Mineral Dissolution Rates Research Articles

珪酸塩鉱物の水溶液に対する溶解速度 –実験値とフィールド値の比較と流動・溶解カイネティックスモデルによる地下水組成の解釈–

珪酸塩鉱物の水溶液に対する溶解速度 –実験値とフィールド値の比較と流動・溶解カイネティックスモデルによる地下水組成の解釈–

An investigation into the leaching behaviour of copper oxide minerals in aqueous alkaline glycine solutions

Copper oxide minerals are normally extracted by acidic leaching followed by copper recovery with solvent extraction and electrowinning. However, copper oxide deposits often containing large amounts of acid consumable gangue leads to a very high acid consumption. In addition, if the oxide deposit also contains precious metals and iron bearing minerals, significant (lime) neutralisation costs are incurred to establish the conditions for subsequent cyanidation, with concomitant production of gypsum, potential formation of silver-locking jarosites and potential gel formation when acids interact with layer silicate minerals. In this study, an alternative aqueous alkaline glycine system has been employed to evaluate the batch leaching behaviour of copper oxide mineral specimens of the minerals azurite, chrysocolla, cuprite and malachite. The effects of glycine concentration and pH were investigated at ambient temperature and atmospheric pressure. Glycine concentration and pH both had a major effect on the copper extraction. Complete extraction of copper from azurite was achieved in <6h when glycine to copper ratio was 8:1. However, further investigation established the optimum leaching conditions as being pH11 and glycine to copper ratio of 4:1. Under such conditions 95.0%, 91.0%, 83. 8% and 17.4% of copper was extracted after 24h from the azurite, malachite, cuprite, and chrysocolla mineral specimens respectively. While the dissolution rates of the copper oxide minerals are markedly slower than acid leaching, the selective dissolution of copper over acid-consuming gangue minerals shows much potential. It was shown through UV–Vis spectroscopy that dissolved copper is in the cupric state and forms a neutral copper-glycinate complex under alkaline conditions. The study has shown that copper extraction from malachite, azurite and cuprite in aqueous alkaline glycine solutions is fast, whereas Cu extraction from chrysocolla was found to be poor and slow.

Experimental identification of CO2–oil–brine–rock interactions: Implications for CO2 sequestration after termination of a CO2-EOR project

Carbon dioxide enhanced oil recovery (CO2-EOR) has been widely applied to the process of carbon capture, utilization, and storage (CCUS). Here, we investigate CO2–oil–water–rock interactions under reservoir conditions (100 °C and 24 MPa) in order to understand the fluid–rock interactions following termination of a CO2-EOR project. Our experimental results show that CO2-rich fluid remained the active fluid controlling the dissolution–precipitation processes in an oil-undersaturated sandstone reservoir; e.g., the dissolution of feldspar and calcite, and the precipitation of kaolinite as well as solid phases comprising O, Si, Al, Na, C, and Ti. Mineral dissolution rates were reduced in the case that mineral surfaces were coated by oil. Mineral wettability and composition, and oil saturation were the main controls on the exposed surface area of grains, and mineral wettability in particular led to selective dissolution. In addition, the permeability of the reservoir decreased substantially due to the precipitation of kaolinite and solid-phase particles, and due to the clogging of less soluble mineral particles released by the dissolution of K-feldspar and carbonate cement, whereas porosity increased. The results provide insight into potential formation damage resulting from CO2-EOR projects.

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Lithologic differences give rise to the differential weatherability of the Earth’s surface and globally variable silicate weathering fluxes, which provide an important negative feedback on climate over geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore water samples with depth were collected from two ridgetops and at three sites along a slope transect in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to determine weathering mechanisms and to estimate weathering rates.Regolith formed on both lithologies is highly leached of most labile elements, although Mg and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and weathering fluxes were determined by integrating mass losses over the thickness of solid-state weathering fronts, and are therefore averages over the timescale of regolith development. Resulting long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9×10−14molm−2s−1, illite: 2.1×10−14molm−2s−1 and kaolinite: 4.0×10−14molm−2s−1. Long-term weathering fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, despite higher abundances of several elements in the granitic regolith. Contemporary weathering fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over space and time between these two watersheds indicate significant lithologic control of chemical weathering mechanisms and rates.

