Evidence of multiple sorption modes in layered double hydroxides using Mo as structural probe

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO ) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms - edge sorption, interfacial dissolution-reprecipitation - are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.This work has been supported by a grant from Labex OSUG@2020 (Investissements d’avenir - ANR10 LABX56). B.M., A.F.-M., L.C., S.G. and F.C. thank the NEEDS program from the CNRS for funding support. B.M. also thanks the financial support from the China Scholarship Council (CSC)

Mécanismes de nucléation des carbonates

Precipitation and dissolution of calcium carbonate (CaCO3) are key processes in both natural and engineered systems due to their intimate association with the Earth’s carbon cycle. Precipitation usually occurs on foreign substrates since they lower the energetic barriers controlling nucleation events. This so-called heterogeneous nucleation results from the interplay between the fluid supersaturation and the interfacial free energies present at the substrate-nucleus-fluid interfaces. Despite the relevance of interfacial energies for the fate of heterogeneous nucleation, the current literature remains scarce in their absolute values, which limits the accuracy of reactive transport modelling. Of particular relevance to the carbon cycle, the formation of biominerals accounts for a major reservoir of the carbonate minerals in the lithosphere. Recent studies have revealed the existence of multistep nucleation pathways that involve formation of amorphous calcium carbonate (ACC), a metastable intermediate during the early stages of biomineral formation. Such amorphous precursors allow molding of the intricate shapes of biominerals, while their stability and crystallization kinetics are effectively controlled by multiple factors. Elucidating the underlying mechanisms is beneficial for the development of biomimetic materials.The first goal of this dissertation is to develop a predictive understanding of interfacial energy values governing CaCO3 heterogeneous nucleation as a function of specific physico-chemical properties of the substrates, such as hydrophobicity. This last was investigated using phlogopite, a common mica, with and without fluorine substitution yielding hydrophobic and hydrophilic substrates. In situ time-resolved Grazing-Incidence Small Angle X-ray Scattering experiments were performed to obtain effective interfacial energy values. Interestingly, the extracted values for both substrates were similar, and thermodynamically these substrates provide a good template for nucleation, but the pathways differ. By ex situ Atomic Force Microscopy characterization, the hydrophilic substrate was shown to promote the formation and stabilization of ACC, whereas the hydrophobic one favored the formation of calcite. These results point to the intrinsic structural flexibility of CaCO3 and its advantage in heterogeneous nucleation processes.The second goal is to provide an atomistic description of the substrate hydrophobicity/hydrophilicity. Water adsorption on phlogopite was studied in situ using Near-Ambient Pressure X-ray Photoelectron Spectroscopy to investigate the effect of fluorine substitution and the influence of different types of counterions (K+, Na+ vs. Cs+). The results of the spectroscopy experiments were further interpreted using molecular dynamics simulations and bond-valence theory. The combination of these techniques shows that the substrate hydrophobicity stems from a competition between two factors: hydration of counterions vs. that of substrate.The final goal is to study the molecular mechanisms by which Mg2+, a common impurity in biogenic amorphous precursors, increases the kinetic persistence of ACC. Inelastic Incoherent Neutron Scattering and X-ray Photon Correlation Spectroscopy were combined to elucidate the nanoscale dynamics of water and ions within ACC. The presence of Mg2+ was shown to enhance the atomic diffusion within the solid while simultaneously increasing the stiffness of the hydrogen bond network. These counter-intuitive results are addressed by considering the different factors included in the pre-exponential