Hydrothermal Medium Research Articles

Changeable hydrothermal media as potential cradle of life on a planet

According to the elaborated systemic concept, a prebiotic microsystem has a chance to be transformed into primary living unit (probiont) only under nonequilibrium oscillating conditions. This transition proceeds through three successive kinds of transformations. First, a prebiotic microsystem must acquire the specific critical properties being at the unstable state of the bifurcate transition. Then it should be relatively stabilized due to balanced oscillations between the initial and the forward states (there appears the paradoxical state of ‘stabilized instability’). The decisive transformation to the living state is connected with the inversion of the ratio of ‘free energy to entropy contribution’ when the free energy input in the microsystem becomes higher than the entropy input. Changeable nonequilibrium conditions are most typical to hydrothermal systems in comparison with other aqueous media of the Earth. In this context the hydrothermal medium seems very suitable to be considered as a potential cradle of life on a planet. To characterize a scale of the thermodynamic and physico-chemical fluctuations some hydrothermal systems in Kamchatka peninsula were explored. The period of the pressure microoscillations in Mutnovsky and Pauzhetsky hydrothermal systems range from 10 to 30 min, and the amplitude is 0.2–0.7 bar. From time to time the amplitude of the fluctuations significantly rises due to spontaneous increase of activity of the nearest volcanoes. Similar results were obtained during the exploration of thermodynamic and chemical fluctuations in Mura hydrothermal field in Slovenia: the period of the water pressure oscillations is 60–70 min, and the average amplitude about 0.5 bar. Temperature and some chemical parameters oscillate in this field with the same period. These data can be used to conduct advanced laboratory experiments on prebiotic organic synthesis under nonequilibrium oscillating conditions. Some universal aspects of this concept may allow us to better understand the origin-of-life process in the Universe.

Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies

Hydrothermal technologies are broadly defined as chemical and physical transformations in high-temperature (200–600 °C), high-pressure (5–40 MPa) liquid or supercritical water. This thermochemical means of reforming biomass may have energetic advantages, since, when water is heated at high pressures a phase change to steam is avoided which avoids large enthalpic energy penalties. Biological chemicals undergo a range of reactions, including dehydration and decarboxylation reactions, which are influenced by the temperature, pressure, concentration, and presence of homogeneous or heterogeneous catalysts. Several biomass hydrothermal conversion processes are in development or demonstration. Liquefaction processes are generally lower temperature (200–400 °C) reactions which produce liquid products, often called “bio-oil” or “bio-crude”. Gasification processes generally take place at higher temperatures (400–700 °C) and can produce methane or hydrogen gases in high yields.

Three stages of the origin of life process: bifurcation, stabilization and inversion

AbstractThe principal succession of transformations of a prebiotic microsystem leading to its transition into the primary living state is theoretically substantiated. For the first stage of the succession, a significant change in the external conditions constrains an organic microsystem to leave the current stable state with the following transition into a new stable state through the unstable critical (bifurcation) point. At the bifurcation point the microsystem acquires the original properties without which life cannot exist (self-maintaining heterogeneous structure, incessant fluctuations and rearrangement of molecules, exchange with the surroundings by matter and energy, etc.). During the second stage its unstable state stabilizes relatively by means of the balanced oscillations around the bifurcation point (the paradoxical state of ‘stabilized instability’ appears). The third stage is characterized with the radical turn in the network of chemical reactions: the free energy contribution begins to prevail over the entropy contribution. As a result, constructive transformations proceed faster than destructive transformations. At this stage the key properties of biological organization appear: the ability to concentrate free energy and information, intensified counteraction to external influences, expedient behaviour and persistent self-renovation. On the early Earth, such successive transformation of organic matter occurred in the changeable conditions of a hydrothermal medium. Some new methods for experimental and theoretical research in the origin of life field are suggested.

Colloidal silica recovery from a hydrothermalhydrothermal heat-transfer medium by membrane filters

A process flowsheet with baromembrane facilities to recover colloidal silica from a hydrothermal heat-transfer medium is proposed. The flowsheet is developed with taking into account data on the kinetics of nucleation and polymerization of orthosilicic acid, the size distribution of colloidal particles, and experiments on the recovery of particles using membrane filters of various pore diameters. The use of membrane filters allows one to obtain amorphous silica with a reduced impurity content and a high specific surface area, as well as an aqueous hydrosol of colloidal silica.

