Experimental study of the effect of pH on the kinetics of montmorillonite dissolution at 25 °C

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 25 °C in batch and stirred flow-through reactors over the pH range of 1-13.5, in KNO₃ solutions. Dissolution rates were obtained based on the release of Si and Al at steady-state under far from equilibrium conditions. Dissolution was non-stoichiometric between pH 5 and 10, due to adsorption/precipitation of Al. Dissolution rates computed from batch and flow-through experiments were consistent, irrespective of the Si and Al concentrations. Sample pre-treatment and the interlayer cation do not affect the steady-state dissolution rate or stoichiometry of cation release. The rate dependence on pH can be described by:

     

Dissolution kinetics of synthetic Na-smectite. An integrated experimental approach

The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 °C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 °C, whereas at 25 °C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 °C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e.g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m² g⁻¹) and used to propose the dissolution rate law for the dissolution reactions as a function of ΔG_r at 25 °C and pH∼9:

     

An experimental study of illite dissolution kinetics as a function of ph from 1.4 to 12.4 and temperature from 5 to 50°C

Dissolution rates of natural illite (Illite du Puy) were measured from Si release rates during closed system experiments at pH ranging from 1.4 to 12.4 and temperatures ranging from 5 to 50°C. Experiments performed at 4<pH<11 exhibited reactive fluid Si/Al concentration ratios that were inconsistent with stoichiometric illite dissolution likely due to secondary phase precipitation. In contrast, after an initial preferential release of aluminum relative to silicon, the reactive fluid Si/Al concentration ratio evolution was consistent with stoichiometric illite dissolution for all experiments conducted at 4>pH>11. Si release rate decreased with time during all experiments; for those experiments performed at 4>pH>11 this observation can be attributed to either 1) changing reactive surface area; 2) the effect of initial fine particle dissolution; or 3) a negative order of the illite dissolution reaction with respect to aqueous Al and/or Si. Measured dissolution rates exhibited a typical variation with pH; rates decrease with increasing pH at acid conditions, minimize at near to neutral pH and increase with increasing pH at basic conditions. An empirical expression describing rates obtained in the present study is given by

     

Kaolinite dissolution and precipitation kinetics at 22 °C and pH 4

Dissolution and precipitation rates of low defect Georgia kaolinite (KGa-1b) as a function of Gibbs free energy of reaction (or reaction affinity) were measured at 22 °C and pH 4 in continuously stirred flowthrough reactors. Steady state dissolution experiments showed slightly incongruent dissolution, with a Si/Al ratio of about 1.12 that is attributed to the re-adsorption of Al on to the kaolinite surface. No inhibition of the kaolinite dissolution rate was apparent when dissolved aluminum was varied from 0 and 60 μM. The relationship between dissolution rates and the reaction affinity can be described well by a Transition State Theory (TST) rate formulation with a Temkin coefficient of 2

     

Dissolution kinetics of fosteritic olivine at 90-150 °C including effects of the presence of CO2

The steady state dissolution rate of San Carlos olivine [Mg_1.82Fe_0.18 SiO₄] in dilute aqueous solutions was measured at 90, 120, and 150 °C and pH ranging from 2 to 12.5. Dissolution experiments were performed in a stirred flow-through reactor, under either a nitrogen or carbon dioxide atmosphere at pressures between 15 and 180 bar. Low pH values were achieved either by adding HCl to the solution or by pressurising the reactor with CO₂, whereas high pH values were achieved by adding LiOH. Dissolution was stoichiometric for almost all experiments except for a brief start-up period. At all three temperatures, the dissolution rate decreases with increasing pH at acidic to neutral conditions with a slope of close to 0.5; by regressing all data for 2 ? pH ? 8.5 and 90 °C ? T ? 150 °C together, the following correlation for the dissolution rate in CO₂-free solutions is obtained:

     

The effects of organic acids on the dissolution of silicate minerals: A case study of oxalate catalysis of kaolinite dissolution

