Characterization of ion transport and -sorption in a carbon based porous electrode for desalination purposes

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although much of the attention and efforts have focused on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high-surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses electric fields to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization (EDI) and electro-dialysis reversal (EDR) where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow-rates. Voltage and electrical current measurements are conducted during the desalination and porous electrode regeneration processes and used to evaluate the percentage of energy recovery.. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption/desorption efficiencies. Correlation of these measurements with an analytical species transport model provides information about the net ionic adsorption/desorption rates in nonsaturated- carbon-electrode scenarios. The results show a strong dependence of the net electrical energy requirements with the number of carbon electrodes regeneration cycles. Finally, a non-dimensional number that compares the convective and electrokinetic transport times is presented. The energy requirements and adsorption/desorption rates analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the re-process of spent nuclear fuels in a near future.