Relative Dissolution Rates Research Articles

A study of the gold colloid dissolution kinetics in oxygenated ammoniacal thiosulfate solutions

Colloidal gold has a characteristic absorption peak at 530 nm that can be used to monitor the extent of gold dissolution in different lixiviant systems using an ultraviolet–visible spectrophotometer. This paper reports the kinetics of the dissolution of gold colloid in ammoniacal thiosulfate solutions using oxygen as a oxidant at pH 9.3–9.5 and temperature 25–48 °C. The relative rates of gold dissolution in different lixiviant systems are in descending order: oxygen–cyanide > copper(II)–ammonia–thiosulfate > oxygen–ammonia–thiosulfate > oxygen–ammonia > copper(II)–ammonia. The predominant gold(I) species in oxygenated thiosulfate solution based on the measured potentials on gold electrode is gold–thiosulfate complex (Au(S 2O 3) 2 3−). However, the measured potentials in the absence of thiosulfate are close to the reported values for the Au(NH 3) 4 3+/Au(NH 3) 2 + couple, which indicates that disproportionation or oxidation of Au(NH 3) 2 + occurs. The initial rates of gold dissolution show reaction orders of ≈0.3 with respect to oxygen and ammonia, and are independent of the concentration of thiosulfate and chloride. The dissolution kinetics follow a shrinking core model with an activation energy of 25–37 kJ mol −1, indicating a mixed chemical-diffusion-controlled reaction.

Solid-state characterization and dissolution properties of Naproxen–Arginine–Hydroxypropyl-β-cyclodextrin ternary system

The effect of ternary complexation of naproxen, a poorly water soluble anti-inflammatory drug, with hydroxypropyl-β-cyclodextrin and the basic aminoacid L-arginine on the drug dissolution properties has been investigated. Equimolar binary (drug–cyclodextrin or drug–arginine) and ternary (drug–cyclodextrin–arginine) systems were prepared by blending, cogrinding, coevaporation, and characterized by differential scanning calorimetry, thermogravimetric analysis, FT-IR spectroscopy, X-ray diffractometry. The dissolution behavior of naproxen from the different products was evaluated by means of a continuous flow through method. The results of solid state studies indicated the presence of strong interactions between the components in ternary coevaporated and coground systems, which were both of totally amorphous nature. In contrast, the presence of either free drug or free arginine was detected when the third component (cyclodextrin or aminoacid) was physically mixed, respectively, to the drug–arginine binary system (as physical mixture, coevaporate, or coground product) or to the drug–cyclodextrin binary system (as physical mixture, coevaporate, or coground product). All ternary combinations were significantly ( P<0.001) more effective than the corresponding binary drug-cyclodextrin and drug–arginine systems in improving the naproxen dissolution rate. The best performance in this respect was given by the ternary coevaporate, with about 15 times increase in terms of both drug relative dissolution rate and dissolution efficiency. The synergistic effect of the simultaneous use of arginine and cyclodextrin on the dissolution rate of naproxen was attributed to the combined effects of inclusion in cyclodextrin and salt formation, as well as to a specific role played by arginine in this interaction.

Use of Dissolved Sulfur Species to Measure Pyrite Dissolution in Water at pH 3 and 6

The oxidative dissolution of pyrite (FeS2 ) is difficult to study because of redox transformations of the iron and sulfur, and adsorptive and precipitative loss of products from solution. Quantitative measurement of pyrite dissolution can perhaps be done most accurately with electrochemical techniques, but monitoring of dissolution products is of interest for interpretation of water-quality data at mine drainage and other field sites where pyrite dissolution is occurring. This l aboratory study was undertaken to assess the usefulness of the sum of sulfate, sulfite, and thiosulfate in solution as a reaction progress variable, or RPV, for abiotic pyrite dissolution. Sulfate was the only sulfur species detected in solution, and thus was chosen as the RPV. Previous investigators have demonstrated that ferrous iron is not reliable as an RPV because of adsorption to the pyrite surface. Results indicated that sulfate is also not a highly accurate RPV. There was a sulfate loss from s olution, perhaps by adsorption, with the extent of loss varying with solution conditions. While not a highly accurate RPV for kinetics modeling, the sulfate concentration provided a measure by which relative rates of abiotic pyrite dissolution could be assessed for pH 3 and 6, and with oxygen and ferric iron as oxidants. Sulfate production was greatest at pH 6 in the presence of oxygen, but no sulfate was produced under anoxic conditions (Fe3+ added as oxidant) at the same pH. The s ulfate release under oxic conditions at this circumneutral pH was an order of magnitude greater than the release at pH 3 under both oxic and anoxic conditions.

