Coupled faradaic-capacitive charge storage in cellulose fiber supported green Fe3O4 nanoparticle activated carbon functionalized electrodes for capacitive deionization: Performance, kinetic study and cost analysis

Capacitive deionization (CDI) is a promising low-energy technology for brackish water desalination; however, its performance is limited by sluggish ion transport, limited active sites, and poor electrode stability. Herein, a green synthesized Fe 3 O 4 nanoparticle-modified activated carbon (Fe 3 O 4 -AC) electrode supported on a three-dimensional cellulose fiber scaffold was developed to enhance Electrosorption performance. The hierarchical porous structure facilitates electrolyte penetration and ion transport, while Fe 3 O 4 nanoparticles provide reversible Fe 2 + /Fe 3+ redox-active sites, synergistically combining electric double-layer capacitance with pseudocapacitive charge storage. FESEM, TEM, XRD, FTIR, Raman, XPS, BET, EDX, and ICP-MS confirmed the successful incorporation of uniformly dispersed Fe 3 O 4 nanoparticles with preserved pore architecture and structural stability. Electrochemical analysis revealed a low charge transfer resistance of 5.67 Ω and an excellent coulombic efficiency of 99.99%. Under optimized conditions (1.2 V), the electrode achieved a salt adsorption capacity of 68.57 mg g −1 , a salt adsorption rate of 1.15 mg g −1 min −1 , and a low specific energy consumption of 1.94 kWh m −3 . Kinetic and mass-transfer analyses followed the pseudo-second-order model (k 2 = 2.20 × 10 −3 g mg −1 min −1 ) with a mass-transfer coefficient of 3.06 × 10 −3 min −1 , demonstrating efficient and sustainable CDI desalination.