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Saldi, Giuseppe D.; Causserand, Carole; Schott, Jacques; Jordan, Guntram (5. December 2021): Dolomite dissolution mechanisms at acidic pH: New insights from high resolution pH-stat and mixed-flow reactor experiments associated to AFM and TEM observations. In: Chemical Geology, Vol. 584, 120521: pp. 1-14
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Recent studies of dolomite surface reactivity have shown that the processes controlling the dissolution of this mineral at the microscopic scale are still not well understood and quantified and not always consistent with the results of macroscopic observations. In the present study dolomite dissolution at pH 3 was investigated using both a macroscopic approach (pH-stat system and mixed-flow reactor) and microscopic characterization methods (atomic force and transmission electron microscopies) to quantify the release rates of Ca and Mg from bulk chemical analyses and determine the micro-topographical and chemo-structural changes that accompany the dissolution reaction. Dolomite dissolution rates from pH-stat and mixed-flow reactor experiments conducted on two distinct dolomite natural samples were found to differ by a factor of 2 between the two samples but falling within the range of values reported in the literature. High-resolution sampling of the aqueous fluid allowed observing frequent fluctuations of the Ca/Mg ratio (1.00 ≤ Ca/Mg ≤ 1.39) and its progressive decrease towards the stoichiometric value of the solid. Such oscillations are related, at the microscopic scale, to the preferential release of Ca from the fresh mineral surface, the concurrent generation of new surface that becomes exposed to the fluid and the temporary surface Mg enrichment occurring as the consequence of the faster departure of Ca ions to the aqueous solution. According to TEM observations, this dissolution mechanism does not bring about any noticeable modification of the dolomite surface structure in contact with the aqueous solution. Atomic force microscopy (AFM) experiments allowed observing the fast nucleation of high density etch pits during dissolution. The fluid effluent from the AFM cell was enriched in Ca (Ca/Mg =3.2–5.5) compared to the solution of the bulk experiments and the dolomite stoichiometry (Ca/Mg =1.01), but there was no evidence of formation of any Mg-rich secondary precipitate, contrary to what reported by previous studies with much higher Ca/Mg values. The increase of the Ca/Mg ratio in these effluents compared to the pH-stat and mixed-flow runs is likely related to the specific hydrodynamic conditions present within the AFM fluid-cell under transport-controlled dissolution regimes. The concentration of Ca aqueous ions preferentially released from the solid surface is likely further increased following their transport across the diffusive layer formed at the solid-fluid interface because of their higher diffusivity compared to aqueous Mg ions. However, the thermodynamic requirements for the formation of Mg-bearing secondary phases are unlikely to be met within this system under the investigated conditions.