesuré.La réduction relative du rayon de pore (ZEMAN, 1983) met en évidence l'effet de la mouillabilité et des charges superficielles. L'étude comparée de membranes d'ultra et microfiltration montre que cette réduction relative du rayon de pore augmente avec le seuil de coupure ou le diamètre de pore. L'effet de la concentration révèle aussi que la résistance hydraulique d'adsorption (MATTHIASSON, 1983) à l'équilibre évolue selon l'isotherme de LANGMUIR. Le modèle cinétique traduisant l'évolution de la résistance d'une membrane d'ultrafiltration proposé par BAKLOUTI et al.(1984), amélioré par AIMAR et al. (1988) puis discuté par RUIZ-BEVIÁ et al. (1997), est complété par un nouvel exposant agissant sur le facteur temps.La comparaison des résistances à l'écoulement de membranes de microfiltration avec celle d'une membrane d'ultrafiltration hydrophile neutre permet de dégager des critères de choix pour l'optimisation du fonctionnement d'un photobioréacteur à membrane utilisable en dépollution d'effluents.Potential applications of microalgae and cyanobacteria for treatment of wastewater effluents using membrane-photobioreactors suffer from limited performance due to fouling effects, mainly attributable to exocellular polysaccharides secreted by these micro-organisms. A membrane photobioreactor is defined as a process associating the culture of photosynthetic micro-organisms with a continuous separation by membrane filtration of the biomass and the water treated. The goal of the present laboratory-scale study was to quantify polysaccharide adsorption effects on organic membranes (ultra and microfiltration) characterised by different materials and surface charges. Sodium alginate was used as the "model adsorbate".Seven plane organic membranes were tested. The influence of membrane cut-off (or of pore diameters) as well as that of the material polyethersuphone (PES), polyacrylonitrile (PAN), polyvinilidene fluoride (PVDF) and of its properties (hydrophobicity, surface charges, …) were assessed. The study consisted of two parts :1. the first part was concerned with the kinetics of alginate adsorption and the influence contact time and solute concentrations on the reduction of pore diameter (ZEMAN, 1983) or on the increase of hydraulic resistance (MATTHIASSON, 1983);2. the second part dealt with adsorption equilibrium (formulations of LANGMUIR and FREUNDLICH).The study constituted the first step of a research program aimed at developing membrane photobioreactors for the treatment of specific industrial effluents. The fluid used to test the membranes was quality II pure water (ISO 3696 norm). Tangential velocities were set to 2.5 m.s-1, corresponding to a Reynolds number of 2500. To represent exopolysaccharides, we used alginic acid at concentrations of 1, 10 and 50 g, neutralised with sodium hydroxide at pH 9. New (or clean) membranes were first characterised through pure water flux measurements. J0, the flux of pure water for a new membrane, was obtained (flowrate / unit of surface area), and then the membrane was kept in contact, for a definite duration, with the alginate solution. After adsorption and rinsing, the pure water flux was measured again. Ja, the pure water flux, was measured through the membrane after adsorption.Adsorption model at equilibrium:The effect of adsorption is quantified under the form of the relative pore size reduction as described by ZEMAN (1983) and included in the relation : ∆r / r=1 - (Ja / Jo)1/4. A variation of this quantification is that of the MATTHIASSON model (1983) applied to the pure water flux, based on DARCY's law expressing the relative value of the hydraulic resistance of the adsorbed layer Ra in relation to the intrinsic resistance of the membrane Rm : Ra / Rm=(Jo / Ja) - 1.To express adsorption phenomena at the solid/liquid interface of membranes, we used LANGMUIR's law together with MATTHIASSON's experimental observation (1983): the relative resistance Ra / Rm due to adsorbed compounds is proportional to the mass "x" of solute adsorbed per unit of membrane surface area, x=Kx.Ra. If one assumes that the mass m of a homogeneous plane membrane per unit of membrane surface area is proportional to its adsorbing surface area Ω per unit of membrane surface area (m=Km.Ω), and if one combines the flux equations expressed by DARCY's and POISEUILLE's laws, then the result is m=K'm.Rm in a homogeneous membrane. Substituting x and m in LANGMUIR's law results in the equilibrium model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) in which c=concentration of adsorbing solute; a and b are coefficients; and Rae is the resistance due to compounds adsorbed at equilibrium. Kinetic model: To show the evolution of membrane resistance with time, we suggest the introduction of an empirical exponent j over the time parameter in the AIMAR et al. model (1988).Results: The effect of changing the alginate concentration reveals that the hydraulic resistance of adsorption, at equilibrium, (MATTHIASSON, 1983) evolves according to LANGMUIR's isotherm. The relative decrease of pore radius ∆r / r in the presence of l g.l-1 of sodium alginate shows that a quasi-plateau is obtained after two hours using the most hydrophobic membrane. The curves ∆r / r=f (t) for five membranes made of different materials, monitored during the transition phase before the plateau with common 1 g.l-1 concentrations, reveal similar adsorption behaviour, characterised by the limiting common value ∆r / r=0.06 ± 0.005. However, the uncharged hydrophilic membrane PAN 3038 stands out owing to a much lower ∆r / r value of 0.09. This peculiar behaviour can also be observed in the influence of the alginate concentration: hydrophobic and charged hydrophilic membranes display a saturation effect with ∆r / r little affected by the increase of alginate concentration, whereas the uncharged hydrophilic membrane PAN 3038 displays a ∆r / r value three to six times lower with great sensitivity to concentration effects at concentrations below 10 g.l-1. The model Rae / Rm=(Jo / Ja) - 1=a.c / (1 + bc) is in agreement with the experimental results obtained with hydrophobic and hydrophilic membranes. The proposed kinetic model shows that time dependence of R (t) does not seem to be linked to the nature of membranes. However, compared with concentration, R (c) is very sensitive to the nature of membranes. A comparative study of ultra and microfiltration membranes shows that the reduction in ∆r / r values increases with molecular weight cut-off (or pore diameter).Criteria for the choice of membranes: A comparative study of three polyacrylonitrile membranes reveals that membrane 3038 PAN (neutral) displays a very interesting, peculiar behaviour: its adsorption, expressed by ∆r / r or Rae/(Rae+Rm) is four to six times weaker than that of the other two. The surface charge of membranes seems to influence the intensity of adsorption in a significant way. Wetability also has a strong influence on adsorption. The sum of resistances Rae + Rm of ultrafiltration membrane 3038 PAN is only four times as great as those of hydrophobic microfiltration membranes. Experimentation already showed that, in the presence of microparticles, interactions between the layer of adsorbed alginate and microparticles will increase the likelihood of fouling of microfiltration membranes, decreasing their resistance down to the level of very little adsorbing ultrafiltration membrane IRIS 3038 (ROSSIGNOL et al., 1999).A culture system of marine microalgae in a membrane photobioreactor using ultrafiltration membrane IRIS 3038 PAN displayed a stable permeation flux during 6 weeks and easy regeneration, which meant adsorption was almost nil. The ability of some microalgae to assimilate ammonia nitrogen, nitrates and phosphates contained in waste water with excellent efficiencies (e.g., Phormidium bohneri: SYLVESTRE et al., 1996) allows one to consider using membrane photobioreactors in the treatment of home or industrial effluents. Other microalgae such as Chlorella salina (GARNHAM et al., 1992) are capable of fixing large amounts of heavy metals (Co, Mn, Zn, etc…); grown in membrane photobioreactors, they could depollute industrial effluents

