Characterization of calcium isotopes in natural and synthetic barite

The mineral barite (BaSO₄) accommodates calcium in its crystal lattice, providing an archive of Ca-isotopes in the highly stable sulfate mineral. Holocene marine (pelagic) barite samples from the major ocean basins are isotopically indistinguishable from each other (δ^44/40Ca = −2.01 ± 0.15‰) but are different from hydrothermal and cold seep barite samples (δ^44/40Ca = −4.13 to −2.72‰). Laboratory precipitated (synthetic) barite samples are more depleted in the heavy Ca-isotopes than pelagic marine barite and span a range of Ca-isotope compositions, Δ^44/40Ca = −3.42 to −2.40‰. Temperature, saturation state, , and aCa²⁺/aBa²⁺ each influence the fractionation of Ca-isotopes in synthetic barite; however, the fractionation in marine barite samples is not strongly related to any measured environmental parameter. First-principles lattice dynamical modeling predicts that at equilibrium Ca-substituted barite will have much lower ⁴⁴Ca/⁴⁰Ca than calcite, by −9‰ at 0 °C and −8‰ at 25 °C. Based on this model, none of the measured barite samples appear to be in isotopic equilibrium with their parent solutions, although as predicted they do record lower δ^44/40Ca values than seawater and calcite. Kinetic fractionation processes therefore most likely control the extent of isotopic fractionation exhibited in barite. Potential fractionation mechanisms include factors influencing Ca²⁺ substitution for Ba²⁺ in barite (e.g. ionic strength and trace element concentration of the solution, competing complexation reactions, precipitation or growth rate, temperature, pressure, and saturation state) as well as nucleation and crystal growth rates. These factors should be considered when investigating controls on isotopic fractionation of Ca²⁺ and other elements in inorganic and biogenic minerals.     

Dissolution of sandstone powders in deionised water over the range 50–350 °C

Sandstone dissolution is a common water–rock reaction in the Earth’s crust, but a thorough understanding of this phenomenon is constrained by poorly determined kinetic data. To this end, kinetic data were determined for the dissolution of arkosic sandstone powders in deionised water (pH was about 7.0–7.3 and electrical conductivity was between 0.95 and 1.00 μS/cm). Release rates of dissolved elements were determined over the range 50–350 °C at 20, 15, and 10 MPa using a column flow-through pressure vessel reactor. The conductivity of the outlet solution, measured at room temperature, is dependent on the charge of major cations such as Na⁺, K⁺, Ca²⁺ and Mg²⁺ at these conditions. The conductivity of the outlet solution was used to determine the steady state of the dissolution of sandstone powders. The pH values of the outlet solutions at the steady state, measured ex situ at room temperature, were about 7.7, 8.3, 8.4, 8.4 and 7.6 at 75, 100, 150, 200 and 250 °C, respectively, at 10 MPa. Silicon, Na, K, Ca, Al and Mg are the major ions found in the solution at low temperatures, but Si is the only major ion retained at higher temperatures (>150 °C). Compared with static experiments, the flowing dissolution experiments occurred at conditions far from equilibrium. The relationship between temperature and dissolution rates of arkosic sandstone powders was described as log R = 0.005469t − 10.50 where R is the dissolution rates of sandstone powders in kg/(m² s), t is temperature in °C which ranged from 100 to 350 °C at 20 and 15 MPa, and the dissolution rates of sandstone powders were measured only for the major dissolved elements without oxygen in the outlet solutions.     

The effect of specific background electrolytes on water structure and solute hydration: Consequences for crystal dissolution and growth

Barium sulfate is used as a model system to illustrate how solution composition can affect processes of crystal dissolution and growth. Rates and modes of reactions as well as morphological features can be modified by the introduction of simple ionic salts (KCl, NaCl, LiCl, CsCl, NaF, NaNO₃), due to the effects of these electrolytes on water structure dynamics and solute hydration. Based on the results of AFM in situ experiments performed at supersaturation (Ω) = 10.6 ± 0.1 and ionic strength (IS) in the range of 0.005-0.1 M we show that growth and dissolution behavior of barite changes under conditions of constant thermodynamic driving force (Ω) and constant IS in a systematic way depending on the specific background electrolyte used to adjust IS. The results are interpreted in terms of the relationships between solution composition, ion properties and the consequent growth and dissolution behavior. Island spreading rate is affected by salt-specific effects on the activation energy barrier of expelling water molecules from solvation shells of barite building units. Dissolution kinetics depends on the balance between the energy expended on breaking solvent structure and the energy gain on hydrating Ba²⁺ and ions, which are specific for different electrolyte solutions. Nucleation rates are determined by the frequency of water exchange around a barium cation which also depends on solution composition. Relating the structure of the solution to its composition can help to understand phenomena such as growth and dissolution in the presence of organic additives or impurity incorporation.     