Most studies agree that the dissolution rate of aluminosilicates in the presence of oxalic and other simple carboxylic acids is faster than the rate with non-organic acid under the same pH. However, the mechanisms by which organic ligands enhance the dissolution of minerals are in debate. The main goal of this paper was to study the mechanism that controls the dissolution rate of kaolinite in the presence of oxalate under far from equilibrium conditions (−29 < ΔG_r < −18 kcal mol⁻¹). Two types of experiments were performed: non-stirred flow-through dissolution experiments and batch type adsorption isotherms. All the experiments were conducted at pH 2.5-3.5 in a thermostatic water-bath held at a constant temperature of 25.0, 50.0 or 70.0 ± 0.1 °C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. At constant temperature, there is a general trend of increase in the overall dissolution rate as a function of the total concentration of oxalate in solution. The overall kaolinite dissolution rates in the presence of oxalate was up to 30 times faster than the dissolution rate of kaolinite at the same temperature and pH without oxalate as was observed in our previous study. Therefore, these rate differences are related to differences in oxalate and aluminum concentrations. Within the experimental variability, the oxalate adsorption at 25, 50, and 70 °C showed the same dependence on the sum of the activities of oxalate and bioxalate in solution. The change of oxalate concentration on the kaolinite surface (C_s,ox) as a function of the sum of the activities of the oxalate and bioxalate in solution may be described by the general adsorption isotherm:

     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

Experimental study of brucite dissolution and precipitation in aqueous solutions: surface speciation and chemical affinity control

Dissolution and precipitation rates of brucite (Mg(OH)₂) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10⁻⁴ to 3 M), saturation index (−12 < log Ω < 0.4) and aqueous magnesium concentrations (10⁻⁶ to 5·10⁻⁴ M). Brucite surface charge and isoelectric point (pH_IEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pH_IEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m²) and lack of dependence on ionic strength predicts the dominance of >MgOH₂⁺ species at pH < 8 and their progressive replacement by >MgOH° and >MgO⁻ as pH increases to 10-12. Rates are proportional to the square of >MgOH₂⁺ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH₂⁺ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity:

     

An experimental study of the dissolution mechanism and rates of muscovite

Steady-state muscovite dissolution rates have been measured at temperatures from 60 to 201 °C and 1 ? pH ? 10.3 as a function of reactive solution K, Si, and Al concentration. The pegmatitic muscovite used in these experiments has a composition consistent with (Na_0.09, K_0.86)Fe_0.05Al_2.92Si_3.05O₁₀(OH_1.95, F_0.06). All experiments were performed in titanium mixed-flow reactors. All experiments were performed at far-from-equilibrium conditions with respect to muscovite. All reactive solutions were undersaturated with respect to secondary product phases other than for some experiments which were supersaturated with respect to bohemite and diaspore; steady-state dissolution was stoichiometric for all experiments that were undersaturated with respect to these phases.The variation of rates with reactive solution composition depends on the solution pH. At pH ? 7 rates were found to decrease significantly with increasing reactive fluid Al activity but be independent of aqueous SiO₂ activity. pH < 7 rates measured in the present study from 60 to 175 °C are consistent with

     

Experimental study of the effect of pH and temperature on the kinetics of montmorillonite dissolution

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 50 and 70 °C in stirred flow-through reactors over the pH range of 1-13.5. Experiments done at very acidic and very basic pH were far from equilibrium. Near neutral pH experiments were closer to equilibrium. The Al/Si release ratio, while initially being incongruent, ultimately approached the stoichiometric value in most of the experiments. Temperature, extreme pH, and time favor congruency. Rates can be described by:

     

Relation between the dissolution rates of single minerals and reservoir rocks in acidified pore waters

A series of kinetic experiments has been carried out to investigate the rates of dissolution (release of Al and Si) of common sandstone minerals in response to acidification of pore waters (pH = 3), using an experimental procedure designed to maximise the proportion of solid to fluid, and to minimise possible damage from agitation. The results have then been compared with those from experiments using disaggregated sandstones from two North Sea reservoirs. Experiments were carried out at 25 °C and 80 °C and in 0.01, 0.1 and 1 M NaCl solutions, with a pH of 3. Hydrochloric acid was used as the source of acidity and rate constants were determined based on both release of Al and Si. Mineral dissolution rates were closely comparable to literature values, despite the different experimental technique, except in the case of smectite where particle aggregation appears to have inhibited reaction. The dissolution rates calculated for reservoir sandstones based on their modal mineralogy and surface areas agree within a factor of 2 with the measured vales. Based on the reaction rates measured here, reservoir rocks rich in feldspar, illite and/or smectite are likely to react most rapidly with acidified pore waters.     

