Modélisation du devenir des produits organiques industriels en milieu aquatique - Revue bibliographique

La nécessité de connaître aussi bien que possible l'impact des produits rejetés dans l'environnement a conduit à la mise au point de modèles mathématiques permettant de mieux comprendre le devenir des produits et de prédire l'exposition à laquelle pourra étre soumis l'environnement.Le présent article fait le point des différents modèles publiés pour ce qui concerne le milieu aquatique et suggère différents critères permettant de classer les modèles suivant une complexité croissante.Reprenant la terminologie de MACKAY (1979), 4 types de modèles sont décrits :- modèles homogènes, équilibrés, conservatifs et stationnaires,- modèles homogènes, non-équilibrés, non-conservatifs et stationnaires,- modèles homogènes, non-équilibrés, non-conservatifs et non-stationnaires,- modèles de dispersion.Assessing the hazard of a chemical to aquatic life is based on comparing the exposure concentration and the « no effects » concentration. The Greater the ratio between these two concentrations, called the margin of safety, the smaller the hazard to aquatic life.For chemicals already in the environment, monitoring programs can lead to an estimate of the exposure concentrations, but for new chemicals, predictive methods are necessary. This leads to the development of mathematical models which give an estimate of the environmental concentrations front the characteristics of the environment, of the chemical and of the quantity released in the environment.This article gives an overview of the different models which have been already published and suggests a classification based on the following criteria :- number of compartments,- closed or open system,- the degradation of the chemical is or is not taken into account,- an equilibrium has or has not been achieved between the compartments,- the situation is or is not at steady-state.Based on these criteria and quoting MACKAY (1979), four types of models are described :- Homogeneous, equilibrium, conservative and steady-state models,- Homogeneous, non-equilibrium, non-conservative and steady-state models,- Homogeneous, non-equilibrium, non-conservative and unsteady-state models,- Dispersion models.A review of these models applied to artificial or natural ecosystems la presented showing the extent to which these models have beau validated.These models are used not only to predict the exposure concentrations but they also help towards understanding the different phenomena which can affect the fate of a chemical. They allow to identify the mort important phenomena and those which have to be extensively studied. They are also useful for organizing experiments.However and though they are more and more used, these models have still to be improved since several fundamental phenomena, such as the sediment/water exchanges, are still badly understood

Metastable and transient states of chemical ordering in Fe-V nanocrystalline alloys

Chemical ordering of the disordered alloys Fe0.78V0.22, Fe0.53V0.47, Fe0.39V0.61, and Fe0.37V0.63 was performed by annealing at temperatures from 723 to 973 K. The initial state of chemical disorder was produced by high-energy ball milling, and the evolution of order was measured by neutron diffractometry and by 57Fe Mössbauer spectrometry. The hyperfine magnetic field distributions obtained from the Mössbauer spectra provided quantitative measurements of the number of antisite Fe atoms in the partially ordered alloys. The long-range order parameters in steady state after long annealing times were used as states of metastable equilibrium for a generally successful comparison with the metastable Fe-V phase diagram calculated by Sanchez et al. [Phys. Rev. B 54, 8958 (1996)]. For the metastable equilibrium state of order in Fe0.53V0.47 at low temperatures, the order parameters were smaller than expected. This corresponded to an abundance of antisite atoms, which were not removed effectively by annealing at the lower temperatures

Structural properties and Raman spectra of columbite-type NiNb2−x_{2-x}Vx_xO6_6 synthesized under high pressure

The complete set of structural parameters of the new series of compounds NiNb$_{2-x}$V$_x$O$_6$ ($0\leq x \leq 2$) with the unusual columbite-type structure is presented here. In the samples containing vanadium, this crystalline structure was stabilized by synthesis in conditions of high pressure and high temperature. Here we report here the first Raman spectrum for the NiV$_2$O$_6$-\textsl{Pbcn} polymorph and extend the list of the previously observed modes for the NiNb$_2$O$_6$. The evolution of the vibrational Raman spectrum produced when the vanadium is substituted for niobium along the series is also presented and discussed. This evolution is interpreted by taking into account the changes in the local structural environment of the niobium/vanadium atoms and its influence over the nickel-oxygen bonds around them. The presence of vanadium atoms favors an increase of the symmetry in the arrangement of oxygen atoms around the nickel-ones; in counterpart, the vanadium is in an octahedral environment which is more distorted than that of the niobium. Because of these apparently subtle differences, the homogeneous distribution of vanadium in the solid solution NiNb$_{2-x}$V$_x$O$_6$ is not possible

Géochimie d'une toposéquence de sols tropicaux du Tchad : utilisation des méthodes statistiques