Seasonal cycle of suspended barite in the mediterranean sea

Biogenic barium (Ba_xs) was measured in suspended particles at the DYFAMED site in the northwestern Mediterranean Sea, on a monthly basis between February and June 2003. The barium content of barite (BaSO₄) micro-crystals was investigated using Scanning Electron Microscopy (SEM). Suspended particles were collected by filtration of small volumes of seawater (∼10 L), as well as large volumes up to 2400 L in March and in May. The Ba_xs profiles obtained from small-volume filtration display the typical mesopelagic maximum reported by earlier studies at ∼200 m depth, with concentrations up to 595 pmol L⁻¹. In addition, suspended Ba_xs was found almost exclusively in the form of micro-crystalline barite, except in February. The Ba_xs profiles obtained from large-volume filtration are consistent with the small-volume filtration findings, but reveal a significant Ba_xs peak of 1698 pmol L⁻¹ in the surface waters in May. Seasonal sampling at the DYFAMED site shows a net increase in barite concentration during phytoplanktonic blooms, confirming the involvement of biological systems in barite formation, as well as the potential role of barite as a primary productivity tracer. In addition, the coincidence between the mesopelagic barite maximum and the oxygen minimum layer suggests that barite is primarily found at depths of intense remineralization, in agreement with the hypothesis that barite forms within microenvironments of decaying organic matter.     

A combined experimental study of vivianite and As (V) reactivity in the pH range 2–11

Four different sets of experiments were completed in order to constrain vivianite [Fe₃(PO₄)₂ · 8H₂O] reactivity under conditions pertinent to As(V)-bearing groundwater systems. Firstly, titration experiments were undertaken in the pH range 4-9 to determine the zero point of charge (ZPC) of vivianite; showing that the ZPC lies at a pH of approximately 5.3. Secondly, the steady state dissolution rates of vivianite far from equilibrium were measured in aqueous solutions in the pH range 2–10 at 18.5 ^°C (±3 °C) using a fluidized bed reactor. The rate of vivianite dissolution, R, is given by     

Ra/Ra and Ra/Ba ratios to track barite formation and transport in the water column

We measured ²²⁸Ra_ex/²²⁶Ra_ex and ²²⁶Ra_ex/Ba_ex ratios in suspended and sinking particles collected at the Oceanic Flux Program (OFP) time-series site in the western Sargasso Sea and compared them to seawater ratios to provide information on the origin and transport of barite (BaSO₄) in the water column. The ²²⁸Ra_ex/²²⁶Ra_ex ratios of the suspended particles down to 2000 m are nearly identical to those of seawater at the same water depth. These ratios are much lower than expected if suspended barite was produced in surface waters and indicate that barite is produced throughout the mesopelagic layer. The ²²⁸Ra_ex/²²⁶Ra_ex activity ratios of sinking particles collected at 1500 and 3200 m varied mostly between 0.1 and 0.2, which is intermediate between the seawater ratio at these depths (<0.03) and the seawater ratios found in the upper 250 m (0.31-0.42). This suggests that excess Ba (i.e., Ba_ex = Ba_total − Ba_lithogenic), considered to be mainly barite, present in the sinking flux is a mixture of crystals formed recently in the upper water column, formed several years earlier in the upper water column, or formed recently in deeper waters. We observe a sizeable temporal variability in the ²²⁸Ra_ex/²²⁶Ra_ex ratios of sinking particles, which indicates temporal variability in the relative proportion of barite crystals originating from surface (with a high ²²⁸Ra_ex/²²⁶Ra_ex ratio) and mesopelagic (with a low ²²⁸Ra_ex/²²⁶Ra_ex ratio) sources. However, we could not discern a clear pattern that would elucidate the factors that control this variability. The ²²⁶Ra/Ba ratios measured in seawater are consistent with the value reported from the GEOSECS expeditions (2.3 dpm μmol⁻¹) below 500 m depth, but are significantly lower in the upper 500 m. High ²²⁶Ra_ex/Ba_ex ratios and elevated Sr concentrations in suspended particles from the upper water column suggest preferential uptake of ²²⁶Ra over Ba during formation of SrSO₄ skeletons by acantharians, which must contribute to barite formation in shallow waters. Deeper in the water column the ²²⁶Ra_ex/Ba_ex ratios of suspended particles are lower than those of seawater. Since ²²⁸Ra_ex/²²⁶Ra_ex ratios demonstrate that suspended barite at these depths has been produced recently and in situ, their low ²²⁶Ra_ex/Ba_ex ratios indicate preferential uptake of Ba over Ra in barite formed in mesopelagic water.     

