Understanding behaviour and performance of flow electrode capacitive deionization (FCDI) during simultaneous selective removal of Chromium (Cr (VI)) and Fluoride from brackish wastewater

In this study, the feasibility of flow electrode capacitive deionization (FCDI) was evaluated by implementing it for simultaneous selective removal of Cr (VI) and F- from brackish wastewater. A systematic investigation was conducted to elucidate the influence of various operational parameters like varying electrode composition, applied voltage, feedwater flowrate, feed concentration of coexisting Cl- on selectivity and removal performance of FCDI system for targeted ions. Findings revealed that current output surges up to 3.75 times and substantial improvement in removal efficiency of Cr (VI) by 30 %, F- by 37 % and Cl- by 39 % with the increase in activated carbon (AC) loading from 2.5 wt% to 7.5 wt%. The removal efficiency of Cr (VI) and F- linearly increases from 60.9 % to 92.5 % and 43.6 % to 80.2 %, respectively with an increase in applied voltage from 0.4 V to 1.2 V. However, further increase in applied voltage resulted in limited improvement in removal efficiency. In spite of low concentration of Cr (VI) and F- than Cl- in feedwater, this study unveiled a persistent selectivity sequence of Cr (VI) > F- > Cl- irrespective of operational conditions. However, there is inherent and universal trade-off relationships between Cr (VI), F- and Cl- selectivity and their removal efficiency. At high feedwater flowrate and low voltage, high ion selectivity of Cr (VI) and F- over Cl- was observed but at cost of low over all ion removal. Varying coexisting Cl- ions concentration significantly impacted the energy consumption as Cl- ion concentration increases from 0 to 600 mg L-1 in feed water, energy consumption in deionization increases from 0.03 to 0.26 kWh m(-3).Notably, prolonged operation yielded enhanced selectivity for Cr (VI) and F- over Cl- due to back diffusion of Cl- ions in feed water compartment from flow electrode chamber, suggesting the potential for optimizing selectivity via strategic adjustments to operational parameters. In conclusion, these findings significantly contribute to the knowledge base for FCDI technology, paving the way for its implementation in the selective removal of Cr (VI) and F- from brackish wastewater.