Radial-flow cell exploiting the activation of commercial carbon felt to improve capacitive deionization for water softening

Exponential growth of interest in capacitive deionization (CDI) technology has been motivated by its low energy consumption and potential use in desalination and water softening. In this work, CDI was applied for water softening using a radial-flow cell designed according to the concept of a “percolation flow cell”, combining flow-through and flow-by configurations to achieve high efficiencies and electrosorption rates. To make this cell economically viable, the electrode was a commercial activated carbon felt (CF) with high specific surface area. The felt was chemically modified to create surface functional groups that could minimize the co-ion repulsion effect, while the use of asymmetric electrodes improved the electrosorption capacity. Considering the variation of mass transfer and residence time along the radial flow, two flow patterns were studied: (i) from the center of the electrode to the edge (CE), and (ii) from the edge to the center of the electrode (EC). For each configuration, evaluation was made of the effects of the applied current and flow rate, enabling determination of the influence of charge and mass transfer processes on the charging efficiency and the ion removal rates. The results indicated that modification of the CF using nitric acid to introduce negative oxygen groups on the surface was highly effective, with a 44.3 % improvement in capacitance, while the use of ethylenediamine led to only modest enhancement. The radial-flow cell showed a significant dependence on the flow direction. The EC flow provided a more uniform ionic flux convergence, which improved charge efficiency and ion transport, with optimal operation achieved at 1.25 A m−2 and 22 mL min−1. Under these conditions, combined with the EC flow direction, the system presented a salt adsorption capacity (SAC) of 25.04 mg g−1 and specific energy consumption of 0.50 J mg−1. The balance between performance and energy efficiency demonstrated the potential of this system for use in scalable water treatment applications.