Mode of occurrence and environmental mobility of oil-field radioactive material at US Geological Survey research site B,Osage-Skiatook Project,northeastern Oklahoma

Two samples of produced-water collected from a storage tank at US Geological Survey research site B, near Skiatook Lake in northeastern Oklahoma, have activity concentrations of dissolved ²²⁶Ra and ²²⁸Ra that are about 1500 disintegrations/min/L (dpm/L). Produced-water also contains minor amounts of small (5–50 μm) suspended grains of Ra-bearing BaSO₄ (barite). Precipitation of radioactive barite scale in the storage tank is probably hindered by low concentrations of dissolved SO₄ (2.5 mg/L) in the produced-water. Sediments in a storage pit used to temporarily collect releases of produced-water have marginally elevated concentrations of “excess” Ra (several dpm/g), that are 15–65% above natural background values. Tank and pit waters are chemically oversaturated with barite, and some small (2–20 μm) barite grains observed in the pit sediments could be transferred from the tank or formed in place. Measurements of the concentrations of Ba and excess Ra isotopes in the pit sediments show variations with depth that are consistent with relatively uniform deposition and progressive burial of an insoluble Ra-bearing host (barite?). The short-lived ²²⁸Ra isotope (half-life = 5.76 a) shows greater reductions with depth than ²²⁶Ra (half-life = 1600 a), that are likely explained by radioactive decay. The ²²⁸Ra/²²⁶Ra activity ratio of excess Ra in uppermost pit sediments (1.13–1.17) is close to the ratio measured in the samples of produced-water (0.97, 1.14). Declines in Ra activity ratio (excess) with sediment depth can be used to estimate an average rate of burial of 4 cm/a for the Ra-bearing contaminant. Local shallow ground waters contaminated with NaCl from produced-water have low dissolved Ra (<20 dpm/L) and also are oversaturated with barite. Barite is a highly insoluble Ra host that probably limits the environmental mobility of Ra at site B.     

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).     

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.     

Empirical prediction of the Pitzer’s interaction parameters for cationic Al species with both SiO2(aq) and CO2(aq): Implications for the geochemical modelling of very saline solutions

The Pitzer’s interaction parameters, λ_N–M, involving the Mth cationic Al species Al³⁺ or AlOH²⁺ or AlO⁺ and the Nth neutral species SiO_2(aq) (at temperatures of 25–300 °C) or CO_2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λ_N–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λ_N–M for both SiO_2(aq) and CO_2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO_2(aq)–Al³⁺ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO₂ solubilities in concentrated solutions of AlCl₃, 0.272 ± 0.010 (2σ), and Al₂(SO₄)₃, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO₂, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study.     

Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity

The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10⁻³ m BaCl₂, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10⁻³ m BaCl₂ and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl₂ shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba²⁺ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba²⁺ ions, respectively. This pattern indicates that most Ba²⁺ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba²⁺ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba²⁺, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl₂-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl₂ solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba²⁺ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba²⁺ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl₂, more Ba²⁺ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba²⁺ ions adsorbed directly on the mica in an amount similar to that in the BaCl₂ solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba²⁺ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface.     

Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental Ba and Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl₂, Na₂SO₄, NaCl and NaHCO₃) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with ¹³³Ba or ²²⁶Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both ¹³³Ba and ²²⁶Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of ¹³³Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The ¹³³Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of ¹³³Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10⁻¹¹ to 1.9 × 10⁻¹⁰ mol m⁻² s⁻¹ (2.4-16 μmol m⁻² d⁻¹), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10⁻¹⁰ mol m⁻² s⁻¹ or ∼49 μmol m⁻² d⁻¹) were determined.Radium uptake by barite was determined by monitoring the decrease of ²²⁶Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W_AB = 3720-6200 J/mol, a₀ = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W_AB = 1240 J/mol, a₀ = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings.     