The effect of Al(OH)4 on the dissolution rate of quartz

The influence of Al(OH)₄⁻ on the dissolution rate of quartz at pH 10-13 and 59-89 °C was determined using batch experiments. Al(OH)₄⁻ at concentrations below gibbsite solubility depressed the dissolution rate by as much as 85%, and this effect was greater at lower pH and higher Al(OH)₄⁻ concentration. Dissolution rates increased with increasing temperature; however, the percent decrease in rate due to the presence of Al(OH)₄⁻ was invariant with temperature for a given H⁺ activity and Al(OH)₄⁻ concentration. These data, along with what is known about Al-Si interactions at high pH, are consistent with Al(OH)₄⁻ and Na⁺ co-adsorbing on silanol sites and passivating the surrounding quartz surface. The observed pH dependence, and lack of temperature dependence, of inferred Al(OH)₄⁻ sorption also supports the assumption that the acid-base behavior of the surface silanol groups has only a small temperature dependence in this range. A Langmuir-type adsorption model was used to express the degree of rate depression for a given in situ pH and Al(OH)₄⁻ concentration. Incorporation of the rate data in the absence of aluminate into models that assume a first-order dependence of the rate on the fraction of deprotonated silanol sites was unsuccessful. However, the data are consistent with the hypothesis proposed in the literature that two dissolution mechanisms may be operative in alkaline solutions: nucleophilic attack of water on siloxane bonds catalyzed by the presence of a deprotonated silanol group and OH⁻ attack catalyzed by the presence of a neutral silanol group. The data support the dominance of the second mechanism at higher pH and temperature.     

Reaction pathways for quartz dissolution determined by statistical and graphical analysis of macroscopic experimental data

In light of recent work on the reactivity of specific sites on large (hydr)oxo-molecules and the evolution of surface topography during dissolution, we examined the ability to extract molecular-scale reaction pathways from macroscopic dissolution and surface charge measurements of powdered minerals using an approach that involved regression of multiple datasets and statistical graphical analysis of model fits. The test case (far-from-equilibrium quartz dissolution from 25 to 300 °C, pH 1-12, in solutions with [Na⁺] ? 0.5 M) avoids the objections to this goal raised in these recent studies. The strategy was used to assess several mechanistic rate laws, and was more powerful in distinguishing between models than the statistical approaches employed previously. The best-fit model included three mechanisms—two involving hydrolysis of Si centers by H₂O next to neutral (>Si-OH⁰) and deprotonated (>Si-O⁻) silanol groups, and one involving hydrolysis of Si centers by OH⁻. The model rate law is

     

Dissolution and carbonation of a serpentinite: Inferences from acid attack and high P-T experiments performed in aqueous solutions at variable salinity

Dissolution experiments on a serpentinite were performed at 70 °C, 0.1 MPa, in H₂SO₄ solution, in open and closed systems, in order to evaluate the overall dissolution rate of mineral components over different times (4, 9 and 24 h). In addition, the serpentinite powder was reacted with a NaCl-bearing aqueous solution and supercritical CO₂ for 24 h at higher pressures (9-30 MPa) and temperatures (250-300 °C) either in a stirred reactor or in an externally-heated pressure vessel to assess both the dissolution rate of serpentinite minerals and the progress of the carbonation reaction. Results show that, at 0.1 MPa, MgO extraction from serpentinite ranges from 82% to 98% and dissolution rate varies from 8.5 × 10⁻¹⁰ mole m⁻² s⁻¹ to 4.2 × 10⁻⁹ mole m⁻² s⁻¹. Attempts to obtain carbonates from the Mg-rich solutions by increasing their pH failed since Mg- and NH₄- bearing sulfates promptly precipitated. On the other hand, at higher pressures, significant crystallization (5.0-10.4 wt%) of Ca- and Fe-bearing magnesite was accomplished at 30 MPa and 300 °C using 100 g L⁻¹ NaCl aqueous solutions. The corresponding amount of CO₂ sequestered by crystallization of carbonates is 9.4-15.9 mole%. Dissolution rate (from 6.3 × 10⁻¹¹ mole m⁻² s⁻¹ to 1.3 × 10⁻¹⁰ mole m⁻² s⁻¹) is lower than that obtained at 0.1 MPa and 70 °C but it is related to pH values much higher (3.3-4.4) than that (−0.65) calculated for the H₂SO₄ solution.Through a thorough review of previous experimental investigations on the dissolution kinetics of serpentine minerals the authors propose adopting: (i) the log rate [mole m⁻² s⁻¹] value of −12.08 ± 0.16 (1σ), as representative of the neutral dissolution mechanism at 25 °C and (ii) the following relationship for the acidic dissolution mechanism at 25 °C:

log rate=-0.45(±0.09)×pH-10.01(±0.30).     

