Enhanced nitrate removal from groundwater using a conductive spacer in flow-electrode capacitive deionization

Flow-electrode capacitive deionization（FCDI） represents a promising approach for ion separation from aqueous solutions. However, the optimization of spacer, particularly for nitrate-contaminated groundwater systems, has often been overlooked. This research comprehensively investigates the influence of using a conductive（carbon cloth, CC） spacer on nitrate removal performance within FCDI system, comparing it to a non-conductive（nylon net, NN） spacer. In both CC and NN FCDI systems, it is unsurprisingly that nitrate removal efficiency improved notably with the increasing current density and hydraulic retention time（HRT）. Interestingly, the specific energy consumption（SEC） for nitrate removal did not show obvious fluctuations when the current density and HRT varied in both systems. Under the auspiciously optimized process parameters, CC-FCDI attained a 20% superior nitrate removal efficiency relative to NN-FCDI, accompanied by a notably diminished SEC for CC-FCDI, registering at a mere 28% of NN-FCDI. This great improvement can be primarily attributed to the decrement in FCDI internal resistance after using conductive spacer, which further confirmed by electrochemical tests such as linear sweep voltammetry（LSV）and electrochemical impedance spectroscopy（EIS）. Upon prolonged continuous nitrate removal at the optimized conditions, the CC-FCDI system achieved a consistent 90% nitrate removal efficiency with a low SEC of 2.7–7.8 k Wh/kg NO3-N, underscoring its steady performance. Overall, this study highlights the pivotal importance of careful spacer design and optimization in realizing energy-efficient groundwater treatment via FCDI.