Influence of etch pit development on the surface area and dissolution kinetics of the orthoclase (001) surface

The (001) orthoclase surface was dissolved at 180°C and at far from equilibrium conditions with an alkaline solution (pH180°C=9) in a titanium open flow reactor. Vertical scanning interferometer (VSI) and atomic force microscope (AFM) surface monitoring were periodically used during the reaction process in order to quantify the surface topography evolution. The dissolution of the (001) orthoclase face occurs with the formation of diamond shape etch pits. Diamond pit diagonals are parallel to the [100] and [010] axes, and the pit walls are parallel to (6 5 6), 65¯6, (6¯511) and 6¯5¯11 planes. The etch pit size and global surface retreat of the (001) surface were found to increase linearly with time. Based on statistical treatments of etch pit development monitoring by AFM, we designed a numerical model aimed at reproducing and quantifying the total surface evolution. Numerical results show that the stabilization of etch pits doubles the calculated dissolution rate, partly due to the intrinsically higher reactivity of pit walls, consistent with a dissolution process in line with the periodic bond chain (PBC) theory. In addition, normalizing the dissolution rate by the initial surface area of the (001) orthoclase surface induces a 20% overestimation of the calculated dissolution rate, while the total surface area of the dissolving face reaches a steady state after a few days of reaction. Additional simulations conducted to assess the impact of defect parameters revealed a weak dependence of the dissolution rate on dislocation density, consistent with previous experimental observations. Overall, the combined effect of the various defect parameters does not affect the dissolution rate by more than an order of magnitude, and probably contributes to a moderate extent to the dispersion of mineral dissolution rate data reported in the literature.

Dissolution of nontronite in chloride brines and implications for the aqueous history of Mars

Increasing evidence suggests the presence of recent liquid water, including brines, on Mars. Brines have therefore likely impacted clay minerals such as the Fe-rich mineral nontronite found in martian ancient terrains. To interpret these interactions, we conducted batch experiments to measure the apparent dissolution rate constant of nontronite at 25.0°C at activities of water (aH2O) of 1.00 (0.01M CaCl2 or NaCl), 0.75 (saturated NaCl or 3.00molkg−1 CaCl2), and 0.50 (5.00molkg−1 CaCl2). Experiments at aH2O=1.00 (0.01M CaCl2) were also conducted at 4.0°C, 25.0°C, and 45.0°C to measure an apparent activation energy for the dissolution of nontronite.Apparent dissolution rate constants at 25.0°C in CaCl2-containing solutions decrease with decreasing activity of water as follows: 1.18×10−12±9×10−14molmineralm−2s−1 (aH2O=1.00)>2.36×10−13±3.1×10−14molmineralm−2s−1 (aH2O=0.75)>2.05×10−14±2.9×10−15molmineralm−2s−1 (aH2O=0.50). Similar results were observed at 25.0°C in NaCl-containing solutions: 1.89×10−12±1×10−13molmineralm−2s−1 (aH2O=1.00)>1.98×10−13±2.3×10−14molmineral m−2s−1 (aH2O=0.75). This decrease in apparent dissolution rate constants with decreasing activity of water follows a relationship of the form: logkdiss=3.70±0.20×aH2O−15.49, where kdiss is the apparent dissolution rate constant, and aH2O is the activity of water. The slope of this relationship (3.70±0.20) is within uncertainty of that of other minerals where the relationship between dissolution rates and activity of water has been tested, including forsteritic olivine (logR=3.27±0.91×aH2O−11.00) (Olsen et al., 2015) and jarosite (logR=3.85±0.43×aH2O−12.84) (Dixon et al., 2015), where R is the mineral dissolution rate. This result allows prediction of mineral dissolution as a function of activity of water and suggests that with decreasing activity of water, mineral dissolution will decrease due to the role of water as a ligand in the reaction.Apparent dissolution rate constants in the dilute NaCl solution (1.89×10−12±1×10−13molmineralm−2s−1) are slightly greater than those in the dilute CaCl2 solutions (1.18×10−12±9×10−14molmineralm−2s−1). We attribute this effect to the exchange of Na with Ca in the nontronite interlayer. An apparent activation energy of 54.6±1.0kJ/mol was calculated from apparent dissolution rate constants in dilute CaCl2-containing solutions at temperatures of 4.0°C, 25.0°C, and 45.0°C: 2.33×10−13±1.3×10−14molmineralm−2s−1 (4.0°C), 1.18×10−12±9×10−14molmineralm−2s−1 (25.0°C), and 4.98×10−12±3.8×10−13molmineralm−2s−1 (45.0°C).The greatly decreased dissolution of nontronite in brines and at low temperatures suggests that any martian nontronite found to be perceptibly weathered may have experienced very long periods of water–rock interaction with brines at the low temperatures prevalent on Mars, with important implications for the paleoclimate and long-term potential habitability of Mars.

