Graphitizing anthracene-based cokes and non-graphitizing saccharose-based chars were processed at temperatures from 450°C to 2900°C at ambient pressure. This offers a whole set of samples that ...greatly differ in structure. Here, their structural evolution was monitored by combining XRD and visible (green) Raman spectroscopy as well as, for the first time, near-infrared Raman and synchrotron-based C-XANES spectroscopies. These different techniques provide complementary information especially regarding the spatial resolution they achieved. However, despite its importance, the quantitative comparison between the structural parameters extracted from these techniques is difficult. Based on a new signal deconvolution procedure to extract quantitative structural information from C-XANES data and the achievement of a new dataset on a complete series of graphitization, the reliability and the precision of the information which can be retrieved from each technique are discussed. C-XANES spectroscopy appears to provide reliable proxies for the extent of aromatic layers of graphitic compounds and an empirical calibration is proposed.
The Longmen Shan (Sichuan, China) is characterised by an unusual morphology which results from a Triassic prism tectonics and a more recent Neogene thick-skin thrusting. Among its specific features ...is the high elevation of the internal zones in continuity with the Songpan Garze fold-and-thrust belt (SG), which is associated with an abrupt 20
km Moho offset between the Sichuan Basin and the Tibetan plateau as revealed by the analysis of teleseismic data acquired by a dense seismic network. The jump in crustal thickness is located at the apex of the Wenchuan shear zone (WSZ) and marks the western boundary of the metamorphosed units of SG characterized by temperatures varying from 590
°C down to 300
°C as commonly observed in mature accretionary wedges. Both the structural style marked by intense shortening in tight kink folds and geophysical data suggest the presence of an horizontal discontinuity at ∼
15
km depth over a thick crust (∼
63
km).
The Longmen Shan east of the WSZ is characterized by thick-skin detachment which thrusts the internal sedimentary units and the Proterozoic basement over the series of the Sichuan Basin deposited on a thinner crust of ∼
44
km thick. The major front is the Beichuan Fault Zone (BFZ) which brought the internal zones onto Triassic and Jurassic series with lower temperatures (less than 400
°C). Locally, temperatures of ∼
425
°C are found just below the klippes.
These results are in agreement with an original contact of the SG zone represented by an accretionary wedge of sediments thrusted over the margin of the continental crust of the Yangtze craton in the early stage (Indosinian) of the evolution. The present-day slow E–W component of the convergence, added to the difference in crustal thickness caused the Yangtze crust to indent the Songpan Tibetan crust which was softened by a high thermal regime. As a response the edge of the Tibet crust was inflated to the bottom (up to 70
km), whereas to the top, the crystalline massif were exhumed and pushed the deformation eastward as emerging and blind thrusts. This configuration reflects a moderate shortening of the crust which behaves as a soft thick material abutting the resistant and cold Yangtze crust.
The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C–S–H), and their structural changes due to aqueous carbonation have been characterized using TEM, ...nitrogen physisorption, and SAXS. Synthetic C–S–H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C–S–H types. When submitting the sample to a CO
2
flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m
2
/g, and its structure, formed by particles of ~5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C–S–H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.
The production of H2 by oxidation of FeO, taken here as model compound for steel slags, has been investigated both in pure water and under acidic aqueous conditions in the 373–573 K temperature ...range. Whereas after 65 h, H2 yield was negligible in pure water at 423 K, the reaction 3 FeO(s) + H2O(l) → Fe3O4(s) + H2(aq) reached near completion at the same temperature within 10 h in a solution containing 0.05 mol/l acetic acid. Increasing acetic acid concentration by one order of magnitude did not yield significantly more H2. At identical initial pH, acetic acid was found to be more efficient than oxalic acid and hydrochloric acid at enhancing H2 production. Acidic conditions increased FeO dissolution kinetics and, consequently, improved H2 yield. The specific efficiency of acetic acid resides in its thermal stability as well as in the potential of ligand-promoted Fe(II) dissolution. We show that the positive kinetics effect of mild acetic acid solutions over H2 yield evidenced on FeO does not apply directly to steel slags which buffer the pH to high values due to the presence of large amounts of CaO.
•Hydrogen production from the FeO – H2O redox system.•Kinetic study performed under hydrothermal conditions.•Influence of pH and ligand-promoted dissolution on FeO oxidation kinetic.•Identified efficiency of dilute acetic acid.•Extrapolation to CaO–FeO mixes, steel slag equivalent system.
•Aqueous Ca(OH)2 carbonation is a powerful method to remove Fe(II) from water.•Portlandite Ca(OH)2 can also act as softening agent prior to carbonation process.•Obtained solid-residues (calcite+FeOOH ...nanoparticles) could find industrial applications.
