Jürgen Sonnefeld

The influence of ionic strength on the dissolution process of silica

Abstract A general dissolution model for oxidic solids in water has been suggested. To confirm this model the dissolution kinetics of well-defined silica spheres is determined dependent on pH and ionic strength at 40°C. By analyzing dissolution data it is possible to study electrical double layer properties. In this approach, the charge state of the oxide is described by application of a diffuse layer model. From a theoretical point of view, some important, general conclusions for the overall rate constant of dissolution, consisting of three partial rate constants, may be drawn. The characteristic parameters such as the pristine point of zero charge, the acidity constants of the surface groups, the saturation concentration, and the partial rate constants, could be determined precisely. Experimental results for two different types of silica are compared. The model is able to elucidate well the dissolution behavior as a function of time, pH and ionic strength.

The kinetics of dissolution of silica ‘Monospher’ into water at different concentrations of background electrolyte

Abstract The kinetics of dissolution of silica “Monospher” into water as regards its dependence on time and pH was investigated at background electrolyte concentrations of 0, 0.001, 0.01 and 0.1 mol 1 −1 . The dissolved amount of silica depicted as a function of time and pH forms a dissolution surface. This surface can be modeled by four deprotonation equilibria and four elementary reactions. The concentration of background electrolyte increases the rate of dissolution and influences the shape of the dissolution surface.

On the influence of background electrolyte concentration on the position of the isoelectric point and the point of zero charge

Abstract In the case of a specific interaction of background electrolyte ions with the surface of a solid in an aqueous electrolyte solution a shift of the characteristic points, the isoelectric point (iep) and the point of zero charge (pzc), in an opposite direction was described by Lyklema (in: Th.F. Thadros (Ed.), Solid/Liquid Dispersions, Academic Press, London, 1987). In the present article equations are derived describing the dependence of the iep on the electrolyte concentration for a specific interaction of one ion only as well as for the case that two ions have specific interactions with the surface. The considerations are made using the triple layer model and a 1-1 electrolyte. Because of transcendental equations an exact analytic solution is not possible for the pzc. Here an approximation published in Janusz et al. (J. Colloid Interface Sci. 187 (1997) 381) is compared with numerical solutions. The ratio between the amount of specifically bonded cations and anions in the inner Helmholtz plane depends on the concentration of potential determining ions and on the electrolyte concentration. As can be shown this ratio is independent of the electrolyte concentration at the iep. Therefore, this ratio is suitable to decide if there is a predominant cation or anion bining.

Uranium release from a natural rock under near -natural oxidizing conditions

SummaryUnderstanding how uranium moves through the soil and groundwater is essential to determine the effectiveness of cleanup technologies. Uranium release and transport in the subsurface under oxic conditions have been reported to be mostly dependent on sorption onto Fe/Mn-oxide and complex interactions with organic substances. Available information in the literature, however, presents evidence of U retardation by natural sands. The aim of this investigation was to characterize U dissolution from a uraninite-containing rock (UO2-rock) in different waters under test conditions relevant to U transport from mine tailings.For this purpose, not shakenbatch experiments were conducted with a constant amount of anUO2-rockand different types of water (deionised, tap and mineral water). For comparison parallel experiments were conducted with 0.1M Na2CO3and 0.1M H2SO4. Furtherdissolution experiments using UO2-rock together with dolomite and pyritewere conducted. The results indicate that carbonate addition enhanced U solubilization, whereas pyrite addition essentially slowed the initial U solubilization. It is shown that SiO2and other rock constituents retard U transport.

The Influence of the Choice of a Suitable Dividing Surface on the Results of Surface Charge Density Calculations

The model of site dissociation of acid surface groups is used in connection with the GouyChapman model of the electrical double-layer for the calculation of theoretical values of the surface charge density of spherical particles in aqueous electrolyte solutions. The influence of a parameter on the particle size dependence of the surface charge density is discussed, is the distance between the plane of the location of the surface charge and the equimolecular dividing surface of the monolayer adsorption of gases.