Hydrothermal syntheses and single crystal structural characterization of M(H2O)6(OPTA)2 [M = Co(II), Ni(II), Zn(II); OPTA = 1-oxopyridinium-2-thioacetato

A new class of compounds of the family M(H2O)6(OPTA)2 (where M = Co(II), Ni(II), and Zn(II); OPTA = 1-oxopyridinium-2-thioacetato) was prepared from the appropriate metal acetates, 1-oxo-pyridinium-2-thioacetic acid (OPTAH), and potassium hydroxide in hydrothermal media and structurally characterized. The structure is constructed from M(H2O)6 2+ and two anions of OPTAH (C7H6NO3S) linked through hydrogen bonding into an extended network.

Synthesis of MCM-41 frameworks incorporating oxygen-sulphur derivates of molybdenum and tungsten

The first examples of mesostructured materials containing Mo- and W-oxo-sulphur species incorporated into a poorly ordered MCM-41 framework have been prepared under hydrothermal condition and alkaline medium. The incorporation of oxygen-sulphur derivates of molybdenum and tungsten not only increases significantly the pore diameter, but also improves the thermal stability of the MCM-41-related mesoporous material.

Size-driven domain reorientation in hydrothermally derived lead titanate nanoparticles

High-resolution transmission electron microscopy studies of hydrothermally derived platelike lead titanate nanoparticles reveal that below a critical size of approximately 70 nm, the single ferroelectric domain polarization axis reorients from perpendicular to parallel to the plate. We suggest that during particle growth, ions in the hydrothermal processing medium compensate for the ferroelectric depolarization energy. When the processing medium is removed by washing and drying, single domain nanoparticles minimize their depolarization energy by c-axis flipping.

Phase transition of γ-Al 2O 3 under hot isostatic pressure

Phase transition of γ-Al 2O 3 was studied under hot isostatic pressure (HIP), using a SUS304 capsule. The transition of γ- to α-phase was accelerated by the pressure. In order to complete the transition, it is necessary to heat γ-Al 2O 3 at high temperature of 1300 °C for a long time. However, applying 200 MPa, γ-Al 2O 3 transformed into α-Al 2O 3 even at 500 °C. The transition temperature increased with the decrease of HIPing pressure. Typical “S” curves were obtained in the kinetic relation between the conversion and time. Avrami plots of the conversion and time revealed that the phase transition proceeded by nucleation process. Hence, the nucleation seems to be induced by the stress at the contact points of powder particles, which is applied by the HIPing pressure through the capsule. On the other hand, particle size was largely increased by the transition. A small amount of water contained in the γ-Al 2O 3 was also considered to play a role of hydrothermal medium for the transition. Therefore, it is deduced that the HIPing pressure induces the nucleation and that a small amount of water in γ-Al 2O 3 accelerates the transport at low temperature.

Surface modification of CaO–Al 2O 3–SiO 2 glass by dissolution of aluminum and silicon components into alkali hydrothermal medium

CaO–Al 2O 3–SiO 2 glass (36:11:53 in molar ratio as a batch composition) was subjected to a hydrothermal treatment in 2 M NaOH at 120 °C for 24 h for making surface-porous glass. The surface-porous structure of the glass with the thickness of about 12 μm was formed by dominant aluminum and small silicon dissolutions to the hydrothermal medium. The porous surface consisted of solid substances with a size of several micrometers connected with each other and spaces with a size ranging from 0.2 to 3 μm. Furthermore, the dispersion of TiO 2 particles on this surface-porous glass by a sol–gel dipping process was successful. It was concluded that the alkaline hydrothermal treatment was effective for the formation of surface-porous glass as a potential support of TiO 2 fine particles.

Low temperature hydrothermal synthesis and formation mechanisms of lead titanate (PbTiO3) particles using tetramethylammonium hydroxide: thermodynamic modelling and experimental verification

Abstract Thermodynamic modeling was used to predict the optimum synthesis conditions for precipitation of the phase-pure lead titanate (PbTiO 3 ) in the Pb–Ti-tetramethylammonium hydroxide (TMAH) system using a newly developed computer program for automatic generation of stability and yield diagrams. The thermodynamic model has been experimentally validated over a wide range of processing conditions. Like KOH-mineralized systems, it was determined that the pH of the hydrothermal reaction medium and the Pb/Ti ratio are critical factors in forming stoichiometric PbTiO 3 powder. Morphological evolution during the reaction suggests that the formation mechanism appears to be controlled by a dissolution and recrystallization process. Two possible growth mechanisms are proposed based on the magnitude of the Pb/Ti ratio. In the case of Pb/Ti ratio=1.1, at the early stage of the reaction (3 h) excess lead species promote the formation of spherical intermediate pyrochlore phase followed by the formation of primary cubic PbTiO 3 crystals. The growth of cubic PbTiO 3 crystals proceeds until the intermediate phase acting as a reservoir to provide precipitating ions is consumed. In case of Pb/Ti ratio=1.25, excess lead condition leads to the formation of a platelet-shaped intermediate pyrochlore phase. These platelet intermediate particles act as a template in which small cubic shaped PbTiO 3 grains grew on the surface of these platelets.

