Kinetics of dissolution of powdered Pentelic marble in under-saturated solutions: the role of particle characteristics

In the present study, the effect of mesopore and particle size distributions on the kinetics of dissolution of powdered Pentelic marble was investigated. Powders obtained by grinding marble slabs in an agate mortar with a pestle (1-3.5 min) were found to be non-porous by nitrogen absorption measurements. These powders, upon dissolution under conditions of constant under-saturation, 25 degrees C, pH 8.25, showed that the kinetics in the absence of mesopores depended on the number of active sites on the exposed surface. Thus, powders consisting of smaller particles, having higher specific surface areas, yielded higher rates of dissolution. Powders, however, which were prepared by grinding marble slabs in a cylinder mill for time periods between 2.0 and 30.0 min, and which exhibited considerable mesoporosity, showed the opposite trend. The rates of dissolution measured for these powders in under-saturated solutions increased with increasing mean particle size and decreased with increasing specific surface area. This finding suggested that the presence of mesopores resulted in lower dissolution rates even though the exposed total surface area was larger. Furthermore, the larger the number of mesopores in a powder sample the slower the corresponding dissolution rates in under-saturated solutions.     

A new model for nanoscale enamel dissolution

The dissolution kinetics of human tooth enamel surfaces was investigated using nanomolar-sensitive constant composition (CC) and in situ atomic force microscopy (AFM) under simulated caries formation conditions (relative undersaturation with respect to hydroxyapatite = 0.902, pH = 4.5). Scanning electron microscopic (SEM) examination of the resulting etched enamel surfaces showed that deminerzalization, initiated at core/wall interfaces of rods, developed anisotropically along the c-axes. After an initial rapid removal of surface polishing artifacts, the dissolution rate decreased as the reaction proceeded in accordance with our recently proposed crystal dissolution model, resulting in hollow enamel cores and nanosized remaining crystallites, resistant to further dissolution. Generally, dissolution of minerals is regarded as a spontaneous reaction in which all the solid phase can be dissolved in undersaturated solutions. However, the dissolution of some biominerals may be suppressed when the crystallites approach nanometer size. This study shows that CC demineralization of enamel in acidic medium follows this new model that can be used to mimic carious lesion formation. In dissolution studies, nanosized enamel crystallites exhibit a remarkable degree of self-preservation in the fluctuating physiological milieu.     

Test of the Epstein-Plesset model for gas microparticle dissolution in aqueous media: effect of surface tension and gas undersaturation in solution

The gas from a free air bubble will readily dissolve in water, driven by two main factors: the concentration (undersaturation) of dissolved gas in the aqueous solution and the surface tension of the gas bubble-water interface via a Laplace overpressure in the bubble that this creates. This paper experimentally and theoretically investigates each of these effects individually. To study the effects of surface tension, single- and double-chain surfactants were utilized to control and define interfacial conditions of the microbubble in saturated solution. To study the effect of undersaturation, solid distearoylphosphocholine lipid was utilized to coat the gas microparticle with, essentially, a wax monolayer and to achieve zero tension in the surface. The experimental work was performed using a micromanipulation technique that allows one to create and micromanipulate single air microparticles (5-50 microm radius range) in infinite dilution and to accurately record the size of the particle as it loses volume due to the dissolution process. The micropipet technique has shown to be an improvement over other previous attempts to measure dissolution time with a 3.2% average experimental error in gas microparticle dissolution time. An ability to study a gas microparticle in infinite dilution in an isotropic diffusion field is in line with the theoretical assumptions and conditions of the Epstein-Plesset model. The Epstein-Plesset model on average underpredicted the experimentally determined dissolution time by 8.6%, where the effect of surface tension was considered with a range of surface tensions from 72 down to 25 mN/m. The Epstein-Plesset model on average overpredicted the dissolution time by 8.2%, where the effect of undersaturation was considered for a microparticle with zero tension in the surface (zero Laplace pressure) and a range of gas saturations from 70% to 100%. Compared to previous attempts in the literature, this paper more appropriately and accurately tests the Epstein-Plesset model for the dissolution of a single microbubble and an air-filled microparticle in aqueous solution.     