Neodymium isotopes in authigenic phases, bottom waters and detrital sediments in the Gulf of Alaska and their implications for paleo-circulation reconstruction

The isotopic composition of neodymium (εNd) extracted from sedimentary Fe–Mn oxyhydroxide offers potential for reconstructing paleo-circulation, but its application depends on extraction methodology and the mechanisms that relate authigenic εNd to bottom water. Here we test methods to extract authigenic εNd from Gulf of Alaska (GOA) sediments and assess sources of leachate Nd, including potential contamination from trace dispersed volcanic ash. We show that one dominant phase is extracted via leaching of core-top sediments. Major and trace element geochemistry demonstrate that this phase is authigenic Fe–Mn oxyhydroxide. Contamination of leachate (authigenic) Nd from detrital sources is insignificant (<1%); our empirical results are consistent with established kinetic mineral dissolution rates and theory. Contamination of extracted εNd from leaching of volcanic ash is below analytical uncertainty. However, the εNd of core-top leachates in the GOA is consistently more radiogenic than bottom water. We infer that authigenic phases record pore water εNd, and the relationships of εNd among bottom waters, pore waters, authigenic phases and detrital sediments are primarily governed by the exposure time of bottom water to sea-floor sediments, rate of exchange across the sediment-water interface and the reactivity and composition of detrital sediments. We show that this conceptual model is applicable on the Pacific basin scale and provide a new framework to understand the role of authigenic phases in both modern and paleo-applications, including the use of authigenic εNd as a paleo-circulation tracer.

Spatial and temporal variations of base cation release from chemical weathering on a hillslope scale

Cation release rates to catchment runoff from chemical weathering were assessed using an integrated catchment model that included the soil's unsaturated, saturated and riparian zones. In-situ mineral dissolution rates were calculated in these zones as a function of pH, aluminum and dissolved organic carbon (DOC) concentrations along a hillslope in Northern Sweden where soil water was monitored over nine years. Three independent sets of mineral dissolution equations of varying complexity were used: PROFILE, Transition-State Theory (TST), and the Palandri & Kharaka database.Normalization of the rate-coefficients was necessary to compare the equations, as published rate-coefficients gave base cation release rates differing by several orders of magnitude. After normalizing the TST- and Palandri & Kharaka-rate coefficients to match the base cation release rates calculated from the PROFILE-equations, calculated Ca2+ and Mg2+ release rates are consistent with mass balance calculations, whereas those of Na+ and K+ are overestimated. Our calculations further indicate that a significant proportion of base cations are released from the organic soils in the near-stream zone, in part due to its finer texture. Of the three sets of rate equations, the base cation release rates calculated from the normalized TST-equations were more variable than those calculated using the other two sets of equations, both spatially and temporally, due to its higher sensitivity to pH. In contrast, the normalized Palandri & Kharaka-equations were more sensitive to variations in soil temperature.

