Degree Of Oversaturation Research Articles

Study of the generation and stability of thorium(IV) colloids by LIBD combined with ultrafiltration

The generation of Th(IV) colloids is investigated by laser-induced breakdown detection (LIBD) combined with ultrafiltration in the concentration range from 10−5 to 10−2 M at pH 3–5 in 0.5 M NaCl. The amount of colloids generated by coulometric titration of initially ionic thorium solutions depends on the total thorium concentration and the degree of over-saturation with regard to the thermodynamic solubility limit of solid Th(IV) hydroxide at given pH. These generated thorium colloids are found to be small in size and remain stable up to 400 days of investigation, without a tendency towards agglomeration or precipitation. A dilution of the suspension at constant pH downgrades colloids to ionic species, while a dilution with 0.5 M NaCl of neutral pH causes an increased over-saturation and colloid generation, leading to an equilibrium between colloids and ionic species. This equilibrium is comparable to the thermodynamic solid–liquid equilibrium of amorphous Th(IV) hydroxide. The present results allow a precise estimation of colloid generation in the dissolution of a Th(IV) hydroxide solid phase. Such a possibility may facilitate the practical solubility assessment of metal hydroxides in natural aquifer systems.

Compositional and Lithological Controls on the PGE-Bearing Sulphide Zones in the Selukwe Subchamber, Great Dyke: a Combined Equilibrium–Rayleigh Fractionation Model

The platinum group element (PGE)-bearing Main and Lower INTRODUCTION Sulphide Zones of the Selukwe Subchamber of the Great Dyke are An increasing number of mafic-layered intrusions are made up of a series of subzones within which the ratios Pd:Pt are reported to have distributions of platinum group elements effectively constant, whereas these ratios vary significantly between (PGE), Au and base metals characterized by vertically the subzones. Fractionation of Pd with respect to Pt varies by a offset profiles. Offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients. The link with sulphide is Great Dyke (Prendergast & Keays, 1989; Wilson et al., indisputable and the control is likely to have been the degree of 1989; Naldrett & Wilson, 1990; Wilson & Tredoux, oversaturation of PGE micro-nuggets in the magma. The apparent 1990). The phenomenon has since been recognized in partition coefficients for Pt and Pd between silicate and sulphide the Munni Munni intrusion of Western Australia (Barnes liquid are dependent on the degree of oversaturation and thereby et al., 1990), the Skaergaard intrusion (Bird et al., 1991) exhibit spurious correlation with the PGE content of the sulphide. and the Rincon del Tigre Complex of eastern Bolivia Modelling replicates the Pt and Pd distribution and ratios only by (Prendergast, 2000). PGE are also postulated to be transdramatically changing the effective partition coefficients. Pyroxene ported in fluids emanating from expulsion of intercumulus compositions (including TiO2) are shown to be strongly dependent liquid from the crystal pile, and there is strong evidence on the incompatible element content of the whole rock, and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the PGE of layered intrusions such as the Skaergaard intrusion subzones. The overall control is Rayleigh fractionation, but constancy (Andersen et al., 1998) and in the Stillwater Complex of the ratio Pd:Pt and the initial pyroxene composition (before re(Boudreau & McCullum, 1986, 1992). In contrast, the equilibration with trapped liquid) within the subzones is indicative mineralized zones in the Great Dyke and Munni Munni of equilibrium crystallization. This layered structure may have been intrusion are located in pyroxenite, and primary magderived from liquid layers in the magma chamber. matic processes have been invoked to explain the min-

Kinetics of solid-phase extraction and solid-phase microextraction in thin adsorbent layer with saturation sorption isotherm

The effects of sorbent saturation in thin adsorbent layers have been much overlooked in earlier research and should be taken into account in both the theory and practice of solid-phase extraction (SPE) and solid-phase microextraction (SPME). The adsorption kinetics of a single analyte into a thin adsorptive layer was modeled for several cases of agitation conditions in the analyzed volume. The extraction process in the adsorbent layer was modeled using a Langmuir isotherm approximated by the linear isotherm at low concentrations and by a saturation plateau at concentrations exceeding the critical saturation concentration. Laplace transformations were used to estimate the equilibration time and adsorbed analyte concentration profile for no agitation, practical and perfect agitation in the analyzed volume. The equilibration time may be significantly reduced at high degrees of oversaturation and/or agitation in the analyzed volume. The resulting models indicated that the adsorbent layer becomes saturated at some critical value of the oversaturation degree parameter. The critical value of the oversaturation parameter is affected by both the concentration of the analyte in the analyzed volume and the sorbent characteristics. It was also shown that the adsorption process is carried out via the propagation of the saturation adsorption boundary toward the inner boundary of the adsorbent layer. These new adsorption models should serve as “stepping stones” for the development of competitive adsorption kinetic models for both SPE and SPME, particularly in cases where fast sampling is used.