term of the nucleation rate equation within the framework of the classical nucleation theory. Overall, the results point to the importance of water as a kinetic stabilizer, and to the existence of steric barriers that lower the crystallization rate.La précipitation et la dissolution du carbonate de calcium (CaCO3) sont des processus clés dans les systèmes naturels en raison de leur association intime avec le cycle du carbone terrestre. La précipitation se produit généralement sur des substrats étrangers en abaissant les barrières énergétiques qui contrôlent la nucléation. Ce processus appelé nucléation hétérogène résulte d'une interaction entre la sursaturation du fluide et les différentes énergies d’interface entre substrat-noyau-fluide. Malgré l’importance des énergies d’interface sur le devenir de la nucléation hétérogène, la littérature actuelle reste rare dans leurs valeurs absolues, limitant la précision de la modélisation du transport réactif. La formation des biominéraux constitue un réservoir majeur des carbonates dans la lithosphère. Des études récentes ont révélé des nucléations par multi-étapes impliquant la formation du carbonate de calcium amorphe (ACC), un intermédiaire métastable durant les premiers stades de la formation des biominéraux. De tels précurseurs amorphes permettent de réaliser les formes complexes des biominéraux, tandis que leur stabilité et leur cinétique de cristallisation sont contrôlées par de multiples facteurs. L'élucidation des mécanismes sous-jacents est bénéfique pour le développement de matériaux biomimétiques.Le premier objectif est de développer une compréhension prédictive des valeurs d'énergie d’interface régissant la nucléation hétérogène du CaCO3 en fonction des propriétés physico-chimiques spécifiques des substrats, comme l'hydrophobicité. Cette dernière est étudiée en utilisant de la phlogopite avec et sans substitution par le fluor produisant des substrats hydrophobes et hydrophiles. La technique de diffusion des rayons X aux petits angles en incidence rasante a été employée in situ pour obtenir des valeurs d’énergie effective d’interface. Il est intéressant de noter que les valeurs extraites pour les deux substrats sont similaires, et thermodynamiquement les deux fournissent un bon modèle pour la nucléation, alors que leurs mécanismes sont différents. La caractérisation ex situ par microscopie à force atomique a montré que le substrat hydrophile favorise la formation et la stabilisation d’ACC, tandis que le substrat hydrophobe favorise la formation de calcite. Ces résultats soulignent la flexibilité structurelle intrinsèque du CaCO3 et son avantage dans les processus de nucléation hétérogènes.Le deuxième objectif est de fournir une description atomistique de l'hydrophobicité du substrat. L'adsorption d'eau sur la phlogopite a été réalisée in situ par spectroscopie de photoélectrons à pression ambiante pour étudier l'effet de la substitution par le fluor et de différents types de contre-ions (K+, Na+ vs. Cs+). Ces résultats ont été interprétés par des simulations de dynamique moléculaire et la théorie de bond-valence. La combinaison de ces techniques montre que l'hydrophobicité du substrat provient d'une compétition entre deux facteurs: l'hydratation des contre-ions par rapport à celle du substrat.Le but final est d'étudier les mécanismes moléculaires par lesquels Mg2+, une impureté chez les précurseurs amorphes biogéniques, augmente la persistance cinétique d’ACC. La technique de diffusion inélastique incohérente des neutrons a été combinée avec la spectroscopie de corrélation de photons X pour élucider la dynamique à l'échelle nanométrique de l'eau et des ions dans les ACC. Les résultats montrent que la présence de Mg2+ augmente la diffusion atomique dans le solide tout en amplifiant la rigidité du réseau des liaisons hydrogène. Ces résultats contre-intuitifs sont abordés en considérant différents facteurs cinétiques inclus dans l’équation décrivant le taux de nucléation au sein de la théorie classique de la nucléation. Dans l'ensemble, ces résultats indiquent l'importance de l'eau comme stabilisant cinétique de la structure amorphe et de l'existence de barrières stériques qui abaissent le taux de cristallisation