Does crystallographic anisotropy prevent the conventional treatment of aqueous mineral reactivity? A case study based on K-feldspar dissolution kinetics

Which conceptual framework should be preferred to develop mineral dissolution rate laws, and how the aqueous mineral reactivity should be measured? For over 30years, the classical strategy to model solid dissolution over large space and time scales has relied on so-called kinetic rate laws derived from powder dissolution experiments. In the present study, we provide detailed investigations of the dissolution kinetics of K-feldspar as a function of surface orientation and chemical affinity which question the commonplace belief that elementary mechanisms and resulting rate laws can be retrieved from conventional powder dissolution experiments. Nanometer-scale surface measurements evidenced that K-feldspar dissolution is an anisotropic process, where the face-specific dissolution rate satisfactorily agrees with the periodic bond chain (PBC) theory. The chemical affinity of the reaction was shown to impact differently the various faces of a single crystal, controlling the spontaneous nucleation of etch pits which, in turn, drive the dissolution process. These results were used to develop a simple numerical model which revealed that single crystal dissolution rates vary with reaction progress. Overall, these results cast doubt on the conventional protocol which is used to measure mineral dissolution rates and develop kinetic rate laws, because mineral reactivity is intimately related to the morphology of dissolving crystals, which remains totally uncontrolled in powder dissolution experiments. Beyond offering an interpretive framework to understand the large discrepancies consistently reported between sources and across space scales, the recognition of the anisotropy of crystal reactivity challenges the classical approach for modeling dissolution and weathering, and may be drawn upon to develop alternative treatments of aqueous mineral reactivity.

Biofilm formation and interspecies interactions in mixed cultures of thermo-acidophilic archaea Acidianus spp. and Sulfolobus metallicus

The understanding of biofilm formation by bioleaching microorganisms is of great importance for influencing mineral dissolution rates and to prevent acid mine drainage (AMD). Thermo-acidophilic archaea such as Acidianus, Sulfolobus and Metallosphaera are of special interest due to their ability to perform leaching at high temperatures, thereby enhancing leaching rates. In this work, leaching experiments and visualization by microscopy of cell attachment and biofilm formation patterns of the crenarchaeotes Sulfolobus metallicus DSM 6482T and the Acidianus isolates DSM 29038 and DSM 29099 in pure and mixed cultures on sulfur or pyrite were studied. Confocal laser scanning microscopy (CLSM) combined with fluorescent dyes as well as fluorescently labeled lectins were used to visualize different components (e.g. DNA, proteins or glycoconjugates) of the aforementioned species. The data indicate that cell attachment and the subsequently formed biofilms were species- and substrate-dependent. Pyrite leaching experiments coupled with pre-colonization and further inoculation with a second species suggest that both species may negatively influence each other during pyrite leaching with respect to initial attachment and pyrite dissolution rates. In addition, the investigation of binary biofilms on pyrite showed that both species were heterogeneously distributed on pyrite surfaces in the form of individual cells or microcolonies. Physical contact between the two species seems to occur, as revealed by specific lectins able to specifically bind single species within mixed cultures.

Experimental study of static and dynamic interactions between supercritical CO2/water and Australian granites

Recent research in Enhanced Geothermal Systems (EGS) have given rise to the interests of a CO2-based EGS concept due to the unique thermo-physical properties of supercritical carbon dioxide (scCO2) in EGS applications. However, available studies related to CO2-based EGS are mostly theoretical investigations and relevant experimental study is highly scarce. To support the development of the new concept, this study conducts both static and dynamic fluid-rock interaction experiments between scCO2/water and three different Australian granites. A tailored fluid-rock integration apparatus was designed to conduct the above investigation. The pulverised granites were exposed to scCO2/water for up to 15days at the simulated reservoir temperatures of 200°C, 250°C and pressures of 20MPa and 35MPa. The results of static fluid-rock interactions show that the elements of Na, Si, K, Ca, Mg, Fe, Al were found dissolved into the scCO2-rich geofluid at an average rate of 4.5, 2.7, 1.6, 0.5, 0.3, 0.2, and 0.1ppm/day, respectively. The dynamic fluid-rock interactions shows that the average rate of mineral dissolution in the pure water was around 183ppm/day of Si, 14ppm/day of Na, 12ppm/day of Al, and 4ppm/day of K, while only 0.4–2.5ppm/day of Si, 0.4–1.6ppm/day of Na, and 0.1–0.3ppm/day of K for using scCO2-rich stream as the geofluid. The typical composition of the trace elements dissolved in both pure water and scCO2-rich geofluids were also identified. Fluid-rock equilibrium analyses shows that the geofluids obtained after the 15days of static fluid-rock interaction may have reached/were approaching geochemical equilibrium for some elements (e.g. Si), whilst for the flow-through experiments the reacted geofluids were far from geochemical equilibrium. The examination of the fluid-rock interaction using the three Australian granites highlighted the importance of mineral composition to fluid-rock interaction. The research provides valuable experimental data and insights for understanding the CO2-based EGS system.

