Visualization and Model Quantification of pH-Controlled Reactive Transport in Porous Media

Abstract:

We present conservative and reactive transport experiments in a refraction index-matched, three-dimensional, water-saturated porous medium, using a pH indicator as the reactant to follow chemical reactions. The measurements were matched with a particle tracking (PT) modeling approach requiring a minimal set of fitting parameters. The magnitude of reaction during transport through the porous medium can be related to the color change of the pH indicator, via detailed high resolution image analysis, with sequential imaging of the flow cell. The setup yields measurements of the temporally evolving spatial (local) concentration field. The current experiments focused on point injection of a pH-sensitive reactive tracer into a macroscopically uniform flow field containing water at a pH different from that of the injected tracer. Parallel experiments employing a conservative tracer demonstrated the transport to be mildly non-Fickian. A PT model was then used to quantify the spatial and temporal migration of both the conservative and reactive tracer plumes. The PT simulations account for non-Fickian transport within a continuous time random walk framework; model parameters related to the non-Fickian transport were determined from the conservative tracer experiments. An additional term accounting for chemical reaction was established solely from analysis of the reactant concentrations, and significantly, no other fitting parameters were used to quantify the reactive transport experiment. The measurements and analysis emphasize the localized nature of transport and reaction, caused by small-scale concentration fluctuations. In particular, an "extinction radius" for pH-controlled reactive transport processes was defined, which delineates the region in which reactions occur rapidly. The small-scale preferential flows, their relation to the non-Fickian transport, and their influence on reaction dynamics in the system, were also identified.