Supercritical Aqueous Fluids Research Articles

A Volcanological Paradox in a Thin-Section: Large Explosive Eruptions of High-Mg Magmas Explained Through a Vein of Silicate Glass in a Serpentinized Peridotite Xenolith (Hyblean Area, Sicily)

Ultramafic magmas (MgO ≥ 18 wt%) are generally thought to be primary mantle melts formed at temperatures in excess of 1600 °C. Volatile contents are expected to be low, and accordingly, high-Mg magmas generally do not yield large explosive eruptions. However, there are important exceptions to low explosivity that require an explanation. Here we show that hydrous (hence, potentially explosive) ultramafic magmas can also form at crustal depths at temperatures even lower than 1000 °C. Such a conclusion arose from the study of a silicate glass vein, ~1 mm in thickness, cross-cutting a mantle-derived harzburgite xenolith from the Valle Guffari nephelinite diatreme (Hyblean area, Sicily). The glass vein postdates a number of serpentine veins already existing in the host harzburgite, thus reasonably excluding that the melt infiltrated in the rock at mantle depths. The glass is highly porous at the sub-micron scale, it also bears vesicles filled by secondary minerals. The distribution of some major elements corresponds to a meimechite composition (MgO = 20.35 wt%; Na2O + K2O < 1 wt%; and TiO2 > 1 wt%). On the other hand, trace element distribution in the vein glass nearly matches the nephelinite juvenile clasts in the xenolith-bearing tuff-breccia. These data strongly support the hypothesis that an upwelling nephelinite melt (MgO = 7–9 wt%; 1100 ≤ T ≤ 1250 °C) intersected fractured serpentinites (T ≤ 500 °C) buried in the aged oceanic crust. The consequent dehydroxilization of the serpentine minerals gave rise to a supercritical aqueous fluid, bearing finely dispersed, hydrated cationic complexes such as [Mg2+(H2O)n]. The high-Mg, hydrothermal solution "flushed" into the nephelinite magma producing an ultramafic, hydrous (hence, potentially explosive), hybrid magma. This hypothesis explains the volcanological paradox of large explosive eruptions produced by ultramafic magmas.

Preparation of Fine-Grained Corundum Powders with Given Properties: Crystal Size and Habit Control

The processes of obtaining fine-grained corundum with a given crystal size and habit are considered. It is shown that thermocouple treatment in aqueous supercritical fluid allows one to obtain corundum from various precursors in a wide range of crystal sizes (0.01–500 μm). Powders with a crystal size in the range 20–500 μm can be obtained directly from hydrargillite prepared in the course of the Bayer Process. The size and habit of the crystals are determined by the parameters of the thermocouple treatment and properties of the process activators used. Powders with a crystal size falling in the range 30–1000 nm can be obtained by using synthesized or commercially available boehmite as a precursor for the thermocouple treatment. The crystal habit is determined by the habit of the boehmite crystals used. New methods of boehmite synthesis have been developed that allow one to obtain nonaggregated crystals with a size of 10 nm with a narrow distribution curve.

Experimental and simulation study of carbon dioxide, brine, and muscovite surface interactions

Capture and subsequent geologic storage of CO2 in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. Subsurface injection of CO2 is also used industrially in enhanced oil and natural gas recovery operations to increase the amount of hydrocarbon that can be economically recovered from a geologic reservoir. The interaction of CO2 and brine species with mineral surfaces controls the ultimate fate of injected CO2 at the nanoscale via surface chemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. High resolution micro X-ray CT scanning, optical contact angle measurements, and large scale molecular dynamics simulations were used to investigate the behavior of supercritical CO2 and aqueous fluids on basal surfaces of muscovite, a common phyllosilicate mineral. Experimental results demonstrate partial wetting by the aqueous phase and a dependence of contact angle upon aqueous phase brine composition. This contrasts with simulation results, which predict that supercritical CO2 forms a non-wetting droplet, separated from direct interaction with the muscovite surface by distinct layers of water and charged species. Simulations with trace amounts of acetate or acetic acid added to the CO2/water/mineral system were used to investigate the potential effect of contamination with small organic molecules. While the observed contact angle was not significantly altered, these simulations demonstrate the influence of pH on species partitioning, with acetic acid molecules partitioning to the CO2/water interface and acetate ions adsorbing to the mineral surface. Similar simulations using hexanoate displayed a greater surfactant effect and significantly increased wetting by the CO2 phase, suggesting that small concentrations of secondary species or contaminants can significantly influence macroscopic wetting behavior.