Effect of pH of Medium on Hydrothermal Synthesis of Nanocrystalline Cerium(IV) Oxide Powders

Well‐crystallized cerium(IV) oxide (CeO2) powders with nanosizes without agglomeration have been synthesized by a hydrothermal method in an acidic medium by using cerium hydroxide gel as a precursor. The relationship between the grain size, the morphology of the CeO2 crystallites, and the reaction conditions such as temperature, time, and acidity of the medium was studied. The experiments showed that with increasing reaction temperature and time, the CeO2 crystallites grew larger. The crystallites synthesized in an acidic hydrothermal medium were larger and had a more regular morphology than the ones synthesized in a neutral or alkaline medium when the reaction temperature and time were fixed. The CeO2 crystallites synthesized in an acidic medium were monodispersed; however, there was vigorous agglomeration among the grains synthesized in a neutral or alkaline medium. It was demonstrated that the hydrothermal treatment was an Ostwald ripening process and the acidity (pH) of the used hydrothermal medium played a key role in the dissolution of smaller grains. It is proposed that the dissolution process can control the kinetics of the growth of larger grains.

The hydrothermal synthesis and structural characterization of a new class of compounds: nickel organoamine-halocadmates

A new class of compounds of the family Ni/Cd/X/LL (X=Cl, Br, I; LL=ethylenediamine, 2,2′-bipyridine, 1,10-phenanthroline) were prepared from mixtures of the metal halide salts, the appropriate organoamine and a source of excess halogen in hydrothermal media and have been structurally characterized. Synthesis and structural descriptions are reported for the compounds [(en)3Ni][CdBr4] (2), [(bpy)3Ni][CdCl4] (3), [(bpy)2Ni(OH2)2][CdBr4] (4), [(bpy)2NiCl2CdI2] (5), [(phen)2NiX2CdX2] (6), [((bpy)2NiCl2CdCl2)2] (7), and [(phen)2NiCl(OH2)CdCl3] (8). As suggested by the formulations the compounds are simple salts (for 2–4), heteronuclear dimetallic molecules (for 5, 6 and 8), and an unusual tetranuclear mixed metal species (for compound 7). The structure of compound 7 is unique among the structures reported here as it contains a five-coordinate Cd(II) center. The structures of compounds 2–6 and 8 exhibit six-coordinate Ni(II) centers and four-coordinate Cd(II) centers in distorted octahedral and tetrahedral geometries, respectively.

Low temperature synthesis of lead titanate by a hydrothermal method

Alkoxide-based hydrothermal powder synthesis of lead titanate was investigated. The objective of this work was to lower the synthesis temperature. By modifying titanium isopropoxide with acetylacetone during solution mixing, the phase-pure lead titanate with perovskite structure was synthesized at temperatures as low as 150 °C. It was determined that the pH of the hydrothermal reaction medium and the initial Pb/Ti ratio are critical factors in forming stoichiometric PbTiO3. When the pH of the initial feedstock is above 14 and the Pb/Ti ratio is greater than 1.5, a phase-pure PbTiO3 can be obtained. The modification of titanium alkoxide gave rise to the formation of a stable complex against hydrolysis and eventually reduced the synthesis temperature significantly. A possible formation mechanism for PbTiO3 is the dissolution-recrystallization from an amorphous precursor to a well-crystalline product as originally proposed by Rossetti et al. Hall–Williamson analysis was also performed on the hydrothermally derived PbTiO3 to interpret the systematic peak broadening and asymmetry for {001} reflections, unlike the commercial PbTiO3. It was observed that the strain in the c-axis direction is much higher than that in the a-axis direction while the domain sizes for both directions are similar. This strain anisotropy exerted in the particles may indicate a unique domain structure in the hydrothermally synthesized particles in which either only 180° domains exist or possibly only a single domain.

The genesis of Lincang germanium deposit — A preliminary investigation

The mechanism of formation of the Lincang germanium deposit is discussed in the light of the spatial distribution of Ge-rich coal and siliceous rocks, the sulfur isotopic composition of pyrite in the Ge-rich coal, the variation of Ge abundance in the coal seams and the geochemical characteristics of the siliceous rocks. The results show that the siliceous rocks intercalated with the coal seams were deposited from a hydrothermal medium through which germanium was enriched in the coal beds. The primary source of germanium is thought to be the Ge-rich granite in the basement of the sedimentary basin.