Mineral dissolution in the Cape Cod aquifer, Massachusetts, USA: I . Reaction stoichiometry and impact of accessory feldspar and glauconite on strontium isotopes, solute concentrations, and REY distribution

To compare relative reaction rates of mineral dissolution in a mineralogically simple groundwater aquifer, we studied the controls on solute concentrations, Sr isotopes, and rare earth element and yttrium (REY) systematics in the Cape Cod aquifer. This aquifer comprises mostly carbonate-free Pleistocene sediments that are about 90% quartz with minor K-feldspar, plagioclase, glauconite, and Fe-oxides. Silica concentrations and pH in the groundwater increase systematically with increasing depth, while Sr isotopic ratios decrease. No clear relationship between 87Sr/86Sr and Sr concentration is observed. At all depths, the 87Sr/86Sr ratio of the groundwater is considerably lower than the Sr isotopic ratio of the bulk sediment or its K-feldspar component, but similar to that of a plagioclase-rich accessory separate obtained from the sediment. The Si-87Sr/86Sr-depth relationships are consistent with dissolution of accessory plagioclase. In addition, solutes such as Sr, Ca, and particularly K show concentration spikes superimposed on their respective general trends. The K-Sr-87Sr/86Sr systematics suggests that accessory glauconite is another major solute source to Cape Cod groundwater. Although the authigenic glauconite in the Cape Cod sediment is rich in Rb, it is low in in-grown radiogenic 87Sr because of its young Pleistocene age. The low 87Sr/86Sr ratios are consistent with equilibration of glauconite with seawater. The impact of glauconite is inferred to vary due to its variable abundance in the sediments. In the Cape Cod groundwater, the variation of REY concentrations with sampling depth resembles that of K and Rb, but differs from that of Ca and Sr. Shale-normalized REY patterns are light REY depleted, show negative Ce anomalies and super-chondritic Y/Ho ratios, but no Eu anomalies. REY input from feldspar, therefore, is insignificant compared to input from a K-Rb-bearing phase, inferred to be glauconite. These results emphasize that interpretation of groundwater chemistry, even in relatively simple aquifers, may be complicated by solute contributions from “exotic” accessory minerals such as glauconite. To detect such peculiarities, groundwater studies should combine the study of elemental concentration and isotopic composition of several solutes that show different geochemical behavior.

Mechanism of Sulfate Segregation during Glass Melting

ABSTRACTSulfate retention in glass during the vitrification process can be as low as 1/3 of the solubility limit, or can exceed the solubility limit if suspended in the glass in the form of droplets. This study is focused on the mechanism of incorporating and segregating sodium sulfate during the melting of an alkali-alumino-borosilicate glass batch. Batches were ramp heated at 4°C/min to temperatures ranging from 600°C to 1050°C and fractured for examination. Observation of the melts showed that as the batch temperature increases and the primary oxo-anionic, predominantly nitrate melt decomposes, the sulfate residue accumulates inside gas bubbles and is transported in them to the melt surface, where it remains segregated. The degree of sulfate incorporation into the final glass depends on the relative rates of sulfate dissolution in the borosilicate melt and sulfate lifting inside bubbles.

