Dissolution kinetics of galena in acid NaCl solutions at 25—75 °C

Experiments on dissolution kinetics of galena were performed in 1 mol l⁻¹ NaCl solutions at pH 0.43–2.45 and 25–75 °C. When the dissolution reaction is far from equilibrium, a linear relation exits between the dissolution rate, r, and the H⁺ ion activity, [H⁺]. The rate law for galena dissolution is given by the following equation: r=k[H⁺]. With respect to H⁺, the dissolution reaction is in the first order. The apparent rate constant, k, has values of 2.34×10⁻⁷ mol m⁻² s⁻¹ at 25 °C, 1.38×10⁻⁶ mol m⁻² s⁻¹ at 50 °C, and 7.08×10⁻⁶ mol m⁻² s⁻¹ at 75 °C. The activation energy of dissolution reaction is 43.54 kJ mol⁻¹. The mechanism of dissolution is suggested to be surface chemical reaction, and the rate determining step is the dissociation of the Pb–S bond of the surface complex, which releases Pb²⁺ into the solution.     

The effect of added dissolved calcium on calcite dissolution kinetics in aqueous solutions at 25°C

The kinetics of calcite dissolution in solutions containing dissolved Ca²⁺ has been investigated at 25°C, using a rotating disc apparatus. In acid solutions no effect of Ca²⁺ in solution is observed. The rate is dependent on the transport of H⁺ to the surface. In neutral to alkaline solutions the dissolution reaction is controlled by mixed kinetics and the conventional empirical representation of the dissolution rate results through the interaction of chemical and transport gradients in the diffusion boundary layer. The chemical reaction rate is a function of the gradient between the equilibrium and the surface concentrations of calcium carbonate, whereas the transport reaction can be described in terms of a series of gradients between the surface and the bulk of dissolved calcium and carbonate species. The presence of dissolved Ca²⁺ decreases the rate of the transport reaction, making the dissolution process more transport-controlled. The chemical rate constant is independent of the Ca²⁺ concentration in solution. The chemical rate constant for Carrara marble dissolution is determined to ∼2·10⁻² cm s⁻¹ and the diffusion coefficient for the transport reaction to ∼7.6·10⁻⁶ cm² s⁻¹.In natural systems with high Ca²⁺ concentrations and in absence of inhibitors of the surface chemical reaction, the dissolution of calcite may approach a transport-controlled reaction, especially in environments with restricted flow.     

Kaolinite and smectite dissolution rate in high molar KOH solutions at 35° and 80°C

Experiments measuring kaolinite and smectite dissolution rates were carried out using batch reactors at 35° and 80°C. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of clay of the ≤2 μm fraction and 80, 160 or 240 ml of 0.1–4 M KOH solution. An untreated but sized kaolinite from St. Austell and two treated industrial smectites were used in the experiments. One smectite is a nearly pure montmorillonite, while the second has a significant component of beidellitic charge (35%). The change in solution composition and mineralogy was monitored as a function of time. Initially, the 3 clays dissolved congruently. No new formed phases were observed by XRD and SEM during the pure dissolution stage. The kaolinite dissolution is characterized by a linear release of silica and Al as a function of the log of time. This relationship can be explained by a reaction affinity effect which is controlled by the octahedral layer dissolution. Far from equilibrium, dissolution rates are proportional to a^0.56±0.12_OH⁻ at 35°C and to a^0.81±0.12_OH⁻ at 80°C. The activation energy of kaolinite dissolution increases from 33±8 kJ/mol in 0.1 M KOH solutions to 51±8 kJ/mol in 3 M KOH solutions. In contrast to kaolinite, the smectites dissolve at much lower rates and independently of the aqueous silica or Al concentrations. The proportionality of the smectite dissolution rate constant at 35 and 80°C was a^0.15±0.06_OH⁻. The activation energy of dissolution appears to be independent of pH for smectite and is found to be 52±4 kJ/mol. The differences in behavior between the two kinds of minerals can be explained by structural differences. The hydrolysis of the tetrahedral and the octahedral layer appears as parallel reactions for kaolinite dissolution and as serial reactions for smectite dissolution. The rate limiting step is the dissolution of the octahedral layer in the case of kaolinite, and the tetrahedral layer in the case of smectite.     

