Non-stoichiometric Dissolution Research Articles

The Dissolution Behavior of Feldspar Minerals in Various Low-Molecular-Weight Organic Acids.

Feldspar is a high-abundance mineral in the earth's crust, and its natural weathering and dissolution processes are an important phenomenon on the earth's surface. This study focused on the dissolution behavior of silicon (Si) and aluminum (Al) in feldspar minerals (microcline and albite) when exposed to low-molecular-weight organic acids (LMWOAs). Various analytical techniques, including atomic absorption spectrophotometer, X-ray diffraction, scanning electron microscope, and Fourier-transform infrared spectroscopy, were employed to investigate these processes. The results revealed that the concentration of Si and Al released from alkali feldspar increased after treatment with LMWOAs, exhibiting non-stoichiometric dissolution. The Si/Al release ratio from feldspar deviated from the expected value of three. Among the LMWOAs tested, oxalic acid was found to be more effective in dissolving aluminum, while citric acid showed greater efficacy in dissolving silicon. Notably, the composite acid demonstrated the highest capacity for feldspar dissolution, with values of 538 μM (Si) and 287 µM (Al) after treatment for 720 h, respectively. The dissolution data for Si and Al in the organic acid solution was fittingly described by a first-order equation, with high correlation coefficients (R2 ≥ 0.992). The characterization of feldspar powders indicated that the (040) crystal plane of feldspar was particularly susceptible to attack by organic acids. In the presence of these acids, the chemical bonds Si (Al)-O, Si-Si(Al), and O-Si(Al)-O shifted to higher wavenumbers. Additionally, the surface corrosion morphology of feldspar exhibited distinct nanostructures, which became more pronounced with increasing exposure time. It was also observed that the reactivity of feldspar increased over time. These findings provide valuable insights into the natural dissolution process of feldspar and offer a new perspective for the study of this phenomenon.

Dissolution behaviors of a visible-light-responsive photocatalyst BiVO4: Measurements and chemical equilibrium modeling.

Despite of the extensive research in semiconductor photocatalysis with respect to material and device innovations, much of the fundamental aquatic chemistry of those new materials that governs their environmental hazard and implications remains poorly understood. BiVO4 has long been recognized as a promising visible-light-responsive photocatalyst. However, the solubility product (Ksp) of BiVO4 and the mechanistic understanding of the non-stoichiometric dissolution of BiVO4 remain unclear. Here, we investigated the solubility of BiVO4 via the observation on its non-stoichiometric dissolution in the pH range of 4-9. Combining dissolution experiments, adsorption behavior and thermodynamic equilibrium calculations, the Ksp of BiVO4 was determined to be 10-35.81±0.51. The solubility and stability of BiVO4 were strongly pH-dependent, with the lowest solubility and highest stability near pH 5. Furthermore, we tested the effect of illumination on the dissolution of BiVO4, which was significantly enhanced by light. Under both dark and illumination conditions, adsorption of dissolved bismuth by BiVO4 solids was the main reason for the non-stoichiometric dissolution of BiVO4, and could be modeled by including an additional surface complexation reaction. Thus, the results highlighted the importance of considering the dissolution of photocatalysts, and presented a feasible method to evaluate environmental stability and risks of other semiconductor materials.

Dissolution kinetics of garnierites from the Falcondo Ni-Laterite deposit (Dominican Republic) under acidic conditions

In this study we tested the dissolution kinetics of three garnierite types (serpentine-, talc- and sepiolite-dominated) at pH of 3 and 5 and room temperature by means of flow-through experiments. The samples selected for the study cover a wide range of mineralogical composition (Xtalc, between 0.34 and 0.78) and Ni content (between 0.8 and 2.4 atoms per formula unit Ni).The variation in the output concentrations of Ni, Mg and Si over time showed a depletion of Si (tetrahedral cation) and Ni (octahedral cation), resulting in non-stoichiometric dissolution reactions. Since the release of Mg was faster than that of Ni and Si, the overall dissolution rates were based on its release (RMg) and normalised to the final specific surface area.The steady-state dissolution rates of these garnierites decreased with increasing pH, and no correlation between RMg, talc content and Ni/Si ratio was found. The dissolution rate-pH dependence was calculated with the equation RMg=kH+×aH+nH+ (RMg is the Mg-based dissolution rate, kH+ is the dissolution rate constant and aH+nH+ is the dependence on the activity of H+), with kH+ and nH+ equal to -10−11.3 and 0.25 (serpentine-dominated experiments), -10−12.1 and 0.19 (talc-dominated experiments), and -10−11.6 and 0.25 (sepiolite-falcondoite). The provided rate laws for the different types of garnierites will allow a more accurate interpretation and understanding of the formation of these weathering profiles through the implementation of geochemical reactive transport modelling. Further work should include integrating data from experiments under neutral or slightly alkaline conditions.