Results of Vertical Scanning Interferometry (VSI) of Dissolved Borosilicate Glass: Evidence for Variable Surface Features and Global Surface Retreat

ABSTRACTTwo disparate reaction mechanisms have been invoked to explain the reactivity of glass in contact with aqueous solution. One model is based on arguments from Transition State Theory (TST), which postulates that glass dissolution rates are surface reaction controlled. Alternatively, the second model argues that release of elements from glass to solution is governed by diffusion through an altered layer that forms on the surface of glass. Vertical Scanning Interferometry (VSI) is a new technique that allows one to observe surface features of specimens exposed to solution and may, potentially, be used to distinguish between competing models. We performed a series of dissolution experiments with a suite of glass compositions from chemically simple (sodium borosilicate) to complex (sixteen component borosilicate). Dissolution rates were determined using single-pass flow-through (SPFT) apparatus at 90°C and pH = 9 and over a solution saturation interval. Upon termination of the experiments, glass coupons were examined by VSI techniques. Effluent chemistry and VSI measurements indicate a nearly constant rate of 2.2 to 3.4 g m-2 d-1 over the solution interval; rates calculated from both methods are identical within experimental uncertainty. We argue that this glass is phase separated, and propose a model for dissolution based on the relative rates of dissolution of the two glass compositions. The data are consistent with a modified version of TST and indicate the potency of VSI methods to elucidate glass reaction mechanisms.

Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation

A theoretical evaluation of the origin and role of stored energy in the phenomenon of mechanically-activated dissolution of finely milled minerals has been undertaken. The changing mechanical behaviour of minerals with decreasing particle size is examined, leading to the treatment of stored energy in terms of the generation and retention of crystalline imperfections (dislocations) during extended milling. Development of amorphism, and polymorphism, is treated as a direct consequence of decreased periodicity in the crystal lattice that is associated with an increased accumulation of dislocations. Quantitative estimates of stored energy are presented and used to quantify changes in dissolution kinetics in terms of a decreased activation energy of dissolution and hence, an increase in the relative rate of dissolution.

Relative Dissolution Rates of Several Hydrated Limes

The relative dissolution rates of several commercial hydrated limes were evaluated. Significant differences in dissolution rates were observed ranging from a time of <10 s for 90% dissolution to 1 min for a 5% slurry. A 2% slurry dissolved faster than a 5% slurry. The speed of lime dissolution may be important for some treatment processes.

Microenvironments of pH in biofilms grown on dissolving silicate surfaces

Differences in pH between silicate–biofilm interfaces and bulk medium (ΔpH=pH interface−pH bulk) were detectable with commercial microelectrodes in cultures grown in unbuffered medium (|ΔpH|=0.27–1.08) for an arthrobacter species, but were generally beneath detection (ΔpH<0.04) for a streptomyces species. Biofilm half-thicknesses developed by Arthrobacter ranged from 1.2 to 11.5 mm, and were highly variable even for replicates. In buffered medium, neither bacterium produced a measurable ΔpH across the biofilms grown on silicates. The silicates consisted of polished hornblende, synthetic Fe-rich glass similar to hornblende in bulk composition, and two commercially available “float glasses,” one low-Fe and one high-Fe. The two species of soil bacteria investigated are both known to accelerate release of Fe from hornblende. For the Arthrobacter, values of |ΔpH| developed on hornblende crystal or glass substrates were generally larger than those developed on either float glass. Differences in ΔpH developed on different substrates could not be related simply to relative rates of dissolution of substrates. Differences between the two bacterial species are probably related to differences in (1) rates of growth, (2) production of low-molecular-weight organic acids, (3) physical characteristics of polysaccharide slimes excreted and/or (4) production of siderophores. Although values of |ΔpH| developed at mineral–water interfaces in natural systems may not be as large as those measured here except for water-saturated systems, it is probable that significant values of ΔpH (≤pH unit) develop where fast-growing, acid-producing microbes colonize slow-dissolving phases in the presence of unbuffered solutions.

Formation of Core/Rim Structures in Ti(C,N)‐WC‐Ni Cermets via a Dissolution and Precipitation Process

The core/rim structures of compounds in the Ti(C,N)‐xWC‐20Ni system have been investigated to determine the effect of WC and nitrogen content on the microstructure of the system. In addition, the relative dissolution rate of WC to Ti(C,N) in the system was studied by analyzing the rim compositions. Variations in the WC content had a much‐lesser influence, as an additive, on the general microstructure than that of other carbides that have been used in previous studies. However, the nitrogen content in Ti(C1−XNX) had a significant effect on its microstructure. The composition of the rim structure was determined by the ratio of solutes that were dissolved in the liquid binder, under the given processing conditions. The dissolution rate of WC was ∼2 and 5 times faster than that of Ti(C,N) in the system at temperatures of 1300° and 1510°C, respectively. The results have been interpreted in terms of phase stability and precipitation phenomena.

