  Electrochemically mediated selective adsorption is an emerging
electrosorption technique that utilizes Faradaically enhanced redox active
electrodes, which can adsorb ions not only electrostatically, but also
electrochemically. The superb selectivity (>100) of this technique enables
selective removal of toxic or high-value target ions under low energy
consumption. Here, we develop a general theoretical framework to describe the
competitive electrosorption phenomena involving multiple ions and surface-bound
redox species. The model couples diffusion, convection and electromigration
with competitive surface adsorption reaction kinetics, consistently derived
from non-equilibrium thermodynamics. To optimize the selective removal of the
target ions, design criteria were derived analytically from physically relevant
dimensionless groups and time scales, where the propagation of the target
anions concentration front is the limiting step. Detailed computational studies
are reported for three case studies that cover a wide range of inlet
concentration ratios between the competing ions. And in all three cases, target
anions in the electrosorption cell forms a self-sharpening reaction-diffusion
wave front. Based on the model, a three-step stop-flow operation scheme with a
pure stripping solution of target anions is proposed that optimizes the ion
adsorption performance and increases the purity of the regeneration stream to
almost 100%, which is beneficial for downstream processing.