Dissolution and solubility of rhomboclase and arsenic-substituted rhomboclase phases at pH 2–10 and 25–45 °C

The synthetic rhomboclase (Rbc) and arsenic-substituted rhomboclase phases (As-Rbc) were colorless well-formed rhombic thin tabular crystals. For their dissolution at initial pH 2–10 and 25–45 °C, the constituents were preferentially dissolved in the order of H5O2+ ≫ AsO4 > SO4 > Fe for ~1 h, H5O2+ ≫ AsO4 > Fe > SO4 from ~1 h to 720 h and finally H5O2+ ≫ AsO4 > SO4 = Fe, suggesting an initial nonstoichiometric dissolution of As-Rbc to form Fe-rich outlayers and a final stoichiometric dissolution. The PHREEQC simulation indicated that the dissolved iron, sulfate and arsenate were present mainly as FeSO4+/Fe(SO4)2-/Fe3+, HSO4−/FeSO4+ and H3AsO40/H2AsO4−, respectively. The SEM observation showed that the tops and edges of the crystals were dissolved and that the crystal surfaces were then gradually smoothed/rounded until all the edges and corners disappeared. For the dissolution at pH 2 and 25 °C for 720–2880 h, the ion-activity products [log IAP], which equal the solubility products [log Ksp] at equilibrium, and the Gibbs free energies of formation [ΔGf°] were estimated to be −8.80 ± 0.01 and −2706.44 ± 1.90 kJ/mol for the Rbc [(H5O2)1.434Fe0.855(SO4)2·2H2O], respectively. For the As-Rbc [(H5O2)y (Fe)z [(AsO4)x (SO4)1-x]2·2H2O], the log IAP and ΔGf° values varied from −10.03 ± 0.03 to −8.89 ± 0.01 and from −2680.51 ± 2.05 kJ/mol to −2548.71 ± 1.94 kJ/mol, respectively. There was a strong negative correlation between the log IAP and the AsO4/(AsO4+SO4) molar ratio of As-Rbc. The results of this study provide new interesting thermodynamic data for the most acidic sulfates Rbc and As-Rbc; these data can be applied in the geochemical modeling and prediction of acid mine drainage systems.

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

The steady-state dissolution rates of sepiolite were measured as a function of pH from mixed-flow reactor experiments at 25 °C in both citrate bearing and citrate-free aqueous solutions. Dissolution at pH ≤ 5.5 and pH ≥ 9.18 was found to be non-stoichiometric, with relatively more Mg being released at low pH, while Si was preferentially released at high pH. The steady-state dissolution rate of sepiolite at 25 °C, based on Si release rates at far from equilibrium conditions, r+ in mol/cm2/s at pH ≥ 4.44 can be described by.r+=10-15.291aH+2/aMg2+1/8,where ai refers to the activity of the subscripted aqueous species. This rate equation is based on a dissolution mechanism by which interstitial Mg is exchanged for 2H+, followed by the rate limiting release of partially detached Si-tetrahedra. At pH ≤ 4.44 and in citrate-free aqueous solutions, sepiolite dissolution rates based on Si release are similar to those measured for amorphous SiO2 despite the fact that the bulk fluid phase was undersaturated with respect to this solid. In contrast, in citrate bearing aqueous solutions, the Si release rate increases with decreasing pH following a trend consistent with the corresponding higher pH values. This difference is attributed to the increase of amorphous SiO2 dissolution rates in citrate bearing aqueous fluids, preventing the formation of substantial SiO2-rich surface layers. To further examine the extent of non-stoichiometric dissolution and its implications on solid phase transformations, batch experiments were performed in citrate bearing aqueous solutions at pH 2.57, 3.40 and 4.31. During these longer term experiments the fluid phase attained saturation with respect to amorphous SiO2. From mass balance calculations, Energy-Dispersive X-ray spectroscopy (EDS) and X-Ray powder Diffraction (XRD), it can be inferred that after 688 hours, 98% of the Mg was removed from the solid structure during pH 2.57 batch experiments, while SiO2 was retained in an amorphous solid phase, demonstrating the independence of the Mg compared to Si release to the aqueous solution.

Evolution of Mg/Ca and Sr/Ca ratios during the experimental dissolution of limestone

