Selectivity and energy recovery in capacitive deionization
Keywords: Mechanical Engineering.

Capacitive deionization (CDI) is an electrosorptive desalination technique where ions are captured via electrostatic interaction in porous, high surface area, and electrically conductive activated carbon (AC) electrodes. The applied bias is typically ~1 V; hence, making CDI compatible with low-voltage sources such as photovoltaics. After electrostatic adsorption, the cell is regenerated by electrostatically releasing ions from the AC electrodes to an effluent waste brine flow. We first present a proof of concept of a hybrid form of inverted capacitive deionization (iCDI) system for nitrate adsorption. Here, we demonstrated the use of functionalized high surface area AC electrodes with the surfactant cetrimonium bromide (CTAB). The hydrophilic head of the surfactant molecule contains quaternary amine groups which have a high affinity for nitrate ions. The functionalized electrodes showed passive adsorption of nitrate via ion exchange of up to ∼80 mg NO3−/g. Unlike a traditional ion exchanger, we repeatedly demonstrated electrostatic regeneration of the functionalized AC electrode by the application of an electrostatic potential which displaces the bound NO3− while leaving an excess of electronic charge on the electrode. We reported retention of the initial passive adsorption of up to 40% after several complete adsorption/desorption cycles. Expanding the hybrid iCDI work, we present a robust full iCDI system (with a capacitive counter electrode) for nitrate removal. The system consisted of two surfactant-treated AC electrodes where the active electrode was functionalized with CTAB and the counter electrode with benzene sulfonate (SDBS). We demonstrated robust operation (adsorption/desorption) of the iCDI system for more than 30 cycles without performance degradation. A high charge efficiency during adsorption was obtained without the use of expensive membranes. Energy consumption was two orders of magnitude lower compared to the hybrid iCDI cell with Faradaic counter electrode. We then showed experimental evidence of competitive adsorption with nitrate and chloride with the iCDI system. The effect of the regeneration voltage and the ratio of feed ion concentration in selectivity was studied. A 6.5-fold selectivity coefficient for nitrate over chloride was demonstrated for a regeneration voltage of 0.4 V. Then, we presented the theoretical framework for a multi-species dynamic model with Langmuir type-equilibria equations for surface groups. The model showed good agreement with nitrate and chloride effluent concentrations measurements at several regeneration voltages. Towards selective removal of ionic species, we present evidence of a new operational scheme to adsorb lead continuously during a traditional CDI operation. AC electrodes were functionalized with a 48-h nitric acid treatment to generate carboxyl surface groups. The CDI cycles consisted of adsorption at constant current followed by desorption at short circuit (0 V) with simultaneous chemisorption of lead by the surface groups. Then, the CDI was regenerated after reconfiguring the applied circuit to facilitate the regeneration of lead ions. Lastly, we presented a multi-species model that predicts the trends observed in the experiments. Finally, we present a study of electrostatic energy transfer from and to a capacitive deionization system. We defined generic energy transfer efficiency metrics for the transfer of energy between a CDI cell to a storage device using a DC/DC converter. We developed an analytical model which was benchmarked with a numerical model. Then, the transfer efficiencies were compared to the overall system efficiency (sometimes imprecisely called energy recovery efficiency). With the analytical model, we demonstrated energy transfer efficiencies of ~90% between a CDI cell and an SC with a buck-boost converter. Overall efficiencies (including CDI cell internal losses during energy transfer) range from 30 to 40%.