In der tropischen Bodenkette von Mindera (Tschad) konnten verschiedene pedologische und geochemische Faziestypen festgestellt werden. Von den höher gelegenen ausgewaschenen Typen zu den angereicherten Typen der tieferen Lagen findet man Kaolinit, konkretionierte Eisen-und Manganhydroxyde, Montmorillonit und schliesslich Kalziumkarbonat. Mit Hilfe statistischer Methoden ist es möglich die Proben in Gruppen zusammenzufassen, die Beziehungen zwischen Hauptund Spurenelementen festzustellen sowie deren Lokalisation in verschiedenen mineralogischen und geochemischen Faziestypen zu erkennen. So erscheint die landschaftliche Verteilung der Elemente sehr nuanciert : die Zusammensetzung der Bestandteile eines Faziestyps ist nicht konstant längs der Bodenkette, die relative Mobilität der Elemente hängt von dieser Fazies und den vorhandenen geochemischen Schranken ab.Геохимия последовательного ряда тропических почв чада. применение статистических методов. В ряде тропических почв Миндеры, территория Чад, различные педологические и геохимические фации располагаются, во времени и пространстве, в определенном порядке. Они характеризуются, от вышележащей выщелоченной до нижележащей замкнутой среды, следующим последовательним рядом : каолинит, конкреционные гидроокиси железа и потом марганца, монтмориллонит и карбонат кальция. Статистические методы (кистевой анализ, факторный анализ) позволяют выделить группы образцов, связь между главными элементами и микроэлементами, также как и их локализацию в различных минералогических и геохимических определенных фациях. Таким образом, хроматографическая схема элементов в ландшафтах представляется нюансированной : состав компонентов одной фации не постоянен в последовательном ряду ; порядок относительной подвижности элементов меняется в зависимости от фаций и природы геохимических барьеров.In the tropical soil catena of Mindera, Chad, several pedological faciès are distributed in space and time. From the leached environments of the upper part of the catena to the confined environments of the lower part, the succession is the following : kaolinite, iron then manganese hydroxide concretions, montmorillonite and calcium carbonate. The statistic methods (factor analysis and cluster analysis) permit the autours to establish groups of samples, relations between major elements and trace elements, as well as their distribution in the different mineralogical and geochemical facies defined. The scheme of the chromatography of elements in the landscapes is thus discriminative : the composition of the constituents of a facies is not constant along the catena ; the order of relative mobility of elements varies as a function of facies and of the nature of geochemical barriers.Dans la chaîne de sols tropicaux de Mindera, au Tchad, différents faciès pédologiques et géochimiques s'ordonnent dans l'espace et dans le temps. Ils sont caractérisés, depuis les milieux lessivés de l'amont aux milieux confinés de l'aval, par la succession suivante : kaolinite, hydroxydes concrétionnés de fer puis de manganèse, montmorillonite et carbonate de calcium. Les méthodes statistiques (analyse de grappe et analyse factorielle) permettent d'établir des groupes d'échantillons, les relations entre éléments majeurs et traces, ainsi que leur localisation dans les différents faciès minéralogiques et géochimiques définis. Le schéma de la Chromatographie des éléments dans les paysages est donc nuancé : la composition des constituants d'un faciès n'est pas constante le long de la toposéquence ; l'ordre de mobilité relative des éléments varie en fonction des faciès et de la nature des barrières géochimiques.Karpoff Anne-Marie, Bocquier Gérard, Isnard Pierre, Tardy Yves. Géochimie d'une toposéquence de sols tropicaux du Tchad. Utilisation des méthodes statistiques. In: Sciences Géologiques. Bulletin, tome 26, n°4, 1973. Sédimentologie et géochimie de la surface. pp. 315-338

Specific heat measurements and structural investigation of CeCu6 - xSnx compounds

International audienceThe evolution of the crystal structure and some magnetic properties of the heavy-fermion material CeCu6 - xSnx (x = 0, 0.25, 0.65, 0.75, 0.85 and 1.0) has been studied by powder neutron diffraction and by specific heat measurements. The substitution of Cu by Sn suppresses the temperature induced orthorhombic to monoclinic transition, known to occur in the pure CeCu6 phase. No structural phase transition has been observed in these samples as a function of x but the cell volume increases considerably in an anisotropic way. Sn occupies preferentially the special Cu crystallographic site which is next to each of the four Ce atoms in the unit cell. The transition to antiferromagnetic order, characterizing the samples with higher x, is sensitive to both x and magnetic field. The results are discussed in the context of the competition between Kondo and RKKY interactions in disordered or not heavy-fermion systems and reveal an interesting interplay between composition, structure and magnetism in CeCu6 - xSnx

