Fluid-solid reaction in partially-saturated media at the pore scale

It is by now well known that pore-scale heterogeneity can lead to mass transfer limitations, resulting in incomplete solute mixing and thereby decreasing reaction rates when compared to laboratory batch experiments. The mixing state of the plume, and therefore the reaction rates, result from a complex interplay of deformation by fluid flow, diffusion, and reactive depletion or production. In partially-saturated systems, such as the vadose zone, the simultaneous presence of air and water further enhances structural heterogeneity, leading to broad flow velocity distributions and resulting in qualitatively different transport dynamics. Despite significant advances in modeling pore-scale reactive mixing, the role of partial saturation in reaction dynamics remains poorly understood. In this work, we focus on linear decay of a transported species upon contact with the water-solid interface. Among other processes, this type of reaction models antibiotic degradation through redox reaction with a mineral phase. We simulate steady-state water flow subject to a frozen spatial configuration of air and water phases obtained experimentally in quasi-2D media. The solid phase is composed of cylindrical pillars with variable radii, characterized by different spatial correlation structures. The flow is simulated using Eulerian methods, while reactive transport simulations employ Lagrangian particle tracking. We find that, while solute dispersion and breakthrough curve width increase dramatically, overall reaction rates are largely insensitive to saturation. We discuss the origins of this counter-intuitive result and how it can be used to model reactive breakthrough. These findings provide new insights into the role of saturation in transport subject to surface reaction, and open up new questions regarding the role of flow structure and reaction kinetics.