X-ray absorption spectroscopy studies in supercritical aqueous solutions

ABSTRACTThe past two decades have witnessed significant advances in the application of X-ray absorption spectroscopy (XAS) to the study of supercritical aqueous fluids. The data obtained using in situ XAS have provided insights into the stability and the structure of metal complexes that are fundamental to understanding natural and industrial hydrothermal processes. Important recent advances using XAS can be attributed to the use of new high temperature and pressure autoclaves designed specifically for the analysis of fluids at extreme temperatures and pressures, improved techniques for the acquisition of X-ray absorption spectra and molecular-level computational modelling used in association with XAS analysis. High-brilliance light sources have not only provided new opportunities for XAS investigations of supercritical fluids, but have also revealed the effects of beam-induced radiolysis of the same fluids. The advent of energy-dispersive and rapid-acquisition XAS holds promise for future studies of beam-induced radiolysis and of the kinetics associated with the formation of metal complexes in high-temperature fluids.

Intermetallic compounds, copper and palladium alloys in Au–Pd ore of the Skaergaard pluton, Greenland

Copper–palladium intermetallic compounds and alloys (2314 grains) from the Au–Pd ore of the Skaergaard layered gabbroic pluton have been studied. Skaergaardite PdCu, nielsenite PdCu3, (Cu,Pd)β, (Cu,Pd)α, (Pd,Cu,Au,Pt) alloys, and native palladium have been identified as a result of 1680 microprobe analyses. The average compositions and various chemical varieties of these minerals are characterized, as well as vertical and lateral zoning in distribution of noble metals. The primary Pd–Cu alloys were formed within a wide temperature interval broadly synchronously with cooling and crystallization of host gabbro and in close association with separation of Fe–Cu sulfide liquid. In the course of crystallization of residual gabbroic melt enriched in iron, noble and heavy metals and saturated with the supercritical aqueous fluid, PGE and Au are selectively concentrated in the Fe–Cu sulfide phase as Pd–Cu and Cu–Au alloys.

Zr/Hf ratio in supercritical chloride fluids: Experimental study of zirconium and hafnium complexation at 450°C and 0.6–1 kbar

The solubility method was used to study the Zr and Hf behavior in supercritical aqueous chloride fluids (m(HCl) = 0.5–1.5, m(NaCl) 1 and 2 mol kg−1 H2O) at 450°C and 0.6–1 kbar. The crystals of natural baddeleyite (Kovdor Deposit) served as a solid phase in all experiments. An increase in HCl and NaCl concentrations leads to the increase of Zr and Hf contents in solutions. Zirconium mainly occurs as ZrOHCl3(aq)0 and NaZrOHCl4(aq)0 complexes, while dominant hafnium species are HfOHCl3(aq)0, HfCl4(aq)0, and NaHfOHCl4(aq)0 or NaHfCl5(aq)0. An increase of acid concentration is accompanied by the decrease of Zr/Hf ratio (Hf enriches fluid), which results from difference in acid-base properties of these elements and may lead to their separation in natural processes.

Complexation of Zr and Hf monomers in supercritical aqueous solutions: Insights from ab initio molecular dynamics simulations

The enhanced mobilization of high field strength elements (HFSE) by certain geological fluids at high pressures, P, and high temperatures, T, as indicated by field observations and recent solubility measurements is most likely related to the type of complex formation in the fluid phase. However, only a few in situ experimental studies have been performed so far to constrain HFSE speciation in supercritical aqueous fluids. Here, we complement these investigations by ab initio molecular dynamics simulations to explore the complexation of Zr4+ and Hf4+ monomers in aqueous solutions at T=1000K and P∼1GPa. Regardless of the type of fluid (basic, neutral, acidic) both Zr and Hf seem to prefer an octahedral coordination. By systematically changing fluid composition and coordinating anions, the structure of various [(Zr,Hf)Clk(OH)m(H2O)n]4−k−m(k=0–6,m=2–6,n=0–4) complexes is investigated. For each complex that is stable on the time scale of the simulation, theoretical X-ray absorption spectra are calculated and compared to experimental data. From this comparison, the most likely complexes in HCl solutions are [(Zr,Hf)Cl3(Cl,OH)2(H2O)]−. The differences between experimental and theoretical XANES spectra present in the case of NaOH solutions indicate HFSE speciation beyond monomers in this system. Charge-neutral [(Zr,Hf)(OH)4(H2O)1–2] complexes, which may be the dominant species in neutral fluids at very low HFSE concentrations, show the lowest coordination (5.2(1)) of all investigated species. Finally, differences in the complexation of Zr and Hf in aqueous solutions at ambient and supercritical conditions and the possible formation of more complex oligomeric species are discussed.