Carbonate mineral nucleation pathways

La précipitation et la dissolution du carbonate de calcium (CaCO3) sont des processus clés dans les systèmes naturels en raison de leur association intime avec le cycle du carbone terrestre. La précipitation se produit généralement sur des substrats étrangers en abaissant les barrières énergétiques qui contrôlent la nucléation. Ce processus appelé nucléation hétérogène résulte d'une interaction entre la sursaturation du fluide et les différentes énergies d’interface entre substrat-noyau-fluide. Malgré l’importance des énergies d’interface sur le devenir de la nucléation hétérogène, la littérature actuelle reste rare dans leurs valeurs absolues, limitant la précision de la modélisation du transport réactif. La formation des biominéraux constitue un réservoir majeur des carbonates dans la lithosphère. Des études récentes ont révélé des nucléations par multi-étapes impliquant la formation du carbonate de calcium amorphe (ACC), un intermédiaire métastable durant les premiers stades de la formation des biominéraux. De tels précurseurs amorphes permettent de réaliser les formes complexes des biominéraux, tandis que leur stabilité et leur cinétique de cristallisation sont contrôlées par de multiples facteurs. L'élucidation des mécanismes sous-jacents est bénéfique pour le développement de matériaux biomimétiques.Le premier objectif est de développer une compréhension prédictive des valeurs d'énergie d’interface régissant la nucléation hétérogène du CaCO3 en fonction des propriétés physico-chimiques spécifiques des substrats, comme l'hydrophobicité. Cette dernière est étudiée en utilisant de la phlogopite avec et sans substitution par le fluor produisant des substrats hydrophobes et hydrophiles. La technique de diffusion des rayons X aux petits angles en incidence rasante a été employée in situ pour obtenir des valeurs d’énergie effective d’interface. Il est intéressant de noter que les valeurs extraites pour les deux substrats sont similaires, et thermodynamiquement les deux fournissent un bon modèle pour la nucléation, alors que leurs mécanismes sont différents. La caractérisation ex situ par microscopie à force atomique a montré que le substrat hydrophile favorise la formation et la stabilisation d’ACC, tandis que le substrat hydrophobe favorise la formation de calcite. Ces résultats soulignent la flexibilité structurelle intrinsèque du CaCO3 et son avantage dans les processus de nucléation hétérogènes.Le deuxième objectif est de fournir une description atomistique de l'hydrophobicité du substrat. L'adsorption d'eau sur la phlogopite a été réalisée in situ par spectroscopie de photoélectrons à pression ambiante pour étudier l'effet de la substitution par le fluor et de différents types de contre-ions (K+, Na+ vs. Cs+). Ces résultats ont été interprétés par des simulations de dynamique moléculaire et la théorie de bond-valence. La combinaison de ces techniques montre que l'hydrophobicité du substrat provient d'une compétition entre deux facteurs: l'hydratation des contre-ions par rapport à celle du substrat.Le but final est d'étudier les mécanismes moléculaires par lesquels Mg2+, une impureté chez les précurseurs amorphes biogéniques, augmente la persistance cinétique d’ACC. La technique de diffusion inélastique incohérente des neutrons a été combinée avec la spectroscopie de corrélation de photons X pour élucider la dynamique à l'échelle nanométrique de l'eau et des ions dans les ACC. Les résultats montrent que la présence de Mg2+ augmente la diffusion atomique dans le solide tout en amplifiant la rigidité du réseau des liaisons hydrogène. Ces résultats contre-intuitifs sont abordés en considérant différents facteurs cinétiques inclus dans l’équation décrivant le taux de nucléation au sein de la théorie classique de la nucléation. Dans l'ensemble, ces résultats indiquent l'importance de l'eau comme stabilisant cinétique de la structure amorphe et de l'existence de barrières stériques qui abaissent le taux de cristallisation.Precipitation and dissolution of calcium carbonate (CaCO3) are key processes in both natural and engineered systems due to their intimate association with the Earth’s carbon cycle. Precipitation usually occurs on foreign substrates since they lower the energetic barriers controlling nucleation events. This so-called heterogeneous nucleation results from the interplay between the fluid supersaturation and the interfacial free energies present at the substrate-nucleus-fluid interfaces. Despite the relevance of interfacial energies for the fate of heterogeneous nucleation, the current literature remains scarce in their absolute values, which limits the accuracy of reactive transport modelling. Of particular relevance to the carbon cycle, the formation of biominerals accounts for a major reservoir of the carbonate minerals in the lithosphere. Recent studies have revealed the existence of multistep nucleation pathways that involve formation of amorphous calcium carbonate (ACC), a metastable intermediate during the early stages of biomineral formation. Such amorphous precursors allow molding of the intricate shapes of biominerals, while their stability and crystallization kinetics are effectively controlled by multiple factors. Elucidating the underlying mechanisms is beneficial for the development of biomimetic materials.The first goal of this dissertation is to develop a predictive understanding of interfacial energy values governing CaCO3 heterogeneous nucleation as a function of specific physico-chemical properties of the substrates, such as hydrophobicity. This last was investigated using phlogopite, a common mica, with and without fluorine substitution yielding hydrophobic and hydrophilic substrates. In situ time-resolved Grazing-Incidence Small Angle X-ray Scattering experiments were performed to obtain effective interfacial energy values. Interestingly, the extracted values for both substrates were similar, and thermodynamically these substrates provide a good template for nucleation, but the pathways differ. By ex situ Atomic Force Microscopy characterization, the hydrophilic substrate was shown to promote the formation and stabilization of ACC, whereas the hydrophobic one favored the formation of calcite. These results point to the intrinsic structural flexibility of CaCO3 and its advantage in heterogeneous nucleation processes.The second goal is to provide an atomistic description of the substrate hydrophobicity/hydrophilicity. Water adsorption on phlogopite was studied in situ using Near-Ambient Pressure X-ray Photoelectron Spectroscopy to investigate the effect of fluorine substitution and the influence of different types of counterions (K+, Na+ vs. Cs+). The results of the spectroscopy experiments were further interpreted using molecular dynamics simulations and bond-valence theory. The combination of these techniques shows that the substrate hydrophobicity stems from a competition between two factors: hydration of counterions vs. that of substrate.The final goal is to study the molecular mechanisms by which Mg2+, a common impurity in biogenic amorphous precursors, increases the kinetic persistence of ACC. Inelastic Incoherent Neutron Scattering and X-ray Photon Correlation Spectroscopy were combined to elucidate the nanoscale dynamics of water and ions within ACC. The presence of Mg2+ was shown to enhance the atomic diffusion within the solid while simultaneously increasing the stiffness of the hydrogen bond network. These counter-intuitive results are addressed by considering the different factors included in the pre-exponential term of the nucleation rate equation within the framework of the classical nucleation theory. Overall, the results point to the importance of water as a kinetic stabilizer, and to the existence of steric barriers that lower the crystallization rate