Enhanced dissolution of minerals: Microtopography and mechanical activation

A theoretical evaluation of the role of microtopography in the phenomenon of mechanically activated dissolution of finely milled minerals has been undertaken. Attention was directed to the role of surface steps produced by fracture and abrasion of milled particles. It was estimated that the activation energy for dissolution at the edge of steps is at least 20% less than that for dissolution from terraces (flat regions) between the edges, leading to enhanced dissolution of steps. The relative rates of dissolution at steps and terraces depend upon the particular mineral/solution process chemistry through the effects of process temperature (T) and the magnitude of the activation energies for dissolution. Enhanced dissolution of particles containing stepped surfaces was modelled by including the fraction, α, of dissolution sites on step edges relative to the total number of surface dissolution sites. The fraction, α, is expected to be mineral-sensitive through the effects of fracture and cleavage behaviour on step formation. The mean diameter, D M, of particles subject to enhanced dissolution by microtopography factors was shown to lie in the micron (μm) to sub-micron range. This range is consistent with particle sizes where enhanced dissolution by mechanical activation has been reported.

準安定相の析出と溶液媒介相転移のカイネティクス

This study summarizes the principal factors on nucleation, growth and polymorphic transformation termed “solution-mediated phase transformation” of some thermodynamically metastable phases in nature. The formation of the metastable phases occurring occasionally in hydrothermal deposits and sedimentary rocks is explained by the empirical Ostwald step rule, where the surface free energy and degrees of supersaturation are important factors. Transformation from a metastable phase to a stable polymorph in a solution can be kinetically analyzed by the relative rates of dissolution and growth of the two phases. The solubility of opal-CT as an example of metastable phase is usually higher than that of quartz as the stable phase, but this relation reverses in the case of nm-size grains according to the mesoscopic effect. In the latter case both solubilities of opal-CT and quartz become distnctly higher, and so the polymorphic relation among the silica minerals in nature is inferred to be in accordance with the Ostwald step rule. In contrast to solid-state transformation, metastably crystallized wurtzite appears to transform readily into stable sphalerite under hydrothermal conditions. This suggests that the solution-mediated transformation can be rapidly attained as compared with the solid-state transformation, showing that the dissolution-precipitation mechanism is predominant in the low-temperature polymorphic transformations.

Toughening of carbon fiber/thermoset composite by the morphology spectrum concept

AbstractMorphology spectrum toughening, a novel technique for toughening brittle carbon fiber/thermoset composites, was applied to a polyetherimide‐modified carbon fiber/dicyanate composite. Composites were prepared by inserting a polyetherimide film between the prepregs of carbon fabric impregnated by dicyanate alone, and by controlling both the relative rates of dissolution of the polyetherimide and the polymerization of the dicyanate by adding zinc stearate catalyst. A composite with 13 wt% polyetherimide afforded a mode I interlaminar fracture toughness of 1.4 kJ/m2, almost 1.8 times higher than that of a control composite containing same composition of polyetherimide. Two effects were observed. First, a thermoplastic continuous morphology formed in the center interply zone, where the concentration of polyetherimide was high, provided good fracture toughness. Second, a thermosetting continuous morphology formed in the fiber‐rich zone, where the concentration of polyetherimide was low, maintained good interfacial shear strength between the fiber and the matrix.

Isotopic ratios and release rates of strontium measured from weathering feldspars

