Precipitation of ordered dolomite via simultaneous dissolution of calcite and magnesite: New experimental insights into an old precipitation enigma

7International audienceIn the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200°C). The temperature and high-carbonate alkalinity have significantly co-promoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200°C from XRD patterns and FESEM observations. Conversely, higher proportion of ordered dolomite, i.e. clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100°C and two kinetic steps were identified (1) proto-dolomite formation after about five days of reaction, characterized by rounded sub-micrometric particles from FESEM observations and by the absence of superstructure ordering reflections at 22.02 (101), 35.32 (015), 43.80 (021), etc. 2thetha on XRD patterns; (2) proto-dolomite to dolomite transformation, probably produced by a coupled dissolution-recrystallization process. Herein, the activation energy was estimated to 29 kJ/mol by using conventional Arrhenius linear-equation. This study provides new experimental conditions to which dolomite could be formed in hydrothermal systems. Temperature and carbonate alkalinity are particularly key physicochemical parameters to promote dolomite precipitation in abiotic systems

Rapid precipitation of magnesite micro-crystals from Mg(OH)2-H2O-CO2 slurry enhanced by NaOH and a heat-ageing step (from 20 to 90°C)

International audienceThis study proposes a simple and novel synthesis route for rhombohedral single crystals (90°C) and its synthesis requires several days or weeks depending on experimental conditions. For this reason, industrial-scale magnesite production has been limited. The proposed magnesite synthesis method, requiring only 48h and moderate temperature, could easily be extrapolated on an industrial scale. Moreover, a simple and novel synthesis route for the production of fine platy particles of hydromagnesite is reported, with synthesis requiring only 5h. Based on their chemical compositions and textural properties, there are potential applications for both minerals, for example as a mineral filler and/or as a flame-retardant

Simultaneous precipitation of magnesite and lizardite from hydrothermal alteration of olivine under high-carbonate alkalinity

13 pagesInternational audienceThe present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in Earth systems (e.g. functioning of hydrothermal fields) or in engineered systems (e.g. ex-situ and in-situ mineral sequestration of CO2). For this case, specific experimental conditions were examined (200°C, saturated vapor pressure ≈ 16bar, solution/solid weight ratio = 15, olivine grain size < 30µm and high-carbonate alkalinity ≈ 1M NaHCO3). Under these conditions, competitive precipitation of magnesite and serpentine (preferentially lizardite type) were clearly determined by using conventional analytic tools (XRD, FESEM, FTIR and TGA); excluding the fate of the iron initially contained in olivine, the alteration reaction for olivine under high-carbonate alkalinity can be expressed as follows: 2〖Mg〗_2 SiO_4+2H_2 O+H〖CO〗_3^-→Mg〖CO〗_3+〖Mg〗_3 〖Si〗_2 O_5 〖(OH)〗_4+〖OH〗^- This reaction mechanism implied a dissolution process, releasing Mg and Si ions into solution until supersaturation of solution with respect to magnesite and/or serpentine. The released iron contained in the olivine has not implied any precipitation of iron oxides or (oxy)hydroxides; in fact, the released iron was partially oxidized (about 50%) via a simple reduction of water (2〖Fe〗^(2+)+〖2H〗_2 O→2〖Fe〗^(3+)+H_2+2〖OH〗^-). In this way, the released iron was incorporated in serpentine (Fe(II) and Fe(III)) and in magnesite (Fe(II). This latter was clearly determined by FESEM/EDS chemical analysis on the single magnesite crystals. The nucleation and epitaxial growth processes at the olivine-fluid interfaces cannot be excluded in our investigated system. The experimental kinetic data fitted by using a kinetic pseudo-second-order model have revealed a retarding process of serpentine formation with respect to magnesite (about three times slower); in fact, the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution. We assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given CO2 pressure is imposed in the system. On the other hand, the reactivity of serpentinized olivine (chrysotile+brucite+small amount of residual olivine) and high-purity chrysotile at the same above investigated conditions; and the olivine serpentinization in initial acid pH ≈ 0.66 are also reported as complementary information in this study. These novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate; this could contribute to a better understanding of fluid-rock interactions in natural active hydrothermal fields on Earth

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)

Sequential precipitation of a new goethite-calcite nanocomposite and its possible application in the removal of toxic ions from polluted water

International audienceThis study proposes a simple and innovative synthesis route for a goethite-calcite nanocomposite. This synthesis is summarized by three sequential precipitation reactions: (1) precipitation of nanosized acicular goethite (α-FeOOH) using a high OH/Fe molar ratio (=5); (2) instantaneous precipitation of portlandite (Ca(OH)2) by adding CaCl2 salt to a goethite alkaline suspension (2NaOH + CaCl2=Ca(OH)2 + 2NaCl) and; (3) sub-micrometric calcite precipitation by injection of CO2 into a goethite-portlandite alkaline suspension (Ca(OH)2 + CO2=CaCO3+H2O). The XRD patterns have confirmed the goethite and calcite mineral composition in the composite precipitated at 30 and 70°C. FESEM and TEM observations have revealed the formation of nanosized goethite particles well dispersed with sub-micrometric calcite particles, leading to an orange-brown colour nanocomposite with high specific surface area of around 92 m2/g for a composite synthesized at 30°C and 45 m2/g for a composite synthesized at 70°C. Both values were determined using the conventional BET method on N2 sorption isotherms. Finally, a goethite/calcite weight ratio equal to 0.8 in the composite was determined by thermogravimetric analysis (TGA). Additionally, some adsorption experiments carried out at two different pH values revealed that the goethite-calcite composite has a good sequestration capacity for Cu>Cd>As(III)>Se(IV)>As(V). Conversely, the Se(VI) did not show any chemical affinity with the goethite-calcite composite under the physico-chemical conditions studied. In practice, the goethite-calcite composite can neutralise acidic wastewater by slight calcite dissolution, enhancing the removal of heavy metals (e.g. Cu and Cd) at the calcite-solution interfaces

