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

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

International audienceThe mobility and availability of the toxic metalloid selenium in the environment is largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe2+, we investigated the long-term (≤ 1 month) kinetics of selenite (Se(IV)O3 -) sorption to montmorillonite in the presence of Fe2+ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe2+, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe2+ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (x-ray absorption near-edge spectroscopy) and EXAFS (extended x-ray absorption finestructure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈ 2 mmol kg-1). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H2 species, and are then 3 available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, the average particle size of this precipitate does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters

Sanctuaire de source, sanctuaire des eaux ou simple sanctuaire en milieu humide? Découverte d'un complexe cultuel antique à Magny-Cours (Nièvre)

In advance of the future extension of the "technopole" and the construction of a service station in the district of Magny-Cours, the undertaking of two evaluations covering 15 hectares and 4 hectares, respectively, has revealed dense occupation of the site from the Neolithic period to the Middle Ages. The most important remains are associated with the Roman period and belong to a complex endowed with public buildings including several temples in an organised arrangement. This paper, preliminary to a larger study that will report on the whole of the site, is concerned specifically with the results obtained in a waterlogged area comprised within a zone that was excluded from the rescue excavations. Located near the cult buildings, this area was found to contain a deposit of ex-votos belonging to the beginning of the Roman era.À l'occasion de la future extension du " technopole " sur la commune de Magny-Cours et la création d'une aire de service, la réalisation de deux diagnostics sur 15 ha et 4 ha a permis la mise en évidence d'une occupation dense du Néolithique au Moyen Âge. La période antique rassemble l'essentiel des vestiges, appartenant à un complexe doté de monuments publics dont plusieurs temples à plan centré. Cet article, préambule à une étude globale qui concernera l'ensemble du site, se limite à la présentation des résultats acquis dans une zone humide exclue de la fouille préventive. Située à proximité d'édifices cultuels, elle a livré un dépôt d'ex-voto remontant au début de la période romaine

Evidence of multiple sorption modes in layered double hydroxides using Mo as structural probe

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO ) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms - edge sorption, interfacial dissolution-reprecipitation - are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.This work has been supported by a grant from Labex OSUG@2020 (Investissements d’avenir - ANR10 LABX56). B.M., A.F.-M., L.C., S.G. and F.C. thank the NEEDS program from the CNRS for funding support. B.M. also thanks the financial support from the China Scholarship Council (CSC)

Bridging the 'gap' between laboratory dissolution and natural chemical weathering

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Dissolution kinetics as a function of the Gibbs free energy of reaction: An experimental study based on albite feldspar

International audienceHere we report on an experimental investigation of the relation between the dissolution rate of albite feldspar and the Gibbs free energy of reaction, ΔGr. The experiments were carried out in a continuously stirred flow-through reactor at 150 °C and pH(150 °C) 9.2. The dissolution rates R are based on steady-state Si and Al concentrations and sample mass loss. The overall relation between ΔGr and R was determined over a free energy range of −150 −25 kJ mol−1, represents the ‘near equilibrium' region where the rates decrease as chemical equilibrium is approached, but with a much weaker dependence on ΔGr. The lowest rate measured in this study, R = 6.2 × 10−11 mol m−2 s−1 at ΔGr = −16.3 kJ mol−1, is more than two orders of magnitude slower than the plateau rate. The data have been fitted to a rate equation (adapted from Burch et al. [Burch, T. E., Nagy, K. L., Lasaga, A. C., 1993. Free energy dependence of albite dissolution kinetics at 80 °C and pH 8.8. Chem. Geol. 105, 137–162]) that represents the sum of two parallel reactions R=k1[1-exp(-ngm1)]+k2[1-exp(-g)]m2, where k1 and k2 are rate constants that have been determined by regression, with values 1.02 × 10−8 and 1.80 × 10−10 mol m−2 s−1, g ≡ |ΔGr|/RT is a dimensionless number, and n, m1, and m2 are adjustable fitted parameters (n = 7.98 × 10−5, m1 = 3.81 and m2 = 1.17). Based on measurements of the temporal evolution of RSi and RAl for each experiment, steady-state dissolution rates appear to be congruent at all ΔGr. In contrast, non-steady-state dissolution is incongruent, and is related to ΔGr. Scanning electron microscopy (SEM) images of post-reaction grain surfaces indicate that dissolution close to equilibrium (ΔGr > −25 kJ mol−1) resulted in the precipitation of a secondary crystalline phase, but there are no indications that this altered the measured R–ΔGr relation

The dependence of albite feldspar dissolution kinetics on fluid saturation state at acid and basic pH: Progress towards a universal relation

International audienceHere we report on two separate ongoing, multi-year investigations on the dependence of the dissolution rate (R) of albite feldspar on fluid saturation state, as defined by the Gibbs free energy of reaction (ΔGr) for dissolution. The investigations are based on dissolution at pH 9.2, 150 °C and pH 3.3, 100 °C. Both studies reveal that the R–ΔGr relation is highly non-linear and sigmoidal. The kinetic data from the first study, being the most complete, can be fitted with a sigmoidal rate curve that is composed of two separate, parallel rate laws that represent distinct mechanisms of dissolution. The switch between one dominant mechanism and the other may be controlled by a critical free energy. The fact that in both studies the same type of sigmoidal R–ΔGr relation exists for dissolution at different pH and temperature condition suggests that this behavior may be universal for albite and other feldspars. Moreover, the experimental data contradict the commonly used R–ΔGr relation that is loosely based on transition state theory (TST). This has important implications with respect to the accuracy of geochemical codes that model water–rock interactions at near-equilibrium conditions

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