Morphology of Zirconia Synthesized Hydrothermally from Zirconium Oxychloride

Monoclinic zirconia has been synthesized hydrothermally from zirconium oxychloride in the range 20–100 MPa, 250–650°C, for run duration from 20 to 100 h and in the presence of NaOH, Na2CO3, Na2SO4, NH4F, HNO3, and H2SO4 additives. Isometric, platelike, and elongated crystal morphologies were obtained depending on the additive. Both spherulitic and isolated textures were encountered with H2SO4. Growth of isometric crystals follows a general dissolution–precipitation mechanism. For H2SO4, two growth steps were identified: an early spherulitic step followed by an isolated crystals step resulting from Ostwald ripening of preexisting spherulites. The formation of spherulites is consistent with the specific properties of the sulfuric hydrothermal medium.

Luminescence of Eu 2+ in strontium aluminates prepared by the hydrothermal method

Preparation of Sr-aluminates in fine powder form, with Al/Sr ratio varying from 2 to 12 has been attempted by the hydrothermal method, starting from Al 2O 3.xH 2O gel containing europium and Sr(OH) 2 at 240 – 250°C. The products are annealed at 850 – 1150°C in N 2 + H 2 atmosphere to yield, efficient blue to green luminescent phosphors. The predominant phase isolated belongs to the family of ‘stuffed’ tridymite-nepheline structures, having the general formula, Sr n Al 2O 3+n where n ⩽ 1. With decreasing n-value, the relative intensity of the 405 nm emission band increases in preference to that of the 510 nm emission. The excitation spectra as well as the EPR results indicate the presence of Eu 2+ in the vacancy-associated nonequivalent sites. In presence of H 3BO 3 in the hydrothermal medium, aluminoborates such as SrAl 5BO 10, Sr 2Al 2B 2O 8 and Sr 3Al 6B 8O 24 are produced, whose only emission maximum around 400 nm remains as a band even at LN 2 temperature.

Hydrothermal analcime synthesis at 10 MPa isobar

AbstractHydrothermal analcime synthesis at 10 MPa isobar and 100, 200, and 300 °C isotherms was performed. As synthesis initial components the minerals nepheline, albite, natrolite, jadeite, wairakite and the mixtures nepheline + albite and nepheline + natrolite were used. 1 M Na2SiO3, 1 M NaCl and 0.1 M NaOH (pH = 14, 7, 13) solutions were applied as the hydrothermal medium. On evaluating the synthesis products by means of X‐ray diffraction analysis and X‐ray microanalysis (EDAX) it was estimated that different analcime structural modifications originate especially at temperatures above 200 °C in neutral and alkaline medium from all the mentioned minerals. Analcime with a higher symmetry of the unit cell of crystals form at temperatures above 250 °C particularly.

Hydrothermal equilibria and crystal chemistry of phases in the oxide-hydroxide-sulfate systems of La, Pr, and Nd

Equilibria of the aqueous Ln + O + SO 4 ( Ln = La, Pr, Nd) system have been investigated at water pressures of 120 ± 20 MPa and temperatures of 450 ± 50°C. At low sulfate contents, the UCl 3-type trihydroxides coexist with the orthorhombic dioxide monosulfates. A second two-phase region, Ln 2O 2SO 4 + Ln(OH)SO 4, appearing at higher sulfate concentrations is followed by a third two-phase region, Ln(OH)SO 4 + Ln 2(SO 4) 3 · nH 2O. Whereas the monohydroxide monosulfates all have the same monoclinic structure, several new hydrate compositions and structures form in hydrothermal media. X-ray diffraction data indicate that the n = 7 hydrates of La and Pr and the monohydrate of La have closely related orthorhombic structures similar to that of the anhydrous La 2(SO 4) 3 phase obtained by decomposition of the hydrates at low temperature. The n = 5 sulfate hydrates of Pr and Nd have also been characterized, and a metastable La 2(OH) 4SO 4 phase has been identified. Lattice parameters and diffraction data obtained by powder and single crystal methods are reported, and the thermal decomposition reactions of the products are described.

The transformation of feldspars and muscovite to clay minerals in (Ca,Mg)-carbonate bearing hydrothermal media

AbstractExperimental transformations of feldspars and muscovites following additions of magnesite and dolomite have been studied at PH2O = 1 kbar, T = 200–600°C. Formation of layer silicates such as smectite, 7 Å (Mg,Al)-serpentine, some mixed-layer phases and other minerals is shown to be a function of the composition of the starting materials, temperature and run duration. It is established that 1 M- and 2 M1-phlogopites are formed from 1 M- and 2 M1-muscovites, respectively, under Mg-bearing hydrothermal conditions. Some causes of variations in the composition of 7 Å (Mg,Al)-serpentines at elevated temperature as a function of the composition of hydrothermal media are given. These data may be used to explain the main characteristics of clay mineral formation from feldspar- and muscovite-bearing sedimentary rocks during their alteration in postdiagenetic and metasomatic processes.