Magnesium (Mg) and strontium (Sr) preserved in speleothems are among the terrestrial climatic proxies. The mechanism of their release from impurities in limestone bedrock is usually neglected in paleoclimatic studies mainly because it is not very well understood; most cation release studies focus on pure minerals instead of limestone. Therefore, the Mg and Sr released from six types of limestone from the Moravian Karst (Czech Republic) were studied experimentally and compared with the dripwater data coming from the Punkva Caves (Moravian Karst) formed in two of the studied limestone types. The dissolution data were used to calibrate a kinetic dissolution model. The results show an incongruent dissolution of Mg-calcite and dolomite, pronounced especially in advanced stages of the dissolution. According to the results, the trace element ratios in solution were dependent mainly on (1) the limestone composition (i.e., the Mg-calcite and dolomite availability and the stoichiometry of individual minerals) and (2) on the extent of limestone-solution interaction (controlled by dissolution dynamics). Modeling indicated that the dynamics depend on the ratios of the area of rock–water and air–water interfaces to the water volume. Moreover, modeling suggests that the dynamics and subsequently also the trace element ratios are influenced by (a) the fresh surface dissolution during the initial stages of interaction and by (b) the preferential release of trace elements from mineral surfaces (nonstoichiometric dissolution). The effects of disturbed surfaces and preferential leaching are limited by available fresh surfaces. Fresh surfaces are formed through rock crushing, e.g., during physical weathering (in the field) or sample preparation (in a laboratory). The dripwaters from Punkva Caves had trace element ratios with values between the ratios resulting from the experimental dissolution of the limestones present on the site, indicating a naturally mixed composition of waters.This study has shown that Mg/Ca and Sr/Ca ratios in dripwaters might be controlled by various climate-independent variables, which raises further questions about utilizing the ratios as paleoclimatic proxies.

Stable and radiogenic strontium isotope fractionation during hydrothermal seawater-basalt interaction

The fluid-rock interactions occurring in hydrothermal systems at or near mid-oceanic ridges (MOR) were studied experimentally by reacting crystalline and glassy basalt with seawater at 250 °C and 290 °C while monitoring the liquid phase Sr isotopic evolution (87Sr/86Sr and δ88/86Sr). The results indicate that seawater Sr was incorporated into anhydrite during the early stages of seawater-basalt interaction. Liquid 87Sr/86Sr values trend towards the basaltic signature as non-stoichiometric basalt dissolution became the dominant process. This suggests that the interplay between fast Sr incorporation into secondary sulfates versus slow and continuous Sr liberation due to basalt dissolution at intermediate temperatures could partly explain previously identified discrepancies between MOR heat budget constraints and the marine 87Sr/86Sr budget. Late-stage anhydrite re-dissolution, likely caused by the liquid phase becoming more reducing through further basalt dissolution, as well as by quenching of the experiments, represents a potential explanation for the low amounts of anhydrite found in naturally altered oceanic basalt samples. Relatively strong decreases in liquid δ88/86Sr values in experiments with crystalline basalt suggest that isotopically light Sr was preferentially released due to non-stoichiometric dissolution. A slight preference of anhydrite for isotopically heavy Sr (εAnhydrite-Liquid88/86=0.034±0.019‰) is indicated by the data, suggesting that changes in MOR spreading rates and Sr removal could be recorded in the isotope compositions of authigenic, sedimentary Sr phases. Such insights will help to constrain the influence of hydrothermal systems on the oceanic stable Sr cycle.

Non-stoichiometric dissolution of sepiolite

Sepiolite is one of the few Mg-phyllosilicates thought to precipitate at Earth’s near-surface conditions, and in hydrothermal systems. Understanding the dissolution mechanisms of sepiolite can elucidate Mg-silicate dissolution processes in general and can provide a better estimation of the availability of divalent cations for CO2 sequestration. Batch experiments containing sepiolite were conducted over a pH range of 3.6-10.4, and the changes in Si/Mg ratio in solution were measured over time. The reaction products were analyzed with scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Under all conditions sepiolite dissolution proved to be non-stoichiometric, where Mg was preferentially released under acidic and neutral conditions, while Si was preferentially released in alkaline solutions. Under acidic and neutral conditions, sepiolite could thus release ample divalent cations for CO2 sequestration.

Dissolution mechanisms of chromitite: Understanding the release and fate of chromium in the environment

An understanding of the formation of toxic hexavalent chromium (Cr6+) in Cr-containing mine tailings and associated soils and sediments, requires an understanding of the underlying dissolution mechanisms of chromitite, a common chromite-bearing rock in both ophiolites suites and ultramafic intrusions. This study will examine dissolution mechanisms of chromitite in various acidic, neutral, and alkaline solutions containing cultivated bacteria, manganese oxides, sulfates, and phosphates. Dissolution of chromitite is non-stoichiometric under acidic, near-neutral, and alkaline pH conditions and involves the release of chromite nanoparticles and complex dissolution/re-precipitation reactions. Chromitite samples are obtained from the Black Thor chromite deposit in Northern Ontario, Canada; part of the “Ring of Fire” intrusive complex. The examined chromitite is composed of chromite, (Fe0.5Mg0.5) (Al0.6Cr1.4)O4 and clinochlore; Mg3[Si4O10(OH)2]·[MgAl1.33(OH)6], the latter phase contains ~3 wt% Cr in the form of chromite nanoparticles. Bulk dissolution data are collected after dissolution experiments with chromitite powders, and the chemical and mineralogical composition of treated chromitite surfaces is characterized with a combination of surface analytical techniques (X-ray photoelectron spectroscopy) and nano- to micro-analytical techniques (scanning electron microscopy, transmission electron microscopy, and focused ion beam technology). In the chromitite systems studied here, the non-stoichiometric dissolution of clinochlore is the dominant reaction, which results in the formation of a hydrous and porous silica precipitate that is depleted in chromite nanoparticles relative to untreated clinochlore. Complete replacement of clinochlore by hydrous silica on the surface of chromitite under acidic conditions promotes the release of chromite nanoparticles and results in higher Cr:Si in solutions and in higher proportions of secondary Cr species on its surface (secondary Cr species are defined as surface terminations that do not occur on an untreated chromite surface, such as -Cr3+-OH2 and -Cr6+-OH). Cultivated bacteria from a sulfide-bearing acid-mine drainage system affect neither the degree of dissolution nor the formation of secondary Cr species, whereas pyrolusite (MnO2) particles, and adsorbed or precipitated Fe- and Al-bearing hydroxide, -sulfate, and -phosphate species, affect release and re-adsorption of chromite nanoparticles and Cr-bearing species during dissolution of chromitite under acidic, neutral, and alkaline conditions. These results show that weathering of chromitite and the release of Cr into the environment are strongly controlled by factors such as dissolution rates of Cr-bearing silicates and chromite, the release of chromite nanoparticles, re-precipitation of amorphous silica, the presence of particles in solution, and the pH-dependence adsorption (or precipitation) of Fe- and Al-bearing hydroxides and sulfates.