Molecular Simulation of Carbon Dioxide, Brine, and Clay Mineral Interactions and Determination of Contact Angles

Capture and subsequent geologic storage of CO2 in deep brine reservoirs plays a significant role in plans to reduce atmospheric carbon emission and resulting global climate change. The interaction of CO2 and brine species with mineral surfaces controls the ultimate fate of injected CO2 at the nanoscale via geochemistry, at the pore-scale via capillary trapping, and at the field-scale via relative permeability. We used large-scale molecular dynamics simulations to study the behavior of supercritical CO2 and aqueous fluids on both the hydrophilic and hydrophobic basal surfaces of kaolinite, a common clay mineral. In the presence of a bulk aqueous phase, supercritical CO2 forms a nonwetting droplet above the hydrophilic surface of kaolinite. This CO2 droplet is separated from the mineral surface by distinct layers of water, which prevent the CO2 droplet from interacting directly with the mineral surface. Conversely, both CO2 and H2O molecules interact directly with the hydrophobic surface of kaolinite. In the presence of bulk supercritical CO2, nonwetting aqueous droplets interact with the hydrophobic surface of kaolinite via a mixture of adsorbed CO2 and H2O molecules. Because nucleation and precipitation of minerals should depend strongly on the local distribution of CO2, H2O, and ion species, these nanoscale surface interactions are expected to influence long-term mineralization of injected carbon dioxide.

In situ XANES Study of Co2+ Ion Adsorption on Fe3O4 Nanoparticles in Supercritical Aqueous Fluids

ABSTRACTIn situ x-ray absorption spectroscopy (XAS) measurements were made on Fe3O4 nanoparticles in supercritical aqueous fluids to 500 °C in order to study their reactivity with Co2+ aqua ions and to investigate the structural properties of the reacted nanoparticles. The analyses of the x-ray absorption near edge structure (XANES) of XAS indicate that reactivity of Fe3O4 nanoparticles with Co2+ ions is minimal to 200 °C but becomes significant in the 250–500 °C temperature range. XANES and angular momentum projected density of states (l-DOS) calculations were carried out using the FEFF8.2 code and analyses were made using multi-peak fitting to determine the origin of the features exhibited in the spectra.

In Situ X-ray Absorption Spectroscopic Study of the Adsorption of Ni2+on Fe3O4Nanoparticles in Supercritical Aqueous Fluids

In situ X-ray absorption spectroscopy (XAS) measurements were made on Fe3O4 nanoparticles in supercritical aqueous fluids to study their reactivity with Ni2+ ions and to investigate the structural properties of the reacted nanoparticles. XAS spectra were measured from Ni2+ added to an aqueous solution containing Fe3O4 nanoparticles at temperatures up to 500 °C. The XANES and the EXAFS data indicate that reactivity of Fe3O4 nanoparticles with Ni2+ ions is negligible to 300 °C but becomes significant in the 400–500 °C temperature range. Analysis of Ni K-edge extended X-ray absorption fine structure (EXAFS) using an atomic cluster model based on the spinel structure shows that Ni2+ adsorbs predominantly on octahedral sites of the surface of nanoparticles in aqueous fluids at 400–500 °C. The EXAFS structural results are consistent with diffusion of Ni into the near-surface region of the Fe3O4 nanoparticles in the aqueous fluid at 400 and 500 °C.

An in situ X-ray spectroscopic study of Mo6+ speciation in supercritical aqueous solutions

In situ XRF and Mo K-edge XAS measurements were made on the ID20-B beam line at the APS on MoO3 in 1 M H2O2 aqueous solution, at temperatures between 400 and 600 °C. The samples were analyzed using a modified Bassett-type hydrothermal diamond anvil cell. Our XRF measurements show that MoO3 is highly soluble in the supercritical H2O2 aqueous fluid. Analysis of XAS spectra shows that the Mo6+ ion exhibits consistent speciation in the H2O2 aqueous solution at temperatures ranging from 400 to 600 °C.

