A spatially resolved surface kinetic model for forsterite dissolution

Abstract

The development of complex alteration layers on silicate mineral surfaces undergoing dissolution is a widely observed phenomenon. Given the complexity of these layers, most kinetic models used to predict rates of mineral-fluid interactions do not explicitly consider their formation. As a result, the relationship between the development of the altered layers and the final dissolution rate is poorly understood. To improve our understanding of the relationship between the alteration layer and the dissolution rate, we developed a spatially resolved surface kinetic model for olivine dissolution and applied it to a series of closed-system experiments consisting of three-phases (water (±NaCl), olivine, and supercritical CO 2) at conditions relevant to in situ mineral carbonation (i.e. 60 °C, 100 bar CO 2). We also measured the corresponding d 26/24 Mg of the dissolved Mg during early stages of dissolution. Analysis of the solid reaction products indicates the formation of Mg-depleted layers on the olivine surface as quickly as 2 days after the experiment was started and before the bulk solution reached saturation with respect to amorphous silica. The d 26/24 Mg of the dissolved Mg decreased by approximately 0.4‰ in the first stages of the experiment and then approached the value of the initial olivine (À0.35‰) as the steady-state dissolution rate was approached. We attribute the preferential release of 24 Mg to a kinetic effect associated with the formation of a Mg-depleted layer that develops as protons exchange for Mg 2+. We used experimental data to calibrate a surface kinetic model for olivine dissolution that includes crystalline olivine, a distinct ''active layer" from which Mg can be preferentially removed, and secondary amorphous silica precipitation. By coupling the spatial arrangement of ions with the kinetics, this model is able to reproduce both the early and steady-state long-term dissolution rates, and the kinetic isotope fractionation. In the early stages of olivine dissolution the overall dissolution rate is controlled by exchange of protons for Mg, while the steady-state dissolution rate is controlled by the net removal of both Mg and Si from the active layer. Modeling results further indicate the importance of the spatial coupling of individual reactions that occur during olivine dissolution. The inclusion of Mg isotopes in this study demonstrates the utility of using isotopic variations to constrain interfacial mass transfer processes. Alternative kinetic frameworks, such as the one presented here, may provide new approaches for modeling fluid-rock interactions.

The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

“…As such, during the early stages of these experiments, the preferential initial release of light Mg is still evident in the fluid phase and negligible isotopic equilibration can occur by the return of material back to the hydromagnesite. Note that a similar initial release of isotopically light divalent metals to the aqueous phase has been observed during forsterite, basaltic glass, and pyrite dissolution (Verney-Caron et al, 2011;Oelkers et al, 2015;Maher et al, 2016;Wolfe et al, 2016). Some preferential departure of light Mg from hydromagnesite could arise from isotopic heterogeneity of the hydromagnesite used in the experiments.…”

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

“…Both Pokrovsky and Schott 25 and Palandri and Kharaka 22 already suggested that for slightly alkaline solutions (e.g., seawater), forsterite dissolution at steady-state is controlled by the decomposition of a protonated surface complex, which is silica-rich and magnesium-deficient. Maher et al 32 postulate that olivine dissolution occurs as a series of boundary layer processes, in which primary dissolution of cations is followed by dissolution of silicic acid ions, which may subsequently repolymerize at the surface. This implies that the measured dissolved silica release rate is a net value, which may not serve as the sole proxy for olivine dissolution.…”

Model‐Based Assessment of the CO2 Sequestration Potential of Coastal Ocean Alkalinization

“…When olivine is added to coastal waters, potential pore water saturation in the seabed and secondary reactions will likely occur (Montserrat et al, ). In addition, the alternating layers (Mg‐poor layers, SiO 4 ‐coatings) generated on the olivine mineral surface can lead to nonstoichiometric dissolution (the number of dissolved atoms cannot be expressed as a ratio presented in the chemical formula) and temporal variations of the area‐normalized olivine dissolution rates (Maher et al, ). Since a detailed understanding of these processes under typical seawater conditions is currently lacking, these complex interactions are ignored in our study.…”