Experimental evidence for calcium-chloride ion pairs in the interlayer of montmorillonite. A XRD profile modeling approach.

Montmorillonite was equilibrated with high normality Cl - solutions to assess the possible presence of MeCl + ion pairs in smectite interlayers which is suggested by chemical modeling of cation exchange experimental studies. Structural modifications induced by the presence of such ion pairs, and more especially those related to smectite hydration properties, were characterized from the modeling of experimental X-ray diffraction (XRD) profiles. As compared to those obtained from samples prepared at low ionic strength, XRD patterns from samples equilibrated in high ionic strength CaCl2 solutions exhibited a small positional shift of 0

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults. To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007. Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems

Initial results from a hydroacoustic network to monitor submarine lava flows near Mayotte Island

In 2019, a new underwater volcano was discovered at 3500 m below sea level (b.s.l.), 50 km east of Mayotte Island in the northern part of the Mozambique Channel. In January 2021, the submarine eruption was still going on and the volcanic activity, along with the intense seismicity that accompanies this crisis, was monitored by the recently created REVOSIMA (MAyotte VOlcano and Seismic Monitoring) network. In this framework, four hydrophones were moored in the SOFAR channel in October 2020. Surrounding the volcano, they monitor sounds generated by the volcanic activity and the lava flows. The first year of hydroacoustic data evidenced many earthquakes, underwater landslides, large marine mammal calls, along with anthropogenic noise. Of particular interest are impulsive signals that we relate to steam bursts during lava flow emplacement. A preliminary analysis of these impulsive signals (ten days in a year, and only one day in full detail) reveals that lava emplacement was active when our monitoring started, but faded out during the first year of the experiment. A systematic and robust detection of these specific signals would hence contribute to monitor active submarine eruptions in the absence of seafloor deep-tow imaging or swath-bathymetry surveys of the active area

Etats d'hydratation d'argiles suivis par analyses vibrationnelles de l'eau et des hydroxyles dans le proche infrarouge : Applications aux systèmes saponite et bentonite.

The study of the feasibility of a deep geological disposal facility conducted by ANDRA - the French national radioactive waste management agency -, requires the knowledge of water status and water content in clays. Thanks to an original lab-built device coupling vibrational spectroscopies and water adsorption isotherms, adsorbed water and clay's structure are described quantitatively and qualitatively. A multidisciplinary approach allows the description of hydration mechanisms and water molecules network in the interlamellar space of synthetic saponites. The effects of density and nature of interfoliar cations and the influence of temperature on hydration are presented. Using mechanisms and important parameters established on saponites, hydration of bentonite MX80 is carried out. In order to describe and quantify simultaneously two different water states, a simple but relevant method of spectra analysis was developed.Dans le cadre de l'étude de faisabilité du stockage en formation argileuse profonde, la disponibilité de l'eau dans les argiles doit être connue. Le statut de l'eau dans un milieu poreux particulier conditionne en effet l'interprétation de nombreuses expériences : altération géochimique, rétention, thermo–hydro–mécanique, modélisations géochimiques, ... . Pour répondre aux préoccupations reliées à la diffusion des polluants à travers des milieux poreux, il est fondamental de décrire correctement les états de l'eau, vecteur des transports. Le but de cette étude est également de clarifier les notions "d'eau libre" et "d'eau liée". Les spectroscopies de vibration sont appliquées à l'étude des états d'hydratation de saponites synthétiques, (Si4-xAlx)Mg3O10(OH)2Mz+x/z, afin de caractériser les molécules d'eau adsorbées et les conséquences de cette adsorption sur la structure des matériaux. La réalisation de ce projet a nécessité la conception, la réalisation et la validation d'un montage expérimental original ; il permet d'enregistrer des spectres proche infrarouge en réflexion diffuse et des spectres de diffusion Raman, les échantillons étant maintenus à température (30°C à 80°C) et pression de vapeur d'eau contrôlées (10-6 L'étude concomitante des spectres de vibration et une approche pluridisciplinaire originale (Diffraction de Rayons X et de neutrons, Analyse Thermique à Vitesse Contrôlée, gravimétrie d'adsorption d'eau et simulations Grand Canonique Monte Carlo) permet de décrire les mécanismes de l'hydratation ainsi que la localisation et l'organisation des molécules d'eau dans l'espace interfoliaire des saponites. Grâce à la diversité des échantillons synthétiques – deux charges (x=0,4 ou 0,7) et trois cations compensateurs (Mz+x/z=Na+ ou K+ ou Ca2+) –, il est possible d'établir l'influence des paramètres charge et nature du cation compensateur sur l'hydratation. Le cation se révèle alors être l'acteur principal de la structuration de l'eau dans l'espace interfoliaire et du gonflement ; par exemple, le potassium K+ s'avère très difficile à extraire de sa cavité ditrigonale lors de l'hydratation, à tel point que, pour la charge x=0,7, il reste localisé au niveau de la substitution au-delà de P/P0=0,90. La conséquence directe sur la quantité d'eau adsorbée est que, à P/P0=0,40 par exemple, la saponite 0,7 K adsorbe quatre fois moins de molécules d'eau que le même échantillon échangé au sodium. Ensuite, l'étude de l'effet de la température sur l'hydratation des saponites sodiques tend à montrer que le film d'eau interfoliaire reste structuré en deux dimensions jusqu'à plus de 95% d'humidité relative à 80°C alors que le passage à un réseau tridimensionnel est observé dès 60% d'humidité relative à 30°C. Une autre conséquence est qu'à P/P0=0,80, la quantité d'eau adsorbée par un même échantillon est trois fois plus grande à 30°C qu'à 80°C. Après avoir extrait les mécanismes et les paramètres principaux de l'hydratation des saponites, l'hydratation d'une argile méthodologique de l'Andra, la bentonite MX80, est également étudiée. L'accent est mis sur l'importance des phases minérales accessoires de cette argile naturelle qui représentent environ 20% en masse. Notamment, la composition en cations de l'espace interfolaire n'est pas à l'équilibre avec les minéraux extérieurs aux particules de smectite ; un enrichissement de l'espace interfoliaire en cations divalents (Ca2+ et/ou Fe2+) est mis en évidence en présence de vapeur d'eau. Les phases accessoires modifient également la porosité intergranulaire et, par conséquent, la quantité d'eau adsorbée à des P/P0 supérieurs à 0,80 varie entre la phase smectique et la bentonite. Enfin, l'acquisition de nombreux spectres de l'eau a nécessité le développement d'une méthode simple de traitement des profils spectraux. Reposant sur la mesure du barycentre du massif d'absorption des combinaisons de l'eau, elle permet de différencier deux types d'eau tout en les quantifiant ; de l'eau adsorbée sur les cations à haut nombre d'onde et de l'eau engagée dans un réseau de liaisons hydrogène appelée eau auto-associée, à plus bas nombre d'onde. Il apparaît que, jusqu'aux alentours de 0,6 en P/P0, c'est l'eau des cations qui est majoritaire. Au-delà, l'eau auto-associée augmente fortement jusqu'à devenir prédominante, la quantité d'eau sur les cations restant quasi-constante

