Hydrodynamic factors influencing mineral dissolution rates

Solution flow profiles near a mineral surface can exert significant influences on the local thermodynamic driving force and, potentially, the rate-controlling kinetic mechanism of its dissolution or precipitation. These influences are investigated here both by in situ digital holographic microscopy of calcite in flowing pure water and by lattice Boltzmann reaction-transport simulations using close to the same macroscopic flow rates and sample geometry as the experiments. The measurements show that the median dissolution rate along the crystal surface decreases significantly with distance along the flow direction. At lower flow rates localized advancement of the surface, rather than retreat, is sometimes observed near the trailing edge of a macroscopic crystal, which suggests some precipitation even though pure water is injected near the crystal's leading edge. The simulations confirm that the lower rates are caused by an accumulation of dissolved species as the solution flows from the leading to the trailing edge. The experiments and simulations both indicate that the effect is lessened as the flow rate increases but never becomes negligible within the range of flow rates examined. This study implies that dissolution rates are significantly influenced by near-surface conditions even under relatively high flow rates where a surface process, not diffusion, controls the rate.