Kyanite far from equilibrium dissolution rate at 0–22 °C and pH of 3.5–7.5

Acta Geochimica - Kyanite is an important and slow-dissolving mineral. Earlier work has measured its dissolution rate at high temperature and acidic pH, but experimental measurements at low...     

Experimental study of uraninite solubility in aqueous HCl solutions at 500°C and 1 kbar

Uraninite solubility in 0.001–2.0 m HCl solutions was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the following U(IV) species dominate in the aqueous solution: U(OH)₄⁰, U(OH)₂Cl₂⁰, and UOH Cl₃⁰ Using the results of uraninite solubility measurement, the Gibbs free energies of U(IV) species at 500°C and 1000 bar were calculated (kJ/mol): −9865.55 for UO₂(aq), −1374.57 for U(OH)₂ Cl₂⁰, and −1265.49 for UOH Cl₃⁰, and the equilibrium constants of uraninite dissolution in water and aqueous HCl solutions were estimated: UO₂(cr) = UO₂(aq), pK ₀ = 6.64; UO₂(cr) + 2HCl⁰ = U(OH)₂ Cl₂⁰, pK ₂ = 3.56; and UO₂(cr) + 3HCl⁰ = UOHcl₃⁰ + H₂O, pK ₃ = 3.05. The value pK ₁ ≈ 5.0 was obtained as a first approximation for the equilibrium UO₂(cr) + H₂O + HCl⁰ = U(OH)₃Cl⁰. The constant of the reaction UO₂(cr) + 4HCl⁰ = UCl₄⁰ + 2H₂O (pK ₄ = 7.02) was calculated taking into account the ionization constants of U Cl₄⁰ and U(OH)₄⁰, obtained by extrapolation from 25 to 500°C at 1000 bar using the BR model. Intense dissolution and redeposition of gold (material of experimental capsules) was observed in our experiments. The analysis and modeling of this phenomenon suggested that the UO_{2 + x} /UO₂ redox pair oxidized Au(cr) to Au⁺(aq), which was then reduced under the influence of stronger reducers.     

Experimental study of Pd hydrolysis in aqueous solutions at 25–70°C

The hydrolysis of the Pd²⁺ ion in HClO₄ solutions was examined at 25–70°C, and the thermodynamic constants of equilibrium K ₍₁₎⁰ and K ₍₂₎⁰were determined for the reactions Pd²⁺ + H₂O = PdOH⁺ + H⁺ and Pd²⁺ + 2H₂O = Pd(OH)₂⁰ + 2H⁺, respectively. The values of log K ₍₁₎⁰ = −1.66 ± 0.5 (25°C) and −0.65 ± 0.25 (50°C) and log K ₍₂₎⁰ = −4.34 ± 0.3 (25°C) and −3.80 ± 0.3 (50°C) were derived using the solubility technique at 0.95 confidence level. The values of log K ₍₁₎⁰ = −1.9 ± 0.6 (25°C), −1.0 ± 0.4 (50°C), and −0.5 ± 0.3 (70°C) were obtained by spectrophotometric techniques. The palladium ion is significantly hydrolyzed at elevated temperatures (50–70°C) even in strongly acidic solutions (pH 1–1.5), and its hydrolysis is enhanced with increasing temperature.     

The inhibition of gypsum and barite nucleation in NaCl brines at temperatures from 25 to 90°C

The inhibition of gypsum and barite nucleation in NaCl brines by phosphonates and polycarboxylates was studied at 25, 50, 70 and 90°C in terms of the prolonged induction period. Hexamethylene-diaminetetra (methylene phosphonic) acid (HDTMP) was found to be an effective inhibitor for the scaling of calcium sulfate dihydrate. Hydroxyethylene-1, 1-diphosphonic acid (HEDP) and phosphinopolycarboxylic acid (PPPC) were found to be effective inhibitors for barium sulfate scaling. The inhibition of nucleation through prolongation of the nucleation induction period likely resulted from an increase in the interfacial tension between the crystal and aqueous solution due to the presence of the inhibitors. It was observed that the inhibition of nucleation depends highly on the lattice cation/anion molar ratio and the pH of the solution as well as the degree of supersaturation and temperature for a given inhibitor.     

