Determination of mineral dissolution regimes using flow-through time-resolved analysis (FT-TRA) and numerical simulation

© 2016 Elsevier B.V. Flow-through time resolved analysis (FT-TRA) involves subjecting small mineral samples (< 10 mg) inserted in a miniature flow-through cell (50 μL) to controlled flows of eluent analyzed on-line by ICP-MS. In this study, FT-TRA is used to empirically determine the dissolution regimes for the two well-studied minerals forsterite and calcite, representing minerals with relatively slow and fast dissolution kinetics. A proportional increase in steady-state effluent [Mg, Si] concentrations with increasing flow-through cell eluent residence times confirms a dominantly surface-controlled dissolution regime for a powdered forsterite sample at pH 2.3, implying that transport limitations are negligible. In contrast, the relationship between flow rates and dissolution rates for single grain calcite samples at pH 2.3-4 reveals that transport limitations affect the rate of calcite dissolution. To provide a quantitative and process-based assessment of the effect of diffusive transport limitations, simulations of the calcite experiments were performed with a high resolution, pore-scale model that considers the geometry of the calcite grain and the FT-TRA flow-through reactor. The pore-scale model reproduces the observed effluent [Ca] concentrations for all experimental conditions using a single set of surface kinetic parameters, by accounting for the formation of a diffusive boundary layer (DBL) that varies in thickness as a function of flow rates. These results demonstrate that combining FT-TRA with pore-scale modeling makes it possible to obtain unprecedented insights not achievable by either method separately, including quantification of DBL thicknesses and the determination of transport controls as a function of pH, flow velocity and residence times.