Precipitation kinetics of U(VI) phases formed in high concentrated saline solutions

The purpose of this paper is to understand the influence of precipitation kinetics on the U(VI) phases solubility and therefore, on the crystallinity of the phases formed. Precipitation kinetics was studied at constant pH value, by considering the influence of the method of pH control and oversaturation degree. Experiments were carried out at room temperature, anaerobic conditions and constant pH value. Characterization of solid phases were performed by scanning microscopy and X-ray diffraction. A comparison between particle size of the phase formed, solubility value obtained and precipitation method used was accomplished in order to develop a model for the U(VI) precipitation kinetics, in high concentrated saline solutions (5 M NaCl).

Methane in the western North Pacific

The concentration of methane in about 400 seawater samples collected in the western North Pacific, mostly from 40°N to 5°S along 165°E was determined. While the concentration of methane in the surface water was slightly greater in the high-latitudes, it did not widely vary with a standard deviation of 0.29 n mol/l for a mean value of 2.49 n mol/l. The 90% confidence limit of the mean was 0.08 n mol/l. The degree of oversaturation in 1991 (31±4%) was not different from that in circa 1970. If we assume that this degree of oversaturation occurs in the entire oceans, the annual flux of methane becomes 6×1012g CH4. Both the concentrations of methane and chlorophylla were higher in the surface 100 m layer. However, the correlation between them was not well in the entire surface waters. This may indicate that the production of methane is not directly related to the photosynthetic process. The concentration of methane decreased gradually with increasing depth down to 1000 m. Its horizontally and vertically uniform concentration in the abyssal water suggests that the turnover time of methane in the oxic pelagic water is in the range between a few years and a few hundred years.

Precipitation of carbonate minerals by microorganisms: Implications for silicate weathering and the global carbon dioxide budget

Direct light and electron microscopic studies show that cyanobacterial cells serve as nucleation sites for carbonate mineral precipitation in a variety of fresh to saline‐alkaline lakes on the Cariboo Plateau in central British Columbia, Canada, and in mineralized crusts on weathered basalt in Iceland. The carbonate minerals found in association with the cyanobacteria were extremely fine‐grained, and invariably occurred on the external surfaces of the cells. Carbonate mineralogy was variable, ranging from calcite to magnesite, depending on differences in lake and groundwater chemistry (i.e., saturation state of the water with respect to individual carbonate minerals). In microcosm experiments, phototrophic cyanobacterial growth increased alkalinity and the degree of oversaturation with respect to calcite. Calculated values for the saturation state of calcite and magnesite in Cariboo Plateau natural waters exhibited two distinct trends, with (1) high magnesite saturation values in areas where the weatherin...

Organic Matter Oxidation and Aragonite Diagenesis in a Coral Reef

ABSTRACT A combination of field and theoretical work is used to study controls on the saturation state of aragonite inside a coral-reef framework. A closed-system ion-speciation model is used to evaluate the effect of organic-matter oxidation on the saturation state of aragonite. The aragonite saturation state initially drops below 1 but becomes oversaturated during sulfate reduction. The C:N ratio of the organic matter affects the degree of oversaturation, with N-poor organic material resulting in a system more corrosive to aragonite. Precipitation of sulfide as FeS strongly affects the aragonite saturation state, and systems with much FeS formation will have a stronger tendency to become oversaturated with respect to aragonite. Both precipitation and dissolution of aragonite are predicted at different stages of the organic reaction pathway if the model system is maintained at aragonite saturation. Field data from a coral-reef framework indicate that the system maintains itself at aragonite saturation, and model-predicted changes in dissolved calcium follow those observed in the interstitial waters of the reef. Aragonite probably acts as a solid-phase buffer in regulating the pH of interstitial waters. Because interstitial water in the reef has a short residence time, the observed equilibration suggests rapid kinetics.

Dynamics of domain structure in uniaxial ferroelectrics

The switching of polarization in ferroelectrics is considered as the first-order phase transition. By the analogy with the theory of crystal growth it is assumed that the switching process is determined by the oversaturation degree which correspond in ferroelectrics to the magnitude of electric field on the domain boundary. It is supposed that the mechanism of domain wall motion in strong field is due to the twodimensional nucleation and in weak field is due to the one-dimensional nucleation. The criterion of strong field IS defined. From this point of view it have been explained the peculiarities of domain structure dynamics in two uniaxial ferroelectrics: lead germanate and gadolinium molybdate.