Experimental determination of the role of diffusion on Li isotope fractionation during basaltic glass weathering

In order to use lithium isotopes as tracers of silicate weathering, it is of primary importance to determine the processes responsible for Li isotope fractionation and to constrain the isotope fractionation factors caused by each process as a function of environmental parameters (e.g. temperature, pH). The aim of this study is to assess Li isotope fractionation during the dissolution of basalt and particularly during leaching of Li into solution by diffusion or ion exchange. To this end, we performed dissolution experiments on a Li-enriched synthetic basaltic glass at low ratios of mineral surface area/volume of solution (S/V), over short timescales, at various temperatures (50 and 90 °C) and pH (3, 7, and 10). Analyses of the Li isotope composition of the resulting solutions show that the leachates are enriched in ⁶Li (δ⁷Li = +4.9 to +10.5‰) compared to the fresh basaltic glass (δ⁷Li = +10.3 ± 0.4‰). The δ⁷Li value of the leachate is lower during the early stages of the leaching process, increasing to values close to the fresh basaltic glass as leaching progresses. These low δ⁷Li values can be explained in terms of diffusion-driven isotope fractionation. In order to quantify the fractionation caused by diffusion, we have developed a model that couples Li diffusion with dissolution of the glassy silicate network. This model calculates the ratio of the diffusion coefficients of both isotopes (a = D₇/D₆), as well as its dependence on temperature, pH, and S/V. a is mainly dependent on temperature, which can be explained by a small difference in activation energy (0.10 ± 0.02 kJ/mol) between ⁶Li⁺ and ⁷Li⁺. This temperature dependence reveals that Li isotope fractionation during diffusion is low at low temperatures (T < 20 °C), but can be significant at high temperatures. However, concerning hydrothermal fluids (T > 120 °C), the dissolution rate of basaltic glass is also high and masks the effects of diffusion. These results indicate that the high δ⁷Li values of river waters, in particular in basaltic catchments, and the fractionated values of hydrothermal fluids are mainly controlled by precipitation of secondary phases.     

The iron-coating role on the oxidation kinetics of a pyritic sludge doped with fly ash

The present study examines the processes that control the oxidation attenuation of a pyrite-rich sludge (72 wt% pyrite) from the Iberian Pyrite Belt by the buffer capacity of a fly ash from Los Barrios power station (S Spain), using saturated column experiments. In addition, in order to understand the behaviour of both materials inside these experiments, a fly-ash leaching test and flow-through experiments with pyritic sludge were carried out. The fly-ash leaching test showed that after leaching this material with a slightly acid solution (Millipore MQ water; pH 5.6) the pH raised up to 10.2 and that the metals released by the fly-ash dissolution did not increase significantly the metal concentrations in the output solutions. The flow-through experiments with the pyritic sludge were performed at pH 9, 22 °C and O₂ partial pressure of 0.21 atm, to calculate the dissolution rate of this residue simulating the fly-ash addition. In the experiments Fe bearing oxyhydroxides precipitated as the sludge dissolved. In two non-stirred experiments the iron precipitates formed Fe-coatings on the pyrite surfaces preventing the interaction between the oxidizing agents and the pyrite grains, halting pyrite oxidation (this process is known as pyrite microencapsulation), whereas in two stirred experiments, stirring hindered the iron precipitates to coat the pyrite grains. Thus, based on the release of S (aqueous sulphate) the steady-state pyritic sludge dissolution rate obtained was 9.0 ± 0.2 × ⁻¹¹ mol m⁻² s⁻¹.In the saturated column experiments, the sludge dissolution was examined at acidic and basic pH at 22 °C and oxygen-saturated atmosphere. In a saturated column experiment filled with the pyritic sludge, pyrite oxidation occurred favourably at pH approx. 3.7. As the leachates of the fly ash yielded high basic pH, in another saturated column, consisting of an initial thick layer of fly-ash material and a layer of pyritic sludge, the pyrite dissolution took place at pH approx. 10.45. In this experiment, iron was depleted completely from the solution and attenuation of the sludge oxidation was produced in this conditions. The attenuation was likely promoted by precipitation of iron-bearing phases upon the pyritic surface forming Fe-coatings (of ferrihydrite and/or Fe(III) amorphous phases) that halted the pyrite oxidation (as in non-stirred flow-through experiments). Results suggest that buffering capacity of fly ash can be used to attenuate the pyrite-rich sludge oxidation.     