Ion correlations drive charge overscreening and heterogeneous nucleation at solid-aqueous electrolyte interfaces.

Classical electrical double layer (EDL) models are foundational to the representation of atomistic structure and reactivity at charged interfaces. An important limitation to these models is their dependence on a mean-field approximation that is strictly valid for dilute aqueous solutions. Theoretical efforts to overcome this limitation are severely impeded by the lack of visualization of the structure over a wide range of ion concentration. Here, we report the salinity-dependent evolution of EDL structure at negatively charged mica-water interfaces, revealing transition from the Langmuir-type charge compensation in dilute salt solutions to nonclassical charge overscreening in highly concentrated solutions. The EDL structure in this overcharging regime is characterized by the development of both lateral positional correlation between adsorbed ions and vertical layering of alternating cations and anions reminiscent of the structures of strongly correlated ionic liquids. These EDL ions can spontaneously grow into nanocrystalline nuclei of ionic compounds at threshold ion concentrations that are significantly lower than the bulk solubility limit. These results shed light on the impact of ion cooperativity that drives heterogeneous nonclassical behaviors of the EDL in high-salinity conditions

Long-Term Effects of Organic Amendments on Soil Organic Matter Quantity and Quality in Conventional Cropping Systems in Switzerland

Increasing soil organic carbon (SOC) in agroecosystems is a promising solution to simultaneously address climate change mitigation, adaptation, and food security. Yet, the best management practices that could achieve these goals remain to be identified. Here, we analyze the long-term effects of application of green manure, cereal straw, farmyard manure, and cattle slurry on SOC in a 37 year long field experiment in Switzerland. The treatment effects were compared against control conditions that received only optimal mineral fertilization. More specifically, this study aimed at evaluating the effect of organic amendments on SOC accumulation and distribution in different soil particle-size fractions by means of a set of indicators about organic matter quality (biological reactivity, humification index) and microbial activity (extracellular enzyme activities). In the absence of organic matter input, application of mineral fertilizers alone resulted in the lowest SOC content and the highest humification index of the bulk soil organic matter. Among the organic amendments, cereal straw, farmyard manure, and cattle slurry promoted a higher SOC content and a lower humification index due to an increase of SOC in the clay-size fraction. The annual C accrual reached 4.4‰ per year over 37 years with farmyard manure. The higher biological reactivity measured for the green manure and cereal straw amendments was associated with higher soil enzymatic activities, while C retention coefficients decreased by at least 2.5 times compared to animal-derived amendments. The low availability of nutrients in green manure and straw amendments as suggested by the high phosphatase and N-acetylglucosaminidase activities may indicate a reduction in C retention of organic matter inputs due to nutrient microbial mining with plant-derived amendments

Ion correlations drive charge overscreening and heterogeneous nucleation at solid-aqueous electrolyte interfaces.