Evaluation of mineral reactive surface area estimates for prediction of reactivity of a multi-mineral sediment

Our limited understanding of mineral reactive surface area contributes to significant uncertainties in quantitative simulations of reactive chemical transport in subsurface processes. Continuum formulations for reactive transport typically use a number of different approximations for reactive surface area, including geometric, specific, and effective surface area. In this study, reactive surface area estimates are developed and evaluated for their ability to predict dissolution rates in a well-stirred flow-through reactor experiment using disaggregated samples from the Nagaoka pilot CO2 injection site (Japan). The disaggregated samples are reacted with CO2 acidified synthetic brine under conditions approximating the field conditions and the evolution of solute concentrations in the reactor effluent is tracked over time. The experiments, carried out in fluid-dominated conditions at a pH of 3.2 for 650h, resulted in substantial dissolution of the sample and release of a disproportionately large fraction of the divalent cations. Traditional reactive surface area estimation methods, including an adjusted geometric surface area and a BET-based surface area, are compared to a newly developed image-based method. Continuum reactive transport modeling is used to determine which of the reactive surface area models provides the best match with the effluent chemistry from the well-stirred reactor. The modeling incorporates laboratory derived mineral dissolution rates reported in the literature and the initial modal mineralogy of the Nagaoka sediment was determined from scanning electron microscopy (SEM) characterization. The closest match with the observed steady-state effluent concentrations was obtained using specific surface area estimates from the image-based approach supplemented by literature-derived BET measurements. To capture the evolving effluent chemistry, particularly over the first 300h of the experiment, it was also necessary to account for the grain size distribution in the sediment and the presence of a highly reactive volcanic glass phase that shows preferential cation leaching.

Chemical models for martian weathering profiles: Insights into formation of layered phyllosilicate and sulfate deposits

Numerical chemical models for water–basalt interaction have been used to constrain the formation of stratified mineralogical sequences of Noachian clay-bearing rocks exposed in the Mawrth Vallis region and in other places on cratered martian highlands. The numerical approaches are based on calculations of water-rock type chemical equilibria and models which include rates of mineral dissolution. Results show that the observed clay-bearing sequences could have formed through downward percolation and neutralization of acidic H2SO4–HCl solutions. A formation of weathering profiles by slightly acidic fluids equilibrated with current atmospheric CO2 requires large volumes of water and is inconsistent with observations. Weathering by solutions equilibrated with putative dense CO2 atmospheres leads to consumption of CO2 to abundant carbonates which are not observed in clay stratigraphies. Weathering by H2SO4–HCl solutions leads to formation of amorphous silica, Al-rich clays, ferric oxides/oxyhydroxides, and minor titanium oxide and alunite at the top of weathering profiles. Mg-Fe phyllosilicates, Ca sulfates, zeolites, and minor carbonates precipitate from neutral and alkaline solutions at depth. Acidic weathering causes leaching of Na, Mg, and Ca from upper layers and accumulation of Mg-Na-Ca sulfate-chloride solutions at depth. Neutral MgSO4 type solutions dominate in middle parts of weathering profiles and could occur in deeper layers owing to incomplete alteration of Ca minerals and a limited trapping of Ca to sulfates. Although salts are not abundant in the Noachian geological formations, the results suggest the formation of Noachian salty solutions and their accumulation at depth. A partial freezing and migration of alteration solutions could have separated sulfate-rich compositions from low-temperature chloride brines and contributed to the observed diversity of salt deposits. A Hesperian remobilization and release of subsurface MgSO4 type solutions into newly-formed depressions could account for formation of some massive layered sulfate deposits through freezing or evaporation. This scenario explains the observed deficiency of salts in Noachian formations, a paucity of Hesperian phyllosilicates, and the occurrence of sulfate deposits in Valles Marineris troughs, chaotic terrains, and some craters of the Hesperian age.

