Time-lapse 3D micro-tomography of calcite column experiments to study pH-dependent dynamic dissolution processes

We investigated the spatiotemporal evolution of calcite dissolution processes using time-lapse high-resolution X-ray CT imaging of live column experiments and real-time data logging of effluent pH and TDS. We evaluated the sensitivity of three pH solutions (7, 5, and 4) to dissolution processes. The results showed spatial variations in dissolution associated with spatial variation in porosity and pore sizes, manifesting as spatial heterogeneity in the dissolution pattern. Dissolution was dominant in larger pores compared to smaller ones. The magnitude of this uneven spatial dissolution exceeded the overall dissolution. However, the total porosity increased linearly over the time of dissolution experiments, with the slope dependent on the pH of the injected solution. Surface area decreased linearly during dissolution, and the decrease in surface area was also found to depend linearly on the increase in total porosity. The temporal evolution of pH-dependent reaction rates demonstrated a decreasing trend that followed a power-law behavior. The time to reach the steady reaction rate, which was evaluated from changes in calcite volume, appeared to be significantly later than the time at which the tracer reached its asymptote. The reaction rate at these two distinct times could differ by an order of magnitude. We emphasize the use of the steady state in mineral volume changes method to determine representative reaction rates in porous media, the significance of which emerges due to the coupling of hydrodynamic and reactive transport processes originating at the pore scale.