Macro Analysis of the Electro-Adsorption Process in Low Concentration NaCl Solutions for Water Desalination Applications

Capacitive deionization (CDI) has become a very attractive desalination technology due to its capability of returning a fraction of the input energy during the regeneration of its adsorbent electrodes. As in any separation technique, analysis of the mass transfer phenomena occurring in this water treatment method is vital to evaluate and extend the performance of a desalination system. This publication proposes a novel method to estimate the net electro-adsorption rate of a CDI cell from a series of low concentration desalination experiments coupled with a one-dimensional electro-adsorption model. In the proposed methodology, a one-dimensional model is presented and two regimes are identified based on the presence or absence of a convection-diffusion layer within the bulk solution, as dictated by the electro-diffusion based Peclet number. For each of these regimes, the net adsorption flux is calculated based on the velocity at which ions are transported toward the electrodes. The proposed model is then solved, first under the assumption of an infinite electrode adsorption capacitance before relaxing this condition, and correlated against the experimental data to assess the global electro-adsorption rate. The analysis in this paper also provides unique physical insight into the macro-scale mass transfer processes that control desalination in CDI.