Multicomponent ionic dispersion during transport of electrolytes in heterogeneous porous media: Experiments and model-based interpretation

Abstract

This study investigates the effects of Coulombic interactions during transport of electrolytes in heterogeneous porous media under steady-state flow and transport conditions. We performed flow-through experiments in a quasi two-dimensional setup using dilute solutions of strong 1:1 and 1:2 electrolytes to study the influence of electrochemical cross-coupling on mass transfer of charged species in saturated porous media. The experiments were carried out under advection-dominated conditions (seepage velocity: 1 and 1.5m/day) in two well-defined heterogeneous domains where flow diverging around a low-permeability inclusion and flow focusing in high-permeability zones occurred. To quantitatively interpret the outcomes of our laboratory experiments in the spatially variable flow fields we developed a two-dimensional numerical model based on a multicomponent formulation and on charge conservation. The results of the multicomponent transport simulations were compared with the high-resolution concentration measurements of the ionic species at the outlet of the flow-through domain. The excellent agreement between the measured concentrations and the results of purely forward numerical simulations demonstrates the capability of the proposed two-dimensional multicomponent approach to describe transport of charged species and to accurately capture the Coulombic interactions between the ions, which are clearly observed in the flow-through experiments. Furthermore, the model allowed us to directly quantify and visualize the ionic interactions by mapping the Coulombic cross-coupling between the dispersive fluxes of the charged species in the heterogeneous domains.