Dissolution kinetics of garnierites from the Falcondo Ni-Laterite deposit (Dominican Republic) under acidic conditions
2022
Villanova-de-Benavent, Cristina | Cama, Jordi | Soler i Canals, Josep M. | Domenech, Cristina | Galí, Salvador | Proenza, Joaquín A.
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.
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