Olivine Dissolution in Seawater: Implications for CO2 Sequestration through Enhanced Weathering in Coastal Environments.

Enhanced weathering of (ultra)basic silicate rocks such as olivine-rich dunite has been proposed as a large-scale climate engineering approach. When implemented in coastal environments, olivine weathering is expected to increase seawater alkalinity, thus resulting in additional CO2 uptake from the atmosphere. However, the mechanisms of marine olivine weathering and its effect on seawater–carbonate chemistry remain poorly understood. Here, we present results from batch reaction experiments, in which forsteritic olivine was subjected to rotational agitation in different seawater media for periods of days to months. Olivine dissolution caused a significant increase in alkalinity of the seawater with a consequent DIC increase due to CO2 invasion, thus confirming viability of the basic concept of enhanced silicate weathering. However, our experiments also identified several important challenges with respect to the detailed quantification of the CO2 sequestration efficiency under field conditions, which include nonstoichiometric dissolution, potential pore water saturation in the seabed, and the potential occurrence of secondary reactions. Before enhanced weathering of olivine in coastal environments can be considered an option for realizing negative CO2 emissions for climate mitigation purposes, these aspects need further experimental assessment.

Long-term flow-through column experiments and their relevance to natural granitoid weathering rates

Four pairs of fresh and partly-weathered granitoids, obtained from well-characterized watersheds—Merced River, CA, USA; Panola, GA, USA; Loch Vale, CO, USA, and Rio Icacos, Puerto Rico—were reacted in columns under ambient laboratory conditions for 13.8yrs, the longest running experimental weathering study to date. Low total column mass losses (<1wt.%), correlated with the absence of pitting or surface roughening of primary silicate grains. BET surface area (SBET) increased, primarily due to Fe-oxyhydroxide precipitation. Surface areas returned to within factors of 2–3 of their original values after dithionite extraction. Miscible displacement experiments indicated homogeneous plug flow with negligible immobile water, commonly cited for column experiments. Fresh granitoid effluent solute concentrations initially declined rapidly, followed by much slower decreases over the next decade. Weathered granitoid effluent concentrations increased modestly over the same time period, indicating losses of natural Fe-oxide and/or clay coatings and the increased exposure of primary mineral surfaces. Corresponding (fresh and weathered) elemental effluent concentrations trended toward convergence during the last decade of reaction. NETPATH/PHREEQC code simulations indicated non-stoichiometric dissolution involving Ca release from disseminated calcite and excess K release from interlayer biotite. Effluent 87Sr/85Sr ratios reflected a progressive weathering sequence beginning and ending with 87Sr/85Sr values of plagioclase with an additional calcite input and a radiogenic biotite excursion proportional to the granitoid ages.Effluents became thermodynamically saturated with goethite and gibbsite, slightly under-saturated with kaolinite and strongly under-saturated with plagioclase, consistent with kinetically-limited weathering in which solutes such as Na varied with column flow rates. Effluent Na concentrations showed no clear trend with time during the last decade of reaction (fresh granitoids) or increased slowly with time (weathered granitoids). Analysis of cumulative Na release indicated that plagioclase dissolution achieved steady state in 3 of the 4 fresh granitoids during the last decade of reaction. Surface-area normalized plagioclase dissolution rates exhibited a narrow range (0.95–1.26 10−13molesm−2s−1), in spite of significant stoichiometric differences (An0.21 to An0.50). Rates were an order of magnitude slower than previously reported in shorter duration experiments but generally 2–3 orders of magnitude faster than corresponding natural analogs. CrunchFlow simulations indicated that more than a hundredfold decrease in column flow rates would be required to produce near-saturation reaction affinities that would start to slow plagioclase weathering to real-world levels. Extending simulations to approximate long term weathering in naturally weathered profiles required additional decreases in the intrinsic plagioclase dissolution and kaolinite precipitation rates and relatively large decreases in the fluid flow rate, implying that exposure to reactive mineral surfaces is significantly limited in the natural environment compared to column experiments.