Sustainable oil-in-water analysis using a supercritical fluid carbon dioxide extraction system directly interfaced with infrared spectroscopy

Abstract A direct aqueous supercritical fluid extraction (SFE) system using carbon dioxide provides a sustainable means by which a vast range of industries may continue to depend on well established infrared (IR) techniques to determine oil-in-water. The SFE-IR method provides an environmentally friendly substitute for current national standard IR reference methods for measuring oil-in-water that rely on using increasingly restricted ozone depleting solvents whose manufacture is being phased out in accordance with international law. The SFE-IR analysis of a 500 mL water sample can be accomplished in 15 min. A rapid on-line SFE-IR calibration method has been implemented. With this calibration method, SFE-IR accuracy for determining diesel oil in 500 mL spiked water samples using single wave number measurement was 86.0%-98.8% with precision (RSD) ranging from 2.5%-7.0%. Using a general purpose calculation which involves measuring infrared absorbance values at three different wave numbers, SFE-IR method accuracy for determining diesel oil in 500 mL spiked water samples was 83.7%-92.2% with RSD 1.0%-9.3%. Data is presented that indicates current long established national standard IR reference methods involving three wave number calculations should be reviewed since, without careful consideration, the inclusion of calculated aromatic hydrocarbon species contributions to final oil-in-water concentration values may provide less accurate results.

Direct aqueous supercritical fluid extraction coupled on-line with liquid chromatography–tandem mass spectrometry for the analysis of polyether ionophore antibiotics in water

A direct aqueous SFE system designed to extract water samples contained in vials has been coupled on-line with a reverse phase LC–MS–MS system using a single 10-port valve. An SFE trap system using C 1 stationary phase connected to a C 18 analytical HPLC column enabled the SFE–LC–MS–MS analysis of three polyether ionophore antibiotics in water using a step gradient. A quantitative SFE–LC–MS–MS method has been developed whereby the progress of SFE can be monitored directly on-line such that ionophore recovery profile data from a single water sample can be obtained. Using a continuous direct aqueous SFE period of 75 min, the SFE–LC–MS–MS recoveries of the ionophores were: monensin 76.2% with RSD 4.1%, lasalocid 84.6% with RSD 3.8% and narasin 91.2% with RSD 3.2%. With positive ion electrospray ionization, the SFE–LC–MS–MS system using a 4 mL water sample provided multiple reaction monitoring (MRM) limits of detection for monensin and lasalocid each equivalent to 90 ng/L whereas 30 ng/L for narasin. A two-way valve controlling carbon dioxide distribution to the SFE vessel has provided a means for the initial investigation of the recovery of ionophore sodium salts from water using static SFE.

Composition of Li-F granite melt and its evolution during the formation of the ore-bearing Orlovka massif in Eastern Transbaikalia