Fresh groundwater is sometimes enriched with dissolved ferrous iron (Fe(II)) that restricts its consumption as potable water because it forms colloidal red matter (mainly ferric oxyhydroxides) under oxic conditions at near neutral pH (>6) conditions. As already demonstrated, natural or synthetic calcite material can be used to accelerate the iron oxidation process from Fe(II) to Fe(III), a process that then enhances its precipitation at the calcite-solution interface as confirmed by in situ atomic force microcopy (AFM) observations in this study. The present study mainly reports on a simplified water treatment method to remove ferrous iron (Fe(II)) from water via aqueous carbonation of calcium hydroxide (Ca(OH)2) at ambient temperature (≈20°C) and moderate CO2 pressure (10bar) conditions. In practice, high concentrations of dissolved iron (up to 100mg/L) can be successfully removed using only 4g of Ca(OH)2 per liter of Fe-rich solution (close to 100% of efficiency) and a short treatment time is required (<1h). This method offers various advantages compared with other calcite-based water treatments. For example, other pre-existent dissolved toxic and eutrophic ions such as As, Cu, Cd, Se, P, S, N, etc. can be simultaneously removed from water during the precipitation of calcite and iron oxyhydroxide nanoparticles (<100nm). Additionally, the dissolution of calcium hydroxide prior to the carbonation process increases the pH (12.4), a process that can act as a softening agent in the water being treated. Finally, the resultant red solid-residue containing mainly calcite and iron oxyhydroxide (FeOOH type) nanoparticles could be reused as pigment or mineral filler powder for industrial applications. This integrated method could be used successfully to remove toxic dissolved ions from water while generating solid residues with industrial uses.
► A simple and innovative synthesis route for goethite–calcite nanocomposite. ► Nanosized goethite particles adhered onto sub-micrometric calcite. ► Goethite–calcite composite has a well ...sequestration capacity for toxic ions.
This study proposes a simple and innovative synthesis route for a goethite–calcite nanocomposite. This synthesis is summarised 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 92m2/g for a composite synthesized at 30°C and 45m2/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.
► Gas–solid carbonation of powdered portlandite is total at high
P
CO
2
, regardless of
T. ► Initial calcite growth rate ranges from 0.27 to 0.35
nm/s for 30
<
T
<
60
°C and
(
P
CO
2
=
20
bar
)
. ► ...Complex processes (calcite dissolution–reprecipitation) occur at high extent of reaction.
The linear growth rate is an essential parameter to describe and simulate the crystal growth processes of solid materials. In the present study, two independent methods were used to estimate how calcite particle size was increasing with reaction time. First, a direct method by using Rietveld refinements of X-ray diffraction patterns quantifies the variation of the coherent domain average size
r with reaction time
t. Second, we used a mass balance method where the formed calcite amount
Mol
CaCO
3
,
t
was determined as a function of time and the linear growth rate was deduced from mass growth rate. For both methods, a kinetic pseudo-second-order expression was successfully used to fit the data and estimate the initial growth rates of nanosized calcite. The results deduced from Rietveld refinements showed that such rates were roughly equivalent for two different temperature conditions, i.e. 0.35
nm/s at 30
°C and 20
bar, and 0.27
nm/s at 60
°C and 20
bar. However, these results were significantly different from those deduced from mass growth rate. For this case, values of 0.09
nm/s at 30
°C and 20
bar, and 0.06
nm/s at 60
°C and 20
bar were determined. This significant discrepancy could be ascribed to other simultaneous processes during crystal growth of calcite such as agglomeration and possibly dissolution–reprecipitation reactions, complicating considerably the measurement of linear growth rate by mass balance. The latter process was possibly enhanced by the release of water into the reactor during the gas–solid carbonation reaction (Ca(OH)
2
+
CO
2
→
CaCO
3
+
H
2O), and suggested to be experimentally evidenced for reaction extents greater than 80%. Taken together, these results suggest that the method based on Rietveld refinements may be more reliable to determine initial linear growth rates for reactions initiated in biphasic (gas–solid) systems, whereas both methods were previously demonstrated to be equivalent for triphasic systems.
In western Vanoise (French Alps), karstic pockets of Triassic‐to‐Jurassic metabauxites embedded in carbonate rocks and containing several generations of metamorphic veins were studied. During ...blueschist facies metamorphism, a cumulative amount of ∼13 vol% of water is inferred to have been produced locally by successive dehydration reactions, and part of this fluid remained in the bauxitic lenses during most of the metamorphic cycle. Field and geochemical evidence show that these rocks have been isolated from large‐scale fluid flow (closed‐system behaviour). It is proposed that the internally derived fluid has promoted the opening of fluid‐filled open spaces (as attested by the euhedral habits of vein minerals) and served as medium for mass transfer from rock to vein. Indeed, the vein infill is obviously the result of chemical interactions, at the millimetre‐to‐centimetre scale, between the rock minerals and the locally produced aqueous fluid. Two vein types can be distinguished based on mineralogical and textural features: (i) some veins are filled with newly formed products of either prograde (chloritoid) or retrograde (chlorite) metamorphic reactions; in this case, fluid‐filled open spaces seem to offer energetically favourable nucleation/growth sites; (ii) the second vein type is infilled with cookeite or pyrophyllite, that were present in the host rock prior to the vein formation. In this closed chemical system, the components for the vein infill minerals have been transferred from rock to vein through the fluid, in a dissolution–transport–precipitation process, possibly stress‐assisted. These different vein generations all contain Al‐rich mineral infills, suggesting that Al was a mobile element (cm scale) during metamorphism. In these HP rocks, fluid flow may have been restricted, and if so mass transfer occurred by diffusion in an almost stagnant fluid. Metamorphic veins can be seen as witnesses of fluid and mass redistribution that partly accommodate the rock deformation (lateral segregation).
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