Coupling between mineral reactions, chemical changes in groundwater, and earthquakes in Iceland

AbstractChemical analysis of groundwater samples collected from a borehole at Hafralækur, northern Iceland, from October 2008 to June 2015 revealed (1) a long‐term decrease in concentration of Si and Na and (2) an abrupt increase in concentration of Na before each of two consecutive M &gt; 5 earthquakes which occurred in 2012 and 2013, both 76 km from Hafralækur. Based on a geochemical (major elements and stable isotopes), petrological, and mineralogical study of drill cuttings taken from an adjacent borehole, we are able to show that (1) the long‐term decrease in concentration of Si and Na was caused by constant volume replacement of labradorite by analcime coupled with precipitation of zeolites in vesicles and along fractures and (2) the abrupt increase of Na concentration before the first earthquake records a switchover to nonstoichiometric dissolution of analcime with preferential release of Na into groundwater. We attribute decay of the Na peaks, which followed and coincided with each earthquake to uptake of Na along fractured or porous boundaries between labradorite and analcime crystals. Possible causes of these Na peaks are an increase of reactive surface area caused by fracturing or a shift from chemical equilibrium caused by mixing between groundwater components. Both could have been triggered by preseismic dilation, which was also inferred in a previous study by Skelton et al. (2014). The mechanism behind preseismic dilation so far from the focus of an earthquake remains unknown.

Production of magnesium-rich solutions by acid leaching of chrysotile: A precursor to field-scale deployment of microbially enabled carbonate mineral precipitation

Carbonate minerals are one of the primary carbon sinks under investigation for sequestering anthropogenic carbon dioxide (CO2). Ultramafic rock has the potential to act as a magnesium source for carbonate precipitation reactions utilizing atmospheric CO2. This study characterizes the release of magnesium from chrysotile tailings from the Clinton Creek Asbestos Mine (Yukon, Canada) by means of a 42-day leaching experiment using sulfuric acid. Low acid concentrations (targeting 8.33% and 16.67% dissolution of chrysotile) resulted in stoichiometric, dissolution. Moderate (33.33% and 66.67% dissolution) and high (100% dissolution) acid concentrations resulted in non-stoichiometric dissolution of chrysotile. Here, the concentration ratio of Mg:Si in solution was greater than expected for moderate acid concentrations and lower than expected for high acid concentrations. The moderate acid reaction system demonstrated that as much as 84% of the magnesium found in chrysotile can be released into solution via chemical weathering. However, at high acid concentrations, Mg values were in fact lower than the moderate acid reactivity, presumably resulting from adsorption of soluble magnesium to amorphous silica, a byproduct of extensive magnesium leaching from chrysotile fibers. The production of magnesium-rich solutions in this experiment demonstrates that a highly concentrated starting material for magnesium carbonate precipitation reactions can be produced from chrysotile-bearing tailings, providing ultra-mafic hosted mines with an important resource for developing technological strategies for reducing their net carbon emissions. This process represents a win-win scenario for the management of chrysotile-bearing mine wastes, in which a hazardous material is transformed into a geologically stable mineral sink for ‘carbon dioxide storage’.

Impact of organic carbon on weathering and chemical denudation of granular basalt

Bench-scale experiments were conducted to determine rates and patterns of coupled organic matter infusion and weathering in a San Francisco volcanic field (Flagstaff, AZ) basalt sample under experimentally-modeled biotic and abiotic condition and to inform larger-scale collaborative studies at the landscape evolution observatory (LEO), Biosphere 2 (Tucson, AZ), where the same basaltic media is being used in a synthetic hillslope experiment. We postulated that mineral transformations depend significantly on the presence of organic carbon compounds including dissolved natural organic matter (DOM), with organic C simultaneously imprinting the chemical and mineralogical properties of primary and secondary solids undergoing incongruent dissolution. The present work reports on solute releases from Flagstaff basalt (FB) along laboratory-controlled gradients in DOM type and concentration. Loamy sand textured FB was subjected to flow-through, saturated column dissolution experiments using influent solutions with and without DOM compounds. Solutions included Ponderosa pine forest soil O-horizon extracts at three target concentrations: 7, 35, and 70mgL−1C, malic acid (MA) solutions at 7, 35, 70, and 140mgL−1C, and a control without DOM but having comparable inorganic solution composition. Chemical denudation rates for FB dissolution products were calculated from the concentration difference between outflow and inflow solutions. In addition, changes in the composition of the solid phase over the course of the experiment were determined using X-ray diffraction (XRD), X-ray fluorescence (XRF), and selective dissolution (SE). Column experiments supported dissolution rates derived from the literature and indicated a potentially strong effect of plant-derived organic ligands on mineral dissolution congruency and secondary phase precipitation. Both malic acid and DOM enhanced basalt dissolution, with malic acid having larger effect on per unit C basis. The largest relative effect of organic ligands was observed for Fe and Ti. Si/Al and Si/Fe chemical denudation rate ratios indicated non-stoichiometric dissolution, consistent with the observed precipitation of secondary phases, as confirmed by XRD and SE. DOM enhanced precipitation of secondary phases but this effect decreased with increase in amount of DOM added. Stoichiometry of the outflow solutions indicated that basaltic glass was preferentially dissolved in agreement with previous modeling predictions. Results suggest that biotic colonization of FB in the large-scale, long-term LEO experiment is likely to strongly influence the rate and congruency of FB weathering reactions and their distribution across the convergent hillslopes.