By the example of the Orlovka massif of Li-F granites in Eastern Transbaikalia, the major- and trace-element (Li, Be, B, Ta, Nb, W, REE, Y, Zr, and Hf) compositions of the parental melt and the character of its variations during the formation of the differentiated rock series were quantitatively estimated for the first time on the basis of electron and ion microprobe analysis and Raman spectroscopy of rehomogenized glasses of melt inclusions in quartz. It was shown that the composition of the Orlovka melt corresponded to a strongly evolved alumina-saturated granitoid magma (A/CNK = 1.12–1.55) rich in normative albite, poor in normative quartz, and similar to ongonite melts. This magma was strongly enriched in water (up to 9.9 ± 1.1 wt %) and fluorine (up to 2.8 wt %). Most importantly, this massif provided the first evidence for high B2O3 contents in melts (up to 2.09 wt %). The highest contents of trace elements were observed in the melt from pegmatoid bodies in the amazonite granites of the border zone: up to 5077 ppm Li, 6397 ppm Rb, 313 ppm Cs, 62 ppm Ta, 116 ppm Nb, and 62 ppm W. Compared with the daughter rock, the Orlovka melt was depleted at all stages of formation in SiO2 (by up to 6 wt %), Na2O (by up to 2.5 wt %), and, to a smaller extent, in Ti, Fe, Mg, Sr, and Ba, but was enriched in Mn, Rb, F, B, and H2O. Two stages were distinguished on the basis of the behavior of trace elements and fluorine in the melts and rocks. The early stage, from the biotite granites of the parental massif to the microcline-albite granites with the pea-shaped quartz is characterized by a decrease in Si, Fe, Ca, Mg, Zr, Hf, and REE and accumulation of Al, Na, Li, Rb, F, Ta, and Nb, which correspond to the ongonite differentiation trend. During the second stage, amazonite-bearing rocks with Li micas and Ta mineralization (columbite-tantalite and microlite) were produced, and the melt was depleted in Al, Na, Li, F, Nb, and Ta. A considerable difference appeared between the compositions of rocks and aluminosilicate melts. There is a paradoxical discrepancy between the high contents of Li, Ta, and Nb in the rock (2289, 446, and 269 ppm, respectively) and the low contents of these elements in the melt from the amazonite granites (554 ppm Li, 23 ppm Ta, and 116 ppm Nb). This discrepancy could be related to quartz crystallization after the fractionation of Li-F micas, albite, topaz, columbite-tantalite, and microlite, which resulted in that the quartz trapped inclusions of already depleted melt. On the other hand, the dramatic depletion of the residual melt in Na, Al, Li, F, Ta, and Nb suggests the possible separation of a specific hydrosaline aluminofluoride melt, which accumulated Nb, Ta, and REE in some experimental systems. This suggestion was supported by the results of the investigation by melt and fluid inclusions in beryl from the pegmatoid bodies of Orlovka, which established the coexistence of two immiscible aluminosilicate melts and a CO2-rich supercritical aqueous fluid (Thomas et al., 2009). One of these melts closely corresponds to the volatile-rich hydrosaline and relatively subalkaline melt with high Li (2.09 wt %) and F contents (2.93 wt %). The Raman spectra of the coexisting fluid indicated the presence of columbite, which implies a Ta content in the fluid of approximately 6500 ppm.

Experimental constraints on Pt, Pd and Au partitioning and fractionation in silicate melt–sulfide–oxide–aqueous fluid systems at 800 °C, 150 MPa and variable sulfur fugacity

We have performed experiments to constrain the effect of sulfur fugacity ( fS 2) and sulfide saturation on the fractionation and partitioning behavior of Pt, Pd and Au in a silicate melt–sulfide crystal/melt–oxide–supercritical aqueous fluid phase–Pt–Pd–Au system. Experiments were performed at 800 °C, 150 MPa, with oxygen fugacity ( fO 2) fixed at approximately the nickel–nickel oxide buffer (NNO). Sulfur fugacity in the experiments was varied five orders of magnitude from approximately log fS 2 = 0 to log fS 2 = −5 by using two different sulfide phase assemblages. Assemblage one consisted initially of chalcopyrite plus pyrrhotite and assemblage two was loaded with chalcopyrite plus bornite. At run conditions pyrrhotite transformed compositionally to monosulfide solid solution (mss), chalcopyrite to intermediate solid solution (iss), and in assemblage two chalcopyrite and bornite formed a sulfide melt. Run-product silicate glass (i.e., quenched silicate melt) and crystalline materials were analyzed by using both electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The measured concentrations of Pt, Pd and Au in quenched silicate melt in runs with log fS 2 values ranging from approximately 0.0 to −5.0 do not exhibit any apparent dependence on fS 2. The measured Pt, Pd and Au concentrations in mss do vary as a function of fS 2. The measured Pt, Pd and Au concentrations in iss do not appear dependent on fS 2. The data suggest that fS 2, working in concert with fO 2, via the determinant role that these variables play in controlling the magmatic sulfide phase assemblage and the solubility of Pt, Pd and Au as lattice bound components in magmatic sulfide phases, is a controlling factor on the budgets of Pt, Pd and Au during the evolution of magmatic systems.