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:

     

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:

     

In situ Atomic Force Microscopy study of dissolution of the barite (0 0 1) surface in water at 30 °C

The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the <hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10⁻² nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10⁻² nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the <hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution.Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10⁻² nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10⁻² nm/s for those from other line defects).The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface.     

Adsorption of Sr and Ba at the cleaved mica-water interface: Free energy profiles and interfacial structure

Monte Carlo simulations show that the adsorption position of the Sr²⁺ or Ba²⁺ ion on the cleaved muscovite surface is determined by the radius of the ion’s first hydration shell, hydrogen bonding of the first shell water molecules with the basal oxygens of muscovite as well as a requirement of minimization of the number of muscovite’s lattice cations in the ion’s first coordination shell. Accordingly, Sr²⁺ or Ba²⁺ adsorbs in ditrigonal cavities at a distance of 1.12 Å or 1.35 Å, respectively, from the basal surface on dehydrated mica and above tetrahedral substitutions at a height of 1.93 ± 0.02 Å or 2.15 ± 0.03 Å, respectively, at the highest simulated water coverage of 28 H₂O per ion. The ion’s displacement from a ditrigonal cavity occurs upon adsorption of 2 H₂O per ion for Sr²⁺ and 3 H₂O per ion for Ba²⁺. At a coverage of 28 H₂O per ion, outer-sphere adsorption of Sr²⁺ or Ba²⁺ at a height of 3.9 ± 0.2 Å or 4.17 ± 0.07 Å, respectively, is possible albeit unfavorable on the free energy scale by 107 ± 7 kJ/mol or 89 ± 13 kJ/mol, respectively, as compared to inner-sphere adsorption. Activation energies for the transformation between inner-sphere and outer-sphere adsorptions are calculated to be 121 ± 3 kJ/mol for Sr²⁺ and 99 ± 10 kJ/mol for Ba²⁺. A comparison of these values with those reported recently for Mg²⁺ and Ca²⁺ results in an adsorption affinity sequence Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ in agreement with the sequence predicted recently for low dielectric constant solids (which include mica) (Sverjensky, 2006). A recent resonant anomalous X-ray reflectivity study of Sr²⁺ adsorption on muscovite (Park et al., 2006) has questioned the common assumption (Stumm, 1992), which is supported by the present simulation results, that inner-sphere adsorption is stronger than outer-sphere adsorption. A modification of the cleaved muscovite surface as a result of Sr²⁺ adsorption in muscovite’s ditrigonal cavities and related destabilization of muscovite’s hydroxyl groups is proposed as a possible reason for this controversy.     

Dissolution of Columbia River Basalt under mildly acidic conditions as a function of temperature: Experimental results relevant to the geological sequestration of carbon dioxide

Increasing attention is being focused on the rapid rise of CO₂ levels in the atmosphere, which many believe to be the major contributing factor to global climate change. Sequestering CO₂ in deep geological formations has been proposed as a long-term solution to help stabilize CO₂ levels. However, before such technology can be developed and implemented, a basic understanding of H₂O–CO₂ systems and the chemical interactions of these fluids with the host formation must be obtained. Important issues concerning mineral stability, reaction rates, and carbonate formation are all controlled or at least significantly impacted by the kinetics of rock–water reactions in mildly acidic, CO₂-saturated solutions. Basalt has recently been identified as a potentially important host formation for geological sequestration. Dissolution kinetics of the Columbia River Basalt (CRB) were measured for a range of temperatures (25–90 °C) under mildly acidic to neutral pH conditions using the single-pass flow-through test method. Under anaerobic conditions, the normalized dissolution rates for CRB decrease with increasing pH (3 ? pH ? 7) with a slope, η, of −0.15 ± 0.01. Activation energy, E_a, has been estimated at 32.0 ± 2.4 kJ mol⁻¹. Dissolution kinetics measurements like these are essential for modeling the rate at which CO₂-saturated fluids react with basalt and ultimately drive conversion rates to carbonate minerals in situ.