Effect of oxalate and pH on chrysotile dissolution at 25 C: An experimental study

The effect of pH on the kinetics of chrysotile dissolution was investigated at 25 °C in batch reactors over the pH range of 1 to 13.5, in oxalic solutions and buffered solutions of inorganic salts. Dissolution rates were obtained based on the release of Si and Mg. Results of the batch with inorganic buffers showed a strong dependence of dissolution rates on pH in the acid range. The logarithm of dissolution rates decreases with the pH with a slope of n = 0.27. Around neutral pH, a minimum is reached. From pH 8 to 12, rates increase again when pH increases, and follow a linear dependence with a shallow slope ( n = 0.06). The Mg/Si ratio shows a non-stoichiometric dissolution reaction with a preferential release of Mg 2+ at acidic pH; it decreases at neutral pH conditions according to Mg solubility. Our results suggest that the relative ease of the breaking of Mg-O bonds compared with Si-O bonds lead to dissolution via a series of steps involving Si and Mg, where Si release is the rate-limiting step. In the presence of 15 mmol L -1 oxalate, an intense catalytic effect from pH 1 to 6 is observed because of the capacity of the oxalate anion to form different complexes with Mg. The ratio of the rates derived from Mg and Si concentrations confirm an enhancement of non-stoichiometric dissolution compared with the series without oxalate. The mechanism of catalysis involves different processes depending on pH: At pH 1, XRD analysis confirms the formation of an amorphous silica phase dissolving all the Mg present in the chrysotile structure. At pH 2, XRD and FTIR results also confirm the precipitation of glushinskite, a magnesium oxalate phase. At pH 3 to 6, the presence of oxalate enhances dissolution almost by an order of magnitude compared with the experiments in inorganic buffered solutions. In this case, the mechanism could be due to the formation of aqueous or surface magnesium oxalate complexes. However, dissolution rates at neutral pH in the presence of oxalate are similar to those obtained in inorganic buffered solutions; the pH dependence at pH 8 to 13 is minimal. The increase in saturation and the drastic decrease in Mg solubility at these pH values could lead to precipitation of secondary phases coating the reactive mineral surface and inhibiting the surface. Results obtained in this study show that chrysotile dissolves faster in acid media and oxalate acts as a strong catalyst increasing the efficiency of magnesium release to solution at ambient temperature. These data may provide an excellent background to design and select optimal conditions in the previous acid treatment for carbon capture processes, as well as help to develop remediation process of asbestos contaminated sites.

Nontronite dissolution rates and implications for Mars

The Fe-rich smectite nontronite M+1.05[Si6.98Al1.02][Al0.29Fe3.68Mg0.04]O20(OH)4 has been detected using orbital data at multiple locations in ancient terrains on Mars, including Mawrth Vallis, Nilli Fossae, north of the Syrtis Major volcanic plateau, Terra Meridiani, and the landing site of the Mars Science Laboratory (MSL), Gale Crater. Given the antiquity of these sites (>3.0Ga), it is likely that nontronite has been exposed to the martian environment for long periods of time and therefore provides an integrated record of processes in near surface environments including pedogenesis and diagenesis. In particular, nontronite detected at Mawrth Vallis is overlain by montmorillonite and kaolinite, and it has been previously suggested that this mineralogical sequence may be the result of surface weathering. In order to better understand clay mineral weathering on Mars, we measured dissolution rates of nontronite in column reactors at solution pH values of 0.9, 1.7, and 3.0, and two flow rates (0.16ml/h and 0.32ml/h). Solution chemistry indicates stoichiometric dissolution at pH=0.9 and non-stoichiometric dissolution at pH=1.7 and 3.0. Mineral dissolution rates based on elemental release rates at pH=1.7 and 3.0 of Ca, Si and Fe follow the order interlayer>tetrahedral>octahedral sites, respectively. The behavior of all experiments suggests far from equilibrium conditions, with the exception of the experiment performed at pH 3.0 and flow rate 0.16ml/h. A pH-dependent dissolution rate law was calculated through Si release from experiments that showed no dependence on saturation (far from equilibrium conditions) under both flow rates and is r=10−12.06 (±0.123)·10−0.297 (±0.058)·pH where r has the units mol mineral m−2s−1. When compared to dissolution rates from the literature, our results indicate that nontronite dissolution is significantly slower than dissolution of the primary phases present in basalt under acidic conditions, suggesting that once nontronite forms it could remain stable at or near the surface of Mars for extended periods of time. Nontronite dissolution rates are faster than dissolution rates of montmorillonite (Rozalén et al., 2008) and kaolinite (Huertas et al., 1999), suggesting that chemical weathering of a mixed clay deposit would enrich the proportions of montmorillonite and kaolinite through the preferential dissolution of nontronite. Vis–NIR analyses of our reacted products and thermodynamic modeling of our experimental conditions both indicate the precipitation of amorphous silica within columns, and amorphous silica has also been observed in association with phyllosilicates on the martian surface (Mustard et al., 2008; Ehlmann et al., 2009; Murchie et al., 2009a). Therefore, chemical weathering of strata containing mixtures of montmorillonite, nontronite and kaolinite provides a potential formation mechanism for the mineralogic stratigraphy observed at Mawrth Vallis and other locations on Mars.

Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite

The wollastonite far from equilibrium dissolution kinetics and the formation and evolution of surface leached layers have been measured in mixed-flow reactors at 25°C, 0.9⩽pH⩽12 and reaction times to more than 3000h. Wollastonite dissolution rate decreases with increasing pH consistent withpH<4.8:r+,Ca,pH≤5(mol/cm2/s)=10-10.00aH+0.46andr+,Si,pH≤5(mol/cm2/s)=10-10.88aH+0.29pH≥4.8:r+,Ca-Si,pH≥5(mol/cm2/s)=10-11.41aH+0.18Dissolution stoichiometry, and the texture, structure and chemistry of wollastonite surfaces strongly depend on solution pH and duration of reaction. At 4.8⩽pH⩽12, after an initial period of non-stoichiometric dissolution, stoichiometric release of Ca and Si was observed. No significant change in the specific surface area of reacted powder was observed using BET and Small Angle X-ray Scattering (SAXS) measurements. In accord with these observations no chemical or structural changes were observed on the surfaces of reacted powders by X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance Infrared Fourier-Transformed (DRIFT) Spectroscopy, SAXS or Transmission Electron Microscopy (TEM). In contrast, at pH<4, preferential calcium release to solution is observed, which produces a thick, highly porous Ca-depleted altered layer affected by an extensive crazing. Both silica and calcium releases decline with time but, whereas Ca release continuously decreases, Si release reaches steady-state after 1000h and continues for more than 2500h of reaction. After 3000h of leaching at pH 1.0–2.2, a completely Ca-free solid is produced.Acid leaching induces drastic molecular and structural rearrangements of wollastonite surfaces that progressively affect the entire grain. After 100h of reaction, most characteristic Si–O and Ca–O bands of crystalline wollastonite disappear and the DRIFT spectra of the solid strongly resembles that of amorphous silica. XPS analysis showed very strong decrease of the Ca/Si surface concentration ratio after 10–100h of reaction at pH 2 and the removal of almost all Ca from the surface after 120h. XPS also evidences a decrease of the O1s peak width reflecting the disappearance of non-bridging oxygens. Condensation reactions and reconstruction of the leached layer is further demonstrated by 29Si MAS NMR spectroscopy which shows, as dissolution proceeds, the progressive formation of less reactive Q3 and Q4 structural units at the expense of initial highly reactive Q2. As a result, wollastonite dissolution as a function of time can be correlated to the relative proportion of the different Qi units.These results demonstrate that the altered layers formed at wollastonite surfaces are initiated by an intensive exchange of protons for Ca2+ (DCa–H∼10−13cm2/s; Kex∼104.9) and that they may not attain a steady-state thickness. This intensive Ca-H exchange together with the abrupt change in dissolution stoichiometry at pH>4 and the stoichiometric dissolution of pseudo-wollastonite strongly suggest that wollastonite leached layers are not formed via stoichiometric dissolution followed by the rapid precipitation of amorphous silica. Such layers have the potential to influence the short and long term weathering rates of important silicates such as pyroxenoids, pyroxenes, hornblendes, plagioclases and natural glasses.

Mechanism of nonoxidizing and oxidative pyrrhotite leaching

Pyrrhotite is one of the main components of sulfide ores. As a result, its behavior has a significant influence on the course of metallurgical processes and on the extraction of nonferrous metals. There is no definitive description of the mechanism of the chemical reactions that occur during nonstoichiometric iron sulfide dissolution. In some studies on pyrrhotite dissolution, pyrite formation is assumed; in others, elemental sulfur is assumed to form. The aim of this study is to clarify the mechanism of the chemical reactions of pyrrhotite dissolution in nonoxidizing acids during oxidative bacterial leaching. In this work, different methods of analysis have been used: scanning electron microscopy, micro X-ray spectral, X-ray diffraction, Mossbauer and IR-spectroscopy analysis. It has been established that acid pyrrhotite dissolution in nonoxidative conditions proceeds in stages, beginning with low-sulfur pyrrhotite dissolution. This occurs with the transition of iron ions to solution and to hydrogen sulfide, with the simultaneous formation of sulfur-rich iron sulfide in the residue from the leaching. Pyrrhotite oxidative bacterial leaching proceeds in two stages. In the first stage, the pyrrhotites are enriched with iron and then, in the second stage, the low-sulfur sulfides dissolve with the formation of iron sulfate (Fe2+). This study proposes a mechanism for sulfide oxidative bacterial leaching that can predict the parameters and the results of the process.

