KOH-activated carbon felt with engineered pyridinic-N sites for enhanced phosphate removal via capacitive deionization

The treatment of phosphorus-containing wastewater has gained increasing attention due to stringent environmental regulations. This study developed a KOH-activated carbon fiber felt (KACFF) electrode for capacitive deionization (CDI) of phosphate, combining experimental characterization with density functional theory (DFT) calculations to unravel atomic-scale adsorption mechanisms. KOH activation at 800 °C engineered a hierarchical porous structure (1409.79 m 2 /g) with enriched pyridinic-N content (36.5 %), achieving a remarkable H 2 PO 4 − adsorption capacity of 15.65 mg/g at pH 4, which represents a 235 % enhancement versus the pristine activated carbon fiber felt (ACFF). Electrochemical tests demonstrated superior performance with a specific capacitance of 173.59 F/g, a 69.4 % improvement over pristine ACFF. DFT analysis revealed that the adsorption was dominated by synergistic ≡N7 → PO 4 3− coordination and N7-H + ···O23 = P hydrogen bonding, while hydrogen bonding from oxygen-containing functional groups on KACFF further strengthened the binding. Independent Gradient Model (IGM) and Reduced Density Gradient (RDG) analyses visually confirmed and quantified these non-covalent interactions. Hirshfeld surface analysis identified the critical interaction sites with localized electron density (N7-O23: 0.486 a.u.). The electrode maintained 91.9 % of its initial capacity over 10 cycles in an actual spent electroless plating solution, offering a high-performance and practical solution for phosphorus recovery.