An experimental study of dissolution kinetics of Calcite, Dolomite, Leucogranite and Gneiss in buffered solutions at temperature 25 and 5°C

Laboratory experiments were carried out continuously for 30–35 days at 25 and 5°C in three different buffer solutions of pH 4.0, 2.2 and 8.4 to calculate dissolution rates of two minerals, calcite (CC) and dolomite (DM) and two rocks, leucogranite (LG) and gneiss (GN) from the Himalayan range. Calculated rates in terms of release of targeted elements versus time (Ca for CC; Mg for DM; Si for LG and GN) demonstrate direct correlation with temperature. Dissolution rates are higher at 25°C compared to 5°C. CC and DM were experimented only at pH 8.4 and results show that both undergo congruent dissolution with CC dissolving ∼5 times faster than DM. Ca and Mg exhibit average apparent activation energies (E _a) of 13.98 and 9.98 kcal mol⁻¹ respectively at pH 8.4 which reflects greater sensitivity of CC dissolution than DM dissolution towards an increase in temperature. Scanning Electron Microscope attached with Energy Dispersive X-Ray Analyser (SEM-EDX) data indicates that dissolution is controlled primarily by surface-reaction processes, with dislocation sites contributing maximum to the dissolution. As compared to CC and DM dissolution, LG and GN undergo relatively slower incongruent dissolution with precipitation of some secondary minerals as revealed from X-ray diffractometer (XRD) results. Rates of dissolution of LG is maximum at pH 2.2, moderate at pH 8.4 and least at pH 4.0, whereas GN shows maximum dissolution at pH 2.2, moderate at pH 4.0 and least at pH 8.4. A comparison in dissolution behavior of LG and GN at experimental conditions reveals that increase in Si-release rate in the temperature range between 5 and 25°C is maximum at pH 8.4 (∼3.4–4.5 times), moderate at pH 4.0 (∼3–1.8 times) and least at pH 2.2 (∼1.0–1.5 times). Within the experimental temperature range, calculated values of E _a for Si release during LG and GN dissolution advocates positive correlation with pH. A substantial decrease in initial values of Brunauer–Emmett–Teller (BET) surface area of DM, LG, and GN has been encountered at the end of the experiment, except for CC for which an increase is observed. The study clearly demonstrates the dissolution behavior of pure minerals and rocks under controlled conditions. The dissolution rates assume enormous significance for the release of trace elements from rocks/minerals to the reacting water.     

Calcium adsorption at the rutile-water interface: a potentiometric study in NaCl media to 250°C

Calcium adsorption by rutile was studied potentiometrically from 25 to 250°C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca²⁺ was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca²⁺ adsorption directly. These experiments revealed that Ca²⁺adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pH_znpc(NaCl) – pH). That is, as temperature increased, Ca²⁺ adsorption commenced at progressively more positive pH_znpc(NaCl) – pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca²⁺ adsorption at all temperatures as a result of either increased competition from Na⁺ or greater complexation of Ca²⁺ by Cl⁻. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H⁺ released / moles Ca²⁺ adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca²⁺ adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca²⁺ waters of hydration, are believed to be primarily responsible for the increase in adsorption with temperature.     

Dielectric loss in ore-forming chromium spinel at temperatures from 20 to 800 °C

Physical, physicochemical, and mineralogical-petrographic methods have been applied to samples of ophiolite-hosted chromite ore from different deposits and occurrences in the Urals. Temperature dependences of dielectric loss obtained for nine chromite ore samples consisting of 95–98% Cr spinel show prominent peaks indicating a relaxation origin of the loss. The analyzed samples have the loss peaks at different temperatures depending mainly on H = (FeO/Fe₂O₃)^? : (FeO/Fe₂O₃)^??, where (FeO/Fe₂O₃)^? and (FeO/Fe₂O₃)^?? are, respectively, the ferrous/ferric oxide ratios in the samples before and after heating to 800 °C, and H is thus the heating-induced relative change in the FeO/Fe₂O₃ ratio. These peak temperatures vary from 550 °C (sample 1, high-Cr chromium spinel with more than 52% Cr₂O₃) to 750 °C (sample 2, aluminous and magnesian spinel with less than 30% Cr₂O₃), and H ranges correspondingly from 1.61 to 5.49. The temperature of the loss peaks is related with H as H = 34.30 ? 11.52N + 1.20N², with an error of σ = 0.19 (N = T · 10^?2, T is temperature in °C).     