A route for long-term DNA preservation through nanoconfinement in smectites

Traces of DNA found in sediments are shifting paradigms in the analysis of past and present ecosystems. DNA is an unstable polymer and conditions at which the millennial stabilization is achieved are unclear. Confinement of DNA in nanopores of clay minerals is a promising route for this long-term stabilization and storage. Using smectites with various layer charges, we measured adsorption capacity for DNA using UV spectroscopy and intercalation capacity using X-ray diffraction. We found that while the smectite adsorption capacity is large, the DNA intercalation, i.e. nanoconfinement, decreases as smectite charge increases. We show that low-charge smectites intercalate DNA at concentrations relevant to aqueous environments even at neutral pH but the nanoconfinement is minimal or absent in high-charge smectites. Different intercalation behaviour in NaCl and CaCl2 solutions imply different mechanisms driven by electrostatic forces. Our results demonstrate that DNA nanoconfinement in smectites is likely an important strategy for DNA preservation and that protocols targeting low-charge smectites might improve the success of ancient and modern DNA extraction even in hot and humid climates so far deemed unfavourable for DNA preservation

Hydration of Na-saturated Synthetic Stevensite, a Peculiar Trioctahedral Smectite

International audienceSmectite interlayer water plays a key role on the mobility of elements and molecules, but also in a variety of geological processes. In contrast to saponite and hectorite, whose layer charge originates from isomorphic substitutions, stevensite layer charge originates from the presence of octahedral vacancies. Despite its common occurrence in lacustrine environments, hectorite hydration has received little attention, compared to saponite and hectorite. Early reports mention a specific hydration behavior, however, with the systematic presence of a low-angle reflection attributed to the regular interstratification of different hydration states. The present study aims at revisiting this specific hydration behavior in more depth. Within this scope, the hydration behavior of the above three smectite varieties are compared using synthetic trioctahedral smectites of similar layer charge, and different compositions of their octahedral sheets. The chemical composition of the octahedral sheet does not appear to influence significantly smectite hydration for saponite and hectorite. Compared to its saponite and hectorite equivalents, H2O content in stevensite is lower by ~2.0 mmol H2O per g of dry clay. Consistent with this lower H2O content, Zn-stevensite lacks a stable monohydrated state, dehydrated layers prevailing from 60 to 0% RH. The presence of the regular interstratification of 0W and 1W layers is responsible for the low-angle reflection commonly observed for stevensite under air-dried conditions. Finally, stevensite identification method based on X-ray diffraction of heated and EG-solvated samples is challenged by the strong influence of octahedral sheet chemical composition (Zn or Mg in the present study) on hectorite swelling behavior. The origin of this effect remains undetermined and further work is needed to propose a more general identification method

Amorphous Calcium–Magnesium Carbonate (ACMC) Accelerates Dolomitization at Room Temperature under Abiotic Conditions

The challenge to produce dolomite CaMg(CO3)2 at low temperature (20–35 °C) over laboratory time scales so far has remained unsuccessful, which has led to long-lasting scientific debates in the last two centuries. This mineral exerts a major control on the natural carbon dioxide sequestration into various sedimentary, basaltic, and mantellic rocks. The present study reports on specific abiotic conditions that allow the precipitation of disordered dolomite, high Mg calcite, and high Ca magnesite at room temperature over time scales of hours to days. Here we show that an amorphous calcium magnesium carbonate (ACMC) phase accelerates dolomitization at room temperature. ACMC is initially precipitated by mixing a carbonate (HCO3–/CO32– = 1; pH ∼10.3 ≈ pKa2) alkaline solution with a Mg-Ca ionic solution (Mg molar fraction between 0 and 1). Then, time-resolved in situ Raman spectroscopy monitored the transformation of ACMC into Mg-rich carbonate minerals. The initial Mg molar fraction controlled both the reaction mechanism (e.g., nature of transient crystalline phases) and the kinetics. Nanosized crystallites with short-range order, called disordered dolomite CaMg(CO3)2, precipitated following a complex reaction pathway. First, nesquehonite (MgCO3·3H2O: nucleation time 2.5 h) and then disordered dolomite (CaMg(CO3)2: nucleation time 3.2 h) followed by monohydrocalcite (CaCO3·H2O: nucleation time 3.4 h) formed from ACMC transformation. Nesquehonite and monohydrocalcite are transient phases that nourish the slow precipitation of disordered dolomite, which reached a spectral equilibrium after 7 days of reaction. The direct transformation of ACMC into disordered dolomite was also measured. Our experimental results demonstrate that disordered dolomite precipitates at room temperature when an ideal Mg/Ca ratio, high carbonate alkalinity, and high ionic concentration are reached in abiotic systems. This result suggests the possibility of a physicochemical rather than biotic control on the formation of disordered dolomite at low temperature in several geosystems

Mineralogical differences in a temperate cultivated soil arising from different agronomic processes and plant K-uptake

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