Measuring silicate mineral dissolution rates using Si isotope doping

New experimental data and quantitative models show that the 29Si doping experimental technique (Gruber, Zhu, and others, 2013, GCA) is robust for measuring silicate mineral dissolution rates even while a Si-containing secondary phase is precipitating. In this study, batch experiments of albite dissolution were conducted under ambient temperature and pH3–7.5, some seeded with kaolinite. Initial solutions of various Si concentrations were doped with 29Si, resulting in a Si isotopic composition highly anomalous to natural Si isotope compositions. The isotopic contrast and precision of isotope fraction analysis to ±0.0005 to ±0.001 allow detection of the dissolution of a minuscule amount of albite in aqueous solutions. Experimental data and quantitative modeling show Si isotope fractionation during albite dissolution ranged from 30εsol-ab −2.870 to 0.804‰, significant for Si biogeochemical cycling, but resulting in only <±0.04% errors in rate determination. The simultaneous precipitation of secondary phases consumed silica, causing slight changes of Si isotope ratios, but the isotopic fractionation due to secondary phase precipitation is negligible for determining albite dissolution rates. Combination of Si isotopes and Si concentrations, precisely measured with the Si isotope dilution method, allowed determination of secondary phase precipitation rates simultaneously. This means that we can now measure rates at circumneutral pH and near equilibrium conditions, even when secondary precipitates are forming. However, while the isotope doping method has greatly improved the precision and sensitivity of rate measurements, the accuracy of rate measurements is still subject to the vagaries of sample preparation and other unknown effects as shown our data near pH5.5. When the solution is very close to equilibrium, the backward reaction becomes important and interpretation of the isotope data would be complicated or impossible.

Hydration effects on gypsum dissolution revealed by in situ nanoscale atomic force microscopy observations

Recent work has suggested that the rates of mineral dissolution in aqueous solutions are dependent on the kinetics of dehydration of the ions building the crystal. Dehydration kinetics will be ultimately determined by the competition between ion–water and water–water interactions, which can be significantly modified by the presence of background ions in solution. At low ionic strength, the effect of electrolytes on ion–water (electrostatic) interactions will dominate (Kowacz et al., 2007). By performing macroscopic and in situ, microscopic (atomic force microscopy) dissolution experiments, the effect of background electrolytes on the dissolution kinetics of gypsum (CaSO4·2H2O) {010} cleavage surfaces is tested at constant, low ionic strength (IS=0.05) and undersaturation (saturation index, SI=−0.045). Dissolution rates are systematically lower in the presence of 1:1 background electrolytes than in an electrolyte-free solution, regardless of the nature of the electrolyte tested. We hypothesize that stabilization of the hydration shell of calcium by the presence of background ions can explain this result, based on the observed correlations in dissolution rates with the ionic surface tension increment of the background ion in solution. Stabilization of the cation hydration shell should favor dissolution. However, in the case of strongly hydrated ions such as Ca2+, this has a direct entropic effect that reduces the overall ΔG of the system, so that dissolution is energetically less favorable. Overall, these results provide new evidence that supports cation dehydration being the rate-controlling step for gypsum dissolution, as proposed for other minerals such as barite, dolomite and calcite.