Praeorbulina-like specimens" in sediments of the Northern Arabian Sea during the last glacial period

Date du colloque : 06/2009International audienc

The fecal microbiota of piglets during weaning transition and its association with piglet growth across various farm environments

This study describes the fecal microbiota from piglets reared in different living environments during the weaning transition, and presents the characteristics of microbiota associated with good growth of piglets after weaning. Fecal samples were collected pre- (d26) and post-weaning (d35) from 288 male piglets in 16 conventional indoor commercial farms located in the West of France. The changes one week after weaning on the most abundant microbial families was roughly the same in all farms: alpha diversity increased, the relative abundance of Bacteroidaceae (-61%), Christensenellaceae (-35%), Enterobacteriaceae (-42%), and Clostridiaceae (-32%) decreased, while the relative abundance of Prevotellaceae (+143%) and Lachnospiraceae (+21%) increased. Among all the collected samples, four enterotypes that were ubiquitous in all farms were identified. They could be discriminated by their respective relative abundances of Prevotella, Faecalibacterium, Roseburia, and Lachnospira, and likely corresponded to a gradual maturational shift from pre- to post-weaning microbiota. The rearing environment influenced the frequency of enterotypes, as well as the relative abundance of 6 families at d26 (including Christensenellaceae and Lactobacillaceae), and of 21 families at d35. In all farms, piglets showing the highest relative growth rate during the first three weeks after weaning, which were characterized as more robust, had a higher relative abundance of Bacteroidetes, a lower relative abundance of Proteobacteria, and showed a greater increase in Prevotella, Coprococcus, and Lachnospira in the post-weaning period. This study revealed the presence of ubiquitous enterotypes among the farms of this study, reflecting maturational stages of microbiota from a young suckling to an older cereal-eating profile. Despite significant variation in the microbial profile between farms, piglets whose growth after weaning was less disrupted were, those who had reached the more mature phenotype characterized by Prevotella the fastest

Improvement of attention span and reaction time with hyperbaric oxygen treatment in patients with toxic injury due to mold exposure

It is, by now, well established that mold toxins (mycotoxins) can cause significant adverse health effects. In this study, 15 subjects who developed an attention deficit disorder (ADD) and slowing of reaction time at the time of exposure to mold toxins were identified. Deficits in attention span and reaction time were documented not only by taking a careful history, but also by performing a Test of Variables of Attention (TOVA). The TOVA test provides an objective measure of these two variables. It was found that mold-exposed subjects show statistically significant decreases in attention span and significant increases in reaction time to stimuli compared to controls. After ten sessions of hyperbaric oxygen treatment (HBOT), a statistically significant improvement was seen in both measures. This preliminary study suggests promising outcomes in treating mold-exposed patients with hyperbaric oxygen