Electrowetting of nonwetting liquids and liquid marbles

Transport of a water droplet on a solid surface can be achieved by differentially modifying the contact angles at either side of the droplet using capacitive charging of the solid-liquid interface (i.e., electrowetting-on-dielectric) to create a driving force. Improved droplet mobility can be achieved by modifying the surface topography to enhance the effects of a hydrophobic surface chemistry and so achieve an almost complete roll-up into a superhydrophobic droplet where the contact angle is greater than 150 degrees . When electrowetting is attempted on such a surface, an electrocapillary pressure arises which causes water penetration into the surface features and an irreversible conversion to a state in which the droplet loses its mobility. Irreversibility occurs because the surface tension of the liquid does not allow the liquid to retract from these fixed surface features on removal of the actuating voltage. In this work, we show that this irreversibility can be overcome by attaching the solid surface features to the liquid surface to create a liquid marble. The solid topographic surface features then become a conformable "skin" on the water droplet both enabling it to become highly mobile and providing a reversible liquid marble-on-solid system for electrowetting. In our system, hydrophobic silica particles and hydrophobic grains of lycopodium are used as the skin. In the region corresponding to the solid-marble contact area, the liquid marble can be viewed as a liquid droplet resting on the attached solid grains (or particles) in a manner similar to a superhydrophobic droplet resting upon posts fixed on a solid substrate. When a marble is placed on a flat solid surface and electrowetting performed it spreads but with the water remaining effectively suspended on the grains as it would if the system were a droplet of water on a surface consisting of solid posts. When the electrowetting voltage is removed, the surface tension of the water droplet causes it to ball up from the surface but carrying with it the conformable skin. A theoretical basis for this electrowetting of a liquid marble is developed using a surface free energy approach.     

Relation of lattice ion solution composition to octacalcium phosphate dissolution kinetics

The kinetics of dissolution of octacalcium phosphate (OCP) has been investigated under conditions of constant relative undersaturation with respect to OCP, σ_OCP=−0.57, at pH 4.500, and ionic strength (I), 0.15 mol l⁻¹. The molar calcium/phosphate ratio (R) in the solutions was varied from 0.1 to 10. The dissolution rate decreased by 160% as R increased from 0.1 to 10. The ζ-potential of the OCP surfaces was measured in solutions equilibrated with respect to OCP at pH values ranging from 5.0 to 11. Under stoichiometric conditions (R=1.33), OCP was positively charged at pH values from 5.0 to 10. As the solution calcium concentration was increased, ζ became more positive over the entire pH range studied. At R=0.1, two isoelectric points were apparent at pH values of about 6.3 and pH 9.5. This behavior may be related to the solubility product (K_sp) of OCP. The relationship between surface characteristics and dissolution rate are discussed in terms of kink density and the kinetic ionic ratio model developed previously (J. Zhang, G.H. Nancollas, J. Colloid Interf. Sci., 200 (1998) 131.     

The seeded growth of calcium phosphates. Surface characterization and the effect of seed material

The kinetics of growth of calcium phosphate on seed materials hydroxyapatite, “amorphous” calcium phosphate, enamel, and dentine has been studied at 25°C and at constant physiological pH in stable supersaturated solutions of calcium phosphate. The grown material was characterized chemically, by x-ray powder diffraction and specific surface area measurements and by scanning electron microscopy. In addition, dissolution kinetic studies have been made of the material grown at different times in the crystal growth experiments and of “amorphous” calcium phosphate. The crystallization experiments were made over a range of initial supersaturation to obtain information about the nature of the calcium phosphate phases formed. The specific surface area of the grown phases show marked changes during the growth experiments. The results of the dissolution experiments indicate that the solid material formed in the early stages of crystallization has a stoichiometry corresponding to OCP.     

Reaction of hydroquinone with hematite; II. Calculated electron-transfer rates and comparison to the reductive dissolution rate

The rate of reaction of hematite with quinones and the quinone moieties of larger molecules may be an important factor in limiting the rate of reductive dissolution of hematite, especially by iron-reducing bacteria. It is possible that the rate of reductive dissolution of hematite in the presence of excess hydroquinone at pH 2.5 may be limited by the electron-transfer rate. Here, a reductive dissolution rate was measured and compared to electron-transfer rates calculated using Marcus theory. An experimental rate constant was measured at 9.5 x 10 (-6) s(-1) and the reaction order with respect to the hematite concentration was found to be 1.1. Both the dissolution rate and the reaction order of hematite concentration compare well with previous measurements. Of the Marcus theory calculations, the inner-sphere part of the reorganization energy and the electronic coupling matrix element for hydroquinone self-exchange electron transfer are calculated using ab initio methods. The second order self-exchange rate constant was calculated to be 1.3 x 10 (7) M(-1)s(-1), which compares well with experimental measurements. Using previously published data calculated for hexaquairon(III)/(II), the calculated electron-transfer rate for the cross reaction with hydroquinone also compares well to experimental measurements. A hypothetical reductive dissolution rate is calculated using the first-order electron-transfer rate constant and the concentration of total adsorbed quinone. Three different models of the hematite surface are used as well as multiple estimates for the reduction potential, the surface charge, and the adsorption density of hydroquinone. No calculated dissolution rate is less than five orders of magnitude faster than the experimentally measured one.     