The mechanism of dissolution of minerals in acidic and alkaline solutions: Part VI a molecular viewpoint

The theory of dissolution proposed here and in this series of papers predicts that the order of reaction with respect to H+ ions in solutions should in the series 0.5, 1.0, 1.5…. Experimentally this is found to be the case. The mechanism of dissolution is based on the removal of material from the surface that results in the formation of cations and anions in parallel ‘partial’ reactions. These reactions result in the charging of the surface and the formation of a potential difference across the solid–solution interface. This potential difference in turn influences the rate of removal from the surface. The purpose of this paper is to discuss in further detail the particular mathematical form of the dependence on potential difference that has been chosen. Three plausible models are examined in order to understand the mechanistic steps of dissolution underlying the mathematical form of the potential dependence: (i) the Marcus model, (ii) ionic transfer model of Fawcett, and (iii) the ‘make-before-break’ model of Gileadi. This mathematical form for the potential dependence implies a somewhat surprising link between the dissolution of a metal and the dissolution of a mineral. This link is strengthened by demonstrating that both the rates of mineral dissolution and metal electrode reactions are correlated with the rate of water self-exchange. Since they are correlated with the same variable, they must be correlated with one another. It is proposed that mineral dissolution reactions and dissolution/deposition reactions at metal electrodes share similar reaction trajectories, and hence the similarity between functional dependencies of the rate of ion formation on potential difference across the Helmholtz layer.

Kinetics of carbonate mineral dissolution in CO2-acidified brines at storage reservoir conditions.

We report experimental measurements of the dissolution rate of several carbonate minerals in CO2-saturated water or brine at temperatures between 323 K and 373 K and at pressures up to 15 MPa. The dissolution kinetics of pure calcite were studied in CO2-saturated NaCl brines with molalities of up to 5 mol kg-1. The results of these experiments were found to depend only weakly on the brine molality and to conform reasonably well with a kinetic model involving two parallel first-order reactions: one involving reactions with protons and the other involving reaction with carbonic acid. The dissolution rates of dolomite and magnesite were studied in both aqueous HCl solution and in CO2-saturated water. For these minerals, the dissolution rates could be explained by a simpler kinetic model involving only direct reaction between protons and the mineral surface. Finally, the rates of dissolution of two carbonate-reservoir analogue minerals (Ketton limestone and North-Sea chalk) in CO2-saturated water were found to follow the same kinetics as found for pure calcite. Vertical scanning interferometry was used to study the surface morphology of unreacted and reacted samples. The results of the present study may find application in reactive-flow simulations of CO2-injection into carbonate-mineral saline aquifers.

The impact of evolving mineral–water–gas interfacial areas on mineral–fluid reaction rates in unsaturated porous media

The distribution and evolution of mineral–water–gas interfacial areas exert a fundamental yet poorly documented control on mineral–fluid reactions in the unsaturated zone. Here, we explore the impact of changing mineral reactive surface area, water content, and gas distribution on the reaction of brucite [Mg(OH)2] with CO2 gas to form hydrated Mg-carbonate minerals in partially water saturated meter-scale column experiments. Brucite surface area, which is inferred to exert a direct control on mineral dissolution rates, demonstrates a complex evolution including roughening, fracturing and passivation that is inconsistent with conventional models of geometric evolution. Mineral–fluid reaction in the interior of single brucite grains maintains surface area at near-constant values despite the decreasing volumetric brucite content, until solid carbonate precipitates passivate brucite surfaces. The evolution of reactive surface area during passivation also does not follow simple geometric processes. A porous amorphous carbonate phase permits ready access to brucite surfaces for reaction until recrystallization of the amorphous carbonate into bladed, low-porosity nesquehonite [MgCO3·3H2O] abruptly quenches reaction. The varied water content of the experiments illustrates that the extent of mineral–gas reaction is limited by the abundance of water available to facilitate precipitation of hydrated carbonate minerals. Conversely, at high bulk water saturation, the development of preferential gas flow paths limited the exposure of reactive minerals to CO2 and reduced the overall extent of reaction. Thus, bulk mineral–fluid reaction rates were reduced at both high and low bulk water contents.