Stoichiometry of smectite dissolution reaction

The dissolution stoichiometry of smectite-rich bentonites SAz-1, STx-1 and SWy-1 was studied at 50°C and pH 2 and 3 using flow-through reactors. In addition to smectite, these samples contain considerable amounts of silica phases (quartz, cristobalite and/or amorphous silica). As a result, the molar Al/Si ratios of the bulk samples are significantly lower than those of the pure smectite.Smectite dissolution was highly incongruent during the first few hundred to few thousand hours of the experiments. Release rates of Si, Mg, Ca and Na underwent a distinct transition from an initial period of rapid release to significantly lower release rate at steady state. A reversed trend was observed for release of Al, which gradually increased from very low starting release rate to higher release rate at steady state. At steady state the ratio of released Al to released Si was found to be constant and independent of the experimental conditions. We suggest that this ratio represents the Al/Si ratio of the smectite itself, and it is not influenced by the presence of accessory phases in the sample.The rapid release of calcium, sodium and magnesium from the interlayer sites is explained by ion-exchange reactions, whereas the fast release of silicon is explained by dissolution of amorphous silica. We interpret the initial slow release of Al as the result of inhibition of smectite dissolution due to coating or cementation of the smectite aggregates by amorphous silica. As the silica is dissolved, the aggregates fall apart and more smectite surfaces are exposed, resulting in an increase in the smectite dissolution rate. Thereafter, the system approaches steady state, in which the major tetrahedral and octahedral cations of smectite are released congruently.     

Transient and quasi-steady-state dissolution of biotite at 22-25°C in high pH, sodium, nitrate, and aluminate solutions

Biotite dissolution under conditions of high pH and high aluminum, sodium, and nitrate concentrations analogous to those found in tank wastes at the Hanford Site was investigated using continuously stirred flow-through reactors at 22 to 25 °C. Experiments were designed to simulate tank leaks into the Hanford vadose zone where Fe(II) from biotite is the dominant reducing agent available to immobilize certain contaminants. Both non-steady-state and steady-state dissolution kinetics were quantified; interest in non-steady-state kinetics derives from the inherently transitory nature of tank leaks. Biotite was conditioned in pH 8 solutions to simulate the alkaline environment of the Hanford sediment, and then reacted in pH 10-14 solutions, some including 0.055 M Al(NO₃)₃ and/or 2 M or 6 M NaNO₃. Initial dissolution transients (intervals of rapid release rates that decay to slower steady-state rates) showed fast preferential release of K followed by near-stoichiometric release of Si, Al, and Mg, and slower release of Fe. Each increase in pH resulted in a second transient with the greatest amounts of Si, Al, and K released at pH 14, followed by pHs 13, 12, 11, and 10. Fe release also was highest at pH 14, but unchanging at pHs 10-13 within experimental error. Transient releases at high pH are attributed to dissolution of amphoteric secondary phases such as ferrihydrite that are inferred from saturation calculations and solid analyses to form during the conditioning interval. Transient release of Si was inhibited by the presence of 0.055 M Al(NO₃)₃; the effects of Al(NO₃)₃ and NaNO₃ on the release rates of Al, Fe, Mg, and K were variable and generally outweighed by the effect of pH. Quasi-steady-state release rates were slowest at pH 11-12 (10^−12.2 mol biotite m⁻² s⁻¹ for Si) and increased in either direction in pH away from this minimum (to 10^−11.5 at pHs 8 and 14 for Si). Fe release rates at high pH were sufficient to account for observed Cr(VI) reduction at Hanford. The net release rates of the major framework cations, from which the biotite dissolution rate is inferred, may reflect the precipitation of secondary phases or the alteration of biotite to vermiculite. The most extensive solid-phase alterations were observed in Na-enriched solutions.     

Effect of pH and organic ligands on the kinetics of smectite dissolution at 25 °C

Forward dissolution rates of Na-Montmorillonite (Wyoming) SWy-2 smectite (Ca_0.06Na_0.56)[Al_3.08Fe(III)_0.38Mg_0.54] [Si_7.93 Al_0.07]O₂₀(OH)₄ were measured at 25 °C in a mixed-flow reactor equipped with interior dialysis compartment (6-8 kDa membrane) as a function of pH (1-12), dissolved carbonate (0.5-10 mM), phosphate (10⁻⁵ to 0.03 M), and nine organic ligands (acetate, oxalate, citrate, EDTA, alginate, glucuronic acid, 3,4-dihydroxybenzoic acid, gluconate, and glucosamine) in the concentration range from 10⁻⁵ to 0.03 M. In organic-free solutions, the Si-based rates decrease with increasing pH at 1 ? pH ? 8 with a slope close to −0.2. At 9 ? pH ? 12, the Si-based rates increase with a slope of ∼0.3. In contrast, non-stoichiometric Mg release weakly depends on pH at 1 ? pH ? 12 and decreases with increasing pH. The empirical expression describing Si-release rates [R, mol/cm²/s] obtained in the present study at 25 °C, I = 0.01 M is given by