The role of organic acids in mineral weathering

Organic acids and their anions (for brevity we shall use the term “acids” to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in solution of ions such as Al³⁺ that themselves affect mineral dissolution rate. In this paper we review the effects of organic acids on the dissolution rates of silicate minerals, particularly feldspars, under conditions approximating the natural weathering environment −25°C, pH 4–7 and with concentrations of organic acids comparable to those measured in soil solutions.

Feldspar dissolution rates far from equilibrium increase with decreasing pH below pH 4–5. They appear to be independent of pH between pH 4–5 and about 8, and above pH 8 feldspar dissolution rates increase with increasing pH.

Small chelating ligands such as oxalate appear to accelerate feldspar dissolution through complexation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enhancement to enhancements of a factor of 15, depending upon the data set, pH, and aluminum content of the mineral: there is a great deal of scatter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic acids do not appear to increase feldspar dissolution rates significantly.

Dissolution rates must decrease as the solution approaches saturation with respect to the primary phase (the chemical affinity effect). Organic acids will influence chemical affinity by complexing Al (and possibly other elements) in solution and hence decreasing the chemical activity of Al³⁺. There are essentially no data on the effect of chemical affinity on feldspar dissolution rate at 25°C and mildly acid pH, so it is hard to evaluate the importance of organic acids in accelerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an important determinant of silicate dissolution rates in nature.

Observed rates of silicate weathering in the field are typically much slower than predicted from laboratory experiments far from equilibrium, suggesting control by transport of solutes between “micropores” and “macropores” (“micropores” include fractures and crystal defects within grains). If such transport is rate-controlling, analysis of the effect of organic acids on weathering rates in nature in terms of dissolution rates far from equilibrium may be misleading.     

Time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species on heterogeneous surfaces,by inverse gas chromatography

A new simple method is developed for measuring surface diffusion coefficients Ds of gases adsorbed on heterogeneous surfaces, using the reversed-flow version of inverse gas chromatography. The Ds values are found in a time-resolved way, together with the corresponding adsorption energy values, the local adsorbed concentrations, and the local adsorption isotherm values. A relative dynamic adsorption rate constant, an adsorption/desorption rate constant, and a surface reaction rate constant are also found in the same experiment, together with the total diffusion coefficient of the gas in the solid bed. The method has been applied for carbon monoxide, oxygen gas, and carbon dioxide as adsorbates on 75% Pt+25% Rh catalyst supported on SiO2, at 593.8 K.     

Thermal decomposition of N-2,4,6-tetranitro-N-methylaniline

Purity determination by calorimetry has been used to determine the rate constant of thermal decomposition of N-2,4,6-tetranitro-N-methylaniline in liquid and solid states. The ratios of the initial rates of decomposition in solid and liquid decrease when temperature increases in the region not far below the melting point. The predicted rate constants in solid state are about 10 times greater than the one estimated by Wiseman. The thermal decomposition process at lower temperatures has been used to determine the rate constant in the decay phase. Activation energy and pre-exponential factor are also presented.     

Mechanism of dissolution of sparingly soluble electrolytes

Recent constant composition dissolution studies of sparingly soluble calcium phosphates have revealed an interesting and unusual behavior in that the rates decreased, eventually resulting in effective suppression, even though the solutions remained undersaturated. Contrary to traditional theories of dissolution, these experimental results indicated the importance of not only the particle size on the dissolution rate but also the participation of critical phenomena. In these theories, it is assumed that when the dissolution reactions are initiated, they continue spontaneously until all solid phase has disappeared. In terms of these mechanisms, there are no critical phenomena in the dissolution mechanism. Although the crystal size decreases during dissolution, when the reaction is controlled by polypitting (formation and growth of pits), the edge free energy increases at the very first stage due to the creation of pits and dissolution steps. The constant composition experimental results demonstrate the development of surface roughness as the dissolution steps are formed, implying an increase of the total edge length during the reactions. In an exactly analogous mechanism to crystal growth, the participation of critical conditions involving dissolution steps is a possibility. In contrast to crystal growth, dissolution is a process of size reduction and, when the particle size is sufficiently reduced, critical phenomena become important so that the influence of size must be taken into consideration. This paper proposes such a model for dissolution reactions, and although these unusual phenomena probably apply to all mineral phases, they are more evident for sparingly soluble electrolytes in which the critical conditions are attained much more readily.     