Classical electrical double layer (EDL) models are foundational to the representation of atomistic structure and reactivity at charged interfaces. An important limitation to these models is their dependence on a mean-field approximation that is strictly valid for dilute aqueous solutions. Theoretical efforts to overcome this limitation are severely impeded by the lack of visualization of the structure over a wide range of ion concentration. Here, we report the salinity-dependent evolution of EDL structure at negatively charged mica-water interfaces, revealing transition from the Langmuir-type charge compensation in dilute salt solutions to nonclassical charge overscreening in highly concentrated solutions. The EDL structure in this overcharging regime is characterized by the development of both lateral positional correlation between adsorbed ions and vertical layering of alternating cations and anions reminiscent of the structures of strongly correlated ionic liquids. These EDL ions can spontaneously grow into nanocrystalline nuclei of ionic compounds at threshold ion concentrations that are significantly lower than the bulk solubility limit. These results shed light on the impact of ion cooperativity that drives heterogeneous nonclassical behaviors of the EDL in high-salinity conditions

Water Adsorption on Mica Surfaces with Hydrophilicity Tuned by Counterion Type (Na, K, Cs) and Structural Fluorination

International audienceThe stability of adsorbed water films on mineral surfaces has far-reaching implications in the Earth, environmental, and materials sciences. Here, we use the basal plane of phlogopite mica, an atomically smooth surface of a natural mineral, to investigate water film structure and stability as a function of two features that modulate surface hydrophilicity: the type of adsorbed counterion (Na, K, Cs) and the substitution of structural OH groups by F atoms. We use molecular dynamics (MD) simulations combined with in situ high-resolution X-ray reflectivity to examine surface hydration over a range of water loadings, from the adsorption of isolated water molecules to the formation of clusters and films. We identify four regimes characterized by distinct adsorption energetics and different sensitivities to cation type and mineral fluorination: from 0 to ½ monolayer film thickness, the hydration of adsorbed ions; from ½ to 1 monolayer, the hydration of uncharged regions of the siloxane surface; from 1 to 1½ monolayer, the attachment of isolated water molecules on the surface of the first monolayer; and for > 1½ monolayer, the formation of an incipient electrical double layer at the mineral-water interface

Electrolytic Sulfuric Acid Production with Carbon Mineralization for Permanent Carbon Dioxide Removal.

Several billion metric tons per year of durable carbon dioxide removal (CDR) will be needed by mid-century to prevent catastrophic climate warming, and many new approaches must be rapidly scaled to ensure this target is met. Geologically permanent sequestration of carbon dioxide (CO2) in carbonate minerals-carbon mineralization-requires two moles of alkalinity and one mole of a CO2-reactive metal such as calcium or magnesium per mole of CO2 captured. Chemical weathering of geological materials can supply both ingredients, but weathering reactions must be accelerated to achieve targets for durable CDR. Here, a scalable CDR and mineralization process is reported in which water electrolysis is used to produce sulfuric acid for accelerated weathering, while a base is used to permanently sequester CO2 from air into carbonate minerals. The process can be integrated into existing extractive processes by reacting produced sulfuric acid with critical element feedstocks that neutralize acidity (e.g., rock phosphorus or ultramafic rock mine tailings), with calcium- and magnesium-bearing sulfate wastes electrolytically upcycled. The highest reported efficiency of electrolytic sulfuric acid production is achieved by maintaining catholyte feed conditions that minimize Faradaic losses by hydroxide permeation of the membrane-separated electrochemical cell. The industrial implementation of this process provides a pathway to gigaton-scale CO2 removal and sequestration during the production of critical elements needed for decarbonizing global energy infrastructure and feeding the world

Role of Impurities in the Kinetic Persistence of Amorphous Calcium Carbonate: A Nanoscopic Dynamics View

International audienc

Surface Wetting Controls Calcium Carbonate Crystallization Kinetics

Because of the widespread presence of foreign substrates in natural settings, mineral precipitation usually occurs via heterogeneous nucleation. This process is controlled by the interplay between the fluid supersaturation and interfacial energies present between the fluid, nucleus, and substrate. Among a number of physicochemical parameters, the surface wetting properties have been shown to be a key parameter controlling heterogeneous nucleation. The present study aims at elucidating the pathway and kinetics of CaCO3 heterogeneous nucleation on a set of phlogopite micas with and without fluorine/hydroxyl substitutions, yielding substrates with contrasting hydrophilicity. Our results show that, irrespective of surface wetting properties, amorphous calcium carbonate (ACC) is formed during the early stages. The surface wetting properties have a strong effect on the crystallization kinetics: ACC precipitates persist longer on the hydrophilic (hydroxylated) surface than on the less hydrophilic (fluorinated) one. We show that this stabilization could have a thermodynamic origin because of the lower interfacial free energy between the hydrated amorphous precursor and the hydrophilic substrate. These results are highly relevant for biomineralization studies, where differences in wetting properties of organic moieties present in calcifying organisms could be used to accelerate or decelerate the crystallization of the initially formed amorphous precursor phase