Magnetic behavior of the NixFe1-xNb2O6 quasi-one-dimensional system: Isolation of Ising chains by frustration

International audiencePhysical properties of the NixFe1-xNb2O6 compounds are investigated combining x-ray and neutron powder diffraction with magnetic and calorimetry measurements as well as 57Fe Mössbauer spectroscopy. This system is known to present quasi-one-dimensional magnetism with the magnetic moments arranged along weakly interacting Ising chains. Partial substitution of the magnetic ion tends to suppress the magnetic ordering observed in the end members of the series. When this happens, the low-temperature magnetic specific heat agrees well with what is expected for isolated Ising chains. The lowest temperature powder neutron-diffraction patterns exhibit evidence for the occurrence of short-range order, and analysis of these diffuse neutron-scattering patterns allow us to obtain information on the magnetic correlations. The suppression of magnetism is consistently interpreted as resulting from the magnetic-cation disorder induced by substitution, which enhances the system's tendency for frustration of geometrical origin

Magnetic order and spin fluctuations in the spin liquid Tb2_2Sn2_2O7_7.

We have studied the spin liquid Tb$_2$Sn$_2$O$_7$ by neutron diffraction and specific heat measurements. Below about 2 K, the magnetic correlations change from antiferromagnetic to ferromagnetic. Magnetic order settles in two steps, with a smeared transition at 1.3(1) K then an abrupt transition at 0.87(2) K. A new magnetic structure is observed, not predicted by current models, with both ferromagnetic and antiferromagnetic character. It suggests that the spin liquid degeneracy is lifted by dipolar interactions combined with a finite anisotropy along $$ axes. In the ground state, the Tb$^{3+}$ ordered moment is reduced with respect to the free ion moment (9 $\mu_{\rm B}$). The moment value of 3.3(3) $\mu_{\rm B}$ deduced from the specific heat is much smaller than derived from neutron diffraction of 5.9(1) $\mu_{\rm B}$. This difference is interpreted by the persistence of slow collective magnetic fluctuations down to the lowest temperatures

Suppression of magnetic ordering in quasi-one-dimensional FexCo1-xNb2O6 compounds

International audienceWe present a systematic investigation of the series of compounds FexCo1-xNb2O6 by means of x-ray and neutron powder diffraction combined with magnetic measurements, carried out in the paramagnetic as well as in the ordered state, to probe the stability of the magnetic ordering against the composition changes in this model Ising system. Fe for Co substitution induces a continuous lattice volume increase, preserving the orthorhombic crystal structure. The unit-cell expansion is anisotropic and occurs mainly in the ab plane. The observed magnetic structures for x=0,0.8, and 1 are described by the propagation vectors (0,1/2,0) and (1/2,1/2,0), and are consistent with the picture of ferromagnetic Ising-type chains of Fe/Co spins antiferromagnetically coupled by weak interchain interactions. We find out that for

Long term surgical results of 154 petroclival meningiomas: A retrospective multicenter study

BACKGROUND: Outcomes of petroclival meningiomas (PCM) (morbidity, permanent cranial nerves deficit, tumor removal and recurrence) are inconsistent in the literature, making it a challenge to predict surgical morbidity. METHODS: A multicenter study of patients with PCMs larger than 2.5cm between 1984 and 2017 was conducted. The authors retrospectively reviewed the patients\u27 medical records, imaging studies and pathology reports to analyze presentation, surgical approach, neurological outcomes, complications, recurrence rates and predictive factors. RESULTS: There were 154 patients. The follow-up was 76.8 months on average (range 8-380 months). Gross total resection (GTR) was achieved in 40 (26.0%) patients, subtotal resection (STR) in 101 (65.6%), and partial resection in 13 (8.3%). Six (2.6%) perioperative deaths occurred. The 5-year, 10-year and 15-year progression-free survival (PFS) of GTR and STR with radiation therapy (RT) was similar (100%, 90% and 75%). PFS of STR without adjuvant radiation was associated with progression in 71%, 51% and 31%, respectively. Anterior petrosectomy and combined petrosectomy were associated with higher postoperative CN V and CN VI deficits compared to the retrosigmoid approach. The latter had a significantly higher risk of CN VII, CN VIII and LCN deficit. Temporal lobe dysfunction (seizure and aphasia) were significantly associated with the anterior petrosectomy approach. CONCLUSIONS: Our study shows that optimal subtotal resection of PCMs associated with postoperative RT or stereotactic radiosurgery results in long-term tumor control to equivalent radical surgery. Case selection and appropriate intraoperative judgement are required to reduce the morbidity