Oil-in-water analysis using supercritical fluid extraction interfaced with fixed wavelength infrared detection

The combination of a direct aqueous supercritical fluid extraction system interfaced to a fixed wavelength infrared detector; measuring CH2 (νasymmetric) absorbance at 2930 cm−1, has been successfully developed for the analysis of oil-in-water. Using an optional, in-line silica gel treatment procedure, method accuracy for determining Brent Delta crude oil in spiked 500 mL water samples was 92.0% to 94.5% with RSD 4.7% to 6.5%. The supercritical fluid extraction-infrared method enables a second analysis of the same water sample without silica gel treatment. For second sets of analyses without silica gel treatment, method accuracy for determining Brent Delta crude oil in spiked 500 mL water samples was 87% to 96.0% with RSD 7.5% to 9.5%. Results of this study indicate that the silica gel treatment procedure reduces the calculated level of Brent Delta crude oil-in-water by 6.6–12.4% relative to samples analysed without silica gel treatment. The results of a study involving Fourier transform infrared spectroscopy indicate a limit of detection for n-decane of approximately 0.5 mg L−1 by measuring CH2 (νasymmetric) absorbance using the supercritical fluid extraction-infrared method. Sample preparation using direct aqueous supercritical fluid carbon dioxide extraction provides an indefinite means for the use of infrared techniques to measure oil-in-water.

Percolation and clustering in supercritical aqueous fluids

Neutron diffraction data for pure supercritical water and supercritical mixtures of water andCO2 are analyzed by using the empirical potential structure refinement Monte Carlosimulation, and molecular configurations compatible with the experimental dataare recorded. The analysis of the distribution functions of water cluster size inthese fluids allows the identification of a percolation line, which separatesgas-like states from liquid-like ones, in pure supercritical water. Solvation ofCO2 in supercritical water inhibits cluster percolation, although the radial distribution functions showliquid-like behavior: this is likely to be due to the excess volume being localized around theCO2 molecules.

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite–quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H 2O with solids up to 30–70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

In situ X-ray absorption spectroscopy measurement of vapour-brine fractionation of antimony at hydrothermal conditions

AbstractDespite the growing geological evidence that fluid boiling and vapour-liquid separation affect the distribution of metals in magmatic-hydrothermal systems significantly, there are few experimental data on the chemical status and partitioning of metals in the vapour and liquid phases. Here we report on an in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of antimony speciation and partitioning in the system Sb2O3-H2O-NaCl-HCl at 400°C and pressures 270—300 bar corresponding to the vapour-liquid equilibrium. Experiments were performed using a spectroscopic cell which allows simultaneous determination of the total concentration and atomic environment of the absorbing element (Sb) in each phase. Results show that quantitative vapour-brine separation of a supercritical aqueous salt fluid can be achieved by a controlled decompression and monitoring the X-ray absorbance of the fluid phase. Antimony concentrations in equilibrium with Sb2O3 (cubic, senarmontite) in the coexisting vapour and liquid phases and corresponding SbIII vapour-liquid partitioning coefficients are in agreement with recent data obtained using batch-reactor solubility techniques. The XAFS spectra analysis shows that hydroxy-chloride complexes, probably Sb(OH)2Cl0, are dominant both in the vapour and liquid phase in a salt-water system at acidic conditions. This first in situ XAFS study of element fractionation between coexisting volatile and dense phases opens new possibilities for systematic investigations of vapour-brine and fluid-melt immiscibility phenomena, avoiding many experimental artifacts common in less direct techniques.

Oxidation of a coal particle in flow of a supercritical aqueous fluid

A study was made of the conversion of single spherical coal particles of diameter 1–5 mm in a supercritical H2O/O2 fluid with an oxygen mass fraction of 0–6.6% in a semibatch reactor at a pressure of 30 MPa and a temperature of 673–1023 K. A decrease in the particle mass was observed in two parallel processes: gasification of coal with water and oxidation of coal with oxygen. An activation energy 19 ± 7 kJ/mole and a pre-exponential factor 10−2±0.4 sec−1 were obtained under the assumption of zero order for the concentration H2O and an Arrhenius dependence for the rate of gasification with water. The oxidation with oxygen at a temperature above 780 K was found to be limited by the rate of O2 diffusion to the coal organic matter. Below 780 K, the rate of heterogeneous oxidation with oxygen is described by a first-order reaction for the concentration of O2 and a zero-order reaction for the concentration of H2O with an activation energy of 150 ± 27 kJ/mole and a pre-exponential factor of 107.6±1.9 cm3/(g · sec).