Based upon dissolution of feldspars under controlled laboratory conditions, we conclude that Sr release, at pH 3, is neither consistently stoichiometric nor constant for the feldspars measured. Bytownite, microcline, and albite all initially release Sr at rates which are 5 (bytownite) to 160 (microcline) times faster than steady-state release rates. The Sr/Si ratios in the early effluents are significantly elevated compared to the bulk mineral values. The 87Sr/ 86Sr measured in effluent early in dissolution is higher than the bulk mineral 87Sr/ 86Sr for bytownite, but lower than bulk mineral ratios for microcline and albite. 87Sr/ 86Sr ratios for the feldspar powders also changed markedly during dissolution of the three phases. In part, nonstoichiometric release of Sr can be explained by the presence of secondary phases (exsolution lamellae or minute quantities of accessory phases) or by surface leaching. Although we infer that these feldspars eventually release Sr with isotopic composition roughly equal to that of the bulk mineral at steady-state, the feldspars dissolve at extremely different rates (bytownite releases Sr at a steady-state rate ∼10 2 to 10 3 times faster than albite and microcline, at pH 3). Therefore, a mixture of these feldspars, or of other minerals exhibiting vast differences in dissolution rate, will release 87Sr/ 86Sr ratios distinctly different from the bulk whole rock. In addition, initial Sr release rates of the minerals (bytownite > microcline > albite) differ from steady-state release rates (bytownite > albite > microcline), complicating analysis of weathering solutions. Log (rate constants) for bytownite, albite, and microcline decrease from −13.5 to −16.4 to −17.2 (mol Sr cm −2 s −1). Interpretation of catchment scale riverine 87Sr/ 86Sr ratios on the basis of whole-rock Sr isotopes is, therefore, problematic at best, and would require normalization of bulk isotopic ratios by relative rates of dissolution of Sr-contributing phases. We also argue that abraded feldspar particles formed naturally, for example, during glaciation, will show this initial transient nonstoichiometric release. However, once the transient release is completed (perhaps 10 2 to 10 3 yr after abrasion), as long as the solution chemistry remains relatively constant, stoichiometric release of cations from feldspars, including Sr, is expected. The most likely way, therefore, to increase riverine fluxes of major cations or radiogenic 87Sr/ 86Sr is to create highly flushed (and, therefore, far from equilibrium) water-rock systems such as glacial sediments and soils, with reactive minerals (e.g., carbonate, plagioclase, or biotite) containing significant radiogenic strontium.

Sorption of inorganic 14C on to calcite, montmorillonite and soil

Although 14C occurs naturally, it is also a waste product of the nuclear industry, and can be important because of its long half-life, high mobility as an anion, and ready incorporation into biota. Some aqueous inorganic species are anionic with migration minimally retarded by most geological and soil materials. Substantial retardation is expected when calcite is present, but there are few data to quantify this effect. The present study measured partition coefficient values, R d (concentration on solids divided by concentration in liquids), of 8–85 l kg −1 for a series of calcite materials and for a carbonated soil. In contrast, R d was zero for montmorillonite. The series of calcite materials varied in particle size. In order to investigate the effects of particle size, dissolution and degassing of 14C and 12C were monitored as pH was slowly decreased. The change in pH with addition of acid was strongly affected by particle size, as expected, but there was no systematic effect of particle size on the relative dissolution rates of 14C vs 12C, or on R d. Apparently, surface area was not a limiting factor in the interaction of 14C with these materials. The 14C in soil behaved most like the very fine calcite, indicating that the specific surface of the soil carbonate was similar to that of the very fine calcite.

Toughened carbon fiber composites with morphology gradient

The morphology and fracture behavior of polyimide modified carbon fiber/dicyanate composites were analyzed. Composites having morphology gradient were prepared by inserting a polyimide film between prepregs of carbon fabric impregnated by dicyanate alone, and controlling the relative rate of dissolution of polyimide and the polymerization of dicyanate by adding zinc stearate catalyst. The composites having morphology gradient showed Gic and Giic values of 2.2 kJ/m2 and 1.4 kJ/m2 respectively, which showed 3–5 times of those values of unmodified carbon fiber/dicyanate composites.