Calcium Carbonate Supersaturation in Soil Solutions of Calciaquolls

AbstractWe found that soil solutions of Calciaquolls in western Minnesota were up to 40‐fold oversaturated with respect to pure calcite. Selective dissolution of soil calcites and x‐ray diffraction showed that the Mg content of these calcites ranged from 1 to 10 mole % MgCO3. This amount of Mg substitution cannot explain the degree of oversaturation observed in the soil solutions. X‐ray peak broadening and scanning electron microscopy (SEM) was used to determine the size and shape of soil calcites. The mean size of soil calcite crystallites was approximately 0.04 to 0.09 µm. Surface energy relationships predict that soil calcites of this size would have solubility products (−log Ksp) near 8.3, not low enough to explain the observed supersaturation. Energy dispersive analysis of x‐rays (EDAX) and SEM observations of these samples showed that the larger soil calcites (0.5–20 µm) were irregularly shaped and did not exist as independent crystals, and were associated with other aluminosilicate solid phases. Soil suspensions of Calciaquolls prepared at 10‐fold oversaturation remained oversaturated for 40 d. Adding pure calcite seeds to soil suspensions and soil solution supernatants induced some precipitation, but the solutions remained oversaturated, attaining CaCO3 ion activity products (−long IAP) ranging from 7.6 to 7.9. These results suggest that the metastable condition of supersaturation is a result of inhibition of the calcite precipitation process by water‐soluble organic components, which are adsorbed and block potential crystal growth sites.

Factors influencing structural evolution in the oxide of hot-pressed Si3N4-Y2O3-SiO2 materials

X-ray diffraction, SEM, EDS and WDS studies were performed of the structure and morphology of the oxide developed on hot-pressed compositions of the Si3N4-Si2N2O-Y2Si2O7 sub-system subjected to short-term (∼30 h) oxidation in air atT=1273 to 1673 K. Viscosity of the oxide glassy phase, ionic mobility and oversaturation degree of the atomic species involved were found to be the most important parameters in the control of structural evolution of the oxide. Clear links appear to exist between compositional parameters of the grain-boundary phase of the hot-pressed materials and the evolution of the crystal phases in the oxide. The ΣMi/Y ratio (ΣMi, total amount of metal impurities) in the grain-boundary phase, seems to play a major role in the transport properties of the oxide glassy phase.

Physico-chemical study of the dead sea waters. I. Activity coefficients of major ions in dead sea water

Conventional single-ion activity coefficients of major ions in Dead Sea brines, and their osmotic coefficients, have been estimated using Pitzer's thermodynamic treatment of mixed electrolyte solutions. For these calculations, higher-order terms have been taken into consideration. It has been shown that Pitzer's equations are applicable to such natural concentrated brines. The problem of gypsum solubility in the Dead Sea was examined. Dead Sea brines were shown to be oversaturated with respect to gypsum, and the degree of oversaturation increased very significantly during the last 20 years.

A Discussion on volcanism and the structure of the Earth - Proposals concerning the variation of volcanic products and processes within the oceanic environment

An attempt is made to fit available petrochemical data on oceanic volcanic rocks into the structural model for the ocean basins presented by the plate tectonic theory. It is suggested that there are three major volcanic regimes: (i) the low-potassic olivine tholeiite association of the axial zones of the oceanic ridges where magmatic liquids are generated at low pressures high in the mantle, (ii) the alkalic (Na > K) associations along linear fractures where liquids generated at greater depth gain easy egress to the surface, (iii) those alkalic associations, rich in incompatible elements, of island groups, remote from fracture zones, where magmas created at depth proceed slowly to the surface and in consequence suffer intense fractionation. There are certain discrepancies in this pattern, notably that there is no apparent relation between rate of sea-floor spreading and degree of over-saturation of the axial zone basalts and that certain areas, such as Iceland, are characterized by excess volcanism. Explanation of these anomalies is sought by examining an oceanic area in an early stage of development—the Red Sea. It is tentatively suggested that the initial split of a contiguous continent might be brought about by the linking of profound fractures, caused by domal uplift related to rising isolated lithothermal systems, and that the present anomalies in oceanic volcanism may reflect the variation in rate of thermal convection within the original isolated lithothermal plumes.

Calcium oxalate crystalluria and urine saturation in recurrent renal stone-formers.

1. The degree of saturation with calcium oxalate has been determined in fresh urine samples from six patients with recurrent calcium oxalate-containing renal stones and six normal control subjects who were studied under the same conditions of diet and fluid intake. 2. The degree of saturation of urine with calcium oxalate was significantly higher in the group of stone-formers than in the control series and more often exceeded the amount needed for spontaneous crystallization of calcium oxalate (formation product). This was accounted for by the higher concentration of calcium and oxalate in the urine of the stone-formers. 3. Crystals of calcium oxalate were observed in all freshly examined urines in which the formation product of calcium oxalate was exceeded. Since the formation product of calcium oxalate was exceeded more often in the urines of stone-formers than in the urines of the control subjects, this accounted for the greater calcium oxalate crystalluria of the stone-formers. 4. Addition of a small quantity of sodium oxalate to the basal diets of the two groups resulted in a greater increase in the urine saturation and calcium oxalate crystalluria of the stone-formers, thus accentuating the difference observed between the two groups when they were given the basal diet. 5. Calcium oxalate crystalluria was related quantitatively to the degree of over-saturation of urine with calcium oxalate, although uric acid solubility may play a small role at low pH values. 6. The results are consistent with a ‘hyperexcretion—crystallization’ mechanism of stone formation.