Solubility kinetics of a synthetic uranium(VI) oxide at different pH and controlled O2 partial pressure and evaluation of substantial interface properties

The solubility behaviour of a synthetic uranium(VI) oxide was studied under inert conditions in aqueous NaClO4 solution at 25 ± 2 °C. The dissolution kinetics were investigated in respect of pH in solution and O2 partial pressure in the atmosphere. The actual uranium concentration was determined by ICP-MS measurements. Under acidic conditions we observed a moderate solubility which increases by lowering of the pH of solution yielding to a solubility constant of log10Ksp = −24.38 ± 0.07. The influence of the ionic strength, I, on the dissolution kinetics could be observed which was evaluated using a theoretical model considering the electrical double layer at the surface of the oxide as well as all possible reaction pathways. As a result we could determine the acid-base equilibrium constants of the surface groups and the overall reaction rate, k+, of dissolution process as a function of pH and I. On this basis we are able to determine the pzc of the uranium oxide, to be 7.28. It is in good accordance with a minimum in the tendency of the solid to dissolve which was observed in the pH range 6 to 8.

Characterizing Uranium Solubilization Under Natural Near Oxic Conditions

A solubilization study for in total 782 days using not shaken batch experiments with uranium-bearing rock and three natural carbonate minerals was conducted to characterize uranium (U) leaching under oxic conditions. Results showed that aqueous U concentration increased continuously with a solubilization rate of 0.16 ▭gm-2h-1 for the first 564 d (1.5 y). After 1.5 y, U concentration reached a maximum value (saturation) and decreased afterwards. The saturation concentration of 54 mgL-1 (mean value) was influenced to variable extent by the presence of carbonate minerals. Dissolution/precipitation, adsorption or ion exchange processes appear to control U solubilization.

Some General Considerations on a Curvature Dependent Surface Tension in the Capillarity of Liquids

Durch sehr Starke Krümmung verursachte qualitative Veränderungen der Oberflächeneigenschaften von Flüssigkeiten sind denkbar. Für eine reine Flüssigkeit kann diesem Effekt durch drei alternative Möglichkeiten Rechnung getragen werden. Entweder man verwendet einen Krümmungsterm bei der Darstellung der inneren Energie oder eine krümmungsabhängige Oberflächenspannung oder die Oberflächendichte zur Beschreibung. Die Bedeutung eines krümmungsabhängigen Terms (dff/dr) in den Gleichungen von KELVIN und YOUNG-LAPLACE ist geklärt worden. Aus der erweiterten Gibbs-Tolman-Theorie haben wir vier Ausdrücke für die Krümmungsabhängigkeit der Oberflächenspannung abgeleitet. Alle diese Ausdrücke beziehen sich auf die nach K O N D O definierte surface of tension. Die Ausdrücke liefern sehr ähnliche Resultate im Bereich kleiner positiver Krümmung. Im Bereich negativer Krümmung und bei sehr großer Krümmung muß man genau prüfen, ob ein bestimmter Ausdruck verwendet werden kann. Keiner der angegebenen Ausdrücke beschreibt die Krümmungsabhängigkeit der Oberflächenspannung im gesamten Krümmungsbereich — oo < c, < oo in physikalisch befriedigender Weise. Die krümmungsabhängigen Ausdrücke für die Oberflächenspannung werden am Beispiel des Gleichgewichtes kleine Tröpfchen Dampf und der Berechnung der freien Bildungsenergie kleiner Cluster geprüft. Die Ergebnisse zeigen, daß in beiden Fällen unterschiedliche Ausdrücke am besten geeignet sind. Die Verwendung einer krümmungsabhängigen Oberflächenspannung bei der Auswertung von Kapillarkondensationsexperimenten zur Bestimmung der Porenradienverteilung poröser Festkörper erweist sich als vorteilhaft, da auf diese Weise ein realistischeres Bild über die Porenstruktur der porösen Stoffe erhalten wird.

Modelluntersuchungen zum Einfloß der Porenlänge auf das Ergebnis der Porenverteilungsberechnung am Beispiel der beidseits offenen Zylinderpore

The complete sorption isotherm is calculated from given pore size distributions. A length-radius-ratio of the pores is assumed. This ratio is shown to influence the shape of the sorption isotherm. The assumption of infinite pore length for the evaluation of sorption experiments to determine the pore size distribution therefore causes an error. Principal assertions about this error are made to help experimentalists for correct valuation. Different techniques for the evaluation of sorption isotherme known from the literature are tested and compared to proof their quality.