Formation of Amorphous Silica Surface Layers by Dissolution-Reprecipitaton During Chemical Weathering: Implications for CO2 Uptake

AbstractChemical weathering reactions at the Earth's surface and upper crust influence the chemical cycle of elements, chemical erosion rates, the quality of potable water resources, soil formation and nutrient availability, and ore genesis. Chemical weathering is also a major process controlling the regulation of the carbon cycle by CO2 consumption and sequestration by carbonation reactions. Based on nanometer-resolution TEM measurements, chemical weathering of silicates in both the laboratory and the field was found to result in the development of a distinct interfacial phase that is amorphous, hydrated, and silica-rich. An abrupt, step function-like change in chemistry and structure delimits the interface with the unaltered parent mineral, suggesting a dissolution-reprecipitation mechanism. The existence of these precipitated silica layers has important and so far unrecognized implications with respect to natural and industrial carbon sequestration processes, as surface silica layers may decrease the amount of atmospheric CO2 consumed during coupled silicate chemical weathering-carbonation reactions

Albite feldspar dissolution kinetics as a function of the Gibbs free energy at high pCO2

We are currently measuring the dissolution kinetics of albite feldspar at 100 °C in the presence of high levels of dissolved CO2 (pCO2 = 9 MPa) as a function of the saturation state of the feldspar (Gibbs free energy of reaction, ∃G). The experiments are conducted using a flow through reactor, thereby allowing the dissolution reactions to occur at a fixed pH and at constant, but variable saturation states. Preliminary results indicate that at far-from-equilibrium conditions, the dissolution kinetics of albite are defined by a rate plateau, with R ≈ 5.0 x 10-10 mol m-2 s-1 at -70 -40 kJ mol-1, the rates decrease sharply, revealing a strong inverse relation between the dissolution rate and free energy. Based on the experiments carried out to date, the dissolution rate-free energy data correspond to a highly non-linear and sigmoidal relation, in accord with recent studies

Albite feldspar dissolution kinetics as a function of the Gibbs free energy at high pCO_2

We are currently measuring the dissolution kinetics of albite feldspar at 100 deg C in the presence of high levels of dissolved CO_2 (pCO_2 = 9 MPa) as a function of the saturation state of the feldspar (Gibbs free energy of reaction, \Delta G). The experiments are conducted using a flow through reactor, thereby allowing the dissolution reactions to occur at a fixed pH and at constant, but variable saturation states. Preliminary results indicate that at far-from-equilibrium conditions, the dissolution kinetics of albite are defined by a rate plateau, with R \approx 5.0 x 10^{-10} mol m^{-2} s^{-1} at -70 -40 kJ mol^{-1}, the rates decrease sharply, revealing a strong inverse relation between the dissolution rate and free energy. Based on the experiments carried out to date, the dissolution rate-free energy data correspond to a highly non-linear and sigmoidal relation, in accord with recent studies

Geological control on 222Rn accumulation as input function for hydrological systems on a loessic aquifer, Argentina

Este trabajo analiza las concentraciones de 222Rn en el acuífero Pampeano en el sudeste bonaerense, y su relación con las características hidrogeológicas del área de estudio. Se tomaron muestras de agua para la determinación de isotopos estables y 222Rn y sedimentos en piezómetros multinivel en diferentes localidades. La identificación de minerales portadores de uranio se realizó con microscopio electrónico de barrido, determinándose la presencia de zircon y monacita. El 222Rn se determinó con un detector RAD7, observándose un rango de actividades de 3.7 a 14 Bq/l. Se reconoció una dependencia del 222Rn con la profundidad y la proximidad del basamento Precámbrico. La actividad de 222Rn aumenta linealmente a razón de 0.08 Bq.l-1.m-1, pero a igual profundidad los valores en pozos cercanos al basamento Precámbrico triplican a los de los pozos que poseen basamento Paleozoico.Centro de Investigaciones Geológica

Dissolution Anisotropy of Pyroxenes: Role of Edges and Corners Inferred from Stochastic Simulations of Enstatite Dissolution

International audienceExperimental studies exhibit a wide variety of dissolution rates for a given mineral depending on the chemical conditions and also on the type of experiment conducted. As a relevant example, studies focused on face-specific dissolution and those focused on powder dissolution can present differences of up to 1 order of magnitude. Linking these two types of experiments is therefore relevant, since experimental conditions can be almost entirely controlled throughout the entire experiment. In this study, we use a stochastic dissolution model based on hydrolysis of atomic bonds of enstatite, the magnesium endmember pyroxene, to simulate the dissolution of different sizes and aspect ratios of enstatite grains. This model, validated in a previous study by a comparison with experimental (face-specific) rate data, is used to understand the evolution of the dissolution rate with time, from the beginning of the dissolution until the entire consumption of the crystal. We show that the behavior of the dissolution is controlled mainly by the aspect ratio of the grain. A simple dissolution model based on face-specific dissolution is then used to compare the results of the simulations obtained with those resulting from the grain dissolution model. The similarity between the results points out that the contribution to the dissolution of edges and corners is only modest for very anisotropic silicates such as pyroxenes, where silicate tetrahedrons are connected through chains running parallel to a given crystallographic axis. This simple model is then extended using both face-specific experimental and model results and compared to existing powder dissolution results. This comparison shows an excellent agreement between face-specific dissolution modeling and powder dissolution experiment, indicating that, for anisotropic minerals such as pyroxenes, face-specific and powder dissolution experiments can be linked, which can be of great interest for future dissolution studies

Examination of crystal dissolution in 3D: A way to reconcile dissolution rates in the laboratory?

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Direct measurement of fungal contribution to silicate weathering rates in soil

International audienceAbstract Chemical weathering produces solutes that control groundwater chemistry and supply ecosystems with essential nutrients. Although microbial activity influences silicate weathering rates and associated nutrient fluxes, its relative contribution to silicate weathering in natural settings remains largely unknown. We provide the first quantitative estimates of in situ silicate weathering rates that account for microbially induced dissolution and identify microbial actors associated with weathering. Nanoscale topography measurements showed that fungi colonizing olivine [(Mg,Fe)2SiO4] samples in a Mg-deficient forest soil accounted for up to 16% of the weathering flux after 9 mo of incubation. A local increase in olivine weathering rate was measured and attributed to fungal hyphae of Verticillium sp. Altogether, this approach provides quantitative parameters of bioweathering (i.e., rates and actors) and opens new avenues to improve elemental budgets in natural settings