Experimental determination of uranium (IV) speciation in HF solutions at 500°C and 1000 bar

Uraninite solubility in HF solutions (0.0001–0.5 m) was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the predominant U(IV) species in aqueous solution are U(OH)₄⁰, U(OH)₃F⁰, and U(OH)₂ F₂⁰. Using the results of uraninite solubility measurement, the Gibbs free energies of the uranium (IV) species were calculated at 500°C and 1000 bar (kJ/mol): −986.55 for UO₂(aq), −1712.42 for U(OH)₃F⁰, −1755.53 for U(OH)₂F₂⁰, and the equilibrium constants of the uraninite solubility in water and HF solutions were estimated: UO₂(κ) = UO₂(aq), which is similar to UO₂(cr) + 2H₂O = U(OH)₄⁰, pK₀ = 6.64; UO₂(cr) + HF⁰ + H₂O = U(OH)₃F⁰, K₁ = 0.0513; UO₂(cr) + 2HF⁰ = U(OH)₂F₂⁰K₂ = 7.00 × 10⁻⁴. Approximate values K₃ = 5.75 × 10⁻³ and K₄ = 6.7 × 10⁻² were obtained for equilibria UO₂(cr) + 4HF⁰ =UF₄⁰ + 2H₂O and UO₂(cr) + 4HF = UF₄⁰ + 2H₂O. Maximum observed in the uranium concentration curve as a function of HF concentration can be explained by the decrease (to < 1) of activity coefficient ratio of HF⁰ to U(OH)₃F⁰ with increasing HF concentrations.     

Coupled alkali-feldspar dissolution and secondary mineral precipitation in batch systems: 1. New experiments at 200 °C and 300 bars

Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems.     

Inner-Helmholtz potential development at the hematite (α-Fe2O3) (0 0 1) surface

Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4-14 range, and develops a positive surface potential below pH 4 due to protonation of μ-OH⁰ sites (pK_1,1,0,int = −1.32). This site is resilient to deprotonation up to at least pH 14 (−pK_−1,1,0,int ? 19). The associated Stern layer capacitance of 0.31-0.73 F/m² is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO₃ etches the (0 0 1) surface, yielding a convoluted surface populated by sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as −0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7-11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm²) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces.     

Pressure solution of quartz aggregates under low effective stress (0.42–0.61 MPa) at 25–45 °C

Dissolution rates of pressure solution (PS) for quartz aggregates in 0.002 M NaHCO₃ solution were experimentally determined under low effective stress conditions of 0.42–0.61 MPa, and low temperatures of 25–45 °C. At temperatures of 25 °C, 35 °C, and 45 °C, the resultant silicon dissolution rates are 4.2 ± 1.2 × 10⁻¹⁵, 6.0 ± 1.0 × 10⁻¹⁵ and 7.8 ± 1.9 × 10⁻¹⁵ mol/cm²/s, respectively. Ratios between these dissolution rates and those of quartz sand at zero effective stress are 4.1 ± 1.2 at 25 °C, 3.0 ± 0.5 at 35 °C, and 2.4 ± 0.6 at 45 °C. As the uniaxial pressure was increased, the dissolution rate of PS also increased, though gradually decreased when the effective stress was kept constant. After the removal of stress, the dissolution rate was observed to increase once again. The activation energy of our PS experiments was determined to be approximately 24 kJ/mol, lower than the amount required for quartz sand dissolution to commence at zero effective stress. Our results clearly show that, even at such low temperature and effective stress, Si released into solution as a result of PS can be detected. This implies that experimental compaction of quartz aggregates can be measured even under such condition.     

An experimental study of hydroxo complex formation in basic and near-neutral solutions of rare-earth elements and yttrium at 25 ºC

We have studied the hydrolytic behavior of Y^3 + and trivalent ions of rare earth elements in aqueous solutions at 25 ºC. The stepwise stability constants of hydroxide complexes were measured by spectrophotometry, using m-cresol purple and 1-(2-pyridylazo)-2-naphthol as pH indicators at an ionic strength no more than 0.0005. The results showed that at pH ranging between 6.0 and 11.0 in freshly prepared solutions of REE trichlorides, lanthanides are presented as Ln3 +, Ln(OH)2 +, Ln(OH)2, and Ln(OH)3. The plots of the formation constants of + 0 monohydroxo complexes of 4f n ions M^3 + versus atomic number Z deviate from smooth ones and consist of four convex curves. This phenomenon is also observed in normalized spectra of REE concentrations in natural objects and is known as the tetrad effect. The obtained data give an insight into the relationship between REE complex formation and REE fractionation in geochemical processes and can be used for physicochemical modeling of geochemical systems.     