Novel Approach to the Analysis of in Vitro–in Vivo Relationships

The objective of this study was to quantify the dependence of degree of in vitro–in vivo correlation on the relative rates of dissolution and intestinal permeation and on the fraction of dose absorbed. The following equation was derived assuming first-order dissolution and permeation after oral drug administration: Fa=fa−1(1−α(α−1)−1(1−Fd) + (α−1)−1(1 −Fd)α), where Fa is the fraction of the total amount of drug absorbed at time t, fa the fraction of the dose absorbed at t=∞, α is the ratio of the first-order permeation rate constant to the first-order dissolution rate constant, and Fd is the fraction of dose dissolved in vitro at time t. This equation was examined in order to pursue a theoretical treatment of in vitro–in vivo correlation. The degree of in vitro–in vivo correlation between Fa and Fd was measured by r2. α was varied between 1000 and 0.001. fa was varied between 0.1 and 1.0. Points employed in the linear regression were geometrically balanced about the derived equation. r2 values decreased as α decreased for all values of fa. r2 values were virtually independent of fa for all values of α, except for 0.01<α<1.0. The slope of the regression was modulated by both α and fa; larger α and smaller fa each increased slope. Application of the equation to a piroxicam data set demonstrated the equation's utility relative to the USP Level A correlation approach. It is concluded that the degree of in vitro–in vivo correlation depends on the relative rates of dissolution and intestinal permeation and on the fraction of dose absorbed and that the derived model merits further study.

Morphological Characteristics of Diamond Populations in Relation to Temperature-Dependent Growth and Dissolution Rates

The octahedron is the growth form expected for a face-centered cubic crystal, and indeed, some diamonds are octahedra; however, most natural diamonds have morphologies that are intermediate between octahedra {111}, dodecahedra {110}, and, more rarely, cubes {100} or twins and aggregates on which elements of these forms are recognizable. Furthermore, the faces frequently are rounded, whereas plane surfaces generally would be expected for growth forms. This has resulted in the widespread belief that diamonds that are not cubes or octahedra have experienced resorption at some stage in their history. This view has been supported by many laboratory experiments involving a variety of oxidizing agents, including gases such as O2, H2O, and CO2, which are likely to have been involved in mantle or emplacement events. It also is known that both relative growth rates and relative dissolution rates of different faces are temperature dependent, so that the observed present morphology, combined where possible with data ...

High pH mobile phase effects on silica-based reversed-phase high-performance liquid chromatographic columns

Aqueous mobile phases above pH 8 often cause premature column failure, limiting the utility of silica-based columns for applications requiring high pH. Previous studies suggest that covalently bound silane ligands are hydrolyzed and removed by high-pH mobile phases. However, we found that the siloxane bonds for certain monomeric silanes are hydrolyzed very slowly from silica supports at pH 9–10. Therefore, bonded-phase packing degradation at high pH is a result mainly of silica support dissolution. The rate of column degradation for C 18 columns is influenced not only by the type and purity of silica support, but also by the nature of the silane stationary phase. We found different rates of degradation for several commercial C 18 columns. The relative rates of silica dissolution for these packings were determined by chemically measuring the silicate formed during column purging at high pH. The type and concentration of mobile phase organic modifier also significantly influences column degradation at high pH. Certain silica-based C 18 packings can be used for long periods at pH 9 without significant changes in chromatographic properties. Results of this study better define the practical utility and limitations of silica-based columns in high pH environments.

Chemical behavior of aluminum and phosphorus during dissolution of glass fibers in physiological saline solutions.

The dissolution of textile glass fibers of four different compositions has been investigated at 37 degrees C. In these glasses, which are isolation type, the P2O5 contents scatter between 0 and 2 wt% and Al2O3 from 0.12 to 3.4 wt%. Both static (30-mg fibers; 250-ml solution) and dynamic (50-mg fibers; 40 ml/day flow rate) conditions with or without bubbling of a gas mixture (95:5, N2-CO2) have been used. Two physiological solutions, one proposed by Kanapilly and the other by Scholtze, were used. After each run (1, 3, 7, 14, and sometimes 30, 62 days) the solutions were analyzed for B and Si by inductively coupled plasma (ICP), the weight losses were determined, and the residual solid were observed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Static runs give a better agreement between measured and calculated weight losses from solution analyses than dynamic experiments. SEM examinations indicate diameter reduction and formation of a hydrated Si-rich layer. Sometimes hollow tubes, suggesting the detachment of these layers, are observed. XPS and energy dispersive X-ray (EDX) analysis indicate the formation of a veneer of calcium phosphate for the most rapidly dissolving glass. In other cases an Al increase is observed at the solid solution interface. Whatever experimental conditions are used, the relative dissolution rates of the four glasses are identical. The kinetics may be modeled with variable dissolution rates from initial high values to final low ones. The latter reflect the very low solubility of the residual product.