Modeling of Compressible Cake Filtration

The transport of suspended solid particles in a liquid through porous media has importance from the viewpoint of engineering practice and industrial applications. Deposition of solid particles on a filter cloth or on a pervious porous medium forms the filter cakes. Following a literature survey, a governing equation for the cake thickness is obtained by considering an instantaneous material balance. In addition to the conservation of mass equations for the liquid, and for suspended and captured solid particles, functional relations among porosity, permeability, and pressure are obtained from literature and solved simultaneously. Later, numerical solutions for cake porosity, pore pressure, cake permeability, velocity of solid particles, concentration of suspended solid particles, and net rate of deposition are obtained. At each instant of time, the porosity decreases throughout the cake from the surface to the filter septum where it has the smallest value. As the cake thickness increases, the trends in pressure variation are similar to data obtained by other researchers. This comparison shows the validity of the theory and the associated solution presented. A sensitivity analysis shows higher pressure values at the filter septum for a less pervious membrane. Finally, a reduction in compressibility parameter provides a thicker cake, causes more particles to be captured inside the cake, and reduces the volumetric filtrate rate. The increase of solid velocity with the reduction in compressibility parameter shows that more rigid cakes compress less.     

Effect of Interfacial Failure on the Viscosity of High-density Polyethylene

The effect of a processing agent (Dynamar) on the viscosity and surface properties of high-density polyethylene (HDPE) has been studied. A capillary rheometer was used to measure the viscosity of HDPE compounds containing various concentrations of Dynamar as a function of time at constant apparent shear rates. The shear rates used are 250 and 500 sec⁻¹. The addition of a small amount of Dynamar leads to a marked reduction in viscosity. The viscosity decreases dramatically initially, then levels off to an equilibrium. The rate of the viscosity reduction and the equilibrium viscosity value depend upon the Dynamar concentration and the shear rate. This phenomenon can be explained by the migration of Dynamar from the bulk to the interface of HDPE melt and die wall, resulting in the formation of a lubrication layer. X-ray photoelectron spectroscopy and scanning secondary ion mass spectrometry analyses of the extrudates from a slit die reveal a low concentration of Dynamar at the surface. Adhesive failure at the Dynamar and HPDE interface is attributed to a reduction in viscosity. © 1997 John Wiley & Sons, Ltd.     

A note regarding the rate-determining step of reduction of solid manganates(V)

Potential pulse chonoamperometry was applied to immobilized microparticles of manganates(V) and MnO₂ in order to compare the nature of the rate determining step in their reduction. Manganates(V) were found to be reduced by an irreversible one-electron surface reaction, whereas MnO₂ reduction is controlled by a reversible reaction coupled to solid-state diffusion. The key importance of the coordination preferences of reacting metal ions on the surface layer of the solid particles is also discussed with respect to known mechanisms of electrochemical dissolution of Fe and Cr oxides. Received: 22 May 1998 / Accepted: 3 July 1998     

XPS and SEM study of calcite bearing rock powders in the case of reactivity measurement with HCl solution

The dissolution of seven natural limestones and calcareous rocks in hydrochloric acid solution was examined to investigate their capability for wet flue gas desulphurisation. All samples were crushed, ground and sieved to a size‐fraction of 150–250 µm. Thereafter they were subjected to a dissolution experiment utilising stepwise titration with hydrochloric acid. The dissolution rates of three calcareous rocks were found to be controlled by reaction kinetics, while the limestones showed mass transfer control. The surface characterisation was implemented before and after dissolution experiments using X‐ray photoelectron spectroscopy and scanning electron microscopy. Additional characterisation was carried out with X‐ray diffraction, X‐ray fluorescence and polarizing microscope. Initial reactivities have been shown to decrease in the order limestone, calcareous rock with high calcium concentration, calcareous rock with low calcium concentration. Coarse grain structure is proposed to decrease the initial reactivity. Copyright © 2011 John Wiley & Sons, Ltd.     