Carbonate dissolution in Mesozoic sand- and claystones as a response to CO2 exposure at 70 °C and 20 MPa

The response to CO₂ exposure of a variety of carbonate cemented rocks has been investigated using pressurised batch experiments conducted under simulated reservoir conditions, 70 °C and 20 MPa, and with a durations of up to14 months. Calcite, dolomite, ankerite and siderite cement were present in the unreacted reservoir rocks and caprocks. Core plugs of the reservoir rocks were used in order to investigate the alterations in situ. Crushing of the caprock samples was necessary to maximise reactions within the relatively short duration of the laboratory experiments. Synthetic brines were constructed for each batch experiment to match the specific formation water composition known from the reservoir and caprock formations in each well. Chemical matched synthetic brines proved crucial in order to avoid reactions due to non-equilibra of the fluids with the rock samples, for example observations of the dissolution of anhydrite, which were not associated with the CO₂ injection, but rather caused by mismatched brines.Carbonate dissolution as a response to CO₂ injection was confirmed in all batch experiments by both petrographical observations and geochemical changes in the brines. Increased Ca and Mg concentrations after 1 month reaction with CO₂ and crushed caprocks are ascribed to calcite and dolomite dissolution, respectively, though not verified petrographically. Ankerite and possible siderite dissolution in the sandstone plugs are observed petrographically after 7 months reaction with CO₂; and are accompanied by increased Fe and Mn contents in the reacted fluids. Clear evidence for calcite dissolution in sandstone plugs is observed petrographically after 14 months of reaction with CO₂, and is associated with increased amounts of Ca (and Mg) in the reacted fluid. Dolomite in sandstones shows only minor dissolution features, which are not clearly supported by increased Mg content in the reacted fluid.Silicate dissolution cannot be demonstrated, either by chemical changes in the fluids, as Si and Al concentrations remain below the analytical detection limits, nor by petrographical changes, as partly dissolved feldspar grains and authigenic analcime are present in the sediments prior to the experiments. It is noteworthy, that authigenic K-feldspar and authigenic albite in sandstones show no signs of dissolution and consequently seem to be stable under the experimental conditions.     

A thermodynamic model for the solubility of barite and celestite in electrolyte solutions and seawater to 200°C and to 1 kbar

This paper describes a model for barite and celestite solubilities in the Na–K–Ca–Mg–Ba–Sr–Cl–SO₄–H₂O system to 200°C and to 1 kbar. It is based on Pitzer's ion interaction model for the thermodynamic properties of the aqueous phase and on values of the solubility products of the solids revised in this work. It is shown how barite and celestite solubilities in electrolyte solutions can be accurately predicted as a function of temperature and pressure from previously determined Pitzer's parameters. The equilibrium constant for the BaSO₄(aq) ion pair dissociation reaction is calculated from recently reported barite solubility in Na₂SO₄ solutions from 0 to 80°C. Pressure corrections are evaluated through partial molal volume calculations and are partially validated by comparing model predictions to measured barite and celestite solubilities in pure water to 1 kbar and in NaCl solutions to 500 bars. The model is then used to investigate the tendency of ion pairing of Ca, Sr and Ba with sulfate in seawater. Finally, the activity coefficient of aqueous barium sulfate in seawater is calculated for temperature, pressure and salinity values found in the ocean and compared to published values.     

Uranium(VI) in aqueous solutions at 25°C and a pressure of 1 bar: Insight from experiments and calculations

The behavior of the 0.1 mNaCl + 0.002 mHCl + 1.9 × 10^?5 mUO₂(NO₃)₂ solution was studied at pH from 2.7 to 11.0, 25°C, and 1 bar in an argon atmosphere. The curve of variations in U concentration exhibits two minima at pH = 6.6 ± 0.7 and 10.0 ± 0.5. These minima are related to the precipitation of schoepite and clarkeite, respectively. The experimental data were used to refine the stability constants of U(VI) (hydroxo) complexes. For the polymer species of U(VI) with charges from +2 to ?1, the method of additivity of thermochemical increments was used, and increments of the linear relation were determined for the calculation of the Gibbs free energies of formation (Δ_fG _298.15 ⁰ ) of respective homologue series. The proposed method was applied to calculate the Δ_fG _298.15 ⁰ of formation of U(VI) (hydroxo)complexes containing from one to five uranium atoms.