Silica- and sulfate-bearing rock coatings in smelter areas: Products of chemical weathering and atmospheric pollution I. Formation and mineralogical composition

Black rock-coatings occur in proximity to smelters and roast yards of the Greater Sudbury area, Ontario, Canada and contain information about the past interactions between surface minerals, and gaseous and particulate atmospheric components, many of which were pollutants. Rock-coatings were collected from various locations within the Sudbury area and are characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron microprobe analysis, infrared spectroscopy and X-ray photoelectron spectroscopy. Acidic fumigations and rain, the result of vast quantities of SO2 released from smelting, increased the chemical weathering rate of exposed rocks in the Sudbury area. Non-stoichiometric dissolution of the silicate minerals under acidic conditions resulted in the accumulation of silicic acid and the subsequent formation of a silica–gel type coating. The silica gel transformed overtime into amorphous silica, opal (opal C and opal-CT) and cristobalite. Dissolution of the underlying rock and also of metal-bearing particles by sulfuric acid resulted in the in situ formation of metal–sulfate-rich layers on the interfaces between the atmosphere and the silica-rich coating (atmosphere–coating interface, ACI) and between the silica-rich coating and the underlying rock (rock–coating interface, RCI). These metal–sulfate-rich layers contain nanometer aggregates of Fe–Cu–sulfate–hydroxide, goldichite, mereiterite, guildite, butlerite and antlerite. The silica-rich matrix also contains a mix of detrital grains from adjacent rocks and soils (feldspar, quartz, hematite, chlorite, montmorillonite) and non-dissolved smelter-derived nano- to micro-size particulates (metal–silicates, metal–oxides, C-spheres). The apparent disequilibrium between the embedded particles and the Fe–Cu-sulfates suggests that trapped nanoparticles were encapsulated into pores which prevented their equilibration with acidic metal–sulfate-bearing fluids. An XPS depth profile indicates a gradual transition from lower to higher concentrations of metals from the coating surface towards the metal–sulfate-rich layer on the ACI, which suggests that the outer surface of the coatings is currently leached on an angstrom scale by surface waters.

CO 2–water–basalt interaction. Numerical simulation of low temperature CO 2 sequestration into basalts

The interaction between CO 2-rich waters and basaltic glass was studied using reaction path modeling in order to get insight into the water–rock reaction process including secondary mineral composition, water chemistry and mass transfer as a function of CO 2 concentration and reaction progress ( ξ). The calculations were carried out at 25–90 °C and pCO 2 to 30 bars and the results were compared to recent experimental observations and natural systems. A thermodynamic dataset was compiled from 25 to 300 °C in order to simulate mineral saturations relevant to basalt alteration in CO 2-rich environment including revised key aqueous species for mineral dissolution reactions and apparent Gibbs energies for clay and carbonate solid solutions observed to form in nature. The dissolution of basaltic glass in CO 2-rich waters was found to be incongruent with the overall water composition and secondary mineral formation depending on reaction progress and pH. Under mildly acid conditions in CO 2 enriched waters (pH <6.5), SiO 2 and simple Al–Si minerals, Ca–Mg–Fe smectites and Ca–Mg–Fe carbonates predominated. Iron, Al and Si were immobile whereas the Mg and Ca mobility depended on the mass of carbonate formed and water pH. Upon quantitative CO 2 mineralization, the pH increased to >8 resulting in Ca–Mg–Fe smectite, zeolites and calcite formation, reducing the mobility of most dissolved elements. The dominant factor determining the reaction path of basalt alteration and the associated element mobility was the pH of the water. In turn, the pH value was determined by the concentration of CO 2 and extent of reaction. The composition of the carbonates depended on the mobility of Ca, Mg and Fe. At pH <6.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg. At pH >8, the mobility of Fe and Mg was limited due to the formation of clays whereas Ca was incorporated into calcite, zeolites and clays. Competing reactions between clays (Ca–Fe smectites) and carbonates at low pH, and zeolites and clays (Mg–Fe smectites) and carbonates at high pH, controlled the availability of Ca, Mg and Fe, playing a key role for low temperature CO 2 mineralization and sequestration into basalts. Several problems of the present model point to the need of improvement in future work. The determinant factors linking time to low temperature reaction path modeling may not only be controlled by the primary dissolving phase, which presents challenges concerning non-stoichiometric dissolution, the leached layer model and reactive surface area, but may include secondary mineral precipitation kinetics as rate limiting step for specific reactions such as retrieved from the present reaction path study.