Fast identification and quantification of BTEX coupling by Raman spectrometry and chemometrics

Monoaromatic hydrocarbons (MAHs) monitoring is of environmental interest since these chemical pollutants are omnipresent. While waiting for robust sensors able to detect hydrocarbons at very low levels, the present study shows how each compound from pure BTEX mixtures can be identified fast and quantified thanks to Raman spectrometry and data processing based on the SIMPLISMA algorithm. A preprocessing module has been created to remove background contributions and a postprocessing program has been added to achieve matching and calibration. A wide range of BTEX concentrations and relative proportions has been investigated in order to determine the limitations of the processing. Output results achieved an accuracy of up to 95%. This method could be extended to other important pollutants such as polyaromatic hydrocarbons (PAHs) and chlorinated hydrocarbon derivatives

La spectroscopie Raman pour l’environnement marin

L’analyse des composés chimiques est au coeur de nombreuses applications en sciences marines et environnementales : surveillance dans le cadre de la Directive cadre sur l’eau (Directive 2000/60/CE) et la Directive cadre stratégie milieu marin (2008/56/CE), études d’impact des activités industrielles marines, surveillance de l’état sanitaire des eaux et produits de la mer, études écosystémiques ou biogéochimiques..

Influence of pH on the interlayer cationic composition and hydration state of Ca-montmorillonite: analytical chemistry, chemical modelling and XRD profile modelling study.

The hydration state of a <2 μm fraction of Ca-saturated SWy-2 montmorillonite was characterised after rapid equilibration (3 hours) under pH-controlled conditions (0.1-12.6 pH range). The solution composition was monitored together with the interlayer composition and X-ray diffraction (XRD) patterns were recorded on oriented preparations. Experimental XRD patterns were then fitted using a trial-and-error procedure to quantify the relative proportions of layers with different hydration states. The montmorillonite is mostly bi-hydrated in basic and near-neutral conditions whereas it is mostly mono-hydrated at low pH. The transition from the bi-hydrated to the mono-hydrated state occurs through very heterogeneous structures. However, the proportion of the different layer types determined from XRD profile modelling and that derived from chemical modelling using Phreeqc2 code strictly coincide. This correlation shows that the hydration modification is induced by a H3O +-for-Ca2+ exchange at low pH, the two species being distributed in different interlayers. This layer-by-layer exchange process occurs randomly in the layer stack. Under alkaline conditions, results from XRD profile modelling and from near infrared diffuse reflectance spectroscopy (NIR-DRS) clearly demonstrate that there is no CaOH +-for-Ca2+ exchange at high pH. The apparent increase in Ca sorption in smectite interlayers with increasing pH is thus probably related to the precipitation of CalciumSilicate-Hydrate (CSH) phases, which also accounts for the decrease in Si concentration under high-pH conditions. This precipitation is thermodynamically favoured