New insights into the Belousov-Zhabotinskii reaction derived from EQCM measurements at a gold electrode

Electrochemical quartz crystal microbalance experiments were used to study the classical Belousov-Zhabotinskii (BZ) homogeneous oscillating system. This system involves 2 × 10(-3) M Ce(III), 0.28 M malonic acid and 0.063 M bromate as the main initial components in 1 M sulfuric acid solution. The gold-evaporated electrodes on a 10 MHz AT-cut quartz crystal were used for potentiometric and amperometric studies while the changes in crystal frequency provided mass data. Under open-circuit conditions, during the BZ reaction, oscillations of the gold electrode potential in the range ca. 0.8 to 1.07 V (SCE) with a period about 80 s occurred. They were accompanied by periodic 10-15 ng [i.e. ca. 45-70 ng cm(-2)] changes in the electrode mass. At more positive potentials a decrease in electrode mass occurred, while the mass increased at more negative potentials. At a constant applied electrode potential, corresponding to either the upper or the lower potential limit attained under open-circuit conditions, periodic pulses of cathodic current occurred and were accompanied by mass changes. A continuous decrease in the electrode mass occurred at 1.06 V. A detailed examination of the gold electrode behavior in the solutions containing individual components of the system using cyclic voltammetry and quartz crystal microgravimetry provided the information needed to interpret the mass changes that occur in the complete system. No significant changes in the electrode mass occurred in sulfuric acid solution in the potential range where current and mass oscillations take place in the full BZ reaction solution. The same result was found in sulfuric acid solutions containing either Ce(III) or malonic acid. Dissolution of gold occurred in a sulfuric acid solution containing bromate or bromide ions. Adsorption of bromide ions on gold electrode occurred in Br(-)-containing sulfuric acid solution at more negative potentials. In the BZ system, dissolution of gold in the presence of oxidizing (bromate) and complexing (bromide) species causes the decrease in the electrode mass that accompanies the positive potential jump under open-circuit conditions, or the current pulse that occurs at more negative applied constant potentials. Cathodic current pulses occurring at a constant electrode potential (either 0.8 or 1.06 V) are associated with the reduction of Ce(IV) formed as a result of periodic homogeneous oxidation of Ce(III) by bromate. Bromide ions formed in the course of the BZ reaction appear to play a significant role in electrode mass changes, causing a mass decrease at more positive potentials due to dissolution of gold, and a subsequent mass increase at more negative potentials due to adsorption processes.     

锌电极在氢氧化钾溶液中阳极惰化时的表面相变的研究

本文试图进一步从动力学方面证明恒电位下的锌电极在KOH溶液中阳极惰化时,阳极溶解电流随时间而缓慢下降是由于“表面相变”所引起。用恒电位法测取了锌电极在不同浓度的KOH溶液中电流对时间的曲线。实验结果与推导的动力学方程式相符,表明在恒电位惰化条件下,锌电极表面上最初形成的[Zn(OH)_2]_(ad)在OH~-参预下徐缓地转变为较不易溶的[ZnO]_(ad),因而导致阳极溶解电流的下降。这一表面相变过程可表示如下:     

Dissolution kinetics of copper(II) oxide in water saturated by sulphur dioxide

The dissolution and the kinetics of dissolution of cooper(II) oxide in water saturated by sulphur dioxide has been studied. In the experiments, the particle size, the flow rate of the gas, the solid to liquid ratio, and the reaction temperature have been chosen as parameters, while the stirring rate was held constant. As a result of present experiments, it was observed that the decrease of the particle size, the solid to liquid ratio, and an increase of the reaction temperature increased the dissolution rate. It was also observed that the flow rate of sulphur dioxide in the range of its flow rate values did not affect the dissolution rate. The reaction kinetics of copper(II) oxide according to the heterogeneous reaction models was examined and it was found that the dissolution rate was controlled by chemical reaction. The calculated activation energy is 66.50 kJmol^?1. © 1994 John Wiley & Sons, Inc.     

Multiparametric dissolution of thorium-cerium dioxide solid solutions

The dissolution of Th(1-x)Ce(x)O(2) solid solutions samples prepared by thermal conversion of oxalate precursors was studied by varying independently several parameters (such as chemical composition, leachate acidity, leaching temperature, firing temperature, and crystallization state). The relative effects of these parameters on the normalized dissolution rate were examined. Either the obtained partial order related to the proton activity (n = 0.50 ± 0.01) or the activation energy (E(A) = 57 ± 6 kJ.mol(-1)) suggested that the dissolution was mainly driven by surface reactions occurring at the solid/liquid interface. Conversely to that observed for Th(1-x)U(x)O(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions, the chemical composition did not induce strong modifications of the chemical durability of the leached samples. While the initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 800 °C, it was mainly independent of the crystallite size (T ≥ 900 °C). The role of crystal defects (then that of preparation conditions) appeared thus important to consider since they contributed to modifications of the normalized dissolution rates of the same order of magnitude than that of the leachate acidity.