Evaluating sensitivity of silicate mineral dissolution rates to physical weathering using a soil evolution model (SoilGen2.25)

Abstract. Silicate mineral dissolution rates depend on the interaction of a number of factors categorized either as intrinsic (e.g. mineral surface area, mineral composition) or extrinsic (e.g. climate, hydrology, biological factors, physical weathering). Estimating the integrated effect of these factors on the silicate mineral dissolution rates therefore necessitates the use of fully mechanistic soil evolution models. This study applies a mechanistic soil evolution model (SoilGen) to explore the sensitivity of silicate mineral dissolution rates to the integrated effect of other soil-forming processes and factors. The SoilGen soil evolution model is a 1-D model developed to simulate the time-depth evolution of soil properties as a function of various soil-forming processes (e.g. water, heat and solute transport, chemical and physical weathering, clay migration, nutrient cycling, and bioturbation) driven by soil-forming factors (i.e., climate, organisms, relief, parent material). Results from this study show that although soil solution chemistry (pH) plays a dominant role in determining the silicate mineral dissolution rates, all processes that directly or indirectly influence the soil solution composition play an equally important role in driving silicate mineral dissolution rates. Model results demonstrated a decrease of silicate mineral dissolution rates with time, an obvious effect of texture and an indirect but substantial effect of physical weathering on silicate mineral dissolution rates. Results further indicated that clay migration and plant nutrient recycling processes influence the pH and thus the silicate mineral dissolution rates. Our silicate mineral dissolution rates results fall between field and laboratory rates but were rather high and more close to the laboratory rates possibly due to the assumption of far from equilibrium reaction used in our dissolution rate mechanism. There is therefore a need to include secondary mineral precipitation mechanism in our formulation. In addition, there is a need for a more detailed study that is specific to field sites with detailed measurements of silicate mineral dissolution rates, climate, hydrology, and mineralogy to enable the calibration and validation of the model. Nevertheless, this study is another important step to demonstrate the critical need to couple different soil-forming processes with chemical weathering in order to explain differences observed between laboratory and field measured silicate mineral dissolution rates.

Measuring mineral dissolution kinetics using on-line flow-through time resolved analysis (FT-TRA): an exploratory study with forsterite

We explore the applicability of on-line flow-through time-resolved analysis (FT-TRA) to measure mineral dissolution rates, determine dissolution rate parameters, and monitor dissolution stoichiometry under both constant and transient eluent conditions. A custom-built automated flow-through system is used to subject mineral samples to controlled eluent flows of constant or varying composition, and the dissolution products are measured on-line with a quadrupole ICP–MS. Because forsterite dissolution has been extensively studied, this mineral provides an ideal benchmark to test the approach.FT-TRA has several advantages compared to the mixed-flow reactors conventionally used to measure mineral dissolution. The mineral dissolution regime (surface vs transport-controlled) can be readily established prior to conducting dissolution experiments, which is important if the goal of the experiment is to determine dissolution rate parameters. Because of the small volume (25μL) of the flow-through reactor, the time to reach steady state concentration in the effluent is shorter and is limited by the intrinsic properties of the mineral, instead of the residence time of the effluent in the reactor. For minerals reaching steady-state dissolution rapidly, dissolution rate parameters (rate constant(s) and reaction order(s)) are established in a few hours, allowing for replications, statistical analysis of the results, and investigation of the underlying causes of variability. The experimental set-up can be adapted for minerals that require longer periods of time to reach steady-state dissolution. In addition, FT-TRA lends itself particularly well for detailed monitoring of dissolution rates and dissolution stoichiometry under transient conditions. For forsterite dissolution under acidic conditions, preferential release of Mg when pH transits to lower values, and preferential release of Si when pH transits to higher values is clearly documented and can be interpreted as indicating changes in the mean depth of the Si-rich surface layer in response to changes in eluent acidity. Episodes of exfoliation of the Si-rich surface layer can also be identified, providing a means to study one of the potential rate limiting steps for CO2 sequestration by carbonation of olivine. The results presented here indicate that FT-TRA with online ICP–MS analysis could be a useful and multifaceted addition to the toolbox available to study mineral dissolution.